nido-Carborane Building-Block Reagents. 1. Polycyclic Arene RR′C₂B₄H₆ Derivatives: Synthesis via Indenyl and Fluorenyl Alkynes and Metal-Promoted Oxidative Fusion

The preparations of C-mono- and C,C′-bis(indenylmethyl) and C-mono- and C,C′-bis(fluorenylmethyl) derivatives of nido-2, 3-dicarbahexaborane(8) (C₂B₄H₈) are described. These carboranes are multifunctional, presenting several aryl and carboranyl sites for metal η⁵ or η⁶ coordination, and are designed as structural modules for use in the construction of multiunit metal sandwich complexes and proposed electron-delocalized solid-state arrays. The carborane syntheses were conducted via the method of Hosmane and Grimes, involving the reaction of alkynes with B₅H₉ in the presence of (C₂H₅)₃N. However, these preparations required a priori the syntheses of the first known difluorenyl and diindenyl alkynes, as well as an improved synthesis of (9-fluorenylmethyl) acetylene. In the formation of the bis(indenylmethyl)acetylene from 1-lithioindene, an unexpected 1 → 2 rearrangement occurred to give the 2-substituted product exclusively. The new alkynes and carboranes were characterized via multinuclear NMR, IR, and mass spectroscopy, and the properties of the rado-RR′C₂B₄H₆ carboranes toward bridge deprotonation, complexation with Fe²⁺, and subsequent fusion to generate R₂R′₂C₄B₈H₈ products were explored. All of the new RR′C₂B₄H₆ species are deprotonated by NaH in THF to give the RR′C₂B₄H₅^- anions, and the monosubstituted anions (R = indenylmethyl, fluorenylmethyl; R′ = H) undergo slow Fe²⁺-promoted fusion to form R₂C₄B₈H₁₀. However, the disubstituted R₂C₂B₄H₅^- anions do not fuse, owing to steric hindrance by the bulky R groups; the diindenyl species forms a complex of the type (R₂C₂B₄H₄)Fe(THF)_x (x = 2 or 3), which does not react further. Treatment of the neutral diindenyl carborane with FeCl₂ gave in low yield an apparent intramolecular sandwich complex in which the iron is proposed to be η⁵-coordinated to the C₅ rings of both indenyl groups.