Bis(ethynyl) - American Chemical Society

4804

Organometallics 2010, 29, 4804–4817 DOI: 10.1021/om100003z

10 ,1000 -Bis(ethynyl)biferrocenyl-Bridged Fe(dppe)Cp* Units: Synthesis, Solid-State Structures, and Electronic Couplings§ Manja Lohan,†,‡ Frederic Justaud,† Thierry Roisnel,† Petra Ecorchard,‡ Heinrich Lang,*,‡ and Claude Lapinte*,† †

Sciences Chimiques de Rennes, Universit e de Rennes 1, UMR CNRS 6226, Campus de Beaulieu, F-35042 Rennes, France, and ‡Fakult€ at f€ ur Naturwissenschaften, Institut f€ ur Chemie, Lehrstuhl f€ ur Anorganische Chemie, Technische Universit€ at Chemnitz, Strasse der Nationen 62, 09111 Chemnitz, Germany Received January 4, 2010

Treatment of 10 ,1000 -bis(ethynyl)biferrocene (1) with 2 equiv of FeCl(η2-dppe)(Cp*) (2) in the presence of [H4N]PF6 produced the vinylidene complex [10 ,1000 -((Cp*)(η2-dppe)FedCdCH)2bfc][PF6]2 (3) (97%, Cp*=η5-C5Me5; dppe=1,2-bis(diphenylphosphino)ethane; bfc=10 ,1000 -biferrocenyl, (η5C5H4)2Fe)2), while neutral tetranuclear 10 ,1000 -((Cp*)(η2-dppe)Fe-CtC)2bfc (4) was obtained from ossbauer, the reaction of 3 with KOtBu (68%). The IR, 1H, 13C, and 31P NMR, cyclic voltammetry, M€ and UV-vis data were obtained for 4. When reacted with 2 equiv of [FcH]PF6 or TCNQ, complex 4 provided 4[X]2 (Fc = 1-ferrocenyl, (η5-C5H5)Fe(η5-C5H4), X = PF6, 89%; X = TCNQ = 7,7,8,8tetracyanoquinodimethane, 91%), and 4[PF6] was obtained by reacting 4 with 4[PF6]2 in a 1:1 molar ratio (76%). Organometallics 4[PF6]3 (93%) and 4[PF6]4 (87%) were obtained from the reaction of 4[PF6]2 with 1 or 2 equiv of Ag[PF6]. For the purpose of reasonable solubility the triflate salts of tri- and tetracationic species 4[OTf]3 (94%) and 4[OTf]4 (95%) were synthesized instead of the hexafluorophosphate salts. The structures of 3, 4, and 4[TCNQ]2 in the solid state were determined by single-crystal X-ray structure analysis. IR and M€ ossbauer spectroscopy of 4[PF6]n (n=0-4) allowed the stepwise observation of the four metal-centered oxidations and established the localized character of the mixedvalence complexes. EPR spectra of 4[PF6]n (n=1, 3) proved the formation of two radicals, one centered on the half-sandwich termini and the other centered on the (ethynyl)biferrocene bridge, indicating that the mediating state is sufficiently low lying to be populated at 66 K. The NIR spectra of the mixedvalent species 4[PF6]n (n=1, 2) and 4[OTf]3 showed the presence of two bands, and their characteristic parameters are in accord with an electron transfer involving intermediate states. Introduction Transition metal complexes have potential applications in the new and fascinating field of molecular electronics.1 This hope is based inter alia on the ability of these compounds to be stable under various oxidation states, allowing intramolecular electron and charge transfers.2 In particular, since the early 1970s biferrocenes (= bfc), in which two ferrocenyl units are connected by forming a fulvalenide bridge, first attracted much attention because they can easily form § Part of the Dietmar Seyferth Festschrift. In honor of Prof. Dr. h.c. mult. Dietmar Seyferth for his outstanding contribution as editor to Organometallics. *Corresponding authors. (H.L.) Phone: þ49-(0)371-531-21210. Fax: þ49-(0)371-531-21219. E-mail: [email protected]. (C.L.) Phone: þ33-(0)2-23-23-5963. Fax: þ33-(0)2-23-23-6939. E-mail: [email protected]. (1) Tour, J. M. Acc. Chem. Res. 2000, 33, 791–803. Caroll, R. L.; Gorman, C. B. Angew. Chem., Int. Ed. 2002, 41, 4379–4400. Robertson, N.; Mc Gowan, G. A. Chem. Soc. Rev. 2003, 32, 96–103. (2) Astruc, D. Electron Transfer and Radical Processes in TransitionMetal Chemistry; VCH: New York, 1995. (3) Togni, A.; Hayashi, T. Ferrocenes: Homogeneous Catalysis-Organic Synthesis-Materials Science; Weinheim, Germany, 1995. Brown, G. M.; Meyer, T. J.; Cowan, D. O.; Le Vanda, C.; Kaufman, F. J.; Roling, P. V.; Rausch, M. D. Inorg. Chem. 1975, 14, 506. Cowan, D. O.; Le Vanda, C.; Park, J.; Kaufman, F. J. Acc. Chem. Res. 1973, 6, 1. Mueller-Westerhoff, U. T. Angew. Chem., Int. Ed. Engl. 1986, 25, 702.

pubs.acs.org/Organometallics

Published on Web 03/15/2010

mixed-valent (MV) Fe(II)-Fe(III) species by electrochemical or chemical oxidation.3 In this respect, bis(alkynyl)biferrocene derivatives constitute attractive electro-active bridging units for connecting transition metal fragments, which can be themselves redox-active. In such molecules, which possess a structural rigidity and various accessible redox states, electron transfer processes should easily take place through the extended delocalized bonding. However, in contrast to ferrocenes, 10 ,1000 -difunctionalized biferrocenyls are less studied; this is probably due to the lack of straightforward synthesis methodologies.4 Recently, we described efficient synthesis methodologies for the preparation of a series of compounds with 10 ,1000 -bis(ethynyl)biferrocenyl as a linking group for gold (Ph3P)Au, ruthenium [(Cp)(Ph3P)2Ru], and osmium [(Cp)(Ph3P)2Os] fragments (Cp = η5-C5H5) together with solid-state structures and electrochemical, UV-vis-NIR, and EPR spectroscopic characterizations.4 This work supported the conclusion that the strongest interaction may occur between one [(Cp)(PPh3)2M] moiety and the biferrocenyl unit through ethynyl fragments, while the biferrocenyl linker can act as a relay, allowing electron transfer from one metal terminus to the other (4) Lohan, M.; Ecorchard, P.; R€ uffer, T.; Justaud, F.; Lapinte, C.; Lang, H. Organometallics 2009, 28, 1878–1890. r 2010 American Chemical Society

Article

Organometallics, Vol. 29, No. 21, 2010

4805

Chart 1

one. Here, taking advantage of the chemical stability of the [(Cp*)(η2-dppe)Fe] terminal building block as iron(II) and iron(III) derivatives (Cp* = η5-C5Me5, dppe = 1,2-bis(diphenylphosphino)ethane), we report the synthesis, isolation, and characterization of the tetrairon complex [(Cp*)(η2-dppe)FeCtC-bfc-CtC-Fe(η2-dppe)(Cp*)][PF6]n (4[PF6]n; n = 0-4) under five discrete oxidation states. Reports on organometallics under more than three oxidation states are not unprecedented, but are still very rare.5,6 In this respect, the ruthenium complexes [{(Cp)(PPh3)(PR3)Ru}2(μ-CtC-CtC)]nþ (R = Ph, Me; n = 0-4) were generated under five oxidation states, but only three of them were isolated,7 and the iron derivatives [{(Cp*)(dippe)Fe}2(μ-CtC-CtC)]nþ (dippe=PPri2CH2CH2PPri2; n = 0-3) were prepared and isolated under four oxidation states.8 An extensive investigation of the properties of all members of this new and large family of molecules, including redox potentials, X-ray determination of molecular structures (only for n=0, 2), M€ ossbauer spectroscopy, and vibrational, electronic, and EPR spectroscopies, evidenced the intricate electronic interactions between the four electro-active metal centers. The spectroscopic and redox properties of this family of molecules will be compared with those of closely related derivatives shown in Chart 1.

Results and Discussion 1. Synthesis and Characterization. The title compound 10 ,1000 ((Cp*)(η2-dppe)FeCtC)2bfc (4) (Cp* = η5-C5Me5; (5) Paul, F.; Lapinte, C. Coord. Chem. Rev. 1998, 178-180, 427–505. (6) Akita, M.; Koike, T. Dalton Trans. 2008, 3523–3530. (7) Bruce, M. I.; Low, P. J.; Costuas, K.; Halet, J.-F.; Best, S. P.; Heath, G. A. J. Am. Chem. Soc. 2000, 122, 1949–1962. (8) Guillemot, M.; Toupet, L.; Lapinte, C. Organometallics 1998, 17, 1928–1930.

dppe=1,2-bis(diphenylphosphino)ethane; bfc=10 ,1000 -biferrocenyl, ((η5-C5H4)2Fe)2) was prepared in a consecutive twostep synthesis methodology involving the formation of the corresponding vinylidene complex 3 followed by its deprotonation as depicted in Scheme 1. Treatment of 10 ,1000 -bis(ethynyl)biferrocene (1) with 2 equiv of FeCl(η2-dppe)(Cp*) (2) and [H4N]PF6 as halide anion abstractor produced in a 1:1 (v/v) mixture of tetrahydrofuran and methanol after 16 h at ambient temperature the expected vinylidene complex [10 ,1000 -((Cp*)(η2-dppe)FedCdC)2bfc][PF6]2 (3) (Scheme 1, reaction (i)), which after appropriate workup could be isolated as a brown solid material in 97% yield (Experimental Section). Compound 3 is the unique product formed in this reaction; that is, no monosubstituted species such as [10 -(Cp*)(η2-dppe)FedCdCH]-1000 -(CtCH)bfc][PF6] or alkoxycarbene derivatives resulting from the nucleophilic addition of methanol were generated. The progress of the reaction could be monitored by IR and 1H NMR spectroscopy. The νCtC vibration of 1 at 2109 cm-1 disappears,4 while simultaneously the νCdC band typical for 3 grows at 1629 cm-1 as the reaction time proceeds. After 16 h, the only characteristic absorption is that for the vinylidene unit. The 1H NMR spectrum of 3 shows a characteristic broad resonance signal at 4.84 ppm for the hydrogen atom bound to the β-carbon atoms of the bis(vinylidene) ligand (Experimental Section). Vinylidene complex 3 formed as a single product could be converted into the corresponding neutral alkynyl system by deprotonation with KOtBu (Scheme 1). Tetranuclear 10 ,1000 ((Cp*)(η2-dppe)FeCtC)2bfc (4) was isolated as a dark red solid in 68% yield. This complex dissolves in organic solvents such as toluene, dichloromethane, and tetrahydrofuran, while it is only slightly soluble in petroleum ether and diethyl

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Scheme 1. Synthesis of [10 ,1000 -((Cp*)(η2-dppe)FedCdC)2bfc][PF6]2 (3) and ((Cp*)(η2-dppe) FeCtC)2bfc (4) from 10 ,1000 -Bis(ethynyl)biferrocene (1)a

a

Key reagents and conditions: (i) (Cp*)(η2-dppe)FeCl (2) (2 equiv), [NH4]PF6, thf/MeOH, 25 C, 16 h; (ii) KOtBu, thf/MeOH, 25 C, 5 h.

ether. Solid samples of 4 are fairly stable toward air and moisture, while solutions containing 4 are stable for long periods of time under argon and readily decompose in air. Complex 4 was characterized by elemental analysis, IR, and 1 H, 13C{1H}, and 31P{1H} NMR spectroscopies. The infrared spectrum displays a weak absorption at 2059 cm-1 corresponding to the CtC stretching mode. Similar values were found for, for example, (Cp*)(η2-dppe)FeCtCPh (8) (2053 cm-1), (Cp*)(η2-dppe)FeCtCFc (7) (2072 cm-1) (Fc = 1-ferrocenyl, (η5-C5H5)Fe(η5-C5H4)), and (Cp*)(η2-dppe)FeCtC-1,4-C6H4X (X = OMe, 1990 cm-1; X = NO2, 2038 cm-1).9-12 The 31P NMR spectrum of 4 indicates the presence of a single phosphorus environment with a resonance signal found at 100 ppm. This differs from 3, where next to the dppe signal at 90 ppm a second resonance, which can be assigned to the PF6counterions, was found at -142 ppm with a JPF coupling constant of 714 Hz. These data indicate that the phosphorus atoms in 4 are more deshielded than in 3, demonstrating that the phosphorus atoms in 3 are somewhat more Lewis basic. 2. Cyclic Voltammetry of 4 and Preparation and Isolation of the Oxidized Forms 4[PF6]n (n=1, 2, 3, 4) and 4[TCNQ]2. The initial scan in the cyclic voltammogram (CV) of 4 from -0.7 to 1.1 V vs SCE shows three fully reversible oxidation waves (Figure 1). The current of the first redox event, which corresponds to the [(Cp*)(η2-dppe)Fe]0/þ redox couple (E01= -0.23 V), is two times more intense that the current corresponding to the two other redox processes (E02 = 0.49 V, E03 = 0.77 V), which can be assigned to the [bfc]0/þ and [bfc]þ/2þ redox couples, respectively. (9) Connelly, N. G.; Gamasa, M. P.; Gimeno, J.; Lapinte, C.; Lastra, E.; Maher, J. P.; Le Narvor, N.; Rieger, A. L.; Rieger, P. H. J. Chem. Soc., Dalton Trans. 1993, 2575–2578. (10) Bruce, M. I.; Low, P. J.; Hartl, F.; Humphrey, P. A.; de Montigny, F.; Jevric, M.; Lapinte, C.; Perkins, G. J.; Roberts, R. L.; Skelton, B. W.; White, A. H. Organometallics 2005, 24, 5241–5225. (11) Sato, M.; Hayashi, Y. Organometallics 1996, 15, 721–728. (12) Denis, R.; Toupet, L.; Paul, F.; Lapinte, C. Organometallics 2000, 19, 4240–4251.

The first two-electron oxidation event of 4 is shifted toward negative potential with respect to the redox potentials of the ruthenium termini of 5. Furthermore in 5 the identical ruthenium centers display two separated oneelectron redox events for the oxidation of the metal termini, which is in contrast to 4, where only one two-electron redox event could be monitored.4 Apparently, the ferrocenyl unit acts as an electron donor with respect to the [(Cp*)(η2dppe)Fe] building block since oxidation of (Cp*)(η2-dppe)FeCtCFc (7)11 is easier than oxidation of the mononuclear compound (Cp*)(η2-dppe)FeCtCPh (8) and the binuclear complex (Cp*)(η2-dppe)FeCtC-C6H4-CtCFe(η2-dppe)(Cp*) (6).9,13 Analysis of the peak currents in the CV of 4 shows that the anodic and cathodic currents are strictly identical, so that the different electron-deficient species are apparently stable on the platinum electrode. The distance between the anodic and cathodic peaks is smaller for the one-electron waves (0.07 V) than for the two-electron wave (0.11 V). This feature is in accord with the overlap of two one-electron waves at two standard potentials, E01 and E02, with a difference between them below the limit of the resolution of cyclic voltammetry. However, it has been shown that the determination of E01 and E02 can be derived from the location of the midpoint between the anodic and cathodic peaks of the two-electron wave and the distance between them (ΔEp) provided the kinetics of the electron transfer processes do not affect the CV response.15 This condition was proven by investigating the variations of ΔEp as a function of the scan rate, and only negligible variations were observed for scan rates below (13) Le Narvor, N.; Lapinte, C. Organometallics 1995, 14, 634–639. (14) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877–910. (15) Myers, R. L.; Shain, I. Anal. Chem. 1969, 41, 980–980. Hapiot, P.; Kispert, L. D.; Konovalov, V. V.; Saveant, J.-M. J. Am. Chem. Soc. 2001, 123, 6669–6667. Andrieux, C. P.; Saveant, J.-M. Electroanal. Chem. 1974, 57, 27–33. Guerro, M.; Carlier, R.; Boubekeur, K.; Lorcy, D.; Hapiot, P. J. Am. Chem. Soc. 2003, 125, 3159–3167.

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4807

Figure 1. Cyclic voltammogram of 4 (V vs SCE, 10-3 M solution in dichloromethane at 298 K, 0.1 M [nBu4N]PF6, scan rate = 0.100 V s-1). Table 1. Electrochemical Dataa for 1 and 4-8

compd 1

[M]

[M(II)]/[M(III)]

bfc/bfcþ

E0 (ΔEp) [V]

E0 (ΔEp) [V]

none

4

2

Cp*(η -dppe)Fe

-0.23 (0.11)

5

Cp(PPh3)2Ru

6

2

Cp*(η -dppe)Fe

0.03 (0.09) 0.22 (0.10) -0.27 (0.06) -0.01 (0.06)

7 8

Cp*(η2-dppe)Fe Cp*(η2-dppe)Fe

-0.33 -0.15 (0.08)

0.47 (0.13) 0.85 (0.13) 0.49 (0.07) 0.78 (0.07) 0.74 (0.12) 0.89b

ref 4 this work 4 13

Fc/Fcþ 0.39

11 12

a Potentials in dichloromethane (0.1 M [nBu4N]PF6; 298 K, platinum electrode, sweep rate 0.100 V s-1) are given in V vs SCE; the ferroceneferrocenium couple [Fc]/[Fcþ] (0.460 V vs SCE) was used as an internal calibrant for the potential measurements.14 b Irreversible.

1 V/s. Thus, ΔEp tends toward a limit (0.11 V) that corresponds to the thermodynamics of the electron transfer which leads to an estimation of the difference ΔE0 = 80 mV (use of a working curve),16 evidencing the equilibrium constant Kc for the comproportionation reaction (eqs 1a and 2a, Kc = 24 at 20 C). The knowledge of Kc allowed the determination of the molar fractions of the species present in solution. In particular, when one equivalent of the neutral complex is reacted with one equivalent of the dioxidized reagent, the molar fraction of the MV species is approximately 0.7.

f½Mred -B-½Mred g þ f½Mox -B-½Mox g2þ Kc

sf 2f½Mred -B-½Mox gþ

ð1aÞ

lnðKc Þ ¼ nFðE 2 0 -E 1 0 Þ=RT ¼ 11 604ðE 2 0 -E 1 0 Þ=T

ð2aÞ

The potential difference (ΔE0) between the two redox events and the comproportionation constant Kc are both (16) Hamon, P.; Justaud, F.; Cador, O.; Hapiot, P.; Rigaut, S.; Toupet, L.; Ouahab, L.; Stueger, H.; Hamon, J.-R.; Lapinte, C. J. Am. Chem. Soc. 2008, 130, 17372–17383.

representative of the thermodynamic stability of the corresponding mixed-valence state relative to other redox states. Their magnitude is determined by the sum of several energetic factors relating to the stability of the reactant and product complexes, and it is important to keep in mind that a single redox event is not diagnostic of the absence of electronic communication between the redox centers.17 Anyway, in the present work precise determination of ΔE is not pivotal, since the direct electronic communication between the iron-based end-groups does not take place (Had ≈ 0, see NIR Spectroscopic section). On the basis of the full reversibility of the CV waves, the oxidized complexes 4[PF6]n (n = 1-4) were considered as accessible synthetic targets. According to a well-established procedure, complex 4 was first reacted with 2 equiv of [FcH]PF6 in tetrahydrofuran (thf) at -60 C. The initial dark red solution turned dark blue, and after completion of the reaction, addition of n-pentane allowed the precipitation of 4[PF6]2. Alternatively, treatment of 4 with 2 equiv of TCNQ provided 4[TCNQ]2 quantitatively. The monocationic complex 4[PF6] was obtained by reacting 1 equiv of the neutral compound 4 with 1 equiv of the dicationic derivative 4[PF6]2 in cold thf (Experimental Section). The redox potential of the redox couple 42þ/3þ being more positive than that of the ferrocene/ferrocenium couple, the tri- and tetracation 4[PF6]n (n = 3, 4) were prepared by reacting 4[PF6]2 with one or two stoichiometric amounts of [AgPF6] in thf at -60 C. Excellent yields of spectroscopically pure materials were obtained by partial precipitation and vacuum drying, and both compounds gave CV responses identical to that obtained for 4. In the solid state, samples of the salts 4[PF6], 4[PF6]2, and 4[PF6]3 were apparently stable for a period of months, while 4[PF6]4 decomposes within a few days when stored under argon at 20 C. The four oxidized complexes 4[PF6]n (n = 1- 4) are paramagnetic and were characterized by FT-IR, UV-vis, NIR, EPR, and M€ ossbauer spectroscopies. As these polynuclear radicals are less stable in solution, only freshly prepared solutions were used for measurement purposes. In addition, an X-ray diffraction analysis was achieved for 4[TCNQ]2. (17) Lapinte, C. J. Organomet. Chem. 2008, 693, 793–801.

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Table 2. Selected Bond Distances (A˚) and Angles (deg) for 3, 4, and 4[TCNQ]2a 3

4

4[TCNQ]2

2.1728(7) 2.1867(7) 1.736 1.909(2) 1.219(3) 1.436(3) 1.464(5) 6.09 5.12 13.64

2.2536(9)/2.2371(9) 2.2413(10)/2.2277(10) 1.782/1.777 1.877(3)/1.855(3) 1.231(5)/1.243(5) 1.421(5)/1.412(5) 1.471(6) 6.08/6.08 5.13 13.88

Bond Distances Fe(1)-P(1)/Fe(3)-P(51) Fe(1)-P(2)/Fe(3)-P(52) Fe(1)-Cp*(1) /Fe(3)-Cp*(3) Fe(1)-C(37)/Fe(3)-C(87) C(37)-C(38)/C(87)-C(88) C(38)-C(39)/C(88)-C(89) C(48)-C(480 )/C(48)-C(89) Fe(1)-Fe(2)/Fe(3)-Fe(4) Fe(2)-Fe(20 )/Fe(2)-Fe(4) Fe(1)-Fe(10 )/Fe(1)-Fe(4)

2.2184(6) 2.2492(6) 1.786 1.753(2) 1.307(3) 1.473(3) 1.425(4) 5.72 5.07 14.05 Bond Angles

P(1)-Fe(1)-P(2)/P(51)-Fe(3)-P(52) 84.70(2) P(1)-Fe(1)-C(37)/P(51)-Fe(3)-C(87) 85.82(7) P(2)-Fe(1)-C(37)/P(52)-Fe(3)-C(87) 88.54(7) Fe(1)-C(37)-C(38)/Fe(3)-C(87)-C(88) 175.0(2) C(37)-C(38)-C(39)/C(87)-C(88)-C(89) 130.1(2) 0 0 154 Cp*(1)-Fe(1)-Fe(1 )-Cp*(1 ) Cp*(1)-Fe(1)-Fe(3)-Cp*(3) 40 Cp*(1)-Fe(1)-Cp‡(2)-Fe(2) Cp*(3)-Fe(3)-Cp‡(4)-Fe(4) ‡ 0 ‡ 00 0 180 Fe(2)-Cp (2 )-Cp (2 )-Fe(2 ) Fe(2)-Cp‡(20 )-Cp‡(40 )-Fe(4) a *Centroid of the C5Me5 ligands. ‡Centroid of the C5H4 ligands.

In the course of this work we found that the solubility of the tri- and tetracations as hexafluorophosphate salts was too low to permit correct NIR measurements. For this purpose, complexes 4[OTf]n (n=3, 4) were prepared by the reaction of 4 with 3 or 4 equiv of [AgOTf] in dichloromethane. After isolation with an almost quantitative yield, these salts were subject to CV and IR analyses and NIR measurements (vide infra) without any further characterization. 3. Molecular Structures of 3, 4, and 4[TCNQ]2. Single crystals suitable for X-ray crystallography were grown by slow diffusion of n-pentane into a benzene or dichloromethane solution containing either 3, 4 or 4[TCNQ]2 at 20 C. Complexes 3 and 4 crystallized in the monoclinic space groups C2/c and P21/c, respectively. These two compounds possess a crystallographic Ci symmetry about the center of the interferrocenyl linkage (C48-C480 ). The salt 4[TCNQ]2 crystallizes in the triclinic space group P1. The crystallographic data are given in Table 8 (Experimental Section), while the molecular structures are shown in Figure 2. Selected bond distances and angles are given in Table 2. As it is invariably observed for all members of the (Cp*)(η2-dppe)Fe family, the two terminal moieties adopt pseudo-octahedral geometries with bond lengths and angles in previously established ranges.5,18 Structural parameters have been reported for a large number of mononuclear vinylidene complexes with many transition metals;19 however, only few precedents are known in the iron series20,21 and only one example of a bimetallic complex containing vinylidene ligands has recently been (18) Hamon, P.; Toupet, L.; Hamon, J.-R.; Lapinte, C. Organometallics 1996, 15, 10–12. (19) Bruce, M. I. Chem. Rev. 1998, 98, 2797–2858. Cadierno, V.; Gamasa, M. P.; Gimeno, J. Eur. J. Inorg. Chem. 2001, 571–591. (20) Argouarch, G.; Thominot, P.; Paul, F.; Toupet, L.; Lapinte, C. C. R. Chim. 2003, 6, 209–222. (21) Gauss, C.; Veghini, D.; Grama, O.; Berke, H. J. Organomet. Chem. 1997, 541, 19–38. (22) Tanaka, Y.; Shaw-Taberlet, J. A.; Justaud, F.; Cador, O.; Roisnel, T.; Akita, M.; Hamon, J.-R.; Lapinte, C. Organometallics 2009, 28, 4656–4669.

86.90(3) 83.15(7) 86.89(8) 177.9(2) 173.4(3) 180

84.02(4)/83.53(3) 89.82(10)/87.43(10) 84.11(11)/96.47(10) 173.1(3)/171.0(3) 175.9(3)/173.3(3) 168 -122 104

163 180

175

Table 3. IR νCtC Vibrations for 4[PF6]n and Closely Related Complexesa compd

KBrb

Nujolc

4 4[PF6] 4[PF6]2 4[TCNQ]2 4[PF6]3 4[OSO2CF3]3 4[PF6]4 4[OSO2CF3]4

2057 2056, 1968 1976 1972 1973 1963 1943 1968

2061 2057, 1974 1978 1961 1984

c

a Absorptions in cm-1. b Solid-state νCtC values obtained in KBr. νCtC values obtained in Nujol between NaCl plates.

Table 4. M€ ossbauer Parameters for 4[PF6]na (Cp*)(η2-dppe)Fe

Fc-Fc

compd

IS

QS

IS

QS

relative surface areas

4 4[PF6]

1.95 1.98 0.95 0.99 1.02

0.54 0.52

2.32 2.24

1:1 1:1:2

4[PF6]2 4[PF6]3

0.26 0.27 0.23 0.22 0.21 0.25

0.96

2.18 2.11 0.42 2.19

1:1 61:19:20

7[PF6]b

0.51 0.50 0.50 0.53

a

The velocity is referenced to the iron metal, data in mm/s. b From ref 11.

reported in the (Cp*)(η2-dppe)Fe series.22 As found in the naphthalene-bridged systems, the FedC bond distance is 0.01 A˚ shorter in 3 than in the related mononuclear vinylidene derivative [(Cp*)(η2-dppe)FedC(H)R][PF6] (R = CH3),23 while the C(37)-C(38) carbon-carbon double bond is longer by 0.01 A˚ in the former. The C(48)-C(48A) carbon-carbon (23) Mahias, V.; Cron, S.; Toupet, L.; Lapinte, C. Organometallics 1996, 15, 5399–5408.

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Organometallics, Vol. 29, No. 21, 2010 Table 7. NIR Data for 4[PF6]n in Dichloromethane

Table 5. EPR Data Determined in thf Glass at 66 K Cp*(η2-dppe)Fe compd 1þ 12þ 4þ 42þ 43þd 5þ 52þ 8þf

g2

g1

2.343 2.338 2.3

2.047 2.0690

2.464

2.033

e

g3

gisoa

Fc-Fc Δgb

g||

g^

Δgc

3.24 3.46 4.30 4.27 4.29 2.852 2.885

1.91 1.86 2.02 2.10 2.05 1.984 1.986

1.33 1.60 2.26 2.17 2.24 0.868 0.899

1.998 1.998 2.0

2.129 2.135 2.1

0.345 0.340 0.3

1.975

2.157

0.307

4809

giso = 1/3(g1 þ g2 þ g3). b Δg = g1 - g3. c Δg = g|| - g^. d Generated using [AgOTf]. e Spectrum obtained with a solid sample (Supporting Information). f From ref 11. a

Table 6. UV-Vis Absorption Spectral Data for 4[PF6]n (n = 0-3) and 4[OTf]4 in Dichloromethane at 298 K compd

absorption λ/nm (10-3 ε/dm3 mol-1 cm-1)

4 4[PF6] 4[PF6]2 4[PF6]3 4[OTf]4

348 (15, sh), 440 (5.8, sh) 345 (15, sh), 430 (5.2, sh), 560 (4.0), 620 (4.5) 345 (17, sh), 420 (3.6, sh), 550 (5.4), 620 (6.4) 345 (17, sh), 434 (9.9), 545 (8.8) 345 (21, sh), 492 (13.0)

bond distance in 3 is 0.03 A˚ shorter than in 4 (C(47)-C(47A)), 4[TCNQ]2 (C(46)-C(98)), and values usually found in the literature, which range between 1.45 and 1.47 A˚.4 This feature is associated with a shortening of the average carbon-carbon distances in the C5 ring attached to the iron vinylidene (1.42 A˚ in 3 vs 1.43 A˚ in 4). In addition, one notes a planar arrangement of the fulvalenide bridge as observed for most biferrocene derivatives.4,24 However, this effect, which probably originates from the withdrawing inductive effect due to the presence of cationic fragments, does propagate along the whole molecule. Indeed, the distance from the iron centers of the ferrocenyl moieties to the centroids of their cyclopentadienyl rings are all equivalent and very similar to those found for 4. Compound 4 and 4[TCNQ]2 exhibit common features; nevertheless several changes in individual bond lengths can be discerned upon oxidation. In particular, the two oneelectron oxidation processes (Figure 1) are evidenced by the typical lengthening of the Fe-P and Fe-Cp* distances,25,26 while examination of the iron to cylopentadienyl ligand bond lengths show that these distances range between 1.64 and 1.65 A˚, very close to the values found for neutral ferrocene (1.65 A˚)27 and quite far from those observed for the ferrocenium ion (1.70 A˚).24 In accordance with the CV data, structural analyses clearly confirmed that the oxidation is apparently centered on the terminal iron centers, while the biferrocenyl moiety is less affected within the limits of the accuracy of the X-ray experiment. Oxidation also results in measurable changes in the Fe-CtC-(C5H4) fragment. The iron-alkynyl and alkynyl-cyclopentadienyl distances shorten by ca. 2.2% and 1.4%, while concomitantly the carbon-carbon triple bonds elongate by ca. 1.5%. (24) Webb, R. J.; Dong, T.-Y.; Pierpont, C. P.; Boone, S. R.; Chadha, R. K.; Hendrickson, D. N. J. Am. Chem. Soc. 1991, 113, 4806–4812. (25) Roger, C.; Toupet, L.; Lapinte, C. J. Chem. Soc., Chem. Commun. 1988, 713–715. (26) de Montigny, F.; Argouarch, G.; Roisnel, T.; Toupet, L.; Lapinte, C.; Lam, S. C.-F.; Tao, C.-H.; Yam, V. W.-W. Organometallics 2008, 27, 1912–1923. (27) Seiler, P.; Dunitz, J. D. Acta Crystallogr. Sect. B 1979, 35.

compd

transition

4[PF6]

LF IVCT IVCT LF IVCT IVCT LF IVCT IVCT LF LF LF

4[PF6]2 4[PF6]3a 4[OTf]4 7þb

vmax (cm-1) ε (M-1 cm-1)

(Δv1/2)exp (cm-1)

4100 (160) 5550 (930) 7560 (2270) 4000 (320) 5550 (3220) 7780 (4930) not observed 5380 (1800) 8920 (3000) 3900 (30) 5250 (65) 7530 (50) 6290 (3400)

1700 1100 2700 2000 1100 2750 1560 3680 1050 1700 2100

(Δv1/2)theoc (cm-1) 3580 4180 3580 4240 3520 4540

a Determined by spectro-electrochemistry (OTTLE cell). ref 11. c Calculated from (Δν1/2)exp = (2310νmax)1/2.

b

From

Comparison of the bond distances in the biferrocenyl moiety reveals that this part of the molecule remains unchanged upon oxidation, or at least, changes are too small to be detected by X-ray crystallography. Despite their different symmetry, the general arrangements of neutral 4 and its salt 4[TCNQ]2 are quite similar, with the terminal iron centers pointing away from the central biferrocenyl bridging unit. The two C5H4 rings of this array are coplanar in 4 and 4[TCNQ]2. The three molecules feature a linear (as a result of the Ci symmetry in 4) or almost linear arrangement, and the dihedral angle Cp*(1)-Fe(1)Fe(10 )-Cp*(10 ) ranges between 168 and 180. Upon two-electron oxidation, the distance between the terminal iron atoms and the iron nucleus of the closest Fc unit shortens slightly, while a small lengthening of the distance between the iron nuclei of the biferrocenyl moiety can be noted. These structural changes might be the result of stronger interactions between the terminal metal groups and the nearest Fc neighbor than between the remote metal centers on one hand and the Fc units on the other hand. The increase of the distance between the terminal iron centers as the oxidation proceeds is also in line with this conclusion. 4. IR Spectroscopy. The five complexes 4[PF6]n (n = 0-4) are characterized by IR absorption bands in the range 1900-2100 cm-1 corresponding to the CtC stretching mode (Table 3). The IR spectrum of novel neutral complex 4 displays one νCtC band at 2061 cm-1, a frequency very close to that found for mono- and binuclear compounds of the (Cp*)(η2-dppe)FeCtC series.12,22 Oxidation of 4 to 4[PF6]2 results in a lowering of the IR stretching frequency (Δν = 83 cm-1, Nujol). It has been found that oxidation provokes a decrease of the IR frequency of the CtC bond, and the magnitude of this shift increases with the electrondonating character of the substituent attached to the iron acetylides.12 According to these data and considering the redox potential of the couple 4/4[PF6], a decrease of ca. 70 cm-1 might be expected for 4[PF6]2 if the biferrocenyl bridge does not convey any electronic interaction between the iron termini.12 The observed Δν value is slightly larger, suggesting that the (Cp*)(η2-dppe)Fe units do not behave as fully insulated mononuclear centers, and a weak interaction between the remote iron centers across the 10 ,1000 -bis(ethynyl)biferrocenyl bridge cannot be definitely excluded. More probably, the shift of the νCtC band can also result from an interaction between the proximal iron centers. Indeed, Sato and co-workers found for the IR spectra of

4810


Lohan et al.

Table 8. Crystal Data and Intensity Collection Data for 3, 4, and 4[TCNQ]2 3

4

4[TCNQ]2

fw 1741.98 1751.21 1901.28 C108H106Fe4P4 C114H100Fe4N6P4 chemical formula C96H96F12Fe4P6 cryst syst monoclinic monoclinic triclinic P1 space group C2/c P21/c a (A˚) 35.8439(12) 12.1381(11) 16.0350(14) b (A˚) 14.0561(5) 10.5715(9) 18.1012(16) c (A˚) 18.9557(7) 34.004(3) 18.2591(16) R (deg) 97.839(5) β (deg) 99.404(2) 97.060(4) 104.244(4) γ (deg) 96.104(5) 9422.0(6) 4330.2(7) 5034.5(8) V (A˚3) 1.228 1.343 1.254 Fcalc (g cm-3) F(000) 3620 1836 1980 cryst dimens(mm) 0.5 0.4 0.12 0.62 0.32 0.17 0.43 0.34 0.3 Z 4 2 2 max. and min. transmn 0.915, 0.720 0.876, 0.636 0.816, 0.742 0.742 0.781 0.679 abs coeff (λ, mm-1) θ range (deg) 2.93-27.48 2.92-27.48 2.39-27.32 range h, k, l -44/46, -18/17, -23/22 -23/22 -15/12, -12/13, -36/44 -20/20, -23/19, -23/23 total reflns 50 222 32 185 68 674 unique reflns 10 241 9831 22 791 0.0361 0.045 0.058 Rint data/restraints/params 10 241/0/503 9831/0/523 22 791/0/1157 1.066 1.055 1.032 goodness-of-fit (S) on F2 0.0424, 0.131 0.0455, 0.1181 0.0578, 0.1399 Ra, Rwpa [I g 2σ(I)] 0.0547, 0.137 0.0616, 0.1277 0.0829/0.1496 Ra, Rwpa (all data) -3 0.852, -0.602 1.201, -0.737 0.563, -0.898 max. and min. peak (e A˚ ) P P P P P a R = n||Fo| - |Fc||/ n|Fo|. Rwp = [ nw(Fo2 - Fc2)2/ nw(Fo2)2]1/2. S = [ nw(Fo2 - Fc2)2/(n - p)]1/2. n = number of reflections, p = parameters used.

complexes [(Cp*)(η2-dppe)FeCtCFc][PF6]n (n = 0, 1) a very similar shift of the CtC stretching vibration upon oneelectron oxidation (Δν = 83 cm-1).11 This feature (also supported by NIR, EPR, and M€ ossbauer spectra) was analyzed in terms of a dissymmetric class III mixed complex with a delocalization of the odd electron over the two iron centers. The IR spectrum of the mixed-valence complex 4[PF6] shows two bands, both slightly shifted with respect to those found for the homovalent complexes 4 and 4[PF6]2, respectively. Apparently, the MV complex features almost independent localized iron(II) and iron(III) alkynyl centers on the IR time scale. In accordance with Sato’s work, comparison of the IR spectra obtained for the three species 4, 4[PF6], and 4[PF6]2 suggests a strong interaction between the remote iron-alkynyl centers and the attached ferrocenyl fragment, while a weak interaction should take place between the two moieties of the molecule across the biferrocenyl linker. Tricationic 4[PF6]3 is characterized by a stretching band at a frequency very weakly shifted with respect to the parent dicationic complex 4[PF6]2, indicating that the terminal iron centers are not involved in this oxidation process, and furthermore, the structure of the alkynyl fragment remains unchanged. In contrast, the fourth oxidation process induced a lowering of the νCtC stretching mode of ca. 30 cm-1 for the hexafluorophosphate salt (Experimental Section). Note that, surprisingly, the frequency of the CtC stretching mode depends on the structure of the counteranion (Table 3). ossbauer spectra 5. 57Fe M€ossbauer Spectroscopy. The M€ of 4[PF6]n (n=0-4) were measured at 80 K and least-squares fitted with Lorentzian line shapes.28 The results are summarized in Table 4. The spectrum of the neutral complex 4 displays two doublets with the same surface area, indicating that two types of Fe(II) nuclei exist in 4. The QS values are

consistent with those of the half-sandwich complex (Cp*)(η2-dppe)Fe-CtC series (≈ 2.0 mm/s)5,20,29 and biferrocenyl derivatives (2.2 mm/s).24 Similar parameters were also found for the ferrocenylethynyl-bridged complex of ruthenium.10 The 57Fe M€ ossbauer spectrum of the doubly oxidized complex 4[PF6]2 also displays two doublets of the same surface areas. The parameters of the doublet assigned to the biferrocenyl moiety are almost unchanged, while the other doublet is now characteristic of the iron(III) termini [(Cp*)(η2-dppe)Fe-CC]þ.5,20,29 The two first oxidations are clearly centered on the iron alkynyl ends and very weakly concern the ferrocenyl moieties. Indeed, the QS value (2.18 mm/s) is only slightly smaller than that found for neutral 4 (2.32 mm/s), indicating that the iron nuclei of the biferrocenyl group sense the oxidation of the terminal iron atoms but are not directly involved in the oxidation processes. The small decrease of the IS values of the two doublets indicates that the electronic density decreases on the four iron atoms. Once again, the data reported by Sato were very ossbauer spectrum of [(Cp*)(η2-dppe) similar.11 The 57Fe M€ þ þ FeCtCFc] (7 ) clearly establishes that the oxidized site in this complex is the iron atom of the half-sandwich unit and not the one in the ferrocenyl part.11 The spectrum of monocation 4[PF6] exhibits three doublets with surface areas in a 1:1:2 ratio. The parameters of the more intense doublet are typical of the iron nuclei in the ferrocenyl environment. The IS and QS values are very close to the average between the values found for compounds 4 and 4[PF6]2. The two small doublets correspond to the halfsandwich termini present in the complex as iron(II) and iron(III) moieties The presence of two distinct doublets for the metal ends is diagnostic of localized valencies with a rate constant for the intramolecular electron transfer in the solid state ke >10-6 s-1. Moreover, the IS and QS parameters of

(28) Varret, F.; Mariot, J.-P.; Hamon, J.-R.; Astruc, D. Hyperfine Interact. 1988, 39, 67–81.

(29) Guillaume, V.; Thominot, P.; Coat, F.; Mari, A.; Lapinte, C. J. Organomet. Chem. 1998, 565, 75–80.

Article


4811

Figure 2. ORTEP drawings of 3 (top; hydrogen atoms, dppe ligands, and counterions are omitted for clarity), 4 (center; hydrogen atoms are omitted for clarity), and 4[TCNQ]2 (bottom; hydrogen atoms are omitted for clarity) with numbering of selected atoms. Thermal ellipsoids are at the 50% probability level.

4812


the terminal metals are characteristic of localized iron(II) and iron(III) centers, showing clearly that these groups do not strongly interact across the bridge. The tricationic species 4[PF6]3 is characterized by a 57Fe M€ ossbauer spectrum showing three doublets with relative surface areas in the 2:1:1 ratio. The IS and QS parameters of the more intense doublet are fully consistent with the (Cp*)(η2-dppe)Fe(III) termini, while the less intense doublets can be assigned to localized iron(II) and iron(III) centers in the biferrocenyl moiety. Indeed, the doublet with the larger QS possesses parameters similar to those found for the biferrocenyl fragment in 4, 4[PF6], and 4[PF6]2. The small QS value found for the other doublet matches quite well with the literature data for ferrocenium and biferrocenium derivatives.24,30,31 57 Fe M€ ossbauer spectra were run for two different samples of 4[PF6]4 independently prepared, but as they gave different spectra, it was concluded that the material partially decomposed before reliable data could be obtained. ossbauer paraFor complexes 4[PF6]n (n = 1-3), the M€ meters are consistent with iron-centered unpaired electrons on the spectroscopic time scale. Indeed, the presence of distinct doublets for the Fe(II) and Fe(III) centers present in the same molecule is diagnostic of electron transfer with exchange rate constants ke Hbc). Nevertheless, the different EPR data obtained for 7þ and 4[PF6]n and the different energies found for the transitions in 4[PF6]2 and 4[PF6]3 suggest that the intrabridge coupling Hbc is sizable. Considering the redox potentials reported in Table 1 and the moderate intensities of the NIR bands reported in Table 7, the MV complexes 4[PF6]n belong to class II of the classification proposed by Robin and Day.49 Brunschwig, Creutz, and Sutin proposed an interesting band shape and intensity analysis for this class of MV complexes, considering systems of increasing complexity from two- to four-state models.50 Their band shape analysis in the class II regime for MV compounds involving intermediate states to mediate the electron transfer established that for a given electronic coupling the bands are narrower than in the two-state treatment, in full agreement with our finding (Table 7). Finally, the variation of the free-energy of 4[PF6] with the reaction coordinates (X) for an electron transfer from one (Cp*)(η2-dppe)Fe unit (X = 0) to the opposite one (X = 1) is schematically represented in Figure 5. In this MV system, the mediating states -Fcþb-Fc- and -Fc-Fcþb- lie below the intersection of the reactant and product diabatic states. Such a rather unusual organization of the energy levels is supported by the experimental observation of the thermal population of the intermediate states. In the region, where X is close to 0.5, one can see that whatever the separation between the two adiabatic states (Had) at the intersection of the reactant and product free-energy curves, these states are too high in energy to allow a direct electron transfer. Similar energy curves can also be drawn to describe the electron transfer in the ruthenium iron MV system 5[PF6] but with (49) Robin, M. B.; Day, P. Adv. Inorg. Chem. Radiochem. 1967, 10, 247–422. (50) Brunschwig, B. S.; Creutz, C.; Sutin, N. Chem. Soc. Rev. 2002, 31, 168–184.

Figure 5. Schematic representation of the diabatic (dashed lines) and adiabatic (solid lines) free-energy curves for the electron transfer in 4[PF6].

two significant differences: (i) The separation between the energy levels of the reactant and product ground states and the intermediate state is much larger, in accord with more positive redox potentials found for the ruthenium termini than for the iron ends. (ii) A much better electronic coupling between the intermediate states in the ruthenium series was evidenced by EPR spectroscopy and a larger separation between the redox events corresponding to the successive one-electron oxidations of the ruthenium centers. As a consequence of these two main differences between the iron and the ruthenium series, the EPR signal did show the spectroscopic signatures of both the ground state and the mediating state in the ruthenium series but an averaged signal.4

Conclusion In this contribution, we have reported on the synthesis of complexes of type [10 ,1000 -((Cp*)(η2-dppe)Fe-CtC)2bfc][X]n (X = PF6, OTf, TCNQ; n = 0, 1, 2, 3, 4) with a biferrocenyl spacer. The new compounds were isolated and characterized under five different oxidation states. In the higher oxidation state, the thermal stability of powdered samples is limited to a few days, but for lower oxidation states (0 < n < 3) the samples are stable for periods of months in the solid state. Comparison of the structural parameters of the neutral and doubly oxidized species revealed that the two first oxidation processes mainly involve the [Fe]-CtC-(C5H4) fragment. In accordance with Sato’s work on compounds 7[PF6]n (n = 0, 1) the IR data reinforced the conclusions drawn from the X-ray analyses. Localization of the electron vacancies on the metal centers was proven by M€ ossbauer spectroscopy for the first four oxidation states of the compound. EPR spectroscopy permitted the characterization of two discrete radicals corresponding to the localization of the unpaired electron on the (Cp*)(η2-dppe)Fe units and the biferrocenyl bridging entity, respectively. The simultaneous observation of these two species shows that low-lying mediating states are thermally populated even at 66 K and demonstrates that a sequential electron transfer, in which the bridging biferrocenyl group becomes oxidized and reduced successively, takes place. EPR spectroscopy also established that the electron exchange rate is slow on the EPR time scale at 66 K. The NIR spectra of the MV species

4816


(n = 1, 2, 3) showed the presence of two bands, and their characteristic parameters are in accord with an electron transfer involving intermediate states. More in-depth analysis of this rich system requires theoretical calculations and will be the subject of further work.

Experimental Section General Data. Manipulations of air-sensitive compounds were performed under an argon atmosphere using standard Schlenk techniques or in an argon-filled Jacomex 532 drybox. All glassware was oven-dried and vacuum or argon flowdegassed before use. Fourier transform infrared (FT-IR) spectra were recorded using a Bruker IFS28 spectrophotometer (range 4000-400 cm-1) as solids dispersed in KBr pellets. UV-visible spectra were recorded on a UVIKON XL spectrometer. 1H, 13C, and 31P NMR spectra were recorded on a Bruker DPX200 NMR multinuclear spectrometer at ambient temperature, unless otherwise noted. Chemical shifts are reported in parts per million (δ) relative to tetramethylsilane (TMS) for 1H and 13C NMR spectra, and external 85% H3PO4 for 31P NMR spectra. Coupling constants (J) are reported in Hertz (Hz), and integrations are reported as number of protons. The following abbreviations are used to describe peak patterns: br = broad, s = singlet, d = doublet, t = triplet, pt = pseudotriplet, q = quartet, m = multiplet. Mass spectra were run with a micrOTOF-W II MS system (Bruker Daltonik GmbH) (capillary voltage 4500 V, nebulizer 0.4 bar, drygas 4 L/min/180 C, scanned mass range 50-2500). EPR spectra were recorded on a Bruker EMX-8/2.7 (X-band) spectrometer. Elemental analyses were conducted on a Thermo-Finnigan Flash EA 1112 CHNS/O analyzer by the Microanalytical Service of the Centre Regional de Mesures Physiques de l’Ouest (CRMPO) at the University of Rennes 1, France, and on a Thermo Flash EA 1112 Series instrument at Chemnitz, Technical University. Melting points of analytically pure samples (sealed off in nitrogen-purged capillaries) were determined using a Gallenkamp MFB 595 010 M melting point apparatus. Spectroelectrochemical measurements were carried out in an OTTLE cell similar to that described previously,54 from solutions in dichloromethane containing 10-1 M NBu4PF6 as supporting electrolyte using a Varian Cary spectrometer. Materials. Reagent grade toluene, tetrahydrofuran, diethyl ether, and n-pentane were dried and deoxygenated by distillation from sodium/benzophenone ketyl. Dichloromethane was distilled under argon from P2O5 and then from Na2CO3. Molecules 10 ,1000 -bis(ethynyl)biferrocene (1),4 (Cp*)(η2-dppe)FeCl (2),51 and FcH[PF6] (ferrocinium hexafluorophosphate)14 were prepared following published procedures. Potassium tertbutoxide (ACROS) and all other chemicals were purchased by commercial suppliers and were used without further purification. Synthesis of [1,1000 -((Cp*)(η2-dppe)FedCdCH)2bifc][PF6]2 (3). To a solution of 658 mg (1.05 mmol) of (Cp*)(η2-dppe)FeCl (2) and 172 mg (1.05 mmol) of [NH4]PF6 dissolved in 60 mL of methanol was added a tetrahydrofuran solution (30 mL) containing 200 mg (0.47 mmol) of 1,1000 -bis(ethynyl)biferrocene (1). The dark suspension was stirred for 16 h at ambient temperature. Then all volatiles of the dark reaction solution were removed under reduced pressure (oil-pump vacuum). The residue was dissolved in 20 mL of dichloromethane, and precipitation of 3 was initiated by addidtion of 20 mL of n-pentane. Afterward, the product was washed twice with n-pentane (10 mL) and dried in an oil-pump vacuum. The title complex was obtained as a brown solid. Yield: 874 mg (0.463 mmol, 97% based on 1). Anal. Calcd for C96H96Fe4P6F12 (1886.9 g/mol): C, 61.10; H, 5.12. Found: C, 60.82; H, 5.08. FT-IR (Nujol, ν/cm-1): 1629 (m, FedCdC). 1H (51) Roger, C.; Hamon, P.; Toupet, L.; Raba^a, H.; Saillard, J.-Y.; Hamon, J.-R.; Lapinte, C. Organometallics 1991, 10, 1045–1054.

Lohan et al. NMR (200 MHz, CDCl3, δ): 1.55 (s, 30 H, C5Me5); 2.5, 2.95 (2 m, 8 H, PCH2); 3.47 (pt, JHH = 1.8 Hz, 4 H, C5H4); 3.94 (pt, JHH = 1.8 Hz, 4 H, C5H4); 4.25 (m, 8 H, C5H4); 4.84 (m, 2 H, CH); 7.25-7.44 (m, 40 H, Ph/Hdppe). 31P{1H} NMR (81 MHz, CDCl3, δ): 90.2 (s, dppe). Synthesis of 1,1000 -((Cp*)(η2-dppe)Fe-CtC)2bifc (4). The base KOtBu (133 mg, 1.02 mmol) was added in a single portion to a tetrahydrofuran solution (30 mL) containing 3 (874 mg, 0.46 mmol). The resulting dark red reaction solution was stirred for 6 h at 25 C. Afterward, all volatiles were evaporated in an oilpump vacuum. The crude residue was extracted six times with toluene (15 mL each). Removal of the solvent under reduced pressure gave 4 as a dark red solid. Yield: 500 mg (0.31 mmol, 68% based on 3). Anal. Calcd for C96H94Fe4P4 3 C6D6 (1679.19 g/mol):a C, 72.95; H, 6.36. Found: C, 72.92; H, 6.3. Mp: 139 C. FT-IR (Nujol, ν/cm-1): 2056 (w, CtC). 1H NMR (200 MHz, CDCl3, δ): 1.54 (s, 30 H, C5Me5); 1.93, 2.8, (2 m, 8 H, PCH2); 3.97 (pt, JHH = 1.8 Hz, 4 H, C5H4); 4.17 (pt, JHH = 1.8 Hz, 4 H, C5H4); 4.24 (pt, JHH = 1.8 Hz, 4 H, C5H4); 4.44 (pt, JHH = 1.8 Hz, 4 H, C5H4); 7.1-7.31 (m, 32 H, Ph, Hdppe); 8.1 (m, 8 H, Ph, Hdppe). 13C{1H} NMR (50 MHz, CDCl3, δ): 10.28 (CH/C5Me5), 30.98 (CH2, dppe), 65.67 (Ci/C5H4), 67.58 (CH/C5H4), 68.31 (CH/C5H4), 69.23 (CH/C5H4), 70.38 (CH/C5H4), 84.41 (Ci/C5H4), 87.25 (C/C5Me5), 113.32 (Fe-CtC), 127.17 (d, JCP = Hz, Cm/C6H5), 128.68, 128.98 (s, Cp/C6H5), 134.21 134.69, (s, Co/C6H5), 138.11, 139.84 (m, Ci/C6H5). 31P{1H} NMR (81 MHz, CDCl3, δ): 101.6 (s, dppe). MS (ESI-TOF) m/z: 798.19 [M]2þ, 589.19 [M - CtC-bfc-CtC-Fe(η2-dppe)(Cp*)]þ, 1005.85 [M - Fe(η2-dppe)(Cp*)]þ. aMolecule 4 was crystallized by slow diffusion from n-pentane in a C6D6 solution containing 4. The obtained crystals contained one equivalent of C6D6. Synthesis of [1,1000 -((Cp*)(η2-dppe)FedCdC)2bifc][PF6]2 (4[PF6]2). To 200 mg (0.125 mmol) of 4 and 87 mg (0.262 mmol) of FcH[PF6] was added 25 mL of tetrahydrofuran at -60 C. The resulting dark reaction solution was stirred for 3 h at the same temperature. Then the reaction solution was concentrated in vacuo to 5 mL. Addition of 50 mL of n-pentane initiated the precipitation of a dark blue solid. Decantation and subsequent washing with 2 3 mL portions of n-pentane followed by 3 mL of diethyl ether were performed. The residue was dried in an oilpump vacuum, affording 4[PF6]2. Yield: 210 mg (0.111 mmol, 89% based on 4). Anal. Calcd for C96H94Fe4P6F12 (1884.97 g/mol): C, 61.16; H, 5.02. Found: C, 61.47; H, 5.08. FT-IR (KBr, ν/cm-1): 1976 (FedCdC). Synthesis of [1,1000 -((Cp*)(η2-dppe)FedCdC)2bifc][TCNQ]2 (4[TCNQ]2). To 50 mg (0.031 mmol) of 4 and 12.8 mg (0.062 mmol) of TCNQ was added 25 mL of tetrahydrofuran at -60 C. The resulting dark green solution was stirred for 3 h at this temperature and was then warmed to 25 C. Thereafter, the reaction solution was concentrated in vacuo to 5 mL. Addition of 50 mL of n-pentane initiated the precipitation of a dark green solid. Decantation, subsequent washing with 2 3 mL portions of n-pentane followed by 3 mL of diethyl ether, and drying the residue under vacuum yielded 4[TCNQ]2 (57 mg, 0.028 mmol, 91% based on 4), C120H102Fe4N8P4 (2003.42 g/ mol). FT-IR (Nujol, ν/cm-1): 1967, 1937 (FedCdC). Synthesis of [1,1000 -((Cp*)(η2-dppe)FedCdC)2bifc][PF6] (4[PF6]). To 30 mg (0.015 mmol) of 4[PF6]2 dissolved in 20 mL of tetrahydrofuran at -40 C was added 25.4 mg (0.015 mmol) of 4. The resulting reaction solution was stirred for 30 min at this temperature. Then it was concentrated in vacuo to ca. 5 mL. Addition of 50 mL of n-pentane initiated the precipitation of a dark solid. Decantation, subsequent washing with 2 3 mL of n-pentane followed by addition of 3 mL of diethyl ether, and drying the residue under vacuum produced 4[PF6]. Yield: 40 mg (0.023 mmol, 76%), C96H94Fe4P5F6 (1740.01 g/mol). FT-IR (KBr, ν/cm-1): 2056 (CtC), 1968 (FedCdC). Synthesis of [1,1 000 -((Cp*)(η 2-dppe)FedCdC)2bifc][PF6 ]3 (4[PF6]3). To a solution of 40 mg (0.021 mmol) of 4[PF6]2 in

Article 20 mL of tetrahydrofuran was added 0.94 mL (c = 0.0276 mmol/mL, 0.0259 mmol) of a tetrahydrofuran solution containing [AgPF6] at -60 C. The resulting dark reaction solution was stirred for 30 min at this temperature. Concentration of the reaction solution in vacuo to ca. 5 mL and addition of 50 mL of n-pentane gave a dark solid. Decantation, subsequent washing with 2 3 mL of n-pentane followed by addition of 3 mL portions of diethyl ether, and drying under vacuum produced 4[PF6 ]3 . Yield: 40 mg (0.019 mmol, 93%), C96H 94 Fe 4P7F18 (2029.94 g/mol). FT-IR (KBr, ν/cm -1): 1973 (FedCdC). Synthesis of [1,1000 -((Cp*)((η2-dppe)FedCdC)2bifc][PF6]4 (4[PF6]4). To 40 mg (0.021 mmol) of 4[PF6]2 in 20 mL of tetrahydrofuran was added 1.88 mL (c = 0.0276 mmol/mL, 0.0519 mmol) of a tetrahydrofuran solution containing [AgPF6] at -60 C. The resulting dark reaction solution was stirred for 30 min at this temperature. Then it was concentrated in vacuo to ca. 5 mL. Addition of 50 mL of n-pentane gave a dark precipitate. Decantation, subsequent washing with 2 3 mL portions of n-pentane followed by addition of 3 mL diethyl ether, and drying under vacuum yielded the desired product, 4[PF6]4. Yield: 40 mg (0.018 mmol, 87%), C96H94Fe4P8F24 (2174.9 g/mol). FT-IR (KBr, ν/cm-1): 1943 (FedCdC). Synthesis of [1,1000 -(η2-dppe(η5-C5Me5)FedCdC)2bifc][OTf]3 (4[OTf]3). To 50 mg (0.031 mmol) of 4 in 20 mL of tetrahydrofuran was added 23.9 mg (0.093 mmol) of [AgOTf] at -60 C. The resulting dark reaction solution was stirred for 30 min at this temperature. Then the solution was concentrated in vacuo to 5 mL. Addition of 50 mL of n-pentane allowed precipitation of a dark solid. Decantation, subsequent washing with n-pentane, and drying under vacuum yielded the desired product, 4[OTf]3. Yield: 60 mg (0.029 mmol, 94%). Synthesis of [1,1000 -(η2-dppe(η5-C5Me5)FedCdC)2bifc][OTf]4 (4[OTf]4). To 50 mg (0.031 mmol) of 4 in 20 mL of tetrahydrofuran was added 31.8 mg (0.124 mmol) of [AgOTf] at -60 C.


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The resulting dark solution was stirred for 30 min at this temperature. Then the solution was concentrated in vacuo to 5 mL. Addition of 50 mL of n-pentane allowed precipitation of a dark solid. Decantation, subsequent washing with n-pentane, and drying under vacuum yielded the desired product, 4[OTf]4. Yield: 65 mg (0.029 mmol, 95%). Crystallography. Crystal data for 3, 4, and 4[TCNQ]2 are presented in Table 8. All data were collected on a APEXII ABSBruker diffractometer with graphite-monochromatized Mo KR radiation (λ = 0.71073 A˚) at 100 K using oil-coated shockcooled crystals. The structures were solved by direct methods using SIR-9752 and refined by full-matrix least-squares procedures on F2 using SHELXL-97.53 All non-hydrogen atoms were refined anisotropically, and a riding model was employed in the refinement of the hydrogen atom positions.

Acknowledgment. We are grateful to the Deutsche Forschungsgemeinschaft and to the Fonds der Chemischen Industrie for generous financial support. Supporting Information Available: Crystallographic files in CIF format for the three reported X-ray crystal structures. This material is available free of charge via the Internet at http:// pubs.acs.org. Crystallographic data for complexes 3, 4, and 4[TCNQ]2 are also deposited with the Cambridge Crystallographic Data Centre (CCDC 757204, 757205, and 757206) and can be obtained free of charge from the CCDC via http://www. ccdc.cam.ac.uk/data_request/cif. Figure showing the IR spectra of 4[PF6]n (n = 0-3) and 4[OTf]4. (52) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115–119. (53) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112–122. (54) Krejcik, M.; Danek, M.; Hartl, F. J. Electroanal. Chem. 1991, 317, 179.

Bis(ethynyl) - American Chemical Society

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