Cryptand complexes with group 16 metals

The crown ethers and cryptands generally form more stable complexes with hard metal ions, such as the group 1, 2, and lanthanide ions. The introduction of softer heteroatoms into the macrocycle can change the selectivity dramatically. There is much interest in tuning macrocycles in this way to favor complexation of certain metal atoms, but O-donor macrocycles will dominate the discussion here. 

The l,l -(OCH2CH20Ts)2-ferrocene derivative was treated with various diaza-[n]-crown-m ethers to give ferrocene cryptands, in which subsequent complexation with Group 1 and 2 metal ions affords large shifts in the redox potentials and consequently to a drastic decrease in the binding strength (up to 10 ) <01CEJ4438>. 

The first cylindrical macrotricyclic ligands synthesized were (52a) and SSa-d).58 70 Cryptands in which the two monocycles are even farther apart as a result of bridging naphthyl, biphenyl and. related groups have also been reported.188,1 9 The smaller macrocycle (52a) forms complexes with a variety of metal cations, including two silver(I) ions.69,70 190 Crystal data results for the latter complex indicate both Ag+ ions are located slightly out of the plane of the macrocycles (undoubtedly the result of macrocyclic size constraints), but within the central main cavity, with an Ag—Ag distance of 3.88 A.191... 

Traces of water in our solutions of monomeric praseodymium cryptate are most probably responsible for the formation of the dimeric complex reported here. Partial hydrolysis of this complex takes place because the excess of (2.2.1) cryptand brings about a pH increase. Incomplete hydrolysis of a lanthanide macrocyclic complex has also been noted by Biinzli et al. [14] who prepared a dimeric praseodymium complex with 1,4,7,10,13-pentaoxacyclododecane (15-crown-5) by dehydrating in vacuo a monomeric species. The metal ions in this dimer are bridged by only one hydroxyl group and by three trifluoroacetate anions. The distance between the two praseodymium ions in the (2.2.1) cryptate reported here is 3.927(1) A this value compares very well with the values reported for the two other dinuclear lanthanide complexes mentioned above [13-14]. 

Introduction.—The ability of certain molecules, such as the macrocyclic crown ethers, e.g. 18-crown-6 (28), and the macrobicyclic cryptands, e.g. [2,2,2] cryptand (29), to form complexes with metal and ammonium cations has been extensively investigated in recent years. Since the original discovery by Pedersen, in 1967, of the crown group,some reviews and many papers have appeared on the syntheses and complexing properties of different classes of ligands, but it is not the intention here to go into detail concerning these aspects. Laboratory syntheses of the polyether class are dependent on the Williamson ether synthesis (Equation 11), but methods for production of commonly used compounds, such as (28), have been improved, and many representatives of both the crown and cryptand groups are now commercially available. 

---