Ruthenium N‐Confused Porphyrins: Selective Reactivity for Ambident 2‐Heteroatom‐Substituted Pyridines Serving as Axial Ligands

Abstract

Three types of ruthenium(II) N‐confused porphyrin (NCP) complexes bearing an axial 2‐thiopyridine, 2‐pyridone, and 2‐iminopyridine moiety at the inner carbon atom, respectively, were synthesized. The unique reactivity of the 2‐substituted pyridine derivatives (2‐X‐pyridine; X=SH, OH, NH2) toward the inner carbon atom of the NCP allows the formation of two types of coordinated products (i. e., pyridine donor versus 2‐heteroatom donors), as inferred from single‐crystal X‐ray structures. The selective reactivity was investigated by using density functional theory (DFT) calculations. Finally, the catalytic activity of these ruthenium complexes was demonstrated through the styrene oxidation reactions. As a result, the ruthenium(II) NCP complex bearing a 2‐thiopyridine moiety, together with aqueous H2O2 as an oxidant showed the highest selectivity for benzaldehyde (benzaldehyde/styrene oxide=20 : 1).

Creation from Confusion and Fusion in the Porphyrin World─The Last Three Decades of N-Confused Porphyrinoid Chemistry

“…281 A distinct coordination mode was found in ruthenium chemistry, where the synergy of axial coordination and innercarbon modification played an important role (Scheme 99). 282,283 The thermal reaction of NCTPP with Ru 3 (CO) 12 followed by treatment with pyridine gave the ruthenium(II) complex 251 in 71% yield. When 251 was treated with 2heteroatom-substituted pyridines, exchange of the axial pyridine ligand and nucleophilic attack on the NCP ligand occurred to give the NCTPP ruthenium complexes of capped-type structures 252−254 in 36−69% yields.…”

“…In the case of ruthenium catalysts, the preferential formation of benzaldehyde was observed, while no marked difference was observed between the NCTPP complex and TPP complex in the same way. 283 The NCP rhenium(V) oxo complexes 302 and NFP rhenium-(VII) oxo complexes 515 showed good catalytic activity in an oxygen atom transfer reaction of pyridine N-oxide, giving the corresponding pyridine in quantitative yields (Scheme 230). Under the best reaction conditions, the turnover numbers (TONs) reached up to 5500 (NCP) and 340000 (NFP), respectively.…”

“… No marked difference was observed between the NCTPP complex and the TPP complex. In the case of ruthenium catalysts, the preferential formation of benzaldehyde was observed, while no marked difference was observed between the NCTPP complex and TPP complex in the same way …”

Nucleophilic Aromatic Substitution (SNAr) and Related Reactions of Porphyrinoids: Mechanistic and Regiochemical Aspects

“…[ 186 ] Miyazaki et al utilized a bioinspired approach in their catalyst design; a penta‐coordinate pyridyl‐NCP metal complex (Scheme 25). [ 187,188 ] The S N Ar of the respective 2‐substituted pyridine attacking the C 21 ‐position varied between the pyridines used; for 2‐mercaptopyridine the thiol was the nucleophile, whereas for 2‐amino and 2‐hydroxypyridine, the pyridyl nitrogen was the “head” of the nucleophile. Interestingly, this was the case for both ruthenium ( 130 – 132 ) and cobalt ( 128 ).…”

Ruthenium(iv) N-confused porphyrin μ-oxo-bridged dimers: acid-responsive molecular rotors

“…The ruthenium( ii ) NCP complex (Ru-1) with an axial carbonyl and pyridine (=py) exhibits catalytic reactivity for alkene epoxidation ( Scheme 1 ). 8 The simultaneous formation of isomers can be anticipated owing to the two stereogenic centers in the NCP planes of Ru 2 -2 and Ru 2 -3, which differ in the relative position of the nitrogen atoms of the confused pyrrole rings in the scaffold. The Ru–μ-oxo dimers showed distinct ring rotation in solution when probed via NMR spectroscopy ( Fig.…”