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Indian Journal of Chemistry
Vol. 38A, December 1999, pp. 1223-122 7
Synthesis and characterisation of lanthanide(III) complexes of 5-methyl2-(2' -pyridyl)benzimidazole and 2-(2' -pyridyl)benzoxazole
P Thakur, V Chakravortty & K C Dash*
Department of Chemi stry, Utkal Uni vers ity, Bhubaneswar 75 1 004 . Indi a
Received 31 May 1999: revised 2(} September 1999
Lanthanide(III) thiocyanate complexes of the type [Ln(NCSMLhln .H20 and fLn (NCSML')21n.HzO where Ln=Y. PrDy. n=2. Ln=Y. Pr- Sm. Eu-Dy, n=3, I) and L & L' are 5-meth yl-2(2' -pyridyl)benzimidazo le (MePB H) and 2-(2'pyrid yl)b.:nzoxazo le(PBOX), respecti ve ly have been synthesised and characterised by va rious physico-chem ica l techniques
such as the conductance and thcrmogravimetric studi es. electroni c. IR. NMR CH and 13C) and fluorescence spectral
investi gations. All the comp lexes show the luminescence of the ligand . The Sill, Eu, Tb. and Dy comp lexes show the
co rresponding m.:tal ion luminescence.
Luminescence spectroscopi c measurements have been
used in coordination chemistry to determin e . the
sy mmetry, di ssociation con stants, number of bonded
wa ter molecules, metal-metal di stances, number of
di stinct metal environments and bonding nature of the
Iigand I. Certain Eu(lIl) complexes are reported to
give laser action in so lution ; the Nd(lll) ion also
provides laser action when present in aprotic solvents,
such as selenium and phosphorus oxychlorides. The
compounds of Tb( III ), Dy(III), Er(lII) and Ho( lII)
have th e hi ghest known magnetic moments, in the
range 9-11 B.M. at room temperature. Several
lanthanides( llI ) ion s are useful as shift reagents in
nuclear magnetic resonance imaging spectroscopy
(MRI) and Gd(II1) has been shown to be an effective
contrast agent for in vivo nuclear magnetic resonan ce
.
.
2-5
IInaglllg .
Trivalent lanthanides behave as hard acids and
these are expected to form stron ger complexes with
li gands containing 0 and N donor atoms. In view of
the many interesting properties of lanthanides, we
report here the sy nthesis and characterisation of
complexes with bidentate biheterocyclic ligands 5methyl-2-(2' -pyridyl)benzimidazole
and
2-(2'pyridyl)benzoxazole.
Materials and Methods
Hydrated metal nitrates (Indian Rare Earths Ltd .,
Udyogmandal 99 .99% pure) were used as such.
Methanolic solutions of liletal thi ocyanates were
obtained by metathesis of the respective metal nitrates
with stoichiometric amounts of KN CS. Phys ical
measurements were carried out as reported earl ieru,7 .
I Hand I.1C NMR spectra of the ligand and its metal
complexes were obtained on a Bruker AC 300 MHz
FTNMR spectrometer. Fluorescence spectra of the
complexes were recorded on a F-30 I 0 Hitachi
spectrofluorometer and electronic spectra of th e
complexes were recorded on a Beckman DU-7
spectrophotometer. The metal content of th e
complexes was determined by ignition to
corresponding metal oxide. A few complexes (S m,
Tb, Y & Gd) were decomposed by repeated treatment
with conc. HNO, and H2S04 and fin ally meta l
contents were estimated complexometrically by
EDT A using xy lenol orange at pH - 6. The ligand
MePBH(L) was prepared by a sli ght modificati on of
the method reported in literatureS and PBOX (L') was
prepared by th e foll ow ing method .
Synthesis of 2-(2 '-pyridyl)benzoxazole(L?
The li gand was prepared by co nd ensing 2aminophenol with pyridene-2-carboxylic acid in
prese nce of polyphosphoric ac id (PPA). After heating
the reaction mixture at 200 C lo r 4 h. The
temperature was brought down to 100°C and th e
contents were poured into a large vo lume of rapidly
stirred water. The resultant slurry was made alkaline
with 50% NaOH . The crude produ ct was collected by
filtration and washed with sufficient amount of water,
dried and crystallized from n-hexane, m.p. 110-111 DC
[Found: C, 73.4, H, 4.0, N, 14.3 . Ca lc . for C I2 HgN20:
C, 73.5 , H, 4.1 , N, 143%]: miz 196 in EI mass
spectrum . IHand I' C NMR (DMSO-d6-CDCI, 300
MHz) ; rin g proton 7.37-8.73 pplll rin g carbons,
161 . 11 (C-2), 150.32 (C-8), 149.91 (C-6'), 145 .19 (CD
1224
[NDIAN J C HEM , SEC. A, D ECE MBER 1999
2'), 141.16 (C-9), 137.25 (C-4'), 125 .86 (C-4, 6),
124.76 (C-3' ), 123 .3 3 (C-5'), 120 .1 4 (C-5) and 111.0
(C-7).
were synthesised by refluxing methanolic solutions of
the li gands (L and L') and the correspondi ng metal
salts in stoichiometric amounts at 70-80°C fo r 4 h.
The so lid complexes were filtered off and washed
with aliquots of methanol followed by diethy lether,
recrystall ised from methanol and finally dried in
Synthesis of camp/exes
The coordination
[Ln(NCSh(L)2]nn .H20
complexes of the types
and [Ln(NCSML')2]n .H 20
vacuo.
Results and Discussion
The li gands 5-methyl··2-(2' -pyridyl)benzimidazole
(MePBH) and 2-(2'-pyridyl)benzoxazo le (PBOX) are
strong che lati ng ligand s similar to 2,2'-bipyridine and .
I, I O-phenanthroline and have two potential donor
PBOX
Table I -Analyti cal and co nducti vity data of lanthanide thiocyanate co mpl exes of L'
Co mpl exes
Colo ur
M.P .
(0C)
% M
%C
'YoH
%N
%S
[Y (NCS)3( L)z2 H zO
White
278d
[Pr(NCSMLh12H zO
>300
(Nd(NCS)3(L)z12H zO
Greeni sh
white
White
ISm(NCSh(Lh12H zO
Dull whit e
290d
[Eu(NCS)3(Lh12H zO
W hi te
>300
[Gd(NCSh(L)z12H zO
White
284d
[Tb(NCSML)z12H zO
White
269d
[Dy(NCSML)z12H zO
White
>300
9.5
(9.9)
17.2
( 17.7)
18.5
( 18.7)
19.2
( 19.3)
19.3
( 19.5)
19.9
(20.0)
20. 1
(20.2)
20.4
(20.5)
49.8
( 49.9)
45.2
(45.5)
45.5
(45. 1)
44.6
(44.7)
44 .5
(44.6)
44.2
( 44.3)
44. 1
(44.3)
44.0
(39.9)
3.6
(3.7)
3.7
(3 .6)
3.3
(3.4)
3.2
(3.3)
3.2
(3 .3)
3.4
(3.3)
3.2
(3.3)
3.2
(3 .3)
18.0
( 18. 1)
16.2
( 16.4 )
16.5
( 16.3)
16. 1
( 162)
16. 1
( 16.2)
15. 1
(16 .1)
15 .9
( 16.0)
15 .9
( 15.9)
13.6
( 13.7)
12.8
( 12.5)
12.8
( 12.4 )
12.5
( 12.3)
12. 1
( 12.3)
12. 1
( 12.2)
12 . 1
( 12.2)
12.0
( 12.0)
>300
1
z
L=5-methyl-2-(2' -py ridyl)benzimidazole, d=decomp. temp. , "=0 - cm mo r
Table 2 -
Molar conduct ivi ty"
acetonitri Ie-acetone
42.06
78.46
58 .93
89.02
62 .05
46.01
54.12
7 1.29
83.58
69.63
47 .29
55.55
87.42
92 .09
680 1
86.04
l
Ana lyti ca l and co nductivity data of lanthanide thiocyanate co mplexes of L'
Co mplexes
Co lour
M.P.
(OC)
%M
%C
% 11
%N
%S
[Y(NCSh( L')z13 HzO
White
>300
[Pr(NCS )JCL'h]3 H 2O
Greenish white
>300
[Nd(NCS)l L'h13 HzO
Pinki sh whit e
260d
[SII1(NCSML'h]3H zO
Cream white
IEu( CSh(L')]I-lzO
Light ye llow
262d
[Gd(NCSh( L') zlll zO
White
289d
ITb( CS)3(L')z]ll zO
Light ye ll ow
292d
[Dy( CS h(L'hlH zO
White
>300
9.8
( 10.0)
18.5
( 18.4 )
18.5
( 18.2)
19.3
( 19. 1)
20.2
(20. 1)
2 1.1
(208)
2 1.4
(2 1.2)
21.2
(2 1.1 )
46.9
(47.0)
42.5
(42.6)
42.5
(42.4 )
42.0
(39.8)
44. 1
( 44.0)
43.5
(43.3)
43.5
(43.4 )
43.2
(43. 1)
3. 1
0.2)
2.8
(2 .5)
2. 8
(2. 7)
2. 7
(2.5)
2.4
(23)
2 .4
(2 .3)
2.3
(2 .2 )
2.4
(2.3)
14. 1
( 14.2)
12.5
( 12.4 )
12.7
( 12.5)
12.8
( 12 .7)
13. 1
( 12.9)
13. 1
( 12.9)
13.3
(13 . 1)
12.9
( 12.8)
13.5
( 13 .9)
12.5
( 12. 3)
12.2
( 12. 1)
12.5
( 12.4 )
12.8
( 12.7)
12.8
( 127)
12.8
( 126)
12.5
( 12.4 )
300
1
L' =2-(2' -pyridy l)benzoxazole, d=dccol11p. temp. "=0 - cm
z 111 0 1- 1
Mo lar conductivity"
aceLOn itri Ie-acetone
43 19
69.44
4209
39.98
52 .73
33.98
82 .90
60.70
42 .84
44 .94
29 .68
67.86
46 .67
34.29
23.50
30.25
THAKUR et at. : LANTHANIDE(III) COMPLEXES OF SUBSTITUTED BENZIMIDAZOLE & BENZOXAZOLE
sites for coordination. These two ligands from well
defined complexes with lanthanide (III) ions. The
characterisation data of the complexes are presented
in Tables I and 2. These complexes are sparingly
so luble in common organic solvents and the molar
conductances of the complexes show them to be nonelectrolytes9 . A higher conductivity value of the
complexes is observed in DMF and DMSO, which
may be due to partial solvolysis of the complexes.
The increase in molar conductivity of the complexes
in the presence of trioctyl phosphineoxide (TO PO)
further suggests coordination of the anion to the metal
Ions.
Electronic and IR spectra
The e lectronic spectra of all complexes were
recorded in methanol. In the UV region an intense
band appears at 275 nm due to the 1t-7t* transition .
The 4ftransitions are normally forbidden, bpt are
ro ll owed when degeneracy in the 4f-orbitals is lifted
due to external crystal fieldl o, ll. The spectra (Tab le 3)
show a shift of the band towards lower energy,
l2
compared with those of the aqua ions owing to
nephelauxetic effect. The bonding parameter (b I /2)
and the covalency parameter (fJ ) are less than unity,
The IR spectra of the complexes sh0w an intense
broad band between 3366 and 3398, which is due to
v(OH) of lattice water. In general , lattice water
absorbs at 3350-3200 cm- I (asymmetric and OH
stretching modes) and at 1630-1600 cm- I (HOH
bending modes). The bending mode of lattice water
which was expected in the 1630-1600 cm- I regi on in
these complexes was not observed because there are
other strong absorptions in thi s region . The absorption
bands in the region 3400-3045 cm- I of the free li gand
(L) and its metal complexes are ass igned to N- H
l4
stretch in g frequencies. Bands observed in the region
I
3 170-3200 cm- are assigned to the aromatic v(C- H).
The bands characteristic of ligand vibratio ns (C=C,
C=N stretching) in the 1625-1500 cm - I range are
shifted towards high energy (ca. 5-20 cm- I ) on
complexation to the lanthanide (III) ions. The in-plane
..
III
C
....c
whi le Sinha ' s parameter (8) is positive, indicating a
moderate covalent character for the bond between the
metals and ligands 13.
700
600
Wave
Table 3 -
Tentati ve
assign ments
Calculated
spectral
parameters
Table 4 - The fluorescence data o fSm(lll) , Eu( III ), Tb(lll ) and
Dy( lll) complexes with PBOX
Comp lex
Pr
Nd
Sm
Eu
Gd
n
Dy
17605
20325
25252
J H4 ~ID2
16878
24371
17547
4 / 9/2
=
0.997
~J po
b I/2=0.041
~ J p2
8=0.341
~ 4C S/2
~ lp l/2
6 f{5I2 ~ 4C S12
~4MI 5I2
18849
17870
20261
22745
28069
27401
21664
23123
f3
IFo~ s Do
b
IF6~5 D2' SCe, SLR
"1-/ 1512 -1"F W2
~ 4 / 1512
(~ 080
Assignments
490
400
445
445
565
595
722
4C)I2~ 6H512
Eu
402
b I/2= 0.074
0= 1.1
f3 = 0.994
b
l12
=
Tb
402
0.055
0=0.60
The covalence factors were ca lculated by the relati on
I/2
Em (nm)
= 0.063
f3 = 0.989
~ 5 DI
~ 5 D2
8S 112~6 p l l2
PBOX
Sm
Ex (nm)
f3 =0.992
I/2
(com plex)/ v (aqu o), b =[1 /2( 1- ,8)] 112 and 8= 100(1 - ,8) /(1
f3
=v
length
Fig. I-Fluorescence spectra of[Tb{N CS)3(PBOXhl
Importantf-fbands and their tentative ass ignments
Complex Am.x (cm- I )
1225
Dy
402
440
590
620
650
720
Ligand
4( ;
5I2~ 6 11712
4 C512~6 H9/2
Ligand
s Do~ 7FI
S DO~ 7F2
5 Do~ 7F3
j Do~ 7 F4
5 D4~7F6
490
540
582
620
SD4~ 7 FJ
480
622
4D7I2 ~6Fsl2
4D7I2~6 F1 3/2
5 D4~ 7Fj
5 D4~ 7 F4
1226
INDIAN J CHEM, SEC. A, DECEMBER 1999
deformation mode of the pyridine ring l5 appears at ca .
640 cm- I. This indicates that the pyridine nitrogens of
the benzimidazole rin g coordinate to lanthanide( lII)
Ions.
In all th e IR spectra of th e complexes of li ga nd L' ,
medium and weak bands are present near 3400 cm- I
which may be du e to lattice water. The prominent IR
absorption frequencies of ligand L' are shifted du e to
coordin ati on and show The bidentate coordination of
this ligand through the N atom of benzoxazo le rin g
and the N atom of the pyridine rin g l6 A new band at
364 cm- I in the spectra of these metal complexes may
be ass igned to the meta l nitrogen stretching
vibrati ons.
A strong band associated with th e vC=N stretchin g
vibrat ion appears in th e region up to 2 100 cm- I. This
band is largely sp lit into two or three pea ks between
2 130 and 2060 cm- I. The splitting probably arises
fro m inequi va lence of the thiocya nate group beca use
of ani on bridgingl 7 interacti ons. Bands around 2050
1
CI11 - arc in fact typical ofN-coo rdinati on.
NMR spectra
The NMR spectra (I H and LIC) reco rded in CDC,",
and DMSO-clr, med ium res pective ly prov ide
co nclu sive ev idence in favour of th e bondin g. The
PM R spectrum of th e free li ga nd L in CDCI} co nsists
of a multiplet at 8 7.05-8.6 ppm . The signal at 8 7.37
ppm corresponds to th e 3',4',5' and 6' protons . The
signal corresponding ( 0 H(4) and H(6) protons is
observed at 8 7.74 ppm. The doubl et obse rved at 8
8.47 and 8.6 ppm corresponds to H( 6) and H(7)
protons, respectively. The 5-CI-I.\ proton appea rs as a
sharp sign let at 8 2.42. However, in co mpl exes
co rrespondin g signals obtained at 8 7.32, 7.63, 8.25
and 8.59 ppm und ergo downfi cld shi ft. Coo rdinati on
res ults in downfield chcmical shifts of the rin g
protons, assigned to th e lowering of electron density
lR
in the rin g system . The proton signal s also und ergo
down field shift an d appear at 82. 0 ppm.
The pea ks observed fo r li ga nd L in CDCI.1 DMSO-d(, med ium at 8.73 , 8.29, 8.00, 7.8, 7.53 and
7. 39 ppm co rres pond to 1-1(6'), 1-1 (4'), H(3), H-5(6),
H(5') and H-4(7), respectively. In co mplexes
differe nce observed is that the signals beco me
broadened. In th e paramagnetic comp lexes the proton
signa Is of Pr" become much weaker and broade ned
and aromatic protons are observed as a mul tipl et in
(he region 7.3-8.6 ppm. The broadenin g may be du e
to spin-spin relaxati on processesl ~)O With th e Gd' !
r
complexes extensive broadenin g of the signals is
observed since Gd 3+ has the maximum number of
unpaired electrons and IS, therefore, stron gly
paramagnetic.
The I}C NMR spectra of the ligand L and its metal
complexes were also recorded in CDC I3- DMSO-d6
medium. In CDCI}, th e signal s observed at 0 150.5 1,
148.40, 13 .71 , 123.23 ppm co rres pond to the C(6),
C(7), C(2), C(4) of th e benzimidazo le moiety whereas
the signa ls at 8 13 1.84, 120 .89 and 11 5.8 1 ppm are
assigned to C(2'), C(6'), C(3', 5') of the pyridine
moiety of the li gand. The signa l due to 5··CH3 is
observed at 21.29 ppm. In meta l co mplexes the
correspondin g signal s obse rved at 148.9, 146 .5,
136.61 , 122.47, 130.49, 132.45, 12 1.39 and 11 4.48
ppm indicate downfi eld shift of th e signals. As
ex pected th e intensity of the J\C signals for the
complexes is very low and suppressed. The 5-CH3
signal s are observed at 0 2 1.37 ill Pr compl exes, and 0
21.5 in case of Nd and 0 2 1.37 in case of other
lanthanides.
The DC NMR spectra of the liga nd L' and its metal
complexes were taken in CDC I,-D MSO-d6 . The
C-4(6) are equivalent, ap pearin g at 125.86 ppm . In the
complexes the intensity of the J3 C signal s for th e
carbon C-2, C-2', C-8 and C-9 is very low and the
signal s are suppressed or even absent indicating
coordin ation of the ligands to the meta l th rough N
ato m of benzoxazo le rin g and N atom of th e pyridine
ring. The TG technique has been used to follow the
thermal behav iour of th e liga nds and the complexes.
The results indicate that the co mpl exes are not
vo latile and their decompositions take pl ace in similar
steps. The final product of th e decomposition is
lanth anide(lll) ox ide.
Fluorescence spectra
The nu oresce nce spectra of the liga nd PBOX and
its metal co mplexes record ed in meth anol at room
temperature, indicate that only Sm( III), Eu( III ),
Tb(lIl) and Dy(lll) com plexes show co rrespondin g
metal luminescence. However, all the co mplexes
ex hibit lum inesce nce of the li gand PBOX by
exc itat ion at 402 nm (Tab le 4).
It ca n be seen that th e emi ss ion ari ses main ly from
a tran sition originatin g at th e 5Do leve l in case of Eu
co mplex and terminatin g in th e 7F I, 7F 2• 7F.1 and 7F4
leve ls. The most in te nse band at 620 nm is due to
5 Do---'t 7F2 transiti on. The transition 7Do 7F, at 650 is
of low intensity and can be reasonabl y taken as
THAK UR et 01.: LANTHANIDE(III) COM PLEXES OF SUBST ITUTED BE ZIMIDA ZOLE & BENZOXAZOLE
forbidden. The 5 Da-+ 7Fa transition observed as a weak
band at 572 nm ex isting as a single peak indicates the
presence of onl y one site for th e Eu3+ ion2 1. The Sm
com plex ex hibits three bands with wea k metal ion
luminescence. For Tb(IIf) complexes, all emi ss ions
arise from the sD4 leve l. The seven 5D 4 -+ 7P! (.1==0-6)
transiti ons may be seen, but th e most intense emi ss ion
invariably occurs in the sD4-+ 7Fs spectral ran ge and in
the present case it occ urs at 540 nm . The intensity of
the 5D4-+ 7Fu, transition is always weak and it is not
observed in the present case. The other three bands at
490, 582, and 620 nm have been assigned to 5D4-+ 7Fu,
5D4-+7F4 amd 5D4 -+7F3. The transition sD4-+ 7F3 5 has
strong magnetic dipole character. A typical spectrum
is displayed in Fig. I.
Acknowledgement
The auth ors are grateful to Dr. S.c. Sawat (In stitute
of Life Sciences, Bhubaneswar) fo r recordin g
fluorescence spectra. Grateful thanks are due to
Co un ci l of Sc ientific and Industri al Research (CS IR),
New Delhi for fundin g th e project.
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