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Subtle differences between Zr and Hf in early/late heterobimetallic complexes with cobalt
Vinay N Setty1, Wen Zhou, Bruce M Foxman
1Department of Chemistry, Brandeis University, 415 South Street, Waltham, Massachusetts 02454, USA.
Inorganic Chemistry
|April 22, 2011
Summary
New cobalt-hafnium complexes show subtle differences from cobalt-zirconium analogues, particularly with N-Mes ligands, impacting reactivity and catalytic activity in Kumada coupling.
Area of Science:
- Organometallic Chemistry
- Coordination Chemistry
- Catalysis
Background:
- Tris(phosphinoamide) ligands offer tunable steric and electronic properties for metal complex synthesis.
- Cobalt-zirconium complexes with these ligands have been previously reported, providing a basis for comparison.
- Understanding the influence of the metal (Hf vs. Zr) on complex properties is crucial for catalyst design.
Purpose of the Study:
- To synthesize and characterize novel phosphinoamide-linked cobalt-hafnium complexes.
- To compare the structural, electronic, and reactivity differences between analogous cobalt-hafnium and cobalt-zirconium complexes.
- To investigate the catalytic activity of the new cobalt-hafnium complexes in Kumada coupling reactions.
Main Methods:
- Synthesis of tris(phosphinoamide) hafnium chloride precursors.
- Formation of phosphinoamide-linked Co/Hf complexes through ligand exchange or related reactions.
- Characterization using spectroscopic techniques (e.g., NMR) and X-ray crystallography.
- Electrochemical studies to investigate reduction products.
- Evaluation of catalytic performance in Kumada coupling.
Main Results:
- Successfully synthesized Co/Hf complexes (4-6) and their reduction products (7, 9).
- Observed minor structural and electronic differences between Hf and Zr complexes with N-(i)Pr ligands.
- Identified more pronounced differences in structure, bonding, and reactivity for Hf/Co and Zr/Co complexes with the N-Mes ligand.
- Reduction product 8 showed significantly different reactivity compared to its Zr congener, particularly in oxidative addition reactions.
- The Co/Hf complex 5 exhibited diminished activity in Kumada coupling compared to Co/Zr analogues.
Conclusions:
- The choice of metal (Hf vs. Zr) and phosphinoamide ligand (N-(i)Pr vs. N-Mes) significantly influences the properties and reactivity of these bimetallic complexes.
- The N-Mes ligand highlights more pronounced differences between Hf and Zr, affecting bonding and reactivity.
- Cobalt-hafnium complexes show potential but reduced efficacy in Kumada coupling compared to their cobalt-zirconium counterparts.
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