The Structure and Fluxionality of Cobalt-Based Tetrahedral Organometallic Clusters

Abstract

<p>A series of trimetal tetrahedra were synthesized with the goal of studying their structures and the mechanistic processes which occur during fluxionality of their ligands. The basic skeleton utilized was a CO₃C tetrahedron, varied to include Mo or W on occasion, but always maintaining the four-vertex core.</p> <p>The bidentate ligands DIPHOS (bis-diphenylphosphinoethane) and ARPHOS (1-diphenylphosphino-2-diphenylarsinoethane) coordinate to tricobalt and mixed-metal clusters, bonding only at the cobalt vertices. The DIPHOS chelate remains tightly bound to the cluster while the carbonyl ligands migrate about the vertices. The ARPHOS ligand cleaves at the As-Co linkage and can move between Co vertices. This process, as well as the concomitant CO migration, is slowed on the NMR timescale at reduced temperature.</p> <p>In a system with a carbonyl ligand bridging two cobalt vertices of a tricobalt cluster, CO₃(CO)₄(C₅H₅)₂C-C₆H₅, the ¹³C NMR spectrum was observed to determine if the EHMO-predicted favoured orientation of the phenyl ring could be frozen out in solution in the NMR experiment. A xylyl ring was incorporated into the system as a probe for loss of mirror symmetry. At a range of temperatures, the clusters did not show cessation of the rotation. The tetrahedral cluster CO₃(CO)₄(C₅H₅)₂C-C₆H₅ crystallizes in the monoclinic space group P2₁2₁2₁, and the phenyl ring was found to be oriented parallel to the bridging CO unit, corresponding to the proposed structure as predicted by EHMO theory.</p> <p>The bulky dinaphthyl fragment CH(4-CIC₁₀H₆)₂ was incorporated into a tricobalt nonacarbonyl cluster as the capping ligand to investigate possible hindrance to exchange of the CO ligands. Cessation of carbonyl exchange was observed on the DDT derivative and other analogues, but the relatively unstable dinaphthyl cluster showed no evidence of this behaviour, attributed to the inability of the naphthyl rings to rotate because of steric restrictions. The crystal structure of CO₃(CO)₉C-CH-(4-CIC₁₀H₆)₂ was solved.</p> <p>The molecules M(CP)(CO)₂Co₂(CO)₆C-CO₂CH(CH₃)₂, (M=Mo, W; CP = (C₅H₅), (C₅H₄CH₃), [C₅(CH₃)₅]) adopt two conformations at reduced temperatures in the NMR experiment, dependent on the position of the cyclopentadienyl ring: one involves the ring in a conventional axial position (accompanying CO ligands equatorial), while in the second isomer, the ring centroid is above the trimetal plane, with the two carbonyl ligands semi-bridged. Isomer interchange occurs rapidly at room temperature. The favoured isomer in the C₅H₅ and C₅H₄CH₃ clusters has the ring below the basal plane, while the larger C₅(CH₃)₅ rings prefer the "up" position, allowing distribution of excess electron density to the remaining vertices via the carbonyl ligands. [C₅(CH₃)₅]Mo(CO)₂CO₂(CO)₆C-CO₂CH(CH₃)₂ crystallizes in the space group P2₁/n, and displays the large C₅(CH₃)₅ ring in the area of least unfavourable interaction with the bulk of the cluster.</p>