Reversible cleavage of carbon-carbon bonds in benzonitrile using nickel(0)

Article abstract of DOI:10.1021/om0008474

The nickel(0) fragment [(dippe)Ni] has been found to π-coordinate to the CN bond of benzonitrile and undergo reversible insertion into the Ph-CN bond.

Full text of DOI:10.1021/om0008474

5544
Organometallics 2000, 19, 5544-5545
Rever sible Clea va ge of Ca r bon -Ca r bon Bon d s in
Ben zon itr ile Usin g Nick el(0)
J uventino J . Garcia† and William D. J ones*,‡
Facultad de Quı´mica, Universidad Nacional Autonoma de Mexico, Mexico, D.F. 04510,
Mexico, and Department of Chemistry, University of Rochester, Rochester, New York 14627
Received October 3, 2000
Summary: The nickel(0) fragment [(dippe)Ni] has been
found to π-coordinate to the CN bond of benzonitrile and
undergo reversible insertion into the Ph-CN bond.
The cleavage of a variety of strong C-X σ-bonds,
including C-H, C-F, C-S, and C-C, has been ac-
complished using low-valent transition metals.1 Of
these, the last bond remains a challenge in substrates
that do not contain ring strain or proximity effects to
bring about the bond cleavage. We report here an
example of C-CN bond cleavage that is both efficient
and reversible.
The nickel dimer [(dippe)NiH]2 has been reported to
be capable of cleaving the C-S bond in a variety of
thiophenes2 and the C-C bond in biphenylene.3 The
hydrido dimer serves as a source of the nickel(0)
fragment [Ni(dippe)], which undergoes oxidative addi-
tion to a variety of substrates. We have discovered that
the reaction of [(dippe)NiH]2 with benzonitrile in THF-
d8 solution leads to the rapid formation of the η2-nitrile
complex (dippe)Ni(η2-NCPh) (1). The solution of the
F igu r e 1. ORTEP drawing of (dippe)Ni(η2-NCPh) (1).
Ellipsoids are shown at the 30% level. Selected distances
(Å) and angles (deg): N(1)-C(1) ) 1.225(6), Ni(1)-N(1) )
1.908(3), Ni(1)-C(1) ) 1.867(4), C(1)-C(2) ) 1.475(6);
N(1)-C(1)-C(2) ) 136.1(4).
shift of the benzonitrile CtN carbon to δ 169.2 (dd, J
) 29.1, 8.8 Hz), compared to δ 119 in the free nitrile.
These data are consistent with an in-plane coordination
of the benzonitrile ligand.
Crystals of 1 were formed by cooling a solution to -30
°C, and the single-crystal X-ray structure was deter-
mined at -80 °C (Figure 1).5 The three-coordinate
Ni(0) complex is essentially planar, with the C and N
atoms of the benzonitrile ligand completing the corners
of the square plane. Other P2Ni0-π-alkyne complexes
are known to adopt a similar geometry.6 The N1-C1-
C2 angle of 150.9° is indicative of a significant amount
of π-back-bonding to the CN group and, as such, adds
enough d8 Ni(II) metallaazacyclopropane character to
the complex to account for the preference for a square-
planar structure. The back-bonding is also evident in
the lengthening of the N1-C1 bond (1.225(6) Å). Also
to be noted is that the phenyl group is coplanar with
the Ni1-N1-C1 plane and the square plane of the
complex, suggesting an electronic preference for this
wine red dihydride complex becomes yellow as the
reaction occurs, and the product can be isolated by
crystallization at low temperature. The 31P NMR spec-
trum (162 MHz) of the product displays two slightly
broadened doublets (δ 66.4 and 78.6) with J P-P ) 68
Hz characteristic of an asymmetric Ni(0) complex. The
1H NMR spectrum shows four distinct methyl reso-
nances for the two types of isopropyl groups, along with
two methylene multiplets and three resonances for the
phenyl ring.4 The IR spectrum in THF solution shows
νC-N at 1745 cm-1, reduced from the free ligand value
of 2235 cm-1. The 13C NMR spectrum shows a downfield
(4) For 1: 1H NMR (THF-d8) δ 1.05-1.18 (m, 20 H), 1.23-1.29 (dd,
4 H), 1.61-1.72 (m, 4 H), 2.09-2.17 (m, 4 H), 7.27 (q, J ) 7.1 Hz, 2
H), 7.32 (t, J ) 7.0 Hz, 1 H), 7.72 (d, J ) 7.0 Hz, 2 H); 31P NMR (THF-
d8) δ 66.4 (d, J ) 68 Hz), 78.6 (d, J ) 68 Hz).
† Universidad Nacional Autonoma de Mexico.
‡ University of Rochester.
(1) For a recent review of these topics, see: Topics in Organometallic
Chemistry. Activation of Unreactive Bonds and Organic Synthesis;
Murai, S., Ed.; Springer-Verlag: Berlin, 1999.
(2) Vicic, D. A.; J ones, W. D. J . Am. Chem. Soc. 1997, 119, 10855-
10856. Vicic, D. A.; J ones, W. D. Organometallics 1998, 17, 3411-
3413. Edelbach, B. L.; Vicic, D. A.; Lachicotte, R. J .; J ones, W. D.
Organometallics 1998, 17, 4784-4794.
(3) Edelbach, B. L.; Vicic, D. A.; Lachicotte, R. J .; J ones, W. D.
Organometallics 1998, 17, 4784-4794. Edelbach, B. L.; Lachicotte, R.
J .; J ones, W. D. Organometallics 1999, 18, 4040-4049.
(5) Crystal data for the X-ray structural determination of 1: C21H37
-
NP2Ni, Mr ) 424.17, hexagonal, a ) b ) 11.3315(4) Å, c ) 30.9995(14)
Å, V ) 3447.1(2) Å3, T ) 193 K, space group P65 (No. 170), Z ) 6,
µ(Mo KR) ) 0.988 mm-1, 15 786 reflections measured, 3294 unique
(Rint ) 0.0464), which were used in calculations. The final wR2(F2)
value was 0.0919 (R1 ) 0.0412, all data).
(6) Edelbach, B. L.; Lachicotte, R. J .; J ones, W. D. Organometallics
1999, 18, 4040. Edelbach, B. L.; Lachicotte, R. J .; J ones, W. D.
Organometallics 1999, 18, 4660-4668.
10.1021/om0008474 CCC: $19.00 © 2000 American Chemical Society
Publication on Web 11/18/2000

Communications
Organometallics, Vol. 19, No. 26, 2000 5545
F igu r e 3. van’t Hoff plot for the equilibrium shown in eq
2.
F igu r e 2. ORTEP drawing of (dippe)Ni(Ph)(CN) (2).
Ellipsoids are shown at the 30% level. Selected distances
(Å) and angles (deg): N(1)-C(7) ) 1.148(3), Ni(1)-C(1) )
Keq over the temperature range 41-91 °C allowed for
the extraction of the thermodynamic parameters ∆H°
) 4.00(22) kcal/mol and ∆S° ) 10.9(6) eu from a van’t
Hoff plot (Figure 3). While a few examples of C-CN
cleavage have been reported,9 the reversibility of this
reaction is not well-documented.10 The present observa-
tions are particularly impressive, considering the strength
of the C-CN bond in benzonitrile (132.7 kcal/mol).11 η2-
Coordination of nitriles is also known.12 Control of the
reversibility might lead to improved regioselectivity in
the hydrocyanation of olefins.
1.935(2), Ni(1)-C(7)
) 1.877(3); C(1)-Ni(1)-C(7) )
89.62(10).
1
geometry. The H NMR spectrum, however, shows no
evidence for hindered rotation around the C1-C2 bond.
If a THF solution of 1 is allowed to stand at room
temperature, a new product is observed to grow in over
several days as the solution turns a pale yellow. The
new product 2 has symmetry similar to that of 1, as
the 1H and 31P NMR spectra also display similar
patterns of resonances.7 The coupling constant J P-P in
the 31P NMR spectrum is now only 20.6 Hz, and the IR
spectrum of 2 shows νC-N at 2108 cm-1. The 13C NMR
spectrum shows the CtN π-coordinated to nickel at δ
138.1 (dd, J ) 80.5, 30.2 Hz). Crystals of 2 were isolated
at room temperature, and a single-crystal X-ray struc-
ture showed the compound to be the C-C cleavage
product (dippe)Ni(Ph)(CN) (Figure 2).8 The C7-N1
distance is only 1.148(3) Å, and the C1-Ni-C7 bond
angle is 89.6(1)°. Note that the phenyl group now rotates
to be perpendicular to the square plane of the complex.
Interestingly, the formation of 2 from 1 did not go to
completion. This observation suggested that perhaps the
reaction was reversible and equilibrium had been
reached (eq 2). Indeed, when isolated crystals of pure 2
Ack n ow led gm en t is made to the U.S. Department
of Energy, Grant No. FG02-86ER13569, to CONACYT,
and to DEGAPA-UNAM for their support of this work.
Su p p or tin g In for m a tion Ava ila ble: Tables giving crys-
tallographic data, intramolecular distances and angles, and
positional and thermal parameters for 1 and 2. This material
is available free of charge via the Internet at http://pubs.acs.org.
OM0008474
(9) For the cleavage of C-CN bonds, see: Abba, M.; Yamamoto, T.
J . Organomet. Chem. 1997, 532, 267. Favero, G.; Movillo, A.; Turco,
A. J . Organomet. Chem. 1983, 241, 251. Morvillo, A.; Turco, A. J .
Organomet. Chem. 1981, 208, 103. Parshall, G. W. J . Am. Chem. Soc.
1974, 96, 2360. Churchill, D.; Shin, J . H.; Hascall, T.; Hahn, J . M.;
Bridgewater, B. M.; Parkin, G. Organometallics 1999, 18, 2403.
Gerlach, D. H.; Kane, A. R.; Parshall, G. W.; J esson, J . P.; Muetterties,
E. L. J . Am. Chem. Soc. 1971, 93, 3543. Burmeister, J . L.; Edwards,
L. M. J . Chem. Soc. A 1971, 1663.
(10) For the formation of C-CN bonds, see: Favero, G.; Gaddi, M.;
Morvillo, A.; Turco, A. J . Organomet. Chem. 1978, 149, 395. Cassar,
L. J . Organomet. Chem. 1973, 54, C57. Tsuji, Y.; Kusui, T.; Kojima,
T.; Sugiura, Y.; Yamada, N.; Tanaka, S.; Ebihara, M.; Kawamura, T.
Organometallics 1998, 17, 4835. Luo, F.-H.; Chu, C.-I.; Cheng, C.-H.
Organometallics 1998, 17, 1025. Huang, J .; Haar, C. M.; Nolan, S. P.;
Marcone, J . E.; Moloy, K. G. Organometallics 1999, 18, 297. Marcone,
J . E.; Moloy, K. G. J . Am. Chem. Soc. 1998, 120, 8527.
(11) Calculated using ∆Hf data from: Afeefy, H. Y.; Liebman, J . F.;
Stein, S. E. Neutral Thermochemical Data. In NIST Chemistry
WebBook; NIST Standard Reference Database 69; Mallard, W. G.,
Linstrom, P. J ., Eds.; National Institute of Standards and Technol-
ogy: Gaithersburg, MD 20899 (http://webbook.nist.gov), 2000.
(12) For η2-nitriles, see: Barrera, J .; Sabat, M.; Harman, W. D. J .
Am. Chem. Soc. 1991, 113, 8178. Chetcuti, P. A.; Knobler, C. B.;
Hawthorne, M. F. Organometallics 1988, 7, 650. Chetcuti, P. A.;
Knobler, C. B.; Hawthorne, M. F. Organometallics 1986, 5, 1913.
Zecchin, S.; Zotti, G.; Pilloni, G. Inorg. Chim. Acta 1979, 33, L117.
Storhoff, B.; Infante, A. J . Inorg. Chem. 1974, 13, 3043. Payne, D. H.;
Frye, H. Inorg. Nucl. Chem. Lett. 1973, 9. 505. Bland, W. J .; Kemmitt,
R. D. W.; Moore, R. D. J . Chem. Soc., Dalton Trans. 1973, 1292.
Krogmann, K.; Mattes, R. Angew. Chem., Int. Ed. Engl. 1966, 5, 1046.
Wright, T. C.; Wilkinson, G.; Motevalli, M.; Hursthouse, M. B. J . Chem.
Soc., Dalton Trans. 1986, 2017.
were redissolved in THF-d8, 1 was seen to be regener-
ated at the expense of 2. The equilibrium constant Keq
was found to be ∼1 at 91 °C. The ratio of 1 to 2 was
found to vary with temperature, and determination of
(7) For 2: 1H NMR (THF-d8) δ 0.92 (dd, J ) 14.7, 7.2 Hz, 6 H), 1.14
(dd, J ) 13.7, 7.1 Hz, 6 H), 1.27 (dd, J ) 13.3, 6.8 Hz, 6 H), 1.45 (dd,
J ) 15.7, 7.2 Hz, 6 H), 1.69-1.89 (m, 4 H), 2.06-2.14 (m, 2 H), 2.37-
2.43 (m, 2 H), 6.67 (t, J ) 7.1 Hz, 1 H), 6.87 (pt, J ) 7.1 Hz, 2 H), 7.34
(pt, J ) 5.6 Hz, 2 H); 31P NMR (THF-d8) δ 71.2 (d, J ) 20.6 Hz), 81.7
(d, J ) 20.6 Hz).
(8) Crystal data for the X-ray structural determination of 2: C21H37
-
NP2Ni, Mr ) 424.17, orthorhombic, a ) 15.9459(6) Å, b ) 14.2501(6)
Å, c ) 19.9039(8) Å, V ) 4522.8(3) Å3, T ) 193 K, space group Pbca
(No. 61), Z ) 8, µ(Mo KR) ) 1.004 mm-1, 19 318 reflections measured,
3253 unique (Rint ) 0.0409), which were used in calculations. The final
wR2(F2) value was 0.0675 (R1 ) 0.0450, all data).