Syntheses and reactivities of new PCsp3P pincer complexes of nickel

Article abstract of DOI:10.1021/om050844x

Heating Ni(II) halides with 1 equiv of Bu^t₂P(CH ₂)₅PBu^t₂ gives the PC_sp3P pincer complexes [{(Bu^t₂P-(CH₂) ₂)₂CH}NiX] (X = Cl, 1a; Br, 1b; I, 1c). These compounds react with MeMgCl or n-BuLi to give, respectively, the methyl species {(Bu ^t₂P(CH₂)₂)₂CH}NiMe, 2, or the hydride species {(Bu^t₂P(CH₂) ₂)₂-CH}NiH, 3, while reaction with NaBPh₄ in a mixture of benzene/acetonitrile gives the cationic species [{(Bu ^t₂P(CH₂)₂)₂CH}Ni (N≡CCH₃)][BPh₄], 4. Complexes 1-3 are inert toward olefins, but react with PhSiH₃ to give (PhSiH)_n. The cationic complex 4 is also inert toward most olefins, but its reaction with excess acrylonitrile results in displacement of coordinated acetonitrile to give [{(Bu^t₂P(CH₂)₂)₂CH}Ni- (N≡CCH=CH₂)][BPh₄], 5. Complexes 1-5 have been characterized by NMR spectroscopy and, in the case of 1, 4, and 5, by X-ray crystallography.

Full text of DOI:10.1021/om050844x

602
Organometallics 2006, 25, 602-608
Syntheses and Reactivities of New PCsp3P Pincer Complexes of
Nickel
Annie Castonguay, Christine Sui-Seng, Davit Zargarian,* and Andre´ L. Beauchamp*
De´partement de Chimie, UniVersite´ de Montre´al, Montre´al (Que´bec), Canada H3C 3J7
ReceiVed October 1, 2005
Heating Ni(II) halides with 1 equiv of But2P(CH2)5PBut2 gives the PCsp3P pincer complexes [{(But2P-
(CH2)2)2CH}NiX] (X ) Cl, 1a; Br, 1b; I, 1c). These compounds react with MeMgCl or n-BuLi to give,
respectively, the methyl species {(But2P(CH2)2)2CH}NiMe, 2, or the hydride species {(But2P(CH2)2)2-
CH}NiH, 3, while reaction with NaBPh4 in a mixture of benzene/acetonitrile gives the cationic species
[{(But2P(CH2)2)2CH}Ni (NtCCH3)][BPh4], 4. Complexes 1-3 are inert toward olefins, but react with
PhSiH3 to give (PhSiH)n. The cationic complex 4 is also inert toward most olefins, but its reaction with
excess acrylonitrile results in displacement of coordinated acetonitrile to give [{(But2P(CH2)2)2CH}Ni-
(NtCCHdCH2)][BPh4], 5. Complexes 1-5 have been characterized by NMR spectroscopy and, in the
case of 1, 4, and 5, by X-ray crystallography.
Chart 1
Introduction
The strongly chelating nature of PCP type pincer ligands and
the rigid geometries they impose on transition metals bestow
high thermal and oxidative stabilities to their complexes and
facilitate novel reactivities.1 Hence, PCP pincer complexes have
been studied intensively during the past decade, and a number
of interesting reactivities2 and highly efficient catalytic reactions3
involving these complexes have been reported. The catalytic
applications of pincer complexes have been reviewed recently.4
The most commonly investigated pincer complexes feature
PCsp2P type ligands consisting of mutually trans PR2 moieties
linked by a m-xylyl group (Chart 1), but the analogous
complexes featuring PCsp3P ligands are receiving increasing
attention and have shown interesting reactivities in their own
rights.5 Noteworthy examples include Pd-PCsp3P complexes
that promote the hydroamination of acrylonitrile6 and the Heck
motivated us to prepare a series of Ni-PCsp3P complexes and
study their reactivities.11 Herein, we report convenient synthetic
routes to a series of Ni-PCsp3P complexes based on the ligand
1,5-bis(di-tert-butylphosphino)pentane and describe a prelimi-
nary account of their reactivities.
vinylation of aryl halides;7 for the latter reaction, the PCsp3
P
Results and Discussion
complexes are believed to be more active than their PCsp2P
counterparts.7,8 Furthermore, a number of spectacular transfor-
mations have been observed with Ru-, Os-, and Ir-PCsp3P
complexes.9
Synthesis, Characterization, and Reactivities of KP,KC,KP-
[{(t-Bu2PCH2CH2)2CH}NiX], 1. Heating mixtures of 1,5-bis-
(di-tert-butylphosphino)pentane and different nickel halides gave
The interesting reactivities promoted by PCsp3P ligands and
the virtually unexplored chemistry of their Ni derivatives10
(5) For some of the original reports on PCsp3P type pincer complexes
see: (a) Al-Salem, N. A.; Empsall, H. D.; Markham, R.; Shaw, B. L.;
Weeks, B. J. Chem. Soc., Dalton Trans. 1979, 1972. (b) Al-Salem, N. A.;
McDonald, W. S.; Markham, R.; Norton, M. C.; Shaw, B. L. J. Chem.
Soc., Dalton Trans. 1980, 59. (c) Crocker, C.; Errington, R. J.; Markham,
R.; Moulton, C. J.; Odell, K. J.; Shaw, B. L. J. Am. Chem. Soc. 1980, 102,
4373. (d) Crocker, C.; Errington, R. J.; Markham, R.; Moulton, C. J.; Shaw,
B. L. J. Chem. Soc., Dalton Trans. 1982, 387. (e) Crocker, C.; Empsall, H.
D.; Errington, R. J.; Hyde, E. M.; McDonald, W. S.; Markham, R.; Norton,
M. C.; Shaw, B. L.; Weeks, B. J. Chem. Soc., Dalton Trans. 1982, 1217.
(f) Briggs, J. R.; Constable, A. G.; McDonald, W. S.; Shaw, B. L. J. Chem.
Soc., Dalton Trans. 1982, 1225.
(6) (a) Seligson, A. L.; Trogler, W. C. Organometallics 1993, 12, 738.
(b) Seligson, A. L.; Trogler, W. C. Organometallics 1993, 12, 744.
(7) Sjo¨vall, S.; Wendt, O. F.; Andersson, C. J. Chem. Soc., Dalton Trans.
2002, 1396.
(8) Ohff, M.; Ohff, A.; van der Boom, M. E.; Milstein, D. J. Am. Chem.
Soc. 1997, 119, 11687.
(1) For recent reviews on pincer complexes see: (a) Albrecht, M.; van
Koten, G. Angew. Chem. Int. Ed. 2001, 40, 3750. (b) van der Boom, M.
E.; Milstein, D. Chem. ReV. 2003, 103, 1759.
(2) (a) Gozin, M.; Weisman, A.; Ben-David, Y.; Milstein, D. Nature
1993, 364, 699. (b) Gozin, M.; Aizenberg, M.; Liou, S.-Y.; Weisman, A.;
Ben-David, Y.; Milstein, D. Nature 1994, 370, 42. (c) van der Boom, M.
E.; Liou, S.-Y.; Ben-David, Y.; Shimon, L. J. W.; Milstein, D. J. Am. Chem.
Soc. 1998, 120, 6531. (d) van der Boom, M. E.; Ben-David, Y.; Milstein,
D. J. Am. Chem. Soc. 1999, 121, 6652. (e) Cohen, R.; van der Boom, M.
E.; Shimon, L. J. W.; Rozenberg, H.; Milstein, D. J. Am. Chem. Soc. 2000,
122, 7723. (f) Ashkenazi, N.; Vigalok, A.; Parthiban, S.; Ben-David, Y.;
Shimon, L. J. W.; Martin, J. M.; Milstein, D. J. Am. Chem. Soc. 2000, 122,
8797. (g) Kanzelberger, M.; Singh, B.; Czerw, M.; Krogh-Jespersen, K.;
Goldman, A. S. J. Am. Chem. Soc. 2000, 122, 11017. (h) Morales-Morales,
D.; Lee, D. W.; Wang, Z.; Jensen, C. M. Organometallics 2001, 20, 1144.
(i) Kanzelberger, M.; Zhang, X.; Emge, T. J.; Goldman, A. S.; Zhao, J.;
Incarvito, C.; Hartwig, J. F. J. Am. Chem. Soc. 2003, 125, 13644.
(3) For instance, Ru-PCP compounds catalyze transfer hydrogenation
of ketones at high turnover numbers: Amoroso, D.; Jabri, A.; Yap, G. P.
A.; Gusev, D. G.; dos Santos, E. N.; Fogg, D. E. Organometallics 2004,
23, 4047, and references therein.
(9) (a) McLoughlin, M. A.; Flesher, R. J.; Kaska, W. C.; Mayer, H. A.
Organometallics 1994, 13, 3816. (b) Kuznetsov, V. F.; Lough, A. J.; Gusev,
D. G. Chem. Commun. 2002, 2432. (c) Gusev, D. G.; Lough, A. J.
Organometallics 2002, 21, 2601. (d) Gusev, D. G.; Fontaine, F. G.; Lough,
A. J.; Zargarian, D. Angew. Chem., Int. Ed. 2003, 42, 216. (e) Zhao, J.;
Goldman, A. S.; Hartwig, J. F. Science 2005, 307, 1080.
(4) Singleton, J. T. Tetrahedron 2003, 59, 1837.
10.1021/om050844x CCC: $33.50 © 2006 American Chemical Society
Publication on Web 12/24/2005

New PCsp3P Pincer Complexes of Nickel
Organometallics, Vol. 25, No. 3, 2006 603
Scheme 1
Chart 2
37-47% yields of orange solids that were identified readily as
our target PCsp3P pincer complexes 1 (Scheme 1; X ) Cl, 1a;
Br, 1b; I, 1c). These reactions also produced unexpected
byproducts that proved difficult to identify, because they were
paramagnetic and only sparingly soluble in common solvents.
The colors of these byproducts, i.e., blue, green, and red for
the Cl, Br, and I derivatives, respectively, would be consistent
n-2 12
with tetrahedral species of the type [LnNiX4-n
]
.
Indeed,
repeated recrystallizations of the byproduct obtained from the
NiCl2 reaction yielded blue microcrystals that were identified
as the tetrahedral, zwitterionic complex [{(t-Bu2PH)(CH2)5(t-
Bu2P)}NiCl3], A (Chart 2).13
The formation of compound A likely arises from the reaction
of 1a with HCl, which is generated in-situ during the cyclo-
metalation reaction; alternatively, protonation of the bis-
(phosphine) ligand might occur prior to its coordination to NiCl2.
Curiously, carrying out the cyclometalation reactions in the
presence of NEt3 did not circumvent the formation of the
byproducts, implying that the phosphorus moieties are proto-
nated in preference to the added base. Moreover, protonating
isolated samples of 1a with HCl (etherate) did not stop at the
singly protonated product (complex A), proceeding instead to
double protonation and formation of a blue solid that was
identified as complex [{(t-Bu2PH)(CH2)2}2CH2][NiCl4], B
(Chart 2).13,14 These observations indicate that the byproducts
Figure 1. ORTEP diagrams for complexes 1a, 1b, and 1c. Thermal
ellipsoids are shown at the 30% probability level. Hydrogens and
methyl groups are omitted for clarity. The iodide atom in 1c is
disordered over two positions (54:46), only one of which is shown
in the ORTEP.
(10) To our knowledge, the only example of a Ni-PCsp3P compound
reported previously is [Ni{1-CH2-2,6-(CH2PPri2)2-3,5-(CH3)2C6H}I]: van
der Boom, M. E.; Liou, S.-Y.; Shimon, L. J. W.; Ben-David, Y.; Milstein,
D. Inorg. Chim. Acta 2004, 357, 4015.
formed during the syntheses of 1 are tetrahedral species
analogous to A and B, but the available experimental evidence
does not allow us to draw firm conclusions on their identities,
structural details, or the pathways that lead to their formation.
Complexes 1 are thermally stable and can be handled in air,
both in the solid state and in solution, without noticeable
decomposition even after weeks. Their 31P{1H} NMR spectra
contain a sharp singlet resonance that confirms the equivalence
of the phosphorus nuclei in accord with a trans geometry; the
chemical shift of this signal (ca. 73-74 ppm) is very close to
that of the analogous PCsp2P complex {2,6-(CH2PBut2)2C6H3}-
NiCl (ca. 74.7 ppm).11a The appearance in the 1H NMR spectra
of the characteristic pattern of virtual triplets for the t-Bu protons
is also consistent with strong coupling of these nuclei to
(11) For examples of Ni-PCsp2P complexes see ref 10 and the following
reports: (a) Moulton, C. J.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1976,
1020. (b) Kennedy, A. R.; Cross, R. J.; Muir, K. W. Inorg. Chim. Acta
1995, 231, 195. (c) Huck, W. T. S.; Snellink-Rue¨l, B.; van Veggel, F. C.
J. M.; Reinhoudt, D. N. Organometallics 1997, 16, 4287. (d) Bachechi, F.
Struct. Chem. 2003, 14, 263. (e) Kozhanov, K. A.; Bubnov, M. P.;
Cherkasov, V. K.; Fukin, G. K.; Abakumov, G. A. Chem. Commun. 2003,
2610. (f) Ca´mpora, J.; Palma, P.; del R´ıo, D.; AÄ lvarez, E. Organometallics
2004, 23, 1652. (g) Ca´mpora, J.; Palma, P.; del R´ıo, D.; Conejo, M. M.;
AÄ lvarez, E. Organometallics 2004, 23, 5653. (h) Groux, L. F.; Be´langer-
Garie´py, F.; Zargarian, D. Can. J. Chem. 2005, 83, 634.
(12) For example, [NiCl3(PPh3)][NEt4] is blue (Smith, M. C.; Davies,
S. C.; Hughes, D. L.; Evans, D. J. Acta Crystallogr. 2001, E57, m509),
[NiBr3(PPh3)][AsPh4] (Hanton, L. R.; Raithby, P. R. Acta Crystallogr. 1980,
B36, 2417) and [NiBr3(PtBu3)][HPtBu3] (Alyea, E. C.; Costin, A.; Ferguson,
G.; Fey, G. T.; Goel, R. G.; Restivo, R. J. J. Chem. Soc., Dalton Trans.
1975, 1294) are green, and [NiI3(PPh3)][AsPh4] is red (Taylor, R. P.;
Templeton, D. H.; Zalkin, A.; Horrocks, W. Inorg. Chem. 1968, 7, 2629).
(13) X-ray diffraction studies carried out on the blue crystals of A and
B have confirmed the empirical formulae and the connectivities in these
compounds, but the poor quality of the data obtained does not allow firm
commentary on the structural details.
(14) The 1H NMR spectrum of this species contained poorly defined,
broad peaks, but its 31P{1H} NMR (CD2Cl2) spectrum showed only a singlet
at 17.4 ppm, which is tentatively assigned to the phosphonium moiety.

604 Organometallics, Vol. 25, No. 3, 2006
Castonguay et al.
Table 1. Crystal Data Collection and Refinement Parameters for Complexes 1a, 1b, 1c, 4, and 5
1a
1b
1c
4
5
chemical formula
fw
T (K)
wavelength (Å)
space group
a (Å)
C21H45ClNiP2
453.67
293 (2)
1.54178
P21/c
12.1335 (2)
14.2189 (2)
14.8382 (2)
90
104.4970 (10)
90
C21H45BrNiP2
498.13
220 (2)
1.54178
P21/c
12.1501 (5)
14.3036 (5)
14.8142 (6)
90
104.337 (3)
90
C21H45INiP2
545.12
220 (2)
1.54178
P212121
11.8960 (5)
14.1665 (5)
15.1062 (6)
90
C47H68NBNiP2
778.48
220 (2)
1.54178
P21/c
15.3450 (3)
14.4944 (3)
19.9357 (5)
90
90.368 (2)
90
C48H68NBNiP2
790.49
200 (2)
1.54178
P21/c
14.3311(8)
18.2163(11)
17.4551(10)
90
100.655(3)
90
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
Z
90
90
4
4
4
4
4
V (Å3)
2478.45 (6)
1.216
33.37
3.76-72.92
0.0611
0.1838
0.0661
0.1882
1.052
2494.38 (17)
1.326
42.07
3.75-72.97
0.0688
0.2088
0.0888
0.2940
1.228
2545.77 (17)
1.422
4433.94 (17)
1.166
15.34
2.88-72.92
0.0639
0.1457
0.1134
0.1641
0.912
4478.3(4)
1.172
15.27
3.14-68.66
0.0402
0.0860
0.0744
0.0959
0.872
F
calcd (g cm-3
)
Μ (cm-1
)
118.06
4.28-72.92
0.0686
0.1720
0.0743
0.1767
1.033
θ range (deg)
R1a [I > 2σ(I)]
wR2b [I > 2σ(I)]
R1 [all data]
wR2 [all data]
GOF
2
2
2
a R1 ) ∑(|Fo| - |Fc|)/∑|Fo|. b wR2 ) {∑[w(Fo - Fc )2]/∑[w(Fo )2]}1/2
.
Table 2. Selected Bond Distances (Å) and Angles (deg) for Complexes 1a, 1b, 1c, 4, and 5
1a (X ) Cl)
1b (X ) Br)
1c (X ) I)
4 (X ) N)
5 (X ) N)
Ni-C(3)
Ni-P(1)
Ni-P(2)
Ni-X
1.979(3)
1.971(4)
1.985(7)
2.2353(17)
2.2295(13)
2.561(3)
2.547(4)
168.4(3)
178.1(5)
169.69(7)
96.49(10)
94.93(13)
93.75(10)
95.37(13)
84.5(3)
1.969(3)
1.982(2)
2.2261(7)
2.2272(7)
1.899(2)
2.2079(8)
2.2130(8)
2.2490(8)
2.2182(12)
2.2123(12)
2.3866(7)
2.2154(10)
2.2142(10)
1.908(3)
C(3)-Ni-X
169.59(11)
169.89(13)
173.98(16)
173.81(9)
P(1)-Ni-P(2)
P(1)-Ni-X
169.70(3)
94.92(3)
170.16(5)
94.48(3)
169.31(4)
95.08(9)
167.84(3)
96.34(6)
P(2)-Ni-X
94.95(3)
95.00(4)
95.56(8)
94.60(6)
P(1)-Ni-C(3)
P(2)-Ni-C(3)
85.24(9)
85.59(9)
85.90(13)
85.22(13)
84.75(12)
84.79(12)
84.64(7)
85.05(7)
85.2(3)
mutually trans phosphorus nuclei. The 13C{1H} NMR spectra
also contained virtual triplets for t-Bu carbons in addition to
low-field triplets (ca. 47-56 ppm; 2JP-C ≈ 9 Hz) attributed to
the central carbon atom of the alkyl chain that is bonded to the
nickel center.
amount of free ligand was detected in both the 1H and 31P NMR
spectra. These observations imply that an initially formed Ni-
Bu intermediate undergoes â-H elimination to give 3, and that
this Ni-H species is itself prone to reductive elimination and
phosphine dissociation.
The Ni-Me compound 2 and the Ni-H derivative 3 could
not be isolated in analytically pure form,16 but their NMR spectra
provided sufficient evidence for a convincing characterization.
For instance, the Ni-CH3 moiety in 2 gave rise to triplet
resonances in the 1H and the 13C{1H} NMR spectra (1H: -0.27
ppm, 3JP-H ) 8 Hz; 13C: -19.07 ppm, 2JP-C ) 19 Hz), whereas
the 31P{1H} NMR spectrum showed a singlet resonance at 80
ppm. In the case of complex 3, the appearance in the 1H NMR
spectrum of a triplet signal at ca. -10 ppm (2JP-H ) 53 Hz) is
characteristic of a hydride moiety. Moreover, the 31P{1H} NMR
spectrum of 3 showed a singlet resonance at ca. 109 ppm,
significantly downfield of the corresponding signals in the Ni-
halide and Ni-Me derivatives, but fairly close to that of the
analogous Pd-H complex reported by Trogler (ca. 106 ppm).6a
Our preliminary investigations of the reactivities of complexes
2 and 3 showed that these compounds are inert toward olefins,
but they catalyze the oligomerization of PhSiH3. Thus, reaction
of excess PhSiH3 (50-100 equiv) with these complexes (ca.
X-ray diffraction studies helped establish the solid-state
structures of 1a, 1b, and 1c. The ORTEP views of these
complexes are shown in Figure 1, the crystal data and collection
details are listed in Table 1, and selected bond distances and
angles are given in Table 2. The Ni center in all three structures
adopts a distorted square-planar geometry defined by two
phosphorus atoms, the central carbon atom of the PCsp3P ligand,
and a halogen atom. All Ni-P bond lengths lie within the
expected range for a trans-P-Ni-P arrangement, while the
P(1)-Ni-P(2) angles of ca. 170° result from a slight tetrahedral
distortion away from the ideal square-planar geometry expected
for the d8 centres. The Ni-C(3) distances of ca. 1.98 Å are
comparable to the mean of all Ni-Csp3 bond lengths reported
to date (1.980 Å).15
We undertook to prepare alkyl derivatives of our pincer
complexes in order to study their reactivities with olefins and
hydrosilanes. Reaction of complex 1a with 1 equiv of MeMgCl
gave the corresponding Ni-Me species 2, whereas the reaction
of 1 with BuLi led to the formation of the Ni-H derivative 3
(Scheme 2). The 1H NMR spectra of the latter reaction mixture
contained signals attributed to 1-butene, while a considerable
(16) The Ni-Me derivative reverts to its precursor Ni-Cl species,
partially or completely, during workup, presumably because of the reverse
metathesis reaction with in-situ-generated MgCl2. Attempts to remove MgCl2
from samples of 2 by washing with water or extraction in hexanes were
not successful. The Ni-H derivative is thermally unstable.
(15) Cambridge Crystallographic Data Center, ConQuest 1.7, 2004.

New PCsp3P Pincer Complexes of Nickel
Organometallics, Vol. 25, No. 3, 2006 605
Scheme 2
Scheme 3
and the emergence of a new resonance at ca. 87 ppm. The
formation of the CH3CNfNi moiety in 4 was also indicated
by the presence in the 1H NMR spectra of a new singlet at 0.70
ppm, indicating a significant upfield shift of this signal relative
to free acetonitrile (ca. 2.1 ppm in CDCl3). Comparison of the
13C chemical shift for the nitrile carbon would have been very
informative in this context, but this signal was not detected in
the 13C{1H} NMR spectrum of complex 4, presumably because
of the coupling with the P nuclei that reduces the intensity of
this signal. The IR spectrum of 4 shows a very weak band at
18
2270 cm-1, which was assigned to νCtN
.
Comparison to the
corresponding band in free MeCN (2254 cm-1) indicates that
the NtC bond order is reinforced upon coordination; we
conclude, therefore, that the Ni-NCMe bond is dominated by
ligand to metal σ-donation, which is known to diminish the
antibonding character of the N lone pair with respect to the
C-N bond.19
0.05 M in C6D6) at room temperature for 24 h gave (PhSiH)n
1
oligomers with ca. 50% conversion. The H NMR spectra of
these reaction mixtures indicated that the oligomers obtained
from the reaction of the Ni-Me derivative 2 contained both
cyclic and linear oligomers (Si-H signals at ca. 5.2-6.0 and
ca. 4.4-4.8 ppm, respectively17), whereas those obtained from
the reaction of the Ni-H derivative 3 showed only the signals
characteristic of the linear oligomers. The halide derivatives
reacted much more sluggishly with PhSiH3; for instance, the
reaction with the Ni-I analogue 1c gave less than 5%
conversion even after heating to ca. 100 °C for 24 h.
Synthesis, Characterization, and Reactivities of KP,KC,KP-
[{(t-Bu2PCH2CH2)2CH}Ni(NtCMe)][BPh4], 4. Reacting com-
plexes 1 with NaBPh4 in a mixture of benzene and acetonitrile
cleaved the Ni-X bond to form the pale yellow, cationic species
4 featuring a Ni-NCMe linkage (Scheme 3). The observation
that halide abstraction does not take place in benzene or CH2-
Cl2 alone underscores the greater solubility of NaBPh4 in MeCN
and the latter’s stabilizing role in such cationic species. Solid
samples of complex 4 can be handled in air indefinitely, but
oxidation can occur if solutions of 4 are left exposed to air over
several weeks. Gradual decomposition was also noticed by 31P-
{1H} NMR spectroscopy when C6D6 solutions of 4 were heated
above 40 °C.
The spectral features of this complex are in agreement with
its crystal structure (Figure 2, Table 1). The more or less
unchanged Ni-P and Ni-C3 bond distances and P-Ni-P and
P-Ni-C3 angles indicate that the slightly distorted square-
planar geometry found for complexes 1 is largely maintained
in 4 (Table 2). The end-bonded acetonitrile molecule has the
expected attachment linearity (Ni-N-C(10) ≈ 177°; N-C(10)-
C(11) ≈ 180°), whereas the Ni-N bond distance in 4 (1.908-
(3) Å) is significantly shorter than the mean value of Ni-NCMe
bond lengths reported in the literature (2.068 Å).20 The fairly
short N-C distance (1.122(4) Å) points to a strong CtN bond,
as inferred from the IR data (vide supra).
The important reactivities of cationic species in catalytic
settings prompted us to investigate the substitution lability of
the acetonitrile moiety in 4 and its reactions with PhSiH3 and
a number of olefins, with the following results. No reaction was
(18) It is noteworthy that the IR spectrum also displayed two equally
weak bands (ca. 2182 and 2304 cm-1) that have not been assigned, but
might arise from overtones of the BPh4 anion or from a complex bearing
two acetonitrile moieties, one engaging in π-back-bonding and the other in
σ-donation with the Ni center.
(19) For comparison, the corresponding IR band for the acetonitrile
moiety in the complex [PPP-Ni(NCMe)]2+ was found to be 2285 or 2319
cm-1, consistent with greater σ-donation and less π-back-bonding in this
dicationic species: Barbaro, P.; Togni, A. Organometallics 1995, 14, 3570.
(20) For examples of structurally characterized Ni-NCMe complexes
see: (a) Jircitano, A. J.; Mertes, K. B. Inorg. Chem. 1983, 22, 1828. (b)
Freeman, G. M.; Barefield, E. K.; Van Derveer, D. G. Inorg. Chem. 1984,
23, 3092. (c) Adhikary, B.; Liu, S.; Lucas, C. R. Inorg. Chem. 1993, 32,
5957.
The conversion of 1 to 4 was signaled by the disappearance
of the 31P{1H} NMR signals for the precursors at ca. 74 ppm
(17) (a) Fontaine, F. G.; Zargarian, D. Organometallics 2002, 21, 401.
(b) Woo, H. G.; Walzer, J. F.; Tilley, T. D. J. Am. Chem. Soc. 1992, 114,
7047. (c) Dioumaev, V. K.; Rahimian, K.; Gauvin, F.; Harrod, J. F.
Organometallics 1999, 18, 2249.

606 Organometallics, Vol. 25, No. 3, 2006
Castonguay et al.
Figure 3. ORTEP diagram for complex 5. Thermal ellipsoids are
shown at the 30% probability level. Hydrogens and methyl groups
of the t-Bu substituents are omitted for clarity.
Figure 2. ORTEP diagram for complex 4. Thermal ellipsoids are
shown at the 30% probability level. Hydrogens and methyl groups
of the t-Bu substituents are omitted for clarity.
the corresponding bond lengths reported for a dicationic,
N-bound methacrylonitrile-Ni complex (1.132(4) and 1.307-
(5) Å, respectively).25
noted when 4 was allowed to react for ca. 18 h in CDCl3 ([4]
≈ 0.02 M) with 100 equiv of PhSiH3, isoprene, styrene,
norbornene, COD, 4-vinylaniline, and methyl methacrylate. The
target Ni-olefin cations were also inaccessible from the reaction
of 1 with NaBPh4 in neat olefin.21 On the other hand, reaction
of 4 with an excess of acrylonitrile led to the reversible exchange
of MeCN (Keq ) 0.98 ( 0.07) to form complex 5 (Scheme 3).
Complex 5 was also prepared directly via the reaction of 1 with
NaBPh4 in neat acrylonitrile and was isolated in ca. 61% yield.
The NMR features of 5 seem to indicate that the acrylonitrile
molecule is linked to the Ni center by its nitrogen lone pair.22
For example, very similar chemical shifts were observed for
the 31P signals in 5 (ca. 90 ppm) and 4 (ca. 88 ppm). In addition,
Conclusion
Formation of the new Ni-PCsp3P pincer complexes 1
proceeds by an uncommon Ni-promoted C-H bond activation
process. Access to complexes 1-5 should facilitate a systematic
exploration of their reactivities. Our preliminary results show
that the Ni-methyl (2) and Ni-hydride (3) species are, as
expected, more reactive toward PhSiH3 than the precursor Ni-
halide complexes (1). The acetonitrile adduct (4) is surprisingly
inert toward a range of olefins, but it does undergo reversible
substitution of the acetonitrile moiety by acrylonitrile. Recent
reports of successful Ni-promoted hydrophosphination25 and
hydroamination26 reactions with methacrylonitrile and other
activated olefins indicate that similar reactivities might be
promoted by our cationic PCsp3P-Ni compounds 4 and 5.
Studies are underway to probe such reactivities.
1
the H and 13C signals for the vinyl moiety in 5 (ca. 4.8, 5.6,
and 5.7 ppm for 1H, and ca. 105 and 143 ppm for 13C) are quite
close to those of the corresponding signals in free acrylonitrile
(ca. 5.6, 6.1, and 6.2 ppm for 1H, and ca. 108 and 137 ppm for
13C). As was the case for complex 4, the 13C signal for NC was
not detected. These observations are consistent with an N-
coordinated acrylonitrile moiety. On the other hand, the νCtN
band in the IR spectrum of 5 appears at ca. 2232 cm-1, almost
identical to that of free acrylonitrile (2231 cm-1 in CCl4
solution). Since a shift to higher frequencies is normally
expected for the νCtN band in N-coordinated acrylonitrile
complexes,23 we invoke some Ni f acrylonitrile π-back-
donation.24
Nevertheless, an X-ray diffraction study carried out on a
single crystal of 5 allowed us to establish that the acrylonitrile
ligand is N-bound in the solid state (Figure 3, Table 1). The
distorted square-planar geometry around the nickel center
resembles those of complexes 1 and 4 (Table 2). The Ni-N
bond distance (ca. 1.90 Å) and the angles Ni-N-C (174°) and
N-C-C (178°) in complex 5 are nearly equivalent to the
corresponding values in complex 4, while the CtN (1.144(3)
Å) and CdC (1.300(3) Å) bond distances are very similar to
Experimental Section
General Comments. All manipulations were carried out under
a nitrogen atmosphere using standard Schlenk techniques and/or
in a nitrogen-filled glovebox, except where noted. Solvents were
purified by distillation from appropriate drying agents before use.
All reagents were used as received from commercial vendors. The
diphosphine ligand But2P(CH2)5PBut2 was prepared according to a
literature procedure27 and distilled before use. The NMR spectra
were recorded at ambient temperature on a Bruker ARX400
instrument. The 1H and 13C NMR spectra were referenced to solvent
resonances, as follows: 7.26 and 77.16 ppm for CHCl3 and CDCl3,
respectively, and 7.15 and 128.06 ppm for C6D5H and C6D6,
respectively. The 31P NMR spectra were referenced to an external
85% H3PO4 sample (0 ppm). The IR spectra were recorded on a
Perkin-Elmer 1750 FTIR (4000-450 cm-1) with samples prepared
v
as KBr pellets. The J term used refers to the apparent coupling
constant of the virtual triplets. The elemental analyses were
performed by the Laboratoire d’Analyse EÄ le´mentaire (Universite´
de Montreal).
[{But2P(CH2)2CH(CH2)2PBut2}NiCl] (1a). 1,5-Bis(di-tert-bu-
tylphosphino)pentane (250 mg, 0.69 mmol) was added to a
suspension of anhydrous nickel(II) chloride (90 mg, 0.69 mmol)
in toluene (15 mL), and the reaction mixture was refluxed for 12
h. Filtration of the final mixture in the air and evaporation of the
(21) To be sure, the 31P{1H} NMR spectrum of the reaction of 1a with
norbornene showed a small new peak (<5%), which might be the target
olefin product, but isolation of this species was not pursued.
(22) For examples of N-bound M-acrylonitrile complexes, see: (a)
Stojcevic, G.; Prokopchuk, E. M.; Baird, M. C. J. Organometallic Chem.
2005, 690, 4349 (M ) Pd). (b) Chong, S. C.; Chong, D.; Lee, S.; Park, Y.
J. Organometallics 2000, 19, 4043 (M ) Ir).
(23) Bryan, S. J.; Huggett, P. G.; Wade, K.; Daniels, J. A.; Jennings, J.
R. Coord. Chem. ReV. 1982, 44, 149.
(24) It should be mentioned, however, that a dynamic exchange between
the N-bound and π-bound forms should also be considered possible in
solution. Consistent with this proposal, a recent report has shown that
N-bound and π-bound Pd-acrylonitrile species can exist in equilibrium:
Groux, L. F.; Weiss, T.; Reddy, D. N.; Chase, P. A.; Piers, W. E.; Ziegler,
T.; Parvez, M.; Benet-Buchholz. J. Am. Chem. Soc. 2005, 127, 1854.
(25) Sadow, A. D.; Haller, I.; Fadini, L.; Togni, A. J. Am Chem. Soc.
2004, 126, 14704.
(26) Fadini, L.; Togni, A. Chem. Commun. 2003, 30.
(27) For an optimized synthesis of this ligand see: Gusev, D. G.; Lough,
A. J. Organometallics 2002, 21, 5091.

New PCsp3P Pincer Complexes of Nickel
Organometallics, Vol. 25, No. 3, 2006 607
1
filtrate gave complex 1a as an orange solid (148 mg, 47%). H
NMR (C6D6): 1.07-1.85 (m, 9H), 1.41 (vt, vJ ) 5.7 Hz, PC(CH3)3,
[(But2P(CH2)2CH(CH2)2PBut2)Ni(NtCCH3)][BPh4] (4). A so-
lution of complex 1a (345 mg, 0.813 mmol) in benzene (10 mL)
was added to a solution of NaBPh4 (293 mg, 0.813 mmol) in
acetonitrile (15 mL), and the mixture was stirred at room temper-
ature for 3 h. Evaporation of the final mixture and extraction of
the solid residues with CH2Cl2 (15 mL) gave a yellow solution,
which was filtered and evaporated to give compound 4 as a pale
yellow solid (373 mg, 63%). 1H NMR (CDCl3): 0.70 (s, CH3CN,
v
1
18H), 1.46 (vt, J ) 6.4 Hz, PC(CH3)3, 18H). H NMR (CDCl3):
1.10-2.10 (m), 1.48 (vt, J ) 5.9 Hz, PC(CH3)3). 13C{1H} NMR
v
(C6D6): 23.44 (vt, vJ ) 9.0 Hz, CH2, 2C), 29.46 (vt, vJ ) 2.4 Hz,
v
PC(CH3)3, 6C), 30.19 (vt, J ) 2.8 Hz, PC(CH3)3, 6C), 34.63 (vt,
v
vJ ) 6.9 Hz, PC(CH3)3, 2C), 35.43 (vt, J ) 5.2 Hz, PC(CH3)3,
2C), 39.43 (vt, vJ ) 10.0 Hz, CH2, 2C), 46.86 (t, J ) 9.7 Hz, CH,
v
1C). 13C{1H} NMR (CDCl3): 23.00 (vt, J ) 9.3 Hz, CH2, 2C),
v
3H), 0.90-2.25 (m, 9H), 1.29 (vt, J ) 7.6 Hz, PC(CH3)3, 18H),
v
v
v
1.34 (vt, J ) 6.4 Hz, PC(CH3)3, 18H), 6.88 (t, J ) 7.6 Hz, Ar,
29.27 (vt, J ) 2.4 Hz, PC(CH3)3, 6C), 30.04 (vt, J ) 2.4 Hz,
4H), 7.05 (t, J ) 7.6 Hz, Ar, 8H), 7.47 (br s, Ar, 8H). 13C{1H}
NMR (CDCl3): 22.32 (vt, vJ ) 11.0 Hz, CH2, 2C), 29.38 (m, PC-
v
PC(CH3)3, 6C), 34.50 (vt, J ) 6.9 Hz, PC(CH3)3, 2C), 35.36 (vt,
vJ ) 5.5 Hz, PC(CH3)3, 2C), 38.95 (vt, J ) 9.7 Hz, CH2, 2C),
v
v
46.70 (t, J ) 9.7 Hz, CH, 1C). 31P{1H} NMR (C6D6): 73.8 (s).
31P{1H} NMR (CDCl3): 74.0 (s). Anal. Calcd for C21H45ClP2Ni:
C, 55.60; H, 10.00. Found: C, 55.43; H, 10.43.
(CH3)3, 6C), 29.61 (m, PC(CH3)3, 6C), 34.94 (vt, J ) 7.2 Hz,
v
PC(CH3)3, 2C), 36.21 (vt, J ) 6.2 Hz, PC(CH3)3, 2C), 39.07 (m,
CH2, 2C), 54.03 (m, CH, 1C), 121.82 (s, Ar, 4C), 125.88 (s, Ar,
8C), 136.10 (s, Ar, 8C), 164.40 (q, J ) 49.0 Hz, Ar, 4C). 31P{1H}
NMR (CDCl3): 88.4 (s). IR (KBr): 2270 cm-1 (µCtN). Anal. Calcd
for C47H68BNP2Ni‚H2O: C, 70.87; H, 8.86; N, 1.76. Found: C,
70.73; H, 9.33; N, 1.54.
[{But2P(CH2)2CH(CH2)2PBut2}NiBr] (1b). The procedure de-
scribed for the preparation of complex 1a was used with nickel(II)
bromide to give complex 1b as an orange solid (152 mg, 44%). 1H
NMR (C6D6): 0.90-1.85 (m, 9H), 1.42 (vt, vJ ) 6.4 Hz, PC(CH3)3,
18H), 1.47 (vt, vJ ) 6.4 Hz, PC(CH3)3, 18H). 13C{1H} NMR
[(But2P(CH2)2CH(CH2)2PBut2)Ni(NtCCHdCH2)][BPh4] (5).
NaBPh4 (77 mg, 0.225 mmol) was added to a solution of complex
1a (102 mg, 0.225 mmol) in neat acrylonitrile (4 mL). The reaction
mixture was stirred for 16 h at room temperature. Evaporation of
the final mixture and extraction of the solid residues with CH2Cl2
(10 mL) gave a yellow solution, which was filtered and evaporated
to give compound 5 as a pale yellow solid (109 mg, 61%). 1H NMR
v
(C6D6): 23.89 (vt, J ) 8.6 Hz, CH2, 2C), 29.63 (s, PC(CH3)3,
v
6C), 30.54 (s, PC(CH3)3, 6C), 35.12 (vt, J ) 5.9 Hz, PC(CH3)3,
v
v
2C), 35.74 (vt, J ) 5.5 Hz, PC(CH3)3, 2C), 39.31 (vt, J ) 9.3
Hz, CH2, 2C), 50.21 (t, J ) 8.6 Hz, CH, 1C). 31P{1H} NMR
(C6D6): 73.5 (s). Anal. Calcd for C21H45BrP2Ni: C, 50.64; H, 9.11.
Found: C, 50.67; H, 9.37.
v
(CDCl3): 0.90-2.30 (m, 9H), 1.30 (vt, J ) 6.7 Hz, PC(CH3)3,
[{But2P(CH2)2CH(CH2)2PBut2}NiI] (1c). The procedure de-
v
18H), 1.34 (vt, J ) 6.7 Hz, PC(CH3)3, 18H), 4.76 (dd, J ) 17.2
scribed for the preparation of complex 1a was used with nickel(II)
Hz, 11.4 Hz, NtCCHdCH2, 1H), 5.57 (d, J ) 11.4 Hz, NtCCHd
CH2, 1H), 5.74 (d, J ) 17.2 Hz, NtCCHdCH2, 1H), 6.87 (t, J )
6.7 Hz, Ar, 4H), 7.02 (t, J ) 7.6 Hz, Ar, 8H), 7.42 (br s, Ar, 8H).
13C{1H} NMR (CDCl3): 22.41 (vt, vJ ) 11.0 Hz, CH2, 2C), 29.38
(vt, vJ ) 2.1 Hz, PC(CH3)3, 6C), 29.66 (vt, vJ ) 2.1 Hz, PC(CH3)3,
1
iodide to give complex 1c as an orange solid (141 mg, 37%). H
NMR (C6D6): 0.80-1.85 (m, 9H), 1.43 (vt, vJ ) 5.1 Hz, PC(CH3)3,
18H), 1.49 (vt, vJ ) 5.1 Hz, PC(CH3)3, 18H). 13C{1H} NMR
v
(C6D6): 24.62 (vt, J ) 8.3 Hz, CH2, 2C), 29.91 (s, PC(CH3)3,
v
6C), 31.19 (s, PC(CH3)3, 6C), 35.83 (vt, J ) 7.2 Hz, PC(CH3)3,
v
v
6C), 35.25 (vt, J ) 7.2 Hz, PC(CH3)3, 2C), 36.48 (vt, J ) 6.6
v
v
2C), 36.11 (vt, J ) 5.5 Hz, PC(CH3)3, 2C), 39.10 (vt, J ) 9.3
Hz, CH2, 2C), 56.02 (t, J ) 8.6 Hz, CH, 1C). 31P{1H} NMR
(C6D6): 74.7 (s). Anal. Calcd for C21H45IP2Ni: C, 46.27; H, 8.32.
Found: C, 46.73; H, 8.66.
v
Hz, PC(CH3)3, 2C), 39.21 (vt, J ) 7.6 Hz, CH2, 2C), 55.63 (m,
CH, 1C), 105.06 (s, NtCCHdCH2, 1C), 121.73 (s, Ar, 4C), 125.66
(s, Ar, 8C), 136.35 (s, Ar, 8C), 142.57 (s, NtCCHdCH2, 1C),
164.39 (q, J ) 48.9 Hz, Ar, 4C). 31P{1H} NMR (CDCl3): 90.1
(s). IR (KBr): 2232 cm-1, ν(CtN). Anal. Calcd for C48H68BNP2-
Ni‚H2O: C, 71.30; H, 8.73; N, 1.73. Found: C, 71.53; H, 8.86; N,
1.64.
[{But2P(CH2)2CH(CH2)2PBut2}NiMe] (2). MeMgCl (0.20 mL
of a 3.0 M solution in THF, 0.60 mmol) was added to a solution
of complex 1a (272 mg, 0.60 mmol) in benzene (10 mL), and the
mixture was refluxed for 1 h. The solvent was then evaporated to
dryness to give a crude sample of complex 2 as a yellow solid.
This sample contained some in-situ-formed MgCl2, which could
not be removed by washing with water or extraction into hexane.
1H NMR (C6D6): -0.27 (t, J ) 7.6 Hz, NiCH3, 3H), 1.29 (vt, J )
6.4 Hz, PC(CH3)3, 18H), 1.30 (vt, J ) 5.1 Hz, PC(CH3)3, 18H).
13C{1H} NMR (C6D6): -19.07 (t, J ) 19.0 Hz, NiCH3, 1C), 27.69
(vt, vJ ) 9.3 Hz, CH2, 2C), 29.78 (vt, vJ ) 2.8 Hz, PC(CH3)3, 6C),
Determination of Keq for the Reaction of 4 with Acrylonitrile.
This experiment was performed by adding different amounts of
acrylonitrile to an NMR sample containing a solution of 4 in CDCl3
1
([4] ) 0.05-0.09 M) and recording successive H and 13P{1H}
NMR spectra until the signal intensities were unchanged from one
spectrum to the next. The Keq values were determined on the basis
of data obtained from eight different experiments; the calculations
were based on the relationship Keq ) [5][MeCN]/[4][acrylonitrile],
using ratios of the NMR signal integrations for these species as an
approximation for concentration ratios. Since the 1H NMR signals
for 4 and MeCN were difficult to measure with precision, the Keq
v
v
30.43 (vt, J ) 2.4 Hz, PC(CH3)3, 6C), 34.52 (vt, J ) 6.2 Hz,
v
PC(CH3)3, 2C), 35.84 (vt, J ) 4.8 Hz, PC(CH3)3, 2C), 38.15 (vt,
vJ ) 10.0 Hz, CH2, 2C), 57.67 (t, J ) 9.7 Hz, CH, 1C). 31P{1H}
1
was determined by combining H and 31P NMR data, as follows:
NMR (C6D6): 80.5 (s).
[{But2P(CH2)2CH(CH2)2PBut2}NiH] (3). n-BuLi (0.05 mL of
a 2.5 M solution in hexanes, 0.125 mmol) was added to a
concentrated solution of 1a (57 mg, 0.125 mmol) in benzene-d6
(0,5 mL). Monitoring by NMR spectroscopy showed that the signals
for 1a were replaced over 30 min by new signals assigned to the
hydride species 3, 1-butene, and free ligand. Evaporation of the
volatiles gave a mixture of complex 3 and the free ligand. 1H NMR
the [5]/[4] ratio was determined from the integration values of the
31P{1H} signals for 5 and 4, whereas the [MeCN]/[acrylonitrile]
ratio was determined from the integration values of the 1H signals
for 5 and acrylonitrile, since the equilibrium mixture must contain
equimolar quantities of 5 and MeCN.
X-ray Crystal Structures of 1a, 1b, 1c, 4, and 5. Single crystals
of these complexes were grown from the following: hexanes
solutions at room temperature (1a, 1b, 1c); slow diffusion of
hexanes into a saturated solution of the complex in benzene-d6 (4);
slow diffusion of diethyl ether into a saturated solution of the
complex in dichloromethane (5). The crystallographic data (Table
1) were collected on a Bruker AXS diffractometer equipped with
a SMART 2K CCD area detector with graphite-monochromatic Cu
KR radiation using SMART. Cell refinement and data reduction
v
(C6D6): -10.14 (t, J ) 53.4 Hz, NiH, 1H), 1.25 (vt, J ) 7.6 Hz,
PC(CH3)3, 18H), 1.26 (vt, vJ ) 6.3 Hz, PC(CH3)3, 18H). 13C{1H}
v
v
NMR (C6D6): 27.37 (vt, J ) 7.9 Hz, CH2, 2C), 29.68 (vt, J )
v
3.5 Hz, PC(CH3)3, 6C), 30.25 (vt, J ) 3.1 Hz, PC(CH3)3, 6C),
33.02 (vt, J ) 8.6 Hz, PC(CH3)3, 2C), 33.58 (vt, J ) 12.4 Hz,
v
v
v
PC(CH3)3, 2C), 38.80 (vt, J ) 10.3 Hz, CH2, 2C), 60.39 (t, J )
6.9 Hz, CH, 1C). 31P{1H} NMR (C6D6): 108.8 (s).

608 Organometallics, Vol. 25, No. 3, 2006
Castonguay et al.
were done using SAINT.28 An empirical absorption correction,
based on the multiple measurements of equivalent reflections, was
applied using the program SADABS.29 The space group was
confirmed by XPREP routine30 in the program SHELXTL.31 The
absolute configuration of the centrosymmetric crystal was deter-
mined using the FALCK parameters. The structures were solved
by direct methods and refined by full-matrix least squares and
difference Fourier techniques with SHELX-97.32 All non-hydrogen
atoms were refined with anisotropic displacement parameters.
Hydrogen atoms were set in calculated positions and refined as
riding atoms with a common thermal parameter. The autoindexing
program for twinned crystals GEMINI33 was used to resolve the
molecular structure of complex 1b. Some disorder was taken into
account for two of the t-Bu groups of complex 1c, as well as the
iodine atom (two positions).
Complete details of the X-ray analyses for complexes 1a, 1b,
1c, 4, and 5 have been deposited at The Cambridge Crystallographic
Data Centre (CCDC 283327-283729 (1), 283330 (4), 284336 (5)).
These data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by e-mailing data_request@ccdc.cam.ac.uk, or
by contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(28) SAINT, Release 6.06, Integration Software for Single-Crystal Data;
Bruker AXS Inc.: Madison, WI, 1999.
(29) Sheldrick, G. M. SADABS, Bruker Area Detector Absorption
Corrections; Bruker AXS Inc.: Madison, WI, 1999.
(30) XPREP, Release 5.10, X-ray data Preparation and Reciprocal Space
Exploration Program; Bruker AXS Inc.; Madison, WI, 1997.
(31) SHELXTL, Release 5.10, The Complete Software Package for Single
Crystal Structure Determination; Bruker AXS Inc.: Madison, WI, 1997.
(32) (a) Sheldrick, G. M. SHELXS97, Program for the Solution of Crystal
Structures; University of Gottingen: Germany, 1997. (b) Sheldrick, G. M.
SHELXL97, Program for the Refinement of Crystal Structures; University
of Gottingen: Germany, 1997.
Acknowledgment. The authors are grateful to NSERC of
Canada for financial support of this work and to Prof. D. Gusev
for his advice on the synthesis of the PCsp3P ligands.
(33) GEMINI 1.02, Release 5, 2000.
OM050844X