Strong Electronic and Magnetic Coupling in M₄ (M = Ni, Cu) Clusters via Direct Orbital Interactions between Low-Coordinate Metal Centers

Abstract

We present an extensive study of the tetranuclear transition metal cluster compounds M 4 (NP t Bu 3 ) 4 and [M4(NP t Bu3)4][B(C6F5)4] (M = Ni, Cu; t Bu = tert-butyl), which feature low-coordinate metal centers and direct metal-metal orbital overlap. X-ray diffraction, electrochemical, magnetic, spectroscopic, and computational analysis elucidate the nature of the bonding interactions in these clusters and the impact of these interactions on the electronic and magnetic properties. Direct orbital overlap results in strongly-coupled, large-spin ground states in the clusters [Ni4(NP t Bu3)4] +/0 and fully delocalized, spin-correlated electrons. Correlated electronic structure calculations confirm the presence of ferromagnetic ground states that arise from direct exchange between magnetic orbitals, and, in the case of the neutral cluster, itinerant electron magnetism similar to that in metallic ferromagnets. The cationic nickel cluster also possesses large magnetic anisotropy, exemplified by a large, positive axial zero-field splitting parameter of D = +7.95 or +9.2 cm -1 , as determined by magnetometry or electron paramagnetic resonance spectroscopy, respectively. The [Ni4(NP t Bu3)4] + cluster is also the first molecule with easy-plane magnetic anisotropy to exhibit zero-field slow magnetic relaxation, and, under a small applied field, it exhibits relaxation exclusively through an Orbach mechanism with a spin relaxation barrier of 16 cm −1 . The S = 1 /2 complex [Cu4(NP t Bu3)4] + exhibits slow magnetic relaxation via a Raman process on the millisecond timescale, supporting the presence of slow relaxation via an Orbach process in the nickel analogue. Overall, this work highlights the unique electronic and magnetic properties that can be realized in metal clusters featuring direct metal-metal orbital interactions between lowcoordinate metal centers.a The standard deviations calculated from the values of interatomic parameters are given in parentheses. The estimated standard uncertainty of each parameter is given in Table S3.

Spectroscopic Investigation of a Metal–Metal-Bonded Fe6 Single-Molecule Magnet with an Isolated S = 19/2 Giant-Spin Ground State

“…Although metal−metal bonded systems have been studied for decades, 81 they have only recently been considered as potential SMMs. 38,39,82,83 The present investigation provides a clear demonstration of the potential of combined wideband EPR methodologies for studies of metal−metal bonded paramagnetic systems, particularly with a view to gaining fundamental microscopic insights that can aid design of future polynuclear SMMs with improved properties.…”

High Spin Cobalt Complexes Supported by a Trigonal Tris(Phosphinimide) Ligand

“…On one hand, negligible exchange coupling was found for a dimeric Dy III 2 complex bridged by two PN ligands . In contrast, PN-bridged, homovalent Ni I 4 and Co I 4 complexes , exhibit well-isolated spin states attributed to the presence of direct metal–metal orbital overlap inferred by short Ni–Ni and Co–Co distances of ∼2.4 Å. Such direct overlap may be operative in 1 despite substantially longer Co–Co distances (2.67 and 2.70 Å).…”

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“…Long und Mitarbeiter:innen haben in den vierkernigen Komplexen [Ni 4 (NP t Bu 3 ) 4 ] +/0 eine starke ferromagnetische Kopplung zwischen den Nickelzentren sogar bei Raumtemperatur beobachtet (Abbildung 6, links). 12) Die magnetische Kopplung erfolgt über einen direkten Austauschmechanismus. Trotz Easy-plane-Anisotropie der einzelnen Nickelzentren zeigt der Komplex bei tiefen Temperaturen eine verlangsamte magnetische Relaxation.…”