Structural, spectroscopic, and magnetic properties of a diphenolate-bridged FeIIINiII complex showing excellent phosphodiester cleavage activity

Polyhedron 31 (2012) 110–117

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Structural, spectroscopic, and magnetic properties of a diphenolate-bridged FeIIINiII complex showing excellent phosphodiester cleavage activity Supriya Dutta a,b,⇑, Papu Biswas c,⇑ a

Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India Department of Chemistry, Budge Budge Institute of Technology, Nischintapur, Budge Budge, Kolkata 700137, India c Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711103, India b

a r t i c l e

i n f o

Article history: Received 14 May 2011 Accepted 6 September 2011 Available online 16 September 2011 Keywords: Macrocyclic compounds Iron(III) Nickel(II) BNPP Hydrolysis of phosphate ester

a b s t r a c t To mimic the phosphate ester hydrolysis behavior of purple acid phosphatases the heterobimetallic complex [(BNPP)FeIIIL(l-BNPP)NiII(H2O)](ClO4) (1) has been synthesized from the precursor complexes [FeIII(LH2)(H2O)2](ClO4)33H2O and [FeIII(LH2)(H2O)Cl](ClO4)22H2O. In these compounds, L2 is the anion of the tetraiminodiphenol macrocyclic ligand (H2L), while LH2 is the zwitterionic form in which the phenolic protons are shifted to the two metal-uncoordinated imine nitrogens, and BNPP is bis(4-nitrophenyl)phosphate. The X-ray crystal structure of compound 1 has been determined. The structure of 1 comprises of two edge-shared distorted octahedrons whose metal centers are bridged by two equatorial phenolate oxygens and two axially disposed oxygens of a BNPP ligand. The internuclear Fe  Ni distance is 3.083 Å. The high-spin iron(III) and nickel(II) in 1 are antiferromagnetically coupled (J = 7.1 cm1; H = 2J S1S2) with S = 3/2 spin ground state. The phosphodiesterase activity of 1 has been studied in 70:30 H2O–(CH3)2SO medium with NaBNPP as the substrate. The reaction rates have been measured by varying pH (3–10), temperature (25–50 °C), and with different concentrations of the substrate and complex at a fixed pH and temperature. Treatment of the rate data, obtained at pH 6.0 and at 35 °C, by the Michaelis–Menten approach have provided the following parameters: KM = 3.6  104 M, Vmax = 1.83  107 M s1, kcat = 9.15  103 s1. As compared to the uncatalyzed hydrolysis rate of BNPP, the kcat value is 8.3  108 times higher, showing that 1 behaves as an excellent model for phosphate ester hydrolysis. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction The purple acid phosphatases (PAPs) represent a type of dinuclear metallohydrolases that contain the heterovalent FeIIIMII (M = Fe, Zn or Mn) active site and catalyze hydrolysis of phosphate esters at pH 4–7 [1–4]. The identity of divalent metal ion in the active site of PAPs depends upon the source (plant, animal or bacteria) from which the enzyme is isolated. For instance, the red kidney bean PAP contains the FeIIIZnII active site, while the PAP isolated from another plant source sweet potato has the FeIIIMnII site. On the other hand, mammalian PAPs isolated from bovine spleen, pig liver, procine uterus fluid (uteroferrin) etc. contain the FeIIIFeII site. Despite the difference of the active site metal contents, the metal-coordinated protein residues of PAPs are conserved and their enzymatic activities also remain largely similar. Further, zin-

⇑ Corresponding authors. Present address: Department of Chemistry, Budge Budge Institute of Technology, Nischintapur, Budge Budge, Kolkata 700137, India (S. Dutta). Tel.: +91 33 2473 4971; fax: +91 33 2668 4564. E-mail address: [email protected] (P. Biswas). 0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2011.09.009

c(II)- substituted mammalian PAPs and iron(II)- substituted kidney bean PAP are kinetically indistinguishable [5,6]. The X-ray structures of the enzymatically active kidney bean PAP [7] and inactive PO4/XO4 (X = As, S, Mo) – ligated derivatives of PAPs [8–12] of different origins have been reported. The crystal structure of the kidney bean PAP has been resolved [7] by including three exogenous OH/H2O ligands in the active site. The iron(III) and zinc(II) sites, which being separated by 3.2–3.3 Å, are bridged by a hydroxide ion as well as a carboxylate oxygen atom of Asp164. A second hydroxide ligand is thought to be terminally bound to iron(III) center, while the zinc(II) is coordinated by a water molecule. The steric environments of the active sites of PAPs have been addressed by various spectroscopic and magnetic measurements [13–15] to ascertain the exogenous ligands present. These studies have confirmed the presence of a l-OH linkage and a water molecule coordinated to the divalent metal site of mammalian PAPs and the kidney bean PAP, although magnetization measurement of the sweet potato PAP seems to indicate the presence of a l-oxo bridge between the iron(III) and manganese(II) [16]. Notwithstanding the evidences obtained from the X-ray structural studies [7,11], the ENDOR measurement carried out for a mammalian PAP has failed

S. Dutta, P. Biswas / Polyhedron 31 (2012) 110–117

to reveal the presence of the terminal hydroxide ligand [17]. The hydrolysis of phosphate esters by PAPs is accompanied by inversion of stereochemistry at the phosphorous atom [18]. The nucleophile is presumably added to the phosphorous atom on the opposite side of the leaving group and thus forming a trigonal bipyramid intermediate. However, the mode of substrate binding and the identity of the attacking nucleophile are still debatable issue [1,3]. Two alternative reaction mechanisms have been proposed. (i) The substrate binds to the MII site in a monodentate form and the terminal hydroxide ligand of the FeIII site acts as the nucleophile [7,19,20]. (ii) The substrate gets attached to the MIIFeIII site as a l-1,3 bridge and is attacked by the sterically-oriented metal-bridged hydroxide [12,16,17,21]. In recent years, a number of FeIIIMII complexes have been synthesized to replicate the structural, spectroscopic, magnetic, and redox features of PAPs and to get an insight of the mechanism of hydrolysis of phosphate ester [22–27]. We report here the structure and properties of the iron(III)-nickel(II) complex [(BNPP)FeIIIL(l-BNPP)NiII (H2O)](ClO4), which in polar solvent dissociates to [(H2O)FeIIIL (l-BNPP)NiII(H2O)]2+ and catalyzes hydrolysis of bis(4-nitrophenyl)phosphate (BNPP) at pH 6.0.

2. Experimental 2.1. Materials Reagent grade chemicals obtained from commercial sources were used as received. Solvents were purified and dried according to standard methods [28]. A Universal buffer system introduced by Ellis [29] was used for this study in the pH range 3–10. Buffers of appropriate pH values were prepared by mixing together a solution containing sodium carbonate, 2-amino-2-methyl propan-1,3-diol, sodium dihydrogen phosphate, and citric acid each of which is 0.4 M with 0.1 M HCl or NaOH in different volume ratio. The precursor complexes [FeIII(LH2)(H2O)2](ClO4)33H2O and [FeIII(LH2) (H2O)Cl](ClO4)22H2O have been prepared by following the methods already described by us [25b]. 2.2. Physical measurements Elemental (C, H, and N) analyses were performed on a Perkin– Elmer 2400 II elemental analyzer. The electrospray ionization mass spectra (ESI-MS) were measured on a Micromass Qtof YA 263 mass spectrometer. IR spectra were recorded using KBr disks on a Shimadzu FTIR 8400S spectrophotometer. The electronic absorption spectra and kinetic measurements over the temperature range 25–50 °C were performed on a Perkin–Elmer 950 UV–Vis–NIR spectrophotometer equipped with thermostated cell and Peltier temperature controller assembly. The 1H NMR and 31P NMR spectra were recorded on a Bruker Avance III 500 MHz NMR spectrometer. Variable temperature (2–300 K) magnetic susceptibility measurements were carried out on a MPMS Quantum Design SQUID magnetometer. Diamagnetic corrections were made using Pascal’s constants [30]. The cyclic voltammetric (CV) and square wave voltammetric (SWV) measurements were carried out using a BAS 100B electrochemical analyzer in the same way as reported earlier [31]. EPR spectrum for a frozen acetonitrile solution (77 K) of 1 (5  104 M) was recorded on a JEOL JES-FA200 ESR spectrometer. 2.3. Preparation of metal complex Caution: Perchlorate salts used in this study are potentially explosive and therefore should be handled with care.

111

2.3.1. [(BNPP)FeL(l-BNPP)Ni(H2O)](ClO4) (1) To an acetonitrile solution of [FeIII(LH2)(H2O)2](ClO4)33H2O (0.421 g, 0.5 mmol) were added solid Ni(ClO4)26H2O (0.18 g, 0.5 mmol), a methanol solution (10 mL) of bis(4-nitrophenyl)phosphate (HBNPP, 0.34 g, 1 mmol), and triethylamine (0.20 g, 2 mmol). On standing at room temperature for overnight period, bright red crystals of 1 were deposited. The product was filtered, washed with ethanol, and recrystallized from acetonitrile-methanol (1:1). Yield 0.54 g (82%). Alternatively, compound 1 can be obtained by slowly adding to a stirred acetonitrile solution (30 mL) of [FeIII(LH2)(H2O)Cl] (ClO4)22H2O (0.37 g, 0.5 mmol) and Ni(ClO4)26H2O (0.18 g, 0.5 mmol), an aqueous solution (15 mL) of NaBNPP (0.90 g, 2.5 mmol). After 1 h, the product that precipitated was filtered, washed with water and ethanol, and recrystallized as stated above. Yield 0.59 g (90%). Anal. Calc. for C48H44ClFeN8NiO23P2: C, 43.87; H, 3.35; N, 8.53. Found: C, 43.76; H, 3.39; N, 8.58%. ESI-MS (positive) in acetonitrile: m/z 1193.84 [FeLNi(BNPP)2]+ (10%); 871.91 [FeNiL(BNPP)(OH)]+ (100%); 428.46 [FeLNi(BNPP)]2+ (50%). FTIR (KBr, m/cm1): 3458(w, br), 1641(m), 1612(w), 1589(m), 1564(m), 1519(s), 1489(w), 1439(w), 1413(w), 1346(s), 1317(w), 1242(w), 1211(m), 1108(s), 1080(s), 918(m), 862(w), 815(w), 764(w), 625(m), 523(w), 430(sh). 1H NMR (500 MHz, (CD3)2SO): d 8.15 (s, 4H, Ar–H); 7.34 (s, 4H, Ar–H). 31P NMR (202 MHz, (CD3)2SO): d 27.0 (coordinated BNPP); 5.35 (PPh3); 11.9 (free BNPP). UV– Vis–NIR (kmax/nm (e/M1 cm1)) in acetonitrile: 1020 (8), 535 (2350), 430 (sh), 350 (12750). 2.4. X-ray structure determinations Crystal suitable for structure determinations of [(BNPP)FeL(lBNPP)Ni(H2O)](ClO4) (1) was mounted on glass fibers and coated with perfluoropolyether oil. Intensity data were collected at 150 K on a Bruker-AXS SMART APEX II diffractometer equipped with a CCD detector with graphite-monochromated Mo Ka radiation (k = 0.71073 Å). The data were processed with SAINT [32], and absorption corrections were made with SADABS [32]. The structure was solved by Direct and Fourier methods and refined by fullmatrix least-squares based on F2 using WINGX software of SHELXTL [33] and SHELX-97 [34]. The non-hydrogen atoms were refined anisotropically, while the hydrogen atoms were placed at geometrically calculated position with fixed isotropic thermal parameters. Wherever the positions of the hydrogen atoms could be detected equivocally, they were referred to as ‘seen’ in the text. The crystallographic data of compound 1 are given in Table 1. 2.5. Kinetic measurements The hydrolytic cleavage of BNPP catalyzed by complex 1 was studied in H2O–(CH3)2SO (70:30 v/v) solvent mixture. The reaction rate was followed by monitoring the increase in absorbance of the hydrolyzed product 4-nitrophenolate at 400 nm (e = 17000 M1 cm1). The pKa value of 4-nitrophenol (6.61) [35] was used to calculate the concentration of the liberated phenolate at the appropriate pH. Rate constants were obtained by the initial rate method and measurements were made over the pH range 3–10 and the temperature ranging from 25 to 50 °C. The pH of the solutions were adjusted with a fixed volume of the Ellis buffer of desired pH and after each kinetic run the pH of the solution was measured by a pH meter. Typically, the solution used for kinetic run was 2  105 M in complex, 2  104 M in NaBNPP, and 5  102 M in buffer, and the ionic strength was maintained at 0.1 M with NaClO4. To compensate the effect of spontaneous hydrolysis of BNPP, a reference cell identical to the sample cell except that it did not contain 1 was used. Reactions were monitored to about 5% cleavage of BNPP.

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Table 1 Crystallographic data for [(BNPP)FeL(l-BNPP)Ni(H2O)](ClO4) (1). 1 Formula Mr Crystal size (mm3) Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) U (Å3) Z D (Mg m3) T (K) l (mm1) No. measured/observed reflections Parameters refined Final R1a, wR2b [I > 2r (I)] R1a, wR2b (all data) Sc

C48H41ClFeN8NiO23P2 1309.84 0.30  0.20  0.04 Monoclinic P21/n 17.2012(17) 15.7559(15) 20.7660(20) 90 91.529(3) 90 5626.0(9) 4 1.546 150(2) 0.785 65408/9911 773 0.0656, 0.1377 0.1510, 0.1715 1.083

a S = [Rw(Fo2  Fc2)2/(N  P)]1/2 where N is the number of data and P the total number of parameters refined. b R1(F) = R||Fo|  |Fc||/R|Fo|. c wR2(F2) = [Rw(Fo2  Fc2)2/Rw(Fo2)2]1/2.

3. Results and discussion 3.1. Synthesis and characterization The heterodinuclear iron(III)–nickel(II) complex [(BNPP)FeL(lBNPP)Ni(H2O)](ClO4) (1) has been obtained either by reacting [Fe(LH2)(H2O)2](ClO4)33H2O with Ni(ClO4)26H2O, HBNPP, and N(C2H5)3 in the molar ratio 1:1:2:4 in acetonitrile–methanol or by treating a 1:1 mixture of [Fe(LH2)(H2O)Cl](ClO4)22H2O and Ni(ClO4)26H2O in acetonitrile with an aqueous solution of 5 equivalents of NaBNPP (Scheme 1). The characterization data of compound 1 is given in Section 2. The ESI-MS of complex 1 in acetonitrile (Fig. S1, Supplementary Information) shows the presence of three major peaks due to three positively charged species, viz. [FeNiL(BNPP)2]+ (10%), [FeNiL(BNPP)(OH)]+ (100%), and [FeNiL(BNPP)]2+ (50%). The isotopic distribution patterns of the m/z values and their relative heights for the observed cluster of peaks and those calculated for the assigned cationic species are in excellent agreement. The FT-IR spectra of the bimetallic complex 1 shows a single mC@N band at 1640 cm1 and mClO4 vibrations at about 1100 and 625 cm1. In compound 1, two medium intensity bands observed at 1210 and 920 cm1 are likely to be due to mP@O and mP–O stretchings of BNPP, while the two strong bands observed at 1520 and 1345 cm1 seem to be due to NO2–Ar symmetric and asymmetric stretching frequencies of BNPP [36]. The electronic absorption spectrum of 1 in acetonitrile exhibits three well-defined bands with their peaks at 350 (12750 M1 cm1), 535 (2350 M1 cm1), and 1020 nm (8 M1 cm1) along with a broad feature observed at about 430 nm. The intense band at 350 nm is due to the macrocyclic ligand-centered p–p⁄ transition, the moderately strong band at 535 nm is due to phenolate ? iron(III) charge transition, while the very weak band observed at 1020 nm is due to 3A2g ? 3T2g transition of octahedral nickel(II). The 1H NMR spectrum of [(BNPP)FeIIIL(l-BNPP)NiII(H2O)]+ (1+) in CD3CN fails to locate the presence of any signal between 250 and 200 ppm due to the fairly large magnetic moment of 1 (6.34 lB) at room temperature. However, when the spectrum is recorded in (CD3)2SO, two slightly broadened singlets of equal

intensity ratio are observed at 7.34 and 8.15 ppm. The observed spectral feature is very similar to that of NaBNPP for which two doublets occur at 7.36 and 8.16 ppm. Quantification of the peak areas of 1+ in (CD3)2SO with respect to known concentration of tert-butyl alcohol used as the internal standard has established that of the two BNPP ligands in 1+ only one BNPP gets dissociated in (CD3)2SO (Fig. S2, Supplementary Information). It is reasonable to believe that the metal-bridged BNPP remains unaffected. 31P NMR spectrum of 1 in (CD3)2SO exhibits two resonances at 27.0 and 11.9 ppm relative to the signal observed at 5.35 ppm for triphenylphosphine used as the internal standard. The spectrum observed for NaBNPP itself in (CD3)2SO shows a single resonance at 12.0. Clearly, the two observed resonances in a highly polar solvent are due to the metal-bridged BNPP (27.0 ppm) and the dissociated BNPP (11.9 ppm). 3.2. Crystal structure of [(BNPP)Fe(L)(l-BNPP)Ni(H2O)](ClO4) A structural projection of 1 is shown in Fig. 1 and the selected bond distances and bond angles of the compound are given in Table 2. In this structure, the central carbon of the lateral propylene unit near the nickel(II) center is disordered over two sites C(100) and C(101) equally. Only C(101) is shown in Fig. 1. The metal centers iron(III) and nickel(II) are triply-bridged by the phenoxo oxygens O(1) and O(2) of L2 and the l-BNPP oxygens O(3) and O(4). Both the metal centers are 6-coordinate with N2O4 donor environments. The coordination sphere [FeN2O4] is comprised of the in-plane donors O(1)O(2)N(3)N(4), while the axial positions are occupied by the oxygens O(3) and O(11) belonging to l-BNPP and monodentate BNPP, respectively. Similarly, for [NiN2O4] the equatorial donors are O(1)O(2)N(2)N(1) and the axial donors are O(4) of l-BNPP and O(19) of a water molecule. All of the Fe–O distances involving the equatorial phenoxides (Fe–O(1)/(2) = 1.994(3) Å) and the axial BNPP linkages (Fe–O(3) = 2.019(4) Å and Fe–O(11) = 1.972(4) Å) are closely similar, while the two equatorial Fe–N (imine) bonds (Fe–N(3) = 2.073(4) Å and Fe–N(4) = 2.065(4) Å) are longer. The trans angles of [FeN2O4] lie in the range 171.2(1)°–172.1(1)° and indicate an overall distorted octahedral geometry of iron(III). In contrast, for [NiN2O4], the in-plane bonds involving the phenoxides (Ni–O(1)/O(2) = 2.063(3) Å) and the imines (Ni–N(1)/N(2) = 1.996 (4) Å) are shorter relative to the axial Ni–O bonds with (l-BNPP) (Ni–O(4) = 2.163(4) Å) and water (Ni–O(19) = 2.103(4) Å). The trans angles in this case lie between 168.7(2)° and 175.9(2)°. Again an irregular octahedral geometry is evident for nickel(II). The internuclear distance between the two metal centers is 3.083(1) Å and the intervening Fe–O(phenoxo)–Ni bridge angle is 98.9(1)°. The dihedral angle between the basal planes of the two metal centers is 12.6°, while the angle between the planes of the two aromatic rings of the macrocycle is 15.7°. The phosphorous atoms P(1) and P(2) of the BNPP ligands are tetrahedrally distorted with O–P–O angles varying from 99.3(2)° to 117.5(2)° for the bridged BNPP P(1) and from 98.9(2)° to 118.2(2)° for the monodentate BNPP P(2). The metal-coordinated P–O bond lengths (av. 1.488 Å) are shorter compared to those of P–OAr distances (av. 1.593 Å). The metal-free P(2)–O(13) bond of BNPP, which is shortest (1.468(4) Å) of all of the P–O bonds, is hydrogen-bonded with the nickel(II)-coordinated water molecule with O(13)  O(19) distance being 2.87 Å. The water molecule is also strongly hydrogen-bonded with the perchlorate oxygen O(22), as indicated by the O(19)  O(22) distance of 2.67 Å (see Table S1). 3.3. Electrochemistry The electrochemical behavior of heterobimetallic complex 1 in acetonitrile (Fig. S3, Supplementary Information) shows a single

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3+

O

N H

OH2 N

N

Fe

H O

N

2+

H

OH2

O

HBNPP

(2eqv) (4eqv)

OH2

N

[Fe(LH2)(H2O)(Cl)]2+ Ni(ClO4)2 6H2O (1 eqv)

(1eqv)

N(C2H5)3

N

Fe

N

N

[Fe(LH2)(H2O)2]3+ Ni(ClO4)2 6H2O

Cl

O

H

O 2N

NO 2 O

NaBNPP (5eqv)

O

P

O

O N

N

O

Fe

Ni

O

N

N

N

O-

N

N

O-

N

OH 2 O

O P

O

L2-

O

O 2N

NO 2

Scheme 1. Schematic representation depicting the preparation of complex 1 using the two precursor iron(III) complexes

O15 N7

C38

O16

C37

C39

C40

C36 O12 O14 C42

O19 O11

C7 C23

C22

C6

C5

N1

C21

O1

Ni O2

N8

C1 C2 C43 C3

C4 C101

O18

C47

P2

O13

C46

C41

C45 C8 N4

C44

O17

C9

Fe N3

N2 C20

C19 C13

C10

C18

C12 C25 O3

O4

C17

C15

C14

P1

C11 C26 C27

O5 C24

N5

C29

C16 C31

C30

O7

C28

O8

O6 C35

C32 C33

C34

O10 N6

O9

Fig. 1. A capped-stick structural motif of the complex cation in [(BNPP)FeL(l-BNPP)Ni(H2O)](ClO4) (1). A methylene carbon is disordered over two sites C100 and C101 with equal occupancy. Only C101 is shown for the sake of clarity.

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Table 2 Selected bond lengths (Å) and angles (°) for [(BNPP)FeL(l-BNPP)Ni(H2O)](ClO4)(1). 1 Bond distances (Å) Fe–O(1) Fe–O(2) Fe–O(3) Fe–N(3) Fe–N(4) Fe–O(11) Ni–O(1) Ni–O(2) Ni–O(4) Ni–O(19) Ni–N(1) Ni–N(2) Fe  Ni

Bond angles (°) 1.996(4) 1.991(3) 2.018(4) 2.073(5) 2.066(5) 1.972(5) 2.064(3) 2.063(4) 2.163(5) 2.101(5) 1.993(4) 2.013(5) 3.083(10)

O(11)–Fe–O(2) O(11)–Fe–O(1) O(2)–Fe–O(1) O(11)–Fe–O(3) O(2)–Fe–O(3) O(1)–Fe–O(3) O(11)–Fe–N(4) O(2)–Fe–N(4) O(1)–Fe–N(4) O(3)–Fe–N(4) O(11)–Fe–N(3) O(2)–Fe–N(3) O(1)–Fe–N(3) O(3)–Fe–N(3) N(4)–Fe–N(3) N(1)–Ni–N(2) N(1)–Ni–O(2) N(2)–Ni–O(2) N(1)–Ni–O(1) N(2)–Ni–O(1) O(2)–Ni–O(1) N(1)–Ni–O(19) N(2)–Ni–O(19) O(2)–Ni–O(19) O(1)–Ni–O(19) N(1)–Ni–O(4) N(2)–Ni–O(4) O(2)–Ni–O(4) O(1)–Ni–O(4) O(19)–Ni–O(4) Fe–O(1)–Ni Fe–O(2)–Ni

97.86(18) 97.45(17) 82.21(14) 171.81(19) 88.66(17) 88.30(15) 87.32(17) 171.22(16) 90.13(16) 86.80(19) 86.83(19) 90.77(17) 172.20(17) 88.15(18) 96.60(18) 98.10(20) 171.84(17) 90.10(20) 93.01(17) 168.80(20) 78.86(14) 87.87(19) 88.20(20) 91.53(15) 90.34(16) 89.89(19) 95.50(20) 90.20(15) 86.33(16) 175.89(19) 98.79(15) 98.97(15)

reversible couple for the iron site (FeIII/FeII) with E1/2 = 150 mV and DEp = 70 mV in the potential window 1.0 to +0.5 V.

tribution of zero-field parameters of the metal ions at low temperature. The theoretical magnetic moment at room temperature expected for a non-interacting dinuclear Fe(III)–Ni(II) system is 6.56 lB. The observed magnetic moment for compound 1 is somewhat lower than the theoretical value, which may be attributed to the antiferromagnetic interaction between the Fe(III) and Ni(II) centers. The local spin-Hamiltonian for 1 is given by Eq. (1)

  1 H ¼ 2J 12 S1  S2 þ D1 S2z;1  S1 ðS1 þ 1Þ 3   1 2 þ D2 Sz;2  S2 ðS2 þ 1Þ þ gbðS1  B þ S2  BÞ 3

ð1Þ

where S1 = 5/2, S2 = 2/2, D1 = DFe, and D2 = DNi. As shown in Fig. 2, nice theoretical fits have been obtained with the parameters J12 = 7.10 cm1, gFe = 1.98, gNi = 2.12, DFe = 1.0 cm1, DNi = 0.2 cm1, and TIP = 200  106 cm3 mol1 using the program JULX [37]. The spin ground state is ST = 3/2. Magnetic exchange interaction between two metal ions can be predicted qualitatively according to Goodenough [38a] and Kanamori [38b] by taking into consideration orbital symmetries of the interacting metal ions and the spin-exchange pathway involved therein. Thus, for heterobimetallic iron(III)–nickel(II) systems (d5–d8) in octahedral environment, the exchange interaction will be ferromagnetic when the bridge angle is close to 90° and antiferromagnetic with larger bridge angle [39]. To obtain further insight to the magnetic property of 1, EPR spectrum of the compound in frozen CH3CN solution has been recorded at 77 K. Unfortunately, due to the dipolar broadening the quality of the spectrum was poor. Nevertheless, the presence of three signals at g  4.2, 5.1, and 9 could be discerned similar to those reported for [FeNi(pmbp)(OPr)2](BPh4)2 by Holman et al [40]. From detailed analysis of EPR spectrum recorded between 2.6 and 80 K, they have attributed the spectral feature observed at g = 5.1 due to the electronic spin ground state S = 3/2, while those observed at g = 4.2 and 9 due to the excited spin state S = 5/2.

3.4. Magnetic properties

6.0

0.6

5.5

0.5

5.0

0.4

4.5

0.3

4.0

0.2

3.5

0.1

3.0

0

50

100

150

200

250

/ emu mol-1

0.7

M

6.5

χ

μeff / μB

Variable-temperature magnetic susceptibility measurements have been carried out for 1 between 2 and 293 K and the plots of vM and leff versus T are shown in Fig. 2. The magnetic moment decreases monotonously from 6.33 lB at 290 K to 3.98 lB at 15 K and then decreases rapidly to 3.19 lB at 2 K, indicating moderately low intramolecular antiferromagnetic exchange interaction with con-

0.0 300

T/K Fig. 2. Molar magnetic susceptibility, vM (D) and effective magnetic moment, leff (s) vs temperature for [(BNPP)FeL(l-BNPP)Ni(H2O)](ClO4) (1). Best fit parameters (—) obtained were J = 7.1 cm1, gFe = 1.98, gNi = 2.12, DFe = 1.0 cm1, DNi = 0.2 cm1.

3.5. Phosphodiesterase activity The catalytic activity of [(H2O)FeL(l-BNPP)Ni(H2O)]2+ (12+) towards the hydrolytic cleavage of BNPP has been ascertained by recording time-interval spectra of a solution of 1 (2  105 M) in (70:30 v/v) H2O–(CH3)2SO containing 10-fold excess of NaBNPP, ionic strength adjusted to 0.1 M (NaClO4) and keeping pH 6 and temperature to 40 °C. As shown in Fig. S4 (Supplementary Information), the intensity of the band at kmax = 400 nm increases with time due to the growing concentration of the hydrolytically generated 4-nitrophenolate (NP). Moreover, the 31P NMR spectrum recorded for a similar solution after 0.5 h of mixing (Fig. S5, Supplementary Information) reveals the presence of two signals at 17.9 and 11.4 ppm against that of PPh3 observed at 5.35 ppm. These two signals are due to BNPP (17.9 ppm) and 4-nitrophenyl phosphate NPP (11.4 ppm). Guo et al. also reported [41] that the hydrolytic cleavage of BNPP by a phenolate bridged dizinc(II) complex produces NPP and NP. The dependence of kobs on pH (Fig. S6, Supplementary Information) and the initial reaction rate (V0) on NaBNPP concentration at pH 6.0 and at 35 °C is shown in Fig. 3. It may be noted that for the complex concentration 2  105 M, saturation kinetics is reached with the substrate concentration ca. 2  103 M. The V0–[NaBNPP] rate profile is typical of the Michaelis–Menten behavior [42] observed for native enzymes. The relevant Michaelis–Menten parameters have been evaluated from non-linear regression analysis of V0 versus [NaBNPP] plot (Fig. 3) as well as least-squares fit of the Lineweaver–Burk linear plot of 1/V0 versus 1/[NaBNPP] (shown in

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8.0x10

-7

1.5x10

-7

1.2x10

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7

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7

7

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-1

-8

7

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0

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V0 / (M s -1)

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0.0

-4

5.0x10

-3

-3

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1.5x10

0.0

-3

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[S] / (M)

0.00

Fig. 3. Dependence of initial reaction rate (V0) on BNPP concentration for the hydrolysis reaction promoted by complex 1. [1] = 2  105 M, [buffer] = 5  102 M, pH = 6, I = 0.1 M (NaClO4) in H2O–(CH3)2SO (70:30 v/v) at 35 °C. Inset shows the Lineweaver–Burk double reciprocal plot.

the inset of Fig. 3). The same results were obtained in both the cases. Thus, at 35 °C, the Michaelis constant KM = 3.6  104 M, the maximum reaction rate Vmax = 1.80  107 M s1, the catalytic rate constant kcat = Vmax/[1] = 9.0  103 s1, and the second order rate constant for BNPP, kBNPP = (kcat/KM) = 25.0 M1 s1 and the substrate-catalyst binding constant (KB), which is approximated as 1/KM, is 2780 M1. Importantly, as compared to the uncatalyzed hydrolysis of BNPP (kuncat = 1.1  1011 s1) [43] the complex catalyzed reaction rate is 8.3  108 times faster. The kcat value indicates that 33 molecules of BNPP are hydrolyzed per hour. Table 3 shows a comparative study of the different hydrolytic parameters associated with phosphate ester hydrolysis for some representative homo- and heterodinuclear compounds derived from 4-methyl2,6-disubstituted phenol systems that are similar to our ligand. To the best of our knowledge, the kcat value as observed by us is 20-folds greater than the only other previously reported FeIIINiII system [22e]. The dinuclear nickel(II) complex reported by Neves and coworkers is the only other complex, to the best of our knowledge that hydrolyzed BDNPP with a kcat value approximately 40 times greater than our observation [45b].The metrical parameters of other homo- and heterobimetallic systems shown in Table 3 are mostly of the same order of magnitude as observed by us. It may be mentioned in this context that BDNPP (bis-2,4-dinitrophenyl phosphate) has been used as substrate in all the cases shown in Table 3, except one, where a dizinc complex was used to hydrolyze BNPP [41]. Hydrolysis of BNPP by complex 1 has

-5

1.30x10

-5

-5

2.60x10 3.90x10 [1] / (M)

-

5.20x10

Fig. 4. Dependence of the pseudo first order rate constant on the concentration of complex 1. [BNPP] = 2  104 M, [buffer] = 5  102 M, pH 5.7, I = 0.1 M (NaClO4) in H2O–(CH3)2SO (70:30 v/v) at 35 °C.

shown a kcat value 1989 folds greater than that reported by Guo and coworkers [41]. Clearly, 12+ is a highly efficient catalyst for the hydrolysis of phosphate diesters. Strictly speaking, the value kuncat = 1.1  1011 s1 refers to the base hydrolysis of BNPP measured at pH 7 and at 25 °C, while the present study has been carried out at pH 6.0 and at 35 °C. Assuming continuation of first order dependence of reaction rate on the concentration of added [OH], the extrapolated value of kuncat at pH 6.0 would be ca. 2  1013 s1. In other words, the value of kcat/kuncat in that case is ca. 4.5  1010, indicating still better catalytic activity of the complex. The dependence of the reaction rate on the concentration of complex 1 has also been studied at pH 6.0 and at 35 °C. From the slope of kobs versus [1] plot (Fig. 4), the second order rate constant kcomplex has been found to be 14.9 M1 s1. It may be noted that kcomplex thus obtained and kBNPP obtained under Michaelis–Menten condition have magnitudes of the same order. The influence of temperature on the hydrolysis reaction rate has been studied under the same condition as stated above at 25°, 35°, 40°, and 50° ± 0.1 °C. The kobs values obtained at pH 6.0 have been subjected to linear fit according to Arrhenius equation [44] to obtain the activation energy Ea. As shown in Fig. 5, the slope of the plot ln(kobs) vs 1/T gives Ea = 22.4 kJ mol1. In order to obtain insight for the complex catalyzed hydrolysis of BNPP and other related phosphate esters further studies are in progress.

Table 3 Comparison of different hydrolytic parameters associated with phosphate ester hydrolysis. Compound III

Substrate II

a

[(BNPP)Fe L(l-NPP)Ni (H2O)](ClO4) [FeIIINiII(BPBPMP)(OAc)2](ClO4)b [FeIIIMnII(BPBPMP)(OAc)2](ClO4)b [FeIII(l-OH)ZnII(L1)]c [Ni2(L2)(OAc)2(H2O)](ClO4)H2Od [Ni2(L3)(OAc)2(CH3CN)](BPh4)e [L4FeIII(l-OAc)2ZnII]+f Zn2L5g

BNPP BDNPP BDNPP BDNPP BDNPP BDNPP BDNPP BNPP

Vmax (M s1)

KM (M) 4

3.6  10 3.85  103 2.10  103 3.55  103 1.19  103 5.67  103 4.20  103 6.15  102

7

1.83  10 1.79  108 1.80  108 1.85  108 5.83  108 3.28  106 9.2  1011

kcat (s1)

Reference

9.15  103 4.47  104 4.51  104 9.02  104 3.43  102 3.86  101 9.13  104 4.60  106

this work [22e] [22d] [45a] [45b] [45b] [45c] [41]

BDNPP = 2,4-bis(dinitrophenyl)phosphate. a L = tetraiminodiphenolate macrocyclic ligand derived from the condensation of 4-methyl-2,6-diformyl phenol with 1,3-diaminopropane. b H2BPBPMP = 2-bis[{(2-pyridylmethyl)-aminomethyl}-6-{(2-hydroxybenzyl)-(2-pyridylmethyl)}-aminomethyl]-4-methylphenol. c H2L1 = 2-bis[{(2-pyridylmethyl)aminomethyl}-6-{(2-hydroxy-6-formylbenzyl)(2-pyridylmethyl)}aminomethyl]-4-methylphenol. d H2L2 = 2-[N-bis-(2-pyridylmethyl)aminomethyl]-4-methyl-6-[N-(2-pyridylmethyl)amino aminomethyl]phenol. e H2L3 = 2-[N-(2-(pyridyl-2-yl)ethyl)(1-methylimidazol-2-yl)aminomethyl]-4-methyl-6-[N-(2-(imidazol-4-yl)ethyl)aminomethyl]phenol. f H2L4 = 2-bis[{(2-pyridylmethyl)aminomethyl}-6-{(2-hydroxybenzyl)(2-pyridylmethyl)}aminomethyl]-4-methylphenol g H3L5 = 2,6-bis(2-pyridylmethyl-2-hydroxyethylamino)methyl-4-methylphenol.

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-7.8

UK; fax: +44 1223 336 033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2011.09.009.

lnkobs

-8.0

References

-8.2

[1] [2] [3] [4]

-8.4 -8.6

[5]

-8.8

[6] -3

3.1x10

-3

3.2x10

-3

3.2x10

1/T

-3

3.3x10 / (K-1)

-3

3.4x10

-3

3.4x10

Fig. 5. Linearization of the observed rate constants for the hydrolysis of BNPP (2  104 M) promoted by complex 1 (2  105 M) at pH 6 as a function of temperature: Arrhenius equation.

[7]

[8]

4. Conclusion Using the mononuclear iron(III) complexes [Fe(LH2)(H2O)2](ClO4)33H2O and [Fe(LH2)(H2O)Cl](ClO4)22H2O as precursors, the heterobinuclear phosphate-bridged complex [(BNPP)FeL(lBNPP)Ni(H2O)](ClO4) (1) have been synthesized to investigate whether it can function as a model for purple acid phosphatases. The structure of 1 has been crystallographically determined. The 1 H and 31P NMR spectra of 1 in (CD3)2SO or D2O-(CD3)2SO have indicated that 1 in solution remains as the species [(Solv)FeL(lBNPP)Ni(H2O)]2+. From temperature-dependent magnetic susceptibility measurements of 1 it has been inferred that the high-spin iron(III) and nickel(II) centers are antiferromagnetically coupled. (H = 2JS1S2) to the S = 3/2 spin ground state with J = 7.1 cm1. The EPR spectrum of 1 in CH3CN at 77 K has shown peaks with g  4.2, 5.1, and 9.0, of which, the peak at g = 5.1 is attributed to S = 3/2 state and those at ca. 4.2 and 9.0 to S = 5/2 state. The catalytic behavior of 1 towards the hydrolytic cleavage of BNPP to give NPP and NP as the hydrolyzed product has been studied in 70:30 H2O–(CH3)2SO at pH 6.0, over the temperature range 25–50 °C and also by varying the concentrations of BNPP and 1. As compared with uncatalyzed hydrolysis of BNPP, the complexcatalyzed hydrolysis rate is at least 8.3  108 times faster, indicating that 1 serves as an efficient model for purple acid phosphatases.

[9] [10] [11] [12] [13] [14]

[15] [16] [17] [18] [19] [20]

[21]

[22]

Acknowledgments We are thankful to Prof. K. Nag, Department of Inorganic Chemistry, Indian Association for the Cultivation of Science for helpful suggestions during work and for preparation of the manuscript. PB acknowledges financial assistance received from the Department of Science and Technology, New Delhi [Grant No. SR/FT/CS022/2009]. Thanks are due to the Department of Science and Technology, Government of India for establishing the National X-ray Diffractometer facility at the Department of Inorganic Chemistry, Indian Association for the Cultivation of Science. Appendix A. Supplementary data CCDC 795725 contains the supplementary crystallographic data for 1. These data can be obtained free of charge via http:// www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,

[23]

[24]

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