Convergent Synthesis of Alkynylbis(bidentate phosphine)ruthenium Dendrimers

Abstract

The “first-generation” alkynylruthenium dendrimers 1,3,5-C6H3(4-C⋮CC6H4C⋮C-trans-[Ru(dppe)2]C⋮C-3,5-C6H3{4-C⋮CC6H4C⋮C-trans-[Ru(4-C⋮CC6H4R)(dppe)2]}2)3 [R = H (19), NO2 (20)], containing nine dialkynylruthenium centers, have been prepared by convergent synthesis. Reaction of 3 equiv of 1-iodo-4-trimethylsilylethynylbenzene with triethynylbenzene, under Sonogashira coupling conditions, followed by deprotection with tetra-n-butylammonium fluoride affords 1,3,5-C6H3(4-C⋮CC6H4C⋮CH)3 (2), which is reacted with cis-[RuCl2(L)2] (L = dppe, dppm) to afford the octopolar, triruthenium dendritic cores 1,3,5-C6H3{4-C⋮CC6H4C⋮C-trans-[RuCl(L)2]}3 [L = dppe (5), L = dppm (6)] via the vinylidene intermediates [1,3,5-C6H3{4-C⋮CC6H4CHC-trans-[RuCl(L)2]}3](PF6)3 [L = dppe (3), L = dppm (4)]. Reaction of 5 with terminal alkynes 4-HC⋮CC6H4R (R = H, NO2, NEt2) affords a series of related dialkynylruthenium zero-generation dendrimers 1,3,5-C6H3{4-C⋮CC6H4C⋮C-trans-[Ru(4-C⋮CC6H4R)(dppe)2]}3 [R = H (7), NO2 (8), NEt2 (9)]. Reaction of 3 equiv of trans-[Ru(4-C⋮CC6H4C⋮CH)(C⋮CPh)(dppe)2] with 1,3,5-triiodobenzene under Sonogashira coupling conditions also affords 7, together with the homo-coupled trans,trans-[(dppe)2(PhC⋮C)Ru(4,4‘-C⋮CC6H4C⋮CC⋮CC6H4C⋮C)Ru(C⋮CPh)(dppe)2]. The first-generation dendrimers 19 and 20 are prepared by coupling core 5 with the dendrons 1-(HC⋮C)C6H3-3,5-{4-C⋮CC6H4C⋮C-trans-[Ru(4-C⋮CC6H4R)(dppe)2]}2 [R = H (17), NO2 (18)]. Thus, reaction of 1-(Me3SiC⋮C)C6H3-3,5-(4-C⋮CC6H4C⋮CH)2 (12), obtained from 1-iodo-3,5-dibromobenzene through a series of Sonogashira coupling and transhalogenation reactions, with cis-[RuCl2(dppe)2] affords 1-(Me3SiC⋮C)C6H3-3,5-{4-C⋮CC6H4C⋮C-trans-[RuCl(dppe)2]}2 (13), which can be reacted with appropriately functionalized terminal alkynes to afford the series 1-(Me3SiC⋮C)C6H3-3,5-{4-C⋮CC6H4C⋮C-trans-[Ru(4-C⋮CC6H4R)(dppe)2]}2 [R = H (14), NO2 (15), NEt2 (16)]. Desilylation of 16 proceeds with decomposition; in contrast, treatment of 14 and 15 with tetra-n-butylammonium fluoride gives 17 and 18, which are coupled with 5 under basic conditions to afford the dendritic complexes 19 and 20 via in situ deprotonation of the vinylidene complex intermediates. A transmission electron micrograph of 19 supported on alumina reveals molecules that are approximately 6 nm in diameter, in agreement with molecular modeling studies.