N-O Ligand Supported Stannylenes: Preparation, Crystal, and Molecular Structures

A new series of tin(II) complexes (1, 2, 4, and 5) were successfully synthesized by employing hydroxy functionalized pyridine ligands, specifically 2-hydroxypyridine (hpH), 8-hydroxyquinoline (hqH), and 10-hydroxybenzo[h]quinoline (hbqH) as stabilizing ligands. Complexes [Sn(mu-kappa(ON)-O-2-OC5H4N)(N{SiMe3}(2))](2) (1) and [Sn-4(mu-kappa(ON)-O-2-OC5H4N)(6)(kappa O-1-OC5H4N)(2)] (2) are the first structurally characterized examples of tin(II) oxypyridinato complexes exhibiting {Sn-2(OCN)(2)} heterocyclic cores. As part of our study, H-1 DOSY NMR experiments were undertaken using an external calibration curve (ECC) approach, with temperature-independent normalized diffusion coefficients, to determine the nature of oligomerisation of 2 in solution. An experimentally determined diffusion coefficient (298 K) of 6.87 x 10(-10) m(2) s(-1) corresponds to a hydrodynamic radius of Ca. 4.95 angstrom. This is consistent with the observation of an averaged hydrodynamic radii and equilibria between dimeric [Sn{hp}(2)](2) and tetrameric [Sn{hp}(2)](4) species at 298 K. Testing this hypothesis, H-1 DOSY NMR experiments were undertaken at regular intervals between 298 K-348 K and show a clear change in the calculated hydrodynamic radii form 4.95 angstrom (298 K) to 4.35 angstrom (348 K) consistent with a tetramer reversible arrow dimer equilibria which lies towards the dimeric species at higher temperatures. Using these data, thermodynamic parameters for the equilibrium (Delta H degrees = 70.4 (+/- 9.22) kJ mol(-1), Delta S degrees = 259 (+/- 29.5) J K-1 mol(-1) and Delta G degrees(298) = -6.97 (+/- 12.7) kJ mol(-1)) were calculated. In the course of our studies, the Sn(II) oxo cluster, [Sn-6(m(3)-O)(6)(OR)(4):{Sn-(II)(OR)(2)}(2)] (3) (R = C5H4N) was serendipitously isolated, and its molecular structure was determined by single-crystal X-ray diffraction analysis. However, attempts to characterise the complex by multinuclear NMR spectroscopy were thwarted by solubility issues, and attempts to synthesise 3 on a larger scale were unsuccessful. In contrast to the oligomeric structures observed for 1 and 2, single-crystal X-ray diffraction studies unambiguously establish the monomeric 4-coordinate solid-state structures of [Sn(kappa(ON)-O-2-OC9H6N)(2))] (4) and [Sn(kappa(ON)-O-2-OC13H8N)(2))] (5).