Substituent effects in formally quintuple-bonded ArCrCrAr compounds (Ar = terphenyl) and related species

The effects of different terphenyl ligand substituents on the quintuple Cr-Cr bonding in arylchromium(1) dimers stabilized by bulky terphenyl ligands (Ar) were investigated. A series of complexes, ArCrCrAr (1-4; Ar = C6H2-2,6-(C6H3-2,6-Pr-i(2))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) angstrom. There also weaker Cr-C interactions [2.294(1)-2.322(2) angstrom] 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 1-4. UV-visible and NMR spectroscopic data indicated that the metal-metal-bonded solid-state structures of 1-4 are retained in solution. Reduction of (4-F3CAr')CrCl (4-F3CAr' = C6H2-2,6-(C6H3-2,6-Pr-i(2))2-4-, CF3) with KC8 gave non-Cr-Cr-bonded fluorine-bridged dimer {(4-F3CAr')Cr(mu-F)(THF)}(2) (5) as a result of activation of the CF3 moiety. The monomeric, two-coordinate complexes [(3,5-(Pr2Ar)-Pr-i*)Cr(L)] (6, L = THF; 7, L = PMe3; 3,5-(Pr2Ar)-Pr-i* = C6H1-2,6-(C6H-2,4,6-Pr-i(3))(2)-3,5-Pr-i(2)) were obtained with use of the larger 3,5-Pr-2(i)-Ar* ligand, which prevents Cr-Cr bond formation. Their structures contain almost linearly coordinated Cr-I atoms, with high-spin 3d(5) configurations. The addition of toluene to a mixture of (3,5-iPr2Ar*)CrCl and KC8 gave the unusual dinuclear benzyl complex [(3,5-(Pr2Ar)-Pr-i*)Cr(eta(3):eta(6)-CH2Ph)Cr(Ar*-1-H-3,5-Pr-i(2))] (8), in which a C-H bond from a toluene methyl group was activated. The electronic structures of 5-8 have been analyzed with the aid of DFT calculations.