Copper complexes dimeric

The acetylide anion 3 is likely to form an alkynyl-copper complex by reaction with the cupric salt. By electron transfer the copper-II ion is reduced, while the acetylenic ligands dimerize to yield the -acetylene 2 ... 

The catalytic asymmetric cyclopropanation of an alkene, a reaction which was studied as early as 1966 by Nozaki and Noyori,63 is used in a commercial synthesis of ethyl (+)-(lS)-2,2-dimethylcyclo-propanecarboxylate (18) by the Sumitomo Chemical Company (see Scheme 5).64 In Aratani s Sumitomo Process, ethyl diazoacetate is decomposed in the presence of isobutene (16) and a catalytic amount of the dimeric chiral copper complex 17. Compound 18, produced in 92 % ee, is a key intermediate in Merck s commercial synthesis of cilastatin (19). The latter compound is a reversible... 

The oxidation of phenol, ortho/meta cresols and tyrosine with Oj over copper acetate-based catalysts at 298 K is shown in Table 3 [7]. In all the cases, the main product was the ortho hydroxylated diphenol product (and the corresponding orthoquinones). Again, the catalytic efficiency (turnover numbers) of the copper atoms are higher in the encapsulated state compared to that in the "neat" copper acetate. From a linear correlation observed [7] between the concentration of the copper acetate dimers in the molecular sieves (from ESR spectroscopic data) and the conversion of various phenols (Fig. 5), we had postulated [8] that dimeric copper atoms are the active sites in the activation of dioxygen in zeolite catalysts containing encapsulated copper acetate complexes. The high substratespecificity (for mono-... 

Fig. 10.6. Dimeric (Ar = 2,6-dimethylphenyl) (a) and monomeric (Ar = 2,4,6-trimethylphenyl) (b) copper complexes with diphenylcarbene. Reproduced from J. Am. Chem. Soc., 126, 10085 (2004), by permission of the American Chemical Society.

Using a triazole-/pyrimidine-containing hybrid ligand, terminally coordinated in a thiocyanate-bridged copper(II) dimer, Haasnoot et al. reported250 the structure of the f3 isomer of complex (288), revealing that the Cu-SCN distance is relatively short compared to other Cu-SCN... 

The neutral 3 dx metallocenes are thus known for x = 3 — 8, but the d9 copper complex has thus far resisted preparation, and the d2 titanocene has been found (54) to be both diamagnetic and dimeric, and is therefore excluded from consideration here. A number of cationic species, corresponding formally to Ti(Cp)2+, and V(Cp)2+, systems are however well known, but it seems very probable that these do not possess pseudo-axial symmetry (see (41) for further discussion), and very recently it has been demonstrated (55) that stable V(Cp)2+ complexes cannot be isolated without the coordination of an additional ligand to the metal. The parent systems are therefore limited to V(Cp)2, Cr(Cp)2, Mn(Cp)2, Fe(Cp)2, Co(Cp)2, and Ni(Cp)2 and the cationic species to Cr(Cp)2+, Fe(Cp)2+, Co(Cp)2+, and Ni (Cp)2+> and the d-d spectra of these systems are now considered individually. 

Figure 3 Dimeric monoalkyl copper complexes, Cu[/j-NRC BuC(H)R]2 5 (R=SiMe3) and Cu2(2-C(SiMe3)2-6-MePy)2 6. Compound 5 Hitchcock, P. B. Lappert, M. F. Layh, M. Dalton Trans. 1998, 1619 - reproduced by permission of The Royal Society of Chemistry. Compound 6 Van den Anckor, T. R. Bhangava, S. K. Mohr, F. Papadopoulos, S. Raston, C. L. Skelton, B. W. White, A. H. Dalton Trans. 2001, 3069 - reproduced by permission of The Royal Society of Chemistry.

Exchange interactions in heterodinuclear transition metal complexes have attracted the attention of many researchers in the last few years. Nickel(II)-copper(II) dimers are, in a sense, the simplest systems to be investigated and several complexes containing paramagnetic nickel(II) and copper(II) ions have been reported, as pure complexes2985-2987 or as impurities in a parent lattice.2959,2987"2991 Magnetic susceptibility or EPR spectroscopy has been used to... 

The ligands (20) contain a coordination site such as pyridyl or amino nitrogen, or a hydroxyl oxygen. Complexes of (20 T = NH2 and n = 2) with iron(II) have been studied recently.65 A copper(II) dimer (with bridging oxime groups) of (20 T = OH, n = 2 or 3) was described by Ablov and coworkers66 in 1972 and its crystal structure has been elucidated67 in 1974. 

Copper-catalyzed oxidations of phenols by dioxygen have attracted considerable interest owing to their relevance to enzymic tyrosinases (which transform phenols into o-quinones equation 24) and laccases (which dimerize or polymerize diphenols),67 and owing to their importance for the synthesis of specialty polymers [poly(phenylene oxides)]599 and fine chemicals (p-benzoquinones, muconic acid). A wide variety of oxidative transformations of phenols can be accomplished in the presence of copper complexes, depending on the reaction conditions, the phenol substituents and the copper catalyst.56... 

A combination of the two techniques was shown to be a useful method for the determination of solution structures of weakly coupled dicopper(II) complexes (Fig. 9.4)[119]. The MM-EPR approach involves a conformational analysis of the dimeric structure, the simulation of the EPR spectrum with the geometric parameters resulting from the calculated structures and spin hamiltonian parameters derived from similar complexes, and the refinement of the structure by successive molecular mechanics calculation and EPR simulation cycles. This method was successfully tested with two dinuclear complexes with known X-ray structures and applied to the determination of a copper(II) dimer with unknown structure (Fig. 9.5 and Table 9.9)[119]. 

An unusual reaction of a diazolium salt was reported during an attempt to form copper complexes of the NHC.100 The isolated products showed ring expansion of the carbene to form six-membered lactams. The authors verified the product by X-ray crystallography and independent synthesis. It is not clear where the extra carbon comes from. The authors consider a possible mechanism involving a carbene dimer formation, but point out that the carbenes are very hindered. Possible reaction with atmospheric carbon dioxide was not considered. Direct oxidation of the carbene to give a urea is also noted. 

The coordinatively unsaturated complexes are even less soluble in organic solvents such as diethyl ether, benzene, and chloroform than their saturated equivalents. The magnetic moment of copper complexes of tridentate formazans with a 2-hydroxyaryl substituent at N1 is very low (1.44 iB) compared to the expected value of copper(n) complexes with an unpaired electron and strong covalent bonding. The low solubility and magnetic moment suggest that the dimerization occurs with the formation of a saturated dinuclear complex 38. 

The saturated pyridine adduct 39, which does not dimerize, has a magnetic moment of 1.7 r . The copper complex 40 has an even higher magnetic moment of 1.81 [% [75],... 

Dimethylphenol is oxidatively polymerized to poly(2,6-dimethyl-1,4-phenyl-ene ether) with a copper-amine complex by a laccaselike reaction. The activated phenols are coupled to form a dimer. The dimer is activated by a mechanism similar to that by which the polymerization proceeds. The effects of the amine ligands are to improve the solubility and the stability of the copper complex as well as the phenol-coordinated complex and to control the redox potential of the copper complex. 

The dimerization of the copper complex takes place to form the bis-Cu(II) compound 3, where the phenolate anion is the bridging ligand, just as proposed by Karlin [75], in the dinuclear complexes that act as the actual catalyst in the active state. 

Interestingly, when using copper(I)triflate, the cyclopentadiene dimer 14 reacts in an intermolecular way, leading to the cydobutane 15 (reaction 5) [22], When the same substrate is transformed in the presence of the triplet sensitizer acetone, an intramolecular [2 + 2] cycloaddition takes place and the cage hydrocarbon compound 16 is formed. Obviously, the formation of a copper complex intermediate involving both alkene double bonds of the substrate is unfavorable in this case. 

Nakajima and co-workers have carried out extensive investigations into the influence of different chiral diamine-copper complexes on the oxidative dimerization of naphthols [146-148]. As emerged from Smrcina s work, the inclusion of an ester moiety on the naphthol precursor is an important factor for optimizing the enantioselectivity. After establishing a catalytic cycle with TMDA as the base and showing that sparteine gave promising results (Scheme 57), they focused their work on other chiral diamines (Table 39) [147]. 

---