Role of Mediator and Effects of Temperature on ortho-C–N Bond Fusion Reactions of Aniline Using Ruthenium Templates: Isolation and Characterization of New Ruthenium Complexes of the in-Situ-Generated Ligands

In this work, ortho-C–N bond fusion reactions of aniline are followed
by the use of two different ruthenium mediators. Reaction of aniline
with [RuIII(terpy)­Cl3] (terpy = 2,2′:6′,2″-terpyridine)
resulted in a trans bis-aniline ruthenium­(II) complex
[1]+ which upon oxidation with
H2O2 produced compound [2]+ of a bidentate ligand, N-phenyl-1,2-benzoquinonediimine,
due to an oxidative ortho-C–N bond fusion
reaction. Complex [1]+ and aniline
(neat) at 185 °C produced a bis-chelated ruthenium complex (3). A previously reported complex [RuII(N-phenyl-1,2-benzoquinonediimine)­(aniline)2(Cl)2] (5) undergoes similar oxidation by air at 185
°C to produce complex [3]. A separate chemical reaction
between aniline and strongly oxidizing tetra-n-propylammonium
perruthenate [(n-pr)4N]+[RuO4]− in air produced a ruthenium complex [4] of a N4-tetraamidophenylmacrocycle ligand via
multiple ortho-C–N bond fusion reaction. Notably,
the yield of this product is low (5%) at 100 °C but increases
to 25% in refluxing aniline. All these complexes are characterized
fully by their physicochemical characterizations and X-ray structure
determination. From their structural parameters and other spectroscopic
studies, complex [2]+ is assigned
as [RuII(terpy)­(N-phenyl-1,2-benzoquinonediimine)­(Cl)]+ whereas complex [4] is described as a ruthenium­(VI)
complex comprised of a reduced deprotonated N-phenyl-1,2-diamidobenzene
and N4-tetraamidophenylmacrocyclic ligand. Complex [2]+ exhibits one reversible oxidation
at 1.32 V and one reversible reduction at −0.75 V vs Ag/AgCl
reference electrode. EPR of the electrogenerated complexes has revealed
that the oxidized complex is a ruthenium­(III) complex with an axial
EPR spectrum at gav= 2.06. The reduced
complex [2], on the other hand, shows a single-line EPR
signal at gav= 1.998. In contrast, complex
[4] shows two successive one-electron oxidation waves
at 0.5 and 0.8 V and an irreversible reduction wave at −0.9
V. EPR studies of the oxidized complexes [4]+ and [4]2+ reveal that oxidations are ligand
centered. DFT calculations were employed to elucidate the electronic
structures as well as the redox processes associated with the above
complexes. Aerial ortho-C–N bond fusion reactions
of aniline using two different mediators, viz. [RuIII(terpy)­Cl3] and [(n-pr)4N]+[RuO4]−, have been followed. It is found that
in the case of oxidizable Ru­(III) mediator complex, C–N bond
fusion is limited only to dimerization reaction whereas the high-valent
Ru­(VII) salt mediates multiple C–N bond fusion reactions leading
to the formation of a novel tetradentate N4-tetraamidophenylmacrocyclic
ligand. Valence ambiguity in the complexes of the resultant redox-active
ligands is scrutinized.

在本项研究中，苯胺的邻位-C-N键融合反应通过两种不同的钌催化剂进行跟踪。苯胺与[RuIII(terpy)-Cl3]（terpy代表2,2′:6′,2″-三吡啶）的反应生成了一种跨双苯胺钌(II)配合物[1]+，该配合物在过氧化氢氧化后，由于氧化性的邻位-C-N键融合反应，产生了具有双齿配体的化合物[2]+，即N-苯基-1,2-苯醌二亚胺。在185°C下，配合物[1]+与苯胺（纯）反应生成了一种双螯合钌配合物（3）。先前报道的配合物[RuII(N-苯基-1,2-苯醌二亚胺)(苯胺)2(Cl)2]（5）在185°C下通过空气氧化，产生了配合物[3]。苯胺与强氧化剂四丙基铵过钌酸盐[(n-pr)4N]+[RuO4]−在空气中的单独化学反应产生了具有N4-四氨基苯基大环配体的钌配合物[4]，这是通过多次邻位-C-N键融合反应实现的。值得注意的是，该产物在100°C时的产率较低（5%），但在回流苯胺中产率可提高至25%。所有这些配合物都通过其物理化学表征和X射线结构分析进行了全面表征。从其结构参数和其他光谱学研究来看，配合物[2]+被认定为[RuII(terpy)-(N-苯基-1,2-苯醌二亚胺)(Cl)]+，而配合物[4]则被描述为包含还原去质子化的N-苯基-1,2-二氨基苯和N4-四氨基苯基大环配体的钌(VI)配合物。配合物[2]+在1.32 V处表现出一个可逆的氧化反应，在-0.75 V vs Ag/AgCl参比电极处表现出一个可逆的还原反应。电生成的配合物的EPR研究表明，氧化态配合物是一个具有轴向EPR谱gav= 2.06的钌(III)配合物。另一方面，还原态配合物[2]在gav= 1.998处显示出单一的EPR信号。相比之下，配合物[4]在0.5和0.8 V处表现出连续的两个一电子氧化波，在-0.9 V处表现出不可逆的还原波。氧化态配合物[4]+和[4]2+的EPR研究表明，氧化过程是以配体为中心的。采用DFT计算来阐明上述配合物的电子结构及其相关的氧化还原过程。通过使用两种不同的催化剂[RuIII(terpy)-Cl3]和[(n-pr)4N]+[RuO4]−跟踪了苯胺的空气介导的邻位-C-N键融合反应。研究发现，对于可氧化的Ru(III)催化剂配合物，C-N键融合仅限于二聚化反应，而高氧化态的Ru(VII)盐介导了多个C-N键融合反应，导致了一种新型四齿N4-四氨基苯基大环配体的形成。对所得氧化还原活性配体配合物的价态不明确性进行了详细审查。