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(μ-κ²ON-OC₅H₄N)(N{SiMe₃}₂)]₂ (1) and [Sn₄(μ-κ²ON-OC₅H₄N)₆(κ¹O-OC₅H₄N)₂] (2) are the first structurally characterized examples of tin(II) oxypyridinato complexes exhibiting {Sn₂(OCN)₂} heterocyclic cores. As part of our study, ¹H 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 × 10⁻¹⁰ m² s⁻¹ corresponds to a hydrodynamic radius of Ca. 4.95 Å. 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, ¹H 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 Å (298 K) to 4.35 Å (348 K) consistent with a tetramer ⇄ dimer equilibria which lies towards the dimeric species at higher temperatures. Using these data, thermodynamic parameters for the equilibrium (ΔH° = 70.4 (±9.22) kJ mol⁻¹, ΔS° = 259 (±29.5) J K⁻¹ mol⁻¹ and ΔG°₂₉₈ = −6.97 (±12.7) kJ mol⁻¹) were calculated. In the course of our studies, the Sn(II) oxo cluster, [Sn₆(m³-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(κ²ON-OC₉H₆N)₂)] (4) and [Sn(κ²ON-OC₁₃H₈N)₂)] (5).