Triorganotin carboxylates

Publications concerning pendant triorganotin polymers are limited to synthetic methods and applications (for a recent review, see reference 5 ). There are also several pertinent reports on the chemistry of monomeric triorganotin carboxylates, alkoxides, and halides (6-8). 

Tin is thought to be an essential trace element for some species, although its precise role remains unknown. Some therapeutic uses of tin compounds have been proposed, and triorganotin carboxylates are effective bacteriocides and pesticides. Tin is, of course, an important component of a number of alloys, with copper in bronze (the Bronze age began about 3500 BC), and with lead in pewter. 

The tin-oxygen interatomic distances present in organotin carboxylates were classified in terms of primary Sn—O covalent bonds ca 2.0 A), slightly longer dative Sn—O bonds (ca 2.2-2.3 A) and Sn- -O secondary interactions (>2.5 A)214. Triorganotin carboxylates can adopt the three idealised structure types 124a-c. 

Triorganotin carboxylates may exist in monomeric or polymeric forms, while diorganotin derivatives may exist as true dicarboxylates or as distannooxane salts [(R2SnO-COR )20] and may further aggregate in a number of ways that influence both solubility and bioavailability. More work on the molecular basis for the activity of tin complexes is needed. 

In the crystalline state, triorganotin carboxylates generally adopt either a polymeric structure with a five-coordinated tin atom (type 383) or a monomeric structure varying from a purely tetrahedral four-coordinate geometry (type 384) to a similar one with a weak additional intramolecular coordination from the carbonyl oxygen to the tin atom (type 385)107,266,778.779... 

Most triorganotin carboxylates are stable to hydrolysis however, R2Sn(02CR )2 andRSn(02CR )3 show a progressive... 

Among organotin carboxylates, clusters and cages are formed mainly in di- and monoorganotin compounds. Among triorganotin carboxylates, the predominant structures are chain and discrete structures, although some macrocycles are also known. ... 

Triorganotin carboxylates, R3Sn02CR, formed generally in the reactions of RsSnOH or (R3Sn)20 with a carboxylic acid R C02H, usually possess two main types of structures (a) chain structures (b) discrete structures. 

Polymeric or chain structures are the most common stmctural types known for triorganotin carboxylates. These are formed in three situations (Figure 2.4.1) ... 

Figure 2.4.2 Representative examples of various structural forms of triorganotin carboxylates (a)-(c) represent discrete structures (d) represents a chain structure where the carboxylate ligand bridges two tin centers (e) represents a chain structure where a heteroatom and a carboxylate ligand bridge two tin centers (f) represents a dimeric structure and (g)-(h) represent macrocyclic structures.

The use of dative bonds to build supramolecular architectures is not limited to the self-assembly of cyclic and cage supermolecules, as illustrated above. Subsequent sections of this book will present numerous other examples, e.g. the self-assembly of triorganotin carboxylates, oxocarboxylates, or phosphinates into intricate supramolecular arrays (Section 3.3). 

Triorganotin carboxylates tend to be monomers (.142 and 143) only with aromatic carboxylic acids (substituted benzoates) or with bulky substituents at tin (e.g. Ph or Cy) [420]. Most other organotin carboxylates are supramolecular aggregates, usually chain polymers with carboxylato bridging. 

A selection of structurally characterized triorganotin carboxylates is listed in Table 3.13, in order of complexity of the organic groups at tin. The Sn-O and Sn O bond lengths and O-Sn O axial bond angles are given. 

Some triorganotin carboxylate structures, in particular those containing two RsSn moieties, deserve additional comment because their structures are more varied. For example, rare types of organotin carboxylates include derivatives of carborane-carboxylic acids [ (l,7-C2BioHn-l-COO)Bu2Sn 20]2 (structure of type A) [451]. The bis(triphenyltin) derivative of phenylmaleic acid, 145, contains only four-coordinate tetrahedral tin (Sn O 2.077 and 2.090 A) and is not associated, but in the bis(triphenyltin) citraconate, 146, one tin atom participates in the supramolecular association, 147, and polymer-chain formation and becomes five-coordinate (trigonal pyramidal Sn-O 2.193 A, Sn O 2.397 A) whereas the second remains four-coordinate (Sn-O 2.089 A), as a part of a dangling side chain [441]. 

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