Preparation and characterization of a new series of Cr(II) hydroborates

Abstract

The synthesis and characterization of a new series of mono-, di-, and trinuclear Cr(II) borohydride compounds is described. The reaction of CrCl2(TMEDA) with two equivalents of NaBH4 afforded the thermally unstable (TMEDA)Cr(BH4)2 (1), which was converted by treatment with pyridine into the octahedral monomeric (Py)4Cr(BH4)2 (2). The reaction proceeds via formation of an intermediate trinuclear complex {[(TMEDA)(Py)Cr(η2-BH4)]2[(Py)2Cr(η2-BH4)2]}(µ,η1-BH4)2 (3), which was isolated and characterized by X-ray crystallography. Reaction of 1 and 2 with both CO2 and RN=C=NR (R = Cy, iPr) afforded hydride insertion and formation of the corresponding diamagnetic lantern-type Cr(II) formate (HCO2)4Cr2Py2 (4) and formamidinate compounds [RNC(H)NR]2Cr2(µ-BH)4 (R = Cy (5a), iPr (5b)), respectively, with supershort Cr—Cr quadruple bonds. The structures of 1, 2, 3, and 5b were elucidated by X-ray analysis. Crystal data are as follows. 1: C6H24N2B2Cr, monoclinic, Cc, a = 8.517(2) Å, b = 15.921(5) Å, c = 9.624(2) Å, β = 115.59(1)°, Z = 4, R = 0.022, Rw = 0.029; 2: C28H44N4B2O2Cr, monoclinic, P21/n, a = 12.021(1) Å, b = 15.555(1) Å, c = 15.723(1) Å, β = 90.13(2)°, Z = 4, R = 0.074, Rw = 0.086; 3: C32H76N8B6Cr3, monoclinic, P21/n, a = 8.515(1) Å, b = 14.525(1) Å, c = 18.286(2) Å, β = 91.38(1)°, Z = 2, R = 0.051, Rw = 0.060; 5b: C21H49N6BCr2, monoclinic, C2/c, a = 17.000(1) Å, b = 9.033(1) Å, c = 19.160(1) Å, β = 105.579(9)°, Z = 4, R = 0.069, Rw = 0.078. Keywords: divalent chromium, borohydride, Cr—Cr quadruple bond.