ic201775c_si_001.pdf (1.29 MB)
Electronic Structure of Nickel(II) and Zinc(II) Borohydrides from Spectroscopic Measurements and Computational Modeling
journal contribution
posted on 2012-03-05, 00:00 authored by Patrick J. Desrochers, Christopher
A. Sutton, Micah L. Abrams, Shengfa Ye, Frank Neese, Joshua Telser, Andrew Ozarowski, J. KrzystekThe previously reported Ni(II) complex, Tp*Ni(κ3-BH4) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate
anion),
which has an S = 1 spin ground state, was studied
by high-frequency and -field electron paramagnetic resonance (HFEPR)
spectroscopy as a solid powder at low temperature, by UV–vis–NIR
spectroscopy in the solid state and in solution at room temperature,
and by paramagnetic 11B NMR. HFEPR provided its spin Hamiltonian
parameters: D = 1.91(1) cm–1, E = 0.285(8) cm–1, g = [2.170(4),
2.161(3), 2.133(3)]. Similar, but not identical parameters were obtained
for its borodeuteride analogue. The previously unreported complex,
Tp*Zn(κ2-BH4), was prepared, and IR and
NMR spectroscopy allowed its comparison with analogous closed shell
borohydride complexes. Ligand-field theory was used to model the electronic
transitions in the Ni(II) complex successfully, although it was less
successful at reproducing the zero-field splitting (zfs) parameters.
Advanced computational methods, both density functional theory (DFT)
and ab initio wave function based approaches, were applied to these
Tp*MBH4 complexes to better understand the interaction
between these metals and borohydride ion. DFT successfully reproduced
bonding geometries and vibrational behavior of the complexes, although
it was less successful for the spin Hamiltonian parameters of the
open shell Ni(II) complex. These were instead best described using
ab initio methods. The origin of the zfs in Tp*Ni(κ3-BH4) is described and shows that the relatively small
magnitude of D results from several spin–orbit
coupling (SOC) interactions of large magnitude, but with opposite
sign. Spin–spin coupling (SSC) is also shown to be significant,
a point that is not always appreciated in transition metal complexes.
Overall, a picture of bonding and electronic structure in open and
closed shell late transition metal borohydrides is provided, which
has implications for the use of these complexes in catalysis and hydrogen
storage.