Tripodal N,P Mixed-Donor Ligands and Their Cobalt Complexes: Efficient Catalysts for Acceptorless Dehydrogenation of Secondary Alcohols

Article abstract of DOI:10.1021/acs.inorgchem.8b00043

A new tetradentate tripodal ligand, ^iPrPPPN^HPy^Me, and the cobalt complexes were synthesized and characterized. The well-defined cobalt complexes efficiently catalyzed acceptorless dehydrogenation of secondary alcohols into ketones.

Full text of DOI:10.1021/acs.inorgchem.8b00043

Communication
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Tripodal N,P Mixed-Donor Ligands and Their Cobalt Complexes:
Efficient Catalysts for Acceptorless Dehydrogenation of Secondary
Alcohols
Shi Xu,†,§ Latifah M. Alhthlol,†,∥,§ Keshav Paudel,†,‡ Eric Reinheimer,⊥ David L. Tyer,† Daniela K. Taylor,†
Amberlynn M. Smith,† Jonathan Holzmann,† Edgar Lozano,† and Keying Ding*,†,‡
‡
†Department of Chemistry and Molecular Biosciences Program, Middle Tennessee State University (MTSU), Murfreesboro,
Tennessee 37132, United States
⊥Rigaku Americas, The Woodlands, Texas 77381, United States
∥King Saud bin Abdulaziz University for Health Sciences, Al Mubarraz, Alahsa 36428, Saudi Arabia
S
* Supporting Information
workers presented an iron catalytic system of CpFe(CO)2Cl2
ABSTRACT: A new tetradentate tripodal ligand,
iPrPPPNHPyMe, and the cobalt complexes were synthesized
and characterized. The well-defined cobalt complexes
efficiently catalyzed acceptorless dehydrogenation of
secondary alcohols into ketones.
(Cp = cyclopentadienyl) and a base for the dehydrogenation of
2-pyridylmethanol derivatives.9 Dehydrogenative coupling by
cobalt catalysts have recently been reported.5a,10
The ligand design plays a key role for such reactions mediated
by base metal catalysts. In our efforts to develop first-row
transition-metal catalysts, we are particularly interested in the
tetradentate tripodal ligand for the following reasons: (1) It may
provide extra stability to the reactive intermediates by enforcing
five or six coordination on the metal center. (2) Because of the
different coordination environments, metal complexes by such
ligands may have catalytic reactivities very different from those of
tridentate pincer ligand systems, which currently dominate the
dehydrogenation of alcohols. Herein, we report an isopropyl-
substituted tris(phosphino)pyridine ligand derivative (abbre-
viated as iPrPPPNHPyMe, 1), which is featured with an N−H
linker connecting the pyridine ring and a phosphino binding
moiety. Its cobalt complexes were synthesized and tested as
efficient precatalysts for the acceptorless and oxidant-free
dehydrogenation of secondary alcohols to afford ketones. To
the best of our knowledge, it is for the first time that base
transition-metal complexes supported by the tetradentate
tripodal ligand are reported for the acceptorless dehydrogenation
of alcohols.
The three-step ligand synthesis route from available starting
materials is described in Scheme 1a. In the initial step, 1,2-
dibromobenzene is activated with 1 equiv of n-butyllithium via
lithium−halogen exchange and subsequently quenched with
diisopropylchlorophosphine to give 2-(diisopropylphosphino)-
phenyl bromide in 68% yield.11 Next, 2 equiv of 2-
(diisopropylphosphino)phenyl bromide reacts with 1 equiv of
n-butyllithium, followed by the addition of phosphorus
trichloride, to afford bis[2-(diisopropylphosphinophenyl)]-
chlorophosphine in a yield of 52%.12 In the final step, 2-amino-
6-methylpyridine is activated with triethylamine and allowed to
react with chlorophosphine to form 1 in good yield (81%).
Detailed synthesis and characterization of 1 are available in the
Supporting Information (SI). The 31P{1H} NMR spectrum of 1
atalytic acceptorless dehydrogenation of alcohols is an
C
atom-economical and environmentally friendly method for
the synthesis of carbonyl compounds and their derivatives
without any oxidants and hydrogen acceptors. This method takes
advantage of inexpensive and readily available alcohols that can
be obtained from biomass resources and fermentation. Because
valuable hydrogen gas is the only byproduct from alcohol
dehydrogenation, alcohols can serve as important hydrogen
carriers for energy-storage applications. Despite the great
importance of this reaction, examples of homogeneous catalytic
acceptorless dehydrogenation of alcohols are dominated by rare,
expensive, and toxic precious metal catalysts including rhodium,1
ruthenium,2 iridium,3 and osmium.4 With increasing concerns on
natural resource depletion and climate change, it is more
desirable to replace precious metal catalysts with ones based on
earth-abundant and less-toxic metals. However, because alcohol
dehydrogenation is a thermodynamically uphill process, this
transformation is still challenging for base-metal catalysts and
such examples are rare. Zhang and Hanson reported a cobalt
precatalyst supported by a bis(phosphino)amine (PNP) pincer
ligand for the dehydrogenation of secondary alcohols. Further
mechanistic study suggests a cobalt(I/III) cycle and metal−
ligand cooperativity (MLC) is not crucial in this reaction.5 Jones
and co-workers developed highly efficient PNP pincer-ligand-
stabilized iron catalysts for the reversible dehydrogenation−
hydrogenation of alcohols and ketones.6 Jones and co-workers
also described a TP′Ni(QR) (Tp′ = tris(3,5-dimethylpyrazolyl)-
borate; Q = quinolinate; R = CF3) catalyst for dehydrogenation−
hydrogenation of alcohols, aldehydes, and ketones.7 An MLC
mechanism is proposed to operate in both iron and nickel
catalytic systems. The Hong group reported an iron(III)
acetylacetonate/1,10-phenathroline system for the dehydrogen-
ation of secondary benzylic alcohols, despite the fact that alkyl
alcohols do not work with this method.8 Nakazawa and co-
Received: January 5, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b00043
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
optimized (Table S2), we examined the reaction scope with a
range of secondary alcohols, as shown in Table 1. Aromatic
Scheme 1. (a) Synthesis of Ligand 1 and Complex 2 and (b) X-
ray Crystal Structure of 2
a
a
Table 1. Dehydrogenation of Secondary Alcohols to Ketones
b
time
(h)
conv
(%)
b
entry
1
R
R′
yield (%)
c
92, 87,
d
p-MeOPh
CH3
24
94
93
2
p-MePh
Ph
CH3
CH3
CH3
CH3
CH3
CH3
Ph
24
48
48
48
24
48
24
48
48
24
100
89
95
84
57
40
86
60
72
80
75
94
3
4
p-FPh
p-CF3Ph
m-MeOPh
o-MePh
Ph
60
5
43
6
90
7
64
8
75
81
9
2-Naph
Ph
CH3
C2H5
10
11
78
1,2,3,4-tetrahydro-1-
naphthol
100
e
12
−(CH2)6−
n-C6H13
Ph
48
24
24
73
72
e
f
f
13
CH3
H
43, 85
99
43, 12
g
a
14
99
50% probability of thermal ellipsoids; H atoms are omitted except for
a
that of the N−H linker.
Reaction conditions: alcohol (0.25 mmol), cobalt catalyst 2 (8.2 mg,
5 mol %), KOtBu (4.2 mg, 15 mol %), toluene (1.5 mL), 125 °C.
b
1
Determined by H NMR spectroscopy using 1,3,5-trimethoxyben-
c
d
contains three sharp resonances at 298 K: a doublet of doublets at
15.0 ppm is assigned to the central phosphine, and two doublets
at 3.1 ppm are assigned to the terminal phosphines in a 1:1 ratio,
with a 31P−31P coupling constant of 168 Hz (Figure S2).
The treatment of 1 with CoCl2 in tetrahydrofuran afforded
[(iPrPPPNHPyMe)CoCl]Cl (2) as a dark-red powder in 86%
isolated yield. As expected, this species is paramagnetic,
displaying broad signals in its 1H NMR spectrum. The magnetic
moment, determined by Evan’s method in solution, was found to
be 1.9 μB in CD2Cl2, consistent with a d7 metal center with one
unpaired electron with a S = 1/2 ground state. To obtain a solid-
state molecular structure, 2 was crystallized as dark-red needles
by vapor diffusion of diethyl ether into a concentrated
dichloromethane solution. The X-ray structure13 revealed a
distorted trigonal-bipyramidal geometry on the Co center, with
the P1 and Cl1 atoms in the axial positions and the terminal
phosphines (P2 and P3) and pyridine N (N1) donor in the
equatorial positions. The second Cl atom (Cl2) is considered to
be in the outer coordination sphere, linking with the N−H group
through hydrogen bonding. A space-filling model of 2 revealed
that the chloride ligand (Cl1) is nestled within a binding pocket
comprised of the phosphine diisopropyl substituents and the 2-
methyl substituent on the pyridine group (Figure S10).
Interestingly, the IR spectrum of 2 displays two intense N−H
stretching bands with an approximate 1:1 ratio at 3570 and 3393
cm−1, respectively (Figure S7). It is known that hydrogen
bonding between N−H and halides will result in a decrease of the
N−H stretching frequencies.14 We postulate that 2 may have two
conformers coexisting in the solid state, and the peak at 3393
cm−1 is tentatively assigned to the N−H stretch for the
conformer with more direct N−H···Cl interaction, while 3570
cm−1 likely belongs to the other conformer that lacks hydrogen-
bonding interaction.
zene as the internal standard. Isolated yield (1 mmol scale). Yield
from the mercury test. Determined by GC−MS. The reaction ran for
48 h. Benzyl benzoate was formed (in benzene).
e
f
g
secondary alcohols with electron-donating groups such as
methoxy (MeO) and methyl (Me) at the para position reacted
smoothly to afford the corresponding ketones with excellent
yields (entries 1 and 2). 1-Phenylethanol required a longer
reaction time (48 h) to attain a comparable yield (entry 3).
Substrates with electron-withdrawing groups like fluorine (F)
and trifluoromethane (CF3) at the para positions showed
diminished activities (entries 4 and 5). This result is in contrast to
Jones’ study on the nickel-based system, where secondary
alcohols bearing electron-donating groups react more slowly
than those with electron-withdrawing substituents.7 To our
delight, the dehydrogenation of sterically hindered ortho-
substituted 1-(2-methylphenyl)ethanol gave a 60% yield (entry
7). Cyclic alcohols 1,2,3,4-tetrahydro-1-naphthol and cyclo-
heptanol were dehydrogenated at the same conditions, affording
α-tetralone and cycloheptanone in 94% and 72% yield,
respectively (entries 11 and 12). The conversion of an aliphatic
alcohol 2-octanol was 43% after 24 h with 2-octanone as the only
product (entry 13). Interestingly, when the reaction time was
extended to 48 h, despite a higher conversion (85%), gas
chromatography−mass spectrometry (GC−MS) analysis re-
vealed mixed products of 2-octanone and 9-methylpentadecan-7-
one with an approximate ratio of 1:2.5 (entry 13d). An
investigation of the selectivity is currently underway. When
benzyl alcohol as an example of the primary alcohol was tested,
instead of benzyl aldehyde, the product was found to be exclusive
benzyl benzoate, an ester that is probably generated through
dehydrogenative alcohol homocouplings (entry 14).
Then we explored the reactivity of 2 for the dehydrogenation
of secondary alcohols. After the reaction conditions were
To understand the catalyst activation process and reaction
mechanism, we performed an initial mechanistic study. The
B
DOI: 10.1021/acs.inorgchem.8b00043
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
addition of 2 and 1 equiv of KOtBu in toluene at room
temperature elicits an immediate white precipitation of
potassium chloride, concurrent with the formation of a new
cobalt complex [iPrPPPNPyMe)CoCl (3)] in good yield (81%;
Scheme 2a). 3 was characterized by 1H NMR and IR
on the yield (Table 1, entry 1g), suggesting a homogeneous
catalytic system.
In conclusion, this study describes the development of a
tetradentate tripodal iPrPPPNHPyMe ligand and the cobalt
complexes as precatalysts for the efficient acceptorless
dehydrogenation of secondary alcohols to afford ketones. The
air-stable precatalysts can be activated by a base via
dehalogenation, providing a convenient way to access catalyti-
cally active species.
Scheme 2. (a) Syntheses of Complexes 3 and 4, (b) Solid-State
a
Structure of 3, and (c) Solid-State Structure of 4
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00043.
Experimental details, additional figures, and other results
(PDF)
Accession Codes
CCDC 1814791−1814793 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
a
50% probability of thermal ellipsoids; the triflate anion in 4 and H
atoms are omitted.
AUTHOR INFORMATION
Corresponding Author
*E-mail: keying.ding@MTSU.edu.
ORCID
spectroscopy, elemental analysis, and X-ray diffraction. The
magnetic moment (1.9 μB) measured by Evan’s method in C6D6
suggests a low-spin CoII center. Notably, the IR spectrum of 3
shows no N−H stretching bands at ca. 3500−3300 cm−1,
indicating N−H deprotonation. The solid-state structure of 3, as
presented in Scheme 2b, reveals the same coordination geometry
around the Co atom as that in 2.13 The main difference arises
from the lengthening of the N1−C1 distance (Δd = 0.04 Å),
concomitant with the shortening of the N2−C1 distance (Δd =
0.04 Å). This change is attributed to dearomatization of the
pyridine ring in 3. In direct contrast to those in 2, N1−C1 is
labeled as a N−C single bond, while N2−C1 is a NC double
bond in 3. Complex 3 with 1 equiv of KOtBu demonstrates
reactivity similar to that of 2 (93% yield of 4′-methoxyaceto-
phenone; see the SI). No conversion was observed in the absence
of base. These observations suggest that 3 is likely involved in the
generation of the actual catalyst that may not require a base as the
cocatalyst because a base is only needed for precatalyst activation
via dehalogenation.15,16
To investigate the possible roles of MLC originating from the
N−H linker, an N−Me-substituted cobalt complex,
[(iPrPPPNMePyMe)CoCl]OTf (4), was synthesized by reacting
3 with an excess amount of MeOTf with a good yield of 72%
(Scheme 2a). The crystal structure of 4 confirmed the installation
of the methyl group (Scheme 2c) on the N atom.13 Similar to 2
and 3, complex 4 features a Co center in a distorted trigonal-
bipyramidal coordination sphere with a triflate counteranion.
The catalytic activity of 4 was compared with those of 2 and 3.
The highest yield of 4′-methoxyacetophenone is 69% for 4 with
various base loadings, which is not substantially lower than that
of 2 or 3 (Table S3). Because the MLC pathway is inaccessible
for complex 4, our results suggest that a non-MLC pathway is
catalytically viable.
■
Keying Ding: 0000-0002-7367-129X
Author Contributions
§These two coauthors contributed equally to the work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was funded by the National Science Foundation
(NSF-RUI; Grant CHE-1465051). A.M.S. and E.L. were
supported by an ACS Project SEED scholarship. We are thankful
for support from FRCAC, URECA, and Clean Energy Fee Funds
from MTSU. Structure data were obtained through an XRD
facility (NSF-MRI; Grant CHE-1626549). We are thankful for
discussions with Prof. Cory Hawkins (Tennessee Tech
University) and contributions from Dr. Irina Novozhilova and
undergraduates Claudia Lopez, Joseph Churchill, and Asfah
Mohammed.
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