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

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_avg), as determined through a single ¹H-¹³C 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_avg 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.

Through-Space Shielding Effects of Metal-Complexed Phenyl Rings

Several naphthalene compounds containing a methyl group in a 1,8-relationship to a metal-complexed phenyl ring bearing various substituents have been synthesized. The through-space shielding effects of the phenyl ring, as a function of substituent and complexing metal species, were monitored by observing the ¹H NMR signal of the methyl group located in the shielding zone of the ring. In all cases, the methyl signal was slightly more downfield in the complexes than in the uncomplexed analogues. A comparison of available crystal structures, however, shows that the phenyl ring is slightly closer to the naphthyl methyl group in the complexes than in their metal-free counterparts. X-ray structures and DFT calculations also reveal a slight elongation in the average length of the carbon-carbon bonds of the phenyl ring upon complexation. The effect of substituents on the signal of the naphthyl methyl group is small but discernible in the uncomplexed derivatives, and consistent with our previous report. A similar trend is absent in the corresponding metal complexes, as exemplified by the chromium series, and the effect of the metal appears to be more dominant than that of the substituents. These observations were found to be in line with NMR shift calculations.

A Convenient Synthetic Route to Benz[cd]azulenes: Versatile Ligands with the Potential To Bind Metals in an η5, η6, or η7Fashion

A facile method for preparing the 2H-benz[cd]azulene system, based upon an elaboration of the guaiazulene framework, is presented. Aerial oxidation to the corresponding 8-(2-propylidene)-benz[cd]azulene, and also cycloaddition reactions with tetracyanoethylene (TCNE), are described. The first X-ray crystal structure of a 2H-benz[cd]azulene, as an eta6-coordinated Cr(CO)3 complex, is reported.