The acetyl CoA synthase paradigm for hybrid bio-organometallics: Quantitative measures for resin-bound Ni-Rh complexes

The active site of Acetyl CoA Synthase utilizes a square planar NiN₂S₂ complex in the form of Ni^II(CGC)²⁻ (CGC = the cysteine-glycine-cysteine tripeptide motif within the protein) to serve as a bidentate sulfur-donor ligand to chelate a second, catalytically active Ni atom responsible for the C-C and C-S coupling reactions for the production of Acetyl CoA. Metalloenzymes, such as this, which house stable catalytic complexes within intricately designed pockets accessible by solvent channels, have inspired design of resin-bound complexes. Through the use of TentaGel S-RAM^® resin beads, the O-Ni(CGC)²⁻ ligand has been synthesized and derivatized with the Rh^I(CO)₂ moiety. The identification of the adduct on these resin beads is afforded by attenuated total reflectance FTIR spectroscopy in the ν(CO) region and compared to solution analogues. The goal of this study is to establish a quantitative measure of the loading of nickel and rhodium on the tripeptide modified resin beads, O-(CGC). The extent of CGC derivatization was determined by Fmoc cleavage of the Fmoc protected O-(CGC). Nickel and rhodium loading were determined by Neutron Activation Analysis. This work provides evidence that the TentaGel S-RAM^® resin beads greatly decrease the air sensitivity of the Ni-Rh complex as compared to the unsupported solution phase analogue. The derivatized beads have also been studied for their ability to withstand a number of physical stresses, i.e., for leaching.