Research Article

A linear cobalt(II) complex with maximal orbital angular momentum from a non-Aufbau ground state

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Science  21 Dec 2018:
Vol. 362, Issue 6421, eaat7319
DOI: 10.1126/science.aat7319

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Cobalt unfettered by its ligand field

Applied magnetic fields induce a field in any compound with unpaired electrons. However, for the induced field to persist once the applied field is gone, the electrons must be configured to manifest orbital angular momentum. Generally, the influence of ligands severely restricts that property in transition metal complexes. Bunting et al. now show that a cobalt ion is just barely affected by two linearly coordinated carbon ligands and, as such, exhibits maximal orbital angular momentum. Although its magnetic properties mainly pertain at very low temperature, its structure offers a more general design principle.

Science, this issue p. eaat7319

Structured Abstract

INTRODUCTION

The magnetic properties of a single metal center are determined by a combination of its total spin S and orbital angular momentum L. Orbital angular momentum gives rise to magnetic anisotropy, an essential property for applications such as information storage and high-coercivity magnets. Unquenched L arises from an odd number of electrons in degenerate orbitals and is typically observed only for free ions, as well as for complexes of the f elements. For the majority of transition metal ions, however, orbital angular momentum is quenched by the ligand field, which removes the requisite orbital degeneracies. Maximal L for a transition metal (L = 3) would require an odd number of electrons in two sets of degenerate orbitals. Such a species would entail a non-Aufbau configuration, wherein the electrons do not fill the d orbitals in the usual order of lowest to highest in energy, and likely exhibit a large magnetic anisotropy.

RATIONALE

Previous efforts have identified the utility of linear coordination environments for isolating iron complexes with unquenched orbital angular momentum and large magnetic anisotropies. Crucially, transition metals in this environment are unaffected by Jahn-Teller distortions that would otherwise remove orbital degeneracies in the case of partially filled d orbitals. Separately, cobalt atoms deposited on a MgO surface—for which one-coordination of the metal is achieved, provided a vacuum is maintained—were shown to have L = 3, giving rise to near-maximal magnetic anisotropy. Calculations on the hypothetical linear molecule Co(C(SiMe3)3)2 (where Me is methyl) also predicted that this system would possess a ground state with L = 3. Empirically, maximal L in a transition metal complex thus requires both a linear coordination environment and a sufficiently weak ligand field strength to allow for non-Aufbau electron filling.

RESULTS

The strongly reducing nature of the carbanion ligand hinders isolation of dialkyl cobalt(II) complexes. However, reducing the basicity of the central carbanion through the use of electron-withdrawing aryloxide groups allowed for the synthesis of the dialkyl cobalt(II) complex Co(C(SiMe2ONaph)3)2, where Naph is a naphthyl group. Ab initio calculations on this complex predict a ground state with S = 3/2, L = 3, and J = 9/2 arising from the non-Aufbau electron configuration (dx2–y2, dxy)3(dxz, dyz)3(dz2)1. Much as for lanthanide complexes, the ligand field is sufficiently weak that interelectron repulsion and spin-orbit coupling play the key roles in determining the electronic ground state. dc magnetic susceptibility measurements reveal a well-isolated MJ = ±9/2 ground state, and simulations of the magnetic data from the calculations are in good agreement with the experimental data. Variable-field far-infrared (FIR) spectroscopy shows a magnetically active excited state at 450 cm−1 that, in combination with calculations and variable-temperature ac magnetic susceptibility experiments, is assigned to the MJ = ±7/2 state. Modeling of experimental charge density maps also suggests a d-orbital filling with equally occupied (dx2–y2, dxy), and (dxz, dyz) orbital sets. As a consequence of its large orbital angular momentum, the molecule exhibits slow magnetic relaxation and, in a magnetically dilute sample, a coercive field of 600 Oe at 1.8 K.

CONCLUSION

Isolation of Co(C(SiMe2ONaph)3)2 illustrates how an extreme coordination environment can confer an f-element–like electronic structure on a transition metal complex. The non-Aufbau ground state enables realization of maximal orbital angular momentum and magnetic anisotropy near the physical limit for a 3d metal. In this respect, the linear L–Co–L motif may prove useful in the design of new materials with high magnetic coercivity.

Linear dialkyl cobalt(II).

(A) Molecular structure of Co(C(SiMe2ONaph)3)2. Purple, gray, turquoise, and red spheres represent Co, C, Si, and O, respectively. Hydrogen atoms have been omitted for clarity. (B) Energy diagram depicting the energy and electron occupations of the 3d orbitals. (C) The calculated splitting of the ground 4Φ state by spin-orbit coupling. The red line is the experimentally determined energy of the MJ = ±7/2 state. (D) Variable-field FIR spectra of Co(C(SiMe2ONaph)3)2­. The top section shows the applied-field spectra (TB) divided by the zero-field spectrum (T0). (E) Variable-field magnetization data for Co(C(SiMe2ONaph)3)2 and Co0.02Zn0.98(C(SiMe2ONaph)3)2 at 1.8 K. μB, bohr magnetons.

Abstract

Orbital angular momentum is a prerequisite for magnetic anisotropy, although in transition metal complexes it is typically quenched by the ligand field. By reducing the basicity of the carbon donor atoms in a pair of alkyl ligands, we synthesized a cobalt(II) dialkyl complex, Co(C(SiMe2ONaph)3)2 (where Me is methyl and Naph is a naphthyl group), wherein the ligand field is sufficiently weak that interelectron repulsion and spin-orbit coupling play a dominant role in determining the electronic ground state. Assignment of a non-Aufbau (dx2–y2, dxy)3(dxz, dyz)3(dz2)1 electron configuration is supported by dc magnetic susceptibility data, experimental charge density maps, and ab initio calculations. Variable-field far-infrared spectroscopy and ac magnetic susceptibility measurements further reveal slow magnetic relaxation via a 450–wave number magnetic excited state.

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