Desrochers UCA Research

MM magnetic dimers

A novel metal-metal dimer is being developed in our laboratory.  These monoanionic species exist as salts, K[X-M(mu-pz)3M-X], where M = Co, Ni, Zn and X = Cl, Br and pz represents any number of different R-substituted pyrazolide bridging groups.  All have structural features and electronic spectra similar to analogous well-characterized TpRMX systems.

Spin Crossover and Chirality in an Iron Heteroscorpionate

In Inorg. Chem. 2022, 61(47), 18907–18922.    doi.org/10.1021/acs.inorgchem.2c02856 

The optical, structural and magnetic properties of iron(II, III) sandwich complexes, Fe(Tp’)2n+ (Tp’ = bis(3,5-dimethylpyrazolyl)benzotriazolylborate), are described. Intensely colored, FeII(Tp’)2 (orange) and FeIII(Tp’)2+ (purple), show strong MLCT bands. Geometric isomerism for M(Tp’)2 is established crystallographically in the racemate of chiral cis-Fe(Tp’)2.   For the first time, paramagnetic 11B NMR describes solution-phase low spin (LS, S = 0) to high spin (HS, S = 2) crossover behavior in Fe(Tp’)2.  Thermochemical parameters for solution-phase SCO of Fe(Tp’)2 demonstrate the endothermic LS to HS conversion and entropic preference of the HS state.  Entropy changes for both Fe(Tp’)2 isomers are significantly larger than for the majority of iron-scorpionate SCO systems. Solid state magnetic and thermochemical measurements show cis-Fe(Tp’)2 to be thermally stable up to 520 K, allowing experimental investigation of a solid-state SCO magnetic hysteresis of over 45 K.  A large solution vs solid state SCO difference was observed: cis-Fe(Tp’)2 shows Tc ~ 270 K (solution) and Tc ~ 385 K (solid), a remarkably wide ΔTc ~115 degrees; trans-Fe(Tp’)2 shows Tc ~278 K (solution) and Tc ~  372 K (solid).  Solid-state Tc values are among the highest seen for iron(II) molecular systems. The large solution/solid ΔTc difference is explained by “anchoring” intermolecular interactions in the solid state that prevent thermal expansion of the LS iron(II) coordination sphere in its transition to the HS state.  DFT calculations, validated against LS cis-Fe(Tp’)2 crystallography and LS to HS SCO thermochemical parameters, demonstrate the role the benzotriazole rings play in its structural and optical properties. Lewis basicity of M(Tp’)2 is shown with the structural characterization of the air stable tin(II) adduct, [cis-Fe(Tp’)2-SnCl2]; tin(II) coordination does not alter the iron(II) spin state. The Tp’ chelate adds functionality (asymmetry, chirality, chemical reactivity) to the array of iron SCO materials for potential incorporation into nanoscale magnetic switches and spintronic devices. 

Paramagnetic Boron-11 NMR

In NMR Spectroscopy in the Undergraduate Curriculum; Soulsby, D. Ed.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013, Ch7.   DOI: 10.1021/bk-2013-1128

The boron-11 nucleus has spin I = 3/2 and is 80% naturally abundant. Routine NMR can be recorded for this nucleus on modern high field instruments. Reaction mixtures across a variety of solvent systems yield meaningful spectra in 5 min or less on our JEOL ECX-300 instrument. The 11B signal in boron-based scorpionate ligands of paramagnetic metal complexes can be exploited to considerable advantage. While the chemical shift is essentially invariant in diamagnetic systems (like zinc-scorpionates), it has proven very sensitive to metal environment in paramagnetic metal systems. The success with this tool in our lab prompted us to post some data with the intention of providing a useful resource for other researchers who also work with metal scorpionates, but who may not have considered the utility of paramagnetic 11B for their own work.  Pyykkoenen et al. (Inorg. Chem. 2020) provided a very detailed theoretical and experimental summary for first metal scorpionates.  These data, some from our lab, and some from other researchers are summarized in the following figure.  The extreme sensitivity of the 11B nucleus to paramagnetic metal environment is evident in these data. 

An extensive catalog of diamagnetic 11B chemical shifts is compiled by the Cole research group at San Diego State University.

Routine measurements with heteronuclear, heterogeneous, and paramagnetic samples

In NMR Spectroscopy in the Undergraduate Curriculum; Soulsby, D. Ed.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013, Ch7.   DOI: 10.1021/bk-2013-1128

High field instrumentation has made pulsed 1H and 13C NMR spectroscopy routine in undergraduate curricula. Less common in undergraduate experiences is NMR spectroscopy of heteronuclear centers (2H, 11B, 19F, 27Al, 31P, or 77Se). Similarly, paramagnetic samples are typically avoided because of their sometimes unpredictable effect on spectra. This summary describes NMR measurements that were performed by undergraduates in teaching and undergraduate research laboratories. An array of heteronuclear, heterogeneous (resin supported), and paramagnetic samples were studied. A primary product of these efforts is the broader training students received in practical NMR instrumentation and applications, complementary to their traditional organic (1H, 13C) NMR spectroscopy experience. A secondary result is the emergent, dynamic NMR spectroscopy environment in the department, leading to yet more novel experiments for both teaching and research applications.

Ring-metathesis yields new heteroscorpionates

In  Inorg. Chem. 2011, 50, 1931–1941   DOI: 10.1021/ic102392x

Coordination behavior of a new heteroscorpionate toward second-row transition metals

In  Inorg. Chem. 2011, 50, 1931–1941   DOI: 10.1021/ic102392x

13C {1H} NMR of Tp′Rh(cod) in protio-THF.  This spectrum is consistent with the structure shown as an inset.  Equivalent 3,5-dimethylpyrazole (pz*) rings are evident in single well-defined resonances (a – d) for each of the five carbon atoms in these rings.  Whether or not the axially positioned Bzt ring is coordinated to Rh(I) is uncertain.  These results suggest that no rapid inter-conversion of pz* and Bzt rings through variable k2/k3 coordination modes is occurring in this system.  The two distinct resonances assignable to the Rh-coordinated alkene carbons (m, doublet, JC-Rh = 13 Hz) and to the -CH2- carbon suggest a highly symmetric environment around the coordinated cod ligand.

Tp*Ni selects for cysteine and selenocysteine over serine and homocysteine

In Inorg. Chem. 2011, 50, 1931–1941   DOI: 10.1021/ic102392x

Bright green trigonal bypramidal Tp*Ni-AA complexes form only with cysteine and selenocysteine, but not with serine or homocysteine.  Theoretical and reactivity evidence indicates this specificity is due to in-plane N2Ni-E (E = S or Se) orbital overlap involving N-lone pairs, Ni d orbital, and E-lone pair.  When E is O, presumably the O lone pair is too spatially constricted or energetically unavailable to participate.  When AA is homocysteine, the more flexible propyl backbone (vs the ethyl backbone of cysteine) discourages optimal in-plane overlap.  Accordingly, serine and homocysteine do not form these green trigonal bipyramidal complexes.

The original green Tp*NiCysEt has a sulfur-to-nickel charge transfer (CT) band near 400 nm.  Successive spectra represent the addition of ¼ equivalent of O(CH3)3+ (as [O(CH3)3][BF4] in acetonitrile), causing a monotonous loss of this CT band (green to blue). The addition of 1 equivalent of triethylamine (inset spectrum) causes the re-emergence of the intense sulfur-to-nickel CT band near 400 nm.  This is interpreted as the re-formation of the nickel-cysteine thiolate bond

Variable-T 31P NMR demonstrates importance of P-Ni-S pi bonding in controlling coordination geometry

In Inorg. Chem. 2007, 46, 9221-9233    DOI: 10.1021/ic701150q

Geometry SP1 can become TBP1, requiring disruption of the trans PS-Ni-S pi overlap, or SP1 can become TBP2, leading to a retention of this favorable pi overlap. Trans pi overlap is not possible when the PN-Ni-N angle is 180° (TBP1). TBP2 is therefore preferred during the scrambling mechanism, keeping the PS-Ni-S angle closer to 180°, so that SP2 is more likely to form. Ultimately, this mechanism explains the sluggishness of PS to scramble with the apical P and further accounts for the continued rapid exchange of the apical P and PN groups long after the resonance assigned to PS has become well resolved.

The complete halide series for Tp*NiX

In Inorg. Chem. 2006, 45, 8930   DOI: 10.1021/ic060843c

Vis-NIR electronic absorption spectra of Tp*NiX in CCl4. Black triangles are centered at calculated spin-allowed (triplet) transition energies for idealized C3V symmetry.  Asterisks mark the 3A2 (3T1, F) --> 3A1 (3T2, F) transition, which is forbidden in C3V symmetry.

Resin-supported functional scorpionates by microwave methods

In Eur. J. Inorg. Chem. 2016, 2465.  DOI: 10.1002/ejic.201501254

Boron-scorpionates are readily prepared by microwave (MW) assisted methods. Such a method is described here, leading to the rapid preparation of a heterogeneous resin-supported scorpionate as its sodium salt, BdPhTpNa [BdPhTp = phenyltris(1-pyrazolyl)borate covalently bound through the phenyl ring to commercial cross-linked polystyrene resin beads].  Formation of a functional heterogeneous scorpionate was confirmed from the subsequent reactions of several metal complexes. This supported ligand system coordinates a variety of transition metal ions including copper(I and II), chromium(III) and rhodium(I). Chromium(III) provided definitive electronic spectral evidence for supported–TpCrIII coordination spheres, including reversible ligand-substitution reactions. The copper(I) case exhibited typical scorpionate CuI reactivity including spectroscopically characterized (IR and 31P NMR) complexes with CO, PPh3, and HCCH. Copper(II) provided EPR evidence for heterogeneous scorpionates. The supported rhodium(I) complex was demonstrated to be a recyclable heterogeneous rhodium–scorpionate catalyst. These results all support the conclusion that the immobilized chelate forms coordinatively unsaturated half-sandwich metal complexes (LMn+) capable of efficient ligand-substitution reactions or catalytic activity.

Electronic Structure of Nickel(II) and Zinc(II) Borohydrides from Spectroscopic Measurements and Computational Modeling

In Inorg. Chem. 2012, 51, 2793-2805.   DOI: 10.1021/ic201775c

The previously reported Ni(II) complex, Tp*Ni(k3-BH4) (Tp* = hydrotris(3,5-dimethyl-pyrazolyl)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 metalborohydrides is provided, which has implications for the use of these complexes in catalysis and hydrogen storage.

Immobilized boron-centered heteroscorpionates: heterocycle metathesis and coordination chemistry

In Inorg. Chem. 2011, 50, 1931–1941   DOI: 10.1021/ic102392x

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, {resin-btz(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 TpRNiCysEt coordination sphere. Bead-Tp'NiCysEt 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-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 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.

Ni-cysteine coordination controlled by Tp*

Tp*NiCysEt owes its deep green color to a dominant S-to-Ni CT band that is controlled by in-plane N-Ni-S pi overlap.

in Inorg. Chem. 1999, 38, 5690-5694  and  Inorg. Chem. 2011, 50, 1931–1941   DOI: 10.1021/ic102392x

A stable monomeric nickel-borohydride

In Inorg. Chem. 2003, 42, 7945. DOI: 0.1021/ic034687a 

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, beta = 107.605(9)°, the space group is monoclinic P21/c, and Z = 4. Infrared measurements confirmed the presence of bridging hydrogen atoms; both ν(B−H)terminal and ν(B−H)bridging are assignable and shifted relative to ν(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.

Tp*NiX readily bind ammonia under mild conditions  

Spectroscopic measurements indicate a clean reversible reaction of Tp*NiX with ammonia.  

Mild heating releases the bound ammonia.

Tp*NiX(s)   +   3NH3(g)  <-->   [Tp*Ni(NH3)3]X(s)