TY - JOUR
T1 - Synthesis, Characterization, and X-ray Crystal Structures of the Divalent Titanium Complex Ti(η2-bh4)2(dmpe)2and The Unidentate Tetrahydroborate Complex V(η1-bh4)2(dmpe)2
AU - Jensen, James A.
AU - Girolami, Gregory S.
PY - 1989/5/1
Y1 - 1989/5/1
N2 - The reaction of the coordination complexes MCl2(dmpe)2, dmpe = l,2-bis(dimethylphosphino)ethane, with LiBH4or NaBH4leads to the divalent transition-metal tetrahydroborate complexes M(BH4)2(dmpe)2, where M is Ti or V, The titanium complex is a thermally robust and oxidatively stable paramagnetic species with very weakly bound bidentate BH4-groups based on infrared spectroscopy. The X-ray crystal structure of Ti(η2-BH4)2(dmpe)2reveals that this compound adopts an eight-coordinate geometry related to a square-prismatic structure that is, however, best described as a trans octahedron with the bidentate BH4-groups occupying the axial sites. The two independent Ti-P distances are 2.627 (1) and 2.625 (1) A, while the bidentate tetrahydroborate groups are bonded to the titanium center with distances of Ti-Hb= 2.04 (2) and 2.09 (2) A and Ti---B = 2.534 (3) A. The H-B-H angles are all near the tetrahedral value of 109°, which is consistent with the weak Ti-BH4interaction deduced by IR spectroscopy. A correlation of the average B-Htand B-HbIR stretching frequencies for a variety of bidentate tetrahydroborate complexes in the literature provides a useful gauge of the relative strengths of M-BH4interactions. Crystal data for C12H40B2P4Ti: space group P21/n,a = 9.404 (2) A, b = 13.088 (2) A, c = 9.466 (2) Á,β = 96.12 (1)°, V= 1158.4 (4) A3, Z = 2, RF= 3.6%, R„F= 3.5% for 2139 reflections and 168 variables. In contrast to the titanium results, the IR spectrum of the vanadium complex is indicative of unidentate BH4-coordination, as shown by the strong BH3deformation mode at 1057 cm-1. The X-ray crystal structure of V(η1-BH4)2(dmpe)2confirms this assignment; this is the first molecule that possesses more than one unidentate BH4-group and is of additional interest since, unlike most other η1-BH4complexes, its electron count is less than 18. The vanadium center adopts a trans octahedral structure, with V-P = 2.499 (1) and 2.506 (1) A, V-H = 1.88 (3) A, V---B = 2.833 (4) A, and V-H-B = 140 (3)°. Crystal data for C12H40B2P4V; space group P21/n, a = 8.469 (2) A, b = 13.735 (4) A, c = 9.666 (7) A, β = 96.14 (3)°, V = 1118 (2) A3, Z = 2, r F = 3.2%, RwF= 3.0% for 1629 reflections and 168 variables. The differences in the structures of Ti(BH4)2(dmpe)2and V(BH4)2(dmpe)2may be ascribed to the smaller atomic radius of vanadium and to the preference of d3V11centers to adopt six-coordinate geometries. Attempts to prepare the analogous chromium(II) complex lead instead to the paramagnetic hydride complex Cr(η1-BH4)H(dmpe)2, which possesses one unidentate BH4-ligand based on its IR spectrum. in the presence of BH3-L, this complex is in equilibrium with Cr(BH4)2(dmpe)2in solution. A comparative summary is presented of the structural parameters of unidentate BH4-complexes of the transition metals, and a reaction coordinate is mapped out for terminal-bridge hydrogen exchange processes involving tetrahydroborate groups.
AB - The reaction of the coordination complexes MCl2(dmpe)2, dmpe = l,2-bis(dimethylphosphino)ethane, with LiBH4or NaBH4leads to the divalent transition-metal tetrahydroborate complexes M(BH4)2(dmpe)2, where M is Ti or V, The titanium complex is a thermally robust and oxidatively stable paramagnetic species with very weakly bound bidentate BH4-groups based on infrared spectroscopy. The X-ray crystal structure of Ti(η2-BH4)2(dmpe)2reveals that this compound adopts an eight-coordinate geometry related to a square-prismatic structure that is, however, best described as a trans octahedron with the bidentate BH4-groups occupying the axial sites. The two independent Ti-P distances are 2.627 (1) and 2.625 (1) A, while the bidentate tetrahydroborate groups are bonded to the titanium center with distances of Ti-Hb= 2.04 (2) and 2.09 (2) A and Ti---B = 2.534 (3) A. The H-B-H angles are all near the tetrahedral value of 109°, which is consistent with the weak Ti-BH4interaction deduced by IR spectroscopy. A correlation of the average B-Htand B-HbIR stretching frequencies for a variety of bidentate tetrahydroborate complexes in the literature provides a useful gauge of the relative strengths of M-BH4interactions. Crystal data for C12H40B2P4Ti: space group P21/n,a = 9.404 (2) A, b = 13.088 (2) A, c = 9.466 (2) Á,β = 96.12 (1)°, V= 1158.4 (4) A3, Z = 2, RF= 3.6%, R„F= 3.5% for 2139 reflections and 168 variables. In contrast to the titanium results, the IR spectrum of the vanadium complex is indicative of unidentate BH4-coordination, as shown by the strong BH3deformation mode at 1057 cm-1. The X-ray crystal structure of V(η1-BH4)2(dmpe)2confirms this assignment; this is the first molecule that possesses more than one unidentate BH4-group and is of additional interest since, unlike most other η1-BH4complexes, its electron count is less than 18. The vanadium center adopts a trans octahedral structure, with V-P = 2.499 (1) and 2.506 (1) A, V-H = 1.88 (3) A, V---B = 2.833 (4) A, and V-H-B = 140 (3)°. Crystal data for C12H40B2P4V; space group P21/n, a = 8.469 (2) A, b = 13.735 (4) A, c = 9.666 (7) A, β = 96.14 (3)°, V = 1118 (2) A3, Z = 2, r F = 3.2%, RwF= 3.0% for 1629 reflections and 168 variables. The differences in the structures of Ti(BH4)2(dmpe)2and V(BH4)2(dmpe)2may be ascribed to the smaller atomic radius of vanadium and to the preference of d3V11centers to adopt six-coordinate geometries. Attempts to prepare the analogous chromium(II) complex lead instead to the paramagnetic hydride complex Cr(η1-BH4)H(dmpe)2, which possesses one unidentate BH4-ligand based on its IR spectrum. in the presence of BH3-L, this complex is in equilibrium with Cr(BH4)2(dmpe)2in solution. A comparative summary is presented of the structural parameters of unidentate BH4-complexes of the transition metals, and a reaction coordinate is mapped out for terminal-bridge hydrogen exchange processes involving tetrahydroborate groups.
UR - http://www.scopus.com/inward/record.url?scp=0000373780&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=0000373780&partnerID=8YFLogxK
U2 - 10.1021/ic00310a019
DO - 10.1021/ic00310a019
M3 - Article
AN - SCOPUS:0000373780
SN - 0020-1669
VL - 28
SP - 2107
EP - 2113
JO - Inorganic Chemistry
JF - Inorganic Chemistry
IS - 11
ER -