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

35Cl solid state NMR (SSNMR), in tandem with 35Cl NQR and density functional theory calculations, was used to characterize chlorine ligands in a series of transition-metal complexes exhibiting structural motifs common to organometallic catalysts. The differentiation of the various chlorine environments was possible, and insight into the origins of the 35Cl electric field gradient tensor parameters was provided. The applicability of 35Cl SSNMR to the study of surface supported transition-metal complexes was demonstrated, validating the use of this technique in the characterization of heterogeneous catalysts

Computational Study and Molecular Orbital Analysis of NMR Shielding, Spin–Spin Coupling, and Electric Field Gradients of Azido Platinum Complexes

¹⁹⁵Pt, ¹⁴N, and ¹⁵N NMR data for five azido (N₃^–) complexes are studied using relativistic density functional theory (DFT). Good agreement with experiment is obtained for Pt and N chemical shifts as well as Pt–N <i>J</i>-coupling constants. Calculated ¹⁴N electric field gradients (EFGs) reflect experimentally observed trends for the line broadening of azido ¹⁴N NMR signals. A localized molecular orbital analysis of the nitrogen EFGs and chemical shifts is performed to explain some interesting trends seen experimentally and in the first-principles calculations: (i) ¹⁴N NMR signals for the Pt-coordinating (N_α) nuclei in the azido ligands are much broader than for the central (N_β) or terminal (N_γ) atoms. The N_β signals are particularly narrow; (ii) compared to N_γ, the N_α nuclei are particularly strongly shielded; (iii) N_β nuclei have much larger chemical shifts than N_α and N_γ ; and (iv) The Pt–N_α <i>J</i>-coupling constants are small in magnitude when considering the formal sp hybridization of N_α . It is found that for N_α a significant shielding reduction due to formation of the dative N_α–Pt bond is counterbalanced by an increased shielding from spin–orbit (SO) coupling originating at Pt. Upon coordination, the strongly delocalized π system of free azide localizes somewhat on N_β and N_γ . This effect, along with rehybridization at N_α upon bond formation with Pt, is shown to cause a deshielding of N_γ relative to N_α and a strong increase of the EFG at N_α . The large 2p character of the azide σ bonds is responsible for the particularly high N_β chemical shifts. The nitrogen s-character of the Pt–N_α bond is low, which is the reason for the small <i>J</i>-coupling. Similar bonding situations are likely to be found in azide complexes with other transition metals

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

Chlorine ligands in a variety of diamagnetic transition-metal (TM) complexes in common structural motifs were studied using ³⁵Cl solid-state NMR (SSNMR), and insight into the origin of the observed ³⁵Cl 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 ³⁵Cl 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 ³⁵Cl 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 ³⁵Cl EFG tensor parameters, suggesting that ³⁵Cl SSNMR is ideal for characterizing chlorine ligands in TM complexes. A detailed localized molecular orbital (LMO) analysis was completed for NbCl₅. 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 ³⁵Cl SSNMR for the structural characterization of WCl₆ 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