Field-Induced Slow Magnetization Relaxation of a Tetrahedral S=2 FeS-Containing Complex.

In the work described herein, the spin relaxation properties of the mononuclear tetrahedral S=2 [Fe{(SPPr)N}] complex (1) were studied by employing static and dynamic magnetic measurements at liquid helium temperatures. In the absence of an external direct current (DC) magnetic field, 1 exhibits fast magnetization relaxation. However, in the presence of external magnetic fields of a few kOe, slow relaxation is induced as monitored by alternating current (AC) magnetic susceptibility measurements up to 10 kHz, in the temperature range 2-5 K.

Magnetic anisotropy and structural flexibility in the field-induced single ion magnets [Co{(OPPh)(EPPh)N}], E = S, Se, explored by experimental and computational methods.

During the last few years, a large number of mononuclear Co(II) complexes of various coordination geometries have been explored as potential single ion magnets (SIMs). In the work presented herein, the Co(II) S = 3/2 tetrahedral [Co{(OPPh)(EPPh)N}], E = S, Se, complexes (abbreviated as CoO2E2), bearing chalcogenated mixed donor-atom imidodiphosphinato ligands, were studied by both experimental and computational techniques. Specifically, direct current (DC) magnetometry provided estimations of their zero-field splitting (zfs) axial () and rhombic () parameter values, which were more accurately determined by a combination of far-infrared magnetic spectroscopy and high-frequency and -field EPR spectroscopy studies.

Effects of the halogenido ligands on the Kumada-coupling catalytic activity of [Ni{ BuN(PPh)-κP}X], X = Cl, Br, I, complexes.

Novel nickel(ii) complexes bearing ( butyl)bis(diphenylphosphanyl)amine and different halogenido ligands, [Ni(P,P)X] = [Ni{ BuN(PPh)-κP}X], (X = Cl, Br, I) are prepared, characterized by IR and NMR spectroscopy, mass spectrometry and X-ray crystallography, and tested as catalysts in the Kumada cross-coupling reaction of model substituted iodobenzenes and -tolylmagnesium bromide. The data obtained together with DFT calculations indicate that these new catalysts operate in the Ni(i)-Ni(iii) mode. The highest catalytic activity and selectivity are exhibited by [Ni(P,P)Cl], which is most easily reduced by the used Grignard reagent to the Ni(i) state.

Electronic Structure of Tetrahedral, = 2, [Fe{(EPPr)N}], E = S, Se, Complexes: Investigation by High-Frequency and -Field Electron Paramagnetic Resonance, Fe Mössbauer Spectroscopy, and Quantum Chemical Studies.

In this work, we assessed the electronic structures of two pseudotetrahedral complexes of Fe, [Fe{(SPPr)N}] ) and [Fe{(SePPr)N}] (), using high-frequency and -field EPR (HFEPR) and field-dependent Fe Mössbauer spectroscopies. This investigation revealed = 2 ground states characterized by moderate, negative zero-field splitting (zfs) parameters . The crystal-field (CF) theory analysis of the spin Hamiltonian (sH) and hyperfine structure parameters revealed that the orbital ground states of and have a predominant d character, which is admixed with d (∼10%).

A Molecular Tetrahedral Cobalt-Seleno-Based Complex as an Efficient Electrocatalyst for Water Splitting.

The cobalt-seleno-based coordination complex, [Co{(SePPr)N}], is reported with respect to its catalytic activity in oxygen evolution and hydrogen evolution reactions (OER and HER, respectively) in alkaline solutions. An overpotential of 320 and 630 mV was required to achieve 10 mA cm for OER and HER, respectively. The overpotential for OER of this -containing complex is one of the lowest that has been observed until now for molecular cobalt(II) systems, under the reported conditions.

Magnetic Properties and Electronic Structure of the = 2 Complex [Mn{(OPPh)N}] Showing Field-Induced Slow Magnetization Relaxation.

The high-spin = 2 Mn(III) complex [Mn{(OPPh)N}] () exhibits field-induced slow relaxation of magnetization ( , , 12869). Magnetic susceptibility and dual-mode X-band electron paramagnetic resonance (EPR) studies revealed a negative value of the zero-field-splitting (zfs) parameter . In order to explore the magnetic and electronic properties of in detail, a combination of experimental and computational studies is presented herein.

Unusual P Hyperfine Strain Effects in a Conformationally Flexible Cu(II) Complex Revealed by Two-Dimensional Pulse EPR Spectroscopy.

Strain effects on and metal hyperfine coupling tensors, , are often manifested in Electron Paramagnetic Resonance (EPR) spectra of transition metal complexes, as a result of their intrinsic and/or solvent-mediated structural variations. Although distributions of these tensors are quite common and well understood in continuous-wave (cw) EPR spectroscopy, reported strain effects on ligand hyperfine coupling constants are rather scarce. Here we explore the case of a conformationally flexible Cu(II) complex, [Cu{PhP(O)NP(O)Ph-κ'}], bearing P atoms in its second coordination sphere and exhibiting two structurally distinct CuO coordination spheres, namely a square planar and a tetrahedrally distorted one, as revealed by X-ray crystallography.

Magnetic Anisotropy of Tetrahedral Co Single-Ion Magnets: Solid-State Effects.

This study reports the static and dynamic magnetic characterization of two mononuclear tetrahedral Co complexes, [Co{PrP(E)NP(E)Pr}], where E = S (CoS) and Se (CoSe), which behave as single-ion magnets (SIMs). Low-temperature (15 K) single-crystal X-ray diffraction studies point out that the two complexes exhibit similar structural features in their first coordination sphere, but a disordered peripheral Pr group is observed only in CoS. Although the latter complex crystallizes in an axial space group, the observed structural disorder leads to larger transverse magnetic anisotropy for the majority of the molecules compared to CoSe, as confirmed by electron paramagnetic resonance spectroscopy.

A Molecular Ni-complex Containing Tetrahedral Nickel Selenide Core as Highly Efficient Electrocatalyst for Water Oxidation.

We report the highly efficient catalytic activity of a transition metal selenide-based coordination complex, [Ni{(SeP Pr ) N} ], (1) for oxygen evolution and hydrogen evolution reactions (OER and HER, respectively) in alkaline solution. Very low overpotentials of 200 mV and 310 mV were required to achieve 10 mA cm for OER and HER, respectively. The overpotential for OER is one of the lowest that has been reported up to now, making this one of the best OER electrocatalysts.

Direct Observation of Very Large Zero-Field Splitting in a Tetrahedral Ni(II)Se4 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.

Spin-relaxation properties of a high-spin mononuclear Mn(III)O6-containing complex.

The magnetic properties of the mononuclear manganese(III) complex [Mn{(OPPh2)2N}3] are investigated by means of magnetometry and dual-mode X-band electron paramagnetic resonance spectroscopy. Slow relaxation of magnetization is induced in the presence of external magnetic fields.

High-frequency EPR study of the high-spin FeII complex Fe[(SPPh2)2N]2.

We report continuous-wave electron-paramagnetic-resonance (EPR) spectra of the high-spin Fe(II) complex Fe[(SPPh(2))(2)N](2) at 275.7 GHz, 94.1 GHz and 9.

Investigating magnetostructural correlations in the pseudooctahedral trans-[Ni(II){(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.

Theoretical analysis of the spin Hamiltonian parameters in Co(II)S4 complexes, using density functional theory and correlated ab initio methods.

A systematic Density Functional Theory (DFT) and multiconfigurational ab initio computational analysis of the Spin Hamiltonian (SH) parameters of tetracoordinate S = 3/2 Co((II))S(4)-containing complexes has been performed. The complexes under study bear either arylthiolato, ArS(-), or dithioimidodiphosphinato, [R(2)P(S)NP(S)R'(2)](-) ligands. These complexes were chosen because accurate structural and spectroscopic data are available, including extensive Electron Paramagnetic Resonance (EPR)/Electron Nuclear Double Resonance (ENDOR) studies.

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.

A bacteria-specific 2[4Fe-4S] ferredoxin is essential in Pseudomonas aeruginosa.

Background: Ferredoxins are small iron-sulfur proteins belonging to all domains of life. A sub-group binds two [4Fe-4S] clusters with unequal and extremely low values of the reduction potentials. These unusual properties are associated with two specific fragments of sequence.

Structurally diverse metal coordination compounds, bearing imidodiphosphinate and diphosphinoamine ligands, as potential inhibitors of the platelet activating factor.

Metal complexes bearing dichalcogenated imidodiphosphinate [R(2)P(E)NP(E)R(2)'](-) 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{(OPPh(2))(OPPh(2))N-O, O}(2)], tetrahedral [Zn{(EPPh(2))(EPPh(2))N-E,E}(2)], E = O, S, and octahedral [Ga{(OPPh(2))(OPPh(2))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{(Ph(2)P)(2)N-S-CHMePh-P, P}X(2)], X = Cl, Br, the corresponding metal salts of all complexes and the (OPPh(2))(OPPh(2))NH ligand were also investigated.

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.

A multifrequency high-field electron paramagnetic resonance study of Co(II)S(4) coordination.

Advanced electron paramagnetic resonance (EPR) methods have been employed in the study of two high-spin cobalt(II) complexes, Co[(SPPh(2))(2)N](2) (Co(Ph,Ph)L(2)) and Co[(SPPh(2))(SP(i)Pr(2))N](2) (Co(iPr,Ph)L(2)), in which the bidentate disulfidoimidodiphosphinato ligands make up for a pseudotetrahedral sulfur coordination of the transition metal. The CoS(4) core in the two complexes has slightly different structure, owing to the different peripheral groups (phenyl or isopropyl) bound to the phosphorus atoms. To determine the zero-field splitting, notoriously difficult for high-spin cobalt(II), the two complexes required different approaches.

A W-band pulsed EPR/ENDOR study of Co(II)S(4) coordination in the Co[(SPPh(2))(SP(i)Pr(2))N](2) complex.

Spin-echo detection at 95 GHz enables an electron-paramagnetic-resonance study of a cobalt complex with a bio-mimetic coordination of the transition metal by four sulfur atoms. A magnetically diluted single crystal of the complex has been investigated in great detail. Electron-nuclear double-resonance signals were observed of ligand nuclei and complete hyperfine tensors of the distinct phosphorus nuclei were derived, assigned and discussed.

Insight into the protein and solvent contributions to the reduction potentials of [4Fe-4S]2+/+ clusters: crystal structures of the Allochromatium vinosum ferredoxin variants C57A and V13G and the homologous Escherichia coli ferredoxin.

The crystal structures of the C57A and V13G molecular variants of Allochromatium vinosum 2[4Fe-4S] ferredoxin (AlvinFd) and that of the homologous ferredoxin from Escherichia coli (EcFd) have been determined at 1.05-, 1.48-, and 1.

Ligands that enforce unnatural stereospinomers.

Computational evidence is provided for the existence of combinations of a stereoisomer and a spin state not previously observed in d(4) and d(8) complexes, achieved by means of encapsulating ligands that enforce a tetrahedral coordination.

Ni[(EPiPr2)2N]2 complexes: stereoisomers (E = Se) and square-planar coordination (E = Te).

The reaction of ((i)Pr 2PE) 2NM.TMEDA (M = Li, E = Se; M = Na, E = Te) with NiBr 2.DME in THF affords Ni[(SeP (i)Pr 2) 2N] 2 as either square-planar (green) or tetrahedral (red) stereoisomers, depending on the recrystallization solvent; the Te analogue is obtained as the square-planar complex Ni[(TeP (i)Pr 2) 2N] 2.

Structural, spectroscopic and magnetic properties of M[R2P(E)NP(E)R'2]2 complexes, M = Co, Mn, E = S, Se and R, R' = Ph or iPr. Covalency of M-S bonds from experimental data and theoretical calculations.

The S/Se-containing bidentate ligands LH of the type R2P(E)NHP(E)R'2, E = S, Se and R, R' = Ph or iPr have been employed to synthesize ML2 (M = Mn, Co) complexes which contain the biologically important MS4 core. Theoretical calculations on the LH and L- forms of the ligands probe the geometric and electronic changes induced by the deprotonation of the LH form, which are correlated with structural data from X-ray crystallography. These results reflect the flexibility of the ligands, which enables them to be rather versatile with respect to the formation of ML2 complexes with varied geometries and MEPNPE metallacycle conformations.

The structure of the 2[4Fe-4S] ferredoxin from Pseudomonas aeruginosa at 1.32-A resolution: comparison with other high-resolution structures of ferredoxins and contributing structural features to reduction potential values.

The structure of the 2[4Fe-4S] ferredoxin (PaFd) from Pseudomonas aeruginosa, which belongs to the Allochromatium vinosum (Alvin) subfamily, has been determined by X-ray crystallography at 1.32-A resolution, which is the highest up to now for a member of this subfamily of Fds. The main structural features of PaFd are similar to those of AlvinFd.