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Research - Quadrupolar Nuclei in Metallocenes

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Solid-State NMR of Quadrupolar Nuclei in Metallocenes and Related Organometallic Compounds

We apply solid-state NMR, powder X-ray diffraction and ab initio calculations to examine the relationships between the molecular structures of metallocenes, dynamic intramolecular motion and anisotropic NMR interaction tensors (such as the chemical shielding and electric field gradient tensors).

M. J. Willans and R. W. Schurko, 2003. A Solid-State NMR and Ab Initio Study of Sodium Metallocenes., J. Phys. Chem. B 107, 5144-5161.

Abstract: Sodium metallocenes (sodocenes) are a fascinating class of molecules that form polymeric chains of repeating [Cp'Na]n units. Herein is a 23Na (I = 3/2) and 13C solid-state NMR study of a variety of sodium metallocenes, where Cp' = Cp (C5H5), CpMe (C5H4Me), CpiPr (C5H4iPr), Cp* (C5Me5). Simulations of 23Na MAS NMR experiments yield the 23Na quadrupolar coupling constants (CQ) and asymmetry parameters (hQ) for all compounds. Values of CQ range from 2.97 to 3.89 MHz for the linear Cp'Na compounds. A new bent base-substituted species, CpNa.THF (THF = tetrahydrofuran), is identified which has CQ = 1.82(2) MHz. The sodium nuclei in sodocenes are extremely shielded with respect to the standard sodium chemical shift range, with chemical shifts (diso) ranging from -45.5 to -61.9 ppm. All of the linear base-free compounds exhibit values of hQ near zero, which reflect the axial symmetry of the sodium environments, while the non-linear CpNa.THF has hQ = 0.39(2), indicative of the electronic asymmetry about the sodium nucleus. Static 23Na NMR experiments reveal subtle instances of 23Na chemical shielding anisotropy, with spans (W) ranging from 9.5 to 12.5 ppm. Variable-temperature 23Na NMR experiments show that the magnitude of CQ(23Na) grows with increasing temperature in the linear metallocenes. Ab initio calculations were performed on a variety of structural models to gain insight into the nature of the electric field gradient (EFG) and chemical shielding (CS) tensors. Rotation of the CpN rings and changes in the inter-ring distances were examined in order to see how these motions affect the 23Na EFG tensors. A combination of experimental and theoretical data indicate that inter-ring distances decrease in the linear sodocenes as the samples are heated. Solid-state 13C CPMAS NMR experiments are used to probe the purity and crystallinity of the metallocene samples, to confirm the existence of the bent ligated metallocene, CpNa.THF (THF = tetrahydrofuran), and to examine the effect of dynamic ring motion on observed carbon chemical shielding tensors of the CpN ring carbons in some species.

Top: Different polymeric metallocene structures.
Bottom: Variable-temperature 23Na MAS NMR spectra of CpNa.

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R. W. Schurko, I. Hung, C. L. B. Macdonald and A. H. Cowley, 2002. Anisotropic NMR interaction tensors in the decamethylaluminocenium cation, J. Am. Chem. Soc. 124, 13204-13214.

Abstract: Solid-state NMR experiments, analytical and numerical simulations of solid NMR powder patterns, ab initio self-consistent field and density functional theory calculations and single crystal X-ray diffraction are used to characterize the molecular structure and anisotropic NMR interaction tensors in the bis(pentamethylcyclopentadienyl)aluminum cation, [Cp*2Al]+. This highly symmetric main group metallocene has a structure analogous to that of ferrocene and the cobaltocenium cation. The single-crystal X-ray diffraction structure is reported for [Cp*2Al][AlCl4]. Solid-state 27Al{1H} magic-angle spinning and static NMR experiments are used to study the aluminum chemical shielding and electric field gradient tensors, revealing axial symmetry in both cases with a large chemical shielding span of W = 81(2) ppm and a small nuclear quadrupole coupling constant, CQ(27Al) = 0.84(10) MHz. Carbon-13 CPMAS NMR experiments in combination with ab initio calculations and simulations of the effects of chemical exchange on 13C static powder patterns reveal dynamic rotation of rings, and suggest a low internal rotational barrier for this process. Theoretical computations of interaction tensors using the Gaussian 98 and Amsterdam Density Functional theory software packages are in good agreement with experiment, and lend insight into the molecular origin of these NMR interactions. Orientations of the NMR tensors determined from theory, the large chemical shielding span and the very small value of CQ(27Al), can all be rationalized in terms of the high molecular symmetry.

Top: 27Al SATRAS MAS NMR spectra of [Cp2Al]+ recorded at a spinning speed of 5 kHz and 9.4 T. Bottom: Molecular orbitals (as determined by calculations using Amsterdam Density Functional (ADF) theory) involved in chemical shielding at the aluminum nucleus.

R. W. Schurko, I. Hung, S. Schauff, C. L. B. Macdonald and A. H. Cowley, 2002. Anisotropic B-11 and C-13 NMR interaction tensors in decamethylcyclopentadienyl boron complexes, J. Phys. Chem. A 106, 10096-10107.

Abstract: Solid-state double-resonance and triple-resonance 13C, 11B and 1H NMR experiments are used to investigate two main group metallocene complexes: the decamethylcyclopentadienyl-borinium cation, [Cp*2B]+, and bis(pentamethylcyclopentadienyl)methylborane, Cp*2BMe. The crystal structure for the latter complex is reported herein. A combination of magic-angle spinning and static 11B{1H} NMR experiments are used to measure 11B nuclear quadrupole coupling constants (CQ) and rare instances of anisotropic boron chemical shielding tensors. Boron-11 nuclear quadrupole coupling constants reflect the higher spherical symmetry of [Cp*2B]+ compared to Cp*2BMe, with CQ(11B) = 1.14 MHz in the former and CQ(11B) = 4.52 MHz in the latter. Chemical shielding tensor spans are measured for [Cp*2B]+ and Cp*2BMe as S = 73.0 and 146.0 ppm, respectively. Hartree-Fock and hybrid density functional theory calculations of electric field gradient and chemical shielding tensors are in quantitative agreement with experiment, and are applied to examine the relationships between the anisotropic NMR interaction tensors and the structure and symmetry of these chemically analogous but structurally dissimilar boron complexes. Variable-temperature 11B MAS NMR, 13C CPMAS NMR and 13C/11B/1H CP-TRAPDOR NMR experiments are applied to make a preliminary investigation of motion of the Cp* rings of [Cp*2B]+.

Top: Crystal structures of the [Cp2B]+ cation and Cp2BCl molecule.  Bottom: Experimental and simulated static 11B NMR spectra of these complexes.

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Last Updated: September 1, 2003
Copyright Rob Schurko, 2003