STUDIES TOWARD HOMOGENEOUS DINITROGEN REDUCTION AND AMMONIA OXIDATION BY BIS(IMINO)PYRIDINE MOLYBDENUM COMPLEXES AND GROUP 4 METALLOCENES

Abstract

Abstract

The zirconocene dinitrogen complex, (η5-C5Me4H)2Zr]2(µ¬2, η2,η2-N2) was synthesized by photochemical reductive elimination from the corresponding zirconium bis(aryl) or aryl hydride complexes, providing a high yielding, alkali metal-free route to strongly activated early metal N2 complexes. Photolysis of zirconium bis(aryl) complex (η5-C5Me4H)2Zr(Tol)2 leads to the reductive coupling of the arenes to form presumed η2 bound intermediate arene complexes. With sufficient mixing in the presence of dinitrogen, these complexes undergo arene displacement to form activated dinitrogen complex (η5-C5Me4H)2Zr]2(µ¬2, η2,η2-N2).

The bis(imino)pyridine molybdenum dinitrogen complex [{(iPrBPDI)Mo(N2)}2(µ2, η1, η1-N2)] has been prepared and its reactivity examined. Exposure of the molybdenum dinitrogen complex to ammonia or hydrazine resulted in the formation of imido (µ-NH) ligands with concomitant reduction of one of the imines of the supporting chelate to form [(iPrBPIBn)Mo(=NArDipp)]2(µ2,η1,η1-NH)2. Experiments utilizing n-propyl amine yielded isolable intermediates in the ligand modification process, leading to the identification of a highly reactive molybdenum hydride complex (iPrBPIABn)Mo(H)(=NC3H7). Upon additional stirring, (iPrBPIABn)Mo(H)(=NC3H7) further reacts to form (iPrBPIBn)Mo(=NArDipp)(=NC3H7), giving insight into the mechanism of ammonia oxidation and the role of the redox and chemically active ligand.

A bis(imino)pyridine molybdenum η6-benzene complex (iPrPDI)Mo(η6-C6H6) has been synthesized and provided an accessible route into studying the chemistry of molybdenum amides, imides and nitrides. The large range of oxidation states available to the [(iPrPDI)Mo] system is possible through a high degree of metal-ligand cooperativity made available by the bis(imino)pyridine ligand.

A bis(imino)pyridine molybdenum complex (iPrPDI)Mo(=NTol)2 was synthesized and used as a platform to quantify nitrogen-hydrogen (N-H) bond dissociation free energies (BDFE’s). Utilizing the electrochemical and acidity studies the BDFE of the N-H bond in (iPrPDI)Mo(-N(H)Tol)(=NTol) was found to be between 75-78 kcal/mol. The anionic form of the aforementioned complex, [K][(iPrPDI)Mo(-N(H)Tol)(=NTol)] was also synthesized and examined. The N-H BDFE of this anionic complex was seen to fall within the range of 69-72 kcal/mol. While the redox-active ligand employed in our studies allowed for the isolation and study of intermediate complexes, the high reduction potentials and amplified basicity did not translate into a large difference in BDFEs between these complexes.