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)(3)Cl, Re(CO)(3)Cl, [Ru(bpy)(2)](2+) and [Cu(PPh3)(2)](+) fragments. The 'free' ligand and the dinuclear [{Cu(PPh3)(2)}(2)(mu-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 Angstrom). Structural preferences and the pi acceptor properties of the conjugated bridging ligand bpip were studied using DFT calculations of the bpip(0/-/2-) chromophore and combined electrochemical and spectroscopic methods; bpip is a stranger pi acceptor than the related 2,5-bis(2-pyridyl)pyrazine but a poorer mediator of metal-metal interaction.