Solid-state 31 P and 13 C NMR of extremely bulky phosphines René T. Boeré, Paul Hazendonk and Dinu Iuga Department of Chemistry and Biochemistry and Department of Physics, The University of Lethbridge, Alberta, Canada Triaryl phosphines are among the most common ligands in coordination chemistry and are of immense importance in all branches of homogeneous catalysis. They are also important reagents in organic and inorganic chemistry. A major theme in the coordination chemistry of phosphines has been their steric bulk. 1 The other major theme has been the basicity of the phosphines, and in particular the way in which the three organic substituents at phosphorus can modify this property. We have recently synthesized and structurally characterized a series of phosphines of differing steric bulk and basicity incorporating the extremely bulky 2,6-diisopropyl group, Dipp. 2 Indeed, the substitution of three aryl groups each bearing two ortho isopropyl groups represent the currently most bulky phosphines ever to be synthesized. The closely related Dipp 3 P prepared in our group and Tripp 3 P prepared by Yoshifuji and Sasaki are the world ’ s bulkiest phosphines. 3 1.(a) Tolman, C. A. Chem. Rev. 1977, 77, (b) Tolman, C. A. J. Am. Chem. Soc. 1970, 92, (c) Andersen, N. G.; Keay, B. A. Chem. Rev. 2001, 101, Boeré, R. T.; Zhang, Y. J. Organomet. Chem. 2005, 690, Sasaki, S.; Sutoh, K.; Murakami, M.; Yoshifuji, M. J. Am. Chem. Soc. 2002, 124, Penner, G. H.; Wasylishin, R. E. Can. J. Chem. 1989, 67, Very high resolution 13 C NMR spectra were obtained for all four compounds. The CH 3 environs of the isopropyl groups lead to four distinct methyl carbon shifts in both Dipp 3 P and DippPPh 2. These compounds differ only in specific details of their other chemical shifts. However, both Tripp 3 P and Dipp 2 PPh display more spectral lines than expected in each region of the spectrum. In the latter, this is easily explained by the low-symmetry environment and the presence of two different molecules in the unit cell. For Tripp 3 P, the para CH 3 confuse the spectra, leading to many overlapping methyl C lines (there are 18 different CH 3 environments!) IntroductionSolid-state 13 C NMR results Acknowledgements References Figure 2: MAS 31 P NMR spectra of the solid phosphines Figure 3: MAS 13 C NMR spectra of the solid phosphines We thank NSERC-Canada and the University of Lethbridge for funding. The Alberta Proteomics Network and WCED supported the acquisition of the spectrometer. Twyla Gietz, Jason Masuda, Sonja Seagrave and Yuankui Zhang prepared the compounds and Masood Parvez performed some of the X-ray crystallographic studies. The solid-state structures of all four phosphines examined in this study have been determined by single-crystal X-ray diffraction. Each have distinct structures that are directly reflected in the solid-state 31 P NMR spectra (shown at right). Thus: Dipp 3 PR3 twinning by merohedry to R322; Z = 3 (one molecule per equiv. position so that the molecule must have a three-fold axis of symmetry). Tripp 3 P, ordered structure; Z = 2 (one molecule per equiv. position, no symmetry applied). Dipp 2 PPhP2 1 /n; Z = 8 (two independent molecules per equiv. position; so symmetry applied.) DippPPh 2 Pbca; Z = 8 (one molecule per equiv. position, no symmetry applied). Figure 1: Molecular structures as found in the crystal structures (H atoms omitted) Dipp 3 PTripp 3 P Dipp 2 PPhDippPPh ppm P NMR spectra of Trip 3 P in the solid-state (MAS) 6 kHz 4 kHz 0 kHz P NMR spectra of Dipp 3 P in the solid-state (MAS) 25 kHz (XY8 1 H dec.) 25 kHz (cw 1 H dec.) 12 kHz 3 kHz 0 kHz 31 P spectra of Dip 2 PPh ppm ppm ppm ppm ppm 33.5 ppm MAS =12kHz MAS =5kHz MAS =3kHz P NMR spectra of DippPPh 2 in the solid-state (MAS) Crystallography Solid-state 31 P NMR Results 1 H ->13 C CP spectra of Dipp 3 P ppm 31 C isotropic peaks of DippPPh 2 1 H ->13 C CP spectrum of Tripp 3 P ppm ppm 1 H ->13 C CP spectra of Dipp 2 PPh The sharpest MAS signal belongs to Tripp 3 P, which also has a cylindrical shielding tensor in the static spectrum that is narrow (σ  ≈ σ  ). Similar shielding tensors were reported previously for other symmetrical ortho substituted triarlyphosphines. 4 By contrast, Dipp 3 P which crystallizes in a higher symmetry space group displays considerably broader lines, and in particular in the static spectrum appears as two different powder-patterns of differing intensity but similar breadth. This seems to reflect twinning by merohedry which was detected in all crystals measured by Dipp 3 P no matter what solvent was used for the recrystallization. The twinning occupancy refined in the single-crystal model to ≈16%. Both of the mixed Dipp/Ph phosphines display more typical spectra for triaryl phosphines. However, Dipp 2 PPh also displays the obvious presence of the two distinct kinds of molecules found in its lattice. These correspond to the peaks centred at and ppm. On of the two components is broadened compared to the other. This mixture of the two environments persists into the static spectrum. While the two molecular structures are virtually superimposable in the two sites, there is a small difference in the twist angle of the unsubstituted rings which may account for this difference. The spectrum of DippPPh 2 has a distinctly asymmetric shielding tensor in the static spectrum, reflecting the very different positions of the three aryl rings which are twisted 12, 52 and 61° from the three-fold axes of the molecules. This CH 3 signal is shifted to lower frequency due to increased diamagnetic shielding (see circled atom in the diagram of the crystal structure) MAS =12kHz MAS =6kHz MAS =3kHz Static