Experimental and Computational Studies of the Mechanisms of Hydroamination/Cyclisation of Unactivated α,ω-Amino-alkenes with CCC-NHC Pincer Zr Complexes

Article abstract of DOI:10.1071/CH15779

Four new CCC-NHC pincer Zr complexes have been synthesised, characterised, and used in mechanistic studies in the hydroamination/cyclisation of unactivated amino-alkenes. These Zr pre-catalysts will cyclise a primary amino-alkene, but no reaction was observed for a secondary amino-alkene even in the presence of a primary amine. The empirical rate law, experimentally determined activation parameters, and kinetic isotope effects (KIEs) are reported. Several possible mechanisms, including amido-versus imido-insertion and concerted-insertion versus [2+2] cycloaddition mechanisms, were modelled computationally at the PBEPBE level of theory with double-zeta quality basis sets. The formation of a catalytically relevant imido complex via the monoamido complexes was accompanied by in situ formation of ammonium salts of the substrates. The experimental and computational data are consistent with an imido-[2+2] cycloaddition mechanism for the CCC-NHC pincer diamido Zr complexes that follow saturation kinetics under catalytically relevant concentrations.

Full text of DOI:10.1071/CH15779

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2016, 69, 573–582
Full Paper
http://dx.doi.org/10.1071/CH15779
Experimental and Computational Studies of the
Mechanisms of Hydroamination/Cyclisation of
Unactivated a,v-Amino-alkenes with CCC-NHC Pincer
Zr Complexes*
A B
,
C
A C D
, ,
Wesley D. Clark,
Katherine N. Leigh, Charles Edwin Webster,
A B D
, ,
and T. Keith Hollis
A
Department of Chemistry, Mississippi State University, Mississippi State, MS 39762,
USA.
B
Department of Chemistry and Biochemistry, The University of Mississippi, Oxford,
MS 38655, USA.
C
Department of Chemistry, The University of Memphis, Memphis, TN 38152, USA.
D
Corresponding authors. Email: ewebster@chemistry.msstate.edu;
khollis@chemistry.msstate.edu
Four new CCC-NHC pincer Zr complexes have been synthesised, characterised, and used in mechanistic studies in the
hydroamination/cyclisation of unactivated amino-alkenes. These Zr pre-catalysts will cyclise a primary amino-alkene, but
no reaction was observed for a secondary amino-alkene even in the presence of a primary amine. The empirical rate law,
experimentally determined activation parameters, and kinetic isotope effects (KIEs) are reported. Several possible
mechanisms, including amido- versus imido-insertion and concerted-insertion versus [2 þ 2] cycloaddition mechanisms,
were modelled computationally at the PBEPBE level of theory with double-zeta quality basis sets. The formation of a
catalytically relevant imido complex via the monoamido complexes was accompanied by in situ formation of ammonium
salts of the substrates. The experimental and computational data are consistent with an imido-[2 þ 2] cycloaddition
mechanism for the CCC-NHC pincer diamido Zr complexes that follow saturation kinetics under catalytically relevant
concentrations.
Manuscript received: 12 December 2015.
Manuscript accepted: 7 March 2016.
Published online: 11 April 2016.
Introduction
the amido-enhanced amido insertion mechanism, illustrate
simplified versions of the mechanisms reported by Schafer
et al. and Sadow et al. respectively.[4b,5d] An imido-[2 þ 2]
cycloaddition mechanism has also been reported (Scheme 1d)
for group 4 complexes, which have not been reported to catalyse
hydroamination/cyclisation of secondary amino-alkenes.[4c,4f]
Computational only studies have also been reported.[6] Some of
us recently reported CCC-NHC pincer Zr complexes and their
hydroamination/cyclisation activity, particularly that the dia-
mido complexes (see preceding paper[7]) are faster than
the monoamido complexes in contrast to reports by others.[5a]
A synergistic rate enhancement with the addition of a
CCC-NHC pincer triiodo Zr complex was recently reported.[8]
Herein, we report mechanistic studies on the hydroamination/
cyclisation of unactivated amino-alkenes with CCC-NHC
pincer Zr complexes employing experimental and computa-
tional techniques.
Pyrrolidines and piperidines are important classes of N-hetero-
cycles present in many biologically active molecules and
pharmaceuticals.[1] Hydroamination/cyclisation of unactivated
amino-alkenes is an atom economical pathway to these mole-
cules.[2] However a high activation barrier (,42 kcal molꢀ1
)
from electrostatic repulsion due to the amine lone pair and the
alkene p-bond prevents the direct reaction from occurring
without a catalyst.[3] A wide variety of hydroamination/
cyclisation catalysts have been reported incorporating chiral or
achiral ligands, and transition or rare earth metals.[4] Accord-
ingly, several mechanistic pathways have been proposed.[4b,4f,5]
The amido insertion mechanism (Scheme 1a) has been reported
for organoactanides which catalyse hydroamination/cyclisation
of primary and secondary amino-alkenes.[5a] Two concerted
proton transfer mechanisms have been reported. Scheme 1b, the
amine-enhanced amido insertion mechanism, and Scheme 1c,
*Dedicated to the memory of Professor B. Bosnich and the clarity of thought he brought to the most significant problems.
Journal compilation Ó CSIRO 2016
www.publish.csiro.au/journals/ajc

574
W. D. Clark et al.
Results and Discussion
solutions were observed at higher temperatures near the previ-
ously reported catalytic temperature of 1608C, but the solutions
were refluxing in the NMR tube and a lock signal was not
obtainable. However, homogenous systems were obtained for
the diiodo complexes 2b, 2c, and diamido complex 4a when 1,2-
dichlorobenzene was used as the solvent. Despite the incorpo-
ration of undecyl substituents into complex 3c, catalytic trials
remained heterogeneous. Complexes 2b, 2c, 3c, and 4a were
evaluated for their activity in the hydroamination/cyclisation of
standard, unactivated a,v-amino-alkenes (Table 1). Results
from these catalytic trials are summarised in Table 1. All four
complexes efficiently converted primary amino-alkene sub-
strate 5 into a cyclised product (entry 1).
A trace quantity of the oxidative amination product 8[9] was
observed when amido complexes 2b, 2c, and 4a were employed.
Curiously, the amount of oxidative amination product observed
was near the catalyst loading levels, which is consistent with its
formation being part of a catalyst decomposition pathway.
However, oxidative amination product 8 was not observed when
using triiodo complex 3c. Such an observation is consistent with
the dimethyl amido ligand contributing to the decomposition.
When secondary amine substrate 6 was evaluated with each pre-
catalyst, no reaction was observed (Table 1, entry 2). The
addition of an exogenous, un-cyclisable primary amine while
attempting to cyclise secondary amino-alkene 6, did not provide
any conversion (Table 1, entry 2, footnote B). Thorough
evaluation of other CCC-NHC pincer Zr complexes as pre-
catalysts for the hydroamination/cyclisation of primary amino-
alkenes has already been reported.[8a,8c] The results in Table 1
are consistent with CCC-NHC pincer Zr complexes 2b, 2c, 3c,
and 4a as active pre-catalysts for the hydroamination/cyclisation
of primary amino-alkenes in line with prior reports.
Synthesis and Characterisation of CCC-NHC Pincer Zr
Complexes
Initial attempts to perform kinetic runs with the previously
reported butyl derivatives at the lower temperatures needed for
NMR observations led to the observation of heterogeneous
mixtures (see below). Therefore, derivatives with longer alkyl
chains were prepared (Scheme 2), which provided improved
solubility. The imidazolium salts 1b (n-hexyl) and 1c (undecyl)
were synthesised in a manner similar to the previously reported
synthesis of imidazolium salt 1a. Imidazolium salts 1b and 1c
were reacted with 1.1 equiv. of Zr(NMe2)4 yielding diiodo
complexes 2b and 2c. Imidazolium salt 1a was reacted with 2.5
equiv. of Zr(NMe2)4 yielding diamido complex 4a as a crys-
talline solid (Scheme 2). Furthermore, triiodo complex 3c was
synthesised by reacting undecyl imidazolium salt 1c with 1.1
equiv. of Zr(NMe2)4 and then excess MeI. The characteristic
imidazolium peak (10–12 ppm) was not observed in the 1H
NMR spectrum for each of the complexes. The 13C NMR spectra
of the complexes contain a
These data are consistent with the formation of CCC-NHC
pincer Zr complexes. Complexes 2b, 2c, 3c, and 4a were iso-
lated as analytically pure solids despite their sensitivity to
moisture.
13
C
resonance near 190 ppm.
NHC
Preliminary Evaluation of a New CCC-NHC Zr Complex
for Activity in Hydroamination/Cyclisation
Initial kinetic trials using the previously reported butyl-CCC-
NHC pincer Zr complexes (2a or 3a) in C6D6 or toluene-d8
resulted in heterogeneous mixtures with solids present at tem-
peratures where a lock signal was obtainable. Homogeneous
(a) Amido insertion
R
(b) Amine-enhanced
amido insertion
R
N
[M]
N
R
N
N
R
H
Rꢁ
Rꢀ
N
[M]
1ꢃ and 2ꢃ
aminoalkenes
1ꢃ and 2ꢃ
aminoalkenes
R
R
N
N
H
R
N
R
H
[M]
N
N
[M]
Rꢀ
Rꢁ
(c) Amido-enhanced amido insertion
(d) Imido-[2ꢂ2] cycloaddition
R
N
R
N
H
N
[M]
N
[M]
N
H
1ꢃ aminoalkenes
2ꢃ aminoalkenes
if 1ꢃ amine is
added.
1ꢃ aminoalkenes
only
R
H2N
N
H
R
[M]
N
[M]
N
N
Scheme 1. Key transition state elements for reported hydroamination/cyclisation mechanisms: (a) Amido
insertion.[5a] (b) Amine-enhanced amido insertion.[4b] (c) Amido-enhanced amido insertion with imido for-
mation.[5d] (d) Imido-[2 þ 2] cycloaddition.[4c]

Mechanism/Hydroamination with CCC-NHC Zr
575
Computational Modelling of Potential Mechanisms with
CCC-NHC Pincer Zr Complexes
Multiple reaction pathways were considered, and each was
modelled computationally at the PBEPBE level of theory with
the LANL2DZ basis set. The alkyl groups on the imidazolyl ring
and the phenyl groups on the substrate were truncated to methyl
groups. Not all potential mechanisms as outlined in Scheme 1
were investigated or reported due to simple scope and limita-
tions of the catalysis eliminating these possibilities.
While experimental and computational studies were occurring
simultaneously, the results of the computational studies are
presented first to establish the scope of potential mechanisms.
CCC-NHC Pincer Zr Complex Disproportionation/
Conproportionation – Experiment and Computation
N
N
1.1 Zr(NMe2)4
I
Zr
I
N
R
N
R
NMe2
The working hypothesis (as described in the preceding paper)
based on no reaction with secondary amines and the qualitative
catalyst concentration experiments was that the monoamido
complex 2a effectively disproportionated to give a bis-amido
complex 4a as illustrated in Scheme 3, which was necessary for
catalysis (imido formation), with concomitant formation of a
triiodo complex 3a. Consistent with this hypothesis was the
observation of complete loss of catalytic activity below a
threshold concentration. The computed free energies of the
species involved in the proposed disproportionation are also
N
N
N
N
2a, R ꢄ n-Bu, ref. [5b]
2b, R ꢄ n-hexyl, 60 %
2c, R ꢄ undecyl, 50 %
N
N
N
N
RI
1. Zr(NMe2)4
2. MeI
N
N
I
Zr
I
N
R
N
R
ꢂ
ꢂ
I
ꢅ
ꢅ
I
I
R
R
3a, R ꢄ n-Bu, ref. [5b]
3c, R ꢄ undecyl, 71 %
1a, R ꢄ n-Bu, ref. [5b]
1b, R ꢄ n-hexyl, 82 %
1c, R ꢄ undecyl, 66 %
included in Scheme 3, and predict a DG8rxn of þ 4.6 kcal molꢀ1
,
which is consistent with the experimental observation of no
observable disproportionation of the mono-amido complex 2a
in solution. For computational simplicity the butyl group was
replaced with a methyl group, as this change was expected to
have little to no impact on the computed results. In addition,
when diamido complex 4a was mixed with triodide complex 3a
(Scheme 3, reverse reaction) at room temperature immediate
conproportionation was observed to produce the monoamido
complex 2a (see Figs S24 and S25 in the Supplementary
Material). Although little diamido 4a would be expected to be
generated in solution starting from mono-amido 2a, the diamido
complex 4a was stable when prepared independently.
2.5 Zr(NMe2)4
N
N
I
Zr
N
R
N
NMe2
NMe2
R
4a, R ꢄ n-Bu, 92 %
Scheme 2. Synthesis of imidazolium salts (1a–c) and CCC-NHC pincer Zr
complexes used in these studies.
Table 1. Evaluation of complexes 2b, 2c, 3c, and 4a as pre-catalysts for
hydroamination/cyclisation
Me
Me
Ph
Ph
5 mol-% precat.
Amido Insertion Mechanism
Ph
Ph
ꢂ
N
N
NHR
Ph
Ph
R
1,2-Cl2C6H4, C6D6, 120ꢃC
This well known mechanism was examined for CCC-NHC
pincer Zr complexes and is illustrated in Scheme 4, since it
would require only one amido ligand as found in catalytically
active 2a.[8b,8c] Simple ligand exchange from pre-catalyst 4
yields intermediate 9, which may be described as the ‘catalyti-
cally active species’ in this cycle. There are two key high energy
transition states in the proposed pathway – the insertion and
protonation steps. The insertion of the alkene into the Zr–N bond
of intermediate 9 through TS-9–10 with DGz ¼ 22.3 kcal molꢀ1
yields intermediate 10. Throughout the discussion TS-9–10 (or,
generally, TS-X-Y) is used to indicate the highest transition
state from intermediate 9 to intermediate 10. But, to be clear, it
may not represent a single step on the potential energy surface
(PES). Intramolecular proton transfer (TS-10–11, Scheme 4)
trace
R ꢄ H, 7
R ꢄ H, 8
R ꢄ H, 5
R ꢄ Bn, 6
Entry
R
Conv. [%], time [h], 7:8A
2c 3c
2b
4a
1
2
H
.98, 3, 35:1 .98, 3, 33:1 .98, 2C,D, – .98, 2, 16:1
0, 12B 0, 12B 0, 12B 0, 12B
Bn
AConversion and 7:8 ratio determined by 1H NMR spectroscopy.
BAdding 20 mol-% n-hexylamine and heating for 24 h at 1208C resulted in
0 % conversion (no reaction).
CHeterogenous (solid present).
DFormation of 7a was not observed.
2
N
N
N
N
N
N
I
I
I
ꢂ
Zr
I
Zr
I
Zr
N
R
N
R
N
R
N
R
N
R
N
NMe2
NMe2
I
R
NMe2
2a
4a
3a
ΔGꢃgas ꢄ 0
28.7
ꢅ24.1
Scheme 3. Computed free energies for the disproportionation of mono-amido CCC-NHC pincer Zr complex
in the gas phase. Computations were performed with R ¼ Me. Experimental results were obtained with R ¼ Bu.
The relative free energies presented in this scheme are for an internally consistent comparison only and do not
correlate with the values below.

576
W. D. Clark et al.
C
C
C
C
I
I
Zr
NMe2
Zr NMe2
C
C
NMe2
NMe2
4
4
C
C
2 H2N
C
N
N
C
ꢄ
H2N
N
N
ꢄ
Zr
Zr
N
N
R
N
Zr
C
2 HNMe2
Zr
N
N
C
R
R
R
C
C
C
H
I
C
Zr
Me2N
I
N
H
Zr
H
N
N
C
TS-12–13
33.3
C
H
H
Me2N
HN
TS-9–10
22.3
12
13.7
H2N
9
1.0
H2N
14
ꢅ3.8
13
10.5
C
C
C
C
H
I
I
C
H
C
C
H
Me2N
Zr
C
Me2N
Zr
N
N
I
I
Zr
N
Zr
N
C
C
TS-13–14
43.3
H
Me2N
H
C
Me2N
C
HN
N
11
ꢅ4.0
10
9.2
Scheme 5. Computationally modelled amine-enhanced concerted amido
mechanism with expanded coordination number. Relative energies (DG8) of
intermediates 12, 13, and 14 are given, and the energies of the transition
states are also included.
TS-10–11
28.2
Scheme 4. Computationally modelled amido insertion mechanism for
CCC-NHC pincer Zr complexes. Relative energies (DG8) of intermediates
9, 10, and 11 are given and the free energy of activation of the key transition
states are also included.
˚
intermediate 13, which has a Zr–I distance of 3.27 A. The free
energy of the transition state for this 1,3 proton transfer between
nitrogen atoms was computed to be þ33.3 kcal molꢀ1, a pro-
hibitively high value. And yet, the next step with concurrent
C–N and C–H bond formation has an even higher free energy of
activation of þ43.3 kcal molꢀ1 on the path to 14. If the CCC-
NHC pincer Zr complexes employed this type of mechanism,
secondary amino-alkenes should be cyclised. The lack of
cyclisation observed for secondary amino-alkenes[8a,8c] and the
high free energies of activation that were computed suggest that
this mechanism is not involved for the CCC-NHC pincer Zr
catalysed reactions.
has a higher free energy of activation (DGz ¼ 28.2 kcal molꢀ1
)
leading to imido intermediate 11, which regenerates catalyti-
cally active 9 upon product dissociation and substrate addition
with concomitant proton transfer.
If this mechanism were operative, secondary amino-alkenes
would be expected to cyclise in the presence of a primary amine,
in a manner similar to that reported by the Sadow group.[5d] An
un-cyclisable primary amine is required as a proton shuttle for
secondary amine substrates in this mechanism. The density
functional theory (DFT) studies are clearly consistent with a
large free energy of activation for this pathway in the current
catalyst system. No reaction was observed with secondary
amines as was previously reported,[8c] not even in the presence
of a primary amine as documented in Table 1, entry 2, footnote B
(see above). Therefore, it seems unlikely that the amido inser-
tion mechanism is operative in CCC-NHC pincer Zr complex
catalysed reactions.
Concerted Imido-[2 1 2] Cycloaddition
This mechanism was also computationally modelled as illus-
trated briefly in Scheme 6 (see the Supplementary Material for a
detailed principle of the microscopic reversibility model of this
mechanism). The lack of cyclisation for a secondary amino-
alkene, even in the presence of a primary amine (Table 1, see
above) was consistent with the formation of a catalytically rel-
evant imido intermediate 15. The concentration corrected
energy for 15 is 3.4 kcal molꢀ1 lower than the diamido pre-
catalyst 4. The imido intermediate 16 is predicted to undergo
cyclisation through a low energy transition state (TS-16–17,
Amine-Enhanced Concerted-Proton Transfer Amido
Mechanism with Expanded Coordination Sphere
z
Zr is well known to expand its coordination sphere. For example,
Cp4Zr is considered 10-coordinate as it has three Z5-Cp (Cp
¼ cyclopentadiene) rings, each occupying three coordination
sites, and one Z1-Cp.[10] No evidence for higher-coordination
numbers have been observed in the CCC-NHC pincer com-
plexes. However, because many reports of the early transition
metal catalysed hydroamination involve Cp ligands and higher
coordinate Zr (typically seven-coordinate) in the catalytic cycle,
expansion to a seven-coordinate species was examined com-
putationally. The results are illustrated in Scheme 5. Interme-
diate 12 was formed by coordination of a substrate molecule
with subsequent proton transfer producing seven-coordinate
DGcomp ¼ þ14.0 kcal molꢀ1) producing azazirconacyclobutane
17 in a reaction step that is initially exergonic by 6.4 kcal molꢀ1
(see Fig. S48, Supplementary Material). When the substrate is
included the energy rises to 12.9 kcal molꢀ1 as indicated for 17
in Scheme 6. Coordination of the substrate is exergonic by
6.8 kcal molꢀ1 (17 versus 18). The proton transfer from N to C
producing intermediate 19 is predicted to be the turnover lim-
iting step with a transition state (TS-18–19) free energy of
activation of þ18.1 kcal molꢀ1, which is the highest computed
transition state value in a highly exergonic process that is
attributable to the seven-coordinate intermediate going to six
coordinate plus the C–H bond formation.

Mechanism/Hydroamination with CCC-NHC Zr
577
C
C
NMe2
Zr
I
C
NMe2
4
NH2
C
C
N
N
ꢄ
Zr
Zr
N
N
R
C
2 HNMe2
R
C
C
C
C
Zr
N
I
H
N
Zr
N
C
I
C
H2N
15
0
TS-19–15
C
C
C
C
20
ꢅ3.0
13.7
N
I
Zr
16
10.6
I
Zr
N
19
C
C
NH
ꢅ14.1
TS-16–17
14.0
TS-18–19
18.1
Turnover
limiting
step
17
12.9
C
C
C
C
Zr
I
I
Zr
N
N
TS-17–18
18
6.1
C
14.4
C
NH2
H2N
Scheme 6. Computationally modelled [2 þ 2] cycloaddition mechanism. Relative energies (DG8) of
intermediates 15–20 are given, and the energies of the transition states are also included.
Intermediate 19 is predicted to undergo proton transfer and
loss of product in an overall endergonic step to yield five-
coordinate intermediate 15. Furthermore, it was predicted that
coordination of the alkene to regenerate the ‘catalytically active
species’ 15, is endergonic by þ14.1 kcal molꢀ1. This prediction,
that the five-coordinate intermediate 15, which is a 16-electron
complex with p-donation from the iodo ligand, is more stable
than the six-coordinate intermediate 16, which is a 16-electron
complex (four electrons from iodo, six from CCC-NHC, two
from alkene, and four from the ‘bent’ imido), is consistent with
the requirement of the gem-dialkyl effect for efficient cyclisa-
tion, and the lack of reactivity towards intermolecular hydro-
amination of alkenes. The origin of the preference ‘not to
coordinate the alkene’ may be found in the requirement of the
imido to bend for alkene coordination, and that the trigonal
bipyramidal configuration avoids two disfavoured trans-influ-
ence interactions, namely, the trans-I–imido and the trans-CAr–
alkene. If the primary cause of the preference was the imido
bending then there would be little impact expected on the
intermolecular reaction, therefore the inference is that the
octahedral geometry at Zr required to assemble all of the
‘substrates’ is disfavoured in the ligand set and coordination
environment. It was also found computationally that the sub-
strate should strongly inhibit the reaction by coordinating to the
catalytically active species 15 to generate 20. The equilibrium is
predicted to strongly favour the bound substrate complex, which
impacts the reaction rates and order in the substrate. If this
mechanism was operative, the following predictions should
hold: 1) A primary kinetic isotope effect should be measureable.
2) Considering the highly ordered state (catalyst, bicyclic
intermediate, and an additional molecule of substrate for pro-
tonation) for the conversion of 18 into 19 a large negative DSz
value would be anticipated. 3) Only primary amines will cyclise.
4) Intermediate 18 would be the potential resting state of the
catalyst. The experimental data presented below are most
consistent with the imido-[2 þ 2] cycloaddition mechanism
being operative for CCC-NHC pincer Zr catalysed hydroamina-
tion cyclisation.
Pre-Catalyst Stability: 1H NMR Investigations
Mono-Amido 2b
The possibility of pre-catalyst decomposition was investi-
gated for diiodo complex 2b. When diiodo complex 2b, primary
substrate 5, and toluene-d8 were combined at room temperature
a homogenous yellow solution was formed that contained no
resonances attributable to an imidazolium salt (no resonances
at .8.5 ppm in the 1H NMR spectrum). However, after sitting at
room temperature for 30 min or heating at 1008C, a precipitate
was observed. Upon dissolving the precipitate with CD2Cl2,
1
resonances were observed in the H NMR spectrum that were
consistent with imidazolium salt formation. However, when the
reaction was performed in 1,2-dichlorobenzene/C6D6 at tem-
peratures .808C the reaction was homogenous and signals
consistent with imidazolium salt formation were not observed.
A weak, broad resonance was observed at 11.23 ppm, but
resonances at 9.04 and 8.56 ppm, which are indicative of the
imidazolium salt, were not observed. These data are consistent

578
W. D. Clark et al.
with an intact pre-catalyst complex and are not consistent with
imidazolium salt formation during catalytic runs under homog-
enous conditions (1,2-dichlorobenzene/C6D6).
1ꢃ amine 5
N
N
N
N
ꢂ ZrIxLn
I
1,2-Cl2C6H4,
C6D6, 110°C
Zr
I
N
R
N
R
ꢅ
N
R
N
ꢂ
ꢂ
5·HCl
2I
I
ꢂ
In an effort to identify the source of the peak at 11.23 ppm,
the reaction was spiked with imidazolium salt 1b, 5ꢁHCl, or
Bu2NHꢁHCl, all of which resulted in the appearance of strong
resonances at 11.40, 9.04, and 8.56 ppm. The peak at 11.23 ppm
was assigned to the N–H signals of the ammonium salt. Pre-
catalyst 2b decomposed in non-polar solvents such as C6D6 and
toluene when combined with a primary amino-alkene. Similar
observations were made for complex 2c. In all cases, however,
cyclisation still occurs, suggesting that the Zr complexes that are
formed from decomposition are catalytically active, or that the
CCC-NHC pincer Zr complex 2b is regenerated at the tempera-
tures of catalysis.
R
Ln ꢄ amido, imido
or amine ligands
3c,
R ꢄ undecyl
1c,
R ꢄ undecyl
Scheme 7. Reaction observed between triiodo complex 3c and substrate 5.
used in an effort to capture the rate of hydroamination/cyclisation
before any side reactions occurred. A plot of the natural loga-
rithm of the change in substrate concentration versus time as
reflected in the integral with diiodo complex 2c was found to be
linear (see Fig. S44a, Supplementary Material). The linearity of
this result is consistent with a first-order dependence upon the
amino-alkene 5 (Fig. S44a). When diamido complex 4a was
evaluated with 5 the plot of substrate concentration (as reflected
in the integral values) versus time was linear (Fig. S44c,
Supplementary Material). Such an observation was consistent
with zero-order dependence upon amino-alkene 5 (Figure
S44c). This observation is consistent with a reversible coordi-
nation of the substrate that is saturated at the relatively high
concentrations of the trials before the turnover limiting step
(Scheme 6) as has been noted previously for hydroamination.[5d]
Each pre-catalyst was independently evaluated over a concen-
tration range. The plot of the kobs values obtained versus the
concentration of each is illustrated in Fig. S44b (Supplementary
Material) for pre-catalyst 2c, and in Fig. S44d (Supplementary
Material) for pre-catalyst 4a. The linearity of each plot with an
intercept near zero (0) is consistent with a first-order depen-
dence with respect to each pre-catalyst. Therefore the empirical
rate laws are:
Diamido 4a
No resonances at greater than 8.5 ppm were observed in the
1H NMR spectra of catalytic runs using diamido complex 4a and
primary amino-alkene 5, which suggests no detectable decom-
position of the Zr complex. However, the characteristic triplet
for the methylene group a to the imidazolyl N (4.03 ppm)
decreased late in the reaction progress with the growth of a peak
at 4.34 ppm, which was assignable to the oxidative amination
product 8 (see Fig. S40, Supplementary Material). No reso-
nances corresponding to the imidazolium salt (11.40, 9.04, and
8.56 ppm, in particular) were observed in catalytic trials using
diamido complex 4a. The data were not consistent with the
presence of an imidazolium salt from decomposition being
present at detectable levels during homogenous catalysis con-
ditions for monoamido or diamido pre-catalysts.
Tri-iodide 3c
Rate ¼ k½2cꢂ½5ꢂ; where k ¼ intrinsic rate constant
ð1Þ
Enhanced rates of catalysis were recently reported when
triodide complex 3a was combined with mono-amido complex
2a, an interesting synergistic effect. Because of the limited
solubility of the butyl derivatives, complex 3c with undecyl
alkyl groups was prepared. A homogenous solution was
obtained at 1108C when the hydroamination/cyclisation experi-
ment was performed with a very dilute initial triiodo complex 3c
concentration ([3c]0 ¼ 0.0016 M) and 1,2-dichlorobenzene. The
high temperature 1H NMR spectrum contained peaks at 11.55,
9.09, and 8.46 ppm, which was consistent with quantitative
formation of imidazolium salt 1c (Scheme 7). Imidazolium salt
1c was only sparingly soluble in 1,2-dichlorobenzene and was
the cause of the heterogeneous nature of the reaction at higher
concentrations. However, the reaction was homogenous when
using the secondary amine 6 with triiodo complex 3c, and
signals corresponding to the imidazolium salt were not
observed. These results were consistent with triiodo complex
3c decomposing in the presence of a primary amino-alkene, but
not a secondary amino-alkene. The ‘synergistic’ enhancement
of rate for hydroamination/cyclisation undoubtedly arose from
the unidentified Zr species (Scheme 7) formed from the decom-
position of triiodo complex 3c.
Rate ¼ k0½4aꢂ; where k0 ¼ intrinsic rate constant ð2Þ
Kinetic Isotope Effects (KIEs)
The CCC-NHC pincer Zr catalysts showed normal KIEs
when the substrate 5 was prepared with deuterium on nitrogen
and cyclised with either 2c or 4a. Detailed kinetic plots may be
found in the Supplementary Material (Figs S46–S47). When
pre-catalyst 2c was employed a large KIE was found at kH/
kD ¼ 4.6 ꢃ 0.1 at 1108C. The KIE measured for pre-catalyst 4a
was just over half of that value at kH/kD ¼ 2.4 ꢃ 0.1 at 1048C.
The slight change in temperature was not expected to have a
significant impact. The observation of significant KIEs is
indicative of breaking of the N–H bond in the transition state,
a common theme in hydroamination/cyclisation.[4b,5a,5d,11]
Activation Parameters
The temperature dependence of the catalysis using com-
plexes 2c or 4a was investigated over a range of 20 or 308C,
respectively. The Eyring plot generated from the data is shown
in Fig. S45a (Supplementary Material) for complex 2c and in
Fig. S45b (Supplementary Material) for complex 4a. The DHz
and DSz values obtained for complex 2c (DHz ¼ 8.7 ꢃ 0.2 kcal
Empirical Rate Law, Kinetic Isotope Effects, and Activation
Parameters
molꢀ1, DSz ¼ ꢀ41 ꢃ 1 eu) and 4a (DHz ¼ 8.3 ꢃ 0.3 kcal molꢀ1
,
Order of Reaction in Substrate and Catalyst
DSz ¼ ꢀ41 ꢃ 1 eu) were within experimental error of each other,
and were close to the range of values previously reported, but
with some differences. The values of DHz were significantly
Kinetic experiments were performed to determine an empiri-
cal rate law for catalysis with complexes 2c and 4a due to their
superior solubility characteristics. The initial rate method was

Mechanism/Hydroamination with CCC-NHC Zr
579
catalytically active species, one scenario for generating a net
zero order in substrate would be – plus one for generating 15 and
minus one for inhibition, 20 (see above). An alternative model
would be that the diamido precatalysts were evaluated under
substrate saturation conditions, which resulted in the zero order
dependence. Despite the fact that Me2NH has a boiling point of
78C it was clearly observable in the spectra during catalysis (see
Fig. S41, Supplementary Material), and, therefore, cannot be
assumed to be in the gas phase. Thus it may also be involved in
the shuttling of protons between cyclised intermediates 17, 18,
and 19, producing a situation that may resemble ‘general acid
catalysis’ regarding the ammonium involved in the protonation
when mono-amido pre-catalysts are used.
N
N
N
N
I
I
Zr
I
Zr
I
N
N
R
N
R
N
R
NH
NMe2
R
NHMe2
A
2c, R ꢄ undecyl
A
H2N
Rꢀ
Rꢀ
ꢅ
A
ꢂ
I
RꢁNH3
B
A
2 NHMe2
N
N
N
N
I
Zr
N
R
N
Zr
N
R
N
At catalytic concentrations the amine substrate is roughly 10
to 20 times the concentration of Me2NH, but for catalysis with
diamido complex 4a, Me2NH may be expected to be twice the
concentration when employing the monoamido complexes.
More importantly, ammonium salts are not formed when dia-
mido pre-catalysts are used. In addition, considering the relevant
pKas of Me2NH (10.73) and, as a surrogate for the substrate,
n-BuNH2 (10.77), they are of comparable strength although the
primary amine is a slightly better base.[12] The acidity of the
ammonium salts is most relevant for the monoamido pre-
catalyst since it is the ammonium cation that is expected to
participate in the protonation steps (18 - 19). Whereas, the
diamido pre-catalysts do not form the ammonium salts, so it is
tempting to invoke the amine pKas of around the low 40s, until
intermediate 18 is given careful consideration. This intermedi-
ate also contains a ‘tetracoordinated’ nitrogen since it is bound
to Zr and may be expected to have an enhanced acidity versus
R
I
N
NMe2
NMe2
4a, R ꢄ n-Bu
R
15
Scheme 8. Generating the catalytically active species from different pre-
catalysts.
lower than for Ti(NMe2)4 (DHz ¼ 26.2 ꢃ 0.7 kcal molꢀ1).[11a]
They were also lower than values determined for a tetravalent U
pre-catalyst (DHz ¼ 13.8 ꢃ 0.3 kcal molꢀ1),[11b] and for a Zr
salicyloxazoline complex (DHz ¼ 17 to 21 kcal molꢀ1).[4f] The
DHz values obtained for complexes 2c and 4a were similar to
values obtained for a Zr cyclopentadienylbis(oxazolinyl)borate
complex (DHz ¼ 6.7 ꢃ 2 eu).[5d] However, the DSz values
obtained for the CCC-NHC Zr complexes 2c and 4a were
significantly higher than values reported for a lanthanide com-
plex (DSz ¼ ꢀ30.1 ꢃ 1 eu) and for the Zr salicyloxazoline
complexes (DSz ¼ ꢀ13 to ꢀ23 eu). They are, however, rather
similar to values reported for a tetravalent U pre-catalyst (DSz
¼ ꢀ43 ꢃ 9 eu) and a (Cp)Zr-bis(oxazolinyl) borate complex
(DSz ¼ ꢀ43 ꢃ 7 eu).[5a] The DHz values obtained in this study
were consistent with small changes in bonding at the transition
state, while the DSz values were consistent with a highly ordered
transition state.
3þ
the free amine. Co(en)3 has a reported acidity of 14.2,[13]
which is anticipated to be similar to an amine bound to the Lewis
acidic, but neutral, Zr centre. While these pKa values are based
on water measurements, the ammonium values are not expected
to differ by more than a few pKa units in DMSO,[14] but may be
10–13 units higher in acetonitrile.[15] The differences in pKas
is expected to be about the same regardless of the solvent. Yet it
is the difference in the transferring agent RNH3þ, which of
necessity undergoes an ‘outer sphere’ transfer, versus the Zr
bound amine (Scheme 8) that likely plays a key role in the
difference in reaction order and isotope effect. This difference in
Interpreting the Experimental and Computational Results
þ
As illustrated in Scheme 8, both monoamido complex 2c and
diamido complex 4a provide access to the ‘catalytically active
species’ 15, which seems the best fit to all of the available data.
But, the reaction conditions that generate it were very different.
Monoamido complex 2c generates an equivalent of a relatively
acidic ammonium salt B during the generation of 15, albeit at a
relatively low concentration. This difference in reaction con-
ditions undoubtedly plays a large role in the differences
observed in reaction order and KIEs when the reaction is initi-
ated with different pre-catalysts 2c and 4a. Such a necessity for
two molecules of substrate to form the active catalyst when
coupled with a molecule of substrate involved in the inhibition
of the catalytically active species (Scheme 6, 20) gives rise to a
net first-order dependence on substrate when monoamido
complexes were employed (see above). Alternatively, it may be
that the monoamido complexes were not saturating the equi-
librium before the turnover limiting step for the monoamido
complexes thus generating pseudo-first order kinetics.
proton transfer agents (RNH3 versus RNH2-Zr) likely gives
rise to a significþant difference in the symmetry of the proton
transfer (RNH3 being much more unsymmetrical) and
accounts for the significant difference in the kH/kD values.
Summary of Observations
To summarise the key experimental observations that must be
accounted for in any mechanism: 1) Hydroamination of primary
amine substrates occurred, but secondary amines were not
cyclised. 2) Secondary amines were not cyclised even in the
presence of a non-cyclisable primary amine thus eliminating the
amine-enhanced amido mechanisms. 3) The reaction was first
order in monoamido pre-catalyst 2c and substrate making it
second order overall. 4) The reaction was first order in diamido
pre-catalyst 4a and zero-order in substrate making it first order
overall. 5) When the amine was deuterated and evaluated for a
KIE, complex 2c was found to have a kH/kD of 4.6, and 4a was
found to have a kH/kD of 2.4. The difference is attributable to the
difference in the acidity and activity of the medium due to the
formation of ammonium salts when monoamido complex 2c
was pre-catalyst, since the most acidic species, the ammonium
In contrast, the diamido complexes were under neutral to
mildly basic conditions as 2 equivalents of dimethyl amine are
generated in the production of 15. Since the diamido complexes
do not need a second substrate molecule to produce the

580
W. D. Clark et al.
salt B (Scheme 8), will deliver the proton in this case, versus the
coordinated amine (18, Scheme 6) for the diamido pre-catalyst
4a. 6) Despite these significant differences the activation para-
meters, DHz and DSz, were found to be coincidentally within
experimental error of one another. While DHz was relatively
small, 8.7 kcal molꢀ1 for 2c and 8.3 kcal molꢀ1 for 4a, which is
consistent with small changes in bond enthalpy, the entropy term
was large and negative for both, DSz ¼ ꢀ41 eu, consistent with a
highly ordered transition state. According to the computed free
energies of activation as presented in Scheme 6, the turnover
limiting step is the protonation of the zircona-azacyclobutane
intermediate 18. This step would be expected to show a primary
KIE on the observed order of magnitudes, and is highly ordered
incorporating a catalyst and a bicyclic intermediate plus a sub-
strate molecules (coordinated for diamido catalyst 4a and as the
ammonium salt for diiodo catalyst 2c) to give the large negative
DSz values and small DHz. 7) The coordinately unsaturated
catalytically active species 15 is subject to substrate inhibition
resulting in a zero order reaction in substrate for pre-catalyst 4a
and first order for pre-catalyst 2c. 8) A disproportionation of
monoamido complexes was considered, but this would make the
reaction second order with respect to [Zr]0 (see Supplementary
Material), which was not observed. In addition, a dispropor-
tionation reaction would make triiodo complex 3c, which
decomposes in the presence of substrate to form imidazolium
salt 1c with observable peaks at 11.55, 9.09, and 8.46 ppm in the
1H NMR spectrum (see above). These peaks are not observed
when using complex 2c. Therefore it was inferred that complex
3c was not being formed at an observable concentration. Thus
the most probable mechanism involves formation of a Zr imido
species and an imido-[2 þ 2] addition mechanism (Scheme 6)
under saturation kinetic conditions in the current study of 4a.
column of activated basic Al2O3. Toluene and CH2Cl2 were
dried using molecular sieves and the SP-1 Solvent System from
LC Technology Solutions Inc. 1,3-Bis(imidazol-10-yl)benzene
and 1,3-bis(30-butylimidazol-10-yl)benzene diiodide (1a) were
prepared according to previously reported procedures.[8b,16]
Synthesis and Characterisation
1,3-Bis(30-hexylimidazol-10-yl)benzene Diiodide (1b)
1,3-Bis(imidazol-10-yl)benzene (1) (3.12 g, 14.7 mmol),
1-iodohexane (45.0 mL, 305 mmol), and CH3CN (100 mL) were
combined in air, degassed, and heated at 1208C for 1 h. The
reaction was cooled to room temperature, and concentrated
under vacuum yielding a yellow solid. The solid was washed
onto a frit with 1 : 1 CH3CN/Et2O (50 mL) and washed with
additional 1 : 1 CH3CN/Et2O (2 ꢄ 50 mL). The resulting white
solid (7.64 g, 82 %) was dried under vacuum at 1108C overnight.
dH (DMSO-d6) 10.01 (s, 2H), 8.45 (s, 2H), 8.38 (s, 1H), 8.15
(s, 2H), 8.05–7.98 (m, 3H), 4.30 (t, 4H, J 7.3), 1.93 (m, 4H),
1.315 (m, 12H), 0.87 (t, 6H, J 6.6). dC (DMSO-d6) 135.7, 135.6,
131.9, 123.5, 122.6, 121.0, 115.7, 49.6, 30.5, 29.0, 25.1, 21.8,
þ
13.8; HRMS m/z 507.1965 [M ꢀ I]þ, calcd for C24H36IN4
507.1979; 379.2847 [M ꢀ 2I ꢀ H]þ, calcd for C24H35N4
379.2856.
1,3-Bis(30-undecyllimidazol-10-yl)benzene
Diiodide (1c)
1,3-Bis(imidazol-10-yl)benzene (1) (1.240 g, 5.898 mmol),
1-iodoundecane (5.055 g, 17.91 mmol), and CH3CN (5 mL)
were combined in air, degassed, and heated at 908C for 5.5 h.
During this time, white crystals formed. The reaction was
allowed to cool to room temperature. The crystals were collected,
washed with acetonitrile (2ꢄ 10mL) and hexane (5ꢄ 10mL),
and dried under vacuum at 1508C for 24 h (3.429g, 75 %).
dH (DMSO-d6) 10.06 (s, 2H), 8.48 (s, 2H), 8.41 (s, 1H), 8.17 (s,
2H), 8.07–7.95 (m, 3H), 4.31 (t, 4H, J 7.1), 1.93 (m, 4H), 1.32–
1.23 (m, 34H), 0.84 (t, 6H, J 6.6). dC (DMSO-d6) 135.63, 135.58,
131.9, 123.5, 122.5, 121.0, 115.6, 49.6, 31.2, 29.0, 28.9, 28.7,
28.6, 28.3, 25.5, 22.0, 13.9. HRMS m/z 647.3553 [Mꢀ I]þ, calcd
for C34H56I4 647.3544.
Conclusions
The possible mechanisms of hydroamination/cyclisation with
CCC-NHC Zr complexes have been experimentally and com-
1
putationally modelled. High temperature H NMR data were
consistent with decomposition of triiodo complex 3c into an
unidentified Zr species and imidazolium salt 1c, which accounts
for its reported synergistic effect. However, high temperature 1H
NMR data were not consistent with the decomposition of
complexes 2b and 4a into an imidazolium salt when using 1,2-
dichlorobenzene as a solvent. Only the [2 þ 2] cycloaddition
mechanism (Scheme 6) was consistent with the reactivity (no
reaction with secondary amino-alkenes even if a primary amino-
alkene was present), experimentally observed primary KIE
effects, substrate and pre-catalyst orders, and experimentally
determined activation parameters.
2-(1,3-Bis(30-hexyl-imidazol-20-ylidene)phenylene)
(dimethylamido)diiodo Zirconium (IV) (2a)
Zr(NMe2)4 (0.248 g, 0.930 mmol), imidazolium salt 1a
(0.559 g, 0.881 mmol), and toluene (25 mL) were heated at
1608C for 4 h. The reaction was allowed to cool to room
temperature. A yellow, crystalline solid appeared. The solid
was collected, washed with toluene (3 ꢄ 5 mL) and Et2O (5 mL),
and dried under vacuum at 1008C overnight (0.368 g, 53 %). dH
(CH2Cl2/C6D6) 7.20 (d, 2H, J 1.8), 7.06 (t, 1H, J 7.8), 6.84 (d,
2H, J 7.8), 4.21 (br s, 2H), 4,07 (br s, 2H), 2.76 (br s, 6H), 1.71
(br s, 4H), 1.26–1.12 (m, 12H), 0.73 (t, 6H, J 6.7). dC (CH2Cl2/
C6D6) 193.6, 164.6, 146.6, 128.9, 121.4, 115.4, 110.3, 52.1,
42.3, 31.77, 31.74, 26.4, 22.8, 14.1. Found: C 40.60, H 5.13, N
8.98. Anal. Calc. for C26H39I2N5Zr: C 40.73, H 5.13, N 9.14 %.
Experimental
General Procedures
All reactions were performed in an inert atmosphere of N2 or Ar
using a glovebox or standard Schlenk techniques unless other-
wise noted. Al2O3 (basic, 50–200 mm) was stored in an oven at
1508C before use. Amine substrates were made according to
previously reported literature procedures and distilled over
CaH2 before use. 1H and 13C NMR spectra were recorded on 300
or 600 MHz Bruker instruments. CHN combustion analysis was
performed by Atlantic Microlabs (Norcross, GA). Chemical
shifts (d) are expressed in ppm and referenced to the residual
solvent peak. Zr(NMe2)4 was freshly sublimed. 1,2-Dichloro-
benzene, C6D6, and CD2Cl2 were dried by passing through a
2-(1,3-Bis(30-undecyl-imidazol-20-ylidene)phenylene)
(dimethylamido)diiodo Zirconium (IV) (2b)
Zr(NMe2)4 (0.248 g, 0.930 mmol), imidazolium salt 1a
(0.686 g, 0.886 mmol), and toluene (10 mL) were heated at
1608C for 4 h. The reaction was allowed to cool to room
temperature. A yellow, crystalline solid appeared. The solid

Mechanism/Hydroamination with CCC-NHC Zr
581
was collected, washed with toluene (5 mL) and Et2O (5 mL), and
dried under vacuum overnight (0.560 g, 70 %). dH (CH2Cl2/
C6D6) 7.23 (d, 2H, J 1.8), 7.06 (t, 1H, J 7.8), 6.84 (d, 2H, J 7.8),
6.82 (d, 2H, J 1.8), 4.22 (br s, 2H), 4.07 (br s, 2H), 2.75 (br s, 6H),
1.72 (br s, 4H), 1.19–1.09 (m, 32H), 0.70 (t, 6H, J 6.7). dC
(CH2Cl2/C6D6) 193.2, 164.2, 146.2, 128.4, 121.0, 114.9, 109.9,
51.7, 41.8, 31.7, 31.4, 29.4, 29.39, 29.33, 29.13, 26.3, 22.5, 13.7.
Found: C 47.45, H 6.33, N 7.57. Anal. Calc. for C36H59I2N5Zr: C
47.68, H 6.56, N 7.72 %.
0.00604 mmol) and the standard solution (0.835 g) were
combined in a screw cap NMR tube. The volume of the total
solution was measured and the resulting solution placed in the
preheated and calibrated NMR probe. The recycle delay (D1)
was set to 5 ꢄ the longest T1 and spectra were collected at reg-
ular time intervals after waiting 10 min to allow for temperature
equilibration.
Computational Methods
Computational models of the catalyst were created based on the
X-ray crystal structure of the CCC-NHC pincer diiodo Zr
complex.[17] In some cases, for computational convenience, the
n-butyl groups were trimmed to methyl groups. The 2,2-dime-
thylpent-4-en-1-amine substrate was used. Geometries were
optimized using the Gaussian09[18] implementation of
PBEPBE[19] density functional theory.[20] The LANL2DZ basis
set and effective core potential (ECP),[21] as modified by Couty
and Hall,[22] was utilised for zirconium. The LANL2DZ(d,p)
basis set/ECP combinations were used for iodine, bromine, and
chlorine.[23] The 6–31G(d’) basis sets[24] were utilised for car-
bon and nitrogen (which have the d polarisation functions taken
from the 6–311G(d) basis sets,[25] rather than the default value of
0.8 used in the 6–31G(d) basis sets), and the 6–31G basis set[26]
was utilised for hydrogen. Each geometry was optimised in the
gas phase and determined to be a minimum energy species or a
transition state using an analytical frequency calculation.
Standard statistical mechanics relationships for an ideal gas at 1
atm and 298 K were used to determine thermodynamic correc-
tions. For comparison purposes, additional SMD SCRF single-
point computations[27] were performed on the optimised
geometries using parameters consistent with toluene as the
solvent; these results yielded qualitatively similar results to
those from the gas phase and are not discussed further. Relative
free energy values include a correction to account for the rela-
tively high substrate concentration under experimental condi-
tions. Using the relationship between free energy and the
equilibrium constant, DG ¼ –RTlnK, where K was the substrate-
to-catalyst ratio of 20, one obtains a concentration correction
factor of ꢀ1.77 kcal molꢀ1 for 298 K. Reported free energies are
gas-phase, concentration corrected values at 298 K.
2-(1,3-Bis(30-undecyl-imidazol-20-ylidene)phenylene)
triiodo Zirconium (IV) (3a)
Zr(NMe2)4 (0.109 g, 0.409 mmol), imidazolium salt 1a
(0.236 g, 0.305 mmol), and toluene (12 mL) were heated at
1608C for 1 h. The reaction was allowed to cool to room tempera-
ture and was then stirred under vacuum for 5 min. The flask was
refilled with Ar and CH3I (3.64 g, 25.6 mmol) was added and the
mixture was stirred at 908C for 2h, after which it was filtered and
concentrated to dryness yielding a yellow solid (0.213 g, 71 %). dH
(C6D6) 7.04 (t, 1H, J 7.7), 6.56 (d, 2H, J 7.8), 6.46 (s, 2H), 5.98
(s, 2H), 4.22 (t, 4H, J 7.6), 1.91 (m, 4H), 1.38–1.29 (m, 32 H),
0.90 (t, 6H, J 6.9). dC (C6D6) 196.2, 165.3, 146.6, 129.1, 120.4,
114.8, 110.3, 52.3, 32.4, 31.7, 30.12, 30.07, 30.0, 29.9, 29.7,
26.9, 23.2, 14.4. Found: C 41.20, H 5.26, N 5.63. Anal. Calc. for
C34H53I3N4Zr: C 41.26, H 5.40, N 5.66 %.
2-(1,3-Bis(30-butyl-imidazol-20-ylidene)phenylene)
bis(dimethylamido)iodo Zirconium (IV) (4a)
Zr(NMe2)4 (0.314 g, 1.17 mmol), imidazolium salt 1a (0.273 g,
0.472 mmol), and THF (3.1 mL) were heated at 1208C for 10 min.
The reaction was allowed to cool to room temperature and sit
undisturbed overnight. During this time, yellow crystals formed.
The mother liquor was decanted. The crystals were washed
with hexane (5 ꢄ 5 mL) and dried under vacuum (0.191 g,
65 %). dH (CH2Cl2, 600 MHz) 7.67 (d, 2H, J 1.7), 7.42 (t, 1H,
J 7.8), 7.27 (d, 2H, J 1.7), 7.26 (d, 2H, J 7.9), 4.01 (t, 4H, J 7.4),
3.01 (s, 12 H), 1.78 (m, 4 H), 1.39 (m, 4H), 0.99 (t, 6 H, J 7.5).
dC (CD2Cl2, 150 MHz) 189.3, 158.4, 148.8, 132.1, 123.2, 116.7,
111.6, 52.1, 38.5, 34.1, 20.4, 14.1.
Alternative Synthesis for 2-(1,3-Bis(30-butyl-imidazol-20-
ylidene)phenylene)bis(dimethyl-amido)iodo Zirconium
(IV) (4a)
Supplementary Material
Spectroscopic data (1H and 13C NMR, and/or exact mass spec-
tra) for the salts, Zr complexes, various experimental and kinetic
runs, and plots of kinetic data, and appendices of computational
and experimental details are available on the Journal’s website.
Zr(NMe2)4 (0.314 g, 1.17 mmol), imidazolium salt 1a
(0.273 g, 0.472 mmol), and THF (2.5 mL) were heated at
1208C for 10 min. The reaction was allowed to cool to room
temperature and sit undisturbed for 13 days. During this time,
large yellow crystals formed. The mother liquor was decanted.
The crystals were washed with hexane (5 ꢄ 5 mL) and dried
under vacuum (0.292 g, 92 %). Found: C 45.93, H 5.97, N 13.17.
Anal. Calc. for C24H37IN6Zr: C 45.92, H 5.94, N 13.39 %.
Acknowledgements
The authors gratefully acknowledge the National Science Foundation (NSF
OIA-1539035, NSF EPSCoR EPS-0903787, NSF CHE-0809732, and NSF
CHE-0955723), the Department of Education (GAANN-P200A120046),
The University of Memphis High Performance Computing Facility, and the
Mississippi State University Department of Chemistry and Office of
Research and Economic Development for funding. K.N.L. conducted the
computational studies, and W.D.C. conducted the experimental studies. The
contributions of each coauthor were equally important for publication.
Representative Kinetic Run
The temperature inside a preheated 1H NMR probe was
calibrated with an 80 % ethylene glycol/DMSO-d6 standard and
T1 relaxation values were determined for relevant resonances
from the standard (1,3,5-trimethoxybenzene) and substrate 5
via the inverse recovery method. A standard solution of pre-
catalyst 2b (0.0091 M) and 1,3,5-trimethoxybenzene (0.066 M)
in 1,2-dichlorobenzene/C6D6 (13 : 1) was prepared and its
density measured (d ¼ 1.258 g mLꢀ1). Substrate 5 (0.0444 g,
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