High-spin enforcement in first-row metal complexes of a constrained polyaromatic ligand: synthesis, structure, and properties

The coordination chemistry of a rigid tetradentate polypyridyl ligand has been developed with first-row transition metals Mn(ii), Fe(ii), Co(ii), Ni(ii), and Zn(ii). The polyaromatic ligand, 2,2'-di([2,2'-bipyridin]-6-yl)-1,1'-biphenyl (5, bpbb), is comprised of 2,2'-bipyridine donors positioned at the 2 and 2' carbons of a biphenyl backbone. Notably, coordination of the typical strong field bipyridine fragments is constrained, weakening the octahedral ligand fields around manganese, iron, and cobalt to give high-spin electronic states. Solution magnetic susceptibility measurements were conducted across the series using the Evans method and variable-temperature solid-state SQUID magnetometry was performed on two Fe(ii) compounds, including a bis(thiocyanato) species. Spin crossover behavior was not observed as the compounds remained high-spin over the entire temperature range. The impact of the biphenyl bridge on M-N bond distances and redox potentials has also been assessed by comparison to relevant first-row metal bis- and tris-bipyridine compounds from the literature.