Syntheses, characterization, and structural studies of copper(I) complexes containing 1,1′-bis(di-tert-butylphosphino)ferrocene (dtbpf) and their application in palladium-catalyzed Sonogashira coupling of aryl halides

Article abstract of DOI:10.1039/c4dt01411a

Four copper(i) complexes [Cu₄(μ₃-Cl) ₄(μ-dtbpf)₂] (1), [Cu₆(μ₃-Br) ₄(μ₂-Br)₂(μ-dtbpf)₃] (2), [Cu₄(μ₂-I)₂(μ₃-I) ₂(μ-dtbpf)₂] (3) and [Cu₂(μ₁- CN)₂(κ²-dtbpf)₂] (4) were prepared using CuX (X = Cl, Br, I, CN) and 1,1′-bis(di-tert-butylphosphino)ferrocene (dtbpf) in 2:1 molar ratio in DCM-MeOH (50:50 V/V) at room temperature. These complexes were characterized by elemental analyses, IR, ¹H and ³¹P NMR, ESI⁺-MS and electronic absorption spectroscopy. Molecular structures of the complexes 1, 2 and 3 were determined crystallographically. Complexes 1 and 3 are tetrameric Cu(i) pseudo-cubane like structures with a Cu₄Cl₄ core, and a stepped cubane-like structure with a Cu₄I₄ core, respectively, whereas complex 2 shows a trimeric copper(i) framework containing two [Cu₃(μ ₃-Br)₂(μ₂-Br)] units that are bridged by three bidentate μ-dtbpf ligands. Each [Cu₃(μ₃-Br) ₂(μ₂-Br)] unit forms a pyramid with one copper atom at the apex and one of the triangular faces capped by one bromine atom. All the complexes were found to be efficient catalysts for the Sonogashira reaction. The coupling products were obtained in moderate to good yields (58-99%) using Pd loadings of 0.2 mol% as well as complex loading of 0.1 mol%. The ³¹P NMR studies show that all the complexes are unstable during the course of tandem catalytic reaction and the dtbpf ligand migrates from copper(I) to palladium(II), which promotes palladium Sonogashira cross-coupling of activated and non-activated aryl halides. the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt01411a

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PAPER
Syntheses, characterization, and structural studies
of copper(I) complexes containing 1,1’-bis(di-tert-
butylphosphino)ferrocene (dtbpf) and their
application in palladium-catalyzed Sonogashira
coupling of aryl halides†
Cite this: Dalton Trans., 2014, 43,
13620
Manoj Trivedi,*a Gurmeet Singh,a Abhinav Kumarb and Nigam P. Rath*c
Four copper(I) complexes [Cu4(μ3-Cl)4(μ-dtbpf)2] (1), [Cu6(μ3-Br)4(μ2-Br)2(μ-dtbpf)3] (2), [Cu4(μ2-I)2(μ3-I)2-
(μ-dtbpf)2] (3) and [Cu2(μ1-CN)2(κ2-dtbpf)2] (4) were prepared using CuX (X = Cl, Br, I, CN) and 1,1’-bis-
(di-tert-butylphosphino)ferrocene (dtbpf) in 2 : 1 molar ratio in DCM–MeOH (50 : 50 V/V) at room temp-
erature. These complexes were characterized by elemental analyses, IR, 1H and 31P NMR, ESI+-MS and
electronic absorption spectroscopy. Molecular structures of the complexes 1, 2 and 3 were determined
crystallographically. Complexes 1 and 3 are tetrameric Cu(I) pseudo-cubane like structures with a Cu4Cl4
core, and a stepped cubane-like structure with a Cu4I4 core, respectively, whereas complex 2 shows a tri-
meric copper(I) framework containing two [Cu3(μ3-Br)2(μ2-Br)] units that are bridged by three bidentate
μ-dtbpf ligands. Each [Cu3(μ3-Br)2(μ2-Br)] unit forms a pyramid with one copper atom at the apex and one
of the triangular faces capped by one bromine atom. All the complexes were found to be efficient cata-
lysts for the Sonogashira reaction. The coupling products were obtained in moderate to good yields
(58–99%) using Pd loadings of 0.2 mol% as well as complex loading of 0.1 mol%. The 31P NMR studies
show that all the complexes are unstable during the course of tandem catalytic reaction and the dtbpf
ligand migrates from copper(I) to palladium(II), which promotes palladium Sonogashira cross-coupling of
activated and non-activated aryl halides.
Received 13th May 2014,
Accepted 21st July 2014
DOI: 10.1039/c4dt01411a
www.rsc.org/dalton
α-arylation of ketones and for certain Pd-catalyzed Suzuki
Introduction
couplings.5,6 The tetraphosphine ligand Fc(P)4 Bu,1,1′,2,2′-tetra-
t
In the last four decades, ferrocenyl diphosphines have received kis(diphenylphosphino)-4,4′-di-tert-butylferrocene acts as an
significant research attention because of their extensive appli- efficient auxiliary in the Sonogashira reaction in the presence
cations as ligands for a wide variety of transition metal cata- of [Pd(η3-C3H5)Cl]2 and copper.7 Various Cu(I) complexes con-
lysed transformations.1 Although there are many different taining phosphine, N-heterocyclic carbene, imidazole, triazole,
types of ferrocenyl diphosphines, the most common are 1,1′- hybrid nitrogen–sulfur ligands have been found to be versatile
bis(diphenylphosphino)ferrocene (dppf)2 and 1,1′-bis(di-tert- catalysts in catalytic reactions.8 We were interested in the struc-
butylphosphino)ferrocene (dtbpf).2i,3 These ligands have sig- tural properties of dtbpf complexes with simple salts, particu-
nificant advantages over other diphosphines which contain larly copper(I) and their applications in cross-coupling
alkyl rather than metallocene-backbones.4 Recently it has been reactions.9 As compared to the previously mentioned 1,1′-bis-
reported that dtbpf is superior to dppf for the Pd-catalyzed (phosphino)metallocene ligands, dtbpf has not been studied
extensively. To date, most of the studies with dtbpf had
focused on catalytic systems in which dtbpf is used as a ligand
aDepartment of Chemistry, University of Delhi, Delhi-110007, India.
for the palladium catalyst.10 With these viewpoints and to
E-mail: manojtri@gmail.com
explore other less expensive metal centres, recently our group11
has explored the interaction of copper(I) thiocyanate with
dtbpf for the construction of a tetranuclear copper(I) complex
with a cubane-like structure and its application in the palla-
dium-catalyzed Sonogashira coupling reaction. To the best of
our knowledge no structural report has been published on
bDepartment of Chemistry, University of Lucknow, Lucknow-226007, India
cDepartment of Chemistry & Biochemistry and Centre for Nanoscience, University of
Missouri-St. Louis, One University Boulevard, St. Louis, MO 63121-4499, USA.
E-mail: rathn@umsl.edu
†Electronic supplementary information (ESI) available. CCDC 1002754 for (1),
1002756 for (2) and 924712 for (3). For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c4dt01411a
13620 | Dalton Trans., 2014, 43, 13620–13629
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3
CuX : dtbpf (X = Cl, Br, I, CN). We decided to extend our 298 K): δ 4.63 (s, 8H, C5H4), 4.44 (s, 8H, C5H4), 1.32 (d, JH–P
=
studies in order to have a better understanding regarding the 13.5 Hz, 72H, CH3). 31P{1H}: δ 31.14 (s) (sharp). UV/Vis: λmax
Cu(I) species present in the solid state as well as in solution. (ε[dm3 mol−1 cm−1]) = 390 (14 770), 288 (34 421). ESI+-MS
Herein we report for the first time syntheses, spectroscopic (m/z): 1344.9 (M+).
characterization, and catalytic properties of four copper(I) com-
Synthesis of [Cu6(μ3-Br)4(μ2-Br)2(μ-dtbpf)3] (2). 1,1′-Bis(di-
plexes [Cu4(μ3-Cl)4(μ-dtbpf)2] (1), [Cu6(μ3-Br)4(μ2-Br)2(μ-dtbpf)3] tert-butylphosphino) ferrocene (474 mg, 1 mmol) was added
(2), [Cu4(μ2-I)2(μ3-I)2(μ-dtbpf)2] (3) and [Cu2(μ1-CN)2(κ2-dtbpf)2] slowly to a solution of CH3OH (15 mL) and CH2Cl2 (15 mL)
(4), respectively.
containing copper(I) bromide (286 mg, 2 mmol). The resulting
solution was refluxed for 24 hours, and the color of the solu-
tion changed from orange to yellowish orange over this period.
The resulting solution was filtered and saturated with hexane
and left for slow evaporation. Yellow crystals suitable for X-ray
studies were obtained after four days. Yield: (1.485 g, 65%).
Anal. Calc. for C78H132Br6P6Cu6Fe3: C, 40.98; H, 5.78. Found:
C, 41.12; H, 5.95. IR(cm−1, KBr): ν = 3422, 3106, 3082, 2945,
2922, 2895, 2864, 2714, 2368, 2345, 2125, 1718, 1475, 1458,
1390, 1365, 1307, 1177, 1155, 1061, 1037, 934, 890, 850, 830,
813, 745, 629, 604, 577, 547, 492, 473, 438. 1H NMR (δ ppm,
400 MHz, CDCl3, 298 K): δ 4.44 (s, 8H, C5H4), 4.29 (s, 8H,
C5H4), 1.35 (d, 3JH–P = 13.92 Hz, 72H, CH3). 31P{1H}: δ 37.39 (s)
(sharp). UV/Vis: λmax (ε[dm3 mol−1 cm−1]) = 467 (40 838), 299
(9930). ESI+-MS (m/z): 2283.5 (M+).
Experimental section
Materials and physical measurements
All the synthetic manipulations were performed under a nitro-
gen atmosphere. The solvents were dried and distilled before
use by following the standard procedures.12 Copper(I) chloride,
copper(I) bromide, copper(I) iodide, copper(I) cyanide and 1,1′-
bis(di-tert-butylphosphino) ferrocene (all from Aldrich) were
used as received. Elemental analyses were performed on a
Carlo Erba Model EA-1108 elemental analyzer and data for
C, H and N analyses were within 0.4% of calculated values.
IR(KBr) was recorded using a Perkin-Elmer FT-IR spectrophoto-
meter. Electronic and emission spectra for 1–4 were obtained
on a Perkin Elmer Lambda-35 and a Horiba Jobin Yvon Fluoro-
log 3 spectrofluorometer, respectively. 1H and 31P NMR spectra
were recorded on a JEOL AL-400 FTNMR instrument using
tetramethylsilane and phosphoric acid as an internal standard,
respectively. Mass spectral data were recorded using a Waters
micromass LCT Mass Spectrometer/Data system. Thermogravi-
metric analysis was carried out in air using a TA DSC Q 200
instrument with a heating rate of 10 °C min−1. Powder X-ray
diffraction data were collected on a Bruker D8 Discover X-ray
diffractometer, with Cu-Kα radiation (λ = 1.5405 Å). Electro-
chemical properties of the complexes 1–4 were measured by
cyclic voltammetry using platinum as the working electrode
and the supporting electrolyte used was [NBu4]ClO4 (0.1 M) in
0.001 M dichloromethane solution of complexes 1–4 versus
Ag/AgCl at a scan rate of 100 m Vs−1. GCMS studies were done
with the Shimadzu-2010 instrument containing a DB-5/RtX-
5MS-30Mt & 60Mt column of 0.25 mm internal diameter.
Synthesis of [Cu4(μ2-I)2(μ3-I)2(μ-dtbpf)2] (3). 1,1′-Bis(di-tert-
butylphosphino) ferrocene (474 mg, 1 mmol) was added
slowly to a solution of CH3OH (15 mL) and CH2Cl2 (15 mL)
containing copper(I) iodide (380 mg,
2
mmol). The
resulting solution was refluxed for 24 hours. Solution color
changed slowly from orange to reddish orange color. The
resulting solution was filtered and saturated with hexane and
left for slow evaporation. Orange crystals suitable for X-ray
studies were obtained after three days. Yield: (1.197 g, 70%).
Anal. Calc. for C52H88P4I4Cu4Fe2: C, 36.49; H, 5.15. Found:
C, 36.63; H, 5.20. IR(cm−1, KBr): ν = 3430, 3097, 2942, 2894,
2862, 1637, 1458, 1389, 1364, 1175, 1157, 1035, 936, 817, 580,
1
544, 493, 470, 439. H NMR (δ ppm, 400 MHz, CDCl3, 298 K):
3
δ 4.45 (s, 8H, C5H4), 4.40 (s, 8H, C5H4), 1.34 (d, JH–P = 8.8 Hz,
72H, CH3). 31P{1H}: δ 28.31 (s) (sharp). UV/Vis: λmax (ε[dm3
mol−1 cm−1]) = 484 (48 568), 272 (12 832). ESI+-MS (m/z):
1710.8 (M+).
Synthesis of [Cu2(μ1-CN)2(κ2-dtbpf)2] (4). 1,1′-Bis(di-tert-
butylphosphino) ferrocene (474 mg, 1 mmol) was added
slowly to a solution of CH3OH (15 mL) and CH2Cl2 (15 mL)
containing copper(I) cyanide (179 mg, 2 mmol). The resulting
M
+ is the mass of the cation.
Syntheses of the complexes
Synthesis of [Cu4(µ3-Cl)4(μ-dtbpf)2] (1). 1,1′-Bis(di-tert-butyl- solution was refluxed for 24 hours. The color of the solution
phosphino) ferrocene (474 mg, 1 mmol) was added slowly to a changed slowly from orange to reddish orange. The resulting
solution of CH3OH (15 mL) and CH2Cl2 (15 mL) containing solution was filtered and saturated with hexane. In a couple of
copper(I) chloride (198 mg, 2 mmol). The resulting solution days reddish orange powder was precipitated which was
was refluxed for 24 hours. Slowly, the color of the solution washed several times with diethyl ether, and vacuum-dried.
changed from orange to yellowish orange. The resulting solu- Yield: (0.789 g, 70%). Anal. Calc. for C54H88N2P4Cu2Fe2: C,
tion was filtered and saturated with hexane and left for slow 57.49; H, 7.81. Found: C, 57.64; H, 7.85. IR(cm−1, KBr): ν =
evaporation. Orange crystals suitable for X-ray studies were 3407, 2925, 2225, 2138, 1541, 1366, 1143, 1157, 1020, 813, 656,
obtained after two days. Yield: (1.075 g, 80%). Anal. Calc. for 602, 493. 1H NMR (δ ppm, 400 MHz, CDCl3, 298 K): δ 4.30
3
C52H88Cl4P4Cu4Fe2: C, 46.40; H, 6.54. Found: C, 46.43; H, 6.85. (s, 8H, C5H4), 4.04 (s, 8H, C5H4), 1.36 (d, JH–P = 8.8 Hz,
IR(cm−1, KBr): ν = 3448, 3356, 3094, 2941, 2895, 2862, 1637, 72H, CH3). 31P{1H}:
δ 66.78 (s) (sharp). UV/Vis: λmax
1475, 1458, 1389, 1365, 1178, 1157, 1037, 932, 831, 815, 741, (ε[dm3 mol−1 cm−1]) = 374 (11 159), 284 (9565). ESI+-MS (m/z):
606, 581, 547, 494, 471. 1H NMR (δ ppm, 400 MHz, CDCl3, 1127.2 (M+).
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X-ray structure determination
nitrogen, formed the coupling products. The progress of the
reaction was monitored by TLC using EtOAc and hexane as the
eluent. After completion, the reaction mixture was treated with
EtOAc (10 mL) and water (3 mL). The organic layer was separ-
ated, and the aqueous layer was extracted with EtOAc (3 ×
5 mL). The combined organic solution was washed with brine
(3 × 5 mL) and water (1 × 5 mL). Drying (anhydrous Na2SO4)
and evaporation of the solvent provided a residue, which was
purified on a short pad of silica gel using EtOAc and hexane as
the eluent.
Intensity data sets for 1, 2 and 3 were collected on Bruker
APEX II and Oxford X-calibur S CCD area detector diffracto-
meters using graphite monochromatized Mo-Kα radiation at
100(2) and 293(2) K. ApexII, SAINT and CrysAlisPro RED soft-
ware packages were used for data collection and data inte-
gration for 1 to 3.13 The structures were solved by direct
methods using SHELXS-97, and refined by full matrix least-
squares with SHELXL-97, refining on F2.13 The non-hydrogen
atoms were refined with anisotropic thermal parameters. All
the hydrogen atoms were treated using appropriate riding
models. Structures 1 and 2 were refined using the solvent
masking routine, SQUEEZE (Platon) as it was difficult to ident-
ify and model the solvent present in the lattice. Despite several
attempts, we are not able to obtain better quality crystals for
complex 3. This resulted in poor crystal data with a higher
R-factor. The crystal undergoes decomposition/solvent loss
during data collection adding to the problem. PLATON was
also used for analyzing the interaction and stacking
distances.14
Results and discussion
Synthesis
All the complexes [Cu4(μ3-Cl)4(μ-dtbpf)2] (1), [Cu6(μ3-Br)4(μ2-
Br)2(μ-dtbpf)3] (2), [Cu4(μ2-I)2(μ3-I)2(μ-dtbpf)2] (3) and [Cu2(μ1-
CN)2(κ2-dtbpf)2] (4) were prepared in high yield by the reaction
of CuX (X = Cl, Br, I, CN) with 1,1′-bis(di-tert-butylphosphino)
ferrocene ligand in a dichloromethane–methanol mixture
(50 : 50 V/V) in 2 : 1 ratio under refluxing conditions
(Scheme 1). Complexes 1–4 were found to be air stable, non-
hygroscopic solids and soluble in dimethylformamide,
Sonogashira catalytic reactions
The reaction of aryl halides (3.38 × 10−3 mol, 1 equivalent), dimethylsulfoxide and halogenated solvents but insoluble in
phenylacetylene (0.75 mL, 6.76 × 10−3 mol, 2 equivalent), and petroleum ether and diethyl ether. The elemental analyses
K2CO3 (0.935 g, 6.76 × 10−3 mol, 2 equivalent) with 0.001 equi- were consistent with their chemical formula. More infor-
valent of complex 1/2/3/4 and 0.002 equivalent of [PdCl(η3- mation about the composition of the complexes was also
C3H5)]2 (0.73 mg) at 120 °C for 20 h in DMF (10 mL), under obtained from ESI+-MS. The positions of different peaks and
Scheme 1 Synthetic routes for the complexes 1–4.
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Fig. 1 Electronic spectra for 1–4 in CH2Cl2.
Fig. 2 Emission spectra for 1–4 in CH2Cl2.
Table 1 Electronic and emission spectral data for complexes 1–4
Absorbance
Emission
λmax/nm
Complexes
λmax/nm (ε/M−1 cm−1
)
[Cu4(μ3-Cl)4(μ-dtbpf)2] (1)
390, 288
467, 299
484, 272
374, 284
341, 417
337, 415
334, 417
338, 419
[Cu6(μ3-Br)4(μ2-Br)2(μ-dtbpf)3] (2)
[Cu4(μ2-I)2(μ3-I)2(μ-dtbpf)2] (3)
[Cu2(μ1-CN)2(κ2-dtbpf)2] (4)
overall fragmentation patterns in the ESI+-MS of the respective
complexes are consistent with their formulations. The IR spec-
trum of complex 4 exhibited a characteristic band corres-
ponding to νCuN of the bridging cyanide group at 2138 cm−1
(see F4, ESI†). However, only one νCuN band agreed well with
the presence of only one type of cyanide bridge between copper(I)
atoms,15 which is in accordance with those reported in the lit-
erature for bridging pseudohalide groups.16 1H and 31P{1H}
NMR spectral data of the complexes are presented in the
Experimental section along with the other data. In 1H NMR
spectra of the complexes 1, 2, 3 and 4, η5-C5H4 protons of the
dtbpf ligand resonated as two broad singlets in the range of δ
4.63–4.04 ppm (see F-5 ESI†). The tert-butyl protons of dtbpf
were observed as doublets at 1.32–1.36 ppm. The 31P NMR
spectra of the complexes 1–4 showed a single resonance
(δ 31.14 (1), 37.39 (2), 28.31 (3), 66.78 (4) ppm) for the dtbpf
ligand which suggested that all the phosphorus atoms were
Fig. 3 Cyclic voltammogram for the complexes 1–4 in CH2Cl2/0.1 M
[NBu4]ClO4 at 100 mV s−1 scan rate.
Table 2 Electrochemical data for 1–4 in dichloromethane solution/
0.1 M [NBu4]ClO4 at 298 K
Ep,a,1 (mV) Ep,c,1 (mV) Ep,c,2 (mV) ΔE (mV) Ip,a/Ip,c
chemically equivalent (see F-6–F-9 ESI†). These chemical shifts Complex-1 +258
+170
+320
+12
−173
—
−714
—
88
111
60
1.11
1.69
0.84
0.61
Complex-2 +431
were within the accepted range and are comparable to that of
Complex-3
Complex-4
+72
+39
the chelating dtbpf ligands.10c,17 All the complexes exhibited
two bands at 374–484 nm and 272–299 nm in dichloro-
methane solution (Fig. 1; Table 1). The lower-energy band in
the range 374–484 nm can be assigned to the d–d transition.
The higher-energy band at 259–275 nm has been assigned to
intraligand charge transfer. All the complexes upon excitation
at their respective lowest energy band maximum (374–484 nm)
were non-emissive; however, on excitation at 272–299 nm
exhibited two broad emissions at 334–341 nm and
415–419 nm, respectively (Fig. 2; Table 1). We tentatively
assigned the emission of these complexes to XLCT, CC, and
LC transitions as the dtbpf ligand does not show luminescence
in the range of 495–900 nm.
−37
76
The electrochemical redox behavior of the complexes 1–4
was studied through cyclic voltammetry (CV) at the Pt electrode
in dichloromethane solution (1 mM) containing 0.1 M tetra-
butylammonium perchlorate (TBAP) as the supporting electro-
lyte (Fig. 3). The Ag/Ag+ in acetonitrile was used as a reference
electrode. The electrochemical parameters derived from these
voltammograms (Ep,a, Ep,c, ΔE) are given in Table 2. It is to be
noted that the peak potential separation (ΔE = Ep,a − Ep,c) is
larger than the expected value for one electron transfer process
(∼60 mV, Ip,a/Ip,c = 1) for these complexes, so one can conclude
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that these complexes exhibit quasi-reversible redox behavior
coupled with some chemical reactions. In comparison with
the ferrocene standard (E° = 0.19 V), the potentials for these
complexes are anodically shifted due to the electron withdraw-
ing effect of the linker groups between the ferrocene units. In
complexes 1 and 3, single cathodic peaks are present at −173
and −714 mV, respectively.
Description of the crystal structures
Three X-ray structures emerged from the current study. Refine-
ment details for all the structures are summarized in Table 1
in the ESI,† and selected bond lengths and angles, and hydro-
gen bond parameters are presented in Tables 2 and 3 in the
ESI,† respectively. Complex 1 crystallizes in the orthorhombic
crystal system, and the structure was solved in the space group
Fddd. The molecular structure of 1 is depicted in Fig. 4.
Complex 1 reveals the tetranuclear copper(I) complex, which
produces an 18-electron configuration at each copper without
the need for any metal–metal bonds. The Cu4C14 core of the
molecule defines a rather distorted “cubane-like” framework,
with the phosphorus atoms being approximately apical,
thereby completing a pseudo-tetrahedral coordination pro-
vided by the bidentate dtbpf ligands in a κ1-manner and the
four bridging chloro groups in a μ3-manner about each Cu(I)
atom. The Cu–Cl bond lengths vary appreciably, ranging from
2.3747(16) Å to 2.5269(16) Å [Cu(1)–Cl(1)#1 = 2.4315(17) Å;
Cl(1)–Cu(1)#2 = 2.5269(16) Å; Cu(1)–Cl(1) = 2.3747(16) Å]. The
tetramer is irregular with respect to the Cu⋯Cu separations
(2.936–3.424 Å) and the Cl⋯Cl separations (3.378–3.953 Å).
The six Cu⋯Cu distances are in the range of 2.936–3.424 Å
(average value = 3.200 Å). All these distances are sufficiently
large to preclude the possibility of any significant direct
Cu⋯Cu interaction. The Cl⋯Cl distances within the [Cu4(μ3-
Cl)4(μ-dtbpf)2] molecule spread over a slightly larger range
than the Cu⋯Cu distances. Angles within the Cu4C14 cube
vary significantly from the idealized value of 90°. The Cu–Cl
and Cu–P bond distances are 2.3747(16)–2.5269(16) Å and
2.1950(17) Å, respectively, and are comparable to other copper
Fig. 5 Projection view of 2 (ellipsoids with 50% probability; hydrogen
atoms are omitted for clarity).
derivatives with ferrocenyl diphosphines.8,9,17,18 The two sub-
stituted Cp rings in the dtbpf ligand in 1 adopt the antiperi-
planar staggered conformation. Crystal packing in complex 1
is stabilized by C–H⋯Cl hydrogen bond interactions (see F-11
ESI†). The contact distances for C–H⋯Cl interactions are in
the range of 2.74–2.90 Å.
ˉ
Complex 2 crystallized in the triclinic space group P1. The
molecular structure of 2 is depicted in Fig. 5. The structure of
complex 2 exhibits a triangular copper(I) framework, where two
[Cu3(μ3-Br)2(μ2-Br)] units are bridged by three bidentate dtbpf
ligands in a κ1-manner. Each [Cu3(μ3-Br)2(μ2-Br)] unit forms a
pyramid with one copper atom at the apex and one of the tri-
angular faces capped by one bromine atom. Atoms Cu(4) and
Cu(5) are tetrahedrally coordinated to the phosphorus donor
[P(4), P(5)] of two bridging dtbpf ligands [Cu(5)–P(5) = 2.2088(12)
Å; Cu(4)–P(4) = 2.2216(13) Å; P(5)–Cu(5)–Br(4) = 113.56(4)°;
P(5)–Cu(5)–Br(6) = 124.58(4)°; P(5)–Cu(5)–Br(5) = 123.64(4)°;
P(4)–Cu(4)–Br(4) = 121.59(4)°; P(4)–Cu(4)–Br(6) = 121.15(4)°;
P(4)–Cu(4)–Br(5) = 124.34(4)°] in a κ1-manner, one double-brid-
ging bromine atom [Br(4)] and two triple-bridging bromine
atoms [Br(5) and Br(6)] whereas, the second phosphorus atom
[P(6)] of the same dtbpf donor additionally coordinates in a κ1-
manner to the Cu(6) atom in a trigonal fashion [Cu(6)–P(6) =
2.2059(13) Å; P(6)–Cu(6)–Br(5) = 128.84(4)°, P(6)–Cu(6)–Br(6) =
131.86(4)°], and two triple-bridging bromine atoms [Br(5) and
Br(6)]. The Cu(2) and Cu(3) of other [Cu3(μ3-Br)2(μ2-Br)] unit
are coordinated tetrahedrally to the phosphorus donor [P(2),
P(3)] of bridging dtbpf ligands [Cu(2)–P(2) = 2.2134(12) Å;
Fig. 4 Projection view of 1 (ellipsoids with 50% probability; hydrogen
atoms are omitted for clarity).
13624 | Dalton Trans., 2014, 43, 13620–13629
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Cu(3)–P(3) = 2.2048(13) Å; P(2)–Cu(2)–Br(3) = 121.51(4)°; P(2)–
Cu(2)–Br(1) = 119.23(4)°; P(2)–Cu(2)–Br(3) = 121.51(4)°, P(3)–
Cu(3)–Br(3) = 127.09(4)°, P(3)–Cu(3)–Br(2) = 129.12(4)°, P(3)–
Cu(3)–Br(1) = 108.39(15)°] in a κ1-manner, one double bridging
bromine atom [Br(3)] and two triple-bridging bromine atoms
[Br(1) and Br(2)], while the remaining Cu(1) atom is trigonally
bonded to the phosphorus donor [P1] of the bridging dtbpf in
a κ1-manner [Cu(1)–P(1) = 2.1998(13) Å; P(1)–Cu(1)–Br(2) =
124.71(4)°, P(1)–Cu(1)–Br(1) = 128.83(4)°, Br(2)–Cu(1)–Br(1) =
106.06(3)°] and two triple-bridging bromine atoms [Br(1) and
Br(2)]. The Cu(I) ions thus come as tetra- and tricoordinate
pairs with distorted tetrahedral or trigonal planar coordi-
nations. As expected, the Cu–P [Cu(6)–P(6) = 2.2059(13) Å,
Cu(1)–P(1) = 2.1998(13) Å] and Cu–Br [Cu(6)–Br(5) = 2.4232(7)
Å, Cu(6)–Br(6) = 2.4413(8) Å, Cu(1)–Br(1) = 2.4064(8) Å, Cu(1)–
Br(2) = 2.3884(8) Å] bond distances of the tricoordinate copper
ions are appreciably shorter than those of the tetrahedrally co-
ordinated Cu–P bond distances [Cu(4)–P(4) = 2.2216(13) Å,
Cu(5)–P(5) = 2.2088(12) Å, Cu(3)–P(3) = 2.2048(13) Å, Cu(2)–P(2) =
Fig. 6 Projection view of 3 (ellipsoids with 50% probability; hydrogen
atoms are omitted for clarity).
(plane II). The dihedral angle between planes I and II is
2.2134(12) Å] and Cu–Br [Br(4)–Cu(4) = 2.4592(8) Å, Br(5)–Cu(4) = 101.31°–102.23°, which is lower than that of [PPh3CuI]4.19
2.6900(9) Å, Br(6)–Cu(4) = 2.7471(8) Å, Br(4)–Cu(5) = 2.4740(8) Atoms Cu(1), I(1), Cu(4), and I(3) are crystallographically copla-
nar. Atoms Cu(2), I(2), Cu(1), and I(1) are not precisely copla-
Å, Br(5)–Cu(5) = 2.6883(8) Å, Br(6)–Cu(5) = 2.5669(8) Å, Br(2)–
Cu(3) = 2.4759(8) Å, Br(3)–Cu(3) = 2.4308(7) Å, Br(1)–Cu(3) = nar. The nonplanarity of this four-membered motif is
3.054(9) Å, Br(1)–Cu(2) = 2.4763(7) Å, Br(3)–Cu(2) = 2.4311(7) Å, conveniently described in terms of the dihedral angles of
135.09° [between I(2)–Cu(2)–I(1) and I(2)–Cu(1)–I(2)] and
Br(2)–Cu(2) = 3.027(7) Å]. Such types of copper(I) complexes
involving tetrahedral and trigonal chromophores and both 133.27° [between Cu(l)–I(2)–Cu(2) and Cu(l)–I(1)–Cu(2)]. Atoms
triple- and double-bridging halides have been found in the tetra- in the step-like Cu4I4 core of the [Cu4(μ2-I)2(μ3-I)2(μ-dtbpf)2]
nuclear ‘step’ [{Cu(PPh3)X}4],18b,19 [{(CuX)2(dppm)}2] (X = I,
molecule have variable coordination numbers. Thus, Cu(1)
Br or Cl)20 and ‘three-runged ladder’ [Cu3(μ3-X)(μ-X)2(μ-dpmt)2] and Cu(2) are of trigonal coordination (CN = 3), while Cu(3)
(X = I, Br, Cl), where dpmt = 2,5-bis[(diphenylphosphino)- and Cu(4) each have an essentially tetrahedral coordination
methyl]thiophene.21 The structure of complex 2 has some
environment (CN = 4). Atoms I(2) and I(4) have an angular geo-
similarities with these open ‘step’ molecules, but differ from metry (CN = 2), while I(1) and I(3) are in pyramidal sites (CN =
them by the virtual planarity of the Cu3X3 framework. The 3). As was previously seen for the Cu4I4 core by Churchill
et al.19 the copper–halogen distances show significant differ-
copper–copper distances are in the range of 2.6980(8) Å–
2.9789(8) Å in 2, shorter than those found in ‘cubane’ ences and increase systematically as the CN of the component
[Cu4X4(PPh3)4] (X = I, Br or Cl) [2.874(5)–3.164(4) Å]18a,b,22 and atoms increases. In the present case, we have observed a
‘step’ [Cu4X4(PPh3)4] (X = I or Br) [2.835(3)–3.448(3) Å]18b,19 and
similar pattern. The four copper atoms are exactly coplanar
are comparable to those found in ‘step’ [{(CuX)2(dppm)}2] (X = and show a parallelogram. These distances in the “Cu4 paralle-
I, Br or Cl) [2.682(7)–3.180(2) Å],22 indicating that there are no logram” are comparable to the reported values. The four
iodine atoms of [Cu4(μ2-I)2(μ3-I)2(μ-dtbpf)2] are precisely copla-
direct Cu⋯Cu interactions. The sum of all bonding angles of
359.8° and 343.2° for the tricoordinate and tetracoordinate nar. Individual I⋯I distances show considerably less variation
copper ions in 2 indicates pyramid formation. The two substi- than do the Cu⋯Cu distances; these are I(1)⋯I(2) = I(3)⋯I(4) =
4.251 Å, I(2)⋯I(3) = I(1)⋯I(4) = 4.115 Å, and I(1)⋯I(3) =
tuted Cp rings in the dtbpf ligand in 2 adopt the antiperipla-
nar staggered conformation as well. Crystal packing in 3.974 Å. These distances are within the range reported for iodo
complex 2 is stabilized by C–H⋯Br hydrogen bond inter- derivatives of phosphine complexes.18,23 Angles within the
four-membered rings defined by Cu(2)–I(2)–Cu(1)–I(1) and Cu(3)–
I(4)–Cu(4)–I(3) vary grossly from 90°. Angles at the copper
actions (see F-15 ESI†). The contact distances for C–H⋯Br
interactions are in the range of 2.90–3.14 Å.
Complex 3 is crystallized in the orthorhombic space group atoms are close to the ideal trigonal or tetrahedral values [I(2)–
Cu(2)–I(1) = 111.9(2)°, I(1)–Cu(1)–I(2) = 102.30(2)°, I(3)–Cu(4)–
Pca21. The molecular structure of 3 is depicted in Fig. 6.
Complex 3 shows the tetranuclear copper(I) complex, which I(4) = 100.62(19)°, and I(4)–Cu(3)–I(3) = 113.0(2)°, respectively]
produces an 18-electron configuration at each copper without while angles at the iodine atoms are quite acute [Cu(2)–I(2)–
Cu(1) = 63.14(18)°, Cu(2)–I(1)–Cu(1) = 60.90(17)°, Cu(3)–I(3)–
the need for any metal–metal bonds. The Cu4I4 core for this
molecule has a step structure, rather than a cubane-like con- Cu(4) = 60.30(17)°, Cu(3)–I(4)–Cu(4) = 63.70(18)°]. Angles between
figuration. The step structure of [Cu4(μ2-I)2(μ3-I)2(μ-dtbpf)2] is atoms in different planes of the step are not dependent upon
the nature of the central atom. Small angles at the iodine
defined by three four-membered systems: Cu(1)–I(l)–Cu(2)–I(2)
[plane I], Cu(1)–I(1)–Cu(4)–I(3), and Cu(3)–I(3)–Cu(4)–I(4) atoms are common to species with a Cu4I4 core, irrespective of
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the gross geometry of the molecule. The Cu–I–Cu angles are Catalytic performances in Sonogashira alkynylationreactions
comparable to those in the other similarly related complexes
with step and cubane-like structures. The reason for small Cu–
I–Cu angles could be due to van der Waals repulsions between
the halogen atoms. The Cu(trigonal)–P distance of 2.176(12)
The comparative catalytic activities of the CuI and the newly
synthesized copper(I) complexes were investigated in Sonoga-
shira alkynylation of activated and non-activated aryl halides
according to the previously established method developed by
Å–2.252(12) Å is slightly shorter than the Cu(tetrahedral)–P dis-
Hierso et al.24 The comparative catalytic performances for the
tance of 2.230(12) Å–2.279(11) Å. The two substituted Cp rings
coupling of the activated and non-activated aryl halides with
phenylacetylene are presented in Table 3. It can be seen that,
without palladium, the system incorporating only either
in the dtbpf ligand in 3 adopt the antiperiplanar staggered
conformation. Crystal packing in complex 3 is stabilized by
C–H⋯I hydrogen bond interactions (see F-19 ESI†). The
complex 1–4 or CuI was inefficient (Table 3, entries 1 & 2). We
contact distances for C–H⋯I interactions are in the range of
have also found that no reaction occurred in the absence of a
copper source and dtbpf (Table 3, entry 3). As shown in
3.00–3.16 Å.
Table 3, we isolated 10–11% yield of the coupling product in
the presence of [Pd(allyl)Cl]2/dtbpf (Table 3, entry 4) with no
formation of homocoupling products under these conditions
Powder X-ray diffraction and thermal gravimetric analysis
indicating that dtbpf and the copper source are crucial to the
cross-coupling reaction. Actually, as shown in Table 3, we
observed a difference in catalytic activity, that is, complexes
1–4 exhibited higher catalytic activity compared to CuI. In the
case of coupling between bromobenzene and phenylacetylene
in the presence of either 0.4 mol% of CuI or CuI/dtbpf and
0.2 mol% of [Pd(allyl)Cl]2, the isolated yield of the alkynylation
product was 20–25%, while alkyne dimerization products were
formed in appreciable amounts (entries 5 & 6). So, the use of
complexes 1–4 in the presence of [Pd(allyl)Cl]2 was more
efficient for this coupling. The use of the complexes 1–4 con-
firmed that the pre-stabilization of copper(I) structures with a
bidentate ferrocenyl phosphine ligand promotes the palladium
cross-coupling of activated and non-activated aryl halides with
phenylacetylene. The desired coupled product in 60–95% yield
was obtained (75–85%, 75–95% and 60–70%, respectively,
All the four complexes (1, 2, 3 and 4) were further studied by
powder X-ray diffraction (PXRD) (see F-22 to F-25 ESI†) and
thermogravimetric analysis (TGA) (see F-26 to F-29 ESI†).
There is a good agreement between the experimental and
calculated powder XRD patterns for the complexes 1, 2, and 3,
thus supporting phase purity. The thermogravimetric (TG)
curves over 30–800 °C indicates that in complex 1, weight loss
takes place in four close and consecutive steps from 30 °C to
784 °C. Weight loss occurred in two steps in complex 2. The
first weight loss is between 30 °C and 425 °C whereas the
second weight loss is finished around 700 °C. There are two
weight loss steps in complex 3. The first one is from 32 °C to
533 °C, the second one is finished around 600 °C. Complex 4
is stable up to ∼250 °C but after that there is a slight weight
increase till 280 °C. Following this, there is a weight loss from
410 °C to 745 °C.
Table 3 Sonogashira coupling of activated and non-activated aryl halides with alkynes RCuCHa in the presence of complexes 1–4
Alkynylation
product yieldb [%]
Alkyne dimerization yieldb [%]
Entry
R
X
Copper source
Palladium source ([Pd(allyl)Cl]2)
1
2
3
4
1
2
3
4
1.
2.
3.
4.
5.
6.
7.
8.
H
H
H
H
H
H
H
H
Br
Br
Br
Br
Br
Br
Br
I
0.1 mol% complexes 1–4
0.4 mol% CuI
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
No
50
60
Traces
No
25
0
0
0
No
50
60
Traces
No
20
0
0
0
No
50
60
Traces
No
25
0
0
0
No
50
60
Traces
No
30
0.2 mol%
0.2 mol%/dtbpf
0.2 mol%
0.2 mol%/dtbpf
0.2 mol%
0.2 mol%
0.2 mol%
0.2 mol%
0.2 mol%
0.2 mol%
10
20
25
70
85
60
68
85
62
10
20
25
78
86
66
75
88
62
11
20
25
85
95
70
84
99
75
10
21
25
70
75
61
72
77
58
0.4 mol% CuI
0.4 mol% CuI
0.1 mol% complexes 1–4
0.1 mol% complexes 1–4
0.1 mol% complexes 1–4
0.1 mol% complexes 1–4
0.1 mol% complexes 1–4
0.1 mol% complexes 1–4
9.
H
Cl
Br
I
10.
11.
12.
CH3
CH3
CH3
Traces
No
Traces
Traces
No
Traces
Traces
No
Traces
Traces
No
Traces
Cl
a All reactions were carried out using 3.38 × 10−3 mol activated and non-activated aryl halides, phenylacetylene (2 equivalent), K2CO3
(2 equivalent), 0.1 mol% of the complexes 1–4/0.4 mol% of CuI with 0.2 mol% of [Pd(allyl)Cl]2 and the catalytic system stirred at 120 °C over 20 h
in 10 mL of DMF under nitrogen. b Isolated yield.
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Scheme 2 The generalized results of Pd/Cu ligand exchange in complexes 1–4.
Table 3 entries 7, 8, and 9) by employing 0.1 mol% of 1 to 4, conducted with the complexes 2–4 in CDCl3. Analogous behav-
0.2 mol% of the palladium source, and the non-activated iour was observed with the complexes 2–4 in CDCl3 (Fig. F32–
bromo, iodo or chlorobenzene. Under the same catalytic con- F34†), showing the total transfer of the ligand from copper to
ditions the electron-rich 4-bromotoluene, 4-iodotoluene and palladium (see F-35 to F-37 ESI†). Similar type of finding was
4-chlorotoluene were efficiently coupled resulting in yields of observed in copper(I) iodide triphosphine adducts used at low
over 58–99% (68–84%, 77–99% and 58–75%, respectively, loading for Sonogashira alkynylation of demanding halide
Table 3 entries 10, 11, and 12). The catalytic activities of substrates, for which a ligand exchange study between copper
various copper(I) complexes such as [Cu4(μ3-SCN)4(κ1-P,P- and palladium have been reported.24 The generalized results
dtbpf)2],11
[Cu4(μ3-Cl)4(μ-dtbpf)2],
[Cu6(μ3-Br)4(μ2- of Pd/Cu ligand exchange are summarized in Scheme 2. The
Br)2(μ-dtbpf)3], [Cu4(μ2-I)2(μ3-I)2(μ-dtbpf)2] and [Cu2(μ1-CN)2(κ2- ligand transfer from the complexes 1–4 might take place due
dtbpf)2] were tested in Sonogashira alkynylation reactions. to the relatively soft nature of the Pd(II) precursor which results
Among them, [Cu4(μ3-SCN)4(κ1-P,P-dtbpf)2] and [Cu4(μ2-I)2(μ3- in the transmetallation reaction to form [PdCl(μ1-C3H5)(κ2-P,P-
I)2(μ-dtbpf)2] are more efficient copper sources as compared to dtbpf)] (see F-38 to F-39 ESI†) when the palladium precursor is
others in catalyzing this reaction.
simply mixed.
Interactions of copper(I) complexes 1–4 with palladium(II)
The stability of all the complexes 1–4 were checked by 31P and
Conclusions
1H NMR experiments with palladium allyl chloride during the
course of tandem catalytic reaction. Upon mixing the complex We have synthesized and structurally characterized a series of
1 with 0.5 equivalent of [Pd(allyl)Cl]2 and heating for 2 h at copper(I) complexes [Cu4(μ3-Cl)4(μ-dtbpf)2] (1), [Cu6(μ3-Br)4(μ2-
120 °C, a portion of the dtbpf ligand is transferred to palla- Br)2(μ-dtbpf)3] (2), [Cu4(μ2-I)2(μ3-I)2(μ-dtbpf)2] (3), and [Cu2(μ1-
dium, giving rise to the palladium complex [PdCl(μ1-C3H5)(κ2- CN)2(κ2-dtbpf)2] (4) in both the solid state and solution. The
P,P-dtbpf)], clearly characterized in the 31P NMR by two sharp interesting features of complexes 1 and 2 are Cu(I) is pseudo-
singlets at 31.14 ppm (complex 1) and 57.27 ppm (palladium cubane with the Cu4Cl4 core and stepped cubane-like structure
complex) of equal intensity (see F-30 ESI†). Further heating the with the Cu4I4 core, whereas complex 3 shows triangular
mixture for 4 h at 120 °C did not modify the equilibrium. copper(I) frameworks, where each [Cu3(μ3-Br)2(μ2-Br)] unit
Upon heating the mixture for 20 h at 120 °C, the total transfer forms a pyramid with one copper atom at the apex and one of
of the ligand from copper to palladium centre was observed in the triangular faces capped by one bromine atom. The newly
complex 1 (see F-31 ESI†). The same kind of experiments was synthesized complexes were successfully used in the Sonoga-
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shira cross-coupling of activated and non-activated aryl halides
with phenylacetylene. While all the complexes screened were
found to be catalytically active, the best results were obtained
for the complexes [Cu4(μ3-SCN)4(κ1-P,P-dtbpf)2] and [Cu4(μ2-
I)2(μ3-I)2(μ-dtbpf)2] in comparison with CuI. The 31P NMR
studies show that complexes 1–4 are unstable during the
course of tandem catalytic reaction and the dtbpf ligand
migrates to palladium(II) from copper(I), which promotes palla-
dium Sonogashira cross-coupling of activated and non-acti-
vated aryl halides. Further studies towards the development of
new copper(I) complexes containing various chiral and achiral
ferrocene phosphine ligands and their catalytic properties are
in progress in our laboratory.
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Acknowledgements
Thanks are due to the Department of Science and Technology,
New Delhi for financial assistance (grant no. SR/FT/CS-104/
2011). We also thank the Director, USIC, and Dr R. Nagarajan,
University of Delhi, Delhi, for providing analytical, spectral,
PXRD and Single Crystal X-ray Diffraction facilities and GC-MS
facility at the AIRF center of JNU, New Delhi. 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 the funding from the
National Science Foundation (CHE0420497) for the purchase
of the APEX II diffractometer.
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