Atom transfer chemistry and electrochemical behavior of Mo(VI) and Mo(V) trispyrazolylborate complexes: new mononuclear and dinuclear species

Metathesis of TpPrMoO2Cl (TpPr=hydrotris(3-isopropylpyrazolyl)borate) with phenol/NEt3 or thiol/NEt3 mixtures results in the formation of air-stable, diamagnetic cis-dioxo-Mo(VI) complexes, TpPrMoO2X (X=OC6H4sBu-2, OC6H4R-3 (R=Me, tBu), OC6H4R-4 (R=Br, Et, OMe), OC6H3tBu2-3,5, OC6H3tBuMe-2,4, SnBu, SsBu). The complexes were characterized by analytical, spectroscopic and mass spectrometric methods, which support the formulations, ligands present [IR: ν(MoO2) approximately 930 and 900 cm−1, ν(BH) 2570–2480 cm−1, OAr 1605–1500 and 1300–1250 cm−1] and assigned molecular symmetries (1H/13C NMR). Electrochemical studies revealed reversible or quasi-reversible (X=OC6H3tBuMe-2,4), one-electron reduction processes with E1/2 values linearly correlated with the electron donor capacity of the ligand X (as expressed by Hammett functions). Oxygen atom transfer and comproportionation resulted in the formation of [Tp*MoO(OPh)]2(μ-O) (Tp*=hydrotris(3,5-dimethylpyrazolyl)borate) in the reaction of Tp*MoO2(OPh) with PEt3 in toluene (in contrast to the formation of phosphine oxide complexes in the TpPr system), while hydrolysis of Tp*MoOCl2 produced [Tp*MoOCl]2(μ-O). Reaction of TpPrMoO2Cl with P(OiPr)3 produced [TpPrMoOCl](μ-O)[TpPrMoO(OH)]. The X-ray crystal structures of [Tp*MoOCl]2(μ-O) and [TpPrMoOCl](μ-O)[TpPrMoO(OH)] are reported. Both compounds exhibit corner shared bioctahedral structures with a single bridging oxo ligand linking TpXMoOX (X=Cl, OH) moieties. Chlorine atom transfer from TpPrMoOCl2 to PPh3 in pyridine (py) or lutidine (lut) produces TpPrMoOCl(py) and TpPrMoOCl(lut), respectively.