A new class of solvatochromic material: Geometrically unsaturated Ni (II) complexes

Article abstract of DOI:10.1016/j.dyepig.2012.05.008

A series of Ni(II) double stranded binuclear complexes were synthesized upon treating the Ni(II) salts with series of diamino-bis-pyridine ligands, L₁H₂-L₄H₂ in 1:1 stiochiometric ratio. The Ni(II) centers exist

Full text of DOI:10.1016/j.dyepig.2012.05.008

Dyes and Pigments 95 (2012) 563e571
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
A new class of solvatochromic material: Geometrically unsaturated Ni (II)
complexes
a
a
b
c
a,
*
P. Mosae Selvakumar , S. Nadella , R. Fröhlich , M. Albrecht , P.S. Subramanian
a
Analytical Science Division, Central Salt and Marine Chemicals Research Institute, G.B.Marg, Bhavnagar, Gujarat, India
Organisch-Chemisches Institut, Universitat Munster, Corrensstrasse 40, D-48149 Munster, Germany
Institut fur Organische Chemie der RWTH e Aachen, 52074 Aachen, Germany
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of Ni(II) double stranded binuclear complexes were synthesized upon treating the Ni(II) salts
with series of diamino-bis-pyridine ligands, L eL in 1:1 stiochiometric ratio. The Ni(II) centers
existing in octahedral geometry, four of the coordination sites are occupied by two ligand strands, while
Received 3 December 2011
Accepted 11 May 2012
Available online 18 May 2012
1
H
2
4 2
H
the remaining two sites are occupied by solvent molecules in each metal centers. The Ni(II) complexes 1a
4 6
e4a and 1be4b are binuclear in nature with ClO and PF counter ions. The crystal structure obtained for
Keywords:
Pyridine ligands
Nickel(II)
Binuclear complexes
Double stranded helicates
Solvent coordination
Solvatochromism
complex 1b ascertains the solvent coordination. Solvatochromism though a well established phenomena,
such observation is reported first time in the present manuscript in the area of helicate chemistry. Thus
the manuscript elicits the solvatochromic property of geometrically unsaturated helicates, by appro-
priately tuning the d-d transition, adapting suitable solvents.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
to-date studies on the helicates gave an understanding on the
architectural aspects, the present decade is believed to bring out
Ever since the terminology “helicate” was coined by Lehn [1e3]
in 1987, varieties of single, double, triple and multistranded helical
architectures with various metal ions were described [4e10]. All
these complexes were mainly investigated due to their interesting
structural features. Only in few cases examples were described
varieties of potential materials.
Colour being fascinating area of research [27e30], chromic
behaviour of diverse organic and inorganic compounds gained
immense research interest in view of the dye and pigments. In this
context complexes of Ni(II) [31e36], Cu(II) [37e40] and Co(II) [41]
are well known for their solvatochromic behaviour. Surprisingly
so far no helicates were reported which possess solvatochromic
behaviour. This opens up a broad field of application which can be
envisaged for coordinatively unsaturated helicates. Their advantage
is that they are usually more stable than corresponding mono-
nuclear compounds. In addition, cooperative or additive effects
may also be expected, if two unsaturated or labile metal units are
fixed with a close distance to each other. The vacant sites can be
filled up by suitable ligands, substrates or solvents. By doing this, it
should be possible to tune the electronic properties of the metal
ions towards achieving molecular recognition properties [42e48],
DNA binding [49,50], appropriate catalysts [51] and chromic
property etc. Having this in mind, the present manuscript describes
the synthesis of a series of coordinatively unsaturated Ni (II) double
stranded binuclear helicates. In this endeavor this present manu-
script demonstrates that this class of compounds represents highly
efficient solvatochromic material which produce different colors
upon changing into different solvents.
[8e22], which possess a high potential for application. Classifica-
tion of the helicates can either be done according to the number of
strands or they can be classified as coordinatively saturated or
unsaturated helicates [21,22]. The metal ion in the unsaturated
helicates possesses labile/vacant sites. In the saturated systems, the
metal ions are usually shielded and no chemical reactivity can
generally be observed at the metal. Here the physical properties of
the metal centers (luminescence [23,24], magnetism [25,26]) are of
special interest. In the unsaturated case the metal ions can be
approached by substrates and special reactivity or properties can be
expected. Further, in the design of complex superstructures, with
augmented number of helical strands, the importance of the
unsaturated helicates was more or less ignored. Hence on
comparison with the number of saturated helicates, the geomet-
rically unsaturated helicates are rare in the literature. While the up-
*
Corresponding author. Tel.: þ91 278 2566970x673; fax: 91 278 2567760.
E-mail address: siva140@yahoo.co.in (P.S. Subramanian).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2012.05.008

564
P.M. Selvakumar et al. / Dyes and Pigments 95 (2012) 563e571
ꢀ
1
2
. Experimental
CH
2
, 2H). IR (n, cm ): 3317 (weak, NH), 1521 (s, intense). ESI [MS]:
m/z calcd (found) for C33
H
28
N
4
(M þ H)þ 481.24 (481.43).
2.1. Materials
2.2.2. Synthesis of double helicate complexes
All chemicals were purchased from sigmaealdrich
&
Co
2.2.2.1. General procedures for the synthesis of complexes. All
complexes 1a, 2a, 3a and 4a were synthesized using 1:1 Ni
and were used as received. All the other reagents and solvents
are commercially available and used without any further
purification.
(ClO
PF complexes 1b, 2b, 3b and 4b were obtained following anion
metathesis using NH PF
[Ni (L (CH CN) (ClO
0.001 mol) dissolved in acetonitrile was mixed with methanolic
solution of [Ni(ClO ].6H O (0.365 g, 0.001 mol) and allowed for
4 2 2
) .6H O and appropriate ligands. Similarly the corresponding
6
Microanalysis of the complexes was done using a PerkineElmer
PE 2400 series II CHNS/O elemental analyzer. Electronic spectra
were recorded on a HP diode array (model 8452A) UV-visible
spectrophotometer. FT-IR spectra for KBr pellets (1% w/w) were
4
6
.
2
1
)
2
3
4
]
4 4 1
) , (1a): The ligand L (0.318 g,
4
)
2
2
1
recorded on a Shimadzu FT-COM 1 spectrophotometer. H NMR
continuous stirring and the resultant light blue precipitate
appeared. The precipitate obtained was separated and dried. Yield:
spectra were recorded on a Bruker 200 MHz and 500 MHz spec-
trometer. TMS was used as internal reference, where non-aqueous
solvents were employed.
85%. Anal. Data: calcd (found)% for C48
H
56Cl
4
N
12Ni
2
O
16 C, 43.80
CN, /nm,
ε/M cm )], 436 (15) , 560 (21), 786 (11), 921 (20). IR (
(43.69); H, 4.29 (4.21); N, 12.77 (12.64). UV-Vis [CH
3
l
ꢀ
1
1
ꢀ1
sh
sh
(
n,
ꢀ
2
2
.2. Synthesis
cm ): 3411(br, NH), 1094 (s, intense), 625 (st, intense). ESI [MS]: m/
þ
z
calcd (found) for
(1051.71).
[Ni (L
C
40
H
44Ni
2
Cl
3
N
8
O
12 (MꢀClO
4
) ; 1051.08
.2.1. Synthesis of ligands
0
N,N -(1,4-phenylenebis (methylene) bis (1-(pyridine-2-yl)
2
1
)
2
(CH
3
CN)
4
]
(PF
6
)
4
,
(1b): The above synthesized
methanamine) (L
carboxaldehyde (0.406 ml, 0.002 mol), and p-xylenediamine
0.318 g, 0.001 mol) were mixed and allowed for constant stirring
1
): 2:1 methanolic solution of pyridine-2-
complex (1a) dissolved in acetonitrile and mixed with methanolic
solution which contains excess amount of ammonium hexaflouro
phosphate and heated to 60 C with continuous stirring for 1 h
ꢁ
(
with gentle heating for 2 h. The characteristic yellow solution for
Schiff base was obtained by condensation. To this yellow color
provided pale blue precipitate. The precipitate obtained was
separated and dried. Yield: 95%. Anal. Data: calcd (found)% for
solution, NaBH
4
was added in excess and the reaction was allowed
48 56 24 2 4
C H F N12Ni P , C, 38.48 (38.41); H, 3.77 (3.72); N, 11.22 (11.18).
ꢀ1
ꢀ1
sh
to run overnight. The color change of the ligand solution from
yellow to colorless indicates the reduction of Schiff base. The
resultant solution was evaporated and extracted using water/
chloroform solvent mixture. Chloroform layer was collected and
evaporated to dryness to get the compound. Yield: 95%. Anal. Data:
UV-Vis [CH
3
CN,
l
/nm, (ε/M cm )]: 567 (40), 796 (20), 925 (26).
ꢀ
1
IR (
n, cm ): 3331(br, NH), 1571 (st, intense), 841 (s, intense). ESI
4
þ
[MS]: m/z calcd (found) for C40
(898.97).
H
44Ni
2
F
6
N
8
P (M-3PF
6
þH) , 898.21
[Ni
2
(L
2
)
2
(CH
3
CN)
4
]
(ClO
4
)
4
,
(2a). The ligand L
2
(0.318 g,
calcd (found)% for C20
1
H
22
N
d
4, C, 75.44 (75.32); H, 6.96 (6.84); N,
, CDCl ): 8.54e8.53 (d, AreCH, 2H),
, 4H),
0.001 mol) dissolved in acetonitrile was mixed with methanolic
solution of [Ni(ClO ].6H O (0.370 g, 0.001 mol) and allowed to
continuous stirring for 3 h provided pale blue precipitate. The
precipitate obtained was separated and dried. Yield: 90%. Anal.
7.60 (17.51). 1H NMR (
)
2
3
4
2
7.62e7.59 (m, AreCH, 2H), 7.37e7.11 (m, Ar, 8H), 3.89 (s, CH
2
ꢀ1
3
.80 (s, CH
2
, 4H), 2.57 (br, NH). IR (
n
, cm ): 3327 (weak, NH), 1588
þ
(
s, br). ESI [MS]: m/z calcd (found) for C20
H
22
N
4
(M þ H) , 319.19
Data: calcd (found)% for C48
H
56Cl
4
Ni
2
N
12
O
CN,
16, C, 43.80 (43.75); H,
ꢀ1
ꢀ1
(
319.57).
4.29 (4.24); N, 12.77 (12.59). UV-Vis [CH
3
l
/nm, (ε/M cm )]:
0
sh
sh
ꢀ1
N,N -(1,3-phenylenebis (methylene)) bis (1-(pyridin-2-yl)
435.06 (57) , 541 (40), 780 (20), 887 (24). IR (
n
, cm ): 3433 (br,
methanamine), (L
m-xylenediamine in-place of p-xylenediamine. Yield 95%. Anal.
Data: calcd (found)% for C20 ; C, 75.44 (75.35); H, 6.96 (6.79);
N, 17.60 (17.54). 1H NMR (
, CDCl ): 8.53e8.52 (d, AreCH, 2H),
, 4H),
2
): The above procedure was repeated using
NH), 1092 (s, intense) 0.618 (s, intesne), ESI [MS]: m/z calcd (found)
þ
for C40
H
(L
44Cl
3
Ni
2
N
8
O
12 (M ꢀ ClO
4
) ; 1051.08 (1051.32).
(2b): The above synthesized
H
22
N
4
[Ni
2
2
)
2
(CH
3
CN) (PF
4
]
6
)
4
,
d
3
complex (2a) was dissolved in acetonitrile and treated with
methanolic solution of excess ammonium hexaflouro phosphate
7
.61e7.56 (t, AreCH, 2H), 7.35e7.10 (m, Ar, 8H), 3.87 (s, CH
2
ꢀ
1
ꢁ
3
.81 (s, CH
2
, 4H), 2.58 (br, NH). IR (
n
, cm ): 3306 (weak, NH), 1592
and heated to 60 C with continuous stirring for 1 h provided pale
þ
(
(
s, br). ESI [MS]: m/z calcd (found) for C20
319.68).
H
22
N
4
(M þ H) , 319.19
blue precipitate. The precipitate obtained was separated and
dried. Yield: 94%. Anal. Data: calcd (found)% for
0
4
,4 -methylenebis (N-(pyridin-2-ylmethyl) aniline), (L
3
): The
was repeated using 4,4 -methylenedianiline
instead of p-xylenediamine. Product was separated as white
powder. Yield 92%. Anal. Data: calcd (found)% for C25 ; C, 78.92
, DMSO-D ):
.51e8.50 (d, 2H), 8.32 (s, 2H), 7.71e7.68 (t, 2H), 7.34e7.33 (d, 2H),
48 56 24 2 4
C H F N12Ni P , C, 38.48 (38.37); H, 3.77 (3.68); N, 11.22
0
ꢀ1
ꢀ1
sh
above procedure for L
1
(11.31). UV-Vis [CH
774 (33), 883 (37). IR (
3
CN,
l
/nm, (ε/M cm )]: 441(83) , 532 (57),
sh
ꢀ1
n
, cm ): 3407 (br, NH), 1545 (st, intense),
H
d
24
N
4
844 (s, intense). MS ESI [MS]: m/z calcd (found) for C40
H
44
1
3þ
(
78.76); H, 6.36 (6.31); N, 14.73 (14.65). H NMR (
6
Ni
2
F
6
N
8
P (M-3PF
(L (CH
0.001 mol) dissolved in acetonitrile was mixed with methanolic
solution of [Ni(ClO ].6H O (0.365 g, 0.001 mol) and allowed to
6
)
; 898.20 (898.97).
8
[Ni
2
3
)
2
3
CN)
4
]
(ClO (3a): The ligand
4
)
4
,
3
L (0.380 g,
7.22e7.20 (t, 2H), 6.86e6.84 (d, 4H, 13), 6.48e6.47 (d, 4H),
6
.16e6.14 (t, 2H, NH), 4.32e4.31 (d, 4H, CH ), 3.55 (s, 2H, CH ); IR (
2
2
n
,
4
)
2
2
ꢀ
1
cm ): 3368 (weak, NH), 1517 (s, intense). ESI [MS]: m/z calcd
continuous stirring for 3 h provided pale blue precipitate. The
precipitate obtained was separated and dried. Yield: 80%. Anal.
þ
(
found) for C25
24
H N
4
(M þ H) , 381.48 (381.12).
0
4
,4 -methylenebis (N-(quinolin-2-ylmethyl) aniline, (L
4
):
quinoline-2-
carboxaldehye (0.314 g, 0.002 mol) was used in place of pyridine-
-carboxaldehyde. The product was separated as pale yellow
powder. Yield 85%. Anal. Data: calcd (found) (%) for C33 .H O;
C, 79.49 (80.10); H, 6.06 (5.92); N, 11.24 (10.58). H NMR Data (
CD Cl ): 8.17e7.43 (m, AreCH, 12H), 7.04e7.00 (d, AreCH, 4H),
.70e6.66 (d, Ar, 4H), 5.16 (br, NH), 4.60e4.58 (d, CH , 4H), 3.76 (s,
Data: calcd (found)% for C58
4.20 (4.17); N, 11.67 (11.61). UV-Vis [DMF, l/nm, (ε/M cm )]: 500
(1047), 770 (11), 956.57 (20). IR (n, cm ): 3360 (br, NH), 1100 (s,
4 2
H60Cl N12Ni O16, C, 48.36 (48.32); H,
ꢀ
1
ꢀ1
Following the above synthetic procedure for L
3
ꢀ1
2
intesne), 625 (s, intesne). ESI [MS]: m/z calcd (found) for
þ
28
H N
4
2
C
50
2þ
H
48Cl
3
N
8
Ni
H] 1176.12 (1176.86).
[Ni (L (CH CN) ] (PF
(3a) dissolved in acetonitrile and mixed with methanolic solution
2
O
12 (MꢀClO
4
) ; 1175.11 (1174.87); [(M ꢀ ClO
4
)þ
1
d
,
2
2
2
3
)
2
3
4
6 4
) (3b): The above synthesized complex
6
2

P.M. Selvakumar et al. / Dyes and Pigments 95 (2012) 563e571
565
which contains excess amount of ammonium hexaflouro phos-
ꢁ
phate and heated to 60 C with continuous stirring for 1 h provided
pale blue precipitate. The precipitate obtained was separated and
dried. Yield: 60%. Anal. Data: calcd (found)(%) for Ni
2 50 48 24 8-
C H F N
P
4
.4H O: C, 39.24 (39.02); H, 3.69 (2.97); N, 7.32 (6.83). IR Data (n,
2
ꢀ
1
cm ): 3322 (br, NH), 1543 (st, intense), 845 (s, intense).UV-Vis
ꢀ
1
ꢀ1
[
DMF,
z calcd (found) for Ni
1189.05).
Ni (L
dissolved in dichloromethane was mixed with methanolic solution
l/nm, (ε/M cm )]:526.38 (132), 979.65 (35). ESI [MS]: m/
3
þ
2
C
50
H F
48 12
N
8
P
2
Na, (M-2PF
6
þNa) : 1189.19
(
[
2
4
)
2
(H
2
O)
2
] (ClO
4
)
4
(4a): The ligand L
4
(0.481 g, 0.001 mol)
of Ni (ClO
4
)
2
.6H
2
O (0.366 g, 0.001 mol) and allowed for continuous
ꢁ
stirring 2 h and warmed at 60 C for 1 h. The color change from
yellow to wine red indicates the complexation. The solution is
evaporated to give reddish brown solid. Yield: 90%. Anal. Data:
calcd (found)(%) for Ni
.00 (3.27); N, 7.41 (6.83). IR Data (
intense), 625 (s, intense). UV-Vis [CH
2 66 4 8 2
C H56Cl N O16.2H O: C, 52.41 (52.54); H,
ꢀ
1
4
n, cm ): 3415 (br, NH), 1098 (s,
ꢀ ꢀ1
1
3
CN,
50.23 (409), 763.36 (3.8), 951.64 (23.7). ESI [MS]: m/z calcd (found)
l/nm, (ε/M cm )]:
5
2
þ
for Ni
2
C
66
H56Cl
3
N
8
O12 (MeClO
4
þH) : 1378.18 (1378.32).
[Ni
2
(L
4
)
2
(H
2
O)
2
] (PF (4b): The above synthesized complex
6 4
)
(
4a) mixed with excess amount of ammonium hexaflouro
ꢁ
phosphate in methanol solution and heated to 60 C with contin-
uous stirring for 1 h provided red coloured precipitate. The
precipitate obtained was separated, washed with cold methanol
and dried. Yield: 60%. Anal Data: calcd (found) (%) for
Scheme 1. Various ligand systems used in the helical construction.
Ni
6.32). IR Data (
intense). UV-Vis [CH
2 66 56 24 8 4 2
C H F N P .2H O: C, 46.78 (46.33); H, 3.57 (3.49); N, 6.61
ꢀ
1
(
n, cm ): 3329 (br, NH), 1513 (st, intense), 835 (s,
obtained from the corresponding ligands L
respectively, with 1:1 Ni(II): ligand ratio. While the perchlorate
complexes 1ae4a were synthesized using Ni(ClO .6H O, the
respective hexafluorophospate complexes of 1be4b were ob-
tained from 1ae4a by anion metathesis approach by using
ammoniumhexafluorophosphate. The MS spectra obtained for
complex 1ae4a and 1be4b indicate the formation of the
respective complexes (S2). Finally, numerous attempts to obtain
suitable single crystals lead to the crystal structure of 1b
1 2 3 4
, L , L and L
ꢀ
1
ꢀ1
3
CN,
l
/nm, (ε/M cm )]: 488.84 (495),
P, (M-
1
3
058.57 (14.2). ESI [MS]: m/z calcd (found) for Ni C H F N
2 66 56 6 8
4
)
2
2
4
þ
PF
CAUTION! Perchlorate salts of metal complexes are potentially
explosive and should be handled with great care.
6
þH) : 1222.31 (1222.58).
2.3. X-ray crystallography
Data set was collected with a Nonius Kappa CCD diffrac-
tometer, equipped with a rotating anode generator. Programs
used: data collection COLLECT [52], data reduction Denzo-SMN
1 3 2 2 6
[Ni(L )(CH CN) ] (PF )4. The IR spectra recorded for all the
perchlorate and hexafluorophosphate complexes confirms their
presence in ionic form. The hexa flurophosphate complexes
[
53], absorption correction Denzo [54], structure solution
ꢀ1
giving single peak at around 840 cm which not only ruled out
SHELXS-97 [55], structure refinement SHELXL-97 [56], graphics
SCHAKAL [57]
CCDC 798728 contains the supplementary crystallographic data
for 1b. These data can be obtained free of charge at www.ccdc.cam.
ac.uk/conts/retrieving.html [or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
any significant interaction with Ni(II) metal center, but also
confirms their existence in the ionic form [59e61]. Similar to the
hexafluorophosphate anion, the perchlorate counter ion in
complex 1ae2a giving characteristic IR bands, respect to their Td
ꢀ1
modes at around 1100 and 625 cm and indicates their existence
in ionic form [62,63]
(
internet.) þ44 (1223) 336-033, E-mail: deposit@ccdc.cam.ac.uk.
3
. Results and discussion
3.1. Crystal structures of complex 1b
The bis-pyridine ligands L
1
, L
2
, L
3
and L
4
were synthesized [58]
The crystal structure of 1b with atom numbering scheme is
depicted in Fig. 1 (i). The molecular structure in the crystal [64]
exhibits C2 symmetry. Fig. 1 shows the dinuclear Ni(II) centers
and characterized starting from their imine precursors following
the standard NaBH reduction. The completion of the reduction
process was monitored using H NMR and the respective CH
4
1
2
and
are wrapped by two ligand strands L
separated by 9.00 Å. They adopt a distorted octahedral geometry
with four PF counter ions residing outside the coordination
1
. The Ni(II) centers are
NH peaks were found appeared, while the corresponding CH]N
peak were disappeared. All these ligands possess two amine and
two pyridyl nitrogens, while the two aminomethyl pyridins are
separated by aromatic spacers as shown in Scheme 1. The attempt
to synthesize triple, double and single stranded helicates with
appropriate 3:2, 2:2 and 1:1 ligand:Ni(II) ratio mainly resulted in
double stranded helicates. Only in few cases triple stranded hel-
icates were obtained. However the present study primarily
explores the coordinatively unsaturated complexes, here we
report only the coordinatively unsaturated double stranded hel-
6
sphere of the metal. While the amino nitrogen (NH17; Ni(1)eNH
(17) ¼ 2.140 Å) of one ligand strand and nitrogen from one
acetonitrile (N41; Ni(1)eN (41) ¼ 2.101 Å) are occupying the axial
sites of Ni (1), the equatorial sites are occupied by two pyridyl
nitrogens of the two different strands (N (24) & N (10); Ni(1)eN
(24) ¼ 2.008; Ni(1)eN (10) ¼ 2.090 Å) and the other two by
amino nitrogen (N8H; Ni(1)eN (8) ¼ 2.127 Å) and acetonitrile
nitrogen (N (31); Ni(1)eN (31) ¼ 2.084 Å). Both the ligand strands
in their neutral form possess two amine and two pyridyl nitrogens
icates. The tetracationic complexes 1ae4a
& 1be4b were

566
P.M. Selvakumar et al. / Dyes and Pigments 95 (2012) 563e571
Fig. 1. (i) Structure of tetracationic complex 1b with atom numbering scheme (ii) Space filling model (iii) Stick representation.
and binds the Ni(II) metal centers. Two cis-arranged acetonitrile
molecules filling up each coordination site, resulted in the dis-
torted octahedral geometry. The two ligands in the dinuclear
coordination compound 1b are binding in different fashion, is
shown in blue and red (Fig. 1 (ii) and iii). This figure illustrates that
one ligand (blue) is coordinating one nickel ion from the “side”
while the other is located in the internal void “in-between” the
two metals. The second ligand (red) is arranged with a curved
shape in order to chelate one of the two nickel atoms. The coor-
dination sites which are occupied by the solvent molecules (green)
are oriented in opposite direction and thus are preventing the
formation of the corresponding triple stranded helicate in this
case. As seen in Fig. 1 (i), four of the six sites in the Ni(II) octahedral
sphere are occupied by the pyridyl and amino nitrogen of the
ligand strands, while the remaining two sites at each Ni(II) center
are occupied by solvent molecules. These solvents are labile and
can easily be replaced by various other solvents or substrates
depending upon the demand. The possibility of functionalization
of this labile site though seems multidirectional, the aim of
the present study limits to explore the solvatochromic property
using different solvents depending on their increasing donor
strength.
3.2. Solvatochromism
The complex (1e3) a, b in the solid state are generally light blue
in color, while the complex 4a, 4b gives an entirely different color
i.e. brownish red in the respective solid state. The complex 1a, 2a
and 3a possessing ClO
in color, while the respective counterparts with PF
and 3b are depicting little intense blue. Though both of them
looking almost similar in color, the subtle change in their intensity
may signify the role of counter ions. This may be due to the
difference in the anions which may have some supramolecular
contact, since they are not directly involved in the coordination.
However the solvent effects being dominant which has direct effect
on the d-d transition, the color changes on changing the solvent are
quite visible. The complexes 4a and 4b though exists in brown
4
counter ions, they are found to be pale blue
anions 1b, 2b
6
4 6
color, the 4a with ClO gives dark brown, while the PF counterpart
gives light brown which may be attributed for the combined role
played by the counter ion as well as quinoline fluorophore
substituted on the ligand moiety. Interestingly, the complexes give
different colors in different solvents. The observed color changes
for 1e3 a, b are depicted in Fig. 2, while the complex 4a, 4b are
presented in Fig. 3. The crystal structure obtained for complex 1b

P.M. Selvakumar et al. / Dyes and Pigments 95 (2012) 563e571
567
Fig. 2. Solvatochromism shown by complexes [Ni
2 1 2 4 4 4 2 1 2 4 6 4 2 2 2 4 4 4 2 2 2 4 6 4 2 3 2 4 4 4
(L ) (S) ],(ClO ) 1a, [Ni (L ) (S) ] (PF ) 1b and [Ni (L ) (S) ](ClO ) 2a, [Ni (L ) (S) ](PF ) 2b and [Ni (L ) (S) ](ClO ) 3a,
[
Ni (L (S) ](PF 3b, where “S” denote the solvents.
2
3
)
2
4
6 4
)
illustrates the coordination of four labile solvent molecules in each
binuclear system. The aim was to replace them by appropriate
solvents simply by dissolving them in the respective solvent.
Complex 1a leads to pink, violet, blue and wine red solutions, while
complex 1b with hexafluorophosphate results in wine red, pale
yellow, pale blue and green colors (Fig. 2). A similar approach to
complex 2a, 2b and 3a, 3b also depict a subtle color change (Fig. 2).
While the complex 1 and 2 behaving almost similar with subtle
difference in their color change, the complex 4a and 4b gives wide
range of solubility with wide range of colors from violet in pyridine
to wine red in tetrahydrofuran (Fig. 3). Such solvatochromic
behaviour was already known in coordination chemistry. However,
the present study is the first in the area of helicate chemistry and
reveals that unsaturated helicates represent a most favorable
platform for such studies. Although a significant number of
unsaturated double stranded binuclear helicates is reported in the
literature, so far no report has been found on the solvatochromic
behaviour.
color of the complexes in different solvents, in which the respective
electronic spectra for 1e3 & 4 were recorded. The electronic spectra
of 4a and 4b were recorded in a wide range of solvents such as THF,
3 3
EtOH, CH OH, CH CN, DMF, DMSO and Pyridine and the respective
data are depicted in Table 1. The complexes 1a and 1b dissolved in
acetonitrile, DMF and DMSO show three main bands in the range of
434e436 nm, 560e590 nm and 920e960 nm as shown in Fig. 4. An
additional weak transition was observed in the range of
775e785 nm. Interestingly the spectra recorded in pyridine shows
two weak transitions, one at 775e785 nm similar as observed for
the other solvents, the second one was detected at 1150 nm. The
peak in the range 560e590 nm, 920e960 nm and the shoulder at
775e785 nm were detected in almost all spectra, while the peak at
434e436 nm appeared as a shoulder only in acetonitrile and pyri-
dine. In other solvents it probably is merged with ligand centered
transitions and/or ligand to metal charge transfer bands, which
appear between 200 and 435 nm.
All these peaks showing bathochromic shift (shift towards
longer wavelength) with increasing Guttmann donor number [65]
of the solvent. The change in the colors can be termed as positive
solvatochromism. A similar spectral behaviour was observed for
complex 2a and 2b, illustrating a negligible effect on the d-d tran-
sition, due to the subtle difference in ligand strength caused by m-
Surprisingly, the elemental data though showed existence of
solvents, the MS spectra did not give any evidence for the existence
of solvent molecules. This may be attributed for the highly labile
coordination nature of the solvents. In the newly reported binu-
clear Ni(II) complexes, the d-d transition on the Ni(II) metal centre
8
with its d configuration required an in-depth analysis by electronic
xylene on L
2 1
over p-xylene on L . The band at 434e436 nm for
spectroscopy (UV-Vis NIR) as discussed below.
complex 1a, 1b and 2b dissolved in acetonitrile and pyridine were
ꢀ1
All the Ni(II) complexes are soluble in a variety of coordinating
solvents, the corresponding changes in the UV-Vis spectrum with
respect to different solvents were studied. Figs. 2 and 3 illustrate the
attributable to
22883e23041 cm
complexes. The increase in the 10Dq suggests that the LFSE of
n
1
in the range of 22675e22988 cm
indicating the respective 10Dq on these
and
ꢀ
1

568
P.M. Selvakumar et al. / Dyes and Pigments 95 (2012) 563e571
2 4 2 4 4 4 2 4 2 4 6 4
Fig. 3. Solvatochromism exhibited by [Ni (L ) (S) ](ClO ) , 4a and [Ni (L ) (S) ] (PF ) 4b, where “S” denote the solvent.
pyridine is stronger than of acetonitrile. Unfortunately in complex
a and 3b, it was not possible to locate the band corresponding to
. It may have merged with the ligand centered transition, which
might have appeared lower than 435 nm where the ligand centered
bands are dominant. The high energy shift could be caused by the
3
n
1
Table 1
Table for UV-Vis spectral data for complexes.
Com Solvent Wavelength, nm (ε)
CH CN
1
combined effect of ligand strength and the solvent strength on n .
The complexes 4a, 4b show an absorption in the range of
500e570 nm and an absorption in the range of 930e1040 nm,
1a
1b
2a
2b
3
436.07 (15), 560.52 (21), 786.75 (11)Sh, 921.14 (20)
Sh
DMF
DMSO
PYR
576.98 (17), 778.90 (10) , 931.90 (20)
Sh
587.52 (16), 785.18 (7) , 953.22 (21)
representing the band which corresponds to
n
2
and
3
n respectively.
434.48 (34)
,
578.86 (20), 778.00 (5)Sh, 950.76 (14),
8
Free nickel(II) ions [66] in d configuration possess Russel
1
142.42 (16).
3
3
1
1
1
3
567.12 (40), 796.40 (19)Sh, 925.58 (26)
Saunders terms F, P, D, G, S. The ground state F of the octa-
3
CH CN
Sh
3
3
3
DMF
DMSO
PYR
574.49 (24), 785.73 (6) , 947.67 (16)
hedral geometry can split into A2g(F), T2g(F) and T1g(F). Similarly
P into T1g(P); D into E1g(D) and T2g(D); G into A1g(G), T1g(G),
2g(G) and Eg(G); and S into
Sh
3
1
3
1
1
1
1
1
575.95 (27), 787.57 (10) , 947.27 (20)
436.47 (22), 589.21 (25), 787.20 (5)Sh, 961.28 (16),
1
1
1
T
A1g(S) once a ligand field is
1
125.42 (16).
3
3
541.23 (40), 780.48 (20)Sh, 887.67 (24),
435.06 (57)
,
introduced. Among this the transitions
A
2g(F)
2g(F) / T1g(F) and A2g(F) / T1g(P) are generally termed spin
allowed transitions. The two spin forbidden transitions are
/
T2g(F),
CH
3
CN
3
3
3
3
Sh
DMF
DMSO
PYR
573.94 (17), 773.78 (6.5) , 932.95 (16.5)
A
Sh
581.45 (19), 781.41 (6.5) , 963.90 (20), 1164.23 (17.6).
437.37 (72)
138.79 (18).
532.04 (57), 774.55 (33)Sh, 883.99 (37)
441.23 (83.5)
,
,
574.26 (25), 781.41 (5.5)Sh, 953.97 (13),
3
2g(F) / 1E1g(D) and A2g(F) / T2g(D). Generally three intense
3
1
A
1
spin allowed and two weak spin forbidden bands are expected for
CH
3
CN
8
Ni (II), d configuration in octahedral geometry [67,68]. Accordingly
DMF
DMSO
Pyr
570.09 (21), 779.32 (10), 944.48 (17.5)
576.08 (19), 781.08 (9), 961.29 (18), 1165.09 (20).
437.50 (75), 580.15 (21), 782.58 (5) , 965.31 (11),
the complex 1a, 1b, 2a and 2b upon dissolving in acetonitrile and
pyridine reveal three bands as stated above along with a shoulder
at 770 nm. The three intense bands represent the three spin
Sh
1140.26 (16).
3
4
a
a
DMF
DMSO
PYR
500.47 (1047), 770.4 (11), 956.57 (20).
512.12 (1742), 788.42 (14), 983.74 (19), 1161.33 (13).
518.09 (770), 791.42 (9.7), 990.46 (14), 1169.29 (7.9).
550.23 (409), 763.36 (3.8), 951.64 (23.7)
489.82 (268)
524.94 (750), 761.91 (12), 975.910 (9), 1134.83 (9)
541.81 (424), 936.11 (4), 1100.13 (12)
543.30 (199), 778.89 (14.7), 994.83 (5.7)
567.02 (492), 807.28 (7.7), 963.05 (11)
488.84 (495), 1058.57 (14.2)
504.46 (649), 974.15 (35)
534.34 (545), 991.62 (16), 1166.49 (16)
494.41 (629), 987.58 (26)
564.54 (351), 993.63 (5.6), 1141.42 (7.6)
535.74 (544), 977.48 (22.3), 1129.32 (19.3)
509.35 (609), 1038.86 (17.1)
3
-
allowed
2g(F) /
d-d
T
T
transitions
2g(F) ¼ 950 nm;
3
from
A
the
ground
state,
3
3
2g(F) / 3T1g(F) ¼ 550 nm;
A
3
CH
THF
CH
3
CN
A
2g(F) /
1g(P) ¼ 360 nm. Weak features, due to the spin
3 1
forbidden transitions
between the
A
2g(F) / E1g(D) also occur, at energies
3
OH
3
3
3
3
A2g(F) / T1g(F) and A2g(F) / T2g(F) transitions,
DMF
DMSO
PYR
while a further week transition was observed at around 1150 nm
3
1
representing A2g(F) / T2g(D) transition. The observation of this
4
b
CH
THF
CH
3
CN
spin forbidden transition is thought to be facilitated by the close
3
3
3
1
g
proximity of A2g / T2g and A2g(F) / E transitions, which leads
3
OH
to a strong mixing through spineorbit coupling.
Based on the above study, the Scheme 2 explains the observed
solvatochromic behaviour of complex 1, and gives the pictorial
representation for all the other complexes reported in the
manuscript.
EtOH
DMF
DMSO
PYR
[sh] shoulder.

P.M. Selvakumar et al. / Dyes and Pigments 95 (2012) 563e571
569
Fig. 4. UV-Vis spectra for complex 1a and 1b in different solvents.
3
Scheme 2. Schematic diagram of solvent coordination fulfilling labile sites in Ni (II) helicates 1. (i) CH CN. (ii) DMF. (iii) DMSO. (iv) Pyridine.

5
70
P.M. Selvakumar et al. / Dyes and Pigments 95 (2012) 563e571
[19] Albrecht M, Janser I, Kamptmann S, Weis P, Wibbeling B, Fröhlich R. Dica-
4
. Conclusion
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chirality of self-assembled systems in solution. Chem Soc Rev 2004;33(6):
In summary, a series of coordinatively unsaturated Ni(II) double
[
[
stranded helicates were synthesized and their solvatochromic
properties were demonstrated for the first time. The described
coordinatively unsaturated helicates possess labile solvent
molecules, which can be substituted upon the change of the solvent
and studied their electronic transitions using electronic spectra.
Our further work in this direction is under progress to use those
labile sites, which provides potential scope for a wide range of
applications in the area of biological science as a marker.
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Appendix A. Supplementary data
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