Anisotropic Fermi couplings due to large unquenched orbital angular momentum: Q-band (1)H, (14)N, and (11)B ENDOR of bis(trispyrazolylborate) cobalt(II)

Electronic Structure of Three-Coordinate FeII and CoII β-Diketiminate Complexes

The β-diketiminate supporting group, [ArNCRCHCRNAr]-, stabilizes low coordination number complexes. Four such complexes, where R = tert-butyl, Ar = 2,6-diisopropylphenyl, are studied: (nacnactBu)ML, where M = FeII, CoII and L = Cl, CH3. These are denoted FeCl, FeCH3, CoCl, and CoCH3 and have been previously reported and structurally characterized. The two FeII complexes (S = 2) have also been previously characterized by Mössbauer spectroscopy, but only indirect assessment of the ligand-field splitting and zero-field splitting (zfs) parameters was available. Here, EPR spectroscopy is used, both conventional field-domain for the CoII complexes (with S = 3/2) and frequency-domain, far-infrared magnetic resonance spectroscopy (FIRMS) for all four complexes. The CoII complexes were also studied by magnetometry. These studies allow accurate determination of the zfs parameters. The two FeII complexes are similar with nearly axial zfs and large magnitude zfs given by D = -37 ± 1 cm-1 for both. The two CoII complexes likewise exhibit large and nearly axial zfs, but surprisingly, CoCl has positive D = +55 cm-1 while CoCH3 has negative D = -49 cm-1. Theoretical methods were used to probe the electronic structures of the four complexes, which explain the experimental spectra and the zfs parameters.

Synthesis, structure, and characterisation of a ferromagnetically coupled dinuclear complex containing Co(II) ions in a high spin configuration and thiodiacetate and phenanthroline as ligands and of a series of isomorphous heterodinuclear complexes containing different Co : Zn ratios

We report the synthesis, crystal structure, and characterisation of a dinuclear Co(II) compound with thiodiacetate (tda) and phenanthroline (phen) as ligands (1), and of a series of metal complexes isomorphous to 1 with different Co : Zn ratios (2, 4 : 1; 3, 1 : 1; 4, 1 : 4; 5, 1 : 10). General characterisation methodologies and X-ray data showed that all the synthesised complexes are isomorphous to Zn(II) and Cu(II) analogues (CSD codes: DUHXEL and BEBQII). 1 consists of centrosymmetric Co(II) ion dimers in which the ions are 3.214 Å apart, linked by two μ-O bridges. Each cobalt atom is in a distorted octahedral environment of the N2O3S type. UV-vis spectra of 1 and 5 are in line with high spin (S = 3/2) Co(II) ions in octahedral coordination and indicate that the electronic structure of both Co(II) ions in the dinuclear unit does not significantly change relative to that of the magnetically isolated Co(II) ion. EPR spectra of powder samples of 5 (Co : Zn ratio of 1 : 10) together with spectral simulation indicated high spin Co(II) ions with high rhombic distortion of the zfs [E/D = 0.31(1), D > 0]. DC magnetic susceptibility experiments on 1 and analysis of the data constraining the E/D value obtained by EPR yielded g = 2.595(7), |D| = 61(1) cm-1, and an intradimer ferromagnetic exchange coupling of J = 1.39(4) cm-1. EPR spectra as a function of Co : Zn ratio for both powder and single crystal samples confirmed that they result from two effective S' = 1/2 spins that interact through dipolar and isotropic exchange interactions to yield magnetically isolated S' = 1 centres and that interdimeric exchange interactions, putatively mediated by hydrophobic interactions between phen moieties, are negligible. The latter observation contrasts with that observed in the Cu(II) analogue, where a transition from S = 1 to S' = 1/2 was observed. Computational calculations indicated that the absence of the interdimeric exchange interaction in 1 is due to a lower Co(II) ion spin density delocalisation towards the metal ligands.

Field- and Temperature-Dependent Paramagnetic Relaxation Enhancements in Co(II) Trispyrazolylmethanes

A comprehensive field- and temperature-dependent examination of nuclear magnetic resonance paramagnetic relaxation enhancements (PREs) for the constitutive protons of [Co(Tpm)2][BF4]2 is presented. Data for an apically substituted derivative clearly establish that bis-Tpm complexes of Co(II) undergo Jahn-Teller dynamics about the molecular threefold axis. PREs from the parent Tpm complex were used to numerically extract the electron relaxation times (T1e). The Tpm complex showed field-dependent behavior, with an approximately 40% higher activation barrier than the related trispyrazolylborate (Tp) complex, based on fits to the T1e vs T, B0 data. Analysis of the field-dependent line widths revealed a surprisingly large contribution from susceptibility (Curie) relaxation (20-35% at the highest field), and a molecular radius (9.5 Å) that is consistent with a tightly associated counterion slowing rotation in solution. Density functional theory showed a shared vibration that is consistent with the Jahn-Teller and appears proportionately higher in energy in [Co(Tpm)2]2+. Complete active-space self-consistent field calculations support ascribing electron relaxation to enhanced mixing of the two Eg orbital sets that accompanies the tetragonal distortion and the differences in electron correlation times to the higher Jahn-Teller activation barrier in [Co(Tpm)2]2+.

Experimental and Theoretical Evidence for an Unusual Almost Triply Degenerate Electronic Ground State of Ferrous Tetraphenylporphyrin

Iron porphyrins exhibit unrivalled catalytic activity for electrochemical CO₂-to-CO conversion. Despite intensive experimental and computational studies in the last 4 decades, the exact nature of the prototypical square-planar [Fe^II(TPP)] complex (1; TPP^2- = tetraphenylporphyrinate dianion) remained highly debated. Specifically, its intermediate-spin (S = 1) ground state was contradictorily assigned to either a nondegenerate ³A_2g state with a (d_xy)²(d_z²)²(d_xz,yz)² configuration or a degenerate ³E_g^θ state with a (d_xy)²(d_xz,yz)³(d_z²)¹/(d_z²)²(d_xy)¹(d_xz,yz)³ configuration. To address this question, we present herein a comprehensive, spectroscopy-based theoretical and experimental electronic-structure investigation on complex 1. Highly correlated wave-function-based computations predicted that ³A_2g and ³E_g^θ are well-isolated from other triplet states by ca. 4000 cm^-1, whereas their splitting Δ_A-E is on par with the effective spin-orbit coupling (SOC) constant of iron(II) (≈400 cm^-1). Therfore, we invoked an effective Hamiltonian (EH) operating on the nine magnetic sublevels arising from SOC between the ³A_2g and ³E_g^θ states. This approach enabled us to successfully simulate all spectroscopic data of 1 obtained by variable-temperature and variable-field magnetization, applied-field ⁵⁷Fe Mössbauer, and terahertz electron paramagnetic resonance measurements. Remarkably, the EH contains only three adjustable parameters, namely, the energy gap without SOC, Δ_A-E, an angle θ that describes the mixing of (d_xy)²(d_xz,yz)³(d_z²)¹ and (d_z²)²(d_xy)¹(d_xz,yz)³ configurations, and the ⟨r_d^-3⟩ expectation value of the iron d orbitals that is necessary to estimate the ⁵⁷Fe magnetic hyperfine coupling tensor. The EH simulations revealed that the triplet ground state of 1 is genuinely multiconfigurational with substantial parentages of both ³A_2g (<88%) and ³E_g (>12%), owing to their accidental near-triple degeneracy with Δ_A-E = +950 cm^-1. As a consequence of this peculiar electronic structure, 1 exhibits a huge effective magnetic moment (4.2 μB at 300 K), large temperature-independent paramagnetism, a large and positive axial zero-field splitting, strong easy-plane magnetization (g_⊥ ≈ 3 and g_∥ ≈ 1.7) and a large and positive internal field at the ⁵⁷Fe nucleus aligned in the xy plane. Further in-depth analyses suggested that g_⊥ ≫ g_∥ is a general spectroscopic signature of near-triple orbital degeneracy with more than half-filled pseudodegenerate orbital sets. Implications of the unusual electronic structure of 1 for CO₂ reduction are discussed.

Paramagnetic Pyrazolylborate Complexes Tp2M and Tp*2M: 1H, 13C, 11B, and 14N NMR Spectra and First-Principles Studies of Chemical Shifts

The paramagnetic pyrazolylborates Tp₂M and Tp*₂M (M = Cu, Ni, Co, Fe, Mn, Cr, V) as well as [Tp₂M]⁺ and [Tp*₂M]⁺ (M = Fe, Cr, V) have been synthesized and their NMR spectra recorded. The ¹H signal shift ranges vary from ∼30 ppm (Cu(II) and V(III)) to ∼220 ppm (Co(II)), and the ¹³C signal shift ranges from ∼180 ppm (Fe(III)) to ∼1150 ppm (Cr(II)). The ¹¹B and ¹⁴N shifts are ∼360 and ∼730 ppm, respectively. Both negative and positive shifts have been observed for all nuclei. The narrow NMR signals of the Co(II), Fe(II), Fe(III), and V(III) derivatives provide resolved ¹³C,¹H couplings. All chemical shifts have been calculated from first-principles on a modern version of Kurland-McGarvey theory which includes optimized structures, zero-field splitting, and g tensors, as well as signal shift contributions. Temperature dependence in the Fe(II) spin-crossover complex results from the equilibrium of the ground singlet and the excited quintet. We illustrate both the assignment and analysis capabilities, as well as the shortcomings of the current computational methodology.

Cobalt(II) "Scorpionate" complexes as electronic ground state models for cobalt-substituted zinc enzymes: Structure investigation by magnetic circular dichroism

Zinc centers in pseudo-tetrahedral geometry are widely found in biology, often with three histidine ligands from protein. The trispyrazolylborate "scorpionate" ligand is used as a model for this tris(histidine) motif, and spectroscopically active Co^II is often used as a substitute for spectroscopically silent Zn^II. In this work, four pseudo-tetrahedral scorpionate complexes with the formula (Tp^t-Bu,Tn)CoL, where Tp^t-Bu,Tn = hydrotris(3-tert-butyl, 5-2'-thienyl-pyrazol-1-yl)borate anion and L = Cl^-, N₃^-, NCO^-, or NCS^-, were studied using variable-temperature, variable-field magnetic circular dichroism (VTVH MCD) spectroscopy. The major goal was to determine the axial and rhombic zero field splitting (ZFS) parameters (D and E, respectively) of these S = 3/2 systems and compare these ZFS parameters to those determined previously by high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy on the same (L = Cl^- and NCS^-) or closely related complexes. Additionally, HFEPR studies were undertaken here on the complexes with L = N₃^-, NCO^-. Crystal structures for these two complexes are also first reported here. The values of D determined by VTVH MCD were + 12.8 and + 3.6 cm^-1 for the L = Cl^- and NCS^- complexes, respectively. These values are in close agreement with those for the same complexes as previously determined by HFEPR. The values of D determined by VTVH MCD were + 3.0 and + 6.6 cm^-1 for the L = N₃^- and NCO^- complexes, respectively. These values were not as close to those determined by HFEPR in the present study, which are 4.2 cm^-1 ≤ |D| ≤ 5.6 cm^-1 in Tp^t-Bu,TnCoN₃, and 8.3 cm^-1 ≤ |D| ≤ 11.0 cm^-1 in Tp^t-Bu,TnCoNCO. The bands in MCD spectra of these complexes were assigned in C_3v symmetry and a complete ligand-field analysis of the MCD data was made using the Angular Overlap Model (AOM), which is compared to previous results.

Coordination [CoII2] and [CoIIZnII] Helicates Showing Slow Magnetic Relaxation

The slow magnetic relaxation of Co^II ions in the elusive intermediate geometry between the trigonal prism and antiprism has been studied on the new [Co₂L₃]⁴⁺ and [CoZnL₃]⁴⁺ coordination helicates [L is a bis(pyrazolylpyridine) ligand]. Solution paramagnetic ¹H NMR and solid-state magnetization measurements unveil single-molecule-magnet behavior with small axial anisotropy, as predicted previously.

Exchangeable proton ENDOR as a probe of the redox-active iron center in activated bleomycin and ferric bleomycin

Activated bleomycin (ABLM) is a drug-Fe(iii)-hydroperoxide complex kinetically competent in DNA attack (via H4' abstraction). This intermediate is relatively stable, but its spontaneous conversion to ferric bleomycin (Fe(iii)·BLM) is poorly characterized because no observable intermediate product accumulates. The Fe(iii)·BLM formed cryophotolytically from ABLM and kept at 77 K was remarkably similar by EPR and ENDOR criteria to Fe(iii)·BLM formed from Fe(iii) + BLM solution. The notable ENDOR criteria were the ENDOR frequencies and features of orientation-selected, strongly hyperfine-coupled, exchangeable protons associated with the environs of the iron within <3.5 Å of paramagnetic Fe(iii) in Fe(iii)·BLM and ABLM. Cryophotolytic conversion of activated bleomycin to its ferric bleomycin product in the frozen solid is a sign that the reaction requires only constrained local proton rearrangements. We have characterized the metal-proton distances and orientations of the protons in that rearrangement, especially noting that these protons are of mechanistic importance in the ambient temperature conversion of ABLM to Fe(iii)·BLM in concert with a directed radical-forming attack on DNA.

Paramagnetic Resonance of Cobalt(II) Trispyrazolylmethanes and Counterion Association

Paramagnetic resonance studies (EPR, ESEEM, ENDOR, and NMR) of a series of cobalt(II) bis-trispyrazolylmethane tetrafluoroborates are presented. The complexes studied include the parent, unsubstituted ligand (Tpm), two pyrazole-substituted derivatives (4Me and 3,5-diMe), and tris(1-pyrazolyl)ethane (Tpe), which includes a methyl group on the apical carbon atom. NMR and ENDOR establish the magnitude of ¹H hyperfine couplings, while ESEEM provides information on the coordinated ¹⁴N. The data show that the pyrazole 3-position is more electron rich in the Tpm analogues, that the geometry about the apical atom influences the magnetic resonance, and that apical atom geometry appears more fixed in Tpm than in Tp. NMR and ENDOR establish that the BF₄^- counterion remains associated in fluid solution. In the case of the Tpm^3,5Me complex, it appears to associate in solution, in the same position it occupies in the X-ray structure.

Magnetic properties of weakly exchange-coupled high spin Co(II) ions in pseudooctahedral coordination evaluated by single crystal X-band EPR spectroscopy and magnetic measurements

We report single-crystal X-band EPR and magnetic measurements of the coordination polymer catena-(trans-(μ2-fumarato)tetraaquacobalt(II)), 1, and the Co(II)-doped Zn(II) analogue, 2, in different Zn:Co ratios. 1 presents two magnetically inequivalent high spin S = 3/2 Co(II) ions per unit cell, named A and B, in a distorted octahedral environment coordinated to four water oxygen atoms and trans coordinated to two carboxylic oxygen atoms from the fumarate anions, in which the Co(II) ions are linked by hydrogen bonds and fumarate molecules. Magnetic susceptibility and magnetization measurements of 1 indicate weak antiferromagnetic exchange interactions between the S = 3/2 spins of the Co(II) ions in the crystal lattice. Oriented single crystal EPR experiments of 1 and 2 were used to evaluate the molecular g-tensor and the different exchange coupling constants between the Co(II) ions, assuming an effective spin S′= 1/2. Unexpectedly, the eigenvectors of the molecular g-tensor were not lying along any preferential bond direction, indicating that, in high spin Co(II) ions in roughly octahedral geometry with approximately axial EPR signals, the presence of molecular pseudo axes in the metal site does not determine preferential directions for the molecular g-tensor. The EPR experiment and magnetic measurements, together with a theoretical analysis relating the coupling constants obtained from both techniques, allowed us to evaluate selectively the exchange coupling constant associated with hydrogen bonds that connect magnetically inequivalent Co(II) ions (|JAB(1/2)| = 0.055(2) cm(–1)) and the exchange coupling constant associated with a fumarate bridge connecting equivalent Co(II) ions (|JAA(1/2)| ≈ 0.25 (1) cm(–1)), in good agreement with the average J(3/2) value determined from magnetic measurements.

Jahn-Teller dynamics in a series of high-symmetry Co(II) chelates determine paramagnetic relaxation enhancements

NMR paramagnetic relaxation enhancements (PREs) of a series of structurally characterized, trigonal bis-trispyrazolylborate (Tp) chelates of high-spin Co(II), spanning 100-850 MHz in field, are reported. Prior knowledge of the metal-nucleus distances allows numerical extraction of position-dependent electron spin relaxation rates (τ(c)(-1)) from direct measurement of the individual PREs of the four symmetry distinct protons in Co(Tp)(2), using available closed-form expressions. The data for this electronically complex system where spin-orbit coupling defines the ground state electronic structure are analyzed in terms of the Solomon-Bloembergen-Morgan (SBM) relations, as well as available zero-field splitting limit theories. A simple angular correction is shown to be sufficient to reconcile the individual τ(c)(T) data for the four classes of protons. The data identify a previously unrecognized dynamic Jahn-Teller effect in these historically important complexes, with a barrier of ~230 cm(-1), pointing to a level of dynamics in trispyrazolylborate chemistry that has not been described before, and further show that it is the Jahn-Teller that is responsible for the PREs in fluid solution. A field-dependent component is also identified for the two protons nearest g(//), which is suggested to arise due to Zeeman mixing of excited state character into the ground level.

EPR-ENDOR characterization of (17O, 1H, 2H) water in manganese catalase and its relevance to the oxygen-evolving complex of photosystem II

The synthesis of efficient water-oxidation catalysts demands insight into the only known, naturally occurring water-oxidation catalyst, the oxygen-evolving complex (OEC) of photosystem II (PSII). Understanding the water oxidation mechanism requires knowledge of where and when substrate water binds to the OEC. Mn catalase in its Mn(III)-Mn(IV) state is a protein model of the OEC's S(2) state. From (17)O-labeled water exchanged into the di-μ-oxo di-Mn(III,IV) coordination sphere of Mn catalase, CW Q-band ENDOR spectroscopy revealed two distinctly different (17)O signals incorporated in distinctly different time regimes. First, a signal appearing after 2 h of (17)O exchange was detected with a 13.0 MHz hyperfine coupling. From similarity in the time scale of isotope incorporation and in the (17)O μ-oxo hyperfine coupling of the di-μ-oxo di-Mn(III,IV) bipyridine model (Usov, O. M.; Grigoryants, V. M.; Tagore, R.; Brudvig, G. W.; Scholes, C. P. J. Am. Chem. Soc. 2007, 129, 11886-11887), this signal was assigned to μ-oxo oxygen. EPR line broadening was obvious from this (17)O μ-oxo species. Earlier exchange proceeded on the minute or faster time scale into a non-μ-oxo position, from which (17)O ENDOR showed a smaller 3.8 MHz hyperfine coupling and possible quadrupole splittings, indicating a terminal water of Mn(III). Exchangeable proton/deuteron hyperfine couplings, consistent with terminal water ligation to Mn(III), also appeared. Q-band CW ENDOR from the S(2) state of the OEC was obtained following multihour (17)O exchange, which showed a (17)O hyperfine signal with a 11 MHz hyperfine coupling, tentatively assigned as μ-oxo-(17)O by resemblance to the μ-oxo signals from Mn catalase and the di-μ-oxo di-Mn(III,IV) bipyridine model.

A hyperactive cobalt-substituted extradiol-cleaving catechol dioxygenase

Homoprotocatechuate 2,3-dioxygenase from Brevibacterium fuscum (HPCD) has an Fe(II) center in its active site that can be replaced with Mn(II) or Co(II). Whereas Mn-HPCD exhibits steady-state kinetic parameters comparable to those of Fe-HPCD, Co-HPCD behaves somewhat differently, exhibiting significantly higher [Formula: see text] and k (cat). The high activity of Co-HPCD is surprising, given that cobalt has the highest standard M(III/II) redox potential of the three metals. Comparison of the X-ray crystal structures of the resting and substrate-bound forms of Fe-HPCD, Mn-HPCD, and Co-HPCD shows that metal substitution has no effect on the local ligand environment, the conformational integrity of the active site, or the overall protein structure, suggesting that the protein structure does not differentially tune the potential of the metal center. Analysis of the steady-state kinetics of Co-HPCD suggests that the Co(II) center alters the relative rate constants for the interconversion of intermediates in the catalytic cycle but still allows the dioxygenase reaction to proceed efficiently. When compared with the kinetic data for Fe-HPCD and Mn-HPCD, these results show that dioxygenase catalysis can proceed at high rates over a wide range of metal redox potentials. This is consistent with the proposed mechanism in which the metal mediates electron transfer between the catechol substrate and O(2) to form the postulated [M(II)(semiquinone)superoxo] reactive species. These kinetic differences and the spectroscopic properties of Co-HPCD provide new tools with which to explore the unique O(2) activation mechanism associated with the extradiol dioxygenase family.