Imidazole/benzimidazole-modified cyclotriphosphazenes as highly selective fluorescent probes for Cu2+: Synthesis, configurational isomers, and crystal structures

Article abstract of DOI:10.1039/c7dt01134b

Configurational isomers (cis and trans) of imidazole- or benzimidazole-modified cyclotriphosphazenes (3a, 4a or 3b, 4b) were designed, synthesized and investigated as fluorescent probes for metal ions. The newly synthesized compounds were characterized by ¹H and ³¹P NMR, and MALDI MS spectrometry. The configurations of geometric isomers were analyzed by X-ray crystallography and ³¹P NMR spectroscopy on addition of CSA. The photophysical behaviour and metal ion selectivity of the compounds were investigated by UV/vis and fluorescence spectroscopy. Among the examined 20 metal ions, the fluorescence emissions of the isomer mixtures were quenched by Cu²⁺ together with Fe²⁺, Fe³⁺, Zn²⁺ and Ni²⁺ ions, but each individual isomer (3a,b and 4a,b) exhibited an on-off-type fluorescence response with high selectivity towards only Cu²⁺ with a low limit of detection ranging from 1.27 μM to 2.04 μM. The complex stoichiometries of 3a,b and 4a,b with Cu²⁺ were determined as 1-:-1 (L/M) using the method of continuous variation (Job's plot) and density functional theory (DFT) calculations; moreover the complex formation of 4a with Cu²⁺ was unambiguously determined by X-ray crystallographic analysis that is consistent with the results obtained by the Job's plots as well as DFT.

Full text of DOI:10.1039/c7dt01134b

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Efficient extraction of sulfate from water using
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Imidazole/Benzimidazole-Modified
Cyclotriphosphazenes
as
Highly Selective Fluorescent Probes for Cu2+: Synthesis, Configurational
Isomers, and Crystal Structures
Aylin Uslu*, Süreyya Oğuz Tümay, Ahmet ꢀenocak, Fatma Yuksel, Elif Özcan, Serkan
Yeꢁilot*
Department of Chemistry, Gebze Technical University, Gebze 41400, Kocaeli, Turkey
∗ Corresponding authors. Tel.: +90 262 6053009 and +90 262 6053014
Eꢂmail addresses: aylin@gtu.edu.tr and yesil@gtu.edu.tr
keywords: Fluorescent probe, copper, imidazole, benzimidazole, phosphazene
ABSTRACT
Configurational isomers (cisꢂ and trans) of imidazoleꢂ or benzimidazoleꢂmodified
cyclotriphosphazenes (3a,4a or 3b, 4b) were designed, synthesized and investigated as
fluorescent probes for metal ions. The newly synthesized compounds were characterized by
1H and 31P NMR; and MALDI MS spectrometry. The configurations of geometric isomers
were analyzed by Xꢂray crystallography and 31P NMR spectroscopy on addition of CSA. The
photophysical behaviour and metal ion selectivity of compounds were investigated by UV/vis
and fluorescence spectroscopies. Among the examined 20 metal ions, the fluorescence
emission of the isomer mixtures were quenched by Cu+2 together with Fe2+, Fe3+, Zn2+ and
Ni2+ ions, but each individual isomer (3a,b and 4a,b) exhibited onꢂoffꢂtype fluorescence
response with high selectivity towards only Cu2+ with the low limits of detection ranging from
1.27 µM to 2.04 µM. The complex stoichiometries of 3a,b and 4a,b with Cu2+ were
determined as 1:1 (L/M) using the method of continuous variation (Job’s plot) and density
functional theory (DFT) calculation, moreover the complex formation of 4a with Cu2+ was
unambiguously determined by Xꢂray crystallographic analysis that is consistent with the
results obtained by the Job's plots as well as DFT.
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1. Introduction
Transitionꢂmetal ions can cause important environmental and health problems because of their
toxicity.1 Among these metal ions, as one of the most abundant transitionꢂmetal ions with zinc
and iron, copper plays a significant role in many fundamental biological and environmental
processes.2 However, excessive copper content in body can cause serious health problem like
liver or kidney damage and high levels of copper is also seriously toxic to environment.3ꢂ5
Copper contamination is still a challenging problem for today and future due to common use
of copper in agriculture and industry. Moreover, maximum acceptable level of copper in
water has been settled 1.3 mg/L (nearly 20µM/L) and 2.0 mg/L (nearly 30µM/L) by U.S.
Environmental Protection Agency (EPA) and the World Health Organization (WHO),
respectively.6,7 Therefore, in past two decades, development of highly sensitive, selective and
simple analytical methods for the detection of Cu2+, especially for the trace amounts, in
human body and environment have attracted great attention.8ꢂ12 Compared to
conventional analytical methods, such as high performance liquid chromatography, mass
spectrometry and atomic absorption spectroscopy, fluorescent probes have received
increasing attention due to their advantages including high sensitivity, low cost, short
response time and easy operation and useful applications in the chemistry, biology and
environment.13ꢂ17 Numerous fluorescence probes exist about Cu2+ detecting but many of these
are lack of selectivity and have relatively high detection limits.18ꢂ22 So, the research and
development of synthesis some highly sensitive and selective fluorescent probes for the
detection of Cu2+ have emerged as an important task in the field of chemical probes in recent
years.18ꢂ25
There is great interest in imidazole/benzimidazole ligands for structural chemistry
because they are very functional fluorophores to develop fluorescent probes. Although the
unsubstituted imidazole/benzimidazole have been known for their strong ability
to bind with many metal ions, their chemically modified derivatives could be employed as
highly selective fluorescent probes for the detecting different metal ions selectively such as
Fe3+, 26ꢂ29 Cr3+,26 Al3+,30 Zn2+,31,32 Ag+ 33 and Cu2+.8,9,11,34ꢂ36 In recent years, there has been
considerable interest in preparation of fluorescent probes containing cyclophosphazene
cores37ꢂ43 due to having hard inorganic rings containing six or eight halogen atoms that can be
easily modified by nucleophilic reaction with the desired groups.44 Furthermore,
chlorocyclophosphazenes are optically inert and their physical and chemical properties can be
tailored by the choice of appropriate substituted groups on phosphorus atoms. At the same
2

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time cyclophosphosphazenes have the ability to engage in coordination to transition metals by
different pendant donor groups attached the phosphazene skeleton and phosphazene systems
possessing nitrogens on the site groups have been prepared that demonstrate coordination
modes using various combinations of nitrogens of ring and/or site groups.45ꢂ47 For example,
the geminal, nonꢂgeminal, tetraꢂ or hexasubstituted pyrazolyl, pyridyloxy, or pyridylaminoꢂ
cyclophosphazene ligands have been extensively examined by the known regioꢂ and stereoꢂ
selectivities involved in the nucleophilic substitution reactions of chlorocyclophosphazenes
for the potential to bind many metal ions.48,49 Specifically, it was found that the pyrazolylꢂ
substituted cyclophosphazene ligands were very suitable and stable to form complex with the
copper ions.50ꢂ52 Inspired by the previously reported with a wellꢂdefined copper complex
based on pyrazolylꢂsubstituted cyclophosphazenes, we decided to exploit a similar approach
to choose the imidazole or benzimidozole groups as the functional fluorophores which are the
analogues to a pyrazole with two adjacent nitrogen atoms for design of new fluorescent
copper probes. To the best of our knowledge, however, no isolated configurational isomers of
phosphazeneꢂbased fluorescent probe for selective detection of Cu2+ has been reported so far.
In the present study, firstly phenyl and phenol groups were attached to
cyclotriphosphazene core in a nonꢂgeminal position in order to generate the configurational
isomers. Then, the imidazole or benzimidazole groups were modified with
cyclotriphosphazene core in the nonꢂgeminal position to synthesize some efficient compounds
possessing appropriate binding sites for transition metal ions to investigate their metal
detecting abilities. We have also desired to obtain both of the configurational isomers
(cis/trans) of these compounds in pure form, so as to able to examine their metal detecting
abilities individually. All synthesized ligands were characterized by the usual spectroscopic
and spectrometric techniques as well as elucidated their stereogenic properties by Xꢂray
crystallography and 31P NMR spectroscopy in the presence of a chiral solvating agent (CSA).
Xꢂray crystallography was also used for determining the structure of ligandꢂCu2+ complex that
could be obtained as a crystal.
2. Results and discussion
2.1. Syntheses and characterization
In this work, some fluorescent efficient cis and trans isomers containing azole side groups
were designed and synthesized using cyclotriphosphazene as a core in order to investigate
their metal detecting abilities as fluorescent probes. Imidazole or benzimidazole groups were
3

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nonꢂgeminally substituted into phosphazene ring to generate the potential metal binding site
because metal binding takes place mostly between nitrogen atoms of nonꢂgeminally
1
substitutens.45,47 Newly synthesized compounds were characterized using mass, 31P and H
1
NMR spectroscopies and Xꢂray crystallography. The mass, 31P and H NMR spectra data of
all the compounds were given in supporting information.
Designed molecules were constructed by a stepwise synthesis. For this purpose, firstly
compound 1, 2,2,4,4ꢂtetrachloroꢂ6,6ꢂdiphenylcyclotriphosphazene [N3P3Cl4(Ph)2], was
prepared as described in the literature.53 Then, the compound 1 was reacted with phenol in 1:2
ratio in THF and presence of sodium hydride for eight hours at room temperature as shown in
Scheme 1. It is known that phenoxy reagent was substituted to cyclophosphazene ring via a
nonꢂgeminal pathway that generates stable configurational diastereoisomers as cis and trans.53
31
The proton decoupled P NMR spectrum of the reaction mixture which was given in Figure
1a showed the formation of two main products which can be referred to as configurational
isomers as expected. From this reaction, a small quantity of pure 2a and a mixture of two
main products (2a and 2b, Scheme 1) could be isolated. However, the compound 2b couldn’t
be isolated in pure form and after that the mixture of 2a and 2b was used for subsequent
reactions.
Scheme 1. The reaction of compound 1 with phenol to give the cis and trans mixture of
compounds 2a and 2b.
4

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31
The proton decoupled P NMR spectrum of the reaction mixture which was given in
Figure 1a showed two AX2 spin systems that leads likely the cis–trans configurational
isomers and the proton decoupled 31P NMR spectrum of 2a in Figure 1b was analyzed as AX2
spin system. The relative amounts of the products found from the 31P NMR spectrum of the
reaction mixture (Figure 1a) were 45% and 55% for 2a and 2b, respectively. The crystal
structure of compound 2a was reported in this work and it was later confirmed that the
configuration of the compound 2a was the cis isomer.
[P(OPh)Cl]
(b)
[P(Ph)2]
ppm
22.0
21.0
20.0
19.0
18.0
17.0
(a)
2a
2b
ppm
22.0
21.0
20.0
19.0
18.0
17.0
Figure 1. The proton decoupled 31P NMR spectra of (a) the reaction mixture of phenol with
N3P3Cl4(Ph)2 and (b) compound 2a.
Compounds 3a,b and 4a,b were obtained as pure form from the reactions of mixture
of 2a,b with imidazole and benzimidazole, respectively. The substitution reactions were
carried out according to Scheme 2. When the mixture of compounds 2a and 2b was treated
with each azole group in a 1:2 molar ratio refluxing in THF and in the presence of
triethylamine, the nonꢂgeminal diphenol diꢂazole compounds, 3a,b or 4a,b were obtained as
shown in Scheme 2.
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Scheme 2. The reactions of the mixture of compounds 2a and 2b with imidazole and
benzimidazole to give compounds 3a,b and 4a,b, respectively.
The formation of two main products (3a and 3b) which were isolated as pure from the
reaction mixture were observed as AX2 spin systems in the proton decoupled 31P NMR
spectrum of reaction mixture of 2a,b with imidazole that was indicative of complete
replacement of chlorine atoms by the azole groups. The relative amounts of each product
determined from this 31P NMR spectrum were about 36% and 64% for 3a and 3b,
respectively. The protonꢂdecoupled 31P NMR spectra of pure compounds 3a and 3b were
observed as AX2 spin systems. Xꢂray crystallography later confirmed that compounds 3a and
3b were the cis and trans compounds, respectively.
The same case also occurred for isomers 4a and 4b that the reaction mixture spectrum
was analyzed as two AX2 spin systems. The protonꢂdecoupled 31P NMR spectra of reaction
mixture and isolated compounds 4a, 4b from this mixture were given in Figure 2 as an
example. The relative proportions observed from the spectrum were 45% and 55% for 4a and
4b, respectively, as shown in Figure 2a. The phosphorus chemical shifts, phosphorusꢂ
phosphorus coupling constants of all synthesized compounds were summarized in Table 4.
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[P(OPh)(C7H6N2)]
[P(Ph)2]
(c)
20.0
15.0
10.0
5.0
ppm
(b)
ppm
20.0
15.0
10.0
5.0
4a
4b
(a)
20.0
15.0
10.0
5.0
ppm
Figure 2. The proton decoupled 31P NMR spectra of (a) the reaction mixture of
benzimidazole with the mixture of 2a,b; (b) compound 4a and (c) compound 4b.
The complexation reactions of obtained ligands 3a,b and 4a,b with Cu(II) were done
in order to understand the correlation between the structure and complexation and only Cu(II)
complex of 4a was obtained as Xꢂray suitable pale blue crystals (compound 5).
2.2. Crystal structure analysis
The molecular structures of six compounds, 2a, 3a, 3b, 4a, 4b and 5 were established by
single crystal xꢂray structural analysis and the selected crystallographic data and refinement
parameters were presented in Table 1. All structures were cyclotriphosphazene derivatives
containing sixꢂmembered P3N3 ring and their crystal structures confirmed the assignments
from those spectroscopic data as shown in Figures 3ꢂ6. The sixꢂmembered
cyclotriphosphazene (P3N3) ring was geminal diꢂsubstituted with phenylꢂ (Ph) and nonꢂ
geminal diꢂsubstituted with phenoxyꢂ (PhO) in compound 2a (Figure 3). Two PhOꢂ groups of
compound 2a were in the same side of phosphazene ring to produce cis configuration. The
substitution of the mixture of compounds 2a and 2b with two imidazole (imꢂ) or
benzimidazole (bimꢂ) units, respectively gave geometrical isomers, 3a,b (Figure 4) and 4a,b
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(Figure 5) in which two PhOꢂ groups and two imidazole or benzimidazole groups were either
on the same or opposite sides of phosphazene rings to produce cis or trans configurations.
Figure 3. View of the molecular structure for compound 2a with the atomꢂnumbering
scheme. Displacement ellipsoids are drawn at the 30% probability level. The hydrogen atoms
have been omitted for clarity.
a)
b)
Figure 4. Views of the molecular structures of (a) compound 3a and (b) compound 3b with
the atom numbering scheme. Displacement ellipsoids are drawn at the 30% probability level.
The hydrogen atoms have been omitted for clarity.
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a)
b)
Figure 5. Views of the molecular structures of (a) compound 4a and (b) compound 4b with
the atom numbering scheme. Displacement ellipsoids are drawn at the 30% probability level.
The hydrogen atoms have been omitted for clarity.
The conformational parameters of cyclotriphosphazene rings of studied compounds
were presented in Table 2. The cyclophosphazene ring has a flattened chair conformation in
compounds 2a and 3a while it has a flattened boat conformation in compounds 3b, 4a and 4b.
The maximum deviation from the plane of the cyclophosphazene ring in compound 2a was
0.0785 (12) Å (on P2 atom). The substitutions with either imidazole or benzimidazole groups
caused to increase on maximum deviation from planarity in compounds 3a,b and 4a,b. The
deviation from planarity of phosphazene ring was larger in benzimidazole substituted
derivatives 4a,b then imidazole substituted derivatives 3a,b; it was 0.1065(16)Å (on P3) for
3a and 0.1018(16)Å (on N3) for 3b while it was 0.1177(13)Å (on P2) for 4a and 0.1487 (15)
(on N1) Å 4b (Table 2). The most deviation was observed in binuclear copper complex
compound 5 in which the max. deviation from planarity was 0.181(4) Å (on P2 atom).
The selected bond parameters of the compounds 2a, 3a,b, 4a,b, and 5 were
summarized in Table 2. The substituent effect on PꢂN bond parameters was observed: the PꢂN
bond lengths involving the gemꢂdiphenyl substituted P1 phosphorus atom [P(Ph)2] were
longer than other PꢂN bond lengths involving the P2 and P3 phosphorus atoms which were
nonꢂgemꢂsubstituted with phenoxy and chlorine, imidazole or benzimidazole groups
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[P(PhO)Cl, P(PhO)(im) and P(PhO)(bim)], because the electronꢂdonating substituents
increase the adjacent PꢂN bond lengths.54
The N–P–N bond angle belong to P1 phosphorus atom [P(Ph)2] was smaller than those
observed for P2 and P3 phosphorus atoms in all compounds. On the contrary, the endocyclic
P–N–P bond angles of N2 nitrogen atom present between P2 and P3 phosphorus atoms
[P(PhO)Cl, P(PhO)(im) and P(PhO)(bim)] was smallest. All these systematic differences
observed between P(Ph)2 and other P(PhO)Cl, P(PhO)(im) or P(PhO)(bim) moieties of all six
cyclotriphosphazene rings were compatible with previously observed bond parameters of
cyclotriphosphazene derivatives54,55 and they can be attributed to electronꢂreleasing character
of phenyl groups.
The crystal structure of compound 5 consists of two cisꢂdibenzimidazole substituted
phosphazene (4a) molecules joined together by Cu2Cl2 bridge (Figure 6). The dinuclear
structure was centrosymmetric and the asymmetric unit contains only one independent Cu(II)
centre and the second one was generated by symmetry operation [symmetry code (#): −x+1,
−y, −z]. Each fiveꢂcoordinated Cu(II) ion was surrounded by two benzimidazole nitrogen
atoms as well as one terminal and two bridging chlorides, which formed a distorted square
pyramidal geometry [Addison parameter (τ): 0.10].56 In the square plane, the Cu(II) was
coordinated by two N atoms and two chlorides in trans arrangement [NNCl(ꢃꢂCl) donor set]
and one (ꢃꢂCl) chloride ligand in the axial position. The CuꢂN bond were 1.990(4)Å and
2.004(4)Å. The equatorial CuꢂCl bonds [2.3037(14)Å and 2.3102(12)Å] were shorter than the
axial one [2.6426(14)Å]. The Cl2 and the centrosymmetric Cl2# ions served to bridge the two
Cu(II) centers and the bridging two chloride ligands were bound asymmetrically to both
copper atoms (Table 2). Two Cu(II) centers were close to each other with a Cu···Cu distance
of 3.5187(9) Å. The Cl2ꢂCu1ꢂCl2# and Cu1ꢂCl2#ꢂCu1# angles for the 4ꢂmembered ring are
almost orthogonal [89.71(4)° and 90.29(5)°]. The N–Cu–N arrangement was almost linear
with an angle of 174.13(15)° and the two coordinated imidazole rings were arranged in almost
coplanar arrangement with respect to the copper atom with only a 6.76° angle between them
(Figure S16 in supporting file). All these parameters were compatible with those previously
observed for similar bis(µꢂchloro)Cu(II) complexes.51,57ꢂ59
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Figure 6. Crystal structures of compound 5 with the atomꢂnumbering scheme. Displacement
ellipsoids are drawn at the 30% probability level. The hydrogen atoms have been omitted for
clarity. [Symmetry code: (#) −x+1, −y, −z]
A close investigation of the crystal structures of compounds 2a, 3a,b, 4a,b, and 5
showed that there are weak CH···N and CH···π interꢂmolecular interactions which were
effective in the stabilization of their unit cell (Figures S17ꢂS22 and Tables S1ꢂS6 in
supporting file). Especially in the crystal packing of compound 5 the weak π···π
intermolecular interactions stabilized tubular channel formation along a direction (Figure S22
in supporting file).
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Table 1. Xꢂray crystallographic data and refinement parameters for compounds 2a, 3a, 3b, 4a, 4b and 5.
5a
C30H26N7O2P3 C30H26N7O2P3 C38H30N7O2P3 C38H30N7O2P3 C76H60Cl4Cu2N14O4P6
Compound
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
2a
C24H20Cl2N3O2P3
546.24
3a
3b
4a
4b
609.49
173(2)
Monoclinic
P21/c
609.49
173(2)
Monoclinic
P21/n
709.60
173(2)
Monoclinic
P21/n
709.60
120(2)
Triclinic
P-1
1688.08
173(2)
Monoclinic
P21/c
120(2)
Monoclinic
P21
7.6214(2)
17.2180(4)
9.4330(2)
15.243(3)
8.429(2)
23.943(6)
10.8586(14)
16.050(2)
17.562(2)
19.144(2)
8.3339(10)
21.679(2)
9.3179(4)
11.1854(5)
16.7744(7)
81.422(2)
89.989(2)
79.340(2)
1698.24(13)
2
8.6840(5)
15.3755(9)
31.2787(18)
b (Å)
c (Å)
92.3800(17)
106.110(16)
106.757(4)°
104.241(8)
92.348(5)
β (°)
Volume (Å3)
Z
1236.78(5)
2
2955.5(12)
4
2930.7(7)
4
3352.5(6)
4
4172.9(4)
2
Density (calc, Mg/m3)
Absorption coeff. (mm-1)
F(000)
1.467
0.485
560
1.370
0.243
1264
1.381
0.245
1264
1.406
0.225
1472
1.388
0.222
736
1.344
0.807
1724
28.31°
22613
6139
0.0481
307
0.0357
0.0732
1.052
24.99
21325
5146
0.1535
379
0.0710
0.1867
1.019
26.37
24599
5990
0.0494
379
0.0411
0.1097
1.034
24.99
31786
5863
0.1560
451
0.0598
0.1676
1.024
28.36
29301
8423
0.0320
451
0.0407
0.1016
1.011
25.00
29425
7017
0.1592
478
0.0667
0.1584
0.980
θ
max (°)
Reflections collected
Independent reflections
Rint (merging R value)
Parameters
R (F2>2σF2)
wR (all data)
Goodness-of-fit on F2
a
The unit cell of 5 had a disordered solvent molecule which couldn’t be modelled.
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Table 2. The selected bond and conformational parameters for compounds 2a, 3a, 3b, 4a, 4b
and 5.
2a
3a
3b
4a
4b
5
Phosphazene Ring
0.0785(12)
(P2)
0.1065(16) 0.1018(16)
0.1177(13)
(P2)
0.1487(15)
(N1)
0.181(4)
(N2)
Max. deviation
(P3)
(N3)
0.151(2)
0.196(3)
0.1565(13)
0.223(2)
0.2489(11)
0.260(3)
Puckering
Amplitude(Q)
Theta(°)
Phi(°)
P1-N1
P1-N3
P2-N1
P2-N2
P3-N2
P3-N3
N1-P1-N3
N1-P2-N2
N2-P3-N3
P1-N1-P2
P2-N2-P3
P1-N3-P3
168.0(8)
284(4)
1.606(3)
1.610(3)
1.564(3)
1.587(3)
1.584(3)
1.566(3)
115.86(14)
117.85(15)
118.67(14)
121.95(15)
120.54(18)
121.64(17)
57.8(9)
191.9(10)
1.603(3)
1.611(4)
1.563(4)
1.583(4)
1.576(3)
1.559(3)
115.44(18)
119.20(19)
118.3(2)
122.0(2)
118.6(2)
122.5(2)
105.0(5)
107.6(5)
62.2(5)
168.6(7)
1.609(3)
1.614(3)
1.567(3)
1.587(3)
1.589(3)
1.565(3)
115.64(15)
118.05(16)
117.94(16)
122.01(18)
119.04(18)
122.6(2)
93.6(3)
232.9(3)
63.9(7)
175.4(8)
1.608(4)
1.614(4)
1.565(4)
1.587(4)
1.589(4)
1.561(4)
116.2(2)
118.2(2)
118.0(2)
121.3(3)
118.1(3)
121.8(2)
1.6091(16)
1.6122(16)
1.5693(16)
1.5841(16)
1.5841(16)
1.5725(16)
116.40(8)
117.50(8)
118.58(8)
122.80(10)
121.92(10)
120.74(10)
1.6064(14)
1.6144(15)
1.5743(15)
1.5814(15)
1.5879(14)
1.5677(15)
115.78(7)
118.57(8)
119.16(8)
121.04(9)
119.63(9)
120.85(9)
Substituent moiety
1.790(3)
1.793(3)
1.581(2)
1.581(2)
1.792(5)
1.7988(18)
1.8039(19)
1.5867(14)
1.5879(14)
1.7016(16)
1.6975(16)
1.796(4)
1.790(3)
1.585(3)
1.589(2)
1.681(3)
1.683(3)
1.7965(17)
1.7932(18)
1.5849(12)
1.5872(13)
1.6904(15)
1.6869(15)
1.781(5)
1.797(5)
1.574(3)
1.575(4)
1.704(4)
1.707(4)
P-C
P-O
P-N
1.789(5)
1.580(3)
1.581(3)
1.700(4)
1.691(4)
2.0153(12)
2.0173(12)
109.11(15)
P-Cl
106.4(2)
97.59(19)
99.25(18)
107.90(8)
97.14(8)
97.36(7)
106.60(16)
99.30(14)
96.99(14)
109.21(8)
96.94(7)
99.65(7)
107.3(2)
98.95(18)
98.63(19)
C-P-C
N-P-O
96.35(10)
97.19(9)
Cl-P-O
Coordination part of compound 5
1.990(4)
2.004(4)
2.3037(14)
2.3102(12)
2.6426(14)
3.5187(9)
174.13(15)
168.24(6)
102.01(5)
89.71(4)
90.99(11)
Cu1-N5
Cu1-N7
Cu1-Cl1
Cu1-Cl2
Cu1-Cl2#
Cu1-Cu1#
N5-Cu1-N7
N5-Cu1-Cl1
89.40(11)
92.74(11)
88.32(12)
90.10(11)
93.11(12)
Cl1-Cu1-Cl2
Cl1-Cu1-Cl2#
Cl2-Cu1-Cl2#
Cu1-Cl2#-Cu1#
N5-Cu1-Cl2
N5-Cu1-Cl2#
N7-Cu1-Cl1
N7-Cu1-Cl2
N7-Cu1-Cl2#
90.29(5)
symmetry code (#) for 5: −x+1, −y, −z
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2.3. Configurational analysis of compounds 3a,b, 4a,b
Tetraꢂcoordinated, pentavalent phosphorus atoms in hexachlorocyclotriphosphazene, N3P3Cl6,
are potential chiral centers as in organic compounds having tetrahedral carbon atoms.48
A
nonꢂgeminal diꢂsubstitution on the N3P3Cl6 generates cis and trans isomers. The trans isomer
containing two equal chiral centers is chiral and exists as two enantiomers (racemate, RR/SS)
while the cisꢂisomer containing two equal chiral centers is not chiral (meso, RS) because of
having a plane of symmetry.48 Considering the cyclotriphosphazene ring is planar, in terms of
centers of chirality its structure was commonly drawn by a stick diagram resulting from
projection of the structure perpendicular to the plane of the ring.48 Therefore, the structures of
compounds 3a,b; 4a,b were given by the stick diagram representations that make easy to
show the configuration on the each phosphorus as stereogenic center without using wedges
and dashes (Figure 7).
Figure 7. The stick diagram representations of compounds 3a,b; 4a,b.
By analogy with previous work on the stereochemistry of disubstituted cis and trans
isomers of N3P3Cl6, in generally the 31P NMR signals on addition of the chiral shift reagent
(CSR) or chiral solvating agent (CSA) of trans isomer split into two lines of equal intensity
corresponding to the separate enantiomers, whereas the NMR signals of cis isomer should be
unaffected except for changes in chemical shift.48 In this study, the stereogenic properties of
the compounds were supported by 31P NMR spectroscopy on addition of a CSA, (R)ꢂ(+)ꢂ
2,2,2ꢂtrifluoroꢂ1ꢂ(90ꢂanthryl)ethanol. It was found that all 31P NMR signals of compounds
3a,b and 4a,b exhibit chemical shift changes indicating complexation with the CSA. On
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addition of a CSA at a 50:1 mol ratio, >P(OPh)(Azoles) peaks of compounds 3b, 4b separate
into two lines of equal intensity consistent with 3b, 4b existing as racemate (RR/SS), but CSA
does not cause extra splitting of the NMR peaks of compound 3a, 4a because they are meso
(RS) diastereoisomers as expected.48 As the example, the protonꢂdecoupled 31P NMR spectra
of a mixture of compounds 3a,b and after added CSA at a 50:1 mol ratio were given in Figure
8. The changes in 31P NMR chemical shifts and separations of signals of compounds 3a,b and
4a,b at a 50:1 mol ratio of CSA were summarized Table 3.
Structures of 3a,b and 4a,b were also confirmed by Xꢂray crystallography that
compounds 3a, 4a are cis diastereoisomers and 3b, 4b are trans diastereoisomers consistent
with the CSA addition results.
>P(OPh)imidazole
>PPh2
(b)
ppm
20.0
15.0
10.0
5.0
0.0
(a)
3a
3b
ppm
20.0
15.0
10.0
5.0
0.0
Figure 8. Protonꢂdecoupled 31P NMR spectra of (a) a mixture of 3a,b and expansion of
signals of the >PPh2 and >P(OPh)(imidazole) moieties; (b) a mixture of 3a,b with added CSA
at ca. 50:1 mole ratio and expansion of signals of the >PPh2 and >P(OPh)(imidazole) moieties
in CDCl3 solution.
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Table 3. 31P NMR parameters of compounds 2a,b;3a,b and 4a,b.a
Compound Chemical shifts / ppm
2J(PP) / Hz
> P(OPh)Az
> P(OPh)Cl
>PPh2
(a) Assignment of signals
16.61
16.66
22.00
22.07
23.34
22.72
22.82
22.32
25.80
24.70
27.40
27.50
27.00
27.30
2a
2bb
2.51
3.16
1.51
2.25
3a
3b
4a
4b
(b) Change of chemical shift (ppb) on addition of CSA in 50:1 mole ratio
ꢂ184
ꢂ43
ꢂ115
ꢂ18
170
172
144
109
27.20
27.20
26.70
27.20
3a
3b
4a
4b
(c) Separation of signals (ppb) on addition of CSA in 50:1 mole ratio
c
c
3a
3b
4a
4b
30
59
c
c
d
54
a202.25 MHz 31P NMR measurements in CDCl3 solutions at 298K analysed as AX2 spin systems. Chemical
b
shifts referenced to external H3PO4; it has been observed in the 31P NMR spectra of reaction mixture; no
c
separation of 31P NMR peaks observed; d peaks have not seen clearly because of background noise.
2.4. Spectral studies
The metal detecting behavior of imidazole, 3 (a mixture of 3a + 3b), and benzimidazole, 4 (a
mixture of 4a + 4b), isomers were initially investigated. The UVꢂVis and fluorescence
spectroscopic experiment of compounds 3 and 4 were measured in acetonitrile: water (1:1)
and acetonitrile with dilute solutions of 50 µM and 10 µM, respectively. All of the spectral
measurements were carried out in a spectroscopic cuvette by using a micropipette at 25 °C. 50
µM compound 3 and 10 µM compound 4 were prepared from their 0.01 M stock solutions.
Compound 3 exhibited absorption bands centered around 260ꢂ267 nm, and maximum
absorption bands of 4 nm were showed at 272ꢂ281 nm. The fluorescence maximums of 3 and
4 were observed at 284 nm and 294 nm, respectively, when excited at 250 nm which
attributed to phenol/phenyl and benzimidazole moieties (Figure 9).60,61 The origin of the
fluorescence signals of compounds 3a-b and 4a-b originated from πꢂπ* transition of
phenol/phenyl and/or benzimidazole moieties as expected.62,63
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To evaluate the fluorescence probe properties of compounds 3 and 4 in acetonitrile: water
(1:1) and acetonitrile, 100 µM different competitive metal ions (Al3+, Li+, Na+, K+, Cs+ , Mg2+,
Ca2+, Ba2+, Hg2+ , Pb2+, Mn2+, Cd2+, Ag+, Ni2+, Cu2+, Zn2+, Co2+, Cr3+, Fe2+ and Fe3+) were
added to solutions of 3 and 4. The UVꢂVis spectra of 3 and 4 nearly unchanged upon the
addition of 100 µM of various metal ions except for Cu2+, Fe2+, Fe3+ and Ni2+ (Figures 10a,b).
In the case of Cu2+, the new absorption bands were observed at 310 nm and 461 nm which can
attributed charge transfer between metal and ligands as expected.64 According to Figure 11a,b,
fluorescence signals of 3 and 4 significantly quenched by upon addition of 100 µM Cu2+,
Fe2+, Fe3+, Zn2+ and Ni2+ when other various metal ions cause minor change.
Figure 9. Absorption and fluorescence responses of compound 3 (50 µM) and 4 (10 µM) in
ACN: Water (1:1) and ACN, respectively (Excitation wavelength= 250 nm).
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Figure 10. Absorption responses of a) compound 3 (50 µM) in ACN: Water (1:1) and b) 4
(10 µM) in ACN upon the addition of 100 µM various metal.
Figure 11. Fluorescence responses of a) compound 3 (50 µM) in ACN: Water (1:1) and b) 4
(10 µM) in ACN upon the addition of 100 µM various metal. Insets: Metal ions selectivity of
compound 3 (50 µM) in ACN: Water (1:1) and b) 4 (10 µM) in ACN. The dark blue bars
represent the fluorescent intensity of 3 and 4. The yellow bars represent the fluorescent
intensity of 3 and 4 and various ions (Excitation wavelength= 250 nm).
The results of UVꢂvis and fluorescence signals of imidazole or benzimidazole isomers (3
or 4) indicated that these forms cannot be used as a potential specific fluorescent probe for
selective detection of any metal ions. After the separated the two configurational isomers (cisꢂ
or trans) of 3 and 4, optical properties of each individual isomer (3a, 3b, 4a or 4b) were reꢂ
investigated by UVꢂvis absorption and fluorescence spectroscopies. The fluorescence
properties of 50 µM 3a, 3b and 10 µM 4a, 4b were studied in various solvents such as water,
cyclohexane (CH), dichloromethane, THF, ethanol, acetonitrile, acetonitrile: water (1:1) and
acetonitrile (5:1). According to Figure 12aꢂd, the maximum fluorescence signals of 50 µM 3a
and 3b were obtained in ACN: Water (1:1) at 284 nm. The fluorescence spectra of 10 µM 4a
and 4b showed that maximum signal at 294 nm were obtained in ACN. For this reason, ACN:
Water (1:1) was chosen as solvent mixture for 3a and 3b and ACN for 4a and 4b which is
also water miscible organic solvent, to use in our study. To investigate concentration effect on
optical properties of 3a, 3b, 4a and 4c different concentration of these compounds were
prepared and 50 µM for 3a and 3b, 10 µM of 4a and 4b chosen for spectral studies (Figure
13a,b). As shown Figure 13, the absorption and fluorescence maxima of 3a and 3b, 4a and 4b
were the identical to their isomer mixtures 3 and 4. The fluorescence probe properties of 3a,
3b, 4a and 4b against competitive metal ions were investigated. For these purpose, 100 µM of
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various metal ions (Al3+, Li+, Na+, K+, Cs+, Mg2+, Ca2+, Ba2+, Hg2+, Pb2+, Mn2+, Cd2+, Ag+,
Ni2+, Cu2+, Zn2+, Co2+, Cr3+, Fe2+ and Fe3+) were added to ACN: Water (1:1) solutions of 3a
and 3b (10 µM), ACN solutions of 4a and 4b (50 µM).
Figure 12. Fluorescence responses of a) 50 µM 3a b) 50 µM 3b c) 10 µM 4a and d) 10 µM
4b in different solvent (Excitation wavelength= 250 nm).
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Figure 13. a) Fluorescence responses b) UVꢂVIS absorption responses of 3a, 3b, 4a and 4b
in ACN: Water (1:1) and ACN solution in different concentrations (Excitation wavelength=
250 nm).
As can be seen Figure 14aꢂd, the UVꢂVis spectra of 3a, 3b, 4a and 4b nearly unchanged
upon the addition of 100 µM of various metal ions except for Cu2+. According to UVꢂVis
spectra, only addition of Cu2+ changed maximum absorption wavelength and new bands
appeared at 310 nm and 461 nm. After addition of 100 µM of Cu2+, maximum fluorescence
signals completely quenched for 4a, 4b and remarkable quenched for 3a, 3b, when other
competitive metal ions nearly did not affect signals (Figure 15aꢂd).
To evaluate the influence of pH with all compounds (3aꢂ4b), whereas other parameters
were kept constant, 0.1 mL of BrittonꢂRobinson buffer systems was used to adjust the pH of
the solvents from 6.09 to 9.91 and plotted in presence of 100 µM Cu2+. The relative
fluorescence intensity of compounds 3aꢂ4b were relatively unaffected by pH 6.09ꢂ8.36 as
seen from Figure S23. Influence of time was investigated and plotted from 5 second to 120
second in presence of 100 µM Cu2+. After 5 second, relative fluorescence responses of all
compound reached maximum value and then nearly did not change after this time as seen
from Figure S24. Therefore, the detection of Cu2+ ion measurements have done after 10
seconds for consistent and accurate results.
In order to receive more information about binding mechanism of compounds 3a, 3b, 4a
and 4b towards Cu2+, fluorescence titration experiments were performed in the 0ꢂ100 µM
Cu2+ (Figure 16aꢂd). According to fluorescence titration curves of compounds 3a and 3b
(Figure 16aꢂb) and 4a and 4b (Figure 16cꢂd), fluorescence maxima at 284 nm for 3a, 3b and
294 nm for 4a, 4b were gradually decreased and finally almost completely quenched. The
highly efficient quenching of Cu2+ proposed that fluorophores showed a specific response to
Cu2+ due to the chelationꢂenhanced fluorescence quenching effect which was previously
reported in the literature.65,66 This could be attributed to the significant binding of Cu(II) in
the nitrogen coordination pocket of fluorophores and could prefer the efficient energy/electron
transfer from paramagnetic Cu(II) to the imidazole units.
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Figure 14. UVꢂVIS absorption responses of a) 3a (50 µM) in ACN: Water (1:1), b) 3b (50
µM) in ACN: Water (1:1), c) 4a (10 µM) in ACN and d) 4b (10 µM) in ACN upon the
addition of 100 µM various metal ions.
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Figure 15. Fluorescence responses of a) 3a (50 µM) in ACN: Water (1:1), b) 3b (50 µM) in
ACN: Water (1:1), c) 4a (10 µM) in ACN and d) 4b (10 µM) in ACN upon the addition of
100 µM various metal ions. Insets: The dark blue bars represent the fluorescent intensity of
compound 3a, 3b, 4a and 4b. The yellow bars represent the fluorescent intensity of compound
3a, 3b, 4a and 4bꢂvarious ions (Excitation wavelength= 250 nm).
The fluorescence titration profiles of 3a, 3b, 4a and 4b with Cu2+ in the range of 0 to 100
µM showed the Cu2+ sensitivity of probes. The detection limit of compounds 3a, 3b, 4a and
4b for Cu2+ ion by fluorescence changes was calculated on the basis of 3σ /K67ꢂ69 and found to
be 1.27 µM, 1.54 µM, 1.89 µM and 2.04 µM pointing to the high detection sensitivity (Figure
16 insets). These results suggested that compounds 3a, 3b, 4a and 4b can be used for the
selective detection of Cu2+ in ACN: Water (1:1) and ACN solutions. Precision of the probe is
an important analytical parameter for probe application. Therefore, ten measurements were
performed for 100 µM of Cu2+ under the same conditions for all compounds. The relative
standard deviation (% RSD) was calculated as %4.86, %4.41, %4.92 and % 3.74 for
compounds 3a, 3b, 4a and 4b, respectively. According to these results, high reproducibility
can be provided for selected probes.
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Figure 16. Fluorescence titration of a) 3a (50 µM) in ACN: Water (1:1), b) 3b (50 µM) in
ACN: Water (1:1), c) 4a (10 µM) in ACN and d) 4b (10 µM) in ACN with various
concentration of Cu2+. (Excitation wavelength= 250 nm). Inset: Calibration curve of 3a, 3b,
4a and 4b versus Cu2+ ions.
A comparison of the UVꢂVis spectra of 10 µM compound 4a + 1 eqv. Cu2+ and
compound 5 which was complex of 1:1 (mole/mole) compound 4a and Cu2+ was presented at
Fig. S29. As shown in Fig. S29, all new absorption bands (310 nm and 461 nm) which can
attributed charge transfer between metal and ligand were observed at both 10 µM compound
4a + 1eqv. Cu2+ and compound 5. According to this result, it can be said that spectroscopic
absorption result of 10 µM compound 4a + 1eqv. Cu2+ is consistent with the crystal structure
of compound 5.
2.5. Metal-Ion complex formation of ligands 3a,b and 4a,b
Job’s plot analyses were used to investigate the stoichiometry between ligands and Cu2+
(Figure 17aꢂd). The results showed that the maximum fluorescent emission intensity of all
ligands and Cu2+ complexes appeared at 0.5, which indicated that 3a, 3b, 4a and 4bꢂCu2+
complexes were formed in 1:1 ratio. Furthermore, the association constants, K, of the target
complexes were then calculated to be 2.81x105 Mꢂ1, 2.74x105 Mꢂ1, 2.46x105 Mꢂ1 and 2.37x105
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Mꢂ1 for compounds 3a, 3b, 4a and 4b, respectively according to BenesiꢂHildebrand equation
(a), as shown in Figure 18aꢂd. In this equation, F is fluorescence signal of compounds after
addition of various amount of Cu2+ at maximum wavelength, F0 is fluorescence signal of
initial compounds at maximum wavelength and Fmax is maximum fluorescence signal of
compounds at maximum wavelength. Although the stereochemistry of compounds were
different, the same binding modes with Cu2+ (1:1 for L/M) determined using Job’s plot and
DFT methods led to similar association constants of the isomers.
(a)
Figure 17. The Job’s plot for a) 3a (50 µM) and Cu2+, b) 3b (50 µM) and Cu2+, c) 4a (10
µM) and Cu2+and d) 4b (10 µM) and Cu2+ (Excitation wavelength= 250 nm).
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Figure 18. Plot of 1/FꢂF0 against 1/[ Cu2+] for a) 3a (50 µM) in ACN: Water (1:1), b) 3b (50
µM) in ACN: Water (1:1), c) 4a (10 µM) in ACN and d) 4b (10 µM) in ACN.
Numerous attempts have been made to prepare Cu2+ complexes of all obtained ligands
(3a, 3b, 4a and 4b) in order to better understand the coordination of ligands with Cu2+ but
only binuclear copper complex of compound 4a was obtained as Xꢂray suitable pale blue
crystal (compound 5, Figure 6). In the crystal structure, cyclotriphosphazene units of
compound 5 linked to “Cu(µꢂCl)2Cu” bridges that the copper(II) ions showed the distorted
squareꢂbased
pyramidal conformation
with coordinated
through two
nongeminal benzimidazole nitrogen atoms. It should be noted that the ring nitrogen atoms of
the cyclophosphazene skeleton did not participate in interaction with Cu+2, though many
cyclophosphazene ligands having exocyclic groups with two adjacent nitrogen atoms
involved in interactions with the ring nitrogen atoms by transition metals.49ꢂ52
Compound 5, which is the complex of 4a with Cu+2, has crystal structure but due to
only limited amount of experimental information available for the complexes of 3a, 3b and 4b
with Cu+2, the exact coordination complex structures were difficult to identify. Firstly,
compound 5 was optimized with DFT m062x and B3LYP/LanL2DZ method and the results
was similar to crystal structure analysis. Therefore, this method was studied for the other
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compounds 3a, 3b and 4b. Since there is possible structures of square planar, tetrahedral,
octahedral and even square based pyramidal structures of transition metal ions with either
solvent and/or inorganic anions in the media, Cu2+ can complete the first coordination shell
with H2O or Cl−. Thus, we tried to understand only the qualitative interaction between the
metal ion and ligand, instead of identifying the true coordination complex formation. We
compared quantum mechanical calculations possible tetrahedral copper complexes formed by
Clꢂ ions for 3a, 3b and 4b. The optimized structures of the possible the Cu2+ complexes were
shown in Figure S25ꢂ28. The data obtained from DFT calculations were given in Table S7.
The crystal structure of the complex (5) has meso configuration with a center of
symmetry because of the structural limitation of the cisꢂconfigured starting material (4a) and a
similar complexation mechanism can be proposed for cisꢂconfigured 3a as shown in the stick
diagram in Figure 19a.
As shown in the stick diagram in Figure 19b, a cyclolinear polymer with coppers
bridges could be proposed for the complex formation of the transꢂconfigured starting
materials (3b and 4b) considering with the trans structures of ligands and 1:1 (L/M) binding
stoichiometry from Job's plot analysis.
Figure 19. The proposed stick diagram representations of complex formations of cis and
trans geometrical isomers with CuCl2.
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3. Experimental
3.1. Materials
The chemicals were obtained commercially and used as received; aluminum chloride (98%),
phenol (99%), benzimidazole (99%), nꢂhexane (99%), dichloromethane (99.0%),
triethylamine (99%), cyclohexane (99%), ethanol (99%), acetonitrile (99%), deuterated
chloroform (for NMR spectroscopy) from Merck, imidazole (99%) from Alfa Aesar, benzene
(>98%) and tetrahydrofuran (>99%) from Aldrich. Chiral solvating agent, (R)ꢂ(+)ꢂ2,2,2ꢂ
trifluoroꢂ1ꢂ(9ꢂanthryl)ethanol (CSA) from Sigma. Tetrahydrofuran, THF (Aldrich) was
distilled over sodium/potassium alloy under an atmosphere of dry argon. Prior to use the oil in
a sodium hydride, 60% dispersion in mineral oil (Merck) was removed by washing with dry
nꢂhexane followed by decantation. All reactions were performed under a dry argon
atmosphere. Hexachlorocyclotriphosphazene which was purified by crystallisation from nꢂ
hexane was obtained from Aldrich. Thin layer chromatography (TLC) and column
chromatography were carried out on Merck silica gel plates (Kieselgel 60, F254 indicator,
0.25 mm) and silica gel (Kieselgel 60, 70–230 mesh) respectively.
3.2. Equipment
Bruker Daltonics Microflex LT MALDIꢂTOF mass spectrometers were used to obtain mass
spectra. NMR (31P, 1H) spectra were recorded on a Varian INOVA 500 MHz spectrometer in
CDCl3 solutions using 85% H3PO4 as external reference for 31P and TMS as internal reference
for 1H. Electronic absorption spectra were recorded with a Shimadzu 2101 UV
spectrophotometer in the UVꢂvisible region. Fluorescence excitation and emission spectra
were recorded on a Varian Eclipse spectrofluorometer using 1 cm pathlength cuvettes at room
temperature and slit withs were all set to 5 nm.
3.3. X-ray crystallography
Intensity data were recorded on a Bruker APEX II QUAZAR diffractometer. Absorption
correction by multiꢂscan has been applied70 and space groups were determined using XPREP
implemented in APEX2.71 Structures were determined using the direct methods procedure in
SHELXSꢂ97 and refined by fullꢂmatrix least squares on F2 using SHELXLꢂ97.72 All nonꢂ
hydrogen atoms were refined with anisotropic displacement factors and CꢂH hydrogen atoms
were placed in calculated positions and allowed to ride on the parent atom. Furthermore, the
unit cell of 5 had a disordered solvent molecule which couldn’t be modelled. Therefore, the
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SQUEEZE command of PLATON73 was used for removing solvent molecule then the rest of
the molecule was refined without solvent. There were two cavities with 381 Å3 volume and
96 void electron count. The final geometrical calculations and the molecular drawings were
carried out with PLATON,73 MERCURY,74 and DIAMOND (Version 3.1)75 programs.
Structure determinations have been deposited with the Cambridge Crystallographic Data
Centre with references CCDC 1525898ꢂ152903 for structures 2a, 5, 4b, 3b, 4a and 3a,
respectively.
3.4. DFT Calculations
The full geometry optimization on the basis of crystal structures of compounds 3a, 3b, 4b and
5 were carried out with the density functional theory method by a hybrid functional B3LYP
functional (Becke’s Three Parameter Hybrid Functional Using the LYP Correlation
Functional)76,77 and Los Alamos National Laboratory 2ꢂDoubleꢂZeta (LanL2DZ) basis set78
have been chosen in the DFT calculations, in this study. LanL2DZ basis set was used in the
calculations in order to make a comparison with the structural parameters of Cu(II) complexes
of molecules. Molecular geometry was restricted and all the calculations for compounds are
performed using Gaussian 0979 program package on a TUBITAK ULAKBIM, High
Performance and Grid Computing Center (TRꢂGrid eꢂinfrastructure).
3.5. Synthesis
2,2,4,4ꢂTetrachloroꢂ6,6ꢂdiphenylcyclotriphosphazene [N3P3Cl4(Ph)2] (1), was prepared as
described in the literature.50
3.5.1. cisꢂ[N3P3Cl2(Ph)2(OC6H5)2] (2a) and transꢂ[N3P3Cl2(Ph)2(OC6H5)2] (2b)
To a stirred solution of 2,2,4,4ꢂtetrachloroꢂ6,6ꢂdiphenylcyclotriphosphazene [N3P3Cl4(Ph)2]
(1) (2.0 g, 4.64 mmol) in 50 mL tetrahydrofuran (THF) was added NaH (0.4 g, 9.28 mmol)
under an argon atmosphere. The solution was cooled to 0°C using an iceꢂbath, phenol (0.88 g,
9.28 mmol) in 20 mL THF was then added dropwise to the stirred mixture under an argon
atmosphere. The mixture was stirred for 24 h at room temperature, then precipitate was
filtered off and the solvent was removed under reduced pressure. From the crude mixture, a
small quantity of compound 2a and the mostly isomer mixture (2a and 2b) were obtained by
column chromatography using dichloromethane/nꢂhexane (1:1) as the mobile phase. 2a and
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Page 29 of 39
Dalton Transactions
View Article Online
DOI: 10.1039/C7DT01134B
2b, 1.2 g, 47%. Compound 2a was crystallized from dichloromethane/nꢂhexane (1:1), 2a, 0.07
1
g, 3%, mp 125°C, M:545.74m/z (calc.545.01), H NMR, CDCl3, 298 K, δ (ppm); 7.21ꢂ7.87
2
(m, ArꢂH), 31P NMR, CDCl3, 298 K, δ (ppm); 16.61 (d, P(OPh)Cl), 22.00 (t, PPh2), J(PP)=
25.80 Hz. Calc. for C24H20Cl2N3O2P3 requires: C, 52.77; H, 3.69; N, 7.69%; found: C, 52.48;
H, 3.53; N, 7.57%
3.5.2.
cisꢂ[N3P3Cl2(Ph)2(OC6H5)2(C3H3N2)2]
(3a)
and
transꢂ
[N3P3Cl2(Ph)2(OC6H5)2(C3H3N2)2] (3b)
A mixture of compounds 2a and 2b (0.50 g, 0.92 mmol) was dissolved in THF (35 ml) and
triethylamine (0.25 ml, 1.84 mmol) was added under an argon atmosphere. Imidazole (0.12 g,
1.84 mmol) dissolved in 15 mL THF was then added dropwise to the stirred solution of
mixture of compounds 2a and 2b under an argon atmosphere. The mixture was refluxed until
TLC indicated complete consumption of the starting compounds 2a and 2b. After refluxing
for 72h, the mixture was cooled to ambient temperature, then the triethylamine hydrochloride
salt formed was filtered off and the solvent was removed under reduced pressure. From the
crude mixture, compounds 3a and 3b were isolated by column chromatography using
tetrahydrofuran/nꢂhexane (2:1) as the mobile phase. Compounds 3a and 3b were crystallized
from dichloromethane/nꢂhexane (2:1); 3a, 0.25 g, 45%, mp 130°C, M:609.84m/z
(calc.609.51); 1H NMR, CDCl3, 298 K, δ (ppm); 6.91ꢂ7.84 (m, ArꢂH), 31P NMR, CDCl3, 298
2
K, δ (ppm); 2.51 (d, P(OPh)Az), 23.34 (t, PPh2), J(PP)= 27.40 Hz; Calc. for C30H26N7O2P3
requires: C, 59.12; H, 4.30; N, 16.09%; found: C, 58.63; H, 4.28; N, 15.88%; 3b, 0.18 g,
32%, mp 150°C, M:609.14m/z (calc.609.51), 1H NMR, CDCl3, 298 K, δ (ppm); 6.89ꢂ7.69 (m,
2
ArꢂH), 31P NMR, CDCl3, 298 K, δ (ppm); 3.16 (d, P(OPh)Az), 22.72 (t, PPh2), J(PP)=27.50
Hz. Calc. for C30H26N7O2P3 requires: C, 59.12; H, 4.30; N, 16.09%; found: C, 58.77; H, 4.17;
N, 15.74%;
3.5.3.
cisꢂ[N3P3Cl2(Ph)2(OC6H5)2(C7H5N2)2]
(4a)
and
transꢂ
[N3P3Cl2(Ph)2(OC6H5)2(C7H5N2)2] (4b)
A mixture of compounds 2a and 2b (0.5 g, 0.92 mmol) was dissolved in THF (35 ml) and
triethylamine (0.25 ml, 1.84 mmol) was added under an argon atmosphere. Benzimidazole
(0.22 g, 1.84 mmol) dissolved in 15 mL THF was then added dropwise to the stirred solution
of mixture of compounds 2a and 2b under an argon atmosphere. The mixture was refluxed
until TLC indicated complete consumption of the starting compounds 2a and 2b. After
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