Cu(II)-mediated intramolecular carbene cation radical formation: Relevance to unimolecular metal - Ligand radical intermediates

Article abstract of DOI:10.1021/ja0120072

We report the syntheses of the photochemically labile 9-diazo-4,5-diazafluorene (1) framework and the corresponding Cu(9-diazo-4,5-diazafluorene)₂(NO₃)₂ compound (2). The X-ray structure of 2 reveals a 6-coordinate, tetragonal geometry with one nitrogen donor of an asymmetrically chelated diazafluorene in the equatorial position and the other defining the weak Jahn-Teller axis. The nitrate counterions bind in a monodentate fashion in the equatorial plane to complete the coordination sphere. Extended Hueckel calculations reveal that the unusual solid-state structure derives from the enlarged bite angle of the fluorene skeleton and steric interactions between the adjacent hydrogen atoms in the higher energy (0.45 eV) symmetrically coordinated state. This is in contrast to Cu(py)₄(NO₃)₂ which is 1.3 eV more stable with the nitrate counterions bound along the Jahn-Telter axis. Electron paramagnetic resonance (EPR) studies in solution reveal that the nitrates dissociate to yield 6-coordinate CuN₂X₂N₂′ structures with either a bound chloride ion (g_x = 2.10, g_y = 2.04, A_z = 2.23, A_z = 177 × 10^-4 cm^-1) or a mixture of counterion and solvent (g_xa = 2.05, g_ya = 2.06, g_za = 2.29, A_za = 170 × 10^-4 cm^-1; g_xb = 2.07, g_yb = 2.08, g_zb = 2.34, A_zb = 155 × 10^-4 cm^-1). Photolyses of 1 and 2 indicate loss of N₂ and formation of either carbene (|D/hc| = 0.408 cm^-1, |E/hc| = 0.0292 cm^-1) or Cu(I)-L⁺ (S = 1/2, g = 2.0019) intermediates, which are identified by EPR, UV-vis, and time-dependent density functional theory methods. The results illustrate the important role redox active transition metals play in determining the nature of fundamental metal-ligand radical intermediates.

Full text of DOI:10.1021/ja0120072

Published on Web 12/13/2001
Cu(II)-Mediated Intramolecular Carbene Cation Radical
Formation: Relevance to Unimolecular Metal-Ligand Radical
Intermediates
Brian J. Kraft, Hilary J. Eppley, John C. Huffman, and Jeffrey M. Zaleski*
Contribution from the Department of Chemistry and Molecular Structure Center,
Indiana UniVersity, Bloomington, Indiana 47405
Received August 20, 2001
Abstract: We report the syntheses of the photochemically labile 9-diazo-4,5-diazafluorene (1) framework
and the corresponding Cu(9-diazo-4,5-diazafluorene)2(NO3)2 compound (2). The X-ray structure of 2 reveals
a 6-coordinate, tetragonal geometry with one nitrogen donor of an asymmetrically chelated diazafluorene
in the equatorial position and the other defining the weak Jahn-Teller axis. The nitrate counterions bind
in a monodentate fashion in the equatorial plane to complete the coordination sphere. Extended Hu¨ckel
calculations reveal that the unusual solid-state structure derives from the enlarged bite angle of the fluorene
skeleton and steric interactions between the adjacent hydrogen atoms in the higher energy (0.45 eV)
symmetrically coordinated state. This is in contrast to Cu(py)4(NO3)2 which is 1.3 eV more stable with the
nitrate counterions bound along the Jahn-Teller axis. Electron paramagnetic resonance (EPR) studies in
solution reveal that the nitrates dissociate to yield 6-coordinate CuN2X2N2′ structures with either a bound
chloride ion (gx ) 2.10, gy ) 2.04, gz ) 2.23, Az ) 177 × 10-4 cm-1) or a mixture of counterion and solvent
(gxa ) 2.05, gya ) 2.06, gza ) 2.29, Aza ) 170 × 10-4 cm-1; gxb ) 2.07, gyb ) 2.08, gzb ) 2.34, Azb
)
155 × 10-4 cm-1). Photolyses of 1 and 2 indicate loss of N2 and formation of either carbene (|D/hc| )
1
0.408 cm-1, |E/hc| ) 0.0292 cm-1) or Cu(I)-L•+ (S ) /2, g ) 2.0019) intermediates, which are identified
by EPR, UV-vis, and time-dependent density functional theory methods. The results illustrate the important
role redox active transition metals play in determining the nature of fundamental metal-ligand radical
intermediates.
photofootprinting agents,10,11 as well as potential antitumor
agents in medicinal chemistry.12-15
Introduction
Diazo compounds are known to release dinitrogen upon
thermal and photochemical activation to produce unstable
carbene intermediates that react with a variety of substrates.1-3
The facile reactivities of these highly energetic compounds have
long been exploited by organic chemists to facilitate various
transformations such as C-H bond insertion, double bond
addition, and double H-atom abstraction.1,3 Recently, the reac-
tivities of organic diazo compounds have also shown potential
in a variety of applied fields, including the generation of
lithographic resists, light-sensitive varnish used in the processing
of silicon chips, for the electronics industry.4,5 Diazo compounds
also have biologically relevant applications including their use
in structural biology as radioactive photoaffinity labels,6-9
The decomposition of diazo groups is known to be facilitated
by the presence of both oxidizing and reducing agents.16
Consequently, their reactivities may be enhanced by interaction
with transition metals in either high or low oxidation states. To
this end, both Cu(I) complexes and salts have been used
extensively in organic syntheses to catalyze decomposition of
the diazo unit for C-C coupling reactions including addition,
insertion, and the synthesis of ylides.3 Diazo compounds have
also been used as reducing agents for Cu(II) salts to generate
catalytically active Cu(I) species in situ.3 This redox partnership
presents opportunities for generating an inter- or intramolecular
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* To whom correspondence should be addressed. E-mail: zaleski@
indiana.edu.
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9
272 VOL. 124, NO. 2, 2002 J. AM. CHEM. SOC.
10.1021/ja0120072 CCC: $22.00 © 2002 American Chemical Society

Intramolecular Carbene Cation Radical Formation
A R T I C L E S
Varian Gemini 2000 NMR spectrometer with the proton solvent as a
reference. Elemental analyses were performed by the Atlantic Mi-
croanalytical Laboratory at Norcross, GA, or at Indiana University using
a Perkin-Elmer Series II CHNS/O Analyzer 2400. Infrared spectra were
recorded either as KBr pellets or in a fluorolube mull on a Nicolet
510P spectrophotometer. Cyclic voltammograms and differential pulse
voltammograms were recorded on a BAS CV-50W voltametric
analyzer. A three-electrode assembly utilizing a glassy carbon working
electrode, a platinum auxiliary electrode, and an SCE reference electrode
was employed without the application of IR compensation. Conductivity
measurements were made using a YSI Model 31A conductance bridge
and a YSI Model 3403 conductivity cell (1 cm-1 cell constant).
Electronic absorption spectra were collected on a Perkin-Elmer Lambda
19 UV/VIS/NIR spectrometer at ambient temperature. Low-temperature
electronic spectral measurements were performed in an Oxford Instru-
ments optical cryostat, which was mounted directly inside the spec-
trometer. Fluorescence measurements were obtained with a Perkin-
Elmer LS 50 B luminescence spectrometer equipped with a Hamatatsu
model R2371 PMT. Quantum yields are given for solutions of matched
optical density at the excitation wavelength and are reported relative
to the fluorescence of phenol. All electron paramagnetic resonance
spectra were recorded at X-band (9.5 GHz) on an ESP 300 Bruker
instrument. Typical EPR conditions: microwave power, 10 mW;
modulation amplitude, 2-20 G; modulation frequency, 100 kHz;
receiver gain, (2-5) × 104. EPR spectra were simulated using SimFonia
(Bruker) for organic radicals and a Monte Carlo method for the copper
signals.35,36 Matrix photolyses were performed directly in the EPR cavity
and the optical dewar with a 150 W Hg source coupled via a liquid
light pipe (Oriel # 77557). Photolyses were run with λ g 455 nm, where
the cutoff wavelength was selected using a series of long wavelength
pass filters (345, 420, and 455 nm). Photolyses at λ ) 457 nm and
λ ) 647 nm were performed with Ar+ and Kr+ ion lasers (Coherent
models I-70 and I-300), respectively, using powers of =300 mW. Solid-
state photolyses were performed with a 1000 W Xe lamp and long
pass filters (295, 345, 420, 455 nm) at 20 °C.
redox switch given the recent interest in radical cations derived
from diaryl diazo compounds.17-20
In addition to the chemical reactivity of diazo compounds
with redox active metals, the geometric and electronic structural
characteristics of metals such as paramagnetism, optical absorp-
tion and emission properties, and radioactivity may enhance the
usefulness of these highly reactive agents for applications in
materials science, medicine, and biochemistry. For example,
transient magnetic materials have been prepared from metal-
diazo coordination polymers whose ground-state paramagnetic
signatures can be photochemically modulated by production of
a ligand-localized diradical that couples to the spin on the metal
center.21,22 The importance of metal-ligand radical intermedi-
ates, however, is not restricted to applications in synthesis3 and
materials,23,24 but extends to DNA degradation agents25-27 and
general bioinorganic chemistry28-31 as well. Here, the emerging
role of metal-ligand radical intermediates for storing redox
equivalents during metalloenzyme activity29,32-34 (e.g., galactose
oxidase) further underscores the importance of fundamental
redox active metal-ligand radical chemistry.
Our interests lie in the function of metal ions in the
determination of the fundamental chemical intermediates pro-
duced upon photolysis of metal-diazo compounds with rel-
evance to materials science and bioinorganic chemistry. Toward
these goals, we have synthesized and structurally characterized
the thermally and photochemically labile 9-diazo-4,5-diazafluo-
rene (1) framework and the corresponding Cu(9-diazo-4,5-
diazafluorene)2(NO3)2 compound (2). Comparison of the photo-
chemical reactivities of 1 and 2 using low-temperature electron
paramagnetic resonance (EPR) and electronic absorption spec-
troscopies reveals that the Cu(II) center strongly influences the
electronic structure of the photogenerated radical intermediates.
Synthetic Methods. All synthetic preparations were performed under
ambient oxygen conditions. Reagents were used as received without
further purification, and all solvents used were spectrochemical grade.
The starting material, 4,5-diazafluorenone-9-hydrazone, was synthesized
from the aqueous oxidation of 1,10-phenathroline followed by the
formation of the hydrazone using hydrazine hydrate, according to
literature procedures.37-39 Caution. Diazo compounds are highly
reactiVe and produce copious amounts of N2 gas upon photo- or thermal
decomposition. Although we did not witness the detonation of solid
samples of these compounds, adequate precautions should be taken
when working with them.
Experimental Section
Physical Measurements. Samples were prepared using volumetric
glassware and standard Schlenk and drybox techniques. Solvents were
degassed either by purging with N2 for >1 h or by several freeze-
1
pump thaw cycles. H NMR spectra were collected on a 300 MHz
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Synthesis of 9-Diazo-4,5-diazafluorene (1). 4,5-Diazafluorenone-
9-hydrazone (1.0 g, 5.1 mmol) was oxidized using a basic solution (10
drops of saturated KOH in water) of yellow HgO (1.2 g, 5.5 mmol) in
benzene (200 mL). The solution was stirred overnight at room
temperature and then filtered through glass wool or filter paper to
remove the insoluble Hg waste. The resulting orange solution was
concentrated to a solid and then recrystallized from a mixture of
dichloromethane and pentane at -20 °C. Yield: 52-65%. Anal. Calcd
for C11H6N4: C, 67.71; H, 2.97; N, 24.28. Found: C, 67.73; H, 3.11;
N, 24.32. EI MS m/z: 194.1 (M+), 166.1 (M+ - N2). 1H NMR
(CDCl3): δ ppm 8.72 (dd, 2H, J ) 1.5, 4.7 Hz), 7.91 (dd, 2H, J ) 1.5,
8.0 Hz), 7.39 (dd, 2H, J ) 4.7, 8.0 Hz). Selected IR data (KBr, cm-1):
3040 (w), 2060 (vs), 1718 (w), 1593 (w), 1560 (w), 1545 (w), 1410
(22) Sano, Y.; Tanaka, M.; Koga, N.; Matsuda, K.; Iwamura, H.; Rabu, P.;
Drillon, M. J. Am. Chem. Soc. 1997, 119, 8246-8252.
(23) Fujita, W.; Awaga, K. J. Am. Chem. Soc. 2001, 123, 3601-3602.
(24) Sakane, A.; Kumada, H.; Karasawa, S.; Koga, N.; Iwamura, H. Inorg. Chem.
2000, 39, 2891-2896.
(25) Maurer, T. D.; Kraft, B. J.; Lato, S. M.; Ellington, A. D.; Zaleski, J. M.
Chem. Commun. 2000, 69-70.
(26) Kraft, B. J.; Zaleski, J. M. New J. Chem. 2001, 25, 1281-1289.
(27) Eppley, H. J.; Lato, S. A.; Ellington, A. D.; Zaleski, J. M. Chem. Commun.
1999, 2405-2406.
(28) Itoh, S.; Kumei, H.; Nagatomo, S.; Kitagawa, T.; Fukuzumi, S. J. Am.
Chem. Soc. 2001, 123, 2165-2175.
(29) Himo, F.; Eriksson, L. A.; Maseras, F.; Siegbahn, P. E. M. J. Am. Chem.
Soc. 2000, 122, 8031-8036.
(30) Kimura, S.; Bill, E.; Bothe, E.; Weyhermueller, T.; Wieghardt, K. J. Am.
Chem. Soc. 2001, 123, 6025-6039.
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Wieghardt, K. J. Am. Chem. Soc. 2000, 122, 9663-9673.
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E. C.; Que, L. J.; Tolman, W. B. J. Am. Chem. Soc. 1997, 119, 8217-
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J., Berliner, J., Eds.; Plenum Press: New York, 1993; Vol. 13, p 1.
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M. Liebigs Ann./Recl. 1997, 1611-1618.
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9
J. AM. CHEM. SOC. VOL. 124, NO. 2, 2002 273

A R T I C L E S
Kraft et al.
Scheme 1. Synthesis of 9-Diazo-4,5-diazafluorene (1) and the
(s), 1394 (s), 1336 (w), 1319 (w), 1296 (m), 1262 (m), 1202 (m), 1186
(s), 1138 (w), 1099 (w), 1065 (m), 1030 (m), 801 (s), 742 (s), 702 (w),
679 (w), 625 (w), 571 (w), 489 (w), 441 (w).
Corresponding Cupric Complex (2)
Synthesis of Bis(9-diazo-4,5-diazafluorene)copper(II) Nitrate (2).
9-Diazo-4,5-diazafluorene (76 mg, 0.39 mmol) was dissolved in 15
mL of methanol under low light conditions. The salt Cu(NO3)2‚2.5H2O
(46 mg, 0.20 mmol) was dissolved in 5 mL of methanol. Both solutions
were filtered through glass wool to remove any undissolved solid, and
the Cu(NO3)2 solution was added to the ligand solution dropwise. The
solution turned from orange to deep green in color and was allowed to
stir for about 1 min. The reaction mixture was then allowed to stand at
4 °C for 12 h. The solution was subsequently filtered to remove the
microcrystalline green solid and washed with several portions of
methanol. Using a more dilute solution, crystals suitable for X-ray
crystallographic analysis could be obtained directly from the reaction
mixture. The solid was dried in vacuo for 2 h. Yield: 94 mg, 82%
based on Cu. Anal. Calcd for C22H12N10O6Cu: C, 45.90; H, 2.02; N,
24.28. Found: C, 45.87; H, 2.10; N, 24.32. Selected IR data (KBr,
cm-1): 2081 (vs), 1589 (w), 1651 (w), 1466 (s), 1419 (m), 1396 (s),
1346 (w), 1262 (s), 1219 (w), 1188 (m), 1072 (w), 1013 (m), 802 (m),
731 (m), 684 (w), 644 (m), 571 (w), 486 (w), 432 (w).
and 6-31G** basis set (0.9613)41 was applied to the vibrational
frequencies obtained from the frequency analysis at the optimized
geometry. Time-dependent DFT (TDDFT) calculations were then
performed with the same functionals and basis set at the optimized C2V
geometry to obtain electronic transition energies.
X-ray Crystallographic Analysis of 2. Data for 2 were collected
on a Picker four-circle goniostat equipped with a Furnas monochromator
(HOG crystal), modified by addition of stepping motors (Slo-Syn) on
each of the four axes, and a fifth motor driving a 20-position filter/
attenuator wheel. The diffractometer control software, PCPS.EXE, was
written by W. E. Streib of the IUMSC. The software for structure
solution and refinement include SHELXTL-PC, and other versions of
SHELX, as well as the XTEL program library. A typical, well-defined
blue-green crystal was removed from the crystallization media and
transferred to the goniostat where it was cooled in the gaseous nitrogen
cold stream. A systematic search of a limited hemisphere of reciprocal
space was used to determine that the crystal possessed no symmetry
or systematic absences, corresponding to one of the triclinic space
groups. Subsequent solution and refinement confirmed the centrosym-
metric choice, P1h. The data were collected using a standard moving
crystal-moving detector technique with fixed backgrounds at each
extreme of the scan. The data were corrected for Lorentz, and
polarization effects and equivalent reflections were averaged. The
structure was readily solved using direct methods (MULTAN78) and
Fourier techniques. Hydrogen atoms were readily located in a difference
Fourier phased on the non-hydrogen atoms. Hydrogen atoms were
assigned isotropic thermal parameters and allowed to vary for the final
cycles of refinement. A final difference Fourier was featureless; the
largest peak of intensity was at 0.65 e/Å3. Atomic coordinates, bond
lengths, and bond angles have been deposited at the Cambridge
Crystallographic Data Center and are available as Supporting Informa-
tion.
Results
Syntheses. The solubility of metal-diazo complexes has been
an impediment to the development of molecular-based applica-
tions of metal-diazo compounds. Polymeric diazo structures
are common among the previous examples of metal-coordinated
diazo compounds.21,22 For our studies, a simple chelating ligand
with an exocyclic diazo group was desired because it minimizes
potential polymeric interactions among multiple metal centers.
Although the 9-diazo-4,5-diazafluorene ligand (1) has been used
in the synthesis of extended organic arrays for molecular racks
(MOLRACs)42 through chemistry at the diazo group, it has not
been utilized previously as an independent ligand.
Compound 1 was synthesized from 4,5-diazafluorenone, using
modified literature procedures.37-39 Briefly, 1,10-phenanthroline
is oxidized under basic conditions to form the 4,5-diazafluorene.
The 4,5-diazafluorenone-9-hydrazone was then prepared by
addition of excess hydrazine hydrate, and subsequent oxidation
with yellow HgO yields 9-diazo-4,5-diazafluorene (Scheme 1).
The copper(II) complex, bis(9-diazo-4,5-diazafluorene)Cu(II)-
(NO3)2 (2), was then synthesized by adding a methanolic
solution of Cu(NO3)2‚2.5H2O to a solution of ligand in a 1:2
ratio in the dark. Green microcrystals of 2 precipitated out of
this solution immediately and could be isolated by filtration in
the dark. Larger, X-ray quality crystals were grown directly from
a more dilute reaction solution.
Computational Methods. Extended Hu¨ckel molecular orbital
(EHMO) calculations of 2 were performed using an internal coordinates
Z-matrix derived from the X-ray crystal structure Cartesian coordinates.
The structure of the complex was modified to increase the short C-H
bonds to at least 0.8 Å using Chem 3-D Pro. Cartesian coordinates of
a comparison complex, Cu(py)4(NO3)2 (py ) pyridine), were obtained
through the Cambridge Crystallographic Database, and H-atoms were
added to its structure in Chem 3-D Pro. Both molecules were oriented
such that their Jahn-Teller (JT) axes were positioned along the z-axis
in the ground-state structure. Dummy atoms defining a vector that
bisects the chelate bite in 2 were added as the rotation/translation axis.
Extended Hu¨ckel calculations and molecular orbital visualization were
performed using WINCACAO. The Hu¨ckel constant (â) was set to
1.75.
(40) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.6; Gaussian,
Inc.: Pittsburgh, PA, 1998.
Geometry optimization for fluorenylidene was performed under a
(41) Foresman, J. B.; Frisch, A. Exploring Chemistry with Electronic Structure
Methods, 2nd ed.; Gaussian: Pittsburgh, 1993.
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F. R.; Kelso, L. S. Angew. Chem., Int. Ed. Engl. 1996, 35, 2485-2487.
C2V symmetry constraint with density functional theory (DFT) and the
UB3LYP combination of functionals using the 6-31G** basis set
employed in Gaussian 98.40 A scaling factor appropriate for the method
9
274 J. AM. CHEM. SOC. VOL. 124, NO. 2, 2002

Intramolecular Carbene Cation Radical Formation
A R T I C L E S
Table 2. Selected Bond Distances (Å) and Angles (deg) for 2
Cu(1)-O(17)
Cu(1)-N(2)
Cu(1)-N(12)
N(15)-C(7)
N(15)-N(16)
O(17)-N(18)
2.014(3)
1.982(3)
2.651(3)
1.319(4)
1.136(4)
1.292(3)
N(2)-Cu(1)-N(12)
79.69(10)
Cu(1)-O(17)-N(18) 126.33(19)
Cu(1)-N(2)-C(3)
Cu(1)-N(2)-C(14)
129.20(22)
113.33(22)
Cu(1)-N(12)-C(11) 150.83(22)
Cu(1)-N(12)-C(13) 94.17(19)
O(17)-Cu(1)-O(17) 180.00
N(16)-N(15)-C(7)
178.5(3)
126.6(3)
O(17)-Cu(1)-N(2)
85.18(10) N(15)-C(7)-C(8)
O(17)-Cu(1)-N(2)a 94.82(10) O(17)-N(18)-O(19) 120.41(27)
O(17)-Cu(1)-N(12) 99.31(9)
a (x - 1, y - 1, z - 1).
renones,47 and diazafluorenone hydrazone.48,49 It is also the first
inorganic structure in this family to possess a thermally and
photochemically reactive diazo group at the 9-position of the
fluorene skeleton. The coordination geometry of 2 is unusual
in that the elongated J-T axis of the molecule lies along the
N-Cu-N vector (Cu(1)-N(12) ) 2.651 Å) and not in the
O-Cu-O axis (Cu(1)-O(12) ) 2.014 Å, O(17)-Cu(1)-O(17)
) 179.96°), which is occupied by nitrate counterions. To our
knowledge, this is one of only two examples of N2O2N2′
coordination to a Cu(II) center with two nitrates bound trans
in the equatorial plane and the nitrogens of the chelate directed
along the J-T axis.50 This N2X2N2′ geometry has been observed
in other Cu(II) complexes with the 4,5-diazafluorene template
and simple anions such as Cl- or Br-,50 while complexes with
transition metals other than Cu(II) (e.g., Re(I),51 Ni(II),52
Co(II)48) typically exhibit a more symmetrical coordination.
Although there is no analogous structure to 2 for comparison
of the distances within the diazo group, the NdN (1.136 Å)
and NdC (1.319 Å) bond lengths are similar to the crystallo-
graphically characterized 9-diazofluorene.53
Figure 1. ORTEP of the X-ray crystal structure of 2. Thermal ellipsoids
are illustrated at 50% probability.
Table 1. Crystallographic Data for 2
empirical formula
formula weight
color of crystal
crystal system
space group
a, Å
C22H12N10O6Cu
575.95
green
triclinic
P1h
8.897(4)
10.178(5)
6.833(3)
104.47(2)
103.31(2)
109.22(2)
531.68
1
b, Å
c, Å
R, deg
â, deg
Extended Hu1ckel Calculations. Extended Hu¨ckel molecular
orbital (EHMO) calculations were performed on 1 and 2 to
provide general insight into the origin of the unusual coordina-
tion geometry and to investigate the electronic structure of the
copper complex. Figure 2 reveals the frontier molecular orbitals
for both 1 and 2 and their relative energies. For 1, two nearly
degenerate orbitals primarily localized on the diazo unit are
observed. The HOMO is π-bonding relative to the CdN bond
at the 9-position and nonbonding between the nitrogens within
the NdN unit. The two lowest energy unfilled MOs localized
on the diazo unit are both CdN nonbonding and NdN
π-antibonding, consistent with the propensity for N2 loss via
the electronic transition between these orbitals (n f π*). For
2, the ligand orbitals are of very similar parentage, but the half-
γ, deg
V, Å3
Z
T, K
105
λ, Å
0.71069
1.799
10.970
0.0389
0.0356
1.508
F
calcd, g/cm3
µ, cm-1
R(Fo)a
Rw(Fo)b
GOF
2
a R ) ∑|Fo| - |Fc|/∑|Fo|. b Rw ) [∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2, where
w ) 1/σ2(|Fo|).
Solid State Characterization. X-ray Crystal Structure. An
ORTEP diagram of 2 is shown in Figure 1, and the compound’s
crystallographic data are given in Table 1. While the unit cell
is composed of only one molecule, stacking of the cells in the
z-direction results in a two-dimensional arrangement of columns
with an interccolumnar separation of 6.83 Å. No close inter-
molecular hydrogen-bonding contacts are evident in the crystal
structure. The closest contact (2.99 Å) involves diazo groups
between two neighboring unit cells.
2
2
filled, primarily dx -y metal-based orbital is now formally the
HOMO as it lies energetically ∼0.3 eV above the filled orbitals
localized on the ligands.
(46) Klein, R. A.; van Belzen, R.; Vrieze, K.; Elsevier, C. J.; Thummel, R. P.;
Fraanje, J.; Goubitz, K. Collect. Czech. Chem. Commun. 1997, 62, 238-
256.
(47) Balagopalakrishna, C.; Rajasekharan, M. V.; Bott, S.; Atwood, J. L.;
Ramakrishna, B. L. Inorg. Chem. 1992, 31, 2843-2846.
(48) Shi, X.-H.; You, X.-Z.; Li, C.; Xiong, R.-G.; Yu, K.-B. Transition Met.
Chem. 1995, 20, 191-195.
The molecular structure of 2 (Figure 1, Table 2) is one of a
relatively small group of metal complexes with diazafluorene-
type ligands, which includes diazafluorenes,43-46 diazafluo-
(49) Lu, Z.-L.; Duan, C.-Y.; Tian, Y.-P.; You, X.-Z.; Fun, H.-K.; Yip, B.-C.;
Hovestreydt, E. Polyhedron 1996, 16, 187-194.
(50) Menon, S.; Rajasekharan, M. V. Polyhedron 1998, 17, 2463-2476.
(51) Yam, V. W.-W.; Wang, K.-Z.; Wang, C.-R.; Yang, Y.; Cheung, K.-K.
Organometallics 1998, 17, 2440-2446.
(43) Thummel, R. P.; Lefoulon, F.; Korp, J. D. Inorg. Chem. 1987, 26, 2370-
2376.
(44) Witte, P. T.; Klein, R.; Kooijman, H.; Spek, A. L.; Polasek, M.; Varga,
V.; Mach, K. J. Organomet. Chem. 1996, 519, 195-204.
(45) Henderson, L. J., Jr.; Fronczek, F. R.; Cherry, W. R. J. Am. Chem. Soc.
1984, 106, 5876-5879.
(52) Xiong, R.-G.; Zuo, J.-L.; Xu, E.-J.; You, X.-Z.; Huang, X.-Y. Acta
Crystallogr., Sect. C 1996, 52.
(53) Tulip, T. H.; Corfield, P. W. R.; Ibers, J. A. Acta Crystallogr., Sect. B
1978, 34, 1549-1555.
9
J. AM. CHEM. SOC. VOL. 124, NO. 2, 2002 275

A R T I C L E S
Kraft et al.
Table 3. Comparison of Bond Distances and Orbital Overlap
Populations for 2 and Cu(py)4(NO3)2 upon Rotation of the Cu(II)
Jahn-Teller Axis
2
Cu(py)4(NO3)2
overlap
populationa
overlap
populationa
bond/step
distance/Å
distance/Å
Cu-O/step 1
2.014
2.454
2.652
2.050
1.982
2.052
0.185
0.028
0.033
0.272
0.358
0.276
2.008
2.448
2.630
2.050
2.066
2.066
0.193
0.031
0.047
0.310
0.312
0.297
Cu-O/step 3
Cu-N (long)/step 1
Cu-N (long)/step 3
Cu-N (short)/step 1
Cu-N (short)/step 3
a Orbital population ) ∑2nCiCjSij represents a decimal percentage of
the total electron density that overlaps between all of the occupied atomic
orbitals on atom x with all of the atomic orbitals on atom y summed over
all occupied MOs.
Figure 2. EHMO description of the frontier molecular orbitals of 1 and 2.
Since degenerate orbitals differ only in phase, only one MO is illustrated
for each pair.
geometry (steps 1-3). The effect is due primarily to an increase
in the bonding orbital overlap populations along the three
molecular axes. This increase in orbital overlap for the O-
elongated geometry in Cu(py)4(NO3)2 is not observed for 2 and
is likely a reflection of the rigidity of the fluorene backbone,
which restricts the orientation of the coordinating lone pairs
and limits the ability of 1 to form strong bidentate σ-overlap.
These results suggest that the lower energy structure of 2 is a
result of a combination of effects: the slightly wider bite angle
of the diimine skeleton and the inability of the planar diaza-
fluorene backbone to reduce steric interactions with the adjacent
ligand.
Solution Structure and Photochemistry. Solution Struc-
ture. Equatorial coordination of the nitrate counterions in 2
raises the potential for solvent molecules to occupy these sites
in solution. Indeed, conductivity measurements confirm that
upon dissolution of 2 in water, the complex forms a 2:1
electrolyte, indicating that water molecules occupy the equatorial
coordination sites left vacant by the solvated counterions.27
The dissociation of the nitrate anions is further confirmed
by solvent-dependent EPR spectra which show drastic variation
when the glassing medium is altered. Two unique spectra are
obtained in aqueous NaCl and H2O:ethylene glycol (EG) media
(Figure 4).55 The EPR spectrum of 2 in aqueous NaCl (Figure
4a) indicates a 6-coordinate solution structure (dx -y ground
state) with rhombic symmetry (gx ) 2.10, gy ) 2.04, gz ) 2.23,
Az ) 177 × 10-4 cm-1), as would be expected for a CuN2X2N2′
system where X ) halogen.50 The g-values correlate well with
those reported from single-crystal EPR studies of Cu(dafone)2X2
(dafone ) 4,5-diazafluorene-9-one and X ) Cl-, Br-), where
the halide ions occupy the two equatorial coordination sites.50
Strong chloride binding is supported by the contrasting EPR
spectrum of 2 in a frozen solution of H2O:EG (1:20 v/v) which
yields a mixture of two copper centers, both exhibiting a more
axial symmetry (Figure 4b). The simulated spin-Hamiltonian
parameters for the first (2a) species (gxa ) 2.05, gya ) 2.06,
gza ) 2.29, Aza ) 170 × 10-4 cm-1) suggest that the structure
has near axial symmetry and N-rich coordination.56-59 Structure
Figure 3. Energy changes associated with alteration of the coordination
environment of 2 as derived from EHMO calculations.
To evaluate the unusual geometric structure of 2, the EHMO
calculations were performed at three different geometries for
both 2 and the model compound Cu(py)4(NO3)2 (Figure 3). For
these calculations, the J-T axis was reoriented from the
N-Cu-N vector to the O-Cu-O axis by rotating the chelating
ligand of 2 until symmetric coordination was achieved. For Cu-
(py)4(NO3)2, the geometry was altered by stepwise elongation
of the Cu-O bond lengths and compression of one of the Cu-N
vectors. For 2, the overall energy of the system decreases by
0.45 eV when the J-T axis is directed along the N-Cu-N
vector (Figure 3), even though the total overlap populations of
the metal-ligand bonding orbitals remain nearly constant (Table
3). These results suggest that the geometry is a product of
unfavorable steric interactions (i.e., between the hydrogen atoms
at the R-carbons of the diimine ligands) rather than more
favorable metal-ligand bonding interactions. This type of steric
interaction is also thought to be partly responsible for the
2
2
(55) Scans from 100 to 10 000 G have revealed no additional features, indicating
that EPR-active spin-coupled dimeric species are not present.
(56) Menon, S.; Rajasekharan, M. V. Inorg. Chem. 1997, 36, 4983-4987.
(57) Pradilla, S. J.; Chen, H. W.; Koknat, F. W.; Fackler, J. P., Jr. Inorg. Chem.
1979, 18, 3519-3522.
(58) Benites, P. J.; Rawat, D. S.; Zaleski, J. M. J. Am. Chem. Soc. 2000, 122,
7208-7217.
(59) Astley, T.; Ellis, P. J.; Freeman, H. C.; Hitchman, M. A.; Keene, F. R.;
Tiekink, E. R. T. J. Chem. Soc., Dalton Trans. 1995, 595-601.
2+
structural diversity observed for Cu(bpy)2 (bpy ) 2,2′-
bipyridine) complexes.54 For Cu(py)4(NO3)2, the overall energy
of the system is lowered considerably (-1.3 eV) upon transition
from the N-elongated structure to the symmetric O-elongated
(54) Hathaway, B. J.; Billing, D. E. Coord. Chem. ReV. 1970, 5, 143-207.
9
276 J. AM. CHEM. SOC. VOL. 124, NO. 2, 2002

Intramolecular Carbene Cation Radical Formation
A R T I C L E S
Figure 5. Electronic absorption spectra of 1 (MeOH) and 2 (H2O).
(λmax ) 480 nm) that is not evident in the spectra of Cu(dafone)2
derivatives (Figure 5).27 In the electronic spectrum of 2, the
n f π* transition from the diazo group remains energetically
unperturbed, but exhibits an increase in the molar absorptivity
due to the presence of two ligands per mole compound. The
Cu(II) d-d transitions are observed as a single broad feature
centered at 640 nm, consistent with a 6-coordinate mixed N,O
coordination environment.61 It is clear from this spectral analysis
that photolysis of both 1 and 2 with λ g 455 nm results in the
initial population of the ligand-based nπ* state. In the previous
report on the photonuclease activity of 2, we noted both the
increased activity of 2 relative to 1 and the evidence for the
formation of Cu(I) during anaerobic photolysis (λ g 455 nm)
of 2 in solution.27 These observations led us to mechanistically
evaluate the effects of Cu(II) upon the intermediates generated
by n f π* photolyses of 1 and 2.
Figure 4. Observed EPR spectra (s) of 2 recorded at 77 K with
corresponding simulations (- - -). (a) Aqueous NaCl (saturated) solution,
and (b) a 20:1 mixture of ethylene glycol:water. Inset: Second derivative
plot of g transition.
2b is also near axial (gxb ) 2.07, gyb ) 2.08, gzb ) 2.34, Azb
)
155 × 10-4 cm-1), but the increased gz and decreased Az values
suggest a structure with increased oxygen coordination.60
Despite the ∼1:1 mixture of 2a:2b, a clearly defined 9-line
superhyperfine splitting pattern of the g transition is also
observed (AN ) 15 G), indicating symmetric coordination of
both diazo-diazafluorene ligands in at least one of the two
structures (Figure 4, inset). On the basis of these results, structure
2a is best characterized as a CuN4O2 system where solvent (EG,
22 M) occupies the axial coordination positions, while 2b more
closely resembles that of [Cu(dafone)2(H2O)2](ClO4)2 with H2O
(2.6 M) in the equatorial positions.50 The structural assignment
for 2a is corroborated by the strong correlation of the spin-
Hamiltonian parameters with crystallographically characterized
Cu(py)4(O2CCF3)2, where the bulky triflate anion occupies the
axial positions.57 From the EMHO calculations, CuN4 coordina-
tion in the equatorial plane is not expected to be the lowest
energy structure for 2. However, the nature of these calculations
does not account for the lability of the nitrate counterions or
the presence of a bulky coordinating solvent, which may provide
sufficient driving force to induce a different solution structure.
Photochemistry. In the solid state, both 1 and 2 are
photochemically unstable and decompose slowly under ambient
light over the course of several days. Infrared measurements of
solid-state photolysis of 2 (fluorolube or Nujol mulls) show
complete disappearance of the diazo stretch in a period of 40
min (λ g 455 nm), suggesting that N2 loss photochemistry
associated with organic diazo analogues is conserved upon
complexation to the copper center.
Low-Temperature Photoreactivity. Matrix Photolysis of
1. Photolysis of 1 (λ g 455 nm) at 4 K in a 2-methyltetrahy-
drofuran (2-MeTHF) glass results in the formation of the triplet
carbene derived from N2 loss (Figure 6a). The zero field
parameters (ZFS) (|D/hc| ) 0.408 cm-1, |E/hc| ) 0.0292 cm-1
)
are within the typical range expected for diaryl carbenes.62
Interestingly, these values are nearly identical to those observed
for fluorenylidene (|D/hc| ) 0.408 cm-1, |E/hc| ) 0.0283
cm-1 63
) but are somewhat different than those reported for the
positional isomers 9-diazo-1,8-diazafluorene64 and 9-diazo-3,6-
diazafluorene65 (|D/hc| ) 0.442 cm-1, |E/hc| ) 0.029 cm-1 and
|D/hc| ) 0.438 cm-1, |E/hc| ) 0.0306 cm-1, respectively).
Comparison of the ZFS parameters supports Schuster’s assertion
that the primary mechanism of the electronic perturbation
introduced by the ring nitrogens is through bond rather than
through space.65
Photolysis of 1 under the same conditions as above results
in the growth of a series of electronic transitions in the visible
region of the spectrum (Figure 6b). Previous reports of the
electronic spectrum of fluorenylidene, both in a 2-MeTHF
matrix at 10 K and in an acetonitrile solution at room
(61) Lever, A. B. P.; Dodsworth, E. S. In Electrochemistry, Charge-Transfer
Spectroscopy, and Electronic Structure; Solomon, E. I., Lever, A. B. P.,
Eds.; John Wiley & Sons: New York, 1999; Vol. 2, pp 227-287.
(62) Sander, W.; Bucher, G.; Wierlacher, S. Chem. ReV. 1993, 93, 1583-1621.
(63) Wasserman, E.; Trozzolo, A. M.; Yager, W. A.; Murray, R. W. J. Chem.
Phys. 1964, 40, 2408-2410.
The electronic absorption spectrum of 1 in solution is
characteristic of an aromatic diazo compound with a weak
(ꢀ ) 41 M-1cm-1) n f π* transition of the diazo group
(60) Hathaway, B. J. In Copper; Wilkinson, G., Ed.; 1987; Vol. 5, pp 662-
(64) Li, Y. Z.; Schuster, G. B. J. Org. Chem. 1986, 51, 3804-3811.
(65) Li, Y. Z.; Schuster, G. B. J. Org. Chem. 1987, 52, 3975-3979.
674.
9
J. AM. CHEM. SOC. VOL. 124, NO. 2, 2002 277

A R T I C L E S
Kraft et al.
Figure 7. TDDFT description of the frontier molecular orbitals of 4,5-
diazafluorenylidene.
Figure 6. Spectral data acquired after the photolysis of 1 (λ g 455 nm) at
4 K in a 2-MeTHF glass. (a) EPR spectrum after 15 min of photolysis. (b)
Difference electronic absorption spectra obtained after 10, 15, 20, 25, and
45 min photolysis periods.
strength. The next lowest energy transition with appreciable
oscillator strength is out-of-plane, half-occupied π (HOMO) f
π* in nature (3A1 r 3B2, ∼425 nm) in which the electron density
at the carbene center is redistributed throughout the ring. Both
the calculated 425 and 601 nm transitions are chemically
reasonable and correspond well to those observed in Figure 6b.
No other spin- or symmetry-allowed transitions with appreciable
oscillator strengths were calculated to lie in this energy region.
The electronic structure calculation and corresponding normal
coordinate analysis provide an opportunity to examine the
molecular origin for the 1260 cm-1 vibration that couples to
temperature, cite absorption maxima at 440 and 470 nm with
the latter being the more intense transition.66-68 Our data closely
parallel these results with a few subtle differences. The first is
the appearance of a broad feature centered at 630 nm that has
not previously been reported. Additionally, there are three
prominent features (432, 457, and 485 nm) in the mid-visible
spectral region. These maxima are spaced at ∼1260 cm-1
,
3
3
suggesting vibrational coupling to a single electronic transition.
We have employed DFT and TDDFT methods to evaluate
the electronic structure and electronic transitions for 4,5-
diazafluorenylidene (Figure 6b) to confirm the identity of the
intermediate observed at 4 K (Figure 6b). The geometric
structure of 4,5-diazafluorenylidene was minimized in C2V
symmetry at the 6-31G** level using hybrid UB3LYP func-
tionals. The calculated electronic structure of 4,5-diazafluo-
renylidene reveals a ground-state triplet with the two unpaired
electrons in half-occupied, σ (in-plane) and π (out-of-plane)
orbitals that are delocalized about the ring system. At the
optimized geometry, TDDFT was used to calculate the five
lowest energy spin-allowed electronic transitions for comparison
to the experimental data. The lowest energy transition (3B2 r
3B2, ∼601 nm) is best described as delocalized π f π* (LUMO)
in origin with considerable electron density in the out-of-plane
orbital at the 9-position (carbene) of the 4,5-diazafluorenylidene
framework (Figure 7). Although spin-allowed, the transition is
symmetry forbidden and therefore possesses low oscillator
the A1 r B2 transition centered near 450 nm. The normal
coordinate analysis reveals six vibrations between 1260 and
1360 cm-1 that involve in-plane C-C breathing motions of the
central ring in combination with C-H out-of-plane bending
motions. By imposing the constraint that the electronic transition
should be y-polarized in accordance with the symmetry selection
rules in C2V, only vibrations of a1 symmetry would yield an
ex
ex
2
allowed vibronic progression (|∫ΨelΨvibMˆ xyzΨel
Ψ
vib
dτ| ).
Since three vibrations of a1 symmetry remain within this energy
region and each involves in-plane C-C displacements of the
central ring system, assignment of the progression to a particular
normal mode cannot be made specifically, rather only in general.
Matrix Photolysis of 2. Photolysis of 2 (λ g 455 nm or λ )
457 nm) at 4 K in a 1:20 H2O:EG glass results in the growth of
an isotropic radical signal concurrent with the loss of both of
the Cu(II) EPR signals from 2a and 2b (Figure 8a). This
spectrum is stable for hours at temperatures as high as 100 K
and is in contrast to the carbene detected in the photolysis of 1
at 4 K (Figure 4a).69 Three possible spin systems are plausible
for a three-electron system. The first involves ferromagnetic
(66) Griller, D.; Hadel, L.; Nazran, A. S.; Platz, M. S.; Wong, P. C.; Scaiano,
J. C. J. Am. Chem. Soc. 1984, 106, 2227-2235.
(67) Griller, D.; Montgomery, C. R.; Scaiano, J. C. J. Am. Chem. Soc. 1982,
104, 6813-6814.
(69) It is possible that a carbene is formed and subsequently reacts; however,
triplet fluorenylidene should, according to published Arrhenius parameters,
be stable in alcohol and glycol matrixes at 4 K. See: Platz, et al. J. Am.
Chem. Soc. 1992, 114, 4(3), 897-905.
(68) Grasse, P. B.; Brauer, B. E.; Zupancic, J. J.; Kaufmann, K. J.; Schuster, G.
B. J. Am. Chem. Soc. 1983, 105, 6833-6845.
9
278 J. AM. CHEM. SOC. VOL. 124, NO. 2, 2002

Intramolecular Carbene Cation Radical Formation
A R T I C L E S
Scheme 2. Potential Photochemical Pathways to Radical Cation 3
radical pair derived from the reaction with matrix is not stable
at T < 130 K.75 The absence of observable hyperfine coupling
on the radical signal at g ) 2.0019 infers a structure with the
radical primarily localized on the carbon at the 9-position of
the fluorene ring.17-20,76,77 The absence of hyperfine coupling
to N2, which is commonly observed in one-electron oxidized
diaryl diazo compounds,17-20,76,77 confirms photoinduced N2 loss
along with reduction of the Cu(II) center and concomitant
appearance of the monoradical signal. Therefore, the spectrum
in Figure 8a is best described as a Cu(I)-L•+ intermediate (3)
derived from the photoinduced oxidation of the diazo unit and
Figure 8. Spectral data following the photolysis of 2 (λ ) 457 nm) at 77
K in a 20:1 ethylene glycol:water glass. (a) EPR spectra after 0, 5, 10, 15,
and 20 min. (b) Difference electronic absorption spectra obtained after 3,
6, 9, and 12 min.
1
subsequent N2 loss to yield an overall S ) /2 spin system
(Scheme 2).
coupling of the ligand diradical to the spin centered on the
Cu(II). This case would yield an S ) /2 spin system (v‚‚‚‚VV)
This interpretation of the EPR results is further supported by
the changes in the electronic absorption spectrum upon pho-
tolysis of 2 (λ g 455 nm) at 4 K in a 1:20 H2O:EG glass.
Difference electronic absorption spectra reveal the growth of a
strong feature at 410 nm, with a shoulder to lower energy
(Figure 8b). This absorption spectrum is nearly identical to those
previously assigned to the carbene cation radicals of diaryl diazo
derivatives formed upon oxidation and subsequent N2 loss.18-20,77
This spectrum is also different than that expected for a simple
9-fluorenyl radical, which exhibits an absorption maximum at
500 nm.68,78 Together the EPR and electronic spectra identify
the photogenerated intermediate at 4 K as a Cu(I)-L•+ species
resulting from intramolecular electron transfer and N2 loss.
3
with an EPR transition centered near g ≈ 4, which is not
observed. The second case can be described as a triplet ligand
diradical antiferromagnetically coupled to Cu(II) (V‚‚‚‚vv), which
should yield an EPR signal with g > g|.70 Last, a Cu(II) center
that interacts with a singlet ligand diradical (v‚‚‚‚vV) would result
in an unperturbed Cu(II) signal (g| > g ).70 Neither of these
latter two cases could be responsible for the observed spectrum,
as they both retain characteristics of a Cu(II) EPR signal. Finally,
a stepwise insertion reaction of the triplet carbene71 with matrix
to yield an uncoupled ligand-matrix radical pair, where the
ligand spin is antiferromagnetically coupled to the Cu(II) center,
could explain the observed EPR spectrum. This intermediate,
however, is unlikely for a number of reasons. First, photolysis
of 2 at 77 K yields the spectrum shown in Figure 8a. After
warming the sample to room temperature anaerobically and
subsequently recording the spectrum at 77 K, the organic radical
vanishes, but the diminished Cu(II) signal remains unperturbed.
Exposure of this sample to O2 results in restoration of the Cu-
(II) signal intensity, thereby documenting the formation of Cu-
(I). Second, photolysis of 1 at 77 K in a 2:1 (v/v) 2-MeTHF:
EG glass does not yield a g ) 2 radical species that can be
detected under our experimental conditions. This observation
is consistent with the previously described reactivity of fluo-
renylidene at low temperatures,72-75 which has shown that the
Discussion
The solid-state structure of 2 is unusual in that the nitrate
counterions, which may be expected to be weakly bound in a
bis(chelate) stoichiometry (i.e., along the J-T axis), provide
stronger equatorial ligation than the diimine ligand. Generation
of a 2:1 electrolyte upon dissolution of 2 reveals the ability to
generate open, and potentially strong, coordination positions for
chemical or biological solution substrates such as DNA.27
(73) Wright, B. B.; Platz, M. S. J. Am. Chem. Soc. 1984, 106, 4175-4180.
(74) Leyva, E.; Barcus, R. L.; Platz, M. S. J. Am. Chem. Soc. 1986, 108, 7786-
7788.
(75) Ruzicka, J.; Leyva, E.; Platz, M. S. J. Am. Chem. Soc. 1992, 114, 897-
905.
(70) Chaudhuri, P.; Verani, C. N.; Bill, E.; Bothe, E.; Weyhermueller, T.;
Wieghardt, K. J. Am. Chem. Soc. 2001, 123, 2213-2223.
(76) Ishiguro, K.; Sawaki, Y.; Izuoka, A.; Sugawara, T.; Iwamura, H. J. Am.
Chem. Soc. 1987, 109, 2530-2531.
(77) Kato, N.; Miyazaki, T.; Fueki, K.; Kobayashi, N.; Ishiguro, K.; Sawaki,
Y. J. Chem. Soc., Perkin Trans. 2 1987, 881-884.
(78) Alberti, A.; Griller, D.; Nazran, A. S.; Pedulli, G. F. J. Am. Chem. Soc.
1986, 108, 3024-3028.
(71) Platz, M. S. In The Chemistry, Kinetics, and Mechanisms of Triplet Carbene
Processes in Low-Temperature Glasses and Solids; Platz, M. S., Ed.;
Plenum: New York, 1990; pp 143-211.
(72) Moss, R. A.; Joyce, M. A. J. Am. Chem. Soc. 1978, 100, 4475-4480.
9
J. AM. CHEM. SOC. VOL. 124, NO. 2, 2002 279

A R T I C L E S
Kraft et al.
Moreover, the fluxionalities of these coordination sites and the
ability of the diazafluorene skeleton to adopt more than one
geometric disposition about the metal give complexes of this
type significant structural flexibility.
metal-ligand radical intermediates generated during turnover
of metalloenzymes such as galactose oxidase. First, the nature
of the intermediate itself is chemically unusual relative to one-
electron, ground-state oxidation pathways, which typically lead
to generation of a Cu(II) center prior to ligand radical formation.
This is due to the disparate ease in oxidizing Cu(I) relative to
stable organic ligands. Second and within this theme, Cu(II)-
phenoxyl models of the active form of galactose oxidase have
been generated electrochemically and show that the ligand
localized π-monoradical is antiferromagnetically coupled to the
From the electronic spectra of 1 and 2 (Figure 5) and the
frontier molecular orbital description (Figure 2), photoexcitation
of 1 and 2 with λ g 455 nm initially populates a ligand localized,
diazo-centered excited state that has propensity for N2 loss due
to the NdN nonbonding and CdN bonding to NdN antibonding
and CdN nonbonding nature of the transition. In the absence
of metal, N2 loss and subsequent formation of a triplet carbene
intermediate are observed. However, in the presence of Cu(II),
an excited state intramolecular redox reaction occurs, which
lone electron in the dx -y orbital to yield a net S ) 0 spin.34
Analogously, the active form of the enzyme is prepared by two
sequential one-electron oxidations of the Cu(I)-tyrosine resting
state by O2 to yield a Cu(II)-tyrosyl radical, where the first
step is oxidation of the Cu(I) center. The negative Cu(II)/Cu(I)
redox potential (∼-0.3 to -0.5 V vs NHE),33 which derives
partially from the distorted square planar geometry that favors
the Cu(II) oxidation state, is therefore responsible for metal
(Cu(II)-tyrosine) and not ligand (Cu(I)-tyrosine•) oxidation
as the first step in the mechanism.
2
2
1
leads directly to a S ) /2 Cu(I)-L•+ (vV‚‚‚‚v) species with-
•+
out detection of the Cu(I)-LdN2 diazo cation radical (4)
(λmax ≈ 680 nm).18-20,76 This reaction pathway is the most viable
as direct photoinduced N2 loss would produce a carbene (5)
that energetically cannot be oxidized by the modest redox
potential of Cu(II).79 Moreover, fluorescence quantum yield
measurements illustrate that Cu(II) significantly quenches the
photochemically prepared excited state (φf1 ) 0.230 vs φf2
)
Conclusions
0.021) by either energy transfer to ligand field states or direct
electron transfer via oxidation of the diazo unit. The viability
of the electron-transfer pathway is clear if the energy of the
charge-separated state is estimated by the ∆E (redox) ap-
Our results show that the unusual structure of 2 in the solid
state derives from the enlarged bite angle of the fluorene
skeleton and steric interactions between the adjacent hydrogen
atoms in the symmetrically coordinated state. This is in contrast
to Cu(py)4(NO3)2 which possesses a more stable structure with
the nitrate counterions bound along the Jahn-Teller axis.
Despite the strong coordination of the counterions, solvent-
dependent EPR studies confirm that the nitrates dissociate to
yield 6-coordinate CuN2X2N2′ structures with either a bound
chloride ion or a mixture of counterion and solvent. Photolyses
of 1 and 2 result in N2 loss and formation of either carbene or
Cu(I)-L•+ intermediates, respectively. The metal-ligand radical
intermediate is chemically unusual relative to one-electron
ground-state oxidation pathways which typically lead to genera-
tion of a Cu(II) center prior to ligand radical cation formation.
Finally, the excited-state redox reaction which leads to the
Cu(I)-L•+ intermediate is different that that observed upon
photolysis of the analogous [Cu(diazodi-4-pyridylmethane)-
(hfac)2]n system, thereby illustrating the important role redox
active transition metals play in determining the nature of
fundamental metal-ligand radical intermediates.
proximation.61,80 The positive Cu2+/Cu+ redox couple (E1/2
)
+0.1 V vs SCE) and the modest 1/1•+ ligand redox potential
(E1/2 ) +1.4 V vs SCE, irreversible) places the charge-separated
state at ∆E ≈ 1.3 V above the ground state. Photoexcitation
with λ ) 457 nm provides ∼2.7 V of excited-state potential
which is sufficient to drive an intramolecular redox reaction.
1
Formation of the S ) /2 Cu(I)-L•+ (vV‚‚‚‚v) intermediate
contrasts the work of Iwamura et al. which demonstrates that
the ligand diradical prepared upon photoinduced (λ ) 532 nm,
2.33 eV) loss of N2 from an analogous [Cu(diazodi-4-
pyridylmethane)(hfac)2]n construct ferromagnetically couples to
3
the Cu(II) spin to yield a local S ) /2 spin (v‚‚‚‚vv) system.22
These spin centers interact ferromagnetically along the poly-
meric chain to produce a transient magnetic material with a net
S ) 33.6 spin state over the sample. Interestingly, no photo-
induced redox process occurs to yield the isoelectronic S )
1
/
Cu(I)-L•+ (vV‚‚‚‚v) intermediate. The difference in the
2
photogenerated intermediates between 2 and the [Cu(diazodi-
4-pyridylmethane)(hfac)2]n system can only be rationalized by
assuming that the Cu(I)-L•+ (vV‚‚‚‚v) state energy lies above
the 532 nm photoexcitation energy. Given the 6-coordinate,
octahedral geometry about the Cu(II) center in [Cu(diazodi-4-
pyridylmethane)(hfac)2]n and the fact that the charge on the
metal is neutralized by the hfac counterions, photoinduced
formation of Cu(I) in the solid state (i.e., absence of solvation)
may indeed require excitation energy that exceeds 2.33 eV.
The formation of intermediate 3 (S ) 1/2 Cu(I)-L•+, vV‚‚‚‚v)
also has general implications to the identities of redox active
Acknowledgment. We thank Aurora Clark and Prof. Joe
Gajewski for helpful discussions. The generous support of the
American Cancer Society (RPG-99-156-01-C), the donors of
the Petroleum Research Fund (PRF #33340-G4), administered
by the American Chemical Society, and the Research Corpora-
tion (Research Innovation Award #RI0102 for J.M.Z.) are
gratefully acknowledged.
Supporting Information Available: Atomic coordinates, bond
lengths, and bond angles (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
(79) Muchall, H. M.; Werstiuk, N. H.; Choudhury, B. Can. J. Chem. 1998, 76,
221-227.
(80) Vlcek, A. A. E. A. Electrochim. Acta 1968, 13, 1063-1078.
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280 J. AM. CHEM. SOC. VOL. 124, NO. 2, 2002