Pyrazolyl- and hydrocarbyl-bridged binuclear complexes of nickel

Article abstract of DOI:10.1021/om960971g

The binuclear nickel complex Ni₂Br₂PMe₃)₃(μ ₂-eta;³:η¹-CH₂-O-C ₆H₄) (1) reacts with pyrazolyllithium (LiPz) to give the μ-pyrazolyl μ-hydrocarbyl compound Ni₂(PMe₃)₂Br(μ₂-η ³:η¹-CH₂-O-C₆H ₄)(μ₂-Pz) (2). X-ray studies show a short Ni-Ni distance (2.710(2) A?) which suggests the existence of some bonding interaction. The bromide ligand of compound 2 can be replaced by anionic (N₃^-, 2,5-dimethylpyrrolide) or neutral (PMe₃, Me₂PCH₂CH₂PMe₂) ligands. Other related complexes containing methyl or tert-butyl substituents on the 3-position or on the 3- and 5-positions of the pyrazolate ligand can be obtained using the corresponding pyrazolylthallium derivatives. The monosubstituted pyrazole complexes exist in solution as equilibrium mixtures of two isomers which differ in the relative position of the pyrazolate substituent. However, only one isomer has been isolated in the solid state.

Full text of DOI:10.1021/om960971g

Organometallics 1997, 16, 2709-2718
2709
P yr a zolyl- a n d Hyd r oca r byl-Br id ged Bin u clea r
Com p lexes of Nick el
J uan Ca´mpora,*,† J orge A. Lo´pez,† Pilar Palma,† Caridad Ru´ız,‡ and
Ernesto Carmona*,†
Departamento de Quı´mica Inorga´nica-Instituto de Investigaciones Qu´ımicas, Universidad de
Sevilla-Consejo Superior de Investigaciones Cientı´ficas, C/ Americo Vespucio s/ n, Isla de la
Cartuja, 41092 Sevilla, Spain, and Instituto de Ciencia de Materiales de Madrid, Consejo
Superior de Investigaciones Cient´ıficas, Campus de Cantoblanco, 28049 Madrid, Spain
Received November 15, 1996X
The binuclear nickel complex Ni2Br2(PMe3)3(µ2-η3:η1-CH2-o-C6H4) (1) reacts with pyra-
zolyllithium (LiPz) to give the µ-pyrazolyl µ-hydrocarbyl compound Ni2(PMe3)2Br(µ2-η3:η1-
CH2-o-C6H4)(µ2-Pz) (2). X-ray studies show a short Ni-Ni distance (2.710(2) Å) which
suggests the existence of some bonding interaction. The bromide ligand of compound 2 can
be replaced by anionic (N3-, 2,5-dimethylpyrrolide) or neutral (PMe3, Me2PCH2CH2PMe2)
ligands. Other related complexes containing methyl or tert-butyl substituents on the
3-position or on the 3- and 5-positions of the pyrazolate ligand can be obtained using the
corresponding pyrazolylthallium derivatives. The monosubstituted pyrazole complexes exist
in solution as equilibrium mixtures of two isomers which differ in the relative position of
the pyrazolate substituent. However, only one isomer has been isolated in the solid state.
Ch a r t 1
In tr od u ction
The study of hydrocarbon- and hydrocarbyl-bridged
di- or polynuclear complexes is a rapidly developing field
of research in organometallic chemistry.1 Much of the
interest in these compounds arises in ascertaining
whether the coordination of an organic fragment to two
or more metal centers may allow the observation of new
reactivity patterns, significantly different from those
present in the related mononuclear complexes. Because
of the possible cooperative effects between the two metal
atoms, the chemical properties of the hydrocarbon-
bridged binuclear complexes have been recognized as
midway between those of mononuclear species and of
high-nuclearity clusters.1a,c
course of these studies,2d are very likely caused by the
mutual action of the two reactive nickel centers. The
bonding mode of the µ2-η3:η1-CH2-o-C6H4 unit was found
to be very sensitive to the number and nature of the
coligands attached to the nickel atoms. Thus, 1 can add
a fourth PMe3 ligand in solution, giving rise to the labile
species (Me3P)2Ni(Br)(µ2-η1:η1-CH2-o-C6H4)Ni(Br)(P-
Me3)2,2b or react with 2 equiv of bis(dimethylphosphino)-
methane (dmpm) to yield Ni2(µ2-dmpm)2(µ2-η1:η1-CH2-
o-C6H4).2a In these two complexes, the benzylic moiety
acts as a η1 ligand.
In recent publications, we have described the chem-
istry of the binuclear nickel complex 1, which contains
the bridging µ2-η3:η1-CH2-o-C6H4 unit.2 The coupling of
As a continuation of these studies, we have probed
the coordination chemistry of pyrazolate-type ligands
in this system. Pyrazole and pyrazolate units have
attracted much attention due to their versatile coordi-
nation chemistry.3 These ligands were judged to be
especially attractive for this study, for they can firmly
hold two metal atoms at short distances, thereby allow-
ing extensive electron delocalization between the two
centers. The pyrazolate (Pz-) group can be monoden-
tate,4 or it can act as a bridging ligand. In the latter
case, two Pz rings can bind two identical5a-d (I, Chart
1) or less frequently two different5e-h metal fragments
acyl and iminoacyl functionalities, and the migration
of an inserted iminoacyl group, observed during the
† Universidad de Sevilla-CSIC.
‡ Instituto de Ciencia de Materiales de Madrid.
X Abstract published in Advance ACS Abstracts, April 15, 1997.
(1) (a) Lotz, S.; Van Rooyen, P.; Meyer, R. Adv. Organomet. Chem.
1995, 37, 219. (b) Beck, W.; Niemer, B.; Wieser, M. Angew. Chem.,
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1986, 86, 339. (d) Moss, J . R.; Scott, L. G. Coord. Chem. Rev. 1984, 60,
171.
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M. L.; Ruiz, C. Organometallics 1992, 11, 11. (b) Ca´mpora, J .;
Gutie´rrez, E.; Monge, A.; Poveda, M. L.; Carmona, E. Organometallics
1992, 11, 2644. (c) Ca´mpora, J .; Gutie´rrez, E.; Monge, A.; Poveda, M.
L.; Ruiz, C.; Carmona, E. Organometallics 1993, 12, 4025.
(4) See for example: (a) Bandini, A.; Banditelli, G.; Minghetti, G.;
Bonati, F. Can. J . Chem. 1979, 57, 3237. (b) Minghetti, G.; Banditelli,
G.; Bonati, F. J . Chem. Soc., Dalton Trans. 1979, 1851. (c) Carmona,
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1991, 10, 3123.
S0276-7333(96)00971-5 CCC: $14.00 © 1997 American Chemical Society

2710 Organometallics, Vol. 16, No. 12, 1997
Ca´mpora et al.
(II). Complexes containing only one bridging Pz-type
ligand are less common and contain usually another
bridging group (III).5i-k In this paper we describe the
synthesis and properties of some complexes of nickel of
type III, in which a hydrocarbyl chain is the second
bridging ligand.
Resu lts a n d Discu ssion
Syn th esis a n d Str u ctu r a l Ch a r a cter iza tion of
th e P yr azolate Com plex Ni2(P Me3)2Br (µ2-η3:η1-CH2-
o-C6H4)(µ2-P z) (2). The reaction of the binuclear
complex 1 with 1 equiv of pyrazolyllithium (LiPz) in
THF leads to the formation of the red crystalline
material 2, which can be isolated in 73% yield (eq 1).
F igu r e 1. ORTEP view and atom-labeling scheme of
compound 2.
Ta ble 1. Selected Bon d Dista n ces a n d An gles for 2
Bond Distances (Å)
Analytical data and NMR studies show the incorpora-
tion of the Pz ring with concomitant dissociation of one
of the PMe3 ligands of 1. Thus, the 31P{1H} NMR
spectrum of 2 consists of a weakly coupled AX spin
system (J PP ) 4.5 Hz), while the 1H and the 13C{1H}
NMR spectra display two doublets of equal intensity in
the region due to the PMe3 ligands (see Experimental
Section). The Pz ring is responsible for three 1H
multiplets at δ 5.91, 6.77, and 7.83 ppm that are
integrated for a single proton each, while the CH2
Ni1-P1
Ni1-N1
Ni1-C1
Ni1-C2
Ni1-C7
Ni2-Br
Ni1 -Ni2
2.132(4)
1.90(1)
2.13(1)
2.08(1)
1.93(1)
2.381(3)
2.710(2)
Ni2-P2
Ni2-N2
Ni2-C1
N1-N2
Cl-C2
2.154(4)
1.91(1)
1.91(1)
1.36(1)
1.43(2)
1.46(2)
C2-C7
Bond Angles (deg)
C2-Ni1-C7
C1-Ni1-C7
C1-Ni1-C2
N1-Ni1-C7
N1-Ni1-C2
N1-Ni1-C1
P1 -Ni1 -C7
P1-Ni1-C2
P1-Ni1-C1
P1-Ni1-N1
N2-Ni2-C1
P2-Ni2-C1
42.3(5)
73.4(5)
39.7(5)
158.4(5)
123.2(5)
87.0(4)
95.0(4)
134.8(3)
162.9(3)
101.9(3)
89.1(5)
88.9(4)
P2-Ni2-N2
Br-Ni2-C1
Br-Ni2-N2
Br-Ni2-P2
Ni1-N1-N2
Ni2-N2-N1
Ni1-C1-Ni2
Ni2-C1-C2
Ni1-C1-C2
Ni1-C2-C1
C1-C2-C7
Ni1-C7-C2
170.2(3)
176.7(4)
91.4(3)
91.0(1)
104.8(7)
116.0(8)
84.2(4)
128.1(9)
68.4(7)
71.9(7)
115.0(1)
74.2(7)
protons of the o-CH2C6H4 unit give an AB system in the
2
1H{31P} NMR spectrum (δ 2.20 and 2.33 ppm, J HH
)
3.4 Hz), which further splits by coupling to only one 31P
nucleus (J H ) 7.4; J H ) 3.4 Hz). In the 13C{1H}
AP
BP
NMR spectrum, the CH2 carbon appears at lower field
than expected for a Ni-η1-benzyl unit (30.5 vs 0-20
ppm2a,6,7) and it is also coupled to only one 31P nucleus
(J CP ) 10 Hz). All these data taken together with the
1
value of 152 Hz found for the J CH coupling constant6
compound; selected bond distances and angles are listed
in Table 1. The central core of the structure of this
complex is composed of two Ni(II) units bridged by the
pyrazolate and the hydrocarbyl CH2-o-C6H4 ligand.
Both nickel atoms are in square-planar environments,
but the coordination of the benzyl-bound Ni atom
deviates considerably from the ideal square-planar
geometry due to its pseudoallylic coordination to the
CH2-o-C6H4 ligand.
An interesting structural feature of compound 2 is the
coordination of the Ni1 atom to C1 instead of C3.
Generally, in η3-benzyl complexes that have an ortho
substituent with respect to the CH2 group, the η3-allylic
coordination involves the less hindered unsubstituted
ortho carbon,2a,6a,b as exemplified by the structure of 1.
The more encumbered coordination mode adopted may
be a consequence of the intrinsic nature of the µ2-Pz
ligand, which due to the shortness of the N-N bond
cannot embrace two metal centers that are too far away
from each other.
suggest the presence in the molecule of 2 of an (η3-
benzyl)nickel moiety and of a bridging pyrazolate ligand.
This assumption has been confirmed by X-ray studies.
Figure 1 shows an ORTEP view of the molecules of this
(5) (a) Coleman, A. W.; Eadie, D. T.; Stobart, S. R.; Zaworotko, M.
J .; Atwood, J . L. J . Am. Chem. Soc. 1982, 104, 922. (b) Marshall, J . L.;
Hopkins, M. D.; Miskowski, V. M.; Gray, H. B. Inorg. Chem. 1992, 31,
5034. (c) Lo´pez, G.; Ruiz, J .; Garc´ıa, G.; Vicente, C.; Casabo´, J .; Molins,
E.; Miravitlles, C. Inorg. Chem. 1991, 30, 2605. (d) J ain, V. K.; Kannan,
S.; Tiekink, E. R. T. J . Chem. Soc., Dalton Trans. 1993, 3625. (e)
Carmona, D.; Mendoza, A.; Ferrer, J .; Lahoz, F. J .; Oro, L. A. J .
Organomet. Chem. 1992, 431, 87. (f) Lo´pez, G.; Ruiz, J .; Garc´ıa, G.;
Vicente, C.; Rodr´ıguez, V.; Sa´nchez, G.; Hermoso, J . A.; Mart´ınez-
Ripoll, M. J . Chem. Soc., Dalton Trans. 1992, 1681. (g) Lo´pez, G.;
Garc´ıa, G.; Sa´nchez, G.; Garc´ıa, J .; Vegas, A.; Mart´ınez-Ripoll, M.
Inorg. Chem. 1992, 31, 1518. (h) Carmona, D.; Lamata, P. M.; Esteban,
M.; Lahoz, F. J .; Oro, L. A.; Apreda, M. C.; Foces-Foces, C.; Cano, F.
H. J . Chem. Soc., Dalton Trans. 1994, 159. (i) Carmona, D.; Ferrer,
J .; Mendoza, A.; Lahoz, F.; Reyes, J .; Oro, L. A. Angew. Chem., Int.
Ed. Engl. 1991, 30, 1171. (j) Beveridge, K. A.; Bushnell, G. W.; Stobart,
S. R.; Atwood, J . L.; Zaworotko, M. J . Organometallics 1983, 2, 1447.
(k) Carmona, D.; Oro, L. A.; Lamata, M. P.; J imeno, M. L.; Elguero,
J .; Belguise, A.; Lux, P. Inorg. Chem. 1994, 33, 2196.
(6) (a) Carmona, E.; Mar´ın, J . M.; Paneque, M.; Poveda, M. L.
Organometallics 1987, 6, 1757. (b) Ca´mpora, J .; Gutie´rrez-Puebla, E.;
Poveda, M. L.; Ruiz, C.; Carmona, E. J . Chem. Soc., Dalton Trans.
1992, 1769.
(7) (a) Carmona, E.; Mar´ın, J . M.; Palma, P.; Paneque, M.; Poveda,
M. L. Organometallics 1986, 4, 2053. (b) Carmona, E.; Mar´ın, J . M.;
Paneque, M.; Poveda, M. L. Inorg. Chem. 1989, 28, 1895.
Compound 2 is a rare example of a binuclear complex
containing µ2-η3:η1 allyl ligands of type A or B.1a On
the assumption that there is no Ni-Ni bonding interac-
tion, 2 would be the first example of a species with
structure A; complexes of type B have been reported.8

Pyrazolyl- and Hydrocarbyl-Bridged Complexes of Ni
Organometallics, Vol. 16, No. 12, 1997 2711
Ni-Ni bonds can be found in a wide variety of
binuclear compounds, in which the metal can have the
oxidation state 0,9a 1+,9b,c,10 or 2+.9d-f Although the
nature of the metal-metal bonding in these compounds
is in general not fully understood on theoretical
grounds,11a the observed Ni-Ni bond lengths have been
rationalized in terms of the electron count at the metal
centers and of the type of bridging ligands.11b Ni-Ni
bond lengths shorter than 2.4 Å12 have been observed
when both metal centers attain a 16-electron configu-
ration (e.g. [Ni(Cl)(µ-C(SiMe3)(PMe3))]2, 2.28(1) Å9c), but
Ni-Ni bond lengths as long as 2.788 Å9f have been found
in binuclear complexes of nickel with 18-electron counts
on both metal centers. Although the relatively short
Ni-Ni distance in 2, 2.710 Å, which is shorter than
twice the van der Waals radius of Ni (1.60 Å13), could
be indicative of a weak bonding interaction, the metal
centers could be also brought into close contact by the
proximity of the N atoms of the bridging pyrazolate
ligand14 and by conformational restrictions arising from
the pseudoallylic fragment. In order to ascertain the
hypothetical metal-metal distance in the absence of the
above ligand strain or bonding effects, we have con-
structed a molecular model15 of the Ni(µ-C6H7)Ni core
using bond lengths and angles derived from Ni(η3-
CH2C6H4-o-CH3)(PMe3)(Cl)6a for the pseudoallyl unit
and from the related binuclear complex Ni(Br)(PMe3)2-
F igu r e 2. Experimental and modeled Ni(µ-C6H7)Ni core
of 2.
tal error. However, Ni1-C1 is significantly shorter in
2; therefore, one reaches the conclusion that it is the
distortion of the pseudoallylic unit of 2 which allows
both nickel atoms to approach. It would appear reason-
able that the distortion of the pseudoallylic unit is a
consequence of electronic effects rather than the imposi-
tion of ligand constraints. Since it has been proposed
that for high-electron-count metals a bridging, π-accep-
tor ligand can increase the metal-metal bonding order
by accepting electron density from the metal-metal
antibonding orbitals,9a,10,16 the relatively short Ni1-C1
distance could be explained as a consequence of the
reinforcement of the retrodonation component of this
bond. Thus, a weak Ni-Ni interaction seems a likely
proposal.
An a logou s Com p lexes w ith Su bstitu ted P yr a -
zola te Liga n d s. As shown in Scheme 1, complexes 3,
which are similar to 2 but contain pyrazolate groups
with Me or t-Bu substituents in the ring 3- or 3,5-
positions, can be readily prepared by the reaction of 1
with the thallium salt of the appropriate pyrazole.
Since thallium pyrazolates can be generated in situ by
addition of TlOEt to a THF solution of the pyrazole,17
this is a simple and convenient synthetic procedure for
the generation of the corresponding thallium pyra-
zolates. Not unexpectedly, in the case of the monosub-
stituted 3-methyl (Pz′) and 3-tert-butyl (Pz*) pyra-
zolates, the reaction results in a mixture of two isomers,
3a /3b and 3d /3e, respectively.
6b
(µ-η1:η1-C6H4-p-CH2)Ni(Br)(PMe3)2 for the Ni-aryl
bond length (Figure 2). As can be seen, there is only a
minor structural change on going from the model to 2.
However, the Ni-Ni distance predicted by the model is
0.13 Å longer than in 2, indicating that some forces
overcome any steric repulsions between the metal
fragments.
Since the Ni2-C1 distance used in the model (1.881
Å) is close to that of 2 (1.91(1) Å), the shorter Ni-Ni
distance of the latter must be attributed to differences
in the pseudoallylic units. Within these, comparison of
the Ni1-C2 or the Ni1-C7 distances of the model and
2 reveals that they are the same within the experimen-
(8) (a) Kreiter, C. G.; Wendt, G.; Kaub, J . Chem. Ber. 1989, 122,
215. (b) Dettlaf, G.; Behrens, U.; Weiss, E. Chem. Ber. 1978, 111, 3019.
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II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Puddephatt, R. J ., Eds.;
Elsevier Science: Oxford, U.K., 1995; Vol. 9, pp 16-18. (b) J archow,
D.; Schutz, H.; Nast, R. Angew. Chem., Int. Ed. Engl. 1970, 9, 71. (c)
Ko¨nig, H.; Menu, M. J .; Dartiguenave, M.; Dartiguenave, Y.; Klein,
H. F. J . Am. Chem. Soc. 1990, 112, 5351. (d) Lawrie, C. J .; Dankosh,
H. E.; Carpenter, B. K. J . Organomet. Chem. 1991, 411, C7. (e) Fackler,
J . P. Prog. Inorg. Chem. 1976, 21, 55. (f) Peng, S. M.; Goedken, V. L.
J . Am. Chem. Soc. 1976, 98, 8500.
(10) Belderra´ın, T. R.; Gutie´rrez, E.; Monge, A.; Nicasio, M. C.;
Paneque, M.; Poveda, M. L.; Carmona, E. Organometallics 1993, 12,
4431.
(11) (a) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry;
Wiley: New York, 1988; p 748. (b) J ones, R. A.; Whittelsey, B. R. Inorg.
Chem. 1986, 25, 282.
(12) Hope, H.; Olmstead, H. H.; Murray, B. D.; Power, P. P. J . Am.
Chem. Soc. 1985, 107, 102.
(13) Bondi, A. J . Phys. Chem. 1964, 68, 44.
(14) Bridging pyrazole ligands allow long metal-metal distances.
For example, in the anionic complex [{Ni(C6F5)2}(µ-OH)(µ-Pz)],5f the
long Ni-Ni distance (3.263 Å) rules out metal-metal bonding.
(15) Molecular models were constructed using a molecular modeling
program: Chem 3D Plus, Cambridge Scientific Computing, Inc.,
Cambridge, MA, 1990.
The Me-substituted pyrazolate (Pz′ and Pz′′) com-
plexes 3a /3b and 3c are red-brown crystalline solids,
with physical appearance similar to that of 2, while the
tert-butyl pyrazolate (Pz* and Pz**) derivatives form
dark purple, nearly black crystals. However, they all
exhibit NMR features similar to those of 2. For in-
stance, they display chemically inequivalent methylene
protons which give rise to two multiplets in the 1.8-
2.5 ppm range in the proton NMR spectra, which arise
from the mutual coupling of these protons and from
coupling to one of the phosphorus nuclei. The corre-
sponding 13C methylene resonance appears at a chemi-
cal shift similar to that of 2 (27-30 ppm); large values
1
2
of J CH (145-157 Hz) and small values of J HH (3.3-
(16) Hahn, J . E. Prog. Inorg. Chem. 1984, 31, 205.
(17) Other azolate derivatives of thallium have been prepared in a
similar manner: (a) Lee, A. G. J . Chem. Soc. (A) 1971, 880. (b) Reedjik,
J .; Roefelsen, G.; Siedle, A. R.; Spek, A. L. Inorg. Chem. 1979, 18, 1947.

2712 Organometallics, Vol. 16, No. 12, 1997
Ca´mpora et al.
3b are significantly shifted upfield as compared to those
on the 5-position (e.g. 7.83 ppm for H(3) in 3b vs 6.77
ppm for H(5) in 3a ). Similar observations on the
chemical shifts of the analogous signals of the 3d /3e
mixture indicate that the major isomer is 3d . Therefore,
the bulkier pyrazole substituent is bonded to the C atom
closer to the η3-benzyl-Ni fragment (Ni1, Figure 1),
avoiding in this way the repulsion with the also steri-
cally demanding Br ligand bonded to the other Ni atom
(Ni2).
The NMR spectra of the 3a /b and 3d /e mixtures are
sharp at room temperature, and therefore no isomer
exchange takes place on the NMR time scale under
these conditions. Since the 3a /3b and 3d /3e isomer
ratios appear to be constant from one preparation to
other, it would appear likely that these isomers are in
equilibrium (eq 2). While the rate of exchange is too
F igu r e 3. Phosphine-pyrazole NOE interaction in com-
pound 2.
Sch em e 1
slow to allow line-broadening observation, it is fast
enough to be detected by spin saturation transfer (SST)
experiments.18a Thus, saturation at room temperature
of the Pz′ methyl signal of one of the isomers of the 3a /
3b mixture causes a ca. 10% intensity decrease of the
analogous signal on the other isomer. In order to
directly observe the transformation between 3a and 3b,
a solid sample obtained by crystallization of a solution
of the isomeric mixture was redissolved in cold CD2Cl2
(-80 °C) and its 1H NMR spectrum registered at various
temperatures. Below -30 °C, pure solutions of 3a were
observed, but at 0 °C, smooth isomerization to 3b occurs,
until the equilibrium concentrations are reached. This
process follows first-order kinetics with k1 ) 5.3 × 10-4
3.9 Hz) for this methylene group also support the
pseudoallylic coordination of the benzylic fragment.
From the 13C{1H} NMR spectra it can also be inferred
that the pseudoallylic Ni atom is situated as well on
the substituted side of the aromatic ring, i.e. close to
the arylic nickel unit, since only the pyrazole C(4)H
resonance can be found in the 95-120 ppm region.
Long-range through-ligand PP coupling (J PP ) 5-6 Hz)
was also observed in the 31P{1H} NMR spectra of 3a -
f.
As mentioned before, the reaction of 1 with the
monosubstituted thallium pyrazolates TlPz′ and TlPz*
affords mixtures of two isomers which differ in the
position of the pyrazole substituent. The assignation
of a precise structure to each isomer can be done with
a combination of chemical shift analysis of the proton
NMR signals of the pyrazole substituents and NOE
experiments.
As shown in Figure 3, a NOE intensity enhancement
of one of the pyrazole proton signals of compound 2 is
to be expected when the phosphine proton resonance
becomes saturated. A similar effect was also detected
on the minor isomer of the 3a /3b mixture, for which
structure 3b can be therefore proposed. Interestingly,
1H and 13C signals corresponding to the substituents
on the 3-position of the pyrazolate ligands of 2 and 3a /
s-1 and k-1 ) 9.8 × 10-4 s-1 18b
.
In a formal sense, the exchange between 3a /3b and
3d /3e isomers can be viewed as a rotation of the
bridging pyrazole ligand through an axis that bisects
the N-N bond, and it can also be detected on those
derivatives that contain symmetrically substituted pyra-
zolate ligands (e.g. 3c) by means of SST experiments.
This type of exchange has also been observed previously
in the Pd complexes [{(C6F5)2Pd}2(µ-L)]22- (L ) Pz, Pz′,
triazolate, indazolate).5c In donor solvents, the analo-
gous Ni compounds5f dissociate into two [(C6F5)2Ni-
(Pz)(S)]- units (S ) CD3COCD3), which then undergo
fast Pz ligand exchange even at -70 °C. Due to the fact
that in complexes with η1-pyrazolate, η1-pyridazine, and
other related ligands the metal atom can readily jump
from one nitrogen atom to the adjacent one,19 it would
appear likely that a similar, solvent-mediated dissocia-
tion process could be responsible for the dynamic
(18) (a) Sandstro¨m, J . Dynamic NMR Spectroscopy; Academic
Press: New York, 1982. (b) Connors, K. A. Chemical Kinetics: The
Study of Reaction Rates in Solution; VCH: New York, 1990.
(19) (a) Kang, S. K.; Albright, T. A.; Mealli, C. Inorg. Chem. 1987,
26, 3158. (b) J ackson, W. G.; Cortez, S. Inorg. Chem. 1994, 33, 1921.
(c) Alvarez, S.; Bermejo, M. J .; Vinaixa, J . J . Am. Chem. Soc. 1987,
109, 5316. (d) Abel, E. W.; Heard, P. J .; Orrell, K. G.; Sik, V. Polyhedron
1994, 13, 2907.

Pyrazolyl- and Hydrocarbyl-Bridged Complexes of Ni
Organometallics, Vol. 16, No. 12, 1997 2713
Sch em e 2
{1H} NMR spectroscopy to produce a new species, 8,
-
which has been isolated as the BPh4 salt (Scheme 3).
1
The H and 13C NMR spectra of 8 are very similar to
those of 2, except for the presence of a new PMe3 ligand,
which is for example evidenced by the observation of a
nickel σ-bonded quaternary aromatic carbon signal as
a doublet of doublet of doublets, with the largest
constant (2J CP ) 72 Hz) being indicative of a trans-type
coupling. Three signals (intensity ratio 1:1:1) are
observed in the 31P{1H} NMR spectrum: two doublets
(δ -7.7, J PP ) 6 Hz; δ -16.4, J PP ) 49 Hz) and a doublet
of doublets (δ -10.0, J PP ) 6 and 49 Hz). Interestingly
enough, these cationic species can delocalize the positive
charge on both metal centers, as described by resonance
forms C and D.
behavior detected in the Pd complexes. However, for
Ni and Pd, dissociation of the Pz ligand from one of the
metal centers usually requires an associative process,20
with participation of a donor molecule to complete the
16-electron shell of the metal center. Since the isomer-
ization kinetics of 3a and 10a was measured in CD2-
Cl2, one has to speculate that either the solvent fulfills
these improbable donor requirements or the exchange
requires at some stage an additional electron output
from the benzyl ligand.21 While we have not pursued
this problem any further, we have studied the influence
of added PMe3 on this isomerization reaction (see
below).
Som e Ch em ica l P r op er ties of Com p ou n d s 2 a n d
3. We have considered the possibility of derivatizing
compounds 2 and 3a -e by replacing the bromine atom
with other substituents. The halide anion can be easily
substituted by other halides or pseudohalides (N3-), or
azolate ligands such as 2,5-dimethylpyrrolyl, giving rise
to compounds 4 and 5 (Scheme 2). NMR data for these
complexes are similar to those already discussed for 2
and merit no further comment, except perhaps to note
that in compound 5 restricted rotation around the Ni-N
bond of the Ni-pyrrolyl linkage makes the two methyl
groups inequivalent. Several attempts aimed at the
preparation of alkyl, aryl, and alkynyl derivatives have
proved unsuccessful, but the reaction of 2 with NaCp
or TlCp gives rise to the bis(cyclopentadienylnickel)
complex 6, independent of the reagent ratio (Scheme 2).
The latter behavior contrasts with that of compound 1,
which selectively reacts with 1 equiv of NaCp to give
Ni(Cp)(PMe3)(µ-CH2-o-C6H4)Ni(PMe3)2Br.6b Compound
6 can also be obtained from 1 using 2 equiv of NaCp
and prolonged reaction times.
A related cationic complex, 9 (Scheme 3), can be
obtained by the reaction of 2 with the chelating diphos-
phine 1,2-bis(dimethylphosphino)ethane (dmpe), and it
is best isolated in a pure, crystalline form, following
-
substitution of the Br- counterion by BPh4
. In a
similar manner, treatment of CD2Cl2 solutions of 3a /
3b with PMe3 results in the formation of the corre-
sponding cationic complexes, once again isolated as
BPh4- salts. Complexes 10a and 10b also interconvert
in solution, but their interchange is somewhat slower
than that observed for the 3a /3b couple, no magnetiza-
tion transfer being detected between the methyl signals
of the former pair of isomers. The crystalline solid
obtained from 10a /10b solutions contains only one of
the isomers, for which structure 10a can be proposed
on the basis of the chemical shift arguments discussed
previously. If a sample of solid 10a is dissolved in CD2-
Cl2 and its proton NMR spectrum is immediately
recorded, only pure 10a is observed. A smooth isomer-
ization process ensues at room temperature, which leads
to the isomeric mixture with 10a and 10b in a ca. 1.1:1
ratio (eq 4). The rate constants for this process have
Compound 2 reacts with an excess of PMe3, at room
temperature, to give the known binuclear species [Ni-
6b
(Br)(PMe3)2(µ-C6H4CH2)]2 (7), together with other un-
characterized products (eq 3). Binuclear 7 has been
reported to form in the reaction of and 1 and PMe32c as
a result of a disproportionation process which appears
to be characteristic of nickel benzyl complexes.6a How-
ever, the low-temperature reaction (-60 °C) of 2 and
equimolecular amounts of PMe3 can be shown by 31P-
(20) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987.
(21) The ability of the benzyl ligand to compensate highly electron-
deficient situations is well-documented. For a recent example see:
Pellechia, C.; Immirzi, A.; Pappalardo, D.; Peluso, A. Organometallics
1994, 13, 3773.
values of k1 ) 2.3 × 10-4 and k-1 ) 2.8 × 10-4 s-1 at 20
°C and are not affected by the presence of the PMe3
scavenger PdCp(η3-C3H7). It is worth mentioning at this
stage that the rate of exchange between 3a and 3b,

2714 Organometallics, Vol. 16, No. 12, 1997
Ca´mpora et al.
Sch em e 3
Sch em e 4
the absence of added PMe3. So far, we are unable to
propose a mechanism for the second situation, but the
difference between the 3a h 3b and the 10a h 10b
exchange rates indicates that the nature of the second-
ary ligands on the arylic nickel atom can influence the
rate of isomerization. Clearly, more detailed research
on the effect of different neutral and anionic coligands
and on the solvent effect will be required to unravel the
nature of this interesting process.
Exp er im en ta l Section
Microanalyses were performed by Pascher Microanalytical
Laboratory, Remagen, Germany, and the Analytical Service
of the University of Seville. The spectroscopic instruments
used were Perkin-Elmer Models 577 and 684 for IR spectra
and Bruker AMX-300 and AMX-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 assignments 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. Lithium pyra-
zolate was obtained by reacting n-BuLi with pyrazole, and
NaCp was prepared from NaH and freshly cracked dicyclo-
pentadiene. Compound 1 was prepared according to the
reported procedure.2b
measured at 0 °C, increases markedly in the presence
of PMe3; the addition of 0.009 mmol of PMe3 makes the
interconversion fast enough to prevent the determina-
tion of the reaction rate by the above-described method,
while line broadening in the NMR spectra due to the
presence of the free ligand makes the use of SST
methods unreliable. A similar observation holds for
isomerization of 10a into 10b. A variety of Ni complexes
containing the PMe3 ligand are known to undergo fast
ligand exchange catalyzed by small amounts of PMe3.22
On these grounds, the pyrazolate “rotation” can be
proposed to proceed as depicted in Scheme 4, at least
in the case of the 10a h 10b interconversion. Added
PMe3 can displace the bridging Pz′ ligand from one of
the Ni atoms; the monodentate Pz′ group then readily
undergoes a facile N1-N2 shift. Rotation about the
Nia-N bond, followed by coordination of the second N
atom of the Pz′ ligand to Nib, would render the other
isomer. As for the 3a h 3b isomerization, a similar
mechanism could be proposed in which the Br- ligand
displaced by the added PMe3 would induce the bidentate
to monodentate change in the coordination of the added
Pz′ ligand. However, neither the isomerization rate of
3a nor that of 10a is affected by the presence of CpPd-
(η3-C3H5), an efficient phosphine sponge.10 Thus, dif-
ferent mechanisms seem to operate in the presence and
Syn t h esis of Ni2(P Me3)2Br (µ2-η3:η1-CH2-o-C6H 4)(µ2-
N2C3H3) (2). To a cold (-60 °C) solution of 1 (0.6 g, 1 mmol)
in THF (50 mL) was added lithium pyrazolate (1 mmol, 2 mL
of a 0.5 M solution in THF). The original red color of 1 changed
to purple. The cooling bath was removed, and when it was
warmed, the solution developed a red coloration again. After
it was stirred at room temperature for 5 min, the reaction
mixture was taken to dryness and the residue extracted with
toluene. This operation was repeated twice, and after filtra-
tion, concentration, addition of some petroleum ether, and
cooling to -20 °C, compound 2 was obtained as red crystals.
Yield: 73%. Anal. Calcd for C16H27N2BrP2Ni2: C, 37.9; H,
5.3; N, 5.5. Found: C, 38.2; H, 5.3; N, 4.7. 1H NMR (CD2Cl2,
2
20 °C): δ 0.99 (d, 9 H, J HP ) 10.2 Hz, PMe3), 1.42 (d, 9 H,
(22) Carmona, E.; Gutie´rrez-Puebla, E.; Mar´ın, J . M.; Monge, A.;
Paneque, M.; Poveda, M. L.; Ruiz, C. J . Am. Chem. Soc. 1989, 111,
2883.
2J HP ) 9.2 Hz, PMe3), 2.20 (dd, 1 H, 2J HH ) 3.4, 3J HP ) 7.8 Hz,
2
3
CH2), 2.33 (pt, 1 H, J HH ≈ J HP ) 3.8 Hz, CH2), 5.91 (dd, 1 H,

Pyrazolyl- and Hydrocarbyl-Bridged Complexes of Ni
Organometallics, Vol. 16, No. 12, 1997 2715
3
1
3J HH ) 2.0, J HP ) 3.2 Hz, H(4) pyraz), 6.77 (d, 1H, J HH ) 0.9
2J CP ) 11, J CH ) 149 Hz, CH2), 104.1 (s, C(4)H pyraz), 125.2,
125.5, 128.4, 144.0 (s, CH aromatics), 129.1 (dd, 2J CP ) 17 and
42 Hz, Cq arom-Ni), 147.8 (s, Cq pyraz), 149.1 (d, J CP ) 3 Hz,
Cq pyraz).
3
Hz, H pyraz), 6.84 (dm, 1 H, J HH ) 6.8 Hz, aromatic), 7.03
3
(m, 2 H, aromatics), 7.79 (m, 1 H, J HH ) 6.6 Hz, aromatic),
7.83 (d, 1 H, J HH ) 2.0 Hz, H pyraz). 31P{1H} NMR (CD2Cl2,
3
Ni2(P Me3)2Br (µ2-η3:η1-CH2-o-C6H4)(µ2-N2C3H2Bu t) (3d /e).
Anal. Calcd for C20H35N2BrP2Ni2: C, 42.7; H, 6.2; N, 5.0.
Found: C, 42.4; H, 6.4; N, 5.1.
20 °C): AX spin system, δA ) -11.8, δX ) -9.8, J AX ) 5 Hz.
13C{1H} NMR (CD2Cl2, 20 °C): δ 15.8 (d, 1J CP ) 32 Hz, PMe3),
1
2
1
15.8 (d, J CP ) 27 Hz, PMe3), 30.4 (d, J CP ) 10, J CH ) 152
2
Hz, CH2), 103.4 (s, C(4)H pyraz), 122.8 (dd, J CP ) 19 and 42
3d : 1H NMR (CD2Cl2, 20 °C) δ 0.90 (s, 9 H, CMe3), 1.02 (d,
Hz, Cq arom-Ni), 124.0, 124.7, 128.8, 144.7 (s, CH aromatics),
9 H, 2J HP ) 10.0 Hz, PMe3), 1.48 (d, 9 H, 2J HP ) 9.1 Hz, PMe3),
2
126.8 (d, J CP ) 5 Hz, Cq arom), 140.0 (s, CH pyraz).
2
3
1.88 (dd, 1 H, J HH ) 3.6, J HP ) 6.4 Hz, CH2), 2.08 (dd, 1 H,
2J HH ) 3.6, 3J HP ) 6.2 Hz, CH2), 5.72 (dd, 1 H, 3J HH ) 1.9, J HP
) 0.7 Hz, H(4) pyraz), 7.38 (dm, 1 H, 3J HH ) 7.5 Hz, aromatic),
Syn t h esis of Ni2(P Me3)2Br (µ2-η3:η1-CH2-o-C6H 4)(µ2-
N2C3HRR′) (R ) H, R′ ) Me, 3a ,b; R ) R′ ) Me, 3c; R ) H,
R′ ) Bu t, 3d ,e; R ) R′ ) Bu t, 3f). The preparation of these
compounds involves the reaction of 1 with the corresponding
thallium pyrazolate salts. A representative example of the
experimental procedure employed to synthesize 3a (b) is as
follows: Thallium 2-methylpyrazolate was generated by the
reaction of 2-methylpyrazole (0.12 mL, 1.5 mmol) with thal-
lium ethoxide (1.5 mmol, 1.5 mL of a 1 M solution in THF) in
THF at low temperature (-40 °C). The cooling bath was then
removed, and the mixture was stirred until it reached room
temperature and added to a stirred solution of 1 (0.9 g, 1.5
mmol) in THF (50 mL) cooled to -60 °C. The resulting red
suspension was slowly warmed and stirred at room temper-
ature for 30 min. The solvent was removed under vacuum
and the residue extracted with Et2O. After centrifugation and
concentration of the solution, cooling to -30 °C furnished red
crystals of complex 3a in 86% yield. Dissolution of these
crystals at room temperature affords equilibrium mixtures of
3a and 3b.
3
6.9-7.0 (m, 3 H, aromatics), 7.64 (dm, 1 H, J HH ) 1.9 Hz, H
pyraz); 31P{1H} NMR (CD2Cl2, 20 °C) δ -12.4 (s, 2 PMe3);
13C{1H} NMR (CD2Cl2, 20 °C) δ 16.2 (d, 1J CP ) 30 Hz, 2 PMe3),
27.4 (d, 2J CP ) 9 Hz, CH2), 30.7 (s, CMe3), 31.3 (s, CMe3), 101.1
(s, C(4)H pyraz), 122.3 (s, Cq arom), 125.1, 126.5, 127.9, 143.7
2
(s, CH aromatics), 131.0 (dd, J CP ) 17 and 46 Hz, Cq arom-
Ni), 140.0 (c, CH pyraz), 162.6 (s, Cq pyraz).
3e: 1H NMR (CD2Cl2, 20 °C) δ 1.02 (d, 9 H, 2J HP ) 10.0 Hz,
PMe3), 1.60 (d, 9 H, 2J HP ) 9.5 Hz, PMe3), 1.82 (s, 9 H, CMe3),
2
3
5
2.13 (ddd, 1 H, J HH ) 3.9, J HP ) 7.1, J HP ) 1.2 Hz, CH2),
2
3
2.49 (pt, 1 H, J HH ≈ J HP ) 3.9 Hz, CH2), 5.72 (m, 1 H, H(4)
3
pyraz), 6.60 (d, 1 H, J HH ) 1.8 Hz, H pyraz), 6.66 (dm, 1 H,
3J HH ) 7.7 Hz, aromatic), 6.9-7.0 (m, 3 H, aromatics); 31P{1H}
NMR (CD2Cl2, 20 °C) AX spin system, δA ) -16.2, δX ) -8.1,
J AX ) 6 Hz; 13C{1H} NMR (CD2Cl2, 20 °C) δ 16.3 (d, J CP ) 27
1
2
Hz, 2 PMe3), 28.8 (d, J CP ) 10 Hz, CH2), 32.1 (s, CMe3), 32.2
(s, CMe3), 101.4 (s, C(4)H pyraz), 124.1, 126.5, 128.5, 144.3 (s,
CH aromatics), 139.5 (s, CH pyraz), 163.5 (s, Cq pyraz).
Ni2(P Me3)2Br (µ2-η3:η1-CH2-o-C6H4)(µ2-N2C3HBu t2) (3f).
Anal. Calcd for C24H43N2BrP2Ni2: C, 46.6; H, 7.0; N, 4.5.
Found: C, 46.1; H, 7.0; N, 4.6. 1H NMR (CD2Cl2, 20 °C): δ
The complexes 3c and 3d /3e were similarly prepared and
isolated as dark red crystals in 70% and 90% yield, respec-
tively. The more soluble compound 3f was crystallized from
petroleum ether as purple crystals in 60% yield.
Ni2(P Me3)2Br (µ2-η3:η1-CH2-o-C6H4)(µ2-N2C3H2Me) (3a /b).
Anal. Calcd for C17H29N2BrP2Ni2: C, 39.2; H, 5.6; N, 5.4.
Found: C, 39.3; H, 5.8; N, 5.7.
3a : 1H NMR (CD2Cl2, 20 °C) δ 0.99 (d, 9 H, 2J HP ) 10.0 Hz,
2
0.89 (s, 9 H, CMe3), 1.02 (d, 9 H, J HP ) 10.1 Hz, PMe3), 1.67
2
(d, 9 H, J HP ) 9.6 Hz, PMe3), 1.88 (s, 9 H, CMe3), 2.03 (m, 1
2
3
H, CH2), 2.07 (dd, 1 H, J HH ) 3.4, J HP ) 7.0 Hz, CH2), 5.56
3
(d, 1 H, J HP ) 0.7 Hz, H(4) pyraz), 6.79-6.90 (m, 4 H,
aromatics). 31P{1H} NMR (CD2Cl2, 20 °C): AX spin system,
δA ) -18.5, δX ) -11.1, J AX ) 6 Hz. 13C{1H} NMR (CD2Cl2,
2
PMe3), 1.44 (d, 9 H, J HP ) 9.2 Hz, PMe3), 1.74 (s, 3 H, Me
pyraz), 2.12 (m, 1 H, CH2), 2.14 (m, 1 H, CH2), 5.68 (m, 1 H,
1
1
20 °C): δ 16.1 (d, J CP ) 31 Hz, PMe3), 16.2 (d, J CP ) 27 Hz,
3
H(4) pyraz), 6.90 (dm, 1 H, J HH ) 7.0 Hz, aromatic), 7.0 (m,
2
1
PMe3), 26.4 (d, J CP ) 11, J CH ) 157 Hz, CH2), 30.8, 32.3 (s,
CMe3), 31.0, 32.0 (s, CMe3), 99.7 (s, C(4)H pyraz), 122.5 (d,
2J CP ) 5 Hz, Cq arom), 125.5, 126.1, 127.6, 142.1 (s, CH
3
2 H, aromatics), 7.70 (d, 1 H, J HH ) 1.5 Hz, H pyraz), 7.70
(dm, 1 H, J HH ) 8.4 Hz, aromatic); 31P{1H} NMR (CD2Cl2, 20
3
°C) AX spin system, δA ) -12.1, δX ) -10.3, J AX ) 5 Hz;
2
aromatics), 127.9 (dd, J CP ) 15 and 46 Hz, Cq arom-Ni),
161.5, 162.7 (s, Cq pyraz).
13C{1H} NMR (CD2Cl2, 20 °C) δ 13.1 (s, Me pyraz), 15.8 (d,
1J CP ) 26 Hz, PMe3), 16.2 (d, J CP ) 23 Hz, PMe3), 29.2 (d,
1
Syn t h esis of Ni2(P Me3)2(N3)(µ2-η3:η1-CH 2-o-C6H 4)(µ2-
N2C3H3) (4). To a solution of 2 (0.5 g, 1 mmol) in THF (50
mL) was added 1 g of solid sodium azide (15 mmol). The
mixture was stirred at room temperature overnight and then
taken to dryness. Extraction with toluene, filtration, concen-
tration of the filtrate, and addition of some petroleum ether
provided red crystals of the complex after cooling to -30 °C
for several hours. Yield: 60%. Anal. Calcd for C16H27N5P2-
Ni2: C, 41.0; H, 5.8; N, 14.9. Found: C, 40.4; H, 5.8; N, 14.1.
IR (Nujol mull): 2046 cm-1 (ν(N3)). 1H NMR (CD2Cl2, 20 °C):
2J CP ) 11, J CH ) 145 Hz, CH2), 103.3 (s, C(4)H pyraz), 124.8,
1
125.1, 128.5, 144.4 (s, CH aromatics), 141.2 (s, CH pyraz),
148.4 (s, Cq pyraz).
3b: 1H NMR (CD2Cl2, 20 °C) δ 1.02 (d, 9 H, 2J HP ) 10.1 Hz,
2
PMe3), 1.53 (d, 9 H, J HP ) 9.3 Hz, PMe3), 2.12 (m, 1 H, CH2),
2
2.29 (pt, 1 H, J HH
≈
3J HP ) 3.7 Hz, CH2), 2.56 (s, 3 H, Me
3
pyraz), 5.67 (m, 1 H, H(4) pyraz), 6.60 (d, 1 H, J HH ) 1.6 Hz,
3
H pyraz), 6.80 (dm, 1 H, J HH ) 7.2 Hz, aromatic), 7.0 (m, 2
H, aromatics), 7.49 (dm, 1 H, 3J HH ) 7.2 Hz, aromatic); 31P{1H}
NMR (CD2Cl2, 20 °C) AX spin system, δA ) -13.6, δX ) -8.6,
J AX ) 4 Hz; 13C{1H} NMR (CD2Cl2, 20 °C) δ 14.7 (s, Me pyraz),
2
2
δ 0.90 (d, 9 H, J HP ) 10.2 Hz, PMe3), 1.41 (d, 9 H, J HP ) 9.2
Hz, PMe3), 2.19 (br s, 1 H, CH2), 2.26 (br s, 1 H, CH2), 5.97 (s,
1 H, H(4) pyraz), 6.80 (s, 1 H, H pyraz), 6.81 (d, 1 H, J HH
1
1
15.6 (d, J CP ) 32 Hz, PMe3), 16.0 (d, J CP ) 22 Hz, PMe3),
29.0 (d, J CP ) 10 Hz, CH2), 104.0 (s, CH pyraz), 124.3, 125.1,
128.6, 144.6 (s, CH aromatics), 139.8 (s, CH pyraz), 149.0 (s,
Cq pyraz).
3
2
)
7.0 Hz, aromatic), 7.03 (m, 2 H, aromatics), 7.64 (s, 1 H, H
pyraz), 7.83 (d, 1 H, J HH ) 7.6 Hz, aromatic). 31P{1H} NMR
3
Ni2(P Me3)2Br (µ2-η3:η1-CH2-o-C6H4)(µ2-N2C3HMe2) (3c).
Anal. Calcd for C18H31N2BrP2Ni2: C, 40.4; H, 5.8; N, 5.2.
Found: C, 40.0; H, 5.9; N, 4.9. 1H NMR (CD2Cl2, 20 °C): δ
(CD2Cl2, 20 °C): δ -10.1 (br s, PMe3), -9.4 (br s, PMe3).
13C{1H} NMR (CD2Cl2, 20 °C): δ 15.2 (d, 1J CP ) 25 Hz, PMe3),
1
17.4 (d, J CP ) 28 Hz, PMe3), 32.3 (s, CH2), 105.2, (s, C(4)H
pyraz), 125.3, 126.3, 130.6, 146.8 (s, CH aromatics), 128.8 (s,
Cq arom), 139.8, 142.1 (s, CH pyraz).
Syn th esis of Ni2(P Me3)2(NC4H2Me2)(µ2-η3:η1-CH2-o-C6H4)-
(µ2-N2C3H3) (5). n-BuLi (1 mmol, 0.62 mL of a 1.6 M solution
in hexanes) was added to a cold (-40 °C) solution of 2,5-
dimethylpyrrole (0.1 mL, 1 mmol) in THF (10 mL). After the
cooling bath was removed, the mixture was warmed to room
temperature and stirred for 15 min. This solution was added
2
2
1.03 (d, 9 H, J HP ) 10.1 Hz, PMe3), 1.55 (d, 9 H, J HP ) 9.4
Hz, PMe3), 1.69 (s, 3 H, Me pyraz), 2.05 (m, 2 H, CH2), 2.53 (s,
3
3 H, Me pyraz), 5.46 (s, 1 H, H(4) pyraz), 6.86 (dm, 1 H, J HH
) 7.0 Hz, aromatic), 6.97 (m, 2 H, aromatics), 7.41 (dm, 1 H,
3J HH ) 7.2 Hz, aromatic). 31P{1H} NMR (CD2Cl2, 20 °C): AX
spin system, δA ) -13.5, δX ) -8.9, J AX ) 5 Hz. 13C{1H} NMR
(CD2Cl2, 20 °C): δ 13.2 (s, Me pyraz), 14.9 (s, Me pyraz), 15.7
1
1
(d, J CP ) 31 Hz, PMe3), 16.6 (d, J CP ) 27 Hz, PMe3), 27.9 (d,

2716 Organometallics, Vol. 16, No. 12, 1997
Ca´mpora et al.
to a cooled (-60 °C) solution of 2 (0.5 g, 1 mmol) in THF (50
mL). The suspension was stirred at room temperature for 2
h. The solvent was stripped off and the residue extracted with
toluene and centrifuged. After partial evaporation of the
solvent, cooling of the resulting solution to -30 °C furnished
dark red crystals of 5 in 60% yield. Anal. Calcd for C22H35N3P2-
Ni2: C, 50.7; H, 6.7; N, 8.1. Found: C, 51.2; H, 6.9; N, 8.1. 1H
0.5 mmol) were mixed and cooled down to -80 °C, THF (10
mL) was introduced into the flask, and after dissolution of the
reagents, PMe3 (0.5 mmol, 0.5 mL of a 1 M solution in THF)
was added. The mixture was stirred at -80 °C for 15 min
and then warmed to room temperature. The solvent was
evaporated under reduced pressure and the solid residue
extracted with CH2Cl2. The solution was filtered and the
filtrate partially concentrated. After addition of some Et2O
and cooling to 0 °C for 3 days, compound 8 was obtained as
red-brown crystals. Yield: 72%. Anal. Calcd for C43H56N2-
BP3Ni2: C, 62.8; H, 6.9; N, 3.4. Found: C, 62.7; H, 7.1; N,
2
NMR (CD2Cl2, 20 °C): δ 0.66 (d, 9 H, J HP ) 9.8 Hz, PMe3),
2
2
1.38 (d, 9 H, J HP ) 9.0 Hz, PMe3), 2.25 (dd, 1 H, J HH ) 3.5,
3J HP ) 7.9 Hz, CH2), 2.37 (d, 1 H, J HH ) 3.5 Hz, CH2), 2.89,
2
2.97 (s, s, 3 H, 3 H, Me pyrrole), 5.68 (s, 1 H, H(4) pyraz), 5.77,
5.81 (s, s, 1 H, 1 H, H pyrrole), 6.59, 6.82 (s, s, 1 H, 1 H, H
2
2.9. 1H NMR (CD2Cl2, 20 °C): δ 0.95 (d, 9 H, J HP ) 9.8 Hz,
3
2
2
pyraz), 6.87 (d, 1 H, J HH ) 7.5 Hz, aromatic), 7.12 (m, 2 H,
PMe3), 1.27 (d, 9 H, J HP ) 8.1 Hz, PMe3), 1.46 (d, 9 H, J HP )
2 3
3
aromatics), 8.26 (d, 1H, J HH ) 7.1 Hz, aromatic). 31P{1H}
9.2 Hz, PMe3), 2.41 (t, 1 H, J HH ≈ J HP ) 3.8 Hz, CH2), 2.44
(dd, 1 H, J HH ) 3.8, J HP ) 7.3 Hz, CH2), 6.16 (pq, 1 H, J HH
2
3
3
NMR (CD2Cl2, 20 °C): AX spin system, δA ) -11.5, δX ) -10.2,
J AX ) 13 Hz. 13C{1H} NMR (CD2Cl2, 20 °C): δ 14.5 (d, 1J CP
)
4
3
≈ J HP ) 2.1 Hz, H(4) pyraz), 6.92 (d, 1 H, J HH ) 2.1 Hz, H
pyraz), 6.96 (m, 4 H, arom BPh4), 7.11 (m, 8 H, arom BPh4),
7.15 (m, 1 H, arom), 7.20 (m, 1 H, arom), 7.40 (m, 8 H, arom
1
29 Hz, PMe3), 16.0 (d, J CP ) 27 Hz, PMe3), 17.5, 18.1 (s, Me
2
pyrrole), 31.0 (d, J CP ) 10 Hz, CH2), 103.2 (s, C(4)H pyraz),
2
3
BPh4), 7.58 (d, 1 H, J HH ) 2.1 Hz, H pyraz). 31P{1H} NMR
105.5, 105.7 (s, CH pyrrole), 122.9 (dd, J CP ) 20 and 39 Hz,
Cq arom-Ni), 123.3, 124.3, 128.9, 145.8 (s, CH aromatics),
(CD2Cl2, 20 °C): AMX spin system, δA ) -9.9, δM ) -12.0, δX
2
) -18.6, J AM ) 6.2, J MX ) 48.6 Hz. 13C{1H} NMR (CD2Cl2,
128.7 (d, J CP ) 4 Hz, Cq arom), 131.2, 133.4 (s, Cq pyrrole),
1
1
138.3 (d, J CP ) 3 Hz, CH pyraz), 140.8 (d, J CP ) 3 Hz, CH
pyraz).
20 °C): δ 14.6 (d, J CP ) 24 Hz, PMe3), 15.8 (d, J CP ) 28 Hz,
PMe3), 16.7 (dd, 1J CP ) 29, 3J CP ) 4 Hz, PMe3), 32.0 (d, 2J CP
)
Syn t h esis of [Cp Ni(P Me3)]2(µ2-η1:η1-CH2-o-C6H 4) (6).
Meth od a . A solution of 2 (0.36 g, 0.71 mmol) in THF (50
mL) cooled to -80 °C was treated with NaCp (1.4 mmol, 3.5
mL of a 0.4 M solution in THF). The resulting mixture was
stirred at room temperature for 30 min and taken to dryness.
Extraction with a mixture of petroleum ether and Et2O (1:1),
centrifugation, and concentration provided crystals of 6 as
green needles after cooling to -30 °C. Yield: 22%.
Meth od b. CpTl (0.54 g, 2 mmol) was added to a cooled
(-80 °C) solution of 2 (0.47 g, 0.9 mmol). After the suspension
was warmed to room temperature, the stirring was continued
overnight. The workup of the reaction mixture was similar
to that in method a. Compound 6 was obtained as green
crystals in 28% yield.
Meth od c. To a cold (-80 °C) stirred solution of 1 (0.74 g,
1.25 mmol) in THF (50 mL) was added CpNa (2.5 mmol, 6.2
mL of a 0.4 M solution in THF). The color of the solution
changed to deep green, and upon gentle warming a further
change to a reddish coloration was observed. The mixture was
stirred at room temperature for 6 h. The solvent was
evaporated under vacuum and the residue extracted with
Et2O. After centrifugation, concentration, and cooling to -20
°C, compound 6 was obtained as green crystals in 48% yield.
10 Hz, CH2), 105.7 (s, C(4)H pyraz), 122.0, 125.9, 136.2 (s, CH
arom BPh4), 124.6, 126.9, 130.6, 144.1 (d, s, s, and s, J CP ) 5
Hz, CH arom), 126.5 (pt, J CP(app) ) 5 Hz, Cq arom), 127.0 (ddd,
J CP ) 72, 37, and 21 Hz, Cq arom-Ni), 139.6, 141.7, (s, CH
pyraz), 164.6 (m, Cq arom BPh4).
Syn th esis of [Ni2(P Me3)(Me2P CH2CH2P Me2)(µ2-η3:η1-
CH2-o-C6H4)(µ2-N2C3H3)]BP h 4 (9). To a cooled (-80 °C)
solution of 2 (0.23 g, 0.46 mmol) in THF (30 mL) was added a
solution of NaBPh4 (0.34 g, 0.46 mmol) and dmpe (0.08 mL,
0.48 mmol) in THF (30 mL). When the temperaure was gently
raised to -10 °C, the solvent was removed in vacuo and the
residue extracted twice with CH2Cl2 and a mixture of THF
and Et2O, respectively. The resulting suspension was centri-
fuged and the solution concentrated and cooled to -30 °C.
Compound 9 was obtained as red-orange crystals in 31% yield.
Anal. Calcd for C43H54N2BP3Ni2: C, 63.3; H, 7.0; N, 3.1.
Found: C, 62.4; H, 7.2; N, 2.5. 1H NMR (CD2Cl2, 20 °C): δ
2
2
0.87 (d, 3 H, J HP ) 11.1 Hz, P-Me), 1.13 (d, 3 H, J HP ) 10.3
2
Hz, P-Me), 1.40-1.70 (bm, 4 H, P-CH2), 1.41 (d, 9 H, J HP
9.2 Hz, PMe3), 1.52 (d, 3 H, J HP ) 9.1 Hz, P-Me), 1.65 (d, 3
)
2
2
2
3
H, J HP ) 9.2 Hz, P-Me), 2.08 (t, 1 H, J HH ≈ J HP ) 3.9 Hz,
2
3
CH2), 2.40 (dd, 1 H, J HH ) 3.9, J HP ) 7.8 Hz, CH2), 6.20 (br
s, 1 H, H(4) pyraz), 6.98 (m, 4 H, arom BPh4), 7.07 (d, 1 H,
3J HH ) 7.9 Hz, arom), 7.13 (m, 8 H, arom BPh4), 7.27 (t, 1 H,
Anal. Calcd for C23H34P2Ni2: C, 56.4; H, 7.0. Found: C,
3J HH ) 7.4 Hz, arom), 7.30 (t, 1 H, J HH ) 7.0 Hz, arom), 7.42
3
2
56.2; H, 7.3. 1H NMR (CD2Cl2, 20 °C): δ 1.01 (d, 9 H, J HP
)
3
9.6 Hz, PMe3), 1.33 (d, 9 H, 2J HP ) 9.1 Hz, PMe3), 2.10 (t, 1 H,
(m, 8 H, arom BPh4), 7.59 (d, 1 H, J HH ) 1.9 Hz, H pyraz).
31P{1H} NMR (CD2Cl2, 20 °C): AMX spin system, δA ) -9.7,
δM ) 32.7, δX ) 36.4, J AM ) 6, J MX ) 29 Hz. 13C{1H} NMR
2J HH ≈ J HP ) 10 Hz, CH2), 2.13 (t, 1 H, J HH ≈ J HP ) 10 Hz,
3
2
3
CH2), 4.79 (s, 5 H, C5H5), 5.44 (s, 5 H, C5H5), 6.41 (tm, 1 H,
1
1
3J HH ) 7.2 Hz, aromatic), 6.57 (tm, 1 H, J HH ) 7.2 Hz,
3
(CD2Cl2, 20 °C): δ 11.4 (d, J CP ) 21 Hz, P-Me), 11.5 (d, J CP
1
1
3
) 23 Hz, P-Me), 12.4 (d, J CP ) 32 Hz, P-Me), 13.0 (d, J CP
aromatic), 6.86 (dm, 1 H, J HH ) 7.5 Hz, aromatic), 7.20 (dm,
1
1
1 H, J HH ) 7.4 Hz, H aromatic). 31P{1H} NMR (CD2Cl2, 20
3
) 33 Hz, P-Me), 15.7 (d, J CP ) 28 Hz, PMe3), 24.8 (dd, J CP
2
1
2
°C): -3.2 (s, PMe3), -2.4 (s, PMe3). 13C{1H} NMR (CD2Cl2,
) 32, J CP ) 11 Hz, P-CH2), 28.4 (dd, J CP ) 31, J CP ) 18
2
2
1
Hz, P-CH2), 31.9 (d, J CP ) 11 Hz, CH2), 105.2 (s, C(4)H
20 °C): δ 9.6 (d, J CP ) 20 Hz, CH2), 17.7 (d, J CP ) 28 Hz,
PMe3). 18.6 (d, 1J CP ) 30 Hz, PMe3), 90.0, 91.3 (s, C5H5), 119.2,
pyraz), 122.2, 126.0, 136.3, (s, CH arom BPh4), 124.5, 126.5,
130.6, 143.6 (d, s, s, and s, J CP ) 5 Hz, CH arom), 124.7 (ddd,
J CP ) 70, 34, and 21 Hz, Cq arom-Ni), 128.1 (pt, J CP(app) ) 5
Hz, Cq arom), 140.3, 141.7 (s, CH pyraz), 164.3 (m, Cq arom
BPh4).
Syn th esis of [Ni2(P Me3)3(µ2-η3:η1-CH2-o-C6H4)(µ2-N2C3-
H2Me)]BP h 4 (10a ,b). Complex 3a /3b (0.46 g, 0.89 mmol) and
NaBPh4 (0.30 g, 0.89 mmol) were cooled to 0 °C, and THF (20
mL) and PMe3 (1 mmol, 1 mL of a 1 M solution in THF) were
added. The mixture was stirred at room temperature for 15
min and then taken to dryness. The residue was extracted
with CH2Cl2, the suspension filtered and the filtrate evapo-
rated under vacuum and extracted with a mixture of Me2CO
and petroleum ether (1:1) to afford red crystals of 10a in 63%
yield. Dissolution of these crystals at room temperature
affords solutions that contain equilibrium mixtures of 10a and
2
121.7, 127.4, 142.8 (s, CH aromatics), 140.7 (d, J CP ) 37 Hz,
Cq arom-Ni), 164.2 (s, Cq arom).
Rea ction of Com p ou n d 2 w ith Excess P Me3. To a
solution of 2 (0.33 g, 0.56 mmol) in THF (50 mL) cooled to -80
°C was added an excess of PMe3 (0.3 mL, 3 mmol). After the
solution was warmed to room temperature, the stirring was
continued overnight. The solvent was stripped off and the
yellow residue extracted with toluene. This solution was
filtered and the filtrate evaporated to dryness. The resulting
yellow microcrystalline residue, identified as [Ni(PMe3)2Br]2-
[µ2-(C6H4-o-CH2)2] (7), was washed with petroleum ether and
dried under vacuum. Yield: 84%. Complex 7 has been
previously prepared by using another synthetic method.2b
Syn th esis of [Ni2(P Me3)3(µ2-η3:η1-CH2-o-C6H4)(µ2-N2C3H3)]-
BP h 4 (8). Solid 2 (0.24 g, 0.47 mmol) and NaBPh4 (0.17 g,

Pyrazolyl- and Hydrocarbyl-Bridged Complexes of Ni
Organometallics, Vol. 16, No. 12, 1997 2717
1
F igu r e 4. Plot of the 3a Pz′ Me H NMR signal decay at 0 °C (A) and representation of ln((C(t) - Ce)/(C0 - Ce)) vs time
(B).
Ta ble 2. Cr ysta l a n d Refin em en t Da ta for 2
10b. Anal. Calcd for C44H58N2BP3Ni2: C, 63.2; H, 7.0. Found:
formula
mol wt
cryst syst
space group
cell dimens
a, Å
Cl6H27BrN2Ni2P2
506.7
monoclinic
P21/n
C, 63.3; H, 7.0.
10a : 1H NMR (CD2Cl2, 20 °C) δ 0.93 (d, 9 H, 2J HP ) 9.8 Hz,
PMe3), 1.32 (d, 9 H, J HP ) 8.1 Hz, PMe3), 1.43 (d, 9 H, J HP
9.0 Hz, PMe3), 1.76 (s, 3 H, Me pyraz), 2.20 (t, 1 H, J HH
2
2
)
≈
2
3J HP ) 4.0 Hz, CH2), 2.29 (dd, 1 H, J HH ) 4.0, J HP ) 7.3 Hz,
CH2), 5.90 (br s, 1 H, H(4) pyraz), 6.92 (m, 4 H, arom BPh4),
7.05 (m, 8 H, arom BPh4), 7.09-7.18 (m, 2 H, arom), 7.33 (m,
8 H, arom BPh4), 7.43 (d, 1 H, 3J HH ) 2.0 Hz, H pyraz); 31P{1H}
2
3
14.970(4)
16.207(4)
9.074(2)
104.96(2)
b, Å
c, Å
â, deg
Z
4
NMR (CD2Cl2, 20 °C) AMX spin system, δA ) -10.4, δM
)
V, Å3
2126.9(9)
1.58
1032
295
Enraf-Nonius
graphite monochromated,
Mo KR (λ ) 0.710 69 Å)
37.9
-12.0, δX ) -18.7, J AM ) 7, J MX ) 47 Hz; 13C{1H} NMR (CD2-
D
calcd, g cm-3
1
Cl2, 20 °C) δ 14.2 (s, Me pyraz), 14.9 (d, J CP ) 24 Hz, PMe3),
F(000)
temp, K
diffractometer
radiation
1
1
16.3 (d, J CP ) 28 Hz, PMe3), 16.4 (dd, J CP ) 31, J CP ) 4 Hz,
2
PMe3), 30.7 (d, J CP ) 10 Hz, CH2), 105.3 (s, C(4)H pyraz),
122.0, 125.9, 136.3, (s, CH arom BPh4), 125.4, 127.0, 130.6,
2
143.3 (d, s, s, and s, J CP ) 5 Hz, CH arom), 125.6 (t, J CP ) 5
µ(Mo KR), cm-1
crystal dimens, mm
2θ range, deg
scan technique
data collected
no. of unique data
no. of obsd rflns
Rint (%)
Hz, Cq arom), 128.7 (ddd, J CP ) 75, 38, and 19 Hz, Cq arom-
Ni), 140.3, (s, CH pyraz), 149.6 (s, Cq pyraz), 164.4 (m, Cq arom
BPh4).
0.1 × 0.3 × 0.08
1-50
ω/2θ
10b: 1H NMR (CD2Cl2, 20 °C) δ 0.98 (d, 9 H, 2J HP ) 9.9 Hz,
PMe3), 1.26 (d, 9 H, J HP ) 8.1 Hz, PMe3), 1.46 (d, 9 H, J HP
(-17,0,0) to (17,19,11)
2
2
)
3732
1953
2.9
2
3
9.2 Hz, PMe3), 2.20 (t, 1 H, J HH ≈ J HP ) 4.0 Hz, CH2), 2.36
2
3
(dd, 1 H, J HH ) 4.0, J HP ) 7.7 Hz, CH2), 2.47 (s, 3 H, Me
3
pyraz), 5.86 (bs, 1 H, H(4) pyraz), 6.75 (d, 1 H, J HH ) 1.8 Hz,
decay, %
std rflns
3
3/48
unit
6.3
6.8
0.03
0.72-1.34
H pyraz), 6.92 (m, 4 H, arom BPh4), 7.05 (m, 8 H, arom BPh4),
7.09-7.18 (m, 2H, arom), 7.33 (m, 8 H, arom BPh4); 31P{1H}
NMR (CD2Cl2, 20 °C) AMX spin system, δA ) -9.3, δM ) -13.7,
δX ) -19.0, J AM ) 7, J MX ) 47 Hz; 13C{1H} NMR (CD2Cl2, 20
weighting scheme
R ) ∑|∆2F|/∑|Fo|
Rw ) (∑w|∆2F|/∑w|Fo|)1/2
max shift/error
abs corr range
1
°C) δ 13.2 (s, Me pyraz), 14.9 (d, J CP ) 24 Hz, PMe3), 16.0 (d,
1J CP ) 29 Hz, PMe3), 16.7 (dd, J CP ) 31, J CP ) 4 Hz, PMe3),
1
31.4 (d, 2J CP ) 10 Hz, CH2), 105.8 (s, C(4)H pyraz), 122.0, 125.9,
slope -(k1 + k-1).9b The graph corresponding to 3a is shown
in Figure 4B. Explicit values for the rate constant are obtained
from the equilibrium constant. Keq was optimized in each case
for the best fit of the experimental points. The following
experiments were carried out.
136.3 (s, CH arom BPh4), 124.7, 127.0, 130.3, 143.9 (d, s, s,
and s, J CP ) 5 Hz, CH arom), 126.5 (t, J CP ) 4 Hz, Cq arom),
127.5 (ddd, J CP ) 75, 37, and 20 Hz, Cq arom-Ni), 142.1 (s,
CH pyraz), 147.7 (d, J CP ) 4 Hz, Cq pyraz), 164.4 (m, Cq arom
BPh4).
Kin etic Mea su r em en ts. A solid sample (10-20 mg) of 3a
or 10a was placed in an NMR tube and weighed by difference.
The tube was placed in a cooling bath (-80 °C for 3a and 0 °C
for 10a ) and the solvent (CD2Cl2, 0.6 mL) was added slowly
from a syringe. The sample was shaken and cooled repeatedly
until the solid completely dissolved and placed in the NMR
probe at the experiment temperature (0 °C for 3a , 20 °C for
10a ). When the kinetic measurement was carried out in the
presence of (η5-Cp)Pd(η3-C3H5), this reagent was previously
dissolved in 0.6 mL of CD2Cl2 and added to the solid sample.
PMe3 was added from
immediately before the sample was placed in the probe. The
intensity of the H signal of the methyl substituent of the Pz′
ligand was used to monitor the relative concentration of each
isomer. Figure 4A shows the intensity decay of this signal
for compound 3a . Representation of ln((C(t) - Ce)/(C0 - Ce))
vs time (t) (where C(t) is the relative intensity of the Pz′ Me
signal of 3a or 10a and C0 and Ce are the same intensities
when t ) 0 and t ) ∞, respectively) gives a straight line with
2
(a) Compound 3a (20 mg, 0.04 mmol): k1 + k-1 ) [1.48(3)]
× 10-3 s-1; Keq ) [3a ]/[3b] ) 1.95; k1 ) 9.8 × 10-4 s-1
.
(b) Compound 3a (18 mg, 0.034 mmol) in the presence of
0.03 mmol of (η5-Cp)Pd(η3-C3H5): k1 + k-1 ) [1.13(2)] × 10-3
s-1; Keq ) [3a ]/[3b] ) 2.19; k-1 ) 7.8 × 10-4 s-1
.
(c) Compound 3a (10 mg, 0.017 mmol) in the presence of
PMe3 (0.9 µL, 0.009 mmol): equilibrium was reached before
the first spectrum could be recorded.
(d) Compound 10a (20 mg, 0.023 mmol): k1 + k-1 ) [7.96-
(9)] × 10-4; Keq ) [10a ]/[10b] ) 0.795; k-1 ) 3.5 × 10-4 s-1
.
(e) Compound 10a (18 mg, 0.021 mmol) in the presence of
0.021 mmol of (η5-Cp)Pd(η3-C3H5): k1 + k-1 ) [5.95(17)] × 10-4
s-1; Keq ) [10a ]/[10b] ) 0.644; k-1 ) 2.3 × 10-4 s-1.
(f) Compound 10a (10 mg, 0.012 mmol) in the presence of
PMe3 (0.8 µL, 0.008 mmol): equilibrium was reached before
the first spectrum could be recorded.
a GC syringe to the NMR tube
1
X-r a y Str u ctu r e Deter m in a tion of Com p ou n d 2.
summary of the fundamental crystal data is given in Table 2.
A red crystal of prismatic shape was coated with epoxy resin
A

2718 Organometallics, Vol. 16, No. 12, 1997
Ca´mpora et al.
and mounted in a Kappa diffractometer. The cell dimensions
were refined by least-squares fitting of the θ values of 25
reflections with a 2θ range of 10-30°. The intensities were
corrected for Lorentz and polarization effects. Scattering
factors for neutral atoms and anomalous dispersion corrections
for Ni and Br were taken from ref 23. The structure was
solved by Patterson and Fourier methods. An empirical
absorption correction24 was applied at the end of the isotropic
refinements. A final refinement was undertaken with unit
weight and anisotropic thermal motion for the non-hydrogen
atoms. The hydrogen atoms were included with fixed isotropic
contributions at their calculated positions. No trend in ∆F vs
Fo or (sinθ)/λ was observed. The final difference synthesis
showed no significant electron density. Most of the calcula-
tions were carried out with the X-Ray 80 system.25
Ack n ow led gm en t. Financial support from the Di-
reccio´n General de Investigacio´n Cient´ıfica y Te´cnica
(Projects PB-90-0890 and PB-91-0612) and the J unta
de Andaluc´ıa is gratefully acknowledged. We also thank
the University of Sevilla for the use of its analytical and
NMR facilities. We thank Professors M. L. Poveda and
R. Andersen for helpful discussions.
Su p p or tin g In for m a tion Ava ila ble: Tables of fractional
coordinates, H atom coordinates, thermal parameters, and
bond lengths and angles for 2 (4 pages). Ordering information
is given on any current masthead page.
OM960971G
(23) International Tables for X-Ray Crystallography; Kynoch
Press: Birmingham, U.K., 1974; Vol. IV, p 72.
(24) Walker, N.; Stuart, D. Acta Crystallogr. 1983, A39, 158.
(25) Stewart, J . M. The X-Ray 80 System; Computer Science Center,
University of Maryland, College Park, MD, 1985.