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45130-G3
Redox Coupling for Multielectron Small-Molecule Activation

Jake D. Soper, Georgia Institute of Technology

Overview and Significance.

The ability to make and break bonds with a high degree of specificity has applications ranging from benchtop synthesis to the production of clean chemical fuels. Most synthetically useful methods for selective transformations of small-molecule substrates rely on transition metal catalysts to mediate transfer of multiple electrons in a single step. Current state-of-the-art inorganic and organometallic catalysts for multielectron bond-making and bond-breaking reactions often feature expensive platinum-group transition metals, and some types of reactions are challenging with current methods. Catalysts based on naturally abundant transition metals may address these limitations. The challenge is in imparting a multielectron redox capacity to metal ions that typically effect only 1e^– redox changes.

Our approach to this problem utilizes redox-active ligands to store and deliver charge to later 3d metal centers for multielectron reactions with small molecules. In this way, four- and five-coordinate manganese, iron and cobalt centers act as surrogates for platinum group metal catalysts, particularly those that rely on oxidative-addition and reductive-elimination steps in catalytic cycles for small-molecule activation and functionalization.

Progress Report.

Chart 1 summarizes the redox-active ortho-catecholate (cat) and ortho-arylamidophenolate (ap^Ar) ligands used in this work. These ligands were selected because their frontier orbitals are close in energy to the manganese, iron and cobalt 3d orbitals, and modification of the ligand can be used to tune steric and electronic properties. This approach has been fruitful. During the grant period we have discovered new stoichiometric and catalytic ligand-mediated multielectron reactions at coordinatively unsaturated first-row transition metals. Selected highlights are presented below.

Cross coupling catalysis. We have prepared and characterized a series of square planar cobalt complexes with two o-arylaminophenolate (ap^Ar) (Ar = Ph, 2,6-ⁱPr₂C₆H₃, 3,5-Cl₂C₆H₃) ligands. Addition of 1.0 equiv Cp*₂Co or Na to blue Co^III(ap^Ar)(isq^Ar) species cleanly generates air-sensitive violet [Co^III(ap^Ar)₂]^– products (Figure 1).

The S = 1 [Co^III(ap^Ar)₂]^– complexes are strong nucleophiles. Reaction of 1.0 equiv of 2,3,4,5,6,6-hexachloro-2,4-cyclohexadien-1-one with solutions of [Co^III(ap^Ar)₂]^– forms Co^IIICl(isq^Ar)₂ (eq 1). The oxidation state of the cobalt(III) center is unchanged in the 2e^– bond-forming reaction because both of the reducing equivalents derive from 1e^– oxidation of the o-amidophenolate chelates.

The nucleophilic nature of the cobalt(III) center in [Co^III(ap^Ar)₂]^– is further revealed in reactions with alkyl halides. Addition of 1.0 equiv CH₂Cl₂ to [Co^III(ap^Ph)₂]^– affords clean conversion to the square pyramidal chloromethyl complex Co^III(CH₂Cl)(isq^Ph)₂ (Scheme 1). Reaction with CH₃I gives a similar conversion to green Co(CH₃)(isq^Ph)₂. Thus, the [Co^III(ap^Ar)₂]^– species have properties reminiscent of "supernucleophilic" cobaloxime(I) complexes. The reaction with CH₂Cl₂ is a remarkable example of nucleophilic attack on an unactivated alkyl halide under extremely gentle conditions.

Addition of 1.0 equiv PhLi to Co^III(CH₃)(isq^Ph)₂ yields [Co^III(ap^Ph)₂]^– and toluene. Such S_N2-type pseudo-reductive elimination closes a catalytic cycle for cross coupling with alkyl halides (Scheme 2). Cobalt catalyzed cross coupling of unactivated alkyl halides with organic nucleophiles for C–C, C–N and C–O bond-forming reactions at sp³-hybridized carbon centers is a focus of our ongoing research.

Selective aerobic oxidations. We have reported the synthesis and characterization of the manganese(III) anions [Mn^III(X₄cat)₂(L)_n]^– [X = Cl, Br; n = 1, L = MeOH, OPPh₃; n = 2, L = acetone, tetrahydrofuran (THF)] as precursors to the square planar [Mn^III(X₄cat)₂]^– core. The axial ligands are substitution labile while the [Mn^III(X₄cat)₂]^– core is preserved in non-aqueous solutions.

Previous reports had speculated that a vacant coordination site was a prerequisite to reaction of [Mn^III(X₄cat)₂(L)_n]^– complexes with O₂. However, we found that the [Mn^III(X₄cat)₂]^– fragment reacts sluggishly with dioxygen. The tris(catecholato) trianion [Mn^III(X₄cat)₃]^3– is the true air-sensitive species, affording [Mn^III(X₄cat)₂(L)₂]^– (L = acetone, THF) and X₄bq with O₂ exposure.

The conversion of X₄catH₂ to X₄bq is an aerobic oxidative dehydrogenation (2H⁺, 2e^–) reaction. We postulated that the [Mn^III(Br₄cat)₂]^– core could catalyze other oxidase-type reactions. Accordingly, quantitative conversion of ^tBu₂catH₂ to ^tBu₂bq is achieved with 0.2 mol % [Mn^III(X₄cat)₂]^– in ca. 400 min at 25 °C (eq 2). Investigations of the reaction mechanism uncovered salient features of these reactions that form the basis for selectivity in other 2e^– reactions using O₂ as the terminal oxidant. Current efforts are pursuing extensions of this aerobic dehydrogenation chemistry to alcohol and amine oxidation, dehydrogenative coupling of amines, and oxidative homocoupling of alkyl-, alkenyl-, alkynl-, and aryl-carbanions.

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