Terphenyl ligand stabilized lead(II) derivatives: Steric effects and lead-lead bonding in diplumbenes

The reaction of PbBr2 with the lithium reagents LiC6H3-2,6-(C6H3-2,6-Pr-2(i))(2) (LiArPr2i) and Et2O.LiC6H3-2,6-(2,6-Pr-i-4-(BuC6H2)-C-t)(2) (Et2O.(LiArPr2But)-Bu-i) furnished the bromide bridged organolead(II) halides {Pb(mu-Br)ArPr2i}(2) (1) and {Pb(mu-Br)(ArPr2But)-Bu-i}(2) (2) as orange crystals. Treatment of 1 with a stoichiometric amount of methylmagnesium bromide resulted in the "diplumbene" (Pr2Ar)-Ar-i(Me)PbPb(Me)ArPr2i (3). The addition of 1 equiv of 4-tert-butylphenylmagnesium bromide to 1 afforded the feebly associated, Pb-Pb bonded species {Pb(C6H4-4-Bu-t)ArPr2i}(2) (4), whereas the corresponding reaction of tert-butylmagnesium chloride and 1 afforded the monomer Pb(But)ArPri2 (5). The reaction of the more crowded aryl lead(II) bromide {Pb(mu-Br)ArPr3i}(2) (Ar* = C6H3-2,6(C6H2-2,4,6-Pr-3(i))(2)) with 4-isopropylbenzylmagnesium bromide or LiSi(SiMe3)(3) yielded the monomers 6, [Pb(CH2C6H4-4-Pr-i)ArPr3i], or 7, [Pb(Si(SiMe3)(3))ArPr3i]. All compounds were characterized with use of X-ray crystallography, H-1, C-13, and Pb-207 NMR (3-7), and UV-vis spectroscopy. The dimeric Pb-Pb bonded (Pb-Pb = 3.1601(6) Angstrom) structure of 3 may be contrasted with the previously reported monomeric structure of Pb(Me)ArPri3, which differs from 3 only in that it has para Pr-i substituents on the flanking aryl rings. The presence of these groups is sufficient to prevent the weak Pb-Pb bonding seen in 3. The dimer 4 displays a Pb-Pb distance of 3.947(1) Angstrom, which indicates a very weak lead-lead interaction, and it is possible that this close approach could be caused by packing effects. The monomeric structures of 6 and 7 are attributable to steric effects and, in particular, to the large size of ArPri3.