Ligand redox effects in the synthesis, electronic structure, and reactivity of an alkyl-alkyl cross-coupling catalyst

Article abstract of DOI:10.1021/ja063334i

The ability of the terpyridine ligand to stabilize alkyl complexes of nickel has been central in obtaining a fundamental understanding of the key processes involved in alkyl-alkyl cross-coupling reactions. Here, mechanistic studies using isotopically labeled (TMEDA)NiMe₂ (TMEDA = N,N,N′,N′-tetramethylethylenediamine) have shown that an important catalyst in alkyl-alkyl cross-coupling reactions, (tpy′)NiMe (2b, tpy′ = 4,4′,4″-tri-tert-butylterpyridine), is not produced via a mechanism that involves the formation of methyl radicals. Instead, it is proposed that (terpyridine)NiMe complexes arise via a comproportionation reaction between a Ni(II)-dimethyl species and a Ni(0) fragment in solution upon addition of a terpyridine ligand to (TMEDA)NiMe₂. EPR and DFT studies on the paramagnetic (terpyridine)NiMe (2a) both suggest that the unpaired electron resides heavily on the terpyridine ligand and that the proper electronic description of this nickel complex is a Ni(II)-methyl cation bound to a reduced terpyridine ligand. Thus, an important consequence of these results is that alkyl halide reduction by (terpyridine)NiR_alkyl complexes appears to be substantially ligand based. A comprehensive survey investigating the catalytic reactivity of related ligand derivatives suggests that electronic factors only moderately influence reactivity in the terpyridine-based catalysis and that the most dramatic effects arise from steric and solubility factors.

Full text of DOI:10.1021/ja063334i

Published on Web 09/19/2006
Ligand Redox Effects in the Synthesis, Electronic Structure,
and Reactivity of an Alkyl-Alkyl Cross-Coupling Catalyst
Gavin D. Jones,† Jason L. Martin,† Chris McFarland,† Olivia R. Allen,†
Ryan E. Hall,† Aireal D. Haley,† R. Jacob Brandon,† Tatyana Konovalova,‡
Patrick J. Desrochers,§ Peter Pulay,† and David A. Vicic*,†
Contribution from the Department of Chemistry and Biochemistry, UniVersity of Arkansas,
FayetteVille, Arkansas 72701, Department of Chemistry, UniVersity of Alabama, Box 870336,
Tuscaloosa, Alabama 35487-0336, and Department of Chemistry, UniVersity of Central
Arkansas, Conway, Arkansas 72035
Received May 12, 2006; E-mail: dvicic@uark.edu
Abstract: The ability of the terpyridine ligand to stabilize alkyl complexes of nickel has been central in
obtaining a fundamental understanding of the key processes involved in alkyl-alkyl cross-coupling reactions.
Here, mechanistic studies using isotopically labeled (TMEDA)NiMe2 (TMEDA ) N,N,N′,N′-tetramethyleth-
ylenediamine) have shown that an important catalyst in alkyl-alkyl cross-coupling reactions, (tpy′)NiMe
(2b, tpy′ ) 4,4′,4′′-tri-tert-butylterpyridine), is not produced via a mechanism that involves the formation of
methyl radicals. Instead, it is proposed that (terpyridine)NiMe complexes arise via a comproportionation
reaction between a Ni(II)-dimethyl species and a Ni(0) fragment in solution upon addition of a terpyridine
ligand to (TMEDA)NiMe2. EPR and DFT studies on the paramagnetic (terpyridine)NiMe (2a) both suggest
that the unpaired electron resides heavily on the terpyridine ligand and that the proper electronic description
of this nickel complex is a Ni(II)-methyl cation bound to a reduced terpyridine ligand. Thus, an important
consequence of these results is that alkyl halide reduction by (terpyridine)NiRalkyl complexes appears to be
substantially ligand based. A comprehensive survey investigating the catalytic reactivity of related ligand
derivatives suggests that electronic factors only moderately influence reactivity in the terpyridine-based
catalysis and that the most dramatic effects arise from steric and solubility factors.
Scheme 1. Metal-Catalyzed Cross-Coupling of Alkyl Nucleophile
with an Alkyl Electrophile
Introduction
Although the metal-catalyzed cross-coupling of an alkyl
nucleophile and an alkyl electrophile (Scheme 1) is perhaps the
most versatile conceivable method for forming a new C(sp3)-
C(sp3) bond in an organic molecule, a number of fundamental
obstacles have hindered the implementation of this method in
synthetic chemistry. First, it is well-known that transition-metal
alkyls are prone to â-hydride elimination reactions. Therefore,
any catalytic process involving activation of alkyl nucleophiles
and/or alkyl electrophiles must take into account this undesired
side reaction. Strategies to suppress the â-hydride elimination
reaction typically involve the use of ligands that never permit
the formation of a metal-alkyl intermediate that has a fully
vacant d-orbital or the use of ligands that produce a metal-
alkyl intermediate that, regardless of electron count, cannot attain
the requisite geometry for â-hydride elimination. These strate-
gies are often difficult to realize, as the nature of the metal
coordination sphere under catalytic conditions is not always
controllable nor predictable. Second, common alkyl electro-
philes, such as alkyl halides, are well-known to oxidize low
valent transition metals by single-electron processes, leading
to the formation of alkyl radicals and X-. The rates of
dimerization and other side reactions of the resulting alkyl
radicals must then not be faster than the radical addition to
substrate or transition metal, or efficient cross-coupling catalysis
will not proceed. Moreover, the ease of alkyl halide reduction
to produce alkyl radicals has serious consequences that need to
be addressed when designing asymmetric versions of the alkyl-
alkyl cross-coupling reactions described in Scheme 1.
Despite the considerations mentioned above, much progress
has been made in recent years in identifying ligands and reaction
conditions that can effectively promote alkyl-alkyl cross-
couplings.1 In 2003, Fu reported a nickel-catalyzed reaction
using the pybox ligand (eq 1, pybox ) bis(oxazolinyl)pyridine)
that successfully achieved Negishi couplings of unactivated
secondary alkyl bromides.1g Conditions were then found to
exploit the C2 symmetry of the pybox ligand, as Fu later reported
the first examples of asymmetric Negishi cross-couplings of
alkyl electrophiles (eqs 2 and 3).1m,2 These experiments were
seminal, as they showed that racemic starting materials could
† University of Arkansas.
‡ University of Alabama.
§ University of Central Arkansas.
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J. AM. CHEM. SOC. 2006, 128, 13175-13183
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A R T I C L E S
Jones et al.
be fully converted to enantiopure products with no evidence of
kinetic resolution during the catalysis. Although no mechanistic
studies to account for the stereo-convergence were reported, the
involvement of radicals in these and other systems has been
suggested.2,3a,b Additionally, to this point, the electrophiles used
for the asymmetric cross-coupling reaction have all been
activated, with success only being reported for the R-bromo
amides or the benzylic halides.1m,2 Moreover, the nickel/pybox-
catalyzed reactions all involve the use of DMA (DMA ) N,N-
dimethylacetamide) or other high boiling additives, thereby
potentially limiting their use. Additives, such as NMI (NMI )
N-methylimidazole), have been reported to enhance the reactiv-
ity of alkylzinc reagents,4 and the successful use of amides in
Negishi cross-couplings has been known for some time.1d,e,i-k,5
Fu showed that the yields for alkyl-alkyl cross-couplings with
Pd/PCyp3 (Cyp ) cyclopentyl) dropped over 20% when NMP
(NMP ) N-methyl-2-pyrrolidone) was removed from the solvent
mixture (eq 4).1f Another striking example of the amide effect
was observed by Organ, who noted that substituting THF for
NMP in the Pd/N-heterocyclic carbene-catalyzed reactions (eq
5) resulted in a drop of overall yield from 75 to 2%.1d We were
therefore pleased to report a Ni/terpyridine-catalyzed version
of the alkyl-alkyl cross-coupling reaction that permitted the
reaction to be run without amide additives in THF solvent (eq
6).1a,b The identification of a ligand system that could catalyze
the Negishi couplings in a low boiling, amide-free solvent thus
prompted us to look more closely at the organometallic features
of the alkyl-alkyl cross-coupling reaction in hopes of obtaining
fundamental information that could serve to increase the current
scope of the chemistry.
One of the most peculiar features of the terpyridyl ligand
system is its ability to stabilize monoalkyl complexes of nickel.
Indeed, reaction of (TMEDA)NiMe2 (1) with a terpyridine
ligand cleanly led to the formation of (terpyridyl)NiMe (2, eq
7). The thermal stability of 2 has then enabled a number of key
studies on alkyl transfer reactions to be performed,1b providing
initial insights into the mechanism of nickel-mediated alkyl-
alkyl cross-coupling reactions. It has been shown that 2 can
effectively transfer its methyl group to alkyl iodides, affording
cross-coupled alkanes in high yields, as exemplified in eq 8
(tpy′ ) 4,4′,4′′-tri-tert-butylterpyridine).1a,b Surprisingly, how-
ever, the closely related Ni(II)-alkyl halide complex 3 did not
react with excess transmetalating agent to afford the same
product (eq 9).1b This result argues against a two-electron
mechanism in a catalytic cycle by which a (terpyridyl)Ni(0)
fragment oxidatively adds an alkyl halide to produce 3 and
subsequently undergoes a simple transmetalation reaction to
afford cross-coupled product.
(1) (a) Anderson, T. J.; Jones, G. D.; Vicic, D. A. J. Am. Chem. Soc. 2004,
126, 8100-8101 [erratum p11113]. (b) Jones, G. D.; McFarland, C.;
Anderson, T. J.; Vicic, D. A. Chem. Commun. 2005, 4211-4213. (c)
Cardenas, D. J. Angew. Chem., Int. Ed. 2003, 42, 384-387. (d) Hadei, N.;
Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. Org. Lett. 2005, 7, 3805-
3807. (e) Hadei, N.; Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. J.
Org. Chem. 2005, 70, 8503-8507. (f) Zhou, J.; Fu, G. C. J. Am. Chem.
Soc. 2003, 125, 12527-12530. (g) Zhou, J.; Fu, G. C. J. Am. Chem. Soc.
2003, 125, 14726-14727. (h) Terao, J.; Todo, H.; Watanabe, H.; Ikumi,
A.; Kambe, N. Angew. Chem., Int. Ed. 2004, 43, 6180-6182. (i)
Giovannini, R.; Stuedemann, T.; Devasagayaraj, A.; Dussin, G.; Knochel,
P. J. Org. Chem. 1999, 64, 3544-3553. (j) Giovannini, R.; Studemann,
T.; Dussin, G.; Knochel, P. Angew. Chem., Int. Ed. 1998, 37, 2387-2390.
(k) Jensen, A. E.; Knochel, P. J. Org. Chem. 2002, 67, 79-85. (l) Netherton,
M. R.; Fu, G. C. AdV. Synth. Catal. 2004, 346, 1525-1532. (m) Fischer,
C.; Fu, G. C. J. Am. Chem. Soc. 2005, 127, 4594-4595. (n) Netherton, M.
R.; Fu, G. C. Top. Organomet. Chem. 2005, 14, 85-108. (o) Netherton,
M. R.; Dai, C.; Neuschuetz, K.; Fu, G. C. J. Am. Chem. Soc. 2001, 123,
10099-10100. (p) Terao, J.; Watanabe, H.; Ikumi, A.; Kuniyasu, H.;
Kambe, N. J. Am. Chem. Soc. 2002, 124, 4222-4223. (q) Hills, I. D.;
Netherton, M. R.; Fu, G. C. Angew. Chem., Int. Ed. 2003, 42, 5749-5752.
(r) Frisch, A. C.; Beller, M. Angew. Chem., Int. Ed. 2005, 44, 674-688.
(2) Arp, F. O.; Fu, G. C. J. Am. Chem. Soc. 2005, 127, 10482-10483.
(3) (a) Castano, A. M.; Echavarren, A. M. Organometallics 1994, 13, 2262-
2268. (b) Gonzalez-Bobes, F.; Fu, G. C. J. Am. Chem. Soc. 2006, 128,
5360-5361.
The unique structure and stability of 2, as well as its probable
involvement in a catalytic cycle, warranted further studies of
this paramagnetic organometallic compound. Magnetic and DFT
studies of 2 interested us, in particular, as we reasoned that if
(4) Inoue, S.; Yokoo, Y. J. Organomet. Chem. 1972, 39, 11-16.
(5) (a) Piber, M.; Jensen, A. E.; Rottlaender, M.; Knochel, P. Org. Lett. 1999,
1, 1323-1326. (b) Dai, C.; Fu, G. C. J. Am. Chem. Soc. 2001, 123, 2719-
2724.
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Ligand Redox Effects on a Cross-Coupling Catalyst
A R T I C L E S
Scheme 2. Possible Reaction Pathways to 2a
trends between the electronic structures of the catalysts with
activities can be observed with terpyridine and other ligands
then one may be able to rationally optimize future catalysts to
ultimately increase the efficiency and scope of the cross-coupling
reaction. Here we report new details on the mechanism of the
formation of 2, EPR and unrestricted DFT analyses of 2, and
the trends observed in the catalytic reactivity of a large pool of
related ligand derivatives.
terpyridine ligand. Indeed, Porschke and co-workers noted that,
upon addition of π-acceptors to 1, ethane can be formed at
temperatures as low as -78 °C.7
Results and Discussion
The formation of 2a from 1 (eq 7) can be envisioned to
proceed by a number of different mechanisms, two of which
are outlined in Scheme 2. The first pathway involves displace-
ment of TMEDA by the terpyridine ligand to produce a five-
coordinate nickel-dialkyl intermediate, which then undergoes
reductive homolysis to produce 2a and a methyl radical (Scheme
2, top). In this mechanism, ethane can then be formed by the
combination of two methyl radicals to produce the half-
equivalent of ethane observed in solution. Alternatively, in a
“nonradical” pathway, addition of terpyridine can slowly lead
to a ligand-induced loss of ethane, in which the resulting Ni(0)
fragment then undergoes a comproportionation reaction with a
remaining Ni(II)-dimethyl species to afford 2a (Scheme 2,
bottom). For each of the Ni(0) and Ni(II) fragments in the
nonradical pathway, L is undefined and may consist of partially
bound diamine, terpyridine, or both. To ascertain whether such
radical or nonradical pathways were involved in the formation
of 2, addition of tpy′ to a mixture of 1 and (TMEDA)Ni(CD3)2
(4) in THF-d8 was performed, and the resulting ethane that was
Notably, the overall reaction described in eq 7 also proceeds
with a net reduction of the organometallic species. A reduction
to an odd-electron species can be confirmed by EPR spectros-
copy, and complex 2a provides a strong unresolved EPR signal
with isotropic g ) 2.021 ( 0.002 at room temperature in THF
solution (Figure 1a). The same solution when frozen at 77 K
exhibits a rhombic signal with g1 ) 2.056, g2 ) 2.021, and g3
) 1.999 (Figure 1b). The giso ) (g1 + g2 + g3)/3 determined
from the rhombic g tensor is similar to the fluid solution g value.
This confirms that the same species was detected in solution
and at 77 K. Interestingly, the g values suggest a more organic-
based radical rather than a metal-centered one, implying that
the charge-transfer state consisting of a Ni(II)-methyl cation
and a reduced ligand (Chart 1) is the ground-state structure; g
values for radicals in extended organic π-systems typically fall
in the range of 2.003-2.005,8 whereas a four-coordinate Ni(I)
complex was previously reported to have a g value close to
2.18.9 Budzelaar and co-workers had observed a similar ligand
reduction phenomenon with cobalt complexes of the tridentate
diiminopyridine ligands.10 Additionally, nickel complexes of
diiminobenzosemiquinonate have been shown to exist in the
ground state with an unpaired electron residing on the reduced
ligand,11 and reduction of bound terpyridine ligands has also
been observed to occur upon addition of terpyridine to ytter-
bocene complexes.12 Perhaps most relevant to this work is Gray
1
2
formed was analyzed by H and H NMR spectroscopy (tpy′
was used for solubility considerations). If ethane was produced
by a combination of two methyl radicals (Scheme 2, top), then
a statistical mixture of CH3-CH3, CH3-CD3, and CD3-CD3
would be expected. CH3-CD3 can be distinguished by its
1
signature H NMR spectrum.6 If, however, the reaction pro-
ceeded in a nonradical pathway (Scheme 2, bottom), then only
CH3-CH3 and CD3-CD3 would be expected, as the ethane
would result from a reductive elimination reaction. The experi-
mental observation is that only CH3-CH3 and CD3-CD3 was
observed (eq 10). Methane might also be expected to form from
a hydrogen atom abstraction from THF solvent, but no methane
nor any of its isotopomers were observed by NMR spectroscopy.
The data thus support a mechanism by which ethane is produced
via a nonradical pathway, triggered by the addition of a
(7) Kaschube, W.; Porschke, K. R.; Wilke, G. J. Organomet. Chem. 1988,
355, 525-532.
(8) Leffler, J. E. An Introduction to Free Radicals; Wiley-Interscience: New
York, 1993.
(9) Ge, P.; Riordan, C. G.; Yap, G. P. A.; Rheingold, A. L. Inorg. Chem. 1996,
35, 5408-5409.
(10) Knijnenburg, Q.; Hetterscheid, D.; Kooistra, T. M.; Budzelaar, P. H. M.
Eur. J. Inorg. Chem. 2004, 1204-1211.
(11) Herebian, D.; Bothe, E.; Neese, F.; Weyhermueller, T.; Wieghardt, K. J.
Am. Chem. Soc. 2003, 125, 9116-9128.
(6) Slaughter, L. M.; Wolczanski, P. T.; Klinckman, T. R.; Cundari, T. R. J.
Am. Chem. Soc. 2000, 122, 7953-7975.
(12) Kuehl, C. J.; Da Re, R. E.; Scott, B. L.; Morris, D. E.; John, K. D. Chem.
Commun. 2003, 2336-2337.
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A R T I C L E S
Jones et al.
overlap of nickel electron density with the LUMO of the
terpyridine ligand (the SOMO in the ligand radical anion). These
ideas are summarized in Figure 2.
From Gray et al.,13 gy ) (ge + 2êFd/∆E), where ge ) free
electron value ) 2.0023, ê ) one-electron spin-orbit coupling
for M+ (Ni: 565 cm-1, Pt: 3400 cm-1),14 and Fd is the nickel
3dyz contribution to the SOMO. Moreover
gy - ge ) 2.021 - 2.0023 ) 0.0187 ) 2êFd/∆E )
2(565 cm-1)Fd/∆E (ref 13)
Then, assuming ∆E ) ∼ 15 000 cm-1 (Figure 2), this gives Fd
) 0.25. This value for Fd seems reasonable, as Figure 3 shows
a small, but clear, contribution of the nickel dyz to the SOMO.
The value is also not so low (<0.1) that there is negligible nickel
contribution, but then not so high (>0.5) that the orbital is more
metal than ligand character. The UV data (Table 1) of 2 are
also consistent with the reduced character of the ligand, as the
spectra are strikingly similar to those of Na(tpy)2.
The unusual electronic structure of 2 suggested by the EPR
data can have large implications for cross-coupling catalysis,
as activities may correlate with the ability of the tridentate
ligands to stabilize a radical anion. To further expound this
intriguing EPR result, DFT calculations (unrestricted B3LYP/
m6-31G*; see Experimental Section) were performed on 2a in
order to determine the spatial distribution of the spin density.
Geometry optimization yielded a planar (except for the methyl
hydrogens), approximately square complex, the geometry of
which agrees well with the X-ray crystal structure.1a The
calculated and measured bond lengths are compared in Table
2.16 A number of population analyses were also included in the
DFT calculations, and all show that the majority of the spin
density in 2a resides on the terpyridine ligand (Table 3). An
isodensity representation of the SOMO of 2a, depicted in Figure
3, also shows that the unpaired electron is delocalized over the
aromatic ligand. The spin density calculations in the planar
conformation are thus in full agreement with the EPR data and
support a ground-state structure that is most properly described
as a nickel(II)-alkyl cation bearing a reduced terpyridine ligand.
Interestingly, the SOMO shows antibonding interactions between
the central nitrogen with nickel, suggesting that, upon oxidation,
this bond should shorten.17 However, this is not observed
experimentally (Table 2), likely due to the short Ni-Ni contacts
observed in the crystal structure of 3.1b The calculations also
show a large amount of radical character at the carbons ortho
and para to the central nitrogen of the terpyridine ring. Kiplinger
has reported alkyl migration to these sites in terpyridine lutetium
complexes.18 Although we have never observed analogous
chemistry with the nickel complexes, we cannot rule out such
species in a catalytic cycle at this time.
Figure 1. X-band EPR spectra of 2a in THF (a) in fluid solution at room
temperature, (b) in frozen glass at 77 K. Parameters: microwave frequency
9.095 GHz; microwave power 5 mW; modulation amplitude 1 G; gain
2 × 103.
Chart 1. Representations of 2a as a Ni(I)-Methyl Complex (left)
and the Charge-Transfer State Comprising a Ni(II)-Alkyl Cation
(right)
and co-workers’ report of ligand reduction in the closely related
(terpyridine)PtCl complex.13
The g tensors of frozen solution EPR spectra are sensitive to
even small contribution from a metal to the singly occupied
molecular orbital (SOMO), with the deviation ∆g ) g -
g(electron) depending on the spin-orbit coupling. The obtained
rhombic spectrum displays g1 > g2 ∼ 2.0 . g3 and g3 < 2.0,
suggesting a minor contribution of the metal d-orbital in the
SOMO.14 Assuming similar electronic factors and a similar
coordinate system as Gray’s description of (tpy)PtCl,13 estimates
of expected g values for nickel can be made based on the
difference between the first ionization energy (IE) of platinum
and nickel and their respective M+1 spin-orbit coupling
values.15 Using the difference in the first IE as a rough estimate
of the difference in energy of the 5d-orbitals on platinum versus
the 3d-orbitals on nickel, then the nickel d-orbitals are roughly
10 000 cm-1 closer in energy than those of platinum to the
terpyridyl π*-orbital. This in turn increases the probability of
We have also determined optimized geometries and energies
of 2a, constraining the N2-Ni-C bond angle to five values
between 180 and 120°, to explore the effect of the catalyst
geometry on the electronic structure. Figure 4 shows that the
N2-Ni-C angle can bend about 15° without raising the energy
more than 1 kcal/mol. However, substantial bends require
(13) Hill, M. G.; Bailey, J. A.; Miskowski, V. M.; Gray, H. B. Inorg. Chem.
1996, 35, 4585-4590.
(16) Because of the large errors in the X-ray structure of 2a, a comparison to
the experimentally derived bond lengths is not appropriate.
(17) We thank a reviewer for pointing this out.
(18) Jantunen, K. C.; Scott, B. L.; Hay, P. J.; Gordon, J. C.; Kiplinger, J. L. J.
Am. Chem. Soc. 2006, 128, 6322-6323.
(14) Goodman, B. A.; Raynor, J. B. AdV. Inorg. Chem. Radiochem. 1970, 13,
135-362.
(15) Huheey, J. E. Inorganic Chemistry, Principles of Structure and ReactiVity,
3rd ed.; Harper and Row: New York, 1983; pp 42-44.
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Ligand Redox Effects on a Cross-Coupling Catalyst
A R T I C L E S
Figure 2. Molecular orbital diagram for platinum and nickel complexes of terpyridine, based on ref 13.
Figure 3. Singly occupied molecular orbital of complex 2a from unrestricted DFT calculations.
Table 1. UV-Visible Data of Reduced Terpyridyl Compounds
for the ligand survey. While solubility effects dominate the
difference in yields observed with ligands 5 and 6,1b we find
here that, upon making the terpyridine ligand more electron rich
as in 7 and 8, a substantial decrease in yields for the cross-
coupling reaction results. However, decreases in yields were
also observed when electron-poor terpyridine ligands, such as
9-11, were used. Steric effects with the terpyridine-based
ligands were quite dramatic, as it was found that placing methyl
groups in the 6 and 6′′ positions of the terpyridine ligand (12)
shuts down the catalysis almost entirely. The π-extended ligands
13 and 14 showed quite respectable yields, but did not
outperform tpy′. The slightly better performance of 13 over 14,
especially in the cross-coupling of alkyl bromides, is not well-
understood at this point and is currently the subject of further
investigation. Connick and co-workers noted that platinum
complexes of 15 and 16 had more negative oxidation potentials
than the analogous terpyridine complex,21 so we were curious
how the cross-coupling catalysis would proceed with these
1
compound
λ
max (nm)
solvent
ꢀ
(L mol-1 cm-
)
×
103
2a
220, 270, 384,
538, 576
THF
10.1, 12.1, 5.63, 1.55,
1.86
2b
220, 270, 380,
544, 584
216, 242, 280,
380, 450, 606
THF
THF
26.6, 25.0, 11.4, 3.41,
3.97
15.7, 22.6, 21.7, 1.51,
1.13, 3.44
Na(tpy)2
10-15 kcal/mol. DFT calculations at these strongly bent
geometries predict that the spin density on the nickel increases
relative to the planar form (see Table 3 for a representative
example). This additional insight provided by the DFT studies
suggests that ligands that effect the positioning of the pendant
alkyl group relative to the plane of the other three nitrogen atoms
can thus, in theory, modulate this charge-transfer chemistry by
allowing or not allowing optimum overlap of the dyz-orbital with
the nitrogen of the central pyridyl ring of the ligand as
represented in the SOMO (Figure 3).
To complement the EPR and DFT studies, comparative cross-
coupling reactions were performed with the ligands shown in
Chart 2. We chose iodo- and bromopropylbenzene as our alkyl
electrophiles and n-pentylzinc bromide as our alkyl nucleophile
(Table 4). Table 4 lists all of the cross-coupling yields obtained
(19) Bessel, C. A.; See, R. F.; Jameson, D. L.; Churchill, M. R.; Takeuchi, K.
J. J. Chem. Soc., Dalton Trans. 1992, 3223-3228.
(20) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.;
Bohmann, J. A.; Morales, C. M.; Weinhold, F. NBO 5.0; University of
Wisconsin, Madison: Theoretical Chemistry Institute, 2001.
(21) Willison, S. A.; Jude, H.; Antonelli, R. M.; Rennekamp, J. M.; Eckert, N.
A.; Bauer, J. A. K.; Connick, W. B. Inorg. Chem. 2004, 43, 2548-2555.
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A R T I C L E S
Jones et al.
Table 2. Calculated Bond Lengths (Å) and Angles (°) Found in 2a
versus Those Found Experimentally in Related Derivatives
bond or
angle
2a
free
terpyridine19
2b
3
(calcd)
(X-ray)
(X-ray)
Ni-C16
Ni-N1
1.935
1.929
1.867
1.929
1.351
1.389
1.364
1.364
1.389
1.351
1.384
1.407
1.384
1.402
1.449
1.392
1.403
1.403
1.392
1.449
1.402
1.384
1.407
1.384
174.7
82.1
1.944(4)
1.903(3)
1.845(3)
1.914(3)
1.356(4)
1.385(4)
1.358(4)
1.355(4)
1.377(4)
1.350(4)
1.364(5)
1.403(5)
1.384(4)
1.403(4)
1.450(4)
1.390(4)
1.393(4)
1.413(4)
1.378(4)
1.455(4)
1.396(4)
1.384(4)
1.399(5)
1.375(5)
177.19 (12)
81.58 (12)
1.921(9)
1.927(9)
1.880(7)
1.923(8)
Figure 4. Optimized relative energies of (tpy)NiMe as a function of the
constrained N2-Ni-C(Me) angle.
Ni-N2
Ni-N3
N1-C1
1.339(9)
1.317(7)
1.353(8)
1.349(7)
1.350(8)
1.344(9)
1.391(10)
1.351(10)
1.388(9)
1.349(8)
1.488(8)
1.398(9)
1.376(9)
1.380(9)
1.396(8)
1.493(8)
1.391(9)
1.386(9)
1.366(10)
1.360(10)
1.336(11)
1.370(11)
1.349(10)
1.328(11)
1.384(10)
1.346(11)
1.370(13)
1.385(13)
1.416(12)
1.364(13)
1.489(11)
1.371(11)
1.402(12)
1.401(12)
1.387(12)
1.465(12)
1.370(13)
1.405(12)
1.410(12)
1.379(13)
N1-C5
N2-C6
N2-C10
N3-C11
N3-C15
C1-C2
C2-C3
C3-C4
C4-C5
C5-C6
C6-C7
C7-C8
C8-C9
C9-C10
C10-C11
C11-C12
C12-C13
C13-C14
C14-C15
N2-Ni-Me
N1-Ni-N2
175.8 (4)
82.1 (3)
Figure 5. ORTEP diagram of (tpy′)NiI2. Ellipsoids shown at the 50% level.
Selected bond distances (Å): I1-Ni1 2.6027(5), I2-Ni1 2.6248(5), Ni1-
N2 1.979(2), Ni1-N1 2.069(2), Ni1-N3 2.092(2). Selected bond angles
(°): N2-Ni1-N1 78.52(9), N2-Ni1-N3 78.27(9), N1-Ni1-N3 156.32-
(9), N2-Ni1-I1 146.59(6), N1-Ni1-I1 98.48(6), N3-Ni1-I1 98.25(6);
N2-Ni1-I2 100.83(6), N1-Ni1-I2 94.97(6), N3-Ni1-I2 93.95(6), I1-
Ni1-I2 112.576(14).
Table 3. Calculated Free Valencies for the Heteroatoms of
Terpyridyl Nickel Complexes
(tpy)NiMe with a
Ni C Angle of 144.1°
Compound 2a
N2
−
−
population analysis
Ni
N2
N1
Ni
N2 N1
that an imine-based moiety is required for catalysis, as ligand
24 (which can potentially form an amido complex) is unproduc-
tive. Dppz (25)22 and dppn (26)23 afforded yields lower than
bathophenanthroline.
Lowdin free valencies
Mulliken free valencies 0.076 0.187 0.063 0.331 0.194 0.064
Natural Spin Densities
from NBO analysis20
0.087 0.179 0.059 0.337 0.187 0.062
0.037 0.245 0.090 0.182 0.273 0.082
The ligand survey also afforded an opportunity to study the
role of the initial nickel source in the cross-coupling reactions.
We found that similar yields were obtained whether a ligand
was added in situ to commercially available Ni(COD)2 or
whether preformed LNiI2 was used as the starting material
(Table 4, entries 2, 20, and 21). One advantage of preparing
preformed nickel dihalide complexes is that they are air-stable
and can be handled and stored outside of a glovebox.24 Their
use also ensures complete ligation of the imines to nickel and
rules out any influence of cyclooctadiene in the interpretation
of results.25 An X-ray structure of (tpy′)NiI2 was obtained to
further support the nature of the nickel diiodides, and the
ligands. While the parent ligand 15 gave yields higher than those
seen with terpyridine, we were disappointed that the tetram-
ethylated ligand 16 showed much lower activity for cross-
coupling catalysis. Again, the steric environment afforded by
the ligand appears to play a major role in catalytic activity, as
it was seen that yields with the bispyrazole system once again
rose when the methyl groups were positioned away from the
metal center as in 17. Given that the sterics in ligands 12 and
16 discourage a flat geometry, it is tempting to infer that any
ligand preventing the flattening of the N3Ni-R coordination
sphere also retards reactivity. Additionally, other ligands deviat-
ing in structure from the planar, aromatic triimines, such as 18-
22, all showed poor activity. Finally, we see that a tridentate
ligand framework is not necessary to achieve catalysis, as the
yields obtained with electron-poor bathophenanthroline (23) are
competitive with those obtained with 14. It appears, however,
(22) Greguric, A.; Greguric, I. D.; Hambley, T. W.; Aldrich-Wright, J. R.;
Collins, J. G. J. Chem. Soc., Dalton Trans. 2002, 849-855.
(23) Yam, V. W.-W.; Lo, K. K.-W.; Cheung, K.-K.; Kong, R. Y.-C. J. Chem.
Soc., Chem. Commun. 1995, 1191-1193.
(24) We found, however, that DMF solutions of (terpyridyl)NiI2 slowly
decomposed to [(terpyridyl)2Ni]I2. Methylene chloride solutions of (terpy-
ridyl)NiI2 were stable over much longer times (days).
9
13180 J. AM. CHEM. SOC. VOL. 128, NO. 40, 2006

Ligand Redox Effects on a Cross-Coupling Catalyst
A R T I C L E S
Chart 2. Ligands Used for Catalytic Studies
Table 4. Effect of Ligand on Yields of Alkyl-Alkyl
Scheme 3. Possible Mechanism for Nickel-Catalyzed Alkyl-Alkyl
Negishi Reactions
Cross-Couplingsa
% yield for
% yield for
Br
entry
ligand
X
)
I
X )
1
2
5
5b
60
54
98
53
38
54
19
44
0
17
16
46
44
3
33
3
33
0
17
4
3
6
4
7
5
8
6
9
7
10
10c
11
11c
12
13
14
15
16
17b
18
19
20
21
22
23
24
25
26
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
31
6
63
58
67
13
63
8
59
37
45
16
31
0
LNiX2 complexes did make a difference, however, in the
reactivity of ligands that were easy to reduce with nickel(0)
(Table 4, entries 7 vs 8 and 9 vs 10). We found that these
electron-poor ligands can be attached to an air-stable source of
nickel(II) (commercially available (DME)NiCl2 (DME ) 1,2-
dimethoxyethane)) without concomitant ligand reduction.
4
1
1
0
3 (2b)
5 (8b)
0
1
56
43
0
3
40
37
Implications
27
23
On the basis of the above studies and previous work,1a,b an
updated mechanism for the nickel-catalyzed alkyl-alkyl Negishi
reactions is proposed in Scheme 3. This mechanism involves
reaction of a (terpyridyl)Ni(alkyl) complex, such as 27, with
an alkyl halide to form 28 and an alkyl radical. The redox
potentials of 2a and 2b were found to be -1.32 and -1.44 V
(vs Ag/Ag+ in THF solution),1b making the alkyl halide
reduction by 2 thermodynamically favorable. What has emerged
as a new, key feature of this current study is that the alkyl halide
reduction by (terpyridine)NiRalkyl is substantially ligand based!
This one-electron, ligand-based redox event is fundamentally a
a Yields calculated by GC relative to an internal standard. b Catalyst used
was LNiI2. c Catalyst used was LNiCl2.
ORTEP diagram is shown in Figure 5. This terpyridine-based
molecule was found to crystallize in the monomeric, five-
coordinate form, similar to (tpy′)NiCl2.26 The use of preformed
(25) In a control experiment, use of Ni(COD)2 with no added ligand gave no
apparent formation of cross-coupled products for both the alkyl bromides
and alkyl iodides.
(26) Suzuki, H.; Matsumura, S.; Satoh, Y.; Sogoh, K.; Yasuda, H. React. Funct.
Polym. 2004, 59, 253-266.
9
J. AM. CHEM. SOC. VOL. 128, NO. 40, 2006 13181

A R T I C L E S
Jones et al.
Table 5. Calculated Free Valencies for Potential Intermediates in a Catalytic Cycle
LNiMe
L
)
ligand 23
Compound 29
Compound 30
population analysis
Ni
N2
N1
Ni
N2
N1
Ni
N2
N1
Lowdin free valencies
Mulliken free valencies
natural spin densities
from NBO Analysis
1.145
1.148
1.244
0.059
0.061
0.003
0.059
0.061
0.003
0.833
0.847
0.938
0.090
0.090
0.143
0.023
0.024
0.019
1.022
1.034
1.131
0.076
0.076
0.015
0.039
0.036
0.008
published procedures. Complexes 2a and 2b1a,b were synthesized
according to previously published procedures. The alkyl-alkyl cross-
coupling reactions in Table 4 were performed by previously published
procedures.1b
UV-Visible Measurements. Solutions of 2a, 2b, and Na(tpy)2 in
THF were prepared in a nitrogen glovebox and placed in quartz cuvettes
(1 cm path length) capped with a Teflon stopper and wrapped tightly
with Parafilm. The UV-visible spectra were collected using a HP
8452A diode array spectrophotometer using a spectral window of 190-
820 nm wavelengths.
Electronic Structure Calculations. Quantum calculations were
performed with the PQS 3.2 software.38 Initial geometries were
constructed using PCModel 8.539 and preoptimized via the MMX force
field. Both constrained and unconstrained geometry optimizations were
performed using the B3LYP exchange-correlation functional40 and the
m6-31G* basis set.41 Note that the most consistent definition of the
SOMO of a doublet state is the charge natural orbital whose occupation
number is close to 1, and this was used in this work. The occupation
number in the case of 2a was exactly 1.
EPR Measurements. Solutions of 2a in THF (10-2-10-3 M) were
prepared under nitrogen in a drybox, placed in quartz EPR tube (o.d.
) 4 mm, WILMAD), degassed before use by three freeze-pump-
thaw cycles, and the tube was sealed. EPR spectra were measured at
X-band (9 GHz) using a Bruker ESP 300 spectrometer equipped with
a Bruker ER035M gaussmeter and a HP 5352B microwave frequency
counter for g value determination.
new concept in cross-coupling chemistry and may provide some
insight into how stereo-convergence might be possible for the
asymmetric versions of this reaction using chiral ligands. We
speculate that alkyl halide reduction by the ligand leaves an
alkyl radical in close proximity to the metal center, where an
oxidative radical addition ensues to afford the nickel(III)-dialkyl
species 29. If L is therefore chiral, enantioselective addition of
the radical to nickel may take place. Fast reductive elimination
of cross-coupled alkane then occurs to release electrons from
the antibonding orbital of 29 leaving 30 as the final nickel-
containing product of a catalytic cycle. We have already shown
that indeed (tpy′)NiI is a viable catalyst for alkyl-alkyl Negishi
reactions.1b Detailed kinetic studies will be necessary to further
support this mechanism, and such studies are being initiated in
our labs. Elements of Scheme 3 may also fit a chain-like
mechanism, similar to what had been proposed by Kochi.27 With
regard to the electronic structures of the proposed intermediates,
we can say that spin-unrestricted DFT analyses of 29 and 30
predict that these compounds should be formulated as Ni(III)
and Ni(I) complexes, respectively (Table 5). The oxidation states
may also be dictated by the nature of the diimine ligand, as we
found that bidentate ligands, such as bathophenanthroline, have
much different electronic characteristics than the terpyridyl-
based counterparts (Table 5).
Synthesis of Na(tpy)2. In a nitrogen-filled glovebox, sodium metal
(32 mg, 1.41 mmol), which had been pressed, was stirred in 6 mL of
THF. A separate solution of tpy (655 mg, 2.81 mmol) in 14 mL of
THF was then added slowly via pipet. Within 10 min, the yellow
solution became dark green in color. After 1 h of stirring, the THF
was removed via high vacuum manifold. The solid was collected and
dried for an additional 2 h, yielding 492 mg (71%) of material. The
solid was used without further purification.
Synthesis of (TMEDA)Ni(CD3)2 (4): (TMEDA)Mg(CD3)2 was
prepared analogously to (TMEDA)Mg(CH3)242 with the substitution of
CD3I for CH3I. Compound 4 was then synthesized in 61% yield from
(TMEDA)Mg(CD3)2 and (TMEDA)Ni(acac)2 as previously reported for
the synthesis of (TMEDA)Ni(CH3)2.43 1H NMR (THF-d8, -40 °C): δ
2.28 (s, 12H), 2.13 (s, 4H). 2H NMR (THF-d8, -40 °C): δ -1.43 (s).
13C NMR (THF-d8, -40 °C): δ 59.3 (s), 46.6 (s), -12.7 (septet, 1JC-D
) 18.5 Hz).
Experimental Section
General Considerations. All manipulations were performed using
standard Schlenk techniques or in a nitrogen-filled drybox, unless
otherwise noted. Solvents were distilled from appropriate drying
reagents, such as Na/benzophenone or CaH2. All reagents were used
as received from commercial vendors. Pentylzinc bromide was pur-
chased from Rieke Metals, Inc. and used without further purification.
Elemental analyses were performed by Desert Analytics. 1H NMR
spectra were recorded at ambient temperature (unless otherwise noted)
on a Bruker Avance 300 MHz (75 MHz for 13C nuclei) spectrometer
and referenced to residual proton or carbon solvent peaks. A Rigaku
MSC Mercury/AFC8 diffractometer was used for X-ray structural
determinations. Ligands 7,28 9,29 10,30 11,28 12,31 13,32 14,32 15,21 16,33
19,36 20,34 21,33 22,35 and 2437 were synthesized according to previously
Isotopic Labeling Study for Scheme 2: To a vial cooled to -30
°C were added (TMEDA)Ni(CH3)2 (11 mg, 0.054 mmol) and (TMED-
A)Ni(CD3)2 (11 mg, 0.054 mmol) in THF-d8 (0.5 mL). This solution
was added to a precooled J. Young type NMR tube. To the NMR tube
was added a solution of tpy′ (43 mg, 0.11 mmol) in THF-d8 (0.5 mL),
and the Teflon cap to the NMR tube was immediately closed. The NMR
tube was shaken vigorously and allowed to warm to room temperature.
(27) (a) Tsou, T. T.; Kochi, J. K. J. Am. Chem. Soc. 1979, 101, 7547-7560.
(b) Tsou, T. T.; Kochi, J. K. J. Am. Chem. Soc. 1979, 101, 6319-6332.
(c) Morrell, D. G.; Kochi, J. K. J. Am. Chem. Soc. 1975, 97, 7262-7270.
(28) Case, F. H. J. Org. Chem. 1962, 27, 640-641.
(29) Case, F. H.; Kasper, T. J. J. Am. Chem. Soc. 1956, 78, 5842-5844.
(30) Hobert, S. E.; Carney, J. T.; Cummings, S. D. Inorg. Chim. Acta 2001,
318, 89-96.
(31) Sato, Y.; Nakayama, Y.; Yasuda, H. J. Organomet. Chem. 2004, 689, 744-
750.
(32) Hung, C.-Y.; Wang, T.-L.; Jang, Y.; Kim, W. Y.; Schmehl, R. H.; Thummel,
R. P. Inorg. Chem. 1996, 35, 5953-5956.
(33) Reger, D. L.; Grattan, T. C. Synthesis 2003, 350-356.
(34) Jameson, D. L.; Goldsby, K. A. J. Org. Chem. 1990, 55, 4992-4994.
(35) Reger, D. L.; Grattan, T. C.; Brown, K. J.; Little, C. A.; Lamba, J. J. S.;
Rheingold, A. L.; Sommer, R. D. J. Organomet. Chem. 2000, 607, 120-
128.
(38) PQS, version 3.2; Parallel Quantum Solutions, 2013 Green Acres Road,
Fayetteville, AR 72703.
(39) PCMODEL for Windows 8.50.0, September 2003, Serena Software.
(40) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
(41) Mitin, A. V.; Baker, J.; Pulay, P. J. Chem. Phys. 2003, 118, 7775-7782.
(42) Anderson, R. A.; Wilkinson, G. Inorg. Synth. 1979, 19, 262-265.
(43) Yamamoto, T.; Yamamoto, A.; Ikeda, S. J. Am. Chem. Soc. 1971, 93,
3350-3359.
(36) Peris, E.; Mata, J.; Loch, J. A.; Crabtree, R. H. Chem. Commun. 2001,
201-202.
(37) Klappa, J. J.; Rich, A. E.; McNeill, K. Org. Lett. 2002, 4, 435-437.
9
13182 J. AM. CHEM. SOC. VOL. 128, NO. 40, 2006

Ligand Redox Effects on a Cross-Coupling Catalyst
A R T I C L E S
After standing for 2 days and being shaken periodically to allow for
adequate mixing, the H NMR spectrum was obtained. No formation
Synthesis of (tpy)NiI2 as a Representative Synthesis of LNiI2
Compounds: To a solution of Ni(COD)2 (475 mg, 1.73 mmol) in 50
mL of THF was added a solution of tpy (411 mg, 1.76 mmol) in 20
mL of THF. This green solution was stirred for 5 min at which point
a solution of I2 (441 mg, 1.74 mmol) in 20 mL of THF was cannulated
resulting in an immediate color change to orange. The reaction was
stirred for 1 h followed by filtering of the orange solid using a glass
frit. The solid was washed with 40 mL of diethyl ether and dried on
the high vacuum manifold for 4 h. Yield 942 mg, 1.73 mmol,
quantitative. Anal. Calcd (found) for C15H11I2N3Ni: C, 32.94 (33.01);
H, 2.24 (2.03).
Synthesis of (4,4′,4′′-Trichloroterpyridine)NiCl2 as a Representa-
tive Synthesis of LNiCl2 Compounds: A mixture of (DME)NiCl2 (150
mg, 0.684 mmol) and 10 (229 mg, 0.683 mmol) in THF (10 mL) was
heated at reflux for 15 h. The resulting light green precipitate was
collected on a glass-fritted funnel and washed with THF, Et2O, and
pentane. The light green solid was dried on a high vacuum manifold,
yielding 294 mg (92%) of product.
1
of CD3CH3 was detected, only CH3CH3. Using the free TMEDA ligand
as the internal standard, the yield of CH3CH3 was 0.0468 mmol, 43%.
It is noted that a control sample of CD3CH3 was easily detected at
1
0.025 mmol. H NMR of CH3CH3 (300.13 MHz, THF-d8, 233 K): δ
0.86 (s). 1H NMR of CD3CH3 (300.13 MHz, THF-d8, 296 K): δ 0.82
(septet, JH-C-C-D ) 1.2 Hz). 2H NMR (proton decoupled) of CD3CH3
(46.07 MHz, THF-d8, 296 K): δ 0.81 (s). 13C NMR (75.47 MHz, THF-
d8, 296 K): δ 6.85 (septet, JC-D ) 0.6 Hz).
Synthesis of Ligand 8: 4,4′,4′′-Trichloro-2,2′:6′,2′′-terpyridine30 (490
mg, 1.46 mmol) and MnCl2(H2O)4 (605 mg, 3.1 mmol) were suspended
in 50 mL of dry MeOH, and then 15 g of freshly distilled piperidine
was added. The mixture was heated to reflux for 20 h. The resulting
mixture was cooled, and the solvents were removed via a rotovap. To
the residue was then added 5 g of KOH in 100 mL of H2O and 100
mL of MeCN. This suspension was stirred open to air for 24 h at room
temperature. The organics were then extracted with CH2Cl2 and dried
with MgSO4. Yield: 511 mg, 73%. 1H NMR (CDCl3, 22 °C): δ 8.34
(d, J ) 6.0 Hz, 2H), 8.16 (d, J ) 2.6 Hz, 2H), 7.87 (s, 2H), 6.69 (dd,
J ) 6.0, 2.8 Hz, 2H), 3.56 (bt, J ) 5.3 Hz, 4H), 3.48 (bt, J ) 4.7 Hz,
8H), 1.73-1.66 (bm, 18H). 13C NMR (CDCl3, 25 °C): δ 156.9, 156.7,
155.8, 155.3, 148.6, 107.9, 105.8, 105.3, 47.7, 47.4, 25.5, 25.3, 24.5,
24.4. HRMS calcd for C30H38N6: 483.3236. Observed: 483.3236
Synthesis of Ligand 17: Synthesis was analogous to that for 2,6-
bis(N-pyrazolyl)pyridine reported by Jameson.34 Under N2 was heated
at 75 °C a 20 mL solution of pyrazole (1.198 g, 17.6 mmol) and
potassium metal (649 mg, 16.6 mmol) in diglyme until the potassium
metal dissolved (1-2 h). 2,6-Dibromo-3,5-dimethylpyridine44 (1.466
g, 5.53 mmol) was added at once and the solution heated to 110 °C for
4 days. Workup was completed as reported by Jameson. Yield 1.136
Acknowledgment. D.A.V. thanks the Chemical Sciences,
Geosciences and Biosciences Division, Office of Basic Energy
Sciences, Office of Science, of the U.S. Department of Energy
(DE-FG02-06ER15801), the Arkansas Biosciences Institute, and
the NIH (RR-015569-06) for support of this work. P.P.
acknowledges support by the National Science Foundation
(CHE-0515922). The authors thank Kevin Shaughnessy for help
with EPR experiments, and Marv Leister for help with 2H NMR
measurements.
Note Added after ASAP Publication. An author’s name was
spelled incorrectly in the version of this paper published ASAP
on September 19, 2006. The corrected version was published
ASAP on September 20, 2006.
1
g, 86%. H NMR (CDCl3, 300.13 MHz, 23 °C): δ 8.24 (dd, J ) 2.5,
0.7 Hz, 2H), 7.75 (dd, J ) 1.6, 0.6 Hz, 2H), 7.64 (t, J ) 0.7 Hz, 1H),
6.45 (dd, J ) 2.6, 0.7 Hz, 2H), 2.59 (d, J ) 0.7 Hz, 6H). 13C NMR
(CDCl3, 75.47 MHz, 23 °C): δ 147.2, 146.5, 141.3, 129.9, 124.5, 106.8,
19.1. Anal. Calcd (found) for C13H13N5; C, 65.18 (65.25); H, 5.67 (5.48).
Supporting Information Available: A crystallographic in-
formation file (CIF) for (tpy′)NiI2. This material is available
free of charge via the Internet at http://pubs.acs.org.
(44) Dunn, A. D.; Guillermic, S. Z. Chem. 1988, 28, 59-60.
JA063334I
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J. AM. CHEM. SOC. VOL. 128, NO. 40, 2006 13183