om801189h_si_001.cif (37.43 kB)

Seven-Membered Cyclic Dialkylstannylene and -Plumbylene Compounds Stabilized by Agostic-type B−H···E Interactions [E = Sn, Pb]

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posted on 13.04.2009 by Keith Izod, Corinne Wills, William Clegg, Ross W. Harrington
The reaction between [[{Me2P(BH3)}(Me3Si)C{(SiMe2)(CH2)}]Li(THF)3]2 and either Cp2Sn or Cp2Pb in toluene cleanly gives the compounds rac-[{Me2P(BH3)}(Me3Si)C{(SiMe2)(CH2)}]2E in moderate yield [E = Sn (10), Pb (11)]. NMR spectra of crude samples indicate that 10 is predominantly formed as the rac isomer; for 11 there is no evidence for the formation of the meso diastereomer at all. Crystallization from diethyl ether yields the solvates 10·Et2O and 11·Et2O; X-ray crystallography reveals that these compounds crystallize as discrete rac-dialkylstannylene and -plumbylene species in which there are two short agostic-type B−H···E contacts. A DFT study suggests that these agostic-type interactions stabilize 10 and 11 by 47.7 and 42.7 kcal mol−1, respectively. Calculations on the corresponding meso diastereomers of 10 and 11 suggest that the tin and lead centers in these compounds have close contacts to just one hydrogen atom of a BH3 group, although a second, weaker B−H···E contact is observed in each case. These contacts afford an overall stabilization of 41.3 and 32.6 kcal mol−1, respectively, for the meso-stannylene and -plumbylene.