Ball-milling Mechanochemical Synthesis of Coordination Bonds: Discrete Units, Polymers and Porous Materials - Ball Milling Towards Green Synthesis: Applications, Projects, Challenges - page 180

dry air and with carefully dried reactants. Under such conditions, [Zn(dab-
co)Cl 2 ] n formed directly, without observable intermediates. 66 That the sen-
sitivity to moisture is largely related to dabco is supported by the observation
that neat grinding of anhydrous ZnCl 2 and bipy to form of a similar zigzag
1D polymer proceeds in one step.
Whereas sample preparation for X-ray powder diffraction analysis often
requires minutes, Raman spectroscopy is much more suited for rapid an-
alysis of reaction mechanisms, as samples can be characterized within sec-
onds without any particular preparation procedure. As a faster alternative to
X-ray powder diffraction, the James group has applied Raman spectroscopy
to monitor the mechanochemical LAG reaction of zinc oxide and imidazole
in the presence of DMF to form the microporous framework known as
ZIF-6. 100 This kinetic study revealed that product formation proceeds in a
manner that highly resembles the kinetics of a second-order reaction in so-
lution. These observations led the authors to propose a ''pseudo-fluid'' model
of the reaction mixture under mechanochemical conditions, in which the
reaction rate for this particular mechanochemical process is controlled largely
by the number of reactive encounters involving surfaces of reactant imidazole
and zinc oxide particles, and in which the role of milling is principally to stir
the reaction mixture and expose fresh reactant surfaces by abrasion. 100 One of
the most surprising aspects of such a pseudo-fluid reaction model, which was
experimentally confirmed by conducting the reaction at different tempera-
tures, is that the reaction rate is highly dependent only on the milling fre-
quency (Figure 7.13d), but not temperature. That mechanochemical milling
reactions by LAG and ILAG can indeed proceed in a manner that resembles
established first- and second-order rate laws in solution was subsequently
confirmed by the Halasz and Fri ˇˇ i ´ groups by quantitative in situ and real
time reaction monitoring using synchrotron X-ray diffraction. 101
An impressive feat of rapid stepwise analysis of mechanochemical re-
actions has very recently been reported by Tr ¨ bs and Emmerling, who con-
ducted X-ray powder diffraction, Raman spectroscopy and even rapid
electron microscopy measurements on the three-component reaction of
bismuth nitrate, terephthalic acid and imidazole. 102 This reaction, which
yields a 3D MOF material, has successfully been monitored with a time
resolution as low as 10 s.
7.8.2 In Situ and Real-time Studies
One of the major drawbacks of mechanochemical milling and grinding chem-
istry is the inability to carry out real-time observation of the transformation of
reactants into products. As pointed out by Drebuschak and co-workers: 103
''It is a challenge to understand the processes taking place in a powder
sample during its grinding in a mill, or compacting, since one can neither
measure local temperature, pressure, shear stresses, nor follow the
changes in the diffraction patterns or vibrational spectra in situ.''