Synthesis and Characterisation of Paddlewheel-Porphyrin Conjugates

Dimolybdenum paddlewheel complexes with a metal-metal quadruple bond are known for their redox activity and photophysical properties, and have been explored in relation to catalysis, macromolecular chemistry, and photovoltaics. Likewise, porphyrins, owing to their high stability, and redox/photophysical properties, have been studied in similar contexts, notably photovoltaics and molecular electronics. Despite the potential for interaction between the paddlewheel δ orbitals and the porphyrin π system, which could give rise to long-range charge-transfer properties, there have been relatively few examples in the literature of paddlewheel complexes bound to porphyrins in single molecule structures. Herein, synthetic procedures for paddlewheel-porphyrin conjugates with carboxylate and formamidinate ancillary ligands have been developed. The natural lability of the carboxylate ancillary ligands gave rise to mixtures of paddlewheel-porphyrin complexes which could not be separated, but the use of formamidinate ligands led to well-defined conjugates which could be isolated cleanly. Two such examples, Mo2(N,N’-di(4-anisyl)formamidinate)3(triphenylcarboxyphenylporphyrin) (Mo2(DAniF)3(A3B)Porphyrin) and [Mo2(N,N’-di(4-anisyl)formamidinate)3]2(μ-trans-diphenyldicarboxyphenylporphyrin) ([Mo2(DAniF)3]2(μ-trans-A2B2Porphyrin)) were characterised by cyclic voltammetry (CV) and UV/Vis spectroscopy, but neither technique provided any evidence of electronic communication between the two components. Supporting density functional theory (DFT) calculations suggest that electron donating N-donor formamidinate ligands raise the energy level of the Mo2 δ system so that it cannot effectively overlap with the porphyrin π system. Further tuning of the electronic structures of the two components is therefore required to promote electronic interaction between paddlewheel complexes and porphyrins and a parallel project stream has been conducted to assess the redox and optical properties of homoleptic fluorinated paddlewheel complexes, which could provide a route to achieve this. Future work aims to develop a second generation of paddlewheel-porphyrin conjugates with maximised electronic communication, and to explore the electrochemical and photophysical properties of these conjugates in the context of molecular electronics and photovoltaics.