Synthesis and Application of Tunable Mo2 and W2 Tetraguanidinate Paddlewheel Complexes

Abstract

Super-electron-donor dimolybdenum and ditungsten tetraguanidinate paddlewheel complexes have proven to be the strongest reducing agents known. Strong single and double electron donors can be used for applications from H2 production to difficult organic transformations like C–Cl bond cleavage and crosscoupling reactions. The goal of this research was to develop accessible syntheses of the ligands and complexes, to investigate high impact applications of the Mo2 and W2 analogues, and to explore new complexes in hopes to tune their reactivity and solubility. Preparation and handling of the super-base, bicyclic guanidinate ligands and complexes have prevented extensive investigations of their applications. New reproducible syntheses for HTEhpp and W2(TEhpp)4Cl2 were developed. Interaction of the paddlewheel complex Mo2(TEhpp)4 and low concentrations of acetic acid were studied using cyclic voltammetry. Experiments show the electron deficient vacant dimetal axial site and the electron rich guanidinate core work together similar to frustrated pairs. Acetic acid protonates the guanidinate while simultaneously coordinating to the metal center. This newly discovered synergistic bonding decreases the electron donor ability of the complex preventing catalysis for the production of H2. By tuning the dimetal center to a more electron rich, 3rd row transition metal, W2(TEhpp)4, the reduction of H+ to H2 is now favored and is catalytic. Computations were used to explore the nature of these interactions. In addition to H2 production, a mechanistic study of C–Cl bond cleavage in dichloromethane by Mo2(TEhpp)4 shows a novel singlet-to-triplet crossover at the transition state. This work shows the potential for this class of complexes to 19 perform electron transfers via multiple mechanisms (e.g. atom transfer and electron transfer). Upwards of 25 new dimetal tetraguanidinate paddlewheel complexes were explored computationally for their electron donor ability. Some of the ligands have synthetic precedent. These and many others will hopefully be part of the next generation of super-electron-donor complexes.