A.C. Matsheku et al.
Polyhedron 205 (2021) 115280
appeared at similar chemical shifts as in the ligand L1. However, a slight
shift of the Me carbon atom was observed in the carbon NMR spectrum
of C1, moving from δ 11.4 ppm in L1 to δ 12.7 ppm in C1. Also, the much
The bond distances and bond angles around the palladium centre in
C3 are very similar to those reported in the literature for related palla-
dium(II) complexes. For instance, the bond lengths Pd(1)-Cl(1) {2.2803
(12) Å} and Pd(1)-N(1) {2.027(4) Å} are in close agreement to Pd(1)-Cl
more intense and broader signal at δ 27.1 ppm and was assigned to C
and C , which implies that these carbon atoms exist in the same chem-
a
b
2
(1) {2.2898(8) Å} and Pd(1)-N(1) {2.032(3) Å} found in [PdCl (N^N)]
ical environment upon coordination with the palladium(II) centre in C1.
Furthermore, two imine signals were seen at δ 173.6 and 172.2 ppm,
(where N^N = 2,6-diMePh-NH-CH
2
-CH
5
H
5
N) [31], Pd(1)-Cl(1) {2.3150
′
(1) Å} and Pd(1)-N(1) {2.074(4) Å} reported for 1,1 -bis(2-pyridyl)
ferrocenepalladium(II) [32], Pd(1)-Cl(1) {2.2970(8) Å} and Pd(1)-N(1)
{2.030(2) Å} found in a 2-(2-pyridyl)imidazolepalladium(II) complex
assigned to C
e
, and the second signal may be due to the formation of cis
–
–
and trans geometric isomers around the C
N double bond. The HR-ESI
◦
mass spectrometry results are also consistent with the formation of C1,
[33]. The bond angles N(2)-Pd(1)-Cl(1) {173.67(10) } and N(2)-Pd(1)-
+
◦
showing [Mꢀ Cl] (Fig. SI-4c).
N(1) {79.98(14) } in C3 are close to those of N(2)-Pd(1)-Cl(1) {173.8
1
◦
The H NMR spectra of complexes C2 and C3 displayed a shift in the
(1) } found in [(8-PQ)Pd(Me)Cl] [34] and N(2)-Pd(1)-N(1) {83.42(11)
◦
imine (N = CH) proton signal upon complexation. For example, this
proton was seen at δ 9.18 ppm in L2 and after complexation, this peak
was seen downfield at δ 8.77 ppm, while in L3 the N = CH proton was
observed at δ 8.06 ppm and experience shielding to δ 7.87 ppm after
complexation. This observation is similar to reports in the literature
after complexation of metals such as Ru, Os, Ir and Rh to Schiff base
ligands, and is said to support that coordination has taken place at the
imine nitrogen atom [28]. In addition, there is a characteristic downfield
shift in the signal of the proton adjacent to the pyridyl nitrogen atom in
the spectrum of C3. The 1 C NMR of C3 also revealed all the charac-
teristic carbon signals (Fig. SI-5). The 31P{ H} NMR spectrum of C2
provides further evidence that the palladium coordinated successfully in
a P^N bidentate manner. In doing so, the palladium centre accepted
electrons from the lone pairs of electrons on the phosphorus atom, thus
de-shielding it, and its signal appeared at δ 27.2 ppm (from δ ꢀ 14.9 ppm,
in L2). This agrees with data reported by Mogorosi and co-workers for
P^N bidentate palladium(II) complexes [29].
} reported for [PdCl
[31].
2
(N^N)] (N^N = 2,6-diMePh-NH-CH
2 5 5
-CH H N)
4. Catalytic studies
4.1. Mizoroki-Heck cross-coupling reactions
The adamantylimine palladium(II) complexes (C1-C3) were
screened as pre-catalysts for Mizoroki-Heck cross-coupling reactions.
Optimum reaction conditions were obtained using iodobenzene (0.66
mmol) and styrene (0.726 mmol) as standard substrate models (Equa-
tion 1). These substrate models were used to establish the best solvent,
temperature, base, reaction time and catalyst loading. This was achieved
by varying these parameters, whilst monitoring the percentage con-
version. The pre-catalysts C1-C3 were used with these optimizations.
3
1
◦
C1 performed well at 110 C, giving 98% conversions to cis- and
trans-stilbene, whereas C2 and C3 reached ≈ 98% and 100% conversions
◦
◦
Infrared spectroscopy was further used to confirm the coordination
of the palladium centre to the imine and pyridyl imine groups of L2- L3
to form complexes C2-C3 (Table 1). In general, the infrared spectra of
respectively at 130 C (Chart SI-1). Above 110 C, C1 gave a marginal
decrease in conversions, however, C1 still performed better than C2 and
C3. The marginal decrease is possibly due to a mixture of the geometric
isomers (cis-C1 and trans-C1) and or decomposition of the catalyst to
palladium(0) species, which are also active towards the cross-coupling
reaction, though evidently not as active as the molecular catalysts. C2
and C3 were stable at high temperatures and were activated to
–
complexes C2 and C3 showed the (C
–
N)imine stretching frequency band
at higher wavenumbers when compared to the ligands, while the ab-
–
N)pyridyl stretching frequency expe-
riences a shift to the lower wavenumber of 1558 cm for C3 from 1583
sorption band assigned to the (C
–
ꢀ 1
ꢀ
1
◦
cm for L3. The former is as a result of the pyridylimine functional
demonstrate good activity above 120 C. These temperature optimiza-
groups and the coordinated palladium atom withdrawing electrons from
tion reactions were conducted in three different solvents, namely N,N-
dimethylformamide (DMF), toluene and 1,4-dioxane. DMF was found to
be the best solvent and was used throughout the catalytic studies.
Organic and inorganic bases such as pyridine, sodium methoxide
(MeONa) and potassium carbonate (K CO ) were employed, but showed
–
the (C
was a shift in the (N
to the lower wavenumbers of 3120 and 3186 cm in the bis(iminohy-
–
N)HAr bond thereby causing the shift [30]. Furthermore, there
ꢀ 1
–
H) stretching frequency band at 3366 cm in L1
ꢀ 1
–
drazone)palladium(II) complex C1. The (C N)Me stretching frequency
–
2
3
bands in these complexes are slightly affected by coordination of
no conversion (Chart SI-2). Triethylamine (Et N) and potassium hy-
3
palladium atom via the amino nitrogen atoms, shifting minimally.
droxide (KOH) gave good activity of up to 100% conversions. However,
KOH is highly hygroscopic and this made it difficult to analyse the
product(s), whereas with triethylamine, the selectivity was almost
exclusively to (E)-stilbenes (up to 99%). The conversion increased by
increasing the catalyst loading and the best conversion of 98% (C1) was
obtained at 0.5 mol % in 3 h.
3
.3. The single crystal X-ray molecular structure of C3
Yellow crystals for the adamantyldiimine palladium(II) complex C3
were grown in DCM using the slow evaporation technique and the
molecular structure is shown in Fig. 4, with residual DCM solvent. C3
formed yellow crystals that crystalize in the triclinic space group P-1 and
the geometry around the palladium centre is a square planar. The N-Pd-
4
.1.1. Mercury poisoning tests
In homogenous catalysis, the molecular catalyst can form heteroge-
neous metal powders or nanoparticles which end up catalysing the re-
actions, instead of the homogenous catalysts [35]. Notably, during the
optimization reactions, there was no decomposition of the palladium
complexes to black colloidal particles, as has been reported for most
palladium(II) complexes used in cross-coupling reactions [30,36]. This
observation suggested that our ligands were successful in stabilizing the
palladium(0) active species in the molecular form, and no decomposi-
tion to form palladium black was seen. To validate this claim, the
mercury drop test was performed, where in typical reactions metallic
mercury was added (Table 2). As such, in separate reaction tubes, the
◦
Cl bond angle ranges from 94 to 175 . The crystallographic data of C3 is
tabulated in SI-Table 1.
Table 1
Infrared spectral data for the ligands L1-L3 and the complexes C1-C3.
ꢀ
1
Compound
Absorption bands (cm
)
ѵ(N–H)
ѵ(C
–
N)imine(Me)
–
ѵ(C
N)imine
–
ѵ(C N)pyridylimine
L1
L2
L3
C1
C2
C3
3366
1598
1583
1614
1580
1586
1602
1550
1565
0
1583
1558
reagents and catalyst precursors (C1-C3) were added together with Hg .
0
3120, 3186
The presence of Hg around metal nanoparticles forms amalgams which
1555
1577
then retard the nanoparticle’s ability to catalyse reactions. This tool is
used to distinguish true homogeneity of molecular catalysts from
8