Imine-enamine tautomeric equilibrium of palladium imidoyl complexes

Article abstract of DOI:10.1021/om9905557

The reaction of benzylpalladium complexes of type trans-[Pd(CH₂C₆H₄Y)(X)(PR₃) ₂] (2) with isocyanides yields imidoyl complexes that exist in solution as equilibrium mixtures of the corresponding imine ([Pd(C(=NR′)CH₂C₆H₄Y)(X)(PR ₃)₂],. 3-im) and enamine ([Pd-(C(NHR′=CHC₆H₄Y)(X)(PR₃) ₂, 3-en) tautomers. While the equilibrium constant is markedly affected by the electronic effect exerted by the substituents at the phenyl ring (Y), the effect of the metal fragment is less pronounced and is dominated by steric factors. Both tautomeric forms can also be found in the solid state, and the X-ray structures of complexes of type 2, 3-im, and 3-en have been determined.

Full text of DOI:10.1021/om9905557

Organometallics 1999, 18, 5225-5237
5225
Im in e-En a m in e Ta u tom er ic Equ ilibr iu m of P a lla d iu m
Im id oyl Com p lexes
J uan Ca´mpora,* Sarah A. Hudson, Philippe Massiot, Celia M. Maya,
Pilar Palma, and Ernesto Carmona*
Departamento de Quı´mica Inorga´nica-Instituto de Investigaciones Quı´micas, Universidad de
Sevilla-Consejo Superior de Investigaciones Cientı´ficas, C/ Ame´rico Vespucio s/ n,
Isla de la Cartuja, 41092 Sevilla, Spain
Luis A. Mart´ınez-Cruz and Angel Vegas*
Instituto de Quı´mica-Fı´sica Rocasolano, Departamento de Cristalografı´a, Consejo Superior de
Investigaciones Cientı´ficas, C/ Serrano 119, E-28006 Madrid, Spain
Received J uly 16, 1999
The reaction of benzylpalladium complexes of type trans-[Pd(CH2C6H4Y)(X)(PR3)2] (2) with
isocyanides yields imidoyl complexes that exist in solution as equilibrium mixtures of
the corresponding imine ([Pd(C(dNR′)CH2C6H4Y)(X)(PR3)2], 3-im ) and enamine ([Pd-
(C(NHR′dCHC6H4Y)(X)(PR3)2, 3-en ) tautomers. While the equilibrium constant is markedly
affected by the electronic effect exerted by the substituents at the phenyl ring (Y), the effect
of the metal fragment is less pronounced and is dominated by steric factors. Both tautomeric
forms can also be found in the solid state, and the X-ray structures of complexes of type 2,
3-im , and 3-en have been determined.
In tr od u ction
equilibrium may sometimes play an important role in
the reactivity of acyl or imidoyl complexes.6
Tautomeric equilibrium is emerging as an important
process in organometallic chemistry.1 Very recently,
equilibria between metal acyl hydrides and hydroxy-
carbene complexes1a and between metal-vinylidene and
-π-alkyne species1b,c have been demonstrated. Similar
equilibria are often implicated in the stabilization of
ligands that contain electronegative atoms.1d-f The keto/
enol or imine/enamine tautomerization of acyl2 or imi-
doyl3 transition metal complexes has been observed in
favorable cases, but in these instances the preference
of the enol or enamine tautomer over the more common
keto or imino form is due to the formation of hydrogen
bonds or to the existence of appreciably acidic C-H
bonds. Although the observation of keto/enol or imine/
enamine tautomerism in simple acyl or imidoyl com-
plexes is still uncommon, it may be related to the more
familiar process well-known for organic ketones4 or
imines.5 As in these organic compounds, the tautomeric
In this contribution we would like to describe the
attainment of simple, tautomeric equilibria in palladium
imidoyl complexes, [Pd]-C(dNR′)CH2Ar, that do not
require any special stabilization of the enamine form,
[Pd]-C(NHR′)dCHAr. In our view, what makes this
system unusual is the existence in solution of both
tautomers in measurable concentrations, therefore al-
lowing the determination of their relative stabilities. A
preliminary report on this work has appeared;7 herein
we address in full the factors that govern such equilibria
and report in addition the crystal structure determina-
tions of the imine and enamine forms of closely related
derivatives.
Resu lts a n d Discu ssion
Syn th esis of P a lla d iu m Im id oyl Com p lexes. Pal-
ladium imidoyl complexes have been prepared (Scheme
2) by reaction of the corresponding benzylpalladium
derivatives 2 with isocyanides. Although the parent
benzyl complex Pd(CH2Ph)Cl(PMe3)2, 2a , had been
previously reported by Milstein and co-workers,8 we
(1) (a) Casey, C. P.; Czerwinski, C. J .; Fusie, K. A.; Hayashi, R. K.
J . Am. Chem. Soc. 1997, 119, 3971. (b) de los Rios, I.; J ime´nez Tenorio,
M.; Puerta, M. C.; Valerga, P. J . Am. Chem. Soc. 1997, 119, 6529. (c)
Wakatsuki, Y.; Koga, N.; Werner, H.; Morkuma, K. J . Am. Chem. Soc.
1997, 119, 360. (d) Albert, J .; Gonza´lez, A.; Granell, J .; Moragas, R.;
Puerta, M. C.; Valerga, P. Organometallics 1997, 16, 3775. (e) Gosh,
P.; Pramanik, A.; Chakravorty, A. Organometallics 1996, 15, 4147. (f)
Fairlie, D. P.; Woon, T. C.; Wickramasinghe, A.; Willis, A. C. Inorg.
Chem. 1994, 33, 6425.
(4) (a) March, J . In Advanced Organic Chemistry; J ohn Wiley: New
York, 1992. (b) Ha¨felinger, G.; Hack, H. In The Chemistry of Enamines;
Patai, S., Rappaport, Z., Eds.; J ohn Wiley: New York, 1994; p 1. (c)
Enamines: Synthesis, Sructure and Reactions; Cook, A. G., Ed.; Marcel
Dekker: New York, 1988.
(2) O’Connor, J . M.; Uhrhammer, R.; Rheingold, A. L.; Roddick, D.
M. J . Am. Chem. Soc. 1991, 113, 4530.
(3) (a) Al´ıas, F. M.; Belderra´ın, T. R.; Paneque, M.; Poveda, M. L.;
Carmona, E. Organometallics 1997, 16, 301. (b) Al´ıas, F. M.; Belder-
ra´ın, T. R.; Paneque, M.; Poveda, M. L.; Carmona, E. Organometallics
1998, 17, 5620. (c) Veya, P.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, L.
Organometallics 1993, 12, 4899. (d) Henderson, W.; Kemmitt, R. D.
W.; Prouse, L. J . S.; Russell, D. R. J . Chem. Soc., Dalton Trans. 1990,
781. (e) Henderson, W.; McKenna, P.; Russell, D. R.; Prouse, L. J . S.
J . Chem. Soc., Dalton Trans. 1989, 345. (f) Bertani, R.; Castellani, C.
B.; Crociani, B.; J . Organomet. Chem. 1984, 269, C15.
(5) (a) de Savignac, A.; Bon, M.; Lattes, A. Bull. Chem. Soc. Fr. 1972,
3167. (b) Albrecht, H.; Hanisch, H.; Funk, W.; Kalas, D. Tetrahedron
1972, 28, 5481. (c) Albrecht, H.; Fischer, S. Tetrahedron 1973, 29, 659.
(d) Albrecht, H.; Funk, W.; Reiner, M. T. Tetrahedron 1976, 32, 479.
(6) Clark, H. C.; Milne, C. R. C.; Payne, N. C. J . Am. Chem. Soc.
1978, 100, 1164.
(7) Ca´mpora, J .; Hudson, S. A.; Carmona, E. Organometallics 1995,
14, 2151.
(8) Milstein, D. J . Chem. Soc., Chem. Commun. 1986, 817.
10.1021/om9905557 CCC: $18.00 © 1999 American Chemical Society
Publication on Web 11/17/1999

5226 Organometallics, Vol. 18, No. 25, 1999
Ca´mpora et al.
to Pd(0)-methyl acrylate complexes.9 The latter can be
generated in situ from [Pd(η3-C3H5)(µ-Cl)]2, NaCp, and
PR3 plus CH2dCHCO2Me (Scheme 1).10 This synthetic
methodology has proved fairly general and provides
access to complexes that contain different substituents
on the aromatic ring and different phosphine coligands.
Good results have been obtained for PMe3, PEt3, and
arylphosphines of type PMe2C6H4-p-X, although the
PMe2C6H4-p-F derivative, 2k , is somewhat unstable in
solution and its isolation in the solid state has proved
unreliable. In general, better yields are achieved when
benzyl chlorides are used instead of bromides, but both
types of reagents appear to be suitable and, in practice,
the use of a particular halide was often imposed only
for reasons of commercial availability. Tables 1 and 2
show analytical and NMR properties for these com-
pounds.
Sch em e 1
The reaction of compounds 2a -l with tert-butyl
isocyanide proceeds readily at room temperature (Scheme
2), yielding the corresponding imidoyl complexes 3a -l
as yellow to orange solids, with the exception of the nitro
derivatives 3e and 3l, which exhibit a remarkable green-
red dichroism in the solid state, and complex 3k , which
was isolated as a yellow oil and purified by chromatog-
raphy. The analogous interaction of 2b with PriNC gives
the expected compound 3m in the form of orange
crystals.
In solution, these imidoyl complexes exist as a mix-
ture of the imino (3-im ) and enamino (3-en ) tautomers,
which can be identified on the basis of their spectro-
scopic properties. The solid-state structures of the
closely related imine and enamine tautomers 3c-im and
3h -en , which differ only in the nature of the PR3 ligands
(PMe3 and PEt3, respectively), have been determined
by X-ray studies (see below). The chloride ligand of
complexes 3a and 3b has been exchanged by Br-, by
metathesis with an excess of KBr, giving rise to com-
plexes 3n and 3o, but as shown in Table 3, this
metathesis has very little effect on the corresponding
tautomeric ratios.
Sch em e 2
1
The H and 13C{1H} NMR spectra of these mixtures
show the expected features for the imidoyl and enamino
functionalities (Tables 4 and 5). The iminoacyl (CdN)
resonance of compounds of type 3-im appears at δ 173-
177 ppm (at somewhat lower field, 184 ppm, in the Pri-
NC complex 3m -im ). The Pd-bonded carbon atom of the
enamines 3-en resonates at higher field, in the 160-
172 ppm region, whereas the â-carbon gives rise to a
singlet signal between 100 and 102 ppm, i.e., the zone
characteristic of organic enamines.11 Although the
quaternary carbon resonances of the imine or enamine
functionalities (M-C) are not appreciably split by
coupling with the phosphorus nuclei, they appear
somewhat broad, making difficult their observation for
the tautomers that are present in a low ratio.
Some of the 1H NMR signals due to the two tautomers
of each complex allow their ready quantification in the
NMR spectra of the mixtures (Table 4). For instance,
(9) Ca´mpora, J .; Graiff, C.; Palma, P.; Carmona, E.; Tiripicchio, A.
Inorg. Chim. Acta 1998, 269, 191.
(10) (a) Binger, P.; Bu¨ch, H. M.; Benn, R. Myntt, R. Angew. Chem.,
Int. Ed. Engl. 1982, 21, 62. (b) Schwager, H.; Benn, R.; Wilke, G.
Angew. Chem., Int. Ed. Engl. 1987, 26, 67.
(11) Chiara, J . L.; Go´mez-Sa´nchez, A. In The Chemistry of Enamines;
Patai, S., Rappaport, Z., Eds.; J ohn Wiley: New York, 1994; p 279.
have found that a convenient preparative route to these
derivatives is the oxidative addition of benzyl halides

Imine-Enamine Tauatomeric Equilibrium
Organometallics, Vol. 18, No. 25, 1999 5227
Ta ble 1. An a lytica l a n d 31P {1H} a n d 1H NMR Da ta for Ben zyl Com p lexes
anal. dataa
1H NMR, δ (J , Hz)b
CH2
c
d
C
H
31P{1H}
-16.1
PR3
CHar
X ) Cl, Y ) H, R ) Me, 2a e
X ) Cl, Y ) CF3, R ) Me, 2bf
40.5(40.5)
6.5(6.5)
1.29 (t, 3.4, Me)
2.66 (t, 8.2)
6.98 (t, 1 H, 7.3)
7.06 (t, 2 H, 7.6)
7.28 (d, 2 H, 7.5)
7.24 (d, 2 H, 7.8)
7.33 (d, 2H, 8.0)
6.95 (br s, 4 H)
6.87 (t, 1 H, 6.3)
7.02 (t, 1 H, 7.4)
7.34 (d, 1 H, 7.9)
7.77 (d, 1 H, 7.5)
7.35 (d, 2 H, 8.1)
7.92 (d, 2 H, 8.1)
7.39 (d, 2 H, 8.2)
7.43 (d, 2 H, 8.3)
7.42 (d, 2 H, 8.1)
7.60 (d, 2 H, 8.1)
7.26 (d, 2 H, 8.5)
7.38 (d, 2 H, 8.4)
6.80 (d, 2 H, 8.1)
7.10 (d, 2 H, 8.1)
37.6(37.1)
5.3(5.3)
-15.4
1.00 (t, 3.2, Me)
2.63 (t, 7.6)
X ) Br, Y ) Br, R ) Me, 2ce
X ) Br, Y ) o-Br, R ) Me, 2d g
31.0(30.7)
30.5(30.7)
4.8(4.7)
4.7(4.7)
-17.7
-16.6
1.12 (br s, Me)
1.26 (t, 3.3, Me)
2.46 (t, 8.0)
2.68 (t, 7.5)
X ) Br, Y ) NO2, R ) Me, 2eh,e
X ) Br, Y ) CN, R ) Me, 2fi,g
X ) Cl, Y ) CF3, R ) Et, 2gj
X ) Br, Y ) Br, R ) Et, 2h j
33.2(32.9)
37.5(37.0)
44.8(44.7)
38.3(38.5)
49.8(49.9)
5.2(5.1)
5.3(5.3)
7.0(6.7)
5.9(6.1)
4.7(4.9)
-20.3
-15.9
14.3
1.36 (t, 3.4, Me)
1.39 (t, 3.4, Me)
1.08 (pq, 7.8, Me)
2.80 (t, 7.8)
2.84 (t, 7.9)
2.71 (t, 7.1)
1.79 (ct, 7.6d,3.6, CH2)
1.06 (pq, 7.8, Me)
1.84 (ct, 7.5d,3.6, CH2)
1.71 (t, 3.3, Me)
12.4
2.71 (t, 7.2)
2.47 (t, 7.5)
X ) Cl, Y ) CF3, PR3 ) PMe2Ph, 2ig
-4.6
7.41 (m, 6 H, Ph)
7.55 (m, 4 H, Ph)
1.62 (t, 2.9, Me)
X ) Cl, Y ) CF3, PR3 ) PMe2Ar, 2jk,g
(Ar ) C6H4-p-NMe2)
50.3(50.7)
46.0(46.1)
5.5(5.8)
4.7(4.7)
5.7
2.51 (t, 7.5)
2.56 (t, 7.2)
6.84 (d, 2 H, 8.0)
7.12 (d, 2 H, 7.9)
2.99 (Me-N)
6.71 (t, 2 H, 8.3d,CHar
)
7.39 (m, 2 H, CHar
1.78 (t, 3.3, Me)
7.42 (m, 6 H, Ph)
7.56 (m, 4 H, Ph)
)
X ) Br, Y ) NO2, PR3 ) PMe2Ph, 2ll,g
-5.6
6.65 (d, 2 H, 8.6)
7.65 (d, 2 H, 8.7)
a
b
d3
g
h
Calculated values in parentheses. Singlets unless otherwise indicated. c Apparent or real J HP
.
J
.
e CDCl3. f C6D6. CD2Cl2. N:
HH
i
j
k
l
3.0(2.9). N: 3.1(3.1). CD3COCD3. N: 4.3(4.2). N: 2.2(2.3).
in acetone-d6, the imino form is characterized by a
singlet resonance at δ 3.5-4.2 ppm due to the methyl-
ene group, while the vinylamino group of the enamine
species gives rise to two signals at δ 4.0-6.0 ppm (broad
Ch a r t 1
4
singlet) and δ 5.4-5.9 ppm (triplet, J HP ca. 3 Hz),
corresponding to the amino (NH) and vinyl (dCH-Ar)
protons, respectively. The integration of directly com-
parable aromatic signals or, in some cases, of the
resonances due to the methyl groups of the phosphine
ligands of both tautomers provides an accurate mea-
surement of the equilibrium constant (Table 3).
The 1H and 13C{1H} spectra of both the imine and the
enamine forms of compounds 3i-l, which bear arylphos-
phines of type PMe2(C6H4-p-X), display two resonances
for the phosphine methyl groups. The diastereotopic
character of these groups reveal that the bulky imidoyl
or R-enamino ligands are not contained in the coordina-
tion plane and that rotation around the Pd-C bond does
not take place readily at room temperature.
In principle both the enamine and the imine species
are expected to occur as mixtures of geometrical isomers
(Chart 1). However, only one species is observed for each
tautomer. In the enamines, the splitting of the vinylic
proton due to coupling to the two 31P nuclei (4J HP ≈ 3
Hz) suggests a z distribution of the aryl and [Pd]
moieties across the CdC bond. This configuration, which
allows for the conjugation of the N lone pair, the double
bond, and the aromatic ring, is also the more favorable
for organic secondary enamines, although for these,
minor amounts of the e-enamine tautomer are fre-
quently present.5a Scheme 3 shows the results of
NOESY experiments for the tautomeric mixtures 3g-
im /3g-en and 3o-im /3o-en . They reveal NOE cross-
peaks between the vinylic and the t-Bu protons, in
accord with a ze configuration, supported in addition by
other NOE relationships, e.g., that between the NH

5228 Organometallics, Vol. 18, No. 25, 1999
Ta ble 2. 13C{1H} NMR Da ta for Ben zyl Com p lexesa
Ca´mpora et al.
b
PR3
CH2
aromatics
121.0, 127.4, 128.9 (1, 2, 2 CH)
X ) Cl, Y ) H, R ) Me, 2a c
X ) Cl, Y ) CF3, R ) Me, 2bd
14.4 (t, 13.2, Me)
18.0 (t, 14.2, Me)
19.2 (t, 16.3)
152.8 (Car
)
21.2 (br s)
129.7 (d, 4,e 2 CH)
134.9 (2 CH)
130.2 (q, 32,e C-CF3)
158.4 (Car
)
X ) Br, Y ) Br, R ) Me, 2cc
X ) Br, Y ) o-Br, R ) Me, 2d f
X ) Br, Y ) NO2, R ) Me, 2ec
X ) Br, Y ) CN, R ) Me, 2ff
14.3 (t, 14.3, Me)
14.4 (t, 14.6, Me)
14.5 (t, 14.7, Me)
14.7 (t, 14.7, Me)
19.3 (br s)
20.5 (br s)
19.7 (br s)
19.9 (br s)
117.3 (Car-Br)
130.4, 131.1 (2,2 CH)
145.9 (Car
125.5 (Car
)
)
126.3, 127.0, 132.1, 133.5 (4 CH)
147.8 (Car
123.1, 129.6 (2, 2 CH)
)
144.4, 156.6 (2 Car
144.4, 156.6 (2 Car
107.5 (C-CN)
119.8 (CN)
)
)
130.3, 131.9 (2 CH)
154.5 (Car
)
X ) Cl, Y ) CF3, R ) Et, 2gd
8.6 (Me)
14.8 (t, 12.4, CH2)
g
125.1 (2 CH)
127.6 (q, 31,e C-CF3)
131.1 (2 CH)
154.6 (Car
117.6 (Car
130.2, 131.8 (2, 2 CH)
146.6 (Car
124.4 (q, 3.7, 2 CH)
125.1 (q, 271.3, CF3)
125.7 (q, 31.9, C-CF3)
129.3 (2 CH)
)
)
X ) Br, Y ) Br, R ) Et, 2h c
8.2 (Me)
14.6 (t, 12.8, CH2)
16.8 (br s)
20.5 (br s)
)
X ) Cl, Y ) CF3, PR3 ) PMe2Ph, 2if
13.1 (t, 14.4, Me)
128.8 (t, 4.6, 4 CH)
130.1 (2 CH)
131.5 (t, 5.8, 4 CH)
134.6 (t, 20.7, 2 Car-P)
13.7 (t, 14.4, Me)
40.2 (Me-N)
112.1 (t, 5.1, 4 CH)
118.6 (t, 24.4, 2 Car-P)
132.8 (t, 6.6, 4 CH)
151.8 (2 Car-N)
14.5 (t, 14.8, Me)
128.8 (t, 4.4, 4 CH)
130.3 (2 CH)
151.9 (Car
)
X ) Cl, Y ) CF3, PR3 ) PMe2Ar, 2jf
(Ar ) C6H4-p-NMe2)
20.9 (br s)
124.4 (q, 3.4,e 2 CH)
125.3 (q, 31.9, C-CF3)
129.3 (2 CH)
125.3 (q, 271.2, CF3)
152.5 (Car
)
X ) Br, Y ) NO2, PR3 ) PMe2Ph, 2lf
23.8 (br s)
122.9, 129.1 (2, 2 CH)
144.3, 156.3 (2 Car
)
131.6 (t, 5.6, 4 CH)
134.4 (t, 20.9, 2 Car-P)
a
b
d
e
g
Singlets unless otherwise indicated. Apparent or real J CP
.
c CDCl3. CD3COCD3. J CF
.
f CD2Cl2. Not observed.
Sch em e 3
3-im /3-en tautomeric mixture, the fact that only one
geometric isomer is always observed for each tautomeric
form and the similarity of the NMR spectra suggest that
the same geometries are adopted by each pair of
tautomers.
The IR spectrum of solid samples of compounds of
type 3 usually display absorption bands due to either
the imine or the enamine tautomer. As found in related
iminoacyl complexes of Ni,12 imidoyl absorptions
(νCdN) occur at ca. 1630 cm-1, and two or more bands
may be observed. Although the band associated with the
vinyl stretch νCdC is difficult to assign, the presence of
the enamine tautomer is unequivocally revealed by the
characteristic N-H absorption at ca. 3400 cm-1. These
IR studies show that when the imine and the enamine
forms have comparable stability, sample crystallization
may result in variable composition mixtures of the
tautomers or even in the unexpected isolation of one
pure tautomer. For example under seemingly similar
resonance and the signals associated with the PR3 alkyl
groups. For the imines, the observation of cross-peaks
between the t-Bu protons and those of the PR3 ligands
and the absence of such effects between the former and
the methylene protons of the iminoacyl ligand are
suggestive of a z-type conformation. Although 2D-
NOESY experiments have not been recorded for every
(12) Carmona, E.; Palma P.; Paneque, M.; Poveda, M. L. Organo-
metallics 1990, 9, 583.

Imine-Enamine Tauatomeric Equilibrium
Organometallics, Vol. 18, No. 25, 1999 5229
Ta ble 3. An a lytica l a n d IR Da ta a n d Im in e:En a m in e Ra tioa for Im id oyl Com p lexes
anal. datab
H
IR
C
N
isolated tautomer
ν(NH)c
ν(CdN)c
Kim/en
X ) Cl, Y ) H, R ) Me, 3a
X ) Cl, Y ) CF3, R ) Me, 3b
X ) Br, Y ) Br, R ) Me, 3c
X ) Br, Y ) o-Br, R ) Me, 3d
X ) Br, Y ) NO2, R ) Me, 3e
X ) Br, Y ) CN, R ) Me, 3f
X ) Cl, Y ) CF3, R ) Et, 3g
X ) Br, Y ) Br, R ) Et, 3h
X ) Cl, Y ) CF3, PR3 ) PMe2Ph, 3i
X ) Cl, Y ) CF3, PR3 ) PMe2Ar, 3j
(Ar ) C6H4-p-NMe2)
46.5(46.2)
7.5(7.3)
6.0(6.2)
5.9(5.6)
6.0(5.6)
5.8(5.9)
6.1(6.1)
7.3(7.3)
6.6(6.7)
5.5(5.6)
6.2(6.3)
2.8(3.0)
2.6(2.6)
2.2(2.4)
2.5(2.4)
5.1(5.0)
5.3(5.2)
2.5(2.3)
2.1(2.1)
2.3(2.1)
5.7(5.6)
im
im + en
im
im
en
en
en
en
en
en
1637
1633, 1610
1635
12.0
0.7
3.5
3.2
<0.1
0.1
<0.1
0.2
0.3
42.3(42.5)
36.8(36.5)
36.6(36.5)
38.3(38.7)
42.4(42.4)
48.3(48.4)
42.4(42.6)
52.8(52.7)
53.0(53.1)
3340
1620
3370
3407
3440
3440
3430
3422
0.4
X ) Cl, Y ) CF3, PR3 ) PMe2Ar, 3k
(Ar ) C6H4-p-F)
0.3
X ) Br, Y ) NO2, PR3 ) PMe2Ph, 3l
X ) Cl, Y ) CF3, R ) Me, 3m d
X ) Br, Y ) H, R ) Me, 3n
X ) Br, Y ) CF3, R ) Me, 3o
49.5(49.3)
41.4(41.4)
42.2(42.1)
39.4(39.3)
5.5(5.5)
6.2(5.9)
6.7(6.6)
5.8(5.7)
4.2(4.1)
2.6(2.7)
2.7(2.7)
2.6(2.4)
en
im
im
im + en
3419
3340
0.1
6.7
10.0
0.8
1640, 1615
1614, 1596
1633, 1610
a
b
d
Acetone-d6. Calculated values in parentheses. c Nujol mull, cm-1
.
PriNC derivative.
Sch em e 4
F igu r e 1. ORTEP view and atom-labeling scheme of
compound 2c.
in a slightly distorted square-planar environment, with
the phosphine ligands occupying mutually trans posi-
tions. The distances and angles in compound 2c are as
expected for a trans-Pd(R)X(PR3)2 complex.13 Its Pd-C
bond length (2.078(13) Å) is within the range expected
for this functionality (2.03-2.07 Å) and very close to the
Pd-C distance found in trans-Pd(Et)Br(PMe3)214 (2.063-
(5) Å). However, the latter compound shows a Pd-Br
bond length that is ca. 0.03 Å longer than that of 2c,
reflecting a somewhat smaller trans influence of the
4-bromobenzyl group as compared to the ethyl ligand.
As can be readily seen from Figures 2 and 3, com-
pounds 3c and 3h crystallize in different tautomeric
forms. Compound 3c appears as the imine tautomer,
conditions, different batches of compound 3b sometimes
afforded pure crystalline samples of either the imine or
the enamine tautomers.
It is worth mentioning at this point that the reaction
of compound 3b with the chelating phosphine 1,2-bis-
(dimethylphosphino)ethane (dmpe) affords a cationic
complex 4 (Scheme 4), for which IR and NMR data
demonstrate the presence of only the enamine form,
both in solution and in the solid state.
Cr ysta l Str u ctu r es of Com p ou n d s 2c, 3c-im , a n d
3h -en . Figures 1, 2, and 3 show the crystal structures
of compounds 2c, 3c-im , and 3h -en , respectively.
Selected bond distances and angles are summarized in
Tables 8-10.
(13) Canty, A. J . In Comprehensive Organometallic Chemistry; Abel,
E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: New York, 1995;
Vol. 9, p 225.
(14) Osakada, K.; Ozawa, Y.; Yamamoto, A. J . Chem. Soc., Dalton
Trans. 1991, 759.
The structures of all three compounds show a Pd atom

5230 Organometallics, Vol. 18, No. 25, 1999
Ta ble 4. 31P {1H} a n d 1H NMR Da ta for Im id oyl Com p lexesa
Ca´mpora et al.
imine
enamine
1H NMR, δ (J , Hz)
But CH2
1H NMR, δ (J , Hz)
CHd
31P{1H}
PR3
CHar
31P{1H}
PR3
But
NH
CHar
b
c
b
c
3a e
-18.0 1.23 (t, 3.6, Me)
1.39 3.82 7.17 (tm, 1H, 7.4)
7.27 (t, 2H, 7.5)
-17.9 1.30 (t, 3.4, Me)
1.38 5.60 (t, 3.1) 4.69 (br s) 6.76 (tm, 1H, 7.2)
7.07 (tm, 2H, 7.8)
7.46 (dm, 2H, 7.5)
1.36 3.94 7.59 (4H)
7.94 (d, 2H, 7.2)
1.42 5.70 (t, 2.9) 5.21 (br s) 7.33 (d, 2H, 8.4)
8.13 (d, 2H, 8.3)
1.40 5.62 (t, 3.2) 4.91 (br s) 7.18 (dm, 2H, 8.7)
7.91 (dm, 2H, 8.6)
1.45 5.93 (t, 3.0) 5.50 (br s) 6.68 (tm, 1H, 8.3)
7.39 (dm, 1H, 7.0)
3be
3ce
3d e
-18.2 1.31 (t, 3.3, Me)
-19.9 1.37 (t, 3.8, Me)
-20.0 1.37 (t, 3.3, Me)
-17.7 1.33 (t, 3.6, Me)
1.36 3.82 7.33 (dm, 2H, 8.6) -19.4 1.37 (t, 3.5, Me)
7.42 (dm, 2H, 8.5)
1.38 4.20 7.11 (tm, 1H, 7.8)
7.28 (tm, 1H, 9.0)
-20.1 1.30 (t, 3.6, Me)
7.55 (dm, 1H, 9.0)
9.10 (dm, 1H, 8.2)
7.60 (dm, 1H, 8.5)
4.04 7.29 (bd, 2H)
7.59 (bd, 2H)
3ee
3fe
-20.0
f
f
-19.4 1.41 (t, 3.8, Me)
-19.9 1.38 (t, 7.5, Me)
1.46 5.88 (t, 3.0) 6.04 (br s) 7.91 (dm, 2H, 8.0)
8.10 (bd, 2H, 8.4)
1.43 5.73 (t, 3.1) 5.52 (br s) 7.33 (d, 2H, 8.8)
8.08 (d, 2H, 8.4)
-18.1 1.41 (t, 3.5, Me)
1.35 3.96 7.54 (d, 2H, 8.3)
7.64 (d, 2H, 8.3)
3ge
9.9 1.09 (pq, 7.0, Me)
f
3.59 7.42 (d, 2H, 8.2)
7.60 (d, 2H, 7.8)
10.0 1.08 (pq, 8.1, Me)
1.75 (m, 1H, CH2)
1.41 5.69 (t, 2.8) 4.44 (br s) 7.31 (d, 2H, 8.1)
8.15 (d, 2H, 8.2)
1.90 (m, 1H, CH2)
-19.9 1.00 (pq, 7.8, Me)
1.75 (m, 1H, CH2)
3h e
3ie
-19.4 1.12 (pq, 7.8, Me)
1.35 3.77 7.26 (d, 2H, 8.0)
7.28 (d, 2H, 8.0)
1.31 5.44 (t, 3.5) 3.99 (br s) 7.11 (d, 2H, 8.0)
7.79 (d, 2H, 8.0)
1.85 (m, 1H, CH2)
-10.5 1.62 (t, 6H, 3.3, Me) 1.28 3.85 7.77 (d, 2H, 7.9)
-9.5 1.56 (t, 6H, 3.6, Me) 1.05 5.59 (t, 3.3) 4.18 (br s) 7.22 (d, 2H, 8.3)
7.49 (m, 6H, Ph)
8.05 (m, 4H, Ph)
1.73 (t, 6H, 3.7, Me)
7.35 (m, 6H, Ph)
7.72 (m, 4H, Ph)
8.04 (d, 2H, 8.3)
3je
-13.1 1.54 (t, 6H, 3.3, Me) 1.30 3.82 7.25 (d, 2H, 8.3)
-11.8 1.47 (t, 6H, 3.6, Me) 1.06 5.58 (t, 3.1) 4.09 (br s) 7.25 (d, 2H, 8.3)
1.63 (t, 6H, 4.1, Me)
2.99 (Me-N)
7.40 (d, 2H, 8.1)
1.65 (t, 6H, 3.6, Me)
2.95 (Me-N)
8.09 (d, 2H, 8.3)
6.77 (d, 4H, 8.7, CH)
7.89 (m, 4H, CH)
6.72 (d, 4H, 8.7, CH)
7.59 (m, 4H, CH)
-9.6 1.61 (t, 6H, 3.7, Me) 1.15 5.60 (t, 3.3) 4.47 (br s) 7.19 (d, 2H, 8.5)
3k e
3le
-10.3 7.25 (d, 4H, 8.8, CH) 1.30 3.90 7.33 (d, 2H, 8.1)
8.16 (m, 4H, CH)
7.43 (d, 2H, 8.2)
1.75 (t, 6H, 3.8, Me)
7.13 (q, 4H, 8.4, CH)
7.76 (m, 4H, CH)
7.99 (d, 2H, 8.2)
-10.8 1.57 (br t, 6H, Me)
f
3.86
f
-11.7 1.64 (t, 6H, 3.6, Me) 1.09 5.72 (t, 3.3) 4.93 (br s) 7.75 (d, 2H, 9.3)
1.76(br t,6H,Me)
1.84 (t, 6H, 3.7, Me)
7.36 (m, 6H, CH)
7.96 (br s, 2H)
7.71 (m, 4H, CH)
-17.1 1.31 (t, 3.8, Me)
3m e,g -16.6 1.29 (t, 3.0, Me)
h
3.82 7.65 (s, 4H)
i
5.52 (t, 2.6) 5.39 (br s) 7.26 (d, 2H, 8.0)
8.04 (d, 2H, 8.1)
3n e
1.31 (t, 3.5, Me)
1.38 3.84 7.17 (tm, 1H, 8.7)
7.27 (tm, 2H, 8.4)
f
f
5.65 (t, 3.4) 4.70 (br s) 6.76 (tm, 1H, 8.0)
7.07 (tm, 2H, 7.9)
7.43 (dm, 2H, 7.3)
1.36 3.96 7.59 (4H)
7.93 (d, 2H, 8.0)
1.42 5.73 (t, 3.2) 5.24 (br s) 7.33 (d, 2H, 8.4)
8.10 (d, 2H, 8.3)
3oe
-20.0 1.39 (t, 3.0, Me)
-19.5 1.38 (t, 3.0, Me)
4J HP. e CD3COCD3. f Not observed. CNPri derivative. Imine:
a
b
c 3
d
g
h
Singlets, unless otherwise indicated. Apparent or real J HP
.
J
HH.
3
3
5
i
3
1.11 (d, J HH ) 6.3, Me), 3.78 (ht, J HH ) 6.3, J HP ) 0.9, CH). Enamine: 1.22 (d, J HH ) 6.4, Me).
whereas the enamine form is adopted by the crystals of
3h . In both cases, the crystalline tautomer obtained is
coincident with the prevailing tautomeric form observed
in solution. The plane formed by the nitrogen atom (N1)
and the R- and â-carbons (C1 and C2, respectively) of
the organic functionality of 3c-im and 3h -en is nearly
perpendicular to the palladium coordination plane. It
seems likely that this arrangement is adopted in order
to minimize steric repulsions, but it could also be argued
that such a spatial distribution could in addition allow
some degree of dπ-π* back-bonding interaction between
the metal center and the organic fragment. This,
however, is not evident at all from other crystallographic
data. Thus, whereas the Pd-C bond distances of
2.012(4) and 2.05(2) Å (3c-im and 3h -en , respectively)
are slightly shorter than the value calculated for a
Pd-C(sp2) bond (2.25 Å) and comparable to the dis-
tances found in palladium aminocarbene complexes,9,15
the CdN (1.24(2) Å) and CdC (1.39(2) Å) distances in
3c-im and 3h -en are very similar to those found in
related organic imines4a and enamines,4b respectively,
indicating that the electron back-donation from the
metal to the ligand is very small, if it occurs at all.
The organic iminoacyl and enamine moieties of 3c-
im and 3h -en are essentially planar and display z and
ze conformations, respectively, as previously deduced
from the spectroscopic data. The aromatic ring of 3h -
en is coplanar with the N(1)-C(1)-C(2) fragment. This
may suggest that the electronic π-systems of the aro-
matic ring, the double bond, and the lone pair of the
nitrogen atom are all conjugated,5b an assumption that
(15) Dixon, K. R.; Dixon, A. C. In Comprehensive Organometallic
Chemistry; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Perga-
mon: New York, 1995; Vol. 9, p 193.

Imine-Enamine Tauatomeric Equilibrium
Organometallics, Vol. 18, No. 25, 1999 5231
Ta ble 5. 13C{1H} NMR Da ta for Im id oyl Com p lexesa
imine
enamine
But
CH2
aromatics
CN
PR3
But
CH
aromatics
121.3 (1CH)
123.6 (2CH)
127.9 (2CH)
CN
b
c
b
PR3
3a d
3b f
14.9 (t,13.5,Me)
31.0 (Me)
56.0 (C-Me)
56.0 (t,12.5) 126.2 (1CH)
128.1 (2CH)
176.9 13.7 (t,14.2,Me)
29.8 (Me)
52.5 (C-Me)
101.8
e
130.5 (2CH)
138.3 (Car
55.4 (t,13.0) 125.3 (2CH)
131.6 (2CH)
)
143.0 (Car
121.4
(q,32,gC-CF3)
123.6 (2CH)
125.2 (2CH)
)
14.7 (t,14.0,Me)
31.3 (Me)
56.4 (C-Me)
175.9 13.6 (t,14.4,Me)
30.6 (Me)
53.7 (C-Me)
101.0
166.2
144.6 (Car
)
149.2 (Car
125.9 (2CH)
131.2 (2CH)
)
3cf
15.6 (t,14.0,Me)
15.8 (t,13.8,Me)
31.4 (Me)
56.8 (C-Me)
54.6 (t,13.3) 120.0 (C-Br)
131.5 (2CH)
e
14.5 (t,14.3,Me)
e
100.7
101.2
e
53.6 (C-Me)
133.2 (2CH)
136.0 (Car
)
139.1 (Car
54.6 (t,13.2) 122.5 (Car
)
)
3d d
31.0 (Me)
174.5 14.5 (t,14.7,Me)
29.5 (Me)
122.3 (Car
)
56.3 (C-Me)
127.1 (1CH)
127.9 (1CH)
132.6 (1CH)
132.7 (1CH)
53.0 (C-Me)
125.5 (1CH)
137.8 (1CH)
147.1 (Car
)
141.9 (Car
)
3ef
3ff
14.5 (t,15.2,Me)
30.0(Me)
54.8 (C-Me)
102.7
101.8
122.7 (2CH)
124.8 (2CH)
140.5 (Car
)
152.7 (C-NO2)
102.0 (C-CN)
121.2 (CN)
15.5 (t,14.1,Me)
31.3 (Me)
57.0 (C-Me)
55.1 (t,13.5) 110.2 (C-CN)
119.7 (CN)
e
14.5 (t,15.1,Me)
30.0 (Me)
54.2 (C-Me) (t,3.5)
170.7
(t, 3.6)
132.1 (2CH)
132.2 (2CH)
123.8 (2CH)
132.3 (2CH)
145.6 (Car
)
150.0 (Car)
124.5 (2CH)
125.2 (2CH)
3gf
8.5 (Me)
14.9 (t,12.8,CH2)
29.8 (Me)
53.6 (C-Me)
102.0
101.0
164.4
159.8
148.9 (Car
)
3h d
8.8 (Me)
16.5 (t,11.5,CH2)
30.7 (Me)
e
e
e
130.2 (2CH)
130.3 (2CH)
131.5 (Car
176.5 8.3 (Me)
14.6 (t,13.1,CH2)
29.7 (Me)
52.6 (C-Me)
113.3 (C-Br)
125.1 (2CH)
130.5 (2CH)
)
142.2 (Car
)
3ih
14.2 (t,13.7,2Me)
14.9 (t,12.9,2Me)
124.6 (d,2CH)
31.1 (Me)
e
130.4 (2CH)
130.7 (2CH)
e
11.3 (t,15.2,2Me)
13.3 (t,15.2,2Me)
125.1 (d,3.6,2CH)
128.7 (t,4.5,4CH)
131.1 (t,5.7,4CH)
29.3 (Me)
53.2 (C-Me)
101.0
121.6 (q,32,gC-CF3)
123.2 (2CH)
163.0
130.0 (2CH)
147.6 (Car)
132.4 (t,6.0,4CH)
135.2 (t,21.3,2Car
174.9 11.7 (t,15.4,2Me)
127.8 (q,32.0,gC-CF3) (t,6.7) 14.3 (t,15.2,2Me)
)
3jf
14.6 (t,13.9,2Me)
15.5 (t,13.5,2Me)
40.2 (4Me-N)
112.6 (m,5.3,4CH)
120.2 (t,24.2,2C-P)
134.5 (t,6.9,4CH)
144.5 (2C-N)
14.2 (t,13.8,2Me)
15.1 (t,13.3,2Me)
130.0 (4CH)
31.4 (Me)
56.8 (C-Me)
54.6 (t,13.9) 124.7 (pq,3.5,g2CH)
29.6 (Me)
101.0
121.5 (q,31.6,gC-CF3) 166.7
(t,4.1)
53.5 (C-Me) (pt,2.7) 123.7 (2CH)
125.3 (pq,3.7,g2CH)
130.1 (q,269.9,CF3)
152.4 (Car
131.5 (2CH)
152.5 (Car
40.2 (4Me-N)
112.6 (m,5.3,4CH)
120.0 (t,25.1,2C-P)
133.3 (t,6.7,4CH)
149.2 (2C-N)
)
)
3k f
31.3 (Me)
57.0 (C-Me)
54.4 (t,14.5) 124.9 (pq,3.6,g2CH)
131.5 (2CH)
173.0 12.0 (t,15.2,2Me)
13.2 (t,15.2,2Me)
116.1 (m,4CH)
29.6 (Me)
53.7 (C-Me) (pt,3.2) 123.9 (2CH)
125.2 (pq,3.6,g2CH)
126.3 (q,269.8,gCF3)
148.5 (Car
101.3
121.9 (q,31.8,gC-CF3) 164.6
144.0 (Car
)
135.8 (4CH)
164.7 (d,249.2,2C-F)
131.9 (t,20.7,2C-P)
134.5 (m,4CH)
)
164.5 (d,248.4,2C-F)
12.0 (t,15.5,2Me)
14.9 (t,2C,15.7,Me)
128.6 (t,4.7,4CH)
130.2 (2CH)
3lh
29.2 (Me)
54.1 (C-Me) (t,3.0)
102.4
122.3 (2CH)
124.5 (2CH)
172
(t,3.6)
140.8 (Car
)
151.4 (C-NO2)
131.2 (t,5.7,4CH)
134.8 (t,21.9,2Car
183.9 13.6 (t,15.0,Me)
)
3m f,i 14.9 (t,14.3,Me)
j
54.4 (t,8.7)
56.1 (pt)
125.6 (2CH)
131.7 (2CH)
k
97.9
123.6 (2CH)
125.3 (2CH)
144.7 (Car
)
149.3 (Car)
3n f
3of
15.6 (t,14.0,Me)
15.5 (t,14.0,Me)
31.4 (Me)
126.8 (1CH)
128.7 (2CH)
131.3 (2CH)
e
139.7 (Car
)
31.3 (Me)
56.8 (C-Me)
54.9 (t,14.1) 125.3 (2CH)
131.7 (2CH)
175.1 14.4 (t,14.3,Me)
e
101.1
123.7 (2CH)
125.3 (2CH)
167.6
53.8 (C-Me)
144.5 (Car
)
149.2 (Car)
a
b
c 3
d
g
h
i
Singlets unless otherwise indicated. Apparent or real J CP
.
J
.
CDCl3. e Not observed. f CD3COCD3. J CF
.
CD2Cl2. CNPri
CP
j
4
k
derivative. Imine: 24.3 (Me), 60.2 (t, J CP ) 6.7, CH). Enamine: 23.3 (Me).

5232 Organometallics, Vol. 18, No. 25, 1999
Ca´mpora et al.
Ta ble 6. Electr on ic a n d Ster ic P r op er ties of
Sch em e 5
P h osp h in e Liga n d s a n d Kim /en Va lu es
b
phosphine
ν
(CO) Ni(CO)3(PR3)a cone angle24
Kim/en
PEt3
PMe3
PMe2-p-C6H4NMe2
PMe2Ph
PMe2-p-C6H4F
2061.7
2064.1
2061.7
2065.3
2066.3
132
118
<0.1 (3g)
0.7 (3b)
0.4 (3j)
0.3 (3i)
0.3 (3k )
∼122
122
∼122
a
b
Calculated from data found in ref 23. For complexes with X
) Cl, Y ) CF3.
cally characterized transition metal η1-imidoyl com-
plexes in the Cambridge Structural Database has shown
that this arrangement is by far the most common, with
33 structures versus only four examples in which the
metal fragment is placed in trans position with respect
to the nitrogen substituent.16 For late transition metals
(Co, Rh, Ni, Pd, and Pt), only the z configuration could
be found. While the widespread occurrence of the z
conformation might have in some cases a kinetic origin
(i.e., stereoespecific insertion of the isocyanide into the
metal-carbon bond), the isomerization of the imidoyl
group from the syn (z in the terminology used in this
paper) to the anti (i.e., e) conformation, which is
observed17 in the splitting of the imidoyl bridges of
Pd2[µ-(C(dNMe)C6F5)]Cl2(CNMe)2 to give trans-
Pd(C(dNMe)C6F5)Cl(CNMe)2, suggests that the ther-
modynamic predominance of the z configuration (syn
conformer) may be due to electronic rather than to steric
factors. In contrast with this allegedly well-defined
stereochemical preference, enamine derivatives and
other complexes that contain R-NHR groups (e.g., cat-
ionic aminocarbenes, vide infra) do not appear to exhibit
such a geometrical choice. As a way of an example, we
have recently reported some enamine complexes of Pd
and Pt in which the z and e conformers exist in
equilibrium in solution.3b
F igu r e 2. ORTEP view and atom-labeling scheme of
compound 3c-im .
P r oton a tion of Im id oyl/En a m in e Liga n d s. Since
η1-imidoyl complexes are readily protonated by ac-
ids,12,18 we have studied this reactivity with the aid of
compounds 3b and 3g. Both react instantly with HBF4
to give the cationic aminocarbene derivatives 5b‚BF 4
and 5g‚BF 4 in quantitative yield (Scheme 5).
F igu r e 3. ORTEP view and atom-labeling scheme of
compound 3h -en .
is in agreement with the nearly planar configuration of
the nitrogen atom.
Interestingly, complex 5g‚BF 4 seems to adopt differ-
ent conformations in the solid state and in solution.
A striking structural difference between compounds
3c-im and 3h -en is the contrasting geometrical disposi-
tion of the t-Bu substituent with respect to the pal-
ladium atom. In the latter complex, the t-Bu group and
the bulky metal fragment are in the e configuration (see
structure ze-3en of Chart 1), possibly to avoid unfavor-
able steric interactions. However, 3c-im displays the
opposite geometry, with the metal fragment and the
nitrogen substituent adopting a z distribution (see
structure z-3im of Chart 1). A survey of crystallographi-
(16) (a) Andrinov, V. G.; Khomutov, M. A.; Zlotina, I. B.; Kolobeva,
E. N.; Struchkov, Yu. T. Koord. Khim. 1979, 5, 283. (b) Beckhaus, R.;
Wagner, T.; Zimmermann, C.; Herdtweck, E. J . Organomet. Chem.
1993, 460, 181. (c) Fandos, R.; Meetsma, A.; Teuben, J . H. Organo-
metallics 1991, 10, 2665. (d) Adams, R. D.; Chodosh, D. F. Inorg. Chem.
1978, 17, 41.
(17) Uso´n, R.; Fornie´s, J .; Espinet, P.; Lalinde, E.; J ones, P.;
Sheldrick, G. M. J . Organomet. Chem. 1985, 288, 249.
(18) (a) Carmona, E.; Palma, P.; Poveda, M. L. Polyhedron 1990, 9,
1447. (b) Maddock, S. M.; Rickard, C. E. F.; Roper, W. R.; Wright, L.
J . J . Organomet. Chem. 1996, 510, 267. (c) Mantovani, A.; Calligaro,
L.; Pasquetto, A. Inorg. Chim. Acta 1983, 76, L145.

Imine-Enamine Tauatomeric Equilibrium
Organometallics, Vol. 18, No. 25, 1999 5233
Ta ble 7. Cr ysta l a n d Refin em en t Da ta for 2
2c
3c
3h
formula
mol wt
crystal system
space group
cell dimensions
a, Å
C
13H24Br2P2Pd
C18H33Br2NP2Pd
591.621
orthorhombic
Pbcn
C24H45Br2NP2Pd
675.77
monoclinic
P212121
508.48
monoclinic
P21/c
11.3000(10)
30.611(3)
19.887(2)
b, Å
12.3990(10)
12.453(1)
16.928(2)
c, Å
14.257(2)
12.946(1)
9.130(2)
R, deg
90
90
90
90
8
90
90
90
4
â, deg
109.260(10)
90
4
1885.7(3)
1.791
5.376
992
γ, deg
Z
V, Å3
4935.00(73)
1.5926
4.1219
3073.6(8)
1.460
3.319
1368
D
calc, g cm-3
abs coeff, mm-1
F(000)
2352.0
temp, K
293(2)
293(2)
293(2)
wavelength, Å
crystal size, mm
θ range for data collection, deg
index ranges
0.71070
0.71070
0.71070
0.21 × 0.20 × 0.18
2.23-27.00
0 e h e 14, 0 e k e 15,
-8 e 1 e 8
2436
0.20 × 0.30 × 0.18
0.17 × 0.22 × 0.19
2.05-24.97
0 e h e 23, 0 e k e 20,
-8 e 1 e 10
3045
2-26
0 e h e 38, 0 e k e 16,
-8 e 1 e 15
5370
no. of reflns collected
no. of indep reflns
2436
2583
3045
[R(int) ) 0.0000]
full-matrix least-
squares on F2
2436//0/163
2.076
R1 ) 0.0775, wR2 ) 0.2156
R1 ) 0.0990, wR2 ) 0.2770
0.723 and -1.662
[R(int) ) 0.0000]
full-matrix least-
squares on F2
3045/0/271
refinement method
full-matrix least-
squares on F2
2583/0/217
no. of data/restraints/params
goodness-of-fit on F2
final R indices [I >2σ(I)]
R indices (all data)
1.133
R1 ) 0.074, Rw ) 0.083
R1 ) 0.0662, wR2 ) 0.1943
R1 ) 0.0892, wR2 ) 0.2258
0.771 and -0.703
largest diff peak and hole, e Å-3
max absorp corr
0.702 and -0.501
1.350
min absorp corr
0.665
av absorp corr
0.963
Ta ble 8. Selected Bon d Dista n ces a n d An gles for
2c
Ta ble 10. Selected Bon d Dista n ces a n d An gles for
3h
bond distances (Å)
bond angles (deg)
bond distances (Å)
bond angles (deg)
Pd(1)-C(1)
2.078(13)
2.307(5)
2.297(6)
2.520(2)
1.48(2)
C(1)-Pd(1)-P(1)
88.7(5)
90.7(6)
Pd(1)-C(1)
2.05(2)
2.332(5)
2.329(5)
2.513(2)
1.35(2)
1.39(2)
1.52(2)
1.44(2)
C(1)-Pd(1)-P(1)
91.3(5)
89.8(5)
Pd(1)-P(1)
Pd(1)-P(2)
Pd(1)-Br(1)
C(1)-C(2)
C(1)-Pd(1)-P(2)
P(1)-Pd(1)-P(2)
C(1)-Pd(1)-Br(1)
P(1)-Pd(1)-Br(1)
P(2)-Pd(1)-Br(1)
C(2)-C(1)-Pd(1)
Pd(1)-P(1)
Pd(1)-P(2)
Pd(1)-Br(1)
C(1)-C(2)
N(1)-C(1)
N(1)-C(9)
C(2)-C(3)
C(1)-Pd(1)-P(2)
P(1)-Pd(1)-P(2)
C(1)-Pd(1)-Br(1)
P(1)-Pd(1)-Br(1)
P(2)-Pd(1)-Br(1)
C(2)-C(1)-Pd(1)
N(1)-C(1)-Pd(1)
N(1)-C(1)-C(2)
C(1)-N(1)-C(9)
177.91(13)
178.2(4)
91.87(11)
88.70(14)
117.4(9)
178.8(2)
171.9(5)
90.16(14)
88.73(13)
126.9(14)
107.6(11)
126(2)
Ta ble 9. Selected Bon d Dista n ces a n d An gles for
3c
127.0(14)
bond distances (Å)
bond angles (deg)
Sch em e 6
Pd(1)-C(1)
2.012(4)
2.317(4)
2.326(4)
2.572(2)
1.58(2)
1.24(2)
1.49(2)
1.50(2)
C(1)-Pd(1)-P(1)
90.5(4)
88.6(4)
178.1(2)
176.5(4)
89.4(1)
Pd(1)-P(1)
Pd(1)-P(2)
Pd(1)-Br(1)
C(1)-C(2)
N(1)-C(1)
N(1)-C(9)
C(2)-C(3)
C(1)-Pd(1)-P(2)
P(1)-Pd(1)-P(2)
C(1)-Pd(1)-Br(1)
P(1)-Pd(1)-Br(1)
P(2)-Pd(1)-Br(1)
C(2)-C(1)-Pd(1)
N(1)-C(1)-Pd(1)
N(1)-C(1)-C(2)
C(1)-N(1)-C(9)
91.4(1)
108.3(8)
135.8(1)
115.7(1)
127.7(1)
Thus, when a solid crystalline sample of this compound
is dissolved in CD2Cl2 at room temperature and the H
1
out with solutions of 5g‚BF 4 allow the identification of
the two rotamers. It is found that whereas that favored
in solution exhibits a z configuration of the t-Bu and
[Pd] fragments across the C-N bond, the one preferred
in the solid state has an e distribution of these groups.
The use of a weaker acid such as HOAc leads to
partial protonation and to the formation of equilibrium
mixtures of the corresponding imine, enamine, and
amidocarbene species (Scheme 7). Figure 4 shows the
NMR is immediately recorded, two sets of signals are
seen, which correspond to two isomers in a ca. 10:1 ratio.
Within 24 h, the signals corresponding to the minor
isomer grow at the expense of the major, inverting the
proportion, until a final equilibrium ratio of ca. 0.05:1
is reached. The initial situation is restored if the sample
is crystallized and dissolved again in CD2Cl2. As rep-
resented in Scheme 6, 2D-NOESY experiments carried

5234 Organometallics, Vol. 18, No. 25, 1999
Ca´mpora et al.
F igu r e 4. 1H spectrum of 3o (0.083 M in CD2Cl2) in the presence of increasing concentrations of acetic acid (0-0.39 M).
Sch em e 7
and therefore, the imidoyl complexes are somewhat
weaker bases than the acetate anion.
Im in e/En a m in e Ta u tom er ism of Im id oyl Com -
p lexes. When imidoyl complexes of type 3 are dissolved
in slightly wet acetone-d6, an equilibrium mixture of the
imine and enamine tautomers is rapidly attained.
Allowing these solutions to stand for extended periods
of time (1-2 days) does not result in any significant
change of the imine/enamine ratio. The values of the
equilibrium constants (Kim/ en ) (imine)/(enamine)) in
acetone-d6 for complexes 3a -o are listed in Table 3. The
addition of anhydrous CD2Cl2 to a pure, crystalline
sample of 3b-en gives a solution that contains the
enamine tautomer exclusively. However, the presence
of small amounts of water (1 µL) catalyzes the conver-
sion of 3b-en into 3b-im , and an equilibrium enamine:
imine ratio of 0.9:1 is reached within 2 h. Accordingly,
as in the case of the related keto/enol or imine/enamine
tautomerism of organic compounds,4,21 a general acid
(path A) or base (path B) catalyzed mechanism could
be proposed for the proton exchange in compounds of
type 3 (Scheme 8). Although the deprotonation of
transition metal acyl derivatives is a well-known pro-
cess,22 we favor path A over path B because the former
appears to be more in accordance with the chemical
properties of palladium imidoyl complexes. For instance,
as already discussed, Pd-imidoyls are readily proto-
nated even by weak acids such as HOAc. In addition,
we have recently shown that the methylene group of
an N,N′-dialkylaminocarbene palladium complex can be
deprotonated with DBU to yield the corresponding
enamine (eq 1).9
effect of successive additions of acetic acid to a solution
of compound 3o in CD2Cl2. The amino carbene complex
5 and the corresponding imidoyl 3im undergo a very
fast exchange and consequently furnish an averaged set
of signals, but the proton exchanges that involve the
enamine tautomer are slow on the NMR time scale. This
is not unexpected19 since proton exchange at nitrogen
is usually much faster than at carbon centers, due to
the higher electronegativity of the former and to the
presence of a lone pair.19 Accordingly, the CH2 signal
of the methylene group of 3o-im , which appears at 3.96
ppm in neat CD2Cl2, is displaced to lower field upon
addition of acetic acid, until it reaches a constant
chemical shift of 4.38 ppm, which corresponds to the
methylene group of the amidocarbene complex 5o‚AcO.
Due to its slow rate of proton exchange, the signals of
the enamine 3o-en remain only slightly broadened and
do not shift on addition of AcOH, allowing the deter-
mination of the concentration of the species involved in
these equilibria.
Similar experiments have been carried out with
complexes 3b, 3g, 3i, and 3j, although reliable equilib-
rium constants could not be calculated, probably due
to ion-pairing effects in the solvent used (CD2Cl2).20
However, from the extrapolation to zero concentration
of AcOH, the equilibrium constant KAcOH can be sti-
mated to be in the range 0.1-0.01 for these complexes,
(1)
Studies on the imine/enamine tautomerism of organic
imines have shown that the electronic properties of the
substituents of the imine/enamine functionality have a
(19) Kramarz, K. W.; Norton, J . R. Prog. Inorg. Chem. 1994, 42, 1.
(20) Ritchie, C. D. In Physical Organic Chemistry; Marcel Dekker:
New York, 1990.
(21) Lowry, T. H.; Schueller Richardson, K. In Mechanism and
Theory in Organic Chemistry; Harper and Row: New York, 1987.

Imine-Enamine Tauatomeric Equilibrium
Organometallics, Vol. 18, No. 25, 1999 5235
Sch em e 8
Ch a r t 2
F igu r e 5. Hammett plot for the imine/enamine ratio in
acetone-d6.
considerable influence on the equilibrium position.5 To
ascertain whether a comparable effect could also be
found in organometallic derivatives, we have prepared
a series of five, closely related iminoacyl complexes (3c,
3e, 3f, 3n , 3o) that contain the same halogen (Br) and
phosphine ligands (PMe3) and differ only in the nature
of the para-substituent on the aromatic ring (Br, NO2,
CN, H, and CF3, respectively). Electron-withdrawing
groups favor the formation of the enamine form in such
a manner that, while for compound 3o a very small
amount on the enamine tautomer can be detected in
solution in acetone-d6 (ca. 9%), for the nitro derivative
the opposite situation holds (ca. 97%). The representa-
tion of the logarithm of the imine/enamine equilibrium
constant in the same solvent versus the Hammet σ
parameter23 (Figure 5) shows an excellent correlation
for the substituents p-H, p-Br, and p-CF3. The groups
The influence of the [Pd] fragment on the value of
Kim/en is less clear than that of the aromatic ring
substituents and requires some additional consider-
ations. It might be argued that an electron-rich metal
center could stabilize the electronically highly delocal-
ized enamine tautomer to a larger extent than the imine
form (see Chart 2), but this hyphotesis fails to provide
an explanation for the im/en ratio of ca. 0.3 found for
compounds 3i, 3j, and 3k , which contain aryl phos-
phines of the type PMe2Ar. In addition, whereas no
correlation can be found between Kim/en and the donor/
acceptor ability of the phosphine (represented by the
value of ν(CO) in Ni(CO)3(PR3),24 see Table 6), a better
defined relationship may exist between Kim/en and the
phosphine cone angle24a,25 since the smaller values of
the equilibrium constant appear associated with the
more sterically demanding phosphines. It is possible
that the steric pressure introduced by the bulky phos-
-
p-CN and p-NO2 deviate considerably, but if the σ
parameter, which takes into account the possible elec-
tronic conjugation with the lone pair of the enamine
nitrogen atom (resonance form C, Chart 2), is used
instead, a very good correlation is found. The effect of
electron-withdrawing substituents on the aromatic ring
of the ligand can be rationalized in terms of the
stabilization of the negative charge on the methyne
group, as represented by the resonance structures B and
D (Chart 2).
(22) (a) Liebeskind, L. S.; Welker, M. E.; Fengl, R. W. J . Am. Chem.
Soc. 1986, 108, 6328. (b) Rusik, C. A.; Collins, M. A.; Gamble, A. S.;
Tonker, T. L.; Templeton, T. L. J . Am. Chem. Soc. 1989, 111, 2250. (c)
Fermin, M.; Thiyagarajan, B.; Bruno, J . W. J . Am. Chem. Soc. 1993,
115, 974.
(24) (a) Tolman, C. A. Chem. Rev. 1977, 77, 313. (b) J olly P. W. In
Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G.
A., Abel, E. W., Eds.; Pergamon Press: New York, 1982; Vol. 6.
(25) White, D.; Coville, N. J . Adv. Organomet. Chem. 1994, 36, 95.
(23) Hansch, C.; Leo, A.; Taft, W. R. Chem. Rev. 1991, 91, 165.

5236 Organometallics, Vol. 18, No. 25, 1999
Ca´mpora et al.
3a . ButNC (0.34 mmol, 0.34 mL of a 1 M solution in toluene)
was added to a solution of 2a (0.15 g, 0.34 mmol) in Et2O (30
mL). The solution was stirred at room temperature for 3 h,
the solvent removed under vacuum, and the residue extracted
with Et2O. Filtration, concentration of the filtrate, and addition
of some petroleum ether provided yellow crystals of the
complex after cooling at -30 °C for several hours. Yield: 71%.
The bromide derivatives 3n and 3o were prepared from the
analogous chlorides, 3a and 3b, by stirring with KBr in
acetone, at room temperature, for 12 h.
phines in the already congested iminoacyl tautomers
(recall that the bulky t-Bu group of the iminoacyl has a
z distribution with regard to the [Pd] moiety) favors
tautomerization to the less hindered enamine tautomer.
In accord with these considerations the substitution of
the t-Bu group of 3b (PR3 ) PMe3; X ) Cl; Y ) CF3) by
a less bulky i-Pr substituent at the isocyanide nitrogen
compound (3m ) leads to a increase of about 1 order of
magnitude in the value of Kim/en (from 0.7 in 3b to 6.7
in 3m , see Table 3).
Tables 1-5 collect analytical and spectroscopic (IR and
NMR) data, for the new compounds described.
4. A solution of Pd(η2-CH2dCH-CO2Me)(PMe3)2 (0.5 g, 1.45
mmol) in THF (30 mL) was treated with ClCH2C6H4-p-CF3
(0.21 mL, 1.45 mmol) and dmpe (1.45 mmol, 2.9 mL of a 0.5
M solution in toluene). The resulting mixture was stirred at
room temperature for 30 min and then taken to dryness.
Extraction with a mixture of toluene-CH2Cl2 (1:1), centrifuga-
tion, and concentration provided pale yellow crystals of 4 after
cooling at -30 °C. Yield: 60%. Anal. Calcd for C22H40ClF3NP3-
Pd: C, 43.3; H, 6.6; N, 2.3. Found: C, 43.6; H, 6.4; N, 2.6. IR
(Nujol mull, cm-1, ν(NH)) 3280 (b). 1H NMR (CDCl3, 20 °C):
Con clu sion s
Palladium imidoyl complexes can exist in tautomeric
imine and enamine forms. As in the related tautomeric
equilibria of organic imines, the exchange is catalized
by small amounts of protic acids present in solution,
including water. While the imine tautomer is in general
more stable, under certain circumstances the enamine
form is preferred. Thus, the presence of electron acceptor
substituents on the imidoyl fuctionality favor the enam-
ine form. Since the influence of the metal fragment on
the equilibrium seems to be small and mostly of steric
origin, this kind of tautomerism resembles the more
general imine/enamine tautomerism of organic imines.
It is reasonable to expect that similar tautomeric
equilibria can be found in other organometallic com-
plexes containing imidoyl or related ligands and that
such equilibria may influence the chemical reactivity
of these complexes.
2
4
δ 1.06 (dd, 3 H, J HP ) 10.2, J HP ) 2.5 Hz, P-Me), 1.41 (s, 9
H, CMe3), 1.45 (dd, 3 H, 2J HP ) 9.7, 4J HP ) 2.4 Hz, PMe3), 1.48
2
(d, 3 H, J HP ) 8.5 Hz, P-Me), 1.65 (m, 2 H, P-CH2), 1.80
2
4
2
(dd, 3 H, J HP ) 11.5, J HP ) 2.3 Hz, P-Me), 1.86 (d, 3 H, J HP
4
) 9.6 Hz, P-Me), 2.20 (m, 2 H, P-CH2), 5.93 (dt, 1 H, J HP
)
4
4
19.4, J HP ) 2.8 Hz, CH), 6.60 (d, 1 H, J HP ) 5.6 Hz, NH),
7.14 (d, 2 H, 3J HH ) 8.6 Hz, CHar), 7.57 (d, 2 H, 3J HH ) 7.4 Hz,
CHar). 31P{1H} NMR (CDCl3, 20 °C): AMX spin system, δA
)
-22.1, δM ) 22.1, δX ) 30.2, J AX ) 364, J AM ) 37, J MX ) 27
Hz. 13C{1H} NMR (CDCl3, 20 °C): δ 11.8 (d, J CP ) 27 Hz,
1
1
1
P-Me), 12.8 (d, J CP ) 29 Hz, P-Me), 14.2 (d, J CP ) 20 Hz,
1
1
P-Me), 15.0 (d, J CP ) 21 Hz, P-Me), 16.0 (d, J CP ) 27 Hz,
Exp er im en ta l Section
1
2
PMe3), 27.9 (dd, J CP ) 31, J CP ) 20 Hz, P-CH2), 28.9 (ddd,
1J CP ) 25, J CP ) 16, J CP ) 5 Hz, P-CH2), 29.3 (s, CMe3),
2
3
Microanalyses were performed by the Analytical Service of
the University of Sevilla and the Instituto de Investigaciones
Qu´ımicas. The spectroscopic instruments used were Perkin-
Elmer models 684 and 883 and Bruker model Vector 22 for
IR spectra and Bruker AMX-300, DRX-400, AMX-500, and
DRX-500 for NMR spectroscopy. The 13C resonance of the
solvent was used as an internal standard, but chemical shifts
are reported with respect to SiMe4. The 13C{1H} NMR assign-
ments were helped in most cases with the use of gate
decoupling techniques. 31P{1H} NMR shifts are referenced to
external 85% H3PO4. All preparations and other operations
were carried out under oxygen-free nitrogen by conventional
Schlenk techniques. Solvents were dried and degassed before
use. The petroleum ether used had a boiling point of 40-60
°C. Phosphines Me2PC6H4-p-NMe2 and Me2PC6H4-p-F were
prepared by reacting Me2PCl with the Grignard reagents
ClMgC6H4-p-NMe2 and ClMgC6H4-p-F, respectively. Com-
pounds Pd(η2-CH2dCH-CO2Me)(PR3)2 (R ) Et, Me2Ph, Me2-
PC6H4-p-NMe2, Me2PC6H4-p-F) were prepared according to the
reported procedure for Pd(η2-CH2dCH-CO2Me)(PMe3)2.9
The preparation of compounds 2a -l involves the reaction
of Pd(η2-CH2dCH-CO2Me)(PR3)2 (R ) Me, Et, Me2Ph, Me2-
PC6H4-p-NMe2, Me2PC6H4-p-F) with the appropriate benzyl
halide. The subsequent treatment of these complexes with the
isonitrile (ButNC or PriNC) affords the corresponding imino-
acyl derivatives. A representative example of the experimental
procedure employed to synthesize 2a and 3a (im /en ) is as
follows.
4
2
53.2 (d, J CP ) 7 Hz, CMe3), 100.6 (s, CH), 120.3 (q, J CF ) 31
Hz, C-CF3), 121.8 (bs, 2 CarH), 124.4 (d, 4J CF ) 3 Hz, 2 CarH),
2
148.7 (s, Car), 169.1 (d, J CP ) 108, CN).
5g‚BF 4. HBF4 (0.6 mmol, 0.12 mL of a 35% solution in
water) was added to a solution of 3g (0.37 g, 0.6 mmol) in Et2O
(30 mL). A white solid precipitated following the addition of
the acid. The suspension was stirred at room temperature for
2 h. The solvent was filtered and the solid recrystallized from
CH2Cl2-Et2O (1:1) and obtained as white crystals in 80% yield.
Complex 5b‚BF 4 was similarly prepared starting from 3b
and HBF4 and isolated as pale yellow crystals in 85% yield.
5b‚BF 4. Anal. Calcd for C19H34BClF7NP2Pd: C, 36.5; H, 5.4;
N, 2.2. Found: C, 36.3; H, 5.4; N, 2.2. IR (Nujol mull, cm-1
,
ν(NH)) 3222 (b). 1H NMR (CD2Cl2, 20 °C): δ 1.30 (t, 6 H, *J HP
) 3.7 Hz, PMe3), 1.69 (s, 9 H, CMe3), 4.19 (bs, 2 H, CH2), 10.62
3
(bs, 1 H, NH), 7.71 (d, 2 H, J HH ) 8.2 Hz, CHar), 7.78 (d, 2 H,
3J HH ) 8.2 Hz, CHar). 31P{1H} NMR (CD2Cl2, 20 °C): δ -15.1
(s, PMe3). 13C{1H} NMR (CD2Cl2, 20 °C): δ 13.9 (t, *J CP ) 16
Hz, PMe3), 29.1 (s, CMe3), 55.1 (s, CH2), 60.7 (s, CMe3), 126.3
(s, 2 CarH), 131.4 (s, 2 CarH), 139.2 (s, Car), 231.0 (s, CN).
5g‚BF 4. Anal. Calcd for C25H46BClF7NP2Pd: C, 42.4; H, 6.5;
N, 2.0. Found: C, 42.1; H, 6.1; N, 2.0. IR (Nujol mull, cm-1
,
ν(NH)) 3303 (b).
1
z-5g‚BF 4. H NMR (CD2Cl2, 20 °C): δ 1.16 (pq, 18 H, *J HP
3
≈ J HH ) 8.0 Hz, Me (PEt3)), 1.68 (m, 6 H, CH2 (PEt3)), 1.72
(s, 9 H, CMe3), 1.79 (m, 6 H, CH2 (PEt3)), 4.38 (s, 2 H, CH2),
3
10.28 (bs, 1 H, NH), 7.58 (d, 2 H, J HH ) 8.0 Hz, CHar), 7.73
3
(d, 2 H, J HH ) 8.1 Hz, CHar). 31P{1H} NMR (CD2Cl2, 20 °C):
2a . To a cold (-30 °C) solution of Pd(η2-CH2dCH-CO2Me)-
(PMe3)2 (0.17 g, 0.5 mmol) in THF (30 mL) was added
ClCH2C6H4-p-CF3 (75 µL, 0.5 mmol). The cooling bath was
removed and the mixture stirred at room temperature for 4
h. The solution was then taken to dryness and the residue
extracted with diethyl ether. After filtration, the solvent was
evaporated and the compound obtained as pale yellow crystals
in 70% yield.
δ 11.7 (s, PEt3). 13C{1H} NMR (CD2Cl2, 20 °C): δ 8.9 (s, Me
(PEt3)), 16.2 (t, *J CP ) 14 Hz, CH2 (PEt3)), 28.9 (s, CMe3), 55.5
1
(s, CH2), 61.0 (s, CMe3), 124.2 (q, J CF ) 272 Hz, CF3), 126.8
4
(q, J CF ) 5 Hz, 2 CarH), 131.2 (s, 2 CarH), 137.0 (s, Car), 228.4
2
(pt, J CP ) 5 Hz, CN).
1
e-5g‚BF 4. H NMR (CD2Cl2, 20 °C): δ 1.07 (pq, 18 H, *J HP
3
≈ J HH ) 8.0 Hz, Me (PEt3)), 1.58 (m, 6 H, CH2 (PEt3)), 1.74

Imine-Enamine Tauatomeric Equilibrium
Organometallics, Vol. 18, No. 25, 1999 5237
(s, 9 H, CMe3), 1.76 (m, 6 H, CH2 (PEt3)), 4.13 (s, 2 H, CH2),
hydrogen atoms were refined with distance restraints for the
C-H distances. No extinction corrections were applied. The
absorption correction was carried out by using DIFABS.29
3
10.06 (bs, 1 H, NH), 7.72 (d, 2 H, J HH ) 8.3 Hz, CHar), 7.79
3
(d, 2 H, J HH ) 8.1 Hz, CHar). 31P{1H} NMR (CD2Cl2, 20 °C):
δ 10.0 (s, PEt3). 13C{1H} NMR (CD2Cl2, 20 °C): δ 8.3 (s, Me
(PEt3)), 15.2 (t, *J CP ) 14 Hz, CH2 (PEt3)), 30.3 (s, CMe3), 49.5
(s, CH2), 64.8 (s, CMe3), 126.8 (s, 2 CarH), 130.9 (s, 2 CarH),
139.3 (s, Car), 240.7 (s, CN).
Ack n ow led gm en t. Financial support from the Di-
reccio´n General de Investigacio´n Cient´ıfica y Te´cnica
(Proyect PB96-0824) and J unta de Andaluc´ıa is grate-
fully acknowledged. S.A.H. thanks the Commission of
the European Union for a grant from the HCM program
(Project No. ERCHBICT920061). C.M.D. thanks the
Direccio´n General de Ensen˜anza Superior for a PFPI
studentship. Thanks are also due to the University of
Sevilla and the Instituto de Investigaciones Qu´ımicas
for free access to the analytical and NMR facilities.
X-r a y Str u ctu r e Deter m in a tion of Com p ou n d s 2c, 3c,
a n d 3h . A summary of the fundamental crystal data is given
in Table 7. Data were collected from single crystals of dimen-
sions (0.22 × 0.19 × 0.20 mm, 2c; 0.20 × 0.30 × 0.18 mm, 3c;
0.20 × 0.30 × 0.18 mm, 3h ) on a PW-1100 diffractometer,
using graphite-monochromated Mo KR radiation. A total of
2546 (2c), 5370 (3c), and 3070 (3h ) independent reflections
were measured, 2436 (2c), 2583 (3c), and 3045(3h ) of which
were considered as observed using the criterion I g 2σ(I).
Scattering factors, dispersion corrections, and absorption
coefficients were taken from International Tables for Crystal-
lography.26 The structure was solved by Patterson methods
and Fourier synthesis, and most of the calculations were
performed by using XRAY8027 and DIRDIF.28 The structure
was refined by least-squares analysis using unit weights with
anisotropic parameters for non-H atoms. The positions of the
Su p p or tin g In for m a tion Ava ila ble: Tables of atomic
coordinates, thermal parameters, and bond lengths and angles
for 2c, 3c, and 3h . This material is available free of charge
via the Internet at http://pubs.acs.org.
OM9905557
(28) Beurskens, P. T.; Bosman, W. P.; Doesburg, H. M.; Gould, R.
O.; Van den Hark, Th. E. M.; Prick, P. A. J .; Noordik, J . H.; Beurskens,
G.; Parthasarathi, V.; Bruins Slot, H. J .; Haltiwanger, R. C.; Smits, J .
M. M. In Dirdif System; Crystallography Laboratory, Toernooivel,
Nijmegen, The Netherlands, 1984.
(29) Walker, N.; Stuart, D. DIFABS. Acta Crystallogr. 1983, A39,
158.
(26) (26)International Tables for Crystallography; C. Kluwer Aca-
demic Publishers: Dordrecht, 1992.
(27) Stewart, J . M.; Machin, P. A.; Dickinson, C. W.; Ammon, H.
L.; Heck, H.; Flack, H. In The X-ray System. Technical Report TR-
446; Computer Science Centre: University of Maryland, 1976.