PREPARATION AND CRYSTALLOGRAPHIC DETERMINATION OF STEREOCHEMISTRY FOR MONOCLINIC AND TRICLINIC CRYSTAL MODIFICATIONS OF 4,6,8-TRIMETHYLAZULENETETRARUTHENIUM ENNEACARBONYL, [(CH3)3C10H5]RU4(CO)9

The reaction of 4,6,8-trimethylazulene with Ru3(CO)12 leads to the formation of 4,6,8-trimethylazulenetetraruthenium enneacarbonyl, [(CH3)3C10H5]Ru4(CO)3, which crystallizes in two forms-a monoclinic and a triclinic modification. The crystal structure of each polymorph has been determined using molybdenum Kα radiation and counter techniques. Monoclinic crystals are obtained as small red parallelepipeds, crystallizing in space group P21/n (C2h6; no. 14) with a = 8.99 ± 0.02 Å, b = 18.56 ± 0.03 Å, c = 14.44 ± 0.02 Å, β = 96.5 ± 0.1°, Pobsd = 2,29 ± 0.02 g cm-1, and Pcalod = 2.293 g cm-3 for Z = 4. Triclinic crystals are obtained as thin red platelets crystallizing in space group A1 (Ci1; no. 2) with a = 15.267 ± 0.010 Å, b = 10.610 ± 0.007 Å, c = 14.862 ± 0.010 Å, α = 92.03 ± 0.05°, β = 101.18 ± 0.05°, γ = 90.08 ± 0.05°, Pobsd=d = 2.35 ± 0.02 g cm-3, and Pcaled = 2.326 g cm-3 for Z = 4. Both structures were refined by the method of least squares, all nonhydrogen atoms having been located in each case. Final discrepancy indices are RF = 9.49% (1844 reflections) for the monoclinic and RF = 6.34% (3158 reflections) for the triclinic modification. The two crystal forms contain experimentally indistinguishable molecular units of 4,6,8-trimethylazulenetetraruthenium enneacarbonyl, separated by normal van der Waals distances. The molecule possesses approximate Cs symmetry, with the four ruthenium atoms defining a distorted tetrahedron in which interatomic distances are found to range from 2.702 to 2.902 Å (±0.005 Å) in the monoclinic determination and from 2.698 to 2.908 Å (±0.003 A) in the triclinic study. The 4,6,8-trimethylazulene ligand is arched across the largest face of the tetrahedral ruthenium cluster (i.e., is directly bonded to three ruthenium atoms) and is bent across C(9)-C(10), C(8)…C(4), and C(7)…C(5) such that there is a dihedral angle of 126° between the planar five-membered ring and the plane defined by C(5), C(6), and C(7). The three ruthenium atoms that are associated with the azulene ligand are each bonded to two terminal carbonyl ligands while the apical ruthenium is bonded to three terminal carbonyl groups. The bonding between the 4,6,8-trimethylazulene ligand and the adjacent three ruthenium atoms is best considered in terms of delocalized “ligand-to-cluster” bonding. © 1969, American Chemical Society. All rights reserved.