Effects of CoS torsion angle variation in a cobalt(II)-thiolate complex: X-ray crystal structure analysis, single-crystal EPR measurements and ligand-field calculations

Structural, magnetic and spectral properties of tetrahedral cobalt(ii) silanethiolates: a variety of structures and manifestation of field-induced slow magnetic relaxation

Blue crystals of five heteroleptic cobalt(ii) silanethiolates 1-5 have been obtained by the reaction of [Co{SSi(tBuO)3}2(NH3)]2 with aminopyridines and aminomethylpyridines at an appropriate molar ratio and their structural, spectral, thermal and magnetic properties have been established and described. All complexes 1-5 contain Co(ii) ions in a tetrahedral CoN2S2 environment formed by (tBuO)3SiS- residues and pyridines and present variable structures. Complexes 1-3 are mononuclear [Co{SSi(tBuO)3}2(L1)2] (L1 = 2-aminopyridine 2AP, 3-aminopyridine 3AP, and 4-aminopyridine 4AP). The application of 3AMP and 4AMP (3-aminomethylpyridine and 4-aminomethylpyridine) allows either dinuclear complex 4 [Co{SSi(tBuO)3}2(μ-3AMP)]2 or 1D coordination polymer 5 with the formula of [Co{SSi(tBuO)3}2(μ-4AMP)]n to be obtained. The molecular structures of 1-5 were determined by single-crystal X-ray and powder diffraction, UV-vis and FTIR spectrocopy for solid samples and their thermal properties were characterized by TG-DSC and TG-FTIR methods. The dc and ac magnetic and EPR studies of polycrystalline samples have been performed. For all complexes, the obtained data show a behavior typical of paramagnetic high-spin Co(ii) ions in a tetrahedral geometry, with a considerable contribution of the ZFS effect in a low temperature range. All complexes were also probed for SIM behavior. The modeling of the magnetic and EPR data was done for samples 1, 3, 4 and 5 to estimate ZFS parameters. The obtained results imply a negative value of the axial parameter D in complex 4 and positive D values for the rest of the compounds. A comparative magneto-structural analysis of complexes 4 and 5 points to the high sensitivity of the single-ion magnetic anisotropy of tetrahedral Co(ii) complexes to subtle changes in the first and second coordination spheres of Co(ii) ions.

Novel tetrahedral cobalt(ii) silanethiolates: structures and magnetism

Three heteroleptic complexes of Co(ii) tri-tert-butoxysilanethiolates have been synthesized with piperidine [Co{SSi(OtBu)₃}₂(ppd)₂] 1, piperazine [Co{SSi(OtBu)₃}₂(NH₃)]₂(μ-ppz)·2CH₃CN 2, and N-ethylimidazole [Co{SSi(OtBu)₃}₂(etim)₂] 3. The complexes have been characterized by a single-crystal X-ray, revealing their tetrahedral geometry on Co(ii) coordinated by two nitrogen and two sulfur atoms. Complexes 1 and 3 are mononuclear, whereas 2 is binuclear. The spectral properties and thermal properties of 1–3 complexes were established by FTIR spectroscopy for solid samples and TGA. The magnetic properties of complexes 1, 2, and 3 have been investigated by static magnetic measurements and X-band EPR spectroscopy. These studies have shown that 1 and 3, regardless of the similarity in structure of CoN₂S₂ cores, demonstrate different types of local magnetic anisotropy. Magnetic investigations of 2 reveal the presence of weak antiferromagnetic intra-molecular Co(ii)–Co(ii) interactions that are strongly influenced by the local magnetic anisotropy of individual Co(ii) ions.

Structural, spectral and thermal properties of three tetrahedral Co(ii) silanethiolates were established by XRD, FTIR for solid samples and TGA. The magnetic properties were investigated by static magnetic measurements and X-band EPR spectroscopy.

A flexible iron(ii) complex in which zero-field splitting is resistant to structural variation

The zero-field splitting parameters D and E in the iron(ii) complex [Fe(C₃S₅)₂]^2– are shown to be remarkably resistant to a twist of the inter-ligand dihedral angle (θ_d) from 90 to 70°.

The relationship between electronic structure and zero-field splitting dictates key design parameters for magnetic molecules. In particular, to enable the directed synthesis of new electronic spin based qubits, developing complexes where zero-field splitting energies are invariant to structural changes is a critical challenge. Toward those ends, we report three salts of a new compound, a four-coordinate iron(ii) complex [Fe(C₃S₅)₂]^2– ([(18-crown-6)K]⁺ (1), Ph₄P⁺ (2), Bu₄N⁺ (3)) with a continuous structural variation in a single parameter, the dihedral angle (θ_d) between the two C₃S₅^2– ligands, as a function of counterion (θ_d = 89.98(4)° for 1 to 72.41(2)° for 3). Electron paramagnetic resonance data for 1–3 reveal zero-field splitting parameters that are unusually robust to the structural variation. Mössbauer spectroscopic measurements indicate that the structural variation in θ_d primarily affects the highest-energy 3d-orbitals (d_xz and d_yz) of the iron(ii) ion. These orbitals have the smallest impact on the zero-field splitting parameters, thus the distortion has a minor effect on D and E. These results represent the first part of a directed effort to understand how spin state energies may be fortified against structural distortions for future applications of qubits in non-crystalline environments.

Discrete and polymeric cobalt organophosphates: isolation of a 3-D cobalt phosphate framework exhibiting selective CO2 capture

Auxiliary ligand assisted control over the structural diversity has been achieved in the case of cobalt(ii) organophosphates.

Multifrequency EPR study of Fe3+ and Co2+ in the active site of desulforedoxin

The understanding of the electronic structure of S > 1/2 transition-metal sites that show a large zero-field splitting (ZFS) of the magnetic sublevels benefits greatly from study by electron-paramagnetic-resonance (EPR) spectroscopy at frequencies above the standard 9.5 GHz. However, high-frequency EPR spectroscopy is technically challenging and still developing. Particularly the sensitivity of high-frequency EPR spectrometers is often too low to apply the technique in the study of transition-metal sites in proteins and enzymes. Here we report a multifrequency EPR study (at 9.5, 94.9, and 275.7 GHz) of the active site of the protein desulforedoxin, both in its natural Fe(3+) form and substituted with Co(2+). The 275.7 GHz EPR spectra made it possible to determine the ZFS parameters of the Fe(3+) site with high precision. No 275.7 GHz spectrum could be observed of the Co(2+) site, but based on 9.5 GHz spectra, its ZFS parameters could be estimated. We find that the typical variation in the geometry of the active site of a protein or enzyme, referred to as conformational strain, does not only make the detection of EPR spectra challenging, but also their analysis. Comparison of the EPR results on the active site of desulforedoxin to those of the closely related active site of rubredoxin illustrates the necessity of explicit quantum-chemical calculations in order to interrelate the electronic and geometric structure of biological transition-metal sites.

Slow magnetic relaxation at zero field in the tetrahedral complex [Co(SPh)4]2-

The Ph(4)P(+) salt of the tetrahedral complex [Co(SPh)(4)](2-), possessing an S = (3)/(2) ground state with an axial zero-field splitting of D = -70 cm(-1), displays single-molecule magnet behavior in the absence of an applied magnetic field. At very low temperatures, ac magnetic susceptibility data show the magnetic relaxation time, τ, to be temperature-independent, while above 2.5 K thermally activated Arrhenius behavior is apparent with U(eff) = 21(1) cm(-1) and τ(0) = 1.0(3) × 10(-7) s. Under an applied field of 1 kOe, τ more closely approximates Arrhenius behavior over the entire temperature range. Upon dilution of the complex within a matrix of the isomorphous compound (Ph(4)P)(2)[Zn(SPh)(4)], ac susceptibility data reveal the molecular nature of the slow magnetic relaxation and indicate that the quantum tunneling pathway observed at low temperatures is likely mediated by intermolecular dipolar interactions.

Tetrahedral and square planar Ni[(SPR(2))(2)N](2) complexes, R = Ph & (i)Pr revisited: experimental and theoretical analysis of interconversion pathways, structural preferences, and spin delocalization

Sulfur-containing mono- or bidentate types of ligands, usually form square planar Ni((II))S(4) complexes. However, it has already been established that the bidentate L(-) dithioimidodiphosphinato ligands, [R(2)P(S)NP(S)R'(2)](-), R, and R' = aryl or alkyl, can afford both tetrahedral and square planar, NiS(4)-containing, homoleptic Ni(R,R')L(2) complexes, owing to an apparent structural flexibility, which has not, so far, been probed. In this work, the literature tetrahedral Ni[R(2)P(S)NP(S)R(2)](2) complexes, R = Ph (Ni(Ph,Ph)L(2), 1(Td)) and R = (i)Pr (Ni(iPr,iPr)L(2), 2) as well as the newly synthesized Ni[(i)Pr(2)P(S)NP(S)Ph(2)](2) complex (Ni(iPr,Ph)L(2), 3), have been studied by UV-vis, IR, and (31)P NMR spectroscopy. Complex 3 was shown by X-ray crystallography to be square planar, and magnetic studies confirmed that it is diamagnetic in the solid state. However, it becomes paramagnetic in solution, as it shows a similar UV-vis spectrum to one of the tetrahedral 1(Td) and 2 complexes. The crystal structure of the potassium salt of the asymmetric ligand, [(i)Pr(2)P(S)NP(S)Ph(2)]K, has also been determined and compared to those of the protonated (i)Pr(2)P(S)NHP(S)Ph(2) ligand and complex 3. All three, 1(Td), 2, and 3, Ni(R,R')L(2) complexes show strong paramagnetic effects in their solution (31)P NMR spectra. The magnetic properties of paramagnetic complexes 1 and 2 in the solid state were investigated on oriented crystals, and their analysis afforded remarkably small values of the spin-orbit coupling constant (lambda) and orbital reduction factor (k) parameters, implying significant delocalization of unpaired electronic density toward the ligands. The above experimental findings are combined with data from standard density functional theory and correlated multiconfiguration ab initio theoretical methods, in an effort to investigate the interplay between the square planar and tetrahedral geometries of the NiS(4) core, the mechanistic pathway for the spin-state interconversion, the degree of covalency of the Ni-S bonds, and the distribution of the spin density in this type of system. The analysis provides justification for the structural flexibility of such ligands, affording Ni(R,R')L(2) complexes with variable metallacycle conformation and NiS(4) core geometries. Of particular importance are the large zero-field splitting values estimated by both experimental and theoretical means, which have not, as yet, been verified by direct methods, such as electron paramagnetic resonance spectroscopy. The findings of our work confirm earlier observations on the feasibility of synthesizing either tetrahedral or square planar NiS(4) complexes containing the same type of ligands. They can also form the basis of investigating structure-properties relationships in other NiS(4)-containing systems.