Synthesis and spectroscopic characterization of 1,2-divanadium(v)-substituted α-Dawson polyoxotungstate-based 1 ∶ 1-type Cp*Rh2+ complex showing three different supporting sites of the Cp*Rh2+ group

Abstract

A novel Dawson polyoxotungstate-based 1 ∶ 1-type Cp*Rh²⁺ complex (Cp* = C₅Me₅), (NBu₄)₆[(Cp*Rh)(α-1,2-P₂W₁₆V₂O₆₂)] 2, was synthesized as an analytically pure, brown powder by 1 ∶ 1 molar ratio reaction in ice-cooled CH₂Cl₂ under air of 1,2-divanadium(V)-substituted α-Dawson polyoxotungstate (NBu₄)₈[α-1,2-P₂W₁₆V₂O₆₂] 1 with the dimeric precursor [Cp*RhCl₂]₂ without the use of AgBF₄. The unprotonated polyoxotungstate support 1 was prepared via synthesis of K₈[α-1,2-P₂W₁₆V₂O₆₂], (NBu₄)₆H₂[α-1,2-P₂W₁₆V₂O₆₂], and deprotonation with aqueous NBu₄OH. Solution (⁵¹V, ³¹P, ¹⁸³W, ¹H and ¹³C) NMR measurements in CD₂Cl₂ of 2 have revealed that it contained three species, 2a with C_s symmetry, 2b with C₁ symmetry and 2c with C_s symmetry, based on different supporting sites of the Cp*Rh²⁺ group on the surface oxygen atoms in the three edge-shared octahedra (B-site, V₂W surface). The Cp*Rh²⁺ group in 2a, 2b and 2c was covalently bound on the V₂W surface, and the distribution of the species in ca. 1 ∶ 2 ∶ 1 ratio was not dependent on the initial concentration of [Cp*RhCl₂]₂ and temperature of the CH₂Cl₂ solution (from room temperature to −90 °C). The brown powder containing 2a, 2b and 2c was readily crystallized in CH₂Cl₂ solution at −20 °C to give dark red crystals, which initially contained one predominant species, but, then, reverted to the 1 ∶ 2 ∶ 1 mixture of the three species during 5 h in CH₂Cl₂ solution at room temperature. The polyoxoanion-supported Cp*Rh²⁺ complex 2 is unique and can be compared with the previous Dawson polyoxotungstate-based organometallic complexes.