A mechanism for the 'solid-solid' reaction between Mn(CO) 4(PPh3)Br and solid phosphines

Article abstract of DOI:10.1016/j.inoche.2004.03.014

The 'solid-solid' reaction between Mn(CO)₄(PPh₃)Br and solid phosphines at temperatures lower than the reactant melting points is shown to occur in the melt.

Full text of DOI:10.1016/j.inoche.2004.03.014

Inorganic Chemistry Communications 7 (2004) 676–678
www.elsevier.com/locate/inoche
A mechanism for the ‘solid–solid’ reaction
between Mn(CO)4(PPh3)Br and solid phosphines
*
*
S. Stanley Manzini , Neil J. Coville
Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Johannesburg 2050, South Africa
Received 18 December 2003; accepted 6 March 2004
Available online 9 April 2004
Abstract
The ‘solid–solid’ reaction between Mn(CO)4(PPh3)Br and solid phosphines at temperatures lower than the reactant melting
points is shown to occur in the melt.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Solid-state reactions; Melt reactions; Manganese; CO substitution
Novel approaches to waste disposal and atom econ-
omy are being incorporated into synthetic chemistry
[1,2]. The simplest approach to these issues is to perform
reactions in the absence of solvents [3,4]. While in
principle this appears a trivial solution to an environ-
mental problem, complicating factors relating to diffu-
sion, heat and mass transfer, etc., have to be addressed.
These factors may yield different reaction mechanisms
and either aid or reduce the activity/selectivity of the
process. Little information is available on the competing
roles of the chemical reaction and diffusion in the melt.
Much work thus needs to be performed to evaluate this
synthetic strategy.
Our approach has been to evaluate solventless reac-
tions via a study of well-established organometallic
chemistry reactions. Others have investigated solventless
chemical reactions by studying organic reactions [3–7].
Recently, we reported that the reaction between
CpM(CO)3Me (M ¼ Mo, W) and PPh3 occurred in the
absence of solvents to yield the CO insertion/Me mi-
gration product CpM(CO)2(PPh3)(COMe) [8]. This
study revealed that the reaction was well behaved and at
low temperature appeared to occur in the solid state.
We now wish to report on an extension of our studies
to an investigation of the solventless CO substitution
reaction of Mn(CO)4(PPh3)Br by phosphines [9]. The
reaction between Mn(CO)4(PPh3)Br and PPh3 was
monitored as a function of reactant ratio (1:1 to 1:10)
and temperature in the absence of solvents. Initially, a
DSC profile of the mixture was recorded (10 °C minꢀ1
)
and revealed only the mp of PPh3 (78 °C) in the region
25–100 °C, suggesting no influence of reagents on each
other. Reactants were mixed together without grinding,
placed in NMR tubes (under argon) and heated in an oil
bath (35–70 °C; below the mp of the individual reac-
tants) at a pre-set temperature. After intervals of time
the NMR tubes were removed from the oil bath, cooled,
and IR spectra recorded (in CHCl3). Calibration curves
permitted the degree of reaction to be determined.
Typical reaction plots were obtained which fitted first
order kinetics for the reaction. Visual inspection of the
reaction revealed solid at all phases during the reaction
and, together with the DSC and kinetic data, suggested
a well-behaved solid state substitution reaction. Foil
wrapped reaction tubes, reactions in air and varying
reactant ratios did not modify the kinetic results.
* Corresponding authors. Tel.: +27-72-192-3141; fax: +27-71-76749
(S.S. Manzini), Tel.: +27-11-717-6738; fax: +27-11-717-6749 (N.J.
Coville).
To further understand the reaction, it was decided to
monitor the reaction under an optical microscope. Re-
actants were mixed together as previously and crystals
E-mail addresses: ngwaneza@yahoo.co.uk (S.S. Manzini),
ncoville@aurum.chem.wits.ac.za (N.J. Coville).
1387-7003/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2004.03.014

S.S. Manzini, N.J. Coville / Inorganic Chemistry Communications 7 (2004) 676–678
677
of the mixture placed on a home built glass heating
device, calibrated with crystals of known melting point.
The device was placed in a dome that was flushed with
inert gas, to ensure that the reactions were performed in
an inert atmosphere.
P(p-OMePh)3, again at the interface between the
touching crystals. If the reactants did not touch,
no melting or reaction occurred. The product
Mn(CO)3(PPh3)[P(p-OMePh)3]Br was character-
ised by IR and NMR spectroscopy.
To our surprise the reaction between Mn(CO)4-
(PPh3)Br and PPh3 was found to commence at 39 °C, a
temperature far below that of either of the melting/de-
composition points of the reactants (78 °C, PPh3; 131–
132 °C, Mn(CO)4(PPh3)Br). An optical microscopy
study was then initiated to fully explore this finding and
our further investigations revealed the following (see
Fig. 1):
(i) Reaction between Mn(CO)4(PPh3)Br and PPh3
occurred at the interface where the two crystalline
materials touched (Fig. 1(a)).
(ii) At the interface the PPh3 melted and reacted with
‘solid’ Mn(CO)4(PPh3)Br (Fig. 1(b)).
(iii) Crystals of both reactants that did not touch, nei-
ther melted nor reacted at the reaction tempera-
tures used, i.e., 35–65 °C (Fig. 1(b)).
(iv) The reaction is thus occurring at about 40 °C be-
low the mp of the reactants; a reaction not de-
tected by DSC data.
(v) As reaction progressed the consumption of PPh3
led to crystallisation, giving the appearance of a
solid–solid reaction (Fig. 1(c)).
(vi) IR analysis of the material at the end of the reac-
tion confirmed the formation of trans-
Mn(CO)3(PPh3)2Br.
(viii) Thus, melting of either reactant will initiate the re-
action.
The above results indicate that the reaction between
Mn(CO)4(PPh3)Br and PPh3, in the absence of solvent,
provides substantive data on the mechanism of the
‘solid–solid’ CO substitution reaction.
When the reactants contact each other at room tem-
perature, no reaction occurs. As the temperature is
raised, reaction of interface molecules takes place.
Normally, this reaction would proceed to completion.
However, there is a complication – the product has a
melting point higher than that of the staring materials.
As it forms it crystallises from the melt and thus blocks
further reaction. This is an important process as it can
limit the reaction even in a well mixed mixture. Further,
reactants that do not touch neither react nor melt and
thus secondary processes are required to lead to their
reaction. A secondary pathway is provided by reactant
diffusion (e.g., PPh3 has a high vapour pressure and
readily sublimes) that permits further reaction. Indeed,
we have observed a fall of rate but continued reaction
with time towards the end of a reaction. Thus, the ki-
netics measured are reflective of kinetic reaction control
in the melt. The measurement of the initial initiation rate
will require techniques with more rapid timescales for
analysis.
(vii) Reaction between Mn(CO)4(PPh3)Br and P(p-
OMePh)3 (mp ¼ 150 °C) was monitored by optical
microscopy at 65 °C and indicated that the
Mn(CO)4(PPh3)Br melted and reacted with ‘solid’
A remarkable feature of the study is the observation
in the DSC of a melting point for reactants in the 1:1
reaction mixture. This finding partly relates to the
Fig. 1. Reaction between Mn(CO)4(PPh3)Br and PPh3 (1:1 ratio, 42 °C) as a function of reaction time: (a) Mn(CO)4(PPh3)Br and PPh3 are seen
separately on the RHS of the picture and are seen to touch on the LHS; (b)–(d) reaction of the touching materials can be seen (see arrows), while the
separated reactants show no reaction.

678
S.S. Manzini, N.J. Coville / Inorganic Chemistry Communications 7 (2004) 676–678
heating rate used (10 °C minꢀ1) but even with a 1:10
mixture, or a slower heating rate (1 °C minꢀ1), similar
DSC traces are observed. Clearly, DSC data must in
future be interpreted circumspectly.
Acknowledgements
We thank The NRF, THRIP, the University of the
Witwatersrand and Sasol for financial support.
The implications from the reaction are that the re-
action is complex. Initial reaction at the reactant–reac-
tant interface could involve a solid–solid reaction. This
is then followed by a reaction in the melt and finally
reaction will be influenced by reactant diffusion.
A preliminary optical microscopy study of the
CpM(CO)3Me (M ¼ Mo, W) and PPh3 reaction [9] also
indicates reactant interface melting prior to reaction to
give CpM(CO)2(PPh3)(COMe). This suggests that the
reaction mechanism described above represents a gen-
eral reaction type and may be applicable to many other
‘solid-state’ organometallic reactions [10]. Indeed, our
data are consistent with the proposal [5,6] that many
reactions previously reported to be solid–solid reac-
tions actually occur in the melt, prior to product
crystallisation.
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