CC BY-NC-ND 4.0 · Organic Materials 2020; 02(02): 182-203
DOI: 10.1055/s-0040-1708501
Review
The Author(s). This is an open access article published by Thieme under the terms of the Creative Commons Attribution-NonDerivative-NonCommercial-License, permitting copying and reproduction so long as the original work is given appropriate credit. Contents may not be used for commercial purposes, or adapted, remixed, transformed or built upon. (https://creativecommons.org/licenses/by-nc-nd/4.0/). (2020) The Author(s).

Metal Salen- and Salphen-Containing Organic Polymers: Synthesis and Applications

Sven M. Elbert
a  Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
,
Michael Mastalerz
a  Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
› Author Affiliations
Further Information

Publication History

Received: 13 December 2019

Accepted after revision: 04 February 2020

Publication Date:
08 June 2020 (online)

 


Abstract

The properties of organic polymeric materials can be chemically fine-tuned by the implementation of functional groups or units within the backbone. Especially the inclusion of coordinated metal centers offers a nearly infinite toolbox to adjust properties and thus potential applications. In particular, salen and salphen complexes are widely known to be highly efficient homogenous catalysts. They are also used as luminescent materials and devices or as supramolecular building blocks. This review focusses on the class of salen- and salphen-containing organic polymers, from 1D to 3D. Besides the comparison of synthetic polymerization methods, properties and applications are discussed, with an emphasis on porous 2D and 3D polymeric metal salphens and salens for heterogeneous catalysis and gas sorption.


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Biosketches

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Sven M. Elbert received his B.Sc. as well as M.Sc. from Ulm University. He conducted his studies for the Master thesis under the supervision of Michael Mastalerz, and got the Barbara-Mez-Starck award for his thesis. He joined the Mastalerz group moving to the Ruprecht-Karls-University of Heidelberg in 2013 to conduct his PhD studies (supported by a scholarship of Studienstiftung des Deutschen Volkes), obtaining the doctoral degree with distinction (summa cum laude) in 2018. Since then, he is the scientific coordinator in the Mastalerz group, focusing his research on porous materials as well as π-extended nonplanar molecules.

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Michael Mastalerz studied Chemistry at the Gerhard-Mercator University of Duisburg and after 9 semesters received his diploma (which was awarded by the University) in 2002. He followed Prof. Dr. Gerald Dyker to the Ruhr-University of Bochum to pursue his PhD studies, receiving his doctoral degree in 2005 with distinction (summa cum laude). After a short stay in industry he joined the group of Prof. Dr. Gregory Fu at the Massachusetts Institute of Technology in the United States in 2006 to work on asymmetric cross-coupling reactions. In 2007 he moved back to Germany to Ulm University affiliated to the group of Prof. Dr. Peter Bäuerle to start his independent career working on porous materials and molecules. He received his venia legendi in February 2013 and subsequently (April 2013) got a position at the Ruprecht-Karls-University of Heidelberg as a full professor. Currently, he is an ERC Consolidator Grant holder. His research focusses on organic cage compounds, porous polymeric materials, supramolecular chemistry, nonplanar extended aromatic molecules as well as the development of new synthetic methods.

Introduction

The imine condensation of two equivalents of salicylaldehyde with one equivalent of 1,2-ethylene- or phenylene diamine is the underlying reaction to synthesize the ligand classes called salens and salphens (sometimes also named salophens) ([Figure 1]). Since the first discovery of the structurally related acacen ligand in 1889 by Combes,[1] numerous N2O2 ligands[2] such as salans,[3] salalens,[4] salamos,[5] or their thio-derivatives[6] have been developed ([Figure 1]). Among all N2O2 ligands, the salens and salphens are probably the most commonly used ones.[7] The most prominent congeners are the Mn(III)-salens developed by Jacobsen (1991) and Katsuki (1994) for the enantioselective epoxidation of unfunctionalized olefins.[8] Inspired by this reaction, metal salen complexes have been successfully used as catalysts for a broad range of asymmetric reactions, such as hydroxylations,[9] cyclopropanations[10] and several other oxidation reactions,[11] aziridinations,[12] epoxide-ring opening reactions,[13] hetero-Diels–Alder reactions,[14] or trimethylsilylcyanations.[7a] [c] [15]

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Figure 1 Molecular structures of selected N2O2 ligands.[1] [8b]

As mentioned above, all the described ligands provide an N2O2 binding site in a square planar geometry, suitable for binding a large variety of metal ions well as ions of main group elements. With a few exceptions, such as the elements of the 7th period or the bigger alkali metals, salphen or salen complexes with nearly all other metals of the periodic system of the elements have been realized ([Figure 2]), such as with Li+,[16] Be2+,[17] Na+,[18] Mg2+,[19] Al3+,[20] Si4+,[21] P3+,[22] Ca2+,[23] Sc3+,[24] Ti4+,[25] V4+,[26] Cr3+,[27] Mn3+,[8b] Fe2+,[28] Fe3+,[29] Co2+,[30] Co3+,[31] Ni2+,[32] Cu2+,[33] Zn2+,[34] Ga3+,[35] Ge2+/4+,[36] Sr2+,[23] Y3+,[37] Zr4+,[38] Nb5+,[39] Mo4+,[40] Tc5+,[41] Ru3+,[42] Rh3+,[43] Pd2+,[44] Ag+,[45] Cd2+,[46] Sb5+,[47] Te4+,[48] La3+,[37] Ce4+,[49] Pr3+,[50] Nd3+,[51] Sm3+,[51] EU3+,[52] Gd3+,[51] Tb3+,[51] Dy3+,[51] Ho3+,[53] Er3+,[51] Tm3+,[53] Yb3+,[53] Lu3+,[53] Hf4+,[54] Ta5+,[55] W6+,[56] Re4+,[57] Os4+,[58] Ir3+,[59] Pt2+,[60] Au3+,[61] Hg2+,[46] Tl3+,[62] Bi3+,[63] Th4+,[64] U6+,[65] Np5+,[66] or Pu4+.[49]

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Figure 2 Periodic table of the elements. The elements which ions have been used for salen and salphen complexes are highlighted in green.

In contrast to metal salens, for which comparably simple chiral diamines can be used to introduce chirality, metal salphen complexes lack of sp3-hybridized centers and chirality can be introduced by the use of more sophisticated chiral building blocks such as biaryls,[67] helicenes,[68] and calixarenes,[69] or upon supramolecular chiral induction.[70] Therefore, salphen complexes are rarely used for enantioselective reactions. Despite this disadvantage, metal salphens were able to catalyze polymerizations[71] or fix CO2 as cyclic carbonates.[72] For the latter, Lewis acidic zinc-salphens have been used, which are able to coordinate reactants to the axial positions of the square-planar coordination sphere and thus activate them for nucleophilic attacks.[73] Besides mononuclear complexes, the formation of salens and salphens was also used to create multinuclear,[74] macrocyclic,[75] or cage-like ligands[76] and their corresponding metal complexes (for selected examples, see [Figure 3]). Such structures have been achieved by dynamic covalent chemistry (DCC).[77]

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Figure 3 Top: macrocyclic [3 + 3]- and [6 + 6]-salphen ligands reported by the groups of Nabeshima[75b] and MacLachlan.[75c] Bottom: cage-like [2 + 3]-salphen ligands reported by Nabeshima and coworkers.[76d] [76e]

Another approach to embed salen or salphen moieties into more complex structures is realized by the decoration of molecules with functional groups enabling additional coordinative binding sites and therefore being able to act as building blocks for supramolecular boxes,[78] coordination cages,[79] coordinatively bound macrocycles,[80] or metal–organic frameworks (MOFs).[81] The immobilization of soluble salens or salphens in polymeric materials such as MOFs nicely allows to exploit the good catalytic activity in a heterogeneous approach.[81s] [t] [v] [x] [z] [aa] [ab] [ac] Similar to that, these MOFs were used for gas separation[81u] or storage,[81y] chiral resolution,[81w] and many more. The insoluble salen-containing MOFs can efficiently be recycled after use, which is a common technique when expanding the application of soluble complexes to the heterogeneous phase by immobilization.[82] However, most MOFs including the aforementioned salen MOFs contain metal nodes which suffer from instability against aqueous acidic or basic media. This limits their potential applications and typically stabilization techniques such as post-synthetic coating or guest addition have to be used to circumvent a quick degradation.[83]

This review highlights salen- and salphen-based materials embedded in fully covalent-bonded polymeric backbones. The objective of this review is to summarize and compare the different synthetic strategies, to examine the advantages and disadvantages of the different approaches based on the resulting material properties, and to give an overview of the structures reported to date. Furthermore, the potential applications of these compounds will be highlighted with selected examples. The review does not cover those salphen-containing superstructures based on the formation of coordinative bonds, which have been discussed in other recent excellent reviews.[84]


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Structural Dimensionality

We arbitrarily divide salen and salphen polymers into 1D, 2D, or 3D by their idealized topology in terms of the propagation of the monomeric building blocks rather than by the real orientation in space by coiling, folding, steric effects, or contortion. First of all, we give an overview of how the different polymers have been synthesized and then discuss their properties.

1D-Salen and Salphen Polymers

The construction of salen- or salphen-containing polymers can in principal be divided into two main synthetic strategies. One strategy is based on the formation of the salen or salphen pocket during the condensation (polymerization) reaction with or without simultaneous complexation of metal ions. The other strategy is to use the preformed salen or salphen moieties, acting as monomers, in polymerization reactions like electropolymerizations,[85] cross-coupling reactions,[86] or comparable polymerization reactions.[87] Both strategies have been applied in the synthesis of 1D salen and salphen polymers.

The first polysalphens were described in the late 1950s by Marvel and Tarköy, synthesized by the condensation of methylene-[88] and sulfone-bridged[89] bissalicylaldehydes with ortho-phenylene diamine ([Scheme 1]). By treatment of the metal-free polymers with THF-solutions of Zn2+, Cu2+, Fe2+, Ni2+, Co2+, and Cd2+ salts, a large variety of metal-containing salphen polymers were obtained. By the assumption that the polymers are pure, monodisperse, and do not contain solvate or other impurities, the polymers had lengths up to 45 repeating units according to the interpretation of elemental analysis results.[88] The thermal stabilities of the polymers were investigated, showing that the coordinated metal centers had a stabilizing effect. Furthermore, an enhancement in thermal stability was achieved by formal exchange of the methylene bridges by sulfone bridges. This was attributed to the higher acidity of the phenolic oxygen atom due to electron-withdrawing effects of the para-substituted sulfone in comparison to the methylene tethers.[88] [89] It must be noted that at that time the analyses of the thermal stability relied on weight losses exclusively. However, in principle the observations could also be attributed to, for example, the release of bound water molecules.

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Scheme 1 Syntheses of the first polysalphens reported by Marvel and Tarköy.[88] [89]

A similar approach based on 2,5-dihydroxyterephthalaldehyde ([Scheme 2]) was reported at the beginning of the 1970s by Manecke and Wille.[90] Here, the structural variety was achieved by using different diamines for the condensation. The conductivity of the fully conjugated 1D-polymers was measured and the authors concluded that the complexation of the metal centers is beneficial for the conductivity by increasing the rigidity of the polymeric backbone. The metalated polymers had up to five times higher conductivities (10−10 S m−1) in comparison to the free ligands, but regarding the nature of the metal used no trend was found.[90b]

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Scheme 2 Polysalens and polysalphens based on the condensation of 2,5-dihydroxyterephthalaldehyde with diamines.[90]

Besides the early examples of salphen polymers synthesized by condensation of amines and aldehydes, the second strategy used was the electropolymerization of presynthesized salen and salphen compounds introduced by Goldsby and coworkers in 1988.[91] This so-called potentiostatic method gave a structurally less defined nickel(II) salen-containing film on the surface of a platinum disc electrode ([Scheme 3]).[91] [92] Later, this approach was reinvestigated by others.[93] Within these studies it was revealed that for the aforementioned film “…the electropolymerization of [Ni(salen)] is ultimately a ligand-based process that takes place through a mixture of o- and p-linking of the phenyl rings…” ([Scheme 3]).[93b] Despite these irregular structures, a clear difference of the chemical as well as physical properties of the synthesized polymer films in comparison to aggregated individual Ni-salen complexes was confirmed by IR, UV/vis, and EPR spectroscopy and the redox and charge conduction behaviors were studied.[93b]

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Scheme 3 Polysalens and polysalphens synthesized by electropolymerization.[91] [92] [93] [94]

To obtain a 1D polymer chain in a regioselective manner, Reynolds and coworkers used 3,4-diaminothiophene as well as terthiophenes to synthesize Ni-and Cu-salphens, which were electropolymerized to give conjugated polythiophenes with salphens[94a] and salphen/crown-ether moieties[94b] ([Scheme 3]). Swager and coworkers followed a slightly different approach to achieve control over the electrochemical properties of the polymers.[94c] They synthesized a Co-salen moiety with thienyl substituents in the para-position to the phenolic oxygen ([Scheme 3]).[94c] The regioselective electropolymerization on the thiophene's vacant α-position was proven by hindered film growth when this position was blocked by methyl substituents.[94c] This detailed structure–activity relationship was further studied on a series of electropolymerized materials derived from poly(3,4-ethylenedioxythiophene) (PEDOT)-substituted Cu(II), Ni(II), and U(VI)O2 salens ([Scheme 3]). The interchain spacing was modulated by the use of different sterically bulky ethylene- and phenylene-diamines. It was found that large chain–chain interactions lower the oxidation potentials of the salen units due to destabilizing interactions of the corresponding HOMOs, which lead to the formation of spin-paired π-dimers upon further oxidation. In contrast to that, sterically shielded, noninteracting chains create radical cations and subsequent dications by oxidation.[95] It is worth mentioning that the potentiostatic method has been used exclusively for the synthesis of 1D polymer films so far. Also other nanostructures such as nanowires[96] or nitric oxide sensors are based on this type of polymerization.[97] Additionally, this approach was used to develop composite materials of metal salphen or salen complexes with multiwalled carbon nanotubes (MWCNTs) as capacitive materials[98] and in the synthesis of high-capacity films (see discussion below).[99]

As known from nature, 1D polymers can also create superstructures of high complexity such as the DNA double helices.[100] Already in 1996, Katz and coworkers synthesized fully conjugated 1D helical ladder polymers based on the condensation of a [6]-helicene bissalicylaldehyde with 1,2-phenylene diamine in the presence of nickel(II) acetate ([Scheme 4]).[68a] According to the integration of characteristic signals in 1H NMR-spectra as well as by gel permeation chromatography (GPC) analyses the average molecular mass of the polymers was estimated to be about 7,000–7,400 Da, which corresponds to approximately eight repeating units. The fully conjugated character of the metal salphen helix was indicated by a bathochromic shift of 114 nm of the metal-to-ligand charge transfer band in comparison to simple nickel(II) salphen complexes in the UV/vis spectra.[68a] At the most redshifted absorption maximum (λ abs = 595 nm), a large circular dichroism (CD) of Δε = 105 was found. Furthermore, by the variation of the substitution pattern on the enantiopure regioisomeric [6]-helicene bissalicylaldehydes, different winding motifs within the corresponding chiral helicenes were realized as depicted in [Scheme 4].[68b]

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Scheme 4 Synthesis of helical polymers from enantiomerically pure bissalicylaldehydes and ortho-phenylene diamine.[68] Sections of the polymeric helices are depicted as MM2-optimized models.

Another example of helical 1D metal salphen polymer was obtained by the condensation of chiral 3,3′-diformylbinaphthol derivatives with various α,ω-diamines and subsequent complexation of Zn2+- or Mn2+-ions ([Scheme 5]).[101] The chiral helices showed clear differences in CD-spectroscopy by decreased Cotton effects at λ = 230 nm for the helical polymers in comparison to mononuclear model compounds. Furthermore, the aforementioned band at λ = 230 nm, which is assigned to the naphthalene subunits, redshifted approximately about Δλ = 30 nm, which is a hint for a close proximity of adjacent naphthalene units as proposed by geometry optimized models (MMFF94 and MM2) ([Scheme 5]). By GPC analyses, average molecular masses of up to 13,000 Da (approximately 10 repeating units) were estimated.[101b] Interestingly, the helical induction of the binaphthol units is not transferred to polymer chains when copolymerized with biphenyl bissalicylaldehyde in various ratios (from 100:0 to 0:100) in the presence of a soluble phenylenediamine and nickel(II) acetate ([Scheme 5]).[67]

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Scheme 5 Synthesis of helical polymers from enantiomerically pure dinaphthyl-bissalicylaldehyde and different α,ω-diamines.[101] Section of the polymeric helix is depicted as a geometry-optimized model (MMFF94, MM2).

Also worth mentioning is a superstructure that was presented by Houjou and coworkers in 2003. Two strands of linear polysalicylimines build a double-helix by the formation of salen-like moieties after complexation of Zn2+, Ni2+, Co2+, and Cu2+ ions. The resulting polymers formed microspheres with diameters between 0.5 and 1.4 µm indicating ordered materials on the macroscopic scale ([Scheme 6]).[102]

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Scheme 6 Left: synthesis of Houjou's linear polyimines interconnected by salen-like moieties.[102] Right: SEM micrographs of Cu(II) polymers creating microspheres. (Parts of the scheme are adapted with permission from Ref. 102. Copyright © 2003 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim.)

In 2002, Lavastre et al. used palladium-catalyzed Sonogashira–Hagihara cross-coupling reactions the first time to synthesize 1D metal salphen polymers ([Scheme 7]).[103] A library of different linear polysalphens with Ni(II)- as well as Zn(II)-metal centers and various dialkynes as linking units (among several polymers without salphen units) were generated. The corresponding polymers emitted light in solution but were unfortunately no potential candidates for organic light-emitting diodes (OLEDs) due to quenched solid-state emission.[103]

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Scheme 7 Synthesis of 1D-polysalphens by palladium-catalyzed Sonogashira–Hagihara cross-coupling.[103] [104]

MacLachlan and coworkers used a similar method to obtain soluble poly(salphen-alkyne)s with Zn2+, Ni2+ as well as VO2+ centers.[104a] GPC analyses revealed high molecular masses of 17,000 Da for the Ni-salphen and up to 84,000 for the VO-salphen polymer ([Scheme 7]). Furthermore, by the variation of the substitution pattern of the halogens at the salphen units, helical or zig-zag-type polysalphen chains with Zn2+, Ni2+, and Cu2+ [104b] as well as with Pt2+ centers have been realized.[105] Furthermore, different external substituents like saccharines were used as well.[105] [106]

An interesting example of a 1D salphen polymer with an ordered 3D structure was presented by Liu and coworkers in 2015 by condensing a bissalicylaldehyde and phenazine diamine with zinc acetate.[107] It is suggested that due to the extended aromatic backbone of the precursors, the resulting polymer chains pack in an ordered fashion to form crystalline supramolecular 3D networks. Crystallinity of the material was proven by scanning electron microscopy and powder X-ray diffraction (PXRD; [Scheme 8]). Interestingly, it was reported that these polymers are permanently porous, having a specific surface area of SABET = 206 m2/g.[107] The non-local density functional theory (NLDFT) pore size distribution showed a maximum in the microporous regime with d pore = 1.4 nm and mesoporous proportions with d pore = 2.5 and 4.4 nm. Unfortunately, no structural model was suggested on how these highly defined pores are generated by the packing of the linear polymer strands in the crystalline state.[107]

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Scheme 8 Top: synthesis of Liu's 1D Zn(II) salphen polymer from a naphthalene bissalicylaldehyde and 2,3-diaminophenazine. Bottom: SEM micrographs of the obtained microspheres (a) and rods (b), and PXRD pattern of the corresponding material (c).[107] (Parts of the scheme are adapted with permission from Ref. 107. Copyright © 2015 The Polymer Society of Korea and Springer.)

Other methods such as the epoxy polymerization[108] or Gilch polymerization[109] have been used to synthesize 1D polysalphens. For the polymers derived from these methods as well as for several ionic polymers,[110] no structural suggestions were made.


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2D-Salen and Salphen Polymers

By structural modification of the molecular building blocks, 2D salen and salphen polymers can be realized. Although the first 1D polymers were reported in 1954 (see above), half a century had to pass until Gothelf and coworkers presented the first 2D salen polymer in 2005.[111] The condensation of a C 3-symmetric trissalicylaldehyde and ethylene diamine[111] gave a metal-free network ([Scheme 9]) with at least locally ordered structures as determined by PXRD. Interestingly, when condensed with chiral ethylene diamines, the presence of Mn(II)-acetate was necessary to observe a comparable PXRD signal pattern.[111] Ding, Wang, and coworkers optimized the reaction conditions of the same reaction in 2017 and synthesized a structurally related material with high crystallinity by solvothermal methods (dioxane, ethanol, acetic acid, 120 °C, 3 d).[112] By NLDFT calculations a pore-size distribution with a maximum diameter of 2.5 nm was found, fitting to the expected mesopore size of 2.9 nm according to the proposed eclipsed alignment of the 2D polymer sheets.[112] After subsequent metalation, the M-salen covalent organic frameworks (COFs) with Cu(II)-, Ni(II)-, Zn(II)-, Co(II)-, and Mn(II)-metal centers were obtained. It is worth mentioning that for both the unsubstituted (pH = 1–13) and Co(II)-salen (pH = 2–13) COFs, high chemical stabilities were reported.

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Scheme 9 Structural motifs of salen-containing 2D polymers and calculated as well as experimentally measured pore sizes.[111] [112]

Liu and Cui just prior to Yang and coworkers synthesized further salen COFs using salicylaldehydes with 1,3,5-triphenylbenzene as the core structure in condensation reactions ([Scheme 10]).[113] Liu and Cui first synthesized chiral Zn-salen-COFs (CCOF-3) and pointed out that the stability of the COFs differs with changing substituents (H vs. tBu) of the salicylaldehyde units with a higher stability of tert-butyl-substituted COFs due to “kinetic blocking.”[113a] After transmetalation,[114] mixed metal networks with Zn(II) and one or two other metal ions were obtained (CCOF-4-M). The exact metal ratios were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES).

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Scheme 10 Structural motifs of Liu and Cui's CCOF-4-M series, and Yang's COF-salen-M polymers with triphenylbenzene core structures and calculated as well as experimentally obtained pore sizes.[113]

Yang and coworkers followed the approach of first synthesizing the metal-free COF, which was postmetalated in a second step with Co2+, Mn3+, Cu2+, and Zn2+ ions to give the corresponding COF-salen-M. For the COF-salen-M series once more a good agreement of an NLDFT calculated pore-size distribution maximum (1.86 nm) with that from the model (calculated pore diameter = 1.9 nm) was found when a fully eclipsed stacking of the 2D polymer sheets was assumed.[113b] The COF-salen-M series did not decompose in a broad pH range (from 1 M HCl to 1 M NaOH solutions) as proven by PXRD analyses of the immersed COF samples.

As it has been mentioned above, the sheets of the 2D salphen and salen COFs pack in an eclipsed fashion, thus in theory making the metal centers inaccessible for substrates of any kind. To avoid such a packing in combination with an ideal close arrangement of salphen metal centers, we synthesized Ni3- and Cu3-MaSOFs ([Scheme 11])[115] based on a triptycene trissalicylaldehyde.[116] Indeed, by single-crystal X-ray diffraction (SCXRD) structure analyses of discrete trinuclear model complexes ([Scheme 11], right), metal-to-metal distances of d M–M = 6.7 Å were found, which according to Zhang should be ideal to bind CO2 synergically.[117] Unfortunately, the corresponding polymers Ni3- and Cu3-MaSOFs did not give any crystalline material with honeycomb 2D-sheets but were rather amorphous ([Scheme 11]). The absence of highly ordered pores was also confirmed by the QSDFT (quenched solid DFT) pore-size analysis revealing that with 1.1–1.2 nm these are nearly half in size of what would have been expected for the idealized structure. Nevertheless, IR comparison of the MaSOFs with their molecular model compounds suggested the existence of well-defined trinuclear units within the polymers causing a high affinity towards CO2, which will be discussed below.

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Scheme 11 Synthesis and structural representation of the M3-MaSOF series. Box: single-crystal X-ray structure of a trinuclear nickel model complex with a metal-to-metal distance of 6.74 Å. (Parts of the scheme are adapted with permission from Ref. 115. Copyright © 2019 American Chemical Society.)

Similar to 1D polysalphens, 2D polysalphens have also been synthesized by palladium-catalyzed cross-coupling reactions.[118] Surprisingly, despite the large toolbox of different coupling reactions as well as an unlimited stock of molecular building blocks, to the best of our knowledge, exclusively the Sonogashira–Hagihara cross-coupling of 1,3,5-triethynylbenzene with halogenated salens or salphens has been used for this purpose so far ([Scheme 12]).[118] The polymers were permanently porous and mainly used in heterogeneous catalysis as discussed below.

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Scheme 12 Overview of 2D salen- and salphen-containing organic polymers made by Sonogashira–Hagihara cross-coupling reactions. Box: cut-out of the connecting unit.[118]

2D salen and salphen polymers were also obtained by radical polymerizations of vinyl- or allyl-substituted salphens[119] and interfacial polymerization[120] by cross-linking of linearly connected polymer chains. These polymers were of amorphous nature and have been used for the catalytic decomposition of H2O2 as in the case of Gupta's N,N-BSPDA-based beads (see discussion below) as well as the activation of molecular oxygen in the aerobic oxidation of cumene.[120]

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Scheme 13 Syntheses and structural motifs of Mastalerz's MaSOF-1 and -2, as well as Fang's JUC-508, JUC-509, and the corresponding metalated networks.[123] [124]

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3D-Salen and Salphen Polymers

The majority of all COF structures are based on 2D-sheets and a much smaller proportion of COFs is three-dimensional.[121] Therefore, it is not surprising that only a few 3D polysalphens have been reported till date. In 2012 Mastalerz synthesized the first 3D salphen frameworks by the condensation of a tetraphenylmethane tetrakis-salicylaldehyde[122] and 1,2-phenylene diamine in the presence of Ni(II)- or Zn(II)-acetate ([Scheme 13]).[123] The resulting MaSOFs were analyzed by IR as well as 13C-CP-MAS spectroscopy, suggesting a high degree of polymerization because nearly no remaining aldehyde moieties were detected. Despite their amorphous nature, both MaSOFs were isostructural in pore size (4.2 Å) and specific surface areas (SABET = 630–647 m2/g) (see discussion below).

In 2019, Fang and coworkers reacted halogenated ortho-phenylene diamines with Mastalerz's tetraphenylmethane-based tetrakis-salicylaldehyde under solvothermal conditions and obtained the highly crystalline COFs JUC-508 (Hal = F) and JUC-509 (Hal = Cl) ([Scheme 13]).[124] These networks retained their crystallinity after complexation with Mn2+, Cu2+, and Eu3+ ions. PXRD analyses in combination with modeling revealed for all COFs noninterpenetrated networks of a dia topology.[124] When an extended tetraphenylmethane core with one more phenyl group[122] or a silicon-centered analogue was used, sevenfold interpenetrated salphen COFs resulted, again with the dia topology ([Scheme 14]).[125] In this case, an in-situ metalation upon condensation in addition to the postmetalation of presynthesized COFs gave networks with high degrees of crystallinity.[125]

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Scheme 14 Cui's COF-1, COF-2, COF-2-Zn, and COF-2-Zn.[124]

It is worth mentioning that an inverse building block design was reported based on tetraaminotetraphenylmethane with a formyl-substituted Mn-salen ([Scheme 15]).[126] The obtained materials showed gel-like behavior and have been used in the asymmetric kinetic resolution of a racemic mixture of R- and S-1-phenylethanol.

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Scheme 15 Synthesis of a chiral Mn-salen imine gel. Inset: photograph of Mn-salen imine gel (0.02 mol/L in DMSO).[126] (Adapted with permission from Ref. 126. Copyright © 2015 The Royal Society of Chemistry.)

Similar to MaSOF-1 and 2 as well as the M3-MaSOFs, the M-MaSOF50/100 series was synthesized via the imine condensation approach in the presence of the corresponding metal salts ([Scheme 16]).[127] For this purpose, a triptycene-based hexakis-salicylaldehyde needed to be synthesized by a sequence of a sixfold Suzuki–Miyaura cross-coupling, a sixfold Duff formylation, and finally a sixfold demethylation. This hexakis-salicylaldehyde was used to synthesize a series of nearly isostructural MaSOFs with Zn(II), Ni(II), Cu(II), Pd(II), and Pt(II) metal centers.[127] Independent from the reaction temperature, the MaSOF50 and MaSOF100 series showed comparable narrow pore-size distributions with micropores of 0.55–0.59 nm in diameter.

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Scheme 16 Syntheses of MaSOF50 and MaSOF100 series.[127]

Besides the polymers obtained by imine condensations, the Sonogashira–Hagihara cross-coupling of tetrakis(4-ethynylphenyl)methane with different halogenated salen complexes was also used to synthesize 3D polymers. In 2013, the Son group synthesized the first 3D Cr3+- and Al3+-salen networks (M-MON series) by cross-coupling methods of tetra(4-ethynylphenyl)methane with trans-cyclohexyldiamine-based diiodo-salens ([Scheme 17]).[128] The insoluble polymers were investigated by 13C-MAS-NMR as well as X-ray photoelectron spectroscopy and PXRD, revealing their amorphous nature due to the irreversibility of the cross-coupling conditions. In accordance with their 2D-analogues, the M-MON series possess permanent porosity and were used as heterogeneous catalysts (see discussed below).

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Scheme 17 Syntheses of 3D salen- and salphen-containing organic polymers by Sonogashira–Hagihara cross-coupling reactions.[128]

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Material Properties and Applications

Depending on the molecular structure, the dimensionality, and the nature of the metal centers of the salen and salphens units, different materials properties result that lead to certain potential applications, which are discussed within the next section.

Conductive Films

Salen or salphen films prepared by electropolymerization commonly find application as conductive materials. One of the first examples was presented by Dahm and Peters in 1994 ([Scheme 3]).[93a] In this study, the catalytic reductions of iodoethane and 2-iodopropane to the corresponding ethyl- and 2-propyl radicals by the polysalphen films were studied and the mixture of subsequent products (different alkanes and the corresponding alkenes) was investigated. A comparison of the product distributions of the nickel(II)-salen coated electrode with those of an uncoated one revealed that the film-coated electrodes exclusively generate products through a radical mechanism while the reaction on the uncoated electrode occurs via a cationic mechanism, demonstrating the significance of the salen polymer film in such transformations.[93a]

In several studies the potential applications of salen or salphen/MWCNT composite materials as supercapacitors have been investigated. A direct relationship of the polysalen/MWCNT-ratio and the capacity of the resulting material was found,[98c] and optimized films possessed between 3.8 and 8 times higher capacities compared to uncoated MWCNTs[98a] [99a] with specific capacitances of up to C = 200 F/g for a poly-Ni(II)salphen coated as well as C = 150 F/g for a poly-Ni(II)salen coated MWCNT electrode.[99b] The latter also gives insight into the modulation of the electronic properties upon incorporation of salen or salphen backbones into conductive polymers. This was nicely exploited, for example, with conductive crown-ether decorated Ni(II)- and Cu(II)-salen films, which have been used for molecular sensing of nanomolar amounts of metal ions (Li+, Na+, Mg2+, Ba2+) and other nonionic species.[94b] In another study, nitric oxide (NO(g)) was detected by a cobalt-salen-based PEDOT-substituted film.[97] For this purpose, the as-mentioned cobalt-salen-based PEDOT polymers were electropolymerized onto an interdigitated microelectrode array (black in [Figure 4], top) and the difference in the resistance (ΔR) between the parent sensor (R 1) and the sensor under a stream of NO gas (R 2) was evaluated (black and red curves in [Figure 4], bottom). The comparison to a similar setup with an electropolymerized PEDOT without salen units (blue curve, [Figure 4], bottom) clearly indicated the role of the Co-salen moieties as the recognition sites of the sensor films.[97]

Zoom Image
Figure 4 Top: schematic representation of interdigitated microelectrode array with an electropolymerized Co(III)-salen PEDOT copolymer as a sensor layer for NO sensing. Bottom: differential resistance (R 2/R 1 , see top figure) against time with various NO(g) concentrations. Black: unconditioned poly-Co(III)-PEDOT film; red: conditioned (at 0.262 V) poly-Co(III)-PEDOT film: blue: PEDOT film without salen moieties.[97] (Adapted with permission from Ref. 97. Copyright © 2006 American Chemical Society.)

#

Heterogeneous Catalysts

The use of salens as well as salphens in homogenous catalysis inevitably led to the development of polymeric salens or salphens as heterogeneous catalysts. As early as in 1983, Wöhrle and coworkers demonstrated that a simple Co(II)-salen polymer can catalyze the valence isomerization of quadricyclane to norbornadiene, which has been proved to be superior for example to Co(II)-porphyrin polymers ([Scheme 18]).[129]

Zoom Image
Scheme 18 Valence isomerization of norbornadiene to quadricyclane catalyzed by a Co(II) salen polymer.[129]

Gupta et al. constructed N,N-bis(3-allyl salicylidene)o-phenylenediamine (N,N-BSPDA) based polymer beads ([Scheme 19]). It is worth mentioning that the polymer beads were among one of the first polymeric salphen materials with permanent porosity proven by gas sorption ([Table 1]).[119a] Depending on the cross-linker, specific surface areas up 380 m2/g have been obtained. After Co(II) complexation, the beads became catalytically active for the decomposition of H2O2.[119a]

Zoom Image
Scheme 19 Gupta's N,N-BSPDA-based beads by radical polymerization. Right: SEM micrograph of the corresponding bead after swelling. (Adapted with permission from Ref. 119a. Copyright © 2003 Elsevier Science B. V.)
Table 1

Porous salen- and salphen-containing organic polymers used as heterogeneous catalysts

Name

Metal

SABET [a] [m2/g]

Catalytic reaction

Yield [%] up to[b]

Ref.

JUC-509-Cu

Cu

957

Dismutation of O2 •−

100[c]

[124]

JUC-509-Mn

Mn

1016

Dismutation of O2 •−

∼50[c]

[124]

JUC-509-Eu

Eu

942

Dismutation of O2 •−

∼10[c]

[124]

CCOF-4-Mn

Zn/Mn

614

Asym. epoxidation of alkenes

79[d] (97% ee)[e]

[113a]

CCOF-4-V

Zn/V

547

Asym. cyanation of aldehydes

79[d] (94% ee)[e]

[113a]

CCOF-4-Co

Zn/Co

628

Asym. Diels–Alder reaction

87[d] (96% ee)[e]

[113a]

CCOF-4-Fe

Zn/Fe

566

Asym. epoxidation of alkenes

71[d] (92% ee)[e]

[113a]

CCOF-4-Cr

Zn/Cr

633

Asym. aminolysis of trans-stilbenes

76[d] (86% ee)[e]

[113a]

CCOF-4-Cr-Mn

Zn/Cr/Mn

616

Alkene epoxidation with subsequent

epoxide aminolysis

84[d] (91% ee)[e]

[113a]

Co/Salen-COF

Co

854

Henry reaction

92[f]

[112]

COF-salen-Co

Co

1065

CO2 addition to epoxides

91[d]

[113b]

COF-salen-Co(III)

Co(III)

836

Epichlorohydrin hydration

98[d]

[113b]

COF-salen-Mn

Mn

961

Styrene epoxidation

74[d]

[113b]

COF-salen-Cu

Cu

1258

CO2 addition to epoxides

20[d]

[113b]

COF-salen-Zn

Zn

1032

CO2 addition to epoxides

90[d]

[113b]

MsMOP-1

Pd

554

Pd-catalyzed cross-coupling reactions

99[d]

[118a]

Cr-MON

Cr

522

CO2 addition to epoxides

66[g]

[128]

Al-MON

Al

650

CO2 addition to epoxides

71[g]

[128]

Co-MON

Co

580

CO2 addition to epoxides

94[g]

[128]

Co-CMP

Co

965

CO2 addition to epoxides

81[h]

[118b]

Al-CMP

Al

798

CO2 addition to epoxides

91[h]

[118b]

Mn-POP

Mn

836

Alkene epoxidation

>99[d]

[118d]

POP-1-Co

Co

370

CO2 addition to epoxides

97[g]

[118e]

POP-1-Cr

Cr

732

CO2 addition to epoxides

98[g]

[118e]

[email protected]

Al

590

CO2 addition to epoxides

99[d]

[110c]

Al-CPOP

Al

136

CO2 addition to epoxides

>99[d]

[132]

[email protected]

Zn

423

CO2 addition to epoxides

97[f]

[133]

N,N′ -BSPDA Bead Type I

56

H2O2 decomposition

[119a]

N,N′ -BSPDA Bead Type II

72

H2O2 decomposition

[119a]

N,N′ -BSPDA Bead Type III

94

H2O2 decomposition

[119a]

N,N′ -BSPDA Bead Type IV

132

H2O2 decomposition

[119a]

N,N′ -BSPDA Bead Type V

380

H2O2 decomposition

[119a]

a Derived from N2 sorption experiments at 77 K.


b For selected examples.


c Clearance rate.


d Determined by GC analyses.


e Determined by HPLC analyses.


f Isolated yield based on benzaldehyde.


g Determined by 1H-NMR analyses.


h Isolated yield.


Mn-POP, a 2D salen polymer obtained via the above-described Sonogashira–Hagihara approach (for the structure see discussion above; [Scheme 12]), was also permanently porous (SABET = 836 m2/g, [Table 1]) and was capable to catalytically transfer olefins to the corresponding epoxides with conversions of up to 99%, similar to Vancheesan's M(III) polysalphens ([Scheme 20]).[118d] [130]

Zoom Image
Scheme 20 Epoxidation of alkenes catalyzed by Mn-salen polymer Mn-POP.[118d] aConditions: Mn-POP (0.1 mol%), O2, isobutyraldehyde (7 eq.), dichloroethane, 25 °C, 5 h. bConversion determined by GC.

A large number of metal-salen and -salphen polymers have been used in the synthesis of cyclic carbonates from epoxides upon CO2 capture, as it is known from the corresponding discrete complexes.[72a] [128] [131] The key advantage to their smaller congeners is their recyclability as a heterogeneous catalyst.[110a] [118e] [119b] [132] For example, Deng et al.'s CMP series (for the structure see discussion above; [Scheme 12]) was able to be used for cyclic carbonate formations in up to 22 cycles even under ambient conditions.[118b]

Polymers such as POP-1-Cr (SABET = 732 m2/g),[118e] POP-1-Co (SABET = 370 m2/g),[118e] Al-CPOP (SABET = 136 m2/g)[132] or Co- (SABET = 580 m2/g),[118b] and Al-CMP (SABET = 798 m2/g)[118b] possess permanent porosity as indicated by nitrogen sorption (see discussion below; [Table 1]). High turnover frequencies (TOF) of up to 1875 h−1  [113b] and turnover numbers (TONs) of over 200 have been achieved for example in addition reactions of CO2 to epoxides.[133] The highest TONs (496) were achieved with an ionic ammonium polymer bearing salen Co(III)-salens[110b] or with POP-1-Cr (693).[118e]

One interesting approach to capture CO2 that is different from the one described above is realized with ionic Zn(II)-salen polymer [email protected], which converts the CO2 to synthetically more valuable formamides in high yields of up to 99% from amines using PhSiH3 as an in-situ reductant ([Scheme 21]).[110c] The high reactivity was attributed to the Lewis acidic nature of the Zn(II) centers in salen and salphen moieties, as has been reported for other Zn(II) salphens too.[72a] [73a] [b] [78a] [134]

Zoom Image
Scheme 21 Different methods to capture CO2.[110c]

It is obvious that asymmetric transformations were realized by using chiral salen polymers. For example, Takata and coworkers reported the asymmetric addition of diethylzinc to aldehydes using a chiral helical Zn(II)-polysalen (structure in [Scheme 5]) in high enantiomeric excess (ee) of 95%, while the use of the related enantiomerically pure mononuclear Zn(II) salphen resulted in a product with an ee of only 5% ([Scheme 22]).[101a] [c]

Zoom Image
Scheme 22 Asymmetric addition of Et2Zn to aldehydes, catalyzed by Takata's poly Zn(II) salen helix.[101a] [101c]

An isostructural chiral helical poly-Mn(III)-salen was used in the asymmetric epoxidation of styrenes, but only delivered a low selectivity of 17% ee,[101b] which is higher than that for the homogenous pendant (1–6% ee), but unfortunately lower than that of other chiral salen complexes (ee up to 98%).[135] This was improved in 2005 by Gothelf and coworkers using also chiral Mn(III) salen polymers (structure in [Scheme 9]; see above), but this time chirality was introduced by enantiomerically pure diamines.[111] The transformation of cis-methyl styrene was performed with a cis/trans-ratio of 12:1 and an ee of 67%. The epoxidation by porous salphen polymers was further improved by Cui and coworkers in 2017.[113a] The porous Mn(III)- (CCOF-4-Mn; SABET = 614 m2/g) alongside the Fe(III)-salen polymers (CCOF-4-Fe; SABET = 566 m2/g; for structures, see [Scheme 10]) were able to convert 2,2-dimethyl-2H-chromene with iodosylbenzene (or derivatives thereof) as an oxidant to the corresponding epoxides with up to 97% ee ([Scheme 23]).[113a] In this publication, the full potential of embedding different metal ions into the pockets of salens (here CCOF-4-M) for different reactions was demonstrated. For instance, other asymmetric transformations were achieved with high yields and high ee values, such as the cyanosilylation of aldehydes (M = V; up to 94% ee), Diels–Alder reactions of 1-amino-substituted butadienes and α-amino acroleins (M = Co; up to 96% ee), aminolysis of trans-stilbene oxide with anilines (M = Cr; up to 96% ee), or epoxidation of alkenes followed by a subsequent ring opening in the presence of anilines to obtain amino alcohols (M = Cr, Mn; up to 91% ee) ([Scheme 24]).

Zoom Image
Scheme 23 Asymmetric epoxidation of styrenes catalyzed by various poly M-salens (for synthesis and structures of the corresponding polymers, see [Scheme 5],[101b] [Scheme 9],[111] and [Scheme 10]).[113a]

All polymers were porous (SABET up to 683 m2/g, [Table 1]) and their reusability was exemplarily proven for the cyanation reaction: no leaching of metal ions was detected by ICP-OES and the crystallinity remained even after five runs as investigated by PXRD.[113a]

Other examples are Co(III)-salen polymers catalyzing the Henry reaction of substituted benzaldehydes and nitromethane[112] or the hydration of epichlorohydrin (EPH).[113b]

Zoom Image
Scheme 24 Asymmetric transformations catalyzed by different members of Cui's CCOF-4-M series (for synthesis and structures, see [Scheme 10]).[113a]

Interestingly, Pd(II)-salen polymers have been reported to catalyze cross-coupling reactions such as Suzuki–Miyaura reactions of various arylhalides with phenylboronic acid or Heck reactions of arylhalides with styrene or ethyl acrylate in yields up to 99%.[118a] This rare and unusual use of metal salphens in C–C cross-coupling reactions[136] underlines that the broad potential of porous polymetal-salphens still needs to be explored.


#

Porous Materials for Gas Sorption and Separation

Most of the above-discussed catalytically active metal salen or salphen polymers are permanently porous, with high surface areas, which is favorable to provide accessible catalytic centers. The data of nitrogen sorption experiments of these salen and salphen polymers are summarized in [Table 1]. Among the metal-containing porous salen and salphen polymers, Yang's COF-salen-Cu (see above; [Scheme 10]) that was used for catalytic CO2 fixation and the corresponding metal-free COF-salen precursor have with SABET = 1,258 and 1,646 m2/g, respectively, the highest reported specific surface areas of all here-discussed polymers ([Scheme 24]).[113b] Besides for heterogeneous catalysis, porous salphen and salen polymers were investigated for gas sorption and separation.

As discussed above, Mastalerz et al. reported 3D porous networks based on a tetraphenylmethane unit as the building block in 2012 (structure in [Scheme 13]). The resulting porous salphen polymers were named MaSOF-1 (M = Ni(II), SABET = 647 m2/g) and MaSOF-2 (M = Zn(II), SABET = 630 m2/g; [Table 2]).[123] Both MaSOFs adsorbed comparable amounts of H2 at 77 K and 1 bar (4.46 mmol/g, 0.90 wt.% and 4.14 mmol/g, 0.90 wt.%) but the zinc-containing MaSOF-2 showed a slightly higher uptake of CO2 (2.23 mmol/g, 9.8 wt.%) than the nickel analogue MaSOF-1 (1.83 mmol/g, 8.1 wt.%), which was attributed to the higher Lewis acidity of the incorporated zinc centers ([Table 2]).[123]

Table 2

Comparison of porous salen- and salphen-containing organic polymers used for gas sorption and separation

Name

Metal

SABET [a] [m2/g]

CO2 uptake[b] [mmol/g] (wt.%)

IAST selectivity

Ref.

CO2/CH4 [c]

CO2/N2 [d]

MaSOF-1

Ni

647

1.83 (8.05)

–[e]

[123]

MaSOF-2

Zn

630

2.23 (9.81)

[123]

Zn-MaSOF100

Zn

638

4.28 (13.6)

10

48

[127]

Zn-MaSOF50

Zn

540

4.02 (12.8)

9.5

45

[127]

Ni-MaSOF100

Ni

816

4.83 (15.6)

9.0

42

[127]

Ni-MaSOF50

Ni

754

4.48 (14.5)

8.8

44

[127]

Cu-MaSOF100

Cu

697

4.66 (14.9)

10.3

52

[127]

Cu-MaSOF50

Cu

725

4.72 (15.1)

9.9

50

[127]

Pd-MaSOF100

Pd

511

4.11 (12.0)

10.5

56

[127]

Pd-MaSOF50

Pd

575

4.24 (12.4)

8.7

43

[127]

Pt-MaSOF100

Pt

572

4.39 (10.9)

9.5

86

[127]

Pt-MaSOF50

Pt

492

4.09 (10.2)

9.5

48

[127]

H2-SOF

116

2.12 (7.84)

10.5

32

[127]

Ni3-MaSOFas

Ni

373

2.36 (10.4)

16

89

[115]

Ni3-MaSOFw

Ni

441

2.45 (10.8)

11

64

[115]

Cu3-MaSOFas

Cu

625

3.39 (14.9)

12

69

[115]

Cu3-MaSOFw

Cu

890

5.02 (22.1)

9

40

[115]

MsMOP-Ni

Ni

1087

1.85 (8.15)

46[f]

[118c]

MsMOP-Zn

Zn

785

1.17 (5.16)

31[f]

[118c]

MsMOP-Pt

Pt

1202

1.67 (7.33)

20[f]

[118c]

CMP

772

1.61 (7.10)

[118b]

Al-CMP

Al

798

1.73 (7.65)

[118b]

Co-CMP

Co

965

1.80 (7.93)

[118b]

[email protected]

Al

590

1.14 (5.04)

[110c]

Al-CPOP

Al

136

0.54 (2.36)[g]

[132]

[email protected]

Zn

423

2.14 (9.40)

[133]

a Derived from N2 sorption experiments at 77 K.


b At 273 K and 1 bar.


c For a 50:50 mixture at 273 K and 0.1 bar.


d For a 20:80 mixture at 273 K and 0.1 bar.


e Not determined.


f Ideal selectivity.


g At 298 K and 1 bar.


For some of the salphens used for CO2 fixation (see above), the corresponding volumetric uptakes of CO2 at 1 bar were determined before catalysis testing. Co- as well as Al-CMP for example takes up 1.80 mmol/g (7.93 wt.%; Co-CMP) and 1.73 mmol/g (7.65 wt.%; Al-CMP) at 298 K, respectively ([Table 2]).[118b] Among those salphen polymers used for heterogeneous catalysis for CO2 addition on epoxides, [email protected] took up the highest amount of CO2 (2.14 mmol/g, 9.40 wt.%). This value is only outperformed by salphen networks designed for gas sorption ([Table 2]).[115] [118c] [123] [127] For example, one of the first porous MaSOFs, MaSOF-2, has already been reported to have an uptake of 2.23 mmol/g (9.8 wt.%) of CO2 at 273 K and 1 bar.[123]

In 2018 the Mastalerz group introduced the next generation of MaSOFs based on a triptycene hexakissalicylaldehyde. In combination with different metal sources, two isostructural series of compounds (M-MaSOF50 and M-MaSOF100) with M = Zn(II), Cu(II), Ni(II), Pd(II) as well as Pt(II) were synthesized at 50 and 100 °C (denoted by the suffix in the name). The BET surface areas measured by nitrogen sorption at 77 K ([Figure 5]) of all compounds range between 492 and 816 m2/g. More importantly, all structures have pronounced and defined pores in the microporous regime with a diameter of approximately 5.7 Å ([Figure 5]), thus making these compounds ideal to compare the influence of the metal centers on gas sorption and selectivity. Significantly higher CO2 uptakes were detected for the M-MaSOF50 and M-MaSOF100 series in comparison to the MaSOFs 1 and −2. For instance, Ni-MaSOF100 adsorbs the highest amount of CO2 (4.83 mmol/g, 15.6 wt.%) at 273 K and 1 bar ([Figure 5] and [Table 2]) within this series.[127] For all MaSOFs in these series, high Henry- and ideal adsorbed solution theory (IAST) selectivities of CO2 against CH4 or N2 were determined with Pt-MaSOF100 having the highest selectivity for CO2/N2 of S IAST = 86 at 0.1 bar at 273 K.[127] This value is one of the highest reported for all polysalphens, higher than those found in the MsMOP-M series (S IDEAL up to 46)[118c] and only outperformed by Ni3-MaSOFas (S IAST = 89 (CO2/N2) at 0.1 bar; see also discussion below).[115]

Zoom Image
Figure 5 Top: nitrogen sorption isotherms (77 K) of MaSOF50 series. Bottom: QSDFT pore-size distributions of the MaSOF50 series.[127] (Adapted with permission from Ref. 127. Copyright © 2018 American Chemical Society.)

The next evolutionary step in the construction of MaSOFs for gas sorption was the development of the M3-MaSOFs (M = Cu and Ni).[115] Based on previous results in the area of MOFs,[117] [137] the idea was to generate networks that contain trinuclear metal units, where the metals are in close proximity to synergically bind CO2. Indeed, model complexes of these trinuclear units revealed metal–metal distances (d = 6.65–6.74 Å) that should be ideal for CO2 sorption (see above; [Scheme 11], [Table 2], and [Figure 6]).[115] For Ni3-MaSOFas, an isosteric heat of adsorption of Q st = 35.6 kJ/mol ([Figure 6] and [Figure 7]) was measured, which is comparable to the values of some of the best-performing MOFs such as Ni-MOF-74 (Q st = 37–42 kJ/mol),[138] HKUST-1 (Q st = 15–35 kJ/mol),[139] or MIL-120 (Q st = 38 kJ/mol).[140] The resulting selectivities are S IAST = 16 (CO2/CH4) and S IAST = 89 (CO2/N2) for Ni3-MaSOFas at 0.1 bar and 273 K ([Table 2] and [Figure 7]).[115] To get a deeper insight into the adsorption processes, DFT calculations (def2-SVPD) on the synergic binding of CO2 were performed ([Figure 7]). A high energy gain for the adsorption of two molecules of CO2 suggests cooperative effects during the adsorption event. This is experimentally supported by a stepwise adsorption of CO2 at 195 K ([Figure 7]). Finally, the potential application in gas separation was shown by calculated breakthrough curves which was found to occur after approximately 10 min for an 80:20 mixture of N2 and CO2 at 1 bar and 1 L/min flow rate with a hypothetical column of 200 × 30 mm and 60 g of substrate.[115]

Zoom Image
Figure 6 Adsorption geometry of one molecule of CO2 on a [2 + 3]-nickel complex calculated by DFT (def2-SVPD level of theory) as a model complex for the M3-MaSOF series.[115] (Adapted with permission from Ref. 115. Copyright © 2019 American Chemical Society.)
Zoom Image
Figure 7 Top left: IAST selectivities of Ni3-MaSOFas (red and green) and Ni3-MaSOFw (black and blue) at 273 K. Top right: CO2 adsorption isotherm of Ni3-MaSOFas at 195 K. Bottom: relative adsorption energies calculated by DFT (def2-SVPD level of theory) of three molecules of N2 and/or CO2 on a [2 + 3]-nickel model complex. (Adapted with permission from Ref. 115. Copyright © 2019 American Chemical Society.)

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#

Miscellaneous Applications

In addition to the as-discussed applications, salen- and salphen-containing polymers were used for example as light-emitting materials in solution.[103] [104] [105] [106] [141] While some of the polymers were only investigated phenomenologically,[103] MacLachlan's linear 1D Zn(II)-containing poly(salphenyleneethynylenes) showed a fluorescence enhancement upon addition of Lewis bases such as pyridine, proving a potential application as a fluorescent sensor for such.[104b] This was further underlined by a series of poly(Pt-salphen)-(para-phenyleneethynylenes) providing low-energy emission (λ Em ∼ 580–700 nm) with a selective fluorescence quenching in the presence of Cu2+ ions, while other main group and transition metals did not influence the emission properties.[105]

Another example for the use of 1D salen polymers is the metal-free polymer P1, which was used to enantio-discriminate chiral α-hydroxyl carboxylic acids. For instance, for mandelic acid an enantiomeric fluorescence difference ratio (ef)[142] of up to 8.41 in preference to the (L)-enantiomer was found ([Figure 8]).[143] When reduced to the corresponding salan polymer, a “turn-off” of the fluorescence was observed, proving again that minor structural changes in the polymeric backbone can tune the corresponding properties dramatically.

Zoom Image
Figure 8 Top: structure of a chiral polymer sensor P1. Bottom: fluorescence enhancement of P1 with (D)- and (L)-mandelic acid. Inset: photographic images of solutions of P1 (left), P1 with (D)-mandelic acid (middle), and P1 with (L)-mandelic acid in toluene/DME.[143] (Adapted with permission from Ref. 143. Copyright © 2012 American Chemical Society.)

Based on defined pores, the COF-1 (SABET = 666 m2/g) and COF-1-Zn networks (SABET = 460 m2/g; see [Scheme 11]) were used as the stationary phase for HPLC to separate mixtures of ethylbenzene and xylene isomers ([Figure 9]).[125] With metal-free COF-1, baseline separation was observed with separation factors α of up to 2.0, resolution factors R of up to 5.2 as well as high effective theoretical plate numbers of up to 10980. These are in the range or even higher than those of, for example, stationary-phase bases on MOFs.[144]

Zoom Image
Figure 9 Representative chromatogram of a column packed with COF-1 ([Scheme 15]) used for the separation of ethylbenzene (EB) and xylene isomers upon different sample loading (depicted on the right).[144] (Adapted with permission from Ref. 144. Copyright © 2019 American Chemical Society.)

#

Conclusions and Outlook

To summarize, salen and salphen units and their corresponding metal complexes have been introduced into 1D, 2D, and 3D polymers and depending on the dimensionality and properties used for several applications. For instance, most 1D salphen polymers are conjugated compounds and thus it is not surprising that these have been investigated as conductive films, for example, to tune the redox properties of electrodes or the resistivity of multiwalled carbon nanotubes. Furthermore, some of the compounds show fluorescence and in combination with various Lewis acidic metal centers have been used for sensing by fluorescence quenching. If chiral salens instead of salphens are embedded into the polymer backbone, the sensing applications are broadened to chiral discrimination. It is worth mentioning that some of the first chiral helical ladder polymers synthesized by Katz and coworkers are based on the formation of metal salphens. There were three main synthetic approaches towards 1D polysalphens established: electropolymerization, condensation of aldehyde and amine moieties to imines, forming the salphen pockets, or cross-coupling reactions. The two latter have been used to make 2D salphen polymers as well. However, electropolymerization has not been used therefor. The vast majority of 2D as well as 3D salphen polymers are porous and therefore it is not surprising that in combination with the embedded metal centers, these have been used for catalytic applications as well as gas sorption and separation of smaller molecules. In both fields there are some materials that are comparable or even superior to similar compounds. For instance, some of the 2D chiral metal salen COFs are excellent catalysts for a number of asymmetric transformations, giving products in high yields and high ee values, thus perfectly combining heterogeneous reactivity with high surface areas. Due to the fact that almost all of the described 2D-salphene polymers are crystalline and being reported that the sheets are packed in an eclipsed fashion, the vast majority of metal salphen centers are not accessible for catalytic reactions or interactions with gases. Therefore, it is just logical that salphen COFs and polymers will be taken to the third dimension. Till date only a handful of 3D salphen polymers are known, but certainly they will be exploited in the future to enhance catalytic activities and gas sorption properties even more. The large plethora of metals that can be complexed by salen and salphen units as well as organic building blocks to construct polymers allows an infinite access of new materials with exciting properties.


#
#

No conflict of interest has been declared by the author(s).

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Figure 1 Molecular structures of selected N2O2 ligands.[1] [8b]
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Figure 2 Periodic table of the elements. The elements which ions have been used for salen and salphen complexes are highlighted in green.
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Figure 3 Top: macrocyclic [3 + 3]- and [6 + 6]-salphen ligands reported by the groups of Nabeshima[75b] and MacLachlan.[75c] Bottom: cage-like [2 + 3]-salphen ligands reported by Nabeshima and coworkers.[76d] [76e]
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Scheme 1 Syntheses of the first polysalphens reported by Marvel and Tarköy.[88] [89]
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Scheme 2 Polysalens and polysalphens based on the condensation of 2,5-dihydroxyterephthalaldehyde with diamines.[90]
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Scheme 3 Polysalens and polysalphens synthesized by electropolymerization.[91] [92] [93] [94]
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Scheme 4 Synthesis of helical polymers from enantiomerically pure bissalicylaldehydes and ortho-phenylene diamine.[68] Sections of the polymeric helices are depicted as MM2-optimized models.
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Scheme 5 Synthesis of helical polymers from enantiomerically pure dinaphthyl-bissalicylaldehyde and different α,ω-diamines.[101] Section of the polymeric helix is depicted as a geometry-optimized model (MMFF94, MM2).
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Scheme 6 Left: synthesis of Houjou's linear polyimines interconnected by salen-like moieties.[102] Right: SEM micrographs of Cu(II) polymers creating microspheres. (Parts of the scheme are adapted with permission from Ref. 102. Copyright © 2003 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim.)
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Scheme 7 Synthesis of 1D-polysalphens by palladium-catalyzed Sonogashira–Hagihara cross-coupling.[103] [104]
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Scheme 8 Top: synthesis of Liu's 1D Zn(II) salphen polymer from a naphthalene bissalicylaldehyde and 2,3-diaminophenazine. Bottom: SEM micrographs of the obtained microspheres (a) and rods (b), and PXRD pattern of the corresponding material (c).[107] (Parts of the scheme are adapted with permission from Ref. 107. Copyright © 2015 The Polymer Society of Korea and Springer.)
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Scheme 9 Structural motifs of salen-containing 2D polymers and calculated as well as experimentally measured pore sizes.[111] [112]
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Scheme 10 Structural motifs of Liu and Cui's CCOF-4-M series, and Yang's COF-salen-M polymers with triphenylbenzene core structures and calculated as well as experimentally obtained pore sizes.[113]
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Scheme 11 Synthesis and structural representation of the M3-MaSOF series. Box: single-crystal X-ray structure of a trinuclear nickel model complex with a metal-to-metal distance of 6.74 Å. (Parts of the scheme are adapted with permission from Ref. 115. Copyright © 2019 American Chemical Society.)
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Scheme 12 Overview of 2D salen- and salphen-containing organic polymers made by Sonogashira–Hagihara cross-coupling reactions. Box: cut-out of the connecting unit.[118]
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Scheme 13 Syntheses and structural motifs of Mastalerz's MaSOF-1 and -2, as well as Fang's JUC-508, JUC-509, and the corresponding metalated networks.[123] [124]
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Scheme 14 Cui's COF-1, COF-2, COF-2-Zn, and COF-2-Zn.[124]
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Scheme 15 Synthesis of a chiral Mn-salen imine gel. Inset: photograph of Mn-salen imine gel (0.02 mol/L in DMSO).[126] (Adapted with permission from Ref. 126. Copyright © 2015 The Royal Society of Chemistry.)
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Scheme 16 Syntheses of MaSOF50 and MaSOF100 series.[127]
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Scheme 17 Syntheses of 3D salen- and salphen-containing organic polymers by Sonogashira–Hagihara cross-coupling reactions.[128]
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Figure 4 Top: schematic representation of interdigitated microelectrode array with an electropolymerized Co(III)-salen PEDOT copolymer as a sensor layer for NO sensing. Bottom: differential resistance (R 2/R 1 , see top figure) against time with various NO(g) concentrations. Black: unconditioned poly-Co(III)-PEDOT film; red: conditioned (at 0.262 V) poly-Co(III)-PEDOT film: blue: PEDOT film without salen moieties.[97] (Adapted with permission from Ref. 97. Copyright © 2006 American Chemical Society.)
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Scheme 18 Valence isomerization of norbornadiene to quadricyclane catalyzed by a Co(II) salen polymer.[129]
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Scheme 19 Gupta's N,N-BSPDA-based beads by radical polymerization. Right: SEM micrograph of the corresponding bead after swelling. (Adapted with permission from Ref. 119a. Copyright © 2003 Elsevier Science B. V.)
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Scheme 20 Epoxidation of alkenes catalyzed by Mn-salen polymer Mn-POP.[118d] aConditions: Mn-POP (0.1 mol%), O2, isobutyraldehyde (7 eq.), dichloroethane, 25 °C, 5 h. bConversion determined by GC.
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Scheme 21 Different methods to capture CO2.[110c]
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Scheme 22 Asymmetric addition of Et2Zn to aldehydes, catalyzed by Takata's poly Zn(II) salen helix.[101a] [101c]
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Scheme 23 Asymmetric epoxidation of styrenes catalyzed by various poly M-salens (for synthesis and structures of the corresponding polymers, see [Scheme 5],[101b] [Scheme 9],[111] and [Scheme 10]).[113a]
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Scheme 24 Asymmetric transformations catalyzed by different members of Cui's CCOF-4-M series (for synthesis and structures, see [Scheme 10]).[113a]
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Figure 5 Top: nitrogen sorption isotherms (77 K) of MaSOF50 series. Bottom: QSDFT pore-size distributions of the MaSOF50 series.[127] (Adapted with permission from Ref. 127. Copyright © 2018 American Chemical Society.)
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Figure 6 Adsorption geometry of one molecule of CO2 on a [2 + 3]-nickel complex calculated by DFT (def2-SVPD level of theory) as a model complex for the M3-MaSOF series.[115] (Adapted with permission from Ref. 115. Copyright © 2019 American Chemical Society.)
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Figure 7 Top left: IAST selectivities of Ni3-MaSOFas (red and green) and Ni3-MaSOFw (black and blue) at 273 K. Top right: CO2 adsorption isotherm of Ni3-MaSOFas at 195 K. Bottom: relative adsorption energies calculated by DFT (def2-SVPD level of theory) of three molecules of N2 and/or CO2 on a [2 + 3]-nickel model complex. (Adapted with permission from Ref. 115. Copyright © 2019 American Chemical Society.)
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Figure 8 Top: structure of a chiral polymer sensor P1. Bottom: fluorescence enhancement of P1 with (D)- and (L)-mandelic acid. Inset: photographic images of solutions of P1 (left), P1 with (D)-mandelic acid (middle), and P1 with (L)-mandelic acid in toluene/DME.[143] (Adapted with permission from Ref. 143. Copyright © 2012 American Chemical Society.)
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Figure 9 Representative chromatogram of a column packed with COF-1 ([Scheme 15]) used for the separation of ethylbenzene (EB) and xylene isomers upon different sample loading (depicted on the right).[144] (Adapted with permission from Ref. 144. Copyright © 2019 American Chemical Society.)