Quasi-aromatic Möbius Metal Chelates

Abstract

We report the design as well as structural and spectroscopic characterizations of two new coordination compounds obtained from Cd(NO)·4HO and polydentate ligands, benzilbis(pyridin-2-yl)methylidenehydrazone (L) and benzilbis(acetylpyridin-2-yl)methylidenehydrazone (L), in a mixture with two equivalents of NHNCS in MeOH, namely [Cd(SCN)(NCS)(L)(MeOH)] (1) and [Cd(NCS)(L)(MeOH)] (2). Both L and L are bound via two pyridyl-imine units yielding a tetradentate coordination mode giving rise to the 12 π electron chelate ring. It has been determined for the first time (qualitatively and quantitatively), using the EDDB electron population-based method, the HOMA index, and the ETS-NOCV charge and energy decomposition scheme, that the chelate ring containing Cd can be classified as a quasi-aromatic Möbius motif. Notably, using the methyl-containing ligand L controls the exclusive presence of the NCS connected with the Cd atom (structure 2), while applying L allows us to simultaneously coordinate NCS and SCN ligands (structure 1). Both systems are stabilized mostly by hydrogen bonding, C-H···π interactions, aromatic π···π stacking, and dihydrogen C-H···H-C bonds. The optical properties have been investigated by diffused reflectance spectroscopy as well as molecular and periodic DFT/TD-DFT calculations. The DFT-based ETS-NOCV analysis as well as periodic calculations led us to conclude that the monomers which constitute the obtained chelates are extremely strongly bonded to each other, and the calculated interaction energies are found to be in the regime of strong covalent connections. Intramolecular van der Waals dispersion forces, due to the large size of L and L, appeared to significantly stabilize these systems as well as amplify the aromaticity phenomenon.

Electron Delocalization in Planar Metallacycles: Hückel or Möbius Aromatic?

“…However, the results of the EDDB orbital-decomposition presented in Figure 5b seem to rationally reconcile this incompatibility between structural, topological, and magnetic characteristics unambiguously identifying the cupraheterobicycle as quasi-aromatic. [28] Indeed, since neither 3d xz nor 3d yz contribute to electron delocalization (acting as doubly-occupied lone-pair-like orbitals) there is no cyclic delocalization in each heterocyclic unit and consequently no diatropic ring current can be observed; this is in full agreement with the previous findings by Krygowski et al [28a] On the other hand, electrostatic interactions with the copper cation (the natural charge on Cu is + 1.308) and noticeable σdelocalization involving 3d x2-y2 orbital and the lone-pairs from heteroatoms (which seem to hold the structure in a plane) support delocalization of π-electrons in both quasi-rings. It is worth noting that, according to the π-EDDB R and LCAO π-MObased electron populations, only a half of the total number of π-electrons in the heterocyclic quasi-rings (6.076 j e j) is delocalized, which gives on average 0.608 j e j per each quasi-ring member; for comparison, the corresponding values for furan and pyrrole are 0.503 j e j and 0.724 j e j , respectively.…”

Structural versatility of the quasi-aromatic Möbius type zinc(ii)-pseudohalide complexes – experimental and theoretical investigations

“…, where L III is formed upon hydrolysis of one of the 2-PyC(Ph) functions of L I , 27 and neutral mononuclear complex [Cd(NCS) 2 (L II )(MeOH)] (5), 24 respectively. The FTIR spectrum of 1 contains a characteristic intense band at 2073 cm À1 attributed to the CN stretching of NCS À .…”

“…32,33 Complexes 2 and 3 each exhibit a salt-like structure (Scheme 1), as opposed to our previously studied Cd II based counterparts. [23][24][25][26][27] In 2, the cationic part exhibits a doubly charged centrosymmetric dinuclear structure, were two Zn II centers are interlinked via two m 1,1 -N 3 À anions and the coordination domain of each metal is lled by the tetracoordinated ligand L I (Fig. 2).…”

“…[23][24][25][26][27] Moreover, we were able to demonstrate for the rst time that the helical motif in the obtained complexes together with the chelate metalloring correspond to a quasi-aromatic Möbius object. 24,27 Herein, we report Zn(NCS) 2 -and Zn(N 3 ) 2 -derived structures with 1,2-diphenyl-1,2-bis((phenyl(pyridin-2-yl)methylene) hydrazono)ethane (L I ) and benzilbis(acetylpyridin-2-yl) methylidenehydrazone (L II ). Using thiocyanate (NCS À ) and azide (N 3 À ) counterions is intriguing and of great interest since both anions are known to be ambidentate ligands, which can bind metal centers in different coordination modes.…”

“…[18][19][20] Recently, we have also directed our attention to Schiff bases comprising two pyridyl-imine functions obtained from benzyldihydrazone. [21][22][23][24][25][26][27] These ligands were found to be efficient for helical structures upon coordination to metal centers. Particularly our comprehensive efforts were directed to various Cd II salts as complexing agents.…”