Structural Diversity of Pentafluorophenylcopper Complexes. First Evidence of π-Coordination of Unsupported Arenes to Organocopper Aggregates Anand Sundararaman, † Roger A. Lalancette, † Lev N. Zakharov, ‡ Arnold L. Rheingold, ‡ and Frieder Ja ¨ kle* ,† Department of Chemistry, Rutgers University-Newark, 73 Warren Street, Newark, New Jersey 07102, and Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0358 Received April 4, 2003 The binding behavior of arenes toward pentafluorophenylcopper was investigated. Crystallization of pentafluorophenylcopper from a mixture of 1,2-dichloroethane and cyclohexane affords the homoleptic tetrameric complex [Cu(C 6 F 5 )] 4 , whereas a π-complex, [Cu(C 6 F 5 )] 4 (η 2 -toluene) 2 , is obtained from toluene solution. The unique coordination of two toluene molecules in [Cu(C 6 F 5 )] 4 (η 2 -toluene) 2 leads to major structural changes as the regular square planar tetramer is distorted toward a butterfly structure with one short and one long diagonal Cu‚‚‚Cu distance of 2.5935(3) and 3.955(1) Å, respectively. The toluene molecules are bound in an unsymmetrical η 2 -coordination mode with the shortest contacts observed between copper and the meta carbon atoms (d(Cu-C) ) 2.271(2), 2.298(2) Å) and slightly longer distances to the para positions (d(Cu-C) ) 2.339(2), 2.455(4) Å). Multinuclear NMR studies show that complexation to the intact tetrameric cluster species also occurs in neat solution of the aromatic species. Strongly coordinating solvents such as acetonitrile or DMSO, however, lead to aggregate breakdown. Introduction Studies on π-arene complexes of d10 transition metals have recently resulted in important new findings that, for example, suggest their use as catalysts for deuterium- exchange reactions 1 and show their potential as com- ponents of supramolecular assemblies and of lumines- cent materials. 2,3 Evidence of such species as inter- mediates in electrophilic aromatic substitution has also been presented. 4 However, although the structures of a number of organomercury complexes with arenes have been reported, 2,5 thus far no crystallographic evidence has been presented for similar interactions between organocopper compounds and unsupported arenes, i.e., of complexes in which chelation does not lead to stabilization of the copper-arene bonding. We are currently studying arylcopper species as part of our efforts directed at the development of new multidentate and polymeric Lewis acids. 6,7 Of particular interest in this area is the development of convenient isolable reagents for transfer of aryl groups with strongly electron-withdrawing substituents. A potential candi- date for these purposes is pentafluorophenylcopper, which can readily be synthesized through metathesis reaction of pentafluorophenyl organometallic reagents with cuprous halides. 8-10 Pentafluorophenylcopper is most conveniently prepared from the Grignard reagent and isolated as a dioxane complex, from which dioxane may subsequently be removed by careful heating to 130 °C. 10 The resulting material has been shown by cryos- copy and vapor pressure osmometry in benzene and by EI-mass spectrometry of a solid sample at 160-190 °C to consist primarily of tetrameric aggregates. 9 The reported formation of a dioxane complex is surprising as, for example, the related complexes mesitylcopper, 11 pentamethylphenylcopper, 12 and thienylcopper 13 co- * To whom correspondence should be addressed. Email: fjaekle@ andromeda.rutgers.edu. † Rutgers University-Newark. ‡ University of California, San Diego. (1) (a) Borovik, A. S.; Bott, S. G.; Barron, A. R. Angew. Chem., Int. 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