A stable monomeric nickel borohydride

by Desrochers, P. J.; LeLievre, S.; Johnson, R. J.; Lamb, B. T.; Phelps, A. L.; Cordes, A. W.; Gu, W. W.; Cramer, S. P.

A stable discrete nickel borohydride complex (Tp*NiBH4 or Tp*NiBD4) was prepared using the nitrogen-donor ligand hydrotris(3,5-dimethylpyrazolyl) borate (Tp*(-)). This complex represents one of the best characterized nickel(II) borohydrides to date. Tp*NiBH4 and Tp*NiBD4 are stable toward air, boiling water, and high temperatures (mp > 230 degreesC dec), X-ray crystallographic measurements for Tp*NiBH4 showed a six-coordinate geometry for the complex, with the nickel(II) center facially coordinated by three bridging hydrogen atoms from borohydride and a tridentate Tp*- ligand. For Tp*NiBH4, the empirical formula is C15H26B2N6Ni, a = 13.469(g) Angstrom, b = 7.740(l) A, c = 18.851(2) Angstrom, beta = 107.605(9)degrees, the space group is monoclinic P2(1)/c, and Z = 4. Infrared measurements confirmed the presence of bridging hydrogen atoms; both nu(B-H)terminal and nu(B-H)bridging are assignable and shifted relative to nu(B-D) of Tp*NiBD4 by amounts in agreement with theory. Despite their hydrolytic stability, Tp*NiBH4 and Tp*NiBD4 readily reduce halocarbon substrates, leading to the complete series of Tp*NiX complexes (X = Cl, Br, I). These reactions showed a pronounced hydrogen/deuterium rate dependence (kH/kD approximate to 3) and sharp isosbestic points in progressive electronic spectra. Nickel K-edge X-ray absorption spectroscopy (XAS) measurements of a hydride-rich nickel center were obtained for Tp*NiBH4, Tp*NiBD4, and Tp*NiCl. X-ray absorption near-edge spectroscopy results confirmed the similar six-coordinate geometries for Tp*NiBH4 and Tp*NiBD4. These contrasted with XAS results for the crystallographically characterized pseudotetrahedral Tp*NiCl complex. The stability of Tp*Ni-coordinated borohydride is significant given this ion's accelerated decomposition and hydrolysis in the presence of transition metals and simple metal salts.

Journal
Inorganic Chemistry
Volume
42
Issue
24
Year
2003
Start Page
7945-7950
URL
https://dx.doi.org/10.1021/ic034687a
ISBN/ISSN
1520-510X; 0020-1669
DOI
10.1021/ic034687a