An Investigation of Chlorine Ligands in Transition-Metal Complexes via ~(35)Cl Solid-State NMR and Density Functional Theory Calculations

摘要

Chlorine ligands in a variety of diamagnetic transitionmetal (TM) complexes in common structural motifs were studied using ~(35)Cl solid-state NMR (SSNMR), and insight into the origin of the observed ~(35)Cl NMR parameters was gained through first-principles density functional theory (DFT) calculations. The WURST-CPMG pulse sequence and the variable-offset cumulative spectrum (VOCS) methods were used to acquire static ~(35)Cl SSNMR powder patterns at both standard (9.4 T) and ultrahigh (21.1 T) magnetic field strengths, with the latter affording higher signal-to-noise ratios (S/N) and reduced experimental times (i.e., <1 h). Analytical simulations were performed to extract the ~(35)Cl electric field gradient (EFG) tensor and chemical shift (CS) tensor parameters. It was found that the chlorine ligands in various bonding environments (i.e., bridging, terminal-axial, and terminal-equatorial) have drastically different ~(35)Cl EFG tensor parameters, suggesting that ~(35)Cl SSNMR is ideal for characterizing chlorine ligands in TM complexes. A detailed localized molecular orbital (LMO) analysis was completed for NbCl_5. It was found that the contributions of individual molecular orbitals must be considered to fully explain the observed EFG parameters, thereby negating simple arguments based on comparison of bond lengths and angles. Finally, we discuss the application of ~(35)Cl SSNMR for the structural characterization of WCl_6 that has been grafted onto a silica support material. The resulting tungsten-chloride surface species is shown to be structurally distinct from the parent compound.