Addition of Alkynes to Zwitterionic μ-Vinyliminium Diiron Complexes

Mar 17, 2010 - Adriano Boni , Tiziana Funaioli , Fabio Marchetti , Guido Pampaloni , Calogero Pinzino , Stefano Zacchini. Journal of Organometallic Ch...
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Organometallics 2010, 29, 1797–1805 DOI: 10.1021/om100037t

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Addition of Alkynes to Zwitterionic μ-Vinyliminium Diiron Complexes: New Selenophene (Thiophene) and Vinyl Chalcogenide Functionalized Bridging Ligands Luigi Busetto,† Fabio Marchetti,‡,§ Filippo Renili,‡ Stefano Zacchini,† and Valerio Zanotti*,† †

Dipartimento di Chimica Fisica e Inorganica, Universit a di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy, and ‡Dipartimento di Chimica e Chimica Industriale, Universit a di Pisa, Via Risorgimento 35, I-56126 Pisa, Italy. § Fabio Marchetti, born in 1974 in Bologna, Italy. Received January 15, 2010

Zwitterionic vinyliminium complexes [Fe2{μ-η1:η3-Cγ(R0 )dCβ(E)CRdN(Me)(R)}(μ-CO)(CO)(Cp)2][SO3CF3] (R = R0 = Me, E = Se, 1a; R = Xyl, R0 = Tol, E = Se, 1b; R = Xyl, R0 = Tol, E = S, 2a; R = Xyl, R0 = Tol, E = S, 2b; Tol = 4-MeC6H4, Xyl = 2,6-Me2C6H3) undergo alkyne addition by different reaction modes. Complexes 1a and 2a undergo 1,3 dipolar cycloaddition with alkynes [HCtCCO2Me and C2(CO2Me)2], affording new 1-(2-amino)-seleno(thio)phene-alkylidene diiron complexes [Fe2{μ-κ1(N):η1(C):η1(C):-Cγ(R0 )CβEC(CO2Me)dC(R00 )CRN(Me)(R)}(μ-CO)(CO)(Cp)2] (R = R0 = Me, E = Se, R00 = CO2Me, 3a; R = R0 = Me, E = Se, R00 = H, 3b; R = Me, R0 = Tol, E = S, R00 = CO2Me, 4). The hemilabile character of the bridging ligand in 3a is investigated by reaction with CNBut, which replaces NMe2 coordination, affording [Fe2{μ-Cγ(Me)CβSeC(CO2Me)dC(CO2Me)CRN(Me)2}(μ-CO)(CO)(CNBut)(Cp)2] (5). Complexes 2a and 2b react with two equivalents of HCtCCO2Me, leading to the formation of [Fe2{μ-κ1(O):η1(C):η3(C)-Cδ(CtCCO2Me)Cγ(R0 )Cβ(SCHdCHCO2Me)CR(O)N(Me)(Xyl)}(μ-CO)(Cp)2] (R0 = Tol, 6a; R0 = Me, 6b). Finally, complexes 1b, 2a, and 2b react with different alkynes, in the presence of NH4PF6, affording the vinyl sulfide and vinyl selenide vinyliminium complexes [Fe2{μ-η1:η3-Cγ(R0 )dCβ(ECR00 dCHCO2Me)CRdN(Me)(Xyl)}(μ-CO)(CO)(Cp)2][PF6] (R0 = Tol, E = S, R00 = H, 7a; R0 = Me, E = S, R00 = H, 7b; R0 = Tol, E = S, R00 = CO2Me, 7c; R0 = Me, E = S, R00 = CO2Me, 7d; R0 = Tol, E = Se, R00 = H, 8a; R0 = Tol, E = Se, R00 = CO2Me, 8b). The molecular structures of 3a, 5, and 6b have been elucidated by X-ray diffraction.

Introduction Zwitterionic organometallic complexes are potential sources of unique reactivity due to the properties associated with the presence of integral opposite charges, linked through a path of covalent bonds and located in separated regions of the complex.1 For instance, zwitterionic complexes combine the advantage of a charged metal center with the solubility of neutral complexes, which can significantly enhance their catalytic activity.2 Further potential applications can be envisaged in the field of optically active compounds and in the development of new approaches to metal-catalyzed 1,3-cycloaddition reactions. We have recently described the synthesis of zwitterionic vinyliminium diiron complexes.3 Some of these species,

reported in Scheme 1, are the compounds investigated in this work. The peculiar character of these complexes consists in the fact that both charges are formally located on the bridging ligand: the negative charge on the S/Se atom and the positive charge on the N atom of the iminium moiety. This is unusual, in that zwitterionic complexes generally display a positive or negative charge located on the metal center. As a consequence, complexes 1 and 2 exhibit some of the properties typical of zwitterionic ligands,4 featuring sites able to bind a metal cation, both as mono and as chelating ligand.5 So far, our investigations have concerned metalation, alkylation, and oxidative dimerization of the zwitterionic complexes,6 but other possible reactions are to be expected in consideration of the dipolar character of the bridging ligand. In particular, cycloaddition with appropriate dipolarophiles

*To whom correspondence should be addressed. E-mail: valerio.zanotti@ unibo.it. (1) Chauvin, R. Eur. J. Inorg. Chem. 2000, 577. (2) (a) Cipot, J.; McDonald, R.; Stradiotto, M. Chem. Commun. 2005, 4932. (b) Lundgren, R. J.; Rankin, M. A.; McDonald, R.; Schatte, G.; Stradiotto, M. Angew. Chem., Int. Ed. 2007, 46, 4732. (c) Cipot, J.; McDonald, R.; Ferguson, M. J.; Schatte, G.; Stradiotto, M. Organometallics 2007, 26, 594. (3) Busetto, L.; Marchetti, F.; Zacchini, S.; Zanotti, V. Organometallics 2006, 25, 4808.

(4) (a) Forgan, R. S.; Davidson, J. E.; Galbraith, S. G.; Henderson, D. K.; Parsons, S.; Tasker, P. A.; White, F. J. Chem. Commun. 2008, 4049. (b) Tasker, P. A.; Tong, C. C.; Westra, A. N. Coord. Chem. Rev. 2007, 251, 1868. (5) Busetto, L.; Marchetti, F.; Mazzoni, R.; Salmi, M.; Zacchini, S.; Zanotti, V. Eur. J. Inorg. Chem. 2009, 1268. (6) Busetto, L.; Dionisio, M.; Marchetti, F.; Mazzoni, R.; Salmi, M.; Zacchini, S.; Zanotti, V. J. Organomet. Chem. 2008, 693, 2383.

r 2010 American Chemical Society

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Scheme 1. Zwitterionic Vinyliminium Complexes Investigated in This Work

Scheme 2 Figure 1. ORTEP drawing of 3a (all H atoms are omitted for clarity). Thermal ellipsoids are at the 30% probability level. Only the main image of the disordered Cp ligand bonded to Fe(2) is represented. Relevant bonding parameters (A˚): Fe(1)-C(11) 1.958(4), Fe(2)-C(11) 1.861(4), Fe(1)-C(13) 2.043(4), 1.978(3), C(13)-C(14) 1.444(5), C(14)-C(15) 1.369(5), C(15)-C(16) 1.419(5), C(16)-C(17) 1.361(5), C(17)-Se(1) 1.868(4), C(14)-Se(1) 1.888(3), C(15)-N(1) 1.460(4), N(1)C(20) 1.493(5), N(1)-C(19) 1.493(5).

(e.g., alkynes) are likely to occur and are the subject of the present report. The aim was to explore new potential routes to heterocycles and other addition products, by reaction of alkynes with bridging ligands in diiron complexes.

Results and Discussion 1,3 Dipolar Cycloaddition with Alkynes. The Se- and S-functionalized vinyliminium complexes 1 and 2 react with activated alkynes [HCtCCO2Me, C2(CO2Me)2] to give the novel 1-(2-amino)-selenophene-alkylidene complexes 3a,b and 1-(2-amino)-thiophene-alkylidene 4, respectively, in about 80% yields (Scheme 2). In Scheme 2, the three carbon atoms of the bridging chain are denoted by greek letters (R, β, and γ) to better identify them as components of the heterocycle products 3 and 4. Complexes 3a, 3b, and 4 have been characterized by spectroscopy and elemental analysis. Moreover, the molecular structure of 3a 3 CH2Cl2 has been determined by X-ray diffraction: the ORTEP molecular diagram is shown in Figure 1 together with most relevant bond lengths and angles. The structure of 3a is composed of a bridging 1-(2-amino)selenophene-alkylidene ligand coordinated to a cis-Fe2(μ-CO)(CO)(Cp)2 core. Coordination of the former occurs via a bridging alkylidene moiety and a terminal amino functionality. Both the bridging carbonyl [Fe(1)-C(11) 1.958(4) A˚; Fe(2)-C(11) 1.861(4) A˚] and bridging alkylidene ligands [Fe(1)-C(13) 2.043(4) A˚; Fe(2)-C(13) 1.978(3) A˚] show a certain degree of asymmetry, with the shorter contacts to the electron richer amino-coordinated Fe(2) center. The highly functionalized selenophene group is almost perfectly planar [mean deviation from the C(14) C(15) C(16) C(17) Se(1) leastsquares plane 0.0220 A˚], in agreement with sp2 hybridization

of the C atoms. Within the five-membered ring, the C(14)C(15) [1.369(5) A˚] and C(16)-C(17) [1.361(5) A˚] interactions display a strong double-bond character, whereas C(15)C(16) [1.419(5) A˚] displays considerably minor π-character. The C(14)-Se(1) [1.888(3) A˚] and C(17)-Se(1) [1.868(4) A˚] interactions are as expected for single C(sp2)-Se bonds. Coordination of N(1) to Fe(2) results in the formation of a second five-membered metallacycle condensed via the C(14)C(15) edge to the selenophene ring, and also this second ring is almost planar [mean deviation from the Fe(2) C(13) C(14) C(15) N(1) least-squares plane 0.0592 A˚]. Condensation of these two nearly coplanar five-membered rings [mean deviation from the common least-squares plane 0.0835 A˚] probably forces a sizable elongation of the C(13)-C(14) contact [1.444(5) A˚] compared to a normal C(sp3)-C(sp2) single bond [1.51 A˚]. Finally, coordination of N(1) to the iron center causes C(15)-N(1) [1.460(4) A˚] to be longer than a C(sp2)-N single bond [1.38 A˚], whereas N(1)-C(19) and N(1)-C(20) [1.493(5) A˚] are longer than the C(sp3)-N single bond [1.47 A˚].7 The NMR data of 3a,b and 4 evidence the presence, in solution, of one single isomeric form for each compound. This is noticeable in the case of 3b, in that the cycloaddition of the primary and asymmetric alkyne HCtCCO2Me could, in theory, produce two regioisomers, as a consequence of two possible modes of inclusion within the fivemembered cycle. Conversely, the [3þ2] cycloaddition is regioselective and the CH termination of the primary alkyne is exclusively bound to the Se atom, as shown by the structure observed in the solid state. In compounds 3a,b, the NMe groups are not equivalent and give rise to distinct resonances in both the 1H and 13NMR spectra. In the 13C NMR spectra of 3a,b and 4, the resonances due to CR, Cβ, and Cγ are in the range typical for vinylalkylidene carbons: the bridging alkylidene carbon (Cγ) exhibits a low-field resonance at about 180.7 ppm, and the CR and Cβ resonances are at about 160 ppm. (7) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A. G.; Taylor, R. J. Chem. Soc., Perkin Trans. 1987, 12, S1–S19.

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Formation of the selenophene/thiophene rings in 3a,b and 4 is clearly the result of a [3þ2] cycloaddition that involves the bridging zwitterionic ligand, in the form of a Cþ-C-X1,3-dipole (X = S or Se), and the alkyne. Thus, the reaction has to be included in the field of 1,3 dipolar cycloaddition reactions, which constitute one of the most powerful protocols in organic synthesis.8 Analogies can be envisages with the 1,3 dipolar cycloaddition of azomethine ylides, in that the positive charge in the dipolar unit is placed on an iminium group,9 and with Huisgen dipolar cycloaddition of azides and alkynes.10 The latter is often referred as the “click reaction” and is the most representative of the synthetic concepts developed by Sharpless.11 The dipole cycloaddition shown in Scheme 2 is far from achieving the status of a click reaction, in that the structure of the dinuclear complex is too complex to meet the requirement of readily available starting materials; neither the efficiency, selectivity, nor reliability seems adequate. Much closer similarities can be found with the cycloadditions involving thiocarbamoyl benzimidazolium (or imidazolinium) salts, which can act as 1,3 dipolar C-C-S species.12 One of the most relevant aspects of the reaction shown in Scheme 2 is the formation of selenophenes and thiophenes by a [3þ2] cycloaddition, which is an uncommon synthetic approach. To the best of our knowledge, examples are limited to the cycloadditions of the thiocarbamoyl azolium salt above-mentioned and the reactions of thiocarbonyl ylides with electron-poor olefinic dipolarophiles.13 A further interesting and unique feature is that the dipolar cycloaddition involves a bridging vinyliminium ligand, which is consequently converted into a bridging alkylidene, connected to a selenophene (thiophene) ring. The latter is further coordinated to one Fe center through a pendant N(Me)R function. Therefore, the observed cycloaddition is the result of a combination of two major features: the dipolar (zwitterionic) character of the ligand and also the versatility of the bridging coordination, which can easily undergo adjustment in response to modifications of the bridging frame. The 1,3 dipolar cycloaddition of alkynes with zwitterionic ligands is also to be compared with other previously reported cyclizations involving alkynes and bridging C3 ligands, in related diiron complexes. This is the reaction of μ-vinylalkylidene complexes [Fe2{μ-η1:η3-CRCHdCH(NMe2)}(μ-CO)(CO)(Cp)2] with alkynes, which leads to the formation of functionalized ferrocenyl products, as shown in Scheme 3.14 In these ferrocene complexes, the polysubstituted cyclopentadienyl ligand results from a [3þ2] cycloaddition of the vinylalkylidene ligand with alkynes. In this case the cycloaddition (8) (a) Huisgen, R. 1,3-Dipolar Cycloaddition Chemistry; Padwa, A., Ed.; Wiley: New York, 1984; Vol. 1 . (b) Harwood, L. M.; Vickers, R. J. Synthetic Applications of 1,3-Dipolar Cycloaddition Chemistry Toward Heterocycles and Natural Products; Padwa, A., Pearson, W. H., Eds.; Wiley: New York, 2002. (9) Gothelf, K. V.; Jørgensen, K. A. Chem. Rev. 1998, 98, 863. (10) (a) Huisgen, R. Pure Appl. Chem. 1989, 61, 613. (b) Huisgen, R.; Szeimies, G.; Moebius, L. Chem. Ber. 1967, 100, 2494. (c) Bastide, J.; Hamelin, J.; Texier, F.; Vo Quang, Y. Q. Bull. Soc. Chim. Fr. 1973, 2555. (11) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40, 2004. (12) (a) Liu, M.-F.; Wang, B.; Cheng, Y. Chem. Commun. 2006, 1215. (b) Cheng, Y.; Liu, M.-F.; Fang, D.-C.; Lei, X.-M. Chem.;Eur. J. 2007, 13, 4282. (c) Li, J.-Q.; Liao, R.-Z.; Ding, W.-J.; Cheng, Y. J. Org. Chem. 2007, 72, 6266. (d) Ma, Y. G.; Cheng., Y. Chem. Commun. 2007, 5087. (13) Komatsu, M.; Choi, J.; Mihara, M.; Oderaotoshi, Y.; Minakata, S. Heterocycles 2002, 57, 1989. (14) Busetto, L.; Marchetti, F.; Mazzoni, R.; Salmi, M.; Zacchini, S.; Zanotti, V. Organometallics 2009, 28, 3465.

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Scheme 3

does not have a dipolar character and shows close analogies with typical cycloadditions of R,β-unsaturated alkylidene ligands with alkynes.15 A further relevant difference is that the reaction shown in Scheme 3 leads to the fragmentation of the dinuclear complexes, whereas the 1,3 dipolar cycloaddition does not, and the resulting five-membered heterocycle remains coordinated as a bridging ligand. Besides the unprecedented nature of the bridging frame in 3a,b and 4, the coordination mode is also uncommon in that ligands containing selenophene or thiophene rings are generally coordinated through the heteroatom and/or the π-bond.16 Conversely, in compounds 3a,b and 4 the heterocycles are connected to the metal centers exclusively through the alkylidene and the N(Me)R group, which are substituents of the five-membered ring. Moreover, the bridging frame might exhibit hemilabile character,17 since the bridging alkylidene moiety appears firmly coordinated, whereas, in theory, the coordination through the pendant amino group should be easier to displace. In order to investigate this point, we examined the reactivity of 3a with isocyanides (CNBut) and phosphines (PMe3), both good candidates to displace the N(Me)R group from metal coordination. However, neither PPh3 nor CNBut reacts with 3a in THF solution at room temperature. Therefore, N-coordination is not very labile; only treatment of 3a in THF at reflux temperature, in the presence of CNBut, produces the expected displacement, affording 5, in high yield (Scheme 4). Complex 5 has been characterized by spectroscopy and X-ray diffraction. The ORTEP molecular diagram is shown in Figure 2, together with most relevant bond lengths and angles. The molecular structure of 5 confirms the displacement of N(1) and the coordination of CNBut to Fe(2), with the Fe2(μ-CO)(CO)(Cp)2 core that retains the cis-conformation. Interestingly, the μ-CO ligand maintains the same asymmetry present in 3a [Fe(1)-C(11) 1.935(7) A˚ and Fe(2)-C(11) 1.847(7) A˚ in 5; Fe(1)-C(11) 1.958(4) A˚ and Fe(2)-C(11) 1.861(4) A˚ in 3a], showing the longer contact to the terminally CO-coordinated Fe(1) center, whereas its sense is reversed concerning the bridging alkylidene ligand [Fe(1)-C(13) 2.002(6) A˚ and Fe(2)-C(13) 2.029(7) A˚ in 5; Fe(1)-C(13) 2.043(4) A˚ and Fe(2)-C(13) 1.978(3) A˚ in 3a]. The selenophene ring is almost perfectly planar [mean deviation from the C(14) C(15) C(16) C(17) Se(1) least-squares plane 0.0189 A˚], as in the parent 3a, and the bonding parameters within the five-membered ring are very similar (15) D€ otz, K. H.; Stendel, J., Jr. Chem. Rev. 2009, 109, 3227. (16) (a) Angelici, R. J. Coord. Chem. Rev. 1990, 105, 61. (b) Chen, J.; Angelici, R. J. Coord. Chem. Rev. 2000, 206-207, 63. (c) Waldbach, T. A.; van Eldik, R.; van Rooyen, P. H.; Lotz, S. Organometallics 1997, 16, 4056. (d) Paneque, M.; Poveda, M. L.; Salazar, V.; Taboada, S.; Carmona, E.; Gutierrez-Puebla, E.; Monge, A.; Ruiz, C. Organometallics 1999, 18, 139. (17) (a) Jeffrey, J. C.; Rauchfuss, T. B. Inorg. Chem. 1979, 18, 2658. (b) Braunstein, P.; Naud, F. Angew. Chem., Int. Ed. 2001, 40, 680. (c) Bader, A.; Lindner, E. Coord. Chem. Rev. 1991, 108, 27. (d) Braunstein, P. J. Organomet. Chem. 2004, 689, 3953. (e) Angell, S. E.; Rogers, C. W.; Zhang, Y.; Wolf, M. O.; Jones, W. E. Coord. Chem. Rev. 2006, 250, 1829.

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Scheme 4

Scheme 5 Figure 2. ORTEP drawing of 5 (all H atoms are omitted for clarity). Thermal ellipsoids are at the 30% probability level. Only the main image of the disordered But group is represented. Relevant bonding parameters (A˚): Fe(1)-C(11) 1.935(7), Fe(2)-C(11) 1.847(7), Fe(1)-C(13) 2.002(6), 2.029(7), C(13)-C(14) 1.505(9), C(14)-C(15) 1.388(9), C(15)-C(16) 1.430(9), C(16)-C(17) 1.353(9), C(17)-Se(1) 1.852(6), C(14)-Se(1) 1.865(6), C(15)-N(1) 1.416(8), N(1)-C(20) 1.451(9), N(1)-C(19) 1.469(10).

to those of the parent compound, with alternated single [C(15)-C(16) 1.430(9) A˚; C(17)-Se(1) 1.852(6) A˚; C(14)Se(1) 1.865(6) A˚] and double bonds [C(14)-C(15) 1.388(9) A˚; C(16)-C(17) 1.353(9) A˚]. The opening of the second fivemembered metallacycle present in 3a as a consequence of displacement of N(1) from Fe(2) in 5 results in some strain, and, thus, C(13)-C(14) [1.505(9) A˚] is rather elongated in 5 compared to the parent 3a [1.444(5) A˚] and as expected for a C(sp3)-C(sp2) single bond [1.51 A˚]. Finally, as consequence of the loss of N-coordination C(15)-N(1) [1.416(8) A˚], N(1)-C(19) [1.469(10) A˚], and N(1)-C(20) [1.451(9) A˚] are shortened compared to 3a [1.460(4), 1.493(5), and 1.493(5) A˚, respectively], approaching the expected values for C(sp2)-N [1.38 A˚] and C(sp3)-N [1.47 A˚] single bonds.7 The NMR spectroscopic data are consistent with the structure shown in Figure 2, and it is reasonable to assume that 5 adopts, in solution, the same conformation shown in the solid, with the selenophene substituent far from the sterically demanding CNBut ligand. The most relevant feature is the equivalence of the NMe2 protons, which give rise to a single signal at 2.80 ppm. Indeed, free rotation around the CR-NMe2 bond, together with inversion at the N atom, provides an exchange mechanism that makes the methyl groups equivalent on the NMR time scale. This was not the case of the parent complex 3a, in which the nitrogen coordination to Fe did not allow rotation around the CR-NMe2 interaction. Addition of Two Alkyne Units at the Bridging Ligand, with CO Bond Cleavage. The cyclization reaction shown in Scheme 2 is not general, and the S(Se)-functionalized vinyliminium diiron complexes can combine with alkynes through different reaction routes. In fact, complexes [Fe2{μ-η1:η3-Cγ(R 0 )dC β (S)C R dN(Me)(Xyl)}(μ-CO)(CO)(Cp)2 ][SO 3 CF 3 ] [R0 = Tol, 2a; R0 = Me, 2b] react with an excess of HCt CCO2Me, in CH2Cl2 solution at room temperature, to form the vinyl sulfide functionalized bridging allylidene complexes 6a and 6b, respectively, obtained in about in 80-90% yields (Scheme 5). Complexes 6a,b have been characterized by spectroscopy and elemental analysis. Moreover, the X-ray molecular structure of 6b has been determined (Figure 3).

Figure 3. ORTEP drawing of 6b (all H atoms are omitted for clarity). Thermal ellipsoids are at the 30% probability level. Only one of the two independent molecules present within the unit cell is represented. Relevant bonding parameters (A˚): Fe(1)-C(12) 1.943(7) and 1.944(7); Fe(2)-C(12) 2.002(6) and 1.993(7); Fe(2)-C(13) 2.024(6) and 2.039(6); Fe(2)-C(14) 2.136(6) and 2.133(6); C(12)-C(13) 1.403(9) and 1.397(9); C(13)-C(14) 1.459(8) and 1.452(8); C(21)-C(22) 1.360(9) and 1.317(9) (for the two independent molecules, respectively).

In this case the reaction with alkynes does not produce the 1,3 dipolar cycloaddition described above. Conversely, two alkyne units are incorporated in the bridging frame: one is bound to the S atom; the second is incorporated in the bridging hydrocarbyl chain as alkynyl group. Interestingly, also a CO is incorporated in the bridging frame; thus the overall result consists of a remarkable growth of the bridging ligand, due to a one-pot formation of several C-C and C-heteroatom bonds. The inclusion of CO and of one alkyne, in the form of an alkynyl substituent, very closely resembles the previously reported acetylide addition to diiron vinyliminium complexes, which is shown in Scheme 6.18 (18) (a) Busetto, L.; Marchetti, F.; Zacchini, S.; Zanotti, V. Eur. J. Inorg. Chem. 2006, 285. (b) Busetto, L.; Marchetti, F.; Zacchini, S.; Zanotti, V. Eur. J. Inorg. Chem. 2007, 1799.

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Organometallics, Vol. 29, No. 7, 2010 Scheme 6

Labeling experiments demonstrated that the reaction shown in Scheme 6 was initiated by acetylide attack at a CO ligand, which was consequently cleaved. Both fragments (C and O), as well as the alkynyl group, were included in the bridging frame (shown as Cδ and O bound to CR, in Scheme 6). A very similar process presumably occurs also in the formation of 6a and 6b, but the architecture of the resulting bridging frame is made more intricate by the addition of an other alkyne unit. Indeed, the complexity of the bridging frame is well evidenced in the X-ray structure of 6b (Figure 3), which is quite similar to that of II, previously reported.18a Thus, the molecule is composed of a Fe2(μ-CO)(Cp)2 core with a bridging amido-vinylsulfido- functionalized allylidene μ-κ 1(O):η1 (C):η 3(C)-{C(CtCCO2 Me)C(Me)C(SCHdCHCO2Me)C(O)N(Me)(Xyl)}. As for most of the bridging vinylalkylidene (allylidene) dinuclear complexes,19 the highly delocalized nature of the π-interaction within the bridging frame is better illustrated by considering several representation forms (e.g., A, B, and C, Scheme 7). These are extreme representations, and a full description of the bonding necessarily involves taking into consideration different contributions of each. Thus, the bridging ligand is σ-coordinated to Fe(1) [Fe(1)-C(12) 1.943(7) and 1.944(7) A˚ for the two independent molecules present within the unit cell; compare 1.923(4) A˚ in II] and η3-coordinated to Fe(2) [Fe(2)-C(12) 2.002(6) and 1.993(7) A˚; Fe(2)-C(13) 2.024(6) and 2.039(6) A˚; Fe(2)-C(14) 2.136(6) and 2.133(6) A˚; compare 2.004(4), 2.030(4), and 2.157(4) A˚ in II] in an allyl-like fashion (see structure A). However, the Fe(2)-C(14) interaction is significantly longer than Fe(2)-C(12) and Fe(2)-C(13), which is consistent with the formulation B, where the C(12)-C(13)-C(14) sequence is viewed as a vinyl-substituted μ-alkylidene. Moreover, since C(12)-C(13) [1.403(9) and 1.397(9) A˚; compare 1.417(5) A˚ in II] is shorter than C(13)C(14) [1.459(8) and 1.452(8) A˚; compare 1.455(6) A˚ in II], a description of the ligand as η1:η2-vinyl connected to an alkynyl and an amido functionality also appears appropriate (structure C). Concerning the vinyl sulfide group, it adopts a Z-configuration with C(21)-C(22) [1.360(9) and 1.317(9) A˚ for the two independent molecules, respectively] in the usual range for a double bond. The spectroscopic data of 6a,b are in agreement with the structure shown in the solid state (Figure 3) and are also consistent with the data reported for the related complex II (Scheme 6). The NMR data indicate that also in solution complexes 6a and 6b exist in a single isomeric form. This point is remarkable in consideration of the complex structure of the bridging frame, containing several insaturations, and of the number of bonds that have been generated in a single step. Thus, the observed assembly and transformation exhibit a considerable regio- and stereoselectivity. For example, S-C bond formation, between the bridging zwitterionic (19) Busetto, L.; Maitlis, P. M.; Zanotti, V. Coord. Chem. Rev. 2010, 254, 470.

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Scheme 7

ligand and the alkyne, occurs selectively at the primary carbon of the alkyne (anti-Markovnikov addition), and the resulting vinyl sulfide group exhibits a Z-configuration, as indicated by the coupling constant of the hydrogen atoms in the SCHdCHCO2Me group. Likewise, the arrangement of the bridging allylidene fragment formed by the incorporation of the CγCtCR unit takes place in a single conformation. 13C NMR resonances due to CR and Cδ evidence the strong similarity to the corresponding carbons in the bridging chain of II (e.g., for 6a at 175.1 and 155.3 ppm, respectively, vs 179.7 and 155.5 of II).18 Despite the complexity of the transformation occurring in the double alkyne addition, it is possible to formulate a hypothesis concerning the formation of 6a,b, based upon the acetylide addition reaction previously reported (Scheme 6). Indeed, it is reasonable to assume a multistep reaction sequence that includes acetylide attack at CO, similarly to that established in the case of the conversion of I to II. Accordingly, the following sequence can be proposed (Scheme 8), in which the initial step is the nucleophilic attack of the thiolate group to the alkyne. This is also consistent with the observation that alkynes with electron-withdrawing groups (e.g., COOMe) are required. The resulting intermediate should give proton abstraction from another alkyne molecule, instead of undergoing cyclization. This sequence should lead to the formation of a cationic intermediate E (Scheme 8), with an acetylide as counteranion. The latter might consequently undergo an intramolecular rearrangement similar to that described in Scheme 6, affording 6a,b as final products. Addition of Alkynes in the Presence of NH4PF6: Synthesis of Vinyl Chalcogenide Vinyliminium Complexes. Proton removal from alkyne reagent is a crucial step in the mechanism formulated in Scheme 8: it is required to form to a vinyl sulfide group, avoiding the formation of the thiophene (selenophene) ring, and it is also necessary to generate a nucleophilic acetylide, which sustains the subsequent addition and rearrangement. Any modification of the reaction conditions able to interfere with this step is expected to produce consequences in the reaction outcome, thus providing more clues on the reaction mechanism. On the basis of these considerations, we investigated the reaction with alkynes in the presence of a proton source, such as NH4PF6, that is more acidic and a better proton supply than primary alkynes. The result, shown in Scheme 9, is the following: complexes 2a,b react with an excess of alkyne (HC2CO2Me), in the presence of NH4PF6 in CH2Cl2 solution, to give the vinyl sulfide species 7a and 7b, respectively, instead of 6a and 6b.

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Organometallics, Vol. 29, No. 7, 2010 Scheme 8

Complexes 7a and 7b have been purified by chromatography on alumina and characterized by IR and NMR spectroscopy and elemental analysis. The IR spectra (in CH2Cl2 solution) show two ν-CO absorptions attributed to terminal and bridging carbonyls (at ca. 1990 and 1830 cm-1, respectively). Further adsorptions are due to the CO2Me group (at ca. 1700 cm-1) and CR-N bond (at ca. 1615 cm-1). The NMR spectra evidence the presence of a single product in a single isomeric form, indicating that the alkyne addition and subsequent protonation take place with the same regioand steroselectivity observed in the formation of 6a and 6b. More in detail, the 1H NMR resonances due to vinyl protons (e.g., for 7b, at 7.37 and 6.25 ppm) display a coupling constant of about 9.5 Hz. These values indicate that the vinyl sulfide group is originated from nucleophilic attack at the primary carbon of the alkyne and that the resulting alkene displays a Z-configuration. Comparison of the reactions shown in Schemes 5 and 9 clearly evidences that the presence of a readily available proton source (NH4PF6) modifies the reaction outcome: alkyne addition at the zwitterionic ligand and protonation to form a vinyl sulfide takes place without requiring proton abstraction from a second alkyne molecule. In the absence of an acetylide counteranion, further additions and rearrangements are blocked. Therefore, this result fits well in the above proposed mechanism, and complexes 7a and 7b are the equivalent of the intermediate E (Scheme 8). The presence of the non-nucleophilic PF6- anion in place of the acetylide anion makes the products more stable and prevents further rearrangements. The overall result shown in Scheme 9 consists in the hydrothiolation of alkynes, which selectively affords antiMarkovnikov products (7a and 7b), in the Z isomeric form. Indeed, sulfur-hydrogen bond addition to alkynes is a well-recognized synthetic method in C-S bond formation, (20) Peach, M. E. In The Chemistry of the Thiol Group; Patai, S., Ed.; Wiley: London, 1974; Vol. 2. (21) (a) Ichinose, Y.; Wakamatsu, K.; Nozaki, K.; Birbaum, J.-L.; Oshima, K.; Utimoto, K. Chem. Lett. 1987, 1647. (b) Benati, L.; Capella, L.; Montevecchi, P. C.; Spagnolo, P. J. Chem. Soc., Perkin Trans. 1995, 1035. (c) Griesbaum, K. Angew. Chem., Int. Ed. Engl. 1970, 9, 273. (22) (a) Truce, W. E.; Simms, J. A. J. Am. Chem. Soc. 1956, 78, 2756. (b) Carson, J. F.; Boggs, L. E. J. Org. Chem. 1966, 31, 2862. (c) Truce, W. E.; Tichenor, G. J. W. J. Org. Chem. 1972, 37, 2391. (d) Katritzky, A. R.; Ramer, W. H.; Ossana, A. J. Org. Chem. 1985, 50, 847. (e) Kondoh, A.; Takami, K.; Yorimitsu, H.; Oshima, K. J. Org. Chem. 2005, 70, 6468.

Busetto et al. Scheme 9

resulting in vinyl sulfides.20 Several procedures have been developed based on free radical21 or nucleophilic addition mechanisms,22 but these methods generally lack complete regio- and stereoselectivity. Conversely, the field of metalcatalyzed hydrothiolation,23 which promises higher selectivity, is rapidly expanding,24 also due to the increasing interest toward vinyl sulfides, as valuable synthetic intermediates. In the mechanism suggested above, alkyne hydrothiolation is initiated by a nucleophilic attack of the thiolate, followed by protonation. An alternative sequence, consisting in protonation of the S atom as initial step, is unlikely, in that protonation of the S atom in the zwitterionic complexes of type 2 is not achieved by using NH4PF6, but requires strong acids, such as HSO3CF3.6 Another point to be remarked is that hydrothiolation is competitive with cyclization: in other words, the initial nucleophilic S addition to the alkyne can be followed by protonation (e.g., to form complexes of the type 7) or by intramolecular attack at the iminium carbon with consequent cyclization (e.g., to form thiophene complexes of the type 4). Protonation seems more favorable, if there is a readily available proton source. This point is evidenced by observing that in the presence of NH4PF6 the reaction of 2 with (CO2Me)CtC(CO2Me), which normally forms the 1,3 cycloaddition product 4 (Scheme 2), affords the vinyl sulfide complex 7c, as shown in Scheme 9. Furthermore, the reaction has a more general character, in that other zwitterionc diiron complexes, containing S or Se, react with different alkynes (both primary and internal alkynes) in the presence of NH4PF6, yielding vinyl sulfide and vinyl selenide products (Scheme 10). Thus, it is appropriate to describe the reaction as a hydrochalcogenation of alkynes,25 which involves the bridging zwitterionic frame. In the case of primary alkynes the addition is regioselective anti-Markovnikov. Complexes 7c,d and 8a,b have been characterized by spectroscopy and elemental analysis. Their spectroscopic features resemble those discussed above for 7a,b. Chemical shifts and coupling constants of the vinyl protons in 8a indicate that the configuration of the vinyl unit is the same as that described for 6a,b and 7a,b (Z-configuration). In 7c, (23) Kondo, T.; Mitsudo, T. Chem. Rev. 2000, 100, 3205. (24) For recent examples see: (a) Field, L. D.; Messerle, B. A.; Vuong, K. Q.; Turner, P. J. Chem. Soc., Dalton Trans. 2009, 3599. (b) Weiss, C. J.; Wobser, S. D.; Marks, T. J. J. Am. Chem. Soc. 2009, 131, 2062. (c) Yang, J.; Sabarre, A.; Fraser, L. R.; Patrick, B. O.; Love, J. A. J. Org. Chem. 2009, 74, 182. (d) Wang, Z.-L.; Tang, R.-Y.; Luo, P.-S.; Deng, C.-L.; Zhong, P.; Li, J.-H. Tetrahedron 2008, 64, 10670. (e) Malyshev, D. A.; Scott, N. M.; Marion, N.; Stevens, E. D.; Ananikov, V. P.; Beletskaya, I. P.; Nolan, S. P. Organometallics 2006, 25, 4462. (25) Beletskaya, I. P.; Ananikov, V. P. Eur. J. Org. Chem. 2007, 3441.

Article

Organometallics, Vol. 29, No. 7, 2010

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Scheme 11a

Scheme 10

two isomers in comparable ratio are observed in solution. These are presumably attributable to E and Z isomers, due to lack of selectivity in the addition of the disubstituted alkyne. Conversely, the NMR spectra of complexes 7d and 8b contain a single set of resonances, indicating that the formation of the S(Se)-vinyl unit is stereoselective. However, NMR experiments did not clarify which configuration, E or Z, is adopted in such cases.

Conclusions The reactions of zwitterionic vinyliminium diiron complexes with alkynes provide multifaceted results, which account for the variety of activation modes offered by dinuclear coordination. Three different transformations have been evidenced: (a) 1,3 dipolar cycloaddition of alkynes with the Cþ-C-X- 1,3 dipole (X = S or Se) in the briding ligand, which yields thiophene or selenophene species; (b) hydrothiolation (hydroselenation) of alkynes obtained by chalcogenide nucleophilic addition in the presence of NH4PF6; (c) addition of two alkyne units, one undergoing hydrothiolation and the second incorporated in the bridging frame as alkynyl unit. Interestingly, the different possibilities can be controlled by choosing the appropriate reaction conditions. This point is well evidenced by the reactions of 2a shown in Scheme 11, since the products depend on the nature of the alkyne (primary and internal alkynes), and by the presence or absence of an external proton source. A further remarkable aspect is that each transformation is selective, and in most of the cases one single product in a single isomeric form is observed. Our findings add a further piece of evidence on the potential of bridging C3 ligands in diiron complexes in providing new synthetic approaches to uncommon species.

Experimental Section General Data. All reactions were routinely carried out under a nitrogen atmosphere, using standard Schlenk techniques. Solvents were distilled immediately before use under nitrogen from appropriate drying agents. Chromatography separations were carried out on columns of SiO2. Glassware was oven-dried before use. Infrared spectra were recorded at 298 K on a PerkinElmer Spectrum 2000 FT-IR spectrophotometer, and elemental analyses were performed on a ThermoQuest Flash 1112 Series EA instrument. All NMR measurements were performed on a Varian Mercury Plus 400 instrument. The chemical shifts for 1H

a

Ancillary Cp and CO ligands are omitted for clarity.

and 13C were referenced to internal TMS. The spectra were fully assigned via DEPT experiments and 1H,13C correlation measured through gs-HSQC and gs-HMBC experiments.26 Unless otherwise stated, NMR spectra were recorded at 298 K. NMR signals due to a second isomeric form (where it has been possible to detect and/or resolve them) are italicized. NOE measurements were recorded using the DPFGSE-NOE sequence.27 All the reagents were commercial products (Aldrich Co.) of the highest purity available and used as received. Complexes 1a,b and 2a,b were prepared by published methods.3 Synthesis of [Fe2{μ-K1(N):η1(C):η1(C)-Cγ(R0 )CβEC(R00 )dC(CO2Me)CrN(Me)(R)}(μ-CO)(CO)(Cp)2] (R = R0 = Me, E = Se, R00 = CO2Me, 3a; R = R0 = Me, E = Se, R00 = H, 3b; R = Xyl, R0 = Tol, E = S, R00 = CO2Me, 4). A solution of 1a (120 mg, 0.254 mmol), in CH2Cl2 (15 mL), was treated with C2(CO2Me)2 (0.10 mL, 0.81 mmol). The resulting mixture was stirred for 20 min. Removal of the solvent and chromatography of the residue on an alumina column, with CH2Cl2 as eluent, gave a brown band of 3a (133 mg, 85%). Crystals suitable for X-ray analysis were obtained by a CH2Cl2 solution layered with pentane, at -20 °C. Anal. Calcd for C24H25Fe2NO6Se: C, 46.94; H, 4.10; N, 2.28. Found: C, 47.02; H, 4.18; N, 2.36. IR (CH2Cl2): ν(CO) 1932 (vs), 1749 (s), 1735 (s), 1697 (m) cm-1. 1H NMR (CDCl3): δ 4.51, 4.38 (s, 10 H, Cp); 3.80, 3.70 (s, 6 H, CO2Me); 3.64 (s, 3 H, CγMe); 2.46, 2.01 (s, 6 H, NMe). 13C NMR (CDCl3): δ 289.1 (μ-CO); 215.1 (CO); 187.7 (Cγ); 168.0, 164.3, 163.2, 160.2 (CR, Cβ, and CO2Me); 137.9, 134.1 (CCO2Me); 87.8, 83.8 (Cp); 59.6, 52.7 (CO2Me); 51.9 (NMe); 47.4 (CγMe). Compounds 3b and 4 were obtained by the same procedure described for 3a, by reacting 1a and 2a with HCtCCO2Me and C2(CO2Me)2, respectively. 3b (yield: 75%). Anal. Calcd for C22H23Fe2NO4Se: C, 47.52; H, 4.17; N, 2.52. Found: C, 47.55; H, 4.06; N, 2.49. IR (CH2Cl2): ν(CO) 1925 (vs), 1750 (s), 1734 (s) cm-1. 1H NMR (CDCl3): δ (26) Wilker, W.; Leibfritz, D.; Kerssebaum, R.; Beimel, W. Magn. Reson. Chem. 1993, 31, 287. (27) Stott, K.; Stonehouse, J.; Keeler, J.; Hwang, T. L.; Shaka, A. J. J. Am. Chem. Soc. 1995, 117, 4199.

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Organometallics, Vol. 29, No. 7, 2010

8.58 (s, 1 H, CH); 4.50, 4.37 (s, 10 H, Cp); 3.87 (s, 3 H, CO2Me); 3.57 (s, 3 H, CγMe); 2.55, 2.22 (s, 6 H, NMe). 13C NMR (CDCl3): δ 291.3 (μ-CO); 215.9 (CO); 187.0 (Cγ); 167.2, 164.4, 163.4 (CR, Cβ, and CO2Me); 138.0, 133.5 (CCO2Me); 87.6, 83.8 (Cp); 57.2 (CO2Me); 51.7, 51.1 (NMe); 48.6 (CγMe). 4 (yield: 80%). Anal. Calcd for C37H35Fe2NO6S: C, 60.59; H, 4.81; N, 1.91. Found: C, 60.66; H, 4.75; N, 1.94. IR (CH2Cl2): ν(CO) 1933 (vs), 1756 (s), 1738 (s), 1695 (m) cm-1. 1H NMR (CDCl3): δ 8.02-6.60 (7 H, Me2C6H3 and MeC6H4); 4.84, 3.96 (s, 10 H, Cp); 4.01, 3.78 (s, 6 H, CO2Me); 2.70, (s, 3 H, NMe); 2.31, 2.24 (s, 6 H, Me2C6H3); 1.98 (s, 3 H, MeC6H4). 13C NMR (CDCl3): δ 276.6 (μ-CO); 218.7 (CO); 180.7 (Cγ); 168.9, 162.2 (CO2Me); 168.7 (Cβ); 156.2 (CR); 147.4 (Cipso-Xyl); 141.3 (Cipso-Tol); 135.8-128.4 (Carom and C-CO2Me); 89.3, 84.6 (Cp); 53.5, 51.7 (CO2Me); 42.4 (NMe); 21.3 (MeC6H4); 18.1, 17.7 (Me2C6H3). Synthesis of [Fe2{μ-Cγ(Me)CβSeC(CO2Me)dC(CO2Me)CrN(Me)2}(μ-CO)(CO)(CNBut)(Cp)2] (5). A solution of 3a (80 mg, 0.130 mmol), in THF (8 mL), was treated with CNBut (0.150 mmol). The mixture was heated at reflux temperature for 15 min; then it was allowed to cool to room temperature. Solvent removal and chromatography of the residue on an Al2O3 column with CH2Cl2 as eluent gave 5. Yield: 77 mg, 85%. Crystals suitable for X-ray analysis were obtained by a CH2Cl2 solution layered with pentane, at -20 °C. Anal. Calcd for C29H34Fe2N2O6Se: C, 49.96; H, 4.92; N, 4.02. Found: C, 50.02; H, 4.86; N, 3.95. IR (CH2Cl2): ν(CtN) 2122 (vs), ν(CO) 1942 (m), 1931 (s), 1762 (vs), 1750 (vs), 1732 (s), 1698 (m) cm-1. 1H NMR (CDCl3): δ 4.73, 4.59 (s, 10 H, Cp); 3.89, 3.76 (s, 6 H, CO2Me); 3.15 (s, 3 H, CγMe); 2.80 (s, 6 H, NMe); 0.96 (s, 9 H, But). 13C NMR (CDCl3): δ 280.1 (μ-CO); 213.2 (CO); 195.4 (Cγ); 168.7, 162.7 (CO2Me); 159.4 (Cβ); 142.3 (CR); 139.3, 124.5 (C-CO2Me); 122.0 (CN); 88.8, 87.6 (Cp); 60.0 (CC3H9); 56.5 (CγMe); 52.4, 51.9 (CO2Me); 44.3 (NMe); 29.9 (CC3H9). Synthesis of [Fe2{μ-K1(O):η1(C):η3(C)-Cδ(CtCCO2Me)Cγ(R0 )Cβ(SCHdCHCO2Me)-Cr(O)N(Me)(Xyl)}(μ-CO)(Cp)2] (R0 = Tol, 6a; R0 = Me, 6b). Compound 2a (100 mg, 0.169 mmol), in CH2Cl2 (15 mL), was treated with HCtC(CO2Me) (0.4 mmol). The solution was stirred for 20 min; then it was filtered through alumina. A brown band was collected by using CH2Cl2 as eluent, affording 6a, upon solvent removal. Yield: 99 mg, 77%. Anal. Calcd for C39H37Fe2NO6S: C, 61.68; H, 4.91; N, 1.84. Found: C, 61.76; H, 5.00; N, 1.79. IR (CH2Cl2): ν(CtC) 2162 (m), ν(CO) 1773 (s), 1698 (vs), ν(CdC) 1564 (w), ν(CRdO) 1512 (w) cm-1. 1H NMR (CDCl3): δ 7.90-6.90 (7 H, Me2C6H3 and MeC6H4); 7.38, 5.58 (d, 2 H, 3JHH = 10 Hz, CH); 4.65, 4.33 (s, 10 H, Cp); 3.82, 3.67 (s, 6 H, CO2Me); 2.39 (s, 3 H, NMe); 2.32 (s, 3 H, MeC6H4); 2.01, 1.41 (s, 6 H, Me2C6H3). 13C NMR (CDCl3): δ 286.6 (μ-CO); 175.1 (CR); 166.3, 153.0 (CO2Me); 155.3 (Cδ) 147.5 (Cβ); 141.7126.8 (Carom); 115.7, 89.3 (CtC); 111.0, 102.1 (CH); 106.7 (Cγ); 86.4, 85.2 (Cp); 52.3 50.9 (CO2Me); 40.0 (NMe); 19.4 (C6H4Me); 18.9, 17.5 (Me2C6H3). Compound 6b was obtained by the same procedure described for 6a, by reacting 2b with HC2(CO2Me). Crystals of 6b suitable for X-ray analysis were obtained by a CH2Cl2 solution layered with pentane, at -20 °C. 6b (yield: 92%). Anal. Calcd for C33H33Fe2NO6S: C, 58.00; H, 4.87; N, 2.05. Found: C, 58.05; H, 4.79; N, 2.00. IR (CH2Cl2): ν(CtC) 2169 (m), ν(CO) 1772 (s), 1697 (vs), ν(CRdO) 1521 (w) cm-1. 1H NMR (CDCl3): δ 7.34, 5.55 (d, 2 H, 3JHH = 9.51 Hz, CH); 7.27-6.80 (3 H, Me2C6H3); 4.60, 4.24 (s, 10 H, Cp); 3.91, 3.61 (s, 6 H, CO2Me); 3.00 (s, 3 H, CγMe); 2.40 (s, 3 H, NMe); 2.00, 1.48 (s, 6 H, Me2C6H3). 13C NMR (CDCl3): δ 287.8 (μ-CO); 175.3 (CR); 166.2, 154.2 (CO2Me); 156.4 (Cδ); 147.1 (Cβ); 142.1 (Cipso-Xyl); 134.8-127.2 (Carom); 115.8, 91.5 (CtC); 110.2, 101.8 (CH); 104.5 (Cγ); 85.4, 84.5 (Cp); 52.4, 50.9 (CO2Me); 39.9 (NMe); 23.0 (CγMe); 19.1, 17.2 (Me2C6H3). Synthesis of [Fe2{μ-η1:η3-Cγ(R0 )dCβ(ECR00 dCHCO2Me)CrdN(Me)(Xyl)}(μ-CO)(CO)(Cp)2][PF6] (R0 = Tol, E = S,

Busetto et al. R00 = H, 7a; R0 = Me, E = S, R00 = H, 7b; R0 = Tol, E = S, R00 = CO2Me, 7c; R0 = Me, E = S, R00 = CO2Me, 6e; R0 = Tol, E = Se, R00 = H, 8a; R0 = Tol, E = Se, R00 = CO2Me, 8b). To a solution of complex 2a (100 mg, 0.169 mmol), in CH2Cl2 (25 mL), were added NH4PF6 (400 mg, 2.454 mmol) and HCtC(CO2Me) (0.035 mL, 0.393 mmol) in the order given. The mixture was stirred for 2 h; then it was filtered on a Celite pad. Removal of the solvent gave a residue that was washed with diethyl ether (2  20 mL). Crystallization from a CH2Cl2 solution layered with diethyl ether, at -20 °C, gave 7a as a brown microcrystalline solid. Yield: 111 mg, 80%. Anal. Calcd for C35H34F6Fe2NO4PS: C, 51.18; H, 4.17; N, 1.71. Found: C, 51.26; H, 4.19; N, 1.79. IR (CH2Cl2): ν(CO) 1990 (vs), 1832 (s), 1702 (s) cm-1. 1H NMR (CDCl3): δ 7.65-7.26 (7 H, Me2C6H3 and MeC6H4); 7.36, 5.93 (d, 2 H, 3JHH = 9.88 Hz, SCHdCH); 5.17, 5.16 (s, 10 H, Cp); 3.69 (s, 3 H, CO2Me); 3.51 (s, 3 H, NMe); 2.55, 2.03 (s, 6 H, Me2C6H3); 2.43 (s, 3 H, MeC6H4). 13C NMR (CDCl3): δ 248.8 (μ-CO); 227.4 (CR); 211.5 (CO); 210.2 (Cγ); 166.2 (CO2Me); 149.2 (Cipso-Tol); 148.0, 114.4 (SCHdCH); 140.4 (Cipso-Xyl); 136.6-126.6 (Carom); 93.3, 88.6 (Cp); 63.7 (Cβ); 51.4 (NMe); 51.3 (CO2Me); 21.0 (MeC6H4) 18.0, 17.6 (Me2C6H3). Complexes 7b-d and 8a,b were prepared by the same procedure described for 7a, by reacting the appropriate alkyne with 2a,b and 1b, respectively. 7b (yield: 82%). Anal. Calcd for C29H30F6Fe2NO4PS: C, 46.74; H, 4.06; N, 1.88. Found: C, 46.77; H, 4.00; N, 1.79. IR (CH2Cl2): ν(CO) 1988 (vs), 1825 (s), 1701 (w) cm-1. 1H NMR (CDCl3): δ 7.45-7.10 (3 H, Me2C6H3); 7.37, 6.25 (d, 2 H, 3 JHH = 9.51 Hz, SCHdCH); 5.63, 4.93 (s, 10 H, Cp); 4.17 (s, 3 H, CγMe); 3.83 (s, 3 H, CO2Me); 3.40 (s, 3 H, NMe); 2.51, 2.01 (s, 6 H, Me2C6H3). 13C NMR (CDCl3): δ 250.4 (μ-CO); 226.5 (CR); 213.0 (CO); 210.5 (Cγ); 166.7 (CO2Me); 147.4, 117.0 (SCHdCH); 140.5 (Cipso-Xyl); 134.1-129.0 (Carom); 92.4, 89.1 (Cp); 66.6 (Cβ); 51.8 (NMe); 50.2 (CO2Me); 39.8 (CγMe); 17.9, 17.8 (Me2C6H3). 7c (yield: 74%). Anal. Calcd for C37H36F6Fe2NO6PS: C, 50.33; H, 4.13; N, 1.59. Found: C, 50.42; H, 4.03; N, 1.63. IR (CH2Cl2): ν(CO) 1998 (vs), 1834 (s), 1728 (s) cm-1. 1H NMR (CDCl3): δ 7.56-7.02 (7 H, Me2C6H3 and MeC6H4); 6.44, 5.95 (s, 1 H, SCdCH); 5.02, 4.99, 4.97, 4.88 (s, 10 H, Cp); 3.78, 3.75, 3.74, 3.73 (s, 6 H, CO2Me); 3.70, 3.57 (s, 3 H, NMe); 2.56, 2.04, 2.02 (s, 6 H, Me2C6H3); 2.44, 2.42 (s, 3 H, MeC6H4). Isomer ratio 1:1. 13C NMR (CDCl3): δ 249.9, 249.2 (μ-CO); 227.7, 226.8 (CR); 210.0, 209.8 (CO); 208.9, 208.6 (Cγ); 163.3, 163.2 (CO2Me); 149.6, 149.2 (Cipso-Tol); 140.4 (Cipso-Xyl); 137.6-124.0 (Carom); 120.3 (SCdCH); 93.1, 93.0, 88.8 (Cp); 64.6, 62.3 (Cβ); 53.7, 53.4 (NMe); 52.3, 52.1, 51.1, 50.5 (CO2Me); 21.2-17.7 (MeC6H4 and Me2C6H3). 7d (yield: 81%). Anal. Calcd for C31H32F6Fe2NO6PS: C, 46.35; H, 4.02; N, 1.74. Found: C, 46.39; H, 3.98; N, 1.80. IR (CH2Cl2): ν(CO) 1997 (vs), 1822 (s), 1737 (s), 1719 (s) cm-1. 1H NMR (CDCl3): δ 7.26-6.87 (4 H, Me2C6H3 and SCdCH); 5.22, 4.67 (s, 10 H, Cp); 3.97 (s, 3 H, CγMe); 3.75, 3.70 (s, 6 H, CO2Me); 3.43 (s, 3 H, NMe); 2.54, 2.06 (s, 6 H, Me2C6H3). 13C NMR (CDCl3): δ 252.0 (μ-CO); 226.5 (CR); 211.0 (CO); 209.8 (Cγ); 169.4, 165.5 (CO2Me); 141.7 (Cipso-Xyl); 134.0-128.6 (Me2C6H3); 113.0 (CH); 91.3, 87.6 (Cp); 69.4 (Cβ); 52.1 (NMe); 51.9, 49.9 (CO2Me); 38.2 (CγMe); 18.1, 17.5 (Me2C6H3). 8a (yield: 88%). Anal. Calcd for C35H34F6Fe2NO4PSe: C, 48.42; H, 3.95; N, 1.61. Found: C, 48.46; H, 4.00; N, 1.55. IR (CH2Cl2): ν(CO) 1998 (vs), 1823 (s), 1712 (w) cm-1. 1H NMR (CDCl3): δ 7.35-6.62 (9 H, Me2C6H3, MeC6H4, and SeCHdCH); 4.71, 4.37 (s, 10 H, Cp); 3.75 (s, 3 H, CO2Me); 2.96 (s, 3 H, NMe); 2.40 (s, 3 H, MeC6H4); 2.11, 1.91 (s, 6 H, Me2C6H3). 13C NMR (CDCl3): δ 252.3 (μ-CO); 229.3 (CR); 209.1 (CO); 207.7 (Cγ); 166.7 (CO2Me); 147.4-121.7 (Carom and SeCHdCH); 91.7, 87.0 (Cp); 68.5 (Cβ); 53.1 (CO2Me); 51.8 (NMe); 21.0 (MeC6H4); 17.7, 17.0 (Me2C6H3). 8b (yield: 79%). Anal. Calcd for C37H36F6Fe2NO6PSe: C, 47.98; H, 3.92; N, 1.51. Found: C, 48.03; H, 3.96; N, 1.46. IR

Article (CH2Cl2): ν(CO) 1997 (vs), 1820 (s), 1737 (s), 1719 (s) cm-1. 1H NMR (CDCl3): δ 7.63-6.85 (3 H, Me2C6H3); 5.46 (s, 1 H, SeCdCH); 4.59, 4.54 (s, 10 H, Cp); 3.75 (s, 6 H, CO2Me); 3.55 (s, 3 H, NMe); 2.43, 2.20 (s, 6 H, Me2C6H3); 2.39 (s, 3 H, C6H4Me). 13 C NMR (CDCl3): δ 253.4 (μ-CO); 229.6 (CR); 208.1 (CO); 206.4 (Cγ); 165.4 165.2 (CO2Me); 147.8-122.3 (Carom and SeC(CO2Me)dCH); 91.8, 88.0 (Cp); 69.1 (Cβ); 53.1 (NMe); 52.2, 51.9 (CO2Me); 21.1-17.7 (MeC6H4 and Me2C6H3). X-ray Crystallography. The diffraction experiments were carried out on a Bruker APEX II diffractometer equipped with a CCD detector using Mo KR radiation. Data were corrected for Lorentz polarization and absorption effects (empirical absorption correction SADABS).28 Structures were solved by direct methods and refined by full-matrix least-squares based on all data using F2.29 All hydrogen atoms were fixed at calculated positions and refined by a riding model. All non-hydrogen atoms were refined with anisotropic displacement parameters, unless otherwise stated. 3a 3 CH2Cl2. Similar U restraints were applied to the C atoms (s.u. 0.01). The Cp ligand bonded to Fe(2) is disordered over two positions. Disordered atomic positions were split and refined isotropically using one occupancy parameter per disordered group. 5. Similar U restraints were applied to the C, O, and N atoms (s.u. 0.01). The But group in the isonitrile ligand is disordered (28) Sheldrick, G. M. SADABS, Program for empirical absorption correction; University of G€ottingen: Germany, 1996. (29) Sheldrick, G. M. SHELX97, Program for crystal structure determination; University of G€ottingen: Germany, 1997.

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over two positions. Disordered atomic positions were split and refined isotropically using one occupancy parameter per disordered group. 6b. Two independent molecules are present within the unit cell, showing the same connectivity, only minor differences in the bonding parameters, and opposite absolute structure. The crystals display combined pseudomerohedral (monoclinic with β approximately 90°, which emulates orthorhombic) and racemic twinning. The appropriate twinning matrix was used during refinement (TWIN 1 0 0 0 -1 0 0 0 -1 -4), and the four twin components refined resulting in the following refined twin component factors: 0.20920, 0.40583, and 0.26416 (the fourth component is the complement at one). Similar U restraints were applied to the C and O atoms (s.u. 0.005), and rigid bond restraints (s.u. 0.005) were applied to all atoms.

Acknowledgment. We thank the Ministero dell’Universit a e della Ricerca (M.I.U.R.) (project: “New strategies for the control of reactions: interactions of molecular fragments with metallic sites in unconventional species”) and the University of Bologna for financial support. Supporting Information Available: Crystallographic data for compounds 3a, 5, and 6b in CIF format. Tables with selected bond lengths and bond angles for 3a, 5, and 6b. Crystal data and experimental details for 3a 3 CH2Cl2, 5, and 6b. This material is available free of charge via the Internet at http:// pubs.acs.org.