Chelation Equilibria and π-Electron Delocalization in Neutral Hypercoordinate Organosilicon Complexes of Pyrithione

Abstract

A series of neutral pyrithionato (OPTO) organosilicon compounds, R 3 Si(OPTO) [R = Me (1), Ph (2)], cis-R 2 Si(OPTO) 2 [R = Me (3), Et (4), i Pr (5), t Bu (6), mesityl (10), allyl (11), p-tolyl (13); R 2 = (CH 2 ) 3 ( 7), (CH 2 ) 4 ( 8), (CH 2 ) 5 (9), Me, allyl (12)], and cis-R 2 Si(OPTO)Cl [R = Me (14), i Pr (15), allyl ( 16), p-tolyl (17), mesityl (18), Ph (19)], have been prepared and characterized by 1 H, 13 C, and 29 Si NMR spectroscopy. X-ray crystallographic analysis reveals four-coordinate silicon atoms in 1, 2, 6, and 10, five-coordinate in 3, 9, 11, 12, and 14−19, six-coordinate in 7 and 8, and primarily six-coordinate with cocrystallized five-coordinate forms in 13. Collectively, a wide range of chelate strengths of the OPTO ligand is observed in these complexes characterized by the Si−S bond length and S−Si−O bite angle in the solid state, which correlates well with the solutionstate 13 C NMR C�S chemical shift. In TBP five-coordinate silicon complexes, the ambidentate potential of the OPTO ligand and πelectron delocalization (π-ED) that occurs within the ligand generally allows the chelate effect to be enhanced with sulfur occupying an equatorial vs axial position. For 8, 9, and 18, reversible chelation equilibria involving Si ← S bond formation and concurrent π-ED have been characterized by variable-temperature 13 C and 29 Si NMR spectroscopy. Solvents of varying dielectric constants were found to have pronounced effects on the 13 C NMR chemical shifts of 1, 15, and pyrithione.

Synthesis and crystal structures of bis[1-oxopyridin-2-olato(1−)]bis(pentafluorophenyl)silicon(IV)–tetrahydrofuran–pentane (2/1/1), bis[1-oxopyridin-2-olato(1−)]bis(p-tolyl)silicon(IV), and dimesitylbis[1-oxopyridin-2-olato(1−)]silicon(IV)

“…There are currently no reported structures of hexacoordinate bis(pentafluorophenyl)silicon(IV) complexes, nor other hexacoordinate dimesitylsilicon(IV) complexes. The related hexacoordinate pyrithione (OPTO) complex, (p-tolyl) 2 -Si(OPTO) 2 , crystallizes with cis aryl groups and primarily with two bidentate OPTO ligands in an S-trans-S arrangement with additional disordered monodentate modes (CSD refcode DEWGAR; Tiede et al, 2022). Mesityl 2 Si(OPTO) 2 is tetracoordinate with two monodentate �O OPTO ligands (CSD refcode DEWSUX; Tiede et al, 2022).…”