Preparation and crystal structure of a non-symmetrical vanadium(II) dimer: tri-μ2-bromido-(hydrogen-tris(3-isopropyl-4-bromopyrazol-1-yl)borato-κ3N,N′,N′′)-tris(tetrahydrofuran-κO)divanadium(II) – tetrahydrofuran (1/1), C34H57BBr6N6O4V2