A 10-Vertex Iron−Dicarbollide System Formed by Insertion of {Fe(CO)₄} into 8-Vertex [<i>closo</i>-1-CB₇H₈]⁻: Synthesis and Reactivity Studies

Reaction of [NBun4][closo-1-CB7H8] with [Fe2(CO)9] in refluxing THF (tetrahydrofuran) gives the neutral ferradicarborane [2,2,2-(CO)3-1-OH-closo-2,1,10-FeC2B7H8] (3), which, upon treatment with PEt3 in the presence of Me3NO, yields [2,2-(CO)2-2-PEt3-1-OH-closo-2,1,10-FeC2B7H8] (4). In 3 and 4 the 10-vertex {FeC2B7} cluster has two carbon vertices in antipodal, four-connected positions and the iron atom in a five-coordinate site adjacent to a carbon vertex that bears an OH substituent. Treatment of 4 in THF with NaH and then R−X (X = Br, I) affords [2,2-(CO)2-2-PEt3-1-OR-closo-2,1,10-FeC2B7H8] (R = Me (5), CH2CHCH2 (6), CH2CCH (7), CH2CCMe (8)). The pendant allyl group in 6 becomes η2-bonded to the iron center upon treatment with Me3NO in CH2Cl2, giving [2-CO-2-PEt3-1,2-σ:η2-OCH2CHCH2-closo-2,1,10-FeC2B7H8] (9), which then with CNXyl (Xyl = C6H3Me2-2,6) in the presence of PEt3 gives [2,2-(CNXyl)2-2-PEt3-1-OCH2CHCH2-closo-2,1,10-FeC2B7H8] (10). Reaction between compound 7 and [Co2(CO)8] in petroleum ether yields [2,2-(CO)2-2-PEt3-1-{OCH2-(μ-η2:η2-CCH)Co2(CO)6}-closo-2,1,10-FeC2B7H8] (12), in which a {C2Co2} tetrahedron is linked to the 10-vertex {FeC2B7} cluster by an −OCH2− bridge. Heating compound 7 with [Co(CO)2(η-C5H5)] in refluxing toluene gives the complex [2,2-(CO)2-2-PEt3-1,4-μ-{OC(H)C(Me)-B}-closo-2,1,10-FeC2B7H7] (13a), which contains a 5-membered ring that involves one carbon vertex and an adjacent boron atom. Compound 13a yields [2-CO-2,2-(PEt3)2-1,4-μ-{OC(H)C(Me)-B}-closo-2,1,10-FeC2B7H7] (13b) upon addition of further PEt3 in the presence of Me3NO. Treatment of compounds 7 and 8 with PEt3 in CH2Cl2 and in the presence of Me3NO affords the zwitterionic ylide species [2-CO-2-PEt3-1,2-μ-{OCH2C{C(R′)PEt3}}-closo-2,1,10-FeC2B7H8] (R′ = H (15), Me (17), respectively). In the latter pair, an −OCH2C{C(R′)PEt3} group bonded to a cage carbon atom is also directly σ-bonded (at the β-position) to the adjacent iron vertex, forming a 5-membered ring. The novel structural features of compounds 4, 10, 12, 13, 15, and 17 have been confirmed by single-crystal X-ray diffraction studies.