Copper(I)/O2 chemistry with imidazole containing tripodal tetradentate ligands leading to μ-1,2-peroxo-dicopper(II) species

Article abstract of DOI:10.1021/ic9017695

Cuprous and cupric complexes with the new imidazolyl containing tripodal tetradentate ligands {L^Mlm, (1 H-imidazol4-yl)-N,N-bis((pyridin-2-yl) methyl)methanamine, and L^Elm, 2-(1 H-imidazol-4-yl)-N,N-bis((pyridin- 2-yl)methyl)ethanami

Full text of DOI:10.1021/ic9017695

Inorg. Chem. 2009, 48, 11297–11309 11297
DOI: 10.1021/ic9017695
Copper(I)/O2 Chemistry with Imidazole Containing Tripodal Tetradentate Ligands
Leading to μ-1,2-Peroxo-Dicopper(II) Species
Yunho Lee,† Ga Young Park,† Heather R. Lucas,† Peter L. Vajda,† Kaliappan Kamaraj,† Michael A. Vance,‡
Ashley E. Milligan,‡ Julia S. Woertink,‡ Maxime A. Siegler,† Amy A. Narducci Sarjeant,†, Lev N. Zakharov,§,^
Arnold L. Rheingold,§,X Edward I. Solomon,*,‡ and Kenneth D. Karlin*,†
†Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218, ‡Department of
Chemistry, Stanford University, Stanford, California 94305, and §Department of Chemistry, University of
Delaware, Newark, Delaware 19716. Current address: Department of Chemistry, Northwestern University,
Evanston IL. ^ Current address: Department of Chemistry, University of Oregon, Eugene OR. X Current
address: Department of Chemistry and Biology, UCSD, La Jolla, CA.
Received September 8, 2009
Cuprous and cupric complexes with the new imidazolyl containing tripodal tetradentate ligands {LMIm, (1H-imidazol-
4-yl)-N,N-bis((pyridin-2-yl)methyl)methanamine, and LEIm, 2-(1H-imidazol-4-yl)-N,N-bis((pyridin-2-yl)methyl)ethana-
mine}, have been investigated to probe differences in their chemistry, especially in copper(I)-dioxygen chemistry,
compared to that already known for the pyridyl analogue TMPA, tris(2-pyridyl)methyl)amine. Infrared (IR) stretching
frequencies obtained from carbon monoxide adducts of [(LMIm)CuI]þ (1a) and [(LEIm)CuI]þ (2a) show that the
imidazolyl donor is stronger than its pyridyl analogue. Electrochemical data suggest differences in the binding constant
of CuII to LEIm compared to TMPA and LMIm, reflecting geometric changes. Oxygenation of [(LMIm)CuI]þ (1a) in
2-methyltetrahydrofuran (MeTHF) solvent at -128 ꢀC leads to an intensely purple colored species with a UV-vis
spectrum characteristic of an end-on bound peroxodicopper(II) complex [{(LMIm)CuII}2(μ-1,2-O2 )]2þ (1bP) {λmax
528 nm}, very similar to the previously well characterized complex [{(TMPA)CuII}2(μ-1,2-O22-)]2þ {λmax=520 nm (ε=
12 000 M-1 cm-1), in MeTHF; resonance Raman (rR) spectroscopy: ν(O-O)=832 (Δ(18O2)=-44) cm-1}. In the
low-temperature oxygenation of 2a, benchtop (-128 ꢀC) and stopped-flow (-90 ꢀC) experiments reveal the
formation of an initial superoxo-Cu(II) species [(LEIm)CuII(O2•-)]þ (2bS), λmax=431 nm in THF) . This converts to the
low-temperature stable peroxo complex [{(LEIm)CuII}2(μ-1,2-O22-)]2þ (2bP) {rR spectroscopy: ν(O-O) = 822
(Δ(18O2)=-46) cm-1}. Complex 2bP possess distinctly reduced Cu-O and O-O stretching frequencies and a
red-shifted UV-vis feature {to λmax=535 nm (ε=11 000 M-1 cm-1)} compared to the TMPA analogue due to a
distortion from trigonal bipyramidal (TBP) to a square pyramidal ligand field. This distortion is supported by the
structural characterization of related ligand-copper(II) complexes: A stable tetramer cluster complex [(μ2-LEIm-)4-
(CuII)4]4þ, obtained from thermal decomposition of 2bP (with formation of H2O2), also exhibits a distorted square
pyramidal Cu(II) ion geometry as does the copper(II) complex [(LEIm)CuII(CH3CN)]2þ (2c), characterized by X-ray
crystallography and solution electron paramagnetic resonance (EPR) spectroscopy.
=
2-
Introduction
those known with the previously well-studied close analo-
gue ligand TMPA (tris(2-pyridylmethyl)amine).1-7 The
study of copper-dioxygen adduct formation, structures,
physical properties, and subsequent reactivity is an area of
active research, since the acquisition of fundamental insights
into this subject is of considerable chemical and bioinorganic
interest.1-3,8
In this Article, we present detailed studies of the copper(I)-
dioxygen reactivity of new complexes containing tripodal
tetradentate ligands LMIm and LEIm (Scheme 1), possessing
one imidazolyl group. We compare and contrast the reactivity
and physical properties of the resulting dioxygen adducts with
ꢀ
(5) Tyeklar, Z.; Jacobson, R. R.; Wei, N.; Murthy, N. N.; Zubieta, J.;
*To whom correspondence should be addressed. E-mail: karlin@jhu.edu.
(1) Hatcher, L. Q.; Karlin, K. D. J. Biol. Inorg. Chem. 2004, 9, 669–683.
(2) Mirica, L. M.; Ottenwaelder, X.; Stack, T. D. P. Chem. Rev. 2004, 104,
1013–1045.
Karlin, K. D. J. Am. Chem. Soc. 1993, 115, 2677–2689.
(6) Baldwin, M. J.; Ross, P. K.; Pate, J. E.; Tyeklar, Z.; Karlin, K. D.;
Solomon, E. I. J. Am. Chem. Soc. 1991, 113, 8671–8679.
(7) Karlin, K. D.; Kaderli, S.; Zuberbuhler, A. D. Acc. Chem. Res. 1997,
ꢀ
€
(3) Lewis, E. A.; Tolman, W. B. Chem. Rev. 2004, 104, 1047–1076.
ꢀ
(4) Jacobson, R. R.; Tyeklar, Z.; Karlin, K. D.; Liu, S.; Zubieta, J. J. Am.
30, 139–147.
Chem. Soc. 1988, 110, 3690–3692.
(8) Itoh, S. Curr. Opin. Chem. Biol. 2006, 10, 115–122.
r
2009 American Chemical Society
Published on Web 11/03/2009
pubs.acs.org/IC

11298 Inorganic Chemistry, Vol. 48, No. 23, 2009
Lee et al.
Scheme 1
Detailed kinetic-mechanistic and structural studies
show that the μ-1,2-end-on peroxo dicopper(II) complex
[{(TMPA)CuII}2(O22-)]2þ (Scheme 1) forms via a reversible
reaction between an initially formed CuII-superoxo inter-
mediate (λmax = 410 nm, also formed reversibly) with a
second TMPA-cuprous complex (Scheme 1).7,9,10 The in-
tensely purple colored [{(TMPA)CuII}2(O22-)]2þ shows
absorption features [λmax nm (ε cm-1 M-1)] at 440 (4 000),
525 (11500), and 590 (7 600) in EtCN at -80 ꢀC, which have
been assigned as ligand(peroxide)-to-metal charge-transfer
(LMCT) bands.6 The formation of an end-on bound dioxy-
gen adduct (as a peroxide species) was confirmed structurally
(Scheme 1), with each copper(II) ion in a trigonal bipyrami-
μ-1,2-peroxodicopper(II) complex analogue of [{(TMPA)-
CuII}2(O22-)]2þ
.
˚
dal coordination environment with Cu Cu=4.359 A and
3 3 3
4
˚
O-O=1.432 A, the latter value typical for peroxides. The
peroxo-dicopper(II) formulation was further confirmed
by resonance Raman (rR) spectroscopy, ν(O-O) = 832
(Δ(18O2) = -44) cm-1.6 In the context of this discussion
of the superoxo-copper(II) or peroxodicopper(II) com-
plexes with TMPA as a ligand, there have been notable
relevant recent advances: (i) The copper(I) carbonyl complex
of a TMPA analogue with Me2N substituents at the 4-pyridyl
position reacts to form a stable -80 ꢀC solution complex
[(Me2N-TMPA)CuII(O2•-)]þ species, spectroscopically pro-
ven to possess an end-on superoxide binding (diagram).11 (ii)
Strikingly, Schindler and co-workers12 crystallized and struc-
turally characterized a related complex [(TMG3tren)CuII-
(O2•-)]þ (diagram). (iii) With all alkylamino containing
N4 tripodal tetradentate ligands (LR1,R2 in diagram), Suzuki
and co-workers13 spectroscopically detailed related super-
oxo-CuII complexes and crystallographically described a
We and others have extensively employed pyridyl donors
as part of tridentate or tetradentate chelating ligands for
copper(I), for their synthetic ease and behavior toward
promoting copper(I)/dioxygen reactivity of considerable
interest.1-3,8,14 Here, our choice of ligands LMIm and LEIm
{LMIm
,
(1H-imidazol-4-yl)-N,N-bis((pyridin-2-yl)methyl)-
methanamine; LEIm, 2-(1H-imidazol-4-yl)-N,N-bis((pyridin-
2-yl)methyl)ethanamine (Scheme 1)} derives from interest in
studying an imidazolyl moiety as a copper ion ligand, which
more closely resembles protein histidine ligand groups (as
linked in the 4(5)-position). In fact, imidazolyl containing
ligands have been studied in copper coordination chemis-
try.15-24 A few examples exist where LCuI/O2 studies
(14) Hatcher, L. Q.; Karlin, K. D. Adv. Inorg. Chem. 2006, 58, 131–184.
(15) Battaini, G.; Monzani, E.; Perotti, A.; Para, C.; Casella, L.; Santa-
gostini, L.; Gullotti, M.; Dillinger, R.; Nather, C.; Tuczek, F. J. Am. Chem.
Soc. 2003, 125, 4185–4198.
(16) Itoh, K.; Hayashi, H.; Furutachi, H.; Matsumoto, T.; Nagatomo, S.;
Tosha, T.; Terada, S.; Fujinami, S.; Suzuki, M.; Kitagawa, T. J. Am. Chem.
Soc. 2005, 127, 5212–5223.
(17) Cheruzel, L. E.; Cecil, M. R.; Edison, S. E.; Mashuta, M. S.; Baldwin,
M. J.; Buchanan, R. M. Inorg. Chem. 2006, 45, 3191–3202.
(18) Cheruzel, L. E.; Mashuta, M. S.; Buchanan, R. M. Chem. Commun.
2005, 2223–2225.
(19) Beretta, M.; Bouwman, E.; Casella, L.; Douziech, B.; Driessen, W. L.;
Gutierrez-Soto, L.; Monzani, E.; Reedijk, J. Inorg. Chim. Acta 2000, 310, 41–50.
(20) Santagostini, L.; Gullotti, M.; Pagliarin, R.; Bianchi, E.; Casella, L.;
Monzani, E. Tetrahedron: Asymmetry 1999, 10, 281–295.
(9) Zhang, C. X.; Kaderli, S.; Costas, M.; Kim, E.-i.; Neuhold,
€
Y.-M.; Karlin, K. D.; Zuberbuhler, A. D. Inorg. Chem. 2003, 42, 1807–
1824.
(10) Fry, H. C.; Scaltrito, D. V.; Karlin, K. D.; Meyer, G. J. J. Am. Chem.
Soc. 2003, 125, 11866–11871.
(11) Maiti, D.; Fry, H. C.; Woertink, J. S.; Vance, M. A.; Solomon, E. I.;
Karlin, K. D. J. Am. Chem. Soc. 2007, 129, 264–265.
€
(12) Wurtele, C.; Gaoutchenova, E.; Harms, K.; Holthausen, M. C.;
Sundermeyer, J.; Schindler, S. Angew. Chem., Int. Ed. 2006, 45, 3867–
3869.
(13) Komiyama, K.; Furutachi, H.; Nagatomo, S.; Hashimoto, A.;
Hayashi, H.; Fujinami, S.; Suzuki, M.; Kitagawa, T. Bull. Chem. Soc. Jpn.
2004, 77, 59–72.

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11299
were carried out,25-29 but they have generally not afforded
new insights with respect to characterization of copper-
dioxygen adducts.
reported in ppm (δ) downfield from an internal TMS (Me4Si)
reference and the residual solvent proton peak.
Infrared Spectra. Infrared spectra were recorded on a
Mattson Instruments 4030 Galaxy series Fourier transform-
infrared (FT-IR) spectrometer. Measuring the solution IR spec-
tra of carbonyl adducts (as a solution in THF) were recorded
using standard solution IR cells. Air sensitive cuprous THF
solutions were prepared in a glovebox (N2 filled, MBraun) and
then removed using 20 mL vials sealed with a 14/20 rubber
septum. Carbon monoxide gas (Airgas Inc., Radnor, PA) was
introduced to the corresponding solution via bubbling for
20-30 s through an 18-gauge, 24-in.-long stainless steel syringe
needle. The resulting solution was transferred to a solution IR
cell by the gastight syringe under the CO atmosphere.
In the present study employing LMIm and LEIm, we have
synthesized new copper(I) complexes (1a and 2a in Scheme 1),
spectroscopically characterized peroxo-dicopper(II) com-
plexes (1bP and 2bP in Scheme 1) and have detected and
partially characterized a superoxo-copper(II) complex with
LEIm (2bS in Scheme 1). Stable copper(II) complexes (1c and 2c
in Scheme 1) have also been synthesized and characterized to
probe the geometric effect of the new ligands on the copper(II)
centers. Insights obtained from these structures aid the inter-
pretation of spectroscopic variations observed for the
peroxo-dicopper(II) species, as detailed below.
Elemental Analyses. Elemental analyses were performed by
Desert Analytics, Tucson, AZ, for air-sensitive samples or by
Quantitative Technologies Inc. (QTI), Whitehouse, NJ, for others.
Mass Spectrometry. Mass spectrometry was conducted at the
mass spectrometry facility either at the Johns Hopkins Univer-
sity or at the Ohio State University. Chemical Iionization (CI)
and fast atomic bombardment (FAB) mass spectra were ac-
quired at the Johns Hopkins University facility using a VG70S
double focusing magnetic sector mass spectrometer (VG Ana-
lytical, Manchester, U.K., now Micromass/Wasters) equipped
with a Xe gas FAB gun (8 kV at 1.2 mA) and an off-axis electron
multiplier and an MSS data system (MasCom, Bremen,
Germany). The resolution of the instrument was set at 10 000
(100 ppm peak width). Samples were mixed with m-nitrobenzyl-
alcohol matrix deposited on the target of a direct insertion probe
for introduction into the source. Nominal mass scan spectra
were acquired with a mass scan range of 10-950 amu using a
magnet scan rate of 25 second/decade. For accurate mass
measurements, a narrower mass scan range was employed, with
the matrix containing 10% PEG mass calibrant. ESI mass
spectra (Johns Hopkins University facility) were acquired using
a Finnigan LCQDeca ion-trap mass spectrometer equipped
with an electrospray ionization source (Thermo Finnigan, San
Jose, CA). Samples were dissolved in CH3OH or CH3CN and
introduced into the instrument at a rate of 10 μL/min using a
syringe pump via a silica capillary line. The heated capillary
temperature was 250 ꢀC, and the spray voltage was 5 kV. High-
resolution electrospray ionization (ESI) mass spectrometry
analyses were performed at the Ohio State University (MS)
facility with a 3 T Finnigan FTMS-2000 Fourier transform mass
spectrometer (FTMS). Samples were sprayed from a commer-
cial Analytical electrospray ionization source and then focused
into the FTMS cell using a home-built set of ion optics.
Experimental Section
General Considerations and Instrumentation. All reagents and
solvents used for this work were commercial products and are of
reagent quality unless otherwise stated. Acetonitrile (CH3CN),
dichloromethane (CH2Cl2), diethylether (Et2O), methanol
(CH3OH), and tetrahydrofuran (THF) were purified and dried
by passing through a double alumina column solvent purifica-
tion system by Innovative Technologies, Inc. Anhydrous
2-methyltetrahydrofuran (MeTHF, packaged under nitrogen
in Sure/Seal bottles, 99þ%) was purchased from Sigma-
Aldrich, Inc. Deoxygenation of these solvents was achieved by
bubbling Ar for 30 min and/or carrying out three freeze/pump/
thaw cycles. Air-sensitive compounds were handled under an
Ar atmosphere using standard Schlenk techniques or within
a MBraun Labmaster 130 inert-atmosphere (N2 atmosphere;
<1 ppm O2, <1 ppm H2O) glovebox. Molecular oxygen
(Airgas Inc., Radnor, PA) was dried by passing it through a
laboratory gas drying unit (W. A. Hammond Drierite Co.,
Xenia, Ohio) and introduced to reaction solutions by bub-
bling through an 18-gauge, 24 inch-long stainless steel syringe
needle. [CuI(CH3CN)4]ClO4 and [CuI(CH3CN)4]B(C6F5)4
were prepared by literature procedures.30,31 KB(C6F5)4
or LiB(C6F5)4 were purchased from Boulder Scientific
Company.
Warning: While we have experienced no problems in working
with perchlorate compounds, they are potentially explosive, and
care must be taken not to work with large quantities.
1H Nuclear Magnetic Resonance (NMR) and 13C NMR. H
1
NMR and 13C NMR spectra were measured on a Varian 400
MHz or Bruker 400 MHz spectrometer, and chemical shifts are
Electrical Conductivity. Electrical conductivity measure-
ments32,33 for the Cu(I) complexes [(TMPA)CuI(CH3CN)]þ,
[(LMIm)CuI]þ (1a), and [(LEIm)CuI]þ (2a) were carried out in
dimethylformamide (DMF) solvent using an Accumet AR20
pH/conductivity meter (Fisher Scientific) with the Accumet
Immersion-type four-cell glass conductivity probe (cell constant
κ = 1.0 cm-1). Air sensitive cuprous DMF solutions (1 mM,
20 mL) were prepared in a glovebox (N2 filled, MBraun) and
then removed using five drum vials sealed with a cap and
electrical tape. The data were collected from measurements with
continuous strong (over the top) Ar flow.
(21) Nie, H. L.; Aubin, S. M. J.; Mashuta, M. S.; Porter, R. A.;
Richardson, J. F.; Hendrickson, D. N.; Buchanan, R. M. Inorg. Chem.
1996, 35, 3325–3334.
(22) Chen, S.; Richardson, J. F.; Buchanan, R. M. Inorg. Chem. 1994, 33,
2376–2382.
(23) Sorrell, T. N.; Vankai, V. A.; Garrity, M. L. Inorg. Chem. 1991, 30,
207–210.
(24) Casella, L.; Gullotti, M.; Pallanza, G.; Ligoni, L. J. Am. Chem. Soc.
1988, 110, 4221–4227.
(25) Zhou, L.; Powell, D.; Nicholas, K. M. Inorg. Chem. 2006, 45, 3840–
3842.
(26) Battaini, G.; Casella, L.; Gullotti, M.; Monzani, E.; Nardin, G.;
Perotti, A.; Randaccio, L.; Santagostini, L.; Heinemann, F. W.; Schindler, S.
Eur. J. Inorg. Chem. 2003, 1197–1205.
(27) Sorrell, T. N.; Allen, W. E.; White, P. S. Inorg. Chem. 1995, 34, 952–
960.
Cyclic Voltammetry. Cyclic voltammetry measurements were
undertaken in CH3CN and DMF using a BAS 100B electro-
chemical analyzer with a glassy carbon working electrode and a
platinum wire auxiliary electrode. Potentials were recorded
versus a Ag/AgNO3þe/l0ectrode. The voltammograms are plotted
(28) Lynch, W. E.; Kurtz, D. M., Jr.; Wang, S.; Scott, R. A. J. Am. Chem.
Soc. 1994, 116, 11030–11038.
versus the Fe(Cp)2
potential which was measured as an
ꢀ
(29) Wei, N.; Murthy, N. N.; Tyeklar, Z.; Karlin, K. D. Inorg. Chem.
external standard. Scans were run at 50-200 mV/s under an
1994, 33, 1177–1183.
(30) Liang, H.-C.; Kim, E.; Incarvito, C. D.; Rheingold, A. L.; Karlin,
K. D. Inorg, Chem. 2002, 41, 2209–2212.
(32) Geary, W. J. Coord. Chem. Rev. 1971, 7, 81–122.
(33) Sorrell, T. N.; Borovik, A. S. J. Am. Chem. Soc. 1987, 109,
4255–4260.
(31) Liang, H.-C.; Karlin, K. D.; Dyson, R.; Kaderli, S.; Jung, B.;
€
Zuberbuhler, A. D. Inorg. Chem. 2000, 39, 5884–5894.

11300 Inorganic Chemistry, Vol. 48, No. 23, 2009
Lee et al.
argon atmosphere using ∼0.1 M [Bu4N][PF6] as the supporting
electrolyte.
4.0 mM. Data acquisition (up to 256 complete spectra, up to 4
different time bases) was performed by using the Kinspec
program (J & M).
X-ray Crystallography. X-ray cCrystallography was perfor-
med at the X-ray diffraction facility either at Johns Hopkins
University or the University of Delaware (by the Prof. Arnold
Rheingold group). A suitable green crystal of [(LMIm)CuII(CH3-
CN)]2þ (1c) was mounted with epoxy cement to the tip of a glass
fiber. Intensity data were collected at 150(2) K with a Bruker
SMART APEX CCD diffractometer with graphite-monochro-
Resonance Raman Experiments. Resonance Raman (rR)
spectra were obtained using a Princeton Instruments ST-135
back-illuminated CCD detector on a Spex 1877 CP triple
monochromator with 1200, 1800, and 2400 grooves/mm holo-
graphic spectrograph gratings. Excitation was provided by a
Coherent I90C-K Krþ ion laser. The spectral resolution was <2
cm-1. Sample concentrations were approximately 3-4 mM with
respect to Cu (1.5-2 mM with respect to the dimer). The
samples were cooled to 77 K in a quartz liquid nitrogen finger
Dewar (Wilmad). Isotopic substitution was achieved by oxygen-
ation with 18O2.
˚
mated [Mo KR radiation (λ=0.710 73 A)]. An absorption correc-
tion was applied using the SADABS program (Sheldrick, G. M.
SADABS (2.01), Bruker/Siemens Area Detector Absorption Cor-
rection Program, Bruker AXS, Madison, WI, 1998). The structure
was solved using direct methods and refined using the Bruker
SHELXTL (version 6.1) software package (Sheldrick, G.M. 2000).
Suitable blue single crystals of [(μ2-LEIm-)4(CuII)4]4þ cluster and
X-Band Electron Paramagnetic Resonance (EPR) Spectra.
X-Band EPR spectra were recorded on a Bruker EMX CW-
EPR spectrometer controlled with a Bruker ER 041 XG micro-
wave bridge operating at X-band (∼9 GHz). The low-tempera-
ture experiments were carried out via either a continuous-flow
He(l) cryostat and ITC503 temperature controller made by
Oxford Instruments, Inc. or N2(l) finger dewar.
[(LEIm)CuII(CH3CN)(-OClO3)]þ (2c OClO3) were mounted in
3
Paratone-N oil on the end of a glass fiber and transferred to the N2
cold stream (110 K) of an Oxford Diffraction Xcalibur3 system
˚
equipped with Enhance optics [Mo KR radiation (λ=0.710 73 A)]
and a CCD detector. The frames were integrated and a face
indexed absorption correction and an interframe scaling correction
were also applied with the Oxford Diffraction CrysAlisRED soft-
ware package (CrysAlis CCD, Oxford Diffraction Ltd., version
1.171.27p5 beta). The structures were solved using direct methods
and refined using the Bruker SHELXTL (version 6.1) software
package (Sheldrick, G.M. 2000).
Synthesis of Ligands. LMIm
. Bis(2-pyridylmethyl)amine
(2.64 g, 13.2 mmol) and glacial acetic acid (2 mL) were added
to 1H-imidazole-4-carbaldehyde (1.3 g, 13.3 mmol) methanol
solution (100 mL). Sodium cyanoborohydride (1.3 g, 20.7 mmol)
was slowly added to the yellow methanol solution with vigorous
stirring. Reaction mixture was stirred at room temperature for
24 h under Ar. After acidification with concentrated hydrochloric
acid, solvent was removed using a rotary evaporator and the
resulting residue was dissolved in dichloromethane and washed
with a saturated sodium carbonate solution. The organic layer
was separated, dried over anhydrous MgSO4, then filtered and
concentrated under vacuum. The oil obtained (0.72 g, 2.45 mmol,
61%) was purified by column chromatography (Al2O3, ethylace-
Low Temperature UV-Visible Spectra. Low-temperature
UV-vis spectra were obtained with either a Cary 50 Bio spectro-
photometer equipped with a fiber optic coupler (Varian) and a
fiber optic dip probe (Hellma, 661.302-QX-UV-2 mm-for-low-
temperature) or a Hewlett-Packard model 8453 diode array
spectrophotometer equipped with a custom-made quartz-wind-
owed vacuum dewar filled with methanol (-80 ꢀC). A low
temperature unit (Neslab ULT-95 low temperature circulator)
is attached to the HP spectrophotometer via copper tubing. The
methanol temperature within the dewar was monitored using a
thermocouple probe (Omega model 651). For the low tempera-
ture measurements with a Cary 50 Bio spectrophotometer, a
hexane/N2(l) bath (-94 ꢀC) or a pentane/N2(l) bath (-128 ꢀC)
was used and the steady temperature was monitored with the type
T thermocouple thermometer (model 650, Omega Engineering,
CT). Air sensitive solutions were prepared in a glovebox (N2
filled, MBraun) and carried out in custom-made Schlenk tubes
designed for the dip probe (Chemglass, JHU-0407-271MS) or
Schlenk cuvettes. The cuvette assembly consisted of a two-
window quartz cuvette (2 mm path) connected, via a 12 cm glass
tube, to a 14/20 female ground glass joint. In order to calculate
absorptivities at low temperature in MeTHF, a solvent contrac-
tion factor was calculated from the volume changes directly
measured at different temperatures (10 mL at room temperature
(RT) vs 8.53 mL at -128 ꢀC).
1
tate/methanol (100:4), Rf =0.3). H NMR (CDCl3): δ 11.6 (s,
1H), 8.42 (d, J=4 Hz, 2H), 7.6 (m, 3H), 7.40 (d, J=7.6 Hz, 2H),
7.05 (m, 2H), 6.89 (s, 1H), 3.74 (s, 4H), 3.64 (s, 2H). 13C NMR
(CDCl3): δ 159.0, 148.7, 136.6, 135.1, 123.4, 122.0, 59.2, 48.5.
FAB mass spectrum: m/z 280.2 (M þ 1)þ.
[4(5)-(2-Phthalimidoethyl)]imidazole (a). Histamine (1.6 g,
14.5 mmol) and phthalic anhydride (2.17 g, 14.7 mmol) were
dissolved in ethanol (100 mL). After glacial acetic acid (20 mL)
was introduced, the reaction mixture was refluxed for 4 h. The
resulting mixture was cooled and solvent and acetic acid were
removed using a rotary evaporator. Upon addition of ethanol
(30 mL) to the resulting residue with vigorous stirring, a white
precipitate formed. The white powder (3.3 g, 13.7 mmol, 95%) was
collected by filtration, washed with cold ethanol, and dried under
vacuum. 1H NMR (CDCl3): δ 13.4 (s, 1H), 8.4 (m, 4H), 8.19 (s,
1H), 7.46 (s, 1H), 4.42 (t, J=7.4 Hz, 2H), 3.47 (t, J=7.4 Hz, 2H).
13C NMR (CDCl3): δ 168.7 (CO), 135.9 (NCN), 135.3 (Im), 134.8
(Ar), 132.6 (Ar), 123.9 (Im), 117.4 (Ph), 26.6 (CH2), 22.1 (CH2).
[4(5)-(2-Phthalimidoethyl)](triphenlymethyl)imidazole
(b).
Stopped-Flow Experiments. Rapid kinetics were followed
Compound a (3.47 g, 14.4 mmol) and triphenylmethylchloride
(4.92 g, 17.6 mmol) were dissolved in DMF (15 mL), triethyla-
mine (2.5 mL) was introduced, and the reaction mixture was
stirred overnight at RT. After DMF was removed with a rotary
evaporator, the resulting residue was dissolved in dichloro-
methane and washed with cold ethylacetate. The product oil
(6.16 g, 12.7 mmol, 88%) was dried under vacuum, its purity was
confirmed by thin layer chromatography (TLC) (silica gel,
ethylacetate/hexane (1:1), Rf = 0.2) and identified by NMR
using
a
Hi-Tech Scientific SF-40 variable-temperature
stopped-flow unit (2 mm or 1 cm path length cell, 2.5 mL
syringe) with Teflon-lined stainless steel plumbing, combined
with a TIDAS-MMS-VIS 500-3 diode array spectrometer
(J&M, 256 diodes, 300-1100 nm, 0.8 ms minimum sampling
time) using fiber bundle light guides connected to a CLH-20
halogen lamp (Zeiss, 20 W/12 V). The two glass coils, containing
Cu(I) and dioxygen solutions, respectively, and the mixing
chamber were immersed in an ethanol bath. This bath was
placed in a Dewar, which was filled with liquid nitrogen for
low-temperature measurements. The ethanol bath was cooled
by liquid nitrogen evaporation, and its temperature was mea-
sured by using a Pt resistance thermocouple and maintained to
0.1 K by using a temperature-controlled thyristor power unit
(both Hi-Tech). The concentrations of (LEIm)Cu(I) solution
used were 0.2 mM. The dioxygen concentration used was
1
spectroscopy. H NMR (CDCl3): δ 7.77 (dd, J=5.8, 3.2 Hz,
2H), 7.70 (dd, J=5.6, 2.8 Hz, 2H), 7.25 (m, 10H), 7.05 (m, 6H),
6.52 (s, 1H), 3.88 (t, J=7 Hz, 2H), 2.86 (t, J=7.2 Hz, 2H).
Triphenylmethylhistamine (c). Compound b (8.22 g, 17.0
mmol) and hydrazine (2.3 mL, 73 mmol) were dissolved in
ethanol (100 mL), and the reaction mixture was stirred over-
night at RT under Ar. After the resulting residue was filtered, the

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11301
solvent was removed by rotary evaporation whereupon the
organic mixture was dissolved in dichloromethane and washed
with 1 N sodium hydroxide. After separation, the product oil
(3.74 g, 10.6 mmol, 62%) was dried over anhydrous MgSO4,
filtered, and concentrated under vacuum. 1H NMR (CDCl3): δ
7.3 (m, 10H), 7.1 (m, 6H), 6.58 (s, 1H), 2.96 (t, J=6.4 Hz, 2H),
2.66 (t, J=6.4 Hz, 2H).
[(LEIm)CuI]B(C6F5)4. LEIm (0.40 g, 1.36 mmol) and [CuI(CH3-
CN)4]B(C6F5)4 (1.23 g, 1.4 mmol) were dissolved and stirred for
1 h in O2-free THF (15 mL) under Ar at RT. The resulting yellow
solution was filtered and transferred to a 100 mL Schlenk flask
by cannula (with filter paper). The complex precipitated as a
yellow powder upon addition of O2-free heptane into the
reaction mixture. The supernatant was decanted, and the result-
ing yellow powder (1.28 g, 1.2 mmol, 88%) was washed two
times with O2-free heptane and dried under vacuum. 1H NMR
(DMSO-d6): δ 12.43 (s, br, 1H), 8.76 (s, 2H), 8.13 (s, 1H), 7.85 (s,
2H), 7.45 (m, 4H), 7.04 (s, 1H), 4.05 (s, br, 4H), 2.75 (s, br, 4H),
1.2 (heptane), 0.8 (heptane). Anal. Calcd for C41H19BCuF20N5-
(1/3C7H16): C, 48.67; H, 2.29; N, 6.55. Found: C, 48.49; H, 2.16;
N, 6.40. FAB mass spectrum: m/z 356.3 (LCuI)þ.
Tetramer [(LEIm-)4(CuII)4](B(C6F5)4)4 Synthesis. The B-
(C6F5)4 salt of [(LEIm)CuI]þ (2a) (180 mg, 0.17 mmol) was stirred
for 1 h in THF (5 mL) under air at RT. The resulting green
solution was layered with diethylether and pentane. After 1-2
days, a green crystalline material was isolated. After this was
washed two times with pentane and was vacuum dried, the yield
of green crystalline product was 137 mg (0.033 mmol, 78%).
UV-vis (MeCN; λmax, nm; ε, M-1 cm-1): 700, 450. Anal. Calcd
for C166H76B4Cl4Cu4F80N20: C, 46.26; H, 1.78; N, 6.50. Found:
C, 46.15; H, 1.58; N, 6.11. X-ray quality green crystals were
obtained from the synthesis.
Triphenylmethyl-LEIm (d). Compound c (3.74 g, 10.6 mmol)
and picolyl chloride hydrochloride (5.61 g, 34.2 mmol; previously
washed with 1 N sodium hydroxide) were dissolved in dichloro-
methane (100 mL). After introduction of triethylamine (3 mL),
the reaction mixture was refluxed under Ar for 3 days. After
cooling to room temperature, more (∼20 mL) of CH2Cl2 was
added and the solution was washed three times with brine. The
organic layer was separated, dried over anhydrous MgSO4, then
filtered, and concentrated under vacuum. The product oil was
obtained (5.03 g, 9.4 mmol, 88.8%) after purification by column
chromatography (Al2O3, ethylacetate/methanol (100:2), Rf =
0.5). 1H NMR (CDCl3): δ 8.36 (dm, J=4 Hz, 2H), 7.42 (t, J=
7.6 Hz, 2H), 7.34 (d, J=7.6 Hz, 2H), 7.2 (m, 10H), 7.0 (m, 8H),
6.48 (s, 1H), 3.76 (s, 4H), 2.82 (t, 2H), 2.77 (t, 2H). 13C NMR
(CDCl3): δ 160.2 (Py), 149.0 (Py), 142.7 (Im), 139.9 (Py), 138.3
(Im), 136.4 (Im), 129.9 (Ph), 128.1 (Ph), 128.0 (Ph), 122.8 (Ph),
121.9 (Py), 118.3 (Py), 75.2 (C), 60.6 (NCH2), 54.6 (CH2), 26.8
(CH2). FAB mass spectrum: m/z 558.3 (M þ Na)þ.
L
EIm. Compound d (2.14 g, 4.0 mmol) was dissolved in ethanol
Detection of Hydrogen Peroxide by Ti(IV)O(SO4). Hydrogen
peroxide produced from the reaction of the peroxo [{(LEIm)-
CuII}2(O22-)]2þ (2bP) with [H(OEt2)2][B(C6F5)4] was deter-
mined spectrophotometrically with titanium(IV) oxysul-
fate.35,36 The B(C6F5)4 salt of [(LEIm)CuI]þ (2a) (70 mg,
0.066 mmol) was dissolved in O2-free THF (5 mL) in the drybox.
The yellow solution turned to a deep blue color corresponding
to an end-on peroxo species after bubbling O2 gas at -94 ꢀC.
After [H(OEt2)2][B(C6F5)4] (67 mg, 0.081 mmol) was intro-
duced to the resulting blue solution, distilled water (10 mL)
and CH2Cl2 (20 mL) were added. To the separated aqueous
layer, a Ti(IV)O(SO4) solution (0.1 mL; titanium(IV) oxysul-
fate, 15% in sulfuric acid) was introduced. The concentration of
the hydrogen peroxide was spectroscopically measured from the
absorption at 408 nm (0.96), and the yield was 28%. The
calibration curve was derived from the absorption measure-
ments of a 2.416 mM H2O2 (29.58%, VWR) stock solution.
Upon addition of 0.1 mL of a Ti(IV)O(SO4) solution to 10 mL of
the aqueous solution with varying concentrations of hydrogen
peroxide (from stock solution: 1, 2, 4, and 10 mL; 0.002 42,
0.004 83, 0.009 66, and 0.0242 mmol, respectively), the absorp-
tion at 408 nm was measured (0.332, 0.553, 1.11, and 2.42,
respectively).
(100 mL). After glacial acetic acid (5% in ethanol) was intro-
duced, the reaction mixture was refluxed for 1 h. After ethanol
was removed using a rotary evaporator, the resulting residue was
dissolved in dichloromethane and washed with a saturated
sodium carbonate solution. The organic layer was separated,
dried over anhydrous MgSO4, then filtered, and concentrated
under vacuum. The oil obtained (0.72 g, 2.45 mmol, 61%) was
purified by column chromatography (Al2O3, ethylacetate/metha-
nol (100:4), Rf=0.3). 1H NMR (CDCl3): δ 13 (s, 1H), 8.44 (dm,
J=4.8 Hz, 2H), 7.56 (s, 1H), 7.45 (dt, J=7.8, 1.7 Hz, 2H), 7.0 (m,
4H), 6.66 (s, 1H), 3.76 (s, 4H), 2.77 (m, 4H). 13C NMR (CDCl3): δ
159.4, 149.0, 136.8, 134.8, 123.4, 122.4, 60.0, 54.1, 23.4. FAB mass
spectrum: m/z 294.3 (M þ 1)þ, 316.4 (M þ Na)þ.
Synthesis of Cu(I) Complexes and Their Reactivity toward
O2. [(LMIm)CuI]ClO4. LMIm (0.10 g, 0.37 mmol) and [CuI-
(CH3CN)4]ClO4 (0.12 g, 0.35 mmol) were dissolved in O2-free
CH3CN (15 mL) in a 50 mL Schlenk flask and stirred for 1 h
under Ar at RT. The resulting yellow solution was filtered and
transferred by cannula (with filter paper) to a 100 mL Schlenk
flask. The complex was precipitated as a yellow powder upon
addition of O2-free diethylether. The supernatant was decanted
and the resulting yellow powder was washed two times with O2-
free diethylether and dried under vacuum to give 0.13 g (0.29
mmol, 83%) of yellow powder product. 1H NMR (nitro-
methane-d3): δ 10.2 (s, 1H), 8.2 (s, 2H), 7.70 (s, 2H), 7.42 (s,
1H), 7.37 (s, 2H), 7.05 (s, 2H), 6.94 (s, 1H), 4.3 (m, 4H), 3.95 (d,
J=14.8 Hz, 2). ESI mass spectrum: m/z 342.31 (LCuI)þ.34
[(LMIm)CuI]B(C6F5)4. In a 50 mL Schlenk flask, LMIm (0.29 g,
1.0 mmol) and [CuI(CH3CN)4]B(C6F5)4 (0.81 g, 0.93 mmol)
were dissolved in O2-free THF (15 mL) and stirred for 1 h under
Ar at RT. The resulting yellow solution was filtered and
transferred by cannula (with filter paper) to a 100 mL Schlenk
flask. The complex precipitated as a yellow powder upon
addition of O2-free heptane into the reaction mixture. The
supernatant was decanted, and the resulting yellow powder
(0.85 g, 0.80 mmol, 86%) was washed two times with O2-free
pentane and dried under vacuum. 1H NMR (DMSO-d6): δ 12.5
(s, br, 1H), 7.5 (m, 10H), 4.05 (s, br, 6H), 3.59 (THF), 1.75
(THF), 1.2 (pentane), 0.8 (pentane).34
Calibration curve of the moles of hydrogen peroxide in aqueous solution
with Ti(IV)O(SO4).
(34) Because of the copper(I)-LMIm complex extreme air sensitivity, even
after many tries we were not able to obtain proper C, H, and N elemental analy-
ses. The expected 1H NMR spectra of these complexes and other clean behavior
attests to their high purity in terms of the chemistry described in this paper.
(35) Chaudhuri, P.; Hess, M.; Mueller, J.; Hildenbrand, K.; Bill, E.;
Weyhermueller, T.; Wieghardt, K. J. Am. Chem. Soc. 1999, 121, 9599–9610.
(36) Eisenberg, G. M. Ind. Eng. Chem., Anal. Ed. 1943, 15, 327–328.

11302 Inorganic Chemistry, Vol. 48, No. 23, 2009
Lee et al.
[H(OEt2)2][B(C6F5)4] Synthesis. [H(OEt2)2][B(C6F5)4] was
prepared by the literature procedure.37 Potassium tetrakis-
(pentafluorophenyl)borate (2.3 g, 3.2 mmol) was dissolved in
diethylether (20 mL). Hydrochloric acid in diethylether (6 mL)
was introduced under Ar at -77 ꢀC. The reaction mixture was
slowly warmed and stirred at RT for 2 h. After filtration with
dried Celite 545, the solvent was evaporated leaving ∼6-7 mL
of solution. The resulting mixture was put in the freezer for 1 h.
After the supernatant was decanted, the colorless crystals ob-
tained were dried under vacuum, giving 1.44 g of product (54%).
Synthesis and Characterization of Cu(II) Complexes. [(LMIm)-
Scheme 2
CuII(CH3CN)]2þ (1c). LMIm (0.45 g, 1.6 mmol) and CuII(ClO4)2
3
6H2O (0.59 g, 1.6 mmol) were dissolved in CH3CN (15 mL) and
stirred for 1 h at room temperature. The resulting blue solution was
filtered and transferred to a 100 mL Schlenk flask with a cannula
(with filter paper). The solution was layered with diethylether and
allowed to stand for 1 h. After the resulting supernatant was
decanted, the resulting powder was washed two times with diethy-
lether and dried under vacuum to give 0.88 g of product
(1.48 mmol, 95%). UV-vis (CH3CN; λmax, nm; ε, M-1 cm-1):
880, 200; 660 (shoulder), 55. Anal. Calcd for C16H17Cl2Cu-
[(LEIm)CuI]þ (2a). For ligand-copper(I) complexes with
nitrogen containing tri- or tetradentate ligands, a dimeric
structure may be possible; there are examples known in
the solid state or in solution,9,29,39 where one N-donor of a
given chelate within [{(L)CuI}2]2þ binds to the “other”
copper(I) moiety. Normally in more polar or high-
dielectric solvents, the dimer “breaks up” to give mono-
nuclear complexes, e.g., [(L)CuI]þ or [(L)CuI(solvent)]þ.9
Solution conductivity measurements can distinguish be-
tween a [(L)CuI]þ or dimer [{(L)CuI}2]2þ formulation,9,40
and we carried out such measurements in dimethylfor-
mamide (DMF) solvent. In fact, 1:1 electrolyte behavior
is observed for both 1a and 2a, consistent with mono-
nuclear formulations.32,33 The molar conductivity values
for [(TMPA)CuI(CH3CN)]þ, [(LMIm)CuI]þ (1a), and
[(LEIm)CuI]þ (2a) are 47, 49, and 45 Ω-1 cm2 mol-1, res-
pectively. Such experiments were not carried out in
2-methyltetrahydrofuran (MeTHF) or THF, which were
used for the dioxygen reactivity studies described below,
since these are not good solvents for conductivity mea-
surements.32 Thus, binuclear structures for 1a or 2a in
these solvents cannot entirely be ruled out.
N5O8 5/2H2O: C, 32.75; H, 3.78; N, 11.93. Found: C, 32.55; H,
3
3.23; N, 11.71. ESI mass spectrum: m/z 342.29 (LCu)þ. An EPR
spectrum of 1c indicates a typical trigonal bipyramidal copper
environment: X-band spectrometer (ν = 9.487 GHz) in DMF/
toluene (1:1) at 4 K; g =1.99, A =70 G; g^=2.21, A^=99 G (See
Figure 7). X-ray quality green crystals were obtained from acet-
onitrile layered with diethylether.
[(LEIm)CuII(CH3CN)]2þ (2c). LEIm (0.50 g, 1.7 mmol) and
CuII(ClO4)2 6H2O (0.63 g, 1.7 mmol) were dissolved and stirred
3
for 1 h in CH3CN (5 mL) at room temperature. The resulting
blue solution was filtered and transferred to a 100 mL Schlenk
flask by cannula (with filter paper). The solution was layered
with diethyl ether. After the supernatant was decanted, the
resulting powder was washed two times with diethyl ether and
dried under vacuum to give 0.79 g of product (1.34 mmol, 81%).
UV-vis (MeCN; λmax, nm; ε, M-1 cm-1): ∼300 (shoulder),
2400; 660, 130; ∼840, 110. Anal. Calcd for C17H23Cl2CuN5O10:
C, 34.50; H, 3.92; N, 11.83. Found: C, 34.88; H, 3.48; N, 11.32.
ESI mass spectrum: m/z 356.35 (LCu)þ. An EPR spectrum of 2c
indicates a typical tetragonal copper environment: X-band
spectrometer (ν = 9.488 GHz) in DMF/toluene (1:1) at 4 K;
g =2.26, A =161 G; g^=2.06 (see Figure 7). X-ray quality green
crystals were obtained from acetonitrile layered with diethyl-
ether.
CO Binding and Electrochemistry of [(LMIm)CuI]þ (1a)
and [(LEIm)CuI]þ (2a). In order to obtain insights into the
solution state structures of copper(I) complexes, as well as
to obtain information concerning the relative amount of
electron donation to the copper(I) ion as a function of
ligand set (i.e., LMIm vs LEIm vs TMPA), carbon mon-
oxide binding to [(LMIm)CuI]þ (1a) and [(LEIm)CuI]þ (2a)
was examined. The two C-O stretching frequencies at
2090 and 2073 cm-1 observed for [(TMPA)CuI(CO)]þ
have previously been attributed to a dynamic equili-
brium involving overall 4- or 5-coordinate complexes, the
former structure possessing an uncoordinated pyridyl
arm.5,41,42 The lower value is ascribed to the pentacoordi-
nate species where ligation of all four N-donors provides
more electron density to the Cu(I) ion, resulting in greater
back-donation to the CO π* orbital (thus weakening
the CO bond and lowering ν(C-O)). An almost iden-
tical behavior is exhibited by [(LMIm)CuI]þ (1a), with CO
vibrational frequencies at 2087 and 2063 cm-1, Table 1
Results and Discussion
Synthesis of Ligands and Copper(I) Complexes. The
imidazolyl containing tetradentate ligands were synthe-
sized by standard methods. LMIm was prepared by the
reductive coupling reaction of PY1 (dipicolylamine) and
1H-imidazole-4-carbaldehyde with sodium cyanoboro-
hydride under acidic conditions, Scheme 2.38 The LEIm
synthesis was more involved, but it was successfully
prepared according to Scheme 2.
The copper(I) complexes were synthesized by the addi-
tion of 1 equiv of the appropriate tripodal ligand to
[CuI(CH3CN)4]Y (Y=ClO4-, B(C6F5)4- 30,31 in CH3CN
)
or THF under argon. They are both yellow air-sensitive
solids. Nujol mull IR spectra reveal absorptions ascribed
to the ligand N-H moiety (Experimental Section). We
were not able to crystallize either [(LMIm)CuI]þ (1a) and
(39) Carrier, S. M.; Ruggiero, C. E.; Houser, R. P.; Tolman, W. B. Inorg.
Chem. 1993, 32, 4889–4899.
(40) Himes, R. A.; Park, G. Y.; Barry, A. N.; Blackburn, N. J.; Karlin,
K. D. J. Am. Chem. Soc. 2007, 129, 5352–5353.
(41) Kretzer, R. M.; Ghiladi, R. A.; Lebeau, E. L.; Liang, H.-C.; Karlin,
K. D. Inorg. Chem. 2003, 42, 3016–3025.
(37) Jutzi, P.; Muller, C.; Stammler, A.; Stammler, H. G. Organometallics
2000, 19, 1442–1444.
(38) Ohtsu, H.; Shimazaki, Y.; Odani, A.; Yamauchi, O.; Mori, W.; Itoh,
S.; Fukuzumi, S. J. Am. Chem. Soc. 2000, 122, 5733–5741.
(42) Fry, H. C.; Lucas, H. R.; Narducci Sarjeant, A. A.; Karlin, K. D.;
Meyer, G. J. Inorg. Chem. 2008, 47, 241–256.

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11303
Table 1. Carbonyl Stretching Frequencies for Copper(I) Carbonyl Complexes in
THF and Cyclic Voltammetry Data for Cuprous Complexes in DMF
nation geometry.43,44 The relatively high redox poten-
tial of [(LEIm)CuI]þ (2a) compared to the TMPA com-
plex and [(LMIm)CuI]þ (1a) likely reflects geometric
changes upon oxidation leading to a (relatively) decreased
binding constant between Cu(II) and LEIm. Such
effects are further clarified by studies on the Cu(II)
complexes of LMIm and LEIm, 1c and 2c, described
below.
a
compound
CO (cm-1
)
E1/2
[(TMPA)CuI(CH3CN)]þ
[(LMIm)CuI]þ (1a)
2090, 2073
2087, 2063
2082
-610
-620
-570
[(LEIm)CuI]þ (2a)
a In millivolts (mV) vs Fe(Cp)2
.
þ/0
Preparation and Characterization of Mononuclear
LCu(II) Complexes. In order to better understand the
nature of the copper(II) ion coordination in the ligand
environments of LMIm and LEIm, we synthesized copper-
(II) complexes by addition of 1 equiv of the appropriate
Scheme 3
tetradentate ligand to CuII(ClO4)2 6H2O in CH3CN or
3
CH3OH (see Experimental Section) at room temperature.
In all cases, diffusion of diethylether allows for the
formation of crystalline materials that could be charac-
terized by X-ray crystallography.
X-ray Structure of Cu(II) Complexes with LMIm and
L
EIm. The X-ray structure of [(LMIm)CuII(CH3CN)]2þ
(1c) as perchlorate salt reveals the coordination geometry
to be a slightly distorted trigonal bipyramid (τ = 0.86,
Figure 1), similar but not identical to a previously re-
ported structure of a very closely related ligand-copper-
(II) complex differing from LMIm only in that the
imidazolyl group is N-methylated; this has τ = 0.96.38
For a perfect trigonal bipyramidal (TBP) geometry, τ=1
while for a square pyramidal geometry, τ=0.45 The struc-
ture of 1c is also similar to TMPA or analogue cupric
complexes with a different fifth ligand such as CH3CN,
and Scheme 3. The five-coordinate isomer of [(LMIm)-
CuI(CO)]þ, designated 5c(1a-CO), has a lower ν(C-O)
value (10 cm-1) than the corresponding isomer for
[(TMPA)CuI(CO)]þ (2063 vs 2073 cm-1, Table 1), indi-
cating that the imidazolyl group in LMIm (and LEIm, see
below) when coordinated to copper(I) is a better donor
than is a pyridyl moiety in these tripodal ligands. With
these observations, we conclude the putative four-coor-
H2O, or Cl- (τ=1-0.96)46,47 or O2 as in [{(TMPA)-
2-
CuII}2(O22-)]2þ (τ = 0.86).5 However, the X-ray struc-
ture of [(LEIm)CuII(CH3CN)(-OClO3)]þ (2c OClO3)
3
(Figure 1) indicates a distortion occurs, with a square
pyramidal arrangement of the five N-donors from
LEIm plus CH3CN; the copper ion is displaced
dinate isomer of LMIm 4c[(LMIm)CuI(CO)]þ4c(1a-CO)
,
˚
(Scheme 3) does not have its imidazolyl group copper
ion bound, since the ν(C-O) value is nearly the same as
that for the four-coordinate TMPA copper-carbonyl
complex (2087 vs 2090 cm-1, Table 1). On the other hand,
for the LEIm complex, only one isomer is observed based
on the observation of a single IR absorption (Table 1); the
band is 8 cm-1 lowered from the 2090 cm-1 value for
[(TMPA)CuI(CO)]þ. Thus, to account for some lowering
of ν(C-O), [(LEIm)CuI(CO)]þ (2a-CO) could be a four-
coordinate species where LEIm acts as a tridentate ligand
but with the imidazolyl rather than pyridyl group bind-
ing. However, we cannot rule out a five-coordinate for-
mulation with all N-donors ligating but where a
geometric/coordination effect leads to less back-donation
to copper(I) in 5c[(LEIm)CuI(CO)]þ then observed for
[(TMPA)CuI(CO)]þ.
0.13 A out of the least-squares plane defined by
N1,N2,N4,N6, toward the axial N3pyridyl ligand
and τ = 0.19 considering only these ligands. There
is also a weakly coordinating perchlorate O-atom
occupying the other axial position, Figure 1. Thus,
one more methylene group (LEIm vs LMIm) consider-
ably changes the local copper(II) coordination geo-
metry.
In fact, this result is not unexpected, and is already
known from studies of a variety of pyridylalkylamine
containing five-coordinate cupric complexes with varying
chelate ring size.46,48-50 Thus the additional methylene
(43) Ambundo, E. A.; Deydier, M.-V.; Grall, A. J.; Aguera-Vega, N.;
Dressel, L. T.; Cooper, T. H.; Heeg, M. J.; Ochrymowycz, L. A.; Rorabacher,
D. B. Inorg. Chem. 1999, 38, 4233–4242.
The electrochemical behavior of [(TMPA)CuI(CH3-
CN)]þ, [(LMIm)CuI]þ (1a) and [(LEIm)CuI]þ (2a) was
measured by cyclic voltammetry in DMF under Ar; the
E1/2 values obtained (vs Fe(Cp)2þ/0) are listed in Table 1.
We find that the E1/2 value for 1a is 10 mV more negative
than for the TMPA complex (Table 1), suggesting an
effect of better donation from the imidazolyl donor.
However, 2a possesses an E1/2 value which is 50 mV more
positive than 1a, indicating [(LMIm)CuI]þ (1a) is a stron-
ger reductant than 2a. Redox potentials are affected by
factors other than only ligand donation, such as coordi-
(44) Rorabacher, D. B. Chem. Rev. 2004, 104, 651–697.
(45) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor,
G. C. J. Chem. Soc., Dalton Trans. 1984, 1349–1356.
(46) Lucchese, B.; Humphreys, K. J.; Lee, D.-H.; Incarvito, C. D.;
Sommer, R. D.; Rheingold, A. L.; Karlin, K. D. Inorg. Chem. 2004, 43,
5987–5998.
(47) Lim, B. S.; Holm, R. H. Inorg. Chem. 1998, 37, 4898–4908.
(48) Wei, N.; Murthy, N. N.; Karlin, K. D. Inorg. Chem. 1994, 33, 6093–
6100.
(49) Karlin, K. D.; Hayes, J. C.; Shi, J.; Hutchinson, J. P.; Zubieta, J.
Inorg. Chem. 1982, 21, 4106–4108.
(50) Schatz, M.; Becker, M.; Thaler, F.; Hampel, F.; Schindler, S.;
ꢀ
Jacobson, R. R.; Tyeklar, Z.; Murthy, N. N.; Ghosh, P.; Chen, Q.; Zubieta,
J.; Karlin, K. D. Inorg. Chem. 2001, 40, 2312–2322.

11304 Inorganic Chemistry, Vol. 48, No. 23, 2009
Lee et al.
solution.) EPR spectroscopic data (Figure 2) provide
compelling support. [(LMIm)CuII(CH3CN)]2þ (1c) exhi-
bits a “reverse” axial EPR spectrum (g =1.99, A =70 G;
g^=2.21, A^=99 G), which is indicative of a system with
a dz2 ground state (g^>g ∼ 2 and A^=60-100ꢀ10-4
cm-1 46,49,51,52
and thus TBP geometry. On the other
)
hand, the EPR spectrum of [(LEIm)CuII(CH3CN)]2þ (2c)
(g =2.26, A =161 G; g^=2.06) shows a typical square
based pyramidal (tetragonal) copper(II) environment,
which is a system with a dx2-y2 ground state revealing a regu-
lar axial (tetragonal) EPR signature (g >2.1>g^>2.0
and A =158-200ꢀ10-4 cm-1).46,48,53
Low-Temperature Formation of Copper-Dioxygen
Adducts: End-on Peroxo Dicopper(II) Complex Formation
with [(LMIm)CuI]þ (1a) and [(LEIm)CuI]þ (2a). In the past
few years we have found that going to relatively nonpolar
solvents such as THF, MeTHF, or even Et2O and toluene
leads to new ligand-Cu(I)/O2 chemistry and insights.
-
For these purposes, copper(I) complexes as B(C6F5)4
salts are employed. When yellow MeTHF solutions of
[(LMIm)CuI]þ (1a) (λmax = 350 nm (sh); ε ∼ 1 500 M-1
cm-1) or [(LEIm)CuI]þ (2a) (λmax=337 nm; ε ∼ 4 100 M-1
cm-1) are oxygenated at -128 ꢀC, intensely purple-
colored solutions form, each having the characteristic
spectrum of an end-on μ-1,2-peroxo-dicopper(II) com-
plex (Scheme 4), with three prominent LMCT bands,
with most intense coming at 525-540 nm accompanied
by a lower energy transition or shoulder at 600-610 nm,
see Figures 3 and 4 and Table 3. The position of the
absorptions for [{(LMIm)CuII}2(O22-)]2þ (1bP) and
[{(LEIm)CuII}2(O22-)]2þ (2bP) compare closely to that
known for the “parent” compound [{(TMPA)CuII}2-
(O22-)]2þ 2,4-6
The absorptivity of the three LMCT
.
absorptions for 2bP match very well to those for
[{(TMPA)CuII}2(O22-)]2þ, however those for 1bP are
about one-third lower in intensity (Figure 5 and
Table 3). We thus conclude that at -128 ꢀC, the
peroxo-dicopper(II) dioxygen adduct 2bP is essentially
fully formed, while the reaction of 1a with O2 does not
go to completion. Consistent with this is the finding
that the 1a/O2 reaction is not even observed at -80 ꢀC
in THF; the binding under these conditions is too weak,
probably for entropic reasons (by comparison to the O2
reactions with 2a or [(TMPA)CuI(CH3CN)]þ). The
oxygenation of 2a occurs very rapidly, even at -128 ꢀC.54
See Table 3, which summarizes all the spectroscopic
data for the complexes with LMIm and LEIm in com-
parison with other copper-dioxygen complexes with
tripodal tetradentate ligands.
Figure 1. ORTEP diagram view of the cationic portions of [(LMIm)CuII-
(CH3CN)]2þ (1c; τ = 0.861, top; 30% probability ellipsoids) and [(LEIm)-
CuII(CH3CN)(-OClO3)]þ (2c OClO3; bottom; 30% probability ellipso-
3
ids). The hydrogen atoms and noncoordinating perchlorate anions are
omitted for clarity.
group on 2c results in a distortion from TBP to a square
pyramidal geometry. As a result of this square pyramidal
distortion, the Nimidazole distance in 2c is shorter than in
˚
the TBP 1c (1.972 vs 2.064 A) suggesting a stronger
Cu(II)-imidazole bond in 2c than in the TBP 1c. Further,
˚
the axial Cu(1)-Npyridine distance (2.183 A) is much
longer than the other Cu-N distances in 2c. In both 1c
and 2c, the Cu(II) ion lies in the plane of the imidazole
ring, as shown by the fact that the sum of the angles
Absorption Spectraof1bP and 2bP. While the absorption
spectrum of [{(LMIm)CuII}2(O22-)]2þ (1bP) shows similar
charge transfer energies compared to [{(TMPA)CuII}2(μ-
1,2-O22-)]2þ, the absorption spectrum of [{(LEIm)CuII}2-
(O22-)]2þ (2bP) reveals a significant red shift (15 nm)
around the coordinating Nimidazole atom is equal to 360ꢀ.
-
In the essentially square pyramidal (without ClO4
,
Figure 1) [(LEIm)CuII(CH3CN)]2þ (2c) molecule, the
Cu-N-C angles are close to each other (see diagram)
and the Cu(II) acceptor orbital has good overlap with the
Nimidazole lone-pair orbital, leading to a stronger bond.
Other selected bond angles and distances are listed in
Table 2.
(51) Barbucci, R.; Bencini, A.; Gatteschi, D. Inorg. Chem. 1977, 16, 2117.
(52) Jiang, F.; Karlin, K. D.; Peisach, J. Inorg. Chem. 1993, 32, 2576–
2582.
Solution Behavior of 1c and 2c: Electron Paramagnetic
Resonance Spectroscopy. In fact, the solid state structures
and difference in coordination geometries described
above for of [(LMIm)CuII(CH3CN)]2þ (1c) and [(LEIm)-
CuII(CH3CN)]2þ (2c) are maintained in solution (Note,
the perchlorate ion is unlikely to be coordinating in
(53) Wei, N.; Murthy, N. N.; Chen, Q.; Zubieta, J.; Karlin, K. D. Inorg.
Chem. 1994, 33, 1953–1965.
(54) Both 1bP and 2bP are stable at -128 ꢀC for more than an hour
and qualitatively have generally similar stability compared to
[{(TMPA)CuII}2(O22-)]2þ, based on observable thermal decomposition
times as measured by absorbance loss.

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11305
Table 2. Selected Bond Distances and Angles for 1c and 2c OClO3
3
[(LMIm)CuII(CH3CN)]2þ (1c)
[(LEIm)CuII(CH3CN)(-OClO3)]þ (2c OClO3)
3
˚
bond distance (A)
˚
bond distance (A)
Cu-N
Cu-N
Cu(1)-N(1)
Cu(1)-N(2)
Cu(1)-N(3)
Cu(1)-N(4)
Cu(1)-N(6)
2.037(3)
2.035(3)
2.039(3)
2.064(4)
1.969(3)
Cu(1)-N(1)
Cu(1)-N(2)
Cu(1)-N(3)
Cu(1)-N(4)
Cu(1)-N(6)
Cu(1)-O(97)
2.005(3)
2.090 (3)
2.183(3)
1.972(3)
2.014(3)
2.770(9)
[(LMIm)CuII(CH3CN)]2þ (1c)
[(LEIm)CuII(CH3CN)(-OClO3)]þ (2c OClO3)
3
N-Cu-N
bond angle (deg)
N-Cu-N
bond angle (deg)
N(6)-Cu(1)-N(2)
N(6)-Cu(1)-N(1)
N(2)-Cu(1)-N(1)
N(6)-Cu(1)-N(3)
N(2)-Cu(1)-N(3)
N(1)-Cu(1)-N(3)
N(6)-Cu(1)-N(4)
N(2)-Cu(1)-N(4)
N(1)-Cu(1)-N(4)
N(3)-Cu(1)-N(4)
96.19(13)
177.60(16)
82.16(12)
96.69(13)
125.94(13)
82.93(13)
99.87(14)
116.08(13)
82.44(13)
112.92(14)
N(4)-Cu(1)-N(1)
N(4)-Cu(1)-N(6)
N(1)-Cu(1)-N(6)
N(4)-Cu(1)-N(2)
N(1)-Cu(1)-N(2)
N(6)-Cu(1)-N(2)
N(4)-Cu(1)-N(3)
N(1)-Cu(1)-N(3)
N(6)-Cu(1)-N(3)
N(2)-Cu(1)-N(3)
161.92(11)
91.52(10)
93.51(11)
94.98(11)
80.63(10)
173.39(10)
97.07(10)
99.57(10)
96.70(10)
81.38(10)
Figure 3. Low-temperature UV-vis spectra of the oxygenation of
[(LMIm)CuI]þ (1a). Spectrum 1: 1a with absorption at 350 nm (1500
M
-1 cm-1). Spectrum 2: [{(LMIm)CuII}2(O22-)]2þ (1bP) with absorption
at 435 (1 700 M-1 cm-1), 528 (6 800 M-1 cm-1), and 605 nm (4 000 M-1
cm-1) in MeTHF at -128 ꢀC.
Figure 2. EPR spectra of [(LMIm)CuII(CH3CN)]2þ (1c) (top) and
[(LEIm)CuII(CH3CN)]2þ (2c) (bottom); X-band spectrometer, ν=9.49
GHz, in DMF/toluene (1:1) at 4 K. 1c: g =1.99, A =70 G; g^=2.21, A^=
99 G. 2c: g =2.26, A =161 G; g^=2.06. See text for discussion.
Scheme 4
in the dominant O22-(π*σ) f Cu(dσ) charge transfer,
Table 3 and Figure 5. The relevant absorption for
[{(TMPA)CuII}2(μ-1,2-O22-)]2þ as measured in the pre-
sent work in MeTHF at -128 ꢀC is λmax=520,55 while for
2bP λmax=535 nm (Figures 4 and 5). As suggested by the

11306 Inorganic Chemistry, Vol. 48, No. 23, 2009
Lee et al.
X-structure of [(LEIm)CuII(CH3CN)]2þ (2c), the Cu(II)
ion in peroxo complex 2bP is expected to be square-
pyramidally distorted, due to the geometry of the LEIm
ligand. Thus, the half-occupied Cu dx2-y2 orbital of 2bP
would be at higher energy than the acceptor dz2 orbital
of the TBP [{(TMPA)CuII}2(μ-1,2-O22-)]2þ, based on
ligand field differences which, by itself, would blue shift
the O22-(π*σ) f Cu(dσ) charge transfer. Instead, a red
shift of the charge transfer is observed for 2bP compared
to [{(TMPA)CuII}2(μ-1,2-O22-)]2þ, which reflects the
increased electron density in the donor peroxo π*σ,
considerably raising the energy of this donor orbital,
resulting in a red shift the O22-(π*σ) f Cu(dσ) charge
transfer.
Resonance Raman Spectroscopy. The resonance Ra-
man spectrum of [{(LEIm)CuII}2(O22-)]2þ (2bP) in THF
with λex=568 nm shows dominant features at 827, 811,
and 539 cm-1, which shift to 776 and 513 cm-1 upon 18O2
substitution (Figure 6). The 827 and 811 cm-1 features are
at energies consistent with those of peroxo ν(O-O)
stretches.1,2,13 Upon 18O2 substitution, a single ν(O-O)
is observed at 776 cm-1 at higher intensity than either the
827 or 811 cm-1 feature. As such, the 827 and 811 cm-1
features are assigned as the result of a Fermi resonance of
ν(O-O) with a local nonenhanced mode of the same
symmetry. The preinteraction ν(O-O) is estimated to be
Figure 4. Low-temperature UV-vis spectra of the oxygenation of
[(LEIm)CuI]þ (2a). Spectrum 1: 2a with absorption at 337 nm (4 100
Figure 5. UV-visible spectra of μ-1,2-peroxo intermediates from LCuI
complexes (L = TMPA, LMIm, and LEIm) reacting with dioxygen at redu-
ced temperature. The ε value scale for the LMIm peroxo complex spectrum
shown here is not to scale (εapparent values are much lower, see text), but
shown here in order to easily compare λmax values.
M
-1 cm-1). Spectrum 2: [{(LEIm)CuII}2(O22-)]2þ (2bP) with absorption
at 445 (2 500 M-1 cm-1), 534 (11000 M-1 cm-1), and 619 nm (8100 M-1
cm-1) in MeTHF at -128 ꢀC.
Table 3. End-On Peroxo Species with PY1 Based Tetradentate Ligands
LMCT; nm (M-1 cm-1
)
solvent, temp
ν(O-O); Δ(18Ο2); cm-1
832; -44a
TMPA6
440 (1 700)
430 (2 100)
435 (1 700)
445 (2 500)
450 (1 100)
445 (2 000)
525 (11 300)
520 (12 000)
528 (6 800)
535 (11 000)
540 (11 100)
532 (9 380)
528 (-)
615 (5 800)
590 (8 000)
605 (4 000)
610 (8 100)
600 (8 700)
590 (7 000)
EtCN, -80 ꢀC
MeTHF, -128 ꢀC
MeTHF, -128 ꢀC
MeTHF, -128 ꢀC
EtCN, -90 ꢀC
EtCN, -80 ꢀC
EtCN, -90 ꢀC
acetone, -90 ꢀC
EtCN, -80 ꢀC
EtCN, -80 ꢀC
EtCN, -90 ꢀC
acetone, -70 ꢀC
TMPA55
LMIm
LEIm
822; -46b
D1 60
TMPAE60
Me2-uns-penp61
uns-penp62
BPIA29
535 (-)
440 (2 000)
535 (11 500)
535 (8 600)
536 (∼5 000)
537 (∼5 000)
∼500
600 (7 600)
600 (∼6 000)
BPQA63
PMEA64
Me1-TPA65
others2,13
440 (-)
610 (-)
∼600
812-847
a In CH2Cl2. b In THF.

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11307
Figure 7. Low-temperature UV-vis spectra in MeTHF at -128 ꢀC.
Spectrum 1: 2a with absorption at 337 nm (4 100 M-1 cm-1). Spectrum 2:
[(LEIm)CuI(CO)]þ without excess CO. Spectrum 3: partial formation of
superoxo species [(LEIm)CuII(O2•-)]þ (2bS) with absorption at 440 nm
and the peroxo complex 2bP. Spectrum 4: 2bP with absorption at 445
(2 500 M-1 cm-1), 535 (11 000 M-1 cm-1), and 610 nm (8 100 M-1 cm-1).
Spectrum 5: thermal decomposition product.
Figure 6. rR spectra of [{(LEIm)CuII}2(O22-)]2þ (2bP) in THF at 77 K
(the red spectrum denotes 16O2, the blue spectrum denotes 18O2, and the
black spectrum is preoxygenation). λex = 568 nm.
at 822 cm-1 56 The feature at 539 cm-1 (Δ(18O2)=-26) is
.
trum (279 cm-1, Figure 6) is consistent with an orienta-
tion of the lobes of the Cu dx2-y2 orbital along the
Cu-Nimidazole bond. The peroxo πσ* to Cu charge trans-
fer into dx2-y2 would result in elongation of the
Cu-Nimidazole bond and thus its rR enhancement. The
square pyramidal environment of each Cu eliminates the
inversion center of the dimer, allowing for enhancement
of the antisymmetric Cu-O stretch in the rR (484 cm-1
(Δ(18O2) = -26), Figure 6). Better orientation of the
Nimidazole ligand lone pair along the lobe of Cu’s dx2-y2
orbital would facilitate its donation thus reducing dona-
tion from the peroxo to the Cu, weakening the Cu-O
bond. This decreased peroxo πσ* donation also results in
more electron density in the antibonding peroxo πσ*
orbital and thus a weaker O-O bond.57
Detection of a Superoxo-Copper(II) Complex with the
LEIm Ligand Complex. We have been able to detect the
initial O2 adduct arising from dioxygen reactivity in the
[(LEIm)CuI]þ (2a) chemistry. This was accomplished in
one manner by first bubbling a -128 ꢀC solution of 2a in
MeTHF with carbon monoxide, which gives the colorless
CO adduct [(LEIm)CuI(CO)]þ (vide supra); the MLCT
band at 337 nm (spectra 1 and 2 in Figure 7) disappears.
After elimination of excess CO by vacuum-purging,
oxygenation of this solution leads to formation of O2
adducts, a mixture of the peroxodicopper(II) species 2bP,
and a new absorption near 440 nm is assigned as the
superoxo complex [(LEIm)CuII(O2•-)]þ (2bS). We have
previously used this “trick” of oxygenation of a solution
of the carbonyl complex in order to stabilize [(Me2N-
TMPA)CuII(O2•-)]þ.11 Additional O2-bubbling leads to
full formation of the peroxo adduct, spectrum 4 in
Figure 7.
assigned as a symmetric ν(Cu-O) stretch based on energy
and 18O2-isotopic shift. These observed vibrations for-
mulate 2bP as an end-on bound μ-1,2 peroxo-dicopper-
(II) complex.6 The antisymmetric ν(Cu-O) is observed
as a weak, broad feature at 484 cm-1 (Δ(18O2)=-26).
Additional isotope-insensitive features are observed at
417, 337, and 279 cm-1 and are assigned as Cu-Npyridine
,
internal TMPA ring mode, and Cu-Nimidizole stretches,
respectively.
The O-O stretching frequency of 2bP is lower (by 10
cm-1) than observed for [{(TMPA)CuII}2(O22-)]2þ (832
cm-1), indicating that 2bP has a weaker O-O bond. The
symmetric Cu-O stretch is also lower than for
[{(TMPA)CuII}2(O22-)]2þ (by 23 cm-1), reflecting a
weaker Cu-O bond 2bP.6 Complex 2bP is likely distorted
from TBP to a more square-pyramidal geometry (as was
observed for 2c, vide supra), resulting in better overlap
between the ligand N’s and Cu’s dx2-y2 orbital. The high
intensity of the Cu-Nimidazole vibration in the rR spec-
(55) Unpublished observations.
(56) Skulan, A. J.; Hanson, M. A.; Hsu, H.-f.; Que, L., Jr.; Solomon, E. I.
J. Am. Chem. Soc. 2003, 125, 7344–7356.
(57) A resonance Raman study on [{(LMIm)CuII}2(O22-)]2þ (1bP) was not
carried out in the context of this report but will be described in a separate
publication.
(58) Species 2bS has an absorption near 440 nm in MeTHF solvent. We
surmise that this difference compared to in THF solvent may be attributed to
a solvent dependence: For [(TMPA)CuII(O2•-)]þ, the related absorption
occurs at 410 nm in EtCN but 425 nm in THF, see ref 10. Further studies are
needed to understand these observations.
ꢀ
(59) Paul, P. P.; Tyeklar, Z.; Jacobson, R. R.; Karlin, K. D. J. Am. Chem.
Soc. 1991, 113, 5322–5332.
ꢀ
(60) Lee, D.-H.; Wei, N.; Murthy, N. N.; Tyeklar, Z.; Karlin, K. D.;
€
Kaderli, S.; Jung, B.; Zuberbuhler, A. D. J. Am. Chem. Soc. 1995, 117,
12498–12513.
A more direct detection of [(LEIm)CuII(O2•-)]þ (2bS)
was possible using a stopped-flow kinetics instrument
with diode array full spectrum monitoring. As has been
previously extensively studied for oxygenation of
[(TMPA)CuI(CH3CN)]þ and study of the transient
superoxo complex [(TMPA)CuII(O2•-)]þ,7,9 millisecond
mixing of [(LEIm)CuI]þ (2a) and dioxygen in THF at
-90 ꢀC readily reveals the formation of 2bS, with strong
absorption at 431 nm.58 The spectroscopic behavior
(61) Weitzer, M.; Schatz, M.; Hampel, F.; Heinemann, F. W.; Schindler,
S. J. Chem. Soc. Dalton Trans. 2002, 686–694.
(62) Schatz, M.; Leibold, M.; Foxon, S. P.; Weitzer, M.; Heinemann, F.
W.; Hampel, F.; Walter, O.; Schindler, S. Dalton Trans. 2003, 1480–1487.
€
(63) Karlin, K. D.; Wei, N.; Jung, B.; Kaderli, S.; Niklaus, P.; Zuberbuh-
ler, A. D. J. Am Chem. Soc. 1993, 115, 9506–9514.
(64) Schatz, M.; Becker, M.; Thaler, F.; Hampel, F.; Schindler, S.;
ꢀ
Jacobson, R. R.; Tyeklar, Z.; Murthy, N. N.; Ghosh, P.; Chen, Q.; Zubieta,
J.; Karlin, K. D. Inorg. Chem. 2001, 40, 2312–2322.
(65) Uozumi, K.; Hayashi, Y.; Suzuki, M.; Uehara, A. Chem. Lett. 1993,
963–966.

11308 Inorganic Chemistry, Vol. 48, No. 23, 2009
Lee et al.
Table 4. Selected Bond Distances of a Tetramer [(μ2-LEIm-)4(CuII)4]4þ Cluster
bond
distance (A)
bond
distance (A)
˚
˚
Cu-N
Cu-N
Cu(1)-N(62)
Cu(1)-N(1)
Cu(1)-N(4)
Cu(1)-N(3)
Cu(1)-N(5)
Cu(2)-N(2)
Cu(2)-N(21)
Cu(2)-N(24)
Cu(2)-N(23)
Cu(2)-N(25)
1.967(3)
1.984(4)
2.046(4)
2.090(3)
2.197(4)
1.957(4)
1.967(4)
2.046(4)
2.095(4)
2.222(4)
Cu(3)-N(41)
Cu(3)-N(22)
Cu(3)-N(44)
Cu(3)-N(43)
Cu(3)-N(45)
Cu(4)-N(42)
Cu(4)-N(61)
Cu(4)-N(64)
Cu(4)-N(63)
Cu(4)-N(65)
1.969(4)
1.973(4)
2.027(4)
2.098(4)
2.185(4)
1.963(3)
1.978(4)
2.039(4)
2.089(3)
2.187(4)
Scheme 5
Figure 8. Time-dependent UV-visible spectra for the oxygenation of
[(LEIm)CuI]þ (2a) in THF at -90 ꢀC as measured using a stopped-flow
kinetics instrument. The inset shows the time dependence of the disap-
pearance of the superoxo complex [(LEIm)CuII(O2•-)]þ (2bS) (431 nm)
and the corresponding formation of the peroxo complex
[{(LEIm)CuII}2(O22-)]2þ (2bP) monitored at 535 nm. See text for further
explanation and discussion.
identified it as a tetranuclear [(μ2-LEIm-)4(CuII)4]4þ clus-
ter, Figure 9. It is constructed from four cupric ions and
four anionic (i.e., with deprotonated imidazolyl) LEIm-
ligands. Each copper center has a distorted square
pyramidal geometry with an average τ = 0.41 (τ = 0.0
for a perfect square pyramid (SP), vide supra). Support-
ing the arguments made above, this is another example of
a copper complex in the LEIm framework which prefers a
(distorted) SP rather than a TBP coordination geometry.
˚
The average Cu Cu distance in the cluster is 5.877 A.
3 3 3
Other selected Cu-N bonds are listed in Table 4. The
presence of the anionic imidazolate group here suggested
to us the probability that hydrogen peroxide was gener-
ated during decomposition of 2bP, Scheme 5, with the
imidazolyl N-H given up to form H2O2. We tried to test
for this possibility using the titanium(IV) oxysulfate
reagent to interrogate the warmed up solution of 2bP;
this should give an optical change at 408 nm when it reacts
with H2O2 (see Experimental Section).35,36 However, no
H2O2 was detected, perhaps due to metal complex
mediated disproportionation chemistry or oxidation of
solvent (which we did not probe). Hydrogen peroxide was
produced in 28% yield when [H(OEt2)2][B(C6F5)4] was
added to the peroxo species 2bP at -94 ꢀC, warming and
workup. Such a result (but in better yield) occurs when
[{(TMPA)CuII}2(O22-)]2þ is protonated with mineral
acids or even phenols.59 In fact, the thermal decomposi-
tion of [{(LMIm)CuII}2(O22-)]2þ (1bP) similarly leads to
the analogue tetramer cluster complex.55
Figure 9. ORTEP view (50% probability ellipsoids) of a tetramer
[(μ2-LEIm-)4(CuII)4]4þ cluster (average τ = ∼0.41). The hydrogen atoms
and perchlorate anions are omitted for clarity.
observed in this experiment (Figure 8) reveals that 2bS
forms within the instrument mixing time (1-2 ms), as the
431 nm absorption is already decreasing in the first few
spectra observed. Thus, from the absorptivity observed
here, we conclude that ε>2 600 M-1 cm-1 at 431 nm for
2bS. At this time, a full stopped-flow kinetic analysis has
not been carried out for the 2a/O2 reaction chemistry.
This will be the subject of future study on a series of
copper(I) complexes with tripodal tetradentate ligands
varying in donor atoms. Closely related to this will be
studies of copper(I)-carbonyl complexes 1a-CO and 2a-
CO in “flash-and-trap” experiments,10 to deduce the
kinetics of formation of superoxo adducts.
Summary and Conclusions
In this Article, we have described the copper(I), copper(II),
and copper(I)-dioxygen chemistry with new tetradentate
ligands possessing one imidazolyl donor (LMIm and LEIm).
The study is directed toward comparisons with our prior
detailed investigations with TMPA-Cu chemistry. LMIm
containing Cu(I)-CO, mononuclear Cu(II), and the pero-
xo complex [{(LMIm)CuII}2(μ-1,2-O22-)]2þ (1bP) all show
Tetramer Cluster Formation. The thermal decomposi-
tion product of [{(LEIm)CuII}2(O22-)]2þ (2bP), following
warming -80 ꢀC solutions in THF, is a green crystalline
material obtained in high yield. X-ray structural analysis

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 11309
structural and/or physical properties very similar to
their TMPA-Cu analogues. However, LEIm containing
Cu(I)-CO, Cu(II) complexes (i.e., 2c), and the peroxo
complex [{(LEIm)CuII}2(μ-1,2-O22-)]2þ (2bP) are different;
2bP possesses significantly weakened Cu-O and O-O bonds
as compared to [{(TMPA)CuII}2(O22-)]2þ. The relative
decreases in bond strength are due to a square pyra-
midal distortion of 2bP, which results in increased overlap
(thus donation) of the ligand N’s with the Cu(II) dx2-y2
orbital resulting in less donation from peroxo to Cu
(decreased Cu-O bond strength) and increased electron
density in the peroxo π*σ orbital (decreased O-O bond
strength).
tive. Also, it will be of interest to compare complexes 1a and
2a in their kinetics and thermodynamics of copper(I)/O2
reactivity giving superoxo-Cu(II) and peroxo-dicopper(II)
complexes. Reactions of peroxo complexes employing tetra-
dentate ligands with varied donor groups (i.e., S or O donors)
will also be tested for their oxidative reactivity toward
exogenous substrates.
Acknowledgment. K.D.K. and E.I.S. are grateful to
the National Institutes of Health (Grants GM 28962
(K.D.K.) and DK 31450 (E.I.S.)) for support of this
research.
Supporting Information Available: Crystallographic informa-
tion files (CIF) for [(LMIm)CuII(CH3CN)]2þ (1c), [(μ2-LEIm-)4-
(CuII)4]4þ cluster, and [(LEIm)CuII(CH3CN)(-OClO3)]þ
Future studies will address the electronic structures of
copper complexes with these varied (in donor atom and
tetradentate chelate architecture) tripodal tetradentate
ligands, both from an experimental and theoretical perspec-
(2c OClO3). This material is available free of charge via the
3
Internet at http://pubs.acs.org.