Synthesis of a bulky bis(imino)pyridine compound: A methodology for systematic variation of steric bulk and energetic implications for metalation

Article abstract of DOI:10.1039/b815987d

A new bis(imino)pyridine compound, 2,6-{(2,6-Me₂-C ₆H₃)NC(t-Bu)}₂C₅H₃N (6), has been synthesized with t-butyl substituents on the imino carbon atoms. The stepwise synthetic method for assem

Full text of DOI:10.1039/b815987d

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis of a bulky bis(imino)pyridine compound: a methodology for
systematic variation of steric bulk and energetic implications for metalation†
Janelle E. Steves, Margaret D. Kennedy, Karen P. Chiang, William S. Kassel, William G. Dougherty,
Timothy J. Dudley and Deanna L. Zubris*
Received 12th September 2008, Accepted 21st October 2008
First published as an Advance Article on the web 18th December 2008
DOI: 10.1039/b815987d
A new bis(imino)pyridine compound, 2,6-{(2,6-Me2-C6H3)NC(t-Bu)}2C5H3N (6), has been synthesized
with t-butyl substituents on the imino carbon atoms. The stepwise synthetic method for assembly of
this compound is novel. Compound 6 and its synthetic precursor, mono(imino)pyridine 3, have been
characterized using single-crystal X-ray diffraction. Metalation attempts of 6 using iron(II) chloride
under forcing conditions does not yield the desired iron(II) chloride complex; the use of refluxing acetic
acid solvent provides a minimal amount of a paramagnetic species that has been characterized by
NMR spectroscopy and magnetic susceptibility (NMR method). Computational methods have been
used to evaluate the relative energies of three conformations of bis(imino)pyridine ligands with varying
alkyl substitution at the imino carbon positions. The relative energies of these closed, open and
open-planar conformations of 6 reveal a thermodynamic argument for the difficulty in metalation of 6,
as compared to related ligands with less steric hindrance at the imino carbon atoms.
or pyrrolyl-8 substituted imino nitrogen atoms. C1-symmetric
Introduction
bis(imino)pyridine ligands have been synthesized;4b,7a,7b,9 their
Bis(imino)pyridine ligands first came into the spotlight ten years
asymmetry is derived from differentially substituted imino nitro-
ago when Brookhart1a and Gibson2a described the synthesis of
gen atoms. Also, on a more limited basis, the central pyridyl ring
iron and cobalt complexes bearing these ligands and their use
has been altered to provide iron(II) pre-catalysts with low activity
for ethylene polymerization.10
as pre-catalysts for polyethylene formation. Follow-up studies by
these research groups, as well as DuPont researchers, expanded
the scope of these initial reports.1b–c,2b–c Iron and cobalt catalysts
incorporating the bis(imino)pyridine ligand have been used for
ethylene oligomerization, homopolymerization, and copolymer-
ization with a-olefins, as described in a 2006 review;3 some
degree of success in a-olefin dimerization, oligomerization and
polymerization has been reported by various groups.4
The backbone and substitution pattern of the bis(imino)-
pyridine ligand have been varied extensively, as described in two
recent reviews.5,6 The two most common substitution patterns
for this ligand (of the general formula: 2,6-{(2-R2,4-R4,6-R3-
C6H2)NCR1}2C5H3N) appear in Fig. 1. Ligand modifications
have largely focused on substitution of the imino nitrogen atoms
(with 2-, 2,6-, and 2,4,6- substituted aryl rings being most
common), and on a more limited basis, the use of alkyl-,7 amino-,
The range of functionality (R1) for the imino carbon atoms
is limited, and to date there are no reports of differential
substitution of these imino carbon atoms to provide C1-symmetric
ligands. Most often, R1 is H or Me, since the corresponding
ligands are synthetically derived from commercially available
2,6-diformylpyridine and 2,6-diacetylpyridine, respectively. The
use of R1 = Ph has been reported by four research groups;
moderate yields are achieved using standard condensation for
ligand synthesis,11a and improved yields are achieved using more
forcing10,11b or atypical7a reaction conditions. One report describes
the installation of heteroatom-substituted imino carbon atoms,
specifically, where R1 = OMe, SMe, O(2,6-Me2C6H3) or S(2,6-
Me2C6H3).12 The latter three substituents yielded iron(II) pre-
catalysts with ethylene polymerization activities comparable to
the prototypical (2,4,6-Me3C6H2)-substituted ketimine analogue,
[2,6-{(2,4,6-Me3C6H2)NCMe}2C5H3N]FeCl2. Aliphatic R1 sub-
stituents with increased steric bulk (relative to R1 = Me) were
singularly reported in 2007; pre-assembled bis(imino)pyridine lig-
ands (where R1 = Me) were treated with lithium diisopropylamide,
followed by alkyl halide, to yield the following: R1 = CH2CH3,
CH(CH3)2, CH2CH2Ph, and CH(CH2Ph)2.13 A notable feature
of the corresponding iron(II) pre-catalysts is that as the steric
bulk of R1 increases, polyethylene molecular weight increases
while maintaining catalyst productivity. The extension of Gibson’s
synthetic methodology to install a tertiary alkyl as R1 was
not reported, though one would anticipate continuation of the
favorable trend of increased polyethylene molecular weight with
comparable catalyst productivity.
Fig. 1 Common substitution patterns for the bis(imino)pyridine ligand.
Villanova University, Department of Chemistry, Mendel Science Center,
Villanova, PA 19085, USA. E-mail: deanna.zubris@villanova.edu;
Fax: +1 610 519 7167; Tel: +1 610 519 4874
† CCDC reference numbers 702075 and 702076. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b815987d
1214 | Dalton Trans., 2009, 1214–1222
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
In order to achieve the synthesis of a bis(imino)pyridine
compound with R1 = t-Bu, we developed a new synthetic route.
In our hands, standard condensation of the appropriate t-butyl
disubstituted 2,6-diketopyridine14 with substituted anilines or
N-amino-pyrroles was unsuccessful. Furthermore, a range of
modified reagents and reaction conditions were employed7a,8c,10,15
but at best the mono(imino)pyridine species was detected. In
developing our current synthetic method, we took a cue from the
t-butyl substituted b-diketiminate ligand employed by Holland
as a component of three coordinate iron(II) complexes.16 We
have utilized an imidoyl chloride as a synthetic intermediate,
similar to their use in the preparation of sterically hindered
b-diketiminate ligands.17,18 Our synthetic route also draws upon
the recent literature for selective mono lithium–halogen exchange
of 2,6-dibromopyridine.19
Scheme 2 Reagents and conditions: (iii) n-BuLi, CH2Cl2, -78 ◦C, 15 min;
1.4 equiv. 2, -78–22 ◦C, 22 h; (iv) n-BuLi, THF, -78 ◦C, 15 min; 1.0 equiv.
2, -78–22 ◦C, 17 h.
via flash silica gel column chromatography. Benzene-d6 is an
optimal solvent for 1H NMR spectroscopic analysis; chloroform-
d gives overlapping pyridyl and phenyl proton resonances. NOE
difference experiments were used to assign the three pyridyl proton
resonances and homonuclear decoupling experiments were used
to detect long range coupling between the methyl (attached to the
1
phenyl rings) and the ortho-phenyl protons. The 13C{ H} NMR
=
spectroscopic C N chemical shift of 174.8 ppm (in benzene-d6)
Results and discussion
-1
=
and a C N infrared stretching frequency of 1647 cm provide
further support for our structural assignment.
Synthesis of bis(imino)pyridine 6
Single crystals of 3 for X-ray diffraction studies were obtained
via slow evaporation of a 2.5% ethyl acetate–hexanes solution
(v/v) of 3 over the course of 1 d. Most notably, the pyridyl and
phenyl rings exhibit a nearly perpendicular arrangement, with a
PlnA–PlnB angle of 110.24(5)◦ (see Fig. 2 and Table 1).
Preparation of t-butyl substituted amide, 1, was achieved in a high
yield by combination of pivaloyl chloride and 2,6-dimethylaniline
in the presence of stoichiometric triethylamine (Scheme 1, step i),
as described previously.20,21 Compound 1 was evaluated via single-
crystal X-ray diffraction; suitable crystals of 1 were obtained from
an anhydrous dichloromethane solution cooled to -15 ◦C for 8 d.
The crystal structure for 1 was independently reported while our
work was in progress; our crystallographic data is consistent with
what appears in the literature.22
Scheme
1
Reagents and conditions: (i) 2,6-dimethylaniline, NEt3,
CH2Cl2, reflux, 1 h; (ii) PCl5, toluene, 22 ◦C, 15 h.
Using literature methods, a toluene solution of compound
1 was treated with phosphorous pentachloride to generate the
t-butyl substituted imidoyl chloride, 2 (Scheme 1, step ii); small-
scale synthesis and characterization of 2 was reported previously.23
Ambient pressure distillation was used to remove toluene and the
POCl3 by-product from the product mixture, followed by standard
vacuum distillation or Ku¨gelrohr distillation to yield 2 as an off-
white semi-solid. Alternatively, we have prepared 2 by stepwise
treatment of a 0 ◦C dichloromethane solution of 1 with 2,6-
lutidene (1.66 equiv.) and oxalyl chloride (1.00 equiv.), similar to
reported conditions for the synthesis of related imidoyl chlorides.24
We utilized Peterson’s method for selective mono-lithiation of
2,6-dibromopyridine19 as a model for our synthesis of brominated
mono(imino)pyridine, 3 (Scheme 2, step iii). While Peterson
demonstrated the successful addition of a variety of electrophiles
to in situ generated 2-bromo-6-lithiopyridine, the use of an imidoyl
chloride, such as 2, as an electrophile was not described.
Fig. 2 X-Ray crystal structure of 3. Hydrogen atoms are omitted for
clarity.
Our initial attempt to prepare the target bis(imino)pyridine
compound followed our methodology for preparation of 3;
namely, lithium–halogen exchange at -78 ◦C using dichloro-
methane solvent, followed by addition of imidoyl chloride, 2, as
electrophile. These reaction conditions yielded product 4 (eqn (1)),
◦
˚
Table 1 Selected bond distances (A) and angles ( ) for 3
Br–C(1)
1.903(2)
1.510(2)
1.272(2)
1.427(2)
110.24(5)
N(1)–C(1)–Br
115.9(1)
117.1(1)
117.9(1)
122.8(1)
121.0(1)
C(5)–C(6)
N(2)–C(6)
N(2)–C(11)
PlnA–PlnBa
N(1)–C(5)–C(6)
C(5)–C(6)–C(7)
C(5)–C(6)–N(2)
C(6)–N(2)–C(11)
Mono-lithiation of
a dichloromethane solution of 2,6-
dibromopyridine at -78 ◦C, followed by addition of 1.4 equiv.
a Where PlnA contains N(1), C(1), C(2), C(3), C(4) and C(5); PlnB contains
C(11), C(12), C(14), C(15), C(16), and C(17).
◦
of 2 and slow warming to 22 C generates 3 (Scheme 2, step iii).
Analytically pure, pale yellow microcrystalline 3 can be obtained
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 1214–1222 | 1215
©

View Article Online
◦
˚
consistent with alkylation of imidoyl chloride, 2. Presumably,
brominated mono(imino)pyridin◦e 3 is not sufficiently soluble in
dichloromethane solvent at -78 C to facilitate lithium–halogen
exchange, so instead, imidoyl chloride, 2, is attacked by n-butyl
lithium. Upon re-examination of NMR spectra for crude samples
of 3, we determined that 4 is also generated as a very minor by-
product in formation of 3.
Table 2 Selected bond distances (A) and angles ( ) for 6
N(1)–C(1)
N(1)–C(5)
C(1)–C(6)
C(5)–C(19)
N(2)–C(6)
N(3)–C(19)
C(6)–C(7)
C(19)–C(20)
1.345(2)
1.342(2)
1.510(2)
1.518(2)
1.275(2)
1.270(2)
1.537(2)
1.533(2)
C(19)–N(3)–C(24)
N(1)–C(1)–C(6)
N(1)–C(5)–C(19)
N(1)–C(1)–C(2)
N(1)–C(5)–C(4)
C(1)–C(6)–N(2)
C(5)–C(19)–N(3)
N(2)–C(6)–C(7)
N(3)–C(19)–C(20)
PlnA–PlnBa
123.9(1)
116.5(1)
115.5(1)
122.5(1)
122.7(1)
123.3(1)
123.2(1)
117.2(1)
118.8(1)
116.22(4)
105.06(4)
C(1)–N(1)–C(5)
C(6)–N(2)–C(11)
118.3(1)
127.2(1)
PlnA–PlnCa
(1)
a Where PlnA contains N(1), C(1), C(2), C(3), C(4) and C(5); PlnB contains
C(11), C(12), C(13), C(14), C(15), and C(16); PlnC contains C(24), C(25),
C(26), C(27), C(28), and C(29).
The use of tetrahydrofuran as solvent, in place of dichloro-
methane, facilitates our desired lithium–halogen exchange for 3.
Treatment of 3 with n-butyl lithium in tetrahydrofuran solvent at
-78 ◦C affords a bright yellow-orange solution immediately upon
addition; quenching with excess methanol provides a pale yellow
solution. Reaction work-up and purification via flash silica gel
column chromatography provides a spectroscopically pure sample
of compound 5 (eqn (2)), consistent with protonation of a lithiated
intermediate.
(2)
Thus, lithiation of a tetrahydrofuran solution of 3 at -78 ◦C,
followed by addition of 1.0 equiv. of 2 and slow warming to 22 ◦C
generates 6 (Scheme 2, step iv). Compound 5 was detected as
the major impurity in this reaction; pale yellow microcrystalline
6 was isolated in 54.2% yield after flash silica gel column
chromatography. We postulate that our moderate yield is due
to some product loss during silica gel column chromatography,
though we found it to be a superior purification method in
Fig. 3 X-Ray crystal structure of 6. Hydrogen atoms are omitted for
clarity.
reported structure for the methyl-substituted analogue, 2,6-{(2,6-
Me2-C6H3)NCMe}2C5H3N25b (vide infra).
Attempted metalation of 6 with FeCl2 and FeCl2·4H2O
1
comparison to recrystallization. The 13C{ H} NMR spectroscopic
=
=
C N chemical shift of 175.9 ppm (in benzene-d6) and the C N
infrared stretching frequency of 1651 cm-1 are consistent with the
presence of a bis(imine).
Literature reports have revealed that as the steric bulk at the
imino carbon atoms increases (where R1 is Me, Et, i-Pr or
Ph, see Fig. 1), longer reaction times or forcing conditions
are required for metalation. Gibson and co-workers have de-
scribed metalation reactions using methyl substituted 2,6-{(2,6-
Me2-C6H3)NCMe}2C5H3N and R1 substituted 2,6-{(2,4,6-Me3-
C6H2)NCR1}2C5H3N (where R1 is Me, Et, i-Pr or Ph), all with the
same metalation agent (FeCl2), solvent (n-butanol), and reaction
temperature (80 ◦C).2,10,13 In all cases, a color change is observed
immediately upon mixing the ligand and FeCl2; the resulting iron
compounds are either blue (where R1 is Me, Et or i-Pr) or green
(where R1 is Ph) in color. Where R1 is methyl, the reaction was
stopped after only 15 min;2 where R1 is phenyl a 30 min reaction
time was used.10 Where R1 is ethyl or isopropyl, a 12 h reaction time
was reported.13 Brookhart described milder conditions for meta-
lation of methyl substituted 2,6-{(2,6-Me2-C6H3)NCMe}2C5H3N
(use of either FeCl2 or FeCl2·4H2O, tetrahydrofuran solvent,
ambient temperature, and several hours reaction time).1
A saturated solution of 6 in anhydrous pentane was cooled
to -15 ◦C for 1 month to produce a single crystal suitable
for X-ray diffraction (Table 2 and Fig. 3). As described in the
literature for related alkyl or aryl substituted bis(imino)pyridine
ligands,25 the pyridyl and phenyl rings are nearly orthogonal, with
a PlnA–PlnB angle of 116.22(4)◦ and a PlnA–PlnC of 105.06(4)◦
(where PlnA contains the pyridyl ring, PlnB contains the phenyl
ring incorporating C(11) through C(16), and PlnC contains the
phenyl ring incorporating C(24) through C(29)). Also, the imino
˚
=
C N bonds are short (1.275(2) and 1.270(2) A, respectively),
as observed for related bis(imino)pyridine ligands. The most
=
striking aspect of this structure is that the imino C N bonds
are nearly orthogonal to the pyridyl ring, as indicated by the
following dihedral angles: N(1)–C(1)–C(6)–N(2), -46.7(2)◦; N(1)–
C(5)–C(19)–N(3), -87.7(1)◦. This conformation differs from the
1216 | Dalton Trans., 2009, 1214–1222
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
Attempted metalation of 6 under the following conditions
gave no evidence of the desired iron(II) chloride metalated
product: FeCl2, n-hexanol solvent, reflux, 48 h; FeCl2, n-butanol
solvent, 80 ◦C, 14 h; FeCl2, tetrahydrofuran solvent, reflux, 24 h;
FeCl2·4H2O, tetrahydrofuran solvent, reflux, 20 h. In each case,
the anticipated color change (to green, blue or purple) was not
detected and unreacted 6 was recovered.
Et, i-Pr or t-Bu): (1) mild conditions are sufficient for metalation
of methyl substituted 2,6-{(2,6-Me2-C6H3)NCMe}2C5H3N with
iron(II) chloride,1 in contrast to t-butyl substituted 6, and (2)
methyl substituted 2,6-{(2,6-Me2-C6H3)NCMe}2C5H3N adopts a
different conformation in its crystalline form25b than t-butyl sub-
stituted 6. For each ligand, the energies of three conformers were
calculated: closed (Fig. 3 gives the prototype for this conformer),
open (the crystal structure of 2,6-{(2,6-Me2-C6H3)NCMe}2C5H3N
gives the prototype for this conformer), and open-planar (the
crystal structure of [2,6-{(2,4,6-Me3-C6H2)NCMe}2C5H3N]FeCl2
gives the prototype for this conformer).2b The closed, open, and
open-planar conformers of 6 are illustrated in Fig. 4.
Based on the ease of metalation of the methyl substituted
ligand and the energies presented in Table 3, the open conformer
appears to be the reactive conformer for metalation. For com-
parison, the open conformers of methyl substituted 2,6-{(2,6-
Me2-C6H3)NCMe}2C5H3N and t-butyl substituted 6 are shown
in Fig. 5. For both the methyl- and ethyl-substituted ligands,
the open conformer is the more thermodynamically stable one.
This agrees well with the previously reported crystal structure
for the methyl-substituted ligand.25b When the size of the alkyl
chain is increased (i.e., i-propyl and t-butyl), marked differences
are observed in relative energies of the reactive conformation. The
closed structure becomes more thermodynamically stable for the
i-propyl and t-butyl substituted species, with a lower calculated
energy for the t-butyl substituted species as compared to the
Under more forcing conditions (FeCl2, acetic acid solvent,
reflux, 3 h to 90 h), a color change was observed (a purple solution),
but only upon concentration in vacuo (after 3 h reflux) or extended
reaction times (>24 h reflux). Similar conditions were described by
Chirik et al. for metalation of phenyl substituted 2,6-{(2,6-i-Pr2-
C6H3)NCMe}2C5H3N (reflux, 4 h).11b For our metalation attempts
using acetic acid solvent, small amounts of a purple, paramagnetic
species were isolated upon reaction work-up (10–15 mg recovery,
where 200–250 mg of 6 was used for metalation). 1H NMR
spectroscopic analysis using CD2Cl2 solvent reveals two broad
downfield signals at 79.04 and 49.38 ppm that we tentatively assign
as pyridyl proton resonances. Other resonances appear in the 12.95
to -32.24 ppm range, with a greater number of signals present
than we would anticipate for a pure monomeric iron(II) chloride
complex of 6. Magnetic susceptibility measurements (Evans NMR
method) give evidence of paramagnetism, though also suggest
that impurities and/or multiple paramagnetic species are present.
Recrystallization attempts using solvents such as acetone-d6 and
CD2Cl2 have led to precipitation of an iron(II) acetate species (pre-
sumably formed under metalation conditions). While these meta-
lation conditions provide reproducible results, the poor recovery
makes this reaction preparatively ineffective. We have considered
the possibility that a given molecule of 6 may coordinate to more
than one metal center, potentially yielding an oligomeric species.
Nonetheless, experiments are under way to obtain X-ray quality
single crystals of this purple, paramagnetic species.
Table 3 Relative energies, in kcal mol-1, of various conformations of 6
optimized at the B3LYP/6–31G(d) level of theory. Energies in parentheses
were calculated at the MP2/cc-pVDZ level. Reorganization energies
(DEreorg) were evaluated in reference to the lowest energy conformation
for a given substitution
Closed
Open
Open-planar
DEreorg
Methyl
Ethyl
i-Propyl
t-Butyl
7.9(2.6)
6.3(3.2)
-4.1(-7.6)
-9.6(-14.4)
0.0
0.0
0.0
0.0
10.0 (10.9)
14.2 (15.6)
14.2 (15.0)
11.1 (12.8)
10.0 (10.9)
14.2 (15.6)
18.3 (22.7)
20.7 (27.2)
Computational studies
Two facts led us to pursue computational studies for ligands of
the form 2,6-{(2,6-Me2-C6H3)NCR1}2C5H3N (where R1 is Me,
Fig. 4 Closed, open, and open-planar conformers of 6 (geometries were calculated at the B3LYP/6–31G(d) level).
Fig. 5 Open conformers of methyl substituted 2,6-{(2,6-Me2-C6H3)NCMe}2C5H3N and t-butyl substituted 6 (geometries were calculated at the
B3LYP/6–31G(d) level).
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 1214–1222 | 1217
©

View Article Online
i-propyl substituted species. This agrees with the crystal structure
for 6 (Fig. 3) and may offer some insight into the difficulty in
metalating this species. The t-butyl-substituted species also shows
a significant geometrical difference compared to the other ligands
in that the imino C–N bonds are almost perpendicular to the plane
formed by the pyridine ring. This is most likely due to the steric
strain the t-butyl groups would experience if the imino nitrogen
atoms were in a configuration similar to that of the other ligands
(i.e., nearly planar with the pyridine ring).
and titrated before use.30 Before use, 2,6-dibromopyridine was
recrystallized using 200 proof absolute ethanol.31
NMR spectra were recorded on a Varian Mercury spectrometer
at 300 MHz (1H) and 75 MHz (13C) at 298 K; all chemical
shifts are referenced relative to the NMR solvent (either residual
protio or 13C signals for the solvent peak(s)). The following
abbreviations are used for NMR splitting patterns: pd (pseudo
doublet), pt (pseudo triplet), and br s (broad singlet). MS data were
obtained using an Applied Biosystems API2000 triple quadrupole
mass spectrometer with electrospray ionization. MS samples,
containing approximately 1 mg mL-1 of analyte in methanol with
0.1% formic acid, were analyzed by direct infusion. Mass scans
were performed from 20–300 m/z, and the signals were time-
averaged over 2 min. Infrared spectra wererecorded using a Perkin-
Elmer Spectrum One FTIR System; samples were prepared by
placing the compounds on a diamond attenuated total reflectance
(ATR) plate in either solid or liquid form. Elemental analyses were
performed at Atlantic Microlab, Inc. in Norcross, Georgia.
Budzelaar and Zhu26 recently published theoretical calculations
on similar species in order to parameterize binding of these ligands
to metal complexes. In their analyses, they broke down differential
binding energies into three terms: s-donation, p-donation, and
reorganization energy associated with the open conformation of
the ligand adopting the open-planar conformation. Their studies
showed that, referenced to a structure almost identical to our
methyl-substitued species, reorganization energies were typically
lower or negligibly higher for structures with various substitutions
on the imino carbons. However, they did not report reorganization
energies based on closed conformations. For the i-propyl and
t-butyl cases, this will significantly increase the reorganization
energy. The t-butyl structure has the highest reorganization energy
of the species considered, suggesting that metalation of this species
will be the most difficult to accomplish.
Multi-step synthesis of 6
Preparation of N-(2,6-dimethyl-phenyl)-2,2-dimethyl-propion-
amide, 1. The synthesis of this sterically hindered amide was
achieved as described by literature methods.20–22 A solution of
2,6-dimethylaniline (24.8 mL, 200 mmol, 1.0 equiv.) and tri-
ethylamine (27.8 mL, 199 mmol, 1.0 equiv.) was prepared using
dichloromethane (320 mL) in a 1 L round bottom flask, equipped
with addition funnel. In a separate 500 mL round bottom flask,
pivaloyl chloride (24.6 mL, 200 mmol, 1.0 equiv.) was dissolved
in dichloromethane (80 mL) whilst stirring. The latter solution
was added to the former solution via the addition funnel, and
slow dropwise addition resulted in a cloudy white suspension. The
reaction mixture was heated to reflux for 1 h, cooled to 22 ◦C
and then extracted with water (2 ¥ 100 mL). The organic phase
was dried over magnesium sulfate, and then filtered to yield a
pale yellow solution. Removal of solvent in vacuo yielded a white
powder (39.36 g, 95.9%). (Found: C 75.77, H 9.43, N 6.86%. Calcd
Conclusions
A sterically hindered bis(imino)pyridine compound, 2,6-{(2,6-
Me2-C6H3)NC(t-Bu)}2C5H3N (6), has been synthesized and char-
acterized using single-crystal X-ray diffraction. Relative energies
of three conformations of this species (closed, open, and open-
planar) and the thermodynamic preference for the closed form
give a potential explanation for the resistance of 6 towards
metalation with iron(II) chloride. Similar calculations are under
way in order to identify new, promising synthetic targets in the
bis(imino)pyridine ligand family.
for C13H19NO: C 76.06, H 9.33, N 6.82%). nmax (ATR)◦/cm-1: 3296
1
=
(N–H), 1646 (C O). H NMR (300 MHz, CDCl3, 22 C) d/ppm:
Experimental
1.37 (9H, s, C(CH3)3), 2.21 (6H, s, CH3), 6.87 (1H, br s, N–H), 7.07
(3H, br s, C6H3). 1H NMR (300 MHz, benzene-d6, 22 ◦C) d/ppm:
1.09 (9H, s, C(CH3)3), 2.05 (6H, s, CH3), 6.03 (1H, br s, N–H),
6.95 (3H, m, C6H3). 13C NMR (75 MHz, CDCl3, 22 ◦C) d/ppm:
18.10 (CH3), 27.56 (C(CH3)3), 39.01 (C(CH3)3), 126.55 (C6H3),
General considerations
All chemical reactions were carried out under an atmosphere
of argon using standard Schlenk techniques27 unless otherwise
noted. Argon gas was purified by passage over DrieriteTM. All
chemicals were purchased from Aldrich and used as received unless
otherwise noted. Chloroform-d, methylene chloride-d2, benzene-
d6, and toluene-d8 were purchased from Cambridge Isotope
=
127.63 (C6H3), 134.26 (C6H3), 135.38 (C6H3), 176.52 (C O). Single
crystals of 1 suitable for X-ray diffraction were obtained from an
anhydrous dichloromethane solution cooled to -15 ◦C for 8 d.
˚
Laboratories and dried over molecular sieves (8–12 mesh, 4 A,
Preparation of N-(2,6-dimethyl-phenyl)-2,2-dimethyl-propion-
imidoyl chloride, 2. The synthesis of this sterically hindered
imidoyl chloride was achieved as described by literature methods.23
Phosphorus pentachloride (11.51 g, 55.3 mmol, 1.0 equiv.) was
slowly added to a yellow slurry of 1 (12.50 g, 60.9 mmol, 1.1 equiv.)
in anhydrous toluene (125 mL) under a positive pressure of
argon. The hydrogen chloride gaseous by-product was bubbled
through a sodium hydroxide solution. The resulting opaque pale
yellow slurry was stirred for 18.5 h at 22 ◦C. The product
mixture was subjected to atmospheric pressure distillation under
argon to remove toluene and the POCl3 by-product, followed by
vacuum distillation to collect the desired product as an off-white
activated) before use. A MBraun Manual Solvent Purification
System (MB-SPS) was used to obtain the following anhydrous
solvents: toluene, tetrahydrofuran, diethyl ether, pentane and
dichloromethane; solvents were submitted to three freeze–pump–
thaw cycles before use when rigorously deoxygenated solvent was
required.28 Anhydrous n-hexanol and n-butanol were purchased
from Aldrich (Sure/Seal bottleTM) and used as received. Molecular
˚
sieves (8–12 Mesh, 4 A) were purchased from J. T. Baker and
activated before use. Silica gel used for flash chromatography29 was
purchased from Aldrich (200–425 mesh). n-BuLi was purchased
from Aldrich as a 1.6 M solution in hexanes (Sure/Seal bottleTM
)
1218 | Dalton Trans., 2009, 1214–1222
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
semi-solid. (11.84 g, 86.9%). (Found: C 69.89, H 8.17, N 6.25%.
Homonuclear decoupling experiments were used to detect long
range coupling between the phenyl-methyl protons and the ortho-
phenyl 1H NMR resonances. Single crystals of 3 suitable for X-ray
diffraction were obtained via slow evaporation of a 2.5% ethyl
acetate–hexanes (v/v) solution over the course of 1 d.
Calcd for C13H18ClN: C 69.79, H 8.11, N 6.26%). nmax (ATR)/cm-1:
1698 (C N). H NMR (300 MHz, CDCl3, 22 ◦C) d/ppm: 1.43
1
=
(9H, s, C(CH3)3), 2.06 (6H, s, CH3), 6.96 (1H, m, C6H3), 7.04
1
(2H, pd, J = 7.4 Hz, C6H3). H NMR (300 MHz, benzene-d6,
22 ◦C) d/ppm: 1.25 (9H, s, C(CH3)3), 2.03 (6H, s, CH3), 6.96
(3H, m, C6H3). 13C NMR (75 MHz, CDCl3, 22 ◦C) d/ppm:
17.67 (CH3), 28.53 (C(CH3)3), 43.85 (C(CH3)3), 123.73 (C6H3),
=
125.89 (C6H3), 127.60 (C6H3), 145.36 (C6H3), 155.45 (C N).
◦
Alternatively, Ku¨gelrohr distillation (95 C, 1 Torr) may be used
to collect the desired product. 31P NMR was used to qualitatively
monitor the absence of the POCl3 by-product. Purification via
flash silica gel chromatography is not viable because it hydrolyzes
2, thus reverting to 1.
(1-Tert-butyl-pentylidene)-(2,6-dimethyl-phenyl)amine, 4.
A
100 mL Schlenk flask was charged with 3 (0.71 g, 2.06 mmol,
1.0 equiv.) and anhydrous dichloromethane (15 mL), then cooled
to -78 ◦C using a dry ice–acetone bath. This solution was treated
with 1.6 M n-butyllithium in hexanes (0.96 mL, 1.54 mmol,
0.75 equiv.). The resulting pale yellow solution was stirred at
-78 ◦C for 15 min. Meanwhile, a 50 mL Schlenk flask was
charged with 2 (0.35 g, 1.58 mmol, 0.77 equiv.) and anhydrous
dichloromethane (5 mL). This solution of 2 was added to the
reaction flask via cannula transfer to yield a yellow solution. The
50 mL Schlenk flask was washed with anhydrous dichloromethane
(2 ¥ 5 mL), and these washes were also transferred to the reaction
flask via cannula transfer. Stirring of the reaction mixture
continued at -78 ◦C and the cold bath was left intact to warm to
22 ◦C over the course of 17.5 h. The resulting opaque pale yellow
slurry was filtered through a fine porosity glass filter, yielding a
transparent yellow filtrate. The solvent was removed in vacuo for
1 h, yielding a yellow oil. The concentrated filtrate was dissolved
in 2.5% ethyl acetate–hexanes (v/v, 1 mL) and dichloromethane
(0.5 mL) and subjected to flash silica gel chromatography
using 2.5% ethyl acetate–hexanes (v/v) followed by 5.0% ethyl
acetate–hexanes (v/v) as eluent. Combined fractions were dried
Preparation of [1-(6-bromo-pyridin-2-yl)-2,2-dimethyl-propyl-
idene]-(2,6-dimethyl-phenyl)amine, 3. A 500 mL Schlenk flask
was charged with 2,6-dibromopyridine (1.22 g, 5.15 mmol,
1.0 equiv.) and anhydrous dichloromethane (90 mL), then cooled
to -78 ◦C using a dry ice–acetone bath. This solution was
treated with 1.6 M n-butyllithium in hexanes (3.3 mL, 5.28 mmol,
1.0 equiv.). The resulting pale yellow solution was stirred at -78 ◦C
for 15 min. Meanwhile, a 250 mL Schlenk flask was charged with
2 (1.58 g, 7.08 mmol, 1.4 equiv.) and anhydrous dichloromethane
(5 mL). This solution of 2 was added to the reaction flask via
cannula transfer to yield a yellow solution. The 250 mL Schlenk
flask was washed with anhydrous dichloromethane (2 mL), and
this wash was also transferred to the reaction flask via cannula
transfer. Stirring of the reaction mixture continued at -78 ◦C and
the cold bath was left intact to warm to 22 ◦C over the course of
22 h. The opaque pale yellow slurry was then filtered through a fine
porosity glass filter, yielding a transparent yellow filtrate. Solvent
was removed in vacuo for 3 h, providing a yellow semi-solid. The
concentrated filtrate was dissolved in 2.5% ethyl acetate–hexanes
(v/v, 2 mL) and dichloromethane (2 mL) and subjected to flash
silica gel chromatography using 2.5% ethyl acetate–hexanes (v/v)
as eluent. Combined fractions were dried in vacuo for 4 h to yield
a pale yellow crystalline solid (1.48 g, 83.3%). (Found: C 62.89, H
6.26, N 7.85%. Calcd for C18H21BrN2: C 62.61, H 6.13, N 8.11◦%).
1
in vacuo for 2.25 h to yield a pale yellow oil (0.11 g, 28.6%). H
NMR (300 MHz, benzene-d6, 22 ◦C) d/ppm: 0.57 (3H, t, J =
7.1 Hz, butyl CH3), 0.89 (2H, sextet, J = 7.1 Hz, CH2), 1.15 (2H,
m, CH2), 1.24 (9H, s, C(CH3)3), 1.96 (2H, m, CH2), 2.04 (6H, s,
CH3), 6.90 (1H, m, C6H3), 7.02 (2H, pd, J = 7.4 Hz, C6H3). 13
C
-1
1
=
max (ATR)/cm : 1647 (C N). H NMR (300 MHz, CDCl3, 22 C)
n
◦
NMR (75 MHz, benzene-d6, 22 C) d/ppm: 13.65, 18.52, 23.62,
d/ppm: 1.38 (9H, s, C(CH3)3), 2.07 (6H, s, CH3), 6.70 (2H, m, Ar),
6.81 (2H, m, Ar), 6.25 (2H, m, Ar). 1H NMR (300 MHz, benzene-
d6, 22 ◦C) d/ppm: 1.37 (9H, s, C(CH3)3), 2.11 (6H, s, CH3), 6.25
(1H, pt, J = 7.7 Hz, Hd), 6.32 (1H, dd, J = 0.8 Hz, 7.7 Hz, He),
6.55 (1H, dd, J = 0.8 Hz, 7.7 Hz, Hc), 6.70 (1H, m, Hb), 6.79
(2H, pd, J = 7.4 Hz, Ha). 1H NMR (300 MHz, toluene-d8; 22 ◦C)
d/ppm: 1.35 (9H, s, C(CH3)3), 2.07 (6H, s, CH3), 6.32 (2H, two
overlapping m, pyr), 6.54 (1H, pd, J = 7.5 Hz, pyr), 6.65 (1H, m,
Hb), 6.74 (2H, pd, J = 7.4 Hz, Ha). 1H NMR (300 MHz, CD2Cl2,
22 ◦C) d/ppm: 1.35 (9H, s, C(CH3)3), 2.03 (6H, s, CH3), 6.68
(1H, m, Hb), 6.75 (1H, m, Hc), 6.79 (2H, pd, J = 7.4 Hz, Ha),
7.25 (1H, m, He), 7.30 (1H, m, Hd). 13C NMR (75 MHz, CDCl3,
22 ◦C) d/ppm: 18.35 (CH3), 28.87 (C(CH3)3), 40.49 (C(CH3)3),
120.04, 122.54, 125.48, 126.96, 127.38, 137.27, 140.78, 147.67,
28.71, 28.93, 30.35, 41.16, 122.44, 124.98, one aromatic resonance
+
=
not detected, 149.38, 178.69 (C N). m/z (ESI) 246.4 ([M + H] ).
(2,6-Dimethyl-phenyl)-(2,2-dimethyl-1-pyridin-2-yl-propylidene)-
amine, 5. Under an argon counter flow, a 100 mL flask was
charged with 3 (0.114 g, 0.33 mmol, 1.0 equiv.) and 1.6 mL
anhydrous tetrahydrofuran, then cooled to -78 ◦C using a
dry ice–acetone bath. This bright yellow solution was treated
with 1.29 M n-butyllithium in hexanes (0.307 mL, 0.40 mmol,
1.2 equiv.). The resulting vibrant yellow-orange solution was
stirred at -78 ◦C for 1 h before the addition of methanol
(0.160 mL, 3.9 mmol, 11.8 equiv.), providing a pale yellow
solution. The removal of solvent in vacuo for 15 min produced a
waxy pale yellow solid. The crude product was dissolved in 10%
ethyl acetate–hexanes (4 mL, v/v) and dichloromethane (2 mL)
and subjected to flash silica gel chromatography using 10% ethyl
acetate–hexanes (v/v) as eluent. Combined fractions were dried
in vacuo for 3.5 h to yield 5 as a yellow oil (0.0503 g, 63.6%).
156.74, 175.21 (C N). 13C NMR (75 MHz, benzene-d6, 22 ◦C)
=
d/ppm: 18.74 (CH3), 29.11 (C(CH3)3), 30.35 (C(CH3)3), 120.23,
123.08, 125.64, 127.03, one aromatic resonance not detected,
=
137.31, 141.20, 148.60, 157.22, 174.81 (C N). NOE difference
-1
1
=
experiments were used to identify the three pyridyl proton signals.
nmax (ATR)/cm : 1645 (C N). H NMR (300 MHz, benzene-d6,
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 1214–1222 | 1219
©

View Article Online
22 ◦C) d/ppm: 1.47 (9H, s, C(CH3)3), 2.19 (6H, s, CH3), 6.30 (1H,
ddd, J = 7.5 Hz, 4.9 Hz, 1.2 Hz, He), 6.56 (1H, dt, J = 7.7 Hz,
1.0 Hz, Hc), 6.66 (1H, td, J = 7.5 Hz, 1.7 Hz, Hd), 6.72 (1H, pt,
J = 7.4 Hz, Hb), 6.82 (2H, pd, J = 7.4 Hz, Ha), 8.21 (1H, ddd,
J = 4.9 Hz, 1.7 Hz, 1.0 Hz, Hf). 13C NMR (75 MHz, benzene-d6,
22 ◦C) d/ppm: 18.92 (CH3), 29.33 (C(CH3)3), 40.87 (C(CH3)3),
121.45, 122.46, 122.77, 125.77, 127.86, one aromatic resonance not
Attempted synthesis of iron(II) chloride complex of 6
General metalation procedure using n-hexanol, n-butanol or
tetrahydrofuran solvent. Under argon, a solution of 6 (1 equiv.)
was treated with a pre-heated solution of FeCl2 or FeCl2·4H2O
(1 equiv., the solution was pre-heated to ensure complete disso-
lution). The reaction mixture was then heated between 14 and
48 h before removal of solvent under vacuum (n-butanol solutions
were heated to 80 ◦C; n-hexanol and tetrahydrofuran solutions
were heated to reflux). In each case, the anticipated color change
(to green, blue or purple) was not detected, neither initially nor
after concentration in vacuo. Inside the glove box, the residue
was extracted with pentane and vacuum filtration provided a
filtrate that was identified as unreacted 6 (structural assignment
=
detected, 134.78, 148.60, 156.97, 176.46 (C N). m/z (ESI) 267.5
([M + H]+).
1
made via H NMR spectroscopy). In each case, the precipitate
isolated during vacuum filtration did not correspond to the desired
iron(II) chloride complex of 6 (1H NMR spectroscopy was used
for analysis).
Preparation of 2,6-{(2,6-Me2-C6H3)NC(t-Bu)}2C5H3N, 6. A
250 mL Schlenk flask was charged with 3 (0.77 g, 2.23 mmol,
1.0 equiv.) and anhydrous tetrahydrofuran (11 mL), then cooled to
-78 ◦C for 20 min using a dry ice–acetone bath. This solution was
treated, dropwise, with 1.6 M n-butyl lithium in hexanes (1.53 mL,
2.45 mmol, 1.1 equiv.). The resulting gold colored solution was
stirred at -78 ◦C for 15 min. Meanwhile, an argon filled 100 mL
round bottom flask was charged with 2 (0.51 g, 2.26 mmol,
0.98 equiv.) and anhydrous tetrahydrofuran (11 mL). This solution
of 2 was added dropwise to the reaction flask via syringe to yield an
orange solution. The 100 mL round bottomed flask was washed
with anhydrous tetrahydrofuran (3 mL), and this wash was also
transferred to the reaction flask via syringe. Stirring of the reaction
mixture continued at -78 ◦C and the cold bath was left intact to
warm to 22 ◦C over the course of 17 h. The solvent was removed
in vacuo for 2.5 h, resulting in an orange semi-solid, which was
then dissolved in 75 mL anhydrous pentane. The opaque yellow
slurry was filtered through a fine porosity glass filter, yielding a
transparent yellow filtrate. The solvent was removed in vacuo for
3 h, yielding a yellow semi-solid. The concentrated filtrate was
dissolved in 1% ethyl acetate–hexanes (5 mL, v/v) and subjected
to flash silica gel chromatography using 1% ethyl acetate–hexanes
(v/v) as eluent. Combined fractions were dried in vacuo for 1 h to
yield 6 as a pale yellow crystalline solid (0.55 g, 54.2%). (Found:
C 81.43, H 8.78, N 8.79%. Calcd for C31H39N3: C 82.07, H 8.66,
General metalation procedure using acetic acid solvent. Under
argon, an acetic acid solution of 6 (1 equiv.) was added to solid
FeCl2 and the reaction mixture was then heated to reflux between
3 and 90 h before removal of solvent under vacuum (with gentle
heating). The anticipated color change was first observed (a purple
solution) either upon concentration in vacuo (after 3 h reflux) or
extended reaction times (>24 h reflux). Inside the glove box, the
residue was extracted with dichloromethane. Using vacuum filtra-
tion, the filtrate was collected and then concentrated to dryness
under vacuum. Next, this residue was treated with pentane, and the
purple, pentane-insoluble precipitate was collected and analyzed
via 1H NMR spectroscopy (in CD2Cl2) and magnetic susceptibility
1
(Evans NMR method). Selected signals, H NMR (300 MHz,
CD2Cl2, 22 ◦C) d/ppm: 79.04 (br s, pyr), 49.38 (br s, pyr),
12.95 (br s), 7.95–0.72 (overlapping signals), -6.74 (br s), -13.25
(br s), -14.26 (br s), -32.34 (br s). The other isolated species from
reaction work-up (dichloromethane-insoluble material; pentane-
soluble material) did not correspond to the desired iron(II) chloride
complex of 6 (1H NMR spectroscopy was used for analysis).
Crystal structure determinations
Crystallographic data are presented in Table 4. Single crystals
R
of 3 and 6, respectively, were mounted using Paratoneꢀ oil onto
-1
1
a glass fiber and cooled to the data collection temperature of
100 K. Data were collected on a Bru¨ker-AXS Platform APEX
=
N 9.26%). nmax (ATR)/cm : 1651 (C N). H NMR (300 MHz,
benzene-d6, 22 ◦C) d/ppm: 1.29 (18H, s, C(CH3)3), 2.09 (12H, s,
CH3), 6.36 (2H, pd, J = 7.5 Hz, Hc), 6.49 (1H, m, Hd), 6.70
(2H, m, Hb), 6.78 (4H, pd, J = 7.5 Hz, Ha). 13C NMR (75 MHz,
˚
II CCD diffractometer with 0.71073 A MoKa radiation for
compound 3. Data were collected on a Bru¨ker-AXS Kappa APEX
˚
◦
II CCD diffractometer with 1.54178 A CuKa radiation for com-
benzene-d6, 22 C) d/ppm: 19.08 (CH3), 29.22 (C(CH3)3), 40.85
pound 6. Unit cell parameters were obtained from 90 data frames,
0.5◦ U, from three different sections of the Ewald sphere. The
data-set was treated with SADABS absorption corrections based
on redundant multi-scan data.32 All non-hydrogen atoms were
refined with anisotropic displacement parameters. All hydrogen
atoms were treated as idealized contribution.
(C(CH3)3), 120.05, 122.65, 125.35, 127.90, 134.42, 148.82, 156.13,
=
175.87 (C N). A saturated solution of 6 in anhydrous pentane was
cooled to -15 ◦C for 1 month to produce a single crystal suitable
for X-ray diffraction.
Computational methods
HF/6–31G(d) optimizations were performed and analytic har-
monic frequencies were calculated to determine the nature of the
stationary points observed. All geometries were then optimized
1220 | Dalton Trans., 2009, 1214–1222
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
Table 4 Selected crystallographic and data collection parameters for compounds 3 and 6
3
6
Empirical formula
Formula weight
Crystal color
Habit
C18H21BrN2
345.28
Colorless
Block
100(2)
0.71073
Monoclinic
P21/n
9.5839(5)
9.2243(5)
18.696(1)
90
91.430(1)
90
1652.3(1)
4
C31H39N3
453.65
Colorless
Block
T/K
100(2)
˚
l/A
1.54178
Monoclinic
P21/c
11.5027(4)
11.6244(4)
20.8562(7)
90
105.597(1)
90
2686.0(2)
4
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
D
calcd/Mg m-3
1.388
2.484
712
1.122
0.496
984
Absorption coefficient/mm-1
F(000)
Crystal size/mm3
0.22 ¥ 0.14 ¥ 0.07
2.18–28.37
-11 ≤ h ≤ 12, -12 ≤ k ≤ 11, -23 ≤ l ≤ 24
11 499
3812 [Rint = 0.0364]
99.2% to H = 25.00◦
Semi-empirical from equivalents
0.8473 and 0.6086
Full-matrix least-squares on F2
3812/0/195
0.27 ¥ 0.24 ¥ 0.15
3.99–68.15
-11 ≤ h ≤ 13, -13 ≤ k ≤ 11, -25 ≤ l ≤ 24
19 189
Hrangef ordatacollection/◦
Index ranges
Reflections collected
Independent reflections
Completeness
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2s(I)]
R indices (all data)
4836 [Rint = 0.0221]
99.5% to H = 67.00◦
Semi-empirical from equivalents
0.9297 and 0.8785
Full-matrix least-squares on F2
4836/0/317
1.048
1.065
R1 = 0.0320, wR2 = 0.0859
R1 = 0.0402, wR2 = 0.0990
R1 = 0.0360, wR2 = 0.0886
R1 = 0.0418, wR2 = 0.1001
-3
-3
˚
˚
Largest diffraction peak and hole
1.041 and -0.331 e A
0.208 and -0.196 e A
at the B3LYP/6–31G(d) level of theory with very little change
observed in any of the geometries. Relative energies are reported
for the B3LYP optimized structures and do not include zero-point
energy corrections. MP2/cc-pVDZ energies were also calculated
at the B3LYP/6–31G(d) optimized geometries. GAMESS33 was
used to optimize structures with the maximum gradient tolerance
set to 0.0006 Hartree/Bohr. Default values were used for all other
parameters.
J. Am. Chem. Soc., 1999, 121, 8728–8740; (c) World Pat., 9 912 981,
1999; Chem. Abstr., 1999, 130, 252793.
3 C. Bianchini, G. Giambastiani, I. G. Rios, G. Mantovani, A. Meli and
A. M. Segarra, Coord. Chem. Rev., 2006, 250, 1391–1418.
4 (a) C. Pellecchia, M. Mazzeo and D. Pappalardo, Macromol. Rapid
Commun., 1998, 19, 651–655; (b) B. L. Small and M. Brookhart,
Macromolecules, 1999, 32, 2120–2130; (c) B. L. Small and A. J.
Marcucci, Organometallics, 2001, 20, 5738–5744; (d) B. L. Small,
Organometallics, 2003, 22, 3178–3183; (e) S. T. Babik and G. Fink,
J. Mol. Catal. A: Chem., 2002, 188, 245–253; (f) S. T. Babik and G.
Fink, J. Organomet. Chem., 2003, 683, 209–219; (g) K. P. Tellman,
V. C. Gibson, A. J. P. White and D. J. Williams, Organometallics, 2005,
24, 280–286.
Acknowledgements
5 V. C. Gibson, C. Redshaw and G. A. Solan, Chem. Rev., 2007, 107,
1745–1776.
6 Q. Knijnenburg, S. Gambarotta and P. H. M. Budzelaar, Dalton Trans.,
2006, 5442–5448.
We are grateful to reviewers for helpful comments. Acknowledge-
ment is made to the Donors of the American Chemical Society
Petroleum Research Fund and Villanova University for support
of this research. M. D. K. acknowledges funding from the United
States Coast Guard. We thank Dr Walter Boyko of Villanova
University for his assistance with NMR spectroscopy.
7 (a) M. A. Esteruelas, A. M. Lopez, L. Mendez, M. Olivan and
E. On˜ate, Organometallics, 2003, 22, 395–406; (b) C. Bianchini, G.
Mantovani, A. Meli, F. Migliacci, F. Zanobini, F. Laschi and A.
Sommazzi, Eur. J. Inorg. Chem., 2003, 1620–1631; (c) K. Lappalainen,
K. Yliheikkila¨, A. S. Abu-Surrah, M. Polamo, M. Leskela¨ and T. Z.
Repo, Z. Anorg. Allg. Chem., 2005, 631, 763–768; (d) Y. Nakayama, K.
Sogo, H. Yasuda and T. Shiono, J. Polym. Sci., Part A: Polym. Chem.,
2005, 43, 3368–3375.
8 (a) L. S. Moody, P. Mackenzie, C. M. Killian, G. G. Lavoie, J. A.
Ponasik Jr., A. G. Barrett, T. W. Smith and J. C. Pearson, World Pat.,
0 050 470, 2000; Chem. Abstr., 2000, 133, 208316; (b) G. J. P. Britovsek,
V. C. Gibson, B. S. Kimberley, S. Mastroianni, C. Redshaw, G. A.
Solan, A. J. P. White and D. J. Williams, J. Chem. Soc., Dalton Trans.,
2001, 1639–1644; (c) C. Amort, M. Malaun, A. Krajete, H. Kopacka,
K. Wurst, M. Christ, D. Lilge, M. O. Kristen and B. Bildstein, Appl.
Organomet. Chem., 2002, 16, 506–516.
References
1 (a) B. L. Small, M. Brookhart and A. M. A. Bennett, J. Am. Chem.
Soc., 1998, 120, 4049–4050; (b) B. L. Small and M. Brookhart, J. Am.
Chem. Soc., 1998, 120, 7143–7144; (c) World Pat., 9 827 124, 1998;
Chem. Abstr., 1998, 129, 122973x.
2 (a) G. J. P. Britovsek, V. C. Gibson, B. S. Kimberley, P. J. Maddox,
S. J. McTavish, G. A. Solan, A. J. P. White and D. J. Williams, Chem.
Commun., 1998, 849–850; (b) G. J. P. Britovsek, M. Bruce, V. C. Gibson,
B. S. Kimberley, P. J. Maddox, S. Mastroianni, S. J. McTavish, C.
Redshaw, G. A. Solan, S. Stro¨mberg, A. J. P. White and D. J. Williams,
9 (a) Z. Ma, W.-H. Sun, Z.-L. Li, C.-X. Shao, Y.-L. Hu and X.-H. Li,
Polym. Int., 2002, 51, 994–997; (b) C. Bianchini, G. Giambastiani, I. R.
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 1214–1222 | 1221
©

View Article Online
Guerro, A. Meli, E. Passaglia and T. Gragnoli, Organometallics, 2004,
23, 6087–6089; (c) B. L. Small, M. J. Carney, D. M. Holman, C. E.
O’Rourke and J. A. Halfen, Macromolecules, 2004, 37, 4375–4386;
(d) A. S. Ionkin, W. J. Marshall, D. J. Adelman, A. L. Shoe, R. E.
Spence and T. Xie, J. Polym. Sci., Part A: Polym. Chem., 2006, 44,
2615–2635.
22 B. T. Gowda, I. Svoboda and H. Fuess, Acta Crystallogr., Sect. E:
Struct. Rep. Online, 2007, 63, o3324.
23 (a) E. Alonso, D. J. Ramo´n and M. Yus, Tetrahedron Lett., 1997, 38,
8903–8906; (b) E. Alonso, D. J. Ramo´n and M. Yus, Tetrahedron, 1998,
12007–12028.
24 (a) P. J. Manley and M. T. Bilodeau, Org. Lett., 2002, 4, 3127–3129;
(b) R. F. Cunico and R. K. Pandey, J. Org. Chem., 2005, 70, 5344–5346.
25 (a) Structural data for 2,6-{(C6H5)NCCH3}2C5H3N: A. Mentes, J.
Fawcett and R. D. W. Kemmitt, Acta Crystallogr., Sect. E: Struct.
Rep. Online, 2001, 57, o424–o425; (b) Structural data for 2,6-{(2,6-
(CH3)2-C6H3)NCCH3}2C5H3N: Y.-B. Huang, X.-L. Ma, S.-N. Zheng,
J.-X. Chen and C.-X. Wei, Acta Crystallogr., Sect. E: Struct. Rep.
Online, 2006, 62, o3044–o3045; (c) Structural data for 2,6-{(2,6-(i-Pr)2-
C6H3)NC(C6H5)}2C5H3N: see ref. 11a.
10 G. J. P. Britovsek, V. C. Gibson, O. D. Hoarau, S. K. Spitzmesser, A. J. P.
White and D. J. Williams, Inorg. Chem., 2003, 42, 3454–3465.
11 (a) N. Kleigrewe, W. Steffen, T. Blo¨mker, G. Kehr, R. Fro¨hlich,
B. Wibbeling, G. Erker, J.-C. Wasilke, G. Wu and G. C. Bazan,
J. Am. Chem. Soc., 2005, 127, 13955–13968; (b) A. M. Archer,
M. W. Bouwkamp, M.-P. Cortez, E. Lobkovsky and P. J. Chirik,
Organometallics, 2006, 25, 4269–4278.
12 T. M. Smit, A. K. Tomov, V. C. Gibson, A. J. P. White and D. J. Williams,
Inorg. Chem., 2004, 43, 6511–6512.
26 D. Zhu and P. H. M. Budzelaar, Organometallics, 2008, 27, 2699–2705.
27 D. F. Shriver and M. A. Drezdon, The Manipulation of Air-Sensitive
Compounds, Wiley, New York, 2nd edn, 1986.
13 S. McTavish, G. J. P. Britovsek, T. M. Smit, V. C. Gibson, A. J. P. White
and D. J. Williams, J. Mol. Catal. A: Chem., 2007, 261, 293–300.
14 A. S. Ionkin and W. J. Marshall, Heteroat. Chem., 2002, 13, 662–666.
15 Y. Chen, C. Qian and J. Sun, Organometallics, 2003, 22, 1231–1236.
16 (a) J. M. Smith, R. J. Lachicotte and P. L. Holland, Chem. Commun.,
2001, 1542–1543; (b) A. R. Sadique, E. A. Gregory, W. W. Brennessel
and P. L. Holland, J. Am. Chem. Soc., 2007, 129, 8112–8121.
17 R. Knorr and A. Weiss, Chem. Ber., 1982, 115, 139–160.
18 P. H. M. Budzelaar, A. B. van Oort and A. G. Orpen, Eur. J. Inorg.
Chem., 1998, 1485–1494.
19 M. A. Peterson and J. R. Mitchell, J. Org. Chem., 1997, 62, 8237–8239.
20 F. El-Zahraa, S. El-Basil, M. El-Sayed, K. M. Ghoneim and M. Khalifa,
Pharmazie, 1979, 34, 12–13.
21 B. T. Gowda, K. M. Usha and K. Jyothi, Z. Naturforsch., A: Phys. Sci.,
2004, 59, 69–76.
28 A. B. Pangborn, M. A. Giardello, R. H. Grubbs, R. K. Rosen and F. J.
Timmers, Organometallics, 1996, 15, 1518–1520.
29 W. C. Still, M. Kahn and A. Mitra, J. Org. Chem., 1978, 43, 2923–2925.
30 W. G. Kofron and L. M. Baclawski, J. Org. Chem., 1976, 41, 1879–1880.
31 W. L. F. Armarego, and D. D. Perrin, Purification of Laboratory
Chemicals, Butterworth-Heinemann, Woburn, MA, 4th edn, 2000.
32 G. M. Sheldrick, SADABS, Program for area detector adsorption
correction, Institute for Inorganic Chemistry, University of Go¨ttingen,
Germany, 1996.
33 M. W. Schmidt, K. K. Baldridge, J. A. Boatz, S. T. Elbert, M. S. Gordon,
J. H. Jensen, S. Koseki, N. Matsunaga, K. A. Nguyen, S. J. Su, T. L.
Windus, M. Dupuis and J. A. Montgomery, J. Comput. Chem., 1993,
14, 1347–1363.
1222 | Dalton Trans., 2009, 1214–1222
This journal is
The Royal Society of Chemistry 2009
©