Anisotropic Chemical Shifts in Cobalt (III) Complexes

Accurate determination of chemical shift tensor orientations of single-crystals by solid-state magic angle spinning NMR

An improved implementation of single-crystal magic-angle-spinning (MAS) NMR is presented which gives access to chemical shift tensors both in orientation (relative to the crystal axis system) and principal axis values. For mounting arbitrary crystals inside ordinary MAS rotors, a mounting tool is described which allows to relate the crystal orientation determined by diffraction techniques to the rotor coordinate system. The crystal is finally mounted into a MAS rotor equipped with a special insert which allows a defined reorientation of the single-crystal by 90°. The approach is based on the idea that the dispersive spectra, which are obtained when applying read-pulses at specific rotor-phases, not only yield the size of the eigenvalues but also encode the orientation of the different chemical shift (rank-2) tensors. For this purpose two 2D-data sets with orthogonal crystal orientation are fitted simultaneously. The presented analysis for chemical shift tensors is supported by an analytical formula which allows fast calculation of phase and amplitude of individual spinning side-bands and by a protocol which solves the problem of finding the correct reference phase of the spectrum. Different rotor-synchronized pulse-sequences are introduced for the same reason. Experiments are performed on L-alanine and O-phosphorylethanolamine and the observed errors are analyzed in detail. The experimental data are opposed to DFT-computed chemical shift tensors which have been obtained by the extended embedded ion method.

Solid-state 13C and 59Co NMR spectroscopy of 13C-methylcobalt(iii) complexes with amine ligands

Five octahedral Co(iii) cations, [trans-Co(en)(2)(X)((13)CH(3))](n+) where en = ethylenediamine, X = CN(-), N(3)(-), NH(3), NO(2)(-) or H(2)O and n = 1 or 2, as well as [Co(NH(3))(5)(13)CH(3)](2+), have been investigated by solid-state (13)C and (59)Co NMR spectroscopy. We show that the determination of the (59)Co nuclear quadrupolar parameters both directly via(59)Co NMR and indirectly via(13)C NMR provide complementary information that is unavailable if one investigates only one nucleus. Specifically, (1)J((59)Co,(13)C) and the orientation of the largest component of the EFG were determined via(13)C NMR spectroscopy, which also established the negative sign of C(Q)((59)Co). Cobalt-59 NMR spectroscopy was used to characterize the cobalt magnetic shielding tensor, to verify the magnitudes of C(Q)((59)Co) and to establish the value of eta(Q), which is difficult to determine indirectly. The measurements show that the EFG tensors are either axially symmetric or close to being so, but there is a wide range of C(Q) values, from -40 MHz for the complex with X = H(2)O to -105 MHz with X = CN(-). The Co chemical shift tensors are approximately axially symmetric with the spans, delta(11)-delta(33), ranging from 3700 to 5600 ppm for X = H(2)O and CN(-), respectively. The latter measurements also established the relative orientations of the Co EFG and chemical shift tensors. Density functional theory calculations of the (59)Co EFG and magnetic shielding tensors as well as of (1)J((59)Co,(13)C) for the NO(2)(-) and N(3)(-) complexes were undertaken. These calculations confirm the experimental observation that the sign of C(Q) is negative and that the largest component of the EFG is along the Co-methyl-carbon bond.

Solid-State 93Nb and 13C NMR Investigations of Half-Sandwich Niobium(I) and Niobium(V) Cyclopentadienyl Complexes

Solid-state 93Nb and 13C NMR experiments, in combination with theoretical calculations of NMR tensors, and single-crystal and powder X-ray diffraction experiments, are applied for the comprehensive characterization of structure and dynamics in a series of organometallic niobium complexes. Half-sandwich niobium metallocenes of the forms Cp'Nb(I)(CO)4 and CpNb(V)Cl4 are investigated, where Cp = C5H5- and Cp' = C5H4R- with R = COMe, CO2Me, CO2Et, and COCH2Ph. Anisotropic quadrupolar and chemical shielding (CS) parameters are extracted from 93Nb MAS and static NMR spectra for seven different complexes. It is demonstrated that 93Nb NMR parameters are sensitive to changes in temperature and Cp' ring substitution in the Cp'Nb(I)(CO)4 complexes. There are dramatic differences in the 93Nb quadrupolar coupling constants (C(Q)) between the Nb(I) and Nb(V) complexes, with C(Q) between 1.0 and 12.0 MHz for Cp'Nb(CO)4 and C(Q) = 54.5 MHz for CpNbCl4. The quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) pulse sequence is applied to rapidly acquire, in a piecewise fashion, a high signal-to-noise ultra-wide-line 93Nb NMR spectrum of CpNbCl4, which has a breadth of ca. 400 kHz. Solid-state 93Nb and 13C NMR spectra and powder XRD data are used to identify a new metallocene adduct coordinated at the axial position of the metal site by a THF molecule: CpNb(V)Cl4.THF. 13C MAS and CP/MAS NMR experiments are used to assess the purity of samples, as well as for measuring carbon CS tensors and the rare instance of one-bond 93Nb, 13C J-coupling, 1J(93Nb,13C). Theoretically calculated CS and electric field gradient (EFG) tensors are utilized to determine relationships between tensor orientations, the principal components, and molecular structures.

Structure and Dynamics of Homoleptic Beryllocenes: A Solid-State9Be and13C NMR Study

The correlation between anisotropic 9Be NMR (quadrupolar and chemical shielding) interactions and the structure and dynamics in [Cp2Be], [Cp2*Be], and [(C5Me4H)2Be] is examined by solid-state 9Be NMR spectroscopy, as well as by ab initio and hybrid density functional theory calculations. The 9Be quadrupole coupling constants in the three compounds correspond well to the relative degrees of spherical ground-state electronic symmetry of the environment about beryllium. Theoretical computations of NMR interaction tensors are in excellent agreement with experimental values and aid in understanding the origins of NMR interaction tensors and their correlation to molecular symmetry. Variable-temperature (VT) 9Be and 13C NMR experiments reveal a highly fluxional structure in the condensed phase of [Cp2Be]. In particular, the pathway by which the Cp rings of [Cp2Be] 'invert' coordination modes is examined in detail using hybrid density functional theory in order to inspect variations of the 9Be NMR interaction tensors. The activation energy for the 'inversion' process is found to be 36.9 kJ mol(-1) from chemical exchange analysis of 13C VT CP/MAS NMR spectra. The low-temperature (ca. -100 degrees C) X-ray crystal structures of all three compounds have been collected and refined, and are in agreement with previously reported structures. In addition, the structure of the same Cp2Be crystal was determined at 20 degrees C and displays features consistent with increased intramolecular motion, supporting observations by 9Be VT NMR spectroscopy.

Anisotropic NMR interaction tensors in the decamethylaluminocenium cation

Solid-state NMR experiments, analytical and numerical simulations of solid NMR powder patterns, ab initio self-consistent field and hybrid 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(2)Al](+). 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(2)Al][AlCl(4)]. Solid-state (27)Al[(1)H] 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 Omega = 83(3) ppm and a small nuclear quadrupole coupling constant, C(Q)((27)Al) = 0.86(10) MHz. Carbon-13 CPMAS NMR experiments in combination with ab initio calculations and simulations of the effects of chemical exchange on (13)C 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 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 C(Q)((27)Al) can all be rationalized in terms of the high molecular symmetry.

Cobalt-59 chemical shift and quadrupolar tensors of simple octahedral cobalt(III) complexes

Analysis of the solid-state powder 59Co NMR spectra of ten simple inorganic cobalt(III) complexes at 11.75, and in most cases, 4.7 T have permitted the assignment of specific ligand planes to ranges of values of the observed chemical shift principal components. The relevant chemical shift components were determined from the simulations of the powder line shapes. These simulations also provided the relative orientations of the chemical shift (CS) and electric field gradient (efg) tensors, as well as magnitude and asymmetry of the 59Co quadrupolar coupling. Using symmetry arguments and ab initio calculations, as appropriate or necessary, the orientations of the efg tensors in the molecular frame were deduced. This allowed the determination of the CS tensors in the molecular frame and thus assignment of the ligand planes responsible for the observed values of chemical shifts.Key words: cobalt, chemical shift, quadrupolar coupling, solid state NMR.

Triple quantum MQMAS spectroscopy of 59Co(I = 7/2) in Na3Co(NO2)6 and trans-Co[(en2)(NO2)2]NO3 interplay between the quadrupole coupling and anisotropic shielding tensors

The purpose of this paper is to investigate the interplay between the chemical shielding anisotropy and quadrupole interaction in MQMAS spectra. 59Co in the compounds Na3Co(NO2)6 and trans-Co[(en2)(NO2)2]NO3 provides model systems for such an investigation. Furthermore, only few results have been reported on the application of the MQMAS method to a spin I = 7/2. The possibilities of the MQMAS spectroscopy for determining the relative orientation of the two tensors and its advantage over previous techniques are discussed. Reported experimental spectra at different spinning speeds of Na3Co(NO2)6 are accurately reproduced by our theoretical simulations. The calculations are based on a recent approach, summarized in the present paper, which allows one to perform efficient simulations of MQMAS spectra including all interactions and their time-dependence throughout the experiment. This is necessary for calculating accurate MQMAS spectra including the spinning sideband pattern. In the case of trans-Co[(en2)(NO2)2]NO3 where the quadrupolar interaction and chemical shielding are stronger and their axes are non-coincident, the MQMAS spectrum is strongly distorted due to the unsufficient spinning speed and RF power. In this case, MAS at different spinning speeds is shown to provide valuable information.

Spin-7/2 nutation and Hahn-echo amplitudes in model compounds and application to the tetrahedral cluster Co4(CO)12

Spin-7/2 central line intensities are calculated for any ratio of the quadrupolar coupling constant to the radiofrequency field for both one-pulse and two-pulse Hahn-echo experiments and tested by 59Co NMR on the model compounds, Na3[Co(NO2)6] and trans-[Co(en)2(NO2)2]NO3. The Hahn-echo sequence is then used to obtain the quadrupole coupling constant and asymmetry parameter on a powder sample of Co4(CO)12. This technique is found to be particularly useful when the NMR spectrum is broad and featureless.

71Ga Chemical Shielding and Quadrupole Coupling Tensors of the Garnet Y(3)Ga(5)O(12) from Single-Crystal (71)Ga NMR

A single-crystal (71)Ga NMR study of the garnet Y(3)Ga(5)O(12) (YGG) has resulted in the determination of the first chemical shielding tensors reported for the (71)Ga quadrupole. The single-crystal spectra are analyzed in terms of the combined effect of quadrupole coupling and chemical shielding anisotropy (CSA). (71)Ga quadrupole coupling and CSA parameters for the two (tetrahedrally and octahedrally coordinated) gallium sites with axial symmetry in YGG (Ga(IV), C(Q) = 13.1 +/- 0.2 MHz and delta(sigma) = 54 +/- 50 ppm; Ga(VI), C(Q) = 4.10 +/- 0.06 MHz and delta(sigma) = 24 +/- 3 ppm) are fully consistent with its cubic crystal structure which supports the reliability of the experimental data. In addition, the (71)Ga and (27)Al isotropic chemical shifts for YGG and YAG give further support to the linear correlation observed earlier between (71)Ga and (27)Al isotropic chemical shifts.