Transition metal complexes of 3-cyano- and 3-nitroformazans

Article abstract of DOI:10.1021/ic7019846

The synthesis and characterization of six transition metal complexes of 3-cyano- and 3-nitroformazans are described. Three different formazans were reacted with nickel(II) to produce complexes with bidentate formazan ligands. Mononuclear NiL₂ (L = deprotonated formazan) or binuclear hydroxo-bridged (LNi)₂(μ-OH)₂ species were produced depending on the steric bulk on the formazan N-aromatic substituents. 1,5-Bis(2-methoxyphenyl)-3-cyanoformazan acts as a tetradentate monoanionic ligand in a copper(II) complex, whereas the analogous 1,5-bis(2-hydroxyphenyl)- 3-cyanoformazan binds as a trianion in a tetradentate manner to Fe(III) and Co(III). Crystal structures - the first examples of metal complexes of cyano- or nitroformazans - as well as the electronic spectra of the complexes are discussed in relation to each other as well as that of the uncoordinated formazans.

Full text of DOI:10.1021/ic7019846

Inorg. Chem. 2008, 47, 1287-1294
Transition Metal Complexes of 3-Cyano- and 3-Nitroformazans
Joe B. Gilroy,† Brian O. Patrick,‡ Robert McDonald,§ and Robin G. Hicks*,†
Department of Chemistry, UniVersity of Victoria, P.O. Box 3065, Victoria, BC, Canada V8W 3V6,
Crystallography Laboratory, Department of Chemistry, UniVersity of British Columbia, VancouVer,
BC, Canada V6T 1Z3, and Crystallography Laboratory, Department of Chemistry, UniVersity of
Alberta, 11227 Saskatchewan Dr. NW, Edmonton, Alberta, Canada T6G 2G2
Received October 8, 2007
The synthesis and characterization of six transition metal complexes of 3-cyano- and 3-nitroformazans are described.
Three different formazans were reacted with nickel(II) to produce complexes with bidentate formazan ligands.
Mononuclear NiL2 (L ) deprotonated formazan) or binuclear hydroxo-bridged (LNi)2(µ-OH)2 species were produced
depending on the steric bulk on the formazan N-aromatic substituents. 1,5-Bis(2-methoxyphenyl)-3-cyanoformazan
acts as a tetradentate monoanionic ligand in a copper(II) complex, whereas the analogous 1,5-bis(2-hydroxyphenyl)-
3-cyanoformazan binds as a trianion in a tetradentate manner to Fe(III) and Co(III). Crystal structures—the first
examples of metal complexes of cyano- or nitroformazans—as well as the electronic spectra of the complexes are
discussed in relation to each other as well as that of the uncoordinated formazans.
Introduction
The ability to tune steric and electronic properties by
modifying R1 and R5 (and, to a lesser extent, through changes
to R2, R3, and R4) has rendered ꢀ-diketimines powerful
additions to the stable of benchmark ligand families. The
impact of ligand skeleton 1 is further demonstrated through
the development of yet more ligand platforms deriVed from
ꢀ-diketimines, such as the 1,3,5-triazapentadienes (2)6 and
the ꢀ-ketimines (3).7
Many of the most popular families of spectator ligands
are chelating, anionic, π-conjugated N-donor ligands. Ligands
of this general type offer a number of advantages, namely,
relatively efficient and versatile synthesis and the ability to
control metal complex properties by the choice of substituents
at the donor N atoms but also on remote sites (via the
π-conjugated framework). Among the many different classes
of conjugated polyamido ligands (e.g., amidinates,1 ami-
notroponiminates,2 radical anions of diiminopyridines,3 and
diazabutadienes4), in the past decade, the ꢀ-diketimines, 1,
have emerged as the most versatile of spectator ligands.5
* Author to whom correspondence should be addressed. E-mail:
rhicks@uvic.ca.
†
University of Victoria.
University of British Columbia.
University of Alberta.
‡
Like the triazapentadienes and ꢀ-ketimines, formazans (4)
are also isostructural to ꢀ-diketimines. However, despite the
fact that synthetic routes to formazans were well-developed
§
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10.1021/ic7019846 CCC: $40.75
Published on Web 01/23/2008
2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 4, 2008 1287

Gilroy et al.
over a half-century ago8 and there are now over 1000
reported derivatives,9 the coordination chemistry of forma-
zans has not received much attention. We recently described
the synthesis and characterization of some boron complexes
of 1,3,5-triaryl formazans.10 Publications describing transition
metal-formazan compounds have appeared sporadically
since the 1940s.11–13 There have been almost no systematic
investigations,14 and many of the reported formazan-metal
complexes remain inadequately characterized.
Scheme 1
accompanying coordination have led to applications as dyes
or as metal-sensing agents.12,20 We are interested in develop-
ing the coordination chemistry of cyano- and nitroformazans,
in part because of the novel reactivity of the corresponding
ꢀ-diketimine complexes and also because we have developed
efficient synthetic routes to 3-cyano- and 3-nitroformazans
with a range of N-aromatic substituents.21 Herein, we
describe the synthesis and characterization of some first-
row transition metal complexes of 3-cyano- and 3-nitrofor-
mazans, with a view to establishing the mode of coordination,
structures, and spectroscopy of the complexes.
The vast majority of ꢀ-diketimine complex chemistry has
been carried out with ligands with the “nacnac” substitution
pattern (1; R2 ) R4 ) Me; R3 ) H). However, there have
been reports of complexes of derivatives of 1 containing
strong electron-withdrawing groups at R3, in particular, cyano
and nitro groups.15,16 The strong electron-withdrawing group
at R3 can significantly alter the catalytic activity of resulting
complexes.17 There is little known about any metal com-
plexes of 3-cyano-18 and 3-nitroformazans19 beyond their
solution spectroscopic properties, although the metal-binding
properties of these ligands and associated color changes
Results and Discussion
Complexes of Bidentate Formazans. The reaction of 1,5-
bis(p-tolyl)-3-nitroformazan (4a) with nickel(II) acetate
hydrate produces a discrete Ni(formazan)2 product 5 (Scheme
1). In contrast, the corresponding reaction using the analo-
gous 3-cyanoformazan in place of 4a yielded completely
insoluble and impure materials. The insolubility may be due
to the formation of an extended (polymeric) structure in
which the cyanoformazan bridges two nickel ions—one via
the formazan nitrogen atoms and the other through the cyano
substituent.
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Pal, A.; Herbst-Irmer, R.; Mosch-Zanetti, N. C. Chem. Commun. 2007,
701.
1
The H and 13C NMR of 5 indicate that all four of the
aromatic groups in the complex are equivalent. The X-ray
crystal structure of 5 (Figure 1) reveals the structure to consist
of two deprotonated formazans chelating to a Ni(II) ion via
the substituent-bearing nitrogens. The bond lengths within
the formazan ligands (Table 1) indicate that the π system is
(8) Nineham, A. W. Chem. ReV. 1955, 55, 355.
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J. Am. Chem. Soc. 2003, 125, 11911. (d) Byrne, C. M.; Allen, S. D.;
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V. M.; Reshetnyak, E. A.; Nikitina, N. A.; Panteleimonov, A. V.;
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R. G. Inorg. Chem. 2008, 4, 1279.
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Bull. Chem. Soc. Jpn. 1984, 57, 1441. (b) Kawamura, Y.; Deguchi,
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Chem. 2001, 40, 5316. (b) Allen, S. D.; Moore, D. R.; Lobkovsky,
E. B.; Coates, G. W. J. Organomet. Chem. 2003, 683, 137. (c)
Shimokawa, C.; Yokota, S.; Tachi, Y.; Nishiwaki, N.; Ariga, M.; Itoh,
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W. B. Inorg. Chem. 2002, 41, 6307.
1288 Inorganic Chemistry, Vol. 47, No. 4, 2008

Transition Metal Complexes of 3-Cyano- and 3-Nitroformazans
Scheme 2
groups to stack over one another in a scissor-type structure.
The proximity of the two ligand planes forces the aromatic
rings sufficiently close together so that they appear to be
repelling one another. These gross structural features are
similar to those found in a Pd(II) complex of a 1,3,5-
triarylformazan 6; in this structure, the larger metal forces
the two ligand planes even farther apart (2.114 Å) and the
Pd square plane is more tilted (43.2°) with respect to the
ligand planes.12
Figure 1. Molecular structure of 5: (a) top view and (b) side view. Thermal
ellipsoids are shown at the 50% probability level. Hydrogen atoms have
been removed for clarity.
A final aspect of the structure of 5 concerns the fact
that the meso carbon (C1) is displaced from the
N1-N2-N3-N4 plane (by 0.238 Å) in the same face in
which the nickel ion is displaced, causing the overall
metallacycle ring to appear as a boat conformation. The
geometry at C1 is also slightly pyramidalized (Σθ )
356.7°). Although we do not have an explanation for the
unusual structure features associated with C1 in this
structure, we do note that these features have been seen
in structures of other chelating formazan complexes.22,23
The reaction of 1,5-bis(mesityl)-3-nitroformazan 4b and
Ni(OAc)2·4H2O does not produce a monometallic complex
analogous to that obtained using formazan 4a, presumably
because the bulkier mesityl groups in 4b preclude formation
of the bis(chelate) structure observed for 5. Instead, a
bimetallic complex, 7, containing two (formazan)Ni(II)
fragments linked by two bridging hydroxo groups, was
obtained (Scheme 2). The bridging hydroxides are confirmed
by a strong, sharp alcohol stretch at 3601 cm-1 in the infrared
Table 1. Selected Bond Distances (Å) and Selected Bond Angles
(deg) for 5, 7, and 8
5
7
8
N2-N1
1.293(3)
1.295(3)
1.337(3)
1.330(3)
1.867(2)
1.880(2)
1.2882(18)
1.2860(18)
1.332(2)
1.288(2)
N3-N4
N1-C1
1.3449(19)
1.8270(15)
N3-C1
1.338(2)
N2-Ni
1.8304(13)
1.8305(13)
2.891
1.891(5)
1.884(5)
118.36(13)
118.22(13)
127.91(11)
128.20(11)
92.54(6)
N4-Ni
Ni-Ni*
2.898
1.854(2)
Ni-O1
Ni-O1*
N2-N1-C1
N4-N3-C1
N1-N2-Ni
N3-N4-Ni
N2-Ni-N4
Ni-O1-Ni*
116.8(2)
116.8(2)
122.97(18)
122.79(18)
85.66(10)
119.32(17)
128.60
100.0(2)
102.81(18)
highly delocalized; within experimental error, there is no
bond-length alternation (as suggested by a single canonical
resonance structure with alternating single and double bonds).
The delocalized structure of coordinated formazans appears
to be the norm.10,12,22 The nickel has a fairly standard square-
planar geometry, although the presence of aryl substituents
on each of the donor atoms on the two chelating ligands
precludes the ligand plane from being coplanar with the
nickel square plane. As a result, the nickel ion resides in
between the planes defined by N1-N4 of each ligand (Figure
1b); these two planes are 1.67 Å apart, and the nickel ion
sits halfway between them (i.e., the nickel atom is 0.83 Å
out of the plane of each of the ligands). The nickel square
plane (N2, N4, Ni, N2*, N4*) is tilted by 37.3° with respect
to the formazan ligand planes, which permits the aromatic
1
spectrum and by a singlet at -6.61 ppm in the H NMR
spectrum.24
The structure of 7 is shown in Figure 2; the molecule
resides on a crystallographic inversion center. The formazan
ligand backbone is again delocalized (Table 1) and in
addition does not show any of the unusual pyramidalization
seen at the C1 carbon in the structure of 5. The nickel ions
are only displaced from the ligand planes by 0.302 Å and
are 2.891 Å apart from one another. The Ni2O2 core of the
structure is planar and diamond-shaped with the nickel ions
separated by 2.89 Å. The Ni2O2 plane is titled with respect
(23) Polyakova, I. N.; Starikova, Z. A.; Olkhovikova, N. B. Kristallografiya
1990, 35, 1133.
(24) (a) Lopez, G.; Garcia, G.; Sanchez, G.; Garcia, J.; Ruiz, J.; Hermoso,
J. A.; Vegas, A.; Martinezripoll, M. Inorg. Chem. 1992, 31, 1518. (b)
Pickel, M.; Casper, T.; Rahm, A.; Dambouwy, C.; Chen, P. HelV.
Chim. Acta 2002, 85, 4337.
(22) Dale, D. J. Chem. Soc. A 1967, 278.
Inorganic Chemistry, Vol. 47, No. 4, 2008 1289

Gilroy et al.
Figure 3. Molecular structure of 8: top view (a) and side view (b).
Thermal ellipsoids are shown at the 50% probability level. Hydrogen
atoms (with the exception of the hydroxyl protons) have been removed
for clarity.
A hydroxide-bridged binuclear nickel complex, 8, also
resulted from the reaction of 3-cyano-1,5-bis(2,6-meth-
ylphenyl)formazan, 4c, with Ni(OAc)2·4H2O (Scheme 3).
Analogously to 7, the bridging OH groups give rise to
characteristic infrared (ν(OH) 3603 cm-1) and proton NMR
spectra (δ(OH) -6.94 ppm). The structure of complex 8
contains bridging oxygens which are disordered in a manner
similar to that described for 7. However, many of the gross
structural features described for complex 7 are also seen in
8 (Figure 3), including (i) a delocalized ligand backbone,
(ii) nickel ions displaced slightly from the ligand planes
(0.211 Å), (iii) an interplanar separation of 0.909 Å between
the two formazan ligands, and (iv) aromatic residues that
are nearly perpendicular to the formazan backbone plane
(85.4°).
Figure 2. Molecular structure of 7: (a) top view, (b) side view, and (c)
longitudinal view of Ni2O2 core showing the two possible positions of the
oxygen atoms. Thermal ellipsoids are shown at the 50% probability level.
Hydrogen atoms (with the exception of the hydroxyl protons) have been
removed for clarity.
Scheme 3
From a steric perspective, the structure of 8 is unsurprising
because the N-aromatic substituents in 4c and 4b both have
ortho-methyl groups. However, the isostructural nature of 8
and 7 is interesting because the crystal structures (and
solution structures) of ligands 4c and 4b are different.
Nitroformazan 4b exists exclusively in the trans-syn, s-trans
(“closed”) structure depicted in Scheme 2, whereas 4c is
predominantly in the trans-syn, s-trans (“open”) conforma-
tion depicted in Scheme 3.21 Some of the structurally
characterized formazan complexes have the formazan ligand
chelating to the metal in an asymmetric manner through one
divalent nitrogen and one trivalent nitrogen, leading to a five-
membered metallacycle ring (9) in which the ligand con-
formation closely resembles an “open” one.13,28 From this
perspective, it is interesting to note that the conformation of
the ligand does not necessarily determine the mode of
chelation; specifically, ligand 4c, which is “open”, produces
a complex in which the formazan has the “closed” structure
as a chelate.
to the formazan backbone plane (N1-N2-C1-N3-N4)
both longitudinally (along the long axis of the complex) and
laterally; in addition, the oxygen atoms are disordered,
leading to two possible relative orientations of the Ni2O2 core
with respect to the plane of the coordinated formazans
(Figure 2c). The N2- and N4-bound mesityl rings have
dihedral angles of 77.36° and 83.26°, respectively, with
respect to the ligand backbone planes. This aromatic sub-
stituent twisting behavior has not been seen before in
formazan chemistry, for the simple reason that there were
essentially no examples of formazans themselves incorporat-
ing ortho-substituted aromatic rings prior to our recent
publication.21 This conformational behavior is, however,
typical in complexes of ꢀ-diketiminates with 2,6-dimeth-
ylphenyl substituents.16,25
(25) (a) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2003,
125, 15286. (b) MacAdams, L. A.; Buffone, G. P.; Incarvito, C. D.;
Rheingold, A. L.; Theopold, K. H. J. Am. Chem. Soc. 2005, 127, 1082.
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2007, 731.
1290 Inorganic Chemistry, Vol. 47, No. 4, 2008

Transition Metal Complexes of 3-Cyano- and 3-Nitroformazans
Figure 4. UV–vis spectra of 5 (black line), 7 (red line), and 8 (blue line)
in CH2Cl2.
Figure 5. Powder EPR spectrum of 10 at room temperature.
Scheme 4
Figure 6. Molecular structure of 10. Thermal ellipsoids are shown at the
50% probability level. Hydrogen atoms are omitted for clarity.
bis(o-hyroxyphenyl) cyanoformazans and present preliminary
explorations of their coordination chemistry here. A cop-
per(II) complex, 10, of 1,5-bis(o-methoxyphenyl)-3-cyano-
formazan was prepared by the deprotonation of ligand 4d
(sodium hydride) followed by reaction with CuCl2 (Scheme
4). The EPR spectrum of a powder sample of 10 (Figure 5)
is typical for a Cu(II) complex, with the unpaired electron
The electronic spectra of complexes 5, 7, and 8 are shown
in Figure 4. It is well-known that free (noncoordinated)
formazans are intensely colored;8 the absorption wavelength
maxima depend on the conformation of the formazan as well
as the substitution pattern. Most “closed” formazans have
intense (ꢀ > 104 L cm-1 mol-1) absorption maxima between
450 and 550 nm, including 4a and 4b;21 in comparison, the
absorption maxima of formazans which adopt the “open
structure” are usually blue-shifted (e.g., 4c, 381 nm).21 We
believe that the lowest energy absorption is therefore charge-
transfer in nature involving the N-aromatic substituent and
the formazan skeleton. The monometallic bis(formazanato)-
Ni(II) 5 has its lowest energy transition maximum at 673
nm. This transition is most likely ligand-based charge transfer
(LLCT) and as such is strongly blue-shifted relative to the
free ligand 4a due to a combination of conformational
locking of the “closed” ligand structure and enhanced
delocalization in the formazan backbone.
The two hydroxo-bridged Ni2 formazan complexes 7 and
8 have nearly identical low-energy maxima near 620 nm.
These maxima are blue-shifted relative to 5 and also have
significantly smaller extinction coefficients. We believe that
the changes in the spectra of 7 and 8 relative to 5 are related
to the structural aspects of the coordinated ligand and not to
differences in the nature of the complexes (mono vs
binuclear). Both 7 and 8 have large torsion angles between
the N-aromatic groups and the formazan backbone as a result
of steric effects of the ortho-methyl substituents. This
twisting of N-aromatic groups has been shown to cause both
a blue shift and a bleaching effect in the absorption spectra
of free formazans,21 similar to the effects observed for the
nickel formazan complexes.
occupying the dx -y orbital.29 Spectral simulation afforded
2
2
g| ) 2.174, and g ) 2.064. The room-temperature magnetic
1
moment of 10 is 2.08 µB as expected for an S ) /2 Cu(II)
ion with g > 2.
X-ray-quality crystals of 10 were grown via diffusion of
hexanes into a dichloromethane solution. The molecular
structure of 10 is shown in Figure 6. The bond lengths within
the formazan ligand backbone indicate that it is fully
delocalized. The 2-methoxyphenyl substituents are twisted
with respect to the formazan plane (N1-N2-C1-N3-N4)
by 23.76° (ring attached to N2) and 25.61° (ring attached to
N3). The geometry around the copper center is irregular and
can be described only as pseudo-five-coordinate; one of the
Cu-O bonds is considerably longer (Cu-O2, 2.479(2)Å)
than the other (Cu-O1, 2.068(2) Å). The copper atom lies
0.487Å above the plane of the formazan skeleton.
The electronic spectrum of 10 is shown in Figure 7 along
with the spectrum of ligand 4d for comparison. The free
ligand has an intense LLCT transition at 464 nm; as was
seen for the nickel formazan complexes, coordination causes
this transition to be red-shifted (λmax 547 nm) due to the
increased delocalization and conformational constraints as-
sociated with coordination.
(26) (a) Renkema, W. E.; Lute, C. N.; Stam, C. H. Acta Crystallogr., Sect.
B 1979, 35, 75. (b) Meuldijk, J.; Renkema, W. E.; Vanherk, A. M.;
Stam, C. H. Acta Crystallogr., Sect. C 1983, 39, 1536.
(27) Balt, S.; Klok, C.; Meuldijk, J.; Deboer, J.; Stam, C. H. Acta
Crystallogr., Sect. C 1985, 41, 528.
Metal Complexes of Tetradentate Formazans. Ad-
ditional functional groups (-OH,26 carboxyl,27 or N-hetero-
cyclic13,28) on the N-aromatic substituents can increase the
denticity of formazan ligands. We recently described21 the
synthesis and characterizization of bis(o-methoxyphenyl) and
(28) Takhirov, T. G.; Aleksandrov, Y. I.; Lipunova, G. N.; Rusinova, L. I.;
Klyuev, N. A.; Kozina, O. A.; Dyachenko, O. A.; Atovmyan, L. O.
Zh. Obshch. Khim. 1989, 59, 2362.
(29) Garribba, E.; Micera, G. J. Chem. Educ. 2006, 83, 1229.
Inorganic Chemistry, Vol. 47, No. 4, 2008 1291

Gilroy et al.
Figure 7. UV–vis spectra of 4d (black line) and 10 (red line) in CH2Cl2.
Scheme 5
Figure 8. Molecular structure of 11a: (a) partial atom labeling scheme
and (b) view down the N30-Fe-N40 axis. Thermal ellipsoids are shown
at the 50% probability level. Hydrogen atoms are removed for clarity.
1,5-Bis(2-hydroxyphenyl)-3-cyanoformazan 4e has also
been employed as a ligand. Mononuclear iron and cobalt
complexes 11a and 11b were prepared by reaction of the
deprotonated ligand with FeCl3 or CoCl2 in the presence of
pyridine (Scheme 5). In the case of cobalt, air oxidation
resulted in isolation of the complex in the Co(III) state. The
room-temperature magnetic moment of 11a is 1.82 µB,
Table 2. Selected Bond Distances (Å) and Selected Bond Angles
(deg) for 9, 11a, and 11b
10 (M ) Cu)
11a (M ) Fe)
11b (M ) Co)
N2-N1
1.297(4)
1.286(4)
1.340(5)
1.354(5)
1.928(3)
1.946(3)
2.068(3)
2.479(2)
2.2147(10)
1.310(3)
1.310(3)
1.352(4)
1.347(4)
1.846(2)
1.849(2)
1.9121(17)
1.9218(18)
1.288(3)
1.286(3)
1.357(4)
1.358(4)
1.856(2)
1.860(2)
1.8979(17)
1.9075(18)
N3-N4
N1-C1
1
indicating that the Fe(III) ion is low-spin. The H NMR
N3-C1
N2-M
spectrum of 11b is consistent with two equivalent pyridine
ligands and two equivalent ortho-substituted aromatic rings.
The structure of 11a is shown in Figure 8. The iron center
is pseudo-octahedral; the four equatorial sites are occupied
by the delocalized and completely planar (including the
ortho-phenyl substituents) tetradentate formazan ligand, while
two pyridine ligands coordinate to the two axial positions.
The iron center lies in the same plane as the formazan. Each
of the coordinated pyridines is oriented such that their ring
planes bisect the N2-Fe-N4 bond angle (i.e., they are
essentially coincident with the axis defined by the cyano
substituent, Figure 8b). The corresponding Co(III) complex,
11b, is isostructural to 11a and as such shares all of the
qualitative structural features described above; pertinent bond
lengths and angles for these two structures are summarized
in Table 2.
The electronic spectra of complexes 11a and 11b and free
formazan 4e are presented in Figure 9. As previously dis-
cussed,21 the uncoordinated ligand has an intense absorption
maximum at 480 nm, and another transition is also apparent
on the low-energy shoulder of this absorption peak. The 480
nm peak appears to remain unchanged in the spectra of the
two complexes 11a and 11b, but the extinction coefficients
of this peak are much smaller in the complexes. The spectra
of the complexes have additional absorption peaks in the
UV (λmax ) 322 nm for 11a and λmax ) 325 nm for 11b)
and in the visible (λmax ) 592 and 631 nm for 11a and λmax
) 608 and 658 nm for 11b) spectra. The qualitative
similarities between the spectra of 11a and 11b suggest that
N4-M
M-O1
M-O2
M-Cl1
M-N30
2.010(2)
1.997(2)
117.6(2)
118.0(2)
128.83(17)
128.45(18)
86.29(8)
86.08(8)
93.49(9)
94.17(7)
1.966(2)
1.954(2)
117.5(2)
M-N40
N2-N1-C1
N4-N3-C1
N1-N2-M
N3-N4-M
N2-M-O1
N4-M-O2
N2-M-N4
O2-M-O1
N2-M-Cl1
N4-M-Cl1
N30-M-N40
118.9(3)
118.5(3)
127.8(3)
127.8(3)
80.10(12)
71.26(10)
87.62(13)
97.47(9)
155.37(9)
103.61(10)
117.8(2)
128.17(17)
127.79(19)
86.42(9)
85.97(9)
94.22(10)
93.41(8)
175.96(8)
177.26(8)
these transitions are ligand-based in character, although the
possibility that some of them may be charge-transfer bands
(ligand-to-metal or metal-to-ligand charge transfer) cannot
be excluded.
Conclusions
We have described preliminary studies of the coordination
chemistry of 3-cyano- and 3-nitroformazan ligands. Previously
described complexes of these ligands have not been well
characterized, particularly with respect to structural data, as there
were no structurally characterized cyano- or nitroformazan
complexes prior to this work. For all of the complexes discussed
herein, the formazans bind to the metal ion via the N1 and N5
atoms to produce a six-membered metallacycle; no five-
1292 Inorganic Chemistry, Vol. 47, No. 4, 2008

Transition Metal Complexes of 3-Cyano- and 3-Nitroformazans
MHz Bruker instrument. EPR spectra were recorded on a Bruker
EMX EPR instrument equipped with an X-band microwave bridge.
Infrared spectra were recorded as KBr pellets using a Perkin-Elmer
Spectrum One instrument. UV–vis spectra were recorded using a
Cary 50 Scan instrument. Elemental analyses were carried out by
Canadian Microanalytical Services Ltd., Vancouver, BC.
X-Ray Structure Determinations. X-ray diffraction data were
collected on a Bruker PLATFORM/SMART 1000 CCD with
graphite-monochromatized Mo KR radiation (λ ) 0.71073 Å). The
crystal structures were solved by direct methods (SHELXS-97);
see Table 3 for crystallographic data.
Figure 9. UV–vis spectra of 4e (black line), 11a (red line), and 11b (blue
Synthesis of Bis(3-nitro-1,5-p-tolylformazanato)nickel (5).
Formazan 4a (0.400 g, 1.3 mmol), nickel acetate tetrahydrate (0.167
g, 0.67 mmol), and 95% ethanol (50 mL) were combined and
allowed to reflux open to the air for 16 h. After cooling to room
temperature, the mixture was filtered and the solid washed with
95% ethanol (3 × 10 mL), affording 5 as a dark green microcrys-
talline solid, yield: 0.362 g (85.5%). X-ray-quality crystals were
grown via diffusion of pentane into a solution of 5 in dichlo-
line) in CH2Cl2.
membered metallaformazan rings (9) were observed. This is
of particular interest with respect to formazan 4c. Even though
it exists as an “open” conformation, which would seem to favor
complex structures analogous to 9, the six-membered chelate
ring is formed. This may in part be due to the particular set of
substituents in these ligands. The strong electron-withdrawing
nature of groups at the C3 position may reduce the coordinating
ability of the N2 and N4 atoms through a combination of
inductive and resonance effects. In a more general context, this
suggests that, although isomerism is a common facet of
formazans, the particular structure adopted by a given formazan
may not have direct implications for its mode of coordination;
the electronic effects of substituents or the presence of additional
donor sites (cf. 4d and 4e) play a more central role in
determining complex structure.
The formazan complexes retain the color characteristics of
free ligands, and in this respect, these ligands are distinct from
the many other benchmark ancillary ligands. However, aside
from this aspect of their chemistry, it remains to be seen whether
formazan ligands are merely “nacnac” mimics or whether they
convey other distinctive properties to the resulting complexes.
A particular avenue of interest to us concerns the possibility of
alternative electronic structure formulations to the generic one
represented by canonical structure 12. We have already
demonstrated that reduction of boron-formazan complexes (12;
E ) B(OAc)2) leads to radical anions 13 (E ) B(OAc)2) in
which the unpaired electron resides on the formazan ligand,
thus justifying its description as a “borataverdazyl”.10 The
formazan complexes described herein are clear examples of the
general structure 12 where E is a transition metal fragment;
attempts to realize “metalloverdazyl”-type systems, 13, are in
progress and will be reported in due course.
1
romethane. Mp.: 358–360 °C. H NMR (CD2Cl2): δ 7.42 (d, 4H,
3
3J ) 8 Hz), 7.00 (d, 4H, J ) 8 Hz), 2.34 (s, 6H). 13C NMR
(CD2Cl2): δ 148.7, 140.5, 129.9, 123.6, 21.5 ppm. FT-IR (KBr):
1525 (m), 1355 (s), 1275 (s), 819 (m) cm-1. UV–vis (CH2Cl2):
λmax 310 nm (ꢀ ) 18 500), 409 nm (ꢀ ) 27 500), 673 nm (ꢀ )
10 000). Anal. calcd for NiC30H28N10O4: C, 55.32; H, 4.33; N,
21.51. Found: C, 55.06; H, 4.22; N, 20.97.
Synthesis of Bis-(1,5-mesityl-3-nitroformazanato)-bis-µ-hy-
droxonickel (7). Formazan 4b (0.610 g, 1.7 mmol), nickel acetate
tetrahydrate (0.430 g, 1.7 mmol), and methanol (100 mL) were
combined and allowed to reflux open to the air for 16 h. After cooling
to room temperature, the mixture was filtered and the solid washed
with methanol (3 × 10 mL), affording 7 as a dark green microcrys-
talline solid, yield: 0.431 g (59.2%). X-ray-quality crystals were grown
via vapor diffusion of pentane into a solution of 7 in dichloromethane.
Mp.: 276–278 °C (dec). 1H NMR (CD2Cl2): δ 6.61 (s, 8H), 2.41 (s,
24H), 2.26 (s, 12H), -6.61 (s, 2H, µ-OH). 13C NMR (CD2Cl2): δ
143.1, 138.4, 130.8, 129.6, 21.5, 18.5 ppm. FT-IR (KBr): 3601 (m)
(OH), 1537 (s), 1371 (s), 1355 (s), 1294 (s), 761 (s) cm-1. UV–vis
(CH2Cl2): λmax 298 nm (ꢀ ) 34 000), 392 nm (ꢀ ) 14 750), 635 nm
(ꢀ ) 2000). Anal. calcd for Ni2C38H46N10O2: C, 53.30; H, 5.42; N,
16.36. Found: C, 53.30; H, 5.48; N, 16.45.
Synthesis of Bis-(3-cyano-1,5-(2,6-dimethylphenyl)formaza-
nato)-bis-µ-hydroxonickel (8). Formazan 4c (0.540 g, 1.8 mmol),
nickel acetate tetrahydrate (0.440 g, 1.8 mmol), and 95% ethanol (100
mL) were combined and allowed to reflux open to the air for 16 h.
After cooling to room temperature, the mixture was filtered and the
solid washed with 95% ethanol (3 × 10 mL), affording 8 as a dark
green microcrystalline solid, yield: 0.440 g (32.2%). X-ray-quality
crystals were grown via diffusion of pentane into a solution of 8 in
dichloromethane. Mp.: 266–268 °C (dec). 1H NMR (CD2Cl2): δ 6.87
(t, 4H, 3J ) 7 Hz), 6.73 (d, 8H, 3J ) 8 Hz), 2.41 (s, 24H), -6.94 (s,
2H, µ-OH). 13C NMR (CD2Cl2): δ 145.2, 130.9, 129.2, 129.1, 18.3
ppm. FT-IR (KBr): 3603 (m) (OH), 2229 (m) (CN), 1358 (s) cm-1
.
UV–vis (CH2Cl2): λmax 271 nm (ꢀ ) 27 500), 307 nm (ꢀ ) 21 500),
620 nm (ꢀ ) 1950). Anal. calcd for Ni2C36H38N10O2: C, 56.88; H,
5.04; N, 18.43. Found: C, 56.99; H, 5.13; N, 18.32.
Copper(3-cyano-1,5-(2-methoxyphenylformazanato)chlor-
ide (10). Formazan 4d (1.00 g, 3.2 mmol) was combined with
freshly distilled tetrahydrofuran (100 mL) under argon and treated
with sodium hydride (0.080 g, 3.3 mmol) before it was allowed to
stir for 16 h, changing in color from red to brown-red. Copper(II)
chloride (0.432 g, 3.3 mmol) was added, and the reaction was once
again left to stir for 16 h. The solvent was then removed and the
Experimental Section
General Considerations. All reactions and manipulations were
carried out under an argon atmosphere using standard Schlenk or
glovebox techniques unless stated otherwise. Solvents were dried
and distilled under argon prior to use. All reagents were purchased
from Aldrich and used as received. The synthesis of formazans
4a-e have been described.21 NMR spectra were recorded on a 300
Inorganic Chemistry, Vol. 47, No. 4, 2008 1293

Gilroy et al.
solid triturated with hexanes (100 mL). The solid was then dissolved
in dichloromethane (3 × 150 mL), leaving behind a small amount
of brown solid byproduct. The solution was filtered before being
concentrated in vacuo, affording 10 as a dark purple solid (green
reflex), yield: 1.00 g (75.9%). X-ray-quality crystals were grown
from the solvent diffusion of hexanes into a dichloromethane
solution of 10. Mp.: 180–182 °C (dec., brown). FT-IR (KBr): 2225
(m) (CN), 1587 (m), 1483 (m), 1344 (s), 752 (s) cm-1. UV–vis
(CH2Cl2): λmax 256 nm (ꢀ ) 13 750), 288 nm (ꢀ ) 14 750), 371
nm (ꢀ ) 6250), 547 nm (ꢀ ) 19 250). Anal. calcd for C19H16N4:
C, 47.18; H, 3.46; N, 17.19. Found: C, 47.21; H, 3.81; N, 16.87.
Iron(3-cyano-1,5-(2-hydroxyphenylformazanato)-bis-pyri-
dine (11a). Formazan 4e (0.250 g, 0.88 mmol) was combined with
freshly distilled tetrahydrofuran (100 mL) under argon and treated
with sodium hydride (0.064 g, 2.7 mmol) before being left to stir
for 16 h, changing in color from red to dark blue. Iron(III) chloride
(0.144 g, 0.88 mmol) and pyridine (0.36 mL, 4.5 mmol) were added,
and the reaction was left to stir for 16 h, at which time the solution
was a dark blue color. The mixture was then filtered, and the solvent
was removed in Vacuo. Trituration with hexanes allowed for 11a
to be isolated as a dark blue solid (bronze reflex), yield: 0.317 g
(72.9%). X-ray-quality crystals were grown from the solvent
diffusion of hexanes into a dichloromethane solution of 11a. Mp.:
270–272 °C. FT-IR (KBr): 2225 (m) (CN), 1577 (m), 1459 (s),
1448 (s), 1300 (s) cm-1. UV–vis (CH2Cl2): λmax 234 nm (ꢀ )
32 500), 320 nm (ꢀ ) 14 000), 484 nm (ꢀ ) 9250), 592 nm (ꢀ )
13 750), 631 nm (ꢀ ) 17 250). Anal. calcd for FeC24H18N7O2: C,
58.55; H, 3.69; N, 19.92. Found: C, 57.63; H, 3.82; N, 19.36.
Cobalt(3-cyano-1,5-(2-hydroxyphenylformazanato)-bis-pyri-
dine (11b). Formazan 4e (0.250 g, 0.88 mmol) was combined with
freshly distilled tetrahydrofuran (100 mL) under argon and treated
with sodium hydride (0.064 g, 2.7 mmol) before being left to stir
for 16 h, changing in color from red to dark blue. Cobalt(II) chloride
(0.115 g, 0.88 mmol) and pyridine (0.36 mL, 4.5 mmol) were added,
and the reaction was left to stir for 16 h, at which time the solution
had changed to a red-brown color. The mixture was then allowed
to stir open to the air, changing in color to dark blue before the
solution was filtered and the solvent removed in Vacuo. Trituration
with hexanes allowed 11b to be isolated as a dark blue solid (bronze
reflex), yield: 0.325 g (74.3%). X-ray-quality crystals were grown
from the solvent diffusion of hexanes into a dichloromethane
1
solution of 11b. Mp.: 262–264 °C. H NMR (CD2Cl2): δ 7.99 (d
of d, 2H, 3J ) 7 Hz, 4J ) 1 Hz), 7.41 (t, 2H, 3J ) 7 Hz), 7.37 (d,
3
3
4
4H, J ) 7 Hz), 7.14 (d of d, 2H, J ) 8 Hz, J ) 1 Hz), 6.98 (t
3
4
3
of d, 2H, J ) 7 Hz, J ) 1 Hz), 6.91 (t, 4H, J ) 7 Hz), 6.53 (t
of d, 2H, J ) 7 Hz, J ) 1 Hz). 13C NMR (CD2Cl2): δ 168.4,
151.6, 147.5, 139.2, 130.4, 125.1, 119.4, 116.4, 116.2 ppm. FT-IR
(KBr): 2219 (m) (CN), 1462 (s), 1448 (s), 1234 (s) cm-1. UV–vis
(CH2Cl2): λmax 242 nm (ꢀ ) 42 500), 325 nm (ꢀ ) 10 000), 483
nm (ꢀ ) 4250), 509 nm (ꢀ ) 4250), 608 nm (ꢀ ) 14 500), 658 nm
(ꢀ ) 24 750). Anal. calcd for CoC24H18N7O2: C, 58.19; H, 3.66;
N, 19.79. Found: C, 57.78; H, 3.79; N, 19.39.
3
4
Acknowledgment. Financial support from the University
of Victoria, The Natural Sciences and Engineering Research
Council of Canada, and the Petroleum Research Fund of the
American Chemical Society is gratefully acknowledged.
Supporting Information Available: Characterization and crys-
tallographic information for compounds 5, 7, 8, 10, 11a, and 11b.
This material is available free of charge via the Internet at http://
pubs.acs.org.
IC7019846
1294 Inorganic Chemistry, Vol. 47, No. 4, 2008