Silver Ion Complexation by a Calix[4]arene bis(Crown Ether): Ambivalence Towards Ether and Polyhapto Coordinations

The synthesis and crystal structure of the Ag + ion complex of calix[4]arene bis(crown-6), BC6, are reported. Ag 2 BC6(NO 3 ) 2 (H 2 O) 1+x (DMF) 0.5 , with x refined to 0.87 (1), crystallizes in the monoclinic space group C2, a = 33.598(2), b = 11.4711(3), c = 13.7755(7) Å, β = 104.585(2)°, V = 5138(2) Å 3 , Z = 4. Refinement led to a final conventional R 1 value of 0.064 for 10401 reflections and 660 parameters. The two silver ions, located in each site of the ditopic calixcrown, are disordered over two positions each, one of them corresponding mainly to a bonding to ether oxygen atoms and the other to a polyhapto bonding with calixarene phenyl rings. Three metal ion positions are located near the crown ether extremity, whereas one of the polyhapto-bonded ions is displaced towards the calixarene cavity. This result provides an illustration of the ambivalent character of silver ions, which may accomodate either ethereal or π-basic coordination sites and of the ability of 1,3-alternate calix[4]crowns to provide both types of environment. Extraction and complexation studies of Ag + in methanol and acetonitrile by BC6 and related calix[4]arene bis(crowns) are also presented. The results show that the 1:1 species only, whose stabilization is purely enthalpic in origin, is present in solution.