Bis-cyclometalated complexes of Pd(II) and Pd(IV) from iminophosphoranes: Synthesis, structure and reactivity This contribution is dedicated to Prof. Maria Pilar García Clemente, who passed away on May 27, 2013.

Article abstract of DOI:10.1016/j.jorganchem.2014.05.024

The mononuclear bis-orthopalladated complexes [Pd(?N){C ₆H₄(PPh₂NPh)-2}] [?N = (2-(pyridin-2-yl)phenyl) 3a, (2-((dimethylamino)methyl)phenyl) 3b, (S)-(2-(1-(dimethylamino)ethyl)phenyl) 3c, (quinolin-8-ylmethyl) 3d, (4-methoxy-2-((triphenyl-λ⁵-phosphanylidene)carbamoyl)phenyl) 3e] have been synthesized as single isomers by reaction of the corresponding chloride dimers [Pd(μ-Cl)(?N)]₂ (1a-1e) with the lithium derivative [Li{C₆H₄(PPh₂NPh)-2}] (2) in Et ₂O. Spectroscopic data show that the two palladated C atoms are in cis disposition, which is confirmed by the X-ray structure of 3a. Oxidation of 3a with PhI(OAc)₂ affords the corresponding Pd(IV) derivative (OC-6-32)[Pd(OAc)₂{C₆H₄(PPh₂NPh)-2} {C₆H₄-(2′-NC₅H₄)-2}] 4a also as a single isomer. 4a undergoes reductive elimination through C-O coupling to give the dimer [Pd(μ-OAc){C₆H₄(PPh₂NPh)-2}] ₂5 and the ortho-acetoxylated phenylpyridine 6. The reactivity of 3b-3e towards oxidants is also discussed.

Full text of DOI:10.1016/j.jorganchem.2014.05.024

Accepted Manuscript
Bis-Cyclometalated Complexes of Pd(II) and Pd(IV) From Iminophosphoranes:
Synthesis, Structure and Reactivity
David Aguilar , Greco González , Pedro Villuendas , Esteban P. Urriolabeitia
PII:
S0022-328X(14)00261-7
10.1016/j.jorganchem.2014.05.024
JOM 18596
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 18 April 2014
Revised Date: 14 May 2014
Accepted Date: 22 May 2014
Please cite this article as: D. Aguilar, G. González, P. Villuendas, E.P. Urriolabeitia, Bis-Cyclometalated
Complexes of Pd(II) and Pd(IV) From Iminophosphoranes: Synthesis, Structure and Reactivity, Journal
of Organometallic Chemistry (2014), doi: 10.1016/j.jorganchem.2014.05.024.
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ACCEPTED MANUSCRIPT
Bis-Cyclometalated Complexes of Pd(II) and Pd(IV) From
Iminophosphoranes: Synthesis, Structure and Reactivity
David Aguilar, Greco González, Pedro Villuendas, Esteban P. Urriolabeitia*
Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), CSIC-Universidad de
Zaragoza, Pedro Cerbuna 12, E-50009 Zaragoza, Spain.
Corresponding Author: Esteban P. Urriolabeitia; Instituto de Síntesis Química y
Catálisis Homogénea, Facultad de Ciencias, Edificio D, planta 4; Campus San
Francisco, Pedro Cerbuna 12, 50009 Zaragoza, Spain. Phone: (+0034)976762302; Fax:
(+0034)976761187; e-mail: esteban@unizar.es.
Present Addresses: Greco González: Department or Organic Chemistry, Stockholm
University, Sweden. David Aguilar: Department of Inorganic Chemistry, University of
Lleida, Spain.
Dedication: This contribution is dedicated to Prof. Maria Pilar García Clemente, who
passed away on May 27, 2013.
MANUSCRIPT JORGANCHEM-D-14-00215
REVISED VERSION
1

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Abstract
The mononuclear bis-orthopalladated complexes [Pd(C^N){C6H4(PPh2=NPh)-2}] [C^N
= (2-(pyridin-2-yl)phenyl) 3a, (2-((dimethylamino)methyl)phenyl) 3b, (S)-(2-(1-
(dimethylamino)ethyl)phenyl) 3c, (quinolin-8-ylmethyl) 3d, (4-methoxy-2-((triphenyl-
λ5-phosphanylidene)carbamoyl)phenyl) 3e] have been synthesized as single isomers by
reaction of the corresponding chloride dimers [Pd(µ-Cl)(C^N)]2 (1a-1e) with the lithium
derivative [Li{C6H4(PPh2=NPh)-2}] (2) in Et2O. Spectroscopic data show that the two
palladated C atoms are in cis disposition, which is confirmed by the X-ray structure of
3a. Oxidation of 3a with PhI(OAc)2 affords the corresponding Pd(IV) derivative (OC-6-
32)[Pd(OAc)2{C6H4(PPh2=NPh)-2}{C6H4-(2'-NC5H4)-2}] 4a also as a single isomer. 4a
undergoes reductive elimination through C-O coupling to give the dimer [Pd(µ-
OAc){C6H4(PPh2=NPh)-2}]2 5 and the ortho-acetoxylated phenylpyridine 6. The
reactivity of 3b-3e towards oxidants is also discussed.
Keywords: orthopalladated, C-O coupling, palladium, high oxidation states,
iminophosphoranes, regioselectivity
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Synopsis
Transmetallation of iminophosphoranes from [Li{C6H4(PPh2=NPh)-2}]2 to chloride-
bridge dimers [Pd(µ-Cl)(C^N)]2 affords mixed bis-cyclopalladated complexes, which
were obtained regioselectively as cis(C,C)-isomers. Complex [Pd{C6H4(PPh2=NPh)-
2}}(C6H4-(2'-NC5H4)-2)] is regioselectively oxidized to the stable Pd(IV) derivative
[Pd(OAc)2{C6H4(PPh2=NPh)-2}(C6H4-(2'-NC5H4)-2)] by treatment with PhI(OAc)2.
This compound undergoes reductive elimination, releasing the acetoxylated
phenylpyridine and keeping the orthopalladated iminophosphorane.
Graphical Abstract (Pictogram)
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Highlights
1.- Transmetallation affords mixed bis(o-palladated) complexes 3a-3e of
iminophosphoranes
2.- cis(C,C)-configuration of these complexes was confirmed by X-ray diffraction study
3.- Bis(o-palladated) complex 3a was oxidized regioselectively to PdIV 4a with
PhI(OAc)2
4.- PdIV complex 4a give acetoxylated C-O coupling product by reductive elimination
5.- Acetoxylation is selectively produced at the phenylpyridine fragment
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1. Introduction
Cyclopalladated complexes are well known organometallic compounds [1], which are
experiencing a "golden age" as useful intermediates in the directed functionalization of
organic molecules [2,3]. In this respect, it is really remarkable that a huge number of
complexes containing one cyclopalladated unit per palladium atom are known, while the
presence of bis-cyclopalladated derivatives (defined as those containing two palladated
ligands on the same palladium atom) is underrepresented. Specific applications of this
type of compounds have been their use as transmetallating reagents [4], in
enantioselective synthesis [5-7], or as compounds with liquid crystal properties [8],
among others. In spite of this apparent indifference, bis-cyclopalladated derivatives are
currently attracting a notable and renewed interest as model intermediate compounds in
organic synthesis involving high oxidation states [9]. This is particularly relevant in the
preparation of bis-aryl derivatives by C-C coupling [10], and in the functionalization of
aryl compounds by C-X (X = halogen) [11], and C-O [12] couplings. In these studies,
stable Pd(II) compounds containing two identical metallated aryl fragments (mostly
robust phenylpyridine) were subjected to oxidation to the corresponding Pd(IV)
complexes, and the subsequent C-C, C-X or C-O bond forming reactions by reductive
elimination were in-depth studied [9-12].
A class of substrates of special interest to be subjected to this methodology are
iminophosphoranes (IM). These are compounds with general formula R3P=NR’, where
R and R' can be alkyl, aryl, acyl, ester, vinyl or cyano groups (among others). The first
synthesis of IM was reported by Staudinger and Meyer in 1919 [13], and since then they
have shown a very interesting behaviour as tunable ligands in coordination chemistry
[14]. We have recently studied several aspects of mono-cyclometallated complexes of
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IM [15-26], such as their use as catalysts [15,16,24], the regioselectivity of the Pd-
induced C-H bond activation in different sites of the IM [17,19-23], their use as
auxiliary species to promote singular bonding modes of allyl ligands [18], or their role
as intermediates in the synthesis of functionalized organic molecules [25,26].
Aiming to expand the scope of the functionalization of IM, we have attempted the
synthesis of biaryl-containing IM from the intramolecular oxidative C-C coupling of
bis-cyclopalladated IM. Organic biaryl derivatives display interesting pharmaceutical
and biological properties [27,28]. Unfortunately, there are not many examples of biaryl
IM. Stalke et al. showed that the lithium derivative [Li(o-C6H4PPh2NSiMe3)]2 reacted
with CuCl2 to yield the product of reductive elimination (Scheme 1) [29], but as far as
we know, there are not other examples of similar couplings.
Scheme 1. Synthesis of a bis-aryl-containing IM.29
To achieve this task, we have first performed the synthesis of bis-cyclopalladated
derivatives containing, at least, one metallated IM. Subsequently, we have studied the
reactivity of these complexes towards oxidants, in order to induce the expected Caryl-
Caryl coupling. In this contribution we report the obtained results in this chemistry.
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2. Experimental Section
2.1. General Methods. Solvents were dried and distilled using standard procedures
before use. All reactions were carried out under Ar atmosphere using standard Schlenk
techniques. Elemental analyses were carried out on a Perkin-Elmer 2400-B
microanalyser. IR spectra were recorded on a Spectrum 100 Perkin Elmer FTIR
spectrophotometer with Universal Attenuated Total Reflectance accesory, which allow
the measurement of the spectra in the 4000-250 cm-1 region. MALDI-TOF mass spectra
were recorded on a Microflex spectrometer (Bruker Daltonic GmBH, Bremen,
Germany). HRMS and ESI (ESI+) mass spectra were recorded using a MicroToF Q,
API-Q-ToF ESI with a mass range from 20 to 3000 m/z and mass resolution 15000
1
13
31
(fwhm). H, C{1H} and P{1H} NMR spectra were recorded using a Bruker AV300
spectrometer (δ in ppm, J in Hz) at 1H operating frequency of 300.13 MHz in CDCl3 as
1
13
deuterated solvent. Other solvents and/or temperatures were specified. H and C{1H}
NMR spectra were referenced to the residual solvent signal, and 31P{1H} was externally
referenced to H3PO4 (85%). Melting points were taken on a Gallenkamp capillary
melting point apparatus (model MPD 350.BM 2.5, Kent, UK). Ph3P=NPh and
PhI(OAc)2 were purchased from commercial sources and used without further
purification. The lithium derivative [Li2(o-C6H4PPh2NPh)2]OEt2 (2) was prepared
following known procedures [30], as well as the starting orthopalladated complexes 1a
[31], 1b [32a], 1c [32b,c], 1d [33] and 1e [17,23].
2.2. General procedure for the synthesis of bis-cyclopalladated compounds 3a-3e
A solution of [Li2(o-C6H4PPh2NPh)2]OEt2 (2) (0.71 mmol) was in situ generated from
treatment of Ph3P=NPh (0.500 g, 1.42 mmol) with PhLi 1.8 M (1.1 mL, 1.98 mmol) in
20 mL Et2O under Ar, according to reported methods [30]. To this solution the
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corresponding dinuclear Pd compound 1a-1e (0.71 mmol) was added, and the mixture
was stirred at 25 ºC for 3 h. The resulting yellow suspension was evaporated to dryness,
and the residue was extracted with CH2Cl2 (2×25 mL). The obtained suspension was
filtered over celite, the solid was discarded and the resulting solution was evaporated to
dryness. The oily residue was treated with Et2O (15 mL) and continuous stirring,
affording complexes 3a-3e as bright yellow solids, which were filtered, washed with
additional Et2O (3×10 mL) and air-dried.
2.3. Synthesis of [Pd(C6H4(PPh2=NPh)-2)(C6H4-(2'-NC5H4)-2)] 3a
1
Yield: 0.405 g (46.5 %). H NMR (CDCl3, 300.13 MHz), δH = 6.69-6.74 (m, 2H, Hp,
NPh + H5’, py), 6.88-7.04 (m, 6H, Hm + Ho, NPh + H4 + H3, PC6H4), 7.07 (td, 1H, H5’’,
C6H4, 3JHH = 7.5, 4JHH = 0.8), 7.17 (td, 1H, H4’’, C6H4, 3JHH = 7.3, 4JHH = 1.1), 7.36-7.44
(m, 5H, H5, PC6H4 + Hm, PPh2), 7.50-7.60 (m, 4H, H4’, py + H6’’, C6H4 + Hp, PPh2),
7.64-7.72 (m, 5H, H3’, py + Ho, PPh2), 7.77 (d, 1H, H3’’, C6H4, 3JHH = 7.5), 7.86 (d, 1H,
3
13
H6’, py, JHH = 6.2), 8.12 (d, 1H, H6, PC6H4, 3JHH = 7.6). C{1H} NMR (CDCl3, 75.47
MHz), δC = 117.88 (s, C3’, py), 120.10 (s, C5’, py), 120.84 (s, Cp, NPh), 122.28 (d, C4,
3
3
C6H4, JPC = 14.7), 123.27, 123.14 (s, C4’, py + C5’’, C6H4), 126.50 (d, Co, NPh, JPC
=
2
10.7), 128.08 (s, Cm, NPh), 128.22 (d, C3, C6H4, JPC = 20.9, + overlapped C6’’, C6H4),
3
1
128.61 (d, Cm, PPh2, JPC = 11.7), 129.31 (s, C4’’, C6H4), 129.89 (d, Ci, PPh2, JPC
=
8

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4
4
87.6), 129.98 (d, C5, C6H4, JPC = 3.3), 132.07 (d, Cp, PPh2, JPC = 2.7), 133.39 (d, Co,
PPh2, 2JPC = 9.7), 137.45 (s, C3’, py), 138.60 (s, C3’’, C6H4), 139.64 (d, C6, C6H4, 3JPC
=
1
15.9), 144.58 (d, C2, C6H4, JPC = 144.7), 149.62 (s, C6’, py), 146.83 (s, C2’’, C6H4),
2
4
149.46 (d, Ci, Ph, JPC = 3.2), 161.44 (d, C1’’, C6H4, JPC = 1.7), 164.58 (s, C2’, py),
2
170.40 (d, C1, C6H4, JPC = 20.2). 31P{1H} NMR (CDCl3, 121.49 MHz), δP = 28.70.
Anal. Calc for [C35H27N2PPd]: C, 68.58; H, 4.44; N, 4.57. Found: C, 68.95; H, 4.20; N,
4.24. IR (ν, cm-1) = 1262 ν(P=N). MS (MALDI +) m/z: 612 (30 %) [M]+ M. p. 249–250
.
°C (dec).
2.4. Synthesis of [Pd(C6H4(PPh2=NPh)-2)(C6H4CH2NMe2-2)] 3b
Yield: 0.526 g (62.5 %). 1H NMR (CDCl3, 300.13 MHz), δH = 2.01 (s, 6H, NMe2), 3.66
(s, 2H, CH2), 6.67-6.71 (m, 1H, (C6H4)’), 6.77 (ddd, 1H, H3, PC6H4, 3JHP = 10.6, 3JHH
7.5, 4JHH = 1.4), 6.85-6.96 (m, 8H, Hm + Hp + Ho, NPh + H4, PC6H4 + 2H, (C6H4)’), 7.20
=
3
4
(tt, 1H, H5, PC6H4, JHH = 7.4, JHH = 1.6), 7.32 (m, 4H, Hm, PPh2), 7.41-7.46 (m, 3H,
3
1H, (C6H4)’ + Hp, PPh2), 7.57 (m, 4H, Ho, PPh2), 7.86 (dd, 1H, H6, PC6H4, JHH = 7.2,
4JHH = 1.7). 13C{1H} NMR (CDCl3, 75.47 MHz), δC = 49.34 (s, NMe2), 72.16 (s, CH2),
120.97 (d, C6’, (C6H4)’, 4JPC = 3.0), 121.74 (s, NPh), 122.15 (d, C4, PC6H4, 3JPC = 14.4),
3
122.50 (s, NPh), 125.40 (s, (C6H4)’), 127.97 (d, Co, NPh, JPC = 9.0), 128.17 (s,
3
(C6H4)’), 128.40 (d, Cm, PPh2, JPC = 11.6, + overlapped C3, PC6H4), 130.01 (d, C5,
4
1
4
PC6H4, JPC = 3.3), 130.27 (Ci, PPh2, JPC = 87.9), 131.32 (d, Cp, PPh2, JPC = 2.7),
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133.45 (d, Co, PPh2, 2JPC = 9.4), 138.50 (s, (C6H4)’), 140.47 (d, C6, PC6H4, 3JPC = 15.7),
1
2
142.63 (d, C2, PC6H4, JPC = 132.6), 148.05 (s, C2’, (C6H4)’), 149.87 (d, Ci, Ph, JPC
=
5.0), 158.08 (s, C1’, (C6H4)’), 167.40 (d, C1, PC6H4, 2JPC = 19.8). 31P{1H} NMR (CDCl3,
121.49 MHz), δP = 29.40. Anal. Calc for [C33H31N2PPd]: C, 66.84; H, 5.27; N, 4.72.
Found: C, 66.49; H, 5.42; N, 4.85. IR (ν, cm-1) = 1257 (νP=N). MS (MALDI +) m/z: 592
(40 %) [M]+. M. p. 201–203 °C (dec).
2.5. Synthesis of [Pd(C6H4(PPh2=NPh)-2)((S)-C6H4CHMeNMe2-2)] 3c
Yield: 0.420 g (48.9 %). 1H NMR (CDCl3, 300.13 MHz), δH = 1.62 (d, 3H, CHMe, 3JHH
3
= 6.5), 1.98 (s, 3H, NMe2), 2.28 (s, 3H, NMe2), 3.66 (q, 1H, CH, JHH = 6.5), 6.75 (td,
3
4
1H, (C6H4)’, JHH = 7.0, JHH = 1.6), 6.77-6.83 (m, 1H, H3, PC6H4), 6.88-7.03 (m, 8H,
3
Hm + Hp + Ho, NPh + H4, PC6H4 + 2H, (C6H4)’), 7.29 (tt, 1H, H5, PC6H4, JHH = 7.4,
4JHH = 1.6), 7.33-7.43 (m, 7H, Hm, PPh2 + Hp, PPh2 + 1H, (C6H4)’), 7.77 (m, 4H, Ho,
3
4
31
PPh2), 7.97 (dd, 1H, H6, PC6H4, JHH = 7.8, JHH = 1.1). P{1H} NMR (CDCl3, 121.49
MHz), δP = 26.67. Anal. Calc for [C34H33N2PPd]: C, 67.27; H, 5.48; N, 4.61. Found: C,
66.90; H, 5.29; N, 4.48. IR (ν, cm-1) = 1274 (νP=N). MS (MALDI+) m/z: 606 (25 %)
[M]+. M. p. 208–210 °C (dec).
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2.6. Synthesis of [Pd(C6H4(PPh2=NPh)-2)(C9H6N-8-CH2)] 3d
Yield: 0.425 g (50.0 %). 1H NMR (CDCl3, 300.13 MHz), δH = 3.89 (br, CH2), 6.37-6.53
3
(m, 5H, NPh), 6.66 (t, 1H, H4, PC6H4, JHH = 7.2), 7.01 (d, 1H, H3, PC6H4, 3JHH = 7.5),
3
7.24 (t, 1H, H5, PC6H4, JHH = 7.5), 7.42-7.79 (m, 15H, 5H, C9H6N + Hm + Hp + Ho,
3
PPh2), 8.27 (d, 1H, H6, PC6H4, JHH = 7.4), 8.98 (br, H1’, C9H6N). 31P{1H} NMR
(CDCl3, 121.49 MHz), δP = 27.47. Anal. Calc for [C34H27N2PPd]: C, 67.95; H, 4.53; N,
4.66. Found: C, 67.83; H, 4.27; N, 4.44. IR (ν, cm-1) = 1266 (νP=N). MS (MALDI+) m/z:
600 (35 %) [M]+. M. p. 142–144 °C (dec).
2.7. Synthesis of [Pd(C6H4(PPh2=NPh)-2)(C6H3-OMe-4-(C(O)N=PPh3)-2)] 3e
1
Yield: 0.492 g, 40.0 %. H NMR (CDCl3, 300.13 MHz), δH = 3.86 (s, 3H, OMe), 6.24
3
3
(t, 2H, Hm, NPh, JHH = 7.7), 6.38 (t, 1H, Hp, NPh, JHH = 7.2), 6.71 (d, 2H, Ho, NPh,
3JHH = 8.0), 6.84-6.97 (m, 3H, 1H, (C6H3)' + 2H, C6H4), 7.25-7.31 (m, 5H, 1H, C6H4 +
11

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Hm, PPh2), 7.40-7.47 (m, 8H, Hp, PPh2 + Hm, PPh3), 7.53-7.74 (m, 15H + 2H, (C6H3)',
3Hp, PPh3 + 6Ho, PPh3 + 4H, Ho, PPh2), 8.04 (dd, 1H, H6, C6H4, 3JHH = 7.5, 4JHH = 2.0).
31P{1H} NMR (CDCl3, 121.49 MHz), δP = 19.66 (PPh2), 24.18 (C(O)N=PPh3). Anal.
Calc for [C50H40N2O2P2Pd]: C, 69.09; H, 4.64; N, 3.22. Found: C, 69.51; H, 4.27; N,
3.05. IR (ν, cm-1) = 1590 (νC=O), 1284 (νP=N). MS (MALDI +): m/z: 868 (20 %) [M]+.
M. p. 198–200 °C (dec).
2.8. Synthesis of (OC-6-32)[Pd(OAc)2(C6H4(PPh2=NPh)-2){C6H4-(2'-NC5H4)-2}] 4a
PhI(OAc)2 (109 mg, 0.326 mmol) was added to a solution of 3a (200 mg, 0.326 mmol)
in 10 mL of CH2Cl2 at room temperature, and the resulting solution was stirred for 1 h.
During this time the color of the solution changed from yellow to orange. After the
reaction time, the solvent was removed under reduced pressure to small volume (1 mL)
and Et2O (4 mL) was added. Further stirring produced the formation of an orange-
yellowish solid 4a, which was filtered, washed with more Et2O (4 mL) and dried in
vacuo. Complex 4a was recrystallized from CH2Cl2/Et2O affording 4a.0.5CH2Cl2,
which was used for analytical and spectroscopic purposes. Complex 4a has to be stored
under Ar at -18 oC.
Yield: 200 mg (83 %). 1H NMR (CD2Cl2, 300.13 MHz, 253 K), δH = 1.81 (3H, s, OAc),
3
1.84 (3H, s, OAc), 5.65 (br, 2H, Ho, NPh), 6.41 (br, 2H, Hm, NPh), 6.54 (1H, d, JHH
=
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7.4, H5’’, C6H4), 6.70 (1H, t, 3JHH = 7.4, Hp, NPh), 6.9-7.1 (3H, m, H4’ py + H5’’ C6H4 +
H6 PC6H4), 7.1-7.4 (7H, m, H4’’ C6H4 + H5 C6H4P + H2’ py, Hm PPh2), 7.4-7.8 (9H, m,
3
H3’ py + H3 C6H4P + H3’’ C6H4 + Ho + Hp PPh2), 8.12 (d, 1H, H6, C6H4P , JHH = 8.0),
8.94 (d, 1H, H6’, py, 3JHH = 4.2). 13C{1H} NMR (CD2Cl2, 75.47 MHz, 253 K), δC = 24.9
(CH3), 115.9 (C3’’, C6H4), 117.7 (Co, PPh2), 122.3 (C4’, py), 125.1 (Cp, NPh), 125.3 (Cp
PPh2), 125.6 (C4’’ C6H4), 126.2 (C2 C6H4P), 127.3 (Cm NPh), 128.4 (Cm PPh2), 128.7
(C2’ py), 129.2 (C5’’ C6H4), 130.1 (C6 C6H4P), 130.3 (Co NPh), 132.2 (C3 C6H4P), 133.1
(C5 C6H4P), 133.5 (C6’’ C6H4), 133.7 (C4 C6H4P), 138.1 (C3’ py), 142.3(C2’’ C6H4), 149.2
(C5’ py), 155.9 (C1’’ C6H4), 156.3 (Ci NPh), 157.8 (C6’ py), 158.1 (Ci PPh2), 166.9 (C1
C6H4P), 175.0 (COO). 31P{1H} NMR (CD2Cl2, 121.49 MHz, 253 K), δP = 59.06. Anal.
Calc for [C39H33N2O4PPd]0.5CH2Cl2: C, 61.33; H, 4.43; N, 3.62. Found: C, 61.68; H,
4.19; N, 3.53. IR (ν, cm-1) = 1616 νa(CO2), 1357 νs(CO2), 1295 ν(P=N). HRMS (ESI-
TOF) m/z: [M-OAc]+ calc for C37H30N2O2PPd 671.1080, found 671.1126; [M-
2OAc+OCH3]+ calc for C36H30N2OPPd 643.1131, found 643.1137.
2.9. X-ray crystallography. X-ray data collection of 3a was performed on an Oxford
Diffraction Xcalibur2 diffractometer using graphite-monocromated Mo-Kα radiation
(λ = 0.71073 Å). A single crystal of 3a was mounted at the end of a quartz fiber in a
random orientation, covered with perfluorinated oil and placed under a cold stream of
N2 gas. A hemisphere of data was collected based on ω-scan or φ-scan runs. The
diffraction frames were integrated using the program CrysAlisRED [34] and the
integrated intensities were corrected for absorption with SADABS [35]. The structure
was solved and developed by Patterson and Fourier methods [36]. All non-H atoms
were refined with anisotropic displacement parameters. The H atoms were placed at
idealized positions and treated as riding atoms. Each H atom was assigned an isotropic
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displacement parameter equal to 1.2 times the equivalent isotropic displacement
2
parameter of its parent atom. The structure was refined to Fo and all reflections were
used in the least-squares calculations [37].
3. Results and Discussion
With notable exceptions [38], the synthesis of bis-cyclopalladated complexes usually
involves a transmetallation step of the metallated ligand from highly charged lithium,
sodium or magnesium derivatives to an electrophilic palladium center [39]. Due to the
easy availability of lithium phosphinimine complex [Li2(o-C6H4PPh2NPh)2]OEt2 (2)
[30,40], the synthesis of the bis-cyclometallated derivatives 3a-3e has been perfomed by
reaction of (2) with the neutral PdII dimers [Pd(µ-Cl)(C^N)]2 1a-1e [C^N = 2-(pyridin-
2-yl)phenyl 1a, 2-((dimethylamino)methyl)phenyl 1b, (S)-2-(1-(dimethylamino)-
ethyl)phenyl 1c, quinolin-8-ylmethyl 1d, 4-methoxy-2-((triphenyl-λ5-phosphanylidene)-
carbamoyl)phenyl) 1e] as shown in Scheme 2. In turn, (2) was prepared by reaction of
Ph3P=NPh with excess of PhLi (1:1.4 molar ratio) in Et2O, following reported
procedures [30]. Different classical C,N-cyclopalladated ligands have been selected as
ancillary groups in the starting materials 1a-1e to provide a wide array of different
electronic and steric environments, from the electron-rich pyridine-containing ligand 1a
to the electron-poor keto-stabilized IM 1e. In all studied cases the workup of the
reaction is very simple, and allows the synthesis of complexes 3a-3e as air-stable bright
yellow solids in moderate yields.
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Scheme 2. Synthesis of bis-cyclopalladated complexes 3a-3e.
The characterization of 3a-3e has been accomplished through their analytic and
spectroscopic data. The elemental analyses and mass spectra (MALDI+) of 3a-3e are in
good agreement with the stoichiometries proposed in Scheme 2. The IR spectra of 3a-3e
show a strong absorption due to the P=N stretch at 1262 (3a), 1257 (3b), 1274 (3c) and
1266 (3d) cm-1, respectively. The position of this band is shifted to low energy with
respect to that found in the free iminophosphorane Ph3P=NPh (1310 cm-1), this fact
strongly suggesting the N-bonding to the Pd center. In the case of 3e two absorption
bands (1309 and 1284 cm-1) assigned to the two P=N moieties present in the structure
are again different from those found the free iminophosphoranes (1330 and 1310 cm-1
respectively). Moreover, the IR spectrum of 3e also shows a strong band at 1590 cm-1,
15

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assigned to the carbonyl stretch νCO, which appears at a wavenumber similar to that
found in other keto-stabilized N-bonded IM [17,23].
31
The P{1H} NMR spectra of 3a-3d show a single peak at δ = 28.70 (3a), 29.40 (3b),
26.67 (3c) and 27.47 (3d) ppm, respectively, which is downfield shifted with respect to
the free iminophosphorane (δ = 3.00 ppm), in keeping with the N-bonding to the Pd
31
atom. On the other hand, the P{1H} NMR of 3e showed two signals at δ = 19.66 and
24.18 ppm. The first one is assigned to the keto-stabilized iminophosphorane ligand
while the second one is due to the semi-stabilized iminophosphorane. In all studied
cases, the presence of a single set of signals shows that 3a-3e have been obtained as
single isomers, this meaning that the reaction takes place with full regioselectivity. The
position of the 31P peaks for the C6H4PPh2=NPh ligand, in the range 24-30 ppm, is
another clear indication of the formation of the neutral bis-cyclometallated complexes.
In fact, chemical shifts for other published complexes containing only one
cyclometallated C6H4PPh2=NPh ligand appear in the range 35-55 ppm [15,40], clearly
shifted to low field, while chemical shift for the bis-cyclopalladated complex
[Pd(C6H4PPh2=NPh)2] is reported at 27.8 ppm [40]. This fact can be explained taking
into account the higher electron-rich nature of the bis-cyclopalladated complex (with
two strong σ-Caryl donors) with respect to those having a single cyclopalladated ligand.
1
The H NMR spectra of 3a-3e show the expected signals for the presence of all
functional groups. In addition, the presence of the cyclopalladated Pd(C6H4) unit is also
clearly inferred from the presence of a deshielded doublet around 8.0-8.2 ppm, assigned
to the H6 proton (ortho to the cyclopalladation position), similar to that reported for the
13
complex [Pd(C6H4PPh2=NPh)2] [40]. The C{1H} NMR spectra of 3a and 3b provide
further evidences for the presence of the σ-aryl Pd-C bond, due to the observation of a
16

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very deshielded peak at 170.40 ppm (3a) or 167.40 ppm (3b), respectively, which
appear very near to that reported for [Pd(C6H4PPh2=NPh)2] (171.2 ppm) [40].
Additional structural information can be obtained from the determination of the X-ray
crystal structure of complex 3a. A draw of the bis-orthopalladated complex is shown in
Figure 1, as well as selected bond distances and angles.
Figure 1. View of complex 3a. Displacement ellipsoids are scaled to 50% probability
level. Selected bond distances (Å) and angles (º): Pd(1)-C(1) = 1.992 (2); Pd(1)-C(12) =
2.003(2); Pd(1)-N(1) = 2.0947(17); Pd(1)-N(2) = 2.1706(17); P(1)-C(17) = 1.785(2);
P(1)-N(2) = 1.5963(17); C(1)-C(6) = 1.417(3); C(6)-C(7) = 1.472(3); C(7)-N(1) =
1.356(3); C(12)-C(17) = 1.414(3); C(1)-Pd(1)-C(12) = 96.75(8); C(1)-Pd(1)-N(1) =
81.27 (8); N(1)-Pd(1)-N(2) = 96.68(6); C(12)-Pd(1)-N(2) = 86.21(7); C(1)-Pd(1)-N(2) =
170.48(7); C(12)-Pd(1)-N(1) = 173.82(7); C(6)-C(1)-Pd(1) = 113.61(14); C(1)-C(6)-
C(7) = 116.67(18); C(7)-N(1)-Pd(1) = 113.92(13); N(1)-C(7)-C(6) = 114.08(17); N(2)-
P(1)-C(17) = 105.95(9).
The Pd atom was located in a slightly distorted square-planar environment, surrounded
by the palladated carbon C(1) and the pyridinic nitrogen N(1), derived from the
17

ACCEPTED MANUSCRIPT
phenylpyridine ligand, and the metalated carbon C(2) and the phosphinimine nitrogen
N(2), from the IM. The slight deviation from the ideal square-planar situation can be
deduced from the value of the angle between the best least-square planes defined by
N(1)-Pd(1)-C(1) and N(2)-Pd(1)-C(12), which amounts 11.4(2)º. The two Pd-Caryl σ-
bonds are in cis(C,C)-arrangement, as it has been found in previous examples [41]. This
particular arrangment is easily explained taking into account the antisymbiotic
behaviour of the soft Pd(II) center [42,43] and the large trans-influence of the Caryl
atoms. In this way, a soft donor (as is the Caryl atom) is more stabilized in trans to a hard
donor (in this case the N atom) than in trans to another soft donor [42]. This reasoning
has been used in several cases through the bibliography to explain geometrical
preferences [43].
The Pd(1)-C(1) bond distance of the phenylpyridine ligand (1.992(2) Å) is identical,
within experimental error, to those found in the dimer [Pd(µ-Cl)(C6H4-2-NC5H4)]2
(1.972(5) and 1.978(4) Å, respectively) [44] while the Pd(1)-N(1) bond distance
(2.0947(17) Å) is clearly longer than those found in the same dimer [2.005(4) and
2.013(5) Å]. The longer distance of the Pd-N bond in 3a with respect to [Pd(µ-
Cl)(C6H4-2-NC5H4)]2 reflects the different influence of the respective trans ligands (C in
3a versus Cl in the dimer). On the other hand, both the Pd(1)-C(12) (2.003 (2) Å) as
well as the Pd(1)-N(2) bond distances (2.1706(17) Å) in the palladated IM are identical,
within experimental error, to the respective distances found in the bis-cyclopalladated
complex [Pd(C6H4PPh2=NPh)2] [40], probably because the trans environments are
similar in both cases (a cyclopalladated ligand). Other internal parameters are as
expected, showing values similar to those found in related complexes [40,44] and do not
deserve further comment.
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Once the synthesis and full characterization of the bis-cyclopalladated complexes 3a-3e
has been accomplished, we have studied their reactivity in different situations in order
to promote the C-C coupling of the two palladated ligands. We have first tested the
reductive elimination from Pd(II), that is, directly from 3a-3e, following procedures
reported in the literature [45-47]. Thermal treatment of 3a-3e at reflux temperature in
ClC6H5 or reaction with CO promoted an unclear decomposition pathway and, although
formation of Pd0 was evident, a very complex mixture of unidentified compounds was
obtained. In the case of treatment of 3a-3e with PPh3 in MeOH, a clean formation of
Pd(PPh3)4 was observed, but in this case the protonated ligands (Ph3P=NPh and HC^N)
were isolated at the end of the reaction, and the C-C coupling product was not observed.
Therefore, even if Pd0 is detected at the end of the reaction, the expected C-C coupling
was not achieved under the studied conditions.
Therefore, we decided to change our strategy, attempting the reactivity of 3a-3e with
oxidants such as PhI(OAc)2. We aimed to promote the oxidation of the square-planar
Pd(II) complexes to the respective octahedral Pd(IV) derivatives, which are more prone
to undergo the C-C coupling [3h].
1
The reaction of 3a with PhI(OAc)2 (1:1 molar ratio) in CD2Cl2 was monitored by H
and 31P NMR spectroscopy at room temperature. Particularly relevant is the information
provided during the reaction by the 31P NMR spectrum, where the disappearance of the
peak at 28 ppm (due to 3a) and the simultaneous grow of another signal at 58 ppm was
evident. No other signals were detected in the spectrum, although their existence as low
intensity transients cannot be completely excluded. After 1 h of reaction at room
temperature, all starting material 3a was cleanly transformed into a single new species
4a. The reaction was then scaled up to a 0.3 mmol scale. An equimolar mixture of 3a
19

ACCEPTED MANUSCRIPT
and PhI(OAc)2 was stirred at room temperature for 1 h, then the solvent was partially
evaporated and 4a was precipitated as a yellow solid by Et2O addition, and
recrystallized from CH2Cl2/Et2O. Complex 4a is stable under Ar at -18 ºC; under these
conditions it does not show changes at least in a period of 1 month. The analytical data
of 4a strongly suggest the incorporation of two acetate units to the molecular formula of
3a. Additional spectroscopic data of 4a suggest a structure as that depicted in Scheme 3.
Scheme 3. Synthesis and suggested structure of stable Pd(IV) derivative 4a
The presence of acetate ligands is clear from the observation of two strong bands in the
IR spectrum of 4a at 1616 and 1357 cm-1, due to the νa(CO2 ) and νs(CO2 ) modes,
-
-
respectively. The separation of these two bands is ∆ν = νa-νs = 259 cm-1, this value
pointing out to an O-monodentate bonding mode when compared with values reported
in the literature (range 228-470 cm-1) for the unidentate coordination [48]. The IR
spectrum of 4a also shows a very strong band at 1295 cm-1, assigned to the ν(P=N
)
stretch, which has been shifted to higher energy after oxidation (1262 cm-1 in 3a)
indicating a more robust P=N bond in 4a compared to 3a.
31
The P NMR spectrum of 4a shows a single peak at 59.06 ppm, which is downfield
shifted with respect to the starting material 3a (28.70 ppm), in good agreement with the
20

ACCEPTED MANUSCRIPT
higher electropositive character of the metal center in 4a [Pd(IV)] with respect to 3a
[Pd(II)]. It is really worthy of note that, although several isomers are possible for 4a,
only a single set of NMR signals is observed, even at low temperature. At 253 K the
molecule behaves as static in the NMR timescale and, therefore, 4a is obtained as a
1
single isomer. The H NMR spectrum of 4a provides additional structural information.
The signal due to H6' proton of the pyridine ring of 4a appears shifted downfield (8.98
ppm) when compared with 3a (7.86 ppm). This large shift suggests that the anisotropic
shielding undegone by the H6' proton in 3a due to their close proximity to the phenyl
ring of the NPh unit (see Figure 1) is not longer active. The observation of new
anisotropic shieldings helps to elucidate the relative disposition of the different groups
in the new octahedral structure. Thus, the ortho and meta protons of the NC6H5 ring
appear in 4a strongly shielded (5.65 ppm Hortho, 6.41 ppm Hmeta) with respect to their
respective positions in 3a (6.88 - 7.04 ppm). Moreover, the H6" proton of the C6H4 unit
of the phenylpyridine ligand is also suffering a strong shielding in 4a (6.57 ppm) with
respect to its position in the precursor 3a (overlapped with other signals between 7.50-
7.60 ppm). This set of shieldings and deshieldings can be explained assuming a
molecular structure for 4a as that depicted in Scheme 3. If the molecular (equatorial)
plane contains the phenylpyridine ligand, one of the acetate ligands and the palladated C
atom of the PC6H4 moiety, then the axial positions are occupied by the other acetate and
by the iminic N atom. In this arrangement the PC6H4 ring, which should be normal to
the molecular plane, shields the H6" proton of the cis C6H4 ring which, in turn, shields
the ortho and meta protons of the iminic NPh unit. The relative disposition of the six
C2N2O2 donor atoms around the Pd atom in 4a is similar to that displayed in other
Pd(IV) compounds characterized by X-ray diffraction methods [12].
21

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Once 4a has been fully characterized, and prompted by its notable stability, a similar
reactivity was attempted with the other derivatives 3b-3e and PhI(OAc)2. As a first step
1
31
a similar monitoring of the reactions by H and P NMR spectroscopy in CD2Cl2 was
performed. The reaction of 3b with PhI(OAc)2 is qualitatively much faster than that
observed for 3a, and in few minutes complex 3b was completely consumed. However,
31
the P NMR spectrum only showed a broad resonance near 0 ppm, likely due to the
starting material Ph3P=NPh (δ = 3.0 ppm) [49], and two very small peaks around 50
ppm. The latter were probably due to Pd(IV) species analogous of 4a, but their amount
decreases along time and finally vanished, even at low temperature. No definite
compound could be isolated from these reaction mixtures. For these reasons and the
similarities between 3b and 3c, complex 3c was not studied. The reaction of 3d with
PhI(OAc)2 also took place much faster than observed for 3a, even at low temperature,
31
and only [Ph3PNH2]+ was detected in the P NMR spectrum (peak at 36.0 ppm). It is
clear that the stability of the presumably formed Pd(IV) intermediates in both cases (3b,
3d) is much lower than for 4a. It is likely the higher robustness of the phenylpyridine
ligand, compared with that of the other classical C,N-orthometalating chelates, the main
responsible of the observed behaviour. In this respect, a close inspection of the recent
literature dealing with organic synthesis mediated by Pd(IV) complexes shows that most
of them use phenylpyridine or analogous rigid-, electron-rich ligands [9-12,50]. Thus,
the correct choice of the ancillary ligand is critical in order to prepare, isolate and
characterize the Pd(IV) intermediates. However, even being important, it is not the only
factor to be taken into account, because the I(III) reagent, the solvent or even the final
charge of the complex play a relevant role [51].
22

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As the final step, we have studied the reductive elimination from 4a. A yellow solution
of 4a in CD3CN was heated at reflux temperature for 2 h, and a visible change of the
color of the solution from bright yellow to orange was observed. The 31P NMR
spectrum of this solution shows the presence of a single resonance at 53.38 ppm,
showing that only one species containing the IM ligand has been formed. Recent work
of Sanford et al. reported the reductive elimination of PdIV(phpy)2(OAc)2 (phpy = C6H4-
2-NC5H4) affording 2-(2-acetoxyphenyl)pyridine and the dimer [Pd(µ-OAc)(phpy)]2 as
the only reaction products [12]. The presence of 2-(2-acetoxyphenyl)pyridine 6 in our
case is inferred from the observation of a clear peak at 214 amu in the ESI+ spectrum of
the CD3CN solution. On the other hand, the position of the 31P chemical shift (53.38
ppm) shows that the P-containing species is still palladated, and matches with that
reported for the acetate-bridge dimer [Pd(µ-OAc)(C6H4PPh2NC6H4Me)]2 (53.69 ppm)
[52]. Other acetate-bridge dinuclear compounds with minor changes with respect to 5
also appear in the same region [52]. Therefore, on the grounds of the NMR and ESI+
data, and taking into account the recent results of Sanford, we propose that the reductive
elimination of 4a in CD3CN takes place affording 5 and 6, as shown in Scheme 4.
Scheme 4. Reductive elimination of 4a in CD3CN to give compounds 5 and 6.
23

ACCEPTED MANUSCRIPT
It is remarkable that, once again, the reaction takes place with full selectivity under the
studied conditions. Following the work of Sanford [12], and due to the fact that 4a
contains two different cyclopalladated ligands, one could expect that, after reductive
elimination, two different acetate-bridge derivatives, 5 and [Pd(µ-OAc)(phpy)]2, could
be formed; and that two different C-O couplings and one C-C coupling are also possible
in order to obtain organic functionalized products. However, the analysis of the reaction
mixtures shows the selective formation of 5 and 6. Therefore, the study of this system
shows that only the C-O coupling in the case of the phenylpyridine ligand is favoured,
and that in no case couplings involving the iminophosphorane (neither C-O, nor C-C)
were detected. Then, the method seems to be adequate for the functionalization of
phenylpyridine, but not for other systems.
4. Conclusions
Bis-aryl Pd(II) complexes containing the metallated iminophosphorane [C6H4PPh2NPh]
and another C^N cyclopalladated ligand have been regioselectively synthesized by
transmetallation reactions from [LiC6H4PPh2NPh]2. The products were obtained as the
cis(C,C)-isomer. The oxidation of the complex [Pd(phpy)(C6H4PPh2NPh)] with
PhI(OAc)2 gave the Pd(IV) derivative [Pd(OAc)2(phpy)(C6H4PPh2NPh)] also as a single
stereoisomer, which could be isolated as a stable complex. The oxidation of other
examples didn't allow the isolation of stable Pd(IV) complexes. The reductive
elimination processes observed on [Pd(OAc)2(phpy)(C6H4PPh2NPh)] gave in a selective
way the acetoxylated phenylpyridine by C-O coupling, while the iminophosphorane
remains palladated.
24

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Acknowledgments
Financial support from the Spanish Ministerio de Economía y Competitividad
(MINECO) under project CTQ2011-22589 and the Regional Government of Aragón
under project E-97 is gratefully acknowledged. P. V. thanks CSIC for a Juan de la
Cierva contract.
Appendix A. Supplementary data
CCDC-983329 (3a) contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
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