E–H (E = B, Si, C) Bond Activation by Tuning ... - ACS Publications

Nov 21, 2017 - ([(Ph3P)2C·PNiPr2]2+) were reported to oxidatively add C−H. (metallocenes) (Scheme 1a)3c and O−H (water and methanol)3d bonds ...
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E−H (E = B, Si, C) Bond Activation by Tuning Structural and Electronic Properties of Phosphenium Cations Nemanja Đorđević,† Rakesh Ganguly,† Milena Petković,‡ and Dragoslav Vidović*,†,§ †

SPMS-CBC, Nanyang Technological University, 21 Nanyang Link, Singapore 638737 Faculty of Physical Chemistry, University of Belgrade, 11000 Belgrade, Republic of Serbia

‡

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S Supporting Information *

ABSTRACT: In this work, strategic enhancement of electrophilicity of phosphenium cations for the purpose of smallmolecule activation was described. Our synthetic methodology for generation of novel two-coordinate phosphorus(III)-based compounds [{C6H4(MeN)2C}2C·PR]2+ ([2a]2+, R = NiPr2; [2b]2+, R = Ph) was based on the exceptional electrondonating properties of the carbodicarbene ligand (CDC). The effects of P-centered substituent exchange and increase in the overall positive charge on small substrate activation were comparatively determined by incorporating the bis(amino)phosphenium ion [(iPr2N)2P]+ ([1]+) in this study. Implemented structural and electronic modifications of phosphenium salts were computationally verified and subsequently confirmed by isolation and characterization of the corresponding E−H (E = B, Si, C) bond activation products. While both phosphenium mono- and dications oxidatively inserted/cleaved the B−H bond of Lewis base stabilized boranes, the increased electrophilicity of doubly charged species also afforded the activation of significantly less hydridic Si−H and C−H bonds. The preference of [2a]2+ and [2b]2+ to abstract the hydride rather than to insert into the corresponding bond of silanes, as well as the formation of the carbodicarbene-stabilized parent phosphenium ion [{C6H4(MeN)2C}2C·PH2]+ ([2·PH2]+) were experimentally validated.

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INTRODUCTION Over the past decade, small-molecule activation by main-group elements gained traction as an alternative to transition-metal complexes. This led to the discovery of abundant, inexpensive, and less toxic main-group-centered species capable of activating enthalpically strong σ bonds.1 For example, frustrated Lewis pairs (FLPs) were shown to activate a variety of small molecules and several of these systems exhibited exceptional catalytic activities.1a,b,c With respect to single-site smallmolecule activation, groundbreaking examples include oxidative additions of E−H (E = H, N, B, Si) bonds at the central element of bis(boryl)stannylene 2a and (alkyl)(amino)carbenes.2b,c However, insertion of isolobal phosphenium cations into E−H bonds remains rather scarce. Apart from intramolecular C−H activation directed by the proximity effect between the 2,4,6-(tBu)3-C6H2 group and the central P atom of transient phosphenium ions,3a,b only two cases of intermolecular insertions have been reported to date. Namely, a phosphenium monocation ([(iPr2N)2P]+, [1]+) and dication ([(Ph3P)2C·PNiPr2]2+) were reported to oxidatively add C−H (metallocenes) (Scheme 1a) 3c and O−H (water and methanol)3d bonds (Scheme 1b), respectively. Interestingly, the monocation [1]+ appeared inert toward O−H bond cleavage, suggesting the importance of increasing the positive charge on the overall activity of these compounds, which was also demonstrated in small-molecule activation by germylenebased systems.2d,e In this report the electrophilicity of phosphenium cations was enhanced by various structural © 2017 American Chemical Society

Scheme 1. (a) Insertion of Aminophosphenium Cation [1][AlCl4] into C−H Bonds of Stannocene and Plumbocene and (b) O−H Bond Activation of Water and Methanol by Phosphenium Dication [(Ph3P)2C·PNiPr2]2+

modifications leading to oxidative addition and/or hydride abstraction of B−H, Si−H, and C−H bonds.

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RESULTS AND DISCUSSION Synthesis of Phosphenium Cations. Recently, our attempts to enhance electrophilic properties of the twocoordinate phosphenium dication [(Ph3P)2C·PNiPr2]2+ by Received: October 7, 2017 Published: November 21, 2017 14671

DOI: 10.1021/acs.inorgchem.7b02579 Inorg. Chem. 2017, 56, 14671−14681

Article

Inorganic Chemistry replacing the amino (NiPr2) substituent by a less electrondonating group (e.g., phenyl) resulted only in the formation of the transient P-centered dication [(Ph3P)2C·PPh]2+, which was subsequently utilized for C−F bond activation of α,α,αtrifluorotoluene and [BArF4] anion (ArF = 3,5-(CF3)2C6H3).4b To accomplish the preparation of isolable phenylphosphenium dication, the carbodiphosphorane (i.e., (Ph3P)2C) ligand was substituted by a more nucleophilic carbodicarbene (i.e., {C6H4(MeN)2C}2C; CDC) analogue.5 The target P-dication [CDC·PPh][BArCl4]2, [2b][BArCl4]2 (ArCl = 3,5-Cl2-C6H3), was synthesized according to the procedure described for [(Ph3P)2C·PNiPr2]2+ (Scheme 2).4 Scheme 2. Synthesis of Amino- and Phenyl-Substituted Dications [2a][BArCl4]2 and [2b][BArCl4]2, as well as an Example of a [4 + 1] Cycloaddition Reaction with [2b]2+ a

Figure 1. Molecular view of P-monocations, P-dication, and [4 + 1] cycloaddition product drawn at 50% probability. H atoms as well as counterions have been omitted for clarity. Selected bond lengths (Å) and angles (deg) are as follows. [2a-Cl]+: C1−P1 1.820(4), P1−Cl1 2.1914(14), P1−N1 1.651(4), N1−P1−C1 105.56(18), N1−P1−Cl1 109.71(13), C1−P1−Cl1 87.49(13). [2b-Cl]+: C1−P1 1.776(6), P1− Cl1 2.120(2), P1−C2 1.836(6), C2−P1−C1 101.7(3), C2−P1−Cl1 100.6(2), C1−P1−Cl1 107.2(2). [2b]2+: C1−P1 1.701(5), P1−C2 1.784(6), C2−P1−C1 110.9(3). [2b(CH 2 CCH 3 )2 ] 2+: C1−P1 1.737(6), P1−C2 1.817(6), C2−P1−C1 108.9(3).

a Reagents and conditions: (a) 3 equiv of RPCl2, benzene, ambient temperature; (b) 2.1 equiv of Na[BArCl4], dichloromethane (DCM), ambient temperature; (c) R = Ph, 2 equiv of AlCl3, 1 equiv of 2,3dimethyl-1,3-butadiene, DCM, ambient temperature.

case of extended π-delocalization across both CDC and Ph substituents. To determine whether [2b]2+ is better described as a phosphenium dication or doubly charged phosphaalkene, we decided to test its reactivity toward 2,3-dimethyl-1,3-butadiene. It has been well-established that conjugated dienes undergo [4 + 2] cycloaddition with cationic phosphaalkenes,8 while treatment of phosphenium salts with these reagents leads to [4 + 1] cycloaddition products.6 The reaction of in situ formed [2b]2+ with 2,3-dimethyl-1,3-butadiene (Scheme 2) yielded a single reaction product, as judged by the 31P NMR spectrum (δP 31 ppm),9 whose identity as the phospholenium salt [CDC· P{(CH2CCH3)2}Ph]2+ ([2b(CH2CCH3)2]2+) was elucidated by X-ray diffraction (Figure 1). Both CCDC−P (1.737(6) Å) and CPh−P (1.817(6) Å) bonds of [2b(CH2CCH3)2]2+ were elongated with respect to the corresponding bonds of [2b]2+ (1.701(5) and 1.784(6) Å, respectively). Similarly, the C1− P1−C2 bond angle of [2b(CH2CCH3)2]2+ (108.9(3)°) appeared sharper than the analogous angle of [2b] 2+ (110.9(3°) due to the planar-to-pyramidal geometry change at the central P atom. Nevertheless, the molecular structure of [2b(CH2CCH3)2]2+ revealed that cycloaddition took place exclusively on the central P atom of [2b]2+, implying its phosphenium character. In order to comparatively determine the effects of not only the Ph for NiPr2 substituent exchange but also the increase in the positive charge on phosphenium-induced small-molecule activation, we also prepared an amino-substituted phosphenium monocation (i.e., [(iPr2N)2P][BArCl4] ([1][BArCl4])) and dication (i.e., [CDC·PNiPr2][BArCl4]2 ([2a][BArCl4]2)). The synthesis of the former phosphenium salt was achieved by abstracting chloride from chlorophosphine (iPr2N)2PCl with an equimolar amount of Na[BArCl4]. On the other hand, the

The coordination of carbodicarbene ligand 2 to the central P atom leading to the formation of the [CDC·P(Ph)Cl]Cl species [2b-Cl]Cl was primarily evident from the 31P NMR spectrum (δP 101 ppm) and subsequently crystallographically confirmed upon replacing the noncoordinating chloride with [SbF6] anion (Figure 1). The sum of the bond angles around the central P of [2b-Cl]+ (309.5(5)°) indicated its pyramidal geometry, while the values for C1−P1 (1.776(6) Å) and P1−Cl1 (2.120(2) Å) bond lengths were similar to the corresponding bond distances (1.7828(13) and 2.1271(5) Å, respectively) previously observed for the carbodiphosphorane-substituted analogue.4b Addition of 2 equiv of Na[BArCl4] to the DCM solution of [2bCl]Cl (Scheme 2) resulted in the disappearance of the 31P NMR signal assigned to [2b-Cl]+ and the appearance of a new signal at δP 393 ppm consistent with the two-coordinate nature of the target dication [2b]2+.6 Its identity was undoubtedly confirmed by single-crystal X-ray diffraction (Figure 1), which revealed shorter C1−P1 (1.701(5) Å) and P1−C2 (1.784(6) Å) bonds in comparison to the precursor [2b-Cl]+ (1.776(6) and 1.836(6) Å, respectively). While contraction of the C1−P1 bond indicated a significant amount of Ccarbone→P electron density flow, a reduced P1−C2 bond length was presumably due to a higher overall positive charge of [2b][BArCl4]2; however, a certain degree of π-conjugation between the Ph group and the central P atom should not be disregarded. Therefore, the CPC fragment of [2b]2+ could be depicted as phosphaalkene-like considering that the Ccarbone−P bond distance is well within the range for a typical C−P double bond of phosphaalkenes (1.65−1.76 Å)7 or is allene-like in the 14672

DOI: 10.1021/acs.inorgchem.7b02579 Inorg. Chem. 2017, 56, 14671−14681

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Inorganic Chemistry doubly charged amino analogue was prepared via initial carbone for chloride displacement to give the corresponding monocation [CDC·P(NiPr2)Cl]+ ([2a-Cl]+) (Figure 1) followed by P−Cl bond cleavage using Na[BArCl4] (Scheme 2). Computational Screening. Previous studies have shown that small-molecule activation by heavier group 14 metallylenes follows a process reminiscent of a transition-metal electrophilic mechanism consistent with electron density transfer from the substrate HOMO to the LUMO of main-group species.2f,g Considering the isolobal analogy between the group 14 tetrelenes and phosphenium ions,8 and the fact that latter species always react as electrophiles,10 we hypothesized that phosphenium cations could mimic their analogues in activation of strong σ bonds by increasing the accessibility of their LUMO. To gain an insight into the electronic properties of the newly prepared P dications [2a]2+ and [2b]2+ and the monocation [1]+, we carried out a theoretical analysis using density functional theory at the B3LYP/6-31G* level.9 According to this investigation, the LUMO of [2b]2+ (−9.20 eV) appeared to be more accessible than an analogous orbital of [2a]2+ (−8.56 eV), which was expected considering the inferior π-donating character of Ph in comparison to the NiPr2 group (Figure 2). The ligand substitution combined with the

Scheme 3. B−H Bond Activation

signals in 31P{1H} (δP 16.1 (NMe3) and 20.3 ppm (Py)) and B{1H} (δB −10.4 and −13.4 ppm, respectively) NMR spectra suggestive of P−B bond formation. Furthermore, protoncoupled 31P NMR spectra revealed broad doublets implying the presence of P−H fragments whose characteristic appearance as a doublet of triplets (1JP−H = 432 Hz and 3JH−H = 2 Hz in both instances) in 1H NMR spectra (δH 6.96 (NMe3) and 7.06 ppm (Py)) indicated the attachment of −BH2·LB moieties directly to the central P.9 These results were consistent with the formation of B−H bond insertion products [(iPr2N)2P(H)(BH2·LB)][BArCl4] ([1(H)(BH2·LB)][BArCl4]; LB = NMe3, Py), whose structural identities were further elucidated by standard crystallographic techniques (Figure 3). The average N−P bond length values for [1(H)(BH2·NMe3)]+ (1.6581(16) Å) and [1(H)(BH2·Py)]+ (1.6453(18) Å) were longer than the average N−P bond distance of the monocation [1] + 11

Figure 2. Relative energy levels of frontier orbitals of phosphenium cations used in this research.

increased overall positive charge also resulted in the stabilization of the HOMO for both phosphenium dications but to a markedly lesser degree than for LUMO(s) (Figure 2), thus reducing the HOMO−LUMO (i.e., singlet−triplet) gap in the following order: [1]+ > [2a]2+ > [2b]2+. Consequently, the frontier orbital separation energy of [2b]2+ (67.5 kcal mol−1) appeared quite comparable to the value reported for cationic germylene (64.1 kcal mol−1), which was demonstrated to undergo oxidative addition of E−H bonds (E = B, Si, H).2d B−H Bond Activation. In order to experimentally assess our hypothesis, we began to study the reactivity of phosphenium salts11 toward hydridic aminoboranes (N = 10.01−7.97, where N is the nucleophilicity parameter of various hydride donors), less nucleophilic silanes (N = 3.58−2.65), and very weak CH-based hydride donors (N = 0.64−0.52).12 Thus, the addition of trimethylamine−borane (H3B·NMe3) or pyridine−borane complexes (H3B·Py) to the DCM solution of P-monocation [1][BArCl4] (Scheme 3) gave rise to broad

Figure 3. Molecular view of B−H bond activation products drawn at 50% probability. Most H atoms as well as counterions have been omitted for clarity. Selected bond lengths (Å) and angles (deg) are as follows. [1(H)(BH2·Py)]+: N1−P1 1.6480(18), N2−P1 1.6427(18), P1−B1 1.942(3), B1−N3 1.578(3), N1−P1−B1 112.17(10), N2− P1−B1 114.24(11), N1−P1−N2 115.19(9). [2b(H)(BH2·NMe3)]2+: P1−C1 1.775(5), P1−B1 1.973(6), P1−C2 1.810(5), B1−N1 1.585(8), C1−P1−B1 108.7(2), C1−P1−C2 112.8(2). [1(H)(BH2· NMe 3 )] + : N1−P1 1.6615(16), N2−P1 1.6547(15), P1−B1 1.9576(19), B1−N3 1.605(2), N1−P1−B1 113.66(8), N2−P1−B1 108.91(8), N1−P1−N2 116.77(8). [2b-H]+: P1−C1 1.807(8), P1− C2 1.838(9), C1−P1−C2 106.0(4). 14673

DOI: 10.1021/acs.inorgchem.7b02579 Inorg. Chem. 2017, 56, 14671−14681

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Inorganic Chemistry

252.6292), respectively, according to the 31P NMR and ESI-MS spectra.9 However, we noticed that significantly higher amounts of hydride abstraction products [2a-H]+ and [2b-H]+ were formed in comparison to the BH insertion products upon replacing H3B·NMe3 with the more hydridic pyridine−borane complex ([2a]2+, 1:10; [2b]2+, 1:4).17 These observations suggested that relative distributions of the B−H bond activation products were mainly governed by electronic factors, presumably due to the relative hydridicity of NMe3 and pyridine-stabilized boranes. Nevertheless, steric influence between phosphenium cations and aminoboranes should not be completely ruled out, due to the relative size of the NMe3 and pyridine substituents (see below). In addition to BH insertion and H abstraction products, the 31 1 P{ H} NMR spectrum of the reaction mixture between [2a][BArCl4]2 and BH3·Py also featured a minor signal at δP −134.4 ppm, which resolved into a triplet upon canceling the proton coupling effect.9 The 1JP−H coupling constant of 212 Hz was indicative of a phosphine-like PH2 fragment,18 whose formation was further evidenced by a signal at m/z 337.1583 (calcd for C19H22N4P+ m/z 337.1582) in the ESI-MS spectrum; its isotopic pattern was in good agreement with [CDC·PH2]+ ([2·PH2]+). Although the mechanistic details regarding the formation of the carbodicarbene-stabilized parent phosphenium ion (PH2)+ in the original reaction remained elusive,2g its existence was further supported by the preparation and structural characterization of [2·PH2]+ in an independent experiment by treating [CDC·PBr2]Br19 with 2 equiv of Na[BArCl4] or AlCl3 in the presence of nBu3SnH (Scheme 4).

(1.6217(10) Å). Furthermore, BH-insertion products featured wider N1−P1−N2 bond angles (NMe3, 116.77(8)°; Py, 115.19(9)°) in comparison to their precursor [1] + (110.65(5)°)9 as a result of P(III) to P(V) conversion. On the other hand, the reaction between the NiPr2substituted dication [2a][BArCl4]2 (δP 328 ppm) and H3B· NMe3 (Scheme 3), apart from the formal oxidative addition product [CDC·P(H)(BH2·NMe3)NiPr2][BArCl4]2 ([2a(H)(BH2·NMe3)][BArCl4]2; δP −15.4 ppm), gave another P−Hbased compound as judged by the corresponding doublet (δP −12.3 ppm) in the 31P NMR spectrum.9 The 1JP−H coupling constant value of 225 Hz for the latter signal indicated the formation of primary phosphine-like species whose identity as the carbodicarbene-stabilized aminohydridophosphenium cation [CDC·P(NiPr2)H]+ ([2a-H]+) was proposed by ESI-MS (calcd for C25H35N5P+ m/z 436.2630, found 436.2632) and subsequently confirmed by treating [2a][BArCl4]2 with 1 equiv of nBu3SnH,9 a well-known hydride source.13 It is noteworthy that the ratio of BH insertion to formal H abstraction product was 1:23. Furthermore, the PH resonance for [2a(H)(BH2· NMe3)]2+ appeared as a doublet of doublets (δH 7.16 ppm, 1 JP−H = 351 Hz and 3JH−H = 4 Hz) in the 1H NMR spectrum, suggesting 1H−1H coupling with only one of the two vicinal BH2 protons, which was also observed for the analogous B−H insertion product [CDC·P(H)(BH2·NMe3)Ph]2+ ([2b(H)(BH2·NMe3)]2+; δH 7.06 ppm, 1JP−H = 383 Hz and 3JH−H = 7 Hz), whose molecular structure was elucidated by single-crystal X-ray diffraction (Figure 3). As expected, oxidative addition of a B−H bond resulted in the elongation of both CCDC−P (1.775(5) Å for [2b(H)(BH2·NMe3)]2+ vs 1.701(5) Å for [2b]2+) and P−CPh (1.810(5) Å for [2b(H)(BH2·NMe3)]2+ vs 1.784(6) Å for [2b]2+) bond distances. Additionally, [2b(H)(BH2·NMe3)]2+ showed a somewhat elongated P−B single bond (1.973(6) Å) in comparison to the analogous products involving [1]+ (1.9576(19) and 1.942(3) Å for [1(H)(BH2· NMe3)]+ and [1(H)(BH2·Py)]+, respectively)14 with the carbodicarbene ligand and NMe3 group in a quasi-trans disposition (Figure 3) due to steric reasons (C−P−B−N dihedral angle 137.7°). Consequently the rotation around the P−B bond was blocked, causing the PH proton to couple only with the vicinal BH proton in the trans disposition according to the placement of H−P−B−H dihedral angle values (78.1 and 163.1°) on the Karplus curve.15 The reaction between Ph dication [2b][BArCl4]2 (δP 393 ppm) and H3B·NMe3, in addition to [2b(H)(BH2·NMe3)]2+, also afforded the formal hydride abstraction product [CDC·P(Ph)H][BArCl4] ([2bH][BArCl4]; d, δP −52.0 ppm, 1JP−H = 226 Hz) as a minor constituent (the ratio of H abstraction vs BH insertion product was 1:9),9 whose spectroscopic and crystallographic16 identification (Figure 3) was achieved by reacting [2b]2+ and the hydride-donating reagent nBu3SnH. The C1−P1 and P1−C2 bond distances (1.807(8) and 1.838(9) Å, respectively) for [2b-H]+ were somewhat longer than analogous bond lengths of the Cl-substituted P-monocation [2b-Cl]+ (1.776(6) and 1.836(6) Å, respectively), mainly due to the stronger electron-withdrawing character of Cl substituent. Additionally, the introduction of BH3·Py to DCM solutions of [2a][BArCl4]2 and [2b][BArCl4]2 (Scheme 3), apart from [2a-H]+ and [2bH]+, expectedly yielded [CDC·P(H)(BH2·Py)NiPr2]2+ ([2a(H)(BH2·Py)]2+; d, δP −1.8 ppm, 1JP−H = 423 Hz; calcd for C30H42BN6P2+ m/z 264.1648, found 264.1658) and [CDC· P(H)(BH2·Py)Ph]2+ ([2b(H)(BH2·Py)]2+; d, δP 13.5 ppm, 1 JP−H = 395 Hz; calcd for C30H33BN5P2+ m/z 252.6280, found

Scheme 4. Independent Synthesis of [2·PH2][AlCl4], Its Molecular View Drawn at 50% Probability, and 31P NMR Spectruma

a Reagents and conditions: (a) 2 equiv of AlCl3, 2 equiv of nBu3SnH, 1,2-difluorobenzene, ambient temperature. Most H atoms and the counterion are omitted for clarity. Selected bond length (Å): P1−C1, 1.811(4).

The CCDC−P bond length (1.811(4) Å) appeared shorter than the analogous bond for NiPr2-substituted P-monocation [2aCl]+ (1.820(4) Å), but longer than the corresponding bond of its phenyl analogue [2b-Cl]+ (1.776(6) Å). While the latter observation was presumably imposed by the difference in electronegativity between the H and the Cl atoms, the former claim was supported by the greater electron-donating and steric character of the NiPr2 group vs H atom(s). Si−H Bond Activation. The electrophilicity of phosphenium cations was further investigated in reactions with significantly less hydridic silanes (Scheme 5).12 While [1][BArCl4] remained inert, the treatment of [2a][BArCl4]2 with 14674

DOI: 10.1021/acs.inorgchem.7b02579 Inorg. Chem. 2017, 56, 14671−14681

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Inorganic Chemistry Scheme 5. C−H Bond Activationa

a

the major component of this particular reaction mixture, which enabled its isolation and characterization by means of multinuclear NMR spectroscopic and single-crystal diffraction methods.22 On the other hand, the product derived from formal C−H bond insertion [CDC·P(H)(C7H7)Ph]2+ ([2b(H)(C7H7)]2+) was quite unstable under the given experimental conditions, and its identity was spectroscopically confirmed (31P: δP −18.9 ppm, 1JP−H = 498 Hz) in a separate reaction employing its respective building blocks [2b-H][BArCl4] and commercially available tropylium tetrafluoroborate ([C7H7][BF4]).9 This example indicated the ability of [2b]2+ to activate saturated C(sp3)−H bonds, which was further utilized in the reaction with xanthene. In this instance, the correlating 31P (δP 4.4 ppm) and 1H (δH 7.65 ppm, 1JP−H = 485 Hz) NMR signals suggested the formation of the C−H bond insertion product [CDC·P(H)(CH{C6H4}2O)Ph]2+ ([2b(H)(CH{C6H4}2O)]2+).9 Spectroscopic characterization of this latest product was only possible in situ due to its conspicuous solution- and solid-state instability, which was also reflected by the appearance of the structural fragments [2b-H]+ and [CH{C6H4}2O]+ (calcd C13H9O+ m/z 181.0648, found 181.0653) in the ESI-MS spectrum.9 In addition, the H atom attached to the C(sp 3 )−PH fragment of [2b(H)(CH{C6H4}2O)]2+ was shifted downfield to 5.79 ppm (xanthene: δH 4.04 ppm) as a result of the increased overall positive charge.

Only trace amounts of this reaction product were observed.

either triethyl- (Et3SiH) or triphenylsilane (Ph3SiH) resulted in a slight conversion (20% based on 31P{1H} NMR spectrum) to the previously observed H-abstraction product [2a-H]+. Furthermore, the 31P{1H} NMR spectrum of reaction mixtures between either Et3SiH or Ph3SiH and the more electrophilic dication [2b][BArCl4]2 yielded complete conversion to [2bH][BArCl4].9 Regarding the 29Si NMR spectrum, no visible signals corresponding to Et3Si or Ph3Si units were observed even after extended scanning time (48 h), suggesting the transient nature of cationic Si-based compounds in noncoordinating solvents such as DCM.20 Nonetheless, this result proposed that P-dications [2a][BArCl4]2 and [2b][BArCl4]2 activate Si−H bonds via H abstraction, unlike their isolobal group 14 analogues that favor oxidative addition. Prompted by the somewhat unexpected affinity of phosphenium ions to undergo H abstraction rather than insertion into Si−H bonds, we attempted the synthesis of insertion product [2b(H)(SiEt3)]2+ by reacting [2b-H][BArCl4] with an excess of [(Et3Si)·toluene][B(C6F5)4] in 1,2-difluorobenzene (Scheme 6). However, according to 31P NMR spectroscopy, there was

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CONCLUSION In summary, we have demonstrated that phosphenium ions can activate an array of E−H (E = Si, B, C) bonds by modifying their electronic/structural properties. While silanes were only activated via hydride abstraction, the B−H and C−H bond activation followed oxidative addition like pathways with the minor appearance of hydride abstraction products. The possibility of incorporating these elementary steps into the corresponding catalytic cycles is currently under investigation.

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EXPERIMENTAL SECTION

General Methods and Instrumentation. All preparations and manipulations were carried out using standard Schlenk techniques and a drybox under an argon atmosphere. Solvents (n-hexane and benzene) were distilled over sodium/benzophenone, while dichloromethane (DCM), 1,2-difluorobenzene, C6D6, CDCl3, CD3CN, and CD2Cl2 were distilled over CaH2. All solvents were stored over 4 Å molecular sieves prior to use. The commercial forms of aluminum trichloride, sodium hexafluoroanitmonate, 2,3-dimethyl-1,3-butadiene, triethylsilane, triphenylsilane, borane−trimethylamine complex, borane−pyridine complex, bis(diisopropylamino)chlorophosphine, tropylium tetrafluoroborate, xanthene, and tributyltin hydride were used without further purification. Dichlorophenylphosphine and cycloheptatriene were distilled prior to use. iPr2NPCl2,23 Na[BArCl4]24 (ArCl = 3,5-Cl2-C6H3), and the ligand carbodicarbene25 were prepared by literature methods. NMR spectra were obtained on Bruker Avance 300, Bruker Avance III 400, and Br̈ker Avance 500 instruments. Chemical shifts (δ) are reported in parts per million (ppm) downfield from the internal standard tetramethylsilane (for 1H and 13C{1H}) and external standard 85% phosphoric acid (for 31P{1H}). Mass spectra were obtained on Waters Q-TOF Premier MS and Agilent Technologies 6230 TOF LC/MS mass spectrometers using the electrospray ionization (ESI) mode. [(iPr2N)2P][BArCl4] ([1][BArCl4]). This compound was formed by adding 0.5 mL of CD2Cl2 to 15 mg (0.06 mmol, 1 equiv) of (iPr2N)2PCl and 38 mg (0.06 mmol, 1 equiv) of Na[BArCl4] (J. Young NMR tube). A light yellow solution of [(iPr2N)2P][BArCl4] ([1][BArCl4]) obtained in this manner was used for further reactions.

Scheme 6. Si−H Bond Activation

no indication of any reactivity between these two substrates, suggesting that formation of the target insertion product might be restricted by thermodynamic (sterics) and/or kinetic (activation energy) factors.21 Thus, these overall experimental observations strongly suggested that hydride abstraction was favored over Si−H bond insertion by [2a]2+ and [2b]2+. C−H Bond Activation. Inspired by the intrinsic electrophilicity of our phosphenium ions, we then attempted the activation of several substrates incorporating slightly hydridic C−H bonds.12 While aminophosphenium ions [1]+ and [2a]2+ were chemically inert toward C−H activation, their counterpart [2b][BArCl4]2 initially reacted with 1,3,5-cycloheptatriene to give reaction products formally derived from [4 + 1] cycloaddition, C(sp3)−H bond insertion, and hydride abstraction (Scheme 6).9 The cycloaddition product [CDC·P({CH}6CH2)Ph][BArCl4]2 ([2b({CH}6CH2)][BArCl4]2) was 14675

DOI: 10.1021/acs.inorgchem.7b02579 Inorg. Chem. 2017, 56, 14671−14681

Article

Inorganic Chemistry

BArCl4). 11B NMR (128 MHz, CD2Cl2, 298 K): δ −6.9 (s, BArCl4). 31P NMR (CD2Cl2, 160 MHz, 298 K): δ 327.7 (s). HR-MS: calcd for [C25H34N5P]2+ (2a2+) m/z 217.6271, found 217.6272. [{C6H4(MeN)2C}2C·PPh][X]2 ([2b][X]2, X = AlCl4, BArCl4). 1H, 13 C{1H}, and 31P NMR spectra of these compounds were virtually identical irrespective of the nature of the counterion; therefore, we only report multinuclear NMR data for [{C6H4(MeN)2C}2C·PPh]2+ ([2b]2+), as NMR spectroscopic details for AlCl4 and BArCl4 anions can be found in numerous published reports. A 20 mg portion (0.04 mmol, 1 equiv) of [2b-Cl]Cl and 11 mg (0.08 mmol, 2 equiv) of AlCl3 were weighed inside the glovebox (J. Young NMR tube) followed by addition of 0.7 mL of CD2Cl2, which resulted in an immediate formation of the target compound. The solvent was removed, and the resulting orange solid was dried in vacuo. Yield: 22 mg (73%). A 0.7 mL portion of 1,2-difluorobenzene was added to a mixture containing 20 mg (0.04 mmol, 1 equiv) of [2b-Cl]Cl and 42 mg (0.06 mmol, 2 equiv) of Na[BArCl4] to form [{C6H4(MeN)2C}2C·PPh][BArCl4]2 ([2b][BArCl4]2) (J. Young NMR tube). The resulting solution was filtered (NaCl removal) and layered with n-hexane to give single crystals of [2b][BArCl4]2. 1H NMR (500 MHz, CD2Cl2, 298 K): δ 3.71 (s, 6H, NCH3), 4.11 (s, 6H, NCH3), 7.38 (br t, 3JHH = 9 Hz, 2H, o-H, Ph), 7.45 (t, 3JHH = 10 Hz, 2H, m-H, Ph), 7.68 (t, 3JHH = 10 Hz, 1H, pH, Ph), 7.75−7.84 (m, 8H). 13C{1H} NMR (100 MHz, CD2Cl2, 298 K): δ 34.5 (s, NCH3), 35.3 (d, 4JPC = 5 Hz, NCH3), 114.1 (d, 4JCP = 5 Hz, CH arom), 129.2 (d, 1JPC = 65 Hz, CCC), 129.9 (s, CH arom), 130.1 (s, CH arom), 131.0 (d, 3JCP = 6 Hz, m-C, Ph), 132.0 (s, CH arom), 132.9 (d, 2JCP = 18 Hz, o-C, Ph), 133.2 (s, p-C, Ph), 136.7 (s, CH arom), 138.2 (d, 1JCP = 54 Hz, ipso-C, Ph), 143.6 (d, 4JCP = 8 Hz, CH arom), 145.3 (d, 2JPC = 26 Hz, NCN). 31P NMR (CD2Cl2, 202 MHz, 298 K): δ 393.4 (s). HR-MS: calcd for [C25H25N4P]2+ (2b2+) m/z 206.0903, found 206.0903. [{C6H4(MeN)2C}2C·P(CH2CCH3)2Ph][AlCl4]2 ([2b(CH2CCH3)2][AlCl4]2). A 20 mg portion (0.04 mmol, 1 equiv) of [2b-Cl]Cl and 11 mg (0.08 mmol, 2 equiv) of AlCl3 were dissolved in 0.5 mL of CD2Cl2 (J. Young NMR tube). Subsequently, 5 μL (0.04 mmol, 1 equiv) of 2,3-dimethyl-1,3-butadiene was added, leading to an immediate color change from orange to light yellow. The solvent was removed, and the resulting beige precipitate was dried in vacuo to afford 25 mg (77%) of [{C6H4(MeN)2C}2C·P(CH2CCH3)2Ph][AlCl4]2 ([2b(CH2CCH3)2][AlCl4]2). Single crystals suitable for Xray analysis were obtained when a dichloromethane solution of [2b(CH2CCH3)2][AlCl4]2 was layered with n-hexane. 1H NMR (500 MHz, CD2Cl2, 298 K): δ 1.85 (s, 6H, CCH3), 3.53 (br m, 4H, CH2), 3.79 (s, 12H, NCH3), 7.64 (m, 13H). 13C{1H} NMR (100 MHz, CD2Cl2, 298 K): δ 15.9 (s, CCH3), 16.1 (s, CCH3), 23.0 (d, 1JCP = 115 Hz, CCC), 34.2 (s, NCH3), 39.0 (d, 1JCP = 56 Hz, CH2), 112.6 (s, CH arom), 127.6 (s, CH arom), 128.9 (s, p-C, Ph), 130.9 (d, 2JCP = 10 Hz, o-C, Ph), 131.4 (d, 3JCP = 8 Hz, m-C, Ph), 131.7 (s, CH arom), 135.8 (br s, ipso-C, Ph), 149.3 (d, 2JCP = 10 Hz, NCN). 31P NMR (CD2Cl2, 202 MHz, 298 K): δ 30.7 (s). 31P{1H} NMR (CD2Cl2, 202 MHz, 298 K): δ 31.1 (s). HR-MS: calcd for [C31H35N4P]2+ (2b(CH2CCH3)22+) m/z 247.1294, found 247.1295. [(iPr2N)2P(H)(BH2·Py)][BArCl4] ([1(H)(BH2·Py)][BArCl4]). A 75 mg portion (0.28 mmol, 1 equiv) of (iPr2N)2PCl and 174 mg (0.28 mmol, 2 equiv) of Na[BArCl4] were weighed into a Schlenk flask followed by addition of 35 mL of 1,2-difluorobenzene. The resulting mixture was stirred for 5 min at room temperature, after which 30 μL (0.28 mmol, 1 equiv) of a borane−pyridine complex was added. After 2 h of stirring at room temperature, NaCl was filtered off and subsequent addition of 80 mL of n-hexane led to the precipitation of a white solid. This precipitate was dried in vacuo to yield 104 mg (40%) of [(iPr2N)2P(H)(BH2·Py)][BArCl4] ([1(H)(BH2·Py)][BArCl4]). Single crystals suitable for X-ray analysis were obtained by layering the dichloromethane solution of [1(H)(BH2·Py)][BArCl4] with n-hexane. 1 H NMR (400 MHz, CD2Cl2, 298 K): δ 0.99 (d, 3JHH = 7 Hz, 12H, (CH3)2CH), 1.26 (d, 3JHH = 7 Hz, 12H, (CH3)2CH), 3.22 (br d, 2H, BH2-Py), 3.49 (sept, 3JHH = 7 Hz, 4H, CH(CH3)2), 7.02 (m, 12H, BArCl4), 7.06 (dt, 1JPH = 432 Hz and 3JHH = 2 Hz, 1H, PH), 7.68 (t, 3 JHH = 7 Hz, 2H, m-H, Py), 8.13 (dd, 3JHH = 7 Hz and 5JHH = 1 Hz, 1H, p-H, Py), 8.40 (d, 3JHH = 6 Hz, 2H, o-H, Py). 13C{1H} NMR (100

Crystals of [1][BArCl4] suitable for single-crystal X-ray diffraction were grown by slow evaporation from 1,2-difluorobenzene solution. The preparation of bis(diisopropyl)phosphenium cation bearing [AlCl4] as counterion ([(iPr2N)2P][AlCl4]) was previously reported.26 1H NMR (300 MHz, CD2Cl2, 298 K): δ 1.44 (d, 3JHH = 7 Hz, 24H, (CH3)2CH), 4.13 (m, 4H, CH(CH3)2), 7.04 (m, 12H). 13C{1H} NMR (75 MHz, CD2Cl2, 298 K): δ 19.5 (s, CH(CH3)2), 24.8 (d, 2JCP = 8 Hz, CH(CH3)2), 123.1 (s, p-C, BArCl4), 132.9 (q, 2JCB = 4 Hz, o-C, BArCl4), 133.1 (s, m-C, BArCl4), 164.7 (q, 1JCB = 49 Hz, ipso-C, BArCl4). 31 P NMR (160 MHz, CD2Cl2, 298 K): δ 300.3 (s). HR-MS: calcd for [C12H28N2P]+ (1+) m/z 231.1985, found 231.1992. [{C6H4(MeN)2C}2C·P(NiPr2)Cl][X] ([2a-Cl][X], X = Cl, SbF6). A 0.50 g portion (1.65 mmol, 1 equiv) of {C6H4(MeN)2C}2C (2) was dissolved in 200 mL of benzene. This solution was then transferred to a Schlenk flask containing 2.5 equiv of iPr2NPCl2. The reaction mixture was stirred overnight followed by filtration. The resulting yellow solid was dried under vacuum to yield 0.76 g (92%) of [{C6H4(MeN)2C}2C·P(NiPr2)Cl]Cl ([2a-Cl]Cl). The most adequate counterion for crystallization of this species was found to be [SbF6], which was quantitatively exchanged with the chloride anion by the addition of 1 equiv of NaSbF6 to a dichloromethane solution of [2aCl]Cl followed by layering with n-hexane ([2a-Cl][SbF6]). 1H NMR (500 MHz, CD2Cl2, 298 K): δ 1.18 (d, 3JHH = 6 Hz, 9H, (CH3)2CH), 1.29 (m, 3H, (CH3)2CH), 3.25 (br m, 2H, CH(CH3)2), 3.81 (s, 12H, NCH3), 7.47−7.65 (m, 8H). 13C{1H} NMR (100 MHz, CD2Cl2, 298 K): δ 18.4 (s, CH(CH3)2), 23.3 (d, 2JCP = 9 Hz, CH(CH3)2), 34.2 (d, 4 JPC = 8 Hz, NCH3), 48.8 (d, 1JPC = 7 Hz, CCC), 111.6 (s, CH arom), 125.5 (s, CH arom), 132.4 (s, CH arom), 153.7 (d, 2JPC = 19 Hz, NCN). 31P NMR (202 MHz, CD2Cl2, 298 K): δ 137.9 (s). HR-MS: calcd for [C25H34N5PCl]+ (2a-Cl+) m/z 470.2240, found 470.2255. [{C6H4(MeN)2C}2C·P(Ph)Cl][X] ([2b-Cl][X], X = Cl, SbF6). The chloride salt of this compound was synthesized by adding 200 mL of a benzene solution of 0.50 g (1.65 mmol, 1 equiv) of 2 to 2.5 equiv of PhPCl2. The reaction mixture was stirred overnight followed by filtration. The resulting light yellow solid was dried under vacuum to yield 0.77 g (97%) of [{C6H4(MeN)2C}2C·P(Ph)Cl]Cl ([2b-Cl]Cl). The most adequate counterion for crystallization of this species was found to be [SbF6], which was quantitatively exchanged with the chloride anion by the addition of 1 equiv of NaSbF6 to a dichloromethane solution of [2b-Cl]Cl followed by layering with nhexane ([2b-Cl][SbF6]). 1H NMR (500 MHz, CDCl3, 298 K): δ 3.64 (s, 12H, NCH3), 7.16−7.53 (m, 13H). 13C{1H} NMR (100 MHz, CDCl3, 298 K): δ 33.7 (d, 4JPC = 8 Hz, NCH3), 54.2 (d, 1JPC = 62 Hz, CCC), 111.6 (s, CH arom), 125.7 (s, CH arom), 128.7 (d, 3JPC = 4 Hz, m-C, Ph), 129.8 (d, 2JPC = 20 Hz, o-C, Ph), 131.7 (s, CH arom), 132.7 (s, p-C, Ph), 136.2 (d, 1JPC = 28 Hz, ipso-C, Ph), 153.2 (d, 2JPC = 18 Hz, NCN). 31P NMR (202 MHz, CD2Cl2, 298 K): δ 101.7 (s). HRMS: calcd for [C25H25N4PCl]+ (2b-Cl+) m/z 447.1526, found 447.1505. [{C6H4(MeN)2C}2C·PNiPr2][BArCl4]2 ([2a][BArCl4]2). A 100 mg portion (0.19 mmol, 1 equiv) of [2a-Cl]Cl and 256 mg (0.40 mmol, 2.1 equiv) of Na[BArCl4] were weighed inside a glovebox (Schlenk flask) followed by addition of 30 mL of dichloromethane. The reaction mixture was stirred for 10 min and then filtered to remove NaCl. Subsequent washing with n-pentane (2 × 10 mL) and drying in vacuo yielded 200 mg (63%) of [{C6H4(MeN)2C}2C· PNiPr2][BArCl4]2 ([2a][BArCl4]2) as an orange solid. Unfortunately, all attempts to crystallize this compound were not successful. 1H NMR (300 MHz, CD2Cl2, 298 K): δ 1.03 (d, 6H, 3JHH = 6 Hz, (CH3)2CH), 1.59 (d, 3JHH = 6 Hz, 6H, (CH3)2CH), 3.17 (br m, 1H, CH(CH3)2), 3.55 (s, 6H, NCH3), 3.63 (s, 6H, NCH3), 4.05 (br m, 1H, CH(CH3)2), 6.94 (m, 8H, p-H, BArCl4), 7.07 (m, 16H, o-H, BArCl4), 7.41 (m, 2H), 7.50 (m, 2H), 7.73 (m, 4H). 13C{1H} NMR (100 MHz, CD2Cl2, 298 K): δ 21.6 (s, CH(CH3)2), 28.3 (d, 3JCP = 12 Hz, CH(CH3)2), 33.4 (d, 4JCP = 6 Hz, NCH3), 33.5 (s, NCH3), 51.4 (s, CH(CH3)2), 61.1 (d, 2JCP = 14 Hz, CH(CH3)2), 85.6 (d, 1JCP = 65 Hz, CCC), 112.7 (s, CH arom), 112.9 (s, CH arom), 123.2 (s, p-C, BArCl4), 129.5 (s, CH arom), 130.1 (s, CH arom), 131.1 (s, CH arom), 132.0 (s, CH arom), 133.1 (m, o-C, m-C, BArCl4), 144.6 (s, NCN), 145.7 (d, 2JCP = 37 Hz, NCN), 164.7 (q, 1JCB = 49 Hz, ipso-C, 14676

DOI: 10.1021/acs.inorgchem.7b02579 Inorg. Chem. 2017, 56, 14671−14681

Article

Inorganic Chemistry MHz, CD2Cl2, 298 K): δ 22.8 (d, 3JCP = 2 Hz, (CH3)2CH), 22.9 (d, 3 JCP = 2 Hz, (CH3)2CH), 48.1 (d, 2JCP = 4 Hz, CH(CH3)2), 123.0 (s, p-C, BArCl4), 127.6 (s, p-C, Py), 132.9 (q, 2JCB = 4 Hz, o-C, BArCl4), 133.1 (s, m-C, BArCl4), 143.3 (d, 3JCB = 2 Hz, m-C, Py), 147.4 (d, 2JCB = 7 Hz, o-C, Py), 164.7 (q, 1JCB = 49 Hz, ipso-C, BArCl4). 11B (128 MHz, CD2Cl2, 298 K): δ - 7.1 (s, BArCl4), - 13.5 (br dd). 11B{1H} NMR (128 MHz, CD2Cl2, 298 K): δ −7.1 (s, BArCl4), −13.4 (br d). 31 P NMR (160 MHz, CD2Cl2, 298 K): δ 20.4 (br dd). 31P{1H} NMR (160 MHz, CD2Cl2, 298 K): δ 20.3 (br d). HR-MS: calcd for [C17H36BN3P]+ ([1(H)(BH2·Py)+]) m/z 324.2740, found 324.2741. [(iPr2N)2P(H)(BH2·NMe3)][BArCl4] ([1(H)(BH2·NMe3)][BArCl4]). A 35 mL portion of 1,2-difluorobenzene was placed in a Schlenk flask containing 75 mg (0.28 mmol, 1 equiv) of (iPr2N)2PCl and 174 mg (0.28 mmol, 2 equiv) of Na[BArCl4]. The resulting mixture was stirred for 5 min at room temperature, after which 21 mg (0.28 mmol, 1 equiv) of the borane−trimethylamine complex was added. The reaction mixture was stirred for an additional 2 h followed by filtration to remove NaCl. A subsequent addition of 80 mL of n-hexane led to the precipitation of a white solid that was dried in vacuo to yield 124 mg (49%) of [(iPr2N)2P(H)(BH2·NMe3)][BArCl4] ([1(H)(BH2· NMe3)][BArCl4]). The crystallization was performed by layering a dichloromethane solution of [1(H)(BH2·NMe3)][BArCl4] with nhexane. 1H NMR (300 MHz, CD2Cl2, 298 K): δ 1.24 (d, 3JHH = 7 Hz, 12H, (CH3)2CH), 1.30 (d, 3JHH = 7 Hz, 12H, (CH3)2CH), 2.40 (br q, 2H, BH2-Py), 2.71 (d, 4JHH = 2 Hz, 9H, N(CH3)3), 3.60 (sept, 3JHH = 7 Hz, 4H, CH(CH3)2), 6.96 (dt, 1JPH = 432 Hz and 3JHH = 2 Hz, 1H, PH), 7.03 (m, 12H, BArCl4). 13C{1H} NMR (100 MHz, CD2Cl2, 298 K): δ 22.9 (d, 3JCP = 4 Hz, (CH3)2CH), 23.5 (d, 3JCP = 3 Hz, (CH3)2CH), 49.0 (d, 2JCP = 3 Hz, CH(CH3)2), 54.4 (d, 3JCP = 8 Hz, NCH3), 123.0 (s, p-C, BArCl4), 132.9 (q, 2JCB = 4 Hz, o-C, BArCl4), 133.1 (s, m-C, BArCl4), 164.7 (q, 1JCB = 49 Hz, ipso-C, BArCl4). 11B NMR (96 MHz, CD2Cl2, 298 K): δ −7.3 (s, BArCl4), −10.4 (br q). 11 1 B{ H} NMR (96 MHz, CD2Cl2, 298 K): δ −7.3 (s, BArCl4), −10.4 (br d). 31P NMR (121 MHz, CD2Cl2, 298 K): δ 15.6 (br dd). 31P{1H} NMR (121 MHz, CD2Cl2, 298 K): δ 16.1 (br d). HR-MS: calcd for [C 15H 40 BN 3P] + ([1(H)(BH 2 ·NMe 3 )+ ]) m/z 304.3053, found 304.3051. Reactions of [2a][BArCl4]2 with H3B·LB (LB = NMe3, Pyridine; General Procedure). A 50 mg portion (0.08 mmol, 2 equiv) of Na[BArCl4] was added to a solution of [2a-Cl]Cl (20 mg, 0.04 mmol, 1 equiv) in CD2Cl2 (0.7 mL) (J. Young NMR tube). The orange solution of phosphenium dication [2a][BArCl4]2 was then treated with 1 equiv (0.04 mmol) of amine−borane H3B·LB (LB = NMe3, Py), and the reaction progress was investigated by means of 31P/31P{1H} NMR spectroscopy (Figures S32−S35 in the Supporting Information) and HR-ES-MS (Figure S36 in the Supporting Information). [{C6H4(MeN)2C}2C·P(H)(BH2·NMe3)NiPr2][BArCl4]2 ([2a(H)(BH2· NMe3)][BArCl4]2). A 35 mL portion of 1,2-difluorobenzene was placed in a Schlenk flask containing 100 mg (0.19 mmol, 1 equiv) of [2a-Cl]Cl and 256 mg (0.40 mmol, 2 equiv) of Na[BArCl4]. The resulting mixture was stirred for 15 min at room temperature, after which 15 mg (0.21 mmol, 1 equiv) of H3B·NMe3 was added. The reaction mixture was stirred for an additional 2 h followed by filtration to remove NaCl. Subsequent addition of 80 mL of n-hexane led to the precipitation of a light yellow solid. The precipitate was isolated by filtration, washed with n-hexane (2 × 10 mL), and then dried in vacuo to yield 123 mg (37%) of [{C6H4(MeN)2C}2C·P(H)(BH2·NMe3)NiPr2][BArCl4]2 ([2a(H)(BH2·NMe3)][BArCl4]2). 1H NMR (400 MHz, CD2Cl2, 298 K): δ 0.88 (d, 3JHH = 6 Hz, 6H, (CH3)2CH), 1.16 (d, 3JHH = 6 Hz, 6H, (CH3)2CH), 2.66 (d, 4JHH = 1 Hz, 9H, N(CH3)3), 3.49 (br s, 12H, NCH3), 3.72 (sept, 3JHH = 6 Hz, 2H, (CH3)2CH), 6.93 (br s, 8H, p-H, BArCl4), 6.94 (m, 16H, BArCl4), 7.16 (dd, 3JHH = 4 Hz and 1JPH = 351 Hz, 1H), 7.05 (br m, 4H), 7.40 (m, 4H). 13C{1H} NMR (100 MHz, CD2Cl2, 298 K): δ 18.5 (s, (CH3)2CH), 22.54 (d, 2JCP = 3 Hz, (CH3)2CH), 23.02 (d, 3JCP = 2 Hz, (CH3)2CH), 32.9 (br s, NCH3), 54.4 (d, 3JCP = 7 Hz, N(CH3)3), 111.9 (s, CH arom), 123.1 (s, p-C, BArCl4), 127.7 (s, CH arom), 131.2 (s, CH arom), 132.9 (q, 2JCB = 4 Hz, o-C, BArCl4), 133.0 (s, m-C, BArCl4), 151.9 (br s, NCN), 164.2 (q, 1JCB = 49 Hz, ipso-C, BArCl4). 11 B NMR (128 MHz, CD2Cl2, 298 K): δ −6.9 (s, BArCl4), −9.4 (br q).

B{1H} NMR (128 MHz, CD2Cl2, 298 K): δ −6.9 (s, BArCl4), −9.3 (br s). 31P NMR (202 MHz, CD2Cl2, 298 K): δ −15.4 (br d). 31P{1H} NMR (160 MHz, CD2Cl2, 298 K): δ −15.3 (br s). The signal corresponding to the central carbon atom (CCC) was not detected. HR-MS: calcd for [C28H46BN6P]2+ ([2a(H)(BH2·NMe3)2+]) m/z 254.1804, found 254.1811. Attempts to isolate [{C6H4(MeN)2C}2C·P(H)(BH2·Py)NiPr2][BArCl4]2, [2a(H)(BH2·Py)][BArCl4]2 from the reaction mixture between [2a][BArCl4]2 and H3B·Py (Figures S34−S36 in the Supporting Information) were not successful. 31P NMR (121 MHz, CD2Cl2, 298 K): δ −1.8 (br d, 1JPH = 423 Hz). 31P{1H} NMR (121 MHz, CD2Cl2, 298 K): δ −1.8 (s). HR-MS: calcd for [C30H42BN6P]2+ ([2a(H)(BH2·Py)2+]) m/z 264.1648, found 264.1658. [{C6H4(MeN)2C}2C·PH2][X] ([2·PH2][X], [X] = [AlCl4], [BArCl4]). All attempts to isolate [{C6H4(MeN)2C}2C·PH2][BArCl4] ([2·PH2][BArCl4]) from the reaction mixture between the in situ formed [2a][BArCl4]2 and H3B·Py have failed. Therefore, we have synthesized this species in a separate procedure using [{C6H4(MeN)2C}2C·PBr2] Br as a P source.19 A 0.7 mL portion of CD2Cl2 was added to 22 mg (0.04 mmol, 1 equiv) of [{C6H4(MeN)2C}2C·PBr2]Br and 47 mg of Na[BArCl4] (0.08 mmol, 2 equiv), followed by addition of 21 μL (0.08 mmol, 2 equiv) of nBu3SnH (J. Young NMR tube). [AlCl4] appeared to be the most suitable anion for crystallization of [2·PH2]+, which was introduced in the following procedure: 0.7 mL of 1,2-difluorobenzene was added to 22 mg (0.04 mmol, 1 equiv) of [{C6H4(MeN)2C}2C· PBr2]Br and 10 mg (0.08 mmol, 2 equiv) of AlCl3. The resulting solution was treated with 21 μL (0.08 mmol, 2 equiv) of nBu3SnH, leading to the formation of [2·PH2][AlCl4]. Crystals suitable for single-crystal X-ray diffraction were grown by layering a 1,2difluorobenzene solution of [2·PH2][AlCl4] with n-hexane. Note: crystals of [2·PH2][AlCl4] were not soluble in common organic solvents (CDCl3, CD2Cl2, CH3CN). 1H NMR (500 MHz, CD2Cl2, 298 K): δ 3.51 (s, 12H, NCH3), 4.15 (d, 2H, 1JPH = 212 Hz), 6.91 (br s, p-H, BArCl4), 7.04 (br s, o-H, BArCl4), 7.34 (m, 4H), 7.44 (m, 4H). 31 P NMR (121 MHz, CD2Cl2, 298 K): δ −133.1 (t, 1JPH = 212 Hz). 31 1 P{ H} NMR (202 MHz, CD2Cl2, 298 K): δ −134.1 (s). HR-MS: calcd for [C19H22N4P]+ (2·PH2+) m/z 337.1582, found 337.1583. Reactions of [2b][BArCl4]2 with BH3·LB (LB = NMe3, Pyridine; General Procedure). A 52 mg portion (0.08 mmol, 2 equiv) of Na[BArCl4] was added to a solution of [2b-Cl]Cl (20 mg, 0.04 mmol, 1 equiv) in CD2Cl2 (0.7 mL) (J. Young NMR tube), leading to the formation of [2b][BArCl4]2. Subsequent addition of 1 equiv (0.04 mmol) of aminoborane H3B·LB (LB = NMe3, Py) led to an immediate color change from orange to light yellow. The reaction progress was investigated by means of 31P/31P{1H} NMR spectroscopy (Figures S47−S50 in the Supporting Information) and HR-ES-MS (Figure S51 in the Supporting Information). [{C6H4(MeN)2C}2C·P(H)(BH2·NMe3)Ph][X]2, [2b(H)(BH2·NMe3)][X]2 (X = AlCl4, BArCl4). A 35 mL portion of 1,2-difluorobenzene was added to a Schlenk flask containing 100 mg (0.21 mmol, 1 equiv) of [2b-Cl]Cl and 268 mg (0.43 mmol, 2 equiv) of Na[BArCl4]. The resulting mixture was stirred for 15 min at room temperature, after which 15 mg (0.21 mmol, 1 equiv) of borane−trimethylamino complex was added. The reaction mixture was stirred overnight followed by filtration to remove NaCl. The resulting solution was treated with 80 mL of n-hexane to cause the precipitation of a light yellow solid, which was isolated by filtration and subsequently dried in vacuo to yield 220 mg (64%) of [{C6H4(MeN)2C}2C·P(H)(BH2· NMe3)Ph][BArCl4]2 ([2b(H)(BH2·NMe3)][BArCl4]2). The most adequate counterion for crystallization of this species was found to be [AlCl4]. Following the procedure described above, [AlCl4] was introduced when AlCl3 was used for preparation of [2b]2+ instead of Na[BArCl4]. Upon isolation, a 1,2-difluorobenzene solution of [{C6H4(MeN)2C}2C·P(H)(BH2·NMe3)Ph][AlCl4]2 ([2b(H)(BH2· NMe3)][AlCl4]2) was layered with n-hexane to afford colorless crystals suitable for X-ray analysis. 1H NMR (500 MHz, CD2Cl2, 298 K): δ 2.39 (br d, 2H), 2.63 (d, 4JHH = 1 Hz, 9H, NMe3), 3.50 (s, 12H, NCH3), 6.93 (t, 4JHH = 2 Hz, 8H, p-H, BArCl4), 7.04 (m, 16H, BArCl4), 7.06 (dd, 3JHH = 7 Hz and 1JPH = 383 Hz, 1H), 7.37−7.61 (m, 13H). 13C{1H} NMR (100 MHz, CD2Cl2, 298 K): δ 24.9 (d, 1JCP = 94 11

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DOI: 10.1021/acs.inorgchem.7b02579 Inorg. Chem. 2017, 56, 14671−14681

Article

Inorganic Chemistry Hz, CCC), 33.3 (s, NCH3), 54.6 (d, 3JCP = 6 Hz, N(CH3)3), 111.9 (s, CH arom), 123.1 (s, p-C, BArCl4), 123.7 (s, p-C, Ph), 127.7 (s, CH arom), 131.3 (d, 1JPC = 14 Hz, ipso-C, Ph), 131.4 (d, 2JPC = 13 Hz, o-C, Ph), 132.9 (q, 2JCB = 4 Hz, o-C, BArCl4), 133.1 (s, m-C, BArCl4), 134.6 (d, 3JCP = 3 Hz, m-C, Ph), 150.8 (d, 2JPC = 8 Hz, NCN), 164.7 (q, 1JCB = 49 Hz, ipso-C, BArCl4). 11B NMR (128 MHz, CD2Cl2, 298 K): δ −7.5 (s, BArCl4), −10.8 (br). 11B{1H} NMR (128 MHz, CD2Cl2, 298 K): δ −7.5 (s, BArCl4), −11.1 (br s). 31P NMR (202 MHz, CD2Cl2, 298 K): δ −30.5 (br d). 31P{1H} NMR (202 MHz, CD2Cl2, 298 K): δ − 30.5 (s). HR-MS: calcd for [C28H37BN5P]2+ ([2b(H)(BH2· NMe3)2+]): m/z 242.6435, found 242.6440. Attempts to isolate [{C6H4(MeN)2C}2C·P(H)(BH2·Py)Ph][BArCl4]2 ([2b(H)(BH2·Py)][BArCl4]2) from the reaction mixture between [2b][BArCl4]2 and H3B·Py (Figures S49−S51 in the Supporting Information) were not successful. 31P NMR (160 MHz, CD2Cl2, 298 K): δ −13.5 (br d, 1JPH = 395 Hz). 31P{1H} NMR (160 MHz, CD 2 Cl 2 , 298 K): δ −13.5 (s). HR-MS: calcd for [C30 H33BN5P] 2+ ([2b(H)(BH2 ·Py) 2+]): m/z 252.6280, found 252.6292. Reactions of [2a][BArCl4]2 with R3SiH (R = Et, Ph). A 51 mg portion (0.08 mmol, 2 equiv) of Na[BArCl4] was added to a solution of [2a-Cl]Cl (20 mg, 0.04 mmol, 1 equiv) in CD2Cl2 (0.7 mL) (J. Young NMR tube), leading to the formation of dication [2a][BArCl4]2. Subsequently, 1 equiv of the corresponding R3SiH (R = Et, Ph) was added, and the reaction mixture was investigated by means of 31 31 1 P/ P{ H} NMR spectroscopy. It is noteworthy that both Et3SiH and Ph3SiH gave almost identical 31P/31P{1H} NMR spectra (Figures S59 and S60 in the Supporting Information). [{C6H4(MeN)2C}2C·P(NiPr2)H][BArCl4] ([2a-H][BArCl4]). A 51 mg portion (0.08 mmol, 2 equiv) of Na[BArCl4] was added to a solution of [2a-Cl]Cl (20 mg, 0.04 mmol, 1 equiv) in CD2Cl2 (0.7 mL) (J. Young NMR tube), followed by addition of 11 μL (0.04 mmol, 1 equiv) of n Bu3SnH. The resulting mixture was analyzed by 31P/31P{1H} NMR spectroscopy and HR-MS. Although it formed as a major product, [{C6H4(MeN)2C}2C·P(NiPr2)H][BArCl4] ([2a-H][BArCl4]) was not isolated due to its high reactivity in solution. 31P NMR (202 MHz, CD2Cl2, 298 K): δ −12.3 (br d, 1JPH = 225 Hz). 31P{1H} NMR (202 MHz, CD2Cl2, 298 K): δ −12.3 (br s). HR-MS: calcd for [C25H35N5P]+ (2a-H+) m/z 436.2630, found 436.2632. Reactions of [2b][BArCl4]2 with R3SiH (R = Et, Ph) and n Bu3SnH (General Procedure). A 52 mg portion (0.08 mmol, 2 equiv) of Na[BArCl4] was added to a solution of [2b-Cl]Cl (20 mg, 0.04 mmol, 1 equiv) in CD2Cl2 (0.7 mL) (J. Young NMR tube) to form the corresponding dication [2b][BArCl4]2. Then, 1 equiv of either silane R3SiH (R = Et, Ph) or stannane nBu3SnH was added, leading to the formation of the single reaction product [{C6H4(MeN)2C}2C· P(Ph)H][BArCl4] ([2b-H][BArCl4]). [{C 6 H 4 (MeN) 2 C} 2 C·P(Ph)H][X] ([2b-H][X], [X] = [AlCl 4 ], [BArCl4]). A 35 mL portion of 1,2-difluorobenzene was added to a Schlenk flask containing 100 mg of (0.21 mmol, 1 equiv) [2b-Cl]Cl and 268 mg (0.43 mmol, 2 equiv) of Na[BArCl4]. The reaction mixture was stirred for 10 min, after which 57 μL (0.21 mmol, 1 equiv) of n Bu3SnH was added, leading to an immediate color change from orange to yellow. After 2 h of stirring at ambient temperature, NaCl was filtered off. The resulting solution was treated with 100 mL of nhexane to afford the formation of [{C6H4(MeN)2C}2C·P(Ph)H][BArCl4] ([2b-H][BArCl4]) as a light yellow precipitate, which was isolated by filtration and subsequently dried under vacuum. Yield: 78 mg (37%). [AlCl4] was found to be the most suitable anion for crystallization of [2b-H]+. This anion was introduced according to the following procedure: 0.7 mL of 1,2-difluorobenzene was added to 20 mg (0.04 mmol, 1 equiv) of [2b-Cl]Cl and 11 mg (0.08 mmol, 2 equiv) of AlCl3 (J. Young NMR tube), followed by addition of 11 μL (0.04 mmol, 1 equiv) of nBu3SnH. The resulting solution was layered with n-hexane to afford single crystals of [{C6H4(MeN)2C}2C· P(Ph)H][AlCl4] ([2b-H][AlCl4]). 1H NMR (400 MHz, CD2Cl2, 298 K): δ 3.53 (s, 12H, NCH3), 5.59 (d, 1H, 1JPH = 226 Hz), 6.94 (br s, 4H, p-H, BArCl4), 7.05−7.44 (m, 21H). 13C{1H} NMR (100 MHz, CD2Cl2, 298 K): δ 33.1 (d, 4JCP = 6 Hz, NCH3), 42.6 (d, 1JCP = 23 Hz, CCC), 110.5 (s, CH arom), 123.0 (s, p-C, BArCl4), 125.4 (s, CH

arom), 128.3 (s, p-C, Ph), 129.1 (d, 3JCP = 5 Hz, m-C, Ph), 129.8 (d, JCP = 15 Hz, o-C, Ph), 132.1 (s, CH arom), 132.9 (q, 2JCB = 4 Hz, o-C, BArCl4), 133.1 (s, m-C, BArCl4), 135.7 (d, 1JCP = 6 Hz, ipso-C, Ph), 155.5 (d, 2JCP = 17 Hz, NCN), 164.7 (q, 1JCB = 49 Hz, ipso-C, BArCl4). 31 P NMR (202 MHz, CD2Cl2, 298 K): δ −52.7 (d, 1JPH = 226 Hz). 31 1 P{ H} NMR (202 MHz, CD2Cl2, 298 K): δ −52.7 (s). HR-MS: calcd for [C25H26N4P]+ (2b-H+) m/z 413.1890, found 413.1892. Reaction of [2b][BArCl4]2 with 1,3,5-Cycloheptatriene (General Procedure). A 52 mg portion (0.08 mmol, 2 equiv) of Na[BArCl4] was added to a solution of [2b-Cl]Cl (20 mg, 0.04 mmol, 1 equiv) in CD2Cl2 (0.7 mL) (J. Young NMR tube) to form the dication [2b][BArCl4]2, which was subsequently treated with 5 μL of 1,3,5-cycloheptatriene (0.05 mmol, 1.2 equiv), and the reaction mixture was investigated by means of 31P/31P{1H} NMR spectroscopy (Figures S20 and S21 in the Supporting Information). [{C6H4(MeN)2C}2C·P{(CH)6CH2}Ph][BArCl4]2 ([2b{(CH)6CH2}][BArCl4]2). A 268 mg portion (0.43 mmol, 2 equiv) of Na[BArCl4] was added to a 1,2-difluorobenzene solution (45 mL) of 100 mg (0.21 mmol, 1 equiv) of [2a-Cl]Cl. The reaction mixture was stirred for 5 min, after which 22 μL (0.21 mmol, 1 equiv) of 1,3,5-cycloheptatriene was added. After 12 h of stirring at ambient temperature, NaCl was removed by filtration. Addition of n-hexane (around 100 mL) to the vigorously stirred solution afforded precipitation of the light yellow solid [{C 6 H 4 (MeN) 2 C} 2 C·P{(CH) 6 CH 2 }Ph][BAr Cl 4 ] 2 ([2b{(CH)6CH2}][BArCl4]2), which was isolated by filtration and dried in vacuo. Yield: 109 mg (31%). Single crystals of [2b{(CH)6CH2}][BArCl4]2 suitable for X-ray diffraction were grown by slow evaporation from 1,2-difluorobenzene solution. Note: once precipitated, [2b{(CH)6CH2}][BArCl4]2 was only sparingly soluble in common organic solvents (CDCl3, CD2Cl2, CD3CN). 1H NMR (300 MHz, CD2Cl2, 298 K): δ 2.37 (br m, 1H), 2.73 (br m, 1H), 3.55 (br s, 12H, NCH3), 5.23 (br m, 1H), 5.58 (br m, 1H), 6.36 (m, 2H), 6.85 (m, 1H), 7.01 (t, 4 JHH = 2 Hz, 8H, p-H, BArCl4), 7.13 (m, o-H, 16H, BArCl4), 7.34−7.82 (m, 14H). 13C{1H} NMR (100 MHz, CD2Cl2, 298 K): δ 123.3 (s, p-C, BArCl4), 125.8 (s), 125.9 (s), 126.9 (s), 127.9 (s), 131.0 (s), 132.1 (br d), 132.9 (br s, o-C, BArCl4), 133.3 (m, m-C, BArCl4), 133.5 (s), 134.6 (s), 135.0 (br d), 135.5 (br t), 137.1 (s), 142.4 (br s), 154.5 (s), 164.7 (q, 1JCB = 49 Hz, ipso-C, BArCl4). 31P NMR (202 MHz, CD2Cl2, 298 K): δ 39.8 (br s). HR-MS: calcd for [C32H33N4P]2+ ([2b{(CH)6CH2}2+]) m/z 252.1216, found 252.1218. Reaction of [2b-H][BArCl4] with Tropylium Tetrafluoroborate ([C7H7][BF4]). A 6 mg portion (0.03 mmol, 1 equiv) of [C7H7][BF4] was added to a CD2Cl2 solution (0.7 mL) of 30 mg (0.03 mmol, 1 equiv) of freshly prepared [2b-H][BArCl4]. The reaction mixture was kept at ambient temperature for 3 h and then analyzed by 31P/31P{1H} NMR spectroscopy. The compound identified by the signal at −18.9 ppm (Figures S79 and S80 in the Supporting Information), presumably the C−H bond insertion product [{C6H4(MeN)2C}2C· P(H)(C7H7)]2+ ([2b(H)(C7H7)]2+) was unstable and was always obtained in a mixture with [2b-H]+. Attempts to improve the ratio between [2b-H]+ and [2b(H)(C7H7)]2+ by modifying reaction conditions as well as amounts of [C7H7][BF4] were unsuccessful. [{C6H4(MeN)2C}2C·P(H)(CH{C6H4}2O)Ph][BArCl4]2 ([2b(H)(CH{C6H4}2O)][BArCl4]2). A 52 mg portion (0.08 mmol, 2 equiv) of Na[BArCl4] was added to a solution of [2b-Cl]Cl (20 mg, 0.04 mmol, 1 equiv) in CD2Cl2 (0.7 mL) (J. Young NMR tube) to form the dication [2b][BArCl4]2, followed by addition of 8 mg (0.04 mmol, 1 equiv) of xanthene. The reaction mixture was then investigated by means of multinuclear NMR spectroscopy and HR-MS. Although the particular reaction mixture showed the presence of a single reaction product, as evidenced by 31P/31P{1H} NMR spectra (Figures S84 and S85), further attempts to isolate [2b(H)(CH{C6H4}2O)][BArCl4]2 were unsuccessful, leading to the formation of a yellow oil which when dissolved in CD2Cl2 showed the presence of multiple compounds. It is also noteworthy that the target compound is moderately stable in solution at ambient temperature over a period of 24 h, after which it decomposes to give a complicated mixture. Therefore, all spectroscopic data were obtained in situ. 1H NMR (300 MHz, CD2Cl2, 298 K): δ 3.10 (s, 12H, NCH3), 5.79 (br s, 1H), 6.92 (t, 4JHH = 2 Hz, 8H, p-H, BArCl4), 7.07 (m, 16H, o-H, BArCl4), 7.65 (d, 1JPH = 485 Hz, 1H, 2

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DOI: 10.1021/acs.inorgchem.7b02579 Inorg. Chem. 2017, 56, 14671−14681

Inorganic Chemistry PH), 7.36−7.82 (m, 21H, CHarom). 13C{1H} NMR (100 MHz, CD2Cl2, 298 K): δ 32.7 (s, NCH3), 110.7 (s, CH arom), 111.9 (s, CH arom), 112.9 (s, CH arom), 117.6 (s, CH arom), 118.03 (s, CH arom), 123.1 (s, p-C, BArCl4), 125.8 (s, CH arom), 126.46 (s, CH arom), 128.3 (s, CH arom), 130.3 (s, CH arom), 131.1 (s, CH arom), 131.3 (s, CH arom), 131.7 (s, CH arom), 132.1 (s, CH arom), 133.0 (br s, o- and m-C, BArCl4), 147.8 (br s, NCN), 164.7 (q, 1JCB = 49 Hz, ipso-C, BArCl4). 31P NMR (160 MHz, CD2Cl2, 298 K): δ 4.4 (br d). 31 1 P{ H} NMR (160 MHz, CD2Cl2, 298 K): δ 4.5 (s). Note: although we were not able to detect the molecular ion of [2b(H)(CH{C6H4}2O)]2+ at the respective m/z value (calcd 327.1738), we managed to clearly observe its fragments [2b-H]+ and [CH{C6H4}2O]+, as well as [{C6H4(MeN)2C}2C·P(CH{C6H4}2O)Ph]+ ([M − H]+) ion. We strongly believe that this fragmentation takes part under HR-MS conditions due to the formation of a more stable (aromatic) ion[CH{C6H4}2O]+. HR-MS: calcd for [C38H34N4OP]+ ([M-H]+) m/z 593.2465, found 593.2469; calcd for [C25H26N4P]+ ([2b-H]+) m/z 413.1890, found 413.1892 (see Figure S72 in the Supporting Information); calcd for [C13H9O]+ ([CH{C6H4}2O]+) m/z 181.0648, found 181.0653. Crystallographic Methods. Single crystals were mounted on quartz fibers, and the X-ray intensity data were collected at 103(2) or 153(2) K on a Bruker X8 APEX system, using Mo Kα radiation, with the SMART suite of programs.27 Data were processed and corrected for Lorentz and polarization effects with SAINT28 and for absorption effects with SABADS.29 Structural solution and refinement were carried out with the SHELXTL suite of programs.30 The structure was solved by direct methods and refined for all data by full-matrix leastsquares methods on F2. All non-hydrogen atoms were subjected to anisotropic refinement. The hydrogen atoms were generated geometrically and allowed to ride on their respective parent atoms; they were assigned appropriate isotopic thermal parameters. Computational Methods. Theoretical analysis was performed with the Gaussian 0931 suite of programs using default convergence criteria for optimization and an ultrafine integration grid. Cartesian coordinates of the structures optimized at the B3LYP/6-31G* level are reported.

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ACKNOWLEDGMENTS

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REFERENCES

We thank Nanyang Technological University (M4080534), the Ministry of Education, Science, and Technological Development of the Republic of Serbia within the framework of a project (M.P., 172040), and Monash University (D.V., SUG) for financial support. Computational analyses were performed on the PARADOX cluster at the Scientific Computing Laboratory of the Institute of Physics Belgrade, supported in part by the Ministry of Education, Science, and Technological Development of the Republic of Serbia under project No. ON171017 (M.P.).

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02579. Multinuclear NMR spectra, as well as crystallographic details and Cartesian coordinates (PDF) Accession Codes

CCDC 1544299−1544309 contain 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 for D.V.: [email protected]. ORCID

Dragoslav Vidović: 0000-0003-4269-3995 Present Address §

School of Chemistry, Monash University, Melbourne, Australia.

Notes

The authors declare no competing financial interest. 14679

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DOI: 10.1021/acs.inorgchem.7b02579 Inorg. Chem. 2017, 56, 14671−14681

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DOI: 10.1021/acs.inorgchem.7b02579 Inorg. Chem. 2017, 56, 14671−14681