Chromium tricarbonyl-facilitated nucleophilic aromatic substitution by metal carbonyl anions. Synthesis and molecular structure of a new class of bimetallic .pi.-arene complexes

The highly nucleophilic metal carbonyl anions (CpFe(CO)/sub 2/)/sup /minus// and ((C/sub 5/Me/sub 5/)Fe(CO)/sub 2/)/sup /minus// (C/sub 5/Me/sub 5/ /identical to/ Cp*) react with (/eta/-XRC/sub 6/H/sub 4/)Cr(CO)/sub 3/ substrates in a previously unknown type of nucleophilic aromatic substitution to form (/eta//sup 6/-(CpFe(CO)/sub 2/)RC/sub 6/H/sub 4/)Cr(CO)/sub 3/ products. A variety of less reactive metal nucleophiles, including (CpMo(CO)/sub 3/)/sup /minus//, (CpNi(CO))/sup /minus//, (Mn(CO)/sub 5/)/sup /minus//, and (Co(CO)/sub 4/)/sup /minus//, fail to participate in the substitution reactions. The structure of (/eta/-(CpFe(CO)/sub 2/)ClC/sub 6/H/sub 4/)Cr(CO)/sub 3/ has been determined by X-ray crystallography. The compound crystallizes in the space group P2/sub 1//n with four molecules in the unit cell of dimensions a = 7.969 (2) /angstrom/, b = 18.982 (4) /angstrom/, c = 10.789 (2) /angstrom/, and /beta/ = 91.45 (3). Full-matrix least-squares refinement yielded R = 0.0353 for 2153 reflections. The structure shows that the conformation of the Cr(CO)/sub 3/ fragment is determined by a cogging of the carbonyl ligands of the CpFe(CO)/sub 2/ and Cr(CO)/sub 3/ units to avoid steric interactions, although /sup 1/H and /sup 13/C NMR studies failed to show a perceptible barrier to Cr(CO)/sub 3/ rotation about the Cr-Ph/sub centroid/ vector. Certain haloarene substrates react predominantly through an apparent electron-transfer pathway to produce (CpFe(CO)/sub 2/)/sub 2/ and (/eta/-RC/sub 6/H/sub 5/)Cr(CO)/sub 3/. The fraction of reduced products formed is dependent on (1) the reducing power of the anion ((Cp*Fe(CO)/sub 2/)/sup /minus//>> (CpFe(CO)/sub 2/)/sup /minus//), (2) the electron-donating ability of the R group (electron donor >> electron acceptor), (3) the substitution pattern of the arene (in general ortho >> metal /congruent/ para), and (4) the identity of the halogen leaving group (I >> Cl > F).