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Mixed Ligated Tris(amidinate)dimolybdenum Complexes as Catalysts for Radical Addition of CCl4 to 1‑Hexene: Leaving Ligand Lability Controls Catalyst Activity Supriya Rej, Moumita Majumdar, Shun Kando, Yoshitaka Sugino, Hayato Tsurugi,* and Kazushi Mashima* Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan S Supporting Information *

ABSTRACT: We synthesized a series of mixed ligated tris(amidinate)dimolybdenum complexes, namely, [Mo2(DAniF)3(L)] [DAniF = N,N′-di(p-anisyl)formamidinate; L = acetate (OAc; 1a), m-diphenylphosphino benzoate (mPPh2Bz; 1b), nicotinate (Nico; 1c), benzoate (Bz; 1d), 3furoate (3-Furo; 1e), isonicotinate (IsoNico; 1f), and trifluoromethanesulfonate (OTf; 1g)], which served as catalysts for radical addition of CCl4 to 1-hexene to give 1,1,1,3tetrachloroheptane. These mixed ligated complexes 1a−g afforded the higher yield of the radical addition product than a homoleptic DAniF complex, [Mo2(DAniF)4] (2). Among them, complexes 1a and 1g gave the radical addition product quantitatively after 9 h with a short induction period. When complexes 1a and 1g were treated with CCl4, we detected the mixedvalence Mo2(II/III) complex, [Mo2(DAniF)3Cl2] (4), in electrospray ionization mass spectrometry measurements, indicating that the leaving nature of the L ligands was a crucial factor for initiating the catalytic reaction: the catalytic activity of the carboxylate-bridged complex 1a and the triflate-bridged complex 1g in the initial 30 min highly depended on the ligand-exchange rate of L, as estimated by monitoring the reaction with CCl4 in pyridine, giving the pyridine adduct complex, [Mo2(DAniF)3Cl(py)] (3).

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INTRODUCTION An intrinsic advantage of dinuclear paddlewheel complexes is that they act as potent and unique scaffolds in which four bridging ligands around the dimetal core finely control not only redox behavior and Lewis acidity1 but also unique coordination environments of axial metal coordination sites.2 Successful and representative examples include paddlewheel complexes of dirhodium(II) and diruthenium(I), which catalyze cyclopropanation, aziridine formation, and C−H functionalization.3−6 In contrast to these noble metal complexes, less attention has been paid to dinuclear complexes of early transition metals as homogeneous catalysts, though multiply bonded paddlewheel complexes of molybdenum have been intensively investigated: Ohshiro et al. reported that regiospecific ring opening of α,β-epoxysilanes by Mo2(OAc)4 to form metal enolate intermediate;7a Kobayashi et al. reported that Mo2(OAc)4 acts as a catalyst for the Aza-Diels−Alder reaction of acyl hydrazones, and the catalytic activity was improved by exposing the Mo2 catalyst to O2 atmosphere for generating high oxidation state dinuclear species;7b Cotton et al. demonstrated that dimolybdenum(II) and ditungsten(II) complexes such as M2(hpp)4 (M = Mo, W; hpp = the 1,3,4,6,7,8-hexahydro-2Hpyrimido[1,2-a]pyrimidine anion) undergo stoichiometric C(sp2)−X (X = Cl, Br, and I) bond reductive cleavage via a single-electron transfer process to give the corresponding oxidized Mo2(II/III) and W2(III/III) complexes Mo2(hpp)4X and W2(hpp)4X2;8 Tsai et al. reported that quintuple-bonded © XXXX American Chemical Society

dimolybdenum complexes undergo cycloaddition reactions with alkynes to give quadruple-bonded dimolybdenum complexes.9 Such redox-active paddlewheel complexes are essentially applicable to the radical-mediated organic transformations. A typical application of metal-mediated radical generation is atom transfer radical addition (ATRA) because of the synthetic utility for the C−C bond formation from simple haloalkanes and alkenes, in which a carbon−halogen bond of polyhalogenated alkanes such as CCl4, CBr4, and CHCl3 added to alkenes in regioselective manner.10 Nowadays, a large number of research groups reported about the ATRA using late transition metals such as Ru, Cu, and Fe.11 However, early transition-metal-catalyzed ATRA reactions were less explored due to the difficulty to control the reversible redox processes.12 Recently, we demonstrated that the quadruply bonded dimolybdenum complexes of the Mo2L4 and [nBu4N]2[Mo2L2Cl4] (L = carboxylate, formamidinate and guanidinate) types are effective catalysts for radical addition and polymerization as well as hydrodehalogenation reactions.13 As part of our continuous studies of the catalytic application of dinuclear complexes of molybdenum, we herein report the synthesis of mixed ligated dimolybdenum complexes with a general formula of Mo2(DAniF)3(L) [DAniF = N,N′-di(panisyl)formamidinate; L = carboxylates or trifluoromethanesulReceived: October 22, 2016

A

DOI: 10.1021/acs.inorgchem.6b02525 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry fonate] and their catalytic performance for radical addition of CCl4 to 1-hexene to yield 1,1,1,3-tetrachloroheptane in regioselective manner. We propose a plausible reaction mechanism based on some controlled experiments, including kinetics and isolations of [Mo2(DAniF)3Cl(py)] (3) and [Mo2(DAniF)3Cl2] (4) with relevance to possible intermediates.

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RESULTS AND DISCUSSION Synthesis and Characterization of Mixed-Ligated Dimolybdenum Complexes. Dimolybdenum complexes 1b−f were synthesized in good yield by treating an acetate complex, [Mo2(DAniF)3(OAc)] (DAniF = N,N′-di(p-anisyl)formamidinate) (1a),14 with the corresponding carboxylic acids, such as m-diphenylphosphino benzoic acid (b),15 nicotinic acid (c), benzoic acid (d), furan-3-carboxylic acid (e), and isonicotinic acid (f), in the presence of sodium methoxide (eq 1). Complexes 1b−f were characterized by 1H Figure 1. ORTEP diagram of [Mo2 (DAniF)3(O2CC5H4N)] (1c). Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level.

Table 1. Selected Bond Lengths (Å) and Angles (deg) of Mo2 Complexes 1c and 1g Mo1−Mo2 Mo1−O1 Mo2−O2 Mo1−N1 Mo1−N3 Mo1−N5 O1−Mo1−N1 O1−Mo1−N3 O1−Mo1−N5

NMR, UV−vis spectroscopy, cyclic voltammetry, and combustion analysis along with X-ray diffraction studies for 1b, 1c, and 1e. The 1H NMR spectrum of complex 1b displayed two singlets at 8.46 (1H) and 8.42 (2H) ppm assignable to the methine protons of two magnetically nonequivalent amidinate ligands, trans and cis to the carboxylate ligand, respectively,16 and two sets of the typical AB pattern due to two kinds of aromatic protons of the amidinate ligands appeared at 6.43 and 6.21 ppm (trans to the carboxylate) and at 6.64 and 6.54 ppm (cis to the carboxylate). In addition, two methoxy resonances due to two kinds of amidinate ligands were observed at 3.71 (12 H) and 3.64 (6 H) ppm. Similarly, the NMR spectral features of complexes 1c−f were consistent with those found for complex 1b as well as the starting complex 1a. In the UV− vis spectra of all newly prepared complexes 1b−f, absorption in the range of 418−453 nm was attributed to a typical δ→δ* transition band of a quadruply bonded Mo2 core.14,16 Figure 1 shows the molecular structure of 1c, and Table 1 summarizes its selected bond lengths and angles. Notably, complex 1c involves a quadruple-bonded Mo2 core supported by three N,N′-di(anisyl)formamidinato ligands and one 3-pyridylcarboxylato ligand. The bond length of Mo1−Mo2 (2.1150(8) Å) in complex 1c is a typical Mo−Mo bond length for quadruply bonded [Mo2]4+ complexes.14,16 The bond lengths of Mo1−O1 (2.150(2) Å) and Mo2−O2 (2.193(2) Å) are in good accordance with those found for carboxylate-bridged dimolybdenum complexes.14,16 The bond lengths of Mo1−N1 (2.162(4) Å), Mo1−N3 (2.127(3) Å), and Mo1−N5 (2.163(4) Å) in complex 1c are also normal for triamidinated dimolybdenum paddlewheel complexes.14,16 The crystal packing of 1c shows that two dimolybdenum cores are in head-to-

1c

1g

2.1150(8) 2.150(2) 2.143(2) 2.162(4) 2.127(3) 2.163(4) 85.9(1) 173.6(1) 88.9(1)

2.096(1) 2.230(5) 2.235(5) 2.137(5) 2.082(6) 2.152(5) 87.1(2) 172.0(2) 86.6(2)

tail arrangement: the nitrogen atom of the pyridine ring interacts with the axial site of the other Mo2 core and vice versa to form a dimer [1c]2 with Mo−N(py) distances of 2.848(5) and 3.011(5) Å.17 Complexes 1b and 1e have almost the same structure as that of 1c, and, hence, we provide the structures of 1b and 1e in Supporting Information.17 Addition of trimethylsilyl trifluoromethanesulfonate to a solution of 1a in toluene led to the formation of triflate complex 1g in 69% yield, accompanied by trimethylsilyl acetate (eq 2). The 1H NMR spectrum of 1g in toluene-d8 displayed

almost the same pattern as observed for 1a, except for no signal due to the acetate ligand, indicating that the acetate ligand of 1a was selectively replaced by a triflate ligand. At room temperature, in CD2Cl2 we observed two singlets at 8.75 (2H) and 8.33 (1H) ppm for (ArN)CH moiety cis and trans to the triflate ligand, respectively. When the measurement temperature was cooled, both signals gradually broadened. At −80 °C, the singlet signal, detected at 8.75 ppm at room temperature, was divided into two broad singlets (8.77 and 8.72 B


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Inorganic Chemistry

Table 2. Radical Additiona of CCl4 to 1-Hexene Catalyzed by Complexes 1a−g and 2

ppm) due to the magnetically inequivalent two (ArN)CH moiety, that is, syn and anti to CF3 group of the triflate ligand. Because the two signals were well-separated at −80 °C, we performed the simulation of the 1H NMR spectra, which provided the thermodynamic parameters (ΔG(303K)‡ = 51.48(1) kJ mol−1, ΔH‡ = 28.43(3) kJ mol−1 and ΔS‡ = −76.06(4) J K−1 mol−1) corresponding to the rotational process of the triflate ligand.17 Complex 1g had an absorption band at 447 nm corresponding to the δ→δ* transition of the quadruply bonded Mo2 core, suggesting that the Mo2 was kept intact. Figure 2 shows the structure of the triflate complex 1g, in which three N,N′-di(anisyl)formamidinato ligands and one

entry

catalyst

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

1a 1b 1c 1d 1e 1f 1g 1a 1a 1a 1a 1g 1g 1g 1g 2

additive

pyridine

pyridine

solvent

yield (%)b

CDCl3 CDCl3 CDCl3 CDCl3 CDCl3 CDCl3 CDCl3 toluene-d8 THF-d8 CD3CN toluene-d8 toluene-d8 THF-d8 CD3CN toluene-d8 CDCl3

86 68 59 63 36 56 87 >99 70 39 60 >99 56 40 61 28

All reactions were performed at 80 °C under Ar with [catalyst]/ [alkene]:[CCl4] = 0.024:0.80:1.04 (in mmol) in sealed J-Young NMR tubes. bThe yield was determined by in situ 1H NMR spectroscopy versus internal standard hexamethylbenzene. a

toluene-d8 (entries 1, 7, 8, and 12), while coordinating solvents lowered the product yield for both catalysts (entries 9, 10, 13, and 14). When pyridine (10 mol %) was added to the catalytic reactions using 1a and 1g, their product yields were decreased to 60% and 61%, respectively (entries 11 and 15), suggesting that coordination of the pyridine blocked the active site. The best catalytic activities of the Mo2 complexes 1a and 1g (turnover number (TON) = up to 33, turnover frequency (TOF) = up to 3.7 h−1) were comparable to those of the previously reported Mo(CO)6-catalyzed ATRA of CCl4 and 1octene (TON = up to 17, TOF = up to 2.3 h−1),12g while the best catalytic activities of Mo2(CO)6Cp2 and Mo2(CO)4(PPh3)2Cp2 were 5 times higher (TON = 328, TOF = up to 16.4 h−1) than our catalytic system, in which other minor isomers such as 1,1,1,5- and 1,1,1,7-tetrachlorononanes were formed from CCl4 and 1-octene;12c however, overall catalytic activity in this Mo2 system was lower than those of well-established late transition-metal catalysts (Cp*RuCl2(PPh3)/AIBN,11a TOF = up to 1856 h−1; [Cp*Ru(MeCN)(κ2-3-Ph2P-2-Me2N-indene)][BF4]/AIBN,11c TOF = up to 82.5 h−1; [Cu(tpma)Cl][Cl]/V-70 (tpma = tris(2pyridylmethyl)amine, V70 = 2,2′-azobis(4-methoxy-2,4-dimethyl valeronitrile),11g TOF = up to 604 h−1). Controlled Reactions Relevant to the Reaction Mechanism. With the best catalysts 1a and 1g in hand, we monitored the time course of the reaction of CCl4 (1.04 mM) with 1-hexene (0.80 mM) using 1a (3 mol %) and 1g (3 mol %) at 80 °C in toluene-d8 by 1H NMR spectra (Figure 3). The plot of the catalytic reaction of 1g clearly suggested spontaneous generation of the catalytically active species without any induction period, presumably due to the lability of the triflate ligand. In contrast, complex 1a gave sigmoidal curve, indicating an induction period due to the gradual dissociation of the acetate ligand prior to generating the

Figure 2. ORTEP diagram of [Mo2(DAniF)3(OSO2CF3)] (1g). Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level. C31 atom is disorderd and has 50% occupancy related to the other disorded atom C31B, which is omitted for clarity.

triflate ligand bridge two molybdenum atoms. The bond length of Mo1−Mo2 (2.096(7) Å) in 1g is almost the same as that (2.1089(8) Å) of 1c (Table 1). The bond lengths for Mo1−O1 (2.230(5) Å) and Mo2−O2 (2.235(5) Å) in 1g are much longer than those of the carboxylate moiety in 1c, consistent with the triflate ligand lability being readily faster than that of the carboxylate ligands. The bond lengths of Mo1−N1 (2.137(5) Å), Mo1−N3 (2.082(6) Å), and Mo1−N5 (2.152(5) Å) in 1g are comparable with those of the amidate ligands in complex 1c. Radical Addition of CCl4 to 1-Hexene Catalyzed by Dimolybdenum Complexes. We examined the catalytic performance of 1a−g and [Mo2(DAniF)4] (2) for radical addition of CCl4 to 1-hexene in CDCl3 at 80 °C for 9 h to give 1,1,1,3-tetrachloroheptane, and the results are summarized in Table 2. All of the mixed-ligated Mo2 complexes were found to be better catalysts for the radical addition compared with complex 2 (entries 17 vs entry 16). Both the acetate complex 1a and the triflate complex 1g afforded the radical addition product in 86% and 87% yields, respectively. Among the carboxylate dimolybdenum complexes we assessed, 1a was the best catalyst, as acetate was less hindered than the other carboxylate ligands. We then performed the catalytic reaction using 1a and 1g in different solvents, and both catalysts afforded the product in good to excellent yield in CDCl3 and C


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Inorganic Chemistry

processes of Mo2 complexes including the formation of complex 4 were crucial for the high catalytic activity. In sharp contrast to the reaction in toluene, reactions of 1a and 1g with excess CCl4 (45 equiv) at 80 °C in pyridine for 5 h a fforded a chloro-pyridin e coordinated complex [Mo2(DAniF)3Cl(py)] (3; eq 4). The 1H NMR spectrum of

Figure 3. Time dependence of the radical addition product yield for the addition of CCl4 to 1-hexene catalyzed by 1a, 1g, and 3. Conditions: 80 °C, 3 mol % of catalyst, toluene-d8.

catalytically active species. The TOFs of 1a and 1g for the initial 30 min were 2.3 and 19.3 h−1, clearly indicating that the lability of carboxylate and triflate ligands affected the initial reaction rate. The retarding effects of pyridine for both complexes was elucidated by a chloro-pyridine complex [Mo2(DAniF)3Cl(py)] (3), whose preparation is discussed in the following section (vide infra); complex 3 showed higher activity in the start-up period of the reaction, but the reaction was significantly suppressed, probably due to the deactivation by the coordination of pyridine. We then conducted reactions of 1a and 1g with excess CCl4 (45 equiv) in toluene at 60 °C for 3 h, and [Mo2(DAniF)3Cl2] (4) was isolated as a dark brown crystalline powder in 78% and 81% yield, respectively (eq 3). Complex 4 was a product of the

3 in toluene-d8 displayed four peaks at 9.53, 8.92, 7.53, and 6.87 ppm due to the pyridine ligand. Singlet methine protons of the amidinate ligands cis and trans to the chloride ligand were observed at 8.95 and 8.44 ppm, respectively. Similarly, two sets of methoxy peaks at 3.24 (3H) and 3.18 (3H) ppm, as well as 3.26 (6H) and 3.23 (6H) ppm, were assignable to the cis and trans amidinate ligands. We also detected trichloromethyl acetate in the ligand exchange reaction by CCl4 in the case of complex 1a.17 We alternatively prepared complex 3 by treating the acetate-ligated complex 1a with trimethylsilyl chloride in the presence of pyridine. The UV−vis spectrum of complex 3 confirmed that the quadruply bonded Mo2 core was intact because of the δ→δ* band (468 nm). Complex 3 was stable only in pyridine and noncoordinating solvents such as toluene, benzene, and dichloromethane, whereas it decomposed slowly in acetonitrile and tetrahydrofuran (THF). Under the reaction condition shown in Table 2, complex 3 afforded the product in rather low yield (40%) because of the retarding effects of pyridine. Figure 4 shows that complex 3 has one chloride atom bound to one molybdenum atom of the Mo2 core, while pyridine coordinates to the other molybdenum atom. The bond length of Mo1−Mo2 (2.113(2) Å) of 3 is slightly longer than that of other quadruply bonded dimolybdenum complexes, presumably due to one open coordination site with three chelating amidate ligands (Table 3).19

replacement of the OAc and OTf attached to the dimolybdenum core by a Cl atom from CCl4 and subsequent oxidation by CCl4. In fact, we observed that the X-band (9.812 GHz) electron paramagnetic resonance (EPR) spectrum of 4 in CH2Cl2 at room temperature displayed a signal at giso = 1.959 with Aiso = 22 G assignable to a singly oxidized [Mo2]5+ species. In addition, electrospray ionization mass spectrometry (ESIMS) of 4 gave its parent peak at m/z 1027.0572. The preparation of complex 4 was previously reported by Cotton et al. by oxidizing [Mo2(DAniF)4] with 1 equiv of [Cp2Fe]Cl in the presence of ClSiMe3.18 When the complex 4 was used as the catalyst for the radical addition reaction under the optimized condition in Table 2, the reaction product was obtained in 9% yield, probably due to the difficulty to regenerate [Mo2]4+ species without organic radicals. In contrast to the recently developed Cu(II)-catalyzed atom-transfer radical addition in the presence of azo compounds,11j,m,n reduction of the complex 4 by AIBN was difficult, and dimerization of the AIBN-derived organic radical mainly occurred before reducing 4, probably due to the difficulty to reduce complex 4 by the AIBN-derived tertiary radical. In fact, when AIBN was used as the radical initiator for the radical addition reaction under our reaction condition, we observed the reaction product in 21% yield, indicating that the redox

Figure 4. ORTEP diagram of [Mo2(DAniF)3Cl(py)] (3). Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level. D


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Inorganic Chemistry

362 nm, indicating that the ligand exchange reaction of the acetate ligand by the chloride from CCl4 proceeded without any detectable intermediates under the UV−vis measurement condition.17 On the other hand, the ligand exchange reaction of 1g into 3 was too rapid to be measured by NMR spectroscopy. Proposed Reaction Mechanism. On the basis of the isolation of the ligand exchanged complex 3 and the oxidized complex 4 together with kinetic studies of the ligand exchange process and UV−vis spectral measurement for 1a, we propose the plausible catalytic cycle for the radical addition reaction catalyzed by 1a, as shown in Scheme 1. The mixed-ligated Mo2

Table 3. Selected Bond Lengths (Å) and Angles (deg) of Complex 3 Mo1−Mo2 Mo1−N7 Mo2−Cl1 Mo1−N1 Mo1−N3

2.113(2) 2.265(9) 2.459(3) 2.194(9) 2.156(9)

N1−Mo1−N7 N3−Mo1−N7 N5−Mo1−N7

91.0(3) 169.7(3) 87.1(3)

We analyzed the conversion of complex 1a to 3 in the presence of excess CCl4 and pyridine by 1H NMR spectroscopy at five different temperatures to determine the pseudo-firstorder rate constant kobs for the ligand exchange process as well as thermodynamic parameters (Figure 5). Increasing the

Scheme 1. Proposed Catalytic Cycle for Radical Addition Reaction

Figure 5. Representative plot of ln[A]0/[A] vs t (top) and Eyring plot (bottom) for exchange reaction of acetate to chloride for complex 1a in pyridine-d5. In the top figure, y in the equations represent ln[A]0/ [A]. First-order rate constants were determined by monitoring 1H NMR spectroscopy: kobs (1 × 10−5 s−1) = 31.8 (353.9 K), 15.3 (347.8 K), 8.58 (342.4 K), 5.02 (337.5 K), and 2.89 (332.6 K).

complex 1a undergoes a substitution of the acetate ligand by the Cl from CCl4 to form trichloromethyl ester, which is generated in situ by Lewis acid-promoted nucleophilic substitution of alkyl halides, giving [Mo2(DAniF)3(μ-Cl)] (A).21 Subsequently, reaction of A with CCl4 produces a [Mo2]5+ complex, [Mo2(DAniF)3Cl2] (4), and ·CCl3 by the reductive cleavage of the C−Cl bond of CCl4. Next step is a radical addition of ·CCl3 to 1-hexene to form secondary carbon radical, which is in similar manner to the generally accepted ATRA mechanism.10,11b,h,i,l,q,12c Then, the secondary carbon radical reacts with the Mo−Cl moiety of 4 to produce the radical addition product and regenerate the catalytically active species A. The regioselectivity of the radical addition product is essentially the same to the previously reported ATRA of CCl4 to α-olefins. Although stabilization of the in situ generated carbon radical is also plausible by species A to form a new [Mo2]5+ complex, [Mo2(DAniF)3Cl(CCl3)], in which ·CCl3 is trapped by A to produce the organometallic species as observed

reaction temperature accelerated the ligand exchange reaction. After the reaction reached the full conversion of 1a, further heating caused no change in the 1H NMR spectrum, indicating that coordination of pyridine suppressed the oxidation of 3. The activation parameters (ΔG(303K)‡ = 110.4(4) kJ mol−1, ΔH‡ = 106.6(2) kJ mol−1, and ΔS‡ = −12.6(6) J K−1 mol−1) were estimated by the Eyring analysis (Figure 5, bottom). The negative value of ΔS‡ implies the ordered transition state for the substitution reaction of the acetate ligand by chloride from CCl4 in the reaction mixture.20 In addition, upon monitoring the reaction of 1a with CCl4 (45 equiv) in pyridine at 80 °C by UV−vis measurement, we observed a single isosbestic point at E


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Inorganic Chemistry for the organometallic-mediated radical polymerization,29 the final C−Cl bond-forming step proceeds through the abstraction of the chloride ligand in complex 4 by the organic radical. The short induction period for 1g compared with 1a−f is ascribed to the faster nucleophilic substitution reaction of Cl3C−Cl by triflate than that by carboxylates. The absence of an induction period for complex 3 further supports this catalytic cycle (Figure 3), though reactivity of 3 was suppressed by the coordination of pyridine.

THF (15 mL), was added 0.50 mL NaOMe solution (0.5 M in methanol). After it was stirred for 2 h, precipitated sodium acetate was removed by centrifugation. To the clear solution was added an excess of m-diphenylphosphino benzoic acid (HO2CC6H4PPh2-3; 91.0 mg, 0.297 mmol). When stirred at room temperature, the color of the mixture changed immediately from yellow to orange. The reaction was continuously stirred at room temperature for 3 h, and then all volatiles were removed under vacuum. The residue was washed with small portions of ethanol (2 × 5 mL) and then dried under vacuum. The resulting orange solid was dissolved in dichloromethane (2 mL), and then hexane (15 mL) was added, giving an orange precipitate. The supernatant was decanted, and the remaining solid was washed with hexane (2 × 15 mL). The orange precipitate was dried under vacuum. Yield: 231 mg (80%). mp 280−282 °C (decomp). 1H NMR (CD2Cl2, 400 MHz, 30 °C): δ 8.46 (s, 1H, NCHN), 8.42 (s, 2H, NCHN), 8.33 (d, 1H, 3J = 6.0 Hz, C6H4PPh2), 8.28 (d, 1H, 3J = 6.0 Hz, C6H4PPh2), 7.45 (t, 1H, 3J = 7.6 Hz, C6H4PPh2), 7.39−7.35 (m, 11H, C6H4PPh2), 6.64 (d, 8H 3J = 8.8 Hz, NAr), 6.54 (d, 8H, 3J = 8.8 Hz, NAr), 6.43 (d, 4H, 3J = 8.8 Hz, NAr), 6.21 (d, 4H, 3J = 8.8 Hz, NAr), 3.71 (s, 12H, OCH3), 3.64 (s, 6H, OCH3). 31P{1H} NMR (CD2Cl2, 400 MHz, 30 °C) δ −5.54 (s). UV/vis (THF): λmax (ε) = 291 (1.07 × 105), 453 nm sh (5.62 × 103 mol−1 dm3 cm−1). Electrochemical potentials: E1/2(Mo24+/5+) = +0.15 V versus Fc/Fc+ couple in dichloromethane. Anal. Calcd for C64H59N6PO8Mo2: C 60.86, H 4.71, N 6.65; Found: C 60.77, H 4.45, N 6.49%. Synthesis of [Mo2(DAniF)3(O2CPy-3)], [Mo2(DAniF)3(O2CPh)], and [Mo2(DAniF)3(O2CFu-3)]. Dimolybdenum complexes 1c, 1d, and 1e were prepared following a similar procedure as described for the synthesis of complex 1b. Complex 1c was obtained in 76% yield (298 mg) as red powder by using nicotinic acid (HO2CPy-3). mp 293−295 °C (decomp). 1H NMR (CD2Cl2, 400 MHz, 30 °C): δ 9.47 (dd, 1H, 4J = 0.8 and 1.6 Hz, Nico), 8.66 (dd, 1H, 3J = 3.2 Hz and 4J = 1.6 Hz, Nico), 8.54−8.51 (m, 1H, Nico), 8.49 (s, 1H, NCHN), 8.46 (s, 2H, NCHN), 7.42−7.39 (m, 1H, Nico), 6.67 (d, 8H, 3J = 8.8 Hz, NAr), 6.58 (d, 8H, 3J = 8.8 Hz, NAr), 6.45 (d, 4H, 3J = 8.8 Hz, NAr), 6.24 (d, 4H, 3J = 8.8 Hz, NAr), 3.71 (s, 12H, OCH3), 3.64 (s, 6H, OCH3). UV/vis (THF): λmax (ε) = 294 (1.00 × 105), 444 nm sh (6.31 × 103 mol−1 dm3 cm−1). Electrochemical potentials: E1/2(Mo24+/5+) = +0.13 V versus Fc/Fc+ couple in dichloromethane. Anal. Calcd for C51H49N7O8Mo2: C 56.72, H 4.57, N 9.08; found: C 56.65, H 4.22, N 9.34%. Complex 1d was obtained in 79% yield (133 mg) as yellow powder by using benzoic acid (BzH). mp > 300 °C. 1H NMR (CD2Cl2, 400 MHz, 30 °C): δ 8.48 (s, 1H, NCHN), 8.45 (s, 2H, NCHN), 8.32− 8.30 (m, 2H, Ph), 7.49−7.47 (m, 3H, Ph), 6.66 (d, 8H, 3J = 8.8 Hz, NAr), 6.58 (d, 8H, 3J = 8.8 Hz, NAr), 6.45 (d, 4H, 3J = 8.8 Hz, NAr), 6.24 (d, 4H, 3J = 8.8 Hz, NAr), 3.71 (s, 12H, OCH3), 3.64 (s, 6H, OCH3). UV/vis (THF): λmax (ε) = 295 (1.02 × 105), 418 nm sh (6.46 × 103, mol−1 dm3 cm−1). Electrochemical potentials: E1/2(Mo24+/5+) = −0.17 V versus Fc/Fc+ couple in dichloromethane. Anal. Calcd for C52H50N6O8Mo2: C 57.89, H 4.67, N 7.79; found: C 57.86, H 4.26, N 7.92%. Complex 1e was obtained in 81% yield (321 mg) as yellow powder by using 3-furanoic acid (HO2CFu-3). mp 296−298 °C (decomp). 1H NMR (CD2Cl2, 400 MHz 30 °C): δ 8.47 (s, 1H, NCHN), 8.46 (s, 2H, NCHN), 8.13 (s, 1H, 3-Fu), 7.47 (br, 1H, 3-Fu), 6.96 (br, 1H, 3-Fu), 6.66 (d, 8H, 3J = 8.8 Hz, NAr), 6.56 (d, 8H, 3J = 8.8 Hz, NAr), 6.44 (d, 4H, 3J = 8.8 Hz, NAr), 6.23 (d, 4H, 3J = 8.4 Hz, NAr), 3.71 (s, 12H, OCH3), 3.64 (s, 6H, OCH3). UV/vis (THF): λmax (ε) = 295 nm (1.05 × 105 mol−1 dm3 cm−1), 448 nm sh (5.86 × 103, mol−1 dm3 cm−1). Electrochemical potentials: E1/2(Mo24+/5+) = −0.14 V versus Fc/Fc+ couple in dichloromethane. Anal. Calcd for C50H48N6O9Mo2: C 56.18, H 4.53, N 7.86; found: C 55.85, H 4.25, N 7.81%. Synthesis of [Mo2(DAniF)3(OSO2CF3)] (1g). In a dry argon-filled Schlenk tube, a yellow colored complex [Mo2(DAniF)3(O2CCH3)] (1a; 0.160 g, 0.156 mmol) was dissolved in toluene (15 mL), and TMSOTf (29.3 μL, 0.162 mmol) was added dropwise at room temperature. The reaction mixture was stirred for 12 h, and then all volatiles were removed under vacuum. The resulting dark yellow colored solid was washed with ethanol (3 × 10 mL) and hexane (10

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CONCLUSION Among the mixed-ligated quadruply bonded dimolybdenum complexes [Mo2(DAniF)3(L)] (1a−g), the triflate-bridged Mo2 complex 1g showed superior activity over carboxylate-bridged Mo2 complex (1a−1f) in the initial reaction period (TOF for initial 0.5 h, 19.3 h−1 for 1g versus 2.3 h−1 for 1a), because the labile triflate ligand promoted facile generation of catalytically active species. The key step was initial displacement of the carboxylate/sulfonate ligands by CCl4 to form a nascent chlorinated Mo2 complex, [Mo2(DAniF)3(μ-Cl)] (A), which has an open coordination site able to homolytically abstract a Cl atom from CCl4 to generate [Mo2(DAniF)3Cl2] (4) and a radical species, ·CCl3. The formation of A was trapped by pyridine to give the chloro-pyridine complex [Mo2(DAniF)3Cl(py)] (3). The use of such efficient catalysts for other organic transformations is currently under investigation in our laboratory.

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EXPERIMENTAL SECTION

General Procedure. All manipulations involving air- and moisture-sensitive organometallic compounds were performed under argon atmosphere using standard Schlenk or glovebox techniques. Anhydrous solvents were purchased from Kanto Chemical and further purified by passage through activated alumina under positive argon pressure as described by Grubbs et al.22 Deuterated solvents (CDCl3, toluene-d8, THF-d8, CD2Cl2) were distilled over CaH2, degassed, and stored under Ar-filled glovebox. Dimolybdenum complexes, [Mo2(OAc)4],23 [Mo2(DAniF)3(OAc)]14 (1a), [Mo2(DAniF)3(4py)]17 (1f), and [Mo2(DAniF)4]1c (2), as well as organic ligands mdiphenylphosphinobenzoic acid (m-PPh2BzH)15 and N,N′-dianisylformamidine24 (DAniFH) were synthesized according to the literature procedures. Other organic substrates were purchased, dried, and deoxygenated by distillation over CaH2. 1H NMR (300 and 400 MHz) spectra were measured on Varian UNITY INOVA-300 and Bruker AVANCEII-400 spectrometers. 1H NMR spectra were referenced to the residual internal solvent, and 31P{1H} NMR spectra was referenced to external reference of 85% H3PO4 at (δ = 0.00). The EPR spectrum was recorded on a Bruker EMX-10/12 spectrometer. UV−vis spectra were measured using Agilent 8453 UV/vis spectroscopy system. Mass spectrometric data were obtained using high-resolution electron impact and ESI techniques on a JEOL SX−102 spectrometer and BRUKER micrOTOF spectrometer, respectively. The cyclic voltammograms (CVs) were recorded using an ALS/CH Instruments electrochemical analyzer model 610D in 0.1 M [nBu4N][PF6] solution in dichloromethane with a glassy carbon working electrode, a platinum wire auxiliary electrode, a Ag wire as reference electrode, and a scan rate of 100 mV/s (CV). All the potential values are referenced to the ferrocene/ferrocenium couple under the same experimental conditions. The elemental analyses were recorded by using a Perkin− Elmer 2400 at the Faculty of Engineering Science, Osaka University. All melting points were measured in sealed tubes under an argon atmosphere. Gas chromatography-MS measurement was performed using a DB-1 capillary column (0.25 mm × 30 m) on a Shimadzu GCMS-QP2010Plus. Synthesis of [Mo2(DAniF)3(O2CC6H4PPh2-3)]. To a yellow solution of [Mo2(DAniF)3(O2CCH3)] (1a; 228 mg, 0.224 mmol) in F


Article

Inorganic Chemistry mL) to remove trimethylsilyl acetate. The yellow colored powders of complex 1g (0.120 g, 69% yield) were obtained after drying. mp 116− 118 °C (decomp). 1H NMR ([D8]Tol, 400 MHz, 30 °C): δ 8.78 (s, 2H, NCHN), 8.26 (s, 1H, NCHN), 6.74 (d, 8H, 3J = 8.8 Hz, NAr), 6.53 (d, 8H, 3J = 8.8 Hz, NAr), 6.49 (d, 4H, 3J = 8.8 Hz, NAr), 6.37 (d, 4H, 3J = 8.8 Hz, NAr), 3.20 (s, 12H, OCH3), 3.11 (s, 6H, OCH3). UV/vis (THF): λmax (ε) = 208 (9.53 × 104), 295 (4.66 × 104), 447 nm sh (1.22 × 103 mol−1 dm3 cm−1). Electrochemical potentials: E1/2(Mo24+/5+) = −0.11 V versus Fc/Fc+ couple in dichloromethane. Anal. Calcd for C46H45N6O9F3Mo2.OC4H8: C 50.94, H 4.53, N 7.13; found: C 50.71, H 4.36, N 7.01%. Synthesis of of [Mo2(DAniF)3Cl(Py)] (3). To a yellow solution of [Mo2(DAniF)3(O2CCH3)] (1a; 0.150 g, 0.148 mmol) in pyridine (8 mL), trimethylsilyl chloride (40.0 μL, 0.315 mmol) was added dropwise with constant stirring, which resulted in the immediate color change of the solution from yellow to wine red. The reaction mixture was stirred for 3 h in room temperature, and then all volatiles were removed under vacuum. Red solid was washed with ethanol (3 × 10 mL) and hexane (10 mL). The red solid product was obtained after drying under vacuum. Yield: 0.118 g (75%). mp 178−180 °C (decomp). 1H NMR ([D8]Tol, 400 MHz, 30 °C): δ 9.53 (d, 1H, 3J = 5.6 Hz, p-Py), 8.95 (s, 2H, NCHN), 8.92 (d, 2H, 3J = 5.6 Hz, o-Py), 8.44 (s, 1H, NCHN), 7.35 (d, 4H, 3J = 8.8 Hz, NAr), 6.74 (t, 2H, 3 J(H,H) = 5.6 Hz, m-Py), 6.60−6.50 (m, 12H, NAr), 6.35 (d, 2H, 3J = 8.8 Hz, NAr), 6.25 (d, 4H, 3J = 8.8 Hz, NAr), 6.06 (d, 2H, 3J = 8.8 Hz, NAr), 3.26 (s, 6H, OCH3), 3.24 (s, 3H, OCH3), 3.23 (s, 6H, OCH3), 3.18 (s, 3H, OCH3). UV/vis (THF): λmax (ε) = 208 (1.27 × 105), 253 (5.72 × 104), 295 (6.80 × 104), 468 nm sh (3.01 × 103 mol−1 dm3 cm−1). Electrochemical potentials: E1/2(Mo24+/5+) = −0.04 V versus F c / F c + co u p l e i n d i c h l o r o m e t h a n e . A n a l . Ca l c d f o r C50H50N7O6ClMo2: C 56.00, H 4.70, N 9.14; found: C 55.24, H 4.38, N 9.19%. The low carbon value is probably due to the contamination of the decomposed species of 3 during the preparation. The 1H NMR spectrum was included in the Supporting Information. General Procedure for Radical Addition of CCl4 to 1-Hexane. In a light-shielded J-Young NMR tube 3 mol % of catalyst (0.024 mmol), 1-hexene (100 μL, 0.80 mmol), CCl4 (100 μL, 1.04 mmol), and hexamethylbenzene as an internal standard (6.6 mg, 0.04 mmol) were added. The reaction mixture was brought to a total volume of 1 mL by the addition of the reaction solvent and heated at 80 °C in an oil bath. The formation of product was monitored by 1H NMR spectroscopy at predetermined intervals. Time Profiles for Radical Addition Reaction. In a J-Young NMR tube 3 mol % of catalyst (0.024 mmol), 1-hexene (100 μL, 0.80 mmol), CCl4 (100 μL, 1.04 mmol), and hexamethylbenzene as an internal standard (6.6 mg, 0.04 mmol) were added. The reaction mixture was brought to a total volume of 1 mL by the addition of toluene-d8. 1H NMR spectra were measured at 80 °C at 10 min time intervals for a period of initial 3 h. The temperatures of the samples in NMR probe were calibrated with ethylene glycol.25 NMR Kinetic Measurements for Ligand Substitution Reaction. In J-Young NMR tubes complexes 1a−1f (0.005 mmol, 0.022 mol % to CCl4), CCl4 (21.9 μL, 0.227 mmol), and hexamethylbenzene as an internal standard were added. The reaction mixture was brought to a total volume of 0.4 mL by the addition of the reaction solvent pyridine-d5. 1H NMR spectra were recorded at different desired temperatures at 5 min time intervals for a period of initial 130 min. The temperatures of the samples in NMR probe were calibrated with ethylene glycol.25 UV−Vis Measurements. Samples (5 mL in pyridine) were prepared in glovebox using a corresponding Mo2 complex (0.1 mM) and CCl4 (4.5 mM), and the mixture was transferred to a quartz cuvette of 0.2 cm thickness with stopcock. The UV−vis spectra of the reaction mixtures were recorded at 80 °C and compared with the standards of complexes of the same Mo2 concentration. X-ray Data Collection and Refinement. Single crystals of 1b, 1c, 1e, 1g, and 3 were grown from THF solution by layering with hexane. A suitable crystal was mounted on a CryoLoop (Hampton Research Corp.) with a layer of mineral oil and placed in a nitrogen stream at 113(2) K. Measurements were performed on a Rigaku Mercury

detector equipped with a Rotating Anode X-ray generator (40 kV, 100 mA). Data for all complexes were collected with graphite monochromated Mo Kα (0.71075 Å) radiation. The frames were indexed, integrated, and scaled, and the data were corrected for Lorentz and polarization effects. An absorption correction was applied. Structures were solved by direct method using SIR97.26 All structures were refined on F2 by full-matrix least-squares methods, using SHELXL-97.27 Measured nonequivalent reflections with I > 2.0σ(I) were used for the structure determination. In complex 1g, C31 atom was found disordered, and it is partially occupied two positions with the ratio of 0.5:0.5. Hydrogen atoms of the ligands were included in the final stages of the refinement and were refined with a typical riding model. ORTEP-III was used to produce the diagrams.28 Pertinent crystallographic data are summarized in Table S2. CCDC 1477111 (1b), 1477717 (1c), 1477118 (1e), 1477116 (1g), and 1477119 (3) contain the supplementary crystallographic data for this paper. Additional crystallographic information is available in the Supporting Information.

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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.inorgchem.6b02525. Crystallographic data for complexes 1b, 1c, 1e, 1g, and 3, cyclic voltammograms of 1a−g and 3, catalyst and solvent screenings of radical addition, kinetic study for the ligand exchange reaction, and variable-temperature NMR spectra of complex 1g (PDF) X-ray crystallographic information (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +81-6-68506245. (K.M.) *E-mail: [email protected]. Phone: +81-6-68506247. (H.T.) Notes

The authors declare no competing financial interest. Crystallographic data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac. uk/data_request/cif.

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ACKNOWLEDGMENTS H.T. acknowledges a financial support by a Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element-Blocks (Grant No. 2401)” (JSPS KAKENHI Grant No. JP 15H00743).

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REFERENCES

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