Catalytic properties of [Pd(COOMe)nX2-n(PPh 3)2] (n = 0, 1, 2; X = Cl, NO2, ONO 2, OAc and OTs) in the oxidative carbonylation of MeOH

Article abstract of DOI:10.1021/ic901569w

cis-[Pd(ONO₂)₂(PPh₃)₂] (1) reacts under mild conditions with CO in methanol (MeOH) in the presence of pyridine (py), yielding trans-[Pd(COOMe)(ONO₂)(PPh₃) ₂] (1a). The use of NEt₃ instead of py leads to a mixture of 1a, trans-[Pd(COOMe)₂(PPh₃)₂] (2), and [Pd(CO)(PPh₃)₃]. Pure 2 was prepared by reacting cis-[Pd(OTs)₂(PPh₃)₂] with CO in MeOH and subsequently adding NEt₃. The nitro complex trans-[Pd(COOMe)(NO ₂)(PPh₃)₂] (3a) was prepared by reacting trans-[Pd(COOMe)Cl(PPh₃)₂] with AgNO₂ or with AgOTs and NaNO₂. New syntheses for 1 and trans-[Pd(NO ₂)₂(PPh₃)₂] (3) are also reported. All complexes have been characterized by IR and ¹H and ³¹P{¹H} NMR spectroscopies. Complexes 1 and 2 exchange irreversibly and quantitatively one nitrato with one carbomethoxy ligand, yielding 1a. 2 in CD₂Cl₂ at 40 °C decomposes with the formation of dimethyl carbonate (DMC), whereas under 4 atm of CO, DMC and dimethyl oxalate (DMO) are formed, ca. 12% each; in the presence of PPh ₃ and in the absence of CO, decomposition occurs at 60 °C with the formation of DMC only, suggesting that decarbonylation involves a five-coordinate intermediate or predissociation of a PPh₃ ligand. The oxidative carbonylation of MeOH does not occur when using NaNO₂ or NaNO₃ as the oxidant and 1, 1a, 3, or 3a as the catalyst precursor. On the contrary, when using benzoquinone (BQ) as the oxidant, these complexes, 2, or [Pd(COOMe)_2-nX_n(PPh₃)₂] (X = Cl, OAc, OTs; n = 1, 2) promote selective catalysis to DMO. After catalysis the precursors are transformed into [Pd(BQ)(PPh₃)₂] ₂ H₂BQ, [Pd(CO)(PPh₃)]₃ and [Pd(CO)(PPh₃)₃]. Also the last with BQ gives selective catalysis to DMO. The solid-state structures of 1 CH₂Cl₂ and 1a have been determined by means of single-crystal X-ray diffraction.

Full text of DOI:10.1021/ic901569w

Inorg. Chem. 2010, 49, 3721–3729 3721
DOI: 10.1021/ic901569w
Catalytic Properties of [Pd(COOMe)nX2-n(PPh3)2] (n = 0, 1, 2; X = Cl, NO2, ONO2,
OAc and OTs) in the Oxidative Carbonylation of MeOH
Emanuele Amadio,† Gianni Cavinato,‡ Alessandro Dolmella,§ and Luigi Toniolo*,†
†
‡
ꢀ
Dipartimento di Chimica, Universita di Venezia, Dorsoduro 2137, 30123 Venezia, Italy, Dipartimento di
§
ꢀ
Scienze Chimiche, Universitadi Padova, via Marzolo 1, 35100 Padova, Italy, and Dipartimento di Scienze
ꢀ
Farmaceutiche, Universita di Padova, via Marzolo 5, 35100 Padova, Italy
Received August 6, 2009
cis-[Pd(ONO2)2(PPh3)2] (1) reacts under mild conditions with CO in methanol (MeOH) in the presence of pyridine
(py), yielding trans-[Pd(COOMe)(ONO2)(PPh3)2] (1a). The use of NEt3 instead of py leads to a mixture of 1a,
trans-[Pd(COOMe)2(PPh3)2] (2), and [Pd(CO)(PPh3)3]. Pure 2 was prepared by reacting cis-[Pd(OTs)2(PPh3)2]
with CO in MeOH and subsequently adding NEt3. The nitro complex trans-[Pd(COOMe)(NO2)(PPh3)2] (3a) was
prepared by reacting trans-[Pd(COOMe)Cl(PPh3)2] with AgNO2 or with AgOTs and NaNO2. New syntheses for 1 and
trans-[Pd(NO2)2(PPh3)2] (3) are also reported. All complexes have been characterized by IR and 1H and 31P{1H}
NMR spectroscopies. Complexes 1 and 2 exchange irreversibly and quantitatively one nitrato with one carbomethoxy
ligand, yielding 1a. 2 in CD2Cl2 at 40 °C decomposes with the formation of dimethyl carbonate (DMC), whereas under
4 atm of CO, DMC and dimethyl oxalate (DMO) are formed, ca. 12% each; in the presence of PPh3 and in the absence
of CO, decomposition occurs at 60 °C with the formation of DMC only, suggesting that decarbonylation involves a five-
coordinate intermediate or predissociation of a PPh3 ligand. The oxidative carbonylation of MeOH does not occur when
using NaNO2 or NaNO3 as the oxidant and 1, 1a, 3, or 3a as the catalyst precursor. On the contrary, when using
benzoquinone (BQ) as the oxidant, these complexes, 2, or [Pd(COOMe)2-nXn(PPh3)2] (X = Cl, OAc, OTs; n = 1, 2)
promote selective catalysis to DMO. After catalysis the precursors are transformed into [Pd(BQ)(PPh3)2]2 H2BQ,
[Pd(CO)(PPh3)]3 and [Pd(CO)(PPh3)3]. Also the last with BQ gives selective catalysis to DMO. The solid-state
3
structures of 1 CH2Cl2 and 1a have been determined by means of single-crystal X-ray diffraction.
3
Introduction
out in the presence of an alkanol, such as, for example, the
alkene-CO copolymerization to polyketones1 and the oxi-
dative carbonylation of alkenes to unsaturated esters or
diesters3 or of alkanols to carbonates and oxalates.2m,3 They
have been proposed as intermediates also in the catalytic
hydrocarboalkoxylation of alkenes to monoesters.4
The interaction of CO with an alkanol on the metal center
providesthe most direct method of synthesis.5 Other methods
include the oxidative addition of chloro or cyano formate
or of phenylcarbonate to palladium(0) complexes,6 the
decarbonylation of alkoxalyl complexes,7 and exchange with
HgCl(COOMe).8
Palladium(II) carboalkoxy complexes are intermediates in
several important catalytic carbonylation reactions carried
*To whom correspondence should be addressed. E-mail: toniolo@
unive.it.
(1) (a) Sen, A., Ed. Catalytic Synthesis of Alkene-CO Copolymers and
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r
2010 American Chemical Society
Published on Web 03/24/2010
pubs.acs.org/IC

3722 Inorganic Chemistry, Vol. 49, No. 8, 2010
Amadio et al.
It has been reported that PdCl29 and Pd(OAc)210 promote
the noncatalytic carbonylation of ethanol (EtOH) or metha-
nol (MeOH) to diethyl carbonate or dimethyl oxalate
(DMO), respectively, with concomitant reduction to palla-
dium metal. The formation of these products occurs through
the intermediacy of PdCOOR species. The presence of PPh3
prevents the formation of palladium black, yielding instead
palladium(0) complexes such as [Pd(CO)(PPh3)]3 and
[Pd(CO)(PPh3)3].5o,p,10 In order to be catalytic in palladium,
some other oxidants, such as oxygen, Cu2þ, Fe3þ, benzoqui-
none (BQ), or an organic nitrite or a combination of them,
must be used.3
operating in the FT mode at 300 and 121.442 MHz for 1H and
1
31P{1H}, respectively. H and 31P{1H} chemical shifts are re-
ported in ppm downfield of the deuterated solvent used as the
internal standard or of externally referenced to 85% H3PO4,
respectively. Gas chromatography (GC) analysis was performed
using a Hewlett-Packard model 6890 chromatograph fitted with
a HP5, 30 m ꢀ 0.32 μm ꢀ 0.25 μm column [detector, flame
ionization; carrier gas, N2, 0.7 mL/min; oven, 40 (3.5 min) to 250 °C
at 15 °C/min]. For detection of methyl formate, a 80/120
1
Carbowax 20M, 6 ft ꢀ /4 in. o.d., 2 mm i.d. glass column was
used (temperature, 50-170 °C, 5 °C/min; detector, FID; carrier
gas, N2, 20 mL/min. For the detection of formaldehyde by the
EPA 8315A method, http://www.epa.gov/epawaste/hazard/
testmethods/sw846/pdfs/8315a.pdf, an Agilent HPLC-UV/vis
instrument was used (wavelength, 360 nm), equipped with a
Zorbax ODS column 5 μm, 4.6 ꢀ 250 mm, at 30 °C.
With the aim of finding whether a NO3- or a NO2- ligand
could act as the oxidant in the oxidative carbonylation of
MeOH, we took into consideration the use of complexes of
the type trans-[Pd(COOMe)2-nXn(PPh3)2] (X = ONO2,
NO2; n = 0, 1, 2). Here, we report their synthesis, reactivity,
and catalytic activity in the oxidative carbonylation of
MeOH. The X-ray diffraction structures of trans-[Pd(COOMe)-
CD2Cl2 and the solvents (Aldrich) for the preparation of the
complexes were used as received. Pd(OAc)2, AgTsO, AgNO3,
AgNO2, PPh3, p-toluensulfonic acid, NEt3, and BQ were also
Aldrich products; only BQ was purified before use (from ethyl
ether). CO (purity higher than 99%) was supplied by SIAD Spa
(Italy). The complexes trans-[Pd(COOMe)(OTs)(PPh3)2],5a
trans-[Pd(COOMe)Cl(PPh3)2],5k trans-[PdCl2(PPh3)2],11 trans-
[PdOAc)2(PPh3)2],12 and cis-[Pd(OTs)2(PPh3)2]13 were prepared
according to methods reported in the literature.
(ONO2)(PPh3)2] and cis-[Pd(ONO2)2(PPh3)2] CH2Cl2 are also
3
reported.
Experimental Section
The reactivity tests under CO pressures higher than 2 atm
were carried out using a stainless autoclave of ca. 60 mL, into
which a glass bottle containing the solvent and reagents was
introduced. The autoclave was first flushed several times with
CO and then taken to the desired pressure and temperature. The
solution was stirred with a magnetic bar. After 1 h of reaction,
the autoclave was rapidly cooled to 0 °C and then slowly
depressurized. The content was analyzed by GC and IR and
NMR spectroscopies.
Instrumentation and Materials. IR spectra were recorded in a
Nujol mull or in KBr on a Nicolet Fourier transform infrared
(FTIR) instrument model Nexus (the IR frequencies reported
below, when not indicated, refer to spectra taken in a Nujol
mull). 1H and 31P{1H} NMR spectra were recorded on a Bruker
AMX 300 spectrometer equipped with a BB multinuclear probe
(5) (a) Amadio, E.; Cavinato, G.; Dolmella, A.; Ronchin, L.; Toniolo, L.;
Vavasori, A. J. Mol. Catal. A: Chem. 2009, 298, 103. (b) Khabibulin, V. R.;
Kulik, A. V.; Oshanina, I. V.; Bruk, L. G.; Temkin, O. N.; Nosova, V. M.;
Ustynyuk, Yu. A.; Bel'skii, V. K.; Stash, A. I.; Lysenko, A. L.; Antipin, M. Yu.
Kinet. Katal. 2007, 48, 228. (c) Giannoccaro, P.; Cornacchia, D.; Doronzo, S.;
Mesto, E.; Quaranta, E.; Aresta, M. Organometallics 2006, 25, 2872. (d)
Cavinato, G.; Vavasori, A.; Toniolo, L.; Benetollo, F. Inorg. Chim. Acta 2003,
343, 183. (e) Dervisi, A.; Edwards, P. G.; Newmam, P. D.; Tooze, R. P.; Coles, S.
J.; Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1999, 1113. (f ) Santi, R.;
Romano, A. M.; Garrome, R.; Millimi, R. J. Organomet. Chem. 1998, 566,
37. (g) Bertani, R.; Cavinato, G.; Facchin, G.; Toniolo, L.; Vavasori, A.
J. Organomet. Chem. 1994, 466, 273. (h) Giannoccaro, P.; Ravasio, M.; Aresta,
M. J. Organomet. Chem. 1993, 451, 243. (i) Cavinato, G.; Toniolo, L.
J. Organomet. Chem. 1993, 444, C65. ( j) Smith, G. D.; Hansom, B. E.; Merola,
J. S.; Waller, F. J. Orgamometallics 1993, 12, 568. (k) Bertani, R.; Cavinato, G.;
Toniolo, L.; Vasapollo, G. J. Mol. Catal. 1993, 84, 165. (l) Cavinato, G.; Toniolo,
L. J. Organomet. Chem. 1990, 398, 187. (m) Sacco, A.; Vasapollo, G.; Nobile,
C. F.; Piergiovanni, A.; Pellinghelli, M. A.; Lanfranchi, M. J. Organomet. Chem.
1988, 356, 397. (n) Vasapollo, G.; Nobile, C. F.; Sacco, A. J. Organomet. Chem.
1985, 296, 435. (o) Rivetti, F.; Romano, U. Chim. Ind. 1980, 62, 7. (p) Rivetti, F.;
Romano, U. J. Organomet. Chem. 1978, 154, 323. (q) Stille, J. K.; Wong, P. K.
J. Org. Chem. 1975, 40, 532. (r) Hidai, M.; Kokura, M.; Uchida, Y.
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T.; Naruto, M.; Ataka, K. J. Am. Chem. Soc. 1973, 95, 3180. (t) van Werner, K.;
Beck, W. Chem. Ber. 1972, 105, 3947. (u) Beck, W.; van Werner, K. Chem. Ber.
1971, 104, 2901.
Preparation of the Complexes. Synthesis of cis-[Pd(ONO2)2-
(PPh3)2] (1). Several procedures for preparing this complex have
already been reported in the literature.14-16
We followed a more direct procedure. To a solution of
0.4 mmol of Pd(AcO)2 in 4 mL of acetone was added at room
temperature under stirring 0.91 mmol of PPh3. A yellow pre-
cipitate formed in a few seconds. To this suspension was added
dropwise at room temperature 160 mg of 65% HNO3. The
mixture was stirred for 30 min, after which the yellow precipitate
was filtered off, washed with ether, and dried under vacuum.
Yield: 93%. Anal. Calcd for C36H30N2O6P2Pd: C, 57.27; H,
4.01; N, 3.71. Found: C, 57.65; H, 3.92; N, 3.87. IR data are
reported in ref 14. 1H NMR in CD2Cl2 (δ): 7.69-7.63 (c m, 30H,
PPh3). 31P{1H} NMR in CD2Cl2 (δ): 33.61 (s).
Synthesis of trans-[Pd(COOMe)(ONO2)(PPh3)2] (1a). A
total of 5 mL of MeOH containing 0.1 mmol of 1, together with
0.1 mmol of PPh3 and 0.12 mmol of pyridine, was added to a
glass bottle introduced into a 60 mL autoclave. After washing
with CO at room temperature, the autoclave was heated at 50 °C
for 2 h under 5 atm of CO. After filtration, 20 mL of diethyl ether
was added under stirring. The suspension that formed after a
few minutes was allowed to stir for about 30 min, after which the
white precipitate was filtered off, washed with ether, and dried
under vacuum. Yield: 77%. Anal. Calcd for C38H33NO5P2Pd:
C, 60.69; H, 4.42; N, 1.86. Found: C, 60.21; H, 4.70; N, 1.78. IR
(6) (a) Nishihara, Y.; Miyasaka, M.; Inoue, Y.; Yamaguchi, T.; Kojima,
M.; Takagi, K. Orgamometallics 2007, 26, 4054. (b) Nishihara, Y.; Miyasaka,
M.; Inoue, Y.; Itazaki, M.; Takagi, K. Org. Lett. 2005, 7, 2639. (c) Hua, R.;
Takeda, H.; Onozawa, S.; Abe, Y.; Tanaka, M. J. Am. Chem. Soc. 2001, 123,
2899. (d) Keim, W.; Becker, J.; Trzeciak, A. M. J. Organomet. Chem. 1989, 372,
447. (e) Otsuka, S.; Nakamura, A.; Yoshida, T.; Naruto, M.; Ataka, K. J. Am.
Chem. Soc. 1973, 95, 3180. (f ) Fitton, P.; Johnson, M. P.; McKeon, J. E. J. Chem.
Soc., Chem. Commun. 1968, 6.
(11) Jenkims, J. M.; Verkade, J. C. Inorg. Synth. 1968, 11, 108.
(12) Stephenson, T. A.; Morehouse, S. M.; Powell, A. R.; Heffer, J. P.;
Wilkinson, G. J. Chem. Soc. 1965, 3632.
(7) (a) Dobrzynsky, E. D.; Angelici, R. J. Inorg. Chem. 1975, 14, 59. (b)
Fayos, J. E.; Dobrzynsky, D.; Angelici, R. J.; Clardy, J. J. Organomet. Chem.
1973, 59, C33.
(13) Cavinato, G.; Vavasori, A.; Toniolo, L.; Dolmella, A. Inorg. Chim.
Acta 2004, 357, 2737.
(14) Jones, C. J.; McCleverty, J. A.; Rothin, A. S.; Adams, H.; Bailey,
M. A. J. Chem. Soc., Dalton Trans. 1986, 2055.
(8) Milstein, D. J. Chem. Soc., Chem. Commun. 1986, 817.
(9) Graziani, M.; Uguagliati, P.; Carturan, G. J. Organomet. Chem. 1971,
27, 275.
(10) Rivetti, F.; Romano, U. J. Organomet. Chem. 1979, 174, 221.
(15) (a) Levison, J. J.; Robinson, S. D. Inorg. Nucl. Chem. Lett. 1968, 4,
407. (b) Levison, J. J.; Robinson, S. D. J. Chem. Soc. A 1971, 762.
(16) Coulson, D. R. Inorg. Synth. 1972, 13, 121.

Article
Inorganic Chemistry, Vol. 49, No. 8, 2010 3723
(Nujol): 1670 s (νCO), 1655 w (νCO), 1465 s (in KBr, νasym NO ),
the corresponding dinitro complex in MeOH, as reported for 1a,
gave unsatisfactory results.
2
1385 s (in KBr), 1284 s (νsym NO ), 1076 br (νCOC), 1018 w (νNO),
2
1
804 w (δONO) cm-1. H NMR in CD2Cl2 (δ): 7.65-7.42 (c m,
30H, PPh3), 2.58 (s, 3H, COOCH3). 31P{1H} NMR in CD2Cl2
(δ): 18.55 (s).
Complex 3a was prepared from a preformed palladium
carbomethoxy complex. To 0.14 mmol of trans-[Pd(COOMe)-
Cl(PPh3)2] dissolved in 3 mL of MeOH was added at room
temperature under stirring for 30 min 0.16 mmol of AgTsO.
After filtration, 0.4 mmol of NaNO2 in 10 mL of water was
added. The white precipitate was collected on a filter, washed
with water and diethyl ether, and dried under vacuum. Yield:
89%. Anal. Calcd for C38H33NO4P2Pd: C, 62.01; H, 4.52; N,
1.86. Found: C, 62.33; H, 4.69; N, 1.90. IR (Nujol): 1657 s (νCO),
In a different procedure, 0.1 mmol of trans-[Pd(COOMe)Cl-
(PPh3)2], suspended in 2 mL of MeOH, was treated with a slight
excess of AgNO3 in 4 mL of CH2Cl2 under stirring at room
temperature. The white precipitate that formed after a few
minutes was filtered off and washed with CH2Cl2. To the
solution were added under stirring 70 mL of diethyl ether/
petroleum ether (2:5). After a few minutes, a white precipitate
formed. The suspension was stirred for a further 30 min and
filtered, and the solid was washed with a mixture of the two
ethers and dried under vacuum. Yield: 92%. The product was
recrystallized from CH2Cl2/petroleum ether.
In another procedure, 0.25 mmol of LiNO3 dissolved in
10 mL of water was added to 0.1 mmol of trans-[Pd(COOMe)-
(OTs)(PPh3)2] dissolved in 2 mL of MeOH at ambient tempera-
ture under stirring. A white precipitate formed immediately. The
suspension was stirred for 30 min and then filtered off. The solid was
washed with diethyl ether and dried under vacuum. Yield: 93%.
Synthesis of trans-[Pd(COOMe)2(PPh3)2] (2). A procedure
for preparing this complex has been reported.5p Following the
procedure for the preparation of 1, but using NEt3 in place of
pyridine after 2 h under 5 atm of CO at room temperature, 2
precipitated in a mixture with 1a, [Pd(CO)(PPh3)3],17 and minor
amounts of unreacted 1.
1413 s (νasym NO ), 1331 s (νsym NO ), 1055 br (νCOC), 821 w
2
2 1
(δONO), 600-550 w (Fw ONO) cm-1. H NMR in CD2Cl2 (δ):
7.63-7.37 (c m, 30H, PPh3), 2.50 (s, 3H, COOCH3). 31P{1H}
NMR in CD2Cl2 (δ): 18.82 (s).
In a procedure different from that of 0.1 mmol of trans-
[Pd(COOMe)Cl(PPh3)2], dissolved in 4 mL of acetronitrile/
CH2Cl2 (1:3), 0.1 mmol of AgNO2 was added at room tempera-
ture under stirring. The suspension that formed was filtered.
After the white solid that precipitated upon the addition of
diethyl ether and petroleum ether was collected on a filter and
the usual workup, the yield was 82%.
Synthesis of cis-[Pd(C2O4)(PPh3)2] (4). This complex was
prepared by treating with oxalic acid the carbonato complex
[Pd(CO3)(PPh3)2],22 in turn prepared by bubbling a mixture of
oxygen and carbon dioxide through a benzene solution of
[Pd(PPh3)4].23
We followed a more direct procedure. A total of 0.1 mmol of
trans-[Pd(OAc)2(PPh3)2] in 50 mL of EtOH was treated with a
slight excess of oxalic acid. After ca. 10 min, an ivory solid
precipitated. The suspension was stirred for a further 2 h. After
50 mL of petroleum ether was added, the solid was collected on a
filter, washed with EtOH and petroleum ether, and dried. Yield:
66%. Anal. Calcd for C38H30O4P2Pd: C, 63.48; H, 4.21. Found:
C, 63.07; H, 3.95. 1H NMR in CD2Cl2 (δ): 7.49-7.28 (c m, 30H,
PPh3). 31P{1H} NMR in CD2Cl2 (δ): 33.94 (s).
Pure complex 2 has been synthesized by treating cis-[Pd-
(OTs)2(PPh3)2] (0.1 mmol), dissolved in 2 mL of MeOH, with
CO (2 atm) at 0 °C for 10 min under stirring, during which
time the solution turned from brownish to yellowish. NEt3
(0.8 mmol) was then added with stirring for a further 10 min.
The light-brown solid that formed was collected on a filter,
washed with MeOH and Et2O, and dried under vacuum. Yield:
87%. Anal. Calcd for C40H36O4P2Pd: C, 64.14; H, 4.84. Found:
C, 63.78; H, 4.94. IR (Nujol): 1631 s (νCO), 1014 br (νCOC) cm-1
.
X-ray Structure Determinations. Crystals of both complexes 1
and 1a, suitable for X-ray analysis, were obtained by slow
recrystallization from CH2Cl2/n-hexane solutions. Complex 1
crystallizes with a dichlorometane molecule and is indicated
1H NMR in CD2Cl2 (δ): 7.68-7.37 (c m, 30H, PPh3), 2.55
(s, 6H, COOCH3). 31P{1H} NMR in CD2Cl2 (δ): 21.76 (s).
Synthesis of [Pd(NO2)2(PPh3)2] (3). For this complex also,
several procedures have already been reported.15,18-21
hereafter as 1 CH2Cl2. The selected specimens of both com-
3
We followed a simpler procedure. A total of 0.1 mmol of
Pd(OAc)2 was stirred in 5 mL of acetonitrile at room tempera-
ture for 10 min, after which 0.22 mmol of AgNO2 was added and
stirred for a further 30 min. The white precipitate that formed
was filtered off. To the solution was added with stirring for
another 0.5 h 0.2 mmol of PPh3 dissolved in 10 mL of diethyl
ether. The resulting suspension was filtered. The white-ivory
solid was washed with ether and dried under vacuum. Yield:
89%. NaNO2 dissolved in water can be used in place of AgNO2.
Yield: 81%. Anal. Calcd for C36H30N2O4P2Pd: C, 59.80; H,
4.18; N, 3.87. Found: C, 60.21; H, 4.24; N 3.57. IR data are
reported in ref 15. 1H NMR in CD2Cl2 (δ): 7.69-7.47 (c m, 30H,
PPh3). 31P{1H} NMR in CD2Cl2 (δ): 16,41 (s).
pounds were lodged into Lindemann glass capillaries and
mounted on the goniometer head of a four-circle Philips
PW1100 diffractometer made available by colleagues of the
CMR-ICIS Institute of Padua, Padua, Italy. Raw diffraction
data were collected at room temperature with the ω-2θ techni-
que, using graphite-monochromated Mo KR radiation (λ =
˚
0.710 73 A). Intensities were corrected for Lorentz and polariza-
tion effects, as well as for absorption (ψ scans).24
Unit cell parameters were determined by the least-squares
refinement of 30 well-centered high-angle reflections. Three
standard reflections were checked every 150 measurements to
assess the crystal stability; no sign of decay was noticed. The
structures were solved with a combination of direct methods and
Fourier difference syntheses (SIR program)25 and subsequently
refined by standard full-matrix least squares based on Fo2 with
the SHELXL-9726 and SHELXTL-NT27 programs. In the last
cycles of refinement, all non-hydrogen atoms were allowed to
The dinitro complex can also be prepared by using 0.1 mmol
of PdCl2 dissolved in 0.2 mL of acetic acid in place of Pd(OAc)2,
following the procedure just reported. Yield: 72%.
Synthesis of trans-[Pd(COOMe)(NO2)(PPh3)2] (3a). Several
attempts to prepare this complex by the direct carbonylation of
(22) Blake, D. M.; Nyman, C. J. J. Am. Chem. Soc. 1970, 92, 5359.
(23) Nyman, C. J.; Wymore, C. E.; Wilkison, G. J. Chem. Soc. A 1968,
561.
(24) North, A. T. C.; Philips, D. C.; Mathews, F. S. Acta Crystallogr.,
Sect. A 1968, 24, 351.
(17) (a) Misono, A.; Uchida, Y.; Hidai, M.; Kudo, K. J. Organomet.
Chem. 1969, 20, P7. (b) Kudo, K.; Hidai, M.; Uchida, Y. J. Organomet. Chem.
1971, 33, 393.
(18) Feltham, R. D.; Elbaze, G.; Ortega, R.; Eck, C.; Dubrawsky, J.
Inorg. Chem. 1985, 24, 1503.
(19) Burmeister, J. L.; Timmer, R. C. J. Inorg. Nucl. Chem. 1966, 28, 1973.
(20) Malatesta, L.; Angoletta, M. J. Chem. Soc. 1957, 1186.
(21) Jain, K. C.; Pandey, K. K.; Agarwala, U. C. Z. Anorg. Allg. Chem.
1981, 472, 217.
(25) SIR-97: Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. J. Appl. Crystallogr. 1999, 32, 115.
(26) Sheldrick, G. M. SHELXL-97, Program for the Refinement of Crystal
€
€
Structures; University of Gottingen, Gottingen, Germany, 1997.
(27) SHELXTL/MT, version 5.10; Bruker AXS Inc.: Madison, WI, 1999.
Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, A46, 467.

3724 Inorganic Chemistry, Vol. 49, No. 8, 2010
Amadio et al.
Table 1. Crystallographic Data of Complexes 1a and 1 CH2Cl2
3
1a
1 CH2Cl2
3
empirical formula
fw
C38H33NO5P2Pd
751.99
C37H32N2O6P2Cl2Pd
839.89
cryst size (mm3)
cryst syst
Space group
0.24 ꢀ 0.20 ꢀ 0.15
0.30 ꢀ 0.25 ꢀ 0.18
monoclinic
P21 (No. 4)
monoclinic
P21/n (No. 14)
˚
a (A)
11.011(3)
19.446(3)
16.276(3)
91.85(3)
3483(1)
11.975(2)
20.127(3)
15.479(3)
95.06(3)
3716(1)
˚
b (A)
˚
c (A)
β (deg)
volume (A )
3
˚
Z
T (K)
4
294
1.434
6.68
1536
4
294
1.501
7.77
1704
calcd density (Mg m-3
)
abs coeff (cm-1
F(000)
)
θ range (deg)
3.04-26.00
6803/0.0377
6306
3.31-25.00
6487/0.0179
5786
indep (unique) reflns/Rint
obsd reflns [I > 2σ(I)]
data/param
R indices [I > 2σ(I)]
R indices (all data)
GOFa on F2
6803/849
6487/451
R1b = 0.0447; wR2c = 0.0980
R1b = 0.0522; wR2c = 0.1050
1.190
R1b = 0.0523; wR2c = 0.1194
R1b = 0.0635; wR2c = 0.1278
1.245
-3
˚
largest difference peak and hole (e A
)
0.697 and -0.690
0.675 and -0.792
P
P
P
P
P
a GOF = [ (w(Fo2 - Fc2)2]/(Nobsns - Nparams)]1/2, based on all data. b R1 = (|Fo| - |Fc|)/ |Fo|. c wR2 = [ [w(Fo2 - Fc2)2]/ [w(Fo2)2]]1/2
.
vibrate anisotropically. Hydrogen atoms were introduced in
calculated positions in their described geometries and refined
as a “riding model”, with fixed isotropic thermal parameters set
at 1.2 times Uequiv of the appropriate carrier atom.
exchange a carbomethoxy ligand and a nitrate ligand,
giving 1a:
cis-½PdðONO2Þ2L2ꢁ þ trans-½PdðCOOMeÞ2L2ꢁ
For the structure of 1a, the Flack parameter28 has also been
refined. The graphical representation of the two complexes have
been obtained through the ORTEP module of the WinGX
software.29 A summary of the main crystallographic data is
shown in Table 1 (see further for the selected bond lengths and
angles). Full listings of the results of the crystallographic
1
2
f 2trans-½PdðCOOMeÞðONO2ÞL2ꢁ
ð2Þ
1a
The reaction has been followed by NMR, by dissolving
equimolar amounts of 1 and 2 in CD2Cl2 at -78 °C under
argon (3 atm). The two 31P{1H} singlets at 33.73 and
21.40 ppm for 1 and 2, respectively, that are still present at
0 °C disappear at 10 °C, to be replaced by one singlet at
18.53 ppm for complex 1a. The transmetalation is irre-
versible (because upon cooling only this last signal
remains present) and occurs with complete retention of
the integrity of the carbomethoxy ligand, as proven by
the 1H NMR spectrum showing that the H(PPh3)/
H(COOMe) ratio is 60:6 before and after transmetala-
tion. The transfer of an aryl or alkyl30-33 group between
palladium atoms is well-known, whereas to the best of our
knowledge, the transfer of a carbomethoxy group from
one palladium center to another has not been reported to
date.
investigations for 1a and 1 CH2Cl2 are available as Supporting
Information.
3
Results and Discussion
1. Synthesis, Characterization, and Reactivity of Com-
plexes [Pd(COOMe)nX2-n(PPh3)2] [X = (ONO2), (NO2);
n = 1, 2]. 1, suspended in MeOH in the presence of
pyridine, reacts with CO under mild conditions, yielding
1a. The formation of the complex occurs through inter-
action of CO with MeOH on the metal center with the
release of one proton. Pyridine is necessary in order to
neutralize the proton that would otherwise reverse the
reaction. As a matter of fact, 1a, suspended in MeOH
containing HNO3, gives back 1.
It was reported that the dinitrato complex 1 is reduced
by CO in CH2Cl2 under ambient conditions, in the
absence of any base, affording reasonably good yields
(49%) of the corresponding dinitro complex 3 after just 1
h of reaction.14 Thus, substitution of a NO3 ligand of
complex 1 with a COOMe one causes stabilization of the
other NO3 ligand toward reduction. That in the present
case the product of reaction (1) is not the corresponding
cis-½PdðONO2Þ2L2ꢁ þ CO þ MeOH
1
py
s
-
trans-½PdðCOOMeÞðONO ÞL ꢁ þ pyHþNO
f
2
2
3
1a
ð1Þ
When NEt3 is used in place of pyridine, the solid that is
recovered after the reaction is a mixture of 1a, the dicarbo-
methoxy complex 2, [Pd(CO)(PPh3)]3,17 and unreacted 1.
Complex 2 does not form via disproportionation of 1a
to 1 and 2, but rather the opposite occurs; i.e., 1 and 2
ꢁ
(30) Albenitz, A. C.; Espinet, P.; Lopez-Cimas, O.; Martin-Ruiz, B.
Chem.;Eur. J. 2005, 11, 242.
(31) Ozawa, F.; Ito, T.; Nakamura, Y.; Yamamoto, A. Bull. Chem. Soc.
Jpn. 1981, 54, 1868.
(28) (a) Bernardinelli, G.; Flack, H. D. Acta Crystallogr., Sect. A 1985, 41,
500. (b) Flack, H. D. Acta Crystallogr., Sect. A 1983, 39, 876.
(29) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
(32) Grushin, V. V.; Marshall, W. J. J. Am. Chem. Soc. 2006, 128, 4632.
(33) Ozawa, F.; Fujimori, M.; Yamamoto, T.; Yamamoto, A. Organo-
metallics 1986, 5, 2144.

Article
Inorganic Chemistry, Vol. 49, No. 8, 2010 3725
reduced nitro complex 3a (or a nitrite complex trans-[Pd-
(COOMe)(ONO)(PPh3)2]) has been unambiguously pro-
Scheme 1. Formation of DMC and DMO from 2a
1
ven by the fact that (i) the IR, H MMR, and 31P{1H}
NMR spectra of complex 1a are identical with those of
the carbomethoxy complex prepared by metathetical
exchange of trans-[Pd(COOMe)Cl(PPh3)2] with AgNO3
or by reaction of the cationic carbomethoxy complex
trans-[Pd(COOMe)(OTs)(PPh3)2] with LiNO3 and (ii)
they differ from the spectra of the carbomethoxy-nitro
complex 3a prepared from trans-[Pd(COOMe)Cl(PPh3)2]
and AgNO2 or from trans-[Pd(COOMe)(OTs)(PPh3)2
and NaNO2.
a For simplicity, the PPh3 ligands are omitted.
(νsym NO ), and 821 (δΟΝΟ) cm-1; in the NO2 wagging
region (Fw2 600-550 cm-1), there are several bands of weak
intensity that prevent an unambiguous assignment. Both
νasym NO and νsym NO have been raised with respect to the
Compared to the IR of trans-[Pd(COOMe)Cl-
(PPh3)2],5k in complex 1a, new absorptions are present,
consistent with an O-coordinated monodentate nitrate [in
the stretching region at 1465 (νasym NO ), 1284 (νsym NO ),
2
2
free ion values(1328 and 1261cm-1).36 Moreover, the band
at 1331 cm-1 is characteristic of the nitrogen-bonded nitro
group.37 This leaves little doubt that the NO2 ligand is
coordinated through the nitrogen atom.
2
2
and 1018 (νNO) cm-1 and in the deformation region at
803 cm-1 (δONO)]14,34 (see also the X-ray structure of 1a).
In addition, there is a band of strong intensity at
1385 cm-1, characteristic of ionic nitrate.35 This band is
present in the spectra taken both in KBr and in a Nujol
mull. Therefore, it cannot be due to the nitrate ligand
displaced, even partially, from palladium(II) by Br- of
KBr in the solid.
The formation of 2 occurs even at -78 °C and has been
followed by 31P{1H} NMR in CD2Cl2 (see the Supporting
Information). Upon admission of CO, the signal at
37.91 ppm of cis-[Pd(OTs)2(PPh3)2] is replaced by several
signals, one of stronger intensity at 23.01 ppm, whose
value suggests that it is relevant to a species having trans
geometry (see later). Upon the addition of MeOH (10%
with respect of CD2Cl2), these signals are immediately
substituted by a signal at 19.04 ppm, related to trans-
[Pd(OTs)(COOMe)(PPh3)2].5a Only upon the addition
of NEt3 (Pd:N = 1:10) is there formation of 2 (the
signal at 20.91 ppm). The complex is stable up to room
temperature.
Complex 2 is formed at -78 °C starting also from 1 or
1a. In this case, 2 is stable up to 0 °C; however, at room
temperature, one displaced nitrato ion reenters the co-
ordination sphere of palladium(II), giving 1a, at variance
with what was observed when starting from cis-[Pd-
(OTs)2(PPh3)2], probably because of the higher coordi-
nating capacity of NO3- compared to that TsO-.
The stability of preformed 2 has also been studied in an
NMR tube in CD2Cl2. In the absence of CO, 2 is stable at
room temperature, but at 40 °C, it decomposes with the
formation of DMC (ca. 10%), suggesting that decarbo-
nylation of one carbomethoxy ligand occurs with the
formation of a (methoxy)(carbomethoxy)palladium(II)
species, leading to DMC. Under 4 atm of CO at 40 °C,
decomposition occurs with the formation of DMC and
DMO, ca. 12% for both. In Scheme 1, the products are
formed via an intramolecular interaction in a species
having the favorable cis geometry.38
The νCO region of 1a is characterized by a band of
strong intensity at 1670 cm-1 and a band of weak
intensity at 1655 cm-1. At 1076 cm-1, a rather broad
band of strong intensity can be attributed to νCOC. Also,
the complex trans-[Pd(COOMe)Cl(PPh3)2] shows two
absorptions in the νCO region, due to the possible
presence of conformational isomers with cis and trans
geometry.5k
Another explanation may be the following: In the
X-ray crystal structure (see the structural characterization
below for a detailed description), two independent, simi-
lar, but not identical, molecules are present in the asym-
metric unit. The two molecules differ for the reciprocal
orientation of the nitrate and COOMe ligands, as well as
for the relative position of the COOMe moiety with
respect to the PPh3 ligands, and appear to be stabilized
by a network of nonbonding contacts. If such arrange-
ments coexist in the conditions under which the IR
spectrum is taken, they might be the cause of the appear-
ance of the two bands in the νCO region.
If two conformational isomers are present in the solid
state, they cannot be distinguished by NMR because the
1H and 31P{1H} NMR spectra in CD2Cl2 show only one
sharp singlet even at -78 °C, at 19.12 and 2.52 ppm,
respectively (18.55 and 2.58 ppm at room temperature).
The IR spectrum of the analogous nitro-carbomethoxy
complex 3a presents bands of strong intensity at 1657 and
1055 cm-1 for νCO and νCOC, respectively. The frequencies
Decarbonylation probably occurs through the avail-
able fifth coordination site of palladium(II) or via predis-
sociation of a PPh3 ligand, which makes another
coordination site available. As a matter of fact, when
the experiment is repeated in the presence of added PPh3
(2 equiv) and in the absence of CO, decarbonylation
occurs with the formation of only DMC (ca. 15%), but
at 60 °C, probably because of the leaving CO competing
(36) Weston, R. E.; Brodasky, T. F. J. Chem. Phys. 1957, 27, 683.
(37) Nakamoto, K., Infrared and Ramam Spectra of Inorganic and
Coordination Compounds, 4th ed.; Wiley: New York, 1986; p 221.
(38) van Leeuwen, P. W. N. M.; Zuideveld, M. A.; Swennenhuis, B. H. G.;
Freixa, Z.; Kamer, P. C. J.; Goubitz, K.; Fraanje, J.; Lutz, M.; Spek, A. L.
J. Am. Chem. Soc. 2003, 125, 5523.
related to the NO2 ligand appear at 1413 (νasym NO ), 1331
2
(34) Rosenthal, M. R. J. Chem. Educ. 1973, 50, 331.
(35) Gatehouse, B. M.; Livingstone, S. E.; Nyholm, R. S. J. Chem. Soc.
1957, 4222.

3726 Inorganic Chemistry, Vol. 49, No. 8, 2010
Amadio et al.
with PPh3 for the fifth coordination site or because the
predissociation equilibrium is less favorable.
Table 2. Oxidative Carbonylation of MeOH
entry
catalyst precursor
TOFDMO, h-1
A comparison of νCO of 1a and 3a gives some insight
into the nature of the trans-[Pd(COOMe)(NO3)] and
trans-[Pd(COOMe)(NO2)] moieties. The nitrate ligand
is coordinated through a Pd-O σ bond. The nitro ligand
is coordinated through a nitrogen atom, the Pd-N bond
has σ and π character, and it is known to exert a
moderately strong trans effect.39 The σ-donating effect
should decrease the νCO stretching frequency, whereas the
π-back-donation effect should increase this frequency.
The fact that νCO of 1a is higher than that of 3a suggests
that the σ effect prevails over the π effect.
A comparison of νCO of 3a and 2 is also interesting. In
the latter complex, νCO is significantly lower. This fact
suggests that (i) the COOMe ligand displays a σ effect
stronger than that of the NO2 ligand and (ii) with the
COOMe ligand also the σ effect prevails over the π effect.
A comparison of the NO2 IR frequencies of 3 and 3a
gives some further insight. If 3 has a trans geometry, the
fact that the frequencies differ little suggests that the
COOMe and NO2 ligands exert a similar effect on the
NO2 ligand in the trans position.
As to the geometry of the complexes, we have observed
that those having trans or cis geometry, as was unam-
biguously established such as, for example, by X-ray
diffraction or by IR studies, show the 31P{1H} NMR
signal in the range 18-25 or 30-38 ppm, respectively
(compare all of the complexes reported here and others;
see, for example, ref 40). Moreover, the 31P MMR
spectrum of the oxalate complex [Pd(C2O4)(PPh3)2)],
having cis geometry both in the solid and in solution,
shows one singlet at 33.94 ppm. On the basis of these
1
2
3
4
5
6
7
8
1a
38
25
19
12
21
34
15
33
11
44
20
38
37
19
1aa
2a
3a
3aa
trans-[PdCl2(PPh3)2]a
trans-[PdCl(COOMe)(PPh3)2]a
trans-[Pd(OAc)2(PPh3)2]a
trans-[Pd(COOMe)(OAc)(PPh3)2]a
cis-[Pd(OTs)2(PPh3)2]a
trans-[Pd(COOMe)(OTs)(PPh3)2]a
[Pd(CO)(PPh3)3]b
9
10
11
12
13
14
[Pd(CO)(PPh3)3]b
[Pd(CO)(PPh3)3]c
a Run conditions: 10-2 mmol of Pd, Pd/PPh3/NEt3/BQ = 1:3:2:100,
5 mL of anhydrous MeOH, 65 atm of CO, 65 °C, 1 h. b Run conditions:
Same as those in footnote a, but without added PPh3 and with
HNEt3þX- (X = Cl, AcO) in place of NEt3. c Run conditions: Same
as those in footnote a, but without added PPh3.
aromatic compounds promoted by Pd(OAc)2 in
CF3COOH as the solvent, occurring with reduction to
palladium metal, can be turned catalytic by the use of
41
NaNO2,3
.
We tested the reactivity of 1, 1a, 3, and 3a with MeOH
under CO in order to check whether (i) these complexes
will give DMC/DMO according to reaction (3) and (ii)
the NO2,3- ligands are able to reoxidize palladium(0) to
palladium(II), so that more than 1 mol of DMC/DMO
per palladium atom will form. We chose the con-
ditions under which the particularly stable trans-[Pd-
(COOMe)Cl(PPh3)2] gives DMC/DMO and palladium-
(0).5i Upon heating of these complexes up to 90 °C in 2 mL
of MeOH in the presence of PPh3 and NEt3 (Pd:P:N =
1:4:10), under 40 atm of CO for 1 h, there occurred the
formation of [Pd(CO)(PPh3)3] (ca. 70%), OPPh3 (OPPh3:
Pd = 0.5:1), and only trace amounts of the expected
DMC/DMO, even with 1a and 3a, which have a pre-
formed carbomethoxy ligand. The same results were
obtained in the presence of added NaNO2,3 (Pd:salt =
1:100). These attempts were repeated by warming of the
autoclave up to 90 °C at a low rate, with the hope of
increasing the possibility of spending enough time in the
range of temperatures more favorable to the formation
of DMO/DMC, albeit unsuccessfully. The addition of
20 equiv of CH3COOH to the system 1/PPh3/NaNO2,3
(1:1:100), under 60-70 atm of CO at 65 °C, led to
decomposition to palladium metal without the formation
of DMC or DMO.
In contrast, when 1, 1a, 2, 3, or 3a was used with BQ,
selective catalysis to DMO was observed, with no other
carbonylation product being formed in detectable
amounts. The reaction conditions (Table 2) were those
experienced by Current except that in his case the con-
centration of the catalyst precursor was 12 times higher
and NEt3 was not used.3d The results obtained with other
catalyst precursors are also reported. The comparison of
their activity is not so straightforward because at the end
of catalysis some decomposition to palladium metal
occurred and the consumption of BQ was higher than
that expected for the amount of DMO formed. Current
observations, we propose that 1a, 1 CH2Cl2, and 2,
3
having trans, cis, and trans geometry in the solid state,
respectively, maintain the same geometry also in solution.
2. Catalytic Properties of Pd(COOMe)nX2-n(PPh3)2]
(X = ONO2, NO2, Cl, OAc and OTs; n = 0, 1, 2). It was
reported that trans-[Pd(OAc)2(PPh3)2] in MeOH under
20-50 atm of CO at room temperature gives trans-[Pd-
(COOMe)(OAc)(PPh3)2], whereas at 50-80 °C, the for-
mation of DMO, together with minor amounts of DMC,
with concomitant reduction to palladium(0) complexes
occurs.5o,p It was also reported that 2 at 50 °C under a CO
atmosphere in MeOH decomposes, yielding oxalate and
palladium(0) complexes.5p The formation of these pro-
ducts can be schematized as follows:
CO, MeOH
½PdðOAcÞ2L2ꢁ
f
½PdðCOOMeÞnðOAcÞ2 - nL2ꢁ
- AcOH
f DMO=DMC þ Pd0n ¼ 1, 2
ð3Þ
It has also been found that a Pd(OAc)2/PPh3 system
(Pd:P = 1:3) can be made catalytic to DMO/DMC by the
use of an oxidant, such as BQ, capable of reoxidizing
palladium(0) to palladium(II), and that CH3COOH has a
beneficial effect of the catalytic activity.3d In principle, the
NO2,3- ligands of 1, 1a, 3, and 3a also can act as oxidants.
As a matter of fact, the stoichiometric carboxylation of
(39) Basolo, F.; Pearson, R. G. Mechanisms of Inorganic Reactions, 2nd
ed.; Wiley: New York, 1967; p 351.
(40) Garrou, Ph. E.; Heck, R. F. J. Am. Chem. Soc. 1976, 98, 4115.
(41) Ugo, R.; Chiesa, A. J. Chem. Soc., Perkin Trans. 1987, 2625.

Article
Inorganic Chemistry, Vol. 49, No. 8, 2010 3727
Figure 1. ORTEP representation of Mol 1 in complex 1a (left) and of complex 1 CH2Cl2 (right), with selected numbering schemes. Ellipsoids are at the
3
50% probability level; hydrogen atoms and the dichloromethane solvation molecule of 1 CH2Cl2 have been omitted for clarity.
3
reported that an insoluble solid was produced by BQ
polymerization promoted by a PPh3-BQ adduct that
formed in situ. He stated that “DMO yields could be
variable, depending on the extent of these unrelated side
reactions”.3d Another BQ-consuming reaction could be
the oxidation of MeOH to formaldehyde, which, in turn,
can give rise to many derivatives (methyl hemiformalde-
hyde and diformaldehyde, formaldehyde oligomers,
methyl formate, 4-hydroxyphenyl formate, pitches).42
However, after the reaction, formaldehyde was present
only in trace amounts, practically as much as when
MeOH was used as the solvent, and no methyl formate
nor trioxane was detected. We did not investigate these
aspects any further because our main purpose was to
establish whether NO2,3- could act as stoichiometric oxi-
dants. The results in Table 2 are only valid mainly as an
indication that (i) 1, 1a, 3, and 3a can be effective catalyst
precursors when used with BQ and (ii) when using them
either as such or in the presence NaNO2,3, catalysis does
not occur because NO2,3- fail to act as reoxidants.
It is interesting to observe that DMO forms selectively
also when a preformed monocarbomethoxy precursor is
used. This suggests that CO and MeOH interact with a
monocarbomethoxy species to form a dicarbomethoxy
one, which gives DMO, before any interaction of MeOH
with any carbomethoxy species, which would give DMC.
It suggests also that under relatively high pressure of CO
the dicarbomethoxy intermediate does not undergo dec-
arbonylation of one carbomethoxy ligand, which would
lead to the formation of DMC, as shown in Scheme 1.
It is worth pointing out that also trans-[Pd-
(COOMe)Cl(PPh3)2] gives good results even though this
complex is stable in the absence of BQ. As a matter of fact,
it can be prepared in high yield by carbonylation of
trans-[PdCl2(PPh3)2] in MeOH in the presence of NEt3
even at 70 °C.5k Therefore, BQ can also change the
properties of the reaction center and, simultaneously,
the mechanism and direction of the reaction.
large amount of 1 or 2 (0.1 mmol) with just 10 mmol of
BQ, enough to ensure catalysis and, hopefully, not en-
ough to cause coprecipitation of the solid mentioned by
Current. After catalysis, the solid recovered by filtration
was recognized by IR as a mixture of [Pd(CO)(PPh3)3]
and [Pd(CO)(PPh3)]3.17b The 31P{1H} NMR spectrum
of the filtered solution showed signals at 33.69 and
22.97 ppm assignable to [Pd(BQ)(PPh3)2]2 H2BQ and
3
the PPh3-BQ adduct, respectively.5b [Pd(CO)(PPh3)3]
was reused as the precursor. In two experiments (entries
12 and 13), it was used in combination with 2 equiv of
HNEt3þX- (X = Cl, AcO) in place of 2 equiv of NEt3 and
in the absence of 1 equiv of added PPh3, which is related to
the conditions of experiments 6 and 8 of Table 2. In
another experiment (entry 14), the precursor was used in
combination with NEt3, again in the absence of 1 equiv of
added PPh3, which is related to the conditions of experi-
ment 2. Again, selective catalysis to DMO was observed,
with TOFs close to those of experiments 6, 8, and 2. These
results, together with those of the attempted catalysis
using NaNO2,3, confirm that in the latter case catalysis
does occur because these salts fail to reoxidize the
palladium(0) species.
Experiments 3 and 14 prove that catalysis occurs even
in the absence of any counteranion such as NO3,2-, Cl-,
AcO-, or TsO-. Therefore, these anions do not play a
crucial role. A detailed investigation on elementary reac-
tions relating to a putative catalytic cycle will be the
subject of forthcoming studies.
3. Structural Characterization of 1a. A list of selected
bond distances and angles for structurally characterized
complexes is reported in Table 3. In 1a, the palladium
atom exhibits a nearly regular square-planar coordina-
tion and the reciprocal orientation of the two PPh3
ligands is almost exactly staggered (Figure 1). The com-
pound crystallizes with two independent molecules in the
asymmetric unit (henceforth, Mol 1 and Mol 2), differing
in the orientation of the nitrate and COOMe ligands. The
arrangement is such that the nitrogen atom and the ester
oxygen atom in one molecule are at opposite sides
with respect to the coordination/basal plane, and their
positions are reversed in the other molecule.
In order to make the recovery of the precursor after
catalysis easier, we carried out a reaction using a relatively
(42) Pearson, D. M.; Waymouth, R. M. Organometallics 2009, 28, 3896.

3728 Inorganic Chemistry, Vol. 49, No. 8, 2010
Amadio et al.
˚
A fitting of eight selected atoms of Mol 1 and Mol
Table 3. Selected Bond Lengths (A) and Angles (deg) for Complexes 1a (Mol 1
2 made with the Mercury software43 gives a root-mean-
and Mol 2) and 1 CH2Cl2
3
˚
square (rms) value of 0.08 A. An examination of the two
1a
overlapped molecules looking down the P1-Pd-P2 axis
reveals that the mean planes Pd, P1, P2, O1, and C37 in
Mol 1 and Mol 2 make an angle of about 35° with each
other, so that the Pd-C37 bond nearly overlaps the
P2-C19 link in Mol 1 and the P1-C13 one in Mol 2.
Among square-planar palladium complexes, -COOR
ligands are scarce. In the CCDC database,44 we found
about 20 structural reports,45 mostly trans-bis-
(phosphine) complexes. In this set, the average Pd-C
Mol 1
Mol 2
1 CH2Cl2
3
Pd-P1
Pd-P2
Pd-O1
Pd-C37
Pd-O4
2.349(3)
2.364(3)
2.150(6)
1.948(8)
2.340(3)
2.367(3)
2.149(6)
1.927(8)
2.272(1)
2.255(1)
2.088(3)
2.111(4)
P-C
1.829(8)
1.80(1)
1.832(8)
1.836(8)
1.804(8)
1.83(1)
1.822(8)
1.819(9)
1.846(8)
1.844(8)
1.812(9)
1.83(1)
1.817(5)
1.826(5)
1.833(5)
1.818(5)
1.831(5)
1.817(5)
˚
distance and Pd-C-O and Pd-C-O angles are 1.985 A,
˚
126.3°, and 112.4°, compared with 1.948(8)/1.927(8) A,
126.4(7)/129(1)°, and 111.9(7)/111(1)° in Mol 1 and Mol
2, respectively.
Compounds most closely resembling 1a are Pd-
P1-Pd-P2
O1-Pd-P1
O1-Pd-P2
O1-Pd-C37
P1-Pd-C37
P2-Pd-C37
O1-Pd-O4
O4-Pd-P1
O4-Pd-P2
176.8(1)
94.6(2)
88.6(2)
175.8(3)
86.8(2)
90.0(2)
176.0(1)
88.1(2)
96.0(2)
177.7(5)
85.9(3)
90.1(3)
96.2(1)
86.6(1)
175.2(1)
˚
(CO2CH3)(OCOCH3)(PPh3)2 (1.983 A, 127.4°, and
110.7°),45d PdCl(CO2Ph)(PPh3)2 (1.961 A, 129.6°, and
˚
109.6°),45b Pd(CO2CH3)(OCOCH3)(Ph2Ppy)2 (py = pyr-
5e
˚
idine; 1.967 A, 127.9°, and 110.5°), and PdCl[(CO2C2-
86.2(1)
172.8(1)
91.0(1)
H4CH2(OH)](PN) [PN = 2-(β-diphenylphosphine)ethyl-
pyridine-N,P; 1.964 A, 124.5°, and 112.0°].5c In the same
˚
˚
set, the shortest Pd-C distance is 1.909 A in PdCl-
[(CO2CH2CH(OH)C2H5)](PN);5c however, in this
at opposite sides with respect to the basal plane. The PPh3
ligands are in a synclinal staggered conformation, and the
C1 and C19 phenyl rings are engaged in a loose π-π
complex, the average Pd-C length (two independent
molecules in the unit cell) is 1.964 A. Not considering
˚
˚
PdCl[(CO2CH2CH(OH)C2H5)](PN), the Pd-C distances
found in 1a are, to the best of our knowledge, the shortest
Pd-C distances so far reported in PdCOOR complexes.
The Pd-O(N) values in 1a are close to those of com-
plexes where the η1-nitrate is trans to a σ-bonded carbon-
donor atom, like in [(S)-9-[1-(dimethylamino)ethyl]-10-
phenanthrenyl-C,N]-(7R-2,3-dimethyl-6-dimethylcarba-
moyl-7-phenyl-7-phosphabicyclo[2.2.1]hept-2-en-7-yl)-
stacking (centroid-to-centroid distance of 3.685 A).
Only a few47 nonchelating bis(triphenylphosphine)-
palladium complexes are reported showing the cis dis-
position, either because of favorable nonbonding inter-
actions or because of the lack of steric repulsions. The
latter seems to be the case for 1 CH2Cl2.
Among known bis(nitrato)phosphine square-planar
3
palladium complexes,14,47 the average Pd-O and O-N
46a
˚
˚
˚
(nitrato-O)palladium(II) (2.150, 2.153, and 2.160 A),
Pd(ONO2)(DMPP)(TMBA) [DMPP = 3,4-dimethyl-1-
distances and Pd-O-N angles are 2.092 A, 1.297 A, and
˚
115.2°, compared with 2.088(3)/2.111(4) A, 1.304(5)/
˚
phenylphosphole; TMBA
R-methylbenzylamine; 2.159 A],
(dimethylamino)ethyl]-3,6-dimethylphenyl](2,3-dimethyl-7-
=
˚
(S)-(þ)-N,N-dimethyl-
1.302(7) A, and 115.2(3)/111.8(4)° for N1 and N2 ni-
trates, respectively. Compounds in the overall better
46b
and (SC,RP)-[2-[1-
resembling 1 CH2Cl2 are Pd(ONO2)2{[CH2[(C6H5)2P]2}2
˚
3
48a
˚
phenyl-6-(propionyl)-7-phosphabornene)(nitrato-O)palla-
(2.103/2.114 A, 1.292/1.289 A, and 114.4/111.2°) and
Pd(ONO2)2(DMPP)2 (2.102/2.124 A, 1.293/1.290 A, and
dium(II) (2.148 A).46c The contacts for which the distance
˚
˚
˚
115.6/113.0°).48b
between interacting atoms is at least 0.1 shorter than the
sum of the pertinent vdW radii are available as Supporting
Information.
The Pd-O distances in 1 CH2Cl2 are the shortest
reported so far in bis(nitrato)bis(phosphine) complexes,
after those of trans-Pd(ONO2)2(OPPh3)(PPh3) (2.030/
3
4. Structural Characterization of 1 CH2Cl2. In
3
14
˚
1 CH2Cl2, the palladium environment is almost regular
2.031 A). A comparison of the Pd-ONO2 bond dis-
3
square-planar, with the two cis-disposed η1-nitrate ligands
tances in 1a and 1 CH2Cl2 indicates that the -COOMe
3
ligand has a stronger trans influence than the PPh3 one.
Longato et al.49 recently reported the structure of the
platinum analogue of 1 CH2Cl2. The platinum derivative
(43) Mercury CSD 2.0: Macrae, C. F.; Bruno, I. J.; Chisholm, J. A.;
Edgington, P. R.; McCabe, P.; Pidcock, E.; Rodriguez-Monge, L.; Taylor,
R.; van de Streek, J.; Wood, P. A. J. Appl. Crystallogr. 2008, 41, 466.
(44) Cambridge Structural Database (version 5.30 of November 2008 þ 2
updates): Allen, F. H. Acta Crystallogr., Sect. B 2002, 58, 380.
(45) (a) Khabibulin, V. R.; Kulik, A. V.; Oshanina, I. V.; Bruk, L. G.;
Temkin, O. M.; Mosova, V. M.; Ustynyuk, Yu. A.; Belsky, V. K.; Stash, A.
I.; Lyssenko, K. A.; Antipin, M. Yu. Kinet. Catal. (Russ.) 2007, 48, 244. (b)
Yasuda, H.; Maki, N.; Choi, J.-C.; Sakakura, T. J. Organomet. Chem. 2003, 682,
66. (c) Gallo, E.; Ragaini, F.; Cenimi, S.; Demartin, F. J. Organomet. Chem.
1999, 586, 190. (d) Del Piero, G.; Cesari, M. Acta Crystallogr., Sect. B 1979, 35,
2411. (e) Zhir-Lebed, L. N.; Kuz'mina, L. G.; Struchkov, Yu. T.; Temkim, O. M.;
Golodov, V. A. Coord. Chem. (Russ.) 1978, 4, 1046.
3
is isostructural and isomorphous with 1 CH2Cl2, and a
fitting of 11 selected atoms in the two complexes made
3
with the Mercury software43 gives a rms value of 0.05 A.
˚
(47) Qin, Z.; Jennings, M. C.; Puddephatt, R. J. Inorg. Chem. 2001, 40, 6220.
(48) (a) Adrian, R. A.; Zhu, S.; Damnels, L. M.; Tiekink, E. R. T.;
Walmsley, J. A. Acta Crystallogr., Sect. E 2006, 62, m1422. (b) Redwine,
K. D.; Wilson, W. L.; Moses, D. G.; Catalano, V. J.; Nelson, J. H. Inorg. Chem.
2000, 39, 3392. (c) Rath, N. P.; Stockland, R. A.; Anderson, G. K. Acta
Crystallogr., Sect. C 1999, 55, 494.
(46) (a) Li, Y.; Ng, K.-H.; Selvaratnam, S.; Tan, G.-K.; Vittal, J. J.;
Leung, P.-H. Organometallics 2003, 22, 834. (b) Li, Y.; Selvaratnam, S.; Vittal,
J. J.; Leung, P.-H. Inorg. Chem. 2003, 42, 3229. (c) Gul, N.; Nelson, J. H.
Organometallics 2000, 19, 91.
(49) Montagner, D.; Zangrando, E.; Longato, B. Inorg. Chem. 2008, 47,
2688.
(50) Arendse, M. J.; Anderson, G. K.; Rath, M. P. Acta Crystallogr., Sect.
C 2001, 57, 237.

Article
Inorganic Chemistry, Vol. 49, No. 8, 2010 3729
Other similar couples in CCDC are given by Pd(ONO2)2-
arbonylation involves a five-coordinate intermediate or a
predissociation of a PPh3 ligand. Potential oxidants, such as
NaNO2 or NaNO3, fail to give the oxidative carbonylation of
MeOH when using 1, 1a, 3, and 3a as catalyst precursors. On
the contrary, when using BQ as an oxidant, these complexes,
2, or trans-[Pd(COOMe)2-nXn(PPh3)2] (X = Cl, OTs, OAc; ,
n =1, 2) promote selective catalysis to DMO. [Pd(CO)-
50
{[CH2[(C6H5)2P]2}2 and Pt(ONO2)2{[CH2[(C6H5)2P]2}2
and by [Pd(ONO2)2(DMPP)2] and [Pt(ONO2)2(DMPP)2].48b
In all cases, the metal-P distances in platinum complexes
are shorter, highlighting the better ability of platinum to
establish π-back bonding with phosphorus.
The network of nonbonding interactions in 1 CH2Cl2
is less extended than that in 1a. The contacts for which the
3
(PPh3)3], [Pd(CO)(PPh3)]3, and [Pd(BQ)(PPh3)2]2 H2B have
3
˚
distance between interacting atoms is at least 0.1 A
shorter than the sum of the pertinent van der Waals radii
are available as Supporting Information.
beenfound after catalysis. [Pd(CO)(PPh3)3] is also an effective
precursor.
Acknowledgment. This work has been supported by the
ꢀ
Conclusions
Ministero dell’Universita e della Ricerca Scientifica e
In summary, we have reported the synthesis and charac-
terization of 1a and 3a, together with the new synthesis of
1-3 and the X-ray structures of 1a and1 CH2Cl2 and the
Tecnologica. The authors gratefully thank Dr. F. Bene-
tollo of the CMR-ICIS Institute (Padua, Italy) for access
to the PW1100 diffractometer and supervision of the
X-ray data collection process and Dr. B. Scantamburlo
of CHELAB srl (Resana, Italy) for analysis of the for-
maldehyde.
3
reactivity of all of them. 1 and 2 interchange a carbomethoxy
ligand and a nitrate ligand, giving 1a quantitatively and
irreversibly. 2 forms even at -78 °C from cis-[PdX2(PPh3)2]
(X = ONO2, OTs) in CD2Cl2/MeOH, under CO, but only
after the addition of NEt3. 2 decomposes at 40 °C, giving
DMC (ca. 10%), whereas under 4 atm of CO, it gives DMC
and DMO (ca. 12% each). In the presence of PPh3 and in
the absence of CO, decomposition occurs at 60 °C with
the formation of 15% of DMC only, suggesting that dec-
Supporting Information Available: Crystallographic data for
the structural analyses, a list of most relevant nonbonding
contacts found in the solid state, as well as MNR spectra
obtained in the synthesis of complex 2. This material is available
free of charge via the Internet at http://pubs.acs.org.