Sterically Demanding Organometallic Complexes Containing Multiple Clusters or Polyphenylated Ligands

Abstract

<p>Sterically demanding organometallic complexes, in which bulky clusters or large polyphenylated groups are incorporated, possess interesting structural character as well as molecular dynamics.</p> <p>Trichloroacetate esters, p-C₆H₄(O₂C-CCI₃)₂ and C(CH₂-O₂C-CCI₃)4, can be utilized as templates in building multi-cobalt cluster systems such as p-C₆H₄(O₂C-CCo₃(CO)₉)₂ and C[CH₂-O₂C-CCo₃(CO)₉]₃(CH₂-O₂C-CCI₃). The crystal structure of C(CH₂-O₂C-CCI₃)₄ has been obtained and a molecular modelling study was performed on the tetra-cluster C[CH₂-O₂C-CCo₃(CO)₉]₄.</p> <p>The 1,1,1-trichloroacetylacetonate complexes of Co(II), Co(IIl), or AI(III) can also be used as templates in the preparation of multi-cluster molecules by reaction with CO₂(CO)₈. The uncomplexed cluster ligand, CH₃C(O)CHC(OH)CCo₃(CO)₉, was structurally characterized. Its direction of enolization was studied by EHMO (Extended Huckel Molecular Orbital) calculations. The tendency to enolize is also found in (benzoylacetone)Cr(CO)₃ which was characterized by x-ray crystallography.</p> <p>The Diels-Alder reaction of tetraphenylcyclopentadienone (tetracyclone) with triphenylcyclopropene yields a ketone adduct, C₇Ph₇HCO, with an endo hydrogen at the methine carbon; thermal elimanation of CO produces heptaphenylcycloheptatriene, C₇Ph₇H, a sterically crowded polyaryl system. This initially formed conformer with a pseudo-equatorial phenyl substituent at the sp³ position is sterically hindered and undergoes a conformational flip which allows the phenyl group to occupy the favored pseudoaxial site. However, the remaining phenyls are sterically encumbered and slowed rotation of these rings can be monitored by ¹H and ¹³C NMR spectroscopy. The intermediacy of the conformer is shown by the formation of isomers of C₇(p-tolyl)₂Ph₅H, arising via a series of [1,5] hydrogen shifts that are possible only for this conformer. Similarly, use of 2,5-dimethyl-3,4-diphenyl-cyclopentadienone yields C₇Me₂Ph₅H in which [1,5] hydrogen shifts also occur.</p> <p>The heptaphenyltropylium ion, C₇Ph₇⁺Brˉ or C₇Ph₇⁺BF₄ˉ, has a planar central seven membered ring. The C₇Ph₇ˉK⁺ anion, which is prepared from C₇Ph₇⁺Brˉ and potassium metal, reacts with Me₃SnCI and SiMe₂HCI to form σ-bonded derivatives, C₇Ph₇-SnMe₃ and C₇Ph₇-SiMe₂H.</p> <p>The reaction of Cr(CO)₆ with C₇Ph₇H, or C₇Me₂Ph₅H, leads to incorporation of a Cr(CO)₃ group on a peripheral phenyl ring rather than on the seven-membered ring. However, the tricyclic ketone, C₇Ph₇HCO, which is the precursor to C₇Ph₇H, reacts with Mo(CO)₆ to give the π-allyl complex, (η⁵-C₅Ph₄OH)Mo(CO)₂(η³-C₃Ph₃H₂), in which the triphenylcyclopropene ring has been opened as seen in its x-ray crystal structure.</p>