Solid-State Chlorine NMR of Group IV Transition Metal Organometallic Complexes

Abstract

Static solid-state (35)Cl (I = (3)/(2)) NMR spectra of the organometallic compounds Cp(2)TiCl(2), CpTiCl(3), Cp(2)ZrCl(2), Cp(2)HfCl(2), Cp*(2)ZrCl(2), CpZrCl(3), Cp*ZrCl(3), Cp(2)ZrMeCl, (Cp(2)ZrCl)(2)mu-O, and Cp(2)ZrHCl (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 (C(Q)) and quadrupolar asymmetry parameter (eta(Q)), are readily extracted from analytical simulations of the spectra. The (35)Cl 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 (35)Cl EFG parameters successfully reproduce the experimental values and trends. The origin of the observed values of C(Q)((35)Cl) 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 (Cp(2)ZrHCl), 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 (35)Cl 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.

Analysis of electric field gradient tensors at quadrupolar nuclei in common structural motifs

“…More recent discussions have noted the connections between the quadrupolar interaction and the local symmetry, coordination numbers and bonding arrangements (27)(28)(29); however, only comparisons of series of structurally and/or chemically similar systems are valid in this context. Therefore, there is a need for the further development for systematic investigations of quadrupolar interactions for a variety of atom types in an assortment of coordination environments, as well as robust computational methods for exploring these systems.…”

On the practical aspects of recording wideline QCPMG NMR spectra

“…Parafilm plugs were used to restrict the sample volume to the middle 5 mm of the rotor to coincide with the coil position, as well as to minimize exposure to air and moisture. The 35 Cl anisotropic NMR parameters of Cp 2 ZrCl 2 have been reported previously [13]. All spectra were externally referenced by setting the 35 Cl resonance of a saturated NaCl(aq) solution to d iso = 0.0 ppm.…”

“…The CPMG refocusing pulse (p2) has traditionally been a 180°-pulse, which maximizes signal refocusing. However, many recent applications of CPMG have been for the examination of wideline spectra which tend to exceed the excitation bandwidth of standard monochromatic pulses, thus requiring acquisition of the complete spectrum in parts/segments [13,23,51]. In this context, the total experimental time will depend on: (1) the number of segments required to cover the complete powder pattern and (2) the time necessary to obtain adequate S/ N for each individual segment.…”

“…This interest stems largely from the ubiquity of quadrupolar nuclei, which represent $75% of the NMR-active nuclei in the Periodic Table and thus constitute integral components of many industrially and biologically relevant substances [1][2][3][4][5][6][7][8][9][10][11][12][13]. Many quadrupolar nuclei (nuclei with spin quantum numbers greater than 1/2) exhibit poor sensitivity due to low gyromagnetic ratios, low natural abundances and/or strong interaction between their electric quadrupole moments with surrounding electric field gradients.…”

“…Since all the intensity is allocated into sharp spikelets, a large gain in signal-to-noise ratio (S/N) is obtained at the expense of resolution, which is dictated by the spikelet separation. The (Q)CPMG pulse sequence has been applied on numerous occasions to facilitate the acquisition of dilute or unreceptive quadrupolar nuclei including 2 H [17], 14 N [18,19], 17 O [20,21], 25 Mg [9,[22][23][24][25], 27 Al [26][27][28], 33 S [29], 35 Cl [13,[30][31][32], 39 K [24,33,34], 47/49 Ti [35][36][37], 53 Cr [11], 55 Mn [38], 59 Co [39], 63/65 Cu [7], 67 Zn [8,24,[40][41][42][43][44][45][46][47], 87 Rb [48], …”