Intermetallic Communication in Titanium(IV) Ferrocenyldiketonates

A tetradentate bis(ferrocenyldiketonate) ligand, Fc2BobH2, is prepared via Claisen condensation of acetylferrocene and 2,2′-biphenyldiacetyl chloride, and is metalated with titanium(IV) isopropoxide to give (Fc2Bob)Ti(OiPr)2 in good yield. The isopropoxide groups are replaced with di(4-nitrophenyl)phosphate groups on treatment with the corresponding acid, and with chlorides on treatment with trimethylsilyl chloride. Metathesis with catechol leads to the bis(o-hydroxyphenoxide) complex rather than the chelating catecholate complex. Hydrolysis selectively gives the μ-oxo trimer (Δ,Δ,Δ)/(Λ,Λ,Λ)-{(Fc2Bob)Ti(μ-O)}3. The solid-state structures of the μ-oxo trimer and the bis(o-hydroxyphenoxide) complex show that the ferrocene substituents are oriented proximal to the biphenyl backbone rather than pointed out toward the exogenous groups. The complexes show dramatic changes in color depending on the bound anions, ranging from the red isopropoxide (λmax = 489 nm) to the green bis(di(4-nitrophenyl)phosphate) (λmax = 653 nm). The oxidation potentials of the ferrocenes show modest shifts based on the titanium environment, but the redox potentials of the two ferrocenes are never separated by more than 60 mV. These results and those of density-functional theory (DFT) calculations indicate that the titanium interacts principally with the lowest unoccupied molecular orbital (LUMO) of the ferrocenyldiketonate and very little with its highest occupied molecular orbital (HOMO).