CCC-NHC Pincer Zr Diamido Complexes: Synthesis, Characterisation, and Catalytic Activity in Hydroamination/Cyclisation of Unactivated Amino-Alkenes,-Alkynes, and Allenes

Article abstract of DOI:10.1071/CH15795

2-(1,3-Bis-3′-butylimidazol-1′-yl-2′-ylidene)phenylene)bis(dimethylamido) iodo zirconium(iv) (3) and 2-(1,3-bis-3′-butylimidazol-1′-yl-2′-ylidene)phenylene)bis (dimethylamido) bromo zirconium(iv) (4), have been prepared via a modification of the solvent and stoichiometry from the previously reported methodology. The reactivity of 3 and 4 in hydroamination/cyclisation is reported. Both diamido complexes have been found to improve catalytic activity as compared with the previously reported mono-amido analogues. Complexes 3 and 4 were observed to be selective for primary amines over secondary amines in hydroamination/cyclisation. The lack of reactivity with secondary amines is consistent with a mechanism involving requisite formation of a Zr-imido intermediate.

Full text of DOI:10.1071/CH15795

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2016, 69, 565–572
Full Paper
http://dx.doi.org/10.1071/CH15795
CCC-NHC Pincer Zr Diamido Complexes:
Synthesis, Characterisation, and Catalytic Activity
in Hydroamination/Cyclisation of Unactivated
Amino-Alkenes, -Alkynes, and Allenes*
A C
,
A C
,
B
Henry U. Valle,
Gopalakrishna Akurathi,
Joon Cho,
A
A
A B D
, ,
Wesley D. Clark, Amarraj Chakraborty, 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,
Mississippi State, MS 38677, USA.
C
These authors contributed equally to this manuscript.
D
Corresponding author. Email: khollis@chemistry.msstate.edu
2-(1,3-Bis-30-butylimidazol-10-yl-20-ylidene)phenylene)bis(dimethylamido) iodo zirconium(IV) (3) and 2-(1,3-bis-30-
butylimidazol-10-yl-20-ylidene)phenylene)bis (dimethylamido) bromo zirconium(IV) (4), have been prepared via a
modification of the solvent and stoichiometry from the previously reported methodology. The reactivity of 3 and 4 in
hydroamination/cyclisation is reported. Both diamido complexes have been found to improve catalytic activity as
compared with the previously reported mono-amido analogues. Complexes 3 and 4 were observed to be selective for
primary amines over secondary amines in hydroamination/cyclisation. The lack of reactivity with secondary amines is
consistent with a mechanism involving requisite formation of a Zr-imido intermediate.
Manuscript received: 18 December 2015.
Manuscript accepted: 21 March 2016.
Published online: 14 April 2016.
Introduction
extensively in the hydroamination/cyclisation of alkynes and
allenes,[9] and the reactions with unactivated amino-alkenes has
been especially popular in recent years.[10] Several groups
worldwide have developed chiral Zr moieties that can enantio-
selectively catalyse the formation of five-, six-, and seven-
membered N-heterocycles (NHCs) (Chart 1).[2a,11] A review of
earth-abundant metals in hydroamination has been recently
reported.[12]
Pincer ligands are a large and important class of organo-
metallic compounds.[13] Examples of early transition metal
complexes with rigid tridentate pincer ligands incorporating
one or more NHCs are sparse.[13] These complexes have proven
to be useful catalytic reagents in various types of reactions. This
activity is due to their strong sigma donor ability and greater
stability compared with their phosphine counterparts.[14] These
properties have led to many applications in catalysis, solar cells,
and biomedical fields.[15] Since the pyridylene moiety coordi-
nates readily, many CNC-NHC pincer complexes have been
reported.[3b,15c,16] However, fewer examples of CCC-NHC
pincer complexes have been reported.
Hydroamination/cyclisation of unactivated amino-alkenes is an
active area of research that is of interest to both the organic and
organometallic chemist.[1] The products of these reactions are
nitrogen-containing building blocks that are found in natural
products, biologically active molecules, and pharmaceuticals.[2]
These desirable moieties have prompted the development
of efficient methods for the formation of C–N bonds, which
typically involve the use of transition-metal catalysts.[2d–f,3]
Methods for the synthesis of nitrogen-containing heterocycles
by transition-metal catalysed intramolecular amine additions
into unactivated amino-alkenes have recently been developed
and have expanded the range of tools available to the synthetic
chemist.[4] Hydroamination of alkenes is attractive as an atom-
economic transformation,[1b,5] however the electrostatic repul-
sion between the nitrogen lone pair and electron rich p-system
of the olefin results in a high activation energy,[6] thus requiring
the use of a catalyst to allow the reaction to occur. Amidates,
diphosphonic amides, biaryl diamines, ureates, and pyridylamido
complexes were reported in the catalytic hydroamination/
cyclisation of unactivated aminoalkenes, and the asymmetric
synthesis of pyrrolidines and piperidines.[3a,7] The use of orga-
nolathanides in hydroaminations/cyclisation has also been
reported.[4c,8] Metal-based catalysts have been employed
The synthesis, characterisation, and catalytic hydroamina-
tion activity of CCC-NHC pincer Zr mono-amido complexes
have recently been reported.[11c,17] In that work it was found
that the mono-amido complexes (Scheme 1, 5, 6) required a
*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

566
H. U. Valle et al.
Mes
Me2N
O
NMe2
NMe2
Ar
Ar
Ar
Zr
P
N
Mes
N
O
N
N
NMe2
NMe2
O
I
I
Me
Me
N
N
NMe2
NMe2
Zr
O
Zr
N
N
Zr
N
P
O
Bu
Bu
NMe2
Mes
Ar
2
R
P
P
N
(NHMe2)x
NMe2
S
N
N
N
NMe2
NMe2
NMe2
NMe2
Si
O
O
Zr
S
Th
Zr
N
Zr
N
NMe2
N
Me2N
NMe2
N
Chart 1. Recently reported catalysts for hydroamination/cyclisation. See text for literature references.
n equiv. Zr(NMe2)4
THF or Tol
N
N
N
N
Zr
L
N
N
ꢁ
ꢁ
N
N
ꢂ X
Bu
Bu
NMe2
X ꢂ
X
Bu
Bu
X ꢀ I,
1
2
n ꢀ 1.5–2.5
L ꢀ NMe2
n ꢀ 1.1
L ꢀ X
Br,
X ꢀ I, 3, 65 %
, 53 %
X ꢀ I,
Br,
5
6
, ref. [11c]
, ref. [11c]
4
Br,
Scheme 1. Synthesis of CCC-NHC pincer Zr complexes.
minimum concentration for catalytic activity and only became
active above 808C. Such a requirement was consistent with
an amido exchange at metal centres to generate the requisite
Zr-imido complex for cyclisation. Therefore it was hypothe-
sised that a diamido complex would provide a more efficient
catalyst and allow a reduction in reaction temperature. Further-
more, in evaluating metals it was found that for the CCC-NHC
complexes thatZrgavethe fastest pre-catalysts. Herein, wereport
an expansion of the previous methodology to form 2-(1,3-bis-30-
butylimidazol-10-yl-20-ylidene)phenylene)bis(dimethylamido)
iodo zirconium(IV) (3), and 2-(1,3-bis-30-butylimidazol-10-yl-
20-ylidene)phenylene)bis(dimethylamido)bromo zirconium(IV)
(4), and their activity in the hydroamination/cyclisation of
unactivated amino-alkenes, -alkynes, and allenes.
reactions, toluene may be the preferred synthetic solvent. Typical
changes in the NMR spectra of the metalation were observed, and
X-ray quality crystals (see below) were readily obtained from the
reaction mixtures. Each was isolated by decanting the mother
liquor from the crystals that were formed upon cooling, washing
with hexanes, and drying. Upon isolation of the crystalline
material the 1H NMR spectra indicated the loss of the imidazolium
proton signal around 11.4–11.6 ppm and the loss of the proton
signalat8.5ppm inthearomaticregion, which was consistentwith
the formation of the CCC-NHC pincer Zr complexes.
X-Ray Crystallography
The previously reported literature[18a] describes the synthesis
and characterisation of the CCC-NHC pincer ZrI(NMe2)2
complex 3. In this previous report, analysis of the crystal data
showed that 9 % of the crystals were the mono-amido structure
and 91 % were the di-amido structure. The crystallographic data
below show 100 % diamido structure and no presence of the
mono-amido. During the synthesis of complex 3 or 4, the
reaction mixtures yielded both cubic and needle crystals
appropriate for single crystal X-ray diffraction. ORTEP illus-
trations of the molecular structure of complex 3 are shown in
Fig. 1 and that of 4 are shown in Fig. 2. The coordination sphere
around the metal centre is the typical distorted octahedron. The
Results and Discussion
Synthesis and Characterisation
The diamido complexes 3 or 4 were readily obtained in pure
form by using excess of the [Zr(NMe2)4] reagent with the
appropriate salt, 1 or 2, and replacing the previously reported
solvent, DCM, with THF or toluene (Scheme 1).[18] This method
provides the diamido complexes through the use of excess
Zr(NMe2)4, whereas use of 1.1 equiv. provided the monoamido
complexes 5 and 6. Due to improved solubility in THF shorter
reaction times and less solvent could be employed during the
synthesis, but both methods were efficient. In some instances
THF remained in the crystalline lattice (see below), so if a
coordinating solvent might interfere with subsequent catalytic
˚
Zr–Ccarbene bonds (C10, C7) in complex 3 are 2.402(3)A and
˚
2.429(3)A, and the Zr–Ccarbene bonds (C10, C7) in complex 4
˚
are 2.407(4)A and 2.384(4)A. The Zr–I bond distance in
˚
˚
complex 3 was 3.0864(3)A, in comparison to the previously
[18a]
˚
reported diamido structure of 3.07615(18)A.

Zr-Diamido Hydroamination
567
(a)
(b)
l1
l1
C22
N3
C1
N1
C21
N6
C10
N2
C7
N4
Zr1
Zr1
N5
N4
C13
N2
N6
N5
C18
C14
C15
C19
C20
C24
C23
C16
Fig. 1. An ORTEP molecular drawing of iodo complex 3 (a) illustrating the meridional tridentate binding and (b) illustrating the meso(cis)
conformation of the butyl groups. Thermal ellipsoids are shown at 50 % probability and hydrogens were omitted for clarity. Selected bond
˚
angles (8), selected bond lengths (A): C(10)–Zr(1)–C(7), 136; N(5)–Zr(1)–I(1), 165; N(6)–Zr(1)–C(1), 150; N(5)–Zr(1)–N(6), 102; Zr(1)–I(1),
3.0864(3); Zr(1)–C(7), 2.429(3); Zr(1)–C(10), 2.402(3); Zr(1)–C(1), 2.325(3); Zr(1)–N(5), 2.053(3); Zr(1)–N(6), 2.068(2).
(a)
(b)
Br1
C7
N3
Br1
C1
N1
C22
N5
C10
N4
Zr1
N2
Zr1
C10 N4
C7
C8
C21
N5
C23
C22
N6
N6
C24
C21
C24
C23
Fig. 2. An ORTEP molecular drawing of bromo complex 4 (a) illustrating the meridional tridentate binding and (b) illustrating the racemic
(trans) conformation of the butyl groups. Thermal ellipsoids are shown at 50 % probability, and hydrogens and the THF are omitted for clarity.
˚
Selected bond angles (8), selected bond lengths (A): C(10)–Zr(1)–C(7), 136; N(5)–Zr(1)–Br(1), 87; N(6)–Zr(1)–C(1), 93; N(5)–Zr(1)–N(6), 100;
Zr(1)–Br(1), 2.8087(5); Zr(1)–C(7), 2.384(4); Zr(1)–C(10), 2.407(4); Zr(1)–C(1), 2.339(4); Zr(1)–N(5), 2.116(3); Zr(1)–N(6), 2.047(3).
Significant differences between the ligand conformations
were noted between the iodo and bromo complexes (Figs 1b and
2b). The butyl groups are in a ‘meso’ conformation for iodo
complex 3 (Fig. 1b), but are in a ‘racemic’ conformation for
bromo complex 4 (Fig. 2b). It may be noted that the iodide is also
bent from the standard octahedral angle, (C(7)–Zr(1)–I(1);
76.78. In addition, there are distortions of the angles around
nitrogen (N6) of the amido group for the iodo complex 3, which
are C(10)–Zr(1)–N(6); 92.6138 and Zr(1)–N(6)–C(22);
143.8658. The bromo complex, 4, has angles of C(10)–Zr(1)–
N(5); 132.784 and Zr(1)–N(5)–C(22); 110.363, much closer to
the normal 1208 of a trigonal planar geometry. This distortion
appears to be due to steric interactions between the large iodo
ligand, the methyls of the amido ligand, and the methylene of the
butyl chain. It may be rationalised based on two reinforcing
factors. First the methylene carbon (C13) of the butyl chain and
sum alone it seems close to the 3608 value for a planar structure.
If the individual angles are examined the large distortions of
143.98 and 97.98 account for the total appearing to be near
planarity. The distortions, therefore, may be entirely explained
on the basis of a steric interaction with the larger iodo ligand
(compare to the Br structure) and not an electronic repulsive
interaction.
Catalyst Optimisation and Amino-Alkene Substrate Scope
The previously employed 5 mol-% loading of pre-catalyst was
selected for evaluation of the diamido complexes 3 or 4 for the
hydroamination of alkenes as illustrated in Scheme 2. Initial
evaluation was made for the formation of pyrrolidines (Table 1,
entry 1) and piperidines (Table 1, entry 5) at different
temperatures. Generally, the diamido complexes were found to
have much faster reaction rates than the previously reported
analogous mono amido complexes.[17] Reaction rates providing
synthetically useful yields were observed at 160 and 408C, and
in some cases with a slow reaction being observed even at room
temperature. Since room temperature reaction rates were slow, it
was decided to evaluate both 40 and 1608C to provide guidance
for future studies. It was determined that by using 5 mol-% Zr-I
˚
the methyl carbon (C22) of the amido group are 3.547A apart,
indicating that they are significantly closer than the Van der
˚
Waals minimum energy distance of 4.0 A for Me–Me interac-
tions.[19] Also, the CMe–I distances are very short at 3.752 A to
˚
˚
C(22) and 3.884 A to C(21). Second, the angles around N(6) are
109.78, 97.98, and 143.98, which sums to 351.58. Based on the

568
H. U. Valle et al.
R
R
R
R
n
5 mol-% cat.
n ꢀ 1, 2
pre-catalyst 3, quantitative conversions were obtain after
40–50 min of heating (Table 1, entry 1 and entry 5 at 1608C).
Since the halogen present in the pre-catalyst had previously
been demonstrated to have a large impact on the reaction rates,
NH
Me
NH2
n
Scheme 2. Hydroamination/cyclisation of unactivated amino-alkenes.
Table 1. Catalyst and substrate scope for intramolecular hydroaminations
Heterocycle
B [8C]
EntryA
Amine
T
Time [h], Conversion [%]
4 (ZrBr)
3 (ZrI)
Ph
Ph
RT
40
72, 11
96, 65
50 min, 100
72, 9
186, 37
1.5, 100
Ph
Ph
NH
Me
NH2
1
160D
40
96, 21
3, 100
240, 8
NH
Me
NH2
2
3
160
10, .98
40
96, 3
240, 4
25, 45
NH
Me
NH2
160
29, 83
NH
Me
40
96, 2
72, 0
NH2
4
5
6
7
8
160
64, 68
24, 15
RT
40
72, 43
95, 44
38, 84
3, 92
Ph
Ph
Ph
Ph
NH
Me
NH2
72, 85
160
40 min, 86
Ph
Ph
24, 0C
24, 0C
24, 0C
24, 0C
Ph
Ph
40
NH
NH2
160
Me
Ph
48, 0C
24, 19
48, 0C
24, 6
Ph
40
NH
NH2
160
Me
40
24, 0C
24, 0C
24, 0C
24, 0C
NH
Me
NH2
160
Ph
Ph
Ph
Ph
NH
Et
40
24, 0C
24, 0C
24, 0C
24, 0C
NH2
NH2
9
160
Ph
Ph
Ph
NH
40
96, 18
7, 98
48, 0
Ph
160
5, .98
10
11
Me
Me
N
Bn
Bn
Ph
Ph
Ph
Ph
40
24, 0C
24, 0C
24, 0C
24, 0C
NH
160
Me
AAll reactions were performed in toluene-d8 with the NMR tube immersed in a 160 or 408C heating bath using 5 mol-% precatalyst (3 or 4). All conversions
were determined by NMR spectroscopy.
BRT ¼ room temperature, ,218C.
CNo reaction.
D,6 % oxidative amination product observed.

Zr-Diamido Hydroamination
569
Zr-Br complex 4 was also evaluated. Generally, the bromide-
containing complex 4 was found to have a slower rate than the
iodide-containing complex 3 (Table 1). Formation of five-
membered rings was efficient as long as the substrate contained
substituents that facilitated ring closure due to the gem-dialkyl
effect (Table 1, entries 1–4 versus 7 and 8). Having only a mono-
substituted substrate (entry 7) or no substituent (entry 8)
produced low conversion to no reaction. While five- and six-
membered rings were formed efficiently, seven-membered ring
precursors (entry 6) did not undergo reaction. A substituent on
the terminus of the alkene (entry 9) also shut down reactivity.
Internal substitution of the alkene (entry 10) resulted in a slower
reaction, but good conversions were obtained at extended
reaction times yielding a tetra-substituted carbon atom (entry
10). Attempted cyclisation of a secondary amine (entry 11) gave
no reaction. Good activity was also noted for 2.5 and 1 mol-%
catalyst loading at the same concentration of Zr pre-catalyst.
It should be noted that for Table 1, entry 1 we were able to
identify a side product at 808C or higher that is the oxidative
amination product that is otherwise obtained in Table 2, entry 1
(see Supplementary Material). This pyrroline product was
present in approximately the same amount as that of the catalyst
present, and, therefore, its production is consistent with a
catalyst decomposition pathway. The diamido complexes 3 and
4 were found to be much more efficient catalysts than their
mono-amido counterparts with reactivity observable at or near
room temperature.
Other Hydroamination Substrates
Further evaluation of the substrate scope using complex3 or 4 was
made using 5 mol-% catalyst loading at room temperature or
1608C (Table 2) for hydroamination/cyclisation of alkyne and
allene substrates. Intramolecular reactivity with alkyne substrates
(entries 1 and 2) was found to be efficient and rapid even at room
temperature for both 3 and 4 producing quantitative yields in
minutes. In contrast, allene substrates gave no reaction even after
extended periods at 1608C (entries 3 and 4). An aniline substrate
(entry 5) was evaluated for the preparation of dihydroindoles. This
substrate reacted slowly with the concomitant formation of the
indole in an ,3.4: 1 ratio, which was the first observation of an
oxidative amination process for these Zr catalysts. Due to the
aromatic nature of indole, the oxidation of indolene was not
surprising. The fate of the hydrogen was not determined for these
reactions. Finally, the intermolecular hydroamination of unac-
tivated alkenes, alkynes, and allenes is also a highly desired pro-
cess, but is difficult to obtain.[20] Intramolecular hydroaminations
of alkenes, alkynes, and allenes have been widely reported by
several groups, but intermolecular reactions are typically slow and
require much longer reaction times. Attempted intermolecular
hydroaminations using 3 or 4 produced no reaction.
Comparison of Hydroamination/Cyclisation Rates
Recalling that the Zr monoamido diiodo complex 5 was the
fastest ofour previously reported complexes, datafor comparison
Table 2. Substrate scope for hydroamination
Product
B [8C]
EntryA
Amine
T
Time [h], Conversion [%]
4 (ZrBr)
3 (ZrI)
Ph
Ph
Ph
Ph
RT
2, 100
1.5, 100
NH2
N
160
30 min, 100
30 min, 100
1
2
NH2
N
RT
RT
1, 99
30 min, 99
Ph
Ph
H
N
NH2
24, 0C
24, 0C
•
3
4
H
H
H
N
RT
24, 0C
41, 0
24, 0C
68, 0
NH2
160
•
H
N
192, 71
192, 21
NH2
ꢁ
5
160
H
N
AAll reactions were performed in toluene-d8 with the NMR tube immersed in a 1608C heating bath using 5 mol-% precatalyst (3 or 4). All conversions were
determined by NMR spectroscopy.
BRT ¼ room temperature.
CNo reaction.

570
H. U. Valle et al.
Table 3. Comparison of CCC-NHC Zr complex rates
catalytic reactions were performed in a screw-cap NMR tube
with toluene-d8, and immersed in an oil bath at the temperature
indicated in Tables 1 and 2. Products were readily identified
based on their 1H NMR signature.
Ph
Ph
Ph
Ph
5 mol-% catalyst
NH
Me
NH2
Synthesis and Characterisation of CCC-NHC Zr Complexes
Method A – THF
Entry
Catalyst
Time [h]
T [8C]
Yield [%]
Preparation of 2-(1,3-Bis(N-butyl-imidazol-2ylidene)pheny-
lene)bis(dimethylamido)(iodo) zirconium(IV) (3). Solid 1,3-bis
(10-butylimidazol-30-yl) benzene diiodide (0.273 g, 0.472 mmol)
was added to a solution of Zr(NMe2)4 (0.314 g, 1.17 mmol) in THF
(3 mL) and stirred for 10 min followed by heating in a 1208C oil
bath for 10 min. The clear, bright yellow solution was allowed to
cool slowly, and crystals formed. The supernatant was decanted.
The product was washed with hexanes (2 ꢀ 2 mL) and dried under
vacuum (0.191 g, 65 %). A crystal suitable for single crystal X-ray
diffraction was selected from this sample. dH (CD2Cl2, 600 MHz)
7.66 (d, J 1.7, 2H), 7.42 (t, J 7.8, 1H), 7.27 (d, J 1.7, 2H), 7.26 (d, J
7.8, 2H), 4.01 (t, J 7.4, 4H), 3.01 (s, 12H), 1.78 (quint, J 7.5, 4H),
1.39 (sext, J 7.5, 4H), 0.99 (t, J 7.4, 6H). dC (150 MHz, CD2Cl2)
189.3, 158.4, 148.8, 132.1, 123.2, 116.7, 111.6, 52.1, 38.5, 34.1,
20.4, 14.1. Anal. Calc. for C24H37IN6Zr: C 45.92, H 5.94, N 13.39.
Found: C 45.93, H 5.97, N 13.17 %.
Preparation of 2-(1,3-Bis(N-butyl-imidazol-2-ylidene)-
phenylene)bis(dimethylamido)(bromo) zirconium(IV) (4). Solid
1,3-bis(10-butylimidazol-30-yl)benzene dibromide (0.229 g,
0.472 mmol) was added to a solution of Zr(NMe2)4 (0.314 g,
1.17 mmol) and THF (3 mL). The mixture was sonicated for
10 min, and heated at 1208C for 10 min. The clear, bright yellow
solution was allowed to cool slowly, and crystals formed.
The supernatant was decanted. The product was washed with
hexanes (2 ꢀ 2 mL) and dried under vacuum (0.103 g, 38 %). A
crystal suitable for single crystal X-ray diffraction was selected
from this sample, which contained THF in the crystalline lattice.
After further drying under high vacuum at 708C, an NMR
sample was taken, which did not contain THF. dH (toluene-d8,
300 MHz) ,7.06 (t, 1H, overlaps toluene), 6.84 (d, J 1.4, 2H),
6.76 (d, J 7.7, 2H), 6.26 (d, J 1.3, 2H), 4.01 (t, J 7.7, 4H), 3.56
(m, 4H, THF), 3.15 (s, 12H), 1.67 (quint, J 8.9, 4H), 1.45 (m, 4H,
THF), 1.28 (sext, J 8.7, 4H), 0.90 (t, J 7.3, 6H).
1
2
3
3, ZrI
96
93
18
40
40
80
65
23
0A
4, ZrBr
5, ZrI2
ARef. [11c].
of the diamido versus monoamido relative rates were collected
in Table 3 for the conversion of the diphenyl substrate.
Both diamido complexes 3 and 4 were observed to have
significant rates of catalysis at 408C with an observed yield of
65 % at 96 h for 3 (entry 1) versus 23 % at 93 h for 4 (entry 2). In
contrast, no reaction was observed when the catalysis with
monoamido diiodo complex 5 was conducted at 808C (entry 3).
Furthermore, as noted in Table 1, entry 1 both diamido com-
plexes were found to have slow conversion at room temperature.
This dramatic difference in the temperature required for initia-
tion of the reaction is the biggest advantage of the diamido
complexes, which has significant implications for stereo-
selective applications. While the diamido complexes are a
significant improvement for the CCC-NHC type catalysts, their
rates of reaction are still significantly less than the best catalysts
as depicted in Chart 1.
Conclusions
The synthesis, characterisation, and the X-ray molecular struc-
tures of CCC-NHC pincer Zr diamido complexes 3 and 4 have
been reported. These complexes were found to have improved
reaction rates for hydroamination in comparison to the previously
reported monoamido complexes. In addition, reactivity was
reported for alkyne hydroamination and the formation of indole
and dihydroindole. The hypothesis that having two amido ligands
would lead to a faster catalyst has been borne out by the results
obtained to date. Furthermore, the lack of reactivity of secondary
amines is consistent with the formation of a Zr-imido in the
catalytic cycle. Work continues to develop improved reaction
rates for hydroamination catalysts based on the CCC-NHC ligand
architecture.
Method B – Toluene
Preparation of 2-(1,3-Bis(30-butyl-imidazol-20-ylidene)phe-
nylene)bis(dimethylamido)(iodo) zirconium(IV) (3). 1,3-Bis
(imidazol-10-yl)benzene diiodide (924 mg, 1.59 mmol),
Zr(NMe2)4 (1.06 g, 3.99 mmol), and toluene (200 mL) were
combined in a storage flask. The resulting mixture was stirred
for 3 h in a 1608C oil bath. After cooling to room temperature,
70 % of the volatiles were removed under reduced pressure
yielding a yellow solid. The precipitate was collected by
vacuum filtration. After drying under reduced pressure, a light
yellow solid was obtained (650 mg, 62 %). dH (CD2Cl2,
300 MHz) 7.68 (d, J 1.5, 2H), 7.42 (t, J 7.5, 1H), 7.29 (d,
J 1.5, 2H), 7.12 (d, J 7.8, 2H), 4.01 (t, J 7.5, 4H), 3.01 (s, 12H),
4.46 (s, 4H), 1.78 (quint, J 7.5 4H), 1.38 (sextet, J 7.5, 4H), 0.98
(t, J 7.2, 6H). dC (75.5 MHz, CD2Cl2) 189.2, 158.3, 148.8, 132.1,
123.2, 116.7, 111.6, 52.0, 38.5, 34.0, 20.4, 14.1.
Experimental
General Procedures
All reactions were done under an inert atmosphere using stan-
dard Schlenk techniques or a glove box unless noted otherwise.
Al2O3 (basic, 50–200 mm) was stored in an oven at 1308C before
use. Amine substrates were made by previously reported
literature procedures and distilled before use.[17] 1H and 13C
{1H} NMR spectra were recorded on 300 or 600 MHz Bruker
instruments. Chemical shifts (d) were expressed in ppm down-
field to TMS at d 0 and referenced to the residual solvent
peak.[21] CD2Cl2, THF, toluene, toluene-d8, and amine sub-
strates were dried by passing through activated alumina.[22] 1,3-
Preparation of 2-(1,3-Bis(30-butyl-imidazol-20-ylidene)-
phenylene)bis(dimethylamido)(bromo) zirconium(IV) (4). 1,3-
Bis(imidazol-10-yl)benzene dibromo (990 mg, 2.04 mmol),
Zr(NMe2)4 (1.36 g, 5.12 mmol), and toluene (200 mL) were
combined in a storage flask. The resulting mixture was stirred
Bis(imidazol-10-yl)benzene,
1,3-bis(30-butylimidazol-10-yl)
benzene diiodide, and 1,3-bis(30-butylimidazol-10-yl)benzene
dibromide were obtained via previously reported proce-
dures.[17,18,23] Zr(NMe2)4 was freshly sublimed before use. All

Zr-Diamido Hydroamination
571
for 16 h in a 1608C oil bath. After cooling to room temperature,
70 % of the volatiles were removed under reduced pressure
yielding a yellow solid. The precipitate was collected by vacu-
um filtration. After drying under reduced pressure, a light
yellow solid was obtained (368 mg, 62 %). dH (500 MHz,
CD2Cl2) 7.56 (s, 2H), 7.42 (t, J 7.5, 1H), 7.13 (s, 2H), 7.12
(s, 2H), 4.18 (t, J 7.5, 4H), 2.94 (s, 12H), 1.84 (quint, J 7.5, 4H),
1.43 (sextet, J 7.5, 4H), 0.99 (t, J 7.5, 6H). dC (125 MHz,
CD2Cl2) 193.0, 165.4, 148.0, 129.0, 121.4, 115.97, 110.4,
51.6, 42.1, 34.0, 20.5, 14.1.
(f) H. Zhai, A. Borzenko, Y. Y. Lau, S. H. Ahn, L. L. Schafer,
Angew. Chem. Int. Ed. 2012, 51, 12219. doi:10.1002/ANIE.
201206826
[3] (a) R. K. Thomson, J. A. Bexrud, L. L. Schafer, Organometallics 2006,
25, 4069. doi:10.1021/OM060545N
(b) K. Riener, S. Haslinger, A. Raba, M. P. Ho¨gerl, M. Cokoja,
W. A. Herrmann, F. E. Ku¨hn, Chem. Rev. 2014, 114, 5215. doi:10.1021/
CR4006439
[4] (a) E. S. Sherman, P. H. Fuller, D. Kasi, S. R. Chemler, J. Org. Chem.
2007, 72, 3896. doi:10.1021/JO070321U
(b) G. Zeni, R. C. Larock, Chem. Rev. 2004, 104, 2285. doi:10.1021/
CR020085H
Typical Catalysis Procedure
(c) S. Hong, T. J. Marks, Acc. Chem. Res. 2004, 37, 673. doi:10.1021/
AR040051R
In a screw cap NMR tube unactivated amino alkene
(,25 mg, 1 equiv.), 2-(1,3-bis(N-butyl-imidazol-2-ylidene)
phenylene)bis(dimethylamido)(iodo) zirconium(IV) (3, 3.9 mg,
0.05 equiv., 5 mol-%) and 0.5 mL of d8-toluene were combined
in a glove box followed by heating in a 1608C oil bath. The
reaction progress was monitored by 1H NMR spectroscopy.
5-Methyl-3,3-diphenyl-3,4-dihydro-2H-pyrrole(Table 3, entry
1). In a screw cap NMR tube 2,2-diphenyl-4-pentyn-1-amine
(0.025 g, 0.106 mmol), 2-(1,3-bis(N-butyl-imidazol-2-ylidene)
phenylene)bis(dimethylamido)(iodo) zirconium (IV) (0.004 g,
0.0054 mmol), and 0.4 mL of toluene-d8 was combined in a glove
box, which was immersed in a 1608C oil bath. The progress of
the reaction was monitored by using 1H NMR spectroscopy. dH
(C6D6, 300 MHz) 7.08–6.95 (m, 10H), 4.38–4.36 (br sextet, J 1.5,
2H), 2.68 (br s, 2H), 1.75–1.74 (br t, J 1.6, 3H).
(d) R. C. Larock, H. Yang, S. M. Weinreb, R. J. Herr, J. Org. Chem.
1994, 59, 4172. doi:10.1021/JO00094A031
[5] J. Hannedouche, E. Schulz, Chem. –Eur. J. 2013, 19, 4972. doi:10.1002/
CHEM.201203956
[6] F. Pohlki, S. Doye, Chem. Soc. Rev. 2003, 32, 104. doi:10.1039/
B200386B
[7] (a) M. A. Antunes, R. F. Munha´, L. G. Alves, L. L. Schafer, A. M.
Martins, J. Organomet. Chem. 2011, 696, 2. doi:10.1016/J.JORGAN
CHEM.2010.08.039
(b) D. C. Leitch, P. R. Payne, C. R. Dunbar, L. L. Schafer, J. Am. Chem.
Soc. 2009, 131, 18246. doi:10.1021/JA906955B
(c) D. C. Leitch, R. H. Platel, L. L. Schafer, J. Am. Chem. Soc. 2011,
133, 15453. doi:10.1021/JA202448B
[8] (a) M. R. Gagne, T. J. Marks, J. Am. Chem. Soc. 1989, 111, 4108.
doi:10.1021/JA00193A056
(b) M. R. Gagne, C. L. Stern, T. J. Marks, J. Am. Chem. Soc. 1992, 114,
275. doi:10.1021/JA00027A036
Crystallographic Data
(c) V. M. Arredondo, S. Tian, F. E. McDonald, T. J. Marks, J. Am.
Chem. Soc. 1999, 121, 3633. doi:10.1021/JA984305D
(d) H. F. Yuen, T. J. Marks, Organometallics 2009, 28, 2423.
doi:10.1021/OM9000023
The X-ray crystal structure data have been deposited with
Cambridge Crystallographic Data Centre (3: CCDC# 982132,
and 4: CCDC# 982133).
[9] (a) J. R. Petersen, J. M. Hoover, W. S. Kassel, A. L. Rheingold,
A. R. Johnson, Inorg. Chim. Acta 2005, 358, 687. doi:10.1016/J.ICA.
2004.09.051
(b) J. M. Hoover, J. R. Petersen, J. H. Pikul, A. R. Johnson, Organo-
metallics 2004, 23, 4614. doi:10.1021/OM049564S
[10] (a) M. J. Lopez-Gomez, D. Martin, G. Bertrand, Chem. Commun.
2013, 49, 4483. doi:10.1039/C3CC41279B
Supplementary Material
1H and 13C NMR data for complexes 3 and 4, along with details
of the X-ray data collection and table of metric data are available
on the Journal’s website.
(b) R. Kinjo, B. Donnadieu, G. Bertrand, Angew. Chem. Int. Ed. 2011,
Acknowledgements
50, 5560. doi:10.1002/ANIE.201100740
(c) S. R. Chemler, Org. Biomol. Chem. 2009, 7, 3009. doi:10.1039/
B907743J
(d) T. E. Mu¨ller, K. C. Hultzsch, M. Yus, F. Foubelo, M. Tada, Chem.
Rev. 2008, 108, 3795. doi:10.1021/CR0306788
(e) H. Kim, T. Livinghouse, D. Seomoon, H. L. Phil, Bull. Korean
Chem. Soc. 2007, 28, 1127. doi:10.5012/BKCS.2007.28.7.1127
(f) H. Kim, T. Livinghouse, J. H. Shim, S. G. Lee, P. H. Lee, Adv.
Synth. Catal. 2006, 348, 701. doi:10.1002/ADSC.200505448
(g) K. C. Hultzsch, F. Hampel, T. Wagner, Organometallics 2004, 23,
2601. doi:10.1021/OM030679Q
(h) I. Bytschkov, S. Doye, Eur. J. Org. Chem. 2003, 935. doi:10.1002/
EJOC.200390149
(i) P. D. Knight, I. Munslow, P. N. O’Shaughnessy, P. Scott, Chem.
Commun. 2004, 894. doi:10.1039/B401493F
The Mississippi State University, Office of Research and Economic
Development and Department of Chemistry, and the National Science
Foundation (OIA-1539035, CHE-0809732) are gratefully acknowledged
for financial support. The authors gratefully acknowledge Dr Theodore
R. Helgert for donations of select substrates for this study.
References
[1] (a) S. Hong, S. Tian, M. V. Metz, T. J. Marks, J. Am. Chem. Soc. 2003,
125, 14768. doi:10.1021/JA0364672
(b) T. E. Mu¨ller, K. C. Hultzsch, M. Yus, F. Foubelo, M. Tada, Chem.
Rev. 2008, 108, 3795. doi:10.1021/CR0306788
(c) J. A. Bexrud, J. D. Beard, D. C. Leitch, L. L. Schafer, Org. Lett.
2005, 7, 1959. doi:10.1021/OL0503992
[2] (a) D. A. Watson, M. Chiu, R. G. Bergman, Organometallics 2006, 25,
4731. doi:10.1021/OM0606791
(j) A. L. Gott, A. J. Clarke, G. J. Clarkson, P. Scott, Organometallics
2007, 26, 1729. doi:10.1021/OM061087L
(k) A. L. Gott, A. J. Clarke, G. J. Clarkson, P. Scott, Chem. Commun.
2008, 1422. doi:10.1039/B718373A
(l) K. Manna, N. Eedugurala, A. D. Sadow, J. Am. Chem. Soc. 2015,
137, 425. doi:10.1021/JA511250M
(b) S. R. Chemler, in Catalytic Methods in Asymmetric Synthesis:
Advanced Materials, Techniques, and Applications (Eds M. Gruttadauria,
F. Giacalone) 2011, pp. 671–688 (John Wiley & Sons, Inc.:
Hoboken, NJ).
(c) A. L. Reznichenko, K. C. Hultzsch, Organometallics 2013, 32,
[11] (a) K. Manna, W. C. Everett, G. Schoendorff, A. Ellern, T. L. Windus,
A. D. Sadow, J. Am. Chem. Soc. 2013, 135, 7235. doi:10.1021/
JA4000189
1394. doi:10.1021/OM3010614
(d) J. C. H. Yim, J. A. Bexrud, R. O. Ayinla, D. C. Leitch, L. L. Schafer,
J. Org. Chem. 2014, 79, 2015. doi:10.1021/JO402668Q
(e) E. Chong, S. Qayyum, L. L. Schafer, R. Kempe, Organometallics
2013, 32, 1858. doi:10.1021/OM3012695
(b) L. Luconi, A. Rossin, A. Motta, G. Tuci, G. Giambastiani, Chem. –
Eur. J. 2013, 19, 4906. doi:10.1002/CHEM.201203644

572
H. U. Valle et al.
(c) W. D. Clark, J. Cho, H. U. Valle, T. K. Hollis, E. J. Valente,
J. Organomet. Chem. 2014, 751, 534. doi:10.1016/J.JORGANCHEM.
2013.11.001
[18] (a) R. J. Rubio, G. T. S. Andavan, E. B. Bauer, T. K. Hollis, J. Cho,
F. S. Tham, B Donnadieu, J. Organomet. Chem. 2005, 690, 5353.
doi:10.1016/J.JORGANCHEM.2005.05.007
(d) H. Kim, P. H. Lee, T. Livinghouse, Chem. Commun. 2005, 5205.
doi:10.1039/B505738H
[12] S. M. Coman, V. I. Parvulescu, Org. Process Res. Dev. 2015, 19, 1327.
doi:10.1021/ACS.OPRD.5B00010
[13] D. J. Morales-Morales, C. Jensen, The Chemistry of Pincer Complexes
(b) J. Cho, T. K. Hollis, E. J. Valente, J. M. Trate, J. Organomet. Chem.
2011, 696, 373. doi:10.1016/J.JORGANCHEM.2010.10.004
[19] (a) T. K. Hollis, J. K. Burdett, B. Bosnich, Organometallics 1993, 12,
3385. doi:10.1021/OM00033A003
(b) L. Pauling, The Nature of the Chemical Bond and the Structure
of Molecules and Crystals: An Introduction to Modern Structural
Chemistry 1960 (Cornell University Press: Ithaca, NY).
[20] (a) T. E. Mu¨ller, Chem. Rev. 1998, 98, 675. doi:10.1021/CR960433D
(b) M. Kawatsura, J. F. Hartwig, J. Am. Chem. Soc. 2000, 122, 9546.
doi:10.1021/JA002284T
2007 (Elsevier Science: Amsterdam).
[14] J. A. Love, M. S. Sanford, M. W. Day, R. H. Grubbs, J. Am. Chem. Soc.
2003, 125, 10103. doi:10.1021/JA027472T
[15] (a) A. Kascatan-Nebioglu, M. J. Panzner, C. A. Tessier, C. L. Cannon,
W. J. Youngs, Coord. Chem. Rev. 2007, 251, 884. doi:10.1016/J.CCR.
2006.08.019
[21] S. Budavari, M. J. O’Neil, A. Smith, P. E. Heckelman, The Merck
Index: An Encylopedia of Chemicals, Drugs, and Biologicals
(11th edn) 1989 (Merck Co.: Rahway, NJ).
(b) X. Zhang, A. M. Wright, N. J. DeYonker, T. K. Hollis, N. I.
Hammer, C. E. Webster, E. J. Valente, Organometallics 2012, 31,
1664. doi:10.1021/OM200687W
[22] A. B. Pangborn, M. A. Giardello, R. H. Grubbs, R. K. Rosen,
F. J. Timmers, Organometallics 1996, 15, 1518. doi:10.1021/
OM9503712
[23] W. D. Clark, G. E. Tyson, T. K. Hollis, H. U. Valle, E. J. Valente, A. G.
Oliver, M. P. Dukes, Dalton Trans. 2013, 42, 7338. doi:10.1039/
C3DT32862G
(c) H. J. Park, K. H. Kim, S. Y. Choi, H. M. Kim, W. I. Lee, Y. K. Kang,
Y. K. Chung, Inorg. Chem. 2010, 49, 7340. doi:10.1021/IC100325C
[16] P. Cao, J. Cabrera, R. Padilla, D. Serra, F. Rominger, M. Limbach,
Organometallics 2012, 31, 921. doi:10.1021/OM200964U
[17] J. Cho, T. K. Hollis, T. R. Helgert, E. J. Valente, Chem. Commun. 2008,
5001. doi:10.1039/B805174G