T. Maity et al. / Catalysis Communications 58 (2015) 141–148
143
water in Cu(II) salt may occur. However, organic modification changes
the nature of IRMOF-3 from hydrophilic to hydrophobic [30]. Owing
to this the framework system retains its structural integrity after
2 2
metal incorporation using CuCl · 2H O. To optimize the extent of mod-
ification, we investigate the functionalization of IRMOF-3, by varying
the amount of pyridine-2-aldehyde and, subsequently, by varying the
amount of copper chloride during Schiff base complex formation stage
(
Figures S5 and S6).
Solid state UV–vis spectra of both IRMOF-3 and IRMOF-3-PA
Figure S7) featured two strong absorption bands (around 251, 280
(
and 258, 335 nm respectively), whereas the later one showed an addi-
tional transition ~382 nm which was attributed to the π–π* transition
of C_N group. IRMOF-3-PA-Cu, however, exhibited an additional
weak yet distinct band at ~743 nm (Figure S7), which could be ascribed
to d–d transition of metal typical for a Cu–Schiff base complex [31].
The FTIR spectra of all three samples were measured (Fig. 1)
and characteristic carboxylato vibration bands found in the range of
−
1
−1
1
430–1380 cm . A moderately intense band appeared at ~3438 cm
−
1
and a relatively strong band at ~1565 cm
could be ascribed to the
2
N\H stretching and bending vibration mode of the \NH group, respec-
tively. IR spectrum of IRMOF-3-PA showed two new extra bands at
Fig. 3. Nitrogen sorption isotherms of (1) IRMOF-3, (2) IRMOF-3-PA and (3) IRMOF-3-PA-
Cu at 77 K.
−
1
−1
~
1651 cm and ~1617 cm ; the former band could be ascribed to
the characteristic vibration band for the azomethine group (NC_N\)
whereas the latter one may due to the vibration mode of the C_N
bond of the pyridine ring. For IRMOF-3-PA-Cu, the vibration bands of
both azomethine group and C_N moiety of the pyridine ring were
shifted to the lower frequency range and overlapped with the asymmet-
ric band of the carboxylato group. This indicated coordination of
azomethine nitrogen with copper metal ion. In addition, the catalyst
that organic functionalization as well as metalation had no effect on
the structural integrity of the framework [30].
Comparison of EDX (Fig. 4C and D) analysis data of 2 and 3 indicates
the presence of copper and chloride (ca. 1:2 ratio) along with zinc and
oxygen in the catalyst. This ratio is close to coordination of copper
with two chloride atoms as shown in Scheme 1.
Upon heating the catalyst showed a small weight loss between room
temperature and 70 °C (Figure S9) in thermogravimetric analysis. This
can be due to the release of adsorbed solvent molecules. Thereafter TG
curve remained flat indicating the thermal stability of the catalyst up
to ~230 °C.
−
1
showed two new bands at 485 and 417 cm , which could be assigned
to the asymmetric Cu\N stretching mode [32]. Two weak peaks at 364
−
1
and 306 cm
were obtained in the far IR range due to symmetric
Cu\N stretching and Cu\Cl (terminal chloro) vibrations, respectively
(Figure S8) [32].
To understand the authenticity and stability of all frameworks,
powder XRD patterns of IRMOF-3, IRMOF-3-PA, and the catalyst were
studied (Fig. 2). Their comparison with simulated IRMOF-3 confirmed
The nitrogen sorption isotherms of IRMOF-3, IRMOF-3-PA and the
catalyst were shown in Fig. 3; their pore diameter plot were given in
Figure S10. BET surface area, pore volume and pore diameter data of
all three species were collated in Table S1 and these values correspond
well with literature data [33]. Gradual decrease of surface area, pore vol-
ume and diameter from IRMOF-3 to IRMOF-3-PA-Cu implied the inner
surface modification of a solid framework.
The principle g values calculated from EPR spectrum of powdered
IRMOF-3-PA-Cu (Figure S11) were in agreement with those reported
for other copper(II) Schiff base complexes [34]. Copper(II) systems
⊥
with g|| N g N 2.0023 (2.42 and 2.06 respectively) suggested that the
unpaired electron occupies the dx2 − y2 orbital, which was the character-
istic of a square planar coordination environment of copper(II) [34].
3.2. Catalytic activity studies of 3
Initially, optimization studies for C–O cross-coupling reaction was
undertaken using phenol and phenyl bromide as substrates under vari-
ous reaction conditions as given in Table S2. Among different solvents,
in DMSO the highest yield of product was obtained (entry 2). However,
ethanol had been selected (entry 14) as solvent for its green nature. It
had been found that O-arylation reaction was much faster with expen-
sive Cs CO
2 3
2 3 3 4 2 3
than with K CO or K PO or Na CO due to the enhanced
basicity of Cs
2
CO (entries 8, 9, 11 and 14) and the catalyst was inactive
3
in the absence of a base (entry 12) [35]. Considering temperature isolat-
ed yield was not up to the mark in low (50 °C) temperature (entry 15).
Thus the optimum condition of the catalytic reaction was as follows:
Fig. 2. PXRD pattern of 1 simulated, 1 synthesized, 2, 3 and 3 recovered.
2 3
Cs CO (base), ethanol (solvent) and reaction temperature 80 °C.