Redox-Active Phosphorus Ligands Bearing a [4Fe−4C] Core

Organometallics 2009, 28, 7055–7058 DOI: 10.1021/om900811u

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Redox-Active Phosphorus Ligands Bearing a [4Fe-4C] Core Substituent Masaaki Okazaki,*,† Ken-ichi Yoshimura, Masato Takano, and Fumiyuki Ozawa* International Research Center for Elements Science (IRCELS), Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan. †Present address: Graduate School of Science and Technology, Hirosaki University, Hirosaki 036-8561, Japan. Received September 16, 2009 Summary: Reaction of [(η5-C5H4Me)4Fe4(HCCH)(HCCBr)](PF6) (1) with HPPh2 in the presence of NEt3, followed by treatment of [Cp2Co], afforded [(η5-C5H4Me)4Fe4(HCCH)(HCC-PPh2)] (2). The electron-rich [4Fe-4C] core substituent leads to the extremely electron-releasing character of the phosphine part, estimated by the JPSe coupling constant of the corresponding selenide [(η5-C5H4Me)4Fe4(HCCH)(HCCP(Se)Ph2)] (3). Reaction of 2 with [AuCl(SMe2)] gave [(η5-C5H4Me)4Fe4(HCCH)(HCC-P(AuCl)Ph2)] (4). The cyclic voltammogram of 4 shows two reversible one-electron oxidation waves, indicating the existence of one- and two-electron oxidized forms.

Scheme 1

Introduction The design of ancillary ligands is crucial for realizing the efficient catalysis of homogeneous transition-metal complexes. Incorporation of a redox-responsive functionality into a framework of ligands enables alternation of reactivity and selectivity of metal centers through the chemical or electrochemical modification of redox states.1 Such a redox-active phosphorus ligand has been developed by the introduction of mononuclear transition-metal complexes.2 Wrighton et al. reported a representative example using a rhodium complex coordinated by 1,10 -bis(diphenylphosphino)cobaltocene that can switch catalytic activity between hydrosilylation and hydrogenation of alkenes upon the redox change of the cobaltocene moiety.2a Considering the multiple redox properties of transition-metal clusters, it is expected that the introduction of polymetallic cores3 could provide a new type of ancillary ligand subject to multistep tuning of catalytic behavior. Herein, we report the synthesis and properties of a redox-active phosphorus ligand bearing a [4Fe-4C] core substituent, its complexation with gold, and the resultant structure.

Results and Discussion Reaction of [(η5-C5H4Me)4Fe4(HCCH)(HCC-Br)](PF6) (1)4 with HPPh2 in the presence of NEt3 afforded [(η5-

C5H4Me)4Fe4(HCCH)(HCC-PPh2)](PF6) ([2](PF6)) in 65% yield (Scheme 1).5 Further treatment of [2](PF6) with [Cp2Co] resulted in a one-electron reduction to give 2 in 97% yield. As expected from the odd number of cluster electrons of [2](PF6), the 31P NMR signal exhibits a characteristic paramagnetic shift (δ -281) and line-broadening (W1/2 = 173 Hz), while the diamagnetic species 2 has a sharp signal at δ 51.6. To estimate the s character of the lone pair orbital of 2, the coupling constant 1JPSe of phosphine selenide [(η5-C5H4Me)4Fe4(HCCH)(HCC-P(Se)Ph2)] (3) was measured.6 Cluster 3 was synthesized by the reaction of 2 with selenium in dichloromethane at room temperature (Scheme 1). Table 1 lists the results in addition to those of other phosphine selenides. The average value of the C-P-C angles is calculated on the basis of X-ray diffraction analysis of 3 (Figure 1). The 1JPSe value of 3 (693 Hz) is significantly smaller than those of R3PdSe (R = Ph, Cy, and tBu), which indicates the smaller s character of the lone pair orbital of 2. Thus, it is considered that 2 exhibits a significant electronreleasing character compared with other well-known tertiary phosphines. The average of the three C-P-C angles is smaller than those of R3PdSe; therefore, the electron-releasing character is mainly attributable to the electron-rich [4Fe-4C] core; a cyclic voltammogram of [(η5-C5H4Me)4Fe4(HCCH)2] shows a reversible one-electron oxidation wave at E1/2 = -0.86 V vs Fc/Fcþ.7 The low value of E1/2 implies an electron-rich [4Fe-4C] core. The strong σ-donor

*To whom correspondence should be addressed. E-mail: mokazaki@ cc.hirosaki-u.ac.jp; [email protected]. (1) (a) Allgeier, A. M.; Mirkin, C. A. Angew. Chem., Int. Ed. 1998, 37, 894. (b) Geiger, W. E. Organometallics 2007, 26, 5738. (2) (a) Lorkovic, I. M.; Duff, R. R., Jr.; Wrighton, M. S. J. Am. Chem. Soc. 1995, 117, 3617. (b) S€ussner, M.; Plenio, H. Angew. Chem., Int. Ed. 2005, 44, 6885. (c) Gregson, C. K. A.; Gibson, V. C.; Long, N. J.; Marshall, E. L.; Oxford, P. J.; White, A. J. P. J. Am. Chem. Soc. 2006, 128, 7410. (3) Construction of functional molecules with polymetallic cores have been reported in the following papers: (a) Albinati, A.; Fabrizi de Biani, F.; Leoni, P.; Marchetti, L.; Pasquali, M.; Rizzato, S.; Zanello, P. Angew. Chem., Int. Ed. 2005, 44, 2. (b) Aranzaes, J. R.; Belin, C.; Astruc, D. Angew. Chem., Int. Ed. 2006, 45, 132. (4) (a) Takano, M.; Okazaki, M.; Tobita, H. J. Am. Chem. Soc. 2004, 126, 9190. (b) Okazaki, M.; Takano, M.; Yoshimura, K. J. Organomet. Chem. 2005, 690, 5318.

(5) Compound [2](PF6) was also synthesized in 46% yield by the reaction of 1 with LiPPh2 in acetonitrile. (6) (a) Allen, D. W.; Taylor, B. F. J. Chem. Soc., Dalton. Trans. 1982, 51. (b) Tsuji, H.; Inoue, T.; Kaneta, Y.; Sase, S.; Kawachi, A.; Tamao, K. Organomeallics 2006, 25, 6142. (7) (a) Okazaki, M.; Ohtani, T.; Inomata, S.; Tagaki, N.; Ogino, H. J. Am. Chem. Soc. 1998, 120, 9135–9138. (b) Okazaki, M.; Ohtani, T.; Takano, M.; Ogino, H. Inorg. Chem. 2002, 41, 6726.

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Table 1. 31P NMR Parameters and C-P-C Angles for R3PdSe 1

J(P-Se)/Hz C-P-C/deg

Ph3PdSe

t

Bu3PdSe

Cy3PdSe

755 105.7

708 110.0

706 108.3

3 693 107.5

Figure 1. ORTEP drawing of 3 with thermal ellipsoids at 50% probability. All hydrogen atoms and methyl groups on η5C5H4Me ligands are omitted for clarity. Selected bond distances (A˚) and angles (deg): Fe1-Fe3 2.5329(12), Fe1-Fe4 2.5062(12), Fe2-Fe3 2.4981(11), Fe2-Fe4 2.5048(11), P-Se 2.1239(14), P-C1 1.841(5), P-C29 1.839(5), P-C35 1.838(5), C1-C2 1.503(6), C3-C4 1.490(8), C1-P-C35 107.7(2), C1-P-C29 112.8(2), C29-P-C35 102.1(2). Scheme 2

2 would allow for the development of wide-scope catalytic systems.8 Reaction of 2 with [AuCl(SMe2)] in dichloromethane at room temperature gave [(η5-C5H4Me)4Fe4(HCCH)(HCC-P(AuCl)Ph2)] (4) in 77% yield (Scheme 2). The 31P NMR signal of 4 appears at δ 62.2. The signal is shifted to the downfield region by 10.6 ppm from 2 upon coordination to the Au(I) center. The structure of 4 was determined by X-ray diffraction analysis (Figure 2). Complex 4 adopts an almost linear geometry, typical for two-coordinated Au(I) species; the angle of P-Au-Cl is 176.56(5)°. The bond distance of Au-P (2.2466(7) A˚) lies within the range observed for [AuCl(PR3)] (2.20-2.25 A˚).9 The cyclic voltammogram of 4 exhibits two reversible oneelectron oxidation waves (Table 2). These oxidation processes are attributable to the removal of electrons from the (8) (a) de Meijere, A.; Diederich, F. Metal-Catalyzed Cross-Coupling Reactions; Wiley-VCH: Weinheim, Germany, 2004; 2 volumes. (b) Grubbs, R. H. Handbook of Metathesis; Wiley-VCH: Weinheim, Germany, 2003; 3 volumes. (9) Based on a survey of the Cambridge Structural Database, CSD version 5.30; The Cambridge Crystallographic Data Centre: Cambridge, U.K., Nov 2008.

Figure 2. ORTEP drawing of 4 with thermal ellipsoids at 50% probability. All hydrogen atoms and methyl groups on η5C5H4Me ligands are omitted for clarity. Selected bond distances (A˚) and angles (deg): Fe1-Fe2 2.5158(7), Fe2-Fe3 2.5134(8), Fe3-Fe4 2.4969(9), Fe4-Fe1 2.5300(8), C1-C2 1.511(4), C3-C4 1.504(4), P-C1 1.840(3), Au-P 2.2466(7), Au-Cl 2.3006(8), P-Au-Cl 176.56(5). Table 2. Cyclic Voltammetry Results for 4a 2þ

[4] /[4] [4]þ/[4]0

þ

Epa, V

Epc, V

E1/2, V

ΔEp, mV

þ0.54 -0.32

þ0.48 -0.38

þ0.51 -0.35

61 65

a Conditions: solution of 4 (0.1 mM) in acetonitrile with 0.1 M [NnBu4]BF4 as supporting electrolyte; scan rate 50 mV s-1. Potentials are given in V vs Fc/Fcþ.

electron-rich [4Fe-4C] core. For instance, the reaction of [2](PF6) with [AuCl(SMe2)] provided a one-electron oxidized form of 4, [(η5-C5H4Me)4Fe4(HCCH)(HCC-P(AuCl)Ph2)](PF6) ([4](PF6)), in 83% yield. The 31P{1H} NMR spectrum of [4](PF6) shows a broad signal at δ -456 (W1/2 = 276 Hz), characteristic of paramagnetism. Although the quality of X-ray diffraction data was low (R1 = 0.091), the structure of the one-electron oxidized form [4](PF6) was determined by X-ray diffraction study (see Supporting Information). The asymmetric unit consists of two independent molecules of [4](PF6). The average of the Au-P bond distances (2.269(3) A˚) is considerably longer than that in the neutral form 4 (2.2466(7) A˚). Removal of one electron from the [4Fe-4C] core weakens the σ-donation of [2](PF6) to the Au(I) center. Rauchfuss and his co-workers described the introduction of functional groups onto the Cp ligand of [Cp4Fe4(CO)4] through deprotonation with lithium diisopropylamide (LDA) followed by treatment with electrophiles.10 The use of PPh2Cl afforded [(η5-C5H4PPh2)Cp3Fe4(CO)4], which further ligated with the Ru(II) center through P-coordination to give [(η5-C5H4PPh2)Cp3Fe4(CO)4RuCl2(cymene)]. Due to the highly electron-rich character of [(C5H4Me)4Fe4(HCCH)2], the methodology involving deprotonation of the cyclopentadienyl ligand10,11 was not operative in our system. The direct introduction of the functional group onto (10) Westmeyer, M. D.; Massa, M. A.; Rauchfuss, T. B.; Wilson, S. R. J. Am. Chem. Soc. 1998, 120, 114. (11) Yeh, W.-Y.; Wu, C.-Y.; Chiou, L.-W. Organometallics 1999, 18, 3547.

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the cluster core presented here is promising for the construction of functional polymetallic molecules with redox responsibility. In conclusion, the PPh2 group was successfully introduced onto the [4Fe-4C] core. The electron-rich [4Fe-4C] core gives rise to an extremely electron-releasing characteristic of 2, which is estimated by the NMR parameter of the JPSe value in the corresponding selenide 3. Ligand 2 can perturb the electronic structure of transition-metal centers through redox processes, which would allow switching of the reactivity and functionality at the metal center.

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 and hexane were distilled from sodium benzophenone ketyl prior to use. [(η5-C5H4Me)4Fe4(HCCH)(HCC-Br)](PF6) (1) was prepared according to the literature method.4 Other chemicals were purchased and used as received. NMR spectra were recorded on a Bruker Avance II 400 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. MS and elemental analysis were performed by the ICR Analytical Laboratory, Kyoto University. Cyclic voltammetry was carried out with a Bioanalytical System ALS-600B electrochemical analyzer. Measurements were made in 0.1 mol dm-3 tetrabutylammonium tetrafluoroborate (TBAB)/acetonitrile solutions with a three-electrode system with a Pt rod working electrode, a Pt coil auxiliary electrode, and an Ag/AgNO3 reference electrode. Synthesis of [(η5-C5H4Me)4Fe4(HCCH)(HCC-PPh2)](PF6) ([2](PF6)). Method A: To a solution of 1 (500 mg, 0.613 mmol) in dichloromethane (30 mL) were added PPh2H (124 mg, 0.666 mmol) and NEt3 (74.0 mg, 0.731 mmol) at room temperature. After stirring for 2 h, volatiles were removed under vacuum. The residue was subjected to silica gel flash column chromatography. Elution with a dichloromethane/acetonitrile (20:1) mixture afforded a brown band. The eluate was concentrated to dryness to give [(η5-C5H4Me)4Fe4(HCCH)(HCC-PPh2)](PF6) ([2](PF6)) as a brown solid. Yield: 366 mg (65%). Method B: To a solution of HPPh2 (110 mg, 0.592 mmol) in hexane (5 mL) was added nBuLi (1.59 M hexane solution, 0.41 mL, 0.65 mmol) at 0 °C. After stirring at room temperature for 30 min, volatiles are removed under reduced pressure. The resultant LiPPh2 was suspended in acetonitrile (7 mL) and added to a solution of 1 (439 mg, 0.538 mmol) in acetonitrile (10 mL) at room temperature. After stirring for 45 min, volatiles were removed under vacuum. The residue was subjected to silica gel flash column chromatography. Elution with a dichloromethane/acetonitrile (20:1) mixture afforded a brown band. The eluate was concentrated to dryness to give [2](PF6) as a brown solid. Yield: 228 mg (46%). Data of [2](PF6): Anal. Calcd for C40H41F6Fe4P2: C, 52.16; H, 4.49. Found: C, 51.92; H, 4.55. Mass (FAB): m/z 776 (Mþ - PF6, 41), 591 (Mþ - PF6 - PPh2, 100). 1H NMR (CD3CN): δ -77.6 (br, W1/2 = 63 Hz, 1H, HCC-P), -67.3 (br, W1/2 = 52 Hz, 2H, HCCH), -6.6, 0.5 (br, W1/2 = 14 Hz, 3H  2 C5H4Me), 1.5 (br, W1/2 = 19 Hz, 6H, C5H4Me), 5.1 (br, W1/2 = 24 Hz, 4H, Ph), 5.2, 5.7, 6.5, 9.7, 10.4, 10.8, 11.7, 13.0 (br, W1/2 = 29 Hz, 2H  8, C5H4Me), 6.7 (br, W1/2 = 22 Hz, 4H, Ph), 7.3 (br, W1/2 = 22 Hz, 2H, Ph). 13C{1H} NMR (CD3CN): δ 1.7, 9.3, 24.1 (C5H4Me), 64.1, 78.6, 92.2, 95.1, 96.3, 97.6, 99.6, 99.9, 139.0, 139.2 (C5H4Me), 127.2, 128.3, 155.0 (Ph). Other 13C NMR signals were not assigned due to the line-broadening. 31 P{1H} NMR (CD3CN): δ -281 (br, W1/2 = 173 Hz). IR (KBr

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pellet, cm-1): 2961 (w), 2925 (w), 1483 (m), 1434 (m), 1373 (m), 1262 (m), 1092 (m), 1029 (m), 844 (vs), 747 (m), 702 (m), 557 (s). Synthesis of [(η5-C5H4Me)4Fe4(HCCH)(HCC-PPh2)] (2). Cobaltocene (53 mg, 0.28 mmol) was added to a solution of [2](PF6) (250 mg, 0.271 mmol) in acetonitrile (12 mL) at room temperature. The solution was stirred for 1 h and was concentrated to dryness under vacuum. The residue was extracted with diethyl ether. Removal of the solvent from the extract under vacuum gave a dark brown solid of [(η5-C5H4Me)4Fe4(HCCH)(HCC-PPh2)] (2). Yield: 204 mg (97%). Data of 2: Anal. Calcd for C40H41Fe4P: C, 61.90; H, 5.32. Found: C, 61.70; H, 5.32. Mass (FAB): m/z 776 (Mþ, 100), 591 (Mþ - PPh2, 82). 1H NMR (C6D6): δ 1.06 (s, 6H, C5H4Me), 1.36, 1.54 (s, 3H  2, C5H4Me), 3.54, 3.58, 3.67, 3.82, 4.04, 4.21, 4.47, 4.89 (m, 2H  8, C5H4Me), 7.04-7.13 (m, 6H, o,p-Ph), 7.46 (m, 4H, m-Ph), 9.89 (s, 2H, HCCH), 10.46 (d, 1H, 3JHP = 6.6 Hz, HCCP). 13C{1H} NMR (C6D6): δ 12.6, 12.8, 13.6 (C5H4Me), 84.4, 85.9, 86.0, 86.6, 86.9, 87.9, 88.4, 88.5, 95.8, 99.2, 100.5 (C5H4Me), 127.6 (m-Ph), 127.8 (p-Ph), 134.9 (d, 2JCP = 20 Hz, o-Ph), 144.3 (1JCP = 32 Hz, ipsoPh), 210.5 (d, 1JPC = 98 Hz, HCCP), 215.5 (HCCH), 216.2 (d, 2 JCP = 9H, HCCP). 31P{1H} NMR (C6D6): δ 51.6. IR (KBr pellet, cm-1): 3059 (w), 2925 (s), 2898 (s), 1482 (m), 1453 (m), 1431 (m), 1371 (w), 1261 (w), 1029 (s), 855 (m), 820 (s), 792 (s), 748 (m), 700 (m). Synthesis of [(η5-C5H4Me)4Fe4(HCCH)(HCC-P(Se)Ph2)] (3). A suspension of 2 (31.8 mg, 0.0410 mmol) and selenium powder (9.7 mg, 0.12 mmol) in dichloromethane (2 mL) was stirred at room temperature for 1 h. Insoluble materials were removed by filtration. Removal of volatiles from the filtrate under vacuum gave 3 as a dark brown solid. Yield: 34.3 mg (98%). Data of 3: Anal. Calcd for C40H41Fe4PSe: C, 56.19; H, 4.83. Found: C, 55.99; H, 5.04. Mass (FAB): m/z 856 (Mþ, 21). 1 H NMR (C6D6): δ 0.88 (s, 6H, C5H4Me), 1.28, 1.39 (s, 3H  2, C5H4Me), 3.50, 3.70, 3.80, 3.81, 4.10, 4.21, 4.75, 5.31 (m, 2H  8, C5H4Me), 7.00-7.03 (m, 6H, o- and p-Ph), 7.85 (m, 4H, m-Ph), 9.89 (s, 2H, HCCH), 10.41 (d, 1H, 3JPH = 11.6 Hz, HCCP). 13 C{1H} NMR (C6D6): δ 12.4, 12.7, 13.6 (C5H4Me), 85.3, 86.75, 86.82, 88.1, 88.2, 88.5, 88.9, 89.4, 97.1, 99.6, 101.0 (C5H4Me), 127.2 (d, 2JPC = 10 Hz, o-Ph), 129.7 (d, 4JPC = 3 Hz, p-Ph), 133.5 (d, 3JPC = 9 Hz, m-Ph), 138.5 (d, 1JPC = 61 Hz, ipso-Ph), 193.1 (d, 1JPC = 17 Hz, HCCP), 215.9 (HCCH), 219.4 (HCCP). 31 P{1H} NMR (C6D6): δ 57.4 (d, 1JSeP = 693 Hz). IR (KBr pellet, cm-1): 3083 (w), 2919 (m), 2278 (w), 1483 (m), 1454 (m), 1434 (m), 1371 (m), 1260 (w), 1087 (m), 1028 (m), 927 (w), 863 (m), 803 (m), 745 (m), 695 (m), 603 (w), 572 (w), 532 (m), 501 (m). Synthesis of [(η5-C5H4Me)4Fe4(HCCH)(HCC-P(AuCl)Ph2)] (4). [AuCl(SMe2)] (8.8 mg, 0.030 mmol) and 2 (23.1 mg, 0.0298 mmol) were dissolved in dichloromethane (1.5 mL), and the solution was stirred at room temperature for 25 min. After removal of volatiles under vacuum, recrystallization of the residue from dichloromethane/hexane gave dark brown crystals of [(η5-C5H4Me)4Fe4(HCCH)(HCC-P(AuCl)Ph2)] (4). Yield: 23.1 mg (77%). Data of 4: Anal. Calcd for C40H41AuClFe4P: C, 47.64; H, 4.10. Found: C, 47.19; H, 4.13. Mass (FAB, m-nitrobenzyl alcohol): m/z 1007 (Mþ - 2H, 71). 1H NMR (CD2Cl2): δ 1.03 (s, 6H, C5H4Me), 1.51, 1.59 (s, 3H  2, C5H4Me), 3.73, 3.84, 3.85, 4.01, 4.19, 4.28, 4.55, 4.78 (m, 2H  8, C5H4Me), 7.38-7.53 (m, 10H, Ph), 9.99 (s, 2H, HCCH), 10.30 (d, 1H, 3JPH = 11.1 Hz, HCCP). 13C{1H} NMR (CD2Cl2): δ 12.3, 12.9, 13.6 (C5H4Me), 85.5, 85.9, 87.02, 87.05, 87.7, 88.4, 89.8, 97.6, 101.4, 102.1 (C5H4Me), 128.4 (d, 3 JPC = 10 Hz, m-Ph), 130.8 (d, 4JPC = 2 Hz, p-Ph), 134.7 (d, 2 JPC = 12 Hz, o-Ph), 135.2 (d, 1JPC = 45 Hz, ipso-Ph), 215.4 (HCCP), 216.7 (HCCH). The 13C NMR signal of the carbon atom connected to the PPh2 moiety was not assigned. 31P{1H} NMR (122 Hz, CD2Cl2): δ 62.2 (s). Synthesis of [(η5-C5H4Me)4Fe4(HCCH)(HCC-P(AuCl)Ph2)](PF6) ([4](PF6)). [AuCl(SMe2)] (19.4 mg, 0.0659 mmol) and [2](PF6) (60.8 mg, 0.0660 mmol) were dissolved in dichloromethane (6 mL), and the solution was stirred at room

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temperature for 30 min. After removal of volatiles, recrystallization of the residue from dichloromethane/hexane gave dark brown crystals of [(η5-C5H4Me)4Fe4(HCCH)(HCC-P(AuCl)Ph2)](PF6) 3 1/4(CH2Cl2) ([4](PF6) 3 1/4(CH2Cl2)). The ratio of [4](PF6) and CH2Cl2 was established by 1H NMR data and confirmed by elemental analysis. Yield: 65.3 mg (83%). Data of [4](PF6) 3 1/4(CH2Cl2): Anal. Calcd for C40.5H42AuCl2F6Fe4P2: C, 41.15; H, 3.56. Found: C, 41.50; H, 3.58. Mass (FAB): m/z 1008 (Mþ - H, 100). 1H NMR (CD3CN): δ -68.9 (br, W1/2 = 60 Hz, 1H, HCCP), -66.4 (br, W1/2 = 51 Hz, 2H, HCCH), -5.7, 1.5 (br, W1/2 = 10 Hz, 3H  2, C5H4Me), 1.0 (br, W1/2 = 18 Hz, 6H, C5H4Me), 5.2 (m, 4H, o-Ph), 6.1, 7.1, 8.0, 9.5, 11.2, 12.3, 13.7, 13.8 (br, W1/2 = 21 Hz, 2H  8, C5H4Me), 6.83 (m, 4H, m-Ph), 7.47 (m, 2H, p-Ph). 13C{1H} NMR (CD3CN): δ 8.2, 21.4 (C5H4Me), 68.7, 69.9, 92.8, 101.1, 104.7, 108.0, 117.4, 121.0 (C5H4Me), 87.0, 107.4, 149.9, 159.8 (ipso-C5H4Me or ipso-Ph), 128.1, 128.8, 131.8 (Ph). Other 13C NMR signals were not assigned due to the line-broadening. 31P{1H} NMR (CD3CN): δ -456 (br, W1/2 = 276 Hz). IR (KBr, cm-1): 3924 (w), 1483 (m), 1436 (m), 1374 (m), 1261 (w), 1094 (m), 1030 (m), 844 (vs), 746 (m), 700 (m), 557 (s).

Okazaki et al. X-ray Structure Analysis. The X-ray diffraction study was performed on a Rigaku Mercury CCD diffractometer with graphite-monochromated Mo KR radiation (λ = 0.71070 A˚). The intensity data were collected at 173 K and corrected for Lorentz and polarization effects and absorption (numerical). The structure was solved by direct methods and refined by fullmatrix least-squares on F2 against all reflections using the program SHELXL-97. Further information is available as CIF files in the Supporting Information.

Acknowledgment. This work was supported by the Ministry of Education, Culture, Sports, Science, and Technology of Japan (Grants-in-Aid for Scientific Research Nos. 17750051, 19655019, 20037036, 20350027, 21655019). Supporting Information Available: ORTEP drawing of [4](PF6) and CIF files giving crystal data for compounds 3, 4, and [4](PF6). This material is available free of charge via the Internet at http://pubs.acs.org.