A Concerted Double Trigonal-Twist in Bis ... - ACS Publications

Nov 16, 2010 - †Department of Chemistry, University College London, 20 Gordon Street, ... Jahangirnagar University, Savar, Dhaka-1342, Bangladesh...
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Organometallics 2010, 29, 6559–6568 DOI: 10.1021/om100894w

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Diphosphine Mobility at a Binuclear Metal Center: A Concerted Double Trigonal-Twist in Bis(dithiolate) Complexes [M2(CO)4(μ-dppm){μ-S(CH2)nS}] (M = Fe, Ru; n = 2, 3) Graeme Hogarth,*,† Shariff E. Kabir,‡ and Idris Richards† †

Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, U.K., and ‡ Department of Chemistry, Jahangirnagar University, Savar, Dhaka-1342, Bangladesh Received September 16, 2010

Heating [M2(CO)6{μ-S(CH2)nS}] (M=Fe, Ru; n=2 (edt), 3 (pdt)) with bis(diphenylphosphino)methane (dppm) in toluene affords the bridged-diphosphine complexes [M2(CO)4( μ-dppm){μ-S(CH2)nS}]. At room temperature, all show two separate environments for the methylene protons of the diphosphine ligand, while at higher temperatures these coalesce to a single peak. This behavior, which interconverts the two sulfur atoms, is ascribed to a concerted double trigonal-twist of the M(CO)2P moieties. No such fluxional behavior was observed for the nonlinked dithiolate complexes [Fe 2 (CO)4 (μ-dppm)(μ-SR)2 ] (R=Me, Ph, p-tolyl). The X-ray structures of [M2(CO)4( μ-dppm)( μ-edt)] (M=Fe, Ru) and [Fe2(CO)4(μ-dppm)( μSMe)2] are presented in order to compare them to the previously reported [M2(CO)4( μ-dppm)( μ-pdt)].

Introduction Dithiolate-bridged diiron complexes, [Fe2(CO)6( μ-dithiolate)], have been known for 45 years1 but are currently the focus of intense interest2-6 resulting from the realization that they closely resemble the two-iron unit of the H-cluster active site of iron-only hydrogenases.7-9 Consequently, the chemistry of [Fe2(CO)6{ μ-S(CH2)nS}] (n = 2, 3; edt, pdt)10,11 and of [Fe2(CO)6{ μ-SCH2N(R)CH2S}]12 have been intensively studied. A key step in the electrocatalytic conversion of protons to hydrogen at the active center of iron-only hydrogenases is believed to be the initial coordination of the proton(s). The hexacarbonyls themselves are not basic enough to bind a proton strongly, and the main approach adopted to circumvent this problem is to substitute one or more carbonyls for more basic phosphine or cyanide ligands. As a result of

this, a large number of phosphine-substituted derivatives of these complexes have been reported.13-36

*To whom correspondence should be addressed. E-mail: g.hogarth@ ucl.ac.uk. (1) King, R. B. J. Am. Chem. Soc. 1962, 84, 2460. (2) Georgakaki, I. P.; Thomson, L. M.; Lyon, E. J.; Hall, M. B.; Darensbourg, M. Y. Coord. Chem. Rev. 2003, 238-239, 255. (3) Evans, D. J.; Pickett, C. J. Chem. Soc. Rev. 2003, 32, 268. (4) Rauchfuss, T. B. Inorg. Chem. 2004, 43, 14. (5) Sun, L.; A˚kermark, B.; Ott, S. Coord. Chem. Rev. 2005, 249, 1653. (6) Liu, X.; Ibrahim, S. K.; Tard, C.; Pickett, C. J. Coord. Chem. Rev. 2005, 249, 1641. (7) Nicolet, Y.; Piras, C.; Legrand, P.; Hatchikian, C. E.; FontecillaCamps, J. C. Structure 1999, 7, 13. (8) Peters, J. W.; Lanzilotta, W. N.; Lemon, B.; Seefeldt, L. C. Science 1998, 282, 1853. (9) Lemon, B. J.; Peters, J. W. Biochemistry 1999, 38, 12969. (10) Seyferth, D.; Womack, G. B.; Gallagher, M. K.; Cowie, M.; Hames, B. W.; Fackler, J. P.; Mazany, A. M. Organometallics 1987, 6, 283. (11) Seyferth, D.; Henderson, R. S. J. Organomet. Chem. 1981, 218, C34. (12) Li, H.; Rauchfuss, T. B. J. Am. Chem. Soc. 2002, 124, 726. (13) Adam, F. A.; Hogarth, G.; Richards, I. J. Organomet. Chem. 2007, 692, 3957. (14) Gao, W.; Ekstr€ om, J.; Liu, J.; Chen, C.; Eriksson, L.; Weng, L.; A˚kermark, B.; Sun, L. Inorg. Chem. 2007, 46, 1981.

(15) Lyon, E. J.; Georgakaki, I. P.; Reibenspies, J. H.; Darensbourg, M. Y. Angew. Chem., Int. Ed. 1999, 38, 3178. (16) Lyon, E. J.; Georgakaki, I. P.; Reibenspies, J. H.; Darensbourg, M. Y. J. Am. Chem. Soc. 2001, 123, 3268. (17) Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Yarbrough, J. C.; Darensbourg, M. Y. J. Am. Chem. Soc. 2001, 123, 9710. (18) Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Mejia-Rodriguez, R.; Chiang, C.-Y.; Darensbourg, M. Y. Inorg. Chem. 2002, 41, 3917. (19) Chong, D.; Georgakaki, I.P.; R. Mejia-Rodriguez, J. SanabriaChinchilla, Soriaga, M.P.; Darensbourg, M.Y. Dalton Trans. 2003, 4158. (20) Mejia-Rodriguez, R.; Chong, D.; Reibenspies, J. H.; Soriaga, M. P.; Darensbourg, M. Y. J. Am. Chem. Soc. 2004, 126, 12004. (21) Gloaguen, F.; Lawrence, J. D.; Rauchfuss, T. B. J. Am. Chem. Soc. 2001, 123, 9476. (22) Gloaguen, F.; Lawrence, J. D.; Rauchfuss, T. B.; Benard, M.; Rohmer, M.-M. Inorg. Chem. 2002, 41, 6573. (23) Nehring, J.; Heinekey, D. M. Inorg. Chem. 2003, 42, 4288. (24) Capon, J.-F.; Gloaguen, F.; Schollhammer, P.; Talarmin, J. J. Electroanal. Chem. 2004, 566, 241. (25) Capon, J.-F.; El Hassnaoui, S.; Gloaguen, F.; Schollhammer, P.; Talarmin, J. Organometallics 2005, 24, 2020. (26) Vijaikanth, V.; Gloaguen, F.; Schollhammer, P.; Talarmin, J. Electrochem. Commun. 2005, 7, 747. (27) Morvan, D.; Capon, J.-F.; Gloaguen, F.; Le Goff, A.; Marchivie, M.; Michaud, F.; Schollhammer, P.; Talarmin, J.; Yaouanc, J.-J.; Pichon, R.; Kervarec, N. Organometallics 2007, 26, 2042. (28) Ezzaher, S.; Capon, J.-F.; Gloaguen, F.; Petillon, F. Y.; Schollhammer, P.; Talarmin, J.; Pichon, R.; Kervarec, N. Inorg. Chem. 2007, 46, 3426. (29) Li, P.; Wang, M.; He, C.; Li, G.; Liu, X.; Chen, C.; A˚kermark, B.; Sun, L. Eur. J. Inorg. Chem. 2005, 44, 2506. (30) Gao, W.; Liu, J.; A˚kermark, B.; Sun, L. Inorg. Chem. 2006, 45, 9169. (31) Gao, W.; Liu, J.; A˚kermark, B.; Sun, L. J. Organomet. Chem. 2007, 692, 1579. (32) Dong, W.; Wang, M.; Liu, T.; Liu, X.; Jin, K.; Sun, L. J. Inorg. Biochem. 2007, 101, 506. (33) Duan, L.; Wang, M.; Li, P.; Wang, N.; Sun, L. Dalton Trans. 2007, 1277. (34) Hogarth, G.; Richards, I. Inorg. Chem. Commun. 2007, 10, 66. (35) Adam, F.I.; Hogarth, G.; Richards, I.; Sanchez, B. E. Dalton Trans. 2007, 2495. (36) Adam, F. I.; Hogarth, G.; Kabir, S. E.; Richards, I. C. R. Chim. 2008, 11, 890.

r 2010 American Chemical Society

Published on Web 11/16/2010

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Organometallics, Vol. 29, No. 23, 2010 Scheme 1

In a recent contribution the dppm-bridged tetracarbonyl complex [Fe2(CO)4( μ-dppm)( μ-pdt)] was prepared upon heating [Fe2(CO)6( μ-pdt)] and dppm in refluxing toluene.13,14 In the solid state, the two sulfur atoms are inequivalent, one lying trans to the diphosphine and the other cis. However, when a d8-toluene solution of [Fe2(CO)4( μ-dppm)( μ-pdt)] was heated while monitoring by 1H NMR spectroscopy the inequivalent methylene protons broadened and collapsed. We attributed these observations to a fluxional process that involved a concerted double trigonal-twist of the two Fe(CO)2P groups.13 In order to fully investigate this process, we have now prepared and examined by high-temperature 1H NMR spectroscopy the series of dppm-bridged complexes [M2(CO)4( μ-dppm){ μ-S(CH2)nS}] (M=Fe, Ru; n=2, 3) with the anticipation that the edt complexes (n=2) would offer extra insight since the methylene protons of the dithiolate ligand would also act as a useful probe, while analysis of related ruthenium complexes would help to shed light on the proposed mechanistic details.

Results and Discussion i. Synthesis and Characterization. Three of the dppmbridged complexes prepared for this study have previously been reported. As detailed in the Introduction, [Fe2(CO)4( μ-dppm)(μ-pdt)] can prepared upon heating [Fe2(CO)6( μ-pdt)] with dppm in toluene,13 a process that occurs via monodentate [Fe2(CO)5(κ1-dppm)( μ-pdt)].13,14 The two ruthenium complexes [Ru2(CO)4( μ-dppm)( μ-pdt)] and [Ru2(CO)4(μ-dppm)( μ-edt)] have previously been prepared in moderate yields upon heating [Ru3(CO)10( μ-dppm)] with the appropriate dithiol.37 We have now found that both ruthenium complexes can be prepared in essentially quantitative yields upon refluxing the parent hexacarbonyls and dppm in toluene for 1-2 h (Scheme 1). The diiron complex [Fe2(CO)4( μ-dppm)( μ-edt)] appears not to have been previously reported. It is prepared in moderate yield upon thermolysis of [Fe2(CO)6( μ-edt)] with dppm in toluene for 12-14 h in the presence of Me3NO. Monitoring the reaction of [Fe2(CO)6( μ-edt)] in the absence of Me3NO revealed the formation of intermediates [Fe2(CO)5(κ1-dppm)( μedt)] and [Fe2(CO)4(κ2-dppm)( μ-edt)] (see Experimental Section), but these were not isolated in pure form. All complexes are air stable in the solid state. The diiron complexes decompose slowly in many organic solvents, but this is sufficiently slow to allow crystallization in air. The diruthenium complexes appear to be indefinitely stable in both the solid state and solution. IR data are very characteristic for all four complexes. For example, [Fe2(CO)4( μ-dppm)( μ-edt)] has carbonyl absorptions at 1991m, 1958vs, 1924s, and 1905sh cm-1, while for the analogous diruthenium complexes they appear at 2004m, 1980vs, 1939s, and 1922sh cm-1. The higher frequencies of the latter (average 17 cm-1) is typical and can be compared to the homologous series of related 1,2-benzenedithiolate complexes [M2(CO)6( μ-SC6H4S)] (M=Fe, Ru, Os) prepared by Riera and (37) Hossain, G. M. G.; Hyder, Md. I.; Kabir, S. E.; Malik, K. M. A.; Miah, Md. A.; Siddiquee, T. A. Polyhedron 2003, 22, 633.

Hogarth et al.

co-workers38 and [M2(CO)6( μ-edt)].11,39-45 As expected, each complex is characterized by a singlet resonance in the 31P NMR spectrum, showing the equivalent nature of the phosphorus atoms. ii. Solid-State Structures. In previous work, single-crystal X-ray structures of [Fe2(CO)4( μ-dppm)( μ-pdt)]13,14 and [Ru2(CO)4( μ-dppm)( μ-pdt)]37 were reported. In order to fully characterize the full series, we have now carried out X-ray analyses of [Fe2(CO)4( μ-dppm)( μ-edt)] and [Ru2(CO)4( μdppm)( μ-edt)]. Large red truncated octahedral crystals of [Fe2(CO)4( μ-dppm)( μ-edt)] were grown upon slow diffusion of methanol into a saturated dichloromethane solution. The molecule crystallizes in the tetragonal space group, P4(2)/ mbc, and the single molecule has a plane of symmetry that contains the three methylene carbons and the two sulfur atoms (Figure 1). The diruthenium complex [Ru2(CO)4( μdppm)( μ-edt)] was also crystallized upon slow diffusion of methanol into a saturated dichloromethane solution. Very large yellow plates resulted, and one of these was cut to give a large yellow block. The molecule crystallizes in the monoclinic space group, Cc, with three independent molecules in the asymmetric unit. This situation is somewhat akin to that found for [Fe2(CO)4( μ-dppm)( μ-pdt)], which also has three independent molecules in the asymmetric unit, but in this case there are also solvent molecules.13 The three independent molecules of [Ru2(CO)4( μ-dppm)( μ-edt)] do not differ significantly, and thus only one is shown (Figure 2). Key structural parameters for both [Fe2(CO)4( μ-dppm)( μ-edt)] and [Ru2(CO)4( μ-dppm)( μ-edt)] together with the corresponding pdt complexes13,14,37 and the parent edt complexes [Fe2(CO)6( μ-edt)]40-43 and [Ru2(CO)6( μ-edt)]39 are given in Table 1. Metric parameters for [Fe2(CO)4( μ-dppm)( μ-edt)] are very similar to those found in the hexacarbonyl [Fe2(CO)6( μedt)]. The iron-iron bond shortens slightly upon addition of the diphosphine and is also somewhat shorter than those found in the [Fe2(CO)4( μ-dppm)( μ-pdt)]. Iron-sulfur bonds also vary only slightly upon diphosphine addition, that to the sulfur trans to the diphosphine being very slightly shorter than that to the cis thiolate sulfur, but these differences are only small and lie within the range of iron-sulfur bonds in [Fe2(CO)6( μedt)]. Hence, it can be seen that the diphosphine does not exert any significant trans influence on the thiolate bridge. The ruthenium-ruthenium bond length in [Ru2(CO)6( μ-edt)] lies in between the range of those found in the three independent molecules of [Ru2(CO)4( μ-dppm)( μ-edt)], and thus it can be concluded that diphosphine addition has no significant effect on this vector. All the ruthenium-sulfur distances in the latter are slightly longer than those in the hexacarbonyl, with those cis to the diphosphine being slightly longer than those trans to it. This slight elongation of the cis thiolate group is reflected in the (38) Cabeza, J. A.; Martinez-Garcia, M. A.; Riera, V.; Ardura, D.; Garcia-Grande, S. Organometallics 1998, 17, 1471. (39) Hanif, K. M.; Kabir, S. E.; Mottalib, M. A.; Hursthouse, M. B.; Malik, K. M. A.; Rosenberg, E. Polyhedron 2000, 19, 1073. (40) Hasan, M. M.; Hursthouse, M. B.; Kabir, S. E.; Malik, K. M. A. Polyhedron 2001, 20, 97. (41) Hughes, D. L.; Leigh, G. J.; Paulson, D. R. Inorg. Chim. Acta 1986, 120, 191. (42) Messelhaeuser, J.; Lorenz, I. P.; Haug, K.; Hiller, W. Z. Naturforsch. Teil B 1985, 40, 1064. (43) Kabir, S. E.; Rahman, A. F. M. M.; Parvin, J.; Malik, K. M. A. Ind. J. Chem., Sect. A 2003, 42, 2518. (44) Adams, R. D.; Yamamoto, J. H. Organometallics 1995, 14, 3704. (45) Adams, R. D.; Chen, L.; Yamamoto, J. H. Inorg. Chim. Acta 1995, 229, 47.

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Figure 1. Two views of the molecular structure of [Fe2(CO)4(μ-dppm)(μ-edt)].

Figure 2. Two views of the molecular structure of one molecule of [Ru2(CO)4(μ-dppm)(μ-edt)].

decreasing angle it subtends with the two ruthenium centers. A similar effect is seen in [Ru2(CO)4( μ-dppm)( μ-pdt)]. The iron-phosphorus bond lengths do not vary significantly between [Fe2(CO)4( μ-dppm)( μ-edt)] and [Fe2(CO)4( μ-dppm)( μ-pdt)], and those in [Ru2(CO)4( μ-dppm)( μ-edt)] are only slightly shorter than in [Ru2(CO)4( μ-dppm)( μ-pdt)]. As can clearly be seen in Figures 1 and 2, diphosphine coordination differentiates the two sulfur atoms: one lying trans and the second cis. In the iron complexes, the P-M-Strans bond angles are similar, varying only between 152.9° and 154.3°, but some more notable differences are seen in the two ruthenium complexes. Thus in [Ru2(CO)4( μ-dppm)( μ-edt)], the P-M-Strans angles vary by nearly seven degrees (146.6-153.3°) in a single independent molecule. The difference is less pronounced in [Ru2(CO)4( μ-dppm)( μ-pdt)] (3.3°) but is also seen in the P-M-Scis bond angles in both complexes. These differences between the iron and ruthenium complexes result from the less planar nature of the M2P2 section of the M2P2C fivemembered ring in the second-row versus the first-row transition metal complexes. Thus in [Fe2(CO)4( μ-dppm)( μ-edt)] all four non-carbon atoms lie within 0.165 A˚ of the plane, while in one molecule of [Ru2(CO)4( μ-dppm)( μ-edt)], one of the phos-

phorus atoms [P(1)] is displaced 0.307 A˚ from the plane. Thus, the M2P2C is less planar in the ruthenium versus the iron complexes. This results from increases in the metal-metal and metal-phosphorus bond lengths in the second-row complexes, while the phosphorus-carbon bonds remain unchanged. The distortion of the five-membered M2P2C ring is proposed to play a key role in the fluxionality of these complexes (see below). We also carried out the X-ray structure of [Fe2(CO)4( μdppm)( μ-SMe)2] (Figure 3) in order to understand why the diphosphine is not fluxional in this and other nonlinked dithiolate complexes (see below). The main feature is the anti arrangement (Scheme 2) of the two methyl groups, being similar to that previously observed in [Fe2(CO)4( μ-dppm)( μ-SAr)2] (Ar=Ph, p-tol).46 This is presumably adopted in order to minimize steric interactions between the two thiolate substituents and also between them and the phenyl rings of the diphosphine. All metric parameters are very similar to those in [Fe2(CO)4( μ-dppm)( μ-edt)], the iron-iron bond length of 2.5082(6) A˚ being about 0.02 A˚ longer. (46) Hogarth, G.; O’Brien, M.; Tocher, D. A. J. Organomet. Chem. 2003, 672, 29.

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Table 1. Selected Bond Lengths (A˚) and Angles (deg) for [M2(CO)4(μ-dppm){μ-S(CH2)nS}] (M = Fe, Ru; n = 2, 3), [Fe2(CO)4(μdppm)(μ-SMe)2], and [M2(CO)6(μ-edt)] compound [Fe2(CO)6(μ-edt)] 293 K 193 Ka,c

a,b

d,e

M-M

M-Strans

2.497(4) 2.502(1) 2.5032(5)

2.245(2) 2.239(1) 2.2493(7) 2.2440(6) 2.2403(5) 2.260(1) 2.259(1) 2.244(1) 2.2480(6) 2.375(2) 2.374(2) 2.381(1) 2.380(1) 2.379(1) 2.378(1) 2.379(1) 2.378(1) 2.392(1) 2.389(1) 2.2472(9) 2.2537(9)

[Fe2(CO)4(μ-dppm)(μ-edt)] [Fe2(CO)4(μ-dppm)(μ-pdt)]f

2.4863(5) 2.5076(9)

[Ru2(CO)6(μ-edt)]g

2.510(1) 2.513(1) 2.6790(8)

[Ru2(CO)4(μ-dppm)(μ-edt)]

2.6830(7) 2.6730(7) 2.6823(8)

[Ru2(CO)4(μ-dppm)(μ-pdt)]h

2.6720(7)

[Fe2(CO)4(μ-dppm)(μ-SMe)2]

2.5082(6)

M-Scis

2.2468(6) 2.2539(7) 2.2507(5) 2.253(1) 2.249(1) 2.253(2) 2.255(2) 2.370(2) 2.370(2) 2.382(1) 2.400(1) 2.394(1) 2.408(1) 2.394(1) 2.411(1) 2.407(1) 2.410(1) 2.2616(9) 2.2787(8)

M-P

M-COap

M-COba

2.2145(4) 2.213(1) 2.210(1) 2.217(1) 2.215(1)

1.778(2) 1.782(5) 1.766(5) 1.774(6) 1.754(5)

1.767(2) 1.767(5) 1.769(5) 1.779(5) 1.771(5)

2.316(1) 2.321(1) 2.319(1) 2.319(1) 2.318(1) 2.319(1) 2.324(1) 2.343(1) 2.2131(9) 2.2235(9)

1.893(5) 1.904(5) 1.886(5) 1.911(6) 1.894(6) 1.904(6) 1.904(5) 1.889(5) 1.771(4) 1.772(3)

1.897(5) 1.876(5) 1.884(5) 1.874(5) 1.889(5) 1.874(5) 1.842(6) 1.870(5) 1.747(3) 1.747(3)

M-Strans-M

M-Scis-M

67.6(2) 67.92(3) 67.71(2)

67.59(2)

67.41(2) 67.41(4)

67.06(2) 67.70(4)

68.01(5) 67.97(5) 68.69(5)

67.69(6) 67.71(6) 68.84(4)

68.60(4)

68.25(4)

68.38(4)

67.64(4)

68.65(4)

67.86(4)

67.96(3)

67.37(3)

67.73(3)

67.07(3)

P-M-Strans

P-M-Scis

154.27(2) 152.90(5) 153.26(5) 152.18(5) 153.66(5)

86.13(2) 85.12(4) 85.56(5) 86.07(5) 83.72(5)

146.56(4) 153.28(4) 148.22(4) 152.46(4) 147.59(5) 152.56(5) 152.45(4) 149.12(4) 156.43(3) 147.69(3)

89.38(4) 86.23(4) 89.08(4) 85.88(4) 88.99(5) 86.25(4) 86.18(4) 84.62(4) 84.57(3) 89.19(3)

a Two molecules in the asymmetric unit; values given are averages. b D. L. Hughes, G. J. Leigh, and D. R. Paulson, Inorg. Chim. Acta 1986, 120, 191. J. Messelhaeuser, I. P. Lorenz, K. Haug, and W. Hiller, Z. Naturforsch. Teil B 1985, 40, 1064. d Cocrystallizes with ferrocene. e S. E. Kabir, A. F. M. M. Rahman, J. Parvin, and K. M. A. Malik, Ind. J. Chem., Sect. A 2003, 42, 2518. f F. A. Adam, G. Hogarth, and I. Richards, J. Organomet. Chem. 2007, 692, 3957. g K. M. Hanif, S. E. Kabir, M. A. Mottalib, M. B. Hursthouse, K. M. A. Malik, and E. Rosenberg, Polyhedron 2000, 19, 1073. h G. M. G. Hossain, Md. I. Hyder, S. E. Kabir, K. M. A. Malik, Md. A. Miah, and T. A. Siddiquee, Polyhedron 2003, 22, 633. c

Scheme 2

Figure 3. Molecular structure of [Fe2(CO)4(μ-SMe)2(μ-dppm)].

iii. Fluxionality. At low temperatures all four linked dithiolate complexes show inequivalent methylene protons of the diphosphine ligand, and upon warming, these broaden and coalesce into a triplet, behavior illustrated (Figure 4) for [Fe2(CO)4( μ-dppm)( μ-edt)] in d8-toluene. At 253 K the methylene protons of the diphosphine appear as a pair of doublets of triplets at δ 3.600 and 3.039, which upon warming collapse (ca. 328 K), reappearing as a triplet at δ 3.54 at 373 K. Concomitant changes are observed for the methylene protons of the edt ligands (multiplets at δ 2.026 and 1.944 at 253 K and a sharp singlet at δ 2.104 at 373 K) and the two

equal-intensity low-field aromatic resonances (multiplets at δ 7.488 and 7.391 at 253 K and a sharp multiplet at δ 7.524 at 373 K). Using the modified Eyring equation47 free energies of activation can be estimated from the methylene protons of the diphosphine (Tc = 328 K, Δν 225 Hz; ΔG# 63.6 ( 0.5 kJ mol-1), the methylene protons of the edt ligand (Tc = 303 K, Δν 33 Hz; ΔG# 63.4 ( 0.5 kJ mol-1), and the low-field aromatic resonances (Tc = 308 K, Δν 39 Hz; ΔG# 64.0 ( 0.5 kJ mol-1). Thus, we conclude that the free energy of activation for [Fe2(CO)4( μ-dppm)( μ-edt)] is around 63.7 ( 1 kJ mol-1. Previous analysis of VT NMR spectra of [Fe2(CO)4( μ-dppm)( μ-pdt)]13 led to the estimation of a free energy of activation of ca. 61.4 ( 1 kJ mol-1. This value was based solely on the coalescence of the methylene protons of the diphosphine since the other regions were less informative. The ruthenium complexes behave analogously. For [Ru2(CO)4( μ-dppm)( μ-edt)] the methylene protons of the diphosphine appear as two sharp and well-resolved doublets of triplets at δ 3.536 and 3.299 at room temperature. The methylene protons of the dithiolate bridge are also well-resolved at this temperature, appearing as multiplets at δ 2.153 and 2.035, and the two low-field aromatic signals appear as multiplets centered at δ 7.495 and 7.252. Upon warming, all signals broaden and coalesce. The presence of the residual toluene methyl resonance makes estimation of the coalescence temperature of the (47) G€ unther, H. NMR Spectroscopy; John Wiley: Chichester, U.K., 1980; p 243.

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Figure 4. Variable-temperature 1H NMR spectra of the diphosphine methylene region of [Fe2(CO)4(μ-dppm)(μ-edt)].

methylene groups of the dithiolate bridge difficult to estimate accurately. Free energies of activation can be estimated from the methylene protons of the diphosphine (Tc =368 K, Δν 95 Hz) and the low-field aromatic resonances (Tc=368 K, Δν 97 Hz) as 74.4 ( 0.5 and 74.3 ( 0.5 kJ mol-1, respectively. Similar changes are observed for [Ru2(CO)4( μ-dppm)( μ-pdt)]. The central methylene group of the dithiolate ligand is partially obscured by the solvent, but the methylene protons of the diphosphine appear as two sharp doublets of triplets at δ 3.692 and 3.015, which collapse upon heating, although even at 373 K they are not fully coalesced. On estimating the coalescence temperature

at >378 K (Δν 271 Hz), a free energy of activation of g73.2 kJ mol-1 can be estimated. Most informative are the low-field aromatic resonances (Tc = 373 K, Δν 133 Hz), which give a free energy of activation of 74.4 ( 0.5 kJ mol-1. All free energies of activation are summarized in Table 2. It is clear from these measurements that that the free energy of activation for the fluxional process does not vary significantly with the nature of the dithiolate ligand. This suggests that this remains innocent throughout and does not rearrange significantly during the process. There is, however, a significant difference between the iron and ruthenium

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Table 2. Free Energies of Activation for the Fluxional Process in [M2(CO)4(μ-dppm){μ-S(CH2)nS}] (M = Fe, Ru; n = 2, 3)

Scheme 5

ΔG# ((1 kJ mol-1) [Fe2(CO)4(μ-dppm)(μ-edt)] [Fe2(CO)4(μ-dppm)(μ-pdt)] [Ru2(CO)4(μ-dppm)(μ-edt)] [Ru2(CO)4(μ-dppm)(μ-pdt)]

63.7 61.4 74.3-74.4 74.4

Scheme 3

Scheme 4

complexes, with free energies of activation in the latter being ca. 10 kJ mol-1 higher. Interestingly, when the nonlinked dithiolate complexes [Fe2(CO)4( μ-dppm)( μ-SAr)2] (Ar = Ph, p-tolyl)46 were heated to 100 °C in d8-toluene, no evidence for any fluxional process was noted, two clearly defined and well-resolved methylene signals being seen throughout. We initially considered that this may be a consequence of the large aryl substituents; however, similar behavior was also noted for [Fe2(CO)4( μdppm)( μ-SMe)2].48 This also shows that the anti arrangement of thiolate bridges is maintained even at higher temperatures. iv. Mechanistic Implications. The exchange of apical and basal carbonyl groups in [Fe2(CO)6( μ-dithiolate)] is well documented,16,18,49-51 and since the metal coordination geometry is best described as a square-based pyramid, this process has been termed pyramidal rotation by Darensbourg.16 We prefer the more widely used term trigonal rotation while accepting that since the geometry is not octahedral this is not a true description (Scheme 3). Activation barriers for this process are affected by the nature of the dithiolate group, although in all cases they are low. For example in [Fe2(CO)6( μ-pdt)] it is estimated at 36 kJ mol-1,51 while for [Fe2(CO)6( μ-edt)] it is considerably higher at 51 kJ mol-1.16 Likewise we use the term trigonal-twist (Scheme 4) in complexes of this type to define a process whereby an Fe(CO)2P subunit undergoes exchange of both phosphine and carbonyl sites. The difference between this and the trigonal-twist process is that the 120° rotation does not have to result in a degenerate state; that is, the substituent can occupy either apical or basal positions. (48) De Beer, J. A.; Haines, R. J.; Greatrex, R.; Greenwood, N. N. J. Chem. Soc. A 1971, 3271. (49) Singleton, M. L.; Jenkins, R. M.; Klemashevich, C. L.; Darensbourg, M. Y. C. R. Chim. 2008, 11, 861. (50) Tye, J. W.; Darensbourg, M. Y.; Hall, M. B. Inorg. Chem. 2006, 45, 1552. (51) Liu, T.; Li, B.; Singleton, M. L.; Hall, M. B.; Darensbourg, M. Y. J. Am. Chem. Soc. 2009, 131, 8296.

The exchange of phosphine ligands between basal and apical sites in mono- and disubstituted derivatives, [Fe2(CO)6-n(PR3)n( μ-dithiolate)] (n=1, 2) has been studied to a far lesser extent. Sun and co-workers have studied the unsymmetrically substituted complex [Fe2(CO)4(PPh3)(PMe3)( μ-edt)].52 In the solid state the bulky PPh3 ligand is coordinated in an apical site, while the smaller PMe3 group occupies a basal site (Scheme 5). Indeed, this trend;for the bulky phosphine to occupy an apical site while smaller more basic phosphines occupy a basal site; appears to be quite general in the solid state. At room temperature two singlets are observed in the 31P NMR spectrum, the higher-field signal (attributed to the PMe3) being quite broad. Upon cooling (213 K), this signal splits into two sharp signals about 10.3 ppm apart (relative intensity ca. 10:1), while the highfield resonance attributed to the PPh3 ligand splits into two signals that are only 0.8 ppm apart. These observations have been attributed to the slowing of the rotation of the Fe(CO)2(PMe3) moiety over this temperature range while the PPh3 ligand is static throughout. Unfortunately no coalescence data are given, and hence a free energy of activation cannot be estimated. Nevertheless, this shows that the phosphine can undergo a trigonal-twist. A number of possible mechanisms can be envisaged in order to account for the observed fluxional behavior documented above. Four of the more probable are shown in Figure 5. We cannot definitively rule out any of these possible pathways but prefer one whereby there is no metal-metal bond scission. On this basis we rule out the pathways shown that involve (i) metal-phosphorus bond cleavage followed by a trigonal-twist and recoordination of the diphosphine, (ii) a double metal-sulfur bond cleavage followed by inversion at sulfur and recoordination of the bridging thiolate ligand, and (iii) a double metal-sulfur bond cleavage resulting in a planar intermediate. The main reasoning behind this is that, as far as we are aware, there is no precedent for the reversible scission-formation of metal-sulfur or metal-phosphorus bonds in such systems, although there is evidence of the former occurring upon reduction.53 We also rule out a process (not shown) that would involve an intermediate with two bridging carbonyl ligands. While such mechanisms are common in binuclear organometallic chemistry, the formation of bridging carbonyls in diiron dithiolate complexes is limited to one-electron oxidation products,54,55 and further there is no room for two bridging carbonyls in the complexes under study here since they already have three bridging groups. Consequently we prefer a process that we describe as a concerted double trigonal-twist. Here, one of the Fe(CO)2P subunits initially rotates, i.e., undergoes a trigonal-twist. Since the second Fe(CO)2P group is unchanged, then this results in a considerable strain being developed within the (52) Li, P.; Wang, M.; He, C.; Liu, X.; Jin, K.; Sun, L. Eur. J. Inorg. Chem. 2007, 3718 . (53) Felton, G. A. N.; Petro, B. J.; Glass, R. S.; Lichtenberger, D. L.; Evans, D. H. J. Am. Chem. Soc. 2009, 131, 11290. (54) Liu, T.; Darensbourg, M. Y. J. Am. Chem. Soc. 2007, 129, 7008. (55) Justice, A. K.; Rauchfuss, T. B.; Wilson, S. R. Angew. Chem., Int. Ed. 2007, 46, 6152.

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Figure 5. Proposed fluxional process for the interconversion of dppm methylene protons in [M 2(CO)4(μ-dppm){μ-S(CH2)nS}] (shown for n = 2).

Fe2PCP ring. This can be relived upon reversing the original twist or via a twisting at the second metal center in the same direction. Hence we term this a concerted double trigonal-twist. Such a process accounts for all of the experimental observations as it interconverts (i) the methylene protons of the diphosphine ligand, (ii) the sulfur-bound methylene protons of the dithiolate bridge, and (iii) the phenyl groups on phosphorus. While the formation of a strained twisted state may seen unusual, we note that in the solid state the dppm ligand in [Fe3(CO)8( μ-CO)2( μdppm)] binds in an axial site at one metal atom but an equatorial site at the second.56 This introduces a significant degree of twisting about the diiron vector such that the P-Fe-Fe-P torsion angle is 30.0°. This compares with values of 0° in [Fe2(CO)4( μdppm)( μ-edt)] and 7.2° (averaged over three independent molecules) in [Ru2(CO)4( μ-dppm)( μ-edt)]. Hence, in some respects the coordination of the diphosphine in [Fe3(CO)8( μ-CO)2( μdppm)] can be viewed as being somewhat akin to that proposed for the transition state of the concerted double trigonal-twist. Further support for this process comes from a recent publication from Richmond and co-workers.57 As part of a study concerning the ortho-metalation and ligand dynamics of (56) Adams, H.; Agustinho, S. C. M.; Chomka, K.; Mann, B. E.; Smith, S.; Squires, C.; Spey, S. E. Can. J. Chem. 2001, 79, 760. (57) Huang, S.-H.; Keith, J. M.; Hall, M. B.; Richmond, M. G. Organometallics, DOI: 10.1021/om100475v.

[Os3(CO)10( μ-dppm)], they calculated an energy barrier for the twisting of the dppm ligand in [Os3(CO)9( μ-dppm)] (from equatorial-equatorial to equatorial-axial) of 67.3 kJ mol-1, close to those measured in this study (Scheme 6). Since the fluxional process does not directly involve any changes to the bridging dithiolate ligand, then one might expect free energies of activation to be insensitive to changes in this group, which is precisely what is seen. The observed differences between iron and ruthenium, the latter being ca. 10 kJ mol-1 higher, are more difficult to understand. Few data are available for the barriers to trigonal rotation of analogous iron and ruthenium complexes, but higher barriers are expected for the heavier elements. The free energies of activation for the concerted double trigonal-twist noted here are significantly higher than those previously reported for related trigonal-twist processes.58-62 For example in [Fe2(CO)5{P(OMe)3}( μ-PPh2)( μ-X)] (X = carbene or acyl) (Scheme 7) the phosphite exchanges between (58) Doherty, S.; Hogarth, G.; Elsegood, M. R. J.; Clegg, W.; Rees, N. H.; Waugh, M. Organometallics 1998, 17, 3331. (59) Cooke, J.; Takats, J. Organometallics 1995, 14, 698. (60) Alex, R. F.; Pomeroy, R. K. Organometallics 1987, 6, 2437. (61) Cooke, J.; McClung, R. E. D.; Takats, J.; Rogers, R. D. Organometallics 1996, 15, 4459. (62) Hyes, S. J.; Gallop, M. A.; Johnson, B. F. G.; Lewis, J.; Dobson, C. M. Inorg. Chem. 1991, 30, 3850.

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Organometallics, Vol. 29, No. 23, 2010 Scheme 6

Hogarth et al. Scheme 9

Scheme 7

Scheme 8 Scheme 10

apical and basal sites with free energies of activation of between 39.7 and 43.5 kJ mol-1,58 while in [Os2Rh(CO)8(η5C5H5)(PMe3)] the free energy of activation for phosphine exchange is 63.2 kJ mol-1.59 This is to be expected due to the requirement of a degree of ring strain being introduced during the process. Unfortunately, as far as we are aware, despite the widespread study of complexes of the type [Fe2(CO)6-n(PR3)n( μ-dithiolate)] (n=1, 2) as models for the active site of the irononly hydrogenase enzyme, free energies of activations for phosphine exchange have not been measured. From the data given in ref 52 (Δν = 1667 Hz) and assuming a coalescence temperature of ca. 253 K, then a free energy of activation ca. 45 kJ mol-1 can be estimated, which is in line with related values for other diiron complexes (see above). A process that has been studied to a greater extent is the interconversion of basal-basal and basal-apical isomers of [Fe2(CO)4(κ2-diphosphine)( μ-dithiolate)].13,28,36 For example, the free energy of activation for this process in [Fe2(CO)4(κ2-Ph2PC6H4PPh2)( μ-pdt)] is estimated at 38 ( 1 kJ mol-1 (Scheme 8). v. Discussion and Conclusions. The realization that the dppm ligand in these complexes was mobile led us to consider whether this process could be more general. A vast number of dimetal-bridged dppm complexes are in the literature,63-65 and in many instances the 1H NMR data are poorly reported. In our own work we have previously studied the bis(phosphido) complexes [Fe2(CO)4( μ-PPh2)2( μ-dppm)] and [Fe2(CO)4( μ-PPh2)( μ-PCy2)( μ-dppm)] by VT 1H and 31P NMR spectroscopy.66 For [Fe2(CO)4( μ-PPh2)2( μ-dppm)] we found that the inequivalent methylene protons of the diphosphine exchanged at higher temperatures, a free energy of activation of 60 ( 1 kJ mol-1 being estimated. Concomitant with this was the collapse of the inequivalent phosphido resonances in the 31P NMR spectrum, a free energy of activation of 62 ( 1 kJ mol-1 being estimated. Clearly these represent the same fluxional process, (63) Puddpehatt, R. J. Chem. Soc. Rev. 1983, 99. (64) Chaudret, B.; Delavaux, B.; Poilblanc, R. Coord. Chem. Rev. 1988, 86, 134. (65) Anderson, G. K. Adv. Organomet. Chem. 1993, 35, 1. (66) Hogarth, G.; Lavender, M. H. J. Chem. Soc., Dalton Trans. 1993, 143.

and at the time we attributed this to the rocking of the phosphido bridges, a process that was proposed to occur via a transition state possessing a plane of symmetry (Scheme 9). At the time we ruled out a mechanism that “involves movement of the diphosphine moiety from one side of the molecule to the other” on the grounds that such a process was unprecedented. However, we now feel that this is fully consistent with the results presented for the linked dithiolate complexes. Interestingly the mixed phosphido-bridge complex [Fe2(CO)4( μPPh2)( μ-PCy2)( μ-dppm)] did not show this behavior. It would interconvert isomers in which the diphosphine lies trans to the μPCy2 and μ-PPh2 ligands. We attributed this to the differing thermodynamic stabilities of these two isomers, presuming that placing the dppm trans to the bulkier μ-PCy2 is thermodynamically preferable. We also note a recent publication that describes the synthesis and spectroscopic properties of [Fe2(CO)4( μ-dppm){μ-SCH2N(Pr)CH2S}].14 Here the crystal structure shows (as expected) that the diphosphine lies trans to one sulfur and cis to the second; however, the methylene protons of the diphosphine are equivalent at room temperature (described as a singlet and most probably not exchanging fast enough at this temperature to observe the coupling constant to phosphorus). Low-temperature data are not reported, so we cannot estimate the free energy of activation, but it suggests that this process is common to [Fe2(CO)4( μ-diphosphine)( μ-dithiolate)] complexes. This then leads us to consider when such a process is not facile in [Fe2(CO)4( μ-dppm)( μ-SAr)2] (Ar=Ph, p-tolyl) or [Fe2(CO)4( μ-dppm)( μ-SMe)2] (Scheme 10). The reason for this pronounced difference between linked and nonlinked dithiolatebridged complexes probably results from the different orientations of the substituents on sulfur, being syn for the linked complexes but anti for the diphosphine-substituted nonlinked

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complexes.46 Hence in the nonlinked complexes, movement of the diphosphine would need to be accompanied by inversion at both sulfur atoms in order to alleviate adverse steric interactions between the aryl groups on the diphosphine and thiolate ligands. Thus, it may be that such an inversion is disfavored. We have also recently noted that a similar double trigonaltwist process occurs in [Fe2(CO)4{μ-Ph2P(CH2)4PPh2}( μpdt)], the free energy of activation being estimated at 67 ( 1 kJ mol-1.36 The somewhat higher activation barrier versus that seen in [Fe2(CO)4( μ-dppm)( μ-pdt)] seems at first sight unusual, while such a process in [Fe2(CO)4( μ-dppm)( μ-pdt)] must be concerted in nature since a transoid dibasal intermediate is inaccessible due to steric strain. In [Fe2(CO)4{μPh2P(CH2)4PPh2}( μ-pdt)], as a result of the highly flexible tetramethylene backbone, this need not be the case and may account for the small, but significant, differences in the free energies of activation. In conclusion we have shown that the diphosphine in complexes of the type [M2(CO)4( μ-dppm){μ-S(CH2)nS}] (M = Fe, Ru) is fluxional and propose that this involves a concerted double trigonal-twist, and it may be that such a process is more common than previously thought for dppm and related diphosphine complexes.

Experimental Section All reactions were carried out under a nitrogen atmosphere in dried degassed solvents, although reaction workup and product crystallization were carried out in air. NMR spectra were run on a Bruker AMX400 spectrometer and referenced internally to the residual solvent peak (1H) or externally to P(OMe)3 (31P). Infrared spectra were run on a Nicolet 205 FT-IR spectrometer in a solution cell fitted with calcium fluoride plates, subtraction of the solvent absorptions being achieved by computation. Elemental analyses were performed in house. Synthesis of [Fe2(CO)4( μ-dppm)( μ-edt)]. A toluene solution (80 cm3) of [Fe2(CO)6( μ-edt)] (0.10 g, 0.27 mmol), dppm (0.114 g, 0.30 mmol), and Me3NO 3 2H2O (0.066 g, 0.60 mmol) was heated at reflux for 12 h. After cooling, removal of volatiles afforded an oily, orange solid. This was washed with 40:60 petrol (3  10 cm3) and dried. The solid was dissolved in a minimum amount of dichloromethane (ca. 10 cm3), and an excess of hexane (ca. 100 cm3) was then added. Removal of volatiles on a rotary evaporator gave a slightly oily, orange solid, which was again washed with 40:60 petrol (3  10 cm3) and dried. This gave a dry orange solid. It was all dissolved in a minimum amount of dichloromethane (ca. 2 cm3) and layered with methanol (ca. 5 cm3). After slow mixing this afforded large truncated octahedral red crystals of [Fe2(CO)4( μdppm)( μ-edt)] as a dry orange solid (0.13 g, 34%). IR ν(CO) (CH2Cl2): 1992s, 1959vs, 1924s, 1905sh cm-1. 1H NMR (d8-toluene), 253 K: δ 7.49 (m, 4H, Ph), 7.39 (m, 4H, Ph), 6.95-6.88 (m, 12H, Ph), 3.60 (dt, 1H, J 14.0, 10.0, PCH2), 3.04 (dt, 1H, J 14.0, 10.8, PCH2), 2.03 (m, 2H, SCH2), 1.94 (m, 2H, SCH2); 373 K: δ 7.52 (s, 8H, Ph), 6.98 (s, 12H, Ph), 3.54 (t, J 10.0, 2H, PCH2), 2.10 (s, 4H, SCH2). 31 1 P{ H} NMR (CDCl3) 298 K: δ 56.8 (s). Anal. Calcd for Fe2P2S2O4C31H26: C, 53.14, H, 3.71. Found: C, 53.67, H, 3.84. When this reaction was carried out without added Me3NO 3 2H 2O, a number of intermediates were clearly observed spectroscopically. Thus, after 30 min complete consumption of [Fe2(CO)6( μ-edt)] was noted. At this stage the IR spectrum was consistent with the formation of both [Fe2(CO)5(κ1-dppm)( μedt)] (2047 cm-1) and [Fe2(CO)4(κ2-dppm)( μ-edt)] (2020 cm-1) in an approximate 6:1 ratio. Both 1H and 31P NMR spectra were complex. It was clear from the latter that [Fe2(CO)5(κ1-dppm)( μ-edt)] was formed as a mixture of isomers (basal and apical) in an approximate 2:1 ratio [major, 56.7 (d), -24.9 (d) ppm JPP 105.0 Hz; minor 53.0 (d), -24.5 (d) ppm, JPP 72.4 Hz]. For

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[Fe2(CO)4(κ2-dppm)( μ-edt)] a singlet was observed at 25.1 ppm. When the reaction was carried out at room temperature in acetonitrile with addition of two equivalents of Me3NO 3 2H2O, immediate consumption of [Fe2(CO)6( μ-edt)] was noted and a similar IR spectrum was seen. The 31P NMR spectrum was similar but also included singlets at 35.4 and 19.8 ppm, which we associate with [{Fe2(CO)5( μ-edt)}2( μ-dppm)] (two isomers). All attempts to isolate these species pure proved unsuccessful. Synthesis of [Ru2(CO)4( μ-dppm)( μ-edt)]. 37 A toluene solution (80 cm3) of [Ru2(CO)6( μ-edt)] (0.10 g, 0.22 mmol) and dppm (0.10 g, 0.26 mmol) was heated at reflux for 2 h, the pale yellow solution darkening slightly over time. An IR spectrum revealed the total consumption of starting materials and clean formation of [Ru2(CO)4( μ-dppm)( μ-edt)]. After cooling, removal of volatiles afforded a dry yellow solid, which was washed with 40:60 petrol (3  10 cm3) and dried (0.164 g, 94%). Slow diffusion of methanol into a saturated dichloromethane solution afforded large yellow crystals. IR ν(CO) (CH2Cl2): 2004s, 1980vs, 1939s, 1922sh cm-1. 1 H NMR (CDCl3) 298 K: δ 7.73-6.96 (m, 20H, Ph), 3.66-3.46 (m, 2H, PCH2), 2.50 (m, 2H, SCH2), 2.39 (m, 2H, SCH2); (d8toluene) 298 K: δ 7.50 (m, 4H, Ph), 7.25 (m, 4H, Ph), 7.01-6.77 (m, 12H, Ph), 3.536 (dt, 1H, J 14.2, 10.1, PCH2), 3.299 (dt, 1H, J 14.2, 10.8, PCH2), 2.15 (m, 2H, SCH2), 2.04 (m, 2H, SCH2). 31P{1H} NMR (CDCl3) 298 K: δ 28.6 (s). Synthesis of [Ru2(CO)4( μ-dppm)( μ-pdt)]. 37 This was carried out as above and gave [Ru2(CO)4( μ-dppm)( μ-pdt)] cleanly in 92% yield. IR ν(CO) (CH2Cl2): 2004s, 1980vs, 1939s, 1922sh cm-1. 1H NMR (d8-toluene) 298 K: δ 7.56 (q, 4H, J 5.4, Ph), 7.21 (q, 4H, J 5.4, 7.09-6.74 (m, 12H, Ph), 3.682 (dt, 1H, J 14.3, 10.3, PCH2), 3.015 (dt, 1H, J 14.3, 10.6, PCH2), ca. 2.09 (2H), 1.94 (m, 2H, SCH2), 1.75 (m, 2H, SCH2). 31P{1H} NMR (CDCl3) 298 K: δ 25.8 (s). X-ray Data Collection and Solution. Single crystals were mounted on glass fibers, and all geometric and intensity data were taken from these samples using a Bruker SMART APEX CCD diffractometer using graphite-monochromated Mo KR radiation (λ = 0.71073 A˚) at 150 ( 2 K. Data reduction was carried out with SAINT PLUS, and absorption correction applied using the program SADABS. Structures were solved by direct methods and developed using alternating cycles of least-squares refinement and difference-Fourier synthesis. All non-hydrogen atoms were refined anisotropically. For [Fe2(CO)4( μ-dppm)( μ-edt)], hydrogen atoms were located from difference maps and refined isotropically, while for [Ru2(CO)4( μ-dppm)( μ-edt)] they were placed in calculated positions (riding model). Structure solution used the SHELXTL PLUS V6.10 program package. Crystallographic data for [Fe2(CO)4( μ-dppm)( μ-edt)]: red truncated octahedron, dimensions 0.42  0.42  0.36 mm, tetragonal, space group P4(2)/mbc, a = b = 17.2280(13) A˚, c = 20.132(3) A˚, V = 5975.1(11) A˚3, Z = 8, F(000) 2864, dcalc = 1.557 g cm-3, μ = 1.254 mm-1; 48 601 reflections were collected, 3790 unique [R(int) = 0.0477], of which 3492 were observed [I > 2.0σ(I)]. At convergence, R1 = 0.0272, wR2 = 0.0686 [I > 2.0σ(I)] and R1 = 0.0298, wR2 = 0.0699 (all data), for 247 parameters. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre, CCDC No. 751121. Crystallographic data for [Ru2(CO)4( μ-dppm)( μ-edt)]: yellow block, dimensions 0.45  0.40  0.36 mm, monoclinic, space group Cc, a = 19.998(4) A˚, b = 52.874(11) A˚, c = 8.7981(19) A˚, β = 96.520(3)o, V=9243(3) A˚3, Z=4, F(000) 4728, dcalc=1.705 g cm-3, μ= 1.255 mm-1; 40 263 reflections were collected, 20 988 unique [R(int) =0.0421], of which 20 988 were observed [I>2.0σ(I)]. At convergence, R1 = 0.0385, wR2 = 0.0871 [I > 2.0σ(I)] and R1 = 0.0414, wR2 = 0.0886 (all data), for 1103 parameters. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre, CCDC No. 751122. Crystallographic data for [Fe2(CO)4( μ-dppm)( μ-SMe)2]: red block, dimensions 0.26  0.25  0.25 mm, triclinic, space group, P1, a = 9.287(1) A˚, b = 10.919(1) A˚, c = 15.839(2) A˚, R = 77.834(2)°, β = 84.137(2)°, γ = 73.505(2)o, V = 1504.0(3) A˚3, Z = 2, F(000) 720, dcalc = 1.551 g cm-3, μ = 1.246 mm-1; 7508

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reflections were collected, 5879 unique [R(int)= 0.0146], of which 5259 were observed [I > 2.0σ(I)]. At convergence, R1= 0.0350, wR2 = 0.1000 [I > 2.0σ(I)] and R1 = 0.0414, wR2 = 0.1261 (all data), for 370 parameters. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre, CCDC No. 798796.

Acknowledgment. We thank The Royal Society (London) for a short visit fellowship to S.E.K., Dr. Abil Aliev for NMR support, Miss Faith Ridley for the preparation of [Fe2(CO)4( μ-dppm)( μ-SAr)2] (Ar = Ph, p-tolyl) and for carrying out NMR experiments on these complexes, Mr. David Unwin for

Hogarth et al.

the preparation of [Fe2(CO)4( μ-dppm)( μ-SMe)2], and Prof. Brian Mann for valuable discussions. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.

Note Added after ASAP Publication. In the version of this paper published on the web on November 16, 2010, the captions within the graphic were incorrect, due to a production error. In the version of the paper that appears as of December 6, 2010, Figure 5 appears correctly.