Nickel Promoted Condensation of Acetamide and Benzonitrile for the

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Nickel Promoted Condensation of Acetamide and Benzonitrile for the Synthesis of an Imidoylamidine (N-NacNac) via Stable Imidoylamide Intermediate Celeo R. Guifarro Calona,*,‡ Alexander S. Filatov,§ Brian Pedro,‡ and Elena V. Rybak-Akimova†,‡ ‡

Tufts University, 62 Talbot Avenue, Medford, Massachusetts 02155, United States SUNY Albany, 1400 Washington Avenue, Albany, New York 12222, United States

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Downloaded by BOSTON UNIV at 09:38:06:048 on June 04, 2019 from https://pubs.acs.org/doi/10.1021/acs.organomet.9b00225.

S Supporting Information *

ABSTRACT: The acetamide-benzonitrile and benzonitrile self-condensation reactions promoted by nickel(II) hydroxy dimer 1, [Ni2(tBuDPA)2(μ-OH)2](ClO4)2 (tBuDPA = bis(2-methylpyridine) tert-butylamine), for the production of imidoylamidine (IDA) ligand, N-benzimidoylbenzamidine, is investigated by UV−vis, ESI-MS, FT-IR, elemental analysis, and single crystal XRD. An imidoylamide (IDD) intermediate 2, [Ni(tBuDPA)(IDD)]ClO4, is trapped and fully characterized, and its reactivity with benzonitrile is confirmed. Kinetic studies gave insight into the mechanism for the formation of 2 and its conversion to 3, [Ni(tBuDPA)(IDA)]ClO4. Formation of 2 is dependent on acetamide concentration, and its reactivity is affected by protic solvents, water, and methanol.

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INTRODUCTION The chemistry of imidoylamidine (IDA or N-NacNac) or 1,3,5-triazapentadiene ligands, all nitrogen analogues of acetylacetone (acac) and β-diketimine (NacNac), have been scantly investigated due to difficulties in developing reliable synthetic methods.1 However, acac2,3 and NacNac4,5 analogues (Figure 1) have been widely reported with broad scope

The yields of the reactions are low due to high byproduct formation and compound instability.1 Many of the aforementioned complications have been eliminated through metal mediated synthesis. Synthesis and evolving coordination chemistry of IDAs was reviewed lately.1 One-pot reactions of metal salts have been effective for the promotion of nitrile couplings with amidoxime,26,32−36 ammonium hydroxide,28,37 guanidine,38 or amidines.39 This approach has facilitated studies in M-IDA catalysis and coordination chemistry, yet most of these methods are limited as procedures are metal dependent, restraining the synthetic scope of ligands and catalysts. In addition, these multistep, multicomponent reactions are also difficult to study mechanistically. Rational approach to ligand and catalyst design relies on structure− reactivity correlations. In order to uncover steric and electronic substituent effects in the chemistry of IDAs and their complexes, versatile and convenient methods of incorporating substituents in the IDA platforms need to be developed. Of the 3D-metals, nickel is one of the most popular “substitutes” for noble metals in catalyzing bond-forming reactions, 40,41 including various nitrile couplings (e.g., reactions with hydrazines,42 pyrazoles,43 or diynes44−46). In biology, nickel(II) catalyzes hydrolysis and nucleophilic transformations of organic substrates, providing metal-bound nucleophile (terminal or bridging HO−); perhaps the bestknown example is enzymatic hydrolysis of urea,47 with related amide hydrolysis documented in synthetic model studies.48 The propensity of nickel(II) to activate nitriles and to support metal-bound hydroxide was demonstrated with biomimetic

Figure 1. Structural comparison of imidoylamidine (IDA), acetylacetone (acac), and β-diketimine (NacNac) ligands.

reactivity. IDA ligands have been demonstrated to bind a plethora of metals6−10 with variable denticity and coordination modes.11−17 Growing interest in IDA complexes is fueled by their potential applications as sensors or catalysts. Recently, MIDA complexes have been successfully probed in pHdependent luminescence18 and phosphorescence systems,19 catalysis of ring-opening polymerization of rac-lactide,20 ethylene polymerization,21,22 copolymerization of ε-caprolactone,23 in synthesis of substituted guanidines,24 atom transfers such as carbene and nitrene,25 Suzuki−Miyaura,26 Heck26,27 and Henry reactions,27,28 and DNA binding or cleavage29 and many additional applications inspired by related chemistry of acac and NacNac systems23,30,31 will likely emerge in near future. Synthesis of IDAs by organic methods are cumbersome, requiring toxic chemicals, anhydrous and harsh conditions. © XXXX American Chemical Society

Received: April 3, 2019

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DOI: 10.1021/acs.organomet.9b00225 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 1. Stepwise Condensation of Benzonitrile with Acetamide Promoted by Nickel Hydroxy Dimer 1

Figure 2. Displacement ellipsoid plot of complex 2. Displacement ellipsoids are drawn at the 40% probability level. Hydrogen atoms are omitted for clarity except those attached to N and O atoms. Selected metrical parameters (bond lengths in Angstroms (Å) and angles in degrees (°)): Ni−O1 2.015, Ni−O2 2.096, Ni−N1 2.009, Ni−N3 2.062, Ni−N4 2.278, Ni−N5 2.101, C2−O1 1.262, C2−N2 1.353, C3−N2 1.370, C3−N1 1.303; O1−Ni−O2 90.44, O1−Ni−N1 84.86, O1−Ni−N3 88.15, O1−Ni−N4 102.41, O1−Ni−N5 173.75, O2−Ni−N1 91.71, O2−Ni−N3 91.04, O2− Ni−N4 163.42, O2−Ni−N5 91.24, N1−Ni−N3 172.51, N1−Ni−N4 99.82, N1−Ni−N5 89.08, N3−Ni−N4 79.08, N3−Ni−N5 97.87, N4−Ni− N5 77.16.

complexes 1-TPA, [Ni2(μ-OH)2(TPA)2](ClO4)2 (TPA = tris(2-pyridylmethyl)amine), and 1, [Ni2(μ-OH)2(tBuDPA)2](ClO4)2, (tBuDPA = bis(2-methylpyridine)tert-butyl amine) (Figure S1). These complexes, enabled the self-condensation of acetonitrile49 and the coupling of acetamide with benzonitrile50 respectively with the reactions yielding IDA as a product either in the form of Ni(IDA)2 (Scheme S1) or 3, [Ni(tBuDPA)(IDA)]ClO4 (Scheme 1). In order to develop versatile, modular, and well-controlled approach for the synthesis of IDAs, we further explored the chemistry of nickel bis-hydroxy dimers, 1-TPA and 1. The selfcondensation of acetonitrile catalyzed by 1-TPA is slow, taking weeks at room temperature and hours to days upon heating.49 To improve reaction rates we replaced hexadendate TPA ligand by pentadentate tBuDPA to afford coordinatively unsaturated complex 1. The substitution showed that the additional cavity at the nickel centers in 1 allows for the timely coupling of acetamide and benzonitrile at room temperature yielding IDA overnight.50 Crystallographic studies identified heteroleptic IDA complex 3, [Ni(tBuDPA)(IDA)](ClO4), as the product of the reaction. Isotopic experiments with 15Nacetamide and unlabeled benzonitrile showed that the central nitrogen of the IDA is donated by acetamide. While, spectroscopic studies (UV−vis, IR, mass spectrometry) showed stepwise nature of the coupling indicated by the initial

formation of an imidoylamide (IDD) intermediate 2, [Ni(tBuDPA)(IDD)](ClO4) (Scheme 1). Complex 2 was trapped but not fully characterized nor tested in the original communication.50 This groundwork exposed a potential pathway for the controlled synthesis of IDAs from 2. It inspired us to further investigate the system with the goal to find a practical pathway to produce metal-free IDAs. Herein, we describe the isolation and characterization of 2; its intermediacy en route to IDA (Scheme 1) is established with successful synthesis of 3. With the aid of kinetic studies, we uncovered factors that control the rates of individual reaction steps, provided mechanistic insights, and allowed for rational reaction optimization.

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RESULTS AND DISCUSSION Synthesis and Characterization of [Ni(tBuDPA)(IDD)(H2O)]ClO4, 2. The synthesis of 2 was done using complex 1 due to its availability and for better data clarity. The perchlorate counterion was deemed best suited because other ions either coordinate to the nickel yielding an inert dimer or their signals masked the signals of the IDD or IDA ligand. Complex 2 is synthesized in 2 h by dissolving 1 in a saturated solution of acetamide (0.24 M) in benzonitrile. Complex 2 is isolated by precipitation with excess diethyl ether followed by vacuum filtration. Single crystals of 2 were grown B


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Organometallics

Scheme 2. Condensation of Amides with Nitriles Promoted by Nickel Complex 1 To Yield Imidoylamide Complex 2R′/R

benzonitrile.50 In the presence of trace amounts of methanol the conversion of 2 to 3 is enhanced significantly, allowing the transformation to be complete overnight (previously reported synthesis of 3 was accomplished by recrystallization of crude 2 from methanol50), at room temperature and with good yield (82%). Addition of excess methanol (>30 equiv per Ni) yields inert nickel(II) methoxy dimer, 1-OMe (Figure S1), lowering the yield of 3. Conversion of 2 to 3 was also observed by FTIR. The spectrum of crystalline 3 (Figure S8) shows characteristic signals corresponding to the IDA ligand consistent with previously reported complexes.32−35,49,55 FTIR of previously described complex 3 prepared from 1, acetamide and benzonitrile,50 also contained minor peaks from solvent impurities at 1689 and 1660 cm−1. The spectra show discernible differences between 2 (Figure S3) and 3 yet appropriate signal identification could not be done due to the heteroleptic nature of the complexes. The identity and purity of 3 prepared from 2 was confirmed by elemental analysis, ESIMS (Figure S9), and X-ray crystallography. The crystal structure of 3 synthesized in this work is identical to the structure reported previously,50 and the data will not be discussed here. The results suggest methanol is a reactant in transforming 2 to 3; further detailed studies support this hypothesis (vide infra). Methanol and other protic solvents must be avoided in the synthesis of pure 2, but methanol additives are beneficial in preparations of IDAs. Determining Reaction Scope by Amide Substitution. In addition to using acetamide plus benzonitrile in the conversion of 1 to 2, the versatility of the reaction was tested using different amides in order to determine whether substituted 2s could be synthesized (Scheme 2). Incorporation of 1,1-dimethylurea in place of acetamide in imidoylamide condensation products was previously reported.50 Four additional amides were tested in the present work: Nbenzylbenzamide, niacinamide, p-toluenesulfonamide, and acrylamide. In the case of acrylamide and niacinamide the reaction mixtures turned purple within an hour, while the reaction mixtures with the other two amides remained blue. Mass spectrometry indicated that the purple solutions contained the expected products with m/z = 486 and m/z = 475 for acrylamide (Figure S10) and niacinamide (Figure S11), respectively. Mass spectrometry confirmed that Nbenzylbenzamide (Figure S12), which differs from the other amides in that it is N-substituted, did not produce a variant of 2 when combined with 1 and benzonitrile or acetonitrile. Instead, benzonitrile hydrolysis was observed affording benzonitrile derived IDD (2Ph/Ph, m/z = 536). Lastly, ptoluenesulfonamide (Figure S13) yield the expected product, m/z = 586 along with 2Ph/Ph. On the basis of these results, it can be said that condensation is hindered by amide Nsubstitution suggesting that deprotonation is involved in amide-nitrile couplings. Bulky, olefinic and sulfonated amides can successfully substitute acetamide to yield substituted 2s.

from diethyl ether diffusion into acetonitrile solution. The purity and identity of the compound was confirmed by elemental analysis and ESI-MS (Figure S2). The FTIR of the purple crystals of 2 (Figure S3) show the disappearance of the non-hydrogen bonded μ-OH in 1 (3641 cm−1, Figure S4).51 The spectrum of 2 also shows additional signal clusters attributed to the IDD ligand (3327 cm−1 νNH, 1597 cm−1 νCN and 1570 cm−1 νCO). Previously reported band at 1668 cm−1,50 which is not observed in the IR of pure crystalline 2, likely originated from an impurity of acetamide in crude imidoylamide complex. Due to the heteroleptic nature of 2 and the overlap of signals between CN stretches (in pyridine groups of tBuDPA and the imine moiety in IDD) appropriate signal assignment could not be done. NMR spectra could not be acquired due to the paramagnetic nature of 2. The chemical structure of 2 was confirmed by single-crystal X-ray crystallography (Figures 2, S5 and Tables S1, S2). The complex has a distorted octahedral geometry with the nickel center coordinated by tBuDPA, an IDD, and a water molecule. The equatorial plane is composed of the two pyridine nitrogen atoms from tBuDPA (N3 and N5) and the nitrogen and oxygen donor atoms of the IDD ligand (O1 and N1), while the tertiary amine donor atom of tBuDPA (N4) and a water molecule (O2) coordinate axially. The bond angles found in the equatorial position are close to 90°. Nickel-donor atom bond lengths are typical of those observed for high-spin Ni(II) complexes.52 In accordance with previously reported NitBuDPA complexes,50,51 the longest Ni−N bond is seen for the tertiary amino group (2.278 Å for Ni−N4), and suggests a weak interaction with this axially coordinated donor atom. Apart from relatively weak coordinating ability of tertiary amine, geometric constraints imposed by the tBuDPA ligand likely contribute to elongating the Ni−N4 bond. Interestingly, even the axial interaction with the water molecule (Ni−O2, 2.096 Å) results in a shorter bond than the axial Ni-tertiary amine (N4) bond. The bond lengths of the IDD backbone are between 1.26−1.37 Å for the C−N bonds, falling within the range of precedent M-IDD complexes.53,54 The shortest bond length is the C−O bond (1.262 Å). Close values of C−N and C−O bond lengths are indicative of charge delocalization in a singly deprotonated IDD ligand. In contrast, protonated IDD ligand free form53 or metal bound,54 such as the one found in copper(II) pyridinophan-N-acetylamidine reported by Mirica et al., showed significant dissymmetry of the backbone, with distinguishable single (C−N, 1.37−1.39 Å) and double (C−O, 1.224 Å; terminal C−N, 1.282 Å) bonds.54 Determining the Intermediacy of 2. In order to test the intermediacy of 2 en route to 3, the reaction of pure, isolated 2 with excess benzonitrile was performed. Following the reaction by ESI-MS showed that 2 is stable in solution for days (Figure S6) and the conversion to 3 is visible in a week (Figure S7) and deemed complete in 3 weeks. The reaction yields 3 identical to the product prepared from 1, acetamide, and C


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Organometallics

Figure 3. (a) UV−vis spectral changes (left panel) and time course (right panel) for the formation of 2 (dotted line) in the reaction of 1 (solid line, 36 mM) and acetamide (140 mM) in 9:1 benzonitrile/acetonitrile at 25 °C. The spectra show an initial rapid decrease of 1 at 626 nm with consequent formation of 2 at 552 nm. (b) Plots of k1obs against acetamide concentration show a positive correlation with increasing acetamide with second-order rate constants for the formation of 2 of k1 = 7.85 × 10−3 M−1 s−1 at 552 nm (dashed line) and disappearance of 1 of k1 = 8.14 × 10−3 M−1 s−1 at 626 nm (solid line).

Figure 4. (a) UV−vis spectral changes (left panel) and time course (right panel) for the formation of 3 in the reaction of 1 (36 mM), acetamide (110 mM), and methanol (2.2 M) in 9:1 benzonitrile/acetonitrile at 25 °C. The spectra show an instantaneous decrease of 1 (solid line, 626 nm) with fast formation of 3 (dashed line, 559 nm) via intermediate 2 (dotted line, 552 nm). (b) Plots of observed rate, k2obs, against methanol concentration show a positive correlation with increasing methanol with second-order rate constant of k2 = 1.13 × 10−5 M−1 s−1 for the formation of 3 at 489 nm.

acetonitrile. Time-resolved UV−vis spectra clearly showed two separate processes: first, amide-assisted condensation, formation of 2 (Figure 3), and second, alcohol-assisted conversion of 2 to 3 (Figure 4). In addition, the reaction is temperature controlled. The first process is slowed down by lowering the temperature. As temperature increases, the second process (Figure S16) and additional reactions (Figure S17) are promoted. Addition of variable amounts of acetamide to benzonitrile solutions of 1 results in clean conversion of 1 to 2 (Figure 3), as evidenced by a decrease of the bands of 1 at 626 nm and concomitant growth of the absorption bands at 552, 774, and 826 nm attributed to 2; the time-resolved spectra showed three isosbestic points at 489 nm, 585 and 712 nm indicative of a single process taking place. Conversion of 1 to 2 was quantitated at 626 nm (disappearance of 1), 774 and 826 nm (formation of 2). Although the formation of 2 is also accompanied by the appearance and growth of intense absorbance peak in near-UV region (centered at λmax = 552 nm), the kinetic traces at this wavelength did not reflect exclusively the formation of 2 because of the overlap with the absorbance of 3 (λmax = 559 nm). The kinetic traces were fitted to a single-exponential equation

Additionally, acetone was found to be unsuccessful as a viable substitute for an amide in the reaction. Complex 2 Is Reversibly Protonated. Deprotonated IDD ligand in 2 undergoes reversible protonation upon reacting with acid in acetonitrile solutions, as demonstrated by UV−vis spectral changes. Incremental additions of triflic acid results in the decrease of absorption with λmax at 552, 774, and 826 nm and concomitant growth of an absorption band at 950 nm; these spectral changes were reversed in a nearly isosbestic manner upon incremental additions of t-butylammonium hydroxide in MeCN/MeOH (2 mL/30 μL), restoring the initial spectrum of 2 (Figures S14, S15). Growth of optical absorbance at ca. 450−500 nm upon base addition can be attributed to partial conversion of 2 into 3 described in detail below; this process is facilitated in basic conditions, especially in the presence of methanol. Kinetic Studies for the Reaction of Acetamide + Benzonitrile + 1. The role of individual components of the reaction of 1, acetamide, and benzonitrile was further probed by kinetic measurements. Benzonitrile and acetamide were the main reactants in the majority of UV−vis studies described below. Acetonitrile was used to dissolve 1; these solutions are relatively stable on the time scale of subsequent kinetic experiments. For kinetic measurements, a 9-fold excess of benzonitrile was added to a concentrated solution of 1 in

( −d[1] /dt = −[1]0 e−k1t + c) D


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Organometallics yielding the observed rate constants, k1obs; the values of k1obs did not depend on the wavelength (Table S3), confirming that one dominant chemical process is being observed over 2 h. The observed rate constants for the first step (conversion of 1 to 2) increased linearly with an increase in concentration of acetamide (Figure S18), in agreement with the first-order in acetamide: k1obs = k1[acetamide]

ization was not attempted. Qualitative observations of methanol-promoted acceleration of the formation of 3 prompted quantitative studies of protic solvent effects in the reaction. Water or methanol additives showed similar behavior: conversion of 1 to 2 was followed by subsequent conversion of 2 to 3 within hours at room temperature, and the rates of both reaction steps could be determined. The effect of methanol in the reaction of acetamide, benzonitrile, and 1 was investigated by keeping the concentration of acetamide constant (1.5 acetamide/Ni), adding varying concentrations of methanol, and following the time-resolved spectra (Figure 4). Spectral changes in the first step are similar to those observed under methanol-free conditions (Figure 3); however, in the presence of methanol, the reaction does not stop at 2. Formation of 2 is now followed by the decrease in absorbance at 774 and 826 nm and a large increase in absorbance in the near-UV region, with a growth of the new peak (λmax = 559 nm), in agreement with conversion of 2 to 3 (Figure 4). Kinetic traces at selected wavelengths were fitted to a single-exponential plus linear equation:

rate = k1[1][acetamide]

It can be concluded that acetamide is an important reactant in converting 1 to 2, and it is directly reacting with 1 (or its derivatives) in a rate-determining step. Insights into the Nitrile Self-Condensation Reaction Promoted by 1. In the absence of acetamide, addition of excess benzonitrile to the solution of 1 in acetonitrile did not cause immediate observable color changes, yet the UV−vis shows very rapid decay in absorbance at 626 nm (Figures S19, S20), which is attributed to the coordination of benzonitrile to the nickel centers. Over longer time (hours to days), visible spectra continue to slowly change, and following the reaction by ESI-MS shows that 1, m/z = 761, has reacted with benzonitrile to yield 2Ph/Ph, m/z = 536 (Scheme 3, Figure

( −d[1] /dt = −[1]0 e−k1t + k 2t + c)

where the single-exponential process (k1obs) corresponds to the condensation of acetamide with benzonitrile that yields 2, while the linear term (k2obs) corresponds to the initial rate in the transformation of 2 to 3 (the second step is far from completion under the conditions and time scale of our kinetic experiments, Figure 4). The second reaction step was best quantitated at 489 nm, where an isosbestic point was observed for the first process. The appropriate initial rates and rate constants are provided in Supporting Information Tables S4− S6. Plotting the k1obs against methanol concentration (M) (Figure S22) showed no correlation, revealing that methanol has no effect in the formation of 2. In contrast, plotting the k2obs versus methanol concentration (Figure S23) show a linear increase, in agreement with the first-order in methanol:

Scheme 3. Self-Condensation of Benzonitrile Promoted by Nickel Hydroxy Dimer 1 To Yield IDD Complex 2Ph/Ph

S21). The formation of 2Ph/Ph is thought to go through the partial hydrolysis of a benzonitrile, which generates a benzamidato that subsequently condenses with a second benzonitrile. Amidato intermediates have been previously proposed to be involved in IDA synthesis.36 Kinetic Studies for the Effect of Methanol in the Reaction of Acetamide + Benzonitrile + 1. In the absence of additives, conversion of 2 to 3, is very slow at room temperature (days to weeks), and its quantitative character-

k 2obs = k 2[MeOH]

rate = k 2[2][MeOH]

The second order rate constant for the formation of 3 in the presence of methanol, measured at 489 nm, is 1.13 × 10−5 M−1 s−1. The effect of methanol in the transformation of 2 to 3 was further confirmed by reacting pure complex 2 with benzonitrile

Figure 5. (a) UV−vis spectral changes (left panel) and time course (right panel) for the formation of 3 (dashed line) in the reaction of 2 (solid line, 40 mM) and methanol (500 mM) in 9:1 benzonitrile/acetonitrile at 25 °C. Spectra show that formation of 3 occurs in one step reflected by an isosbestic point at 697 nm and a fast increase in absorbance in the near-UV with an observed rate of k2obs = 3.93 × 10−5 s−1 (489 nm). Complex 2 disappears at an observed rate of k2obs = 4.90 × 10−5 s−1 (774 nm). (b) Plots of complex concentration (M) against time (s) show 2 (hollow diamond) disappears at similar rate as 3 (filled diamond) forms with initial rates of k2O = 1.96 × 10−6 M s−1 (774 nm) and 1.57 × 10−6 M s−1 (489 nm) respectively. E


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Organometallics Scheme 4. One Possible Mechanism for the Formation of 2 from 1 via Acetamide-Nitrile Condensationa

a Crystallographically characterized compounds are framed; hypothetical, unobserved intermediates are shown in brackets. Monomer−dimer equilibria are possible at various reaction steps.

Scheme 5. One Possible Mechanism for the Formation of 2 from 1 via Nitrile Self-Condensationa

a Characterized compounds are framed; hypothetical, unobserved intermediates are shown in brackets. Monomer−dimer equilibria are possible at various reaction steps.

reaction yielding 2Ph/Ph (m/z = 536) and the second reaction yielding 3 (m/z = 535). The data suggests that addition of water slows down the formation of 2 due to competition with acetamide for the metal centers in 1. Coordination of water to the nickel centers favor benzonitrile hydrolysis resulting in an amide (or amidato) that consequently couples with a second benzonitrile yielding 2Ph/Ph. We also observe competition for the water substrate between 1 and 2. Upon its formation or a derivative, 2 reacts with water and benzonitrile to yield 3. These results show that although water significantly slows down the first process, it enhances the second process (Figure S28). Overall, both methanol and water additives facilitate conversion of 2 to 3, but water also promotes side reactions; therefore, methanol is a more attractive additive for synthetic applications. It should be kept in mind that water effects may influence the outcome of benchtop condensations of nitriles with amides, where conversion of 2 to 3 or 2Ph/Ph may be promoted by moisture in air. Insights into the Mechanism. The above-mentioned studies provide insights into the mechanism of nitrile condensations promoted by 1. Complex 2 is an intermediate in the reaction pathway of IDA products. The first reaction step (formation of 2) involves both a nitrile and an amide (or amidato); rapid (although relatively minor) visible spectral changes are observed upon addition of benzonitrile to solutions of 1, and the rate of formation of 2 depends linearly on the amide concentration. In the absence of added amide, the reaction is much slower and proceeds via nitrile hydrolysis forming an amide or amidato that subsequently condenses with a second nitrile molecule. The presence of NH groups in the added amide is necessary and indicates deprotonation is required. Nucleophilic reactivity of hydroxide groups in 1 is established by the ability of this complex to promote benzonitrile hydrolysis. The exact sequence of events cannot be determined from the available experimental data, but reasonable pathways can be proposed. One possibility (in fact, a limiting case scenario) is the concerted “bimetallic” mechanism previously proposed by some of us.50

in the presence of methanol. The UV−vis spectra (Figure 5) of the reaction shows growth at 559 nm with a concomitant decrease at 774 and 826 nm. The spectra also show a tight isosbestic point at 697 nm demonstrating that one chemical process had taken place. Initial rate of this reaction is close to the initial rate of the second process in the reaction starting from 1 (Table S5). In the time span of 2 h, the purple solution had become gray-yellow and ultimately changed into dark yellow-brown overnight. The ESI-MS of the reaction kept overnight showed that 2 had converted to 3, same to the reactions performed with 1 (Figures S24, S25). These results further confirmed that methanol enhances the transformation of 2 to 3, and that 2 is an intermediate on the pathway from 1 to 3. Kinetic Studies for the Effect of Water in the Reaction of Acetamide + Benzonitrile + 1. Significant effects of methanol prompted us to probe possible effects of the most common protic solvent, water. Studies of the effect of water in the reaction of acetamide, benzonitrile, and 1 was performed by keeping the concentration of acetamide constant (1.5 acetamide/Ni), adding varying concentrations of water, and following time-resolved spectra. We found that conversion of 1 to 2 is slowed by water (Figure S26). The observed rate constant, k1obs, decreased linearly with an increase in concentration of water, in agreement with the following rate law: k1obs = k1[acetamide]/[H 2O] rate = k1[1][acetamide] /[H 2O]

The values of the second-order rate constants at different concentrations of water are provided in Supporting Information Table S7. To better understand the effect of water the reaction was also followed by ESI-MS. The ESI-MS of the reaction performed with added water (Figure S27) shows that overnight, two processes occurred: the self-condensation of benzonitrile and the conversion of IDD to IDA; with the first F


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Organometallics Scheme 6. Proposed Mechanism for the Alcohol Assisted Formation of 3 from 2a

a

Crystallographically characterized compounds are framed; hypothetical, unobserved intermediates are shown in brackets.

nucleophilic attack of methanol to the amide moiety of IDD. Benzonitrile is weakly bound to nickel(II); either metalcoordinated or free nitrile may undergo a nucleophilic attack. Methyl acetate will not remain coordinated and may be replaced by nitrile solvent, methanol, or water in solution. Water-assisted IDA formation likely proceeds via a similar pathway, although competing pathways are also accessible, notably, nitrile hydrolysis in the presence of water, which eventually leads to a mixture of products (resulting from the condensation of nitrile with an externally added amide, or an amide produced via nitrile hydrolysis). In synthetic applications, methanol as an additive is preferred.

In Scheme 4, 1 promotes acetamide-benzonitrile coupling to yield 2. One benzonitrile coordinates to one of the nickel centers and acetamide binds to the second nickel. The bridging hydroxide deprotonates the amide, which in turn, couples with the electrophilic carbon of the nitrile. This bimetallic pathway is similar to one of the proposed mechanisms of urea hydrolysis by urease, where a general base (depicted as a terminal hydroxide, but other options are possible) abstracts a proton from one of the urea nitrogen atoms.47 Alternatively, reversible dissociation of hydroxy-bridged dimer into monomeric species may occur, which would be facilitated by nitrile coordination to the nickel centers and by reversible proton transfer from/to water molecules in the solvent. Protonation of the hydroxy bridge may also occur as incoming amide is deprotonated prior to or during intramolecular condensation with nitrile at a single nickel center. In Scheme 5, the self-condensation of benzonitrile catalyzed by 1 goes through a bimetallic pathway. The IDD is the result of the nucleophilic attack of the bridging hydroxide to the electrophilic carbon in the nitrile moiety. In turn, it yields an amidato that subsequently couples with a nearby nitrile. Alternatively, dissociation of the hydroxy-dimer into monomeric species may occur. In this case, amide anion is the result of the nucleophilic attack of a terminal hydroxide to the electrophilic carbon of a bound or uncoordinated nitrile. Similar mechanisms have been proposed for IDD forming Cu systems.54 Nickel-bound terminal hydroxide as a nucleophile was implicated in a number of hydrolytic metalloenzymes and their models, including some proposed mechanisms of urease,47 and synthetic models of carbonic anhydrase,56,57 to name just a few examples. However, steric protection of the terminal hydroxide is usually needed to prevent dimerization; distant t-butyl substituent at the amine nitrogen in tBuDPA supporting ligand is unlikely to provide sufficient shielding on its own. Hydrated species of monomeric Ni(II)(tBuDPA) with coordinated acetonitrile has been documented.51 Qualitative observations on the behavior of monomeric nickel(II) complexes with tBuDPA ligand in nitrile solvents in the presence of hydroxide ions demonstrated the formation (and precipitation) of hydroxy-bridged dimer, 1, thus providing indirect support to the bimetallic mechanism. The second reaction step, formation of IDA (3) from IDD (2) (Scheme 6), is assisted by methanol or water, and likely proceeds via aminal intermediates (which were not directly observed in our work). Similar alcohol-assisted reactions are well-precedent,58,59 and examples include alcohol assisted Schiff base rearrangement60 and nucleophilic additions of methanol to cyanamides bound to copper(II),61,62 which supports the reasoning that IDA may be formed after the

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CONCLUSION Herein, we reported a benign method for the successful synthesis of IDD and IDA ligands using a Ni(II) template. Previously described complex 250 was isolated and characterized, and its intermediacy for the formation of IDA was established. The crystal structure of 2 has a distorted octahedral geometry with the nickel center facially bound by tBuDPA, an IDD, and a water solvent molecule. The Ni−OH2 bond length is significantly shorter than the apical Ni−Namine bond, implying its elongation is due to steric constraints on the t-butyl moiety in tBuDPA. Kinetic studies on its generation and its subsequent reactivity were done and from it plausible mechanisms were proposed. In the presence of 1, formation of IDDs is the result of the condensation of an amide or amidato with a nitrile molecule. The amide involved in the condensation is either generated in situ, as the product of a partially hydrolyzed nitrile, or can be directly added to speed the reaction. For synthetic purposes, protic solvents, such as water and methanol, should be avoided for the production of 2 as these can induce alternative reaction pathways or can further react with 2 yielding side products. Albeit, once isolated the addition of methanol to a solution of 2 and a nitrile is recommended for the speedy formation of 3. Despite recent advances most synthetic procedures for IDAs are limited in scope, and catalyst design tends to be restricted to one-pot synthesis with methodologies confined by the metal of choice. The identification of a simple and effective procedure for the tailored synthesis of IDAs is pressing,19 as this would open a new frontier in chemistry (facilitating ligand and catalyst design, the exploration of IDA and M-IDA reactivity, and the discovery of new and exciting chemistries) with big impact for industrial and pharmaceutical62,63 applications. In an attempt to find a solution, we identified a bioinspired method that can allow for the controlled synthesis of unsymmetrical IDAs from 2. Furthermore, the method can be adapted for the synthesis of IDDs, a class of ligands less G


Article

Organometallics

package.67 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms bonded to N1 and O2 were located on a difference Fourier map and refined with restraining equivalent isotropic displacement parameters to be 1.2 times the Ueq value of the respective atom. All other hydrogen atoms were included at idealized positions for structure factor calculations. Selected crystallographic data is summarized in Tables S1 and S2. Kinetic Studies. The condensation reaction of 1, acetamide and benzonitrile was investigated by executing various UV−vis studies. In general, the reactions were performed by dissolving 69 mg of 1 (0.08 mmol) in 0.2 mL acetonitrile followed by the addition of variant and subsequently diluting the mixture to 2.2 mL with additional benzonitrile. The reactions were followed by UV−vis for 2 h with 60 s intervals at 25 °C. To understand dependence on acetamide varying amounts of saturated solution of acetamide in benzonitrile (0.5−2 mL, 0.12−0.48 mmol) were added. Similarly, the effect of polar protic solvents was investigated. This was accomplished by adding varying amounts of either water (0.72−7.2 μL, 0.04−0.40 mmol) or methanol (3−65 μL, 0.08−1.60 mmol) while keeping the concentration of acetamide (0.24 mmol) constant. Temperature dependent reactions were performed by keeping the concentration of acetamide constant (0.24 mmol) and varying the temperature (10 °C − 75 °C). Furthermore, the effect of methanol on the reaction starting from 2 was also investigated. The reaction was done by dissolving 53 mg of 2 (0.09 mmol) in 0.2 mL acetonitrile followed by the addition of methanol (45 μL, 1.1 mmol) and diluting the mixture to 2.2 mL with additional benzonitrile. The reaction was followed by UV−vis for 40 min with 60 s intervals at 25 °C. Additionally, in order to probe whether complex 2 could be protonated titrations were performed. An acid titration was done by dissolving 70 mg of 2 (0.12 mmol) in 2 mL acetonitrile followed by the addition of trifluoromethanesulfonic acid aliquots (3−10 μL, 0.03−0.11 mmol). The solution was immediately back-titrated by adding aliquots (2−30 μL, 0.05−0.75 mmol) of nonstandardized 25 M solution of tetramethylammonium hydroxide in methanol.

commonly explored than IDAs, but that have been demonstrated to be useful in catalysis.53 Details on further reaction optimization for the synthesis of symmetrical and unsymmetrical IDAs and successful ligand liberation are forthcoming.

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EXPERIMENTAL SECTION

Materials and Methods. Chemicals were obtained from commercial sources and used as received. Ethanol was dried and distilled using Mg/I2 under argon64 prior to usage for the synthesis of 1. C, H, and N elemental analysis were carried out by Galbraith Lab, Inc. Infrared spectra of the solid were recorded, in KBr pellets, on a Nicolet MAGNA-IR 760. UV−vis spectra were acquired on a JASCO V-570 spectrophotometer. Kinetic fits were done manually utilizing Igor Pro. MS analysis was performed by electrospray mass spectrometry using a Finnigan LTQ (San Jose, CA) LCQ ion trap mass spectrometer in the positive ion detection mode. NMR spectra were measured on Bruker AVANCE 300 spectrometer at ambient temperature. Anaerobic experiments were performed on an argon filled vacuum atmosphere glovebox. All experiments were carried out under aerobic conditions unless otherwise specified. Caution! Perchlorate salts are all potentially explosive and should be handled with care. Synthesis of [Ni(tBuDPA)(IDD)(H2O)]ClO4, 2. The synthesis of 2 was done in the glovebox by dissolving 5.61 g of 1 (6.5 mmol) in 60 mL saturated solution (0.24 M) of acetamide (0.71 g) in benzonitrile (50 mL) at ambient temperature. The reaction mixture was left stirring for ∼2.5 h. Complex 2 was precipitated out in 500 mL diethyl ether and a pale purple powder was collected by filtration and washed with acetonitrile twice. Purple crystals were grown from acetonitrile by diethyl ether diffusion. Yield: 5.65 g (73.1%). Calcd. for C25H32ClN5NiO6: C, 50.66; H, 5.44; N, 11.82; Ni, 9.90. Found: C, 50.50; H, 4.99; N, 11.92; Ni, 10.7%. ESI-MS m/z = 474, [Ni(tBuDPA)(IDD)]+; UV−vis (MeCN) λmax/nm (ε/M−1 cm−1) 552 (9), 774 (6.7), 826 (8.0), 1133 (7.1); FT-IR (KBr) ν/cm−1: 3499 (νOH), 3327 (νNH), 2967, 1598 (νCO), 1569 (νCN), 1485, 1450, 1415, 1379, 1335, 1304, 1223, 1189, 1109, 1023, 928, 768, 728, 623. Synthesis of [Ni(tBuDPA)(IDA)]ClO4, 3. The synthesis of 3 can be accomplished by a variety of routes, and only the two most effective are discussed herein. Complex 3 is successfully synthesized with same results starting either from 1 (Route 1) or 2 (Route 2). Route 1. In the glovebox, 2.00 g 1 (2.32 mmol) were dissolved in 50 mL of saturated solution of acetamide in benzonitrile and 5 mL of methanol subsequently added. The green/blue mixture was stirred for 1 day, at which point the product was precipitated in 200 mL diethyl ether. The yellow powder was collected by vacuum filtration. Yield: 2.42 g (82%). Route 2. Complex 2 (0.53 g, 0.89 mmol) was dissolved in 10 mL benzonitrile with subsequent addition of 0.25 mL methanol. The mixture was stirred for 1 day at which point the brown solution was precipitated in 100 mL diethyl ether and a yellow powder was collected by filtration. The yellow powder was crystallized from methanol by diethyl ether diffusion. Yield: 0.48 g (85.2%). Calcd. for C30H33ClN6NiO4: C, 56.67; H, 5.23; N, 13.22; Ni, 9.23. Found: C, 56.91; H, 5.40; N, 13.58; Ni, 9.67%. ESI-MS m/z = 535, [Ni(tBuDPA)(IDA Ph/Ph)]+; UV−vis (MeCN) λmax/nm (ε/M−1 cm−1) 558 (60), 314 (3,619), 241 (15,350); FT-IR (KBr) ν/cm−1: 3360 (νNH), 3333 (νNH), 3069, 2971, 1607 (νCN), 1588, 1545, 1460, 1395, 1318, 1293, 1253, 1198, 1161, 1090, 973, 935, 858, 803, 768, 709, 622. X-ray Structure Determination. The X-ray intensity data were measured at 100 K on a Bruker SMART APEX CCD or Bruker D8 Quest X-ray diffractometer system equipped with a Mo-target X-ray tube. The frames were integrated with the Bruker SAINT software package65 using a narrow frame integration algorithm. The data were corrected for absorption effects using the empirical method (SADABS).66 The structures were solved by direct methods and refined using the Bruker SHELXTL (Version 6.14) software

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.9b00225. Experimental details, structural comparison of nickel dimers, reaction scheme for the self-condensation of acetonitrile promoted by Ni(TPA) dimer, ESI-MS, FTIR, and UV−vis spectra, XRD crystallography, and kinetic data (PDF) Accession Codes

CCDC 1895052 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 [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

Celeo R. Guifarro Calona: 0000-0002-5957-5066 Alexander S. Filatov: 0000-0002-8378-1994 Funding

This work was supported by the NSF (CHE 0750140) and the DOE (DE-FG02-06ER15799 to ERA). The NMR facility, the kinetic instrumentation, and the ESI-MS spectrometer at Tufts H


Article

Organometallics

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University were supported by the NSF Grants CHE-MRI 0821508, CHE-CRIF 0639138, and CHEM-MRI 0320783. Notes

The authors declare no competing financial interest. † Deceased March 11, 2018

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ACKNOWLEDGMENTS AF is very grateful to the University at Albany for supporting the X-ray center at the Department of Chemistry. Dedicated to the memory of Elena V. Rybak-Akimova, educator, mentor, and friend.

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