Supramolecular Structures Of Ni-Ii And Cu-Ii With The Sterically Demanding Schiff Base Dyes Driven By Cooperative Action Of Preagostic And Other Non-Covalent Interactions

This work reports on synthesis and extensive experimental and theoretical investigations on photophysical, structural and thermal properties of the Ni-II and Cu-II discrete mononuclear homoleptic complexes [Ni(L-I,L-II)(2)] and [Cu(L-I,L-II)(2)] fabricated from the Schiff base dyes o-HOC6H4-CH=N-cyclo-C6H11 (HLI) and o-HOC10H6-CH=N-cyclo-C6H11 (HLII), containing the sterically crowding cyclohexyl units. The six-membered metallocycles adopt a clearly defined envelope conformation in [Ni(L-II)(2)], while they are much more planar in the structures of [Ni(L-I)(2)] and [Cu(L-I,L-II)(2)]. It has been demonstrated by in-depth bonding analyses based on the ETS-NOCV and Interacting Quantum Atoms energy-decomposition schemes that application of the bulky substituents, containing several C-H groups, has led to the formation of a set of classical and unintuitive intra- and inter-molecular interactions. All together they are responsible for the high stability of [Ni(L-I,L-II)(2)] and [Cu(L-I,L-II)(2)]. More specifically, London dispersion dominated intramolecular C-H center dot center dot center dot O, C-H center dot center dot center dot N and C-H center dot center dot center dot H-C hydrogen bonds are recognized and, importantly, the attractive, chiefly the Coulomb driven, preagostic (not repulsive anagostic) C-H center dot center dot center dot Ni/Cu interactions have been discovered despite their relatively long distances (similar to 2.8-3.1 angstrom). All the complexes are further stabilized by the extremely efficient intermolecular C-H center dot center dot center dot pi(benzene) and C-H center dot center dot center dot pi(chelate) interactions, where both the charge-delocalization and London dispersion constituents appear to be crucial for the crystal packing of the obtained complexes. All the complexes were found to be photoluminescent in CH2Cl2, with [Cu(L-II)(2)] exhibiting the most pronounced emission - the time-dependent density-functional-theory computations revealed that it is mostly caused by metal-to-ligand charge-transfer transitions.