Synthesis and reactivity of iron and cobalt complexes in a sulfur-rich coordination environment and organonickel complexes containing n-heterocyclic carbene and cyclopentadienyl ligands

Abstract

Many metalloenzymes contain first-row transition metals in a sulfur-rich coordination environment. For example, the active sites of nitrogenases and [FeFe]-hydrogenases that are responsible for N 2 reduction to ammonia, and H 2 generation in nature, respectively, consist of multiple iron atoms bound through cysteine sulfur donors, inorganic sulfide ions, and/or CO and CN- ligands. Moreover, some reactions mediated by transition metal ions involve homolysis of metal-carbon bonds and the formation of organic radicals. Using a so-called "synthetic analog" approach, we seek to develop small molecule mimics of enzyme active sites to understand electronic structure, and chemical reactivity, including the identification of catalytic intermediates relevant to the storage and utilization of non-fossil derived energy sources such as H 2 and NH 3. In this work, we examine the coordination chemistry and reactivity of iron and cobalt ligated by the S3- ligand, phenyltris((tert- butylthio)methyl)borate, [PhTt tBu]-. This sterically encumbered ligand often enforces monomeric coordination complexes in a pseudo-tetrahedral coordination geometry while providing metal ions with a sulfur-rich coordination environment. The first portion of this work focuses on the synthesis, characterization, and reactivity of new [PhTt tBu]Fe-alkyl/aryl complexes, and [PhTt tBu]Co-alkyl, thiolato, and anilido compounds. Reactivity studies include carbonylation reactions with CO, and thermally or photochemically promoted M-X (X = C or S) bond cleavage reactions to generate organic radicals. The last portion of this work highlights advances in the synthesis, characterization, and photochemical reactivity of nickel complexes supported by cyclopentadienyl (Cp) and N-heterocyclic carbene (NHC) ligands, i.e. CpNi(NHC)(X) (X = alkyl/aryl, thiolato, and azido ligands). While different in both metal and supporting ligand structure, in some cases the reactivity of the organometallic nickel systems directly mirrors that of the cobalt and iron thioether complexes.