Date of Award
Doctor of Philosophy (PhD)
James R. Gardinier
Chae S. Yi
Pincer ligands are uninegative tridentate metal-coordinating agents of the form [XZY]- where Z is the central, anchoring Lewis donor while X and Y are flanking Lewis donors. Ever since initial reports of transition metal pincer complexes were published in the late 1970's, there has been burgeoning interest in such complexes because of their desirable robust nature, generally simple syntheses, and the spectacular chemical transformations that they can mediate. In this research project, two new sets of pincer ligands with a diarylamido anchor and either two pyrazolyl nitrogenous flanking donors (NNN pincer) or one pyrazolyl and one diphenylphosphine donor (NNP pincer) have been prepared and their late transition metal complexes have been studied.
First, for tricarbonylrhenium(I) complexes it was demonstrated that the NNN pincers bind in bidentate or fac- tridentate modes. By increasing steric bulk at the 3-pyrazolyl position near the metal, the fac-Re(CO)3 moiety distorts the ligand to enhance ligand-centered reactivity.
Second, for carbonylrhodium(I) complexes, (NNN)Rh(CO), substitution at the para-aryl positions predictably modulates the electronic properties and chemical reactivity. Oxidative addition reactions of the (NNN)Rh(CO) with iodoalkanes proceed about three orders of magnitude faster than those reported for the Monsanto catalyst, [Rh(CO)2I2]-.
Third, there is also interest in metal complexes of redox-active ligands because it is hoped that one could use the ligand as an electron reservoir to help arbitrate difficult multi-electron processes. For (NNN)RhIIIXYZ, varying non-pincer ligands(X, Y, and Z) changes the (NNN)/(NNN)+ oxidation potential by 700 mV. An empirical ligand additivity model was discovered that predicts the half wave potential of the ligand-based redox couple. Such a model is envisioned to be important for future considerations when designing complexes for exothermic electron transfer reactions.
Finally, a comparison of related (NNN)Rh(CO), (NNP)Rh(CO) and (PNP)Rh(CO) complexes revealed that substitutions of pyrazolyl for diphenylphosphine primarily impacts sterics (not electronics), thereby affecting kinetics of reactions. The PPh2 moiety permits the isolation of a coordinatively-unsaturated 16-electron rhodium(III) complex that showed metal ligand cooperativity in its reactions with HI. The hemilability of the (NNP)Rh fragment was also demonstrated by reactions with t-BuNC.