1,1′-Bis(di-tert-butylphosphino)ferrocene copper(i) complex catalyzed C-H activation and carboxylation of terminal alkynes

Article abstract of DOI:10.1039/c5dt03794h

Four copper(i) complexes, [CuBr(dtbpf)] (1), [CuI(dtbpf)] (2), [Cu₄(μ₂-I)₂(μ₃-I)₂(μ-dtbpf)₂] (3) and [Cu₆(μ₃-I)₆(μ-dtbpf)₂]·2CH₃CN (4), were prepared using CuX (X = Br, I) and 1,1′-bis(di-tert-butylphosphino)ferrocene (dtbpf). These complexes have been characterized by elemental analyses, IR, ¹H and ³¹P NMR, ESI-MS and electronic absorption spectroscopy. Molecular structures of the complexes 2 and 4 were determined crystallographically. Complex 2 is the first monomeric isolated Cu(i) complex of dtbpf with the largest P-Cu-P bite angle (120.070(19)°) to date. Complex 4 shows a centrosymmetrical dimeric unit with two [Cu₃(μ₃-I)₃] motifs bridged by two bidentate dtbpf ligands in the κ¹-manner. Each [Cu₃(μ₃-I)₃] motif unites to form a pyramid with one copper atom at the apex and one of the triangular faces capped by an iodine atom. All the complexes were found to be efficient catalysts for the conversion of terminal alkynes into propiolic acids with CO₂. Owing to the excellent catalytic activity, the reactions proceeded at atmospheric pressure and ambient temperature (25 °C). The catalytic products were obtained in moderate to good yields (80-96%) by using complex loading to 2 mol%. To the best of our knowledge, this is the first example of an active ferrocenyl diphosphine Cu(i) catalyst for the carboxylation of terminal alkynes with CO₂.

Full text of DOI:10.1039/c5dt03794h

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1,1′ꢀbis(diꢀtertꢀbutylphosphino) ferrocene copper(I) complex
catalyzed CꢀH activation and carboxylation of terminal alkynes
Manoj Trivedi,*a Gurmeet Singh,a* Abhinav Kumar,b Nigam P. Rath*c
Received 00th January 20xx,
Accepted 00th January 20xx
Four copper(I) complexes [CuBr(dtbpf)] (
1), [CuI(dtbpf)] (2), [Cu4(ꢁ2ꢀI)2(ꢁ3ꢀI)2(ꢀꢀdtbpf)2] (3) and
DOI: 10.1039/x0xx00000x
[Cu6(ꢁ3ꢀI)6( ꢀdtbpf)2].2CH3CN ( ), were prepared using CuX (X= Br, I) and 1,1′ꢀbis(diꢀtertꢀ
ꢀ
4
www.rsc.org/
butylphosphino) ferrocene (dtbpf). These complexes have been characterized by elemental analyses,
1
IR, H and 31P NMR, ESIꢀMS and electronic absorption spectroscopy. Molecular structures of the
complexes
Cu(I) complex of dtbpf with largest PꢀCuꢀP bite angle (120.070(19)°) to date. Complex
centrosymmetrical dimeric unit with two [Cu3( 3ꢀI)3] motifs bridged by two bidentate dtbpf ligands in
1ꢀmanner. Each [Cu3(
3ꢀI)3] motifs unites to form a pyramid with one copper atom at the apex and
2
and
4
were determined crystallographically. Complex
2
is the first monomeric isolated
4
shows a
ꢀ
κ
ꢀ
one of the triangular faces capped by an iodine atom. All the complexes were found to be efficient
catalysts for the conversion of terminal alkynes into propiolic acids with CO2. Owing to the excellent
catalytic activity, the reactions proceed at atmospheric pressure and ambient temperature (25°C). The
catalytic products were obtained in moderate to good yields (80ꢀ96%) by using complex loading to 2
mol%. To the best of our knowledge, this is the first example of an active ferrocenyl diphosphines
Cu(I) catalyst for the carboxylation of terminal alkynes with CO2.
CO2 which causes global warming and a consequent series of
environmental problems.8 One of the ways to mitigate this
problem is to use carbon dioxide as an C1 building block in
Introduction
Ferrocenyl diphosphines have received considerable attention
because of their extensive applications as ligands for a wide
variety of transition metal catalyzed transformations.1 There are
many different types of ferrocenyl diphosphines, the most
common are 1,1′ꢀbis(diphenylphosphino) ferrocene (dppf)2 and
1,1′ꢀbis(diꢀtertꢀbutylphosphino) ferrocene (dtbpf).2i,3 These
ligands have significant advantages over other diphosphines
which contain alkyl rather than metalloceneꢀbackbones.4 The
advantage of these ligands is that they are capable of stabilizing
the metal centre in a variety of states of charge and geometries
due to their numerous possible modes of coordination. Recently
it has been reported that dtbpf is superior to dppf for the Pdꢀ
catalyzed αꢀarylation of ketones and for certain Pdꢀcatalyzed
Suzuki couplings.5,6 Various Cu(I) complexes containing
organic synthesis because it is an abundant, renewable carbon
source and an environmentally friendly chemical reagent.9 The
utilization, as opposed to the storage of CO2, is indeed more
attractive especially if the conversion process to useful bulk
products is an economical one. Significant efforts have been
devoted towards exploring the technologies for CO2
transformation, whereby harsh and severe reaction conditions
are one of the major limitations for their practical
applications.9,10 Therefore, the development of efficient catalyst
systems for CO2 utilization under mild conditions is highly
desired, especially for real world applications.
One of the best strategies for CO2 conversion is the
synthesis of propiolic acids through the CꢀH bond activation of
terminal alkynes with CO2 as a C1 building block11 because the
alkynyl carboxylic acid products can serve as an important
synthetic intermediates12 for further applications in medicinal
chemistry as well as organic synthesis13 to give coumarins,
flavones, aminoalkynes, alkynylarenes, and arylidene
oxindoles.14 Several procedures and catalysts, including both
homoꢀ11cꢀe,15 and heterogeneous catalytic11a,16 systems, have
been developed in this area, but either reusability problems or
synthetic complications limit further application of these
catalytic systems. The design and synthesis of efficient,
inexpensive, and easily prepared catalysts for this type of
reactions are only at the beginning and urgently require further
developments. In the past decades, several interesting systems
phosphine,
Nꢀheterocyclic carbene, imidazole, triazole, hybrid
nitrogenꢀsulfur ligands have been found to be versatile
catalysts.7
Currently, an urgent issue of our decade is the emission of
a.Department of Chemistry, University of Delhi, Delhiꢀ110007, INDIA. Email:
manojtri@gmail.com, gurmeet123@yahoo.com.
b.Department of Chemistry, University of Lucknow, Lucknowꢀ226007, INDIA.
c. Department of Chemistry & Biochemistry and Centre for Nanoscience, University
of MissouriꢀSt. Louis, One University Boulevard, St. Louis, MO 63121ꢀ4499, USA.
Email: rathn@umsl.edu.
†
Electronic Supplementary Information (ESI) available: CCDC reference
numbers 1401905 ( ), 1401906( ). For ESI and crystallographic data in CIF or
2
4
other electronic format see DOI: 10.1039/b000000x/
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have been reported for metalꢀmediated reductive carboxylation measurements were performed against Fc+/Fc couple at a scan
of alkynes17, allenes18, and alkyenes19 with CO2 to form rate of 100 mVsꢀ1. GCMS studies were done with the
carboxylic acids or esters. However, most of those systems Shimadzuꢀ2010 instrument containing a DBꢀ5/RtXꢀ5MSꢀ30Mt
need either a stoichiometric amount of transition metals as & 60Mt column of 0.25mm internal diameter. M+ is the mass of
reactants or an excess amount of organometallic reagents for the cation.
Syntheses of the complexes 1, 2 and 4
Complexes 1, 2 and 4 were prepared as follows: 1 mmol of 1,1′ꢀ
bis(diꢀtertꢀbutylphosphino) ferrocene was dissolved in CH3OH (15
transmetallation processes. An alternative possibility to achieve
the catalytic synthesis of carboxylic acid from CO2 is by direct
CꢀH bond activation and carboxylation. Recently, Nolan’s
group reported a goldꢀcatalyzed CO2 carboxylation of CꢀH
bonds of highly activated arenes and heterocycles.20 The state
of the art for the carboxylation of terminal alkynes has recently
mL), and CH2Cl2 (15 mL) and
a copper(I) bromide/iodide (For
complex 1 & 2 (1 mmol); 4 (6 mmol)) was then added. The resulting
solution was refluxed for 24 hours. The resulting solution was
filtered and saturated with hexane/CH3CN and left for slow
evaporation.
been published by Gooßen et al. 11
This article suggests that it
.
is possible to perform carboxylations of terminal alkynes with
various copper and silver complexes, even at ppm loadings.
This motivated us to study the application of these compounds
in carbon dioxide fixation reaction. Because of our interests in
the area of copper chemistry, we decided to investigate the
Orange block crystals for complex
studies were obtained after three days.
Synthesis of [CuBr(dtbpf)] (1). Yield: (0.432 g, 70%). Anal.
2 and 4 suitable for Xꢀray
Calc. for C26H44BrP2CuFe: C, 50.56; H, 7.13. Found: C, 50.88;
H, 7.15. IR(cmꢀ1, KBr):
ν = 3420, 3100, 3080, 2940, 2920,
interaction of CuX (X=Br, I) complexes with 1,1′ꢀbis(diꢀtert
ꢀ
2890, 2860, 2360, 2340, 2120, 1720, 1470, 1453, 1380, 1360,
1302, 1180, 1150, 1060, 1040, 938, 898, 850, 829, 810, 740,
butylphosphino) ferrocene (dtbpf). Herein, we also report
copper(I) complexes containing 1,1′ꢀbis(diꢀtertꢀbutylphosphino)
ferrocene (dtbpf) catalyzed transformation of CO2 to carboxylic
acids through CꢀH bond activation and carboxylation of
terminal alkynes. To the best of our knowledge, this is the first
example of an active ferrocenyl diphosphines Cu(I) catalyst for
the carboxylation of terminal alkynes with CO2. Various
propiolic acids were synthesized in good to excellent yields
under ambient conditions. This catalytic system is a simple and
economically viable protocol with great potential for practical
applications.
630, 601, 580, 548, 490, 471, 440. 1H NMR (
δ
ppm, 400 MHz,
5.27 (s, 4H, C5H4), 4.44 (s, 4H, C5H4), 1.31
(m, 36H, CH3). 31P{1H}:
36.04 (s) (sharp). UV/Vis: λmax
[dm3 molꢀ1 cmꢀ1]) = 367(12770), 240(14620). ESIꢀMS (m/z):
537.6 (M+).
CDCl3, 298K):
δ
δ
(
ε
Synthesis of [CuI(dtbpf)] (2). Yield: (0.531 g, 80%). Anal.
Calc. for C26H44IP2CuFe: C, 46.98; H, 6.62. Found: C, 47.02;
H, 6.85. IR (cmꢀ1, KBr):
ν
= 3410, 3098, 2940, 2880, 1640,
1455, 1390, 1360, 1180, 1160, 1040, 940, 815, 591, 540, 491,
460, 438. 1H NMR (
ppm, 400 MHz, CDCl3, 298K): 4.41 (s,
4H, C5H4), 4.40 (s, 4H, C5H4), 1.46 (m, 36H, CH3). 31P{1H}:
19.85 (s) (sharp). UV/Vis: λmax
[dm3 molꢀ1 cmꢀ1]) = 364
(27666), 238(9505). ESIꢀMS (m/z): 537.2 (M+).
Synthesis of [Cu4(µ2ꢀI)2(µ3ꢀI)2( ꢀdtbpf)2] (3). This is
1ꢀ
prepared following our earlier reported method.24d
Synthesis of [Cu6(µ3ꢀI)6( ꢀdtbpf)2].2CH3CN (4). Yield:
δ
δ
δ
Experimental section
(
ε
Materials and Physical Measurements
κ
P,P
All the synthetic manipulations were performed under nitrogen
atmosphere. The solvents were dried and distilled before use by
following the standard procedures.21 Copper(I) iodide,
Copper(I) bromide, 1,1′ꢀbis(diꢀtertꢀbutylphosphino) ferrocene,
ꢀ
(1.197 g, 85%). Anal. Calc. for C56H94N2P4I6Cu6Fe2: C, 30.92;
H, 4.32; N, 1.29 . Found: C, 31.03; H, 4.48; N, 1.30. IR (cmꢀ1,
Phenylacetylene,
1ꢀChloroꢀ4ꢀethynylbenzene,
4ꢀ
KBr):
1361, 1180, 1160, 1040, 930, 817, 580, 544, 493, 470, 438. H
NMR ( ppm, 400 MHz, CDCl3, 298K): 4.73 (s, 8H, C5H4),
4.40 (s, 8H, C5H4), 1.36 (m, 72H, CH3), 1.22(s, 6H, CH3).
31P{1H}: [dm3 molꢀ1 cmꢀ1])
28.07 (s) (sharp). UV/Vis: λmax
= 330(67129), 232 (18362). ESIꢀMS (m/z): 2173.2 (M+).
ν = 3410, 3098, 2940, 2880, 2860, 1630, 1460, 1385,
1
Ethynylanisole, cyclohexylacetylene, Ethynyltrimethylsilane, 4ꢀ
Ethynyltoluene (all Aldrich) and 4ꢀNitrophenylacetylene (TRC)
were used as received. Elemental analyses were performed on a
Carlo Erba Model EAꢀ1108 elemental analyzer and data of C,
H and N was within ±0.4% of calculated values. IR(KBr) was
δ
δ
δ
(ε
recorded using PerkinꢀElmer FTꢀIR spectrophotometer. Xꢀray structure determination
Electronic and emission spectra for and were recorded on
2 and 4 were collected on a Bruker APEX
1
,
2
4
Intensity data sets for
II CCD area detector diffractometer using graphite
monochromatized MoꢀK radiation at 100(2) K. ApexII, and
SAINT software packages were used for data collection and
data integration for The structures were solved by
and 4 22
direct methods using SHELXSꢀ97, and refined by full matrix
leastꢀsquares with SHELXLꢀ2014.22 The nonꢀhydrogen atoms
were refined with anisotropic thermal parameters. All the
hydrogen atoms were treated using appropriate riding models.
PLATON was also used for analyzing the intermolecular
interactions and stacking distances.23
a Perkin Elmer Lambdaꢀ35 and Horiba Jobin Yvon Fluorolog 3
spectrofluorometer, respectively. 1H and 31P NMR spectra were
recorded on a JEOL ALꢀ400 FTNMR instrument using
tetramethylsilane and phosphoric acid as an internal standard,
respectively. Mass spectral data were recorded using a Waters
Spectrometer/Data
Electrochemical properties of the complexes
investigated using cyclic voltammetry. Platinum was used as
the working electrode and tetrabutylammonium perchlorate (0.1
M) in dichloromethane solution was used as supporting
α
2
.
micromass
LCT
Mass
system.
1, 2 and 4 were
electrolyte in 0.001 M solution of complexes 1, 2 and 4. The
2 | J. Name., 2012, 00, 1-3
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General Experimental Procedure for Carboxylation of Terminal
Alkynes
CuI/complex 1ꢀ4 (2 mol%) and Cs2CO3 (1.5 mmol) were added
to DMF (5 mL) in the reaction tube (10 mL). A CO2 (balloon)
and 1 mmol of terminal alkynes were introduced into the
reaction mixture under stirring. The progress of the reaction
was monitored by TLC. After completion, the reaction mixture
was diluted with water (15 mL) and the solid residue was
separated via centrifugation. The mixture was washed with
CH2Cl2 and the aqueous layer was acidified with concentrated
HCl to pH=1 at low temperature, then extracted with ethyl
acetate. The combined organic layers were washed with
saturated NaCl solution, dried over Na2SO4 and filtered. The
solvent was removed in vacuum to afford the acid products.
Results and discussion
Synthesis
All the complexes [CuBr(dtbpf)] (
I)2( 3ꢀI)2( ꢀdtbpf)2] ( ) and [Cu6(
were prepared in high yield by the reaction of CuX (X= Br, I)
with 1,1′ꢀbis(diꢀtertꢀbutylphosphino) ferrocene ligand in a
dichloromethane: methanol mixture (50:50 V/V) in 1:1, 2:1 and
1
), [CuI(dtbpf)] (
2), [Cu4(ꢁ2ꢀ
ꢁ
ꢀ
3
ꢁ
3ꢀI)6( ꢀdtbpf)2].2CH3CN (4)
ꢀ
Scheme 1. Synthetic routes for the complexes 1ꢀ4.
in excess (6: 1) under reflux (Scheme 1). Complex
isolated in good yield following our earlier reported method.24d
Complexes were found to be air stable, nonꢀhygroscopic
3 was
1ꢀ4
solids and soluble in DMF, DMSO and halogenated solvents
but insoluble in petroleum ether and diethyl ether. The positions
of different peaks and overall fragmentation patterns in the ESIꢀ
MS of the respective complexes are consistent with their
1
formulations. In H NMR spectra of the complexes
η
1
,
2
and
4,
5ꢀC5H4 protons of dtbpf ligand were observed as two broad
singlets in the range of
δ 5.27ꢀ4.40 ppm (See Fꢀ1 to Fꢀ3
supporting material). The tertꢀbutyl protons of dtbpf were
observed as a multiplet at 1.31ꢀ1.46 ppm. The 31P NMR spectra
of the complexes showed a single resonance (
1
,
2
and
4
δ
36.04( ), 19.85( ), 28.07 (4) ppm) for the dtbpf ligand which
1
2
suggested that all the phosphorus atoms were chemically
equivalent (See Fꢀ4 to Fꢀ6 supporting material). These chemical
shifts were within the accepted range and are comparable to
that of the chelating dtbpf ligands (δ
P for dtbpf is 28.2).21 In the
Fig. 1. Electronic spectra for 1, 2 and
4 in CH2Cl2.
electronic absorption spectra all the complexes exhibited two
bands at 328ꢀ367 nm and 230ꢀ241 nm in dichloromethane
solution (Fig. 1). The lowerꢀenergy band can be assigned to the
MLCT transition. The higherꢀenergy band at 230ꢀ241 nm has
been assigned to intraligand charge transfer. All the complexes
were non emissive on excitation at 328ꢀ367 nm however, on
excitation at 230ꢀ241 nm exhibited two broad emissions at 334ꢀ
341 nm and 415ꢀ419 nm, respectively (Fig. 2). We tentatively
assigned the emission of these complexes to XLCT, CC, and
LC transitions as dtbpf ligand does not show luminescence in
the range of 495ꢀ900 nm.
The electrochemical properties of the complexes 1, 2 and
were studied through cyclic voltammetry (CV) (Fig. 3) and the
pertinent electrochemical data are presented in Table 1. The
4
complex
complexes
1
exhibits two quasiꢀreversible redox waves while
and exhibit one quasiꢀreversible redox peak and
2
4
one single reduction peaks with onset of ꢀ0.198 and ꢀ0.224 V,
respectively with respect to Fc+/Fc.
Description of the Crystal Structures
Two Xꢀray structures emerged from the current study.
Structural data and refinement details for both the structures are
summarized in Table 1 in the SI, and hydrogen bond
parameters are presented in Table 2 in the SI, respectively.
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polynuclear copper(I) complexes with dtbpf ligand.24c,24d The
bite angle for complex [P(1)ꢀCu(1)ꢀP(2)] is 120.070(19)°. The
2
bite angle (βn) of diphosphines is defined as the selective
chelation angle (PꢀMꢀP bond angle) that is determined by the
diphosphine ligand backbone (Fig. 5). The bite angle seems to
be influenced by several factors such as the central metal atom
of the metallocene, groups on the phosphorus atoms, and the
complexing metal.
βn
P
P
M
Fig. 2. Emission spectra for 1, 2 and
4 in CH2Cl2.
Fig. 5. Bite angle of a diphosphine ligand bound to metal.
A comparison of the PꢀMꢀP bite angle of several of these
complexes is given in Table 2. To the best of our knowledge,
this is the largest bite angle found to date in complexes
containing different ferrocenyldiphosphines.1c,1f,24,27ꢀ41 The
plane of the bridge defined in
2 by the copper centre and the
iodide atoms is almost perpendicular to the plane defined by the
two phosphorus atoms. The dihedral angle between these two
planes (β) is 89.95°. The two substituted Cp rings in the dtbpf
ligand in
[Cp(centroid)····Fe····Cp(centroid) = 176.97°]. Crystal packing
in complex is stabilized by CꢀHꢁꢁꢁI weak interactions (See Fꢀ7
supporting material). The contact distances for CꢀHꢁꢁꢁI
interactions are in the range of 3.00ꢀ3.32Å. Complex
crystallized in the triclinic space group P 1. The structure of
complex exhibits a centrosymmetrical dimeric unit with two
[Cu3( 3ꢀI)3] units bridged by two bidentate dtbpf ligands in
1ꢀ
manner (Fig. 6). Each [Cu3( 3ꢀI)3] units forms a pyramid with
2 adopt the antiperiplanar staggered conformation
2
4

4
ꢀ
κ
ꢀ
Fig. 3. Cyclic voltammogram for the complexes 1, 2 and 4 in CH2Cl2/0.1 M [NBu4]ClO4
one copper atom at the apex and one of the triangular faces
capped by one iodine atom. Cu(2) is trigonally coordinated to
the phosphorus donor [P(2)] of two bridging dtbpf ligands in
at 100 mV s−1 scan rate.
Complex
2 crystallizes in the orthorhombic crystal system, and
κ
1ꢀmanner and two tripleꢀ bridging iodine atoms [I(1) and I(2)]
the structure was solved in space group Pna21 (Fig. 4).
whereas Cu(1) exhibits trigonal bipyramidal coordination and
bonded to the phosphorus donor [P(1)] in
1ꢀmanner, threeꢀ
κ
Table 1. Electrochemical data for 1, 2 and
4
in dichloromethane solution/0.1 M
[NBu4]ClO4 at 298 K.
bridging iodine atoms[I(1), I(2)and I(3)] and one Cu(3) atom.
The Cu(3) atom is octahedrally bonded to four threeꢀbridging
iodine atoms[I(1), I(2), I(3)and I(3)#1] and two copper atoms
[Cu(1) and Cu(3)]. The CuꢀCu distances are in the range of
2.6661(8) Åꢀ2.9161(6) Å which are shorter than those found in
‘cubane’ [Cu4X4(PPh3)4] (X = I, Br or Cl)25 [2.874(5)–3.164(4)
Å] and ‘step’ [Cu4X4(PPh3)4] (X = I or Br) [2.835(3)ꢀ3.448(3)
Oxidation
Reduction
aEpa (1),
aEpa (2),/V
bEpc (1), bEpc (2),/V
vs. Fc+/Fc (in CH2Cl2)
vs. Fc+/Fc (in CH2Cl2)
Complexꢀ1
Complexꢀ2
Complexꢀ4
+0.106
+0.462
+0.483
+0.41
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ0.498
+0.389
+0.373
ꢀ0.025
ꢀ0.198
ꢀ0.224
ꢀ
b
aEpa is the anodic peak potential ; Epc is the cathodic peak potential
Å] and are comparable to [Cu4(ꢁ2ꢀI)2(ꢁ3ꢀI)2(ꢀ
ꢀdtbpf)2].24d The
Xꢀray structure for
2 reveals that the copper(I) atom is bonded
CuꢀIꢀCu angles are comparable to those in the other similarly
related complexes with step and cubaneꢀlike structures. The
reason for small CuꢀIꢀCu angles could be due to van der Waals
repulsions between the halogen atoms.
to two phosphorus atoms of the dtbpf ligand in a trigonal planar
environment completed by an iodide [Cu(1)ꢀI(1) 2.5293(3) Å].
The CuꢀP distances [Cu(1)ꢀP(1) 2.2540(5) Å, Cu(1)ꢀP(2)
2.2584(5) Å] are slightly longer than those reported for
4 | J. Name., 2012, 00, 1-3
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Table 2. Bite angle of metal complexes of ferrocenyldiphosphines.
Complex
[Cu2(µꢀCl)2(
2ꢀP,PꢀBꢀdppf)2]
Bite angle(°)
111.24(4)/
111.2(1)/ 111.26(4)
111.58(5)
111.06(5)
115.85(3)
112.13(4)
117.8(1)
Reference
1f, 27
κ
27
[Cu2(µꢀBr)2(
[Cu2(µꢀI)2(
[Cu2(µꢀCN)2(
[Cu2(µꢀSCN)2(
κ
2ꢀP,PꢀBꢀdppf)2]
2ꢀP,PꢀBꢀdppf)2]
2ꢀP,PꢀBꢀdppf)2]
2ꢀP,PꢀBꢀdppf)2]
1f, 28
27
κ
κ
1f, 29
1f, 30
1f, 30
1d, 31
1d, 32
1d, 33
1d, 34
1d, 33
1d, 35
1d, 36
1d, 36
1d, 37
κ
[{Cu(µꢀNO3ꢀO)(dppfꢀP,P′)}2]
[{Cu(µꢀO2CHꢀO,O′)(dppfꢀP,P′)}2]
[PdCl2(dppf)]
110.8(1)°
97.98(4)
[PdCl2(dppr)]
100.02(2)
101.29(4)
98.95(9)/99.64(9)
97.74(3)
[PdCl2(dppo)]
[PdCl2(dmpf)]
[PdCl2(depf)]
[PdCl2(dippf)]
103.59(4)
103.78(5)
109.98(2)
102.45(3)
104.22(5)
104.28(5)
103.75(5)
112.82(3)
116.36(8)
101.37(3)
[PtCl2(dippf)]
[ZnCl2(dippf)]
Fig. 6. Molecular structure of 4. Selected bond lengths [Å] and angles [°]: I(1)ꢀCu(2)
2.5665(4), I(1)ꢀCu(3) 2.7372(4), I(1)ꢀCu(1) 2.7647(4), I(2)ꢀCu(2) 2.5984(4), I(2)ꢀ
Cu(1) 2.7130(4), I(2)ꢀCu(3) 2.7160(4), I(3)ꢀCu(3) 2.6596(4), I(3)ꢀCu(1) 2.8190(4),
Cu(1)ꢀP(1) 2.2402(7), Cu(1)ꢀCu(3) 2.5969(5), Cu(1)ꢀCu(2) 2.9161(6), Cu(2)ꢀP(2)
2.2182(8), P(1)ꢀC(11) 1.887(3), P(1)ꢀC(15) 1.896(3), P(2)ꢀC(19) 1.885(3), P(2)ꢀC(23)
1.888(3), C(1S)ꢀN(1S) 1.137(6), C(1S)ꢀC(2S) 1.457(6), Cu(2)ꢀI(1)ꢀCu(3) 75.241(12),
Cu(2)ꢀI(1)ꢀCu(1) 66.202(13), Cu(3)ꢀI(1)ꢀCu(1) 56.326(11), Cu(2)ꢀI(2)ꢀCu(1)
66.562(13), Cu(2)ꢀI(2)ꢀCu(3) 75.105(13), Cu(1)ꢀI(2)ꢀCu(3) 57.155(12), Cu(3)#1ꢀI(3)ꢀ
Cu(3) 61.350(15), Cu(3)ꢀI(3)ꢀCu(1) 56.499(11), P(1)ꢀCu(1)ꢀCu(3) 174.12(3), P(1)ꢀ
Cu(1)ꢀI(2) 120.48(2), P(1)ꢀCu(1)ꢀI(1) 122.51(2), I(2)ꢀCu(1)ꢀI(1) 97.066(14), I(2)ꢀCu(1)ꢀ
I(3) 99.118(13), I(1)ꢀCu(1)ꢀI(3) 97.146(12), P(1)ꢀCu(1)ꢀCu(2) 114.06(2), Cu(3)ꢀCu(1)ꢀ
Cu(2) 71.731(15), P(2)ꢀCu(2)ꢀI(1) 127.48(2), P(2)ꢀCu(2)ꢀI(2) 126.46(2), I(1)ꢀCu(2)ꢀI(2)
105.262(15), I(3)ꢀCu(3)ꢀI(2) 103.126(14), I(3)ꢀCu(3)ꢀI(1) 101.743(14), I(2)ꢀCu(3)ꢀI(1)
97.653(13), N(1S)ꢀC(1S)ꢀC(2S) 179.7(6).
[PdCl2(dcypf)]
1d, 38
1d, 39
1d, 40
41
[PdCl2(dtbpf)]
[PdBr(4ꢀCNꢀC6H4)(dtbpf)]
[Rh(NBD)(dtbpf)][ClO4]
[Cu2(ꢀ
2ꢀSCN)2(k2ꢀP,Pꢀdppdtbpf)2]
[Cu(k2ꢀP,Pꢀdppdtbpf)(CH3CN)2]PF6
[Pd(k2ꢀP,Pꢀdppdtbpf)Cl2]
41
41
The comparative catalytic activities of the CuI and Cu(I)
complexes (1ꢀ4) were determined for the fixation of CO2 with
terminal alkynes into propiolic acids.In the initial investigation,
the carboxylation of phenylacetylene was selected as a model
reaction to study the influence of various solvents, loadings and
reaction time on the reaction. It can be seen that, without
copper source, the system was inefficient (Table 3, entry 1). We
have also found that no reaction occurred in the presence of
dtbpf ligand (entry 2). Table 3 shows the generation of 3ꢀ
phenylpropiolic acid from CO2 (1 atm) and phenylacetylene at
room temperature in the presence of CuI as well as complex
1ꢀ
4
. The yield in DMF was found to be higher in comparison to
that obtained in DMSO, CH3CN, chloroform, and NMP (entry
4, 7ꢀ10). We have also checked the effect of catalyst loadings
on the outcome of the reaction in the presence of 1, 2, 3 and 5
mol% of CuI (entry 3ꢀ6). For 2 mol% of CuI, 3ꢀphenylpropiolic
acid was obtained in good yields (50%). However, no
significant increase was observed with increase in the catalyst
amount to 5 mol%. The plot of yield vs. time for fixation of
Fig. 4. Molecular structure of 2. Selected bond lengths [Å] and angles [°]: I(1)ꢀCu(1)
2.5293(3), Cu(1)ꢀP(1) 2.2540(5), Cu(1)ꢀP(2) 2.2584(5), P(1)ꢀC(15) 1.8895(19), P(1)ꢀ
C(11) 1.898(2), P(2)ꢀC(23) 1.8901(19), P(2)ꢀC(19) 1.8903(19), P(1)ꢀCu(1)ꢀP(2)
120.070(19), P(1)ꢀCu(1)ꢀI(1) 119.785(15), P(2)ꢀCu(1)ꢀI(1) 120.135(15).
CO2 with phenylacetylene with complex
7. As shown in Table 3, complexes
2
is presented in Fig.
The two substituted Cp rings in the dtbpf ligand in
antiperiplanar staggered conformation. Crystal packing in
complex is stabilized by CꢀHꢁꢁꢁI and CꢀHꢁꢁꢁN weak
4 adopt the
1ꢀ4 exhibited higher
catalytic activity compared to CuI. The catalytic activity
decreased in the following order: . The high product
yields observed in as compared to others may be due to larger
bite angle which suggests
4
2>4>3>1
interactions (See Fꢀ11 supporting material).
2
Catalytic Performances for the conversion of terminal
alkynes into propiolic acids with CO2
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It is known that copper acetylide is the key intermediate for
copperꢀcatalyzed CꢀH activation of terminal alkynes and the
CuꢀC bond is active for CO2 insertion.26
Table 4. Synthesis of propiolic acid derivatives from CO2 and terminal alkynes with
complex 2.
2 mol% Complex 2
+ CO2
R
R
COOH
HCl
Entry
Alkyne
Product
Yield
(%)b
Fig. 7. Plots of 3ꢀphenylpropiolic acid yield vs. time for the CO2 fixation reaction of
phenylacetylene and CO2 in the presence of complex 2 as the catalyst.
1.
2.
3.
4.
94
95
96
90
COOH
that the PꢀCuꢀP bite angle plays a crucial role in catalytic
activity. The yield dropped to 30% when the base was changed
to K2CO3 under similar conditions (entry 16).
Cl
COOH
Cl
Table 3. Synthesis of 3ꢀphenylpropiolic acid from CO2 and Phenylacetylene with
catalysts CuI and complexes 1ꢀ4.a
H3CO
COOH
H3CO
CuI/Complex 1ꢀ4
+ CO2
COOH
HCl
H3C
H3C
COOH
Entry
1.
Catalyst
No catalyst
Solvent
DMF
DMF
DMF
DMF
DMF
DMF
CH3CN
DMSO
NMP
CHCl3
DMF
DMF
DMF
DMF
DMF
DMF
Yield (%)b
0
5.
80
O
2.
Dtbpf
0
O2
N
O2
N
3.
4.
1 mol% CuI
2 mol% CuI
21
50
50
52
20
35
10
10
80
96
90
92
0
OH
5.
3 mol% CuI
6.
5 mol% CuI
O
7.
2 mol% CuI
6.
7.
85
90
8.
2 mol% CuI
OH
9.
2 mol% CuI
2 mol% CuI
10.
11.
12.
13.
14.
c15.
d16
2 mol% complex 1
2 mol% complex 2
2 mol% complex 3
2 mol% complex 4
2 mol% complex 4
2 mol% complex 4
CH3
Si
CH3
O
H3C
H3C
Si
OH
CH3
CH3
65
aReaction conditions: alkyne (1.0 mmol), complex 2 (2 mol%), Cs2CO3 (1.5
mmol), CO2 (1.0 atm), 25°C, DMF (5 mL), 24 h. bYield of isolated product.
aReaction conditions: Phenylacetylene (1.0 mmol), catalyst, Cs2CO3 (1.5 mmol),
CO2 (1.0 atm), 25°C, solvent (5 mL), 24 h. bYield of isolated product. cIn absence
of CO2. dK2CO3.
This indicate that Cs2CO3 is a superior base for this direct
carboxylation reaction of terminal alkynes with CO2. With the
optimized conditions (1.5 equiv Cs2CO3, 1 atm CO2, DMF, 2
mol% catalyst, 25°C) in hand, several typical alkyne substrates
were subjected to this carboxylation reaction (Table 4). Under
the standard conditions, the corresponding products were
obtained in excellent yields (80ꢀ96%) when aromatic alkynes
with either electronꢀdonating (–CH3, –OCH3) or electronꢀ
withdrawing (–Cl, –NO2) substituent’s were employed.
However, for nonꢀphenylacetyleneꢀbased alkynes such as
cyclohexylacetylene or ethynyltrimethylsilane, the desired yield
was 85ꢀ90%.
A
possible reaction mechanism for copperꢀcatalyzed
carboxylation of terminal alkynes with CO2 is proposed in Fꢀ
18, supporting material. The copper center activates the
terminal alkyne with a base to form the copper acetylide
intermediate. Afterwards CO2 inserts into the CꢀCu bond to
form the carboxylic acid products, as previously proposed.11c,11e
Conclusions
In summary, we have synthesized and characterized four Cu(I)
complexes containing dtbpf ligand in the solid as well as in
solution state. We have successfully developed a process where
6 | J. Name., 2012, 00, 1-3
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3
4
K.M. Clapham, A.S. Batsanov, R.D.R. Greenwood, M.R.
Bryce, A.E. Smith, B. Tarbit, J. Org. Chem. 2008, 73, 2176ꢀ
2181.
L.E. Hagopian, A.N. Campbell, J.A. Golen, A.L. Rheingold,
C. Nataro, J. Organomet. Chem. 2006, 691, 4890ꢀ4900.
G.A. Grasa, T.J. Colacot, Org. Lett. 2007, 9, 5489ꢀ5492.
T.J. Colacot, H.A. Shea, Org. Lett. 2004, 6, 3731ꢀ3734.
copper(I) complexes containing dtbpf ligand catalyze the
transformation of CO2 to carboxylic acids through CꢀH bond
activation of terminal alkynes. Various propiolic acids were
synthesized in good to excellent yields under ambient
conditions. The most remarkable advantage of this mild
reaction system is its tolerance toward a wide substrate scope.
This protocol opens up access to a pool of highly functionalized
propiolic acids from CO2.
5
6
7
(
a
) V. Rampazzi, A. Massard, P. Richard, M. Picquet, P.L.
Gendre, J.ꢀC. Hierso, ChemCatChem 2012, 4, 1828ꢀ1835; (
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A.J. Blake, N.R. Brooks, N.R. Champness, L.R. Hanton, P.
Hubberstey, Martin Schröder, Pure and Appl. Chem., 1998,
Acknowledgements
70, 2351ꢀ2357; (
Young, T.S. Andy Hor, Dalton Trans. 2010, 39, 2631ꢀ2636;
) S.ꢀQ. Bai,L. Jiang, J.ꢀL. Zuo, T.S. Andy Hor, Dalton
Trans. 2013, 42, 11319ꢀ11326; ( ) S.ꢀQ. Bai, A.M. Yong, J.J.
Hu, D.J. Young, X. Zhang, Y. Zong, J. Xu, J.ꢀL. Zuo, T.S.
Andy Hor, CrystEngComm 2012, 14, 961ꢀ971;( M.
c) S.ꢀQ. Bai, J.Y. Kwang, L.L. Koh, D.J.
This project was financially supported by the Department of
Science and Technology, New Delhi, India (Grant No.
SR/FT/CSꢀ104/2011). Special thanks are due to Professor P.J.
Sadler, University of Warwick, UK and Professor Josef Michl,
University of Colorado, USA for their kind encouragement. We
acknowledge funding from the National Science Foundation
(CHE0420497) for the purchase of the APEX II diffractometer.
(
d
e
,
f)
Beaupérin, E. Fayad, R. Amardeil, H. Cattey, P. Richard, S.
Brandès, P. Meunier, J.ꢀC. Hierso, Organometallics 2008,
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