A new Organopalladium compound containing four Iron (III) Porphyrins for the selective oxidation of alkanes/alkenes by t-BuOOH

Article abstract of DOI:10.1007/s12039-016-1032-1

Two iron(III) tetraphenyl porphyrin catalytic units are connected by an azo-link to form the dimeric compound A. The compound A was then reacted with Pd ²⁺ to make a tetrameric iron(III) porphyrin complex B with all four iron(III) catalytic sites open to the substrates and reactants. Both the compounds were characterized spectroscopically and the results of homogeneous oxidation of some alkanes and alkenes with t-BuOOH in presence of catalytic quantities of A and B have indicated remarkable improvement in selectivity and efficiency of A over the monomeric catalyst and B over A. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s12039-016-1032-1

c
J. Chem. Sci. Vol. 128, No. 3, March 2016, pp. 383–389. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-016-1032-1
A new Organopalladium compound containing four Iron (III)
Porphyrins for the selective oxidation of alkanes/alkenes by t-BuOOH
MANOJ KUMAR SINGH and DEBKUMAR BANDYOPADHYAY∗
Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016, India
e-mail: dkbp@chemistry.iitd.ac.in
MS received 14 August 2015; revised 6 November 2015; accepted 10 December 2015
Abstract. Two iron(III) tetraphenyl porphyrin catalytic units are connected by an azo-link to form the dimeric
compound A. The compound A was then reacted with Pd2+ to make a tetrameric iron(III) porphyrin complex B
with all four iron(III) catalytic sites open to the substrates and reactants. Both the compounds were character-
ized spectroscopically and the results of homogeneous oxidation of some alkanes and alkenes with t-BuOOH in
presence of catalytic quantities of A and B have indicated remarkable improvement in selectivity and efficiency
of A over the monomeric catalyst and B over A.
Keywords. Iron(III) porphyrin; catalysis; t-BuOOH; cycloalkenes/alkanes.
1. Introduction
plan is based on the known rich chemistry of cyclopal-
ladated azobenzene compounds.12 Thus, the most eas-
ily available and degradable compound TPPFeCl was
selected for the present study. Two such units are
planned to couple by an azo-linkage to get compound
A. Compound A is thus nothing but a p–substituted
azobenzene, which on reaction with Na2PdCl4 should
give B. Herein we report the synthesis, spectroscopic
characterization and the remarkable improvement of the
catalytic ability of A and B with respect to the monomer
which is TPP(p-NO2)FeCl.
The importance of metalloporphyrin-catalysed oxidation
reactions in biology and their potential as industrial cat-
alysts have led to a considerable volume of work in
these reactions.1 The properties of such catalysts are
tuned by attaching different substituents to the macro
cycle.2 Such catalysts are then successfully used for
selective oxidation reactions of various organic sub-
strates by peracids and hydroperoxides.3 Mechanistic
studies have shown that the epoxidation of alkenes
by hydroperoxides, hydrogen peroxides and m-CPBA
passes through higher valent metal-oxo intermediates.4
It has been observed that these monomeric catalysts
were degraded by the formation of μ-oxo dimer (eq. 1).5
2. Experimental
2.1 Materials and synthesis
m-Cresol was distilled under argon before use. Other
organic solvents for reactions were distilled over ap-
propriate drying reagents under argon or obtained as
dehydrated reagents from Merck (India) Chemicals.
Deuterated solvents for NMR, pyrrole, p-nitrobenzal-
dehyde, tert-butyl hydroperoxide (t-BuOOH) and ole-
fines were obtained from Sigma-Aldrich Chemical
Pvt. Ltd (U.S.A.). Ferrous chloride tetrahydrate was
obtained from LobaChemie. SnCl2.2H2O, acetic anhy-
dride and hydrochloric acid were obtained from Quali-
gens Chemicals. Isoquinoline, ethanol and m-cresol
were obtained from Spectrochemicals.
(1)
In order to avoid this, attempts have been made
to anchor monomers into solid supports or to make
covalent linkages.6,7 In such studies, instead of using
tetraphenyl porphyrin, the tetrapyridyl porphyrins were
used in the development of a series of interesting cat-
alyst frameworks.8,9 In this process of modification,
including ours, the materials were found to be amor-
phous in nature.10 Attempts were also made to make
the materials crystalline in order to improve their cat-
alytic efficiency.11 In order to stop two catalytic units to
destroy each other by μ-oxo-dimerization, we thought
of a very simple compound depicted in scheme 1. The
2.1a Synthesis of 5-(4-nitro phenyl),10,15,20 triph-
enylporphyrin: Sodium nitrite (20 mg, 0.29 mmol) to a
solution of tetra phenyl porphyrin (TPPH2) (100 mg,
∗For correspondence
383

384
Manoj Kumar Singh and Debkumar Bandyopadhyay
Scheme 1. The synthetic route and the possible structures of the two new catalysts compound A and compound B.
0.163 mmol) in TFA (10 mL) was added. The reaction 6-collidine (100 μL, 97.7 mg,) and mixture was
mixture was stirred for 3 min at room temperature allowed to reflux under argon. Solid FeCl2.4H2O (310
and was quenched by adding 100 mL of ice cold water. mg, 1.559 mmol) was added to the refluxing reaction
The reaction mixture was extracted with dichlorome- mixture. The refluxing was continued for 1 hr, then
thane (6×25 mL) and the organic layer was washed first again FeCl2.4H2O (310 mg, 1.559 mmol) was added
with saturated aqueous sodium bicarbonate and then to the reaction mixture and reaction was continued for
with water and finally dried over anhydrous sodium sul- another 2 h. The reaction mixture was then brought to
phate. The dried solution was passed through a silica gel room temperature. Dichloromethane was added to the
column. The desired compound 5-(4-nitrophenyl)-10, reaction mixture and the contents were slowly poured
15,20-triphenyl porphyrin was eluted in 1:1dichloro- to a 500 mL separating flask containing 250 mL of
methane-hexane mixture. The solution was then evap- distilled water. The organic layer was taken immedi-
orated to dryness under vacuum and the yield was ately without any shaking then this organic layer was
65 mg (59%) yield. This compound has following washed with distilled water (6×250 mL). The organic
spectroscopic features: UV-Visible (CH2Cl2)λmax/nm layer was dried over anhydrous Na2SO4for overnight
(ε/M−1cm−1): 418 (4.69×105), 515 (2.65×104), 550 and was decanted and was evaporated to dryness under
(1.36×104), 590 (1.24×104), 645 (9.35×103); 1HNMR reduced pressure. The crude solid was purified by silica
(300MHz, CDCl3): δ -2.79 (s, 2H, NH pyrrole), 7.76- gel column chromatography and the desired compound
7.78 (t, 9H, m-&p-phenyl), 8.20-8.23 (m, 6H, o- was eluted from column by using 3% methanol in
phenyl), 8.39-8.42 (d, 2H, m-nitrophenyl), 8.63-8.66 (d, dichloromethane. The characteristic spectroscopic fea-
2H, o-nitrophenyl), 8.73-8.75 (s, 2H, β pyrrole), 8.86- tures: UV-visible (CH2Cl2)λmax (nm): 378 (1.09×105),
8.90 (s, 6H, β pyrrole); FTIR (KBr, cm−1): ν 3442 (w), 416 (ε = 2.03×105), 510 (2.55×104); FTIR (KBr,
1515 (s), 1343 (s). The compound was finally charac- cm−1): ν 3442 (w), 1519 (s), 1335 (s), 998 (s, Fe–N
terized by ESI-MS: C44H29N5O2[M](m/z) = 660.2370 stretch). This compound was characterized by follow-
([M+H]+, observed), 660.2394 (calculated).
ing: ESI-MS: C44H27N5O2FeCl [M] (m/z) = 713.1441
([M-Cl]+, observed), 713.1514 (calculated).
2.1b Synthesis of 5-(4-nitro phenyl),10,15,20 triph-
enylporphyrin iron(III) chloride: In a solution of 5- 2.1c Synthesis of 4,4ꢁ-bis[5-(10,15,20-triphenyl)por-
(4-nitro phenyl), 10,15,20 triphenylporphyrin (100 mg) phyrinyl] azobenzene iron(III) chloride: A solution of
in dimethyl formamide (100 mL) was added 2,4, 1 g of NaOH in 1 mL of water and 50 mg of 5-(4-nitro

Organopalladium compound containing four iron(III) porphyrin
385
phenyl),10,15,20 triphenylporphyrin iron(III)chloride 2400 model while Cl analysis is performed by gravi-
in 8 mL of benzene were heated to 80◦C. The solution metric method using AgNO3.
was stirred vigorously, and KOH (2 g) and zinc powder
(0.5 g) were added to it. After the mixture was stirred
2.2b General procedure for the oxidation of alkenes/
alkane: The oxidations of cycloalkenes/alkane (400 mM)
for 10 h, zinc powder (0.5 g) was added again. The reac-
tion mixture was then heated to 90◦C and was stirred
were carried out at room temperature in a 4 mL screw-
for 10 h. This hot solution was filtered and solid was
capped vial fitted with PTFE septa. In all the experi-
washed with 5 mL of methanol. Air was passed through
ments the iron(III) porphyrin catalyst in mM conc. were
this solution for 5 h. The resultant solution was neu-
taken in 2.0 mL dichloromethane under argon. The oxi-
tralize by HCl solution (1 N) and filtered. An excess
dation reactions were initiated by adding the oxidants
of dichloromethane (50 mL) was added and the organic
(10mM) at the end to the vial. This was followed by
layer was separated and dried by Na2SO4. The solvent
stirring the reaction mixture with a small magnetic bar
was evaporated to dryness and to this residue dichloro-
and the reactions were carried out under argon. After
the reaction was over, 2 μL dodecane was added to this
reaction mixture as an internal standard. An aliquot
(∼1μL) was withdrawn after regular intervals using a
methane added again and solvent was removed under
vacuum and powder solid product was collected. The
product was purified by silica gel column chromatog-
raphy and the desired compound was eluted by 10%
microlitre syringe from the reaction mixture for anal-
methanol-dichloromethane mixture. The characteristic
ysis. At the end of the reaction the solid particles
spectroscopic features of the compound: UV-Visible
(catalyst) were separated by filtration and the product
analysis for cycloalkenes/alkane oxidation was per-
formed using Perkin-Elmer AutoSystemXL gas chro-
matography equipped with flame ionization detector
(CH2Cl2)λmax/nm (ε/M−1cm−1): 416 (3.37×105), 511
(3.95×104); FTIR (KBr, cm−1): ν 3442 (w), 1628 (s,
-N=N- stretch), 999 (s, Fe–N stretch). It was char-
acterized by: ESI-MS: C88H54N10Fe2Cl2 [M] m/z =
(FID) and carbowax capillary column of 30 m length.
681.1540 ([M-2Cl]2+(observed), 681.1614 (calculated).
Tetraphenyl porphyrin is synthesized according to the
procedure reported in the literature.
2.1d Synthesis of 4,4ꢁ-bis[5-(10,15,20-triphenyl)por-
phyrinyl]azobenzene iron(III) chloride coordinated to
palladium(II): A solution of Na2PdCl4 (7.5 mg, 0.02
mmol) in 5 mL of methanol was added to a solution
of 4,4ꢁ-bis[5-(10,15,20-triphenyl)porphyrinyl] azoben-
zene iron(III) chloride (30 mg, 0.02 mmol) in 5 mL of
dichloromethane. The solution was stirred for 30 min at
room temperature, and the precipitated compound was
filtered by G-4 and was thoroughly washed with water
and then with methanol. The solid residue (yield = 22
mg, 66%) was collected and was dried under vacuum.
This product has following characteristic spectroscopic
features: UV-Visible (CH2Cl2)λmax/nm (ε/M−1cm−1):
378 (2.33×105), 418 (ε = 4.22×105), 510 (5.99×104);
FTIR (KBr, cm−1): ν 3430 (w), 1597 (s, -N=N- stretch),
999 (s, Fe–N stretch), 726 (orthometallation), 555 (ν Pd–
C) and 460 (ν Pd–N); and fragmented ESI-MS: [C88
H53N10Fe2Pd]2+ m/z = 733.1933 (observed), 733.6094
(calculated).
3. Result and Discussion
The TPP(p-NO2) was prepared by controlled reaction
of NaNO2 with TPPH2. The iron insertion to this was
done by conventional treatment of this porphyrin with
FeCl2.4H2O in DMF to evolve TPP(p-NO2)FeCl. The
4,4ꢁ-bis[5-(10,15,20-triphenyl)porphyrinyl] azobenzene
iron(III) chloride (Compound A) was synthesized from
5-(4-nitro phenyl),10,15,20 triphenylporphyrin by react
ing it with zinc dust and KOH in benzene (scheme 1).
The dimer has soret band very similar to that of 5-
(4-nitro phenyl),10,15,20 triphenylporphyrin iron(III)
chloride monomer. The two iron porphyrin units are
connected by azo linkage and this has changed the
extinction coefficient per iron from 2.03×105 in mono-
mer to 1.68×105 M−1 cm−1 in dimer (Supporting infor-
mation: figures S1–S2). The ESI-Mass of the monomer
gives parent ion peak at 713.1441 (figure S3) which
indicates that chloride ion is dissociated which is com-
mon in iron(III) porphyrin compounds.13 The dimer
2.2 Characterization and catalytic experiments
2.2a Characterization techniques: UV-Visible spectra exhibited a parent ion peak at m/z = 681.1540 (C88H54
were recorded by Agilent Technologies UV 8453 model. N10Fe2Cl2 [M-2Cl]2+ = 681.1614) (Mass spectra,
Fourier transformed infrared spectra (FTIR) were re- figure S5) due to dissociation of chloride ions. This
corded on KBr pellets using Agilent Technologies Cary can be rationalized by the generation of two positive
660 model. Elemental analysis (C, H, N) was measured charges on dissociation of two chloride ions from the
by Perkin Elmer instrument series II CHNS/O Analyser dimer.14 The 4,4ꢁ-bis[5-(10,15,20-triphenyl)porphyri-

386
Manoj Kumar Singh and Debkumar Bandyopadhyay
In order to demonstrate the catalytic activity of the
nyl] azobenzene iron(III) chloride coordinated to Pd(II)
complex (Compound B) was synthesized by reacting iron(III)porphyrin monomer, dimer and tetramer the
Compound A and Na2PdCl4 in 50% dichloromethane in oxidation of norbornene by t-BuOOH in dichlorome-
methanol (scheme 1). The MALDI-TOF mass spectra thane was undertaken and the results are summarized in
show the major peak at 733.1512 (figure S6). This is due table 1.
to the fragment containing iron(III) porphyrin dimer
The monomer shows ∼7% conversion of norborne-
with one Pd only with all chlorides detached from the neto 2,3-exoepoxy norbornane. The dimeric iron(III)
metal ions (Fe3+ and Pd2+). The loss of one mass unit porphyrin (compound A) gives higher conversion up
in this spectrum also signifies the formation of a C-Pd to 46% with 99% selectivity while tetrameric iron(III)
linkage as expected.15 In order to reinforce our analysis porphyrin (compound B) gives highest conversion up
and observation the Cope compound (AzoBz2Pd2Cl2) to 95% with 99% selectivity. This result indicates that
was prepared.12,16 The ESI-Mass of this compound B is the most efficient catalyst for the selective oxida-
showed the parent ion peak at ∼365, which is along the tion of norbornene. In selecting the best reaction condi-
line of our expectation (figure S7).17
tion, the range of substrate concentration has also been
The FT-IR of compound A shows a characteristic optimized. The catalytic oxidation of some other cyclo-
-N=N- azo stretching frequency18 at 1628 cm−1 and alkenes/and cyclohexane by t-BuOOH has now been
that for the Fe-N stretching frequency at 999 cm−1. studied in presence of catalytic quantities of A and the
Compound A forms azobenzene type of complex as results are shown in table 2.
shown in B, where the frequency of –N=N- was
Cyclohexene was oxidized to cyclohexene-1-one
shifted to 1597 cm−1. Similar shift is observed in case and cyclohexene-1-ol with 72% and 23% respectively
of cyclopalladated azobenzene complexes.19 The com- (table 2, entry No. 2). Cyclooctene have better con-
pound B exhibited other important IR peaks at 555 version to cyclooctene epoxide with 34% yield with
cm−1 (ν Pd–C), 726 cm−1 (orthometallation) and 460 respect to monomer. Cyclohexane has been oxidized to
cm−1 (ν Pd–N).20 Elemental analysis of both A and B cyclohexan-1-one and cyclohexan-1-ol with 50% selec-
shows the content of C, H, N to be 74.47, 4.08, 10.39% tivity. Cyclooctane has been oxidized to cyclooctane
and 67.02, 3.53, 8.49, respectively (theoretical values, 1-one (15%) and cyclooctane 1-ol (13%) with 52%
for C88H54N10Fe2Cl2: C 73.70%, H, 3.80%, N 9.77% selectivity. Styrene has been oxidized to styrene epox-
and for C176H106N20Fe4Pd2Cl6: C 67.07%, H 3.45%, N ide (56%) with 72 % selectivity.
8.89%). In both these molecules the percentage of chlo-
In presence of Compound B, the oxidation of cis-2,
ride is most important and this is done gravimetrically. 3-norbornene was increased significantly from 7% for
The content of chloride ion in compound A and B was monomeric catalyst to 95% for the tetrameric one at
observed to be 5.02% and 6.95% (calculated values: room temperature. The oxidation of norbornene with
4.94% and 6.75%), respectively. In electronic absorp- comparable concentration of Pd2+ (table 1, entry no. 7 and
tion spectra, the extinction coefficients per iron for A 8) shows that Pd2+ is acting simply as a linker and has
and B are changed as expected, because the azo bond is almost no catalytic role in B. For comparative study the
coordinated to Pd (figure S9).These data and the known oxidation of other cycloalkenes and cyclohexane were
azobenzene chemistry of palladium support the most performed and the results are summarized in table 3.
rational structure of Pd tetramer to be like that given in
It was observed that norbornene and cyclooctene
give stereospecific epoxide products with 95% and 82%
scheme 1.21
Table 1. Oxidation of norbornene to 2,3-exoepoxy norbornane with t-BuOOH in CH2Cl2 at 25 2◦Ca.
Entry
Substrate conc. (mM)
Time (h)
Catalysts Conc. (mM)
Catalysts
Yield (%)b
1
2
3
4
5
6
7
8
200
400
200
400
200
400
200
400
24
24
24
24
24
24
24
24
0.313
0.313
0.328
0.328
0.317
0.317
0.317
0.317
Monomer
Monomer
Compound A
Compound A
Compound B
Compound B
Na2PdCl4
4
7
28
46
72
95
<1
<1
Na2PdCl4
[a] Solvent (2 mL), Oxidant(t-BuOOH) = 10 mM and dodecane (5mM) used as internal standard kept in Teflon-lined screw-
cap vial and was sealed under dry argon atmosphere. The mixture was stirred at room temperature for 24h. [b] conversion in
[%] was determined by GC. The yields are w.r.t. oxidant and average of duplicate sets.

Organopalladium compound containing four iron(III) porphyrin
Table 2. Compound A catalyzed oxidation of alkenes/alkanesa.
387
Entry
Reactant
Product
Yieldb (%)
Selectivityc (%)
1
2
3
4
5
6
Norbornene
Cyclohexene
Cyclooctene
Cyclohexane
Cyclooctane
Styrene
2,3-exoepoxy norbornane
Cyclohexen1-one (72%) + Cyclohexene1-ol (23%)
Cyclooctane oxide
Cyclohexan1-one and cyclohexan1-ol
Cyclooctan 1-one (15%) and Cyclooctan 1-ol (13%)
Styrene epoxide (56%)
46
99
34
10
28
78
99
75
99
50d
52e
72f
[a] Catalyst A = 0.328 mM , Oxidant(t-BuOOH) = 10 mM , Substrate Conc. = 400 mM, dichloromethane (2.0 mL) and
dodecane (5mM) were taken in a teflon-lined screw cap vial and it was sealed under dry argon atmosphere. The mixture was
stirred at room temperature for 24 h. [b] conversion [%] and [c] selectivity [%] were determined by GC. The yields are w.r.t.
oxidant and average of duplicate sets. [d] cyclohexane 1-one (5%) and cyclohexan-1-ol (5%). [e] cyclooctan-1-one (15%)
and cyclooctan-1-ol (13%). [f] The by-products were benzaldehyde [12%] and benzeneacetaldehyde [10%].
Table 3. Compound B catalyzed oxidation of alkenes/alkanesa.
Entry
Reactant
Product
Yieldb (%)
Selectivityc (%)
1
2
3
4
5
6
Norbornene
Cyclohexene
Cyclooctene
Cyclohexane
Cyclooctane
Styrene
2,3-exoepoxy norbornane
Cyclohexen1-one (70) +Cyclohexe 1-ol (26)
Cyclooctane oxide
95
99
82
18
42
99
99
73
99
Cyclohexan1-one and Cyclohexan1-ol
Cyclooctan 1-one and Cyclooctan 1-ol
Styrene epoxide (77%)
60d
55e
77f
[a] Catalyst B (0.317 mM ), Oxidant t-BuOOH) (10 mM), Substrate Conc. (400 mM), dichloromethane (2.0 mL) and dode-
cane (5mM) were taken in a screw-capped vial and it was sealed under dry argon atmosphere. The mixture was stirred at
room temperature for 24h. [b] conversion [%] and [c] selectivity [%] were determined by GC. The yields are w.r.t. oxi-
dant and average of duplicate sets. [d] cyclohexane1-one (11%) and cyclohexan-1-ol (7%). [e] cyclooctan-1-one (23%) and
cyclooctan-1-ol (19%). [f] The by-products were benzaldehyde [9%] and benzeneacetaldehyde [13%].
yields, respectively. Styrene was oxidized to styrene
epoxide (77%) with 77% selectivity.
The time bound plot for exo-2,3-epoxy norbornane
formation (figure 1) indicates that there is no co-opera-
tivity of four iron(III) catalytic sites for the substrate
binding and in their subsequently oxidation reaction.
In these reactions the stability of the catalyst A and
B were also checked under the oxidizing conditions.
Thus after the completion of the first reaction, new
batches (aliquot) of oxidants were added to the same
pot and the product yields were measured. The UV-
Visible spectrum before and after the reaction did not
show any observable change. The recovered catalyst
also did not show any degradation of original catalyst
or transformation to C-Pd bond oxidation product on
TLC. The process of oxidation in a separate set of sim-
ilar experiment was repeated for 3 times for A and 5
times for B (table S1 in Supporting Information). The
yields of product and the reaction time remained almost
Figure 1. Plot of % yield of exo-2,3-epoxy norbornane vs
time (h) for Catalyst B with t-BuOOH in dichloromethane.
The error range is shown with each data point.
unchanged, which indicates that the catalysts were sta- in the molecular frame. The structure provides rigid-
ble under the reaction conditions, unlike the monomeric ity to the complex B while compound A has flexible
one, which gets decayed substantially even after the cis-trans orientation. This has indeed provided the
first cycle. The complex B is more stable over A due to remarkable improvement in selectivity and catalytic
the formation of strong C-Pd bond and azo N-Pd bond efficiency of catalyst B over catalyst A. Probable

388
Manoj Kumar Singh and Debkumar Bandyopadhyay
Scheme 2. The probable scheme for the iron(III) porphyrin catalyzed oxidation reactions
of cycloalkenes/alkanes by t-BuOOH.
pathway for the catalytic oxidation reaction is presented Supplementary Information
in scheme 2.
Electronic spectra, ESI-Mass, Maldi TOF spectra and
stability of the catalysts under oxidizing condition
(table S1) are given in supporting information. Sup-
plementary Information is available at www.ias.ac.in/
chemsci.
4. Conclusions
New iron(III) porphyrin catalysts have been prepared.
These catalysts selectively oxidize several alkenes/al-
kanes. The tetrameric catalysts were highly efficient
and non-degradable up to at least five cycles in one pot.
The dimeric and tetrameric compounds are soluble in
organic solvents. The spectroscopic methods are used
to establish the structure of the catalysts. We could not
get single crystals of any of these catalysts. The work
is in progress, so that more accurate rationalization of
the structure of the catalysts can be ascertained and
whether more such catalysts with variable linkers can
be prepared.
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Acknowledgements
We thank the Department of Science and Technology
(DST, India) (Project no. SR/S1/IC-48/2010) for fund-
ing and UGC for the fellowship to MKS.
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Organopalladium compound containing four iron(III) porphyrin
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A, two +ve charges are generated in the molecule, so
expectedly the parent ion peak should come at (M-
2Cl)/2 which is clearly visible at 681.1540 (observed),
681.1614 (calculated). We also see the small peak for
the dimer at 1361.2811
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Cope compound
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Z J, Liu Z and Zhang W J 2015 Mater. Chem. A 3
4954
is given in figure S8. Our present compound B is noth-
ing but a derivative of I. Thus we prepared I and took
its ESI mass and have observed the major peak at ∼365
(given in supporting information figure S7). The mass of
I (C24H18N4Pd2Cl2) is 645, but we get the major peak
at ∼365 (C12H9N2Pd + 2K+). In I the C12H9N2Pd frag-
ment has no charge, so it picks up two K+ from the K+-
formate matrix used during performing the ESI-mass.
Thus we conclude that similar thing happens in case of
B and because it has already two charges on the iron cen-
tres, no additional charge (K+) is required for the major
peak (∼733) to appear
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