Tris(pyrazolyl)borate carbosilane dendrimers and metallodendrimers

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( (OPr)-Pr-i) (2) ( 1), which was then reacted with potassium pyrazolate and pyrazole to give the compound K[CH2 = CHCH(2)Bpz(3)] ( 2). The new allyl- containing scorpionate anion of 2 acts as a bi- or tri-dentate ligand, as shown by the mononuclear complexes [CH2 = CHCH(2)Bpz(3)M(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(mu- 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) B-3( (OPr)-Pr-i) (2)] (4) ( G( 0) - B-4), Si[( CH2) 3SiMe{( CH2) 3SiMe2( CH2) B-3( (OPr)-Pr-i) (2)} (2)] (4) ( G( 1) - B-8), and Si[(CH2)(3)SiMe{(CH2)(3)SiMe[(CH2) 3SiMe2( CH2) B-3( (OPr)-Pr-i) (2)] (2)} (2)] (4) ( G( 2)- B-16) selectively in the anti- Markovnikov direction. Further reactions of G( 0) - B-4, G( 1) - B-8 and G( 2) - B-16 with potassium pyrazolate and pyrazole rendered the corresponding polyanionic dendrimers K-4[ Si{(CH2)(3)SiMe2(CH2) (3)Bpz(3)} (4)] ( G( 0) -( Bpz(3)) (4)), G( 1) -( Bpz(3)) (8), and G( 2) -( Bpz(3)) (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) -( Bpz(3)) (4), G( 1) -( Bpz(3)) (8), and G( 2) -( Bpz(3)) (16) with [{Rh(mu- Cl)( nbd)} (2)] to give the neutral rhodadendrimers [ Si{( CH2) 3SiMe2( CH2) (3)Bpz(3)Rh( nbd)} (4)] G( 0) -( Bpz(3)Rh) (4), G( 1) -( Bpz(3)Rh) (8) and G( 2) -( Bpz(3)Rh)(16) as stable solids in excellent yields. Following this protocol, mixed rhodium/ iridium metallodendrimers can be prepared.