Copper bimetallic complexes

A new application of bis(oxazolines) ligands was reported by Reiser et al. who obtained some excellent results, such as that depicted in Scheme 2.14 for the 1,4-addition of ZnEt2 to 2-cyclohexenone. These authors used a bimetallic complex in which the substrate was locked in a two-point binding mode via a zinc atom and a copper atom. 

PhI=NTs in MeCN affords a copper species that is indistinguishable by ultraviolet-visible (UV-vis) spectroscopy from an identical solution derived from Cu(OTf)2. Given the strong oxidizing nature of PhI=NTs, it seems likely that both catalysts proceed through a Cu(II) species. Beyond this, little can be said with certainty. If nitrenoid formation proceeds by a two-electron oxidation of the catalyst, one would need to invoke Cu(IV) as an intermediate in this process (77). This issue is resolved if one invokes the intervention of a bimetallic complex in the catalytic cycle. However, attempted observation of a nonlinear effect revealed a linear relationship between ligand enantiopurity and product ee (77, 78). 

Serendipitous formation of bimetallic complexes 55, 56 with a /7 -coordi-nated phosphonio-benzophospholides was observed in the reaction of the zwitterion 26 with [M2(CO)io] (M=Mn, Re) [43]. Complexes 56a,b were characterised by spectroscopic data and single-crystal X-ray diffraction studies whose results pointed to a low degree of d(M) r (L) charge-transfer and thus low metallacycle character, suggesting that, similar as in the copper r-complex 51, the coordinated double bond acts predominantly as n-donor rather than an as n-acceptor [43]. 

While today the construction of such bidentate phosphine ligands with the use of an assembly metal is referred to as supramolecular chemistry, this is not to say that there are no examples in older literature utilizing this principle. Actually, there are many and using as a search term hetero bimetallic complexes (a named coined in the early 1980s) leads us to a plethora of complexes. For instance in complex 29, reported by Rauchfuss in 1982, one would consider copper as the assembly metal and platinum as the potential catalytic metal [70]. 

Reaction of 124 with copper(i) proceeded readily and gave the desired double-helix complex 125, but in rather modest yield ca. 15%). Transformation of this bimetallic complex 125 into the knot-like structure required the interconnection of opposite ends of the intertwined threads, as is shown in Scheme 4.41. To achieve this goal, 125 was treated with the diiodo derivative of hexaethylene glycol 126. This reaction turned out to be non-selective and a... 

A series of hexa-amines (27) has been reported for which the ionization behavior has been determined, together with the microscopic or intrinsic protonation constants for the different chemical environment and ionic strengths.37 A study carried out on copper(II) complexes of hexa-amines (27) has revealed how a preference for bimetallic, bis-tridentate coordination of the hexa-amine ligands in this series may be promoted either by extension of the bridging polymethylene chain (disfavoring chelation), or by A-methylation of the terminal amine groups (steric effects).38... 

S. Chandra, D. Jain, and A. K. Sharma, EPR, mass, electronic, IR spectroscopic and thermal studies of bimetallic copper(II) complexes with tetraden-tate ligand, 1,4-diformyl piperazine bis(carbohydrazone), Spectrochimica Acta Part A, vol. 71, no. 5, pp. 1712-1719, 2009. 

As mentioned before, a worthwhile model must possess a good accessibility of both the Cu(I) and Cu(II) oxidation states, and retain in solution not only the bimetallic unit but also any bridging group. To detect whether a proposed model of a Type III copper centre satisfies these requirements many studies on the cyclic voltammetry and/or the differential pulse polarography have been carried out and reported [157-210]. In some cases authors started with a di-copper(II) complex, in some other with a di-copper(I) complex and even eventually with a mixed-valence Cu(II)-Cu(I) dinuclear complex. 

Reaction with O2 (and CO) is shown to be reversible in the solid state although in solution (MeNOz) oxygenation of the bimetallic complex is more complicated. A report on the reaction of blue oxidases with Og indicates that the optical and e.p.r. absorptions correlate well. The reaction time with fully reduced fungal laccase is 1 s and for the Rhus enzyme it is 10 s. Since only one line of the e.p.r. spectrum may be observed because of overlapping copper signals, identification of the oxygen species involved is diflBcult but the most likely candidate at present is the ion 0. ... 

---