Photolysis of Zn(8-quinolinolate)2(H2O)2 in non-aqueous solution. Photoactivation of hydrolysis

Article abstract of DOI:10.1016/j.inoche.2014.04.013

The octahedral complex Zn(8-quinolinolate)₂(H₂O) ₂ in non-aqueous solvents (e.g. chloroform, cyclohexane) undergoes a photolysis: Zn(8-quinolinolate)₂(H₂O)₂ → Zn(OH)₂ + 2 8-quinolinole. It is suggested that this photohydrolysis is facilitated by hydrogen bonding. The irradiation (λ > 340 nm) is absorbed by the ligand chromophore. The IL excitation is associated with a charge shift from the coordinated phenolate to the nitrogen of the pyridine ring. As a consequence, the phenolate loses its coordinating ability. The subsequent deactivation is accompanied by the reversal of the charge shift. The regenerated phenolate may now undergo protonation from an adjacent water ligand in competition with the re-coordination to Zn^{2 +}. This system represents a simple model for activation of water which is coordinated to zinc(II). In distinction to the enzymatic function the hydrolysis in our system is initiated by a light switch.

Full text of DOI:10.1016/j.inoche.2014.04.013

Inorganic Chemistry Communications 45 (2014) 82–83
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Photolysis of Zn(8-quinolinolate) (H O) in non-aqueous solution.
2
2
2
Photoactivation of hydrolysis
Arnd Vogler
Institut für Anorganische Chemie, Universität Regensburg, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
2 2 2
The octahedral complex Zn(8-quinolinolate) (H O) in non-aqueous solvents (e.g. chloroform, cyclohexane)
Received 10 March 2014
Accepted 11 April 2014
Available online 21 April 2014
2 2 2 2
undergoes a photolysis: Zn(8-quinolinolate) (H O) → Zn(OH) + 2 8-quinolinole. It is suggested that this
photohydrolysis is facilitated by hydrogen bonding. The irradiation (λ N 340 nm) is absorbed by the ligand chro-
mophore. The IL excitation is associated with a charge shift from the coordinated phenolate to the nitrogen of the
pyridine ring. As a consequence, the phenolate loses its coordinating ability. The subsequent deactivation is
accompanied by the reversal of the charge shift. The regenerated phenolate may now undergo protonation from
an adjacent water ligand in competition with the re-coordination to Zn2+. This system represents a simple model
for activation of water which is coordinated to zinc(II). In distinction to the enzymatic function the hydrolysis in
our system is initiated by a light switch.
Keywords:
Photochemistry
Hydrolysis
Zinc complexes
8-Qinolinolate complexes
©
2014 Elsevier B.V. All rights reserved.
Zinc enzymes play a very important role in a variety of biological
processes [1]. They catalyze, for example, the hydrolysis of peptides
carboxypeptidase) or the reversible hydration of carbon dioxide
carboanhydrase). In these reactions water coordinated to zinc is
activated (Eq. (1)).
100 °C, e.g. 160 °C [9]. The aquo complex dissolves in various organic sol-
vents. The compound is solvatochromic [3] since the excitation is associ-
ated with a charge redistribution in the oxinate ligands [4–8]. It is rather
surprising that the aquo complex is soluble even in alkanes [10] but its
solubility in all solvents is quite low [10]. The absorption and emission
(
(
2 2 2
spectrum of Zn(oxinate) (H O) in chloroform and cyclohexane (Fig. 1;
Zn−OH22þ⇄Zn−OHþ þ Hþ
absorption λmax = 345 and 384 nm, emission λmax = 540 nm) are rather
similar.
ð1Þ
The irradiation of these solutions with λ N 320 nm leads to the disap-
pearance of the complex as indicated by the loss of its absorption, e.g. in
Mechanistic studies are made more difficult by the lack of simple
probes. Unfortunately, suitable chromophores as indicators general-
ly are not available. While the catalytic function of hydrolases is
based on thermal processes, proper models could be operated photo-
chemically. Accordingly, such a photo-induced function would be
stimulated by a light switch [2]. As a suitable model for an artificial
CHCl
imately φ = 10
3
(Fig. 2). A rough estimate yielded a quantum efficiency of approx-
−3
at λirr = 436 nm. At later stages of the photolysis
the solution becomes cloudy. Colloidal particles cause the scattering of
light which leads to an increase of the optical density over the whole
UV/visible spectrum growing towards shorter wavelengths. When the
photolysis is carried out in cyclohexane, the solution becomes cloudy
already at the beginning of the irradiation. The photolysis can be driven
to completion. After standing overnight the colloidal particles separate
as a solid. The residual colorless and transparent solution shows
only the absorption spectrum of 8-quinolinol with λmax = 245 nm;
photohydrolase we selected the complex Zn(oxinate)
structure (1)) with oxinate = 8-hydroxyquinolinolate.
Oxinate complexes have been utilized for the qualitative and quan-
2 2 2
(H O)
(
titative analysis of metal ions for many decades [3]. Moreover, the optical
and photochemical properties of metal oxinates have been studied in
quite some detail [4–8].
ε = 5 × 104 and 320 nm; ε = 3 × 103 [11]. The solid residue was dis-
Irradiations were performed with a high-pressure mercury lamp
Osram HBO W/2 as the light source. Monochromatic light was obtained
by using Schott PIL/IL interference and Schott cutoff filters. Generally,
the preparation of zinc oxinate is carried out in an aqueous medium [3,
solved in very little hydrochloric acid and upon addition of 8-
quinolinol in ethanol transformed to zinc oxinate as indicated by its ab-
sorption spectrum. It is assumed that the colloidal particles and the final
solid residue consist of ZnO or Zn(OH) .
2
These observations lead to the conclusion that the photolysis proceeds
according to the Eq. (2) or (3).
9
]. This leads to the formation of the aquo complex Zn(oxinate)
2 2 2
(H O)
while its dehydration to Zn(oxinate) requires temperatures above
2
E-mail address: arnd.vogler@chemie.uni-regensburg.de.
ZnðoxinateÞ ðH OÞ →ZnðOHÞ þ 2 8−quinolinol
2
2
2
2
ð2Þ
http://dx.doi.org/10.1016/j.inoche.2014.04.013
1387-7003/© 2014 Elsevier B.V. All rights reserved.

A. Vogler / Inorganic Chemistry Communications 45 (2014) 82–83
83
H
O
H
N
O
Zn
O
N
H
O
H
Structure 1.
Fig. 2. Spectral variations during the photolysis of 4 × 10− M Zn(oxinate)
4
(H
O)
in chlo-
2
2
2
roform at r.t., 1-cm quartz cell, λirr N 320 nm, irradiation time: (a) 0 min, (d) 12 min.
ZnðoxinateÞ ðH OÞ →ZnO þ H O þ 2 8−quinolinol
ð3Þ
2
2
2
2
Generally, zinc(II) complexes are not light sensitive because elec-
tronic excitation does not involve transitions with the participation of
metal-based molecular orbitals since the 3d10 shell is too low and the
empty 4s and 4p orbitals are too high in energy. Accordingly, the photo-
sensitivity of zinc(II) complexes depends on the presence of suitable li-
gands. They should provide chromophores in the UV/vis region and
luminescent and/or reactive intraligand (IL) excite states. The complex
context, it might be suspected that it is simply the amount of energy
which facilitates this photolysis. However, when the complex is heated
up to 200 °C, it loses only the water ligands under formation of
Zn(oxinate) indicating that the photolysis is a quite selective process
2
which is determined by the nature of the reactive excited state. In this
case, it can be only of the IL (or ILCT) type [17]. At this point, it should
be mentioned that biologically active ligands which are related to
Zn(oxinate)
The yellow fluorescence of Zn(oxinate)
oxinate IL type and a characteristic feature of many oxinate complexes
4,6–8]. However, the photoreaction (Eq. (1)/(2)) is apparently a com-
(H O)
2 2 2
represents a rather intriguing case of this type.
2+
8
-quinolinolate are apparently coordinated to Zn in natural systems
2
(H O) (Fig. 1) is of the
2
2
[
18].
In conclusion, we observed a photochemical hydrolysis: Zn(8-
quinolinolate) (H O) → Zn(OH) + 2 8-quinolinol. This rather inter-
[
2
2
2
2
peting process of deactivation of this IL excited state. It can be rational-
ized by the following considerations. In reality, the IL excitation of
oxinate is accompanied by a charge shift from the phenolate oxygen
to the pyridine moiety [5,6,8,12]. In this ILCT state the coordinating abil-
ity of the phenolate function is lost and phenolate is then detached from
zinc. The subsequent deactivation involves a reversal of the initial
charge shift and hence a regeneration of the phenolate function. This
in turn, may lead to the re-coordination of the phenolate to zinc, but
as a competing process the phenolate abstracts a proton. The resulting
oxinateH (8-quinolinol) is finally released. Since this photoreaction
takes place in non-aqueous solution, the proton, which is transferred
to the phenolate oxygen, must originate from the coordinated water li-
gands. This should be facilitated by the adjacent location of the proton
donor and acceptor site (structure (1)) as confirmed by X-ray analysis
esting example of photoactivation of a water ligand coordinated
to Zn(II) is based on a charge shift induced by IL excitation of the
quinolinolate ligand. This observation can be viewed as a photochemical
analog of thermal hydrolysis as catalyzed by zinc enzymes in nature. Al-
though our observations are based on a very simple system, they may
stimulate more studies in this research field.
Acknowledgement
Financial support by DFG (grant Vo 211/19-1) is greatly appreciated.
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Fig. 1. Absorption (ab) and emission (em) spectrum of a 10− M Zn(oxinate)
4
(H
O)
so-
2
2
2
lution in cyclohexane, 1-cm quartz cell, r.t., emission intensity in arbitrary units.