Solid State NMR Spectroscopy as a Precise Tool for Assigning the Tautomeric Form and Proton Position in the Intramolecular Bridges of o-Hydroxy Schiff Bases

Abstract

Two analogous Schiff bases, (S,E)-2-((1-hydroxy-3-methyl-1,1-diphenylbutan-2-ylimino)methyl)phenol (1) and (S,Z)-2-hydroxy-6-((1-hydroxy-3-methyl-1,1-diphenylbutan-2-ylamino)methylene)cyclohexa-2,4-dienone (2), exist in the solid state as phenol-imine and keto-amine tautomers, respectively. Their crystal structures were solved using the X-ray diffraction method. Sample 1 forms orthorhombic crystals of space group P2(1)2(1)2(1), while 2 forms monoclinic crystals of space group P2(1). In each sample, one molecule is in the asymmetric unit of the crystal structure. One-dimensional and two-dimensional solid state NMR techniques were used for structure assignment and for inspection of the (13)C and (15)N δ(ii) of the chemical shift tensor (CST) values. NMR study indicates that the span (Ω = δ(11)-δ(33)) and the skew (κ = 3(δ(22)-δ(iso)/Ω) are extremely sensitive to change in the tautomeric form of the Schiff bases. Theoretical calculations of NMR shielding parameters for 1 and 2 and a model compound with reduced aliphatic residue were performed using the GIAO method with B3LYP functional and 6-311++g(d,p) basis sets. From comparative analysis of the experimental and theoretical parameters, it was concluded that the position of hydrogen in the intramolecular bridge has tremendous influence on (13)C and (15)N CST parameters. Inspection of Ω and κ parameters allowed for the establishment of the nature of the hydrogen bonding and the assignment of the equilibrium proton position in the intramolecular bridges in the solid state.

Bidentate Schiff bases derived from (S)-α-methylbenzylamine as chiral ligands in the electronically controlled asymmetric addition of diethylzinc to aldehydes

“…3). The appropriate molecular calculation studies are currently in progress also for other SB ligands 37 . The tautomerization of Schiff bases was described in literature and is important phenomenon for understanding many physicochemical properties, such as photo-and thermochromism as well as the biological activity 38 .…”

NMR crystallography of molecular organics

“…Analogous work on estimating "ADPs" from 1 H shifts [230] is discussed in Section 3.2.3. 13 C and 15 N shift tensors calculated on isolated molecules have also been shown to be quite sensitive, via the σ 22 principal value, to the position of hydrogen within an intramolecular O...H…N bond in a pair of Schiff bases [231].…”

The structures of two aldazines: [1,1′‐(1E,1′E)‐hydrazine‐1,2‐diylidenebis(methan‐1‐yl‐1‐ylidene)dinaphthalen‐2‐ol] (Lumogen) and 2,2′‐(1E,1′E)‐hydrazine‐1,2‐diylidenebis(methan‐1‐yl‐1‐ylidene)diphenol (salicylaldazine) in the solid state and in solution

“…could explain the anomalies of the signals of carbon atoms involved in the pseudo six‐membered ring (CH, C1 and C2) modifying the position of the proton involved in the HB. This has been demonstrated for Schiff bases by Potrzebowski et al . These authors carried out a series of calculations moving the H along the intramolecular HB finding that its positions is determinant to explain the experimental results.…”

“…More recently, Potrzebowski et al . reported the case of two very similar Schiff bases one existing in the conventional phenol‐imine tautomer a and the other in the less usual keto‐amine tautomer c (there is an OH group ortho to the carbonyl) . Compound 4 was also studied by NMR concluding that both tautomers a and c were present .…”