Organometallic Polymers with Transition Metals in the Main Chain.

Since the mid 1990s soluble, well-characterized high molecular weight metal-containing and metallosupramolecular polymers have become readily available for the first time, even in some cases, with narrow molecular weight distributions and controlled architectures such as block copolymers. This has led to a rapidly expanding interest in their properties and uses. The review provides a survey of the range of applications for these new materials, which combine the processing advantages of polymers with the functionality provided by the presence of metal centers.

Metallopolymers: New Multifunctional Materials

The appearance of ferrocene in the middle of the 20th century has revolutionized organometallic chemistry and is now providing applications in areas as varied and sometimes initially unexpected as optical and redox devices, battery and other materials, sensing, catalysis, including asymmetric and enantioselective catalysis, and medicine. The author presents here a general, although personal, view of ferrocene's chemistry, properties, functions, and applications through a literature survey involving both historical and up-to-date trends and including examples of his group's research in a number of these areas. The review gathers together general features of ferrocene chemistry and representative examples of the salient aspects. Its focus is on ferrocene's basic properties, ferrocene-containing ligands, the ferrocene/ferricinium redox couple, ferrocene mixed-valence and average-valence systems, the ferricinium/ferrocene redox shuttle in catalysis, ligand-exchange reactions, ferrocene-containing polymers, ferrocene-containing structures for cathodic battery and other materials, ferrocenes in supramolecular ensembles, liquid crystals, and nonlinear optical materials, ferrocene-containing stars and their electrostatic effects, ferrocene-containing dendrons, dendrimers, and nanoparticles (NPs) and their application in redox sensing and catalysis, and ferrocenes in nanomedicine.

Why is Ferrocene so Exceptional

The incorporation of metal centers into the backbone of polymers has led to the development of a broad range of organometallic and coordination compounds featuring properties that are relevant for potential applications in diverse areas of research, ranging from energy storage/conversion to bioactive or self-healing materials. In this review, the basic concepts and synthetic strategies leading to these types of materials as well as the scope of available characterization techniques will be summarized and discussed.

Synthesis and characterization of metallo-supramolecular polymers

Electron and energy transfer are ubiquitous in biological, chemical, and physical processes, which has led to extensive multidisciplinary research efforts to elucidate the factors influencing mechanistic pathways. Of considerable importance in these studies have been investigations of intramolecular electron and energy transfer within polymetallic assemblies as a result of the diversity (coordination number, ligand environment, stereochemistry, and redox characteristics) provided by the metal centers in such structures.

Because of their multicomponent nature, these structures have considerable design potential to exploit the cooperation between the metals and/or other redox-active centers. Novel photochemical molecular devices (PMDs) may be constructed which are capable of performing useful light- and redox-induced functionsincluding artificial photosynthesis and photoinduced energy- and electron-transfer processes in light-harvesting “antenna” systems. Metallosupramolecular assemblies have also been designed to mimic the photoinduced charge separation function in photosynthetic organisms, in an attempt to elucidate the complex electron- and energy-transfer mechanisms which occur in natural systems. The possibility of multiple electron transferby absorption of several photons by linked chromophores, or the design of systems which generate more than one electron upon absorption of one photonhas significant implications in catalytic schemes, as well as understanding long-range electron transfer in biological systems, and the conductivity of “molecular wires”. In molecules involving delocalized unpaired electrons, polarizability may be present so that the species exhibit interesting nonlinear optical or magnetic properties.

Polypyridyl complexes of the d^6 metals Fe^II, Ru^II, and OsII have attracted particular attention as the basis of these assemblies due to a combination of favorable photophysical and redox characteristics, the longevity of their excited states, and their chemical inertness in a variety of oxidation states. An important feature of these complexes is the capability of systematic variation of the ground- and excited-state properties by the judicious choice of the coordinating ligands.

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Intervalence charge transfer (IVCT) in trinuclear and tetranuclear complexes of iron, ruthenium, and osmium.

Oligo- and poly-(1,1′-dihexylferrocenylenes), more soluble than unsubstituted oligo- and poly-(1,1′-ferrocenylenes), were synthesized by reaction of dihexylfulvalene dianion with [FeCl2(thf)2](thf = tetrahydrofuran). Electrochemical properties of pentamers and a hexamer were revealed and compared with theory for a system with a linear combination of redox centres. Charge-transfer complexes of poly(dihexylferrocenylene)(M= 5000, M/Mn= 1.2) with tetracyanoethylene and 7,7,8,8-tetracyanoquino-dimethane exhibit photoconductivity.

Synthesis, electrochemistry and photoconductivity of oligo- and poly-(1,1′-dihexylferrocenylenes)

Interaction over three redox centers at conjugated oligomeric ferrocenes with N = 2 to 6 nuclei

Redox Properties of Hepta(1,1′-dihexylferrocenylene)

Ferrocene, 1,1,″-biferrocene and 1,1″ : 1′″,1″″-terferrocene derivatives involving a (cycopentadiene)Fe(CO)3 moiety were synthesized; the ν(CO) peak frequency shifts during their redox processes were monitored by the IRAS technique. All the complexes give two ν(CO) peaks, which shift to higher wavenumbers (12—18 cm−1) by the oxidation from the most reduced (all Fe’s are Fe(II)) to the most oxidized (all Fe’s are Fe(III)) forms. The biferrocene derivative in the monocationic form gives peaks at wavenumbers between those for neutral and dicationic forms, probably because the rate of electron-transfer affording two electronic isomers, Red–Ox–[FeCO] and Ox–Red–[FeCO], where Red, Ox, and [FeCO] refer to reduced and oxidized forms of a ferrocene unit, and a [(η4-cyclopentadiene)Fe(CO)3] moiety, respectively, is slower than or comparable to the time scale of IR (1011—1012 s−1). The terferrocene derivative shows ν(CO) frequency changes corresponding to the three-step 1e− oxidation processes. The shift is most sig...

IR Sensing of the Electronic Structure in the Mixed-Valence States of Iron Carbonyl-Attached Biferrocene and Terferrocene.

Summary form only given. Polyfluorene and polyindene are /spl pi/-conjugated organic polymers with both /spl eta/sub 5// - or /spl eta/sub 6// /spl pi/-coordination abilities to transition metal centers. It is interesting to understand how the electrical, optical and magnetic properties of the /spl pi/-conjugated systems in such polymers can be perturbed by the electronic interaction with d-electrons (orbitals) of transition metals. We have previously reported the electrochemical synthesis of poly(9-n-hexylfluorene) (PHF) and poly(l-n-hexylindene) (PHI), which are soluble in less polar organic solvents such as benzene or dichloromethane and thus utilizable as reagents in homogeneous reactions. In this paper we present a study on the synthesis and physical properties of transition metal complexes of PHF and PHI. Reaction of PHF with Mn(CO)/sub 5/Br in the presence of AICl/sub 3/ gives PHF-Mn(CO)/sub 3/ where PHF behaves as /spl eta/sub 5//-ligands. PHF-Mn(CO)/sub 3/ shows a shift of absorption edge in the visible spectrum to the longer wavelength and an increase in electric conductivity compared to free PHF. Synthesis and physical properties of other polymer complexes, PHF-CO(C/sub 5/Me/sub 5/)/sup +/ and PHI-Rh(C/sub 5/Me/sub 5/)/sup +/ will be also reported.

Takashi Hirao

Papers

Synthesis, electrochemistry and photoconductivity of oligo- and poly-(1,1′-dihexylferrocenylenes)

Interaction over three redox centers at conjugated oligomeric ferrocenes with N = 2 to 6 nuclei

Redox Properties of Hepta(1,1′-dihexylferrocenylene)

IR Sensing of the Electronic Structure in the Mixed-Valence States of Iron Carbonyl-Attached Biferrocene and Terferrocene.

Synthesis and physical properties of transition metal complexes of poly(9-n-hexylfluorene) and poly(l-n-hexylindene)