Eleftherios Ferentinos

Conversion of tetrahedral to octahedral structures upon solvent coordination: studies on the M[(OPPh2)(SePPh2)N]2 (M = Co, Ni) and [Ni{(OPPh2)(EPPh2)N}2(dmf)2] (E = S, Se) complexes

The synthesis of the M[(OPPh(2))(SePPh(2))N](2), M = Co (1), Ni (2) complexes was accomplished by metathetical reactions between the corresponding M(II) salts and the deprotonated form of the dichalcogenated imidodiphosphinato ligand [(OPPh(2))(SePPh(2))N](-). X-Ray crystallography revealed a pseudo-tetrahedral MO(2)Se(2) coordination sphere, owing to the asymmetric (O,Se) nature of the chelating ligand. Slow diffusion of the coordinating solvent dimethylformamide into dichloromethane solutions of Ni[(OPPh(2))(SPPh(2))N](2) or 2, afforded the pseudo-octahedral trans-[Ni{(OPPh(2))(EPPh(2))N}(2)(dmf)(2)], E = S (3), Se (4) complexes, respectively. UV-vis spectra provided evidence that, in solution, complexes 3 and 4 revert to the corresponding pseudo-tetrahedral complexes, most likely due to the removal of the dmf molecules from the coordination sphere. The IR spectra of all complexes reflect the structural features observed by X-ray crystallography. The magnetic properties of the S = 3/2 complex 1, as well as the S = 1 complexes 2, 3 and 4, were extensively studied, and the magnitude of their g and zero-field splitting D parameters was estimated. The reported structures establish a structural transformation of tetrahedral to octahedral geometry of Ni(II) complexes bearing asymmetric imidodiphosphinate ligands, upon recrystallization from coordinating solvents. The structural correlations between the Ni(II) coordination spheres are aided by DFT and ab initio multi-configuration MCSCF calculations, which investigate the corresponding interconversion pathways. In addition, the calculations provide descriptions of the bonding interactions in the octahedral Ni(II) complexes, as well as predictions of their D values.

Investigating Magnetostructural Correlations in the Pseudooctahedral trans-[NiII{(OPPh2)(EPPh2)N}2(sol)2] Complexes (E = S, Se; sol = DMF, THF) by Magnetometry, HFEPR, and ab Initio Quantum Chemistry

In this work, magnetometry and high-frequency and -field electron paramagnetic resonance spectroscopy (HFEPR) have been employed in order to determine the spin Hamiltonian (SH) parameters of the non-Kramers, S = 1, pseudooctahedral trans-[Ni(II){(OPPh(2))(EPPh(2))N}(2)(sol)(2)] (E = S, Se; sol = DMF, THF) complexes. X-ray crystallographic studies on these compounds revealed a highly anisotropic NiO(4)E(2) coordination environment, as well as subtle structural differences, owing to the nature of the Ni(II)-coordinated solvent molecule or ligand E atoms. The effects of these structural characteristics on the magnetic properties of the complexes were investigated. The accurately HFEPR-determined SH zero-field-splitting (zfs) D and E parameters, along with the structural data, provided the basis for a systematic density functional theory (DFT) and multiconfigurational ab initio computational analysis, aimed at further elucidating the electronic structure of the complexes. DFT methods yielded only qualitatively useful data. However, already entry level ab initio methods yielded good results for the investigated magnetic properties, provided that the property calculations are taken beyond a second-order treatment of the spin-orbit coupling (SOC) interaction. This was achieved by quasi-degenerate perturbation theory, in conjunction with state-averaged complete active space self-consistent-field calculations. The accuracy in the calculated D parameters improves upon recovering dynamic correlation with multiconfigurational ab initio methods, such as the second-order N-electron valence perturbation theory NEVPT2, the difference dedicated configuration interaction, and the spectroscopy-oriented configuration interaction. The calculations showed that the magnitude of D (∼3-7 cm(-1)) in these complexes is mainly dominated by multiple SOC contributions, the origin of which was analyzed in detail. In addition, the observed largely rhombic regime (E/D = 0.16-0.33) is attributed to the highly distorted metal coordination sphere. Of special importance is the insight by this work on the zfs effects of Se coordination to Ni(II). Overall, a combined experimental and theoretical methodology is provided, as a means to probe the electronic structure of octahedral Ni(II) complexes.

Direct Observation of Very Large Zero-Field Splitting in a Tetrahedral NiIISe4 Coordination Complex

The high-spin (S = 1) tetrahedral Ni(II) complex [Ni{(i)Pr2P(Se)NP(Se)(i)Pr2}2] was investigated by magnetometry, spectroscopic, and quantum chemical methods. Angle-resolved magnetometry studies revealed the orientation of the magnetization principal axes. The very large zero-field splitting (zfs), D = 45.40(2) cm(-1), E = 1.91(2) cm(-1), of the complex was accurately determined by far-infrared magnetic spectroscopy, directly observing transitions between the spin sublevels of the triplet ground state. These are the largest zfs values ever determined--directly--for a high-spin Ni(II) complex. Ab initio calculations further probed the electronic structure of the system, elucidating the factors controlling the sign and magnitude of D. The latter is dominated by spin-orbit coupling contributions of the Ni ions, whereas the corresponding effects of the Se atoms are remarkably smaller.

Structurally Diverse Metal Coordination Compounds, Bearing Imidodiphosphinate and Diphosphinoamine Ligands, as Potential Inhibitors of the Platelet Activating Factor

Metal complexes bearing dichalcogenated imidodiphosphinate [R2P(E)NP(E)R2′]− ligands (E = O, S, Se, Te), which act as (E,E) chelates, exhibit a remarkable variety of three-dimensional structures. A series of such complexes, namely, square-planar [Cu{(OPPh2)(OPPh2)N-O, O}2], tetrahedral [Zn{(EPPh2)(EPPh2)N-E,E}2], E = O, S, and octahedral [Ga{(OPPh2)(OPPh2)N-O,O}3], were tested as potential inhibitors of either the platelet activating factor (PAF)- or thrombin-induced aggregation in both washed rabbit platelets and rabbit platelet rich plasma. For comparison, square-planar [Ni{(Ph2P)2N-S-CHMePh-P, P}X2], X = Cl, Br, the corresponding metal salts of all complexes and the (OPPh2)(OPPh2)NH ligand were also investigated. Ga(O,O)3 showed the highest anti-PAF activity but did not inhibit the thrombin-related pathway, whereas Zn(S,S)2, with also a significant PAF inhibitory effect, exhibited the highest thrombin-related inhibition. Zn(O,O)2 and Cu(O,O)2 inhibited moderately both PAF and thrombin, being more effective towards PAF. This work shows that the PAF-inhibitory action depends on the structure of the complexes studied, with the bulkier Ga(O,O)3 being the most efficient and selective inhibitor.

The [Fe{(SePPh2)2N}2] Complex Revisited: X-ray Crystallography, Magnetometry, High-Frequency EPR, and Mössbauer Studies Reveal Its Tetrahedral FeIISe4 Coordination Sphere: The [Fe{(SePPh2)2N}2] Complex Revisited: X-ray Crystallography, Magnetometry, High-Frequency EPR, and Mössbauer Studies Reveal Its Tetrahedral FeIISe4

The synthesis and characterization of a preparation of the [Fe{(SePPh 2) 2 N} 2 ] complex (1 Td) is described. X-ray crystallography shows that this system contains a tetrahedral FeSe 4 coordination sphere. The structural features of 1 Td are compared with those of similar first row transition element complexes, including a recently reported preparation of [Fe{(SePPh 2) 2 N} 2 ] (1 SP) exhibiting a highly unusual square planar FeSe 4 coordination sphere. The electronic structure of 1 Td was elucidated by magnetometry, high-frequency EPR, and Mossbauer spectroscopic studies, which reveal zero-field splitting (ZFS) parameters typical of high spin S = 2 tetrahedral Fe II sites. Accurate ZFS parameters (D = +8.22 cm −1 , E/D = 0.104) were obtained by analysis of the EPR spectra and compared with those of the analogous [Fe{(SPPh 2) 2 N} 2 ] complex. The findings of this work call for a thorough structural and physicochemical characterization of the literature 1 SP system.

The [Fe{(SePPh : The [Fe{(SePPh2)2N}2] Complex Revisited: X-ray Crystallography, Magnetometry, High-Frequency EPR, and Mössbauer Studies Reveal Its Tetrahedral FeIISe4

The synthesis and characterization of a preparation of the [Fe{(SePPh 2) 2 N} 2 ] complex (1 Td) is described. X-ray crystallography shows that this system contains a tetrahedral FeSe 4 coordination sphere. The structural features of 1 Td are compared with those of similar first row transition element complexes, including a recently reported preparation of [Fe{(SePPh 2) 2 N} 2 ] (1 SP) exhibiting a highly unusual square planar FeSe 4 coordination sphere. The electronic structure of 1 Td was elucidated by magnetometry, high-frequency EPR, and Mossbauer spectroscopic studies, which reveal zero-field splitting (ZFS) parameters typical of high spin S = 2 tetrahedral Fe II sites. Accurate ZFS parameters (D = +8.22 cm −1 , E/D = 0.104) were obtained by analysis of the EPR spectra and compared with those of the analogous [Fe{(SPPh 2) 2 N} 2 ] complex. The findings of this work call for a thorough structural and physicochemical characterization of the literature 1 SP system.

The [Fe{(SePPh 2 ) 2 N} 2 ] Complex Revisited : X‐ray Crystallography, Magnetometry, High‐Frequency EPR, and Mössbauer Studies Reveal Its Tetrahedral Fe II Se 4 Coordination Sphere

The synthesis and characterization of a preparation of the [Fe{(SePPh 2) 2 N} 2 ] complex (1 Td) is described. X-ray crystallography shows that this system contains a tetrahedral FeSe 4 coordination sphere. The structural features of 1 Td are compared with those of similar first row transition element complexes, including a recently reported preparation of [Fe{(SePPh 2) 2 N} 2 ] (1 SP) exhibiting a highly unusual square planar FeSe 4 coordination sphere. The electronic structure of 1 Td was elucidated by magnetometry, high-frequency EPR, and Mossbauer spectroscopic studies, which reveal zero-field splitting (ZFS) parameters typical of high spin S = 2 tetrahedral Fe II sites. Accurate ZFS parameters (D = +8.22 cm −1 , E/D = 0.104) were obtained by analysis of the EPR spectra and compared with those of the analogous [Fe{(SPPh 2) 2 N} 2 ] complex. The findings of this work call for a thorough structural and physicochemical characterization of the literature 1 SP system.