Divalent palladium and platinum complexes containing rigid bidentate nitrogen ligands and electrochemistry of the palladium complexes

Article abstract of DOI:10.1021/om960578a

The synthesis and characterization of divalent palladium and platinum complexes of the type PdX₂(Ar-BIAN) (X = Cl, Br, I, OC(O)Me), PdCl₂(Ph-BIC) and PtCl₂(Ar-BIAN) is described. These complexes contain the rigid bidentate nitrogen ligands bis(arylimino)-acenaphthene (Ar-BIAN; Ar = Ph, p-MeC₆H₄, p-MeOC₆H₄, o,o′-Me₂C₆H₃, o,o′-i-Pr₂C₆H₃) or bis(phenylimino)camphane (Ph-BIC), which act as σ-donor ligands to the metal center, as was deduced from the observed shifts in the IR and NMR spectra of the complexes. Electrochemical reduction of PdCl₂(Ar-BIAN) complexes in THF or DMF occurs via two one-electron reductions and affords the complex Pd^ICl(Ar-BIAN)^.-, which slowly produces some Pd⁰(Ar-BIAN) complex. Pd^ICl(Ar-BIAN)^.- reacts with iodomethane, whereas with iodobenzene or bromobenzene no reaction was observed. Pd^ICl(Ar-BIAN)^.- reacts with free Ar-BIAN or the alkenes dimethyl fumarate, dimethyl maleate, and methyl acrylate, giving complexes of the formulas Pd^ICl(Ar-BIAN)₂^.- and Pd^ICl(Ar-BIAN)(alkene)^.-, respectively. A two-electron reduction of the latter afforded Pd⁰(Ar-BIAN)₂^2- and Pd⁰(Ar-BIAN)(alkene)^2-, respectively, whose further oxidation in two one-electron steps produces Pd⁰(Ar-BIAN)₂ and Pd⁰(Ar-BIAN)(alkene). The Pd⁰(Ar-BIAN) complex which is slowly formed from Pd^ICl(Ar-BIAN)^.- reacts with alkene but directly affords the complexes Pd⁰(Ar-BIAN)(alkene)^.- because Pd⁰(Ar-BIAN)(alkene) complexes are formed at a potential more negative than their first reduction potential. Reoxidation of Pd⁰(Ar-BIAN)(alkene)^.- affords Pd⁰(Ar-BIAN)(alkene) complexes. The results of the electrochemical experiments corroborate earlier mechanistic proposals of exchange of Ar-BIAN ligands in Pd⁰(Ar-BIAN)(alkene) complexes and homogeneous hydrogenation of electron-poor alkenes by Pd⁰(Ar-BIAN)(alkene) complexes.

Full text of DOI:10.1021/om960578a

Organometallics 1997, 16, 317-328
317
Diva len t P a lla d iu m a n d P la tin u m Com p lexes Con ta in in g
Rigid Bid en ta te Nitr ogen Liga n d s a n d Electr och em istr y
of th e P a lla d iu m Com p lexes1
Rob van Asselt,† Cornelis J . Elsevier,*,† Christian Amatore,*,‡ and
Anny J utand*,‡
Anorganisch Chemisch Laboratorium, J . H. van’t Hoff Research Institute, Universiteit van
Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands, and
De´partement de Chimie, Ecole Normale Supe´rieure, CNRS URA 1679, 24 Rue Lhomond,
75231 Paris Cedex 05, France
Received J uly 11, 1996X
The synthesis and characterization of divalent palladium and platinum complexes of the
type PdX2(Ar-BIAN) (X ) Cl, Br, I, OC(O)Me), PdCl2(Ph-BIC) and PtCl2(Ar-BIAN) is
described. These complexes contain the rigid bidentate nitrogen ligands bis(arylimino)-
acenaphthene (Ar-BIAN; Ar ) Ph, p-MeC6H4, p-MeOC6H4, o,o′-Me2C6H3, o,o′-i-Pr2C6H3) or
bis(phenylimino)camphane (Ph-BIC), which act as -donor ligands to the metal center, as
was deduced from the observed shifts in the IR and NMR spectra of the complexes.
Electrochemical reduction of PdCl2(Ar-BIAN) complexes in THF or DMF occurs via two one-
electron reductions and affords the complex PdICl(Ar-BIAN)•-, which slowly produces some
Pd0(Ar-BIAN) complex. PdICl(Ar-BIAN)•- reacts with iodomethane, whereas with iodoben-
zene or bromobenzene no reaction was observed. PdICl(Ar-BIAN)•- reacts with free Ar-
BIAN or the alkenes dimethyl fumarate, dimethyl maleate, and methyl acrylate, giving
complexes of the formulas PdICl(Ar-BIAN)2 and PdICl(Ar-BIAN)(alkene)•-, respectively.
•-
A two-electron reduction of the latter afforded Pd0(Ar-BIAN)22- and Pd0(Ar-BIAN)(alkene)2-,
respectively, whose further oxidation in two one-electron steps produces Pd0(Ar-BIAN)2 and
Pd0(Ar-BIAN)(alkene). The Pd0(Ar-BIAN) complex which is slowly formed from PdICl(Ar-
BIAN)•- reacts with alkene but directly affords the complexes Pd0(Ar-BIAN)(alkene)•- because
Pd0(Ar-BIAN)(alkene) complexes are formed at a potential more negative than their first
reduction potential. Reoxidation of Pd0(Ar-BIAN)(alkene)•- affords Pd0(Ar-BIAN)(alkene)
complexes. The results of the electrochemical experiments corroborate earlier mechanistic
proposals of exchange of Ar-BIAN ligands in Pd0(Ar-BIAN)(alkene) complexes and homo-
geneous hydrogenation of electron-poor alkenes by Pd0(Ar-BIAN)(alkene) complexes.
In tr od u ction
the in situ reduction of the catalyst precursor, leading
to a formal 14-electron zerovalent complex which un-
dergoes oxidative addition of one of the reacting sub-
strates. In situ reduction of Pd(II) precursors by Al(i-
Bu)2H3h or Li2(cyclooctatetraene)6 prior to further
reactions has been reported. The formation of a Pd(0)
complex from the Pd(OC(O)Me)2/PPh3 system has been
elucidated by electrochemistry.7
Divalent palladium and platinum complexes are valu-
able catalyst precursors and have been used in a wide
variety of reactions, including carbonylation of alkenes,
organic halides, and nitroaromatic compounds,2 C-C
cross coupling,3 allylic alkylation,4 and Heck type reac-
tions.5 A general feature of most of these reactions is
We have recently demonstrated the activity of diva-
lent palladium complexes containing the rigid bidentate
nitrogen ligands bis(arylimino)acenaphthene (Ar-BIAN)
in catalytic cross coupling reactions of organic halides
with organomagnesium and -zinc compounds, whereas
similar reactions employing organotin reagents were
more effectively catalyzed by zerovalent complexes of
the type Pd(Ar-BIAN)(alkene).8 The use of such ze-
rovalent catalyst precursors limits the applicability of
the coupling reaction, as now the first step of the
reaction is oxidative addition of the organic halide to a
16-electron Pd(Ar-BIAN)(alkene) species. Because of
* To whom correspondence should be addressed.
† Universiteit van Amsterdam.
‡ Ecole Normale Supe´rieure.
X Abstract published in Advance ACS Abstracts, December 1, 1996.
(1) Rigid Bidentate Nitrogen Ligands in Organometallic Chemistry
and Homogeneous Catalysis. 11. Part 9: Reference 9. Part 10: van
Belzen, R.; Klein, R. A.; Smeets, W. J . J .; Spek, A. L.; Benedix, R.;
Elsevier, C. J . Recl. Trav. Chim. Pays-Bas 1996, 115, 275.
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Kumada, M. Acc. Chem. Res. 1982, 15, 395. (d) Suzuki, A. Pure Appl.
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L. H.; Brubaker, C. H., J r. Organometallics 1986, 5, 482. (f) Yamamoto,
A. Organotransition Metal Chemistry; Wiley: New York, 1986; pp 374-
399. (g) Stille, J . K.; Simpson, J . H. J . Am. Chem. Soc. 1987, 109, 2138.
(h) Negishi, E.; Takahashi, T.; Baba, S.; Van Horn, D. E.; Okukado,
N. J . Am. Chem. Soc. 1987, 109, 2393. (i) Ali, H. M.; Brubaker, C. H.,
J r. J . Mol. Catal. 1990, 60, 331. (j) Wright, M. E.; Lowe-Ma, C. K.
Organometallics 1990, 9, 347. (k) Hatanaka, Y.; Hiyama, T. Synlett.
1991, 845.
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Taylor, R. J . J . Organomet. Chem. 1990, 370, 397. (b) Brown, J . M.;
Cooley, N. A. Organometallics 1990, 9, 353.
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11, 3009.
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(b) van Asselt, R.; Elsevier, C. J . Tetrahedron 1994, 50, 323. (c) van
Asselt, R.; Elsevier, C. J . J . Am. Chem. Soc. 1994, 116, 977.
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S0276-7333(96)00578-X CCC: $14.00 © 1997 American Chemical Society

318 Organometallics, Vol. 16, No. 3, 1997
van Asselt et al.
Sch em e 1
subsequent reaction(s) of the complexes monitored by
cyclic voltammetry. Furthermore, only the palladium
complex and the reacting species are present during the
reaction, whereas in the case of chemical reduction
always several reagents are present, which might give
rise to side reactions.
In this paper we report the results of our studies on
the electrochemical behavior of PdCl2(Ar-BIAN) com-
plexes and the reactivity of the complexes formed after
reduction toward alkenes, free Ar-BIAN, and some
organic halides. Furthermore, the synthesis and char-
acterization of these and related PdX2(Ar-BIAN) (X )
Cl, Br, I, OC(O)Me), PdCl2(Ph-BIC) (Ph-BIC ) bis-
(phenylimino)camphane) and PtCl2(Ar-BIAN) complexes
1-5 are described.
steric and electronic reasons, such a complex can be
anticipated to be much less reactive than an in situ
generated Pd(Ar-BIAN) complex and cross coupling is
only observed for relatively reactive organic halides,8a,b
which are able to undergo the oxidative addition reac-
tion.9 This is evidenced by our observations that e.g.
benzyl chloride and -bromostyrene do not give ap-
preciable conversion in the Pd(Ar-BIAN)-catalyzed cross
coupling with organotin reagents. On the other hand,
ready conversion of these substrates was achieved with
organomagnesium and -zinc reagents in the presence
of 1 mol % of a PdCl2(Ar-BIAN) catalyst,8a,b as in these
latter cases a transient reactive 14-electron Pd0(Ar-
BIAN) complex is formed in situ.
In view of the high reactivity of 14-electron zerovalent
PdL2 complexes toward oxidative addition, we have
investigated the possibilty of in situ generation of low-
coordinated Pd0(Ar-BIAN) complexes and subsequent
reactions of such species. Available methods for the in
situ generation of these complexes are (Scheme 1) (i)
chemical reduction of divalent palladium or platinum
complexes, e.g. with organolithium or -aluminium com-
pounds10 or with fluoride anion,11 (ii) photochemical
extrusion of CO2 or N2 from M(II) complexes containing
oxalate12 or azide13 ligands, respectively, and (iii) elec-
trochemical reduction of the divalent halide com-
plexes.14,15
We have used electrochemistry to generate and study
reduced Pd(Ar-BIAN) complexes, as this method has
several important advantages as compared to the other
methods. First, the palladium dihalide complexes which
are conveniently synthesized in high yields can be used
directly, whereas for photochemical reduction of Pd-
oxalate or Pd-azide complexes additional synthetic
modifications are required.12,13 Second, the potential of
the reduction can be controlled and the formation and
Exp er im en ta l Section
All syntheses were performed in air, unless noted otherwise.
1H NMR spectra were recorded on a Bruker AMX 300 (300.13
MHz) and a Bruker AC 100 (100.13 MHz) spectrometer and
13C NMR spectra on a Bruker AMX 300 spectrometer (75.48
MHz). Chemical shift values are in ppm relative to TMS as
external standard with high-frequency shifts denoted as
positive. IR spectra were recorded on a Perkin-Elmer 283
spectrophotometer. Mass spectra were obtained on a Varian
MAT 711 double-focusing spectrometer and were performed
by the Institute for Mass Spectroscopy, Universiteit van
Amsterdam, and all obtained data agreed well with simulated
spectra of the respective complexes. Elemental analyses were
carried out by Dornis und Kolbe, Mikroanalytisches Labora-
torium, Mu¨lheim a.d. Ruhr, Germany. PdCl2(PhCN)2,16 PtCl2-
(SEt2)2,17 K[PtCl3(C2H4)]‚H2O,18 and Ar-BIAN and Ph-BIC19
were synthesized by following reported procedures. PdCl2-
(MeCN)2 was synthesized in a way similar to that reported
for PdCl2(PhCN)2.
(9) van Asselt, R.; Vrieze, K.; Elsevier, C. J . J . Organomet. Chem.
1994, 480, 27.
(10) (a) Negishi, E.; Takahashi, T.; Akiyoshi, K. J . Chem. Soc., Chem.
Commun. 1986, 1338. (b) Urata, H.; Suzuki, H.; Moro-oka, Y.; Ikawa,
T. J . Organomet. Chem. 1989, 364, 235.
(11) Mason, M. R.; Verkade, J . G. Organometallics 1992, 11, 2212.
(12) (a) Paonessa, R. S.; Prignano, A. L.; Trogler, W. C. Organome-
tallics 1985, 4, 647. (b) Trogler, W. C. In Excited States and Reactive
Intermediates; ACS Symposium Series 307, Lever, A. B. P., Ed.;
American Chemical Society: Washington, DC, 1986; pp 188-191.
(13) Knoll, H.; Stich, R.; Hennig, H.; Stufkens, D. J . Inorg. Chim.
Acta 1990, 178, 71.
P d Cl2(P h -BIAN) (1a ). Meth od A. To a solution of 2.02
g of PdCl2(PhCN)2 (5.3 mmol) (or 1.37 g (5.3 mmol) of PdCl2-
(MeCN)2) in 100 mL of dichloromethane was added 1.81 g of
Ph-BIAN (5.4 mmol), and the mixture was stirred at 20 °C.
After 1 h the suspension was evaporated and the resulting
product washed subsequently with methanol (2 × 10 mL) and
diethyl ether (3 × 20 mL). The product was dried in vacuo,
(16) Kharasch, M. S.; Seyler, R. C.; Mayo, F. R. J . Am. Chem. Soc.
1938, 60, 882.
(14) (a) Amatore, C.; Azzabi, M.; J utand, A. J . Organomet. Chem.
1989, 363, C41. (b) Amatore, C.; Azzabi, M.; J utand, A. J . Am. Chem.
Soc. 1991, 113, 1670. (c) Amatore, C.; Azzabi, M.; J utand, A. J . Am.
Chem. Soc. 1991, 113, 8375. (d) Amatore, C.; J utand, A.; Khalil, A. F.;
Nielsen, M. F. J . Am. Chem. Soc. 1992, 114, 7076.
(15) (a) Paumard, E.; Mortreux, A.; Petit, F. J . Chem. Soc., Chem.
Commun. 1989, 1380. (b) Davies, J . A.; Staples, R. J . Polyhedron 1991,
10, 909.
(17) Kaufmann, G. B.; Cowan, D. O. Inorg. Synth. 1960, 6, 211.
(18) Chock, P. B.; Halpern, J .; Paulik, F. E. Inorg. Synth. 1973, 14,
90.
(19) (a) Matei, I.; Lixandru, T. Bul. Inst. Politeh. Iasi 1967, 13, 245;
Chem. Abstr. 1969, 70, 3623. (b) Normant, M. H. C. R. Seances Acad.
Sci., Ser. C 1969, 268, 1811. (c) van Asselt, R.; Elsevier, C. J .; Smeets,
W. J . J .; Spek, A. L. Benedix, R. Recl. Trav. Chim. Pays-Bas 1994,
113, 88.

Divalent Palladium and Platinum Complexes
Organometallics, Vol. 16, No. 3, 1997 319
giving 2.56 g of PdCl2(Ph-BIAN) as an orange powder (95%).
Analytically pure samples were obtained by recrystallization
from dichloromethane/hexane.
mL). The combined filtrates were evaporated to about 5 mL,
15 mL of hexane was added, and the mixture was cooled to
-20 °C. After several hours the product was filtered off and
dried in vacuo, giving 0.50 g of a red product (85%).
Anal. Found (calcd for C26H20Br2N2Pd): C, 49.58 (49.83);
H, 3.13 (3.22); N, 4.41 (4.47). IR (cm-1, KBr): 1617, (CdN).
P d Br 2(o,o′-i-P r 2C6H3-BIAN) (2e; orange) was synthesized
in the same way as 2b in 78% (method C) or 81% (method D)
yield. Anal. Found (calcd for C36H40Br2N2Pd): C, 56.41
(56.38); H, 5.32 (5.26); N, 3.73 (3.65). IR (cm-1, KBr): 1620,
(CdN). 13C NMR (ppm, CDCl3): 176.0, C1; 125.6, C2; 126.5,
C3; 129.7, C4; 133.1, C5; 132.0, C6; 147.1, C7; 142.2, C8; 140.4,
C9; 124.8, C10; 129.7, C11; 30.2, CH (i-Pr); 24.7, 24.2, CH3 (i-
Pr).
P d I2(P h -BIAN) (3a ). To an orange solution of 15.1 mg of
PdCl2(Ph-BIAN) (1a ) (0.025 mmol) in 10 mL of dichlo-
romethane was added a solution of 0.15 g of NaI (1.0 mmol)
in 5 mL of acetone, and the solution immediately turned dark
red. After it was stirred at 20 °C over 4 h, the solution was
evaporated to dryness. The solid was extracted at 0 °C with
dichloromethane (2 × 10 mL), and the extracts were evapo-
rated to dryness. After washing of the solid with 5 mL of
diethyl ether and drying in vacuo, a dark purple-red product
was obtained.
Meth od B. A suspension of 1.28 g of PdCl2 (7.2 mmol) in
100 mL of acetonitrile was warmed to 70 °C, to give a red
solution. Then 2.60 g of Ph-BIAN (7.8 mmol) was added and
the mixture refluxed for 1 h. After the mixture was cooled to
20 °C, the solvent was evaporated to about 10 mL and 50 mL
of diethyl ether was added. The solid product was filtered off
and washed with diethyl ether (3 × 30 mL). Then the product
was redissolved in dichloromethane, this solution was filtered
through Celite filter aid to remove metallic palladium, and the
filtrate was evaporated to 5 mL. After addition of hexane the
solid was filtered out and dried in vacuo, yielding 2.81 g of an
orange solid (76%).
Anal. Found (calcd for C24H16Cl2N2Pd): C, 55.65 (56.55);
H, 3.25 (3.16); N, 5.13 (5.50). IR (cm-1, KBr): 1630, (CdN);
348, (PdsCl). 13C NMR (ppm, DMSO-d6; the atom-number-
ing scheme is shown in Table 1): C1, not observed; 128.3, C2;
124.8, C3; 128.9, C4; 132.1, C5; 130.9, C6; 145.1, C7; 155.3, C8;
122.6, C9; 129.2, C10; 125.1, C11. MS (m/z): 510 (calcd 510).
P d Cl2(p-Tol-BIAN) (1b; orange-brown) was synthesized in
the same way as 1a (method A) by addition of 0.95 equiv of
p-Tol-BIAN to a filtered solution of PdCl2(PhCN)2 in dichlo-
romethane (87% yield). The product obtained in this way is
analytically pure but may be recrystallized by slow evaporation
of a solution of 1b in DMF to give orange-brown plates. Anal.
Found (calcd for C26H20Cl2N2Pd): C, 58.95 (58.07); H, 3.86
(3.75); N, 4.99 (5.21). IR (cm-1, KBr): 1620, (CdN); 350,
(PdsCl). MS (m/z): 538 (calcd 538).
P d Cl2(p-MeOC6H4-BIAN) (1c; red) was synthesized and
purified in the same way as 1b, in 88% yield. Anal. Found
(calcd for C26H20Cl2N2O2Pd): C, 54.84 (54.81); H, 3.65 (3.54);
N, 4.75 (4.92). IR (cm-1, KBr): 1620 sh, (CdN); 360,
(PdsCl). MS (m/z): 570 (calcd 570).
P d Cl2(o,o′-Me2C6H3-BIAN) (1d ; orange) was synthesized
in 80% yield as described in method B. Anal. Found (calcd
for C28H24Cl2N2Pd): C, 59.39 (59.44); H, 4.29 (4.28); N, 4.99
(4.95). IR (cm-1, KBr): 1618, (CdN); 345, (PdsCl).
P d Cl2(o,o′-i-P r 2C6H 3-BIAN) (1e) was synthesized via
method B as a yellow-orange solid (84%). Anal. Found (calcd
for C36H40Cl2N2Pd): C, 63.60 (63.77); H, 6.04 (5.95); N, 4.51
(4.13). IR (cm-1, KBr): 1617, (CdN); 342, (PdsCl). 13C
NMR (ppm, CDCl3): C1,2,6,7,8, not observed; 125.7, C3; 129.1,
C4; 132.3 C5; 139.8, C9; 124.1, C10; 128.9, C11; 29.4, CH (i-Pr);
23.7, 23.5, CH3 (i-Pr).
P d I2(p-Tol-BIAN) (3b) was obtained as a dark red product
in the same way as 3a . MS (m/z): 720 (calcd for C26H20I2N2-
Pd: 720).
P d(OC(O)Me)2(o,o′-i-P r 2C6H3-BIAN) (4e). A 0.26 g amount
of o,o′-i-Pr2C6H3-BIAN (0.52 mmol) was added to a solution of
0.11 g of Pd(OC(O)Me)2 (0.49 mmol) in 15 mL of acetonitrile,
and the mixture was heated to 70 °C. After 1 h the brown-
red solution was evaporated to dryness and the product
washed with diethyl ether (2 × 10 mL). The resulting product
was dissolved in 50 mL of dichloromethane and filtered
through Celite filter aid. The filtrate was evaporated to about
3 mL, and 20 mL of hexane was added. The product was
obtained as an orange-brown solid in 70% yield (0.25 g) by
filtration and drying in vacuo. Anal. Found (calcd for C40
-
H
46N2O4Pd): C, 66.90 (66.25); H, 6.76 (6.40); N, 3.95 (3.86).
IR (cm-1, KBr): 1629, (CdO). 13C NMR (ppm, CDCl3): 174.8,
C1; 125.2, C2; 126.5, C3; 129.7, C4; 133.2, C5; 132.0, C6; 147.4,
C7; 140.8, C8; 141.2, C9; 125.0, C10; 129.6, C11; 29.9, CH (i-Pr);
25.1, 24.2, CH3 (i-Pr); 177.4, (OC(O)Me); 22.8 (OC(O)Me).
Rea ction of P d (OC(O)Me)2(o,o′-i-P r 2C6H3-BIAN) (4e)
w ith HCl. To an orange solution of 22.3 mg of 4e (0.031
mmol) in 0.5 mL of CDCl3 was added 1 drop of concentrated
aqueous hydrochloric acid (excess), and the mixture was
vigorously shaken for a few seconds. The yellow CDCl3
solution was removed from the biphasic system, filtered, and
P d Cl2(P h -BIC) (1f) was synthesized from PdCl2 in 77%
yield via method B and recrystallized from DMF to yield yellow
needles. Anal. Found (calcd for C22H24Cl2N2Pd): C, 53.93
(53.52); H, 4.65 (4.90); N, 5.80 (5.67). IR (cm-1, KBr): 1657,
1634, (CdN); 345, (Pd-Cl). 1H NMR (ppm, DMSO-d6): 7.2-
7.4 m (10 H), Ph; 1.02 s, 0.85 s, 0.41 s, H8,9,10; other signals
overlapped by the signals of H2O and DMSO.
1
analyzed directly by H NMR spectroscopy without isolation.
The spectrum of the product formed was identical with that
of 1e obtained by the reaction of o,o′-i-Pr2C6H3-BIAN with
PdCl2 as described above.
P d Br 2(p-Tol-BIAN) (2b). Meth od C. To a solution of 0.21
g of PdCl2 (1.2 mmol) and 1.22 g of NaBr (11.9 mmol) in 20
mL of acetonitrile, warmed to 70 °C, was added 0.46 g og p-Tol-
BIAN (1.3 mmol), and the mixture was stirred at 70 °C. After
1 h the mixture was evaporated, and the solid product was
extracted with dichloromethane (3 × 30 mL). The combined
dichloromethane layers were washed with water (3 × 25 mL),
dried on MgSO4, and filtered through Celite filter aid. After
evaporation of the solvent to approximately 3 mL, 25 mL of
hexane was added. The product was filtered off, washed with
10 mL of hexane, and dried in vacuo, to yield 0.56 g (75%) of
a red product.
Meth od D. A solution of 0.25 g of PdBr2 (0.94 mmol) and
0.35 g of p-Tol-BIAN (0.97 mmol) in 20 mL of acetonitrile was
stirred at 70 °C. After 30 min the solution was evaporated to
dryness and the product washed with diethyl ether (2 × 10
mL). The product was redissolved in 50 mL of dichlo-
romethane, this solution was filtered through Celite filter aid,
and the residue was washed with dichloromethane (2 × 20
P tCl2(P h -BIAN) (5a ). Meth od E. A solution of 0.29 g of
Ph-BIAN (0.87 mmol) in 30 mL of dichloromethane was added
to a solution of 0.32 g of K[PtCl3(C2H4)]‚H2O (0.83 mmol) in
10 mL of methanol. After the mixture was stirred at 20 °C
for 15 min, charcoal was added; this mixture was stirred for 5
min and subsequently filtered. The residue was washed with
10 mL of dichloromethane, and the filtrates were evaporated
to dryness. The product was washed with diethyl ether (2 ×
5 mL), and after drying in vacuo 0.32 g of a brown-red product
was obtained (65%).
Meth od F . To a suspension of 0.72 g of PtCl2(SEt2)2 (1.6
mmol) and 0.61 g of Ph-BIAN (1.8 mmol) in 30 mL of acetone
was added approximately 5 mL of dichloromethane, and a clear
solution was obtained. This solution was transferred to a
stainless steel autoclave, pressurized with ethene (5 bar), and
stirred at 30 °C. After 16 h the ethene pressure was released
and the reaction mixture concentrated to ca 5 mL in vacuo.
The product was precipitated by the addition of 15 mL of
hexane, washed with diethyl ether (2 × 10 mL), and dried in

320 Organometallics, Vol. 16, No. 3, 1997
van Asselt et al.
1/2
vacuo to yield 0.57 g of a brown-red solid (59%). Anal. Found
(calcd for C24H16Cl2N2Pt): C, 49.26 (48.17); H, 2.61 (2.70); N,
4.75 (4.68). IR (cm-1, KBr): 1618 sh, (CdN); 348, (PtsCl).
MS (m/z): 598 (calcd 598).
iPd nPd[Pd]DPd
)
) B
1/2
iFe
[Fe]DFe
From expressions A and B one gets
P tCl2(p-Tol-BIAN) (5b; brown-red) was obtained in the
same way as described for 5a (method A) in 87% yield. Anal.
Found (calcd for C26H20Cl2N2Pt): C, 48.42 (49.85); H, 2.90
(3.22); N, 4.12 (4.47). IR (cm-1, KBr): 1630 sh, (CdN); 339,
(PtsCl). MS (m/z): 626 (calcd 626).
B2
nPd[Pd]
)
A
[Fe]
Electr och em ica l Setu p a n d P r oced u r e for Cyclic Vol-
ta m m etr y. Cyclic voltammetry was performed as described
previously.14 A home-built potentiostat equipped with positive
feedback for ohmic-drop compensation was used.20 The refer-
ence electrode was an SCE (Tacussel) separated from the
solution by a bridge (3 mL) filled with an n-Bu4NBF4 solution
identical with that used in the cell. All potentials given here
refer to this reference electrode. All the experiments were
performed at 20 °C under an atmosphere of argon. Solvents
were distilled (THF from sodium benzophenone ketyl and DMF
from calcium hydride) and degassed prior to use.
In a typical procedure 12.2 mg of PdCl2(Ph-BIAN) (0.024
mmol) was dissolved in 12 mL of a solution of 1.48 g of n-Bu4-
NBF4 (4.5 mmol) in 15 mL of THF (0.30 M; 3 mL of the latter
solution was introduced into the bridge compartment of the
reference SCE electrode). Under standard conditions, cyclic
voltammograms were obtained at a scan rate of 0.2 V/s,
reduction first (0 to -2.0 V) followed by oxidation (-2.0 to +1.0
V) and finally reduction (to 0 V), unless noted otherwise in
the text. After cyclic voltammograms of the starting complex
were recorded, reagents were added to the cell and voltam-
mograms recorded (reduction first), to monitor the reaction(s)
of the electrogenerated Pd(Ar-BIAN) complex with the added
reagents. Experiments on the influence of the scan rate on
the voltammograms were all performed for scan rates between
0.1 and 100 V/s.
and thus:
(iPd)2(ilimFe)[Fe]
(iFe)2(ilimPd)[Pd]
B2[Fe]
A[Pd]
nPd
)
)
On the basis of the chronoamperometric value at ) 200
ms, one obtains nPd ) 1.2 ( 0.2 and nPd ) 1.2 ( 0.2 when
)
100 ms. The diffusion coefficient, DPd, for 1a can be deter-
mined from the value of the diffusion coefficient for the
ferrocene, DFe ) 7.3 × 10-6 cm2 s-1
ilimPd[Fe]DFe
,
as follows:
21
) DPd ) 2.16 × 10-6 cm2 s-1
ilimFe[Pd]nPd
Under these conditions, the characteristic time of the voltam-
metry at the ultramicroelectrode is T ) r2/D ) 115 ms,21
value which fits the gap examined by chronoamperometry.
From these results, one concludes that the first reduction of
1a involves one electron.
a
Resu lts a n d Discu ssion
Syn th esis a n d Ch a r a cter iza tion of th e Com -
p lexes 1-5. Dichloro(Ar-BIAN)palladium complexes 1
have been synthesized in good yields by substitution of
weakly coordinating ligands from suitable precursors,
such as PdCl2(PhCN)2 and PdCl2(MeCN)2 (eq 1). PdCl2-
Deter m in a tion of th e Nu m ber of Electr on s In volved
in th e F ir st Red u ction of P d Cl2(P h -BIAN) (1a ). Electron
consumption in transient electrochemistry was determined by
following a method previously described, which combines the
use of chronoamperometry and steady-state voltammetry at
an ultramicroelectrode.21 Thus, 11.4 mg of PdCl2(Ph-BIAN)
(1a ; 0.022 mmol, [Pd] ) 1.83 mM) and 4.9 mg of ferrocene (Fe-
(
5-C5H5)2; 0.026 mmol, [Fe] ) 2.16 mM) were dissolved in 12.0
mL of a 0.30 M solution of n-Bu4NBF4 in THF. The limiting
current for the steady-state voltammetry, performed at a gold
ultramicroelectrode (10 m diameter) with a scan rate of 20
mV s-1, was ilim ) (0.175 V)G ) 1.82 nA for 1a (where G )
Pd
10.44 nA V-1 is the gain of the potentiostat) and ilim ) (0.69
Fe
V)G ) 7.20 nA for the ferrocene. Chronoamperometry at
)
200 ms (Ered ) -0.58 V and Eox ) +0.75 V) performed at a
Pd
Fe
gold electrode (0.5 mm diameter) revealed currents of iPd
)
(0.50 V)G ) 2.62 A for 1a (where G ) 5.25 A V-1 is the gain
of the potentiostat) and iFe ) (0.99 V)G ) 5.19 A for the
ferrocene. At ) 100 ms, iPd ) (0.71 V)G ) 3.72 A for 1a
and iFe ) (1.36 V)G ) 7.14 A for ferrocene (all data are the
average of two or three independent determinations).
(COD) (COD ) (Z,Z)-1,5-cyclooctadiene) could also be
used as a precursor, but substitutiton of dimethyl sulfide
ligands from PdCl2(SMe2)2 by Ar-BIAN ligands was only
very slow. Alternatively, the same complexes were
obtained by direct reaction of an Ar-BIAN ligand with
PdCl2 in acetonitrile, via in situ formed PdCl2(MeCN)2.
When the latter method is applied, metallic palladium
and impurities from the PdCl2 are easily removed by
dissolution of the product in dichloromethane and
filtration through Celite filter aid. This method was less
suitable for the synthesis of 1b and 1c, as these
complexes did not dissolve well enough in common
organic solvents to allow further purification. PdCl2-
At an ultramicroelectrode, at low scan rate, the expression
for the current is ilim ) 4rFnCD (r ) radius of the electrode; C
) concentration of the species; n ) number of electrons; D:
diffusion coefficient of the species). Therefore, one has ilim
Pd
) 4rFnPd[Pd]DPd for 1a and, since nFe ) 1 ilim ) 4rF[Fe]DFe
Fe
for the ferrocene, so that
ilim
ilim
nPd[Pd]DPd
[Fe]DFe
Pd
Fe
)
) A
The expression for the current of the chronoamperometry is I
) nFSC -1/2D1/2 -1/2 (S ) electrode surface; ) step duration),
so that
(20) Amatore, C.; Lefrou, C.; Pflu¨ger, F. J . Electroanal. Chem.
Interfacial Electrochem. 1989, 270, 43.
(21) Amatore, C.; Azzabi, M.; Calas, P.; J utand, A.; Lefrou, C.; Rollin,
Y. J . Electroanal. Chem. Interfacial Electrochem. 1990, 288, 45.

Divalent Palladium and Platinum Complexes
Organometallics, Vol. 16, No. 3, 1997 321
Ta ble 1. 1H NMR Da ta for P d X2(Ar -BIAN) a n d P tCl2(Ar -BIAN) Com p lexes 1-5a
H3
H4
H5
H9,10,11
R
1a
6.81 d
7.2
7.3-7.7 m (12 H)
8.13 d
8.4
7.3-7.7 m (12 H)
1bb
1cb
1d
1e
6.66 d
7.1
6.76 d
7.0
6.62 d
7.3
6.54 d
7.3
6.81 d
7.3
6.48 d
7.3
6.66 d
7.5
6.74 d
7.2
7.65 pst
7.65 pst
8.30 d
8.2
8.30 d
8.0
8.15 d
8.3
8.13 d
8.3
8.11 d
8.3
8.14 d
8.3
8.08 d
8.6
8.09 d
8.3
7.43 d, 7.30 d
8.4
7.38 d, 7.14 d
8.1
2.44 s, p-Me
3.88 s, p-MeO
2.44 s, o,o′-Me2
7.52 dd
7.3, 8.3
7.5 m (4 H)
7.3 m (6 H)
7.5 m (4 H), H11
7.37 d (7.7 Hz), H10
7.38 d, 7.26 d
3.53 sep (6.8 Hz), CH (i-Pr)
1.52 d, 0.99 d (6.8 Hz), CH3 (i-Pr)
2.48 s, p-Me
2b
2e
7.50 pst
8.1
7.5 m (4 H)
7.5 m (4 H), H11
7.38 d (7.6 Hz), H10
7.3-7.7 m (12 H)
3.51 sep (6.8 Hz), CH (i-Pr)
1.50 d, 0.96 d (6.8 Hz), CH3 (i-Pr)
3a
7.3-7.7 m (12 H)
7.49 pst
3b
4ec
5a
7.42 d, 7.21 d
8.0
7.5 m (4 H), H11
7.34 d (7.6 Hz), H10
7.3-7.6 m (12 H)
2.50 s, p-Me
6.63 d
7.3
6.96 d
7.3
7.05 d
7.1
7.5 m (4 H)
8.11 d
8.3
8.27 d
8.1
8.26 d
8.1
3.71 sep (6.8 Hz), CH (i-Pr)
1.57 d, 0.93 d (6.8 Hz), CH3 (i-Pr)
7.3-7.6 m (12 H)
7.4-7.7 m (10 H)
5b
7.4-7.7 m (10 H)
2.50 s, p-Me
a
Recorded at 300.13 MHz in CDCl3 at 20 °C, unless indicated otherwise. Coupling constants (Hz) are given below the chemical shifts.
b
Abbreviations used: s ) singlet, d ) doublet, dd ) doublet of doublets, pst ) pseudo triplet, sep ) septet, m ) multiplet. In DMSO-d6.
c 1.50 s, OC(O)Me.
(Ph-BIC) (1f) was obtained similarly to the PdCl2(Ar-
BIAN) analogues 1a -e.
The dibromo(Ar-BIAN)palladium complexes 2 were
synthesized in a comparable way, either by reaction of
Ar-BIAN with PdCl2 in the presence of an excess of
sodium bromide (about 10 equiv relative to palladium)
or by reaction with PdBr2 (eq 2).
in acetonitrile. This complex reacted with aqueous
concentrated hydrochloric acid to give quantitatively the
dichloro complex 1e (eq 3).
Synthesis of the dichloro(Ar-BIAN)platinum com-
plexes 5 was achieved by reaction of K[PtCl3(C2H4)]‚H2O
with the appropriate Ar-BIAN ligand (eq 4). There was
no evidence for the formation of any five-coordinate
PtCl2(Ar-BIAN)(C2H4) complex22 under the conditions
described here. No PtCl2(Ar-BIAN) complexes were
obtained when Ar-BIAN was reacted with K2PtCl4,
which was used for the synthesis of PtCl2(phen),23 cis-/
trans-PtCl2(SEt2)2, and trans-PtCl2(MeCN)2.24 How-
ever, formation of PtCl2(Ph-BIAN) (5a ) was achieved
when the reaction of Ph-BIAN with PtCl2(SEt2)2 was
carried out under ethene presssure (eq 4), probably via
an in situ formed Pt(ethene) complex. Addition of
dimethyl fumarate (DMFU) to a mixture of Ph-BIAN
and PtCl2(SEt2)2 did not lead to the formation of 5a , and
only starting materials were recovered after 6 h at 20
°C.
The diiodo complexes 3 were obtained by halogen
metathesis of the dichloro complexes 1 with sodium
iodide, and the complexes obtained show the same
spectral data as those obtained by oxidative addition of
diiodine to Pd(Ar-BIAN)(alkene) complexes.9 The best
results were obtained when the reactions were per-
formed on a small scale (10-50 mg of Pd complex) with
a large excess of NaI (g20 equiv relative to Pd), since
otherwise mixtures of complexes were obtained, most
likely PdCl2(Ar-BIAN), PdI2(Ar-BIAN), and PdClI(Ar-
BIAN).
1
From the H NMR data of the complexes 1-5 (Table
1) it appears that the signals of the Ar-BIAN ligands
(22) Maresca, L.; Natile, G.; Cattalini, L. Inorg. Chim. Acta 1975,
14, 79.
(23) (a) Palocsay, F. A.; Rund, J . V. Inorg. Chem. 1969, 8, 524. (b)
Rund, J . V.; Palocsay, F. A. Inorg. Chem. 1969, 8, 2242.
(24) Fanizzi, F. P.; Intini, F. P.; Maresca, L. J . Chem. Soc., Dalton
Trans. 1990, 199.
Pd(OC(O)Me)2(o,o′-i-Pr2C6H3-BIAN) (4e) was obtained
by reaction of o,o′-i-Pr2C6H3-BIAN with Pd(OC(O)Me)2

322 Organometallics, Vol. 16, No. 3, 1997
van Asselt et al.
reaction with organolithium, -magnesium, or -zinc
reagents failed. In all cases mixtures of unknown
products were formed, which is most likely due to a
combination of reactions at the metal center and at the
ligand and decomposition. It has been reported before
that reaction of PtCl2(bpy) (bpy ) 2,2′-bipyridine) with
MeLi was not an efficient route for the synthesis of Pt-
methyl complexes.27 Only in the case of the reaction of
PdCl2(Ar-BIAN) with an excess of tetramethyltin at 60
°C in toluene was conversion to Pd(Me)Cl(Ar-BIAN)
observed (eq 5). At 20 °C no reaction occurred, and at
have shifted to higher frequency upon coordination,
especially H4 and H5, by about 0.1-0.3 ppm for Pd and
0.1-0.4 ppm for Pt. This observation is consistent with
donation of electron density from the Ar-BIAN ligand
to the metal center, similar to the observations made
for Pd(Ar-BIAN)(alkene) complexes.25 The effect of
coordination on the chemical shift of H3 is less clear,
which might be due to the combined effect of coordina-
tion to the metal center, inducing a shift to higher
frequency, and reorientation of the aromatic ring due
to the presence of the halide ligands in the coordination
plane, leading to increased anisotropic shielding of H3
and a concomitant low-frequency shift.19c,25 The per-
pendicular orientation of these aromatic rings in the
complexes appears also from the observed low-frequency
singlet at 1.50 ppm for the acetate moiety in Pd(OC-
(O)Me)2(o,o′-i-Pr2C6H3-BIAN) (4e), which must be as-
cribed to anisotropic shielding of this methyl group by
the aromatic ring of the o,o′-i-Pr2C6H3 substituent. For
complexes bearing ligands with ortho-substituted aro-
matic groups on the Ar-BIAN ligands, i.e. 1d , 1e, 2e,
and 4e, the observed shift of the o-CHR2-Ar (R ) H,
Me) resonance by 0.3-0.7 ppm to higher frequency
seems to indicate that there is some interaction between
the halide or acetate ligand and this proton.
60 °C excess SnMe4 (g5 equiv) and reactions lasting
several hours (g4 h) were necessary to obtain complete
conversion. The products synthesized in this way show
1H NMR spectral data identical with the complexes
obtained by the reaction of Pd(Me)Cl(COD) with Ar-
BIAN.8c
At this point it becomes clear that the observed
behavior of the PdCl2(Ar-BIAN) complexes, when dis-
solved in DMF, can be ascribed to the formation of a
[PdCl(Ar-BIAN)(DMF)]+Cl- species. In previous papers
we described the necessity of strongly polar aprotic
solvents such as DMF and HMPA (hexamethylphos-
phoric triamide) in Pd(Ar-BIAN)(DMFU)-catalyzed cross-
coupling reactions of organic halides (RX) with organ-
otin reagents (R′4Sn).8a,b Probably, the Pd(R)X(Ar-
BIAN) complex formed after oxidative addition of RX
to Pd(Ar-BIAN)(DMFU) is partially solvated in DMF
solution to give [Pd(R)(Ar-BIAN)(DMF)]+X-, which ac-
celerates the rate-determining transmetalation step.
The faster reaction of the ionic [Pd(R)(Ar-BIAN)(DMF)]+
complex with R′4Sn, as compared to the neutral Pd(R)X-
(Ar-BIAN) complex, can be explained by invoking (i) the
increased accessibility of palladium, (ii) the increased
electrophilicity of palladium, facilitating attack at the
tin bound C atom in R′4Sn, and (iii) the presence of a
noncoordinating halide anion, which, by precoordination
to tin, might facilitate the transfer of an organic group
from tin to palladium.8b
Shifts to higher frequency have also been observed
in the 13C NMR spectroscopy of some complexes, espe-
cially for the imine carbon C1 and the naphthene C
atoms C3,4,5
.
Unfortunately, the solubility of most
Electr och em ica l Beh a vior of P d IICl2(Ar -BIAN)
Com p lexes 1a -c. In the cyclic voltammogram of PdII-
Cl2(Ar-BIAN) complexes 1a -c, 2 mM in THF (or DMF),
two reduction peaks were observed, a reversible one at
ca. -0.4 V vs SCE (R1) and an irreversible one at about
-1.1 V (R2) (Figure 1a, Table 2).28 After reduction of
the complex at R2, a weak oxidation peak was observed
on the reverse scan that might be ascribed to species
generated during the two-step reduction of PdCl2(Ar-
BIAN) complexes, i.e. Cl- or a low-valent palladium
complex. Determination of the number of electrons21
involved in the first reduction step of PdIICl2(Ph-BIAN)
(1a ) by using double-step chronoamperometry and
complexes is rather low, which precluded the collection
of 13C NMR data for the entire series of complexes.
In the IR spectra of the complexes (as KBr pellets)
the CdN stretching vibration is observed at 1620-1660
cm-1, i.e. shifted by 10-30 cm-1 to lower frequency as
compared to the free ligands. For the dichloro com-
plexes 1 and 5 one M-Cl stretching vibration is observed
in the region 340-360 cm-1
. The broadness of this
signal is probably due to two overlapping M-Cl vibra-
tions, as for related cis-MCl2(NN) complexes (M ) Pd,
Pt; NN ) R-diimine ligand) two close-lying M-Cl
vibrations were reported.26
Tr a n sm eta la tion Rea ction s. Attempted substitu-
tion of one or two of the halide atoms by organic groups
in PdCl2(Ar-BIAN) (1a ) or PtCl2(Ar-BIAN) (5a ) by
(27) Chaudhury, N.; Puddephatt, R. J . J . Organomet.Chem. 1975,
84, 105.
(28) In DMF, an additional small reversible reduction peak was
observed at -0.70 V (10-20% relative to R1). As the ratio of this signal
did not increase at lower scan rates, it is not due to a species formed
upon the first reduction of PdCl2(Ph-BIAN). The most likely explana-
tion is a thermal reaction of PdCl2(Ph-BIAN) with DMF with formation
of the cationic complex PdCl(Ph-BIAN)(DMF)+Cl- coordinated by DMF.
In agreement with this assumption, we observed that the current of
this peak decreased when n-Bu4NCl (5-60 equiv relative to the
palladium complex) was added to the solution.
(25) van Asselt, R.; Elsevier, C. J .; Smeets, W. J . J .; Spek, A. L.
Inorg. Chem. 1994, 33, 1521.
(26) (a) van der Poel, H.; van Koten, G.; Vrieze, K. Inorg. Chem.
1980, 19, 1145. (b) Navarro-Ranninger, M. C.; Camazon, M. J .; Alvarez-
Valdes, A.; Masaguer, J . R.; Martinez-Carrera, S.; Garcia-Blanco, S.
Polyhedron 1987, 6, 1059. (c) Crociani, B.; Di Bianca, F.; Bertani, R.;
Zanotto, L. Inorg. Chim. Acta 1988, 141, 253. (d) Zaghal, M. H.; Qaseer,
H. A. Transition Met. Chem. 1991, 16, 39.

Divalent Palladium and Platinum Complexes
Organometallics, Vol. 16, No. 3, 1997 323
Ta ble 2. Electr och em ica l Da ta for P d (Ar -BIAN) Com p lexes a n d P h -BIANa
Epc(R0)/Epc(O0) (V)
Ph-BIAN
-1.27/-1.14
O0′/-0.93
Epc(R1)/Epc(O1) (V)
Epc(R2) (V) irr
PdCl2(Ph-BIAN) (1a )
-0.38/-0.29
-0.46/-0.39
-0.41/-0.31
-0.40/-0.35
-0.50/-0.42
-1.03
-1.11
-1.09
-1.14
-1.19
PdCl2(Ph-BIAN) (1a )b
PdCl2(p-Tol-BIAN) (1b)
PdCl2(p-Tol-BIAN) (1b)b
PdCl2(p-MeOC6H4-BIAN) (1c)b
Epc(R3)/Epc(O3) (V)
Epc(R4)Epc(O4) (V)
Pd(p-Tol-BIAN)(DMFU)
Pd(p-Tol-BIAN)(MA)
Pd(Me)I(p-Tol-BIAN)
-0.79/-0.71
-0.66/-0.58
-0.74/-0.59
-1.28/-1.19
-1.18/-1.10
-1.40/-1.32
a
The mechanisms of the reduction of the different complexes are similar. For simplification, their respective first and second reduction
potential peaks have the same notation, although their potentials differ. All peaks were reversible except when stated otherwise (irr )
irreversible). Oxidation and reduction peak potentials were determined versus SCE. Recorded as 2.0 mM solutions in THF at a scan
b
rate of 0.2 V s-1, at a gold-disk electrode (i.d. ) 5 mm) at 20 °C unless otherwise noted. Recorded in DMF under the same conditions.
below allow to discriminate between these species:
PdIICl2(Ph-BIAN)•- + 1e- f
PdICl(Ph-BIAN)•- + Cl- or
Pd0(Ph-BIAN)Clxx- + (2 - x)Cl-
x ) 0-2
The observation of two reduction peaks contrasts with
that made for PdCl2(PPh3)2 under similar conditions,14a-c
where only one irreversible reduction peak involving two
electrons was detected with generation of chloride-
ligated palladium(0) complexes.32
Electr och em ica l P r op er ties of P d 0(Ar -BIAN)-
F igu r e 1. Cyclic voltammetry performed in THF (0.3 M
n-Bu4NBF4) at a stationary gold-disk electrode (i.d. ) 0.5
mm) with a scan rate of 0.2 V s-1, at 20 °C: (a) PdCl2(Ph-
BIAN) (2 mM); (b) Ph-BIAN (2 mM).
(a lk en e) Com p lexes. In order to investigate the role
of R-diimine-ligated palladium(0) complexes in the
catalytic hydrogenation of alkenes, the electrochemical
properties of some Pd0(Ar-BIAN)(alkene) complexes
have been studied. The electrochemical data concerning
steady-state voltammetry at a ultramicroelectrode re-
vealed that one electron was involved in this step. Since
the reduction peak was reversible, the first reduction
step affords a radical anion:
(29) (a) The reduction peak was not completely reversible, since on
the reverse scan the two oxidation peaks O0 and O0′ were observed
(Table 1). The oxidation current peak of O0 was found to be smaller
relative to that of O0′ when the scan rate was increased; i.e., the
reduction peak was less reversible at high scan rate. This is charac-
teristic of an electron transfer that affords a species involved in an
equilibrium:
PdIICl2(Ph-BIAN) + 1e- / PdIICl2(Ph-BIAN)•- (6)
Epc(R1) ) -0.41 V
Ph-BIAN + 1e- / Ph-BIAN•- R0/O0
Ph-BIAN•- / [dimer]2-
The cyclic voltammogram of solely the ligand Ph-
BIAN19 in solution exhibited one reduction peak at
-1.27 V, which was partially reversible at the scan rate
of 0.2 V s-1 (Table 2, Figure 1b).29a Therefore, we can
assume that the first reduction at R1 is located on the
Ph-BIAN ligand (which is therefore more easily reduced
when ligated to the palladium29b), similar to the reduc-
tion reported for PtIICl2(R-diimine) complexes.30 Fur-
thermore, it is expected that the LUMO of complexes
of the type cis-MIICl2(diimine) (M ) Pd, Pt) is the *-
orbital of the diimine ligand.19c,31
The current of the second irreversible reduction peak
was comparable to that of the first one. Since the
diffusion coefficients are expected to be close for the
neutral and the monoreduced complexes, the second
reduction also involves one electron. This reduction
may afford several different species by cleavage of one
or two Pd-Cl bonds. Further experiments reported
[dimer]2- f [dimer]•- + 1e- O0′
For an example of reversible dimerization, see: Smith, C. R.; Utley,
J . H. P.; J . Chem. Res., Synop. 1982, 18. (b) The reduction of the ligand
Ph-BIAN on the palladium is fully reversible, whereas it was not for
the free ligand.29a When ligated on the palladium, the ligand Ph-
BIAN•- cannot be involved in the equilibrium reported above in ref
29a.
(30) (a) Braterman, P. S.; Song, J . I.; Vogler, C.; Kaim, W. Inorg.
Chem. 1992, 31, 222. (b) Braterman, P. S.; Song, J . I.; Wimmer, F. M.;
Wimmer, S.; Kaim, W.; Klein, A.; Peacock, R. D. Inorg. Chem. 1992,
31, 5084. (c) Vogler, C.; Schwederski, B.; Klein, A.; Kaim, W. J .
Organomet. Chem. 1992, 436, 367. (d) Klein, A.; Hausen, H. D.; Kaim,
W. J . Organomet. Chem. 1992, 440, 207.
(31) (a) Martin, M.; Krogh-J espersen, M. B.; Hsu, M.; Tewksbury,
J .; Laurent, M.; Viswanath, K.; Patterson, H. Inorg. Chem. 1983, 22,
647. (b) Kamath, S. S.; Uma, V.; Srivastava, T. S. Inorg. Chim. Acta
1989, 161, 49.
(32) In this case the reduction afforded three chloride-ligated
palladium(0) complexes involved in fast equilibrium [Pd0(PPh3)2Clx]n
,
x-
depending on the chloride ion and palladium concentration (n ) 1,
x )1, 2; n ) 2, x ) 2).14

324 Organometallics, Vol. 16, No. 3, 1997
van Asselt et al.
Ta ble 3. Electr och em ica l Da ta for Com p lexes
Obta in ed u p on Red u ction of P d Cl2(Ar -BIAN) (1a ,b)
in th e P r esen ce of Iod om eth a n e, Alk en es, a n d
P h -BIANa
direct scan
Epc(R5) Epc(R6)/
pc(O6) (V)
reverse scan
Epc(O4) Epc(O3)/
pc(R3) (V)
reacn
(V)
E
(V)
E
1a + MeI
-1.37
1a + DMFU
1a + DMFUb
1a + DMM
1a + DMMb
1a + MeAC
1a + MeACb
1a + Ph-BIAN
-1.24 -1.44/-1.30
-1.38
-1.13
-1.23
-1.12
-1.30
-1.18
-1.24
-1.15
-0.80/-0.69
n.d./-0.70
-1.21 -1.59/-1.46
-1.40 -1.70/-1.50
-1.28 n.d.
-1.31
-0.79/-0.68
n.d./-0.70
-0.87/-0.78
n.d./-0.80
-0.78/-0.80
-1.30
F igu r e 2. Cyclic voltammetry of Pd0(p-Tol-BIAN)(MA) (2
mM) in THF (0.3 M n-Bu4NBF4) at a stationary gold-disk
electrode (i.d. ) 0.5 mm) with a scan rate of 0.2 V s-1, at
20 °C.
1b + MeI
-1.02
1b + DMFUb
1b + DMFUb
-1.31 -1.42/-1.34
-1.32
-1.19
-1.25
-0.80/-0.71
n.d./-0.71
a
For simplification, the first and second reduction potential
peaks have the same notation, although their potentials differ.
All peaks were reversible except when stated otherwise. Oxidation
and reduction peak potentials were determined versus SCE.
the reduction of the zerovalent complexes Pd0(p-Tol-
BIAN)(DMFU) (DMFU ) dimethyl fumarate) and Pd0-
(p-Tol-BIAN)(MA)25 (MA ) maleic anhydride) have been
collected in Table 2. All palladium complexes showed
two reversible reduction peaks at R3 and R4 involving
one electron each (Figure 2). For these two complexes,
the first reduction is located on the Ar-BIAN ligand, as
in PdIICl2(Ar-BIAN) complexes, whereas the second
reduction is located on the alkene ligand, as reported
for comparable M0(R-diimine)(alkene) complexes (M )
Pd, Pt).33
Recorded as 2.0 mM solutions in THF at a scan rate of 0.2 V s-1
,
at a gold-disk electrode (i.d. ) 5 mm) at 20 °C unless stated
otherwise. Recorded in DMF under the same conditions.
b
corresponding to similar reactions but different com-
plexes have the same notation in every column, al-
though their respective potential values are different).
Typical cyclic voltammograms are represented in Fig-
ures 3 (reduction in the presence of the alkene) and 4
(reduction in the presence of Ph-BIAN). In all cases,
when PdIICl2(Ar-BIAN) was reduced in the presence of
an alkene or Ph-BIAN, the corresponding cyclic voltam-
mogram exhibited, besides the two one-electron-reduc-
tion peaks R1 and R2 of the PdCl2(Ar-BIAN) complex
(which remained similar to those observed in the
absence of alkene), two additional reduction peaks at
R5 and R6. The latter was due to the reduction of the
free alkene.34 Since some free alkene could be detected
on the cyclic voltammogram, it implies that the reaction
of the palladium complexes generated by the reduction
of PdIICl2(Ar-BIAN) with the alkene was not quantita-
tive within the time scale of the cyclic voltammetry.
When the scan direction after reduction was reversed
at a potential more negative than -1.4 V (after peak
R6), two oxidation peaks were observed at O4 and O3
(Figures 3a,c and 4a). By performing successive cyclic
scans, we observed that the oxidation peak O3 was
reversible (Figures 3a and 4a). We also observed that
peak R4, which is the reverse peak of O4, was located
at a slightly less negative potential than that observed
for peak R5 (Figures 3a and 4a).
A detailed investigation of the mechanism could be
achieved in the case of the reduction of PdCl2(p-Tol-
BIAN) in the presence of DMFU, since an authentic
sample of Pd0(p-Tol-BIAN)(DMFU) was available (Table
2). By comparison of the electrochemical data from
Tables 2 and 3, we noticed that the second reduction
peak R4 of Pd0(p-Tol-BIAN)(DMFU) was not observable
when PdCl2(p-Tol-BIAN) was reduced in the presence
of DMFU, demonstrating that Pd0(p-Tol-BIAN)(DMFU)
was not formed. Indeed, the reduction potential ob-
served at R5 (-1.31 V) was different from that observed
Pd0(p-Tol-BIAN)(DMFU) + 1e- /
Pd0(p-Tol-BIAN)(DMFU)•- R3/O3 (7)
Pd0(p-Tol-BIAN)(DMFU)•- + 1e- /
Pd0(p-Tol-BIAN)(DMFU)2- R4/O4 (8)
The equations for the reduction of Pd0(p-Tol-BIAN)-
(MA) are similar. The less negative reduction potential
observed for the coordinated ligand as compared to free
Ar-BIAN can be ascribed to donation of charge density
from the Ar-BIAN ligand to the palladium and concomi-
tant lowering of the ligand-centered LUMO, which is
in agreement with the observation that the reduction
potentials for the complex containing the more electron-
withdrawing alkene (MA) are less negative than those
of the DMFU complex.
No oxidation peaks of the complexes Pd0(Ar-BIAN)-
(alkene) were observed up to +1.5 V.
In the presence of excess DMFU, neither the potential
nor the current of the reduction of peaks R3 and R4 of
Pd0(p-Tol-BIAN)(DMFU) was affected. In the presence
of MA, substitution of DMFU by MA was observed, and
in the presence of DMM (dimethyl maleate) no changes
in the cyclic voltammogram were observed, as was
expected on the basis of earlier results.25
In Situ Rea ction of Alk en es or P h -BIAN w ith th e
Low -Va len t P a lla d iu m Com p lexes F or m ed u p on
Red u ction of P d IICl2(Ar -BIAN) Com p lexes. The
reduction of complexes 1a and 1b has been performed
in the presence of 1 equiv of either alkenes (DMFU,
DMM, MeAc (methyl acrylate)) or Ph-BIAN. The cor-
responding electrochemical data are collected in Table
3 (for simplification, all reduction or oxidation peaks
(34) The reduction peak potentials (in THF) of the alkenes employed
in this work are (corresponding oxidation peak potentials in paren-
theses): MA, -0.82 V (irr); DMFU, -1.44 V (-1.30 V); DMM, -1.55
V (-1.41 V); MeAC, -2.21 V (irr).
(33) Ito, N.; Saji, T. Aoyagui, S. Bull. Chem. Soc. J pn. 1985, 58, 2323.

Divalent Palladium and Platinum Complexes
Organometallics, Vol. 16, No. 3, 1997 325
F igu r e 3. Cyclic voltammetry performed in THF (0.3 M n-Bu4NBF4) at a stationary gold-disk electrode (i.d. ) 0.5 mm)
with a scan rate of 0.2 V s-1, at 20 °C: (a) reduction of PdCl2(p-Tol-BIAN) (2 mM) in the presence of DMFU (2 mM); (b)
same experiment as in (a) but the scan direction after reduction was reversed at -1.4 V, just after R2; (c) reduction of
PdCl2(Ph-BIAN) (2 mM) in the presence of DMM (2 mM).
and so the peak current of R2 should have a double
magnitude in the presence of DMFU. This was not
observed for DMFU (Figure 3a), for DMM (Figure 3c),
or for Ph-BIAN (Figure 4a). Moreover, as mentioned
above, the second reduction peak of Pd0(p-Tol-BIAN)-
(DMFU) at R4 was never observed on the first reduction
scan.
A second hypothesis would be that the two-step
reduction of PdCl2(p-Tol-BIAN) at R2 affords the mono-
valent palladium complex PdICl(p-Tol-BIAN)•- (reaction
9), which reacts with the olefin to produce the complex
PdICl(p-Tol-BIAN)(DMFU)•-, which is reduced at R5.
PdCl2(p-Tol-BIAN) + 2e- f
PdICl(p-Tol-BIAN)•- + Cl- at (R1 + R2) (9)
F igu r e 4. Cyclic voltammetry performed in THF (0.3 M
n-Bu4NBF4) at a stationary gold-disk electrode (i.d. ) 0.5
mm) with a scan rate of 0.2 V s-1, at 20 °C: (a) reduction
PdICl(p-Tol-BIAN)•- + DMFU f
of PdCl2(Ph-BIAN) (2 mM) in the presence of Ph-BIAN (2
mM); (b) same experiment as in (a) but the scan direction
after reduction was reversed at -1.1 V, just after R2; (c)
same experiment as in (b) but the potential was held for a
PdICl(p-Tol-BIAN)(DMFU)•- (10)
PdICl(p-Tol-BIAN)(DMFU)•- + 2e- f
while at -1.1 V.
Pd0(p-Tol-BIAN)(DMFU)2- + Cl- at R5 (11)
for R4 (-1.28 V) for the reduction of Pd0(p-Tol-BIAN)-
Note that in this framework, the peak R5 must involve
(DMFU). However, on the reverse scan we observed two
the exchange of two electrons because it yields the
dianion oxidized at O4. However, its magnitude is less
than that of R2 (1e) because reaction 10 is not quantita-
oxidation peaks at -1.19 and -0.80 V (Table 3) that
correspond to the oxidation peaks O4 (-1.19 V) and O3
(-0.80 V) of Pd0(p-Tol-BIAN)(DMFU)2- (Table 2, reac-
tive, as indicated by the presence of the free olefin
tions 7 and 8). Furthermore, the oxidation peak O3 was
reduced at R6. Therefore, the complex Pd0(p-Tol-BIAN)-
reversible (Figure 3a) as in the case of Pd0(p-Tol-BIAN)-
(DMFU)2- is generated at R5, and on the reverse scan
(DMFU) at R3 ) -0.71 V (Tables 2 and 3). The fact
one can observe its two reversible sequential oxidations:
that the oxidation peaks characteristic of Pd0(p-Tol-
BIAN)(DMFU)2- were observed on the reverse scan
Pd0(p-Tol-BIAN)(DMFU)2-
Pd0(p-Tol-BIAN)(DMFU)•- + 1e- O4/R4 (8′)
/
demonstrates that the complex Pd0(p-Tol-BIAN)(DMFU)
has been generated during the reduction of PdCl2(p-Tol-
BIAN) in the presence of DMFU. This second conclu-
sion seems to be in contradiction with that mentioned
above.
Pd0(p-Tol-BIAN)(DMFU)•-
/
Pd0(p-Tol-BIAN)(DMFU) + 1e- O3/R3 (7′)
A first hypothesis, i.e., that the reduction of PdCl2-
(p-Tol-BIAN) affords directly and quantitatively the
zerovalent palladium complex Pd0(p-Tol-BIAN) (or Pd0-
(p-Tol-BIAN)Clxx-; x ) 1, 2), which could react with
DMFU to afford Pd0(p-Tol-BIAN)(DMFU), can be easily
rejected. Indeed, the first reduction of the latter occurs
at -0.79 V, i.e. at a potential less negative than that
where it would be produced at R2. This would imply
that the second reduction peak of the bivalent complex
PdIICl2(p-Tol-BIAN) at R2 should involve two electrons
However, for a given scan rate, when the scan
direction was reversed at -1.2 V, i.e. after the reduction
at R2 but before the reduction at R5, the oxidation peak
O3 was already observed but its magnitude was much
lower as compared to the situation where the scan
direction was reversed at -1.7 V (i.e. after the reduction
at R5) (compare parts a and b of Figure 3). This shows
that some complex Pd0(p-Tol-BIAN)(DMFU)•- has been
generated during the time required to scan the potential

326 Organometallics, Vol. 16, No. 3, 1997
van Asselt et al.
Sch em e 2
between peaks R2 and O3. We have demonstrated above
(by ruling out the first hypothesis) that the complex Pd0-
(p-Tol-BIAN) was not produced directly and quantita-
tively at R2. Therefore, PdICl(p-Tol-BIAN)•-, which is
quantitatively produced at R2, slowly affords Pd0(p-Tol-
BIAN):
PdICl(p-Tol-BIAN)•-
f
Pd0(p-Tol-BIAN) + Cl- slow (12)
Its reaction with DMFU affords the complex Pd0(p-Tol-
BIAN)(DMFU), which is immediately reduced to Pd0-
(p-Tol-BIAN)(DMFU)•-, since the first reduction poten-
tial of Pd0(p-Tol-BIAN)(DMFU) is located at R3, i.e.
before the potential R2 where it starts to be produced:
Pd0(p-Tol-BIAN) + DMFU + 1e- f
Pd0(p-Tol-BIAN)(DMFU)•- R2 (13)
Sch em e 3
However, this overall processs is slow. The time
elapsed between R2 and R5 is very short compared to
that elapsed between R2 and O3. Hence, the complex
Pd0(p-Tol-BIAN)(DMFU)•- cannot be formed in ap-
preciable quantity during such a short time, which is
the reason that we do not observe its reduction peak R4
and also that the reduction peak R2 always involves
nearly one electron and not two.
In the case of the reduction of PdIICl2(Ph-BIAN) in
the presence of Ph-BIAN (1 equiv) reported in Figure
4, the reduction at R5 was necessary to produce the Pd0-
2-
(Ph-BIAN)2 complex oxidized at O4 and O3 (by reac-
tions similar to reactions 10 and 11), since the current
of O3 decreased drastically when the scan direction was
reversed at -1.1 V just after R2 instead of -2 V
(compare parts a and b of Figure 4). However, when
the potential was held at -1.1 V for a while, just before
inversion, the oxidation peak O3 was observed with a
higher magnitude on the reverse scan (Figure 4c).
During the duration of the potential hold, the following
reactions had time to proceed:
PdICl(Ph-BIAN)•-
f
Pd0(Ph-BIAN) + Cl- slow (12′)
Pd0(Ph-BIAN) + Ph-BIAN + 1e- f
•-
Pd0(Ph-BIAN)2 (13′)
•-
Pd0(Ph-BIAN)2
/
Pd0(Ph-BIAN)2 + 1e- O3/R3 (7′′)
The general mechanism of the reactions of alkenes
(or Ph-BIAN) with the low-valent palladium complexes
generated from the reduction of the bivalent PdIICl2-
(Ar-BIAN) can be summarized as in Scheme 3.
In Situ Rea ction of Or ga n ic Ha lid es w ith th e
P a lla d iu m Com p lexes F or m ed u p on Red u ction of
P d IICl2(Ar -BIAN) Com p lexes. The cyclic voltammo-
gram of the reduction of PdIICl2(Ar-BIAN) remained
These results show that the reaction of Pd0(Ar-BIAN)
with Ar-BIAN is slower than that with the alkenes.
Therefore, we can assume that the reduction of PdII-
Cl2(Ph-BIAN)•- afforded the monovalent palladium spe-
cies PdICl(Ph-BIAN)•- with one electron still located on
the ligand. The zerovalent palladium complex Pd0(Ph-
BIAN) (or Pd0(Ph-BIAN)Clxx-) is not produced directly
from the reduction of PdIICl2(Ph-BIAN)•- but is formed
from the PdICl(Ph-BIAN)•- complex35 by a slow in-
tramolecular reaction in which the electron is trans-
ferred from the ligand to the palladium(I). The overall
reduction of PdIICl2(Ph-BIAN) might be summarized as
in Scheme 2.
(35) The PdICl(Ph-BIAN)2 complex contains two paramagnetic
•-
entities but could not be characterized by ESR spectroscopy. Indeed,
the complex was produced during a short time, i.e., during the
voltammetric scan. In order to observe electrogenerated species in the
cavity of an ESR instrument, special equipment is required, which is
not available in our institutes.

Divalent Palladium and Platinum Complexes
Organometallics, Vol. 16, No. 3, 1997 327
potential than -1.4 V at which MePdIII(p-Tol-BIAN)2-
is formed. Moreover, the oxidation peak O4 of MePdII-
Cl(p-Tol-BIAN)2- should then be observed on the re-
verse scan, but it was not (Figure 5a). Therefore, we
can discriminate between reactions 15 and 16 and
consider that the reduction peak R5 most probably
corresponds to reaction 15.
Im p lica tion s to Ca ta lytic a n d Stoich iom etr ic
Rea ction s of P d 0(Ar -BIAN) Com p lexes. The in situ
observation of complexes of the type Pd0(Ar-BIAN)2,
which could not be synthesized or isolated in any other
way, lends support to our proposed mechanism for the
substitution of a coordinated Ar-BIAN ligand in a Pd0-
(Ar-BIAN)(MA) complex for another Ar′-BIAN ligand as
detailed elsewhere.25 This reaction may occur via initial
dissociation of the alkene and formation of a Pd0(Ar-
BIAN)(Ar′-BIAN) complex, which loses one of the two
Ar-BIAN ligands to generate the new Pd0(Ar′-BIAN)-
(MA) complex or to regenerate the starting complex.37
Furthermore, the observation of zerovalent Pd0(Ar-
BIAN)(alkene) complexes containing DMM and MeAc,
complexes which could not be isolated from reaction of
Pd0(DBA)2 or Pd03(TTAA)3 (DBA ) dibenzylideneac-
etone, TTAA ) tritoluylideneacetylacetone) with Ar-
BIAN in the presence of these alkenes, shows that these
alkenes are able to stabilize Pd0(Ar-BIAN)38 fragments
in solution. Similar species are expected to be formed
in solution during the hydrogenation of these alkenes
by Pd0(Ar-BIAN)(alkene) complexes, a reaction which
was shown to be homogeneous for alkenes containing
at least one electron-withdrawing substituent.39 For the
isolation of zerovalent complexes of the type Pd(Ar-
BIAN)(alkene), strongly electron-withdrawing alkenes
such as DMFU, MA, fumaronitrile, and tetracyanoeth-
ylene were necessary, whereas for alkenes containing
less electron-withdrawing substituents starting materi-
als and/or decomposition products were recovered after
attempted isolation. The electrochemical results show
that such complexes can be formed in solution, although
they are not isolable.
F igu r e 5. Cyclic voltammetry performed in THF (0.3 M
n-Bu4NBF4) at a stationary gold-disk electrode (i.d. ) 0.5
mm) with a scan rate of 0.2 V s-1, at 20 °C: (a) reduction
of PdCl2(p-Tol-BIAN) (2 mM) in the presence of MeI (2
mM); (b) Reduction of MePdI(p-Tol-BIAN) (2 mM).
unchanged in the presence of 1 equiv of PhBr or PhI,
demonstrating that no reaction occurred with either PdI-
Cl(Ar-BIAN)•- or Pd0(Ar-BIAN) during the time scale
of the cyclic voltammetry. This poor reactivity contrasts
with that of Pd0(PPh3)2 generated from the reduction
of PdIICl2(PPh3)2, which was found to be very reactive
with PhI and a little less with PhBr.14c However, a
reaction was observed with methyl iodide, a more
reactive organic halide. The cyclic voltammogram of the
reduction of PdIICl2(p-Tol-BIAN) in the presence of 1
equiv of MeI is given in Figure 5a. By comparison with
the cyclic voltammogram of MePdIII(p-Tol-BIAN)9 given
in Figure 5b, we can conclude that the latter was not
formed, since we did not observe its second reduction
peak at R4 (see Tables 2 and 3). In contrast, an
additional reduction peak was observed at R5 partially
overlapping with peak R2 (Figure 5a) that might result
from the oxidative addition of PdICl(p-Tol-BIAN)•- to
MeI.
PdICl(p-Tol-BIAN)•- + MeI f
MePdIIIICl(p-Tol-BIAN)•- (14)
Con clu sion
A series of divalent palladium and platinum com-
plexes containing the rigid Ar-BIAN and Ph-BIC ligands
has been synthesized and characterized; the PdIICl2(Ar-
BIAN) complexes show interesting electrochemical be-
havior. The two one-electron reductions of PdIICl2(Ar-
BIAN) complexes result in the formation of PdICl(Ar-
BIAN)•- complexes, which are able to react with alkenes,
Ar-BIAN ligands, and MeI but not with aryl halides
during the time scales investigated. The PdICl(Ar-
BIAN)•- complexes slowly afford some Pd0(Ar-BIAN)
complex which coordinates alkenes or Ar-BIAN, giving
Pd0(Ar-BIAN)(alkene) and Pd0(Ar-BIAN)2 complexes,
MePdIIIICl(p-Tol-BIAN)•- + 1e f
MePdIICl(p-Tol-BIAN)•- + I- at R5 (15)
or
MePdIIIICl(p-Tol-BIAN)•- + 2e f
MePdIICl(p-Tol-BIAN)2- + I- at R5 (16)
When the scan direction was reversed after R5, an
oxidation peak was observed at +0.485 V, featuring the
oxidation of iodide ions liberated in reaction 15 or 16.
The two reduction steps of MePdIII(p-Tol-BIAN) were
found to be reversible and of course did not produce any
iodide anion, the oxidation of the latter being not
observed on the reverse scan (compare parts a and b of
Figure 5).
(36) The reduction peak potentials of ArPdX(PPh3)2 complexes are
in the following order: I < Br < Cl.14d The same order was found for
PdX2(PPh3)2.14c
(37) In view of some current studies involving Ar-BIAN and similar
rigid ligands we cannot, however, exclude the possibility that one
nitrogen of the Ar-BIAN dissociates, followed by addition of another
Ar′-BIAN molecule, generating as an intermediate a Pd(Ar-BIAN)(Ar′-
BIAN)(alkene) complex in which two BIAN ligands are (monodentate)
coordinated.
The reduction of the complex MePdIICl(p-Tol-BIAN)•-
is expected to occur at a more negative potential than
that of MePdIII(p-Tol-BIAN)•-.36 Were reaction 16
operating, it would imply that MePdIICl(p-Tol-BIAN)2-
would be produced at R5 (-1.02 V), i.e. at a less negative
(38) Complexes of the formula “Pd0(Ar-BIAN)” are expected to be
extremely reactive toward oxidative addition, due to the coordinative
unsaturation and the bent geometry of the complex.15b
(39) van Asselt, R.; Elsevier, C. J . J . Mol. Catal. 1991, 65, L13.

328 Organometallics, Vol. 16, No. 3, 1997
van Asselt et al.
supporting the proposed mechanisms for ligand ex-
change reactions25 and homogeneous alkene hydrogena-
tion reactions.39 In neither case can the complexes
Pd0(Ar-BIAN)(alkene), Pd0(Ar-BIAN)2, and MePdIII(Ar-
BIAN) be obtained directly by reduction in cyclic vol-
tammetry, because they are all generated at a potential
which is more negative than their first reduction step.
In all cases they are then generated in a state corre-
sponding to their first or second reduction. Neverthe-
less, electrochemistry has provided evidence that species
which cannot be generated via synthetic routes, such
as Pd0(Ar-BIAN)(MeAc) and Pd0(Ar-BIAN)2, are indeed
accessible. In contrast to the reduction of PdIICl2-
(PPh3)2, which affords quantitatively the zerovalent
complex Pd0(PPh3)2 ligated by chloride anions, the
reduction of PdIICl2(Ar-BIAN) complexes does not afford
directly and quantitatively Pd0(Ar-BIAN) complexes.
This is due to the ability of the Ar-BIAN ligand to accept
one electron, whereas this is not the case for the
triphenylphosphine ligand, where the electrons are not
transferred to the ligand but remain on the palladium
center.
Ack n ow led gm en t. R.v.A. and C.J .E. thank Prof. K.
Vrieze for his support of this work and the Netherlands
Foundation for Chemical Research (SON) and the
Netherlands Organization of Scientific Research (NWO)
for financial support of R.v.A.; C.A. and A.J . acknowl-
edge support of this work by the CNRS (URA 1679 and
MESR (ENS)).
OM960578A