Klaus Eichele: Publication Abstracts 2000 [ Go Home ]

Phosphorus-31 1D NMR spectra of a stationary powder sample of a phosphole tetramer containing two phosphorus spin pairs have been obtained at 4.7 T and 9.4 T. In order to separate ³¹P-³¹P spin-spin coupling from anisotropic chemical shielding, 2D spin-echo NMR spectra have been acquired. Phosphorus-31 CPMAS NMR experiments indicate that the two spin pairs of the tetramer are equivalent and each may be treated as an isolated spin pair. Within a given spin pair, the difference between the isotropic chemical shifts of two directly bonded phosphorus nuclei is 1.7 ppm. As well, they are spin-spin coupled by both the indirect and direct interactions, ¹J(³¹P, ³¹P) = -362 Hz and R_DD = 1.80 kHz, respectively. The principal components and relative orientation of the two phosphorus chemical shielding tensors have been determined using the dipolar-chemical shift technique; however, since the dipolar tensor is axially symmetric, ambiguities in the chemical shielding tensor orientation relative to the molecular framework result. Using ab initio calculations and simulations of the 2D spin-echo spectra, many of these ambiguities have been resolved. The spans and skews of the phosphorus shielding tensors for all four three-coordinate phosphorus nuclei are the same within experimental error, 115 ppm and 0.70, respectively. Combined experimental and theoretical results indicate that the phosphorus shielding tensor orientations are dictated by the local environment. For both shielding tensors, the most shielded component, d₃₃, is approximately 78° from the P-P bond and in the phosphole ring plane. The relative orientation of the d₃₃ components is described by a dihedral angle of 82°, similar to the dihedral angle of approximately 76° defining the twist of the phosphole rings about the bridging P-P bond.

In refluxing toluene the tetradentate ligand (Ph₂PCH₂CH₂OCH₂-)₂ (1) reacts with Cl₂Ru(PPh₃)₃ to give the stable h⁴-(O,O,P,P)-chelated ruthenium(II) complex trans- Cl₂Ru(Ph₂PCH₂CH₂OCH₂-)₂ (2). With carbon monoxide no transformation to an h³-(O,P,P)- or h²-(P,P)-chelated complex accompanied by an uptake of CO takes place. However, if [Cl₂Ru(CO)₂]_n is treated with 1 in a mixture of dichloromethane and 2-methoxyethanol, the h³-(O,P,P)-coordinated ruthenium(II) complex trans-Cl₂Ru(CO)(Ph₂PCH₂CH₂OCH₂-)₂ (3) is formed. Attempts to eliminate carbon monoxide from complex 3 to give 2 failed. Also, 3 does not react with further carbon monoxide to form an h²-(P,P)-chelated dicarbonylruthenium(II) complex. Complexes 2 and 3 have been characterized by X-ray structural analyses.

The principal components of the carbon, nitrogen, and selenium chemical shift (CS) tensors for several solid selenocyanate salts have been determined by NMR measurements on stationary or slow magic-angle-spinning powder samples. Within experimental error, all three CS tensors are axially symmetric, consistent with the expected linear geometry of these anions. The spans (W) of the carbon and selenium CS tensors for the selenocyanate anion (SeCN^-) are approximately 300 and 800 ppm, respectively, much less than the corresponding values for carbon diselenide (CSe₂). This difference is a consequence of the difference in the CS tensor components perpendicular to the C infinity symmetry axes in these systems. Ab initio calculations show that the orbital symmetries of these compounds are a significant factor in the shielding. For CSe₂, efficient mixing of the s and p orbitals results in a large paramagnetic contribution to the total shielding of the chemical shielding tensor components perpendicular to the molecular axis. Such mixing is less efficient for the SeCN^-, resulting in a smaller paramagnetic contribution and hence in greater shielding in directions perpendicular to the molecular axis.

Phosphorus chemical shift and spin-spin coupling tensors have been characterized for tetramethyldiphosphine disulfide (TMPS) by analysis of ³¹P CP NMR spectra obtained at 4.7 T for a single crystal. In addition, ³¹P CP NMR spectra of stationary powder and magic angle spinning (MAS) samples have been acquired at two applied magnetic fields (4.7 and 9.4 T) and analyzed independently using the dipolar-chemical shift method. A 2D spin-echo NMR spectrum was also obtained to independently determine the effective ³¹P-³¹P dipolar coupling constant. The crystal structure of TMPS (space group C2/m) consists of six molecules per unit cell. For two of the six molecules, the two phosphorus nuclei are related by an inversion center (site 1), while the remaining four molecules possess mirror planes containing the S-P-P-S bonds (site 2). The differences between the two sites are very subtle, as revealed by a redetermination of the X-ray crystal structure. The phosphorus chemical shift tensors obtained from both single-crystal and dipolar-chemical shift NMR methods are in excellent agreement. For site 1, d₁₁ = 91 ppm, d₂₂ = 75 ppm, and d₃₃ = -63 ppm with an error of ±2 ppm for each component. The principal components of the phosphorus chemical shift tensor at site 2 are very similar; d₁₁ = 92 ppm, d₂₂ = 74 ppm, and d₃₃ = -59 ppm, again with errors of ±2 ppm. The phosphorus chemical shift tensors for both sites are oriented such that the direction of highest shielding is closest to the P-S bond while the direction of least shielding is perpendicular to the plane containing the S-P-P-S bonds. Ab initio (RHF and DFT) calculations of the phosphorus chemical shift tensors for both sites are in good agreement with experiment.

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