Isolation and Computational Studies of a Series of Terphenyl Substituted Diplumbynes with Ligand Dependent Lead–Lead Multiple-Bonding Character

A series
of formally triply bonded diplumbyne analogues of alkynes
of the general formula ArPbPbAr (Ar = terphenyl ligand with different
steric properties) was synthesized by two routes. All diplumbyne products
were synthesized by a simple reduction of the corresponding Pb­(II)
halide precursor ArPb­(Br) by DIBAL-H with yields in the range 8–48%.
For one of the diplumbynes ArPri4PbPbArPri4 (ArPri4 = C6H3-2,6-(C6H3-2,6-Pri2)2) it was shown that reduction of ArPri4Pb­(Br) using a magnesium­(I) beta-diketiminate afforded
a much improved yield in comparison (29 vs 8%) to that obtained by
reduction with DIBAL-H. The more sterically crowded diplumbyne ArPri8PbPbArPri8 (ArPri8 = C6H-3,5-Pri2-2,6-(C6H2-2,4,6-Pri3)2) displayed a shortened Pb–Pb bond
with a length of 3.0382(5) Å and wide Pb–Pb–C angles
of 114.73(7)° and 116.02(6)° consistent with multiple-bond
character with a bond order of up to 1.5. The others displayed longer
metal–metal distances and narrower Pb–Pb–C angles
that were consistent with a lower bond order that approached one.
Computational studies of the diplumbynes yielded detailed insight
of the unusual bonding and explained their similar electronic spectra
arising from the flexibility of the C–Pb–Pb–C
core in solution. Furthermore, the importance of London dispersion
interactions for the stabilization of the diplumbynes was demonstrated.