Agostic-Type B−H···Pb Interactions Stabilize a Dialkylplumbylene. Structure of and Bonding in [{nPr2P(BH3)}(Me3Si)C(CH2)]2Pb

The reaction between Cp2Pb and 1 equiv of [[{nPr2P(BH3)}(Me3Si)C(CH2)]Li(THF)2]2 in toluene affords the dialkylplumbylene [{nPr2P(BH3)}(Me3Si)C(CH2)]2Pb (10) in excellent yield; compound 10 may readily be separated into its two diastereomeric forms by a simple crystallization procedure. X-ray crystallography shows that rac- and meso-10 crystallize as discrete dialkylplumbylenes in which there are either two (rac-10) or one (meso-10) short agostic-type B−H···Pb contacts; for meso-10 there is a secondary, weak B−H···Pb contact, which affords this diastereomer a further, weak stabilization. Multielement and variable-temperature NMR studies indicate that, while rac-10 is essentially static in solution, meso-10 undergoes dynamic exchange between the free and bound BH3 groups. Unusually, the methylene backbone protons in both diastereomers (at low temperature for meso-10) lie within the range 2.5−6.5 ppm, possibly due to the close proximity of the 6s lone pair on the lead center. DFT studies indicate that there is significant delocalization of B−H σ-bonding electron density into the vacant lead 6p orbital, furnishing an overall stabilization of 40.6 and 30.3 kcal mol−1 for rac- and meso-10, respectively.