Method of establishing the Lewis acidity of a metal fragment based on the relative binding strengths of Ar-BIAN ligands (Ar-BIAN = bis(aryl)acenaphthenequinonediimine)

Article abstract of DOI:10.1021/om034370i

The relative coordination strengths of a series of differently substituted Ar-BIAN ligands (Ar-BIAN = bis(aryl)acenaphthenequinonediimine) to a series of palladium complexes in both the formal 0 and 2 oxidation states have been determined. In all cases a good to excellent linearity of log K_eq with respect to the Hammett σ constants of the substituents on the aryl fragments of the ligands was observed. The resulting ρ constant is proposed to be a good indication of the Lewis acidity of the metal fragment, a physical quantity for which experimental parameters have been determined only for a limited class of compounds. The obtained parameters allow a comparison not only of different olefin complexes among themselves but also with respect to different metal fragments such as Pd(OAc)₂, Pd(Me)Cl, and a π-allyl complex. The Lewis acidity of the olefin complexes is extremely variable and ranges from the less acidic (Pd(Ar-BIAN)(DMFU); DMFU = dimethyl fumarate) to two of the most acidic (Pd(Ar-BIAN)(TCNE) and Pd(Ar-BIAN)(FN); TCNE = tetracyanoethylene, FN = fumarodinitrile) complexes among those examined. A cationic π-allyl complex has the highest Lewis acidity among the complexes examined. The importance of steric effects is examined in some cases.

Full text of DOI:10.1021/om034370i

Organometallics 2004, 23, 995-1001
995
Meth od of Esta blish in g th e Lew is Acid ity of a Meta l
F r a gm en t Ba sed on th e Rela tive Bin d in g Str en gth s 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)
Michela Gasperini and Fabio Ragaini*
Dipartimento di Chimica Inorganica, Metallorganica e Analitica and ISTM-CNR,
Via G. Venezian 21, I-20133 Milano, Italy
Received December 15, 2003
The relative coordination strengths of a series of differently substituted Ar-BIAN ligands
(Ar-BIAN ) bis(aryl)acenaphthenequinonediimine) to a series of palladium complexes in
both the formal 0 and 2 oxidation states have been determined. In all cases a good to excellent
linearity of log Keq with respect to the Hammett σ constants of the substituents on the aryl
fragments of the ligands was observed. The resulting F constant is proposed to be a good
indication of the Lewis acidity of the metal fragment, a physical quantity for which
experimental parameters have been determined only for a limited class of compounds. The
obtained parameters allow a comparison not only of different olefin complexes among
themselves but also with respect to different metal fragments such as Pd(OAc)2, Pd(Me)Cl,
and a π-allyl complex. The Lewis acidity of the olefin complexes is extremely variable and
ranges from the less acidic (Pd(Ar-BIAN)(DMFU); DMFU ) dimethyl fumarate) to two of
the most acidic (Pd(Ar-BIAN)(TCNE) and Pd(Ar-BIAN)(FN); TCNE ) tetracyanoethylene,
FN ) fumarodinitrile) complexes among those examined. A cationic π-allyl complex has the
highest Lewis acidity among the complexes examined. The importance of steric effects is
examined in some cases.
Ch a r t 1
In tr od u ction
Given the high importance that nitrogen ligands,
especially chelating ones, have gained in homogeneous
catalysis1 and their traditional role in coordination
chemistry, it is surprising that very little quantitative
information exists on their relative binding strength to
transition metals. A recent review on the role of nitrogen
ligands in homogeneous catalysis1a mentions the bond
dissociation energies of the adducts of some aliphatic
2
amines with BMe3 as the only quantitative data
relevant to the coordination strength of nitrogen ligands,
but these data are hardly of any use to predict the
strength of the interaction between the usually em-
ployed ligands (most of which contain sp2-hybridized
nitrogen atoms) and a transition metal. Several papers,
especially in the older literature, have been devoted to
phenanthroline (Phen) and bipyridine (Bipy) com-
plexes,3 but all of the available data were measured in
aqueous solution, where protonation of the ligand
becomes important. Thus, this information cannot be
transferred to the low- to medium-polarity, often anhy-
drous, solvents commonly employed for catalytic reac-
tions. The effect of steric hindrance on the identity of
the most stable complex has been examined for some
R-DAB complexes,4 but electronic effects were not
investigated.
Very recently,5 we improved the synthesis of the Ar-
BIAN class of ligands6 (see Chart 1 for the ligands
employed in the present study), extending it to aryl
groups bearing strongly electron withdrawing substit-
* To whom correspondence should be addressed. E-mail:
fabio.ragaini@unimi.it.
(1) (a) Togni, A.; Venanzi, L. M. Angew. Chem., Int. Ed. Engl. 1994,
33, 497-526; Angew. Chem. 1994, 106, 517-547. (b) Fache, F.; Schulz,
E.; Tommasino, M. L.; Lemaire, M. Chem. Rev. 2000, 100, 2159-2231.
(c) Fache, F.; Dunjic, B.; Gamez, P.; Lemaire, M. Top. Catal. 1997, 4,
201-209. (d) Ghosh, A. K.; Mathivanan, P.; Cappiello, J . Tetrahedron
Asymm. 1998, 9, 1-45.
(2) (a) Brown, H. C.; Bartholomay, H., J r.; Taylor, M. D. J . Am.
Chem. Soc. 1944, 66, 435-442. (b) Brown, H. C.; Sujishi, S. J . Am.
Chem. Soc. 1948, 70, 2878-2881. (c) Brown, H. C. J . Chem. Educ. 1959,
36, 424-431.
(3) (a) McWhinnie, W. R.; Miller, J . D. Adv. Inorg. Chem. Radiochem.
1969, 12, 135-197. (b) For a more recent reference see: Steinhouser,
R. K.; Kolopajlo, L. H. Inorg. Chem. 1985, 24, 1845-1852.
(4) (a) van der Poel, H.; van Koten, G.; Kokkes, M.; Stam, C. H.
Inorg. Chem. 1981, 20, 2941-2950. (b) van der Poel, H.; van Koten,
G.; Vrieze, K. Inorg. Chim. Acta 1981, 51, 241-252. (c) van der Poel,
H.; van Koten, G.; Vrieze, K. Inorg. Chim. Acta 1981, 51, 253-262.
(d) For more information on the chemistry of R-DAB ligands see: van
Koten, G.; Vrieze, K. Adv. Organomet. Chem. 1982, 21, 151-239 and
references therein.
(5) Gasperini, M.; Ragaini, F.; Cenini, S. Organometallics 2002, 21,
2950-2957.
10.1021/om034370i CCC: $27.50 © 2004 American Chemical Society
Publication on Web 01/27/2004

996 Organometallics, Vol. 23, No. 5, 2004
Gasperini and Ragaini
uents, and we measured the equilibrium constants for
reactions 1 and 2 (DMFU ) dimethyl fumarate, Tol-
BIAN ) 4-MeC6H4-BIAN).
complexes, Pd(Ar-BIAN)(MA) (2), Pd(Ar-BIAN)(FN) (3),
and Pd(Ar-BIAN)(TCNE) (4) (MA ) maleic anhydride,
FN ) fumarodinitrile, TCNE ) tetracyanoethylene), two
“standard” palladium(II) complexes, Pd(Ar-BIAN)(Me)-
(Cl) (5) and [Pd(Ar-BIAN)(COD-OMe)][PF6] (6) (COD-
OMe ) η1:η2-C8H12OMe), and one π-allylic complex,
[Pd(Ar-BIAN)(η3-CH2C(CH3)CH2)][PF6] (7). The proce-
dure is the same as that previously employed.5 In short,
when possible, the complex bearing the Tol-BIAN ligand
was prepared, a known amount was dissolved in CDCl3,
and a known amount of another Ar-BIAN ligand was
CDCl3
Pd(Tol-BIAN)(DMFU) + Ar-BIAN y20 °C
z
Pd(Ar-BIAN)(DMFU) + Tol-BIAN (1)
CDCl3
Pd(3,5-Me2C6H3-BIAN)(OAc)2 + Ar-BIAN y20 °Cz
Pd(Ar-BIAN)(OAc)2 + 3,5-Me2C6H3-BIAN (2)
1
added. The reaction was followed by H NMR at 20 °C,
The data obtained were normalized to Ph-BIAN as a
reference by dividing the individual Kobs by the corre-
sponding Kobs value for the reaction in which Ar ) Ph,
so that a set of Keq values was obtained which refers to
the equilibria in eqs 3 and 4.
monitoring the signals of the methyl group of free and
coordinated Tol-BIAN until no further variation in the
relative ratio was observed. The relative amount of the
second Ar-BIAN ligand to be added is a function of its
coordinating strength. The only difference with respect
to the procedure reported in the previous paper is that
now we prepared a stock solution in CDCl3 of each
complex and measured the amount of added complex
by volume, rather than weighing individual amounts
of complex in each NMR tube. In general, excesses up
to 6 times the stoichiometric amounts were necessary
for the less coordinating ligands in order to displace the
equilibrium to the point that a reliable integration of
the NMR signals could be obtained, and in several cases
even this was not enough, so that the corresponding
data could not be measured. This is especially the case
for the allylic complex and for the complex with TCNE.
In the case of the complex with maleic anhydride, we
observed a broadening of the signals when some of the
less coordinating ligands were added, leading to non-
baseline separation of the signals for the free and
coordinated Tol-BIAN, so that again the corresponding
data could not be measured. In the case of complexes
5, the signals of the methyl groups of free and coordi-
nated Tol-BIAN were partly overlapped, but the corre-
sponding signals for the 3,5-Me2C6H3-BIAN analogue
were not, allowing for the determination of the corre-
sponding equilibrium constant. The same situation had
been earlier encountered with the palladium acetate
complex.5 In the case of complex 6, the signals of the
aliphatic region of the alkyl ligand overlapped with
those of both Tol-BIAN and 3,5-Me2C6H3-BIAN and the
signals of free and coordinated 4-MeOC6H4-BIAN were
too close to allow a separate integration. Fortunately,
the position of the methoxy protons of the COD-OMe
moiety shifted enough upon exchange of coordinated
4-MeOC6H4-BIAN with other Ar-BIANs that the corre-
sponding signals could be employed to assess the
position of the equilibrium.
CDCl3
Pd(Ph-BIAN)(DMFU) + Ar-BIAN y20 °Cz
Pd(Ar-BIAN)(DMFU) + Ph-BIAN (3)
CDCl3
Pd(Ph-BIAN)(OAc)2 + Ar-BIAN y20 °Cz
Pd(Ar-BIAN)(OAc)2 + Ph-BIAN (4)
Other chelating nitrogen ligands were also investi-
gated, and it was found that phenanthroline and 2,2′-
bipyridine always bind more strongly than all Ar-BIAN
ligands, but Ph-DAB (Ph-DAB ) phenyldiazabutadiene)
is a much weaker ligand. Most interestingly, a good
correlation was found for all Ar-BIAN ligands, with the
exception of 2,6-iPrC6H3-BIAN, between the log Keq
values for the reactions in eqs 3 and 4 and either the
Hammett σ constants7 of the substituents on the Ar-
BIAN ligands or the pKa values of the anilines employed
in the syntheses of the same ligands.8 The F values for
the two reactions, however, were very different (-1.57
and -3.44, respectively).
In the present work, we have extended the series of
complexes for which the equilibrium constants have
been measured, including several olefin complexes as
well as both neutral and cationic palladium(II) com-
plexes and a π-allylic system. The results allow us to
discuss the binding properties of the ligands and the
Lewis acidity of the metal fragment in a new quantita-
tive light.
Resu lts
In this work we measured the relative coordination
equilibria of different Ar-BIAN ligands in three olefinic
(6) (a) Compounds of the family Ar-BIAN (Ar-BIAN ) bis(aryl)-
acenaphthenequinonediimine) have been known for some time6b,c but
have been brought to general attention only in recent years by Elsevier
and his group.6d Since then, they have found widespread use as ligands
for palladium, ruthenium, and nickel and the corresponding complexes
have been employed as catalysts for a wide variety of reactions. For a
list of applications see ref 5 and references therein. (b) Dvolaitzky, M.
C. R. Acad. Sci. Paris, Ser. C 1969, 268, 1811-1813; Chem. Abstr. 1969,
71, 61566b. (c) Matei, I.; Lixandru, T. Bul. Ist. Politeh. Iasi 1967, 13,
245-255; Chem. Abstr. 1969, 70, 3623m. (d) van Asselt, R.; Elsevier,
C. J .; Smeets, W. J . J .; Spek, A. L.; Benedix, R. Recl. Trav. Chim. Pays-
Bas 1994, 113, 88-98.
(7) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165-195.
(8) (a) Sheppard, W. A. J . Am. Chem. Soc. 1962, 84, 3072-3076.
(b) Tickle, P.; Briggs, A. G.; Wilson, J . M. J . Chem. Soc. B 1970, 65-
71. (c) Bolton, P. D.; Hall, F. M. J . Chem. Soc. B 1970, 1247-1251. (d)
Kvita, V.; Sauter, H.; Rihs, G. Helv. Chim. Acta 1985, 68, 1569-1575.
(e) Gross, K. C.; Seybold, P. G.; Peralta-Inga, Z.; Murray, J . S.; Politzer,
P. J . Org. Chem. 2001, 66, 6919-6925.
In order for the equilibrium data to be significant, it
is essential that no other product apart from the starting
and final complexes and the two free ligands is formed
during the reaction. In our previous work, an unknown
decomposition product had been observed when Pd(Tol-
BIAN)(DMFU) was employed and it had been found
that decomposition could be avoided, at least on the time
scale of the equilibration experiment, by adding an
excess of dimethyl fumarate to the solution. We have
now identified the decomposition product as Pd(Tol-
BIAN)Cl2, clearly deriving from a reaction of the start-
ing complex with the chloroform solvent. With the more
tightly bonding olefins employed in this work, however,

Lewis Acidity of Ar-BIAN Complexes
Organometallics, Vol. 23, No. 5, 2004 997
Ta ble 1. Rela tive Coor d in a tion Str en gth s of Ar -BIAN Liga n d s to Differ en t Meta l F r a gm en tsa
Keq
[Pd(Ar-BIAN)- [Pd(Ar-BIAN)(η3-
Pd(Ar-BIAN)- Pd(Ar-BIAN)- Pd(Ar-BIAN)- Pd(Ar-BIAN)-
(COD-OMe)]-
CH2C(CH3)CH2]-
b
Ar
pKa
σc
(MA) (2)
(FN) (3)
(TCNE) (4)
(Me)(Cl) (5)
[PF6] (6)
[PF6] (7)
4-MeOC6H4
4-MeC6H4
3,5-Me2C6H3
Ph
4-ClC6H4
3-CF3C6H4
4-CF3C6H4
4-NCC6H4
3,5-Cl2C6H3
5.29 -0.27
5.12 -0.17
4.91 -0.14
4.70
2.27
10.3
4.17
7.24
3.11
2.43
1.00
6.78
6.24
4.08
1.00
0.39
0.10
0.08
0.02
0.012
0.008
0.008
10.6
7.01
7.37
1.00
0.38
0.043
0.021
13.3
4.88
4.58
3.98
3.20
2.75
1.74
2.37
0.00
0.23
0.43
0.54
0.66
0.74
0.86
-
1.00
0.50
0.054
0.021
1.00
0.26
0.037
0.029
1.00
0.15
3,5-(CF3)2C6H3 1.15
2,6-iPr2C6H3
4.41
0.34
0.138
0.96
a
b
Data obtained at 20 °C in CDCl3. pKa of the aniline employed in the synthesis of the Ar-BIAN ligand. c The sum of the individual
σ values is reported when two substituents are present on the phenyl ring.
Ta ble 2. Va lu es of GBIAN a n d of th e An gu la r
Coefficien t of th e Cor r ela tion log Keq vs An ilin e
p Ka Va lu es for Differ en t Meta l F r a gm en tsa
slope of log
metal
fragment
Keq vs aniline R2(aniline
FBIAN R2(σ)
pKa
pKa)
Pd(DMFU)b
Pd(Me)(Cl)
Pd(MA)
Pd(FN)
Pd(TCNE)
-1.57 0.926
-2.71 0.989
-2.77 0.961
-3.21 0.992
-3.16 0.992
-3.44 0.935
-3.47 0.983
0.40
0.744
0.888
1.01
1.13
0.88
1.11
1.43
0.893
0.955
0.983
0.979
0.956
0.878
0.980
0.991
b
Pd(OAc)2
Pd(COD-OMe)+
F igu r e 1. Plot of log Keq for the exchange reaction of 5d
with Ar-BIAN as a function of the Hammett σ constants
for the substituents on the aryl group of Ar-BIAN. Data
are given in Tables 1 and 2.
Pd(η3-CH2C(CH)3CH2)+ -3.88 0.998
a
b
Data obtained at 20 °C in CDCl3. Data from ref 5.
this product was not formed on the time scale of the
experiment in most cases. On the other hand, in those
cases in which the exchange reaction was very slow and
decomposition occurred, olefin addition could not be
employed to stabilize the complexes, because this ad-
dition broadened the signals and a baseline separation
for the signals of free and coordinated Tol-BIAN was
no longer observed.
All the obtained data have been normalized to the
equilibrium starting from the Ph-BIAN complex by
dividing the observed Kobs value by the Kobs value for
the reaction between the actually employed complex and
Ph-BIAN. In the following, only the normalized data,
Keq, are reported and discussed. The results for all
equilibria for which completely reliable data could be
obtained are reported in Table 1. The corresponding
Hammett σ value and the pKa value of the aniline
employed in the synthesis of the ligands are also shown.
Where two substituents were present, the sum of the
individual σ constants is reported and was employed in
the following fitting. A graphical representation of the
data is shown in Figure 1 and Figures S1-S5 (Support-
ing Information). A good to excellent (R2 up to 0.998)
linear relationship was observed between the equilib-
rium constant and the Hammett σ constant. A usually
somewhat lower correlation also exists between the
same data and the aniline pKa value. The values for the
F constants (hereafter referred as FBIAN), the slope of the
log Keq/aniline pKa plot, and the corresponding R2 values
are reported in Table 2. In the case of complexes 2, 3,
5, and 7, the equilibrium constant of the sterically
hindered 2,6-iPr2C6H3-BIAN ligand was also measured.
F igu r e 2. Plot of log Keq for the exchange reaction of 5d
with Ar-BIAN as a function of the pKa of the aniline
employed in the synthesis of Ar-BIAN. Data are given in
Tables 1 and 2. The open circle corresponds to the data for
2,6-iPr2C6H3-BIAN and was not included in the fitting.
Obviously, the corresponding data cannot be analyzed
in terms of the Hammett σ constants, which do not exist
for ortho substituents, but a comparison within the log
K
eq vs aniline pKa correlation is possible. Figures 2 and
3 and Figures S6-S9 (Supporting Information) show the
fitting obtained, excluding the point for 2,6-iPr2C6H3-
BIAN, but the corresponding data are also shown with
a different marker to allow a comparison.
Discu ssion
The first observation which emerges from the data is
that the existence of a linear relationship between the
relative binding strength of Ar-BIAN ligands and either
the Hammett σ constant or, to a lower extent, the
aniline pKa is a general and up to now unrecognized
phenomenon. No exception was found. The second

998 Organometallics, Vol. 23, No. 5, 2004
Gasperini and Ragaini
series concerns strong Lewis acids and the only transi-
tion-metal compounds included are metal chlorates or
triflates. This approach is unlikely to be successful for
the type of compounds discussed in the present paper.
On the other hand, we recognize that our approach
would not be suitable for most of the Lewis acids to
which the fluorescence method can be applied, since
complexes with different numbers of ligands would
surely be formed. An experimental Lewis acidity scale
that may allow a comparison between different transi-
tion-metal fragments independent of the oxidation state
and the other ligands is thus still missing.11 The FBIAN
scale allows for the first time such a comparison. It
should also be noted that, although in the present paper
we only measured constants for palladium complexes,
there is no reason for which a comparison between the
FBIAN constants even of complexes of different metals
should not be performed, making its use much more
general. The only limitation is represented by the fact
that the metal fragment to be examined must have two
vacant cis coordination sites. However, considering that
bidentate ligands are those most commonly employed
in homogeneous catalysis and that most metal-based
reactions require the availability of such two vacant cis
positions, it emerges that the large majority of the most
interesting cases will fulfill the requirement to be
analyzed by this method.
F igu r e 3. Plot of log Keq for the exchange reaction of 2d
with Ar-BIAN as a function of the pKa of the aniline
employed in the synthesis of Ar-BIAN. Data are given in
Tables 1 and 2. The open circle corresponds to the data for
2,6-iPr2C6H3-BIAN and was not included in the fitting.
observation is that, although more basic ligands coor-
dinate more strongly in all cases, the slope of the line
describing the correlation is highly variable. It is
immediately evident that the metal fragments which
are intuitively more electron poor have more negative
values of FBIAN. Although this may have been qualita-
tively foreseen, no experimental quantitative data have
ever been reported in the literature supporting this
observation. Most importantly, the fact that FBIAN can
be considered as a measure of the Lewis acidity of the
metal fragment allows for the first time a direct
comparison of metal fragments bearing completely
different ligands and having different oxidation states.
In the literature, very little experimental quantitative
information exists on the Lewis acidity of transition-
metal complexes. One approach is based on the mea-
surement of the acidity of a coordinated water molecule.9
However, stable water complexes (which also need to
be water soluble) of the typical metal complexes which
are commonly employed as catalysts are very rare, and
the method is of limited application. An electrophilicity
scale has been determined from kinetic data for the
nucleophilic attack of different nucleophiles on metal
π-complexes.10 However, what is measured in these
systems is the electrophilicity of the whole complex, and
it is not obvious how the same metal fragment would
behave when coordinated to a ligand very different from
a polyolefin or arene. Note that, according to IUPAC,
Lewis acidity is defined as a thermodynamic phenom-
enon, whereas electrophilicity is a kinetic one; thus, this
last approach gives in any case different, at least from
a quantitative point of view, information with respect
to that here discussed. A spectroscopic series has been
reported, based on the fluorescence maxima of 10-
methylacridone-metal ion complexes.11 However, this
A few comments on this more generalized use of FBIAN
are worth making.
The determination of FBIAN requires the determination
of a series of equilibrium constants, rather than a single
measure. However, once the parent compound is syn-
thesized, all equilibrium constants can be determined
almost in parallel, typically in a few hours of NMR time.
Different metal complexes will be differently sensitive
to steric hindrance. However, the use of a series of
ligands having very similar, if not identical, steric
requirements will elide the steric factor, so that a purely
electronic effect is observed.
It may be suggested that a monodentate ligand may
be of more general use than a bidentate one. However,
there are few complexes in which a clean substitution
of only one monodentate nitrogen ligand is likely to be
observed, whereas clean substitution of a chelating
ligand is probably much more frequent.
Other ligands may be employed in place of Ar-BIANs.
However, Ar-BIAN ligands have some advantages that
should be considered. First, although not commercially
available (at least at the moment), they are very easily
synthesized5 in multigram amounts in a short time from
inexpensive materials. Moreover, they are obtained in
high purity without the use of column chromatography.
As a comparison, a few substituted bipyridines and
phenanthrolines are commercially available, but they
would be completely insufficient for the determination
of a meaningful series of equilibria and the synthesis
of the missing ligands is much more experimentally
demanding than that of Ar-BIANs.12 With respect to
other chelating Schiff bases, Ar-BIAN ligands are also
chemically robust.
(9) For some examples see: (a) Dadci, L.; Elias, H.; Frey, U.; Ho¨rnig,
A.; Koelle, U.; Merbach, A. E.; Paulus, H.; Schneider, J . S. Inorg. Chem.
1995, 34, 306-315. (b) Hofmann, A.; J aganyi, D.; Munro, O. Q.; Liehr,
G.; van Eldik, R. Inorg. Chem. 2003, 42, 1688-1700. (c) Minsker, K.
S.; Ivanova, S. R.; Babkin, V. A. Dokl. Akad. Nauk 1995, 343, 646-
650; Chem. Abstr. 1997, 127, 56353.
(10) (a) Kane-Maguire, L. A. P.; Honig, E. D.; Sweigart, D. A. Chem.
Rev. 1984, 84, 525-543 and references therein. (b) Mayr, H.; Patz, M.
Angew. Chem., Int. Ed. Ingl. 1994, 33, 938-957; Angew. Chem. 1994,
106, 990-1010.
(11) (a) Another Lewis acid scale reported by the same authors and
based on EPR data suffers from the same limitations as that based on
fluorescence data but also requires a sophisticated apparatus and is
of more limited application.11c,d (b) Fukuzumi, S.; Ohkubo, K. J . Am.
Chem. Soc. 2002, 124, 10270-10271. (c) Ohkubo, K.; Menon, S. C.;
Orita, A.; Otera, J .; Fukuzumi, S. J . Org. Chem. 2003, 68, 4720-4726.
(d) Fukuzumi, S.; Ohkubo, K. Chem. Eur. J . 2000, 6, 4532-4535.
Coming back to the data collected in the present work,
they can be employed to give an experimental answer,
or at least a different contribution, to a longstanding
question.

Lewis Acidity of Ar-BIAN Complexes
Organometallics, Vol. 23, No. 5, 2004 999
In general, two limiting structures are possible for a
metal-olefin complex in which the olefin bears electron-
withdrawing substituents, A and B. The first neglects
polarizing the metal. To the best of our knowledge, such
an “electronic saturation” effect is unprecedented and
some caution should be taken in generalizing it until
more data are available.
Overall, the traditional qualitative picture that pal-
ladium complexes of olefins with strongly electron-
withdrawing substituents such as TCNE should be
considered as palladium(II) complexes is confirmed, but
a warning may be raised against extending this as-
sumption too far, as the Lewis acidity of the dimethyl
fumarate complex is much lower than that of typical
palladium(II) complexes.
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 metallacyclopropane with the metal in
the +2 oxidation state.
The trends in the “classical” palladium(II) complexes
are not surprising. The strongly donating methyl group
dominates when the Pd(Cl)(Me) and Pd(OAc)2 moieties
are compared, despite the fact that the chloride ligand
is expected to donate to a lesser extent than an acetate
group. If the chloride is replaced by a neutral olefin
ligand, as in 6, and a cationic complex is generated, then
the Lewis acidity of the metal fragment increases
markedly, which was also expected.
On the basis of the spectroscopic features of Pd(Ar-
BIAN)(olefin) complexes, it was suggested that reso-
nance structure B may be more important than struc-
ture A in describing the bonding in these complexes.13
Similar arguments had been earlier applied to the
related Pd(R-DAB)(olefin) complexes.4 In general, the
problem highlighted is an old one and dates back to the
first olefin complexes of metals in a low oxidation
state.14 In our earlier paper,5 we could only compare the
data for Pd(Ar-BIAN)(DMFU) and Pd(Ar-BIAN)(OAc)2
and noted that a large difference existed between the
two. A more general conclusion was demanded for the
moment at which data on a more extensive set of
compounds would be available. Now that these data are
available, we can conclude the following. The Lewis
acidity of the olefin complexes is extremely variable and
range from the less acidic (Pd(Ar-BIAN)(DMFU)) to two
of the most acidic complexes (Pd(Ar-BIAN)(TCNE) and
Pd(Ar-BIAN)(FN)) among those examined. This is even
more impressive when we consider that complexes of
the type here examined with olefins bearing no electron-
withdrawing group are not stable enough to be isolated
but are surely formed in solution during the catalytic
reactions which involve them: for example, hydrogena-
tion.15 The FBIAN values for the TCNE and FN series
are very close, with that for the FN complexes being
surprisingly more negative than that for the TCNE
series. One would expect the contrary to occur. However,
the values are close and the reverse order is observed
when the slopes of the log Keq vs aniline pKa plots for
the two series are compared. The only conclusion that
can be drawn from these values is that there appears
to be a limiting value to the amount of charge that
palladium can transfer to the olefin in such complexes
and increasing the electron-withdrawing power of the
olefin over a certain limit is ineffective in further
The value observed for the π-allylic complex is also
interesting. The standard rules for attributing oxidation
states result in a negatively charged allylic fragment,
bound to a palladium(II) moiety. However, the well-
known fact that allylic ligands can be attacked easily
by nucleophiles supports an alternative description, in
which a cationic allyl fragment is bound to a palladium-
(0) moiety. The observed FBIAN value is the most negative
of all and clearly shows that the traditional “formal”
rules are quite adequate even in this case. It is worth
making a comparison between these allyl complexes 7
and the COD-OMe complexes 6. Both are cationic
complexes, and if we consider the limiting resonance
structure for 7, the metal coordinates in both cases to
an η1-alkyl and an η2-olefin group. However, the FBIAN
values differ and indicate that an allyl moiety with-
draws more charge from palladium than two separate
alkyl and olefin groups.
For some of the series, the value of the equilibrium
constant for the sterically hindered 2,6-iPr2C6H3-BIAN
ligand could also be determined. We have previously
reported5 that, in the Pd(DMFU) series, the correspond-
ing point in the Keq vs aniline pKa was clearly below
the line individuated by the other points, indicating that
this ligand coordinates less strongly in this system than
would be expected on the basis of its basicity, clearly
for steric reasons. The corresponding constant for the
Pd(OAc)2 moiety could not even be determined, because
the exchange reaction was too slow, apparently again
for steric reasons. Exchange was attempted with this
ligand even for the systems reported in this study. As
was expected, in some cases the reaction did not reach
completion before decomposition begun to be observed.
However, in four cases a clean reaction was observed.
In two of these, the Pd(FN) and Pd(Cl)(Me) series, the
point for 2,6-iPr2C6H4-BIAN falls quite below the line
individuated by the other ligands (Figure S6 (Support-
ing Information) and Figure 2), evidencing again the
onset of steric effects. However, in the other two cases
(namely the complexes with maleic anhydride and the
allylic complex, 2 and 7, respectively, Figure 3 and
Figure S9 (Supporting Information)) the point corre-
sponding to 2,6-iPr2C6H4-BIAN nicely fit the general
(12) Moreover, it is expected that the sensitivity of the equilibrium
constant to a substituent on the ring of a pyridine ring should be much
higher than that observed for the Ar-BIAN ligands, in which the aryl
group lies more or less perpendicularly to the -NdCCdN- moiety.
Since the sensitivity displayed by Ar-BIAN ligands is already such that
in several cases the coordinating strength of the more electron-poor
ligands could not be determined, the measurement of accurate equi-
librium constants with different phenanthrolines or bipyridines would
probably be confined to a limited range of substituents having σ
constants close to each other. This would strongly affect the reliability
of the correlation in a negative way.
(13) van Asselt, R.; Elsevier, C. J .; Smeets, W. J . J .; Spek, A. L.
Inorg. Chem. 1994, 33, 1521-1531.
(14) Malatesta, L.; Cenini, S. Zerovalent Compounds of Metals;
Academic Press: London, 1974.
(15) van Asselt, R.; Elsevier, C. J . J . Mol. Catal. 1991, 65, L13-
L19.

1000 Organometallics, Vol. 23, No. 5, 2004
Gasperini and Ragaini
freeze-pump-thaw cycles, followed by storage under an N2
atmosphere. The purification over basic alumina is essential
in order to eliminate 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 the collection of 1H NMR spectra of the equilibrat-
ing mixtures. Elemental analyses and mass spectra were
recorded in the analytical laboratories of Milan University. Pd-
(Tol-BIAN)(MA) (2b), Pd(Tol-BIAN)(FN) (3b), and Pd(Tol-
BIAN)(TCNE) (4b) were prepared as reported in the litera-
ture.13
correlation (in the case of 7 it is even slightly above the
line, but the distance is within the general uncertainty).
This indicates that steric effects are not always impor-
tant, even with an apparently very sterically hindered
ligand. It can be noted that the series for which the
steric effect is small are the only ones in which the
constant ligand in the series binds asymmetrically with
respect to the coordination plane. This should permit
some torsion of the aryl groups of the Ar-BIAN ligand,
which minimizes steric repulsion. Such a torsion effect
can indeed be observed when the previously reported
X-ray crystal structures of 2l6d and 5l are compared.13
The fact that the allylic complex is the one for which
the least steric effect is observed may appear surprising,
given the importance asymmetric addition reactions
have on this kind of complex and the traditional role
steric effects have on these reactions. However, we note
that the complex chosen has unusually small terminal
groups on both ends of the allyl moiety (actually, no
asymmetric synthesis would be possible on this allyl
group) and steric effects are likely to become important
even in this series if the terminal CH2 groups are
substituted with more bulky substituents.
P d (3,5-Me2C6H4-BIAN)(Me)(Cl) (5c). This complex was
prepared by the method reported for 5a and 5l.16
Yield: 85.0%. Anal. Calcd for C29H27N2PdCl: C, 63.86; H, 4.99;
1
N, 5.14. Found: C, 63.79; H, 5.15; N, 5.10. H NMR (CDCl3,
Con clu sion s
δ): 8.07 (d, J ) 8.6 Hz, 1H, H5), 8.04 (d, J ) 8.6 Hz, 1H, H5A),
7.50 (pst, 1H, H4), 7.46 (pst, 1H, H4A), 7.19 (d, J ) 7.3 Hz,
1H, H3), 7.10 (s, 1H, H11), 7.04 (s, 1H, H11A), 6.97 (s, 2H,
H9), 6.83 (s, 2H, H9A), 6.64 (d, J ) 7.3 Hz, 1H, H3A), 2.44 (s,
6H, CH3), 2.41(s, 6H, CH3), 0.94 (s, 3H, CH3-Pd). 13C{1H}
NMR (CDCl3, δ): 146.9, 146.1, 140.2, 139.2, 131.4, 131.1, 130.7,
129.5, 129.0, 126.9, 126.5, 125.4, 125.3, 119.2, 119.1, 21.8, 4.1.
[P d (COD-OMe)(p-MeOC6H4-BIAN)](P F 6) (6a ) a n d [P d -
(COD-OMe)(Tol-BIAN)](P F 6) (6b). They were prepared by
the method reported for 6l.17
In this work we have measured the relative coordina-
tion ability of different Ar-BIAN ligands toward several
palladium complexes, chosen so as to represent all the
commonly encountered ones. A linear correlation be-
tween the relative coordination strength and either the
Hammett σ constant of the substituents on the ligands
or the pKa of the aniline employed in their synthesis
was always found, but the slope of the plots is highly
variable. This allowed us to define a new experimental
parameter, FBIAN, which is a measure of the Lewis
acidity of the metal fragment investigated. Until now,
an experimental measure of this kind was limited to
few classes of compounds and only calculations could
be used in a general way. However, the charge distribu-
tion generated by calculations strongly relies on the
method and the parameters employed in the calculation
itself, so that a comparison between calculations re-
ported in different papers is seldom possible. Finally,
it is worth mentioning that the method here described
only employs apparatus that is available in any research
laboratory. No sophisticated technique is needed, so that
anybody can measure the parameter for any compound
he/she is interested in. The next step will be trying to
correlate the FBIAN values to other physical or chemical
properties, especially to catalytic activity, a field where
the Lewis acidity of the metal is well-known to play an
important role.
6a . Yield: 73.4%. Anal. Calcd for C35H35N2O3PdPF6: C,
1
53.68; H, 4.51; N, 3.58. Found: C, 53.62; H, 4.51; N, 3.57. H
NMR (CDCl3, δ): 8.05 (d, J ) 8.3 Hz, 2H, H17), 7.48 (pst, 2H,
H16), 7.39 (d br, 4H, H10), 7.11 (d, J ) 8.9 Hz, 4H, H11), 6.85
(d, J ) 7.3 Hz, 2H, H15), 5.73 (m, 1H, H4), 5.23 (m, 1H, H5),
3.92 (s, 6H, OCH3), 3.47 (m, 1H, H8), 2.86 (br, 1H, H6), 2.73
(s, 3H, COD-OCH3), 2.54 (br, 2H, H6A, H1), 2.27 (m, 3H, H2,
H3, H3A), 1.95 (m, 2H, H7, H7A), 1.47 (m, 1H, H2A). 13C{1H}
NMR (CDCl3, δ): 159.5, 138.9, 132.2, 131.6, 128.9, 126.2, 122.4,
115.9, 109.1, 108.5, 81.3, 56.2, 56.1, 33.5, 31.7, 28.4, 26.7.
6b. Yield: 70.5%. Anal. Calcd for C35H35N2OPdPF6: C,
Exp er im en ta l Section
Gen er a l P r oced u r es. The synthesis of the ligands was
carried out as previously reported.5 NMR spectra were re-
corded under N2 on a Bruker AC 300 FT, operating at 300
MHz for 1H, at 75 MHz for 13C, and at 121 MHz for 31P, at 20
°C. The 1H NMR and 13C NMR signals of the compounds
described in the following have been attributed by HSQC
techniques and by comparison with the spectra of previously
reported members of the same series of compounds. CDCl3 was
purified by passing it through a basic alumina column, drying
with activated molecular sieves, and degassing by three
1
55.97; H, 4.70; N, 3.73. Found: C, 55.95; H, 4.81; N, 3.48. H
NMR (CDCl3, δ): 8.04 (d, J ) 8.3 Hz, 2H, H17), 7.46 (pst, 2H,
(16) van Asselt, R.; Gielens, E. E. C. G.; Ru¨lke, R. E.; Vrieze, K.;
Elsevier, C. J . J . Am. Chem. Soc. 1994, 116, 977-985.
(17) Bellachioma, G.; Binotti, B.; Cardaci, G.; Carfagna, C.; Mac-
chioni, A.; Sabatini, S.; Zuccaccia, C. Inorg. Chim. Acta 2002, 330, 44-
51.

Lewis Acidity of Ar-BIAN Complexes
Organometallics, Vol. 23, No. 5, 2004 1001
Ta ble 3
H16), 7.43 (d, J ) 8.3 Hz, 2H, H15), 7.34 (br, 4H, H10), 6.79
(d, J ) 7.2 Hz, 4H, H11), 5.72 (m, 1H, H4), 5.24 (m, 1H, H5),
3.47 (m, 1H, H8), 2.87 (m, 1H, H6), 2.68 (s, 3H, COD-OCH3),
2.57 (m, 1H, H1), 2.52 (br, 1H, H6A), 2.49 (s, 6H, CH3), 2.27
(br, 3H, H2, H3, H3A), 1.95 (m, 2H, H7, H7A), 1.46 (m, 1H,
H2A).
1H NMR of
amt of
ligand (ppm)
complex
Pd(Ar-BIAN)(L)
(10-3 mmol) coord
free
Pd(Tol-BIAN)(MA) (2b)
Pd(Tol-BIAN)(FN) (3b)
Pd (Tol-BIAN) (TCNE) (4b)
[Pd (Tol-BIAN)(η3-C4H7)][PF6] (7b)
Pd(3,5-(CH3)2C6H3-BIAN)-
(CH3)(Cl) (5c)
5.859
5.325
4.072
3.529
4.523
2.50
2.50
2.53
2.51
2.41,
2.46
2.46
2.46
2.46
2.38
[CH2dC(CH3)CH2OP (NMe2)3][P F 6] (8). This product was
prepared by reaction of 2-methyl-2-propen-1-ol, CCl4, and tris-
(dimethylamino)phosphine following the procedure reported
for related compounds.18 Anal. Calcd for C10H25N3OP2F6: C,
31.67; H, 6.64; N, 11.08. Found: C, 31.33; H, 6.44; N, 10.80.
1H NMR (CDCl3, δ): 5.1 (d, J H-P ) 14.4 Hz, 2H, CH2), 4.59 (d,
J H-P ) 7.1 Hz, 2H, CH2O), 2.83 (d, J H-P ) 10.1 Hz, 18H,
N(CH3)2), 1.83 (s, 3H, CH3). 31P NMR (CDCl3, 298 K): δ 37.1
(s, P(NMe2)3), -143.7 (sept., J P-F ) 712 Hz, PF6-).
2.44
Exch a n ge Exp er im en ts. A solution of a known amount
of Pd(L)(Ar-BIAN) in 0.6 mL of CDCl3 was introduced in an
NMR tube. A spectrum was recorded, after which the second
ligand (1 equiv with respect to the complex for ligands having
electron-donating substituents on the aryl ring and 2-6 equiv
for ligands having electron-withdrawing substituents) was
added to the NMR tube. A spectrum was recorded immediately
and then every 15 min at 20 °C, until two consecutive spectra
did not show any further variation. The value of the ratio
between the integrated intensities of the peaks due to the
methyl groups of coordinated and free ligand allows further
calculations to obtain the equilibrium constant. The mathe-
matical procedure was illustrated in a previous publication.5
The identity and amounts of complexes 2-5 and 7 employed
and the position of the signals for the free and coordinated
ligands used for the integration are reported in Table 3.
In the case of complexes 6, compound 6a (3.793 × 10-3
mmol) was employed as a starting material and the signal for
the methyl group on the COD-OMe moiety was used for the
quantification of the starting and exchanged complexes. The
following values were observed for the aforementioned sig-
nals: 6a , 2.73 ppm; 6b, 2.68 ppm; 6c, 2.71 ppm; 6d , 2.63 ppm;
6e, 2.79 ppm; 6f, 2.63 ppm; 6g, 2.64 ppm.
[P d (Tol-BIAN)(η3-CH2C(CH3)CH2)][P F 6] (7b). This com-
pound was prepared by the method reported for 7l.19 The
required 8 was prepared as described above.
Yield 50.1%. Anal. Calcd for C30H27N2F6PPd: C, 54.03; H, 4.08;
N, 4.20. Found: C, 54.44; H, 4.28; N, 3.81. H NMR (CDCl3,
Ack n ow led gm en t. We thank Dr. A. Caselli for help
in discussing NMR spectra and the MIUR (Programmi
di Ricerca Scientifica di Rilevante Interesse Nazionale)
for financial support.
1
δ): 8.10 (d, J ) 8.3 Hz, 2H, H9), 7.53 (pst, 2H, H8), 7.41 (m,
8H, H11, H12), 7.18 (d, J ) 7.3 Hz, 2H, H7), 3.42 (s, 2H, H1syn
,
H3syn), 3.30 (s, 2H, H1anti, H3anti), 2.52 (s, 6H, CH3), 2.16 (s,
3H, (CH2)2CCH3). 13C{1H} NMR (CDCl3, δ): 146.6, 138.8,
136.4, 132.3, 131.7, 131.0, 128.9, 126.0, 120.9, 64.0, 23.9, 21.7.
Su p p or tin g In for m a tion Ava ila ble: Plots of log Keq vs
Hammett σ values and of log Keq vs pKa values of the aniline
employed in the synthesis of the Ar-BIAN ligands for the
reactions in eqs 3 and 4. This material is available free of
charge via the Internet at http://pubs.acs.org.
(18) (a) Castro, B.; Selve, C. Bull. Soc. Chim. Fr. 1971, 2296-2298.
(b) Castro, B.; Selve, C. Bull. Soc. Chim. Fr. 1971, 4368-4373.
(19) Mechria, A.; Rzaigui, M.; Bouachir, F. Tetrahedron Lett. 2000,
41, 7199-7202.
OM034370I