Development of Bi-metallic Catalysts for Multi-electron Chemistry: of Consequence to Sustainable Energy

Abstract

The new ligand L1 (21), 1-N,1-N-bis(pyridine-2-ylmethyl)-3-N-(pyridine-2-ylmethylidene)benzene-1,3-
diamine, was synthesised as a platform to study bi-metallic first row transition complexes containing
redox-active ligands. The asymmetric ligand L1 (21) contains a redox-active α-iminopyridine unit
bridged to redox-inert or “innocent” bis(2-pyridylmethyl)amino counterpart and offers two distinct coordination
sites. The co-ordination chemistry of L1 (21) with Fe2+, Cu2+, and Zn2+ was examined.
Reaction with Zn2+ afforded the asymmetric binuclear complex [(L1)Zn2Cl4] (C1), whereas the symmetric
[(L1)2Fe2(OTf)2](OTf)2 (C2), [(L1)2Fe2(CH3CN)2](PF6)4 (C3) and [(L1)2Cu2](OTf)4 (C4) complexes were
isolated in reactions with iron and copper salts. Both metal- and ligand-centered redox processes are
available to complexes C2, C3 and C4 and were investigated by cyclic voltammetry. Complexes C2 and
C3 favor ligand-centered reduction while C4 favors metal-centered reduction. EPR, Mössbauer
spectroscopy and magnetic susceptibility studies establish that complexes C2, C3 and C4 are
paramagnetic.
Ferrous complexes C2 and C3 were studied as potential C-H oxidation catalysts. Difficult C-H
oxidation transformations are known to be facilitated by synthetic and biomimetic iron species. The C2
and C3 complexes provide a unique contrast non-heme enzyme active sites and biomimetic complexes
due to the large Fe…Fe inter-nuclear distances of > 7Å. Complexes C2 and C3 provided evidence of
moderate catalytic activity toward a range of substrates of varying bond strengths including cyclohexene
(80 kcal mol-1), 9,10-dihydroanthracene (76 kcal mol-1), xanthene (75.5 kcal mol-1) and triphenylmethane
(81 kcal mol-1). The oxidising strength of complexes C2 and C3 toward C-H bonds were found to be
limited by bond strength with both C2 and C3 proving to be incapable of oxidising cyclohexane (99 kcal
mol-1) and adamantane (96 kcal mol-1). The oxidation pathway was investigated by employing deuterated
substrates in place of cyclohexene, DHA and xanthene. The increased strength of the C-D bond over a CH
bond (1.4 kcal mol-1) should induce decreased reactivity and hence product formation if the hydrogen
abstraction pathway is being accessed. Decreased reactivity was observed when the deuterated versions
of cyclohexene, DHA and xanthene were employed illustrating the hydrogen abstraction pathway is
indeed the mechanistic means by which complexes C2 and C3 oxidise C-H bonds.
The structural identity of the active species formed by C3 during C-H oxidation, was investigated in
the absence of substrate which exposed the formation of a transient species identified by λmax 430 nm in
the UV-Vis spectrum - labelled C7. Complex C7 decays to the di-ferric species C8. Spectroscopic
studies in the presence of substrate uncovered the existence of two active species with two distinct rates
of oxidation. Exposing complexes C7 and C8 to DHA unveiled complex C7 as the lesser active species,
while complex C8 was unreactive toward DHA oxidation. The structural identity of the more reactive
species has been elusive to the studies presented herein.
A new bi-macrocyclic ligand scaffold which might more readily favour the formation of two-electron
mixed valence species was designed. A series of five structures with varying M…M inter-nuclear
distances were designed and the synthetic routes toward their formation are discussed accompanied by
calculated geometry optimised structures as full characterisation of the final complexes are yet to be
obtained.