The Question of cis versus trans Configuration in Octahedral Metal Diketonates: An In-Depth Investigation on Diorganobis(4-acyl-5-pyrazolonato)tin(IV) Complexes

Diorganotin(IV) derivatives of 4-acyl-5-pyrazolones Q2SnR2 [HQ = 3-methyl-1-phenyl-4-R-5-pyrazolone: HQnPe, R = neopentylcarbonyl; HQiPr, R = isopropylcarbonyl; HQtBu, R = tert-butylcarbonyl; HQCy, R = cyclohexylcarbonyl; HQCp, R = cyclopentylcarbonyl; HQEtCp, R = (ethylcyclopentyl)carbonyl] have been synthesized and characterized spectroscopically (IR, far-IR, 1H, 13C and 119Sn NMR) and structurally (X-ray). Steric bulkiness in the acyl fragment of HQtBu induces partial dissociation in solution of (QtBu)2SnMe2, which exists as an equilibrium mixture of six- and five-coordinate tin species. Single-crystal X-ray structure determinations of several representative complexes are presented. Whereas the dialkyltin(IV) complexes are always trans-octahedral, a diphenyltin(IV) derivative exists in the solid state that has the two phenyl groups in a cis arrangement. DMol3 and CASTEP DFT studies of this unprecedented cis, and the corresponding trans configurations, show the former to be slightly more stable, in agreement with the diffraction study. Solid-state 119Sn cross-polarization magic-angle spinning NMR spectroscopic data show that the isotropic chemical shifts (delta iso,mas) and chemical-shift anisotropies/spans (Delta delta/Omega) that characterize the diphenyl(IV) derivatives are markedly removed from values measured for the corresponding dimethyl and di-n-butyl derivatives, thereby adding further evidence for a stable cis configuration in the diphenyl(IV) systems. This is supported by GIPAW DFT calculation of the 119Sn isotropic shifts and chemical-shift anisotropies/spans from fully relaxed structures by using the CASTEP code, which provides a direct link between the cis diphenyl(IV) arrangement and the reported differences in the 119Sn NMR spectroscopic parameters.