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-HOC₆H₄—CH=N—cyclo-C₆H₁₁ (HL ^I) and o-HOC₁₀H₆—CH=N—cyclo-C₆H₁₁ (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 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.