Supramolecular structures of Ni and Cu 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 and Cu discrete mononuclear homoleptic complexes [Ni( )] and [Cu( )] fabricated from the Schiff base dyes -HOCH-CH=N--CH (H ) and -HOCH-CH=N--CH (H ), containing the sterically crowding cyclo-hexyl units. The six-membered metallocycles adopt a clearly defined envelope conformation in [Ni( )], while they are much more planar in the structures of [Ni( )] and [Cu( )]. 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( )] and [Cu( )]. More specifically, London dispersion dominated intramolecular C-H⋯O, C-H⋯N and C-H⋯H-C hydrogen bonds are recognized and, importantly, the attractive, chiefly the Coulomb driven, preagostic (not repulsive anagostic) C-H⋯Ni/Cu interactions have been discovered despite their relatively long distances (∼2.8-3.1 Å). All the complexes are further stabilized by the extremely efficient intermolecular C-H⋯π(benzene) and C-H⋯π(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 CHCl, with [Cu( )] 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.