Active site models for the CuA site of peptidylglycine α-hydroxylating monooxygenase and dopamine β-monooxygenase

Article abstract of DOI:10.1021/ic301272h

A mononuclear copper(II) superoxo species has been invoked as the key reactive intermediate in aliphatic substrate hydroxylation by copper monooxygenases such as peptidylglycine α-hydroxylating monooxygenase (PHM), dopamine β-monooxygenase (DβM), and tyramine β-monooxygenase (TβM). We have recently developed a mononuclear copper(II) end-on superoxo complex using a N-[2-(2-pyridyl)ethyl]-1,5- diazacyclooctane tridentate ligand, the structure of which is similar to the four-coordinate distorted tetrahedral geometry of the copper-dioxygen adduct found in the oxy-form of PHM (Prigge, S. T.; Eipper, B. A.; Mains, R. E.; Amzel, L. M. Science2004, 304, 864-867). In this study, structures and physicochemical properties as well as reactivity of the copper(I) and copper(II) complexes supported by a series of tridentate ligands having the same N-[2-(2-pyridyl) ethyl]-1,5-diazacyclooctane framework have been examined in detail to shed light on the chemistry dictated in the active sites of mononuclear copper monooxygenases. The ligand exhibits unique feature to stabilize the copper(I) complexes in a T-shape geometry and the copper(II) complexes in a distorted tetrahedral geometry. Low temperature oxygenation of the copper(I) complexes generated the mononuclear copper(II) end-on superoxo complexes, the structure and spin state of which have been further characterized by density functional theory (DFT) calculations. Detailed kinetic analysis on the O₂-adduct formation reaction gave the kinetic and thermodynamic parameters providing mechanistic insights into the association and dissociation processes of O ₂ to the copper complexes. The copper(II) end-on superoxo complex thus generated gradually decomposed to induce aliphatic ligand hydroxylation. Kinetic and DFT studies on the decomposition reaction have suggested that C-H bond abstraction occurs unimolecularly from the superoxo complex with subsequent rebound of the copper hydroperoxo species to generate the oxygenated product. The present results have indicated that a superoxo species having a four-coordinate distorted tetrahedral geometry could be reactive enough to induce the direct C-H bond activation of aliphatic substrates in the enzymatic systems.

Full text of DOI:10.1021/ic301272h

Article
pubs.acs.org/IC
Active Site Models for the CuA Site of Peptidylglycine
α‑Hydroxylating Monooxygenase and Dopamine β‑Monooxygenase
Atsushi Kunishita,† Mehmed Z. Ertem,‡ Yuri Okubo,† Tetsuro Tano,† Hideki Sugimoto,† Kei Ohkubo,†
Nobutaka Fujieda,† Shunichi Fukuzumi,†,§ Christopher J. Cramer,*,‡ and Shinobu Itoh*,†
†Department of Material and Life Science, Division of Advanced Science and Biotechnology, Graduate School of Engineering, Osaka
University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
‡Department of Chemistry, Supercomputing Institute, and Chemical Theory Center, University of Minnesota, 207 Pleasant Street SE,
Minneapolis, Minnesota 55455, United States
§Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
S
* Supporting Information
ABSTRACT: A mononuclear copper(II) superoxo species has
been invoked as the key reactive intermediate in aliphatic
substrate hydroxylation by copper monooxygenases such as
peptidylglycine α-hydroxylating monooxygenase (PHM), dop-
amine β-monooxygenase (DβM), and tyramine β-monooxygenase
(TβM). We have recently developed a mononuclear copper(II)
end-on superoxo complex using a N-[2-(2-pyridyl)ethyl]-1,5-
diazacyclooctane tridentate ligand, the structure of which is similar
to the four-coordinate distorted tetrahedral geometry of the
copper-dioxygen adduct found in the oxy-form of PHM (Prigge,
S. T.; Eipper, B. A.; Mains, R. E.; Amzel, L. M. Science 2004, 304,
864−867). In this study, structures and physicochemical proper-
ties as well as reactivity of the copper(I) and copper(II)
complexes supported by a series of tridentate ligands having the same N-[2-(2-pyridyl)ethyl]-1,5-diazacyclooctane framework
have been examined in detail to shed light on the chemistry dictated in the active sites of mononuclear copper monooxygenases.
The ligand exhibits unique feature to stabilize the copper(I) complexes in a T-shape geometry and the copper(II) complexes in a
distorted tetrahedral geometry. Low temperature oxygenation of the copper(I) complexes generated the mononuclear copper(II)
end-on superoxo complexes, the structure and spin state of which have been further characterized by density functional theory
(DFT) calculations. Detailed kinetic analysis on the O2-adduct formation reaction gave the kinetic and thermodynamic
parameters providing mechanistic insights into the association and dissociation processes of O2 to the copper complexes. The
copper(II) end-on superoxo complex thus generated gradually decomposed to induce aliphatic ligand hydroxylation. Kinetic and
DFT studies on the decomposition reaction have suggested that C−H bond abstraction occurs unimolecularly from the superoxo
complex with subsequent rebound of the copper hydroperoxo species to generate the oxygenated product. The present results
have indicated that a superoxo species having a four-coordinate distorted tetrahedral geometry could be reactive enough to induce
the direct C−H bond activation of aliphatic substrates in the enzymatic systems.
Amzel and his co-workers, CuA site exhibits a distorted trigonal
planar geometry with three histidine imidazole ligands, whereas
CuB site has a distorted tetrahedral geometry consisting with
two histidine imidazole ligands, one methionine thioether
ligand, and a water molecule as shown in Figure 1A.18,19
It has been suggested that the CuA site is associated with
electron transfer from ascorbate (physiological reductant) to
the CuB site acting as the reaction center for dioxygen
activation and subsequent substrate hydroxylation. Recently,
Amzel and his co-workers also determined the crystal structure
of the oxy-form of PHM to demonstrate that CuB exhibits a
INTRODUCTION
■
Dioxygen activation at a mononuclear copper reaction center
has attracted much recent attention because of its strong
relevance to biological and synthetic oxidation/oxygenation
reactions.1−12 For instance, peptidylglycine α-hydroxylating
monooxygenase (PHM), dopamine β-monooxygenase (DβM),
and tyramine β-monooxygenase (TβM) have been reported to
catalyze aliphatic C−H bond hydroxylation of their respective
substrates (dopamine, peptide hormones, and tyramine,
respectively) using a mononuclear copper active-oxygen
species.1,13−16 These enzymes possess a similar active site
consisting of two different mononuclear copper reaction
centers (designated as CuA and CuB) separated by ∼11 Å.17
According to the X-ray structure of resting PHM reported by
Received: June 15, 2012
Published: August 21, 2012
© 2012 American Chemical Society
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Figure 1. (A) Active site structure of PHM.17 (B) The copper-superoxo species involved in oxy-PHM.20.
distorted tetrahedral geometry including an end-on superoxo
species (η1-O2•−) as shown in Figure 1B.20 The recent
enzymatic studies as well as the theoretical studies have
suggested that the mononuclear copper(II) superoxo species
(Figure 1B) could be a key reactive intermediate that is directly
involved in the C−H bond activation of the sub-
strates.4,13,15,19,21−26 However, little is known about the
physicochemical properties and reactivity of such an end-on
superoxo copper(II) species having a distorted tetrahedral
geometry.
gave a mixture of the mononuclear copper(II)-superoxo
complex and a dinuclear copper(II) side-on peroxo complex,
(μ-η2:η2-peroxo)dicopper(II), in solution. Thus, they re-
examined the spectroscopic features of the mononuclear
copper(II) side-on superoxo complex using the more sterically
−
demanding ligand HB(3-Ad-5-iPr-pz)3 (Ad = adamantyl) to
demonstrate that the complex has a diamagnetic ground state
(S = 0) and exhibits weak absorption bands at 383, 452, 699,
and 980 nm with extinction coefficients smaller than 300 M−1
cm−1.29
In the synthetic modeling studies, a great deal of effort has
been made to obtain insights into the structure, physicochem-
ical properties, and reactivity of mononuclear copper(II) active-
oxygen complexes.27−42 With respect to the mononuclear
copper(II)-superoxo complex, Kitajima and co-workers re-
ported the first crystal structure of a side-on superoxo
copper(II) complex SS supported by a sterically demanding
tridentate ligand, which exhibits a five-coordinate square
pyramidal structure as shown in Figure 2 (left).28 However, it
More recently, Schindler et al. reported a mononuclear
copper(II) end-on superoxo complex [CuII(η1-O2)-
(TMG3tren)]+ (SE) supported by tris(tetramethylguanidino)-
tren (TMG3tren) tetradentate ligand (Figure 2, right).35 In this
case, the complex exhibits a five-coordinate trigonal bipyramidal
geometry with an end-on superoxide as one of the axial ligands.
The end-on superoxo complex SE exhibits a relatively intense
absorption band at 447 nm (ε = 3400 M−1 cm−1) together with
broad absorption bands at 680 and 780 nm. Subsequent NMR
analyses and theory clearly indicated that the compound has a
triplet ground state (S = 1).40
With respect to reactivity of the mononuclear copper(II)
superoxo complexes, no experimental data are available for
Kitajima’s side-on superoxo complex, [CuII(η2-O2)([HB(3-tBu-
5-iPrpz)3])]+ (SS). In contrast, the end-on superoxo complex,
[CuII(η1-O2)(TMG3tren)]+ (SE), has been shown to react with
hydrogen atom donors such as phenols and TEMPO-H
(2,2,6,6-tetramethylpyperidine-1-hydroxide) to generate a
hydroperoxo intermediate LCuIIOOH.37 In this case, they
observed aliphatic ligand hydroxylation of one of the methyl
groups of the tetramethylguanidino substituent.37 The same
ligand hydroxylation took place, when the copper(II) complex
of the same ligand was reacted with H2O2 in the presence of a
base or the copper(I) complex was treated with PhIO.37
However, decomposition of the superoxo complex itself did not
Figure 2. Structurally characterized copper(II)-superoxo complexes:
[CuII(η2-O2)([HB(3-tBu-5-iPrpz)3])]+ (SS)28 and [CuII(η1-O2)-
(TMG3tren)]+ (SE).35
was later found that the reaction of the corresponding
copper(I) complex [CuI([HB(3-tBu-5-iPrpz)3])] with O2
Scheme 1. Formation of End-on Superoxo Copper(II) Complex 2RO2
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afford the hydroxylated product.37 On the basis of these results,
they concluded that the end-on superoxo complex itself was not
directly involved in the aliphatic C−H bond activation, but
instead a putative copper(II) oxyl radical species LCuIIO•
generated by O−O bond homolysis of the hydroperoxo
complex LCuIIOOH may be the real active-oxygen species.37
Recently, we have succeeded in developing a new type of
end-on superoxo copper(II) complex 2Phe(X)O2 having a four-
coordinate distorted tetrahedral geometry (Scheme 1).43 The
structure of 2Phe(X)O2 is more similar to the suggested structure
of the active oxygen species of PHM (Figure 1B) as compared
to Schindler’s end-on superoxo complex SE having trigonal
bipyramidal structure (Figure 2).43 The compound is generated
by the reaction of copper(I) complexes supported by tridentate
ligand LPhe(X) (Chart 1, X = OMe, H, NO2) with O2 at low
temperature (Scheme 1).43 The superoxo complex has been
characterized by UV−vis [λmax = 397 nm (ε = 4200 M−1 cm−1),
570 (850), and 705 (1150)] and resonance Raman
spectroscopic methods (νO−O = 1033 and νCu−O = 457 cm−1,
shift to 968 and 442 cm−1, respectively, upon 18O2
substitution).43 Moreover, the superoxo complex 2Phe(X)O2
has been shown to have a triplet ground state (S = 1) as
confirmed by electron paramagnetic resonance (EPR) and to
exhibit aliphatic ligand hydroxylation reactivity.43
Our success may rely on the unique structural features of the
supporting ligand, a N-[2-(2-pyridyl)ethyl]-1,5-diazacyclooc-
tane derivative, which enforces the copper(II) complexes taking
a mononuclear structure with a distorted tetrahedral geometry.
So far, N-substituted 1,5-diazaoctane (DACO) derivative
ligands have been demonstrated to provide copper(II)
complexes having structural features deviating from the normal
tetragonal geometry of copper(II).38,44−48 In this study, the
structure, physicochemical properties, and reactivity of the
copper(I) and copper(II) complexes including the superoxo
complexes 2RO2 supported by LPhe(X) and LiPr (Scheme 1) have
been examined in detail to shed light on the chemistry of
resting states, reduced states, and oxidized states of the
corresponding enzymes.
at temperature T, and C0 and d0 are the known values at room
temperature.50 Because of the small solubility of O2 in liquids, the
density of the solution can be assumed equal to that of pure solvent.
For several common solvents, densities were experimentally
determined in a wide range of temperatures and fit satisfactorily
with the following equation, dT = a − bT (T in K; the parameters a
and b are found in the literatures).51
1-[2-(4-Methylphenyl)ethyl]-1,5-diazacyclooctane. A mixture
of DACO (1,5-diazacyclooctane)·2HBr (7.0 g, 25 mmol) and KOH
(2.8 g, 50 mmol) in CH3CN-H2O (5: 1, 120 mL) was stirred for 1 h at
room temperature. Then, 2-(4-methylphenyl)ethyl bromide (5.0 g, 25
mmol) and triethylamine (13 g, 100 mmol) were added to the
solution, and the resulting solution was refluxed for 14 h. The solvent
was removed under reduced pressure, and the resulting residue was
dissolved in water (10 mL), to which NaOH(aq) was added slowly
until the pH of the solution became 14. The aqueous solution was
extracted with CHCl3 (20 mL × 3), and the combined organic layer
was dried over Na2SO4. After removal of Na2SO4 by filtration,
evaporation of the solvent gave dark brown oil. Purification by
Kugelrohr distillation (200 °C, 5.0 Torr) afforded the pure product as
a colorless oil in a 19% isolated yield. 1H NMR (CDCl3, 300 MHz) δ
1.61 (4 H, pentet, J = 5.7 Hz, −CH2−), 2.31 (3 H, s, −CH3), 2.57−
2.71 (8 H, m, −CH2−), 2.84 (4 H, t, J = 5.7 Hz, −CH2−), 7.09 (4 H,
br s, ArH); HRMS (FAB+) m/z = 232.1933, calcd for C15H24N2 =
232.1940.
1-[2-(4-Methylphenyl)ethyl]-5-[2-(2-pyridyl)ethyl]-1,5-diaza-
cyclooctane (LPhe(Me)). To a methanol solution (40 mL) containing
1-[2-(4-methylphenyl)ethyl]-1,5-diazacyclooctane (680 mg, 3.1
mmol) and 2-vinylpyridine (1.5 g, 15.5 mmol) was added acetic acid
(1.0 g, 15.5 mmol), and the mixture was refluxed for 1 day. Removal of
the solvent by evaporation gave brown oil, from which ligand LPhe(Me)
was isolated by alumina column chromatography (eluent: AcOEt) in
56%. 1H NMR (CDCl3, 300 MHz) δ = 1.65 (4 H, pentet, J = 5.7 Hz,
−CH2−), 2.31 (3 H, s, −CH3), 2.72−2.79 (12 H, m, −CH2−), 2.93 (4
H, s, −CH2−), 7.09−7.19 (6 H, m, ArH), 7.57 (1 H, dt, J = 1.8 Hz and
J = 7.5 Hz, PyH4), 8.52 (1 H, d, J = 4.8 Hz, PyH6); HRMS (FAB+) m/
z = 338.2603, calcd for C22H32N3 = 338.2596.
1-[2-(4-Chlorophenyl)ethyl]-1,5-diazacyclooctane. This com-
pound was prepared by a similar procedure as described for the
synthesis of 1-[2-(4-methylphenyl)ethyl]-1,5-diazacyclooctane using 2-
(4-chlorophenyl)ethyl bromide instead of (2-(4-methylphenyl)ethyl
bromide in a 14% isolated yield. 1H NMR (CDCl3, 300 MHz) δ = 1.61
(4 H, pentet, J = 5.7 Hz, −CH2−), 2.63−2.76 (8 H, m, −CH2−), 2.84
(4 H, t, J = 5.7 Hz, −CH2−), 7.12 (2 H, d, J = 8.4 Hz, ArH), 7.25 (2
H, d, J = 8.4 Hz, ArH); HRMS (FAB+) m/z = 253.1474, calcd for
C14H22N2Cl1 = 253.1472.
EXPERIMENTAL SECTION
■
Materials and Methods. The reagents and the solvents used in
this study, except the ligands and the copper complexes, were
commercial products of the highest available purity and were further
purified by the standard methods, if necessary.49 Ligands 1-(2-
phenethyl)-5-[2-(2-pyridyl)ethyl]-1,5-diazacyclooctane (LPhe(H)), 1-[2-
(4-nitrophenyl)ethyl]-5-[2-(2-pyridyl)ethyl]-1,5-diazacyclooctane
(LPhe(NO2)), and 1-[2-(4-methoxyphenyl)ethyl]-5-[2-(2-pyridyl)ethyl]-
1,5-diazacyclooctane (LPhe(OMe)) and their copper(I) complexes
(1Phe(H), 1Phe(NO2), and 1Phe(OMe)) and copper(II) complexes
(2Phe(NO2)Cl and 2Phe(NO2)OAc) were prepared according to the
reported procedures.43 FT-IR spectra were recorded on a Jasco FTIR-
4100, and UV−visible spectra were taken on a Hewlett-Packard 8453
photo diode array spectrophotometer equipped with a Unisoku
thermostatted cell holder USP-203 designed for low temperature
1-[2-(4-Chlorophenyl)ethyl]-5-[2-(2-pyridyl)ethyl]-1,5-diaza-
cyclooctane (LPhe(Cl)). This compound was prepared by a similar
procedure as described for the synthesis of LPhe(Me) using 1-[2-(4-
chlorophenyl)ethyl]-1,5-diazacyclooctane instead of 1-[2-(4-
1
methylphenyl)ethyl]-1,5-diazacyclooctane in 65%. H NMR (CDCl3,
300 MHz) δ = 1.64 (4 H, pentet, J = 5.7 Hz, −CH2−), 2.71−2.77 (12
H, m), 2.92 (4 H, s), 7.07−7.26 (6 H, m, ArH), 7.58 (1 H, td, J = 1.6
Hz and J = 7.5 Hz, PyH4), 8.52 (1 H, d, J = 4.2 Hz, PyH6); HRMS
(FAB+) m/z = 357.1972, calcd for C21H28Cl1N3 = 357.1975.
1-Isopropyl-5-[2-(2-pyridyl)ethyl]-1,5-diazacyclooctane
(LiPr). 1-Isopropyl-1,5-diazacyclooctane was prepared according to the
reported procedures.52 To a methanol solution (20 mL) containing 1-
isopropyl-1,5-diazacyclooctane (0.33 g, 2.1 mmol) and 2-vinilpyridine
(0.67 g, 6.3 mmol) was added acetic acid (0.38 g, 6.3 mmol), and the
mixture was refluxed for 2 days. The solvent was removed by
evaporation, and the resulting residue was dissolved in water (15 mL),
to which NaOH(aq) was added slowly until the pH of the solution
became 14. The aqueous solution was extracted with CH2Cl2 (20 mL
× 5), and the combined organic layer was dried over Na2SO4. After
removal of Na2SO4 by filtration, the solvent was evaporated to give a
brown oil, from which LiPr was isolated by alumina column
chromatography (eluent: from AcOEt:hexane = 1: 1 to AcOEt only)
1
measurements. H NMR spectra were recorded on a JEOL LMN−
ECP300WB, a JEOL ECP400, a JEOL ECS400, or a Varian UNITY
INOVA 600 MHz spectrometer. ESI-MS (electrospray ionization mass
spectra) measurements were performed on a PE SCIEX API 150EX, a
PerSeptive Biosystems Mariner Biospectrometry workstation, or a
Micromass LCT spectrometer. Elemental analyses were performed on
a Perkin-Elmer or a Fisons instruments EA1108 Elemental Analyzer.
Dioxygen concentrations in organic solvents (acetone, tetrahy-
drofuran (THF), and propionitrile) at various temperatures (−70 ∼
−90 °C) were calculated by using the equation, CT = C0 (dT/d0),
where CT and dT are the O2 concentration and the density of solution
1
in 77%. H NMR (CDCl3, 600 MHz) δ = 0.98 (6 H, d, J = 6.8 Hz),
1.65 (4 H, pentet, J = 6.0 Hz), 2.74 (4 H, t, J = 6.0 Hz), 2.84 (4 H, t, J
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= 6.0 Hz), 3.00 (4 H, m), 3.12 (1 H, septet, J = 6.8 Hz), 7.10 (1 H,
ddd, J = 7.6, 5.0, and 1.1 Hz), 7.19 (1 H, br d, J = 7.6 Hz), 7.58 (1 H,
td, J = 7.6 and 1.1 Hz), 8.52 (1 H, ddd, J = 5.0, 1.9, and 0.9 Hz);
HRMS (FAB, pos): m/z = 262.2274. Calcd for C16H28N3 262.2283.
Since all the copper(I) complexes are sensitive to air, all the
manipulations for the syntheses, crystallization, and stock solution
preparations of copper(I) complexes were carried out under Ar
atmosphere in a glovebox (Miwa DBO-1KP, [O2] < 1 ppm).
[CuI(LPhe(Me))](PF6) (1Phe(Me)). Ligand LPhe(Me) (67.4 mg, 0.2 mmol)
was treated with an equimolar amount of [CuI(CH3CN)4]PF6 (74.0
mg, 0.2 mmol) in CH2Cl2−CH3CN (7: 1, 4 mL). After stirring the
mixture for 5 min at room temperature, insoluble material was
removed by filtration. Addition of ether (100 mL) to the filtrate gave a
pale yellow powder that was precipitated by standing the mixture for
several minutes. The supernatant was then removed by decantation,
and the remained pale yellow solid was washed with ether three times
(50 mL × 3) and dried to give complex 1Phe(Me) in 62%. Single crystals
of 1Phe(Me) were obtained by vapor diffusion of ether into a CH2Cl2−
CH3CN (10 : 1) solution of the complex. FT-IR (KBr) 840 cm−1
(PF6−); HRMS (FAB+): m/z 400.1813, calcd for C22H31CuN3
400.1814; Anal. Calcd for [CuI (LP h e ( M e ) )]PF6 ·H2 O
(C22H33CuF6N3O1P1): C, 46.85; H, 5.90; N, 7.45. Found: C, 46.64;
H, 5.50; N, 7.77.
[CuII(LiPr)(N3)](BF4) (2iPrN3). To a methanol solution (30 mL)
containing LiPr (50 mg, 0.19 mmol) was added CuSO4·5H2O (48 mg,
0.19 mmol), when the color of the solution turned dark blue. After
stirring for 5 min, NaN3 (16 mg, 0.25 mmol) was added to give a
green solution. After stirring for 2 min, NH4BF4 (70 mg, 0.67 mmol)
was added to the solution, and the mixture was stirred for additional 2
min. Volume of the solvent was reduced under reduced pressure to
give dark green powder, which was collected by filtration and dried (69
mg, 80%). Single crystals were obtained by vapor diffusion of ether
into an acetone solution of the complex. FT-IR (KBr) 2037 cm−1
(N3−), 1057 cm−1 (BF4−); Anal. Calcd for: [CuII(LiPr)(N3)]BF4
(C16H27BCuF4N6): C, 42.35; H, 6.00; N, 18.52. Found: C, 42.29;
H, 5.74; N, 18.25; ESI-MS (pos) m/z = 324.2 calcd for: C16H27CuN3:
324.2.
[CuII(LiPr)(Cl)](PF6) (2iPrCl). To an ethanol solution (50 mL)
containing LiPr (70 mg, 0.27 mmol) was added CuCl2 (36 mg, 0.27
mmol), when the color of the solution turned greenish blue. After
stirring for 5 min, NH4PF6 (130 mg, 0.81 mmol) was added to the
solution, and the mixture was stirred for additional 2 min. The volume
of the solvent was reduced under reduced pressure to give a green
powder, which was collected by filtration and dried (96 mg, 71%). FT-
−
IR (KBr) 841 cm−1(PF6 ); Anal. Calcd for: [CuII(LiPr)Cl]PF6·0.5H2O
(C16H28ClCuF6N3O0.5P): C, 37.36; H, 5.49; N, 8.17; Found: C, 37.65;
H, 5.32; N, 7.85; ESI-MS (pos) m/z = 359.1 calcd for: C16H27ClCuN3:
359.1. Single crystals of 2iPrCl were obtained as a BPh4 salt
[CuII(LiPr)(Cl)](BPh4) by adding NaBPh4 into a CH3CN solution
of the PF6 salt [CuII(LiPr)(Cl)](PF6) obtained above.
[CuI(LPhe(Cl))](PF6) (1Phe(Cl)). This compound was prepared by a
similar procedure as described for the synthesis of 1Phe(Me) in 51%
using LPhe(Cl) instead of LPhe(Me). Single crystals of 1Phe(Cl) were
obtained by diffusion of n-hexane into a CH2Cl2−CH3CN (10 : 1)
[CuII(LiPr)(NO2)](BF4) (2iPrNO2). This compound was prepared as a
blue powder by a similar procedure described for the synthesis of
−
solution of the complex. FT-IR (KBr) 840 cm−1 (PF6 ); HRMS
(FAB+): m/z 420.1279, calcd for C21H28CuN3Cl1 420.1267; Anal.
Calcd for [CuI(LPhe(Cl))]PF6·H2O (C21H28CuF6N3Cl1P1): C, 44.63; H,
4.99; N, 7.39. Found: C, 44.53; H, 4.98; N, 7.42.
2
iPrN3 by using NaNO2 instead of NaN3 (79%). Single crystals were
obtained by vapor diffusion of ether into an acetone solution of the
complex. FT-IR (KBr) 1046 cm−1 (BF4−), Anal. Calcd for:
[CuIILNO3]PF6·0.4H2O (C16H27.8BCuF4N4O2.4): C, 41.33; H, 6.03;
N, 12.05. Found: C, 41.56; H, 5.83; N, 11.78; ESI-MS (pos) m/z =
324.2 calcd for: C16H27CuN3: 324.2.
[CuI(LiPr)](PF6) (1iPr). To an acetonitrile solution (30 mL)
containing LiPr (50 mg, 0.19 mmol) was added [CuI(CH3CN)4]PF6
(71 mg, 0.19 mmol), when the color of the solution turned light
yellow. After stirring 5 min, the volume of the solvent was reduced
under reduced pressure, and diethyl ether (5 mL) was added to the
residue to give a light yellow powder of the product (69 mg, 76%).
Single crystal was obtained by vapor diffusion of ether into a CH3CN
X-ray Structure Determination. Each single crystal obtained was
mounted on a loop with mineral oil, and all X-ray data were collected
on a Rigaku R-AXIS RAPID diffractometer using filtered Mo-Kα
radiation. The structures were solved by direct methods (SIR 200853)
and expanded using Fourier techniques. The non-hydrogen atoms
were refined anisotropically by full-matrix least-squares on F2. The
hydrogen atoms were attached at idealized positions on carbon atoms
and were not refined. All structures in the final stages of refinement
showed no movement in the atom positions. The calculations were
performed using Single-Crystal Structure Analysis Software, version
3.8 (Rigaku Corporation: The Woodlands, TX, 2000−2006).
Crystallographic parameters are summarized in Table 1. Atomic
coordinates, thermal parameters, and intramolecular bond distances
and angles are deposited in the Supporting Information (CIF file
format).
EPR Measurements. EPR spectra of copper(II) complexes were
recorded on a JEOL X-band spectrometer (JES-RE1XE) with an
attached variable temperature apparatus. The EPR spectra were
measured in frozen acetone at 77 K. The magnitude of modulation was
chosen to optimize the resolution and the signal-to-noise (S/N) ratio
of the observed spectra under nonsaturating microwave conditions.
The g values and the hyperfine coupling constants were calibrated with
a Mn2+ marker.
Kinetic Measurements. Kinetic measurements for the oxygen-
ation reaction of copper(I) complexes, 1Phe(X) (X = OMe, Me, H, Cl,
NO2) and 1iPr, and the decomposition reaction of the generated
copper(II) superoxo complexes, 2Phe(X)O2 and 2iPrO2, were performed
using a Hewlett-Packard 8453 photo diode array spectrophotometer
with a Unisoku thermostatted cell holder designed for low temperature
measurements (USP-203, a desired temperature can be fixed within
0.5 °C) in an appropriate solvent (3 mL) at low temperature. For the
oxygenation reaction of the copper(I) complexes, O2 gas was rapidly
introduced to a solution of the copper(I) complex in a UV cell (1.0 cm
path length) through a silicon rubber cap by using a gastight syringe,
and the increase of the absorption band due to the superoxo complex
−
solution of the complex. FT-IR (KBr) 842 cm−1 (PF6 ); ESI-MS (pos)
m/z = 324.2 calcd for: C16H27BCuN6: 324.2; Anal. Calcd for
[CuI(LiPr)]PF6·0.05[Cu(CH3CN)4]PF6 (C16.4H27.6Cu1.05F6.3N3.2P1.05):
C, 40.32; H, 5.69; N, 9.17. Found: C, 40.28; H, 5.40; N, 8.96.
[CuII(LPhe(NO2))(NO2)](BF4) (2Phe(NO2)NO2). Ligand LPhe(NO2) (50
mg, 0.14 mmol) was treated with an equimolar amount of
CuIISO4·5H2O (34 mg, 0.14 mmol) in CH3OH (20 mL). After
stirring the mixture for 3 min at room temperature. NaNO2 (12 mg,
0.18 mmol) and NH4BF4 (42 mg, 0.40 mmol) were added to the
solution continuously. Insoluble material was then removed by
filtration. Addition of ether (100 mL) to the filtrate gave a blue
powder that was precipitated by standing the mixture for several
minutes. The supernatant was then removed by decantation, and the
remaining blue solid was washed with ether three times and dried to
give the complex in 87%. Single crystals of 2Phe(NO2)NO2 were
obtained by vapor diffusion of n-hexane into CH2Cl2 solution of the
complex. FT-IR (KBr) 1053 cm−1 (BF4−); Anal. Calcd for
[CuII(LPhe(NO2))(NO2)](BF4)·H2O (C22H32BCuF4N5O5): C, 43.27;
H, 5.19; N, 12.02. found: C, 43.54; H, 5.16; N, 12.08.
[CuII(LiPr)(OAc)](BF4) (2iPrOAc). To an ethanol solution (30 mL)
containing LiPr (59 mg, 0.22 mmol) was added Cu(OAc)2·H2O (45
mg, 0.22 mmol), when the color of the solution turned dark blue. After
stirring 5 min, NH4BF4 (70 mg, 0.67 mmol) was added to the solution,
and the mixture was stirred for 2 min. The volume of the solvent was
reduced under reduced pressure to give a blue powder, which was
collected by filtration and dried (73 mg, 69%). Single crystals were
obtained by vapor diffusion of ether into an acetone solution of the
−
complex. FT-IR (KBr) 1608 cm−1 (CO), 1055 cm−1 (BF4 ); Anal.
Calcd for: [CuII(LiPr)(OAc)]BF4·0.5H2O (C18H31BCuF4N3O2.5): C,
45.06; H, 6.51; N, 8.76; Found: C, 45.01; H, 6.21; N, 8.95; ESI-MS
(pos) m/z = 382.8 calcd for: C18H30CuN3O2: 383.2.
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molecular partition functions (with all frequencies below 50 cm−1
replaced by 50 cm−1 when computing free energies), and for
determining the reactants and products associated with each
transition-state structure (by following the normal modes associated
with imaginary frequencies). Partition functions were used in the
computation of 298 K thermal contributions to free energy employing
the usual ideal-gas, rigid-rotator, harmonic oscillator approximation.58
Solvation effects associated with acetone as solvent were accounted for
by using the SMD continuum solvation model.59 Free energy
contributions were added to single-point M11-L electronic energies
computed with the SDD basis set on Cu and the 6-311+G(2df,p) basis
set on all other atoms to arrive at final, composite free energies.
Addition of triplet molecular oxygen to the closed shell singlet
Cu(I) complexes may in principle generate either open shell singlet or
triplet products. These species were characterized by weak coupling
between two different spin centers, and the singlet states were thus not
well described by a single determinant as is implicit in the standard
Kohn−Sham density functional formalism. In such instances, Kohn−
Sham DFT is not directly applicable,58,60−62 and we adopt the
Yamaguchi broken-spin-symmetry (BS) procedure63,64 to compute the
energy of the spin-purified low-spin (LS) state as
Table 1. X-ray Crystallographic Data of Copper(I)
Complexes 1Phe(Me) and 1iPr
1Phe(Me)
1iPr
formula
formula weight
crystal system
space group
a, Å
C22H31N3CuPF6
546.02
C16H27N3CuPF6
469.92
monoclinic
P21/c (#14)
7.805(9)
19.739(17)
15.504(17)
90
orthorhombic
Pbca (#61)
10.6388(4)
13.7319(5)
26.100(1)
90
b, Å
c, Å
α, deg
β, deg
94.66(5)
90
90
γ, deg
V, Å3
90
2381(4)
4
3813.0(3)
8
Z
F(000)
1128.00
1.523
1936.00
1.637
D
calcd, g/cm−3
T, K
153
103
crystal size, mm
μ (MoKα), cm−1
2θmax, deg
0.40 × 0.30 × 0.20
10.468
0.20 × 0.10 × 0.10
12.921
BSE HS⟨S2⟩ − LS⟨S2⟩ − HSE BS⟨S2⟩ − LS⟨S2⟩
)
(
)
(
LSE =
HS⟨S2⟩ − BS⟨S2⟩
54.9
55.0
no. of reflns measd
no. of reflns obsd
no. of variables
21644
34770
where HS refers to the single-determinantal high-spin coupled state
that is related to the low-spin state by spin flip(s) and ⟨S2⟩ is the
expectation value of the total spin operator applied to the appropriate
determinant. This broken-symmetry DFT approach has routinely
proven effective for the prediction of state-energy splittings in metal
coordination compounds.65−68 Time-dependent DFT (TDDFT)
calculations were performed to predict the UV/visible electronic
excitations of postulated intermediates. The B3LYP69 density
functional, the Stuttgart [8s7p6d | 6s5p3d] ECP10MWB contracted
pseudopotential basis set on Cu, and the 6-311+G(d) basis set56 on all
other atoms were used for the TDDFT calculations with G09 default
nonequilibrium solvation effects computed using the SMD continuum
solvation model for acetonitrile or acetone as solvent.70
5402 (all reflections)
329
4364 (all reflections)
244
a
R1
0.0487
0.0261
b
wR2
0.0761
0.0588
GOF
1.003
1.008
a
b
2
2
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = [∑w(Fo − Fc2)2/∑w(Fo )2]1/2
was monitored. The pseudo-first-order rate constants for the
formation and decomposition of the copper(II) superoxo complexes
were determined from the plots of ln(ΔA) vs time based on the time
course of the absorption change at λmax due to the copper(II) superoxo
complexes.
Ligand Hydroxylation Reaction. Typically, 1Phe(H) (10.6 mg,
0.02 mmol) was dissolved into deaerated acetone (10 mL) under
anaerobic conditions at room temperature, and then the solution was
cooled to −80 °C and kept under O2 atmosphere for 24 h. A mixture
of the hydroxylated ligand and the original ligand was obtained after an
ordinary workup treatment of the reaction mixture with NH4OHaq
and following extraction by CH2Cl2. The yield of hydroxylated ligand
was determined as 34% based on the copper(I) starting material by
using an integral ratio in the 1H NMR spectrum between the benzylic
proton (-CH-OH-) at δ = 4.63 (dd, J = 3.5 and 7.1 Hz) of the
hydroxylated ligand and the pyridine protons at the 6-position (δ =
8.55) from both the hydroxylated ligand and the original ligand.
HRMS (FAB, pos) m/z = 340.2389, calcd for C21H30ON3 340.2389.
The yields of ligand hydroxylation of other ligands LPhe(X) were
determined similarly by 1H NMR as 33, 32, 31, and 29% for X = OMe,
Me, Cl, and NO2, respectively; the benzylic proton (-CH-OH-) for the
hydroxylated ligand: δ = 4.59 (dd, J = 3.5 and 7.5 Hz) for X = OMe; δ
= 4.57 (dd, J = 3.5 and 7.1 Hz) for X = Me; δ = 4.60 (dd, J = 3.5 and
7.0 Hz) for X = Cl; δ = 4.75 (dd, J = 3.5 and 7.1 Hz) for X = NO2.
Computational Methods. All geometries were fully optimized at
the M11-L54 level of density functional theory (DFT) using the
Stuttgart [8s7p6d | 6s5p3d] ECP10MWB contracted pseudopotential
basis set on Cu55 and the 6-31G(d) basis set56 on all other atoms. In
addition, 3 uncontracted f functions having exponents 5.100, 1.275,
and 0.320 were placed on Cu. The grid = ultrafine option in Gaussian
0957 (which was used for all calculations, see ref (1) in Supporting
Information for full citation of ref 56) was chosen for integral
evaluation and an automatically generated density-fitting basis set was
used within the resolution-of-the-identity approximation for the
evaluation of Coulomb integrals. The nature of all stationary points
was verified by analytic computation of vibrational frequencies, which
were also used for the computation of zero-point vibrational energies,
RESULTS AND DISCUSSION
■
Synthesis and Characterization. Copper(I) Complexes.
The tridentate ligands LPhe(X) (X = OMe, Me, H, Cl, NO2)
were prepared by Michael addition of 1-(2-p-substituted-
phenethyl)-1,5-diazacyclooctane to 2-vinylpyridine in refluxing
methanol in the presence of excess amount of acetic acid.71
Ligand LiPr was prepared in a similar manner by using 1-
isopropyl-1,5-diazacyclooctane instead of the p-substituted
phenetyl derivative as indicated in Scheme 2.
Scheme 2. Synthesis of Supporting Ligands LR
Treatment of the ligand with [CuI(CH3CN)4](PF6) in
CH3CN−CH2Cl2 under anaerobic conditions gave the
corresponding mononuclear copper(I) complex, 1Phe(X) and
1iPr. The crystal structures of 1Phe(OMe), 1Phe(H), and 1Phe(NO2)
have already been reported in our previous communication,43
and those of 1Phe(Me), 1Phe(Cl), and 1iPr have been determined in
this study as shown in Figure 3 (1Phe(Me) and 1iPr) and
Supporting Information, Figure S1 (1Phe(Cl)). The crystallo-
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Figure 3. ORTEP drawings of (A) 1Phe(Me) and (B) 1iPr showing 50% probability thermal ellipsoids. The counteranion and the hydrogen atoms are
omitted for clarity.
Table 2. Selected Bond Lengths (Å) and Angles (deg) of 1Phe(Me) and 1iPr
1Phe(Me)
Cu(1)−N(1)
1.899(3)
Cu(1)−N(2)
2.184(4)
Cu(1)−N(3)
1.968(4)
N(1)−Cu(1)−N(2)
N(2)−Cu(1)−N(3)
101.30(14)
90.33(14)
N(1)−Cu(1)−N(3)
168.27(14)
1iPr
Cu(1)−N(1)
Cu(1)−N(3)
N(1)−Cu(1)−N(2)
N(2)−Cu(1)−N(3)
1.9080(14)
1.9865(14)
103.15(6)
90.62(5)
Cu(1)−N(2)
2.1783(13)
165.82(6)
N(1)−Cu(1)−N(3)
graphic data and the selected bond lengths and angles are
standard state conditions (e.g., 1 M CO solution). However, at
25 °C the concentration of CO in acetone will be much less
than this value, thereby disfavoring binding. At low temper-
atures, decreased entropic influence and greater CO solubility
lead to complexation. Thus, it could be said that our ligands LR
highly stabilizes copper(I) complexes with the three-coordinate
T-shape structure without any external coligand. This structural
feature of the copper(I) complexes is somewhat different from
that of the CuB site of reduced PHM, where the copper(I)
center exhibits a distorted four-coordinate tetrahedral geometry
with H2O or CO as the external coligand.18,72
Copper(II) Complexes. A series of copper(II) complexes
having a different counteranion (Y) have been prepared to
examine the structural features of the copper(II) complexes
supported by the ligand LR. The crystal structures of
2Phe(NO2)Cl and 2Phe(NO2)OAc supported by LPhe(NO2) have
already been reported in our previous Communication,43 and
that of 2Phe(NO2)NO2 has been determined in the present study
(see Supporting Information, Figure S3, Table S3 and S4). The
copper(II) complexes of the isopropyl ligand LiPr (2iPrCl,
2iPrOAc, 2iPrN3, and 2iPrNO2) have also been determined in
this study (Figure 4), and the crystallographic data and the
selected bond lengths and angles are summarized in Tables 3
and 4, respectively. All the copper(II) complexes exhibit a
mononuclear structure having the counteranion as an external
coligand.
summarized in Tables 1 and 2 for 1Phe(Me) and 1iPr and
Supporting Information, Tables S1 and S2 for 1Phe(Cl)
,
respectively.
All the copper(I) complexes 1Phe(X) (X = OMe, Me, H, Cl,
and NO2) and 1iPr exhibit a similar three-coordinate T-shape
structure ligated by the three nitrogen atoms N(1), N(2), and
N(3) of the supporting ligand.43 The sum of the three angles
around the copper center [∠N(1)−Cu(1)−N(2), ∠N(1)−
Cu(1)−N(3), and ∠N(2)−Cu(1)−N(3)] is equal to 360°,
indicating that the copper ion and the three nitrogen atoms
exist in the same plane. It should be noted that the copper(I)
complexes do not involve a CH3CN molecule as an external
coligand even though the complexes were prepared using
[CuI(CH3CN)4]+ as the metal source and were crystallized in a
CH3CN-containing solvent (see Experimental Section). In
general, copper(I) complexes with an ordinary didentate or
tridentate ligand do involve one or more CH3CN external
coligand(s) so as to take on a favorable tetrahedral geometry
(assuming that they are prepared in a CH3CN-containing
solvent system). Consistent with experiment, our DFT
calculations at the M11-L level of theory predict that the free
energy of binding of CH3CN to the 1Phe(H) system proceeds
with an unfavorable free energy change of +4.9 kcal/mol at 25
°C.
Such a weak binding ability of the copper(I) complexes 1R
toward external substrate was also seen in the treatment with
CO, which is a well-known substrate exhibiting strong binding
to ordinary copper(I) complexes. For instance, little spectral
changes were observed upon addition of CO gas to an acetone
solution of 1R at 25 °C, even though CO can form an adduct at
very low temperature (−85 °C) as shown in Supporting
Information, Figure S2. The calculated free energy change for
CO binding to 1Phe(H) was found to be −4.2 kcal/mol assuming
The structure of the metal center of 2iPrCl is essentially the
same with that of 2Phe(NO2)Cl,43 having a highly distorted
tetrahedral geometry with the N3Cl donor set (Figure 4A). The
τ4 values for 2iPrCl and 2Phe(NO2)Cl, a simple geometry index for
four-coordinate complexes proposed by Houser and his co-
workers, are 0.57 and 0.54, respectively; τ4 = [360° − (α + β)]/
141, where α and β are the two largest θ angles in the four-
coordinate species.73 The values of τ4 will range from 1.00 for a
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substituents may prevent formation of a Cu2Cl2 (doubly
chloride-bridged) dimer.
The acetate complex 2iPrOAc also shows a monomeric
structure as in the case of 2Phe(NO2)OAc, in which the acetate
anion looks like it acts as a didentate ligand. However, the
distance of Cu−O(2) (dCu−O(2) = 2.588(3) Å) is significantly
larger than that of Cu−O(1) (dCu−O(1) = 1.973(3) Å) (Table
4). Thus, the structure of 2iPrOAc can be regarded as a
distorted tetrahedron with the N3O donor set having a weakly
interacting acetate O(2) atom (so-called capped tetrahedron),
where the τ4 value is 0.45.
Recently, Amzel and co-workers reported the crystal
structures of PHM soaked with NaNO2 and NaN3.74 They
unambiguously demonstrated that only the CuB site of PHM
can bind NO2− and N3− anions, consistent with the conclusion
that CuB is the oxygen- and substrate-binding site whereas CuA
is the electron acceptor site from ascorbate.17,20 In this study,
the crystal structures of our model compounds 2iPrNO2
(2Phe(NO2)NO2) and 2iPrN3 were compared to those of the
−
−
active sites of the resting state of PHM having NO2 and N3
coordinating counteranion.
The overall structure of 2iPrNO2 is similar to that of 2iPrOAc
(Figure 4B), where the distance of Cu−O(1) (dCu−O(1)
2.009(3) Å) is much shorter than that of Cu−O(2) (dCu−O(2)
=
=
2.513(4) Å) (Figure 4C and Table 4). Thus, the structure of
2iPrNO2 can also be described as distorted tetrahedron with a
weakly interacting O(2) atom (capped tetrahedron).
Figure 4. ORTEP drawings of (A) 2iPrCl, (B) 2iPrOAc, (C) 2iPrNO2,
and (D) 2iPrN3 showing 50% probability thermal ellipsoids. The
uncoordinated counteranion and the hydrogen atoms are omitted for
clarity.
2
Phe(NO2)NO2 exhibits a similar structure to that of 2iPrNO2;
dCu−O(1) = 2.018(4) Å and dCu−O(2) = 2.432(4) Å; (Supporting
−
Information, Figure S3). The binding of NO2 in the CuB site
of the nitrite-soaked PHM (copper-to-nitrite oxygen distances
are 1.9 and 2.6 Å) is comparable to those of our model
compounds.74
perfect tetrahedral geometry to zero for a perfect square planar
geometry.73 The isopropyl (iPr) and the phenetyl (Phe(X))
Table 3. X-ray Crystallographic Data of Copper(II) Complexes 2iPrY
2iPrCl
2iPrOAc
2iPrNO2
2iPrN3
formula
formula weight
crystal system
space group
a, Å
C40H47BClCuN3
679.64
C18H30BCuF4N3O2
470.80
C16H27BCuF4N4O2
C16H27BCuF4N6
453.78
457.76
triclinic
monoclinic
P21/n (#14)
11.9825(5)
12.6970(5)
14.8164(6)
90
orthorhombic
monoclinic
Pn (#7)
7.4111(5)
14.733(1)
9.2356(6)
90
P1 (#2)
P21212 (#18)
9.7137(6)
̅
11.1894(5)
11.3597(6)
14.2362(8)
103.344(1)
90.250(1)
102.542(1)
1715.8(2)
2
b, Å
26.834(2)
c, Å
7.4613(5)
α, deg
90
β, deg
109.849(2)
90
90
98.941(1)
90
γ, deg
V, Å3
90
2120.3(1)
4
1944.8(3)
996.2(1)
2
Z
4
F(000)
718.00
980.00
948.00
470.00
D
calcd, g/cm−3
1.315
1.475
1.563
1.513
T, K
113
113
103
113
crystal size, mm
μ (MoKa), cm−1
2θmax, deg
0.32 × 0.16 × 0.03
7.474
0.52 × 0.28 × 0.09
10.833
0.10 × 0.10 × 0.10
11.800
0.24 × 0.21 × 0.06
11.467
54.9
54.9
54.9
54.9
no. of reflns measd
no. of reflns obsd
no. of variables
16970
20291
18830
9544
7975 [I > 1.00σ(I)]
460
4039 [I > 1.00σ(I)]
310
4450 (all reflections)
7450 [I > 1.00σ(I)]
281
280
a
R1
0.0504
0.0566
0.0538
0.1330
1.163
0.0454
b
wR2
0.1239
0.1871
0.0732
GOF
0.997
0.978
1.047
a
b
2
2
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = [∑w(Fo − Fc2)2/∑w(Fo )2]1/2
.
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Table 4. Selected Bond Lengths (Å) and Angles (deg) of Copper(II) Complexes 2iPrY
2iPrCl
Cu(1)−N(1)
1.976(4)
Cu(1)−N(2)
1.996(4)
Cu(1)−N(3)
2.036(4)
Cu(1)−Cl(1)
2.2345(11)
131.36(11)
98.87(16)
90.61(15)
Cl(1)−Cu(1)−N(1)
Cl(1)−Cu(1)−N(3)
N(1)−Cu(1)−N(3)
96.54(10)
99.45(10)
148.55(14)
Cl(1)−Cu(1)−N(2)
N(1)−Cu(1)−N(2)
N(2)−Cu(1)−N(3)
2iPrOAc
2iPrNO2
2iPrN3
Cu(1)−N(1)
Cu(1)−N(3)
Cu(1)−O(2)
O(1)−Cu(1)−N(1)
O(2)−Cu(1)−N(3)
N(1)−Cu(1)−N(2)
N(2)−Cu(1)−N(3)
2.022(4)
2.036(3)
2.588(3)
92.17(12)
95.45(11)
98.78(14)
90.65(13)
Cu(1)−N(2)
Cu(1)−O(1)
2.000(4)
1.973(3)
O(1)−Cu(1)−N(2)
O(1)−Cu(1)−O(2)
N(1)−Cu(1)−N(3)
148.12(13)
56.29(10)
148.54(14)
Cu(1)−N(1)
Cu(1)−N(3)
Cu(1)−O(2)
O(1)−Cu(1)−N(1)
O(2)−Cu(1)−N(3)
N(1)−Cu(1)−N(2)
N(2)−Cu(1)−N(3)
2.025(4)
2.042(3)
2.513(4)
92.10(13)
95.41(13)
98.77(14)
90.64(13)
Cu(1)−N(2)
Cu(1)−O(1)
1.980(4)
2.009(3)
O(1)−Cu(1)−N(2)
O(1)−Cu(1)−O(2)
N(1)−Cu(1)−N(3)
149.66(14)
54.32(12)
147.19(14)
Cu(1)−N(1)
1.979(3)
Cu(1)−N(2)
2.024(3)
Cu(1)−N(3)
2.016(3)
Cu(1)−N(4)
1.983(4)
N(4)−Cu(1)−N(1)
N(4)−Cu(1)−N(3)
N(1)−Cu(1)−N(3)
91.85(12)
94.80(12)
158.38(11)
N(4)−Cu(1)−N(2)
N(1)−Cu(1)−N(2)
N(2)−Cu(1)−N(3)
141.73(11)
95.66(12)
91.79(11)
Table 5. UV-vis Data, EPR Parameters, and Redox Potentials of Copper(II) Complexes 2iPrY
a
b
c
complex
iPrCl
λmax (nm) ε (M−1 cm−1
)
EPR parameter
E1/2 (V)
ΔEp (V)
2
643 (260)
854 (300)
655 (310)
745 (300)
650 (340)
725 (350)
435 (2220)
643 (690)
∼780 (shoulder)
g∥ = 2.22
g⊥ = 2.09
g∥ = 2.23
g⊥ = 2.06
g∥ = 2.22
g⊥ = 2.08
g∥ = 2.22
g⊥ = 2.09
A∥ = 144 G
0.015
0.11
0.14
0.12
0.15
2iPrOAc
2iPrNO2
2iPrN3
A∥ = 156 G
A∥ = 160 G
A∥ = 153 G
−0.22
−0.07
−0.03
a
b
c
In CH3CN. In acetone. In CH3CN containing 0.1 M tetra-n-butylammonium perchlorate (TBAP) at 25 °C. Working electrode glassy carbon,
counter electrode Pt, reference electrode Ag/0.01 M AgNO3, scan rate 50 mV/s.
The azide complex 2iPrN3 also exhibits a mononuclear
structure with a highly distorted tetrahedral geometry having a
band due to the azide ligand at 435 nm (Supporting
Information, Figure S7). To explore the electronic-structures
in detail, time-dependent DFT (TD-DFT) calculations employ-
ing the hybrid B3LYP density functional and accounting for
nonequilibrium solvation were undertaken. The calculated
spectra and the predicted λmax values for the 2iPrY complexes
were found to be in good agreement with the experimental
UV−vis data and are listed in the Supporting Information along
with the description of molecular orbitals involved in the
individual excitations (Supporting Information, Figures S16−
S19 and Tables S5−S8). Analysis of the TD-B3LYP excitations
reveals a complex pattern for the broad peaks in the 600−800
nm range whereas the LMCT bands at higher energies could be
easily assigned as in the case of 2iPrN3. All of the excitations
take place into the same acceptor orbital, which is the lowest
unoccupied molecular orbital (LUMO) in the beta manifold,
but the individual occupied orbitals from which the excitations
occur are diverse as illustrated in the Supporting Information,
−
monodentate N3 ligand; dCu−N(4) = 1.983(4) Å, τ4 = 0.42
(Figure 4D). The structural feature of 2iPrN3 is also similar to
that of the CuB site in the NaN3-soaked PHM; four-coordinate
tetrahedral geometry with dCu−N(4) = 2.0 Å.74 All these results
suggest that the copper(II) complexes supported by LiPr and
LPhe(X) are good structure models for the CuB site of PHM
having the copper(II) oxidation state.
To obtain information about the copper(II) complexes in
solution, UV−vis and EPR spectra as well as cyclic voltammetry
of 2iPrY were measured as summarized in Table 5. The spectra
and cyclic voltammograms are presented in Supporting
Information (Supporting Information, Figure S4 ∼ S15).
The copper(II) complexes exhibit at least two broad and
weak d-d bands around 650 nm and above 700 nm (the d-d
bands of 2iPrCl are exceptionally well separated, see Supporting
Information, Figure S4), and 2iPrN3 shows a distinct LMCT
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Tables S5−S8. The LUMOs exhibit significant Cu d character
with smaller contributions from π* orbitals of nitrogen atoms
of 2iPr and coordinating atoms of the anionic ligands. For the
electronic excitations in the 600−800 nm range, the individual
occupied orbitals from which the excitations occur display
significant amplitude on Cu centers (and to a lesser amount on
2iPr ligands) as well as on the π* orbitals of the coordinated
anionic ligands. On the other hand, the peaks in the 400−430
nm range for the 2iPrN3 involve electronic excitations from
molecular orbitals with mainly N3-π* character to LUMO
orbitals confirming the LMCT assignment.
The EPR spectra of the copper(II) complexes are presented
in Supporting Information, Figures S8∼S11, and the EPR
parameters are summarized in Table 5. The complexes exhibit
relatively small g∥ (∼2.22) and A∥ values as compared to those
of ordinary copper(II) complexes having tetragonal geometry,
suggesting that the tetrahedral distortion of the four-coordinate
structure is maintained in solution.
Figure 5. UV−vis spectral change for the oxygenation reaction of 1iPr
(0.2 mM) in acetone at −85 °C (50 s interval). Inset: First-order plot
based on the absorption changes at 395 nm.
16O2 and 18O2 derivatives clearly indicated the existence of the
O−O stretching vibration band, its intensity was significantly
weaker as compared to the Cu−O band.43 In this study, we
have re-examined the resonance Raman spectrum of the
superoxo copper(II) species using 2iPrO2 (isopropyl derivative)
using different excitation laser light at λex = 442 and 647 nm.
However, no improvement of the peak intensity was obtained
for the O−O stretching vibration band. The O−O moiety of
the end-on superoxide ligand may take some different
geometries relative to the ligand (LiPr) backbone, causing the
broadening of the Raman peak due to O−O stretching.
Kitajima and co-workers reported a mononuclear copper(II)-
chloride complex supported by HB(3,5-iPr2pz)3 [hydrotris(3,5-
diisopropyl-1-pyrazolyl)borate] having a tetrahedral geome-
try.75 This compound was, however, easily converted to a five-
coordinate copper(II)-Cl complex having additional dimethyl-
formamide (DMF) or dimethylsulfoxide (DMSO) solvent
molecules, when the four-coordinate copper(II)-Cl complex
was treated with a small amount of DMF or DMSO.75 With our
copper(II)-chloride complex 2iPrCl, on the other hand, no
spectral change was observed even in the presence of large
excess (5.0 M) of coordinating solvent such as DMF, DMSO,
and CH3CN. Thus, the four-coordinate copper(II) complex
2iPrCl strongly resists the binding of external fifth ligand. In
other words, the copper(II) complexes supported by ligand LR
strongly favor the four-coordinate structure.
The fine-structure EPR study indicated that 2Phe(H)O2 has a
spin triplet ground state (S = 1).43 This assignment is
supported by DFT calculations. Theoretical calculations at
the M11-L level of theory show that both end-on and pseudo-
side-on geometries are possible for 2Phe(H)O2 species, with the
former isomer being slightly (∼0.6 kcal/mol) more stable than
the latter. For both isomers, the triplet spin state was found to
be the ground state with the singlet structures higher in energy
by about 7.0 kcal/mol (Supporting Information, Figure S25).
The formation of 2iPrO2 shown in Figure 5 follows first-order
kinetics in the presence of an excess amount of O2 (9.26 mM in
acetone at −85 °C; O2 concentrations were determined as
described in Experimental Section), and the pseudo-first-order
rate constant kobs was determined from the first-order plot
shown in the inset of Figure 5. Then, the rate-dependence on
the O2-concentration was examined as shown in Figure 6,
The copper(II) complexes 2iPrY exhibited a quasi-reversible
CuII/CuI redox couple (E1/2) around 0 V vs Ag/0.01 M AgNO3
in CH3CN. The slight difference in E1/2 among the complexes
may be due to the different electronic effects of the
coordinating counteranions Y.
Formation and Characterization of Copper(II) Super-
oxo Complex. Reversible O2-Binding to Copper(I) Complex.
As we have already reported in the previous Communication,
the copper(I) complexes supported by the phenetyl derivative
LPhe(H) reacted with O2 in a 1:1 ratio (determined by
manometry) at a low temperature (−85 °C) to produce
copper(II) superoxo complexes 2Phe(H)O2, exhibiting character-
istic absorption bands at 397 nm (ε = 4200 M−1 cm−1), 570 nm
(850), and 705 nm (1150) (Scheme 1).43 In this study,
essentially the same spectra were obtained in the reactions of
other phenetyl derivatives 1Phe(X) (X = OMe, Me, Cl, and NO2)
and isopropyl derivative 1iPr with O2 under the same
experimental conditions, demonstrating that the same copper-
(II)-superoxo complexes 2Phe(X)O2 (X = OMe, Me, Cl, and
NO2) and 2iPrO2 were generated in all the ligand systems. A
typical example of the UV−vis spectral change for the
oxygenation reaction of 1iPr is shown in Figure 5, where an
intense LMCT band at 395 nm (ε = 5810 M−1 cm−1) readily
appeared together with the broad absorption bands around 585
(980) and 723 nm (1240).
In the resonance Raman spectrum excited at λex = 406.7 nm,
the superoxo complex 2Phe(H)O2 exhibited O−O and Cu−O
stretching vibrations at 1033 cm−1 and 457 cm−1, respectively,
which shifted to 968 cm−1 and 442 cm−1 upon 18O2-
substitution.43 Although the difference spectrum between the
Figure 6. Dependence of the observed first-order rate constant (kobs
)
on the dioxygen concentration for the formation of 2iPrO2 in acetone
at −90 °C.
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Figure 7. Plots of ln(kon/T), ln(koff/T), and ln Keq vs 1/T for the reaction of 1iPr and O2 to give 2iPrO2 in acetone.
Table 6. Kinetic and Thermodynamic Parameters for the Formation of Copper(II)-Superoxo Complexes (1:1-Adduct) upon
Low Temperature Oxygenation of Copper(I) Complexes
activation
parameters (kon
activation parameters
thermodynamic
b
a
b
a
b
a
ligand
LiPr
solvent
acetone
kon (M−1 s−1
)
)
koff (s−1
)
(koff
)
Keq (M−1
)
parameters (Keq)
(4.0 0.1) × 10−1
(7.6 0.2) × 10−1
(1.18 0.01) × 104
(9.5 0.4) × 104
ΔH⧧ = 15.5 0.9
ΔS⧧ = −164
ΔH⧧ = 24.4 1.3
ΔS⧧ = −110
ΔH⧧ = 31.6 0.5
ΔS⧧ = 10
ΔH⧧ = 17.1 0.6
ΔS⧧ = −52
(3.8 0.7) × 10−4
(1.1 0.1) × 10−3
(1.59 0.01) × 101
(7.0 0.3) × 10−2
ΔH⧧ = 53.5 1.2
ΔS⧧ = −14.4 6.2
ΔH⧧ = 33.9 1.1
(1.05 0.1) × 103
(7.04 0.1) × 102
(7.42 0.04) × 102
(1.35 0.04) × 106
ΔH0 = −38.0 1.3
5
ΔS0 = −150
7
LiPr
THF
ΔH0 = −9.50 0.31
ΔS0 = 2.57 1.60
ΔH0 = −29.8 0.2
7
ΔS⧧ = −13
6
tmpa77
Me6tren77
EtCN
ΔH⧧ = 61.5 0.5
ΔS⧧ = 18
ΔH⧧ = 62.0 0.6
ΔS⧧ = 76
3
3
ΔS0 = −108
1
EtCN
ΔH0 = −44.9 0.2
ΔS0 = −128
3
3
1
a
b
At −90 °C. Units: ΔH⧧ and ΔH0 in kJ mol−1 ΔS⧧ and ΔS0 in J K−1 mol−1.
where the plot of kobs against [O2] gave a linear correlation with
a distinct intercept on the y-axis (Figure 6 shows the rate-
dependence on [O2] at −90 °C. The rate dependence on [O2]
at −85, −80, −75, and −70 °C are shown in Supporting
Information, Figure S20). Existence of such an intercept in the
plot indicates that the O2-binding is reversible (eq 1),
pyrydylmethyl)amine] and the Me6tren [tris(2-
dimethylaminoethyl)amine] in propionitrile reported by
Schindler, Zuberbuhler, and co-workers are also included in
̈
Table 6.77 Similar results for the tmpa system were also
reported by Karlin, Zuberbuhler, and co-workers.78 However,
̈
the O2-adduct formation reaction of 1iPr in propionitrile was
too slow, competing with the self-decomposition reaction of
generated superoxo complex 2iPrO2. Thus, the O2-binding
process of 1iPr in propionitrile could not be examined
accurately.
kon
iPr
1
+ O2 XooY 2iPrO2
koff
(1)
and the association rate constant kon and the dissociation rate
constant koff of O2 are equal to the slope and the intercept,
respectively (eq 2).76
The association of O2 to 1iPr (kon process) in both acetone
and THF are significantly slower than those of the copper(I)
complexes of tmpa and Me6tren in propionitrile. In the tmpa
and Me6tren ligand systems, it was suggested that the solvent
molecule EtCN binds strongly to the copper(I) ion and thus
dissociation of EtCN from the copper(I) complex is rate-
limiting for the O2-adduct formation (dissociative mechanism).
This is reflected in the positive or small negative ΔS⧧ values for
tmpa and Me6tren systems, respectively (Table 6). On the
other hand, our ligand system (LiPr) exhibits very large negative
ΔS⧧ values both in acetone and THF, suggesting that the
association of O2 to the copper(I) ion is rate-limiting
(associative mechanism), and the transition-state structure of
the O2-adduct is highly restricted (Table 6). This is the reason
for the much smaller kon values in the LiPr ligand system.
With respect to the koff process, the positive ΔS⧧ values in
the tmpa and Me6tren systems suggest that dissociation of O2
from the superoxo complex is rate-limiting and the resulting
copper(I) complex quickly binds to EtCN. This may be due to
the stronger binding of the nitrile solvent to copper(I). On the
other hand, the negative ΔS⧧ values in the LiPr system indicate
that association of the solvent molecules (acetone and THF) to
kobs = kon[O2] + koff
(2)
Reversibility of the O2-binding has been confirmed by the
separate experiments demonstrating that O2 was removed by
bubbling with Ar and applying a vacuum and then reforming
the superoxo complex by exposure to O2 (Supporting
Information, Figure S21). Then, the equilibrium constant Keq
can be determined by eq 3,
Keq = kon/koff
(3)
and the kinetic and thermodynamic parameters were
determined from the linear plots of ln(kon/T), ln(koff/T), and
ln Keq against 1/T as presented in Figures 7, respectively. The
kinetic and thermodynamic parameters for the reactions in
THF were determined in the similar manner (Supporting
Information, Figures S22 and S23), and data are listed in Table
6 together with those of the reactions of 1iPr in acetone. The
kinetic and thermodynamic parameters for the (1: 1) O2-adduct
formation reactions between the copper(I) complexes
supported by the tripodal tetradentate ligands tmpa [tris(2-
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copper(I) complexes is rate-limiting rather than O2-dissocia-
tion.
With regard to the thermodynamic parameters, the LiPr
system in THF is exceptional, showing a small negative ΔH0
and positive ΔS0. This clearly indicates that the binding energy
of end-on superoxide to copper(II) ion in 2iPrO2 is weaker as
compared with those in the other systems. This may be due to
the existence of some interaction between a THF molecule and
the superoxo complex 2iPrO2. On the other hand, the
thermodynamic parameters in acetone are similar to those of
tmpa and Me6tren ligand systems, where there may be no
solvent coordination to the five-coordinate cupric ion in the
superoxo copper(II) complexes. This may indicate that there is
no solvent coordination to the copper(II) ion in 2iPrO2 in
acetone neither. The stability of the four-coordinate structure in
the copper(II) oxidation state in the present ligand system has
been well demonstrated above.
Figure 8. UV−vis spectral change for the decomposition (ligand
hydroxylation) of 2Phe(H)O2 (0.2 mM) in acetone at −65 °C. Inset:
First-order plot based on the absorption changes at 395 nm.
Reactivity of Copper(II) Superoxo Complexes. Ali-
phatic Ligand Hydroxylation. The copper(II) superoxo
complex 2Phe(H)O2 has previously been shown to undergo an
aliphatic ligand hydroxylation at its benzylic position to give
alkoxo-copper(II) complexes 3Phe(H) (Scheme 3).43 Isotope
ligand hydroxylation process as ΔH⧧ = 19 0.1 kJ mol−1 and
ΔS⧧= −223 0.6 J K−1 mol−1.
Electronic effects of the p-substituent (X) on the
hydroxylation reaction were also examined using a series of
p-substituted phenetyl derivatives 2Phe(X)O2 (X = OMe, Me, H,
Cl, NO2). The plot of log kdec against the Hammett σp
constants gave a negative small ρ value (−0.63; R = 0.88;
Figure 9), suggesting that the reactive species exhibits a weakly
Scheme 3. Aliphatic Ligand Hydroxylation in End-on
Superoxo Copper(II) Complexes 2Phe(X)O2
labeling experiments using 18O2 unambiguously indicated that
the oxygen atom of the hydroxyl group comes from molecular
43
oxygen used for the preparation of 2Phe(H)O2. The H NMR
analysis of organic product obtained by demetalation from the
final product complex using NH4OH(aq) demonstrated that
the phenetyl side arm was hydroxylated in ∼35% yield based on
the starting material.43
1
Figure 9. Hammett plot for the ligand hydroxylation in acetone at −60
°C.
In this study, the decomposition (aliphatic ligand hydrox-
ylation) process has been examined in more detail to get
mechanistic insight into the C−H bond activation process
(Scheme 3). Figure 8 shows the spectral change for the
decomposition reaction of 2Phe(H)O2 at −65 °C as a typical
example. As clearly shown in the inset of Figure 8, the reaction
obeyed first-order kinetics, suggesting that the ligand
hydroxylation reaction is a unimolecular (intramolecular)
process. The first-order kinetics was further confirmed by the
fact that identical first-order rate constants were obtained in the
copper concentration range of 0.1−0.4 mM. The decom-
position reactions of all other ligands LR also exhibited the first-
order kinetics. In the reaction of perdeuterated phenetyl
derivative LPhe(H)-d4 (ethylene linker moiety is fully deuterated)
appreciable amount of kinetic isotope effect (KIE) value [KIE =
4.1; kdec(H) = 2.5 × 10−4 s−1 and kdec(D) = 0.60 × 10−4 s−1] was
obtained, indicating that the reaction involves the benzylic C−
H bond activation. Furthermore, analysis of the rate-depend-
ence on the reaction temperature (Eyring plot, Supporting
Information, Figure S24) gives activation parameters for the
nucleophilic nature and that the ligand hydroxylation reaction
involves hydrogen atom abstraction from the benzylic position
of the ligand side arm.
To provide microscopic insight into the details of the
mechanistic pathway and various potential intermediates of the
ligand hydroxylation, DFT calculations at the M11-L level of
theory were undertaken (see Computational Methods section
for full details). To begin, 1Phe(H) and O2 were chosen as the
reactants, and all free energy changes in the proposed
mechanisms are reported with respect to separated 1Phe(H)
and O2 (Figure 10).
The proposed mechanism for the ligand hydroxylation starts
with the formation of 2Phe(H)O2 which can be viewed as a
copper(II) superoxo complex based on the extent of localized
spin density on the copper and oxygen centers. Two
conformers, namely, end-on and pseudo side-on (Figure 11a
and 11b, respectively), were located for 2Phe(H)O2 that are close
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Figure 10. Proposed mechanism for the hydroxylation reaction of 1Phe(H). Relative free energies at 298.15 K are reported in units of kcal/mol.
in energy, with the end-on structure being slightly more stable
(∼0.6 kcal/mol). For both conformers the triplet spin state was
found to be the ground state with broken symmetry singlet
structures lying about 7.0 kcal/mol higher in energy as stated
above.
intermediate III abstracts the hydrogen atom of the hydroxyl
group at the benzylic position, TS-IV (Figure11i) and generates
the same final product II as in the first path. This step proceeds
with a ΔG⧧ of 8.2 kcal/mol relative to III. A comparison of the
free energy changes indicates that Path 1 is favored significantly
over Path 2 as the rate determining step (TS-II) in Path 1 has a
ΔG⧧ of 22.5 kcal/mol compared to 33.2 kcal/mol (TS-III) for
Path 2, all taken relative to separated reactants. Both pathways
lead to the same intermediate II which has a triplet ground
state and can be described as a supported Cu(II) center with an
additional unpaired electron delocalized over the two oxygen
atoms. Subsequent reduction of intermediate II by unreacted
copper(I) complex 1Phe(H) and hydroxide transfer leads to the
formation of the Cu(II)-alkoxide product 3Phe(H) and Cu(II)-
hydroxide byproduct, which is quite exergonic with a predicted
free energy change of −35.7 kcal/mol (Scheme 4). Invocation
of this comproportionation step implies a maximum yield for
ligand hydroxylation of 50%. As the redox step follows the rate
determining H-atom abstraction step, the second-order
character of the redox step does not affect the kinetics.
In addition to these considerations of the reactions of the
mononuclear complex, we also investigated the possible
formation and reactivity of dinuclear species. Our calculations
indicate that the formation of dinuclear trans-peroxo (4Phe(H)
trans-peroxo; this compound has a geometry that is actually
intermediate between side-on and end-on peroxo, see
Supporting Information, Figure S25) and bis-μ-oxo (4Phe(H)
bis-μ-oxo) species are possible with standard-state ΔG values
As a next step, we considered hydrogen atom abstraction
from the benzylic position by the copper(II) superoxo unit and
we located a transition state structure, TS-I (Figure 11c), which
has a free energy of activation of 21.8 kcal/mol including
tunneling effects.79 The resulting intermediate, I (Figure 11d),
features a copper(II) hydroperoxo complex with a nearby
radical center at the benzylic position. Following generation of
intermediate I, we have identified two possible pathways for the
hydroxylation step. Path 1 involves the oxygenation of the
benzylic position by the proximal oxygen to the copper center
with concomitant O−O bond scission. The located transition
state structure on this path, TS-II (Figure 11e), has ΔG⧧ = 8.3
kcal/mol relative to the preceding intermediate and generates
the final product, II (Figure 11f). This step is highly exergonic
with a free energy change of −59.9 kcal/mol. Path 2 starts with
the hydroxylation of the benzylic position by the hydroxo unit
of the copper(II) hydroperoxo intermediate I also with
concomitant O−O bond scission. The free energy of activation
associated with the relevant transition state (TS) structure, TS-
III (Figure 11g), is 19.0 kcal/mol relative to I and the resulting
product, III (Figure 11h), with hydroxylated benzylic position
and a copper(II) oxyl group is 42.0 kcal/mol more stable than
I. In the final step of this path, the copper(II) oxyl group in
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Figure 11. M11-L optimized structures of several complexes potentially relevant to the C−H bond activation reaction mechanism.
Scheme 4. Reduction of Intermediate II by Copper(I) Starting Material 1Phe(H) (The Final Step of the Reaction)
of −4.5 and 13.0 kcal/mol, respectively. The corresponding TS
structures for hydrogen atom abstraction from the benzylic
positions are associated with ΔG⧧ values of 23.1 and 16.4 kcal/
mol, respectively (Figure 12). While these absolute values are
lower than those predicted for the unimolecular pathway, it
must be recalled that these are standard-state free energies, and
are thus only subject to direct comparison when all species are
present at 1 M concentration. As the concentration of 1Phe(H) is
orders of magnitude smaller than 1 mol/L under experimental
conditions, the formation and reactivity of dinuclear species is
rendered unfavorable, although at much higher concentrations
the bis-μ-oxo pathway might well begin to contribute and
ultimately dominate the reactivity. In any case, theory is
consistent with the observation that the ligand hydroxylation
reaction displays first order kinetics and unchanging rate
constant over a copper concentration range of 0.1−0.4 mM.
On the basis of these theoretical results, we conclude that
ligand hydroxylation proceeds by hydrogen atom abstraction
from the benzylic position initiated by the copper(II) superoxo
unit of 2Phe(H)O2 followed by oxygenation of the benzylic
position by the oxygen proximal to the copper center in I with
concomitant O−O bond scission. Including an estimation for
tunneling effects, we predict a ΔG⧧ of 20.9 kcal/mol for the key
hydrogen abstraction step by 2Phe(H)O2, and we calculated the
KIE for this step to be 5.6 at −65 °C, which is in fair agreement
with the experimental predictions of about 16.0 kcal/mol for
ΔG⧧ with a KIE of 4.1. The smaller KIE observed
experimentally may indeed be due to some kinetic complexity
introduced by the “rebound”-like step of benzylic oxygenation
(I to TS-II in Figure 10) having an activation free energy
similar to that for C−H abstraction. At the M-11 L level, we
predict the rebound to have a somewhat higher free energy of
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Figure 12. Reactivities of possible dinuclear species, 4Phe(H) trans-peroxo and 4Phe(H) bis-μ-oxo. The relative free energies at 298.15 K are reported
in units of kcal/mol.
activation, but the difference of 3 kcal/mol is likely within the
error of the various approximations in the theoretical model.
itself is an active species to induce the aliphatic ligand
hydroxylation reaction. DFT studies at the M11-L level support
that; at low copper concentrations, C−H bond abstraction
occurs unimolecularly from the superoxo complex with
subsequent rebound of the copper hydroperoxo species to
generate an oxygenated ligand.
To date, the reactivity of several mononuclear copper(II)
superoxo complexes has been examined to assess possible
involvement of such species in related enzymatic reactions, but
none of the superoxo copper(II) model complexes has been
shown to exhibit direct C−H bond activation of organic
substrates.37,41,42 Recently, mononuclear copper(II) superoxo
species has been invoked as the key intermediate in the
enzymatic reactions catalyzed by PHM and DβM. The present
model studies indicate that such a superoxo species could be
reactive enough to induce the direct C−H bond activation of
aliphatic substrates.
CONCLUSION
■
In this study, structures and physicochemical properties as well
as reactivity of the copper(I) and copper(II) complexes
supported by the new tridentate ligands LR have been examined
in detail in order to gain insight into the chemistry in the active
sites of mononuclear copper monooxygenases. The ligand
exhibits the unique feature that in the solid state it stabilizes the
copper(I) complexes in a T-shaped geometry and the
copper(II) complexes in a distorted tetrahedral geometry.
The copper(II) complexes also adopt the four-coordinate
structure in solution as demonstrated by the EPR spectra and
by the observation that the copper(II) complexes exhibit the
same spectra both in the absence and in the presence of highly
coordinating co-solvents such as DMSO and DMF in CH2Cl2.
Such a distorted tetrahedral structure of the copper(II)
complexes resembles the structure of the CuB site of the
resting state of PHM.74
ASSOCIATED CONTENT
* Supporting Information
■
S
Low temperature oxygenation of the copper(I) complexes
generated mononuclear copper(II) end-on superoxo complexes
having an intense absorption band at ∼395 nm and broad
bands at 585 and 723 nm (Figure 5) and isotope sensitive O−
O and Cu−O stretching vibrations of the end-on superoxo
species. The superoxo complex has a triplet ground state, which
has also been well confirmed by parallel mode EPR43 and DFT
calculations. Detailed kinetic analysis for the O2-adduct
formation reaction gave kinetic and thermodynamic parameters
providing mechanistic insights into the association and
dissociation processes of O2 to the copper complexes.
Crystallographic data in CIF format. Further details are given in
Figures S1−S24 and Tables S1−S8. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: shinobu@mls.eng.osaka-u.ac.jp (S.I.), cramer@umn.
edu (C.J.C.).
Notes
The authors declare no competing financial interest.
The copper(II) end-on superoxo complex gradually decom-
poses to induce aliphatic ligand hydroxylation to give an
alkoxocopper(II) product (Scheme 3). Kinetic studies on the
decomposition reaction suggested that the superoxo complex
ACKNOWLEDGMENTS
■
This work was partially supported by a Grant-in-Aid for
Scientific Research on Innovative Areas “Molecular Activation
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(34) Schatz, M.; Raab, V.; Foxon, S. P.; Brehm, G.; Schneider, S.;
Reiher, M.; Holthausen, M. C.; Sundermeyer, J.; Schindler, S. Angew.
Chem., Int. Ed. 2004, 43, 4360.
Directed toward Straightforward Synthesis” from the Ministry
of Education, Culture, Sports, Science and Technology, Japan
(SI, No. 22105007). We are also grateful for financial support
from the U.S. National Science Foundation (C.J.C., M.Z.E.,
CHE0952054). The authors acknowledge Dr. Minoru Kubo
and Prof. Takashi Ogura of University of Hyogo for their help
in the trial of resonance Raman measurements of 2iPrO2.
(35) Wurtele, C.; Gaoutchenova, E.; Harms, K.; Holthausen, M. C.;
̈
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