Synthesis and Reactivity of Group Six Metal PCP Pincer Complexes: Reversible CO Addition Across the Metal–Caryl Bond

In the present study, Cr­(III) complexes of the type trans-[Cr­(PCPNEt-iPr)­(solvent)­Cl2] (solvent = CH3CN, THF), the Cr(0) complex [Cr­(κ3P,CH,P-P­(CH)­PNEt-iPr)­(CO)3] which features an η2-Caryl–H agostic bond as well as seven coordinate cationic chloro carbonyl Mo­(II) and W­(II) complexes of the type [M­(PCPNEt-iPr)­(CO)3Cl] featuring PCP pincer ligands based on a 1,3-diaminobenzene scaffold were prepared and characterized. The seven coordinate chloro tricarbonyl complexes exhibit fluxional behavior in solution due to rapid CO ligand interconversions. Another interesting aspect is a rapid and reversible addition of one CO ligand across the metal–Cipso bond. The mechanism of the dynamic process of the chloro carbonyl complexes was investigated by means of DFT calculations. The Mo­(II) and W­(II) tricarbonyl complexes could be reduced to the respective anionic Mo(0) and W(0) complexes [Mo­(PCPNEt-iPr)­(CO)3]− and [Mo­(PCPNEt-iPr)­(CO)3]−. These air sensitive compounds are readily protonated by MeOH to form agostic and hydride species of the types [Mo­(κ3P,CH,P-P­(CH)­PNEt-iPr)­(CO)3] and [W­(PCPNEt-iPr)­(CO)3H]. Structures of representative complexes were determined by X-ray single crystal analyses.

本研究中，我们合成并表征了以下几类配合物：具有反式结构的[Cr(PCPNEt-iPr)(溶剂)Cl₂]（溶剂=乙腈(CH₃CN)、四氢呋喃(THF)）的三价铬(Cr(III))配合物；带有η²-芳基C-H agostic键的零价铬(Cr(0))配合物[Cr(κ³P,CH,P-P(CH)PNEt-iPr)(CO)₃]；以及基于1,3-二氨基苯骨架的PCPN钳形配体构建的七配位阳离子型氯代羰基钼(II)和钨(II)配合物，其构型为[M(PCPNEt-iPr)(CO)₃Cl]（M=Mo, W）。
这类七配位三羰基氯代配合物在溶液中因CO配体的快速互变而表现出流变行为。另一有趣的现象是，一个CO配体可快速且可逆地加成至金属-ipso碳键之上。我们通过密度泛函理论(DFT)计算，对这类氯代羰基配合物的动态过程机理展开了研究。
前述钼(II)和钨(II)三羰基配合物可被还原为对应的零价钼和钨阴离子配合物[Mo(PCPNEt-iPr)(CO)₃]⁻与[W(PCPNEt-iPr)(CO)₃]⁻。这些对空气敏感的化合物可被甲醇(MeOH)快速质子化，生成分别具有agostic结构与氢化物结构的两类配合物：[Mo(κ³P,CH,P-P(CH)PNEt-iPr)(CO)₃]和[W(PCPNEt-iPr)(CO)₃H]。代表性配合物的结构通过X射线单晶衍射分析得以确定。