Synthesis, structure, and electrochemistry of mononuclear and face-to-face binuclear orthometalated complexes of palladium(II) with N-monodentate or N(1),N(3)-bridging 1,3-di-p-tolyltriazenido ligands. Dependence on geometrical arrangement of the electronic communication between two equivalent redox sites

Article abstract of DOI:10.1021/om010075r

The synthesis, electrochemistry, and structural characterization of the mononuclear complex [Pd{C₆H₄N(H)N=C(CH₃)C₅H₄N }(p-tolN-N=Np-tol)] (1) containing the monodentate 1,3-di-p-tolyltriazenido ligand is described. Compound 1 is an example of a stable species containing a Pd-N amido bond cis to a Pd-C aryl bond. Kinetic parameters for the dynamic intramolecular N(1)-N(3) exchange of the monodentate ligand in complex 1 have been calculated. The cis and trans isomers of the orthometalated face-to-face complex [{Pd(C₆H₄N=NC₆H₅)(μ-p-tolNNNp-tol )}₂] (2) have also been prepared, and the crystal structure of the trans isomer is reported. There are noticeable differences in the electrochemical behavior of the mononuclear and binuclear species. From the electrochemical experiments on both isomers of 2 it is possible to recognize different redox sites, to calculate the electronic coupling between them, and to suggest where the reversible electron transfers occur. Each isomers of 2 undergoes two one-electron oxidations and two one-electron reductions. The electronic coupling (ΔE = 0.40 V) at oxidizing potentials is identical for both isomers of complex 2, suggesting that the oxidations occur on the Pd(μ-triazenido)₂Pd framework which is common to both isomers. By contrast, the electronic coupling at reducing potentials is greater for cis-2 (ΔE = 0.33 V) than for trans-2 (ΔE = 0.25 V), suggesting that the reduction processes occur on the orthopalladated fragments, which are arranged differently on the two isomers. Thus, electronic communication between two equivalent redox centers in the same molecule depends not only on the nature of the bridging ligand but also on the geometrical arrangement of the redox centers.

Full text of DOI:10.1021/om010075r

Organometallics 2001, 20, 3223-3229
3223
Articles
Syn th esis, Str u ctu r e, a n d Electr och em istr y of
Mon on u clea r a n d F a ce-to-F a ce Bin u clea r
Or th om eta la ted Com p lexes of P a lla d iu m (II) w ith
N-Mon od en ta te or N(1),N(3)-Br id gin g
1,3-Di-p-tolyltr ia zen id o Liga n d s. Dep en d en ce on
Geom etr ica l Ar r a n gem en t of th e Electr on ic
Com m u n ica tion betw een Tw o Equ iva len t Red ox Sites
G. Garc´ıa-Herbosa,*,† N. G. Connelly,‡ A. Mun˜oz,† J . V. Cuevas,†
A. G. Orpen,‡ and S. D. Politzer‡
Departamento de Quı´mica, Facultad de Ciencias, Universidad de Burgos,
09001 Burgos, Spain, and School of Chemistry, University of Bristol,
Bristol, U.K. BS8 1TS
Received J anuary 30, 2001
The synthesis, electrochemistry, and structural characterization of the mononuclear
complex [Pd{C6H4N(H)NdC(CH3)C5H4N}(p-tolN-NdNp-tol)] (1) containing the monodentate
1,3-di-p-tolyltriazenido ligand is described. Compound 1 is an example of a stable species
containing a Pd-N amido bond cis to a Pd-C aryl bond. Kinetic parameters for the dynamic
intramolecular N(1)-N(3) exchange of the monodentate ligand in complex 1 have been
calculated. The cis and trans isomers of the orthometalated face-to-face complex [{Pd(C6H4Nd
NC6H5)(µ-p-tolNNNp-tol)}2] (2) have also been prepared, and the crystal structure of the
trans isomer is reported. There are noticeable differences in the electrochemical behavior of
the mononuclear and binuclear species. From the electrochemical experiments on both
isomers of 2 it is possible to recognize different redox sites, to calculate the electronic coupling
between them, and to suggest where the reversible electron transfers occur. Each isomers
of 2 undergoes two one-electron oxidations and two one-electron reductions. The electronic
coupling (∆E ) 0.40 V) at oxidizing potentials is identical for both isomers of complex 2,
suggesting that the oxidations occur on the Pd(µ-triazenido)2Pd framework which is common
to both isomers. By contrast, the electronic coupling at reducing potentials is greater for
cis-2 (∆E ) 0.33 V) than for trans-2 (∆E ) 0.25 V), suggesting that the reduction processes
occur on the orthopalladated fragments, which are arranged differently on the two isomers.
Thus, electronic communication between two equivalent redox centers in the same molecule
depends not only on the nature of the bridging ligand but also on the geometrical arrangement
of the redox centers.
In tr od u ction
trans to a Pd-C bond have been reported.4,5 The
number of related cis complexes, which are the accepted
intermediates in the mechanism of C-N bond formation
through reductive elimination, has recently been in-
creased by the use of the chelating ancillary ligand dppf
{1,1′-bis(diphenylphosphino)ferrocene}.6 In addition to
the role of palladium amido complexes in organic
synthesis, we have found that arylamido ligands coor-
dinated to palladium undergo oxidative C-C couplings
and formation of tetranuclear complexes with unusual
electrochemical properties.7 The goal of this work was
The chemistry of the metal-nitrogen amido bond,
where M is a late transition metal, continues to attract
attention due, among other reasons, to the role of such
compounds in stoichiometric or catalytic C-N bond
formation.1,2 Whereas bridging amido complexes of
palladium(II) were reported by Okeya in 1982,3 only a
few complexes containing terminal Pd-N amido bonds
* Corresponding author. E-mail: gherbosa@ubu.es.
† Universidad de Burgos.
‡ University of Bristol.
(1) Hartwig, J . F. Angew. Chem., Int. Ed. 1998, 37, 2046.
(2) Mu¨ller, T. E.; Beller, M. Chem. Rev. 1998, 98, 675.
(3) Okeya, S.; Yoshimatsu, H.; Nakamura, Y.; Kawaguchi, S. Bull.
Chem. Soc. J pn. 1982, 55, 483.
(4) Espinet, P.; Garc´ıa, G.; Herrero, F. J .; J eannin, Y.; Philoche-
Levisalles, M. Inorg. Chem. 1989, 28, 4207.
(5) Driver, M. S.; Hartwig, J . F. J . Am. Chem. Soc. 1995, 117, 4708
(6) Driver, M. S.; Hartwig, J . F. J . Am.Chem. Soc. 1997, 119, 8232.
10.1021/om010075r CCC: $20.00 © 2001 American Chemical Society
Publication on Web 06/20/2001

3224 Organometallics, Vol. 20, No. 15, 2001
Garc´ıa-Herbosa et al.
orange 5 min later, was left to stir for 90 min. An orange solid
was then collected by filtration and washed with methanol (2
× 10 mL). Anal. Calcd (found) for PdC27H26N6: C, 60.0 (60.4);
to prepare species containing both Pd-N amido and
Pd-C bonds using orthopalladated complexes [Pd-
{C6H4N(H)NdC(CH3)C5H4N}Cl]8 and [{Pd(C6H4Nd
NC6H5)(µ-Cl)}2]9 and the 1,3-di-p-tolyltriazenido group
as the amido ligand. In this paper we report the
synthesis, structural characterization, and electrochemi-
cal behavior of the mononuclear complex [Pd{C6H4N-
(H)NdC(CH3)C5H4N}(p-tolN-NdNp-tol)] (1) and the
binuclear complexes cis- and trans-[{Pd(C6H4NdNC6H5)-
(µ-p-tol-NNN-p-tol)}2] (2).
Electronic communication between redox sites in
binuclear complexes is a subject of great interest not
only from the theoretical point of view but also for the
potential applications in molecular electronics. Many
current articles stress the importance of the bridge
connecting the redox sites.10-14 Electrochemical studies
on complexes cis-2 and trans-2 indicate that electronic
communication between chemically equivalent redox
centers depends not only on the nature of the bridge
but also on the geometrical arrangement of the redox-
active fragments.
1
H, 4.8 (4.9); N, 15.6 (15.8). H NMR (CDCl3, 300.13 MHz, 20
°C): δ 13.14 (s, 1H, NH), 8.04 (d, 2H, J (HH) ) 8.4 Hz, o-C6H4,
p-tol), 7.68 (m, 1H, H6′), 7.66 (d, 2H, J (HH) ) 8.3 Hz, o-C6H4,
p-tol′), 7.46 (td, 1H, J (HH) ) 8.0, 1.6 Hz, H4′), 7.14(d, 2H,
J (HH) ) 8.3 Hz, m-C6H4, p-tol′), 7.08 (d, 2H, J (HH) ) 8.4 Hz,
m-C6H4, p-tol), 6.92 (m, 1H, H5′), 6.65 (d, 1H, J (HH) ) 8.0 Hz,
H3′), 6.52 (dd, 1H, J (HH) ≈ 7.1, 0.9 Hz, H6), 6.16 (td, 1H, J (HH)
≈ 7.2, 1.3 Hz, H5), 6.02 (dd, 1H, J (HH) ≈ 7.7, 1.0 Hz, H3), 5.94
(td, 1H, J (HH) ≈ 7.9, 0.9 Hz, H4), 2.35 (s, 3H, Me, p-tol′), 2.32
(s, 3H, Me, p-tol), 1.93 (s, 3H, Me, hydrazone). (Proton
numbering of orthometalated ring: Hn. Proton numbering of
pyridine ring: Hn′.)
Crystals of 1‚0.5CH2Cl2 were grown by the slow diffusion
of Et2O into a solution of the complex in CH2Cl2 under nitrogen
at room temperature and protected from light.
Syn th esis of tr a n s- a n d cis-[{P d (C6H4NdNC6H5)(µ-p-
tolNNNp-tol)}2], tr a n s a n d cis-2. To a mixture of [{Pd-
(C6H4NdNC6H5)(µ-Cl)}2] (800 mg, 1.24 mmol) and 1,3-di-p-
tolyltriazene (556.5 mg, 2.48 mmol) in CH2Cl2 (100 mL) was
added a solution (0.53 M) of sodium methoxide (4.7 mL, 2.48
mmol) in methanol. After stirring for 1 h at room temperature
the dark brown mixture was filtered through Celite. Addition
of n-hexane and concentration to induce precipitation afforded
a mixture of cis-2 and trans-2 (ca. 1:1 by 1H NMR spectros-
copy), which was washed with n-hexane (2 × 10 mL) and dried
in vacuo. Yield: 1306 mg (95%).
The cis-2 and trans-2 isomers were separated by chromato-
graphing 100 mg of the mixture on a silica gel-n-hexane
column. Elution of the first band (dark brown) with CH2Cl2-
n-hexane (3:7) gave a solution, which was evaporated to low
volume in vacuo to yield 37 mg (yield 74%) of trans-2 as a black
solid. Anal. Calcd (found) for Pd2C52H46N10: C, 61.0 (60.4); H,
4.5 (4.5); N, 13.7 (13.6). 1H NMR (CDCl3, 80 MHz): δ 7.54 (m,
14H, aromatic), 6.99 (m, 20H, aromatic), 2.24 (s, 6H, Me, p-tol),
2.16 (s, 6H, Me, p-tol′). Cyclic voltammetry of trans-2 (CH2-
Cl2, Pt-disk electrode): E°1 ) -1.24 V, E°2 ) -1.03 V, E°3 )
0.91 V, E°4 ) 1.31 V versus SCE.
Exp er im en ta l Section
Gen er a l In for m a tion . All reactions and manipulations
were routinely performed under a nitrogen or argon atmos-
phere using standard Schlenk techniques. Solvents were dried
and distilled prior to use. The complexes [Pd{C6H4N(H)Nd
C(CH3)C5H4N}Cl],8 [{Pd(C6H4NdNC6H5)(µ-Cl)}2],9 and 1,3-di-
p-tolyltriazene15 were prepared according to published meth-
ods. All the other reagents and chemicals were reagent grade
and were used as received from commercial suppliers. Elemen-
tal analyses (C, H, N) were made on a Perkin-Elmer 2400
instrument. 1H NMR spectra were obtained on a Varian VXR-
200S, a Bruker AF-80, or a Bruker AC-300 spectrometer and
were referenced to internal TMS and reported on the δ scale.
Cyclic voltammetry and differential pulse voltammetry were
carried out using EG&G 273 and EG&G VersaStat poten-
tiostats in conjunction with a three-electrode cell using 0.1 M
[NBun4][PF6] solutions in CH2Cl2 or tetrahydrofurane, a Pt
bead or Pt disk electrode, and the saturated calomel electrode
(SCE) as reference. Under the conditions used, E°′ for the one-
electron oxidation of [Fe(η-C5Me5)2], added to the test solutions
as an internal calibrant, is -0.08 V.
X-r a y Cr yst a l St r u ct u r e Det er m in a t ion s of 1 a n d
tr a n s-2. Crystal data for the structure determinations are
collected in Table 1.
Syn th esis of [P d {C6H4N(H)NdC(CH3)C5H4N}(p-tolN-
NdNp-tol)], 1. To a suspension of [Pd{C6H4N(H)NdC(CH3)-
C5H4N}Cl] (300 mg, 0.85 mmol) in acetone (75 mL) was added
184.7 mg (0.85 mmol) of 1,3-di-p-tolyltriazene. After stirring
the mixture for 5 min at room temperature a solution (0.53
M) of sodium methoxide (1.6 mL, 0.85 mmol) in methanol was
added. The mixture, which was first blue and then reddish-
Well-shaped crystals of trans-2‚CH2Cl2, suitable for X-ray
analysis, were obtained by slow crystallization of the complex
from CH2Cl2-n-hexane solutions under nitrogen at room
temperature.
Elution of the second band (dark green) with the same
mixture of solvents and similar subsequent treatment of the
solution afforded 18 mg (yield 36%) of cis-2 as black micro-
crystals. Anal. Calcd (found) for Pd2C52H46N10: C, 61.0 (60.5);
1
H, 4.5 (4.6); N, 13.7 (13,6). H NMR (CDCl3, 80 MHz): δ 7.57
(m, 11H, aromatic), 6.90 (m, 23H, aromatic), 2.25 (s, 6H, Me,
p-tol), 2.19 (s, 6H, Me, p-tol′).
Kin etic Stu d ies by Va r ia ble-Tem p er a tu r e NMR Sp ec-
tr oscop y. Spectra were obtained using a Varian VXR-200S
spectrometer equipped with
a variable-temperature unit,
calibrated against a standard Varian methyl alcohol sample.
The two-site exchange of the triazenido ligand was studied by
means of line shape analysis (LSA) of the temperature-
dependent resonances corresponding to the methyl protons of
the p-tolyl groups. The spectra were recorded with good
homogeneity at different temperatures and simulated using
the program DNMR6 [DNMR6; Quantum Chemical Program
Exchange (QCPE633); Indiana University, Bloomington, IN,
1995]. The acceptable value of k at each temperature was that
for which the simulated and experimental spectra coincided.
Thermodynamic activation parameters were calculated ac-
cording to Arrhenius and Eyring equations. The ∆Hq and ∆Sq
values were obtained from least-squares treatment of the
corresponding rate data. The small magnitude of the chemical
shift separation, ν1-ν2, acted as a source of error. Maximum
errors of ca. (1.1 kJ mol-1 in ∆Hq and EA and ca. (3.5 J K-1
mol-1 in ∆Sq were estimated. The effect of the uncertainty in
(7) Espinet, P.; Alonso, M. Y.; Garc´ıa-Herbosa, G.; Ramos, J . M.;
J eannin, Y.; Philoche-Levisalles, M. Inorg. Chem. 1992, 31, 2502.
(8) Garc´ıa-Herbosa, G.; Mun˜oz, A.; Miguel, D.; Garc´ıa-Granda, S.
Organometallics 1994, 13, 1775.
(9) Cope, A. C.; Siekman, R. W. J . Am. Chem. Soc. 1965, 87, 3272.
(10) Astruc, D. Acc. Chem. Res. 1997, 30, 383.
(11) Fraysse, S.; Coudret, C.; Launay, J . P. Eu. J . Inorg. Chem. 2000,
7, 1581.
(12) Higuchi, H.; Ishikura, T.; Miyabayashi, K.; Miyake, M.; Yama-
moto, K. Tetrahedron Lett.1999, 40, 9091.
(13) Kheradmandan, S.; Heinze, K.; Schmalle, H. W.; Berke, H.
Angew. Chem., Int. Ed. 1999, 38, 2270.
(14) Ortega, J . V.; Hong, B.; Ghosal, S.; Hemminger, J . C.; Breedlove,
B.; Kubiak, C. P. Inorg. Chem. 1999, 38, 5102.
(15) Furniss, B. S.; Hannaford, A. J .; Smith, P. W. G.; Tatchell, A.
R. Vogel’s Textbook of Practical Organic Chemistry, 5th ed.; Longman
Scientific and Technical: Essex, England, 1989.

Orthometalated Complexes of Palladium(II)
Organometallics, Vol. 20, No. 15, 2001 3225
Ta ble 1. Cr ysta l a n d In ten sity Da ta for [P d (C6H4NHNdC(CH3)C5H4N)(p-tol-NNN-p-tol)]‚0.5CH2Cl2 (1) a n d
for [{P d (C6H4NdNC6H5)}2{µ-(pTolNNNp-tol)}2]‚CH2Cl2 (tr a n s-2)
crystal data for 1
crystal data for trans-2
empirical formula
color; habit
C
27.5H26Cl N6Pd
empirical formula
color; habit
C53H48Cl2 N10Pd2
brown flat cuboids
0.5 × 0.5 × 0.5 mm
monoclinic
C2/c (No. 15)
a ) 22.052(4) Å
b ) 22.922(5) Å
c ) 10.263(2) Å
â ) 90.85(1)
5187(2) Å3
8
black irregular blocks
0.7 × 0.5 × 0.5 mm
triclinic
cryst size
cryst size
cryst syst
space group
unit cell dimens
cryst syst
space group
unit cell dimens
P1h
a ) 12.673(5) Å
b ) 13.581(5) Å
c ) 15.893(7) Å
R ) 94.79(3)°
â ) 94.99(4)°
γ ) 112.45(3)°
2498(2) Å3
volume
Z
fw
density(calcd)
abs coeff
F(000)
582.4
volume
Z
fw
density(calcd)
abs coeff
F(000)
1.492 Mg/m3
0.85 mm-1
2368
2
1108.7
1.474 Mg/m3
0.874 mm-1
1124
Data Collection
diffractometer used
radiation
temperature
Siemens R3m/V
Mo KR (λ ) 0.71073 Å)
292(2) K
3.56-50°
Wyckoff ω
variable; 1.50-14.65
deg/min in ω
Data Collection
diffractometer used
radiation
temperature
2θ range
Siemens R3m/V
Mo KR (λ ) 0.71073 Å)
292(2) K
2θ range for data collection
scan type
scan speed
3.0-50.0°
standard reflns
index ranges
3 every 50 reflns
-26 < h < 26, -27 < k < 0,
0 < l < 12
scan type
scan speed
Wyckoff ω
variable; 1.50-14.65
deg/min in ω
no. of reflns collected
no. of ind reflns
profile fitting
6188
scan range (ω)
standard reflns
index ranges
2.70°
4575 [R(int) ) 0.0484]
3 every 50 reflns
0 < h < 15, -16 < k < 14,
-18 < l < 18
not used
abs corr
semiempirical
no. of reflns collected
no. of ind reflns
no. of obsd reflns
abs corr
9221
Solution and Refinement
8780 [R(int) ) 6.88]
5840 (F > 4.0σ(F))
semiempirical
0.2005/0.2388
refinement method
no. of data/restraints/params
goodness-of-fit on F2
final R indices [I>2σ(I)]
R indices (all data)
largest diff peak
full-matrix least-squares on F2
4573/3/323
1.180
min./max. transmn
R1 ) 0.0859, wR2 ) 0.1371
R1 ) 0.1817, wR2 ) 0.1699
0.913 e Å-3
Solution and Refinement
refinement method
quantity minimized
absolute structure
extinction corr
full-matrix least-squares
∑w(Fo - Fc)2
N/A
largest diff hole
-0.619 e Å-3
N/A
hydrogen atoms
riding model, fixed
isotropic U
weighting scheme
no. of params refined
final R indices (obsd data)
R indices (all data)
goodness-of-fit
w-1 ) σ2(F) + 0.0004F2
604
R ) 5.21, wR ) 5.39
R ) 8.60, wR ) 5.93
1.48
largest and mean ∆/σ
data-to-param ratio
largest diff peak
0.001, 0.000
9.7:1
0.76 e Å-3
largest diff hole
-0.72 e Å-3
the evaluation of the line-width is a minimum at the coales-
cence temperature because the broadening of the lines due to
the exchange is a maximum. For that reason, the ∆Gq value
was calculated from the rate data at the coalescence temper-
between the metal and the uncoordinated N(3) atom
(2.836 Å), whereas for 1 the Pd-N(3) distance is 3.030
Å (Table 2). Creswell et al. (see below) explained the
observed dynamic behavior of trans-[PdCl(PPh3)2-
(p-tolNNNp-tol)] in solution on the basis of this short
Pd‚‚‚N(3) distance.17 Taking into account the higher
steric strain in trans-[PdCl(PPh3)2(p-tolNNNp-tol)], which
contains the bulky PPh3 ligands, why is the distance
Pd‚‚‚N(3) longer in complex 1? A possible answer
requires a closer look at the structure of 1 as presented
in Scheme 2.
The plane of the tolyl group attached to N(3) is at an
angle of 5.7° to the plane defined by the three nitrogen
atoms, whereas the ring bonded to N(1) is 27.3° out of
this plane. This could be a result of the proximity of
the N(1) ring substituent to the metal atom in that if it
were planar with respect to the N atoms, then an ortho
ature. The estimated error for ∆Gq was (1.9 kJ mol-1
.
Resu lts a n d Discu ssion
Syn th esis. As shown in Scheme 1, treatment of [Pd-
{C6H4N(H)NdC(CH3)C5H4N}Cl] with 1,3-di-p-tolyltria-
zene and sodium methoxide in acetone affords 1 as a
stable orange solid in good yield. Similarly, treatment
of [{Pd(C6H4N dNC6H5)(µ-Cl)}2] with 1,3-di-p-tolyltria-
zene and sodium methoxide in dichloromethane affords
2 as a mixture of cis and trans isomers, which can be
separated by chromatography.
X-r a y Str u ctu r e of 1. Figure 1 shows a thermal
ellipsoid plot of complex 1. Monodentate triazenido
complexes are rare, and the Cambridge Structural
Database shows, to date, only one such palladium
complex, trans-[PdCl(PPh3)2(p-tolNNNp-tol)].16a In that
case there is a noticeably short nonbonded contact
(16) (a) Bombieri, G.; Immirzi, A.; Toniolo, L. Inorg. Chem. 1976,
15, 2428. (b) Peregudov, A. S.; Kravtsov, D. N.; Drogunova, G. I.;
Starikova, Z. A.; Yanovsky, A. I. J . Organomet. Chem. 2000, 597, 164.
(17) Creswell, C. J .; Queiro´s, M. A. M.; Robinson, S. D. Inorg. Chim.
Acta 1982, 60, 157.

3226 Organometallics, Vol. 20, No. 15, 2001
Garc´ıa-Herbosa et al.
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for Com p lex 1a
Pd(1)-C(15)
Pd(1)-N(1)
N(1)-N(2)
N(2)-N(3)
N(4)-N(5)
N(5)-C(21)
N(6)-C(23)
1.842(13)
2.034(7)
1.294(9)
1.296(9)
1.344(10)
1.298(11)
1.348(12)
Pd(1)-N(5)
Pd(1)-N(6)
N(1)-C(8)
N(3)-C(1)
N(4)-C(20)
N(6)-C(27)
C(1)-C(6)
1.969(7)
2.125(8)
1.433(10)
1.421 (11)
1.430(12)
1.338(13)
1.334(13)
C(15)-Pd(1)-N(5)
N(5)-Pd(1)-N(1)
N(5)-Pd(1)-N(6)
N(2)-N(1)-C(8)
C(8)-N(1)-Pd(1)
N(2)-N(3)-C(1)
C(21)-N(5)-N(4)
N(4)-N(5)-Pd(1)
83.8(5) C(15)-Pd(1)-N(1)
99.2(4)
176.9(3) C(15)-Pd(1)-N(6) 162.2(4)
78.5(4) N(1)-Pd(1)-N(6)
113.4(8) N(2)-N(1)-Pd(1)
118.9(6) N(1)-N(2)-N(3)
109.9(8) N(5)-N(4)-C(20)
98.6(3)
125.4(6)
116.3(8)
109.2(9)
122.6(9) C(21)-N(5)-Pd(1) 119.9(8)
117.4(7) C(27)-N(6)-C(23) 118.8(10)
C(27)-N(6)-Pd(1) 128.6(8) C(23)-N(6)-Pd(1) 112.5(8)
a
Symmetry transformations used to generate equivalent
atoms: #1 -x, y, -z+3/2.
F igu r e 1. Thermal ellipsoid plot of 1.
Sch em e 1
F igu r e 2. Thermal ellipsoid plot of trans-2.
Sch em e 3
The canonical form 1-ii accumulates negative charge
on the uncoordinated N(3) atom, enhancing repulsions
with the filled dz orbital (see Schemes 2 and 3). The
Sch em e 2
2
contribution of canonical form 1-ii also accounts for the
surprisingly almost identical bond distances N(1)-N(2)
[1.294(9) Å] and N(2)-N(3) [1.296(9) Å]. Other mono-
dentate triazenido complexes always show the expected
shorter distance for N(2)-N(3) than for N(1)-N(2).16b
In the complex trans-[PdCl(PPh3)2(p-tolNNNp-tol)] the
N(1)-N(2) and N(2)-N(3) distances are 1.336 and 1.286
Å, respectively, indicating a higher contribution of the
canonical form 1-i when compared with complex 1.
These results are consistent with the presence or
absence of π-acceptor ancillary ligands such as PPh3.
Canonical form 1-i must be favored by the presence of
π-acceptor ligands. In good agreement with such an
assessment, EHMO calculations18 on the simplified
model of complex 1, [Pd(NHCHCHdNNHCHCH)(HNN-
hydrogen would be close to the electron density in the
filled metal dz orbital. The N(1)-N(2)-N(3) bond angle
2
[116.3(8)°, Table 2] is slightly larger than that of trans-
[PdCl(PPh3)2(p-tolNNNp-tol)] (113.0°). Both the bond
angles and the nonbonded Pd‚‚‚N(3) distances indicate
larger repulsions in complex 1. Such repulsions could
be explained by describing the bond in terms of the
resonance structures 1-i and 1-ii (Scheme 3).
(18) Mealli, C.; Proserpio, D. M. J . Chem. Educ. 1990, 67, 399.

Orthometalated Complexes of Palladium(II)
Organometallics, Vol. 20, No. 15, 2001 3227
Ta ble 3. Selected Bon d Len gth s (Å) a n d An gles (d eg) for tr a n s-2‚CH2Cl2
Pd(2)-Pd(1)
Pd(2)-N(9)
Pd(1)-N(4)
N(1)-N(2)
N(3)-C(1)
N(5)-N(6)
N(7)-C(47)
N(9)-C(35)
2.895 (2)
2.047 (5)
2.145 (4)
1.308 (9)
1.431 (10)
1.304 (9)
1.467 (9)
1.442 (8)
Pd(2)-N(3)
Pd(2)-C(29)
Pd(1)-N(7)
N(1)-C(8)
N(4)-N(5)
N(6)-C(15)
N(8)-C(46)
N(10)-C(34)
2.145 (5)
1.972 (6)
2.044 (6)
1.398 (9)
1.280 (8)
1.444 (9)
1.395 (10)
1.410 (8)
Pd(2)-N(6)
Pd(1)-N(1)
Pd(1)-C(41)
N(2)-N(3)
N(4)-C(22)
N(7)-N(8)
N(9)-N(10)
C(1)-C(2)
2.036 (5)
2.025 (5)
1.968 (6)
1.300 (9)
1.434 (10)
1.258 (8)
1.266 (9)
1.362 (12)
Pd(1)-Pd(2)-N(3)
76.0(2)
88.5(2)
102.2(2)
112.0(2)
90.2(2)
Pd(1)-Pd(2)-N(6)
79.7(2)
108.2(2)
168.0(2)
171.5(3)
78.4(2)
N(3)-Pd(2)-N(6)
N(3)-Pd(2)-N(9)
Pd(1)-Pd(2)-C(29)
N(6)-Pd(2)-C(29)
Pd(2)-Pd(1)-N(1)
N(1)-Pd(1)-N(4)
N(1)-Pd(1)-N(7)
Pd(2)-Pd(1)-C(41)
N(4)-Pd(1)-C(41)
Pd(1)-N(1)-N(2)
N(2)-N(1)-C(8)
Pd(2)-N(3)-N(2)
N(2)-N(3)-C(1)
Pd(1)-N(4)-C(22)
Pd(1)-Pd(2)-N(9)
N(6)-Pd(2)-N(9)
N(3)-Pd(2)-C(29)
N(9)-Pd(2)-C(29)
Pd(2)-Pd(1)-N(4)
Pd(2)-Pd(1)-N(7)
N(4)-Pd(1)-N(7)
N(1)-Pd(1)-C(41)
N(7)-Pd(1)-C(41)
Pd(1)-N(1)-C(8)
N(1)-N(2)-N(3)
Pd(2)-N(3)-C(1)
Pd(1)-N(4)-N(5)
N(5)-N(4)-C(22)
79.3(2)
87.7(2)
74.7(2)
110.6(2)
101.8(2)
91.3(2)
167.7(2)
113.0(2)
171.9(3)
127.6(4)
112.2(5)
123.5(4)
112.1(5)
119.5(4)
78.2(3)
120.2(5)
116.1(5)
120.6(5)
127.4(4)
111.4(5)
NH)], indicate that N(3) supports a net negative charge
of -0.82, whereas in the related triphenylphosphine
simplified model complex, trans-[PdCl(PH3)2(HNNNH)],
the net charge is -0.74.
intramolecular associative mechanism through a five-
coordinate intermediate, in agreement with those pro-
posed for the complexes trans-[PdXL2(κ1-RNNNR)] (X
) Cl, Br; L ) phosphine or arsine, R ) p-tol or
p-MeOC6H4).16a,17,20 Whereas the activation barriers for
trans-[PdXL2(κ1-RNNNR)] are in the range 33.7 e EA
e 42.3 kJ ‚mol-1, the larger value for complex 1 could
be explained by two effects. First, the Pd‚‚‚N(3) distance
in 1 (3.030 Å) is much longer than 2.836 Å, the distance
in trans-[PdCl(PPh3)2(p-tolNNNp-tol)]. Second, the trans
effect for the halide is stronger than for the N-donors.
X-r a y Str u ctu r e of 2. Figure 2 shows a thermal
ellipsoid plot of trans-2. This compound is dimeric with
the palladium atoms in a square-planar geometry. The
two palladium centers are bridged by two triazenido
ligands arranged cis to one another. Each metal atom
is also chelated by an orthometalated azobenzene
ligand, forming a five-membered ring. These rings are
essentially planar, with the metalated benzene ring
lying in the same plane as the chelate ring. The other
“free” benzene ring is twisted relative to the chelate ring.
The azobenzene ligands are arranged trans with respect
to one another.
In compound trans-2 the Pd‚‚‚Pd distance (Table 3)
is 2.895(2) Å. Although a shorter Pd‚‚‚Pd distance,
2.747(1) Å, has been reported for two palladium atoms
bridged by two triazenido ligands in [{Pd2(µ-ArNN-
NAr)2}2(µ-Cl)4],19 both are formally nonbonding given
the 16-electron, square-planar coordination environ-
ment of the Pd(II) centers.
As for other binuclear complexes bridged by triazenido
ligands, the palladium atoms are not in the plane of the
triazenido nitrogen atoms, causing the metal square
planes to be mutually twisted. The degree of twist can
be measured by the torsion angles N(1)-Pd(1)-Pd(2)-
N(3) and N(4)-Pd(1)-Pd(2)-N(6), which are 26.8° and
26.4°, respectively. The dihedral angle between the
square planes surrounding the palladium atoms is 42.1°.
Dyn a m ic Beh a vior . The NMR spectrum shows that
complex 1 exhibits dynamic behavior at room temper-
ature, and the coalescence of the signals due to the
methyl substituents of the p-tolyl groups takes place at
328 ( 1 K. This can be explained by the N(1) and N(3)
atoms bonding alternately to the metal. The activation
parameters calculated as indicated above are EA ) 54.5
( 1.1 kJ ‚mol-1, ∆Gq ) 74.1 ( 1.9 kJ ‚mol-1, ∆Hq ) 51.7
( 1.1 kJ ‚mol-1, and ∆Sq ) -197 ( 3.5 J ‚mol-1‚K-1. The
large negative value of ∆Sq is consistent with an
Electr och em istr y
The mononuclear orthopalladated complexes [Pd-
{C6H4N(H)NdC(CH3)C5H4N}L][BF4] {L ) PPh3, P(OR)3,
4-CH3C5H4N, C4H8S (tetrahydrothiophene)} show ir-
reversible oxidations in the range 1.2-1.5 V versus
SCE.21 The binuclear face-to-face orthometalated dipal-
ladium complex containing the same CNN tridentate
ligand [(Pd{C6H4N(H)NdC(CH3)C5H4N})2(µ-Ph2PCH2-
8
PPh2)][BF4]2 is irreversibly oxidized at 1.20 V.21 At-
tempts to isolate and characterize any product through
chemical or electrochemical oxidation were unsuccessful
probably due to the high potential required to remove
electrons from this dicationic complex. To lower the
oxidation potential while keeping the face-to-face struc-
ture, we attempted to replace the neutral Ph2PCH2PPh2
bridge with the anionic di-p-tolyl triazenido ligand. As
described above, such attempts led to 1. Cyclic voltam-
metry of 1 showed an irreversible oxidation wave at 0.3
V, in agreement with previous findings that the forma-
tion of Pd-N amido bonds leads to low oxidation
potentials.7 However, despite the accessible potential,
all attempts to isolate oxidation products were unsuc-
cessful.
The cyclic voltammogram of trans-2 in dichloromethane
(Figure 3a) shows four reversible waves, two oxidations
at 0.91 and 1.31 V and two reductions at -1.03 and
(20) Toniolo, L.; Immirzi, A.; Croatto, U.; Bombieri, G. Inorg. Chim.
Acta 1976, 19, 209.
(19) Cuevas, J . V.; Garc´ıa-Herbosa, G.; Mun˜oz, A.; Hickman, S. W.;
Orpen, A. G.; Connelly, N. G. J . Chem. Soc., Dalton Trans. 1995, 4127.
(21) Garc´ıa-Herbosa, G.; Mun˜oz, A.; Maestri, M. J . Photochem.
Photobiol. A: Chem. 1994, 83, 165.

3228 Organometallics, Vol. 20, No. 15, 2001
Garc´ıa-Herbosa et al.
F igu r e 4. Electronic coupling (communication) between
redox sites in cis-2 and trans-2. (∆E in volts).
C(orthometalated) bond, two Pd-N(sp2) bonds, and one
Pd-N(sp3-amido) bond. The observed shift for the first
oxidation potential from 0.31 V in mononuclear complex
1 to 0.91 V in binuclear complexes 2 indicates that the
energies of the HOMOs are very different in the two
types of complexes. Strictly, only potentials of reversible
systems can be directly compared, as the oxidation
potential of a chemically irreversible system will also
depend on the rate of the reaction following electron
transfer. However, the very large difference of 0.7 V
cannot be ascribed simply to the fact that one of the
two oxidation processes is chemically irreversible.
The observed pattern in the CVs of 2 corresponds to
binuclear complexes with two equivalent halves, and
each half undergoes two reversible electron transfers.
Why are the CVs of the cis and trans isomers coincident
apart from the second reduction wave? Although one
might expect more differences because the energy levels
should be different for both isomers, the energies of the
HOMO and LUMO seem to be almost identical for both
isomers. The separation between the two oxidation
waves (∆E ) 0.40 V) is a measure of the electronic
communication (coupling) between the two halves of the
mixed valence species 2+, and it is identical in both
isomers (see Figure 4).
However, as observed in Figure 3c, the separation
between the two reduction waves is different for cis-2
(∆E ) 0.33 V) and trans-2 (∆E ) 0.25 V). This suggests
that oxidations occur successively on each half of the
Pd(µ-triazenido)2Pd framework (which is identical in
both isomers), leading to identical oxidation potentials.
On the other hand, the reductions occur successively on
each half of the chelate azobenzene orthopalladated
(CN)Pd‚‚‚Pd(CN) framework in the cis-2 case, which is
different from the (CN)Pd‚‚‚Pd(NC) framework in the
trans-2 case. The assignment of azobenzene-based re-
ductions when the ligand is orthopalladated has been
reported.23
F igu r e 3. (a) Cyclic voltammogram of trans-2 in dichlo-
romethane, 0.1 M [NBun4][PF6] at a Pt-disk electrode, scan
rate: 200 mV s-1. (b) Cyclic voltammogram of a mixture
of cis-2 and trans-2 in tetrahydrofuran; other conditions
as in (a). (c) Differential pulse voltammogram of an
equimolecular mixture of cis-2 and trans-2 in tetrahydro-
furan; other conditions as in (a). Potentials (E) are referred
to the saturated calomel electrode.
-1.24 V. The four waves have the same limiting current
and are therefore associated with the same number of
electrons. Although controlled-potential coulometry has
not been carried out, the monoelectronic nature of the
waves is proposed on the basis of electrochemical studies
in structurally related face-to-face compounds.22
The isomer cis-2 has a similar CV in CH2Cl2 except
that the potential for the second reduction wave differs
from that of trans-2; the difference is more clearly seen
in the CV (Figure 3b) and DPV (Figure 3c) of a mixture
of the two isomers in thf. The first reduction waves of
the two isomers are coincident in both solvents. (The
oxidation waves occur at potentials too positive to be
observed in thf.)
In summary, one-half of the redox site of the Pd(µ-
triazenido)2Pd framework “feels” the other half to the
(22) Cotton, F. A.; Matusz, M.; Poli, R.; Feng, X. J . Am. Chem. Soc.
1988, 110, 1144.
(23) Bag, K.; Misra, T. K.; Sinha, C. Polyhedron 1998, 17, 4109.
The coordination around palladium in complexes 1
and 2 is almost identical. It consists of one Pd-

Orthometalated Complexes of Palladium(II)
Organometallics, Vol. 20, No. 15, 2001 3229
same extent in cis-2+ and trans-2+, but the first ortho-
palladated chelate fragment “feels” the second to dif-
ferent extents in cis-2- and trans-2-. This effect is
comparable to the magnetic coupling constants between
nuclei in NMR spectroscopy, a parameter largely used
to distinguish between isomers. The different electronic
coupling between the two equivalent redox sites in
“(CN)Pd‚‚‚Pd(CN)” or in “(CN)Pd‚‚‚Pd(NC)” can only be
assigned to the geometrical arrangement of the redox
sites, as the bridges are identical in both isomers. Such
dependence, of the electronic communication between
the two redox sites, not with the bridges but with their
geometrical arrangement is, to the best of our knowl-
edge, here reported for the first time.
activation energy observed in the fluxional behavior of
1 [corresponding to the N(1) and N(3) atoms bonding
alternately to the metal] when compared with that of
related complexes if it is accepted that exchange takes
place through an associative mechanism.16a,17,20 Bi-
nuclear complexes 2 show very different electrochemical
behavior when compared with 1. They show rich revers-
ible electrochemistry and also an important shift of the
oxidation potentials to higher values. The different
electronic coupling between the equivalent reduction
sites in trans-2 and cis-2 has been ascribed to the
geometrical arrangement of the reduction sites as the
bridges are identical in both isomers.
Ack n ow led gm en t. The authors gratefully acknowl-
edge the Spanish Direccio´n General de Ensen˜anza
Superior (PB97-0470-C02-02) and the J unta de Castilla
y Leo´n (BU08/99) for financial support.
Con clu sion s
Orthopalladated complexes react with diaryltriaz-
enide ligands to give stable mononuclear and binuclear
compounds containing Pd-N(amido) and Pd-C(aryl)
bonds in the cis arrangement, which is prone to undergo
reductive elimination.1 In monodentate triazenido com-
plexes the Pd-N(3) distance depends on the donor-
acceptor properties of the ancillary ligands coordinated
to palladium. This dependence can explain the higher
Su p p or tin g In for m a tion Ava ila ble: Tables of atomic
coordinates and equivalent isotropic displacement parameters,
bond distances, bond angles, hydrogen coordinates, and aniso-
tropic parameters for the structural analysis of 1 and trans-2
are available free ofcharge via the Internet at http://pubs.acs.org.
OM010075R