Exploring the Reactivity of Four-Coordinate PNPCoX with Access to Three-Coordinate Spin Triplet PNPCo

The compounds (PNP)CoX, where PNP is (tBu2PCH2SiMe2)2N- and X is Cl, I, N3, OAr, OSO2CF3 and N(H)Ar, are
reported. Some of these show magnetic susceptibility, color, and 1H NMR evidence of being in equilibrium between
a blue, tetrahedral S = 3/2 state and a red, planar S = 1/2 state; the equilibrium populations are influenced by
subtle solvent effects (e.g., benzene and cyclohexane are different), as well as by temperature. Attempted oxidation
to Co(III) with O2 occurs instead at phosphorus, giving [P(O)NP(O)]CoX species. The single O-atom transfer reagent
PhIO likewise oxidizes P. Even I2 oxidizes P to give the pendant phosphonium species (tBu2P(I)CH2SiMe2NSiMe2CH2PtBu2)CoI2 with a tetrahedral S = 3/2 cobalt; the solid-state structure shows intermolecular PI···ICo
interactions. Attempted alkyl metathesis of PNPCoX inevitably results in reduction, forming PNPCo, which is a spin
triplet with planar T-shaped coordination geometry with no agostic interaction. Triplet PNPCo binds N2(weakly) and
CO (whose low CO stretching frequency indicates strong PNP → Co donor power), but not ethene or MeCCMe.

本文报道了一系列通式为(PNP)CoX的有机金属配合物，其中PNP为配体(tBu₂PCH₂SiMe₂)₂N⁻，X分别为氯(Cl)、碘(I)、叠氮基(N3)、芳氧基(OAr)、三氟甲磺酰氧基(OSO₂CF₃)以及芳基氨基(N(H)Ar)。部分该类配合物的磁化率、颜色以及氢核磁共振(¹H NMR)谱学证据表明，其在蓝色四面体配位自旋态S=3/2与红色平面配位自旋态S=1/2之间存在动态平衡；平衡状态下的物种布居会受到细微的溶剂效应（如苯与环己烷的溶剂环境存在差异）以及温度的共同影响。尝试以氧气(O₂)将该配合物中的钴中心氧化至三价钴(Co(III))时，反应并未发生在金属钴位点，而是选择性发生在磷原子上，生成了[P(O)NP(O)]CoX型配合物。单原子氧转移试剂亚碘酰苯(PhI=O)同样可实现磷原子的氧化。即便是碘单质(I₂)也能氧化磷原子，生成带有膦鎓侧基的配合物(tBu₂P(I)CH₂SiMe₂NSiMe₂CH₂PtBu₂)CoI₂，其中心钴为四面体配位的自旋态S=3/2；该配合物的固态结构表征显示其存在分子间PI···ICo相互作用。对PNPCoX进行烷基复分解反应的尝试均不可避免地发生了还原反应，生成了PNPCo物种；该物种为自旋三重态，具有平面T型配位几何结构，且不存在agostic相互作用。自旋三重态的PNPCo可与弱配位的氮气(N₂)以及一氧化碳(CO)结合（CO的低伸缩振动频率表明PNP配体具有极强的给电子能力），但无法与乙烯(ethene)或2-丁炔(MeCCMe)发生配位作用。