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Discovery of Molecular Oxidation Catalysts Using
Metallo-Enzyme Mimicry and Combinatorial Chemistry
Stephen B. Colbran & D. Brynn Hibbert, School of Chemistry,
University of New South Wales, Sydney, NSW 2052, Australia
Aims
• To synthesize and screen a library of molecules combining an organometallic N-heterocyclic
carbene (NHC) centre and one or more ‘classical’ metal sites as homogeneous catalysts for
selective oxidations of organic substrates.
Results
• Routes to the precursor polytopic ligands with the NHC and bis(pyridylmethyl)amine (bpa) donor
domains to make multi-metallic complexes were developed.
• Problems of metal selectivity between the ligand domains restricted the library of complexes to
the novel bi- and tri-metallic systems 1 and 2.
• Serial screens for catalysis of styrene oxidation using series 1 and 2 were conducted. Catalytic
activity above that found for the monomeric constituent metal centres was not uncovered.
• Density functional calculations of M(IV)=O (M = Ti–Cu) species were
undertaken and point to peak activity for the latter transition metals ...
NiIV, CuIV (Zn is inactive and does form a M=O species). The reactivity
scales with increasing radical character for the oxo ligand
(i.e. as the metal centre becomes more MIII–O• like)
M1
N
N
N
N
N
N
N
1. M1{(NHC)--(bpa)}Ma
M1 = RhCl(cyclooctadiene)
Ma = Co2+, Ni2+, Cu2+, Zn2+
N
N
N M1
N
N
Ma
N
Ma
Mb N
N
N
M1 = PdCl: Ma,b = Co2+, Cu2+, Zn2+
Ma,b = PdCl: M1 = RhCl, PdCl, AgCl
2. M1{(py-NHC2)--(bpa)2}Ma,Mb