Alkynyl-Linked Poly(cobaltacarboranes). Directed Synthesis, Structures, and Electrochemistry of Linear Complexes and Benzene-Centered Pinwheels¹

Abstract

A series of B(5)- and B(7)-alkynyl-substituted derivatives of CoC2B3, CoC2B4, and Co2C2B3 cobaltacarboranes has been prepared and linked via metal-promoted cross-coupling reactions to generate several types of polynuclear molecular complexes, which were structurally characterized and studied via electrochemical techniques. The target compounds prepared include linear dicobalt species containing BC⋮CC⋮CB linkages, e.g., [Cp*Co(2,3-Et2C2B4H3-7-C⋮C)]2 (8) and [Cp*Co(2,3-Et2C2B4H3-5-C⋮C)]2 (17), trigonally symmetrical 1,3,5-triethynylbenzene-anchored tricobalt and hexacobalt complexes, e.g., {Cp*Co(2,3-Et2C2B4H3-5-C⋮C)}3C6H3 (19), {Cp*Co(2,3-Et2C2B4H3-7-C⋮C)}3C6H3 (22), and 1,3,5-[Cp*2Co2(2,3-Et2C2B3H2-5-C⋮C)]3C6H3 (21), and novel cobaltacarborane−Co2C2 mixed-cluster systems, e.g., Cp*Co(2,3-Et2C2B4H3)-7-C2HCo2(CO)6 (5) and {Cp*Co[2,3-Et2C2B4H3-7-(C2Co2CO)6]}3C6H3 (23), a nonacobalt complex. The new compounds were obtained in all cases as air-stable crystalline solids and were characterized by multinuclear NMR and mass spectra, supplemented by X-ray diffraction data for 8, 19, and 21. Electrochemical data on the alkynyl-linked compounds indicate that metal−metal communication in oxidized and reduced species occurs to a limited extent in most cases and to a considerable degree (Robin−Day class III) in the C2B3-bridged triple-decker systems.

Synthesis, Structures and Reactivity of Mono‐ and Diborylacetylenes

“…[7] Figure 2. Molecular structure of 5b in the solid state; hydrogen atoms are omitted for clarity; selected bond lengths [Å ] (8) The synthesis of compounds 6Ϫ8 was achieved by the reaction of the [(diisopropylamino)boryl]acetylenes 5a,b with 2 equiv. of HCl and 1 equiv.…”

“…of catechol, dithiocatechol or 2-hydroxythiophenol, respectively, in THF (Scheme 4). Compounds 7a and 7b are colorless solids, whereas 6a,b [8] and 8a,b are pale-yellow liquids, which are readily soluble in common organic solvents. Scheme 4 The 11 B NMR spectra exhibit broad signals at δ ϭ 24 (6a), 23 (6b), 48 (7a), 47 (7b), 38 (8a) and 37 (8b) ppm.…”

“…In refluxing toluene the mono-and diborylacetylenes 1Ϫ4 form the corresponding benzene derivatives 17Ϫ20 in the presence of a catalytic amount of Co 2 (CO) 8 (Scheme 8).…”

“…6a: [8] 7.11, 7.19 (m, 4 H, C 6 H 4 114.8, 123.9, 125.4, 126.3, 126.7, 156.4 (C 6 to a solution of (η 5 -cyclopentadienyl)bis(ethene)cobalt (0.36 g, 2 mmol) in 40 mL of hexane. The reaction mixture was stirred at room temp.…”

“…Hydrolysis of 13a−15a yields the (η 4 -1,3-di-tert-butylcyclobutadiene)cobalt complex 16. Catalytic trimerizations of mono-(1, 2) and diborylacetylenes (3,4) with Co 2 (CO) 8 lead to isomeric mixtures of triboryl-(17/17Ј, 18/18Ј) and hexaborylbenzene derivatives (19,20), respectively. The new compounds are characterized by NMR spectroscopy and mass spectrometry as well as by X-ray structure analyses for 1b, 5b, 6b, 14a, 15a and 16.…”