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

This study reports the static and dynamic magnetic characterization of two mononuclear tetrahedral CoII complexes, [Co{iPr2P(E)NP(E)iPr2}2], where E = S (CoS4) and Se (CoSe4), 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 iPr group is observed only in CoS4. 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 CoSe4, as confirmed by electron paramagnetic resonance spectroscopy. Static magnetic characterization indicates that both CoS4 and CoSe4 show easy-axis anisotropy, with comparable D values (∼-30 cm-1). Moreover, alternating-current susceptibility measurements on these CoII complexes, magnetically diluted in their isostructural ZnII analogues, highlight the role of dipolar magnetic coupling in the mechanism of magnetization reversal. In addition, our findings suggest that, despite their similar anisotropic features, CoS4 and CoSe4 relax magnetically via different processes. This work provides experimental evidence that solid-state effects may affect the magnetic behavior of SIMs.

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{(SePi Pr2 )2 N}2 ], (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-2 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. In addition, this molecular complex exhibits an exceptionally high mass activity (111.02 A g-1 ) and a much higher TOF value (0.26 s-1 ) at a overpotential of 300 mV. This bifunctional electrocatalyst enables water electrolysis in alkaline solutions at a cell voltage of 1.54 V.

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 = 45.40(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.

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. For Co(Ph,Ph)L(2), the study of the X-band EPR spectrum of a single crystal as a function of temperature revealed a nearly axial character of the zero-field splitting (ZFS; E/D approximately -0.05). For Co(iPr,Ph)L(2), the combination of the EPR spectra at 9, 95, and 275 GHz revealed a rhombic character of the ZFS (E/D approximately -0.33). The energy difference between the Kramers doublets in Co(Ph,Ph)L(2) and Co(iPr,Ph)L(2) amounts to 24 cm(-1) and 30 cm(-1), respectively. From the X-band EPR spectra of diamagnetically diluted single crystals at fields up to 2.5 T for Co(Ph,Ph)L(2) and 0.5 T for Co(iPr,Ph)L(2), the effective g tensors and cobalt hyperfine tensors have been determined, including the direction of the principal axes in the cobalt sites. The values of the EPR observables are discussed in relation to the structural characteristics of the first (CoS(4)) and second coordination sphere in the complexes.

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.