A Trithiol Protio-Ligand and Its Fixation to the Periphery of a Carbosilane Dendrimer as Scaffolds for Polynuclear Rhodium and Iridium Complexes and Metallodendrimers

Tris(methylenethiol)methane derivatives containing an allyl or benzyl linker (ROCH2C(CH2SH)3; R = Bn(6a), CH2CHCH2(6b)), which could allow their fixation to carbosilane dendrimers, were synthesized through a multistep synthesis. Grafting the periphery of the core
dendrimer Si[(CH2)3Si(Me)2H]4 was achieved with the tosyl derivative CH2CHCH2OCH2C(CH2OTs)3 by a hydrosilylation reaction to give selectively Si[(CH2)3Si(Me)2(CH2)3OCH2C(CH2OTs)3]4 (G(0)OTs-12). This dendrimer was converted into the thiol-functionalized
carbosilane dendrimer Si[(CH2)3Si(Me)2(CH2)3OCH2C(CH2SH)3]4 (G(0)SH-12) by reduction of
the thiocyanate intermediate Si[(CH2)3Si(Me)2(CH2)3OCH2C(CH2SCN)3]4 with LiAlH4. The
trithiol compounds 6a and 6b protonated the complexes [Rh(μ-OMe)(cod)]2 and [Rh(acac)diolefin] (M = Rh, diolefin = cod, nbd, tfb) to give the corresponding trinuclear complexes
[Rh3{μ-ROCH2c(ch2s)3}(diolefin)3] (7−9). Their structure can be described as an adamantane-like entity, in which the metals are held together by the thiolate arms of the ligand, with
the sulfurs bridging the metal atoms in a μ2 fashion, as shown for [Rh3{μ-BnOCH2C(CH2S)3}(nbd)3] by X-ray diffraction methods. Carbonylation of 7−9 under atmospheric pressure gave
the carbonyl complexes with the thiolate tripod ligands [M3{μ-ROCH2C(CH2S)3}(CO)6]
(M = Rh, Ir), which were reacted with phosphorus donor ligands (PR3 = PPh3, P(OMe)3) to
give [M3{μ-ROCH2C(CH2S)3}(CO)3(PR)3] as a sole isomer of averaged C3v symmetry. The
functional dendrimers were metalated by reaction of the core molecule Si[(CH2)3Si(Me)2(CH2)3OCH2C(CH2SH)3]4 (G(0)SH-12) by applying the synthetic protocols above-described for
trinuclear complexes. Thus, the reactions of G(0)SH-12 with the complexes [M(acac)(diolefin)]
(M = Rh, Ir; diolefin = cod, nbd, tfb) gave the metallodendrimers Si[(CH2)3Si(Me)2(CH2)3OCH2C(CH2S)3{M(diolefin)}3]4 (G(0)M(diolefin)-12) as insoluble solids. Soluble metallodendrimers, Si[(CH2)3Si(Me)2(CH2)3OCH2C(CH2S)3{Rh(CO)2}3]4 (G(0)Rh(CO)2-12) and Si[(CH2)3Si(Me)2(CH2)3OCH2C(CH2S)3{M(CO)(PPh3)}3]4 (G(0)M(CO)(PPh3)-12) were obtained either by
carbonylation of the diolefin compounds or by the reaction of G(0)SH-12 with [M(acac)(CO)2]
(M = Rh, Ir), and by further reaction with PPh3, respectively.