31p chemical shift anisotropies of trimethyl- and triphenylphosphine- substituted group 6 metal pentacarbonyl complexes

Article abstract of DOI:10.1139/v98-167

The ³¹P chemical shift tensor components and anisotropies of the trimethyl- and triphenylphosphine complexes of the group 6 metal pentacarbonyls, M(CO)₅PR₃ (M = Cr, Mo, W and R = Me, Ph), have been measured using solid-sta

Full text of DOI:10.1139/v98-167

1280
31P chemical shift anisotropies of trimethyl- and
triphenylphosphine-substituted Group 6 metal
pentacarbonyl complexes
Jordan H. Wosnick, Frederick G. Morin, and Denis F.R. Gilson
Abstract: The 31P chemical shift tensor components and anisotropies of the trimethyl- and triphenylphosphine
complexes of the group 6 metal pentacarbonyls, M(CO)5PR3 (M = Cr, Mo, W and R = Me, Ph), have been measured
using solid-state CP-MAS 31P NMR spectroscopy. For the trimethylphosphine derivatives, the chemical shift tensors
have near axial symmetry and the shift tensor components are in reasonable agreement with the calculated values for
the chromium and molybdenum complexes. In the triphenylphosphine complexes, the tensors are asymmetric due to the
different torsion angles of the phenyl rings. The trend to higher shielding of the isotropic 31P chemical shifts on
descending group 6 arises from changes in the perpendicular components of the shift tensor. The one-bond coupling
1
constants, J(95/97Mo–31P), for the trimethyl- and triphenylphosphine complexes are 129 and 133 Hz, respectively.
Key words: chemical shift anisotropy, phosphines, chromium, molybdenum, tungsten.
Résumé : Faisant appel à la spectroscopie RMN «CP-MAS» à l’état solide du 31P, on a mesuré les composantes du
tenseur du déplacement chimique du 31P et les anisotropies des complexes triméthyl- et triphénylphosphines de
pentacarbonyles de métaux du groupe 6, M(CO)5PR3 (M = Cr, Mo, W et R = Me, Ph). Pour les dérivés de la
triméthylphosphine, les tenseurs du déplacement chimique présentent une symétrie pratiquement axiale et les
composantes du tenseur du déplacement sont en bon accord avec les valeurs calculées pour les complexes du chrome
et du molybdène. Pour les dérivés de la triphénylphosphine, les tenseurs sont asymétriques à cause des différents
angles de torsion des noyaux phényles. La tendance vers des blindages plus importants des déplacements chimiques
isotropes du 31P lorsqu’on descend dans le groupe 6 provient de changements dans les composantes perpendiculaires du
1
tenseur du déplacement. Les constantes de couplage à travers une liaison, J(95/97Mo–31P), des complexes triméthyl- et
triphénylphosphines sont respectivement 129 et 133 Hz.
Mots clés : anisotropie du déplacement chimique, phosphines, chrome, molybdène, tungstène.
[Traduit par la Rédaction] Wosnick et al. 1283
components of the chemical shift tensor provide more infor-
mation than do the isotropic chemical shifts.
31P NMR spectroscopy has become a common tool in re-
cent years for studying the structural and electronic proper-
ties of organometallic complexes containing phosphine
ligands. Direct information about the tensor properties of the
chemical shift is not available from solution spectra but can
be regained from solid-state spectra under either non-
spinning conditions or with magic-angle spinning, where the
well-known “powder pattern” is replaced by a sharp isotro-
pic peak surrounded by spinning side bands and the chemi-
cal shift tensor information can be recovered from the
relative intensities of the spinning side-band manifolds at
low spinning speeds (1). This is desirable because the
Theoretical methods, particularly Density Functional The-
ory (DFT) approaches, are now capable of calculating the
chemical shift tensors of molecules containing “heavy” nu-
clei (2), and the purpose of the present study was to deter-
mine the chemical shift tensor components for a series of
phosphine-substituted group 6 metal pentacarbonyl com-
plexes of the form M(CO)5PR3 (M = Cr, Mo, W and R =
Me, Ph) and to compare these values with the results of the
DFT predictions of Kaupp (3) and of Ruiz-Morales and
Ziegler (4). The known trend (5) of the 31P isotropic chemi-
cal shifts in the direction of increased shielding on going
from chromium to tungsten was interpreted by Kaupp as
arising from changes in the perpendicular components of the
shift tensor. Supporting evidence for this has been reported
recently for Group 6 metal pentacarbonyl complexes with
the ligand 5-phenyldibenzophosphole (PhDBP) (5).
Wasylishen and co-workers (6, 7) have suggested that
PhDBP resembles triphenylphosphine as a ligand except that
two of the phenyl groups are constrained to one plane and
the 31P chemical shift anisotropy reflects this difference in
local symmetry. We have examined the 31P chemical shift
anisotropies of the triphenylphosphine complexes for
Received March 23, 1998.
J.H. Wosnick, F.G. Morin, and D.F.R. Gilson.1 Department
of Chemistry, McGill University, 801 Sherbrooke Street
West, Montreal, QC H3A 2K6, Canada.
1
Author to whom correspondence may be addressed.
Telephone: (514) 398-6908. Fax: (514) 398-3797.
E-mail: gilson@omc.lan.mcgill.ca
Can. J. Chem. 76: 1280–1283 (1998)
© 1998 NRC Canada

Wosnick et al.
1281
Table 1. Solid state 31P chemical shift tensor components and anisotropies,a isotropic chemical shifts, and
coupling constants for the complexes M(CO)5PR3 (M = Cr, Mo, W; R = Me, Ph). Chemical shift values
are in ppm, referenced to 85% H3PO4, and all coupling constants are in Hz.
δiso
soln
10
0.3
15.0
14
δiso
solid
8
δ11
δ22
δ33
∆δa
1JMP
Cr(CO)5PMe3
Cr(CO)5PMe3
Cr(CO)5PMe3
38
29
–42
–63
–45.1
–52
–56
–68
–45
–42
–67
–14
–7.4
–20
–7.9
–17
–75.5
b
c
32
45.1
13
13
1
32
45.1
–6
–6
1
6.1
–36
0
75
94.2
49
71.1
22
–95
–90.2
–55.5
–59.5
–69
–69
–8.5
–67
–102
–107.8
–83.5
–86.2
–54
Mo(CO)5PMe3
–15
–17
128
129
b
c
Mo(CO)5PMe3
Mo(CO)5PMe3
W(CO)5PMe3
–22
–10.9
–35
–22
55
64.5
38
6.1
–31
–36
54
220
b
W(CO)5PMe3
Cr(CO)5PPh3
Cr(CO)5PPh3
0
100
106.8
78
85.5
52
c
Mo(CO)5PPh3
Mo(CO)5PPh3
W(CO)5PPh3
36
133
c
50.4
22
19
236,
244e
Cr(CO)5PhDBP c
Mo(CO)5PhDBP d
W(CO)5PhDBP d
Mo(CO)5MeDBP f
a
46.8
28.0
7.7
127
104
69
41
24
–13
–23
–28
–44
–36
–40
–112
–108
–64
120
123
16.5
112
–84.5
∆δ is defined as δ – (δ + δ )/2.
33
11
22
b
c
d
Calulated values from ref. 3.
Calculated values from ref. 4.
From ref. 6.
e Solution value from ref. 17.
f
From ref. 7.
comparison with the PhDBP results and the calculations of
Ruiz-Morales and Ziegler (4).
were curve-fitted using the program Peakfit (Jandel
Scientific) and the chemical shift tensor components were
calculated from the spinning side-band intensities using the
Herzfeld–Berger Analysis program of Eichele and
Wasylishen (15).
The complexes M(CO)5PPh3 (M = Cr, Mo, W) were pre-
pared by the trimethylamine – N-oxide promoted substitu-
tion of the triphenylphosphine on the metal hexacarbonyl
(8), and purified by recrystallization from a 2:1 ethanol–
chloroform mixture. The trimethylphosphine complexes
were prepared by displacement of the bromide ion from the
tetraethylammonium bromopentacarbonylmetallate salt (9,
10) using trimethylphosphine – silver iodide adduct as the
source of phosphine (11, 12). These complexes were puri-
fied by repeated sublimations or by crystallization from hex-
ane solution at –20°C, the isolation of the compounds being
complicated by their low melting points and tendency to
form oils. Infrared and solution 31P and 1H nmr spectroscopy
were used to confirm the identity and purity of the products
by comparison with the literature spectra (5, 13, 14).
CP-MAS nmr spectra were recorded on a Chemagnetics
CMX-300 spectrometer operating at 121.279 MHz, using
high-power proton decoupling, contact times of 2 ms, and
recycle delays of 2–180 s, depending on the complex stud-
ied. Samples were packed into 7.5 mm bullet-type zirconia
rotors. Different spinning rates, ranging from 0.3 to 2.5 kHz,
were used to locate isotropic peaks and the chemical shifts
were referenced to 85% H3PO4. Line broadenings of be-
tween 0 and 25 Hz were applied to the spectra. The spectra
The phosphine complexes of the Group 6 metals are espe-
cially easy to study via 31P NMR because, while each metal
has nmr-active isotopes, none are present in a natural abun-
dance large enough to seriously complicate the 31P spectra
with scalar coupling or quadrupole–dipole effects. The
chemical shift tensor values, isotropic chemical shifts, and
coupling constants for the six complexes studied are pre-
sented in Table 1, together with the calculated shift tensor
values (3, 4) and the results reported for the PhDBP and
MeDBP complexes (6, 7). The standard deviations in the δii
values, from the measurements at different spinning rates,
were ±2 ppm.
The CP-MAS spectra showed only one chemical shift for
all compounds except the trimethylphosphine(pentacarbo-
nyl)molybdenum complex, where two shifts were observed
separated by about 2 ppm. This difference is probably due to
the presence of two crystallographically nonequivalent phos-
phorus atoms in the asymmetric unit cell, although the re-
ported X-ray crystal structure (16) indicated only one
molecule in the asymmetric unit. Unfortunately,
© 1998 NRC Canada

1282
Can. J. Chem. Vol. 76, 1998
Fig. 1. 31P CP-MAS spectra of the triphenylphosphine complexes
of Group 6 metal pentacarbonyls. Spinning speed: 1600 Hz for
all spectra. Asterisks indicate the isotropic shift position.
and 97Mo, respectively, and the former is probably negative
in sign (20). These values were obtained at 77 K and the
room temperature quadrupole coupling constants would be
lower in frequency. Thus, neglecting ∆J and using the low-
temperature χ values, the calculated second-order splittings
would be –70 and 6.1 Hz for 97Mo and 95Mo, respectively,
compared with the experimental values of –55 and 4.5 Hz.
The differences could be due to the lower quadrupole cou-
pling constants at high temperature and a finite anisotropy in
the spin–spin coupling constant. The splittings in the spectra
of the trimethylphosphine complex were not sufficiently
well resolved for a full analysis and only the J value could
be determined.
The experimentally determined chemical shift tensors for
the trimethylphosphine complexes show near-axial symme-
try (δ11 ≈ δ22), the deviations being attributable to solid state
effects, and the values are in quite good agreement with the
theoretical predictions (3) if the δ11 and δ22 values are aver-
aged and compared with the calculated perpendicular com-
ponent. The lack of agreement for the tungsten complex is
not unexpected and was attributed by Kaupp to the neglect
of spin-orbit contributions in the calculations (3). The agree-
ment between the observed and calculated shift tensor com-
ponents confirms the assignment of δ33 as lying along the
bond direction in both the trimethyl- and triphenylphosphine
complexes. The chemical shift tensors of the triphenylpho-
sphine compounds deviate from axial symmetry, in contrast
to free triphenylphosphine, which has an axially symmetric
chemical shift tensor (21, 22). This asymmetry is attributed
to the breakdown in local symmetry caused by the different
torsional angles of the phenyl rings. For the chromium and
molybdenum complexes, the δ22 component shows the larg-
est difference between experimental and calculated values
(4) but the spans and shift anisotropies, ∆δ, are in good
agreement.
The influence of the metal atom on the phosphorus chemi-
cal shift anisotropy can be clearly seen in Table 1 and Fig. 1.
The most noticeable trend is the marked decrease in the
spans, (δ11 – δ33), and anisotropies of the chemical shift ten-
sors on moving from chromium to tungsten. This decrease is
due almost entirely to changes in δ11 and δ22. For example, in
the trimethylphosphine complexes, where δ11 and δ22 change
by nearly 70 ppm, δ33 varies by only 14 ppm. For the
triphenylphosphine series, δ11 and δ22 decrease by about
20 ppm from Cr to Mo and by about 30 ppm from Mo to W.
In contrast, δ33 for these complexes remains almost constant
at –17 ± 3 ppm. It is this behaviour which results in the in-
creased shielding observed for the solution chemical shift on
going from chromium to tungsten. The DFT calculations of
the shift tensor components for the series of trimethyl- and
triphenylphosphine pentacarbonyl complexes the Group 6
metals showed good agreement with experiment, a result
that should encourage further work, both experimental and
theoretical.
decomposition of the sample prevented the determination of
the crystal structure of this polymorph.
The molybdenum and tungsten complexes showed addi-
tional splittings due to couplings from the isotopes with
spin, 95Mo (15.7%, I = 5/2), 97Mo (9.46%, I = 5/2), and
183W (14.4%, I = 1/2), but no splitting from 53Cr (9.55%, I =
3/2) was observed in the spectra of either of the chromium
complexes. The solid state value for 1J(P–W) in the tungsten
triphenylphosphine complex was slightly lower than the
value in solution (17).
Couplings to quadrupolar nuclei are not always observed
in solution state spectra, due to rapid relaxation, but can be
observed in solid state spectra under magic-angle-spinning
conditions (18). There is an additional line displacement that
depends upon the residual dipole–quadrupole coupling inter-
action, d, given by eq. [1].
[1]
d = − (3χD′/20νs)(3 cos2β −1 + η sin2 β cos (2α))
where D′ = (D – ∆J/3), D is the dipolar coupling constant, –
(µ0/4π)γIγSh/(2πr3), ∆J is the anisotropy in the spin cou-
pling, χ is the nuclear quadrupole coupling constant, and η is
the asymmetry in the electric field gradient. The angles β
and α are the polar and azimuthal angles relating the inter-
nuclear vector and the principal axes of the electric field
gradient. The local symmetry around the metal–phosphorus
bond is such that η is assumed to be zero and β and α are
taken to be 0° and 90°, respectively, and eq. [1] is much
simplified. The two molybdenum isotopes have very similar
magnetogyric ratios and, therefore, have spin–spin coupling
constants that cannot be easily distinguished, but the nuclear
quadrupole moment of 97Mo is much greater, 0.255 × 10–28
m2 versus –0.022 × 10–28 m2 for 95Mo, and so, in principle,
the second-order splittings can be analysed to give the
quadrupole coupling constants and the anisotropy in J. The
direct dipole coupling constants, calculated using a bond
length of 2.508 Å (16), are –202 and –206 Hz for 97Mo and
95Mo, respectively. The quadrupole coupling constants of
triphenylphosphine(pentacarbonyl)molybdenum have been
measured (19) to be 1.972 MHz and 22.783 MHz for 95Mo
Wasylishen and co-workers (6, 7) have suggested that the
PhDBP ligand is analogous to triphenylphosphine and we
are now able to compare the 31P chemical shift tensor com-
ponents of both ligands when bonded to the Group 6 metal
carbonyls, Table 1. The changes in the δii values are almost
constant (within 5 ppm, and close to the experimental error)
regardless of the metal atom; for PhDBP, the δ11 value is less
© 1998 NRC Canada

Wosnick et al.
1283
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© 1998 NRC Canada

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