Microgels DOI: 10.1002/ange.200803232 AWater Soluble Diruthenium–Gold Organometallic Microgel** Manuel Serrano Ruiz, Antonio Romerosa,* Benjamín Sierra-Martin, and Antonio Fernandez- Barbero Selection of the combination of metal, ligand set, and spacer groups that are most appropriate to form a coordination complex with a desired function are of paramount importance in supramolecular chemistry. In particular, the establishment of reproducible methods to accomplish controlled self- organization of molecules to form polymers and homo- or heterometallic coordination aggregates is an important field of research. [1] Although important advances have already been made, few metallopolymers, which are one of the most exciting classes of functional materials, are water soluble. An important example of a water-soluble polymer is the poly- ferrocenylsilane-b-polyaminomethacrylate copolymer de- scribed by Manners and co-workers, [2] as part of their ongoing study on metallocene-based polymers. [3] Recent examples also include the water-soluble metallopolymer obtained by reaction of bipyridyl-appended poly(p-phenyleneethynylene) (PPEs) with metal ions in organic and aqueous solution. [4] Other examples of ligands that afford coordination polymers with various topologies and applications are ferrocenyl groups bearing bipyridine (bpy) [5] or carboxylate moieties. [6] A recent example of a non-water-soluble multimetallic polymer is [Sm(H 2 O) 5 ][Ru 2 (bpy) 2 (CN) 7 ], in which the CN ligands bridge the samarium and ruthenium metal centers. [7] The first air-stable water-soluble multimetallic polymer that includes mixed P,N ligands as metal-coordinating spacers has been recently reported by us. [8] This heterobimetallic complex is based on two metal-containing moieties, [CpRu] + (Cp = cyclopentadienyl) and [AgCl 2 ]  , and is bridged by the cagelike water-soluble monodentate phosphine 1,3,5-triaza-7- phosphaadamantane (pta) in an unprecedented P,N coordi- nation mode. More recently, the synthesis of silver coordina- tion polymers containing pta bridging molecules in a triden- tate P,N,N’ coordination mode [9] has been reported and several examples of pta N coordination have been pre- sented. [10] Therefore the pta molecule could be an excellent ligand from which to obtain water-soluble Ru–Au polymers which could have interesting and useful properties for a variety of applications such as magnetism, [11] nonlinear optics, [12] electrocatalysis, [13] photocatalysis, [14] photovoltaic, [15] template formation of ordered networks, [16] advanced elec- trode materials, [17] and conjugated coordination polymers. [18] Herein, we describe the first water-soluble, air-stable heterobimetallic polymeric structure based on two metal- containing moieties [CpRuCNRuCp] + and [Au(CN) 4 ]  , bridged by pta in the P,Ncoordination mode. Interestingly, this complex display gel-like properties [19] in water, specifi- cally a thermally controlled volume transition. To the best of our knowledge, this is the first example of an coordination polymer network that is sensitive to its environment. The physical and chemical properties of this complex make it a promising material for industrial and biological applications, for example, smart catalysis, drug delivery, or chemical sensing. The first strategy we attempted to obtain a water soluble Ru–Au polymer was similar to that used for the synthesis of [{CpRu(pta) 2 (DMSO-kS)}{AgCl 2 }] 1 . [8] The complex [CpRuCl(pta) 2 ](1) was reacted in a straightforward manner with AuCl 3 in water, EtOH, and DMSO, and in the presence of NaBF 4 and NaCF 3 SO 3 . This resulted in the swift formation of a stoichiometric amount of metallic gold. A second strategy, in which the [Au(CN) 4 ]  moiety (which is very stable under a wide variety of conditions) [20] was used, was then tested. The reaction of 1 with K[Au(CN) 4 ] led to the partial reduction of gold together with the formation of several new species. A careful analysis of the reaction products suggested that the presence of the chloride ion in 1 could give rise to the formation of unstable gold species which finally decompose to metallic gold. To avoid the presence of the chloride ion in the reaction, the complex [RuCp(CN)- (pta) 2 ](2) was prepared by substitution of 1 with KCN at room temperature in water (Scheme 1). Slow crystallization of a diluted H 2 O solution of [Ru(CN- kC)Cp(pta) 2 ](2) in air give colorless crystals suitable for X- ray diffraction. [21] The X-ray crystal structure of complex 2 shows that it is a mononuclear complex in the solid state and is similar to 1, except that the chloride ligand has been Scheme 1. Formation of 2 by substitution of Cl  in 1 by CN  ions. [*] Dipl.-Chem. M. Serrano Ruiz, Prof. Dr. A. Romerosa rea de Quimica Inorganica, Universidad de Almería 04071 Almería (Spain) Fax: (+ 34) 950-015-008 E-mail: romerosa@ual.es Dr. B. Sierra-Martin, Prof. Dr. A. Fernandez-Barbero Departamento de Física Aplicada, Universidad de Almería 04071 Almería (Spain) Fax: (+ 34) 950-015-909 [**] Funding was provided by Junta de Andalucía through PAI (research teams QM-317 and FQM-230) and Excellence Projects: FQM-02353 and FQM-03092, the MCRTN program AQUACHEM (MRTN-CT- 2003-503864), COST Actions D17, D29, D45, and MCYT (Spain) projects CTQ2006-06552/BQU, and MAT2006-13646-C03-02. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200803232. Angewandte Chemie 8793 Angew. Chem. 2008, 120, 8793 –8797 # 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim