Spin-State Tuning at Pseudo-tetrahedral d⁶ Ions: Spin Crossover in [BP₃]Fe^II–X Complexes

Abstract

Low-coordinate transition-metal complexes that undergo spin crossover remain rare. We report here a series of four-coordinate, pseudo-tetrahedral P 3 Fe II −X complexes supported by tris(phosphine)borate P 3 ([PhBP 3 R ] − ) and phosphiniminato X-type ligands (−NPR 3 ′) that, in combination, tune the spin-crossover behavior of the system. Most of the reported iron complexes undergo spin crossover at temperatures near or above room temperature in solution and in the solid state. The change in spin state coincides with a significant change in the degree of π-bonding between Fe and the bound N atom of the phosphiniminato ligand. Spin crossover is accompanied by striking changes in the ultraviolet−visible (UV-vis) absorption spectra, which allows for quantitative modeling of the thermodynamic parameters of the spin equilibria. These spin equilibria have also been studied by numerous techniques including paramagnetic nuclear magnetic resonance (NMR), infrared, and Mossbauer spectroscopies; X-ray crystallography; and solid-state superconducting quantum interference device (SQUID) magnetometry. These studies allow qualitative correlations to be made between the steric and electronic properties of the ligand substituents and the enthalpy and entropy changes associated with the spin equilibria.

Light Enhanced Fe-Mediated Nitrogen Fixation: Mechanistic Insights Regarding H2 Elimination, HER, and NH3 Generation

“…The theoretical framework for the use of this equation is described by Pfirrman et al 9 Similar estimations of singlet-triplet gaps have been estimated for various systems. [9][10][11][12][13] One should note that such a large singlet triplet splitting (corresponding to a thermal energy of E/k = 2717 K) can be hardly detected by usual static magnetic susceptibility measurements as the thermal population of the triplet at 298 K is only about 0.034 % and the quintet is 6.7•10 -10 %. 9 Overlap of three aromatic signals with toluene-d8 hampered their analysis and therefore only ten out of the thirteen signals, expected in Cs symmetry, could be tracked reliably over the measured temperature range.…”

The Effect of Ligand Design on Metal Ion Spin State—Lessons from Spin Crossover Complexes

“…That is, the low-spin state of these complexes is stabilized by electron-donating, not electron-withdrawing, substituents. Another recent study of different tetrahedral iron(II) complexes [(MeC{CH2PR 1 2}3)FeN=PR 2 3] (Scheme 8) showed a dependence of T½ on both R 1 and R 2 , but deconvolution of steric and electronic substituent effects in that series is not so straightforward [94]. However, data from the solid dinuclear complexes [Fe2(tpa)2(μ-dabq R )][BAr F ]2 showed T½ decreasing as the electronegativity of 'R' increases, in the order X = H > Br > Cl > F [95].…”

“…Scheme 8. Other families of substituted complexes with available, comparative SCO data [94][95][96].…”

“…In contrast, complexes substituted at the pyrazole donors, [Fe(1-bpp H,R2 )2] 2+ , exhibited a weaker negative linear free energy relationship between T½ and Scheme 8. Other families of substituted complexes with available, comparative SCO data [94][95][96].…”

Cobalt and Vanadium Trimetaphosphate Polyanions: Synthesis, Characterization, and Electrochemical Evaluation for Non-aqueous Redox-Flow Battery Applications

“…The spin equilibrium was modeled by using the absorbance intensities at various temperatures and fitting the data to the Boltzmann distribution. 27,40,42,43 Values of ∆H, and ∆S were extracted from the UV-vis data and determined to be, 56.02 ± 4.77 JK −1 mol −1 , and 18.49 ± 2.09 kJmol −1 , respectively. These data allowed for calculation of the high-spin percentage at any given temperature, where the high-spin S = 2 percentage at 300 K was determined to be 34.0 ± 7.2%.…”