Structural studies in main group chemistry : XI. Tin-119m mössbauer investigations of triorganotin derivatives of substituted pyridines

Tin-119m Mössbauer and selected infrared and ¹H NMR data are reported for twenty four triorganotin derivatives of substituted pyridines and related ligands, and conclusions concerning structure inferred. Mössbauer quadrupole splittings in the ranges 3.09–3.38 mm sec^?1 and 2.92–3.22 mm sec^?1, respectively, for the 2- and 3-pyridone derivatives indicate five-coordinate trans-R₃SnX₂ trigonal bipyramidal configurations at tin with planar R₃Sn moieties. In the case of the derivatives of 2-pyridone, the extra coordination to give linear chain structures arises from bridging oxygen atoms, but bridging via either oxygen or the pyridyl nitrogen atom may occur in the derivatives of 3-pyridone. Similar oxygen-bridged chain structures are also postulated for the triorganotin derivatives of 2-pyridylalcohol and 2-pyridylaldehyde oxime, but lower splittings values of 2.63(5)–2.93 mm sec^?1 indicate some relaxation from planarity of the R₃Sn group towards tetrahedral. The splittings are observed to decrease in the order Me > Et > Pr, illustrating that the deviation from planarity and the bond distance of the coordinate O?Sn bond increase with the steric bulk of R. Much lower splittings of 1.60–2.27 mm sec^?1 are observed for the derivatives of 2-thiopyridone and 8-hydroxyquinoline, which, with the presence of three tin—carbon stretching modes in the infrared spectrum for the methyltin compounds, are consistent with the alternative five-coordinate cis-R₃SnX₂ geometry at tin. The very high splitting values, together with the low values of the anti-symmetric OCO stretching vibration of the carboxylate group and the multiple tin—carbon stretching modes observed for the methyltin compounds in the infrared, have been interpreted in terms of the six-coordinate mer-R₃SnX₃ stereochemistry for the triorganotin esters of 2-pyridylcarboxylic and 2,6-pyridyldicarboxylic acids. The low and almost constant value of the two-bond coupling constant ²J(¹HC¹¹⁹Sn) for the methyltin derivatives indicate that n solution either four or five-coordinate cis-R₃SnX₂ structures are adopted.