Hydrogen Bonding and Electron Transfer between Dimetal Paddlewheel Compounds Containing Pendant 2‑Pyridone Functional Groups

The compounds M₂(TiPB)₃(HDON) (TiPB = 2,4,6-triisopropylbenzoic acid; H₂DON = 2,7-dihdroxy-1,8-napthyridine; M = Mo (1a) or W (1b)) and Mo₂(TiPB)₂(O₂CCH₂Cl)­(HDON) (1c) which contain a pendant 2-pyridone functional group have been prepared. These compounds are capable of forming self-complementary hydrogen bonds, resulting in the formation of “dimers of dimers” ([1a–c]₂) in CH₂Cl₂ solutions. Electrochemical studies reveal two successive one-electron redox processes for [1a–c]₂ in CH₂Cl₂ solutions that correspond to successive oxidations of the dimetal core, indicating stabilization of the mixed-valence state. Only small changes in the value of K_c are observed upon changing the ancillary ligand or metal, implying that proton coupled mixed valency is responsible for the stabilization. Dimethylsulfoxide (DMSO) disrupts the hydrogen bonding interactions in these compounds, and a single oxidation process is observed in DMSO which shifts to lower potential as the number of HDON ligands increases. Further substitution of carboxylate ligands with HDON leads to the formation of Mo₂(TiPB)₂(HDON)₂ (2) and Mo₂(HDON)₄ (3), which adopt trans-1,1 and cis-2,2 regioisomers in the solid-state. ¹H NMR spectroscopy indicates that there are at least two regioisomers present in solution for both compounds. The lowest energy transition in the electronic absorption spectra of these compounds corresponds to a M₂-δ → HDON-π* transition. The electrochemical, spectroscopic and structural results were rationalized with the aid of density functional theory (DFT) calculations.