Spin-orbit coupling, an interaction with critical consequences for a variety of magnetic and photophysical properties, is generally regarded as an intrinsic property of an individual atom only related to the size of that atom’s nucleus. However, in some cases, heavy diamagnetic atoms may enhance the spin-orbit coupling derived properties of neighboring lighter, paramagnetic metal centers. Studies to this end have been largely limited to the effect of heavy halides on such centers, and, while halides provide a convenient series of isoelectronic, isostructural ligands of varying sizes, their highly ionic interactions limit the degree to which they may “donate” spin-orbit coupling character to their lighter partners. We have sought to extend this investigation to ligands containing heavy main group donor atoms, in particular those of Group 15. Of particular interest are ligands containing bismuth, the largest of the pnictogens and the heaviest stable element. Its poor donor strength and propensity for shedding or rearranging its bonds have traditionally rendered bismuth a less attractive donor atom than its lighter analogs and, to date, there are no crystallographically characterized complexes containing a formal bond between bismuth and a paramagnetic metal center (one complex containing a long-range Bi—Mn2+ interaction has been reported.) Though the synthesis of complexes featuring such an interaction presents a unique set of challenges, it also offers the opportunity to not only expand our knowledge of the coordination chemistry of bismuth but to better elucidate the role of very heavy main group donor atoms in paramagnetic systems. In pursuit of this unusual synthetic target, we have focused our efforts on enhancing the donor strength of the bismuth center as well as on suppressing ligand scrambling and other routes to decomposition or undesired reactivity. Additionally, we have sought not only the synthesis of a bismuth ligated complex, but also the synthesis and characterization of the series of analogous complexes ligated by the lighter pnictogens (P, As, Sb) so as to form a more complete picture of the interplay of donor size and donor strength down the periodic table.

Toward the synthesis of bismuth ligated paramagnetic complexes

ACS Fall 2021

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