Redox Potential Tuning of Dimolybdenum Systems through Systematic Substitution by Guanidinate Ligands

We report the synthesis and characterization of a series of dimolybdenum paddlewheel complexes of the type Mo₂(DAniF)_4-(hpp) (n = 1-3), where DAniF is the anion of N,N'-di-p-anisyl-formamidine and hpp is the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine. The effect on the electronic structure of these tetragonal paddlewheel dimolybdenum compounds was studied upon systematic substitution of formamidinate ligands by the more basic guanidinates. Mo-Mo distances in the paddlewheel structures decreased upon guanidinate ligand substitution, and were found to be 2.0844(6) and 2.0784(6), for Mo₂(DAniF)₃(hpp) (1) and trans-Mo₂(DAniF)₂(hpp)₂ (2), respectively. Electrochemical studies show that the half-wave potential of the Mo₂⁵⁺/Mo₂⁴⁺ couple shifts cathodically upon ancillary ligand substitution ranging from -0.286 V for the tetraformamidinate complex to -1.795 V for the tetraguanidinate analogue and with redox potentials of -0.75, -1.07, and -1.14 V for 1, 2, and 3 (Mo₂(DAniF)(hpp)₃), respectively. The presence of a second redox event assigned to the Mo₂⁶⁺/Mo₂⁵⁺ couple was not observed until two guanidinate ligands were introduced. Raman spectroscopy shows that the v(M-M) stretch gets systematically strengthened upon formamidinate ligand substitution by the guanidinate ligand hpp. The induced delta bond destabilization by the basic hpp ligand was measured using DFT calculations by tracking the energy of the frontier orbitals. The decrease in the HOMO-LUMO energy gap was supported by the red shift in the UV-vis spectra of the compounds: 412, 442, and 450 nm for 1, 2, and 3, respectively.