Hyperfine Interact (2008) 185:47–56 DOI 10.1007/s10751-008-9810-x
Dimeric Fe (II, III) complex of quinoneoxime as functional model of PAP enzyme: Mössbauer, magneto-structural and DNA cleavage studies Sunita Salunke-Gawali · Khursheed Ahmed · François Varret · Jorge Linares · Santosh Zaware · Sadgopal Date · Sandhya Rane
Published online: 11 October 2008 © Springer Science + Business Media B.V. 2008
Abstract Purple acid phosphatase, (PAP), is known to contain dinuclear Fe2 +2,+3 site with characteristic Fe+3 ← Tyr ligand to metal charge transfer in coordination. Phthiocoloxime (3-methyl-2-hydroxy-1,4-naphthoquinone-1-oxime) ligand L, mimics (His/Tyr) ligation with controlled and unique charge transfers resulting in valence tautomeric coordination with mixed valent diiron site in model compound Fe-1: [μ-OH-Fe2 +2,+3 (o-NQCH3ox ) (o-NSQCH3ox )2 (CAT) H2 O]. Fe-2: [Fe+3 (o-NQCH3ox ) ( p-NQCH3ox )2 ]2 a molecularly associated dimer of phthiocoloxime synthesized for comparison of charge transfer. 57 Fe Mössbauer studies was used to quantitize unusual valences due to ligand in dimeric Fe-1 and Fe-2 complexes which are supported by EPR and SQUID studies. 57 Fe Mössbauer spectra for Fe-1 at 300 K indicates the presence of two quadrupole split asymmetric doublets due to the differences in local coordination geometries of [Fe+3 ]A and [Fe+2 ]B sites. The hyperfine interaction parameters are δA = 0.152, (EQ )A = 0.598 mm/s with overlapping doublet at δB = 0.410 and (EQ )B = 0.468 mm/s. Due to molecular association tendency of ligand, dimer Fe-2 possesses 100% Fe+3 (h.s.) hexacoordinated configuration with isomer shift δ = 0.408 mm/s. Slightly distorted octahedral symmetry created by NQCH3ox ligand surrounding Fe+3 (h.s.) state generates small field gradient indicated by quadrupole split EQ = 0.213 mm/s. Decrease of isomer shifts together with variation of quadrupole splits with temperature in Fe-1 dimer compared to Fe-2 is
S. Salunke-Gawali · K. Ahmed · S. Zaware · S. Rane (B) Department of Chemistry, University of Pune, Pune 411 007, India e-mail:
[email protected] S. Salunke-Gawali · F. Varret · J. Linares Laboratoire de Magnétisme et d’Optique, CNRS, UMR 8634, Université de Versailles, 45 Avenue des Etats-Unis, 78 035 Versailles Cedex, France K. Ahmed Department of Chemistry, Abeda Inamdar Senior College, Pune 411001, India S. Date Department of Physics, University of Pune, Pune 411007, India
48
S. Salunke-Gawali et al.
result of charge transfers in [Fe2 +2,+3 SQ] complexes. EPR spectrum of Fe-1 shows two strong signals at g1 = 4.17 and g2 = 2.01 indicative of S = 3/2 spin state with an intermediate spin of Fe+3 (h.s.) configuration. SQUID data of χmcorr .T were best fitted by using HDVV spin pair model S = 2, 3/2 resulting in antiferromagnetic exchange (J = −13.5 cm−1 with an agreement factor of R = 1.89 × 10−5 ). The lower J value of antiferromagnetic exchange leads to Fe+3 μ-(OH) Fe+2 bridging in Fe-1 dimer instead of μ-oxo bridge. The intermolecular association through H-bonds may lead to weakly coupled antiferromagnetic interaction between two Fe-2 molecules having Fe+3 (h.s.) centers. Using S = 5/2, 5/2 spin pair model we obtained bestfitted parameters such as J = −12.4 cm−1 , g = 2.3 with R = 3.58 × 10−5 . Synthetic strategy results in non-equivalent iron sites in Fe-1 dimer analogues to PAP enzyme hence its reconstitution results in pUC-19 DNA cleavage activity, as physiological functionality of APase. It is compared with nuclease activity of Fe-2 RAPase. Keywords Acid phosphatase (APase) · Valence tautomers · Reconstituted acid phosphatase (RAPase) · Mössbauer · Magnetostructural studies
1 Introduction In case of non-heme proteins like PAP, iron is taking part in the hydrolysis of phospho monoesters with a pH optimum below 7. The PAP enzyme isolated from mammals have a dinuclear iron center in their active site with two accessible oxidation states, in the inactive form Fe+3,+3 and the catalytically active form Fe+2,+3 [1]. Mössbauer spectra of ufPAP [2] and bsPAP [3] revealed that the iron centers are in distinct asymmetric coordination environments, which were confirmed from their X-ray crystallographic studies. The dinuclear mixed valent model complexes containing different set of ligands with radical ligation are rare and rather difficult to synthesize. The extent of the degree of delocalization can be tuned by asymmetric set of ligands in these complexes. In recent reports diiron complexes as models of PAP having a central bridging phenolate moiety with terminal phenolate are mimicked [4]. In the present report using molecularly associated dimer of phthiocoloxime ligand, we have tried to mimic NO coordinations of His/Tyr residue in native PAP with controlled and unique charge transfers. These coordination syntheses between phthiocoloxime (L) and Fe2+ , Fe+3 resulted in valence tautomeric coordination with mixed valent non-equivalent diiron sites in Fe-1: [μ-OH–Fe2 +2,+3 (o-NQCH3ox ) (o-NSQCH3ox )2 (CAT) H2 O]. Hydrogen bonding tendency of ligand gives rise to molecular association of Fe-2 monomers resulting it in dimer as [Fe+3 (o-NQCH3ox ) ( p-NQCH3ox )2 ]2 .
2 Experimental a. The ligand L (3-methyl-2hydroxy-1,4-naphthoquinone-1-oxime) was prepared from reported procedure [5]. Fe-1 and Fe-2 complexes were synthesized using FeSO4 .7H2 O and FeCl3 in M:L ratios of 1:2 and 1:3 respectively under N2 atmosphere. In case of Fe-1, 4% ascorbic acid was used during synthesis. The bluish green precipitate of Fe-1 and dark brown precipitate of Fe-2 were
PAP model: Mössbauer, magneto-structural and DNA cleavage studies
49
separated out immediately after washing successively with ether and dried under vacuum. Yields of the products were ∼80%. b. Reconstitution of Fe-1 and Fe-2 in APase from Solanum tuberosum (i.e. RAPases of Fe-1 and Fe-2) were prepared using the procedure given in the literature [6]. c. pUC-19 DNA cleavage activity: The DNA cleavage activity of Fe-1, Fe-2 RAPases and native APase was determined according to literature [7]. The intensities of the bands were measured by gel documentation system AlphaImager 2200. 2.1 Physicochemical techniques Elemental analyses were performed for C, H, N on EA1108 Elemental Analyzer. Metal ion analysis of RAPase Fe-1 and Fe-2 were performed on a Varian 220 AAS with Graphite furnace. Mössbauer spectra were recorded on a constant-acceleration spectrometer, with a 25 mCi source of 57 Co in rhodium matrix [8]. Magnetic measurements were carried out on powder samples of typical weight 15–20 mg using a Quantum Design SQUID magnetometer (MPMS model) operating between 4 and 300 K [9]. EPR spectra of complexes were recorded on a Varian E-112 spectrometer using 100 kHz field modulation at X-band (9.5 GHz) frequency.
3 Results and discussion 3.1 Synthetic strategy The oxime derivative of hydroxy-naphthoquinone (Scheme 1) is a radical ligand having following valence tautomeric forms (Scheme 1) and recently we have shown that its methyl derivative also perform radical due to H-bondings [10]. In Fe-1 synthesis, Fe+2 to L ratio is taken as 1:2. Two semiquinone ligands by intramolecular charge transfer may end up into the valence tautomers such as, Fe+2
NSQox + NSQox CAT + NQox
(1)
The charge transfers are facilitated by metal ion because the energies of metal and ligand orbitals are comparable [11]. Due to MLCT characteristic green complex is formed with [Fe+3 (CAT) (NSQox )]A site as follows; Fe+2 + 2 NSQox → Fe+3 (CAT) (NSQox ) A (2) Radical form of quinone ligand possesses hydrophilic character [12]. The intermolecular dynamics in the radical co-ordination sphere appears to be dependent on solvent which is formerly established by us with comparable thermal energies of NSQOX ligand and water [13]. Back co-ordination M → L is facilitated due to low ionization potential of iron ion which finally stabilizes Fe+3 (CAT) species in co-ordination sphere as in Eq. 2. The first dissociation constant for Fe+3 (H2 O)6 species is three, hence nucleophilic hydroxide ion forms even at low pH at Fe+3 site, generating [μ(OH)–Fe+3 (CAT)(NSQox ) (H2 O)]− species. However hydroxyl group bridges second iron site with [Fe+2 /(o-NQox )(o-NSQox )]+ B formulation according to Eq. 1. Finally the electronic structure of Fe-1 is resulted as dimer due to coupled
50
S. Salunke-Gawali et al.
Scheme 1 Valence tautomers of 3-methyl-2hydroxy-1, 4-naphthoquinone-1-oxime
NO
NOH
NOH OH
+1e R
-1e
OH
-
+1e
OH
-
-1e-
-
R
R OH
O
O
CAT
2-HNSQox
2-HNQox
+1e-1e
-
NOH
NOH O
R
O
+1e-
R
-1eOH
OH
4-HNSQox
4-HNQox
R= CH3
O
HO CH3
O
N
H O
N
H
O +3
HO
Fe
Fe
OH2
+2
O
CH3 O
OH H
N
Fe3+ O
O
O
N
N
O N
CH3
OH
.
.
HO
N O O
H3C
O
OH H3C
H3C OH
CH3
OH OH
(a)
(b)
Fig. 1 a Probable electronic structure of Fe-1 b Probable electronic structure of Fe-2
reactions shown in Eqs. 1 and 2 with the following valence tautomeric coordinations such as, Fe − 1:
μ − OH − Fe2 +2,+3 (o − NQCH3ox ) (o − NSQCH3ox )2 (CAT) H2 O
1:3::M:L ratio in Fe-2 resulted in octahedral complex. As iron is in known stable oxidation state +3, it is not leading to charge transfers in complex formation and there is no possibility of hydrolysis due to anhydrous FeCl3 . Here the ligand coordinates in innocent 2-oxido, 3-methyl-1,4-naphthoquinone oxime form performing neutral Fe-2 complex viz. [Fe+3 (o-NQCH3ox ) ( p-NQCH3ox )2 ]2 . Hence the probable electronic structures of Fe-1 and Fe-2 are shown in Fig. 1a, b which are further supported by infra spectroscopic and susceptibility studies.
PAP model: Mössbauer, magneto-structural and DNA cleavage studies
(a)
(b) 102
100
100 %Transmision
%Transmission
51
98
96
98 96 94 92
94 90 -4
-2
0 V mm/s
2
4
-4
-2
0 V mm/s
2
4
Fig. 2 Mössbauer spectra of a Fe-1 and b Fe-2 at 78 K Table 1 Mössbauer data of complexes Fe-1 and Fe-2 Sample
T (K)
δ (1) mm/s
EQ mm/s
a
δ (2) mm/s
EQ mm/s
% δ (1): % δ (2)
Fe-1
78 300 78 300
0.224(1) 0.152(3) 0.519(1) 0.408(1)
0.612(1) 0.598(5) 0.213(4) 0.213(0)b
0.280(2) 0.248(8) 0.424(2) 0.714(1)
0.539(3) 0.410(2)
0.240(4) 0.468(2)
0.480(1) 0.680(2)
47:53 42:58 100 100
Fe-2
Figures in parenthesis indicate standard deviations a Full width at half maxima b Fixed
value
3.2 Mössbauer spectroscopy The Mössbauer spectra of Fe-1 and Fe-2 at 78 K are displayed in (Fig. 2a and b). The least square fitted parameters are presented in Table 1. Mössbauer spectra of Fe-1 dimer consist of two quadrupole split doublets, labeled (1) (2), due to differences in local coordination geometries of [Fe+3 ]A and [Fe+2 ]B respectively. Striking feature is line broadening at site-B which is well fitted by Lorentzian line shape and accordingly can be assigned to the magnetic relaxation of the electronic spin. However usually in such a case one observes a decrease of the line width at high temperature. The present observation of broader line at 300 K can be attributed to the effect of temperature on the electron transfer between metal-semiquinone or metal-metal centers. From infra magnetostructural data ferric distorted octahedral site is composed of redox tautomers such as [Fe+3 (CAT) (o-NSQox )]A which gives rise to lowering of spin state at high spin [Fe+3 ]A site such as (S* = 2) which in turn antiferromagnetically coupled with [Fe+2 ]B site via μ-(OH) bridge, so there is decrease of unpaired electrons in eg orbital at [Fe+3 ]A site, which are taking part in bonding with sp3 d2 hybridization. Removal of d electron density from the metal ion which in turn decreases shielding of the s electrons. This effect increases s electron density at nucleus and decreases δ. Strong π acceptor character of quinoneoxime ligand resulted in Fe+3 (CAT) formation of site A (Eq. 2), it is reflected in lowering of isomer shift such as δ(1) = 0.152 mm/s (300 K) and δ(1) = 0.224 mm/s (78 K) and
52
S. Salunke-Gawali et al.
(a) 4.0
(b) 6 5
3.2
.T
corr
2.4
3
χm
χ mcorr .T
4
2 1.6 1 0
0.8 0
50
100
150
T (K)
200
250
300
0
50
100
150 200 T (K)
250
300
corr .T vs T for Fe-1 b χ corr .T vs T for Fe-2 complex. (filled diamond) experimental, Fig. 3 Plots of a χm m (solid line) best fitted data using parameters discussed in text
reaches to nearly in low spin Fe+3 range [14] of Fe+3 (CAT) type complexes. The high electric field gradient of Fe+3 site (EQ = 0.598 mm/s at 300 K) is attributed to distorted octahedral coordination [15] and corresponds to Fe+3 (h.s.)-SQ complexes [16]. The second [Fe+2 (h.s.)]B site is comprised of square pyramidal geometry with (o-NQCH3ox )(o-NSQCH3ox ) coordinations with effective spin S* = 3/2. It shows isomer shift δ(2) = 0.410 at 300 K and δ(2) = 0.539 mm/s at 78 K together with EQ = 0.468 mm/s at 300 K and EQ = 0.240 mm/s at 78 K. Fe+2 (l.s.) configuration with innocent ligation shows δ = 0.296 mm/s and EQ = 0.567 mm/s [17]. However S = 3/2 for Fe+3 with radical coordination in square pyramidal geometry resulted in δ = 0.2 mm/s with EQ = 2.2 mm/s [18]. Such intermediate state S = 3/2 in five coordinated Fe+3 complexes with non-innocent iminobenzo-semiquinone ligation has δ value ∼0.24 mm/s. All these literature reports suggest that [Fe]B site in Fe-1 is not Fe+3 with intermediate S = 3/2 spin or Fe+2 with low spin but it is due to lowering of spin concentration at Fe+2 (h.s.) configuration having non-innocent ligation and antiferromegnatic exchange with (Fe+3 )A site. The quadrupole split in Fe-1 is large (EQ )B = 0.47 mm/s because it is penta-coordinated with square pyramidal geometry. Boillot and coworkers have shown that Fe+3 (CAT) Fe+2 (SQ) equilibrium shifted to right with more e− donating substituent [19]. Similar observation is obtained with e− donor R = CH3 substituted ligand L which performs [Fe+3 (CAT)]A : [Fe+2 (NSQ)]B sites quantitized by Mössbauer spectra as 42: 58. To support the present picture, the low temperature magnetic Mössbauer studies are in progress. In case of Fe-2, 100% Fe+3 (h.s.) hexacoordinated configuration shows isomer shift δ = 0.408 mm/s [20]. Slightly distorted octahedral symmetry created by NQCH3ox ligand surrounding Fe+3 (h.s.) state generates small field gradient indicated by quadrupole split EQ = 0.213 mm/s. The innocent coordination viz. NQCH3ox of redox active ligand L in Fe-2 perform similar Fe+3 (CAT) effect of innocent ligation with 100% high spin fraction at 300 K as reported by Gütlich and coworkers in hexacoordinated [(TPA) Fe+3 (CAT)] BPh4 complex [19] with δ = 0.412 mm/s. For latter complex isomer shift raises to 0.526 mm/s at 75.4 K for Fe+3 (h.s.) fraction. Analogues to this, isomer shift of Fe-2 raises to 0.519 mm/s at 78 K.
PAP model: Mössbauer, magneto-structural and DNA cleavage studies
1
2
3
4
5
6
7
8
9
53
Reaction Conditions: Lane 1: DNA Marker Lane 2: DNA Lane 3: DNA + Fe-1 RAPase + H2O2 Lane 4: DNA + Fe-1 RAPase Lane 5: Blank Lane 6: DNA + Fe-2 RAPase + H2O2 Lane 7: DNA + Fe-2 RAPase Lane 8: DNA; Lane 9: DNA + 10 μg E1-APase
Fig. 4 Photograph of DNA cleavage activity by reconstituted enzyme using Fe-1/Fe-2 RAPases compared to native enzyme E1 -APase Table 2 Composition of different forms of pUC-19 DNA after its cleavage by Fe-1, Fe-2 RAPases and native APase (E1 ) Lane 1: DNA marker Lane 2: DNA Lane 3: DNA + Fe-1 + H2 O2 Lane 4: DNA + Fe-1 Lane 5: Blank Lane 6: DNA + Fe-2 + H2 O2 Lane 7: DNA + Fe-2 Lane 8: DNA Lane 9: DNA + E1 -APase 10 μg
Form-I
Form-III
Form-I
Total % cleavage
– 88.2 17.8 23.9 – 91.5 93.4 84.3 11.5
– 0.0 0.0 0.0 – 0.0 0.0 0.0 0.0
–
– – 75.4% 69.3% – 1.7% 0.4% – 76.00%
6.8 82.2 76.1 – 8.5 6.4 12.5 88.5
3.3 Magnetic susceptibility measurements The χmcorr .T variations with temperatures for Fe-1 and Fe-2 complexes are shown in Fig. 3a, b. In case of Fe-1 dimer decrease of μeff. from 5.52 B.M. at 284 K to 2.88 B.M. at 5.19 K (not shown in figure), at the same time decrease of χmcorr .T from 3.82 cm3 mol−1 K at 284 K to 1.04 cm3 mol−1 K at 5.19 K clearly indicates antiferromagntic interactions between [Fe+3 ]A and [Fe+2 ]B sites of dimer. The redox isomers ligated at [Fe+3 ]A and [Fe+2 ]B sites as established from supra discussions generate S = 2, 3/2 spin pair at active sites respectively. χmcorr .T data were best fitted by using HDVV spin pair S = 2, 3/2 model. For the best fit the following parameters, J = −13.5 cm−1 , g = 2.05 were used (Fig. 3a) with an agreement factor R = 1.89 × 10−5 . The lower J value of antiferromagnetic exchange is indicative of Fe+3 μ(OH)Fe+2 bridging in Fe-1 dimer instead of μ-oxo bridge. It is seen that μeff value at 278.9 K for Fe-2 is 6.40 B.M. which decreases with decrease of temperature up to 0.708 B.M. at 5.16 K. In Fig. 3b χmcorr .T at temperature 278.9 K equals to 5.12 cm3 mol−1 K which also drastically decreases with temperature up to 0.0626 cm3 mol−1 K at 5 K. Both observations show antiferromagnetic exchange between the two centers. The ligand (L) shows the tendency of molecular association as dimers through
54
S. Salunke-Gawali et al.
H-bonding and π -stackings, hence in Fe-2 monomer complex intermolecular association through H-bonds may lead to weakly coupled antiferromagnetic interaction between two Fe-2 molecules having Fe+3 (h.s.) centers. Similar behaviour is reported by Hendrickson [21] on model compound of Acid Phosphatse enzyme. We have used S = 5/2, 5/2 spin pair model and obtained best fitted parameters such as J = −12.4 cm−1 , g = 2.3 with R = 3.58 × 10−5 . Such molecular association in quinone complexes via intermolecular H-bonding is reported by us in copper complexes [9]. 3.4 EPR spectral studies EPR spectra of Fe-1 show two strong signals at g1 = 4.17 and g2 = 2.01. g ∼ 4 signal is characteristic of S = 3/2 spin state as an intermediate spin of Fe+3 (h.s.) configuration. In supra SQUID data fittings we have seen that such intermediate spin is result of Fe+2 (o-NSQCH3ox ) interaction also which is populated throughout variation of temperature. Second broad g2 = 2.01 signal is due to exchange coupled interaction. In case of Fe-2 complex only one very broad exchange coupled signal is seen at g = 2.05 and 2.16 in 300 and 77 K EPR spectra respectively (not shown) that may be due to electron transfers through H-bonding between the molecularly associated two monomeric units. 3.5 DNA cleavage activity Increase of PAP level in breast cancer cells lead to its physiological function with DNA [22]. Such DNA cleavage activity is shown by RAPase of Fe-1 while Fe-2 RAPase indicates absence of such activity. The DNA cleavage activity of native APase from Solanum tuberusosum is also compared with cleavage activities of reconstituted dimers Fe-1 and Fe-2. The results are shown in Fig. 4 and Table 2. Fe-1 RAPase transforms supercoiled circular Form-I directly to nicked circular Form-II without formation of linear Form-III in both oxidizing H2 O2 medium and without H2 O2 medium [7]. RAPase of Fe-1 dimer exhibits increase in ∼6–7% nuclease activity in presence of H2 O2 compared to absence of H2 O2 in medium. The electronic structure of Fe-1 dimer compound shows that it possesses radical coordination while later Fe-2 performs dimerization via H-bondings but it doesn’t have radical coordination. Such difference confirms oxidative path of DNA cleavage by Fe-1 and also its redox activity due to non-innocent nature in Fe-1 complex that is equally responsible for DNA cleavage with formation of Fe+3 –OOH or a Fe+4 = O species [23]. Acknowledgements SYR is grateful to the CSIR, New Delhi, India [01(1686)/00/EMR-II) for the grant. S S-G is thankful to Ministry of Research of France for awarding visiting fellowship. Khursheed Ahmed is thankful to UGC for teacher fellowship.
References 1. Vincent, J.B., Olivier-Lilley, G.L., Averill, B.A.: Proteins containing oxo-bridged dinuclear iron centers: a bioinorganic perspective. Chem. Rev. 90, 1447–1467 (1990)
PAP model: Mössbauer, magneto-structural and DNA cleavage studies
55
2. Rodriguez, J.H., Ok, H.N., Xia, Y.M., Debrunner, P.G., Hinrichs, B.E., Meyer, T., Packard, N.H.: Mössbauer spectroscopy of the spin-coupled Fe3+ –Fe2+ center of reduced uteroferrin. J. Phys. Chem. (Article) 100(16), 6849–6862 (1996) 3. Averill, B.A., Davis, J.C., Burman, S., Zirino, T., Sanders-Loehr, J., Loehr, T.M., Sage, J.T., Debrunner, P.G.: Spectroscopic and magnetic studies of the purple acid phosphatase from bovine spleen. J. Am. Chem. Soc. 109(12), 3760–3767 (1987) 4. Jarenmark, M., Carlsson, H., Nordlander, E.: Asymmetric dinuclear metal complexes as models for active sites in hydrolases and redox enzymes. C. R Chimie. 10, 433–462 (2007) 5. Thube, D.R., Todkary, A., Rane, S.Y., Joshi, K., Gejji, S.P., Salunke, S.A., Varret, F., Marrot, J.: Theoretical and experimental investigations on the structure and vibrational spectra of 2-hydroxy-3-methyl-1, 4-naphthoquinone-1-oxime. J. Mol. Struct. (THEOCHEM) 622, 211–219 (2003) 6. Merkx, M., Averill, B.A.: Ga3+ as a functional substitute for Fe3+ : preparation and characterization of the Ga3+ Fe2+ and Ga3+ Zn2+ forms of bovine spleen purple acid phosphatase. Biochemistry 37, 8490–8497 (1998) 7. Kumbhar, A.S., Damle, S.G., Dasgupta, S.T., Rane, S.Y., Kumbhar Jr., A.S.: Nuclease activity of oxo-bridged diiron complexes. J. Chem. Research (S) 98–99 (1999) 8. Gawali-Salunke, S., Maurin, I., Boukheddaden, K., Codjovi, E., Hashimoto, K., Tokoro, H., Ohkoshi, S., Varret, F.: Magnetic and Mössbauer investigation of photomagnetic Prussian Blue analogue Na0.45 Co[Fe(CN)6 ]0.79 ·3.4H2 O: relaxation of the thermally quenched state, dehydration effect on the thermal hysteresis loop. J. Phys. Chem. B 109, 8251–8256 (2005) 9. Salunke-Gawali, S., Rane, S.Y., Puranik, V.G., Guyard-Duhayon, C., Varret, F.: Three dimensional hydrogen-bondings network in a copper complex of 2-Hydroxy-1, 4-naphthoquinone: structural, spectroscopic and magnetic properties. Polyhedron 23, 2541–2547 (2004) 10. Rane, S.Y., Todkary, A.V., Dalvi, R., Salunke-Gawali, S., Linares, J., Varret, F., Marrot, J., Yakhmi, J.V., Bhadbhade, M., Sriniwas, D., Gejji, S.P.: SOM assembly of hydroxynaphthoquinone and its oxime: polymorphic X-ray structures and EPR studies. Spectrochim. Acta Part A 79, 130–138 (2006) 11. Pierpont, C.G.: Unique properties of transition metal quinone complexes of the MQ3 series. Coord. Chem. Rev. 219–221, 415–433 (2001) 12. Rakhimov, R.R., Benetis, N.P., Lund, A., Hwang, J.S., Prokof’ev, A.I., Lebedev, Y.S.: Intramolecular and reorientation dynamics of bis(triphenylphosphine)-3,6-di-tert-butyl-4,5-dimethoxy-osemiquinone complex of copper(I) in viscous media. Chem. Phys. Lett. 255(1–3), 156–162 (1996) 13. Rane, S.Y., Gawali, S.D., Padhye, S.B., Kumbhar, A.S., Bakare, P.P.: Thermal studies of metal-quinone complexes: role of aqua colligation with Cu(II) coordination of 2-hydroxy-1, 4-naphthoquinone. J. Therm. Anal. & Cal. 55, 249–258 (1999) 14. Simaan, A.J., Boillot, M.-L., Carrasco, R., Cano, J., Girerd, J.-J., Mattioli, T.A., Ensling, J., Spiering, H., Gütlich, P.: Electronic, vibrational, and structural properties of a spin-crossover catecholato-iron system in the solid state: theoretical study of the electronic nature of the doublet and sextet states. Chem. Eur. J. 11(6), 1779–1793 (2005) 15. Boudalis, A.K., Dahan, F., Bousseksou, A., Tuchagues, J.P., Perlepes, S.P.: Use of the di2-pyridyl ketone/3,5-di-tert-butylcatechol blend in iron(III) chemistry: a cationic tetranuclear cluster and an anionic trinuclear complex. Dalton Trans. 17, 3411–3418 (2003) 16. Boone, S.R., Purser, G.H., Chang, H.R., Lowery, M.D., Hendrickson, D.N., Pierpont, C.G.: Magnetic exchange interactions in semiquinone complexes of iron. Structural and magnetic properties of tris(3,5-di-tert-butylsemiquinonato)iron(III) and tetrakis(3,5-di-tertbutylsemiquinonato)tetrakis(3,5-di-tert-butylcatecholato) tetrairon(III). J. Am. Chem. Soc. 111(6), 2292–2299 (1989) 17. Boudalis, A.K., Clemente-Yuan, J.M., Dahan, F., Tuchagues, J.P.: New Poly-Iron(II) Complexes of N4 O dinucleating schiff bases and pseudohalides: syntheses, structures, magnetic and Mössbauer properties. Inorg. Chem. 43(4), 1574–1586 (2004) 18. Blanchard, S., Bill, E., Weyhermuller, T., Wieghardt, K.: N,N-coordinated radical anions of S-Methyl-1-phenyl-isothiosemicarbazide in two five-coordinate ferric complexes [FeIII (LMe )2 X] (X = CH3 S− , Cl− ). Inorg. Chem. 43(7), 2324–2329 (2004) 19. Floquet, S., Simaan, A.J., Rivière, E., Nierlich, M., Thuéry, P., Ensling, J., Gütlich, P., Girerd, J.J., Boillot, M.L.: Spin crossover of ferric complexes with catecholate derivatives. Single-crystal X-ray structure, magnetic and Mössbauer investigations. Dalton Trans. 9, 1734–1742 (2005) 20. Hummel, H., Bill, E., Weyhermuller, K., Wieghardt, K.: The molecular and electronic structure of the asymmetric, trinuclear complex [LFeIII (μ-O) (μ-piv)2 FeIII (μ-OH)
56
S. Salunke-Gawali et al.
(μ-piv)2 FeIII (piv)2 ]·C6 H5 CH3 (L = 1,4,7-trimethyl-1,4,7-triazacyclononane, piv = pivalate (1−)). Inorg. Chim. Acta. 291, 258–265 (1999) 21. Nie, H., Aubin, S., Mashuta, M.S., Wu, J., Richardson, J.F., Hendrickson, D.N., Buchanan, R.M.: Synthesis, crystal structure, and properties of a polyimidazole diiron(III) complex., A new model of the active site of purple acid phosphatase. Inorg. Chem. 34(9), 2382–2388 (1995) 22. Allen, S.H., Nuttleman, P.R., Ketcham, C.M., Roberts, R.M.: Purification and characterization of human bone tartrate-resistant acid phosphatase. J. Bone Miner. Res. 4(1), 47–55 (1989) 23. Tsubouchi, A., Bruice, T.C.: Remarkable (.apprx.1013) rate enhancement in phosphonate ester hydrolysis catalyzed by two metal ions. J. Am. Chem. Soc. 116(25), 11614–11615 (1994)
