Syntheses and Investigations of Bi- and Trimetallic Organotransition Metal Clusters

Abstract

<p>The reaction of dicobalt octacarbonyl with a series of 1,1,1-trichloro-2,2-bis(aryl)ethanes yields the corresponding Co₃(CO)₉[μ³-CCHRR'] tetrahedral clusters, where R= R' = 4-methoxyphenyl or R = 4-chlorophenyl, R' = 2-chlorophenyl. The nine-fold degeneracy of the cobalt carbonyl ligands in these clusters is split on the NMR time-scale at low temperature. In order to investigate the fluxional processes, the three axial carbonyl ligands were replaced by the tripodal moiety HC(Ph₂P)₃. The molecular structure of (HC (Ph₂P)₃)CO₃ (CO)₆[μ³-CCH₃] was determined by using X-ray crystallography. These data, in conjunction with the variable-temperature NMR results revealed the existence of several independent fluxional processes.</p> <p>A series of enantiomerically pure dicobalt μ-alkyne clusters which possess diastereotopic cobalt vertices were synthesized by the treatment of [CO₂ (CO)₆] [HC≡C-CH₂OH] with menthol or borneol, or by the reaction of Co₂ (CO)₈ with 2-endo-propynyl-borneol; the latter cluster was characterized X-ray crystallographically. Carbonyl substitution by phosphines occurs with some degree of diastereoselectivity. Moreover, diastereoselective replacement of a metal vertex by isolobal groups (C₅H₄R)M(CO)₂, where R = H or CH₃ and M = Mo or W, has been observed by using NMR spectrosopy.</p> <p>The reaction of [(C₅H₅)Mo(CO)₂]₂ with 2-endo-propynyl-borneol yields the crystallographically characterized tetrahedral cluster (R*C≡≡CMe)Mo₂ (CO)₄ (C₅H₅)₂, where R* is the 2-norbornenyl group. Protonation of this cluster and the above-mentioned hexacarbonyldicobalt species leads to metal stabilized cations which maintain their terpenoid skeletons and are stable towards carbocationic rearrangement. High field NMR studies on the mixed metal cationic clusters [M-Co (CO)₃ (HC≡C-CR₂)]⁺, where M = (C₅H₅)Mo(CO)₂, (C₅H₄Me)W(CO)₂ or (C₅H₄Me)Mo(CO)₂, reveal that the capping vinylidene moiety leans preferentially towards one vertex. NMR data, together with EHMO calculations, on the related trimetallic cluster cation [(C₅H₅)Mo(CO)₂CO₂(CO)₆CCR₂]⁺ demonstrate that the positive charge is better tolerated by the (C₅H₅)Mo(CO)₂ fragment than at the Co(CO)₃ vertex.</p>