Lewis Acid Chemistry of a Cationic [CCH] Subunit in a

Feb 22, 2013 - Abstract. Abstract Image. Reaction of [(η5-C5H4Me)4Fe4(HCCH)(HCCBr)](PF6) ([2](PF6)) with AgPF6 in acetonitrile gave ...
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Lewis Acid Chemistry of a Cationic [CCH] Subunit in a Bisdisphenoidal Eight-Atom Tetrairon−Tetracarbon Cluster Masaaki Okazaki,*,† Wataru Taniwaki,† Kazuki Miyagi,‡ Masato Takano,‡ Satoshi Kaneko,† and Fumiyuki Ozawa*,‡ †

Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki University, Bunkyo-cho, Hirosaki 036-8561, Japan ‡ International Research Center of Elements Science, Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan. S Supporting Information *

ABSTRACT: Reaction of [(η 5 -C 5 H 4 Me) 4 Fe 4 (HCCH)(HCCBr)](PF6) ([2](PF6)) with AgPF6 in acetonitrile gave [(η 5 -C 5 H 4 Me) 4 Fe 4 (HCCH)(HCCNCMe)](PF 6 ) 2 ([3](PF6)2). The X-ray diffraction study revealed that the cationic [CCH] subunit is stabilized through coordination of the acetonitrile molecule to the cationic carbon atom. As a synthon for a donor-free [(η5-C5H4Me)4Fe4(HCCH)(HCC)]2+, [(η5C5H4Me)4Fe4(HCCH)(HCCL)](PF6)2 ([6](PF6)2, L = pyrazine) was synthesized by the reaction of [2](PF6) with AgPF6 in the presence of pyrazine. Treatment of [6](PF6)2 with tertiary amines in acetonitrile led to deprotonation of acetonitrile to form [(η 5 -C 5 H 4 Me) 4 Fe 4 (HCCH)(HCCCH2CN)](PF6) ([11](PF6)). Treatment of [6](PF6)2 with maleimide in the presence of tertiary amine in acetonitrile allowed the functionalization of the coordinated acetonitrile through the nucleophilic attack of maleimide at the nitrile carbon atom.

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INTRODUCTION The ethynyl cation, formulated as [HCC]+, is 230 and 544 kJ/mol less stable than the methyl and tert-butyl cations, respectively.1 The extreme instability of the ethynyl cation can be accounted for by considering the positive charge located at the more electronegative sp orbital. Thus, alkynyl cations cannot be generated in situ, even with efficient leaving groups such as diazonium (N2+).2 Several groups have reported the observation of alkynyl cations using mass spectroscopy (MS).3 In 1988, Hanack and Vermehren et al. succeeded in the transient generation of an alkynyl cation via nuclear decay of a tritium atom in a tritiated organic molecule.4 Tritium is converted to the 3He isotope, which does not form a stable bond with the carbon atom. The dissociation of helium generates the alkynyl cation, which is quickly trapped by benzene to give a phenyl-substituted alkyne. Our recent effort has focused on stabilization of the cationic ethynyl moiety formulated as [CCH]+ on polymetallic cores. Electron-deficient neutral or cationic species can be stabilized on transition metals through π-back-donation from the metals to the electron-deficient species.5 Dolby and Robinson reported Friedel−Crafts-type reactions between [Co3(CO)9(μ3-CCl)] and arenes (ArH) in the presence of 1 equiv of aluminum chloride to give [Co3(CO)9(μ3-CAr)].6 The reaction can be rationalized by assuming the μ3 carbocation, or at least a species with a great deal of positive charge, at the apical carbon atom.7a © 2013 American Chemical Society

The reactivity of the CO adduct, formulated as [Co3(CO)9(μ3C·CO)], toward nucleophiles was reported by Seyferth.7b,c The unprecedented pyramidal carbocation would be stabilized by πback-donation from the tricobalt core. We have previously reported the formation and structure of the acetylenecoordinated tetrairon cluster [(η5-C5H4Me)4Fe4(HCCH)2] (1) by the reaction of [(η5-C5H4Me)4Fe4(CO)4] with LiAlH4.8 Further treatment of the one-electron-oxidized form [1](PF6) with 1 equiv of N-bromosuccinimide (NBS) led to the selective bromination of an acetylenic proton to give [(η5C5H4Me)4Fe4(HCCH)(HCCBr)](PF6) ([2](PF6)).9 This article reports on the abstraction of the bromo group from bromoacetylene by AgPF6. Coordination of acetonitrile or pyrazine to the resulting cationic [CCH] subunit was confirmed by an X-ray diffraction study. The cationic carbon atom activates an acetonitrile molecule under significantly mild conditions to allow deprotonation and functionalization of the acetonitrile molecule, which indicates the high Lewis acidity of the cationic [CCH] subunit.

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RESULTS AND DISCUSSION Reaction of [2](PF6) with AgPF6 in acetonitrile gave [(η5C5H4Me)4Fe4(HCCH)(HCCNCMe)](PF6)2 ([3](PF6)2) in Received: January 17, 2013 Published: February 22, 2013 1951

dx.doi.org/10.1021/om400035q | Organometallics 2013, 32, 1951−1957

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77% yield (Scheme 1). The 1H NMR spectrum of [3](PF6)2 in acetonitrile-d3 showed no signal of coordinated acetonitrile, due

orbital. The bonding character around the acetonitrilium moiety indicates sp hybridization of the CN part: the C1−N bond distance is 1.414(13) Å, which indicates the coordination of acetonitrile to the [CCH] subunit. The C5−N bond distance (1.112(13) Å) is close to that of the typical carbon−nitrogen triple bond in nitriles (1.157(5) Å).10 The C1−N−C5 and N− C5−C6 bond angles are 178.5(10) and 178.9(12)°, respectively. Reaction of [2](PF6) with AgPF6 in dichloromethane resulted in precipitation of a brown solid (Scheme 2). The 1H NMR

Scheme 1

Scheme 2 to the exchange with the acetonitrile-d3 to form [(η5C5H4Me)4Fe4(HCCH)(HCCNCCD3)](PF6)2. Instead, a signal for the free acetonitrile was observed at δ 1.98. Moreover, the 2H NMR spectrum exhibits a signal of the coordinated acetonitrile-d3 at δ −4.8. The unusually high-field shift of the signal is consistent with the structure of [3](PF6)2, in which acetonitrile is bound to the cationic [CCH] subunit surrounded by the paramagnetic tetrairon core. The structure of [3]2+ is depicted in Figure 1. The cluster consists of a puckered rhombus of four iron atoms: the bond

spectrum of the residue in acetonitrile-d3 indicates the formation of [3](PF6)2 and a new compound in a molar ratio of 1:1.2. The species was also formed in the reaction of [1](PF6) with N-fluorobenzenesulfonimide (NFSI) and tentatively characterized as the fluoroacetylene-coordinated cluster [(η 5 -C 5 H 4 Me) 4 Fe 4 (HCCH)(HCCF)](PF 6 ) ([4](PF6)). The 1H NMR and ESI-mass spectroscopic data support the structure of [4](PF6): the 1H NMR spectrum of [4](PF6) shows two singlet signals at δ −74.0 (2H, HCCH) and −54.7 (1H, HCCF). The signal of HCCF was shifted to the high-field region in comparison with that of HCCBr in [2](PF6) (δ −61.0),9 explainable as due to the diamagnetic deshielding effect of fluorine. The number of the signals assigned to the η5C5H4Me ligands is consistent with the structure. The mass spectrum shows a molecular ion peak (M+) at m/z 610, which is consistent with the formula of [4]+. The formation of the HCCF moiety indicates the electron deficiency of the [CCH] subunit. Coordination of acetonitrile to the cationic [CCH] subunit in [3](PF6)2 is facile, as indicated by NMR analysis (vide supra), and is not effective in compensating for the electron deficiency. The cluster [3](PF6)2 is extremely air-sensitive and quickly decomposed to form [(η5-C5H4Me)4Fe4(HCCH)(μ3CO)(μ3-CH)]+ ([5]+) as a main product.11 To expand this chemistry, we have synthesized an easily accessible synthon for a donor-free [(η5-C5H4Me)4Fe4(HCCH)(HCC)]2+ that can be handled by using standard Schlenk techniques. AgPF6 was added to an acetonitrile solution of [2](PF6) in the presence of pyrazine. After filtration of insoluble materials, concentration under vacuum and recrystallization of the residue from acetonitrile/diethyl ether gave dark green crystals of [(η5C5H4Me)4Fe4(HCCH)(HCCNC4H4N)](PF6)2 ([6](PF6)2) in 79% yield (Scheme 3). The cluster [6](PF6)2 was adequately characterized by elemental, spectroscopic, and X-ray diffraction analyses (Figure 2). The pyrazine moiety of [6](PF6)2 was quickly replaced with pyridazine and pyridine to give the corresponding products [(η5-C5H4Me)4Fe4(HCCH)(HCCL)](PF6)2 (L = pyridazine ([7](PF6)2), pyridine ([8](PF6)2)), which confirms the high lability of the pyrazine moiety. The reaction tendency is consistent with the pKa values of protonated pyrazine (0.6), pyridazine (2.33), and pyridine (5.17).10 Reactions of [6](PF6)2 with water and methanol in acetonitrile gave [5](PF6)

Figure 1. ORTEP drawing of [3]2+. Thermal ellipsoids are drawn at the 30% probability level. The methyl groups on the cyclopentadienyl ligands and hydrogen atoms, except for acetylenic hydrogens, are omitted for clarity. Selected bond lengths (Å) and angles (deg) of [3]2+: Fe1−Fe3, 2.4878(19); Fe1−Fe4, 2.4689(18); Fe2−Fe3, 2.4584(19); Fe2−Fe4, 2.474(2); Fe1−C1, 1.921(9); Fe1−C3, 1.979(10); Fe1−C4, 1.987(10); Fe2−C2, 1.942(10); Fe2−C3, 2.003(10); Fe2−C4, 1.984(10); Fe3−C1, 1.971(9); Fe3−C2, 1.984(9); Fe3−C3, 1.929(10); Fe4−C1, 1.975(9); Fe4−C2, 1.973(10); Fe4−C4, 1.947(9); C1−C2, 1.484(14); C3−C4, 1.501(14); C1−N, 1.414(13); C5−N, 1.112(13); C5−C6, 1.443(15); C1−N−C5, 178.5(10); N−C5−C6, 178.9(12).

distances Fe1−Fe3 (2.4878(19) Å), Fe1−Fe4 (2.4689(18) Å), Fe2−Fe3 (2.4584(19) Å), and Fe2−Fe4 (2.474(2) Å) are in the range typically expected for iron−iron single bonds. The interatomic distances of Fe1−Fe2 (3.271 Å) and Fe3−Fe4 (3.263 Å) indicate that there is no iron−iron bond between them. The bond distances of C1−C2 (1.484(14) Å) and C3− C4 (1.501(14) Å) are close to that of a typical carbon−carbon single bond in a hydrocarbon (1.541(3) Å),10 which is attributable to electron donation from the carbon−carbon πbonding orbital to the tetrairon core and back-donation from the tetrairon core to the empty carbon−carbon π*-antibonding 1952

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of [6](PF6)2 with NaBPh4 in acetonitrile gave a mixture of [(η5C 5H4 Me)4 Fe 4(HCCH)(HCCPh)](PF6 ) ([10](PF6 )) and [5](PF6) in a molar ratio of 6:1 (Scheme 4). The minor formation of [5](PF6) can be explained by the reaction of [6](PF6)2 with a trace amount of water. The cluster [10](PF6) was independently synthesized by the reaction of [2](PF6) with PhMgBr and isolated in 29% yield. Despite the high reactivity toward nucleophiles, [6](PF6)2 can be handled using standard Schlenk techniques. These experimental results show that [6](PF6)2 is a promising candidate as a synthon for the donorfree [(η5-C5H4Me)4Fe4(HCCH)(HCC)]2+. Carbon−hydrogen bond activation of acetonitrile has been achieved at the cationic [CCH] subunit under mild basic conditions (Scheme 5). An acetonitrile solution of [6](PF6)2

Scheme 3

Scheme 5

Figure 2. ORTEP drawing of [6]2+. Thermal ellipsoids are drawn at the 30% probability level. The methyl groups on the cyclopentadienyl ligands and hydrogen atoms, except for acetylenic hydrogens, are omitted for clarity. Selected bond lengths (Å) of [6]2+: Fe1−Fe3, 2.5249(7); Fe1−Fe4, 2.4723(7); Fe2−Fe3, 2.4649(7); Fe2−Fe4, 2.5017(7); Fe1−C1, 1.972(4); Fe1−C3, 2.009(4); Fe1−C4, 1.998(4); Fe2−C2, 1.916(4); Fe2−C3, 1.976(4); Fe2−C4, 1.992(3); Fe3−C1, 1.988(4); Fe3−C2, 2.018(3); Fe3−C4, 1.921(4); Fe4−C1, 2.003(4); Fe4−C2, 2.007(3); Fe4−C3, 1.942(3); C1−C2, 1.481(5); C3−C4, 1.490(5); N1−C1, 1.510(5).

and NiPr2Et was stirred at room temperature for 30 min. After removal of volatiles under reduced pressure, recrystallization of the residue from acetonitrile/diethyl ether gave dark brown crystals of [(η5-C5H4Me)4Fe4(HCCH)(HCCCH2CN)](PF6) ([11](PF6)) in 95% yield. The less basic NEt3 was also effective in affording [11](PF6) in 88% yield. Treatment of [11](PF6) with [Cp2Co] underwent one-electron reduction to give the diamagnetic species [(η5-C5H4Me)4Fe4(HCCH)(HCCCH2CN)] (11) in 52% yield. The NMR signals of the CH2CN group in 11 were observed at δ(1H) 3.04 and δ(13C) 45.2 (CH2) and 118.1 (CN), the assignment of which was confirmed by two-dimensional NMR techniques (COSY, HSQC, and HMBC). The structure of [11]+ is illustrated in Figure 3. The cyanomethyl group is bonded to one of the acetylenic carbons. The C1−C5 distance is 1.540(4) Å, which is typical for carbon−carbon single bonds (1.541(3) Å).10 The C5−C6 distance is 1.456(5) Å, which is also typical for carbon−carbon single bonds next to a nitrile group (1.466(5) Å).10 The C6−N bond distance (1.151(5) Å) and the C5−C6−N bond angle (178.0(5)°) are consistent with the existence of an sphybridized cyano group. A plausible mechanism for the formation of [11]+ involves the initial replacement of pyrazine with acetonitrile to form [3]2+ (Scheme 6). Coordination of acetonitrile enhances the acidity of the methyl group, which is deprotonated with tertiary amines. The subsequent 1,3-shift of the cyanomethyl group affords [11]+. The introduction of a functional group to acetonitrile was achieved at the cationic [CCH] subunit by reaction with a nucleophile. Treatment of [6](PF6)2 with maleimide in the presence of NiPr2Et in acetonitrile resulted in the nucleophilic attack of maleimide at the nitrile carbon atom to give [12](PF6) in 87% yield (Scheme 7). The structure of [12]+ is illustrated in Figure 4. The N1−C5 bond distance (1.270(4) Å) is close to that of typical carbon−

and [(η 5-C 5H4Me) 4Fe 4(HCCH)(HCCOMe)](PF 6) ([9](PF6)), respectively, in good yields (Scheme 4). The reaction Scheme 4

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Figure 4. ORTEP drawing of [12]+. Thermal ellipsoids are drawn at the 30% probability level. The methyl groups on the cyclopentadienyl ligands and hydrogen atoms, except for acetylenic hydrogens, are omitted for clarity. Selected bond lengths (Å) and angles (deg) of [12] +: Fe1−Fe3, 2.4735(5); Fe1−Fe4, 2.4772(5); Fe2−Fe3, 2.4935(5); Fe2−Fe4, 2.4823(5); Fe1−C1, 1.981(2); Fe1−C3, 2.002(2); Fe1−C4, 1.996(2); Fe2−C2, 1.927(2); Fe2−C3, 1.994(2); Fe2−C4, 1.986(2); Fe3−C1, 1.997(2); Fe3−C2, 1.999(3); Fe3−C3, 1.945(3); Fe4−C1, 2.001(2); Fe4−C2, 1.997(2); Fe4−C4, 1.949(3); C1−C2, 1.515(3); C3−C4, 1.474(4); N1−C1, 1.414(3); N1−C5, 1.270(4); N2−C5, 1.446(4); C5−C6, 1.480(4); C1−N1−C5, 130.7(2); N1−C5−N2, 114.1(2); N1−C5−C6, 133.2(3); N2−C5− C6, 112.6(2).

Figure 3. ORTEP drawing of [11]+. Thermal ellipsoids are drawn at the 30% probability level. The methyl groups on the cyclopentadienyl ligands and hydrogen atoms, except for acetylenic hydrogens, are omitted for clarity. Selected bond lengths (Å) and angles (deg) of [11] + : Fe1−Fe3, 2.4910(6); Fe1−Fe4, 2.4608(6); Fe2−Fe3, 2.4759(7); Fe2−Fe4, 2.4909(6); Fe1−C1, 1.954(3); Fe1−C3, 1.986(3); Fe1−C4, 1.989(3); Fe2−C2, 1.952(3); Fe2−C3, 1.977(4); Fe2−C4, 1.976(3); Fe3−C1, 1.997(3); Fe3−C2, 1.997(3); Fe3−C3, 1.935(3); Fe4−C1, 2.026(3); Fe4−C2, 1.992(3); Fe4−C4, 1.943(4); C1−C2, 1.487(4); C1−C5, 1.540(4); C3−C4, 1.491(4); C5−C6, 1.456(5); C6−N, 1.151(5); C1−C5−C6, 115.6(3); C5−C6−N, 178.0(5).

Scheme 6 Reactivity studies of [6]2+ related to Fischer−Tropsch synthesis are now in progress.

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CONCLUSION In conclusion, we have succeeded in the generation of a cationic [CCH] subunit, which is stabilized by bridging coordination to a tetrairon core. The resulting cationic species activates acetonitrile, which allows deprotonation and nucleophilic attack reactions. The α-cyano carbanion is important and is widely employed in organic synthesis;15 however, its poor acidity16 has meant that deprotonation of alkanenitrile under mild conditions is still limited. Direct addition of acetonitrile to aldehydes or imines catalyzed by a cationic ruthenium complex has been reported by Shibasaki and co-workers.17 They proposed a plausible catalytic cycle involving the initial coordination of acetonitrile to a cationic ruthenium complex. The coordinated acetonitrile is deprotonated by 1,8-diazabicycloundec-7-ene (DBU). DBU was essential for this reaction, because less basic amines such as pyridine, NEt3, and NiPr2Et were not operative. Fan and Ozerove also reported a related catalytic reaction using a nickel complex and DBU.18 Deprotonation of acetonitrile by tertiary amines such as NEt3 and NiPr2Et under mild conditions (room temperature, 30 min) was unprecedented, which indicates the high Lewis acidity of the cationic [CCH] subunit on the tetrairon core.

Scheme 7

nitrogen double bonds (1.32 Å). The C1−N1−C5 bond angle is 130.7(2)°. The N1−C1 (1.414(3) Å), C5−C6 (1.480(4) Å), and N2−C5 bond distances (1.446(4) Å) indicate the singlebond character of each bond.10 These structural features are in good agreement with formation of the imino group derived from the nucleophilic attack of maleimide at the nitrile carbon atom. It should be noted that the tetrairon cluster [3]2+ or [6]2+ can be regarded as one of the key intermediates in Fischer− Tropsch synthesis.12 Liu and Hu carried out a theoretical study showing that the barrier for the C/CH coupling leading to the [CCH] moiety is only 0.43 eV on the stepped surface13 which is generally modeled by butterfly type tetrametallic clusters.14

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EXPERIMENTAL SECTION

General Procedures. All reactions were performed under a dry nitrogen or argon atmosphere using standard Schlenk techniques. Acetonitrile and dichloromethane were distilled from CaH2 and stored over activated molecular sieves (MS4A). Diethyl ether, tetrahydrofuran, and hexane were distilled from sodium benzophenone ketyl prior to use. [(η5-C5H4Me)4Fe4(HCCH)(HCCBr)](PF6) ([2](PF6)) was prepared according to the literature method.9 Pyrazine, NiPr2Et, and NEt3 were distilled from CaH2. Other chemicals were purchased and 1954

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Article

C5H4Me), 2.3 (s, 6H, C5H4Me), 4.8, 5.3 (s, 2H × 2, NC4H4N), 6.0, 6.4, 9.3, 9.4, 9.6, 10.6, 12.8, 13.4 (s, 2H × 8, C5H4Me). 13C{1H} NMR (CD3CN): δ 2.2, 16.7, 23.9 (C5H4Me), 68.0, 81.7, 95.0, 101.3, 102.0, 105.9, 110.8, 114.2, 120.5, 125.4, 157.9, (C5H4Me), 93.4, 160.9 (HCCNC4H4N). IR (KBr pellet, cm−1): 3121 (w), 3100 (w), 2961 (w), 2924 (w), 2853 (w), 1725 (m), 1482 (m), 1452 (m), 1420 (m), 1372 (m), 1170 (w), 1123 (w), 1094 (w), 1024 (m), 827 (vs), 740 (s), 554 (s). Reaction of [6](PF6)2 with Pyridazine. An NMR tube was charged with [6](PF6)2 (5 mg, 5 μmol). Acetonitrile-d3 (0.6 mL) and hexamethyldisiloxane (6 μL, internal standard) were added to the tube. After measurement of the 1H NMR spectrum, pyridazine (10 μL, excess) was added to the solution. Monitoring of the reaction by 1H NMR spectroscopy indicated the quantitative formation of [(η5C5H4Me)4Fe4(HCCH)(HCC(1,2-N2C4H4))](PF6)2 ([7](PF6)2). Dissociation of pyrazine was also confirmed by a 1H NMR spectrum. Synthesis of [(η5-C5H4Me)4Fe4(HCCH)(HCC(1,2-N2C4H4)](PF6)2 ([7](PF6)2). To a solution of [2](PF6) (54 mg, 0.066 mmol) and AgPF6 (66 mg, 0.26 mmol) in acetonitrile (10 mL) was added pyridazine (0.5 mL, excess). After the mixture was stirred at room temperature for 30 min, insoluble materials were removed by filtration through a Celite pad. After removal of volatiles under reduced pressure, the residue was washed with dichloromethane and dried under vacuum to give a brown solid of [7](PF6)2. Yield: 49 mg (77%). Anal. Calcd for C32H35F12Fe4N2P2: C, 40.00; H, 3.67. Found: C, 39.72; H, 3.76. Mass (FAB, m-nitrobenzyl alcohol): m/z 816 (M+ − PF6, 2), 591 (M+ − 2PF6, 9). 1H NMR (CD3CN, 300 MHz): δ −71.5 (s, 2H, HCCH), −52.7 (s, 1H, HCC(1,2-N2C4H4)), −8.4, −2.6 (s, 3H × 2, C5H4Me), 2.2 (s, 6H, C5H4Me), 3.9, 5.0, 7.3, 10.0 (m, 1H × 4, N2C4H4), 5.7, 6.0, 7.7, 8.9, 9.2, 10.3, 12.4, 12.6 (s, 2H × 8, C5H4Me). 13 C{1H} NMR (75.5 MHz, CD3CN): δ 3.7, 17.4, 28.2 (C5H4Me), 70.6, 93.2, 95.0, 100.3, 105.8, 116.1, 123.6, 127.0, 144.9, 161.5 (C5H4Me or N2C4H4), 101.1, 119.7, 156.3 (ipso-C5H4Me). IR (KBr, cm−1): 3121 (w), 2927 (w), 1655 (br), 1573 (w), 1484 (m), 1455 (m), 1423 (w), 1390 (w), 1374 (m), 1030 (m), 844 (vs), 785 (m), 725 (w), 558 (m). Reaction of [6](PF6)2 with Pyridine. An NMR tube was charged with [6](PF6)2 (5 mg, 5 μmol), hexamethyldisiloxane (6 μL, internal standard), and acetonitrile-d3. After measurement of the 1H NMR spectrum, pyridine (10 μL, excess) was added to the solution. Monitoring of the reaction by 1H NMR spectroscopy indicated the quantitative formation of [(η 5 -C 5 H 4 Me) 4 Fe 4 (HCCH)(HCC(NC5H5)](PF6)2 ([8](PF6)2). Dissociation of pyrazine was also confirmed by a 1H NMR spectrum. [8](PF6)2 was identified by comparison with spectroscopic data previously reported by us. Reaction of [6](PF6)2 with Water. To a solution of [6](PF6)2 (34 mg, 0.035 mmol) in acetonitrile (10 mL) was added water (20 μL, excess). After the mixture was stirred for 30 min at room temperature, volatiles were removed under vacuum. The residue was washed with diethyl ether and dried under vacuum to give [(η 5 C5H4Me)4Fe4(HCCH)(μ3-CH)(μ3-CO)](PF6) ([5](PF 6)) as a greenish brown solid. Yield: 24 mg (91%). [5](PF6) was identified by comparison with spectroscopic data previously reported by us. Reaction of [6](PF6)2 with Methanol. To a solution of [6](PF6)2 (867 mg, 0.902 mmol) in acetonitrile (25 mL) was added methanol (2 mL). After the mixture was stirred for 2 h at room temperature, volatiles were removed under vacuum. The residue was extracted with dichloromethane through a Celite pad. Evaporation of the solvent gave a dark brown solid of [(η5-C5H4Me)4Fe4(HCCH)(HCCOMe)](PF6) ([9](PF6)). Yield: 678 mg (98%). 1H NMR (CD3CN): δ −70.1 (s, 2H, HCCH), −65.3 (s, 1H, HCCOMe), −5.0, −3.6, −2.3 (s, 3H × 3, C5H4Me, OMe), 7.8 (s, 6H, C5H4Me), 4.1, 4.8, 6.2, 6.6, 6.9, 9.0, 11.7, 12.3 (s, 2H × 8, C5H4Me). 13C{1H} NMR (125.8 MHz, acetonitriled3): δ −4.5, 15.6, 19.7, 46.7 (C5H4Me or OMe), 57.5, 74.4, 87.8, 91.2, 105.0, 105.3, 123.3, 127.7 (C5H4Me). IR (KBr, cm−1): 3429 (vs), 1482 (w), 1455 (w), 1372 (w), 1186 (w), 1149 (w), 1030 (w), 843 (vs), 830 (vs), 557 (m). Mass (ESI, acetonitrile): m/z 622 (M+, 100). Anal. Calcd for C29H34OFe4PF6: C, 45.41; H, 4.47. Found: C, 45.40; H, 4.68.

used as received. NMR spectra were recorded on a Bruker Avance II 400 or JEOL JNM-ECA500 spectrometer. Chemical shifts are reported in δ, referenced to 1H (of residual protons) and 13C signals of deuterated solvents as internal standards or to the 31P signal of 85% H3PO4 as an external standard. IR spectra were recorded on a JASCO FT/IR-410 spectrometer. Elemental analyses were performed using a Vario MICRO cube instrument. MS spectra were recorded with a Shimadzu LCMS-2010EV instrument. Synthesis of [(η5-C5H4Me)4Fe4(HCCH)(HCCNCMe)](PF6)2 ([3](PF6)2). [(η5-C5H4Me)4Fe4(HCCH)(HCCBr)](PF6) ([2](PF6); 184 mg, 0.226 mmol) and AgPF6 (68 mg, 0.27 mmol) were dissolved in acetonitrile (10 mL). After the mixture was stirred at room temperature for 30 min, insoluble materials were removed by filtration and the filtrate was evaporated to dryness. The residue was washed with dichloromethane and dried under vacuum to give a brown solid of [(η5-C5H4Me)4Fe4(HCCH)(HCCNCMe)](PF6)2 ([3](PF6)2). Yield: 160 mg (77%). Anal. Calcd for C30H34F12Fe4NP2: C, 39.08; H, 3.72. Found: C, 38.57; H, 4.05. 1H NMR (CD3CN): δ −73.4 (s, 2H, HCCH), −54.0 (s, 1H, HCCN), −4.5, −4.0 (s, 3H × 2, C5H4Me), 1.3 (s, 6H, C5H4Me), 5.0, 6.7, 8.0, 8.9, 9.5, 10.9, 11.4, 11.5 (br, 2H × 8, C 5 H 4 Me). 2 H NMR (CH 3 CN): δ −4.8 (s, HCCNCCD3 ). 13 C{ 1 H} NMR (CD3 CN): δ 6.4, 16.8, 22.0 (C5H4Me), 85.7, 92.4, 97.1, 97.5, 98.9, 108.9, 112.0, 116.4 (C5H4Me). IR (KBr pellet, cm−1): 3101(m), 2925 (m), 2252 (w), 1720 (s), 1482 (m), 1451 (m), 1372 (m), 1273 (m), 1235 (m), 840 (vs), 739 (sh), 636 (w), 557 (s), 542 (sh), 482 (w). The 1H NMR spectrum of [3](PF6) 2 is shown in Figure S1 (Supporting Information). Single crystals of [3](PF6)(OTf)·CH3CN, synthesized by the reaction of [2](PF6) with AgOTf, were obtained by cooling an acetonitrile/dichloromethane solution at −30 °C. Reaction of [2](PF6) with AgPF6 in Dichloromethane. [(η5C5H4Me)4Fe4(HCCH)(HCCBr)](PF6) ([2](PF6); 29 mg, 0.036 mmol) and AgPF6 (9.0 mg, 0.036 mmol) were suspended in dichloromethane (5 mL). After the mixture was stirred at room temperature for 30 min, volatiles were removed under reduced pressure and extracted with acetonitrile. The solvent was removed under vacuum. The 1H NMR spectrum of the residue indicated the formation of [3](PF6)2 and [(η5-C5H4Me)4Fe4(HCCH)(HCCF)](PF6) ([4](PF6)) in a molar ratio of 1:1.2. Due to contamination of impurities, isolation of [4](PF6) was not achieved. Data for [4](PF6) are as follows. 1H NMR (CD3CN): δ −74.0 (s, 2H, HCCH), −54.7 (s, 1H, HCCF), −2.7, −1.8 (s, 3H × 2, C5H4Me), −0.5 (s, 6H, C5H4Me), 4.4, 5.2, 6.1, 6.9, 8.1, 8.4, 8.8, 10.1 (s, 2H × 8, C5H4Me). Mass (ESI, acetonitrile): m/z 610 (M+, 100). Synthesis of [(η5-C5H4Me)4Fe4(HCCH)(HCCF)](PF6) ([4](PF6)). A solution of [1](PF6) (104 mg, 0.141 mmol) and N-fluorobenzenesulfonimide (57.9 mg, 0.184 mmol) in acetonitrile (7 mL) was stirred at room temperature for 2 days. Volatiles were removed under vacuum. The residue was extracted with dichloromethane. After removal of volatiles under vacuum, recrystallization of the residue from dichloromethane/diethyl ether gave a dark green solid of [4](PF6). Yield: 101 mg. An analytically pure sample was not obtained due to contamination of [(η5-C5H4Me)4Fe4(HCCH)(μ3-CH)(μ3-CO)](PF6) ([5](PF6)) and impurities derived from N-fluorobenzenesulfonimide. The 1H NMR spectrum of [4](PF6) is shown in Figure S2 (Supporting Information). Synthesis of [(η5-C5H4Me)4Fe4(HCCH)(HCCNC4H4N)](PF6)2 ([6](PF6)2). The cluster [2](PF6) (1.44 g, 1.77 mmol), pyrazine (0.571 g, 7.13 mmol), and AgPF6 (482 mg, 1.91 mmol) were placed in a Schlenk flask. Acetonitrile (15 mL) was added to the flask by syringe, and the solution was stirred at room temperature for 30 min. After the insoluble materials were filtered off through a Celite pad, volatiles were removed under vacuum. The residue was washed with dichloromethane. Recrystallization from acetonitrile (10 mL)/diethyl ether (30 mL) at room temperature gave a greenish brown solid of [6](PF6)2. Yield: 1.35 g (79%). Single crystals for an X-ray diffraction study were obtained in the same manner. Anal. Calcd for C32H35F12Fe4N2P2: C, 40.00; H, 3.67. Found: C, 39.60; H, 3.87. FAB-MS: m/z 815 (M+ − PF6, 17), 591 (M+ − 2PF6 − C4H4N2, 100). 1H NMR (CD3CN): δ −69.2 (s, 2H, HCCH), −50.7 (s, 1H, HCCN), −6.3, −2.9 (s, 3H × 2, 1955

dx.doi.org/10.1021/om400035q | Organometallics 2013, 32, 1951−1957

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Reaction of [6](PF6)2 with NaBPh4. An acetonitrile (20 mL) solution of [6](PF6)2 (105 mg, 0.109 mmol) and NaBPh4 (112 mg, 0.327 mmol) was stirred at 30 °C for 1 h. After removal of volatiles under vacuum, the residue was extracted with dichloromethane through a Celite pad. The volatiles were moved under vacuum, and the residue was washed with water and diethyl ether and dried under vacuum. Recrystallization from dichloromethane/hexane at room temperature gave a dark brown solid of [(η5-C5H4Me)4Fe4(HCCH)(HCCPh)](PF6) ([10](PF6)) and [5](PF6) in a molar ratio of 6:1. Synthesis of [(η5-C5H4Me)4Fe4(HCCH)(HCCPh)](PF6) ([10](PF 6 )). To a diethyl ether solution (30 mL) of [(η 5 C5H4Me)4Fe4(HCCH)(HCCBr)](TFPB) (319 mg, 0.208 mmol) was added PhMgBr (1.0 M hexane solution, 280 μL, 0.280 mmol). After the mixture was stirred at room temperature overnight, volatiles were removed under vacuum. The residue was dissolved in dichloromethane and treated with nBu4NPF6 (150 mg, 0.387 mmol) in order to exchange the counteranion. After the mixture was stirred at room temperature for 10 min, volatiles were removed under vacuum. The residue was extracted with diethyl ether and dichloromethane. The diethyl ether extract was concentrated to dryness under vacuum. For the resulting residue, the procedure for the counteranion exchange was repeated in the same manner. All dichloromethane extracts were combined, and the solvent was removed under vacuum. The residue was subjected to silica gel flash chromatography. The first greenish brown fraction was collected with a toluene/acetonitrile mixture (5/1) as an eluent. Removal of solvents gave a greenish brown solid of [10](PF6). Yield: 49 mg (29%). 1H NMR (CD3CN): δ −68.8 (s, 1H, HCCPh), −67.6 (s, 2H, HCCH), −7.4, −1.4 (s, 3H × 2, C5H4Me), 0.91 (s, 6H, C5H4Me), 2.5, 4.1 (s, 2H × 2, o,m-Ph), 4.4, 5.7, 6.6, 7.4, 8.0, 9.0, 10.0, 10.9 (s, 2H × 8, C5H4Me), 8.9 (s, 1H, p-Ph). 13C{1H} NMR (CD3CN): δ 4.4, 14.7, 23.1 (C5H4Me), 73.8, 75.7, 88.9, 92.7, 94.2, 99.4, 102.3, 105.1, 106.1, 106.7, 114.1, 123.0, 137.1, 141.1 (C5H4Me or Ph). Mass (FAB): m/z 668 (M+ − PF6). Anal. Calcd for C34H36F6Fe4P: C, 50.23; H, 4.46. Found: C, 49.90; H, 4.58. Reaction of [6](PF6)2 with Acetonitrile in the Presence of NiPr2Et. To a solution of [6](PF6)2 (100 mg, 0.104 mmol) in acetonitrile (5.0 mL) was added NiPr2Et (28 μL, 0.160 mmol) at room temperature. After the mixture was stirred for 30 min, volatiles were removed under reduced pressure. Recrystallization of the residue from tetrahydrofuran/diethyl ether at −30 °C gave greenish brown crystals of [(η5-C5H4Me)4Fe4(HCCH)(HCCCH2CN](PF6) ([11](PF6)). Yield: 77 mg (95%). 1H NMR (CD3CN, 400 MHz): δ −71.2 (s, 2H, HCCH), −69.4 (s, 1H, HCCCH2), −25.4 (s, 2H, HCCCH2), −4.74, −1.15 (s, 3H × 2, C5H4Me), 2.57 (s, 6H, C5H4Me), 4.62, 5.75, 6.53, 8.08, 8.82, 9.35, 10.49, 11.24 (s, 2H × 8, C5H4Me). 13C{1H} NMR (CD2Cl2, 400 MHz): δ 4.8 (CH2CN), 15.7, 17.5, 19.0 (C5H4Me), 14.4, 23.2, 57.1, 66.2, 77.3, 89.7, 95.1, 97.8, 98.8, 103.0, 106.5, 111.8, 121.4, 124.1, 146.6 (C5H4Me, HCCH, HCCCH2CN). IR (KBr, cm−1): 3208 (m), 3107 (vw), 2925 (w), 2241 (vw), 1483 (m), 1457 (m), 1411 (m), 1373 (m), 1180 (vw), 1135 (vw), 1065 (w), 1031 (m), 845 (vs), 557 (s), 467 (w). Mass (FAB, o-nitrophenyl octyl ether): m/z 776 (M+, 2), 631 (M+ − PF6, 100). Anal. Calcd for C30H33NFe4PF6: C, 46.44; H, 4.29; N, 1.81. Found: C, 46.15; H, 4.45; N, 1.77. Exchange of the Counteranion in [11](PF6) with BPh4− for Xray Diffraction Study. A solution of [11](PF6) (0.176 g, 0.227 mmol) and NaBPh4 (0.252 g, 0.736 mmol) in acetonitrile (20 mL) was stirred at room temperature for 3 h. After removal of volatiles under vacuum, recrystallization of the residue from acetonitrile/diethyl ether gave dark green crystals, which were washed with water and diethyl ether. Recrystallization of the residue from acetonitrile/diethyl ether gave dark green crystals of [(η5-C5H4Me)4Fe4(HCCH)(HCCCH2CN)](BPh4) ([11](BPh4)). Yield: 0.110 g (51%). Single crystals for X-ray diffraction analysis were obtained by recrystallization from tetrahydrofuran/diethyl ether. Reaction of 6 with Acetonitrile in the Presence of Triethylamine. To a solution of [6](PF6)2 (115 mg, 0.120 mmol) in acetonitrile (10 mL) was added triethylamine (25 μL, 0.18 mmol). The solution was stirred at room temperature for 30 min and evaporated to dryness in vacuo. Recrystallization of the residue from

acetonitrile/diethyl ether gave a brown solid of [11](PF6). Yield: 82 mg (88%). Synthesis of [(η5-C5H4Me)4Fe4(HCCH)(HCCCH2CN)] (11). To a suspension of [11](PF6) (50 mg, 64 mmol) in toluene (2.0 mL) was added [Cp2Co] (12 mg, 64 mmol). After the mixture was stirred at room temperature for 30 min, insoluble materials were removed by filtration. Volatiles were removed in vacuo, and the resulting solid was washed with diethyl ether followed by recrystallization from tetrahydrofuran/diethyl ether to yield 21 mg (52%) of 11. Anal. Calcd for C30H33NFe4: C, 57.11; H, 5.27; N, 2.22. Found: C, 56.93; H, 5.43; N, 2.30. 1H NMR (benzene-d6, 400 MHz) δ: 1.34 (s, 6H, C5H4Me), 1.46, 1.53 (s, 3H × 2, C5H4Me), 3.04 (s, 2H, CH2CN), 3.27 (s, 2H, C5H4Me), 3.44 (s, 2H, C5H4Me), 3.64 (s, 2H × 3, C5H4Me), 3.83 (s, 2H × 2, C5H4Me), 4.12 (s, 2H, C5H4Me), 9.87 (s, 2H, HCCH), 10.41 (s, 1H, HCCC). 13C{1H} NMR (benzene-d6, 400 MHz): δ 12.8, 13.0, 13.3 (C5H4Me), 45.2 (CH2), 83.5, 84.3, 85.4, 85.5, 86.0, 87.3, 87.4, 87.6, 97.7, 99.9, 101.0 (C5H4Me), 118.1 (CN), 208.0 (HCC−CH2), 211.1 (HCCCH2), 214.0 (HCCH). IR (KBr, cm−1): 3208 (m), 3107 (vw), 2925 (w), 2241 (vw), 1483 (m), 1457 (m), 1411 (m), 1373 (m), 1180 (vw), 1135 (vw), 1065 (w), 1031 (m), 845 (vs), 557 (s), 467 (w). Mass (FAB, o-nitrophenyl octyl ether): m/z 631 (M+). Synthesis of [(η5-C5H4Me)4Fe4(HCCH)(μ3-CH)(μ3-CNC(Me)NC4H2O2)](PF6)2 ([12](PF6)). To a solution of [6](PF6)2 (244 mg, 0.254 mmol) in acetonitrile (5 mL) was added a solution of maleimide (349 mg, 3.60 mmol) and NiPr2Et (48 μL, 0.28 mmol) in acetonitrile (5 mL). After the mixture was stirred at room temperature for 30 min, volatiles were removed under vacuum. Recrystallization of the residue from acetonitrile/diethyl ether gave a brown solid of [12](PF6). Single crystals for X-ray diffraction analysis were obtained by recrystallization from acetonitrile/diethyl ether. Yield: 192 mg (87%). Anal. Calcd for C34H36F6Fe4N2O2P·1/3C4H10O: C, 47.27; H, 4.42; N, 3.12. Found: C, 47.52; H, 4.29; N, 2.93. ESI-MS: m/z 728 (M+, 100). 1H NMR (500 MHz, CD3CN): δ −70.7 (s, 2H, HCCH), −63.7 (s, HCCN), −9.9, −9.7, −1.4 (s, 3H × 3, NCMe, C5H4Me), 4.9 (s, 6H, C5H4Me), 5.7 (s, 2H, NC4H2O2), 3.7, 5.5, 5.6, 7.0, 10.3, 10.7, 11.2, 12.0 (s, 2H × 8, C5H4Me). 13C{1H} NMR (CD3CN): δ −0.4, 14.7, 22.4, 38.5 (C5H4Me, NCMe), 55.7, 80.7, 84.4, 98.5, 98.6, 105.6, 127.3, 147.7 (C5H4Me), 133.1 (CH of maleimide), 156.8 (CO of maleimide), 157.5 (NC). IR (KBr, cm−1): 3102 (w), 2925 (w), 1714 (s), 1638 (w), 1482 (w), 1451 (w), 1389 (w), 1369 (sh), 1348 (m), 1167 (w), 1115 (w), 1030 (w), 843 (vs), 693 (w), 556 (s). Crystal Structure Determinations. Data were collected at 173 K on a Rigaku RAXIS RAPID imaging plate diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.71069 Å). Data were corrected by Lorentz, polarization, and absorption effects. The structure was solved by direct methods (SHELXS-97) and refined on F2 by full-matrix least squares (SHELXL-97) with non-H atoms anisotropic and H atoms included in riding mode.19 The checkCIF report in [3](PF6)(OTf) contains several alerts of level A and B. However, these alerts can be explained reasonably due to unresolved disorder of a η5-C5H4Me ligand and a PF6− counteranion. These alerts do not affect the validity of the X-ray results. Crystallographic data are given in Table S1 in the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

Figures giving 1H NMR spectra of [3](PF6)2 and [4](PF6) and a table and CIF files giving crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.O.) and ozawa@scl. kyoto-u.ac.jp (F.O.). Notes

The authors declare no competing financial interest. 1956

dx.doi.org/10.1021/om400035q | Organometallics 2013, 32, 1951−1957

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ACKNOWLEDGMENTS This research was supported by the Collaborate Research Program of Institute for Chemical Research, Kyoto University (grant 2012-11). M.O. acknowledges the funding program for Next Generation World-Leading Researchers and Grant for Hirosaki University Institution Research.

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REFERENCES

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