Copper bridging complexes, structures

A structural study of receptors 51, 52, and 49, showed three different modes of azide complexation to the binuclear copper(II) host (123) 1,1 cascaded, 1,3 cascaded, and noncascaded, respectively. These structures indicate that the nature of the macrocyclic framework of the receptor is important in determining the mode of anion coordination. Figure 5(a and b) shows the crystal structures of 52, 2 Cu(II)-azide, and 52, 2Cu(II)-chloride, respectively, for comparison (129). As can be seen, it is the length of the azide bridge that makes cascade complexation possible, whereas for the smaller mononuclear chloride anion this obviously cannot occur. Receptor 49 has also been shown to cascade bind pyrophosphate [as its bis-copper(II) complex] (130) and sulfate [as its bis-iron(II) complex](131). At about the same time, Nelson and co-workers (132) published a similar bis-copper(II) complex structure that cascaded an azide anion. 

Another illustration of the structural changes that may result as a consequence of alkyl substitution at the 3-position of the pyrazolyl group is provided by the structures of the dimeric copper(I) complexes [Tp]Cu 2 (36), [TpMe2]Cu 2 (36), [TpPh2]Cu 2 (37), and [TpBut]Cu 2 (37), which differ in the manner in which the tris(pyrazo-lyDhydroborato ligand bridges the two copper centers (Fig. 16). 

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