EPR, ENDOR, and Electronic Structure Studies of the Jahn–Teller Distortion in an Fe^VNitride

Abstract

The recently synthesized and isolated low-coordinate FeV nitride complex has numerous implications as a model for high-oxidation states in biological and industrial systems. The trigonal [PhB(tBuIm)3FeV≡N]+ (where (PhB(tBuIm)3– = phenyltris(3-tert-butylimidazol-2-ylidene)), (1) low-spin d3 (S = 1/2) coordination compound is subject to a Jahn–Teller (JT) distortion of its doubly degenerate 2E ground state. The electronic structure of this complex is analyzed by a combination of extended versions of the formal two-orbital pseudo Jahn–Teller (PJT) treatment and of quantum chemical computations of the PJT effect. The formal treatment is extended to incorporate mixing of the two e orbital doublets (30%) that results from a lowering of the idealized molecular symmetry from D3h to C3v through strong “doming” of the Fe–C3 core. Correspondingly we introduce novel DFT/CASSCF computational methods in the computation of electronic structure, which reveal a quadratic JT distortion and significant e–e mixing, thus reaching a new level of synergism between computational and formal treatments. Hyperfine and quadrupole tensors are obtained by pulsed 35 GHz ENDOR measurements for the 14/15N-nitride and the 11B axial ligands, and spectra are obtained from the imidazole-2-ylidene 13C atoms that are not bound to Fe. Analysis of the nitride ENDOR tensors surprisingly reveals an essentially spherical nitride trianion bound to Fe, with negative spin density and minimal charge density anisotropy. The four-coordinate 11B, as expected, exhibits negligible bonding to Fe. A detailed analysis of the frontier orbitals provided by the electronic structure calculations provides insight into the reactivity of 1: JT-induced symmetry lowering provides an orbital selection mechanism for proton or H atom transfer reactivity.

EPR/ENDOR and Theoretical Study of the Jahn–Teller-Active [HIPTN3N]MoVL Complexes (L = N–, NH)

“…In part, a low value can be understood by noting that in the idealized C 3v symmetry of {[ Mo ]N} − Telser and McGarvey showed how this symmetry-lowering mixes the e b 3 E and 4 E orbitals, 48 which diminishes the covalency parameter, k , 18 for {[ Mo ]N} − . However, it would seem that this low value also reflects limitations to the two-orbital model.…”

Magnetization Slow Dynamics in Ferrocenium Complexes

“…The electronic structure of this latter class of complexes can lead to remarkable magnetic properties, such as spin‐crossover, single‐molecule magnet (SMM), or photoinduced SMM properties for example for iron(II) tris(carbene)borate complexes . More recently, we have found that the low spin d 3 ( S = ) Mn IV complex PhB(MesIm) 3 Mn≡N, which has an 2 E ground state, displays slow relaxation of the magnetization, and thus can be classified as an SMM . Theoretical investigations revealed that this behavior originates from an anisotropic ground doublet that is stabilized by spin‐orbit coupling.…”

Characterization of an Fe≡N–NH2Intermediate Relevant to Catalytic N2Reduction to NH3

“…The 1 Hα pattern resembles that of a heme hydroperoxy Fe-O-OH proton, 18 an appealing analogy to the Fe-N-NH 2 unit in 2 , and those analyses provide a good starting point for analysis of the 1 Hα hyperfine tensor; simulation of the 1 Hα pattern (Figure 4, blue) yields a slightly rhombic tensor of A = +[18.0, 10.5, 8.0] MHz (see Fig 4 caption), whose anisotropic contribution corresponds to an Fe-Hα distance of d > 3.1 Å, with the Fe-Hα vector rotated away from g 1 , which coincides approximately with the Fe-N bond and the molecular pseudo C 3 axis. 19 In short, the analysis requires a non-linear Fe-N-N geometry (Fig S8). …”