Recyclable scavengers for photo-cyclopropanation via an in situ crystallization process

Article abstract of DOI:10.1039/c6dt02604d

The palladium(ii) cyclophane systems, constructed by previously reported proof-of-concept self-assembly, represent a crucial landmark in the field of effective and recyclable scavenging of triiodide (I₃^?) in the photo-cyclopropanation of alkenes with CH₂I₂. The scavenger's driving force behind photo-cyclopropanation is the efficient in situ crystallization of triiodide-exchanged species. The exact quantitative photoreaction yields according to the mole ratios of the cyclophane system are impressive. The recycling behavior can be ascribed to the rigidity and stability of the four-layered tripalladium(ii)cyclophane.

Full text of DOI:10.1039/c6dt02604d

Dalton
Transactions
View Article Online
View Journal
PAPER
Recyclable scavengers for photo-cyclopropanation
via an in situ crystallization process†
Cite this: DOI: 10.1039/c6dt02604d
Haeri Lee, Eunkyung Choi, Tae Hwan Noh and Ok-Sang Jung*
The palladium(II) cyclophane systems, constructed by previously reported proof-of-concept self-assembly,
represent a crucial landmark in the field of effective and recyclable scavenging of triiodide (I3−) in the
photo-cyclopropanation of alkenes with CH2I2. The scavenger’s driving force behind photo-cyclopropana-
tion is the efficient in situ crystallization of triiodide-exchanged species. The exact quantitative photoreac-
tion yields according to the mole ratios of the cyclophane system are impressive. The recycling behavior
can be ascribed to the rigidity and stability of the four-layered tripalladium(II)cyclophane.
Received 30th June 2016,
Accepted 19th September 2016
DOI: 10.1039/c6dt02604d
www.rsc.org/dalton
throids and a number of quinolone based antibiotics (cipro-
floxacin, sparfloxacin).13–18
Introduction
The idea of an efficient reaction strategy via an effective sca-
Recently, an intriguing new palladium(II)cyclophane-based
venger has intrigued synthetic chemists over the last molecular system was formulated in our laboratory19,20 via a
decade.1–4 In synthetic chemistry, a scavenger is a fascinating suitable combination of interligand transannular π⋯π inter-
chemical substance that eliminates undesirable products in a actions and a square planar palladium(II) geometry; this
wide range of task-specific chemical reactions.5 Any novel system might prove integral to donor–acceptor electron trans-
molecular architecture’s scavenging role is a prerequisite to fer processes, through-space effects, molecular strain, topo-
the understanding and control of forward reactions’ driving logy, helicity, and binding selectivity. Research on the thermo-
forces, in particular, equilibrium reactions. For instance, dynamics and kinetics of such a metallacyclophane system
photo-cyclopropanation and the Simmons–Smith reaction would certainly provide important insights into the evolution
using CH2I2 produce iodide/triiodide (I−/I3−) as byproducts.6 and emergence of delicate molecular applications,21–26 and
Thus, the introduction of the aqueous solutions of sodium thereby, would establish a basis for the relationships between
thiosulfate, sodium bicarbonate, or tin metal as scavengers physicochemical phenomena and molecular structures too.27
into the reaction solution is an essential factor for obtaining
The comprehensive scavenging procedure for photo-cyclo-
exclusive products.7–10 Reactions without scavengers do not propanation using CH2I2 remains unexplored. In this context,
give rise to desirable cyclopropane products, but instead to a in order to understand the detailed operation principle of the
mixture of alkene isomers and exchanged halomethane scavengers behind photo-cyclopropanation, highly specific and
−
species.3 Additionally, I−/I3 species have been employed as focused research is needed on the quantitative change process
useful redox mediators for regenerative dye-sensitized solar of the unique metallacyclophane module during the photo-
cells (DSSCs).11,12 Thus, information on the recognition and reaction. This paper reports advanced proof-of-concept experi-
−
abstraction of the I−/I3 chemical species is a significant ments on a recyclable and effective triiodide scavenger for
specific process. Furthermore, cyclopropanation reactions are photo-cyclopropanation of alkenes with CH2I2 in the presence
important chemical processes in the fields of drug synthesis, of a four-layered tripalladium(II)cyclophane molecular system
ring expansion, and electrophilic addition, as cyclopropane via efficient in situ crystallization. This molecular module, sig-
moieties contribute to many useful compounds such as pyre- nificantly, represents the most efficient and perfect recyclable
triiodide scavenger for photo-cyclopropanation (Scheme 1).
Department of Chemistry and Chemistry Institute for Functional Materials,
Experimental
Materials and measurements
Pusan National University, Busan 46241, Korea. E-mail: oksjung@pusan.ac.kr
†Electronic supplementary information (ESI) available: HR-ESI-TOF-MS data;
NMR spectra; UV spectra; SEM/EDS data; powder X-ray diffraction results. CCDC
All chemicals and solvents employed in the syntheses were pur-
chased from Aldrich, and were used without further purification.
1453026, 1453027 and 1457749–1457752. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c6dt02604d
This journal is © The Royal Society of Chemistry 2016
Dalton Trans.

View Article Online
Paper
Dalton Transactions
Scheme 1 Stepwise synthetic procedure and recyclable photoreaction scavengers for four-layered tripalladium(II)cyclophanes (PdCl2, yellow;
Pd(NO3)2, red; L1, black; L2, orange; L3, green).
N,N′,N″-Tris(3-pyridinylpropyl)-1,3,5-benzenetricarboxylate (L2) gave a pure product (2.21 g, 84%). Mp 96 °C. Elemental
and N,N′,N″-tris(3-(1-imidazolyl)propyl)-1,3,5-benzenetricarbox- analysis: found: C, 68.50; H, 5.07; N, 8.03. Calc. for
1
amide (L3) were synthesized according to the procedure avail- C30H27N3O6: C, 68.56; H, 5.18; N, 8.00% δ H NMR (300 MHz;
3
able in the literature.19,20 1H (300 MHz) and 13C (75 MHz) Me2SO-d6) 8.50 (3H, d, J = 4.70 Hz, py–H), 8.44 (3H, s, Ar–H),
NMR spectra for ligand characterization and substitution 7.71 (3H, t, 3J = 7.63 Hz, py–H), 7.34 (3H, d, 3J = 7.63 Hz, py–H),
reactions were recorded on a Varian Mercury Plus 300. 7.23 (3H, dd, 3J = 4.70 Hz, 7.63 Hz, py–H), 4.68 (6H, t, 3J = 6.46
1H (500 MHz) and 13C (125 MHz) NMR spectra for new tripalla- Hz, –CH2–), 3.20 (6H, t, 3J = 6.46 Hz, –CH2–). δ 13C NMR
dium(II)cyclophanes were recorded on a Varian USA Unity- (75 MHz; Me2SO-d6) 163.89, 157.61, 149.17, 136.50, 133.33,
Inova. Infrared spectra were obtained on a Nicolet 380 FT-IR 130.97, 123.39, 121.76, 64.55, 36.38. νmax/cm−1 1726vs (CO2R),
spectrophotometer using samples prepared as KBr pellets. 1591s, 1473m, 1437m, 1242s, 993m, 741m.
Raman spectra were obtained on a FT-Raman Module with
Synthesis of [Pd3Cl6(L1)2]·6MeOH (1(Cl)6)
A methanol solution (10 mL) of L1 (21 mg, 0.04 mmol) was
adaptation (Nd:YAG laser, 1000 mW) at Pusan Center, KBSI.
Thermal analyses were performed under N2 at a scan rate of
10 °C min−1 using a Labsys TG/DSC-1600. Electrospray ioniza-
slowly diffused into an N,N-dimethylformamide solution
tion time of flight mass spectrometry (ESI-TOF-MS) was per-
formed on a Synapt G2 mass spectrometer (Waters) at Ochang
Center, KBSI. Samples were injected as a mixed Me2SO and
acetonitrile solution (v/v = 1 : 250). Elemental analyses were
performed on crystalline samples by the KBSI Pusan Center
using a Vario-EL III analyzer. SEM images and EDX were
obtained on a Tescan VEGA 3.
(5 mL) of K2PdCl4 (20 mg, 0.06 mmol). Yellow block crystals of
1(Cl)6 were obtained (29 mg, 74%). Mp 364 °C dec. Elemental
analysis: found: C, 44.90; H, 4.50; N, 4.76. Calc. for C60H54Cl6–
N6O12Pd3·6CH3OH: C, 44.65; H, 4.43; N, 4.73% δ 1H NMR
3
(300 MHz; Me2SO-d6) 9.25 (6H, d, J = 4.89 Hz), 8.46 (6H, s),
3
8.01 (6H, t, 3J = 7.04 Hz), 7.75 (6H, d, J = 8.22 Hz), 7.56 (6H, t,
3J = 7.04 Hz), 5.01 (12H, m), 4.47 (12H, m). νmax/cm−1 3077br,
3033br, 2960w, 1730vs (CO2R), 1670m, 1606m, 1488m, 1438m,
1243s, 993m.
Synthesis of N,N′,N″-tris(2-pyridinylethyl)-1,3,5-
benzenetricarboxylate (L1)
Synthesis of [Pd3(NO3)6(L1)2]·3H2O (1(NO3)6)
Triethylamine (2.30 mL, 16.5 mmol) was slowly added into a
stirred mixture of 2-pyridineethanol (1.69 mL, 15.0 mmol) and Method 1. A dimethyl sulfoxide (Me2SO) solution of L1
1,3,5-benzenetricarbonyl trichloride (1.35 g, 5.0 mmol) in (0.04 mmol, 21 mg) was drop-wise added to a Me2SO solution
chloroform (100 mL) at 60 °C. The reaction mixture was of Pd(NO3)2 (0.06 mmol, 14 mg) at 70 °C. After vigorous stir-
refluxed for 24 h. The solution was filtered, and then the fil- ring and maintaining the same temperature for 5 h, the solu-
trate was washed with 0.5 N aqueous NaOH solution several tion was filtered, after which ethyl acetate was added. Yellow
times. The chloroform layer was dried over anhydrous mag- block crystals suitable for X-ray crystallography were obtained
nesium sulfate and filtered. Evaporation of the chloroform in 6 days (27 mg, 69%). Method 2. An aqueous solution of
Dalton Trans.
This journal is © The Royal Society of Chemistry 2016

View Article Online
Dalton Transactions
Paper
AgNO3 (0.12 mmol, 20 mg) was slowly added to a mixed analysis: found: C, 43.42; H, 5.37; N, 10.97. Calc. for
solution of H2O and acetone (Me2CO) (v/v = 1 : 1) in which 1
C114H120N30O36Pd3·12C2H6–SO·4H2O: C, 43.44; H, 5.28;
(Cl)6 (0.02 mmol, 39 mg) was dispersed. The solution was N, 11.01% δ 1H NMR (500 MHz, Me2SO-d6) 9.91 (6H, d, 3J =
3
stirred at 60 °C for 1 h, and then the AgCl was filtered off 5.38 Hz), 8.29 (6H, t, J = 7.83 Hz), 8.27 (6H, s), 8.20 (6H, s),
through 0.22 μm nylon (PA) syringe filters. The remnant 1 8.08 (6H, d, 3J = 7.83 Hz), 7.91 (6H, s), 7.84 (6H, t, 3J = 5.87 Hz),
(NO3)6 solution was evaporated using a rotary evaporator, and 7.71 (6H, s), 7.50 (6H, s), 6.84 (6H, s), 5.09 (6H, m), 4.37 (12H,
then re-dissolved in Me2SO. Ethyl acetate was slowly diffused m), 4.12 (6H, m), 4.01 (12H, m), 2.94 (6H, m), 2.76 (6H, m),
into the resulting solution to obtain yellow block crystals suit- 1.95 (6H, m). νmax/cm−1 3430br, 1722vs (CO2R), 1652vs
able for X-ray crystallography in 6 days (26 mg, 64%). Mp 304 °C (CONHR), 1540s, 1384vs (NO3−), 1245s, 1108m, 1022m.
dec. Elemental analysis: found: C, 39.92; H, 3.59; N, 8.66. Calc. ESI-TOF-HR-MS m/z, found: 1339.2925. Calc. for [3·(NO3)6
for C60H54N12O30Pd3·3C2H6SO: C, 40.10; H, 3.67; N, 8.50% 2NO3−]2+: 1339.2915, found: 872.1980. Calc. for [3·(NO3)6
δ 1H NMR (300 MHz, Me2SO-d6) 9.55 (6H, m), 8.47 (6H, m), 8.28 3NO3−]3+: 872.1980.
−
−
(6H, m), 7.96 (6H, m), 7.84 (6H, m), 4.90 (12H, m), 4.43 (12H,
Synthesis of [Pd3(L1)(L3)3](NO3)6·40H2O (4·(NO3)6)
A mixture of L1 (0.02 mmol, 10 mg) and L3 (0.06 mmol, 32 mg)
m). νmax/cm−1 3438br, 1727vs (CO2R), 1608m, 1572s, 1502m,
1440m, 1384vs (NO3−), 1244s, 995m, 968m, 771w, 739w.
in a H2O/acetone solution (v/v = 1 : 1) was drop-wise added to a
H2O/acetone solution (v/v = 1 : 1) of Pd(NO3)2 (0.06 mmol,
Synthesis of [Pd3(L1)2(L2)2](NO3)6·10Me2SO·12H2O (2·(NO3)6)
Method 1. A Me2SO solution of mixed L1 (0.04 mmol, 21 mg) 14 mg) at 60 °C. After vigorous stirring at 60 °C for 5 h, the
and L2 (0.04 mmol, 23 mg) was drop-wise added to a Me2SO solution was slowly evaporated. Colorless block crystals suit-
solution of Pd(NO3)2 (0.06 mmol, 14 mg) at 70 °C. After vigor- able for X-ray crystallography were obtained in 5 days (51 mg,
ous stirring and maintaining the same temperature for 5 h, 72%). Mp 394 °C dec. Elemental analysis: found: C, 37.20;
ethyl acetate was slowly diffused into the resulting solution. H, 5.74; N, 14.10. Calc. for C111H126N36O33Pd3·40H2O: C, 37.74;
1
Colorless block crystals suitable for X-ray crystallography were H, 5.88; N, 14.28% δ H NMR (300 MHz, Me2SO-d6) 9.76 (3H,
obtained in 5 days (62 mg, 77%). Method 2. An aqueous solu- d, 3J = 5.28 Hz), 9. 07 (3H, s), 8.73 (3H, s), 8.64 (3H, s), 8.32
tion of AgNO3 (0.12 mmol, 20.2 mg) was slowly added to a (3H, s), 8.21 (3H, t, 3J = 8.22 Hz), 8.11 (3H, s), 8.06 (3H, s), 8.02
mixed solution of H2O and Me2CO (v/v = 6 : 1) in which 1(Cl)6 (3H, s), 7.88 (3H, s), 7.85 (3H, t, 3J = 5.87 Hz), 7.74 (3H, s), 7.62
(0.02 mmol, 39 mg) was dispersed. The solution was stirred at (3H, s), 7.53 (9H, s), 7.46 (6H, m), 7.14 (3H, s), 7.06 (3H, s),
60 °C for 1 h, and then the AgCl was filtered off through 4.96 (3H, m), 4.76 (6H, m), 4.48 (3H, m), 4.25 (3H, m) 3.93
0.22 μm nylon (PA) syringe filters. The remnant 1(NO3)6 solu- (18H, m), 3.17 (9H, m), 2.76 (6H, m), 2.27 (9H, m), 1.84 (12H,
tion was evaporated using a rotary evaporator and then redis- m). νmax/cm−1 1726vs (CO2R), 1652vs (CONHR), 1589s, 1570m,
solved in Me2SO. The resulting solution, after the addition of 1473m, 1437m, 1386vs (NO3−), 1342m, 1242s, 987m.
L2 (0.04 mmol, 23 mg), was stirred at 70 °C for 5 h. Following ESI-TOF-HR-MS m/z, found: 1342.3391. Calc. for [4·(NO3)6
−
−
−
slow diffusion of ethyl acetate into the resulting solution, 2NO3−]2+: 1342.3313, found: 874.2275. Calc. for [4·(NO3)6
− 3+
yellow crystals suitable for X-ray crystallography were obtained 3NO3
in 4 days (55 mg, 68%). Mp 397 °C dec. Elemental analysis: 4NO3
found: C, 45.14; H, 5.24; N, 6.34. Calc. for C126H120N18–
]
874.2247, found: 640.1753. Calc. for [4·(NO3)6
]
− 4+ 640.1714.
Photo-cyclopropanation of 1-methylcyclohexene with
diiodomethane in the presence of 2·(NO3)6
O42Pd3·10C2H6SO·12H2O: C, 45.25; H, 5.31; N, 6.51%
1H NMR (500 MHz, Me2SO-d6) 10.02 (6H, d, 3J = 5.87 Hz),
δ
9.45 (6H, s), 9.40 (6H, d, 3J = 5.38 Hz), 8.18 (6H, t, 3J = 7.84 Hz), A crystalline solid of 2·(NO3)6 (4.2 μmol, 16.9 mg) was added to
8.16 (6H, s), 8.06 (12H, m), 7.96 (6H, s), 7.84 (6H, t, 3J = a chloroform solution of a 1-methylcyclohexene (25.0 μmol,
6.85 Hz), 7.80 (6H, t, 3J = 7.34 Hz), 5.52 (6H, m), 4.76 (12H, m), 2.96 μL) and CH2I2 (37.5 μmol, 3.02 μL) mixture. The resulting
4.69 (6H, m), 4.24 (6H, m), 4.09 (6H, m), 2.99 (6H, m), 2.81 solution was placed in an LZC-4 V photoreactor and exposed
(6H, m), 2.34 (6H, m), 1.99 (6H, m). δ 13C NMR (125 MHz, to 350 nm UV-radiation at ambient temperature for the pro-
1
Me2SO-d6) 165.17, 163.94, 157.96, 151.83, 151.50, 148.44, posed time. The yields were monitored by using the H NMR
142.45, 141.56, 134.50, 132.83, 131.33, 130.63, 128.48, 128.05, spectra. As the reaction proceeded, the crystalline solid
126.64, 66.24, 65.79, 40.86, 38.64, 29.63. νmax/cm−1 3558br, was predominantly recrystallized as red block crystals of
3428br, 3033br, 2958br, 1726vs (CO2R), 1384vs (NO3−), 1243s. [Pd3(L1)2(L2)2](I3)3·(NO3)3·10CHCl3 (2·(I3)3·(NO3)3). The as-
ESI-TOF-HR-MS m/z, found: 1375.2598. Calc. for [2·(NO3)6
2NO3−]2+: 1375.3578, found: 896.1780. Calc. for [2·(NO3)6
3NO3−]3+: 896.1750.
−
−
synthesized in situ crystals are highly moisture- and air-
sensitive. Additional photoreactions at various mole ratios
(1-methylcyclohexene : CH2I2 : 2·(NO3)6
= 6 : 9 : 0–7) were
attempted. For the recyclability experiments, anion exchange
of 2·(I3)3·(NO3)3 in H2O/Me2CO (v/v = 1 : 1) with AgNO3 was per-
Synthesis of [Pd3(L1)2(L3)2](NO3)6·12Me2SO·4H2O (3·(NO3)6)
3·(NO3)6 was obtained in the same manner as the above formed. The resulting pale yellow 2·(NO3)6 solid was employed
2·(NO3)6, except for using L3 instead of L2. Pale yellow in the next photoreaction of 1-methylcyclohexene with CH2I2.
block crystals suitable for X-ray crystallography were For 2·(I3)3(NO3)3, Mp 472 °C dec. Elemental analysis: found:
obtained in 3 days (68 mg, 82%). Mp 392 °C dec. Elemental C, 32.20; H, 2.17; N, 3.97. Calc. for C126H120N15O33Pd3I9·10CHCl3:
This journal is © The Royal Society of Chemistry 2016
Dalton Trans.

View Article Online
Paper
Dalton Transactions
C, 32.49; H, 2.61; N, 4.18% δ 1H NMR (300 MHz, Me2SO-d6) tridentate N-donors is depicted in Scheme 1. The key driving
10.02 (6H, d, 3J = 5.87 Hz), 9.45 (6H, s), 9.40 (6H, d, 3J = force behind this proof-of-concept experiment was the innate
5.87 Hz), 8.18 (6H, m), 8.15 (6H, s), 8.06 (12H, m), 7.96 (6H, s), transannular π⋯π interactions between the central phenyl
7.83 (12H, m), 5.51 (6H, m), 4.74 (18H, m), 4.20 (6H, m), 4.09 moieties, showing thermodynamic stability. The practical for-
(6H, m), 2.99 (6H, m), 2.80 (6H, m), 2.34 (6H, m), 1.99 (6H, m). mation of 4·(NO3)6 containing an inner-long ligand (L3) can be
νmax/cm−1 1716vs (CO2R), 1606m, 1487m, 1440m, 1385vs ascribed to the slightly stronger Lewis basicity of the imidazo-
(NO3−), 1334m, 1246s, 1113m, 1024m, 988m, 738w, 719w. lyl group than that of the pyridyl moiety.28 The formation of
ESI-TOF-HR-MS m/z, found: 1327.2427. Calc. for [2·(I3)3·(NO3)3 the final products provided strong evidence that the present
− 2L2 − 2NO3−]2+: 1328.1557, found: [2·(I3)3·(NO3)3 − 3NO3−]3+: coordination-driven self-assembly via either a direct reaction
1214.9011, found: 802.0461. Calc. for [2·(I3)3·(NO3)3 − 2L2
3NO3− − Pd2+ + 2H+]3+: 801.7862.
−
or a stepwise reaction is an efficient method for the construc-
tion of desirable hetero tripalladium(II) cyclophanes.
All of the products including 1(Cl)6 and 1(NO3)6 were iso-
lated as single crystals suitable for X-ray single crystallography.
The nitrate complexes were soluble in dimethyl sulfoxide
(Me2SO), N,N-dimethylformamide, and a mixture of H2O and
acetone, but were insoluble in common organic solvents such
as chloroform, tetrahydrofuran, and toluene. Their thermo-
gravimetric analyses indicated that 2·(NO3)6, 3·(NO3)6, and
4·(NO3)6 are thermally stable up to 397, 392, and 394 °C,
respectively, and that solvate molecules evaporate in the
40–210 °C temperature range. The tripalladium(II) cyclo-
phanes’ high-temperature collapse can be attributed to the
strong triple π⋯π interactions. The tripalladium(II) cyclophane
skeletons along with the respective ligand species were
detected in the high resolution-ESI-TOF mass data (Fig. S1†).
For 2·(NO3)6, peaks at 896.1780 and 1375.2598, assigned to
[2·(NO3)3]3+ and [2·(NO3)4]2+, respectively, were observed. In the
case of 3·(NO3)6, the peaks were detected at 872.1980 and
1339.2925 and assigned to [3·(NO3)3]3+ and [3·(NO3)4]2+,
respectively, and in the case of 4·(NO3)6, the 640.1753,
874.2275, and 1342.3391 peaks were assigned to [4·(NO3)2]4+,
[4·(NO3)3]3+, and [4·(NO3)4]2+, respectively. All experimental iso-
topic distributions of the suggested palladium-containing
species matched the theoretical isotopic patterns, which were
simulated by using ChemCalc software.29
X-ray crystallographic measurement
The details on their crystal and structure data are summarized
in Table S1,† and the relevant bond distances and angles are
listed in Table S2.† X-ray data were collected on a Bruker
SMART automatic diffractometer with graphite-monochro-
mated Mo Kα (λ = 0.71073 Å) and a CCD detector at −25 °C.
Thirty-six frames of two-dimensional diffraction images were
collected and processed to obtain the cell parameters and the
orientation matrix. The data were corrected for Lorentz and
polarization effects. Absorption effects were corrected by the
multi-scan method (SADABS).40 The structures were solved by
direct methods (SHELXS 2014/7) and refined by full-matrix
least squares techniques (SHELXL 2014/7).41,42 Refinement
details are included in the ESI.† The non-hydrogen atoms
were refined anisotropically, and hydrogen atoms were placed
at the calculated positions and refined only for the isotropic
displacement parameters. CCDC 1457749, 1457750, 1453027,
1453026, 1457751, and 1457752 for 1(Cl)6, 1(NO3)6, 2·(NO3)6,
2·(I3)3(NO3)3, 3·(NO3)6, and 4·(NO3)6 contain(s) the supplemen-
tary crystallographic data for this paper.
Results and discussion
Synthesis
In order to elucidate the behaviors of 2·(NO3)6, 3·(NO3)6,
and 4·(NO3)6 in Me2SO-d6, their 1H NMR spectra were scruti-
The self-assembly of K2PdCl4 with N,N′,N″-tris(2-pyridinyl- nized (Fig. S2–S4†). All of the products had even in solution
ethyl)-1,3,5-benzenetricarboxylate) (L1) at room temperature diastereotopic protons in the ethylene and propylene groups,
produced two-layered tripalladium(II)cyclophanes, [Pd3Cl6(L1)2] due to the strong molecular rigidity resulting from the intra-
(1(Cl)6). The subsequent anion exchange of 1(Cl)6 with AgNO3 molecular π⋯π interactions. The 1H NMR spectra of the
at 60 °C for 2 h yielded the corresponding nitrate analogue, present four-layered tripalladium(II)cyclophanes showed that
[Pd3(NO3)6(L1)2] (1(NO3)6). The further in situ reaction of the chemical shifts corresponding to the central phenyl
1(NO3)6 with 2 moles of L2, 2 moles of L3, and 3 moles of L3 moieties were significantly upfield-shifted relative to their free
(L2 = N,N′,N″-tris(3-pyridinylpropyl)-1,3,5-benzenetricarboxylate; ligands, indicating the existence of intramolecular π⋯π inter-
L3 = N,N′,N″-tris(1-(3-imidazolylpropyl))-1,3,5-benzenetricarbox- actions in the solution. Their resonance signals were assigned
amide) gave rise to the four-layered trimetallacyclophanes by 1H–1H COSY measurement (Fig. S2–S4†).
[Pd3(L1)2(L2)2](NO3)6 (2·(NO3)6), [Pd3(L1)2(L3)2](NO3)6 (3·(NO3)6),
Crystal structures
and [Pd3(L1)(L3)3](NO3)6 (4·(NO3)6), respectively. The direct
reaction of palladium(II) nitrate with 2L1 + 2L2, 2L1 + 2L3, and The crystal structures of 1(Cl)6 and 1(NO3)6 consist of three
1L1 + 3L3 in the same molar ratios produced the same palladium(II) ions, two L1, six coordinated anions (chloride or
2·(NO3)6, 3·(NO3)6, and 4·(NO3)6, respectively.
nitrate), and solvate molecules. The local geometry around the
A crucial strategy for the construction of various four- palladium(II) centers is the typical square planar arrangement
layered tripalladium(II) cyclophanes reflecting the strategy of with two nitrogen donors from two L1 (Pd–N = 1.99(1)–2.031(9) Å)
the self-assembly of square planar palladium(II) with com- and two coordinated anions (Pd–Cl = 2.282(5)–2.314(5) Å,
bined inner-short (L1)/outer-long (L2 or L3) C3-symmetric Pd–ONO2 = 1.99(1)–2.02(1) Å) in the trans position. 1(Cl)6 and
Dalton Trans.
This journal is © The Royal Society of Chemistry 2016

View Article Online
Dalton Transactions
Paper
1(NO3)6 consist of a cubic close-packed arrangement in ab′ca′
bc′⋯ layers along the b-axis; the a–c and a′–c′ layers designate
P- and M-helices,30 respectively (Fig. S5†). Solvate molecules
remain in the vacancies within the unit cell, and thus they
exist as 1(Cl)6·6MeOH and 1(NO3)6·6Me2SO.
The X-ray structures of 2·(NO3)6, 3·(NO3)6, and 4·(NO3)6 indi-
cated that all four-layered tripalladium(II)cyclophanes consist
of three palladium(II) ions, four mixed L1 and L2 or L1 and L3,
six nitrate anions, and solvate molecules. The local geometry
around the palladium(II) centers is the typical square planar
arrangement (Pd–N = 1.96(1)–2.08(1) Å) of the four N-donors
from four ligands (Fig. S6†). The four pyridyl/imidazolyl
groups around the palladium(II) ions are twisted from the pal-
ladium(II) plane to form the propeller-type conformation. The
inner-short ligands are coordinated to the central palladium(II)
in a trans fashion, likewise the outer-long ligands.
Helicity was induced by means of the C3-symmetric triden-
tate conformational restrictions involved in the coordination
with the central palladium(II) ions. All of the tripalladium(II)-
cyclophanes indicated the existence of their strong trans-
annular π⋯π interactions (see Table 1), which are comparable
to that (3.3–3.4 Å) of graphite.31 All of the 2·(NO3)6, 3·(NO3)6,
and 4·(NO3)6 crystals consist of racemic helices, in which the
molecules align in a columnar fashion in the (101), (001), and
(010) directions, respectively, with intermolecular π⋯π inter-
actions to be 3.41(1), 3.71(1), and 3.47(2) Å, respectively.32 All
of the intermolecular interacting columns of 2·(NO3)6 and
3·(NO3)6 showed racemic P- and M-helices. The 4·(NO3)6
columns consisted of two kinds of chiral P- and chiral
M-helices (Fig. S7†). The nitrate anions and solvate molecules,
meanwhile, existed in between the molecular pillars. The
solvent accessible void volume is 6686.6 Å3 (35.4%) for
2·(NO3)6, 8474.6 Å3 (44.0%) for 3·(NO3)6, and 3828.8 Å3 (24.6%)
for 4·(NO3)6.
2·(I3)3·(NO3)3 crystals formed from in situ crystallization via
the photoreaction is isostructural to 2·(NO3)6 except for the
counter anions. The I–I distance of the range of 2.798(6)–
−
2.860(3) Å is similar to the bond distances of the known I3
species.33–35 2·(I3)3·(NO3)3 molecules align in two kinds of
chiral P- and M-helical columns in contrast to 2·(NO3)6 (Fig. 1).
The distance of the intermolecular π⋯π interaction within a
column is 3.80(1) Å. Within the void, disordered nitrate anions
and solvate molecules exist in the crystalline state. These
solvents were distinguished by 1H NMR spectra.
Triiodide scavenger for photo-cyclopropanation
In a control experiment, the thermal reaction of CH2I2 with
alkenes in the presence of 2·(NO3)6 did not yield any cyclopro-
panation products. Thus, the photo-cyclopropanation of
1-methylcyclohexene with CH2I2 under 2·(NO3)6 was attempted
(Fig. 2). Initially, the reaction solution in the 6 : 9 : 1 mole
ratios of 1-methylcyclohexene, CH2I2, and 2·(NO3)6 in CDCl3
was subjected to 350 nm UV irradiation at room temperature,
and the reaction efficiencies were monitored by the 1H NMR
spectra. Even though the crystalline 2·(NO3)6 was insoluble in
chloroform, the compound became soluble under UV
This journal is © The Royal Society of Chemistry 2016
Dalton Trans.

View Article Online
Paper
Dalton Transactions
Fig. 3 Product yields of photo-cyclopropanation (350 nm UV radiation)
according to the mole ratios of 1-methylcyclohexene : diiodomethane :
2·(NO3)6 = 6 : 9 : x after completion of the photoreaction in CDCl3. The
inset shows the packing structures with highlighted anions (NO3−, red;
I3− purple).
Fig. 1 Crystal structures of 2·(NO3)6 (left) and 2·(I3)3·(NO3)3 (right)
showing side views highlighting intramolecular π⋯π interactions (a) and
packing systems designating the helicity of each column consisting of
two kinds of chiral P-, M-helices (2·(NO3)6) and racemic P- or M-helices
(2·(I3)3·(NO3)3) (b). Red and blue arrows denote left-handed M-helices
and right-handed P-helices, respectively (Pd2+, dark green; C, gray;
N, blue; O, red).
ratios of 1-methylcyclohexene, CH2I2, and 2·(NO3)6. That is, the
addition of 2·(NO3)6 enhanced the formation of the cyclopro-
pane product at a mole ratio of up to 6 (1-methylcyclohexene) :
9 (CH2I2) : 1 (2·(NO3)6), and then further addition of 2·(NO3)6
(6 (1-methylcyclohexene) : 9 (CH2I2) : >1 (2·(NO3)6)), after which
the product yields gradually decreased, until finally, the reac-
tion yield at the mole ratio of 6 : 9 : 6 approximated that of the
photoreaction without 2·(NO3)6. Thus, the 6 : 9 : 1 mole ratio
gave rise to the best yield. This yield trend was found to be
closely related to the quantity of the generated I2 + I− during
the photoreaction. That is, 2·(NO3)6 acted as a scavenger
−
for I3
, through in situ crystallization, resulting in the
formation of anion-exchanged 2·(I3)3·(NO3)3 (Fig. S9†). The
photoreaction at the ratio of 6 : 9 : >1 formed 2·(I3)n·(NO3)6−n
(0 < n < 3), which were soluble in the reaction system. Thus,
the presence of more than 1 mole of 2·(NO3)6 was ineffective.
−
The formation of I3 can be reasonably explained by the
following eqn (1)–(3):36,37
hν
2C7H12 þ 2CH2I2 ꢀ! 2C8H14 þ 2I2
ð1Þ
hν
CH2I2 þ H2O ꢀ! CH2ðOHÞ2 þ Hþ þ 2Iꢀ
ð2Þ
ð3Þ
ꢀ
2I2 þ 2Iꢀ
2I
!
3
Fig. 2 1H NMR monitoring of photo-cyclopropanation (350 nm UV
In a subsequent control experiment, the reaction of CH2I2
with 1-methylcyclohexene in CDCl3 without 2·(NO3)6 did not
yield the product efficiently; rather, only a partially photo-
decomposed species from CH2I2 molecules and 1-methyl-
cyclohexene along with a maximum 20% yield of the cyclopro-
radiation) in a mole ratio of 1-methylcyclohexene : CH2I2 : 2·(NO3)6
=
6 : 9 : 1 at 0 h, 2 h, 10 h, and 36 h in CDCl3 (dagger, CDCl3; blue peak,
1-methylcyclohexene; red peak, 1-methylnorocarane).
irradiation under the conditions of the photo-cyclopropana- panation product could be produced. The reaction equations
tion reaction. The photo-cyclopropanation was performed for explained the relationship between the mole ratios and the
up to 36 h (max. yield 88%). In order to elucidate the mole reaction yield. Trace byproducts such as CH3CHO, CH2ClI, and
ratio effects of 2·(NO3)6 on the photoreaction, various reactant CHCl2I obtained from the reactions of the active species were
mole ratios were employed (Fig. 3 and S8†). The most striking observed by 1H NMR and gas chromatography (Fig. S10†).
−
result was the dependence of the reaction yields on the mole Next, in order to investigate the recyclability of the I3 scaven-
Dalton Trans.
This journal is © The Royal Society of Chemistry 2016

View Article Online
Dalton Transactions
Paper
ger, anion exchange of 2·(I3)3·(NO3)3 with AgNO3 in a mixed driving force to the in situ crystallization of 2·(I3)3·(NO3)3
solution of H2O and Me2SO (v/v = 19 : 1) or H2O and acetone during the photo-cyclopropanation under the given reaction
(v/v = 1 : 1) was confirmed by UV-Vis and infrared spectra. conditions. The Raman spectra of the 2·(I3)3·(NO3)3 crystals
According to the UV-Vis spectra of 2·(I3)3·(NO3)3 in a mixed clearly showed that the strong bands at 113, 144 and 168 cm−1
−
−
solution of H2O and Me2SO, I3 species represent the absor- correspond to the characteristic modes of the I3 species
bance at 293 nm and 369 nm. The absorbance was gradually (Fig. 5).34,39 These results indicated, conclusively, that 2·(NO3)6
decreased during the anion exchange (Fig. S11†). Even after acts as an effective I3 scavenger in the photoreaction. Of
−
the whole process of anion exchange, the virtually unchanged course, single-crystal X-ray diffraction measurement had
peaks of the IR bands and the 1H NMR spectra of 2·(NO3)6 already shown that the skeletal structure of 2·(I3)3·(NO3)3 is pre-
indicated that the skeletal structure remained (Fig. S12†). served both before and after photo-irradiation. Other sub-
According to the powder XRD pattern, meanwhile, the anion strates such as cyclohexene, 1-octene, and trans-4-octene were
exchange produced silver(I) iodide, owing to the equilibrium of employed, resulting in similar cyclopropanation outcomes.
−
I−/I3 in the solution (Fig. S13†).38 The product-yield plot in The imidazole-moiety-containing palladium(II) cyclophanes,
Fig. 4 shows that the exchanged 2·(NO3)6 is perfectly recyclable, 3·(NO3)6 and 4·(NO3)6 were inactive in the photoreaction,
as explained by the rigidity and stability of the four-layered tri- owing to their insolubility under the reaction conditions.
palladium(II)cyclophane irrespective of the anions.
What is the critical driving force behind the efficiency and
recyclability of photo-cyclopropanation? We ascribed the Conclusions
A direct reaction followed by a stepwise reaction of palladium(II)
ions with an appropriate combination of C3-symmetric tri-
dentate ligands containing a central phenyl moiety efficiently
allow for the construction of rigid four-layered tripalladium(II)
cyclophanes via the concepts of cyclophanes’ transannular
π⋯π interactions and inner-short/outer-long spacer lengths.
For the photo-cyclopropanation reaction via in situ crystalliza-
tion, 2·(NO3)6 is the most efficient and recyclable triiodide sca-
venger, as the detailed scavenger data, including mole ratios,
establish. This conceptually-advanced synthetic methodology
can be helpful in enabling the understanding of the skeletal
structures’ stability and in controlling their physicochemical
properties including the relevant structural thermo-dynamics
and solubility. The scavenger reaction, intriguingly, can be
advantageous in formulating new, task-specific scavengers.
These results will help in realizing potential applications to
Fig. 4 Product yield of photo-cyclopropanation (350 nm UV radiation)
small-molecule-accommodating sensing systems, starch ana-
lysis, separators, and catalysts.
in a mole ratio of 1-methylcyclohexene : diiodomethane : 2·(NO3)6
=
6 : 9 : 1 using the as-synthesized crystal, 2·(NO3)6 (black line) and the
recyclable anion-exchanged sample, 2·(NO3)6 (red line).
Acknowledgements
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korean Government
[MEST] (2016R1A2B3009532 and 2016R1A5A1009405).
Notes and references
1 Q. Lin, X. Bu, C. Mao, X. Zhao, K. Sasan and P. Feng, J. Am.
Chem. Soc., 2015, 137, 6184–6187.
2 J. D. Froehlich and C. P. Kubiak, J. Am. Chem. Soc., 2015,
137, 3565–3573.
3 W. Kim, T. Tachikawa, G. Moon, T. Majima and W. Choi,
Angew. Chem., Int. Ed., 2014, 53, 14036–14041.
4 P. H. Seeberger, Nat. Chem., 2009, 1, 258–260.
5 T. Hamasaki, T. Kashiwagi, T. Imada, N. Nakamichi,
S. Aramaki, K. Toh, S. Morisawa, H. Shimakoshi,
Fig. 5 Raman spectra of 2·(NO3)6 (bottom) and 2·(I3)3·(NO3)3 (top). The
inset shows the 250–0 cm−1 magnification range.
This journal is © The Royal Society of Chemistry 2016
Dalton Trans.

View Article Online
Paper
Dalton Transactions
Y. Hisaeda and S. Shirahata, Langmuir, 2008, 24, 7354– 25 T. R. Cook and P. J. Stang, Chem. Rev., 2015, 115, 7001–
7364. 7045.
6 A. N. Tarnovsky, V. Sundström, E. Åkesson and T. Pascher, 26 T. H. Noh, E. Heo, K. H. Park and O.-S. Jung, J. Am. Chem.
J. Phys. Chem. A, 2004, 108, 237–249. Soc., 2011, 133, 1236–1239.
7 D. C. Blomstrom, K. Herbig and H. E. Simmons, J. Org. 27 A. Sørensen, A. M. Castilla, T. K. Ronson, M. Pittelkow and
Chem., 1965, 30, 959–964.
J. R. Nitschke, Angew. Chem., Int. Ed., 2013, 52, 11273–
8 N. J. Pienta and P. J. Kropp, J. Am. Chem. Soc., 1978, 100,
655–657.
11277.
28 C. Laurence and J.-F. Gal, Lewis Basicity and Affinity Scales:
9 P. J. Kropp, N. J. Pienta, J. A. Sawyer and R. P. Polniaszek,
Tetrahedron, 1981, 37, 3229–3236.
Data and Measurement, John Wiley & Sons, New York,
2010.
10 P. J. Kropp, Acc. Chem. Res., 1984, 17, 131–137.
11 M. Wang, N. Chamberland, L. Breau, J.-E. Moser,
29 L. Patiny and A. Borel, J. Chem. Inf. Model., 2013, 53, 1223–
1228.
R. Humphry-Baker, B. Marsan, S. M. Zakeeruddin and 30 K. H. Park, T. H. Noh, Y.-B. Shim and O.-S. Jung, Chem.
M. Grätzel, Nat. Chem., 2010, 2, 385–389. Commun., 2013, 49, 4000–4002.
12 G. Boschloo and A. Hagfeldt, Acc. Chem. Res., 2009, 42, 31 P. Delhaes, Graphite and Precursors (World of Carbon),
1819–1826. Gordon and Breach Science Publishers, New York, 2001.
13 H. Lebel, J.-F. Marcoux, C. Molinaro and A. B. Charette, 32 M. Shibahara, M. Watanabe, T. Iwanaga, K. Ideta and
Chem. Rev., 2003, 103, 977–1050. T. Shinmyozu, J. Org. Chem., 2007, 72, 2865–2877.
14 J. L. Kane, K. M. Shea, Jr., A. L. Crombie and 33 P. K. Bakshi, M. A. James, T. S. Cameron and O. Knop,
R. L. Danheiser, Org. Lett., 2001, 3, 1081–1084. Can. J. Chem., 1996, 74, 559–573.
15 C. Borie, L. Ackermann and M. Nechab, Chem. Soc. Rev., 34 T. J. Marks and D. W. Kalina, Extended Linear Chain
2016, 45, 1368–1386. Compounds, Plenum, New York, USA, 1982.
16 S. Hanessian, D. Andreotti and A. Gomtsyan, J. Am. Chem. 35 A. J. Blake, W.-S. Li, V. Lippolis, M. Schröder,
Soc., 1995, 117, 10393–10394.
17 S. Hanessian, D. Andreotti and A. Gomtsyan, J. Am. Chem.
Soc., 1996, 118, 2537.
F. A. Devillanova, R. O. Gould, S. Parsons and C. Radek,
Chem. Soc. Rev., 1998, 27, 195–206.
36 W. M. Kwok, C. Y. Zhao, X. G. Guan, Y. Li, Y. Du and
D. L. Phillips, J. Chem. Phys., 2004, 120, 9017–9032.
18 R. R. R. Taylor, H. C. Twin, W. W. Wen, R. J. Mallot,
A. J. Lough, S. D. Gray-Owen and R. A. Batey, Tetrahedron, 37 M. Martino, P. S. Liss and J. M. C. Plane, Environ. Sci.
2010, 66, 3370–3377. Technol., 2005, 39, 7097–7101.
19 H. Lee, T. H. Noh and O.-S. Jung, Angew. Chem., Int. Ed., 38 J.-L. Burgot, Ionic Equilibria in Analytical Chemistry,
2013, 52, 11790–11795. Springer Science & Business Media, New York, USA, 2012.
20 H. Lee, T. H. Noh and O.-S. Jung, Angew. Chem., Int. Ed., 39 A. A. Bernussi and G. M. Gualberto, J. Raman Spectrosc.,
2016, 55, 1005–1009. 1987, 18, 93–95.
21 R. Gleiter and H. Hopf, Modern Cyclophane Chemistry, 40 G. M. Sheldrick, SADABS: A program for Empirical Absorption
Wiley-VCH, New York, 2004.
Correction of Area Detector Data, University of Göttingen,
22 A. V. Davis, R. M. Yeh and K. N. Raymond, Proc. Natl. Acad.
Sci. U. S. A., 2002, 99, 4793–4796.
Germany, 1997.
41 G. M. Sheldrick, SHELXS-97:
A Program for Structure
23 M. J. Levin and P. J. Stang, J. Am. Chem. Soc., 2000, 122,
7428–7429.
Solution and Refinement, University of Göttingen, Germany,
1997.
24 L. Xu, Y.-X. Wang and H.-B. Yang, Dalton Trans., 2015, 44, 42 G. M. Sheldrick, Acta Crystallogr., Sect. C: Cryst. Struct.
867–890.
Commun., 2015, 71, 3–8.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2016