Terphenyl ligand stabilized lead(II) derivatives: steric effects and lead-lead bonding in diplumbenes.
The reaction of PbBr(2) with the lithium reagents LiC(6)H(3)-2,6-(C(6)H(3)-2,6-Pr(i)(2))(2) (LiArPr(i)(2)) and Et(2)O.LiC(6)H(3)-2,6-(2,6-Pr(i)-4-Bu(t)C(6)H(2))(2) (Et(2)O.LiArPr(i)(2)Bu(t)) furnished the bromide bridged organolead(II) halides [Pb(mu-Br)ArPr(i)(2)](2) (1) and[Pb(mu-Br)ArPr(i)(2)Bu(t)](2) (2) as orange crystals. Treatment of 1 with a stoichiometric amount of methylmagnesium bromide resulted in the "diplumbene" Pr(i)(2)Ar(Me)PbPb(Me)ArPr(i)(2) (3). The addition of 1 equiv of 4-tert-butylphenylmagnesium bromide to 1 afforded the feebly associated, Pb-Pb bonded species [Pb(C(6)H(4)-4-Bu(t))ArPr(i)(2)](2) (4), whereas the corresponding reaction of tert-butylmagnesium chloride and 1 afforded the monomer Pb(Bu(t))ArPr(i)(2) (5). The reaction of the more crowded aryl lead(II) bromide [Pb(mu-Br)ArPr(i)(3)](2) (Ar = C(6)H(3)-2,6(C(6)H(2)-2,4,6-Pr(i)(3))(2)) with 4-isopropyl-benzylmagnesium bromide or LiSi(SiMe(3))(3) yielded the monomers 6, [Pb(CH(2)C(6)H(4)-4-Pr(i))ArPr(i)(3)], or 7, [Pb(Si(SiMe(3))(3))ArPr(i)(3)]. All compounds were characterized with use of X-ray crystallography, (1)H, (13)C, and (207)Pb NMR (3-7), and UV-vis spectroscopy. The dimeric Pb-Pb bonded (Pb-Pb = 3.1601(6) A) structure of 3 may be contrasted with the previously reported monomeric structure of Pb(Me)ArPr(i)(3), 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) A, 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 ArPr(i)(3).
Hino
Shirley
S
Department of Chemistry, University of California-Davis, One Shields Avenue, Davis, CA 95616, USA.
Olmstead
Marilyn
M
Phillips
Andrew D
AD
Wright
Robert J
RJ
Power
Philip P
PP