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Substituent Effects in Formally Quintuple-Bonded ArCrCrAr Compounds (Ar = Terphenyl) and Related Species

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posted on 24.12.2007 by Robert Wolf, Chengbao Ni, Tailuan Nguyen, Marcin Brynda, Gary J. Long, Andrew D. Sutton, Roland C. Fischer, J. C. Fettinger, Matthew Hellman, Lihung Pu, Philip P. Power
The effects of different terphenyl ligand substituents on the quintuple Cr−Cr bonding in arylchromium(I) dimers stabilized by bulky terphenyl ligands (Ar) were investigated. A series of complexes, ArCrCrAr (14; Ar = C6H2-2,6-(C6H3-2,6-iPr2)2-4-X, where X = H, SiMe3, OMe, and F), was synthesized and structurally characterized. Their X-ray crystal structures display similar trans-bent C(ipso)CrCrC(ipso) cores with short Cr−Cr distances that range from 1.8077(7) to 1.8351(4) Å. There also weaker Cr−C interactions [2.294(1)−2.322(2) Å] involving an C(ipso) of one of the flanking aryl rings. The data show that the changes induced in the Cr−Cr bond length by the different substituents X in the para positions of the central aryl ring of the terphenyl ligand are probably a result of packing rather than electronic effects. This is in agreement with density functional theory (DFT) calculations, which predict that the model compounds (4-XC6H4)CrCr(C6H4-4-X) (X = H, SiMe3, OMe, and F) have similar geometries in the gas phase. Magnetic measurements in the temperature range of 2−300 K revealed temperature-independent paramagnetism in 14. UV−visible and NMR spectroscopic data indicated that the metal−metal-bonded solid-state structures of 14 are retained in solution. Reduction of (4-F3CAr‘)CrCl (4-F3CAr‘ = C6H2-2,6-(C6H3-2,6-iPr2)2-4-CF3) with KC8 gave non-Cr−Cr-bonded fluorine-bridged dimer {(4-F3CAr‘)Cr(μ-F)(THF)}2 (5) as a result of activation of the CF3 moiety. The monomeric, two-coordinate complexes [(3,5-iPr2Ar*)Cr(L)] (6, L = THF; 7, L = PMe3; 3,5-iPr2Ar* = C6H1-2,6-(C6H-2,4,6-iPr3)2-3,5-iPr2) were obtained with use of the larger 3,5-Pri2-Ar* ligand, which prevents Cr−Cr bond formation. Their structures contain almost linearly coordinated CrI atoms, with high-spin 3d5 configurations. The addition of toluene to a mixture of (3,5-iPr2Ar*)CrCl and KC8 gave the unusual dinuclear benzyl complex [(3,5-iPr2Ar*)Cr(η36-CH2Ph)Cr(Ar*-1-H-3,5-iPr2)] (8), in which a C−H bond from a toluene methyl group was activated. The electronic structures of 58 have been analyzed with the aid of DFT calculations.

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