Modular O- vs. N-coordination of pyridylidene amide ligands to iron determines activity in alcohol oxidation catalysis

Abstract

A family of polydentate pyridine-substituted pyridylidene amide (PYA) complexes bound to iron(II) was developed. Variation of the coordination set from NN-bidentate PYA to tridentate pincer-type pyPYA2 systems (pyPYA2 = 2,6-bis(PYA)pyridine)...

Pincer and Macrocyclic Pyridylidene Amide (PYA) AuIII Complexes

“…The spectra of [Au­( o -PYA)­Cl]­[PF 6 ] 2 show an additional band at 1572 cm –1 that is not present for the meta and para variants (Figure S21). This band at 1572 cm –1 has previously been assigned to that of the C–O stretch within the ortho -ligand and used to distinguish between κ 3 - N , N , N and κ 3 - O , N , O connectivity within the complexes of zinc­(II) and cobalt­(II). , For [Au­(o-PYA)­Cl]­[PF 6 ] 2 , the signal at 1572 cm –1 therefore suggests the presence of κ 3 - O , N , O coordination.…”

“…This large angle is typical for considerable X-type character of the PYA donor site (Scheme ), which suggests that the gold-coordinating N amides are predominantly anionic and π-basic in nature . Angles approaching 0° have been reported in complexes when the x -PYA ligand features predominantly a neutral, L-type resonance structure …”

“…Alternatively, the two isomers may originate from two, nonexchanging coordination modes of the PYA pincer ligand through κ 3 - N , N , N- and κ 3 - O , N , O -coordination. A tendency for the o -PYA ligand to κ 3 - O , N , O -coordinate has been established in related complexes of Co, Ni, and also Ru. , Support for such ambivalent coordination came from solid-state infrared (IR) spectroscopy. All gold­(III)–PYA complexes exhibit two characteristic signals in the1500–1750 cm –1 region, presumably associated with amide carbonyls and PYA–imine units in the κ 3 - N , N , N linkage isomer.…”

“…The pyridylidene–nitrogen can also be varied in ortho -, meta -, or para -positions. Notably, the meta -position cannot be represented in a totally neutral form, making this type a formally mesoionic N-donor system, which is closely related to mesoionic N-heterocyclic carbenes. , The unique ligand flexibility imparted by these PYA systems has been applied successfully to a range of catalytic transformations. ,− …”

“…These modifications have been shown to effect chelation and electronic effects . Recently, a series of N , N , N -tridentate, pincer-type bis -PYA ligands was developed, ,, which were shown to directly influence the reduction potentials within respective ruthenium­(II) complexes …”