Homo- and heterodinuclear complexes of the tetrakis(pyrazolyl)borate ligand

Abstract

Monometallic complexes of the tetrakis(pyrazolyl)borate ligand [ML₂{B(pz)₄}] {M = Rh, Ir; L₂ = η-cod, η-nbd, (CO)₂, (CO)(PPh₃)} have two free pyrazolyl rings which can be coordinated to a second ML₂ unit to give the dimeric compounds [L₂M{μ-B(pz)₄}ML₂]⁺, and to a metal halide to give heterobimetallic species [L₂M{μ-B(pz)₄}M′Cl₂]. ¹H NMR spectroscopy shows that [(η-cod)Rh{μ-B(pz)₄}Rh(η-cod)]⁺1⁺, [(η-nbd)Rh{μ-B(pz)₄}Rh(η-nbd)]⁺2⁺, [(η-cod)Ir{μ-B(pz)₄}Ir(η-cod)]⁺3⁺ and [(CO)₂Rh{μ-B(pz)₄}Rh(CO)₂]⁺4⁺ are fluxional at room temperature. Cooling a solution of [(η-cod)Rh{μ-B(pz)₄}Rh(η-cod)]⁺1⁺ to −90 °C slows the fluxional process, which involves inversion of the two B-(N–N)₂-M six-membered rings. Attempts to synthesise the asymmetric complexes [(η-cod)Rh{μ-B(pz)₄}Rh(η-nbd)]⁺7⁺, [(η-cod)Rh{μ-B(pz)₄}Ir(η-cod)]⁺8⁺ and [(η-cod)Rh{μ-B(pz)₄}Rh(CO)₂]⁺9⁺ produced a mixture of [L₂M{μ-B(pz)₄}ML₂]⁺, [L′₂M′{μ-B(pz)₄}M′L′₂]⁺ and the desired species. The heterobimetallic complexes [L₂Rh{μ-B(pz)₄}M′Cl₂] (M′ = Co, L₂ = η-cod 10; M′ = Co, L₂ = η-nbd 11; M′ = Co, L = CO 12; M′ = Co, L₂ = (CO)(PPh₃) 13; M′ = Zn, L₂ = η-cod 14; M′ = Zn, L₂ = η-nbd 15; M′ = Pd, L₂ = η-cod 16) possess square planar Rh(I) linked to square planar Pd(II) or tetrahedral Zn(II) and Co(II) centres. The paramagnetic Co(II) complexes give ¹H NMR spectra with signals shifted over a range of 75 ppm. The UV-Vis spectra of 10–13 show four bands, one MLCT at Rh and three d-d transitions arising from the spltting of the ⁴T₁(P) excited state due to approximate C_2v symmetry at Co.