Ligand steric bulk: A neglected factor in the formation of CrCr supershort contacts

Abstract

The ability of three center chelating ligands to form dichromium units and to enforce short and very short CrCr contacts is examined in a consistent series of cyclohexyl amidinate chromium complexes, with the aim of understanding the factors intrinsic in the nature of the bridging ligands which are able to promote or disfavor dinuclear aggregation and to determine the extent of intermetallic separation. The present study describing the crystal structures of the dinuclear [(CyNC(H)NCy) 2Cr] 2·C 7H 8 ( 1) and mononuclear (CyNC(CH 3)NCy) 2Cr ( 2), {CyNC[Ph( o-CH 2NMe 2)]NCy} 2Cr·2THF ( 3) and [(Me 3Si)NC(Ph)N(SiMe 3)] 2Cr ( 4) indicates that the steric interaction between the lateral cyclohexyl and the central amidinate group is capable of determining the nuclearity (monomeric versus dimeric) and probably the extent of intermetallic separation. Crystal data are as follows. 1: C 52H 84N 8Cr 2·toluene, M=1017.41, monoclinic, P2 1/ c, a=13.682(5), b=17.446(3), c=24.521(3) Å, β=90.02(5)°, V=5853(3) Å 3, Z=4, T=−30 °C, Mo Kα, R=0.104, R w=0.077 for 4030 reflections out of 10319 unique; 2: C 28H 50N 4Cr, M=494.73, orthorhombic, Pnma, a=8.104(1), b=26.572(4), c=12.533(2) Å, V=2699(1) Å 3, Z=4, T=−157 °C, Mo Kα, R=0.054, R w=0.069 for 1580 reflections out of 2752 unique; 3: C 44H 68N 6Cr·2THF, M=853.16, triclinic, P 1 , a=12.293(5), b=20.437(5), c=11.071(4) Å, α=94.55(3), β=111.72(3), γ=73.86(3)°, V=2481(1) Å 3, Z=2, T=−157 °C, Mo Kα, R=0.093, R w=0.081 for 6492 reflections out of 8710 unique.