Zinc Complexes with Cyanoxime: Structural ... - ACS Publications

Article Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX

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Zinc Complexes with Cyanoxime: Structural, Spectroscopic, and Catalysis Studies in the Pivaloylcyanoxime−Zn System Adedamola A. Opalade,† Anirban Karmakar,‡ G. M. D. M. Rúbio,‡ Armando J. L. Pombeiro,‡ and Nikolay Gerasimchuk*,† †

Department of Chemistry, Temple Hall 431, Missouri State University, Springfield, Missouri 65897, United States Centro de Química Estrutural (CQE), Instituto SuperiorTécnico (IST) University of Lisbon, Avenida Rovisco Pais, 1049-001 Lisboa, Portugal

‡

S Supporting Information *

ABSTRACT: Reaction of 2-hydroxyimino-4,4-dimethyl-3-oxo-pentanenitrile (common abbreviation HPiCO, pivaloyl-cyanoxime) with zinc sulfate in an aqueous solution results in the formation of the two new complexes: [Zn(PiCO){H(PiCO)2}(H2O)] (I) and tetranuclear Zn complex [Zn4(μ3−OH)2(PiCO)6 (H2O)4] (II). Both complexes were characterized by elemental analysis, IR− and UV−visible spectra, DSC/TGA studies, and X-ray analysis. In complex II, the PiCO− cyanoxime anion adopts three bidentate binding modes: O-monodentate, chelating (κ2), and bridging (η2) coordinations. Also, the ligand represents the mixture of two diasteromers (cis−anti and cis−syn) that form five- and sixmembered chelate rings with Zn atoms and cocrystallize in one unit cell at population of 0.57−0.43. There are two crystallographically different Zn-centers in the ASU, and two μ3-bridging hydroxo-groups arrange via inversion center the formation of an elegant tetranuclear complex. Each Zn atom has a molecule of coordinated water and is in the distorted octahedral environment. Because of the structural flexibility and multidentate propensity of the pivaloyl-cyanoxime, complex II may act as a structural model of naturally occurring Zn-containing enzymes. Indeed, compound I exhibits an efficient catalytic performance for transesterification reaction of various esters in ethanol under mild reaction conditions. Therefore, obtained results allow assignment of observed activity as green catalysis.

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INTRODUCTION The chemistry, preparation, crystal structure, and properties of multinuclear 3d transition metal complexes have received considerable attention due to their probable application and role in multi-metal-centered catalysis both in biological and industrial reactions.1−7 Their magnetic and optical properties also make these types of compounds very important.8−11 The study of multinuclear 3d metal complexes with primarily O- or N-based ligation is today an area of modern science with interfaces in different facets of science.12 The chemistry of Zn(II) multimetallic systems has not been fully explored. These are significant, nevertheless, because of their importance as a precursor to fabrication of ZnO-based materials, modeling of active sites in enzymes, polymerization catalyst, molecular material with interesting properties, and reagents in organic synthesis.13−17 Thus, the development of catalyst-promoted atom-economical and environmentally tolerable processes is a topic of great interest in modern organic chemistry, e.g., in ester formation.18 An attractive method of ester synthesis concerns transesterification endorsed by a metal catalyst.19 Transesterifications are important reactions in organic synthesis and industrial and academic laboratories20 and have important applications in polyester synthesis and biodiesel production.21 There are many catalysts available for transesterification, namely, Ti(OiPr)4,22 and diverse catalysts such as BuSn(OH)3, © XXXX American Chemical Society

Al(OR)3, Mg−Al−O−t-Bu hydrotalcite, amberlyst-15, BF3· OEt2, and so on.23 Several metal complexes and metal organic frameworks (MOFs) can also catalyze such reactions, but usually, they require high reaction temperature and acidic reaction conditions.24,25 Therefore, the development of new types of catalysts based on cheap, nontoxic, environmentally tolerable (green catalysts), and less moisture sensitive compounds is highly desirable for practical utility. Zinc is especially fit for this purpose considering its crucial catalytic role in a variety of enzymes in biological systems.26 Due to cited potential applications of these polymetallic systems, there is a need for controlled synthesis methods for preparation of these compounds, as well as a need for different polydentate ligands that are able to act as bridges in such systems. So far, polymetallic Zn complexes have been obtained using ligands such as salicylaldo(keto)oximes,27 pyridyl oximes,28 and aliphatic dioximes.29 Chemical structures of the oximes and their derivatives used for the preparation of multiZn complexes are presented in Scheme 1 with corresponding literature citations highlighted in blue. Thus, a 36-membered macrocyclic hexaoxime ester derivative with Zn3La core/shell tetranuclear cluster structure was obtained by condensation of Received: August 14, 2017

A

DOI: 10.1021/acs.inorgchem.7b01891 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

photoluminescence with maxima ∼375 nm upon maximum excitation at 314 nm.12 In terms of applications of the multimetallic Zn(II) complexes, there have been various Zn(II) complexes with different ligands that have been reported for different applications. Recently, Badetti et al. reported how the utilization of trimetallic Zn(II) complexes of two modified tris(2-pyridylmethyl)amine ligands and amino acids for the determination of amino acids ee.54 Also, multimetallic zinc complexes have been shown to be very promising catalysts for ring opening polymerization (ROP) of esters.55 Recently, alkyl Zn(II) complexes with dpg, derived from the Ph2C(X)−dpgH (X = OH, NH2) were reported to give tetranuclear ring complexes [RZn(dpg)]4 (R = Me, Et, 2-CF3C6H4, and 2,4,6F3C6H2). The complexes were reported to be active for ROP of ε-Cl, rac-LA, and δ-VL in the absence of benzyl alcohol.56 Heterometallic complexes of Zn have also been reported for their catalytic activities toward transesterification reaction. Two novel one-dimensional heterometallic complexes represented as [Zn2Co(O2CPh)6]n(bpa)n and [Zn2Cd(O2CPh)6]n(bpa)n, containing two bridging ligands: 1,2-Bis(4-pyridyl)ethane (bpa) and benzoate were used as catalysts for transesterification of esters by methanol using different substrates at room temperature under mild conditions.57 Also, an octanuclear Zn(II) complex of meta-bis(imidazolylmethyl)-benzene ligand was reported as a stable but very active catalyst for the transesterification reaction of a broad range of substrates and alcohols; displaying enhanced efficiency and very high yields25l. The enhanced catalytic activity of the multimetallic Zn(II) complexes over monometallic analogues to aid chemical transformation could be based on the cooperative and synergistic effect of the metal centers, due to their close proximity thus favoring multisite interactions with the substrate. This could lead to the substrate activation. Metal centers could also alter the electronic properties of its neighbors by playing the role of a ligand toward them.58 There are only two reports containing preparation and some properties of Zn-cyanoximates,38,39 but no crystallographic characterization was carried out. Cyanoximes represent an important subclass of oximes40 that has received intense studies in the last decades because of their greater acidity, diversity of donor atoms for metal ions binding, and, subsequently, excellent properties as polydentate ligands.41 Complexes of cyanoximes exhibited a variety of practically interesting properties.42 The pivaloylcyanoxime, 2-hydroxyimino-4,4-dimethyl-3-oxo-pentanenitrile (HPiCO), was first described in 1993 by Fedorenko43 and represents a rare example of sterically hindered chelating oximes. However, this cyanoxime is insufficiently studied with only a handful of publications. Thus, the Tl+ and Ag+ PiCO-based complexes have been structurally characterized before. The crystal structure of the Ag-PiCO compound is a layered 2D coordination polymer where the PiCO− anion displayed bridging mode between four different silver(I) atoms, while the crystal structure of Tl−PiCO complexes have been reported as coordination polymer with three different binding modes of the PiCO− anion.42f,44,59 In this work, we report the first detailed investigations of the Zn−PiCO− system and present (1) syntheses and characterization of the first Zn-cyanoxime complexes, (2) crystallographic data for a new Zn(II) complex that turned out to be an interesting tetranuclear compound of [Zn4(μ3-OH)2(PiCO)6 (H2O)4] composition, and (3) results of significant catalytical

Scheme 1. Oximes Used in Earlier Preparation of Zn-Based Complexes

dialdehyde with diamine using La3+ (core metal) and Zn2+ (shell metal) as a novel core/shell template.30 The first example of a 12-metallacrown-4 with an “anti-metallacrown” structure type inv-(OH)2[12-MCZn2+N(pko)−4] (a tetranuclear species [Zn4(OH)2(O2CMe)2(pko)4]) made by using an oxime based ligand, bis(2-pyridyl)ketone oxime (later Hpko), was reported to have a Zn4O2 core structure31 similar to Fe4O2 core structure.32 A synthesis route to cationic pentanuclear ZnII/pko clusters and to a new type of 12-MCZnIIN(pko)-4 compound featuring SCN−, PhCO2−, N3−, and acac (−1) ion and NCO− coligands was reported, and this pathway provided access to six new polymetallic Zn complexes of which four were tetranuclear and two were pentanuclear.33 Three tetranuclear ZnII complexes of the general formula [Zn4(μ3-OH)2(LR1− 3N,N,O)4(LR1−3N,N)2] were also reported from the reaction of ZnCl2 with respective 2-pyridyl ketone oximes, HLR1−3 (R1−3 = H, Ph, and Py) where the complexes with R = Ph and Py have hydroxyl group encapsulated in the cavity of the tetranuclear ZnII cluster.34 The synthesis of octanuclear 12membered metallacrown, [Zn2]{[Zn2-(pko)4][12-MCZn(II)N(shi)-4](CH3OH)2}, was also performed using ZnCl2 dissolved in methanol, salicylhydroxamic acid, and 2-pyridyl-ketonoxime, deprotonated by NaOH in methanol.35 Solvothermal synthesis of Zn complex of pyridine-2-amidoxime, (py)C(NH2)NOH resulted in the formation of [Zn4(OH)2{(py)C(NH2)NO}4Cl2]·3MeCN where each ZnII ion is coordinated by a μ3-bridging hydroxide ion.36 The reaction of syn-2-pyridinealdoxime (HL) with ZnII metal salt in a mixed C2H5OH/ CH3CN solution resulted in a new complex with the tetranuclear unit [Zn4(L)4Cl2(μ3-OH)2·4CH3CN].37 When pyridine-2-carbaldehyde oxime (later as paoH) reacted with Zinc(II) benzoate in the absence and in the presence of azide ions, the clusters [Zn 12 (OH) 4 (O 2 CPh) 16 (pao) 4 ] and [Zn4(OH)2(pao)4(N3)2] were obtained. Zn4(OH)2(pao)4(N3)2] has an inverse 12-metallacrown-4 configuration with two μ3-OH accommodated in the metallacrown ring while the [Zn12(OH)4(O2CPh)16(pao)4] can be described as consisting of a central {Zn4(μ3-(pao)4)}4+ cubane subunit linked to four {Zn2(μ−OH)}3+ subunits via a hydroxide group of the latter, which becomes the μ3-type. The [Zn12(OH)4(O2CPh)16(pao)4] was found to exhibit B


Article

Inorganic Chemistry Scheme 2. Preparation of the Cyanoxime and Its Zn Complexes

later on referred to as tetra-Zn complex II (Scheme 2). Elemental analysis for C42H64N12O18Zn4 calculated (found), %: C, 39.21 (39.25), H, 5.01 (4.98), N, 13.06 (13.22). When bulk precipitate was triturated with ether (not dry, technical grade, commonly contains up to 4% of moisture) for several hours at room temperature, tetra-Zn complex II [Zn4(μ3-OH)2(PiCO)6(H2O)4] was obtained again as well as a small amount of the protonated pyvaloylcyanoxime HPiCO. Similarly, when bulk precipitate was dissolved in CH3CN and reprecipitated by an addition of ether (via vapor deposition method in closed vial), tetra-Zn complex II was obtained once more as well as a free HPiCO cyanoxime. More specifically, 200 mg of the bulk precipitate was dissolved in 2 mL of acetonitrile and filtered over glass wool. The clear filtrate was collected in a glass tube that then was inserted into a largemouth screw-cap tube containing ether and sealed up for several weeks. The inner test tube then contained the above mixture of tetraZn complex II and HPiCO, which was washed from the complex by CHCl3. An identity of the oxime was confirmed by TLC (Rf = 0.67; CH3OH/benzene = 1:9) and thermal measurements: The compound melts at ∼150 °C followed by its rapid decomposition. Also, an examination of crystalline samples of the HPiCO side-byside under the microscope has confirmed the same crystal habits. Also, a suitable single crystal specimen of HPiCO after ether trituration was inspected using a single crystal diffractometer (Figure S1), and determined unit cell parameters matched the recently published data for the structure of HPiCO.45 Hence, both solvent treatments of the complex I lead to the same outcome (see Results and Discussion below). A series of reactions that describes observed chemistry in the PiCO−−Zn system is presented below. In water/CH3CN:

activity study for another complex [Zn(PiCO){H(PiCO)2} (H2O)] obtained in this system.

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EXPERIMENTAL PART

Synthesis. The preparation of the HPiCO was carried out according to previously published procedure,43,45 while the reactions in the cyanoxime−Zn system are presented in Scheme 2 and in a set of equations below. For the preparation of Zn(II) complex, 0.300 g (1.95 mM) of white powdery HPiCO was placed in a beaker containing 5 mL of deionized water, to which 1.95 mL of 1 M KOH solution was added at once. The color quickly changes to yellow following a complete dissolution of the cyanoxime (eq 1). Then, 0.2797 g (0.97 mM) of ZnSO4·7H2O was dissolved in 5 mL of deionized water, and the solution was added dropwise under continuous stirring to the yellow PiCO− cyanoximate solution prepared above (eq 3). The mixture was allowed to stir continuously for ∼5 h until precipitate appeared in the system. The mixture was gravity-filtered through the paper filter leaving behind some bright-yellow precipitate (later cited as the compound I). The bulk precipitate was weighed, and the yield was 26% (0.2049 g). The precipitate was left in the filter to dry at room temperature for a week, while clear bright-yellow filtrate was collected in a plastic centrifuge tube and kept in a vacuum desiccator charged with concentrated sulfuric acid to expedite the crystal growth. Elemental analysis for C21H30N6O7Zn calculated (found), %: C, 46.37 (47.07), H, 5.56 (5.65), N, 15.45 (15.40). This composition corresponds to the formula of I as [Zn(PiCO){H(PiCO)2} (H2O)]. Small plate-like bright yellow crystals were formed from the aqueous yellow solution in the plastic centrifuge tube after ∼2 months, and the crystals were harvested for structural studies. This compound will be C


Article

Inorganic Chemistry

source. The first full-sphere data set obtained from four runs in ω scanning mode allowed only determination of the atoms’ connectivity. We used 60 s/frame exposure time with 0.5° step at 120 K and still were able to obtain the high R(int) value of ∼0.26 from a small and poorly diffracting crystal. Therefore, the second data set was collected using the high-brilliance photon source at Lawrence Berkeley National Laboratory (LBNL). The crystal was placed in the MiTeGen cryoloop, and intensity data were collected at 150 K on a D8 goniostat equipped with a Bruker PHOTON100 CMOS detector. The Si[111] channel cut crystal was used as monochromator. The Beamline 11.3.1 at the Advanced Light Source (LBNL) with synchrotron radiation tuned to λ = 0.7749 Å was used for data collection. Thus, frames were measured for a duration of 2 s for data at 0.5° intervals of ω with a maximum 2θ value of ∼60° using ω−φ scans. The data frames were collected using the program suite on APEX 2 and processed using the program SAINT routine within the package. The data were corrected for absorption and beam corrections based on the multiscan technique as implemented in SADABS. A series of DISP cards had to be used for the final refinement accounting for the elements scattering factors at 0.7749 Å crystal irradiation wavelength (Supporting Information page 3). Hydrogen atoms on methyl groups were placed in their idealized positions and refined isotropically, while H-atoms on two water molecules and bridging hydroxyl group were found on the difference map. The structure was successfully solved using the synchrotron data set by direct methods and will be presented and discussed herein. Crystal and refinement data are presented in Table 1, while selected bond lengths and angles are summarized in Table 2 with geometries around metal centers presented in Figure 3. The molecular structure of tetra-Zn complex II is shown in Figure 1. The complex is registered as CCDC 1562109, and its PLATON check CIF report is shown in the Supporting Information. Powder Diffraction Data. The XRD patterns were recorded at room temperature from 8 to 15 mg amounts of powdery samples adhered by vacuum grease to the plastic MiTeGen loop. The Agilent single crystal diffractometer equipped with Cu tube (λ = 1.54056 Å) operated in the powder Θ−2Θ mode was used to record patterns to 60° in 2Θ. Results are presented in Figure S3. Caution: Although we have not encountered any problems during many years of laboratory work and handling cyanoxime compounds, it is an absolute necessity to wear protective gloves at all times when working with them due to their pronounced biological activity.

HPiCO (colorless) + KOH → K+PiCO− (bright‐yellow) + H 2O (1) Hydrolysis of the anion:

H 2O + 2PiCO− ↔ OH− + H(PiCO)2−

(2)

Complexation reaction:

[Zn(H 2O)6 ]2 + + PiCO− + H(PiCO)2− → [Zn(PiCO){H(PiCO)2 }(H 2O)] + 6H 2O off ‐ yellow bulk precipitate,compound I

(3)

Trituration with ether:

4[Zn(PiCO){H(PiCO)2 }(H 2O)] → [Zn4(μ3‐OH)2 (PiCO)6 (H 2O)4 ] + 6HiPCO bright ‐ yellow plates,tetra ‐ Zn ‐ compound II

+ 4CH3CN + 2H 2O

(4)

Overall reaction:

8K+PiCO− + 4ZnSO4 ·7H 2O → [Zn4(μ3 ‐OH)2 (PiCO)6 (H 2O)4 ] + 2HPiCO + 4K 2SO4 + 22H 2O

(5)

Physical and Analytical Methods. Elemental analyses for all obtained PiCO−−Zn system compounds were analyzed for elemental C−H−N content at Atlantic Microlab at Norcross, GA, USA. Differential Scanning Calorimetry−Thermogravimetric Analysis (DSC−TGA). The TG/DSC measurements of I, II, and HPiCO were carried out under N2 (UHP grade) flow using a TA Q− 600 instrument. Results are presented in Figure 1.

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RESULTS AND DISCUSSION Preparation. The cyanoximes are much more acidic than their other related family members aldoximes and ketoximes due to the strong electron-withdrawing effect of the nitrile group.40 Being easily ionizable in aqueous and nonaqueous solutions, cyanoximes form a large array of metal complexes.41,42,46 Recently, we reported the pKa value for the HPiCO as equal to 4.91 ± 0.01.45 For comparison, the acidity of the above aldoximes and ketoximes (including conventional aliphatic and aromatic α-dioximes) is in the pKa range of 9− 12.47 In this context, it was unexpected to find that to this date the HPiCO cyanoxime despite presence of the t-butyl group attached to the core is one of the top four most acidic cyanoximes after benzoylcyanoxime HBCO46a (pKa = 4.27 ± 0.01), and two isomeric 3-pyridyl and 4-pyridyl cyanoximes41a (pKa = 3.25 and 4.00 respectively). Deprotonation of HPiCO leads to the formation of a yellow anion that exhibits strong solvatochromism.45 In general, the reaction of room temperature hydrolysis of cyanoximes was found to generate rather unique “acid-salt monoanions” of HL2− composition,40 in which H atom can be either symmetrically (equidistantly), or asymmetrically bound to both anions. This was also the situation in the case of the PiCO− anion (eq 2) which led to the formation of bulk precipitate I of [Zn(PiCO){H(PiCO)2}] composition in eq 3, with this product characterized by well-

Figure 1. Molecular structure of the tetranuclear zinc complex II; an ORTEP drawing at 50% probability level. Well-modeled geometry with two chelating diastereomers of PiCO− ; labeled L1−L4 are differently coordinated to metal centers anions. IR and UV−Visible Spectroscopy. The IR spectra of compound I, tetra-Zn complex II, and the initial HPiCO ligand were recorded at ambient temperature using Bruker Vertex 70 Spectrophotometer in 13 mm KBr disks pressed at 9 tons of pressure delivered by a Carver hydraulic press. Electronic spectra of the ligand and its zinc complexes were recorded in solid state with the help of Cary 100 Bio spectrophotometer equipped with the solid-state accessory by Labsphere (Figure S2). X-ray Analysis. Single Crystal Data. Two low- temperature data sets were collected for a small bright-yellow plate of the Zn complex II: one using an APEX 2 diffractometer with monochromated Mo radiation (λ = 0.71073 Å) and the second using the synchrotron D


Article

Inorganic Chemistry Table 1. Crystal and Refinement Data for the Tetranuclear Zn−Hydroxo-Cyanoximate II chemical formula formula weight, g/mol temperature wavelength crystal size, mm crystal habit, color crystal system space group unit cell dimensions

volume, Å3 Z density (calculated; g/cm3) absorption coefficient, mm−1 F(000) Θ range for data collection index ranges reflections collected independent reflections max. and min transmission absorption method structure solution technique structure solution program refinement method refinement program function minimized data/restraints/parameters goodness-of-fit on F2 Δ/σmax final R indices weighting scheme largest diff. peak and hole, e·Å−3 RMS deviation from mean, e·Å−3

C42H64N12O18Zn4 1286.59 150(2) K 0.77490 Å (synchrotron data; Si [111] filter) 0.005 · 0.030 · 0.040 clear bright yellow plate triclinic P1̅, no. 2 a = 11.1704(8) Å b = 11.2712(8) Å c = 11.5404(9) Å 1424.82(18) 1 1.499 2.194 664 2.61 to 31.15° −14 ≤ h ≤ 14, −15 ≤ k ≤ 15, −15 ≤ l ≤ 15 30299 7069 [R(int) = 0.0573] 0.9870 and 0.9610 multiscan, from equivalents direct methods Bruker SHELXTL v6.14 Full-matrix least-squares on F2 Bruker SHELXTL v6.14 Σ w(Fo2 − Fc2)2 7069/134/469 1.026 0.001 5481 data; I > 2σ(I) all data

R1 = 0.0316, wR2 = 0.0601 R1 = 0.0508, wR2 = 0.0649

w = 1/[σ2(Fo2) + (0.0191P)2 + 0.1749P] where P = (F02 + 2Fc2)/3 0.351 and −0.455 0.066

and 1.64% weight loss (Figure 1A). Then, molecules of coordinated water leave the system at 134 °C (calculated/ found total % of water loss: 5.01/4.78), after which two molecules of the HPiCO cyanoxime sublime from 165 to 192 °C (calculated/found % of weight loss: 29.63/30.18). Pure uncomplexed cyanoxime undergoes sublimation within 176− 216 °C with the maximum rate of the process at 193 °C (Figure S5). The tentative decomposition path for the complex I is outlined in Figure S5. Tetra-Zn complex II experiences continuous loss of four water molecules ending at ∼154 °C: calculated/(found) 5.59% (5.41%) respectively (Figure 1B). The intermediate products formed between 400−1000 °C are unknown but may include remains of the decarboxylation process, loss of cyanogen (CN)2, and so on. The final decomposition product for both complexes at ∼600 °C appeared to be ZnO (Figure S6). Vibrational and Electronic Spectra. Bands in the IR spectrum of the pure HPiCO cyanoxime and its anion in the bulk precipitate I and tetra-Zn complex II showed a considerable decrease in the ν(CO) vibrational frequency in the latter, which evidence coordination of the anion to metal center through the oxygen atom of the carbonyl group (Figures S7 and S8). The increase in the frequency of the as,sν(N−O) vibrations indicates involvement of the oxime fragment into

matching elemental analysis and IR spectra. Redissolving of complex I in CH3CN with subsequent reprecipitation with ether has led to further nucleation of the complex into tetrameric, tetra-Zn complex II. It was established in the past that in the presence of water the HL2− acid-salts of cyanoximes monoanions undergo dissociation to their initial components or exist under equilibrium conditions: HL 2− ↔ HL + L−

α = 82.712(3)° β = 88.895(3)° γ = 81.342(3)°

(6)

In this case, only deprotonated anionic cyanoxime L− is able to coordinate to the metal center because of its much higher nucleophilicity, while the protonated cyanoxime HL is left behind and can be easily separated from the mixture via extraction or trituration with the solvent (eq 4). Therefore, an overall reaction that describes preparation of the tetranuclear Zn−cyanoximates complex in the system with 1:2 stoichiometric ratio between metal ion and PiCO− can be presented as in eq 5. Thermal Analysis. The thermal decomposition patterns of I and tetra-Zn complex II are completely different for both complexes (Figure 1) and also different from that for the pure cyanoxime (Figure S5). The decomposition pattern of I is such that at first there is a loss of one molecule of water (presumably from the H{PiCO}2− anion) at 103 °C with 1.65% calculated E


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Inorganic Chemistry

Table 2. Selected Bonds and Angles in the Structure of Anionic Cyanoxime Ligands in the Tetranuclear Zn−Hydroxo Complex II ligand

bonds, Å (Figure 3) O1−N1 N1−C1 C1−C2 C1−C3 C3−O2 C2−N2 O3−N3 N3−C8 C8−C9 C8−C10 C10−O4 C9−N4 O5a−N5a N5a−C15a C15a−C17a C15a−C16a C17a−O6a C16a−N6a O5−N5 N5−C15 C15−C17 C15−C16 C17−O6 C16−N6

L1

L2

diastereomeric mixture: L3

L4

= = = = = = = = = = = = = = = = = = = = = = = =

geometry (in Scheme 3)

1.314 1.312 1.434 1.474 1.230 1.148 1.291 1.317 1.439 1.489 1.230 1.143 1.296 1.342 1.446 1.413 1.236 1.433 1.286 1.325 1.446 1.434 1.241 1.144

binding mode (in Scheme 4)

trans−anti, oxime form

O-monodentate, 2

cis−anti, nitroso form

N,O five-membered chelate + bridging, 8

cis−anti, nitroso form

N,O five-membered chelate + bridging, 8

cis−syn, nitroso form

O,O six-membered chelate + bridging, 10

Metal Centers Zn1−O1 Zn1−N3 Zn1−O8W Zn2−O7B Zn2−O9W

= = = = =

1.9987(12) 2.1201(15) 2.1746(15) 2.0710(14) 2.1071(14)

Zn1−O7B = 2.0235(13) Zn1−O4 = 2.1332(13) Zn2−O7B = 2.0652(13) Zn2−O3 = 2.0760(13) Zn−L Distances in Diastereomers

Zn1−O5 = 2.164(4) N5A−Zn2 = 2.133(4) O6A−Zn2 = 2.325(6)

Zn1−O5A = 2.219(5) O5−Zn2 = 2.170(3) O6−Zn2 = 2.088(4)

coordination either via O- or N-binding41a,e,42g,48 (Table S1). Both were confirmed by the X-ray analysis results and are presented below. The IR spectrum of I to some extent resembles that of the pure ligand HPiCO (Figure S9) with only few new bands related to anionic cyanoxime in its nitroso form,45 while two isolated complexes I and tetra-Zn complex II possess yellow color with a broad band with a shoulder at ∼420 nm which belongs to the n → π* ligands transition45,46e,48a (Figure S10). Both these Zn complexes represent nonfluorescent compounds. X-ray Powder Diffraction. The two XRD patterns for I and II have some similarities with some peaks present in both profiles but do not match exactly (Figure S3). Therefore, two Zn(II) complexes obtained with PiCO− are not isostructural. Structural Studies. We were successful in growing crystals only for tetra-Zn complex II suitable for X-ray analysis, despite numerous attempts to obtain diffracting crystalline material for precipitating in bulk complex I. The crystal structure of the uncomplexed cyanoxime was recently reported45 and evidence the adoption of trans−anti geometry by the ligand (Scheme 3). Displayed here are four principal geometrical isomers which have been observed for other cyanoximes and their anions in the recent past41c,48a as these compounds represent ampolydentate ligands.40,41e,49 This term was coined to broaden the

Scheme 3. Geometrical Isomerism Observed in Cyanoximes

concept of the bidentate ligands such as classical NCX− and XCN− anions (X = O, S, Se, and Te) when a new series of the cyano-methanides, cyano-amides, and cyanoximes and their numerous complexes were discovered and studied.60 Specifically, for the PiCO− anion the most straightforward ways of coordination are presented in Scheme 4. As an illustration for this concept, we should mention that the variety of binding modes of other cyanoximes has been welldocumented in the past.41e,49 Thus, coordination type 1 was observed for the benzoylcyanoxime anion BCO− in its macrocyclic complex,50 while types 3 and 4 were found in abundance in a variety of Tl(I) complexes.41a−c,51 Binding modes 5 and 7 were observed in transition metal complexes,46a,b,d,49 types 6 and 8 in Ag(I) cyanoximates,42b,d,f,g and coordination types 2 and 11 in organometallic antimony,52 tin,46c and tellurium53 compounds (Scheme 4). More specifically, for the PiCO− anion its ampolydentate behavior F


Article

Inorganic Chemistry Scheme 4. Possible Binding Modes of the PiCO− Anion in Complexes

t-butyl group: see Supporting Information page 3 for detail), and EADP for one of the carbon atoms that showed rather a “flat” feature. There are in total four different binding modes of the PiCO− anion in this structure which are marked as L1−L4 in Figure 2 and subsequently explained in Table 2. Fragments of crystal structure with unobstructed views of different parts (L1, L2 and L3, L4) together with the numbering scheme can be found in Figure 2. First, a mixture of two diastereomers in one chelating cyanoxime is cocrystallized in one unit cell and statistically distributed throughout the crystal. Thus, PiCO− adopts cis− anti geometry (Scheme 3) that is reflected in binding mode 8 (Scheme 4; L3 in Figure 2B) and cis−syn geometry (Scheme 3) shown in binding mode 10 (Scheme 4; L4 in Figure 2B) at the ratio 0.57:0.43. It should be noted that it is not two positional disorders but indeed a cocrystallized mixture of two geometrical isomers that have similar lattice energies and crystal packing. The presence of aforementioned four different binding modes of the same cyanoxime anion in one complex is rather remarkable in itself. The geometry of the PiCO− anion in the monodentate O-binding mode in L1 (Figure 2A) is very similar to that in the structure of uncomplexed HPiCO.45 There are two kinds of chelating + bridging cyanoximes L2, L3 and L4 (Figure 2). The first two have formed five-membered metallocycles with the values of “bite angles” O4−Zn1−N3 = 75.16° and O6a−Zn2−N5a = 71.31°, while cis−syn diastereomer L4 forms six-membered chelate and has a “biting angle” O6−Zn2−O5 equal to 80.31°. There are two crystallographically independent Zn1 and Zn2 atoms in the ASU of the structure of the tetra-Zn complex II and both possess distorted octahedral geometry with metal centers environment depicted in (Figure 3). The average Zn- -Zn distance is 3.273 Å and is large to consider any intermetallic interactions. An inversion center generates tetra-zinc complex (Figure 1) in which the hydroxo-group O(7)−H acts as a μ3bridge. The distorted trigonal-pyramidal geometry around O(7) in this group is shown in Figure 4. It is quite similar to the geometry of some active catalysts and Zn-based enzymes, for which, presented in this work, tetra-Zn complex II may act as a structural model. This complex, however, is stable at ambient conditions to reaction with CO2 present in the air. There are other important features in the structure of tetraZn complex II: intramolecular and intermolecular H-bonding. The geometry of a strong intramolecular H-bond is best seen in an unobstructed view in Figure S12. There is a very pronounced “tilt” of the monodentate coordinated PiCO− anion (as L1, Figure 1) bound to Zn1 toward coordinated to Zn2 water molecule O9w (Figure S12). The intermolecular Hbonding has two parts. One is the bond between the CN-group and water molecule O8w (Figure S13). The geometry of this medium strength H-bond is best seen viewing the unit cell content (Z = 2) in the Figure S13, namely, these H-bonds are holding molecules of the tetra-Zn complex II in a chain structure along c-direction as it is shown in Figure S14. The second part of intermolecular H-bonding represents 2.164 Å bond between the oxygen atom of the keto-group and μ3bridging hydroxo group O7 (Figure S15). The crystal packing diagram of the tetra-Zn complex II is displayed in Figure S16 and shows intermolecular van der Waals contacts between hydrophobic t-butyl groups directed at each other. These contacts are responsible for the formation of layers of the complex in the structure. On the basis of results of the thermal analysis, IR-spectra, and data of elemental analysis,

was crystallographycally confirmed as coordination modes 2−4 in its Tl complex,44a mode 7 in Pd, Pt complexes,45 and binding mode 12 in Ag complex42f (Scheme 4). The IR spectroscopy evidence the adoption of modes 2 and 11 in organoantimony(V)43 and organotin(IV)43 compounds, respectively. At the same time, uncomplexed protonated cyanoxime HPiCO has trans−anti geometry45 shown in Scheme 3. The crystal structure of tetra-Zn complex II is shown in Figure 1, and there are several interesting features that have to be discussed. First, the successful structure solution and refinement demonstrates overall quite reasonable connectivity of atoms, but five of them show rather large thermal displacement parameters unexplainable in comparison with those of other adjacent atoms in the same molecule (Figure S11). It is interesting to mention that the Platon checkCIF report for this nonmodeled structure did not contain A- or Btype alerts. Nevertheless, the structure did not look right or completely acceptable, and needed improvement. It was possible to find on a difference map Q-peaks that correspond to two alternative structures one of the cyanoxime anions: syn and anti diastereomers. It was necessary to apply a series of restrains to keep all atoms connected in proper order with correct geometry of functional groups affected. More specifically, these were DFIX commands for one of the water molecules (O8W H8Wa, O9W H9 Wb with 0.85 Å bond distance), RIGU (for two alternative positions of the cyanoxime including hydrogen atoms attached to the carbon atoms of the G


Article

Inorganic Chemistry

Figure 2. Fragments of the crystal structure of tetra-Zn complex II showing the atomic numbering sheme in monodentate coordinated L1 (trans− anti), bridging-chelating L2 (cis−anti) (A), and in well-modeled geometry of two diastereomers L3 (cis−anti, solid bonds) and L4 (cis−syn, open bonds) in the complex (B).

transesterification reaction, and the obtained reaction yield was only 15% (entry 14, Table 3). When we used 3 mol % catalyst I, a conversion of 94% of methyl-4-nitrobenzoate into ethy-l-4-nitrobenzoate was reached after 24 h of reaction time (entry 5, Table 4). The increase of the catalyst amount enhances the product yield from 65 to 94% for the corresponding amounts of 1.0 and 3.0 mol % catalyst I (entries 6 and 5, respectively, Table 4), but a further increase in the catalyst amount to 7 mol % improves the yield only by 1% (entries 6−7, Table 4). Increasing the reaction temperature from room temperature to 75 °C (entries 10, 11, and 5 Table 4) resulted in yield improvement from 10 to 94%, but a further rise in the temperature to 100 °C (entry 12, Table 4) led to a decrease in the overall yield. Extending the reaction time to 48 h did not lead to a significant increase of the yield. The high conversion rate suggests that I act as an efficient catalyst for the transesterification reaction. The product yield increases with time (Figure 5) until 24 h, but after 12 h, it already reached 85%

the suggested structure of complex I is shown in the Supporting Information, below Table S1. Catalytic Activity in the Transesterification Reaction. We have tested the catalytic activity of our compound I toward the transesterification reaction of methyl-4-nitrobenzoate. In a typical reaction, a mixture of this ester and catalyst I in EtOH (2 mL) was added into a capped glass vessel and the resulting mixture was stirred at 75 °C for 24 h (Scheme 5). The solvent was evaporated in vacuum, leading to a crude mixture of products which was analyzed by 1H NMR spectroscopy (Figure S17). The effects of temperature, reaction time, amount of catalyst, and the alcohol were investigated, and the obtained results are presented in Table 3. A blank test was carried out with methyl4-nitrobenzoate in the absence of any metal catalyst at 75 °C in ethanol, and no considerable amount of ethy-l-4-nitrobenzoate was detected after a reaction time of 24 h (entry 13, Table 4). We have also checked the activity of ZnSO4·7H2O in the H


Article

Inorganic Chemistry

Figure 3. Donor atoms environment and geometry of two independent zinc(II) atoms in the ASU in the structure of tetranuclear complex: A, bond length (Å); B, valence angles (°). Hydrogen atoms on water molecules and bridging hydroxyl groups are shown while all other atoms were removed for clarity. Symmetry code for #1: −x, −y, −z. Figure 4. Geometry of μ3-bridging hydroxo-group in the tetranuclear complex: two orthogonal views. A, bond lengths (Å); B, angles (o); C, intermetallic distances. Symmetry transformation for #1 position: 1 − x, 1 − y, −z.

(entry 4). Thus, we have obtained the best reaction yield by using 3 mol % of catalyst at 75 °C for 24 h. When 1-propanol was used instead of ethanol the reaction yield decreased from 94 to 87% (entries 8 vs 5, Table 4). The reaction conversion further decreased to 17% with the use of 2-propanol, a secondary alcohol (entry 9, Table 3). Hence, for further studies we have used ethanol as the alcohol. Moreover, we have performed control experiments using different ratios of water and ethanol mixture and the results indicate that the reaction yield significantly decreases upon increasing the water concentration (entries 15−17, Table 3). We also investigated the reaction of various substituted methyl benzoates in ethanol. Those with the strongest electronwithdrawing nitro substituent underwent the fastest transesterification (entries 1−2, Table 4) whereas that with the electron-donating methoxy group experienced the slowest conversion (entry 5, Table 4). Ethyl-4-hydroxybenzoate also converted to methyl-4-hydroxybenzoate upon reaction with methanol, with a yield of 54% (entry 10, Table 4). We also tested some aliphatic esters and obtained yields are in the range of 21−63% (entries 8−9 and 11−12, Table 4). To check the integrity of the complex catalyst after the catalytic reaction, we have performed the transesterification reaction with a low boiling ester, e.g., ethyl acetate, and isolated the catalyst by taking the reaction solution to dryness. FT-IR of complex I

Scheme 5. Transesterification Reaction Used as Probe in Catalysis Studies of Activity of Zn−Cyanoximates

after the reaction was identical to the initial one indicating that the structure of the solid was retained (Figure S18A). We tested the recyclability of complex I by using ethyl acetate as substrate. For this purpose, upon completion of a reaction cycle, we separated the catalyst by evaporation of the solvent, washing with dichloromethane, and drying at room temperature before further use. We repeatedly recycled catalyst I, and the catalytic system essentially maintained the full activity for at least four consecutive cycles as shown in Figure S18B. The transesterification of methyl-4-nitrobenzoate with ethanol has not been broadly studied. Very few Zn(II) compounds were reported to catalyze this reaction. For example, the 3D zinc(II) MOF of 5-benzamidoisophthalate I


Article

Inorganic Chemistry

Table 3. Optimization of Parameters of the Transesterification Reaction of Methyl-4-Nitrobenzoate in Alcohols with Complex I as Catalysta entry

catalyst

time (h)

amount of catalyst (mol %)

T (°C)

alcohol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

I I I I I I I I I I I I blank ZnSO4·7H2O I I I

1 3 6 12 24 24 24 24 24 24 24 24 24 24 24 24 24

3.0 3.0 3.0 3.0 3.0 1.0 7.0 3.0 3.0 3.0 3.0 3.0

75 75 75 75 75 75 75 75 75 RT 50 100 75 75 75 75 75

EtOH EtOH EtOH EtOH EtOH EtOH EtOH 1-PrOH 2-PrOH EtOH EtOH EtOH EtOH EtOH H2O/EtOH (1:3) H2O/EtOH (1:1) H2O/EtOH (3:1)

3.0 3.0 3.0 3.0

yield (%)b 51 63 67 85 94 65 95 87 17 10 63 77 15 15 0.9 no reaction

TONc 17 21 22 28 31 65 13 29 6 3 21 26 5 5 0.3

Reaction conditions: methyl ester (0.5 mmol), ethanol (2.0 mL) and catalyst I at 75 °C for 24 h. bCalculated by 1H NMR method. cNumber of moles of ethyl ester per mole of catalyst.

a

Table 4. Transesterification Reaction of Various Methyl/ Ethyl Esters in Ethanol/Methanol Catalysed with the Zn− Cyanoximate Complex Ia entries

substrate

1 2 3 4 5 6 7 8 9 10 11 12

methyl-3-nitrobenzoate methyl-2-nitrobenzoate methyl-4-aminobenzoate methyl-4-iodobenzoate methyl-4-methoxybenzoate methyl-3-hydroxybenzoate methyl-2,4-dihydroxybenzoate methyl butyrate vinyl acetate ethyl-4-hydroxybenzoate ethyl acetate ethyl butyrate

alcohol

ethanol

methanol

yield (%)b

TON

91 89 72 47 10 22 18 33 21 54 63 37

30 29 24 16 3 7 6 11 7 18 21 12

a

Reaction conditions: 3.0 mol % of catalyst, methyl/ethyl ester (0.5 mmol) and ethanol/methanol (2.0 mL) at 75 °C for 24 h. bCalculated by 1H NMR method.

Figure 5. Plot of product yield vs time for the transesterification reaction of methyl-4-nitrobenzoate and ethanol catalyzed by I.

led to an overall yield of 91% after 30 h of reaction time at 75 °C.25 Other 3D frameworks of zinc(II) constructed with Zn(II) and 2-propionamidoterephthalate ligand led to an overall yield of 96% in the same reaction after 24 h at the higher temperature of 100 °C. Nevertheless, our catalyst I leads to a yield of 94% after 24 h reaction time at 75 °C.25 The above two examples show that our catalyst appears to be more efficient than other reported Zn(II) compounds (Figure 5). In addition, the transesterification reactions of phenyl or substituted phenyl acetate in different alcohols catalyzed by zinc compounds, to afford the corresponding substituted phenols and acetates, have been explored well.25 For example, the transesterification of phenyl acetate and methanol, in the presence of a dinuclear Zn(II) complex constructed by benzoate and quinoxaline ligands takes ca. 7 days for complete conversion to the corresponding products at 50 °C.25 The same research group has reported the use of a 1D Zn(II) coordination polymer of benzoate and 1,2-bis(4-pyridyl)ethane

ligands, which takes 1.5 days for the complete conversion of phenyl acetate to phenol and methyl acetate at same reaction temperature.25 They have also reported polymer-supported Zn catalysts which are more efficient for such a reaction (required 0.8 days at room temperature for complete conversion).25 Moreover, a mononuclear Zn(II) complex of 2,2′-dipyridylamine also effectively catalyzes the transesterification of phenyl acetate at room temperature, with complete conversion into the corresponding products in 0.25 days.25a A 1D Zn(II) coordination polymer of meta-bis(imidazolylmethyl)benzene ligand also catalyzes the reaction of phenyl acetate and benzyl alcohol in toluene with a 91% reaction yield at 120 °C in 5 h.25 The mechanism of the metal-catalyzed transesterification conceivably involves an electrophilic activation of the carbonyl moiety of the substrate upon binding to the metal center of the catalyst.19 Accordingly, the Lewis acidity of the metal center is relevant and a possible mechanism is shown in Figure 6. The J


Article

Inorganic Chemistry Accession Codes

CCDC 1562109 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Figure 6. Proposed catalytic cycle for the transesterification reaction catalyzed by I.

Nikolay Gerasimchuk: 0000-0003-4867-6475 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are very thankful to Dr. Jeannette Krause and Dr. Allen Oliver for the data collection at the synchrotron facility through the SCrALS (Service Crystallography at the Advanced Light Source) program at Beamline 11.3.1 at the Advanced Light Source (ALS) in the Lawrence Berkeley National Laboratory. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE−AC02− 05CH11231. We thank Bruker Analytical X-ray Instruments, Inc., Madison, WI, for APEX2 v2014.11.0 and SAINT v8.34A data collection and data processing programs, and Professor G. M. Sheldrick, University of Göttingen, Germany, for the use of the SADABS v2014.5 semi-empirical absorption and beam correction program. We also thank instructors Dr. Bruce Noll (Bruker AXS, Inc.) and Dr. Roger Sommer (North Caroline State University) at the American Crystallographic Association Small Molecules summer school for assistance with the structure refinement. This work has been supported by the Foundation for Science and Technology (FCT) (project UID/ QUI/00100/2013), Portugal. A.K. expresses his gratitude to the FCT for postdoctoral fellowship (ref. No. SFRH/BPD/76192/ 2011). N.G. is grateful to Dr. Sergey Lindeman (Marquette University, Milwaukee, WI, USA) for recording XRD powder diffraction patterns on a single crystal diffractometer in his laboratory and to Ms. Janet Dyer for invaluable technical help with the manuscript preparation.

ESI-MS spectrum (Figure S17) of the reaction mixture shows a peak at m/z 765.16, which can be accounted for by the formation of [Zn(PiCO){H(PiCO)2}·4-NO2PhCOOEt·EtOH· H]+. This suggests that 4-nitroethyl benzoate and ethanol add to the Zn center as we are proposing in the reaction mechanism. Future Work. The observations and results obtained from the studies of these compounds has fostered our interest for further exploration of other analogues, for probable activity in catalyzing useful reactions like epoxidation, transamidation, ring opening polymerization, C−C bond forming reactions, and so on.

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CONCLUSIONS First, zinc cyanoxime-based complexes were obtained in a Zn− PiCO− system and were characterized using spectroscopic methods, thermal analysis, and X-ray crystallography. Second, the crystal structure of the tetra-Zn complex II showed the formation of the tetra-zinc complex with μ3-bridging OHgroups, with cyanoxime anions demonstrating three different binding modes, as well coexistence of the oxime- and nitrosoanions. Third, there is a rare phenomenon in the structure of II of cocrystallization of the two dieastereomers of the cyanoxime to same metal centers at 0.57 (cis−anti) to 0.43 (cis−syn) ratio. Fourth, Zn(II) compound I acts as a good homogeneous catalyst for the transesterification reactions of methyl carboxylate esters. It effectively catalyzes the reaction of various methyl benzoate esters with ethanol producing the corresponding ethyl benzoate esters (and methanol) in high yields which depend on the electrophilicity of the benzoate substituent, under relatively mild conditions. The process has “green” features that deserve to be further explored for other reactions.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01891. Crystallographic information, photograph of solid-state accessory, applied restraints for final structure refinement, XRD patterns for compounds I and II, DSC/TGA profiles, IR spectra and vibrational frequencies, reflectance spectrum, complex II structures, unit cell structures, elemental analyses, 1H NMR, FT-IR, and ESI-MS spectra (PDF) K


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