Insight into copper-oxygen species proposed as intermediates in oxidation catalysis is provided by the identification of a Cu(II)-superoxide complex supported by a sterically hindered, pyridinedicarboxamide ligand. A tetragonal, end-on superoxide structure is proposed based on DFT calculations and UV-vis, NMR, EPR, and resonance Raman spectroscopy. The complex yields a trans-1,2-peroxodicopper(II) species upon reaction with [(tmpa)Cu(CH 3 CN)]OTf, and, unlike other known Cu(II)-superoxide complexes, acts as a base rather than an electrophilic (Hatom abstracting) reagent in reactions with phenols.An important first step in copper-promoted aerobic oxidations in biology1 and catalysis2 is the formation of a 1:1 Cu/O 2 adduct, in which the O 2 molecule is activated for subsequent reactions, either with substrate or to form different copper-oxygen species. In one approach aimed at understanding such adducts, synthetic 1:1 Cu/O 2 complexes have been targeted for detailed structural, spectroscopic, and reactivity studies.3 To date, three types have been identified: (a) end-on, triplet Cu(II)-superoxos supported by tetradentate tripodal4 or, in one case, tridentate5 N-donor ligands, (b) side-on, singlet Cu(II)-superoxos supported by facially coordinating tris(pyrazolyl)hydroborates,6 and (c) side-on, singlet Cu(III)-peroxos supported by strongly electron-donating, bidentate β-diketiminates or anilido-imines.3 , 7 A key finding from reactivity studies of type (a) compounds is that they are electrophilic, with the ability to perform biologically relevant H-atom abstractions from phenols and weak C-H bonds; 4b , 5 investigations of the reactivity of type (b) compounds have not been reported, and type (c) compounds are relatively unreactive with external organic substrates. Herein we report that in seeking to expand the repertoire of available 1:1 Cu/O 2 structures for comparative evaluations, we have discovered an end-on Cu(II)-superoxide complex that displays unique characteristics, including a tetragonal geometry and non-electrophilic reactivity.Inspired by a recent report,8 we prepared complex 1 (Scheme 1) by treating N,N′-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide9 with NaOMe followed by CuCl 2 in the presence of CH 3 CN.10 In methanol or THF solution, 1 is green, whereas it is red-brown in the presence of CH 3 CN or pyridine. Consistent with this solvatochromism (Figure S2), attempts to obtain crystals of 1 suitable for X-ray crystallographic analysis were complicated by apparent CH 3 CN lability. The addition of 4-tBu-pyridine, however, yielded X-ray quality dark red crystals of 2 (Scheme 1). The complex is square planar with a geometry similar to other known 2,6-pyridinedicarboxamide Cu(II) complexes.11 wtolman@umn.edu. Supporting Information Available: Experimental procedures, spectra, computational details (PDF), and CIF. This material is available free of charge via the Internet at http://pubs.acs.org. NIH Public AccessAuthor Manuscript J Am Chem Soc. Author manuscript; available in PMC 2011 Nov...

The greenhouse gas N 2 O is converted to N 2 by a µ-sulfido-tetracopper active site in the enzyme nitrous oxide reductase (N 2 OR) via a process postulated to involve µ-1,3 coordination of N 2 O to two Cu(I) ions. In efforts to develop synthetic models of the site with which to test mechanistic hypotheses, we have prepared a localized mixed valent Cu(II)Cu(I) 2 cluster bridged in µ-η 2 :η 1 :η 1 fashion by disulfide, [L 3 Cu 3 (µ 3 -S 2 )]X 2 (L = 1,4,7-trimethyl-triazacyclononane, X = O 3 SCF 3 − or SbF 6 − ). This cluster exhibits spectroscopic features similar to those of the active site in N 2 OR and reacts with N 2 O to yield N 2 in a reaction that models the function of the enzyme. Computations implicate a transition state structure that features µ-1,1-bridging of N 2 O via its O-atom to a [L 2 Cu 2 (µ-S 2 )] + fragment and provide chemical precedence for an alternative pathway for N 2 O reduction by N 2 OR.Nitrous oxide (N 2 O) is an important greenhouse gas and component of the global nitrogen cycle. 1 Its reduction to dinitrogen (N 2 ) is thermodynamically favorable (E° = 1.76 V), making it attractive as an environmentally benign oxidant, yet its utility in this regard is limited by high kinetic barriers that limit reaction rates. Transition metals facilitate the reduction of N 2 O, although in most heterogeneous catalytic systems high temperatures are required 2 and homogeneous processes that operate under mild conditions generally use highly reducing lowvalent metal complexes. [3][4][5] In Nature, conversion of N 2 O to N 2 and H 2 O is catalyzed under ambient conditions during microbial dentrification by the metalloenzyme nitrous oxide reductase, N 2 OR. 6 X-ray crystallographic, 7 spectroscopic, and theoretical studies 8 have identified the active site of N 2 OR as a µ-sulfido-tetracopper cluster without precedent in biology or synthetic chemistry, which cycles through tetracopper(I) and mixed-valent states during catalysis. 9 A provocative mechanism for N 2 O reduction has been suggested that involves µ-1,3-coordination and bending of N 2 O between two of the copper ions in the fully reduced (all copper(I)) cluster, with the µ-sulfide acting to facilitate electron delocalization during the redox process. (Figure 1). The structures and spectroscopic properties of these complexes are similar to each other and to those of others with antiferromagnetically coupled (µ-η 2 :η 2 -disulfido)dicopper(II) cores. 11b, 12 For example, they are EPR silent and exhibit an intense S 2 2− → Cu(II) charge transfer transition at ~395 nm (ε 15,000 M −1 cm −1 , Figure 1c, red line), excitation into which (λ ex 406.7 or 457.0 nm) results in resonance enhancement of a peak in the Raman spectrum at ~431 cm −1 (Δ 34 S = 19 cm −1 ) attributable to an S-S stretching mode.Monitoring the reactions by UV-vis spectroscopy revealed the formation and subsequent decay (at room temperature, t 1/2 ~ 45 min) of an intermediate with λ max = 634 (1a) or 631 (1b) nm, respectively, the lifetime of which can be extended significa...

We report the results of a comprehensive investigation of the recently discovered stereoselective and controlled polymerization of racemic lactide (D,L-LA) using an initiator prepared in situ from indium(III) chloride (InCl(3)), benzyl alcohol (BnOH), and triethylamine (NEt(3)). Linear relationships between number-average molecular weight (M(n)) and both monomer to alcohol concentration ratio and monomer conversion are consistent with a well-controlled polymerization. Studies on polymerization kinetics show the process to be first-order in [InCl(3)](0) and zero-order in both [BnOH](0) and [NEt(3)](0). The rate of D,L-LA conversion is also dependent on the indium(III) halide (i.e., t(1/2)(InCl(3)) approximately = 43 min versus t(1/2)(InBr(3)) approximately = 7.5 h, 21 degrees C, CD(2)Cl(2), [D,L-LA](0)/[BnOH](0) approximately = 100, [D,L-LA](0) = 0.84 M, [InX(3)](0)/[BnOH](0) = 1) and lactide stereoisomer (i.e., k(obs)(D,L-LA) approximately = k(obs)(meso-LA) > k(obs)(L-LA)). A model system that polymerizes D,L-LA with the same high degree of stereoselectivity was developed using 3-diethylamino-1-propanol (deapH) in lieu of BnOH and NEt(3). The product of the reaction of deapH with InCl(3) was identified as [InCl(3)(deapH)(H(2)O)](2) by elemental analysis, X-ray crystallography, and NMR and FTIR spectroscopies. An anhydrous version of the complex was also isolated when care was taken to avoid adventitious water, and was shown by pulsed gradient spin-echo (PGSE) NMR experiments to adopt a dinuclear structure in CD(2)Cl(2) solution under conditions identical to those used in its stereoselective polymerization of D,L-LA. The combined data suggest that the initiating species for the InCl(3)/BnOH/NEt(3) system is similar to [InCl(3)(deapH)(H(2)O)](2) and of the type [InCl((3-n))(OBn)(n)](m). With this information we propose a mechanism that rationalizes the observed stereocontrol in D,L-LA polymerizations. Finally, in an exploration of the scope of the InCl(3)/BnOH/NEt(3) system, we found this system to be effective for the polymerization of other cyclic esters, including epsilon-caprolactone and several substituted derivatives.

Copper(I) complexes of a diketiminate featuring CF 3 groups on the backbone and dimethylphenyl substituents (4) and a nitroformazan (5) were synthesized and shown by spectroscopy, X-ray crystallography, cyclic voltammetry, and theory to contain Cu(I) sites electron deficient relative to those supported by previously studied diketiminate complexes comprising alkyl or aryl backbone substituents. Despite their electron poor nature, oxygenation of LCu(CH 3 CN) (L = 4 or 5) at room temperature yielded bis(hydroxo)dicopper(II) compounds and at − 80 °C yielded bis(μ-oxo)dicopper complexes that were identified on the basis of UV-vis and resonance Raman spectroscopy, spectrophotometric titration results (2:1 Cu/O 2 ratio), EPR silence, and DFT calculations. The bis (μ-oxo)dicopper complex supported by 5 exhibited unusual spectroscopic properties and decayed via a novel intermediate proposed to be a metallaverdazyl radical complex, findings which highlight the potential for the formazan ligand to exhibit 'non-innocent' behavior.