High Magnetic Anisotropy of a Square-Planar Iron-Carbene Complex

Among Earth-abundant catalyst systems, iron-carbene intermediates that perform C-C bond forming reactions such as cyclopropanation of olefins and C-H functionalization via carbene insertion are rare. Detailed descriptions of the possible electronic structures for iron-carbene bonds are imperative to obtain better mechanistic insights and enable rational catalyst design. Here, we report the first square-planar iron-carbene complex ((PDPPh)-P-Mes)Fe(CPh2), where [(PDPPh)-P-Mes](2-) is the doubly deprotonated form of [2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine]. The compound was prepared via reaction of the disubstituted diazoalkane N2CPh2 with ((PDPPh)-P-Mes)Fe(thf) and represents a rare example of a structurally characterized, paramagnetic iron-carbene complex. Temperature-dependent magnetic susceptibility measurements and applied-field Miissbauer spectroscopic studies revealed an orbitally near-degenerate S = 1 ground state with large unquenched orbital angular momentum resulting in high magnetic anisotropy. SpinHamiltonian analysis indicated that this S = 1 spin system has uniaxial magnetic properties arising from a ground M-s = +/- 1 non-Kramers doublet that is well-separated from the M-s = 0 sublevel due to very large axial zero-field splitting (D = -195 cm(-1), E/D = 0.02 estimated from magnetic susceptibility data). This remarkable electronic structure gives rise to a very large, positive magnetic hyperfine field of more than +60 T for the 37 Fe nucleus along the easy magnetization axis observed by Mossbauer spectroscopy. Computational analysis with complete active space self-consistent field (CASSCF) calculations provides a detailed electronic structure analysis and confirms that ((PDPPh)-P-M-P-es)Fe(CPh2) exhibits a multiconfigurational ground state. The majority contribution originates from a configuration best described as a singlet carbene coordinated to an intermediate-spin Fe-II center with a (d(xy))(2){(d(xy)),(d(z)(2))}(3)(d(yz))(1)(d(x)(2)-y(2))(0) configuration featuring near-degenerate d(xy) and d(z)(2) orbitals.