M.M. Ibrahim et al. / Journal of Molecular Structure 985 (2011) 191–201
193
complex 3 (0.43 g, 0.5 mmol)in aqueous methanol. After stirring
R
for 2.0 h, a white precipitate was obtained, filtered, and dried in va-
R
N
R
N
cuo. Yield: 0.29 g, 84% of {[L1Zn–OH](PF6)]ꢁCH3OHꢁH2O} 5.
N
N
N
N
N
Y
Zn
2.2.8. Syntheses of hydroxotris(N-methylbenzimidazolylmethyl)amine
zinc(II)hexaflurophosphate 6
The zinc(II)-bound hydroxo complex 6 was obtained in a similar
fashion to complex 5. Yield: 0.31 g, 79% of {[L2Zn–OH](PF6)]ꢁ-
CH3OHꢁ2H2O} 6.
X
[L1Zn-X]Y
[L2Zn-X]Y
X = Cl, Y = Cl
(1)
(2)
2.2.9. Synthesis of bis[thiocyanatotris(benzimidazolylmethyl)amine
zinc(II)]tetrathio-cyanatozinc(II) 7
X = H2O, Y = (PF6)2
X = OH, Y = PF6
(3)
(4)
Stoichiometric amounts of zinc thiocyanate (0.91 g, 5.0 mmol)
and NTB (2.04 g, 5.0 mmol) were dissolved separately in hot abso-
lute ethanol. The mixed hot solution was stirred for 2 h. The vol-
ume was reduced, followed by cooling. A white precipitate was
obtained, dried in vacuuo. Single crystals suitable for X-ray crystal-
lography were obtained by slow diffusion of diethyl ether into
aqueous acetonitrile solution of that complex. Yield: 23 mg, 63%
of {[L1Zn(NCS)]2[Zn(NCS)4]} 7.
5
( )
6
( )
Scheme 3. The zinc(II) model complexes with their different coligands.
3. Results and discussion
3.1. Characterization of the zinc(II) model complexes
The syntheses of the ligands L1 and L2 were carried in micro-
wave at 500 W and 120 °C for 10–15 min. These ligands were used
to emulate the N3 coordination environment provided by the three
histidine protein residues in the hydrolytic zinc enzyme, carbonic
anhydrase. The zinc(II) ion coordinates to both ligands via three
pyridinic nitrogen donors from the benzimidazole rings and too
weak coordination to the tertiary nitrogen atom with one chloride
atom, water molecule, or hydroxide anion, forming [LZn(Cl)](PF6)]
1, 2 (L = L1 and L1); [LZn–H2O](PF6)2 3, 4 and [LZn–OH](PF6) 5, 6,
respectively (Scheme 4). The aqua zinc(II) complexes complexes
3 and 4 were obtained by the reaction of the chloro complexes 1
and 2 with AgPF6 in aqueous acetone. Like all other complexes with
LZn unit, the resulting complexes 1–4 are anionic as evidenced by
its reasonable solubility in protic solvents.
2.3. X-ray determination of 7
X-ray measurement of complex 7 was performed at 123 K on a
Rigaku RAXIS-RAPID Imaging Plate diffractometer with graphite
monochromated Mo K
a radiation. Data have been corrected for
Lorentz and polarization effects. The structure was solved by direct
methods [30] and expanded by using Fourier techniques [31]. Non-
hydrogen atoms were refined anisotropically. Hydrogen’s were in-
cluded but not refined. All calculations were carried out using the
teXsan crystallographic software package developed by Molecular
Structure Corporation (1985 and 1999).
2.3.1. 1H NMR measurements
To investigate the coordination behavior towards zinc(II) ions,
1H NMR analysis of the nature of L1 or L2/Zn2 + binding in a mixed
solvent of DMSO-d6 and D2O (40 v/v %) and I = 0.1 M NaNO3 were
undertaken as a function of pD and at different zinc-to-ligand ra-
tios (R = [Zn2+]o/[L]o). The chemical shifts are recorded relative to
the resonance signal of tetramethyl silane (TMS) as an internal
standard signals. The pD was adjusted using portable pH-meter
using standard NaOD and DNO3, so that the effect of dilution was
neglected.
The chemical analyses and some physical properties of the iso-
lated pure complexes are listed in Tables 1 and 2. The analytical re-
sults demonstrate that all the complexes have (1:1) metal: ligand
stoichiometry. The microcrystalline complexes are stable as solids
or in solution under the atmospheric conditions. All the investi-
gated compounds behave as 1:1 and 1:2 electrolytes with molar
conductance values rang of 121–139
X
ꢀ1 cm2 molꢀ1 suggesting
that in solution the PFꢀ6 ions are outside the coordination sphere.
Table 3 lists the observed FT-IR frequencies and band assign-
ments of the ligand L1 and chloro and aqua complexes 1 and 3 in
the solid state using KBr method. The IR spectrum of 1 showed
one peak at 3234 cmꢀ1 and four peaks in the region of 1248–
1213 cmꢀ1, which were assigned as hydrogen bond stretching
and bending, respectively. The peak at 3234 was absent in the case
of the aqua complexes 3 and 4. Whereas in the region of 1248–
1213 cmꢀ1, it showed two peaks. This may be due to the intramo-
lecular hydrogen bonding in complex 1 between the hydrogen
atoms of the benzene rings and coordinated chloride ligand [32].
In complex 3, the coordinated oxygen is blocked by two hydrogen
atoms, avoiding hydrogen bonding. This confirms that in complex
1, the extra peaks are from hydrogen bond stretching and bending.
In solution, these extra peaks were absent.
2.4. Fixation of CO2 by using complex 3
The reaction of 3 with CO2 was monitored from the product of
the bicarbonate complex using 13C NMR. To compare the effect of
the presence and absence of base, we prepared two solutions for
each complex, one of them contain 3 + Et3N and the others without
Et3N. CO2 was introduced to each solution at the same time using a
Y-shape tube (Scheme 3). For 13C NMR measurements, samples
were made as follows: complex 3 (ca. 100 mM) with or without
equimolar amounts of Et3N was dissolved into DMSO-d6. CO2
was bubbled for 4 h to each solution. The chemical shifts of 13C
NMR in DMSO-d6 were recorded vs. TMS. For IR measurements,
samples were made as follows: after the equimolar base was dis-
solved into a CH3CN solution of 3 (ca. 50 mM), the solutions were
suspended as a result of the formation of the monomeric zinc(II)
bound hydroxo complex [L1Zn–OH]PF6 5. Slow evaporation of the
suspended solutions by bubbling CO2 gas. A solid compound was
produced, which was assigned by using IR measurements as
hydrogen carbonate complex.
The effect of zinc(II) binding on the ring vibration of C–C–N–C in
the benzimidazole ring of the ligand systems has been investigated.
The observed bands at 1495–1475, 1048, and 503 cmꢀ1 for both 1
and 3 were assigned as ring stretching, ring bending, and ring out
of plane deformation, respectively [33–38]. These bands were
shifted by 7–18, 17, and 25 cmꢀ1 to high frequencies upon
complexation with zinc(II) ions. The skeletal modes of these