Characterization of indirect ³¹P-³¹P spin-spin coupling and phosphorus chemical shift tensors in pentaphenylphosphinophosphonium tetrachlorogallate, [Ph₃P-PPh₂][GaCl₄]

Phosphorus chemical shift and 31 P, 31 P spin-spin coupling tensors have been characterized for pentaphenylphosphinophosphonium tetrachlorogallate, [Ph 3 P-PPh 2 ][GaCl 4 ], using solid-state 31 P NMR spectroscopy. Spectra obtained with magic-angle spinning yield the isotropic value of the indirect spin-spin coupling, | 1 J( 31 P, 31 P) iso |, 323 ± 2 Hz, while 2D spin-echo and rotational resonance experiments provide the effective dipolar coupling constant, R eff , 1.70 ± 0.02 kHz, and demonstrate that J iso is negative. Within experimental error, the effective dipolar coupling constant and J iso are unchanged at –120°C. The anisotropy in 1 J( 31 P, 31 P), ΔJ, has been estimated by comparison of R eff and the value of the dipolar coupling constant, R DD , calculated from the P—P bond length as determined by X-ray diffraction. It is concluded that |ΔJ| is small, with an upper limit of 300 Hz. Calculations of 1 J( 31 P, 31 P) for model systems H 3 P-PH + 2 and (CH 3 ) 3 P-P(CH 3 ) + 2 using density functional theory as well as multiconfigurational self-consistent field theory (H 3 P-PH + 2 ) support this conclusion. The experimental spin-spin coupling parameters were used to analyze the 31 P NMR spectrum of a stationary powder sample and provide information about the phosphorus chemical shift tensors. The principal components of the phosphorus chemical shift tensor for the phosphorus nucleus bonded to three phenyl groups are δ 11 = 36 ppm, δ 22 = 23 ppm, and δ 33 = –14 ppm with an experimental error of ±2 ppm for each component. The components are oriented such that δ 33 is approximately perpendicular to the P—P bond while δ 11 forms an angle of 31° with the P—P bond. For the phosphorus nucleus bonded to two phenyl groups, the principal components of the phosphorus chemical shift tensor are δ 11 = 23 ppm, δ 22 = –8 ppm, and δ 33 = –68 ppm with experimental errors of ±2 ppm. In this case, δ 33 is also approximately perpendicular to the P—P bond; however, δ 22 is close to the P—P bond for this phosphorus nucleus, forming an angle of 13°. The dihedral angle between the δ 33 components of the two phosphorus chemical shift tensors is 25°. Results from ab initio calculations are in good agreement with experiment and suggest orientations of the phosphorus chemical shift tensors in the molecular frame of reference.Key words: Nuclear magnetic resonance spectroscopy, phosphorus chemical shift tensors, 31 P- 31 P J-coupling tensors, density functional theory, multiconfigurational self-consistent field theory, phosphinophosphonium salts.

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