Organotin compounds intramolecular coordination

Since the dominant higher-coordination chemistry is that of tin, we survey this first. The range of structural studies is so wide that it is possible only to generalize and look at recent and representative examples. Intramolecular coordination in organotin compounds has been extensively reviewed up to 1992113. 

Few X-ray diffraction data are available for the organotin alkoxides. Trimethyltin methoxide in the crystal is a five-coordinate linear polymer 47,380 and both tin atoms in the methanol solvate, CH2(SnPh2OMe)2-MeOH, are five coordinate, one by intramolecular coordination, and the other by solvation 48.381 Dimethyltin dimethoxide 49,382 dimethyltin diphenoxide 50,383 and isopropyltin triisopropoxide 51384 are five-coordinate oxygen-bridged dimers. All these compounds are monomers in solution. 

SCHEME 19. Intramolecular cyclization of organotin compounds bearing 0,C,0-coordinating ligand... 

C. Divalent Organotin Compounds Stabilized by Intramolecular Coordination... 

Recently the first examples of divalent organotin compounds in which intramolecular coordination as such is sufficient for stabilization were reported. These compounds are bis(2-[(dimethylamino)methyl]phenyl tin(II), 6 (74) bis(2-[l-(dimethylamino)ethyl]phenyl tin(II), 7 (78) bis[8-(dimethylamino)-l-naphthyl]tin(II), 8 (75) bis([2-(dimethylamino) phenyl (trimethylsilyl)methyl tin(II), 9 (78) and bis[(2-pyridyl)bis(tri-methylsilyl)methyl]tin(II), 10 (79) and are schematically shown in Fig. 5. 

Attempts have been made to correlate the stabilization properties of organotins with their reactivity toward HC1 (75,76), and the stabilizing effectiveness of bis(4-ketopentyl)tin bis ( isooctyl mercaptoacetate) has been found to be significantly greater than that of the corresponding di-n-butyltin compound (77). This difference was attributed to intramolecular coordination of tin with the C=0 groups of bis(4-ketopentyl)-tin dichloride, an effect that should make this dichloride a weaker dehydrochlorination catalyst than n-Bu2SnCl2 (77). 

The first item concerns the first article verifying the intramolecular-coordination bond in cyclometalation reactions [92]. Published in 1966, this article reported on cyclometalation reactions with main group metal compounds, which are organotin compounds. The intramolecular-coordination bond is verified with IR spectrum studies. Unfortunately, this study was not widely known, because it was published in a Japanese academic journal. 

All stable derivatives contain a radical with at least two nitrogen atoms, occupying 1,3-positions relative to each other. Nitrogen in any other position is not able to stabilize the organotin compound (pyrazole) or to destabilize it (1,2,3-benztriazole). Thus the crystal stabilization is probably due to the coordinating action of the second nitrogen atom. Since intramolecular coordination is sterically impossible, some kind of association has to be involved. 

The Sn chemical shift also provides effective means of studying covalent solute-solvent interaction, autoassociation, dissociation, and intramolecular coordination. For example, the Sn chemical shift is nearly concentration independent when tin (IV) and organotin compounds such as Me2SnCl2... 

---