Reversible C–C Bond Formation between Redox-Active Pyridine Ligands in Iron Complexes

This manuscript describes the formally iron­(I) complexes
LMeFe­(Py-R)2 (LMe = bulky β-diketiminate;
R = H, 4-tBu), in which the basal pyridine ligands preferentially
accept significant unpaired spin density. Structural, spectroscopic,
and computational studies on the complex with 4-tert-butylpyridine (tBupy) indicate that the S = 3/2 species is a resonance hybrid between descriptions as (a)
high-spin iron­(II) with antiferromagnetic coupling to a pyridine anion
radical and (b) high-spin iron­(I). When the pyridine lacks the protection
of the tert-butyl group, it rapidly and reversibly
undergoes radical coupling reactions that form new C–C bonds.
In one reaction, the coordinated pyridine couples to triphenylmethyl
radical, and in another, it dimerizes to give a pyridine-derived dianion
that bridges two iron­(II) ions. The rapid, reversible C–C bond
formation in the dimer stores electrons from the formally reduced
metal as a C–C bond in the ligands, as demonstrated by using
the coupled diiron­(II) complex to generate products that are known
to come from iron­(I) precursors.

本手稿报道了一系列形式上为铁(I)的配合物LMeFe(Py-R)₂（其中LMe为位阻型β-二亚胺配体(β-diketiminate)，R为H或4-叔丁基(4-tBu)），该类配合物的基面吡啶配体可优先接受显著的未成对自旋密度。针对4-叔丁基吡啶(tBupy)修饰的该配合物所开展的结构、光谱与计算研究表明，自旋多重度S=3/2的物种存在两种共振杂化形式：(a) 高自旋铁(II)(high-spin iron(II))与一个吡啶阴离子自由基发生反铁磁耦合(antiferromagnetic coupling)；(b) 高自旋铁(I)(high-spin iron(I))。当吡啶配体不含叔丁基保护基团时，其会快速且可逆地发生自由基偶联反应，生成新的C-C键。在一类反应中，配位吡啶可与三苯甲基自由基(triphenylmethyl radical)发生偶联；在另一类反应中，配位吡啶会发生二聚，生成可桥联两个铁(II)离子的吡啶衍生双阴离子(dianion)。该二聚体中快速可逆的C-C键形成过程，可将源自形式上被还原的金属的电子以C-C键的形式储存在配体中，这一点可通过如下实验证实：利用偶联得到的双铁(II)配合物可生成已知源自铁(I)前驱体的产物。