2-Pyridonate (Opy) rhodium(I) carbonyl complexes. Crystal structure of the unusual tetranuclear complex [Rh4(µ-Opy)4(µ-CO)2(CO)4]

Abstract

Reaction of [Rh₂(µ-Cl)₂(CO)₄] with 2-hydroxypyridine (HOpy) and potassium hydroxide in methanol yields the tetranuclear complex [Rh₄(µ-Opy)₄(µ-CO)₂(CO)₄](1), but the binuclear compound [Rh₂(µ-Opy)₂(CO)₄](2) is obtained when the reaction is performed under one atmosphere of carbon monoxide. Both complexes interconvert: by addition of trimethylamine oxide [(2)→(1)], or bubbling of carbon monoxide [(1)→(2)]. The related complex [Rh₂(µ-Opy)(µ-dppm)₂(CO)₂][ClO₄](3)[dppm = bis(diphenylphosphino)methane] can be prepared by reaction of (2) with dppm and perchloric acid. Crystals of (1) are monoclinic, space group P2₁/n, with a= 19.429 3(9), b= 8.782 7(2), c= 8.439 9(2)Å, β= 98.912(3)°, and Z= 2. The structure was solved by Patterson and Fourier methods and refined by least-squares calculations to R= 0.058. The structure of the tetranuclear complex consists of two binuclear units, centrosymmetrically related, and joined together through 2-pyridonate groups. In each binuclear moiety a 2-pyridonate group and a carbonyl ligand bridge the two rhodium atoms, which are directly bonded [Rh–Rh 2.619 7(8)Å].