Mapping the Binding Modes of Hemilabile Pincer–Crown Ether Ligands in Solution Using Diamagnetic Anisotropic Effects on NMR Chemical Shift

Abstract

A protocol for identifying ligand binding modes in a series of iridium pincer complexes bearing hemilabile aza-crown ether ligands has been developed using readily accessible NMR methods. The approach was tested on a collection of 13 structurally diverse pincer-crown ether complexes that include several newly characterized species. New synthetic routes enable facile interconversion of coordination modes and supporting ligands. Detailed structural assignments of five complexes reveal that the difference in chemical shift (Δδ) between geminal protons in the crown ether is influenced by diamagnetic anisotropy arising from halides and other ligands in the primary coordination sphere. The average difference in chemical shift between diastereotopic geminal protons in the crown ether macrocycle (Δδ), as determined through a single H-C HSQC experiment, provides information on the pincer ligand binding mode by establishing whether the macrocycle is in close proximity to the metal center. The Δδ values for binding modes that involve chelating ether(s) bound to iridium are roughly 2-fold larger than those for tridentate complexes with no Ir-O bonds.

The Trans Effect in Electrocatalytic CO2 Reduction: Mechanistic Studies of Asymmetric Ruthenium Pyridyl-Carbene Catalysts

“…The diagnostic signals for the methyl protons appear at 4.56 ppm for N -Ru-CO 2+ and at 2.86 ppm for C -Ru-CO 2+ . The 1.7 ppm difference provides further evidence that the N -methyl proton chemical shift nicely reflects the molecular geometry based on anisotropic effects . The signal attributed to the carbene carbon of N -Ru-CO 2+ (203.4 ppm) is shifted downfield compared to C -Ru-CO 2+ (195.3 ppm).…”

“…The 1.7 ppm difference provides further evidence that the N-methyl proton chemical shift nicely reflects the molecular geometry based on anisotropic effects. 41 The signal attributed to the carbene carbon of N-Ru-CO 2+ (203. 20,26 so this solvent was utilized in our detailed mechanistic analysis of this complex and the newly isolated N-trans isomer N-Ru-MeCN 2+ .…”

Silver(I) and Copper(I) Complexes of Semi‐Bulky Nitrogen‐Confused C‐Scorpionates

“…These exceptional resonances are shifted upfield from those in the free ligand by |Δ δ | = 0.10 (H α ), 0.03 (CHMe 2 ), and 0.005 ( i Pr CH 3 ) ppm, respectively. The significant anomalous upfield shift of the former is thought to arise from close ion‐pair contact with the triflate ion oxygen atoms, since similar behavior has been observed in solutions of other metal complexes and because the triflate ion is often found to be in close contact with the methine hydrogen of the ligands in the solid state (including that of 1a· acetone) . It is noted that in solution the triflate ion in [Ag( x L iPr2 )](OTf) is not bound to the metal, rather it is displaced by CD 3 CN.…”

Decarbonylative ether dissection by iridium pincer complexes

“…S54-S58 †). 20 A strong correlation between the phosphorus and one (and only one) methyl of a methoxyethyl group in a 1 H-31 P HMBC experiment provides additional evidence for ether ligation (Fig. S57 †).…”