Inorganic Chemistry Communications 38 (2013) 1–4

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Inorganic Chemistry Communications

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Copper(II) complexes of piperazine-derived tetradentate ligands andtheir chiral diazabicyclic analogues for catalytic phenol oxidativeC–C coupling

Ivan Castillo a,⁎, Viridiana Pérez a, Iván Monsalvo b, P. Demare b, Ignacio Regla b

a Instituto de Química, Universidad Nacional Autónoma de México, Circuito Exterior, Ciudad Universitaria, México, D. F. 04510, Méxicob Facultad de Estudios Superiores Zaragoza, Universidad Nacional Autónoma de México, Batalla del 5 de Mayo y Fuerte de Loreto, México, D. F. 09230, México

⁎ Corresponding author. Tel.: +52 55 56224449; fax: +E-mail address: joseivan@unam.mx (I. Castillo).

1387-7003/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.inoche.2013.10.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 24 August 2013Accepted 2 October 2013Available online 11 October 2013

Keywords:CopperPiperazineHeterocyclesChiral ligandsOxidative coupling

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 resultsin the hydroxo-bridged dicopper complexes [(L1)Cu2(μ-OH)(H2O)Cl3] (3), and [(L2)Cu2(μ-OH)(H2O)Cl3](4). Both chiral complexeswere characterized spectroscopically, and 3 in the solid state byX-ray crystallography,confirming they are structurally related to their previously reported copper acetate analogues (1 and 2) dueto their hydroxo-bridged bimetallic core. The achiral ligand analogues N,N′-bis(2-picolyl)piperazine (L3) andN,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 ofwhich features a bridging hydroxo ligand; instead, complex 5was 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 3and 4 have low activity; the achiral complexes 5 and 6, lacking a bridging hydroxo ligand, are inactive inthis reaction.

© 2013 Elsevier B.V. All rights reserved.

Recent advances in the development of copper complexes that carryout selective oxidation reactions draw inspiration from the coordinationenvironment in the active sites of various metalloenzymes. These in-clude the dicopper tyrosinase [1], chatecol oxidase [2], and to a lesserextent methanemonooxygenase [3]. Histidine asN-donor is ubiquitousin such non-heme metalloenzymes, which has led to the design andsynthesis of nitrogen-rich ligands featuring pyridyl [4], pyrazolyl [5],imidazolyl [6], and bezimidazolyl groups [7]. Novel approaches for thedevelopment of catalysts inspired on the aforementioned enzymeshave recently relied on the use of chelating ligands that provide a suffi-ciently preorganized coordination environment [8], which can befurther tuned by the electronic properties of the donor groups toharness the synergistic reactivity of the bimetallic sites.

In this context, we recently reported the synthesis of novelbinucleating ligands with pyridyl and benzimidazolyl groupsappended to the nitrogen atoms of the enantiomerically pure(1S,4S)-2,5-diazabicyclo[2.2.1]heptane, resulting in the tetradentateligands L1 and L2 (Scheme 1) [9]. Both compounds give rise to chiralhydroxo-bridged dicopper complexes irrespective of the amount ofcopper(II) acetate employed in the reactions, with Cu2+ ions at adistance of approximately 3.7 Å. We herein extend this work to the

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analogous hydroxo-bridged dicopper complexes obtained fromcopper(II) chloride, as well as the achiral versions of the ligandsbispicolyl- and bis(2-methylbenzimidazolyl)piperazine derivativesL3 and L4, differing only in the bridgehead methylene group of thediazabicycle. The latter compounds were also employed for thepreparation of the corresponding dicopper complexes with CuCl2.The combined results of their competence as catalysts for oxidativephenol coupling reactions are presented.

Recently, we reported the synthesis and characterization of theproducts of the reactions between L1 and L2, and copper(II) acetate. Inboth 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-bridgedbimetallic species. This was extended to CuCl2 as a source of Cu2+ ionsto gauge the effect of the chloride counterions in the formation of thedinuclear complexes. Replacement of acetate for chloride resulted alsoin the formation of bimetallic complexes [(L1)Cu2(μ-OH)(H2O)Cl3] (3)and [(L2)Cu2(μ-OH)(H2O)Cl3] (4), as determined in acetonitrile solutionby ESI mass spectrometry: for 3, a peak at m/z= 495 consistent withthe cation [(L1)Cu2(μ-OH)Cl2]+ was detected, while a similar speciesat m/z=601 assigned to [(L2)Cu2(μ-OH)Cl2]+ was present in solutionsof 4; these bimetallic formulations are also consistent with combustionanalysis [10]. Further characterization included IR spectroscopy, which

Scheme 1. Ligands employed in this work.

Fig. 1. Mercury diagram of 3 at the 50% probability level; hydrogen atoms and twomolecules 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–N12.015(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–N1690.09(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–Cl296.86(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).

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is not as informative as for 1 and 2 due to the lack of acetate ligands, butnevertheless confirmed the presence of the ligandswith a pyridyl C=Nband at 1474cm−1 for 3, and a benzimidazolyl C=Nband 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 couplingbetween the two S=½Cu2+ ions. Ultimately, complex 3was character-ized in the solid state by X-ray crystallography, confirming its bimetallicnature. Single crystals were obtained in the chiral space group P21 froma concentratedMeOH/H2O solution by slow evaporation, with onemol-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 apseudo-C2 axis passing through the bridgehead C21 and the hydroxoO1 atoms; lack of a strict C2-symmetry is due to the presence of onemolecule of water as ligand towards Cu2 instead of the anionic chloridebound to Cu1 (Fig. 1). The coordination geometry around each cupricion is square pyramidal with a slight distortion; in the case of Cu1, thesquare 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-2methylbenzimidazolyl groups to (1S,4S)-2,5-diazabicyclo[2.2.1]heptane wasreported in a biphasic CH2Cl2/H2O system to afford L1 and L2 ingood yields (Scheme 1). Replacement of the diazabicycle withpiperazine in the above procedure resulted in the achiral analogues L3

and L4 [12]. Initial characterization by 1H NMR spectroscopy confirmedthe presence of the signals corresponding to the piperazine framework,with a broad singlet at δ 2.47 and 2.54ppm for L3 and L4, respectively.Singlets consistent with methylene groups were observed at δ 3.57and 3.81 ppm, while an additional signal at 3.86 ppm was attributedto the benzimidazole N-methyl resonance of L4; in contrast, the methy-lene signals of the pendant 2-pyridyl and 2-benzimidazolyl groups ofchiral L1 and L2 give rise to diastereotopic signals with geminal coupling(J=14.30 and 13.35Hz). Finally, the aromatic signals of the pyridyl andbenzimidazolyl groups were observed between δ 7.01–8.45 and 7.25–7.76 ppm for L3 and L4. Electron-ionization mass spectrometry and IRspectroscopy confirmed the identity of the achiral ligands. The formertechnique 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 benzimidazolylC=N band at 1612 cm−1 for L4.

A contrasting behavior of the piperazine-derived ligands L3 and L4

was initially inferred from ESI-MS measurements in the reactions with2 equivs. of CuCl2. The ethanolicmixtures gave rise apparently tomono-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

in H2O for 5 and DMSO for 6 due to their poor solubility in less polarorganic solvents. Partial solvolysis of the complexes may occur underthese conditions, giving rise to the observed spectra consistent withisolated Cu2+ centers in axial environments (g = 2.07 and g|| = 2.24for 5; g = 2.20 and g|| = 2.04 for 6). This observation is indicative ofoligomeric or polymeric structures in the solid state, as confirmedfor 5 by X-ray crystallography. Slow evaporation of a concentratedMeOH/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 triclinicspace group P-1, with one half of the molecule generated by the centerof symmetry. The piperazine backbone adopts a chair conformation,resulting in a distal arrangement of pyridne fragments, with N1 andthe symmetry-related N1* donor atoms facing in opposite directions(Fig. 2). This geometric arrangement gives rise to a complex wheretwo cupric ions are bound to two bidentate clefts on opposite sidesof the piperazine, with a Cu…Cu distance of 6.886(1) Å that does notaccommodate a bridging hydroxo ligand as observed for the chiralanalogues 1–4. Bridging interactions through Cl2 are present, generat-ing a coordination polymer as depicted in Fig. 2 that results inpentacoordinate Cu2+ ions bound to the pyridine N1, N8, the terminalCl1 ligand, and the bridging Cl2 and symmetry-related Cl2* ligands;in the polymer the Cu–Cu distance is 3.568(1) Å. The coordinationgeometry 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 potentialcatalysts: 5 or 10% mol of the complexes was added to 1mmol of thephenol in 15 mL of acetonitrile; compressed air was bubbled throughthe solutions for several hours, and the progress was monitored byTLC (Scheme 2). Acetate-containing complexes 1 and 2 were the mostactive,with phenol consumed after 4h; 3 and 4 required longer reactiontimes, and not all of the phenol was consumed even after bubblingwithpure O2 for 4h; finally, the achiral compexes 5 and 6 failed to react withthe phenol. In all cases the isolated product corresponded to thebiphenol in Scheme 2, which forms by oxidative C–C coupling at theavailable phenolic ortho-C–H bond; the biphenol was characterized by1H NMR spectroscopy and IE-MS. Apparently, the hydroxo-bridgedbimetallic core is required for oxidative C–C coupling to occur in thesesystems, highlighting the importance of not only the chiral natureof 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

Scheme 2. Oxidative C–C coupling.

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–N82.082(3), Cu1–Cl12.245(1), Cu1–Cl2 2.301(1), Cu1–Cl2* 2.670(1); Cu1–Cl2–Cu1* 91.4(1),N1–Cu1–N880.3(1),N1–Cu1–Cl1 154.8(1),N1–Cu1–Cl2 93.4(1),N1–Cu1–Cl2* 99.5(1),N8–Cu1–Cl1154.8(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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with the chiral complexes 1 and 2 result in incomplete conversion to adark brown material that appears to be oligomeric in nature dueto the broad resonances observed in 1H NMR spectra of the isolatedproduct. Intriguingly, chloroform solutions are optically active, hintingat an asymmetric catalytic C–C coupling process. This possibility iscurrently being explored in our laboratory, and the results will beinformed in due course.

Acknowledgments

The authors thank María de la Paz Orta for combustion analysis,Rubén A. Toscano for crystallographic work, CarmenMárquez, EréndiraGarcía Ríos and Javier Pérez for mass spectroscopic assistance, MaríaGregoria Medina-Pintor from CIQ-UAEM for IR data, DGAPA-UNAM(IN211509) for financial support, and Conacyt grant 101855 for partialfinancial support.

Appendix A. Supplementary material

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.inoche.2013.10.002.

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[10] Synthesis of complex (3): L1 (105mg, 0.38mmol) was dissolved in 20mL of ethanoland CuCl2•2H2O was added (128 mg, 0.75 mmol). After stirring for 3 h volatilematerials were evaporated under reduced pressure and the green solidwas trituratedwith diethylether (10mL); washing with further diethylether resulted in pale greenmicrocrystals 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, 1284,1258, 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]D25=−225. Complex (4): Analogous procedure with L2

(140mg, 0.36mmol) and CuCl2•2H2O (123mg, 0.72mmol) afforded 4 as pale greenmicrocrystals (218mg) 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, 431cm−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]D25=−208.

[11] Crystallographic data: Crystals were mounted on a glass fiber for data collection on aBruker Smart X-ray diffractometer equipped with an Apex CCD area detector withgraphite-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 leastsquares on F2 with SHELXTL [15]. All non-hydrogen atoms were refined anisotrop-ically, while hydrogen atomswere refined as riding. CCDC 957111 & 957112 containthe supplementary crystallographic data for this paper. These data can be obtainedfree of charge fromThe Cambridge Crystallographic Data Center viawww.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),γ = 90.00°, V = 1105.7(2) Å3. T = 123(2) K, Z = 2, Dcalc = 1.757 g cm−3,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,μ = 2.692 mm−1, 1821 independent reflections, final R1 [I ≥ 2σ(I)] = 0.0360,wR(F2)=0.0766, S=0.972.

[12] Synthesis of N, N′-bis(2-picolyl)piperazine (L3): 2-Chloromethylpyridine hydro-chloride (1.90 g, 11.6 mmol), piperazine (0.50 g, 5.80 mmol), and NaOH (1.85 g,46.4 mmol) were placed in a 500 mL round bottom flask and dissolved in 15 mLof distilled water; after cooling to ca. 0 °C with an ice bath 15 mL of CH2Cl2 wasadded, and the biphasic mixture was allowed to warm to room temperature andstirred for 48 h. Progress was monitored by TLC on silica gel with CH2Cl2/MeOH(95:5) as mobile phase. Phases were separated and the aqueous layer extractedwith 3 × 10 mL CH2Cl2; the combined organic phases were dried with sodiumsulfate, and concentrated under reduced pressure. Column chromatography onsilica gel with the same mobile phase afforded L3 as colorless crystals. Yield:1.40 g (90%). Mp 90–91 °C. IR (KBr): 2937, 2799, 2755, 1583, 1484, 1434, 1420,1343, 1163, 1132, 1014, 923, 895, 839, 755, 726, 599 cm−1. 1H NMR (CDCl3,300 MHz, 20 °C): d 8.44 (d, 3 J = 4.8 Hz, 2 H, Py), 7.52 (t, 3 J = 7.8 Hz, 2 H,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),149.2 (Py), 136.3 (Py), 123.2 (Py), 122.0 (Py), 64.6 (NCH2), 53.2 (CH2)ppm. EI-MS: m/z = 268 [L3] + (8%), 176 [L3–CH2Py] + (60%). Synthesisof N, N′-bis(1-methyl-2-methylbenzimidazolyl)piperazine (L4). Analogousto L3 with 1-Methyl-2-chloromethylbenzimidazole (1.93 g, 10.7 mmol),piperazine (0.46 g, 5.35 mmol), and NaOH (0.80 g, 20.1 mmol). Columnchromatography on silica gel with CH2Cl2 as mobile phase afforded L4 as

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, 3241,3050, 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 (200mg, 0.75mmol) was dissolved in 20mL of ethanoland CuCl2•2H2O was added (254 mg, 1.49 mmol). After stirring for 3 h volatilematerials were evaporated under reduced pressure and the turquoise oily residuetriturated with diethylether (10 mL); washing with further diethylether resultedin 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,

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 procedurewith L4 (220mg, 0.59mmol) and CuCl2•2H2O (201mg, 1.18mmol) afforded 359mgof6 as a pale green solid (95% yield). Single crystals were obtained from a concentratedMeOH/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.

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[15] G.M. Sheldrick, SHELXTL, v. 6.10, University of Göttingen, distributed by Bruker-AXSInc., Madison, WI, 2000.