Tetranuclear complexes containing bimetallic tetracarboxylates and ferrocene - Models for subunits of one-dimensional organometallic polymers

The tetranuclear compounds M₂(O₂C-R)₄L₂ [M = Rh, R = nC₇H₁₅, L = ferrocenyl-4-pyridylacetylene (FPA) (1a); M = Rh, R = CH₃, L = FPA (1b); M = Mo, R = CF₃, L = FPA (2b); M = Rh, R = nC₇H₁₅, L = 4-ferrocenylpyridine (FP) (3a); M = Rh, R = CH₃, L = FP (3b); M = Mo, R = nC₇H₁₅, L = FP (4a); M = Mo, R = CF₃, L = FP (4b)] have been synthesized by the reaction of bimetallic tetracarboxylates with L in a 1:2 ratio, in a noncoordinating solvent. The reaction of Mo₂(O₂C-nC₇H₁₅)₄ with FPA in a 1:2 molar ratio leads to a trinuclear compound Mo₂(O₂C-nC₇H₁₅)₄(FPA) (2a). A soluble oligomer [Rh₂ (O₂C-nC₇H₁₅)₄(BPEF)]_n [5, BPEF = 1,1′-bis(4-pyridylethynyl)ferrocene] is obtained by the reaction of Rh₂(O₂C-nC₇H₁₅)₄ with BPEF in a 1:1 ratio in CH₂Cl₂ or CHCl₃, whereas the reaction of Mo₂(O₂C-nC₇H₁₅)₄ with BPEF leads to a pentanuclear compound [Mo₂ (O₂C-nC₇H₁₅)₄]₂(BPEF) (6), even when BPEF is in excess. ¹H NMR, Raman, UV/Vis absorption spectroscopy and cyclic voltammetry indicate that the metal-metal bonds are weakened by the metal-axial ligand interaction and the donor-acceptor interactions occurring between the building blocks of the multinuclear compounds and oligomers. The interaction between the bimetallic centers and the ferrocene moiety is enhanced by shortening the distances between them. This shortening is achieved by modification of the axial ligand from FPA to FP. A significant electronic communication between the ferrocene units is not observed.