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-n(hpp)n (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.

Hydrogen gas generation using a metal-free fluorinated porphyrin.

Free-base meso-tetra(pentafluorophenyl)porphyrin, 1, is electrocatalytically active for hydrogen gas generation in the presence of p-toluenesulfonic acid. The electrochemical potential of hydrogen evolution (-1.31 V vs. Fc/Fc+ in THF) is comparable to those of metal containing electrocatalysts such as metallated porphyrins or other metallated macrocycles. Combining experimental observations and DFT computations, we propose the most favorable hydrogen generation mechanism to be a (1) reduction, (2) protonation, (3) reduction, (4) protonation (E-P-E-P) pathway.