DRIFTS and optical microscopic study of the solventless reaction between (η5-C5H5)M(CO)3CH3 (M = Mo, W) and L (L = P(p-FC6H4)3, P(C 6H5)3)

Article abstract of DOI:10.1021/om0343869

The reaction between solid (η⁵-C₅H ₅)M(CO)₃CH₃ and solid phosphines is shown to occur in the melt phase (optical microscopy), and a DRIFTS study of the melt reveals a well-behaved reaction following pseudo-first-order kinetics.

Full text of DOI:10.1021/om0343869

2048
Organometallics 2004, 23, 2048-2052
DRIF TS a n d Op tica l Micr oscop ic Stu d y of th e
Solven tless Rea ction betw een (η5-C5H5)M(CO)3CH3 (M )
Mo, W) a n d L (L ) P (p-F C6H4)3, P (C6H5)3)
Muhammad D. Bala, Asheena Budhai, and Neil J . Coville*
Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand,
J ohannesburg 2050, South Africa
Received December 18, 2003
The reaction between solid (η5-C5H5)M(CO)3CH3 and solid phosphines is shown to occur
in the melt phase (optical microscopy), and a DRIFTS study of the melt reveals a well-
behaved reaction following pseudo-first-order kinetics.
In tr od u ction
regimes between 25 and 500 °C, (v) the ability to
perform reactions under pressure, and (vi) the use of
small samples of reagents. As will be shown in this
study, DRIFTS can also be used to study reactions in
the melt phase.
In particular, our study has thus entailed the reaction
of solid phosphines (e.g. P(p-FC6H4)3) with (η5-C5H5)M-
(CO)3CH3 (M ) Mo, W) at temperatures below the
melting points of the reactants and products. An optical
microscopic study was used to monitor the physical
changes as the reaction progressed.
Solvent-free synthesis has received increased atten-
tion, due partly to environmental concerns1 but also
because of the potential novel products and mechanisms
that can be generated when reactions are conducted in
the absence of solvents.2 While solventless synthesis has
been well studied in organic chemistry3,4 and materials
science,5 the study of solvent-free organometallic reac-
tions is still relatively unexplored and the potential this
method has in organometallic synthesis has yet to be
fully established.6
In a continuation of our studies on organometallic
transformations in the absence of solvents, we recently
reported on the migratory insertion of (η5-C5H5)M(CO)3-
Me (1: M ) Mo, W) in the presence of phosphines
(Scheme 1). Some of the reactions appeared to occur in
the solid state.7 However, it is difficult to imagine a
mechanism that would allow for an insertion reaction
between two solids. To further explore this reaction, we
have now studied the reaction using diffuse reflectance
infrared Fourier transform spectroscopy (DRIFTS) and
optical microscopy.
DRIFTS is a technique that can be used to study
chemical reactions when the reactants are in the form
of powders or crystalline solids and is thus well suited
to the study of the migration/insertion reaction under
investigation.8
Exp er im en ta l Section
The metal complexes (η5-C5H5)M(CO)3Me (M ) Mo, W) were
prepared by adaptation of literature procedures,9 and the new
products (η5-C5H5)M(CO)2P(p-FC6H4)3(COMe) (M ) Mo, W)
were characterized by elemental analysis and IR and NMR
spectroscopy.
Details on the equipment and methods used for analytical
and spectroscopic studies (DRIFTS, NMR, DSC, etc.) and the
results from such studies are available as Supporting Informa-
tion.
DRIF TS Stu d ies. The samples for DRIFTS studies were
prepared by mechanical grinding all reactants to a fine powder
(40-80 µm particle size) using a pestle and mortar. Typically
1-2 wt % of the metal complex (η5-C5H5)M(CO)3Me (M ) Mo,
W) in KBr (as a diluent) was ground together. A similar weight
percentage of the phosphine diluted with KBr was also ground
together. No reaction occurred between KBr and either of the
reactants due to the grinding process.10 Similar results were
obtained when NaCl and Al2O3 were used as diluents.
The finely ground phosphine/KBr mixture was first loaded
into the DRIFTS cell and measured as the background, and
then the separately ground complex/KBr and phosphine/KBr
mixtures were thoroughly mixed in the required molar ratio
to give a homogeneous powder (no grinding). This homoge-
The DRIFTS technique offers the following advan-
tages: (i) in situ measurements, (ii) facile data collec-
tion, (iii) the ability to work under inert reaction
conditions, (iv) the ability to work in temperature
(1) Anastas, P. T.; Warner, C. Green Chemistry, Theory and Practice;
Oxford University Press: London, 1998.
(2) (a) Cheng, L.; Coville, N. J . Organometallics 1996, 15, 867. (b)
Coville, N. J .; Cheng, L. J . Organomet. Chem. 1998, 571, 149. (c)
Coville, N. J .; Levendis, D. C. Eur. J . Inorg. Chem. 2002, 3067.
(3) See, for example, the following reviews: (a) Paul, I. C.; Curtin,
D. Y. Acc. Chem. Res. 1973, 6, 217. (b) Toda, F. Acc. Chem. Res. 1995,
28, 480. (c) Metzger, J . O. Angew. Chem., Int. Ed. 1998, 37, 2975. (d)
Toda, F.; Tanaka, K. Chem. Rev. 2000, 100, 1025. (e) Cave, G. W.;
Raston, C. L.; Scott, J . L. Chem. Commun. 2001, 2159.
(4) Patil, A. O.; Curtin, D. Y.; Paul, I. C. J . Am. Chem. Soc. 1984,
106, 348.
(5) (a) Thadhani, N. N. Prog. Mater. Sci. 1993, 37, 117. (b) Moore,
J . J .; Feng, H. J . Prog. Mater. Sci. 1995, 39, 243.
(6) Kaupp, G.; Schmeyers, J .; Boy, J . Chemosphere 2001, 43, 55.
(7) Adeyemi, O. G.; Coville, N. J . Organometallics 2003, 22, 2284.
(8) (a) Fuller, E. L.; Smyre, N. R. Appl. Spectrosc. 1990, 44, 451. (b)
Allen, T. J . Vibr. Spectrosc. 1992, 3, 217. (c) Ferraro, J . R.; Rein, A. J .
Applications to Chemical Systems. In Fourier Transform Spectroscopy;
Ferraro, J . R., Basile, L. J ., Eds.; Academic Press: Orlando, FL, 1985;
Vol. 4,
p 253. (d) Krishnan, K. In Fourier Transform Infrared
Spectroscopy, Industrial Chemical and Biochemical Applications;
Theophanides, T., Ed.; D. Reidel: Dordrecht, The Netherlands, 1984;
p 148. (e) Korte, E. H.; Ro¨seler, A. In Infrared and Raman Spectroscopy,
Methods and Applications; Schrader, B., Ed.; Wiley-VCH: Weinheim,
Germany, 1995; p 598. (f) Ryczkowski, J . Catal. Today 2001, 68, 263.
(9) Piper, T. S.; Wilkinson, G. J . Inorg. Nucl. Chem. 1956, 3, 104.
(10) Braga, D.; Maini, L.; Polito, M.; Grepioni, F. Chem. Commun.
2002, 2302.
10.1021/om0343869 CCC: $27.50 © 2004 American Chemical Society
Publication on Web 03/19/2004

Solventless (η5-C5H5)M(CO)3CH3-Phosphine Reaction
Organometallics, Vol. 23, No. 9, 2004 2049
Sch em e 1. Mech a n ism of th e Migr a tor y-In ser tion Rea ction
Figu r e 1. DRIFTS study of the reaction of (η5-C5H5)W(CO)3-
CH3 with P(p-FC6H4)3 at 60 °C (M:P 1:10 mol/mol).
F igu r e 2. Comparison of rates of conversion to product
as a function of time for the reaction of (η5-C5H5)Mo(CO)3-
CH3 with P(p-FC6H4)3 at 40 and 60 °C (M:P 1:10 mol/mol).
neous mixture was then loaded into the DRIFTS cell and an
initial IR spectrum recorded. The heating control was then
adjusted to the required temperature (40-60 °C) and the FTIR
program set to record repeated spectra (at preset time inter-
vals). Reactant peaks decreased in intensity, while simulta-
neously new peaks associated with product formation were
formed. The rate of disappearance of reactant peaks was
monitored by the disappearance of the CO absorption at ca.
Arrhenius plots based on the data in Table 1 gave the
calculated activation energies for the reaction.
Op tica l Micr oscop y Stu d y. A homemade glass heating
device with a dome for studies under an inert atmosphere,
precalibrated with crystals of known melting points, was used
for the study.11 Attached above this cell was an optical
microscope fitted with a digital camera. Crystals or fine
powders of the reactants were placed on the glass surface. The
heating device was then adjusted to heat at a certain rate to
a preset temperature, while changes in the reactants were
monitored and recorded on a PC monitor attached to the
camera. Results from this study are presented in Figure 4.
2020 cm-1
.
A typical set of data for the reaction is presented in Figure
1. As can be seen, the raw data show good behavior. Spectral
kinetic information was extracted from the raw data. Figure
2 illustrates a plot of reactant conversion with time at 40 and
60 °C (reaction of (η5-C5H5)M(CO)3CH3 with P(p-FC6H4)3; M:P
molar ratio of 1:10).
Attempts were made to monitor the reaction by spreading
of the reactants on KBr (in CH2Cl2; followed by solvent
removal). However, difficulty was experienced in monitoring
the reaction under the optical microscope, and this approach
was not pursued further.
If it is assumed that the first part of the reaction at 40 °C
and the reactions at 60 °C occur via a first-order rate law (i.e.
the first 15 min of the reaction), then the plots shown in Figure
3 are obtained.
Rate constants for 1 were calculated for a range of reaction
conditions (40-60 °C, varying reactant ratios, Mo and W), and
the data are presented in Table 1.
(11) Bogadi, R.; Levendis, D. C.; Coville, N. J . J . Am. Chem. Soc.
2002, 124, 1004.

2050 Organometallics, Vol. 23, No. 9, 2004
Bala et al.
and the rate constants for the reactions are shown in
Table 1. The following points should be noted.
(i) Reactions with Mo are faster than with W. How-
ever, rate constants are only 2-3 times greater. Activa-
tion energies (Ea) for the reactions are as follows: (η5-
C5H5)W(CO)3CH3/P(p-FC6H4)3, 50 kJ mol-1; (η5-C5H5)Mo-
(CO)3CH3/P(p-FC6H4)3, >60 kJ mol-1
.
(ii) At 1:1 and 1:10 ratios of reactants (M ) Mo, W),
the rate increases with temperature.
(iii) At 40 °C the reaction between (η5-C5H5)W(CO)3-
CH3 and P(p-FC6H4)3 revealed a decrease in reaction
rate with increasing reactant ratio.
(iv) The reaction between (η5-C5H5)Mo(CO)3Me and
PPh3 gave a rate constant of 4.34 × 10-5 s-1 (1:1 ratio,
40 °C), which is comparable to the value obtained in ex
situ studies, as reported earlier (1.81 × 10-5 s-1).6 The
difference could be due to experimental issues associated
with small temperature differences relating to the
different techniques. The data do suggest that the
presence of KBr does not significantly affect the reac-
tion.
Figu r e 3. First-order plots ofthe reaction of(η5-C5H5)M(CO)3-
CH3 with P(p-FC6H4)3 at a 1:10 M:P molar ratio.
Ta ble 1. Da ta for th e Rea ction of
(η5-C5H5)M(CO)3CH3 w ith P (p-F C6H4)3 Mon itor ed
by in Situ DRIF TS
(v) The conclusion of point iv is further suggested by
reactions performed with other diluents (NaCl and
Al2O3), in which similar rate constant variations, under
similar reaction conditions, were noted (ca. 4 × 10-5 s-1).
M
temp, °C M:P mol/mol time,a 10-4k, s-1 conversn, %b
s
Mo
40
50
60
40
50
60
40
40
40
50
60
1:1
1:1
1:1
1:10
1:10
1:10
1:1
1500
2160
1500
1620
2520
1500
1500
2520
2160
2820
2460
5.31
6.36
20.3
7.87
9.38
25.2
7.05
3.94
2.33
2.52
7.59
77
84
85
41
83
96
50
35
35
44
89
(vi) The latter result indicates that rapid in situ data
can readily be obtained by the DRIFTS technique, in
agreement with data obtained by less accurate and more
time-consuming approaches.
To rationalize the kinetic data and determine a
possible mechanism for the reaction, an optical micro-
scopy study was initiated. Remarkably, the reaction of
(η5-C5H5)W(CO)3CH3 with P(p-FC6H4)3 was found to
commence at temperatures of about 41 °C, a tempera-
ture far below that of the melting points of the reactants
((η5-C5H5)W(CO)3CH3, mp 142 °C; P(p-FC6H4)3, mp )
80 °C). DSC thermographs of reactant mixtures in the
absence of diluent do indicate a lowering of the melting
points, in line with this effect (see the Supporting
Information).
W
1:5
1:10
1:10
1:10
a
Period of reaction within which the first-order rate law was
applicable. Note that all the systems attained 99% conversion over
b
extended reaction periods. Determined by DRIFTS.
Resu lts a n d Discu ssion
The DRIFTS technique allowed the progress of the
reaction of (η5-C5H5)M(CO)3CH3 with solid phosphines
(e.g. P(p-FC6H4)3) to be monitored. Typical raw data for
the reaction of (η5-C5H5)W(CO)3CH3 with P(p-FC6H4)3
at 60 °C (a temperature well below the melting points
of the reactants) is presented in Figure 1. The reactant
can be seen to be consumed (peak at 2021 cm-1) as the
product forms (peaks at 1857 and 1629 cm-1). The peak
at 1937 cm-1 results from both product and reactant
contributions. The data for the reaction at 60 °C (1:10
ratio) has been plotted in Figure 2. At 60 °C the reaction
is well behaved and a 96% conversion occurred within
the first 25 min. If first-order kinetics are assumed, the
plot shown in Figure 3 can be obtained (k ) 25.2 × 10-4
s-1).
Figure 4 shows photographs of the reaction as a
function of time. Figure 4a shows the reactants at room
temperature prior to reaction. The reactants have been
arranged such that separated and contiguous reagents
can be seen. When the samples are heated, at just over
40 °C, melting rapidly occurs for the reactants in contact
with each other but not for the reactants separated from
each other (Figure 4b,c). Progression of the reaction
resulted in a consumption of reactants and crystalliza-
tion of material out of the melt (Figure 4d). The overall
appearance was that of a solid to solid reaction. IR and
NMR analyses confirmed the formation of the product
(η5-C5H5)W(CO)2(P(p-FC6H4)3)COCH3 in the reaction.
Similar results were obtained for the other reactions.
At 40 °C the conversion versus time plot shows that
there are at least two distinct reactions that can be
detected (Figure 2). The first part of the reaction is
observed to occur within the first 15-30 min and obeys
A mechanism for the reaction can be offered. Physical
contact between the two reactants (η5-C5H5)M(CO)3CH3
and P(p-FC6H4)3 results in formation of a eutectic
solution with a melting point lower than that of the
separate reactants.12 This melt permits intimate mixing
a first-order rate law (1:10 ratio; k ) 7.87 × 10-4 s-1
,
Figure 3, Table 1). This is followed by a deceleration in
rate (k ) 3.37 × 10-5 s-1). At 40 °C, even after 5 h, only
63% of the reactants were converted to product.
Further results were obtained from reactions in which
the temperature and mole ratio of reactants were varied,
(12) (a) Rothenberg, G.; Downie, A. P.; Raston, C. L.; Scott, J . L. J .
Am. Chem. Soc. 2001, 123, 8701. (b) Brown, W. E.; Dollimore, D.;
Galwey, A. K. In Comprehensive Chemical Kinetics; Bamford, C. H.,
Tipper, C. F. H., Eds.; Elsevier: Amsterdam, 1980; Vol. 22, p 247.

Solventless (η5-C5H5)M(CO)3CH3-Phosphine Reaction
Organometallics, Vol. 23, No. 9, 2004 2051
F igu r e 4. Interface melting in the reaction of (η5-C5H5)W(CO)3CH3 with P(p-FC6H4)3 at 40 °C: (a) room temperature; (b)
40 °C/1 min; (c) 40 °C/2 min; (d) 40 °C/10 min.
of reagents. At the reaction temperature (>40 °C) the
methyl migration/insertion occurs in the melt by the
classical mechanism that occurs in the solution phase
(as indicated by kinetic studies).
The results thus explain the data obtained when the
reaction was monitored ex situ7 and implies that these
earlier reactions also took place in the melt. The
excellent kinetic data for what appears to be a solid-
state reaction can now be rationalized.
The actual initiation of the reaction, i.e., the method
by which the reactants contact each other, is still open
to speculation. The reagents have low melting points
and high vapor pressures (all starting materials sub-
lime), and this could provide a method for additional
reactant contact at a surface.
However, a complicating issue is the precipitation of
the product from the melt. The optical microscopy study
clearly shows the crystallization of product in the low-
temperature reactions (Figure 4d). This is also indicated
by the change in kinetics with time at 40 °C (see above)
that indicates a diffusion-controlled reaction.
be explained by the lowered ability to form a melt,
resulting in the greater difficulty of the W complex to
make intimate contact with the phosphine.
Similar results were obtained for reactions carried out
with PPh3.
Finally, it is worth noting that initial results from an
ongoing study in our laboratory of CO substitution
reactions by phosphines (L ) PPh3, P(p-OMePh)3) on
Mn(CO)4(PPh3)Br also indicates reactant interface melt-
ing prior to reaction and generation of the product Mn-
(CO)3(PPh3)(L)Br.13 This implies that this type of reac-
tion is quite general, with implications for many other
so-called “solid-solid” reactions.14
Con clu sion s
The insertion/migration reaction described above
reveals that a combination of DRIFTS and optical
(13) Manzini, S. S.; Coville, N. J . Inorg. Chem. Comm., in press.
(14) (a) Toda, F.; Yagi, M.; Kiyoshige, K. J . Chem. Soc., Chem.
Commun. 1988, 958. (b) Toda, F.; Tanaka, K.; Iwata, S. J . Org. Chem.
1989, 54, 3007. (c) Toda, F.; Tatsuya, S. J . Chem. Soc., Perkin Trans.
1 1989, 209. (d) Toda, F.; Takumi, H.; Masafumi, A. J . Chem. Soc.,
Chem. Commun. 1990, 1270. (e) Goud, B. S.; Desiraju, G. R. J . Chem.
Res., Synop. 1995, 244. (f) Im, J .; Kim, J .; Kim, S.; Hahn, B.; Toda, F.
Tetrahedron Lett. 1996, 38, 451. (g) Schmeyers, J .; Toda, F.; Boy, J .;
Kaupp, G. J . Chem. Soc., Perkin Trans. 2 1998, 989. (h) Kaupp, G.;
Schmeyers, J .; Kuse, A.; Atfeh, A. Angew. Chem., Int. Ed. 1999, 38,
2896.
The effect of diffusion was also noted for the reaction
of (η5-C5H5)W(CO)3CH3 with different amounts of P(p-
FC6H4)3 at low temperature. The unexpected lowering
of reaction rate with increased ratio of P(p-FC6H4)3 can

2052 Organometallics, Vol. 23, No. 9, 2004
Bala et al.
microscopy studies can provide information on environ-
mentally benign solventless reactions. The DRIFTS data
were collected for a reaction that comprises a mixture
of KBr together with a melt of the organometallic
reactants, and the DRIFTS technique allowed for the
facile collection of kinetic data. This implies that
DRIFTS can be used to monitor reactions not only in
the solid phase but also in the liquid (melt) phase,
extending the range of the technique.
bonyls to give high yields of pure products have been
demonstrated.
Ack n ow led gm en t. We wish to thank the NRF,
THRIP, and the University of the Witwatersrand for
financial support.
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental results and a figure giving DSC thermograms. This
material is available free of charge via the Internet at
http://pubs.acs.org.
In this specific example, solventless reactions of
phosphines with transition-metal cyclopentadienyl car-
OM0343869