D.D. Narulkar et al. / Inorganica Chimica Acta 427 (2015) 248–258
253
Table 1
species respectively. For 5 and 6, the mass peaks at 264.1 (calc. m/z
264.0) and 278.1 (calc. m/z 278.0) in the ESI-MS spectra are observed
for [Ni(bqenH2)(bpy)]2+ and [Ni(bqenMe2)(bpy)]2+ species.
UV–Vis data of compounds 1–6.
Compound
3A2g
?
3T1g(F), nm
3A2g 3T2g, nm
?
(e/dm3 molꢁ1 cmꢁ1
)
(e/dm3 molꢁ1 cmꢁ1
)
3.3. Infrared spectroscopy
1
2
3
4
5
6
528 (8)
528 (9)
489 (21)
552 (11)
489 (29)
528 (10)
872 (9)
872 (8)
793 (8)
872 (9)
793 (9)
872 (12)
The Infrared (IR) spectrum of bqenMe2 shows absence of N-H
vibration that is observed at ꢂ3385 cm1ꢁ for bqenH2 (Fig. 3). This
observation indicates that, the H atoms on two N atoms in bqenH2
are replaced by the –CH3 groups. The presence –CH3 groups was
further confirmed by the use of 1H and 13C NMR spectroscopy
(Figs. S1–S4 in supplementary information). For compounds 1, 3
and 5, the N-H stretching vibrations occur at ꢂ3265, 3269 and
3228 cmꢁ1 respectively.
The N–H stretching vibrations in these three compounds are
shifted to the lower frequencies as compared to that observed for
the free ligand. This observation reveals that the ligand bqenH2 is
coordinated to the Ni(II) [36,37]. Further, no such bands were
observed for compounds 2, 4, and 6 indicating the absence of N-H
bonds in these compounds. Compounds 1 and 2 exhibit broad peaks
at ꢂ3547 cmꢁ1 and ꢂ3405 cmꢁ1 respectively which are assigned to
the O-H stretching vibrations of water. When 1 and 2 were dis-
solved in CH3CN, the coordinated water molecules are exchanged
with CH3CN ligands [38]. The complete disappearance of -OH vibra-
tions in 3–6 indicates the substitution of two H2O molecules (which
may be present as labile ligands) by bidentate phen and bpy in 3–6.
The presence of aromatic -C@N functionality is observed at
ꢂ1526 cmꢁ1 for both the ligands while it is shifted to lower fre-
quency of ꢂ 1518 cmꢁ1 in all the compounds. This observation is
not unusual as the two N donor atoms are coordinated to metal cen-
ter [26,39,40]. The presence of perchlorate anions in 1–6 was
revealed from the appearance of strong and medium absorption
peaks at ꢂ1093 and 621 cmꢁ1 respectively [36,39].
Fig. 5. CV (solid line) and DPV (dotted line) of 2 recorded at scan rate of 100 mV sꢁ1
in DMSO containing 0.1 M of TBAPF6 as supporting electrolyte.
(see Figs. S1–S4 in the supporting information). The reaction of
bqenH2 and bqenMe2 with Ni(ClO4)2ꢀ6H2O in CH3CN afforded com-
pounds 1 and 2 respectively in good yields. Our efforts to obtain
the single crystals of compound 1 and 2 suitable for X-ray diffrac-
tion studies were not fruitful. Complex 1 was reacted with auxil-
iary bidentate N-donor ligands such as phen and bpy in CH3CN
resulting in the exchange of weakly coordinating solvent mole-
cules (CH3CN or H2O) to obtain 3 and 5. Under identical reaction
conditions, the compounds 4 and 6 were prepared using 2 as a
starting material. The single crystals of 3 and 4 were isolated on
slow diffusion of diethyl ether into their solutions and directly
used for X-ray data collection, however we were unable to grow
the single crystals of 5 and 6. The synthetic methodology adopted
for the preparation of 1–6 is shown in Scheme 3.
3.4. UV–Vis spectroscopy
The electronic spectrum of nickel(II) ion in an octahedral envi-
ronment is expected to show three d–d bands assignable for
the 3A2g ? 3T2g 3A2g ? 3T1g(F) and 3A2g ? 3T1g(P) transitions. The
,
overlaid UV–Vis spectra of compounds 1–6 are shown in the
Fig. 4 and Fig. S6 while the data for the intense d–d bands observed
at different wavelengths in CH3CN is summarized in the Table 1.
The d–d band assigned to 3A2g ? 3T1g(F) transition is observed in
the region of 489–553 nm on the other hand the peak due to
3A2g ? 3T2g transition is observed in the wavelength range 793–
872 nm [41]. Both the bands are very weak in intensity and are
observed only at higher concentrations of the compounds in CH3-
CN. The tailing of a charge transfer band hinders the observation
of third d–d band assigned to the 3A2g ? 3T1g(P) transition in all
six compounds [42].
3.2. ESI-Mass spectrometry
Compounds 1 and 2 were characterized by using ESI-Mass spec-
trometry in CH3CN (see Fig. 2). The ESI-MS spectrum of 1, shows
prominent mass peaks at m/z 227.0 (calc. m/z 227.1) and 471.0
(calc. m/z 471.1) which are assigned to the [Ni(bqenH2)(CH3CN)2]2+
and [Ni(bqenH2)(ClO4)]+ species respectively while the mass peak
observed at m/z 371.1 (calc. m/z 371.0) is attributed to the
[Ni(bqenH)]+ species. On other hand, the ESI-MS spectrum of 2
exhibits prominent mass peaks at m/z 220.5 (calc. m/z 220.6),
241.0 (calc. m/z 241.1) and 499.1 (calc. m/z 499.1) which are
assigned to the [Ni(bqenMe2)(CH3CN)]2+, [Ni(bqenMe2)(CH3CN)2]2+
and [Ni(bqenMe2)(ClO4)]2+ species respectively. Similarly, we have
extended the ESI-MS spectrometry to the remaining complexes 3–6
(Fig. S5 in the supporting information).
The d–d absorption bands for 1 and 2 are similar in terms of
their intensities and energies. Compounds 3 and 4 differ slightly
in their absorption patterns from those of 5 and 6 which clearly
suggests an influence of ligands (phen and bpy) on the crystal
fields. The high-intensity bands observed in the UV region of
200–320 nm are assigned to the intra-ligand transitions. The band
at ꢂ272 nm in 3 and 4 is assigned to the
from the coordination of the nickel to 1,10-phenanthroline [43].
The
p⁄ transition due to bipyridine ligand is observed at
p–
p⁄ transition that arises
p
–
284 nm in compound 5 and at 296 nm in compound 6. The bands
in the region of 290–320 nm are assigned to the n–p⁄ transitions.
3.5. Cyclic and differential pulse voltammetry
The ESI-MS mass spectra of 3 and 4 show prominent mass peaks
at m/z 276.0 (calc. m/z 276.1) and 290.0 (calc. m/z 290.1) which are
assigned to the [Ni(bqenH2)(phen)]2+ and [Ni(bqenMe2)(phen)]2+
Compounds 1–6 were characterized by using cyclic voltammetry
(CV) and differential pulse voltammetry (DPV) to explore their