PNNL

Abstract

The synthesis of a series of PEtPNRR' (PEtPNRR' = Et2PCH2CH2P(CH2NRR?)2, R = H, R? = Ph or 2,4-difluorophenyl; R = R? = Ph or iPr) diphosphine ligands containing mono- and di-substituted pendant amine groups, and the preparation of their corresponding molybdenum bis(dinitrogen) complexes trans-Mo(N2)2(PMePh2)2(PEtPNRR') is described. In situ IR and multinuclear NMR spectroscopic studies monitoring the stepwise addition of HOTf to trans-Mo(N2)2(PMePh2)2(PEtPNRR') complexes in THF at -40 °C show that the electronic and steric properties of the R and R? groups of the pendant amines influence whether the complexes are protonated at Mo, a pendant amine, a coordinated N2 ligand, or a combination of these sites. For example, complexes containing mono-aryl substituted pendant amines are protonated at Mo and pendant amine to generate mono- and dicationic Mo?H species. Protonation of the complex containing less basic diphenyl substituted pendant amines exclusively generates a monocationic hydrazido (Mo(NNH2)) product, indicating preferential protonation of an N2 ligand. Addition of HOTf to the complex featuring more basic diisopropyl amines primarily produces a monocationic product protonated at a pendant amine site, as well as a trace amount of dicationic Mo(NNH2) product that contain protonated pendant amines. In addition, trans-Mo(N2)2(PMePh2)2(depe) (depe = Et2PCH2CH2PEt2) without a pendant amine was synthesized and treated with HOTf, generating a monocationic Mo(NNH2) product. Protonolysis experiments conducted on select complexes in the series afforded trace amounts of NH4+. Computational analysis of the series of trans-Mo(N2)2(PMePh2)2(PEtPNRR') complexes provides further insight into the proton affinity values of the metal center, N2 ligand, and pendant amine sites to rationalize the differing reactivity profiles. This research was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Computational resources provided by the National Energy Research Scientific Computing Center (NERSC) at Lawrence Berkeley National Laboratory. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.