Stephen R. Allen

Reactions of co-ordinated ligands. Part 27. Formation of η3-allyl and σ-vinyl complexes by nucleophilic attack on cationic molybdenum monoacetylene complexes. Molecular structures of [Mo(CO)(PEt3)(η3-anti-1-MeC3H4)(η5-C9H7)] and [Mo{P(OMe)3}3{σ-(E)-CHCHBut}(η-C5H5)]

Reaction of [MoLL1(η-MeC2R)A][BF4][R = Me, Et, or Pri; L = L1= P(OMe)3; LL1= Ph2PCHCHPPh2; L = PEt3, L1= CO; A =η-C5H5 or η5-C9H7] with NaBH4 or K[BHBus3] in tetrahydrofuran affords the anti-substituted η3-allyl complexes [MoLL1(η3-CH2CHCHR)A]. The structure of the complex [Mo(CO)(PEt3)(η3-anti-1-MeC3H4)(η5-C9H7)] was established by single-crystal X-ray crystallography. Crystals are monoclinic, space group P21/n with a= 10.038(2), b= 17.291(4), c= 11.346(3)A, β= 95.000(18)°, and Z= 4. Using 3 887 reflections measured on a four-circle diffractometer at room temperature, the structure has been refined to R 0.0344, R′ 0.0459. The molecule adopts an exo-conformation with the methyl substituent of the allyl ligand in an anti-position cis to the carbonyl ligand. Treatment of the complex [Mo{P(OMe)3}2(η-HC2But)(η-C5H5)][BF4] with NaBH4 or K[BHBus3] in the presence of excess P(OMe)3 gives the σ-vinyl complex [Mo{P(OMe)3}3{σ-(E)–CHCHBut}(η-C5H5)], structurally identified by n.m.r, spectroscopy and X-ray crystallography. Crystals are monoclinic, space group P21/c with a= 11.731(3), b= 12.796(3), c= 18.827(4)A, β= 92.33(2)°, and Z= 4. Using 7 437 intensity data, measured at 195 K on a four-circle diffractometer, the structure has been refined to residuals R= 0.026, R′= 0.026. The molecule shows typical four-legged ‘piano-stool’ geometry with the vinyl group adopting E stereochemistry, σ-bonded to the molybdenum atom. In contrast, the reaction of K[BHBus3] with [Mo{P(OMe)3}2(η-HC2Pri)(η-C5H5)][BF4] in tetrahydrofuran at low temperature affords the allyl complex [Mo{P(OMe)3}2(η3-1,1-Me2C3H3)(η-C5H5)]. The n.m.r. spectra and mechanism of formation of these compounds are discussed.

Reactions of co-ordinated ligands. Part 24. The reaction of bis-(but-2-yne)carbonyl(η-cyclopentadienyl or η5-indenyl)molybdenum tetrafluoroborate with acetonitrile and phosphines; crystal structures of but-2-ynecarbonyl(η5-indenyl)(triethylphosphine)molybdenum tetrafluoroborate and but-2-yne(η5-indenyl)bis(trimethylphosphine)molybdenum tetrafluoroborate

Reaction of the compounds [Mo(CO)(RC2R1)2(η-C5H5 or η5-C9H7)][BF4] with refluxing acetonitrile in the presence of the respective acetylene affords the compounds [Mo(NCMe)(RC2R1)2(η-C5H5 or η5-C9H7)][BF4] where R = R1= Me; R = But, R1= H; R = Ph, R1= Me; R = Pr1, R1= H. Treatment of [Mo(CO)(RC2R1)2(η-C5H5 or η5-C9H7)][BF4] at room temperature in methylene chloride with PEt3, PPh3, or P(C6H11)3 gives the monoacetylene compounds [Mo(CO)L(RC2R1)(η-C5H5 or η5-C9H7)][BF4][L = PEt3, PPh3, P(C6H11)3; R = R1= Me; R = But, R1= H; R = Pri, R1= H; R = Ph, R1= Me; or R = R1= C6H4Me-4]. Reaction of the acetonitrile complexes with the phosphines PEt3, PMe3, PMePh2, Ph2PCHCHPPh2, (Ph2PCH2)2, or (Me2PCH2)2 results in the displacement of both the acetonitrile and one acetylene ligand to form complexes of the type [MoL2(RC2R1)-(η-C5H5 or η-5-C9H7)][BF4]. X-Ray single-crystal crystallographic studies were carried out on the compounds [Mo(CO)(PEt3)(MeC2Me)(η5-C9H7)][BF4] and [Mo(PMe3)2(MeC2Me)(η5-C9H7)][BF4]. Crystals of the former are monoclinic, space group P21/c, Z= 4, in a unit cell of dimensions a= 9.158(2), b= 14.726(2), c= 16.581(3)A, and β= 94.12(2)°. Crystals of the latter complex are also monoclinic, space group P21, Z= 2, in a unit cell of dimensions a= 8.661 (2), b= 10.293(2), c= 13.580(2)A, and β= 106.42(2)°. The structures have been refined to R 0.041 (R′ 0.045) and R 0.039 (R′ 0.048) for 4 329 and 2 052 respective reflections with I > 3σ(I). The molybdenum co-ordination in each cation may be described in terms of a pseudo-octahedral structure. The but-2-yne ligand adopts a similar conformation in both complexes lying approximately coplanar with either the Mo–CO or Mo–PMe3 bond. Variable-temperature 1H and 13C n.m.r. data are interpreted in terms of rotation of the acetylene. Examination of the 13C chemical shifts of the acetylene contact carbons suggests that in these complexes the acetylene ligand can act as a four-electron donor.

Reactions of co-ordinated ligands. Part 33. Mononuclear η2-vinyl complexes: synthesis, structure, and reactivity

Treatment of the four-electron alkyne cation [Mo(η2-PhC2Ph){P(OMe)3}2(η-C5H5)][BF4] with K[BHBus3] affords the η2-vinyl or metallacyclopropene complex [[graphic omitted]HPh}{P(OMe)3}2(η-C5H5)](2). The related complexes [[graphic omitted]HPh}{P(OMe)3}2(η-C5H5)](3),[[graphic omitted]H(C6H4Me-4)}{P(OMe)3}2(η-C5H5)](4), [[graphic omitted]H(C6H4Me-4)}{P(OMe)3}2(η-C5H5)](5), and [[graphic omitted]Ph2}{P(OMe)3}2(η-C5H5)](6) are obtained on reaction of the corresponding lithium diarylcuprate with the respective alkyne cation [Mo(η2-R1C2R2){P(OMe)3}2(η2-C5H5)][BF4](R1= But, R2= H; R1= Pri, R2= H; R1= Me, R2= Ph). The structures of (2), (3), and (6) have been determined by single-crystal X-ray diffraction studies. The molecules show close similarities; each has a molybdenum atom to which a vinyl moiety is co-ordinated via one short and one long Mo–C bond. These molecules may be described either as η2(3e)-vinyl or metallacyclopropene complexes. The orientation of the C2 vinyl group relative to the Mo{P(OMe)3}2(η-C5H5) fragment is discussed in terms of the torsion angles. In (2) and (3), Cα of the vinyl group lies closer to the plane of the η-C5H5, ligand than does Cβ, whereas in (6) the reverse orientation is observed. The solution n.m.r. spectra of (2) and (3) have distinct 31P environments, whereas in (6) the phosphorus environments are equivalent. This is discussed in terms of a rotational movement, an extended Huckel molecular orbital analysis suggesting that the orientation and fluxional behaviour of the η2-vinyl ligands parallel those of the related alkyne complexes. Reaction of [Mo(η2-PhC2CH2Ph){P(OMe)3}2(η-C5H5)][BF4] with K[BHBus3] affords a separable mixture of isomeric complexes [[graphic omitted]HPh}{P(OMe)3}2(η-C5H5)], which differ only in the orientation of the η2-vinyl moiety. The complex (6) slowly rearranges in solution to an η3-allylic complex this being explained in terms of an η2 to σ change in the bonding mode of the vinyl ligand. A similar transformation is suggested to explain the formation of η3- allylic complexes on reaction of [Mo(η2-RC2H){P(OMe)3};2(η-C5H5)][BF4](R = But or Pri) with LiCuMe2. Extension of the dimethyl- or diphenyl-cuprate reactions to the cations [Mo(η2- R1C2R2){P(OMe)3}2(η-5H5)][BF4] provides evidence for competing reaction pathways involving either direct attack on the metal centre or on a co-ordinated alkyne carbon. The regioselectivity of the latter reaction is discussed in terms of steric and electronic effects.

Reactions of co-ordinated ligands. Part 28. The reaction of nucleophilic reagents with the cationic bis(acetylene) molybdenum complexes [Mo(CO)(RC2R′)2(η-C5H5)][BF4]; X-ray crystal structures of [Mo{C(Me)C(Me)C(O)C(Me)CHMe}(CO)2(η-C5H5)] and [Mo{η3:η2-HButCCHC–O–C(O)C(But)CH}(CO)(η-C5H5)]

Reaction of [Mo(CO)(η2-MeC2Me)2(η-C5H5)][BF4] with lithium dimethylcuprate affords [Mo(CO)(η2-MeC2Me)(σ-CMeCMe2)(η-C5H5)], whereas reaction with K[BHBus3] followed by treatment with CO gives [[graphic omitted])C(Me)CHMe}(CO)2(η-C5H5)], which has been identified by X-ray crystallography. Crystals are monoclinic, space group P21/n, Z= 4, in a unit cell with a= 6.495(3), b= 16.902(9), c= 14.002(7)A, β= 97.62(4)°. The structure has been refined to R= 0.025 (R′= 0.024) for 2 136 reflections (293 K) to 2θ < 55°(Mo-KαX-radiation). The molybdenum atom is co-ordinated by an η-C5H5 ligand, two terminal linear carbonyl ligands, and a bidentate ligand derived from one CO, two but-2-yne molecules, and a hydride ligand. Reaction of CO with [Mo(CO)(η2-MeC2Me)(σ-CMeCMe2)(η-C5H5)] affords an analogous complex. A similar reaction between K[BHBus3] and [Mo(CO)(η2-EtC2Me)2(η-C5H5)][BF4] gives two analogous isomeric complexes, both isomers containing the vinyl group, C(Me)CHEt. In contrast [Mo(CO)(η2-ButC2H)2(η-C5H5)][BF4] affords the lactone complex [Mo{η3:η2-HButCCH[graphic omitted]H}(CO)(η-C5H5)] characterised by X-ray crystallography. Crystals are monoclinic, space group P21/c(no. 14), Z= 4, in a unit cell with a= 11.935(7), b= 11.673(9), c= 13.674(9)A, β= 105.93(5)°. The structure has been refined to R= 0.053 (R′= 0.052) for 2 526 reflections (230 K) to 2θ⩽ 50°(Mo-KαX-radiation). The molybdenum atom is bound to an η-cyclopentadienyl ligand and a linear terminal CO. The remainder of the co-ordination sphere is taken up by an η3 : η2 vinyl-substituted, unsaturated γ-lactone derived from one hydride, two carbonyls, and two ButC2H ligands. The structures of these complexes and the mechanism of their formation are discussed.

Reactions of co-ordinated ligands. Part 39. The synthesis of carbyne complexes from alkyne molybdenum and tungsten cations; formation and crystal structures of [Mo(CCH2But){P(OMe)3}2(η-C5H5)] and [Mo{C(SiMe3)CH2}{P(OMe)3}2(η5-C9H7)]

Thermolysis of the σ-vinyl complexes [M{σ-(E)-CHCHBut}{P(OMe)3}3(η5-CmHn)](M = Mo or W; m=n= 5; m= 9, n= 7), formed by reaction of hydride-anion donors with the cations [M(η2-ButC2H){P(OMe)3}2(η5-CmHn)][BF4], leads to formation of the carbyne complexes [M(CCH2But){P(OMe)3}2(η5-CmHn)]. The η-cyclopentadienylmolybdenum carbyne complex was structurally characterised by X-ray crystallography. The molecule adopts a ‘three-legged piano-stool’ geometry with one of the legs being a MoC triple bond [1.796(2)A]. A similar reaction (–78 °C) between [Mo(η2-PriC2H){P(OMe)3}2(η-C5H5)][BF4] and K[BHBus3] in the presence of P(OMe)3 afforded the η3-allyl complex [Mo(η3-CH2 CH CMe2){P(OMe)3}2(η-C5H5)], whereas the same cation with NaBH4–P(OMe)3 gave the carbyne complex [Mo(CCH2Pri){P(OMe)3}2(η-C5H5)]. Possible mechanisms for these carbyne-forming reactions are discussed. In contrast, reaction of the trimethylsilyl-substituted cation [Mo(η2-Me3SiC2H){P(OMe)3}2(η5-C9H7)][BF4] with K[BHBus3] or Li(CuPh2) affords the η2-vinyl complexes [[graphic omitted]HR}{P(OMe)3}2(η5-C9H7)](R = H or Ph). The phenyl-substituted η2-vinyl is also formed on reaction of [Mo(η2-Me3SiC2Ph){P(OMe)3}2(η5-C9H7)][BF4] with K[BHBus3], the corresponding reaction with Li(CuPh2) affording [[graphic omited]Ph2}{P(OMe)3}2(η5-C9H7)]. The η2-vinyl species [graphic omitted]HR}(R = H or Ph) thermally rearrange to the carbyne complexes [Mo(CCH2SiMe3){P(OMe)3}2(η5-C9H7)] and [Mo{CCH(Ph)SiMe3}{P(OMe)3}2(η5-C9H7)]. These trimethylsilyl-substituted carbynes are desilylated in NaF–H2O–MeCN to form [Mo(CCH3){P(OMe)3}2(η5-C9H7)] and [Mo(CCH2Ph){P(OMe)3}2(η5-C9H7)]. The molecular geometry of [[graphic omitted]H2}{P(OMe)3}2(η5-C9H7)] was established by X-ray crystallography. The η2-vinyl C–C bond lies nearly parallel to the Mo–P(1) vector the P(1)–Mo–C(8)–C(7) torsion angle being 13.8(2)°, with the carbene carbon C(8) nearer than C(7) to the plane of the indenyl ligand. The Mo–C(8) bond length is 1.955(3)A, whereas Mo–C(7) is 2.258(4)A.

The role of metallacyclopropenes in the formation of alkylidyne complexes; evidence for a 1,2-trimethylsilyl shift and the X-ray crystal structure of [MoC(SiMe3)CH2{P(OMe)3}2(η5-C9H7)]

Reaction of [Mo{P(OMe)3}2(Me3SiC2H)(η5-C9H7)][BF4] with K[BHBus3] affords the metallacyclopropene [[graphic omitted]H2{P(OMe)3}2(η5-C9H7)], structurally identified by X-ray crystallography, which rearranges quantitatively into the alkylidyne complex [MoC.CH2SiMe3{P(OMe)3}2(η5-C9H7)].

Reactions of co-ordinated ligands. Part 29. The formation of a 1,3-diene at a molybdenum centre by linking of but-2-yne with an alkene; X-ray crystal structures of [Mo(η2-o-Ph2PC6H4CHCH2)(η2-MeC2Me)(η-C5H5)][BF4] and [Mo(NCMe)(η4-o-MeCHCMe–CHCHC6H4PPh2)(η-C5H5)][BF4]

Reaction of [Mo(CO)(η2-RC2R′)2(η-C5H5)][BF4](R = R′= Me; R = H, R′= But) with o-diphenylphosphinostyrene (dpps) in CH2Cl2 affords the alkyne–alkene complex [Mo(dpps)(η2-RC2R′)(η-C5H5)][BF4]. The structure of [Mo(η2-dpps)(η2-MeC2Me)(η-C5H5)][BF4] was established by X-ray crystallography. Crystals are orthorhombic, space group Pna21 with Z= 4 in a unit cell of dimensions a= 22.324(5), b= 9.249(3), c= 14.566(3)A. The structure has been refined to R= 0.040 (R′= 0.041) for 2 228 reflections at 293 K. The molybdenum atom is bonded to an η-C5H5, an η2-bonded but-2-yne, and chelating o-diphenylphosphinostyrene ligands, where the C–C vectors of the co-ordinated alkyne and alkene lie essentially parallel to the Mo–P axis. In refluxing acetonitrile the but-2-yne cation is transformed via C–C bond formation and a 1,3-H shift into a 1,3-diene complex [Mo(NCMe)(η4-MeCHCMe–CHCHC6H4PPh2-o)(η-C5H5)][BF4] identified by X-ray crystallography. Crystals are monoclinic, space group P21/c with Z= 4 in a unit cell of dimensions a= 10.836(2), b= 8.551 (2), c= 30.896(6)A, and β= 94.80(2)°. The structure has been refined to R= 0.029 (R′= 0.030) for 3 828 reflections at 293 K. The cation contains a molybdenum atom bound to an η-C5H5 ligand, an N-bonded acetonitrile, and a ligand formally derived from but-2-yne and o-diphenylphosphinostyrene, which is η4-bonded to the Mo via a 1,3-diene moiety. The co-ordinated acetonitrile is displaced by ButCN, CO, and P(OMe)3 to form the cations [Mo(L)(η4-MeCHCMe–CHCHC6H4PPh2-o)(η-C5H5)][BF4][L = ButCN, CO, or P(OMe)3]. The nitrile-substituted cations exist in solution as a mixture of two conformational isomers, whereas the CO- and P(OMe)3-substituted cations are present in only one isomeric form. The conformations of these species are discussed in terms of frontier molecular orbitals, and the nature of the 1,3-H shift process has been examined using deuterium-labelled o-diphenylphosphinostyrene.

Reactions of co-ordinated ligands. Part 30. The transformation of methylenecyclopropanes into cationic η4-trimethylenemethanemolybdenum complexes, reactions with nucleophilic reagents, and the molecular structure of [Mo{η4-C(CH2)3}(CO)2(η-C5Me5)][BF4]

Reaction of [Mo2(CO)6(η-C5Me5)2] with methylenecyclopropane and AgBF4 in CH2Cl2 affords the cationic trimethylenemethane complex [Mo{η4-C(CH2)3}(CO)2(η-C5Me5)][BF4]. Methylenecyclopropane and 2,2-dimethylmethylenecyclopropane react with [Mo(NCMe)2(CO)2(η-C5H5)][BF4] to give [Mo{η4-C(CH2)3}(CO)2(η-C5H5)][BF4] and [Mo{η4-C(CH2)2CMe2}(CO)2(η-C5H5)][BF4] respectively. A single-crystal X-ray diffraction study of [Mo{η4-C(CH2)3}(CO)2(η-C5Me5)][BF4] confirmed that ring opening of methylenecyclopropane had occurred. The complex crystallises in the centrosymmetric orthorhombic space group Pbca with a= 12.822(2), b= 12.311 (3), c= 22.660(4)A, and Z= 8 ion pairs. The structure has been solved by conventional methods and refined by full-matrix least squares to R= 0.0674 for 2 747 observed reflections at 268 ± 1 K. In the cation the trimethylenemethane ligand adopts an orientation that is syn with respect to the OC–Mo–CO angle. It is pyramidal with the CH2 groups bent towards the molybdenum atom by an average of 12.4°. There is intramolecular congestion involving the C5Me5 and C(CH2)3 ligands that may contribute towards the observed asymmetric bonding of the former to the metal atom. Extended Huckel molecular-orbital calculations suggest, that the observed syn stereochemistry is electronically preferred, and that the barrier to rotation of the η4-C(CH2)3 ligand relative to a Mo(CO)2(η-C5H5)+ fragment is high. The stereochemistry of the ring-opening reaction is disrotatory-out as exemplified by the conversion of cis-and trans-2,3-dimethylmethylenecyclopropane into syn,syn-dimethyl and syn,anti-dimethyl-trimethylene-methane complexes. The reaction of these cationic η4-trimethylenemethane complexes with the nucleophiles BH4–, OH–, CuMe2–, and SPh– affords η3-allylic complexes derived from attack on the peripheral carbons.

σ-η2 Transformation of an electron-rich thioarsenic heterocyclic ligand: X-ray crystal structure of [Mo(η2-AsSCH2CH2S)(CO)2(η-C5H5)]

Photolysis of the complex [Mo(σ-[graphic omitted])(CO)3(η-C5H5)] causes elimination of carbon monoxide and formation of [Mo(η2-[graphic omitted])(CO)2(η-C5H5)] in which the ring behaves as a di-hapto, formally three-electron donor.

Reactions of co-ordinated ligands. Part 32. The reaction of sulphur nucleophiles with cationic molybdenum alkyne complexes: alkyne rotation and the molecular structures of the complexes [Mo(SC6H4NO2-p){P(OMe)3}(η2-MeC2Me)(η-C5H5)] and [Mo(SC6H4SPh-o)(CO)(η2-MeC2Me)(η-C5H5)]

Treatment of [Mo{P(OMe)3}2(η2-MeC2Me)(η-C5H5)][BF4] with NaSR (R = Me, p-NH2C6H4, p-MeOC6H4, p-MeC6H4, C6H5, or p-NO2C6H4) leads to displacement of P(OMe)3 and formation of [Mo(SR){P(OMe)3}(η2-MeC2Me)(η-C5H5)]. Similar reactions afforded [Mo(SePh){P(OMe)3}-(η2-MeC2Me)(η-C5H5)] and [Mo(SMe){P(OMe)3}(η2-ButC2H)(η-C5H5)]. Barriers to rotation of the but-2-yne ligand present in these complexes are related to electronic effects of the substituents on the thiolate ligand. The molecular structure of [Mo(SC6H4NO2-p){P(OMe)3}(η2-MeC2Me)-(η-C5H5)] has been established by single-crystal X-ray crystallography. Crystals are monoclinic, space group P21/n with a= 8.968(3), b= 20.104(9), c= 11.452(5)A, β= 95.77(3), and Z= 4. Using 3 601 reflections measured on a four-circle diffractometer at 200 K, the structure has been refined to R 0.028, R′ 0.032. The molecule adopts the familiar three-legged piano stool geometry, in which the but-2-yne ligand is orientated such that the central C–C bond lies approximately parallel to the Mo–P vector. In contrast, [Mo(CO)(η2-MeC2Me)2(η-C5H5)][BF4] reacts with NaSMe to give the complexes [Mo(SMe)(CO)(η2-MeC2Me)(η-C5H5)] and [Mo2(CO)4(µ-MeC2Me)(η-C5H5)2], whereas the corresponding reaction with [Mo(NCMe)(η2-MeC2Me)2(η-C5H5)][BF4] affords [Mo(SMe)-(η2-MeC2Me)(η-C5H5)]. Reaction of sodium dimethyldithiocarbamate with [Mo(NCMe)(R1C2R2)2-(η-C5H5)][BF4](R1= R2= Me or R1= But, R2= H) gives the monoalkyne complexes [Mo(S2C–NMe2)(η2-R1C2R2)(η-C5H5)]. In contrast the anion [SC6H4SPh-o]– affords a separable mixture of [Mo(SC6H4SPh-o)(η2-R1C2R2)2(η-C5H5)] and [[graphic omitted]Ph-o)(η2-R1C2R2)(η-C5H5)]. Reaction of the S,S′-bidentate species with carbon monoxide gives [Mo(SC6H4SPh-o)(CO)(η2-MeC2Me)-(η-C5H5)], which was structurally identified by X-ray crystallography. Crystals are orthorhombic, space group P212121(no. 19) with Z= 4 in a unit cell of dimensions a= 9.852(3), b= 11.486(4), and c= 18.476(6)A. From 1 714 independent intensities measured at 292 K the structure has been refined to R 0.059, R′ 0.041. The molecule adopts a piano stool geometry with the but-2-yne lying parallel to the Mo–CO vector. The dithioaryl ligand is monodentate.