Routine Paramagnetic ¹¹B NMR from NMR Spectroscopy in the Undergraduate Curriculum ACS Symposium Series v. 1128; American Chemical Society: Washington, DC, 2013, Ch7, p 182. DOI: 10.1021/bk-2013-1128 The boron heteroatom in paramagnetic TpNiX (X = Cl, Br, I, NO₃, BH₄, or a second Tp^R) provided undergraduate researchers the opportunity to develop a unique ¹¹B NMRchemical shift range that was very sensitive to both the identity of X and the metal coordination geometry. The influence of the nickel(II) paramagnet (S = 1) on the Tp boron atom is clearly evident. Typical through-space nickel-boron distances in TpNiX are ~3 Å (X = Cl, Br, BH₄). Chemical shifts for four-coordinate TpNiX geometries vary widely with X. Five-coordinate TpNiX geometries (or fluxional variations) give chemical shifts near -25 ppm and six-coordinate octahedral TpNiX cases give chemical shifts near -36 ppm over a wide range of N-donors (including a second Tp, 3 NCCH₃, or 3 NH₃). In contrast the boron chemical shift in the closed-shell zinc(II) cases, TpZnX, is insensitive to X, such that the boron chemical shifts of TpZnX (X= Cl and I) and Zn(Tp)₂ only differ by 0.5 ppm. The broad utility of scorpionates in paramagnetic transition metal complexes motivates development of meaningful B-11 chemical shift libraries of these complexes. The nickel(II) examples demonstrate the usefulness of this method for rapidly identifying metal-scorpionate coordination geometries in reaction mixtures. The combination of such trends and the general excellent sensitivity of ¹¹B has made this tool a quick and routine confirmatory measurement for existing and newly prepared metal scorpionates in our work. For example, this trend was useful in identifying in situ formation of Tp’NiNO₃ (¹¹B d ~ 25 ppm), a product of this new heteroscorpionate but one that proved difficult to isolate from reaction mixtures

A Rhodium(I) Heteroscorpionate Catalyst for Phenylacetylene Polymerization Rhodium(I) scorpionates have demonstrated activity in the polymerization of phenylacetylene, a bright orange conjugated polymer with useful electrical and optical properties. Recently we reported the preparation of scorpionates (Tp’) anchored to polystyrene synthesis beads (bead-Tp’ in Inorg. Chem. 2011, p. 1931). These supported chelates offer the versatility of scorpionates to rapid-throughput combinatorial methods. This motivation led to the preparation of Tp’Rh(cod), where Tp’ represents the new tridentate chelate, hydrobis(3,5-dimethylpyrazolyl)(benzotriazolyl)borate. The catalytic activity of Tp’Rh(cod) toward phenylacetylene polymerization was compared to the established analogue TpRh(cod) (Tp = hydrotris(3,5-dimethylpyrazolyl)-borate). Marked differences in catalytic activity of these two complexes are ascribed to the variable hapticity (k² vs k³) preferences of the two scorpionates. Completely different rates of activity were also noted when a more electron-rich monomer (p-H₃C-PhC≡CH) replaced phenylacetylene. The present results for homogeneous samples of Tp’Rh(cod) will help describe subsequent activity studies in heterogeneous supported systems, bead-Tp’Rh(cod). Here, the brightly colored polymer-product will allow rapid optical screening of favorable supported-catalyst candidates.

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, TpNi(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, TpZn(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 TpMBH₄ 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-Tp’K, {resinbtz(H)B(pz)₂}K. Significantly, bead-Tp’K readily bound nickel(II) from simple salts with minimal leaching of the nickel ion. Bead-Tp’NiNO₃ reacts further with cysteine thiolate (ethyl ester), imparting the deep green color to the beads characteristic of a Tp^RNiCysEt coordination sphere. Bead-Tp’NiCysEt exhibited an oxygen sensitivity similar to TpNiCysEt in solution (Inorg. Chem. 1999, p 5690) and also independently verified for a selenocystamine analogue, TpNiSeCysAm. Addition of fresh cysteine thiolate ethyl ester to oxidized bead-Tp’NiCysEt reproduced the original green color. Heterocycle metathesis was also used to prepare KTp’ as a white solid. Reaction with nickel(II) gave (Tp’)₂Ni, separable into two different isomers. The air-sensitive molybdenum(0) complex, [PPh₄][Tp’Mo(CO)₃], was also prepared and the C_s 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 TpmNiCl₂(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/NiX₄^2- 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 NiX₄^2-) 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 p 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 TpNiCys-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 TpNiX, 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 TpNiCl has been previously reported; that for TpNiBr 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. TpNiX contains a four-coordinate nickel(II) ion (3d⁸) with approximate C_3v point group symmetry about the metal and a resulting S = 1 high-spin ground state. As a consequence of sizable zero-field splitting (zfs), TpNiX 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 TpNiX 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 TpNiX 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). TpNiCysEt 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)°. [TpNiCys^-][K⁺] and Tp^PhMeNiCysEt show magnetic and electronic characteristics similar to TpNiCysEt, so that the trigonal bipyramidal coordination geometry confirmed for TpNiCysEt 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 m_B, 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 Tp^X^- 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 TpNiCysEt and O2 fit the following composite rate law: rate = k1[TpNiCysEt] + k2[O2][TpNiCysEt] 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 Tp^PhMe^-) 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 (TpNiBH4 or TpNiBD4) 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. TpNiBH4 and TpNiBD4 are stable toward air, boiling water, and high temperatures (mp > 230 °C dec). X-ray crystallographic measurements for TpNiBH4 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 TpNiBH4, 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 TpNiBD4 by amounts in agreement with theory. Despite their hydrolytic stability, TpNiBH4 and TpNiBD4 readily reduce halocarbon substrates, leading to the complete series of TpNiX 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 TpNiBH4, TpNiBD4, and TpNiCl. X-ray absorption near-edge spectroscopy results confirmed the similar six-coordinate geometries for TpNiBH4 and TpNiBD4. These contrasted with XAS results for the crystallographically characterized pseudotetrahedral TpNiCl 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 239^th American Chemical Society National Meeting, San Francisco, CA, March 2010, INOR 230. Nickel(II) demonstrates a high affinity for nitrogen-donor bases. Accordingly, TpNi⁺ reversibly binds N-donors according to the reaction: TpNiX + 3 N-donor à [TpNi(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 TpNiX/acetonitrile systems yielded thermodynamic parameters for these equilibria. We also describe reversible ammonia binding at TpNiBH₄, an interesting case because the product, [TpNi(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.

Routine Paramagnetic ¹¹B NMR from NMR Spectroscopy in the Undergraduate Curriculum ACS Symposium Series v. 1128; American Chemical Society: Washington, DC, 2013, Ch7, p 182. DOI: 10.1021/bk-2013-1128 The boron heteroatom in paramagnetic TpNiX (X = Cl, Br, I, NO₃, BH₄, or a second Tp^R) provided undergraduate researchers the opportunity to develop a unique ¹¹B NMRchemical shift range that was very sensitive to both the identity of X and the metal coordination geometry. The influence of the nickel(II) paramagnet (S = 1) on the Tp boron atom is clearly evident. Typical through-space nickel-boron distances in TpNiX are ~3 Å (X = Cl, Br, BH₄). Chemical shifts for four-coordinate TpNiX geometries vary widely with X. Five-coordinate TpNiX geometries (or fluxional variations) give chemical shifts near -25 ppm and six-coordinate octahedral TpNiX cases give chemical shifts near -36 ppm over a wide range of N-donors (including a second Tp, 3 NCCH₃, or 3 NH₃). In contrast the boron chemical shift in the closed-shell zinc(II) cases, TpZnX, is insensitive to X, such that the boron chemical shifts of TpZnX (X= Cl and I) and Zn(Tp)₂ only differ by 0.5 ppm. The broad utility of scorpionates in paramagnetic transition metal complexes motivates development of meaningful B-11 chemical shift libraries of these complexes. The nickel(II) examples demonstrate the usefulness of this method for rapidly identifying metal-scorpionate coordination geometries in reaction mixtures. The combination of such trends and the general excellent sensitivity of ¹¹B has made this tool a quick and routine confirmatory measurement for existing and newly prepared metal scorpionates in our work. For example, this trend was useful in identifying in situ formation of Tp’NiNO₃ (¹¹B d ~ 25 ppm), a product of this new heteroscorpionate but one that proved difficult to isolate from reaction mixtures

A Rhodium(I) Heteroscorpionate Catalyst for Phenylacetylene Polymerization Rhodium(I) scorpionates have demonstrated activity in the polymerization of phenylacetylene, a bright orange conjugated polymer with useful electrical and optical properties. Recently we reported the preparation of scorpionates (Tp’) anchored to polystyrene synthesis beads (bead-Tp’ in Inorg. Chem. 2011, p. 1931). These supported chelates offer the versatility of scorpionates to rapid-throughput combinatorial methods. This motivation led to the preparation of Tp’Rh(cod), where Tp’ represents the new tridentate chelate, hydrobis(3,5-dimethylpyrazolyl)(benzotriazolyl)borate. The catalytic activity of Tp’Rh(cod) toward phenylacetylene polymerization was compared to the established analogue TpRh(cod) (Tp = hydrotris(3,5-dimethylpyrazolyl)-borate). Marked differences in catalytic activity of these two complexes are ascribed to the variable hapticity (k² vs k³) preferences of the two scorpionates. Completely different rates of activity were also noted when a more electron-rich monomer (p-H₃C-PhC≡CH) replaced phenylacetylene. The present results for homogeneous samples of Tp’Rh(cod) will help describe subsequent activity studies in heterogeneous supported systems, bead-Tp’Rh(cod). Here, the brightly colored polymer-product will allow rapid optical screening of favorable supported-catalyst candidates.

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, TpNi(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, TpZn(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 TpMBH₄ 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-Tp’K, {resinbtz(H)B(pz)₂}K. Significantly, bead-Tp’K readily bound nickel(II) from simple salts with minimal leaching of the nickel ion. Bead-Tp’NiNO₃ reacts further with cysteine thiolate (ethyl ester), imparting the deep green color to the beads characteristic of a Tp^RNiCysEt coordination sphere. Bead-Tp’NiCysEt exhibited an oxygen sensitivity similar to TpNiCysEt in solution (Inorg. Chem. 1999, p 5690) and also independently verified for a selenocystamine analogue, TpNiSeCysAm. Addition of fresh cysteine thiolate ethyl ester to oxidized bead-Tp’NiCysEt reproduced the original green color. Heterocycle metathesis was also used to prepare KTp’ as a white solid. Reaction with nickel(II) gave (Tp’)₂Ni, separable into two different isomers. The air-sensitive molybdenum(0) complex, [PPh₄][Tp’Mo(CO)₃], was also prepared and the C_s 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 TpmNiCl₂(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/NiX₄^2- 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 NiX₄^2-) 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 p 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 TpNiCys-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 TpNiX, 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 TpNiCl has been previously reported; that for TpNiBr 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. TpNiX contains a four-coordinate nickel(II) ion (3d⁸) with approximate C_3v point group symmetry about the metal and a resulting S = 1 high-spin ground state. As a consequence of sizable zero-field splitting (zfs), TpNiX 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 TpNiX 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 TpNiX 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). TpNiCysEt 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)°. [TpNiCys^-][K⁺] and Tp^PhMeNiCysEt show magnetic and electronic characteristics similar to TpNiCysEt, so that the trigonal bipyramidal coordination geometry confirmed for TpNiCysEt 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 m_B, 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 Tp^X^- 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 TpNiCysEt and O2 fit the following composite rate law: rate = k1[TpNiCysEt] + k2[O2][TpNiCysEt] 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 Tp^PhMe^-) 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 (TpNiBH4 or TpNiBD4) 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. TpNiBH4 and TpNiBD4 are stable toward air, boiling water, and high temperatures (mp > 230 °C dec). X-ray crystallographic measurements for TpNiBH4 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 TpNiBH4, 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 TpNiBD4 by amounts in agreement with theory. Despite their hydrolytic stability, TpNiBH4 and TpNiBD4 readily reduce halocarbon substrates, leading to the complete series of TpNiX 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 TpNiBH4, TpNiBD4, and TpNiCl. X-ray absorption near-edge spectroscopy results confirmed the similar six-coordinate geometries for TpNiBH4 and TpNiBD4. These contrasted with XAS results for the crystallographically characterized pseudotetrahedral TpNiCl 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 239^th American Chemical Society National Meeting, San Francisco, CA, March 2010, INOR 230. Nickel(II) demonstrates a high affinity for nitrogen-donor bases. Accordingly, TpNi⁺ reversibly binds N-donors according to the reaction: TpNiX + 3 N-donor à [TpNi(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 TpNiX/acetonitrile systems yielded thermodynamic parameters for these equilibria. We also describe reversible ammonia binding at TpNiBH₄, an interesting case because the product, [TpNi(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.