Tungsten-183 NMR study of cis-[W(CO)4(PPh3)(4-RC 5H4N)]; Effect of varying the σ donor strength of the pyridine

Article abstract of DOI:10.1002/mrc.3925

Tungsten-183 NMR data are reported for the complexes cis-[W(CO) ₄(PPh₃)(4-RC₅H₄N)] (R = H, Me, Ph, COMe, COPh, OMe, NMe₂, Cl, NO₂). The ¹⁸³W chemical shift (obtained by indirect detection using ³¹P) is found to correlate with the Hammett σ function for the group R, with ¹⁸³W shielding increasing approximately linearly with the donor strength of the pyridine over a range of 93 ppm. The X-ray structures of cis-[W(CO)₄(PPh₃)(4-MeOC₅H₄N)] and cis-[W(CO)₄(PPh₃)(4-PhCOC₅H₄N)] are also reported. Copyright

Full text of DOI:10.1002/mrc.3925

Research Article
Received: 8 November 2012
Revised: 20 December 2012
Accepted: 21 December 2012
Published online in Wiley Online Library: 13 February 2013
(wileyonlinelibrary.com) DOI 10.1002/mrc.3925
Tungsten-183 NMR study of cis-[W(CO)4(PPh3)
(4-RC5H4N)]; effect of varying the s donor
strength of the pyridine
Laurence Carlton,* Lebohang V. Mokoena and Manuel A. Fernandes
Tungsten-183 NMR data are reported for the complexes cis-[W(CO)4(PPh3)(4-RC5H4N)] (R = H, Me, Ph, COMe, COPh, OMe, NMe2,
Cl, NO2). The 183W chemical shift (obtained by indirect detection using 31P) is found to correlate with the Hammett s function
for the group R, with 183W shielding increasing approximately linearly with the donor strength of the pyridine over a range of
93 ppm. The X-ray structures of cis-[W(CO)4(PPh3)(4-MeOC5H4N)] and cis-[W(CO)4(PPh3)(4-PhCOC5H4N)] are also reported.
Copyright © 2013 John Wiley & Sons, Ltd.
Keywords: 183W NMR; 31P NMR; tungsten complexes; pyridines; substituent influence; Hammett sigma
suppliers and (with the exception of 4-chloropyridine which
was distilled) used without further purification; diglyme was
Introduction
Complexes of the type [W(CO)5(4-RC5H4N)] and [W(CO)4(PPh3)
(4-RC5H4N)], where R is an extended chain or rigid oligoacetylide
rod, have received attention in connection with potential applica-
tion in nonlinear optics[1] and, when the substituent R forms a
bridge to a second metal atom, in the development of materials
that make use of metal–metal interactions.[2–4] Variants, in which
the phosphine carries a pyridine substituent, have also been
reported.[5,6] Our interest in such compounds lies in the relationship
between ligand properties and NMR parameters, more specifically
the electron-donating ability of the ligand and the 183W chemical
shift. A linear pyridine-terminated bridge requires a 4-substituted
pyridine, which has the advantage that electronic effects arising
from the substituent can be studied in isolation from steric effects.
Ligand substituents are known to exert significant influence on
metal chemical shifts, which have been found to correlate with
substituent parameters (e.g. Hammett s) for complexes [Rh(Z5-
C5H4R)(CO)2][7] and [Rh(acacF){(4-RC6H4)2P(CH2)4P(4-RC6H4)2}],[8]
where R is varied. The correlation between ligand substituent
properties and metal chemical shift has been found to extend
to the reactivity of the metal complex, where it has been shown by
von Philipsborn that metal chemical shifts can be used to predict
reaction rates.[9] Although 183W NMR has found use in the study of
a variety of organotungsten complexes including diene,[10]
cyclopentadienyl,[11] trispyrazolylborate,[12] imido,[13] carbene[14]
and vinylidene[14] derivatives, it has not previously been the subject
of a study focusing specifically on ligand substituents. Here, we
report 183W chemical shifts for complexes cis-[W(CO)4(PPh3)
(4-RC5H4N)] where R is varied from strongly electron-donating
(NMe2) to strongly electron-withdrawing (NO2). This work extends
an earlier 183W NMR study of complexes [W(CO)4(PPh3)(PR3)].[15]
distilled from sodium, and acetonitrile and benzene were dried
over calcium hydride. [W(CO)5(PPh3)] was prepared by the
method of Magee and co-workers,[16] and cis-[W(CO)4(PPh3)
(CH3CN)] was prepared by the method of Adams and Perrin,[17]
with slight modification (solvent 100% CH3CN; temperature
80 ꢀC; Me3NO.2H2O 1.2 eq added over 10 min, stirred 40 min).
All operations were performed under an argon atmosphere.
Preparation of cis-[W(CO)4(PPh3)(4-RC5H4N)]
The following procedure was used for the preparation of all
complexes except the 4-nitropyridine complex, which could not
be isolated in pure form (it readily decomposed) and was prepared
in situ in C6D6 with heating to 60–70ꢀC for 1–2 min. A mixture
of cis-[W(CO)4(PPh3)(CH3CN)] (~0.05 g) and 4-RC5H4N (1–3 eq) in
benzene was heated to 80ꢀC for 10–30min during which time a
color change was observed, in some cases, to orange or red. The
solution was cooled to room temperature, concentrated and
treated with hexane to give the product as crystals or crystalline pow-
der in yields of 52–76% as follows: cis-[W(CO)4(PPh3)(4-RC5H4N)],
R = Me2N, yellow crystalline powder (52%. Calc. for C29H25N2O4PW:
C, 51.19; H, 3.70; N, 4.12. Found: C, 50.90; H, 3.82; N, 4.34%); R = MeO,
yellow crystals (65%. Calc. for C28H22NO5PW: C, 50.39; H, 3.32;
N, 2.10. Found: C, 50.28; H, 3.29; N, 2.14%); R = Me, yellow micro-
crystals (59%. Calc. for C28H22NO4PW: C, 51.63; H, 3.40; N, 2.15.
Found: C, 51.75; H, 3.47; N. 2.04%); R = Ph, yellow powder (63%.
Calc. for C33H24NO4PW: C, 55.56; H, 3.39; N, 1.96. Found:
C, 55.86; H, 3.60; N, 1.93%); R = H,[18] yellow crystals (76%); R = Cl,
yellow needles (74%. Calc. for C27H19ClNO4PW: C, 48.27; H, 2.85;
Experimental
*
Correspondence to: Laurence Carlton, Molecular Sciences Institute, School of
Chemistry, University of the Witwatersrand, P O Wits, 2050 South Africa.
E-mail: laurence.carlton@wits.ac.za
Materials
Tungsten hexacarbonyl, triphenylphosphine, pyridines and
trimethylamine oxide hydrate were purchased from various
Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand,
Johannesburg, South Africa
Magn. Reson. Chem. 2013, 51, 199–202
Copyright © 2013 John Wiley & Sons, Ltd.

L. Carlton, L. V. Mokoena and M. A. Fernandes
N, 2.08. Found: C, 47.86; H, 3.08; N, 2.02%); R = PhCO, red crystals
(71%. Calc. for C34H24NO5PW: C, 55.08; H, 3.26; N, 1.89. Found:
C, 55.36; H, 3.10; N, 2.02%); R =MeCO, orange powder (67%. Calc.
for C29H22NO5PW: C, 51.27; H, 3.26; N, 2.06. Found: C, 50.80;
H, 3.33; N, 2.00%).
Table 1. Crystal data and structure refinement
cis-[W(CO)4(PPh3) cis-[W(CO)4(PPh3)
(MeOC5H4N)]
(4-PhCOC5H4N)]
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
C28H22NO5PW
667.29
C34H24NO5PW
741.36
NMR
293(2)
293(2)
0.71073
0.71073
Spectra were recorded on a Bruker DRX 400 spectrometer operating
at 400.13 MHz (1H), 161.98 MHz (31P) and 16.62 MHz (183W).
183W–31P spectra were obtained by indirect detection using
the pulse sequence:
Triclinic
Monoclinic
P2(1)/c
Space group
P1
Unit cell dimensions
a = 8.5174 (11) (Å)
b = 16.817 (2) (Å)
c = 18.828 (2) (Å)
a = 86.580 (2)ꢀ
b = 79.468 (2)ꢀ
g = 88.780 (2)
2646.6(5)
4
a = 10.6505(17)
b = 21.677(4)
c = 13.834(2)
a = 90ꢀ
À
Á
Â
À
ÁÃ
À
Á
À
Á
p=2 31P ꢁ 1= 2J 183W;31P ꢁ p=2 183W ꢁ t ꢁ p 31P ꢁ t
À
Á
Â
À
ÁÃ
À
Á
½19ꢂ
ꢁ p=2 183W ꢁ 1= 2J 183W;31P ꢁ Acq 31P
b = 108.263(3)ꢀ
g = 90ꢀ
ꢀ
with broadband H decoupling throughout and 183W decoupling
omitted during acquisition. A spectral width in f2 (31P) of
6 ppm and an acquisition time of 1.05 s gave a digital resolution
of 0.47 Hz per point. In f1 (183W), a spectral width of 40 ppm and a
time domain of 200 were used; after zero filling, this gave a
digital resolution in f1 of 0.65 Hz per point. To eliminate the
possibility of a folded signal, spectra were first recorded (20–30
increments only) with a spectral width in f1 of up to 2000 ppm.
183W chemical shifts were referenced to Ξ (183W) = 4.15 MHz.[15]
1
Volume (Å3)
3033.0(9)
4
Z
Density (calcd) (Mg/m3)
1.675
1.624
Absorption coefficient
4.463
3.903
(mmꢁ1
)
Crystal size (mm3)
θ range for data
collection (ꢀ)
0.38 ꢃ 0.18 ꢃ 0.12 0.44 ꢃ 0.36 ꢃ 0.15
1.10 to 27.00
1.81 to 28.00
Index ranges
ꢁ10 ≤ h ≤ 10
ꢁ16 ≤ k ≤ 21
ꢁ24 ≤ l ≤ 23
16731
ꢁ13 ≤ h ≤ 14
ꢁ28 ≤ k ≤ 23
ꢁ18 ≤ l ≤ 17
20354
X-ray crystallography
Reflections collected
Intensity data were collected on a Bruker SMART 1K CCD area
detector diffractometer with graphite monochromated Mo Ka
radiation (50 kV, 30 mA). The collection method involved o-scans of
width 0.3ꢀ. Data reduction was carried out using the program
SAINT+,[20] and absorption corrections were made using the
program XPREP.[21] The crystal structures were solved by direct
methods using SHELXTL.[22] Non-hydrogen atoms were first refined
isotropically followed by anisotropic refinement by full matrix least-
squares calculations based on F2 using SHELXTL. Hydrogen atoms
were first located in the difference map then positioned geometri-
cally and allowed to ride on their respective parent atoms. Crystal
and structure refinement data are given in Table 1; selected
bond lengths and angles for cis-[W(CO)4(PPh3)(4-MeOC5H4N)] (2)
and cis-[W(CO)4(PPh3)(4-PhCOC5H4N)] (7) are given in Table 2.
Independent reflections
11357 [R
7291 [R
(int) = 0.0405]
98.4
(int) = 0.0493]
99.7
Completeness to θmax (%)
Max and min transmission
0.6927 and 0.2806
0.5921 and
0.2785
Data/restraints/parameters
Goodness of fit on F2
11357/0/651
0.964
7291/0/379
0.932
Final R indices [I > 2s (I)]
R1 = 0.0367
wR2 = 0.0807
R1 = 0.0631
wR2 = 0.0949
R1 = 0.0328
wR2 = 0.0531
R1 = 0.0676
wR2 = 0.0592
R indices (all data)
Largest diff. peaks (e. Åꢁ3
)
1.124 and ꢁ1.099 0.829 and ꢁ0662
C1–O1 (the bond angles N–W–C1, C1–W–C3 and C1–W–C4 differ
by ~3–4ꢀ) with other differences being minor.
Results and Discussion
The complexes cis-[W(CO)4(PPh3)(4-RC5H4N)] (R = H, Me, Ph,
COMe, COPh, OMe, NMe2, Cl) are prepared in moderate to good
yield by the reaction of cis-[W(CO)4(PPh3)(CH3CN)] with the 4-R
pyridine in benzene at 80 ꢀC (Scheme 1). The 4-nitropyridine
complex was prepared in situ. The cis geometry was confirmed
by X-ray diffraction studies of two of the complexes (R = OMe,
COPh), whose structures are shown in Figs 1 and 2.
The OMe complex crystallizes with two conformational isomers
in the unit cell, differing by 3.4ꢀ in the P–W–C2 bond angle. If a dif-
ference of this magnitude can be induced solely by crystal packing
forces, then differences between the 4-MeOC5H4N complex and
the 4-PhCOC5H4N complex (see following text), which are of the
same order of magnitude, may have a similar origin and be unre-
lated, in any direct way, to the substituent on pyridine. The
geometry at the metal in the 4-MeOC5H4N and 4-PhCOC5H4N
complexes is slightly distorted octahedral, the most significant dif-
ference between the structures being in the position of carbonyl
NMR parameters for the nine complexes are given in Table 3,
where the tungsten chemical shift can be seen to increase as
the substituent on pyridine is varied from electron donating to
electron withdrawing. The phosphorus chemical shift also varies
but in a manner that is less closely related to the substituent on
pyridine, and the tungsten–phosphorus spin coupling constant
shows a small but significant increase as the substituent on
pyridine becomes less electron rich.
The means chosen by which to quantify the electron-donating/
withdrawing ability of the substituent R on pyridine is the
Hammett s function, introduced in 1937 by Hammett[25] in order
to account for the influence of a substituent on a benzene ring
on rate and equilibrium constants for reactions involving an
attached functional group and further developed by Jaffé,[26]
McDaniel and Brown[27] and Hansch et al.[28] The Hammett s
values of Jaffé are derived from a wider range of experimental
data than those of McDaniel and Brown, whose data were
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Copyright © 2013 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2013, 51, 199–202

Tungsten-183 NMR study
Table 2. Selected bond lengths (Å) and angles (ꢀ); the unit cell
of cis-[W(CO)4(PPh3)(4-MeOC5H4N)] contains two nonidentical
structures A, A0
cis-[W(CO)4(PPh3)
cis-[W(CO)4(PPh3)
(4-MeOC5H4N)]
(4-PhCOC5H4N)]
A
A0
P1
W–P
2.5527(14)
2.298(5)
2.022(7)
1.997(6)
1.964(7)
2.025(6)
87.42(11)
93.66(18)
177.19(19)
96.56(18)
87.80(17)
92.6(2)
2.5523(16)
2.284(4)
2.020(7)
1.986(7)
1.957(6)
2.025(7)
86.71(12)
96.3(2)
2.5566(11)
2.281(3)
O1
W–N
C1
W1
W–C1
2.012(4)
C3
N41
O3
W-C2
1.982(5))
1.965(4)
C4
O4
W–C3
C2
O2
W–C4
2.033(4)
P–W–N
P–W–C1
P–W–C2
P–W–C3
P–W–C4
N–W–C1
N–W–C2
N–W–C3
N–W–C4
C1–W–C2
C1–W–C3
C1–W–C4
C2–W–C3
C2–W–C4
C3–W–C4
85.39(8)
96.57(14)
173.82(13)
96.18(12)
85.62(11)
95.39(14)
90.01(14)
177.23(14)
96.88(12)
87.96(19)
82.18(16)
167.67(15)
88.60(16)
90.85(17)
85.53(15)
173.79(19)
97.20(19)
85.28(18)
92.3(2)
Figure 2. Structure of cis-[W(CO)4(PPh3)(4-PhCOC5H4N)]. Thermal ellipsoids
are shown at the 50% probability level.
89.9(2)
90.0(2)
Table 3. NMR data for complexes [W(CO)4(PPh3)(4-RC5H4N)]
175.9(2)
94.3(2)
175.5(2)
95.7(2)
Complexa
Ξ(183W)b
d(183W)c
d(31P)d
J(183W,31P)e
87.4(2)
89.1(3)
[W(CO)4(PPh3)
(4-Me2NC5H4N)]
[W(CO)4(PPh3)
(4-MeOC5H4N)]
[W(CO)4(PPh3)
(4-MeC5H4N)]
[W(CO)4(PPh3)
(4-PhC5H4N)]
[W(CO)4(PPh3)
(C5H5N)]
4.155857
1411
31.25
237
86.4(3)
85.1(3)
172.9(2)
86.1(3)
171.9(2)
86.3(3)
4.155957
4.155994
4.156001
4.156033
4.156091
4.156102
4.156113
4.156243
1435
1444
1446
1454
1468
1470
1473
1504
32.45
32.38
32.69
32.77
33.58
32.98
32.90
33.19
240
240
241
241
242
242
243
243
91.5(4)
89.8(3)
86.6(3)
86.9(3)
PPh3
PPh3
OC
OC
CO
CO
OC
OC
4-RC5H4N
C6H6 80oC
[W(CO)4(PPh3)
(4-ClC5H4N)]
W
W
N
NCCH3
[W(CO)4(PPh3)
(4-PhCOC5H4N)]
[W(CO)4(PPh3)
(4-MeCOC5H4N)]
[W(CO)4(PPh3)
(4-NO2C5H4N)]
CO
CO
R
Scheme 1. R = H, Me, Ph, COMe, COPh, OMe, NMe2, Cl, NO2.
aSolution ~0.01–0.015 M in C6D6 at 300 K.
bResonant frequency in a field in which the protons of TMS resonate
at exactly 100 MHz.
cChemical shift in ppm relative to Ξ (183W) = 4.15 MHz. To convert to a scale
relative to W(CO)6 {Ξ (183W) = 4.151888 MHz[10]} subtract 455 ppm; to
convert to a scale relative to WF6 {Ξ (183W) = 4.161780 MHz[23]} subtract
2831 ppm; to convert to a scale relative to WO24ꢁ[10] subtract 3939 ppm.
dChemical shift in ppm relative to 85% H3PO4 (external standard).
eCoupling constants (absolute magnitude) in Hz. The sign of J is
probably positive.[23,24]
P1
O1
C1
W1
C3
N41
O3
C4
restricted to dissociation constants of substituted benzoic acids.
With regard to the substituent groups chosen for this study, the
Hammett s values of Jaffé and of McDaniel and Brown (whose
values, with the exception of that for COPh, are those used by
Hansch et al.) are in quite good agreement except for the value
of NMe2, which is given as ꢁ0.60 by Jaffé and ꢁ0.83 by McDaniel
and Brown. In Fig. 3, data points for the complex with R = NMe2
are given for both these values.
O4
C2
O2
Figure 1. Structure of cis-[W(CO)4(PPh3)(4-MeOC5H4N)]. Thermal ellipsoids
are shown at the 50% probability level.
Magn. Reson. Chem. 2013, 51, 199–202
Copyright © 2013 John Wiley & Sons, Ltd.
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L. Carlton, L. V. Mokoena and M. A. Fernandes
Conclusion
Tungsten-183 chemical shifts measured from complexes
cis-[W(CO)4(PPh3)(4-RC5H4N)] show a good correlation with the
electron-donating/withdrawing properties of the substituent R
as quantified by the Hammett substituent parameter sp.
Supplementary data
Supplementary data for complexes cis-[W(CO)4(PPh3)(4-RC5H4N)]
(R = MeO, COPh) have been deposited as CCDC 910115 and 910116,
respectively. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Figure 3. Plot of d(183W) against Hammett sp for complexes cis-[W(CO)4
(PPh3)(4-RC5H4N)], with R identified for each data point. Values of sp are
those of McDaniel and Brown[27] and Hansch et al.[28] except for NMe2*,
which is given by Jaffé.[26] Estimated accuracy ꢄ1 ppm for d(183W), ꢄ0.02
units for sp.[27]
Acknowledgements
We thank the University of the Witwatersrand and the NRF for the
financial support and Mr M Philpott and Ms T von Reiche of the
ARC-Institute for Soil, Climate and Water for the elemental analysis.
Figure 3 shows a plot of d(183W) against Hammett s (sp indicating
a para substituent) for the complexes cis-[W(CO)4(PPh3)(4-RC5H4N)],
with R identified for each data point. When the Jaffé value for
the NMe2 substituent (rather than the value of McDaniel and
Brown) is used, the data points show a good linear correlation
between d(183W) and s. From the most electron-donating (NMe2)
to the most strongly electron-withdrawing (NO2) substituent, the
tungsten chemical shift increases by 93ppm, demonstrating the
high sensitivity of the tungsten nucleus to changes occurring at a
distance of five bonds. In view of the position of the substituent
(4-position) and the minimal structural changes on going from a
small (OMe) to a bulky (COPh) substituent (Table 2; Figs 1 and 2),
the influence exerted by the substituent is considered to be almost
entirely electronic.
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shift are (from a simplified form of the Ramsey equation[29]) ΔEꢁ1
,
where ΔE is the energy difference between the highest occupied
and lowest unoccupied molecular orbitals, and <rꢁ3>, where r
is the average radius of a d orbital.[30] The ability of a ligand to
increase the d orbital radius, r, is related to its polarizability
(hard/soft property). For ligands that create a low ΔE, such as
pyridine, which is a good s donor and poor p acceptor,
polarizability effects will largely determine the value of the metal
chemical shift. The polarizability of 4-RC5H4N will increase as
R becomes more strongly electron donating (reflected in
the value of Hammett sp) and will therefore (via <rꢁ3>, which
will decrease) lead to a lower chemical shift as observed for
cis-[W(CO)4(PPh3)(4-RC5H4N)].
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The tungsten chemical shift reference of 4.15 MHz[15] was
introduced in the hope that its use might alleviate a situation
in which three reference standards are commonly used and
overcome difficulties associated with the measuring of signals
from them. The 183W chemical shifts (relative to 4.15 MHz) in Table 3
can be converted into shifts relative to W(CO)6 by subtracting
455 ppm, into shifts relative to WF6 by subtracting 2831 ppm, and
into shifts relative to WO24ꢁ by subtracting 3939 ppm.
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Magn. Reson. Chem. 2013, 51, 199–202