Isomeric olefin tetracarbonyl complexes of tungsten(I): An infrared spectroelectrochemical study at low temperatures

Article abstract of DOI:10.1021/om700384g

In situ electrolysis within an optically transparent thin-layer electrochemical (OTTLE) cell was applied at 293-243 K in combination with FTIR spectroscopy to monitor spectral changes in the carbonyl stretching region accompanying oxidation of four tetrac

Full text of DOI:10.1021/om700384g

4066
Organometallics 2007, 26, 4066-4071
Isomeric Olefin Tetracarbonyl Complexes of Tungsten(I): An
Infrared Spectroelectrochemical Study at Low Temperatures
Marcin Go´rski,† Frantisˇek Hartl,‡ and Teresa Szyman´ska-Buzar*,†
Faculty of Chemistry, UniVersity of Wrocław, ul. F. Joliot-Curie 14, 50-383 Wrocław, Poland, and
Homogeneous and Supramolecular Catalysis, Van’t Hoff Institute for Molecular Sciences, UniVersity of
Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands
ReceiVed April 20, 2007
In situ electrolysis within an optically transparent thin-layer electrochemical (OTTLE) cell was applied
at 293-243 K in combination with FTIR spectroscopy to monitor spectral changes in the carbonyl
stretching region accompanying oxidation of four tetracarbonyl olefin complexes of tungsten(0), viz.,
trans-[W(CO)4(η2-ethene)2], trans-[W(CO)4(η2-norbornene)2], [W(CO)4(η4-cycloocta-1,5-diene)], and
[W(CO)4(η4-norbornadiene)]. In all cases, the one-electron-oxidized radical cations (17-electron complexes)
have been identified by their characteristic ν(CtO) patterns. For the bidentate diene ligands, the cis
stereochemistry is essentially fixed in both the 18- and 17-electron complexes. The radical cation of the
trans-bis(ethene) complex was observed only at 243 K, while at room temperature it isomerized rapidly
to the corresponding cis-isomer. The thermal stability of the three studied radical cations in the cis
configuration correlates with the relative strength of the W-CO bonds in the positions trans to the olefin
ligand, which are more affected by the oxidation than the axial W-CO bonds. For the bulky norbornene
ligands, their trans configuration in the bis(norbornene) complex remains preserved after the oxidation
in the whole temperature range studied. The limited thermal stability of the radical cations of the trans-
bis(alkene) complexes is ascribed to dissociation of the alkene ligands. The spectroelectrochemical results
are in very good agreement with data obtained earlier by DFT (B3LYP) calculations.
mentioned the occurrence of oxidative trans f cis isomerization
also for several bis(alkene) tetracarbonyl complexes of tungsten-
(0), though without convincing spectroelectrochemical
evidence.5 The closed-shell neutral bis(alkene) tetracarbonyl
complexes can exist in two geometrical forms (trans and cis),
of which only the thermodynamically more stable trans-isomers
have been isolated so far.2 The labile cis-bis(alkene) isomers,
usually detectable as products of photolysis at low temperatures,
have nevertheless been supposed to be active species in
homogeneous catalysis with group 6 transition metal carbonyl
complexes.2 In the particular case of tungsten(0) complexes
under study in our laboratories, attention should be paid to the
role of electron transfer in their transformation to catalytically
active species, following the principles of electron-transfer chain
catalysis in organotransition metal chemistry.6-8 Combined
experimental and theoretical efforts are highly desirable to
explain and predict structural changes and to obtain deeper
mechanistic insights. For example, density functional theory
(DFT) calculations indicate that the 17-electron tungsten(I)
complex cis-[W(CO)4(η2-ethene)2]+ is ca. 10 kJ mol-1 more
stable than its trans-isomer.9 On the other hand, DFT calcula-
tions point to facile stepwise transformation of the corresponding
Introduction
The chemistry of group 6 metal penta- and tetracarbonyl
complexes with olefin ligands has been widely examined since
their initial IR spectroscopic characterization by Stolz et al. in
1963.1 On the contrary, their redox behavior has remained
largely unexplored.2 A particularly intriguing topic in the area
of redox activation is the electron-transfer-induced geometrical
isomerization of di- and trisubstituted pseudooctahedral carbonyl
complexes.3,4 In a brief report from 1993, Wilgocki et al.
* Corresponding author. E-mail: tsz@wchuwr.chem.uni.wroc.pl. Fax:
+48 71 328 23 48. Tel: +48 71 375 7221.
† University of Wrocław.
‡ University of Amsterdam.
(1) (a) Stolz, I. W.; Dobson, G. R.; Sheline, R. K. Inorg. Chem. 1963,
2, 1264-1267. (b) Bachmann, C.; Demuynck, J.; Veillard, A. J. Am. Chem.
Soc. 1978, 100, 2366-2369. (c) Grevels, F.-W.; Lindemann, M.; Benn,
R.; Goddard; R.; Kru¨ger, C. Z. Naturforsch., B: Anorg. Chem., Org. Chem.
1980, B35, 1298-1309. (d) Pope, K. R.; Wrighton, M. S. Inorg. Chem.
1985, 24, 2792-2796. (e) Grevels, F.-W.; Jacke, J.; O¨ zkar, S. J. Am. Chem.
Soc. 1987, 109, 7536-7537. (f) Daniel, C.; Veillard, A. Inorg. Chem. 1989,
28, 1170-1173. (g) Takeda, H.; Jyo-o, M.; Ishikawa, Y.; Arai, S. J. Phys.
Chem. 1995, 99, 4558-4565. (h) Pidun, U.; Frenking, G. Organometallics
1995, 14, 5325-5336. (i) Jaroszewski, M.; Szyman´ska-Buzar, T.; Wilgocki,
M.; Zio´łkowski, J. J. J. Organomet. Chem. 1996, 509, 19-28. (j)
Szyman´ska-Buzar, T.; Jaroszewski, M.; Downs, A. J.; Greene, T. M.; Morris,
L. J. J. Organomet. Chem. 1997, 531, 207-216. (k) Szyman´ska-Buzar, T.;
Kern, K. J. Organomet. Chem. 1999, 592, 212-224. (l) Guillemot, G.;
Solari, E.; Floriani, C. Organometallics 2000, 19, 5218-5230. (m) Del Rio,
D.; Schubert, G.; Pa´pai, I.; Galindo, A. J. Organomet. Chem. 2002, 663,
83-90.
(5) Wilgocki, M.; Szyman´ska-Buzar, T.; Jaroszewski, M.; Zio´łkowski,
J. J. In Molecular Electrochemistry of Inorganic, Bioinorganic and
Organometallic Compounds; Pombeiro, A. J. L., McCleverty, J. A., Eds.;
Kluwer Academic Publishers: Dordrecht, 1993; pp 573-582.
(6) Chanon, M.; Tobe, M. L. Angew. Chem., Int. Ed. Engl. 1982, 21,
1-23.
(7) Astruc, D. Angew. Chem., Int. Ed. Engl. 1988, 27, 643-660.
(8) Pombeiro, A. J. L. New J. Chem. 1997, 21, 649-660.
(9) Handzlik, J.; Hartl, F.; Szyman´ska-Buzar, T. New J. Chem. 2002,
26, 145-152.
(2) Szyman´ska-Buzar, T. Coord. Chem. ReV. 2006, 250, 976-990, and
references therein.
(3) Bond, A. M.; Colton, R. Coord. Chem. ReV. 1997, 166, 161-180,
and references therein.
(4) Pombeiro, A. J. L.; Guedes da Silva, M. F. C.; Lemos, M. A. N. D.
A. Coord. Chem. ReV. 2001, 219-221, 53-80, and references therein.
10.1021/om700384g CCC: $37.00 © 2007 American Chemical Society
Publication on Web 06/30/2007

Isomeric Olefin Tetracarbonyl Complexes of Tungsten(I)
Organometallics, Vol. 26, No. 16, 2007 4067
Chart 1 Schematic Molecular Structures of Studied Olefin
Complexes 1-4
Table 1. Cyclic Voltammetric Data for Complexes 1-4a
compd
E1/2ox (V)
Ep,a - Ep,c (mV)
1
2
3
4
0.73
0.42
0.53
0.56b
90
80
80
b
-
Figure 1. Cyclic voltammogram of trans-[W(CO)4(η2-ethene)2]
(1) in dichloromethane. The redox couple at -1.33 V, denoted by
an asterisk, is the cobaltocene/cobaltocenium internal reference.
Conditions: Pt disk microelectrode, V ) 100 mV s-1, room
temperature.
a Conditions: 10-3 M complex in dichloromethane containing 10-1
M
Bu4NPF6; V )100 mV s-1; T ) 293 K; Pt disk microelectrode. Ep,a value
for irreversible oxidation. Ep,a - Ep,c ) 80 mV for the cobaltocene/
cobaltocenium internal standard.
b
dicationic tungsten(II) complex to tungstacyclopentane and
butylidene species active in catalytic olefin metathesis.2
In the course of our exploration of olefin rearrangement in
the coordination sphere of tungsten carbonyls, we have focused
on the processes induced by electrochemical oxidation of the
metal center. We have been interested in the influence of the
olefin nature on the oxidation potentials of the parent complexes
and stability of the electron-deficient cationic products. In the
present infrared spectroelectrochemical study, we describe one-
electron oxidation of four selected tungsten(0) tetracarbonyl bis-
(alkene) and diene complexes with trans and cis mutual
positions of the W-(η2-CdC) bonds, respectively (Chart 1). The
neutral complexes with chelating diene ligands served as
convenient models for the nonaccessible cis-isomers of the bis-
(alkene) tetracarbonyls.
Figure 2. Cyclic voltammogram of [W(CO)4(η4-norbornadiene)2]
(4) in dichloromethane. The redox couple at -1.33 V, denoted by
an asterisk, is the cobaltocene/cobaltocenium internal reference.
Conditions: Pt disk microelectrode, V ) 100 mV s-1, room
temperature.
Results and Discussion
Electrochemistry. Cyclic voltammetric study of the four
studied olefin complexes (Chart 1) was carried out in dichlo-
romethane. The results are summarized in Table 1.
monitoring (see below). A new cathodic peak appeared during
the reverse cathodic sweep at -0.07 V (at room temperature),
which is comparable with the cyclic voltammogram of complex
1. In this case, however, the reduction of the secondary anodic
product is strongly affected by its adsorption at the polished Pt
disk microelectrode (Figure 2). It is noteworthy that the internal
reference, the cobaltocenium/cobaltocene redox couple, re-
mained unaffected by the irreversible voltammetric response
of complex trans-1, as testified by ∆Ep ) 80 mV at V ) 0.1 V
At room temperature, the cyclic voltammograms of the bis-
(norbornene) (trans-2) and cyclooctadiene (3) complexes show
a fully reversible anodic wave even at the limiting scan of 0.02
V s-1. Comparison with the standard ferrocene/ferrocenium and/
or cobaltocenium/cobaltocene redox couples used as internal
references confirms diffusion control of the electron transfer.
On the other hand, oxidation of bis(ethene) complex trans-1
becomes reversible only at V g 2 V s-1. At slower scans, the
cathodic-to-anodic peak current ratio, Ip,c/Ip,a drops below 1, viz.,
0.96 at 1 V s-1, 0.93 at 0.5 V s-1, and 0.84 at 0.1 V s-1. With
the decreasing scan rate, a new cathodic waves grows on the
reverse scan at Ep,c ) 0.05 V vs Fc/Fc+ (Figure 1). Fully
reversible oxidation of complex 1 was also achieved at
temperatures below 0 °C.
In contrast to complexes trans-1, trans-2, and 3, the anodic
response of norbornadiene complex 4 is totally irreversible at
room temperature (Figure 2), in agreement with an earlier report
by Kochi et al.10 No change in this behavior was observed even
at 205 K. The absence of the corresponding cathodic counter-
peak implies a rapid chemical transformation of the 17e product
4+, which was also confirmed by infrared spectroelectrochemical
s-1
.
Norbornadiene complex 4 and cycloocta-1,5-diene complex
3 with the cis arrangement of the W-(η2-CdC) bonds oxidize
at similar anodic potentials around 0.56 V vs Fc/Fc+ (Table 1).
The reversible oxidation of bis(norbornene) complex trans-2
lies at a slightly lower anodic potential, viz., 0.46 V, while for
bis(ethene) complex trans-1 the anodic wave has a distinctly
higher potential, viz., 0.73 V. The more difficult oxidation of
trans-1 is probably a result of higher π-acceptor ability of the
coordinated ethene compared to the other olefin ligands. This
explanation is further supported by the higher CO-stretching
frequencies of complex trans-1 than those obtained for com-
plexes trans-2, 3, and 4 (Table 2). In addition, the W-C(ethene)
bond in complex trans-1, determined by X-ray diffraction
analysis, is ca. 0.1 Å shorter compared to W-C(diene) bond
lengths in complexes 3 and 4 and slightly shorter than reported
(10) Hershberger, J. W.; Klinger, R. J.; Kochi, J. K. J. Am. Chem. Soc.
1982, 104, 3034-3043.

4068 Organometallics, Vol. 26, No. 16, 2007
Go´rski et al.
Table 2. Infrared Spectral Data for Complexes 1-4 and
Products of Their One-Electron Oxidationa
compd
T (K)
ν(CtO) (cm-1
)
trans-1
trans-1
trans-1 b
trans-1+
trans-1+b
cis-1b
cis-1+
cis-1+b
trans-2
trans-2
trans-2+
trans-2+
3
293
243
2062 (vw), 1994 (vw), 1956 (vs)
2061 (vw), 1992 (vw), 1955 (vs)
2055 (0), 1996 (0.04), 1968 (1)
2131 (vw), 2066 (sh, m-w), 2042 (s), 2014 (vs)
2114 (0), 2051 (0.001), 2041 (0.88), 2040 (1)
2042 (0.21), 1971 (0.27), 1967 (1), 1938 (0.66)
2125 (m), 2055 (sh), 2035 (vs)
2108 (0.25), 2055 (0.21), 2052 (0.48), 2037 (1)
2041 (vw), 1974 (w), 1928 (vs)
2040 (vw), 1972 (w), 1926 (vs)
243
293
293
243
293
243
293
243
293
243
293
243
243
2111 (vw), 2045 (w), 2013 (vs)
2110 (vw), 2042 (w), 2014 (vs)
2036 (m), 1942 (vs, br), 1879 (s)
2034 (m), 1941 (vs, br), 1871 (s)
2110 (m), 2046 (m,br), 2013 (vs)
2110 (m), 2046 (m,br), 2010 (vs)
2036 (m), 1941 (vs, br), 1878 (s)
2034 (m), 1940 (vs, br), 1870 (s)
2112 (m), 2064 (m), 2036(sh), 2018 (s)
3
3+
Figure 4. IR spectrum of [W(CO)4(η4-norbornadiene)]+ (4+).
Conditions: 1e oxidation of parent complex 4 within an OTTLE
cell in dichloromethane at 243 K.
3+
4
4
4+
stability of 3+. The intensities of the ν(CtO) bands of 3+ are
much lower than those of precursor 3, which applies also to
the other studied W(0) and W(I) complexes (see below). It is
noteworthy that the broad middle band has a fairly low intensity
in the IR spectrum of 3+, while it is dominant in the IR spectrum
of 3. The observed high intensity of the lowest-energy ν(Ct
O) band in 3+ has correctly been predicted by DFT (B3LYP)
calculations9 on related bis(ethene) cation cis-3+. The calcula-
tions have revealed that the intense b1 and weaker b2 absorptions
in cis-1 interchange their energetic positions in cis-1+ (two b
modes, local C2 symmetry2), which probably accounts also for
the different ν(CtO) intensity patterns of complexes 3 and 3+
(Figure 3).
a All experimental IR spectra were recorded in dichloromethane contain-
b
ing 2.5 × 10-1 M Bu4NPF6. DFT data reported by Handzlik et al. in ref
9; relative band intensities are given in parentheses.
The irreversible oxidation of norbornadiene complex 4, as
observed by cyclic voltammetry (see above), gave hardly any
chance to characterize 4+ by IR spectroscopy. Indeed, at room
temperature no carbonyl product was detected during the anodic
electrolysis of 4. The decarbonylation of 4+ was also evidenced
by CO gas development at the anode. The presence of free CO
in the solution was indicated by a weak absorption at ca. 2140
Figure 3. IR spectra of [W(CO)4(η4-cycloocta-1,5-diene)] (3)
(dashed line) and 1e-oxidized 3+ (full line). Conditions: oxidation
of 3 within an OTTLE cell in dichloromethane at 243 K.
cm-1 15
Surprisingly, when the oxidation of complex 4 in the
.
thin solution layer was performed at 243 K in a potential step,
we observed a small amount of 4+ (Figure 4). The assignment
has been based on comparison with the IR spectrum of the stable
W(I) radical cation 3+, the latter showing similar ν(CtO)
wavenumbers and intensity pattern with the broad middle
absorption band at 2046 cm-1. This absorption is resolved in
the case of 4+ into two bands of medium intensities and a very
close average wavenumber value (Table 2).
for the closely related trans-bis(cyclooctene) tungsten(0)
tetracarbonyl.2,11-14
Infrared Spectroelectrochemistry. Infrared spectra of the
diene tungsten(0) complexes 3 and 4 with the cis arrangement
of the W-(η2-CdC) bonds (local C2V symmetry2) exhibit three
ν(CtO) bands in the range 2040-1870 cm-1 (Table 2), the
middle one with a shoulder on the high-frequency side. These
absorption bands correspond to a1(1), close-lying a1(2) + b1, and
b2 stretching modes, all being IR-active. Monitoring the fully
reversible 1e oxidation of cycloocta-1,5-diene complex 3 carried
out in the OTTLE cell at 243 K has revealed a large
high-frequency shift of the ν(CtO) bands (Figure 3). The shift
magnitude (>70 cm-1) corresponds to predominantly tungsten-
localized W(0) f W(I) oxidation and greatly diminished W f
CO π-back-donation. A very similar result has been obtained
at room temperature (Table 2), in agreement with the inherent
A similar large shift to higher ν(CtO) frequencies was
observed in the IR spectra recorded during the W(0) f W(I)
oxidation of the bis(norbornene) complex trans-2 (Table 2). The
neutral parent complex exhibits a very intense ν(CtO) band at
1926 cm-1 (243 K), corresponding to the e mode in the local
D2d symmetry,2 if only the carbonyl groups are considered
(Figure 5). The weak bands of trans-2 at 2040 and 1972 cm-1
represent a1 and b2 vibrational modes, respectively.9 It is
important to stress the very low-intensity a1 mode (IR-inactive
in D2d), which is characteristic for trans-[W(CO)4(η2-alkene)2]
complexes. In contrast, a medium intensity of the symmetric
(11) Szyman´ska-Buzar, T.; Kern, K.; Downs, A. J.; Greene, T. M.;
Morris, L. J. Parsons, S. New J. Chem. 1999, 23, 407-416.
(12) Grevels, F.-W.; Jacke, J.; Betz, P.; Kru¨ger, C; Tsay, Y.-H.
Organometallics 1989, 8, 293-298.
(13) Dalla Riva Toma, J. M.; Toma, P. H.; Fanwick, P. E.; Bergstrom,
D. E. Byrn, S. R. J. Crystallogr. Spectrosc. Res. 1993, 23, 41-47.
(14) Grevels, F.-W.; Jacke, J.; Kru¨ger, C; Klotzbu¨cher, W. E.; Mark,
F.; Skibbe, V. Schaffner, K. Organometallics 1999, 18, 3278-3293.
(1)
all-carbonyl stretch (IR-active a1 mode) was observed for
complexes 3 and 4 with the cis arrangement (Figures 3 and 4),
which also agrees with the DFT (B3LYP) data calculated9 for
the trans-1 and cis-1 isomers. The oxidation of complex trans-2
(15) Dubost, H. Chem. Phys. 1976, 12, 139-151.

Isomeric Olefin Tetracarbonyl Complexes of Tungsten(I)
Organometallics, Vol. 26, No. 16, 2007 4069
Figure 7. IR spectrum of trans-[W(CO)4(η2-ethene)2]+ (trans-1+).
Conditions: 1e oxidation of parent complex trans-1 within an
OTTLE cell in dichloromethane at 243 K.
Figure 5. IR spectra of trans-[W(CO)4(η2-norbornene)2] (2)
(dashed line) and 1e-oxidized 2+ (full line). Conditions: oxidation
of 2 within an OTTLE cell in dichloromethane at 243 K.
(Figure 3) with the carbonyl ligands in two different configura-
tions, (i) mutually trans and (ii) trans to the diene CdC bonds.
This comparison points to fast trans f cis isomerization of
trans-1+ following the one-electron oxidation of trans-1. From
preceding DFT (B3LYP) calculations9 it is known that the cis-
bis(ethene) cationic complex (cis-1+) is thermodynamically more
stable by ca. 10 kJ mol-1 than trans-1+. The four computed
ν(CtO) frequencies and relative intensities (in parentheses) of
cis-4+ are 2108 (0.25), 2055 (0.21), 2052 (0.48), and 2037 (1)
cm-1.9 The DFT data thus reproduce very well the experimental
IR spectrum.
According to the reversible cyclic voltammetric response of
trans-1 (see above), lowering the temperature of the OTTLE
cell was expected to facilitate IR spectroscopic detection of the
initially formed cation trans-1+. Figure 7 shows ν(CtO) bands
in the IR spectrum recorded at 243 K, with frequencies and
intensities that indeed significantly differ from the band pattern
obtained at room temperature: 2131 (vw), 2066 (sh, m-w), 2042
(s), 2014 (vs) cm-1. This experimental result agrees appreciably
with the DFT data computed for trans-1+ (the intensities are
given in parentheses): 2114 (0), 2051 (0.001), 2041 (0.88), and
2040 (1) cm-1. In the experimental spectrum, the two intense
bands at 2042 and 2014 cm-1 probably correspond to the low-
energy b3 and b2 modes with nearly identical theoretical9
wavenumbers, reminiscent of the single e mode in parent
complex trans-1. Cation trans-1+ differs in this respect from
trans-2+ (Figure 5) but resembles another W(I) tetracarbonyl
with linear alkene ligands, viz., trans-[W(CO)4(η2-but-1-ene)2]+
(ν(CtO) at 2127 (vw), 2058 (sh, m), 2035 (vs), and 2005 (vs)
cm-1).16 The latter radical complex is fully stable at 223 K,
like trans-2+. In contrast, oxidation of trans-1 at 243 K produces
also [W(CO)6] (ν(CtO) at 1975 cm-1; omitted in Figure 7),
indicating dissociation of the ethene ligands, the weakest
σ-donors in the series (Table 2). This behavior is explained
below.
Figure 6. IR spectra of trans-[W(CO)4(η2-ethene)2] (trans-1)
(dashed line) and 1e-oxidized cis-[W(CO)4(η2-ethene)2]+ (cis-1+)
(full line). Conditions: oxidation of trans-1 and concomitant
isomerization of trans-1+ within an OTTLE cell in dichloromethane
at 293 K.
to trans-2+ was fully reversible at 243 K. The radical cationic
product exhibits the same ν(CtO) band intensity pattern (Figure
5), thereby excluding any trans f cis isomerization induced
by the electron transfer. Again, the trans arrangement of the
norbornene ligands in trans-2+ has been supported by the very
low intensity of the ν(CtO) band at 2110 cm-1 (Raman-active
a mode, D2 local symmetry), in line with the DFT calculations9
on the closely related cation trans-1+ (Table 2). At room
temperature, primary oxidation product trans-2+ lives only a
few minutes and could be detected by IR spectroscopy shortly
after a rapid potential step. Again, no trans f cis isomerization
was observed.
In marked contrast to bis(norbornene) complex trans-2, IR
spectroelectrochemical monitoring of one-electron oxidation of
bis(ethene) derivative trans-1 revealed a more complex behavior.
The IR spectrum of the fairly stable product obtained upon
oxidation of trans-1 at room temperature displays one strong
band at 2035 cm-1 with a shoulder at ca. 2055 cm-1 and,
importantly, a medium-intensity band at 2125 cm-1 (Figure 6).
It resembles the IR spectrum of cycloocta-1,5-diene cation 3+
(16) The cationic complex [W(CO)4(η2-but-1-ene)2]+ was originally
described as the cis-isomer, on the basis of the separation of all four CO-
stretching modes and a large ∆Ep value from slow-scan thin-layer cyclic
voltammetry at low temperatures.9 However, the comprehensive results
presented in this work for complexes 1-4, in particular the characteristic
very low intensity of the IR-forbidden highest-frequency all-CO stretch for
trans-[W(CO)4(η2-alkene)2]0/+ and the absence of trans f cis isomerization
for cation trans-2+ above 243 K, have prompted us to rectify the assignment
of the cationic oxidation product as trans-[W(CO)4(η2-but-1-ene)2]+. The
latter species decomposes at room temperature but does not isomerize,
similar to trans-2+.

4070 Organometallics, Vol. 26, No. 16, 2007
Go´rski et al.
As calculated earlier,9 the four ν(CtO) bands of bis(ethene)
complex cis-1 (a1(1), a1(2), b1, and b2 modes) shift upon the one-
electron oxidation to higher frequencies by 66, 84, 70, and 114
cm-1, respectively. The vibration of the two CO ligands in trans
positions to the ethene ligands corresponds to the b mode at
2052 cm-1 in cis-1+ (C2 symmetry) and to the b2 mode at 1938
cm-1 in neutral complex cis-1 (C2V symmetry). The energetic
separation between these two modes, ∆ν ) 114 cm-1 for cis-
1/cis-1+, increases significantly to 175 cm-1 and 194 cm-1 for
the couples 3/3+ and 4/4+. On the other hand, much smaller
wavenumber differences have been determined for vibrations
of two mutually trans carbonyl ligands in the redox couples
trans-1/trans-1+ (∆ν ) 59 cm-1) and trans-2/trans-2+ (∆ν )
88 cm-1). From the comparison of these ∆ν values it is clear
that the one-electron oxidation of a tetracarbonyl W(0) complex
containing two mutually cis-arranged olefin ligands removes
the electron density to a higher extent from π-orbitals represent-
ing the W-CO bonds trans to the W-olefin bonds than from
the W-CO π-bond of the mutually trans carbonyl ligands. For
this reason, the lability of the carbonyl ligands in the oxidized
complexes should correlate with the ∆ν value increasing in the
order cis-1+ < 3+ < 4+. This reasoning may partly explain the
observed high reactivity of the 17e complex with chelating
norbornadiene (4+) compared with the fairly high stability of
the cationic bis(ethene) complex cis-1+. The decarbonylation
of 4+ (detectable in situ at 243 K) was indeed confirmed by IR
spectroscopic monitoring.
The ∆ν separation of the e mode of trans-1 (D2d) and the b2
mode of trans-1+ (D2), both corresponding to vibrations of the
two mutually trans CO ligands, amounts 59 cm-1 (243 K). This
value is thus much smaller than those characterizing the
vibration of the CO ligands in trans positions to the olefin
ligands in complexes cis-1, 3, and 4 and their 1e-oxidized forms
(see above). These data suggest that the one-electron oxidation
of bis(olefin) complexes trans-1 and trans-2 removes the
electron density mainly from the π-orbitals of the tungsten-
olefin bonds, inducing dissociation of the olefin ligands. This
behavior corresponds nicely with the formation of [W(CO)6]
in the course of the oxidation of complex trans-1 at 243 K,
where no trans f cis isomerization takes place. For the redox
couple trans-2/trans-2+, the larger ∆ν value of 88 cm-1 for the
e and b2 modes, respectively, is fully in line with the higher
stability of trans-2+ compared to trans-1+. Norbornene is also
a stronger σ-donor than ethene, as testified by the smaller ν-
(CtO) wavenumbers of trans-2 compared to those of trans-4
(Table 2), capable of better stabilizing the W(I) oxidation state.
At the same time, the greater steric demands of the mutually
cis-arranged norbornene ligands in hypothetical cis-2+ compared
to the smaller ethene in cis-1+ can be considered as a plausible
explanation for the completely hindered trans f cis isomer-
ization of 1e-oxidized trans-2+.
cis-isomer affects mainly the W-CO bonds trans to the
W-olefin bonds. The lability of these carbonyl ligands ought
to be higher in 17-electron diene complexes 3+ and 4+ than the
carbonyls in bis(alkene) complexes trans-1+ and trans-2+. The
comparison of the ∆ν(CtO) shifts has further revealed that in
the latter two cations the oxidation mainly weakened the
W-alkene bonds. The analysis of the bonding situation is
consistent with the facile decarbonylation of 4+ even at low
temperatures and observation of [W(CO)6] in the IR spectra of
trans-1+.
At room temperature, cation trans-1+ undergoes spontaneous
trans f cis isomerization, in agreement with higher thermo-
dynamic stability of cis-1+ as computed by DFT. The absence
of trans f cis isomerization for trans-2+ has been tentatively
ascribed to steric hindrance of the bulky norbornene ligands in
mutual cis positions.
Experimental Section
General Procedures. Standard inert-atmosphere techniques were
used for all syntheses and sample manipulations. Dichloromethane
(Acros; analytical quality) was dried over P2O5 and freshly distilled
under nitrogen. The supporting electrolyte Bu4NPF6 (Aldrich) was
recrystallized twice from absolute ethanol and dried overnight under
vacuum at ca. 80 °C. [W(CO)]6 (Merck) was used as received.
Olefins (Sigma-Aldrich) were dried over CaH2 and distilled under
nitrogen prior to use.
Syntheses. The preparation of the studied olefin complexes
(Chart 1) trans-[W(CO)4(η2-ethene)2] (trans-1),11 trans-[W(CO)4-
(η2-norbornene)2] (trans-2),18 [W(CO)4(η4-cycloocta-1,5-diene)]
(3),17 and [W(CO)4(η4-norbornadiene)] (1)17 was based on literature
procedures. All compounds were prepared by photochemical
substitution of two CO ligands in [W(CO)6] by olefin in n-heptane.
Complexes 2, 3, and 4 were purified by sublimation under vacuum
at 80 °C, and complex 1 by column chromatography on silica, using
1
n-heptane as the eluent. Their identity was verified by H NMR
and IR spectroscopy.
Cyclic Voltammetry. Conventional cyclic voltammograms of
10-3 M complexes 1-4 in dichloromethane containing 10-1 M Bu4-
NPF6 were recorded with a PAR model 283 potentiostat, using an
airtight and light-protected single-compartment cell placed in a
Faraday cage. A Pt disk (0.4 mm diameter) working electrode was
polished with a 0.25 µm diamond paste between scans. Coiled Pt
and Ag wires served as an auxiliary and pseudoreference electrode,
respectively. All electrode potentials are reported against the
standard ferrocene/ferrocenium (Fc/Fc+) couple. Ferrocene (BDH)
or cobaltocenium hexafluorophosphate (Aldrich) were added as
internal standards.19,20 After the scans at room temperature, the cell
was cooled by a dry ice-acetone slurry.
Spectroelectrochemistry. Infrared spectroelectrochemistry was
carried out with a cryostatted optically transparent thin-layer
electrochemical (OTTLE) cell equipped with a Pt minigrid working
electrode and CaF2 windows.21 The course of the anodic electrolyses
was monitored with thin-layer cyclic voltammetry and infrared
spectroscopy, using a Bio-Rad FTS-7 FTIR spectrometer (16 scans,
spectral resolution of 2 cm-1). The OTTLE cell potential was
controlled by a PA4 potentiostat (EKOM, Polna´, Czech Republic).
The electrolyzed dichloromethane solutions contained 5 × 10-3
Conclusions
These results document that infrared thin-layer spectroelec-
trochemistry at variable temperatures is a valuable tool to study
the anodic processes in tungsten(0) carbonyls with olefin and
diene ligands in different (trans and cis) arrangements.
(17) King, R. B.; Fronzaglia, A. Inorg. Chem. 1966, 5, 1837-1846.
(18) Go´rski, M.; Kochel, A.; Szyman´ska-Buzar, T. Organometallics 2004,
23, 3037-3046.
(19) Gritzner, G.; Ku˚ta, J. Pure Appl. Chem. 1984, 56, 462-466.
(20) Stojanovic, R. S.; Bond, A. M. Anal. Chem. 1993, 65, 56-64.
(21) (a) Hartl, F.; Luyten, H.; Nieuwenhuis, H. A.; Schoemaker, G. C.
Appl. Spectrosc. 1994, 48, 1522-1528. (b) Mahabiersing, T.; Luyten, H.;
Nieuwendam, R. C.; Hartl, F. Collect. Czech. Chem. Commun. 2003, 68,
1687-1709.
The IR spectroscopic data show that the one-electron oxida-
tion of the neutral parent complexes can be generally assigned
as W(0) f W(I). However, the magnitude of the high-frequency
ν(CtO) shifts depends on the mutual positions of the carbonyl
and olefin ligands, suggesting that significant electron density
is removed from different W-olefin and W-carbonyl π-orbit-
als. In particular, the one-electron oxidation of a tetracarbonyl

Isomeric Olefin Tetracarbonyl Complexes of Tungsten(I)
Organometallics, Vol. 26, No. 16, 2007 4071
M W(0) complex under study and 3 × 10-1 M Bu4NPF6 as
the form of the grant no. 3 T09A 103 28. Mr. T. Mahabiersing
(University of Amsterdam) helped us with the IR spectroelec-
trochemical experiments.
supporting electrolyte. The thin-layer cyclic voltammograms were
recorded at a scan rate of 2 mV s-1
.
Acknowledgment. We are grateful to the Polish State
Committee for Scientific Research for support of this work in
OM700384G