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Nucleophilicity of Oximes Based upon Addition to a Nitrilium closoDecaborate Cluster Dmitrii S. Bolotin,† Valeria K. Burianova,† Alexander S. Novikov,† Marina Ya. Demakova,† Carla Pretorius,‡ Pennie Petrus Mokolokolo,‡ Andreas Roodt,*,‡ Nadezhda A. Bokach,† Vitaliy V. Suslonov,§ Andrey P. Zhdanov,∥ Konstantin Yu. Zhizhin,∥ Nikolay T. Kuznetsov,∥ and Vadim Yu. Kukushkin*,† †

Institute of Chemistry, Saint Petersburg State University, Universitetskaya Nab., 7/9, Saint Petersburg 199034, Russian Federation Department of Chemistry, University of the Free State, P.O. Box 339, Bloemfontein 9300, South Africa § Center for X-ray Diffraction Studies, Saint Petersburg State University, Universitetskii Pr., 26, Saint Petersburg 198504, Russian Federation ∥ N. S. Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences, Leninsky Pr., 31, Moscow 119991, Russian Federation ‡

S Supporting Information *

ABSTRACT: Three types of oxime species, i.e., 4-morpholylcarbamidoxime (hydroxyguanidine), phenylacetamidoxime and benzamidoxime (amidoximes), and cyclohexanone oxime and benzophenone oxime (ketoximes), react at room temperature with the 2-nitrilium closo-decaborate clusters, leading to 2-iminium closo-decaborates (14 examples; 57−94%). These species were characterized by ICPMS-based boron elemental analysis, HRESI−-MS, molar conductivity, IR, 1 H{11B}, and 11B{1H} NMR spectroscopies, and additionally by single-crystal Xray diffraction (for six compounds). On the basis of kinetic data, ΔH⧧, ΔS⧧, and ΔG⧧ of the additions were determined, showing a 4 order-of-magnitude decrease in reactivity from the hydroxyguanidine to the aromatic ketoxime as entering nucleophiles. The results of DFT calculations indicate that the mechanism for these reactions is stepwise and is realized through the formation of the orientation complex of the nitrone form, R2R3CN+(H)O−, of oximes with [B10H9NCEt]−, giving further an acyclic intermediate (the rate-determining step), followed by proton migration, leading to the addition product. The calculated overall activation barrier for these transformations is consistent with the experimental kinetic observations. This work provides, for the first time, a broad nucleophilicity series of oximes, which is useful to control various nucleophilic additions of oxime species.

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INTRODUCTION

of reactions does not affect the cluster unit but includes substitution of one or two hydrides by NR3,7 OH (water and OH−),8 OR and OR2,9 MeCO2,10 phen,10a NCO,11 CO,11 and also by RCN.7 When the closo-decaborates are functionalized with a nitrile group, they could be easily derivatized further by nucleophilic addition to the nitrile C atom. Indeed, linkage of the CN group to the closodecaborate cluster facilitates addition of HO-,12 HN-,13 and HC-nucleophiles14 to the C atom of the CN moiety and also promotes, as we found, 1,3-dipolar cycloaddition of nitrones15 and azides16 to this bond. Nucleophilic addition of oximes to nitrilium closo-decaborate clusters was not previously explored, albeit this type of addition is known in coordination chemistry when an oxime adds to a metal-activated nitrile ligand.17 In metal-free chemistry,

In the past 15 years, polyhedral borane clusters have been a subject of significant attention due to useful properties of these boron-containing species in inorganic1 and coordination chemistry,2 and also in view of their potential application in pharmacology, where they are generally used in boron neutroncapture cancer therapy3 and also as antiviral agents,4 inhibitors of platelet aggregation,3e and modulators of important hormone receptors.3e Although the chemistry of a closo-dodecaborate cluster and its derivatives has drawn a lot of attention, the chemistry of its lighter congener, viz. closo-decaborate, has been studied in a substantially lesser extent. Two types of reactions leading to functionalization of closodecaborate clusters are known. Reactions of the first type proceed via redox destruction of the cluster moiety and include either reduction to the nido-decaborane [B10H14] or 6substituted nido-boranes [B10H13X] (X = OH, F, Cl, Br, I),5 or oxidative coupling leading to [B20H18]4−.6 The second type © XXXX American Chemical Society

Received: August 25, 2016

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DOI: 10.1021/acs.organomet.6b00678 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

from the reaction mixtures in the range of 1−15 min as the colorless solids, and these species were isolated in 57−94% yields (Scheme 1 and Table 2). Compounds (Ph3PCH2Ph)[7− 8] and (Ph3PCH2Ph)[12] remain in solutions, and all attempts of their isolation as solids gave oily residues contaminated with some impurities, including MeCN and the corresponding oxime. Despite these synthetic difficulties, we succeeded in situ characterization of (Ph3PCH2Ph)[7−8] and (Ph3PCH2Ph)[12] by HRESI−-MS, and 1H{11B} and 11B{1H} NMR techniques. Amidoxime derivatives (Ph3PCH2Ph)[2−3], (Ph3PCH2Ph)[5−6], (Ph3PCH2Ph)[10−11], and (Ph3PCH2Ph)[13−14] undergo hydrolysis in undried solvents (MeCN, MeOH) at RT over a 1 month period, whereas aliphatic ketoxime derivative (Ph3PCH2Ph)[7] hydrolyzes at RT for 1 week. Inspection of the HRESI−-MS data indicated that the hydrolytic reactions lead to (Ph3PCH2Ph)[B10H9NH3],7 (Ph3PCH2Ph)[B 10 H 9 N(H)C(OH)R 1 ] (Table 7S), and the parent R2R3CNOH (Scheme 2, a−e). In addition, the amidoxime derivatives generate 3-substituted 5-ethyl-1,2,4-oxadiazoles detected in trace amounts by HRESI+-MS (f); the latter reaction between nitriles and amidoximes has been studied21 by us earlier. Hydroxyguanidine derivatives (Ph3PCH2Ph)[1], (Ph3PCH2Ph)[4], (Ph3PCH2Ph)[9], and (Ph3PCH2Ph)[12] decompose in solutions at RT for approximately 2 weeks, giving a broad mixture of products, whereas aromatic ketoxime derivative (Ph3PCH2Ph)[8] in solutions at RT decomposes after 4 days. In the cases of (Ph3PCH2Ph)[2−3], (Ph3PCH2Ph)[5−7], (Ph3PCH2Ph)[10−11], and (Ph3PCH2Ph)[13−14], the hydrolysis in a MeOH/H2O (10/1, v/v) mixture completes after 4 h at 60 °C. A plausible mechanism of hydrolysis includes, first, nucleophilic attack of H2O on the imine C atom (Scheme 2, a), followed by the elimination of the oxime (b) or O-alkanoyl oxime (c), leaving the iminol or the ammonium salt, respectively. The iminol undergoes hydrolysis to the ammonium salt (d),12 whereas O-alkanoyl oximes hydrolyze to the parent oxime and aliphatic carboxylic acid (e). In addition, O-alkanoyl amidoximes undergo heterocyclization to 1,2,4-oxadiazoles22 formed as minor products (f).17b Analytical and Spectroscopic Data. Clusters (Ph3PCH2Ph)[1−6], (Ph3PCH2Ph)[9−11], and (Ph3PCH2Ph)[13− 14] give satisfactory ICPMS-based B elemental analysis for the proposed formulas. These species were also characterized by HRESI−-MS, molar conductivity, IR, 1H{11B}, and 11B{1H} NMR spectroscopies, and additionally by single-crystal X-ray diffraction study (for (Ph3PCH2Ph)[3−6], (Ph3PCH2Ph)[10], and (Ph3PCH2Ph)[13]). Molar conductivities of (Ph3PCH2Ph)[1−6], (Ph3PCH2Ph)[9−11], and (Ph3PCH2Ph)[13−14] are in the range 93.0−110.8 S cm−1 mol−1, which is somehow lower than that expected for 1:1 electrolytes (120− 160 S cm−1 mol−1 in MeCN23). However, many other 1:1 electrolyte closo-decaborate species in MeCN exhibit conductivities in the range 119−127 S cm−1 mol−1,14 and our experimental values are well coherent with these data. Clusters

nucleophilic addition of oximes to the CN bond of nitriles proceeds, on the one hand, exclusively for nitriles RFCN featuring very strong electron-withdrawing groups, typically perfluoroalkyls, accomplishing O-iminoacyl oximes at low temperatures. This reaction gives RFC(NH)ONCRR′, which then easily splits to the parent substrates even on gentle heating.18 On the other hand, the irreversible addition can be realized for strongly activated N-alkyl and N-aryl nitrilium salts [RCNR′]+ featuring a 1+ positive charge.19 Owing to the general importance of nucleophilic addition of oximes to unsaturated substrates17b,20 and, in particular, their addition to the CN functionalized closo-decaborate clusters, we decided to study the possibility of integration of oximes with a nitrilium closo-decaborate cluster bearing, in contrast to nitrilium salts [RCNR′]+, a 1− negative charge and where the nitrile group is linked to the boron cluster unit. Driven by the success of our synthetic experiments, facile occurrence, and high selectivity of this reaction, we undertook a systematic mechanistic study of the addition that included both kinetic work and theoretical considerations, and as a result, we obtained a broad series of nucleophilicity for oximes that includes a wide-ranging variation of their structural types. This series, in spite of the rich and versatile chemistry of RR′C NOH species, has never been established in the past, and it should thus allow for better control of various additions of oximes, including also the rational tuning of reactants and reaction conditions. All of our results are consistently disclosed in the sections that follow.

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RESULTS AND DISCUSSION Nucleophilic Addition of Oximes to Nitrilium closoDecaborates. As the starting materials for the study of the nucleophilic addition of oxime species to 2-nitrilium closodecaborate, we addressed on the one hand three types of oxime species, i.e., (i) the hydroxyguanidine (OX1; Figure 1 and

Figure 1. Oxime species addressed in this study.

Table 1), (ii) the aliphatic and aromatic amidoximes (OX2 and OX3, respectively), and (iii) the aliphatic and aromatic ketoximes (OX4 and OX5, respectively). On the other hand, the nitrilium clusters (Ph3PCH2Ph)[B10H9NCR1], abbreviated as (Ph3PCH2Ph)[NC1−4] (R1 = Me NC1, Et NC2, nPr NC3, i Pr NC4), were taken as a substrates. The reaction between (Ph3PCH2Ph)[NC1−4] and any one of OX1−5 at room temperature (RT) in 1:1.05 (OX1−4) or 1:2 (OX5) molar ratios in MeCN proceeds for 1 min (OX1− 3) or 1 h (OX4−5) and leads to 2-iminium closo-decaborates (Ph 3 PCH2 Ph)[1−14]. Compounds (Ph 3 PCH 2Ph)[1−6], (Ph3PCH2Ph)[9−11], and (Ph3PCH2Ph)[13−14] precipitate

Table 1. Oxime Species Numbering and Corresponding Types of Oxime Species oxime nos. substituents type of oxime

OX1

OX2

OX3

OX4

OX5

OC4H8N/NH2 hydroxyguanidine

PhCH2/NH2 aliphatic amidoxime

Ph/NH2 aromatic amidoxime

(CH2)5 aliphatic ketoxime

Ph/Ph aromatic ketoxime

B


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Organometallics Scheme 1. Nucleophilic Addition of the Oxime Species to (Ph3PCH2Ph)[NC1−4]

Table 2. Compound Numbering and Yields nos.

R1

(Ph3PCH2Ph)[1] (Ph3PCH2Ph)[2] (Ph3PCH2Ph)[3] (Ph3PCH2Ph)[4] (Ph3PCH2Ph)[5] (Ph3PCH2Ph)[6] (Ph3PCH2Ph)[7]

Me Me Me Et Et Et Et

R2 NC4H8O CH2Ph Ph NC4H8O CH2Ph Ph (CH2)5

R3

yield (%)

nos.

R1

R2

R3

yield (%)

NH2 NH2 NH2 NH2 NH2 NH2

64 93 78 68 85 94 >99 (NMR)

(Ph3PCH2Ph)[8] (Ph3PCH2Ph)[9] (Ph3PCH2Ph)[10] (Ph3PCH2Ph)[11] (Ph3PCH2Ph)[12] (Ph3PCH2Ph)[13] (Ph3PCH2Ph)[14]

Et n Pr n Pr n Pr i Pr i Pr i Pr

Ph NC4H8O CH2Ph Ph NC4H8O CH2Ph Ph

Ph NH2 NH2 NH2 NH2 NH2 NH2

>99 (NMR) 57 74 78 >99 (NMR) 92 88

Scheme 2. Plausible Mechanisms for Hydrolysis of the Clusters

Figure 2. Molecular structures of [3]− (left), [4]− (center), and [10]− (right) showing the atomic numbering scheme. Thermal ellipsoids are given at the 50% probability level.

group, and weak to medium band at 3296−3215 cm−1, assignable to the N−H stretches of the N−H···N moiety, were observed. Weak to medium bands in the region of 3063− 2849 cm−1 are assignable to the C−H stretches.24 The spectra also all display one strong band in the 2475−2459 cm−1 region specific for ν(B−H) of the closo-decaborate cluster.7 In addition, the spectra display a strong to very strong band at 1628−1611 cm−1, which is characteristic for ν(CN) of the oxime and the imine moieties.24 The 1H{11B} NMR spectra (Supporting Information) of (Ph3PCH2Ph)[1−14] display a set of signals from [Ph3PCH2Ph]+ (7.91−7.88, 7.73−7.66, 7.64−7.56, 7.37−7.35, 7.29− 7.22, 7.00−6.97, 4.75−4.66 ppm). A characteristic feature of the 1H{11B} NMR spectra is the presence of a low-field broad singlet at 8.52−8.21 ppm, which is characteristic of the Nimine−

(Ph 3 PCH 2 Ph)[1−6], (Ph 3 PCH 2 Ph)[9−11], and (Ph 3 PCH2Ph)[13−14] are stable in the solid state at RT but decompose at 104−157 °C. The negative mode high-resolution ESI mass spectra of (Ph3PCH2Ph)[1−14] (Supporting Information) exhibit sets of peaks corresponding to the molecular ions [M]− and the [2 M + Ph3PCH2Ph]− ions. Specific features of the mass spectra of (Ph3PCH2Ph)[1−14] is the presence of intensive peaks of fragmentation ions [M − R2R3CN]•−, which probably derived from the homolytic splitting of the N−O bond under the experimental conditions. In the IR spectra of (Ph3PCH2Ph)[1−6], (Ph3PCH2Ph)[9− 11], and (Ph3PCH2Ph)[13−14] (Supporting Information), medium to strong bands at 3497−3441 and 3360−3327 cm−1, which can be attributed to the N−H stretches of the NH2 C


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Organometallics H···Noxime resonance. The spectra of (Ph3PCH2Ph)[1−6] and (Ph3PCH2Ph)[9−14] display broad singlets at 6.13−5.25 ppm from two protons assignable to NH2 resonances. Another characteristic feature of the 1H{11B} NMR spectra is the presence of two broad singlets at 3.46−3.34 and 3.13−3.24 ppm, each with integral intensity of one H assignable to the axial hydrides of the closo-decaborate cluster. The signals of the residual hydrides are located at 0.99−0.87, 0.53−0.40, 0.30− 0.16 (broad singlets, each with integral intensity of two hydrides assignable to the B(3)H and B(5)H, B(6)H and B(9) H, B(7)H and B(8)H), and 0.21−0.08 ppm (broad singlet with integral intensity of one hydride assignable to the B(4)H). The 11 1 B{ H} NMR spectra of (Ph3PCH2Ph)[1−14] are typical for 2-substituted closo-decaborates.12,25 The 13C{1H} NMR spectra were not measured due to poor solubility of the products in MeCN. X-ray Structure Determination. In the molecular structures of (Ph3PCH2Ph)[3]·MeCN, (Ph3PCH2Ph)[4], (Ph3PCH2Ph)[5]·1/2MeCN, (Ph3PCH2Ph)[6]·MeCN, (Ph3PCH2Ph)[10], and (Ph3PCH2Ph)[13], the closo-decaborate cluster exhibits the usual bicapped square bipyramidal geometry (Figure 2). 7,12,14−16,25 The N(1)−B(2) bond lengths (1.5291(19)−1.540(4) Å) are typical for closo-decaborate bound imines.7,12,14−16,25 The N(1)−C(1) bonds are double bonds (1.283(2)−1.2926(15) Å),7,12,25 whereas the N(2)− C(2) and N(3)−C(2) distances are intermediate between single and double bonds (1.296(2)−1.305(4) and 1.331(2)− 1.3381(15) Å, respectively).17b The O(1)−N(2) distances (1.4563(16)−1.4726(18) Å) are longer than the single N−O bond in various amidoximes, which is a characteristic feature of O-iminoacylamidoximes.21b,26 The O-iminoacylamidoximes are in the E-configuration around the N(1)−C(1) bond, and the O(1) and the N(3) atoms are in the sin position at the N(2)−C(2) bond, which is common for N-bound O-iminoacylamidoximes to various metal centers.21b,26 The E-form of the imine part of the molecule provides an H atom for intramolecular H-bonding between this iminium H atom and the oxime N atom (N(1)···N(2) 2.543− 2.599 Å; N(1)−H···N(2) 109.40−113.21°); the same type of hydrogen bonding was previously observed when iminoacylated oximes are bound to metals.21b,26 All bond lengths and angles in the Ph3PCH2Ph+ counterion are typical for this cation.27 We have defined the energy for hydrogen bonding N−H···N in the equilibrium structure of [3]− in acetonitrile solution (25−26 kJ/mol) from a theoretical point of view using DFT calculations, followed by the topological analysis of the electron density distribution within the formalism of QTAIM;28 for details, see the Supporting Information (this approach has already been successfully used by us in studies of noncovalent interactions and properties of coordination bonds in various transition metal complexes21c,29). We assume that, despite a moderate strength, the hydrogen bonding N−H···N is sufficiently strong to control the reaction channel and determine the configuration of the product (see the Theoretical Study of the Nucleophilic Addition section, later). Kinetics. The kinetic measurements were conducted for the addition of any one of OX1−5 to the 2-propanenitrilium closodecaborate, (Ph3PCH2Ph)[NC2]. Only one addition reaction was observed spectrophotometrically (Figure 3), for all the different entering nucleophiles R2R3CNOH studied, as defined by eq 1, ensuring that it can be evaluated by a single exponential to determine the observed pseudo first-order rate constant kobs for all the reactions. The rates varied from slow

Figure 3. UV−vis spectral changes of absorbance for the addition of the oxime nucleophiles to (Ph3PCH2Ph)[NC2] (MeCN; 25 °C). Insets indicate fits of Abs vs time data to a first-order exponential30 at 330 nm. (a) Slower − conventional UV/vis data of OX4; Δt = 1 min, ttotal = 180 min. [OX4] = 10−2 M, [(Ph3PCH2Ph)[NC2]] = 5 × 10−5 M; (b) Faster − diode-array stopped-flow data for OX2: Δt = 500 ms, ttotal = 10 s. [OX2] = 5 × 10−3 M, [(Ph3PCH2Ph)[NC2]] = 10−4 M.

(OX5, UV/vis scanning spectrophotometer) to progressively faster (OX1, stopped-flow). k1, K1

(Ph3PCH 2Ph)[NC2] + OX1−5 XooooY (Ph3PCH 2Ph)[4−8] k −1

(1)

The concentration dependence of the pseudo first-order rate constant (kobs) for the addition of the oximes R2R3CNOH to (Ph3PCH2Ph)[NC2] is given by eq 2.31 The kinetics was monitored under conditions where [R2R3CNOH] ≫ [(Ph3PCH2Ph)[NC2]], with the typical substrate concentration range 5 × 10−5 to 1 × 10−5 M and oxime range from 7 × 10−4 to 5 × 10−2 M. kobs = k1[OX1−5] + k −1

(2)

The second-order rate constants for the slow and fast reactions, respectively, obtained from typical plots as illustrated in Figure 4, are reported in Table 3. Rate constants of the reverse reactions are from 10−7 to 10−10 s−1 and are all approximate zero within esd. This implies that the dissociative reverse elimination reactions are very slow, and negligible in comparison with the forward ones and may be omitted in further discussions. Analysis of the kinetic data indicates that the strong +M donor NR2 groups at the oxime CNOH moiety significantly accelerate the reaction due to a stepwise reduction of ΔG⧧ of the reactions. On the other hand, the reaction is accelerated by the methyl substituents at the oxime moiety in comparison with the phenyl groups, and changes in the reaction rates logically D


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Organometallics

Figure 5. Eyring plots [ln(k/T) vs 1/T] for the addition of OX1−5 to (Ph3PCH2Ph)[NC2] in MeCN.

The consistent negative values of the entropy of activation (ΔS⧧(k1), Table 4) for all the entering nucleophiles also indicate that the addition of the oximes to the nitrile group of the boron cluster is in agreement with an associative mechanism. The contribution of the activation entropy to the overall ΔG⧧298 for the nucleophilic addition reaction of OX5 to (Ph3PCH2Ph)[NC2] is less than 20%, suggesting that the transition state for this reaction is significantly dependent both on bond breaking/ formation and, in addition, ordering is less important. However, by going from OX4 to OX3, the bond breaking/formation becomes less important and the ordering in the transition state increases to 44% and 61%, respectively. Comparable large contributions of the activation entropy to the overall ΔG⧧298 are observed for OX2 and OX1. Moreover, the definite “break” in reactivity of more than 2 orders-of-magnitude between the groups OX1−3 vs OX4−5 (i.e., 0.152 ± 0.002 M−1 s−1 for OX4 vs 16.21 ± 0.09 M−1 s−1 for OX3) is clear (Figure 5). As mentioned above, and in conjunction with the corresponding activation enthalpy values, it is in good agreement with the electronic characteristics of the substituents on the entering oximes (Figure 1, Table 3). The amidoximes (OX1−3) are clearly more reactive, indicating a probable stabilization of the transition state by the NH2-moiety, compared to the aliphatic (OX4) or aromatic (OX5) ketoximes. The progressive 16-fold increased reactivity of OX1 (hydroxyguanidine) to OX2 (benzyl) and OX3 (phenyl) (250 ± 3 M−1 s−1 vs 77 ± 1 vs 16.21 ± 0.09) is in line with the increased electron-donating ability of the substituents on the amidoximes. Similarly, the 7-fold increase in the forward rate constant upon replacing the two phenyl groups in OX5 with the aliphatic chain in OX4 agrees with the increased electron-donating ability of the (CH2)5 entity. Theoretical Study of the Nucleophilic Addition. With an aim to further understand the mechanism for nucleophilic addition of oximes to a 2-nitrilium closo-decaborate cluster, theoretical DFT calculations were performed for the reaction [NC2]− + OX3 → [6]−. Three possible reaction paths have

Figure 4. Plot of kobs vs entering [nucleophile] for the addition reaction of OX2 to (Ph3PCH2Ph)[NC2] at various temperatures in MeCN, yielding linear plots; [(Ph3PCH2Ph)[NC2]] = 5 × 10−5 M (λ = 330 nm).

are less prominent than upon the case of introduction of the NR2 group at the oxime moiety. Comparison of the obtained rate constants obtained with those reported for the addition of the ketoxime (PhCH2)2C NOH to [PtCl5(NCEt)]− giving [PtCl5(HNC(Et)ON C(CH2Ph)2]−32 clearly indicates that the closo-decaborate cluster activates the nitrile moiety significantly more than even the platinum(IV) center. It is clear from Table 3 that a more than 4 orders-ofmagnitude difference in reactivity is observed for OX1−5, indicating that the different oxime types of nucleophiles add at significantly different rates to the corresponding 2-propanenitrilium closo-decaborate cluster. This is manifested in the approximate 30 kJ mol−1 decrease in the ΔH⧧, translating to about 20 kJ mol−1 in the free energy of activation, ΔG⧧. This indicates the significant dependence of the rate on the entering group and points to an associative type of process for the nucleophilic addition reactions reported here. This observation is also qualitatively concluded from the time required for the syntheses of the different complexes as illustrated in the synthetic part below. The fact that the forward rate constant (k1) for the addition of the five different entering nucleophiles to the same nitrilium substrate varies by more than 4 orders-of-magnitude indicates the significant dependence on the entering group and a clear evidence favoring an associative type of process. Activation Parameters. The individual kinetic data points for each of the five entering nucleophiles were fitted to a global model, yielding values for the activation parameters that were in excellent agreement with those obtained from the classic Eyring plots (Figure 5);33 the collected data are given in Table 4.

Table 3. Rate Constants for the Reactions between (Ph3PCH2Ph)[NC2] and the Oximes in MeCN at Different Temperatures nucleophile

R1/R2

k1 (M−1 s−1), 35 °C

k1 (M−1 s−1), 25 °C

k1 (M−1 s−1), 15 °C

k1 (M−1 s−1), 5 °C

OX5 OX4 OX3 OX2 OX1

Ph/Ph (CH2)5 Ph/NH2 PhCH2/NH2 OC4H8N/NH2

0.097 ± 0.003 0.317 ± 0.004 24.9 ± 0.2 91 ± 2 483 ± 3

0.022 ± 0.002 0.152 ± 0.002 16.21 ± 0.09 77 ± 1 250 ± 3

0.015 ± 0.002 0.075 ± 0.006 12.0 ± 0.2 41 ± 1 175 ± 3

0.005 ± 0.002 0.036 ± 0.006 7.6 ± 0.4

E


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Organometallics

Table 4. Activation Parameters for the Nucleophilic Addition of R2R3CNOH to (Ph3PCH2Ph)[NC2] in Acetonitrile at 25 °C nucleophile

R1/R2

k1 (M−1 s−1)

OX5 OX4 OX3 OX2 OX1

Ph/Ph (CH2)5 Ph/NH2 PhCH2/NH2 OC4H8N/NH2

0.022 ± 0.002 0.152 ± 0.002 16.21 ± 0.09 77 ± 1 250 ± 3

ΔH⧧(k1) (kJ mol−1)

ΔS⧧(k1) (J K−1 mol−1)

± ± ± ± ±

−49 ± 13 −116 ± 8 −134 ± 7 −105 ± 9 −73 ± 8

67 43 26 31 37

4 2 2 3 2

ΔG⧧(k1) (kJ mol−1)

fraction of ΔS⧧(k1) to ΔG⧧(k1)298 (%)

± ± ± ± ±

18 44 61 50 37

82 78 66 62 59

6 3 2 9 8

Scheme 3. Possible Reaction Paths for the Nucleophilic Addition

H2O elimination and generation of [6]−. The overall activation barrier in this case is lower (147.3 kJ/mol), but still not allowing the proceeding of the reaction under normal conditions. Finally, the third route is the stepwise nucleophilic addition of the nitrone form, R1R2CN+(H)O−, of the oxime (Int1-C) to [NC2]−, which occurs through the endoergonic (+48.5 kJ/mol) formation of the orientation complex OC-C, giving first the acyclic intermediate Int2-C (the ratedetermining step), followed by the proton migration and generation of [6]− (Route C, Scheme 3). The overall activation barrier for these transformations is only 58.6 kJ/mol, and it is in excellent agreement with experimental kinetic observations (Table 4). Thus, Route C was found to be the preferred and most probable of all three possible reaction paths, of which the appropriate energy profiles are shown in Figure 6. It is noteworthy that we were unable to locate any intermediates on the PES for the stepwise associative nucleophilic addition and

been found on the potential energy surface (PES) (Scheme 3). The first route is the concerted nucleophilic addition that proceeds via a four-membered transition state (TS) through the endoergonic (+23.4 kJ/mol) formation of the orientation complex of the oxime with the 2-propanenitrilium closodecaborate cluster (OC-A) and gives first the acyclic form of the product [6] − (intermediate Int-A), which further exoergonically (−11.7 kJ/mol) transforms to the cyclic form by the formation of hydrogen bonding N−H···N (Route A, Scheme 3). The overall activation barrier for these transformations is extremely high (203.3 kJ/mol), and Route A seems unrealistic. The second reaction path is the concerted water-assisted nucleophilic addition via a six-membered TS (Route B, Scheme 3). The process begins from the endoergonic (+10.9 kJ/mol) water molecule association with OX3 giving Int-B, and subsequently the addition of this supramolecular entity to nitrilium cluster [NC2]−, followed by F


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Organometallics

Figure 7. Rate constants for oximes OX1−5 relative to that for OX5. Figure 6. Energy profiles of “reactants → products” transformations for nucleophilic addition.

Further development of kinetics and reaction mechanisms of addition of oxime to a nitrile group coupling at other centers and nucleophilic addition of other nucleophiles to 2-nitrilium closo-decaborates is underway in our group.

five-membered TS for the concerted nucleophilic addition of Int1-C to [NC2]−, and these routes were excluded from consideration. The stepwise ordering of the reactants in the different transition states in Route C may account for the large negative activation entropy observed from the kinetic studies.

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EXPERIMENTAL SECTION

Materials and Instrumentation. Solvents were obtained from commercial sources and used as received. Benzyltriphenylphosphonium 2-nitrilium-closo-decaborates,37 (Ph3PCH2Ph)[NC1−4], and also R2R3CNOH38 (OX1−5) were synthesized according to the literature methods. Melting points were measured on a Stuart SMP30 apparatus in capillaries and are not corrected. Elemental analysis for boron was performed by the FSUE IREA 291 Center (Moscow) on an aiCAP 6300 Duo ICP spectrometer using H3BO3 as an internal standard. Electrospray ionization mass spectra were obtained on a Bruker micrOTOF spectrometer equipped with an electrospray ionization (ESI) source. The instrument was operated both in negative and in positive ion mode using an m/z range of 50− 3000. The nebulizer gas flow was 0.4 bar, and the drying gas flow was 4.0 L/min. For ESI, the clusters were dissolved in MeCN. In the isotopic pattern, the most intensive peak is reported. Molar conductivities of 1 × 10−4 M solutions in MeCN were measured on a Mettler Toledo FE30 conductometer using an Inlab710 sensor. Infrared spectra (3600−500 cm−1) were recorded on a Shimadzu IR Prestige-21 instrument in KBr pellets. 1H{11B} and 11B{1H} NMR spectra were measured on a Bruker Avance 400 spectrometer in CD3CN at 25.0 °C; residual solvent signals were used as the internal standard for 1H{11B} NMR, whereas BF3·Et2O was used as the external standard for 11B{1H} NMR. X-ray Structure Determination. Single-crystal X-ray diffraction experiment was carried out using Agilent Technologies Xcalibur and Supernova diffractometers with monochromated Mo Kα or Cu Kα radiation, respectively. Crystals were fixed on a micro mount, placed on diffractometers, and measured at the temperature of 100 K. The unit cell parameters (Table 6S) were refined by least-squares techniques in the 2θ range of 5−55° for Mo Kα and 7−145° for Cu Kα. The structure has been solved by the SHELXS39 structure solution program using Direct Methods and refined with the SHELXL40 refinement incorporated in the OLEX2 program package.41 The carbon-bound H atoms were placed in calculated positions and were included in the refinement in the “riding” model approximation, with Uiso(H) set to 1.2Ueq(C) and C−H 0.97 Å for the CH2 groups, Uiso(H) set to 1.5Ueq(C) and C−H 0.96 Å for the CH3 groups, Uiso(H) set to 1.2Ueq(C) and C−H 0.93 Å for the CH groups, Uiso(H) set to 1.2Ueq(N) and N−H 0.88 Å for the NH groups, and Uiso(H) set

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CONCLUSIONS The results of this work could be considered from two main perspectives. First, we observed a new route for functionalization of the closo-decaborate cluster via previously unreported addition of various types of oximes to the CN bond. This novel integration opens up an easy access to closo-decaboratebound O-iminoacyl oximes, which, in particular, are of potential interest in boron neutron-capture cancer therapy. By contrast to reversible additions of oximes to RFCN, this type of reaction is irreversible and proceeds easily despite the 1− overall negative charge of the cluster. We found that the nucleophilic addition of different types of oximes to 2-nitrilium closo-decaborate cluster proceeds under the same conditions at substantially different rates. The significant 4 order-of-magnitude variation in rate constants is worth noting and yields a convenient reactivity probe in this series of oximes via a straightforward kinetic study. The theoretical DFT study (M06-2X/6-311+G* level of theory) suggests that the most plausible mechanism of this reaction is the stepwise addition of the nitrone form of an oxime to the CN moiety (Route C, Scheme 3). Second, this work provides, for the first time, a broad series of nucleophilicity of oximes with wide-ranging variation of their structural types (Figure 7). Although the obtained series follow general principles of physical organic chemistry, the quantitative data are certainly helpful for planning physical chemistry experiments. Moreover, metal-free34 and metal-involving35 nucleophilic additions of oximes comprise a field of intensive studies, and we believe that our nucleophilicity series will be useful to control various nucleophilic additions of oxime species. A similar methodology was previously developed using the reaction of KSeCN with a range of PX3 to order the nucleophilicity in a wide range of tertiary phosphine ligands.36 G


Article

Organometallics

15 min, whereupon the solid was filtered off, washed with one 200 μL portion of cold (−20 °C) MeCN, and dried at 80 °C for 12 h. Compounds (Ph3PCH2Ph)[7−8] and (Ph3PCH2Ph)[12] were not isolated from their reaction mixtures and were characterized in situ by HRESI−-MS, 1H{11B} and 11B{1H} NMR after 5 min ((Ph3PCH2Ph)[12]) or 1 h (for (Ph3PCH2Ph)[7−8]) after dissolution of the starting compounds in CD3CN instead of MeCN.

to 1.2Ueq(B) and B−H 1.12 Å for the BH groups. The unit cells of (Ph3PCH2Ph)[10] and (Ph3PCH2Ph)[13] also contain disordered molecules of acetonitrile (total potential solvent accessible void 124 and 121 Å3, and electron count per cell equals 13 and 16, respectively) that have been treated as a diffuse contribution to the overall scattering without specific atom positions by SQUEEZE/PLATON.42 Empirical absorption correction was applied in the CrysAlisPro43 program complex using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm. Supplementary crystallographic data for this paper have been deposited at the Cambridge Crystallographic Data Centre (1498798−1498799 and 1498801−1498804) and can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif. Kinetic Equipment and Procedures. Exploratory UV/visible measurements were performed on a Varian Cary 50 Conc UV−visible spectrophotometer with thermostated automated multicell changers (10 cells virtually simultaneously monitored), equipped with a Julabo F12-mV temperature cell regulator (accurate within 0.1 °C) in 1.000 ± 0.001 cm quartz tandem cuvette cells. The more rapid reactions (t1/2 < 20 s) were first evaluated on a third-generation Hi Tech SF61DX2 Stopped-Flow System equipped with a diode array (dead time < 5 ms; yielding 400 nm spectral width scans collected at 99% (NMR). HRESI−-MS (m/z): 189.2166 ([M − (CH2)5CN]•−, calcd 189.2164), 285.2991 ([M]−, calcd 285.2980), 924.7396 ([2 M + Ph3PCH2Ph]−, calcd 924.7396). 1 H{11B} NMR (δ): 8.21 (s, br, 1H, NH), 7.89 (t, 3H, Ph3PCH2Ph), 7.73−7.69 (m, 6H, Ph3PCH2Ph), 7.64−7.59 (m, 6H, Ph3PCH2Ph), 7.35 (t, 1H, Ph3PCH2Ph), 7.23 (t, 2H, Ph3PCH2Ph), 7.00 (d, 2H, Ph3PCH2Ph), 4.75 (d, 2H, Ph3PCH2Ph), 3.41 (s, br, 1H, B(1)H), 3.18 (s, br, 1H, B(10)H), 2.93 (q, 2H, CH2), 2.60−2.57 (m, 2H, CH2), 2.32−2.29 (m, 2H, CH2), 1.75−1.53 (m, 6H, CH2), 1.18 (t, 3H, CH3), 0.93 (s, br, 2H, B(3)H and B(5)H), 0.49 (s, br, 2H, B(6)H and B(9) H), 0.21 (s, br, 2H, B(7)H and B(8)H), 0.15 (s, br, 1H, B(4)H). 11 1 B{ H} NMR (δ): −0.04, −3.62 (B(1) and B(10)), −14.82 (B(2)), −24.89 (B(3) and B(5)), −26.02 (B(6) and B(9)), −28.83 (B(4), (B(7), and B(8)).

(Ph3PCH2Ph)[5]. Yield: 85% (109.2 mg). mp: 151 °C (dec). Anal.Calcd for C36H46N3B10OP: B, 16.00. Found: B, 16.1. HRESI−-MS (m/z): 189.2169 ([M − PhCH2C(NH2)N]•−, calcd 189.2164), 322.2928 ([M]−, calcd 322.2934), 998.7285 ([2 M + Ph3PCH2Ph]−, calcd 998.7308). ΛM (CH3CN, 1 × 10−4 M): 101.0 S cm−1 mol−1. IR (KBr, selected bonds, cm−1): 3441(m), 3343(s), 3267(w) ν(N−H); 3063(w), 2936(w-m), 2901(w) ν(C−H); 2473(s) ν(B−H); 1624(vs) ν(CN). 1H{11B} NMR (δ): 8.26 (s, br, 1H, NH), 7.90 (t, 3H, Ph3PCH2Ph), 7.73−7.69 (m, 6H, Ph3PCH2Ph), 7.62−7.57 (m, 6H, Ph3PCH2Ph), 7.38−7.35 (m, 3H, Ph3PCH2Ph and PhCH2), 7.32−7.23 (m, 5H, Ph3PCH2Ph and PhCH2), 6.98 (d, 2H, Ph3PCH2Ph), 5.72 (s, br, 2H, NH2), 4.68 (d, 2H, Ph3PCH2Ph), 3.50 (s, 2H, CH2), 3.34 (s, br, 1H, B(1)H), 3.13 (s, br, 1H, B(10)H), 2.92 (q, 2H, CH2), 1.22 (t, 3H, CH3), 0.88 (s, br, 2H, B(3)H and B(5)H), 0.42 (s, br, 2H, B(6)H and B(9)H), 0.16 (s, br, 2H, B(7)H and B(8)H), 0.08 (s, br, 1H, B(4) H). 11B{1H} NMR (δ): −0.19, −3.76 (B(1) and B(10)), −15.01 (B(2)), −24.90 (B(3) and B(5)), −25.95 (B(6) and B(9)), −28.71 (B(7) and B(8)), −29.33 (B(4)). Crystals of (Ph3PCH2Ph)[5]·1/ 2MeCN suitable for X-ray diffraction were obtained by slow evaporation of MeCN solution.

(Ph3PCH2Ph)[8]. Yield: >99% (NMR). HRESI−-MS (m/z): 189.2168 ([M − Ph2CN]•−, calcd 189.2164), 369.2987 ([M]−, calcd 369.2984), 1092.7416 ([2 M + Ph3PCH2Ph]−, calcd 1092.7409). 1 H{11B} NMR (δ): 8.52 (s, br, 1H, NH), 7.88 (t, 3H, Ph3PCH2Ph), 7.72−7.67 (m, 6H, Ph3PCH2Ph), 7.63−7.43 (m, 13H, Ph3PCH2Ph and Ph), 7.40−7.31 (m, 4H, Ph3PCH2Ph and Ph), 7.22 (t, 2H, Ph3PCH2Ph), 6.99 (d, 2H, Ph3PCH2Ph), 4.74 (d, 2H, Ph3PCH2Ph), 3.46 (s, br, 1H, B(1)H), 3.24 (s, br, 1H, B(10)H), 2.87 (q, 2H, CH2), 1.23 (t, 3H, CH3), 0.99 (s, br, 2H, B(3)H and B(5)H), 0.53 (s, br, 2H, B(6)H and B(9)H), 0.30 (s, br, 2H, B(7)H and B(8)H), 0.21 (s, br, 1H, B(4)H). 11B{1H} NMR (δ): 0.04, −3.54 (B(1) and B(10)), −14.60 (B(2)), −24.80 (B(3) and B(5)), −25.78 (B(6) and B(9)), −28.63 (B(4), (B(7), and B(8)). I


Article

Organometallics

6.10 (s, br, 2H, NH2), 4.66 (d, 2H, Ph3PCH2Ph), 3.34 (s, br, 1H, B(1) H), 3.15 (s, br, 1H, B(10)H), 2.99−2.95 (m, 2H, CH2), 1.87−1.77 (m, 2H, CH2), 1.06 (t, 3H, CH3), 0.90 (s, br, 2H, B(3)H and B(5)H), 0.41 (s, br, 2H, B(6)H and B(9)H), 0.16 (s, br, 2H, B(7)H and B(8)H), 0.08 (s, br, 1H, B(4)H). 11B{1H} NMR (δ): −0.19, −3.65 (B(1) and B(10)), −14.92 (B(2)), −24.92 (B(3) and B(5)), −25.92 (B(6) and B(9)), −28.67 (B(7) and B(8)), −29.29 (B(4)).

(Ph3PCH2Ph)[9]. Yield: 57% (74.2 mg). mp: 104 °C (dec). Anal.Calcd for C34H49N4B10O2P: B, 15.79. Found: B, 15.9. HRESI−MS (m/z): 203.2323 ([M − OC4H8NC(NH2)N]•−, calcd 203.2323), 331.3146 ([M]−, calcd 331.3147), 1016.7688 ([2 M + Ph3PCH2Ph]−, calcd 1016.7735). ΛM (CH3CN, 1 × 10−4 M): 93.0 S cm−1 mol−1. IR (KBr, selected bonds, cm−1): 3468(m), 3356(m-s), 3283(m) ν(N− H); 3053(w), 2968(w-m), 2928(m), 2901(w-m), 2860(w-m) ν(C− H); 2471(s) ν(B−H); 1611(s) ν(CN). 1H{11B} NMR (δ): 8.25 (s, br, 1H, NH), 7.91 (t, 3H, Ph3PCH2Ph), 7.73−7.69 (m, 6H, Ph3PCH2Ph), 7.62−7.57 (m, 6H, Ph3PCH2Ph), 7.36 (t, 1H, Ph3PCH2Ph), 7.25 (t, 2H, Ph3PCH2Ph), 6.99 (d, 2H, Ph3PCH2Ph), 5.28 (s, br, 2H, NH2), 4.69 (d, 2H, Ph3PCH2Ph), 3.65−3.62 (m, 4H, CH2), 3.34 (s, br, 1H, B(1)H), 3.13−3.11 (m, 5H, CH2 and B(10)H), 2.91−2.87 (m, 2H, CH2), 1.78−1.68 (m, 2H, CH2), 1.00 (t, 3H, CH3), 0.88 (s, br, 2H, B(3)H and B(5)H), 0.42 (s, br, 2H, B(6)H and B(9) H), 0.15 (s, br, 2H, B(7)H and B(8)H), 0.08 (s, br, 1H, B(4)H). 11 1 B{ H} NMR (δ): −0.25, −3.79 (B(1) and B(10)), −15.06 (B(2)), −24.99 (B(3) and B(5)), −25.80 (B(6) and B(9)), −28.63 (B(7) and B(8)), −29.36 (B(4)).

(Ph3PCH2Ph)[12]. Yield: >99% (NMR). HRESI−-MS (m/z): 203.2328 ([M − OC4H8NC(NH2)N]•−, calcd 203.2323), 331.3160 ([M]−, calcd 331.3147). 1H{11B} NMR (δ): 8.29 (s, br, 1H, NH), 7.90 (t, 3H, Ph3PCH2Ph), 7.73−7.68 (m, 6H, Ph3PCH2Ph), 7.63−7.57 (m, 6H, Ph3PCH2Ph), 7.35 (t, 1H, Ph3PCH2Ph), 7.24 (t, 2H, Ph3PCH2Ph), 6.99 (d, 2H, Ph3PCH2Ph), 5.25 (s, br, 2H, NH2), 4.72 (d, 2H, Ph3PCH2Ph), 4.02 (hept, 1H, CH), 3.64−3.62 (m, 4H, CH2), 3.36 (s, br, 1H, B(1)H), 3.13−3.11 (m, 5H, CH2 and B(10)H), 1.19 (d, 6H, CH3), 0.89 (s, br, 2H, B(3)H and B(5)H), 0.44 (s, br, 2H, B(6)H and B(9)H), 0.17 (s, br, 2H, B(7)H and B(8)H), 0.10 (s, br, 1H, B(4)H). 11B{1H} NMR (δ): −0.25, −3.79 (B(1) and B(10)), −15.06 (B(2)), −24.99 (B(3) and B(5)), −25.80 (B(6) and B(9)), −28.63 (B(7) and B(8)), −29.36 (B(4)).

(Ph3PCH2Ph)[10]. Yield: 74% (97.0 mg). mp: 157 °C (dec). Anal.Calcd for C37H48N3B10OP: B, 15.67. Found: B, 15.9. HRESI−-MS (m/z): 203.2326 ([M − PhCH2C(NH2)N]•−, calcd 203.2321), 336.3087 ([M]−, calcd 336.3091), 1026.7605 ([2 M + Ph3PCH2Ph]−, calcd 1026.7622). ΛM (CH3CN, 1 × 10−4 M): 99.3 S cm−1 mol−1. IR (KBr, selected bonds, cm−1): 3447(m), 3343(m-s), 3277(w) ν(N−H); 3055(w), 2934(w-m), 2901(w) ν(C−H); 2469(s) ν(B−H); 1620(vs) ν(CN). 1H{11B} NMR (δ): 8.25 (s, br, 1H, NH), 7.90 (t, 3H, Ph3PCH2Ph), 7.73−7.68 (m, 6H, Ph3PCH2Ph), 7.62−7.56 (m, 6H, Ph3PCH2Ph), 7.38−7.35 (m, 3H, Ph3PCH2Ph and PhCH2), 7.32−7.23 (m, 5H, Ph3PCH2Ph and PhCH2), 6.98 (d, 2H, Ph3PCH2Ph), 5.71 (s, br, 2H, NH2), 4.68 (d, 2H, Ph3PCH2Ph), 3.50 (s, 2H, CH2), 3.34 (s, br, 1H, B(1)H), 3.13 (s, br, 1H, B(10)H), 2.90−2.86 (m, 2H, CH2), 1.78−1.69 (m, 2H, CH2), 1.00 (t, 3H, CH3), 0.88 (s, br, 2H, B(3)H and B(5)H), 0.42 (s, br, 2H, B(6)H and B(9)H), 0.16 (s, br, 2H, B(7) H and B(8)H), 0.08 (s, br, 1H, B(4)H). 11B{1H} NMR (δ): −0.14, −3.69 (B(1) and B(10)), −15.06 (B(2)), −24.94 (B(3) and B(5)), −25.91 (B(6) and B(9)), −28.72 (B(7) and B(8)), −29.31 (B(4)). Crystals of (Ph3PCH2Ph)[10] suitable for X-ray diffraction were obtained by slow evaporation of MeCN solution.

(Ph3PCH2Ph)[13]. Yield: 92% (120.6 mg). mp: 141 °C (dec). Anal.Calcd for C37H48N3B10OP: B, 15.67. Found: B, 15.9. HRESI−-MS (m/z): 203.2329 ([M − PhCH2C(NH2)N]•−, calcd 203.2321), 336.3092 ([M]−, calcd 336.3091), 1026.7620 ([2 M + Ph3PCH2Ph]−, calcd 1026.7622). ΛM (CH3CN, 1 × 10−4 M): 95.3 S cm−1 mol−1. IR (KBr, selected bonds, cm−1): 3466(m), 3348(m-s), 3265(w) ν(N−H); 3030(w), 2976(w), 2936(w-m), 2901(w-m) ν(C−H); 2475(s) ν(B− H); 1620(vs) ν(CN). 1H{11B} NMR (δ): 8.29 (s, br, 1H, NH), 7.90 (t, 3H, Ph3PCH2Ph), 7.73−7.68 (m, 6H, Ph3PCH2Ph), 7.62−7.56 (m, 6H, Ph3PCH2Ph), 7.38−7.29 (m, 6H, Ph3PCH2Ph and PhCH2), 7.29− 7.25 (t, 2H, Ph3PCH2Ph), 6.98 (d, 2H, Ph3PCH2Ph), 5.68 (s, br, 2H, NH2), 4.68 (d, 2H, Ph3PCH2Ph), 4.01 (hept, 1H, CH), 3.51 (s, 2H, CH2), 3.34 (s, br, 1H, B(1)H), 3.13 (s, br, 1H, B(10)H), 1.20 (d, 6H, CH3), 0.88 (s, br, 2H, B(3)H and B(5)H), 0.42 (s, br, 2H, B(6)H and B(9)H), 0.16 (s, br, 2H, B(7)H and B(8)H), 0.08 (s, br, 1H, B(4)H). 11 1 B{ H} NMR (δ): −0.21, −3.54 (B(1) and B(10)), −15.05 (B(2)), −24.91 (B(3) and B(5)), −25.96 (B(6) and B(9)), −28.72 (B(7) and B(8)), −29.38 (B(4)). Crystals of (Ph3PCH2Ph)[13] suitable for Xray diffraction were obtained by slow evaporation of MeCN solution.

(Ph3PCH2Ph)[11]. Yield: 78% (100.2 mg). mp: 110 °C (dec). Anal. Calcd for C36H46N3B10OP: B, 16.00. Found: B, 15.9. HRESI−-MS (m/ z): 203.2326 ([M − PhC(NH2)N]•−, calcd 203.2321), 322.2930 ([M]−, calcd 322.2934), 998.7304 ([2 M + Ph3PCH2Ph]−, calcd 998.7308). ΛM (CH3CN, 1 × 10−4 M): 96.7 S cm−1 mol−1. IR (KBr, selected bonds, cm−1): 3497(m), 3350(m), 3296(w) ν(N−H); 3057(w), 2963(w), 2930(w), 2884(w) ν(C−H); 2463(s) ν(B−H); 1622(vs) ν(CN). 1H{11B} NMR (δ): 8.36 (s, br, 1H, NH), 7.90 (t, 3H, Ph3PCH2Ph), 7.73−7.64 (m, 8H, Ph3PCH2Ph and Ph), 7.61−7.56 (m, 7H, Ph3PCH2Ph and Ph), 7.50 (t, 2H, Ph), 7.37 (t, 1H, Ph3PCH2Ph), 7.25 (t, 2H, Ph3PCH2Ph), 6.98 (d, 2H, Ph3PCH2Ph),

(Ph3PCH2Ph)[14]. Yield: 88% (113.0 mg). mp: 106 °C (dec). Anal.Calcd for C36H46N3B10OP: B, 16.00. Found: B, 15.8. HRESI−-MS (m/z): 203.2329 ([M − PhC(NH2)N]•−, calcd 203.2321), 322.2936 ([M]−, calcd 322.2934), 998.7313 ([2 M + Ph3PCH2Ph]−, calcd 998.7308). ΛM (CH3CN, 1 × 10−4 M): 96.7 S cm−1 mol−1. IR (KBr, selected bonds, cm−1): 3497(m), 3345(m-s), 3283(w-m) ν(N−H); 3049(w), 2972(w), 2924(w-m), 2880(w) ν(C−H); 2473(s) ν(B−H); 1624(vs) ν(CN). 1H{11B} NMR (δ): 8.40 (s, br, 1H, NH), 7.90 (t, 3H, Ph3PCH2Ph), 7.73−7.65 (m, 8H, Ph3PCH2Ph and Ph), 7.61−7.56 (m, 7H, Ph3PCH2Ph and Ph), 7.50 (t, 2H, Ph), 7.36 (t, 1H, J


Article

Organometallics Ph3PCH2Ph), 7.25 (t, 2H, Ph3PCH2Ph), 6.98 (d, 2H, Ph3PCH2Ph), 6.06 (s, br, 2H, NH2), 4.68 (d, 2H, Ph3PCH2Ph), 4.08 (hept, 1H, CH), 3.33 (s, br, 1H, B(1)H), 3.15 (s, br, 1H, B(10)H), 1.28 (d, 6H, CH3), 0.90 (s, br, 2H, B(3)H and B(5)H), 0.42 (s, br, 2H, B(6)H and B(9)H), 0.17 (s, br, 2H, B(7)H and B(8)H), 0.09 (s, br, 1H, B(4)H). 11 1 B{ H} NMR (δ): −0.18, −3.57 (B(1) and B(10)), −15.03 (B(2)), −24.96 (B(3) and B(5)), −25.98 (B(6) and B(9)), −28.62 (B(7) and B(8)), −29.32 (B(4)).

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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.6b00678. Tables of crystal data; HRESI−-MS, IR, 1H{11B}, and 11 1 B{ H} NMR spectra of (Ph3PCH2Ph)[1−6], (Ph3PCH 2 Ph)[9−11], and (Ph 3 PCH 2 Ph)[13−14] and HRESI−-MS data of (Ph3PCH2Ph)[7−8]; HRESI−-MS data for hydrolysis products; kinetic details; and computational details (PDF) The text file of all computed molecule Cartesian coordinates in the format for convenient visualization (XYZ) X-ray crystallographic data of (Ph3PCH2Ph)[3]·MeCN, (Ph3PCH2Ph)[4], (Ph3PCH2Ph)[5]·0.5MeCN, (Ph3PCH2Ph)[6]·MeCN, (Ph3PCH2Ph)[10], and (Ph3PCH2Ph)[13] (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.R.). *E-mail: [email protected] (V.Yu.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.S.B. and A.S.N. are grateful to the Russian Foundation for Basic Research for support (projects 16-03-00573 and 16-3360063). The kinetic part of this work was conducted under Saint Petersburg State University Project 12.37.214.2016. A.R., C.P., and P.P.M. also acknowledge the South African National Science Foundation (SA-NRF: GUN 92196), SASOL, and PETLabs Pharmaceuticals for support. Opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the SA-NRF.

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REFERENCES

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