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,II))(2)] and [Cu(L (I,II))(2)] fabricated from the Schiff base dyes o-HOC(6)H(4)—CH=N—cyclo-C(6)H(11) (HL (I)) and o-HOC(10)H(6)—CH=N—cyclo-C(6)H(11) (HL (II)), 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,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,II))(2)] and [Cu(L (I,II))(2)]. 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 CH(2)Cl(2), 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.