Carbene Transfer from a Pyridine Dipyrrolide Iron–Carbene Complex: Reversible Migration of a Diphenylcarbene Ligand into an Iron–Nitrogen Bond

The reaction between the square-planar, paramagnetic
iron–carbene
complex (MesPDPPh)­Fe­(CPh2) (where
[MesPDPPh]2– is the doubly
deprotonated form of 2,6-bis­(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)­pyridine) and two equivalents of carbon monoxide
or 2,6-dimethylphenylisocyanide, CNdmp, resulted in carbene insertion
into a Fe–Npyrrole bond of the MesPDPPh ligand. Crystallographic studies established complete scission
of the Fe–Npyrrole bond in the resulting low-spin
FeII complexes. Experiments using 1-adamantylisocyanide,
CN1Ad, demonstrated that binding of two exogenous ligands
was required to promote carbene migration and allowed the isolation
of diamagnetic (MesPDPPh)­Fe­(CPh2)­(CN1Ad). Computational studies by density functional theory (DFT)
and complete-active-space self-consistent field (CASSCF) calculations
support that (MesPDPPh)­Fe­(CPh2)­(CN1Ad) possesses a similar electronic structure to porphyrin
iron carbenes, best described as containing a neutral Fischer-type
carbene coordinated to a low-spin FeII center. DFT calculations
also suggested that carbene migration is triggered by an increase
in carbene electrophilicity upon binding of a second isocyanide ligand.
Thermolysis of the carbene insertion product obtained following the
addition of CNdmp to (MesPDPPh)­Fe­(CPh2) revealed that the insertion is fully reversible and provided the
ketenimine N-(2,2-diphenylethenylidene)-2,6-dimethylbenzenamine
as the major product.

平面正方形顺磁性铁-卡宾配合物(MesPDPPh)Fe(CPh2)（其中[MesPDPPh]²⁻为2,6-双(5-(2,4,6-三甲基苯基)-3-苯基-1H-吡咯-2-基)吡啶的双去质子化形式）与两当量一氧化碳或2,6-二甲基苯基异氰（CNdmp）发生反应，卡宾插入至MesPDPPh配体的Fe-吡咯氮键中。晶体学研究证实，所得低自旋Fe(II)配合物中Fe-Npyrrole键完全断裂。采用1-金刚烷基异氰（CN1Ad）的实验表明，需结合两个外源性配体才能促进卡宾迁移，并可分离得到反磁性的(MesPDPPh)Fe(CPh2)(CN1Ad)。通过密度泛函理论（DFT）与完全活性空间自洽场（CASSCF）计算的结果支持：(MesPDPPh)Fe(CPh2)(CN1Ad)具有与卟啉铁卡宾类似的电子结构，可被最优描述为中性费舍尔型卡宾与低自旋Fe(II)中心配位的物种。DFT计算还表明，卡宾迁移是由第二个异氰配体结合后卡宾亲电性提升所触发的。将CNdmp与(MesPDPPh)Fe(CPh2)反应得到的卡宾插入产物进行热解，结果显示该插入反应完全可逆，主要产物为烯酮亚胺N-(2,2-二苯基乙烯亚基)-2,6-二甲基苯胺。