Dimolybdenum Centre Research Articles

C≡N and N≡O Bond Cleavages of Acetonitrile and Nitrosyl Ligands at a Dimolybdenum Center to Render Ethylidyne and Acetamidinate Ligands.

Extended reduction of [Mo2Cp2(μ-Cl)(μ-PtBu2)(NO)2] (1) with Na(Hg) in acetonitrile (MeCN) at room temperature resulted in an unprecedented full cleavage of the C≡N bond of a coordinated MeCN molecule to yield the vinylidene derivative Na[Mo2Cp2(μ-PtBu2)(μ-CCH2)(NO)2], which upon protonation with (NH4)PF6 gave the ethylidyne complex [Mo2Cp2(μ-PtBu2)(μ-CMe)(NO)2] [Mo1-Mo2 = 2.9218(2) Å] in a selective and reversible way. Controlled reduction of 1 at 273 K yielded instead, after protonation, the 30-electron acetamidinate complex [Mo2Cp2(μ-PtBu2)(μ-κN:κN'-HNCMeNH)(μ-NO)]PF6 [Mo1-Mo2 = 2.603(2) Å], in a process thought to stem from the paramagnetic MeCN-bridged intermediate [Mo2Cp2(μ-PtBu2)(μ-NCMe)(NO)2], followed by a complex sequence of elementary steps including cleavage of the N≡O bond of a nitrosyl ligand.

Mild N-O Bond Cleavage Reactions of a Pyramidalized Nitrosyl Ligand Bridging a Dimolybdenum Center.

Complex [Mo2Cp2(μ-PCy2)(μ-NO)(NO)2] (1) was prepared by reacting [Mo2Cp2(μ-H)(μ-PCy2)(CO)4] with 2 equiv of [NO]BF4 and then treating the resulting product [Mo2Cp2(μ-PCy2)(CO)2(NO)2](BF4) with NaNO2 at 323 K, and it was shown to display a bridging nitrosyl ligand with significant pyramidalization at the N atom, a circumstance related to an unusual behavior concerning degradation of the bridging nitrosyl. Indeed, complex 1 reacts with HBF4·OEt2 to give the nitroxyl-bridged derivative [Mo2Cp2(μ-PCy2)(μ-κ(1):η(2)-HNO)(NO)2](BF4), is reduced by Zn(Hg) in the presence of trace H2O to give the amido complex [Mo2Cp2(μ-PCy2)(μ-NH2)(NO)2], and reacts with excess P(OPh)3 to give the phosphoraniminato-bridged derivative [Mo2Cp2(μ-PCy2){μ-NP(OPh)3}(NO)2].

Mild P–P Bond Cleavage in the Methyldiphosphenyl Complex [Mo2Cp2(μ-PCy2)(μ-κ2:κ2-P2Me)(CO)2] To Give Novel Phosphide-Bridged Trinuclear Derivatives

Reactions of the title diphosphenyl complex with [Fe2(CO)9] and [W(CO)4(THF)2] gave the trinuclear species [Mo2FeCp2(μ3-P)(μ-PCy2)(μ3-PMe)(CO)5] and [Mo2WCp2(μ3-P)(μ-PCy2)(μ3-PMe)(CO)6] following from formal insertion of the 14-electron fragments Fe(CO)3 and W(CO)4, respectively, in the P-P bond of the diphosphenyl ligand and formation of a new heterometallic bond [Mo-Fe = 2.9294(6) Å and Mo-W = 3.146(1) Å]. Reactions of the diphosphenyl complex with the tetrahydrofuran adducts [MLn(THF)] (MLn = MnCp'(CO)2, W(CO)5) led instead to trinuclear diphosphenyl complexes [Mo2MCp2(μ-PCy2)(μ3-κ(2):κ(2):κ(1)-P2Me)(CO)2Ln] following from coordination in each case of the corresponding 16-electron fragment MLn to the lone-pair-bearing P atom of the P2Me ligand. However, these diphosphenyl complexes were unstable and decomposed at room temperature or under mild heating by the release of methylphosphinidene (PMe), to give the corresponding derivatives [Mo2MCp2(μ3-P)(μ-PCy2)(CO)2Ln] displaying trigonal-planar phosphide ligands, giving rise to strongly deshielded (31)P NMR resonances (δP ca. 1100 ppm), while being involved in strong π bonding with the unsaturated Mo2 center of these molecules [Mo-Mo = 2.749(1) Å and Mo-P = ca. 2.30 Å when M = W]. An isolobal analogy could be established between the P→MLn fragments in these products and a carbyne ligand (CR), supported by density functional theory calculations on the tungsten compound, which also enabled an easy interpretation and prediction of their chemical behavior. Thus, the manganese complex could be reversibly carbonylated (pCO = ca. 3 atm, 293 K) to give the corresponding electron-precise pentacarbonyl [MnMo2Cp2Cp'(μ3-P)(μ-PCy2)(CO)5] [Mo-Mo = 3.1318(7) Å], a process also involving a trans-to-cis rearrangement of the Mo2Cp2 subunit. On the other hand, decarbonylation of the tungsten complex was accomplished in a refluxing toluene solution to give the hexacarbonyl [Mo2WCp2(μ3-P)(μ-PCy2)(μ-CO)(CO)5], a derivative containing an unsaturated 30-electron dimolybdenum center with an intermetallic triple bond.

Chemistry of Unsaturated Group 6 Metal Complexes with Bridging Hydroxy and Methoxycarbyne Ligands. 3. Formation and Cleavage of C−C and C−O Bonds in the Reactions of the Complexes [Mo2Cp2(µ-COMe)(µ-COR)(µ-PCy2)]BF4(R = Me, Et)

The reactions of the 30-electron complexes [Mo2Cp2(µ-COMe)(µ-COR)(µ-PCy2)]BF4 (Cp = η5-C5H5; R = Me, Et) with CO, CNtBu, or tetraethylpyrophosphite (tedip) lead to the new 32-electron complexes [Mo2Cp2{µ-η2:η2-C(OMe)C(OR)}(µ-PCy2)(CO)2]BF4, [Mo2Cp2{µ-η2:η2-C(OMe)C(OMe)}(µ-PCy2)(CNtBu)2]BF4, and [Mo2Cp2{µ-η2:η2-C(OMe)C(OMe)}(µ-PCy2)(µ-tedip)]BF4, respectively. These products contain a bonded dialkoxyacetylene molecule resulting from the coupling of two alkoxycarbyne ligands, induced by the addition of the donor molecules to the unsaturated dimetal center of the starting material, and this coupling can be fully reversed in the dicarbonyl product upon photochemical treatment. The structure of the diisocyanide and tedip complexes, both displaying quite short intermetallic lengths (ca. 2.66 Å), was confirmed crystallographically. The structure of the dicarbonyl derivative, which in solution exhibits cis and trans isomers, was optimized using DFT methods, which also led to the prediction of short intermetallic distances (2.75 Å). Demethylation of the dicarbonyl complexes leads to the new carboxycarbyne derivatives [Mo2Cp2{µ-C(CO2R)}(µ-PCy2)(CO)2] (R = Me, Et) as a result of the unexpected 1,2-shift of the methoxyl group occurring in the ketenyl intermediate (not detected) presumably formed first. This proposal is supported by DFT calculations, these predicting a moderate activation barrier (ΔG⧧298 = 78 kJmol−1) for the spontaneous transformation (ΔG298 = −51 kJ mol−1) of the ketenyl intermediate into the carbyne complex finally isolated. The bis(methoxycarbyne) complex is also reactive toward the 16-electron fragment Ru(CO)4, to give the 46-electron cluster [Mo2RuCp2(µ-COMe)2(µ-PCy2)(CO)4]BF4, which concentrates most of its electronic unsaturation at the dimolybdenum center (Mo−Mo = 2.686(1) Å), in spite of the rearrangement of the carbyne ligands, both of them now bridging one of the Mo−Ru edges, according to an X-ray diffraction study.

Dithiolene transfer from nickel to a dimolybdenum centre: the first dithiolene alkyne complex

Transfer of dithiolene ligands from [Ni(S 2C 2Ph 2) 2] to the dimolybdenum complex [Mo 2(μ-C 2R 2)(CO) 4Cp 2] (R=CO 2Me, Cp=η-C 5H 5) affords the first example of a dithiolene alkyne complex, [Mo 2(μ-C 2R 2)(μ-S 2C 2Ph 2) 2Cp 2], together with [Mo 2(μ-SCRCR)(μ-SCPhCPh)Cp 2] in which sulfur transfer from dithiolene to alkyne has occurred.

Room temperature ring opening of a 1,3-dithiole-2-thione at a dimolybdenum centre

The reaction of [Mo 2(CO) 4Cp 2] ( 1; Cp=η-C 5H 5) with 4,5-bis(carbomethoxy)-1,3-dithiole-2-thione at room temperature affords two products, both of which have been crystallographically characterised. In the first, [Mo 2{μ-SCS 2C 2(CO 2Me) 2}(CO) 5Cp 2] ( 2), the Mo≡Mo bond has been completely cleaved and the complex consists of the heterocycle η 2-side-bound through the thione function to a CpMo(CO) 2 unit, with an additional CpMo(CO) 3 group attached to the thione sulfur atom. In the second complex, [Mo 2{μ-SCSC(CO 2Me)C(CO 2Me)S}(CO) 3Cp 2] ( 3) the MoMo bond has been retained and partial ring opening of the heterocycle by CS bond cleavage has occurred, the latter being bonded as a thiolate ligand to one molybdenum and through the thione group to both metals.

Phosphorus–carbon bond activation of PMe3 at a dimolybdenum center: synthesis and structure of [Cp*Mo(μ-O2CMe)]2(μ-PMe2)(μ-Me)

The reaction of Mo2(µ-O2CMe)4 with KCp* in the presence of PMe3 yields [Cp*Mo(µ-O2CMe)]2(µ-PMe2)(µ-Me) as a result of cleavage of the P–CH3 bond.

Desulfurisation of trithiocarbonates at a dimolybdenum centre: an unexpected insertion into a co-ordinated alkyne

On reaction with the dimolybdenum alkyne complex [Mo2{µ-C2(CO2Me)2}(CO)4(η-C5H5)2], dialkyl trithiocarbonates (RS)2CS were dismantled into sulfido (µ-S), thiolate (µ-SR) and CSR fragments; remarkably the last of these inserts into the middle of the alkyne to produce a dimetalla-allyl species with a µ-C(CO2Me)C(SR)C(CO2Me) ligand.

Phosphido-bridged dimolybdenum complexes with sulfide and thiolate ligands as precursors to mixed-metal clusters

Treatment of [Mo 2(μ-H)(μ-PPh 2)(CO) 4(η-C 5H 5) 2] with an excess of a thiol, RSH, in refluxing toluene produced the novel quadruply-bridged compounds [Mo 2(μ-S) 2(μ-SR)(μ-PPh 2)(η-C 5H 5) 2] (7a, R = Et; 7b, R = Pr 1; 7c, R = Bu 1; 7d, R = p-C 6H 4Me; 7c R = C 12H 25) in moderate to good yield. The sulfur ligands in the product are formed by dealkylation of the thiol at the dimolybdenum centre. The reaction is proposed to proceed via the unstable intermediate species [Mo 2(μ-SR)(μ-PPh 2)(CO) 2(η-C 5H 5) 2], which could not be isolated in a pure form. Reaction of 7a-d with [Ru 3(CO) 12] in refluxing tetrahydrofuran (thf) produced good yields of the mixed-metal clusters [Mo 2Ru 2( μ 3S) 2( μ-SR)( μ-PPh 2)(CO) 4( η-C 5H 5) 2] 9; the X-ray crystal structure of the Et-substituted complex 9a has been determined. The cluster consists of a tetrahedral metal core with both Mo 2Ru faces capped by triply bridging sulfides. The phosphido group bridges the MoMo bond, whereas the thiolate ligand rated to the RuRu edge.

Desulfurisation and ring opening of cyclic trithiocarbonates at a dimolybdenum centre

The dimolybdenum alkyne complexes [Mo 2 (µ-R 1 CCR 1 )(CO) 4 (η-C 5 H 5 ) 2 ] (R 1 CO 2 Me, Ph) react with 1,3-dithiole-2-thiones by cleavage of the CS bond, ring opening of the resulting carbene and coupling with the alkyne ligand; one of the products, [Mo 2 (µ-S)(µ-SCR 2 CR 2 SCCR 1 CR 1 )(η-C 5 H 5 ) 2 ] (R 1 = R 2 = CO 2 Me), is structurally characterised.

Dimolybdenum complexes with sulfide and thiolate ligands as precursors to mixed-metal clusters: crystal structure of [Mo2Ru2(µ3-S)2(µ-SPri)2(CO)4(η-C5H5)2

The reaction of [Mo2(CO)4(µ-C2Me2)(η-C5H5)2]1 with thiols, RSH, in boiling toluene provides a useful new route to the known compounds [Mo2(µ-S)2(µ-SR)2(η-C5H5)2](R = Et 2a or Pri2b) in 40–45% yield. Both compounds exist as two isomers which were separated by chromatography for R = Et. The sulfur ligands in these complexes are formed by dealkylation of the thiols at the dimolybdenum centre. Reaction of 2 with [Ru3(CO)12] in refluxing tetrahydrofuran produced high yields of the mixed-metal clusters [Mo2Ru2(µ3-S)2(µ-SR)2(CO)4(η-C5H5)2]3; the crystal structure of the Pri-substituted complex 3b has been determined. The cluster consists of a tetrahedral metal core with both Mo2Ru faces capped by triply bridging sulfides. One of the thiolate ligands still bridges the molybdenum–molybdenum edge, but the other has migrated to the ruthenium–ruthenium edge.

Disproportionation of hydrazine by [Mo2(η-C5H5)2(µ-Cl)(µ-SMe)3] and formation of an Mo2(µ-NH2) amido bridge

Cleavage of the N–N bond of hydrozine by the dimolybdenum centre in [Mo2(η-C5H5)2(µ-Cl)(µ-SMe)3] 1 gives rise to the amido-bridged complex [Mo2(η-C5H5)2(µ-NH2)-(µ-SMe)3] 2.

Reactions of co-ordinated ligands. Part 53. Synthesis of dimolybdenum and ditungsten µ-σ,η2-(4e)-vinylidene and -allenylidene complexes; crystal structures of [N(PPh3)2][Mo2{µ-σ:η2-(3e)-C2Ph}(CO)4(η5-C9H7)2], [Mo2{µ-σ:η2-(4e)-CC(Ph)(CH2)4OMe}(CO)4(η-C5H5)2] and [Mo2{µ-σ:η2-(4e)-CCCMe2}(CO)4(η-C5H5)2

Addition of RCCLi to [Mo2(CO)4(η-C5H5)2] affords the anionic 1 : 1 adducts Li[Mo2{µ-σ:η2-(3e)-C2R}(CO)4(η-C5H5)2], the structure of the analogous η5-indenyl analogue being confirmed by a single-crystal X-ray diffraction study with the relatively stable complex [N(PPh3)2][Mo2{µ-σ:η2-(3e)-C2Ph}(CO)4(η5-C9H7)2]. The same anionic species are also formed on deprotonation (LiBut) of the transversally bridged alkyne complexes [Mo2(µ-RC2H)(CO)4(η-C5H5)2]. Variable-temperature NMR studies suggest that the µ-acetylide ligand in these anions readily switches from one metal centre to the other. Protonation of Li[Mo2{µ-σ:η2-(3e)-C2Ph}(CO)4(η-C5H5)2] affords a relatively unstable ‘side-on’ bonded vinylidene [Mo2{µ-σ:η2-(4e)-CCHPh}(CO)4(η-C5H5)2], which in solution at room temperature rearranges into [Mo2(µ-PhC2H)(CO)4(η-C5H5)2]. Reaction of Li[Mo2{µ-σ:η2-(3e)-C2Ph}(CO)4(η-C5H5)2] with MeOSO2CF3 in tetrahydrofuran (thf) affords via ring opening of the thf, the crystallographically identified stable complex [Mo2{µ-σ:η2-(4e)-CC(Ph)(CH2)4OMe}(CO)4(η-C5H5)2]. This is the first structurally characterised ‘side-on’ bonded vinylidene. The vinylidene fragment donates four electrons to the dimolybdenum centre with Mo(1)–C(1) 1.909(5), Mo(2)–C(1) 2.179(5) and Mo(2)–C(2) 2.443(6)A. The corresponding alkylation reaction in dichloromethane affords [Mo2{µ-σ:η2-(4e)-CC(Ph)Me}(CO)4(η-C5H5)2], and in this way a wide range of ‘side-on’ bonded vinylidene complexes were synthesised, extension of the synthetic procedure affording the corresponding ditungsten systems. Variable-temperature NMR studies show that in solution some of these complexes exhibit interesting dynamic behaviour and these observations are discussed. In solution the compound [Mo2{µ-σ:η2-(4e)-CC(Ph)Me}(CO)4(η-C5H5)2] undergoes an unusual thermal rearrangement into [Mo2{µ-σ:η3-CHC(Ph)CH2}(CO)4(η-C5H5)2], and it is shown that a variety of β-substituted vinylidenes undergo analogous rearrangement reactions to form µ-allylidene complexes. Protonation of the adduct Li[Mo{µ-σ:η2-(3e)-C2C(Me)CH2}(CO)4(η-C5H5)2] formed between CH2C(Me)CCLi and [Mo2(CO)4(η-C5H5)2] affords the first ‘side-on’ bonded allenylidene [Mo2{µ-σ:η2-(4e)-CCCMe2}(CO)4(η-C5H5)2], which was structurally characterised by X-ray crystallography. The molecule contains an asymmetrically bridged allenylidene, the first such molecule to be structurally identified. The Mo(1)–C(1) distance of 1.912(3)A indicates the presence of a MoC double bond.

The interconversion of oxo and imido ligands at a dimolybdenum centre: X-ray crystal structure of [Mo(η-C 5H 4Me)(NPh)(μ-NPh)] 2

The reversible interconversions of oxo and arylimido ligands and a metathesis reaction between arylimido ligands and arylisocyanates are described. The compound [Mo(η-C 5H 4Me)(NPh)(μ-NPh)] 2 contains linear terminal MoNPh ligands and a planar Mo 2(μ-N) 2 fragment.

Catalyst design. The activation of a trinuclear metal cluster complex by metal atom substitution

Investigations of the compound Mo/sub 2/(CO)/sub 4/(C/sub 5/H/sub 5/)/sub 2/ have shown that it will react with alkynes to give products formed by the addition and coupling of two, three, and four alkyne molecules at the dimolybdenum center. The authors have investigated the reaction of Ru/sub 3/(CO)/sub 9/(..mu../sub 3/-CO)(..mu../sub 3/-S) with HC/sub 2/Ph under similar conditions and have found that the reaction terminates with the introduction of one HC/sub 2/Ph unit. They have now investigated the reactivity of the heteronuclear cluster Mo/sub 2/Ru(CO)/sub 7/(C/sub 5/H/sub 5/)/sub 2/(..mu../sub 3/-S) (1) formed by the substitution of two ruthenium carbonyl units in Ru/sub 3/(CO)/sub 9/(..mu../sub 3/-S) by two cyclopentadienyl molybdenum carbonyl units, with HC/sub 2/Ph, and have found that the trinuclear center in 2 is active toward HC/sub 2/Ph oligomerization. Details of this study are described in this report.

Pathways for carbon–chain growth reactions at a dimolybdenum centre

Reaction of [Mo2(CO)4(η5-C9H7)2] with MeC2Me forms, via competitive reaction paths, the dinuclear complexes [Mo2(µ-MeC2Me)(CO)4(η5-C9H7)2] and [Mo2{µ-(σ,σ-η4-C4Me4)}(CO)4(η5-C9H7)(η3-C9H7)]; the latter on heating decarbonylates to give [Mo2{µ-(σ,σ-η4-C4Me4)}(µ-CO)(η5-C9H7)2], which reacts with MeC2Me to form the ‘fly-over’ complex [Mo2{µ-(σ,η3 : η3,σ-C6Me6)}(η5-C9 H7)2].

‘Side-on’ co-ordination of vinylidene at a dimolybdenum centre

Ethyne reacts with [Mo2(CO)4(η-C5Me5)2] under u.v. irradiation to yield [Mo2(CO)4(µ-η1,η2-CCH2)(η-C5Me5)2], containing vinylidene bound side-on to the dimolybdenum centre as a four-electron ligand; this new species undergoes thermal isomerisation to µ-ethyne, reacts with diazomethane to give µ-allene, and is step-wise protonated to afford µ-vinyl then µ-ethylidene.

Formation of an alkyne and a 1,3-diene at a dimolybdenum centre by isomerisation of a µ-allylidene complex; synthesis and X-ray structure of [Mo3(µ3-{η4-C·CH·CMe·CH})(CO)4(η-C5H5)3

Thermolysis of [Mo2(µ-CH·CH·CMe2)(CO)4(η-C5H5)2] in toluene–hexane gives [Mo2(µ-HC2Pri)(CO)4(η-C5H5)2], [Mo2(µ-{η4-CH2·CH·C(Me)CH2})(CO)2(η-C5H5)2], alkyne and isoprene complexes, respectively, and in addition the trinuclear complex [Mo3(µ3-{η4-C·CH·CMe·CH})(CO)4(η-C5H5)3], idnetified by X-ray crystallography.

Linking of four acetylene molecules at a dimolybdenum center and the protonation of the resulting metallacyclononatetraene to form a cation containing a bridging MoHC system

Linking of four acetylene molecules at a dimolybdenum center and the protonation of the resulting metallacyclononatetraene to form a cation containing a bridging MoHC system