Chelation and Stereodynamic Equilibria in Neutral Hypercoordinate Organosilicon Complexes of 1‑Hydroxy-2-pyridinone

A series
of neutral organosilicon compounds, R3Si­(OPO) (R = Me (1), Et (2), Ph (3)), cis-R2Si­(OPO)2 (R = Me (4), Et (5), iPr (6), tBu (7), Ph (9)),
(CH2)3Si­(OPO)2 (8),
and cis-R2Si­(OPO)Cl (R = Me (10), Et (11)) (OPO = 1-oxo-2-pyridinone) have been prepared
and fully characterized. X-ray crystallographic analyses show 1 to be tetracoordinate, 3, 7, and 10 to be pentacoordinate, and 4–6, 8, and 9 to be hexacoordinate.
In the hexacoordinate structures, a mixture of diastereomers is observed
in the form of C/N site disorder in each OPO ligand. Variable-temperature 13C and 29Si NMR studies indicate reversible Si←OC
bond dissociation occurring in all pentacoordinate and hexacoordinate
complexes to a varying degree with greater tendency toward dissociation
in hydrogen-bonding donor solvents. Significant weakening of the dative
Si←OC bond in 3 is observed in the cocrystallized
adduct solvate 3·Ph3SiOH·1/2C5H12, providing structural evidence
for the decrease in coordination number of the OPO ligand by hydrogen-bonding
donors. In the hexacoordinate complexes, increasing steric bulk of
ancillary ligands also was found to promote dissociation. 1H and 13C VT-NMR studies of 4, 6, 8, and 9 indicate stereoisomerization
equilibria concurrent with Si←OC bond dissociation proposed
to occur through trigonal-bipyramidal intermediates.