|
Electronic Structure of
Nickel(II) and Zinc(II) Borohydrides from Spectroscopic
Measurements and Computational Modeling
 Inorganic Chemistry 2012, 51, 2793-2805. DOI:
10.1021/ic201775c
The previously reported
Ni(II) complex, Tp*Ni(k3-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 analog. The
previously unreported complex, Tp*Zn(k2-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 wavefunction 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(k3-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.
|
|
Coordination behavior of a
new heteroscorpionate toward second-row transition metals
Spectral evidence supports the coordination behavior of Tp′
shown with molybdenum(0) and rhodium(I) below.
Clean room temperature NMR measurements suggest no rapid
interconversion of Bzt and pyrazole rings
 is occurring in these
systems.
|
|
 Boron-scorpionates anchored to polymer
supports
Inorganic Chemistry 2011, 50, 1931 – 1941. DOI:
10.1021/ic102392x
The preparation of a resin-supported boron-scorpionate ligand and
its nickel(II) coordination complexes are reported.
The supported ligand is prepared as its potassium salt, making it
a general reagent suitable for chelation of any
transition metal ion. Resin-immobilized
benzotriazole (Bead-btz) reacted
cleanly with KTp* (Tp* =
hydrotris(3,5-
dimethylpyrazolyl)borate) by
heterocycle metathesis in warm dimethylformamide (DMF) to yield bead-Tp0K,
{resinbtz(
H)B(pz*)2}K. Significantly, bead-Tp0K readily bound nickel(II) from
simple salts with minimal leaching of the nickel
ion. Bead-Tp0NiNO3 reacts
further with cysteine thiolate (ethyl ester),
imparting the deep green color to the beads
characteristic of a TpRNiCysEt
coordination sphere. Bead-Tp0NiCysEt
exhibited an oxygen sensitivity similar to
Tp*NiCysEt in
solution (Inorg. Chem. 1999, p 5690) and also independently verified for a selenocystamine
analogue,
Tp*NiSeCysAm.
Addition of fresh cysteine thiolate ethyl ester to oxidized bead-Tp0NiCysEt reproduced the original
green color.
Heterocycle metathesis was also used to prepare KTp0
as a white solid. Reaction with nickel(II) gave
(Tp0)2Ni,
separable into two different isomers. The air-sensitive molybdenum(0) complex, [PPh4][Tp0Mo(CO)3], was
also prepared and the Cs complex
symmetry demonstrated by infrared and 13C NMR spectroscopies. Immobilized
TpmMo(CO)3 was prepared from the previously reported
resin-supported tris(pyrazolyl)methane. In contrast to its
weak coordination of nickel(II) (Inorg. Chem. 2009, p 3535), bead-Tpm proved a strong chelate
toward this second
row metal. The supported scorpionates described
here should find use in studies of selective metal-protein binding,
metalloprotein modeling, and heterogeneous catalysis, and render such
scorpionate applications amenable to
combinatorial methods.
|
|
 Polymer supported tris(pyrazolyl)methane, minimum steric constraints
Inorganic Chemistry 2009, 48, 3535-3541 DOI: 10.1021/ic8015645
Single-scorpionates
of nickel(II), TpRNiX or TpmRNiX,
are kinetic products whose preparation has generally required
considerable steric
constraints on the ligands (i.e., R ) phenyl, tert-butyl,
or isopropyl) to prevent formation of
intractable two-ligand
products like (TpR)2Ni. It is well
established that the facial tridentate chelates hydrotris(3,5-
dimethylpyrazolyl)borate (Tp*-), tris(3,5-dimethylpyrazolyl)methane
(Tpm*), and trispyrazolylmethane
(Tpm), all readily
form two-ligand
complexes as thermodynamic products. For the first time we report a route to
the single-ligand
complex TpmNiX2(OH2)n (X
) Cl and Br). We also report a novel method for
making single-ligand nickel(II) scorpionate
complexes using preformed tetrahalonickelate(II) ion in nitromethane. The complex Tpm*NiCl2(OH2)n was also
prepared
here for the first time
utilizing an alternative method first reported by Zargarian
and co-workers (Inorg. Chim. Acta
2006, 2592).
TpmNiX2(OH2)n are kinetic products, and although they are stable
indefinitely in the solid state, they
readily convert to
the thermodynamic product (Tpm)2Ni2+
in solution over the course of several hours at room
temperature and in a matter of
minutes at 100 °C. The new nitromethane/NiX42- method
offers an alternative route
to monoscorpionates of first row transition metals, for
which tetrahalometallate ions are common. HOCH2Tpm
(2,2,2-tris(pyrazolyl)ethanol) was covalently attached to polystyrene
synthesis beads and found to bind nickel(II)
(from NiX42-)
in a manner similar to Tpm. Solid state electronic
spectra of supported-TpmNiCl2 are comparable to
those measured for their
homogeneous complexes. Covalently supported scorpionates are expected to
further
extend the utility of
this rich ligand class in areas of heterogeneous catalysis and metal-protein
interactions.
|
|
 Phosphine-nickel-cysteine complexes
Inorganic Chemistry 2007, 46, 9221 - 9233 DOI: 10.1021/ic701150q
The effect of
chelating phosphines was tested on the structure
and pH-dependent stability of nickel-cysteine
binding.
(1,2-Bis(diphenylphosphino)ethane (dppe)
and 1,1,1-tris[(diphenylphosphino)methyl]ethane (triphos) were used with
three different
cysteine derivatives (L-cysteine, Cys; L-cysteine
ethyl ester, CysEt; cystamine,
CysAm) to prepare
complexes of the form (dppe)NiCysRn+ and (triphos)NiCysRn+ (n )
0 for Cys; n ) 1 for CysEt
and CysAm).
Similar 31P {1H} NMR spectra for all
(dppe)NiCysRn+ confirmed their
square-planar P2NiSN coordination
spheres.
The structure of [(dppe)NiCysAm]PF6 was also confirmed
by single-crystal X-ray diffraction methods. The (triphos)-
NiCysAm+ and (triphos)NiCysEt+ complexes were
fluxional at room temperature by 31P NMR. Upon cooling to -80
°C, all gave spectra
consistent with a P2NiSN coordination
sphere with the third phosphorus uncoordinated.
Temperature-dependent
31P NMR spectra
showed that a trans P-Ni-S ð interaction controlled the scrambling of
the coordinated triphos. In aqueous media, (dppe)NiCys was protonated at pH ~ 4-5, leading to
possible formation
of a nickel-cysteinethiol
and eventual cysteine loss at pH < 3. The importance of N-terminus
cysteine in such
complexes was
demonstrated by preparing (dppe)NiCys-bead
and trigonal-bipyramidal Tp*NiCys-bead
complexes,
where Cys-bead represents cysteine anchored to polystyrene
synthesis beads and Tp*-
) hydrotris(3,5-
dimethylpyrazolyl)borate.
Importantly, results with these heterogeneous systems demonstrated the
selectivity of
these nickel centers for
cysteine over methionine and serine and most specifically for N-terminus
cysteine. The
role of Ni-S
pi bonding in nickel-cysteine
geometries will be discussed, including how these results suggest a
mechanism for the movement
of electron density from nickel onto the backbone of coordinated cysteine.
|
|
Electronic
structure of half-sandwich nickel(II)-scorpionates
Inorganic Chemistry 2006, 45, 8930 - 8941 DOI: 10.1021/ic060843c
A series of
complexes of formula Tp*NiX,
where Tp*- ) hydrotris(3,5-dimethylpyrazole)borate
and X = Cl, Br, I,
has been
characterized by electronic absorption spectroscopy in the visible and
near-infrared (NIR) region and by
high-frequency and -field
electron paramagnetic resonance (HFEPR) spectroscopy. The crystal structure
of Tp*NiCl
has been previously
reported; that for Tp*NiBr
is given here: space group = Pmc21,
a = 13.209(2) Å, b =
8.082(2) Å, c ) 17.639(4) Å, a = b = g = 90°, Z = 4. Tp*NiX contains a
four-coordinate nickel(II) ion (3d8) with
approximate C3v
point group symmetry about the metal and a resulting S = 1 high-spin ground
state. As a
consequence of
sizable zero-field splitting (zfs), Tp*NiX complexes are “EPR silent” with use of conventional
EPR;
however, HFEPR allows
observation of multiple transitions. Analysis of the resonance field versus
the frequency
dependence of these
transitions allows extraction of the full set of spin Hamiltonian parameters.
The axial zfs
parameter for Tp*NiX displays pronounced
halogen contributions down the series: D = +3.93(2), -11.43(3), -22.81(1)
cm-1, for X = Cl, Br, I,
respectively. The magnitude and change in sign of D observed for Tp*NiX reflects the
increasing bromine and iodine
spin-orbit contributions facilitated by strong covalent interactions with
nickel(II).
These spin
Hamiltonian parameters are combined with estimates of 3d energy levels based
on the visible-NIR
spectra to yield
ligand-field parameters for these complexes following the angular overlap
model (AOM). This
description of
electronic structure and bonding in a pseudotetrahedral
nickel(II) complex can enhance the
understanding of similar sites
in metalloproteins, both native nickel enzymes and nickel-substituted zinc
enzymes.
|
|
Nickel-cysteine selectivity
Inorganic Chemistry 1999, 38, 5690 - 5694 DOI: 10.1021/ic990059a
Monomeric
five-coordinate nickel-cysteine complexes were prepared using anionic tris(3,5-disubstituted pyrazolyl)-
borates (Tp* -
and TpPhMe-) and l-cysteine
(ethyl ester and amino acid forms). Tp*NiCysEt crystallizes with a
single methanol of
solvation in the monoclinic space group P21: a =
7.8145(18), b = 24.201(6), c = 7.9925(14)
Å; b = 117.991(16)°. [Tp*NiCys-][K+] and TpPhMeNiCysEt
show magnetic and electronic characteristics similar
to Tp*NiCysEt, so that the
trigonal bipyramidal coordination geometry confirmed for Tp*NiCysEt in the solid
state likely applies to
all three. All three complexes have high spin magnetic ground states at room
temperature
(meff ) 2.9-3.2 mB, S =1).
Their electronic spectra are dominated by sulfur to nickel charge-transfer
bands (388-
430 nm in
chloroform) with energies that correlate to respective thiolate basicities and TpX- donor strengths. The
Tp* derivatives
undergo a rapid reaction with molecular oxygen. Stoichiometric,
infrared, and electronic
spectroscopy measurements are
consistent with formation of a sulfinate as a
result of reaction with dioxygen.
Kinetics
measurements for the reaction of Tp*NiCysEt and O2 fit the following composite rate law: rate
=
k1[Tp*NiCysEt]
+ k2[O2][Tp*NiCysEt] with k1 = 0.013(1) min-1 and k2 = 4.8(1) M-1 min-1 at 22 °C. Increased
nucleophilicity of the
nickel-sulfur center enhanced by electron donation from Tp*- (vs
TpPhMe-) and encouraged
by a trigonal
bipyramidal geometry (vs square planar Ni(CysEt)2)
is hypothesized as the reason for the susceptibility
of Tp*NiCys complexes to oxygen.
|
|
Stable nickel-borohydrides
Inorganic Chemistry 2003, 42, 7945 - 7950 DOI: 10.1021/ic034687a
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 °C 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(9)
Å, b = 7.740(1) Å, c =
18.851(2)
Å, b = 107.605(9)°,
the space group is monoclinic P21/c, and Z =
4.
Infrared measurements confirmed the
presence of
bridging hydrogen atoms; both n(B-H)terminal and n(B-H)bridging are assignable and
shifted relative to
n(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 ~ 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.
|
|
Reversible nickel-ammonia binding, storage
“Exchange
equilibria of variable nitrogen-donors at nickel(II)-scorpionates”
Kristin A. Thorvilson, Adeniyi Osinowo, Patrick J.
Desrochers
239th
American Chemical Society National Meeting, San Francisco, CA, March 2010,
INOR 230.
Nickel(II) demonstrates a
high affinity for nitrogen-donor bases.
Accordingly, Tp*Ni+ reversibly
binds N-donors
according
to the reaction: Tp*NiX + 3 N-donor à [Tp*Ni(N-donor)3]X, where N-donor = imidazole,
acetonitrile,
or ammonia and X = Cl-, Br-, I-, and BH4-
and Tp* = the scorpionate
hydrotris(3,5-dimethylpyrazolyl)borate.
For
all N-donors studied, this reaction is exothermic, reflecting the exchange of
stronger nickel-N-donor for weaker
nickel-X bonds. Variable
temperature 11B NMR of the Tp*NiX/acetonitrile systems yielded thermodynamic parameters
for these
equilibria. We also describe
reversible ammonia binding at Tp*NiBH4,
an interesting case because the
product, [Tp*Ni(NH3)3][BH4],
incorporates reactive hydridic B-H and protic N-H
groups in a single solid. This is
reminiscent
of magnesium based hydrogen-storage materials incorporating a similar Mg-NH3—BH4
arrangement
(Soloveichik, et al. Inorg. Chem. 2008, p.
4290). These materials are expected to have applications to solid-state
ammonia storage and
ammonia sensors.
|
Rhodium(I) scorpionates
have demonstrated activity in the polymerization of phenylacetylene,
a 