Experimental and Theoretical Investigation of a Series of Novel Dimanganese(III) μ‑Hydroxo Bisporphyrins: Magneto–Structural Correlation and Effect of Metal Spin on Porphyrin Core Deformation

The synthesis, structure, and properties of a new family of five ethane-bridged dimanganese­(III) μ-hydroxo bisporphyrins with the same core structure but different counteranions are reported here. Additions of 10% Brønsted acids such as HI, HBF4, HSbF6, HPF6, and HClO4 to a dichloromethane solution of the dichloro dimanganese­(III) bisporphyrin produces complexes having a remarkably bent μ-hydroxo group with I3–, BF4–, SbF6–, PF6–, and ClO4– as counteranions, respectively. The X-ray structures of all complexes have been determined, which have revealed the presence of two equivalent high-spin manganese­(III) centers with equally distorted porphyrin rings in the complexes, in sharp contrast with the case for the diiron­(III) μ-hydroxo bisporphyrin analogues. 1H NMR spectra have shown highly deshielded meso resonances, unlike the case for the diiron­(III) analogues, where the meso resonances are highly shielded. The variable-temperature magnetic data have been subjected to a least-squares fit which provides a moderate antiferromagnetic coupling through the hydroxo bridge between two zero-field split Mn­(III) centers with coupling constant (J) values ranging from −29.5 to −38.6 cm–1. Fairly good correlations are observed for J with Mn–O­(H) distances and Mn–O­(H)–Mn angles for all the complexes except for that having an I3– counteranion. DFT calculations support the stabilization of two equivalent high-spin Mn­(III) porphyrin cores in the complexes and have also explored the role of metal spin in controlling porphyrin ring deformation. Unlike diiron­(III) μ-hydroxo bisporphyrin complexes, the dimanganese­(III) analogues do not have easily accessible spin states of the metal attainable by subtle environmental perturbations and, therefore, can only stabilize the high-spin state with a variety of counteranions.