CarbyneâCarbyne Coupling and H-Shifts in Reactions of the

Article pubs.acs.org/Organometallics

Carbyne−Carbyne Coupling and H‑Shifts in Reactions of the Unsaturated Methoxy- and Hydroxycarbyne Complexes [Mo2Cp2(μCOR)(μ-CPh)(μ-PCy2)]+ with CO and Isocyanides M. Angeles Alvarez, M. Esther García, Sonia Menéndez, and Miguel A. Ruiz* Departamento de Química Orgánica e Inorgánica/IUQOEM, Universidad de Oviedo, E-33071 Oviedo, Spain S Supporting Information *

ABSTRACT: The unsaturated methoxycarbyne complex [Mo2Cp2(μ-COMe)(μ-CPh)(μ-PCy2)](CF3SO3) (Cp = η5-C5H5; Mo−Mo = 2.4707(3) Å) reacted with CO (293 K, 40 bar) or CNR (233 K, R = tBu, Xyl) to give the corresponding methoxyalkynebridged derivatives [Mo2Cp2{μ-η2:η2-C(OMe)CPh}(μ-PCy2)L2](CF3SO3) following from a reductive C−C coupling between methoxycarbyne and benzylidyne ligands (L = CO, CNR). This coupling could be fully reversed for the dicarbonyl product upon photolysis in tetrahydrofuran solution. The related hydroxycarbyne complex [Mo2Cp2(μ-COH)(μ-CPh)(μ-PCy2)]BF4 reacted analogously with CO (293 K, 4 bar) to give the hydroxyalkyne-bridged derivative [Mo2Cp2{μ-η2:η2-C(OH)CPh}(μPCy2)(CO)2]BF4 (Mo−Mo = 2.6572(5) Å) as a result of C−C coupling between hydroxycarbyne and benzylidyne ligands, but this process could not be reversed photochemically. The latter complex could be prepared more efficiently via protonation of the ketenyl precursor [Mo2Cp2{μ-C(Ph)CO}(μ-PCy2)(CO)2] with HBF4·OEt2 in dichloromethane solution. The hydroxycarbyne complex also reacted with CNtBu and CNXyl to give C−C coupled products, but different than anticipated: in both cases this reaction yielded selectively the corresponding aminoalkyne-bridged derivatives [Mo2Cp2{μ-η2:η2-C(NHR)CPh}(μ-PCy2)(CNR)2]BF4 (Mo−Mo = 2.6525(5) Å when R = tBu), as a result of H-transfer from hydroxycarbyne to isocyanide ligands and subsequent C−C coupling between aminocarbyne and benzylidyne ligands.

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INTRODUCTION The coordinative and electronic unsaturation of binuclear cyclopentadienyl complexes having metal−metal triple bonds provides them with a high reactivity under mild conditions, thus allowing the synthesis of molecules that cannot be readily prepared using more conventional (i.e., electron-precise species) synthetic routes.1 Some time ago we reported that the 30-electron alkoxycarbyne complexes [Mo2Cp2(μ-COMe)(μ-COR)(μ-PCy2)]BF4 (R = Me, Et) underwent easy C−C coupling between their carbyne ligands upon addition of simple donors (L) such as CO and isocyanides, to give the corresponding alkyne-bridged derivatives [Mo2Cp2{μ-η2:η2C(OMe)C(OR)}(μ-PCy2)L2]BF4, a transformation that could be reversed for the dicarbonyl complexes under photochemical conditions.2 Several aspects in these reactions are of interest: First of all, even if carbyne−carbyne coupling reactions are themselves well documented at polynuclear species,3 and have been postulated as the critical step in the reductive and related couplings of CO and CNR ligands at different mononuclear complexes,4 only a few examples have been reported previously where these transformations can be reversed to yield the carbyne precursors, all of them occurring at polynuclear Fe3 (COR/CR coupling),5 OsW2 (CR/CR coupling),6 and Fe4 clusters (CH/CH couplings).7 Thus, our reactions proved at the time that two metal sites are enough to achieve such a coupling in a reversible way. Second, it should be noted that carbyne−carbyne coupling reactions are of general interest in © 2015 American Chemical Society

the context of the Fischer−Tropsch chemistry, since the latter involves C−C couplings among different hydrogenation products of CO, notably methyne and methylene groups,8 but perhaps even hydroxycarbyne ligands.9,10 Finally, our coupling reactions yielded rare metal-bound alkyne molecules which are themselves unstable in the free state, such as dimethoxyacetylene.11 In a recent preliminary study, we could extend this chemistry to the benzylidyne- and methoxycarbynebridged complex [Mo 2 Cp 2 (μ-COMe)(μ-CPh)(μ-PCy2 )](CF3SO3) (2), a species easily prepared upon alkylation of the carbonyl-bridged precursor [Mo2Cp2(μ-CPh)(μ-PCy2)(μCO)] (1) (Scheme 1). Indeed this cation underwent related benzylidyne−methoxycarbyne couplings upon addition of CO and CNtBu, a process being also reversible for the carbonyl derivative. 12 A natural question emerging from these preliminary studies was whether similar C−C couplings might be induced when using hydroxycarbyne instead of alkoxycarbyne ligands, but little could be anticipated about this possibility since, as we have noted previously, the chemistry of hydroxycarbyne ligands has been little explored at all, mainly because of the low stability even when bound to several metal centers.10 Rearrangement into the corresponding hydride/ carbonyl isomers is a common degradation path for hydroxycarbyne complexes. For instance, the hydroxy- and Received: February 27, 2015 Published: April 30, 2015 1681

DOI: 10.1021/acs.organomet.5b00166 Organometallics 2015, 34, 1681−1691

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Organometallics

salts were very similar to each other, thus indicating little anion−cation interaction, in spite of the unsaturated nature of these 30-electron complexes. Therefore, the geometrical information obtained from salt 2′ can be reliably considered as valid for other salts of the same cation, and even for the related hydroxycarbyne complex 3. The structure of the cation of 2′ in the crystal (Figure 1 and Table 1) is built from two MoCp moieties bridged symmetri-

Scheme 1

methoxycarbyne-bridged complex [Mo 2 Cp 2 (μ-COH)(μCOMe)(μ-PCy2)]BF4 rearranges into its hydride carbonyl isomer rapidly above 253 K,2a,13 thus precluding any C−C coupling studies on this substrate. Recently, however, we reported that complex [Mo2Cp2(μ-COH)(μ-CPh)(μ-PCy2)]BF4 (3), which is the hydroxycarbyne analogue of compound 2, could be conveniently prepared upon oxidation of the carbonylbridged complex 1 in the presence of BH3·NH2tBu as H source (Scheme 1), and would last in solution for several hours at room temperature.14 This circumstance should enable us to explore some of its reactivity, particularly its C−C coupling chemistry, which is the main goal of the present investigation. We here report completed C−C coupling studies of compound 2 and its extension to the hydroxycarbyne analogue 3 by analyzing their behavior toward CO and isocyanides. As it will be shown below, we have observed for the first time that the hydroxycarbyne ligand can also couple to another carbyne, but also is involved in easy H-shift reactions eventually resulting in coupling between benzylidyne and aminocarbyne ligands.

Figure 1. ORTEP drawing (30% probability) of the cation in compound 2′, with H atoms and cyclohexyl rings (except the C1 atoms) omitted for clarity.

Table 1. Selected Bond Lengths (Å) and Angles (deg) in Compound 2′ Mo1−Mo2 Mo1−P1 Mo2−P1 Mo1−C1 Mo2−C1 Mo1−C13 Mo2−C13 C1−O1 O1−C2 C13−C14

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RESULTS AND DISCUSSION Structure of the Starting Methoxy- and Hydroxycarbyne Complexes. In our preliminary study, we described the preparation of the methoxycarbyne complex 2 upon reaction of the neutral carbonyl-bridged complex 1 with methyl triflate in toluene solution (Scheme 1).12 Although the corresponding spectroscopic data were fully consistent with the proposed structure (see below), in the context of its C−C coupling chemistry it was of interest to get metric information on the relevant interatomic distances, so an X-ray diffraction study of this complex was undertaken. We were unable to grow suitable crystals of the triflate salt 2, but succeeded with the corresponding tetrafluoroborate salt [Mo2Cp2(μ-COMe)(μCPh)(μ-PCy2)]BF4 (2′), a product easily prepared analogously upon reaction of 1 with (Me3O)BF4 in dichloromethane solution. We noticed that a side product was obtained in variable amounts in both of the above reactions. This side product has been now identified as the oxo carbene cation cis[Mo2Cp2(μ-η1:η3-CHPh)(O)(μ-PCy2)(CO)]+, which follows from competitive oxidation of 1 and subsequent H capture by the resulting radical 1+ from trace amounts of water in the solvent,14 and its formation can be minimized by using reduced volumes of solvent (see the Experimental Section). We have also prepared the tetraarylborate salt [Mo2Cp2(μ-COMe)(μCPh)(μ-PCy2)]BAr′4 (2″) upon cation exchange in 2 with NaBAr′4 in dichloromethane solution (Ar′ = 3,5-C6H3(CF3)2, see the Experimental Section). Spectroscopic data for all these

2.4707(3) 2.4103(8) 2.4221(8) 2.017(3) 1.997(3) 2.004(3) 2.002(3) 1.315(4) 1.453(3) 1.444(4)

Mo1−P1−Mo2 Mo1−C1−Mo2 Mo1−C13−Mo2 P1−Mo1−C1 P1−Mo1−C13 P1−Mo2−C1 P1−Mo2−C13 C1−Mo1−C13 C1−Mo2−C13 C1−O1−C2

61.50(2) 76.0(1) 76.2(1) 88.3(1) 87.8(1) 88.5(1) 87.5(1) 91.3(1) 91.9(1) 117.5(2)

cally by dicyclohexylphosphide, benzylidyne, and methoxycarbyne ligands, with a short intermetallic length of 2.4707(3) Å comparable to the one measured in the precursor 1 (2.464(1) Å),12,15 or in the related bis(methoxycarbyne) complex [Mo2Cp2(μ-COMe)2(μ-PCy2)]BF4 (2.474(1) Å).2a This is a characteristic feature of different 30-electron carbyne-bridged complexes of the type [M2Cp2(μ-PR′2)(μ-CR)(μ-Y)]n+ (Y = 2or 3-electron donor; n = 0, 1), for which metal−metal triple bonds should be formulated according to the 18 electron rule formalism,1a a view also supported by DFT calculations.13,14 The methoxycarbyne ligand adopts the characteristic coplanar conformation relative to the Mo2C ring which allows for the operation of some π bonding interaction reinforcing the O− C(carbyne) bond in detriment of the Mo−C bond (Figure 2).13,16 This is consistent with the short C1−O1 separation of 1.315(4) Å (cf. 1.453(3) Å for the single O1−C2 bond), itself comparable to those measured in [Mo2Cp2(μ-COMe)2(μPCy2)]BF4 (ca. 1.32 Å).2a Since this effect cannot occur in the benzylidyne ligand (aryl ring not coplanar with Mo2C ring), we would have anticipated slightly higher values for the 1682


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molecules of donors such as CO and isocyanides, and this triggers a reductive C−C coupling reaction between the carbyne ligands that renders a dimetal-bound alkyne molecule. The first reaction takes place slowly at room temperature under pressure (ca. 40 bar) to give the alkyne-bridged dicarbonyl derivative [Mo2Cp2{μ-η2:η2-C(OMe)CPh}(μ-PCy2)(CO)2]BF4 (4), whereas reactions with alkyl or aryl isocyanides CNR occur readily even at 233 K to give the analogous bis(isocyanide) derivatives [Mo2Cp2{μ-η2:η2-C(OMe)CPh}(μPCy2)(CNR)2]BF4 (R = tBu (5a), Xyl (5b), Scheme 2), with

Figure 2. Canonical forms in alkoxycarbyne-bridged binuclear complexes.

corresponding Mo−C(OMe) lengths (compared to the Mo− C(Ph) ones), but the actual difference in 2 is not significant (ca. 2.01 vs 2.00 Å). Yet, these distances are shorter than typical single-bond lengths in bridging carbonyls (ca. 2.10 Å in 1), in agreement with the formal bond order of 1.5 to be proposed for the carbyne Mo−C bonds. Finally, we note that the bridging carbyne ligands in 2 define a Mo2(μ-C)2 dihedral angle of ca. 131°, close to the one measured in the mentioned bis(methoxycarbyne) complex, and are also placed at a relatively short nonbonding distance from each other (2.88 Å), a circumstance that should allow for development of a similar C− C coupling chemistry, as indeed is the case. Spectroscopic data in solution for 2 (Table 2 and Experimental Section) are fully consistent with the structure

Scheme 2

Table 2. Selected IRa and 31P{1H} NMR Datab for New Compounds compound [Mo2Cp2(μ-CPh)(μ-PCy2)(μ-CO)] (1)c [Mo2Cp2(μ-COMe)(μ-CPh)(μ-PCy2)] (CF3SO3) (2) [Mo2Cp2(μ-COMe)(μ-CPh)(μ-PCy2)]BF4 (2′) [Mo2Cp2(μ-COMe)(μ-CPh)(μ-PCy2)]BAr′4 (2″) [Mo2Cp2(μ-COH)(μ-CPh)(μ-PCy2)]BF4 (3)d [Mo2Cp2{μ-η2:η2-C(OMe)CPh}(μ-PCy2) (CO)2](CF3SO3) (4) [Mo2Cp2{μ-η2:η2-C(OMe)CPh}(μ-PCy2) (CNtBu)2](CF3SO3) (5a) [Mo2Cp2{μ-η2:η2-C(OMe)CPh}(μ-PCy2) (CNXyl)2](CF3SO3) (5b) [Mo2Cp2{μ-η2:η2-C(OH)CPh}(μ-PCy2) (CO)2]BF4 (6) [Mo2Cp2{μ-η2:η2-C(NHtBu)CPh}(μ-PCy2) (CNtBu)2]BF4 (7a) [Mo2Cp2{μ-η2:η2-C(NHXyl)CPh}(μ-PCy2) (CNXyl)2]BF4 (7b)

ν(CX) 1686 (s)

δP 228.5 265.6

no intermediates being detected even when using 1 equiv of reagent. An important difference in the above coupling reactions is that the first one can be fully reversed by removing the CO ligands. Thus, complex 2 was recovered quantitatively upon irradiation of dicarbonyl 4 with visible−UV light in tetrahydrofuran solution at 285 K for a few minutes. All the above complexes were generated as mixtures of geometrical isomers (Figure 3), with the major species being in

265.3 265.9

1986 (w, sh), 1962 (s) 2137 (s), 2118 (m, sh) 2122 (m, sh), 2088 (s) 1995 (w, sh), 1961 (s) 2106 (s), 2049 (w, sh) 2094 (m, sh), 2073 (s)

254.8 155.3e 170.9f 167.8g 152.0 172.1 176.7

a

Recorded in dichloromethane solution, unless otherwise stated, with C−O and C−N stretching bands [ν(CX)] in cm−1. bRecorded in CD2Cl2 at 121.50 MHz and 295 K. cData taken from ref 15. dData taken from ref 14. eThe minor isomers cis give resonances at 186.2 and 190.7 ppm (see text). fThe minor cis-syn isomer gives a resonance at 179.0 ppm (see text). gThe minor isomers give resonances at 177.4 (cis-syn) and 180.4 ppm (cis-anti, see text). Figure 3. Geometrical isomers present in the solutions of compounds 4 and 5 (L = CO, CNR).

in the solid and moreover comparable to those measured for 3, [Mo2Cp2(μ-COMe)2(μ-PCy2)]BF4, and related carbyne complexes, then deserving no detailed analysis. We just note that the carbyne ligands give rise to strongly deshielded 13C NMR resonances at 422.0 (CPh) and 365.6 ppm (COMe) as expected, but just to a single 1H and 13C NMR resonance for the strictly inequivalent Cp rings. The latter indicates fast rotation of the OMe group around the O−C(carbyne) bond, a circumstance commonly found in methoxycarbyne-bridged complexes.13 Addition Reactions of the Methoxycarbyne Complex 2. Due to its unsaturated nature, complex 2 readily adds two

all cases the one with a transoid arrangement of the terminal CO or CNR ligands. The ratio of isomers trans/cis-syn/cis-anti for the dicarbonyl complex 4 was ca. 10:1:1, and only limited spectroscopic data could be obtained for the very minor isomers (see the Experimental Section). The isocyanide complex 5a was obtained as a mixture of just two isomers in a relative ratio trans/cis of ca. 2:1, with little influence of reaction conditions (temperature, reagent concentration) on this ratio, and the minor isomer presumably has a cis-syn 1683


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Ph)15 and therefore suggestive of the alkyne acting as a fourelectron donor to the dimetal center. Alkoxyalkyne-bridged complexes still are relatively rare species. Actually, compound 5a remains as the unique structurally characterized example of a complex with an alkoxyalkyne ligand coordinated in a μ-η2:η2 tetrahedrane-like fashion. We can also quote one example of an asymmetric μη1:η2 coordination mode, found in the heterometallic complex [CoWCp(O)(μ-PPh2){μ-C(p-tol)C(OMe)}(CO)(PPh2H)]BF4,17 and a few clusters displaying triply bridging μ3-η1:η2:η1alkoxycarbyne ligands.5,18 In contrast, more examples of mononuclear complexes are known, most of them made through electrophile-induced carbyne−carbonyl coupling or alkylation of ketenyl precursors.4 Interestingly, all structurally characterized mononuclear alkoxyalkyne complexes display also four-electron donor ligands,19 with W−C lengths (1.99−2.03 Å) expectedly shorter than those in 5b (2.12−2.27 Å), but comparable alkyne C−C lengths (in the range 1.18−1.34 Å, to be compared with 1.328(8) Å in 5b). Spectroscopic data for compounds 4 and 5 (Table 2 and Experimental Section) are fully consistent with the retention of the dimetal-bound alkyne ligands in solution, which give rise to diagnostic 13C NMR resonances at ca. 140 and 125 ppm corresponding to the bridgehead carbons of the COMe and CPh groups, respectively, located more than 200 ppm upfield from the corresponding resonances in the carbyne precursors. The 31P NMR resonances for the major isomers (trans) are found in the range 155−170 ppm, some 100 ppm more shielded than the PCy2 resonance in the precursor 2, which is consistent with the reduction of the intermetallic bond order from 3 to 2, and therefore with the four-electron contribution proposed for the alkyne ligand (for instance, the 31P chemical shift of 1 is reduced from 228.5 to 117.6 ppm upon addition of a single CO molecule to give the 32-electron dicarbonyl [Mo2Cp2(μ-CPh)(μ-PCy2)(CO)2]).2b The minor cis isomers give rise to a single Cp resonance in each case, and display 31P NMR resonances more deshielded than the corresponding trans isomers. This is a general trend found previously for pairs of cis and trans isomers of the type [Mo2Cp2(μ-PR2)(μX)(CO)2] (X = PR2, CRCR′2),20 and also holds for isomers of the related dialkoxyacetylene complexes [Mo2Cp2{μ-η2:η2C(OMe)C(OR)}(μ-PCy2)(CO)2]BF4 (R= Me, Et).2 The IR spectrum of 4 displays C−O stretching bands with the pattern (weak and strong, in order of decreasing frequencies) characteristic of M2(CO)2 oscillators with almost antiparallel carbonyl ligands,21 as expected for the major trans isomer. The same holds for the C−N stretching bands in compound 5b, with a quite dominant proportion of the trans isomer. In contrast, the IR spectrum of 5a displays a strong C−N stretch at 2137 cm−1 with a shoulder of medium intensity at lower frequency (2118 cm−1), which we attribute to the presence of significant amounts (ca. 35%) of the minor cis-syn isomer in the corresponding reactions mixtures, as noted above. Synthesis and Structural Characterization of the Hydroxyalkyne-Bridged Complex 6. Reaction of the hydroxycarbyne complex 3 with CO was faster than that of its methoxycarbyne analogue 2, and proceeded even under a moderate CO overpressure of ca. 4 bar to give the hydroxyalkyne-bridged complex [Mo2Cp2{μ-η2:η2-C(OH)CPh}(μ-PCy2)(CO)2]BF4 (6), following analogously from C−C coupling between hydroxycarbyne and benzylidyne ligands (Scheme 3). However, since 3 is typically generated in situ along with small amounts of some side products

geometry, by analogy with the structure found for the related compound 7a (see below). In contrast, three isomers were obtained for the xylylisocyanide complex 5b, with a significant influence of reaction conditions on their ratio, it being ca. 10:4:2 for reactions carried out at room temperature, whereas only the trans and cis-syn isomers were obtained at 203 K (ratio ca. 20:1). Since no conversion between isomers was observed in solution at room temperature, we conclude that the above differences have a kinetic origin, but we have not investigated this matter in detail. We note, however, that the related aminoalkyne complexes 7 also display similar isomerism (see below), while the related dimethoxyacetylene complex [Mo2Cp2{μ-η2:η2-C(OMe)C(OMe)}(μ-PCy2)(CNtBu)2]BF4 was formed as a single isomer (trans) from the corresponding bis(methoxycarbyne) precursor.2 Structure of the Alkyne-Bridged Complexes 4 and 5. The solid-state structure of the isocyanide complex 5a was determined during our preliminary study of this chemistry12 and corresponds to the major (trans) isomer exhibited by all these products (Figure 4). The cation is built from two

Figure 4. ORTEP drawing (30% probability) of the cation in compound 5a, with H atoms, cyclohexyl rings, and tBu groups (except the C1 atoms) omitted for clarity.12 Selected bond lengths (Å) and angles (deg): Mo(1)−Mo(2) = 2.6377(6), Mo(1)−C(1) = 2.115(6), Mo(2)−C(1) = 2.227(5), Mo(1)−C(3) = 2.269(6), Mo(2)−C(3) = 2.149(6), C(1)−C(3) = 1.328(8), Mo(1)−C(20) = 2.075(6), Mo(2)−C(25) = 2.038(6), Mo(1)−P(1) = 2.379(1), Mo(2)−P(1) = 2.380(2); C(20)−Mo(1)−Mo(2) = 98.0(2), C(25)−Mo(2)− Mo(1) = 87.7(2).

MoCp(CNtBu) moieties arranged in a transoid disposition, bridged symmetrically by a dicyclohexylphosphide ligand, and by a (methoxy)phenylacetylene ligand placed almost perpendicularly to the intermetallic vector (twist angle ca. 82°). The overall structure is very similar to the one previously determined by us for the dimethoxyacetylene complex [Mo2Cp2{μ-η2:η2-C(OMe)C(OMe)}(μ-PCy2)(CNtBu)2]BF4 (twist angle 78°),2 and the metric parameters are also comparable. In particular, the intermetallic separation of 2.6377(6) Å (vs 2.6550(5) Å) is just marginally shorter than the values of ca. 2.66 Å measured for the 32-electron carbyne complexes [Mo2Cp2(μ-CX)(μ-PCy2)(CO)2] (X = CO2Me,2b 1684


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Figure 5. ORTEP drawing (30% probability) of the cation in compound 6, with H atoms and cyclohexyl rings (except the C1 atoms) omitted for clarity.

(notably, the oxo carbene complex cis-[Mο2Cp2(μ-η1:η3CHPh)(O)(μ-PCy2)(CO)]+),14 it was difficult to get very pure samples of 6 out from these reaction mixtures. A more convenient preparation of 6 was implemented via protonation of the ketenyl complex [Mo2Cp2{μ-C(Ph)CO}(μ-PCy2)(CO)2] with HBF4·OEt2 in dichloromethane solution. This route exploits the nucleophilic nature of the ketenyl oxygen atom and has been widely used in mononuclear complexes to yield hydroxy- and alkoxyalkyne derivatives.4a In our case, this reaction expectedly takes place with retention of the overall stereochemistry (trans) of the ketenyl-bridged precursor, to yield complex 6 specifically as the isomer trans. We examined the reversibility of the C−C coupling leading to 6 by inspecting the photochemical decarbonylation of the latter species. Unfortunately, all photochemical experiments carried out on 6 gave mixtures of products that could not be identified, but certainly contained no significant amounts of the hydroxycarbyne complex 3. The carbyne−carbyne coupling leading to 6 seems to lack precedent in the case of hydroxycarbyne ligands. The wellestablished “reductive coupling” method developed by Lippard and co-workers on mononuclear complexes does not work to build hydroxyalkyne ligands (ynols), which are instead generated by hydrolyzing the corresponding silylalkyne complexes.4b Alternatively, hydroxyalkyne complexes have been generated through acid-mediated carbyne−carbonyl coupling, with this presumably involving ketenyl intermediates.4c Still another possible synthetic route relies on the reaction of CO with molecules already having carbyne and hydride ligands, as illustrated by the reaction of the cation [Co3Cp3(μ3CH)2(μ-H)]+ with CO to yield the ethynol-bridged cluster [Co3Cp3(μ3-CH)(μ-η1:η2:η1-C(OH)CH)]+.22 It should be noted that the critical C−C coupling step in the reductive coupling method has been proposed to follow two possible pathways: (a) sequential formation of two carbyne ligands, followed by carbyne−carbyne coupling, and (b) sequential formation of carbyne and ketenyl ligands, followed by electrophile attack on the latter. It is worth noting that both routes are efficient synthetic strategies to build the hydroxyalkyne-bridged complex 6. The structure of 6 in the crystal (Figure 5 and Table 3) is comparable to that of the methoxyalkyne complex 5a discussed above and needs not a very detailed discussion. The metal skeleton is now built from two MoCp(CO) fragments arranged in a transoid conformation and symmetrically bridged by a PCy2 ligand, and by a hydroxyalkyne ligand with its C−C bond defining an angle of ca. 78° relative to the intermetallic vector (cf. 82° in 5a). The most significant interatomic distances are

Table 3. Selected Bond Lengths (Å) and Angles (deg) in Compound 6 Mo1−Mo2 Mo1−P1 Mo2−P1 Mo1−C1 Mo2−C2 Mo1−C3 Mo2−C3 Mo1−C4 Mo2−C4 C3−O3 C3−C4 C4−C5

2.6572(5) 2.3949(11) 2.4071(11) 1.970(5) 2.004(5) 2.304(4) 2.124(4) 2.143(4) 2.300(4) 1.351(5) 1.335(6) 1.466(6)

Mo1−P1−Mo2 Mo1−C3−Mo2 Mo1−C4−Mo2 P1−Mo1−C1 P1−Mo2−C2 P1−Mo1−C3 P1−Mo2−C3 P1−Mo1−C4 P1−Mo2−C4 O3−C3−C4 C3−C4−C5 C3−O3−H3 Mo1−Mo2−C2 Mo2−Mo1−C1

67.19(3) 73.6(1) 73.4(1) 88.1(1) 86.6(1) 104.6(1) 110.2(1) 108.9(1) 103.4(1) 134.1(4) 135.3(4) 130(6) 89.2(1) 82.6(1)

analogous to those found in 5a, such as the short Mo−Mo and C−C separations of 2.6572(5) and 1.335(6) Å respectively, or the dissymmetric Mo−C(alkyne) lengths in the range 2.12− 2.30 Å. Only one other hydroxyalkyne-bridged binuclear complex appears to have been structurally characterized so far, it being the heterometallic complex [MoW(μ-PPh2){μ-C(OH)C(ptol)}(η5-C2B9H11)(η7-C7H7)(CO)], a species formed upon reaction of a carbyne-bridged dicarbonyl precursor with the secondary phosphine PHPh2.23 The coordination of the hydroxyalkyne ligand in this heterometallic complex, however, is much more asymmetric than the one observed in 6, and it can be viewed as intermediate between the μ-η2:η2 coordination mode found in 6 and a μ-η1:η2 mode (Figure 6). Other coordination modes of ynol ligands structurally characterized so far are the μ3-η1:η2:η1 mode found in the mentioned Co3 cluster,22 and the η2-mode identified in a few mononuclear tungsten complexes.19c,24

Figure 6. Extreme coordination modes in four-electron-donor bridging hydroxyalkyne and aminoalkyne ligands (X = OH, NR2). 1685


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Structural Characterization of Aminoalkyne Complexes 7. The structure of the CNtBu derivative 7a in the crystal (Figure 7 and Table 4) is built up from two cisoid

Spectroscopic data for 6 in solution are fully consistent with the structure found in the crystal and not very different from those of the methoxyalkyne complex 4 (Table 2 and Experimental Section). The transoid arrangement of the carbonyl ligands is evidenced by the appearance of two C−O stretches with the expected pattern which, when combined with the distinct substituents at the alkyne ligand, renders inequivalent pairs of Cp and CO ligands, readily apparent in the 1H and 13 C NMR spectra. The OH proton gives rise to a characteristically deshielded NMR resonance at 9.74 ppm, and the retention of the μ-η2:η2 coordination of the hydroxyalkyne ligand in solution is indicated by the observation of similarly shielded resonances for the bridgehead carbon atoms, which appear at 139.6 (COH) and 90.0 (CPh) ppm. Although these shifts differ from each other by more than observed in compounds 4 and 5 (δC ca. 140 and 125 ppm for the COMe and CPh resonances, respectively), the difference still is far from the one observed for the asymmetrically bridged MoW complex mentioned above (δ C 193.1 and 85.5 ppm, respectively).23 Reactions of the Hydroxycarbyne Complex 3 with Isocyanides. Compound 3 reacted readily with isocyanides to give also C−C coupled products, but not the expected ones following from coupling between hydroxycarbyne and benzylidyne ligands. Reactions with CNtBu and CNXyl proceeded in both cases with consumption of 3 equiv of reagent and release of CO, and without any observable intermediates, to yield the corresponding aminoalkyne derivatives [Mo2Cp2{μ-η2:η2-C(NHR)CPh}(μ-PCy2)(CNR)2]BF4 as unique products (R = tBu (7a), Xyl (7b); Scheme 4).

Figure 7. ORTEP drawing (30% probability) of the cation in compound 7a, with H atoms, tBu groups, and cyclohexyl rings (except their C1 atoms) omitted for clarity.

Table 4. Selected Bond Lengths (Å) and Angles (deg) in Compound 7a Mo1−Mo2 Mo1−P1 Mo2−P1 Mo1−C1 Mo2−C2 Mo1−C3 Mo2−C3 Mo1−C4 Mo2−C4 C1−N1 C2−N2 C3−N3 C3−C4 C4−C5 N3−C29

Scheme 4

2.6525(5) 2.378(1) 2.372(1) 2.069(5) 2.059(4) 2.119(4) 2.282(4) 2.245(4) 2.176(4) 1.162(5) 1.150(5) 1.363(5) 1.338(6) 1.483(6) 1.476(5)

Mo1−P1−Mo2 Mo1−C3−Mo2 Mo1−C4−Mo2 P1−Mo1−C1 P1−Mo2−C2 P1−Mo1−C3 P1−Mo2−C3 P1−Mo1−C4 P1−Mo2−C4 N3−C3−C4 C3−C4−C5 C3−N3−C29 C3−N3−H3 C1−N1−C21 C2−N2−C25 Mo1−Mo2−C2 Mo2−Mo1−C1

67.91(3) 74.0(1) 73.7(1) 95.3(1) 91.9(1) 106.3(1) 101.4(1) 106.6(1) 109.2(1) 142.8(4) 141.0(4) 127.4(4) 119(3) 169.4(4) 170.2(4) 98.3(1) 95.7(1)

CpMo(CN t Bu) fragments symmetrically bridged by a dicyclohexylphosphide ligand, and by an aminoalkyne ligand placed perpendicular to the intermetallic vector, as found for the alkoxy- and hydroxyalkyne complexes 5a and 6 discussed above (twist angle ca. 82°), with Mo−C distances in the range 2.12−2.28 Å. The terminal isocyanide ligands are almost parallel to each other (Mo−Mo−C angles ca. 97°) and placed close to the CPh group, thus defining an overall conformation of type cis-syn (Figure 3), but this seems to have little effect on the dimensions within the central Mo2C2 core of the molecule. As found for the mentioned alkyne-bridged complexes, the short interatomic Mo−Mo length of 2.6525(5) Å is consistent with the formulation of a double intermetallic bond and a fourelectron donation of the alkyne ligand to the dimetal center. The overall structure thus can be related to that of the dimethoxyacetylene-bridged complex [Mo2Cp2{μ-η2:η2-C(OMe)C(OMe)}(μ-PCy2){μ-(EtO)2POP(OEt)2}]BF4 (Mo− Mo = 2.655(1) Å, twist angle 86°), with the latter having a cisoid conformation imposed by the bridging diphosphite

The process likely involves an H-transfer from the hydroxycarbyne ligand to an isocyanide molecule to yield an aminocarbyne intermediate at some stage of the reaction (see below), a critical step which we have reported recently for the isoelectronic ditungsten hydroxycarbyne complex [W2Cp2(μCOH)(μ-PPh2)2]BF4.25 Besides, we note that compounds 7a and 7b are generated as different stereoisomers, with the CNtBu complex being obtained as the cis-syn isomer whereas the CNXyl derivative is formed selectively as the trans isomer, a matter also to be discussed later on. We finally note that some aminoalkyne complexes have been previously generated through acid-induced coupling between carbyne and isocyanide ligands at mononuclear centers,4a,26 although polynuclear species have been typically prepared previously upon reaction of the free ligands (ynamines) with suitable precursors.27 1686


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Organometallics ligand.2b We finally note that the NHtBu substituent of the alkyne ligand adopts a twisted conformation relative to the Mo2C plane precluding any significant participation of the lone electron pair of nitrogen in any π bonding interaction, which is consistent with the C3−N3 length of 1.363(5) Å, only a bit below the reference figure for a single N−C(sp) bond (1.40 Å),28 and some 0.1 Å shorter than the N2−C29 length of 1.476(5) Å, which corresponds to a N−C(sp3) bond. Compound 7a appears to be the first complex structurally characterized as having an aminoalkyne ligand in the μ-η2:η2 coordination mode. The few previous examples of these ligands bridging two metal atoms rather correspond to the asymmetric μ-η1:η2 coordination mode,29 already discussed in the context of hydroxyalkyne ligands (Figure 6). This is so even when bridging equivalent metal fragments, and there is a marked preference for the NR2-bearing C atom to be terminally bound to one of the metal atoms as found, for instance, in the family of complexes of type [M2{μ-C(NR2)CMe}(CO)8] (M = Mn, Re; R = Me, Et).29a We can also quote a few crystallographic studies on mononuclear aminoalkyne complexes, all of them revealing the presence of η2-bound ligands acting as fourelectron donors.30 Spectroscopic data in solution for compounds 7a and 7b (Table 2 and Experimental Section) indicate that these complexes bear similar ligands, but arranged differently in space. The cisoid arrangement of the MoCp(CNR) fragments of 7a in the crystal lattice certainly is retained in solution, since the NMR data indicate the presence of equivalent pairs of Cp, CN, and tBu resonances. In contrast, 7b displays separated NMR resonances for the pairs of Cp, CN, and Xyl groups, thus indicating a trans structure in solution. In both cases, the 13C NMR resonance of the NHR-bearing C atom appears just a few ppm above the resonance of the Ph-bearing C atom, and both of them are found around 145 ppm, therefore consistent with the retention of the symmetrical μ-η2:η2 coordination mode of the ynamine ligand found in the crystal lattice. Other spectroscopic data are similar to those of the methoxyalkyne complexes 5 and deserve no further comment. We just note that the patterns of the C−N stretching bands in compounds 7 are fully consistent with their different stereochemistry, with medium and strong intensities (in order of decreasing frequencies) for the trans isomer (7b) and the opposite pattern for the cis isomer (7a). Reaction Pathways to Aminoalkyne Complexes. As noted above, no intermediates were observed in the reactions of the hydroxycarbyne complex 3 that yield the aminoalkyne derivatives 7, these necessarily being multistep processes requiring in each case the incorporation of three isocyanide molecules and release of carbon monoxide. Even when using just 1 equiv of isocyanide, only partial formation of the corresponding complex 7 was observed, along with unreacted 3. Yet, a sensible reaction pathway can be envisioned on the basis of previous studies on reactions of isocyanides with the related hydroxycarbyne complex [W2Cp2(μ-COH)(μ-PPh2)2]BF4,25 and with the methoxycarbyne analogues [M2Cp2(μ-COMe)(μ-PR2)2]BF4 (M = Mo, R = Et; M = W, R = Ph).31 Reaction should be initiated with the nucleophilic attack of an isocyanide molecule to the unsaturated substrate 3 via its less hindered position (between the COH and CPh ligands) and transient displacement of the hydroxycarbyne ligand to a terminal position (A in Scheme 5). This leaves COH and CNR ligands in a cisoid position and might be followed by H-transfer to the unsaturated dimetal center to give a hydride intermediate B

Scheme 5. Pathways in Reactions of the Hydroxycarbyne Complex 3 with Isocyanides

analogous to the hydride complex [W2Cp2(H)(μ-PPh2)2(CO)(PH2Cy)]BF4 isolated in the reaction of the W2 hydroxycarbyne complex with PH2Cy. This should be followed by hydride transfer to the isocyanide ligand to yield an aminocarbyne intermediate C keeping a cisoid disposition relative to the carbonyl ligand, which might then undergo CO substitution (perhaps in an associative way) by a second isocyanide molecule to yield a cisoid isocyanide intermediate D. The nucleophilic attack of a third isocyanide molecule is now likely to occur opposite the terminal aminocarbyne ligand, since this allows the concerted displacement of the latter back to a bridging position and the C−C coupling with the benzylidyne ligand that leads to the final alkyne-bridged product 7. Clearly, the above pathway leads to a transoid arrangement of the terminal isocyanide ligands, and therefore it should be dominant in the reaction of 3 with CNXyl. However, to explain the formation of 7a specifically as the cis-syn isomer, it has to be assumed that some fast cis/trans isomerization occurs at some stage, as observed previously in reactions of the mentioned [M2Cp2(μ-COMe)(μ-PR2)2]BF4 complexes. Such an isomerization might take place at intermediate D to give trans-D which, after an analogous nucleophilic attack of the third isocyanide molecule, would lead to the final product 7 specifically in the cis-syn form (Scheme 5). At present, however, it is not clear why such an intermediate would rearrange rapidly (before attack of the third isocyanide molecule) only when R = t Bu. We note, however, that although cisoid structures for group 6 metal complexes of the type [M2Cp2(μ-PR2)2L2] (L= CO and related ligands) generally are thermodynamically disfavored with respect to their trans isomers,20,32 we have previously observed that the rate of the corresponding isomerization for the dicarbonyl complexes is strongly dependent on the particular complex.20b Based on these precedents, we conclude that the observed isomerism for complexes 7 follows 1687


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relative to internal tetramethylsilane (1H, 13C) or external 85% aqueous H3PO4 (31P). Coupling constants (J) are given in hertz. Preparation of [Mo2Cp2(μ-COMe)(μ-CPh)(μ-PCy2)](CF3SO3) (2). Neat CF3SO3Me (120 μL, 1 mmol) was added to a toluene solution (5 mL) of compound 1 (0.050 g, 0.079 mmol), and the mixture was stirred at room temperature for 4 h to give a yelloworange solution. Solvent was then removed under vacuum, and the residue was washed with petroleum ether (2 × 5 mL) to give compound 2 as a yellow powder (0.050 g, 79%). Anal. Calcd for C32H40F3Mo2O4PS: C, 48.01; H, 5.04. Found: C, 48.18; H, 5.12. 1H NMR: δ 7.35 [false t, 3JHH = 7, 2H, H3(Ph)], 7.20 [t, 3JHH = 7, 1H, H4(Ph)], 6.62 [false d, 3JHH = 7, 2H, H2(Ph)], 6.22 (s, 10H, Cp), 4.11 (s, 3H, OMe), 2.30−0.90 (m, 18H, Cy), 0.65, 0.42 (2m, 2 × 2H, Cy). 13 C{1H} NMR: δ 422.0 (s, μ-CPh), 365.6 (d, JCP = 14, μ-COMe), 160.0 [s, C1(Ph)], 128.9 [s, C3(Ph)], 128.3 [s, C4(Ph)], 122.2 [s, C2(Ph)], 121.3 (q, 1JCF = 320, CF3), 98.8 (s, Cp), 70.0 (s, OMe), 41.8 [d, 1JCP = 18, C1(Cy)], 41.0 [d, 1JCP = 17, C1(Cy)], 33.4, 33.2 [2s, C2(Cy)], 27.0, 26.9 [2d, 3JCP = 6, C3(Cy)], 25.8 [s, 2C4(Cy)]. Preparation of [Mo2Cp2(μ-COMe)(μ-CPh)(μ-PCy2)]BF4 (2′). Solid [Me3O]BF4 (0.100 g, 0.676 mmol) was added to a dichloromethane solution (1 mL) of compound 1 (0.050 g, 0.079 mmol), and the mixture was stirred at room temperature for 3 h to give a yellow solution, which was filtered using a cannula. Solvent was then removed under vacuum from the filtrate, and the residue was washed with petroleum ether (3 × 5 mL) to give compound 2′ as a yellow powder (0.049 g, 85%). The crystals used in the X-ray diffraction study were grown by the slow diffusion of a layer of toluene into a concentrated dichloromethane solution of the complex at 253 K. Anal. Calcd for C31H40BF4Mo2OP: C, 50.43; H, 5.47. Found: C, 50.35; H, 5.40. 1H NMR: δ 7.35 [false t, 3JHH = 7, 2H, H3(Ph)], 7.20 [t, 3JHH = 7, 1H, H4(Ph)], 6.68 [false d, 3JHH = 7, 2H, H2(Ph)], 6.21 (s, 10H, Cp), 4.10 (s, 3H, OMe), 2.20−1.10 (m, 18H, Cy), 0.65, 0.42 (2m, 2 × 2H, Cy). Preparation of [Mo2Cp2(μ-COMe)(μ-CPh)(μ-PCy2)](BAr′4) (2″). A suspension of compound 2 in toluene was prepared as described above from compound 1 (0.050 g, 0.079 mmol). Solvent was then removed under vacuum, and the residue was dissolved in dichloromethane (5 mL). Solid Na(BAr′4) (0.071 g, 0.08 mmol) was added, and the mixture was stirred for 5 min and then filtered. After removal of solvent from the filtrate, the residue was dissolved in dichloromethane/petroleum ether (1/2) and chromatographed on alumina at 253 K. Elution with dichloromethane/petroleum ether (3/1) gave a yellow fraction yielding, after removal of solvents, compound 2″ as a yellow powder (0.040 g, 70%). Anal. Calcd for C63H52BF24Mo2OP: C, 49.96; H, 3.46. Found: C, 49.67; H, 3.30. 1H NMR: δ 7.72 [m, 8H, H2(Ar′)], 7.56 [s, 4H, H4(Ar′)], 7.35 [false t, 3JHH = 7, 2H, H3(Ph)], 7.25 [t, 3JHH = 7, 1H, H4(Ph)], 6.62 [false d, 3JHH = 7, 2H, H2(Ph)], 6.18 (s, 10H, Cp), 4.05 (s, 3H, OMe), 2.60−0.80 (m, 18H, Cy), 0.62, 0.43 (2m, 2 × 2H, Cy). Preparation of [Mo2Cp2{μ-η2:η2-C(OMe)CPh}(μ-PCy2)(CO)2](CF3SO3) (4). A dichloromethane solution (10 mL) of compound 2 (0.050 g, 0.063 mmol) was placed in a high-pressure autoclave and stirred at room temperature under CO (40 bar) for 72 h to give a red solution. Solvent was then removed under vacuum, and the residue was crystallized from dichloromethane−petroleum ether to give compound 4 as a red microcrystalline solid (0.050 g, 93%). This product was shown by NMR to contain a mixture of isomers trans and cis (two isomers) in a ratio of ca. 10:1:1. Anal. Calcd for C34H40F3Mo2O6PS: C, 47.69; H, 4.71. Found: C, 47.55; H, 4.55. Data for Isomer trans-4. 1H NMR (400.13 MHz): δ 7.51−7.10 (m, 5H, Ph), 5.95, 5.83 (2s, 2 × 5H, Cp), 3.99 (s, 3H, OMe), 2.50− 0.20 (m, 22H, Cy). 13C{1H} NMR (100.61 MHz): δ 218.0 (d, 2JCP = 12, MoCO), 217.5 (d, 2JCP = 11, MoCO), 169.5 [s, C1(Ph)], 138.0 (s, COMe), 130.0 [s, C3(Ph)], 129.6 [s, C4(Ph)], 129.4 [s, C2(Ph)], 125.6 (s, CPh), 121.3 (q, 1JCF = 320, CF3), 94.8, 93.9 (2s, Cp), 64.0 (s, OMe), 49.9 [d, 1JCP = 20, C1(Cy)], 45.0 [d, 2JCP = 18, C1(Cy)], 35.2 [d, 2JCP = 4, C2(Cy)], 34.1 [d, 2JCP = 5, C2(Cy)], 33.1, 32.0 [2s, C2(Cy)], 28.4 [d, 3JCP = 12, 2C3(Cy)], 27.8 [d, 3JCP = 11, 2C3(Cy)], 26.2, 26.0 [2s, C4(Cy)]. Data for Minor cis isomers. 1H NMR (400.13 MHz): δ 6.00, 5.96 (2s, 2 × 10H, Cp), 4.10, 3.58 (2s, 2 × 3H, OMe); other resonances

from kinetic rather than thermodynamic effects. In fact, the formation of trans and cis-syn isomers for the carbonyl and isocyanide derivatives of the methoxycarbyne complex 2 can be understood analogously. Initial attack of a ligand (CO or isocyanide) would yield methoxycarbyne intermediates with geometries analogous to those of the intermediates A or D depicted in Scheme 5, which in turn would yield mixtures of isomers trans and cis-syn if attack of the second ligand molecule occurs at a rate comparable to the rate of the cis/trans isomerization of these intermediates. The substantial variation for compound 5b of the ratio trans/cis-syn with the reaction temperature, noted earlier, is fully consistent with this kinetic influence on the isomer distribution in all these reaction. Finally, we note that formation of the minor cis-anti isomers in the case of compounds 4 and 5b, however, is indicative of the operation of a third reaction pathway in the above reactions, but presently we will not speculate on this additional path.

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CONCLUSION Addition of two molecules of carbon monoxide to the unsaturated methoxy- and hydroxycarbyne-bridged benzylidyne complexes [Mo2Cp2(μ-COR)(μ-CPh)(μ-PCy2)]+ (R = H, Me) takes place at room temperature and induces a C−C reductive coupling between the carbyne ligands bound to these cations. This yields methoxy- and hydroxyalkyne ligands coordinated to the dimetal center in a nearly symmetrical μ-η2:η2 fashion, but the coupling process could be reversed only for the methoxyalkyne complex. An analogous behavior is observed upon addition of isocyanides to the methoxycarbyne complex, a reaction taking place rapidly even at 233 K. In contrast, addition of isocyanides to the hydroxycarbyne-bridged complex triggers an H-shift reaction to the nitrogen atom of the added ligand, likely involving a metal-mediated formation of an aminocarbyne intermediate that eventually evolves through reductive C−C coupling between aminocarbyne and benzylidyne ligands, to yield aminoalkyne-bridged complexes also coordinated to the dimetal center in the symmetrical μ-η2:η2 fashion. All these products display stereoisomerism derived from the different disposition of the added ligands with respect to the distinct substituents of the alkyne being generated, which seems to be governed by kinetic factors.

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EXPERIMENTAL SECTION

General Procedures and Starting Materials. All manipulations and reactions were carried out under a nitrogen (99.995%) atmosphere using standard Schlenk techniques. Solvents were purified according to literature procedures and distilled prior to use.33 Compounds 1,15 [Mo2Cp2{μ-C(Ph)CO)}(μ-PCy2)(CO)2],15 and Na(BAr′4) (Ar′ = 3,5-C6H3(CF3)2),34 were prepared as described previously. Compound 3 was prepared in situ from 1 and [FeCp2]BF4 in the presence of BH3NH2tBu,14 and all other reagents were obtained from the usual commercial suppliers and used as received, unless otherwise stated. Petroleum ether refers to that fraction distilling in the range 338−343 K. Filtrations were carried out through diatomaceous earth unless otherwise indicated. Chromatographic separations were carried out using jacketed columns refrigerated by tap water (ca. 288 K) or by a closed 2-propanol circuit kept at the desired temperature with a cryostat. Commercial aluminum oxide (activity I, 70−290 mesh) was degassed under vacuum prior to use. The latter was mixed afterward under nitrogen with the appropriate amount of water to reach the activity desired (activity IV, unless otherwise stated). IR stretching frequencies of CO ligands were measured in solution using CaF2 windows. NMR spectra were routinely recorded at 300.13 (1H), 121.50 (31P{1H}), and 75.47 MHz (13C{1H}) at 295 K in CD2Cl2 solution unless otherwise stated. Chemical shifts (δ) are given in ppm, 1688


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Organometallics

η1:η3-CHPh)(O)(μ-PCy2)(CO)]BF4 and its neutral derivative cis[Mo2Cp2(μ-CPh)(O)(μ-PCy2)(CO)],14 along with other minor species, and thus was a less convenient preparative procedure. Anal. Calcd for C32H38BF4Mo2O3P: C, 49.26; H, 4.91. Found: C, 49.13; H, 4.83. 1H NMR (400.13 MHz): δ 9.74 (s, br, 1H, OH), 7.58 [false d, 3 JHH = 7, 2H, H2(Ph)], 7.52 [false t, 3JHH = 7, 2H, H3(Ph)], 7.43 [t, 3 JHH = 7, 1H, H4(Ph)], 5.87, 5.75 (2s, 2 × 5H, Cp), 2.50−1.10 (m, 22H, Cy). 13C{1H} NMR (100.61 MHz): δ 218.6, 218.4 (2d, 2JCP = 12, MoCO), 163.8 [s, C1(Ph)], 139.6 (s, COH), 129.9 [s, C3(Ph)], 129.4 [s, C2(Ph)], 129.1 [s, C4(Ph)], 95.1, 94.3 (2s, Cp), 90.0 (s, CPh), 48.7 [d, 1JCP = 21, C1(Cy)], 45.7 [d, 1JCP = 20, C1(Cy)], 35.0 [d, 2JCP = 5, C2(Cy)], 34.4 [d, 2JCP = 4, C2(Cy)], 33.0, 32.5 [2s, C2(Cy)], 27.9 [d, 3JCP = 12, C3(Cy)], 27.9, 27.8 [2d, 3JCP = 11, C3(Cy)], 27.7 [d, 3JCP = 10, C3(Cy)], 26.1, 26.0 [2s, C4(Cy)]. Preparation of cis-syn-[Mo2Cp2{μ-η2:η2-C(NHtBu)CPh}(μPCy2)(CNtBu)2]BF4 (7a). Neat CNtBu (30 μL, 0.27 mmol) was added to a dichloromethane solution (5 mL) of compound 3 (ca. 0.050 g, 0.070 mmol), and the mixture was stirred at room temperature for 5 min to give a red-purple solution. Solvent was then removed under vacuum, and the residue was washed with petroleum ether (2 × 5 mL) to give compound 7a as a purple microcrystalline solid (0.059 g, 89%). The crystals used in the X-ray diffraction study were grown by the slow diffusion of layers of toluene and petroleum ether into a concentrated dichloromethane solution of the complex at 253 K. Anal. Calcd for C44H65BF4Mo2N3P: C, 55.88; H, 6.93; N, 4.44. Found: C, 55.72; H, 6.80; N, 4.32. 1H NMR (400.13 MHz): δ 7.36−7.30 (m, 3H, Ph), 7.18 [false d, 3JHH = 7, 2H, H2(Ph)], 5.26 (s, 10H, Cp), 4.00 (s, NH), 2.00−1.00 (m, 22H, Cy), 1.10 (s, 18H, tBu), 1.37 (s, 9H, tBu). 13C{1H} NMR (100.61 MHz): δ 168.5 (s, MoCN), 158.2 [s, C1(Ph)], 148.6 (s, CNH), 146.0 (s, CPh), 134.8 [s, C4(Ph)], 132.1 [s, C3(Ph)], 128.2 [s, C2(Ph)], 92.0 (s, Cp), 59.4 [s, 2C1(tBu)], 58.9 [s, C1(NHtBu)], 51.1 [d, 1JCP = 20, C1(Cy)], 44.4 [d, 1JCP = 18, C1(Cy)], 33.8, 33.3 [2s, C2(Cy)], 30.5, 30.4 [2s, C2(tBu)], 28.4 [d, 3JCP = 11, C3(Cy)], 28.2 [d, 3JCP = 12, C3(Cy)], 26.8, 26.6 [2s, C4(Cy)]. Preparation of trans-[Mo2Cp2{μ-η2:η2-C(NHXyl)CPh}(μ-PCy2)(CNXyl)2]BF4 (7b). Neat CNXyl (0.020 g, 0.15 mmol) was added to a dichloromethane solution (5 mL) of compound 3 (ca. 0.030 g, 0.041 mmol), and the mixture was stirred at room temperature for 5 min to give a red-purple solution. Solvent was then removed under vacuum, and the residue was washed with petroleum ether (2 × 5 mL) to give compound 7b as a purple microcrystalline solid (0.040 g, 92%). Anal. Calcd for C56H65BF4Mo2N3P: C, 61.72; H, 6.01; N, 3.86. Found: C, 61.40; H, 5.85; N, 3.65. 1H NMR: δ 7.30−5.89 (m, 14H, Xyl and Ph), 5.37, 5.19 (2s, 2 × 5H, Cp), 4.60 (s, 1H, NH), 2.40−0.90 (m, 22H, Cy), 1.39, 1.28, 1.27 (3s, 3 × 6H, Me). 13C{1H} NMR: δ 156.1 [s, C1(Ph)], 144.9 (s, CNH) 142.0 (s, CPh), 136.9, 135.9 (2s, MoCN), 135.0 [s, C1(NHXyl)], 130.4−126.9 (m, Ph and Xyl), 118.0, 117.7 [2s, C2(Xyl)], 93.5, 91.8 (2s, Cp), 54.0 [d, 1JCP = 24, C1(Cy)], 43.4 [d, 1JCP = 13, C1(Cy)], 34.4 [d, 2JCP = 3, C2(Cy)], 34.1, 33.9 [2s, C2(Cy)], 33.6 [d, 2JCP = 3, C2(Cy)], 28.8 [d, 3JCP = 12, 2C3(Cy)], 28.2, 28.1, 27.7 (3s, Me), 27.3 [d, 3JCP = 11, C3(Cy)], 26.3 [d, 3JCP = 12, C3(Cy)], 26.5, 25.7 [2s, C4(Cy)]. X-ray Crystal Structure Determination for Compound 2′. Data collection was performed at 100(2) K on an Oxford Diffraction Xcalibur Nova single crystal diffractometer, using Cu Kα radiation. Images were collected at a 65 mm fixed crystal−detector distance, using the oscillation method, with 1° oscillation and variable exposure time per image (10−40 s). Data collection strategy was calculated with the program CrysAlis Pro CCD.35 Data reduction and cell refinement were performed with the program CrysAlis Pro RED.35 An empirical absorption correction was applied using the SCALE3 ABSPACK algorithm as implemented in the program CrysAlis Pro RED. Using the program suite WinGX,36 the structure was solved by Patterson interpretation and phase expansion using SHELXL97, and refined with full-matrix least-squares on F2 using SHELXL97.37 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were geometrically placed and refined using a riding model. X-ray Crystal Structure Determination for Compound 6. Data collection was performed at 100(2) K on a Nonius Kappa CCD

were obscured by those of the major isomer. 31P{1H} NMR: δ 190.7, 186.2 (2s); individual assignment of the resonances of these minor isomers could not be done. Preparation of [Mo 2 Cp 2 {μ-η 2 :η 2 -C(OMe)CPh}(μ-PCy 2 )(CNtBu)2](CF3SO3) (5a). Neat CNtBu (30 μL, 0.27 mmol) was added to a dichloromethane solution (5 mL) of compound 2 (0.050 g, 0.063 mmol) cooled at 233 K, and the mixture was stirred at that temperature for 5 min to give a red-purple solution. Solvent was then removed under vacuum, and the residue was washed with petroleum ether (2 × 5 mL) to give compound 5a as a red microcrystalline solid (0.058 g, 95%). This product was shown by NMR to contain a mixture of isomers trans and cis-syn in a ratio of ca. 2:1. Anal. Calcd for C42H58F3Mo2N2O4PS: C, 52.17; H, 6.05; N, 2.90. Found: C, 52.24; H, 5.98; N, 2.81. Data for trans-5a. 1H NMR (400.13 MHz): δ 7.41 [false t, 3JHH = 7, 2H, H3(Ph)], 7.24−7.11 (m, 3H, Ph), 5.49, 5.34 (2s, 2 × 5H, Cp), 3.92 (s, 3H, OMe), 2.50−1.10 (m, 22H, Cy), 1.21, 0.85 (2s, 2 × 9H, t Bu). 13C{1H} NMR (100.61 MHz): δ 184.0, 172.5 (2d, 2JCP = 4, MoCN), 160.9 [s, C1(Ph)], 143.6 (s, COMe), 130.4 [s, C4(Ph)], 130.2 [s, C3(Ph)], 128.9 [s, C2(Ph)], 126.0 (s, CPh), 121.3 (q, 1JCF = 320, CF3), 92.1, 91.5 (2s, Cp), 61.9 (s, OMe), 59.4, 58.9 [2s, C1(tBu)], 51.6 [d, 1JCP = 21, C1(Cy)], 44.7 [d, 1JCP = 18, C1(Cy)], 35.2 [d, 2JCP = 5, C2(Cy)], 34.0, 33.7, 33.2 [3s, C2(Cy)], 30.6, 30.4 [2s, C2(tBu)], 28.5 [d, 3JCP = 12, C3(Cy)], 28.4, 27.8 [2d, 3JCP = 11, C3(Cy)], 26.7 [d, 3JCP = 10, C3(Cy)], 26.5, 26.4 [2s, C4(Cy)]. Data for cis-syn-5a. 1H NMR (400.13 MHz): δ 5.36 (s, 10H, Cp), 3.67 (s, 3H, OMe), 1.13 (s, 18H, tBu); other resonances of this isomer were masked by those of the major isomer. 31P{1H} NMR: δ 179.1 (s). Preparation of [Mo 2 Cp 2 {μ-η 2 :η 2 -C(OMe)CPh}(μ-PCy 2 )(CNXyl)2](CF3SO3) (5b). Neat CNXyl (0.032 g, 0.24 mmol) was added to a dichloromethane solution (5 mL) of compound 2 (0.050 g, 0.063 mmol) cooled at 203 K, and the mixture was stirred at that temperature for 30 min to give a purple solution. Solvent was then removed under vacuum, and the residue was washed with petroleum ether (2 × 5 mL) to give compound 5b as a red microcrystalline solid (0.059 g, 88%). This product was shown by NMR to contain a mixture of the isomers trans and cis-syn in a ratio of ca. 20:1. If the above reaction is carried out at room temperature, then small amounts of a third isomer (cis-anti) were formed, and the trans/cis-syn/cis-anti ratio was then ca. 10:4:2. Anal. Calcd for C50H58F3Mo2N2O4PS: C, 56.50; H, 5.50; N, 2.64. Found: C, 56.13; H, 5.25; N, 2.45. Data for trans-5b. 1H NMR: δ 7.40−6.90 (m, 11H, Ph and Xyl), 5.58, 5.57 (2s, 2 × 5H, Cp), 3.97 (s, 3H, OMe), 2.50−0.50 (m, 22H, Cy), 2.15, 1.69 (2s, 2 × 6H, Me). 13C{1H} NMR (100.61 MHz): δ 178.5, 176.7 (2s, MoCN), 168.4 [s, C1(Ph)], 142.9 (s, COMe), 135.0 (s, CPh), 133.3 [s, C2(Xyl)], 132.7 [s, C2(Xyl)], 130.3−127.0 (m, Ph and Xyl), 127.9 [s, C1(Xyl)], 121.3 [q, 1JCF = 320, CF3], 92.4, 92.0 (2s, Cp), 62.6 (s, OMe), 52.3, 51.6 [2d, 1JCP = 18, C1(Cy)], 34.7, 34.3 [2d, 2 JCP = 4, C2(Cy)], 34.2 [d, 2JCP = 5, C2(Cy)], 33.7 [s, C2(Cy)], 28.4 [d, 3JCP = 10, 2C3(Cy)], 28.1 [d, 3JCP = 12, 2C3(Cy)], 26.1, 26.0 [2s, C4(Cy)], 18.8, 18.5 (2s, Me). Data for cis-syn-5b. 1H NMR: δ 5.60 (s, 10H, Cp), 3.61 (s, 3H, OMe), 1.61 (s, 12H, Me). 31P{1H} NMR: δ 177.4 (s). Data for cis-anti-5b. 1H NMR: δ 5.33 (s, 10H, Cp), 4.07 (s, 3H, OMe), 1.99 (s, 12H, Me). 31P{1H} NMR: δ 180.4 (s). Other resonances of the minor isomers masked by those of the major isomer. Preparation of [Mo2Cp2{μ-η2:η2-C(OH)CPh}(μ-PCy2)(CO)2]BF4 (6). Neat HBF4·OEt2 (12 μL, 0.087 mmol) was added to a dichloromethane solution (5 mL) of [Mo2Cp2{μ-C(Ph)CO)}(μPCy2)(CO)2] (0.050 g, 0.072 mmol), and the mixture was stirred for 5 min to give a deep red solution. After removal of solvent under vacuum, the residue was washed with petroleum ether (2 × 5 mL) to give compound 6 as a red powder (0.040 g, 88%). The crystals used in the X-ray diffraction study were grown by the slow diffusion of a layer of toluene into a concentrated dichloromethane solution of the complex at 253 K. This complex could also be obtained by placing a dichloromethane solution of compound 3 in a Schlenk tube equipped with a Young’s valve and stirring it at room temperature under a moderate CO pressure (ca. 4 bar) for 24 h, but this yielded a material contaminated with the known oxo carbene complex cis-[Mo2Cp2(μ1689


Article

Organometallics

(5) Nuel, D.; Daham, F.; Mathieu, R. Organometallics 1985, 4, 1436− 1439. (6) Chi, Y.; Shapley, J. R. Organometallics 1985, 4, 1900−1901. (7) (a) Okazaki, M.; Suto, K.; Kudo, N.; Takano, M.; Ozawa, F. Organometallics 2012, 31, 4110−4113. (b) Okazaki, M.; Takano, M.; Ozawa, F. J. Am. Chem. Soc. 2009, 131, 1684−1685. (c) Okazaki, M.; Ohtani, T.; Ogino, H. J. Am. Chem. Soc. 2004, 126, 4104−4105. (8) (a) Qi, Y.; Yang, J.; Chen, D.; Homen, A. Catal. Lett. 2015, 145, 145−161. (b) Filot, A. W.; van Santen, R. A.; Hensen, E. J. M. Angew. Chem., Int. Ed. 2014, 53, 12746−12750. (c) Maitlis, P. M. In Greener Fischer−Tropsch Processes for Fuels and Feedstocks; Maitlis, P. M., de Klerk, A., Eds.; Wiley-VCH: Weinheim, Germany, 2013; Chapter 12. (d) Maitlis, P. M.; Zanotti, V. Chem. Commun. 2009, 1619−1634. (9) (a) Nicholas, K. M. Organometallics 1982, 1, 1713−1715. (b) Muetterties, E. L.; Stein, J. Chem. Rev. 1979, 79, 479−490. (10) (a) Alvarez, M. A.; García, M. E.; Riera, V.; Ruiz, M. A.; Robert, F. Organometallics 2002, 21, 1177−1183. (b) Alvarez, M. A.; García, M. E.; Riera, V.; Ruiz, M. A. Organometallics 1999, 18, 634−641. (c) Alvarez, M. A.; Bois, C.; García, M. E.; Riera, V.; Ruiz, M. A. Angew. Chem., Int. Ed. Engl. 1996, 35, 102−104. (11) Messeguer, A.; Serratosa, F. Tetrahedron Lett. 1973, 14, 2895− 2898. (12) Alvarez, M. A.; García, M. E.; Martínez, M. E.; Menéndez, S.; Ruiz, M. A. Organometallics 2010, 29, 710−713. (13) García, M. E.; García-Vivó, D.; Ruiz, M. A.; Alvarez, S.; Aullón, G. Organometallics 2007, 26, 4930−4941. (14) Alvarez, M. A.; García, M. E.; García-Vivó, D.; Menéndez, S.; Ruiz, M. A. Organometallics 2013, 32, 218−231. (15) Alvarez, M. A.; García, M. E.; García-Vivó, D.; Martínez, M. E.; Ruiz, M. A. Organometallics 2011, 30, 2189−2199. (16) García, M. E.; García-Vivó, D.; Ruiz, M. A.; Alvarez, S.; Aullón, G. Organometallics 2007, 26, 5912−5921. (17) Amin, E. A. E.; Jeffery, J. C.; Walters, T. M. Chem. Commun. 1990, 170−172. (18) (a) Churchill, M. R.; Fettinger, J. C.; Keister, J. B.; See, R. F.; Ziller, J. W. Organometallics 1985, 4, 2112−2116. (b) Boyar, E.; Deeming, A. J.; Felix, M. S. B.; Kabir, S. E.; Adatia, T.; Bhusate, R.; McPartlin, M.; Powell, H. R. J. Chem. Soc., Dalton Trans. 1989, 5−12. (19) (a) Kreissl, F. R.; Sieber, W. J.; Hofmann, P.; Riede, J.; Wolfgruber, M. Organometallics 1985, 4, 788−792. (b) Mayr, A.; McDermott, G. A.; Dorries, A. M.; van Engen, D. Organometallics 1987, 6, 1503−1508. (c) Mayr, A.; Bastos, C. M.; Chang, R. T.; Haberman, J. X.; Robinson, K. S.; Belle-Oudry, D. A. Angew. Chem., Int. Ed. Engl. 1992, 31, 747−749. (20) (a) García, M. E.; Riera, V.; Ruiz, M. A.; Rueda, M. T.; Sáez, D. Organometallics 2002, 21, 5515−5525. (b) Alvarez, M. A.; García, M. E.; Martínez, M. E.; Ramos, A.; Ruiz, M. A.; Sáez, D. Inorg. Chem. 2006, 45, 6965−6978. (c) Alvarez, M. A.; García, M. E.; Ramos, A.; Ruiz, M. A.; Lanfranchi, M.; Tiripicchio, A. Organometallics 2007, 26, 5454−5467. (21) Braterman, P. S. Metal Carbonyl Spectra; Academic Press: London, U.K., 1975. (22) Volhardt, K. P. C.; Wolfgruber, M. Angew. Chem., Int. Ed. Engl. 1986, 25, 929−931. (23) Brew, S. A.; Dosset, S. J.; Jeffery, J. C.; Stone, F. G. A. J. Chem. Soc., Dalton Trans. 1990, 3709−3718. (24) (a) Fischer, E. O.; Friedrich, P. Angew. Chem., Int. Ed. Engl. 1979, 18, 327−328. (b) Jeffery, J. C.; Laurie, J. C. V.; Moore, I.; Stone, F. G. A. J. Organomet. Chem. 1983, 258, C37−C40. (25) Cimadevilla, F.; García, M. E.; García-Vivó, D.; Ruiz, M. A.; Graiff, C.; Tiripicchio, A. Organometallics 2013, 32, 4624−4635. (26) Filippou, A. C.; Lungwitz, B.; Kociok-Köhn, G. Eur. J. Inorg. Chem. 1999, 1905−1910 and references therein. (27) Adams, R. D.; Daran, J. C.; Jeannin, Y. J. Cluster Sci. 1992, 3, 1− 54. (28) Cordero, B.; Gómez, V.; Platero-Prats, A. E.; Revés, M.; Echeverría, J.; Cremades, E.; Barragán, F.; Alvarez, S. Dalton Trans. 2008, 2832−2838.

single crystal diffractometer, using graphite-monochromated Mo Kα radiation. Images were collected at a 35 mm fixed crystal−detector distance, using the oscillation method, with 1° oscillation and 40 s exposure time per image. Data collection strategy was calculated with the program Collect.38 Data reduction and cell refinement were performed with the programs HKL Denzo and Scalepack.39 A semiempirical absorption correction was applied using the program SORTAV.40 The structure was solved and refined as indicated for 2′, and the compound was found to crystallize with one molecule of toluene. The hydroxyl H(3) atom was located in the Fourier maps, but free refinement of all positional parameters proved to be unstable, so eventually it was refined isotropically with the O(3)−H(3) distance restrained at the fixed value of 0.96(2) Å. X-ray Crystal Structure Determination for Compound 7a. Diffraction data were collected on a Kappa-Appex-II Bruker diffractometer using graphite-monochromated Mo Kα radiation at 100 K. The software APEX41 was used for collecting frames with the ω/ϕ scan measurement method. The SAINT software was used for data reduction,42 and a multiscan absorption correction was applied with SADABS.43 Structure solution and refinements were performed as described for 2′. One of the cyclopentadienyl groups of the molecule was found to be disordered over two positions and satisfactorily modeled with 0.6/04 occupancies. These carbon atoms were refined isotropically to prevent their temperature factors from becoming nonpositive definite. The amino hydrogen atom H(3) was located in the Fourier map and refined isotropically.

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ASSOCIATED CONTENT

S Supporting Information *

A PDF file containing a table with crystal data for compounds 2′, 6, and 7a and a CIF file containing full crystallographic data for these compounds (CCDC 1048097−1048099). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00166.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the DGI of Spain for financial support (Project CTQ2012-33187) and the Consejeriá de Educación of Asturias for a grant (to S.M.). We also thank the Universidad de Santiago de Compostela and the Universidad de Oviedo (X-ray units) for collection of the diffraction data.

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REFERENCES

(1) (a) García-Vivó, D.; Ramos, A.; Ruiz, M. A. Coord. Chem. Rev. 2013, 257, 2143−2191. (b) Winter, M. J. Adv. Organomet. Chem. 1989, 29, 101. (c) Curtis, M. D. Polyhedron 1987, 6, 759−782. (2) (a) Alvarez, C. M.; Alvarez, M. A.; García, M. E.; García-Vivó, D.; Ruiz, M. A. Organometallics 2005, 24, 4122−4124. (b) García, M. E.; García-Vivó, D.; Ruiz, M. A. Organometallics 2008, 27, 543−554. (3) (a) Ritleng, V.; Chetcuti, M. J. Chem. Rev. 2007, 107, 797−858. (b) Shapley, J. R.; Comstock, M. C. Coord. Chem. Rev. 1995, 143, 501−533. (c) Adams, R. D.; Horvath, I. T. Prog. Inorg. Chem. 1985, 33, 127. (d) Stone, F. G. A. Angew. Chem., Int. Ed. Engl. 1984, 23, 89−99. (4) (a) Mayr, A.; Bastos, C. M. Prog. Inorg. Chem. 1992, 40, 1−98. (b) Carnahan, E. M.; Protasievicz, J. D.; Lippard, S. J. Acc. Chem. Res. 1993, 26, 90−97. (c) Fillipou, A. C. In Organic Synthesis via Organometallics; Helmchen, G., Dibo, J., Flubacher, D., Wiese, B., Eds.; F. Vieweg & Sons Verlagsgesellschaft: Braunschweig, Germany, 1997. 1690


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