Magneto-Structural Correlations in Pseudotetrahedral Forms of the [Co(SPh)4]2- Complex Probed by Magnetometry, MCD Spectroscopy, Advanced EPR Techniques, and ab Initio Electronic Structure Calculations.

The magnetic properties of pseudotetrahedral Co(II) complexes spawned intense interest after (PPh4)2[Co(SPh)4] was shown to be the first mononuclear transition-metal complex displaying slow relaxation of the magnetization in the absence of a direct current magnetic field. However, there are differing reports on its fundamental magnetic spin Hamiltonian (SH) parameters, which arise from inherent experimental challenges in detecting large zero-field splittings. There are also remarkable changes in the SH parameters of [Co(SPh)4]2- upon structural variations, depending on the counterion and crystallization conditions. In this work, four complementary experimental techniques are utilized to unambiguously determine the SH parameters for two different salts of [Co(SPh)4]2-: (PPh4)2[Co(SPh)4] (1) and (NEt4)2[Co(SPh)4] (2). The characterization methods employed include multifield SQUID magnetometry, high-field/high-frequency electron paramagnetic resonance (HF-EPR), variable-field variable-temperature magnetic circular dichroism (VTVH-MCD), and frequency domain Fourier transform THz-EPR (FD-FT THz-EPR). Notably, the paramagnetic Co(II) complex [Co(SPh)4]2- shows strong axial magnetic anisotropy in 1, with D = -55(1) cm-1 and E/D = 0.00(3), but rhombic anisotropy is seen for 2, with D = +11(1) cm-1 and E/D = 0.18(3). Multireference ab initio CASSCF/NEVPT2 calculations enable interpretation of the remarkable variation of D and its dependence on the electronic structure and geometry.

[1]  A. Schnegg,et al.  A linear single-molecule magnet based on [Ru(III)(CN)6]3-. , 2011, Chemical communications.

[2]  A. Ozarowski,et al.  High-frequency and high-field electron paramagnetic resonance (HFEPR): a new spectroscopic tool for bioinorganic chemistry , 2014, JBIC Journal of Biological Inorganic Chemistry.

[3]  Valérie Vallet,et al.  A two-step uncontracted determinantal effective Hamiltonian-based SO–CI method , 2000 .

[4]  F. Neese,et al.  Magnetic Circular Dichroism Evidence for an Unusual Electronic Structure of a Tetracarbene-Oxoiron(IV) Complex. , 2016, Journal of the American Chemical Society.

[5]  S. Grimme,et al.  A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. , 2010, The Journal of chemical physics.

[6]  A. Schnegg,et al.  THz Electron Paramagnetic Resonance / THz Spectroscopy at BESSY II , 2016 .

[7]  E. Pardo,et al.  Cytosine Nucleobase Ligand: A Suitable Choice for Modulating Magnetic Anisotropy in Tetrahedrally Coordinated Mononuclear CoII Compounds. , 2017, Inorganic chemistry.

[8]  K. Dunbar,et al.  Ligands effects on the magnetic anisotropy of tetrahedral cobalt complexes. , 2014, Chemical communications.

[9]  S. Sottini,et al.  A multifrequency high-field electron paramagnetic resonance study of Co(II)S(4) coordination. , 2010, Inorganic chemistry.

[10]  Frank Neese,et al.  An improvement of the resolution of the identity approximation for the formation of the Coulomb matrix , 2003, J. Comput. Chem..

[11]  H. Masuda,et al.  Effects of CoS torsion angle variation in a cobalt(II)-thiolate complex: X-ray crystal structure analysis, single-crystal EPR measurements and ligand-field calculations , 1995 .

[12]  Silvia Gómez-Coca,et al.  Large magnetic anisotropy in mononuclear metal complexes , 2015 .

[13]  J. Behrends,et al.  Frequency domain Fourier transform THz-EPR on single molecule magnets using coherent synchrotron radiation. , 2009, Physical chemistry chemical physics : PCCP.

[14]  M. Murrie.  Cobalt(II) single-molecule magnets. , 2010, Chemical Society reviews.

[15]  M. Dressel,et al.  High‐frequency/high‐field EPR spectroscopy of the high‐spin ferrous ion in hexaaqua complexes , 2005, Magnetic resonance in chemistry : MRC.

[16]  Frank Neese,et al.  All-Electron Scalar Relativistic Basis Sets for Third-Row Transition Metal Atoms. , 2008, Journal of chemical theory and computation.

[17]  E. Solomon,et al.  Magnetic circular dichroism spectroscopy as a probe of the geometric and electronic structure of non-heme ferrous enzymes , 1995 .

[18]  S. Sottini,et al.  Magnetic Anisotropy of Tetrahedral CoII Single-Ion Magnets: Solid-State Effects. , 2016, Inorganic chemistry.

[19]  J. Telser,et al.  Measuring giant anisotropy in paramagnetic transition metal complexes with relevance to single-ion magnetism. , 2016, Dalton transactions.

[20]  A. Schnegg,et al.  Simulating Frequency-Domain Electron Paramagnetic Resonance: Bridging the Gap between Experiment and Magnetic Parameters for High-Spin Transition-Metal Ion Complexes. , 2015, The journal of physical chemistry. B.

[21]  Instrumentation for millimeter-wave magnetoelectrodynamic investigations of low-dimensional conductors and superconductors , 1999, cond-mat/9907310.

[22]  W. Wernsdorfer,et al.  Slow Magnetic Relaxation in Weak Easy-Plane Anisotropy: the Case of a Combined Magnetic and HFEPR Study. , 2017, Inorganic chemistry.

[23]  D. Coucouvanis,et al.  Tetrahedral mercaptide complexes. Crystal and molecular structures of [(C6H5)4P]2M(SC6H5)4 complexes (M = cadmium(II), zinc(II), nickel(II), cobalt(II), and manganese(II)) , 1978 .

[24]  B. Roos,et al.  The complete active space SCF (CASSCF) method in a Newton–Raphson formulation with application to the HNO molecule , 1981 .

[25]  J. Perdew,et al.  Density-functional approximation for the correlation energy of the inhomogeneous electron gas. , 1986, Physical review. B, Condensed matter.

[26]  D. Stalke,et al.  Triimidosulfonates as Acute Bite-Angle Chelates: Slow Relaxation of the Magnetization in Zero Field and Hysteresis Loop of a Co(II) Complex. , 2015, Chemistry.

[27]  B. Roos,et al.  A complete active space SCF method (CASSCF) using a density matrix formulated super-CI approach , 1980 .

[28]  H. Görls,et al.  Cobalt(II)-Based Single-Ion Magnets with Distorted Pseudotetrahedral [N2O2] Coordination: Experimental and Theoretical Investigations. , 2016, Inorganic chemistry.

[29]  F. Neese,et al.  A fully variational spin-orbit coupled complete active space self-consistent field approach: application to electron paramagnetic resonance g-tensors. , 2013, The Journal of chemical physics.

[30]  D. M. Barnes,et al.  Low-spin hexacoordinate Mn(III): synthesis and spectroscopic investigation of homoleptic tris(pyrazolyl)borate and tris(carbene)borate complexes. , 2013, Inorganic chemistry.

[31]  F. Neese,et al.  A four-coordinate cobalt(II) single-ion magnet with coercivity and a very high energy barrier , 2016, Nature Communications.

[32]  J. Pilbrow.  Effective g values for S = 3/2 and S = 5/2 , 1978 .

[33]  N. Guihéry,et al.  Giant Ising-type magnetic anisotropy in trigonal bipyramidal Ni(II) complexes: experiment and theory. , 2013, Journal of the American Chemical Society.

[34]  D. Coucouvanis.  Fe-M-S complexes derived from MS42- anions (M = molybdenum, tungsten) and their possible relevance as analogs for structural features in the molybdenum site of nitrogenase , 1981 .

[35]  Joseph M. Zadrozny,et al.  Slow magnetic relaxation at zero field in the tetrahedral complex [Co(SPh)4]2-. , 2011, Journal of the American Chemical Society.

[36]  W. Wernsdorfer,et al.  Cyanide Single-Molecule Magnets Exhibiting Solvent Dependent Reversible "On" and "Off" Exchange Bias Behavior. , 2015, Journal of the American Chemical Society.

[37]  Hess,et al.  Relativistic electronic-structure calculations employing a two-component no-pair formalism with external-field projection operators. , 1986, Physical review. A, General physics.

[38]  N. Hirota,et al.  ESR and Magnetic Susceptibility Studies on High-Spin Tetrahedral Cobalt(II)–Thiolate Complexes: An Approach to Rubredoxin-Type Active Sites , 1991 .

[39]  R. Cimiraglia,et al.  Multireference perturbation configuration interaction V. Third-order energy contributions in the Møller–Plesset and Epstein–Nesbet partitions , 2002 .

[40]  Hans W. Horn,et al.  Fully optimized contracted Gaussian basis sets for atoms Li to Kr , 1992 .

[41]  Joseph M. Zadrozny,et al.  Slow magnetic relaxation in the tetrahedral cobalt(II) complexes [Co(EPh)4]2− (EO, S, Se) , 2013 .

[42]  J. Dawson,et al.  Bioinorganic applications of magnetic circular dichroism spectroscopy: Copper, rare-earth ions, cobalt and non-heme iron systems , 1984 .

[43]  L. Kuo,et al.  Ground term splitting of high-spin cobalt(2+) ion as a probe of coordination structure. 1. Dependence of the splitting on coordination geometry , 1985 .

[44]  Frank Neese,et al.  First-principles calculations of magnetic circular dichroism spectra. , 2008, The Journal of chemical physics.

[45]  Frank Neese,et al.  The ORCA program system , 2012 .

[46]  Rotating cavity for high-field angle-dependent microwave spectroscopy of low-dimensional conductors and magnets , 2005, cond-mat/0505513.

[47]  N. Guihéry,et al.  Universal Theoretical Approach to Extract Anisotropic Spin Hamiltonians. , 2009, Journal of chemical theory and computation.

[48]  A. Schäfer,et al.  Fully optimized contracted Gaussian basis sets of triple zeta valence quality for atoms Li to Kr , 1994 .

[49]  Frank Neese,et al.  MCD C-Term Signs, Saturation Behavior, and Determination of Band Polarizations in Randomly Oriented Systems with Spin S >/= (1)/(2). Applications to S = (1)/(2) and S = (5)/(2). , 1999, Inorganic chemistry.

[50]  F. Neese,et al.  Magnetic circular dichroism and computational study of mononuclear and dinuclear iron(iv) complexes , 2015, Chemical science.

[51]  F. Neese,et al.  Direct Observation of Very Large Zero-Field Splitting in a Tetrahedral Ni(II)Se4 Coordination Complex. , 2015, Journal of the American Chemical Society.

[52]  J. Sutherland,et al.  Magnetic circular dichroism of biological molecules. , 1980, Annual review of biophysics and bioengineering.

[53]  Frank Neese,et al.  First principles approach to the electronic structure, magnetic anisotropy and spin relaxation in mononuclear 3d-transition metal single molecule magnets , 2015 .

[54]  F. Neese,et al.  Magneto-Structural Correlations in a Series of Pseudotetrahedral [Co(II)(XR)4](2-) Single Molecule Magnets: An ab Initio Ligand Field Study. , 2015, Inorganic chemistry.

[55]  P Kuske,et al.  Brilliant, coherent far-infrared (THz) synchrotron radiation. , 2003, Physical review letters.

[56]  T. Volkert,et al.  MAGNETIC CIRCULAR DICHROISM SPECTROSCOPY AS A PROBE OF GEOMETRIC AND ELECTRONIC STRUCTURE OF COBALT(II)-SUBSTITUTED PROTEINS : GROUND-STATE ZERO-FIELD SPLITTING AS A COORDINATION NUMBER INDICATOR , 1997 .

[57]  A. Thomson,et al.  A novel, general method of analyzing magnetic circular dichroism spectra and magnetization curves of high-spin metal ions: Application to the protein oxidized rubredoxin, Desulfovibrio gigas , 1999 .

[58]  Stefan Grimme,et al.  Effect of the damping function in dispersion corrected density functional theory , 2011, J. Comput. Chem..

[59]  E. Pidcock,et al.  Optical Detection of Spin Polarization in Single-Molecule Magnets [Mn12O12(O2CR)16(H2O)4] , 2002 .

[60]  Joseph M. Zadrozny,et al.  Slow magnetization dynamics in a series of two-coordinate iron(II) complexes , 2013 .

[61]  Celestino Angeli,et al.  Introduction of n-electron valence states for multireference perturbation theory , 2001 .

[62]  W. Wernsdorfer,et al.  What Controls the Sign and Magnitude of Magnetic Anisotropy in Tetrahedral Cobalt(II) Single-Ion Magnets? , 2016, Inorganic chemistry.

[63]  Arthur Schweiger,et al.  EasySpin, a comprehensive software package for spectral simulation and analysis in EPR. , 2006, Journal of magnetic resonance.

[64]  N. Kojima,et al.  Metal-thiolate bonding properties: single-crystal ESR, susceptibility, and polarized absorption evidence for a strong .pi. interaction in tetrakis(thiophenolato)cobaltate(II) , 1992 .

[65]  A. Thomson,et al.  The nature of the exchange coupling between high-spin Fe(III) heme o3 and CuBII in Escherichia coli quinol oxidase, cytochrome bo3: MCD and EPR studies. , 2004, Journal of the American Chemical Society.

[66]  F. Neese,et al.  Theoretical analysis of the spin Hamiltonian parameters in Co(II)S4 complexes, using density functional theory and correlated ab initio methods. , 2011, Inorganic chemistry.

[67]  E. L. Bominaar,et al.  Theoretical analysis of the three-dimensional structure of tetrathiolato iron complexes. , 2004, Inorganic chemistry.

[68]  Joseph M. Zadrozny,et al.  A mononuclear transition metal single-molecule magnet in a nuclear spin-free ligand environment. , 2014, Inorganic chemistry.

[69]  R. Boča,et al.  Zero-field splitting in pseudotetrahedral Co(II) complexes: a magnetic, high-frequency and -field EPR, and computational study. , 2013, Inorganic chemistry.

[70]  A. Becke,et al.  Density-functional exchange-energy approximation with correct asymptotic behavior. , 1988, Physical review. A, General physics.

[71]  M. Murrie,et al.  3d single-ion magnets. , 2015, Chemical Society reviews.

[72]  D. Coucouvanis,et al.  TETRAHEDRAL MERCAPTIDE COMPLEXES. CRYSTAL AND MOLECULAR STRUCTURES OF ((C6H5)4P)2M(SC6H5)4 COMPLEXES (M = CADMIUM(II), ZINC(II), NICKEL(II), COBALT(II), AND MANGANESE(II)) , 2002 .

[73]  A. Ozarowski,et al.  Multi-frequency, high-field EPR as a powerful tool to accurately determine zero-field splitting in high-spin transition metal coordination complexes , 2006 .

[74]  A. Thomson,et al.  Magnetic circular dichroism of symmetry and spin forbidden transitions of high-spin metal ions , 2000 .

[75]  F. Weigend,et al.  Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy. , 2005, Physical chemistry chemical physics : PCCP.

[76]  D. Hendrickson,et al.  High frequency electron paramagnetic resonance (HFEPR) study of a high spin Co(II) complex , 2007 .

[77]  Christopher J. Chang,et al.  Slow magnetic relaxation in a pseudotetrahedral cobalt(II) complex with easy-plane anisotropy. , 2012, Chemical communications.