Dynamic behaviour of octahedral complexes of manganese(I). X-Ray crystal structure of fac-[Mn(η¹-1,8-naphthyridine)-η²-1,8-napht hyridine)(CO)₃]ClO₄ · CH₂Cl₂

The addition of 1,8-naphthyridine (naph) to a solution of [Mn(OClO₃)(naph)(CO)₃] at room temperature affords the octahedral complex [Mn (η¹-naph)(η²-naph)(CO)₃]ClO ₄ (1). This complex can also be obtained when naph is added to a solution of [Mn(OClO₃) (CO)₅]. When naph is reacted with [Mn(OClO₃)(CO)₅] at - 80°C and with 1 : 1 molar ratio, [Mn(η¹-naph)(CO)₅]ClO₄ (3) is obtained. This complex changes into (1) at temperatures above 0°C. The crystal structure of [Mn(η¹-naph)(η²-naph) (CO)₃]ClO₄ (monoclinic, space group P2₁/a, a = 16.006(3) Å, b = 12.024(2) Å, c = 13.004(2) Å, β = 109.85 (2)°, Z = 4) shows that the manganese atom displays a slightly distorted octahedral coordination being linked to three facial carbonyl ligands, to two nitrogen atoms of a bidentate 1,8-naphthyridine (η²-naph) and to another nitrogen atom of a monodentate 1,8-naphthyridine (η¹-naph). A detailed variable temperature ¹H NMR study shows this structure persist in solution at low temperature. At higher temperature the complex is fluxional and there is a rapid exchange of the coordination modes of the two naph ligands: the bidentate becomes monodentate and the monodentate bidentate. The mechanism is intramolecular, the rate-limiting step being the opening of the four-membered ring formed by the metal and the η2-naph.