Multiple C–H Bond Activations and Ring-Opening C–S Bond

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Multiple C−H Bond Activations and Ring-Opening C−S Bond Cleavage of Thiophene by Dirhenium Carbonyl Complexes Richard D. Adams,* Poonam Dhull, and Jonathan D. Tedder Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States

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S Supporting Information *

ABSTRACT: The reaction of Re2(CO)8(μ-C6H5)(μ-H) (1) with thiophene in CH2Cl2 at 40 °C yielded the new compound Re2(CO)8(μ-η2-SC4H3)(μ-H) (2), which contains a bridging σ−π-coordinated thienyl ligand formed by the activation of the C−H bond at the 2 position of the thiophene. Compound 2 exhibits dynamical activity on the NMR time scale involving rearrangements of the bridging thienyl ligand. The reaction of compound 2 with a second 1 equiv of 1 at 45 °C yielded the doubly metalated product [Re2(CO)8(μ-H)]2(μ-η2-2,3-μ-η2-4,5C4H2S) (3), formed by the activation of the C−H bond at the 5 position of the thienyl ligand in 2. Heating 3 in a hexane solvent to reflux transformed it into the ring-opened compound Re(CO)4[μ-η5-η2-SCC(H)C(H)C(H)][Re(CO)3][Re2(CO)8(μ-H)] (4) by the loss of one CO ligand. Compound 4 contains a doubly metalated 1-thiapentadienyl ligand formed by the cleavage of one of the C−S bonds. When heated to reflux (125 °C) in an octane solvent in the presence of H2O, the new compound Re(CO)4[η5-μ-η2-SC(H)C(H)C(H)C(H)]Re(CO)3 (5) was obtained by cleavage of the Re2(CO)8(μ-H) group from 4 with formation of the known coproduct [Re(CO)3(μ3-OH)]4. All new products were characterized by single-crystal X-ray diffraction analyses.

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by a combination of IR, 1H NMR, mass spectrometry, and single-crystal X-ray diffraction analyses. An ORTEP diagram of the molecular structure of 2 is shown in Figure 1. Compound 2 contains a μ-η2-thienyl ligand formed by the reductive elimination of benzene from 1 and the oxidative addition of the C−H bond at the 2 position of the thiophene to the two Re atoms. The bridging thienyl ligand exhibits the classical σ + π coordination mode typically observed for bridging alkenyl ligands, 9 and both Re atoms in 2 have 18-electron configurations. The structure of 2 resembles that of the naphthyl compound Re2(CO)8(μ-η2-C10H7)(μ-H) that was obtained from the reaction of 1 with naphthalene, which contains a bridging η2-naphthyl ligand in a similar σ + π coordination mode.10 By contrast, the phenyl ligand is coordinated to the metal atoms in 1 as a bridging ligand by using only one C atom (Scheme 2). The S1−C2 bond distance in 2, 1.746(4) Å, next to the coordinated double bond is significantly longer than the S1−C5 bond distance, 1.704(6) Å, which is connected to the uncoordinated C−C double bond (C4−C5) in the bridging thienyl ligand. The C−S bond distances in thiophene are 1.714(1) Å.11 The coordinated C−C “double” bond, C2−C3 = 1.392(6) Å, has increased in length, while the uncoordinated C−C double bond, C4−C5 = 1.338(8) Å, has decreased in length compared to that of the

INTRODUCTION The activation of aromatic C−H bonds in arenes and heteroarenes by metal complexes has attracted considerable attention in recent years and led to important new routes to their functionalization.1,2 Sulfur-containing heteroarenes, such as thiophene, benzothiophene, and dibenzothiophene, are of interest because they are found in fossil fuels and they contribute to environmental pollution if the sulfur is not removed before the fuels are used. 3 Thiophenes are conveniently desulfurized by transition-metal catalysts4 and on metal surfaces.5 In recent studies, we have found that multiple additions of triosmium carbonyl clusters to furan can produce multiple CH activations that can lead to ring opening and decarbonylation of the furan ring, Scheme 1.6 We have also found that the dirhenium complex Re2(CO)8(μ-C6H5)(μ-H) (1) can engage in multiple CH activation reactions with arenes including reactions with itself (Scheme 2).7 We have now found that multiple additions of dirhenium carbonyl complexes to thiophene also produce a series of novel, multiple C−H bond activations in thiophene that ultimately lead to an opening of the thiophene ring by the cleavage of one of the C−S bonds under mild conditions.

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RESULTS AND DISCUSSION

The reaction of 18 with thiophene, SC4H4, in a CH2Cl2 solution at 40 °C for 6 h yielded the new compound Re2(CO)8(μ-η2SC4H3)(μ-H) (2) in 76% yield. Compound 2 was characterized © XXXX American Chemical Society

Received: April 19, 2018

A

DOI: 10.1021/acs.inorgchem.8b01091 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1

free molecule, CC = 1.370 (2) Å. Compound 2 also contains one bridging hydrido ligand H2 at −12.46 ppm. The mechanism for the formation of 2 from 1 is probably similar to the mechanism of exchange of benzene in 1 that was reported previously.10 The 13C NMR spectrum of 2 exhibits four resonances located at 148.09, 144.10, 128.61, and 113.46 ppm for the bridging thienyl ligand. The resonance at 113.46 ppm was not observed in the 13C DEPT (polarization transfer) spectrum, confirming that this one is the resonance of the bridging C2 atom. Interestingly, the carbonyl region of the spectrum of 2 exhibits only three resonances: two sharp singlets of intensity 2 at 186.68 and 180.58 ppm and a broad resonance of intensity 4 at 183.90 ppm. Suspecting dynamical activity, variable-temperature 13C NMR spectra were obtained for 2. A stacked plot of the 13C NMR spectra of 2 at temperatures from −80 to +40 °C is shown in Figure 2. At −80 °C, four CO resonances of equal intensity are observed at 187.21, 184.52, 183.56, and 181.37 ppm. For the structure of 2 as found in the solid state, all of the CO ligands are inequivalent. Assuming that the structure in solution is the same as that found in the solid state, the 13C NMR spectrum of 2 should show eight CO resonances in the absence of any dynamical averaging. Thus, the observed spectrum at −80 °C is indicating than an averaging process is already occurring and is rapid on the NMR time scale even at −80 °C. This could be explained by windshield-wiper-like flipping of the thienyl ligand between the two Re atoms, which would interchange the enantiomers of the molecules 2 and 2*, as shown at the top of Scheme 3. In this process, the atom C3 simply moves back and forth between the metal atoms Re1 and Re2. We could call this

Scheme 2

Figure 1. ORTEP diagram of the molecular structure of 2 showing 25% thermal ellipsoid probability. Selected interatomic bond distances (Å) are as follows: Re1−Re2 = 3.061(2), Re1−H2 = 1.910(5), Re2− H2 = 1.781(5), Re1−C2 = 2.186(4), Re2−C2 = 2.474.(4), Re2−C3 = 2.623(4), C2−C3 = 1.392(6), C3−C4 = 1.440(7), C4−C5 = 1.338(8), S1−C5 = 1.704(6), S1−C2 = 1.746(4).

Figure 2. Plot of the 13C NMR spectra in the CO ligand region for compound 2 recorded in a CD2Cl2 solvent. B

DOI: 10.1021/acs.inorgchem.8b01091 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Scheme 3. Mechanism To Explain the Dynamical Averaging of the CO Ligands in 2 Observed by 13C NMR Spectroscopy

a π−π flip process. In this process, the CO ligands on each Re atom will be averaged into four pairs by the dynamically produced mirror symmetry. A transition state, such as I, in which only the atom C2 of the thienyl ligand is coordinated in an η1 fashion to the metal atoms in a bridging position could be a reasonable intermediate in such a process. The 13C NMR spectrum for the CO ligands should exhibit four resonances, as observed. However, as the temperature is raised from −80 to +40 °C, the resonances at 184.52 and 183.56 ppm broaden, coalesce, and merge into a broad singlet of intensity 4 at +40 °C. This is indicative of the existence of another higher-energy dynamical process that averages two of the pairs of CO ligands but leaves the remaining two pairs unexchanged with each other. A higher-energy exchange process that could explain this is also shown in Scheme 3. By rotating the bridging thienyl ligand in I by 180°, C11 in the back is exchanged with C14 in the front and C21 in the back is exchanged with C24 in the front but an equivalent pair, C12 and C22, are not exchanged with the other pair, C13 and C23. From the exchange rate calculated to be 852/sec at the coalescence temperature of 10 °C, ΔG283⧧ of activation was calculated to be 11.9(3) kcal/mol for the higher-temperature exchange process. In recent studies, we have also observed that the η1-bridging phenyl ligand in the complex Os3(CO)10(μ-AuPPh3)(μ-η1-C6H5) also undergoes a similar rotational process perpendicular to the Os−Os bond.12

The broadness in all of the CO resonances at +40 °C could signal the onset of yet another dynamical process that leads to an averaging of all CO resonances, but higher-temperature spectra could not be recorded in the solvent that was used. Compound 2 (70% yield), together with a small amount of compound Re(CO)4[μ-η5-η2-SCC(H)C(H)C(H)][Re(CO)3][Re2(CO)8(μ-H)] (4; see below; 4.3% yield), can also be obtained conveniently from the reaction of Re2(CO)8(μ-H)[μη2-C(H)C(H)Bun] with thiophene when heated to 80 °C for 7 h in a solution in toluene solvent. Interestingly, the reaction of compound 2 with an additional quantity of 1 yielded two products, [Re2(CO)8(μ-H)]2(μ-η22,3-μ-η2-4,5-C4H2S) (3) in 53% yield and 4 in 24% yield, after 7 h at 45 °C by the loss of benzene from 1 and the addition of a second 1 equiv of Re2(CO)8 to 2. Compound 3 was independently converted to 4 in 70% yield by heating a solution in hexane solvent to reflux for 6 h. The molecular structure of 3 was confirmed by single-crystal X-ray diffraction analysis, and an ORTEP diagram of its molecular structure is shown in Figure 3. Compound 3 contains a 2,5-doubly-CH activated, dimetalated thiophene (thiendiyl) ligand bridged between two HRe2(CO)8 groups. In 3, both of the formal “double” bonds of the original thiophene molecule are coordinated to a HRe2(CO)8 subunit of the complex in the η1-η2-(σ + π)-bonding mode; see a further discussion of the C

DOI: 10.1021/acs.inorgchem.8b01091 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. ORTEP diagram of the molecular structure 3 showing 40% thermal ellipsoid probability. Selected interatomic bond distances (Å) are as follows: for molecule 1, Re1−Re2 = 3.0558(3), Re1−H2 = 1.89(6), Re2−H2 = 1.82(6), Re3−Re4 = 3.0549(3), Re3−H5 = 1.81(8), Re4−H5 = 1.90(7), Re1−C2 = 2.157(5), Re2−C2 = 2.433(5), Re2−C3 = 2.561(5), Re3−C5 = 2.162(5), Re4−C5 = 2.476(5), Re4−C4 = 2.543(5), S1−C5 = 1.761(5), S1−C2 = 1.771(5), C2−C3 = 1.392(7), C3−C4 = 1.453(7), C4−C5 = 1.405(7); for molecule 2, Re5−Re6 = 3.0453(3), Re7−Re8 = 3.0797(3), Re5−C6 = 2.167(5), Re6−C6 = 2.445(4), Re6−C7 = 2.569(5), Re7−C9 = 2.158(5), Re8−C9 = 2.473(5), Re8−C8 = 2.520(5), S2−C6 = 1.751(5), S2−C9 = 1.763(5), C6−C7 = 1.394(7), C7−C8 = 1.464(7), C8−C9 = 1.392(7).

bonding described below. Compound 3 contains an approximate C2 symmetry in the solid state. Thus in solution, the two hydrido ligands, −13.31 ppm, are equivalent and the two CH protons, 6.31 ppm, are equivalent, as observed in its 1H NMR spectrum. The Re−Re bond distances in 3, Re1−Re2 = 3.0558(3) Å and Re3−Re4 = 3.0549(3) Å, are similar to those in 2. Both C−S bonds in the thiendiyl ligand have increased in length compared to those found in 2 [S1−C2 = 1.771(5) Å and S1−C5 = 1.761(5) Å; molecule 2, S2−C6 = 1.751(5) Å and S2−C9 = 1.763(5) Å], presumably because of the additional disruption in the π-bonding network of the original thiophene ring by the metal atoms; see the molecular orbital (MO) calculations described below. Overall, the formation of compound 3 from 1 can be viewed as another example of a two-step “double” CH activation7 of the original thiophene molecule, with the CH activations occurring sequentially at the 2 and 5 positions of the thiophene ring. Compound 4 was also characterized by single-crystal X-ray diffraction analysis. An ORTEP drawing of the molecular structure of 4 is shown in Figure 4. Compound 4 contains a 1thiapentadienyl ligand that is doubly metalated by Re atoms at the 2 and 5 positions. It bridges all four of the Re atoms. The S and four C atoms of the original thiophene ligand are π-bonded to one metal atom Re4: Re4−C2 = 2.392(3) Å, Re4−C3 = 2.317(3) Å, Re4−C4 = 2.296(3) Å, Re4−C5 = 2.467(3) Å, and Re4−S1 = 2.4971(7) Å. Atoms Re1 and Re2 form a Re2(CO)8 subunit that bridges the S1−C2 edge of the metalated thiapentadienyl ligand: S1−C2 = 1.796(3) Å, Re1−S1 = 2.4831(7) Å, and Re2−C2 = 2.190(3) Å. The Re1−Re2 bond is long, Re1−Re2 = 3.29628(18) Å, because it contains a single bridging hydrido ligand, −14.91 ppm. Atom Re3 was inserted into the S1−C5 bond, Re3−C5 = 2.192(3) Å and Re3−S1 = 2.4629(7) Å. Assuming that the S atom donates two electrons

Figure 4. ORTEP diagram of the molecular structure of 4 showing 50% thermal ellipsoid probability. Selected interatomic bond distances (Å) are as follows: Re1−Re2 = 3.29628(18), Re1−H2 =1.73(4), Re2− H2 = 1.92(4), Re1−S1 = 2.4831(7), Re2−C2 = 2.190(3), Re3−C5 = 2.192(3), Re3−S1 = 2.4629(7), Re4−C4 = 2.296(3), Re4−C3 = 2.317(3), Re4−C2 = 2.392(3), Re4−C5 = 2.467(3), Re4−S1 = 2.4971(7), S1−C2 = 1.796(3), C2−C3 =1.400(4), C3−C4 = 1.458(4), C4−C5 = 1.379(4).

to Re1, two electrons to Re3, and one electron to Re4, then all of the metal atoms in 4 have 18-electron configurations. D

DOI: 10.1021/acs.inorgchem.8b01091 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 4

Figure 5. ORTEP diagram of the molecular structure of 5 showing 60% thermal ellipsoid probability. Selected interatomic bond distances (Å) are as follows: Re1−S1a = 2.4826(8), Re1−C5a = 2.198(7), Re2−C2a = 2.281(4), Re2−C3a = 2.300(5), Re2−S1a = 2.5183(9), Re2−C4a = 2.289(4), Re2−C5a = 2.460(8), S1−C2a = 1.754(4), C2a−C3a = 1.398(5), C3a−C4a = 1.456(5), C4a−C5a = 1.362(8).

Studies of the insertion of metal atoms into the C−S bonds of mononuclear metal thiophene complexes have indicated that S-coordinated intermediates are involved.15 Sattler and Parkin observed the formation of an η5-thiapentadienyl ligand in the complex (η5-C4H5S)W(PMe3)2(η2-CH2PMe2) formed by hydride transfer and insertion of the W atom of the complex W(PMe3)4(η2-CH2PMe2)H into a C−S bond of thiophene.16 Finally, when compound 4 was heated to reflux (125 °C) in octane solvent for 5 h in the presence of small amounts of H2O, the new compound Re(CO)4[η5-μ-η2-SC(H)C(H)C(H)C(H)]Re(CO)3 (5) was obtained in 77% yield together with the known compound [Re(CO)3(μ3-OH)]4, in 16% yield.17 The molecular structure of 5 is shown in Figure 5. Compound 5 contains a η5-thiapentadienyl ligand that is metalated only at the 5 position because one H atom was added to the atom C2a when the Re2−C2 bond in 4 was cleaved. It is π-bonded to Re2 and σ-bonded by the S atom and the terminal atom C5 to the atom Re1 [Re1−S1a = 2.4826(8) Å and Re1−C5a = 2.198(7) Å]. All of the metal atoms in 5 have 18-electron configurations. The disappearance of the HRe2(CO)8 fragment from 4 and the formation of [Re(CO)3(μ3-OH)]4 indicate that a simple water addition induced cleavage of the pendant HRe2(CO)8 group. When the reaction was performed in the absence of added water, these products were not formed. It is known that water reacts with Re2(CO)10 to yield [Re(CO)3(μ3-OH)]4. Removal

The insertion of metal-containing groups into the C−S bonds of thiophenes has been observed on a number of previous occasions.13,14 Some years ago, Angelici showed that Re2(CO)10 reacts with thiophenes to yield a ring-opened thiophene complex under the influence of UV irradiation.14 One product, Re2(CO)7(μ-2,5-Me2H2C4S), contains a monometalated η5-dimethylthiapentadienyl ligand coordinated to one Re atom and a Re(CO)4 group bridging the opened edge of the original 2,5-Me2C4S ligand similarly to that in 4. A possible pathway for the transformation of 3 to 4 is shown in Scheme 4. This pathway begins by an isomerization to a plausible intermediate I by shifting one of the Re2 groups from one of the C−S edges of the thiophene ligand, Re1−Re2, to the C2−S edge, as shown in Scheme 4. This could be facilitated by the formation of a bonding interaction to electrons located on the S atom (see below) to one of the Re atoms, e.g., Re2, step 1. The “double” bond between C2 and C3 is then released from coordination to Re1, and that bond is then added to Re4, step 2, which could, in turn, induce elimination of a CO ligand from Re4, step 3. Atom Re3 then inserts into the C5−S bond of the ring, step 4, and the bridging hydrido ligand is shifted to atom C5 to form a C−H bond and the Re3−Re4 bond is broken. Finally, a bond is established between Re4 and the S atom, and the transformation to compound 4 is completed. It is possible that these steps may occur in a different order. E

DOI: 10.1021/acs.inorgchem.8b01091 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 6. Selected MOs of thiophene including the HOMO−HOMO−3 of thiophene, with an ISO value equal to 0.03. The energy in electronvolts is given under the orbital. Atom color code: S, yellow; C, gray; H, white.

of the bridging HRe2(CO)8 fragment from 4 results in a slight shortening of the associated S−C bond distance, S1a−C2a = 1.754(4) Å in 5. The other bond distances and angles in 5 are similar to those in 4 and in Angelici et al.’s compound Re2(CO)7(μ-2,5-Me2H2C4S).14a The manganese homologue of 5 has been synthesized by a different method and was structurally characterized. It is structurally similar to that of 5.18 In order to obtain a better understanding of the bonding in these multicenter polynuclear metal coordination complexes, PBEsolD3 ADF DFT MO calculations were performed on thiophene, 3, and 4 (see the Supporting Information). To begin, we performed geometry-optimized MO calculations of the free molecule of thiophene. Selected MOs for the structure of thiophene are shown in Figure 6. Our geometry-optimized structure of thiophene compares favorably with the experimental structure11 and with that of other geometry-optimized calculations (see Table 1).19

HOMO and HOMO−1 are the two expected single-node out-of-plane π orbitals. The out-of-plane component on the S atom in HOMO−1 could be viewed as a “lone-pair-like” pair of electrons at that site, thus giving it the potential for ligation. HOMO−2 is a delocalized, multiple-node σ-type orbital that could contain some in-plane “lone-pair-like” character on the S atom. HOMO−3 is the fully delocalized out-of-plane π orbital. Other MOs of thiophene are shown in the Supporting Information. Selected MOs that emphasize the bonding between the thiendiyl ligand and the Re atoms of the geometry-optimized structure of 3 are shown in Figure 7. The thiendiyl ligand in the HOMO is related to the HOMO−1 of thiophene (see above). The HOMO−1 of 3 shows the nature of the interactions of a πlike orbital closely related to the HOMO of thiophene with the two Re2 groups. HOMO−8 shows some σ-bonding interactions of the Re atoms with the C atoms at the 2 and 5 positions of the ring. HOMO−12 shows interactions of the Re atoms with the out-of-plane π-like “lone” pair of electrons on the S atom of the HOMO−1 of thiophene. This interaction could represent a first stage of the transformation, step 1, shown in Scheme 4, which ultimately leads to the opening of the thiophene ring. HOMO−19 shows a highly delocalized orbital distribution above and below the plane of the thiendiyl ligand that is most closely related to the HOMO−3 of thiophene itself. Selected MOs that focus on the π-bonding interactions between the metalated thiapentadienyl ligand and the Re atoms of the geometry-optimized structure of 4 are shown in Figure 8.

Table 1. Bond Distances for the DFT Geometry-Optimized Structure of Thiophene

a

bond

distance (Å)a

distance (Å)b

distance (Å)c

S−C1 C1−C2 C2−C3

1.720 1.372 1.420

1.714(1) 1.370(2) 1.424(2)

1.736 1.367 1.429

This work. bExperimentally determined values.11 cB3LYP.18

Figure 7. Selected MOs for 3 including the HOMO, HOMO−1, HOMO−8, HOMO−12, and HOMO−19 (as viewed from the top). The ISO value equals to 0.03. The energy in electronvolts is also given under the orbital. Atom color code: Re, orange; S, yellow; O, red; C, gray; H, white. F

DOI: 10.1021/acs.inorgchem.8b01091 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 8. Selected MOs for compound 4. The ISO value equals to 0.04. The energy in electronvolts is given under the orbital. Atom color code: Re, orange; S, yellow; O, red; C, gray. for 2. IR (CH2Cl2): νCO 2112(w), 2087(m), 2014(vs), 1993(sh), 1961(s) cm−1. 1H NMR (CD2Cl2): δ 8.49−8.51 (d, 1H, H5, 3J = 5.1 Hz), 7.39−7.41 (d, 1H, H3, 3J = 3.3 Hz), 7.21−7.24 (dd, 1H, H4, 3J = 5.1 Hz, 4J = 3.3 Hz), −12.46 (s, 1H, H2, hydride). 13C NMR at 20 °C (CD2Cl2, 400 MHz): δ 186.68, 183.90, 180.58, 148.09, 144.10, 128.61, 113.46. 13C NMR (CD2Cl2, DEPT, 400 MHz): δ 148.09, 144.104 and 128.611. 13C NMR at −80 °C (CD2Cl2, 400 MHz): δ 187.21, 184.52, 183.56, 181.37, 147.34, 134.98, 128.84, 116.76. EI/MS: m/z 680 (M+), 652 (M+ − CO). This isotope distribution pattern is consistent with the presence of two Re atoms. Reaction of 2 with 1. A 45.0 mg amount (0.066 mmol) of 2 and 50.0 mg (0.074 mmol) of 1 were dissolved in 25 mL of methylene chloride. The solution was then heated to reflux for 7 h. After the reaction mixture was cooled, the solvent was removed in vacuo. The residue was extracted in CH2Cl2 and separated by TLC by using hexane solvent to yield a yellow band of 3 (45.0 mg; 53% yield) and a yellow band of 4 (20.0 mg; 24% yield). Spectral data for 3. IR (CH2Cl2): νCO 2110(w), 2089(m), 2024(vs), 1997(sh), 1984(sh), 1969(s) cm−1. 1H NMR (CD2Cl2): δ 6.31 (s, 2H, H3, H4), −13.31 (s, 2H, hydride ligands). EI/MS: m/z 1278 (M+), 1250 (M+ − CO). Spectral data for 4. IR (CH2Cl2): νCO 2119(w), 2095(m), 2043(vs), 2108(m), 2001(m), 1987(sh), 1961(s) cm−1. 1H NMR (acetone-d6): δ 6.93−6.95 (d, 1H, H3, 3J = 5.4 Hz), 5.69−5.75 (dd, 1H, H4, 3J = 5.7 Hz, 4J = 12 Hz), 5.44−5.48 (d, 1H, H5, 3J = 11.7 Hz), −14.91 (s, 1H, H2, hydride). EI/MS: m/z 1250 (M+), 1222 (M+ − CO). This isotope distribution pattern is consistent with the presence of four Re atoms. Conversion of 3 to 4. A solution of 30.0 mg (0.023 mmol) of 3 in 25 mL of hexane was heated to reflux for 6 h. After the reaction mixture was cooled, the solvent was removed in vacuo. The residue was extracted in CH2Cl2 and separated by TLC by using hexane to give a yellow band of 4 (20.0 mg; 70% yield). Synthesis of 2 and 4 from the Reaction of Re2(CO)8(μ-H)[μ-η2C(H)C(H)Bun] with Thiophene. A 100.0 mg amount (0.147 mmol) of Re2(CO)8(μ-H)[μ-η2-C(H)C(H)Bun] and 0.3 mL (3.75 mmol) of thiophene, C4H4S, were dissolved in 25 mL of toluene. The solution was then heated to 80 °C for 7 h. After the reaction mixture was cooled, the solvent was removed in vacuo. The residue was extracted in CH2Cl2 and separated by TLC by using hexane to give a yellow band of 2 (70.0 mg; 70% yield) and yellow band of 4 (8.0 mg; 4.3% yield). Reaction of 4 with H2O. A total of 10.0 mg (0.008 mmol) of 4 and 0.05 mL (2.78 mmol) of water were dissolved in 10 mL of octane. The solution was then heated to reflux for 5 h. The solution was then cooled, and the solvent was removed in vacuo. The residue was extracted in CH2Cl2 and separated by TLC by using hexane to give a

The extended π-like orbitals of the dimetalated thiapentadienyl ligand shown in HOMO−17 and HOMO−18 are related to the HOMO−3 of the original molecule of thiophene.

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CONCLUSIONS In this study, we have shown that thiophene undergoes facile CH activation reactions with compound 1 at the 2 position and at both the 2 and 5 positions of the ring by the reaction of 2 with a second 1 equiv of 1 to yield 3. The doubly activated thiendiyl ligand in 3 undergoes a facile opening of the thiendiyl ring by cleavage of one of its C−S bonds to yield a doubly metalated thiapentadienyl ligand in compound 4 that retains all four of the original Re atoms. Finally, when heated in the presence of water, the HRe2(CO)8 group in 4 was cleaved from the molecule and the remaining dirhenium portion was isolated as the free molecule 5.

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EXPERIMENTAL SECTION

General Data. All reactions were performed under a nitrogen atmosphere. Reagent-grade solvents were dried by the standard procedures and freshly distilled prior to use. IR spectra were recorded on a Nicolet IS10 mid-infrared FT-IR spectrophotometer. 1H NMR spectra were recorded on a Varian Mercury 300 spectrometer operating at 300 MHz. Variable-temperature 13C NMR spectra were recorded on a Bruker Avance III-HD spectrometer operating at 400 MHz. Mass spectrometry (MS) measurements performed by a directexposure probe using electron impact ionization (EI) were made on a VG 70S instrument. Re2(CO)10 and thiophene were obtained from STREM and Sigma-Aldrich and used without further purification. Re2(CO)8(μ-H)[μ-η2-C(H)C(H)Bun]20 and 18 were prepared according to previously reported procedures. Product separations were performed by thin-layer chromatography (TLC) in air on Analtech 0.25 and 0.5 mm silica gel 60 Å F254 glass plates. Reaction of 1 with Thiophene. A 20.0 mg amount (0.029 mmol) of 1 and 0.2 mL (2.50 mmol) of thiophene were dissolved in 1.6 mL of CD2Cl2 in a 5 mm NMR tube. The NMR tube was evacuated and filled with nitrogen. The NMR tube was then allowed to stand for 6 h at 40 °C. A 1H NMR spectrum obtained after this period showed a new hydride resonance at −12.46 ppm. The contents were then transferred to a flask, and the solvent was removed in vacuo. The residue was extracted in CH2Cl2 and separated by TLC by using hexane to give a yellow band of 2 (15.0 mg; 76% yield). Spectral data G

DOI: 10.1021/acs.inorgchem.8b01091 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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yellow band of 5 (4.0 mg; 77% yield) and [Re(CO)3(OH)]4 (3.0 mg; 16% yield). Spectral data for 5. IR (CH2Cl2): νCO 2095(m), 2043(vs), 1995(m), 1966(sh), 1944(s) cm−1. 1H NMR (CD2Cl2): δ 6.67−6.71 (t, 1H, H3, 3J = 6 Hz), 5.92−5.94 (d, 1H, H2, 3J = 6 Hz), 5.60−5.66 (dd, 1H, H4, 3J = 5.4 Hz, 4J = 12 Hz), 5.40−5.44 (d, 1H, H5, 3J = 12 Hz). EI/MS: m/z 652 (M+), 626 (M+ − CO). Computational Analyses. All DFT calculations were performed with the Amsterdam density functional theory ADF2014 program library by using the PBEsol-D3 functional with ZORA scalar relativistic correction and valence triple-ζ + 1 polarization, a relativistically optimized (TZP) Slater-type basis set with small frozen cores. For additional details, see the Supporting Information.

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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.inorgchem.8b01091. Details of the syntheses and characterizations of the new compounds, computational details, and selected MO diagrams (PDF) Accession Codes

CCDC 1838479−1838482 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected] (R.D.A.). ORCID

Richard D. Adams: 0000-0003-2596-5100 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Professor Vitaly Rassolov and Dr. Mark D. Smith for helpful discussions. We thank Dr. Perry Pellechia for recording the VT 13C NMR spectra of compound 2 for us. This research was supported by a grant from the National Science Foundation (Grant CHE-1464596).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b01091 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.8b01091 Inorg. Chem. XXXX, XXX, XXX−XXX