A Bioinspired Dinickel(II) Hydrolase: Solvent Vapor-Induced

Article pubs.acs.org/IC

A Bioinspired Dinickel(II) Hydrolase: Solvent Vapor-Induced Hydrolysis of Carboxyesters under Ambient Conditions Suman K. Barman,† Francesc Lloret,§ and Rabindranath Mukherjee*,†,‡ †

Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, India Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741 246, India § Departament de Química Inorgànica/Instituto de Ciencia Molecular (ICMOL), Universitat de València, Polígono de la Coma, s/n, 46980 Paterna, València, Spain ‡

S Supporting Information *

ABSTRACT: From the perspective of synthetic metallohydrolases, a phenoxo-bridged dinickel(II) complex [NiII2(L)(H2O)2(CH3OH)][ClO4]·CH3OH (1) (H3L = 2,6-bis[{{(5-bromo-2-hydroxybenzyl)(N′,N″-(dimethylamino)ethyl)}amino}methyl]-4methylphenol) has been synthesized and structurally characterized. The presence of a vacant coordination site and a weakly bound water molecule provides the scope for substrate binding to act as a metallohydrolase model. Ethyl acetate vapor diffusion at 298 K to a CH3CN/CH3OH solution of 1 results in the formation of a pentanuclear acetato-bridged complex [NiII5(H2L)2(μ3OH)2(μ-O2CCH3)4][ClO4]2·CH3CO2C2H5 (2), demonstrating for the first time the metal-coordinated water-promoted hydrolysis of a carboxyester at room temperature. When the crystals of 1, moistened with a few drops of ethyl acetate, were kept for ethyl acetate vapor diffusion, it transforms into a monoacetato-bridged complex [NiII2(HL)(μ-O2CCH3)(H2O)2][ClO4]· 4H2O (3). This kind of solvent (vapor)-induced single-crystal-to-single-crystal structural transformation concomitant with the hydrolysis of external substrate (ethyl acetate) is unprecedented. Reaction of H3L with 2 equiv of NiII(O2CCH3)2·4H2O, followed by the usual workup, and recrystallization from CH2Cl2 led to the isolation of [NiII2(H2L)(μ-O2CCH3)2][ClO4]· CH2Cl2·2H2O (4). Complex 4 is structurally different from 3, confirming that the reaction of NiII(O2CCH3)2·4H2O with H3L is a different phenomenon from the hydrolysis of ethyl acetate, promoted by NiII-coordinated water in 1. Complex 1 is also capable of hydrolyzing ethyl propionate to a propionato-bridged complex [NiII2(HL)(μ-O2CCH2CH3)(H2O)2][ClO4] (5). For the hydrolytic phenomena mentioned above, the coordinated ligand donor sites (phenolate and tertiary amine) provide a microenvironment around the dinickel(II) center to facilitate efficient stoichiometric hydrolysis of ethyl acetate and ethyl propionate under ambient conditions. Temperature-dependent magnetic studies of dimeric complexes 1, 4, and 5 reveal the presence of moderate antiferromagnetic coupling: J = −25.0(1) cm−1 for 1, J = −20.0(1) cm−1 for 4, and J = −18.80(8) cm−1 for 5. For pentanuclear complex 2, three types of magnetic-exchange interactions, two ferromagnetic (Ja = +16.02 cm−1, and Jb = +9.02 cm−1) and an antiferromagnetic (Jc = −49.7 cm−1), have been identified.

■

INTRODUCTION In nature, there are many enzymes that catalyze hydrolysis of phosphate esters or carboxyesters. Considerable attention has therefore been paid to the development of efficient lowmolecular weight synthetic hydrolase models.1,2 Dinuclear metal centers have been found in a number of metallohydrolases.3,4 The cooperation of the catalytic center (primary coordination sphere) and secondary coordination sphere (the amino acids those are not directly bonded with the catalytic center) allows a native metalloenzyme to promote biological transformations with remarkable efficiency and enantioselectivity, under ambient conditions.5 The catalytic efficiency for metallohydrolases depends on several factors: (i) choice of metal ion, (ii) availability of labile and/or vacant © XXXX American Chemical Society

coordination site(s) and sutitable disposition of the labile and/ or vacant sites so that the nucleophile can approach the substrate effectively, (iii) availability of basic functionalities like amines or carboxylates around the catalytic centers that play vital roles in the generation of an effective nucleophile at close to neutral pH by consuming the liberated protons,6 (iv) electrophilic activation of the substrate in the ground state or stabilization of the transition state by metal-ion coordination, hydrogen bonding, or proton transfer, and (v) releasing the product(s) and/or leaving groups at a reasonable rate. Biomimetic studies of dinuclear metallohydrolases have Received: August 7, 2016

A

DOI: 10.1021/acs.inorgchem.6b01895 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

structural point of view. Temperature-dependent magnetic studies of 1, 2, 4, and 5 have also been performed.

improved our understanding of the mechanisms of the functioning of the enzymes.7 Nevertheless, much remains to be learned regarding the molecular-level understanding of the subtle controlling factors of their functional efficiency. To design synthetic models that more closely mimic enzyme active sites, a few interesting synthetic model systems have been explored with hydrogen-bond donors suitably placed close to the primary coordination sphere so that H-bonding interaction subtly fine-tunes required substrate binding and/or release of the product.8,9 There are reports of shifting of pHmax to higher pH values with a H-bonding donor substitutent compared to its unsubstituted analogue.8a,c Thus, regulation of the strength and positioning of H-bonding donor and acceptor groups is of paramount importance and needs to be investigated further. Along this line of thought, we paid attention to a ligand system in which primary coordination sites on their own can provide a H-bonding acceptor or donor source, when required. From this perspective, herein we present a phenolate-based binucleating ligand providing terminal tricoordination, H3L (Figure 1),

■

EXPERIMENTAL SECTION

Reagents and Materials. All reagents were obtained from commercial sources and used as received. Solvents were dried and/ or purified as reported previously.10 The ligand H3L was synthesized following a reported procedure.11 Synthesis of [NiII2(L)(H2O)2(CH3OH)][ClO4]·CH3OH (1). The ligand H3L (0.090 g, 0.132 mmol) was dissolved in a CH2Cl2/CH3OH solution [8 mL, 1:7 (v/v)]. Solid NaOH (0.016 g, 0.397 mmol) was dissolved in CH3OH (2 mL, 2 drops of water added), and the solution was added to the ligand solution and the mixture stirred for 5 min. Then solid [NiII(H2O)6][ClO4]2 (0.096 g, 0.262 mmol) was added pinch by pinch, generating a green solution. The mixture was stirred for 6 h. To this reaction mixture was added water (0.3 mL), and the mixture was kept for slow evaporation to yield green crystals, suitable for X-ray diffraction. Yield: 0.060 g, 46%. Anal. Calcd (%) for C33H51Br2ClN4Ni2O11: C, 39.94; H, 5.18; N, 5.65. Found: C, 39.77; H, 5.05; N, 5.58. IR (KBr, cm−1, selected bands): 3447 [ν(OH) of water], 1091, 625 [ν(ClO4−)]. Electronic spectrum (CH3CN) λmax, nm (ε, M−1 cm−1): 300 (9900), 380 (sh) (740), 666 (16), 860 (sh) (20), 940 (20). Synthesis of [NiII5(H2L)2(OH)2(O2CCH3)4][ClO4]2·CH3CO2C2H5 (2). Complex 1 (0.010 g) was dissolved in a CH3CN/CH3OH solution [1.5 mL, 1:1 (v/v)], and the solution was kept for vapor diffusion with ethyl acetate. After 2 weeks, green crystals of 2, suitable for X-ray diffraction studies, were isolated. Yield: 0.004 g, 40%. Anal. Calcd (%) for C74H104Br4Cl2N8Ni5O26: C, 40.37; H, 4.58; N, 5.09. Found: C, 40.21; H, 4.42; N, 5.15. IR (KBr, cm−1, selected bands): 3446 [ν(OH) of water/OH], 1583 [ν(O2CCH3)asym], 1477 [ν(O2CCH3)sym], 1088, 624 [ν(ClO4−)]. Synthesis of [NiII2(HL)(O2CCH3)(H2O)2][ClO4]·4H2O (3). Single crystals of complex 1 (0.003 g, 0.003 mmol), moistened with a small amount (0.01 mL, 0.10 mmol) of ethyl acetate, were kept for vapor diffusion. The crystals of 1 transformed to crystals of 3. IR (KBr, cm−1, selected bands): 3430 [ν(OH) of water/phenol], 1570 [ν(O2CCH3)asym], 1475 [ν(O2CCH3)sym], 624, 1107 [ν(ClO4−)]. Electronic spectrum (CH3CN) λmax, nm (ε, M−1 cm−1): 300 (9470), 374 (700), 668 (40), 866 (sh) (36), 920 (35). Synthesis of [NiII2(H2L)(O2CCH3)2][ClO4]·CH2Cl2·2H2O (4). The ligand H3L (0.102 g, 0.150 mmol) was dissolved in a CH2Cl2/ CH3OH solution [8 mL, 1:7 (v/v)]. Solid Ni(O2CCH3)2·4H2O (0.075 g, 0.301 mmol) was added to the mixture while it was being continuously stirred, and then NaClO4·H2O (0.021 g, 0.150 mmol) was added. The resulting reaction mixture was stirred for 3 h and kept for slow evaporation. The green product that formed was filtered and dried under vacuum. Yield: 0.105 g, 70%. X-ray-quality crystals were obtained by diffusion of n-hexane into a CH2Cl2/CH3OH [2:1 (v/v)] solution of 4. Anal. Calcd (%) for C36H53Br2Cl3N4Ni2O13: C, 38.15; H, 4.71; N, 4.94. Found: C, 38.28; H, 4.61; N, 4.85. IR (KBr, cm−1, selected bands): 3456 [ν(OH) of water/phenol], 1578 [ν(O2CCH3)asym], 1471 [ν(O2CCH3)sym], 630, 1103 [ν(ClO4−)]. Electronic spectrum (CH3CN) λmax, nm (ε, M−1 cm−1): 300 (7950), 380 (485), 675 (16), 876 (sh) (16), 964 (20). Synthesis of [NiII2(HL)(O2CCH2CH3)(H2O)2][ClO4] (5). Complex 1 (0.05 g, 0.05 mmol) was suspended in ethyl propionate (2 mL) and the mixture stirred for 6 h. Solvent was then removed under vacuum. The solid was recrystallized from a DMF/Et2O solvent [5 mL, 1:4 (v/ v)]. Yield: 0.033 g, 65%. X-ray-quality single crystals were obtained by vapor diffusion of Et2O into a DMF/CH3CN [1:2 (v/v)] solution of the complex. Anal. Calcd (%) for C34H49Br2ClN4Ni2O11: C, 40.74; H, 4.93; N, 5.59. Found: C, 41.01; H, 4.83; N, 5.42. IR (KBr, cm−1, selected bands): 3450 [ν(OH) of water/phenol], 1585 [ν(O2CCH3)asym], 1477 [ν(O2CCH3)sym], 624, 1108 [ν(ClO4−)]. Caution! Perchlorate salts of compounds containing organic ligands are potentially explosive! Physical Measurements. Elemental analyses were conducted using a Thermo Quest EA 1110 CHNS-O instrument. Spectroscopic

Figure 1. Ligand pertinent to this work.

where (i) a -CH2CH2- spacer will provide suitable flexibility and a -NMe2 group can act as a base, if required, and (ii) terminal metal-coordinated phenolate can act as a H-bond acceptor for the neighboring coordinated water and can also act as a H-bond donor, when it exists as a terminal metal-coordinated phenol. With the ligand H3L, we have synthesized [NiII2(L)(H2O)2(CH3OH)][ClO4]·CH3OH (1). The presence of vacant and labile coordination sites makes it a suitable bioinspired complex for hydrolytic activity. In addition to that, strong intramolecular H-bonding interaction between a tightly bound water and terminal phenolate at the other nickel(II) center makes the NiII-coordinated water more nucleophilic. In fact, we observed vapor-induced hydrolysis of ethyl acetate at 298 K by a CH3CN/CH3OH solution of 1 to produce [NiII5(H2L)2(μ3OH)2(μ-O2CCH3)4][ClO4]2·CH3CO2C2H5 (2). Interestingly, when the crystals of 1 moistened with a few drops of ethyl acetate were kept for ethyl acetate vapor diffusion, 1 transforms to a monoacetato-bridged complex [NiII2(HL)(μ-O2CCH3)(H2O)2][ClO4]·4H2O (3). This kind of solvent (vapor)induced single-crystal-to-single-crystal structural transformation with simultaneous hydrolysis of ethyl acetate is a rare phenomenon in coordination chemistry. From a mechanistic point of view, another dinickel(II) complex with two acetate bridges, [NiII2(H2L)(μ-O2CCH3)2][ClO4]·CH2Cl2·2H2O (4), has been synthesized, from the direct reaction of H3L and NiII(O2CCH3)2·4H2O, and NaClO4·H2O. Notably, 1 is also capable of hydrolyzing ethyl propionate at 298 K, with successful synthesis of [NiII2(HL)(μ-O2CCH2CH3)(H2O)2][ClO4] (5). The comparative ease of hydrolysis of ethyl acetate and ethyl propionate has been investigated. Mechanistic insight into and stoichiometry of the reactions mentioned above are considered. Synergistic adjustment of the ligand periphery during hydrolysis of carboxyesters has been discussed from a B


Article

Inorganic Chemistry measurements were taken using the following instruments: IR (KBr, 4000−600 cm−1), Bruker Vector 22; electronic, Agilent 8453 diode array spectrophotometer. For the IR spectra of neat ethyl acetate, one drop of ethyl acetate was adsorbed on solid KBr and then its spectrum was recorded. For the mixture containing 1 and ethyl acetate (or ethyl propionate), after desired time intervals, a portion of the mixture was air-dried and IR spectra were recorded. Variable-temperature magnetic susceptibility measurements in the solid state were performed with a Quantum Design (València, Spain) SQUID magnetometer at 0.01 T for temperatures of 50 K. The effective magnetic moment was calculated from the equation μeff = 2.828(χMT)1/2, where χM is the corrected molar susceptibility. Susceptibilities were corrected for diamagnetic contributions.12 Crystal Structure Determination. X-ray data of 1−5 were recorded on a Bruker SMART APEX CCD diffractometer at 100(2) K, with graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation [Tables S1−S5; CCDC-1496651 (1), CCDC-1496674 (2), CCDC1496680 (3), CCDC-1496742 (4), and CCDC-1496758 (5)]. For data reduction, the “Bruker Saint Plus” program was used. Data were corrected for Lorentz and polarization effects; empirical absorption correction (SADABS) was applied. Structures were determined by SIR-97 and refined by full-matrix least-squares methods based on F2 using SHELXL-97, expanded by Fourier difference syntheses, and refined with SHELXL-2014,13a incorporated in a WinGX 2014.1 crystallographic collective package.13b Hydrogen atoms were placed in idealized positions and treated using riding model approximation with displacement parameters derived from those of the atoms to which they were bonded. All non-hydrogen atoms were refined with anisotropic thermal parameters by full-matrix least-squares procedures on F2. For 3 and 4, the squeeze filter of PLATON14 was employed because of the presence of severely disordered solvent molecules and counteranions. For 3, the squeeze process corresponded to 81 electrons per unit cell or 40.5 electrons (four water molecules) per cation in a volume of 353 Å3, of a unit volume of 2174 Å3. For 4, the squeeze process resulted in 1337 electrons per unit cell or 111.4 electrons per cation (corresponding to one ClO4−, a CH2Cl2, and two water molecules) in a volume of 4116 Å3, of a unit volume of 8557.60 Å3. Potentiometric Measurements. Potentiometric titrations were conducted at 303 K using a Metrohm 794 Basic Titrino instrument connected to a Metrohm AG 9101 Herisau pH glass electrode and a ground-joint diaphragm. Prior to the experiment, calibration was done with aqueous buffer solutions at pH 4.00 and 7.00. Complex solutions were prepared in a CH3CN/H2O solvent [50% (v/v)] because of the low solubility of the complexes in pure water, and the ionic strength was adjusted to 0.04 M by adding appropriate amounts of NaClO4. A pKw value of 15.40 was used for the CH3CN/H2O [50% (v/v)] mixture.15 Computations were performed with the HYPERQUAD 2000 program, and species distributions were calculated using HySS.16 To determine the pKa values of the coordinated water molecules in 1, a typical pH-metric titration was performed as follows. A 0.4 mM solution (in 50% CH3CN/H2O) of the complex was titrated with a 50 mM NaOH solution. The ionic strength (I) of the medium was maintained at 0.04 N NaClO4.

Figure 2. Perspective view of the cation of [NiII2(L)(H2O)2(CH3OH)][ClO4]·CH3OH (1). Only donor atoms are labeled. All the hydrogen atoms have been omitted for the sake of clarity.

Table 1. Selected Bond Lengths (angstroms) and Angles (degrees) of [NiII2(L)(CH3OH)(H2O)2][ClO4]·CH3OH (1) Ni1−N1 Ni1−N2 Ni1−O1 Ni1−O2 Ni1−O3 Ni1−O4

2.061(3) 2.124(3) 2.112(2) 2.022(3) 2.197(3) 2.045(3)

Ni2−N3 Ni2−N4 Ni2−O1 Ni2−O5 Ni2−O6

2.049(3) 2.093(3) 1.997(2) 1.979(2) 2.031(3)

Ni1···Ni2

3.581

Ni1−O1−Ni2 O2−Ni1−O1 O2−Ni1−O3 O2−Ni1−O4 O2−Ni1−N1 O2−Ni1−N2 O3−Ni1−O1 O3−Ni1−O4 O3−Ni1−N1 O3−Ni1−N2 O4−Ni1−O1 O4−Ni1−N1 N1−Ni1−N2 N1−Ni1−O1 N2−Ni1−O1 N2−Ni1−O4 O5−Ni2−O1 O5−Ni2−O6 O5−Ni2−N3 O5−Ni2−N4 O6−Ni2−O1 O6−Ni2−N3 O6−Ni2−N4 N3−Ni2−O1 N3−Ni2−N4 N4−Ni2−O1

121.22(11) 92.27(10) 79.77(10) 90.38(11) 91.50(11) 171.63(11) 171.35(10) 86.28(10) 91.57(11) 92.13(11) 90.53(10) 176.22(11) 86.58(11) 92.02(11) 95.93(10) 90.38(11) 100.96(10) 89.08(10) 92.09(11) 158.73(11) 94.67(10) 166.92(11) 88.65(12) 97.89(11) 85.54(12) 100.30(11)

distance of 3.581 Å and a Ni1−O1−Ni2 angle of 121.21(11)°. In addition to the bridging phenolate O1, Ni1 is terminally coordinated by two tertiary alkyl amines N1 and N2 and a phenolate O2 from the ligand. The O3 and O4 atoms of two water molecules complete the pseudooctahedral geometry around Ni1. On the other hand, the geometry around Ni2 is best described as close to a square pyramid (τ = 0.138). Two tertiary alkyl amines N3 and N4, a phenolate O5, and a CH3OH coordination O6 form the basal plane. The bridging phenolate O1 occupies the apical position of the square pyramid. Notably, Ni1 has two labile coordination sites occupied by two water molecules, and Ni2 has a labile site occupied by a CH3OH molecule. These kinds of H2O/ CH3OH-coordinated metal centers are ideal for any complex to act as a synthetic hydrolase.18,19 A water molecule is strongly bound to Ni1 (Ni−O4 = 2.045 Å), and there is a vacant site at Ni2, which is cis-oriented to the O4 water molecule. Strong binding of this water (O4) to the metal center is expected to decrease its pKa (see the potentiometry section). Moreover, there is strong intramolecular hydrogen bonding between this

■

RESULTS AND DISCUSSION Synthesis and Structure of [NiII2(L)(H2O)2(CH3OH)][ClO4]·CH3OH (1). Reaction of H3L with 2 equiv of [NiII(H2O)6][ClO4]2 and 3 equiv of NaOH afforded isolation of [NiII2(L)(H2O)2(CH3OH)][ClO4]·CH3OH (1). IR spectra of 1 displayed characteristic absorptions due to stretching vibrations of ClO4− at 1091 and 625 cm−1. X-ray structural analysis of 1 reveals an asymmetric17 dinickel(II) structure (Figure 2 and Table 1). While Ni1 is six-coordinate with a N2O4 coordination sphere, Ni2 attains five-coordinate N2O3 coordination. The central phenoxide of deprotonated H3L holds two metal centers with a Ni1···Ni2 C


Article

Inorganic Chemistry

susceptible to nucleophilic attack. Indeed, solvent (vapor)induced hydrolysis of ethyl acetate has been observed, when a CH3CN/CH3OH solution [1:1 (v/v)] of 1 was subjected to CH3CO2C2H5 vapor diffusion (Scheme 1). In fact, after a few days, a new complex, [NiII5(H2L)2(μ3-OH)2(μ-O2CCH3)4][ClO4]2·CH3CO2C2H5 (2), crystallized. The presence of acetate and perchlorate is revealed by IR spectroscopy. The NiII-coordinated water molecule in 1 brings about hydrolysis of CH3CO2C2H5 with formation of CH3CO2H and C2H5OH. Acetate ions from acetic acids are utilized in the bridging of two nickel(II) ions, and the released protons are consumed by the ligand periphery. One of the -NMe2 units N4 of each ligand consumes a proton and in turn remains uncoordinated, and a terminal phenolate of each ligand exists in the phenol form (Figure 4), authenticated by the X-ray

water molecule and the phenolate coordinated to Ni2 (Figure 3 and Table 2). The strong hydrogen bonding along O4−H100···

Figure 3. Perspective view of the C−H···O and O−H···O interaction present in 1.

Table 2. Hydrogen Bonding Parameters of [NiII2(L)(CH3OH)(H2O)2][ClO4]·CH3OH (1) D−H···A

H···A (Å)

D···A (Å)

D−H···A (deg)

O4−H100···O5 O6−H678···O2 O3−H102···O9 C10−H10A···O8 C6−H6B···O7 C5−H5A···O7

1.720 1.70 1.933 2.70 2.590 2.679

2.595 2.514 2.793 3.649 3.525 3.548

177.50 175.44 170.80 166.28 162.10 149.33

Figure 4. Perspective view of the cation [NiII5(H2L)2(μ3-OH)2(μO2CCH3)4][ClO4]2·CH3CO2C2H5 (2).

structure of 2. Thus, both the bromophenolate and -NMe2 units of the ligand play a significant role by consuming protons, liberated during hydrolysis. Here the primary coordination sphere provides subtle coordination adjustment and acts like a pseudo-microenvironment through H-bonding, which stabilizes product formation (see below). X-ray structural analysis shows that 2 is a pentanuclear nickel(II) complex, bridged by the acetate and hydroxide ions (Figure 4), and selected metric parameters are listed in Table 3. In 2, two dinuclear units are linked through a unique nickel(II) ion, Ni1. This Ni1 is bridged with other nickel(II) ions by acetate and hydroxo (μ3-bridging mode) groups, in addition to the terminal phenolate bridge. Each of the nickel(II) centers is six-coordinate with distorted octahedral geometry. The Ni2 center is coordinated by bridging phenolates O1 and O2 and tertiary amine N3 from the ligand. The tertiary amine nitrogen (N4) that belongs to the N,N-dimethyl unit is protonated and

O5 (H100···O5 = 1.720 Å) also makes the water molecule more nucleophilic. Thus, 1 is expected to be a potential candidate for hydrolytic activity. Analysis of crystal packing diagrams shows that there are additional H-bonding interactions in 1. Significant O−H····O interaction is also observed involving CH3OH coordinated to Ni2 and phenolate (O2) coordinated to Ni1 (H678····O2 = 1.70 Å) and water coordinated to Ni1 and perchlorate oxygen (H102···O9 = 1.933 Å). Extensive C−H···O interaction20,21 is also observed between perchlorate oxygen and C−H of the methylene spacer (2.59−2.70 Å) of the ligand. Hydrolysis of Ethyl Acetate. Synthesis and Structure of [NiII5(H2L)2(μ3-OH)2(μ-O2CCH3)4][ClO4]2·CH3CO2C2H5 (2). As discussed above, 1 is expected to be a potential candidate for hydrolytic activity. It has a strongly NiII-coordinated water molecule, which is activated by metal-ion coordination and participates in a strong H-bonding interaction, making it Scheme 1. Hydrolysis of Ethyl Acetate by 1

D


Article

Inorganic Chemistry

the phenolic H123 is strongly H-bonded to another bridging acetate O7 with a H125···O7 length of 1.660 Å. Extensive C− H···O interactions19,20 are also observed between perchlorate oxygen and C−H of the methylene spacer (2.341−2.617 Å) of the ligand and methyl C−H of the acetate (2.708 Å). Solvent (Vapor)-Induced Single-Crystal-to-Single-Crystal Structural Transformation with Concomitant Hydrolysis of Ethyl Acetate. Synthesis and Structure of [NiII2(HL)(μO2CCH3)(H2O)2][ClO4]·4H2O (3). During hydrolysis of ethyl acetate and formation of 2, it is reasonable to assume that at some stage one of the NiII-coordinated water molecules would have caused hydrolysis of an ethyl acetate molecule to generate a dimeric complex bridged by an acetate. Unfortunately, this “dimeric species” could not be crystallized from such a solution, as all such attempts always led to the formation of 2. Interestingly, when the crystals of 1, moistened with a few drops of ethyl acetate, were kept for ethyl acetate vapor diffusion, they transform into a monoacetato-bridged complex [NiII2(HL)(μ-O2CCH3)(H2O)2][ClO4]·4H2O (3) (Figure 5 and Table 4). The presence of acetate and perchlorate is revealed by IR spectroscopy.

Table 3. Selected Bond Lengths (angstroms) and Angles (degrees) of [NiII5(H2L)(OH)2(O2CCH3)4][ClO4]2· CH3CO2C2H5 (2) Ni1−O7 Ni1−O2 Ni1−O4

2.108(3) 2.063(3) 2.071(3)

Ni2−N3 Ni2−O1 Ni2−O2 Ni2−O8 Ni2−O6 Ni2−O4

2.124(4) 2.010(3) 2.062(3) 2.058(3) 2.115(3) 2.093(3)

Ni3−N1 Ni3−N2 Ni3−O1 Ni3−O3 Ni3−O5 Ni3−O4

2.100(4) 2.136(4) 1.992(3) 2.127(3) 2.039(3) 2.079(3)

Ni1···Ni2 Ni2···Ni3 Ni1···Ni3

3.123 2.995 3.897

Ni1−O2−Ni2 Ni1−O4−Ni2 Ni2−O1−Ni3 Ni3−O4−Ni2 Ni3−O4−Ni1 O7−Ni1−O2 O4−Ni1−O2

98.45(11) 97.18(12) 96.90(12) 91.77(11) 139.78(14) 88.89(11) 78.28(11)

O1−Ni2−O2 O1−Ni2−O8 O1−Ni2−O6 O1−Ni2−O4 O1−Ni2−N3 O2−Ni2−O8 O2−Ni2−O6 O2−Ni2−O4 O2−Ni2−N3 O8−Ni2−O6 O8−Ni2−O4 O8−Ni2−N3 O6−Ni2−O4 O6−Ni2−N3 O4−Ni2−N3 O1−Ni3−O3 O1−Ni3−O5 O1−Ni3−O4 O1−Ni3−N1 O1−Ni3−N2 O3−Ni3−O5 O3−Ni3−O4 O3−Ni3−N1 O3−Ni3−N2 O5−Ni3−O4 O5−Ni3−N1 O5−Ni3−N2 O4−Ni3−N1 O89−Ni3−N2 N1−Ni3−N2

102.55(11) 167.33(11) 84.32(11) 83.26(11) 90.72(13) 89.74(11) 169.03(11) 77.81(11) 90.55(12) 83.08(11) 96.52(11) 92.26(12) 94.73(11) 97.98(12) 165.33(12) 86.93(11) 90.30(12) 84.05(11) 91.15(13) 176.63(13) 177.07(12) 91.41(11) 86.30(13) 91.32(13) 89.26(11) 92.79(13) 91.40(14) 174.79(13) 98.89(13) 85.86(14)

Figure 5. Perspective view of the cation [NiII2(HL)(μ-O2CCH3)(H2O)2][ClO4]·4H2O (3).

The X-ray structure of 3 reveals that the dinuclear unit in 3 is bridged by the acetate (O4, O5) and by the phenolate O1. The Ni···Ni distance is 3.562 Å, and the Ni1−O1−Ni2 bond angle is 120.19(11)°. Both metal centers are six-coordinate with a N2O4 coordination environment. For Ni1, the pseudooctahedral arrangement is provided by two alkyl amines (N1 and N2), a central bridging phenolate (O1), a terminal phenolate (O2), an acetate (O4), and another oxygen (O3) from the coordinated water molecule. The coordination sphere for Ni2 consists of two alkyl amines (N3 and N4), a central bridging phenolate (O1), a terminal phenol (O7), an acetate (O5), and a coordinated water (O6). Unlike O2, O7 is protonated, and this leads to the lengthening of the bond with the nickel(II) center [Ni1−O2 = 2.047(2) Å vs Ni2−O7 = 2.126(3) Å].7f H101 of the coordinated water (O6) is in a strong intramolecular H-bonding interaction (Figure S2 and Table S7) with the phenolate (O2) coordinated to the other nickel(II) center (H101···O2 = 1.659 Å). H103 of the other coordinated water (O3) is involved in intermolecular Hbonding with the phenolate (O2) of a neighboring molecule (H101···O2 = 1.945 Å). On the other hand, H104 of the coordinated water (O3) is involved in intermolecular Hbonding with the acetate oxygen (H104···O4 = 2.012 Å). There are extensive H-bonding interactions present involving perchlorate oxygens and O−H of coordinated water (H102··· O555 = 2.171 Å), phenolic−OH (H100···O100 = 1.823 Å),

a Symmetry transformations used to generate equivalent atoms: 1 − x, 2 − y, −z.

hence remains uncoordinated. The proton source is due to the generation of acetic acid during the hydrolysis of ethyl acetate by 1 (Scheme 1). The remaining coordination sites of Ni2 are satisfied by a bridging hydroxo group (O4) and two bridging acetates (O8 and O6). The coordination sphere for Ni3 consists of two tertiary amines (N1 and N2), a bridging phenolate (O1), a μ3-bridging hydroxo group (O4), a coordinated acetate (O5), and another phenolate (O3). Because of the presence of the H atom on the phenolic O3, the interaction with the metal center (Ni3) is weaker than that of a phenolate. This leads to the lengthening of the bond [Ni3−O3 = 2.127(3) Å],7f compared to that of the bridging phenolate [Ni3−O1 = 1.992(3) Å]. Analysis of the crystal packing diagram shows that appreciable H-bonding interactions are present in 2 (Figure S1). H atom H125, attached to the uncoordinated and protonated N4, is strongly H-bonded to the bridging acetate (O6) with a H125···O6 length of 1.860 Å (Table S6). Similarly, E


Article

Inorganic Chemistry Table 4. Selected Bond Lengths (angstroms) and Angles (degrees) of [NiII2(HL)(O2CCH3)(H2O)2][ClO4]·4H2O (3) Ni1−N1 Ni1−N2 Ni1−O1 Ni1−O2 Ni1−O3 Ni1−O4

2.085(3) 2.177(3) 2.067(2) 2.047(2) 2.129(2) 2.073(2)


2.089(3) 2.154(3) 2.043(2) 2.048(2) 2.055(3) 2.126(3)

Ni1···Ni2

3.562

Ni1−O1−Ni2 O2−Ni1−O1 O2−Ni1−O3 O2−Ni1−O4 O2−Ni1−N1 O2−Ni1−N2 O3−Ni1−O1 O3−Ni1−O4 O3−Ni1−N1 O3−Ni1−N2 O4−Ni1−O1 O4−Ni1−N1 O4−Ni1−N2 N1−Ni1−N2 N1−Ni1−O1 N2−Ni1−O1 O5−Ni2−O1 O5−Ni2−O6 O5−Ni2−O7 O5−Ni2−N3 O5−Ni2−N4 O6−Ni2−O1 O6−Ni2−O7 O6−Ni2−N3 O6−Ni2−N4 O7−Ni2−N3 O7−Ni2−N4 N3−Ni2−O1 N3−Ni2−N4 N4−Ni2−O1

120.19(11) 91.48(9) 82.46(9) 91.56(9) 91.10(10) 170.26(10) 172.26(9) 83.92(9) 92.55(10) 88.89(10) 91.46(9) 175.26(9) 91.97(10) 84.79(11) 92.38(10) 97.49(10) 87.96(9) 94.32(10) 82.61(9) 91.99(10) 174.92(11) 95.41(9) 81.52(10) 166.59(10) 90.05(11) 87.58(10) 95.49(10) 96.64(10) 83.21(11) 94.22(10)

Figure 6. Perspective view of the cation [NiII2(H2L)(μ-O2CCH3)2][ClO4]·CH2Cl2·2H2O (4).

Table 5. Selected Bond Lengths (angstroms) and Angles (degrees) of [NiII2(H2L1)(μ-O2CCH3)2][ClO4]·CH2Cl2· 2H2O (4)

and C−H of the terminal phenol ring (H047···O100 = 2.499 Å). Synthesis and Structure of [NiII2(H2L)(μ-O2CCH3)2][ClO4]·CH2Cl2·2H2O (4). Complexes 2 and 3 showed acetato-bridged nickel(II) complexes, and acetate(s) was obtained from ethyl acetate hydrolysis. We next investigated the product formed from direct reaction of H3L and Ni(O2CCH3)2·4H2O to determine whether structures similar to 2 or 3 are formed. In fact, the reaction of H3L with 2 equiv of NiII(O2CCH3)2·4H2O, followed by addition of 1 equiv of NaClO4·H2O, and recrystallization from CH2Cl2 led to the isolation of [NiII2(H2L)(μ-O2CCH3)2][ClO4]·CH2Cl2·2H2O (4), which is notably different from both 2 and 3. This observation proves that the formation of 2 and 3 is not caused by the presence of free nickel(II) and acetate ions in solution [formed by probable hydrolysis of ethyl acetate by free nickel(II) ions] but is caused by hydrolysis of ethyl acetate, promoted by NiII-coordinated water in 1. The presence of acetates and perchlorates is characterized by their IR stretching vibrations. X-ray structural study reveals that the dinuclear unit in 4 (Figure 6 and Table 5) is bridged by an endogenous phenoxide O1 and two exogenous acetates with a Ni1···Ni1# separation of 3.405 Å and a Ni1−O1−Ni1# angle of 115.1(6)°. The asymmetric unit of this complex consists of two molecules. Because bond lengths, bond angles, and the geometry around the NiII centers are quite similar for both molecules (Table 5), structural parameters for only one of them are discussed. Both metal centers in 4 have a N2O4 coordination environment with


2.081(13) 2.170(11) 2.016(9) 2.129(11) 2.031(11) 2.067(11)


2.079(13) 2.147(12) 2.037(9) 2.103(11) 2.064(9) 2.030(11)

Ni1···Ni1a

3.405

Ni2···Ni2a

3.401

Ni1−O1−Ni1a O2−Ni1−O1 O2−Ni1−O3 O2−Ni1−O4 O2−Ni1−N1 O2−Ni1−N2 O3−Ni1−O1 O3−Ni1−O4 O3−Ni1−N1 O3−Ni1−N2 O4−Ni1−O1 O4−Ni1−N1 O4−Ni1−N2 N1−Ni1−N2 N1−Ni1−O1 N2−Ni1−O1

115.1(6) 167.9(4) 83.8(4) 79.1(4) 89.3(4) 93.4(5) 95.3(5) 94.0(4) 170.0(4) 90.1(4) 88.9(4) 91.8(4) 171.0(5) 83.0(4) 93.0(5) 98.7(4)

Ni2−O5−Ni2b O6−Ni2−O5 O6−Ni2−O7 O6−Ni2−O8 O6−Ni2−N3 O6−Ni2−N4 O7−Ni2−O5 O7−Ni2−O8 O7−Ni2−N3 O7−Ni2−N4 O8−Ni2−N3 O8−Ni2−N4 N3−Ni2−O5 N3−Ni2−N4 N4−Ni2−O5

112.9(6) 169.1(4) 80.5(4) 84.0(4) 89.3(4) 92.9(5) 88.6(4) 95.2(4) 91.0(4) 170.9(5) 169.9(4) 90.3(4) 92.0(5) 82.5(4) 98.0(4)

Symmetry transformations used to generate equivalent atoms: −x, −x + y, 1.66 − z. bSymmetry transformations used to generate equivalent atoms: *x − y, −y, 1.33 − z. a

pseudo-octahedral geometry. The six-coordination environment of the nickel(II) centers is satisfied by two tertiary amines (N1 and N2), a bridging phenolate (O1), a terminal phenol (O2), and two acetates (O3 and O4). The presence of terminal phenolic−OH (O2) is reflected by the lengthening of the bond, compared to that of the bridging phenolate O1: Ni1−O2 = 2.129(11) Å versus Ni1−O1 = 2.016(9) Å.7f Hydrolysis of Ethyl Propionate. Synthesis and Structure of [NiII2(HL)(μ-O2CCH2CH3)(H2O)2][ClO4] (5). When 1 was suspended in ethyl propionate and stirred for 6 F


Article

Inorganic Chemistry

propionate (O4), and a coordinated water (O3). On the other hand, the coordination sphere for Ni2 consists of two alkyl amines (N3 and N4), a central bridging phenolate (O1), a terminal phenolic-OH (O6), a propionate (O5), and a coordinated water (O7). Unlike O2, O6 is protonated, and this leads to the lengthening of the bond to the nickel(II) center [Ni1−O2 = 2.059(3) Å vs Ni2−O6 = 2.220(4) Å].7f As in 3, extensive H-bonding interactions are present in 5 (Figure S3 and Table S8). The coordinated water (O7) is interacting (intramolecular) with the phenolate oxygen (H103···O2 = 1.809 Å). The hydrogens of another water molecule (O3) are involved in intermolecular H-bonding with the phenolate oxygen of a neighboring molecule (H101···O2 = 1.964 Å) and propionate oxygen (O4) (H102···O4 = 2.170 Å). There are extensive H-bonding interactions present involving perchlorate oxygens and O−H of coordinated water (H104··· O11 = 2.419 Å, and H104···O8 = 2.437 Å) and phenolic-OH (H100···O9 = 2.639 Å, and H100···O9 = 2.234 Å). Mechanistic Insights into Carboxyester Hydrolysis. We have already discussed that 1 exhibits solvent (vapor)induced stoichiometric hydrolysis of ethyl acetate to produce [NiII5(H2L)2(μ3-OH)2(μ-O2CCH3)4][ClO4]2·CH3CO2C2H5 (2). This type of vapor-induced hydrolysis of ethyl acetate at 298 K by a hydroxo-bridged dicopper(II) complex has been reported by us.22 Meyer and co-workers also reported the hydrolysis of ethyl acetate starting with a hydroxo-coordinated dinickel(II) complex but at a higher temperature (348 K).7b Borovik and co-workers also demonstrated hydrolysis of ethyl acetate by a hydroxo-bridged dicobalt(II) complex at room temperature.6 However, hydrolysis of an unactivated carboxyester like ethyl acetate at room temperature by a MIIcoordinated water molecule acting as a nucleophile has not so far been reported. Synthesis of monoacetato-bridged dimer [NiII2(HL)(μO2CCH3)(H2O)2][ClO4]·4H2O (3) has been accomplished via single-crystal-to-single-crystal structural transformation. From a mechanistic perspective, one may consider that during the course of the reaction one of the NiII-coordinated water molecules in 1 brings about hydrolysis of one ethyl acetate molecule to produce a monoacetato-bridged dimeric core 3. Notably, the proton liberated is consumed by a terminal phenolate ion, which now coordinates in its phenol form. The space-filling model of 1 shows that the metal center Ni2 is exposed (Figure 8) and can be approached by an external substrate. Prior to hydrolysis, ethyl acetate is expected to be coordinated to the metal center. To establish this, we performed systematic IR spectroscopic studies. In fact, 10 mg of 1 was suspended in 1 mL of ethyl acetate and held at 298 K. After 1 day, the IR spectrum displayed a new peak at 1736 cm−1 (Figure S4 and Table 7), suggestive of CO coordination. Complex 1 does not exhibit any peak at this position. Free ethyl acetate (without 1) showed the stretching vibration of the C O group at 1768 cm−1 (Figure S4 and Table 7). A shift of the CO streaching vibration to a lower energy implies coordination of ethyl acetate to the nickel(II) center in 1. After 7 days, IR spectra showed strong vibration for coordinated acetate at 1572 and 1474 cm−1 (Figure S4 and Table 7). These experiments attest to the formation of acetatobridged dimer 3. Additionally, a peak was also observed at 1731 cm−1, which indicates coordination of ethyl acetate to 3. In 3, there is an activated water (Ni2−O6 = 2.055 Å) strongly bound to Ni2 and another water weakly bound to Ni1 (Ni1−O3 = 2.129 Å). Thus, in the presence of excess ethyl acetate, the ester

h at 298 K, a propionate-bridged dinickel(II) complex [NiII2(HL)(μ-O2CCH2CH3)(H2O)2][ClO4] (5) was isolated. Here an ethyl propionate molecule is hydrolyzed to produce a propionate-bridged dimeric structure (Figure 7 and Table 6),

Figure 7. Perspective view of the cation [NiII2(HL)(μ-O2CCH2CH3)(H2O)2][ClO4] (5).

and the liberated proton is consumed by one of the terminal bromophenolates, O6. The presence of propionate and perchlorates is revealed by their IR stretching vibrations. Table 6. Selected Bond Lengths (angstroms) and Angles (degrees) of [NiII2(HL)(O2CCH2CH3)(H2O)2][ClO4] (5) Ni1−N1 Ni1−N2 Ni1−O1 Ni1−O2 Ni1−O3 Ni1−O4

2.087(4) 2.203(4) 2.069(3) 2.059(3) 2.160(4) 2.052(3)

Ni2−N3 Ni2−N4 Ni2−O5 Ni2−O6 Ni2−O7

2.076(4) 2.140(4) 2.019(3) 2.220(4) 2.056(4)

Ni1···Ni2

3.552

Ni1−O1−Ni2 O2−Ni1−O1 O2−Ni1−O3 O2−Ni1−O4 O2−Ni1−N1 O2−Ni1−N2 O3−Ni1−O1 O3−Ni1−O4 O3−Ni1−N1 O3−Ni1−N2 O4−Ni1−O1 O4−Ni1−N1 O4−Ni1−N2 N1−Ni1−N2 N1−Ni1−O1 N2−Ni1−O1 O5−Ni2−O1 O5−Ni2−O6 O5−Ni2−O7 O5−Ni2−N3 O5−Ni2−N4 O6−Ni2−O1 O6−Ni2−O7 O6−Ni2−N3 O6−Ni2−N4 O7−Ni2−N3 O7−Ni2−N4 N3−Ni2−O1 N3−Ni2−N4 N4−Ni2−O1

119.39(15) 93.24(12) 82.05(14) 93.27(13) 90.27(14) 168.61(14) 173.46(13) 83.39(14) 93.52(15) 88.25(15) 92.39(12) 174.93(14) 91.51(15) 84.36(16) 91.03(14) 96.88(14) 88.18(13) 79.94(14) 95.07(15) 92.39(14) 174.22(14) 167.99(13) 85.35(15) 86.42(14) 95.30(15) 167.76(15) 87.76(16) 95.88(14) 84.00(15) 96.66(14)

The X-ray structure of 5 reveals that the two nickel(II) centers are bridged by the phenolate O1 and a propionate, with a Ni···Ni distance of 3.552 Å and a Ni1−O1−Ni2 angle of 119.39(15)°. Both metal centers are six-coordinate with a N2O4 coordination environment. For Ni1, the pseudo-octahedral arrangement is provided by two alkyl amines (N1 and N2), a central bridging phenolate (O1), a terminal phenolate (O2), a G


Article

Inorganic Chemistry

molecular event of binding of esters followed by nucleophilic attack by NiII-coordinated H2O is justified (Scheme 2). Scheme 2. Binding of Carboxyester to Ni(II) and Subsequent Hydrolysis

Figure 8. Space-filling model for the structure of 1, showing the vacant site at the nickel(II) center.

Table 7. IR Spectral Data of Neat Ethyl Acetate and Complex 1 with Ethyl Acetate at Different Time Intervals neat ethyl acetate complex 1 and ethyl acetate complex 1 and ethyl acetate

time (days)

ν (cm−1)

− 1 7

1768 1736 1572, 1474

Potentiometric Titration. To evaluate the pKa values of the coordinated water molecules in 1, potentiometric titration was performed. The titration experiment leads to the consumption of 2 mol of NaOH per mole of the complex (Figure S6). This indicates the presence of two deprotonation equilibria as shown in Scheme 3. After treatment of the data, the pKa values were calculated as 6.80 ± 0.12 (pKa1) and 9.33 ± 0.06 (pKa2). pKa1 corresponds to the formation of monohydroxo species “(H2O)NiII(OH)NiII(H2O)2” (RH in Scheme 3), while pKa2 accounts for the formation of species “(H2O)NiII(OH)NiII(OH)(H2O)” (R in Scheme 3). The species distribution plot for 1 is displayed in Figure S7, which reveals that below pH 6.8 the aqua species RH2 is the major species in solution. In the pH range of 6.8−9.3, the monohydroxo species (RH) predominates. Above pH 9.3, the bis-hydroxo species R is the major species present in solution. Magnetic Properties. To gain insight into the type and extent of magnetic interaction present in 1, 2, 4, and 5, temperature-dependent (2−300 K) magnetic susceptibility measurements were taken. The χMT value for 1 at 300 K is found to be 2.14 cm3 mol−1 K (Figure 9) and gradually decreases with a decrease in temperature, approaching zero below 10 K. This indicates the presence of antiferromagnetic interaction between the nickel(II) ions in 1. To evaluate the antiferromagnetic-exchange coupling, the data were analyzed according to the following Hamiltonian (eq 2):

is expected to displace the loosely bound water, can coordinate to the NiII center, and become hydrolyzed by the activated water. This leads to the formation of 2 with subsequent structural rearrangement. The formation of pentanuclear species 2 is not well-understood, but the stoichiometry of the reaction can be described by eq 1. 5[Ni II 2(L)(H 2O)2 (CH3OH)][ClO4 ] + 8CH3CO2 C2H5 + 2H 2O → 2[Ni II 5(H 2L)2 (μ3‐OH)2 (μ‐O2 CCH3)4 ][ClO4 ]2 + 8C2H5OH + 5CH3OH + [H4L][ClO4 ]

(1)

Notably, 1 is also capable of hydrolyzing ethyl propionate. When solid 1 was mixed with ethyl propionate and stirred for 6 h at 298 K, a propionato-bridged dinickel(II) complex, [NiII2(HL)(μ-O2CCH2CH3)(H2O)2][ClO4] (5), was isolated. This complex is structurally similar to 3 but with a propionate bridge, instead of acetate bridge like that in 3. For a comparative study, with ethyl acetate, a similar procedure was employed; i.e., 1 was mixed with ethyl acetate, followed by stirring for 6 h. The IR spectrum was recorded and compared with that obtained by using ethyl propionate. After 6 h, the product obtained from the “1/ethyl propionate” mixture showed strong stretching vibrations for coordinated propionate at 1585 cm −1 [ν(O 2 CCH 3 ) asym ] and 1477 cm −1 [ν(O2CCH3)sym]. The product obtained from the “1/ethyl acetate” mixture showed similar IR spectra as observed with only 1 (Figure S5), indicating no ethyl acetate hydrolysis. However, after 7−10 days, the “1/ethyl acetate” mixture shows a vibrational peak for acetate coordination. This demonstrates that hydrolysis of ethyl propionate is easier than that of ethyl acetate. Considering the enhanced electron-releasing power (through an inductive effect) of ethyl (ethyl propionate) over methyl (ethyl acetate), it is reasonable to assume stronger binding in the case of ethyl propionate. Thus, the expected

H = −JS1S2 + D(S1z 2 + S2z 2 + 4/3) + βHg (S1 + S2) (2)

where D is the single-ion zero-field splitting parameter for nickel(II), H is the applied magnetic field, J is the magneticexchange integral, and β is the Bohr magneton. The best-fit parameters are as follows: J = −25.0(1) cm−1, g = 2.16(1), and D = ±1.0(3) cm−1. Thus, two nickel(II) centers in 1 are interacting antiferromagnetically through the phenoxo bridge [Ni1−O1−Ni2 = 121.22(11)°]. H


Article

Inorganic Chemistry Scheme 3. Proposed Equilibria in Solution for 1, with a Change in pHa

a

The number of hydrogens corresponds to the number of protons that could be deprotonated.

Scheme 4. Ni5 Skeleton Showing the Possible MagneticExchange Interactions in 2

Heisenberg−Dirac−van Vleck (HDvV) Hamiltonian as described in eq 3: H = Hexchange + Hzfs + HZeeman ̂ S3a ̂ ) − J (S1̂ S2̂ + S1̂ S2a ̂ ) − J (S1̂ S3̂ + S1̂ S3a ̂ ) Hexchange = −Ja (S2̂ S3̂ + S2a b c

Figure 9. Temperature dependence of magnetic data for 1.

2

2

2

2

2

Hzfs = D(Sẑ 1 + Sẑ 2 + Sẑ 3 + Sẑ 2a + Sẑ 3a − 10/3)

Figure 10 depicts the temperature-dependent magnetic susceptibility plot for 2. The χMT value for 2 at 300 K is

̂ + S3a ̂ ) HZeeman = gβH(S1̂ + S2̂ + S3̂ + S2a

(3) II

where g and D for all Ni ions were considered identical to avoid overparametrization. The best-fit parameters are as follows: Ja = +16.02 cm−1, Jb = +9.02 cm−1, Jc = −49.7 cm−1, g = 2.06, and D = 9.91 cm−1. The observed parameters indicate moderate ferromagnetic interaction between the phenoxobridged centers Ni2−Ni3 (+16.02 cm−1), Ni1−Ni2 (+9.02 cm−1), Ni2a−Ni3a (+16.02 cm−1), and Ni1−Ni2a (+9.02 cm−1). The Ni2−O1−Ni3 angle along the phenoxo bridge and the Ni2−O4−Ni3 angle along the hydroxo bridge are 96.90° and 91.77°, respectively. Similarly, from structural analysis, the Ni1−O2−Ni2 angle is 98.45° (along the phenoxo bridge) and the Ni1−O4−Ni2 angle is 97.18° (along the hydroxo bridge), which were observed between the Ni1 and Ni2 centers. Because these bridging angles are close to 90°, orthogonality of the magnetic orbitals results in the observed ferromagnetism. Here the effects of phenoxo−hydroxo bridging angles predominate over the magnetic exchange through syn-syn bridging acetates. The bridging angles (through phenoxo and hydroxo) between Ni2 and Ni3 are closer to 90°, compared to those between Ni1 and Ni2. This is reflected in comparatively stronger ferromagnetic interaction between Ni2 and Ni3. Between Ni1 and Ni3, there is only a hydroxo bridge. The absence of any additional bridging (like phenoxo or acetate) groups increases the angle between Ni1 and Ni3 to 139.78(14)° (Ni1−O4−Ni3). Thus, the larger bridging angle along the hydroxo bridge leads to reasonable antiferromagnetic interaction between Ni1 and Ni3 with a Jc of −49.7 cm−1. Recently, two pentanuclear nickel(II) clusters have been reported23 with similar topologies, like that of 2, however, differing in the bridging groups. The exchange integral (J) values were lower than those of 2, and in both reported cases, two magnetic-exchange values were observed.22 The magnetic behavior of 4 (Figure S8) and 5 (Figure S9) is quite similar. At 300 K, both of them show a χMT value of ∼2.2

Figure 10. Temperature dependence of magnetic data for 2.

4.96 cm3 mol−1 K, which is slightly lower than the spin-only value expected for a cluster of five noninteracting S = 1 NiII ions. A further decrease in temperature leads to a decrease in the χMT value until a minimum at 38 K (3.86 cm3 mol−1 K) is reached. This indicates the presence of antiferromagnetic interaction. However, below 38 K, a sharp increase in χMT is observed up to 10 K (4.69 cm3 mol−1 K). Thus, the overall magnetic behavior in the temperature range of 10−300 K denotes the existence of both ferromagnetic and antiferromagnetic interactions in 2. Below 10 K, the χMT values decrease sharply to 3.76 cm3 mol−1 K at 2 K. The sudden decrease in χMT below 10 K can be attributed to zero-field splitting within the ground state, Zeeman effects, and/or weak antiferromagnetic intermolecular interactions.23a Given the large Ni2··· Ni3a/Ni3···Ni2a distance of 6.397 Å, the exchange interaction between them can be neglected (Scheme 4). In addition to that, because of the symmetry of 2, the magnetic interactions can be expressed in terms of three exchange (J) values as shown in Scheme 4. For the magnetic data fitting of 2, the zero-field and Zeeman splitting effects were included in the isotropic I


Article

Inorganic Chemistry cm3 mol−1 K, and with a decrease in temperature, it decreases and approaches zero below 10 K. This indicates the existence of antiferromagnetic interaction between the nickel(II) ions. Magnetic data for both the complexes were analyzed according to the Hamiltonian in eq 2. This leads to the following parameters: g = 2.20(1), D = 5.2(1), and J = −20.0(1) for 4, and g = 2.18(1), D = 4.8(2), and J = −18.80(8). These values are in the ranges observed for phenoxo−acetato-bridged dinickel(II) systems reported by us10a and others.18a,24

■

AUTHOR INFORMATION

Corresponding Author

■

*E-mail: [email protected] or [email protected]. Telephone: +91-33-2597437. Fax: +91-33-2597436.

SUMMARY AND CONCLUDING REMARKS In this work, synthesis of a dinickel(II) complex [NiII2(L)(H2O)2(CH3OH)][ClO4]·CH3OH (1) with readily accessible labile coordination sites has been achieved. One of the water molecules is activated not only because of its strong coordination to the NiII center but also because of strong intramolecular H-bonding interaction, thereby making it more nucleophilic. Ethyl acetate vapor diffusion to a CH3CN/ CH3OH solution of 1 results in the formation of a pentanuclear acetato-bridged complex [NiII5(H2L)2(μ3-OH)2(μ-O2CCH3)4][ClO4]2CH3CO2C2H5 (2), demonstrating for the first time the metal-coordinated water-promoted hydrolysis of a carboxyester at room temperature. When the crystals of 1, moistened with a few drops of ethyl acetate, were kept for ethyl acetate vapor diffusion, they transform to a monoacetato-bridged complex [NiII2(HL)(μ-O2CCH3)(H2O)2][ClO4]·4H2O (3). When suspended in ethyl propionate and stirred, 1 afforded propionatobridged complex [NiII2(HL)(μ-O2CCH2CH3)(H2O)2][ClO4] (5). To rationalize the formation of 2 and 5, the diacetatobridged complex was isolated, [NiII2(H2L)(μ-O2CCH3)2][ClO4]·CH2Cl2·2H2O (4), from direct reaction between Ni(O2CCH3)2·4H2O and the ligand H3L. This kind of solvent (vapor)-induced structural transformation with concomitant hydrolysis of carboxyesters RCO2C2H5 (R = CH3 and CH3CH2) under ambient conditions is unprecedented. To shed light on the mechanistic insights into the hydrolysis of carboxyesters, IR spectroscopic studies confirm the coordination of the esters to the nickel(II) center for nucleophilic attack by the NiII-coordinated water. Temperature-dependent magnetic studies of dimeric complexes 1, 4, and 5 reveal antiferromagnetic coupling. For 2, three types of magneticexchange interactions are observed (two are ferromagnetic, and one is antiferromagnetic), and overall, the magnetic-exchange interaction is antiferromagnetic.

■

data for 4 (Figure S8) and 5 (Figure S9); crystal data and structural refinement for 1−5 (Tables S1−S5, respectively); and H-bonding parameters for 2 (Table S6), 3 (Table S7), and 5 (Table S8) (PDF) Crystallographic information (CIF)

ORCID

Rabindranath Mukherjee: 0000-0003-0739-5896 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is supported by a J. C. Bose fellowship from the Department of Science & Technology (DST), Government of India, to R.M. R.M. sincerely thanks DST for this fellowship. S.K.B. gratefully acknowledges the award of SRF by the Council of Scientific & Industrial Research, Government of India.

■

REFERENCES

(1) Chin, J. Developing artificial hydrolytic metalloenzymes by a unified mechanistic approach. Acc. Chem. Res. 1991, 24, 145−152. (2) (a) Breslow, R. Artificial Enzymes; Wiley-VCH: Weinheim, Germany, 2005. (b) Penkova, L. V.; Maciag, A.; Rybak-Akimova, E. V.; Haukka, M.; Pavlenko, V. A.; Iskenderov, T. S.; Kozłowski, H.; Meyer, F.; Fritsky, I. O. Efficient Catalytic Phosphate Ester Cleavage by Binuclear Zinc(II) Pyrazolate Complexes as Functional Models of Metallophosphatases. Inorg. Chem. 2009, 48, 6960−6971 and references therein. (3) Sträter, N.; Lipscomb, W. N.; Klabunde, T.; Krebs, B. Two-Metal Ion Catalysis in Enzymatic Acyl- and Phosphoryl-Transfer Reactions. Angew. Chem., Int. Ed. Engl. 1996, 35, 2024−2055. (4) (a) Göbel, M. W. Binuclear Metal Complexes as Efficient Intermediaries in Biochemically Relevant Hydrolysis Reactions. Angew. Chem., Int. Ed. Engl. 1994, 33, 1141−1143. (b) Williams, N. H.; Takasaki, B.; Wall, M.; Chin, J. Structure and Nuclease Activity of Simple Dinuclear Metal Complexes: Quantitative Dissection of the Role of Metal Ions. Acc. Chem. Res. 1999, 32, 485−493. (c) Mitić, N.; Smith, S. J.; Neves, A.; Guddat, L. W.; Gahan, L. R.; Schenk, G. The Catalytic Mechanisms of Binuclear Metallohydrolases. Chem. Rev. 2006, 106, 3338−3363. (d) Schenk, G.; Mitić, N.; Gahan, L. R.; Ollis, D. L.; McGeary, R. P.; Guddat, L. W. Binuclear Metallohydrolases: Complex Mechanistic Strategies for a Simple Chemical Reaction. Acc. Chem. Res. 2012, 45, 1593−1603. (e) Daumann, L. J.; Schenk, G.; Ollis, D. L.; Gahan, L. R. Spectroscopic and mechanistic studies of dinuclear metallohydrolases and their biomimetic complexes. Dalton Trans. 2014, 43, 910−928. (5) (a) Zhao, M.; Wang, H.-B.; Ji, L.-N.; Mao, Z.-W. Insights into metalloenzyme microenvironments: biomimetic metal complexes with a functional second coordination sphere. Chem. Soc. Rev. 2013, 42, 8360−8375; Chem. Soc. Rev. 2013, 42, 8360−8375. (b) de Souza, B.; Kreft, G. L.; Bortolotto, T.; Terenzi, H.; Bortoluzzi, A. J.; Castellano, E. E.; Peralta, R. A.; Domingos, J. B.; Neves, A. Second-CoordinationSphere Effects Increase the Catalytic Efficiency of an Extended Model for FeIIIMII Purple Acid Phosphatases. Inorg. Chem. 2013, 52, 3594− 3596. (c) Cook, S. A.; Borovik, A. S. Molecular Designs for Controlling the Local Environments around Metal Ions. Acc. Chem. Res. 2015, 48, 2407−2414. (6) Zinn, P. J.; Sorrell, T. N.; Powell, D. R.; Day, V. W.; Borovik, A. S. Acetonitrile Hydration and Ethyl Acetate Hydrolysis by PyrazolateBridged Cobalt(II) Dimers Containing Hydrogen-Bond Donors. Inorg. Chem. 2007, 46, 10120−10132.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01895. Perspective views of C−H···O, N−H···O, and O−H···O interactions present in 2 (Figure S1), C−H···O and O− H···O interactions present in 3 (Figure S2), and O−H··· O interactions present in 5 (Figure S3); IR spectra showing coordination of ethyl acetate in 1 during hydrolysis (Figure S4); IR spectra showing sluggish hydrolysis of ethyl acetate compared to that of ethyl propionate (Figure S5); titration curve obtained by titrating 1 with NaOH in a CH3CN/H2O solution [50:50 (v/v)] (Figure S6); species distribution curve for 1 in a CH3CN/H2O solution [50:50 (v/v)] as a function of pH (Figure S7); temperature dependence of magnetic J


Article

Inorganic Chemistry (7) (a) Frey, S. T.; Murthy, N. N.; Weintraub, S. T.; Thompson, L. K.; Karlin, K. D. Hydrolysis of Unactivated Esters and Acetonitrile Hydration by a Hydroxo−Dicopper(II) Complex. Inorg. Chem. 1997, 36, 956−957. (b) Meyer, F.; Kaifer, E.; Kircher, P.; Heinze, K.; Pritzkow, H. Cooperative Transformations of Small Molecules at a Dinuclear Nickel(II) Site. Chem. - Eur. J. 1999, 5, 1617−1630. (c) Subat, M.; Woinaroschy, K.; Anthofer, S.; Malterer, B.; König, B. 1,4,7,10-Tetraazacyclododecane Metal Complexes as Potent Promoters of Carboxyester Hydrolysis under Physiological Conditions. Inorg. Chem. 2007, 46, 4336−4356. (d) Zhou, Y.-H.; Zhao, M.; Li, J.H.; Mao, Z.-W.; Ji, L.-N. Carboxylic ester hydrolysis catalyzed by a host−guest system constructed by cyclodextrin dimer and zinc complex. J. Mol. Catal. A: Chem. 2008, 293, 59−64. (e) Xue, S.-S.; Zhao, M.; Ke, Z.-F.; Cheng, B.-C.; Su, H.; Cao, Q.; Cao, Z.-K.; Wang, J.; Ji, L.-N.; Mao, Z.-W. Mao. Enantioselective Hydrolysis of Amino Acid Esters Promoted by Bis(β-cyclodextrin) Copper Complexes. Sci. Rep. 2016, 6, Article no. 22080, doi: 2208010.1038/srep22080. (f) Piovezan, C.; Silva, J. M. R.; Neves, A.; Bortoluzzi, A. J.; Haase, W.; Tomkowicz, Z.; Castellano, E. E.; Hough, T. C. S.; Rossi, L. M. Design of a Dinuclear Nickel(II) Bioinspired Hydrolase to Bind Covalently to Silica Surfaces: Synthesis, Magnetism, and Reactivity Studies. Inorg. Chem. 2012, 51, 6104−6115. (8) (a) Comba, P.; Gahan, L. R.; Mereacre, V.; Hanson, G. R.; Powell, A. K.; Schenk, G.; Zajaczkowski-Fischer, M. Spectroscopic Characterization of the Active FeIIIFeIII and FeIIIFeII Forms of a Purple Acid Phosphatase Model System. Inorg. Chem. 2012, 51, 12195− 12209. (b) Bosch, S.; Comba, P.; Gahan, L. R.; Schenk, G. Dinuclear Zinc(II) Complexes with Hydrogen Bond Donors as Structural and Functional Phosphatase Models. Inorg. Chem. 2014, 53, 9036−9051. (c) Comba, P.; Gahan, L. R.; Hanson, G. R.; Mereacre, V.; Noble, C. J.; Powell, A. K.; Prisecaru, I.; Schenk, G.; Zajaczkowski-Fischer, M. Monoesterase Activity of a Purple Acid Phosphatase Mimic with a Cyclam Platform. Chem. - Eur. J. 2012, 18, 1700−1710. (d) Dalle, K. E.; Daumann, L. J.; Schenk, G.; McGeary, R. P.; Hanton, L. R.; Gahan, L. R. Polyhedron 2013, 52, 1336−1343. (9) (a) Rivas, J. C. M.; Salvagni, E.; Prabaharan, R.; de Rosales, R. T. M.; Parsons, S. Zinc(II) complexes with intramolecular amideoxygen coordination as models of metalloamidases. Dalton Trans. 2004, 172− 177. (b) Rivas, J. C. M.; Salvagni, E.; Parsons, S. Investigating the effect of hydrogen bonding environments in amide cleavage reactions at zinc(II) complexes with intramolecular amide oxygen co-ordination. Dalton Trans. 2004, 4185−4192. (10) (a) Mandal, S.; Balamurugan, V.; Lloret, F.; Mukherjee, R. Syntheses, X-ray Structures, and Physicochemical Properties of Phenoxo-Bridged Dinuclear Nickel(II) Complexes: Kinetics of Transesterification of 2-Hydroxypropyl-p-nitrophenylphosphate. Inorg. Chem. 2009, 48, 7544−7556. (b) Arora, H.; Barman, S. K.; Lloret, F.; Mukherjee, R. Isostructural Dinuclear Phenoxo-/AcetatoBridged Manganese(II), Cobalt(II), and Zinc(II) Complexes with Labile Sites: Kinetics of Transesterification of 2-Hydroxypropyl-pnitrophenylphosphate. Inorg. Chem. 2012, 51, 5539−5553. (11) Mondal, A.; Sarkar, S.; Chopra, D.; Guru Row, T. N.; Pramanik, K.; Rajak, K. K. Family of Mixed-Valence Oxovanadium(IV/V) Dinuclear Entities Incorporating N4O3-Coordinating Heptadentate Ligands: Synthesis, Structure, and EPR Spectra. Inorg. Chem. 2005, 44, 703−708. (12) O’Connor, C. J. Magnetochemistry - advances in theory and experimentation. Prog. Inorg. Chem. 1982, 29, 203−283. (13) (a) Sheldrick, G. M. SHELXL-2014, Program for Crystal Structure Refinement; University of Gottingen: Gottingen, Germany, 2014. (b) Farrugia, L. J. WinGX version 2014.1, an Integrated Systems of Windows Programs for the Solution, Refinement and Analysis of SingleCrystal X-ray Diffraction Data; Department of Chemistry, University of Glasgow: Glasgow, U.K., 2014. (14) Spek, A. L. Single-crystal structure validation with the program. J. Appl. Crystallogr. 2003, 36, 7−13. (15) Herrador, M. A.; Gonzalez, A. G. Potentiometric titrations in acetonitrile−water mixtures: evaluation of aqueous ionisation constant of ketoprofen. Talanta 2002, 56, 769−775.

(16) (a) Gans, P.; Sabatini, A.; Vacca, A. Investigation of equilibria in solution. Determination of equilibrium constants with the HYPERQUAD suite of programs. Talanta 1996, 43, 1739−1753. (b) Alderighi, L.; Gans, P.; Ienco, A.; Peters, D.; Sabatini, A.; Vacca, A. Hyperquad simulation and speciation (HySS): a utility program for the investigation of equilibria involving soluble and partially soluble species. Coord. Chem. Rev. 1999, 184, 311−318. (17) Belle, C.; Pierre, J.-L. Asymmetry in Bridged Binuclear Metalloenzymes: Lessons for the Chemist. Eur. J. Inorg. Chem. 2004, 35, 4137−4146. (18) (a) Greatti, A.; Scarpellini, M.; Peralta, R. A.; Casellato, A.; Bortoluzzi, A. J.; Xavier, F. R.; Jovito, R.; de Brito, M. A.; Szpoganicz, B.; Tomkowicz, Z.; Rams, M.; Haase, W.; Neves, A. Synthesis, Structure, and Physicochemical Properties of Dinuclear NiIIComplexes as Highly Efficient Functional Models of Phosphohydrolases. Inorg. Chem. 2008, 47, 1107−1119. (b) Neves, A.; Lanznaster, M.; Bortoluzzi, A. J.; Peralta, R. A.; Casellato, A.; Castellano, E. E.; Herrald, P.; Riley, M. J.; Schenk, G. An Unprecedented FeIII(μOH)ZnII Complex that Mimics the Structural and Functional Properties of Purple Acid Phosphatases. J. Am. Chem. Soc. 2007, 129, 7486−7487. (19) (a) Hendry, P.; Sargeson, A. M. Metal ion promoted reactions of phosphate derivatives. Prog. Inorg. Chem. 1990, 38, 201−258. (b) Hegg, E. L.; Burstyn, J. N. Toward the development of metalbased synthetic nucleases and proteases: a rationale and progress report in applying the principles of coordination chemistry. Coord. Chem. Rev. 1998, 173, 133−165. (c) Krämer, R. Bioinorganic models for the catalytic cooperation of metal ions and functional groups in nuclease and peptidase enzymes. Coord. Chem. Rev. 1999, 182, 243− 261. (20) Brammer, L. In Perspectives in Supramolecular Chemistry Crystal Design: Structure and Function; Desiraju, G. R., Ed.; Wiley: Chichester, U.K., 2003; Vol. 7, pp 1−75. (21) (a) Balamurugan, V.; Hundal, M. S.; Mukherjee, R. Hydrogen Bonding Using Inorganic Supramolecular Synthons: Lamellar, Stitched Stair-Case, Linked-Ladder, and Helical Structures. Chem. - Eur. J. 2004, 10, 1683−1690. (b) Mishra, H.; Mukherjee, R. Half-sandwich η6benzene Ru(II) complexes of phenolate-based pyridylalkylamine/ alkylamine ligands: Synthesis, structure, and stabilization of oneelectron oxidized species. J. Organomet. Chem. 2007, 692, 3248−3260. (22) Mukherjee, J.; Mukherjee, R. Reaction with dioxygen of a Cu(I) complex of 1-benzyl-[3-(2′-pyridyl)]pyrazole triggers ethyl acetate hydrolysis: acetato-/pyrazolato-, dihydroxo- and diacetato-bridged Cu(II) complexes. Dalton Trans. 2006, 1611−1621. (23) (a) Papatriantafyllopoulou, C.; Stamatatos, T. C.; Wernsdorfer, W.; Teat, S. J.; Tasiopoulos, A. J.; Escuer, A.; Perlepes, S. P. Combining Azide, Carboxylate, and 2-Pyridyloximate Ligands in Transition-Metal Chemistry: Ferromagnetic NiII5 Clusters with a Bowtie Skeleton. Inorg. Chem. 2010, 49, 10486−10496. (b) Ghosh, A. K.; Shatruk, M.; Bertolasi, V.; Pramanik, K.; Ray, D. Self-Assembled Tetra- and Pentanuclear Nickel(II) Aggregates From Phenoxido-Based Ligand -Bound {Ni2} Fragments: Carboxylate Bridge Controlled Structures. Inorg. Chem. 2013, 52, 13894−13903. (24) Massoud, S. S.; Ledet, C. C.; Junk, T.; Bosch, S.; Comba, P.; Herchel, R.; Hošek, J.; Trávníček, Z.; Fischer, R. C.; Mautner, F. A. Dinuclear metal(II)-acetato complexes based on bicompartmental 4chlorophenolate: syntheses, structures, magnetic properties, DNA interactions and phosphodiester hydrolysis. Dalton Trans. 2016, 45, 12933−12950.

K


A Bioinspired Dinickel(II) Hydrolase: Solvent Vapor-Induced

Recommend Documents