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

Abstract

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 bpip^0/–/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.