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was not fully dissolved forming an inhomogeneous mixture,
irradiation of the system with 365 nm light for 20 min led to
a clear solution (Fig. 5a and b). Irradiation of the solution with
440 nm light led to the re-formation of the cloudy mixture
(Fig. 5c).
Further UV-Vis measurements regarding the solubility of the
isomers were conducted. Increasing the concentration of trans-
5-Pd from 10ꢁ5 M up to 3 ꢂ 10ꢁ4 M in DMF/H2O 1 : 1, a cloudy
mixture was formed showing a UV-Vis spectrum (Fig. 6) similar
to that of trans-5-Pd at a lower concentration (Fig. 3). However, it
was characterized by lower intensity absorptions and shied
baseline, indicating the inhomogeneity of the sample. Upon
irradiation of the suspension, the catalyst was gradually dis-
solved and aer about 20 min a spectrum corresponding to that
of cis-5-Pd was obtained.
It is noted, that upon irradiation no apparent Pd black
formation was observed, indicating the photostability of 5-Pd.
The photostability of the complex was also conrmed by 1H
NMR spectroscopy (Fig. 7 and S1, ESI†). Upon irradiation of
a sample of trans-5-Pd for 2 hours, a clean mixture of trans-5-Pd
and cis-5-Pd was formed in a ratio of 1 : 1.7 without any sign of
degradation.
Fig. 8 Monitoring the Suzuki coupling reaction of (a) 4-iodobenzo-
nitrile and (b) 4-iodoanisole with phenylboronic acid catalyzed by
trans- (black squares – 1.5 mol%) and cis-5-Pd (red circles – 1.5 mol%,
blue triangles – 0.6 mol%). Reaction conditions: phenylboronic acid
(0.1 mmol), aryl halide (0.1 mmol), K2CO3 (0.3 mmol), 5-Pd, DMF/H2O
1 : 1 (3 mL), lmax ¼ 365 nm, P ¼ 10 W, rt.
2.2 Evaluation of catalytic activity
Trans-5-Pd (1.5 mol%) showed low activity in the Suzuki
coupling reaction of phenylboronic acid and 4-iodobenzonitrile
(1 : 1) that is consistent with its moderate solubility in the
reaction medium. Aer 6 hours in the darkness, only about 20%
conversion into 4-cyanobiphenyl was observed (Fig. 8a). In all
the reactions, the cis-form of the catalyst was generated by the
irradiation (lmax ¼ 365 nm, P ¼ 10 W) of a solution of parent 5-
Pd for 30 min at room temperature prior to catalysis. Following
catalyst addition, the reaction mixture was continuously irra-
diated to maintain the concentration of the cis-form high.
Gratifyingly, when the soluble cis-rich 5-Pd (1.5 mol%) was used
as a catalyst, a remarkable increase in the activity was observed.
In 2 hours, 50% conversion was obtained as compared to 6% by
trans-5-Pd (Fig. 8a). Furthermore, the catalyst loading could be
decreased to 0.6 mol% without any loss of activity.
Time-course studies of the reaction catalyzed by cis-5-Pd up
to 6 hours revealed that a steep increase in conversion in the
rst 2 h was followed by a plateau when 1.5 mol% catalyst was
used, or a slight further increase up to 70% at a catalyst loading
of 0.6 mol% (Fig. 8a). Along with the proceeding of the cross-
coupling, the reaction mixture became increasingly dark, indi-
cating gradual loss of metal. Probably, at higher catalyst
concentrations, enhanced Pd aggregation led to loss of activity,
while at low catalyst loading slower aggregation preserved
activity. No such darkening of the reaction mixture was
observed in the case of the less active trans-5-Pd catalyst. Based
on the observed photostability of the complex, the metal loss
likely occurs in the catalytic cycle, upon contact with the
substrate molecules. Nevertheless, it is clear that the increased
catalytic activity of cis-5-Pd is strongly dependent on its light-
induced dissolution during the reaction allowing a more effi-
cient reaction to take place. Employing 4-iodoanisole in the
coupling, similar activity trends of the isomers were found
(Fig. 8b).
To exclude a potential photoredox mechanism in the reac-
tion, cis-5-Pd was generated in DMF/H2O (1 : 1), then the clear
catalyst solution was added to the reactants and the mixture was
stirred at room temperature in the darkness. Samples were
taken aer 1 h and 2 h reaction times. Aer 1 h, 11%, while aer
2 h 18%, conversion was obtained. These values are signi-
cantly higher than those obtained with trans-5-Pd, while much
lower than with continuously irradiated reaction mixtures. The
lower conversion can be explained with the favoured thermal
cis-to-trans isomerization of the complex leading to catalyst
precipitation and hence to the loss of activity.
In an attempt to increase the yield of the Suzuki coupling
reaction by reducing the metal loss, the concentration of the
starting materials was decreased by 50% (0.05 mmol/3 mL)
compared to what was used initially (0.1 mmol/3 mL), while the
concentration of the catalyst was kept at 0.6 mol%. This dilution
resulted in slower product formation (1 h: 7%; 2 h: 24%)
compared to the original conditions (1 h: 31%; 2 h: 53%), while
the metal loss was visibly decreased.
Since no considerable Pd loss was observed at lower reactant
concentrations, we explored the possibility of increasing the
reaction rate by increasing the intensity of the light source from
10 W to 30 W, while keeping the conditions otherwise identical.
Gratifyingly, increased light intensity promoted the conversion
23424 | RSC Adv., 2021, 11, 23419–23429
© 2021 The Author(s). Published by the Royal Society of Chemistry