HALF-SANDWICH RHENIUM(III) ACYL COMPLEXES - SYNTHESIS, STRUCTURE AND CONVERSION TO RHENIUM(I) AND RHENIUM(III) ISONITRILE COMPLEXES

The synthesis, structure and reactions of rhenium(III) eta(1)-acyl com plexes bearing a cyclopentadienyl (Cp) or a pentamethylcyclopentadieny l (Cp) ligand are described. Oxidation of the rhenium(I) acyl complex Li[CpRe(CO)(2)(eta(1)-PhCO)] (1) with one equivalent of PhICl(2) affo rds the rhenium(III) acyl complex trans-CpRe(CO)(2)(eta(1)-PhCO)Cl (tr ans-2a). The diphenylcarbene complex Cp(CO)(2)Re=CPh(2) (3) is also fo rmed in this reaction. Similarly, oxidation of 1 with Br-2 gives cis/t rans-CpRe(CO)(2)(eta(1)-PhCO)Br (cis/trans-2b), which can also be obta ined from the alkoxycarbene complex Cp(CO)(2)Re=C(OEt)Ph (4) with one equivalent of Br-2. Oxidation of Li[CpRe(CO)(2)(eta(1)-PhCO)] (5) wit h one equivalent of PhICl(2), Br-2 or I-2 affords selectively the rhen ium(III) acyl complexes cis/trans-CpRe(CO)(2)(eta(1)-PhCO)X (X=Cl-I) [cis/trans-(6a-6c)]. Cleavage of the Re-C-acyl bond of trans-2a with P hICl(2) and of cis/trans-2b with Br-2 offers a convenient way to the r henium(III) dihalo complexes cis/trans-CpRe(CO)(2)Cl-2 (cis/trans-7a) and cis/trans-CpRe(CO)(2)Br-2 (cis/trans-7b) respectively. Reduction o f cis/trans-7a with sodium/amalgam gives in the presence of RNC, the r henium(I) isonitrile complexes CpRe(CO)(2)(CNR) (8:R=Et; 9:R=Pr-i; 10: R=(t)Bu). Oxidation of 8-10 with one equivalent of bromine leads selec tively to the rhenium(III) isonitrile complexes [trans-CpRe(CO)(2)(CNR )Br]Br (11:R=Et, 12: R=Pr-i, 13: R=(t)Bu). The crystal structure of th e acyl complex trans-2b and the carbonyl complex [Cp(2)Re-2(CO)(4)(mu -Br)][Re-2(mu-Br)(3)(CO)(6)] (14) are described, complex 14 being form ed by the thermal decomposition of cis/trans-6b.