Acid - Base reactions of methylnickel hydroxo, alkoxo, and amide complexes with carbon acids. Studies on the reactivity of noncyclic nickel enolates

Article abstract of DOI:10.1021/om7007259

Basic organonickel hydroxo, alkoxo, or amido complexes of composition Ni(Me)(X)(dippe) (X = OH, t-BuO, cyclo-NC₄H₈; dippe = i-Pr₂PCH₂CH₂Pi-Pr₂) react with enolizable ketones, esters, and nitriles, producing the corresponding enolate complexes. In the case of the hydroxo and alkoxo complexes, these reactions may lead to equilibrium mixtures, allowing a comparison of their respective basicities, while the amido compound reacts clean and quantitatively in all cases, allowing the isolation of the corresponding enolates. While the ketone derivatives display oxygen-bound enolate ligands, the ester and nitrile enolates bind the metal center through the carbon atom. The acetophenone enolate complex has strong nucleophilic properties and rapidly reacts with aldehydes (PhCHO) or CO₂, affording the corresponding aldolate and carboxylate addition products. 2007 American Chemical Society.

Full text of DOI:10.1021/om7007259

5712
Organometallics 2007, 26, 5712-5721
Acid-Base Reactions of Methylnickel Hydroxo, Alkoxo, and Amide
Complexes with Carbon Acids. Studies on the Reactivity of
Noncyclic Nickel Enolates
Juan Ca´mpora,*,† Inmaculada Matas,† Pilar Palma,† Eleuterio AÄ lvarez,† Claudia Graiff,‡ and
Antonio Tiripicchio‡
Instituto de InVestigaciones Qu´ımicas, Consejo Superior de InVestigaciones Cient´ıficas-UniVersidad de
SeVilla, AVenida Ame´rico Vespucio 49, 41092 SeVilla, Spain, and Dipartamento di Chimica Generale ed
Inorganica, Chimica Analitica, Chimica Fisica, UniVersita` di Parma, Via G. P. Usberti 17/A,
I-43100 Parma, Italy
ReceiVed July 19, 2007
Basic organonickel hydroxo, alkoxo, or amido complexes of composition Ni(Me)(X)(dippe) (X )
OH, t-BuO, cyclo-NC4H8; dippe ) i-Pr2PCH2CH2Pi-Pr2) react with enolizable ketones, esters, and nitriles,
producing the corresponding enolate complexes. In the case of the hydroxo and alkoxo complexes, these
reactions may lead to equilibrium mixtures, allowing a comparison of their respective basicities, while
the amido compound reacts clean and quantitatively in all cases, allowing the isolation of the corresponding
enolates. While the ketone derivatives display oxygen-bound enolate ligands, the ester and nitrile enolates
bind the metal center through the carbon atom. The acetophenone enolate complex has strong nucleophilic
properties and rapidly reacts with aldehydes (PhCHO) or CO2, affording the corresponding aldolate and
carboxylate addition products.
The chemistry of late transition metal enolates has attracted
in the reactivity of enolates,1a,2,6 enhancing their reactivity and
stereoselectivity (Scheme 1), but a precoordinative step is not
a strict requisite for these reactions. Our group has carried out
several studies on O-bound metallacyclic enolate complexes of
much attention in the last decades due to its implication in many
metal-mediated organic transformations.1 In part, the develop-
ment of these synthetic methodologies has been facilitated by
the previous understanding of the chemistry of this kind of
compounds. Studies on late transition metal enolates have
focused mainly on C-C bond-forming reactions, such as aldol-
type condensations,2,3 cross-coupling reactions with aryl halides,4
and, more recently, conjugated additions to R,â-unsaturated
ketones.5 In general, it is widely admitted that precoordination
of the electrophile to the metal center plays an important role
nickel of composition Ni(OC(dCRR′)-o-C6H4)(dippe), where
the enolate functionality is rigidly held apart from the metal
center.7 This situation prevents the interaction of the electrophilic
reagent with the metal center, as well as the migration of the
latter from one oxygen atom to another in the corresponding
addition products. The latter effect leads to some interesting
differences in the nature of the final products generated in the
addition reactions of metallacyclic and nonmetallacyclic eno-
lates. Rather curiously, most nickel enolate complexes reported
to date have cyclic structures displaying O-bonded coordina-
tion,7,8 with the exception of the noncyclic cyclopentadienyl
* Corresponding author. E-mail: campora@iiq.csic.es.
† Universidad de Sevilla.
‡ Universita` di Parma.
(1) (a) Sodeoka, M.; Hamashima, Y. Bull. Chem. Soc. Jpn. 2005, 78,
941. (b) Palomo, C.; Oiarbide, M.; Garc´ıa, J. M. Chem. Soc. ReV. 2004,
33, 65. (c) Taggi, A. E.; Hafez, A. M.; Lectka, T. Acc. Chem. Res. 2003,
36, 1. (d) Ikeda, S. Miyashita, H.; Taniguchi, M.; Kondo, H.; Okano, M.;
Sato, Y.; Odashima, K. J. Am. Chem. Soc. 2002, 124, 12060. (e)
Montgomery, J. Acc. Chem. Res. 2000, 33, 467. (f) Paterson I. In
ComprehensiVe Organic Synthesis; Trost, B. M., Fleming, I., Heathcock,
C. H., Eds.; Pergamon: Oxford, 1991; Vol. 2, pp 301-319.
(2) (a) Burkhardt, E. R.; Doney, J. J.; Bergman, R. G.; Heathcock, C.
H. J. Am. Chem. Soc. 1987, 109, 2022. (b) Slough, G. A.; Bergman, R. G.;
Heathcock, C. H. J. Am. Chem. Soc. 1989, 111, 938. (c) Burkhardt, E. R.;
Bergman, R. G.; Heathcock, C. H. Organometallics 1990, 9, 30. (d) Slough,
G. A.; Ashbaugh, J. R.; Zannoni, L. A. Organometallics 1994, 13, 3587.
(3) (a) Jung, C. K.; Krische, M. J. J. Am. Chem. Soc. 2006, 128, 17051.
(b) Cuperly, D.; Petrignet, J.; Cre´visy, C.; Gre´e, R. Chem.-Eur. J. 2006,
12, 3621. (c) Hmashima, Y.; Sasamoto, N.; Hotta, D.; Somei, H.;
Umebayashi, N.; Sodeoka, M. Angew. Chem., Int. Ed. 2005, 44, 1525. (c)
Jang, H. Y.; Krische, M. J. Chem.-Eur. J. 2004, 19, 3953. (d) Fuji, A.;
Hagiwara, E.; Sodeoka, M. J. Am. Chem. Soc. 1999, 121, 5450. (e) Fujimura,
O. J. Am. Chem. Soc. 1998, 120, 10032.
(5) (a) Suzuki, T.; Sodeoka, M. AdV. Synth., Catal. 2005, 347, 1576. (b)
Hamashima, Y.; Hotta, D.; Sodeoka. M. J. Am. Chem. Soc., 2002, 124,
11240. (c) Watanabe, M.; Ikagawa, A.; Wang, H.; Murata, K.; Ikariya, T.
J. Am. Chem. Soc., 2004, 126, 11148.
(6) (a) Veya, P.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Organome-
tallics 1993, 12, 4892. (b) Veya, P.; Floriani, C.; Rotzinger, F. P.; Chiesi-
Villa, A.; Rizzoli, C. Organometallics, 1995, 14, 4101. (c) Cozzi, P. G.;
Veya, P.; Floriani, C.; Rotzinger, F. P.;Chiesi-Villa, A.; Rizzoli, C.
Organometallics 1995, 14, 4992.
(7) (a) Ca´mpora, J.; Maya, C. M.; Palma, P.; Carmona, E.; Gutie´rrez-
Puebla, E.; Ruiz, C. J. Am. Chem. Soc. 2003, 125, 1482. (b) Ca´mpora, J.;
Maya, C. M.; Palma, P.; Carmona, E.; Gutie´rrez-Puebla, E.; Ruiz, C.; Graiff,
C.; Tirippichio, A. Chem.-Eur. J. 2005, 11, 6889. (c) Ca´mpora, J.; Maya,
C. M.; Palma, P.; Carmona, Graiff, C.; Tiripicchio, A. Chem. Commun.
2003, 1742. (d) Ca´mpora, J.; Matas, I.; Maya, C. M.; Palma, P.; AÄ lvarez,
E. Organometallics 2006, 25, 3124. (e) Maya, C. M.; Ca´mpora, J.; Carmona,
E.; Matas, I.; Palma, P.; Gutie´rrez-Puebla, E.; Monge. A. Chem.-Eur. J.
2007, 13, 3675.
(8) (a) Amarsighe, K. K. D.; Chowdhury, S. K.; Heeg, M. J.; Mont-
gomery, J. Organometallics 2001, 20, 370. (b) Hratchian, H. P.; Chowdhury,
S. K.; Gutie´rrez-Garc´ıa, V. M.; Amarasinghe, K. K. D.; Heeg, M. J.;
Schlegel, H. B.; Montgomery, J. Organometallics 2004, 23, 4636. (c) Ketz,
B. E.; Ottenwaelder, X. G.; Waymouth, R. M. Chem. Commun. 2004, 5693.
(d) Martin, J. D.; Abboud, K. A.; Dahmen, K.-H. Inorg. Chem. 1998, 37,
5811. (e) Braunstein, P. Chem. ReV. 2006, 106, 134.
(4) (a) Su, W.; Raders, S.; Verkade, J. G.; Liao, X.; Hartwig, J. F. Angew.
Chem., Int. Ed. 2006, 45, 5852. (b) Lu, G.; Portscheller, J. L.; Malinakova
M. C. Organometallics 2005, 24, 945. (c) Willis, M. C.; Taylor, D.;
Gillmore, A. T. Org. Lett. 2004, 6, 4755. (d) Culkin, D. A.; Hartwig, J. F.
Organometallics 2004, 23, 3398. (e) Liu, X.; Hartwig, J. F. J. Am. Chem.
Soc. 2004, 126, 5182. (f) Culkin, D. A.; Hartwig, H. F. Acc. Chem. Res.
2003, 36, 234.
10.1021/om7007259 CCC: $37.00 © 2007 American Chemical Society
Publication on Web 09/27/2007

ReactiVity of Noncyclic Nickel Enolates
Organometallics, Vol. 26, No. 23, 2007 5713
Scheme 1
Results and Discussion
Synthesis and Characterization of the Acetophenone
Enolate Complex 3. The starting point for the present work is
the preparation of an example of a noncyclic ketone enolate
complex, following our original procedure, i.e., fluoride dis-
placement from 1 with a lithium ketone enolate. Acetophenone
lithium enolate was generated according to the classic method
by treating acetophenone with LDA in thf, and the resultant
solution was reacted with 1. NMR monitoring of this reaction
by 31P{1H} NMR revealed the formation of a single product,
3, characterized by two singlet resonances at δ 64.7 and 76.6
ppm, that could be isolated in 45% yield as a orange crystalline
solid, stable in the solid state and in solution for extended periods
of time at room temperature. The NMR spectra of 3 are fully
consistent with the O-bound enolate structure shown in eq 1.
Thus, the terminal methylene group gives rise to two singlets
at δ 4.56 and 4.85 ppm in its 1H NMR spectrum. The
corresponding 13C resonance appears at δ 80.1 ppm and the
1JCH coupling constant, 153 Hz, is consistent with its sp2
character. The absence of resonances in the proximity of 200
ppm rules out the presence of a carbonyl group, but instead the
oxygen-bound quaternary carbon atom resonates at δ 166.3 ppm.
In addition, the IR spectrum displays a strong absorption at
1590 cm-1, which can be assigned to the CdCH2 bond stretch.
These features are comparable to those of the cyclic O-enolate
derivatives Ni(Cp)(PR3)(CH2COR),2c which display a C-bonded
coordination mode. This is in contrast with the relatively large
number of nonmetallacyclic enolate derivatives reported for
Pd,4d,9 Rh,2b-d,10 Ru,5c,11 and other late transition elements.12
Open structures allow facile changes of the enolate fragment
coordination mode, which are not limited to the well-known σ
C- and O-bound modes, but include also η3 (pseudoallylic)2a,d,10,13
or bridging (in binuclear species),9b,14 which facilitate access
to many different reactivity paths.
Recently, we have shown that the reaction of the fluoro
complex Ni(Me)(F)(dippe) (1) with lithium hydroxide, alkox-
ides, or amides cleanly leads to the corresponding monomeric
complexes Ni(Me)(X)(dippe) (2, X ) OH, OR, NR2), providing
a convenient route to these otherwise difficult to prepare
species.15 As a continuation of our previous work, herein we
describe the use of these highly basic16 compounds as precursors
for the generation of noncyclic enolate complexes from eno-
lizable ketones, esters, and nitriles and the reactivity of the
ketone enolates toward benzaldehyde, CO, and CO2. In
contrast with aldol-type addition reactions, there is little
information about the reactivity of transition metal enolates
toward carbon dioxide,17 despite the importance of this kind
of process for carbon dioxide assimilation in biological sys-
tems.18
Ni(OC(dCH2)-o-C6H4)(dippe),7a as well as those of O-bound
acetophenone enolates of Ti,6a Zr,19 and Rh.2b The formulation
of enolate complex 3 has been confirmed by a single-crystal
diffraction study, illustrated by the ORTEP diagram shown in
Figure 1.
(9) (a) Wanat, R. A.; Collum, D. B. Organometallics 1986, 5, 120. (b)
Veya, P.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C.; Organometallics
1993, 12, 4899. (c) Vicente, J.; Arcas, A.; Ferna´ndez-Herna´ndez, J. M.;
Bautista, D.; Jones, P. J. Organometallics 2005, 24, 2516. (d) Vicente,
J.; Arcas, A.; Ferna´ndez-Herna´ndez, J. M.; Bautista, D. Organometallics
2001, 20, 2767. (e) Ruiz, J.; Rodr´ıguez, V.; Cutillas, N.; Pardo, M.; Pe´rez,
J.; Lo´pez, G.; Chaloner, P.; Hitchcock, P. B. Organometallics 2001, 20,
1973.
(10) Slough, G. A.; Ashbaugh, J. R.; Aukamp, A. M. Organometallics
1994, 13, 890.
(11) Hartwig, J. F.; Andersen, R. A.; Bergman, R. G. Organometallics
1991, 10, 3326.
(12) (a) Suzaki, Y.; Osakada, K. Organometallics 2006, 25, 3251. (b)
Stack, J.; Doney, J.; Bergman, R. G.; Heatchcock, C. H. 1990, 9, 453.
(13) Zuidveld, M. A.; Kamer, P. C. J.; van Leeuwen, P. N. W. M.;
Kleusener, P. A. A.; Stil, H. A.; Roebeck, C. F. J. Am. Chem. Soc. 1998,
120, 7977.
(14) (a) Albe´niz, A. C.; Catalina, N. M.; Espinet, P.; Redo´n, R.
Organometallics 1999, 18, 5571. (b) Murahashi, T.; Kurosawa, H. J.
Organomet. Chem. 1999, 574, 142.
(15) (a) Ca´mpora, J.; Matas, I.; Palma, P.; Graiff, C.; Tiripicchio, A.
Organometallics 2005, 24, 2827. (b) Ca´mpora, J.; Matas, I.; Palma, P.;
AÄ lvarez, E.; Graiff, C.; Tiripicchio, A. Organometallics 2007, 26, 3840.
(16) (a) Bryndza, H. E.; Tam, W. Chem. ReV. 1988, 88, 1163. (b) Fulton,
R.; Holland, A. W.; Fox, D. J.; Bergman, R. G. Acc. Chem. Res. 2002, 35,
44.
(17) (a) Braunstein, P.; Matt, D.; Nobel, D. J. Am. Chem. Soc. 1988,
110, 3207. (b) Braunstein, P.; Matt, D.; Dusausoy, Y.; Fischer, J.; Mitschler,
A.; Ricard, L. J. Am. Chem. Soc. 1981, 103, 5115.
The molecule displays slightly distorted square-planar ge-
ometry with the enolate ligand lying perpendicular to the
coordination plane, in contrast to the related hydroxide, alkoxide,
and amide complexes.15 The Ni-O bond distance, 1.9043(17)
Å, is somewhat longer than that in Ni(Me)(OH)(dippe) (1.877-
(4) Å),15b probably due to steric effects. Whereas the olefinic
C16-C17 bond length (1.343(4) Å) is close to the standard
CdC bond length (ca. 1.40 Å), the C-O bond distance (1.318-
(3) Å) lies in between the typical values for C-O (1.43 Å) and
CdO (1.20 Å) bonds, suggesting a significant delocalization
of the π system. As expected, the two Ni-P distances display
appreciably different lengths, due to the weaker trans influence
of the enolate fragment as compared with the methyl group. In
complex 3 the dNi-P1-dNi-P2 difference (0.096 Å) is similar to
that of fluoride complex 1 (0.090 Å)15a and larger than in the
related hydroxide (0.057 Å)15a or methoxide (0.065 Å)
complexes,15b suggesting that the trans influence of the enolate
ligand is small, more similar to that of fluoride than to other
covalently bound O-donor anionic ligands.
Equilibria Involving Hydroxide, Alkoxide, and Amido
Complexes and Weak Carbon Acids. Once we established
the identity of complex 3 and its stability in solution, we wished
(18) (a) Schloss, J. V.; Hixon, M. S. Enol Chemistry and Enzymology.
In ComprehensiVe Biological Catalysis, A Mechanistic Reference, Vol. II;
Sinott, M., Ed.; Academic Press: London, 1998. (b) Nicole, J.; Silverman,
D. H. Biotin-Dependent Enzymes. In ComprehensiVe Biological Catalysis,
A Mechanistic Reference, Vol. I; Sinott, M., Ed.; Academic Press: London,
1998.
(19) Howard, W. A.; Parkin, G. J. Am. Chem. Soc. 1994, 116, 606.

5714 Organometallics, Vol. 26, No. 23, 2007
Ca´mpora et al.
Scheme 2
to investigate the ability of complexes of type Ni(Me)(X)(dippe)
(X ) OH, 2a, t-BuO, 2b, and cyclo-NC4H8, 2c) to deprotonate
different weakly acidic organic compounds (ketones, esters, and
nitriles), as a measure of their relative basicities.
Addition of acetophenone or tetralone to thf solutions of 2a
leads to incomplete conversion to the enolate complexes 3 and
4, respectively (eq 2). The relative concentration of the
equilibrium components can be readily determined by 31P{1H}
NMR, using an external reference of C6D6 for the lock signal.
Given the structural similarity of these ketones, it is not
surprising that the equilibrium constants, averaged over several
measurements, amount to experimentally indistinguishable
values (2(1) × 10-2 and 5(2) × 10-2 for 3 and 4, respectively,
see Experimental Section). The less acidic ketone pinacolone
The spectroscopic features of complexes 4 and 5 are
comparable to those of 3, indicating the presence of O-bound
enolate groups. For instance, their IR spectra exhibit absorptions
at 1600 cm-1 (4) or 1581 cm-1 (5), attributable to the ν(CdC)
vibration, but no carbonyl bands are observed in the proximity
1
of 1700 cm-1, while the H NMR spectrum shows the olefin
resonances at δ 5.34 ppm in the case of 4 and δ 4.05 and 4.21
for 5.
The difference between the chemical shifts of the R and â
13C resonances of the enolate functionality has been correlated
to the degree of polarization of the CdC bond of enolates
displaying related structures, providing some indication of their
relative reactivity.22 It is interesting to note that this parameter
is appreciably larger for compound 5 (102 ppm) than for 3 (86.2
ppm) and 4 (62.9 ppm), as it would be expected for the higher
basicity of the alkyl-substituted enolate.
The facile deprotonation of ketones by the amide complex
2c prompted us to examine the reaction of this compound with
weaker acids, such as esters (methyl and phenyl acetate) and
nitriles (acetonitrle). The outcome of these reactions is shown
in Scheme 3.
Treatment of a thf solution of complex 2c with a stoichio-
metric amount of methyl acetate or acetonitrile affords com-
plexes 6 and 7, displaying C-bound enolate ligands. These
compounds have been isolated in 40-60% yield as orange
solids, thermally stable in the solid state and in solution. At
variance with the bis(cyanomethyl) complexes Ni(CH2CN)2L2
(L ) PPh3, dppe or bipy), which decompose above -40 °C,23
7 is stable in C6D6 up to 60 °C.
does not react appreciably with 2a, even for a ketone/hydroxide
reagent ratio as high as 35:1, providing an upper limit of 7 ×
10-5 for Keq. In contrast, the tert-butoxide derivative 2b reacts
quantitatively with acetophenone or tetralone to afford 3 and
4. 31P NMR monitoring shows that the reaction is slower in the
latter case, requiring 5 h to complete the transformation. These
reactions can be used for preparative purposes, allowing the
isolation of the two enolate complexes in 45% and 40% yield,
respectively. Compound 2b reacts more slowly with pinacolone
to afford the enolate 5, taking 24 h to approach a situation of
apparent equilibrium, with Keq ≈ 6, although some decomposi-
tion takes place during this time. Heating the reaction mixture
to 50 °C accelerates the reaction, but does not avoid the
decomposition processes. Considering the pKa values of
acetophenone20a (24.7) and pinacolone20b (27.7) in dmso, the
K
eq value for the 2b + acetophenone reaction can be estimated
to be about 6 × 103, i.e., 5 orders of magnitude larger than the
value measured for the hydroxide 2a. The relative values of
the two Keq for acetophenone are consistent with the above limit
given for the reaction of 2a with pinacolone (Keq(2a)/Keq(2b)
< 7 × 10-5/6 ≈ 10-5). These comparisons provide a clear
indication of the much stronger basicity of the alkoxide as
compared to the hydroxide complex.21
As mentioned, the NMR spectra of 6 and 7 indicate the
1
presence of C-bound enolate ligands. Thus, the H and 13C
resonances corresponding to the Ni-CH2 groups appear in the
high-field region of the spectra and exhibit the expected
couplings with the 31P nuclei, e.g., at δ 17.4 ppm in the 13C
NMR spectrum with trans and cis 2JCP couplings of 53 and 11
Hz for 6. The 31P{1H} NMR spectrum is also a useful diagnostic
tool for the binding mode of the enolate, since the C-enolate
derivatives provide an electronically more symmetric environ-
ment for the dippe ligand than O-enolates containing one Ni-
CH3 and one Ni-O bonds. Accordingly, a smaller separation
of the two phosphorus signals is observed for 6 and 7 (∆δ ) 8
ppm) than for the O-enolates 3-5 (∆δ ≈ 12 ppm). The IR
spectra of 6 and 7 display characteristic ν(CdO) and ν(CtN)
bands at 1681 and 2181 cm-1, respectively. These frequencies
are somewhat decreased with respect to the analogous absorp-
In spite of the low thermal stability of 2c, it proves to be a
preparatively useful reagent and reacts rapidly and quantitatively
with acetophenone, tetralone, or pinacolone to afford the
corresponding enolates in 60% (3), 50% (4), and 60% (5)
isolated yield (Scheme 2). These experiments highlight the
expected trend for the basicities of the three complexes 2, which
decreases in the order 2c > 2b > 2a.
(20) (a) Bordwell, F. G.; et al. Acc. Chem. Res. 1988, 21, 456, 463. (b)
Bordwell, F. G.; et al. Can. J. Chem. 1990, 68, 1714.
(21) It is interesting to note that the basicities of the hydroxide 2a and
the tert-butoxide 2b differ more than one would expect on the basis of the
pKa of water and t-BuOH, which are 15.5 and 17.0 in aqueous medium,
i.e., a difference of less than 2 orders of magnitude. In dmso, the pKa of
water is even larger than that of t-BuOH (31.2 and 29.4, repectively),
suggesting that hydroxides should be even stronger bases than alkoxides in
aprotic solvents. The comparatively low basicity of the hydroxide complex
could be due to the high thermodynamic stability of the partially ionic Ni-
OH bond, which includes a strong electrostatic interaction between the metal
center and the small hydroxo ligand.
(22) (a) Hlavinka, M. L.; Hagadorn, J. R Organometallics 2006, 25, 3501.
(b) Rodriguez-Delgado, A.; Chen, E. Y.- X. J. Am. Chem. Soc. 2005, 127,
961.
(23) Alburquerque, P. R.; Pinhas, A. R.; Krause Bauer, J. A. Inorg. Chim.
Acta 2000, 298, 239.

ReactiVity of Noncyclic Nickel Enolates
Organometallics, Vol. 26, No. 23, 2007 5715
Scheme 3
derivatives often display relatively short C-C(O) and long Cd
O bonds, presumably as a consequence of the â effect.2a,c,9c
The preference of the ester and nitrile enolate ligands in 6
and 7 for binding the metal through the carbon atom contrasts
with the behavior of nickel ketone enolates, which tend to adopt
the opposite coordination mode. The spectral and structural data
for 6 and 7 suggest that these compounds are best described as
â-functionalized alkyl derivatives, displaying essentially covalent
Ni-C bonds, while ketone enolates feature a polar Ni-O bond
with a large ionic contribution.25 Very likely, the O-bonded
coordination mode is stabilized by the capability of the enolate
ligand to disperse the partial negative charge originated in the
highly polarized Ni-O bond. It is well known that this capability
is less pronounced for esters or nitriles than for ketones (this
being the reason usually invoked to explain the lower acidity
of the former26), and this may help to explain the different
preferences for the O- and C-coordination modes of the
corresponding nickel enolate complexes. In any case, it is worth
mentioning here that the preference of nickel ketone enolates
for the O- over the C-bonded coordination mode is probably
very small, as in some cases the two may be nearly energetically
equivalent,7a,b or even the latter may be more stable.2c
Figure 1. ORTEP view of 3. Selected bond lengths (Å) and angles
(deg): Ni-O1, 1.9043(17); Ni-C1, 1.955(3); Ni-P1, 2.2145(9);
Ni-P2, 2.1177(8); O1-C16, 1.318(3); C16-C17, 1.343(4); O1-
Ni-C1, 88.45(11); C1-Ni-P2, 90.63(9); O1-Ni-P1, 91.85(6);
P2-Ni-P1, 89.15(3).
As shown in Scheme 3, phenyl acetate reacts with 2c,
affording a different kind of product, the phenoxide complex
8, which has been isolated in 55% yield as a red crystalline
material. Monitoring the reaction by 31P NMR allowed the
detection of a transient species, 8′, whose characteristic signals
fade away in the course of the reaction. These consist of two
singlets at δ 71.8 and 80.3, whose position and small separation
(8.5 ppm) are strongly reminiscent of those of the ester enolate
6 (∆δ ) 8.3 ppm), pointing out an analogous C-enolate structure
for 8′. This result can be explained as shown in Scheme 4.
Phenyl acetate is deprotonated by 2c in the same fashion as
methyl acetate (path A). However, for the more reactive phenyl
acetate, the acid-base reaction competes with the nucleophilic
attack of 2c on the aryloxycarbonyl group, which leads to the
phenoxide 8 (path B). The latter compound is very stable, and
its formation behaves as an effectively irreversible process,
driving the formation of 8 at the expense of 8′.
Carbonylation of Nickel O-Enolates. We have previously
shown that the carbonylation of nickelacyclic enolates cleanly
leads to the corresponding enol lactones and Ni(CO)2(dippe).7b,e
Apart from being a synthetically useful reaction, it proved to
be an auxiliary tool for the characterization of enolate complexes
that are difficult to isolate, allowing fixing their structures in
organic products readily purified by standard chromatographic
Figure 2. ORTEP view of 6. Selected bond lengths (Å) and angles
(deg): Ni-C1A, 1.9894(18); Ni-C2A, 2.0056(19); Ni-P1, 2.1669-
(5); Ni-P2, 2.1724(5); O1A-C3A, 1.212(4); C2A-C3A, 1.524-
(3); O2A-C3A, 1.368(4); C1A-Ni-C2A, 90.14(9); C1A-Ni-
P1, 90.36(7); C2A-Ni-P2, 91.19(6); P1-Ni-P2, 88.400(18).
tions of free methyl acetate (1744 cm-1) and acetonitrile (2254
cm-1). Frequency shifts of ν(CdO) of a comparable magnitude
are often observed for C-enolate complexes2a,c,24 and have been
attributed to the decrease of the CdO bond order as a result of
the mesomeric interaction of the M-C and CdO bonds (â
effect).
The crystal structure of compound 6 is shown in Figure 2.
The methyl and the enolate fragments were found disordered,
exchanging their mutual positions, with one of the two possible
orientations (shown in Figure 2) being slightly more abundant
than the other (64:34 ratio). The bond distances and angles are
unexceptional and show no important deviations from their
normal values. The two Ni-C carbon bonds have negligible
differences, and the Ni-P bond lengths are the same within
the experimental error. The enolate fragment is oriented
perpendicularly to the coordination plane, and the C(2)-C(3)
and C(3)dO distances (1.524(3) and 1.212(4) Å) can be
considered normal. In contrast, other transition metal C-enolate
(25) (a) Holland, P. L.; Andersen, R. A.; Bergman, R.; Huang, J.; Nolan,
S. P. J. Am. Chem. Soc. 1997, 119, 12800. (b) Holland, P. L.; Andersen, R.
G.; Bergman, R. G. Comments Inorg. Chem. 1999, 21, 115.
(26) Rablen, P.; Bentrup, K. H. J. Am. Chem. Soc., 2003, 125, 2148,
and references therein.
(24) Belderrain, T. R.; Knight, D. A.; Irvine, D. J.; Paneque, M.; Poveda,
M. L.; Carmona, E. J. Chem. Soc., Dalton Trans. 1992, 1491.

5716 Organometallics, Vol. 26, No. 23, 2007
Ca´mpora et al.
Scheme 4
plexes displaying C-bound enolate ligands display a rather
complex reactivity toward aldehydes, which involves the
successive addition of two aldehyde molecules and leads to
mixtures of two isomeric â-hydroxy esters.2c In order to check
whether noncyclic O-enolates of nickel follow a classic addition
pathway to aldehydes, we have investigated the reaction of
complex 3 with benzaldehyde. Treatment of a solution of 3 in
C6D6 with an equimolar amount of PhCHO led to the partial
conversion of the former into a new product, 11, characterized
by a pair of doublets in the 31P{1H} spectrum at δ 63.5 and
75.1 ppm (3JPP ) 7 Hz), whose separation of 12 ppm is
consistent with the presence of an O-bonded aldolate ligand.
This compound gradually decays in solution, affording Ni(Me)-
(OH)(dippe), 2a, as the major phosphorus-containing product,
together with some Ni(Me)2(dippe), Ni(dippe)2, and minor
amounts of unidentified species. 1H NMR and GC-MS analysis
of the solution indicates the presence of trans-chalcone in the
mixture. Under these reaction conditions, the formation and
decomposition of 11 takes place at roughly similar rates,
preventing the buildup of a concentration sufficient to allow a
univocal identification. However, as shown in Scheme 6,
treatment of 3 with a 2-fold excess of PhCHO caused its
quantitative conversion into 11. Under these conditions, useful
1H and 13C{1H} NMR could be recorded, which confirm the
identity of the product as an aldolate complex. The key feature
of this compound is the presence of a chiral NiOC(Ph)HR unit.
The asymmetry of this group causes the neighboring CH2
protons to become diastereotopic, giving rise to two multiplets
at δ 3.32 and 3.47 in the proton spectrum. The characteristic
signals of the alkoxo R-methyne are a somewhat broadened 1H
signal at 5.70 ppm and a doublet at δ 73.0 (2JCP ) 5 Hz) in the
13C{1H} NMR spectrum. These features are nearly identical to
those originated by the alkoxo moiety in the complex Ni(CH3)-
(OC(Ph)(Me)H)(dippe), recently reported by us.15b The vicinal
1H coupling scheme and assignation of 13C signals has been
further confirmed by the 2D COSY and 1H-13C NMR hetero-
correlation spectra.
techniques. The carbonylation of noncyclic enolates follows the
same path and can be similarly used for the identification of
enolate derivatives. Thus, compound 3 rapidly reacts with CO
to afford acetophenone enol acetate, PhC(dCH2)OCOCH3, and
Ni(CO)2(dippe). In order to check whether the reaction proceeds
through CO insertion into the Ni-C or the Ni-O bond, a
solution of 3 in toluene-d8 was treated with ca. 0.75 equiv of
CO at -60 °C. However, the reaction directly afforded 0.25
equiv of Ni(CO)2(dippe) and the enol acetate, leaving the
remaining starting material unaltered. No intermediates could
be detected, showing that these are too unstable under the
experimental conditions, immediately evolving into the reductive
elimination products.
The carbonylation of 3 can be efficiently performed using
samples of the enolate complex generated in situ from 2c and
acetophenone. This stimulated us to apply this procedure to the
investigation of the reaction of the amide reagent with the
unsymmetric ketone 4-phenylbutan-2-one, where two regioiso-
meric enolates can possibly be formed (Scheme 5). When the
reaction is carried out at -78 °C, 31P NMR monitoring confirms
the formation of a mixture of two enolate complexes, 9a,b in a
6:1 ratio. Immediate carbonylation of the reaction mixture at
-78 °C leads to the corresponding Ni(CO)2(dippe) and the
enolate acetates 10a,b, which were obtained as an insoluble
mixture by spinning band chromatography. The H and 13C
1
On standing at room temperature, the C6D6 solution of 11,
containing only excess PhCHO, decays in the same fashion
observed for partially converted mixtures, affording the hy-
droxide 2a and trans-2-chalcone as the main product, the latter
in 54% yield as determined by GC, with an estimated half-life
of ca. 5 h. The fact that the formation of the R,â-unsaturated
ketone takes place under rigorously anhydrous conditions, and
that no free aldol or acetophenone could be detected by 1H NMR
or GC, suggests that this process takes place through the
intramolecular abstraction of an acidic methylene proton by the
alkoxo oxygen, with concomitant elimination of the hydroxide
complex 2a, as indicated in Scheme 6. However, more complex
intermolecular processes involving the rapid dehydration of free
aldol are also conceivable and cannot be ruled out at this stage.
Whatever the exact mechanism of this transformation is, the
generation of the hydroxide 2a, together with the ability of this
complex to react with acetophenone to regenerate the enolate,
implies that this compound should be able to catalyze the aldol
condensation of acetophenone and benzaldehyde. In fact,
compound 2a, as well as the amide 2c, efficiently catalyzes the
condensation of acetophenone and other ketones with aromatic
aldehydes.28
NMR spectra of this mixture show that the major isomer
corresponds to the terminal enol ester 10a, and hence the main
enolate complex must be 9a.
On standing at room temperature, the mixture of enolate
complexes gradually equilibrates, eventually attaining the op-
posite isomer ratio (i.e., 9a:9b 1:6). Thus, it can be concluded
that the less hindered enolate is formed under kinetic control
conditions, while thermodynamic control favors the most
substituted isomer. This isomerization process is likely to be
catalyzed by the pyrrolidine generated in the reaction or by
traces of free ketone. The selectivity control observed in these
reactions is analogous to that observed in the formation of
alkaline enolates from the corresponding amides.27
Reaction of Enolate Complex 3 with Benzaldehyde and
Carbon Dioxide. As already mentioned, the aldol reactivity of
transition metal enolates has attracted much attention because
of its possible application to asymmetric synthesis. Several
catalytic processes of this kind, which are proposed to proceed
via late transition metal enolate complexes, have been reported,3
and in some cases, both enolate and aldolate complexes have
been isolated.2b In our previous work, we found that nickela-
cyclic enolates react with enolizable and nonenolizable alde-
hydes, but these reactions lead to addition products containing
substituted enolate functionalities, rather than classical aldolate
complexes.7a,b On the other hand, cyclopentadienylnickel com-
In order to gain more insight into the nucleophilic reactivity
of the enolate complexes, the reaction of complex 3 with carbon
dioxide was also investigated. Bubbling dry CO2 (1 equiv)
through a solution of the enolate complex 3 causes the latter to
(27) Carey, F. A.; Sundberg, R. J. AdVanced Organic Chemistry, Part
B, 3rd ed.; Plenum Press: New York, 1990.
(28) Ca´mpora, J.; Matas, I. To be submitted.

ReactiVity of Noncyclic Nickel Enolates
Organometallics, Vol. 26, No. 23, 2007 5717
Scheme 5
Scheme 6
be quantitatively converted into a new compound, 12, as
deduced by 31P{1H} NMR monitoring of this reaction. Again,
the spectrum features two 31P resonances (δ 67.0 and 78.0)
separated by 11 ppm, which is consistent with the simultaneous
presence of one Ni-C and one Ni-O bond. Full NMR data
have been obtained for 12 that are in agreement with the
formulation represented in Scheme 7. Thus, the CH2 unit of
the â-ketocarboxylate ligand gives rise to a single 1H resonance
at δ 4.03 ppm and at δ 51.2 ppm in the 13C{1H} NMR spectrum.
The presence of two carbonyl groups is evinced by two 13C
resonances at low field (δ 169.9 and 194.5 ppm).
The reactivity of enolate 3 toward carbon dioxide differs from
that of alkoxide and amide complexes, which experience the
formal insertion of the heterocumulene in the Ni-X bond (X
) O, N).15b In contrast, the formation of complex 12 can be
envisaged as a nucleophilic attack of the â carbon of the enolate
ligand to the carbon atom of CO2, which leads to C-C bond
formation. Therefore, the reactions of 3 with benzaldehyde and
CO2 can be considered as closely related reactions, exemplifying
the characteristic reactivity of enolate complexes where the
center of the nucleophilic reactivity sits in a position remote
from the metal atom.
Attempts to isolate complex 12 were thwarted by its low
stability in solution. After 24 h, the H NMR spectrum of the
1
C6D6 solution of 12 shows the presence of ca. 0.5 equiv of
acetophenone, which gradually increases over several days. At
the same time an insoluble material begins to precipitate. After
a few days, crystals of the insoluble product 13 were collected
and submitted for X-ray structure determination. As shown in
Figure 3, this compound is a dimer comprising two identical
binuclear fragments, linked by a bridging diphosphine ligand.
Each binuclear moiety features a doubly deprotonated benzoy-
lacetic acid unit, which coordinates to one Ni atom as a
carboxylate and to the second Ni center as a 1,3-dionate ligand.
Remarkably, each metal fragment retains its original methyl
group.
The release of acetophenone during the formation of 13
implies a partial loss of carbon dioxide. The reversibility of the
enolate carboxylation reaction has been fully demonstrated for
palladium phosphinoenolate derivatives.17 A possible mechanism
for this process is shown in Scheme 8. The decarboxylation of
12 would restore compound 3, the basic enolate complex
deprotonating the acidic methylene group of a second molecule
of 12. This reaction releases acetophenone and gives rise to a
transient binuclear species, 14, which rapidly condensates into
the tetranuclear product 13, with elimination of 1 equiv of the
diphosphine.
As a final comment, it is worth comparing the reactivity
of enolate 3 and that of its closely related cyclic analogue
Ni(OC(dCH2)-o-C6H4)(dippe). The rigid structure of the latter
compound makes it unlikely that the enolate attack on the
aldehyde might involve the previous coordination of the
aldehyde to the metal center (see Scheme 1). On the contrary,
precoordination of the aldehyde to the nickel center becomes
possible for the noncyclic enolate 3. Such precoordination
increases the electrophilicity of the aldehyde and brings it to a
favorable position for an intramolecular nucleophilic attack of
the enolate ligand. Therefore, the reaction of compound 3 with
benzaldehyde was expected to be appreciably faster than that
of the cyclic enolate. However, both enolate complexes react
at similar rates under similar experimental conditions (C6D6,
room temperature, ca. 0.16 M of each reagent).29 Therefore,
aldehyde precoordination does not appear to be important for
the reactivity of these nickel enolates. The facile reaction of 3
with carbon dioxide provides further support for this conclusion,
Figure 3. ORTEP view of 13. Selected bond lengths (Å) and angles
(deg): Ni1-O3, 1.9457(16); Ni2-O2, 1.9009(16); Ni2-O1,
1.9173(16); O1-C18, 1.300(3); O2-C16, 1.282(3); C16-C17,
1.442(3); C17-C18, 1.367(3); O3-Ni1-C1, 88.83(9); C1-Ni1-
P2, 88.77(8); O3-Ni1-P1, 91.64(5); P2-Ni1-P1, 89.93(3); O2-
Ni2-O1, 93.57(7); O2-Ni2-C25, 85.28(8); O1-Ni2-P3, 91.12-
(5); C25-Ni2-P3, 91.19(7).
(29) For the cyclic enolate, the reaction takes ca. 4 h to complete under
these conditions.

5718 Organometallics, Vol. 26, No. 23, 2007
Ca´mpora et al.
Scheme 7
Scheme 8
performed by the Microanalytical Service of the Instituto de
Investigaciones Qu´ımicas (Sevilla, Spain). Infrared spectra were
recorded on a Bruker Vector 22 spectrometer, and NMR spectra
on Bruker DRX 300 and 400 MHz spectrometers. The 1H and 13C-
{1H} NMR resonances of the solvent were used as the internal
standard, but the chemical shifts are reported with respect to TMS.
31P NMR resonances are referenced to external 85% H3PO4. GC
analyses were performed in a Agilent model HP 6890 chromato-
graph and GC-MS on a Thermoquest Automass Multi gas chro-
matograph-mass spectrometer, both of them equipped with a
Technokroma HP1 column. Carbon dioxide was dried by storage
in a Fischer-Porter reactor containing P2O5. Nickel complexes 2a,
2b, and 2c were prepared as reported previously.15 For the present
work, solutions of 2c in thf were generated as follows.
since CO2 coordination to the four-coordinated Ni(II) center
appears unlikely to play any important role.
Conclusions
In summary, the synthesis and reactivity of some nickel alkyl-
enolate complexes has been investigated. Ketone enolates can
be generated from the fluoro precursor 1 and lithium enolates
or by acid-base exchange reactions of the alkyl-hydroxo,
-alkoxo, and -amido Ni(II) complexes 2a-c with enolizable
ketones. The positions of the equilibria associated with the latter
acid-base exchanges confirm the expected basicity order: Ni-
OH < Ni-OR < Ni-NR2. Nickel amides behave as highly
basic reagents, reacting rapidly and quantitatively not only with
ketones but also with less acidic carbon acids such as esters
and nitriles. While the ester and nitrile enolates adopt the
C-bonded coordination mode, the more polar O-bonded tautomer
is preferred in the case of the ketone enolates, presumably as a
result of the ability of the latter ligands to disperse negative
charge.
The methylnickel enolates are highly reactive compounds that
undergo clean reactions with CO, CO2, and aldehydes. The
carbonylation reaction leads to the corresponding enol acetates,
and this reaction has been used to explore the regioselectivity
of the deprotonation of a nonsymmetric ketone with the amide
complex 2c. Like lithium amides, the nickel reagent affords
preferentially the less hindered enolate complex under kinetic
control conditions (-78 °C), but this slowly equilibrates at room
temperature, with the most stable product displaying the most
substituted double bond.
Preparation of a thf Solution of 2c. A solution of 0.5 mmol of
lithium pyrrolidinide, prepared by adding 0.3 mL of a 1.6 M hexane
solution (0.5 mmol) of nBuLi to a cooled solution (-78 °C) of 42
µL of pyrrolidine (0.5 mmol) in 2 mL of thf and allowing it to
warm to room temperature, was added to a solution of 176 mg
of the fluoro complex 1 (0.5 mmol) in 3 mL of thf, cooled to
-78 °C. The color changed from orange to dark red, indicating
the formation of 2c. The solution was allowed to warm to room
temperature and then cooled again to -78 °C and used. Compound
2c is thermally unstable, and this solution cannot be stored.
Synthesis of Ni(dippe)(CH3)(OC(CH2)Ph) (3). Method A. A
solution of lithium enolate of acetophenone (1 mmol) was prepared
by reacting equimolar amounts of lithium diisopropylamide and
acetophenone at -78 °C in 5 mL of thf and then allowed to warm
to room temperature. This solution was added dropwise to a stirred
solution of compound 1 (335 mg, 1 mmol) in thf (5 mL) cooled to
-78 °C. The cooling bath was removed and the mixture stirred at
room temperature. The solvent was removed under vacuum and
the residue extracted with 5 mL of thf. This solution was
concentrated and treated with ca. 2 mL of diethyl ether. Then the
solid residue was separated by centrifugation. After cooling the
solution to -20 °C, orange crystals were obtained. Yield: 200 mg
(45%).
Method B. A 0.96 mL amount of a 1.7 M solution of LinBu
(0.5 mmoL) in hexane was added to 3 mL of thf at -78 °C
containing 48 µL of tert-butanol (0.5 mmoL). The mixture was
stirred and allowed to warm to room temperature, then added to a
solution of 176 mg of Ni(dippe)(Me)(F) (0.5 mmoL) in 3 mL of
thf stirred at -78 °C. The mixture was warmed to room temperature
and cooled again at -78 °C. Then, 58 µL (0.5 mmol) of
acetophenone was added. The cooling bath was removed and the
mixture allowed to reach the room temperature. The solvent was
removed under reduced pressure and the residue extracted with 3
mL of thf. The product was obtained in a 45% yield after adding
some diethyl ether and cooling at -30 °C.
The reaction of the acetophenone enolate 3 with PhCHO or
CO2 proceeds similarly, giving rise to the thermally unstable
addition products 11 and 12. The nickel aldolate 11 decomposes
in solution, eliminating hydroxide complex 2a and giving trans-
chalcone. Complex 12 experiences a slow decarboxylation
process to afford the tetranuclear compound 13, containing
doubly deprotonated phenylacetate ligands. The fact that
compound 3 does not react more rapidly with PhCHO than the
cyclic enolate Ni(OC(dCH2)-o-C6H4)(dippe) suggests that the
previous coordination of the aldehyde to the metal center does
not play an important role in either of these reactions.
Method C. To a thf solution of 2c (0.5 mmol) cooled to
-78 °C was added 58 µL (0.5 mmol) of acetophenone. The dark
red color of the solution lightened. The cooling bath was removed,
the solution was allowed to reach room temperature, the solvent
was removed under vaccuum, the solid residue was extracted with
3 mL of thf, and solid LiF was removed by centrifugation. The
product was isolated as orange crystals in 60% yield (270 mg) after
addition of diethyl ether to this solution and cooling to -30 °C.
Experimental Section
General Considerations. All preparations were carried out under
oxygen-free nitrogen by conventional Schlenk techniques. Solvents
were rigorously dried and degassed before use. Microanalyses were

ReactiVity of Noncyclic Nickel Enolates
Organometallics, Vol. 26, No. 23, 2007 5719
Table 1. Equilibrium Constants for the Neutralization of 2a
with Ketones
Anal. Calcd for C23H42ONiP2: C, 60.68; H, 9.30. Found: C, 60.49;
1
H, 9.05. IR (Nujol mull): ν(CdC) 1590 cm-1. H NMR (C6D6,
400 MHz): δ 0.40 (pt, 3H, J*HP ≈ 5.2 Hz, Ni-CH3), 0.83 (m,
2a + acetophenone
2a + R-tetralone
3
3
2H, CH2), 0.87 (dd, 6H, JHP ) 13.3 Hz, JHH ) 7.0 Hz,
ketone
equiv
ketone
Keq(average) equiv
3
3
PCHMeMe), 0.98 (dd, 6H, JHP ) 12.7 Hz, JHH ) 7.0 Hz,
PCHMeMe), 1.03 (m, partially obscured by peaks at 0.98 and 1.12,
Keq
Keq
Keq(average)
3
6
10
20
3.1 × 10-2 2(1) × 10-2
5
10
15
3.2 × 10-2 5(2) × 10-2
3
3
CH2), 1.12 (dd, 6H, JHP ) 15.5 Hz, JHH ) 7.2 Hz, PCHMeMe),
1.31 (dd, 6H, JHP ) 14.9 Hz, JHH ) 7.2 Hz, PCHMeMe), 1.70
(m, 2H, PCHMe2), 1.85 (m, 2H, PCHMe2), 4.56 (s, 1H, OCCHH),
2.0 × 10-2
5.1 × 10-2
3
3
1.5 × 10-2
7.3 × 10-2
0.7 × 10-2
3
4.85 (s, 1H, OCCHH), 7.12 (m, 1H, p-CH, Ph), 7.26 (t, 2H, JHH
3
probably due to its low stability. IR (Nujol mull): ν(CdC) 1581
cm-1. 1H NMR (C6D6, 300 MHz): δ 0.47 (pt, 3H, J*HP ) 5.3 Hz,
) 7.6 Hz, m-CH, Ph), 8.19 (d, 2H, JHH ) 7.2 Hz, o-CH, Ph).
13C{1H} NMR (C6D6, 100 MHz): δ -0.1 (dd, JCP ) 71 and 35
2
3
3
1
2
Ni-CH3), 0.82 (dd, 6H, JHP ) 13.1 Hz, JHH ) 7.0 Hz,
Hz, Ni-CH3), 17.1 (dd, JCP ) 19 Hz, JCP ) 11 Hz, CH2), 16.9
(pt, J*CP ≈ 16 Hz, CH2), 18.6 (s, PCHMeMe), 18.8 (s, PCHMeMe),
3
3
PCHMeMe), 0.98 (dd, 6H, JHP ) 12.5 Hz, JHH ) 7.1 Hz,
3
3
2
2
PCHMeMe), 1.10 (dd, 6H, JHP ) 15.5 Hz, JHH ) 7.2 Hz,
19.5 (d, JCP ) 5 Hz, PCHMeMe), 20.0 (d, JCP ) 3 Hz,
3
3
PCHMeMe), 24.2 (m, CH2), 24.4 (d, 1JCP ) 14 Hz, PCHMe2), 25.6
PCHMeMe), 1.41 (dd, 6H, JHP ) 15.1 Hz, JHH ) 7.1 Hz,
PCHMeMe), 1.54 (s, 9H, CMe3), 1.67 (m, 2H, PCHMe2), 1.89 (m,
2H, PCHMe2), 4.05 (s, 1H, OCdCHH), 4.21 (s, 1H, OCdCHH).
13C{1H} NMR (C6D6, 75 MHz): δ 0.5 (dd, 2JCP ) 69, 35 Hz, Ni-
1
(d, JCP ) 27 Hz, PCHMe2), 80.1 (s, OCdCH2), 126.3 (s, CH,
4
Ph), 126.5 (s, CH, Ph), 127.6 (s, CH, Ph), 145.2 (d, JCP ) 4 Hz,
Cq, Ph), 166.3 (s, OCdCH2). 13C NMR (gated, C6D6, selected
1
2
data): 80.1 (d, JCH ) 153 Hz, OCdCH). 31P{1H} NMR (C6D6,
1
CH3), 16.8 (dd, JCP ) 18 Hz, JCP ) 12 Hz, CH2), 18.0 (s,
2
PCHMeMe), 18.3 (s, PCHMeMe), 19.2 (d, JCP ) 6 Hz, PCH-
162 MHz): δ 64.7, 76.6.
MeMe), 19.5 (d, 2JCP ) 3 Hz, PCHMeMe), 23.8 (d, 1JCP ) 14 Hz,
Synthesis of Ni(dippe)(CH3)(OC(CH(C6H4-o-CH2CH2) (4).
Method A. A solution of 0.5 mmol of lithium tert-butoxide in 3
mL of thf (see synthesis of 3, method B) was added to a cold
(-78 °C) solution of 176 mg of 1 (0.5 mmol) dissolved in 3 mL
of thf. The mixture was allowed to reach room temperature and
cooled again at -78 °C. Then, 66 µL (0.5 mmol) of R-tetralone
was added. After stirring at room temperature for 5 h, the solvent
was removed under reduced pressure and the solid was extracted
with 3 mL of thf. The insoluble solid (LiF) was removed by
centrifugation, some diethyl ether was added, and the solution was
cooled to -30 °C, to yield the product in 40% yield (190 mg).
1
PCHMe2), 25.1 (d, JCP ) 27 Hz, PCHMe2), 30.0 (s, CMe3), 37.7
(s, CMe3), 75.4 (s, OC ) CH2), 177.5 (s, OCdCH2). 13C NMR
(gated, C6D6, selected data): δ 75.4 (d, 1JCH ) 152 Hz, OCdCH).
31P{1H} NMR (C6D6, 121 MHz): δ 64.8, 76.6.
Measurement of Enolate Formation Equilibrium Constants.
In a typical experiment, the hydroxide complex 2a (30 mg, 85 µmol)
was dissolved in 0.5 mL of thf and placed in a NMR tube. Then,
1 equiv of acetophenone was added, and the solution monitored
by 31P NMR spectroscopy, which indicated that the equilibrium
was immediately reached. Increasing amounts of acetophenone were
added (up to 20 equiv). The ratios of the enolate and hydroxide
complexes were determined by integration of the 31P NMR spectra.
Table 1 shows the values of the equilibrium constants (Keq)
obtained. Neutralization experiments using 2b were carried out in
a similar manner, using samples of tert-butoxide complex generated
in situ from 1 and LiO-t-Bu. These reactions proceeded more slowly
and were monitored until no further change was observed. For
acetophenone and tetralone, the equilibrium was fully shifted to
the enolate complexes and the constants could not be determined.
During the slow neutralization of 2b by pinacolone, some decom-
position took place, but an equilibrium constant Keq ≈ 6 could be
estimated.
Method B. A solution of 2c (0.5 mmol) in 3 mL of thf was
cooled to -78 °C, and 66 µL (0.5 mmol) of R-tetralone was added.
The dark red color characteristic of 2c immediately changed to
bright orange. The mixture was warmed to room temperature, the
solvent removed under reduced pressure, and the solid extracted
with 3 mL of thf. After centrifugation, diethyl ether was added
and the solution was cooled to -30 °C to afford 4 in 50% yield.
Anal. Calcd for C23H42ONiP2: C, 62.39; H, 9.22. Found: C, 62.08;
1
H, 8.92. IR (Nujol mull): ν(CdC) 1600 cm-1. H NMR (C6D6,
300 MHz): δ 0.42 (pt, 3H, J*HP ≈ 5.1 Hz, Ni-CH3), 0.83 (dd,
3
3
6H, JHP ) 13.1 Hz, JHH ) 7.0 Hz, PCHMeMe), 0.97 (dd, 6H,
3JHP ) 12.4 Hz, 3JHH ) 7.1 Hz, PCHMeMe), 1.10 (dd, 6H, 3JHP
)
Synthesis of Ni(dippe)(CH3)(CH2CO2CH3) (6). A solution of
0.5 mmol of 2c in 3 mL of thf, prepared as described before, was
cooled at -78 °C, and 39 µL (0.5 mmol) of methyl acetate was
added. The dark red solution lightened immediately. The cooling
bath was removed and the mixture stirred while warming to room
temperature. Then, the solvent was removed under reduced pressure
and the solid extracted with 3 mL of Et2O. Then the solid residue
was separated by centrifugation. The product was obtained in 60%
yield as orange crystals after partial evaporation of the solvent and
cooling at -20 °C. Anal. Calcd for C18H40O2NiP2: C, 52.84; H,
9.85. Found: C, 52.03; H, 10.24. IR (Nujol mull): ν(CdO) 1681
cm-1. 1H NMR (C6D6, 400 MHz): δ 0.53 (dd, 3H, 3JHP ) 8.1, 4.3
3
3
15.5 Hz, JHH ) 7.1 Hz, PCHMeMe), 1.36 (dd, 6H, JHP ) 14.9
3
Hz, JHH ) 7.1 Hz, PCHMeMe), 1.65 (m, 2H, CH), 1.88 (m, 2H,
CH), 2.73 (m, 2H, CH2), 2.98 (t, 2H, 3JHH ) 7.5 Hz, CH2), 5.36 (t,
3
1H, JHH ) 4.2 Hz, OCdCH), 7.16 (s, 2H, CarH), 7.40 (m, 1H,
CarH), 8.37 (d, 1H, JHH ) 7.3 Hz, CarH). 13C{1H} NMR (C6D6,
3
75 MHz): δ -0.2 (dd, 2JCP ) 71, 35 Hz, Ni-CH3), 16.5 (dd, 1JCP
2
) 18 Hz, JCP ) 11 Hz, CH2 (dippe)), 18.1 (s, PCHMeMe), 18.3
(s, PCHMeMe), 19.0 (d, 2JCP ) 5 Hz, PCHMeMe), 19.5 (d, 2JCP
)
3 Hz, PCHMeMe), 23.7 (s, PCHMe2), 24.5 (s, CH2), 25.1 (d, 1JCP
) 27 Hz, PCHMe2), 30.7 (s, CH2), 95.3 (s, OCdCH), 123.3 (s,
CarH), 125.3 (s, CarH), 125.6 (s, CarH), 126.2 (s, CarH), 137.8 (s,
Car), 140.3 (s, Car), 158.2 (s, OCCH). 13C NMR (gated, C6D6,
selected data): δ 95.3 (d, 1JCH ) 152 Hz, OCdCH). 31P{1H} NMR
(C6D6, 162 MHz) δ 64.4, 76.3.
3
3
Hz, Ni-CH3), 0.79 (dd, 6H, JHP ) 12.8 Hz, JHH ) 7.0 Hz,
3
3
PCHMeMe), 0.84 (dd, 6H, JHP ) 11.9 Hz, JHH ) 7.3 Hz,
3
PCHMeMe), 0.94 (m, 2H, CH2), 1.02 (dd, 6H, JHP ) 15.1 Hz,
Synthesis of Ni(dippe)(CH3)(OC(CH2)C(CH3)3) (5). A solution
of 2c (0.5 mmol) in 3 mL of thf was cooled at -78 °C, and 62 µL
(0.5 mmol) of pinacolone was added. The color of the solution
changed from red to orange. After warming at room temperature,
the solvent was removed under reduced pressure and the solid
residue was extracted in 3 mL of hexane. Then the solid residue
was separated by centrifugation. The product crystallized out from
this solution at -78 °C as a dark orange solid in a 60% yield. No
satisfactory analytical data could be gathered for this compound,
3JHH ) 7.2 Hz, PCHMeMe), 1.17 (dd, 6H, 3JHP ) 14.9 Hz, 3JHH
)
7.2 Hz, PCHMeMe), 1.83 (m, 2H, PCHMe2), 2.07 (m, 2H,
3
PCHMe2), 2.23 (dd, 2H, JHP ) 10.6, 6.3 Hz, CH2CO2CH3), 3.72
(s, 3H, CH2CO2CH3). 13C{1H} NMR (C6D6, 75 MHz): δ -1.0
(dd, 2JCP ) 66, 27 Hz, Ni-CH3), 17.4 (dd, 2JCP ) 53, 12 Hz, CH2-
CO2CH3), 18.2 (s, PCHMeMe), 19.7 (s, PCHMeMe), 21.5 (dd, 1JCP
) 22 Hz, 2JCP ) 18 Hz, CH2), 24.5 (pt, J*CP ≈ 20 Hz, PCHMe2),
49.1 (s, CH2CO2CH3), 180.2 (s, CH2CO2CH3). 31P{1H} NMR
(C6D6, 162 MHz): δ 71.4, 79.5.

5720 Organometallics, Vol. 26, No. 23, 2007
Ca´mpora et al.
Synthesis of Ni(dippe)(CH3)(CH2CN) (7). A solution of 0.5
mmol of 2c (0.5 mmol) in 3 mL of thf was cooled to -78 °C, and
26 µL (0.5 mmol) of acetonitrile was added. The color changed
from dark red to orange. The solvent was removed under vacuum,
the residue extracted with 5 mL of diethyl ether, and then the solid
residue separated by centrifugation. Upon filtration and evaporation,
the product was obtained as an orange solid. Yield: 30%. 1H NMR
changing the color from orange to pale yellow. The mixture was
warmed at room temperature and the solvent was removed under
vacuum. The residue was extracted in ether (5 mL). Compounds
10a31 and 10b were separated from Ni(dippe)(CO)2 by spinning
band chromatography using a 10:1 hexane/ether mixture as eluent.
It was not possible to resolve the mixture of 10a and 10b, which
were found to be in a 4:1 ratio. In a similarly performed experiment,
a solution containing 9a and 9b was allowed to stand for several
hours at room temperature. The inversion of the isomer ration was
established by 31P{1H} NMR.
3
(C6D6, 300 MHz): δ 0.43 (dd, 3H, JHP ) 8.1 and 4.1 Hz, Ni-
CH3), 0.80 (dd, 12 H, 3JHP ) 12.2 Hz, 3JHH ) 7.0 Hz, PCHMeMe),
1.05 (dd, 12 H, 3JHP ) 15.2 Hz, 3JHH ) 7.7 Hz, PCHMeMe), 1.25
3
(dd, 2H, JHP ) 11.0 and 4.9 Hz, NiCH2CN), 1.80 (m, 4H,
Compound 10a: 1H NMR (CDCl3, 300 MHz): δ 2.10 (s, 3H,
2
3
3
PCHMeMe). 13C{1H} NMR (C6D6, 100 MHz): δ -6.5 (dd, JCP
CH3), 2.53 (t, 2H, JHH ) 7.8 Hz, CH2), 2.80 (t, 2H, JHH ) 7.8
Hz, CH2), 4.74 (s, 1H, dCHH), 4.75 (s, 1H, dCHH), 7.26 (m,
5H, CarH). 13C NMR (CDCl3, 100 MHz): δ 21.0 (s, CH3), 32.9
(s, CH2), 35.0 (s, CH2), 101.8 (s, dCH2), 126.1 (s, CarH), 128.3 (s,
CarH), 128.4 (s, CarH), 140.9 (s, Car), 155.7 (s, OCdCH), 169.2
(s, O2CCH3).
2
) 61, 14 Hz, Ni-CH2CN), 1.6 (dd, JCP ) 67, 26 Hz, Ni-CH3),
2
18.6 (s, PCHMeMe), 19.8 (d, JCP ) 5 Hz, PCHMeMe), 20.1 (d,
2JCP ) 4 Hz, PCHMeMe), 21.9 (pt, J*CP ≈ 20 Hz, CH2), 24.8 (d,
1JCP ) 6 Hz, PCHMe2), 27.9 (d, 1JCP ) 11 Hz, PCHMe2). 31P{1H}
NMR (C6D6, 162 MHz): δ 73.3, 81.3.
Synthesis of Ni(dippe)(CH3)(OPh) (8). A solution of 2c (0.5
mmol) in 3 mL of thf, prepared as described above, was cooled at
-78 °C, and 63 µL (0.5 mmol) of phenyl acetate was added. After
reaching room temperature, solvent was removed under reduced
pressure, and the residue was extracted in 5 mL of diethyl ether.
After centrifugation, the solution was cooled at -20 °C. The product
crystallized as red crystals. Yield: 55%. Anal. Calcd for C21H40-
Compound 10b: 1H NMR (CDCl3, 300 MHz): δ 1.93 (s, 3H,
3
CH3), 2.17 (s, 3H, CH3), 3.27 (d, 2H, JHH ) 7.3 Hz, CH2), 5.19
(t, 1H, 3JHH ) 6.9 Hz, dCH), 7.26 (m, 5H, CarH). 13C NMR (CDCl3,
100 MHz): δ 19.6 (s, CH3), 20.8 (s, CH3), 31.8 (s, CH2), 115.7 (s,
dCH), 126.1 (s, CarH), 128.3 (s, CarH), 128.4 (s, CarH), 140.1 (s,
Car), 145.6 (s, OCdCH2), 169.0 (s, O2CCH3).
Reaction of Enolate Complex 3 with Benzaldehyde. Char-
acterization of Ni Aldolate 11. A 21 µL portion of benzaldehyde
(0.2 mmol) was added to a solution of complex 3 (48 mg, 0.1 mmol)
in 0.5 mL of C6D6. 31P and 1H NMR monitoring revealed
quantitative formation of aldolate 11. The reaction mixture was
allowed to stand at room temperature for 2 days. 31P NMR indicated
total consumption of 11 to give Ni(Me)(OH)(dippe) as the major
P-containing product. The yield of trans-chalcone in the mixture,
determined by GC, was 54%. Compound 11: 1H NMR of (C6D6,
300 MHz): δ -0.23 (s, 3H, Ni-CH3), 0.75 (d, 6H, PCHMeMe),
1
ONiP2: C, 58.77; H, 9.39. Found: C, 58.26; H, 9.47. H NMR
(C6D6, 300 MHz): δ 0.29 (pt, 3H, J*HP ≈ 5.0 Hz, Ni-CH3), 0.70
3
3
(m, 2H, CH2), 0.80 (dd, 6 H, JHP ) 13.2 Hz, JHH ) 7.0 Hz,
3
3
PCHMeMe), 0.93 (dd, 6 H, JHP ) 12.6 Hz, JHH ) 7.1 Hz,
PCHMeMe), 1.07 (dd, 6H, JHP ) 15.6 Hz, JHH ) 7.2 Hz,
PCHMeMe), 1.28 (dd, 6H, JHP ) 14.9 Hz, JHH ) 7.1 Hz,
PCHMeMe), 1.61 (m, 2H, PCHMe2), 1.80 (m, 2H, PCHMe2), 6.78
3
3
3
3
3
3
(t, 1H, JHH ) 7.0 Hz, p-CH, Ph), 7.30 (d, 2H, JHH ) 7.6 Hz,
o-CH, Ph), 7.42 (t, 2H, JHH ) 7.4 Hz, m-CH, Ph). 13C{1H} NMR
(C6D6, 75 MHz): δ 0.0 (dd, 2JCP ) 72, 35 Hz, Ni-CH3), 16.1 (dd,
3
0.99 (m, 12H, PCHMeMe), 1.39 (dd, 6H, JHP ) 14.2, 6.3 Hz,
1JCP ) 18 Hz, JCP ) 11 Hz, CH2), 18.0 (s, PCHMeMe), 18.2 (s,
PCHMeMe), 1.53 (m, 2H, PCHMe2), 1.98 (m, 2H, PCHMe2), 3.32
2
2
2
PCHMeMe), 19.0 (d, JCP ) 5 Hz, PCHMeMe), 19.5 (s, PCH-
(d, 1H, JHH ) 9.9 Hz, Ni-OCH(Ph)CHH(COPh)), 3.47 (pt, 1H,
1
1
MeMe), 23.7 (d, JCP ) 14 Hz, PCHMe2), 25.1 (d, JCP ) 27 Hz,
PCHMe2), 112.3 (s, p-CH, Ph), 121.9 (s, o-CH, Ph), 128.8 (s, m-CH,
Ph), 169.4 (s, O-C, Ph). 31P{1H} NMR (C6D6, 121 MHz): δ 64.0,
76.5.
J*HH ) 10.5 Hz, Ni-OCH(Ph)CHHC(O)Ph), 5.70 (bs, 1H, Ni-
3
OCH(Ph)), 7.00 (d, 1H, JHH ) 7.1 Hz, CarH), 7.34 (t, 2H, JHH
)
3
7.0 Hz, CarH), 7.86 (d, 2H, JHH ) 7.0 Hz, CarH), 8.14 (s, 2H,
CarH). 13C{1H} NMR (C6D6, 75 MHz): δ -2.9 (dd, 2JCP ) 70, 35
2
Carbonylation of Enolate 3. A solution of 2c (0.5 mmol) in 3
mL of thf was cooled to -78 °C. Then, 58 µL (0.5 mmol) of
acetophenone was added. After reaching room temperature, the
mixture was brought back to -78 °C and CO was bubbled in for
5 min, turning the initial orange color of the solution to pale yellow.
Solvent was removed under reduced pressure when room temper-
ature was reached, and the oily residue was extracted with 5 mL
of ether and filtered. Acetophenone acetate30 was separated from
Ni(dippe)(CO)2 by spinning band chromatography, with a eluent
Hz, Ni-CH3), 16.1 (pt, J*CP ) 11 Hz, CH2), 17.9 (d, JCP ) 11
2
Hz, PCHMeMe), 18.2 (d, JCP ) 16 Hz, PCHMeMe), 19.2 (s,
PCHMeMe), 19.4 (s, PCHMeMe), 23.1 (d, 1JCP ) 13 Hz, PCHMe2),
24.9 (pt, J*CP ≈ 26 Hz, CH2), 53.4 (s, Ni-OCH(Ph)CH2(COPh)),
3
73.0 (d, JCP ) 5 Hz, Ni-OCH(Ph)), 125.1 (s, CarH), 126.6 (s,
CarH), 127.8 (s, CarH), 131.3 (s, CarH), 139.0 (s, Car), 153.4 (s,
Car), 199.5 (s, Ni-OCH(Ph)CH2(COPh)). 31P{1H} NMR (C6D6,
2
121 MHz): δ 63.5 (d, JPP ) 7 Hz), 76.1 (d).
Reaction of Enolate Complex 3 with Carbon Dioxide.
Synthesis of Complexes 12 and 13. A 6.3 mL (0.28 mmol) sample
of dry CO2 was carefully bubbled into a solution of 129 mg (0.28
1
mixture of hexane/ether (19:1). H NMR (CDCl3, 400 MHz): δ
2
2.27 (s, 3H, CH3), 5.01 (d, 1H, JHH ) 2.2 Hz, OCdCHH), 5.47
(d, 1H, 2JHH ) 2.2 Hz, OCdCHH), 7.32 (m, 3H, CarH), 7.45 (m,
2H, CarH).
mmol) of the enolate complex 3 in 0.8 mL of C6D6. 31P{1H}, H,
1
and 13C{1H} NMR monitoring of the mixture revealed quantitative
formation of complex 12. Insoluble crystals of the tetranuclear
complex 13 were obtained when the solution was allowed to stand
at room temperature for several days.
This reaction was also carried out starting from an isolated sample
of enolate complex 3: to this compound (115 mg, 25 µmol), which
was dissolved in 0.7 mL of toluene-d8 in a NMR tube, was added
0.75 equiv (4.2 mL) of dry CO at -60 °C. A 31P NMR spectrum
of the reaction mixture indicated the presence of the starting material
and Ni(CO)2(dippe) in a 1:0.25 ratio.
CarbonylationoftheEnolatesof4-Phenylbutan-2-one: CH3CO2C-
(dCH2)(CH2)2Ph (10a) and CH3CO2C(dCHCH2Ph)CH3 (10b).
4-Phenylbutan-2-one (72 µL, 0.5 mmol) was added to 3 mL of a
cooled (-78 °C) thf solution containing 0.5 mmol of 2c. The 31P-
{1H} NMR spectrum of the reaction mixture displayed characteristic
signals corresponding complexes 9a and 9b as the only P-containing
products (9a/9b ) 6:1). CO was bubbled for 5 min at -78 °C,
3
12: 1H NMR (C6D6, 300 MHz): δ 0.21 (t, 3H, JHP ) 4.8 Hz,
3
Ni-CH3), 0.85 (m, 12H, PCHMeMe), 1.06 (dd, 6H, JHP ) 15.8
3
3
Hz, JHH ) 7.2 Hz, PCHMeMe), 1.20 (dd, 6H, JHP ) 15.4 Hz,
3JHH ) 7.1 Hz, PCHMeMe), 1.67 (m, 4H, PCHMe2), 4.03 (s, 2H,
OC(O)CH2C(O)Ph), 7.11 (m, 3H, CarH), 8.27 (m, 2H, CarH). 13C-
{1H} NMR of 3 (C6D6, 75 MHz): δ -0.7 (dd, JCP ) 67 and 36
2
1
2
Hz, Ni-CH3), 16.2 (dd, JCP ) 25 Hz, JCP ) 11 Hz, CH2), 18.1
2
(s, PCHMeMe), 18.2 (s, PCHMeMe), 19.0 (d, JCP ) 5 Hz,
PCHMeMe), 19.6 (s, PCHMeMe), 23.9 (d, 1JCP ) 14 Hz, PCHMe2),
25.4 (d, 1JCP ) 29 Hz, PCHMe2), 51.2 (s, OC(O)CH2C(O)Ph), 125.7
(30) Harrison, J. J. J. Org. Chem. 1979, 44, 3578.
(31) Boaz, N. W. Tetrahedron Lett. 1998, 39, 5505.

ReactiVity of Noncyclic Nickel Enolates
Organometallics, Vol. 26, No. 23, 2007 5721
Table 2. Summary of Crystallographic Data and Structure Refinement Results for 3, 6, and 13.
3
6
13
formula
fw
cryst syst
space group
a, Å
C23H42NiOP2
455.22
monoclinic
P21/c
14.496(5)
10.941(3)
16.481(5)
90.0
105.09(3)
90.0
2524(1)
4
C18H40NiO2P2
409.15
monoclinic
P21/n
10.1804(7)
16.4024(11)
13.0782(9)
90.0
98.6330(10)
90.0
C72H136Ni4O8P6
1550.47
monoclinic
P21/n
14.2577(11)
15.5013(12)
18.3160(15)
90.0
96.4120(10)
90.0
4022.7(5)
2
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
2159.1(3)
4
F(000)
984
1.198
0.906
2.36; 28.12
293
888
1.259
1.054
2.94; 28.70
100
1668
1.280
1.089
2.32; 28.67
100
D
calc, Mg m-3
µ, mm-1
θ
min; θmax, deg
temp, K
no. refins collected
no. reflns used; R(int)
no. reflns obsd [I > 2σ(I)]
no. of params
14 396
5543; 0.029
3711
7715
5438; 0.020
3332
245
0.0276
0.0702
1.026
0.415, -0.309
25 484
9358; 0.016
8252
452
0.0406
0.1165
1.032
1.310, -0.641
253
R1(F) [I > 2σ(I)]a
wR2(F2)b (all data)
Sc (all data)
0.0371
0.1026
0.943
largest diff peak and hole, e Å-3
0.377; -0.297
b
2
2
2
c
2
2
a R1(F) ) ∑(|Fo| - |Fc|)/∑|Fo| for the observed reflections [F2 > 2σ(F2)]. wR2(F2) ) {∑[w(Fo - Fc )2]/∑w(Fo )2}1/2. S ) {∑[w(Fo - Fc )2]/(n -
p)}1/2 (n ) number of reflections, p ) number of parameters).
2
(s, CarH), 129.2 (s, CarH), 131.7 (s, CarH), 138.1 (s, Car), 169.9 (s,
OC(O)CH2C(O)Ph), 194.5 (s, OC(O)CH2C(O)Ph). 31P{1H} NMR
(C6D6, 121 MHz): δ 67.0, 78.0. 13‚2thf: IR (Nujol mull): ν(Cd
O) + ν(CdC) 1586, 1533 cm-1. Anal. Calcd for C72H136Ni4O8P6:
C, 55.78; H, 8.84. Found: C, 55.66; H, 8.73.
least-squares techniques (SHELXTL-6.14),35 minimizing w[Fo
-
Fc2]2. One isopropyl group of the diphosphine ligand of compound
3 was observed disordered in two positions with occupancy factors
first refined and later fixed to 0.60 and 0.40. The -CH3 and -CH2-
COOCH3 groups on the nickel metal of compound 6 were also
observed disordered, both in two positions with occupancy factors
first refined and later fixed to 0.64 and 0.36. All the non-hydrogen
atoms were refined with anisotropic displacement parameters. The
hydrogen atoms were included from calculated positions and refined
as riding contributions with isotropic displacement parameters 1.5
(methyl groups) times those of their respective attached carbon
atoms.
X-ray Crystal Structure Analyses of 3, 6, and 13. A single
crystal of each representative compound (yellow prism 3, orange
plate 6, and orange block 13) of suitable size was mounted on a
glass fiber using glue (3) or using perfluoropolyether oil (FOMBLIN
140/13, Aldrich) in the cold N2 stream of the a low-temperature
device attachment (6 and 13). A summary of crystallographic data
and structure refinement is reported in Table 2. Intensity data for
3 were collected on a Bruker AXS SMART 1000 single-crystal
diffractometer equipped with a CCD area detector using a graphite
monochrotomator λ(Mo KR1) ) 0.71073 Å, whereas the data
collections for complexes 6 and 13 were performed on a Bruker-
AXS X8Kappa diffractometer equipped with an APEXII CCD area
detector, using a graphite monochromator λ(Mo KR1) ) 0.71073
Å and a Bruker Cryo-Flex low-temperature device. The data
collection strategy used in all instances was phi and omega scans
with narrow frames. Instrument and crystal stability were evaluated
from the measurement of equivalent reflections at different measur-
ing times, and no decay was observed. The data were reduced
(SAINT)32 and corrected for Lorentz and polarization effects, and
a semiempirical absorption correction was applied (SADABS).33
The structures were solved by direct methods (SIR-2002)34 and
Patterson methods and refined against all F2 data by full-matrix
Acknowledgment. Financial support from the DGI (Project
CTQ2006-05527/BQU) and Junta de Andaluc´ıa (Project P06-
FQM-01704) is gratefully acknowledged. I.M. is thankful for a
research fellowship from the Consejo Superior de Investiga-
ciones Cient´ıficas (I3P program).
Supporting Information Available: X-ray crystallographic file
in CIF format is available free of charge via the Internet at
http://pubs.acs.org.
OM7007259
(34) Burla, M. C.; Camalli, M.; Carrozzini, B.; Cascarano, G. L.;
Giacovazzo, C.; Polidori, G.; Spagna, R. SIR2002: the program. J. Appl.
Crystallogr. 2003, 36, 103.
(32) SAINT 6.02; Bruker-AXS, Inc.: Madison, WI, 1997-1999.
(33) Sheldrick, G. SADABS; Bruker AXS, Inc.: Madison, WI, 1999.
(35) SHELXTL 6.14; Bruker AXS, Inc.: Madison, WI, 2000-2003.