Synthesis, Characterization, and Catalytic Application of Mononuclear and Dendritic Cationic CuIIminopyridine-Ligated Complexes in Aryl Iodide Hydroxylation

Article abstract of DOI:10.1002/ejic.201600417

A series of mononuclear (C1–C4) and dendritic G1 (DC1–DC4) and G2 (DC5–DC8) cationic Cu^Iiminopyridine complexes of the general formula [Cu{(R-C₅H₃N)CH=N(nPr)-κ²-N,N}₂][BF₄] (C1: R = 6-Me; C2: R = H; C3: R = 6-Br; C4: R = C₄H₃) and [DAB-G_x-PPI-(Cu{(R-C₅H₃N)-κ²-N,N}_y)_z][BF₄]_n[DAB: 1,4-diaminobutane; PPI: poly(propyleneimine); for G1: x = 1, y = 4, z = 2, n = 2; for G2: x = 2, y = 8, z = 4, n = 4; DC1/DC5: R = 6-Me; DC2/DC6: R = H; DC3/DC7: R = 6-Br; DC4/DC8: R = C₄H₃] have been prepared and characterized by a range of spectroscopic and analytical techniques. The mononuclear and dendritic complexes were found to be active catalysts for the hydroxylation of 4-iodotoluene to p-cresol in DMSO/H₂O mixtures. A positive dendritic effect on catalytic activity was observed. Furthermore, our catalyst system was found to be active for the hydroxylation of 4-iodotoluene in neat water. The active catalyst could be recycled twice before catalyst deactivation was observed. HRTEM analysis revealed that catalyst deactivation arose as a result of metal agglomeration. A series of poisoning experiments provided evidence for the mediation of hydroxylation by a homogeneous active species for both classes of pre-catalysts, and a radical-trapping experiment in combination with our experimental observations provided evidence that the reaction proceeds through a similar mechanism to that reported for Cu-catalyzed halide exchange.

Full text of DOI:10.1002/ejic.201600417

DOI: 10.1002/ejic.201600417
Full Paper
Dendritic Catalysts
Synthesis, Characterization, and Catalytic Application of
Mononuclear and Dendritic Cationic CuI Iminopyridine-Ligated
Complexes in Aryl Iodide Hydroxylation
Nomvano Mketo,[a] Johan H. L. Jordaan,[b] Anine Jordaan,[c] Andrew J. Swarts*[d] and
Selwyn F. Mapolie*[a]
Abstract: A series of mononuclear (C1–C4) and dendritic G1 A positive dendritic effect on catalytic activity was observed.
(DC1–DC4) and G2 (DC5–DC8) cationic CuI iminopyridine com- Furthermore, our catalyst system was found to be active for the
plexes of the general formula [Cu{(R-C5H3N)CH=N(nPr)-κ2- hydroxylation of 4-iodotoluene in neat water. The active catalyst
N,N}2][BF4] (C1: R = 6-Me; C2: R = H; C3: R = 6-Br; C4: R = could be recycled twice before catalyst deactivation was ob-
C4H3) and [DAB-Gx-PPI-(Cu{(R-C5H3N)-κ2-N,N}y)z][BF4]n [DAB: 1,4- served. HRTEM analysis revealed that catalyst deactivation arose
diaminobutane; PPI: poly(propyleneimine); for G1: x = 1, y = 4, as a result of metal agglomeration. A series of poisoning experi-
z = 2, n = 2; for G2: x = 2, y = 8, z = 4, n = 4; DC1/DC5: R = 6- ments provided evidence for the mediation of hydroxylation by
Me; DC2/DC6: R = H; DC3/DC7: R = 6-Br; DC4/DC8: R = C4H3] a homogeneous active species for both classes of pre-catalysts,
have been prepared and characterized by a range of spectro- and a radical-trapping experiment in combination with our ex-
scopic and analytical techniques. The mononuclear and den- perimental observations provided evidence that the reaction
dritic complexes were found to be active catalysts for the hy- proceeds through a similar mechanism to that reported for Cu-
droxylation of 4-iodotoluene to p-cresol in DMSO/H2O mixtures. catalyzed halide exchange.
Introduction
efficient Pd/phosphine catalyst systems being reported.[3] De-
spite their catalytic efficiency, these systems suffer from high
cost, air- and moisture-sensitivity, toxicity issues as well as se-
lectivity problems associated with the formation of C–O cou-
pling byproducts. These considerations have led to the devel-
opment of homogeneous and heterogeneous copper-based
catalyst systems that display comparable activity to their noble
metal counterparts.[4] In addition, a number of examples display
enhanced selectivity and stability without the need to operate
under anaerobic moisture-free conditions, with many of the sys-
tems displaying recyclability.[4a,4b]
Dendrimers are hyperbranched macromolecules consisting
of a core with repeating branch units extending radially out-
ward, the number of which determines the dendrimer genera-
tion.[5] The incorporation of metals within the framework of
dendrimers has led to the development of the so-called metal-
lodendrimers, which have found widespread application as cat-
alysts, sensors, drug-delivery agents, and as imaging and radio-
pharmaceutical agents.[6] In particular, the tunable size of met-
allodendrimers has been exploited in catalysis, because nanom-
eter-sized metallodendrimers allow for the separation and re-
covery of the catalyst post-reaction. In recent years our research
group has focused on the development of peripherally modi-
fied metallodendrimers for a variety of applications, including
catalysis.[7] Herein we report the synthesis and characterization
of mononuclear and dendritic (G1 and G2) cationic CuI com-
plexes bearing iminopyridyl ligands and their application as cat-
alysts in the hydroxylation of 4-iodotoluene to p-cresol, an im-
portant feedstock in the preparation of antioxidants.[8] We also
Phenolic compounds are important synthons for the prepara-
tion of pharmaceuticals, polymeric materials, and natural prod-
ucts.[1] Industrially, the global demand for phenol is met by the
Hock process, which proceeds by the peroxidation of cumene,
itself obtained by benzene propylation. The drawbacks of the
process are low product yields combined with the formation
of large amounts of byproducts and low energy efficiency. To
circumvent these drawbacks a number of catalyst systems have
been reported that promote phenol synthesis by the hydroxyl-
ation of substituted and unsubstituted aryl halides. The initial
report by Buchwald and co-workers employed a Pd/phosphine
catalyst system.[2] This seminal work has led to a number of
[a] Stellenbosch University, Department of Chemistry and Polymer Science,
Private Bag X1, Matieland, 7602 Stellenbosch, South Africa
E-mail: smapolie@sun.ac.za
www.sun.ac.za
[b] Focus Area for Chemical Resource Beneficiation: Laboratory for Analytical
Services, North-West University,
Hoffman Street, Potchefstroom 2520, South Africa
[c] Focus Area for Chemical Resource Beneficiation: Laboratory for Electron
Microscopy, North-West University,
Hoffman Street, Potchefstroom 2520, South Africa
[d] Focus Area for Chemical Resource Beneficiation: Catalysis and Synthesis
Research Group, North-West University,
Hoffman Street, Potchefstroom 2520, South Africa
E-mail: Andrew.Swarts@nwu.ac.za
www.nwu.ac.za
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejic.201600417.
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demonstrate their catalytic efficiency and recyclability in neat 1:4 ratio (Scheme 2).[10] The complexes were isolated as micro-
water and provide evidence for the presence of a homogene- crystalline (C1–C4) or amorphous solids (DC1–DC8) in good-to-
ous active species during catalysis.
excellent yields (78–94 %).
All the mononuclear and dendritic complexes were stable at
room temperature in the solid state, but some decomposition
was observed in solution after prolonged periods of time. The
cationic CuI complexes display solubility in solvents such as di-
chloromethane, acetonitrile, and dimethyl sulfoxide, whereas
they are insoluble in alkanes, alcohols, and ethers. Both classes
of complexes were characterized by a range of spectroscopic
and analytical techniques. In the FTIR spectra of the complexes,
the imine absorption band shows a shift to lower wavenumbers
in the range 1624–1600 cm–1. In addition, a strong absorption
band is observed in the range 1029–1051 cm–1, which was as-
Results and Discussion
Synthesis and Characterization
Monofunctional (L1–L4) and multifunctional (DL1–DL8) N-pro-
pyliminopyridine ligands were prepared by Schiff-base conden-
sation of various pyridine- and quinolinecarbaldehydes with
propylamine and first- (G1) and second-generation (G2) DAB-
PPI [DAB: 1,4-diaminobutane; PPI: poly(propyleneimine);
Scheme 1a,b]. Ligands L2, DL2, and DL6 have been reported
previously,[9] whereas all the other ligands are novel. The li-
gands were isolated as colorless or orange-yellow oils and sol-
ids that display solubility in most organic solvents with the ex-
ception of alkanes. The ligands were characterized by a range
of spectroscopic and analytical techniques. In brief, FTIR spec-
troscopy showed the appearance of the imine absorption band
in the range 1647–1650 cm–1.
1
signed to B–F stretching vibrations.[11] The H and 13C{1H} NMR
spectra show the imine resonance as a singlet in the range
δ = 8.53–9.19 and 161.21–162.32 ppm, respectively, downfield-
shifted in comparison with the free ligand. Both analytical tech-
niques confirmed the coordination of the metal to the li-
gand.[10,12]
For the mononuclear complexes (C1–C4), ESI-MS (positive
1H NMR spectroscopic analysis confirmed imine formation, mode) analysis showed parent ions that correspond to the pro-
observed as a singlet resonance in the range δ = 8.29– ton adduct of the cationic portion of the ion pair. In addition,
8.55 ppm. The mononuclear (C1–C4) and dendritic (DC1–DC8) mass fragments arising from the sequential loss of the imino-
cationic CuI N-propyliminopyridine-ligated complexes were pre- pyridyl ligand are observed in the spectra. For the dendritic
pared by treating the mono- or multifunctional ligands with complexes (DC1–DC8), the situation was a bit more complex.
tetrakis(acetonitrile)copper(I) tetrafluoroborate in a 2:1, 1:2, or In the spectra of the G1 and G2 metallodendrimers (DC1–DC8),
Scheme 1. Synthesis of mono- and multifunctional iminopyridyl ligands.
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Table 1. Crystallographic data pertaining to complex C2.
C2
Empirical formula
Temperature [K]
C18H24BCuF4N4
100(2)
Crystal system
Space group
a [Å]
b [Å]
c [Å]
monoclinic
P21/n
10.2859(8)
14.0850(11)
13.8959(11)
90.00
α [°]
ꢀ [°]
93.1460(10)
90.00
2010.2(3)
4
γ [°]
V [Å3]
Z
F(000)
920
1.476
1.133
Dcalcd. [g cm–3
]
μ [mm–1
]
Reflections [Fo > 4(Fo)]
Parameters
G.O.F.
4184
255
1.064
R1 (I > 2δ)
wR2
0.0315
0.0774
Table 2. Selected bond lengths and angles for complex C2.
Bond lengths [Å]
Bond angles [°]
Cu1–N1
Cu1–N1′
Cu1–N2
Cu1–N2′
2.080(1)
2.062(1)
1.985(1)
2.002(1)
N1–Cu1–N1′
N1–Cu1–N2
N1–Cu1–N2′
N1′–Cu1–N2
N1′–Cu1–N2′
N2–Cu1–N2′
118.09(5)
81.54(5)
114.30(5)
126.35(5)
81.37(5)
Scheme 2. Synthesis of mononuclear and multinuclear cationic CuI imino-
pyridine complexes.
138.71(5)
118.09 and 114.30°, respectively. The Cu1–N1 and Cu1–N2 bond
lengths of 2.080(1) and 1.985(1) Å, respectively, are within the
reported range for complexes of the type [L2Cu]+X–.[10,15]
Houser and co-workers have developed a simple geometric in-
both the parent ion (see Figure S1 in the Supporting Informa-
tion) as well as additional mass fragments due to the formation
of both doubly, [M – (BF4)2]2+, and triply charged, [M – (BF4)4]3+
species are observed (see Figure S2).
,
The UV/Vis absorption spectra of the complexes C1–C4 and
DC1–DC8 were recorded in acetonitrile solutions (10–5
M
). For
both classes of compounds, high-energy bands are observed in
the range 235–255 and 278–311 nm, which correspond to li-
gand-centered π→π* and n→π* transitions, respectively. Visi-
ble bands are observed in the range 470–530 nm, which are
characteristic of metal-to-ligand charge transfer (MLCT) proc-
esses (see Figure S3 in the Supporting Information). These spec-
tral features are characteristic of CuI complexes bearing N-N
chelating ligands.[13] Finally, elemental analysis confirmed the
bulk purity of the mononuclear and dendritic complexes. Espe-
cially in the case of the dendritic complexes, we observed the
inclusion of trace dichloromethane solvent molecules. This sol-
vent inclusion behavior has previously been observed by
us[9c,9d] and others.[14]
Single crystals of complex C2 suitable for X-ray crystallo-
graphic analysis were grown by vapor diffusion of diethyl ether
into a concentrated dichloromethane solution of the complex.
Selected crystallographic data and metric parameters are tabu-
lated in Table 1 and Table 2, respectively.
The coordination sphere of the CuI center is distorted tetra-
hedral as a result of ligand coordination (Figure 1). This is evi-
denced by the N1–Cu1–N1′ and N1–Cu1–N2′ bond angles of
Figure 1. Molecular structure of complex C2. Ellipsoids are drawn at the 50 %
probability level. Hydrogen atoms have been omitted for clarity.
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dex parameter, τ4, for four-coordinate complexes.[16] The calcu-
lated value places the geometry of the metal center between
two extremes, perfect tetrahedral (τ4 = 1.00) and perfect
square-planar (τ4 = 0). For complex C2, τ4 = 0.67, which corre-
lates to a see-saw geometry about the metal center (for see-
saw geometry, τ4 = 0.64). Similarly, calculating τ4 for analogous
[L2Cu]+X– complexes reported by Haddleton et al.[10] we ob-
tained values of 0.72 and 0.69.
Table 3. Effect of catalyst precursor and reaction parameters on the catalytic
hydroxylation of 4-iodotoluene.[a]
Entry Pre-catalyst x mol [%] DMSO/H2O [y:z] Time [h] Conv.[b] [%]
1[c]
[CuCl(SIPr)]
0.05
1:1 (1 mL)
24
82 (isolated
yield)
0
2
–
–
4
4
4
4
4
4
4
4
2
2
2
2
1
1
1
1
2
2
2
2
1
1
1
1
1
1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
1:1
0:1
0:1
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
1
1
1
1
1
1
1
1
12
24
3
4
5
6
C1
C2
C3
C4
C3
C3
C3
C3
DC1
DC2
DC3
DC4
DC5
DC6
DC7
DC8
DC1
DC2
DC3
DC4
DC5
DC6
DC7
DC8
DC6
DC6
80
78
100
78
0
100
100
0
100
100
100
100
100
100
100
100
38
100
43
32
70
100
52
7[d]
8[e]
9[f]
10[g]
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27[h]
28[h]
Copper-Catalyzed Hydroxylation of 4-Iodotoluene
The mono- and multinuclear cationic CuI iminopyridine com-
plexes (C1–C4, DC1–DC8) were evaluated as catalyst precursors
in the hydroxylation of 4-iodotoluene (Scheme 3).
Scheme 3. Catalytic hydroxylation of 4-iodotoluene to p-cresol.
As the starting point, we evaluated our pre-catalysts under
previously reported conditions after having established that in-
deed a catalyst is required for the reaction (Table 3, entry 2).[4b]
For the mononuclear complexes C1, C2, and C4, comparable
catalytic activity was observed after 3 hours of reaction, with
conversion of 4-iodotoluene of around 80 % (Table 3, entries 3,
4, and 6). In contrast, complex C3 displayed quantitative con-
version to p-cresol after 3 hours of reaction (Table 3, entry 5).
This marked difference in activity may be attributed to differen-
ces in the solubility of the pre-catalysts en route to forming the
active species. The observed catalytic activity of complex C3 for
the hydroxylation of 4-iodotoluene is higher than that disclosed
in a number of previous literature reports on homogeneous
catalyst systems: You and co-workers reported a CuI/phen-
anthroline/KOH catalyst system that gave a 92 % optimized
69
55
100
[a] Reagents and conditions: 1 mmol 4-iodotoluene, 4 mmol (4 equiv.) base,
4 mol-% C1–C4, 2 mol-% DC1–DC4, or 1 mol-% DC5–DC8 pre-catalyst. [b]
Conversion was determined by GC–MS with 1,4-dioxane as internal standard
employing a calibration curve. The data reported are the average conversions
obtained from two independent runs. [c] Data taken from Reference.[20] [d]
Reaction performed at 90 °C. [e] Reaction performed under argon. [f] Reac-
tion performed with 4 equiv. KOH as base. [g] Bromobenzene employed as
substrate under the optimized reaction conditions. [h] Reaction performed in
neat water as solvent at 100 °C.
Although a number of reports have shown significant
yield of p-cresol after 24 hours,[4j] Yang et al. reported a CuI/8- amounts of the symmetric biaryl ether byproduct being formed
hydroxyquinoline N-oxide/CsOH catalyst system capable of during aryl halide hydroxylation reactions,[4f,4i] our catalyst sys-
forming p-cresol in an optimized yield of 82 % after 18 hours,[4i] tem displayed excellent chemoselectivity, as determined by
and Taillefer and co-workers also obtained p-cresol in an opti- GC–MS, for the formation of p-cresol regardless of the type of
mized yield of 82 % by employing a CuI/1,3-diphenylpropane- catalyst precursor or reaction parameters (see Figure S4 in the
1,3-dione/CsOH catalyst system.[4f] The enhanced catalytic ac- Supporting Information). We probed the effect of various reac-
tivity observed for our catalyst system relative to previous re- tion parameters on the conversion of 4-iodotoluene employing
ports may be attributed to the nature of the catalyst precursor complex C3 as the catalyst precursor. Decreasing the reaction
employed. The catalyst systems reported in the literature all temperature (Table 3, entry 7) to 90 °C resulted in the complete
employed a copper precursor containing a coordinating coun- quenching of the reactivity with no aryl halide conversion being
ter ion. In our case, our complex ion pairs consist of the nonco- observed. In contrast, performing the reaction in the absence
ordinating anion BF4. This noncoordinating anion facilitates ac- of air (Table 3, entry 8) had no effect on the conversion, thereby
cess of the aryl halide into the copper coordination sphere than eliminating any effect the presence of oxygen may have on the
the coordinating anions, thereby promoting the oxidative addi- reaction. The nature of the base could also be varied because
tion step in the case of our catalyst systems. This effect of the employing 4 equiv. of KOH instead of NaOH as base under the
copper precursor has been observed previously in copper-cata- optimized conditions (Table 3, entry 9) also gave 100 % conver-
lyzed alcohol oxidation reactions.[17] Although our catalyst sys- sion of 4-iodotoluene. Finally, we evaluated the use of bromo-
tem displayed high activity, it still fell short of the catalytic ac- benzene as substrate in the hydroxylation reaction under the
tivity reported by Cazin and co-workers.[18] They found that a optimized conditions (Table 3, entry 10); no conversion to
CuI NHC complex was capable of hydroxylating iodobenzene phenol was observed, highlighting the limitation of the catalyst
with TON values of 1640.
system in terms of aryl bromides and chlorides.
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Next, we evaluated the use of G1 (DC1–DC4) and G2 (DC5–
Next, we investigated the recyclability of the catalyst system
DC8) CuI iminopyridine metallodendrimers in the catalytic hy- in neat water. Complex DC6 was active for two successive runs,
without the addition of any additional base or catalyst precur-
sor (Figure 2). It should be noted that no attempts were made
to characterize the catalyst during the recycling experiment.
The utility of our method lies in the simple extraction of the
aqueous catalyst mixture with ethyl acetate to remove unre-
acted aryl halide and allow efficient recycling of the active cata-
lyst. The activity decreased significantly for the third run (38 %
conversion) and complete catalyst deactivation was observed
for the fourth run (0 % conversion).
droxylation reaction under the optimized reaction conditions
(Table 3 entries 11–18). It should be noted that the dendritic
complexes were evaluated at the same metal loading as their
mononuclear counterparts. Our initial results showed that both
the G1 and G2 metallodendrimers are more active than their
mononuclear counterparts (e.g., entry 3 vs. entry 11). This may
be attributed to greater stabilization of the active copper spe-
cies by the dendritic ligand in comparison with the monofunc-
tional analogue. This stabilizing effect has been observed previ-
ously.[7c] Decreasing the reaction time to 1 hour allowed us to
investigate the effect of the metallodendrimer and dendrimer
generation on catalysis in more detail (Table 3, entries 19–26).
For the G1 metallodendrimers the observed catalytic activity
decreases in the order DC2 > DC3 > DC1 > DC4. In the case of
the G2 metallodendrimers, the observed catalytic activity de-
creases in the order DC6 > DC5 ≈ DC8 > DC7.
Regardless of dendrimer generation, the unsubstituted deriv-
ative (DC2 and DC6) significantly outperformed the other ana-
logues. This is in contrast to what was observed for the mono-
nuclear analogues (see above). We attribute this contrasting
trend in reactivity to the steric environment created by the den-
drimer scaffold in which substrate approach is inhibited by
more sterically bulky ligands (e.g., DL1, DL3, and DL4 vs. DL2),
which results in the observed decrease in activity.
Figure 2. Effect of recyclability on the catalytic activity of complex DC6 in
neat water.
We observed a positive dendritic effect when comparing the
G1 and G2 metallodendrimers (e.g., cf. entry 19 with entry 23
and entry 22 with entry 26). Again, this increase in catalytic
activity may be attributed to the enhanced stabilization af-
forded by the G2 dendritic ligand in comparison with the G1
analogue.
To probe the mode of catalyst deactivation we analyzed the
catalyst solution by HRTEM and energy-dispersive X-ray spec-
troscopy (EDS) analysis. Our results showed the formation of
large copper aggregates (>100 nm), which indicates that cata-
lyst deactivation is a result of complex decomposition accom-
panied by metal agglomeration (see Figure S6 in the Support-
ing Information). Next, we performed a series of experiments
to gain an insight into the reaction mechanism and the nature
of the active species formed during the catalysis. First, we inves-
tigated whether the iminopyridine CuI complexes serve as a
reservoir of copper nanoparticles or whether a heterogeneous
active species is formed during the reaction. This was accom-
plished by performing the hydroxylation reaction in the pres-
ence of additives that are known to poison and deactivate cop-
per nanoparticles (Scheme 4).[19]
Recent advances in hydroxylation catalysis have led to the
development of catalyst systems capable of phenol synthesis in
neat water. Typically, these catalyst systems employ a Cu source
in conjunction with hydrophilic ligands and/or a phase-transfer
catalyst like tetrabutylammonium bromide (TBAB).[4h] Although
these catalyst systems are efficient they still require special li-
gands or additives and should be used under an inert atmos-
phere. With these drawbacks in mind, we evaluated the use of
the G2 unsubstituted iminopyridyl CuI metallodendrimer DC6
as pre-catalyst for the hydroxylation of 4-iodotoluene in neat
water. When performing the reaction at 100 °C in air and in the
absence of a phase-transfer agent, the observed conversion
after 12 hour of reaction was 55 % (Table 3, entry 24, and Fig-
ure S5 in the Supporting Information). On increasing the reac-
tion time to 24 h, 100 % conversion could be obtained (Table 3,
entry 25). In this regard, the observed conversion is identical to
that reported by Yang et al.[4h] Importantly, the observed cata-
lytic activity was not dependent on the solubility of the pre-
catalyst or the substrate in water, because both components
used in the current study are insoluble in water. We speculate
that some sort of biphasic catalyst system may be forming in
which the catalytically active species is dispersed within the
molten organic substrate. This and other possibilities are cur-
rently being investigated.
Scheme 4. Catalytic hydroxylation of 4-iodotoluene to p-cresol in the pres-
ence of poison additives.
The G1 CuI metallodendrimer DC2 was employed as catalyst
precursor under the optimized reaction conditions (Table 4). A
Hg drop-test showed only partial inhibition of catalyst activity
(Table 4, entry 1), in contrast to the complete inhibition that
is normally observed when colloidal metal is present during
catalysis.[20] In addition, common poisons like pyridine (Table 4,
entry 2) and triphenylphosphine (Table 4, entry 3) in stoichio-
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metric and nonstoichiometric amounts had no inhibiting effect Conclusions
on catalytic activity.[21]
We have prepared and characterized a series of mononuclear
and dendritic cationic CuI complexes ligated by substituted imi-
nopyridine or quinoline ligands. All the mononuclear (C1–C4)
and G1 (DC1–DC4) and G2 (DC5–DC8) dendritic complexes dis-
played high activity in the hydroxylation of 4-iodotoluene with
the observed catalytic activity being significantly higher than
that disclosed in previous literature reports. A positive dendritic
effect was observed when comparing the different classes of
pre-catalysts, with an increase in catalytic activity observed with
increasing dendrimer generation. Furthermore, the G2 unsub-
stituted CuI metallodendrimer DC6 displayed high catalytic ac-
tivity and recyclability when the hydroxylation was performed
in neat water and did not require a phase-transfer agent, inher-
ent water solubility of the catalyst, or an inert atmosphere for
catalysis. HRTEM analysis showed that metal agglomeration in
neat water leads to catalyst deactivation. A series of poisoning
experiments were conducted, the results of which suggest that
the metallodendrimer pre-catalysts form homogeneous active
species during catalysis. The results of a radical-trapping experi-
ment combined with our experimental observations suggest
that the catalysis proceeds by a CuI/CuIII cycle. Current work in
our laboratories is directed towards the isolation and characteri-
zation of key intermediates in the catalytic cycle and the prepa-
ration of well-defined copper nanoparticles for catalysis.
Table 4. Catalytic results of poisoning experiments employing DC2 as pre-
catalyst.
Entry
Poisoning additive (amount)
Conv. [%][a]
1
2
3
Hg (one drop)
Pyridine (150 equiv. per metal atom)
PPh3 (0.5 equiv. per metal atom)
85
100
100
[a] Conversion was determined by GC–MS with 1,4-dioxane as internal stan-
dard employing a calibration curve. The data reported are the average con-
versions obtained from two independent runs.
The results of the poisoning experiments indicate that the
active species in aryl halide hydroxylation is molecular in na-
ture. Combined with the results of our hydroxylation experi-
ments in water we can conclude that a homogeneous active
species that is molecular in nature is responsible for the cata-
lytic activity observed for both mononuclear and dendritic imi-
nopyridine CuI complexes and that heterogeneous copper is
formed only when the catalyst is deactivated either intention-
ally during work-up or after prolonged reaction in neat water.
We believe that the hydroxylation reaction mediated by com-
plexes C1–C4 share mechanistic features with the copper-cata-
lyzed halide exchange of aryl halides for which a number of
mechanistic scenarios to account for aryl halide activation have
been proposed.[22] These include the formation of a four-mem-
bered transition state, oxidative addition to form a CuIII interme-
diate, copper-promoted nucleophilic aromatic substitution
(SNAr), and single-electron-transfer processes such as radical-
nucleophilic aromatic substitution (SRN1). The order of reactivity
observed experimentally for our mono- and multinuclear cata-
lyst systems, Ar-I > Ar-Br, does not support an SNAr mecha-
nism.[22] To probe whether an SRN1 mechanism may be opera-
tive we performed the hydroxylation reaction with complex C3
as pre-catalyst in the presence of 1 equiv. of (2,2,6,6-tetrameth-
ylpiperidin-1-yl)oxyl (TEMPO) as radical-trapping agent
(Scheme 5). Under these reaction conditions substrate conver-
sion to p-cresol of 70 % was observed. This is in contrast to
what was observed for copper-catalyzed C–N and C–C bond-
formation reactions that operate by single-electron-transfer
processes in which radical-trapping experiments resulted in the
Experimental Section
General: All manipulations involving air- and/or moisture-sensitive
compounds were performed in a nitrogen-filled glove box or under
purified dry nitrogen using standard Schlenk techniques. All rea-
gents, with the exception of the poly(propyleneimine) dendrimers,
were obtained from Sigma–Aldrich and used as received. The G1
and G2 poly(propyleneimine) dendrimers were purchased from
SymoChem (Netherlands). Solvents were purchased from Sigma–
Aldrich or Kimix Chemicals (South Africa) and were heated at reflux
over appropriate drying agents and distilled under nitrogen prior
to use. IR spectra were recorded as neat oils or solids with a Perkin–
Elmer Paragon 1000 PC FT-IR spectrometer using an ATR accessory.
NMR spectra were recorded with a Varian Gemini 2000 spectrome-
ter (1H at 300/400/600 MHz, 13C at 75 MHz). Chemical shifts are
reported in ppm and referenced to the residual protons of the deu-
teriated solvents with tetramethylsilane (TMS) as internal standard.
ESI-MS (positive mode) analyses were performed with a Waters API
Quattro Micro spectrometer with direct injection of dichloro-
methane solutions. The MS settings were as follows: capillary volt-
age 3.5 KV, cone voltage 15 RF1 40, cone gas 50L/h, desolvation
temperature 400 °C, and desolvation gas 500L/h. Melting point de-
terminations were performed with a Stuart Scientific SMP3 melting-
point apparatus. GC analyses were performed with an Agilent 6890
chromatograph equipped with a mass selective (5973) detector and
an Agilent HP5-MS01 column (30 m × 0.32 mm × 0.25 μm). Elemen-
tal analyses were performed at the micro-analytical laboratory of
the Department of Chemistry at the University of Cape Town and
the Mass Spectrometry Unit at the University of Kwazulu-Natal.
total inhibition of catalysis.[23] Thus, we can exclude an SRN
1
mechanism being operative. Although our efforts to date to
isolate and characterize a catalytically competent CuIII species
have been unsuccessful, we propose that aryl halide hydroxyl-
ation proceeds by a CuI/CuIII catalytic cycle analogous to that
reported for copper-catalyzed halide-exchange reactions.[24]
Synthesis of the Monofunctional Ligands L1, L3, and L4
Synthesis of [(6-Methylpyridin-2-yl)methylidene]propan-1-
amine (L1): Propylamine (0.16 mL, 1.96 mmol) was added dropwise
to a stirred slurry of 6-methylpyridine-2-carbaldehyde (200 mg,
Scheme 5. Radical-trapping experiment to probe whether an SRN1 mecha-
nism is operative.
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Full Paper
1.65 mmol) and magnesium sulfate (2–3 g) in diethyl ether (10 mL)
at 0 °C. Following the complete addition of amine, the reaction
mixture was stirred for 4 h at room temperature. After the allotted
time, the reaction mixture was filtered and the solvent removed.
The resulting oil was purified by dissolving in diethyl ether at –4 °C
to remove any unreacted aldehyde. The diethyl ether was decanted,
the solvent removed, and the product isolated as a colorless oil,
(729.01): calcd. C 70.87, H 8.07, N 18.69; found C 70.19, H 8.24, N
19.10.
DAB–G1–PPI-[(6-bromopyridin-2-yl)methanimine]4 (DL3): Pre-
pared according to the same procedure outlined for DL1 by em-
ploying 6-bromopyridine-2-carbaldehyde as reagent, yield 504 mg,
1
91 %. H NMR (300 MHz, CDCl3): δ = 8.30 (s, 4 H, 1-H), 7.93 (d, J =
6 Hz, 4 H, 3-H), 7.59 (t, J = 9 Hz, 4 H, 4-H), 7.50 (d, J = 6 Hz, 4 H, 5-
H), 3.68 (t, J = 6 Hz, 8 H, 7-H), 2.50 (t, J = 6 Hz, 8 H, 9-H), 2.40 (t, J =
6 Hz, 4 H, 10-H), 1.83 (m, 8 H, 8-H), 1.41 (m, 4 H, 11-H) ppm. 13C{1H}
NMR (75 MHz, CDCl3): δ = 160.09 (C-1), 155.31 (C-2), 141.04 (C-6),
138.37 (C-4), 128.53 (C-5), 119.18 (C-3), 58.99 (C-7), 53.62 (C-8), 51.18
1
yield 187 mg, 70 %. H NMR (300 MHz, CDCl3): δ = 8.38 (s, 1 H, 1-
H), 7.78 (d, J = 6 Hz, 1 H, 3-H), 7.60 (t, J = 9 Hz, 1 H, 4-H), 7.16 (d,
J = 9 Hz, 1 H, 5-H), 3.65 (t, J = 6 Hz, 2 H, 7-H), 2.58 (s, 3 H, CH3),
1.78 (m, 2 H, 8-H), 1.06 (t, J = 6 Hz, 3 H, 9-H) ppm. 13C{1H} NMR
(75 MHz, CDCl3): δ = 162.06 (C-1), 158.04 (C-2), 154.02 (C-6), 136.71
(C-3), 124.20 (C-5), 118.23 (C-4), 63.29 (C-7), 24.30 (CH3), 23.80 (C-8),
(C-9), 27.75 (C-10), 24.75 (C-11) ppm. FTIR (ATR, neat): ν = 1648 (C=
˜
N) cm–1. MS (ESI, + mode): m/z = 673 [M + H]+. C40H48Br4N10
(988.49): calcd. C 48.60, H 4.89, N 14.17; found C 49.00, H 5.47, N
14.59.
11.80 (C-9) ppm. FTIR (ATR, neat): ν = 1651 (C=N) cm–1. MS (ESI, +
˜
mode): m/z = 162.2 [M + H]+. C10H14N2 (162.23): calcd. C 64.01, H
7.47, N 11.94; found C 64.65, H 7.16, N 11.81.
DAB–G1–PPI-[(quinolin-2-yl)methanimine]4 (DL4): Prepared ac-
cording to the same procedure outlined for DL1 by employing
quinoline-2-carbaldehyde as reagent, yield 441 mg, 90 %. 1H NMR
(300 MHz, CDCl3): δ = 8.55 (s, 4 H, 1-H), 8.12 (d, J = 9 Hz, 4 H, 4-H),
8.12 (d, J = 4 Hz, 4 H, 4′-H), 8.11 (d, J = 8 Hz, 4 H, 6′-H), 8.09 (d, J =
8 Hz, 4 H, 3-H), 7.70 (t, J = 8 Hz, 4 H, 2′-H), 7.53 (t, J = 8 Hz, 4 H, 3′-
H), 3.76 (t, J = 8 Hz, 8 H, 7-H), 2.56 (t, J = 6 Hz, 8 H, 9-H), 2.44 (t, J =
6 Hz, 4 H, 10-H), 1.89 (m, 8 H, 8-H), 1.45 (m, 4 H, 11-H) ppm. 13C{1H}
NMR (75 MHz, CDCl3): δ = 162.44 (C-1), 154.76 (C-2), 147.68 (C-6),
136.48 (C-4), 129.54 (C-5), 129.36 (C-2′), 128.78 (C-6′), 127.63 (C-4′),
127.26 (C-3′), 118.33 (C-3), 59.59 (C-7), 54.08 (C-8), 51.67 (C-9), 28.20
[(6-Bromopyridin-2-yl)methylidene]propan-1-amine (L3): Pre-
pared according to the same procedure outlined for L1 by employ-
ing 6-bromopyridine-2-carbaldehyde as reagent, yield 88 %. 1H
NMR (300 MHz, CDCl3): δ = 8.29 (s, 1 H, 1-H), 7.94 (d, J = 6 Hz, 1 H,
3-H), 7.58 (t, J = 9 Hz, 1 H, 4-H), 7.47 (d, J = 6 Hz, 1 H, 5-H), 3.61 (t,
J = 6 Hz, 2 H, 7-H), 1.73 (m, 2 H, 8-H), 0.93 (t, J = 6 Hz, 3 H, 9-
H) ppm. 13C{1H} NMR (75 MHz, CDCl3): δ = 159.96 (C-1), 155.32 (C-
2), 141.02 (C-6), 130.36 (C-4), 128.52 (C-5), 119.19 (C-3), 62.71 (C-7),
23.28 (C-8), 11.23 (C-9) ppm. FTIR (ATR, neat): ν = 1649 (C=N) cm–1
.
˜
MS (ESI, + mode): m/z = 227.1 [M + H]+. C9H11BrN2 (227.10): calcd.
C 47.60, H 4.85, N 12.33; found C 47.78, H 4.81, N 12.02.
(C-10), 25.21 (C-11) ppm. FTIR (ATR, neat): ν = 1643 (C=N) cm–1. MS
˜
(ESI, + mode): m/z = 874 [M + H]+. C56H60N10 (873.14): calcd. C
[(Quinolin-2-yl)methylidene]propan-1-amine (L4): Prepared ac-
cording to the same procedure outlined for L1 by employing quin-
oline-2-carbaldehyde as reagent, yield 80 %. 1H NMR (300 MHz,
CDCl3): δ = 8.55 (s, 1 H, 1-H), 8.21 (d, J = 9 Hz, 1 H, 4-H), 8.16 (d, J =
4 Hz, 1 H, 4′-H), 8.12 (d, J = 8 Hz, 1 H, 6′-H), 7.86 (d, J = 8 Hz, 1 H,
3-H), 7.74 (t, J = 8 Hz, 1 H, 2′-H), 7.57 (t, J = 8 Hz, 1 H, 3′-H), 3.76 (t,
J = 8 Hz, 2 H, 7-H), 1.76 (m, 2 H, 8-H), 1.00 (t, J = 4 Hz, 3 H, 9-
H) ppm. 13C{1H} NMR (75 MHz, CDCl3): δ = 162.64 (C-1), 155.19 (C-
2), 148.08 (C-6), 136.38 (C-4), 129.99 (C-5), 129.78 (C-2′), 128.88 (C-
6′), 127.89 (C-4′), 127.64 (C-3′), 118.70 (C-3), 63.71 (C-7), 24.23 (C-8),
75.86, H 6.79, N 15.75; found C 74.70, H 7.06, N 16.37.
DAB–G2–PPI-[(6-methylpyridin-2-yl)methanimine]8 (DL5): Pre-
pared according to the same procedure outlined for DL1 by em-
ploying 6-methylpyridine-2-carbaldehyde as reagent with a reac-
tion time of 72 h, yield 850 mg, 95 %. 1H NMR (300 MHz, CDCl3):
δ = 8.46 (s, 8 H, 1-H), 7.89 (d, J = 6 Hz, 8 H, 3-H), 7.71 (t, J = 9 Hz,
8 H, 4-H), 7.25 (d, J = 9 Hz, 8 H, 5-H), 3.77 (t, J = 6 Hz, 16 H, 7-H),
2.69 (s, 24 H, CH3), 2.64 (t, J = 6 Hz, 24 H, 9,9′-H), 2.51 (t, J = 6 Hz,
4 H, 10-H), 1.96 (m, 8 H, 7′-H), 1.69 (m, 24 H, 8,8′-H), 1.49 (m, 4 H,
11-H) ppm. 13C{1H} NMR (75 MHz, CDCl3): δ = 162.04 (C-1), 157.86
(C-2), 153.96 (C-6), 136.57 (C-3), 124.07 (C-5), 118.21 (C-4), 59.54 (C-
7,7′), 54.13 (CH3), 52.24 (C-8,8′), 51.67 (C-9,9′), 28.20 (C-10), 24.59 (C-
12.21 (C-9) ppm. FTIR (ATR, neat): ν = 1646 (C=N) cm–1. MS (ESI, +
˜
mode): m/z = 198.3 [M + H]+. C13H14N2 (198.26): calcd. C 74.03, H
6.64, N 13.29; found C 74.75, H 6.74, N13.22.
Synthesis of Multifunctional G1 (DL1–DL4) and G2 (DL5–DL8)
Dendritic Iminopyridyl Ligands
11) ppm. FTIR (ATR, neat): ν = 1648 (C=N) cm–1. MS (ESI, + mode):
˜
m/z = 1599 [M + H]+. C96H136N22 (1598.25): calcd. C 70.72, H 8.37,
N 18.81; found C 70.25, H 8.65, N 19.48.
Synthesis of DAB–G1–PPI-[(6-methylpyridin-2-yl)methanimine]4
(DL1): A solution of DAB-(NH2)4 first-generation dendrimer (180 mg,
0.56 mmol) in toluene (10 mL) was added to a stirred slurry of 6-
methylpyridine-2-carbaldehyde (263 mg, 2.17 mmol) and anhy-
drous MgSO4 (3 g) in diethyl ether (15 mL). The reaction mixture
was stirred for 48 h at room temperature. After the allotted time
the reaction mixture was filtered and the solvent removed to yield
a colorless oil. The oil was dissolved in dichloromethane (20 mL)
and extracted with deionized water (5 × 30 mL portions) to remove
any unreacted dendrimer. The organic phase was dried with MgSO4,
filtered, and the solvent removed. The product was isolated as a
colorless oil, yield 310 mg, 76 %. 1H NMR (300 MHz, CDCl3): δ = 8.46
(s, 4 H, 1-H), 7.89 (d, J = 6 Hz, 4 H, 3-H), 7.71 (t, J = 9 Hz, 4 H, 4-H),
7.25 (d, J = 9 Hz, 4 H, 5-H), 3.79 (t, J = 6 Hz, 8 H, 7-H), 2.71 (t, J =
6 Hz, 8 H, 9-H), 2.65 (t, J = 6 Hz, 4 H, 10-H), 2.54 (s, 12 H, CH3), 1.97
(m, 8 H, 8-H), 1.06 (m, 4 H, 11-H) ppm. 13C{1H} NMR (75 MHz, CDCl3):
DAB–G2–PPI-[(6-bromopyridin-2-yl)methanimine]8 (DL7): Pre-
pared according to the same procedure outlined for DL1 by em-
ploying 6-bromopyridine-2-carbaldehyde as reagent with a reaction
time of 72 h, yield 1.043 g, 88 %. 1H NMR (300 MHz, CDCl3): δ =
8.29 (s, 8 H, 1-H), 7.95 (d, J = 6 Hz, 8 H, 3-H), 7.58 (t, J = 9 Hz, 8 H,
4-H), 7.49 (d, J = 6 Hz, 4 H, 5-H), 3.49 (t, J = 6 Hz, 16 H, 7-H), 2.51
(t, J = 6 Hz, 24 H, 9,9′-H), 2.39 (t, J = 6 Hz, 4 H, 10-H), 1.82 (m, 24
H, 8,8′-H), 1.58 (m, 8 H, 7′-H), 1.38 (m, 4 H, 11-H) ppm. 13C{1H} NMR
(75 MHz, CDCl3): δ = 160.48 (C-1), 155.71 (C-2), 141.46 (C-6), 138.80
(C-4), 128.97 (C-5), 119.63 (C-3), 59.44 (C-7,7′), 53.03 (C-8,8′), 51.64
(C-9,9′), 28.16 (C-10), 25.20 (C-11) ppm. FTIR (ATR, neat): ν = 1649
˜
(C=N) cm–1. MS (ESI, + mode): m/z = 1058 [M + H]2+. C88H112Br8N22
(2117.21): calcd. C 49.92, H 5.33, N 14.99; found C 49.98, H 5.71, N
15.64.
δ = 162.04 (C-1), 157.86 (C-2), 153.96 (C-6), 136.57 (C-3), 124.07 (C- DAB–G2–PPI-[(quinolin-2-yl)methanimine]8 (DL8): Prepared ac-
5), 118.21 (C-4), 59.54 (C-7), 54.13 (CH3), 52.24 (C-8), 51.67 (C-9), cording to the same procedure outlined for DL1 by employing
28.20 (C-10), 24.59 (C-11) ppm. FTIR (ATR, neat): ν = 1648
quinoline-9-carbaldehyde as reagent with a reaction time of 72 h,
˜
1
(C=N) cm–1. MS (ESI, + mode): m/z = 730 [M + H]+. C44H60N10 yield 982 mg, 93 %. H NMR (300 MHz, CDCl3): δ = 8.52 (s, 8 H, 1-
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H), 8.13 (d, J = 9 Hz, 8 H, 4-H), 8.12 (d, J = 4 Hz, 8 H, 4′-H), 8.09 (d, reagent. Isolated as a dark-brown crystalline solid, yield 130 mg,
1
J = 8 Hz, 8 H, 6′-H), 7.73 (d, J = 8 Hz, 8 H, 3-H), 7.71 (t, J = 8 Hz, 8
H, 2′-H), 7.53 (t, J = 8 Hz, 8 H, 3′-H), 3.73 (t, J = 8 Hz, 16 H, 7-H),
2.56 (t, J = 6 Hz, 24 H, 9,9′-H), 2.41 (t, J = 6 Hz, 4 H, 10-H), 1.88 (m,
24 H, 8,8′-H), 1.59 (m, 8 H, 7′-H), 1.38 (m, 4 H, 11-H) ppm. 13C{1H}
NMR (75 MHz, CDCl3): δ = 161.95 (C-1), 154.36 (C-2), 147.26 (C-6),
135.99 (C-4), 129.13 (C-5), 128.95 (C-2′), 128.80 (C-6′), 127.46 (C-4′),
127.20 (C-3′), 117.93 (C-3), 59.20 (C-7,7′), 52.76 (C-8,8′), 51.27
83 %, m.p. 175–179 °C. H NMR (400 MHz, [D6]DMSO): δ = 9.19 (s,
2 H, 1-H), 8.89 (d, J = 7 Hz, 2 H, 4-H), 8.31 (d, J = 7 Hz, 2 H, 4′-H),
8.24 (d, J = 7 Hz, 2 H, 6′-H), 8.15 (d, J = 7 Hz, 2 H, 3-H), 7.69 (t, J =
8 Hz, 2 H, 2′-H), 7.64 (t, J = 8 Hz, 2 H, 3′-H), 3.83 (t, J = 7 Hz, 4 H, 7-
H), 1.53 (m, 4 H, 8-H), 0.74 (t, J = 7 Hz, 6 H, 9-H) ppm. 13C{1H} NMR
(150 MHz, [D6]DMSO): δ = 150.16 (C-1), 144.80 (C-2), 138.26 (C-6),
131.99 (C-4), 130.13 (C-5), 128.75 (C-2′), 128.10 (C-6′), 127.01 (C-4′),
(C-9,9′), 27.82 (C-10), 24.92 (C-11) ppm. FTIR (ATR, neat): ν = 1644 126.43 (C-3′), 122.98 (C-3), 60.97 (C-7), 23.58 (C-8), 10.99 (C-9) ppm.
˜
(C=N) cm–1. MS (ESI, + mode): m/z = 1887 [M + H]+. C120H136N22 UV/Vis: λmax = 250 (π→π*), 311 (n→π*), 530 (MLCT) nm. FTIR (ATR,
(1888.53): calcd. C 71.28, H 6.82, N 14.99; found C 71.79, H 7.15, N
15.21.
Synthesis of Cationic Mononuclear Iminopyridyl CuI Complexes
C1–C4
neat): ν = 1620 (C=N), 1049 (B–F) cm–1. MS (ESI, + mode): m/z =
˜
461 [M – BF4 + H]+. C26H28BCuF4N4 (546.88): calcd. C 57.10, H 5.16,
N 10.24; found C 56.94, H 4.98, N 9.96.
Synthesis of Cationic First-Generation (DC1–DC4) and Second-
Generation (DC5–DC8) Multinuclear Iminopyridyl CuI Com-
plexes
Synthesis of [Cu{(6-Me-C5H3N)CH=N(nPr)-κ2-N,N}2][BF4] (C-1): A
solution of ligand L1 (85 mg, 0.574 mmol) in dichloromethane
(5 mL) was added to a stirred solution of tetrakis(acetonitrile)cop-
per(I) tetrafluoroborate (90 mg, 0.287 mmol) in dichloromethane
(10 mL). The resulting red-brown solution was stirred for 2 h at
room temperature. After the allotted time the solvent volume was
reduced to approximately 2 mL and the reaction mixture layered
with diethyl ether. Slow diffusion of diethyl ether resulted in the
formation of a red-brown crystalline solid. The solid was washed
with cold diethyl ether and dried in vacuo, yield 116 mg, 85 %, m.p.
[DAB–G1–PPI-(Cu{(6-Me-C5H3N)-κ2-N,N}4)2][BF4]2 (DC1): A solu-
tion of ligand DL1 (105 mg, 0.1435 mmol) in dichloromethane
(5 mL) was added to a stirred solution of tetrakis(acetonitrile)cop-
per(I) tetrafluoroborate (90 mg, 0.287 mmol) in dichloromethane
(10 mL). The resulting red-brown solution was stirred for 12 h at
room temperature. After the allotted time the solvent volume was
reduced to approximately 2 mL and the reaction mixture layered
with diethyl ether. Slow diffusion of diethyl ether resulted in the
formation of a deep-red amorphous solid. The solid was washed
with cold diethyl ether and dried in vacuo, yield 139 mg, 94 %, m.p.
147–149 °C. 1H NMR (400 MHz, [D6]DMSO): δ = 8.86 (s, 4 H, 1-H),
8.09 (d, J = 6 Hz, 4 H, 3-H), 7.86 (t, J = 9 Hz, 4 H, 4-H), 7.67 (d, J =
6 Hz, 4 H, 5-H), 3.89 (t, J = 6 Hz, 8 H, 7-H): δ = 2.21 (s, 12 H, CH3),
1.54–2.73 (br. signals, 24 H, 8,9,10,11-H) ppm. 13C{1H} (150 MHz,
[D6]DMSO): δ = 162.00 (C-1), 157.60 (C-2), 149.87 (C-6), 138.08 (C-3),
127.66 (C-5), 124.52 (C-4), 57.50 (C-7), 56.60 (C-8), 54.00 (C-9), 50.98
(C-10), 47.47 (C-11), 23.99 (CH3) ppm. UV/Vis: λmax = 247 (π→π*),
1
85–87 °C. H NMR (300 MHz, [D6]DMSO): δ = 8.86 (s, 1 H, 1-H), 8.10
(t, J = 9 Hz, 2 H, 4-H), 7.88 (d, J = 6 Hz, 2 H, 3-H), 7.67 (d, J = 9 Hz,
2 H, 5-H), 3.76 (t, J = 6 Hz, 4 H, 7-H), 2.18 (s, 6 H, CH3), 1.55 (m, 4
H, 8-H), 0.81 (t, J = 6 Hz, 6 H, 9-H) ppm. 13C NMR (150 MHz,
[D6]DMSO): δ = 162.28 (C-1), 157.60 (C-2), 149.74 (C-6), 138.64 (C-3),
127.82 (C-5), 124.48 (C-4), 56.64 (C-7), 28.48 (C-8), 23.97 (CH3), 10.90
(C-9) ppm. UV/Vis: λmax = 250 (π→π*), 283 (n→π*), 477 (MLCT) nm.
FTIR (ATR, neat): ν = 1620 (C=N), 1030 (B–F) cm–1. MS (ESI, + mode):
˜
m/z = 475 [M – BF4 + H]+. C20H28BCuF4N4·0.3CH2Cl2 (500.29): calcd.
C 48.54, H 5.74, N 11.14; found C 48.80, H 5.46, N 11.30.
283 (n→π*), 470 (MLCT) nm. FTIR (ATR, neat): ν = 1619 (C=N), 1031
˜
(B–F) cm–1
44H60B2Cu2F8N10 (1029.72): calcd. C 51.32, H 5.87, N 13.60; found
C 51.04, H 5.61, N 13.36.
. MS (ESI, + mode): m/z = 428 [M – .
(BF4)2]2+
[Cu{(C5H4N)CH=N(nPr)-κ2-N,N}2][BF4] (C-2): Prepared according to
the procedure outlined for complex C1, employing ligand L2 as
reagent. Isolated as a red crystalline solid, yield 100 mg, 78 %, m.p.
C
1
[DAB–G1–PPI-(Cu{(C5H4N)-κ2-N,N}4)2][BF4]2 (DC2): Prepared ac-
cording to the procedure outlined for complex DC1, employing li-
gand DL2 as reagent. Isolated as a brown amorphous solid, yield
105–108 °C. H NMR (400 MHz, [D6]DMSO): δ = 8.87 (d, J = 8 Hz, 2
H, 6-H), 8.53 (s, 2 H, 1-H), 8.19 (d, J = 8 Hz, 2 H, 3-H), 8.03 (t, J =
7 Hz, 2 H, 4-H), 7.74 (t, J = 7 Hz, 2 H, 5-H), 3.76 (t, J = 7 Hz, 4 H, 7-
H), 1.56 (m, 4 H, 8-H), 0.81 (t, J = 7 Hz, 6 H, 9-H) ppm. 13C{1H} NMR
(150 MHz, [D6]DMSO): δ = 161.55 (C-1), 150.30 (C-2), 148.84 (C-4),
138.38 (C-6), 128.14 (C-3), 126.87 (C-5), 60.57 (C-7), 23.31 (C-8), 10.92
(C-9) ppm. UV/Vis: λmax 247 (π→π*), 278 (n→π*), 477 (MLCT) nm.
1
121 mg, 87 %, m.p. 150–155 °C. H NMR (400 MHz, [D6]DMSO): δ =
8.83 (d, J = 6 Hz, 4 H, 6-H), 8.50 (s, 4 H, 1-H), 8.15 (d, J = 6 Hz, 4 H,
3-H), 7.98 (t, J = 9 Hz, 4 H, 4-H), 7.71 (t, J = 9 Hz, 4 H, 5-H), 3.76 (t,
J = 6 Hz, 8 H, 7-H), 1.68–2.65 (br. signals, 24 H, 8,9,10,11-A) ppm.
13C{1H} NMR (150 MHz, [D6]DMSO): δ = 161.59 (C-1), 150.42 (C-2),
148.80 (C-4), 138.36 (C-6), 128.11 (C-3), 126.87 (C-5), 57.55 (C-7,7′),
56.40 (C-8,8′), 53.94 (C-9,9′), 50.66 (C-10), 30.72 (C-11) ppm. UV/Vis:
λmax = 248 (π→π*), 277 (n→π*), 470 (MLCT) nm. FTIR (ATR, neat):
FTIR (ATR, neat): ν = 1625 (C=N), 1032 (B–F) cm–1. MS (ESI, + mode):
˜
m/z = 361 [M – BF4 + H]+. C18H24BCuF4N4·0.5CH2Cl2 (489.26): calcd.
C 45.42, H 5.15, N 11.45; found C 45.63, H 4.88, N 11.79.
[Cu{(6-Br-C5H3N)CH=N(nPr)-κ2-N,N}2][BF4] (C-3): Prepared accord-
ing to the procedure outlined for complex C1, employing ligand L3
as reagent. Isolated as a dark-brown crystalline solid, yield 139 mg,
ν = 1622 (C=N), 1032 (B–F) cm–1. MS (ESI, + mode): m/z = 401 [M –
˜
(BF4)2]2+. C40H52B2Cu2F8N10 (973.61): calcd. C 49.35, H 5.38, N 14.39;
found C 49.04, H 5.12, N 14.06.
1
80 %, m.p. 175–177 °C. H NMR (400 MHz, [D6]DMSO): δ = 8.89 (s,
2 H, 1-H), 8.13 (t, J = 8 Hz, 2 H, 4-H), 8.04 (d, J = 7 Hz, 2 H, 6-H),
8.01 (d, J = 8 Hz, 2 H, 5-H), 3.81 (t, J = 7 Hz, 4 H, 7-H), 1.63 (m, 4 H,
8-H), 0.87 (t, J = 7 Hz, 6 H, 9-H) ppm. 13C{1H} NMR (150 MHz,
[D6]DMSO): δ = 161.21 (C-1), 151.68 (C-2), 141.40 (C-6), 141.20 (C-4),
132.08 (C-5), 126.29 (C-3), 61.15 (C-7), 23.73 (C-8), 11.21 (C-9) ppm.
UV/Vis: λmax = 235 (π→π*), 288 (n→π*), 477 (MLCT) nm. FTIR (ATR,
[DAB–G1–PPI-(Cu{(6-BrC5H3N)-κ2-N,N}4)2][BF4]2 (DC3): Prepared
according to the procedure outlined for complex DC1, employing
ligand DL3 as reagent. Isolated as a maroon amorphous solid, yield
1
166 mg, 90 %, m.p. 169–171 °C. H NMR (400 MHz, [D6]DMSO): δ =
8.86 (s, 4 H, 1-H), 8.12 (d, J = 6 Hz, 4 H, 3-H), 8.00 (t, J = 9 Hz, 4 H,
4-H), 7.97 (d, J = 6 Hz, 4 H, 5-H), 3.95 (t, J = 6 Hz, 8 H, 7-H), 2.16–
2.72 (br. signals, 24 H, 8,9,10,11-H) ppm. 13C{1H} NMR (150 MHz,
[D6]DMSO): δ = 161.04 (C-1), 151.36 (C-2), 141.45 (C-6), 132.04 (C-4),
126.15 (C-5), 121.50 (C-3), 57.66 (C-7,7′), 50.12 (C-8,8′), 47.88
neat): ν = 1616 (C=N), 1029 (B–F) cm–1. MS (ESI, + mode): m/z =
˜
519 [M – BF4 + H]+. C18H22BBr2CuF4N4 (604.55): calcd. C 35.76, H
3.67; N: 9.27; found C 35.50, H 3.34, N 8.98.
[Cu{(C9H6N)CH=N(nPr)-κ2-N,N}2][BF4] (C-4): Prepared according to (C-9,9′), 30.11 (C-10), 28.10 (C-11) ppm. UV/Vis: λmax = 255 (π→π*),
the procedure outlined for complex C1, employing ligand L4 as
288 (n→π*), 470 (MLCT) nm. FTIR (ATR, neat): ν = 1624 (C=N), 1030
˜
Eur. J. Inorg. Chem. 2016, 3781–3790
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© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
(B–F) cm–1
.
MS (ESI,
+
mode): m/z
=
557 [M
–
(BF4)2]2+
.
[DAB–G2–PPI-(Cu{(C9H6N)-κ2-N,N)}8)4][BF4]4 (DC8): Prepared ac-
cording to the procedure outlined for complex DC1, employing li-
gand DL8 as reagent with a reaction time of 24 h. Isolated as a
purple amorphous solid, yield 165 mg, 92 %, m.p. 175–179 °C. 1H
NMR (400 MHz, [D6]DMSO): δ = 9.21 (br. s, 8 H, 1-H), 8.87–8.31 (br.
m, 16 H, 4,4′-H), 8.16 (br., 16 H, 2′,6′-H), 7.68 (br., 16 H, 3,3′-H), 3.85
(br., 24 H, 7,7′-H), 1.70–3.44 (br. signals, 56 H, 8,8′,9,9′,10,11-H) ppm.
13C{1H} NMR (150 MHz, [D6]DMSO): δ = 150.85 (C-1), 144.93 (C-2),
138.96 (C-6), 131.92 (C-4), 130.13 (C-5), 128.96 (C-2′), 128.39 (C-6′),
127.44 (C-4′), 126.69 (C-3′), 123.50 (C-3), 64.57 (C-7,7′), 57.30
C40H48B2Br4Cu2F8N10·2CH2Cl2 (1459.06): calcd. C 34.57, H 3.59, N
9.60; found C 34.29, H 3.38, N 9.45.
[DAB–G1–PPI-(Cu{(C9H6N)-κ2-N,N}4)2][BF4]2 (DC4): Prepared ac-
cording to the procedure outlined for complex DC1, employing li-
gand DL4 as reagent. Isolated as a maroon amorphous solid, yield
1
166 mg, 90 %, m.p. 169–171 °C. H NMR (400 MHz, [D6]DMSO): δ =
9.20 (br. s, 4 H, 1-H), 8.87–8.31 (br. m, 8 H, 4,4′-H), 8.16 (br., 8 H, 2′
,6′-H), 7.68 (br., 8 H, 3,3′-H), 3.85 (br., 8 H, 7-H), 1.70–3.44 (br. signals,
24 H, 8,9,10,11-H) ppm. 13C{1H} NMR (150 MHz, [D6]DMSO): δ =
150.66 (C-1), 144.91 (C-2), 138.54 (C-6), 131.02 (C-4), 130.11 (C-5),
128.75 (C-2′), 128.32 (C-6′), 127.41 (C-4′), 126.70 (C-3′), 123.50 (C-3),
64.55 (C-7), 57.31 (C-8), 31.00 (C-9), 25.99 (C-10), 24.90 (C-11) ppm.
UV/Vis: λmax = 248 (π→π*), 289 (n→π*), 470 (MLCT) nm. FTIR (ATR,
(C-8,8′), 30.90 (C-9,9′), 26.01 (C-10), 24.92 (C-11) ppm. UV/Vis: λmax
=
248 (π→π*), 307 (n→π*), 521 (MLCT) nm. FTIR (ATR, neat): ν = 1615
˜
(C=N), 1051 (B–F) cm–1. MS (ESI, + mode): m/z = 714 [M – (BF4)4 +
H]3+. C120H136B4Cu4F16N22·2CH2Cl2 (2657.77): calcd. C 57.93; 5.51, N
12.39; found C 57.70, H 5.46, N 12.11.
neat): ν = 1620 (C=N), 1030 (B–F) cm–1. MS (ESI, + mode): m/z =
˜
X-ray Crystal Structure Determination: A single-crystal of com-
plex C2 was mounted on a nylon loop and centered in a stream of
cold nitrogen at 100(2) K. Crystal evaluation and data collection
were performed with a Bruker–Nonius SMART Apex II CCD diffrac-
tometer with Mo-Kα radiation (λ = 0.71073 Å). Data collection, re-
duction, and refinement were performed by using SAINT[25] and
SADABS,[26] which form part of the APEX II software package. The
structures were solved by direct methods and refined by full-matrix
least-squares on F2 using SHELX-97[27] within the X-Seed graphic
user interface.[28] All non-hydrogen atoms were refined anisotropi-
cally and all hydrogen atoms were located using calculated posi-
tions and riding models.
557 [M – (BF4)2]2+. C56H60B2Cu2F8N10 (1173.84): calcd. C 57.30, H
5.15, N 11.93; found C 57.01, H 4.98, N 11.72.
[DAB–G2–PPI-(Cu{(6-Me-C5H3N)-κ2-N,N)}8)54][BF4]4 (DC5): Pre-
pared according to the procedure outlined for complex DC1, em-
ploying ligand DL5 as reagent with a reaction time of 24 h. Isolated
as a deep-red amorphous solid, yield 141 mg, 89 %, m.p. 166–
169 °C. 1H NMR (400 MHz, [D6]DMSO): δ = 8.87 (s, 8 H, 1-H), 8.08
(br., 8 H, 3-H), 7.85 (br., 8 H, 4-H), 7.67 (br., 8 H, 5-H), 3.90 (br., 24 H,
7,7′-H), 2.20 (s, 24 H, CH3), 1.58–3.63 (br. signals, 56 H, H8,8′,9,9′
,10,11-H) ppm. 13C{1H} NMR (150 MHz, [D6]DMSO): δ = 162.32 (C-1),
157.64 (C-2), 149.77 (C-6), 138.68 (C-3), 127.86 (C-5), 124.52 (C-4),
57.51 (C-7,7′), 56.62 (C-8,8′), 54.14 (C-9,9′), 50.78 (C-10), 47.07 (C- CCDC 1449644 (for C2) contain the supplementary crystallographic
11), 24.00 (CH3) ppm. UV/Vis: λmax = 247 (π→π*), 284 (n→π*), 470 data for this paper. These data can be obtained free of charge from
(MLCT) nm. FTIR (ATR, neat): ν = 1624 (C=N), 1032 (B–F) cm–1
.
.
The Cambridge Crystallographic Data Centre.
˜
MS (ESI,
+
mode): m/z
=
734 [M
–
(BF4)4
+
H]3+
Catalytic Hydroxylation of 4-Iodotoluene to p-Cresol: A mixture
of 4-iodotoluene (1.0 mmol), CuI catalysts (4, 2 or 1 mol-%), and
NaOH (4.0 mmol) was stirred in DMSO/H2O (1:1, total volume 3 mL)
solution at 120 °C for the desired reaction time. The reaction mix-
ture was cooled to room temperature and carefully acidified with
C96H136B4Cu4F16N22·0.5CH2Cl2 (2242.12): calcd. C 51.69, H 6.16, N
13.74; found C 51.41, H 5.97, N 13.53.
[DAB–G2–PPI-(Cu{(C5H4N)-κ2-N,N)}8)4][BF4]4 (DC6): Prepared ac-
cording to the procedure outlined for complex DC1, employing li-
gand DL6 as reagent with a reaction time of 24 h. Isolated as a
brown amorphous solid, yield 120 mg, 80 %, m.p. 155–159 °C. 1H
NMR (400 MHz, [D6]DMSO): δ = 8.88 (br., 8 H, 6-H), 8.53 (br., 8 H, 1-
H), 8.20 (br., 8 H, 3-H), 8.03 (m, 8 H, 4-H), 7.75 (m, 8 H, 5-H), 3.75
(br., 24 H, 7,7′-H), 1.68–2.65 (br. signals, 56 H, 8,8′,9,9′,10,11-H) ppm.
13C{1H} NMR (150 MHz, [D6]DMSO): δ = 161.87 (C-1), 150.41 (C-2),
149.21 (C-4), 138.77 (C-6), 128.40 (C-3), 127.02(C-5), 64.16 (C-7,7′),
57.59 (C-8,8′), 46.58 (C-9,9′), 36.99 (C-10), 24.82 (C-11) ppm. MS (ESI,
+ mode): m/z = 871 [M – (BF4)4 + H]2+. UV/Vis: λmax = 248 (π→π*),
0.1
M HCl solution (30–40 mL). The acidified solution was extracted
with EtOAc (10 mL) and the organic layer separated. An aliquot
(3 mL) was dried with MgSO4, filtered, and a sample (1 mL) analyzed
by GC–MS with 1,4-dioxane as internal standard.
Hydroxylation in Neat Water and Recycling Experiments: A mix-
ture of 4-iodotoluene (1.0 mmol), DC6 (1 mol-%, 20.8 mg), and NaOH
(4.0 mmol) was stirred in neat water (total volume 3 mL) at 100 °C
for 1 h. The reaction mixture was cooled to room temperature and
extracted with EtOAc (10 mL). The EtOAc layer was separated and
dried with MgSO4. An aliquot (5 mL) was filtered and a sample (1 mL)
analyzed by GC–MS with 1,4-dioxane as internal standard.
277 (n→π*), 470 (MLCT) nm. FTIR (ATR, neat): ν = 1622 (C=N), 1032
˜
(B–F) cm–1. C88H120B4Cu4F16N22·0.5CH2Cl2 (2129.91): calcd. C 49.91,
H 5.73, N 14.47; found C 49.68, H 5.55, N 14.20.
Supporting Information (see footnote on the first page of this
article): FTIR, NMR, MS (ESI), and UV/Vis spectra, GC–MS chromato-
grams, and HR-TEM images of representative samples.
[DAB–G2–PPI-(Cu{(6-Br-C5H3N2)-κ2-N,N)}8)4][BF4]4 (DC7): Pre-
pared according to the procedure outlined for complex DC1, em-
ploying ligand DL7 as reagent with a reaction time of 24 h. Isolated
as a maroon amorphous solid, yield 170 mg, 87 %, m.p. 171–173 °C.
1H NMR (400 MHz, [D6]DMSO): δ = 8.85 (br., 8 H, 1-H), 8.11 (br., 8
H, 3-H), 8.02 (br., 8 H, 4-H), 7.96 (br., 8 H, 5-H), 3.95 (br., 24 H, 7,7′-
H), 1.70–2.72 (br. signals, 56 H, 8,8′,9,9′10,11-H) ppm. 13C{1H} NMR
(150 MHz, [D6]DMSO): δ = 161.46 (C-1), 151.63 (C-2), 141.48 (C-6),
132.24 (C-4), 126.22 (C-5), 121.59 (C-3), 57.68 (C-7,7′), 50.82 (C-8,8′),
47.58 (C-9,9′), 30.71 (C-10), 28.19 (C-11) ppm. UV/Vis: λmax = 230
Acknowledgments
The authors acknowledge the financial support by SASOL, Stel-
lenbosch University, South Africa and North-West University,
Potchefstroom, South Africa. The Focus Area for Chemical Re-
source Beneficiation and the Catalysis and Synthesis Research
Group is acknowledged for infrastructure support.
(π→π*), 288 (n→π*), 470 (MLCT) nm. FTIR (ATR, neat): ν = 1617 (C=
˜
N), 1031 (B–F) cm–1. MS (ESI, + mode): m/z 906 [M – (BF4)4 + H]3+
.
Keywords: Homogeneous catalysis · Hydroxylation ·
Nanoparticles · Dendrimers · Copper
C88H112B4Br8Cu4F16N22·CH2Cl2 (2803.54): calcd. C 38.13, H 4.10, N
10.99; found C 37.85, H 3.98, N 11.20.
Eur. J. Inorg. Chem. 2016, 3781–3790
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© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
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Received: April 11, 2016
Published Online: July 18, 2016
Eur. J. Inorg. Chem. 2016, 3781–3790
www.eurjic.org
3790
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim