Article pubs.acs.org/Organometallics
Photochemical Alkyne Insertions into the Iron−Thiocarbonyl Bond of [Fe2(CS)(CO)3(Cp)2] Fabio Marchetti,‡ Stefano Zacchini,† and Valerio Zanotti*,† †
Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Via Moruzzi 13, I-56124 Pisa, Italy
‡
S Supporting Information *
ABSTRACT: Internal alkynes (RCCR) react with [Fe2(CS)(CO)3(Cp)2] (1), under UV radiation, to give the complexes [Fe2{μ-η1:η3-C(R)C(R)C S}(μ-CO)(CO)(Cp)2] [R = Et, 2a; R = Ph, 2b; R = CO2Me, 2c], as a result of CO displacement and alkyne insertion into the metal−CS bond of 1. In addition, the reaction of 1 with EtCCEt affords the metallacycle species [Fe2{μ-η2:η2-C(S)C(Et)C(Et)C(O)}(CO)2(Cp)2] (3) as secondary product. The molecular structures of 2a and 3 have been elucidated by X-ray diffraction studies. Compound 1 reacts with HCCCO2Me, affording the complex [Fe2{μ-η1:η3-C(H)C(CO2Me)CS}(μCO)(CO)(Cp)2] (2d) in modest yield. Complexes 2a,b undergo selective methylation at the sulfur atom, generating the cationic complexes [Fe2{μ-η1:η3-C(R)C(R)C(SMe)}(CO)(μ-CO)(Cp)2][CF3SO3] [R = Et, 4a; R = Ph, 4b]. The X-ray structure of 4a has also been determined. Finally, 4b undergoes nucleophilic addition of hydride and cyanide (from NaBH4 and NBun4CN, respectively) to form the vinylalkylidene complexes [Fe2{μ-η1:η3-C(Ph)C(Ph)C(X)(SMe)}(CO)(μ-CO)(Cp)2] [X = H, 5; X = CN, 6].
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INTRODUCTION Insertion of alkynes into the metal−carbon bond of transitionmetal carbenes (alkylidenes) is a key step in several synthetic protocols that make use of alkynes for the construction of cyclic and heterocyclic molecules,1 including the classic Dotz benzannulation.2 By contrast, insertion of alkynes into the metal−carbon bond of bridging alkylidenes in dinuclear complexes has been not developed, so far, into relevant synthetic methods, in spite of the fact that the reaction provides a convenient route to the formation of bridging vinylalkylidene complexes.3 One example is shown in Scheme 1 (entry a).4 The reactions usually require photolytic conditions, to remove one CO, or the presence of labile ligands (e.g., acetonitrile) as the equivalent of coordinative unsaturation, suggesting that the insertion presumably takes place via preliminary coordination of the alkyne. On the other hand, insertion of alkynes into the metal−carbon bond of bridging ligands is not limited to μcarbenes, but is extended to other bridging C1 ligands such as CO (Scheme 1, entry b),5 isocyanides (CNR),6 and carbynes (CR),7 resulting in a variety of C−C bond forming reactions that have no counterparts in the chemistry of mononuclear complexes. In particular, we have found that insertion of alkynes into bridging aminocarbynes in diiron complexes leads to the formation of bridging vinyliminium complexes8 (Scheme 1, entry c), which, in turn, show a remarkable and unique reactivity and provide unprecedented routes to C−C and C− heteroatom bond formation.9 On the basis of these results we decided to extend our investigations to the insertion of alkynes into bridging thiocarbonyl diiron complexes. Indeed, thiocarbonyl exhibits similar or even superior properties as a ligand compared to CO; © XXXX American Chemical Society
Scheme 1. Alkyne Insertion into the M−C Bond of Bridging Ligands: (a) Insertion into Bridging Alkylidene to Form μVinylalkylidene;4 (b) Insertion into Bridging CO;5a (c) Insertion into μ-Aminocarbyne8
however thiocarbonyl complexes are, by far, less common compared to metal carbonyls.10 The reason is mainly due to the lack of a direct source of CS and of straight synthetic methods. In spite of these limitations, a number of bridging thiocarbonyl Received: May 2, 2016
A
DOI: 10.1021/acs.organomet.6b00349 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
diffraction: the ORTEP molecular diagrams are shown in Figure 1 and 2, respectively, and relevant bond lengths and angles are reported in Tables 1 and 2.
complexes are known, including the diiron complex [Fe2(CS)(CO)3(Cp)2] (1).11 To the best of our knowledge, neither 1 nor other bridging thiocarbonyl complexes have been reported to undergo alkyne insertion into the metal−CS bond. Herein, we report on our investigations concerning the reactions of alkynes with the diiron bridging thiocarbonyl complex 1. The study is part of a more general effort directed toward two major goals. One is to exploit the peculiar reactivity of bridging ligands for the construction of unusual molecular fragments by assembling simple units, in this case CS and alkynes.9 The second point is the use of complexes containing iron, which is expected to provide sustainable alternatives to metal-mediated reactions based on rare and/or toxic metals.12
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RESULTS AND DISCUSSION Complex [Fe2(CS)(CO)3(Cp)2] (1) reacts with disubstituted alkynes RCCR (R = Et, Ph, CO2Me) under hν radiation, to give 2a−c as major products (45−65% yields; see Scheme 2). Scheme 2 Figure 1. ORTEP drawing of [Fe2{μ-η1:η3-C(Et)C(Et)CS}(μCO)(CO)(Cp)2] (2a). Thermal ellipsoids are at the 30% probability level.
The carbon atoms of the bridging frame have been numbered in Scheme 2 in order to make clearer data assignments in the following sections. The synthesis of 2a−c is the result of alkyne insertion into the metal−carbon bond of a bridging CS, promoted by release of CO under photolysis. Optimal reaction time ranges between 10 and 16 h. The synthesis of 2a has also evidenced the formation of a byproduct, which was isolated by chromatography (25% yield) and identified as the dimetallacycle complex 3 (Scheme 2). Formation of 3 can be considered as the result of insertion of both alkyne and CO into the iron-bridging thiocarbonyl bond. By contrast with 2a−c, there is no loss of CO in this case. So far, it appears difficult to formulate a reasonable reaction sequence leading to 3. We might observe that the peculiar character of the reaction, which is the incorporation of CO into a five-membered metallacycle, is not unique, in that it is frequently observed upon fragmentation of diiron complexes containing bridging vinyliminium ligands.13 Incorporation of CO is also observed in a few cases upon reaction of the bridging ligand without Fe−Fe fragmentation, such as in the formation of [Fe 2 {μ-η 1 :η 2 -C(X)S(Me)CH 2 C(O)}(CO)2(Cp)2]14 and [Fe2{μ-κ1(O):η1(C):η3(C)-C(N(Me)(Xyl))C(H)C(Me)C(O)OMe}(μ-CO) (Cp)2].15 Interestingly, the formation of 3 is restricted to the reaction of 1 with EtCCEt, in that no traces of a complex analogous to 3 have been detected in the reactions with C2Ph2 or C2(CO2Me)2. Compounds 2a−c and 3 have been purified by column chromatography on alumina and characterized by spectroscopy (IR and NMR) and elemental analysis. Moreover, the structures of 2a and 3 have been determined by X-ray
Figure 2. ORTEP drawing of [Fe2{μ-η2:η2-C(S)C(Et)C(Et)C(O)}(CO)2(Cp)2] (3). Thermal ellipsoids are at the 30% probability level.
Compound 2a is composed of a Fe2(μ-CO)(CO)(Cp)2 fragment and a bridging μ-η1:η3-C(Et)C(Et)CS ligand. The former shows a cis geometry of the Cp ligands, as previously found in analogous diiron and diruthenium complexes.6,8,16 The bridging ligand is closely related to the one reported for the complexes [Fe2{μ-η1:η3-C(R)C(R)C O}(μ-CO)(CO)(Cp)2],5c where the main difference is the replacement of the sulfur with oxygen. Thus, the bonding in the ligand can be described by two resonance forms (Scheme 3): in the first (a), the compound is composed of a dimetallacyclopententhione ring with the C(2)−C(3) double bond η2coordinated to Fe(1); conversely, the form (b) comprises a bridging alkylidene with a thioketene substituent [C(2)− C(1)−S(1)], which is η2-bonded through C(1)−C(2) to Fe(1). The structural data suggest that the actual bonding situation lies somewhere between these two resonance forms; that is, the “allylic” form (c) has some validity, but is B
DOI: 10.1021/acs.organomet.6b00349 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Table 1. Selected Bond Lengths (Å) and Angles (deg) for [Fe2{μ-η1:η3-C(Et)C(Et)CS}(μ-CO)(CO)(Cp)2] (2a) Fe(1)−Fe(2) Fe(2)−C(20) Fe(2)−C(12) Fe(1)−C(12) Fe(2)−C(3) Fe(1)−C(3) Fe(1)−C(1) Fe(1)−C(12)−Fe(2) Fe(1)−C(3)−Fe(2) Fe(1)−C(1)−C(2) Fe(1)−C(1)−S(1)
2.5325(7) 1.757(3) 1.906(3) 1.922(3) 1.975(3) 2.022(3) 1.889(3) 82.84(12) 78.63(10) 75.64(16) 143.20(18)
C(20)−O(20) C(12)−O(12) C(1)−S(1) C(1)−C(2) C(2)−C(3) Fe(1)−C(2)
1.131(4) 1.182(4) 1.605(3) 1.431(4) 1.407(4) 2.068(3)
S(1)−C(1)−C(2) C(1)−C(2)−C(3) C(2)−C(3)−Fe(2)
140.1(2) 116.6(2) 121.59(19)
Table 2. Selected Bond Lengths (Å) and Angles (deg) for [Fe2{μ-η2:η2-C(S)C(Et)C(Et)C(O)}(CO)2(Cp)2] (3) Fe(1)−Fe(2) Fe(2)−C(20) Fe(1)···C(20) Fe(2)···C(10) Fe(1)−C(10) Fe(2)−C(4) Fe(2)−C(1) Fe(1)−C(1) Fe(1)−C(1)−Fe(2) Fe(2)−C(4)−C(3) Fe(2)−C(4)−O(4) O(4)−C(4)−C(3) C(4)−C(3)−C(2) C(1)−C(2)−C(3)
2.6364(5) 1.762(2) 2.792(3) 2.901(24) 1.766(2) 2.0028(16) 1.9672(16) 1.9979(16) 83.35(6) 114.43(11) 123.42(13) 122.08(15) 114.62(13) 114.08(14)
Fe(1)−S(1) C(20)−O(20) C(10)−O(10) C(4)−O(4) C(3)−C(4) C(2)−C(3) C(1)−C(2) C(1)−S(1) C(2)−C(1)−S(1) C(2)−C(1)−Fe(2) C(2)−C(1)−Fe(1) C(1)−S(1)−Fe(1) Fe(1)−Fe(2)−C(20) Fe(1)−C(20)−Fe(2)
2.2802(8) 1.141(2) 1.141(3) 1.213(2) 1.476(2) 1.348(2) 1.470(2) 1.7155(15) 121.54(12) 116.13(10) 124.98(11) 57.96(6) 75.93(7) 66.33(6)
Å] interactions have the characters of double bonds, and the Fe(2)−C(3)−C(3)−C(2)−C(1) ring is almost perfectly planar [mean deviation from the least-squares plane 0.0267 Å]; moreover, O(4) lies very close to the plane of the ring [dihedral angle C(2)−C(3)−C(4)−O(4) 179.15(16)°]. As a consequence of the η2-coordination of the thioketone double bond, the C(1)−S(1) interaction [1.7155(15) Å] is longer than a pure double bond, and some deviation from the plane of the ring is observed [dihedral angle C(3)−C(2)−C(1)−S(1) 171.11(12)°]. Finally, the Fe(2)−C(4) [2.0028(16) Å], Fe(2)−C(1) [1.9672(16) Å], and Fe(1)−C(1) [1.9979(16) Å] interactions suggest an acyl character for the former and a bridging alkylidene nature for C(1). Spectroscopic data of the complexes 2a−c and 3 are detailed in the Experimental Section and are consistent with the structures established by X-ray diffraction. The 1H NMR spectra of 2a−c show a single set of resonances, indicating the presence in solution of a single isomeric form. Salient features for the complexes of type 2 include the 13C NMR resonances of the bridging carbons: C1, C2, and C3 (for 2a at 290.0, 64.1, and 206.0 ppm, respectively). The resonance due to the thiocarbonyl carbon (C1) occurs at significantly higher fields with respect to the parent compound 1,11 whereas C2 and C3 signals are in the typical regions for olefinic carbon and a bridging alkylidene carbon, respectively. The different nature assumed by the bridging frame in 3 is evidenced by the resonances of the corresponding carbons: C1, C2, C3, and acyl carbon (at 237.1, 188.6, 151.7, and 259.0 ppm, respectively). The IR ν CO absorption pattern [ν(CO) 1972 (vs), 1938 (s), 1591 (m)] also evidences the incorporation of a CO into the bridging frame, as well as the absence of any bridging CO ligand.
Scheme 3
considerably closer to (a) than (b). In fact, both C(1)−S(1) [1.605(3) Å] and C(2)−C(3) [1.407(4) Å] interactions are mainly double bonds, and the similar values for Fe(1)−C(2) [2.068(3) Å] and Fe(1)−C(3) [2.022(3) Å] are in agreement with a η2-coordinated alkene. Nonetheless, also the C(1)−C(2) interaction [1.431(4) Å] shows a partial double-bond character, suggesting that also form (b) has to be considered. Finally, it is noteworthy that the Fe(1)−C(1) interaction [1.889(3) Å] is quite short for a pure σ-Fe−C(sp2) interaction, indicating a good degree of π-back-donation. Compound 3 represents a rare case in which the Fe2(CO)2(Cp)2 moiety does not contain any bridging carbonyl, even though C(20)O(20) displays some semibridging character [Fe(1)···C(20) 2.792(3) Å; Fe(2)−C(20) 1.762(2) Å]. Interestingly, the Fe(1)−Fe(2) interaction [2.6364(5) Å] shows a significant lengthening compared to the one found in 2a [2.5325(7) Å], probably because of the presence of only a single bridging ligand. Moreover, the Cp ligands adopt a trans geometry relative to the Fe(1)−Fe(2)−C(1) plane. Concerning the bridging μ-η2:η2-C(S)C(Et)C(Et)C(O) ligand, this can be described as a 1-metalla-5-thioxo-3-cyclopenten-2-one composed by the Fe(2)−C(4)−C(3)−C(2)−C(1) ring, which is η2-coordinated to Fe(1) via C(1)−S(1). In agreement with this, both C(4)−O(4) [1.213(2) Å] and C(2)−C(3) [1.348(2) C
DOI: 10.1021/acs.organomet.6b00349 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
C3−H). All these findings suggest that the unique reactivity with methylpropiolate has to be associated with the activation of the triple bond due to conjugation with the carboxylate group. In complexes of type 2, the bridging fragment containing a conjugated thiocarbonyl function is potentially reactive toward electrophilic attack. In order to investigate this point, reactions of 2a,b with CF3SO3CH3 have been performed. Complexes 2a,b react with equimolar amounts of CF3SO3CH3, in CH2Cl2, yielding the cationic vinyl-thiocarbene complexes 4a and 4b, respectively (Scheme 5).
The reactions reported in Scheme 2 represent a unique example of alkyne insertion into the metal−carbon bond of a bridging CS. Indeed, even considering compounds in which the CS is bound to a single metal, insertion reactions involving alkynes are uncommon. Compared to CO, thiocarbonyl ligands display an enhanced propensity for entering into migratory insertion reactions, but this behavior is mostly restricted to sigma alkyl or aryl complexes.17 Insertion involving alkynes and CS would likely proceed through alkynyl or alkenyl intermediates, which remain difficult to observe,18 although these species are proposed as intermediates in the formation of metallacyclobutenethiones, metallabenzenes, or cyclopentadienethione complexes.19 Conversely, migratory insertion reaction involving bridging CS takes advantage of the stabilization provided by multisite coordination and provides a unique opportunity to observe CS−RCCR coupling as well as the resulting bridging thiocarbonyl−alkenyl fragment. Indeed, this latter molecular fragment is unique in terms of composition and coordination mode, in that few similarities can be envisaged with mononuclear complexes containing α,β-unsaturated thioacyl (thiocinnamoyl complexes)18,20 and, to a lesser extent, with thioketene bridging diiron complexes.21 We aimed to extend our investigation on the reactivity of 1 with alkynes; thus the reactions with monofunctionalized acetylenes have been examined. In principle, the incorporation of an asymmetrically substituted alkyne might afford isomeric forms, due to head-to-head or head-to-tail insertion modes. Indeed, the reaction of 1 with HCCCO2Me (see Scheme 4) leads to one single insertion product (2d), although isolated in yields not exceeding 40%, which is analogous to the complexes 2a−c described above.
Scheme 5
Complexes 4a,b have been spectroscopically characterized, and the molecular structure of 4a has been ascertained by X-ray diffraction: the ORTEP molecular diagram is shown in Figure 3, while relevant distances and bond angles are reported in
Scheme 4
A relevant point concerning the formation of 2d is that we have selectively obtained one of the two possible regioisomers generated by alkyne insertion. Indeed, complex 2d consists of a single isomeric form in which the alkyne primary carbon is bound to the Fe atom instead of the thiocarbonyl (see Scheme 4). This has been easily ascertained by 1H NMR spectroscopy, in that the C3H resonance has a distinctive methylidene proton character (δ 12.80 ppm). The other isomer would display a C2H resonance at about 4 ppm, as found in related complexes obtained by insertion of primary alkynes into a bridging aminocarbyne ligand (shown in Scheme 1, c).8 In that case the insertion reaction exhibits the opposite regioselectivity: the primary alkyne carbon, rather than the substituted one, is selectively bound to the carbyne carbon (C1). The reactions of 1 with HCCSiMe3 and HCCPh failed, and no stable products could be isolated. On the other hand, the reaction with HCCTol (Tol = 4-C6H4Me) allowed recovering in low yield a product whose IR (bands at 1977 and 1790 cm−1, in CH2Cl2) and 1H NMR spectra suggest the formula [Fe2{μ-η1:η3-C3(Tol)C2(H)C1S}(μ-CO)(CO)(Cp)2] (absence of typical low-field resonance attributable to
Figure 3. ORTEP drawing of [Fe2{μ-η1:η3-C(Et)C(Et)C(SMe)}(CO)(μ-CO)(Cp)2][SO3CF3] (4a). Thermal ellipsoids are at the 30% probability level.
Table 3. The molecule maintains the Fe2(μ-CO)(CO)(Cp)2 core present in the parent compound 2a. The bridging μ-η1:η3C(Et)C(Et)C(SMe) ligand, which directly derives from methylation of 2a, is closely related to the one previously reported for the bridging vinyliminium complexes [Fe2{μ-η1:η3C(R′)C(R″)CN(Me)(R)}(μ-CO)(CO)(Cp)2][SO3CF3]. C(1) shows partially the character of a thiocarbene, and, in fact, Fe(1)−C(1) [1.805(9), 1.856(10) Å, for the two independent molecules, respectively] is significantly shorter than in 2a [1.889(3) Å]. The C(2)−C(3) interaction [1.396(12), 1.420(13) Å] shows clearly some double-bond character, whereas C(1)−C(2) [1.434(13), 1.451(14) Å] is longer. D
DOI: 10.1021/acs.organomet.6b00349 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Table 3. Selected Bond Lengths (Å) and Angles (deg) for [Fe2{μ-η1:η3-C(Et)C(Et)C(SMe)}(CO)(μCO)(Cp)2][SO3CF3] (4a)a Fe(1)−Fe(2) Fe(2)−C(20) Fe(1)−C(12) Fe(2)−C(12) Fe(1)−C(1) Fe(1)−C(2) Fe(1)−C(3) Fe(2)−C(3) C(20)−O(20) C(12)−O(12) C(2)−C(3) C(1)−C(2) C(1)−S(1) S(1)−C(11) Fe(1)−C(3)−Fe(2) Fe(2)−C(3)−C(2) C(1)−C(2)−C(3) C(2)−C(1)−Fe(1) C(2)−C(1)−S(1) Fe(1)−C(1)−S(1) C(1)−S(1)−C(11) a
molecule 1
molecule 2
2.5503(17) 1.753(12) 1.982(10) 1.868(11) 1.805(9) 2.096(9) 2.039(8) 1.983(8) 1.139(13) 1.178(13) 1.396(12) 1.434(13) 1.661(9) 1.800(13) 78.7(3) 119.6(6) 114.5(8) 79.8(5) 130.8(7) 148.2(7) 104.1(5)
2.546(22) 1.762(12) 2.008(10) 1.888(10) 1.856(10) 2.060(9) 2.021(9) 1.975(10) 1.150(12) 1.143(11) 1.420(13) 1.451(14) 1.602(11) 1.757(15) 79.1(4) 118.8(7) 115.8(8) 76.0(6) 127.6(8) 153.0(6) 102.1(7)
Scheme 6
regioselective mode. In particular, NOE experiments conducted on 5 (in CDCl3 solution) have suggested that the C1-bound hydrogen is trans-located with respect to the C2−Ph ring. Indeed, a significant NOE effect was detected between one Cp (δ = 4.71 ppm) and C1-H (δ = 4.00 ppm); otherwise no NOE effect was found between S-Me and the Cp ligands. In both 5 and 6, the 13C NMR resonance of C1 appears greatly high field shifted compared to the parent complex 4b (5: Δδ = 214.0 ppm; 6: Δδ = 239.5 ppm), as a consequence of the loss of alkylidene character of C1. Nucleophilic addition at the C1 carbon (thiocarbene carbon) is accompanied by a rearrangement in the coordination mode of the resulting bridging fragment, which adopts a vinylalkylidene nature. This type of rearrangement is rather common in related vinyliminium complexes upon addition of nucleophiles (see Scheme 7).25 However, the similarity is not
Two independent molecules are present in the crystal.
The spectroscopic data of 4a,b are consistent with the structure found in the solid. Evidence for the presence of the SMe group is given by the 1H NMR resonance at ca. 3.50 ppm and by the corresponding 13C NMR resonance at about 22 ppm. In complex 4a, the resonances due to the C1, C2, and C3 are found at δ 280.7, 65.1, and 214.0 ppm, respectively. These values are comparable with those of the precursor 2a, indicating that methylation at the S atom does not significantly affect the bridging coordination mode. The resonance due to C1 undergoes some upfield shift, Δδ = 9.3 ppm, consistently with the carbene character of C1. As for the precursor complex 2, the nature of the bridging ligand and its coordination mode is unique. A related η1 thiabutadienyl ligand (RSCCH CPh2, R = nPr) is reported, but this involves a mononuclear ruthenium complex,22 and, to the best of our knowledge, there is no example of bridging coordination. As evidenced in the discussion of the structural data, the Fe− C1 interaction in 4a,b has some metal−carbene character. Like classic Fischer alkoxycarbenes, thiocarbene complexes are expected to react with nucleophiles, with some differences due to the nature of the heteroatoms.23 For example, the iron complex [FeCp(CO)2(CHSMe)]+ is known to undergo nucleophilic addition by a variety of nucleophiles, and the overall positive charge of the complex certainly contributes to enhance its reactivity.24 On the basis of these considerations, it is reasonable to predict that complexes of type 4 are susceptible to nucleophilic addition. The reaction of 4b with an excess of NaBH4, in THF solution, confirms this hypothesis, resulting in the formation of the bridging vinyl-alkylidene 5, in good yield (Scheme 6). Similarly, 4b reacts with NBut4CN, in CH2Cl2 solution, affording 6 (Scheme 6). Compounds 5 and 6 have been fully characterized by IR and NMR spectroscopy and elemental analysis (see Experimental Section). The NMR spectra contain single sets of resonances, indicating that the nucleophilic addition to 4b takes place in a
Scheme 7. Comparison with Vinyliminium Complexes21a
a
Ancillary ligands are omitted for clarity of presentation.
complete in that bridging vinyliminium complexes, in some cases, undergo addition at the α position (C2 carbon), affording bis-alkylidene derivatives, which is not observed in the reaction of 4b (Scheme 7).25 The reason seems to be steric rather than electronic, in that addition at the C2 carbon is observed only in the presence of sterically demanding substituents (such as xylyl) acting as a protecting group on the C1 carbon. This is not the case of 4b, in which C1 is sterically accessible by the incoming nucleophiles. A final observation concerns the nature of the bridging fragment resulting from the nucleophilic addition, which can be described as a μ-alkylidene containing a vinylthioether E
DOI: 10.1021/acs.organomet.6b00349 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
used as received. [Fe2(CO)4(Cp)2] was purchased from Strem and used as received. Compound [Fe2(CS)(CO)3(Cp)2] (1) was prepared by published methods.11b Synthesis of [Fe2{μ-η1:η3-C3(Et)C2(Et)C1S}(μ-CO)(CO)(Cp)2] (2a) and [Fe2{μ-η2:η2-C(S)C(Et)C(Et)C(O)}(CO)2(Cp)2] (3). A solution of [Fe2(CS)(CO)3(Cp)2] (1) (500 mg, 1.35 mmol), in THF (25 mL), was treated with EtCCEt (8.1 mmol), and the mixture was irradiated by a UV lamp for 15 h. The resulting solution was filtered on alumina; then the volatile materials were removed under reduced pressure. Hence, the solid residue was charged on an alumina column. Subsequent chromatography afforded two bands: elution with neat diethyl ether gave an emerald green fraction corresponding to compound 3, whereas 2a was obtained as a greenishbrown band by using a mixture (1:2 v/v) of CH2Cl2 and Et2O as eluent. Crystals of 2a and 3 suitable for X-ray analysis were obtained respectively by a CH2Cl2 solution layered with pentane and by a diethyl ether solution layered with pentane at −20 °C. 2a (yield: 52%). Anal. Calcd for C19H20Fe2O2S: C, 53.81; H, 4.75. Found: C, 53.77; H, 4.70. IR (CH2Cl2): ν(CO) 1978 (vs), 1789 (s). IR (KBr pellets): ν(CS) 1373 (s) cm−1. 1H NMR (CDCl3): δ 4.97, 4.57 (s, 10 H, Cp); 4.34, 4.25 (m, 2 H, C3CH2); 2.53, 1.44 (m, 2 H, C2CH2); 1.75 (t, 3 H, 3JHH = 7.32 Hz, C3CH2CH3); 1.25 (t, 3 H, 3JHH = 7.32 Hz, C2CH2CH3). 13C NMR (CDCl3): δ 290.0 (C1); 262.0 (μCO); 210.3 (CO); 206.0 (C3); 88.4, 87.7 (Cp); 64.1 (C2); 43.1 (C3CH2); 23.6 (C2CH2); 18.8 (C3CH2CH3); 11.7 (C2CH2CH3). 3 (yield: 25%). Anal. Calcd for C20H20Fe2O3S: C, 53.13; H, 4.46. Found: C, 53.12; H, 4.51. IR (CH2Cl2): ν(CO) 1972 (vs), 1938 (s), ν(CC) 1617 (m), ν(CO) 1591 (m) cm−1. 1H NMR (CDCl3): δ 4.42, 4.34 (s, 10 H, Cp); 2.84 (m, 2 H, CH2CH3); 2.42, 2.21 (m, 2 H, CH2CH3); 1.44, 1.06 (t, 6 H, 3JHH = 6.95 Hz, CH2CH3). 13C NMR (CDCl3): δ 259.9 (CO); 237.1 (C1); 225.2, 221.8 (CO); 188.6, 151.7 (C2 and C3); 91.5, 85.9 (Cp); 25.1, 20.1 (CH2CH3); 15.1, 14.1 (CH2CH3). Synthesis of [Fe2{μ-η1:η3-C3(R)C2(R′)C1S}(μ-CO)(CO)(Cp)2] [R = R′ = Ph, 2b; R = R′ = CO2Me, 2c; R = H, R′ = CO2Me, 2d]. These products were obtained by the same procedure described for 2a, by allowing 1 to react with the appropriate alkyne. 2b (yield: 65%; color: green; reaction time: 16 h). Anal. Calcd for C27H20Fe2O2S: C, 62.34; H, 3.88. Found: C, 62.28; H, 3.76. IR (CH2Cl2): ν(CO) 1984 (vs), 1796 (s). IR (KBr pellets): ν(CS) 1355 (s) cm−1. 1H NMR (CDCl3): δ 8.05−7.01 (m, 10 H, Ph); 4.89, 4.72 (s, 10 H, Cp). 13C NMR (CDCl3): δ 285.2 (C1); 260.8 (μ-CO); 210.1 (CO); 196.5 (C3); 155.3, 134.7 (ipso-Ph); 130.0, 127.8, 127.5, 127.1, 126.4, 125.2 (Ph); 90.1, 88.8 (Cp); 79.6 (C2). 2c (yield: 45%; color: brown; reaction time: 10 h). Anal. Calcd for C19H16Fe2O6S: C, 47.14; H, 3.33. Found: C, 47.30; H, 3.11. IR (CH2Cl2): ν(CO) 1992 (vs), 1819 (s), 1741 (m), 1715 (m) cm−1. 1H NMR (CDCl3): δ 5.10, 4.86 (s, 10 H, Cp); 3.81, 3.80 (s, 6 H, CO2Me). 13C NMR (CDCl3): δ 281.5 (C1); 256.1 (μ-CO); 208.8 (CO); 182.5, 172.5 (CO2Me); 86.0, 85.6 (Cp); 51.9, 51.8 (CO2Me) (C2 and C3 not observed). 2d (yield: 41%; color: brown; reaction time: 15 h). Anal. Calcd for C17H14Fe2O4S: C, 47.93; H, 3.31. Found: C, 48.01; H, 3.28. IR (CH2Cl2): ν(CO) 1984 (vs), 1797 (s), 1720 (m) cm−1. 1H NMR (CDCl3): δ 12.80 (s, 1 H, C3H); 5.08, 4.69 (s, 10 H, Cp); 3.75 (s, 3 H, CO2Me). 13C NMR (CDCl3): δ 280.5 (C1); 258.5 (μ-CO); 209.2 (CO); 180.8 (CO2Me); 156.1 (C3); 88.2, 87.1 (Cp); 67.9 (C2); 52.5 (CO2Me). Synthesis of [Fe2{μ-η1:η3-C3(R)C2(R)C1(SMe)}(CO)(μ-CO)(Cp)2][CF3SO3] [R = Et, 4a; R = Ph, 4b]. Complex 2a (95 mg, 0.224 mmol) was dissolved in CH2Cl2 (10 mL) and treated with CF3SO3CH3 (0.03 mL, 0.27 mmol). The solution was stirred for 30 min and then filtered on an alumina pad. Solvent removal and chromatography of the residue on an alumina column with a 1:1 mixture of methanol and THF afforded a band corresponding to 4a. Crystallization from a CH2Cl2 solution layered with diethyl ether, at −20 °C, gave dark green crystals, suitable for X-ray diffraction. Yield: 111 mg, 84%. Anal. Calcd for C21H23F3Fe2O5S2: C, 42.88 ; H, 3.94. Found: C, 42.92; H, 3.98. IR (CH2Cl2): ν(CO) 2009 (vs), 1826 (s) cm−1. 1H NMR (CDCl3): δ 5.40, 5.19 (s, 10 H, Cp); 4.33, 3.90 (m, 2
functional group. Vinylthioethers continue to attract interest due to many possible applications and the lack of easy synthetic procedures.26 It has to be remarked that the relative complexity of the bridging fragments in 5 and 6 is simply the result of a three-step assembly procedure: (i) alkyne insertion into the Fe−CS bond; (ii) S methylation, (iii) nucleophilic addition. All of these steps are made possible and assisted by bridging coordination, providing a further example of an unconventional synthetic approach based on the use of diiron complexes.
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CONCLUSIONS Alkyne−thiocarbonyl migratory insertion has been observed for the first time in a bridging thiocarbonyl complex, namely, in [Fe2(CS)(CO)3(Cp)2] (1). The reaction provides novel and unprecedented elements to understand the chemistry of bridging CS, in comparison with that of terminally bonded thiocarbonyls. Moreover, the insertion results in the formation of a bridging fragment (SC−CRCR) displaying peculiar features, in terms of both coordination mode and π bond delocalization. The bridging ligand can be further transformed upon methylation of the S atom and subsequent addition of nucleophiles (hydride, cyanide) at the thiocarbonyl carbon. Each step is accompanied and made possible by changes in the coordination mode. The overall sequence, shown in Scheme 8, Scheme 8. Step-by-Step Assembling of Small Molecular Units at a Diiron Center
offers an interesting example of assembling small molecular units that takes place on a diiron frame, providing new synthetic approaches to molecular fragments otherwise difficult to obtain.
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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 deactivated alumina (4% w/w water). Glassware was ovendried 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. ESI MS spectra were recorded on a Waters Micromass ZQ 4000 with samples dissolved in CH3CN. All NMR measurements were performed on a Mercury Plus 400 instrument. The chemical shifts for 1 H and 13C were referenced to internal tetramethylsilane. The assignment of spectra was assisted by DEPT experiments, and 1 13 H, C correlation measured using gs-HSQC and gs-HMBC experiments. All NMR spectra were recorded at 298 K. NOE measurements were recorded using the DPFGSE-NOE sequence. All the reagents were commercial products (Aldrich) of the highest purity available and F
DOI: 10.1021/acs.organomet.6b00349 Organometallics XXXX, XXX, XXX−XXX
Organometallics
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H, C3CH2); 3.49 (s, 3 H, SMe); 2.76 (m, 2 H, C2CH2); 1.70 (m, 3 H, C3CH2CH3); 1.24 (m, 3 H, C2CH2CH3). 13C NMR (CDCl3): δ 280.7 (C1); 250.0 (μ-CO); 214.0 (C3); 207.3 (CO); 91.6, 89.4 (Cp); 65.1 (C2); 45.2 (C3CH2); 23.5 (SMe); 22.8 (C2CH2); 18.8 (C3CH2CH3); 14.5 (C2CH2CH3). Complex 4b was prepared by the same procedure described for 4a, by allowing 2b to react with CF3SO3CH3. 4b (yield: 75%; color: dark green): Anal. Calcd for C29H23F3Fe2O5S2: C, 50.90; H, 3.39. Found: C, 50.93; H, 3.35. IR (CH2Cl2): ν(CO) 2012 (vs), 1843 (s) cm−1. 1H NMR (CDCl3): δ 7.63−6.86 (m, 10 H, Ph); 5.59, 5.17 (s, 10 H, Cp); 3.51 (s, 3 H, SMe). 13C NMR (CDCl3): δ 277.1 (C1); 246.2 (μ-CO); 207.2 (CO); 204.4 (C3); 153.4, 132.5 (ipso-Ph); 129.1, 128.9, 128.1, 127.6, 127.1, 126.6, 126.2 (Ph); 93.5, 88.4 (Cp); 72.9 (C2); 22.1 (SMe). Synthesis of [Fe2{μ-η1:η3-C3(Ph)C2(Ph)C1(X)(SMe)}(CO)(μCO)(Cp)2] [X = H, 5; X = CN, 6]. Complex 2b (0.130 mmol) was dissolved in THF (10 mL) and treated with NaBH4 (10 mg, 0.263 mmol) under stirring for 30 min. Then, the solvent was removed and the solid residue dissolved in CH2Cl2 (10 mL) and filtered on an alumina pad. The product was obtained as a brown powder upon removal of the solvent. Yield: 77%. Anal. Calcd for C28H24Fe2O2S: C, 62.71; H, 4.51. Found: C, 62.68; H, 4.46. IR (CH2Cl2): ν(CO) 1960 (vs), 1772 (s) cm−1. 1H NMR (CDCl3): δ 8.14−6.78 (m, 10 H, Ph); 4.71, 4.53 (s, 10 H, Cp); 4.00 (s, 1 H, C1H); 2.39 (s, 3 H, SMe). 13C NMR (CDCl3): δ 269.5 (μ-CO); 215.3 (CO); 194.4 (C3); 157.0, 142.9 (ipso-Ph); 131.5−124.2 (Ph); 89.8, 85.1 (Cp); 89.0 (C2); 63.1 (C1); 18.5 (SMe). Compound 6 was obtained by the same procedure described for 5, by allowing 2b to react with NBun4CN. 6 (yield: 65%; color: brown). Anal. Calcd for C29H23Fe2NO2S: C, 62.06; H, 4.13. Found: C, 62.01; H, 4.18. IR (CH2Cl2): ν(CN) 2193 (w), ν(CO) 1971 (vs), 1793 (s) cm−1. 1H NMR (CDCl3) δ 7.92−6.70 (m, 10 H, Ph); 4.92, 4.60 (s, 10 H, Cp); 2.33 (s, 3 H, SMe). 13 C NMR (CDCl3): δ 264.4 (μ-CO); 213.7 (CO); 195.1 (C3); 156.1 (ipso-Ph); 139.3, 131.3, 128.1, 127.9, 127.6, 127.3, 126.9, 124.5 (Ph); 123.0 (CN); 90.6, 86.9 (Cp); 90.2 (C2); 37.6 (C1); 17.8 (SMe). X-ray Crystallography. Crystal data and collection details for 2a, 3, and 4a are reported in Table S1 (Supporting Information). The diffraction experiments were carried out on a Bruker SMART 2000 diffractometer equipped with a CCD detector using Mo Kα radiation. Data were corrected for Lorentz polarization and absorption effects (empirical absorption correction SADABS).27 Structures were solved by direct methods and refined by full-matrix least-squares based on all data using F2.28 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. Two independent molecules are present in the asymmetric unit of 4a. The Cp ligand bound to Fe(1) in 2a and the ones bound to Fe(3) and Fe(4) in 4a, as well as one of the two [CF3SO3]− anions, are disordered. Disordered atomic positions were split and refined isotropically using similar distance and similar U restraints and one occupancy parameter per disordered group.
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ACKNOWLEDGMENTS We thank the Ministero dell’Università e della Ricerca Scientifica e Tecnologica (MIUR), the University of Bologna, and the University of Pisa for financial support.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00349.
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Article
Crystallographic parameters for 2a, 3, and 4a (PDF) Crystallographic data (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. G
DOI: 10.1021/acs.organomet.6b00349 Organometallics XXXX, XXX, XXX−XXX
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