2,5-Bis(1-phenyliminoethyl)pyrazine (bpip): a conjugated metal–metal bridging acceptor ligand and its homodinuclear complexes with low-valent metal centres

Complexes of 2,5-bis(1-phenyliminoethyl)pyrazine (bpip), a symmetrical bis(bidentate) ligand which acts through two different chelate donor centres, one imine and one azine nitrogen atom per metal chelate site, have readily been obtained with Cr(CO)4, Mo(CO)4, W(CO)4, Mn(CO)3Cl, Re(CO)3Cl, [Ru(bpy)2]2+ and [Cu(PPh3)2]+ fragments. The ‘free’ ligand and the dinuclear [{Cu(PPh3)2}2(µ-bpip)][BF4]2 were characterised crystallographically. The low-energy molecular conformation of free bpip is qualitatively different from the planar syn/trans/syn arrangement required in the bpip-bridged dicopper(I) compound (Cu· · ·Cu distance 6.944 Å). Structural preferences and the π acceptor properties of the conjugated bridging ligand bpip were studied using DFT calculations of the bpip0/–/2– chromophore and combined electrochemical and spectroscopic methods; bpip is a stronger π acceptor than the related 2,5-bis(2-pyridyl)pyrazine but a poorer mediator of metal–metal interaction.