Phosphine Complexes of Trivalent Early Transition Metals. X-ray Crystal Structure of the d¹-d¹Dimer [TiCl₃(dippe)]₂and Synthesis of the Alkyl CrMeCl₂(dippe)(thf)

Treatment of TiCl₃(thf)₃(thf = tetrahydrofuran) with the chelating phosphine 1,2-bis(diisopropylphosphino)ethane (dippe) gives two products: the purple dimer [TiCl₃(dippe)]₂and the green monomer TiCl₃(dippe)(thf). The two compounds may be readily interconverted by loss or addition of the thf ligand. The dimer is coupled antiferromagnetically, and its X-ray crystal structure reveals an octahedral geometry about each titanium(III) center and a Ti…Ti distance of 3.44 Å. The dippe ligands each chelate a titanium atom and are trans to the bridging chlorides; the dimer thus possesses approximate D_2hsymmetry. Reactions of MCl₃(thf)₃, where M is vanadium or chromium, with dippe yield the monomeric thf adducts MCl₃(dippe)(thf). The bromo-vanadium analogue VBr₃(dippe)(thf) and the chromium acetonitrile adduct CrCl₃(dippe)(MeCN) are also described. Addition of dippe to CrMeCl₂(thf)₃gives the chromium(III) alkyl CrMeCl₂(dippe)(thf) which has been shown to be a slow ethylene polymerization catalyst at room temperature. NMR and EPR studies of the dippe adducts are described. Crystal data for [TiCl₃(dippe)]₂·PhMe: orthorhombic, space group Pbcn, with a = 15.586 (5) Å, b = 14.442 (4) Å, c = 21.639 (6) Å,V = 4871 (2) ų, Z = 4, R_F= 6.4%, R_wF= 6.6% for 196 variables and 1231 data with I > 2.58σ(I).