Vibronic coupling, the interaction between molecular vibrations and electronic states, is a fundamental effect that profoundly affects chemical processes. In the case of molecular magnetic materials, vibronic, or spin-phonon, coupling leads to magnetic relaxation, which equates to loss of magnetic memory and loss of phase coherence in molecular magnets and qubits, respectively. The study of vibronic coupling is challenging, and most experimental evidence is indirect. Here we employ far-infrared magnetospectroscopy to directly probe vibronic transitions in [Yb(trensal)] (where H3trensal = 2,2,2-tris(salicylideneimino)trimethylamine). We find intense signals near electronic states, which we show arise due to an “envelope effect” in the vibronic coupling Hamiltonian, which we calculate fully ab initio to simulate the spectra. We subsequently show that vibronic coupling is strongest for vibrational modes that simultaneously distort the first coordination sphere and break the C3 symmetry of the molecule. With this knowledge, vibrational modes could be identified and engineered to shift their energy towards or away from particular electronic states to alter their impact. Hence, these findings provide new insights towards developing general guidelines for the control of vibronic coupling in molecules.

High-field electron paramagnetic resonance spectroscopy shows that the structurally distorted Mn(III) ion in Na5[Mn(L-tart)2]·12H2O (1; L-tart = L-tartrate) has a significant negative axial zero-field splitting and a small rhombic anisotropy (∼1% of D). Alternating-current magnetic susceptibility measurements demonstrate that 1, which contains isolated Mn(III) centers, displays slow relaxation of its magnetization under an applied direct-current magnetic field.

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.