Radical Substitution at Boron in Small\nCobaltacarboranes. Conversion of η⁵-C₅Me₅ to η⁴-C₅Me₅R\nComplexes¹

Abstract

As an alternative approach to metal-promoted cross-coupling reactions for the preparation\nof B-substituted organo derivatives of small cobaltacarboranes, synthesis via radical\nsubstitution reactions has been explored. Reduction of neutral Cp*Co^III(2,3-Et₂C₂B₄H₃-5-I)\n(<b>1</b>) and Cp*Co^III(2,3-Et₂C₂B₄H₄) (<b>4</b>) to generate the respective 19-electron anionic Co(II)\ncomplexes <b>1</b>^- and <b>4</b>^-, followed by radical reactions with nucleophiles and electrophiles, led\nto substitution at boron and/or the Cp* (η⁵-C₅Me₅) ligand. Reaction of <b>1</b> with Rieke Mg* in\nTHF formed Cp*Co(Et₂C₂B₄H₃)-5-O(CH₂)₄-(η⁴-C₅Me₅)CoH(Et₂C₂B₄H₃-5-I) (<b>2</b>), whose two\ncobaltacarborane units are linked through a tetramethyleneoxy linking group. In contrast,\n<b>1</b>^- and <b>4</b>^- reacted with MeOSO₂CF₃ to afford (η⁴-C₅Me₆)CoH(Et₂C₂B₄H₃-5-I) (<b>3</b>) or (η⁴-C₅Me₆)CoH(Et₂C₂B₄H₃-5-Me) (<b>5</b>) and (η⁴-C₅Me₆)CoH(Et₂C₂B₄H₄) (<b>6</b>). The somewhat reactive<b>\n1</b>^- was stabilized with the introduction of 18-crown-6 to form the paramagnetic salt K(18-crown-6)⁺Cp*Co^II(Et₂C₂B₄H₃-5-I)^-<b></b>(<b>10</b>). Treatment of <b>1 </b>and <b>10</b> with the nucleophile LiNMe₂\nin the presence of K(Hg) gave respectively Cp*Co(Et₂C₂B₄H₃-5-NMe₂) (<b>8</b>) and Cp*Co(Et₂C₂B₄H₃-7-NMe₂) (<b>11</b>). Compound <b>8</b> was decapped on exposure to air to give Cp*Co(Et₂C₂B₃H₄-5-NMe₂) (<b>9</b>); its apically substituted isomer <b>11</b> cannot undergo deborylation. A\n<i>tert</i>-butoxide group was introduced at B(5) in moderate yield by reaction of K O-<i>t</i>-Bu with\n<b>1</b>^-, affording <b>12</b>. Treatment of <b>12</b> with acidified methanol produced the B(5)−OH derivative\n<b>14</b>. X-ray diffraction analyses confirmed the structures of <b>2</b>, <b>10</b>, and <b>11</b>.