A Bis(azo-imine)palladium(II) System with 10 Ligand π Electrons. Synthesis, Structure, Serial Redox, and Relationship to Bis(azooximates) and Other Species

Article abstract of DOI:10.1021/ic950361t

The first azo-imine chelate system, Pd(N (H)C (R)NNPh)₂ (Pd(RA)₂), has been isolated in the form of diamagnetic solids by the 6e^--6H⁺ reduction of bis(phenylazooximato)palladium(II), Pd(N (O)C (R)NNPh)₂ (abbreviated Pd(RB)₂), with ascorbic acid in a mixed solvent (R = Ph, α-naphthyl). Selected spectral features are described. The X-ray structures of Pd(PhA)₂ and Pd(PhB)₂ have revealed trans-planar geometry consistent with metal oxidation state of +2. Bond length trends within the chelate rings are rationalized in terms of steric and electronic factors. In Pd(PhA)₂ a total of 10 ligand π electrons are present, each formally monoanionic ligand contributing five. Model EHMO studies have revealed that the filled HOMO (a_u) in Pd(RA)₂ is a bonding combination of two ligand π* orbitals with large azo contributions. The LUMO (b_g¹³) is roughly the corresponding antibonding combination. The outer π-electron configuration of Pd(RA)₂ is (a_u)²(bg)⁰. Four successive voltammetric responses, two oxidative and two reductive, are observed. The E_1/2 range is - 1.3 to +0.8 V vs SCE for Pd(PhA)₂ in a 1:9 MeCN-CH₂Cl₂ mixture (Pt electrode). EPR and electronic spectra of the electrogenerated one-electron-oxidized complex Pd(PhA)₂⁺ are described. The azo-imine system is compared with imine-imine and azo-azo systems. Crystal data for the complexes are as follows. Pd(PhA)₂: crystal system monoclinic; space group C2/c; a = 18.167(5) A?, b = 7.420(3) A?, c = 16.527(6) A?; β= 92.70(3)°; V = 2225(1) A?³ Z = 4; R = 2.61%, R_w = 3.58%. Pd(PhB)₂: crystal system monoclinic; space group P2₁/n; a = 5.735(5) A?, b = 10.797(6) A?, c = 18.022(11) A?; β = 97.73(6) A?; V = 1105(1) A?³ Z = 2; R = 3.37%; R_w = 3.40%.

Full text of DOI:10.1021/ic950361t

2442
Inorg. Chem. 1996, 35, 2442-2447
A Bis(azo-imine)palladium(II) System with 10 Ligand π Electrons. Synthesis, Structure,
Serial Redox, and Relationship to Bis(azooximates) and Other Species
Chandan Kumar Pal, Surajit Chattopadhyay, Chittaranjan Sinha, and
Animesh Chakravorty*
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science,
Calcutta 700 032, India
ReceiVed March 24, 1995X
The first azo-imine chelate system, Pd(N(H)C(R)NNPh)2 (Pd(RA)2), has been isolated in the form of diamagnetic
solids by the 6e--6H+ reduction of bis(phenylazooximato)palladium(II), Pd(N(O)C(R)NNPh)2 (abbreviated Pd-
(RB)2), with ascorbic acid in a mixed solvent (R ) Ph, R-naphthyl). Selected spectral features are described.
The X-ray structures of Pd(PhA)2 and Pd(PhB)2 have revealed trans-planar geometry consistent with metal oxidation
state of +2. Bond length trends within the chelate rings are rationalized in terms of steric and electronic factors.
In Pd(PhA)2 a total of 10 ligand π electrons are present, each formally monoanionic ligand contributing five.
Model EHMO studies have revealed that the filled HOMO (au) in Pd(RA)2 is a bonding combination of two
ligand π* orbitals with large azo contributions. The LUMO (bg) is roughly the corresponding antibonding
combination. The outer π-electron configuration of Pd(RA)2 is (au)2(bg)0. Four successive voltammetric responses,
two oxidative and two reductive, are observed. The E1/2 range is -1.3 to +0.8 V vs SCE for Pd(PhA)2 in a 1:9
MeCN-CH2Cl2 mixture (Pt electrode). EPR and electronic spectra of the electrogenerated one-electron-oxidized
complex Pd(PhA)2+ are described. The azo-imine system is compared with imine-imine and azo-azo systems.
Crystal data for the complexes are as follows. Pd(PhA)2: crystal system monoclinic; space group C2/c; a )
18.167(5) Å, b ) 7.420(3) Å, c ) 16.527(6) Å; â ) 92.70(3)°; V ) 2225(1) Å3; Z ) 4; R ) 2.61%, Rw ) 3.58%.
Pd(PhB)2: crystal system monoclinic; space group P21/n; a ) 5.735(5) Å, b ) 10.797(6) Å, c ) 18.022(11) Å;
â ) 97.73(6) Å; V ) 1105(1) Å3; Z ) 2; R ) 3.37%; Rw ) 3.40%.
Introduction
along with X-ray structures of one complex and its azooxime
precursor. The electronic structure, spectra, and electroactivity
of the species are scrutinized.
Metal chelates of conjugated ligands are of abiding interest
in inorganic research, especially those bearing one or more π*
orbitals lying low enough to promote properties like successive
electron transfer, intense color, and oxidation state ambivalence.
The dithiolenes,1 dioxolenes,2 diimines,3-8 and tetraazenides9
constitute good examples.10 In the present work, we introduce
the azo-imine system in the form of electroneutral diamagnetic
bis chelates of palladium. The synthetic method is described
Results and Discussion
A. Synthesis and Spectral Characterization. The two
azo-imine complexes reported here belong to the type 1
structure and are generally abbreviated as Pd(RA)2, the R
* Corresponding author. Telefax: +91-33-473-2805. E-mail: icac@
iacs.ernet.in.
X Abstract published in AdVance ACS Abstracts, March 1, 1996.
(1) (a) Burns, R. P.; McAuliffe, C. A. AdV. Inorg. Chem. Radiochem.
1979, 22, 303. (b) McCleverty, J. A. Prog. Inorg. Chem. 1968, 10,
49.
(2) (a) Lange, C. W.; Conklin, B. J.; Pierpont, C. G. Inorg. Chem. 1994,
33, 1276. (b) Bhattacharya, S.; Pierpont, C. G. Inorg. Chem. 1992,
31, 35 and references therein. (c) Masui, H.; Lever, A. B. P.; Auburn,
P. R. Inorg. Chem. 1991, 30, 2402. (d) Bag, N.; Pramanik, A.; Lahiri,
G. K.; Chakravorty, A. Inorg. Chem. 1992, 31, 40.
(3) Stufkens, D. J. Coord. Chem. ReV. 1990, 104, 39.
(4) van Koten, G.; Vrieze, K. AdV. Organomet. Chem. 1982, 21, 151.
(5) (a) DeArmond, M. K.; Hanck, K. W.; Wertz, D. W. Coord. Chem.
ReV. 1985, 64, 65. (b) Vlcek, A. A. Coord. Chem. ReV. 1982, 43, 39.
(6) (a) Ghosh, P.; Chakravorty, A. Inorg. Chem. 1984, 23, 2242. (b)
Dodsworth, E. S.; Lever, A. B. P. Chem. Phys. Lett. 1986, 124, 152.
(7) (a) Hitoshi, M.; Lever, A. B. P.; Dodsworth, E. S. Inorg. Chem. 1993,
32, 258 and references therein. (b) Braterman, P. S.; Song, J.-I.;
Wimmer, F. M.; Wimmer, S.; Kaim, W.; Klein, A.; Peacock, R. D.
Inorg. Chem. 1992, 31, 5084.
(8) (a) Cloke, F. G. N.; Dalby, C. I.; Daff, P. J.; Green, J. C. J. Chem.
Soc., Dalton Trans. 1991, 181. (b) Gardiner, M. G.; Hanson, G. R.;
Henderson, M. J.; Lee, F. C.; Raston, C. L. Inorg. Chem. 1994, 33,
2456.
substituent being Ph or R-naphthyl (Np). In 1, the dotted
semicircle represents π electrons in the NNCN frame. The
synthetic precursor to 1 is the bis(arylazooximate) Pd(RB)2,
2.11,12 Attempted isolation of 1e--1H+ reduction products of
Pd(RB)2 in a manner similar to that recently described13 for
Pt(RB)2 did not succeed, but the search led us to Pd(RA)2.
(11) Bandyopadhyay, P.; Mascharak, P. K.; Chakravorty, A. J. Chem. Soc.,
Dalton Trans. 1981, 623.
(9) (a) Trogler, W. C. Acc. Chem. Res. 1990, 23, 426. (b) Lee, S. W.;
Miller, G. A.; Campana, C. F.; Trogler, W. C. Inorg. Chem. 1988,
27, 1215.
(12) Bandyopadhyay, D.; Bandyopadhyay, P.; Chakravorty, A.; Cotton, F.
A.; Falvello, L. R. Inorg. Chem. 1984, 23, 1785.
(13) Pal, C. K.; Chattopadhyay, S.; Sinha, C.; Chakravorty, A. Inorg. Chem.
1994, 33, 6140.
(10) Ghosh, B. K.; Chakravorty, A. Coord. Chem. ReV. 1989, 95, 129.
0020-1669/96/1335-2442$12.00/0 © 1996 American Chemical Society

A Bis(azo-imine)palladium(II) System
Table 1. Spectral Data
Inorganic Chemistry, Vol. 35, No. 9, 1996 2443
UV-vis:a
λ
max, nm
IR νN-H
,
1H NMRb
δ(N-H), ppm
complex
(ꢀ, M-1 cm-1
)
cm-1
Pd(PhA)2
740 (17 900),
465sh (3200),
350 (16 400)
3370c
6.766
3365d
3340c
Pd(NpA)2
730 (16 500),
465sh (4000),
380 (10 400)
6.822
3347d
a In dichloromethane. Superscript sh ) shoulder. b In CDCl3;
multiplets in the region δ 7.2-8.6, aromatic protons. c In KBr disk.
d In CCl4 solution.
Figure 2. ORTEP plot and atom-labeling scheme for Pd(PhA)2. All
non-hydrogen atoms are represented by their 50% probability ellipsoids.
Figure 1. Electronic spectrum of Pd(PhA)2 in dichloromethane
solution.
The synthetic procedure consists of heating Pd(RB)2 with
excess ascorbic acid in a mixed-solvent medium. Chromato-
graphic workup of the reaction mixture afforded Pd(RA)2 as a
dark solid in good yield. The synthetic reaction, eq 1, consumes
Figure 3. ORTEP plot and atom-labeling scheme for Pd(PhB)2. All
non-hydrogen atoms are represented by their 50% probability ellipsoids.
Table 2. Selected Bond distances (Å) and Angles (deg) and Their
Estimated Standard Deviations for Pd(PhA)2 and Pd(PhB)2
Pd(RB)2 + 6e- + 6H+ f Pd(RA)2 + 2H2O
(1)
Pd(PhA)2
Distances
Pd(PhB)2
2(dN-O-) + 4e- + 6H+ f 2(dNH) + 2H2O (2)
Pd-N(1)
1.967(2)
1.973(2)
1.332(3)
1.342(3)
1.320(3)
0.89(2)
2.039(4)
2.018(5)
1.276(6)
1.375(7)
1.329(8)
Pd-N(3)
N(3)-N(2)
N(2)-C(1)
C(1)-N(1)
N(1)-H(1)
N(1)-O(1)
6e- and 6H+, of which 4e- and 6H+ are used in the oximato
f imine conversion, eq 2. The remaining 2e- go to the
azo-imine frame in Pd(RA)2; Vide infra. The free azo-imine
ligand PhNdNC(R)dNH is not known, and we have not
succeeded in isolating it either by demetalation of Pd(RA)2 or
by reduction of free arylazooximes, PhNdNC(R)dNOH. The
azo-imine system reported here is apparently formed and
stabilized only in the chelated state.
We have tried to synthesize R ) alkyl species. Upon ascorbic
acid reduction of Pd(MeB)2, a violet solution was obtained,
displaying electronic spectra very similar to that of Pd(PhA)2.
The complex however decomposes progressively in solution,
and we have not succeeded in isolating it.
The Pd(RA)2 complexes are diamagnetic both in the solid
state and in solution. Selected spectral data are given in Table
1. The yellow-green solutions of Pd(RA)2 in dichloromethane
absorb strongly in the visible region (Figure 1). The N-H
stretch occurs as a sharp feature of moderate intensity in the
range 3340-3370 cm-1 in both the solid and solution phases.
The strong N-O stretch near 1240 cm-1 characteristic of Pd-
(RB)211 (no νN-H) is absent for Pd(RA)2. In 1H NMR (CDCl3)
the aromatic protons of Pd(RA)2 afford sharp multiplets. The
N-H signal is observed as a relatively broad (14N quadrupole
moment) singlet near δ 6.8 (width at half-height 0.06 ppm).
1.258(7)
Angles
N(3)-Pd-N(1)
N(3)-Pd-N(1A)
Pd-N(1)-C(1)
N(2)-C(1)-N(1)
N(3)-N(2)-C(1)
Pd-N(3)-N(2)
N(1)-Pd-N(1A)
N(3)-Pd-N(3A)
N(1)-Pd-N(3A)
N(3)-Pd-N(1A)
Pd-N(1)-H(1)
Pd-N(1)-O(1)
77.1(1)
76.6(2)
103.0(1)
114.9(2)
118.4(2)
111.3(2)
118.1(1)
171.2(1)
179.6(1)
103.0(1)
103.0(1)
123.4(8)
103.4(2)
114.3(4)
115.9(5)
116.0(4)
117.2(3)
180.0(1)
180.0(1)
103.4(2)
103.4(2)
124.0(4)
B. Structure. a. Trans-Planar Geometry and Metal Oxi-
dation State. The X-ray structure of Pd(PhA)2 and for
comparison that of Pd(PhB)2 have been determined. Molecular
views are shown in Figures 2 and 3, and selected bond
parameters are listed in Table 2. Both complexes have trans-
planar geometry.
In Pd(RA)2 (Figure 2), the metal atom lies on a crystal-
lographic 2-fold axis of symmetry making the halves of the

2444 Inorganic Chemistry, Vol. 35, No. 9, 1996
Pal et al.
Table 3. Relative Percent Atomic Contributionsa to the MO’s of 5
molecule equivalent. The chelate ring is excellently planar
(mean deviation 0.02 Å), and the dihedral angle between the
two of them is 11.6°. The PdN4 coordination sphere is
approximately planar (mean deviation 0.07 Å), the dihedral
angle between the planes of Pd, N(1), N(3) and Pd, N(1A),
N(3A) being 9.1°. The phenyl rings attached to C(1) and N(3)
are inclined to the plane of the chelate ring by 16.2 and 32.0°,
respectively. All hydrogen atoms in the structure including the
N-H hydrogen were observed directly in difference Fourier
maps.
largest
MO
Pd 2N(3) 2N(2) 2N(1) 2C(1) contributor
HOMO (au)
-11.79 eV
LUMO (bg) 31
1
47
29
13
9
azo-π*
azo-π*
36
16
12
4
-11.18 eV
a These do not add up to 100% since <1% contributions are not
listed.
The bond lengths within the chelate rings will now be
examined. Between Pd(PhB)2 and Pd(PhA)2 the N(2)-N(3)
length increases by 0.05 Å, N(2)-C(1) decreases by 0.03 Å,
and N(1)-C(1) remains virtually unaffected (Table 2). In the
HOMO of Pd(RA)2 (in Pd(RB)2 the corresponding orbital is
the LUMO), the pairwise π interactions between adjacent
bonded atoms within each chelate ring are antibonding in N(2)-
N(3) and N(1)-C(1) but bonding in N(2)-C(1); Vide infra.
Also, the relative weightage of these interactions are in the order
N(2)-N(3) > N(2)-C(1) > N(1)-C(1). This is in qualitative
agreement with the observed trend.
In Pd(PhB)2, the metal atom lies on a crystallographic center
of symmetry making the PdN4 coordination sphere exactly
planar. The chelate ring is an excellent plane (mean deviation
0.01 Å) and the oximato oxygen atom also lies on this plane.
The phenyl rings pendent from C(1) and N(3) are inclined to
the chelate plane by 28.5 and 66.3°, respectively.
In its electroneutral form, the azo-imine ligand frame NdN-
CdN is expected to have four bonding π electrons correspond-
ing to the two double bonds. Structure 3 depicts this case. Since
the complex has no net charge, the metal oxidation state in 3 is
zero and the total number of azo-imine electrons is 8 (two
ligands). Here the coordination geometry would be pseudot-
etrahedral.
C. Molecular Orbitals. In order to gain insight into the
frontier orbitals of Pd(RA)2, extended Hu¨ckel calculations
were performed on the simplified model 5 assumed to have
In practice however Pd(PhA)2 and by inference Pd(NpA)2
are very nearly planar. This, coupled with the observed
diamagnetism, shows that the formal metal oxidation state in
Pd(RA)2 is +2. This requires each ligand to be in the formally
monoanionic radical form, RA•-, corresponding to the popula-
tion of the lowest azo-imine π* orbital. This is indicated in
structure 4 where the dotted semicircle represents five ligand π
electrons. Diamagnetism is the result of trans-metal π*-π*
interaction; Vide infra. In Pd(RB)2, 2, the ligand is simply the
nonradical oximato monoanion PhNdNC(R)dN-O- and the
metal is bivalent. There is no question of oxidation state
ambiguity here.
(i) exact planarity (C2h symmetry) and (ii) H atoms in place of
the R and Ph substituents at C(1) and N(3). Relative percent
atomic contributions of the HOMO and the LUMO are given
in Table 3.
The HOMO 6 is constituted of the bonding combination of
two groups of identical atomic orbitals, each chelate ring contrib-
b. Bond Length Trends. The average Pd-N length in Pd-
(PhA)2 is shorter than that in Pd(PhB)2 by 0.06 Å (Table 2).
The nonbonded O(1)‚‚‚C(8A) distance in Pd(PhB)2 is 3.130(8)
Å, which equals the sum (3.1 Å) of van der Waals radii of O
and C.14 A closer approach (required for shorter Pd-N bonds)
of the two chelate rings in Pd(PhB)2 can occur only at the
expense of considerable steric repulsion. In Pd(PhA)2, however,
the rings can come closer since the radii sum of C and H is 2.7
Å.14 In Pd(PhA)2, the existing N(1)H‚‚‚C(8A) distance is much
larger, 3.05(2) Å. We also note that the HOMO (Vide infra) of
Pd(RA)2 involves a bonding interaction between chelate π
frames and the ligand anionic charge lies in the chelate ring,
while in Pd(RB)2 the pendent oximato oxygen atoms hold at
least a considerable part of the charge. These electronic factors
could also contribute toward the relative shortness of Pd-N
bonds in Pd(PhA)2, but the steric factor is no doubt far more
important.
uting one such group. The latter is essentially the lowest
unoccupied π* orbital of the electroneutral (4πe) azo-imine
frame, NdN-CdN. The N(2)-N(3) interaction contributes
76% to the HOMO which may therefore be roughly described
as a bonding combination of the azo-π* pair. The HOMO has
au symmetry. A metal d contribution is evidently absent, but
there is a small pz contribution (not shown in 6). Qualitatively,
the LUMO, 7, is the antibonding counterpart of 6. It has bg
representation, and the metal dxz and dyz orbitals (not shown in
7) are significant contributors. The contribution of the azo-π*
pair is 52%.
The trans-metal interaction between the two chelate rings is
appreciable, the HOMO-LUMO energy gap in the model being
∼1 eV. Thus two antibonding electrons of the two RA•-
fragments pair up, occupying the HOMO in 5. The intense Pd-
(14) Huheey, J. E. Inorganic Chemistry Principles of Structure and
ReactiVity, 3rd ed.; Harper and Row: New York, 1983; p 258.

A Bis(azo-imine)palladium(II) System
Table 4. Electrochemical Data
Inorganic Chemistry, Vol. 35, No. 9, 1996 2445
cyclic voltammetric data:a E1/2
V (∆Ep, mV)b
,
complex
Pd(PhA)2
Pd(NpA)2
0.73 (190), 0.50 (220), -0.74 (180), -1.21 (240)
0.66 (80), 0.51 (140), -0.72 (120), -1.18 (160)
a Conditions: solvent, 10% acetonitrile-90% dichloromethane;
supporting electrolyte, TEAP (0.1 M); working electrode, platinum;
b
reference electrode, SCE; solute concentration, ∼10-3 M. E1/2 is
calculated as the average of anodic (Epa) and cathodic (Epc) peak
potentials; ∆Ep ) Epa - Epc.
Figure 5. (a) X-band EPR spectra of electrogenerated Pd(PhA2)+ in
1:1 dichloromethane-toluene solution at 298 K (fluid) and at 77 K
(glass). Instrument settings: power, 30 dB; modulation, 100 kHz;
sweep center, 3200 G; sweep width, 1000 G; sweep time, 240 s;
microwave frequency, 9.1 GHz. The apparent difference in the intensity
of the signal at the two temperatures was due to the difference in
receiver gain used (3.2 × 103 at 77 K and 1.6 × 103 at 298 K). (b)
Electronic spectrum of electrogenerated Pd(PhA2)+ in 1:1 dichlo-
romethane-acetonitrile solution.
Figure 4. Cyclic (s) and differential pulse (- - -) voltammograms of
a 10-3 M solution of Pd(PhA)2 in 1:9 acetonitrile-dichloromethane
(0.1 M TEAP) at a platinum electrode with scan rates of 50 mV s-1
(CV) and 10 mV s-1 (DPV). The marked current ranges are 10 µA
(CV) and 20 µA (DPV).
reduction potential drives each azo-imine frame to accept an
extra electron spontaneously: the result is electroneutral Pd-
(RA)2.
b. Pd(RA)2+. One-electron coulometric oxidation at 0.6 V
afforded a brown solution which displayed four voltammetric
responses (three reductions and one oxidation), coinciding with
those of Pd(PhA)2. One-electron reduction of the oxidized
(RA)2 band in the visible region (Figure 1) can be assigned to
the allowed HOMO (au) f LUMO (bg) transition.
+
solution at 0.3 V regenerated Pd(PhA)2. Evidently Pd(PhA)2
For comparison, EHMO computations were also performed
retains the gross structure of the parent complex.
on the precursor complex Pd(RB)2 (R and Ph replaced by H).15
+
It has not been possible to isolate Pd(PhA)2 as a pure salt,
2
Here the HOMO (ag) is 90% metal (primarily dz , dxy) in
but it has been spectrally studied in electrogenerated solutions.
The complex is paramagnetic and displays (Figure 5a) a strong
EPR signal both in fluid solution (g ) 2.001) and in the glassy
state at 77 K (g ) 1.998). Extending the orbital description of
character and the LUMO is similar in composition to the HOMO
of 5.
D. Voltammetry. a. Serial Redox. In mixed dichlo-
romethane-acetonitrile solution, the Pd(RA)2 complexes display
four successive one-electron responses at a platinum electrode,
the E1/2 values lying in the range -1.3 to +0.8 V vs SCE (Table
4). The voltammograms of Pd(PhA)2 are displayed in Figure
4. Two of the steps correspond to oxidation and two to
reduction of Pd(RA)2, eq 3. This is apparently consistent with
+
Pd(PhA)2, we propose that Pd(PhA)2 has the π configuration
(au)1(bg)0. The HOMO (au) has large azo-π* character (Vide
supra), and this is consistent with the lack of resolvable
hyperfine structure of the EPR signal. The azo 14N coupling
constant is small (4.8 G), and it is usually difficult to resolve it
because of dominant anisotropic contributions, as has been
repeatedly documented in literature.13,16 The electronic spectrum
+
of Pd(PhA)2 in solution has a strong band at 1600 nm (ꢀ )
9420 M-1 cm-1) (Figure 5b). There are only relatively weak
absorptions near 740 nm, where the parent complex absorbs
strongly (Figure 1). The origin of the 1600 nm band is unclear
at present. It is tempting to assign it to au f bg excitation
strongly shifted to lower energy upon cation formation (Pd-
(PhA)2 to Pd(PhA)2+). Significantly the intensity of the 1600
the outermost π configuration (au)2(bg)0 of Pd(RA)2. However,
it would be improper to assign electronic structures to the Pd-
z
(RA)2 (z ) (1, (2) species till these are individually
+
nm band in Pd(PhA)2 is nearly half that of the 740 nm band
characterized. Unfortunately bulk electrogeneration of the
species by constant-potential coulometry has not succeeded so
far due to instability, except in the case of Pd(RA)2+; see below.
The one-electron couple Pd(PhB)2-Pd(PhB)2- has a E1/2 of
-0.50 V vs SCE in dichloromethane.11 Here the redox electron
occupies an orbital like 6 which is the LUMO in Pd(PhB)2.
The above couple formally corresponds to the couple Pd-
(PhA)22+-Pd(PhA)2+ (E1/2 ) 0.73 V) in eq 3. The bischelated
azo-imine function is thus far more easily reducible than the
azo-oxime function so chelated. We can now look back at
the synthetic reaction, eqs 1 and 2. Once the oximato function
is reduced to the imine, dN-O- f dNH, the highly augmented
in Pd(PhA)2.
E. Isoelectronic Systems. The azo-imine frame, 8, intro-
duced in this work is isoelectronic with the known imine-
imine,3,4 9, and azo-azo,9 10, framessall three being aza-
substituted butadienes. However unlike 8, neither 9 nor 10 is
known to afford electroneutral bis chelates of palladium (or
platinum).17 The reported systems are of the types such as (i)
(16) (a) Krejcik, M.; Zails, S.; Klima, J.; Sykora, D.; Matcheis, W.; Klein,
A.; Kaim, W. Inorg. Chem. 1993, 32, 3362. (b) Kaim, W.; Kohlmann,
S. Inorg. Chem. 1986, 25, 3442. (c) Johnson, C. S.; Chang, R. J.
Chem. Phys. 1965, 43, 3183.
(17) Such chelates (pseudotetrahedral) exist for nickel(0): (a) Servaas, P.
C.; Stufkens, D. J.; Oskam, A. Inorg. Chem. 1989, 28, 1774. (b) van
Koten, G.; Overbosch, P.; Overbeek, O. Inorg. Chem. 1982, 21, 2373.
(15) Pal, C. K.; Chakravorty, A. Unpublished.

2446 Inorganic Chemistry, Vol. 35, No. 9, 1996
Pal et al.
Experimental Section
Materials. Sodium tetrachloropalladate and (phenylazo)benzal-
doxime and (phenylazo)-1-naphthaldoxime were prepared by reported
methods.21,22 Solvents and chemicals used for syntheses were of
analytical grade. Supporting electrolyte (tetraethylammonium perchlo-
rate, TEAP) and solvents for electrochemical work were obtained as
before.23
19
Pd0(ligand)(olefin)18 and PdII(ligand)Cl2 in the case of 9 and
Physical Measurements. A Hitachi 330 spectrophotometer was
used to record UV-vis-near-IR spectra. IR spectra were recorded
with a Perkin-Elmer 783 IR spectrophotometer. A Varian E-109C
spectrometer fitted with a quartz dewar was used for EPR studies. DPPH
(g ) 2.0037) was used to calibrate the spectra. 1H NMR spectra in
CDCl3 were recorded with a Varian VXR 300S spectrometer. TMS
was used as internal standard. A Perkin-Elmer 240C elemental analyzer
was used to collect microanalytical data (CHN). Electrochemical
measurements were performed under nitrogen atmosphere on a PAR
Model 370-4 electrochemistry system as reported earlier.24 All
potentials reported in this work are referenced to the saturated calomel
electrode (SCE) and are uncorrected for junction contribution.
Synthesis of Parent Complexes. trans-Bis[(phenylazo)benzal-
doximato]palladium(II), Pd(PhB)2. The complexes were prepared
by following the reported procedures.11 Bis[(phenylazo)-1-naphthal-
doximato]palladium(II), Pd(NpB)2, was prepared by reaction of Na2-
PdCl4 with the ligand (phenylazo)-1-naphthaldoxime in a similar way.
The complex, hitherto unreported, was synthesized in 60% yield. Anal.
Calcd for C34H24N6O2Pd: C, 62.34; H, 3.69; N, 12.83. Found: C,
62.21; H, 3.78; N, 12.96. UV-vis (CH2Cl2), λmax in nm (ꢀ in M-1
cm-1): 625 (2528), 580 (sh) (2325), 418 (5351). Sharp νNO in IR (KBr
9
(ii) PdII(ligand)(PR3)2 in the case of 10. In (i) the ligand is
present in the electroneutral 4πe form 11 while in (ii) it is
formally dianionic, occurring in the 6πe form 12a, for which
a simplified valence-bond description is 12b.
It seems that the larger the number of aza atoms, the larger
are the number of π electrons and the associated anionic charge
that can be easily sustained: 11 (diaza), 4πe (charge 0); 4
(triaza), 5πe (1-); 12 (tetraaza), 6πe (2-). Last, we note that
the azo-imine system has a formal similarity to the dioxolene
system, the state of the ligand in Pd(RA)2 corresponding to the
semiquinone (sq•-) stage. The diamagnetic planar bis chelates20
of the type PdII(sq)2 are then distant relatives of Pd(RA)2.
disk) at 1240 cm-1
.
Synthesis of Azo-Imine Complexes. The azo-imine complexes
were prepared by the same general procedure from the respective
precursors. Details are given below for Pd(PhA)2.
trans-Bis[(phenylazo)benzaldiminato]palladium(II), Pd(PhA)2.
To a 1:1 dichloromethane-acetonitrile solution (20 mL) of Pd(PhB)2
(0.140 g, 0.25 mmol) was added ascorbic acid (0.130 g, 0.75 mmol) in
2 mL of water, and the resultant solution was refluxed for 2.5 h. The
deep violet solution containing the product was evaporated in vacuo.
The dark residue was washed thoroughly with water and dried. The
brown mass thus obtained was chromatographed over a silica gel (60-
120 mesh) column. The yellowish-green band containing the product
was eluted with petroleum ether (boiling range 40-60 °C) and benzene
(1:4). Evaporation of solvent afforded pure product which was dried
over anhydrous CaCl2. Yield: 80 mg, 60%. Anal. Calcd for C26H22N6-
Pd: C, 59.49; H, 4.19; N, 16.03. Found: C, 59.64; H, 4.27; N, 15.96.
Anal. Calcd for Pd(NpA)2, C34H26N6Pd: C, 65.33; H, 4.18; N, 13.45.
F. Concluding Remarks. The main findings of this work
will now be recapitulated. The first isolation of chelated azo
imines has been achieved in the form of the spin-paired Pd-
(RA)2, 4, incorporating 10 ligand π electrons (R ) Ph,
R-naphthyl). The synthetic precursor is the bis(azooximate) Pd-
(RB)2, 2. The oximate to imine reduction dN-O- f dN-H
is associated with a large shift of reduction potentials to higher
values, causing spontaneous addition of one electron to each
chelated azo-imine frame.
The Pd-N lengths in Pd(PhA)2 are significantly shorter than
those in Pd(PhB)2 owing to steric interaction between an
oximato oxygen and a pendent Ph carbon in the latter complex.
The trend of C-N and N-N lengths within the chelate rings
are consistent with the nature of the HOMO’s of the two
complexes.
Found: C, 65.18; H, 4.26; N, 13.52.
+
Pd(PhA)2
. A 20 mL 1:1 dichloromethane-acetonitrile (0.1 M
TEAP) solution of Pd(PhA)2 (7.29 mg, 0.014 mmol) was oxidized
coulometrically at a constant potential of +0.64 V vs SCE under a
dinitrogen atmosphere. Electrolysis was stopped when 1.34 C (cor-
responding to the transfer of one electron) had passed. The initial
yellow-green color of the solution changed to dark brown after
electrolysis. This solution was subjected to spectral characterization
by UV-vis-near-IR and EPR spectroscopy. The oxidized solution
was reduced at +0.30 V vs SCE under nitrogen atmosphere. This
regenerated the parent complex, Pd(PhA)2.
In Pd(RA)2, both the HOMO (fully occupied) and the LUMO
are constituted primarily of azo-π* orbitals. Observed serial
redox and low-energy electronic transitions may be associated
with these orbitals. In EPR-active Pd(PhA)2+, electrogenerated
in solution, the HOMO is singly occupied.
Molecular Orbital Calculations. Extended Hu¨ckel calculations
were performed on an IBM PC AT using the ICON software package
originally developed by Hoffmann.25 The orthogonal coordinate system
chosen for calculations is defined in structure 5. The averaged
experimental bond distances and angles were used in our calculations.
The azo-imine frame can be looked upon as the intermediate
member in the azo-azo, azo-imine, imine-imine sequence.
The known palladium chemistry, including the first azo-imine
results of this work, suggests that the chelate frame can sustain
more ligand π electrons and anionic charge as the number of
nitrogen atoms increases: imine-imine < azo-imine < azo-
azo. Ongoing work concerns synthesis and characterization of
azo-imine chelates of other transition metals.
(21) Mahapatra, A. K.; Bandyopadhyay, D.; Bandyopadhyay, P.; Chakra-
vorty, A. Inorg. Chem. 1986, 25, 2214 and references therein.
(22) Kalia, K. C.; Chakravorty, A. J. Org. Chem. 1970, 35, 2231.
(23) (a) Goswami, S.; Chakravarty, A. R.; Chakravorty, A. Inorg. Chem.
1981, 20, 2246. (b) Datta, D.; Mascharak, P. K.; Chakravorty, A.
Inorg. Chem. 1981, 20, 1673.
(24) Chandra, S. K.; Basu, P.; Ray, D.; Pal, S.; Chakravorty, A. Inorg.
Chem. 1990, 29, 2423.
(25) (a) Hoffmann, R. J. Chem. Phys. 1963, 39, 1397. (b) Ammeter, J.
H.; Bu¨rgi, H.-B.; Thibeault, J. C.; Hoffmann, R. J. Am. Chem. Soc.
1978, 100, 3686.
(18) Cavell, K. J.; Stufkens, D. J.; Vrieze, K. Inorg. Chim. Acta 1980, 47,
67.
(19) (a) van der Poel, H.; van Koten, G.; Vrieze, K. Inorg. Chem. 1980,
19, 1145. (b) van der Poel, H.; van Koten, G.; Vrieze, K.; Kokkes,
M.; Stam, C. H. J. Organomet. Chem. 1979, 175, C21.
(20) Fox, G. A.; Pierpont, C. G. Inorg. Chem. 1992, 31, 3718.

A Bis(azo-imine)palladium(II) System
Inorganic Chemistry, Vol. 35, No. 9, 1996 2447
Table 5. Crystallographic Data for Pd(PhA)2 and Pd(PhB)2
Table 6. Atomic Coordinates (×104) and Equivalent Isotropic
Displacement Coefficients (Å2 × 103) for Pd(PhA)2
Pd(PhA)2
Pd(PhB)2
x
y
z
U(eq)a
empirical formula
fw
C26H22N6Pd
524.9
C26H20N6O2Pd
554.9
Pd
0
2888(1)
2685(3)
2666(3)
2897(3)
2556(3)
2250(3)
1715(4)
1326(5)
1504(5)
2089(5)
2482(5)
2913(3)
3905(4)
3899(4)
2959(4)
2006(4)
1979(3)
2500
34(1)
41(1)
40(1)
37(1)
38(1)
40(1)
51(1)
62(1)
62(1)
69(1)
58(1)
35(1)
40(1)
48(1)
52(1)
50(1)
43(1)
space group
a, Å
b, Å
C2/c
P21/n
N(1)
N(2)
N(3)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
C(9)
C(10)
C(11)
C(12)
C(13)
36(1)
-1220(1)
-1051(1)
-612(1)
-701(1)
-1372(2)
-1452(2)
-870(2)
-205(2)
-116(2)
-1663(1)
-1622(1)
-2209(1)
-2846(2)
-2889(2)
-2304(1)
3688(1)
3522(1)
2753(1)
4012(1)
4882(1)
5155(2)
5967(2)
6510(2)
6254(2)
5447(2)
2197(1)
1484(1)
927(1)
18.167(5)
7.420(3)
16.527(6)
92.70(3)
2225.4(13)
4
5.735(5)
10.797(6)
18.022(11)
97.73(6)
1105.4(13)
2
c, Å
â, deg
V, Å3
Z
T, K
λ, Å
296
0.710 73
1.567
8.61
0.8801/1
2.61
296
0.710 73
1.667
8.78
0.9468/1
3.37
F
calcd, g cm-3
µ, cm-1
transm coeffa
R,b %
Rw,c %
3.58
1.72
3.40
1.24
1077(2)
1790(2)
2353(2)
GOFd
a Maximum value normalized to 1. b R ) ∑||Fo| - |Fc||/∑|Fo|. c Rw
2
2
) [∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2, where w-1 ) σ2(|Fo|) + g|Fo| ; g )
0.0002 for trans-Pd(PhA)2 and g ) 0.0001 for trans-Pd(PhB)2. d The
goodness of fit is defined as [w(|Fo| - |Fc|)2/(no - nv)]1/2 where no and
nv denote the numbers of data and variables, respectively.
a Equivalent isotropic U defined as one-third of the trace of the
orthogonalized Uij tensor.
Table 7. Atomic Coordinates (×104) and Equivalent Isotropic
Displacement Coefficients (Å2 × 103) for Pd(PhB)2
x
y
z
U(eq)a
The C-H distance was taken as 0.96 Å. The atomic parameters and
Hii values for C, O, H, N, and Pd were taken from literature.26
Pd
0
0
0
33(1)
51(2)
36(1)
34(2)
34(1)
35(2)
30(2)
40(2)
44(2)
46(2)
37(2)
32(2)
45(2)
39(2)
39(2)
39(2)
40(2)
38(2)
X-ray Structure Determination. Single crystals of Pd(PhA)2 and
Pd(PhB)2 were grown by slow diffusion of hexane into respective
dichloromethane solutions of the complexes at 298 K (Pd(PhA)2, brown,
prismatic 0.32 × 0.34 × 0.45 mm3; Pd(PhB)2, dark, prismatic 0.08 ×
0.06 × 0.20 mm3). Cell parameters were obtained by least-squares
fits of 30 machine-centered reflections obtained from rotation photo-
graph. Systematic absences afforded the space group C2/c (or Cc; the
structure was successfully solved in in C2/c) for Pd(PhA)2 and P21/n
for Pd(PhB)2. Data were collected at 296 K by the ω-scan method
over the 2θ range 2-50° on a Nicolet R3m/V diffractometer with
graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å). Two
check reflections measured after every 98 reflections showed no
significant change in intensity over 25 h (Pd(PhA)2) and 23 h (Pd-
(PhB)2) of exposure to X-rays. Data were corrected for Lorentz-
polarization effects and absorption (azimuthal scan).27 Of the 2560
and 1938 unique reflections, 2233 and 1158 with I > 3σ(I) were used
for respective structure solution of Pd(PhA)2 and Pd(PhB)2 (heavy-
atom method). All non-hydrogen atoms were made anisotropic. All
hydrogen atoms in the structure were directly located from difference
Fourier maps. Least-squares refinements were performed by full-matrix
procedures. All calculations were done on a MicroVAX II computer
with programs of SHELXTL-PLUS.28 Significant crystal data are listed
in Table 5, and atomic coordinates and isotropic thermal parameters
are listed in Tables 6 and 7.
O(1)
N(1)
N(2)
N(3)
C(1)
C(2)
C(3)
C(4)
C(5)
C(7)
C(8)
C(6)
C(9)
C(10)
C(11)
C(12)
C(13)
-3727(8)
-3017(8)
-3180(8)
-1237(8)
-4158(11)
-6289(9)
-6643(10)
-8642(11)
-10324(11)
-7987(9)
-397(10)
-9997(10)
1727(10)
-1095(4)
-118(6)
1951(4)
1752(4)
949(6)
1170(5)
2333(5)
2602(5)
1708(6)
257(5)
2822(5)
542(6)
3354(5)
4445(5)
4934(7)
4344(5)
3289(5)
-1079(3)
-739(2)
-461(2)
-45(2)
-862(3)
-1401(3)
-1719(3)
-2223(3)
-2410(3)
-1599(3)
400(3)
-2098(3)
278(3)
2417(10)
1083(9)
-969(10)
-1739(10)
667(3)
1168(3)
1303(3)
908(3)
a Equivalent isotropic U defined as one-third of the trace of the
orthogonalized Uij tensor.
Acknowledgment. We thank the Department of Science and
Technology, New Delhi, and the Council for Scientific and
Industrial Research for financial support. Affiliation with the
Jawaharlal Nehru Centre for Advanced Scientific Research,
Bangalore, India, is acknowledged.
(26) (a) Tatsumi, K.; Hoffmann, R. J. Am. Chem. Soc. 1981, 103, 3328.
(b) Hoffmann, D. M.; Hoffmann, R.; Fisel, C. R. J. Am. Chem. Soc.
1982, 104, 3858. (c) Tatsumi, K.; Hoffmann, R.; Yamamoto, A.;
Stille, J. K. Bull. Chem. Soc. Jpn. 1981, 54, 1857.
(27) North, A. C. T.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr.
1968, A24, 351.
Supporting Information Available: For Pd(PhA)2 and Pd(PhB)2,
complete bond distances (Tables S1 and S5) and angles (Tables S2
and S6), anisotropic thermal parameters (Tables S3 and S7), and
hydrogen atom positional parameters (Tables S4 and S8) (6 pages).
Ordering information is given on any current masthead page.
(28) Sheldrick, G. M. SHELXTL-PLUS 88: Structure Determination
Software Programs; Nicolet Instrument Corp.: Madison, WI, 1988.
IC950361T