Sterically Driven Pathways in Schiff Base Formation: tert-Butyl Effects on Hindered Imines

Abstract

The ZnCl₂/AcOH-mediated condensation of acenaphthenequinone with 2,4,6-tri-tert-butylaniline was explored to prepare a sterically demanding bidentate α-diimine ligand. Contrary to expectation, the reaction consistently afforded 1,2-bis[(2,4-di-tert-butylphenyl)imino]acenaphthene as the major product, together with a mono-substituted imino-ketone, while the fully substituted 1,2-bis[(2,4,6-tri-tert-butylphenyl)imino]acenaphthene was detected only in trace amounts. Spectroscopic analysis (nuclear magnetic resonance, UV/vis, Fourier transform infrared spectroscopy and single-crystal X-ray diffraction, supported by density functional theory calculations, showed that both ligands possess nearly identical electronic structures. However, the additional ortho tert-butyl group in the tri-substituted analogue induces severe steric congestion, twisting the aryl–imine geometry, diminishing π-conjugation, and facilitating acid-promoted tert-butyl elimination. In contrast, the di-substituted ligand adopts a near-planar geometry, enabling straightforward synthesis, crystallization, and full characterization. These results demonstrate that steric effects override electronic factors in dictating the reactivity and stability of highly hindered Schiff bases, providing mechanistic insight and practical design principles for the development of robust, sterically encumbered ligands in coordination chemistry and catalysis.

Keywords

Schiff bases - Steric hindrance - tert-Butyl elimination - Bidentate acenaphthenes - Sterically encumbered ligands

2,4,6-Tri-tert-butylaniline is a model for extreme steric congestion in aromatic amines, the three ortho- and para-tertiary butyl groups forcing its –NH₂ substituent out of the ring plane and interrupting conjugation ([Fig. 1a,b]). This distortion profoundly alters the molecule’s electronic structure, solubility, and reactivity: the amino lone pair is shielded, basicity is suppressed, and the compound exhibits enhanced hydrophobicity relative to simpler anilines.[1] [2] [3] Remarkably, the N-centered radical derived from this hindered aniline ([Fig. 1c]) persists for hours to days, as the tert-butyl groups sterically block dimerization and other decay pathways.[1] [4] Thermal stability is also increased: the neutral amine resists degradation to elevated temperatures, and even when integrated into organometallic ligands it enhances the persistence of otherwise sensitive complexes.[1]

Fig. 1 (a) Structure of aniline. (b) Resonance structures of tri-tert-butylaniline. (c) Structure of N-centered radical derived from tri-tert-butylaniline.

Early synthetic studies by Bartlett, Roha and Stiles demonstrated that the extreme steric bulk of 1,3,5-tri-tert-butylbenzene—the precursor to both tri-tert-butylaniline and tri-tert-butylphenol—fundamentally changed its electrophilic substitution patterns and drastically reduced amine basicity.[5] Saleh and Tashtoush later provided a systematic framework for tert-butyl deprotection in aromatic systems, showing that Lewis acids, mineral acids, metal oxides, and heteropoly acids can selectively cleave tert-butyl groups under controlled conditions; catalyst choice, temperature, and presence of alkyl acceptor govern yield and regioselectivity.[6]

Under strongly acidic or photochemical/radical conditions, tert-butyl groups in aromatic substrates frequently exit via unexpected pathways. In superacid media or acidic zeolites tert-butyl groups ionize to tert-butyl cations then eject isobutylene in reverse Friedel–Crafts reactions—a process confirmed by solid-state nuclear magnetic resonance (NMR) studies.[7] [8] Under UV photolysis, neophyl-type rearrangements have been observed: UV irradiation of 2,4,6-tri-tert-butylnitrosobenzene yields a tert-butylphenyl radical, followed by intramolecular hydride (1,2-) shift from an ortho tert-butyl such that radical rerecombination or trapping leads to rearranged products.[9] Analogous behavior in related hindered phenols reinforces that tert-butyl groups, though robust, can migrate or cleave under energetic stimulus.[10]

The steric profile of the tert-butyl groups also complicates spectroscopic characterization. In 2,4,6-tri-tert-butylaniline, hindered rotation broadens and decoalesces NMR signals; the proximity of ortho tert-butyl substituents to –NH₂ induces distinctive coupling patterns and chemical shifts, often requiring 2D NMR (COSY, HSQC) or variable-temperature experiments to assign peaks clearly.[3] Mechanistic insights have been derived via isotopic labelling: Eastman and Stack showed reversible tert-butyl “scrambling” in mixed acid systems through ipso-attack and exchange of ortho protons, tracked by NMR.[7] Similarly, in lutetium mono(anilide) complexes of the tri-tert-butyl derivative, isotopic studies trace direct aryl-transfer rearrangements mediated by intramolecular ligand migration.[1] Complementary spectroscopic tools such as ESR for radical intermediates[11] or solid-state NMR for surface-bound cations[8] further delineate key mechanistic steps.

Synthetically, 2,4,6-tri-tert-butylaniline is prepared most often via catalytic hydrogenation of preformed tri-tert-butylnitrobenzene—typically obtained by Friedel–Crafts tert-butylation of mesitylene analogues—with recent advances using non-noble metal catalysts affording ~90% yields at modest pressure and temperature (~110 °C, 40 bar H₂).[12] Alternative multicomponent strategies now bypass nitroarene precursors entirely: for example, a protocol combining aryl boronic acids, tert-butyl nitrite, and alkyl bromides delivers heavily ortho-substituted anilines in one step.[2] Once synthesized, the compound is routinely characterized by NMR, Fourier transform infrared spectroscopy (FTIR), mass spectrometry, and occasionally X-ray crystallography, which reveals the pronounced geometric distortion of the aniline nitrogen and the broadened C–N–C bond angles wrought by steric repulsion.[1] In coordination chemistry, this bulky aniline acts as a “shielding” ligand to isolate reactive centers: for example, it stabilizes monomeric anilide complexes and prevents oligomerization, or enforces monomeric iminophosphorane formation over polymeric phosphazenes in phosphorus chemistry.[1] [4]

In an advanced application, Chen and colleagues developed an acenaphthyl-based α-diimine Pd catalyst bearing ortho-tert-butyl-substituted N-aryl ligands for aerobic, phosphine-free direct C–H heteroarylation. The system exhibits excellent scope (various heterocycles with bromides), low catalyst loadings (0.05–0.1 mol%), and operational simplicity—but condensation of 2,4,6-tri-tert-butylaniline with acenaphthenequinone was thwarted, likely due to steric-strain–induced ZnCl₂-promoted de-tert-butylation. Only a single isomer was isolated (by ¹H NMR), and the precise mechanism (including de-alkylation vs. rearrangement) remains unresolved.[13] [14] [15] In another work from Pappalardo and coworkers,[16] acenaphthene α-diimine ligand symmetry and ortho-bulk programmed polypropylene microstructure. C_2v Ni(II) complexes (2,6- ⁱ Pr₂Ph and 2,6-Me₂Ph) yielded syndiotactic chains via chain-end control; C₂ variants introduce enantiomorphic site-control, increasing isotactic (mm) and suppressing syndiotactic (rr). Bulky tert-butyl elevates 1,3-insertions (slower insertion/chain-running). rac- versus meso-5 proves symmetry gating (meso, C_s , restores rr). Across systems: single-site dispersities; predominant 1,2-insertion despite poor region-regularity.

Here, in this study, the synthesis and characterization of an imine-based bidentate ligand 1,2-bis[(2,4,6-tri-tert-butylphenyl)imino]acenaphthene was targeted using acenaphthenequinone (a diketone) and the bulky aniline derivative 2,4,6-tri-tert-butyl aniline. Given the synthetic challenges associated with sterically hindered anilines, particular attention was paid to the formation process, including the observed loss of tert-butyl groups. Potential experimental factors contributing to this degradation were thoroughly investigated, providing valuable insights into the design and stability of sterically demanding ligands in catalyst development. A plausible mechanistic pathway was also proposed. The findings from the work not only clarify the synthetic limitations imposed by extreme ortho-steric congestion but also provide a mechanistic and strategic framework for the rational design of robust, sterically encumbered ligands in advanced catalytic systems.

The ZnCl₂/AcOH-mediated condensation of acenaphthenequinone with 2,4,6-tri-tert-butylaniline consistently yielded a red-orange ligand fraction containing four components described by thin-layer chromatography (TLC): trace 1,2-bis[(2,4,6-tri-tert-butylphenyl)imino]acenaphthene, dominant 1,2-bis[(2,4-di-tert-butylphenyl)imino]acenaphthene, a mono-substituted imino-ketone byproduct, and residual diketone. Despite expectations, the intended tri-tert-butyl ligand appeared only in trace, while the di-tert-butyl product was major and reproducible (n = 3), indicating that severe ortho-steric congestion disrupts conventional Schiff base formation, diverting the reaction toward elimination and substitution pathways.

In the synthetic protocol ([Scheme 1]), the appropriate literature protocols[17] [18] adapted to target achieve the synthesis of 1,2-bis[(2,4,6-tri-tert-butylphenyl)imino]acenaphthene. Under nitrogen atmosphere, acenaphthenequinone and anhydrous ZnCl₂ in glacial AcOH/toluene were heated to 50–60 ^oC, followed by the addition of 2,4,6-tri-tert-butylaniline and reflux for 1 h. The crude orange-red solid, obtained by filtration and washing, was treated with aqueous sodium oxalate in dichloromethane (DCM), separated the layers, dried the organic layer, and concentrated under reduced pressure. The product was initially assumed to be the target ligand 1,2-bis[(2,4,6-tri-tert-butylphenyl)imino]acenaphthene ([Scheme 1], desired product shown in blue) but spectroscopic and crystallographic analysis revealed loss of one ortho tert-butyl group, affording the 1,2-bis[(2,4-di-tert-butylphenyl)imino]acenaphthene derivative ([Scheme 1], isolated product shown in red) and a mono-substituted imino-ketone 2-[(2,4,6-tri-tert-butylphenyl)imino)acenaphthylen-1(2H)-one ([Scheme 1], by-product shown in purple). This reproducible product profile (n = 3) implicates steric effects as the driving force in product distribution. This unanticipated de-alkylation was confirmed using ¹H and ¹³C NMR spectroscopy ([Figs. 2], [Fig. 3] and S1–S6) and further confirmed via X-ray crystallography ([Figs. 4] and S7, and Tables S1–S3), marking a significant deviation from the intended ligand framework. In the case of non-templated protocol (reaction without ZnCl₂) trials with acenaphthenequinone in glacial AcOH, heated to 50–60 °C and followed by the addition of 2,4,6-tri-tert-butylaniline and reflux for 1 h, 5 h and overnight (three trials), the diketone and aniline starting materials were observed and collected almost quantitatively upon isolation with the column chromatography, leaving no observation of any acenaphthene derivatives. Comparing with this work, the two reported studies[13] [16] showed the synthesis of metal ion complexes (Pd(II) and Ni(II), respectively) with the tert-butyl-based acenaphthene ligands via the templation method. Although both studies are highly important with the complex-based contribution, the researchers unfortunately did not conduct a detailed investigation of the ligand only; therefore, this study specifically fills the gap left by those two works.

Scheme 1 Syntheses of compounds: The desired product, 1,2-bis[(2,4,6-tri-tert-butylphenyl)imino]acenaphthene (titled in blue), 1,2-bis[(2,4-di-tert-butylphenyl)imino]acenaphthene (titled in red), and 2-[(2,4,6-tri-tert-butylphenyl)imino)acenaphthylen-1(2H)-one (titled in purple).

Fig. 2 Annotated and stacked ¹H NMR spectra (500 MHz, CDCl₃, 298 K) of 1,2-bis[(2,4-di-tert-butylphenyl)imino]acenaphthene (all from three different trials) and 2,4,6-tri-tert-butylaniline (top).

Fig. 3 Annotated and stacked ¹³C NMR spectra (125 MHz, CDCl₃, 298 K) of 1,2-bis[(2,4-di-tert-butylphenyl)imino]acenaphthene (below) and 2,4,6-tri-tert-butylaniline (top).

Fig. 4 Crystal structure and the ORTEP representation of 1,2-bis[(2,4-di-tert-butylphenyl)imino]acenaphthene (hydrogen atoms are omitted for the sake of clarity).

It is proposed that the steric hindrance arising from the four bulky ortho tert-butyl groups impedes condensation of two equivalents of 2,4,6-tri-tert-butylaniline, promoting loss of one tert-butyl group from each side to form a more sterically feasible di-substituted product. This phenomenon is consistent with previously reported sterically driven deviations in Schiff base formations involving hindered aromatic amines.[13] The NMR spectra of crude and purified products ([Figs. 2], [3], and S1–S6) consistently confirmed the absence of one ortho tert-butyl signal and the presence of characteristic imine resonances, with no C=O signals, supporting condensation with a substituted aniline. The NMR of commercial 2,4,6-tri-tert-butylaniline (Figs. S5 and S6) confirmed its integrity, ruling out reagent impurity. TLC of the crude mixture from all three trials revealed the presence of four distinct components. Subsequent flash column chromatography led to the isolation of the following species: (i) trace amount (<1 mg) of 1,2-bis[(2,4,6-tri-tert-butylphenyl)imino]acenaphthene; (ii) a major product of 1,2-bis[(2,4-di-tert-butylphenyl)imino]acenaphthene; (iii) a byproduct of 2-[(2,4,6-tri-tert-butylphenyl)imino]acenaphthylen-1(2H)-one; and (iv) unreacted acenaphthenequinone in minor quantities.

The observed elimination and steric restriction phenomena highlight the importance of molecular crowding effects in designing reactions involving highly hindered aryl amines. This case serves as a compelling example of how steric size can dramatically redirect the expected course of imine-forming condensation reactions. The mono-substituted byproduct likely results from selective condensation of one equivalent of aniline with acenaphthenequinone, reducing steric congestion and allowing formation of an unsymmetrical imine. Both ¹H and ¹³C NMR spectroscopic analyses (Figs. S3 and S4) confirmed their structure, including a ¹³C resonance at 189.87 ppm, characteristic of the C=O in the acenaphthenone moiety.

A plausible mechanistic pathway ([Scheme 2]) for tert-butyl loss can be proposed using acid-catalyzed elimination via carbocation formation. In such route, activation of the tert-butyl group by the Lewis acid (ZnCl₂) or the protic acidic medium (AcOH) leads to protonation of the tert-butyl group, forming a good leaving group ( ^t BuOH or isobutylene). The tert-butyl carbocation ( ^t Bu⁺) formation occurs, particularly favored at ortho position due to steric crowding or adjacent imine/nitrogen lone pair stabilizing the transition state. The resulting aryl cation or de-alkylated aromatic ring may be stabilized by delocalization or further coordination to Zn²⁺. Isobutylene gas or tert-butanol may be released and driven out by reflux. Efforts are currently underway to elucidate the full mechanism of this tert-butyl loss and to optimize reaction conditions for the synthesis of 1,2-bis[(2,4,6-tri-tert-butylphenyl)imino]acenaphthene ligand.

Scheme 2 Proposed mechanism for the tert-butyl cation loss as isobutylene from the desired product, 1,2-bis[(2,4,6-tri-tert-butylphenyl)imino]acenaphthene to form 1,2-bis[(2,4-di-tert-butylphenyl)imino]acenaphthene.

Spectroscopic characterization of pure 1,2-bis[(2,4-di-tert-butylphenyl)imino]acenaphthene shows UV/Vis absorptions (2.0 × 10^–4 M in DCM) at λ_max = 425 nm (ε = 3.85×10³ L·mol⁻¹·cm⁻¹) and 248 nm (ε = 1.89 × 10⁴ L·mol⁻¹·cm⁻¹), consistent with extended π-conjugation ([Fig. 5a]). The FTIR ([Fig. 5b]) confirms disappearance of strong C=O (~1750 cm^–1) and appearance of medium C=N (1580–1720 cm^–1) bands. The absorption profile, with a main band at 430–450 nm, corresponds to π–π* transitions of the acenaphthene–diimine core, as reported for conjugated aromatic imines.[19] [20] [21] [22] A weaker ~300 nm band reflects localized aromatic transitions. Compared to the tri-tert-butyl analogue, the di-substituted derivative is expected to show a blue shift in λ_max due to reduced steric hindrance and enhanced planarity, improving π-overlap. The tri-substituted form should exhibit a twisted geometry from ortho repulsion, reducing conjugation and producing red-shifted/broadened absorption. Solid-state FTIR corroborates complete imine formation: sharp C=N bands at 1625–1640 cm^–1, absence of 1650–1700 cm^–1 C=O stretches, loss of broad N–H (3300–3400 cm^–1), and presence of aliphatic C–H stretches (2950–2850 cm^–1). Subtle changes in C–H intensity/shape between derivatives reflect tert-butyl number and arrangement, with the tri-substituted expected to have broader aliphatic bands from restricted vibrational freedom. These UV/Vis and FTIR data confirm steric modulation of conjugation efficiency, consistent with NMR-based structural conclusions and observed product distributions.

Fig. 5 (a) UV/vis spectrum of 1,2-bis[(2,4-di-tert-butylphenyl)imino]acenaphthene (molarity (M) = 2.0 × 10^–4 in dichloromethane (DCM). (b) FTIR spectrum of 1,2-bis[(2,4-di-tert-butylphenyl)imino]acenaphthene.

The density functional theory (DFT) computational studies using the B3LYP/6-31G(d,p) level with Gaussian 0.9 software[23] (geometry optimizations performed without symmetry constraints and Avogadro 2[24] was employed to produce figures) on both ligands ([Figs. 6]–[8], [Table 1]) show nearly identical HOMO/LUMO energies: 1,2-bis[(2,4-di-tert-butylphenyl)imino]acenaphthene, HOMO = –0.201 eV, LUMO = –0.077 eV, gap = 0.124 eV; 1,2-bis[(2,4,6-tri-tert-butylphenyl)imino]acenaphthene, HOMO = –0.201 eV, LUMO = –0.075 eV, gap = 0.126 eV. Thus, the 6-position tert-butyl group minimally affects frontier orbital energetics, indicating similar electronic stabilization for both derivatives.

Fig. 6 Frontier molecular orbitals, electrostatic potential surface, and dipole moment visualization of 1,2-bis[(2,4-di-tert-butylphenyl)imino]acenaphthene. (a) HOMO and LUMO orbitals. (b) Molecular Electrostatic Potential (MEP) surface mapped using Gasteiger charges at an isosurface value of 0.03 a.u., highlighting regions of electron-rich (red) and electron-deficient (blue) character. (c) Visualization of the molecular dipole moment direction (red arrow), indicating the net charge separation within the molecule due to asymmetric electronic distribution imposed by the sterically hindered tert-butyl substituents and the amino group.

Fig. 7 Frontier molecular orbitals, electrostatic potential surface, and dipole moment visualization of 1,2-bis[(2,4,6-tri-tert-butylphenyl)imino]acenaphthene. (a) HOMO and LUMO orbitals. (b) Molecular electrostatic potential (MEP) surface mapped using Gasteiger charges at an isosurface value of 0.03 a.u., illustrating electron-rich (red) and electron-deficient (blue) regions. The electrostatic profile highlights the shielding effect of the tert-butyl groups and localized negative potential near the amino nitrogen. (c) Dipole moment vector (red arrow) indicating the net polarization across the molecule, directed from the amino-substituted region toward the aromatic ring, consistent with the electronic asymmetry introduced by steric crowding.

Fig. 8 Optimized geometries, bond lengths, and dihedral angles of imine-based ligands containing 1,2-bis[(2,4-di-tert-butylphenyl)imino]acenaphthene (left) and 1,2-bis[(2,4,6-tri-tert-butylphenyl)imino]acenaphthene (right) substituents based on DFT calculations. For clarity, hydrogen atoms are omitted in both structures. (a) Optimized structure of the ligand containing di-tert-butyl substituted derivative. Bond distances between the aromatic ring and adjacent atoms are shown in angstroms (Å), with color coding distinguishing single (blue) and double (orange) bonds. (b) Corresponding geometry for the tri-tert-butyl-substituted derivative. Bond length variations around the nitrogen and phenyl core suggest local geometric perturbations arising from steric congestion.

Table 1 Density functional theory (DFT) computed orbital energies (eV) of optimized geometry of 1,2-bis[(2,4-di-tert-butylphenyl)imino]acenaphthene (observed) and 1,2-bis[(2,4,6-tri-tert-butylphenyl)imino]acenaphthene (expected) derivatives based on B3LYP/6-31G(d,p) calculations

Compound	E HOMO (eV)	E LUMO (eV)	ΔE (HOMO–LUMO) (eV)
1,2-Bis[(2,4-di-tert-butylphenyl)imino]acenaphthene	–0.201	–0.077	–0.124
1,2-Bis[(2,4,6-tri-tert-butylphenyl)imino]acenaphthene	–0.201	–0.075	–0.126

HOMO densities are delocalized over the aromatic ring and partly toward the amino nitrogen; LUMOs are localized on the benzene π-system ([Figs. 6a] and [Fig. 7a]). No significant orbital density is associated with the 6-tert-butyl group, confirming its minimal role in frontier orbital topology. Electrostatic potential maps ([Figs. 6b] and [Fig. 7b]) show electron-rich regions near the aniline nitrogen and aromatic rings, and electron-deficient zones at the periphery. In the tri-substituted ligand, tert-butyl groups provide additional steric/electronic shielding and slightly increased polarization ([Figs. 6c] and [Fig. 7c]).

The DFT-optimized geometries ([Fig. 8]) highlight steric effects: the 1,2-bis[(2,4-di-tert-butylphenyl)imino]acenaphthene is relatively planar, with a 128.01° dihedral between imine and aryl ring (from the actual crystal structure data (as two ligands in the unit cell), the dihedral angles are 105.4 and 131.2° for one and 115.2 and 133.5° for the other; average C=N bond length of 1.274 Å ( from the bond lengths of 1.276, 1.271, 1.277, and 1.273 Å, respectively) compared to the DFT analysis of C=N bond lengths of 1.279 Å), allowing partial π-conjugation ([Fig. 8a]). The tri-substituted derivative ([Fig. 8b]) shows a twisted geometry (101.79° dihedral angle and 1.276 Å C=N bond length; not obtained the actual crystal structure for this derivative) from 6-tert-butyl hindrance, reducing conjugation and introducing torsional strain. This is accompanied by slight bond elongation at the aryl–imine linkage, indicating geometric strain. Thus, while electronic profiles are similar, steric effects dictate synthetic outcomes. The tri-substituted ligand’s twist and steric bulk raise the activation barrier for condensation and increase susceptibility to tert-butyl loss under acid/heat, consistent with known tert-butyl rearrangements/eliminations in aromatic systems.[7] [8] In contrast, the di-substituted ligand’s lower steric demand supports smooth imine formation and stability.

Single-crystal X-ray analysis further supports these conclusions. Crystallization attempts for both ligands succeeded only for the di-tert-butyl derivative 1,2-bis[(2,4-di-tert-butylphenyl)imino]acenaphthene ([Fig. 4]), yielding a planar acenaphthene backbone with E-configured imines, in excellent agreement with NMR and DFT data. The tri-substituted ligand failed to crystallize, likely due to its trace yield (<1 mg) and increased steric hindrance preventing lattice packing. This lack of crystalline material reinforces that steric congestion hampers both formation and isolation. Instead, isolable fractions contained the di-tert-butyl product, unreacted starting materials, and side products.[25] The DFT-optimized structure corroborated the experimentally observed bond lengths and angles, further validating the structural model.[23] [26] These findings not only provide definitive structural confirmation of the di-substituted species but also lend insight into the synthetic challenges encountered with the tri-substituted analogue.

In summary, this work demonstrated that ZnCl₂/AcOH-mediated condensation of acenaphthenequinone with 2,4,6-tri-tert-butylaniline was dominated by steric, rather than electronic, control, reproducibly affording 1,2-bis[(2,4-di-tert-butylphenyl)imino]acenaphthene as the major product, alongside a mono-substituted imino-ketone, with the fully substituted tri-tert-butyl analogue forming only in trace amounts. Comprehensive NMR, UV/Vis, FTIR, and single-crystal X-ray analyses, supported by DFT calculations, revealed that both derivatives possessed nearly identical frontier orbital energies and electronic distributions, but the additional tert-butyl group in the tri-substituted analogue induces severe ortho-steric congestion, twisting the aryl–imine geometry, reducing π-conjugation, and facilitating acid-promoted tert-butyl elimination. The di-substituted ligand adopts a more planar, conjugated structure, enabling smooth synthesis, crystallization, and full characterization, while the tri-substituted form remains synthetically inaccessible under these conditions. These findings not only elucidate the mechanistic basis of steric hindrance-driven deviations in Schiff base formation but also provide a strategic framework for designing and stabilizing sterically demanding ligands in coordination chemistry and catalysis.

All obtained precursors/chemicals/starting materials and reagents were purchased from commercial suppliers and used without further purification. ZnCl₂ was dried by heating it under vacuum and was subsequently stored under nitrogen in a desiccator. All glassware and magnetic stirring bars were dried with a torch for 5–10 min and let to cool under vacuum before use. For the synthesis of 1,2-bis[(2,4,6-tri-tert-butylphenyl)imino]acenaphthene (the desired product), the literature procedures[17] [18] [27] [28] [29] [30] [31] [32] [33] [34] were reviewed, revised and newly modified or those synthetic routes have been adopted and modified where necessary. All details for synthetic procedures are described in Section B of the Supporting Information. TLC was performed on silica gel 60 F₂₅₄ (E. Merck). Flash chromatography was performed using Silicycle UltraPure Flash Silica Gel (60 Å, 40–63 μm). Column chromatography was carried out either on silica gel 60F (Merck 9385, 0.040–0.063 mm) or run on the Selekt Flash Chromatography Instrument by Biotage. All NMR spectra were recorded on a Bruker Avance NEO 500 with a working frequency of 500 MHz. Chemical shifts were reported in ppm relative to the signals corresponding to the tetramethylsilane (TMS, δ = 0.00 ppm) or residual non-deuterated solvents (CDCl₃: δ 7.26 ppm for ¹H NMR, and 77.16 ppm for ¹³C NMR), which served as a shift reference. Coupling constants, J, are reported in hertz (Hz). UV/Vis absorbance spectrum was collected at room temperature (RT) in CH₂Cl₂) on a UV–1280 Shimadzu spectrophotometer. Solution of the sample for UV/Vis studies was measured in diluted solutions (2.0 × 10^–4 M). The FTIR spectrum was recorded on a Fourier Transform Shimadzu IRSpirit-T spectrophotometer. The ligand for FTIR studies was used from the purified and NMR confirmed solid sample, and the spectrum was collected at RT after running a background spectrum. The single crystal X-ray determination measurements were performed on an XtaLAB Synergy, Dualflex, HyPix diffractometer with Cu Kα radiation (λ = 0.71075 Å) at 297 K in a stream of cooled nitrogen gas. All reaction vessels were flame-dried under vacuum and filled with nitrogen prior to use. All reactions were performed under a nitrogen atmosphere as a routine practice, not as an essential requirement. To remove solvents and other volatile impurities under reduced pressure, a Heidolph Hei-VAP Expert equipped with a vacuum pump was used.

The detailed synthetic procedures and structural characterization data for the intermediates and desired compounds are presented in the Supporting Information. Some important details and structural determination data for the product(s) are presented below.

1,2-Bis[(2,4-di-tert-butylphenyl)imino]acenaphthene (Exclusively Observed Product)

¹H NMR (500 MHz, CDCl₃) δ 8.85 (d, J = 8.2, 2H), 7.54 (s, 2H), 7.36 (t, J = 7.8, 2H), 7.23 (dd, J = 8.0 and 2.2 Hz, 2H), 6.85 (d, J = 8.2 Hz, 2H), 6.83 (d, J = 7.8 Hz, 2H), 1.41 (s, 18H), 1.39 (s, 18H); ¹³C NMR (125 MHz, CDCl₃) 159.06, 148.06, 147.34, 141.79, 138.69, 131.31. 129.59, 128.54, 127.77, 123.84, 123.70, 123.46, 118.53, 35.83, 34.83, 31.85, 29.96.

2-[(2,4,6-Tri-tert-butylphenyl)imino]acenaphthylen-1(2H)-one (observed byproduct)

¹H NMR (500 MHz, CDCl₃) δ 8.14 (d, J = 8.0 Hz, 2H), 7.93 (d, J = 8.4 Hz, 1H), 7.78 (t, J = 7.6 Hz, 1H), 7.45 (s, 2H), 7.33 (t, J = 7.8 Hz, 1H), 5.88 (d, J = 7.0 Hz, 1H, 1.43 (s, 9H), 1.21 (s, 18H); ¹³C NMR (125 MHz, CDCl₃) 189.87, 161.21, 148.06, 145.87, 142.68, 136.65, 132.24, 131.04, 130.94, 128.71, 128.65, 128.28, 128.14, 123.77, 122.48, 122.11, 36.17, 34.95, 31.92, 31.54.

2,4,6-Tri-tert-butylaniline (commercially purchased aniline derivative – starting material)

¹H NMR (500 MHz, CDCl₃) δ 7.25 (s, 2H), 4.04 (s, 2H, –NH₂), 1.48 (s, 18H), 1.31 (s, 9H); ¹³C NMR (125 MHz, CDCl₃) 141.19, 139.22, 133.62, 122.02, 34.91, 34.56, 31.86, 30.46.

Contributors’ Statement

M. Mustafa Cetin: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing.

Conflict of Interest

The authors declare that they have no conflict of interest.

Acknowledgment

The author (M.M.C.) thanks Professor Michael Findlater, Dr. Kotono Babaguchi, Vahap Gazi Fidan and Mine Cengiz Cetin for their support in the discussion of the results obtained. Financial support from the TÜBİTAK research grant (Project No: 122N411) is gratefully acknowledged. He also thanks the R&D facility of Istinye University and Department of Chemistry at Istinye University for use of the instrumental and/or computer facilities.

Author Contributions

M.M.C. is the only author who created, wrote, and contributed to this work. All data were generated in-house, and no paper mill was used. The author agrees to be accountable for all aspects of work ensuring integrity and accuracy.

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Publication History

Received: 13 August 2025

Accepted after revision: 17 September 2025

Accepted Manuscript online:
23 September 2025

Article published online:
22 October 2025