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A series of Ni( ii ), Pd( ii ) and Pt( ii ) complexes [ML][PF 6 ] 2 [L = L 1 , M = Ni ( 1 ), Pd ( 2 ), Pt ( 3 ); L = L 2 , M = Ni ( 4 ), Pd ( 5 ), Pt ( 6 )] and [Pt(L 2 )(acac)] ( 7 ) have been prepared by the reactions of two tetradentate macrocyclic amine-NHC ligand precursors, [H 2 L 1 ][PF 6 ] 2 and [H 2 L 2 ][PF 6 ] 2 , with Ni(OAc) 2 ·4H 2 O, Pd(OAc) 2 and Pt(acac) 2 in the presence of NaOAc. Complex 7 is isolated along with 6 from the same reaction between [H 2 L 2 ][PF 6 ] 2 and Pt(acac) 2 . There are two atropisomers in 1–3 and two achiral conformers in 4–6 . The crystal structures of 1–3 and one conformer of 4–6 ( 4a–6a ) have been determined by single-crystal X-ray diffraction studies. The metal ion is found to reside in the cavity of the macrocyclic ring and adopts a square-planar configuration. Detailed NMR studies including variable-temperature NMR spectroscopy reveal a dynamic interconverting process between two atropisomers of 1–3 in the solutions via a ring twisting mechanism. Two conformers in the equilibrated solution of 4–6 , probably arising from the orientation of two amine N–H bonds with respect to the coordination plane, exchange slowly. Time-dependent 1 H NMR spectra show that one conformer ( 4a–6a ) in solution converts into the other ( 4b–6b ) via the inversion of the nitrogen atom. 

Abstract 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(PPh₃)₂X₂(Co‐X; X=Cl, Br, I) reveal rarely observed spin‐phonon coupling as avoided crossings between magnetic andu‐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 theS=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) andgvalues. 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 inCo‐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 spin‐phonon coupling may lead to magnetic relaxation and develop guidance to control such coupling.