Metal—metal interaction in ligand-bridged dimolybdenum(0,0) and -(I,0) complexes with very small frontier orbital gaps: electrochemistry and spectroscopic properties of three neighboring oxidation states☆

Abstract Complexes (μ-L)[Mo(CO) 2 (P n Bu 3 ) 2 ] 2 with cis -carbonyl ligands and symmetrically bridging bis(α-diimine) chelate ligands L = 2,2′-bipyrimidine (bpym) and 2,5-bis(2-pyridyl)pyrazine (2,5-bppz) were synthesized and studied in different oxidation states by cyclic voltammetry, 1 H NMR, EPR, IR and UV-Vis-NIR absorption spectroscopy. The combination of two very electron rich d 6 metal centers with one π accepting bridging ligand results in rather small HOMO-LUMO energy gaps, as evident from differences of about 1 V between the potentials for reversible oxidation and reduction and from intense charge transfer absorption at about 1 eV, i.e. in the near-infrared (NIR) region. Whereas the shifts of the carbonyl vibrational bands are about equal for the reduction (low-energy shift) and the oxidation (high-energy shift) of the more stable bpym complex, the EPR results reveal occupation of the α-diimineπ ∗ orbitals in the anionic forms and an Mo(I)/Mo(0) mixed-valent state with a comproportionation constant K c greater than 10 8 for the cation {(bpym)[Mo(CO) 2 (P n Bu 3 ) 2 ] 2 } + . Electro transitions observed by UV-Vis-IR spectroelectrochemistry confirm these assignments of the redox orbitals; the maximum of the weak metal-to-metal charge transfer band of the mixed-valent complex {(bpym)[Mo(CO) 2 (P n Bu 3 ) 2 ] 2 } + was found at 3700 mm