Synthesis, Coordination Chemistry and Bonding of Strong N‐Donor Ligands Incorporating the 1H‐Pyridin‐(2E)‐Ylidene (PYE) Motif

Abstract

A range of N-donor ligands based on the 1H-pyridin-(2E)-ylidene (PYE) motif have been prepared, including achiral and chiral examples. The ligands incorporate one to three PYE groups that coordinate to a metal through the exocyclic nitrogen atom of each PYE moiety, and the resulting metal complexes have been characterised by methods including single-crystal X-ray diffraction and NMR spectroscopy to examine metal-ligand bonding and ligand dynamics. Upon coordination of a PYE ligand to a proton or metal-complex fragment, the solid-state structures, NMR spectroscopy and DFT studies indicate that charge redistribution occurs within the PYE heterocyclic ring to give a contribution from a pyridinium-amido-type resonance structure. Additional IR spectroscopy and computational studies suggest that PYE ligands are strong donor ligands. NMR spectroscopy shows that for metal complexes there is restricted motion about the exocyclic C-N bond, which projects the heterocyclic N-substituent in the vicinity of the metal atom causing restricted motion in chelating-ligand derivatives. Solid-state structures and DFT calculations also show significant steric congestion and secondary metal-ligand interactions between the metal and ligand C-H bonds.

Charged Behaviour from Neutral Ligands: Synthesis and Properties of N‐Heterocyclic Pseudo‐amides

“…From structural data it is clear that for 2 and 3 the N(1)C(2) bond distance is more typical of an imine double bond (1.28 Å) than a single C(sp 2 )N(sp 3 ) bond (1.416 Å) 20. Furthermore, for 3 all related protonated and metal complex derivatives exhibit only the E isomer, and isomerisation is not observed in the NMR timescale 4c. In contrast the molecular structures of [ i Pr N Ph (H)][I] and related derivatives (see the Supporting Information) show E , Z or both isomers, indicating that rotation about the CN bond of 6 can occur.…”

“…In order to gauge the contribution of the azolium–amido resonance structure across the series 2 , 3 , and 6 the metrical data of 7 , 11 , [ i Pr N Ph (H)][I] (see the Supporting Information) and analogous compounds of 2 and 3 ,4b, c can be compared. The data indicate that the azolium–amido form is more significant for 6 and its derivatives than for 2 and 3 .…”

“…The data indicate that the azolium–amido form is more significant for 6 and its derivatives than for 2 and 3 . For example, the N(1)C(2) bond lengths are consistently longer for 6 ‐derived compounds and complexes, including 1.365(5) for 11 , compared to 1.324(7) Å for the analogous complex of 2 , and 1.3719(17) and 1.328(7) Å for [ i Pr N Ph (H)][I] and the analogous derivative of 3 , respectively 4c. However, there is significant asymmetry between the N‐substituent NC bonds, where for 7 N(1)C(2) and N(1)C(9) are 1.339(2) and 1.411(1), respectively, and for 11 , 1.365(5) and 1.422(5) Å.…”

Filling a Niche in “Ligand Space” with Bulky, Electron‐Poor Phosphorus(III) Alkoxides

“…The electronic properties of ligands have been assessed using a variety of methods, all of which have positive and negative aspects. A convenient approach, which we have used previously to assess the donor properties of very electron‐rich ligands, has been proposed by Gusev et al ,,,. This involves geometry optimizations and vibrational frequency analyses of [(η 5 ‐C 5 H 5 )Ir(CO)L] complexes (in which L is the desired ligand), using DFT methods to obtain CO stretching frequencies and C−O bond lengths that vary as a function of the donor/acceptor properties of L. The use of this complex, as opposed to other commonly used gauges of electronic properties {for example, [Ni(L)(CO) 3 ]}, allows the comparison of a large range of different ligand classes using the same scale.…”

One-pot synthesis of new $N$-(1-methylpyridin-4(1$H)$-ylidene)amine ligands for palladium-catalyzed Heck coupling reaction

“…2 In addition, steric properties of PYE around metal can be altered by synthesizing various libraries of its derivatives. 3 It can resonate between zwitterion pyridinium amido and neutral N -(1-methylpyridin-4(1H) -ylidene)amine ( Figure 1) similar to NHCs and thus can be categorized as a flexible electron donor and nitrogen donor ligand. 4 As it possesses an exocyclic nitrogen atom, it can provide a better environment to metal for coordination.…”