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

Redn. of Cr(I) dimeric chlorides gave substituted bis-terphenylyl dichromium complexes bound by formally quintuple Cr-Cr bond; the effects of terphenyl ligand substituents on the quintuple Cr-Cr bonding in arylchromium(I) dimers were investigated. Redn. of chloro-bridged Cr(II) dimers [(4'-X-Ar1)Cr(μ-Cl)]2 by KC8 gave metal-metal-bonded complexes [(4'-X-Ar1)CrCr(4'-X-Ar1)] (1-4; 4'-X-Ar1 = 4'-X-2,2'',6,6''-tetraisopropyl-1,1':3',1''-terphenyl-2'-yl, X = H, SiMe3, OMe, F), whereas similar redn. of [(4'-CF3-Ar1)Cr(μ-Cl)]2 gave [(4'-CF3-Ar1)Cr(μ-F)(THF)]2 (5) as a result of activation of the CF3 moiety. Redn. of monomeric Cr(II) chloride of the larger Ar* ligand, which prevents Cr-Cr bond formation, Ar*CrCl (Ar* = 2,2'',4,4',4'',6,6',6''-hexaisopropyl-1,1':3',1''-terphenyl-2'-yl) gave monomeric complexes [Ar*Cr(L)] (6, 7; L = THF, PMe3) and unusual dinuclear benzyl Cr(II)-Cr(0)-triple-decker [Ar*Cr(η3:η6-CH2C6H5)Cr(Ar*H)] (8). The x-ray crystal structures of the complexes 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. Weaker Cr-C interactions [2.294(1)-2.322(2) Å] involving an C(ipso) of one of the flanking aryl rings are also present. 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 d. functional theory (DFT) calcns., which predict that the model compds. (4-XC6H4)CrCr(C6H4-4-X) (X = H, SiMe3, OMe, and F) have similar geometries in the gas phase. Magnetic measurements in the temp. range of 2-300 K revealed temp.-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 soln. The structures of monomers 6 and 7 contain almost linearly coordinated Cr(I) atoms, with high-spin 3d5 configurations. The electronic structures of 5-8 were analyzed with the aid of DFT calcns. [on SciFinder(R)]