We have demonstrated that a bipyrimidinylplatinum-polyoxometalate, [Pt(Mebipym)Cl2]+[H4PV2Mo10O40]-, supported on silica is an active catalyst for the aerobic oxidation of methane to methanol in water under mild reaction conditions. Further oxidation of methanol yields acetaldehyde. The presence of the polyoxometalate is presumed to allow the facile oxidation of a Pt(II) intermediate to a Pt(IV) intermediate and to aid in the addition of methane to the Pt catalytic center.

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...

Hybrid compounds with two functional centers consisting of a metallosalen moiety (M[bond]salen; M = Mn, Co, Ni, and Pd) connected by an alkylene bridging group to a lacunary Keggin type polyoxometalate were synthesized and characterized. In these metallosalen-polyoxometalate compounds (M[bond]salen[bond]POM) it was shown by the use of a combination of UV[bond]vis, (1)H NMR, EPR, XPS, and cyclic voltammetry measurements that the polyoxometalate exerts a significant intramolecular electronic effect on the metallosalen moiety leading to formation of an oxidized metallosalen moiety. For the Mn[bond]salen[bond]POM, the metallosalen center is best described as a metal[bond]salen cation radical species; that is, a localized "hole" is formed on the salen ligand. For the other M[bond]salen[bond]POM compounds, the metallosalen moiety can be described as a hybrid of a metal[bond]salen cation radical species and an oxidized metal[bond]salen species, that is, a delocalized "hole" is formed at the metallosalen center. It is proposed that these oxidized metallosalen centers are best characterized as stabilized charge transfer (metallosalen donor[bond]polyoxometalate acceptor) complexes despite the relatively large distance between the two functional centers.

In order to assess supporting ligand effects on S-S bond activation, a series of [Cu 2 (μ-η 2 :η 2 -S 2 )] 2+ complexes supported by various β-diketiminate or anilido-imine ligands (L) were synthesized via the reaction of Cu(I) precursors LCu(CH 3 CN) with S 8 . For the cases where L = β-diketiminate, the syntheses were complicated by formation of clusters [Cu(SR)] 4 , where "SR" represents the ligand functionalized by sulfur at the central methine position. The [Cu 2 (μ-η 2 :η 2 -S 2 )] 2+ products were characterized by X-ray crystallography and electronic absorption and resonance Raman spectroscopy. Correlations among the Cu-S, Cu-Cu, and S-S distances and the ν(S-S) values were observed and interpreted within the framework of a previously described bonding picture. Comparison of these data to those for other relevant species revealed a remarkable degree of S-S bond activation in the compounds supported by the β-diketiminate and anilido-imine ligands, which through strong electron donation increase backbonding from the copper ions into the S-S σ* orbital and cause S-S bond weakening. Reactions of one of the complexes supported by an anilido-imine ligand with PPh 3 and xylyl isocyanide were explored, revealing facile transfer of sulfur to PPh 3 but only displacement of sulfur to yield a LCu(I)-CNAr (Ar = xylyl) complex with the isocyanide.

A phenanthroline ligand has been covalently modified at the 2 and 9 positions by an aminophenylhexamolybdate substituent. The 1H NMR spectrum indicated a strong electron-withdrawing effect of the hexamolybdate (Mo6O19(2-)) moiety on the phenanthroline ligand. UV-vis and cyclic voltammetry showed extended conjugation of the hybrid phenanthroline-polyoxometalate compound and the possibility of easy oxidation of the extended phenanathroline ligand. Further EPR experiments provided strong evidence for an intramolecular charge-transfer process with the formation of a phenanthroline cation radical and a reduced hexamolybdate.