Cationic Organoscandium β-Diketiminato Chemistry: Arene Exchange Kinetics in Solvent Separated Ion Pairs

Abstraction of methide from the β-diketiminato supported organoscandium complex [LScMe2]2 using the trityl borate activator [Ph3C][B(C6F5)4] in arene solvents gives solvent separated ion pairs in which the arene (C6H5Br, 1a; C6H6, 1b; C7H8, 1c; 1,3,5-Me3C6H3, 1d) is coordinated to the cationic scandium center in an η6 bonding mode. L1 incorporates methyl groups in the 2,4 positions of the ligand backbone and bulky 2,6-diisopropylphenyl groups on the nitrogen atoms. The relative binding strength of the arenes is C6H5Br ≪ C6H6 < 1,3,5-Me3C6H3 < C7H8. Ion pairs 1a and 1c have been characterized crystallographically, and the C6H5Br derivative is notable for its η6 bonding mode in preference to the more common η1 bonding mode via the halogen atom. The kinetics of displacement of mesitylene by toluene (1d → 1c) yield activation parameters of ΔH⧧ = 21.4(6) kcal mol-1 and ΔS⧧ = 6(1) cal mol-1 K-1. In combination with the observed lack of dependence of [toluene] on the rate of displacement, these data suggest a mechanism involving partial dissociation of the coordinated arene, followed by attack of the incoming arene. This chemistry has relevance to the role of these solvent separated ion pairs in olefin polymerization processes and presents a rare opportunity for the detailed study of these ephemeral species.

在芳烃溶剂中，以三苯甲基四(五氟苯基)硼酸盐[Ph3C][B(C6F5)4]作为活化剂，从β-二亚胺配位的有机钪配合物[LScMe2]2中夺取甲基负离子，可得到溶剂分离离子对。该类离子对中，芳烃（C6H5Br，1a；C6H6，1b；C7H8，1c；1,3,5-三甲基苯，1d）以η6配位模式与阳离子钪中心相结合。配体L1在其骨架的2,4位引入甲基，氮原子上连有大位阻的2,6-二异丙基苯基基团。上述芳烃的相对结合强度顺序为：C6H5Br ≪ C6H6 < 1,3,5-三甲基苯 < C7H8。离子对1a和1c已通过单晶X射线衍射完成结构表征，该溴苯衍生物以η6配位模式成键，而非更为常见的通过卤原子的η1配位模式，这一特性尤为引人注目。对均三甲苯被甲苯取代的动力学过程（1d → 1c）进行研究，得到的活化焓ΔH⧧ = 21.4(6) kcal·mol⁻¹、活化熵ΔS⧧ = 6(1) cal·mol⁻¹·K⁻¹。结合观测到的反应速率与甲苯浓度无关这一现象，上述数据表明该反应的机理为：配位芳烃先发生部分解离，随后由外来芳烃发起亲核进攻。这类化学过程与溶剂分离离子对在烯烃聚合过程中的作用密切相关，同时也为深入研究这类转瞬即逝的活性物种提供了难得的契机。