Selective Dimerization of Propylene with Ni-MFU-4l

Article abstract of DOI:10.1021/acs.organomet.7b00178

We report the selective dimerization of propylene to branched hexenes using Ni-MFU-4l, a solid catalyst prepared by cation exchange. Analysis of the resulting product distribution demonstrates that the selectivity arises from 2,1-insertion and slow product reinsertion, mechanistic features reproduced by a molecular nickel tris-pyrazolylborate catalyst. Characterization of Ni-MFU-4l by X-ray absorption spectroscopy provides evidence for discrete, tris-pyrazolylborate-like coordination of nickel, underscoring the small-molecule analogy that can be made at metal-organic framework nodes.

Full text of DOI:10.1021/acs.organomet.7b00178

Communication
pubs.acs.org/Organometallics
Selective Dimerization of Propylene with Ni-MFU‑4l
Robert J. Comito,† Eric D. Metzger,† Zhenwei Wu,‡ Guanghui Zhang,‡ Christopher H. Hendon,†
Jeffrey T. Miller,‡ and Mircea Dinca*,†
̆
†Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139,
United States
‡Davidson School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, Indiana 47907, United
States
S
* Supporting Information
ABSTRACT: We report the selective dimerization of propylene to
branched hexenes using Ni-MFU-4l, a solid catalyst prepared by cation
exchange. Analysis of the resulting product distribution demonstrates
that the selectivity arises from 2,1-insertion and slow product
reinsertion, mechanistic features reproduced by a molecular nickel
tris-pyrazolylborate catalyst. Characterization of Ni-MFU-4l by X-ray
absorption spectroscopy provides evidence for discrete, tris-pyrazolyl-
borate-like coordination of nickel, underscoring the small-molecule
analogy that can be made at metal−organic framework nodes.
he nodes of metal−organic frameworks (MOFs)
Trepresent promising targets for industrially significant
single-site heterogeneous catalysis. These secondary building
units (SBUs) are often capable of cation exchange with
preservation of their local coordination structure,1,2 offering a
predictable strategy for installing a discrete active site in a solid,
high-surface-area medium. Along these lines, we recently
reported the selective dimerization of ethylene to 1-butene
using Ni-MFU-4l (1),3 prepared by replacing Zn2+ ions with
Figure 1. Structural analogy between Ni-MFU-4l and TpMesNiCl
(structures abbreviated for clarity).
Ni2+ in Zn5Cl4(BTDD)3 (MFU-4l, H2BTDD = bis(1H-1,2,3-
triazolo[4,5-b][4′,5′-i])dibenzo[1,4]dioxin)).4 The cation ex-
change, performed by soaking MFU-4l in a Ni2+ solution at
room temperature, preserves the crystallinity and high surface
area of the parent material, suggesting that Ni2+ has been
incorporated into an SBU with an otherwise preserved
structure.4
model with the reported crystal structure of scorpionate 2
confirmed a close structural analogy between 1 and 2 (Table
S3.1 in the Supporting Information): both feature a
pseudotetrahedral arrangement of three nitrogen atoms and
one chloride. The most significant difference is a slightly wider
bite angle for DFT-1, with ∠N−Ni−N = 97.06° versus 92.88°
for 2. Notably, the porous structure of MFU-4l is well
preserved in DFT-1 (Figure S3.1 in the Supporting
Information).
Here, we evaluate 1 as a heterogeneous catalyst for the
selective dimerization of propylene, a reaction for which
molecular complexes provide the best chain selectivity and
regioselectivity.5−7 Industrially, this process continues to rely
on soluble nickel catalysts, despite the large reactor volumes
and challenging catalyst recovery associated with homogeneous
catalysis.8−11 We reasoned that Ni-MFU-4l would be a selective
catalyst for this reaction on the basis of the proposed structural
analogy between 1 and molecular tris-pyrazolylborate (Tp)
complexes, given the selective olefin dimerization activity of
catalysts such as TpMesNiCl (2).12 Because MFU-4l coordinates
Zn2+ tetrahedrally with three azolate nitrogens, it serves as a
good template for scorpionate-like coordination of Ni2+ ions,
installed through cation exchange (Figure 1).13 Indeed,
computational modeling of Ni-MFU-4l within the Kohn−
Sham DFT construct (DFT-1, Figure 1) and comparison of the
To verify the Ni2+ coordination environment, 1 was
examined by X-ray absorption spectroscopy (XAS). The XAS
spectra of Tp2Ni and anhydrous NiCl2 were also collected for
comparison. The edge energy obtained for Ni-MFU-4l agrees
well with both of these standards (Table S2 in the Supporting
Information), consistent with a Ni(II) oxidation state in 1.
Furthermore, Ni-MFU-4l displays a more intense pre-edge
feature in comparison to either NiCl2 or Tp2Ni, consistent with
Received: March 7, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00178
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
a lower-symmetry, pseudotetrahedral coordination environ-
ment (Figure S4.2 in the Supporting Information). Fitting the
X-ray absorption fine structure (EXAFS) data resulted in three
Ni−N bonds (1.99 Å, Table S4.2 in the Supporting
Information) and one Ni−Cl bond (2.14 Å), similar in length
to those of DFT-1 (1.95 and 2.12 Å, respectively) (Table 1).
resulting product distribution could be analyzed by quantitative
13C NMR. This analysis reveals a modest selectivity for
branched products, which are valuable as antiknock additives
for gasoline,8−11 with a 2.8:1 ratio of dimers 3, 4, and 6 to 5
(Scheme 1). The low overall concentration of chain-walking
products suggests that reinsertion of the products is slow, also
consistent with the high C6:C9 selectivity. Consequently, the
resulting product distribution can be analyzed as a product of
primary insertion selectivity.15−17 Indeed, we recently showed
that ethylene dimerization with 1 proceeds through such a
coordination−insertion mechanism.14 Parsing the product
distribution into an insertion tree indicates a mechanistic
preference for 2,1- over 1,2-insertion of propylene (Scheme 1).
In this analysis, regioselectivity increases with the steric size of
the alkyl group on nickel (H < 1-propyl < 2-propyl).
Significantly, this branch distribution is well reproduced by 2
under analogous conditions, including a comparable step-
specific 2,1-insertion selectivity and low isomerization rate
(Figure S6.1 in the Supporting Information).
a
Table 1. Fitting Results of the EXAFS Spectra
sample
Tp2Ni
scattering pair coordination no. bond length (Å) σ2 (Å)
Ni−N
Ni−Cl
Ni−N
Ni−Cl
6
6
3
1
2.06
2.40
1.99
2.14
0.006
0.009
0.006
0.003
NiCl2
Ni-MFU-4l
a
The average error in bond length is 0.01 Å and in σ2 is 0.002 Å2. The
fitting ranges for Tp2Ni and Ni-MFU-4l are Δk = 3.0−11.0 Å−1 and
ΔR = 1.0−2.0 Å. For NiCl2, the ranges are Δk = 3.0−10.0 Å−1 and ΔR
= 1.4−2.4 Å.
In summary, we have shown that Ni-MFU-4l serves as a
selective propylene dimerization catalyst whose selectivity can
be understood by structural and mechanistic analogy to
molecular Ni(II) tris-pyrazolylborate complexes. This work
underscores the molecule-like reactivity and tunability of MOF
nodes that underlie catalyst development and selectivity for α-
olefin dimerization, which represents a longstanding goal of
heterogeneous catalysis.
Fitting a different coordination number and a different
combination of scattering atoms both gave significantly worse
fits, allowing us to rule out other potential coordination modes
for Ni2+ in 1.
The competence of 1 and 2 as propylene dimerization
catalysts was evaluated in the presence of modified methyl-
aluminoxane-12 (MMAO-12) and 6 bar of applied propylene
pressure, under slurry-phase and semibatch conditions as in the
ethylene dimerization with 1.3 On a total Ni basis, 1 is less
active than 2 (490 versus 2000 (mol of product)/((mol of Ni)
h), respectively), and both catalysts are considerably less active
toward propylene than toward ethylene.12,14 These differences
in reactivity can be explained on the basis of the greater steric
hindrance of propylene in comparison to ethylene and the
differences in the steric environment of the active sites in 1 and
2. Nevertheless, the dimer to trimer ratios (C6:C9) are high
and nearly identical for the two catalysts (17:1). Neither MFU-
4l nor MMAO-12 shows oligomerization activity in the absence
of nickel.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00178.
Experimental details, gas chromatograms, NMR data,
computational methods, and XAS data (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail for M.D.: mdinca@mit.edu.
■
To gain insight into the C6 regioselectivity, propylene
dimerization was repeated using C6D6 as solvent, so that the
a
Scheme 1. Product Distribution and Insertion Tree for the Dimerization of Propylene by 1
a
Yields were determined by NMR.
B
DOI: 10.1021/acs.organomet.7b00178
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Organometallics
Communication
ORCID
Eric D. Metzger: 0000-0001-6597-1981
Christopher H. Hendon: 0000-0002-7132-768X
Mircea Dinca: 0000-0002-1262-1264
̆
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported through a Research Agreement
with Saudi Aramco, a Founding Member of the MIT Energy
Initiative. Fundamental studies of cation exchange in MOFs
were supported through a CAREER award to M.D. from the
National Science Foundation (DMR-1452612). E.D.M. ac-
knowledges the Department of Defense (DoD) for support
through the National Defense Science and Engineering
Fellowship (NDSEG) Program. Use of the Advanced Photon
Source was supported by the U.S. Department of Energy,
Office of Basic Energy Sciences, under contract no. DE-AC02-
06CH11357. MRCAT operations, beamline 10-BM, are
supported by the Department of Energy and the MRCAT
member institutions.
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DOI: 10.1021/acs.organomet.7b00178
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