Organometallics 2010, 29, 2430–2445 - Chemistry - University of ...

Article Organometallics, Vol. 29, No. 11, 2010 2431
spectrum, assigned to the ortho and meta protons of the η 2 -C,C
adduct. These observations suggest rapid equilibrations of isomers
A2, A3, A4,andA5 (eq 3). As the temperature of the sample
increased to -20 °C, the η 2 -arene complexes completely disappeared,
leaving only A1. When the temperature reached 60 °C, a
new set of doublets appeared in the 31 P{ 1 H} spectrum that
belonged to the C-CN activation product A6. When the mixture
was cooled back to room temperature, none of the η 2 -arene
complexes were observed to reappear in the NMR spectrum
(Supporting Information, Figure SI-1).
Figure 1. ORTEP drawing of [Ni(dippe)H] 2 . Ellipsoids are
shown at the 50% probability level. Selected bond lengths (Å):
Ni(1)-Ni(1)A, 2.3737(5); Ni(1)-H(1), 1.64(6); Ni(1)-H(2),
1.51(7); Ni(1)-P(1), 2.1291(6); Ni(1)-P(2), 2.1212(6). Selected
angles (deg): P(1)-Ni(1)-P(2), 91.78(2); P(2)-Ni(1)-Ni(1)A,
136.28(2); P(2)-Ni(1)-H(1), 106(2); H(1)-Ni(1)-H(2), 76(3).
were undertaken to examine the energetics, intermediates,
and reaction pathways of the C-CN bond activation. Again,
it was found that formation of an η 2 -arene complex is crucial
prior to C-C bond cleavage. This complex is fluxional and
can exist as a stable species in reactions with polycyclic
aromatic nitriles. The transition states leading to C-CN
oxidative addition products showed very similar geometries
to that seen for PhCN, 7 but an additional fluxional pathway
has been identified.
Results and Discussion
Structure of the [Ni(dippe)H] 2 Complex. The red dimer
[Ni(dippe)H] 2 is readily prepared from the reaction of Ni-
(dippe)Cl 2 and LiHBEt 3 . 8 Recrystallization from hexanes
allowed the X-ray structure of the nickel dimer to be
obtained (Figure 1). At each Ni center, two P atoms and
one bridging H atom are essentially planar, with the second
bridging H atom out of the plane. The twist angle between the
P1-Ni1-P2 and P1A-Ni1A-P2Aplanesis80° and compares
with values of 80° in [Ni(dtbpe)H] 2 , 9 63° in [Ni(dcypp)H] 2 , 10 and
75° in [Ni(dippp)H] 2 . 11 The angle between the Ni1-H1-Ni1A
and Ni1-H2-Ni1A planes is approximately 156°. The distance
between the two Ni atoms is 2.37 A˚ , indicating the presence of a
Ni-Ni bond between the two formally Ni I centers. The Ni I -Ni I
distances in the other hydride dimers average 2.44 Å.
Reactions with Dicyanobenzenes. The reactionof [Ni(dippe)H] 2
with 1,2-dicyanobenzene at -60 °C initially formed both
η 2 -nitrile (A1)andη 2 -arene complexes (eq 2, A2-A5). The major
product A1 showed a pair of doublets in the 31 P{ 1 H} NMR
spectrum at δ 62.7 and 73.5 (J P-P = 62 Hz). The minor product
showed a singlet at δ 57.0 in the 31 P{ 1 H} NMR spectrum and two
broad upfield peaks at δ 5.83 (2H) and 6.28 (2H) in the 1 HNMR
(8) Vicic, D. A.; Jones, W. D. J. Am. Chem. Soc. 1997, 119, 10855.
Compare to [Ni(dcype)H] 2 : Jonas, K.; Wilke, G. Angew. Chem., Int. Ed.
Engl. 1970, 9, 312.
(9) Bach, I.; Goddard, R.; Kopiske, C.; Seevogel, K.; P€orschke, K.-R.
Organometallics 1999, 18, 10.
(10) Barnett, B. L.; Kruger, C.; Tsay, Y. H.; Summerville, R. H.;
Hoffmann, R. Chem. Ber. 1977, 110, 3900.
(11) Fryzuk, M. D.; Clentsmith, G. K. B.; Leznoff, D. B.; Rettig, S. J.;
Geib, S. J. Inorg. Chim. Acta 1997, 265, 169.
Upon adding a second equivalent of [Ni(dippe)H] 2 to A1,
a new pair of broad doublets appeared in the 31 P{ 1 H} NMR
spectrum (C 6 D 6 )atδ 62.0 and 72.5 (J P-P = 70 Hz), which
was assigned to a bis-η 2 -nitrile complex (A7). 6b After sitting
at room temperature for 1 h, a new species (A8) appeared
in the 31 P{ 1 H} NMR spectrum with characteristics of two
different Ni centers, each displaying a pair of doublets, both
Ni(II) (δ 63.4 and 77.5, J P-P = 25 Hz) and Ni(0) (δ 63.1 and
74.5, J P-P = 71 Hz) within the same molecule, indicating
only one C-CN bond in A7 was cleaved (eq 4). About 50%
A6 was also observed. Single-crystal structures of A6 and A8
were obtained as shown in Figure 2.
A computational study was undertaken to examine the
fluxionality of the η 2 -arene complexes and the pathway
leading to the C-CN oxidative addition product. The
[Ni(dmpe)] fragment was used as a model of the active
species [Ni(dippe)] in the experiment. 12 Initially the ground
(12) Atesin, T. A.; Li, T.; Jones, W. D. J. Am. Chem. Soc. 2007, 129,
7562.