Synthesis of Ar-BIAN ligands (Ar-BIAN = bis(aryl)acenaphthenequinonediimine) having strong electron-withdrawing substituents on the aryl rings and their relative coordination strength toward palladium(0) and-(II) complexes

Article abstract of DOI:10.1021/om020147u

The synthesis of Ar-BIAN ligands (Ar-BIAN = bis(aryl)acenaphthenequinonediimine) having strong electron-withdrawing substituents on the aryl ring is reported. Most of these derivatives had escaped isolation in a pure form up to now. A quantitative scale of coordination strength of the newly synthesized ligands in the complexes Pd⁰(L)(DMFU) (DMFU = dimethylfumarate) and Pd^II(L)(OAc)₂ has been measured. The series also includes some previously known Ar-BIAN ligands, phenanthroline, bipyridine, and Ph-DAB (Ph-DAB = diphenyldiazabutadiene). A good correlation is observed for the Ar-BIAN ligands between the Hammet σ constants of the substituents of the aryl group and the relative binding constant with respect to Ph-BIAN. The values for the p constant for the Pd(L)(DMFU) and Pd(L)(OAc)₂ series are respectively -1.57 and -3.44, indicating that electron-rich ligands bind more strongly in both series, but the effect is much stronger in the Pd(II) series. The observed effect is relevant to the question of the effective oxidation state of palladium in the Pd(L)(DMFU) complex. Phenanthroline and bipyridine are stronger ligands than any Ar-BIAN compound, whereas Ph-DAB is the weakest ligand of all. The sterically hindered 2,6-Pr₂ⁱC₆H₆-BIAN binds less strongly than its basicity would suggest.

Full text of DOI:10.1021/om020147u

2950
Organometallics 2002, 21, 2950-2957
Syn th esis of Ar -BIAN Liga n d s (Ar -BIAN )
Bis(a r yl)a cen a p h th en equ in on ed iim in e) Ha vin g Str on g
Electr on -With d r a w in g Su bstitu en ts on th e Ar yl Rin gs
a n d Th eir Rela tive Coor d in a tion Str en gth tow a r d
P a lla d iu m (0) a n d -(II) Com p lexes
Michela Gasperini, Fabio Ragaini,* and Sergio Cenini
Dipartimento di Chimica Inorganica, Metallorganica e Analitica and INSM-CNR,
V. G. Venezian 21, I-20133 Milano, Italy
Received February 21, 2002
The synthesis of Ar-BIAN ligands (Ar-BIAN ) bis(aryl)acenaphthenequinonediimine)
having strong electron-withdrawing substituents on the aryl ring is reported. Most of these
derivatives had escaped isolation in a pure form up to now. A quantitative scale of
coordination strength of the newly synthesized ligands in the complexes Pd0(L)(DMFU)
(DMFU ) dimethylfumarate) and PdII(L)(OAc)2 has been measured. The series also includes
some previously known Ar-BIAN ligands, phenanthroline, bipyridine, and Ph-DAB (Ph-DAB
) diphenyldiazabutadiene). A good correlation is observed for the Ar-BIAN ligands between
the Hammet σ constants of the substituents of the aryl group and the relative binding
constant with respect to Ph-BIAN. The values for the F constant for the Pd(L)(DMFU) and
Pd(L)(OAc)2 series are respectively -1.57 and -3.44, indicating that electron-rich ligands
bind more strongly in both series, but the effect is much stronger in the Pd(II) series. The
observed effect is relevant to the question of the effective oxidation state of palladium in
the Pd(L)(DMFU) complex. Phenanthroline and bipyridine are stronger ligands than any
Ar-BIAN compound, whereas Ph-DAB is the weakest ligand of all. The sterically hindered
2,6-Pri C6H3-BIAN binds less strongly than its basicity would suggest.
2
Sch em e 1
In tr od u ction
Compounds of the family Ar-BIAN (Ar-BIAN ) bis-
(aryl)acenaphthenequinonediimine) (Scheme 1) have
been known for some time,1,2 but have been brought to
general attention only in recent years by Elsevier and
his group.3 Since then, they have found widespread use
as ligands for palladium, ruthenium, and nickel and the
corresponding complexes have been employed as cata-
lysts for a wide variety of reactions, such as alkene
hydrogenation,4 polymerization,5 aziridination,6 and
cyclopropanation,6 alkene-CO copolymerization,7 alkyne
coupling in the presence of halogens or organic halides
and tin compounds8 or just tin compounds,9 selective
hydrogenation of alkynes to alkenes,10 allylic aminations
of olefins by nitroarenes in the presence of CO,11,12 the
synthesis of pyrroles and oxazines from dienes, ni-
troarenes, and CO,12 the reduction of nitroarenes to
anilines by CO/H2O,13 the cross-coupling reaction be-
tween organic halides and organomagnesium, -zinc, and
-tin reagents,14 the Suzuki-Miyaura cross coupling,15
* Corresponding author. Fax: (39)-02-50314405. E-mail: fabio.
ragaini@unimi.it.
(1) Dvolaitzky, M. C. R. Acad. Sci. Paris, Ser. C 1969, 268, 1811;
Chem. Abstr. 1969, 71, 61566b.
(2) Matei, I.; Lixandru, T. Bul. Ist. Politeh. Iasi 1967, 13, 245; Chem.
Abstr. 1969, 70, 3623m.
(3) van Asselt, R.; Elsevier, C. J .; Smeets, W. J . J .; Spek, A. L.;
Benedix, R. Recl Trav. Pays Bas 1994, 113, 88.
(4) van Asselt, R.; Elsevier, C. J . J . Mol. Catal. 1991, 65, L13.
(5) (a) Ittel, S. D.; J ohnson, L. K.; Brookhart, M. Chem. Rev. 2000,
100, 1169, and references therein. (b) Svejda, S. A.; Brookhart, M.
Organometallics 1999, 18, 65. (c) Tempel, D. J .; J ohnson, L. K.; Huff,
R. L.; White, P. S.; Brookhart, M. J . Am. Chem. Soc. 2000, 122, 6686.
(6) Llewellyn, D. B.; Adamson, D.; Arndtsen, B. A. Org. Lett. 2000,
2, 4165.
(7) Groen, J . H.; Delis, J . G. P.; van Leeuwen, P. W. N. M.; Vrieze,
K. Organometallics 1997, 16, 68.
(8) (a) van Belzen, R.; Hoffmann, H.; Elsevier, C. J . Angew. Chem.,
Int. Ed. Engl. 1997, 36, 1743. (b) van Belzen, R.; Klein, R. A.; Kooijman,
H.; Veldman, N.; Spek, A. L.; Elsevier, C. J . Organometallics 1998,
17, 1812.
(9) Shirakawa, E.; Yoshida, H.; Nakao, Y.; Hiyama, T. J . Am. Chem.
Soc. 1999, 121, 4290.
(10) van Laren, M. W.; Elsevier: C. J . Angew. Chem., Int. Ed. 1999,
38, 3715.
(11) (a) Cenini, S.; Ragaini, F.; Tollari, S.; Paone, D. J . Am. Chem.
Soc. 1996, 118, 11964. (b) Ragaini, F.; Cenini, S.; Tollari, S.; Tummo-
lillo, G.; Beltrami, R. Organometallics 1999, 18, 928.
(12) Ragaini, F.; Cenini, S.; Borsani, E.; Dompe´, M.; Gallo, E.; Moret,
M. Organometallics 2001, 20, 3390.
(13) (a) Ragaini, F.; Cenini, S.; Tollari, S. J . Mol. Catal. 1993, 85,
L1. (b) Ragaini, F.; Cenini, S.; Gasperini, M. J . Mol. Catal. (A) 2001,
174, 51.
10.1021/om020147u CCC: $22.00 © 2002 American Chemical Society
Publication on Web 06/08/2002

Synthesis of Ar-BIAN Ligands
Organometallics, Vol. 21, No. 14, 2002 2951
the synthesis of 4-quinolones and 2,3-dihydroquinolones
from 2′-nitrochalcones and CO,16,17 and the synthesis
of indoles from o-nitrostyrenes and CO.17,18 For some
of these syntheses, the use of the Ar-BIAN ligands was
instrumental to achieving high performances of the
catalytic system. With respect to most diimine ligands,
BIAN derivatives are more rigid, this rigidity both
imposing the correct geometry for coordination and most
of all imparting a high chemical stability both with
respect to hydrolysis and rupture of the central C-C
bond. The latter is a common problem with most diimine
ligands and prevents their use as ligand for many
catalytic systems when long catalyst lives are a requi-
site, as is always the case for industrial applications.
Despite the utility of these ligands, only Ar-BIAN
compounds having electron-releasing or moderately
electron-withdrawing groups on the aryl ring have been
reported in the literature. Among those reported, the
one having the most electron-withdrawing substituent
is 4-CF3C6H4-BIAN5b (pKa of 4-CF3C6H4NH3+ ) 2.75;19
all pKa values quoted in the following are measured in
water at 25 °C), which was synthesized by refluxing
acenaphthenequinone and 4-CF3C6H4NH2 in toluene
with a Dean-Stark apparatus for 5 days.20 The same
procedure failed to afford the corresponding ligand with
rare molecules, even if the field is not limited to the
BIAN skeleton.
Given the high importance that bidentate chelating
nitrogen ligands have gained in homogeneous cataly-
sis,21 it is surprising that very little quantitative infor-
mation exists on their relative binding strength to
transition metals. Several papers, especially in the older
literature, have been devoted to phenanthroline (Phen)
and bipyridine (Bipy) complexes,22 but all of the avail-
able data were measured in aqueous solution, where
protonation of the ligand becomes important. Thus this
information is difficult to transfer to the low- to medium-
polarity, often anhydrous, solvents commonly employed
for catalytic reactions. The effect of steric hindrance on
the identity of the most stable complex has been
examined for some R-DAB complexes,23 but electronic
effects were not investigated. To the best of our knowl-
edge, no quantitative binding strength scale exists for
chelating nitrogen ligands toward a transition metal
and in an organic solvent. We have thus managed to
measure such a scale for our new ligands, together with
others already known in the literature, and the results
are also reported here.
Resu lts a n d Discu ssion
a 4-nitro group (pKa of 4-NO2C6H4NH3 ) 1.0021). The
+
Syn t h esis of Ar -BIAN Liga n d s w it h Ar ) 4-
NO2C6H 4-, 4-NCC6H 4-, 3,5-Cl2C6H 3-, 3,5-(NO2)2-
C6H3-. The ZnCl2 complexes of the ligands having the
4-NO2-, 4-CN-, 3,5-Cl2-, and 3,5-(NO2)2- substituents
on the aryl group of the starting aniline (the pKa values
for the corresponding anilinium salts are respectively
1.00,24 1.74,25 2.37,26 and 0.22927) can be easily synthe-
sized by the known procedure employing the reaction
of a 15% molar excess of the aniline with acenaphthene-
quinone and an almost 3-fold molar excess of ZnCl2 in
refluxing acetic acid.2,3 The desired complex precipitates
in high yields from the hot solution that is filtered while
hot. During this work we have noted that the excess
aniline is partly transformed into the corresponding
acetanilide by reaction with the solvent. Especially the
derivative of 3,5-dicloroaniline is slightly soluble and
partly coprecipitates. To remove these impurities it is
convenient to suspend the precipitate in hot acetic acid
for at least 15 min and filter it again, rather then simply
washing it with hot acetic acid, as originally reported.
The formation of acetanilides has other important
consequences and will be further discussed below.
The well-dried precipitate is then suspended in
CH2Cl2, and an aqueous solution of sodium or potassium
oxalate is added in a separating funnel. Shaking for a
few minutes results in complete dissolution of the
colored complex, with concomitant formation of colorless
ZnCl2 or NiBr2 complexes of the latter can be obtained
if the corresponding salts are present in the reaction
mixture (acetic acid as solvent),1-3 but previous at-
tempts to liberate the free ligand from the templating
metal by refluxing the complex with ethanolic K2CO3
(for ZnCl2) or treating it with sodium cyanide (for NiBr2)
led to extensive hydrolysis.1,3
Most of the catalytic systems previously mentioned
are sensitive to the electron-donating or -withdrawing
power of the substituents on the aryl ring, and electron-
withdrawing substituents are beneficial in some cases,
for example in the technologically important polymer-
ization of ethylene by nickel catalysts.5b In this paper
we report that simply applying our previously reported11b
protocol for the liberation of Ar-BIAN ligands from
ZnCl2 allows the effective synthesis (without hydrolysis)
not only of 4-NO2C6H4-BIAN but also of other BIANs
that are even more electron-poor, such as the derivative
having two nitro groups in the 3 and 5 positions
(Scheme 1). In some cases, a modification of the previ-
ously reported procedure for the synthesis of the ZnCl2
complex was required, which is also discussed. It should
be noted that chelating diimine ligands having strong
electron-withdrawing groups on the aryl rings are very
(14) (a) van Asselt, R.; Elsevier: C. J . Organometallics 1992, 11,
1999. (b) van Asselt, R.; Elsevier, C. J . Tetrahedron 1994, 50, 323. (c)
van Asselt, R.; Elsevier: C. J . Organometallics 1994, 13, 1972.
(15) Grasa, G. A.; Hillier, A. C.; Nolan, S. P. Org. Lett. 2001, 3, 1077.
(16) (a) Tollari, S.; Cenini, S.; Ragaini, F.; Cassar, L. J . Chem. Soc.,
Chem. Commun. 1994, 1741. (b) Annunziata, R.; Cenini, S.; Palmisano,
G.; Tollari, S. Synth. Commun. 1996, 26, 495.
(22) (a) McWhinnie, W. R.; Miller, J . D. Adv. Inorg. Chem. Radio-
chem. 1969, 12, 135. (b) For a more recent reference see: Steinhouser,
R. K.; Kolopajlo, L. H. Inorg. Chem. 1985, 24, 1845.
(17) Ragaini, F.; Sportiello, P.; Cenini, S. J . Organomet. Chem. 1999,
577, 283.
(18) Cenini, S.; Bettetini, E.; Fedele, M.; Tollari, S. J . Mol. Catal.
1996, 111, 37.
(19) Sheppard, W. A. J . Am. Chem. Soc. 1962, 84, 3072.
(20) The compound 3,5-(CF3)2C6H3-BIAN was mentioned in a table
in a paper,9 but its synthesis was not described.
(21) (a) Togni, A.; Venanzi, L. M. Angew. Chem., Int. Ed. Engl. 1994,
33, 497. (b) Fache, F.; Schulz, E.; Tommasino, M. L.; Lemaire, M. Chem.
Rev. 2000, 100, 2159. (c) Fache, F.; Dunjic, B.; Gamez, P.; Lemaire,
M. Top. Catal. 1998, 4, 201. (d) Ghosh, A. K.; Mathivanan, P.;
Cappiello, J . Tetrahedron Asymm. 1998, 9, 1.
(23) (a) van der Poel, H.; van Koten, G.; Kokkes, M.; Stam, C. H.
Inorg. Chem. 1981, 20, 2941. (b) van der Poel, H.; van Koten, G.; Vrieze,
K. Inorg. Chim. Acta 1981, 51, 241. (c) van der Poel, H.; van Koten,
G.; Vrieze, K. Inorg. Chim. Acta 1981, 51, 253. (d) For more information
on the chemistry of R-DAB ligands see: van Koten, G.; Vrieze, K. Adv.
Organomet. Chem. 1982, 21, 151, and references therein.
(24) Tickle, P.; Briggs, A. G.; Wilson, J . M. J . Chem. Soc. (B) 1970,
65.
(25) Fickling, M. M.; Fisher, A.; Mann, B. R.; Packer, J .; Vaughan,
J . J . Am. Chem. Soc. 1959, 81, 4226.
(26) Bolton, P. D.; Hall, F. M. J . Chem. Soc. (B) 1970, 1247.
(27) Kvita, V.; Sauter, H.; Rihs, G. Helv. Chim. Acta 1985, 68, 1569.

2952 Organometallics, Vol. 21, No. 14, 2002
Ta ble 1. 1H NMR Sp ectr a of Ar -BIAN Liga n d sa
Gasperini et al.
R
H1
H2
H3
H4
H5
7.81 d
(J ) 8.5)
8.4 d (J ) 8.3)
H6
H7
H8
4-CN
6.88 d
(J ) 7.2)
6.93 d
(J ) 7.1)
6.86 d
(J ) 7.2)
7.03 d
(J ) 7.5)
6.89 d
(J ) 7.2)
6.92 d
7.49 pst
8.02 d
(J ) 8.3)
8.04 d
(J ) 8.2)
7.98 d
(J ) 8.3)
8.02 d
(J ) 8.3)
8.05 d
(J ) 8.3)
7.9 d
7.24 d
(J ) 8.5)
7.28 d
) H5
) H5
) H4
4-NO2
7.50 pst
7.81 pst
7.54 pst
7.52 pst
7.40 pst
) H4
7.44 s
) H4
) H4
) H4
(J ) 8.3)
7.57 d
3-CF3
7.63 pst
7.35 d
(J ) 7.5)b
7.05 d
(J ) 7.6)b
7.29 t
3,5-Cl2
3,5-(CF3)2
(J ) 1.9)
7.63 s
(J ) 1.9)
7.83 s
c
3,5-Me2
6.75 s
6.9 s
(J ) 6.9)
(J ) 8.2)
a
Recorded in CDCl3 at room temperature; 300 MHz. Values are in ppm relative to TMS, J values are in Hz. 3,5-(NO2)2C6H3-BIAN was
too insoluble to allow recording of the 1H NMR spectrum. b CF3 group at position 7 in the drawing. Assignment was made by two-dimensional
1H NMR (COSYDFTP), but the attribution of the H4 and H6 signals is not unequivocal and may be reversed. c In addition to the signals
reported in the table, a singlet at δ 2.38 was observed for the methyl groups.
Ta ble 2. IR Da ta a n d Mp ’s for Ar -BIAN Liga n d s
ZnC2O4. Oxalate is preferred with respect to carbonate,
as the former is less basic, but, at the same time, zinc
oxalate is less soluble than zinc carbonate. The free
ligand is isolated in a pure form from the CH2Cl2 layer.
In general, no hydrolysis was observed, as indicated in
particular by the absence of any absorption over 1700
cm-1 in the IR spectrum of the free ligands. Only in the
case of 3,5-Cl2C6H3-BIAN was a small amount of the
monoimine detected after decoordination. This impurity
is present in a small amount and does not prevent
employing the crude ligand in most cases. A complete
purification can be achieved by column chromatography
on silica. In the case of 3,5-(NO2)2C6H3-BIAN, not only
its zinc complex but even the pure ligand was only
slightly soluble in CH2Cl2. In this case decoordination
was best preceded by an irradiation of the biphasic
mixture by ultrasound (in an ultrasound cleaning bath)
in order to disaggregate the solid particles at best. The
ligand was finally recovered by filtration. It is only
sparingly soluble in most solvents.
R
ν
CdNa
other IR dataa
mp (°C)
4-CN
4-NO2
4-CF3
3-CF3
3,5-Cl2
3,5-(NO2)2
1671, 1646
1674, 1654
1665, 1645
1669, 1644
1673, 1649
1669, 1646
2224 (νCN
)
290
>300
241
130
214
1504, 1345 (νNO2)
1638, 1616, 1539, 1533,
1361, 1343 (νNO2
>300
)
3,5-(CF3)2
3,5-Me2
1675, 1652
1665, 1635
186
219
a
In Nujol, values in cm-1
.
the dissolution of ZnCl2), the corresponding complexes
all precipitated from the hot solution in good yields. The
free ligands were obtained as described before without
any problem.
The spectroscopic data and melting points for the
ligands obtained are reported in Tables 1 and 2. The
1H NMR spectrum of 4-CF3C6H4-BIAN was identical to
that reported in ref 5b. Also included in the table are
the data for 3,5-(CH3)2C6H3-BIAN, which has not been
reported in the literature before. This ligand was
synthesized following the standard protocol (i.e., without
toluene) in order to use it in the relative coordination
strength studies, as detailed below.
The limit of the ZnCl2-promoted synthesis of the
Ar-BIAN ligands (not of the following decoordination)
was reached with the very acidic 2,6-(NO2)2C6H3NH2
(pKa ) -5.3724) and C6F5NH2 (pKa ) -0.3628). With
these amines, no condensation product could be obtained
by the procedures reported above.
Com m en ts on th e Role of Zn Cl2 a n d th e F or m a -
tion of Aceta n ilid es. Transition metal salts have long
been used as promoters for the formation of imines from
amines and ketones.29 Although a catalytic amount is
often sufficient in the case of monoimine formation, a
Syn t h esis of Ar -BIAN Liga n d s w it h Ar
)
4-CF 3C6H4-, 3-CF 3C6H4-, 3,5-(CF 3)2C6H3-. When
the previously described protocol for the synthesis of
ZnCl2(Ar-BIAN) complexes is applied to 3-CF3C6H4NH2
(pKa ) 3.2019), an acceptable yield of 70% in the desired
complex is still obtained, but with its 4 isomer (pKa )
2.7519), the yields drop to about 40%, and with the 3,5-
disubstituted aniline (pKa ) 1.1527), the hot solution was
perfectly homogeneous and only traces of 3,5-(CF3)2C6H3-
BIAN were obtained by workup, the reaction affording
a mixture of the monoimine and 3,5-(CF3)2C6H3NHC-
(O)CH3. The pKa of 3,5-(CF3)2C6H3NH2 is comprised
between those of 3,5-Cl2C6H3NH2 and 4-NO2C6H4NH2,
both of which afford the corresponding complex in good
yields. Thus the failure to obtain the desired complex
cannot be related to the poor basicity of the aniline
and must be connected with its high solubility in the
reaction medium. Indeed, when the reactions of the CF3-
containing anilines were repeated by adding some
toluene to the reaction mixture (2 mL for 7.5 mL of
AcOH; such a low amount of toluene does not prevent
(28) Yates, K.; Shapiro, S. A. Can. J . Chem. 1972, 50, 581.
(29) (a) Sollenberger, P. Y.; Martin, R. B. In The Chemistry of the
Amino Group; Patai, S., Ed.; Interscience Publishers: London, 1968;
p 349, and references therein. (b) van Koten, G.; Vrieze, K. Adv.
Organomet. Chem. 1982, 21, 151, and references therein.

Synthesis of Ar-BIAN Ligands
Organometallics, Vol. 21, No. 14, 2002 2953
stoichiometric amount is necessary to obtain diimines,
the metal remaining linked to the so formed diimine,
which acts as a ligand. This last promoting effect has
been generally attributed to a “templating” effect or to
the stabilization of the diimine by its coordination to
the metal, adding a thermodynamical bias toward the
condensation. However, the results obtained in this
work and some other unpublished results obtained in
our group when the analogous condensation employing
camphorquinone in place of acenaphthenequinone was
studied show that, at least in these cases, the key point
is not the formation of the complex as such, but the fact
that these complexes are usually less soluble in the
reaction medium than the starting reagents. Precipita-
tion of the final product thus appears to be the driving
force of the double condensation. If formation of the
complex were determining, then the complexes should
be obtained by workup of the reaction solutions even
when they do not spontaneously precipitate. However,
in our experience this never happens. In all cases in
which the final complex did not precipitate, it was
formed in only minute amounts. This observation has
important consequences for the design or modification
of the synthesis of chelating diimines. Indeed a low
solubility of the resulting complex with respect to the
starting materials should be looked for in selecting both
the metal salt and the solvent mixture. We have
successfully applied this principle to the synthesis of the
CF3-containing BIAN ligands. Indeed, toluene dissolves
well the intermediate monoimine and the starting
quinone, but acts as a precipitating agent toward the
Ar-BIAN complexes. Although we have not indepen-
dently optimized the addition of toluene to all reagent
combinations, it is likely that the addition of toluene is
also beneficial even in the synthesis of other Ar-BIAN
ligands. It should be also noted that now that the
solubility issue has been evidenced, it is no longer
further stresses the importance of the insolubility of the
BIAN-metal salt complex in order to drive the reaction
in the correct direction.
Finally, a comment on the solubility of the 3,5-
(CF3)2C6H3-BIAN ligand is worthwhile. This molecule
and especially its complexes display a very different
solubility in different solvents with respect to complexes
derived from non-fluorinated BIAN ligands. For ex-
ample, 0.05 mmol of ZnCl2(Ph-BIAN) requires 2.5, 10,
and 38 mL, respectively, of CH2Cl2, THF, and methanol
to completely dissolve. The same molar amount of the
corresponding complex ZnCl2(3,5-(CF3)2C6H3-BIAN) re-
quires 21.7, 3, and 3 mL of the same solvents to dissolve.
Thus the fluorinated complex is almost 10 times less
soluble in dichloromethane, but 12 times more so in
methanol.
We are now exploring the use of the “solubility
criterion” to synthesize other chelating diimine ligands
that have eluded isolation up to now.
Rela tive Bin d in g Str en gth of th e Liga n d s. Dur-
ing our previous studies on the use of Ru(CO)3(Ar-BIAN)
complexes to catalyze the allylic amination of olefins by
11b
nitroarenes,
we noticed that Ph-BIAN coordinates
to the Ru(CO)3 fragment more strongly than the more
basic Tol-BIAN, and indirect evidence based on chem-
ical reactivity indicates12 that the even more basic
4-MeOC6H4-BIAN is the most weakly coordinating lig-
and in the series. This suggests that some of the newly
reported compounds may be quite strong ligands for low-
valent metal complexes. Unfortunately, several theo-
retical and technical problems render the Ru(CO)3(Ar-
BIAN) system unsuitable for the determination of even
relative coordination constants in an accurate and quan-
titative way. We thus decided to examine the relative
coordinating ability of the new ligands and of others
previously reported toward two palladium complexes,
respectively in the 0 and 2 formal oxidation states.
As a palladium(0) series we chose the complexes
Pd(L)(DMFU) (L ) bidentate chelating nitrogen ligand,
DMFU ) dimethylfumarate). Several complexes of the
kind Pd(Ar-BIAN)(olefin) have been reported by Elsevi-
er and were shown to undergo both olefin and ligand
exchange.31 However, the only system for which a
quantitative measure of the ligand exchange was re-
corded was the Pd(Tol-BIAN)(maleic anhydride) + 2,6-
surprising that 2,6-Pri C6H3-BIAN can be obtained from
2
the quinone and the aniline in acetic acid even in the
absence of metal salts. Indeed it is the only Ar-BIAN
among those reported to be sufficiently insoluble in
acetic acid to precipitate almost quantitatively from the
hot solution.30
Concerning the formation of the acetanilides, a fur-
ther observation was made. In an attempt to further
increase the yield of the synthesis of 3,5-(CF3)2C6H3-
BIAN, the reaction time for the reaction also including
toluene was increased from 45 min to 2 h. In the lat-
ter case, the initially formed precipitate completely
redissolved as the reaction proceeded. After 2 h, the
solution did not contain any more 3,5-(CF3)2C6H3-
BIAN, and even the monoimine could not be detected.
The only products found were acenaphthenequinone,
3,5-(CF3)2C6H3NHC(O)CH3, and 3,5-(CF3)2C6H3NH2. The
water formed during the formation of the acetanilide
clearly had hydrolyzed the BIAN ligand. This indicates
that in a homogeneous system the acetanilide, not the
BIAN ligand, is the thermodynamically favored product,
although not necessarily the kinetic one. This fact
Pri C6H3-BIAN one. In this work, we selected DMFU
2
as the olefin because it is the least electron-withdrawing
(and thus the least perturbing for the oxidation state
of the metal) among all of the ones that allowed the
complex to be isolated (electron-withdrawing groups on
the olefin are anyway required to this aim). The corre-
sponding complexes with Phen (1,10-phenanthroline)
and Bipy (2,2′-bipyridine) have also been reported in the
literature.32 The complex Pd(Tol-BIAN)(DMFU) has
been reported to decompose in solution.33 We noted that
addition of excess DMFU (5 equiv with respect to the
complex) to the solution strongly stabilizes the complex,
and decomposition is only observable after several
hours, a time longer than that required for our experi-
ments.
(30) It can be speculated that the solubility of the other Ar-BIAN
compounds in acetic acid is largely due to the formation of hydrogen
bonds between the acidic hydrogen and the imine nitrogens. Since the
latter are strongly shielded in the bis-isopropyl derivative, this
interaction is lacking or is very weak and the ligand precipitates.
However, this explanation does not limit the validity of the concepts
expressed above.
(31) van Asselt, R.; Elsevier, C. J .; Smeets, W. J . J .; Spek, A. L.
Inorg. Chem. 1994, 33, 1521.
(32) Ito, Ts.; Hasegawa, S.; Takahashi, Y.; Ishii, Y. J . Organomet.
Chem. 1974, 73, 401.
(33) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165.

2954 Organometallics, Vol. 21, No. 14, 2002
Gasperini et al.
Ta ble 3. Rela tive Coor d in a tion Str en gth of
Sever a l Ch ela tin g Nitr ogen Liga n d s tow a r d
P a lla d iu m (0) a n d -(II) Com p lexes
aniline
pKa
Keq
Keq
Pd(L)(DMFU)a Pd(L)(OAc)2
b
ligand
σ
4-MeOC6H4-BIAN
4-MeC6H4-BIAN
3,5-Me2C6H3-BIAN
Ph-BIAN
4-ClC6H4-BIAN
3-CF3C6H4-BIAN
4-CF3C6H4-BIAN
4-NCC6H4-BIAN
3,5-Cl2C6H3-BIAN
4-O2NC6H4-BIAN
3,5-(CF3)2C6H3-BIAN
2,6-Pri2C6H3-BIAN
Phen
5.29 -0.27
5.12 -0.17
4.91 -0.14
6.85
2.56
35.3
1.70
2.22
1.00
0.10
0.011
0.0064
0.0018
0.0051
0.0020
0.0037
4.58
3.98
3.20
2.75
1.74
2.37
1.00
1.15
4.41
0.00
0.23
0.43
0.54
0.66
0.74
0.78
0.86
1.00
0.85
0.21
0.38
0.077
0.12
0.090
0.11
0.18
F igu r e 1. Plot of log(Keq) for the reactions in eqs 2
(squares) and 4 (circles) as a function of the Hammet σ
constants for the substituents on the aryl group of Ar-
BIAN. The lines drawn correspond respectively to the
functions log(Keq) ) -1.57σ + 0.200 (R2 ) 0.926) and log-
(Keq) ) -3.44σ - 0.054 (R2 ) 0.935).
66.4
45.1
0.046
>100
>100
<0.001
Bipy
Ph-DAB
a
The Keq value refers to the reaction in eq 2, at 20 °C. bThe Keq
value refers to the reaction in eq 4, at 20 °C.
Competition experiments were run by reacting pre-
formed Pd(Tol-BIAN)(DMFU) with an equimolar amount
of a different ligand in CDCl3 at 20 °C as in eq 1.
CDCl3
Pd(Tol-BIAN)(DMFU) + L y20 °C
z
Pd(L)(DMFU) + Tol-BIAN (1)
1
The evolution of the reaction was followed by H NMR.
With the exception of the case of 2,6-Pri C6H3-BIAN, all
2
reactions reached equilibrium in less than 20 min.
Within this time frame, only the peaks attributable to
the starting and final complexes and to the two free
1
ligands were observed in the H NMR spectrum, indi-
F igu r e 2. Plot of log(Keq) for the reactions in eqs 2
(squares) and 4 (circles) as a function of the pKa values for
the substitued anilines employed in the synthesis of the
corresponding Ar-BIAN. The lines drawn correspond re-
spectively to the functions log(Keq) ) 0.40pKa - 1.64 (R2 )
0.893) and log(Keq) ) 0.88pKa - 4.01 (R2 ) 0.878).
cating that no other compound was formed. The position
of the equilibrium was best evaluated by comparing the
integrated intensity of the signals for the methyl groups
of free and coordinated Tol-BIAN, which are well
separated. To have a more interesting set of data we
also included Ph-BIAN, 4-MeOC6H4-BIAN, 4-ClC6H4-
withdrawing ones, but 2,6-Pri C6H3-BIAN binds more
BIAN, 2,6-Pri C6H3-BIAN, Phen, Bipy, and Ph-DAB
2
2
weakly than its basicity would suggest, clearly for steric
reasons. The reaction between Pd(Tol-BIAN)(DMFU)
(Ph-DAB ) diphenyldiazabutadiene) in our series. As
mentioned before, the reaction with 2,6-Pri C6H3-BIAN
2
and 2,6-Pri C6H3-BIAN stopped at a 72:28 molar ratio
2
was slower than the others, likely because of steric
reasons, and stabilized only after 1 h, time at which a
very small, but detectable amount of an unidentified
decomposition product was also present. Thus the
datum for this ligand is affected by an error which is
not present in the others. The data for 3,5-(NO2)2C6H3-
BIAN could not be determined due to insufficient
solubility of the ligand.
between the starting and the final complex, which is in
good qualitative agreement with the 65:35 value re-
ported by Elsevier31 for the related reaction in which
maleic anhydride was employed as the olefin and which
represents the only other available data for a reaction
of this kind.
Interestingly, a good correlation is found between the
relative coordination constants of the Ar-BIAN ligands
The data obtained have been normalized to Ph-BIAN
as a reference, so that they now refer to the equilibrium
in eq 2, and are reported in Table 3.
(with the exception of 2,6-Pri C6H3-BIAN) and both the
2
Hammet σ constants33 of the substituents on the aryl
fragment (Figure 1, R2 ) 0.926; additivity of the
constants was supposed for the disubstituted aryls) or
the pKa of the aniline employed in the synthesis of the
ligands (Figure 2, R2 ) 0.893). That a good agreement
exists in both cases is not unexpected, because a good
linear correlation exists between the pKa values of sub-
stituted anilines and the Hammet σ constants of the
substituents.34 The correlation with the Hammet σ
constants was not significantly improved (R2 ) 0.929)
CDCl3
Pd(Ph-BIAN)(DMFU) + L y20 °C
z
Pd(L)(DMFU) + Ph-BIAN (2)
It is immediately clear that both Phen and Bipy are
quite stronger ligands toward the Pd(DMFU) moiety
than any Ar-BIAN ligands. Conversely Ph-DAB is the
weakest ligand of all. Contrary to what is observed for
the ruthenium complexes mentioned above, electron-
donating substituents on the aryl ring of the BIAN
ligands stabilize the complex with respect to electron-
(34) Gross, K. C.; Seybold, P. G.; Peralta-Inga, Z.; Murray, J . S.;
Politzer, P. J . Org. Chem. 2001, 66, 6919.

Synthesis of Ar-BIAN Ligands
Organometallics, Vol. 21, No. 14, 2002 2955
if only the 4-substituted Ar-BIAN ligands were consid-
ered, supporting additivity of the constants for this
system.
thermodynamic equilibrium constant is so small in this
case. It is more likely that the failure to observe the
new complex is due to an extremely slow reaction. Such
a behavior strongly points to an associative mechanism
for this substitution. In all cases, only signals attribut-
able to the starting and final complexes and to the two
free ligands were observed in the time frame examined,
as also confirmed by comparison of the observed signal
with those reported in the literature for the previously
reported compounds. Moreover, only two signals (for the
starting and final complex) were observed for the methyl
protons of the acetate groups. Independent measure-
ment of the relative coordination constants based on the
ratio between the integrated intensities of these acetate
signals gave values within (4% of those calculated from
the ligand methyl signals, further supporting the as-
sumption that no other species is present during the
reaction.38
To examine the sensitivity of the relative coordinating
constant to the electronic influence of the substituents
as a function of the oxidation state of the metal, we
performed an analogous series of experiments for a
typical palladium(II) complex. Palladium acetate was
selected as the fixed moiety because of the sufficient
solubility and stability of all of the complexes involved.35
The complexes Pd(OAc)2(L) with L ) 2,6-Pri C6H3-
2
BIAN,36 Phen,37 and Bipy37 have been previously re-
ported in the literature.
For this second system, Tol-BIAN could not be em-
ployed as a standard ligand, because the signals for the
methyl protons of free and coordinated Tol-BIAN are
almost degenerate. To solve this problem, the previously
unreported 3,5-Me2C6H3-BIAN ligand and its palladium
acetate complex were synthesized following standard
procedures (see Experimental Part). In this case, the
signals of the methyl groups shifted enough upon
coordination of the ligand that a good separation was
observed. Only one signal for the four methyl groups
was observed for either the free or the coordinated
ligand. No other notable feature is present in the 1H
NMR spectra of Pd(OAc)2(Tol-BIAN) and Pd(OAc)2(3,5-
Me2C6H3-BIAN) except for a strong shift to high fields
of the signals of the acetate protons. This shift has been
earlier observed in the related complex with 2,6-Pri2C6H3-
BIAN and explained with the position of the acetate
groups, placed over the plane (in the shield cone) of the
aryl groups of the BIAN ligand.36 The latter are almost
perpendicular to the acenaphthene plane. The effect is
observed during the exchange reactions even for all
other Ar-BIAN complexes, but not for the Phen and Bipy
ones, which give signals in the expected range.
Again, data have been normalized to Ph-BIAN as a
reference, and the results reported in Table 3 refer to
the equilibrium in eq 4.
CDCl3
Pd(Ph-BIAN)(OAc)2 + L y20 °Cz
Pd(L)(OAc)2 + Ph-BIAN (4)
Even for this system, a good correlation exists between
the Keq value and the Hammet σ constants for the
substituents (Figure 1, R2 ) 0.935). As for the Pd(0)
series, the correlation with the pKa values of the
substituted anilines is a bit worse (Figure 2, R2 ) 0.878),
but still fair. It should be noted that the points that
deviate more are those at the extremes of the series and
are the ones for which a small amount of either free
ligand or coordinated ligand is present. Under these
conditions, even a small imprecision in the determina-
tion of the absolute integral of the minor signal results
in a much larger relative error in the Keq value.
The comparison of the data for the two series in
Figure 1 immediately reveals that the second line has
a much larger slope. The values for the F constant for
the Pd(L)(DMFU) and Pd(L)(OAc)2 series are respec-
tively -1.57 and -3.44, indicating that more electron-
rich ligands bind more strongly in both series, but the
effect is much larger (about 75 times) in the Pd(II)
series. The observed effect is relevant to the question
of the effective oxidation state of palladium in the
Pd(L)(olefin) complexes. In general, two limiting struc-
tures are possible for a metal-olefin complex in which
the olefin bears electron-withdrawing substituents, A
and B. The first neglects back-donation and considers
the metal in the zero oxidation state, whereas the
second implies a complete (formal) donation of two
electrons to the olefin, with the formation of a metal-
lacyclopropane with the metal in the +2 oxidation state.
The reaction in eq 3 was found to be slower than that
in eq 1, and the onset of the equilibrium took 3-4 h at
20 °C. Moreover, the equilibrium was found to be much
more sensitive to electronic factors than the one involv-
ing the Pd(0) complex, and in the case of the weakest
ligands it was necessary to add a 4-fold molar excess of
ligand in order to obtain an amount of final complex
that could be reliably quantified.
CDCl3
Pd(3,5-Me2C6H3-BIAN)(OAc)2 + L y20 °Cz
Pd(L)(OAc)2 + 3,5-Me2C6H3-BIAN (3)
On the other hand, in this series an equimolar amount
of Phen or Bipy with respect to the starting complex
gave an essentially quantitative substitution, so that
only a lower limit can be estimated for their relative
coordination constants. At the opposite side, Ph-DAB
did not displace 3,5-Me2C6H3-BIAN from palladium(II)
at a measurable extent (at least under the time frame
examined), even when it was present in a 4-fold molar
excess. 2,6-Pri C6H3-BIAN did not give any detectable
2
reaction even after 3 days. It is unlikely that the
(35) Several of the PdCl2(L) complexes, on the other hand, were not
completely soluble under the reaction conditions.
(36) van Asselt, R.; Elsevier, C. J .; Amatore, C.; J utand, A. Orga-
nometallics 1997, 16, 317.
(37) Milani, B.; Alessio, E.; Mestroni, G.; Sommazzi, A.; Garbassi,
F.; Zangrando, E.; Bresciani-Pahor, N.; Randaccio, L. J . Chem. Soc.,
Dalton Trans. 1994, 1903.
On the basis of the spectroscopic features of Pd(Ar-
BIAN)(olefin) complexes, it was suggested that reso-

2956 Organometallics, Vol. 21, No. 14, 2002
Gasperini et al.
was then added. The solution was reflux heated for 45 min
and then filtered on a Buckner while hot. It is important not
to exceed this reaction time (vide supra). The solids obtained
in the synthesis of the derivatives with the 4-nitro, 4-cyano,
3,5-dinitro, and 3,5-dichloro were then suspended in hot acetic
acid (5 mL) for 15 min and filtered again. The solid was washed
on the filter with diethyl ether to help remove the acetic acid
and dried in vacuo. Care must be taken to eliminate any
residue of the acetic acid employed as solvent in the synthesis,
as this has a strongly negative effect on the effectiveness of
the separation in the following step. In the case of the CF3-
containing products, the purification with acetic acid was not
performed, but the residue was washed with diethyl ether.
Decoor d in a tion of th e Liga n d s. The complex (10 mmol)
was suspended in CH2Cl2 (200 mL) in a separating funnel, and
a solution of sodium or potassium oxalate (30 mmol) in water
(20 mL) was added. After shaking for 5 min a white precipitate
of Zn(C2O4) was present, suspended in the aqueous phase. The
phases were separated and the organic layer was washed with
water (2 × 20 mL), dried with Na2SO4, filtered, and evaporated
to dryness, affording the analytically pure ligands in almost
quantitative yields (with respect to the intermediate complex).
A larger excess of oxalate can also be used, in which case
Zn(C2O4) is partly solubilized in the aqueous phase as
[Zn(C2O4)2]2-, but the efficiency of the decoordination of the
Ar-BIAN ligand is not altered. In the case of ZnCl2(3,5-(NO2)2-
BIAN), the biphasic suspension was irradiated with ultrasound
in an ultrasound bath for 30 min before shaking it, to
disaggregate the very insoluble particles of the complex, which
also enclose some coprecipitated ZnCl2. In this case the ligand
was only sparingly soluble in CH2Cl2 and was recovered by
filtration. To this aim, it was essential to use a large excess of
oxalate (a saturated solution) during the decoordination
procedure, so that all of the zinc remained in solution and was
not collected together with the free ligand. Only in the case of
3,5-Cl2C6H3-BIAN was the crude ligand purified by flash
chromatography (silica, toluene/hexane, 4:1).
nance structure B may be more important than struc-
ture A in describing the bonding in these complexes.31
Similar arguments had been earlier applied to the
related Pd(R-DAB)(olefin) complexes.23 In general, the
problem highlighted is an old one and dates back to the
first olefin complexes of metals in a low oxidation
state.39 Our data indicates that, whichever description
is given, there is large difference in the electrophilicity
of a Pd(olefin) moiety with respect to that of a “true”
palladium(II) compound. Although a larger set of data
for more series of compounds would be required in order
to check if they all divide in two well-separated groups,
to the best of our knowledge, this is the first time that
equilibrium constant data are used to shed light on this
problem.
The fact that strongly donating compounds are better
ligands than weakly donating ones with the same
geometrical features should not be considered to imply
that Phen and Bipy are more basic than all Ar-BIAN
ligands. The different angle between the lone pairs on
the nitrogen atoms of these last two compounds with
respect to that in Ar-BIAN ligands surely plays a role
in determining the stability of the corresponding com-
plexes. Anyway, the much larger stability of Phen and
Bipy palladium complexes with respect to Ar-BIAN ones
is a new and unexpected fact that will have to be
considered in the study of those catalytic systems in
which both kinds of ligands can be employed
Exp er im en ta l P a r t
Gen er a l P r oced u r es. The syntheses of the ligands were
generally performed under a dinitrogen atmosphere, but can
also be performed in the air without problems. The decoordi-
nation reactions were always performed in the air. All organic
reagents were commercial products and were used as received.
Dry ZnCl2 was stored in an oven at 120 °C and quickly weighed
in the air just before use. NMR spectra were recorded on a
Bruker AC 300 FT (300 MHz) at RT. IR spectra were recorded
on a FTS-7 Bio Rad FT-IR spectrometer. Elemental analyses
and mass spectra were recorded in the analytical laboratories
of Milan University. All ligands gave an observable M+ peak
in the EI mass spectrum. Except for the spectrum of crude
3,5-Cl2C6H3-BIAN, the peak corresponding to the mass of the
monoimine was never detected. No absorption was observed
in the region 1700-1750 cm-1 of the IR spectra of the ligands,
again with the exception of that of crude 3,5-Cl2C6H3-BIAN.
Monoimines, on the other hand, always have an absorption
in this region, and the lack of such an absorption is the most
sensitive test for their absence. Pd(Tol-BIAN)(DMFU) was
prepared as reported in the literature.31
Total yields (after decoordination) and the elemental analy-
ses are given in the following for the individual ligands.
4-NO2C6H 4-BIAN: 85%. Anal. Calcd for C24H14N4O4: C,
68.2; H, 3.3; N, 13.3. Found: C, 68.0; H, 3.6; N, 13.0.
4-NCC6H4-BIAN‚H2O: 70%: Anal. Calcd for C26H16N4O:
C, 78.0; H, 4.0; N, 14.0. Found: C, 78.1; H, 4.2; N, 13.8.
4-CF 3C6H4-BIAN: 73%. Anal. Calcd for C26H14N2F6: C,
66.7; H, 3.0; N, 6.0. Found: C, 66.5; H, 3.1; N, 5.8.
3-CF 3C6H4-BIAN: 85%. Anal. Calcd for C26H14N2F6: C,
66.7; H, 3.0; N, 6.0. Found: C, 66.5; H, 3.0; N, 5.9.
3,5-(NO2)2C6H3-BIAN‚2H2O: 70%. Anal. Calcd for
24H16N6O10: C, 52.6; H, 3.0; N, 15.3. Found: C, 52.4; H, 2.7;
C
N, 15.0.
3,5-Cl2C6H3-BIAN: 85% (crude product; 70% after chro-
matographic purification on silica, eluent toluene/hexane, 4:1).
Anal. Calcd for C24H12N2Cl4: C, 61.3; H, 2.6; N, 6.0. Found:
C, 61.1; H, 2.5; N, 5.8.
Syn th esis of Zn Cl2(Ar -BIAN) Com p lexes. The interme-
diate ZnCl2(Ar-BIAN) complexes were prepared similarly to
what is reported in the literature.1-3 In a flask equipped with
a side port and a reflux condenser, acenaphthenequinone (500
mg, 2.74 mmol) and dry ZnCl2 (1.0 g, 7.34 mmol) were
suspended in glacial acetic acid (7.5 mL). In the case of the
CF3-containing anilines, toluene (2 mL) was also added. The
flask was heated to about 50-60 °C, and the aniline (6.3 mmol)
3,5-(CF 3)2C6H3-BIAN: 65%. Anal. Calcd for C28H12N2F12
C, 55.6; H, 2.0; N, 4.6. Found: C, 55.7; H, 2.3; N, 4.3.
:
3,5-Me2C6H3-BIAN: 98%. Anal. Calcd for C28H24N2: C,
86.6; H, 6.2; N, 7.2. Found: C, 86.3; H, 6.4; N, 7.6.
Syn th esis of P d (OAc)2(Ar -BIAN) (Ar ) 4-tolyl, 3,5-
Me2C6H3). The synthesis of these complexes was performed
by the procedure reported in ref 36. The proton numbering
for the NMR data is the same as in Table 1.
Ar ) 4-tolyl. Yield: 74.3%. 1H NMR (CDCl3, 298 K) δ, ppm:
7.18 (d, J ) 7.3 Hz, 2H, H1), 7.53 (pst, 2H, H2), 8.12 (d, J )
8.3 Hz, 2H, H3), 7.33 (d, 8.1 Hz, 4H, H4,8), 7.54 (d, J ) 8.1 Hz,
4H, H5,7), 2.46 (s, 6H, CH3Ar), 1.59 (s, 6H, CH3C(O)O). Anal.
Calcd for C30H26N2O4Pd: C, 61,7; H, 4.4; N, 4.8. Found: C,
61.4; H, 4.2; N, 4.5.
(38) The largest deviations are observed at the extremes of the
series, in accord with the following about the experimental error.
Although the values calculated by integrating the acetate signals are
slightly different from the ones calculated from the arylmethyl groups,
the relative coordination strength order is unchanged, as is the F value
(-3.48). The ratio determined from the arylmethyl groups is deemed
to be more reliable because the corresponding signals are 2 times more
intense and thus more precisely integrated.
1
Ar ) 3,5-Me2C6H3. Yield: 78.4%. H NMR (CDCl3, 298 K)
δ, ppm: 7.18 (d, J ) 7.4 Hz, 2H, H1), 7.53 (pst, 2H, H2), 8.11
(d, J ) 8.3 Hz, 2H, H3), 7.18 (s, 4H, H4,8), 7.13 (s, 2H, H6),
(39) Malatesta, L.; Cenini, S. Zerovalent Compounds of Metals;
Academic Press: London, 1974.

Synthesis of Ar-BIAN Ligands
Organometallics, Vol. 21, No. 14, 2002 2957
2.42 (s, 12H, CH3Ar), 1.60 (s, 6H, CH3C(O)O). Anal. Calcd for
of Pd(L)(DMFU) formed. The amount of free L remained is
obtained by the difference with the initially added one. It
should be noted that the amount of reagents weighed in each
experiments may differ slightly from the molar amount stated
above, and calculations were performed on the actually added
amounts. The equilibrium constant for the reaction in eq 1 is
calculated from the ratio (eq 5):
C
32H30N2O4Pd: C, 62,7; H, 4.9; N, 4.6. Found: C, 62.8; H, 5.1;
N, 4.9.
Exch a n ge Exp er im en ts. All NMR spectra were recorded
under a N2 atmosphere. CDCl3 was purified by passing it
through a basic alumina column, drying with activated mo-
lecular sieves, and degassing by three freeze-pump-thaw
cycles, followed by storage under a N2 atmosphere. The
purification over basic alumina is essential in order to elimi-
nate traces of HCl that are present in commercial CDCl3 and
that may react with the ligands, altering the equilibrium
position. A delay of 10 s was always used during collection of
1H NMR spectra.
P d (L)(DMF U) Ser ies. Pd(Tol-BIAN)(DMFU) (5.0 mg,
8.18 × 10-3 mmol) and DMFU (5.9 mg, 4.09 mmol) were
weighed in a NMR tube and dissolved in CDCl3 (0.6 mL). In
some cases, hexamethylbenzene (3.5 mg, 2.15 × 10-2 mmol)
was also added as internal standard. A spectrum was recorded,
after which the second ligand (8.18 × 10-3 mmol) was added.
A spectrum was recorded immediately and then every 10
minutes at 20 °C, until two consecutive spectra did not show
any further modification. In all cases in which hexamethyl-
benzene was present as an internal standard, the sum of the
integrated intensities of the peaks due to the methyl groups
of coordinated (δ ) 2.50 ppm) and free (δ ) 2.46 ppm) Tol-
BIAN was constant. The value of the ratio between the inte-
grated intensities of these two signals allows the calculation
of the molar amounts of Pd(Tol-BIAN)(DMFU) and Tol-BIAN
present at equilibrium. The latter is equal to the molar amount
Keq(Tol-BIAN) ) [Pd(L)(DMFU)] [Tol-BIAN]/
[Pd(Tol-BIAN)(DMFU)] [L] (5)
where molar amounts can be employed instead of concentra-
tions. The equilibrium constants for the reaction in eq 2 were
calculated by dividing the corresponding values relative to Tol-
BIAN by the value calculated from eq 5 for Ph-BIAN (0.39).
P d (L)(OAc)2 Ser ies. The reactions were performed as
detailed above (Pd(OAc)2(3,5-Me2C6H3-BIAN): 5.0 mg, 8.16 ×
10-3 mmol), but hexamethylbenzene was omitted and 1 mL of
CDCl3 had to be employed in order to dissolve the reagents
completely. The amount of ligand used was 8.16 × 10-3 mmol,
except for 4-NO2C6H4-BIAN, 3,5-(CF3)2C6H3-BIAN, 3,5-Cl2C6H3-
BIAN, 4-NCC6H4-BIAN, 2,6-Pri C6H3-BIAN, and Ph-DAB, for
2
which it was 3.26 × 10-2 mmol.
Ack n ow led gm en t. We thank MURST (Programmi
di Ricerca Scientifica di Rilevante Interesse Nazionale)
for financial support.
OM020147U