Copper(II) complexes of piperazine-derived tetradentate ligands and their chiral diazabicyclic analogues for catalytic phenol oxidative C-C coupling

Article abstract of DOI:10.1016/j.inoche.2013.10.002

Reaction of the chiral ligands (1S,4S)-2,5-bis(6-methylpyridyl)- diazabicyclo[2.2.1]heptane (L¹) and (1S,4S)-2,5-bis(1-methyl-2- methylbenzimidazolyl)-diazabicyclo[2.2.1]heptane (L²) with CuCl ₂ results in the hydroxo-bridged dicopper complexes [(L ¹)Cu₂(μ-OH)(H₂O)Cl₃] (3), and [(L²)Cu₂(μ-OH)(H₂O)Cl₃] (4). Both chiral complexes were characterized spectroscopically, and 3 in the solid state by X-ray crystallography, confirming they are structurally related to their previously reported copper acetate analogues (1 and 2) due to their hydroxo-bridged bimetallic core. The achiral ligand analogues N,N′-bis(2-picolyl)piperazine (L³) and N,N′-bis(1-methyl- 2-methylbenzimidazolyl)piperazine (L⁴) were employed to obtain the corresponding complexes with CuCl₂, affording the chloro-bridged [(L³)(CuCl)₂(μ-Cl)₂]_n (5) and [(L⁴)(CuCl)₂(μ-Cl)₂] (6), neither of which features a bridging hydroxo ligand; instead, complex 5 was structurally characterized as a coordination polymer. The acetate-derived complexes 1 and 2 are active in oxidative C-C coupling of 2,4-di-tert-butylphenol, while 3 and 4 have low activity; the achiral complexes 5 and 6, lacking a bridging hydroxo ligand, are inactive in this reaction.

Full text of DOI:10.1016/j.inoche.2013.10.002

Inorganic Chemistry Communications 38 (2013) 1–4
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Copper(II) complexes of piperazine-derived tetradentate ligands and
their chiral diazabicyclic analogues for catalytic phenol oxidative
C–C coupling
a
b
b
b
Ivan Castillo a, , Viridiana Pérez , Iván Monsalvo , P. Demare , Ignacio Regla
⁎
a
Instituto de Química, Universidad Nacional Autónoma de México, Circuito Exterior, Ciudad Universitaria, México, D. F. 04510, México
Facultad de Estudios Superiores Zaragoza, Universidad Nacional Autónoma de México, Batalla del 5 de Mayo y Fuerte de Loreto, México, D. F. 09230, México
b
a r t i c l e i n f o
a b s t r a c t
Reaction of the chiral ligands (1S,4S)-2,5-bis(6-methylpyridyl)-diazabicyclo[2.2.1]heptane (L1) and
(1S,4S)-2,5-bis(1-methyl-2-methylbenzimidazolyl)-diazabicyclo[2.2.1]heptane (L2) with CuCl2 results
in the hydroxo-bridged dicopper complexes [(L1)Cu2(μ-OH)(H2O)Cl3] (3), and [(L2)Cu2(μ-OH)(H2O)Cl3]
(4). Both chiral complexes were characterized spectroscopically, and 3 in the solid state by X-ray crystallography,
confirming they are structurally related to their previously reported copper acetate analogues (1 and 2) due
to their hydroxo-bridged bimetallic core. The achiral ligand analogues N,N′-bis(2-picolyl)piperazine (L3) and
N,N′-bis(1-methyl-2-methylbenzimidazolyl)piperazine (L4) were employed to obtain the corresponding com-
plexes with CuCl2, affording the chloro-bridged [(L3)(CuCl)2(μ-Cl)2]n (5) and [(L4)(CuCl)2(μ-Cl)2] (6), neither of
which features a bridging hydroxo ligand; instead, complex 5 was structurally characterized as a coordination poly-
mer. The acetate-derived complexes 1 and 2 are active in oxidative C–C coupling of 2,4-di-tert-butylphenol, while 3
and 4 have low activity; the achiral complexes 5 and 6, lacking a bridging hydroxo ligand, are inactive in
this reaction.
Article history:
Received 24 August 2013
Accepted 2 October 2013
Available online 11 October 2013
Keywords:
Copper
Piperazine
Heterocycles
Chiral ligands
Oxidative coupling
© 2013 Elsevier B.V. All rights reserved.
Recent advances in the development of copper complexes that carry
out selective oxidation reactions draw inspiration from the coordination
environment in the active sites of various metalloenzymes. These in-
clude the dicopper tyrosinase [1], chatecol oxidase [2], and to a lesser
extent methane monooxygenase [3]. Histidine as N-donor is ubiquitous
in such non-heme metalloenzymes, which has led to the design and
synthesis of nitrogen-rich ligands featuring pyridyl [4], pyrazolyl [5],
imidazolyl [6], and bezimidazolyl groups [7]. Novel approaches for the
development of catalysts inspired on the aforementioned enzymes
have recently relied on the use of chelating ligands that provide a suffi-
ciently preorganized coordination environment [8], which can be
further tuned by the electronic properties of the donor groups to
harness the synergistic reactivity of the bimetallic sites.
In this context, we recently reported the synthesis of novel
binucleating ligands with pyridyl and benzimidazolyl groups
appended to the nitrogen atoms of the enantiomerically pure
(1S,4S)-2,5-diazabicyclo[2.2.1]heptane, resulting in the tetradentate
ligands L1 and L2 (Scheme 1) [9]. Both compounds give rise to chiral
hydroxo-bridged dicopper complexes irrespective of the amount of
copper(II) acetate employed in the reactions, with Cu2+ ions at a
distance of approximately 3.7 Å. We herein extend this work to the
analogous hydroxo-bridged dicopper complexes obtained from
copper(II) chloride, as well as the achiral versions of the ligands
bispicolyl- and bis(2-methylbenzimidazolyl)piperazine derivatives
L3 and L4, differing only in the bridgehead methylene group of the
diazabicycle. The latter compounds were also employed for the
preparation of the corresponding dicopper complexes with CuCl2.
The combined results of their competence as catalysts for oxidative
phenol coupling reactions are presented.
Recently, we reported the synthesis and characterization of the
products of the reactions between L1 and L2, and copper(II) acetate. In
both cases dicopper complexes [(L2)Cu2(μ-OH)(H2O)(OAc)2Cl] (1)
and [(L1)Cu2(μ-OH)(H2O)(MeOH)(OAc)2]OAc (2) were obtained [9],
even when only one equiv. of Cu2+ was employed in the reactions,
indicating that these ligands favor the formation of hydroxo-bridged
bimetallic species. This was extended to CuCl2 as a source of Cu2+ ions
to gauge the effect of the chloride counterions in the formation of the
dinuclear complexes. Replacement of acetate for chloride resulted also
in the formation of bimetallic complexes [(L1)Cu2(μ-OH)(H2O)Cl3] (3)
and [(L2)Cu2(μ-OH)(H2O)Cl3] (4), as determined in acetonitrile solution
by ESI mass spectrometry: for 3, a peak at m/z = 495 consistent with
the cation [(L1)Cu2(μ-OH)Cl2]+ was detected, while a similar species
at m/z=601 assigned to [(L2)Cu2(μ-OH)Cl2]+ was present in solutions
of 4; these bimetallic formulations are also consistent with combustion
analysis [10]. Further characterization included IR spectroscopy, which
⁎
Corresponding author. Tel.: +52 55 56224449; fax: +52 55 56162217.
E-mail address: joseivan@unam.mx (I. Castillo).
1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2013.10.002

2
I. Castillo et al. / Inorganic Chemistry Communications 38 (2013) 1–4
Fig. 1. Mercury diagram of 3 at the 50% probability level; hydrogen atoms and two
molecules of water are omitted for clarity. Selected bond lengths (Å), and angles (°):
Cu1–O1 1.918(2), Cu2–O1 1.937(2), Cu1–Cl1 2.298(1), Cu1–O2 2.200(2), Cu1–N1
2.015(2), Cu1–N16 2.091(2), Cu2–Cl2 2.529(1), Cu2–Cl3 2.295(1), Cu2–N8 2.025(2),
Cu2–N19 2.084(2); Cu1–O1–Cu2 148.61(11), O1–Cu1–N1 171.24(9), O1–Cu1–N16
90.09(8), O1–Cu1–O2 97.46(8), O1–Cu1–Cl1 89.68(6), N1–Cu1–N16 82.39(9), N1–Cu1–
O2 87.59(8), N1–Cu1–Cl1 95.77(6), N16–Cu1–O2 94.05(8), N16–Cu1–Cl1 159.42(6),
O2–Cu1–Cl1 106.38(6), O1–Cu2–N8 167.11(8), O1–Cu2–N19 89.99(8), O1–Cu2–Cl2
96.86(6), O1–Cu2–Cl3 90.23(6), N8–Cu2–N19 81.03(8), N8–Cu2–Cl2 93.70(6), N8–Cu2–
Cl3 94.50(6), N19–Cu2–Cl2 99.22(6), N19–Cu2–Cl3 157.27(6), Cl2–Cu2–Cl3 103.32(2).
Scheme 1. Ligands employed in this work.
is not as informative as for 1 and 2 due to the lack of acetate ligands, but
nevertheless confirmed the presence of the ligands with a pyridyl C=N
band at 1474cm−1 for 3, and a benzimidazolyl C=N band at 1617cm−1
for 4. Both complexes are EPR silent at room temperature and 77K ex-
cept for small signals that were attributed to paramagnetic (monomer-
ic) impurities; this is consistent with antiferromagnetic coupling
between the two S=½ Cu2+ ions. Ultimately, complex 3 was character-
ized in the solid state by X-ray crystallography, confirming its bimetallic
nature. Single crystals were obtained in the chiral space group P21 from
a concentrated MeOH/H2O solution by slow evaporation, with one mol-
ecule of 3 and two molecules of water present in the asymmetric unit.
The symmetry of the ligand is reflected in the complex, which has a
pseudo-C2 axis passing through the bridgehead C21 and the hydroxo
O1 atoms; lack of a strict C2-symmetry is due to the presence of one
molecule of water as ligand towards Cu2 instead of the anionic chloride
bound to Cu1 (Fig. 1). The coordination geometry around each cupric
ion is square pyramidal with a slight distortion; in the case of Cu1, the
square pyramid is defined by N1, N16, O1, and Cl1 as basal ligands,
and a molecule of water (O2) in the axial position. Cu2 has a similar co-
ordination environment defined by N8, N19, O1, and Cl3 in basal posi-
tions, and Cl2 in the axial position [11].
Introduction of 2-methylpyridyl and 1-methyl-2methylben
zimidazolyl groups to (1S,4S)-2,5-diazabicyclo[2.2.1]heptane was
reported in a biphasic CH2Cl2/H2O system to afford L1 and L2 in
good yields (Scheme 1). Replacement of the diazabicycle with
piperazine in the above procedure resulted in the achiral analogues L3
and L4 [12]. Initial characterization by 1H NMR spectroscopy confirmed
the presence of the signals corresponding to the piperazine framework,
with a broad singlet at δ 2.47 and 2.54 ppm for L3 and L4, respectively.
Singlets consistent with methylene groups were observed at δ 3.57
and 3.81 ppm, while an additional signal at 3.86 ppm was attributed
to the benzimidazole N-methyl resonance of L4; in contrast, the methy-
lene signals of the pendant 2-pyridyl and 2-benzimidazolyl groups of
chiral L1 and L2 give rise to diastereotopic signals with geminal coupling
(J=14.30 and 13.35Hz). Finally, the aromatic signals of the pyridyl and
benzimidazolyl groups were observed between δ 7.01–8.45 and 7.25–
7.76 ppm for L3 and L4. Electron-ionization mass spectrometry and IR
spectroscopy confirmed the identity of the achiral ligands. The former
technique revealed peaks at m/z=268 and 374; the latter is character-
ized by a pyridyl C=N band at 1484cm−1 for L3, and a benzimidazolyl
C = N band at 1612 cm−1 for L4.
in H2O for 5 and DMSO for 6 due to their poor solubility in less polar
organic solvents. Partial solvolysis of the complexes may occur under
these conditions, giving rise to the observed spectra consistent with
isolated Cu2+ centers in axial environments (g = 2.07 and g|| = 2.24
for 5; g = 2.20 and g|| = 2.04 for 6). This observation is indicative of
oligomeric or polymeric structures in the solid state, as confirmed
for 5 by X-ray crystallography. Slow evaporation of a concentrated
MeOH/H2O solution gave rise to single crystals that revealed a polymer-
ic structure [(L3)(CuCl)2(μ-Cl)2]n. Complex 5 crystallizes in the triclinic
space group P-1, with one half of the molecule generated by the center
of symmetry. The piperazine backbone adopts a chair conformation,
resulting in a distal arrangement of pyridne fragments, with N1 and
the symmetry-related N1* donor atoms facing in opposite directions
(Fig. 2). This geometric arrangement gives rise to a complex where
two cupric ions are bound to two bidentate clefts on opposite sides
of the piperazine, with a Cu…Cu distance of 6.886(1) Å that does not
accommodate a bridging hydroxo ligand as observed for the chiral
analogues 1–4. Bridging interactions through Cl2 are present, generat-
ing a coordination polymer as depicted in Fig. 2 that results in
pentacoordinate Cu2+ ions bound to the pyridine N1, N8, the terminal
Cl1 ligand, and the bridging Cl2 and symmetry-related Cl2* ligands;
in the polymer the Cu–Cu distance is 3.568(1) Å. The coordination
geometry around Cu1 is best described as a distorted square pyramid
(τ = 0.31) [14].
Oxidative phenol coupling. 2,4-di-tert-butylphenol has been exten-
sively used as a model substrate in oxidative C–C coupling reactions.
This served as a test of the reactivity of the bimetallic complexes 1–6.
A general procedure was applied for all cupric complexes as potential
catalysts: 5 or 10% mol of the complexes was added to 1 mmol of the
phenol in 15 mL of acetonitrile; compressed air was bubbled through
the solutions for several hours, and the progress was monitored by
TLC (Scheme 2). Acetate-containing complexes 1 and 2 were the most
active, with phenol consumed after 4h; 3 and 4 required longer reaction
times, and not all of the phenol was consumed even after bubbling with
pure O2 for 4h; finally, the achiral compexes 5 and 6 failed to react with
the phenol. In all cases the isolated product corresponded to the
biphenol in Scheme 2, which forms by oxidative C–C coupling at the
available phenolic ortho-C–H bond; the biphenol was characterized by
1H NMR spectroscopy and IE-MS. Apparently, the hydroxo-bridged
bimetallic core is required for oxidative C–C coupling to occur in these
systems, highlighting the importance of not only the chiral nature
of L1 and L2, but also their capability to induce dicopper complex forma-
tion. Initial reactivity tests for the oxidative C–C coupling of 2-naphthol
A contrasting behavior of the piperazine-derived ligands L3 and L4
was initially inferred from ESI-MS measurements in the reactions with
2 equivs. of CuCl2. The ethanolic mixtures gave rise apparently to mono-
metallic complexes detected at [(L3)CuCl]+ at m/z = 366, and [(L4)
CuCl]+ at m/z=472. Nonetheless, the isolated complexes were formu-
lated as the bimetallic species [(L3)(CuCl2)2] (5) and [(L4)(CuCl2)2] (6)
based on combustion analysis [13]. Solution EPR data were obtained

I. Castillo et al. / Inorganic Chemistry Communications 38 (2013) 1–4
3
Fig. 2. Mercury diagram of a fragment of polymeric 5 at the 50% probability level; hydrogen atoms are omitted for clarity. Selected bond lengths (Å), and angles (°): Cu1–N1 2.019(3), Cu1–N8
2.082(3), Cu1–Cl1 2.245(1), Cu1–Cl2 2.301(1), Cu1–Cl2* 2.670(1); Cu1–Cl2–Cu1* 91.4(1), N1–Cu1–N8 80.3(1), N1–Cu1–Cl1 154.8(1), N1–Cu1–Cl2 93.4(1), N1–Cu1–Cl2* 99.5(1), N8–Cu1–Cl1
154.8(1), N8–Cu1–Cl2 173.2(1), N8–Cu2–Cl2* 89.9(1), Cl1–Cu1–Cl2 94.2(1), Cl1–Cu1–Cl2* 104.7(1), Cl2–Cu1–Cl2* 88.6(1).
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Scheme 2. Oxidative C–C coupling.
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with the chiral complexes 1 and 2 result in incomplete conversion to a
[10] Synthesis of complex (3): L1 (105 mg, 0.38 mmol) was dissolved in 20mL of ethanol
dark brown material that appears to be oligomeric in nature due
and CuCl2•2H2O was added (128 mg, 0.75 mmol). After stirring for 3 h volatile
materials were evaporated under reduced pressure and the green solid was triturated
to the broad resonances observed in 1H NMR spectra of the isolated
with diethylether (10 mL); washing with further diethylether resulted in pale green
microcrystals of 3 (180 mg) in 86% yield. Mp N 250 °C (dec. 215 °C). IR (KBr): 3473,
3106, 3075, 3040, 3016, 2992, 2935, 1607, 1572, 1474, 1446, 1376, 1313, 1284,
product. Intriguingly, chloroform solutions are optically active, hinting
at an asymmetric catalytic C–C coupling process. This possibility is
currently being explored in our laboratory, and the results will be
informed in due course.
1258, 1213, 1156, 1107, 1056, 1018, 944, 845, 810, 773, 724, 649, 609, 564, 525,
471, 434 cm−1. Anal. Calcd for C17H23Cl3Cu2N4O2: C, 37.20; H, 4.22; N, 10.21. Found:
C, 37.17; H, 3.77; N, 9.76. [α]2D5 = −225. Complex (4): Analogous procedure with L2
(140 mg, 0.36 mmol) and CuCl2•2H2O (123 mg, 0.72 mmol) afforded 4 as pale green
microcrystals (218 mg) in 92% yield. Mp N 250 °C (dec. 160 °C). IR (KBr): 3430, 3098,
3065, 3026, 2997, 2946, 1617, 1506, 1457, 1429, 1368, 1327, 1295, 1248, 1214, 1122,
1065, 1011, 933, 831, 752, 698, 575, 546, 431 cm−1. Anal. Calcd for C23H35Cl3Cu2N6O5:
C, 38.96; H, 4.98; N, 11.85. Found: C, 39.29; H, 4.54; N, 11.56. [α]2D5 = −208.
Acknowledgments
The authors thank María de la Paz Orta for combustion analysis,
Rubén A. Toscano for crystallographic work, Carmen Márquez, Eréndira
García Ríos and Javier Pérez for mass spectroscopic assistance, María
Gregoria Medina-Pintor from CIQ-UAEM for IR data, DGAPA-UNAM
(IN211509) for financial support, and Conacyt grant 101855 for partial
financial support.
[11] Crystallographic data: Crystals were mounted on a glass fiber for data collection on a
Bruker Smart X-ray diffractometer equipped with an Apex CCD area detector with
graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) out to θ = 25.37°.
The structures were solved by direct methods and refined by full-matrix least
squares on F2 with SHELXTL [15]. All non-hydrogen atoms were refined anisotrop-
ically, while hydrogen atoms were refined as riding. CCDC 957111 & 957112 contain
the supplementary crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Center via www.ccdc.cam.ac.
uk/data_request/cif. Data for 3: Mol. wt. = 584.86, monoclinic, space group P21,
a = 8.9494(11), b = 14.2560(17), c = 9.6659(12) Å, α = 90.00, β = 116.2840(10),
Appendix A. Supplementary material
γ = 90.00°, V = 1105.7(2) Å3. T = 123(2) K, Z = 2, Dcalc = 1.757 g cm−3
,
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.inoche.2013.10.002.
μ = 2.318 mm−1, 3922 independent reflections, final R1 [I ≥ 2σ(I)] = 0.0204,
wR(F2) = 0.0501, S = 1.041. Data for 5: Mol. wt. = 267.61, triclinic, space group P-1,
a = 7.5827(13), b = 7.8437(13), c = 9.3439(16) Å, α = 91.844(3), β = 113.625(2),
γ = 101.213(2)°, V = 495.68(2) Å3. T = 298(2) K, Z = 2, Dcalc = 1.793 g cm−3
,
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wR(F2) = 0.0766, S = 0.972.
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1343, 1163, 1132, 1014, 923, 895, 839, 755, 726, 599 cm−1. 1H NMR (CDCl3,
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Py), 7.30 (d, 3 J = 7.8 Hz, 2 H, Py), 7.03 (m, 2 H, Py), 3.57 (s, 4 H, NCH2),
2.47 (s, 8 H, CH2) ppm. 13C{1H} NMR (CDCl3, 75 MHz, 20 °C): d 158.5 (Py),
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4
I. Castillo et al. / Inorganic Chemistry Communications 38 (2013) 1–4
off-white crystals. Yield: 1.60 g (80%). Mp 233–237 °C. IR (KBr): 3355, 3241,
1367, 1307, 1277, 1238, 1156, 1112, 1049, 1019, 908, 821, 770, 726, 682, 625,
496, 455, 417, 378, 308 cm−1. Anal. Calcd for C16H20Cl4Cu2N4O (5•H2O): C, 34.61;
H, 3.99; N, 10.09. Found: C, 34.93; H, 3.86; N, 10.02. Complex (6): Analogous procedure
with L4 (220mg, 0.59mmol) and CuCl2•2H2O (201mg, 1.18mmol) afforded 359mg of
6 as a pale green solid (95% yield). Single crystals were obtained from a concentrated
MeOH/H2O solution by slow evaporation. Mp N 250 °C (dec. 246 °C). IR (KBr): 3295,
3060, 3028, 2947, 2925, 2857, 1614, 1506, 1482, 1460, 1426, 1388, 1332, 1302,
1276, 1251, 1227, 1127, 1084, 1047, 998, 952, 830, 749, 705, 642, 524, 438, 403,
282 cm−1. Anal. Calcd for C22H37Cl4Cu2N6O5.5 (6•5.5H2O): C, 35.59; H, 5.02; N, 11.32.
Found: C, 35.93; H, 4.54; N, 10.80.
3050, 2943, 2915, 2817, 1612, 1475, 1453, 1401, 1332, 1298, 1238, 1143,
1126, 1000, 769, 750, 681, 622, 580 cm−1. 1H NMR (CDCl3, 300 MHz, 20 °C):
d 7.75 (d, 3 J = 7.2 Hz, 2 H, BzIm), 7.34 (m, 2 H, BzIm), 7.30 (m, 4 H, BzIm), 3.86 (s,
6 H, NCH3), 3.81 (s, 4 H, NCH2), 2.54 (s, 8 H, CH2) ppm. 13C{1H} NMR (CDCl3,
75 MHz, 20 °C): d 151.2 (BzIm), 142.1 (BzIm), 136.4 (BzIm), 122.7 (BzIm), 122.0
(BzIm), 119.6 (BzIm), 109.3 (BzIm), 55.6 (NCH2), 53.2 (NCH3), 30.3 (CH2) ppm.
EI-MS: m/z = 374 [L4] + (5%), 229 [L3-(1-Me-2–CH2BzIm)] + (65%).
[13] Synthesis of complex (5): L3 (200 mg, 0.75 mmol) was dissolved in 20mL of ethanol
and CuCl2•2H2O was added (254 mg, 1.49 mmol). After stirring for 3 h volatile
materials were evaporated under reduced pressure and the turquoise oily residue
triturated with diethylether (10 mL); washing with further diethylether resulted
in pale green microcrystals of 5 (304 mg) in 75% yield. Mp N 250 °C (dec. 234 °C).
IR (KBr): 3459, 3105, 3071, 3042, 2995, 2971, 2934, 1703, 1610, 1573, 1475, 1445,
[14] A.W. Addison, T.N. Rao, J. Reedijk, J. van Rijn, G.C. Verschoor, J. Chem. Soc. Dalton
Trans. (1984) 1349–1356.
[15] G.M. Sheldrick, SHELXTL, v. 6.10, University of Göttingen, distributed by Bruker-AXS
Inc., Madison, WI, 2000.