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Inorg Chem. Author manuscript; available in PMC 2010 July 20.
Published in final edited form as:
PMCID: PMC2878392
NIHMSID: NIHMS116573

Intermediates in Reactions of Copper(I) Complexes with N-Oxides: From Formation of Stable Adducts to Oxo Transfer

Abstract

Reactions of Cu(I) complexes of bidentate N-donor supporting ligands with pyridine- and trimethylamine-N-oxides or PhIO were explored. Key results include the identification of novel Cu(I)-N-oxide adducts, aryl substituent hydroxylation, and bis(μ-oxo)dicopper complex formation via a route involving oxo transfer.

Because copper-promoted oxidation reactions play important roles in biology1 and catalysis,2 great effort has been expended to uncover mechanistic information and, in particular, to identify possible copper-oxygen intermediates.3 Of notable interest are [CuO]+ species (Cu(III)-O2− ↔ Cu(II)-O•), which have been invoked in catalysis by enzymes4 and synthetic systems5,6 and characterized by theory4b-d,7 and in the gas phase.7b For example, ligand-supported [CuO]+ species have been postulated in order to rationalize ligand hydroxylations observed in the decay of Cu(II)-OOH complexes,5 the O2-induced decarboxylation of Cu(I)-α-ketocarboxylates,6 and reactions of Cu(I) complexes with iodosylbenzene (PhIO).5 A [CuO]+ intermediate has also been invoked as the active species responsible for regiospecific arene hydroxylations by Cu/Me3NO systems,8 via a mechanism proposed on the basis of theory to involve O-N bond homolysis from an isolated Cu(II)-ONMe3 complex.9 While a number of such Cu(II)-N-oxide complexes are known,10 to our knowledge Cu(I) variants have not been reported. Such Cu(I)-N-oxide adducts are of interest because they might provide a [CuO]+ or related species through heterolytic N-O bond cleavage, as seen in a number of reactions of N-oxides with other metal centers (e.g. Fe11 and Ru12). We therefore sought to investigate the reactions of pyridine- and trialkylamine-N-oxides with Cu(I) complexes, using a range of supporting ligands with variable electronic and steric profiles that have been previously used in O2 reactivity investigations.6,13 Herein we report initial results of this study, including the isolation and full characterization of the first examples of Cu(I)-N-oxide complexes, demonstration of oxo transfer from an N-oxide resulting in the formation of a bis(μ-oxo)dicopper core, and ligand oxidation from a related reaction with iodosylbenzene (PhIO).

Reactions of [(L1)Cu(O3SCF3)]6 with substituted pyridine-N-oxides (1 equiv) or [(L)Cu(CH3CN)] (L = L3 or L4)13,14 with Me3NO (1 equiv) in THF yielded the adducts 15, respectively (Scheme 1). The products were isolated as red-brown crystalline solids in good yields (67−95%) and were characterized by 1H and 13C{1H} NMR spectroscopy, CHN analysis, and X-ray crystallography (Figures 1 and S1-S3). All the structures feature 3-coordinate Cu(I) ions and ‘bent’ coordination of the N-oxide ligands (Cu-O-N angles 120−122°). Little ‘activation’ of the N-oxide is evident from the N-O distances, which for 1−3 (1.33−1.34 Å) are similar to those seen in Cu(II)-pyridine-N-oxide complexes,10e,f and for 4 and 5 (1.40−1.41 Å) are similar to that of free Me3NO (1.404(5) Å).15 The pyridine-N-oxide ring planes in 13 are parallel to the dimethylphenyl substituent in L1, consistent with π-stacking interactions. In solution, however, the pyridine-N-oxides undergo rapid fluxional processes, as indicated by the presence of broad peaks in the room temperature 1H NMR spectra that were found in the case of 3 to split and sharpen upon cooling to −78 °C (Figure S4). Complexes 13 are quite stable in solution, showing no signs of decay by NMR spectroscopy in THF-d8 solution after 1 d at 80 °C (sealed tubes). Under the same conditions, complexes 4 and 5 decompose to paramagnetic species that have yet to be identified.

Scheme 1
Reactions of Cu(I) complexes with pyridine- and trimethylamine-N-oxides.
Figure 1
Representations of the X-ray structures of (a) the cationic portion of 3 and (b) 4, showing all nonhydrogen atoms as 50% thermal ellipsoids. Selected interatomic distances (Å) and angles (deg) are as follows. 3: Cu1-O1, 1.9048(19); Cu1-N1, 2.066(2); ...

Working under the hypothesis that the stability of 13 is due to a relatively low degree of electron donation from the ligand L1, we turned our attention to an analog of L1 containing an electron donating p-Me2N- group on the pyridyl ring (L2). Unfortunately, reaction of [(L2)Cu(CH3CN)](O3SCF3) with Me3NO resulted in extensive ligand redistribution and redox processes as indicated by the formation of a complex mixture of products, including the known complex [Cu(Me3NO)4]2+ 16 and [(L2)2Cu](O3SCF3) (identified by X-ray crystallography; Figure S5). By using PhIO instead, oxo transfer was observed, yielding a dicationic tricopper cluster (with two CF3SO33 counterions) featuring hydroxylated forms of L2 coordinated to 4-coordinate Cu(II) ions bridged by a central IO3 ion (Figure 2; 57% isolated yield, also characterized by CHN analysis and ESIMS; Figure S6). While the overall structure of this cluster is unique, the observation of ligand aryl group hydroxylation by PhIO has parallels in the literature,5 and similarly may be attributed to a [CuO]+ intermediate or some type of CuOIPh species.

Figure 2
A drawing and a representation of the X-ray crystal structure of the dicationic portion of the tricopper cluster resulting from reaction of [(L2)Cu(CH3CN)](O3SCF3) with PhIO. Selected interactomic distances (Å) and angles (deg) are as follows: ...

In contrast to the formation of stable Cu(I)-ONMe3 adducts when using L3 and L4, the Cu(I) complex of the more electron donating ligand L5 reacted rapidly with Me3NO at room temperature to give a brown solution, from which a few crystals of the known17 complex [(L5)2Cu2(μ-OH)2] were obtained; no Cu(I)-ONMe3 adduct was observed. When the reaction was performed at −78 °C, smooth conversion to an intermediate occurred over ~1 h that was identified as the bis(μ-oxo)dicopper complex [(L5)2Cu2(μ-O)2] (6) on the basis of comparison of its UV-vis (λmax = 423 nm, ε ~ 16,000 cm−1M−1) and resonance Raman (ν(Cu2O2) = 608 cm−1, λex = 457.9 nm) spectroscopic properties to a sample prepared independently from O2 and to data in the literature18 (Figure S7). To our knowledge, this is the first report of a bis(μ-oxo)dicopper complex derived from an oxo transfer reagent.5,19 An adduct akin to 4 and 5 is a likely intermediate in the reaction. It is tempting to speculate that the [Cu2(μ-O)2]2+ core is formed via dimerization of a [CuO]+ species, but we recognize that other routes are equally viable, such as ones involving dimerization of Cu(I)-ONMe3 adducts prior to O-N bond cleavage.

In conclusion, explorations of the reactivity of Cu(I) complexes of ligands L1 and L3-L5 with pyridine- and trimethylamine-N-oxides have led to (a) the isolation of novel, stable Cu(I)-N-oxide adducts for L1, L3, and L4, and (b) observation of oxo transfer to the Cu(I) complex of L5 to yield a bis(μ-oxo)dicopper core. Reaction of the Cu(I) complex of L2 with PhIO yields a unique tricopper cluster derived from aryl substituent hydroxylation. The processes of Cu(I)-N-oxide adduct formation, arene hydroxylation, and oxo transfer to yield a [Cu2(μ-O)2]2+ core that we have observed provide important precedence for possible steps in mechanisms of Cu/O mediated reactions.

Supplementary Material

1_si_001

2_si_002

ACKNOWLEDGMENT

We thank Joe B. Gilroy and Robin G. Hicks for providing a sample of ligand L4, L. Que, Jr., for access to the resonance Raman facility, and the NIH (grant GM47365 to W. B. T.) for financial support of this research.

Footnotes

Supporting Information Available: Experimental details, Figures S1-S7 (PDF) and CIFs. This information is available free of charge via the Internet at http://pubs.acs.org/

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