Large separation of magnetic levels and slow relaxation in metal complexes are desirable properties of single-molecule magnets (SMMs). Spin-phonon coupling (interactions of magnetic levels with phonons) is ubiquitous, leading to magnetic relaxation and loss of memory in SMMs and quantum coherence in qubits. Direct observation of magnetic transitions and spin-phonon coupling in molecules is challenging. We have found that far-IR magnetic spectra (FIRMS) of Co(PPh 3 ) 2 X 2 (Co-X; X = Cl, Br, I) reveal rarely observed spin-phonon coupling as avoided crossings between magnetic and u-symmetry phonon transitions. Inelastic neutron scattering (INS) gives phonon spectra. Calculations using VASP and phonopy programs gave phonon symmetries and movies. Magnetic transitions among zero-field split (ZFS) levels of the S = 3/2 electronic ground state were probed by INS, high-frequency and -field EPR (HFEPR), FIRMS, and frequency-domain FT terahertz EPR (FD-FT THz-EPR), giving magnetic excitation spectra and determining ZFS parameters (D, E) and g values. Ligand-field theory (LFT) was used to analyze earlier electronic absorption spectra and give calculated ZFS parameters matching those from the experiments. DFT calculations also gave spin densities in Co-X, showing that the larger Co(II) spin density in a molecule, the larger its ZFS magnitude. The current work reveals dynamics of magnetic and phonon excitations in SMMs. Studies of such couplings in the future would help to understand how spinphonon coupling may lead to magnetic relaxation and develop guidance to control such coupling.

A combination of inelastic neutron scattering (INS), far-IR magnetospectroscopy (FIRMS), and Raman magneto-spectroscopy (RaMS) has been used to comprehensively probe magnetic excitations in Co(AsPh 3 ) 2 I 2 (1), a reported singlemolecule magnet (SMM). With applied field, the magnetic zero-field splitting (ZFS) peak (2D′) shifts to higher energies in each spectroscopy. INS placed the ZFS peak at 54 cm −1 , as revealed by both variable-temperature (VT) and variable-magnetic-field data, giving results that agree well with those from both far-IR and Raman studies. Both FIRMS and RaMS also reveal the presence of multiple spin−phonon couplings as avoided crossings with neighboring phonons. Here, phonons refer to both intramolecular and lattice vibrations. The results constitute a rare case in which the spin− phonon couplings are observed with both Raman-active (g modes) and far-IR-active phonons (u modes; space group P2 1 /c, no. 14, Z = 4 for 1). These couplings are fit using a simple avoided crossing model with coupling constants of ca. 1−2 cm −1 . The combined spectroscopies accurately determine the magnetic excited level and the interaction of the magnetic excitation with phonon modes. Density functional theory (DFT) phonon calculations compare well with INS, allowing for the assignment of the modes and their symmetries. Electronic calculations elucidate the nature of ZFS in the complex. Features of different techniques to determine ZFS and other spin-Hamiltonian parameters in transition-metal complexes are summarized.

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Two-dimensional (2D) coordination polymer (CP) [Co III (CN) 6 ] 2 [Co II (TODA)] 3 •7H 2 O (TODA = 1,4,10-trioxa-7,13diazacyclopentadecane, Co-TODA) was reported earlier to show field-induced slow magnetic relaxation, displaying single-ion magnet (SIM) behaviors. Most SIMs are molecular compounds with fewer adopting coordination polymer (CP) or metal−organic framework (MOF) structures. In the current work, magnetic and phonon properties of Co-TODA have been studied by advanced spectroscopies and computations. The combined use of far-IR magneto-spectroscopy (FIRMS) and variable-temperature (VT) high-frequency and -field electron paramagnetic resonance (HFEPR) gives spin Hamiltonian (SH) parameters: Axial zerofield splitting (ZFS) parameter D as +38.0(1.0) ≤ D ≪ +40.2(1.0) cm −1 and rhombic ZFS parameter E as 0 ≪ |E| ≤ 7.3(1.0) cm −1 , showing that Co-TODA has the easy-plane magnetic anisotropy. Two Co II centers in the CP, as determined by synchrotron single-crystal X-ray diffraction at 15(2) K, show similar magnetic properties indistinguishable in FIRMS at 5.3(3) K or in HFEPR at 5−150 K. Ab initio calculations explore the origin of the magnetic anisotropy and magnetostructural correlations. VT inelastic neutron scattering (INS) spectra of Co-TODA have been obtained to show the phonon properties of the CP. Density functional theory (DFT) calculations, giving both a calculated INS spectrum and spin distributions in Co-TODA, demonstrate that, compared with other high-spin Co II complexes, the larger the spin density on a metal ion, the larger the ZFS in the complex. Pulsed X-band EPR studies probe relaxations of the Co II ions from the M S = +1/2 to −1/2 state in the ground Kramers doublet (KD), yielding spin−lattice (T 1 ) and spin−spin relaxation (T 2 ) times. The work reported here highlights the versatility and power of the spectroscopic techniques and computations in the characterization of magnetic and phonon properties of a CP and the understanding of its magnetic anisotropy.

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