Synthesis and spectral characterization of diorganodiaminosilanes [(ArNH)2SiPhMe] (Ar = 2,6-Pr2C6H3; 2,4,6-Me3C6H2) and lithium silylamide [(2,6-Et2C6H3NLi)(2,6-Et2C6H3NH)SiPh2]

Aminosilanes bearing bulky substituents on nitrogen centers, [(ArNH)₂SiPhMe] (Ar = 2,6-ⁱPr₂C₆H₃ (1), 2,4,6-Me₃C₆H₂ (2)) and half-sandwich lithium silylamide [(2,6-Et₂C₆H₃NLi)(2,6-Et₂C₆H₃NH)SiPh₂] (3) have been prepared and characterized by elemental analysis, IR, EI mass and NMR (¹H and ²⁹Si) spectroscopic studies. The solid state structures of 2 and 3 have been determined by single crystal X-ray diffraction studies. The molecule 2 has a C₁ symmetry due to the steric crowding, and the two N-H protons are approximately trans to each other. The amido nitrogen atoms in 2 show significant deviation from trigonal-planar geometry, and as a result, the observed Si-N bonds are marginally longer than those observed in aminosilanes with planar nitrogen atoms. The molecule 3 exists as discrete dimer with an inversion center. The Li ion in 3 forms intramolecular π-complex with the neighboring aryl (2,6-Et₂C₆H₃) group, to form a half-sandwich lithium silylamide.     

Synthesis of neutral and cationic monocyclopentadienyl alkyl niobium and tantalum complexes

Compound [NbCp′Me₄] (Cp′ = η⁵-C₅H₄SiMe₃, 1) reacted with several ROH compounds (R = tBu, SiiPr₃, 2,6-Me₂C₆H₃) to give the derivatives [NbCp′Me₃(OR)] (R = tBu 2a, SiiPr₃2b, 2,6-Me₂C₆H₃2c). The diaryloxo tantalum compound [TaCp^∗Me₂(OR)₂] (Cp^∗ = η⁵-C₅Me₅, R = 2,6-Me₂C₆H₃3) was obtained by reaction of [TaCp^∗Cl₂Me₂] with 2 equiv of LiOR (R = 2,6-Me₂C₆H₃). Abstraction of one methyl group from these neutral compounds 1-3 with the Lewis acids E(C₆F₅)₃ (E = B, Al) gave the ionic derivatives [NbCp′Me₂X][MeE(C₆F₅)₃] (X = Me 4-E. X = OR; R = SiiPr₃5b-E, 2,6-Me₂C₆H₃5c-E. E = B, Al) and [TaCp^∗Me(OR)₂][MeE(C₆F₅)₃] (R = 2,6-Me₂C₆H₃6-E; E = B, Al). Polymerization of MMA with the aryloxoniobium compound 2c and Al(C₆F₅)₃ gave syndiotactic PMMA in a low yield, whereas the tetramethylniobium compound 1 and the diaryloxotantalum derivative 3 were inactive.     

Synthesis and structures of crystalline lithium 1-azaallyls and a 1,3-diazaallyl derived from Li{CH(SiMe3)(SiMe3−n(OMe)n)} (n=1 or 2) and Li{CH(SiMe2OMe)2} and RCN (R=Bu, Ph, 2,5-Me2C6H3, or Ad)

Crystalline [Li{N(SiMe₂OMe)C(^tBu)C(H)(SiMe₃)}]₂ (5), [Li{N(SiMe₂OMe)C(Ph)C(H)(SiMe₃)}]₂ (6), [C(C₆H₃Me₂-2,5)C(H)(SiMe₃)}(TMEDA)](7), [Li{N(SiMe(OMe)₂)C(^tBu)C(H)(SiMe₃)}(THF)]₂ (8), Li{N(SiMe(OMe)₂)C(Ph)C(H)(SiMe₃)}(TMEDA) (9) and [Li{N(SiMe₂OMe)C(^tBu)C(H)(SiMe₂OMe)}]₂ (10) were readily obtained at ambient temperature from (i) [Li{CH(SiMe₃)(SiMe₂OMe)}]₈ (1) and an equivalent portion of RCN (R=^tBu (5), Ph (6) or 2,5-Me₂C₆H₃ (7)); (ii) [Li{CH(SiMe₃)(SiMe(OMe)₂)}]_∞ (2) and an equivalent portion of ^tBuCN (8) or PhCN (9); and (iii) [Li{CH(SiMe₂OMe)₂}]_∞ (3) and one equivalent of ^tBuCN (10). Reactions (i) and (ii) were regiospecific with SiMe_3−n(OMe)_n>SiMe₃ in 1,3-migration from C (in 1 or 2)→N. The 1-azaallyl ligand was bound to the lithium atom as a terminally bound κ¹-enamide (8 and 10), a bridging η³-1-azaallyl (6), or a bridging κ¹-enamide (5). The stereochemistry about the CC bond was Z for 5, 8 and 10 and E for 7. X-ray data are provided for 5, 6, 7, 8 and 10 and multinuclear NMR spectra data in C₆D₆ or C₆D₅CD₃ for each of 5-10.     

Triphenylpyrylium salt-sensitized photoreactions of 1,4-diaryl-2,3-dioxabicyclo[2.2.2]octanes through competitive single electron-transfer pathway and proton-catalyzed pathway

2,4,6-Triphenylpyrylium tetrafluoroborate (TPPBF₄)-sensitized photoinduced electron-transfer (PET) reactions of 1,4-diaryl-2,3-dioxabicyclo[2.2.2]octanes 5 (a: Ar¹ = Ar² = p-MeOC₆H₄, b: Ar¹ = Ar² = p-MeC₆H₄, c: Ar¹ = Ar² = Ph) underwent novel fragmentation through their radical cations to give 1,4-diarylbutan-1,4-diones 6 accompanied by elimination of ethylene. On the other hand, 4-aryl-cyclohex-3-en-1-ones 7, p-substituted phenols 8, and 4-aryl-4-aryloxycyclohexanones 9 were produced through proton-catalyzed pathways when the PET reactions of 5 were performed in the absence of a certain base such as 2,6-di-tert-butylpyridine (DTBP). Particularly, the formation of 9 is consistent with the novel cationic rearrangement involving nucleophilic O-1,2-aryl shifts and C-1,4-aryl shifts.     

X-ray and multinuclear NMR study of the mixed aggregate [(Ph2P(O)N(CH2Ph)CH3) · LiOC6H2-2,6-{C(CH3)3}2-4-CH3) · C7H8]2: a model for the directed metalation of phosphinamides

The addition of LiBuⁿ to a toluene solution of Ph₂P(O)N(CH₂Ph)CH₃1 and 2,6-di-tert-butyl-4-methylphenol 5 leads to the formation of the mixed dimer [(Ph₂P(O)N(CH₂Ph)CH₃) · LiOC₆H₂-2,6-{C(CH₃)₃}₂-4-CH₃) · C₇H₈]₂6. The single crystal X-ray structure shows that two lithium aryloxide moieties dimerize giving rise to a Li₂O₂ core in which each lithium atom is additionally coordinated to a phosphinamide 1 ligand. The multinuclear magnetic resonance study (¹H, ⁷Li, ¹³C, ³¹P) indicates that the solid-state structure is preserved in toluene solution. Complex 6 may be considered as a model for the pre-complexation step preceding the metalation of phosphinamides by an organolithium base.     

Synthesis and characterization of aluminum complexes of 2-pyrazol-1-yl-ethenolate ligands

The synthesis and the characterization of some new aluminum complexes with bidentate 2-pyrazol-1-yl-ethenolate ligands are described. 2-(3,5-Disubstituted pyrazol-1-yl)-1-phenylethanones, 1-PhC(O)CH₂-3,5-R₂C₃HN₂ (1a, R = Me; 1b, R = Bu^t), were prepared by solventless reaction of 3,5-dimethyl pyrazole or 3,5-di-tert-butyl pyrazole with PhC(O)CH₂Br. Reaction of 1a or 1b with (R¹ = Me, Et) yielded N,O-chelate alkylaluminum complexes (2a, R = R¹ = Me; 2b, R = Bu^t, R¹ = Me; 2c, R = Me, R¹ = Et). Compound 1a was readily lithiated with LiBuⁿ in thf or toluene to give lithiated species 3. Treatment of 3 with 0.5 equiv of MeAlCl₂ or AlCl₃ yielded five-coordinated aluminum complexes [XAl(OC(Ph)CH{(3,5-Me₂C₃HN₂)-1})₂] (4, X = Me; 5, X = Cl). Reaction of 5 with an equiv of LiHBEt₃ generated [Al(OC(Ph)CH{(3,5-Me₂C₃HN₂)-1})₃] (6). Complex 6 was also obtained by reaction of 3 with 1/3 equiv of AlCl₃. Treatment of 5 with 2 equiv of AlMe₃ yielded complex 2a, whereas with an equiv of AlMe₃ afforded a mixture of 2a and [Me(Cl)AlOC(Ph)CH{(3,5-Me₂C₃HN₂)-1}] (7). Compounds 1a, 1b, 2a-2c and 4-6 were characterized by elemental analyses, NMR and IR (for 1a and 1b) spectroscopy. The structures of complexes 2a and 5 were determined by single crystal X-ray diffraction techniques. Both 2a and 5 are monomeric in the solid state. The coordination geometries of the aluminum atoms are a distorted tetrahedron for 2a or a distorted trigonal bipyramid for 5.     

Synthetic access to optically active isoflavans by using allylic substitution

A general approach to the (S)- and (R)-isoflavans was invented, and efficiency of the method was demonstrated by the synthesis of (S)-equol ((S)-3), (R)-sativan ((R)-4), and (R)-vestitol ((R)-5). The key step is the allylic substitution of (S)-6a (Ar¹=2,4-(MeO)₂C₆H₃) and (R)-6b (Ar¹=2,4-(BnO)₂C₆H₃) with copper reagents derived from CuBr·Me₂S and Ar²-MgBr (7a, Ar²=4-MeOC₆H₄; 7b, 2,4-(MeO)₂C₆H₃; 7c, 2-MOMO-4-MeOC₆H₃), furnishing anti S_N2′ products (R)-8a and (S)-8b,c with 93-97% chirality transfer in 60-75% yields. The olefinic part of the products was oxidatively cleaved and the Me and Bn groups on the Ar¹ moieties was then removed. Finally, phenol bromide 9a and phenol alcohols 9b,c underwent cyclization with K₂CO₃ and the Mitsunobu reagent to afford (S)-3 and (R)-4 and -5, respectively.     

ipso-and para-Functionalization of meta-terphenyl ligands with substituted methyl groups: Unusual head-to-tail coupling of terphenyl moieties

The synthesis and characterization of several ipso-functionalized derivatives of the bulky terphenyl group are described. These include the primary alcohol Ar′CH₂OH (1), the bromo derivative Ar′CH₂Br (2), and the terphenyl formate Ar′CH₂OC(O)H (3). The alcohol 1 was obtained by treatment of LiAr′ with formaldehyde, and 1 was readily converted to the bromo derivative 2 using HBr. The reaction of 1 with formic acid afforded 3 in good yield. Attempts to form the Grignard derivative of 1, i.e., Ar′CH₂MgBr, resulted in a head-to-tail reaction of the terphenyl benzyl units to yield an unusual coupled product 4. An approach to the avoidance of this coupling involved the synthesis of the terphenyl derivatives and , bearing methyl groups in the para positions of the central aryl ring, which could be prepared in good yield, and converted to their respective lithium salts 7 and 8 without complication . The compounds were characterized by ¹H and ¹³C NMR spectroscopy, IR spectroscopy (1) and X-ray crystallography (2, 4, 5 and 6).     

Synthesis and characterisation of complexes of the 2,6-diphenoxyphenyl ligand

The use of the 2,6-diphenoxyphenyl ligand has facilitated the stabilisation of lithium, silane and stannane complexes. The ortho-metallation reaction between 1,3-(PhO)₂C₆H₄ and ⁿBuLi yields 2,6-(PhO)₂C₆H₃Li (1); the crystallographically characterised dimer [2,6-(PhO)₂C₆H₃Li(OEt₂)]₂ ([1.Et₂O]₂) can be obtained by the crystallisation of 1 from diethyl ether. The reaction between 1 and Me₃ECl gives rise to the structurally authenticated complexes 2,6-(PhO)₂C₆H₃EMe₃ [E = Si, 2; E = Sn, 3].     

Lutetium gets a crown: Synthesis, structure and reaction chemistry of the separated ion pair complex, [Li(12-crown-4)2][(C5Me5)2LuMe2]

Reaction of (C₅Me₅)₂Lu(Me)(μ-Me)Li(THF)₃ (2) with excess 12-crown-4 affords the new separated ion pair complex, [Li(12-crown-4)₂][(C₅Me₅)₂LuMe₂] (3), in excellent yield. This complex reacts with 2,6-diisopropylaniline and phenylacetylene to give the methyl amide complex [Li(12-crown-4)₂][(C₅Me₅)₂Lu(Me)(NH-2,6-ⁱPr₂C₆H₃)] (4) and the bis(acetylide) complex [Li(12-crown-4)₂][(C₅Me₅)₂Lu(C≡C-Ph)₂] (5), respectively. Attempts to promote methane loss from complexes 3 and 4 to generate a lutetium methylidene or imido complex, respectively, were unsuccessful. The ability of the bis(acetylide) complex 5 to act as a π-tweezer complex was also explored. Reaction between [Li(12-crown-4)₂][(C₅Me₅)₂Lu(C≡C-Ph)₂] (5) and CuSPh gave only intractable lutetium products and the copper(I) species [Li(12-crown-4)₂][Cu(C≡C-Ph)₂] (8). The new lutetium complexes have been characterized by elemental analysis and NMR spectroscopy. Finally, the X-ray crystal structures of (C₅Me₅)₂Lu(Me)(μ-Me)Li(THF)₃ (2), [Li(12-crown-4)₂][(C₅Me₅)₂LuMe₂] (3), [Li(12-crown-4)₂][(C₅Me₅)₂Lu(Me)(NH-2,6-ⁱPr₂C₆H₃)] (4), [Li(12-crown-4)₂][(C₅Me₅)₂Lu(C≡C-Ph)₂] (5), and [Li(12-crown-4)₂][Cu(C≡C-Ph)₂] (8) are also reported.     

Synthesis, characterisation and solid state structures of α-diimine cobalt(II) complexes: Ethylene polymerisation tests

A series of cobalt(II) compounds of the type [CoX₂(α-diimine)] were synthesised by direct reaction of anhydrous CoCl₂ or CoI₂ and the corresponding α-diimine ligand, in CH₂Cl₂: [CoI₂(o,o′,p-Me₃C₆H₂-DAB)] (1), [CoI₂(o,o′-ⁱPr₂C₆H₃-DAB)] (2), (where Ar-DAB = 1,4-bis(aryl)-2,3-dimethyl-1,4-diaza-1,3-butadiene), and [CoCl₂(o,o′,p-Me₃C₆H₂-BIAN)] (3), [CoCl₂(o,o′-ⁱPr₂C₆H₃-BIAN)] (4), and [CoI₂(o,o′-ⁱPr₂C₆H₃-BIAN)] (5) (where Ar-BIAN = bis(aryl)acenaphthenequinonediimine). All compounds were characterised by elemental analyses, IR, mass spectrometry, and X-ray diffraction whenever possible. The crystal structures of compounds 2-4 showed, in all cases, distorted tetrahedral geometries about the Co, built by two halogen atoms and two nitrogen atoms of the α-diimine ligand. Compounds 3 and 4, as well as [CoCl₂(o,o′,p-Me₃C₆H₂-DAB)] (1a), and [CoCl₂(o,o′-ⁱPr₂C₆H₃-DAB)] (2a), were activated by methylaluminoxane (MAO) and tested as catalysts for ethylene polymerisation, showing low catalytic activities. Selected polyethylene (PE) samples were characterised by ¹H and ¹³C NMR and FT-IR spectroscopies, and by differential scanning calorimetry (DSC), revealing branching microstructures (2.5-5.5%).     

Synthesis and crystal structures of novel lithium- and palladium-1-azaallyls

Treatment of (RH₂C)₂C₅H₃N-2,6 (R=SiMe₃) with BuⁿLi followed by addition of Me₃SiCl gave the tetrasilyl pyridine derivative (R₂HC)₂C₅H₃N-2,6 1 in high yield. Further lithiation of 1 with BuⁿLi and reaction of the intermediate with PhCN led to the new lithium-1-azaallyl [Li{N(R)C(Ph)C(R)(C₅H₃N-2,6)(CHR₂)}]₂2, while metallation of the previously described di-lithium compounds [Li{N(R)C(R^′)CH}₂(C₅H₃-2,6)]Li(tmen)_n (R=SiMe₃, R^′=Bu^t, n=1 or R=SiMe₃, R^′=Ph, n=2) with PdCl₂(PhCN)₂ yielded the novel metallacycles [Pd{{N(H)(R)C(R^′)CH}{N(SiMe₂CH₂)C(R^′)CH}C₅H₃N-2,6}] 3 (R^′=Bu^t) and [Pd{{N(R)C(R^′)CH}{N(R)(H)C(R^′)CH}C₅H₃N-2,6}₂] (R^′=Ph) 4 in moderate to low yield. Compound 3 is unusual in being the first example of a crystallographically characterised PdNSiC heterocycle which is believed to be formed via an intramolecular CH-activation of a trimethylsilyl group by Pd(II). All four compounds were fully characterised by NMR-spectroscopy, microanalysis (not 4) and X-ray diffraction.     

(P-Bis(pentafluorophenyl) substituted) PCP-pincer Ru(II) complexes: A theoretical study of the molecular structure and electronic properties

The differences between the molecular structures of the PCP-pincer complex [RuCl{C₆H₃(CH₂P(C₆H₅)₂)₂-2,6}(PPh₃)] ([RuCl(PCP^H)(PPh₃)], 1) and its tetrakis-pentafluorophenyl substituted analogue [RuCl{C₆H₃(CH₂P(C₆F₅)₂)₂-2,6}(PPh₃)] ([RuCl(PCP^F20)(PPh₃)], 2) have been rationalised by performing calculations on the cations [Ru(PCP^H)(PPh₃)]⁺ (1cat) and [Ru(PCP^F20)(PPh₃)]⁺ (2cat). The molecular interactions between the chloride ligand and the axial rings, as found in 1 and 2, respectively, have been studied computationally in the model systems [(C₆X₅PH₂)₂Cl⁻] (X = H, F). The calculations on 2cat show that in 2 it is most likely the attractive electrostatic interaction between the chloride ligand and the fluorinated phenyl rings that forces the C_ipso atom to occupy an axial position rather than an equatorial one in the observed (X-ray of 2) square pyramidal arrangement. In 1, however, repulsive steric hindrance forces the PPh₃ ligand to take the apical position. The applicability of the TD-DFT method for the calculation of the electronic spectra of the PCP-pincer compounds 1 and 2 has been tested. The results indicate that the excitation energies calculated for both complexes are in a reasonable agreement with the experimental absorption maxima. However, for 1, all the calculated transition energies are underestimated.     

Rare earth metal complexes based on β-diketiminato and novel linked bis(β-diketiminato) ligands: Synthesis, structural characterization and catalytic application in epoxide/CO2-copolymerization

Mesityl substituted β-diketiminato lanthanum and yttrium complexes [(BDI)Ln{N(SiRMe₂)}₂] (BDI = ArNC(Me)CHC(Me)NAr, Ar = 2,4,6-Me₃C₆H₂, Ln = La, R = Me (1), H (2a); Ln = Y, R = H (2b)) can be prepared via facile amine elimination starting from [La{N(SiMe₃)₂}₃] and [Ln{N(SiHMe₂)₂}₃(THF)₂] (Ln = Y, La), respectively. The X-ray crystal structure analysis of 1 revealed a distorted tetrahedral geometry around lanthanum with a η²-bound β-diketiminato ligand. A series of novel ethylene- and cyclohexyl-linked bis(β-diketiminato) ligands [C₂H₄(BDI^Ar)₂]H₂ and [Cy(BDI^Ar)₂]H₂ [Ar = Mes (=2,4,6-Me₃C₆H₂), DEP (=2,6-Et₂C₆H₃), DIPP (=2,6-i-Pr₂C₆H₃)] were synthesized in a two step condensation procedure. The corresponding bis(β-diketiminato) yttrium and lanthanum complexes were obtained via amine elimination. The X-ray crystal structure analysis of the ethylene-bridged bis(β-diketiminato) complex [{C₂H₄(BDI^Mes)₂}YN(SiMe₃)₂] (3b) and cyclohexyl-bridged complexes [{Cy(BDI^Mes)₂}LaN(SiHMe₂)₂] (7) and [{Cy(BDI^DEP)₂}LaN(SiMe₃)₂] (8) revealed a distorted square pyramidal coordination geometry around the rare earth metal, in which the amido ligand occupies the apical position and the two linked β-diketiminato moieties form the basis. The geometry of the bis(β-diketiminato) ligands depends significantly on the linker unit. While complexes with an ethylene-linked ligand adopt a cisoid arrangement of the two aromatic substituents, complexes with cyclohexyl linker adopt a transoid arrangement. Either one (3b) or both (7, 8) of the β-diketiminato moieties are tilted out of the η² coordination mode, resulting in close Ln?C contacts. The β-diketiminato and linked bis(β-diketiminato) complexes were moderately active in the copolymerization of cyclohexene oxide with CO₂. A maximum of 92% carbonate linkages were obtained using the ethylene-bridged bis(β-diketiminato) complex [{C₂H₄(BDI^Mes)₂}LaN(SiHMe₂)₂] (4).     

Tricyclic ligands on cyclam core and X-ray crystallographic structure of hexachloro-(1,11:4,8-bis(pyridine-2,6-diyl-bis(2-(N-(2-formidoylethylene)carbamoyl)ethylene))-1,8-diazonia-4,11-diazacyclotetradecane) - dimanganese(II) dimethanol dihydrate solvate

The synthesis of tricyclic compounds on functionalized cyclam core is described. The addition of four methyl acrylate molecules and consecutive condensation of this derivative with ethylenediamine resulted in formation of 1,4,8,11-tetrakis(2-(N-(2-aminoethyl)carbamoyl)ethyl)-1,4,8,11-tetraazacyclotetradecane (3). Compound 3 was the substrate for further condensation with dialdehydes: iso-phthaldialdehyde and 2,6-pyridinedicarbaldehyde, resulting in spontaneous macrocycle ring closure to give tricyclic derivatives: 1,11:4,8-bis(benzene-1,3-diyl-bis(2-(N-(2-formidoylethylene)carbamoyl)ethylene))-1,4,8,11-tetraazacyclotetradecane (4) in the reaction of 3 with iso-phthaldialdehyde and three isomers: 1,4:8,11-bis(pyridine-2,6-diyl-bis(2-(N-(2-formidoylethylene)carbamoyl)ethylene))-1,4,8,11-tetraazacyclotetradecane (5A), 1,11:4,8-bis(pyridine-2,6-diyl-bis(2-(N-(2-formidoylethylene)carbamoyl)ethylene))-1,4,8,11-tetraazacyclotetradecane (5B), and 1,8:4,11-bis(pyridine-2,6-diyl-bis(2-(N-(2-formidoylethylene)carbamoyl)ethylene))-1,4,8,11-tetraazacyclotetradecane (5C) when 2,6-pyridinedicarbaldehyde was used. The compounds 4, 5B, and 5C were identified crystallographically. The isolated 5A converted in solution into the mixture of 5B and 5C as monitored by the ¹H NMR spectroscopy. The tricycle 5 is able to accept two manganese(II) metal ions by reacting with manganese(II) dichloride with simultaneous diprotonation of 5. Structure of the resulting Mn₂(5BH₂)Cl₆·(CH₃OH)₂(H₂O)₂ was determined crystallographically.     

Coordinating properties of [M(CO)5(CN)] [M=Cr; Mo; W] ligands: formation of ion pairs or dinuclear cyanide-bridged complexes, spectroscopic and X-ray diffraction studies

The reaction of sodium cyanopentacarbonylmetalates Na[M(CO)₅(CN)] (M=Cr; Mo; W) with cationic Fe(II) complexes [Cp(CO)(L)Fe(thf)][O₃SCF₃], [L=PPh₃ (1a), CN-Benzyl (1b), CN-2,6-Me₂C₆H₃ (1c); CN-Bu^t (1d), P(OMe)₃ (1e), P(Me)₂Ph (1f)] in acetonitrile solution, yielded the metathesis products [Cp(CO)(L)Fe(NCCH₃)][NCM(CO)₅] [M=W, L=PPh₃ (2a), CN-Benzyl (2b), CN-2,6-Me₂C₆H₃ (2c); CN-Bu^t (2d), P(OMe)₃ (2e), P(Me)₂Ph (2f); M=Cr, L=(PPh₃) (3a), CN-2,6-Me₂C₆H₃ (3c); M=Mo, L=(PPh₃) (4a), CN-2,6-Me₂C₆H₃ (4c)]. The ionic nature of such complexes was suggested by conductivity measurements and their main structural features were determined by X-ray diffraction studies. Well-resolved signals relative to the [M(CO)₅(CN)] moieties could be distinguished only when ¹³C NMR experiments were performed at low temperature (from −30 to −50 °C), as in the case of [Cp(CO)(PPh₃)Fe(NCCH₃)][NCW(CO)₅] (2a) and [Cp(CO)(Benzyl-NC)Fe(NCCH₃)][NCW(CO)₅] (2b). When the same reaction was carried out in dichloromethane solution, neutral cyanide-bridged dinuclear complexes [Cp(CO)(L)FeNCM(CO)₅] [M=W, L=PPh₃ (5a), CN-Benzyl (5b); M=Cr, L=(PPh₃) (6a), CN-2,6-Me₂C₆H₃ (6c), CO (6g); M=Mo, L=CN-2,6-Me₂C₆H₃ (7c), CO (7g)] were obtained and characterized by infrared and NMR spectroscopy. In all cases, the room temperature ¹³C NMR measurements showed no broadening of cyano pentacarbonyl signals and, relative to tungsten complexes [Cp(CO)(PPh₃)FeNCW(CO)₅] (5a) and [Cp(CO)(CN-Benzyl)FeNCW(CO)₅] (5b), the presence of ¹⁸³W satellites of the ¹³CN resonances (J_CW ∼ 95 Hz) at room temperature confirmed the formation of stable neutral species. The main ¹³C NMR spectroscopic properties of the latter compounds were compared to those of the linkage isomers [Cp(CO)(PPh₃)FeCNW(CO)₅] (8a) and [Cp(CO)(CN-Benzyl)FeCNW(CO)₅] (8b). The characterization of the isomeric couples 5a-8a and 5b-8b was completed by the analyses of their main IR spectroscopic properties. The crystal structures determined for 2a, 5a, 8a and 8b allowed to investigate the geometrical and electronic differences between such complexes. Finally, the study was completed by extended Hückel calculations of the charge distribution among the relevant atoms for complexes 2a, 5a and 8a.     

Chiral memory effects in catalytic hydrogenations with dynamically chiral ligands

The low barrier for interconversion of chiral conformations of the dynamically chiral 2,2′-biphenyl ligand NMe₂C₆H₄C₆H₄PCy₂ is raised upon coordination. The individual enantiomers of the planar chiral arene-tethered complex Ru(η⁶:η¹- NMe₂C₆H₄C₆H₄PCy₂)Cl₂ (1), however, do not undergo racemization readily. A second source of chirality, such as a chiral diamine, can be included by conversion of 1 into a dicationic analogue [Ru(η⁶:η¹-NMe₂C₆H₄C₆H₄PCy₂)((1S,2S)-DPEN)](SbF₆)₂ (2), which is a catalyst precursor for the hydrogenation of aryl ketones. Two epimers of 2, R_Ar,S,S and S_Ar,S,S, are formed when starting from racemic 1; this 1:1 mixture of diastereomers catalyzed the asymmetric hydrogenation of acetophenone. The enantiomerically pure diastereomers were obtained from resolved 1 and used separately to catalyze the reaction. Each diastereomer showed different selectivity, with S_Ar,S,S-2 being the more selective (61% ee for the hydrogenation of acetophenone). Our studies suggest that ruthenium hydride formation is accompanied by a decrease in hapticity of the η⁶-arene and probable detachment of the ring from the metal. Nevertheless, the original conformational chirality of the biphenyl ligand appears to be at least partially retained during the catalysis.     

Rare-earth metal dichloride and bis(alkyl) complexes containing amidinate-amidopyridinate ligands: synthesis,structure, and reactivity

A reaction of anhydrous yttrium chloride with an equimolar amount of lithium amidinateamidopyridinate obtained in situ by metallation of N,N’-bis(2,6-dimethylphenyl)-N-{6-[(2,6-dimethylphenyl)amino]pyridin-2-yl}acetimidamide ((2,6-Me₂C₆H₃)NH(2,6-C₆H₃N)N(2,6-Me₂C₆H₃)C(Me)=N(2,6-Me₂C₆H₃), L¹H) (1) with n-butyllithium in THF at–70 °C was used to synthesize the yttrium dichloride complex (L¹)YCl₂(THF)₂ (2). The lutetium bis(alkyl) complex, namely, N’-(2,6-diisopropylphenyl)-N-(2,6-dimethylphenyl-N-{6-[(2,6-dimethylphenyl)amido]pyridin-2-yl}acetimidoamidinatebis(trimethylsilylmethyl)lutetium (4), was obtained by the reaction of N’-(2,6-diisopropylphenyl)-N-(2,6-dimethylphenyl)-N-(6-((2,6dimethylphenyl)amino)pyridin-2-yl)acetimidamide ((2,6-Me₂C₆H₃)NH(2,6-C₆H₃N)N-(2,6-Me₂C₆H₃)C(Me)=N(2,6-Pr ₂ ⁱ C₆H₃), L²H (3)) with an equimolar amount of Lu(CH₂SiMe₃)₃(THF)₂. Complex 4 was found to be very stable and did not show indications of C—H-activation and other kinds of disintegration in benzene or toluene solution even upon prolonged heating at 60 °C. The reaction of complex 4 with an equimolar amount of 2,6-diisopropylaniline in toluene solution at room temperature led to the formation of the lutetium alkyl-anilide complex (L²)Lu(CH₂SiMe₃)(NH-2,6-Pr ₂ ⁱ C₆H₃) (5). A three-component system 4—AlBu ₃ ⁱ —[X][B(C₆F₅)₄] ([X] = [Ph₃C], [PhNHMe₂], the molar ratio of 1: 10: 1) was found to catalyze polymerization of isoprene.