Synthesis and characterization of palladium complexes of 3-nitroformazans

Article abstract of DOI:10.1016/j.ica.2008.02.005

3-Nitroformazans react with bis(1,1,1,5,5,5-hexafluroacetylacetonato)palladium to afford monometallic complexes. The complexes were characterized via X-ray crystallography, cyclic voltammetry, and electronic spectroscopy. The nature of the aryl substituents on the formazan ligands had a profound effect on the structure and spectroscopy of the complexes, with o-substituted aromatic rings twisted with respect to the ligand, while the p-tolyl substituted derivative adopted a planar structure. The complexes undergo irreversible reductions near -1 V versus Fc/Fc⁺.

Full text of DOI:10.1016/j.ica.2008.02.005

Available online at www.sciencedirect.com
Inorganica Chimica Acta 361 (2008) 3388–3393
www.elsevier.com/locate/ica
Synthesis and characterization of palladium complexes
of 3-nitroformazans
Joe B. Gilroy a, Michael J. Ferguson b, Robert McDonald b, Robin G. Hicks a,*
a Department of Chemistry, University of Victoria, P.O. Box 3065, Victoria, BC, Canada V8W 3V6
b Crystallography Laboratory, Department of Chemistry, University of Alberta, 11227 Saskatchewan Dr. NW, Edmonton,
Alberta, Canada T6G 2G2
Received 17 January 2008; accepted 4 February 2008
Available online 8 February 2008
This manuscript is being submitted as a contribution to the Special Issue dedicated to Dante Gatteschi (Andrea Dei acting as guest Editor).
Abstract
3-Nitroformazans react with bis(1,1,1,5,5,5-hexafluroacetylacetonato)palladium to afford monometallic complexes. The complexes
were characterized via X-ray crystallography, cyclic voltammetry, and electronic spectroscopy. The nature of the aryl substituents on
the formazan ligands had a profound effect on the structure and spectroscopy of the complexes, with o-substituted aromatic rings twisted
with respect to the ligand, while the p-tolyl substituted derivative adopted a planar structure. The complexes undergo irreversible reduc-
tions near ꢀ1 V versus Fc/Fc+.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Palladium complexes; Formazans; Verdazyl radicals; X-ray crystal structure; Cyclic voltammetry
1. Introduction
R
R
N
O
N
N
O
O
Stable radicals based on organic [1] and main group
systems [2] have become increasingly common in recent
years. Traditionally, nitroxide 1, and nitronyl nitroxide
2 radicals have been recognized as the most stable
classes of organic radicals. However, verdazyl radicals 3
have emerged as a useful class of organic radicals,
and their stability shown to rival that of the nitroxides.
R
1
2
R
E
R
N
N
N
N
Type I E = CH2
Type II E = CO
R'
3
The magnetochemistry of verdazyl radicals [3,4] and
their transition metal complexes [5–9] has been studied
extensively. These radicals have also been used as electron
donors [10,11], and as mediators of living radical polymer-
ization [12]. As their utility increases, so to does interest in
creating new derivatives, and ultimately correlating struc-
ture and properties. In this context, we have been interested
*
Corresponding author. Tel.: +1 250 721 7165; fax: +1 250 721 7147.
E-mail address: rhicks@uvic.ca (R.G. Hicks).
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.02.005

J.B. Gilroy et al. / Inorganica Chimica Acta 361 (2008) 3388–3393
3389
in making verdazyl radicals in which inorganic elements are
placed in the heterocyclic skeleton. Previous efforts have
focused on ‘‘phosphaverdazyl” radicals 4 [13] and 5
[13,14]; more recently we described the reactions of forma-
zans 6 with boron triacetate to form boratatetrazines 7,
which were converted to borataverdazyl radical anions 8,
isolobal to type I verdazyl radicals [15].
were purified by recrystallization in saturated methanolic
solution and analyzed by standard spectroscopic tech-
niques. Triarylformazan complexes of this type have been
reported previously, but were isolated in low yields as side
products during the preparation of bis(formazanato)palla-
dium complexes and no structural information was
described [18]. We note that complex 9c represents the
first complex of formazan 6c, where 1,5-di-iso-propylphe-
nyl substituents have been incorporated. This ligand may
have utility in the field of catalysis where the majority of
studies involving diketiminate ligands rely on the steric
bulk associated with the 2,6-di-iso-propylphenyl N-sub-
stituents [19].
O
O
R
Me
Me
Me
P
Me
N
N
N
N
N
N
N
N
P
Ph Ph
Ph
5
4
2.2. X-ray structures
OAc
AcO
OAc
AcO
X-ray quality crystals of 9a were obtained via slow cool-
ing of a saturated methanolic solution. The molecular
structure of 9a is shown in Fig. 1. The repeat unit incorpo-
rates two crystallographically independent molecules (see
Table 1 for details), however only one (molecule A) will
be discussed in detail as they are structurally very similar.
The trifluoromethyl groups in molecule B are highly
disordered. The bond lengths within the formazan back-
bone indicate that it is fully delocalized (N2–N1
R
H
R
R
B
R
R
B
R
1N
2N
N5
N4
N
N
N
N
N
N
N
N
3
R'
6
R'
R'
7
8
The borataverdazyl radical anions were realized via
reductive pathways. It occurred to us that formazan
ligands with strong electron-withdrawing substituents
might render resulting complexes more easily reduced to
verdazyl radicals. To this end we recently described the
synthesis and characterization of formazans with cyano
or nitro groups at the 3-position [16] and reported some
preliminary transition metal complex chemistry of some
of these ligands [17]. Herein, we report the synthesis and
characterization of monometallic palladium–formazan
complexes of some 3-nitroformazans with different aro-
matic N-substituents.
˚
˚
˚
1.279(2) A, N4–N3 1.277(2) A, N1–C1 1.333(3) A, N3–C1
˚
1.327(3) A), consistent with other complexes of 3-nitrofor-
mazans [17]. The p-tolyl substituents are twisted with
respect to the formazan plane (N2–N1–N3–N4) at torsion
angles of 21.00° (ring attached to N2) and 23.46° (ring
attached to N4). The geometry around the palladium cen-
ter is square planar, however the square plane lies at an
angle of 27.71° relative to the formazan ligand backbone.
˚
As a result the palladium ion lies 0.699 A out of the plane,
likely due to steric repulsion between the hfac ligand and
the 1,5-p-tolyl substituents of the formazan. The meso car-
bon (C1) is also displaced from the formazan ligand plane
2. Results and discussion
˚
by 0.171 A, consistent with other metal complexes of
formazans where the metal ion is significantly displaced
from the plane of the ligand [17,18] (see Table 2).
2.1. Synthesis
X-ray quality crystals of 9b were obtained by slow
evaporation of a dichloromethane solution. The molecular
structure (Fig. 2) of 9b is qualitatively similar to 9a based
on the following features: (i) a delocalized ligand back-
Palladium formazan complexes 9a–9c were prepared by
refluxing one equivalent of the appropriate formazan 6a–
6c [16] with bis(1,1,1,5,5,5-hexafluoroacetylacetonato)pal-
ladium in methanol for 2 h (Scheme 1). The complexes
˚
˚
bone (N2–N1 1.280(4) A, N4–N3 1.277(4) A, N1–C1
˚
˚
1.329(4) A, N3–C1 1.339(4) A), (ii) the palladium ion is
˚
displaced from the formazan plane (0.452 A), (iii) the
F3C
R
CF3
R
square plane of the palladium is not coplanar with the
R
H
R
formazan ligand backbone (27.17°), and (iv) the formazan
N
N
N
N
O
O
˚
Pd(hfac)2
Pd
takes on a boat conformation (C1 displaced by 0.127 A).
MeOH
N
N
N
N
However, the incorporation of ortho-substituents causes
the N-aryl rings to twist relative to the formazan ligand
plane by 62.55° (ring attached to N2) and 56.40° (ring
attached to N4) in 9b. The twisting of the N-substituents
observed in 9b, is consistent with other formazan metal
complexes where the N-substituents are ortho-substituted
[17].
Δ
NO2
6a-c
NO2
9a R = p-tolyl
9b R = mesityl
9c R = 2,6-di-iso-propylphenyl
Scheme 1. Synthesis of complexes 9a–c.

3390
J.B. Gilroy et al. / Inorganica Chimica Acta 361 (2008) 3388–3393
Fig. 1. Molecular structure of 9a top view (left) and side view (right). Thermal ellipsoids shown at 50% probability level. Hydrogen atoms are removed for
clarity.
Table 1
2.3. Solution properties
˚
Selected bond distances (A) and bond angles (°) for 9a and 9b
Atoms
9a
9b
The NMR spectra of 9a–c are consistent with delocal-
ization of the formazan ligand in solution as the aryl sub-
stituents are equivalent. Complexes 9a–c do not isomerize
in solution, as observed in a similar diketiminate complex
10, which rapidly converts to four member metallacycle
100 at room temperature in solution [20]. It is not clear
whether the lack of isomerism is due to increased ligand
strength of the formazan compared to the diketiminate,
or whether the introduction of the fluorine atoms into the
acetylacetonate backbone increases the electron affinity of
the palladium center, thereby increasing the strength of
the Pd–N bonds.
Molecule A
Molecule B
N2–N1
N4–N3
N1–C1
N3–C1
N2–Pd
N4–Pd
O3–Pd
O4–Pd
N1–N2–Pd
N3–N4–Pd
N2–Pd–N4
N2–Pd–O3
N4–Pd–O4
O3–Pd–O4
C4–C3–O3
C4–C5–O4
1.279(2)
1.277(2)
1.333(3)
1.284(2)
1.276(2)
1.327(2)
1.280(4)
1.277(4)
1.329(4)
1.339(4)
1.959(3)
1.963(3)
2.030(2)
2.042(2)
125.2(2)
125.4(2)
89.48(11)
89.51(11)
90.46(10)
90.48(9)
128.4(3)
129.1(3)
1.327(3)
1.332(2)
1.9859(15)
1.9811(15)
2.0464(13)
2.0310(13)
122.91(13)
123.80(13)
86.71(6)
93.75(6)
89.88(6)
89.42(5)
129.58(18)
128.78(17)
1.9912(14)
1.9866(15)
2.0394(12)
2.0460(12)
123.86(12)
124.10(12)
88.15(6)
90.57(6)
91.12(5)
89.99(5)
129.62(17)
129.23(17)
O
N
O
N
O
N
O
Pd
R
R
Pd
R
N
R
10
10'
Table 2
Crystallographic data for 9a and 9b
Compound
9a
9b
3-Nitroformazans and their complexes are intensely col-
oured due to strongly absorbing ligand-based chromoph-
ores [16,17]. This is also the case for 9a–c, whose
electronic spectra are shown in Fig. 3. The low energy tran-
sitions in formazans (and their complexes) has been
assigned to a charge-transfer band involving the formazan
backbone and the N-aryl substituents. The major absorp-
tions for 9a–c are red-shifted relative to the corresponding
free ligands [16], due to increased delocalization within the
formazan backbone associated with fixed conformation of
the formazanato anion. The electronic spectrum of 9a is
significantly red shifted relative to 9b and 9c due to further
delocalization associated with the planarity of the formaz-
anato ligand. The interaction of the N-aromatic groups in
9b and 9c are attenuated by twisting of the o-substituted
aryl substituents, observed in the solid state structures.
The close agreement of the electronic spectra of 9b and
Formula
Formula Weight
Dimensions (mm)
PdC20H15F6N5O4
609.77
PdC24H23F6N5O4
665.87
0.53 ꢁ 0.41 ꢁ 0.30
0.35 ꢁ 0.15 ꢁ 0.06
11.8370(17)
13.2420(19)
17.729(3)
monoclinic
90
99.234(2)
90
2743.0(7)
P21/c (No. 14)
4
0.756
193.15
12120
0.0000
0.0545
0.1335
˚
a (A)
11.7149(9)
˚
b (A)
13.8658(11)
16.2625(13)
triclinic
87.9598(10)
72.3284(10)
66.5383(9)
˚
c (A)
System
a (°)
b (°)
c (°)
3
˚
V (A )
2297.9(3)
ꢀ
Space group
P1 (No. 2)
Z
4
l (mmꢀ1
T (K)
)
0.894
193.15
10380
0.0108
0.0247
0.0646
Independent
Reflections Rint
R1
wR2

J.B. Gilroy et al. / Inorganica Chimica Acta 361 (2008) 3388–3393
3391
Fig. 2. Molecular structure of 9b top view (left) and side view (right). Thermal ellipsoids shown at 50% probability level. Hydrogen atoms are removed for
clarity.
of ꢀ915 mV, while 9b and 9c are reduced at ꢀ1155 mV
40000
and ꢀ1180 mV respectively. The fact that 9a is somewhat
35000
easier to reduce than 9b or 9c is consistent with the
hypothesis made from the electronic spectra, whereby
30000
9b
25000
20000
15000
10000
5000
0
increased planarity leads to increased delocalization
lowering the energy of the lowest unoccupied molecular
orbital (LUMO). Collectively the electrochemical
properties of 9a–9c contrast the electrochemical (and
chemical) reduction of the boratetrazines 7, which are
reduced at higher potentials and lead to stable (but highly
reactive) borataverdazyls [15]. The irreversibility of the
reduction wave indicates that the putative ‘‘palla-
daverdazyl” radical anion is unstable even on the electro-
chemical timescale.
9a
9c
-5000
250
350
450
550
650
750
Wavelength (nm)
The finding that 9a–9c are so difficult to reduce and do
not lead to stable radical anions may seem curious, given
the presence of strong electron-withdrawing groups on
both the formazan as well as the ancillary (hfac) ligand
on the palladium ion; the boratetrazines, which do not pos-
sess strong electron-withdrawing substituents, are compa-
rably easier to reduce and produce stable (isolable)
radical anions. The differences between the redox proper-
ties of 7 and 9 likely have to do with the nature of the for-
mazan interaction with B or Pd. The BN bonds in 7 are
likely to be relatively more covalent than the PdN bonds
in 9. The radical anions of 7 are isolobal to Type I verdaz-
yls and the electron deficient nature of the boron center
serves to accommodate the negative charge associated with
chemical reduction, enhancing the stability of the resulting
radical anions. In contrast, reduction of the palladium for-
mazan complexes 9 (and indeed likely other transition
metal formazan complexes) to give the radical anion 9Åꢀ
would require the additional charge to be accommodated
substantially by the formazan ligand, thereby rendering
the formazan a radical dianion. The electrochemical data
suggest that, even with the presence of the nitro substitu-
ent, the high negative charge associated with the reduced
species is clearly a situation that does not lend itself to
chemical stability. This suggests that for ‘‘metalloverdaz-
yls” to be realized, metal fragments should be targeted
which can interact with formazans in a more covalent man-
ner or which are redox-active.
Fig. 3. Electronic spectra of 9a (dashed line), 9b (black line), and 9c (grey
line).
9c strongly suggest that they are isostructural, i.e., the 2,6-
di-iso-propyl substituents in 9c are twisted relative to the
backbone as observed in 9b.
2.4. Electrochemistry
The cyclic voltammograms of 9a–c are shown in Fig. 4.
All three CVs indicate that the complexes are irreversibly
reduced. Complex 9a is irreversibly reduced at a potential
40 μA
-2000
-1500
-1000
-500
0
Potential (mV)
Fig. 4. Cyclic voltammograms of 9a (top), 9b (middle), and 9c (bottom)
vs. Fc/Fc+.

3392
J.B. Gilroy et al. / Inorganica Chimica Acta 361 (2008) 3388–3393
PdC20H15F6N5O4: C, 39.39; H, 2.48; N, 11.49. Found C,
F3C
R
CF3
R
39.39; H, 2.46; N, 11.57%.
F3C
R
C
F3
O
O
O
O
3.2.2. (1,1,1,5,5,5-Hexafluoroacetylacetonato)(1,5-mesityl-
3- nitroformazanato)palladium (9b)
PdII
PdII
2
e-
R
Compound 6b (0.136 g, 0.38 mmol), bis-(1,1,1,6,6,6-
hexafluoroacteylacetonato)palladium (0.200 g, 0.38 mmol),
and methanol (20 mL) were combined and allowed to
reflux in air for 2 h. The solvent was removed in vacuo,
and the complex was recrystallized by slow cooling of a sat-
urated methanolic solution. Compound 9b was isolated by
filtration as an orange microcrystalline solid, yield 0.220 g
(86.9%). X-ray quality crystals were obtained via slow
evaporation of a dichloromethane solution of 9b in an
N
N
N
N
N
N
N
N
NO2
NO2
9 -
9
1
3. Experimental
3.1. General considerations
NMR tube. M.p. 198–200 °C. H NMR (CD2Cl2): d 6.93
(s, 4H), 6.06 (s, 1H), 2.31 (s, 6H), 2.26 (s, 12H). 13C
NMR (CD2Cl2): d 177.0 (q, 2J = 36 Hz), 151.5, 144.7,
139.6, 131.9, 129.4, 116.9 (q, 1J = 285 Hz), 92.6, 21.3,
18.5 ppm. FT-IR (KBr): 1631 (s), 1610 (s), 1544 (s), 1452
All reagents were purchased from Aldrich and used as
received. Ligands 6a–c were prepared according to litera-
ture procedures [16]. NMR spectra were recorded on a
500 MHz Bruker instrument. Infrared spectra were
recorded as KBr pellets using a Perkin Elmer Spectrum
One instrument. Electronic spectra were recorded using a
Cary 50 Scan instrument in dichloromethane solutions.
Cyclic voltammetry experiments were performed with a
Bioanalytical Systems CV50 voltammetric analyzer at a
scan rate of 250 mV/s. Typical electrochemical cells con-
sisted of a three-electrode setup including a glassy carbon
working electrode, platinum counter electrode, and silver
reference. Acetonitrile solutions of analyte (ꢂ1 mM) and
electrolyte (0.1 M Bu4N+BF4ꢀ) were referenced against
an internal standard (ꢂ1 mM Fc). Elemental analyses were
carried out by Canadian Microanalytical Services Ltd.,
Vancouver, BC.
(s), 1402 (s), 1307 (s), 1254 (s), 1220 (s), 1149 (s) cmꢀ1
.
UV–Vis (CH2Cl2): kmax 250 nm (e = 33750), 344 nm
(e = 16750), 473 nm (e = 5250). Anal. Calc. for
PdC24H23F6N5O4: C, 43.29; H, 3.48; N, 10.52. Found C,
43.74; H, 3.71; N, 10.53%.
3.2.3. (1,1,1,5,5,5-Hexafluoroacetylacetonato)(1,5-(2,6-di-
iso- propylphenyl)-3-nitroformazanato)palladium (9c)
Compound 6c (0.252 g, 0.58 mmol), bis-(1,1,1,6,6,6-
hexafluoroacteylacetonato)palladium (0.300 g, 0.58 mmol),
and methanol (30 mL) were combined and allowed to
reflux in air for 2 h. The solvent was removed in vacuo,
and the complex was recrystallized from saturated metha-
nolic solution. 9c was isolated by filtration as large orange
1
crystals, yield 0.270 g (62.1%). M.p. 222–224 °C (dec.). H
3
NMR (CD2Cl2): d 7.42 (t, 2H, J = 8 Hz), 7.24 (d, 4H,
3J = 8 Hz), 6.05 (s, 1H), 3.21 (septet, 4H, J = 7 Hz), 1.29
3
3
3
3.2. Synthesis of Pd complexes
(d, 12H, J = 7 Hz), 1.21 (d, 12H, J = 7 Hz). 13C NMR
(CD2Cl2): d 176.9 (q, 2J = 36 Hz), 144.4, 142.7, 130.2,
1
3.2.1. (1,1,1,5,5,5-Hexafluoroacetylacetonato)(1,5-p-tolyl-
3-nitroformazanato)palladium (9a)
124.3, 116.8 (q, J = 285 Hz), 92.7, 29.7, 24.7, 23.1 ppm.
FT-IR (KBr): 2966 (w), 1630 (s), 1549 (s), 1466 (s), 1407
Compound 6a (0.240 g, 0.81 mmol), bis-(1,1,1,6,6,6-
hexafluoroacteylacetonato)palladium (0.420 g, 0.81 mmol),
and methanol (30 mL) were combined and allowed to
reflux in air for 2 h. The solvent was removed in vacuo,
and the complex was recrystallized from saturated metha-
nolic solution. Compound 9a was isolated by filtration as
(s), 1310 (s), 1257 (s), 1214 (s), 1150 (s), 786 (m) cmꢀ1
.
UV–Vis (CH2Cl2): kmax 248 nm (e = 30750), 341 nm
(e = 14750), 473 nm (e = 5250). Anal. Calc. for
PdC30H35F6N5O4: C, 48.04; H, 4.70; N, 9.34. Found C,
48.24; H, 4.71; N, 9.38%.
a
dark purple microcrystalline solid, yield 0.233 g
3.3. X-ray structure determinations
(47.2%). X-ray quality crystals were obtained by cooling
a dilute methanolic solution of 9a in a ꢀ20 °C freezer for
72 h. M.p. 186–188 °C. 1H NMR (CD2Cl2): d 7.82 (d,
X-ray diffraction data were collected on a Bruker
PLATFORM/SMART 1000 CCD with graphite-mono-
chromatized Mo Ka radiation (k = .71073A). The crystal
3
3
˚
4H, J = 8 Hz), 7.26 (d, 4H, J = 8 Hz), 6.12 (s, 1H), 2.42
(s, 6H). 13C NMR (CD2Cl2): d 176.1 (q, 2J = 36 Hz),
146.7, 141.3, 129.9, 125.3, 116.9 (q, 1J = 285 Hz), 92.7,
21.6 ppm. FT-IR (KBr): 1631 (s), 1601 (w), 1533 (s), 1458
(s), 1394 (s), 1293 (s), 1257 (s), 1152 (s), 817 (m), 802 (m)
structures were solved by direct methods (SHELXS-97).
Acknowledgements
cmꢀ1
344 nm (e = 14000), 540 nm (e = 10500). Anal. Calc. for
.
UV–Vis (CH2Cl2): kmax 250 nm (e = 30000),
Financial support from the University of Victoria, The
Natural Sciences and Engineering Research Council of

J.B. Gilroy et al. / Inorganica Chimica Acta 361 (2008) 3388–3393
3393
[7] J.B. Gilroy, B.D. Koivisto, R. McDonald, M.J. Ferguson, R.G.
Hicks, J. Mater. Chem. 16 (2006) 2618.
[8] T.M. Barclay, R.G. Hicks, M.T. Lemaire, L.K. Thompson, Inorg.
Chem. 42 (2003) 2261.
[9] T.M. Barclay, R.G. Hicks, M.T. Lemaire, L.K. Thompson, Chem.
Commun. (2000) 2141.
Canada, and the Petroleum Research Fund of the Ameri-
can Chemical Society is gratefully acknowledged.
Appendix A. Supplementary material
[10] J.B. Gilroy, S.D.J. McKinnon, B.D. Koivisto, R.G. Hicks, Org. Lett.
9 (2007) 4837.
[11] S. Nakatsuji, A. Kitamura, A. Takai, K. Nishikawa, Y. Morimoto,
N. Yasuoka, H. Kawamura, H. Anzai, Naturforsch B 53 (1998)
495.
[12] E.K.Y. Chen, S.J. Teertstra, D. Chan-Seng, P.O. Otieno, R.G. Hicks,
M.K. Georges, Macromolecules 40 (2007) 8609.
[13] R.G. Hicks, R. Hooper, Inorg. Chem. 38 (1999) 284.
[14] R.G. Hicks, L. Ohrstrom, G.W. Patenaude, Inorg. Chem. 40 (2001)
1865.
CCDC 678948, 678949 and contain the supplementary
crystallographic data for compounds 9a, 9b. These data
can be obtained free of charge from The Cambridge Crys-
tallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this
article can be found, in the online version, at
doi:10.1016/j.ica.2008.02.005.
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