CCC-N-heterocyclic carbene pincer complexes: Synthesis, characterization and hydroamination activity of a hafnium complex

Article abstract of DOI:10.1016/j.jorganchem.2010.10.004

Our methodology for the stoichiometric preparation of CCC-NHC pincer complexes of Zr has been extended to Hf. The CCC^Bu-NHC pincer Hf complex has been characterized by X-ray crystal structure analysis. Catalytic activity in the intramolecular h

Full text of DOI:10.1016/j.jorganchem.2010.10.004

Journal of Organometallic Chemistry 696 (2011) 373e377
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
CCCeN-heterocyclic carbene pincer complexes: Synthesis, characterization
and hydroamination activity of a hafnium complex
,
b
a
Joon Cho a, T. Keith Hollis a , Edward J. Valente , Jaclyn M. Trate
*
a Department of Chemistry and Biochemistry, The University of Mississippi, 407 Coulter Hall, University, MS 38677, USA
b Department of Chemistry, University of Portland Diffraction Facility, 112A Swindells Hall, University of Portland, Portland, OR 97203, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Our methodology for the stoichiometric preparation of CCCeNHC pincer complexes of Zr has been
extended to Hf. The CCCBueNHC pincer Hf complex has been characterized by X-ray crystal structure
analysis. Catalytic activity in the intramolecular hydroamination/cyclization of unactivated alkenes is
reported and compared to the recently reported Zr analog. An improved, scaled-up CuO-catalyzed aryl
amination of 1,3-dibromobenzene and an improved salt formation methodology for preparation of bis
(butyl-imidazolium)benzene are reported also.
Received 31 August 2010
Received in revised form
1 October 2010
Accepted 1 October 2010
Available online 4 November 2010
Ó 2010 Elsevier B.V. All rights reserved.
Keywords:
Homogeneous catalysis
N-Heterocyclic carbene
Pincer complex
Hydroamination
Pyrrolidines
Piperidines
1. Introduction
Pincer ligand metal complexes have been extensively studied
and applied in catalytic reactions because they possess a unique
The catalytic synthesis of CeN bonds is a long standing chal-
lenge in synthetic chemistry. The addition of NeH bonds to unac-
tivated alkenes is an atom-economical approach to this challenge.
The original contribution of Marks and Gagne with lanthanides
sparked much exploration in this area [1]. Metals ranging from Pt,
Pd, Rh, Ir, Au, Zr, and Ti have been reported to catalyze this addition
reaction [2], and many groups have made recent contributions [3].
In addition, several asymmetric variants of this reaction have been
reported [4].
Following the first reports of stable carbenes, pushepull car-
benes by Bertrand et al. and N-heterocyclic carbenes (NHCs) by
Arduengo et al. [5], various metal complexes incorporating NHC
ligands have been successfully synthesized and applied in disparate
coordination environment [13]. Several groups have reported NHC
pincer complexes containing an aromatic ring as a central donor
group as illustrated in Fig. 1. Due to the ease of coordination of the
nitrogen in the pyridine moiety, many pyridinylene [14] and 2,6-
lutidinyl [15] bridged CNCeNHC pincer complexes (A) have been
reported for a wide range of transition metals. Fewer examples
have been reported for xylene-bridged CCCeNHC pincer complexes
B, n ¼ 1 [16]. We have recently reported several syntheses of early
and late transition metal complexes [17e19] of the phenylene-
bridged architectures (B, n ¼ 0). Others have also begun reporting
examples based on this ligand architecture [20]. We recently
reported
a highly efficient stoichiometric synthesis of the
(CCCBueNHC)ZrI2(NMe2) complex and its effectiveness as a catalyst
for hydroamination/cyclization of unactivated alkenes [17].
There are several examples of Hf NHC complexes, and a few have
been crystallographically characterized [21]. Fryzuk reported the
crystallographic characterization of an Hf(diamido-NHC) pincer
complex, [22] and Cavell reported a unique C-atom bridged carbene
Hf pincer complex [23]. We report herein the successful extension
of our Zr methodology to the preparation of an Hf analog and
compare its reactivity to the Zr complex. Additionally, improved
synthetic methods for the preparation of the ligand precursors are
described.
scientific arenas [6,7]. The stronger
s donating ability with almost
no -backbonding has led to NHCs place as phosphine complements
p
[7], and as a result, they are now common ligands in a variety of
transition metal catalyzed reactions [8] such as hydrogenation [9],
carbonecarbon bond formation [10], carbonenitrogen bond
formation [11] and olefin metathesis [12].
* Corresponding author. Tel.: þ1 662 915 5337; fax: þ1 662 915 7300.
E-mail address: hollis@olemiss.edu (T.K. Hollis).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.10.004

374
J. Cho et al. / Journal of Organometallic Chemistry 696 (2011) 373e377
Scheme 2. Metal complex synthesis.
constraints of the pincer structure. The trans IeHfeN angle of 175ꢀ
was near ideal. The HfeI distance trans to the amido ligand was
significantly longer than that trans to the aryl ligand (2.993(2) Å vs.
2.861(2) Å). Full metric data was included in the electronic sup-
porting information. Additional data regarding data collection are
included in Table 1.
Fig. 1. Schematic of aryl-bridged CCCeNHC pincer complexes.
2. Results and discussion
2.1. Improved ligand precursor synthesis
We report here a 10-fold scale-up of the synthesis of di(imida-
zolyl)benzene (Scheme 1, step 1). Filtration and concentration of
the crude reaction mixture led to crystallization of the product in
some cases. Pure material was routinely obtained on a 35 g scale.
Use of MeCN as solvent for the bis(alkylation) procedure (Scheme 1,
step 2) lead to faster reaction times and more efficient isolation
procedures. Bis(imidazolium) salt 1 was routinely isolated in >90%
yield by simple concentration and filtration.
2.4. Hydroamination/cyclization catalysis
Initial studies for hydroamination/cyclization were conducted
with the unactivated primary alkenyl amine, 2,2-diphenyl-4-pen-
tenamine, depicted in Table 2. At 5 mol% catalyst loading the
production of the pyrrolidine product was clean and efficient.
Therefore, the catalyst loading was decreased by a factor of w2 in
two additional experiments. Each time, complex 3 was found to be
active and efficient yet required an increased reaction time to
achieve the same level of conversion. The time required for
conversion increased by a factor of w2 with each reduction of
catalyst loading by an equivalent factor. Such an anecdotal obser-
vation is consistent with a reaction that is first order. Significantly,
pincer complex 3 was active even at 1 mol% catalyst loading.
Based on the data in Table 2, 5 mol% catalyst loading (entry 1)
was chosen as optimal for evaluation of a series of substrates.
2.2. Synthesis and characterization of CCCBueNHC Hf pincer
complex 3
We recently reported that use of sublimed Zr(NMe2)4 allowed
the efficient isolation of analytically pure CCCBueNHC Zr complex 2
[17]. With appropriate technical preparations the use of a stoi-
chiometric amount of Hf(NMe2)4 was employed to selectively and
efficiently metallate ligand precursor 1 yielding the CCCBueNHC Hf
complex 3 (Scheme 2). Upon cooling the reaction a crystalline
precipitate formed. This material proved to be analytically pure and
was sufficient for single crystal X-ray diffraction analysis.
2.3. X-ray diffraction analysis of the molecular structure of
CCCBueNHC Hf complex 3
An ORTEPÒ plot of the molecular structure of Hf complex 3 is
presented in Fig. 2 along with selected metric data [24]. The coor-
dination sphere was a distorted octahedron. The values of the bond
distance and angles were within normal ranges and were typical of
those previously reported for this class of molecules [22]. All of the
average HfeC distances were shorter than for the Zr complex. The
bond distances for HfeI and HfeN5 did not differ significantly from
the Zr complex. The C12eHfeN5 angle was 99ꢀ compared to 107ꢀ in
the Zr analog. The angle of the carbenes around the metal center
(C3eHfeC4) would be expected to be 180ꢀ in an idealized octa-
hedron compared to the observed value of 138ꢀ, which is due to the
Fig. 2. ORTEPÒ plot at 50% thermal ellipsoids of CCCBueNHC Hf pincer complex 3.
Selected structural bond distances (Å) and angles (deg): HfeC12, 2.25(3) Å; HfeC3,
2.34(3) Å, HfeC4, 2.35(3) Å; HfeI1, 2.993(2) Å; HfeI2, 2.861(2) Å; HfeN5, 1.98(3) Å
C12eHfeC3, 68.2(10)ꢀ; C12eHfeC4, 69.4(10)ꢀ; C12eHfeI1, 80.1(6)ꢀ; C12eHfeI2, 164.1
(6)ꢀ; C12eHfeN5, 98.6(11)ꢀ; C3eHfeC4, 137.6(11)ꢀ; I1eHfeI2, 84.58(6)ꢀ; N5eHfeI1,
175.1(9)ꢀ.
Scheme 1. Improved ligand precursor syntheses.

J. Cho et al. / Journal of Organometallic Chemistry 696 (2011) 373e377
375
Table 1
Table 3
Crystal data and structure refinement for CCCBueNHC Hf pincer complex, 3.
Substrate survey of hydroamination catalytic activity at 5 mol% 3 in tolꢁd8 at 160 ꢀC.
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
C22H31HfI2N5
797.81
101(2) K
0.71073
Monoclinic
P2(1)/n
Entry
Amine
Heterocycle
Hfa
Zrb
Time conv (%) Time
Conv (%)
1
4.5 h >98%
50 min >98%
12.8134(8)
b (Å)
13.4801(6)
c (Å)
15.4005(10)
90
95.054(5)
90
a
b
g
(ꢀ)
(ꢀ)
(ꢀ)
2
3
14 h >98%
3 h
>98%
Volume (Å3)
2649.7(3)
Z
4
Density (calculated, Mg/m3)
Absorption coefficient (mmꢁ1
F(000)
2.000
6.284
1504
0.20 ꢂ 0.20 ꢂ 0.20
3.02 to 30.34
ꢁ18 ꢃ h ꢃ 13,
ꢁ18 ꢃ k ꢃ 18,
ꢁ21 ꢃ l ꢃ 20
14894
)
Crystal size (mm)
q
28 h 83%
18 h
92%
range for data collection (ꢀ)
Index ranges
Reflections collected
Independent reflections
7163 [R(int) ¼ 0.0552]
4
5
6
48 h 81%
12 h 93%
48 h ec
38 h
88%
90%
ec
Completeness to
q
¼ 30.34ꢀ
89.9%
Absorption correction
Max. and min. transmission
Refinement method
Semi-empirical from equivalent
1 and 0.34895
Full-matrix least-squares on F2
7163/168/271
1.026
R1 ¼ 0.1466, wR2 ¼ 0.3374
R1 ¼ 0.1830, wR2 ¼ 0.3539
7.154 and ꢁ0.3146
Data/restraints/parameters
Goodness-of-fit on F2
2 h
Final R indices [I > 2
s(I)]
R indices (all data)
Largest diff. peak and hole (e Åꢁ3
)
49 h
Table 3 depicts these results and includes the data for Zr analog 2
for comparison [18]. The general reactivity patterns found with Zr
complex 2 were also observed for Hf analog 3. Pyrrolidines (entries
1e4) and piperidines (entry 5) were prepared in high yields,
whereas the 7-membered heterocycle did not form (entry 6). A
significant “gem-dialkyl” effect was observed when the rate of
mono-substituted substrate was compared with the di-substituted
substrates (entry 7 vs. entry 1) [25]. The extra substituent
dramatically enhanced the rate of cyclization. Terminal substitution
of the alkene prevented reaction completely (entry 8). Internal
substitution of the alkene slowed but did not stop the reaction
(entry 9), which is promising for asymmetric applications. Only
primary amines underwent cyclization, since the dialkene (entry
10) underwent monocyclization only. The Hf catalyst was consis-
tently slightly slower than the Zr catalyst in all examples evaluated.
Both catalysts produced product at synthetically useful rates.
7
8
30 h 19%
11 h
39 h
18%
48 h ec
ec
9
35 h >98%
8 h
1 h
>98%
>98%
10
5 h >98%
Table 2
Evaluation of catalyst loading for CCCBuꢁNHC Hf complex 3.
a
All conversions determined by 1H NMR.
All Zr data are from Ref. [17].
No reaction.
b
c
Entry
Catalyst loading (mol%)
Temp (ꢀC)
Time (h)
Conv (%)a
3. Conclusions
1
2
3
5
2.5
1
160
160
160
4.5
7
12
>98
>98
>98
The highly efficient stoichiometric synthesis of early transition
metal CCCeNHC pincer complexes has been extended to an Hf
analog, which was structurally similar to the Zr complex. The Hf
a
All conversions determined by 1H NMR.

376
J. Cho et al. / Journal of Organometallic Chemistry 696 (2011) 373e377
complex was found to be catalytically active for hydroamination/
cyclization of unactivated alkenylamines, but it was slower than the
Zr analog. Large scale synthesis of the CCCBueNHC ligand precursor
was accomplished.
placed in a 0.2e0.3 mm nylon loop. It extinguished fairly well under
crossed polarizers. The loop was glued to a stout glass fiber, and
mounted on a pin placed in a goniometer head. The crystal was flash
frozen in the cold stream of the Oxford Cryostream to 101K. The
crystallographic properties and data were collected using Mo K
a
4. Experimental section
radiation and the charge-coupled area detector (CCD) detector on an
Oxford Diffraction Systems Gemini S diffractometer at 101(2) K [28].
A preliminary set of cell constants was calculated from reflections
observed on three sets of 5 frames which were oriented approxi-
mately in mutually orthogonal directions of reciprocal space. This
crystal was the third specimen examined, and each was fairly
mosaic (above 1.3ꢀ), and reflections were significantly broadened,
but intensity did extend to below 1.0 Å. Data collection was carried
4.1. General considerations
All experiments were performed with Schlenk techniques under
an atmosphere of Ar or in an Ar filled glovebox except as noted. Hf
(NMe2)4 was sublimed prior to use. Toluene, DMSO and CH2Cl2
were dried by passing through activated alumina [26]. MeOH and
MeCN were used as received. The NMR spectra were recorded on
Bruker 300 and 500 MHz instruments.
out using Mo Ka radiation (graphite monochromator) with 8 runs
consisting of 350 frames with a frame time of 50.0 s and a crystal-to-
CCD distance of 50.000 mm. The runs were collected by omega
scans of 1.0ꢀ width, and at detector positions of 28.781 and
4.2. Preparation of bis(imidazolium) precursors
ꢁ30.499ꢀ in 2
q. The intensity data were corrected for absorption
4.2.1. 1,3-Di-(imidazolyl)benzene
with an empirical correction [29]. Final cell constants were calcu-
lated from 4975 stronger reflections from the actual data collection
after integration. See Table 1 for crystal and refinement information.
The structure was solved using direct methods in SHELXS-97,
and refined using SHELXL-97 [30]. The space group P2(1)/n was
determined based on the cell, intensity statistics, systematic
absences, and successful solution structure and refinement. Non-H
atoms from a single molecule were located in the initial E-map.
They were refined at first with isotropic vibrational factors, then
with anisotropic vibrational factors. H-atoms were then observable
in difference electron density maps. Hydrogen atoms were placed
in ideal positions; all were refined as riding atoms with relative
isotropic displacement parameters. Soft restraints were used to
keep some light atoms positive definite. The final full-matrix least-
squares refinement converged to R1 ¼ 0.1466 (5038 reflections, F2,
1,3-Dibromobenzene (25 mL, 207 mmol), imidazole (35g,
514 mmol), K2CO3 (72g, 520 mmol), CuO (2g, 25 mmol), and DMSO
(200 mL) were combined in a 1 L round bottom flask. No precau-
tions for excluding air were used. The solution was heated at 150 ꢀC
for 48 h. The reaction was cooled, checked by TLC, and filtered
through FlorosilÒ with MeOH. The resulting solution was concen-
trated, and crystallization yielded dark-yellow crystals. The crystals
were washed with cold CH2Cl2 and dried yielding 17g of pale yellow
crystals. A second batch of crystals was isolated similarly from the
mother liquor, and it was purified by silica gel chromatography
(10:1) eluting with CH2Cl2/MeOH (10:1) providing off-white crys-
tals (19.4g) for a combined yield of 36.4 g, 83%. Alternatively, after
concentration the crude product was purified by silica gel chro-
matography yielding 35.4g, 81% in a separate experiment. Spec-
troscopic data was identical to previously published data [27].
I > 2
s
(I)), and wR2 ¼ 0.3539 for all 7163 data, 271 parameters, 168
restraints (to keep light non-H atoms positive definite), goodness-
of-fit (S) 1.026.
4.2.2. 1,3-Bis (1-butyl-imidazolium-3-yl) benzene iodide 1
1,3-Bis-(imidazolyl) benzene (1.04 g, 4.95 mmol), butyl iodide
(11.30 mL, 99.00 mmol) and MeCN (50 mL) were combined. The
resulting mixture was stirred at reflux overnight. After cooling to
room temperature, the reaction mixture was concentrated under
reduced pressure to give a yellow solid (2.68 g, 94%). Spectroscopic
data was identical to previously published data [27].
The cell was of the same general type and space group as the
comparable Zr/I structure [18]. The structure is of the hafnium
carbene complex with a non-chiral carbene ligand, two iodides
(one axial and one equatorial), and an axial dimethylamido group.
The butyl side chains are ordered. There are no close contacts that
meet the definition of hydrogen-bonds, and most other contacts are
of the H.H type.
4.3. Preparation of CCCeNHC Hf complex
All calculations were performed using Pentium computers using
the current SHELX suite of programs.
4.3.1. 2-(1,3-Bis(N-butyl-imidazol-2-ylidene)phenylene)bis
(dimethylamido)(iodo)Hafnium (IV), 3
Acknowledgements
1,3-Bis(1-butyl-imidazol-3-yl) benzene diiodide
1 (0.70 g,
1.22 mmol), Hf(NMe2)4 (0.48 g, 1.34 mmol) and toluene (40 mL)
were combined and heated for 16 h in a 160 ꢀC oil bath. The reaction
was cooled to room temperature. During this period a solid
precipitated. The precipitate was collected and dried under reduced
pressure yielding a light yellow solid (0.67 g, 69%): 1H NMR (CD2Cl2)
The authors gratefully acknowledge the National Science
Foundation (CHE-0317089, CHE-0809732, EPS-0903787, MRI-
0421319 (NMR), MRI-0618148 (X-ray) and the University of Mis-
sissippi for financial support.
d
7.52 (s, 2H), 7.31 (t, J ¼ 7.8 Hz, 1H), 7.15 (d, J ¼ 7.8 Hz, 2H), 7.10 (s,
Appendix A. Supplementary data
2H), 4.4e4.6 (br m, 2H), 4.3e4.4 (br m, 2H), 3.07 (s, 6H), 1.94 (m,
4H), 1.47 (sex, J ¼ 7.2 Hz, 4H), 1.00 (t, J ¼ 7.2 Hz, 6H)); 13C{1H}
CCDC 791008 (compound 3) contains the supplementary crys-
tallographic data for this paper. These data can be obtained free of
charge from the Cambridge Crystallographic Data Centre via http://
www.ccdc.cam.ac.uk/data_request/cif.
(75.5 MHz, CD2Cl2):
d 201.0, 168.9, 146.8, 129.2, 122.0, 116.0, 110.8,
52.3, 43.1, 34.1, 20.3, 14.2. Anal. Calcd For C22H31I2N5Hf: C, 33.12; H,
3.92; N, 8.78. Found: C, 33.43; H, 3.90; N, 8.81.
4.4. X-ray structure determination for compound C22H31HfI2N5, 3
Appendix. Supplementary material
4.4.1. Data collection
A thin, almost colorless crystal of dimensions approximately
0.20 ꢂ 0.20 ꢂ 0.20 mm, was retrieved from fluorocarbon oil, and
Supplemental data associated with this article can be found in
the online version at doi:10.1016/j.jorganchem.2010.10.004.

J. Cho et al. / Journal of Organometallic Chemistry 696 (2011) 373e377
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