Transition Metal Polyynes

POLYMERS WITH METAL-CARBON a-BONDS A. Transition Metal Polyynes 

Alkynes have two mutually perpendicular sets of n orbitals. In mononuclear complexes, alkynes often coordinate like olefins. Bonding is typically best represented as direct n-bonding to the metal. This forms a jt-bonded alkyne (106), rather than as a metallocyclopropene (107). 

Even so, there are examples that are consistent with the structure being of the metallocyclopropane variety. One ready measure of the amount of each type of bonding is the location of the alkyne stretching fiequency in the IR, This absorption appears about 2200 cm in alkyne complexes while the band appears about 1750 cm for the metallacyclopropene complexes. Alkyne complexes can also act as four-electron donors. Back-donation into the metallic n orbitals is negligible in these cases, but sigma-donation occurs. Finthermore, alkynes form a number of din-uclear complexes where each perpendicular n system can be considered to formally bond to a separate metal atom. Thus care must be exercised in assigning the type of bonding present. 

Current research includes the incorporation of thiophene and pyridine rings into transition metal-containing acetyUde polymers. Kohler and coworkers have studied the photoluminescence of various platimun polyacetylides. Altering the conjugation of the spacers between the acetylide units in polymers 111-113 

Polymers containing cyclodiborazane groups in their backbones have been synthesized via hydroboration of monomers of platinum and palladium acetylides functionalized with cyano groups. These polymers (114) showed good solubility in organic solvents and possessed values between 6000 and 6300. 

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Figure 3.2 General skeleton of transition metal polyyne polymers...

The synthesis, optical, and structural properties of another Pt(II) polyyne-containing biphenyl moiety, P12, were reported recently.44 The system has also been extended to the Au(I) and Hg(II) congeners (see Sections IV and V). The influence of the metal center on the spatial extent of S and T excited states was characterized in detail. The ligand-based phosphorescence emissions can be harvested by the heavy-atom effect of these transition metals, which facilitates efficient intersystem crossing from the S] state to the T] state. 

It is conceivable that a parallel bundle of linear polyyne chains (the carbon allotrope named chaoite or carbyne) may form, via thermally-allowed (4 + 2)-n-electron cycloadditions, six-membered rings which lead to graphene sheets and/or nanotubes. However, the mechanism of nanotube formation in a plasma produced by arc discharge appears to require the presence of a transition metal atom at the rim of the growing nanotube [77 j. 

Rigid-Rod Transition Metal Acetylide Polymers 1171 Table 5.4 Third-order nonlinear optical coefficients for some Pt-polyynes [84]... 

Despite their obvious similarity to alkenes and alkynes, transition-metal alkylidene and alkylidyne complexes have not been used as building blocks for the synthesis of lowdimensional materials analogous to polyenes and polyynes. We have begun to explore the syntheses, structures, and properties of conjugated complexes and polymers derived from metal-alkylidyne complexes as part of our effort to develop the chemistry of transition-metal analogues of conjugated organic compounds. 

The third class of polymers that we will investigate in this chapter is polyynes which are prepared by the involvement of acetylene fimctional groups in the polymerization reaction. These polymers are also called rigid-rod organometallic polymers. In these types of polymers various late transition metal ions alternate with a rigid organic unit. 

Synthesis of Polyynes. Conjugated diyne and polyyne units, which exhibit unusual electrochemical, optical, and structural properties, can be efficiently constructed from 1-haloalkynes via homologation by one acetylene unit with TIPS-acetylene through a transition metal-catalyzed cross-coupling reaction, 2 which allows an access to a new class of polyyne framework. 

The interest in transition metals with <7-bound alkynyl ligands continues, since metal acetyiides are potential nonlinear optical, conducting, or liquid crystalline materials. Their chemistry and applications have been reviewed in 2003 and 2004. Another review dealing on organoiron polymers includes also rigid-rod iron acetyiides. " The chemistry of metal complexes with 1,3-diynes and polyynes was surveyed in 2001. " ... 

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