Stabilisation of true π-electron–π-electron interactions in an inorganic cocrystal

Abstract Using the 1:2 condensate of hydrazine and 4-methylimidazole-5-carboxaldehyde as an N-donor ligand (L), two green copper(II) dinuclear complexes [Cu 2 L 4 ](PF 6 ) 4 ·2H 2 O ( 1 ) and [Cu 2 L 3 ](NO 3 ) 4 ·H 2 O ( 2 ) are synthesised. In the X-ray crystal structures, both the compounds contain the centrosymmetric cation [Cu 2 L 3 ] 4+ where the metal is pentaco-ordinated with a square pyramidal geometry. The solid state X-band EPR at 4 K is axial with g ||  = 2.20 and g ⊥  = 2.06 for 1 , and, g ||  = 2.19 and g ⊥  = 2.05 for 2 , indicating a d x 2 - y 2 ground state. Variable temperature (2–300 K) magnetic susceptibility measurements on 1 reveal that the exchange coupling constant between the two copper(II) centres is negligible. In 1 , the cation is sandwiched between two unco-ordinated ligands. The free ligand stacks at distances of ∼3.6 A more or less parallel to the co-ordinated ligand, with several interatomic distances less than 3.50 A indicating π–π stacking. Currently, by π–π stacking, interactions between solely arenes are implicated. But here is a case of hetero-olefin–hetero-olefin interactions. Thus complex 1 represents interactions among π electrons in the true sense. These observations are backed by DFT calculations, which show that the stacking energy is −19.5 kcal mol −1 in the gas phase. The DFT calculations also indicate that the observed stacking in 1 is not a result of tolerated short contacts. Since this stacking does not occur in 2 , the counter anions seem to play a definite role in the formation of the cocrystal obtained in 1 .