1,2-Borotropic shifts and B–N bond cleavage reactions in molybdenum hydrotris(3-isopropylpyrazolyl)borate chemistry: Mixed-valence MoVIMo2V and pyrazole-rich oxo-MoIV complexes

Abstract Red–black [Tp i Pr∗ Mo V O] 2 (μ-O)(μ-Mo VI O 4 ) ( 1 , Tp i Pr∗ = hydrobis(3-isopropylpyrazolyl)(5-isopropylpyrazolyl)borate) has been isolated as a by-product in the synthesis of NEt 4 [Tp i Pr Mo(CO) 3 ] (Tp i Pr = hydrotris(3-isopropylpyrazolyl)borate) and characterized by spectroscopic and X-ray crystallographic techniques. The trinuclear, mixed-valence complex contains two distorted octahedral anti -Tp i Pr∗ Mo V O centers bridged by bent oxo (Mo–O–Mo av. 158.7°) and tetrahedral κO,κO′-molybdate ligands. The complex contains a six-membered, non-planar Mo 3 (μ-O) 3 core and two 1,2-borotropically-shifted Tp i Pr∗ ligands (with the shifted pyrazolyl trans to Mo V =O). Aerial decomposition of solid NEt 4 [Tp i Pr Mo(CO) 3 ] produces sky-blue, diamagnetic Tp i Pr MoO( i Prpz)( i PrpzH) ( 2 , i Prpz – = 3-isopropylpyrazolate, i PrpzH = 3-isopropyl-2 H -pyrazole). Molecules of 2 feature a tridentate fac -Tp i Pr ligand and mutually cis terminal oxo (Mo O = 1.665(2) A) and monodentate i Prpz − and i PrpzH ligands. The latter are formed by B–N bond cleavage of Tp i Pr . The complex can also be synthesized by reacting NEt 4 [Tp i Pr Mo(CO) 3 ] with excess 3-isopropylpyrazole and dioxygen at 100 °C. Cleavage of the B–N bond(s) of Tp i Pr was also observed in the formation of Tp i Pr MoO(SPh)( i PrpzH) ( 3 ) as a by-product in the synthesis of Tp i Pr MoO 2 (SPh). In the monohydrate, 3 exhibits a distorted octahedral geometry defined by a tridentate fac -Tp i Pr ligand and mutually cis terminal oxo (Mo O = 1.676(3) A) and monodentate SPh − and i PrpzH ligands. The pyrazole β-NH group is observed to participate in a hydrogen-bond to the lattice water molecule. The complex can be synthesized in high yield by reducing Tp i Pr MoO 2 (SPh) by HSPh or PPh 3 in the presence of excess 3-isopropylpyrazole.