Reversible fixation of carbon dioxide at nickel(0) centers: A route for large organometallic rings, dimers, and tetramers

The reaction between bis(cycloocta-1,5-diene)nickel(0), carbon dioxide and benzaldehyde-N-furfurylideneimine (A) in 1,4-dioxane or THF results in the formation of the 24-membered organometallic macrocycles of the type [(A)Ni( -CH(R-t)-N(R-2)-COO-)](6)(solv)(n) (R-1: phenyl, R-2: furfurylidene, solv: 1,4-dioxane in 1 a, THF in 1 b). According to the X-ray analyses, six monom eric nickelacyclic units are connected through six Ni-mu (2)-OCO-Ni bridges in these macrocycles. The cavities of the metallomacrocycles (diameter: 9. 410(1) Angstrom in 1 a, 9.250(1) Angstrom in 1 b) each contain one solvent molecule. Reaction of 1 b with Me3P results in the displacement of the peri pheral ligands A by the phosphine to form the 24-membered organometallic ma crocycle 1 c. Both 1 a and 1 b isomerize in benzene to form the di-meric co mplex [(A)Ni(-CH(R-1)-N(R-2)-COO-)](2)(solv)(n) (2). The X-ray crystal stru cture reveals that 2 consists of the same monomeric units as found in I a a nd I b. However two Ni-mu (2)-O-Ni bonds link the carboxylato groups. The s olvent-dependent isomerization of lb yielding 2 is a reversible reaction. F urthermore, the macrocycle lb partially eliminates carbon dioxide above 20 degreesC, followed by elimination of half of the monodentately coordinated Schiff base ligands to form the planar tetrameric complex 6. This is also a reversible process.