Poly(3-alkylthiophene)s and Poly(3-alkylthiophene ketone)s Via Coupling of Organomercurials and a Dicarbenium Ion Stabilized by a Molybdenum Dimer

The synthesis of polythiophenes, polymers that are electrically conductive, has mainly been limited to methods which employ nucleophilic reagents or may result in conjugation breaking defects. A new chemical synthesis of poly(3-alkylthiophene)s, the first to make well-characterized polymers using organomercurials, was demonstrated. Polythiophene and poly(3-methylthiophene) were produced as insoluble red powders by a palladium catalyzed coupling with copper. With side chain substitution, soluble regiorandom poly(3-alkylthiophene) (alkyl = butyl, octyl, ethyl acetate, hexyl acetate), as well as the copolymers poly(3-butylthiophene-co-3-methylthiophene), poly(3-butylthiophene-co-ethyl thiophene-3-acetate), and three mixtures of poly(3-octylthiophene-co-thiophene) (80/20; 60/40; 50/50) were synthesized and characterized. These materials had M$\sb{\rm W}$ = 5300-83000 and conductivities up to 2 S/cm. In addition, this method offers many advantages: exclusive $\alpha$-$\alpha\sp\prime$ linkages, wide variety of functional groups tolerated, facile copolymerization, and alternating copolymerization with CO to give a new class of materials, the poly(3-alkylthiophene ketone)s (alkyl = methyl, butyl, ethyl acetate). These polymers, having M$\sb{\rm W}$ = 3000-10800, were irreversibly reduced in THF around $-$1.0 V vs SCE. The chemistry of $\rm Cp\sb2Mo\sb2(CO)\sb4$(alkyne) compounds was also examined to model their use in organometallic polymers. Treatment of $\rm Cp2Mo\sb2(CO)\sb4(HOCH\sb2CCCH\sb2OH)$ with $\rm HBF\sb4{\cdot}Et\sb2O$ in $\rm CH\sb2Cl\sb2$ produced the dication $\rm\lbrack Cp\sb2Mo\sb2(CO)\sb4 (CH\sb2CCCH\sb2)\rbrack\lbrack BF\sb4\rbrack\sb2$ which was recrystallized from MeCN and characterized by single-crystal x-ray diffraction. This compound was found to undergo nucleophilic addition with ($\sp{\rm i}$Pr)$\sb2$NH and MeOH, but reduction with Na/Hg did not produce oligomers cleanly. This behavior was rationalized by an EHMO analysis, which indicated that all the positive charge of the carbenium ion is transferred to the metal atoms, and the regioselectivity of the reactions with nucleophiles is frontier-orbital controlled. Masked dicarbenium ions, $\rm Cp\sb2Mo\sb2(CO)\sb4$ $\rm (MeCO\sb2CH\sb2CCCH\sb2O\sb2CMe)$ and $\lbrack\rm Cp\sb2Mo\sb2(CO)\sb4(LCH\sb2CCCH\sb2L)\rbrack\lbrack BF\sb4\rbrack\sb2$ (L = tert-butylpyridine) were found to undergo limited substitution