Directing aryl-I versus aryl-Br bond activation by nickel via a ring walking process

Article abstract of DOI:10.1021/ic702289n

Activation of a strong aryl-Br bond of a halogenated vinylarene by nickel(0) is demonstrated in the presence of aryl-I containing substrates. η²-Coordination of Ni(PEt₃)₂ to the C=C moiety of halogenated vinylarenes is kinetically preferable and is followed by an intramolecular aryl-halide bond activation process. This ring- walking process is quantitative and proceeds under mild reaction conditions in solution. Mechanistic studies indicate that the metal insertion into the aryl-halide bond is not the rate-determining step. The reaction obeys first-order kinetics in the η²-coordination complexes with almost identical activation parameters for Br and I derivatives. The ring-walking process is kinetically accessible as shown by density functional theory (DFT) calculations at the PBE0/SDB-cc-pVDZ//PBE0/SDD level of theory.

Full text of DOI:10.1021/ic702289n

Inorg. Chem. 2008, 47, 5114-5121
Directing Aryl-I versus Aryl-Br Bond Activation by Nickel via a Ring
Walking Process
Olena V. Zenkina,† Amir Karton,† Dalia Freeman,† Linda J. W. Shimon,‡ Jan M. L. Martin,*,†
and Milko E. van der Boom*,†
Departments of Organic Chemistry and Chemical Research Support, The Weizmann Institute of
Science, 76100 RehoVot, Israel
Received November 21, 2007
Activation of a strong aryl-Br bond of a halogenated vinylarene by nickel(0) is demonstrated in the presence of
2
aryl-I containing substrates. η -Coordination of Ni(PEt3)2 to the CdC moiety of halogenated vinylarenes is kinetically
preferable and is followed by an intramolecular aryl-halide bond activation process. This “ring-walking” process is
quantitative and proceeds under mild reaction conditions in solution. Mechanistic studies indicate that the metal
insertion into the aryl-halide bond is not the rate-determining step. The reaction obeys first-order kinetics in the
2
η -coordination complexes with almost identical activation parameters for Br and I derivatives. The ring-walking
process is kinetically accessible as shown by density functional theory (DFT) calculations at the PBE0/SDB-cc-
pVDZ//PBE0/SDD level of theory.
Introduction
date, selective activation of relatively strong chemical
bonds in the presence of weaker ones is extremely
rare.5,19–25 In particular, possible routes to selectively
activate distinctly different aryl-halide bonds may be
available by unimolecular reaction channels, which direct
the metal center to the desired functional group as shown
by van Koten et al.24,25 We recently demonstrated that the
reaction of Pt(PEt3)4 with a halogenated vinylarene or azoben-
zene system results in selective η2-CdC or η2-NdN coordina-
tion, respectively, followed by “ring-walking” of the metal
Metal-mediated aryl-halide bond activation is a key
step in many carbon-carbon bond formation processes;
examples include Stille, Suzuki, Heck, and Sonogashira
reactions.1–5 Numerous experimental and theoretical stud-
ies have addressed the intriguing mechanism underlying
aryl-halide oxidation addition by group 10 metals in
solution.6–18 Regardless of the tremendous progress made
in this field, many challenging open questions remain. To
* To whom correspondence should be addressed. E-mail: gershom@
weizmann.ac.il (J.M.L.M.), milko.vanderboom@weizmann.ac.il (M.E.B.).
Fax: +972-8-934-4142.
(14) Sundermann, A.; Uzan, O.; Martin, J. M. L. Chem.sEur. J. 2001, 7,
1703.
†
Department of Organic Chemistry.
(15) Hartwig, J. F. Angew. Chem., Int. Ed. 1998, 37, 2046.
(16) Ohff, M.; Ohff, A.; van der Boom, M. E.; Milstein, D. J. Am. Chem.
Soc. 1997, 119, 11687.
‡
Department of Chemical Research Support.
(1) 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, 2006.
(17) Zenkina, O.; Altman, M.; Leitus, G.; Shimon, L. J. W.; Cohen, R.;
van der Boom, M. E. Organometallics 2007, 26, 4528.
(18) Strawser, D.; Karton, A.; Zenkina, O. V.; Iron, M. A.; Shimon,
L. J. W.; Martin, J. M. L.; van der Boom, M. E. J. Am. Chem. Soc.
2005, 127, 9322.
(2) Negishi, E. I. Handbook of Organopalladium Chemistry for Organic
Synthesis; Wiley: New York, 2002.
(3) Hegedus, L. S. Transition Metals in the Synthesis of Complex Organic
Molecules; University Science Books: Mill Valley, CA, 1994.
(4) Netherton, M. R.; Fu, G. C. AdV. Synth. Catal. 2004, 346, 1525.
(5) van der Boom, M. E.; Milstein, D. Chem. ReV. 2003, 103, 1759.
(6) Norrby, P.-O.; Ahlquist, M. Organometallics 2007, 26, 550.
(7) Lam, K. C.; Marder, T. B.; Lin, Z. Organometallics 2007, 26, 758.
(8) Lucassen, A. C. B.; Shimon, L. J. W.; van der Boom, M. E.
Organometallics 2006, 25, 3308.
(19) Espino, G.; Kurbangalieva, A.; Brown, J. M. Chem. Commun. 2007,
1742.
(20) Blum, J.; Berlin, O.; Milstein, D.; Ben-David, Y.; Wasserman, B. C.;
Schutte, S.; Schumann, H. Synthesis 2000, 4, 571.
(21) Littke, A. F.; Dai, C.; Fu, G. C. J. Am. Chem. Soc. 2000, 122, 4020.
(22) van der Boom, M. E.; Liou, S.-Y.; Ben-David, Y.; Gozin, M.; Milstein,
D. J. Am. Chem. Soc. 1998, 120, 13415.
(9) Gatard, S.; Celenligil-Cetin, R.; Guo, C. Y.; Foxman, B. M.; Ozerov,
O. V. J. Am. Chem. Soc. 2006, 128, 2808.
(23) Fahey, D. R. J. Am. Chem. Soc. 1970, 92, 402.
(24) Slagt, M. Q.; Rodriguez, G.; Grutters, M. M. P.; Gebbink, R. J. M. K.;
Klopper, W.; Jenneskens, L. W.; Lutz, M.; Spek, A. L.; van Koten,
G. Chem.sEur. J. 2004, 10, 1331.
(10) Grushin, V. V.; Marshall, W. J. J. Am. Chem. Soc. 2006, 128, 4632.
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(25) Rodriguez, G.; Albrecht, M.; Schoenmaker, J.; Ford, A.; Lutz, M.;
Spek, A. L.; van Koten, G. J. Am. Chem. Soc. 2002, 124, 5127.
5114 Inorganic Chemistry, Vol. 47, No. 12, 2008
10.1021/ic702289n CCC: $40.75  2008 American Chemical Society
Published on Web 05/17/2008

Directing Aryl-I Wersus Aryl-Br Bond ActiWation
Scheme 1. Selective η2-(CdC) Coordination of Ni(PEt3)2 to Compounds 1 and 2 is Followed by Activation of the Aryl-Halide Bond
center and rate-determining aryl-halide oxidative addition.17,18
Intramolecular pathways to arene functionalization have
synthetic utility and can be utilized to selectively activate
aryl-halide bonds.24–30 For example, Yokozawa et al.
recently observed that nickel-catalyzed chain growth po-
lymerization of poly(3-hexylthiophene) and poly(p-phe-
nylene) proceeds by intramolecular transfer of the Ni(0)
catalyst over the conjugated system, followed by selective
activation of only one of the C-Br bonds.26–28
We report here on the exclusive activation of a strong
aryl-Br bond of a halogenated vinylarene by nickel(0) in
the presence of aryl-I containing substrates. The experi-
mental findings and mechanistic interpretation are supported
by density functional theory (DFT) calculations at the PBE0/
SDB-cc-pVDZ//PBE0/SDD level of theory.
followed by intermolecular activation of the aryl-halide bond.
No free ligands (1, 2) or Ni(PEt3)x (x ) 2-4) were observed
1
during the course of the reactions by H and 31P{1H} NMR
spectroscopy, indicating that the equilibrium is shifted toward
complexes 3 and 4. Alternatively, the reaction may involve an
intramolecular metal ring-walking process.17,18,29,30,32–36 In
order to distinguish between these distinctly different pathways,
compound 2 or PhI (0.5 equiv) was added to a toluene-d8
solution containing complex 3 and 2 equiv of PEt3. Notably,
compound 2 or PhI is present in a large excess, as compared
with ligand 1 and Ni(PEt3)x, if these compounds will indeed be
generated by a dissociation process. Subsequently, in situ
31P{1H} NMR follow-up experiments at 278 K show the
exclusive formation of complex 5. No formation of complexes
4 and 6 or activation of PhI was observed (Scheme 2). PhI
undergoes oxidative addition with Ni(PEt3)4 within ∼10 min
under similar reaction conditions, whereas the complete trans-
formation of complex 3 f 5 takes ∼4.5 h. Likewise, complex
4 forms almost exclusively complex 6 in the presence of
compound 1.
The reaction of Ni(PEt3)4 with compounds 1 and 4-R-PhI
with R ) H, OMe, or CF3 (molar ratio 1:1:1) in toluene-d8
at -65 °C for 2 h, followed by stirring at room temperature
for an additional 2 h, resulted in metal insertion into the
aryl-Br bond to generate complex 5 as the major compound
(96%, R ) H; 97%, R ) OMe; 86%, R ) CF3). Metal
insertion into the aryl-I bonds was also observed by
31P{1H} NMR: 4% for NiI(C6H5)(PEt3)2, 3% for NiI(4-
MeOC6H4)(PEt3)2, and 14% for NiI(4-CF3C6H4)(PEt3)2. The
latter three complexes were identified by the addition of
authentic samples to the product solutions. 31P{1H} NMR
spectroscopy at –2 °C showed the initial quantitative
formation of complex 3. Neither continuous stirring for an
additional 48 h nor heating the product solution at 45 °C for
Results and Discussion
The reaction of an equimolar amount of Ni(PEt3)4 with
compounds 1 or 2 in dry toluene-d8 at 271 K for ∼80 min
results in the quantitative formation of complexes 3 or 4,
respectively (Scheme 1). Compounds 3 and 4 were charac-
terized in situ in the presence of 2 equiv of PEt3 by low
1
temperature H, 13C{1H}, and 31P{1H} NMR spectroscopy.
Assignments in the 13C{1H} NMR spectra were made using
13C-DEPT-135 NMR spectroscopy. Although compounds 3
and 4 could not be isolated, their spectroscopic characteristics
are nearly identical to those of isolated platinum com-
plexes.17,18 Prolonged reaction times (∼14 h) or increasing
the reaction temperature results in the quantitative formation
of complexes 5 and 6 by metal insertion into the aryl-halide
bond. For instance, this reaction goes to completion in ∼1.5
h at room temperature. The new Ni(II) complexes (5, 6) have
been isolated by evaporation of all the volatiles under vacuum
and by washing with cold pentane as yellow solids (80%),
1
and they were fully characterized by a combination of H,
13C{1H}, and 31P{1H} NMR spectroscopy, elemental analysis,
and mass spectrometry.31 In addition, complex 5 has been
characterized by single-crystal X-ray crystallography (Figure
1). Evidently, selective η2-(CdC) coordination of Ni(PEt3)2
to compounds 1 and 2 is kinetically preferable over activation
of the aryl-halide bond.
Mechanistically, the transformations of complexes 3 f 5 and
4 f 6 may proceed via reversible Ni(PEt3)2 dissociation
(26) Yokoyama, A.; Yokozawa, T. Marcomolecules 2007, 40, 4093.
(27) Miyakoshi, R.; Shimono, K.; Yokoyama, A.; Yokozawa, T. J. Am.
Chem. Soc. 2006, 128, 128.
Figure 1. ORTEP diagram of complex 5 with thermal ellipsoids set at
50% probability. Hydrogen atoms are omitted for clarity. Selected bond
lengths (Å) and angles (deg): Ni(1)-Br(1) 2.366(2), Ni(1)-P(2)
2.209(2), Ni(1)-P(3) 2.221(2), Ni(1)-C(1) 1.888(8), C(4)-C(7) 1.484(12),
C(7)-C(8) 1.317(11), C(8)-C(9) 1.500(12), C(1)-Ni(1)-Br(1) 171.5(2),
P(2)-Ni(1)-P(3) 168.02(11).
(28) Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. J. Am. Chem. Soc. 2005,
127, 17542.
(29) Keane, J. M.; Harman, W. D. Organometallics 2005, 24, 1786.
(30) Harman, W. D. Coord. Chem. ReV. 2004, 248, 853.
(31) For analogous Pd complexes, see: Burdeniuk, J.; Milstein, D. J.
Organomet. Chem. 1993, 451, 213.
Inorganic Chemistry, Vol. 47, No. 12, 2008 5115

Zenkina et al.
Scheme 2. Exclusive Formation of Complex 5 from 3 in the Presence of Compound 2 (0.5 equiv)a
a
The addition of PhI instead of compound 2 to the reaction mixture also results in exclusive formation of complex 5. Performing this reaction with
complex 4 and compound 1 results in the formation of complex 6. The experiments were performed in the presence of 2 equiv of PEt3.
Scheme 3. Directing Aryl-Halide Bond Activation via Ring-Walkinga
the metal center for electronic reasons (e.g., compare
Hamment constants for PhBr, σp ) 0.23 vs PhI, σp ) 0.18).
In situ 31P{1H} NMR follow-up measurements in the
presence of unreacted ligands 1 and 2 show the formation
of complexes 5 and 6 with similar first-order rate constants:
k(aryl-Br) ) 1.2 × 10-4 s-1 and k(aryl-I) ) 1.5 × 10-4 s-1, as
shown in Scheme 4B. The final product distribution 5:6 )
a The formation of complex 5 was observed in the presence of 4-CF3PhI.
1.5:1 is identical to the ratio of the kinetic complexes (3, 4).
The quantitative formation of complex 3 was observed by 31P{1H} NMR
Apparently, also under these reaction conditions, no ligand
spectroscopy prior to formation of complex 5.
(1, 2) scrambling occurs. The small difference between the
rate constants (k(aryl-Br)/k(aryl-I) ≈ 0.8) indicates that aryl-halide
4 h changed the product distribution, demonstrating that
complex 5 does not react with the aryl iodides to generate
bond activation is not the rate-determining step. Furthermore,
repeating this experiment with the addition of complex 3 (1
the observed minor products of aryl-I bond activation. It is
equiv) to the solution containing compounds 1 and 2 and
remarkable that even nickel insertion into the aryl-Br bond
complexes 3 and 4 (3:4 ) 1.5:1) afforded a product
occurs as a major pathway in the presence of the highly
reactive 4-CF3PhI. Apparently, the metal center is trapped,
distribution of 5:6 ) 2.5:1. It again appears that ligand (1,
2) exchange and/or dissociation does not occur, which is fully
in agreement with a ring-walking process. In addition, a
reaction of free ligands (1, 2) with complexes 3 and 4 to
afford 5 and 6 directly can also be excluded.
The Eyring plots for both transformations (3 f 5 and
does not dissociate, and is directed toward the aryl-Br
moiety because of the η2-coordination to the sCdCs moiety
of ligand 1. The formation of 5 is clearly an intramolecular
process. A reaction between two complexes 3 to afford free
ligand 1 and a bimetallic complex, followed by the formation
4 f 6) are identical within experimental errors (Figure
of 5 (from a bimetallic species), would be hampered by the
2). The very similar reaction profiles confirm that the aryl-
presence of the aryl iodides. It is plausible that PhI, 4-
halide activation process by nickel is not rate determining
MeOPhI, or 4-CF3PhI reacts with complex 3 or a transient
in the temperature range 265–285 K. This is in sharp
intermediate complex. One possible route may involve
contrast to our findings with platinum, in which a
reversible PEt3 dissociation from complex 3, followed by
reaction of the unsaturated metal complex with the aryl
iodide. Indeed, performing the abovementioned reaction of
1 + 4-CF3PhI with Ni(PEt3)4 in the presence of 10 equiv of
significant halide effect has been observed on the reaction
kinetics.18 The low entropy values, ∆S‡, (i) exclude
metal–ligand dissociation as the slow step and are
consistent with either (ii) metal–ligand dissociation fol-
PEt3 increased the yield of complex 5 from 86% to 98%, as
lowed by aryl-halide oxidative addition as the rate
determining step or (iii) an intramolecular process. The
competition experiments rule out possibilities i and ii.
Only an intramolecular process is fully in agreement with
all our observations.
31P{1H} NMR follow-up experiments were performed for
the thermolysis of complex 3 in two different solvents and
in the presence of excess PEt3 (Figure 3). The solvent polarity
(acetone vs toluene with dielectric constants of εr ) 21 vs
2.4, respectively)37 or the presence of 2–10 equiv of PEt3
does not affect the reaction kinetics. A “Meisenheimer-type”
attack of the nickel center on the aryl-halide moiety or an
judged by 31P{1H} NMR spectroscopy (Scheme 3). A similar
pathway seems to be operating for competing aryl-Br
oxidative addition reactions with platinum and an azobenzene
ligand.17 The intramolecular nature of the process 3 f 5 is
unambiguously demonstrated by the lack of metal transfer
between ligands 1 and 2 and the abovementioned aryl-Br
activation of ligand 1 as a major reaction pathway in the
presence of aryl iodides (Schemes 2 and 3).
Further insight into the reaction pathway (3 f 5, 4 f 6)
and the rate-determining step is provided by the following
competition experiments: the addition of a toluene-d8 solution
of compounds 1 and 2 (both 2 equiv) to a toluene-d8 solution
of Ni(PEt3)4 (1 equiv) at 278 K resulted in the initial
formation of complexes 3 and 4 in an approximately 1.5:1
ratio, as judged by 31P{1H} NMR spectroscopy (Scheme 4A).
The nature of the product ratio is not entirely clear; however,
compound 1 is expected to coordinate slightly stronger to
(32) Stanger, A.; Weismann, H. J. Organomet. Chem. 1996, 515, 183.
(33) Stanger, A.; Vollhardt, K. P. C. Organometallics 1992, 11, 317.
(34) Stanger, A. Organometallics 1991, 10, 2979.
(35) Siegel, J. S.; Baldridge, K. K.; Linden, A.; Dorta, R. J. Am. Chem.
Soc. 2006, 128, 10644.
(36) Zingales, F.; Chiesa, A.; Basolo, F. J. Am. Chem. Soc. 1966, 88, 2707.
5116 Inorganic Chemistry, Vol. 47, No. 12, 2008

Directing Aryl-I Wersus Aryl-Br Bond ActiWation
Scheme 4a
a (A) The competitive aryl-I and aryl-Br bond activation is controlled by η2-coordination of the metal center to compounds 1 and 2. (B) In situ 31P{1H}
NMR follow-up measurements of the conversions of 3 f 5 (black line, R2 ) 0.988) and 4 f 6 (red line, R2 ) 0.990) in toluene-d8 at 278 K in the presence
of free ligands (1, 2) and PEt3.
Figure 2. (A) In situ variable temperature 31P{1H} NMR follow-up measurements of the conversion of 3 f 5 (black lines) and 4 f 6 (red lines) in
toluene-d8 (15 mM). (B) Eyring plots of the conversion of 3 f 5 (black line, R2 ) 0.999) and 4 f 6 (red line, R2 ) 0.996).
Figure 3. Representative in situ 31P{1H} NMR follow-up measurements of the thermolysis of complex 3 at 271 K.
electron-transfer process as the rate-limiting step would have
been affected by the solvent.38–40 Tsou and Kochi established
that the oxidative addition of Ni(PEt3)4 to aryl halides
proceeds via an electron transfer process.40 Such a mecha-
nism is likely to be involved in various reactions such as
the Ni-catalyzed Kharasch addition.41 Therefore, the ther-
molysis of complex 3 has also been followed by 31P{1H}
NMR spectroscopy in the presence of 1 equiv of n-Bu4NI.
If the reaction proceeds by intramolecular electron transfer
in complex 3,40–42 then ionic bromide should be formed as
an intermediate, and the presence of iodide should lead to
the formation of complex 6. However, we only observed
quantitative formation of complex 5. In addition, we did not
observe organic products derived from aryl radicals. The
reaction 3 f 5 is also not affected by the concentration
within the range 15–80 mM, as expected for a first-order
process. In addition, acetone is known to coordinate to
unsaturated metal complexes,43 which probably does not
occur here or is not observable. Apparently, the transition
(37) CRC Handbook of Chemistry and Physics, 83rd ed.; Lide, D. R., Ed.;
CRC Press: Boca Raton, FL, 2002-2003.
(38) Broxton, T. J.; Chung, R. P.-T. J. Org. Chem. 1990, 55, 3886.
´
(39) Mucsi, Z.; Szabo´, A.; Hermecz, I.; Kucsman, A.; Csizmadia, I. G J. Am.
Chem. Soc. 2005, 127, 7615.
(40) Tsou, T. T.; Kochi, J. K. J. Am. Chem. Soc. 1979, 101, 6319.
(41) Gossage, R. A.; Gossage, R. A.; van der Kuil, L. A.; van Koten, G.
Acc. Chem. Res. 1998, 31, 423.
(42) Hill, R. H.; Becalska, A.; Chiem, N. Organometallics 1991, 10, 2104.
(43) For example, see: Esteruelas, M. E.; Go´mez, A. V.; Lahoz, F. J.; Lo´pez,
A. M.; On˜ate, E.; Oro, L. A. Organometallics 1996, 15, 3423.
Inorganic Chemistry, Vol. 47, No. 12, 2008 5117

Zenkina et al.
Summary and Conclusions
In conclusion, selective nickel η2-coordination to a
sCHdCHs moiety is kinetically preferable over the oxida-
tion addition of aryl-I and aryl-Br bonds. The overall
process (3 f 5 and 4 f 6) proceeds via an intramolecular
ring-walking mechanism. The aryl-halide bond activation
by the nickel center is most probably not rate determining.
It has been demonstrated that the coordination of the nickel
center to the stilbazole model system can be used to direct
the selectivity of aryl-halide bond activation processes.
Selective activation of a strong aryl-Br bond occurs in the
presence of relatively weak aryl-I bonds, as a result of
metal–ligand interactions prior to the actual bond activation
process. It is remarkable that selective aryl-Br activation
by the d10 metal center occurs in the presence of a much
weaker aryl-I bond (e.g., compare bond dissociation ener-
gies (BDEs) for Ph-Br ) 80.5 kcal/mol vs Ph-I ) 65.5
kcal/mol).37 These findings and the mechanistic insight
presented here, albeit with a model system, may have
implications for future development of practical organic
reactions involving selective functionalization of aryl-halides
and other functional aryls by platinum group metals.
Figure 4. Reaction profile (∆G298 kcal/mol, PCM(Benzene)-PBE0/SDB-
cc-pVDZ//PBE0/SDD) for the ring-walking of Ni(PEt3)2 over the bromo-
and iodo-stilbazole substrates (1, 2). All H atoms have been omitted for
clarity. (Atomic color scheme: C, gray; Ni, green; N, blue; Br or I, purple;
P, orange.)
state of the slow step is nonpolar and most likely involves a
system having two PEt3 ligands bound to the metal center
because solvent coordination and the presence of excess of
PEt3 appear not to play an experimentally observable role.
DFT calculations were performed to gain additional
mechanistic insight into the experimental findings. The
potential energy surfaces (PES) are simulated in benzene as
the solvent for both systems (1 f 5, 2 f 6) and are shown
in Figure 4 along with a schematic illustration of the local
minima and transition states that were located. As expected,
the calculated structures located on the PES are qualitatively
very similar to those reported for the related platinum
system.18 Seven local minima were located on the PES. The
local minimum (3′, 4′), the reaction entry channel, corre-
sponds to the experimentally observed η2-coordination of
Ni(PEt3)2 to the sCHdCHs moiety of ligands 1 and 2.
Three local minima involve η2-coordination of the Ni(PEt3)2
moiety to the CdC bonds of the phenyl ring (R1-R3). The
two remaining local minima are the cis (Cis prod) and trans
oxidative addition products (5′, 6′). The transition states for
ring-walking of Ni(PEt3)2 (TS(R1-R2), TS(R2-R3)) and
for the oxidative addition step (TS(O.A.)) were also located.
Ligands 1 and 2 have very similar reaction profiles, with
the exception of the expected higher barrier for aryl-Br
activation: ∆(TS(O.A.)) ≈4.2 kcal/mol. Conversion of R1
to R2 via the transition state, TS(R1-R2), occurs with
relatively similar high energies for both the bromine (3′) and
Experimental Details
General Procedures. All reactions were carried out under an
N2-filled M. Braun glovebox with H2O and O2 levels <2 ppm.
Solvents were reagent grade or better, dried, distilled, and degassed
before introduction into the glovebox, where they were stored over
activated 4 Å molecular sieves. Deuterated solvents were purchased
from Aldrich and were degassed and stored over 4 Å activated
molecular sieves in the glovebox. Reaction flasks were washed with
deionized (DI) water, followed by acetone, and then oven-dried
prior to use. Mass spectrometry was carried out using a Micromass
Platform LCZ 4000 mass spectrometer. Elemental analyses were
performed by H. Kolbe, Mikroanalytisches Laboratorium, Mülheim
an der Ruhr, Germany. Ligands 1, 2, and Ni(PEt3)4 were prepared
by published procedures.44
Spectroscopic Analysis. The 1H, 13C, and 31P NMR spectra were
recorded at 500.132, 125.77, and 202.46 MHz, respectively, on a
Bruker Avance 500 NMR spectrometer. All chemical shifts (δ) are
reported in parts per million and coupling constants (J) in hertz.
1
The H and 13C NMR chemical shifts are relative to tetramethyl-
silane; the resonance of the residual protons of the solvent was
used as the internal standard h1 (7.15 ppm, benzene; 7.09 ppm,
toluene; 3.58 and 1.73 ppm, tetrahydrofuran) and all-d solvent peaks
(128.0 ppm, benzene; 20.4 ppm, toluene; tetrahydrofuran, 67.6 and
25.3 ppm), respectively. 31P{1H} NMR chemical shifts are relative
to 85% H3PO4 in D2O at δ ) 0.0 (external reference), with shifts
downfield of the reference considered positive. All measurements
were carried out at 298 K unless otherwise stated. Screw-cap 5
mm NMR tubes were used in NMR follow-up experiments. Et3PO
was used as the internal standard. Assignments in the 1H and
13C{1H} NMR were made using 1H{31P} and 13C-DEPT-135 NMR.
the iodine (4′) systems: ∆G‡
) 24.2 and 25.5 kcal/mol,
298K
respectively. For both systems (3′, 4′), this seems to be the
RDS for the overall reaction. Moreover, the RDS for the
bromine (3′) system is lower by 1.3 kcal/mol than that for
the iodine system (4′). The transition state, TS(3′, 4′-R1),
has not been located; however, the computational results
suggest that a ring-walking process is kinetically accessible
and in good agreement with the experimental findings.
31P{1H} NMR Follow-up Measurements of the Formation
of Complex 3. Ni(PEt3)4 (30 mg, 0.056 mmol) was dissolved in
0.3 mL of toluene-d8 and loaded in a 5 mm screw-cap NMR tube.
Subsequently, a solution of 4-[2-bromo-phenyl)-vinyl]-pyridine (1)
(44) (a) Rodriguez, J. G.; Martin-Villamil, R.; Lafuente, A. Tetrahedron
2003, 59, 1021. (b) Schunn, R. A. Inorg. Chem. 1976, 15, 208.
5118 Inorganic Chemistry, Vol. 47, No. 12, 2008

Directing Aryl-I Wersus Aryl-Br Bond ActiWation
(15 mg, 0.057 mmol) in 0.4 mL of toluene-d8 was added at -75
°C. The tube was sealed, shaken, and immediately transferred into
the precooled NMR machine at -75 °C. Follow-up 31P{1H} NMR
spectroscopy showed the selective formation of complex 3 and the
concurrent disappearance of the starting materials. The reaction was
completed after 4.5 h, yielding spectroscopically pure complex 3.
The formation of free PEt3 (2 equiv) was observed during the
reaction. No intermediates were observed. Complex 3 is stable in
the temperature range -73 to -8 °C. For instance, it is stable in
solution for at least 2 days at -40 °C and ∼1 h at -8 °C.
Performing the same reaction at -2 °C also resulted in the
quantitative formation of complex 3 (after ∼80 min). However,
prolonged reaction times also showed the formation of complex 5.
For 3: 1H NMR (acetone-d6 -60 °C, TMS): δ 0.84 (m, 18H,
R2 ) 0.994). Reactions were also monitored in acetone-d6 (0.080
mM, kac271K ) 3.23 × 10-5 ( 2.8 × 10-7 s-1, R2 ) 0.996), at
lower concentrations (15 mM, k271K ) 3.65 × 10-5 ( 2.8 × 10-7
s-1, R2 ) 0.996) and with excess of PEt3 (15 mM, 10 equiv of
PEt3, k271K ) 3.47 × 10-5 ( 1.0 × 10-7 s-1, R2 ) 0.999).
Performing the reaction at various temperatures affords the fol-
lowing first-order rate constants: 15 mM, k265K ) 1.2 × 10-5
(
5.2 × 10-8 s-1, R2 ) 0.999; 15 mM, k278K ) 8.71 × 10-5 ( 4.2 ×
10-7 s-1, R2 ) 0.999; 15 mM, k284K ) 2.02 × 10-4 ( 2.4 × 10-6
s-1, R2 ) 0.997.
31P{1H} NMR Follow-up of the Formation of Complex 5
from Complex 3 in the Presence of Tetrabutylammonium
Iodide. A screw-cap NMR tube containing a solution of Ni(PEt3)4
(3.5 mg, 0.0066 mmol) and tetrabutylammonium iodide (2.4 mg,
0.0065 mmol) in 0.4 mL of dry toluene-d8 was cooled to –60 °C.
Subsequently, a solution of 4-[2-bromo-phenyl)-vinyl]-pyridine (1)
(1.7 mg, 0.0065 mmol) in 0.3 mL of toluene-d8 was added. The
tube was shaken and immediately transferred into the precooled (5
°C) NMR machine. Quantitative formation of complex 3 was
observed by 31P{1H} NMR spectroscopy after ∼5 min. Complex
3 undergoes selective aryl-halide oxidative addition to afford
complex 5. The reaction progress was monitored by 31P{1H} NMR
spectroscopy showing the disappearance of complex 3 and the
concurrent formation of complex 5. No other products such as
complex 6 were observed. Complex 5 was identified by the addition
of authentic sample. The reaction was completed in about 4 h.
Formation of Complex 4. Ni(PEt3)4 (7.8 mg, 0.015 mmol) was
dissolved in 0.3 mL of toluene-d8 and loaded in a 5 mm screw-cap
NMR tube, which was equipped with a septum and was tightly
closed using insulating tape and cooled until -60 °C. A solution
of 4-[2-iodo-phenyl)-vinyl]-pyridine (4.5 mg, 0.015 mmol) in 0.4
mL of toluene-d8 (∼30 °C) was added dropwise via a syringe into
the sealed screw-cap NMR tube. Next, the tube was shaken and
immediately transferred into the precooled (-2 °C) NMR machine.
After ∼5 min, the formation of complex 4 became visible by
31P{1H} NMR (28% conversion of starting material). After 1.4 h
at -2 °C, 97% conversion was observed. Complex 4 was the only
observable product. No intermediates were observed during the
reaction. Complex 4 slowly converts selectively to complex 6. For
4: 1H NMR (THF-d8, -13 °C, TMS): δ 0.93 (m, 18H, PCH2CH3),
1.14 (m, 12H, PCH2CH3), 3.41 (m, 2H, CHdCH, 3JHH ) 5.5 Hz),
6.97 (br, 2H, ArH), 7.00 (d, 2H, ArH, 3JHH ) 8.0 Hz), 7.35 (d, 2H,
3
PCH2CH3), 1.51 (m, 12H, PCH2CH3), 3.60 (t, 2H, CHdCH, JHH
3
) 5.5 Hz), 7.63 (m, 4H, ArH, JHH ) 8.3 Hz), 7.64 (d, 2H, ArH,
3JHH ) 4.9 Hz), 8.59 (d, 2H, PyrH, 3JHH ) 4.9 Hz). 13C{1H} NMR
(acetone-d6): δ 7.77 (m, PCH2CH3) 16.15 (m, PCH2CH3), 49.56
(d, η2-(CHdCH), 2JPC ) 17.7 Hz), 50.68 (d, η2-(CHdCH), 2JPC
)
16.9 Hz), 121.72, 122.39 (s, Cq, CsBr), 129.02, 131.56, 135.69
(Cq, s), 144.16 (Cq, s), 150.31. 31P{1H} NMR (toluene-d8): q, AB
2
2
system δA ) 16.9 (1P, JPP ) 39.7 Hz), δB ) 18.3 (1P, JPP
)
39.7 Hz).
Formation of Complex 5. A solution of Ni(PEt3)4 (30 mg, 0.056
mmol) in THF (1 mL) was added to a solution of 4-[2-bromo-
phenyl)-vinyl]-pyridine (1) (15 mg, 0.057 mmol) in THF (0.5 mL)
at room temperature. After 2 days, the volatiles were removed under
vacuum followed by washing of the residue with dry, cold pentane
(1 mL, -40 °C). The resulting solid was dissolved in a minimum
quantity (∼0.5 mL) of dry THF followed by a dropwise addition
of dry pentane (∼3 mL). Orange crystals, suitable for X-ray
diffraction, were obtained upon slow evaporation of the solvent at
1
room temperature (∼80% yield). For 5: H NMR (C6D6): δ 1.01
(m, 18H, PCH2CH3), 1.39 (m, 12H, PCH2CH3), 6.76 (d, 1H,
3
CHdCH, JHH ) 16.3 Hz), 6.88 (br, 2H, ArH), 7.05 (d, 1H,
3
3
CHdCH, JHH ) 16.7 Hz), 7.45 (d, 2H, ArH, JHH ) 7.6 Hz),
3
3
7.07 (d, ArH, 2H, JHH ) 7.7 Hz), 8.56 (d, 2H, JHH ) 5.8 Hz,
PyrH). 13C{1H} NMR (C6D6): δ 8.06 (s, PCH2CH3), 14.67 (t,
1
PCH2CH3, JPC ) 14.3 Hz), 120.33, 122.98, 124.92, 129.61 (s, Cq),
1
133.78, 137.29, 144.62 (s, Cq), 150.52 (s), 162.39, (t, Cq, JPC
)
66.6 Hz). 31P{1H} NMR (C6D6): δ 11.24 (s, 2P). Calcd m/e: 555.14,
found (M+ – Br) ) 475.22. Anal. Calcd for C25H40BrNNiP2: C,
54.09; H, 7.26; N, 2.52. Found: C, 54.31; H, 7.11; N, 2.52.
3
ArH, JHH ) 8.2 Hz), 8.11 (br, 2H, PyrH). 13C{1H} NMR (THF-
d8, -13 °C): δ ) 8.19 (br, PCH2CH3), 17.18 (ddd, 1JPC ) 7.9 Hz,
31P{1H} NMR Follow-up of the Formation of Complex 5
from Complex 3. A solution of Ni(PEt3)4 (30 mg, 0.056 mmol) in
dry toluene-d8 (0.4 mL) was added to a solution of 4-[2-bromo-
phenyl)-vinyl]-pyridine (1) (15 mg, 0.057 mmol) in dry toluene-d8
(0.3 mL). The tube was shaken and immediately transferred into
the precooled (-75 °C) NMR machine. Formation of complex 3
was observed by 31P{1H} NMR spectroscopy after ∼10 min (46%
selective conversion of starting material). Nearly quantitative
formation of complex 3 (98%) was observed after 4.5 h. The
transformation 3 f 5 was monitored by VT 31P{1H} NMR spec-
troscopy. The temperature was raised stepwise from -68 °C by
10 °C with time intervals of 30 min. At -8 °C, the formation of
complex 5 was observed (∼5% after 10 min). Follow-up 31P{1H}
NMR measurements were performed at -2 °C and showed the
formation of complex 5 and the concurrent disappearance of
complex 3. No intermediates were observed during the reaction.
After ∼13.4 h, 83% of the starting material was selectively
converted into compound 5. Raising the temperature to 25 °C for
∼2 h resulted in the quantitative formation of complex 5. First-
1
3JPC ) 2.0 Hz, PCH2CH3), 49.56 (d, η2-(CHdCH), JPC ) 17.9
Hz), 51.59 (d, η1-(CHdCH), 1JPC ) 17.2 Hz), 85.23 (br, Cq, CsI),
119.17, 126.82, 137.13, 137.13 (Cq, s), 148.23 (Cq, d), 149.35, 155.2
(Cq, br). 31P{1H} NMR (toluene-d8): q, AB system, δA ) 17.0 (1P,
2
2JPP ) 39.6 Hz), δB ) 18.3 (1P, JPP ) 39.6 Hz).
31P{1H} NMR Follow-up Experiment of the Formation of
Complex 6 from Complex 4. A solution of complex 4 (15 mM)
was monitored by 31P{1H} NMR spectroscopy at -2 °C. Formation
of complex 6 became visible after ∼30 min. 31P{1H} NMR follow
up measurements show the quantitative formation of complex 6
and the concurrent disappearance of complex 4 (k271K ) 3.97 ×
10-5 ( 2.5 × 10-7 s-1, R2 ) 0.997). After ∼12 h, 90% conversion
of starting materials was observed. The reaction was followed at
various temperatures: 15 mM, k268K ) 2.07 × 10-5 ( 2.4 × 10-7
s-1, R2 ) 0.995; 15 mM, k275K ) 5.70 × 10-5 ( 2.5 × 10-7 s-1
,
R2 ) 0.999; 15 mM, k285K ) 2.6 × 10-4 ( 4.2 × 10-6 s-1, R2 )
0.999). For 6: 1H NMR (C6D6): δ 0.80 (m, 18H, PCH2CH3)
3
1.52–1.43 (m, 12H, PCH2CH3), 6.76 (dd, CHdCH, 1H, JHH
)
order linear fitting yielded k271K ) 3.55 × 10-5 ( 1.1 × 10-7 s-1
,
16.3 Hz), 7.09 (d, 1H, CHdCH, JHH ) 16.1 Hz), 7.16 (br, ArH,
3
Inorganic Chemistry, Vol. 47, No. 12, 2008 5119

Zenkina et al.
2H), 7.19 (d, 2H, ArH, 3JHH ) 8.1 Hz), 7.51 (d, 2H, ArH, 3JHH
)
Competition Experiments of Ni(PEt3)4 with Iodobenzene
and Ligand 1. Ni(PEt3)4 (3.5 mg, 0.0066 mmol) was dissolved in
0.3 mL of dry toluene-d8 and loaded in a 5 mm screw-cap NMR
tube, which was equipped with a septum and was tightly closed
using insulating tape. After cooling to -65 °C, a solution of 4-[2-
bromo-phenyl)-vinyl]-pyridine (1) (1.7 mg, 0.0065 mmol) and
iodobenzene (1.35 mg, 0.0066 mmol) in 0.4 mL of dry toluene
was added. Next, the tube was shaken and immediately transferred
into the precooled (-2 °C) NMR machine. After 5 min, 31P{1H}
NMR measurements showed the formation of complex 3 and the
concurrent disappearance of Ni(PEt3)4. Quantitative formation of
complex 3 was observed after ∼80 min. Prolonged reaction times
resulted in the formation of complex 5 and the concurrent
disappearance of compound 3. After 14 h, 31P{1H} NMR analysis
showed full conversion of Ni(PEt3)4, free PEt3, and the formation
of complexes 5 (96%) and NiI(C6H5)(PEt3)2 (4%). No intermediates
were observed during the reaction. The product distribution is stable
at room temperature for at least 48 h and for 4 h at 45 °C.
Apparently, complexes 5 and NiI(C6H5)(PEt3)2 are thermodynami-
cally stable and do not interconvert.
Competition Experiments of Ni(PEt3)4 with 1-Iodo-4-meth-
oxy-benzene and Ligand 1. A cold solution (-65 °C) of Ni(PEt3)4
(3.5 mg, 0.0066 mmol) in 0.3 mL of toluene-d8 was mixed with a
cold solution of 1-iodo-4-methoxy-benzene (1.54 mg, 0.0066 mmol)
and 4-[2-bromo-phenyl)-vinyl]-pyridine (1) (1.7 mg, 0.0065 mmol)
in 0.4 mL of toluene-d8. The reaction mixture was kept at -65 °C
for 2 h and then allowed to attain room temperature (∼2 h). 31P{1H}
NMR measurements showed the formation of complexes 5 (97%)
and NiI(4-MeO-C6H4)(PEt3)2 (3%). No other products were ob-
served.
Competition Experiments of Ni(PEt3)4 with 1-Iodo-4-triflu-
oromethyl-benzene and Ligand 1. A cold solution (-65 °C) of
Ni(PEt3)4 (3.5 mg, 0.0066 mmol) in 0.3 mL of dry toluene-d8 was
mixed with a cold solution of 1-iodo-4-trifluoromethyl-benzene
(1.79 mg, 0.0066 mmol) and 4-[2-bromo-phenyl)-vinyl]-pyridine
(1) (1.87 mg, 0.0072 mmol) in 0.4 mL of toluene-d8. The reaction
mixture was kept at -65 °C for 2 h and then allowed to attain
room temperature (∼2 h). 31P{1H} NMR analysis showed full
conversion of Ni(PEt3)4, free PEt3, and the formation of complexes
5 (86%) and NiI(4-CF3-C6H4)(PEt3)2 (14%). No other products
were observed. Repeating this experiment with the addition of 10
equiv of PEt3 (7.7 mg, 0.060 mmol) to the reaction mixture
suppressed the formation of NiI(4-CF3sC6H4)(PEt3)2 to 2%.
X-ray Analysis of Complex 5. Crystal data: C25H40NP2BrNi,
M ) 555.14, orthorhombic, space group Pna21, a ) 16.112(3), b
) 14.717(3), c ) 11.515(2) Å, Z ) 4, Fcalcd ) 1.350 g cm-3, 16 170
reflections measured, 2747 unique (Rint ) 0.098, 2θmax ) 50.7°),
final R ) 0.046 (Rw ) 0.091) for 2622 reflections with I > 2σ(I)
and R ) 0.061 (Rw ) 0.098) for all data.
7.5 Hz), 9.70–9.56 (br, 2H, PyrH). 13C{1H} NMR (C6D6): δ 8.11
(s, PCH2CH3), 16.04 (t, PCH2CH3, 1JPC ) 13.3 Hz), 123.89, 124.98,
126.96, 128.82, 130.20 (s, Cq), 133.14, 137.24 (s), 149.38 (br, Cq),
1
165.14 (t, Cq, JPC ) 66.8 Hz). 31P{1H} NMR (C6D6): δ 11.81(s,
2P). Calcd m/e: 602.14, found ) 602.85. Anal. Calcd for
C25H40INNiP2: C, 49.87; H, 6.7. Found: C, 49.46; H, 6.81.
Competition Experiments of Ni(PEt3)4 with 4-[2-Bromo-
phenyl)-vinyl]-pyridine (1) and 4-[2-Iodo-phenyl)-vinyl]-pyri-
dine (2). A screw-cap NMR tube containing a solution of Ni(PEt3)4
(3.5 mg, 0.0066 mmol) in 0.3 mL of dry toluene-d8 was cooled to
-60 °C. A solution of 4-[2-bromo-phenyl)-vinyl]-pyridine (1) (3.4
mg, 0.013 mmol) and 4-[2-iodo-phenyl)-vinyl]-pyridine (2) (4.0
mg, 0.013 mmol) in 0.4 mL of toluene-d8 (-30 °C) was injected
into the NMR tube (twofold excess of each ligand). The tube was
shaken and immediately transferred into the precooled (5 °C) NMR
machine. Complexes 3 and 4 in a ratio of 1.5:1 were observed by
31P{1H} NMR spectroscopy after ∼5 min. No Ni(PEt3)x (x ) 3, 4)
was observable. Both complex 3 and complex 4 undergo selective
aryl-halide oxidative addition. The reaction progress was monitored
by 31P{1H} NMR spectroscopy showing the disappearance of
complexes 3 and 4 and the concurrent formation of complexes 5
and 6 (ratio 1.5:1). No intermediates were observed. The reaction
was nearly completed in ∼4 h with k278k ) 1.2 × 10-4 ( 3.1 ×
10-6 s-1 for 3 f 5 (R2 ) 0.988) and k278k ) 1.5 × 10-4 ( 3.1 ×
10-6 s-1 for 4 f 6 (R2 ) 0.990). Complexes 3–6 were identified
by addition of authentic samples to the reaction mixture. The
reaction was repeated; however, after the formation of complexes
3 and 4 (ratio 1.5:1), 1 equiv of complex 3 was added to the reaction
mixture, resulting in a ratio 3:4 ) 2.5:1. Quantitative formation of
complexes 5 and 6 (ratio 2.5:1) was observed after 12 h at 0 °C.
Formation of Complex 5 from Complex 3 in the Presence
of 4-[2-Iodo-phenyl)-vinyl]-pyridine (2). A screw-cap NMR tube
containing a solution of Ni(PEt3)4 (3.5 mg, 0.0066 mmol) in 0.3
mL of dry toluene-d8 was cooled to -60 °C. Subsequently, a
solution of 4-[2-bromo-phenyl)-vinyl]-pyridine (1) (1.7 mg, 0.0065
mmol) in 0.3 mL of toluene-d8 was added. The reaction progress
was monitored by 31P{1H} NMR spectroscopy at –2 °C. The
Ni(PEt3)4 was consumed after ∼15 min, and quantitative formation
of compound 3 was observed. Subsequently, a solution of 4-[2-
iodo-phenyl)-vinyl]-pyridine (2) (0.93 mg, 0.0030) in 0.2 mL of
toluene-d8 was added to the reaction mixture at –60 °C. The tube
was shaken and immediately transferred into the precooled (5 °C)
NMR machine. The reaction progress was monitored by 31P{1H}
NMR spectroscopy showing the exclusive formation of complex 5
(93% conversion after ∼4 h) and the concurrent disappearance of
complex 3. Formation of complexes 4 and 6 was not observed.
Formation of Complex 6 from Complex 4 in the Presence
of 4-[2-Bromo-phenyl)-vinyl]-pyridine (1). A screw-cap NMR
tube containing a solution of Ni(PEt3)4 (3.5 mg, 0.0066 mmol) in
0.3 mL of toluene-d8 was cooled to -60 °C. Subsequently, a
solution of 4-[2-iodo-phenyl)-vinyl]-pyridine (2) (1.86 mg, 0.0061)
in 0.3 mL of toluene-d8 was added. The reaction progress was
monitored by 31P{1H} NMR spectroscopy at -2 °C. The Ni(PEt3)4
was consumed after ∼20 min, and quantitative formation of
compound 4 was observed. Subsequently, a solution of 4-[2-bromo-
phenyl)-vinyl]-pyridine (1) (0.85 mg, 0.0033 mmol) in 0.2 mL of
toluene-d8 was added to the reaction mixture. The tube was shaken
and immediately transferred into the precooled (5 °C) NMR
machine. The reaction progress was monitored by 31P{1H} NMR
spectroscopy showing the formation of complex 6 (92% after ∼4
h) and the concurrent disappearance of complex 4. Traces of
complex 5 (<5%) were observed.
Computational Details
All calculations were carried out using Gaussian 03,
Revision C.01.45 The PBE0 DFT exchange-correlation
functional was used for the investigation. This functional,
also known as PBE1PBE, is the hybrid variant (25% HF
exchange) of PBE (Perdew, Burke, and Ernzerhof’s)46
nonempirical GGA functional. With this functional, two basis
set-RECP (relativistic effective core potential) combinations
were used. The first, denoted SDD, is a combination of the
Huzinaga–Dunning double-ꢀ basis set on lighter elements
with the Stuttgart-Dresden basis set-RECP combination on
transition metals.47 The second donated SDB-cc-pVDZ
5120 Inorganic Chemistry, Vol. 47, No. 12, 2008

Directing Aryl-I Wersus Aryl-Br Bond ActiWation
corresponds to the Dunning cc-pVDZ basis set48 on the main
group elements and the Stuttgart-Dresden basis set-RECP
on the transition metals with additional 2f1g-type polarization
exponents given in ref 49. Geometries were optimized using
the former basis set with the default pruned (75 302) grid,
whereas the binding energies were calculated with the latter
basis set and the “ultrafine” grid (i.e., a pruned (99 590) grid).
This level of theory is conventionally denoted as PBE0/SDB-
cc-pVDZ//PBE0/SDD. Bulk solvent effects50 were approx-
imated by single point PBE0/SDB-cc-pVDZ energy calcu-
lations using a polarized continuum (overlapping spheres)
model (PCM)51–53 with benzene as the solvent. The con-
nectivity of the transition states was confirmed by performing
intrinsic reaction coordinate (IRC) calculations.54–56 Equi-
librium geometries were verified to have all real harmonic
frequencies and transition states to have one imaginary
frequency.
Acknowledgment. This research was supported by the
Helen and Martin Kimmel Center for Molecular Design, the
Lise Meitner-Minerva Center for Computational Quantum
Chemistry, the Israel Science Foundation (ISF), the German
Federal Ministry for Education and Research (BMBF), and
the Minerva Junior Research Groups (MJRG) program.
M.E.vd.B. is the incumbent of the Dewey David Stone and
Harry Levine Career Development Chair, while J.M.L.M is
the Baroness Thatcher Professor of Chemistry. We thank Mr.
D. Strawser for performing preliminary experiments.
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Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene,
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Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin,
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Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
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Supporting Information Available: Crystallographic data (CIF)
for complex 5. This material is available free of charge via the
Internet at http://pubs.acs.org.
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Inorganic Chemistry, Vol. 47, No. 12, 2008 5121