Metal Cluster Stabilized Fluorenyl, Indenyl, and Cyclopentadienyl Antiaromatic Cations:  An NMR and X-ray Crystallographic Study

Abstract

Treatment of fluorenone, 2,3-diphenylindenone, tetraphenylcyclopentadienone, or 2,5-diethyl-3,4-diphenylcyclopentadienone with ((trimethylsilyl)ethynyl)lithium gives, after hydrolysis, the analogous alkynol; subsequent addition of dicobalt carbonyl and then fluoroboric acid yields the corresponding fluorenyl, indenyl, or cyclopentadienyl cation stabilized by complexation to a tricarbonylcobalt moiety. Variable-temperature NMR data on these cluster cations, and on their bis(diphenylphosphino)methane derivatives, reveal that the barrier to migration of the cationic center between cobalt cluster vertices increases in the order fluorenyl < indenyl < cyclopentadienyl and suggest that the cations with more antiaromatic character have the greatest need for charge delocalization onto the metal center. Replacement of a Co(CO)3 cationic fragment by an Fe(CO)3 unit yields the mixed-metal species [((fluorenyl)CCSiMe3)FeCo(CO)6] (22) and [((2,3-diphenylindenyl)CCSiMe3)FeCo(CO)6] (27). In these structural models for the cationic complexes, the Fe−C(9) distance in 22 is 2.626(11) Å, while in the indenyl system 27 the Fe−C(1) distance is 2.347(7) Å, again indicating that the 8π indenyl cation interacts more strongly with the metal center than does the 12π fluorenyl cation.

Solvent‐Mediated Generation of Cobalt‐Cluster‐Stabilised Propargyl Cations and Radicals: Allyl Migration versus Peroxide Formation

“…the resulting terminal alkyne-dicobalt complex to give [Co 3 (CO) 9 (9H-fluorenylmethylcarbynyl)] (24); moreover, dimerisation of the (benzyldimethylsilyl)ethynyl-9H-fluorenyl moiety led to the propargyl-allene 26. In contrast, protonation of 5 in THF yielded [{Co 2 (CO) 6 …”

“…Moreover, such stabilisation by a metal allows the convenient generation of otherwise inaccessible species such as the non-classical bornyl or fenchyl cations, [7,8] or the antiaromatic indenyl or fluorenyl cations. [9] This phenomenon, together with the known enhanced stabilisation of b-silyl cations, [10] has been elegantly exploited by Schreiber in a number of cyclisations, [11] as depicted in Scheme 2. An additional factor in this process is the bending of the alkyne upon complexation to a {Co 2 (CO) 6 } moiety such that the originally linear C À C C À C unit now makes angles of approximately 135-1458 at the central carbon atoms, thus permitting the formation of cycloalkynes containing as few as six ring carbon atoms.…”

The alkyne-anion promoted ring-contraction of hexachlorotropone: synthesis and structure of [trimethylsilyl(pentachlorobenzoyl)ethyne]-hexacarbonyldicobalt

“…These have encompassed cobalt and molybdenum complexes of formally nonclassical systems, such as the 2-bornyl or 2-fenchyl cations [1][2][3][4][5], cobalt and iron derivatives of the cyclopentadienyl, indenyl and fluorenyl cations which possess 4p, 8p and 12p electrons, respectively, and are formally antiaromatic [6], and also chromium-stabilized benzyl cations [7]. In such cases, the cationic carbon leans toward the relatively electron-rich metal so as to alleviate the electron deficiency by direct overlap with a filled metal orbital [8].…”

“…Our goal was to prepare a system of the type [(C 7 Cl 6 )ML n ] þ , where ML n is an organometallic moiety, such as (RCBC)Co 2 (CO) 6 or ferrocenyl, both of which are known to stabilize a neighbouring carbocationic site [8,15]. In cases where the ligand itself is stable as a cation, such as in the (C 6 H 5 )Cr(CO) 3 -pyrylium salt, 2 [16], or the ferrocenyl-tropylium cation, 3 [17], the cyclic p system retains its planarity since it has no need for additional stabilization from the metal center.…”

“…Crystallographic collection and refinement parameters for6 The solution was then allowed to stir for 24 h at room temperature. After removal of solvent, the residue was subjected to flash chromatography, using hexanes as eluent, to give dark red crystals of 6 in essentially quantitative yield 1.…”