Solid-State Chlorine NMR of Group IV Transition Metal Organometallic Complexes
Aaron J. Rossini, Ryan W. Mills, Graham A. Briscoe†, Erin L. Norton, Stephen J. Geier, Ivan Hung, Shaohui Zheng, Jochen Autschbach and Robert W. Schurko
Static solid-state 35Cl (I = 3/2) NMR spectra of the organometallic compounds Cp2TiCl2, CpTiCl3, Cp2ZrCl2, Cp2HfCl2, Cp*2ZrCl2, CpZrCl3, Cp*ZrCl3, Cp2ZrMeCl, (Cp2ZrCl)2μ-O, and Cp2ZrHCl (Schwartz’s reagent) have been acquired at 9.4 T with the quadrupolar Carr−Purcell Meiboom−Gill (QCPMG) sequence in a piecewise manner. Spectra of several samples have also been acquired at 21.1 T. The electric field gradient (EFG) tensor parameters, the quadrupolar coupling constant (CQ) and quadrupolar asymmetry parameter (ηQ), are readily extracted from analytical simulations of the spectra. The 35Cl EFG and chemical-shift tensor parameters are demonstrated to be sensitive probes of metallocene structure and allow for differentiation of monomeric and oligomeric structures. First-principles calculations of the 35Cl EFG parameters successfully reproduce the experimental values and trends. The origin of the observed values of CQ(35Cl) are further examined with natural localized molecular orbital (NLMO) analyses. The combination of experimental and theoretical methods applied to the model compounds are employed to structurally characterize Schwartz’s reagent (Cp2ZrHCl), for which a crystal structure is unavailable. Aside from a few select examples of single-crystal NMR spectra, this is the first reported application of solid-state 35Cl NMR spectroscopy to molecules with covalently bound chlorine atoms. It is anticipated that the methodology outlined herein will find application in the structural characterization of a wide variety of chlorine-containing transition-metal and main-group systems.