Intramolecular B-B Linkage Between Polyhedral Cages in a Commo-metallacarborane

Alternative Title

Synthesis and structure of a fluxional metal-boron cluster, [η⁵-C₅(CH₃)₅]₂Co₃(CH3)₄C₄B₈H₇

Abstract

The reaction of Li+C5(CH3)5 -, CoCl2, and Na+(CH3)2C2B4H 5 - in cold THF forms primarily closo-1,2,3-[ηs-C5-(CH3) 5]Co(CH3)2C2B4H 4 (I), the triple-decker complex 1,7,2,3-[η5-C5(CH3)5] 2Co2(CH3)2C2B 3H3 (II), a structurally novel complex, [η5-C5(CH3)5]2Co 3(CH3)4C4B8H7 (III), and traces of other products in a combined isolated yield of ∼50%. Complexes I and II are C5(CH3)5 analogues of C5H5-substituted cobalta carboranes reported earlier, but III has no previous counterpart. An X-ray diffraction study of III disclosed a structure consisting of two identical [η5-C5(CH3)5]Co(CH 3)2C2B4H3 units face coordinated to a third cobalt atom, with a direct B-B bond [1.758 (5) Å] between the two ligands; the linked boron atoms have no terminal hydrogen atoms in the solid-state structure. The “extra” hydrogen atom was not directly located, but its general position was indicated by a 3.5° fold in the molecule which creates a larger interligand pocket on one side than on the other. In the crystal, the unique hydrogen is proposed to reside in the larger pocket (in the vicinity of the central cobalt atom), but 11B and 1H FT NMR and infrared evidence indicates that, in solution, this hydrogen atom tautomerizes between equivalent terminal positions on the (formerly) linked boron atoms on the two ligands, with cleavage of the B-B link. Possible mechanisms of formation and stereochemical implications of the structure and fluxional behavior of III are presented. The chemistry of complex I resembles that of its C5H5-substituted counterpart; thus, degradation of I in basic CH3CN produces nido-1,2,3-[η5-C5(CH3) 5]Co(CH3)2C2B3H 5 in 93% yield. Crystal data for III: space group P1, Z = 2, a = 8.555 (2) Å, b = 12.599 (2) Å, c = 16.268 (9) Å, α = 102.76 (4)°, β = 92.71 (3)°, γ = 99.15 (4)°, V = 1682 Å3, R = 0.073 for the 3739 reflections for which Fo 2 > 3σ(Fo 2). © 1981 American Chemical Society.

Department(s)

Chemistry

International Standard Serial Number (ISSN)

0002-7863

Document Type

Article - Journal

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 1981 American Chemical Society (ACS), All rights reserved.

Publication Date

01 Jan 1981

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