Linearly coordinated transition metals have garnered interest in the field of single-molecule magnetism due to their ability to show unquenched orbital angular momentum and, consequently, very large magnetic anisotropy. Prior to this work, maximal orbital angular momentum for transition metals had only been observed for cobalt adatoms on an MgO surface. Calculations on a hypothetical complex Co(C(SiMe3)3)2 suggested that a dialkyl cobalt(II) complex could have a similar electronic structure to the cobalt adatom, however initial synthetic attempts indicated that the [C(SiMe3)3]− anion was too reducing for cobalt(II). Using electron withdrawing aryl oxide groups we reduced the reducing strength of the central carbanion and isolated the first dialkyl cobalt(II) complex, Co(C(SiMe2OPh)3)2. However, due to a combination of weak Co–C bonds, long-range Co…O interactions, and non-covalent interligand interactions the C–Co–C was bent and Co(C(SiMe2OPh)3)2 displayed unremarkable magnetic properties. Using a bulkier aryl group that allowed for stronger interligand interactions we isolated a complex with a linear C–Co–C axis, Co(C(SiMe2ONaph)3)2, where Naph is naphthyl. The molecule exhibits a non-Aufbau ground state, (dx2-y2,dxy)3(dxz,dyz)3(dz2)1, which arises from an extremely weak and high symmetry ligand field. The electronic structure is confirmed by dc magnetic susceptibility, ab initio calculations, and experimental charge density maps. The electronic structure has the maximal orbital angular momentum for a transition metal complex, L = 3, and magnetic anisotropy that is near a physical limit for transition metals. We directly measure the energy of the first magnetic excited state using variable-field far-IR spectroscopy and, in combination with ac susceptometry, find that the complex has the largest spin-reversal barrier (450 cm−1) for any transition metal-containing molecule.

Linear cobalt(II) dialkyl complex with a non-Aufbau ground state and very large magnetic anisotropy

2019 Fall National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.