Tris(pyrazolyl)borate carbosilane dendrimers and metallodendrimers

Abstract

A modified tris(pyrazolylborate) ligand has been prepared in two steps. First, reaction of triisopropylborate with allylmagnesium bromide and further treatment with benzoyl chloride gave CH₂ = CHCH₂B(OⁱPr)₂ (1), which was then reacted with potassium pyrazolate and pyrazole to give the compound K[CH₂ = CHCH₂Bpz₃] (2). The new allyl-containing scorpionate anion of 2 acts as a bi- or tri-dentate ligand, as shown by the mononuclear complexes [CH₂ = CHCH₂Bpz₃M(LL)] (M = Rh, LL = nbd, 3; LL = tfb, 4; LL = (CO)(PPh₃), 5; M = Ir, LL = cod, 6), obtained from reactions of the chlorido-bridged dinuclear complexes [{M(µ-Cl)(LL)}₂] with 2. Furthermore, the borate 1 represents a key material to achieve the attachment of tris(pyrazolyl)borate groups to the peripheries of carbosilane dendrimers. Thus, the platinum-catalyzed hydrosilylation reactions of compound 1 with the dendritic cores Si[(CH₂)₃SiMe₂H]₄ (G(0)–(SiH)₄), (G(1)–(SiH)₈), and (G(2)–(SiH)₁₆) gave the corresponding borate-containing dendrimers Si[(CH₂)₃SiMe₂(CH₂)₃B(OⁱPr)₂]₄ (G(0)–B₄), Si[(CH₂)₃SiMe{(CH₂)₃SiMe₂(CH₂)₃B(OⁱPr)₂}₂]₄ (G(1)–B₈), and Si[(CH₂)₃SiMe{(CH₂)₃SiMe[(CH₂)₃SiMe₂(CH₂)₃B(OⁱPr)₂]₂}₂]₄ (G(2)−B₁₆) selectively in the anti-Markovnikov direction. Further reactions of G(0)–B₄, G(1)–B₈ and G(2)–B₁₆ with potassium pyrazolate and pyrazole rendered the corresponding polyanionic dendrimers K₄[Si{(CH₂)₃SiMe₂(CH₂)₃Bpz₃}₄] (G(0)–(Bpz₃)₄), G(1)–(Bpz₃)₈, and G(2)–(Bpz₃)₁₆, respectively, which contain 4, 8, and 16 tris(pyrazolyl)borate groups symmetrically located around the dendritic peripheries. These unusual polyanionic dendrimers are excellent scaffolds to support metal centres, as shown by the reactions of G(0)–(Bpz₃)₄, G(1)–(Bpz₃)₈, and G(2)–(Bpz₃)₁₆ with [{Rh(µ-Cl)(nbd)}₂] to give the neutral rhodadendrimers [Si{(CH₂)₃SiMe₂(CH₂)₃Bpz₃Rh(nbd)}₄] G(0)–(Bpz₃Rh)₄, G(1)–(Bpz₃Rh)₈ and G(2)–(Bpz₃Rh)₁₆ as stable solids in excellent yields. Following this protocol, mixed rhodium/iridium metallodendrimers can be prepared.