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-Pri2)2 (LiArPri2) and Et2O·LiC6H3-2,6-(2,6-Pri-4-ButC6H2)2 (Et2O·LiArPri2But) furnished the bromide bridged organolead(II) halides {Pb(μ-Br)ArPri2}2 (1) and{Pb(μ-Br)ArPri2But}2 (2) as orange crystals. Treatment of 1 with a stoichiometric amount of methylmagnesium bromide
resulted in the “diplumbene” Pri2Ar(Me)PbPb(Me)ArPri2 (3). The addition of 1 equiv of 4-tert-butylphenylmagnesium
bromide to 1 afforded the feebly associated, Pb−Pb bonded species {Pb(C6H4-4-But)ArPri2}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(μ-Br)ArPri3}2 (Ar* = C6H3-2,6(C6H2-2,4,6-Pri3)2) with 4-isopropyl-benzylmagnesium bromide or LiSi(SiMe3)3 yielded the monomers 6, [Pb(CH2C6H4-4-Pri)ArPri3], or 7, [Pb(Si(SiMe3)3)ArPri3]. All compounds were characterized with use of X-ray crystallography, 1H, 13C, and 207Pb NMR (3−7), and
UV−vis spectroscopy. The dimeric Pb−Pb bonded (Pb−Pb = 3.1601(6) Å) 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 Pri
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) Å, 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.