Metallomimetic C-F Activation Catalysis by Simple Phosphines.

Delivering metallomimetic reactivity from simple p-block compounds is highly desirable in the search to replace expensive, scarce precious metals by cheap and abundant elements in catalysis. This contribution demonstrates that metallomimetic catalysis, involving facile redox cycling between the P(III) and P(V) oxidation states, is possible using only simple, cheap, and readily available trialkylphosphines without the need to enforce unusual geometries at phosphorus or use external oxidizing/reducing agents. Hydrodefluorination and aminodefluorination of a range of fluoroarenes was realized with good to very good yields under mild conditions. Experimental and computational mechanistic studies show that the phosphines undergo oxidative addition of the fluoroaromatic substrate via a Meisenheimer-like transition state to form a fluorophosphorane. This undergoes a pseudotransmetalation step with a silane, via initial fluoride transfer from P to Si, to give experimentally observed phosphonium ions. Hydride transfer from a hydridosilicate counterion then leads to a hydridophosphorane, which undergoes reductive elimination of the product to reform the phosphine catalyst. This behavior is analogous to many classical transition-metal-catalyzed reactions and so is a rare example of both functional and mechanistically metallomimetic behavior in catalysis by a main-group element system. Crucially, the reagents used are cheap, readily available commercially, and easy to handle, making these reactions a realistic prospect in a wide range of academic and industrial settings.

Isoprene polymerization mediated by vanadium-[ONNO] complexes.

A series of vanadium complexes bearing dianionic tetradentate amine-bisphenolate [ONNO] ligands, V([double bond, length as m-dash]O)X[ONNO(Me)] (X = Cl (1); O(i)Pr (2)), VCl2[ONNO(Me)] (3) and V(O(i)Pr)2[ONNO(R)] (R = Me (4), (t)Bu (5), Cl (6)), displaying various electronic and steric properties have been prepared. The molecular structures of two of these complexes, namely V[ONNO(R)](O(i)Pr)2 with R = (t)Bu (5) or Cl (6), are reported. Activated with dialkylmagnesium, all complexes lead to modest isoprene homo-polymerization activities at 50 °C. Quantitative polymerizations were observed using 4 and 6 as pre-catalysts combined with Al(i)Bu3. The resulting polyisoprene microstructure was composed of ca. 70% 3,4 enchainments, the remaining 30% 1,4 enchainments being a mixture of cis and trans stereoisomers. 6 leads to a more active catalyst than 4. ß-Hydride abstraction occurs during the reaction.

Novel aspects of the transamination reaction between Ti(NMe2)4 and primary amines.

A detailed study of the transamination reaction between Ti(NMe2)4 and primary amines RNH2 is reported Alkylamines (1-adamantylamine, t-butylamine) and triphenylsilylamine yield dimers of the type [Ti(µ-NR)(NMe2)2]2 (R = 1-adamantyl (1), (t)Bu (2), SiPh3 (3)). The experimental conditions and the nature of the amine are important issues that determine the scope and yield of the reaction. When reacted under the same conditions, aniline PhNH2 does not yield the expected dimer, but instead the trimer [Ti3(µ(2)-NPh)3(µ(3)-NPh)(NMe2)4(NHMe2)] (5) and tetramer [Ti4(µ-NPh)5(NMe2)6] (6) complexes were characterized. The reaction of Ti(NMe2)4 and 2,6-diisopropylaniline (Ar*NH2) is essentially an equilibrium reaction of great complexity in which multiple species can coexist in solution. However, under certain circumstances, we were able to isolate and/or characterize several new complexes such as [Ti(µ-NAr*)(NMe2)2]2 (7), [(Me2N)2Ti(µ-NAr*)2Ti(NMe2)(NHAr*)] (8), [Ti(NMe2)3]2(µ-NAr*) (9), [(Ar*NH)2Ti(µ-NAr*)2Ti(NMe2)2] (10), [(Me2N)(Me2NH)Ti(µ-NAr*)2Ti(NMe2)(=NAr*)] (11), Ti(NHAr*)4 (12), and [(Ar*NH)2Ti(µ-NAr*)2Ti(NMe2)(NHAr*)] (13). Complexes 8 and 11 are tautomers. Alternatively, complex 7 was prepared by deprotonation of NHMe2 ligands in Ti(=NAr*)Cl2(NHMe2)2 with KN(SiMe3)2. Addition of pyridine (py) to the reaction of Ti(NMe2)4/Ar*NH2 mixtures or to 7 or 8 allows characterization of transient [Ti(µ-NAr*)(NMe2)2]2·py (14) and trapping of the intermediates [(Me2N)(py)2Ti(µ-NAr*)2Ti(NMe2)(=NAr*)] (15), Ti(=NAr*)(NHAr*)(NMe2)(py)2 (16), and Ti(=NAr*)(NHAr*)2(py)2 (17). The crystal structures of 15 compounds were determined by X-ray diffraction studies.

Tight encapsulation of a "naked" chloride in an imidotitanium hexanuclear host.

Treating the imidotitanium dimer [Ti(µ-NAr)(NMe2)2]2 (Ar = 2,6-(i)Pr2C6H3) with excess Me3SiCl affords the hexanuclear complex [{Ti(═NAr)Cl2}6(Cl)](-)[Q](+). The self-assembled hexameric cage arrangement encapsulates a chloride ion guest that provides evidence of new host-guest chemistry in this area, while the cationic part is composed of mixtures of the Q(+) cations Me2NHSiMe3(+) and Me2N(SiMe3)2(+).

Imido-bridged homo- and heterobimetallic complexes.

Transamination reactions of primary amines with group 4 and 5 amido precursors M(NMe(2))(4) have been studied to prepare homo- and heterobimetallic complexes [(Me(2)N)(2)M(1)(µ-NR(1))(µ-NR(2))M(2)(NMe(2))(2)(NHMe(2))(x)] (x = 0, 1) with two identical or distinct bridging imido ligands.

Synthesis and structure of early transition metal NHC complexes.

The synthesis and structure of several new early transition metal (Ti, Zr, Hf, and V) NHC complexes supported by one or two IMes ligands is described. Reaction of M(NMe2)4 with IMes.HCl gives compounds VCl2(NMe2)(IMes)2 (2), and MCl2(NMe2)2(IMes) (M = V (3), Zr (4), Hf (5)). Treatment of TiCl3(THF)3 with 2 equiv. of IMes and 1 equiv. of LiNMe2 affords the titanium(III) compound TiCl2(NMe2)(IMes)2. Reaction of VCl3(THF)3 with 2 equiv. of IMes in toluene produces the trivalent complex VCl3(IMes)2 (6), while VCl2(Py)4 gives the divalent compound VCl2(IMes)(Py)3 (7), and ZrCl4(THF)2 leads to ZrCl4(IMes)2 (8). Imido compounds M(N-2,6-iPr2-C6H3)Cl2(NHMe2)(IMes) (M = V (9), Zr (10)) are prepared from V(NR)Cl2(NHMe2)2 or ZrCl2(NMe2)2(IMes) synthons. Ionic imido complexes [M(N-2,6-iPr2-C6H3)Cl3(IMes)]-[HIMes]+ (M = V (12), Ti (13)) are obtained in modest yields from the treatment of [M(NR)Cl2(NHMe2)2] with 2 equiv. of IMes. Treatment of a toluene solution of V(NMe2)4 and an amide RC(O)NH2 (R = tBu, Ph) in the presence of an excess of Me3SiCl produces the terminal oxo complex V(O)Cl2(NHMe2)2 (13), which reacts with two equiv. of IMes to give the bis-NHC oxo-complex V(O)Cl2(IMes)2 (14). All new compounds have been characterised by various spectroscopic techniques, elemental analyses, and magnetism studies for paramagnetic compounds. The solid-state structure of 12 of these complexes and of IMes.HCl have further been confirmed by X-ray diffraction analyses.

Reaction of p-toluenesulfonylamide and M(NMe2)4 (M = Ti, V): generation of electron-deficient imido complexes of early transition metals.

p-Toluenesulfonamide (p-TsNH2) was successfully employed as an imido ligand precursor in the synthesis of highly air- and moisture-sensitive titanium(IV) and vanadium(IV) complexes. Reaction of M(NMe2)4 (M = Ti, V) with TsNH2 in toluene afforded [M(micro-NTs)(NMe2)2]2 dimer complexes (M = Ti (1), V (2)). By contrast, the reaction carried out in dichloromethane led to [Ti[micro-N,O-NTs]Cl(NMe2)(NHMe2)2]2 (3) and [Ti[micro-N,O-NTs]Cl2(NHMe2)2]n (4) through solvent activation. The same reaction of M(NMe2)4/TsNH2 conducted in the presence of an excess of trimethylchlorosilane produced [V(=NTs)Cl2(NHMe2)2] (5) and [(Me2HN)Cl2Ti(micro2-N-NTs-kappa2N,O)2TiCl2(NHMe2)2] (6). Alternatively, compound 6 has also been prepared from TiCl2(NMe2)2 and TsNH2. 1 was reacted with trimethylchlorosilane to afford the amide complex [Ti[micro-N,O'-N(SiMe3)Ts-kappa3N,O,O']Cl2(NMe2)]2 (7) in which the tosylimide bond has been silylated. Compounds 1-6 represents the first examples of sulfonylimido complexes for titanium and vanadium.

Vanadium(V) complexes of a chelating dianionic [ONNO]-type amine bis(phenolate) ligand: synthesis and solid state and solution structures.

The reaction between VO(OR)(3) (R = (i)()Pr, (t)()Bu, CH(2)CF(3)) and the chelating dianionic bis(phenoxy)amine ligand [ONNO]H(2) affords a mixture of two isomers (A and B in a ratio A:B approximately 3:1) formulated as VO(OR)[ONNO] (1a-c) (R = (i)()Pr (1a), (t)Bu (1b), CH(2)CF(3) (1c)). Multinuclear and NOESY NMR spectroscopy experiments were able to determine the structure in solution of the complexes. Both isomers have the symmetry-related phenolate groups in a trans configuration, the difference arising from the different configuration of the oxo and alkoxo ligands being located either cis (in isomer A) or trans (in isomer B) to the tripodal amino nitrogen donor atom and the (dimethylamino)ethyl sidearm respectively for the oxo and the alkoxo ligands. Crystals of isomer A (cis-1a) were obtained, and the structure determination confirms the arrangement of the ligands around the vanadium center. Analogue complexes VO(X)[ONNO] (X = Cl (2); X = N(3) (3)) were prepared by reacting equimolar amount of [ONNO]H(2) and VO(X)(n)(OR)(3-n) (X = Cl, R = Et, n = 1; X = N(3), R = (i)Pr, n = 2) at ambient temperature. Compounds 2 and 3 were further characterized by NMR spectroscopy experiments and X-ray structure determination. For both 2 and 3, a single isomer is obtained, having a trans-(O,O) configuration for the phenolate groups and a trans configuration of the oxo ligand in respect to the tripodal amino nitrogen donor atom. Finally, complex 2 could also be obtained by chlorination of 1a or 3 using a large excess of ClSiMe(3) in refluxing toluene.

Synthesis and crystal structure of unprecedented phosphine adducts of d(1)-aryl imido-vanadium(IV) complexes.

The reaction of the imido precursor [V(NAr)Cl(2)](n)() (1) (Ar = 2,6-i-Pr(2)C(6)H(3)) with 3 equiv of PMe(2)Ph yields the monomeric complex [V(=NAr)Cl(2)(PMe(2)Ph)(2)] (2). Reacting 1 with 1.5 equiv of dmpe or 1 equiv of dppm affords the dimeric complexes [V(=NAr)Cl(2)(dmpe)](2)(mu-P,P'-dmpe) (3) and [V(=NAr)Cl(2)(dppm)](2) (4), respectively. Complexes 2-4 have been fully characterized by spectroscopic methods, magnetism studies, and X-ray crystallography.

Reactivity of vanadocene with a nitrile -C identical to N bond activated by a tris(fluorophenyl)borane as Lewis acid: formation of borane adducts of vanada(IV)azirine complexes--EPR evidence for an intramolecular C-F...V interaction.

Reaction of vanadocene [V(Cp)2] with "activated" nitrile R1CN.L (L: Lewis acid), obtained by the reaction of borane adducts (L = BR3; R = C6F5, 2,6-F2C6H3, 3,4,5-F3C6H2) with nitriles (CH3CN, F3CC6H4CN), yields the borane adduct of vanada(IV)azirine complexes [V(Cp)2(eta 2-R1C = N.L)]. EPR studies of a fluid solution were conducted on these complexes. A doublet of octets due to the coupling of one unpaired electron of the vanadium with the 51V (I = 7/2) nucleus and to an additional hyperfine coupling to the ortho-F atom borne by the phenyl ring of the borane was elucidated by means of the different Lewis acids used in this work. This EPR behaviour gives evidence for the presence of a C-F...V interaction in a fluid solution with L = B(C6F5)3 and B(2,6-F2C6H3)3. In contrast, the expected eight line EPR pattern is observed with L = B(3,4,5-F3C6H2)3, in which no ortho-F atoms are present in the phenyl ring. A model can be drawn to take into account this flexibility and V...F distances between V and ortho-F atoms are in the expected range for such an interaction.

Synthesis and structure of a series of new d(1)-Aryl imido-vanadium(IV) complexes stabilized by n-donor ligands.

A family of new coordination vanadium(IV) compounds supported by a terminal or bridged aryl imido ligand are reported. Reaction of V(NMe(2))(4) with anilines ArNH(2), where Ar = 2,6-i-Pr(2)-C(6)H(3), 2,6-Me(2)-C(6)H(3), Ph, 2,6-Cl(2)-C(6)H(3), and C(6)F(5), afforded the diamagnetic imido-bridged complexes [V(NAr)(NMe(2))(2)](2) (1a-e). Chlorination of 1a-e with trimethylchlorosilane afforded complexes 2a-e formulated as [V(=NAr)Cl(2)(NHMe(2))(x)()](n)(). One-pot reaction of V(NMe(2))(4) with ArNH(2) in the presence of an excess of trimethylchlorosilane gave the five-coordinate compound [V(=NAr)Cl(2)(NHMe(2))(2)] (3a-e). Reaction of 3a-e with pyridine, bipyridine (bipy), or N,N,N',N'-tetramethylethylenediamine (tmeda) gave respectively the six-coordinate tris- or bis(pyridine) adducts [V(=NAr)Cl(2)(Py)(3)] (4a-e) or [V(=NAr)Cl(2)(Py)(2)(NHMe(2))] (5a), bipyridine complexes [V(=NAr)Cl(2)(bipy)(NHMe(2))] (5a-e) and [V(=NAr)Cl(2)(bipy)(Py)] (9a), and tmeda adduct [V(=NAr)Cl(2)(tmeda)(NHMe(2))] (10a). Moreover, five-coordinate complexes free of NHMe(2) ligands, such as [V(=NAr)Cl(2)(Py)(2)] (5a), [V(=NAr)Cl(2)(bipy)] (8a), and [V(=NAr)Cl(2)(tmeda)] (11a), were directly prepared starting from precursors 2a-e. All compounds were totally characterized by spectroscopic methods (IR, (1)H NMR for diamagnetic complexes, and EPR for paramagnetic complexes), elemental analysis, magnetism, and single-crystal X-ray diffraction studies for 1b, 3a, 3d, 4b, 4d, 7c, 10a, and 11a.