The Coordination Behavior of Oxygen-depleted Calixarenes towards d10 Noble Metal Ions

A calix[4]arene, in which two of the phenol functions are replaced by pyrazole units, [H-2(bpzCal)], was investigated as a ligand for Cu+, Ag+ and Au+ ions. Using [Cu(MeCN)(4)]BF4 and AgSbF6 as the precursors, complexes [MH2(bpzCal)]X (M = Cu, X = BF4; M = Ag, X = SbF6) were formed, where the calixarene ligands adopt a 1,3-alternate structure and the metal ions are coordinated linearly by the two pyrazolyl donors. [CuH2(bpzCal)]BF4 displayed a - for copper(I) complexes - unusual stability towards O-2, which is due to the steric protection of the Cu-I center. By contrast a dinuclear copper(I) complex [Cu-2(bpzCal)] that was obtained through treatment of [H-2(bpzCal)] with two equivalents of Cu(HMDS) is rather sensitive towards O-2. The preparation of a gold complex required the employment of a gold precursor, which contains one labile and one stabilizing neutral ligand, namely [(PPh3)Au(NCMe)]SbF6, which led to the formation of [(PPh3)AuH2(bpzCal)]SbF6. In this complex [H-2(bpzCal)] acts only as a monodentate ligand for the gold center. Taken together, the results demonstrate the potential of [H-2(bpzCal)] in providing rather different coordination spheres for metal ions.