Kinetics of the substitution of norbornadiene in tetracarbonyl(norbornadiene)molybdenum(0) by 2,2'-bipyridine

Article abstract of DOI:10.1515/znb-2001-0310

The kinetics of the thermal substitution of norbornadiene (nbd) by 2,2'-bipyridine (2,2'-bipy) in (CO)₄Mo(C₇H₉) was studied by quantitative FT-IR and UV-VIS spectroscopy. The reaction rate exhibits first-order dependence o

Full text of DOI:10.1515/znb-2001-0310

Kinetics of the Substitution of Norbornadiene in
Tetracarbonyl(norbomadiene)molybdenum(0) by 2,2’-Bipyridine
Ceyhan Kayran and Eser Okan
Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey
Reprint requests to Assoc. Prof. Dr. C. Kayran. E-mail: ckayran@metu.edu.tr
Z. Naturforsch. 56 b, 281-286 (2001); received November 23, 2000
2,2’-Bipyridine, Norbornadiene, Molybdenum
The kinetics of the thermal substitution of norbornadiene (nbd) by 2,2’-bipyridine (2,2’-
bipy) in (CO)4Mo(CyH9) was studied by quantitative FT-IR and UV-VIS spectroscopy. The
reaction rate exhibits first-order dependence on the concentration of the starting complex, and
the observed rate constant depends on the concentration of both leaving nbd and entering
2,2’-bipy ligand. The mechanism was found to be consistent with the previously proposed one,
where the rate determining step is the cleavage of one of the two Mo-olefin bonds. The reaction
was performed at four different temperatures (35 - 50 °C) and the evaluation of the kinetic data
gives the activation parameters which now support an associative mechanism in the transition
states.
Introduction
1 , 1 0 phenanthroline) results in stable tetracarbonyl
derivatives [11]. This reaction is thought to proceed
via a two step process: initial photochemical for-
mation of M(CO)5 (N-N) (N-N = diimine ligands),
and subsequent thermal extrusion of a second CO
ligand to form M(CO)4 (N-N).
The tetracarbonyl(77-diene) complexes of the
group 6 elements have been known for over three
decades [1] and used as M(CO)4 transfer reagents
to prepare a variety of M(CO)4L2 compounds (L =
donor ligand). Furthermore, M(CO)4 (77-diene) com-
plexes have been shown to be the intermediates
in the catalytic transformations of dienes using
M(CO)ö [2]. The lability of the coordinated dienes
is very important in designing homogeneous cat-
alytic processes. Despite of this, very little work
M(CO)6 + N-N
M(CO)s(N-N) + CO
M(CO)5(N-N) -A+ M(CO)4(N-N) + CO
N-N = 1,4-diazabutadiene, 2,2’-bipyridine,
or pyridine-2-carbaldehyde
The formation of chelate complexes M(CO)4-
has been performed on the displacement of dienes in (NN) was followed by UV-VIS spectroscopy and
M(CO)4 (?7-diene) complexes and most of the avail-
able studies deal with the displacement of diene
by two monodentate ligands [3, 4]. These studies
kinetic data were reported [12 - 14]. The unsta-
ble intermediates M(CO)s(NN) could not be iso-
lated, but the formation of the chelate complexes
revealed that the cleavage of one of the M-olefin M(CO)4 (NN) was assumed to proceed through
bonds is the rate determining step in the proposed the formation of M(CO)s(NN) [15, 16]. Later on,
mechanism [5, 6 ]. The substitution of diene in the M(CO)5 (N-N) (NN = 1,4-diazabutadiene) com-
same complex with a bidentate ligand, has recently plexes were isolated and the kinetics of the ring-
been studied using bis(diphenylphosphino)alkane closure reactions were studied [17]. This type of
as a ligand and provided more information about the chelate ring-closure reaction is very fast in the
mechanism [7 - 10]. At this point, we thought that case of 2,2’-bipyridine, and therefore M(CO)s(NN)
it is interesting to change the donor atoms of the en- (NN = 2,2’-bipyridine) could not be isolated
tering bidentate ligand from phosphorus to nitrogen [18 - 20]. The displacement of one bidentate ligand
(norbornadiene) by another (2 ,2 ’-bipyridine) may
and see whether there will be any change in the re-
action mechanism. It is well established that irradia- give valuable insight into the substitution kinetics.
tion of group 6 metal carbonyl solutions containing Therefore, in this work, we studied the ligand dis-
an excess of adiimine ligand (e. g. 2 ,2 ’-bipyridine or placement kinetics of norbornadiene (nbd) in tetra-
0932-0776/01/0300-0281 $ 06.00 © 2001 Verlag der Zeitschrift
ftir Naturforschung, Tübingen • www.znaturforsch.com
K
Brought to you by | University of Sydney
Authenticated
Download Date | 9/24/15 12:01 AM

282
C. Kayran and E. Okan • Kinetics of the Substitution of Norbomadiene
carbonyl^2 2 -nbd)molybdenum(O) with 2 ,2 ’-bipyr- bottom flask. The solution is irradiated with a 200 W Hg
lamp for approximately 30 min with continued purging
(gentle N2 stream) and constant stirring. Upon long irra-
diation the solution turns orange and precipitation starts.
Cooling in an ice bath for 15 min yields the orange colored
product. It is purified by washing the residue with n-hex-
ane several times. (60%) (17). - IR( 1,2-dichloroethane):
i/ = 2015(C=0), 1905.0, 1876.8, 1831.8 cm -1. 13C{1H}
NMR (100 MHz, dö-dmso): 6 = 155.01 (C=N), 153.73
(HC=N), 141.37 (HC=), 128.73 (HC=), 126.60 (HC=).
idine (2 ,2 ’-bipy) according to the following reac-
tion:
M(CO)4(nbd) + 2,2’-bipy
M(CO)4(2,2’-bipy) + nbd
The process was studied by quantitative FT-IR
spectroscopy. Since both the reactant and the prod-
uct give sharp bands of the CO stretching, the inten-
sity of which is generally very sensitive to changes
in concentration, the use of IR spectroscopy enables
one to follow the reaction precisely and to control
the material balance and the yield at any conver-
sion up to 80 - 90%. For comparison the kinetics
were also followed by UV-VIS spectroscopy in the
presence of a 1 0 -fold excess of 2 ,2 ’-bipy.
Kinetic measurements
All spectral measurements were recorded in 1,2-di-
chloroethane with a Specac variable temperature IR cell
with 0.109 mm path length and calcium fluoride windows.
The molar extinction coefficients of 1834 L em -1 mol-1
at 2039 cm-1 for 1, and 1115 L cm - 1 mol-1 at
2015 cm-1 for 2 were determined from 1,2-dichloro-
ethane solutions of the pure complexes by plotting the
absorbance of the i^co IR band, with the highest fre-
quency, versus the concentration of the respective com-
plex in the standard solutions prepared. By using these
values, the concentration of 1 and 2 during the reaction of
Mo(CO)4(nbd) with 2,2’-bipy could be determined from
the measured IR absorbances at any stage of the reaction.
Samples of the solution for IR measurements were
taken periodically. The initial concentration of 1 was
Co(l) = 8-10-3 molar when it was added into the pre-
heated solution of 2,2’-bipy in 1,2-dichloroethane. The
reaction rates were determined by following the disap-
pearance of the highest frequency peak of the starting
material, since it was the only distinct peak which does
not overlap with the peak of the product. The graphical
evaluation of the data gave the rate constants. To study
the dependence of the rate constants on the concentration
of the entering and leaving ligands, kinetic experiments
were performed first, by varying the concentration ra-
tio of 2,2’-bipy to 1 ([2,2’-bipy]/[l] = 0, 10, 20, 40, 60,
80) with no added nbd; second, keeping the concentra-
tion ratio 2,2’-bipy to 1 constant ([2,2’-bipy]/[l] = 1) and
varying the concentration ratio of nbd to 1 ([nbd]/[l] = 0,
2, 4, 6).
Experimental Section
All reactions and manipulations were carried out ei-
ther in vacuum or under a dry and deoxygenated nitrogen
atmosphere. Solvents were distilled after refluxing over
metallic sodium or phosphorus pentoxide under nitrogen
for 3 - 4 d and stored until used. Hexacarbonylmolyb-
denum(O), norbomadiene and 2,2’-bipyridine were pur-
chased from Aldrich Chemical Co., Ltd, Dorset, England
and used without further purification. NMR spectra were
recorded on a Bruker DPX 400 Spectrometer. Infrared
spectra were recorded from 1,2-dichloroethane solution
using a Perkin Elmer PC-16 FTIR or a Nicolet 510 FTIR
spectrometers.
Tetracarbonyl(r f 2-norbomadiene)molybdenum(0):
Mo(CO)4(nbd) (I)
1 was prepared according to the procedure described
in the literature [21, 22] with minor modifications. 6 g
(22.7 mmol) of Mo(CO)6 is dissolved in 70 ml of isooc-
tane and 8 ml (78.4 mmol) of nbd is added with stirring
and refluxed for 3 h until the color of the solution turns
to yellow. The reaction solution is evaporated to dryness
and the residue is washed several times with n-hexane
in order to remove the starting material. Crystallization
yields yellow crystals (70%). - IR (1,2-dichloroethane):
v (C=0) 2039, 1959.0, 1913.0 cm“ 1. - 13C-{1H}NMR
(100 MHz, C7D8): 6 = 78.5 (=CH-), 48.8 (-CH2), 64.8
(CH), 217.70 (C=0), 213.80 (C=0).
The activation parameters were obtained from the tem-
perature dependence of the rate constant in the range of
35 - 50 °C by an increment of 5 °C.
Results and Discussion
Tetracarbonyl(2,2 ’-bipyridine)molybdenum:
Mo(CO)4(2,2’-bipy) (2)
The IR absorption spectrum of Mo(CO)4 (7y2:2-
nbd) dissolved in 1,2-dichloroethane (Fig. la)
shows three absorption bands in the CO stretch-
ing region corresponding to a local C2V symme-
try of its Mo(CO)4 moiety (2Ai + Bi + B2) [23].
A hexane solution (30 ml) containing Mo(CO)6
(0.017 mmol) and 2,2’-bipy (0.017 mmol) is deoxy-
genated (gentle N2purging for 10 min) in a 100 ml round-
Brought to you by | University of Sydney
Authenticated
Download Date | 9/24/15 12:01 AM

C. Kayran and E. Okan • Kinetics of the Substitution of Norbomadiene
283
■
1.2
I '
■
■
.
■
"
*
(a)
♦
a
■
0
0.8
■
♦
■
♦
♦
0.4
0.2
♦
♦
*
♦
♦
♦
n i
0
1000
2000
3000
4000
5000
t(s)
t(s)
Fig. 2. Time dependent behavior of concentrations of
Mo(CO)4(r?2:2-nbd) and Mo(CO)4(2,2’-bipy): (a) Con-
centration versus time plot (♦: Mo(CO)4(?722-nbd);
Mo(CO)4(2,2’-bipy)); (b) plot of the first order reaction
kinetics for the substitution reaction.
2080
2030
1980
1930
1880
1830
1780
wavenumber (cm1)
Fig. 1. IR spectra of (a) Oo(CO)4(r?22-nbd), (b) Mo-
(CO)4(ry22-nbd) and Mo(CO)4-(2,2’-bipy) during the sub-
stitution reaction and (c) Mo(CO)4(2,2’-bipy) in 1,2-
dichloroethane.
bands of Mo(CO)4 (?72:2-nbd) are gradually replaced
by the bands of Mo(CO)4 (2 ,2 ’-bipy) during the re-
action (Fig. 1 a-c). When the reaction is conducted
at 45 °C isosbestic points indicate a straightforward
conversion of the reactant into product without side
or subsequent reactions [25].
Two of the four bands expected obviously over-
lap. The 13C-1 H-NMR spectrum of Mo(CO)4 (r?2:2-
nbd) shows three signals for the nbd ligand and
two for the carbonyl groups. The carbonyl signal
at lower field is assigned to the CO groups trans
to each other [24]. Thermal substitution of nbd in
Mo(CO)4 (7?2:2-nbd) by 2,2’-bipy yields the complex
Mo(CO)4 (2 ,2 ’-bipy). The IR absorption spectrum
ofMo(CO)4 (2,2’-bipy) again shows four absorption
bands in the CO stretching region (C2Vsymmetry
for the Mo(CO)4 part). The ^C^H-NMR spectrum
of Mo(CO)4 (2 ,2 ’-bipy) gives two signals in a 1:1
ratio for two carbonyl groups and five signals for
the 2 ,2 ’-bipy system indicating that two aromatic
rings remain equivalent.
An inspection of the spectra in Fig. 1 shows that
the highest frequency bands of the reactant and
product at 2043.0 and 2014 cm-1 , respectively, do
not overlap and remain well resolved during the
whole reaction. Therefore, these two IR bands are
selected to follow the consumption of the educt, 1 ,
and the growth of the product 2 .
Fig. 2a shows a plot of concentration versus time
for the thermal reaction at 45 °C for which the time
resolved spectra are given in Fig. 1. This graph
shows an exponential decay for 1 and an exponen-
The kinetics of the substitution was followed by tial growth for 2 (Fig. 2a). The logarithmic plot of
quantitative FT-IR spectroscopy. In the CO stretch- the concentration of 1 against time gives a straight
ing region of the IR spectrum the three absorption line (Fig. 2b) for the reaction. This indicates that
Brought to you by | University of Sydney
Authenticated
Download Date | 9/24/15 12:01 AM

284
C. Kayran and E. Okan • Kinetics of the Substitution of Norbomadiene
100
200
300
400
500
600
700
Concentration of 2,2-bipy (mM)
Fig. 3. Variations of the observed rate constant with the
concentration of 2,2’-bipyridine at 45 °C.
By application of the well known steady state ap-
proximation for intermediates I and II the rate is
found to be
Rate = kobs[1],
kik_i
k obs — k i
k5
k_i +k5 + k3[NN] -
1+k6[NN]/k_5[nbd]
The proposed mechanism was shown to be in agree-
ment with the experimental results. When the com-
plex is dissolved in 1 ,2 -dichloroethane and left at
45 °C in the absence of the entering ligand 2,2’-bipy
for about 2 - 3 h, there is no noticeable change in
the concentration of the complex. The limit of k^s
approaches zero, when the concentration 2 ,2 ’-bipy
goes to zero as the proposed mechanism predicts:
the displacement of nbd from Mo(CO)4 (?72:2-nbd)
by 2 ,2 ’-bipy obeys pseudo-first order kinetics with
a correlation coefficient around 0.99. The slope of
the straight line gives the observed rate constant,
lim(NN)-*o(kobs) = 0.
As the concentration of the entering ligand in-
creases gradually, k^s increases and reaches a finite
value at very high concentrations (Fig. 3):
kobs (S )•
From previous studies [7 - 9] we assume that dis-
sociation of nbd from the complex is the rate de-
termining step: One of the two olefin-metal bonds
lini(NN)^oo(kobs) = ki = 1.24 • 10 3.
The effect of the leaving group nbd on the rate of
is cleaved to yield intermediate I (Scheme 1). The the substitution reaction was also studied by varying
next steps involve associative or dissociative pro- the concentration of nbd but keeping the concentra-
tion of 2,2’-bipy constant (8 -10“ 3 molar). The plot
cesses to form III or the very labile intermediate
II, which immediately reacts with 2,2’-bipy to form of kobs versus nbd is curved downward, with posi-
the product. Dissociation of the olefin from inter- tive intercept, indicating the presence of a complex
mechanism [27] in which the increasing concentra-
product as well. There is no evidence for the forma- tion of nbd causes a decrease in kobS. This clearly
suggests that the substitution rate of nbd in 1 is
mediate III and ring closure of 2,2’-bipy give the
tion cis-M(CO)4 (772-nbd)2 from intermediate I upon
reversible coordination of a second nbd molecule, retarded by the presence of excess nbd. As the con-
even though similar complexes have already been centration of nbd goes to infinity, k^s approaches a
limit value which can be obtained from Fig. 4:
isolated [26].
Brought to you by | University of Sydney
Authenticated
Download Date | 9/24/15 12:01 AM

C. Kayran and E. Qkan • Kinetics of the Substitution of Norbomadiene
285
Table 1. The rate constants (kobs) obtained at different
temperatures and free energy of activation values deter-
mined from IR and UV-VIS data.
"4sec 1]
UV-VIS
AG* [kJ/mol]
IR
kobs [10'
IR
T(K)
UV-VIS
2.24
3.48
4.62
5.47
2.00
2.91
3.70
4.70
97.2
98.4
98.8
99.7
308
313
318
323
96.9
97.3
98.2
99.3
20
30
40
Concentration of nbd (mM)
Fig. 4. Variations of the observed rate constant with the
concentration of norbomadiene at 45 °C.
-14.2
3.03
3.08
3.13
3.18
3.23
3.28
1000/T (K'1)
Fig. 6. Eyring plot for the substitution of nbd in Mo-
(CO)4(r?2“-nbd) in the presence of a 10 fold excess of
2,2’-bipy (followed by IR spectroscopy).
ditions. The observed rate constant values (Fig. 5)
as well as the free energy of activation calculated
are given in Table 1. From the Eyring plot the en-
thalpy and the entropy of activation are found to be
AH* = 46.5(±7) kJ-mol- 1 and Z\S^ = -86.9(±18)
Jm ol_1 K_1, respectively (Fig. 6 ). The negative
value of AS* is consistent with an Sn2 or asso-
ciative mechanism in the transition state [27].
For comparison with the IR spectroscopic results,
the formation of 2 (ca. 6 -1 0 - 4 molar in hexane so-
lution) was also monitored by quantitative UV-VIS
spectroscopy in the presence of a 1 0 -fold excess of
2,2’-bipyridine. From the increase in the concentra-
tion of product as a function of time kobs is found
for a pseudo-first order reaction. The rate of forma-
tion of 2 (A = 470 nm), is comparable with the result
obtained from FT-IR for the disappearance of 1 (Ta-
ble 1). The activation parameters, calculated using
UV data, -dH7', and Z\S^are 43.8(±3) kJ-mol- 1 and
-96(± 10) J mol- 1 K-1, respectively. The results
compare well with those obtained by quantitative
IR spectroscopy.
Time (s)
Fig. 5. First order kinetics for the displacement reaction
of nbd from Mo(CO)4(ry22-nbd) by 2,2’-bipy in the tem-
perature range 35 - 50 °C.
kik_i
-i + k 3[NN]
= 9.3 • 10'
lim[nbd]—>oo(kobs) —ki
On the other hand, as the concentration of nbd
goes to zero, kobS approaches another limit value
which can also be seen from Fig. 4:
kik_i
= 1.25-10'
lim[nbd]—>o(kobs) — ki
k_1+ k5+ k3[NN]
In order to evaluate the activation parameters
thermal substitution reactions were performed at
four different temperatures (35, 40, 45, 50 °C) in
the presence of 1 0 -fold excess (8 .0 -1 0 ~ 2 molar) of
2 ,2 ’-bipyridine to provide pseudo-first order con-
In a dissociative Sn I mechanism, the rate de-
Brought to you by | University of Sydney
Authenticated
Download Date | 9/24/15 12:01 AM

286
C. Kayran and E. Okan • Kinetics of the Substitution of Norbomadiene
termining step involves only bond breaking, while
in an associative mechanism ( S n 2 ), the rate deter-
mining step involves both bond breaking and bond
formation [28]. It is expected that the enthalpy of
activation, Z\H^, would approach the M-nbd bond
energy for a predominantly dissociative processes
and be rather independent of the nature of the enter-
ing ligand, whereas for an associative process Z\H^
is expected to be smaller than the M-nbd bond en-
ergy. Single Mo-nbd bond energy is reported to be
111.6 kJ m o r 1 [29], but AH¥ = 46.5 kJ-mol" 1 is
found experimentally. This also clearly indicates
that substitution reaction of Mo(CO)4(?72:2-nbd)
with 2 ,2 ’-bipy has an Sn2 or associative mechanism
in the transition state.
Acknowledgements
The authors thank Prof. Dr. Saim Ozkar for his enthu-
siasm and suggestions in this work. Support of this work
by TÜBITAK under Grant No. TBAG-1226 is gratefully
acknowledged.
[1] a) E. O. Fischer, W. Fröhlich, Chem. Ber. 92, 2995 [11] W. Strohmeier, V. Hobe, Chem. Ber. 94, 2031
(1959). b) C. G. Kreiter, Adv. Organomet. Chem. (1961).
26, 297 (1986); M. Kotzian, C. G. Kreiter, S. Özkar, [12] M. J. Schadt, N. J. Gresalfi, A. J Lees, J. Chem.
Soc., Chem. Commun. 506 (1984).
S. Özkar, J. Organomet. Chem. C13, 152 (1978); [13] M. J. Schadt, N. J. Gresalfi, A. J. Lees, Inorg. Chem.
J. Organomet. Chem. 29, 229 (1982); C. G Kreiter,
26,620(1987).
[14] M. J. Schadt, N. J. Gresalfi, A. J. Lees, Inorg. Chem,
24,2942(1985).
[15] M. J. Schadt, A. J. Lees, Inorg. Chem. 25, 672
I. Fischler, M. Butzwait, E. A. Koemer von Gustorf,
J. Organomet. Chem. 105, 325 (1976); S. Özkar,
N. B. Peynircioglu, Inorg. Chim. Acta 119, 127
(1986); H. Werner, R. Prinz, Chem. Ber. 100, 265
(1967).
(1986).
[2] M. S. Wrighton, M. A. Schröder, J. Am. Chem. Soc.
95, 5764 (1973); D. Ritvelde, L. Wilputte-Steinert,
[16] L. Chan, A.J. Lees, J. Chem. Soc., Dalton Trans.
515 (1987).
J. Organomet. Chem. 118, 191 (1976); M. J. Mir- [17] F. W. Grevels, C. Kayran, S. Özkar, Organometallics
bach, T. N. Phu, A. Sams, J. Organomet. Chem. 236,
309 (1982); M. A. Schröder, M. S. Wrighton, J. Am.
Chem. Soc. 96, 6235 (1974); W. Jennings, B. Hill,
13,2937(1994).
[18] D. M. Manuta, A. J Lees, J. Chem. Ednc. 64, 637
(1987).
J. Am. Chem. Soc. 92, 3199 (1970); W. Abdelqader, [19] H. Drechthom, Chem. Ber. 100, 228 (1967).
[20] H. Brunner, W. A. Hermann, Chem. Ber. 105, 770
(1972).
S. Özkar, N. B. Peynircioglu, Z. Naturforsch. 48b,
539 (1993).
[3] F. Zingales, M. Graziani, U. Belluco, J. Am. Chem.
Soc. 89, 256(1967).
[4] D. T. Dixon, J. A. S. Howell, J. Chem Soc., Dalton
Trans. 1307(1984).
[5] F. Zingales, F. Canziani, F. Basolo, J. Orgonomet.
Chem. 7,461 (1967).
[6] D. T. Dixon, P. M. Burkinshaw, J. A. S. Howell,
J. Chem Soc., Dalton Trans. 2237 (1980).
[7] C. Kayran, E. Okan, S. Özkar, O. Öztürk, S. Sal-
damli, A. Tekkaya, C. G Kreiter, Pure Appl. Chem.
69, 193 (1997).
[21] H. Werner, R. Prinz, Chem. Ber. 100, 265 (1967).
[22] M. A. Bennett, L. Pratt, G. Wilkinson, J. Chem. Soc.
(London) 75, 2037(1961).
[23] M. Y. Darensbourg, D. J. Darensbourg, J. Chem.
Educ. 47, 33 (1970).
[24] M. Kotzian, C. G. Kreiter, S. Özkar, J. Organomet.
Chem. 229, 29 (1982).
[25] B. S. Creaven, F. W. Grevels, C. Lang, Inorg. Chem.
28,2231 (1989).
[26] S. Zhang, I. H. Wang, P. H. Werner, C. B. Dobson,
G. R. Dobson, Inorg. Chem. 31, 3482 (1992).
[8] C. Kayran, C. Kreiter, F. Kozanoglu, S. Ozkar, S. Sal- [27] R. J. Angelici, J. R. Graham, J. Am. Chem. Soc. 87,
5586(1965).
damli, A. Tekkaya, Inorg. Chim. Acta 284, 229
(1999).
[28] F. Basolo, Chem. Rev. 52, 459 (1953).
[29] J. A. Connor, J. P. Day, E. M. Jones, G. K. McEwen,
J. Chem. Soc., Dalton Trans. 347 (1973).
[9] A. Tekkaya, C. Kayran, S. Özkar, Inorg. Chem. 33,
2439(1994).
[10] A.Tekkaya, S. Özkar, J. Organomet. Chem. 590, 208
(1999).
Brought to you by | University of Sydney
Authenticated
Download Date | 9/24/15 12:01 AM