Electronic and Molecular Structures of Heteroradialenes: A Combined Synthetic, Computational, Spectroscopic, and Structural Study Identifying IR Spectroscopy as a Simple but Powerful Experimental Probe

Abstract

The vicinity of a hydrogen bond donor (O-H) and a hydrogen bond acceptor (C=O or C=N-R) in salicylaldehydes and ortho-Schiff bases results in significant structural variations compared to the monosubstituted derivatives that are reflected in the electronic structure and thus in the spectroscopic properties. This interplay between intramolecular hydrogen bonding and multicenter π-electron delocalization is the origin of the concept of resonance-assisted hydrogen bonding (RAHB). Herein, the complexity is extended from one hydrogen bond donor-acceptor pair in salicylaldehyde and ortho-Schiff bases to three hydrogen bond donor-acceptor pairs in 2,4,6-tricarbonyl-and 2,4,6-triimine-substituted phloroglucinols (1,3,5-trihydroxybenzene), respectively. To evaluate the changes in the molecular and electronic structures, we have performed a comprehensive computational, spectroscopic, and structural study starting from monosubstituted benzene derivatives as references over ortho-disubstituted derivates to the sixfold-substituted derivatives. Whereas in salicylaldehydes, ortho-Schiff bases, and 2,4,6-tricarbonyl-phloroglucinols the phenolic O-protonated tautomers represent the energy minima, the N-protonated tautomers represent the energy minima in 2,4,6-triiminephloroglucinols. The analysis provides a keto-enamine resonance structure with six exocyclic double bonds to be dominant for these species reminiscent of [6]radialenes, which were termed heteroradialenes. These heteroradialenes are non-aromatic alicycles. However, the predominance of this resonance structure does not represent a sudden change going from the 2,4,6-tricarbonyl-to the 2,4,6-triimine-phloroglucinols, but a gradual increase of analogous resonance structure contributions is observed even in salicylaldehyde and ortho-Schiff bases demonstrating some hetero-orthoquinodimethane character. These changes are, besides in the molecular structures, well reflected in the IR spectra, which can therefore be used as a simple tool to probe the electronic structures in these systems. Interruption of the delocalized π system supporting the intramolecular hydrogen bond, i. e. going from 2,4,6-triimine-to 2,4,6-triamine-substituted phloroglucinols, reestablishes an O-protonated aromatic phloroglucinol system.

Aromatic Versus Heteroradialene Character in Extended Thiophloroglucinol Ligands and their Trinuclear Nickel(II) Complexes

“…The synthesis of the trinuclear Ni II 3 complexes [(bert tBu 2 )Ni 3 ], [(bert Me )Ni 3 ], [(bertdien)Ni 3 ], and [(habbi)-Ni 3 ] was reported recently. The extended phloroglucinol ligands H 6 feld Me , [38] H 3 felddien, [81] and H 3 O 3 [83] have also been reported. The separation of the thiourea byproduct by column chromatography proved to be not applicable for H 6 bert tBu 2 , H 6 bert Me , and H 3 bertdien but for H 6 habbi.…”

“…[68] The synthesis of extended thiophloroglucinols was based on the reaction of the trialdehyde 2,4,6-triformyl-1,3,5-tris(dimethyldithiocarbamoyl)benzene with six equivalents of a primary amine, which results in simultaneous Schiff-base formation and nucleophilic cleavage of the dithiocarbamate groups with concomitant formation of a thiourea byproduct, which exhibits similar solubility behavior to the extended thiophloroglucinol ligands. 2) As we have already synthesized and characterized the O analog H 3 O 3 , [83] the comparison of H 3 S 3 with H 3 O 3 without perturbation of the pendant arms allowed us to evaluate changes in the electronic structure with respect to the heteroradialene contribution of O versus S. [68] Thus, the first three ligands were prepared only in situ and directly transformed into their Ni II 3 complexes.…”

“…Evaluation of the data in Table 2 (Figure 2 b) shows two unresolved multiplets at about 11.4 and about 11.0 ppm assigned to the NH resonances and four doublets between 8.25 and 8.09 ppm assigned to the vinylic protons. [83] This pattern of signals is characteristic for N-protonated tris-Nsalicylalkylidenes in the heteroradialene formulation as a mixture of C 3h and a C s isomers (Scheme 2). [80-82, 89, 90] One set of signals can be assigned to the C 3h -symmetric isomer, and three sets of signals to the three inequivalent pendent …”

“…[38,[80][81][82][83] The band around 1610 cm À1 is assigned to a combination of the stretches of the exocyclic C=C double bonds with the stretches of the CÀN single bond of the C Ar =CÀN unit. [38,[80][81][82][83] The band around 1610 cm À1 is assigned to a combination of the stretches of the exocyclic C=C double bonds with the stretches of the CÀN single bond of the C Ar =CÀN unit.…”

“…[69][70][71][72][73] For example, nature utilizes strong metal-sulfur covalent bonds in iron-sulfur clusters and blue copper centers for biological electron-transfer sites. [38,[80][81][82][83][84] This ketoenamine resonance structure is reminiscent of [6]radialenes and has been termed heteroradialene. [38,[80][81][82][83][84] This ketoenamine resonance structure is reminiscent of [6]radialenes and has been termed heteroradialene.…”