Unexpected orthopalladation of acetophenonephenylhydrazone

Article abstract of DOI:10.1016/S0020-1693(02)01054-X

A singular hydrazonato complex of palladium has induced the unexpected orthometallation of acetophenonephenylhydrazone on the phenyl group. This is the first X-ray characterized example of an orthopalladated E-arylhydrazone of general formula ArC(R″)=N-NHAr′ where the metallated aryl is Ar′ although palladation on Ar is not hindered. Structural evidence suggests that the Pd-N(amido) bond that is present in the starting material induced the palladation of the unexpected phenyl.

Full text of DOI:10.1016/S0020-1693(02)01054-X

Inorganica Chimica Acta 338 (2002) 260Á264
/
www.elsevier.com/locate/ica
Note
Unexpected orthopalladation of acetophenonephenylhydrazone
Arancha Carbayo a, Gabriel Garc´ıa-Herbosa a,*, Santiago Garc´ıa-Granda b,
Daniel Miguel c
a Departamento de Qu´ımica, Facultad de Ciencias, Universidad de Burgos, 09001 Burgos, Spain
b Departamento de Qu´ımica F´ısica y Anal´ıtica, Facultad de Qu´ımica, Universidad de Oviedo, 33071 Oviedo, Spain
c Departamento de Qu´ımica Inorga´nica, Facultad de Ciencias, Universidad de Valladolid, 47005 Valladolid, Spain
Received 18 February 2002; accepted 29 April 2002
Abstract
A singular hydrazonato complex of palladium has induced the unexpected orthometallation of acetophenonephenylhydrazone on
the phenyl group. This is the first X-ray characterized example of an orthopalladated E-arylhydrazone of general formula ArC(Rƒ)ꢀ
/
Nꢁ
/
NHAr? where the metallated aryl is Ar? although palladation on Ar is not hindered. Structural evidence suggests that the Pdꢁ
/
N(amido) bond that is present in the starting material induced the palladation of the unexpected phenyl.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Palladium; Arylhydrazones; Orthopalladation
1. Introduction
hydrazone ligands, four different orthopalladation
modes can be predicted for arylketones and arylalde-
hides of arylhydrazones (see Scheme 1).
Whereas structures with Ar orthopalladated (E) and
with Ar? orthopalladated (Z) have been reported, no
examples of Ar orthopalladated (Z) and of Ar? ortho-
palladated (E) have been described at the moment.
When the iminic nitrogen further coordinates to palla-
dium, the palladacycle thus formed has been denomi-
Organometallic complexes of palladium are a group
of chemicals of great interest as deduced from the huge
number of references and books devoted to this subject.
Particular attention is centered in their applications to
organic synthesis [1]. Palladation reactions have been
widely studied, and N-donor ligands capable of giving
metallacycles have been thoroughly explored during the
last decades and more recently with regard to the
synthesis, reactivity, structural and mechanistic aspects
nated endo (the Cꢀ
/
N bond inside the palladacycle) or
exo (the CꢀN bond outside the palladacycle) [5]. In that
/
[2Á
/
4]. Most of the N-donor ligands, like amines, imines
H activation and, in
work, the use of a large variety of substituted benzili-
denearylhydrazones allowed isolation and 1H NMR
characterization of examples with endo and exo co-
ordination modes. For the Ar orthopalladated cases, the
arylhydrazone adopted the E form, the only allowing
endo cyclopalladation. On the contrary, the Z isomeric
form was reported to be adopted by all the Ar?
cyclopalladated cases, although both forms E and Z
were able to undergo exo cyclopalladation. Sterical
reasons were used to explain this preference. Although
not in a large extent, metallation on the Ar? group was
and azobenzenes are limited to Cꢁ
/
these cases, the products of their palladation reaction
can be quite precisely anticipated. However, the task
become less easy when the ligands have two or more
distinct Cꢁ
have competitive Nꢁ
case of arylhydrazone ligands ArC(Rƒ)ꢀ
/
H bonds to activate or, even more, when they
H bonds to deprotonate. This is the
NNHAr?,
/
/
where Ar (the C-bonded aryl group) and Ar? (the N-
bonded aryl group) represent differently substituted (R
or R?) phenyl groups. As orthopalladation can occur on
each aryl and Z or E forms can be adopted by
reported to be more stable than on Ar for ArC(H)ꢀ
/
NNHAr?. However, in the two X-ray structurally
characterized examples, the Ar group is 2,4,6-trimethyl-
phenyl, obviously unable to orthometallate [5]. A search
* Corresponding author
E-mail address: gherbosa@ubu.es (G. Garc´ıa-Herbosa).
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 0 5 4 - X

A. Carbayo et al. / Inorganica Chimica Acta 338 (2002) 260Á
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261
2.2. Reagents and materials
PPh3 was obtained from Aldrich Qu´ımica S.A.
Complexes [Pd{C6H4C(Me)ꢀNꢁNHPh}(m-Cl)]2 [15]
and NꢁNHPh}{N(Ph)ꢁNꢀ
[Bu4N][Pd{C6H4C(Me)ꢀ
/
/
/
/
/
/
C(Me)C6H4}(m-Cl)PdCl] [11] were prepared by pub-
lished procedures. All reactions were carried out using
Schlenk techniques under nitrogen atmosphere. Solvents
were distilled and dried over calcium dihydride (hexane
and methylene chloride) or drierite (acetone), and
degassed by standard methods prior to use.
2.2.1. Preparation of trans-[PdCl(PPh3)2{C6H4NHꢁ
NꢀC(Me)Ph}]×0.5(Me)2CO (1)
[Bu4N][Pd{C6H4C(Me) ꢀ N ꢁNHPh}{N(Ph) ꢁ
/
/
/
/
/
/
N ꢀ
/
C(Me)C6H4}(m-Cl)PdCl] (100 mg, 0.1 mmol) was dis-
solved in acetone (50 ml) and PPh3 (104 mg, 0.4 mmol)
was added to the red solution. The mixture was refluxed
under nitrogen atmosphere for 6 h. Under these condi-
tions, a yellow solid appeared; this was isolated by
filtration, washed with acetone and dried in vacuo (10
mg, 0.011 mmol, 11%). Characterization data are as
Scheme 1.
for palladated complexes of arylhydrazones in the
Cambridge Crystallographic Data Base shows that
acetophenonephenylhydrazone always appears metal-
lated or cyclometallated at the Ar, i.e. at the C-bonded
follows. Anal. Calc. for PdC51.5H46N2P2ClO0.5 (Mrꢀ
/
904.75): C, 68.37; H, 5.12; N, 3.10. Found: C, 68.22;
1
H, 4.97; N, 3.13%. H NMR (CDCl3): d 2.04 (s, 3H,
phenyl group and under the E form [6Á9]. No one
/
CH3COCH3), 2.15 (s, 3H, CH3), 5.93 (m, 1H, agostic
aromatic), 6.57 (m, 3H, aromatics), 7.37 (m, 35H,
aromatics), 8.02 (s, 1H, NH). 13C{1H, 31P} (CDCl3): d
crystal structure appeared palladated on the Ar? (the N-
bonded aryl) group when the Ar (the C-bonded aryl)
group is able to orthopalladate.
206.36 (Cꢀ/O), 146.55, 139.71, 137.67, 134.75 (PPh3),
Another important feature of arylhydrazones,
134.67 (PPh3), 134.58, 131.14, 130.83 (agostic C), 130.53
(PPh3), 130.17 (PPh3) 128.29, 128.23 (PPh3), 128.16
(PPh3), 128.09, 127.29, 125.30, 123.92, 119.79, 114.03,
28.2 (CH3COCH3), 11.68 (CH3). IR: 3293 cmꢁ1
ArC(R)ꢀ
NꢁH bond giving hydrazonato complexes. Examples of
palladium amido complexes of acetophenonephenylhy-
drazone showing both NꢁH and CꢁH (at the C-bonded
Ar group) deprotonated bonds have been reported [8,9].
H bond to facilitate or not the
/
NNHAr?, is their possibility to activate the
/
/
/
(nNH), 1711 cmꢁ1 (nCꢀ
/
O), 1571 cmꢁ1 (nCꢀ
/N).
The importance of the Nꢁ
/
2.2.2. Preparation of trans-
[PdCl(PPh3)2{C6H4C(Me)ꢀ
To a stirred suspension of [Pd{C6H4C(Me)ꢀ
orthopalladation of the N-bonded aryl group has been
also pointed out [10].
/
Nꢁ
/
NHPh}] (2)
/
Nꢁ
/
NHPh}(m-Cl)]2 (100 mg, 0.14 mmol) in methylene
chloride (50 ml) under nitrogen atmosphere was added
PPh3 (73 mg, 0.28 mmol). In 30 min, the complex
dissolved to give a yellow solution. After stirring for 2 h,
the solution was filtered through kieselgur to remove
any trace of solid and the solvent was removed in vacuo
to saturation. An excess of hexane (25 ml) was added.
Under these conditions, a yellow solid appeared; this
2. Experimental
2.1. Instrumentation
was isolated by filtration, washed with hexane (2ꢂ
/
15
1H NMR spectra were recorded on a Bruker AC 80
instrument. 13C NMR was recorded on a Bruker 200
instrument. 1H chemical shifts were measured with TMS
as internal reference. 13C chemical shifts were measured
with reference to the residual solvent resonances. IR
spectra were recorded as KBr disk on a Nicolet Impact
410 instrument. Microanalyses were performed by the
SCAI ( Servicio Central de Apoyo a la Investigacio´n) de
la Universidad de Burgos.
ml), crystallized from methylene chlorideÁhexane and
/
dried in vacuo to provide 192 mg (0.22 mmol, 85%) of
the complex 2. Characterization data are as follows.
Anal. Calc. for PdC50H43N2P2Cl (Mrꢀ875.71): C,
/
68.58; H, 4.95; N, 3.20. Found: C, 68.32; H, 4.76; N,
1
3.11%. H NMR (CDCl3): d 2.35 (s, 3H, Me), 6.58 (m,
2H, aromatics), 7.06 (m, 4H, aromatics), 7.52 (m, 33H,
aromatics), 9.11 (s, 1H, NH). IR: 3197 cmꢁ1 (nNH),
1598 cmꢁ1 (nCꢀ
/
N).

262
A. Carbayo et al. / Inorganica Chimica Acta 338 (2002) 260Á264
/
2.3. X-ray diffraction study of 1
3. Results and discussion
Crystals were grown from methylene chlorideÁ
/
hex-
Here we report the first crystal structure that shows
an E arylhydrazone of general formula ArC(Rƒ)ꢀ
/
Nꢁ
/
ane. Data were collected on a EnrafÁNonius CAD4
/
NHAr? orthopalladated on Ar?, the N-bonded aryl,
when both aryls are unsubstituted and plainly able to
orthometallate phenyls. Neither electronic nor steric
effects due to substituents on the aromatic rings can
be invoked to explain the preference of one phenyl over
the other.
diffractometer. The structure was solved by Patterson
methods, phase expansion, and subsequent Fourier
maps with DIRDIF [20]. Full-matrix least-squares refine-
ment was made with SHELX-93 [21]. After anisotropic
refinement, several peaks were found near the inversion
center. These were identified as corresponding to a
molecule of acetone which was refined as a rigid model
As displayed in Scheme 2, the reaction of the anionic
binuclear
NHPh}{m-N(Ph)ꢁ
with an excess of triphenylphosphine afforded trans-
[PdCl(PPh3)2{C6H4NHꢁNꢀC(Me)Ph}] (1) as yellow
complex
[NBu4][Pd{C6H4C(Me)ꢀ
/
Nꢁ
[11]
/
with occupancyꢀ
/
0.5 (this being consistent with analy-
tical and H NMR data). Perspective drawing in Fig. 1
was made with PLATON [22]. C50H43ClN2P2Pd×0.5
1; aꢀ
1
/
NꢀC(Me)C6H4}(m-Cl)PdCl]
/
/
/
/
¯
Me2CO, Mꢀ
/
904.74, triclinic, space group P
/
/
microcrystalline solid in low yield. Such reaction was
carried out in order to test the starting complex towards
˚
˚
˚
9.671(6) A, bꢀ
81.31(4)8, bꢀ80.26(3)8, gꢀ
Zꢀ2, Dcalc
1.36 mg mꢁ3, crystal size 0.24ꢂ
0.18 mm3, F(000)ꢀ
900, Tꢀ293(2) K, Mo Ka radia-
0.71073 A. Also 8737 reflections were collected,
giving 8616 independent reflections after merging
(Rint 0.0437), with 6858 reflections having Iꢀ 2s(I).
The U range for data collection was from 0.97 to 26.018.
The final indices [Iꢀ 2s(I)] were R1ꢀ0.0532, wR2ꢀ
0.1399.
/
11.278(3) A, cꢀ
/
21.38(3) A, aꢀ
/
3
˚
/
/
74.66(4)8, Vꢀ
/
2203(4) A ,
reductive elimination reaction, that should lead to Cꢁ
bond formation, as can be expected for a palladium
N(amido) bonds
/N
/
ꢀ
/
/
0.20ꢂ
/
/
/
compound containing Pdꢁ
/
C and Pdꢁ
/
˚
tion, lꢀ
/
on a cis arrangement [12,13]. After separation of pure 1
by filtration, the mother liquors of the reaction contain
ꢀ
/
/
a mixture of 1 and trans-[PdCl(PPh3)2{C6H4C(Me)ꢀ
/
Nꢁ
/
NHPh}] (2) (the isomer orthometallated on the C-
bonded aryl group). The synthesis of 2, as shown on
Scheme 2, has been carried out only for comparative
purposes since the synthesis and structure of the related
/
/
/
trans-[PdCl(PPh3)2{C6H4C(Ph)ꢀ
[14] and trans-[PdCl(PEt3)2{C6H4C(Me)ꢀ
[6] have been reported. Dimeric [Pd{C6H4C(Me)ꢀ
/
Nꢁ
/
NHC6H4-p-Cl}]
NꢁNHPh}]
Nꢁ
/
/
/
/
NHPh}(m-Cl)]2 [15] cleanly reacts with an excess of
triphenylphosphine to afford high yield of trans-
[PdCl(PPh3)2{C6H4C(Me)ꢀ
/
Nꢁ
/
NHPh}] (2). Formation
of 1 from the precursor amido complex requires a
proton and the source of Hꢃ is unknown. It surely
accounts for the low observed yield. From these results
it is clear that the unexpected activation of the N-
bonded phenyl group has been very likely promoted by
Fig. 1. Perspective view of the molecule of 1, showing the atom
numbering. Hydrogen atoms are omitted for clarity. Selected bond
˚
P(1) 2.326(2), Pd(1)ꢁP(2) 2.338(2),
lengths (A) and angles (8): Pd(1)ꢁ
/
/
Pd(1)ꢁ
N(2)ꢁC(7) 1.289(6), C(2)ꢁ
Cl(1)ꢁPd(1)ꢁP(1) 87.37(8), P(1)ꢁ
C(1) 90.73(14), Cl(1)ꢁPd(1)ꢁC(1) 176.77(12), P(1)ꢁ
177.25(4).
/
C(1) 2.019(4), Pd(1)ꢁ
N(1) 1.390(6), Cl(1)ꢁ
Pd(1)ꢁC(1) 91.79(14), P(2)ꢁ
Pd(1)ꢁ
/
Cl(1) 2.386(16), N(1)ꢁ
/
N(2) 1.356(5),
P(2) 90.06(8),
Pd(1)ꢁ
P(2)
/
/
/
Pd(1)ꢁ
/
/
/
/
/
/
/
/
/
/
/
Scheme 2.

A. Carbayo et al. / Inorganica Chimica Acta 338 (2002) 260Á
/264
263
the previous deprotonation of one of the Nꢁ
/
H bonds
leading to formation of a PdꢁN(amido) bond. The
/
observed stability of both isomers, 1 and 2, indicates
that formation of each one is not a matter of thermo-
dynamic but kinetic reasons. We only have to find the
right pathway to build the desired chemical bond. In this
case, the palladation of the N-bonded phenyl ring has
been achieved even starting from the acetophenonephe-
nylhydrazone palladated in the C-bonded phenyl group.
It means that the Pdꢁ
/
N(amido) bond has induced
special reactivity on the N-bonded phenyl ring that,
finally, orthometallates.
1
The H NMR spectrum of 1 shows a singlet at 8.02
ppm due to the proton of the Nꢁ
/
H group whereas for
complex 2 it appears at 9.11 ppm. Although it is well
known that the chemical shift for protons belonging to
Nꢁ/H groups is strongly dependent on the solvent, the
observed shift to highfields for 1 can be assigned in this
case to the relatively short distance HÁ Á ÁPd as discussed
below. The multiplet at 5.93 ppm is assigned to the
Scheme 3.
proton attached to C(6)*
an weak agostic PdÁ Á ÁHꢁC interaction.
The IR spectrum of 1 shows the Cꢀ
cmꢁ1 and NꢁH at 3293 cmꢁ1. The IR spectrum of 2
shows the Cꢀ H at 3197
N band at 1598 cmꢁ1 and Nꢁ
cmꢁ1
/
see Fig. 1*
/
and involved on
ally avoided. Disposition A, which strains PdÁ Á ÁH
interaction, is that found. This behavior suggests an
interesting question: what is hindering the PdÁ Á ÁN(i-
imino) interaction avoiding a five member palladacycle?
One answer is that palladium (II) is not prone to
pentacoordination as it has been found for the complex
/
/
N band at 1571
/
/
/
.
A thermal ellipsoid drawing of 1 is provided in Fig. 1.
Complex 1 shows square planar coordination. The
largest deviation from the mean plane determined by the
similar to 2, trans-[PdCl(PEt3)2{C6H4C(Me)ꢀ
NHPh}], where the imino nitrogen lies far from
˚
palladium (4.260 A) avoiding repulsive interaction
/
Nꢁ
/
˚
C(1) is 0.032(1) A
five atoms Pd(1)ꢁ
/
P(1)ꢁ
/
P(2)ꢁ
/
Cl(1)ꢁ
/
between the filled 4dz2-orbital and the lone pair on the
imino nitrogen [6]. Our answer is that the structure
found is that which prevents approaching of the other
phenyl ring to palladium that could lead to a tridentate
CfflNfflC dicyclopalladated acetophenonephenylhydra-
zone whose structural formula is shown in Scheme 3.
Such structure has not been reported so far and its
absence is consistent with the transphobia effect reported
for palladium, according to which, arylpalladium com-
plexes show preference to coordinate hard ligands in
and affects to Pd(1). Another plane in the complex is
that defined by the ten atoms Pd(1)ꢁC(1) to C(6)ꢁC(7)ꢁ
N(1)ꢁN(2) where the largest deviation from the mean
/
/
/
/
˚
plane, which affects to C(7), is 0.043(4) A. Both planes
are almost perpendicular with an angle of 88.8(1)8.
Two agostic PdÁ Á ÁH interactions emerge from the
analysis of the structure. One involving PdÁ Á ÁHꢁ
/
C(6)
˚
1.62 A. The
N(1) and the calculated rbp
which shows a calculated parameter rbp
ꢀ
/
other one involves PdÁ Á ÁHꢁ
/
˚
is 1.5 A. Both values are in the range of weak agostic
interactions [16].
In complex 1, the hydrazone shows the E form (both
position trans to the Pdꢁ/C(aryl) bond [17,18]. We have
not found in the Structural Data Base of Cambridge any
structure of a ligand containing the two trans dicyclo-
palladated aryl groups on such tridentate CfflNfflC
fashion. In fact this is uncommon for any metal. The
synthesis and structural characterization of platinum
complexes with 2,6-diphenylpyridine acting as tridentate
CfflNfflC ligand has been reported recently [19], but it
has been pointed out that the behavior of palladium and
platinum towards tridentate ligands can be divergent
leading to very different results [4]. This does not mean
that the dipalladated complex shown in Scheme 3
cannot exist. It just means that kinetic barriers are
very important and that the right pathway leading to its
formation has not yet been discovered, this representing
an interesting synthetic challenge.
aryl groups are trans with regard to the Cꢀ
/
N double
bond), the same form found in the amido precursor and
in all complexes shown in Scheme 2. Although the E
form is the commonly found for metallated arylhydra-
zones [6Á9,14] the Z form has been that chosen by those
/
arylhydrazones palladated on the N-bonded aryl Ar?
mentioned above [5]. Thus complex 1 is, to the best of
our knowledge, the first X-ray characterized case where
an arylhydrazone (E) has been palladated on the N-
bonded aryl Ar?. As shown in Scheme 3, two structural
possibilities (rotational conformers A and B) can be
anticipated for the acetophenonephenylhydrazone (E
isomer) palladated on the N(bonded) aryl. Disposition
B, which allows for PdÁ Á ÁN(imino) interaction is actu-

264
A. Carbayo et al. / Inorganica Chimica Acta 338 (2002) 260Á264
/
[6] J. Dehand, J. Fischer, M. Pfeffer, A. Mitschler, M. Zinsius, Inorg.
Chem. 15 (1976) 2675.
4. Supplementary material
[7] G.I. Borgne, S.E. Bouaud, D. Grandjean, P. Braunstein, J.
Dehand, M. Pfeffer, J. Organomet. Chem. 136 (1977) 375.
[8] P. Espinet, M.Y. Alonso, G. Garc´ıa-Herbosa, J.M. Ramos, Y.
Jeannin, M. Philoche-Levisalles, Inorg. Chem. 31 (1992) 2501.
[9] P. Espinet, G. Garc´ıa, F.J. Herrero, Y. Jeannin, M. Philoche-
Levisalles, Inorg. Chem. 28 (1989) 4207.
Crystallographic data (excluding structure factors) for
the structure reported in this paper has been deposited
with the Cambridge Crystallographic Data Centre as
supplementary publication no. CCDC-166143. Copies
of the data can be obtained free of charge on application
to the Director, CCDC, 12 Union Road, Cambridge
[10] G. Garc´ıa-Herbosa, A. Munoz, D. Miguel, S. Garc´ıa-Granda,
˜
Organometallics 13 (1994) 1775Á1780.
/
CB2 1EZ, UK (fax: ꢃ44-1223-336-033; e-mail: depos-
/
[11] A. Carbayo, G. Garc´ıa-Herbosa, D. Miguel, Eur. J. Inorg. Chem.
(2001) 2361.
itccdc.cam.ac.uk).
[12] M.S. Driver, J.F. Hartwig, J. Am. Chem. Soc. 119 (1997) 8232.
[13] J.F. Hartwig, Angew. Chem., Int. Ed. 37 (1998) 2090.
[14] J. Granell, R. Moragas, J. Sales, M. Font-Bard´ıa, X. Solans, J.
Organomet. Chem. 431 (1992) 359.
Acknowledgements
[15] B.N. Cockburn, D.V. Howe, T. Keating, B.F.G. Johnson, J.
Lewis, J. Chem. Soc., Dalton Trans. (1973) 404.
[16] R.H. Crabtree, E.M. Holt, M. Lavin, S.M. Morehouse, Inorg.
Chem. 24 (1985) 1986.
The authors gratefully acknowledge to the Spanish
Direccio´n General de Ensenanza Superior (PB97-0470-
˜
C02, BQU2000-0219) and the Junta de Castilla y Leo´n
(BU08/99) for financial support.
[17] J. Vicente, A. Arcas, D. Bautista, P.G. Jones, Organometallics 16
(1997) 2127.
[18] J. Vicente, J.A. Abad, A.D. Frankland, M.C.R. de Arellano,
Chem.-Eur. J. 5 (1999) 3066.
[19] G.W.V. Cave, F.P. Fanizzi, R.J. Deeth, W. Errington, J.P.
Rourke, Organometallics 19 (2000) 1355.
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