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†

Abstract

The dinuclear complex [{Mo(Tp^Me,Me)(NO)Cl}₂(µ-L²)] 2 [L² 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(L¹)] [L¹ is 2-bromo-7-{2-(4-pyridyl)ethen-1-yl}fluorenone], and the dinuclear complex [{Mo(Tp^Me,Me)(NO)Cl}₂(µ-L³)] 3 [L³ 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.