Synthesis and Characterization of Dendritic Necklaces: A Class of Outer‐Sphere–Outer‐Sphere Connected Dendronized Organoplatinum Polymers

Abstract

Some years ago Newkome and Vögtle proposed the concept of dendritic networks [1] whereby simple dendrimer/dendron subunits could be connected together to form macromolecular assemblies with nanoscopic dimensions.[2] At about the same time, Schlüter and co-workers began to report their pioneering works on the synthesis and characterization of a special kind of dendritic networks, also known as cylindrical dendronized polymers, [3,4] based on an inner-sphere-innersphere connection [2] approach, that is, the resulting dendronized polymers may be viewed as constructed from the connection of individual dendrons through their internal focal-point functional groups. A large number of such innersphere-inner-sphere connected dendronized polymers were reported. Some of them were shown to have cylindrical geometry [4] while some were known to self-assemble into spherical nanostructures.[5] To our surprise, examples of dendritic networks created by the alternative outer-sphere-outer-sphere connection strategy [2] are less studied. A few examples were already reported but invariably they all involve a random polymerization of dendrimer subunits that have a large number of reactive functional groups on the dendrimer surface.[6] We report herein a new variant of the outer-sphere-outer-sphere connection strategy that allows us to prepare a novel class of nanoscopic dendritic networks (hereafter called dendritic necklaces because of their structural resemblance to untied necklaces) by the controlled polymerization of dendrimer subunits containing only two reactive surface functional moieties (Figure 1). [7] We also show that the resulting dendritic necklaces, in contrast to randomly connected dendritic networks, can be fully characterized by solution techniques such as NMR, MS, GPC and laser light scattering (LLS) to enable a thorough assessment of their structural identity and purity. Employing organoplatinum coordination chemistry as a method for the polymerization of bis(ethynyl) surface-functionalized G1-G3 dendritic beads 1-3, we demonstrate that higher-order 1D dendritic necklaces with high degree-of-polymerization values (DP, from 30-880) could be prepared in high yields. We also describe a general synthetic protocol for the construction of dendrimers that contain a limited number of functional groups on the surface sector.Because bis(ethynyl) polyether-based dendrimers readily form trans-dialkynyl organoplatinum(ii) complexes upon treatment with trans-[(Et 3 P) 2 PtCl 2 ], this reaction was chosen to link the dendrimers 1-3 together.[8] To avoid random polymerization of the dendritic beads, only two of the surface moieties were functionalized with "reactive" long chain 10-(4-ethynylphenoxy)decyloxyl residues while the rest were decorated with "inert" 3-(4-tert-butylphenoxy)propoxy groups.The synthesis of the target dendritic monomers 1-3 required the availability of three bromo-dendrons 10, 11, and 12[9] (Scheme 1). Hence, the G1-dendrimer 4 and G2-

Synthesis of amphiphilic dendrons and their interactions in aqueous solutions with cetyltrimethylammonium p-toluenesulfonate (CTAT)

“…To this aim, a variety of molecular building blocks have already been combined. Recently, dendrimers and their segments, dendrons, have fostered scientific interest as a class of molecular building blocks in the construction of supramolecular systems [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16].…”

Site Selectivity in the Reactions of the Hexanuclear Platinum Cluster [Pt6(μ‐PtBu2)4(CO)6][CF3SO3]2

“…[2] The synthetic procedures employ different types of inorganic or organometallic "bricks", from simple mononuclear L n M fragments [1][2][3] to dinuclear, [4] metal-chain, [5] porphyrin, [1a, 6] polyoxometalate, [7] carborane, [8] C60 [9] or dendrimeric [10] units, generally connected by organic bridging ligands or spacers. Owing to the peculiar redox properties of transition-metal clusters, [11] there is also a growing interest in the construction of rigid-rod or dendrimeric structures in which two [12] or more [13,14] cluster units are connected by conjugated or insulating spacers.…”

Synthesis of Organoplatinum Poly(dendrimer)s: Pronounced Effect of Size and Geometry of Small Organoplatinum Linkers on the Copolymerization Efficiency with Bifunctional Dendritic Macromonomers

“…SEC calculations also confirmed that the DP values of the long‐ trans ‐ Pt ‐ S/L ‐ G n poly(dendrimer)s were consistently higher than those of the trans ‐ Pt ‐ S/L ‐ G n poly(dendrimer)s. Fourth, both third‐generation macromonomers S ‐ G3 and L ‐ G3 , previously found to have problems to undergo copolymerization, could form organoplatinum poly(dendrimer)s with good DP values with the longer rod‐liked linker 3 . It should be pointed out here that although the SEC calculated DP values are in the order of 10 2 , the actual DP values can be 1.5–14 times higher as the hyperbranched poly(dendrimer)s have a much larger mass per unit length than the polystyrene standards 4b. Unfortunately, we were unable to obtain their absolute molecular weights by laser light‐scattering studies as they tended to form large aggregates in relatively concentrated solutions 4c.…”

“…The copolymerization of S ‐ G n / L ‐ G n with organoplatinum complexes 2 or 3 is a step‐growth reaction and involves the polycondensation of the terminal acetylene moiety on the dendrimer surface with the platinumchlorine bond. The reactions were conducted under reaction conditions (8.7 m M ; CuI, i Pr 2 NH, CHCl 3 , 40 °C, 48 h) that were identical to those involving trans ‐[Pt(PEt 3 ) 2 Cl 2 ] ( 1 )4b in order to evaluate the structural effect of the platinum linkers on the formation of the poly(dendrimer)s (Scheme ). After polymerization, the yellow products, cis ‐ Pt ‐ S ‐ G n , cis ‐ Pt ‐ L ‐ G n , long ‐trans ‐ Pt ‐ S ‐ G n and long ‐trans ‐ Pt ‐ L ‐ G n were isolated by passing them through a short column of alumina and washing with CHCl 3 to remove the copper salt, followed by precipitation in MeOH.…”

“…We reported earlier that poly(dendrimer)s with a high degree of polymerization (DP) value (30–1000 based on laser light scattering data) could be obtained from the copolymerization of surface functionalized bis(acetylene) dendritic macromonomers and trans ‐[Pt(PEt 3 ) 2 Cl 2 ] ( 1 ) 4b. To further elaborate on the general efficiency of this synthetic strategy, we later modified the backbone structure of the dendritic macromonomers and found that structurally more rigid Fréchet type oligo(benzyl ether) dendrimers S ‐ G n ( n =1–3) tended to give organoplatinum poly(dendrimer)s trans ‐ Pt ‐ S ‐ G n with higher DP values than structurally more flexible oligo(3‐phenylpropyl ether) dendrimers L ‐ G n ( n =1–3) 4c.…”