Complexes containing redox-active fluorenone-based ligands linked to redox-active tris(pyrazolyl)boratomolybdenum fragments: assignment of ligand-centred and metal-centred redox processes by EPR and UV/VIS/NIR spectroelectrochemistry

The dinuclear complex [{Mo(Tp(Me,Me))(NO)Cl}(2)(mu-L-2)] 2 [L-2 is the new bridging ligand 2,7-bis{2-(4-pyridyl)ethen-1-yl}fluorenone] contains two redox-active molybdenum(I) centres linked by a bridging ligand which is itself redox-active by virtue of the fluorenone spacer unit. The complex undergoes four one-electron reductions of which two are metal-centred Mo(0)/Mo(I) couples and two are reductions of the fluorenone unit. Also studied were the mononuclear analogue [Mo(Tp(Me,Me))(NO)Cl(L-1)] [L-1 is 2-bromo-7-{2-(4-pyridyl)ethen-1-yl}fluorenone], and the dinuclear complex [{Mo(Tp(Me,Me))(NO)Cl}(2)(mu-L-3)] 3 [L-3 is the bridging ligand 2,7-bis{2-(4-pyridyl)ethen-1-yl}fluorene] which lacks the ligand-centred redox activity. A combination of EPR and UV/VIS/NIR spectroelectrochemical techniques was used and showed how the separate metal-centred and ligand-centred reduction processes lead to quite distinct and characteristic spectroscopic signatures, such that it was possible to assign the sequence of reduction sites as ligand-metal-metal-ligand. The initial reduction of the bridging ligand in 2 results in a much larger separation between the two Mo(0)/Mo(I) couples (240 mV) than occurs in complexes where the bridging ligand is not redox-active.