Chiral tripodal ligand containing three N-heterocyclic donor functions and its copper complexes: Crystallization of [LCui]2 2+/[L2CuII]2+ stereoisomers and tyrosinase activity

Article abstract of DOI:10.1021/ic301391s

A novel chiral ligand system L containing one pyridyl and two imidazolyl donor functions has been synthesized and investigated with respect to its Cu^I and Cu^II coordination chemistry. Reaction with [Cu(MeCN)₄]PF₆ and [Cu(MeCN)₄]OTf led to the dimeric complexes [LCu]₂X₂ (1, X = PF₆; 2, X = OTf) with the ligands L in different configurations (R,S). The ligand matrix formed in these complexes can also host a Cu^II ion instead of two Cu^I ions so that mixed crystals of [L₂Cu]X₂ and [LCu]₂X₂ can be produced. The pure compounds [L ₂Cu]X₂ (3, X = PF₆; 4, X = OTf) can be obtained by treatment of 1 and 2 with O₂ in acetonitrile, respectively. From the corresponding solution 3 crystallizes with the two L molecules in different configurations, while 4 crystallizes with the ligands in (S,S) or (R,R) configurations, respectively. Crystals containing the analogous diastereomers of 3 were obtained, besides those isolated previously, when this compound was synthesized by reaction of 1 with AgPF₆. On treating 2 with O ₂ as the oxidant in acetonitrile, besides formation of 4, additional evidence for oxygenation of L to L^ox, where one of the original phenyl units corresponds to a phenolate function, was found: The dinuclear complex [L^oxCu(OH)(OTf)CuL](OTf) (5) was isolated as the final product of O₂ activation and conversion, which resembles the one of tyrosinase. In acetonitrile 5 reacts further to give 4 and [L^ox ₂Cu₂](OTf)₂ (6), and hence, product mixtures are obtained. In CH₂Cl₂ decomposition can be avoided, and hence, changing the solvent from acetonitrile to CH₂Cl₂ leads to selective formation of 5.

Full text of DOI:10.1021/ic301391s

Article
pubs.acs.org/IC
Chiral Tripodal Ligand Containing Three N‑Heterocyclic Donor
Functions and Its Copper Complexes: Crystallization of [LCuI]22+/
[L2CuII]2+ Stereoisomers and Tyrosinase Activity
Aline Arnold, Christian Limberg,* and Ramona Metzinger
Humboldt-Universitat zu Berlin, Institut fur Chemie, Brook-Taylor-Straβe 2, 12489 Berlin, Germany
̈
̈
S
* Supporting Information
ABSTRACT: A novel chiral ligand system L containing one
pyridyl and two imidazolyl donor functions has been
synthesized and investigated with respect to its CuI and CuII
coordination chemistry. Reaction with [Cu(MeCN)4]PF6 and
[Cu(MeCN)4]OTf led to the dimeric complexes [LCu]2X2 (1,
X = PF6; 2, X = OTf) with the ligands L in different
configurations (R,S). The ligand matrix formed in these
complexes can also host a CuII ion instead of two CuI ions so
that mixed crystals of [L2Cu]X2 and [LCu]2X2 can be
produced. The pure compounds [L2Cu]X2 (3, X = PF6; 4, X
= OTf) can be obtained by treatment of 1 and 2 with O2 in acetonitrile, respectively. From the corresponding solution 3
crystallizes with the two L molecules in different configurations, while 4 crystallizes with the ligands in (S,S) or (R,R)
configurations, respectively. Crystals containing the analogous diastereomers of 3 were obtained, besides those isolated
previously, when this compound was synthesized by reaction of 1 with AgPF6. On treating 2 with O2 as the oxidant in
acetonitrile, besides formation of 4, additional evidence for oxygenation of L to Lox, where one of the original phenyl units
corresponds to a phenolate function, was found: The dinuclear complex [LoxCu(OH)(OTf)CuL](OTf) (5) was isolated as the
final product of O2 activation and conversion, which resembles the one of tyrosinase. In acetonitrile 5 reacts further to give 4 and
[Lox2Cu2](OTf)2 (6), and hence, product mixtures are obtained. In CH2Cl2 decomposition can be avoided, and hence, changing
the solvent from acetonitrile to CH2Cl2 leads to selective formation of 5.
tripodal ligands that contain some of the donor functions
approved in biomimetic or bioinspired chemistry, namely,
imidazolyl, pyrazolyl, and pyridyl residues. Recently, we
reported the synthesis of such a ligand and its employment
in biomimetic iron chemistry.13 Here we describe a further
representative and considering the His3 ligand spheres in
various type 3 copper enzymes, like the ones mentioned above,
its CuI and CuII coordination chemistry was investigated.
INTRODUCTION
■
Histidine is one of the most abundant amino acids in the
coordination spheres of metal ions belonging to nonheme
metalloenzymes.1−3 In proteins like carbonic anhydrase2 or
hemerythrin3 even three histidine-based imidazole residues are
binding to zinc and iron, respectively, and examples where this
is the case in copper proteins include tyrosinase,1,4 catecholox-
idase,5 and hemocyanin.6 Consequently, a lot of research has
been devoted in the last decades to the design and employment
of multipodal ligands with N-heterocyclic donor functions for
biomimetic chemistry. This has led, for instance, to the
development of the pyrazole-based Tp (tris(pyrazolyl)borate)7
ligands and also to tris(imidazolyl)methanes8 featuring three
imidazole units. In addition, pyridyl-based ligands are
frequently employed in bioinorganic contexts, the most
prominent example being the tetrapodal TPA (tris-
(pyridylmethyl)amine), where three pyridyl donors are linked
to an amine.9 The reaction pockets of metalloenzymes are
intrinsically chiral, and this also translates to the first ligand
sphere of the metal centers. Although certain protein functions
do not utilize chirality in the immediate and remote
surroundings of their metal centers directly during substrate
conversion, in other cases, like cytochromes P450,10 PAM,11
tyrosinase,1,4 or the lipoxygenases,12 it allows for stereoselective
reactions. We therefore contemplated the synthesis of chiral
EXPERIMENTAL SECTION
■
General Procedures. All manipulations were carried out in a
glovebox or else by means of Schlenk-type techniques involving the
use of a dry argon atmosphere. 1H and 13C NMR spectra were
1
recorded on a Bruker DPX 300 NMR spectrometer. H NMR spectra
were calibrated against the residual proton and 13C NMR spectra
against natural abundance 13C resonances of the deuterated solvents.
Microanalyses were performed on a HEKAtech Euro EA 3000
elemental analyzer. Infrared (IR) spectra were recorded using samples
prepared as KBr pellets with a Shimadzu FTIR-8400S spectrometer.
Materials. Solvents were purified, dried, degassed, and stored over
molecular sieves prior to use. [Cu(MeCN)4]PF6 and [Cu(MeCN)4]-
OTf were synthesized from Cu2O and hexafluorophosphoric acid or
Received: June 28, 2012
Published: October 30, 2012
© 2012 American Chemical Society
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trifluoromethanesulfonate acid in acetonitrile and recrystallized twice
from acetonitrile/diethyl ether.
= 89%) of 2 was isolated. Crystals suitable for single-crystal X-ray
diffraction studies could be obtained by layering an acetonitrile
solution of 1 with diethyl ether. H NMR (400 MHz, CDCl3): δ =
1
Synthesis of L. A 11.4 g (0.049 mol) amount of 1-methyl-4,5-
diphenylimidazole dissolved in 30 mL of thf was treated with 19.4 mL
(2.5 M in hexane, 0.049 mol) of n-BuLi at −78 °C. The mixture was
stirred at this temperature for 30 min, and subsequently, a solution of
9.1 g (0.049 mol) of (1-methylimidazol-2-yl)(pyridine-2-yl)methanon
(I) in 30 mL of thf was added. Overnight the mixture was warmed to
room temperature and neutralized with 2.5 M sulfuric acid. The
organic layer was separated and washed twice with H2O. Aqueous
layers were extracted twice with 10 mL of CH2Cl2, and all combined
organic phases were dried over MgSO4. Solvent was removed, and
12.6 g (0.030 mol, η = 61%) of the alcohol II was obtained as a yellow
solid. This was dissolved in 30 mL of thf, and 1.36 g (0.057 mol, 1.9
equiv) of NaH was added carefully. After stirring for 7 days at room
temperature the mixture was refluxed for 6 h. A 3.55 mL (0.057 mol,
1.9 equiv) amount of MeI was added, and after stirring for 3 h the
solvent was removed in vacuo. A 30 mL amount of CH2Cl2 as well as
30 mL of H2O were added and the layers separated. The aqueous layer
was extracted twice with 20 mL of CH2Cl2, and all combined phases
were dried over MgSO4. Solvent was removed in vacuo, the yellow
solid obtained was purified over a column of silica gel (ethylacetate:
triethylamine = 9:1), and the solvent was removed at 45 °C in vacuo.
A 1.53 g (3.513 mmol, η = 12% related to II) amount of L was
3.26 (s, 3H, OCH3), 3.28 (s, 3H, Ph-Im NCH3), 3.54 (s, 3H, Im
NCH3), 6.99 (s, 1H, Im CH-5), 7.07 (d, 1H, 3JH,H = 0.8 Hz, Im CH-4),
3
7.22−7.27 (m, 5H, Ph CH), 7.33 (q, 1H, JH,H = 4.4 Hz, Py CH-5),
7.37−7.43 (m, 5H, Ph CH), 7.88 (d, 2H, 3JH,H = 4.4 Hz, Py CH-3/4),
3
8.53 (1H, d, JH,H = 4.8 Hz, Py CH-6). 13C NMR (CDCl3): δ = 33.1
(OCH3), 35.2 (Ph-Im NCH3), 52.8 (Im NCH3), 80.2 (CqOCH3),
119.6 (Ph CH), 121.4 (Py CH-3), 122.8 (Ph CH), 123.6 (Py CH-5),
124.9 (Im CH-5), 126.6 (Im CH-4), 127.5 (Ph CH), 127.7 (2C, Ph
CH), 128.7 (2C, Ph CH), 129.1 (Ph CH), 129.3 (2C, Ph CH), 131.1
(2C, Ph CH), 132.3 (Ph-Im Cq-5), 133.2 (Ph-Im Cq-4), 136.7 (Ph-Im
CqCH), 137.2 (Py CH-4), 143.7 (2C, Im Cq-2, Ph-Im Cq-2), 149.7 (Py
CH-6). IR (KBr): 3119 (w), 3058 (w), 2957 (m), 2934 (w), 1599(m),
1535 (w), 1490 (m), 1472 (m), 1457 (m), 1443 (m), 1267 (vs), 1223
(m), 1151 (w), 1111 (w), 1075 (m), 1057 (w), 1030 (s), 990 (m), 915
(w), 776 (m), 770 (m), 756 (m), 705 (w), 636 (s), 573 (w), 517 (m).
ESI-MS (pos, MeCN): m/z = 1024.2736 (calcd for [L2Cu2CN]+
1024.2741). Anal. Calcd for C56H50Cu2F6N10O8S2 (1296.27 g/mol):
C, 51.89; H, 3.89; N, 10.81; S, 4.95. Found: C, 52.18; H, 3.86; N,
10.81; S, 5.00.
Synthesis of [L2Cu](PF6)2 (3). Method a: 120 mg (0.093 mmol) of
1 was dissolved in acetonitrile, and the argon atmosphere in the
Schlenk tube was exchanged by O2. Within 3 min the reaction mixture
turned green, and after stirring overnight it was layered with diethyl
ether. 3meso (92 mg, 0.075 mmol, η = 80%) could be obtained in the
form of bright green crystals, which were suitable for single-crystal X-
ray diffraction studies. In addition to these crystals, 18 mg of a light
green solid could be obtained which could not be identified
unambiguously but possibly belongs to a copper complex with the
oxygenated ligand Lox. Method b: After dissolution of 10 mg (0.015
mmol) of 1 and 6.7 mg (0.015 mmol) of L in 1 mL of acetonitrile and
treatment with 3.9 mg (0.015 mmol) of AgPF6 the solution
immediately turned green and elemental silver precipitated. After
filtration the solution was layered with diethyl ether, resulting in
formation of bright green (3meso) and grass green (3rac) crystals
(together 17 mg, 0.014 mmol, η = 93%) which were suitable for X-ray
diffraction studies. IR (KBr), 3meso: 3168 (w), 3147 (w), 3062 (w),
2959 (w), 1603 (m), 1591 (m), 1535 (w), 1497 (m), 1463 (m), 1445
(m), 1437 (m), 1399 (w), 1268 (w), 1178 (w), 1112 (m), 1085 (m),
1053 (m), 1026 (m), 994 (m), 916 (w), 897 (w), 877 (m), 839 (vs),
787 (m), 772 (s), 757 (w), 712 (w), 701 (m), 667 (w), 558 (s). IR
(KBr), 3rac: 3148 (w), 3050 (w), 3000 (w), 2964 (w), 2837 (w), 1585
(m), 1574 (w), 1505 (m), 1497 (m), 1457 (m), 1445 (m), 1436 (m),
1399 (w), 1321 (w), 1285 (w), 1225(w), 1184 (w), 1164 (w), 1131
(w), 1108 (m), 1081 (m), 1056 (w), 1025 (w), 1005 (m), 993(m),
921 (w), 897 (w), 877 (m), 837 (vs), 798 (m), 776 (s), 755 (m), 734
(w), 705 (m), 698 (m), 663 (w), 557 (s). Anal. Calcd for
C54H50CuF12N10O2P2 (1224.51 g/mol): C, 52.97; H, 4.12; N, 11.44.
Found: C, 53.01; H, 4.06; N, 11.69.
Synthesis of [L2Cu](OTf)2 (4). Method a: 12 mg (0.032 mmol) of
2 was dissolved in acetonitrile, and the argon atmosphere in the
Schlenk tube was exchanged by O2. Within 3 min the reaction mixture
turned green, and after stirring overnight it was layered with diethyl
ether. A 9 mg (0.007 mmol, η = 49%) amount of 4rac could be
obtained in the form of grass green crystals, which were suitable for
single-crystal X-ray diffraction studies. Method b: 10 mg (0.015 mmol)
of 2 and 6.7 mg (0.015 mmol) of L were dissolved in 1 mL of
acetonitrile and treated with 4.2 mg (0.015 mmol) of AgOTf. The
solution immediately turned green along with precipitation of
elemental silver. After filtration the green solution was layered with
diethyl ether, which led to formation of 16 mg (0.013 mmol, η = 84%)
4rac in the form of grass green crystals. Method c: 20 mg of L (0.046
mmol) and 8.3 mg (0.023 mmol) Cu(OTf)2 were dissolved in
acetonitrile. After layering the resulting green solution with diethyl
ether grass green crystals of 4rac (26 mg, 0.021 mmol, η = 92%) could
be obtained. IR (KBr) 4rac: 3148 (w), 3050 (w), 3000 (w), 2964 (w),
2837 (w), 1585 (m), 1574 (w), 1505 (m), 1497 (m), 1457 (m), 1445
(m), 1436 (m), 1399 (w), 1321 (w), 1285 (w), 1225(w), 1184 (w),
1
obtained as a white solid. H NMR (400 MHz, CDCl3): δ = 3.20 (s,
3H, Ph-Im NCH3), 3.53 (s, 3H, OCH3), 3.62 (s, 3H, Im), 6.89 (d, 1H,
3JH,H = 0.8 Hz, Im CH-5), 7.02 (d, 1H, 3JH,H = 0.8 Hz, Im CH-4), 7.06
3
3
(d, 1H, JH,H = 7.2 Hz, Ph CH), 7.12 (t, 2H, JH,H = 7.2 Hz, Ph CH),
7.1 (d*t, 1H, Py CH-5), 7.32 (m, 2H, Ph CH), 7.41 (m, 5H, Ph CH),
3
7.69 (m, 1H, Py CH-4), 7.76 (d, 1H, JH,H = 8.0 Hz, Py CH-3), 8.53
(1H, d, 3JH,H = 4.4 Hz, Py CH-6). 13C NMR (400 MHz, CDCl3): δ =
32.7 (Ph-Im CH3), 35.0 (Im CH3), 54.4 (OCH3), 82.9 (Cq), 122.9 (Py
CH-5), 123.4 (Im CH-5), 125.0 (Py CH-3), 126.0 (Ph CH), 126.7 (Im
CH-4), 126.7 (Ph CH), 127.9 (Ph CH), 128.6 (Ph CH), 128.9 (Ph
CH), 131.1 (Ph-Im Cq), 131.1 (Ph CH), 135.0 (Ph Cq), 135.9 (Ph
Cq), 136.4 (Py CH-4), 145.4 (Ph-ImCq-2), 146.1 (Im Cq-2), 147.5 (Py
CH-6), 158.9 (Py Cq). IR (KBr): 3111 (w), 3054 (w), 2988 (w), 2947
(m), 2922 (m), 2824 (w), 1648 (w), 1601 (m), 1586 (m), 1568 (m),
1504 (m), 1465 (m), 1444 (m), 1430 (m), 1388 (w), 1320 (w), 1281
(m), 1253 (w), 1239 (w), 1150 (w), 1098 (m), 1078 (s), 1073 (s),
1057 (m), 1029 (m), 983 (m), 951 (w), 933 (w), 905 (w), 833 (w),
792 (m), 777 (s), 756 (m), 749 (m), 733 (m), 721 (m), 701 (s), 698
(s), 682 (m), 669 (m), 647 (w), 682 (m). ESI-MS (pos, MeCN): m/z
= 436.2116 (calcd for [LH]+ 436.2137).
Synthesis of [LCu]2(PF6)2 (1). A 59 mg (0.135 mmol) amount of
L was added to 50 mg (0.135 mmol) of [Cu(MeCN)4]PF6 in 4 mL of
thf, and the reaction mixture was stirred overnight. The precipitate was
separated by filtration and washed with 2 mL of thf as well as with 5
mL of diethyl ether. After drying 80 mg of 1 could be isolated in the
form of a light yellow solid (0.062 mmol, η = 92%). Crystals suitable
for single-crystal X-ray diffraction studies could be grown by layering
an acetonitrile solution with diethyl ether, but due to the high
sensitivity of 1 toward oxidants these contained 5% of cocrystallized
3
meso. 1H NMR (400 MHz, CDCl3): δ = 3.24 (s, 3H, OCH3), 3.29 (s,
3H, Ph-Im NCH3), 3.55 (s, 3H, Im NCH3), 7.00 (s, 1H, Im CH-5),
7.08 (s, 1H, Im CH-4), 7.22−7.42 (m, 11H, Ph CH and Py CH-5),
3
7.90 (m, 2H, Py CH-3/4), 8.53 (1H, d, JH,H = 2.8 Hz, Py CH-6). IR
(KBr): 3151 (w), 3132 (w), 2965 (m), 2914 (w), 1603 (m), 1535 (w),
1492 (m), 1474 (m), 1442 (w), 1427 (w), 1380 (w), 1369 (m), 1287
(w), 1247 (m), 1216 (w), 1128 (w), 1107 (m), 1073 (m), 1060 (m),
1048 (m), 978 (w), 909 (w), 875 (m), 839 (vs), 808 (m), 767 (m),
751 (w), 710 (w), 557 (s). ESI-MS (pos, MeCN): m/z = 1024.2766
(calcd for [L2 Cu2 CN]+ 1024.2741). Anal. Calcd for
C54H50Cu2F12N10O2P2 (1288.06 g/mol): C, 50.35; H, 3.91; N,
10.87. Found: C, 50.83; H, 3.97; N, 10.55.
Synthesis of [LCu]2(OTf)2 (2). Treatment of 50 mg (0.115 mmol)
of L with 43 mg (0.115 mmol) of [Cu(MeCN)4]OTf dissolved in 2
mL of thf led to instantaneous precipitation of a light yellow solid.
This was isolated by filtration and washed once with 2 mL of thf as
well as with 2 mL of diethyl ether. After drying 66 mg (0.051 mmol, η
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1164 (w), 1131 (w), 1108 (m), 1081 (m), 1056 (w), 1025 (w), 1005
(m), 993(m), 921 (w), 897 (w), 877 (m), 837 (vs), 798 (m), 776 (s),
755 (m), 734 (w), 705 (m), 698 (m), 663 (w), 557 (s). ESI-MS (pos,
MeCN): m/z = 959.3544 (calcd for [L2CuCN]+ 959.3439; 1082.2923
(calcd for [L2CuOTf]+). Anal. Calcd for C56H50CuF6N10O8S2
(1232.72 g/mol): C, 54.56; H, 4.09; N, 11.36; S, 5.20. Found: C,
54.10; H, 3.97; N, 11.60; S, 5.12.
Scheme 1. Synthesis of Ligand L
Synthesis of a Solid Solution of 2 and 4meso. A 0.7 mg (0.0005
mmol) amount of 2 and 6 mg (0.0049 mmol) of 4 were dissolved in
acetonitrile and layered with diethyl ether. Among grass green crystals
of 4rac yellow green crystals of a solid solution of 2 and 4meso were
formed. The ratio of 2 and 4meso in the crystal measured by single-
crystal X-ray diffraction was 34:66 (see Supporting Information).
Synthesis of [LCu(OH)(OTf)CuLox](OTf) (5). Method a: 15 mg
(0.0116 mmol) of 2 was suspended in 0.5 mL of dichloromethane, and
the argon atmosphere in the Schlenk tube was exchanged by O2.
Overnight the reaction mixture turned brown. Subsequently, it was
layered with diethyl ether at −30 °C, which lead to precipitation of
13.5 mg (0.0102 mmol, η = 88%) of 5 in form of a brown powder.
Method b: 109 mg (0.084 mmol) of 2 was dissolved in 2 mL of
acetonitrile, and the argon atmosphere in the Schlenk tube was
exchanged by O2. Within 10 min the reaction mixture turned green,
and after stirring for another hour it was layered with diethyl ether.
From such solutions crystals of 5 and 4rac grew at room temperature,
which were separated mechanically. Brown crystals of 5 were suitable
for single-crystal X-ray diffraction studies. IR (KBr) 5: 3607 (m), 3124
(w), 3058 (w), 2972 (w), 2939 (w), 2843 (w), 1601 (w), 1585 (m),
1506 (m), 1470 (m), 1461 (m), 1384 (w), 1272 (vs), 1266 (vs), 1243
(s), 1224 (m), 1149 (s), 1108 (m), 1079 (m), 1056 (w), 1030 (vs),
1003 (m), 993 (m), 920 (w), 858 (w), 792 (m), 778 (m), 753 (m),
733 (w), 715 (m), 707 (m), 668 (w), 637 (vs), 619 (w), 572 (w), 517
(m). ESI-MS (pos, MeCN): m/z = 498.1363 (calcd for [LCu]+
498.1455); 513.1223 (calcd for [LoxCu]+ 513.1326); 948.3281 (calcd
for [LLoxCu]+ 948.3385). Anal. Calcd for C56H50Cu2F6N10O10S2
(1328.27 g/mol): C, 50.64; H, 3.79; N, 10.55; S, 4.83. Found: C,
50.30; H, 3.78; N, 10.76; S, 4.87.
methylate this acidic function. This finally gave the potential
ligand L containing three different N-donor functions
connected to a methoxymethine unit. The central carbon
atom in L is thus chiral, and the compound is obtained in the
form of a racemic mixture, which was employed without prior
separation into the pure enantiomers. Nevertheless, chirality
was found to have interesting implications on complex
formation and crystallization.
Complex Synthesis. Treatment of L with [Cu(MeCN)4]-
PF6 led to a product which proved very sensitive to air and was
identified as [LCu]2(PF6)2 (1) by elemental analysis, mass
spectrometry, and NMR spectroscopy (see eq 1). Analogously,
reaction with [Cu(MeCN)4]OTf led to [LCu]2(OTf)2 (2).
2L + 2[Cu(MeCN)4]X → [LCu]2(X)2
(X = PF (1), OTf(2))
(1)
6
Crystals of 2 suitable for single-crystal X-ray diffraction could
be grown by carefully adding a diethyl ether phase on top of a
saturated solution of 2 in acetonitrile, and the result of a
corresponding analysis is shown in Figure 1. The complex
Synthesis of [(Lox)2Cu2](OTf)2 (6). 6 is formed by reaction of 2
and O2 in acetonitrile as described in the discussion. It also forms if
acetone is used as the solvent for this reaction, and in contrast to
acetonitrile this allows a more facile separation of 6 from the other
products because it is not soluble. Therefore, we only report here the
synthesis in acetone. A 19 mg (0.0147 mmol) amount of 2 was
suspended in 0.5 mL of acetone, and the argon atmosphere in the
Schlenk tube was exchanged by O2. Overnight the reaction mixture
turned brown, and it was stirred for another 4 days. This led to
precipitation of 6 as a green powder, which was filtered off and dried in
vacuum. A 7 mg (0.0053 mmol, η = 36%) amount of 6 could be
obtained. Crystals suitable for X-ray diffraction studies could be
obtained by layering a solution of 6 in acetonitrile with diethyl ether.
IR (KBr) 6: 3450 (m), 3136 (w), 3058 (w), 3048 (w), 2959 (w), 2942
(w), 2835 (w), 1599 (w), 1586 (m), 1507 (m), 1465 (m), 1448 (m),
1437 (m), 1407 (w), 1284 (s), 1267 (s), 1236 (vs), 1179 (m), 1163
(s), 1137 (m), 1107 (m), 1075 (m), 1051 (w), 1027 (s), 1004 (m),
994 (m), 924 (w), 846 (m), 792 (m), 788 (m), 775 (m), 759 (m), 732
(w), 709 (m), 668 (w), 638 (vs), 619 (w), 585 (w), 572 (w), 518 (m).
ESI-MS (pos, MeCN): m/z = 513.1230 (calcd for [LoxCu]+
513.1326); 933.3323 (calcd for [(Lox)2Cu]+ 933.3514); 1054.2454
(calcd for [(Lox)2Cu2CN]+ 1054.2465). Anal. Calcd for
C56H48Cu2F6N10O10S2 (1322.25 g/mol): C, 50.71; H, 3.65; N,
10.56; S, 4.84. Found: C, 51.00; H, 3.67; N, 10.70; S, 4.94.
2+
Figure 1. Molecular structure of the cation [LCu]2 of 2. Hydrogen
atoms are omitted for clarity. Selected bond lengths [Angstroms] and
angles [degrees]: N1−Cu 2.000(3), N3−Cu 1.976(3), N5−Cu
1.942(3), Cu−Cu 2.8653(9); N1−Cu−N3 92.32(11), N1−Cu−N5
137.83(11), N3−Cu−N5 127.89(11).
RESULTS AND DISCUSSION
crystallizes as a dimer: The two imidazole-based arms of L
coordinate to one copper(I) center, while the pyridyl donor
function binds to a second LCuI entity, whose pyridyl unit
forms a bridge to the first one. Within one molecule one of the
ligands has an (R) configuration, the other one possesses an (S)
configuration.
■
Ligand Synthesis. Ligand synthesis started from a ketone I
(see Scheme 1) that had been prepared before by Canty et al.14
and contains a pyridyl as well as an imidazolyl moiety already.
Reaction of I with deprotonated 2,3-diphenyl-1-methylimida-
zole yielded in the alcohol II. As the alcohol function was
anticipated to disturb coordination via the three N-donor
functions, II was further reacted with NaH and MeI in order to
Complex 2 crystallizes in the centrosymmetric space group
P-1 with the inversion center located directly between the two
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Figure 2. Molecular structures of the cation [LCu]22+ within 2 (a), solid solution of [LCu]22+ and [L2Cu]2+ with PF6− counterions (b), and [L2Cu]2+
within 3 (c). Hydrogen atoms are omitted for clarity.
copper ions. Due to the inversion center between both metal
ions the complex possesses the ligand in both configurations.
Each copper center is coordinated by three nitrogen atoms in a
trigonal planar fashion. The angles at the copper atoms vary
between 92.32° (N1−Cu−N3) and 137.83° (N1−Cu−N5),
indicating a strongly distorted coordination sphere. The Cu−
Npyridyl bond (Cu−N5 1.942(3) Å) is slightly shorter than the
Cu−Nimidazolyl (Cu−N3 1.976(3) Å) and the Cu−
N(phenyl)imidazolyl bonds (Cu−N1 2.000(3) Å).
obtained of 3. Instead of bright green they were grass green,
and X-ray analysis gave a surprise: Indeed, a complex with the
composition [L2Cu]X2 had been formed, too, but unlike in the
case of 3 the ligands L within the complex cation [L2Cu]2+
contained identical configurations (both the (S,S) and the
(R,R) combinations were found in the crystal). This is due to
the fact that 4 crystallizes in the acentric space group Cc with
the complex molecule lying on a general position. The opposite
enantiomer in the unit cell is generated due to the C-glide
plane. Copper centers are coordinated distorted tetrahedrally
(see Figures 3 and 4); the corresponding diastereomers will be
Dosy NMR experiments indicate that complex 2 in solution
exists maily in its monomeric form (ca. 90%); only 10%
remains dimeric (see Supporting Information).
An interesting phenomenon could be observed: for complex
syntheses starting materials were employed that were not
completely free of copper(II) impurities (readily formed in
contact with air). In this case the [LCu]2(X)2 products
cocrystallized with [L2Cu]X2 (X = PF6 (3), OTf (4)) in the
form of a solid solution (ratio 95: 5). This becomes possible as
the ligand matrix, as depicted in Figure 1, without significant
rearrangement can also bind a copper(II) ion in its center
instead of two copper(I) ions in the periphery as shown in
Figure 2 for the case of X = PF6. The copper(II) ion is bound in
a square planar fashion by four imidazole units of the two
ligand molecules. The remaining two donor functions from the
pyridyl residues seem to complete an octahedral coordination
sphere, but a Cu−N distance of 2.8486(34) Å indicates, if at all,
only very weak bonds. By comparison, the bond distances
between the imidazole-based nitrogen atoms and the copper
ion (Cu−N2 2.004(3) Å, Cu−N3 2.320(3) Å) are in the typical
range for coordinative Cu−N bonds. Therefore, the coordina-
tion sphere basically has to be regarded as square planar.
Complex Oxidation. In the solid state complexes 1 and 2
contain two CuI centers, which are bound by three N-
heterocyclic donor functions, each with a Cu···Cu distance of
2.8 Å, and this situation resembles the one of dicopper enzymes
like tyrosinase, catecholoxidase, or hemocyanine, although the
Cu···Cu separation in the enzymes is somewhat larger. As these
Figure 3. Molecular structure of the cation [L2Cu]2+ of 4rac. Hydrogen
atoms are omitted for clarity. Selected bond lengths [Angstroms] and
angles [degrees]: N1−Cu 1.972(2), N3−Cu 1.9760(19), N5−Cu
1.941(2), N7−Cu 1.9269(19); N1−Cu−N3 141.67(8), N1−Cu−N5
93.28(8), N1−Cu−N7 98.89(8), N3−Cu−N5 100.45(8), N3−Cu−
N7 92.49(8), N5−Cu−N7 141.16(8).
enzymes readily react with O2 to give CuII −peroxido moieties,
2
we tested the O2 reactivity of 1 and 2 in the next step. Reacting
1 dissolved in acetonitrile with O2 the colorless solution
gradually turned green, and layering of the resulting solution
with diethyl ether yielded bright green crystals, the analysis of
which proved exclusive formation of 3. Accordingly, X-ray
analysis provided the molecular structure of pure 3, as it had
been deduced already as part of the solid solution obtained as
described above. While the observations made in the course of
the reaction of 2 with O2 were basically the same, the crystals
obtained of the resulting product 4 differed in color from those
Figure 4. Illustration of the two possible structures of the cation
[L2Cu]2+: (a) the “meso” form and (b) the “rac” form ((R,R) shown).
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distinguished throughout the rest of the discussion by the
superscript “rac”, while the (S,R) configuration will be
designated with “meso”.
Again the two ligand molecules coordinate in a bidentate
fashion via the imidazole-based nitrogen atoms, while the
pyridyl units remain pending. The angles at the copper center
vary between 92.68(12)° and 141.86(11)°.
Thus, setting out from almost identical starting situations in
1 and 2 that only differ in the weakly coordinating
counteranions and possess meso configurations reaction with
O2 leads to 3meso in the case of 1 and to 4rac in the case of 2.
Obviously, the anions play a decisive role, although they do not
interact significantly with complex cations of either 1 and 2 or
products 3 and 4. To examine whether they exert their
influence in cooperation with O2 or its reduced species formed
in the course of the oxidation reaction, an alternative synthetic
route was tested using AgPF6 and AgOTf as single-electron
oxidants, respectively: 2 equiv of those were reacted with
[LCu]2X2 dissolved in acetonitrile in the presence of an
additional equivalent of L. As expected, this led to precipitation
of silver and clean, almost quantitative formation of 3 and 4,
respectively. Again, the resulting solutions were layered with
diethyl ether to crystallize the products. While crystallization of
4 again led to grass green crystals of 4rac, a mixture of bright
green and grass green crystals (ratio 4:1) was obtained in the
case of 3. As anticipated, the bright green crystals turned out to
be crystals of 3meso, while the grass green crystals, as already
expected based on the color, corresponded to 3rac, which was
confirmed by means of X-ray analysis (see Supporting
Information). If such crystals of 3rac were added to the solution
of 3 during the crystallization process as seeds, 3 crystallized in
the form of 3rac even quantitatively. However, if these crystals
of 3rac were then recrystallized in the absence of seeds, 3meso
was isolated almost exclusively. This shows that there must be
Figure 5. NMR spectra of (a) L and (b) 3 + L (molecular ratio 4:1) in
d3-MeCN.
the ligands or even external substrates in a biomimetic
fashion.15 Some of these systems16 modeled the function of
tyrosinase, which promotes ortho-hydroxylation of tyrosine via
a mechanism shown in a simplified way in Scheme 2.
Investigating the reactions of 1 or 2 with O2 in acetonitrile
leading to 3 or 4, respectively, formation of byproducts was
observed, and in the case of 2, it proved possible to adjust the
experimental conditions such that these became the major
products, so that an adequate characterization could be
performed. Even crystalline material could be obtained, and
single-crystal X-ray analysis identified the product as [LoxCu-
(OH)(OTf)CuL](OTf) (5) (Figure 6). In contrast to 2, which
is an oxidation product, 5 is the product of an oxygenation:
compared to 2 it contains two additional O atoms, which are
part of the hydroxide and alkoxide ligands, Figure 6. Since in
the ESI mass spectrum a signal for the cationic fragment
[LoxCu]+ could be detected, it could be shown that the O atom
of the alkoxide ligand originates from O2: When the reaction
was performed with 18O2 instead of 16O2 a shift by two mass
units was observed for this signal (m/z = 513.12 with 16O and
515.12 with 18O). Unfortunately, a complex cation containing
the Cu(OH)Cu unit, for example, [LoxCu(OH)(OTf)CuL]+,
could not be detected by mass spectroscopy, so that we only
can assume that the O atom of the hydroxide ligand comes
from O2, too, which is reasonable. In this case, after activation
O2 has exploited its four oxidation equivalents to oxidize the
two CuI centers to the oxidation state +II and oxygenate the
C−H bond of a phenyl residue of L to give the aryloxide ligand
Lox. This process resembles the tyrosinase reactivity as shown in
Scheme 2.
Each of the copper atoms in 5 is coordinated by two nitrogen
and three oxygen atoms in a slightly distorted square pyramidal
fashion. The basal planes of the two square pyramids formed by
two nitrogen and two oxygen atoms are linked by an edge. The
Cu2O2 core formed by the two copper centers and the two
oxygen atoms of the bridging hydroxyl group (whose presence
was also confirmed by IR spectroscopy, vOH 3606 cm−1) as well
as the phenolate function of Lox is butterfly shaped with a
torsion angle of 17° between Cu−O−Cu−O. The two apical
positions are occupied by two oxygen atoms of a triflate anion
with binding parameters that are expected for such bonds. Also,
the other Cu−N and Cu−O bond lengths are in the expected
an equilibrium in solution containing [LCu]2+, free L, 3meso
,
and 3rac, and obviously from this mixture 3meso crystallizes first
either because it is the major, thermodynamically most stable
species or because it crystallizes best. In the case of 4 obviously
4rac is favored.
To test whether ligand exchange is fast on the NMR time
scale, L was added to solutions of 3 and 4 in d3-acetonitrile, and
1
a H NMR spectrum was recorded. Sharp signals observed for
L beside the very broad paramagnetically shifted signals of 3 or
4 indicated that the equilibrium was slow on the NMR time
scale, Figure 5.
Unlike 3, 4 crystallized only in the form of one diastereomer,
although a variety of synthetic and crystallization routes were
tested: For instance, 4 was not only synthesized by the methods
mentioned before but also by reacting 2 equiv of L with
Cu(OTf)2, and crystallization was performed at various
temperatures, in different solvents like CH2Cl2 or acetone, as
well as by evaporation of the solvent from saturated solutions of
4. Independent of these variations, pure 4 always crystallized, if
at all, as 4rac though. 4meso could only be obtained in the form of
solid solutions with 2, but such solid solutions are only stable
up to a CuII ratio of 66%. If this CuII ratio is exceeded, 4rac is
formed.
Tyrosinase Activity. With the background described above
a lot of bioinorganic chemistry15 has been reported in the past,
setting out with CuI in N donor ligand environments for
simulation of the histidine-rich coordination spheres often
found in copper-based oxygenases/oxidases. Subsequent treat-
ment with O2 then often led to functional systems that oxidized
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Scheme 2. Simplified Illustration of the Mechanism of the Ortho-Hydroxylation Process Catalyzed by the Enzyme Tyrosinase
Figure 6. Molecular structure of the cation of [LoxCu(OH)(OTf)-
CuL](OTf), 5. Hydrogen atoms are omitted for clarity. Selected bond
lengths [Angstroms] and angles [degrees]: N1−Cu1 1.956(5), N5−
Cu1 1.942(5), N3−Cu2 2.014(5), N7−Cu2 1.934(5), O1−Cu1
1.923(4), O2−Cu1 1.952(4), O1−Cu2 1.920(4), O2−Cu2 2.008(4),
O3−Cu1 2.418(4), O4−Cu2 2.5031(44), S−O3 1.446(5), S−O4
Figure 7. Molecular structure of the cation of [Lox2Cu2](OTf)2, 6.
1.453(4), S−O5 1.436(5); N1−Cu1−N5 91.3(2), N1−Cu1−O1
Hydrogen atoms are omitted for clarity. Selected bond lengths
170.42(19), N1−Cu1−O2 92.70(18), N5−Cu1−O1 97.44(19),
[Angstroms] and angles [degrees]: N1−Cu1 1.937(5), N3−Cu1
1.944(5), N6−Cu1 2.365(5), O1−Cu1 1.978(4), O1′−Cu1 1.954(4);
N1−Cu1−N3 87.7(2), N1−Cu1−N6 106.26(19), N3−Cu1−O11
93.49(18), N1−Cu1−O1 90.42(19), N1−Cu1−O1′ 158.62(17), N3−
Cu1−O1 178.08(19), N3−Cu1−O1′ 103.27(19), N6−Cu1−O1
87.35(16), N6−Cu1−O1′ 91.53(17), Cu1−O1−Cu1′ 101.58(18),
O1−Cu1−O1′ 78.42(18).
N5−Cu1−O2 175.75(18), Cu1−O1−Cu2 101.36(19), Cu1−O2−
Cu2 97.31(17), N3−Cu2−N7 88.9(2), N3−Cu2−O1 97.11(18),
N3−Cu2−O2 170.03(18), N7−Cu2−O1 173.65(19), N7−Cu2−O2
96.40(18), O1−Cu1−O2 78.66(17), O1−Cu2−O2 77.35(16).
ranges for N-chelating coordination complexes. The magnetic
moment μeff/Cu of 5 was determined by the Evans method and
found to be 1.38 μB/Cu. Compared to other complexes
containing two copper centers bridged by phenoxido or
hydroxido ligands this magnetic moment indicates a moderate
antiferromagnetic coupling between the unpaired electrons.17
This is consistent with other spectroscopic results obtained
investigating 5; the signals in its NMR spectrum appear
paramagnetically shifted and broad, and the EPR spectrum
shows a typical CuII signal (see Supporting Information).
In acetonitrile 5 undergoes further reactions to give 4 and a
product that has been identified as [Lox2Cu2](OTf)2 (6)
spectroscopically and also by means of a single-crystal X-ray
analysis (Figure 7). 6 is a dinuclear CuII complex containing the
ligand Lox, in which the two copper atoms are bridged by two
oxygen atoms belonging to the phenolate function of Lox.
Each copper center is coordinated in a slightly distorted
square pyramidal fashion by these two oxygen atoms as well as
by three nitrogen atoms of an acetonitrile molecule and two
imidazolyl units of Lox. In contrast to 5, the Cu2O2 core in 6 is
planar and the Cu−O−Cu angles are somewhat larger (both
101.58°) than those in 5 (97.31° and 101.36°). Both the Cu−
O−Cu angle and the core structure have an influence on the
coupling of the unpaired electrons in dinuclear copper cores.18
Therefore, the magnetic moment μeff/Cu of 6 differs from that
of 5 and was determined to be 0.94 μB, which is a typical value
for complexes where two copper centers are strongly
antiferromagnetically coupled through hydroxido and phenox-
ido ligands.17b,18b Consistent with this, 6 shows only a very
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Scheme 3. Reactions within the [LCuI]2(OTf)2/O2 System
small signal in the EPR spectrum at 77 K (see Supporting
Information). The signals in the NMR spectrum appear only
slightly paramagnetically shifted.
AUTHOR INFORMATION
Corresponding Author
*E-mail: christian.limberg@chemie.hu-berlin.de.
■
Due to its tendency to undergo further reactions it is difficult
to isolate 5 from reactions in acetonitrile in the pure form,
although crystallization has only been achieved from acetoni-
trile solutions. However, performing the reaction of 2 with O2
in CH2Cl2 leads to selective formation of 5 (Scheme 3).
Remarkably, reaction of 1 and O2 does not show any variations
with the solvent; in acetonitrile as well as in dichloromethane it
leads to 3 as the major reaction product, and ligand
oxygenation occurs only to a small extent.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the Humboldt Universitat zu Berlin for
̈
financial support as well as to Bayer Services GmbH & Co.
OHG, BASF AG, and Sasol GmbH for the supply of chemicals.
We thank Dr. Beatrice Braun and Dr. Stefan Mebs for some of
the crystal structure analyses as well as Prof. Dr. Ulli Englert
from RWTH Aachen for help with solving the solid solution.
We also thank Dr. Michael John from Georg-August-
CONCLUSIONS
■
A novel, chiral ligand has been presented that was employed for
synthesis of [LCu]2X2 and [L2Cu]X2 compounds (X = PF6,
OTf). Interestingly, the binding matrix, formed by two ligands
with different configurations, is equally suited to coordinate
either two CuI ions or one CuII ion, so that solid solutions can
be generated. On the other hand, CuII ions can also coordinate
two molecules of L with identical configurations, and
corresponding complex cations are part of an equilibrium in
solution. Which of the diastereomers crystallizes from a given
solution is sensitively determined by the counteranions, the
existence of other compounds in solution, and the presence of
seeds. The reactivity of [LCu]2X2 compounds in contact with
O2 was found to depend on the X counterions, too. In the case
of X = PF6 almost exclusively the CuII oxidation product
[L2Cu]X2 was formed, while for X = OTf an additional product
could be isolated that in dichloromethane solution represents
the main product: [LoxCu(OH)(OTf)CuL](OTf), whose
formation can be explained via an initial O2 activation followed
by C−H oxygenation. Hence, [LCu]2(OTf)2 shows tyrosine
activity. Future research will build upon this and aim at the
oxidation of exogenous substrates. Successful systems will then
also be investigated with enantiomerically pure L to test its
potential to induce stereoselective oxygenations.
Universitat Gottingen for DOSY NMR measurements, C.
̈
̈
Matlachowski for measurement of the EPR spectra, C.
Jankowski for preparation of starting materials, and the
members of the Cluster of Excellence, Unifying Concepts in
Catalysis, for valuable discussions.
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