Synthesis and Reactivity of Transition Metal Complexes Bearing the Tridentate Bis(2-mercapto-p-tolyl)amine ([SNS]H3) Ligand
The work described herein focuses on the synthesis and characterization of new heterobimetallic complexes containing the redox-active W[SNS]2 metalloligand and investigation into their electronic properties and reactivity. Most recent studies have explored the redox nature of the [SNS]H3 scaffold through the synthesis and reactivity of a novel set of square-planar nickel complexes.
Chapters 2 and 3 describe a modular synthetic approach towards generating a new series of heterobimetallic complexes with the general formula W[SNS]2M(L) ([SNS] = bis(2-mercapto-p-tolyl)amine; M = Ni, Pd, or Pt; and L = dppe, depe, dmpe, dppp, PR’2NRPR’2 (R = phenyl, benzyl; R’=phenyl), DPEphos or dppf). The complexes were prepared by a salt metathesis of Cl2MII(L) with the previously reported W[SNS]2 coordination complex under reducing conditions. X-ray diffraction analysis revealed interesting coordination geometries about the appended Group 10 metal centers moving from Pt and Pd (pseudo-square planar) to the first row Ni (pseudo-tetrahedral) analogue. These complexes demonstrate formal metal–metal bond formation across the series with a tunable first oxidation potential up to 600 mV.
Chapter 4 investigates the use of W[SNS]2Ni(dppe) as a catalyst for the electrochemical reduction of protons to hydrogen. This complex was found to catalytically generate hydrogen with an overpotential of 700 mV, a TOF of 14 sec–1, and a Faradaic yield of 80 ± 3 % using 4-cyanoanilinium tetrafluoroborate in non-aqueous solutions.
Chapter 5 demonstrates the effect of exchanging the nickel center of the heterobimetallic complexes discussed in Chapters 2 and 3 with other first row transitions metal ions (i.e. cobalt and copper). Analysis into the observed metal–metal distances reveal stark differences across the series. Additionally, the copper ion containing complexes demonstrate dynamic behavior in solution.
Chapter 6 investigates the synthesis and reactivity of a series of monomeric square-planar nickel complexes of the [SNS] scaffold to demonstrate the ligand as redox, proton, and hydrogen atom non-innocent.
Appendix A illustrates the electrochemical responses observed for the monoanionic complexes from Chapter 6 in the presence of CO2 and CO. Appendices B and C describe the synthesis and characterization of a five-coordinate cobalt and a heterotrimetallic tungsten-nickel complex, respectively.