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)(2)H](4) was achieved with the tosyl derivative CH2=CHCH2OCH2C(CH2OTs)(3) by a hydrosilylation reaction to give selectively Si[(CH2)(3)Si(Me)(2)(CH2)(3)OCH2C(CH2OTs)(3)](4) (G(O)(OTs-12)). This dendrimer was converted into the thiol-functionalized carbosilane dendrimer Si[(CH2)(3)Si(Me)(2)(CH2)(3)OCH2C(CH2SH)(3)](4) (G(0)(SH-12)) by reduction of the thiocyanate intermediate Si[(CH2)(3)Si(Me)(2)(CH2)(3)OCH2C(CH2SCN)(3)](4) with LiAlH4. The trithiol compounds 6a and 6b protonated the complexes [Rh(mu-OMe)(cod)](2) and [Rh(acac)-diolefin] (M = Rh, diolefin = cod, nbd, tfb) to give the corresponding trinuclear complexes [Rh-3{mu-ROCH(2)c(ch(2)S)(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 mu(2) fashion, as shown for [Rh-3{mu-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 [M-3{mu-ROCH2C(CH2S)(3)}(CO)(6)] (M = Rh, Ir), which were reacted with phosphorus donor ligands (PR3 = PPh3, P(OMe)(3)) to give [M-3{mu-ROCH2C(CH2S)(3)}(CO)(3)(PR)(3)] as a sole isomer of averaged C-3 nu symmetry. The functional dendrimers were metalated by reaction of the core molecule Si[(CH2)(3)Si(Me)(2)- (CH2)(3)OCH2C(CH2SH)(3)](4) (G(0)(SH-12)) by applying the synthetic protocols above-described for trinuclear complexes. Thus, the reactions of G(O)SH-12 with the complexes [M(acac)(diolefin)] (M = Rh, Ir; diolefin = cod, nbd, tfb) gave the metallodendrimers Si[(CH2)(3)Si(Me)(2)(CH2)(3)-OCH2C(CH2S)(3){M(diolefin)}3]4 (G(0)(M(diolefin)-12)) as insoluble solids. Soluble metallodendrimers, Si[(CH2)(3)Si(Me)(2)(CH2)(3)OCH2C(CH2S)(3){Rh(CO)(2)}(3)](4) (G(0)(Rh(CO)2-12)) and Si[(CH2)(3)-Si(Me)(2)(CH2)(3)OCH2C(CH2S)(3){M(CO)(PPh3)}(3)](4) (G(O)(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.