Tetrahedral and Square Planar Ni[(SPR₂)₂N]₂ complexes, R = Ph & ⁱPr Revisited: Experimental and Theoretical Analysis of Interconversion Pathways, Structural Preferences, and Spin Delocalization

Abstract

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.

Cu2ZnSnS4 Nanorods Doped with Tetrahedral, High Spin Transition Metal Ions: Mn2+, Co2+, and Ni2+

“…104 An example of such EPR silent system is Ni 2+ , a 3d 8 non-Kramer's system with an integer-spin ground state S = 1. 105 For this reason, EPR measurements were not performed on the Ni doped nanocrystals.…”

Zero-field splitting in nickel(II) complexes: A comparison of DFT and multi-configurational wavefunction calculations

“…First, we consider recent work on nickel(II) complexes. Maganas et al 27 investigated tetrahedral and square planar complexes with bidentate dithioimidophosphinato ligands. For one of the complexes, the ZFS was calculated using the DFT (B3LYP/TZVP, CP method for the SOC, UNO for the SSC), yielding D = +58 cm −1 and E = 0.…”

“…13,14 We also reported a combination of molecular dynamics (MD) simulations and ZFS calculations yielding a time correlation function for the fluctuations of the ZFS in aqueous nickel(II) solution. 15,16 The development of the theoretical tools to calculate the ZFS parameters by quantum chemistry technique over the last decade has been quite impressive, with Frank Neese and his group as the leading team, 10,11,[17][18][19][20][21][22][23][24][25][26][27][28][29][30] but also with important contributions from other authors. [31][32][33][34][35][36][37] The development has followed two routes, making use of either efficient density functional theory (DFT) techniques or of high-end wavefunction methods based on multi-configurational SCF techniques (such as complete active space self-consistent field (CASSCF) and related extensions).…”

“…38,39 The application of these recent computational tools to mononuclear nickel(II) complexes have been described in some papers. Maganas et al 27 reported a theoretical study of ZFS in the tetrahedral and square planar nickel(II) complexes, using both the DFT and CASSCF-type methods. Desrochers et al 40 used similar methods and studied nickel(II) borohydrides.…”

First principles approach to the electronic structure, magnetic anisotropy and spin relaxation in mononuclear 3d-transition metal single molecule magnets

“…In fact, DFT approaches have been quite controversial in their ability to predict the magnetic properties of transition metal compounds. Although they have been reasonably successful for Mn II [151][152][153] and Mn III [12,154] compounds, they have failed remarkably for T d Co II [155] and O h V III [156] and Ni II [157] complexes. There are, also, cases, such as the [Cr(H 2 O) 6 ] 2+ complex [158], in which DFT approaches led to correct results, but for incorrect reasons.…”