Solid-state 31P and 13C NMR of extremely bulky phosphines

1
Solid-state 31P and 13C 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
Introduction
Solid-state 13C NMR results
Solid-state 31P NMR Results
Figure 3 MAS 13C NMR spectra of the solid
phosphines
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 Dipp3P prepared in our group and Tripp3P
prepared by Yoshifuji and Sasaki are the worlds
bulkiest phosphines.3
Figure 2 MAS 31P NMR spectra of the solid
phosphines
31P NMR spectra of Dipp3P in the solid-state (MAS)
-52.7
31P NMR spectra of Trip3P in the solid-state (MAS)
-51.9
1H-gt13C CP spectra of Dipp3P
6 kHz 4 kHz 0 kHz
-48.5
nMAS12kHz
1H-gt13C CP spectrum of Tripp3P
nMAS6kHz
150 140 130 120 110
30 20
ppm
nMAS3kHz
Crystallography
1H-gt13C CP spectra of Dipp2PPh
Static
This CH3 signal is shifted to lower frequency due
to increased diamagnetic shielding (see circled
atom in the diagram of the crystal structure).
160 140
120 35.5
25 ppm
Figure 1 Molecular structures as found in the
crystal structures (H atoms omitted)
-100
-50
0
-100 ppm
-50
0
-50
0
31C isotropic peaks of DippPPh2
31P spectra of Dip2PPh
31P NMR spectra of DippPPh2 in the solid-state
(MAS)
160 150 140 130 120
40 30
20 ppm
25 kHz (XY8 1H dec.) 25 kHz (cw 1H
dec.) 12 kHz 3 kHz 0 kHz
-20.7
Very high resolution 13C NMR spectra were
obtained for all four compounds. The CH3 environs
of the isopropyl groups lead to four distinct
methyl carbon shifts in both Dipp3P and DippPPh2.
These compounds differ only in specific details
of their other chemical shifts. However, both
Tripp3P and Dipp2PPh 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
Tripp3P, the para CH3 confuse the spectra,
leading to many overlapping methyl C lines (there
are 18 different CH3 environments!)
nMAS12kHz
-31.5 ppm
-28.5 ppm
-32.7 ppm
33.5 ppm
nMAS5kHz
Dipp3P
Tripp3P
-20.8 ppm
nMAS3kHz
50 0
-50 -100
ppm
References
The sharpest MAS signal belongs to Tripp3P, which
also has a cylindrical shielding tensor in the
static spectrum that is narrow (s? s??).
Similar shielding tensors were reported
previously for other symmetrical ortho
substituted triarlyphosphines.4 By contrast,
Dipp3P 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 Dipp3P 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, Dipp2PPh also
displays the obvious presence of the two distinct
kinds of molecules found in its lattice. These
correspond to the peaks centred at -31.5 and
-32.7 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 DippPPh2 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.

1. (a) Tolman, C. A. Chem. Rev. 1977, 77, 313-348.
   (b) Tolman, C. A. J. Am. Chem. Soc. 1970, 92,
   2956-2965. (c) Andersen, N. G. Keay, B. A. Chem.
   Rev. 2001, 101, 997-1030. .
2. Boeré, R. T. Zhang, Y. J. Organomet. Chem. 2005,
   690, 2651-2657..
3. Sasaki, S. Sutoh, K. Murakami, M. Yoshifuji,
   M. J. Am. Chem. Soc. 2002, 124, 14830-14831.
4. Penner, G. H. Wasylishin, R. E. Can. J. Chem.
   1989, 67, 1909-1913.

Dipp2PPh
DippPPh2
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 31P NMR spectra (shown at
right). Thus Dipp3P R3 twinning by merohedry to
R322 Z 3 (one molecule per equiv. position so
that the molecule must have a three-fold axis of
symmetry). Tripp3P , ordered structure Z
2 (one molecule per equiv. position, no symmetry
applied). Dipp2PPh P21/n Z 8 (two independent
molecules per equiv. position so symmetry
applied.) DippPPh2 Pbca Z 8 (one molecule per
equiv. position, no symmetry applied).
Acknowledgements
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.