Crown block metals

Where the lanthanide ionic radius and the macrocyclic cavity are incompatible, though in hydrous conditions the crown ether is likely to be displaced by water ligands, the crown may still be present in the structure of the crystal as a hydrogen-bonded adduct. This behaviour is seen in [Gd(N03)3(H20)3]-(18-crown-6).445 This type of compound is quite well known in the case of s block metals also, e.g. [Mg(H20)6]Cl2 (12-crown-4)454 and, a more subtle case, [Ca(nitrobenzoate)2(benzo-15-crown-5)]-3H20(benzo-15-crown-5)455 in which an apparent 2 1 complex has only half its crown ligand coordinated to Ca2+. 

E R. E. Mulvey, s-Block metal inverse crowns synthetic and structural synergism in mixed alkali metal-magnesium (or zinc) amide chemistry. Chem. Commun. 1049-56 (2001). 

Fig. 12 Evolution of ligands containing sp3 oxygen donor atoms. Only crown-ethers (e.g. 8, 18-crown-6) and cryptands (e.g. 9, 2.2.2-crypt) form stable complexes with s-block metal ions...

Takeda, Y., Ikeo, N., and Sakata, N. Thermodynamic study of solvent extraction of 15-crown-5- and 16-crown-6-s-block metal ion complexes and tetraalkylammonium ions with picrate anions into chloroform. Talanta 1991, 38, 1325-1333. 

Crown ether complexes of the -block metals number in the many hundreds, " and reviews focused on them, including their use in separation chemistry and selective ion extractions, are extensive. Growing interest has been expressed in the use of macrocyclic ethers in the design of electroactive polymers. ... 

K(18-crown-6)]2K[Bi(SCN)6], which exists in two isomeric forms." In the yellow form, octahedral [Bi(SCN)6] " anions are surrounded by four K[K(18-crown-6)]2 units in a layered arrangement all the thiocyanates are A -bound to potassium. In the yellow form, the thiocyanatobismuth anions have four 5-bonded and two trans Ai-bonded thiocyanate ligands, and are arranged in parallel columns separated by K+ cations each potassium has two trans 5-bound and four Ai-bound thiocyanate ligands. Two [K(18-crown-6)] units are located between the columns." Structurally characterized selenocyanate complexes of the i-block metals are much rarer, but as befits the softer nature of selenium relative to sulfur, all known examples are Ai-bound." " " ... 

Cation polarizability is also a factor in selectivity. Kodama et al studied the complexation of aza-18-crown-6 with alkali, alkaline earth, and some d-block metals. This ligand showed higher selectivity toward the more polarizable, soft d-block metal ions than the hard, nonpolarizable alkali and alkaline earth metal cations. 

Te donor atoms, and mixed donor derivatives where the donor types are both chalcogens. Therefore, while crown ethers themselves are excluded, mixed donor S/O, Se/O, and other macrocycles are included, whereas those containing, for example, S/N or S/P donor atoms are excluded. In addition, we have tried to present an overview of the coordination chemistry of the hetero-crown ethers toward a range of acceptors grouped under (i) d- and f-block metal ions, and (ii) p-block elements (groups 13-17), emphasizing in particular the variety of coordination modes observed. It is not our intention here to revisit work described prior to 2000 unless it is particularly relevant to more recent work. 

The formation of such complexes is analogous to the formation of those of s- and p-block metals and discussed in previous chapters, e.g. [K(18-crown-6)], ... 

Beside four-coordinate Be2+ structures, a couple of five-coordinate structures are known, e.g., Dehnicke s [BeCl(12-crown-4)]+ (179). The combination of beryllium cations and different crown ethers is of particular interest, not only because the Nobel Prize was awarded in part for the development of crown ethers (15), but also because crown ethers are common building blocks in all kinds of chemistry (180,181), with a wide range of applications. Since Be2+ is the smallest metal ion, the binding modes of Be2+ and crown ethers can lead to unexpected structural motifs. 

Block copolymers of (R,S)-(3-butyrolactone and eCL have been synthesized by combining the anionic ROP of the first monomer with the coordinative ROP of the second one (Scheme 15) [71]. The first step consisted of the synthesis of hydroxy-terminated atactic P(3BL by anionic polymerization initiated by the alkali-metal salt of a hydroxycarboxylic acid complexed with a crown ether. The hydroxyl end group of P(3BL could then be reacted with AlEt3 to form a macroinitiator for the eCL ROP. 

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