Masked Radicals: Iron Complexes of Trityl, Benzophenone, and Phenylacetylene

We report the first
Fe–CPh3 complex and show
that the long Fe–C bond can be disrupted by neutral π-acceptor
ligands (benzophenone and phenylacetylene) to release the triphenylmethyl
radical. The products are formally iron­(I) complexes, but X-ray absorption
spectroscopy coupled with density functional and multireference ab initio calculations indicates that the best description
of all the complexes is iron­(II). In the formally iron­(I) complexes,
this does not imply that the π-acceptor ligand has radical character,
because the iron­(II) description arises from doubly occupied frontier
molecular orbitals that are shared equitably by the iron and the π-acceptor
ligand, and the unpaired electrons lie on the metal. Despite the lack
of substantial radical character on the ligands, alkyne and ketone
fragments can couple to form a high-spin iron­(III) complex with a
cyclized metalladihydrofuran core.

本研究首次报道了Fe–CPh3配合物，并证实中性π-受体配体(π-acceptor ligand)（二苯甲酮与苯乙炔）可断裂较长的Fe–C键，进而释放三苯甲基自由基。所得产物形式上为铁(I)配合物，但结合X射线吸收光谱法(X-ray absorption spectroscopy)、密度泛函(density functional)与多参考从头算(multireference ab initio calculations)的结果表明，所有配合物的最佳价态描述应为铁(II)。对于形式上的铁(I)配合物，这一描述并不意味着π-受体配体带有自由基特征：铁(II)的价态归属源于铁与π-受体配体公平共享的双占据前线分子轨道(frontier molecular orbitals)，而未成对电子仅位于金属中心。尽管配体上几乎不存在显著的自由基特征，炔烃与酮片段仍可发生偶联反应，生成带有环化金属二氢呋喃(metalladihydrofuran)核的高自旋铁(III)配合物。