The first "kuhn verdazyl" ligand and comparative studies of its PdCl2 complex with analogous 6-oxoverdazyl ligands

Article abstract of DOI:10.1039/c3dt52191e

The synthesis and characterization of two new N,N′-diarylverdazyl radical ligands and their corresponding PdCl₂ complexes are described. One of the two radicals is of the "Kuhn verdazyl" structure type and was made by adaptation of standard syn

Full text of DOI:10.1039/c3dt52191e

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The first “Kuhn verdazyl” ligand and comparative
studies of its PdCl2 complex with analogous
Cite this: DOI: 10.1039/c3dt52191e
6-oxoverdazyl ligands†
Cooper W. Johnston,a Stephen D. J. McKinnon,a Brian O. Patrickb and
Robin G. Hicks*a
The synthesis and characterization of two new N,N’-diarylverdazyl radical ligands and their corresponding
PdCl2 complexes are described. One of the two radicals is of the “Kuhn verdazyl” structure type and was
made by adaptation of standard synthetic procedures for this class of verdazyl. The N,N’-diphenyl-6-oxo-
verdazyl was prepared by hydrolysis of a related tetrazane; the resulting N,N’-diphenylcarbohydrazide
was condensed with pyridinecarboxaldehyde and then oxidized to the verdazyl according to standard
protocols. Square planar PdCl2 complexes of both verdazyls were prepared by reactions of the radicals
with PdCl2 in acetonitrile solution. The structural, spectroscopic, and electrochemical properties of the new
verdazyl ligands and their Pd complexes are reported; generally the distinct ligand-centred properties
associated with each verdazyl type carry over into the properties of the complexes. The electrochemical
studies reveal ligand-centred oxidation and reduction processes; despite the minimal extent of spin de-
localization onto Pd in the metal complexes, large shifts in oxidation and reduction potentials (relative to
those of the free verdazyl ligands) are discussed.
Received 11th August 2013,
Accepted 21st September 2013
DOI: 10.1039/c3dt52191e
www.rsc.org/dalton
1,5-dialkyl-6-oxoverdazyls 1 in which the C3 substituent (R′)
has a donor site (e.g. pyridine, imidazole) which creates a che-
Introduction
The ‘spin-active’ and redox-active nature of radical ligands has lating site for metals (e.g. 1a). Coordination complexes of other
drawn attention to the magnetic, redox, and chemical reacti- verdazyl subclasses – 1,5-diaryl-6-oxoverdazyls 2 and 1,3,5-
vity of metal–radical complexes. Radicals such as phenoxyls1 triaryl verdazyls 3 containing a saturated carbon at C6 (“Kuhn”
and nitroxides2 have a rich coordination chemistry, and com- verdazyls, so named here after their discoverer17) are as yet
plexes of other radical types (e.g. thiazyls,3 aminyls4,5) are cur- unknown. At first glance the structural differences between 1,
rently under development. Among the various known classes 2 and 3 may seem to be relatively minor (particularly between
of stable radicals,6 verdazyls are notable as the only radical 1 and 2 which differ in the nature (alkyl vs. aryl) of the N-sub-
type whose general stability rivals that of the more well-known stituents). However, the physicochemical properties of verdazyl
nitroxides.7 The coordination chemistry of verdazyls has flou- are sensitive to their substituents. For example, whereas most
rished in the past 15 years, motivated principally by interest in radicals of general structure 1 are orange, 1,5-diaryl-6-oxover-
the magnetic properties of metal–verdazyl complexes.8–13 More dazyls 2 are deep red and 1,3,5-triaryl verdazyls 3 are green.
recently the redox activity of verdazyls14 and their metal com- The redox properties of each verdazyl type are also distinc-
plexes15,16 has been revealed, highlighting these radicals as a tive;14 the oxidation and reduction potentials of 1–3 both span
new entry into the (expanding) redox-active ligand family.
a potential range of over 0.5 V. Derivatives of 3 are generally
Verdazyl radicals can be subdivided into three broad struc- the easiest to oxidize and hardest to reduce and derivatives of
ture types. All known metal–verdazyl complexes are based on 2 are relatively difficult to oxidize and easy to reduce. The
chemical properties of different verdazyl types are also distinct
from one another. For example, whereas derivatives of 1 can
be used in controlling radical polymerization,18 Kuhn
verdazyls 3 are ineffective.19 We became interested in exploring
how the differences in steric profiles and physicochemical pro-
perties of verdazyls of type 1–3 would be expressed in their
respective coordination complexes. In particular the coordi-
nation chemistry of Kuhn verdazyls 3 in which the tetrazine
aDepartment of Chemistry, University of Victoria, PO Box 3065 STN CSC, Victoria,
BC V8W 3V6, Canada. E-mail: rhicks@uvic.ca
bCrystallography Laboratory, Department of Chemistry, University of British
Columbia, Vancouver, BC V6 T 1Z1, Canada
†Electronic supplementary information (ESI) available: Crystallographic data (cif
format) for 2a, 2a·PdCl2 and 3a·PdCl2. CCDC 943810–943812. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/c3dt52191e
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heterocycle is directly bound to a metal are unknown,
although “spin labelled” complexes in which Kuhn verdazyls
are remotely attached to a ligand have been reported.20 Herein
we report the first transition metal complexes of 1,5-diaryl-
6-oxoverdazyl 2a and Kuhn verdazyl 3a and compare the pro-
perties of these complexes with those of the analogous 1,5-
dialkyl-6-oxoverdazyl ligand 1a.10
Results and discussion
Ligand and complex synthesis
Scheme 2 Synthesis of N,N’-diphenyl-6-oxoverdazyl 2a and its corresponding
Ligand 1a9 and its PdCl2 complex10 were made as previously
reported. Kuhn verdazyl 3a was made by adaptation of estab-
lished protocols for the synthesis of this class of verdazyl
(Scheme 1); thus, reaction of formazan 4 with formaldehyde
under basic conditions yielded radical 3a as a grass-green
solid which is very acid-sensitive (Kuhn verdazyls are prone to
acid-induced disproportionation).7 This sensitivity may con-
tribute to our inability to obtain adequate microanalytical data
for this radical. Reaction of 3a with PdCl2 in hot acetonitrile
gave 3a·PdCl2 as a dark green crystalline solid.
The precursor to desired verdazyl 2a, i.e. tetrazane 7, could
not be made using the conventional Milcent-based procedure
(i.e., careful reaction of phosgene with a hydrazine, followed by
treatment of the chloroformylhydrazone intermediate with a
monosubstituted hydrazine21,22). We then targeted N,N′-diphe-
nyl carbohydrazide 6 as a precursor to tetrazane 7, by analogy
to the conventional syntheses of N,N′-dialkyl 6-oxotetra-
zanes.7,23 A recent report claimed that desired N,N′-diphenyl
carbohydrazide 6 could be made by metal-catalyzed bis-aryla-
tion of carbohydrazide (H2NNHC(O)NHNH2),24 but these
results could not be reproduced in our hands. We were able
to make carbohydrazide 6 by hydrolysis of tetrazane 522
(Scheme 2). Subsequent condensation of 6 with 2-pyridine-
carboxaldehyde gave tetrazane 7 which was then oxidized with
silver oxide/celite to afforded the desired radical as a dark red
compound. The PdCl2 complex of this radical was made using
the same procedures as for 3a·PdCl2.
PdCl2 complex.
X-ray structures
The structures of ligand 1a9 and its PdCl2 complex10 has been
reported. We have been able to obtain X-ray quality crystals of
ligand 2a, but not 3a. The structure of 2a is shown in Fig. 1.
Bond parameters within the tetrazine ring are typical for
1,5-diaryl-6-oxoverdazyls.21,24,25 Torsion angles of the aromatic
substituents with respect to the tetrazine ring are 36.3° (for
the N-phenyl ring attached to N2), 33.1° (N-phenyl ring N3)
and 8.1° (for the 2-pyridyl attached to C2).
The structures of 2a·PdCl2 and 3a·PdCl2 are presented in
Fig. 2 and 3 respectively; selected bond lengths for these com-
plexes, along with those of 1a·PdCl2 (provided for comparative
purposes) are found in Table 1. All three complexes consist of
a square-planar Pd(II) ion chelated by a verdazyl radical
in the N,N′-bidentate mode. It is worth noting that the
pendant N-phenyl groups do not cyclometallate in either case.
This contrasts the norm in square planar Pd(II) complexes
containing bipyridine ligands with ortho-phenyl substituents,
in which the ligand adopts an NNC tridentate binding mode
i.e., 8.26
Fig. 1 Structure of 2a. Hydrogen atoms removed for clarity. Thermal ellipsoids
drawn at 50% probability. Selected bond lengths (Å) C1–O1 1.225(5); C1–N2
1.363(5); C1–N3 1.389(5); C2–N4 1.333(5); C2–N1 1.337(5); C2–C15 1.477(5);
N1–N2 1.369(5); N3–N4; 1.376(5).
Scheme 1 Synthesis of Kuhn verdazyl 3a and its PdCl2 complex.
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contacts in the solid state (Fig. 2), whereas 3a·PdCl2 (and
1a·PdCl2) do not, although PdCl2 complexes of other deriva-
tives of 1 do adopt this dimeric motif. The presence/absence
of weak Pd–Pd interactions is presumably influenced by inter-
molecular packing effects.
The effect of the N3-phenyl substituent on the structures of
the complexes of 2a and 3a is substantial. In 1a·PdCl2 the
N-isopropyl group adjacent to the palladium ion adopts an
orientation to minimize its steric interactions with PdCl2 unit,
which results in ligand 1a possessing an essentially planar
conformation as bound to Pd.10 For 2a·PdCl2 the N-phenyl
group forces a slight torsion of the tetrazine ring relative to the
pyridine (18.2°) but the trivalent nitrogen (N3) remains essen-
tially planar (sum of angles = 359.3°). In 3a·PdCl2 the tetrazine
and pyridine rings remain coplanar, but the geometry at N3 is
modestly pyramidalized (349°) (the remote trivalent nitrogen,
N2, is trigonal planar (359.9°)). As a result the N3–C(Ph) bond
is tilted by 59.8° with respect to the plane defined by the
conjugated (N2–N1–C2–N3–N3) subunit of the tetrazine ring.
The displacement of the saturated carbon C1 from this plane
is a normal structural feature of Kuhn verdazyls.7
Fig. 2 Structure of 2a·PdCl2. Left: partial atom labelling scheme. Right: perpen-
dicular view showing Pd–Pd interaction. Hydrogen atoms removed for clarity.
Thermal ellipsoids drawn at 50% probability.
EPR spectroscopy
We previously reported the solution EPR spectra of ligand 1a
and its PdCl2 complex.10 The spectra of 2a and 2a·PdCl2 are
presented in Fig. 4, while the spectra of 3a and 3a·PdCl2 are
shown in Fig. 5. Spectral data for all ligands and complexes
are compiled in Table 2. The hyperfine coupling constants
(obtained from spectral simulation) for the two ligands 2a and
3a are representative of each verdazyl structure type. The spin
distribution is found predominantly on the four tetrazine
nitrogen atoms. Previous EPR/ENDOR studies on related diaryl
verdazyls have identified small but non-negligible spin density
on the N-phenyl substituents,21,27 but the magnitude of the
coupling to the aromatic protons is too small to be observed in
the present compounds.
Fig. 3 Structure of 3a·PdCl2. Left: partial atom labeling scheme. Right: perpen-
dicular view. Hydrogen atoms removed for clarity. Thermal ellipsoids drawn at
50% probability.
Table 1 Selected bond lengths for Pd–verdazyl complexes
10
Bond
1a·PdCl2
2a·PdCl2
3a·PdCl2
Pd1–N4
Pd1–N5
N3–N4
N1–N2
C2–N1
C2–N4
C1–O1
2.064(2)
2.029(2)
1.374(3)
1.353(3)
1.313(3)
1.341(4)
1.211(4)
2.064(7)
2.076(7)
1.348(10)
1.358(11)
1.322(13)
1.371(12)
1.220(12)
2.0253(12)
2.0310(13)
1.3813(17)
1.3353(18)
1.3361(19)
1.3413(19)
—
The bond metrics within the verdazyl ring of the complexes
are broadly comparable (Table 3); all three complexes show
desymmetrization of the C2–N1/C2–N4 bonds in the ligand to
varying degrees, with the CN bond adjacent to Pd being
slightly longer than the remote CN bond. There are also small
perturbations in the NN bonds in all cases, but all fall within
ranges seen for other metal–verdazyl complexes. 2a·PdCl2
Fig. 4 (a) Experimental (blue) and simulated (red) EPR spectra of 2a in CH2Cl2
solution at room temperature. (b) Experimental (blue) and simulated (red) EPR
spectra of 2a·PdCl2 in CH2Cl2 solution at room temperature. Microwave frequen-
adopts a dimeric structure in the solid state via weak Pd–Pd cies for 2a and 2a·PdCl2 are 9.8457 and 9.8462 GHz, respectively.
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Fig. 6 Electronic spectra of 1a (black line), 2a (red line), and 3a (green line) in
MeCN.
Fig. 5 (a) Experimental (blue) and simulated (red) EPR spectra of 3a in CH2Cl2
solution at room temperature. (b) Experimental spectra of 3a·PdCl2 in CH2Cl2
solution. Microwave frequencies for 3a and 3a·PdCl2 are 9.8464 and 9.8470 GHz,
respectively.
Table 2 EPR parameters
2a
2a·PdCl2
3a
3a·PdCl2
g
2.0028
4.57
4.57
6.49
6.49
2.0087
4.74
4.79
5.76
8.04
2.0042
5.78
5.78
5.87
5.87
2.0120
—
—
—
—
a(N)
a(N)
a(N)
a(N)
Fig. 7 Electronic spectra of 1a·PdCl2 (black line), 2a·PdCl2 (red line), and
3a·PdCl2 (green line). Spectra recorded in CH2Cl2 solution.
spectra of the PdCl2 complexes of 1a and 3a are, to a good
approximation, red-shifted from the spectra of the corres-
ponding free ligand. However the spectrum of 2a·PdCl2 con-
sists of a multiple absorption bands between 600–700 nm,
some of which may be charge-transfer in nature.
The EPR spectra of the Pd complexes of 2a and 3a show the
expected shift in g-value which arises from spin density on the
palladium nucleus (see below). The g-values of the complexes
are similar to those of other verdazyl–Pd complexes.10 Consist-
ent with the EPR spectra of verdazyl complexes of other metal
ions,12 metal ion coordination causes modest changes in the
spin distribution in the tetrazine ring of the verdazyl for
2a·PdCl2. As we found previously for other Pd–verdazyl com-
plexes,10 any coupling to the palladium nucleus (105Pd (22.3%
abundance), I = 5/2) is too small to be observed in the
spectrum of 2a·PdCl2. Unfortunately, no fine structure is
evident at all in the spectrum of 3a·PdCl2, precluding detailed
analysis. However the magnitudes of the shift in g-value of all
three Pd complexes are similar, suggesting similar (small)
magnitudes of spin density on the Pd ion for each case.
Electrochemistry
The cyclic voltammograms of ligands 1a, 2a, and 3a and their
corresponding PdCl2 complexes are presented in Fig. 8: perti-
nent electrochemical parameters are compiled in Table 3. The
voltammograms of all three ligands contain reversible oxi-
dation and reduction processes. The oxidation and reduction
potentials for each fall within the normal range for the general
verdazyl structure type (1, 2, or 3) of which they are representa-
tives. Thus, the Kuhn verdazyl 3a is considerably easier to
oxidize and harder to reduce than the two oxoverdazyls 1a and
2a which we have previously rationalized as arising from the
Electronic spectroscopy
The UV-visible spectra of ligands 1a, 2a, and 3a are presented electron-withdrawing carbonyl groups in the latter two.14 The
in Fig. 6. The spectra of the three ligands have distinct low- difference between oxidation and reduction potentials of a
28,29
energy absorption maxima (410, 518, and 693 nm, respectively) given radical – known as its cell potential, Ecell
– have also
which are characteristic of each verdazyl type (1, 2 or 3). The been previously discussed14 for general derivatives of 1, 2, and
significant spectral differences between 1a and 2a clearly 3: qualitatively the values of Ecell correlate with the degree of
implicate the involvement of the N-aryl groups (the only struc- delocalization. The two N,N′-diaryl verdazyls (2a and 3a), in
tural difference between 2a and 1a) in the lowest energy tran- which spin delocalization onto the N-aryl rings is evident, have
sitions. The UV-visible spectra of the three Pd complexes are somewhat smaller Ecell values than that of 1a (Table 3), which
shown in Fig. 7. The lowest energy transitions in the electronic does not have N-aryl substituents. The differences between 2a
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with the extent of delocalization (basically, the size/space of
the conjugated framework encompassing spin density) of an
organic radical.29 The spectroscopic data for the Pd complexes
presented herein suggest minimal spin delocalization onto the
Pd ion, yet the complexes have much smaller Ecell values than
the free ligands. The origin of the effect of the Pd ion on the
verdazyl ligand’s redox properties may, therefore, be inductive
in nature – an effect with differing magnitude of consequences
for the reduction and oxidation processes of the verdazyl.
Conclusions
Herein we have presented two new verdazyl radical ligands,
thereby expanding the toolkit of verdazyl coordination chemi-
stry, including the first Kuhn verdazyl capable of binding
directly to transition metals. The distinctive spectroscopic and
particularly the redox properties associated with each ligand
type carry over into their respective Pd complexes. The ligand-
centred redox events in the metal complexes cannot be ration-
alized based on delocalization arguments and point to an
inductive “metal-as-substituent” effect which will require a
larger array of complexes to better understand.
Fig. 8 Cyclic voltammograms of (a) 1a, (b) 1a·PdCl2 (c) 2a, (d) 2a·PdCl2, (e) 3a,
(f) 3a·PdCl2. 1 mM analyte, 0.1 M Bu4NBF4 electrolyte, CH2Cl2 solutions, scan
rate 100 mV s−1
.
Table 3 Electrochemical data. Potentials are in V vs. Fc+/Fc in CH2Cl2 solution
Cpd
E°red
E°rox
Ecell
Experimental section
General considerations
1a
1a·PdCl2
2a
2a·PdCl2
3a
−1.37
−0.44
−1.00
−0.16
−1.30
−0.41
+0.20
+0.67a
+0.47
+0.81
−0.22
+0.42
1.57
1.11b
1.47
0.97
1.08
0.83
All solvents and reagents were purchased from Aldrich and
used as received without further purification unless otherwise
stated. 1,3,5-Triphenyl 6-oxotetrazane22 and 2-pyridinecarbox-
aldehyde phenylhydrazone30 were synthesized using literature
methods.
3a·PdCl2
a Irreversible process; anodic peak potential given. Ecell calculated
using anodic peak potential.
b
1H and 13C NMR spectra were recorded on a Bruker AC300
(300 MHz) instrument. FT-IR spectra were recorded on a
Perkin-Elmer Spectrum One FT-IR spectrometer as pressed
and 3a are consistent with the larger degree of N-aryl substitu- KBr discs. EPR spectra were recorded on a Bruker EMX instru-
ent spin delocalization.21 ment (9.85 GHz) with samples prepared as dilute (∼10−4 M)
The voltammograms of each of the PdCl2 complexes also and deoxygenated (purging with Ar) solutions. A DPPH radical
consist of a ligand-centred oxidation and reduction. All such standard (g = 2.0036) was used as a field reference. The EPR
processes are reversible except for the oxidation of 1a·PdCl2 spectra were simulated using the WinSim 2002 program.
(Fig. 8). Each redox process for a given complex is shifted to UV-Vis spectra were recorded on a Perkin-Elmer Lambda 1050
more positive potential relative to the corresponding process spectrometer. Melting points were determined using a Gallen-
for the free ligand. In other words, all of the verdazyl radicals kamp melting point apparatus and are uncorrected. Elemental
become easier to reduce and harder to oxidize upon coordi- analyses were performed by Canadian Microanalytical Services
nation to Pd(II). We have noted similar behaviour in Zn(II) Ltd, Delta, British Columbia, Canada. CV experiments were
verdazyl complexes.16 One noteworthy feature of the voltam- performed using a Bioanalytical Systems E2 Epsilon Electro-
mograms of the Pd complexes is that, for each of the verdazyls, chemical Analyzer with a cell consisting of a glassy carbon
the magnitude of the shift of the reduction potential upon working electrode, platinum wire counter electrode, and
coordination is substantially larger than the shift of the oxi- a silver wire reference electrode. Crystallographic data are
dation potential. The verdazyl reduction potentials shift by summarized in Table 4.
0.84–0.93 V, while the shift in oxidation potentials is smaller
1,5-Diphenyl-3-(2-pyridyl)formazan
(4). 2-Pyridinecarbox-
and more variable (0.34–0.64 V). A consequence of this obser- aldehyde phenylhydrazone (2.19 g, 11.1 mmol) was added to a
vation is that the Ecell values for the complexes are significantly mixture of EtOH (10 mL), H2O (70 mL), and CH2Cl2 (120 mL).
smaller than the values of the free ligands (Table 3). As dis- The solution was cooled to −5 °C and Na2CO3·H2O (5.57 g,
cussed above, arguments have been put forth correlating Ecell 44.9 mmol) and nBu4NBr (0.44 g, 1.4 mmol) were added.
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1,5-Diphenyl-3-(pyridin-2-yl)verdazyl (3a). To 1,5-diphenyl-3-
(pyridin-2-yl)formazan 4 (522 mg, 1.73 mmol) was added DMF
(25 mL). A 37% solution of formaldehyde (1.9 mL, 26 mmol)
was added and the resulting solution was left to stir for 1 h.
A solution of 2 M NaOH (5.5 mL, 11 mmol) was added drop-
wise. The solution was allowed to stir at RT and open to air for
23 h. It was poured into Et2O (200 mL), washed with H2O
(5 × 50 mL), and the organic layer was stored on Na2CO3. The
solvent was removed in vacuo and the dark residue was
chromatographed on deactivated neutral alumina (hexanes–
CH2Cl2–NEt3, 14 : 6 : 1) yielding 3a as a dark green solid
(81.5 mg, 15.0%). FT-IR (KBr): 3057 (w), 2957 (w), 1685 (w),
1588 (s), 1567 (m), 1495 (s), 1476 (s), 1457 (m), 1393 (m),
1368 (m), 1314 (m), 1304 (m), 1270 (m), 1216 (w), 1147 (m),
1130 (m), 1076 (w), 1028 (w), 994 (w), 940 (w), 894 (w), 789 (m),
748 (s), 687 (m), 650 (w), 628 (w), 622 (w), 612 (w), 603 (m)
cm−1. UV-Vis (MeCN): λmax (nm) (ε, M−1 cm−1), 279 (1.4 × 104),
304 (1.4 × 104), 380 (4.5 × 103), 435 (4.8 × 103), 693 (2.6 × 103).
MS (ESI): m/z LR-MS 315 (M + H+, 100%), HR-MS: theor
314.14057 (M+), expt 314.13997 (M+, 100%). Anal. Calc. for
C19H16N5: C, 72.59; H, 5.13; N, 22.28. Found: C, 72.47; H, 5.08;
N, 20.87. MP: 80–84 °C.
Table 4 Crystallographic data
2a
2a·PdCl2
3a·PdCl2
Empirical
formula
Formula wt
T (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C19H14N5O
C19H14N5OPdCl2 C19H16N5PdCl2
328.35
90
0.71073
505.65
90
0.71073
491.67
90
0.71073
Monoclinic
P21/c (#14)
8.4974(8)
14.1240(13)
15.4290(15)
90
99.586(5)
90
1825.8(3)
4
13.24
1.789
31 632
5304
244
Orthorhombic Triclinic
ˉ
Pna21 (#33)
11.2176(4)
19.5159(6)
7.0585(2)
90
P1 (#2)
9.421(1)
10.034(1)
11.028(1)
77.160(7)
77.700(6)
72.661(6)
958.1(2)
2
b (Å)
c (Å)
α (°)
β (°)
90
90
γ (°)
V (Å3)
1545.26(8)
4
0.93
1.411
18 023
2837
Z
μ (cm−1
)
12.68
1.753
9294
2439
253
1.20
0.088
0.156
ρcalc (g cm−3
)
Data collected
Unique data
Parameters
g.o.f.
226
1.13
1.09
0.023
0.050
943812
R1
0.073
0.202
943810
wR2 (all data)
CCDC#
943811
[1,5-Diphenyl-3-(pyridin-2-yl)verdazyl]palladium(II) chloride
(3a·PdCl2). PdCl2·2H2O (37.4 mg, 0.175 mmol) was added
to MeCN (20 mL) and the solution was heated to reflux.
A solution of 3a (59.4 mg, 0.189 mmol) in MeCN (5 mL)
was added dropwise. The resulting solution was stirred for
15 min at reflux. The solution was cooled and left to stand.
A dark green crystalline solid was collected by filtration
(42 mg, 48%). FT-IR (KBr): 3466 (br, m), 3047 (s), 2920 (w),
1607 (m), 1584 (m), 1487 (s), 1458 (s), 1424 (s), 1392 (m),
1344 (m), 1288 (m), 1266 (m), 1243 (m), 1199 (m), 1161 (m),
1146 (m), 1136 (m), 1108 (m), 1100 (m), 1077 (w), 1050 (w),
1035 (w), 1027 (w), 1003 (w), 969 (w), 951 (m), 906 (w), 766 (s),
755 (s), 700 (m), 680 (m), 655 (m), 629 (m), 602 (w), 575 (w),
A solution of benzenediazonium chloride (made by mixing
H2O (10 mL), aniline (1.20 mL, 13.0 mmol), and 37% HCl
(4 mL, 46.6 mmol) followed by cooling to −5 °C; a solution of
NaNO2 (1.04 g, 15.1 mmol) at −5 °C was added dropwise to it
and the resulting solution was allowed to stir for 30 minutes)
was added dropwise to the biphasic solution producing an
gradual colour change from yellow to blood red. The solution
was stirred for 1 h at −5 °C. The solution added to a separatory
funnel and CH2Cl2 (100 mL) was added to the funnel. The
organic layer was washed with H2O (5 × 100 mL) and stored on
MgSO4. The solvent was removed in vacuo and the deep red
residue was purified by column chromatography on alumina
(hexanes–CH2Cl2, 8 : 2) followed by a second column on silica
(hexanes–CH2Cl2–EtOAc, 7 : 2 : 1). The product was taken up in
EtOAc and the solvent was removed in vacuo, this was repeated
535 (w) cm−1. UV-Vis (MeCN): λmax (nm) (ε, M−1 cm−1
)
314 (9.8 × 103), 489 (5.3 × 103), 733 (1.7 × 103). Anal. Calc. for
C19H16N5PdCl2: C, 46.41; H, 3.28; N, 14.24. Found: C, 46.35;
H, 3.15; N, 14.32. MP: 202–208 °C.
1
until a dark red solid was obtained (1.30 g, 39.0%). H NMR
(300 MHz, CD2Cl2): δ 15.61 (s, 1H, NH), 14.25 (s, 1H, NH), 8.76
(ddd, 1H, J = 5, 2, 1 Hz), 8.71 (ddd, 1H, J = 5, 2, 1 Hz), 8.27 (dt,
1H, J = 8, 1 Hz), 8.14 (dt, 1H, J = 8, 1 Hz), 7.95 (dd, 2H, J =
8, 2 Hz), 7.91 (td, 1H, J = 8, 2 Hz), 7.79 (td, 1H, J = 8, 2 Hz), 7.74
(dd, 4H, J = 8, 2 Hz), 7.35 (m, 15H), 7.04 (tt, 1H, J = 7, 2 Hz)
ppm. 13C NMR (300 MHz, CD2Cl2): δ 155.5, 153.8, 153.0, 149.7,
148.1, 147.5, 144.0, 143.3, 141.9, 137.3, 136.7, 129.9, 129.8,
129.5, 128.2, 124.1, 123.3, 123.2, 122.9, 122.7, 121.7, 119.3,
115.3 ppm. FT-IR (KBr): 3369 (w), 3049 (m), 1599 (m), 1584 (s),
1565 (m), 1501 (s), 1470 (s), 1450 (s), 1430 (s), 1351 (s),
1240 (s), 1076 (s), 1058 (s), 1036 (s), 994 (m), 926 (w), 894 (w),
792 (m), 767 (m), 754 (s), 741 (s), 696 (m), 685 (s), 653 (m),
633 (m), 588 (m), 546 (w), 501 (w) cm−1. UV-Vis (CH2Cl2):
λmax (nm) (ε, M−1 cm−1) 262 (1.4 × 104), 308 (1.9 × 104), 464
(1.9 × 104). MS (ESI): m/z LR-MS 324 (M + Na+, 100%), HR-MS:
theor 302.14057 (M+H)+, expt 302.13969 (M + H+, 100%). MP:
106–109 °C.
2,4-Diphenylcarbonohydrazide (6). 2,4,6-Triphenyl tetrazane
5 (2.480 g, 7.507 mmol) was combined with 80 mL of 37% HCl
and 30 mL H2O. The solution was stirred for 24 h and then
refluxed for 2 h. After cooling, the solution was added to a
separatory funnel and was washed with CH2Cl2 (3 × 50 mL).
CH2Cl2 (60 mL) was added to the funnel and the solution was
rendered basic using 2 M NaOH until pH 14 was reached. The
aqueous phase was then extracted with CH2Cl2 (4 × 60 mL)
and the organic layer was dried using MgSO4. The solvent was
removed in vacuo leaving a yellow oil which solidified. The
yellowish solid was collected (1.375 g, 75.6%). 1H NMR (300 MHz,
DMSO-d6): δ 7.29 (dd, 4H, J = 8, 1 Hz), 7.20 (t, 4H, J = 7 Hz),
6.95 (2H, tt, J = 7, 1 Hz), 5.24 (4H, s) ppm. 13C NMR (300 MHz,
DMSO-d6): δ 160.8, 145.5, 127.9, 123.0, 121.5 ppm. FT-IR (KBr):
3342 (s), 3313 (s), 3282 (s), 3063 (m), 3039 (m), 1653 (s), 1620
(s), 1585 (s), 1494 (s), 1363 (s), 1305 (s), 1179 (m), 1091 (m),
1076 (m), 1024 (m), 921 (s), 901 (s), 775 (s), 766 (s), 748 (s),
Dalton Trans.
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693 (s), 658 (m), 603 (m), 536 (m) cm−1. UV-Vis (MeCN): 608 (w), 508 (w) cm−1. UV-Vis (MeCN): λmax (nm) (ε, M−1 cm−1):
λmax (ε, M−1 cm−1) 266 (1.2 × 104). MS (ESI): m/z LR-MS 283 (1.4 × 104), 307 (1.4 × 104), 478 (3.5 × 103), 563 (2.4 × 103),
265 (M + Na+, 100%), HR-MS: theor 243.12459 (M+H+), expt 645 (1.7 × 103). Anal. Calc. for C19H14N5Cl2OPd: C, 45.13;
243.12385 (M + H+, 100%). Anal. Calc. for C13H14N4O: C, 64.45; H, 2.79; N, 13.85. Found: C, 44.70; H, 2.56; N, 13.73. MP: >320 °C.
H, 5.82; N, 23.13. Found: C, 64.47; H, 5.65; N, 23.22. MP:
74–77 °C.
Acknowledgements
2,4-Diphenyl-6-(pyridin-2-yl)-1,2,4,5-tetrazinan-3-one (7). To
MeOH (15 mL) was added 6 (317 mg, 1.31 mmol), 2-pyridine-
carboxaldehyde (0.11 mL, 1.2 mmol) followed by pyridinium
tosylate (108 mg, 0.430 mmol). The bright yellow solution was
stirred at room temperature for 48 h. It was then poured into
H2O (90 mL) and stirred for 3 h to form a white precipitate
which was collected by filtration. The product was washed
with H2O and dried overnight to yield a white solid (371 mg,
94.7%). 1H NMR (300 MHz, DMSO-d6): δ 8.56 (ddd, 1H, J =
5, 2, 1 Hz), 7.84 (td, 1H, J = 8, 2 Hz), 7.64–7.53 (mult, 5H), 7.39
(ddd, 1H, J = 8, 5, 1 Hz), 7.30 (tt, 4H, J = 8, 2 Hz), 7.05 (tt, 2H,
J = 7, 1 Hz), 6.49 (d, 2H, J = 10 Hz), 5.40 (t, 1H, J = 10 Hz) ppm.
13C NMR (300 MHz, DMSO-d6): δ 156.8, 156.3, 148.7, 142.8,
137.2, 128.0, 123.8, 123.3, 122.7, 121.1, 73.4 ppm. FT-IR (KBr):
3222 (m), 3066 (w), 3029 (w), 1653 (s), 1596 (s), 1570 (w),
1484 (s), 1455 (m), 1440 (m), 1360 (s), 1299 (s), 1111 (w),
1090 (w), 1031 (w), 997 (m), 898 (s), 796 (m), 751 (s), 706 (m),
693 (s), 647 (w), 593 (w), 568 (m) cm−1. UV-Vis (MeCN): λmax
(ε, M−1 cm−1) 263 (2.3 × 104). MS (ESI): m/z LR-MS 354
(M + Na+, 100%), HR-MS: theor 332.15114 (M+H)+, expt 332.15031
(M + H+, 100%). Anal. Calc. for C19H17N5O: C, 68.87; H, 5.17;
N, 21.13. Found: C, 69.28; H, 4.99; N, 21.25. MP: 143–147 °C.
1,5-Diphenyl-3-(pyridin-2-yl)6-oxoverdazyl (2a). To MeOH
(5 mL) was added 7 (97.0 mg, 0.293 mmol), celite (92.0 mg),
and silver oxide (102 mg, 0.439 mmol). The resulting solution
was stirred for 1 h. The solvent was removed in vacuo and the
residue was purified by chromatography using silica gel
(hexanes-EtOAc-CH2Cl2, 2:1:1) and the major red fraction was
collected (56 mg, 58%). FT-IR (KBr): 3089 (w), 3065 (w), 3013
(w), 1743 (w), 1696 (s), 1589 (m), 1570 (m), 1485 (s), 1457 (m),
1401 (m), 1368 (m), 1298 (m), 1258 (m), 1238 (m), 1136 (m),
1125 (m), 994 (w), 1088 (w), 1044 (w), 1024 (w), 897 (w), 798
(w), 758 (s), 713 (w), 692 (s), 654 (m), 631 (m), 619 (m), 612
(m), 601 (s) cm−1. UV-Vis (MeCN): λmax (nm) (ε, M−1 cm−1): 269
(1.5 × 104), 312 (1.3 × 104), 416 (2.1 × 103), 518 (2.5 × 103).
MS (ESI): m/z LR-MS 351 (M + Na+, 100%), HR-MS: theor
329.12766 (M+H+), expt 329.12661 (M + H+, 100%). Anal. Calc.
for C19H14N5O: C, 69.50; H, 4.30; N, 21.33. Found: C, 69.43;
H, 4.16; N, 21.37. MP: 206–209 °C.
We thank the University of Victoria and the Natural Sciences
and Engineering Research Council of Canada for support.
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