Chalcogen Impact on Covalency within Molecular [Cu3(µ3-E)]3+ Clusters (E = O, S, Se): A Synthetic, Spectroscopic, and Computational Study.

Reaction of the tricopper(I)-dinitrogen tris(ß-diketiminate) cyclophane, Cu3(N2)L, with O-atom-transfer reagents or elemental Se affords the oxido-bridged tricopper complex Cu3(µ3-O)L (2) or the corresponding Cu3(µ3-Se)L (4), respectively. For 2 and 4, incorporation of the bridging chalcogen donor was supported by electrospray ionization mass spectrometry and K-edge X-ray absorption spectroscopy (XAS) data. Cu L2,3-edge X-ray absorption data quantify 49.5% Cu 3d character in the lowest unoccupied molecular orbital of 2, with Cu 3d participation decreasing to 33.0% in 4 and 40.8% in the related sulfide cluster Cu3(µ3-S)L (3). Multiedge XAS and UV/visible/near-IR spectra are employed to benchmark density functional theory calculations, which describe the copper-chalcogen interactions as highly covalent across the series of [Cu3(µ-E)]3+ clusters. This result highlights that the metal-ligand covalency is not reserved for more formally oxidized metal centers (i.e., CuIII + O2- vs CuII + O-) but rather is a significant contributor even at more typical ligand-field cases (i.e., Cu3II/II/I + E2-). This bonding is reminiscent of that observed in p-block elements rather than in early-transition-metal complexes.

A Tricopper(I) Complex Competent for O Atom Transfer, C-H Bond Activation, and Multiple O2 Activation Steps.

Oxygenation of a tricopper(I) cyclophanate (1) affords reactive transients competent for C-H bond activation and O atom transfer to various substrates (including toluene, dihydroanthracene, and ethylmethylsulfide) based on 1H NMR, gas chromatography/mass spectrometry (MS), and electrospray ionization (ESI)/MS data. Low product yields (<1%) are determined for C-H activation substrates (e.g, toluene, ethylbenzene), which we attribute to competitive ligand oxidation. The combined stopped-flow UV/visible, electron paramagnetic resonance, ESI/MS, 1H NMR, and density functional theory (DFT) results for reaction of 1 with O2 are consistent with transient peroxo- and di(oxo)-bridged intermediates. DFT calculations elucidate a concerted proton-coupled electron transfer from toluene to the di(µ-oxo) intermediate and subsequent radical rebound as the C-H activation mechanism. Our results support a multicopper oxidase-like mechanism for O2 activation by 1, traversing species similar to the coplanar Cu3O2 unit in the peroxy and native intermediates.

Countercations and Solvent Influence CO2 Reduction to Oxalate by Chalcogen-Bridged Tricopper Cyclophanates.

One-electron reduction of Cu3EL (L3- = tris(ß-diketiminate)cyclophane, and E = S, Se) affords [Cu3EL]-, which reacts with CO2 to yield exclusively C2O42- (95% yield, TON = 24) and regenerate Cu3EL. Stopped-flow UV/visible data support an A→B mechanism under pseudo-first-order conditions ( kobs, 298K = 115(2) s-1), which is 106 larger than those for reported copper complexes. The kobs values are dependent on the countercation and solvent (e.g., kobs is greater for [K(18-crown-6)]+ vs (Ph3P)2N+, and there is a 20-fold decrease in kobs in THF vs DMF). Our results suggest a mechanism in which cations and solvent influence the stability of the transition state.

A family of tri- and dimetallic pyridine dicarboxamide cryptates: unusual O,N,O-coordination and facile access to secondary coordination sphere hydrogen bonding interactions.

A series of tri- and dimetallic metal complexes of pyridine dicarboxamide cryptates are reported in which changes to the base and metal source result in diverse structure types. Addition of strong bases, such as KH or KN(SiMe3)2, followed by divalent metal halides allows direct access to trinuclear complexes in which each metal center is coordinated by a dianionic N,N,N-chelate of each arm. These complexes bind a guest K(+) cation within the central cavity in a trigonal planar coordination environment. Minor changes to the solvent and equivalents of base used in the syntheses of the triiron(II) and tricobalt(II) complexes affords two trinuclear clusters with atypical O,N,O-coordination by each pyridine dicarboxamide arm; the amide carbonyl O atoms are oriented toward the interior of the cavity to coordinate to each metal center. Finally, varying the base enables the selective synthesis of dinuclear nickel(II) and copper(II) complexes in which one pyridine dicarboxamide arm remains protonated. These amide protons are at one end of a hydrogen bonding network that extends throughout the internal cavity and terminates at a metal bound hydroxide, carbonate, or bicarbonate donor. In the dinickel complex, the bicarbonate cannot be liberated as CO2 either thermally or upon sparging with N2, which differs from previously reported monometallic complexes. The carbonate or bicarbonate ligands likely arise from sequestration of atmospheric CO2 based on the observed reaction of the di(hydroxonickel) analog.

Modeling biological copper clusters: synthesis of a tricopper complex, and its chloride- and sulfide-bridged congeners.

The synthesis and characterization of a family of tricopper clusters housed within a tris(ß-diketimine) cyclophane ligand (H3L) that bear structural similarities to biological copper clusters are reported. In all complexes, each Cu atom is held within the N2-chelate of a single ß-diketiminate arm. Reaction of L(3-) with CuCl affords an anionic complex containing a µ3-chloride donor in the central cavity, whereas there is no evidence for bromide incorporation in the product of the reaction of L(3-) with CuBr (Cu3L). Cu3L reacts with elemental sulfur to generate the corresponding air-stable mixed-valent (µ3-sulfido)tricopper complex, Cu3(µ3-S)L, which represents the first example of a sulfide-bridged copper cluster in which each metal center is both coordinatively unsaturated and held within a N-rich environment. The calculated LUMO is predominantly Cu-S π* in character and delocalized over all three metal centers, which is consistent with the isotropic ten-line absorption (g ∼ 2.095, A ∼ 33 G) observed at room temperature in EPR spectra of the one-electron chemically reduced complex, [Cu3(µ3-S)L](-).