Probing the interactions of transition-metal complexes bound at the ortho position of substituted biphenyls

Article abstract of DOI:10.1021/om034363u

(4,4′-dimethylbipyridyl)NiMe₂ was found to react thermally with 2-iodo-2′,4′,6′-trimethylbiphenyl (6) to produce a C - I insertion that is extremely distorted from square planarity, with the C - Ni - I plane intersecting the N - Ni - N plane by 33.4°. It was also found that [(dippe)EhCl]₂ reacts with 6 to produce an η³-benzylic complex by way of a rhodium migration to the methylated ring of the biphenyl unit.

Full text of DOI:10.1021/om034363u

Organometallics 2004, 23, 3071-3074
3071
Notes
P r obin g th e In ter a ction s of Tr a n sition -Meta l Com p lexes
Bou n d a t th e Or th o P osition of Su bstitu ted Bip h en yls
Gavin D. J ones, Thomas J . Anderson, Ninca Chang, R. J acob Brandon,
Grace L. Ong, and David A. Vicic*
Department of Chemistry and Biochemistry, University of Arkansas,
Fayetteville, Arkansas 72701
Received December 11, 2003
Summary: (4,4′-dimethylbipyridyl)NiMe2 was found to
react thermally with 2-iodo-2′,4′,6′-trimethylbiphenyl (6)
to produce a C-I insertion that is extremely distorted
from square planarity, with the C-Ni-I plane intersect-
ing the N-Ni-N plane by 33.4°. It was also found that
[(dippe)RhCl]2 reacts with 6 to produce an η3-benzylic
complex by way of a rhodium migration to the methyl-
ated ring of the biphenyl unit.
activation. If one could arrest the attachment of a metal
to the 2-position of a biphenyl, more detailed informa-
tion concerning the interactions of a metal with the
organic ligand could be obtained. One obvious modifica-
tion of a biphenyl substrate that would remove any
proximal C-H bonds is the replacement of the 2′,6′-aryl
hydrogens with methyl groups. Here we report the
reactivity of 2-iodo-2′,4′,6′-trimethylbiphenyl (6) with a
group 10 metal and describe some intimate details about
the metal environment upon activation of the C-I bond
of this sterically demanding substrate. Additionally we
show that, depending on accessible oxidation states,
metal migrations can still occur despite the absence of
any available aryl C-H bonds.
In tr od u ction
Biphenyls substituted at the position ortho to the
aryl-aryl linkage are important synthetic precursors,
as they have recently been used to prepare higher order
polycycles. Larock, in particular, has developed pal-
ladium-catalyzed methods that can convert o-iodobiaryls
such as 1 to a number of fused polycycles proceeding
through oxidative addition of the C-I bond, followed by
palladium migrations via C-H bond activation and
subsequent C-C bond coupling to afford fused poly-
cycles such as 2 (eq 1).1 This palladium migration
Resu lts a n d Discu ssion
Reaction of (bpy′)NiMe2 (7; bpy′ ) 4,4′-dimethylbi-
pyridyl) with 6 for 2.5 h at 50 °C in benzene led to a
complete color change of the solution from dark green
to pink with formation of the C-I insertion product 8
(eq 3). Analysis of this new product by NMR spectros-
chemistry has also been observed during the palladium-
catalyzed olefination of the asymmetrical o-iodobiphenyl
3 to afford the mixture of Heck products 4 and 5 (eq
2).2 Of note was the unexpected migration of the
palladium metal to the opposite ring of the biphenyl
substrate. Clearly, the close proximity of aryl C-H
bonds to transition metals bound in the 2-position of
biaryls facilitates, and even promotes, a C-H bond
activation step that normally would not readily occur.
We wondered what would happen if a transition
metal, bound to the 2-position of a biphenyl substrate,
did not have any readily available aryl C-H bonds for
* To whom correspondence should be addressed. E-mail: dvicic@
uark.edu.
(1) Campo, M. A.; Huang, Q.; Yao, T.; Tian, Q.; Larock, R. C. J . Am.
Chem. Soc. 2003, 125, 11506-11507.
(2) Campo, M. A.; Larock, R. C. J . Am. Chem. Soc. 2002, 124,
14326-14327.
10.1021/om034363u CCC: $27.50 © 2004 American Chemical Society
Publication on Web 05/12/2004

3072 Organometallics, Vol. 23, No. 12, 2004
Notes
F igu r e 2. Alternate view of 8, showing the distortion of
the square plane around the nickel atom.
The structure of 8 is also interesting in terms of the
distortion from square planarity that ensues upon C-I
bond activation (Figure 2). The C1-Ni1-I1 plane
intersects the N1-Ni1-N2 plane by 33.4°, which is to
our knowledge the largest deviation from the square
plane shown by a four-coordinate NiII complex contain-
ing a Ni-C bond. Tetrahedral NiII organometallic
complexes are attractive synthetic targets, as high-spin
methylnickel intermediates have been postulated in the
A cluster of acetyl-coenzyme A synthase.3-7 However,
recent attempts at preparing high-spin NiII methyl
complexes have proved unsuccessful.8,9
Interestingly, in the solution phase, complex 8 was
found to adopt a symmetrical conformation. Three
methyl resonances could be observed in the 1H NMR
spectrum of 8, along with a total of eight broad signals
for aromatic protons. Assuming that rotation of the
aryl-aryl bond is restricted by geometrical constraints,
these solution data are at least consistent with a
tetrahedral arrangement of the ligands around nickel,
with the methylated ring of the biphenyl symmetrically
overlapping the bpy′ ligand. Although a tetrahedral NiII
complex would formally be high spin and paramagnetic,
it is not uncommon for paramagnetic nickel compounds
to show broad, but distinguishable, resonances in the
1H NMR spectra.10 The solution data could also be
consistent with equilibration of the iodide ligand to
ether side of the metal via the fast equilibrium process
outlined in eq 4. Such a process would produce sym-
metrical spectra similar to those seen for η3-benzyl and
allyl cation complexes of nickel11,12 and would not
require full rotation around the aryl-aryl bond. To
investigate further the possibility of a high-spin, tetra-
F igu r e 1. ORTEP diagrams of 8, showing both a side-on
view relative to the bpy′ ligand (top structure) and an
overhead view (bottom structure). Ellipsoids are shown at
the 50% level, with hydrogens omitted for clarity. Selected
bond lengths (Å): I(1)-Ni(1) ) 2.4793(6), Ni(1)-N(1) )
1.915(3), Ni(1)-N(2) ) 1.962(3), Ni(1)-C(1) ) 1.890(3).
Selected bond angles (deg): N(1)-Ni(1)-N(2) ) 82.8(1),
N(1)-Ni(1)-C(1) ) 94.6(1), N(1)-Ni(1)-I(1) ) 154.29(7),
N(2)-Ni(1)-C(1) ) 158.8(1), N(2)-Ni(1)-I(1) ) 99.2(1),
C(1)-Ni(1)-I(1) ) 92.2(1).
copy reveals that solutions of 8 can only be kept for a
limited time, as the C-I insertion product decomposes
readily into unidentified material. However, layering a
cold THF solution of 8 with pentane at -30 °C afforded
crystalline material fit for X-ray analysis, and the
ORTEP diagrams are shown in Figure 1.
(3) Wang, H.; Ralston, C. Y.; Patil, D. S.; J ones, R. M.; Gu, W.;
Verhagen, M.; Adams, M.; Ge, P.; Riordan, C.; Marganian, C. A.;
Mascharak, P.; Kovacs, J .; Miller, C. G.; Collins, T. J .; Brooker, S.;
Croucher, P. D.; Wang, K.; Steifel, E. I.; Cramer, S. P. J . Am. Chem.
Soc. 2000, 122, 10544-10552.
Compound 8 exhibits a number of interesting struc-
tural features in the solid state. The methylated ring
of the biphenyl ligand lies almost parallel to the bpy′
ligand, with the two aromatic moieties assuming a novel
“stacking”-type conformation (Figure 1). One can infer
from the different views of the solid-state structure how
a metal complex which has activated the C-I bond of
an o-iodobiphenyl is in a particularly good position to
activate the 2′- and 6′-positions of the biphenyl sub-
strate. Simple rotation of the biaryl C-C bond is all that
is needed to place any substituent on those positions in
close proximity to the metal center.
(4) Tan, X.; Sewell, C.; Yang, Q.; Lindahl, P. A. J . Am. Chem. Soc.
2003, 125, 318-319.
(5) Seravalli, J .; Kumar, M.; Ragsdale, S. W. Biochemistry 2002, 41,
1807-1819.
(6) Loke, H.-K.; Tan, X.; Lindahl, P. A. J . Am. Chem. Soc. 2002,
124, 8667-8672.
(7) Lindahl, P. A. Biochemistry 2002, 41, 2097-2105.
(8) Holland, P. L.; Cundari, T. R.; Perez, L. L.; Eckert, N. A.;
Lachicotte, R. J . J . Am. Chem. Soc. 2002, 124, 14416-14424.
(9) Schebler, P. J .; Mandimutsira, B. S.; Riordan, C. G.; Liable-
Sands, L. M.; Incarvito, C. D.; Rheingold, A. L. J . Am. Chem. Soc. 2001,
123, 331-332.
(10) Mindiola, D. J .; Waterman, R.; J enkins, D. M.; Hillhouse, G.
L. Inorg. Chim. Acta 2003, 345, 299-308.

Notes
Organometallics, Vol. 23, No. 12, 2004 3073
hedral nickel species, the magnetic susceptibility of 8
in benzene solution was determined by the Evans NMR
method.13 Large magnetic susceptibilities were found
(consistently greater than µeff ) 3.0 µB), but it is
extremely difficult to interpret these data, as 8 is not
completely stable in the solution phase (see above).
We have also found that, depending on the identity
of the transition metal bound to the o-iodobiphenyl,
rearrangements can still occur despite the absence of
any available aryl C-H bonds. Upon reaction of [(dippe)-
RhCl]2 (9; dippe ) 1,2-bis-(diisopropylphosphino)ethane)
with the lithiated biphenyl 10, the expected product 11
is not observed (eq 5). Rather, complex 13, which
F igu r e 3. ORTEP diagram of 13. Ellipsoids are shown at
the 50% level. Hydrogens are omitted for clarity. Selected
bond lengths (Å): Rh-P(1) ) 2.210(4), Rh-P(2) )
2.234(3), Rh-C(8) ) 2.21(1), Rh-C(9) ) 2.35(2), Rh-C(13)
) 2.17(1). C(8)-C(9) ) 1.41(2), C(8)-C(13) ) 1.43(2).
Selected bond angles (deg): P(1)-Rh-P(2) ) 86.8(1), P(1)-
Rh-C(8) ) 132.9(3), P(1)-Rh-C(9) ) 166.3(3), P(1)-Rh-
C(13) ) 101.1(4), P(2)-Rh-C(8) ) 133.2(3), P(2)-Rh-C(9)
) 106.9(3), P(2)-Rh-C(13) ) 171.2(4), C(8)-Rh-C(9) )
35.9(4), C(8)-Rh-C(13) ) 38.0(5), C(9)-Rh-C(13) )
65.3(5).
Ta ble 1. Cr ysta l P a r a m eter s for Com p ou n d s 8 a n d
13
8
13
chem formula
formula wt
C
27H27IN2Ni
C29H47P2Rh
560.52
565.13
cryst dimens (mm)
color, habit
cryst syst
0.20 × 0.25 × 0.30 0.10 × 0.20 × 0.25
red, prism
triclinic
0.710 70
21.26
orange, prism
triclinic
0.710 70
7.41
wavelength, Å
µ (cm-1
)
max-min transmissn 0.653-0.498
1.0000-0.4432
P1h, 2
space group, Z
a, Å
b, Å
P1h, 2
8.854(13)
11.312(13)
12.50(2)
78.76(12)
78.19(12)
80.52(9)
1191.7(29)
1.574
8.86(10)
8.97(17)
18.26(18)
98.4(5)
95.920(12)
101.5(3)
1393.7(34)
1.336
c, Å
R (deg)
â (deg)
γ (deg)
V, Å3
F
calcd, g cm-1
temp, °C
-100
-100
R indices (I > 2σ(I))
R indices (all data)
goodness of fit
2θ range, deg
no. of data collected
no. of unique data
Rint
0.0445, 0.0765
0.0697, 0.0834
1.125
4.58-55.7
8084
0.1384, 0.3014
0.1751, 0.3214
1.217
4.56-55.7
8335
5477
0.0235
5856
0.1225
contains a rhodium atom bound to the methylated
biphenyl ring, was the only apparent new product. A
reasonable mechanism for this transformation is likely
to involve activation of a benzylic C-H bond to produce
the rhodium hydride 12, followed by reductive elimina-
tion of the stronger aryl C-H bond to afford 13. The
migration process is extremely fast, as no hydride
signals could be detected by 1H NMR spectroscopy. Such
a benzylic C-H bond activation process is similar to
those recently reported by Catellani, who found that a
constrained palladium complex can also activate ortho-
substituted aryls, although an η1- instead of an η3-
metalated product was formed.14
Compound 13 was also structurally characterized
(Table 1), and although evidence for twinning was
found, all atoms could be refined anisotropically. The
ORTEP diagram of the new product is shown in Figure
3. X-ray analysis reveals that, upon migration to the
benzylic carbon atom, the rhodium atom prefers to bind
(11) Lee, B. Y.; Bazan, G. C.; Vela, J .; Komon, Z. J . A.; Bu, X. J .
Am. Chem. Soc. 2001, 123, 5352-5353.
(12) Carmona, E.; Palma, P.; Poveda, M. L. Polyhedron 1990, 9,
757-761.
(14) Catellani, M.; Motti, E.; Ghelli, S. Chem. Commun. 2000, 2003-
2004.
(13) Sur, S. K. J . Magn. Reson. 1989, 82, 169-173.

3074 Organometallics, Vol. 23, No. 12, 2004
Notes
Syn th esis of (bp y′)Ni((η1-2)-2′,4′,6′-tr im eth ylbip h en yl)-
(I) (8). 7 (100 mg, 0.37 mmol) and 1 equiv of 6 were dissolved
in 20 mL of benzene and heated to 50 °C for 2.5 h. The benzene
was then removed on a vacuum line, and the residue was
washed with pentane and filtered, leaving a burgundy solid.
Yield: 32%. 1H NMR (THF-d8): δ 8.31 (br s, 2H), 7.83 (s, 2H),
7.56 (d, J ) 6.30 Hz, 1H), 7.14 (br s, 2H), 6.83 (t, J ) 6.69 Hz,
1H), 6.70 (t, J ) 6.21 Hz, 1H), 6.55 (s, 2H), 6.47 (d, J ) 6.74
Hz, 1H), 2.37 (s, 6H), 2.02 (s, 3H), 1.73 (s, 6H). Anal. Calcd
for C27H27IN2Ni: C, 57.05 (57.38); H, 5.02 (4.82).
to the biphenyl substrate in an η3 fashion, thereby
increasing its electron count to 16 electrons. Interest-
ingly, only one other η3-benzylic complex of rhodium has
been structurally characterized, namely (η3-CH2C6Me6)-
Rh(P(O-i-Pr)3)2.15 While this phosphite complex was
prepared by addition of a hydride source to an arene
complex of rhodium, we report here that 13 can be
prepared directly from a C-H bond activation process.
Syn th esis of (d ip p e)Rh (η3-bip h en yl) (13). Butyllithium
(15.35 mL of a 1.02 M solution in hexanes) was added to a
stirred solution of 6 (5.046 g, 15.6 mmol) in pentane at 0 °C.
The solution was then warmed to room temperature and
stirred for 3 h, during which time a white precipitate formed.
The white precipitate was filtered, washed with pentane, and
dried, affording 10 in 73.4% yield. Freshly prepared 10 (130
mg, 6.4 mmol) was then added to a stirred solution of 9 (246
mg, 0.31 mmol) in 50 mL of THF and stirred at room
temperature for 1 h. The solvents were removed on a vacuum
line, and the residue was extracted with pentane and filtered.
The pentane was removed on a vacuum line, affording crude
13 (230 mg, 66%). The residue was dissolved in pentane and
cooled to -35 °C overnight, leaving pure 13 as orange crystals
Con clu sion s
A better picture is now emerging of the interactions
of transition metals, bound to the 2-position of a biaryl
unit, with substituents on the 2′- and 6′-positions of the
adjacent ring. The steric interactions are large enough
that metals with available oxidation states can activate
benzylic C-H hydrogens, affording products in which
the metal has migrated to the other arene ring. In the
absence of readily available oxidation states, the inher-
ent geometry of the biaryl ligand causes severe distor-
tion of the ligand environment of the organometallic
complex, due to the projection of the remaining arene
ring back toward the metal center.
1
(80 mg, 23% yield). H NMR (C6D6): δ 7.67 (d, J ) 7.58, 2H),
7.33-7.12 (m, 3H), 6.46 (s, 1H), 5.16 (s, 1H), 2.65 (m, 1H), 2.45
(s, 3H), 2.22-0.69 (m, 9H), 2.09 (s, 3H), 1.28 (dd, J ) 7.29,
15.42, 3H), 0.59 (dd, J ) 7.08, 15.42, 3H). 31P{1H} NMR
(C6D6): δ 97.80 (dd, J Rh-P ) 242.5, J P-P ) 18.5 Hz), 89.09 (dd,
J Rh-P ) 177.1, J P-P ) 19.6 Hz). Anal. Calcd for C29H47P2Rh:
C, 62.14 (61.75); H, 8.45 (8.51).
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All manipulations were per-
formed using standard Schlenk techniques or in a nitrogen-
filled glovebox, unless otherwise noted. Solvents were distilled
from Na/benzophenone or CaH2. All reagents were used as
received from commercial vendors unless otherwise noted.
Aluminum oxide (activated, neutral, Brockmann I, ∼150 mesh)
was dried at 200 °C under vacuum for 2 days prior to use.
Elemental analyses were performed by Desert Analytics. 1H
NMR spectra were recorded at ambient temperature (unless
otherwise noted) on a Bruker Avance 300 MHz spectrometer
and referenced to residual proton solvent peaks. 31P{1H} NMR
spectra were recorded on the Bruker Avance spectrometer
operating at 121.44 MHz and referenced to an 85% phosphoric
acid external standard set to 0 ppm. A Rigaku MSC Mercury/
AFC8 diffractometer was used for X-ray structural determina-
tions. 6,16 7,17 and 918,19 were prepared by previously published
procedures.
Ack n ow led gm en t. D.A.V. thanks the University of
Arkansas, the NSF (Grant No. CHE-REU 0243978), the
Arkansas Biosciences Institute, and the NIH (Grant No.
RR-15569) for support of this work. Lee M. Daniels of
Rigaku/MSC, Inc., is acknowledged for help with ana-
lyzing the X-ray data for compound 13.
Su p p or tin g In for m a tion Ava ila ble: Crystal structure
analysis reports, along with tables giving all X-ray data; these
data are also available as CIF files. This material is available
free of charge via the Internet at http://pubs.acs.org.
OM034363U
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J ones, T.; Albright, T. A. J . Am. Chem. Soc. 1989, 111, 5709-5721.
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