Aminotroponiminates as ligands for potential metal-based nitric oxide sensors

Article abstract of DOI:10.1021/ic000344q

A family of new fluorescently labeled ligands, H(R)DATI, was prepared to develop transition-metal-based NO sensing strategies. The ligands are composed of aminotroponiminates (ATIs) with a dansyl fluorophore on one of the imine nitrogen atoms and an alkyl substituent, either i-Pr (8), t-Bu (9), or Bz (10), on the other. Bis(chelate) Co²⁺ ([Co((i-Pr)DATI)₂] (12), [Co((t-Bu)DATI)₂] (14), [Co((Bz)DATI)₂] (15)) and Zn²⁺ ([Zn((i-Pr)DATI)₂] (13)) complexes were prepared and characterized by X-ray crystallography. The bis(ATI) complex [Co(i-Pr₂ATI)₂] (11) was also prepared and its X-ray crystal structure determined. Cyclic voltammetry reveals reversible redox waves at -2.57 and -0.045 V (vs Cp₂Fe/Cp₂Fe⁺) in THF for the Co²⁺/Co⁺ and Co³⁺/Co²⁺ couples, respectively, of 11. Only a Co²⁺/Co⁺ wave at -2.09 V is observed for 12. When excited at 350 nm, the H(R)DATI ligands and the diamagnetic Zn²⁺ complex 13 fluoresce around 500 nm, whereas the paramagnetic Co²⁺ complexes quench the fluorescence. These air-stable cobalt compounds react with nitric oxide to dissociate a DATI ligand and form neutral dinitrosyl complexes, [Co(NO)₂((R)DATI)]. The release of the fluorophore-containing ligand is accompanied by an increase in fluorescence intensity, thus providing a strategy for fluorescent NO sensing. Linking two DATI moieties via a tetramethylene chain affords the ligand H₂DATI-4 (18). The Co²⁺ complex [Co(DATI-4)] (19) reacts more readily with NO than the bis(DATI) compounds and also displays an increase in fluorescence intensity upon NO binding.

Full text of DOI:10.1021/ic000344q

Inorg. Chem. 2000, 39, 4081-4092
4081
Aminotroponiminates as Ligands for Potential Metal-Based Nitric Oxide Sensors
Katherine J. Franz, Nisha Singh, Bernhard Spingler, and Stephen J. Lippard*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
ReceiVed March 30, 2000
A family of new fluorescently labeled ligands, HRDATI, was prepared to develop transition-metal-based NO
sensing strategies. The ligands are composed of aminotroponiminates (ATIs) with a dansyl fluorophore on one of
the imine nitrogen atoms and an alkyl substituent, either i-Pr (8), t-Bu (9), or Bz (10), on the other. Bis(chelate)
Co2+ ([Co(i-PrDATI)2] (12), [Co(t-BuDATI)2] (14), [Co(BzDATI)2] (15)) and Zn2+ ([Zn(i-PrDATI)2] (13)) complexes
were prepared and characterized by X-ray crystallography. The bis(ATI) complex [Co(i-Pr2ATI)2] (11) was also
prepared and its X-ray crystal structure determined. Cyclic voltammetry reveals reversible redox waves at -2.57
and -0.045 V (vs Cp2Fe/Cp2Fe+) in THF for the Co2+/Co+ and Co3+/Co2+ couples, respectively, of 11. Only a
Co2+/Co+ wave at -2.09 V is observed for 12. When excited at 350 nm, the HRDATI ligands and the diamagnetic
Zn2+ complex 13 fluoresce around 500 nm, whereas the paramagnetic Co2+ complexes quench the fluorescence.
These air-stable cobalt compounds react with nitric oxide to dissociate a DATI ligand and form neutral dinitrosyl
complexes, [Co(NO)2(RDATI)]. The release of the fluorophore-containing ligand is accompanied by an increase
in fluorescence intensity, thus providing a strategy for fluorescent NO sensing. Linking two DATI moieties via
a tetramethylene chain affords the ligand H2DATI-4 (18). The Co2+ complex [Co(DATI-4)] (19) reacts more
readily with NO than the bis(DATI) compounds and also displays an increase in fluorescence intensity upon NO
binding.
Introduction
to a volume containing approximately 2 million synapses.9 The
development of NO sensors that could monitor the passage of
Since the discovery in the 1980s that nitric oxide (NO) is
the endothelium-derived relaxing factor (EDRF),1-3 postulated
biological roles for NO have continued to proliferate. For
example, in addition to cardiovascular signaling, NO also seems
to function as a neurotransmitter that may be important in
memory and as a weapon to fight infection when released by
immune system macrophages.4 Uncovering these roles and
deciphering their implications are complicated by the array of
reactions that this gaseous molecule undergoes. In a biological
environment, NO can react with a host of targets, including
dioxygen, oxygen radicals, thiols, amines, and transition metal
ions.4 Some of the products formed, such as NO2 and NO+, are
pathophysiological agents, whereas others, such as S-nitroso
thiols, may in fact themselves be NO-transfer agents.5 Transition
metal centers, especially iron in oxyhemoglobin,6 can rapidly
scavenge free NO, thereby altering the amount available for
signaling purposes.
NO through these synapses and around various scavengers is
crucial for understanding NO neurochemistry.
A variety of analytical methods are available to monitor
aspects of NO in biology, each having its merits and limita-
tions.10,11 The Griess assay, for instance, is useful for estimating
total NO production, but it is not very sensitive, cannot give
real-time information, and only measures the stable oxidation
product nitrite.12 Although more sensitive and selective for NO,
the chemiluminescent gas-phase reaction of NO with ozone
requires purging aqueous samples with an inert gas to strip NO
into an analyzer.13 It too is therefore incapable of monitoring
intracellular NO. Electrochemical sensing using microsensors
provides in situ real-time detection of NO;14-16 the only spatial
information obtained, however, is directly at the electrode tip
and is therefore influenced by the placement of the probe.
These methods and others10 continue to be invaluable for
studying NO-related biochemistry; however, to achieve more
detail, new sensors with improved properties are needed. In
The concentration-dependent lifetime of NO and its ability
to diffuse freely through cellular membranes further complicate
the delineation of these various processes. With a lifetime of
up to 10 min under some conditions7 and a diffusion range of
100-200 µm for NO,8 its biological action can be distant from
its point of origin. A diffusional spread of 200 µm corresponds
(9) Wood, J.; Garthwaite, J. Neuropharmacology 1994, 33, 1235-1244.
(10) Feelisch, M.; Stamler, J. S. In Methods in Nitric Oxide Research;
Feelisch, M., Stamler, J. S., Eds.; John Wiley & Sons: New York,
1996; pp 303-307.
(11) Methods in Enzymology; Packer, L., Ed.; Academic Press: Boston,
MA, 1996; Vol. 268, pp 58-258.
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10.1021/ic000344q CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/11/2000

4082 Inorganic Chemistry, Vol. 39, No. 18, 2000
Franz et al.
particular, the challenge calls for noninvasive techniques that
provide both temporal and spatial data and remain unbiased by
the probe itself.
Scheme 1
Fluorescence methods are ideally suited to meet these
requirements,17,18 as has been amply demonstrated for sensing
intracellular Ca2+ 19,20
Fluorescent NO sensors include DAF
.
(diaminofluorescein)21-23 and DAN (2,3-diaminonaphthalene),24
the aromatic vicinal diamines of which react with nitrosating
agents (NO+ or NO2) to afford fluorescent triazole compounds.
DAF compounds can report intracellular NO,22,23 but their
sensing ability relies on NO autoxidation products and not direct
detection. A rhodamine-type fluorescent NO indicator similarly
senses autoxidation products.25
Fluorescent nitric oxide cheletropic traps (FNOCTs) are
fluorescent versions of molecules that have been used as EPR
spin probes and do react directly with NO.26,27 The initially
formed nitroxide radical species formed are not fluorescent,
however. Addition of a common biological reductant such as
ascorbic acid is required to reduce the nitroxide and display
increased fluorescence intensity.25,27
The heme domain in soluble guanylate cyclase (sGC) is an
NO receptor for signal transduction and therefore an attractive
candidate for biomimetic NO sensors. One approach is to outfit
fiber optic probes with fluorescent-dye-labeled heme domains
of cytochrome c′ 28,29 or sGC.30 NO reacts with heme domains
to change the fluorescence response of the reporter dye, which
is ratiometrically measured against standards and gives a linear
response selective for NO.28-30 A quinoline-pendant cyclam has
been designed as a fluorescent model of sGC, the quinoline
mimicking the distal histidine of sGC and the cyclam represent-
ing the porphyrin.31 The Fe2+ center shifts the fluorescence
emission of the pendant quinoline, and addition of NO alters
the quinoline-Fe interaction, resulting in a decrease in fluo-
rescence intensity.31
exploits the fluorescence-quenching properties of metal ions with
partly filled d shells.33 In terms of a molecular switch, the
paramagnetic metal complexes are therefore in the “off”
position. Subsequent addition of the desired analyte displaces
a fluorescent ligand from the quenching metal center, thereby
turning “on” fluorescence. A similar ligand-dissociation strategy
has been employed to monitor pH changes by fluorescence. In
this case, a Ni2+ cyclam is appended with an alkylamine side
chain that is, in turn, linked to an anthracene fragment.34
Changes in pH affect the binding affinity of the anthracene-
tagged amine for the Ni2+ center, thereby altering the fluores-
cence intensity.34
Here we report the syntheses and characterizations of fluo-
rescently labeled aminotroponiminate ligands, HRDATI and H2-
DATI-4, together with their Zn2+ and Co2+ complexes. As
expected, the Zn2+ complexes are fluorescent, whereas the Co2+
compounds display only residual emission intensity. The cobalt
complexes report the presence of NO by releasing one of their
fluorescent moieties upon formation of neutral dinitrosyl species.
We are interested in harnessing the properties of transition
metal-nitrosyl adducts to trigger a positiVe change in fluores-
cence. Our strategy, recently described in preliminary form,32
(17) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd ed.;
Kluwer Academic/Plenum Publishers: Boston, MA, 1999.
(18) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A.
J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997,
97, 1515-1566.
(19) Minta, A.; Kao, J. P. Y.; Tsien, R. Y. J. Biol. Chem. 1989, 264, 8171-
8178.
Results and Discussion
(20) Tsien, R. Y. Biochemistry 1980, 19, 2396-2404.
(21) Kojima, H.; Sakurai, K.; Kikuchi, K.; Kawahara, S.; Kirino, Y.;
Nagoshi, H.; Hirata, Y.; Nagano, T. Chem. Pharm. Bull. 1998, 46,
373-375.
(22) Kojima, H.; Nakatsubo, N.; Kikuchi, K.; Kawahara, S.; Kirino, Y.;
Nagoshi, H.; Hirata, Y.; Nagano, T. Anal. Chem. 1998, 70, 2446-
2453.
Syntheses. Scheme 1 outlines the four-step synthetic route
that provides gram quantities of HRDATI ligands in 20-50%
yields. These ligands are derived from the N,N′-disubstituted
aminotroponimine (HR2ATI) class of ligands and the dansyl
(5-(dimethylamino)naphthalenesulfonamide) fluorophore, widely
used for sensor design and labeling of peptides and amino
acids.17 A variety of alkyl substituents can be used to derivatize
the ligands; here we chose isopropyl, tert-butyl, and benzyl. A
methyl-substituted analogue, HMeDATI, was also prepared, but
the yield was considerably lower and its metal complexes were
not as crystalline as others in the class.
Preparation of the 4-methoxybenzyl (PMB)-protected 1 allows
for the asymmetric derivatization of ATI bearing a deactivating
tosyl group on one nitrogen atom. Attempts to synthesize
N-alkyl-2-amino intermediates such as 5-7 directly from
2-aminotropolone, which can easily be prepared from 2-(tosyl-
(23) Kojima, H.; Urano, Y.; Kikuchi, K.; Higuchi, T.; Hirata, Y.; Nagano,
T. Angew. Chem., Int. Ed. 1999, 38, 3209-3212.
(24) Miles, A. M.; Wink, D. A.; Cook, J. C.; Grisham, M. B. In Methods
in Enzymology; Packer, L., Ed.; Academic Press: Boston, MA, 1996;
Vol. 268, pp 105-121 and references therein.
(25) Rieth, R.; Sasamoto, K. Anal. Commun. 1998, 35, 195-197.
(26) Ba¨tz, M.; Korth, H.-G.; Sustmann, R. Angew. Chem., Int. Ed. Engl.
1997, 36, 1501-1503.
(27) Meineke, P.; Rauen, U.; de Groot, H.; Korth, H.-G.; Sustmann, R.
Chem.sEur. J. 1999, 5, 1738-1747.
(28) Barker, S. L. R.; Kopelman, R.; Meyer, T. E.; Cusanovich, M. A.
Anal. Chem. 1998, 70, 971-976.
(29) Barker, S. L. R.; Clark, H. A.; Swallen, S. F.; Kopelman, R.; Tsang,
A. W.; Swanson, J. A. Anal. Chem. 1999, 71, 1767-1772.
(30) Barker, S. L. R.; Zhao, Y.; Marletta, M. A.; Kopelman, R. Anal. Chem.
1999, 71, 2071-2075.
(31) Katayama, Y.; Takahashi, S.; Maeda, M. Anal. Chim. Acta 1998, 365,
159-167.
(32) Franz, K. J.; Singh, N.; Lippard, S. J. Angew Chem., Int. Ed. 2000,
39, 2120-2122.
(33) Bergonzi, R.; Fabbrizzi, L.; Licchelli, M.; Mangano, C. Coord. Chem.
ReV. 1998, 170, 31-46.
(34) Fabbrizzi, L.; Licchelli, M.; Pallavicini, P.; Parodi, L. Angew. Chem.,
Int. Ed. 1998, 37, 800-802.

Metal-Based NO Sensors
Inorganic Chemistry, Vol. 39, No. 18, 2000 4083
oxy)tropolone and ammonia, resulted in mixtures of products
that included significant amounts of the undesired N,N′-dialkyl
ATI ligands. Protecting one nitrogen with PMB provides a
convenient solution to the problem. Activation of the tropolone
oxygen of 1 with Me3OBF4 followed by reaction with the
appropriate alkylamine easily and cleanly affords the N-PMB-
2-alkyl intermediates 2-4. Cleavage of the methoxybenzyl
protecting group with trifluoroacetic acid (TFA) supplies the
desired free amino functionality in 5-7. Deprotonation with
sodium hydride followed by reaction with dansyl chloride (DNS-
Cl) furnishes the final ligands 8-10 as yellow-orange powders
after purification by flash chromatography.
Moderate yields of the recrystallized metal complexes 11-
15 are obtained by deprotonating 2 equiv of the ligand with
potassium or sodium hydride in THF, followed by addition of
anhydrous CoCl2 or ZnCl2. A report of [Co(R2ATI)2] com-
pounds with ethyl and aryl substituents claims that the com-
plexes are sensitive to dioxygen and react slowly with some
chlorinated solvents.35 The isopropyl derivative 11 is not oxygen
sensitive, nor does it appear to react with chlorinated solvents,
although these were limited in use as a precaution. Even though
all of the complexes are air stable, they are susceptible to slow
hydrolysis by trace moisture, so they were synthesized under a
dry nitrogen atmosphere. The complexes are readily soluble in
organic solvents such as THF, CH2Cl2, and benzene. The DATI
complexes are insoluble in diethyl ether and pentane, but [Co-
(i-Pr2ATI)2] (11) is very soluble in diethyl ether.
Figure S1 (Supporting Information). The N-S distance of 1.603-
(1) Å is slightly shorter than the average N-S distance of 1.623-
(3) Å in the Co complex 14. Otherwise, the bond distances and
angles are unremarkable and consistent with those in similar
sulfonamide ligands.37
The synthesis and characterization of [Co(i-Pr2ATI)2] (11)
permit a comparison between the geometric features of the
parent ATI ligand environment and those of the new sulfona-
mide-appended DATI ligands. The Co2+ center in 11 has nearly
perfect tetrahedral angles, a common geometry for Co2+ 38
.
Monoanionic, bidentate ligands often form Co3+ octahedral tris-
(chelate) complexes due to favorable ligand field stabilization.38
The less bulky methyl-substituted HMe2ATI ligand prefers such
a [Co(ATI)3] complex, whereas the bulkier diethyl, dibenzyl,
and di-p-tolyl ATI ligands favor bis(chelate) Co2+ species.35
The syntheses of several [Co(R2ATI)2] complexes were reported
previously,35 but not of the isopropyl derivative discussed here.
Moreover, no crystal structures of these compounds are avail-
able.
The dihedral angle Θ, measured between the planes of the
two five-membered chelate rings, is 89.5° in 11, nearly that of
an ideal tetrahedron. The Θ angle measures the degree of twist
in related tropocoronand (H2TC-n,m) complexes, where two ATI
rings are tethered by methylene linker chains of varying lengths
(n,m).39 The most nearly tetrahedral Θ value, 84.5°, in the series
occurs for [Co(TC-6,6)].39 The average Co-N bond distance
of 1.980(3) Å in 11 is quite similar to the value of 1.971(4) Å
recorded for [Co(TC-6,6)].39
The synthesis of the linked-DATI ligand 18 follows a route
similar to that described above for the HRDATI ligands. The
The electron-withdrawing sulfonamide moiety in the DATI
ligands slightly affects the Co-N bond lengths by increasing
the average distance to 1.999(3) Å in 12, with similar values
for the other derivatives, as shown in Table 2. The coordination
chemistry of complexes with sulfonamides is not well devel-
oped, but there are some examples of both octahedral40-42 and
tetrahedral37,43 Co complexes having one or more such ligands.
The average Co-N distance revealed in this study is similar to
the value of 2.00 Å reported for pseudotetrahedral Co2+
complexes of bidentate pyridyl/sulfonamide type ligands.37,43
A Θ angle of 85° was reported for one of these complexes,37
which is larger than those in 12, 14, and 15, where Θ ) 76.1,
81.4, and 73.8°, respectively. The variations in these values
reflect the steric requirements of the different R substituents,
isopropyl, tert-butyl, and benzyl, respectively. The dansyl groups
in 15 are parallel and planar, with an average distance between
the ring planes of 3.5(1) Å, a value in the range for π-π
stacking interactions.44 The dansyl groups are not similarly
aligned in the other bis(chelate) structures.
first step involves linking two ATI rings with a polymethylene
chain, in a manner analogous to the preparation of the
tropocoronand ligands.36 Activation of “4-dimer-tropolone” with
Me3OBF4 followed by addition of (4-methoxybenzyl)amine
afforded 16 in high yield. Cleaving the 4-methoxybenzyl
protecting groups with TFA provided the reactive amine
functionalities in 17. Slow addition of the deprotonated 17 to a
solution of excess dansyl chloride at 4 °C gave a modest yield
of 18 and suppressed formation of monodansylated product. A
slurry of this mustard yellow ligand in THF/CH3CN became a
clear, orange solution after double deprotonation with KH. The
use of [Co(CH3CN)4](PF6)2 as the Co source provided the
desired monometallic species [Co(DATI-4)] (19).
Structural Studies. The molecular structures of 11, 12, 14,
and 15 are displayed in Figure 1 as ORTEP diagrams. Single-
crystal X-ray diffraction results are provided in Table 1, and
selected bond distances and angles are contained in Table 2. A
study was also undertaken of one of the free ligands, the tert-
butyl derivative 9, an ORTEP diagram of which appears in
The coordination environment of Zn2+ in 13 is also distorted
tetrahedral, as shown in Figure S4 (Supporting Information),
with Θ ) 81.5°. As was found in the Co compounds, the
sulfonamide groups lengthen the average Zn-N bond length
to 2.01(1) Å, compared with 1.981(3) Å for pseudotretrahedral
(37) Otter, C. A.; Couchman, S. M.; Jeffery, J. C.; Mann, K. L. V.; Psillakis,
E.; Ward, M. D. Inorg. Chim. Acta 1998, 278, 178-184.
(38) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry, 5th ed.;
Wiley-Interscience: New York, 1988; pp 724-741.
(39) Jaynes, B. S.; Doerrer, L. H.; Liu, S.; Lippard, S. J. Inorg. Chem.
1995, 34, 5735-5744.
(40) Borsari, M.; Menabue, L.; Saladini, M. J. Chem. Soc., Dalton Trans
1996, 4201-4205.
(41) Haider, S. Z.; Malik, K. M. A.; Ahmed, K. J.; Hess, H.; Riffel, H.;
Hursthouse, M. B. Inorg. Chim. Acta 1983, 72, 21-27.
(42) Castresana, J. M.; Elizalde, M. P.; Arrieta, J. M.; Germain, G.; Declerq,
J.-P. Acta Crystallogr., Sect. C 1984, C40, 763-765.
(43) Uhlig, V. E.; Do¨ring, M. Z. Anorg. Allg. Chem. 1982, 492, 52-62.
(44) Liu, Z.-H.; Duan, C.-Y.; Hu, J.; You, X.-Z. Inorg. Chem. 1999, 38,
1719-1724.
(35) Eaton, D. R.; McClellan, W. R.; Weiher, J. F. Inorg. Chem. 1968, 7,
2040-2046.
(36) Zask, A.; Gonnella, N.; Nakanishi, K.; Turner, C. J.; Imajo, S.; Nozoe,
T. Inorg. Chem. 1986, 25, 3400-3406.

4084 Inorganic Chemistry, Vol. 39, No. 18, 2000
Franz et al.
Figure 1. ORTEP diagrams showing selected atom labels and 50% probability ellipsoids for all non-hydrogen atoms of [Co(i-Pr2ATI)2] (11),
[Co(i-PrDATI)2] (12‚CH2Cl2), [Co(t-BuDATI)2] (14‚CH2Cl2‚THF), [Co(BzDATI)2] (15‚0.5CH2Cl2‚0.5THF), and [Co(DATI-4)] (19). Solvent molecules
are omitted.
Table 1. Summary of X-ray Crystallographic Data
[Co(i-Pr2ATI)2]
[Co(i-PrDATI)2]
[Co(t-BuDATI)2]
[Co(BzDATI)2]
11
12‚CH2Cl2
14‚C4H8O‚CH2Cl2
15‚0.5C4H8O‚0.5CH2Cl2
empirical formula
fw
crystal system
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
CoC26H38N4
465.53
triclinic
CoC45H48Cl2N6O4S2
930.84
triclinic
CoC51H54Cl2N6O5S2
1024.95
triclinic
CoC54.5H48ClN6O4.5S2
1052.94
orthorhombic
Pbca
17.3577(5)
22.5064(7)
25.2381(8)
P1h
P1h
P1h
9.4007(3)
10.7556(4)
12.9497(5)
76.923(1)
76.880(1)
85.408(1)
1241.53(8)
2
12.1739(2)
13.2855(3)
14.8238(3)
102.769(1)
101.401(1)
94.309(1)
2274.34(8)
2
13.616(2)
14.060(2)
15.382(2)
85.40(1)
63.75(1)
73.341(7)
2526.4(7)
2
9859.5(5)
8
1.371
0.541
188(2)
Z
F
calcd (g cm-3
)
1.245
0.711
188(2)
1.359
0.635
188(2)
1.347
0.580
188(2)
µ (mm-1
temp (K)
)
no. of tot. reflns
data/restraints/parameter
no. of obsd reflnsa
R1b
7736
5416/0/432
4835
0.0370
0.0881
14 424
10 060/58/559
7829
0.0545
0.1353
16 026
11 167/0/604
8809
0.0404
0.1111
59 920
11 703/102/658
7733
0.0447
0.1037
wR2c
crystal size (mm)
0.30 × 0.20 × 0.12
0.54 × 0.24 × 0.20
0.50 × 0.45 × 0.22
0.40 × 0.30 × 0.15
max/min residuals (eÅ-3
)
0.273/-0.578
0.840/-0.711
0.516/-0.689
0.562/-0.458
a Observation criterion: I > 2σ(I). b R1 ) Σ||Fo| - |Fc||/Σ|Fo|. c wR2 ) [Σw(Fo - Fc2)2/Σw(Fo )2]1/2
.
2
2
[Zn(TC-6,6)].45 The N-S and S-O bond lengths are compa-
rable to the values found in other 4-coordinate zinc complexes
containing sulfonamide ligands.46-50
An ORTEP diagram of the linked-DATI Co2+ complex 19
is also included in Figure 1, and selected bond distances and
angles are contained in Table 2 for comparison with its bis-

Metal-Based NO Sensors
Inorganic Chemistry, Vol. 39, No. 18, 2000 4085
Table 2. Selected Bond Distances (Å) and Angles (deg)a
[Co(i-Pr2ATI)2] (11)
Co1-N1
Co1-N2
Co1-N3
1.981(2)
1.983(2)
1.975(2)
Co1-N4
Co-Nave
1.980(2)
1.980(3)
N1-Co1-N2
N3-Co1-N4
N1-Co1-N4
N2-Co1-N3
81.80(6)
81.66(6)
122.50(6)
126.72(6)
N2-Co1-N4
N1-Co1-N3
Θ
128.15(6)
121.86(6)
89.5
[Co(i-PrDATI)2] (12‚CH2Cl2)
Co1-N1
Co1-N2
Co1-N3
Co1-N4
Co-Nave
2.001(2)
2.000(2)
1.999(2)
1.995(2)
1.999(3)
N1-S1
N4-S2
S1-O1
S1-O2
S2-O3
S2-O4
1.620(2)
1.623(2)
1.447(2)
1.437(2)
1.440(2)
1.439(2)
N1-Co1-N2
N3-Co1-N4
N1-Co1-N4
N2-Co1-N3
80.75(9)
80.33(9)
138.2(1)
122.6(1)
N2-Co1-N4
N1-Co1-N3
Θ
120.9(1)
120.0(1)
76.1
Figure 2. Cyclic voltammogram of [Co(i-Pr2ATI)2] (11) in THF with
0.5 M (n-Bu4N)(ClO4) at a scan speed of 100 mV/s. Potentials are
reported vs Cp2Fe/Cp2Fe+.
[Co(t-BuDATI)2] (14‚CH2Cl2‚THF)
Co1-N1
Co1-N2
Co1-N3
Co1-N4
Co-Nave
1.977(2)
2.014(2)
2.005(2)
1.980(2)
1.99(2)
N1-S1
N4-S2
S1-O1
S1-O2
S2-O3
S2-O4
1.625(2)
1.620(2)
1.436(2)
1.445(2)
1.441(2)
1.445(2)
N1-Co1-N2
N3-Co1-N4
N1-Co1-N4
N2-Co1-N3
80.64(7)
81.03(7)
127.17(7)
126.86(7)
N2-Co1-N4
N1-Co1-N3
Θ
122.20(7)
125.48(7)
81.4
[Co(BzDATI)2] (15‚0.5CH2Cl2‚0.5THF)
Co1-N1
Co1-N2
Co1-N3
Co1-N4
Co-Nave
1.986(2)
1.998(2)
1.990(2)
2.008(2)
1.99(1)
N1-S1
N4-S2
S1-O1
S1-O2
S2-O3
S2-O4
1.618(2)
1.615(2)
1.441(2)
1.439(2)
1.444(2)
1.446(2)
N1-Co1-N2
N3-Co1-N4
N1-Co1-N4
N2-Co1-N3
80.24(8)
80.39(7)
121.45(8)
116.61(8)
N2-Co1-N4
N1-Co1-N3
Θ
132.29(8)
133.72(8)
73.8
Figure 3. Cyclic voltammogram of [Co(BzDATI)2] (15) in THF with
0.5 M (n-Bu4N)(ClO4) at a scan speed of 100 mV/s. Potentials are
reported vs Cp2Fe+/Cp2Fe.
stacked at an average aryl-aryl distance of 3.63(9) Å. The
average Co-N bond distance of 1.98(1) Å is similar to the
values for the [Co(RDATI)2] compounds but longer than the
value of 1.88(2) Å found in [Co(TC-4,4)],39 consistent with
the electron-withdrawing properties of the sulfonamide func-
tionality.
[Co(DATI-4)] (19)
Co1-N1
Co1-N2
Co1-N3
Co1-N4
Co-Nave
1.986(3)
1.963(2)
1.996(3)
1.977(2)
1.98(1)
N1-S2
N4-S1
S1-O1
S1-O2
S2-O3
S2-O4
1.607(3)
1.618(3)
1.440(2)
1.442(2)
1.439(2)
1.437(2)
Electrochemistry. Representative cyclic voltammograms of
[Co(i-Pr2ATI)2] (11) and [Co(BzDATI)2] (15) are shown in
Figures 2 and 3, respectively. Half-wave potentials for these
compounds are presented in Table 3, along with values recorded
for 12, 13, and a series of [Co(TC-n,m)] compounds. Compound
11 displays a reversible oxidation at -0.045 V and a reversible
reduction at -2.57 V in THF versus Cp2Fe/Cp2Fe+. These
potentials describe a Co2+ compound that is more difficult both
N1-Co1-N2
N3-Co1-N4
N1-Co1-N4
N2-Co1-N3
79.78(10)
79.93(10)
123.78(10)
103.94(10)
N2-Co1-N4
N1-Co1-N3
Θ
136.82(10)
141.36(11)
62.2
a Numbers in parentheses are estimated standard deviations of the
last significant figures. Atoms are labeled as indicated in Figure 1. The
angle Θ is the dihedral angle between the planes of the two
five-membered chelate rings.
(chelate) relatives. The Co2+ center has distorted tetrahedral
geometry, with Θ ) 62.2°. The tetramethylene linker chain
restricts the N2-Co1-N3 angle to 103.94(10)°, very similar
to the 102.1(1)° angle observed in the related macrocyclic
tropocoronand compound [Co(TC-4,4)].39 The opposing angle
N1-Co-N4, however, is not restrained in 19 and opens to
123.78(10)°, larger than the restrained 101.3(1)° angle in [Co-
(TC-4,4)].39 Similar to those in 15 described above, the dansyl
groups in 19 also align in a parallel-planar fashion, with the
two six-membered aromatic rings attached at the S atoms
(45) Doerrer, L. H.; Lippard, S. J. Inorg. Chem. 1997, 36, 2554-2563.
(46) Romero, J.; Garc´ıa-Va´squez, J. A.; Dura´n, M. L.; Castin˜eiras, A.;
Sousa, A.; Garnovskii, A. D.; Garnovskii, D. A. Acta Chem. Scand.
1997, 51, 672-675.
(47) Quinzani, O. V.; Tarulli, S.; Piro, O. E.; Baran, E. J.; Castellano, E.
E. Z. Naturforsch. B 1997, 52, 183-187.
(48) Baenziger, N. C.; Modak, S. L.; Fox, C. L. J. Acta Crystallogr., Sect.
C 1983, C39, 1620-1623.
(49) Denmark, S. E.; O’Conner, S. P.; Wilson, S. R. Angew. Chem., Int.
Ed. 1998, 37, 1149-1151.
(50) Koike, T.; Kimura, E.; Nakamura, I.; Hashimoto, Y.; Shiro, M. J.
Am. Chem. Soc. 1992, 114, 7338-7345.

4086 Inorganic Chemistry, Vol. 39, No. 18, 2000
Table 3. Cyclic Voltammetric Dataa
Franz et al.
d
complex
Co2+/Co+ Co3+/Co2+
Ep
a
[Co(TC-3,3)]b
[Co(TC-4,4)]
[Co(TC-4,5)]
[Co(TC-5,5)]
[Co(TC-6,6)]
-2.12
-2.22
-2.35
-2.40
-2.46
-0.49
-0.41
-0.38
-0.24
-0.26
0.49c
0.54c
0.52c
0.49c
0.55c
[Co(i-Pr2ATI)2] (11)
-2.57
-0.045
[Co(BzDATI)2] (15)
[Co(i-PrDATI)2] (12)
[Zn(i-PrDATI)2] (13)
-1.98
-2.09
0.47, 0.73, 1.08c
0.47, 0.84, 1.06
0.47, 0.93
a All potentials are given in volts and referenced to Cp2Fe/Cp2Fe+
at 0.0 V. Data were recorded for THF solutions unless otherwise noted.
b Results for all [Co(TC-n,m)] compounds are taken from ref 51.
c CH2Cl2. d Ep refers to an irreversible process.
a
to oxidize and to reduce than its macrocyclic analogue [Co-
(TC-6,6)], for which the values are -0.26 and -2.46 V,
respectively. This observation continues a trend established in
the tropocoronand series that compounds with higher Θ values
are more difficult both to oxidize and to reduce.51
Figure 4. Comparison of the fluorescence emission spectra of 46 µM
CH2Cl2 solutions of Hi-PrDATI (8) and [Co(i-PrDATI)2] (12). Excitation
) 350 nm.
The [Co(RDATI)2] compounds have only one quasi-reversible
wave, that for a reduction, as displayed in Figure 3 for the benzyl
derivative 15. The Co2+/Co+ half-wave potential occurs at -2.0
V for 12 and 15. This value is significantly more positive than
that found for 11. An irreversible wave at 0.47 V was confirmed
to be that for ligand oxidation, not a metal-centered process,
by observation of a similar wave for the Zn complex 13. At
least two other irreversible processes occur for 12 and 15, as
indicated in Table 3; for 12, the half-wave potentials are 0.47,
0.84, and 1.06 V. Complexes of the chelating ligand 2-(2-p-
toluenesulfonamido)pyridine also give similar irreversible elec-
trochemical oxidations, believed to be ligand based, at 0.51,
0.74, and 0.96 V versus Cp2Fe/Cp2Fe+.37 The Co2+/Co3+ couple
must therefore occur at a potential more positive than 0.47 V
but is unobservable. The DATI complexes are thus easier to
reduce and more difficult to oxidize than their parent ATI and
TC-n,m compounds, a property that is consistent with the
electron-withdrawing environment provided by the sulfonamide
functionality.
Fluorescence. When excited at 350 nm, ligand 8 displays a
broad emission band centered around 500 nm due to the dansyl
fluorophore. A plot of concentration versus fluorescence
intensity (Figure S7, Supporting Information) reveals that the
optimal emission intensity occurs at 50 µM. At higher concen-
trations, the intensity decreases because of inner filter effects.17
Plots for the Zn complex are shown in Figure S8 (Supporting
Information) and show a similar optimal concentration range.
The chelated Zn2+ complex 13 does not shift the emission
wavelength, λmax ) 506 cm-1, compared with that of the free
ligand. This result contrasts with the blue shift of about 25 cm-1
observed upon zinc complexation by a dansyl-pendant cyclen
ligand.52
ion therefore provides an efficient pathway for quenching the
fluorescence of the DATI ligands, whereas the filled d-shell
Zn2+ ion remains fluorescent.
NO Reactivity of [Co(RDATI)2]. The reactions of 12, 14,
and 15 in CH2Cl2 solutions with NO are slow, but after several
hours of exposure to an NO atmosphere, two new bands appear
at 1838 and 1760 cm-1 in the IR spectrum of 20a. For 20b and
20c, the corresponding values are 1833, 1760 cm-1 and 1833,
1755 cm-1, respectively. The appearance of these NO stretching
bands indicates the formation of dinitrosyl complexes, examples
of which display symmetric and asymmetric stretching modes
between 1750-1798 and 1820-1876 cm-1 53-60
.
In addition to the similarity of our νNO bands to those in these
examples, they are suggestive of those at 1809 and 1730 cm-1
for the nondansylated parent complex, [Co(NO)2(i-Pr2ATI)]
(21). The shift to higher energy in the spectra of 20a-c is
expected owing to the electron-withdrawing sulfonamides of
the DATI ligands. Although we were unable to obtain crystal
structures of 20a-c, the structure of the related molecule
[Co(NO)2(i-Pr2ATI)]61 strongly suggests that the geometries
of 20a-c will be pseudotetrahedral. Because of the lack
of appreciable reactivity differences across the series of
[Co(RDATI)2] compounds, we chose to focus on [Co(i-PrDATI)2]
(12) for further studies.
The 1H NMR spectrum of paramagnetic 12 reveals broad but
well-defined resonances at 92.3, 87.2, 14.3, -1.3, -36.7, -74.2,
(53) Field, J. S.; Wheatley, P. J.; Bhaduri, S. J. Chem. Soc., Dalton Trans.
1974, 74-79.
(54) Haymore, B. L.; Huffman, J. C.; Butler, N. E. Inorg. Chem. 1983,
22, 168-170.
(55) Kaduk, J. A.; Ibers, J. A. Inorg. Chem. 1977, 16, 3283-3287.
(56) Reichert, B. E. Acta Crystallogr., Sect. B 1976, B32, 1934-1936.
(57) Roustan, J.-L.; Ansari, N.; Le Page, Y.; Charland, J.-P. Can. J. Chem.
1992, 70, 1650-1657.
(58) Aresta, M.; Ballivet-Tkatchenko, D.; Bonnet, M. C.; Faure, R.;
Loiseleur, H. J. Am. Chem. Soc. 1985, 107, 2994-2995.
(59) Hendrickson, A. R.; Ho, R. K. Y.; Martin, R. L. Inorg. Chem. 1974,
13, 1279-1281.
Complexation of ligands 8, 9, and 10 by Co2+ results in
significant fluorescence quenching. Solutions of 40 µM 12, 14,
and 15 in CH2Cl2 display only 5-6% of the fluorescence
intensity of the free ligands, as illustrated in Figure 4 for 12.
Fluorescence quenching by transition metals can occur by energy
or electron transfer between the excited state of the fluorophore
and empty or partially filled d orbitals on the metal.33 The Co2+
(60) Martin, R. L.; Taylor, D. Inorg. Chem. 1976, 15, 2970-2977.
(61) The pseudotetrahedral dinitrosyl structure of 21 was determined in
space group Iba2 with a ) 13.711(2) Å, b ) 24.639(6) Å, c ) 8.997-
(2) Å, V ) 3039.4(11) Å3, Z ) 8, T ) 188(2) K, R1 ) 0.125, wR2
) 0.227. The crystal quality precluded better refinement of the
structure.
(51) (a) Doerrer, L. H.; Bautista, M. T.; Lippard, S. J. Inorg. Chem. 1997,
36, 3578-3579. (b) Doerrer, L. H. Ph.D. Dissertation, Massachusetts
Institute of Technology, 1997.
(52) Koike, T.; Watanabe, T.; Aoki, S.; Kimura, E.; Shiro, M. J. Am. Chem.
Soc. 1996, 118, 12696-12703.

Metal-Based NO Sensors
Inorganic Chemistry, Vol. 39, No. 18, 2000 4087
and -91.7 ppm in C6D6. By comparison with reported spectra
of [Co(R2ATI)2] complexes, we assign the 87.2, -91.7, and
92.3 ppm resonances to the R, â, and γ tropolone ring protons,
respectively.35 In the presence of NO, these peaks are replaced
by two sets of resonances in positions expected for diamagnetic
compounds. One set of signals corresponds to the spectrum of
the free ligand Hi-PrDATI, and the other is consistent with the
dinitrosyl-mono(DATI) formulation assigned to 20a (eq 1). The
NO
[Co(i-PrDATI)2]
[Co(NO)2(i-PrDATI)] + Hi-PrDATI
8
e-, H+
12
20a
(1)
reaction was slow, but after 4 days, 20a and Hi-PrDATI formed
in a 1:1 ratio. The yield (75%) was calculated by comparing
peak integrations of the products against those of acetone added
as an internal standard. A small amount of 12 was still present
in the solution. The source of the protons was assumed to be
adventitious water in the solvent. Addition of 1 equiv of water
helped to drive the reaction to completion, as evidenced by
increased intensity of the νNO bands in the IR spectrum after
shorter reaction times when wet solvent was used. Similar
reactivity was observed during the reaction of [Co(i-Pr2ATI)2]
with NO, the formation of the dinitrosyl compound 21 coincid-
ing with the release of the free ligand H-i-Pr2ATI. Unlike the
case for the DATI compounds, this reaction proceeded within
minutes rather than hours and the product was not stable under
excess NO.
Figure 5. Emission spectra of a 45 µM CH2Cl2 solution of
[Co(i-PrDATI)2] (12) after exposure to NO gas. The lowest intensity
trace is that of 12 in an N2 atmosphere, excited at 350 nm. The other
spectra were recorded, in increasing order of intensity, 5 min, 3 h, 4.5
h, and 6 h after addition of NO gas to the headspace of the sample
cell. About an 8-fold increase in intensity was observed from that of
the starting material. A control cell that was exposed to air over the
same periods of time showed no change its fluorescence emission.
O2 stable, they do not tolerate moisture and will slowly
hydrolyze if solutions are exposed to air for too long a period
of time (several days). They are similarly sensitive to the addi-
tion of protons from other sources. Figure S9 (Supporting
Information) shows the fluorescence response of a CH2Cl2 solu-
tion of 12 following the addition of 2 equiv of HBAr′4‚2Et2O.
The response is similar to that given in Figure 5 after the
addition of NO but reaches an 8-fold increase in half the time.
To test the hypothesis that liberation of free ligand in the
reaction of 12 with NO accompanies the positive fluorescence
response, emission spectra of 12 were recorded after incremental
additions of a solution of Hi-PrDATI (Figure S10, Supporting
Information). As expected, the emission at 505 nm increases
with increasing amounts of added free ligand. The most intense
emission recorded occurs when approximately 1 equiv of ligand
is added to the cobalt complex, giving a 5-6-fold increase in
fluorescence over that of 12 alone. The Co2+ center therefore
does not exert a quenching effect on other fluorophores in
solution but only affects the fluorophore to which it is ligated.
Since the intensity increase seen here is comparable to the
changes observed after addition of NO to 12, it is reasonable
to conclude that the amount of Hi-PrDATI formed in the NO
reaction is sufficient to elicit the fluorescence response observed
in Figure 5. The key event responsible for the fluorescence
enhancement, therefore, is dissociation of a fluorophore-
containing ligand upon formation of the metal-nitrosyl adduct.
It is also possible that the dinitrosyl 20a itself may contribute
to the fluorescence response. Empty or partially filled d orbitals
of a metal provide an efficient energy-transfer-quenching
pathway for excited-state fluorophores.33 Metal ions with filled
d shells, such as Zn2+ or Cu+, do not provide such an energy-
transfer pathway, and fluorescence can occur.65 The compound
[Zn(i-PrDATI)2] is one such example. Formation of a diamag-
netic {Co(NO)2}10 moiety may also remove a fluorescence-
quenching pathway.
Known cobalt dinitrosyl complexes are 4-coordinate, pseudo-
tetrahedral species designated as {Co(NO)2}10 in the Enemark-
Feltham notation for metal nitrosyl compounds. In this notation,
n (10 in this case) denotes the number of electrons in the
{MNO}n unit as the sum of the d electrons of the metal plus
the electrons in π* orbitals of NO.62 By formal count, a Co2+
species such as 12 would bind two molecules of NO to give n
) 9, not 10, d electrons. In some reported examples, {Co-
(NO)2}10 forms from Co2+ starting materials, the metal center
providing the needed electron by disproportionation of [CoIIL2]
to give [CoI(NO)2L] and [CoIIIL3].59,60,63 Owing to the steric
bulk of the DATI ligands, it is unlikely that tris(chelate) Co3+
1
complexes could form. The H NMR spectrum of the reaction
mixture of 12 and NO discussed above reveals only two new
species, which together account for at least 75% of the starting
material. Unreacted starting material comprises another fraction,
eliminating the possibility of a Co3+ byproduct in the reaction.
Although the nature of the reductant remains unknown, reductive
nitrosylation is well documented.64
Fluorescence Response of [Co(i-PrDATI)2] (12) + NO. As
expected from the unfilled d shell of the Co2+ starting
complex,33 the fluorescence of the DATI ligands is quenched
in 12. The dissociation of a fluorescent ligand upon NO binding
to the Co center, however, provides a mechanism for sensing
NO. As shown in Figure 5, a CH2Cl2 solution of 12 displays a
slow but steady increase in emission at 505 nm in the presence
of NO. After 6 h, the fluorescence intensity increases by
approximately 8-fold over that of the starting complex.
This 8-fold increase in the presence of NO contrasts sharply
with the lack of response when 12 is exposed to air (data not
shown). The Co2+ complexes are remarkably air stable, both
in solution and in the solid state. Although 12, 14, and 15 are
(62) Enemark, J. H.; Feltham, R. D. Coord. Chem. ReV. 1974, 13, 339-
406.
(63) Del Zotto, A.; Mezzetti, A.; Rigo, P. Inorg. Chim. Acta 1990, 171,
61-69.
(64) Scheidt, W. R.; Ellison, M. K. Acc. Chem. Res. 1999, 32, 350-359.
(65) Fabbrizzi, L.; Licchelli, M.; Pallavicini, P. Acc. Chem. Res. 1999, 32,
846-853.

4088 Inorganic Chemistry, Vol. 39, No. 18, 2000
Franz et al.
Figure 7. In situ IR reaction spectra of [Co(DATI-4)] (19) in CH2Cl2
at -78 °C as NO gas was introduced into the headspace of the reaction
flask. The growth of a nitrosyl stretch at 1660 cm-1 tracks the formation
of N2O at 2223 cm-1. Both bands maximize within 20 min and then
are stable at -78 °C for the remainder of the monitoring time, slightly
more than 1 h. The cold bath was then removed. As the reaction mixture
warmed to room temperature, the band at 1660 cm-1 was replaced by
bands at 1837 and 1760 cm-1, and the reaction spectra (not shown)
appeared analogous to those displayed in Figure 6.
Figure 6. In situ IR reaction spectra of [Co(DATI-4)] (19) in CH2Cl2
at room temperature as NO gas was introduced into the headspace of
the reaction flask. The bands at 1837 and 1760 cm-1 indicate the
development of a dinitrosyl species. A residual amount of N2O is
observed at 2223 cm-1 (band not labeled), but this band does not grow
in to an appreciable intensity. The reaction was monitored for 20 h,
but no significant changes were observed after the 2 h period represented
in this figure.
NO Reactivity of [Co(DATI-4)] (19). The NO-induced
fluorophore dissociation observed upon the reaction of 12 with
NO led to the design and synthesis of a linked-DATI ligand. In
this construct, one arm of the ligand can dissociate from the
metal center but remain part of the complex. Such a strategy
could possibly facilitate a reversible NO-binding complex. The
reaction of 19 with NO proceeds 50 times faster than that of
compounds 12, 14, and 15, perhaps due to geometric differences.
The linker chain in 19 imposes some distortion at the metal
center, forcing it even further away from an idealized tetrahedral
environment compared to those of 12, 14, and 15. NO may
therefore have better access to the metal center in 19, and release
of the strain would drive the Co center to adopt a more favorable
pseudotetrahedral geometry.
Scheme 2
The faster reaction time made 19 amenable to in situ IR
monitoring of its reaction with NO. Figure 6 displays the
changes seen in the NO stretching frequency region when a
CH2Cl2 solution of 19 at room temperature was anaerobically
exposed to a stream of NO gas. Two new bands appeared at
1835 and 1760 cm-1, very closely matching the signature
frequencies of 20b at 1833 and 1760 cm-1. The formation of
these characteristic bands indicates formation of a Co dinitrosyl
compound that has an environment almost identical to that of
20b. In the solid state, these bands appear at 1827 and 1751
cm-1 and shift to 1793 and 1719 cm-1 upon isotopic labeling
with 15NO, in agreement with the values 1794 and 1720 cm-1
calculated by assuming a classic diatomic oscillator model.
When the reaction of 19 with NO was followed at -78 °C,
however, new bands at 1660 and 2223 cm-1 appeared simul-
taneously, as displayed in Figure 7. Both bands reached their
maximum intensity in about 15 min, after which they remained
stable at -78 °C for at least 45 min. When the reaction mixture
was warmed to ambient temperature, the 1660 cm-1 signal
disappeared and the characteristic dinitrosyl bands at 1835 and
1760 cm-1 appeared (not shown). Cooling the solution back to
-78 °C did not cause the 1660 cm-1 band to return.
product. Once the stable, tetrahedral dinitrosyl adduct forms, it
does not react further with NO or NO-derived species.
To obtain a pseudotetrahedral coordination environment, one
of the DATI arms must dissociate from the metal center, as
1
shown in Scheme 2. The H NMR spectrum of the reaction
mixture of 19 and NO is difficult to interpret because of
extensive signal overlap, but it clearly contains only diamagnetic
signals and the methyl proton resonances of the dansyl group
are no longer equivalent, as they are in the spectrum of ligand
18.
Fluorescence Response of [Co(DATI-4)] (19) + NO. As
was the case for the [Co(RDATI)2] compounds, the Co2+ center
in 19 quenches the fluorescence of the DATI-4 fluorophore.
Under an NO atmosphere, a solution of 19 rapidly displays an
increase in fluorescence at 505 nm, however, as shown in Figure
8. The first emission spectrum shown in Figure 8a is that of
the starting compound 19. The next spectrum was recorded 3
min after NO addition, by which time the emission intensity
had already doubled. The response continued to rise over the
course of several hours, affording a greater than 4-fold increase
in fluorescence intensity compared to that of the starting
material. The initial response is much faster than those for the
The band at 2223 cm-1 is the signature of N2O and was also
observed during NO disproportionation reactivity by Mn and
Fe tropocoronand complexes.66,67 In both of these cases, the
first step in the reaction with NO was formation of a mononi-
trosyl adduct. It is possible that the band at 1660 cm-1 cor-
responds to a 5-coordinate mononitrosyl species that is stable
at low temperature and upon warming decays to a {Co(NO)2}10
(66) Franz, K. J.; Lippard, S. J. J. Am. Chem. Soc. 1998, 120, 9034-9040.
(67) Franz, K. J.; Lippard, S. J. J. Am. Chem. Soc. 1999, 121, 10504-
10512.

Metal-Based NO Sensors
Inorganic Chemistry, Vol. 39, No. 18, 2000 4089
of one DATI ligand upon coordination of two nitrosyl ligands
to the metal center. Removal of a fluorophore from the
quenching metal center revives fluorescence, thereby reporting
the presence of NO. An additional contribution to the fluores-
cence enhancement may come from DATI bound to the
dinitrosylmetal center itself, which has been characterized as
{Co(NO)2}10.
The linked-DATI ligand DATI-4 represents a second-genera-
tion NO sensor. When two DATI ligands are connected via a
methylene chain, the dissociative fluorescence enhancement is
retained while the ligand components remain intact. Although
this system does not exhibit reversible NO binding, the design
idea provides the possibility of creating such a desirable sensor
property in the future. These results offer a paradigm for NO
sensor designs, the selective displacement of a fluorescent
moiety from the metal center upon NO binding.
Figure 8. (a) Fluorescence emission spectra showing the increase in
intensity at 505 nm (excitation ) 350 nm) when a 40 µM CH2Cl2
solution of [Co(DATI-4)] (19) was exposed to 1 atm of NO gas. The
lowest intensity spectrum is that of the starting material; the next
spectrum was recorded 3 min after addition of NO to the headspace.
After 6 h (top spectrum), a greater than 4-fold increase in intensity
over that of the starting material was observed. (b) Fluorescence
emission spectra of a 40 µM CH2Cl2 solution of 19, unaffected by
exposure to air over a period of 6 h.
Experimental Section
General Considerations. Tetrahydrofuran (THF), diethyl ether, and
pentane were purified by passage through columns of alumina under
N2.68 Dichloromethane (CH2Cl2), chlorobenzene, and triethylamine
(Et3N) were distilled from CaH2 under N2. 2-(Tosyloxy)tropone,69 H-i-
Pr2ATI,70 2,2′-(pentamethylenediamino)bis(2,4,6-cycloheptatrien-1-one)
(“4-dimer-tropolone”),36 [Co(CH3CN)4](PF6)2,71 HBAr′4‚2Et2O72 and
[Co(DATI-4)] (19)32 were prepared as described in the literature. Nitric
oxide (Matheson, 99%) and 15NO (Aldrich, 99%) were purified of
higher nitrogen oxides by passage through a column of NaOH pellets
and a mercury bubbler and kept over mercury in gas storage bulbs.
Analysis by GC of the NO used in the experiments revealed no
contaminants, such as NO2 or N2O, at the limit of the thermal
conductivity detector, about 30 nM. All other reagents were obtained
commercially and not further purified. Silica gel 60 (230-400 mesh,
EM Science) or activated basic alumina (150 mesh, Brockmann I) was
used for column chromatography. UV-visible spectra were recorded
on a Hewlett-Packard 8435 spectrophotometer. Standard IR spectra were
recorded on a Bio-Rad FTS-135 instrument; solid samples were
prepared as pressed KBr disks and solution samples were prepared in
an airtight Graseby-Specac solution cell with CaF2 windows. In situ
IR sample monitoring was performed with a ReactIR 1000 instrument
from ASI Applied Systems equipped with a 1-in.-diameter, 30-reflection
silicon ATR (SiComp) probe optimized for maximum sensitivity.
Reaction protocols were as described previously.67 Fluorescence
emission spectra were recorded at 25 ( 1 °C on a Hitachi F-3010
fluorescence spectrophotometer. Mass spectra were determined in a
3-nitrobenzyl alcohol matrix with a Finnegan 4000 mass spectrometer
using 70-eV impact ionization. Melting points were measured on a
Thomas-Hoover capillary melting point apparatus. NMR spectra were
recorded on a Bruker AC 250-MHz or a Varian Mercury 300-MHz
spectrometer at ambient probe temperature and referenced to the internal
1H and 13C solvent peaks.
analogous reactions with the first-generation [Co(RDATI)2]
compounds, which only show appreciable increases after several
hours.
To estimate the sensitivity of 19, aliquots of NO gas were
injected into 2 mL of a 35 µM CH2Cl2 solution of 19 at 25 °C.
Addition of 2 µL of NO caused only a slight fluorescence
enhancement, whereas addition of 5 µL of NO doubled the
fluorescence intensity within 1 h (Figure S11, Supporting
Information). These results place the detection limit of 19 at
50-100 µM, a value that needs to be lowered for biological
applications.
Creation of a slightly strained geometry at the Co center by
linking the DATI ligands does not jeopardize the air stability
of 19. Figure 8b demonstrates the lack of a fluorescence
response when 19 is exposed to air. In the solid state and in
solution, 19 appears to be stable to O2 indefinitely. The
concentration of NO involved in regulatory functions such as
vasodilation and neurotransmission is estimated to be 0.1-10
µM.7 Because the reaction of NO with O2 to form NO2 is second
order in [NO], the lifetime of NO at these functional concentra-
tions can be up to 10 min in an aerobic environment.7 Sensors
that react with O2 to give a fluorescence response are therefore
of limited value for these applications.
Summary and Conclusions
Synthetic Procedures. 2-((4-Methoxybenzyl)amino)tropone (1).
To a 150-mL ethanol slurry of 2-(tosyloxy)tropone (21 g, 76 mmol)
were added via syringe Et3N (12.5 mL, 90 mmol) and (4-methoxy-
benzyl)amine (11.9 mL, 90 mmol). The mixture was heated to reflux
under Ar. The green solution obtained within 1.5 h turned golden yellow
after another 3 h. Solvents were removed by rotary evaporation, and
the solid residue was treated with aqueous NaHCO3, extracted into CH2-
Cl2 (3×), and dried over MgSO4. Filtration and evaporation yielded a
brownish crystalline solid, which was stirred in 50 mL of diethyl ether
for 0.5 h and isolated by filtration to give 16.1 g (88%) of 1 as a mustard
yellow, microcrystalline powder. TLC: Rf ) 0.20 (5:1 hexane/ethyl
A new route to asymmetrically derivatized aminotroponimi-
nate ligands has been developed. Protecting one of the imino
nitrogens with a 4-methoxybenzyl group provides a facile and
general way to functionalize the ATI core with a sulfonamide
group, specifically a dansyl fluorophore. This synthetic route
provides a new family of fluorescent ligands for development
as potential NO sensors. As anticipated, the filled d-shell Zn2+
complex remains fluorescent, whereas Co2+ quenches fluores-
cence of the DATI ligands.
The [Co(DATI)2] compounds 12, 14, and 15 represent our
initial attempts at developing metal-based, fluorescent NO
sensors. These molecules address one of the inherent challenges
of designing an NO sensor, namely, discrimination of NO over
O2. The complexes are indefinitely dioxygen stable and display
significantly enhanced fluorescence in the presence of NO. The
sensing behavior of [Co(DATI)2] results from the dissociation
(68) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518-1520.
(69) Doering, W. v. E.; Hiskey, C. F. J. Am. Chem. Soc. 1952, 74, 5688.
(70) Dias, H. V. R.; Jin, W.; Ratcliff, R. E. Inorg. Chem. 1995, 34, 6100-
6105.
(71) Goldstein, A. S.; Drago, R. S. Inorg. Chem. 1991, 30, 4506-4510.
(72) Brookhart, M.; Grant, B.; Volpe, A. F. J. Organometallics 1992, 11,
3920-3922.

4090 Inorganic Chemistry, Vol. 39, No. 18, 2000
Franz et al.
1
acetate + 10% Et3N, SiO2). H NMR (300 MHz, C6D6): δ 7.70 (1H,
9.3 Hz), 5.96 (1H, d, J ) 10.2 Hz), 3.41 (1H, septet, J ) 6.3 Hz), 0.95
(6H, d, J ) 6.3 Hz). IR (neat on NaCl plates, cm-1): 3859, 3848,
2967, 2930, 1610, 1591, 1417, 1400, 1367, 1337, 1284, 1245, 1216,
1170, 1133, 941, 881, 849, 803, 750, 699. HRMS (FAB (+)), m/z:
calcd for C10H14N2, 162.1157; found, 162.1152(5).
br s, NH), 7.36 (1H, d, J ) 11.7 Hz), 6.85-6.75 (3H, m), 6.71-6.63
(3H, m), 6.22 (1H, t, J ) 9.5 Hz), 6.08 (1H, d, J ) 10.2 Hz), 3.79
(2H, d, J ) 6 Hz), 3.26 (3H, s). IR (KBr, cm-1): 3310, 2962, 2907,
1602, 1545, 1505, 1394, 1358, 1249, 1172, 1028, 854, 834, 767, 734,
668, 408. Mp: 98-100 °C. HRMS (FAB (+)), m/z: calcd for C15H15-
NO2, 241.1103; found, 241.1108(7).
N-Amino-2-(tert-butylamino)troponimine (6). A portion of 3 (4.54
g, 15.31 mmol) was slowly added to 5 mL of rapidly stirring
trifluoroacetic acid chilled over an ice-water slush bath. The resulting
red solution was stirred for 2.5 h with slow warming to ambient
temperature. The reaction was quenched with ice, and the sample was
treated with 20 mL of aqueous 2 M NaOH and extracted into CHCl3
(3×). The brown solution was dried over Na2CO3 and filtered, and the
filtrate was evaporated to a brown oil. The product was purified by
flash column chromatography on alumina (1:4 ethyl acetate/hexane +
N-((4-Methoxybenzyl)amino)-2-(isopropylamino)troponimine (2).
Under an N2 atmosphere, finely divided Me3OBF4 (5.62 g, 38 mmol)
was added to a 75-mL CH2Cl2 solution of 1 (8.3 g, 34 mmol), and the
mixture was stirred for 2 h. The resulting dark golden solution was
chilled over an ice-water slush bath, and a portion of isopropylamine
(6.4 mL, 41 mmol) was added via syringe to give a clear, red solution,
which was then stirred for 2 h. Aqueous NaHCO3 was added, the layers
were separated, and the organic phase was dried over Na2CO3. Filtration
and evaporation yielded a brownish yellow oil, which was recrystallized
from diethyl ether at 0 °C to give 6.36 g (66%) of 2 as a yellow powder.
TLC: Rf ) 0.5 (5:1 hexane/ethyl acetate + 10% Et3N, SiO2). 1H NMR
(250 MHz, C6D6): δ 7.22 (2H, d, J ) 8.5 Hz), 6.80 (2H, d, J ) 8.6
Hz), 6.71-6.58 (2H, m), 6.32 (1H, d, J ) 11.0 Hz), 6.22-6.09 (2H,
m), 4.35 (2H, s), 3.61-3.51 (1H, m), 3.30 (3H, s), 1.03 (6H, d, J )
6.3 Hz). IR (KBr, cm-1): 2962, 1591, 1512, 1380, 1302, 1250, 1174,
1118, 1031, 945, 828, 815, 747, 701, 540. Mp: 50-52 °C. HRMS
(FAB (+)), m/z: calcd for C18H22N2O, 282.1732; found, 282.1729(9).
N-((4-Methoxybenzyl)amino)-2-(tert-butylamino)troponimine (3).
Finely divided Me3OBF4 (5.1 g, 35 mmol) was added to a 50-mL CH2-
Cl2 solution of 1 (7.93 g, 33 mmol), and the mixture was stirred for 2
h under N2. The resulting dark golden solution was chilled over an
ice-water slush bath, and a portion of tert-butylamine (7.6 mL, 73
mmol) was added via syringe to give a dark, red-brown solution, which
was stirred for 4 h. Aqueous NaHCO3 was added, the layers were
separated, and the organic phase was dried over Na2CO3. Filtration
and evaporation yielded a brown solid, which was recrystallized from
CH2Cl2/diethyl ether to give 4.91 g (50%) of 3 as a yellow, crystalline
solid. TLC: Rf ) 0.3 (5:1 hexane/ethyl acetate + 10% Et3N, SiO2). 1H
NMR (300 MHz, C6D6): δ 8.49 (1H, br s, NH), 7.39 (2H, d, J ) 8.7
Hz), 6.92-6.87 (2H, m), 6.75-6.53 (3H, m), 6.40 (1H, d, J ) 9.9
Hz), 6.16 (1H, t, J ) 9.2 Hz), 4.52 (2H, s), 3.53 (3H, s), 1.19 (9H, d,
J ) 7.5). IR (KBr, cm-1): 3201, 2969, 2832, 1611, 1588, 1459, 1368,
1284, 1229, 1174, 1032, 924, 859, 814, 770, 698, 558, 518. Mp: 103-
105 °C. HRMS (FAB (+)), m/z: calcd for C19H24N2O, 296.1889; found,
296.1883(9).
1
10% Et3N; Rf ) 0.28) to give 1.75 g (63%) of 6 as a golden oil. H
NMR (300 MHz, C6D6): δ 7.97 (1H, br s, NH), 6.58 (1H, t, J ) 10.1
Hz), 6.33-6.27 (3H, m), 6.06-5.99 (1H, m), 1.17 (9H, s). IR (neat on
NaCl plates, cm-1): 2976, 2834, 1612, 1590, 1539, 1503, 1467, 1415,
1398, 1368, 1359, 1294, 1235, 1212, 1135, 1060, 1034, 880, 849, 802,
774, 750, 697. HRMS (FAB (+)), m/z: calcd for C11H16N2, 176.1313;
found, 176.1318(5).
N-Amino-2-(benzylamino)troponimine (7). A portion of 4 (5.45
g, 16 mmol) was slowly added to 5 mL of rapidly stirring trifluoroacetic
acid chilled over an ice-water slush bath. The resulting red solution
was stirred for 2.5 h with slow warming to ambient temperature. The
reaction was quenched with ice, and the sample was treated with 20
mL of aqueous 2 M NaOH and extracted into CHCl3 (3×). The yellow
solution was dried over Na2CO3 and filtered, and the filtrate was
evaporated to yield 2.73 g (81%) of 7 as a yellow powder. TLC: Rf )
0.16 (5:1 hexane/ethyl acetate + 10% Et3N, SiO2). 1H NMR (300 MHz,
C6D6): δ 7.17-7.04 (4H, m), 6.45 (2H, t, J ) 10.4 Hz), 6.37-6.29
(1H, m), 6.11 (1H, d, J ) 11.1 Hz), 6.05-5.97 (2H, m), 4.13 (2H, s).
IR (KBr, cm-1): 3286, 3246, 3003, 2828, 1588, 1508, 1456, 1351,
1244, 1102, 1028, 874, 808, 756, 492. Mp: 63-66 °C. HRMS (FAB
(+)), m/z: calcd for C14H14N2, 210.1157; found, 210.1163(6).
Hi-PrDATI (8). A 20-mL THF solution of 5 (2.74 g, 17 mmol) was
transferred via cannula under Ar to a portion of NaH (811 mg, 20 mmol,
60% dispersion in paraffin) that had been washed with hexanes and
dried in vacuo. The effervescence dissipated over 1 h, and the reaction
mixture was stirred for an additional 1 h. The reaction flask was placed
over an ice-water slush bath, a 15-mL THF solution of dansyl chloride
(5.4 g, 20 mmol) was added dropwise under Ar, and the reaction mixture
was stirred overnight. The solvent was removed in vacuo to leave a
dark brown residue, which was treated with aqueous NaHCO3, extracted
into CH2Cl2 (3×), and dried over Na2CO3. After filtration and
evacuation, the brown solid was washed with 15 mL of diethyl ether
(3×) and collected by filtration. The product was purified by flash
column chromatography on silica gel (4:3 hexane/ethyl acetate + 10%
Et3N; Rf ) 0.20) to give 1.55 g (23%) of 8 as a light orange powder.
1H NMR (300 MHz, C6D6): δ 9.21 (1H, d, J ) 8.7 Hz), 8.80 (1H, dd,
J ) 7.2, 1.5 Hz), 8.71 (1H, d, J ) 11.4 Hz), 8.4-8.36 (1H, m), 7.83
(1H, br s, NH), 7.34 (1H, t, J ) 8.1 Hz), 7.18-7.12 (1H, m), 6.85
(1H, d, J ) 7.5 Hz), 6.73 (1H, td, J ) 10.3, 1.2 Hz), 6.59 (1H, t, J )
10.2 Hz), 6.16 (1H, t, J ) 9.4 Hz), 6.01 (1H, d, J ) 10.8 Hz), 2.96-
2.85 (1H, m), 2.49 (6H, s), 0.52 (6H, d, J ) 6.3 Hz). 13C NMR (75
MHz, CDCl3): δ 155.2, 154.6, 151.6, 139.1, 138.9, 138.3, 130.4, 130.1,
129.3, 127.7, 127.3, 125.5, 124.3, 123.3, 121.1, 115.3, 114.8, 45.6,
44.8, 22.0. IR (KBr, cm-1): 3236, 2974, 2830, 1600, 1533, 1519, 1438,
1339, 1283, 1216, 1124, 1059, 947, 871, 846, 800, 751, 737, 708, 622,
568, 532, 464. UV-vis (CH2Cl2) λ, nm (ꢀ, M-1 cm-1): 263 (35 400),
359 (16 500), 433 (20 500). Mp: 163-165 °C. Anal. Calcd for
C22H25N3SO2: C, 66.81; H, 6.37; N, 10.62; S, 8.11. Found: C, 66.62;
H, 6.55; N, 10.48; S, 7.96.
N-((4-Methoxybenzyl)amino)-2-(benzylamino)troponimine (4).
Finely divided Me3OBF4 (4.18 g, 28 mmol) was added to a 50-mL
CH2Cl2 solution of 1 (6.20 g, 26 mmol), and the mixture was stirred
for 2 h under N2. The resulting dark golden solution was chilled over
an ice-water slush bath, and portions of benzylamine (3.1 mL, 28
mmol) and Et3N (3.9 mL, 28 mmol) were added via syringe to give an
orange solution, which was stirred for 4 h. Aqueous NaHCO3 was
added, the layers were separated, and the organic phase was dried over
Na2CO3. Diethyl ether was added after filtration to afford 6.25 g (73%)
of 4 as a yellow, crystalline solid. TLC: Rf ) 0.24 (5:1 hexane/ethyl
1
acetate + 10% Et3N, SiO2). H NMR (300 MHz, C6D6): δ 7.27 (2H,
d, J ) 7.5 Hz), 7.18-7.04 (5H, m), 6.78-6.75 (2H, m), 6.65-6.54
(2H, m), 6.26 (2H, t, J ) 11.5 Hz), 6.10 (1H, t, J ) 9.3 Hz), 4.32 (2H,
s), 4.23 (2H, s), 2.28 (3H, s). IR (KBr, cm-1): 3263, 3016, 2826, 1611,
1586, 1530, 1497, 1452, 1383, 1272, 1241, 1179, 1105, 1027, 824,
752, 729, 420. Mp: 104-106 °C. HRMS (FAB (+)), m/z: calcd for
C22H22N2O, 330.1732; found, 330.1739(9).
N-Amino-2-(isopropylamino)troponimine (5). A portion of 2 (6.1
g, 21.5 mmol) was slowly added to 6 mL of rapidly stirring
trifluoroacetic acid chilled over an ice-water slush bath. The resulting
red solution was stirred for 4 h with slow warming to ambient
temperature. The reaction was quenched with ice, and the sample was
treated with 20 mL of aqueous 2 M NaOH and extracted into CHCl3
(3×). The brown solution was dried over Na2CO3 and filtered, and the
filtrate was evaporated to a brown oil. The product was purified by
flash column chromatography on alumina (3:2 hexane/ethyl acetate +
Ht-BuDATI (9). This compound was obtained from 6 by a procedure
analogous to that described for the synthesis of 8. The product was
purified by flash column chromatography on silica gel (3:2 hexane/
ethyl acetate + 10% Et3N; Rf ) 0.23) to give 1.2 g (42%) of 9 as a
yellow-orange, crystalline powder. Crystals suitable for X-ray crystal-
1
1
10% Et3N; Rf ) 0.25) to afford 2.57 g (88%) of 5 as a golden oil. H
lography were grown by slow evaporation of CH2Cl2. H NMR (300
NMR (300 MHz, C6D6): δ 6.58 (1H, td, J ) 10.2, 1.4 Hz), 6.34 (1H,
td, J ) 9.9 Hz, 1.2 Hz), 6.23 (1H, d, J ) 11.4 Hz), 6.03 (1H, t, J )
MHz, C6D6): δ 9.19 (1H, d, J ) 8.7 Hz), 8.79 (1H, dd, J ) 6, 1.3
Hz), 8.72 (1H, d, J ) 11.1 Hz), 8.40 (1H, d, J ) 8.7 Hz), 8.13 (1H, br

Metal-Based NO Sensors
Inorganic Chemistry, Vol. 39, No. 18, 2000 4091
s, NH), 7.33 (1H, t, J ) 8.1 Hz), 7.18-7.13 (1H, m), 6.84 (1H, d, J )
7.5 Hz), 6.73 (1H, td, J ) 9.98, 1.3 Hz), 6.63-6.52 (2H, m), 6.17
(1H, t, J ) 8.4 Hz), 2.51 (6H, s), 0.78 (9H, s). 13C NMR (75 MHz,
CDCl3): δ 155.6, 154.6, 151.6, 139.0, 138.2, 130.3, 130.0, 129.2, 127.4,
127.3, 125.4, 124.2, 123.4, 121.1, 116.5, 114.7, 52.6, 45.6, 28.81. IR
(KBr, cm-1): 3197, 2981, 2788, 1598, 1518, 1275, 1223, 1190, 1123,
946, 868, 796, 770, 745, 682, 628, 598, 590, 573, 472. UV-vis (CH2-
Cl2) λ, nm (ꢀ, M-1 cm-1): 263 (36 600), 359 (17 700), 433 (21 500).
Mp: 201-202 °C. Anal. Calcd for C23H27N3SO2: C, 67.45; H, 6.65;
N, 10.26. Found: C, 67.12; H, 6.44; N, 10.13.
683, 626, 609, 590, 570, 550, 494, 470. UV-vis (CH2Cl2) λ, nm (ꢀ,
M-1 cm-1): 254 (59 500), 366 (29 400), 405 (14 400). Dec: 184-
187 °C. Anal. Calcd for ZnC44H48N6S2O4‚C6H5Cl (the X-ray crystal
structure and the 1H NMR spectrum both confirm the presence of one
molecule of chlorobenzene per Zn in the material): C, 62.11; H, 5.52;
N, 8.69. Found: C, 61.82; H, 5.62; N, 8.54.
[Co(t-BuDATI)2] (14). The preparation of 14 followed a procedure
similar to that described for 12 using KH (24 mg, 0.60 mmol), 9 (223
mg, 0.55 mmol), and CoCl2 (39 mg, 0.30 mmol). X-ray-quality crystals
were grown from pentane diffusion into a 1:1 CH2Cl2/THF solution of
the complex. Yield: 107 mg, 45%. IR (KBr, cm-1): 2973, 2766, 1610,
1579, 1502, 1477, 1438, 1400, 1311, 1291, 1203, 1129, 1057, 864,
787, 744, 628, 590, 570, 479. UV-vis (CH2Cl2) λ, nm (ꢀ, M-1 cm-1):
254 (57 700), 361 (27 300). Dec: 209-217 °C. Anal. Calcd for
CoC46H52N6S2O4: C, 63.07; H, 5.98; N, 9.59. Found: C, 63.36; H,
6.13; N, 9.68.
[Co(BzDATI)2] (15). The preparation of 15 followed a procedure
similar to that described for 12 using KH (53 mg, 1.33 mmol), 10 (537
mg, 1.21 mmol), and CoCl2 (86 mg, 0.67 mmol). X-ray-quality crystals
were grown from pentane diffusion into a 1:1 CH2Cl2/THF solution of
the complex. Yield: 660 mg, 60%. IR (KBr, cm-1): 2948, 2827, 1609,
1576, 1508, 1452, 1407, 1314, 1288, 1212, 1128, 1058, 868, 787, 745,
697, 628, 572, 479. UV-vis (CH2Cl2) λ, nm (ꢀ, M-1 cm-1): 253
(59 500), 363 (27 600). Dec: 155-160 °C. Anal. Calcd for
CoC52H48N6S2O4: C, 66.16; H, 5.12; N, 8.90. Found: C, 66.17; H,
5.21; N, 8.52.
1,4-Bis((N-(4-methoxybenzyl)-N′-troponimino)butane (16). A por-
tion of “4-dimer-tropolone” (10.2 g, 34 mmol) was treated with 2.1
equiv of finely divided Me3OBF4 (10.7 g, 72 mmol) in 100 mL of
CH2Cl2 under an N2 atmosphere. After the mixture was stirred
overnight, a cream-colored precipitate was collected by filtration and
rinsed with ether. The solid was taken up in 15 mL of CHCl3, and the
solution was treated with 2 M aqueous NaOH (3×). The mixture was
dried over MgSO4 and filtered, and the filtrate was evaporated to
dryness. The resulting solid was redissolved in 200 mL of EtOH, and
(4-methoxybenzyl)amine (8.62 mL, 66 mmol) was added via syringe.
The reaction mixture was stirred overnight under N2. Then the solvent
volume was reduced by half in vacuo, and a light yellow precipitate
was collected by filtration and washed with diethyl ether. Yield: 13.8
g, 76%. TLC: Rf ) 0.35 (5:1 hexane/ethyl acetate + 10% Et3N, Si2O3).
1H NMR (250 MHz, C6D6): δ 7.21 (4H, d, J ) 8.5 Hz), 6.81 (4H, d,
J ) 8.5 Hz), 6.71-6.59 (4H, m), 6.27 (2H, d, J ) 11.0 Hz), 6.19-
6.09 (4H, m), 4.32 (4H, s), 3.29 (6H, s), 3.02-2.98 (4H, m), 1.60-
1.55 (4H, m). IR (KBr, cm-1): 2935, 1613, 1593, 1535, 1510, 1479,
1461, 1438, 1409, 1390, 1293, 1277, 1246, 1168, 1104, 1037, 996,
883, 814, 744, 609. Mp: 126-131 °C. HRMS (FAB (+)), m/z: calcd
for C34H38N4O2, 534.2994; found, 534.2983(17).
1,4-Bis(N-amino-N′-troponimino)butane (17). A portion of 16 (5.2
g, 9.7 mmol) was slowly added to 8 mL of rapidly stirring trifluoroacetic
acid chilled over an ice-water slush bath. The resulting red solution
was stirred for 6 h with slow warming to ambient temperature. The
reaction was quenched with ice, the sample was treated with 20 mL of
aqueous 2 M NaOH, and the mixture was extracted into 20 mL of
CHCl3 (3×). The brown solution was dried over Na2CO3, filtered, and
evaporated. Recrystallization from boiling CH2Cl2/diethyl ether yielded
2.15 g (75%) of 17 as a light orange, microcrystalline powder. 1H NMR
(250 MHz, C6D6): δ 6.59 (2H, t, J ) 10.1 Hz), 6.36 (2H, t, J ) 10.0
Hz), 6.19 (2H, d, J ) 11.1 Hz), 6.05 (2H, t, J ) 9.3 Hz), 5.94 (2H, d,
J ) 10.4 Hz), 2.82-2.77 (4H, m), 1.40-1.35 (4H, m). IR (KBr, cm-1):
3453, 3291, 2941, 1608, 1591, 1507, 1480, 1461, 1397, 1344, 1243,
1130, 1087, 1033, 876, 855, 813, 753, 697, 604. Mp: 133-141 °C.
HRMS (FAB (+)), m/z: calcd for C18H22N4, 294.1844; found,
294.1838(9).
H2DATI-4 (18). To a 200-mL solution of 17 (1.00 g, 3.4 mmol) in
freshly distilled THF at 4 °C was added dropwise a portion of n-BuLi
(4.46 mL, 7.13 mmol, 1.6 M solution in hexanes), causing the yellow-
orange solution to darken to brownish yellow. After 2 h of stirring,
this solution was transferred dropwise via cannula to a 150-mL THF
solution of dansyl chloride (2.3 g, 8.5 mmol), also at 4 °C. The resulting
brown solution was dried in vacuo after 4 h, and the residue was treated
with aqueous NaHCO3, followed by extraction into CHCl3 (3×). The
HBzDATI (10). This compound was obtained from 7 by a procedure
analogous to that described for the synthesis of 8. The product was
purified by flash column chromatography on silica gel (2:1 hexane/
ethyl acetate + 10% Et3N; Rf ) 0.24) to give 1.7 g (32%) of 10 as a
1
yellow-orange powder. H NMR (300 MHz, C6D6): δ 9.21 (1H, d, J
) 8.7 Hz), 8.85 (1H, dd, J ) 8.4, 1.3 Hz), 8.74 (1H, d, J ) 11.1 Hz),
8.39 (1H, d, J ) 8.1 Hz), 8.11 (1H, br s, NH), 7.32 (1H, t, J ) 8.1
Hz), 7.20-7.16 (1H, m), 6.97-6.95 (3H, m), 6.82 (1H, d, J ) 6.9
Hz), 6.73-6.66 (3H, m), 6.40 (1H, t, J ) 9.4 Hz), 6.07 (1H, t, J ) 9.4
Hz), 6.01 (1H, d, 10.8 Hz), 3.46 (2H, d, J ) 6.3 Hz), 2.48 (6H, s). 13
C
NMR (75 MHz, CDCl3): δ 156.0, 155.2, 151.7, 139.2, 138.6, 138.5,
135.6, 130.4, 130.1, 129.5, 129.2, 128.1, 127.9, 127.5, 127.0, 126.2,
125.0, 123.4, 121.1, 115.6, 114.9, 47.3, 45.6. IR (KBr, cm-1): 3276,
2939, 2825, 1601, 1524, 1434, 1346, 1228, 1211, 1125, 864, 793, 735,
620, 590, 574, 505. UV-vis (CH2Cl2) λ, nm (ꢀ, M-1 cm-1): 261
(34 100), 357 (16 100), 431 (20 500). Mp: 168-170 °C. Anal. Calcd
for C26H25N3SO2: C, 70.40; H, 5.68; N, 9.47. Found: C, 70.51, H,
5.96; N, 8.87.
[Co(i-Pr2ATI)2] (11). A portion of NaH (80 mg, 2.0 mmol of a
60% dispersion in mineral oil) was washed with hexanes and added to
a 15-mL yellow THF solution of H-i-Pr2-ATI (350 mg, 1.71 mmol).
Effervescence subsided after stirring under N2 for 1 h. Anhydrous CoCl2
(129.84 mg, 1.0 mmol) was added, causing the orange solution rapidly
to turn dark brown. After 4 h of stirring, the solvent was removed in
vacuo, the product was extracted into CH2Cl2, the extract was filtered,
and the filtrate was dried in vacuo. Dark brown rhombic plates suitable
for X-ray crystallography were grown from slow evaporation of Et2O.
Yield: 310 mg, 78%. IR (KBr, cm-1): 2966, 2925, 2863, 2345, 1686,
1654, 1585, 1497, 1468, 1416, 1379, 1362, 1305, 1265, 1225, 1168,
1151, 1121, 1072, 1019, 884, 801, 766, 670, 531. UV-vis (CH2Cl2)
λ, nm (ꢀ, M-1 cm-1): 275 (44 000), 388 (36 000), 410 (37 000). Dec:
140-145 °C. Anal. Calcd for CoC26H38N4: C, 67.08; H, 8.23; N, 12.03.
Found: C, 67.06; H, 8.28; N, 11.97.
[Co(i-PrDATI)2] (12). A portion of KH (48 mg, 1.19 mmol) was
added to a 10-mL THF slurry of 8 (430 mg, 1.09 mmol) under N2. A
clear, orange solution was obtained after the effervescence had subsided,
about 45 min. The color immediately turned dark amber upon the
addition of anhydrous CoCl2 (77.6 mg, 0.60 mmol). The solvent was
evacuated after 4 h of stirring, the product was extracted into CH2Cl2,
and the extract was filtered through Celite. X-ray-quality crystals were
grown from pentane diffusion into a 1:1 CH2Cl2/THF solution of the
complex. Yield: 363 mg, 80%. IR (KBr, cm-1): 2971, 2788, 1612,
1579, 1507, 1477, 1449, 1312, 1284, 1206, 1129, 1059, 867, 791, 741,
627, 569, 498, 471. UV-vis (CH2Cl2) λ, nm (ꢀ, M-1 cm-1): 256
(55 900), 359 (28 200). Dec: 185-188 °C. Anal. Calcd for
CoC44H48N6S2O4: C, 62.32; H, 5.71; N, 9.91. Found: C, 62.47; H,
5.56; N, 9.78.
[Zn(i-PrDATI)2] (13). A portion of KH (22 mg, 0.56 mmol) was
added to a 10-mL THF slurry of 8 (202 mg, 0.51 mmol) under N2. A
clear, orange solution was obtained after the effervescence had subsided,
about 45 min. The color lightened to yellow-orange upon the addition
of anhydrous ZnCl2 (38.2 mg, 0.28 mmol). The workup followed the
procedure described for 12. X-ray-quality crystals were grown from
diethyl ether diffusion into a chlorobenzene solution of the complex.
Yield: 280 mg, 64%. 1H NMR (300 MHz, C6D6): δ 9.34 (2H, d, J )
8.4 Hz), 8.36 (2H, d, J ) 7.5 Hz), 7.90 (2H, d, J ) 8.7 Hz), 7.73 (2H,
d, J ) 10.2 Hz), 7.27 (2H, t, J ) 8.1 Hz), 6.68 (4H, m), 6.43 (4H, m),
6.21 (2H, d, J ) 10.0 Hz), 5.94 (2H, t, J ) 9.0 Hz), 3.75 (2H, m, J )
6.6 Hz), 2.36 (12H, s), 1.69 (6H, d, J ) 6.6 Hz), 1.27 (6H, d, J ) 6.3
Hz). IR (KBr, cm-1): 2972, 2869, 2786, 1615, 1581, 1513, 1481, 1452,
1409, 1315, 1286, 1211, 1129, 1060, 913, 877, 855, 794, 742, 703,

4092 Inorganic Chemistry, Vol. 39, No. 18, 2000
Franz et al.
organic phase was dried over Na2CO3, filtered, and dried in vacuo.
The brown-orange powder obtained was stirred in 50 mL of THF for
1 h, collected by filtration, and washed with 20 mL of diethyl ether
(2×) to give a mustard yellow powder. The product was purified by
flash chromatography on deactivated alumina (3:2 ethyl acetate/hexanes,
mV/s, revealing linear plots of anodic and cathodic peak currents versus
the square root of the scan rate, as expected for reversible couples.
The peak-to-peak ∆Ep separations were around 100 mV, similar to the
value obtained for the Cp2Fe/Cp2Fe+ couple under the same conditions.
X-ray Crystallography. Single crystals were mounted on quartz
fibers with Paratone N (Exxon) and transferred rapidly to the -85 °C
cold stream of a Bruker (formerly Siemens) CCD X-ray diffraction
system with graphite-monochromatized Mo KR radiation (λ ) 0.710 73
Å) controlled by a pentium-based PC running the SMART software
package.73 The temperature of the crystals was maintained with a Bruker
LT-2A nitrogen cryostat. The general procedures for data collection
and reduction followed those reported elsewhere.74 Empirical absorption
corrections were calculated and applied by the SADABS program.75
Structures were solved by the direct methods program XS, and
refinements were carried out with XL, both part of the SHELXTL
program package.76 Non-hydrogen atoms were refined by a series of
least-squares cycles. Hydrogen atoms were assigned idealized positions
and given thermal parameters that were 1.2 times those of the carbon
atoms to which they were attached. All structure solutions were checked
for higher symmetry with the PLATON program.77 Enantiomorph
checks were carried out by using the Flack parameter refinement in
the XL program.78 A disordered molecule of CH2Cl2 in the structure
of 12‚CH2Cl2 was modeled over three positions with a ratio of 20:50:
30 and refined with appropriate restraints in XL.76 In the structure of
13‚C6H5Cl, a molecule of chlorobenzene was modeled as disordered
over two positions with a ratio of 50:50 and refined with appropriate
restraints. The solvent disorder in the structure containing 15 was
successfully modeled as 0.5 CH2Cl2 and 0.5 THF and refined with
appropriate restraints. Crystallographic data for 11, 12, 14, and 15 are
summarized in Table 1; the data for 19 were published previously.32
Selected bond distances and angles for 11, 12, 14, 15, and 19 are given
in Table 2, and ORTEP plots showing 50% thermal ellipsoids are
displayed in Figure 1 and Figures S2, S3, S5, and S6 (Supporting
Information). Crystallographic data and ORTEP plots for 9 and 13 are
also included as Supporting Information in Table S1 and Figures S1
and S4, respectively.
1
10% MeOH, 10% Et3N, Si2O3; Rf ) 0.26). Yield: 663 mg, 26%. H
NMR (300 MHz, CDCl3): δ 8.53 (2H, d, J ) 9 Hz), 8.43 (4H, t, J )
8.3 Hz), 8.11 (2H, d, J ) 11.1 Hz), 7.99 (1H, br s, NH), 7.54 (2H, t,
J ) 8.0 Hz), 7.37-7.28 (6H, m), 7.04 (2H, d, J ) 7.5 Hz), 6.84 (2H,
t, J ) 9.6 Hz), 6.66 (2H, d, J ) 10.8 Hz), 3.22 (4H, m), 2.81 (12H, s),
1.58 (4H, m). 13C NMR (75 MHz, CDCl3): δ 155.3, 154.9, 151.6,
139.1, 138.6, 138.4, 130.2, 129.9, 129.4, 127.7, 127.3, 125.9, 124.6,
123.4, 120.8, 115.0, 114.8, 139.1, 138.6, 138.4. IR (KBr, cm-1): 3280,
2937, 2790, 1600, 1516, 1476, 1433, 1358, 1285, 1199, 1129, 1093,
1061, 1044, 988, 939, 856, 794, 738, 711, 680, 623, 600, 572, 541,
491, 462. HRMS (FAB (+)), m/z: calcd for C42H44N6S2O4, 760.2865;
found, 760.2887(21).
[Co(NO)2(RDATI)] (20; a ) i-Pr, b ) t-Bu, c ) Bz). The
procedures for 20b and 20c were analogous to the one described here
for 20a. In a glovebox, a portion of [Co(i-PrDATI)2] (12) (50 mg, 0.059
mmol) was dissolved in 10 mL of wet CH2Cl2 (1.8 µL of H2O, 1.8
equiv) under an N2 atmosphere in a 25-mL flask equipped with a
gastight Teflon stopcock. The resulting red-brown solution was purged
with NO for 10 min. No apparent color change was detected. The flask
was sealed, and the contents were stirred for 1 week, after which the
solvent was evacuated to dryness and the flask was returned to the
N2-filled glovebox. The 1H NMR spectrum of the crude reaction mixture
exhibited resonances for Hi-PrDATI along with a new set of diamagnetic
1
resonances assigned to 20a in a 1:1 ratio for a 75% yield. H NMR
(300 MHz, C6D6): δ 9.00 (1H, d, J ) 8.7 Hz), 8.75-8.69 (1H, m),
8.54 (1H, d, J ) 7.5 Hz), 8.40-8.32 (1H, m), 7.94 (1H, d, J ) 10.2
Hz), 7.85 (br s, NH), 7.36-7.32 (2H, m), 6.73 (1H, d, J ) 6.9 Hz),
6.40 (1H, t, J ) 9.5 Hz), 6.31-6.23 (1H, m), 6.00-5.94 (1H, m), 3.43-
3.35 (1H, m), 2.41 (6H, s), 0.64 (6H, d, J ) 6.0 Hz). IR (KBr, cm-1):
1826 (νNO), 1751 (νNO), 1579, 1576, 1507, 1448, 1355, 1313, 1286,
1208, 1129, 1059, 869, 790, 746, 626, 572, 496, 472. IR (CH2Cl2,
cm-1): 1838 (νNO), 1760 (νNO).
[Co(NO)2(i-Pr2ATI)] (21). A portion of 11 (36 mg, 0.077 mmol)
was dissolved in 5 mL of THF in a 25-mL, thick-walled, round-bottom
flask equipped with a screw-top Teflon stopcock and a sidearm for
attachment to a high-vacuum manifold. The resulting red-brown solution
was subjected to three freeze-pump-thaw cycles before NO gas (0.34
mmol, 4.4 equiv) was transferred under reduced pressure to the solution.
Submerging the reaction flask in a liquid N2 bath facilitated the complete
transfer of NO. No apparent color change was observed. 1H NMR (250
MHz, C6D6): δ 6.83 (2H, t, J ) 10.0 Hz), 6.42 (2H, d, J ) 11.2 Hz),
6.23 (1H, t, J ) 10.2 Hz), 3.80 (2H, m, J ) 6.1 Hz), 0.91 (12H, d, J
) 6.2 Hz). Dark red-black crystals of 21 were grown from slow
evaporation of diethyl ether in a glovebox. IR (KBr, cm-1): 1810, 1734
(νNO), 1591, 1507, 1416, 1382, 1269, 1230, 1171, 1121, 1072, 966,
815, 722, 468. Because of the mixture of products (21 and H-i-Pr2-
ATI) present, elemental analysis was not performed.
Electrochemistry. Cyclic voltammograms were recorded with an
EG&G model 263 potentiostat. The standard three-electrode setup
consisted of an Ag/AgNO3 reference electrode (0.01 M in acetonitrile),
a platinum wire auxiliary electrode, and a 1.75 mm2 platinum disk
working electrode. Samples were measured at ambient temperature
under a blanket of Ar in freshly distilled THF with 0.5 M (Bu4N)-
(ClO4) as supporting electrolyte. All spectra were externally referenced
to the ferrocene/ferrocenium (Cp2Fe/Cp2Fe+) couple under identical
conditions, for which E1/2 ) 133 mV vs Ag/AgNO3 in THF. Scan
rate profiles were acquired for each sample, at scan speeds of 50-100
Acknowledgment. This work was supported by a grant from
the National Science Foundation and a fellowship to N.S. from
the Undergraduate Research Opportunity Program of MIT. B.S.
thanks the Swiss National Science Foundation for postdoctoral
support.
Supporting Information Available: Figures S1-S6, displaying
fully labeled ORTEP diagrams for 9, 11, 12, 13, 14, and 15,
respectively. Figures S7 and S8, showing fluorescence spectra of 8
and 13, respectively, at varying concentrations, Figures S9 and S10,
displaying fluorescence spectra of 12 after addition of H+ and
Hi-PrDATI, respectively, and Figure S11, showing the fluorescence
spectra of 19 after the additions of incremental amounts of NO. X-ray
crystallographic files in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org.
IC000344Q
(73) SMART, Version 5.05; Bruker AXS, Inc.: Madison, WI, 1998.
(74) Feig, A. L.; Bautista, M. T.; Lippard, S. J. Inorg. Chem. 1996, 35,
6892-6898.
(75) Sheldrick, G. M. SADABS: Area Detector Absorption Correction;
University of Go¨ttingen: Go¨ttingen, Germany, 1996.
(76) Sheldrick, G. M. SHELXL97-2: Program for the Refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(77) Spek, A. L. PLATON: A Multipurpose Crystallographic Tool; Utrecht
University: Utrecht, The Netherlands, 1998.
(78) Flack, H. D. Acta Crystallogr., Sect. A 1983, A39, 876-881.