RESUMEN

Three molybdenum(VI) dioxido complexes [MoO2(L)2] bearing Schiff base ligands were reacted with B(C6F5)3 to afford the corresponding adducts [MoO{OB(C6F5)3}(L)2], which were fully characterized. They exhibit Frustrated Lewis-Pairs reactivity when reacting with silanes. Especially, the [MoO{OB(C6F5)3}(L)2] complex with L=2,4-dimethyl-6-((phenylimino)methyl)phenol proved to be active as catalyst for the hydroalkylation of aryl alkenes with organohalides and for the Atom-Transfer Radical Addition (ATRA) of organohalides to aliphatic alkenes. A series of gem-dichloride and gem-dibromide compounds with potential for further derivatization were synthesized from simple alkenes and organohalides, like chloroform or bromoform, using low catalyst loading.

RESUMEN

Two novel iminophenolate ligands with amidopropyl side chains (HL2 and HL3) on the imine functionality have been synthesized in order to prepare dioxidomolybdenum(VI) complexes of the general structure [MoO2L2] featuring pendant internal hydrogen bond donors. For reasons of comparison, a previously published complex featuring n-butyl side chains (L1) was included in the investigation. Three complexes (1-3) obtained using these ligands (HL1-HL3) were able to activate dioxygen in an in situ approach: The intermediate molybdenum(IV) species [MoO(PMe3)L2] is first generated by treatment with an excess of PMe3. Subsequent reaction with dioxygen leads to oxido peroxido complexes of the structure [MoO(O2)L2]. For the complex employing the ligand with the n-butyl side chain, the isolation of the oxidomolybdenum(IV) phosphino complex [MoO(PMe3)(L1)2] (4) was successful, whereas the respective Mo(IV) species employing the ligands with the amidopropyl side chains were found to be not stable enough to be isolated. The three oxido peroxido complexes of the structure [MoO(O2)L2] (9-11) were systematically compared to assess the influence of internal hydrogen bonds on the geometry as well as the catalytic activity in aerobic oxidation. All complexes were characterized by spectroscopic means. Furthermore, molecular structures were determined by single-crystal X-ray diffraction analyses of HL3, 1-3, 9-11 together with three polynuclear products {[MoO(L2)2]2(µ-O)} (7), {[MoO(L2)]4(µ-O)6} (8) and [C9H13N2O]4[Mo8O26]·6OPMe3 (12) which were obtained during the synthesis of reduced complexes of the type [MoO(PMe3)L2] (4-6).

Asunto(s)

RESUMEN

Lewis acid-base pair chemistry has been placed on a new level with the discovery that adduct formation between an electron donor (Lewis base) and acceptor (Lewis acid) can be inhibited by the introduction of steric demand, thus preserving the reactivity of both Lewis centers, resulting in highly unusual chemistry. Some of these highly versatile frustrated Lewis pairs (FLP) are capable of splitting a variety of small molecules, such as dihydrogen, in a heterolytic and even catalytic manner. This is in sharp contrast to classical reactions where the inert substrate must be activated by a metal-based catalyst. Very recently, research has emerged combining the two concepts, namely the formation of FLPs in which a metal compound represents the Lewis base, allowing for novel chemistry by using the heterolytic splitting power of both together with the redox reactivity of the metal. Such reactivity is not restricted to the metal center itself being a Lewis acid or base, also ancillary ligands can be used as part of the Lewis pair, still with the benefit of the redox-active metal center nearby. This Minireview is designed to highlight the novel reactions arising from the combination of metal oxido transition-metal or rare-earth-metal compounds with the Lewis acid B(C6 F5 )3 . It covers a wide area of chemistry including small molecule activation, hydrogenation and hydrosilylation catalysis, and olefin metathesis, substantiating the broad influence of the novel concept. Future goals of this young and exciting area are briefly discussed.

RESUMEN

The reaction of a molybdenum(VI) oxido imido complex with the strong Lewis acid B(C6 F5 )3 gave access to the Lewis adduct [Mo{OB(C6 F5 )3 }(NtBu)L2 ] featuring reversible B-O bonding in solution. The resulting frustrated Lewis pair (FLP)-like reactivity is reflected by the compound's ability to heterolytically cleave Si-H bonds, leading to a clean formation of the novel cationic MoVI species 3 a (R=Et) and 3 b (R=Ph) of the general formula [Mo(OSiR3 )(NtBu)L2 ][HB(C6 F5 )3 ]. These compounds possess properties highly unusual for molybdenum d0 species such as an intensive, charge-transfer-based color as well as a reversible redox couple at very low potentials, both dependent on the silane used. Single-crystal X-ray diffraction analyses of 2 and 4 b, a derivative of 3 b featuring the [FB(C6 F5 )3 ]- anion, picture the stepwise elongation of the Mo=O bond, leading to a large increase in the electrophilicity of the metal center. The reaction of 3 a and 3 b with benzaldehyde allowed for the regeneration of compound 2 by hydrosilylation of the benzaldehyde. NMR spectroscopy suggested an unusual mechanism for the transformation, involving a substrate insertion in the B-H bond of the borohydride anion.

RESUMEN

Activation of molecular dioxygen at a molybdenum(IV) imido compound led to the isolation and full characterization of a remarkably stable transition-metal imidoperoxido complex.

RESUMEN

Synthesis of molybdenum(vi) dioxido complexes 1-3, coordinated by one or two functionalized iminophenolate ligands HL1 or HL2, bearing a donor atom side chain or a phenyl substituent, respectively, allowed for systematic investigation of the oxygen atom transfer (OAT) reactivity of such complexes towards phosphanes. Depending on stoichiometry and employed phosphane (PMe3 or PPh3), different molybdenum(iv) and molybdenum(v) complexes 4-7 were obtained. Whereas molybdenum(iv) complexes 4 and 5, bearing a terminal PMe3 ligand, readily reacted with molecular O2 to form oxido peroxido complexes 8 and 9, phosphane free µ-oxido bridged dinuclear molybdenum(v) complexes 6 and 7 proved to be stable towards oxidation with molecular O2 under ambient conditions. Single-crystal X-ray diffraction analyses revealed different isomeric structures in the solid state for dioxido complexes 1 and 2 in comparison with oxido phosphane complex 5, dinuclear oxido µ-oxido complex 6 and oxido peroxido complexes 8 and 9, pointing towards an isomeric rearrangement during OAT. Compounds 1 and 2 were furthermore tested for their ability to catalyze the aerobic oxidation of PMe3 and PPh3. A significant difference in catalytic activity has been observed in the oxidation of PMe3, where complex 1 bearing donor atom functionalized ligands led to higher conversion and selectivity than complex 2 coordinated by phenyl iminophenolate ligands. In the oxidation of PPh3, complex 2 leads to higher conversion compared to 1. In a control experiment, phenyl-based dinuclear µ-oxido complex 7, derived from complex 2, was found to be catalytically active, which suggests a lower energy barrier for disproportionation into [MoO(L)2] and [MoO2(L)2] in comparison with methoxypropylene based compound 6, a prerequisite for subsequent reactivity toward molecular O2.

RESUMEN

The synthesis of oxidorhenium(V) complexes 1-3 coordinated by tetradentate iminophenolate ligands H2L1-H2L3 bearing backbones of different rigidity (alkyl, cycloalkyl, and phenyl bridges) allows for the formation of distinct geometric isomers, including a symmetric trans-oxidochlorido coordination motif in complex 3. The complex employing a cycloalkyl-bridged ligand (2) of intermediate rigidity exhibits an interesting solvent- and temperature-dependent equilibrium between a symmetric (trans) isomer and an asymmetric (cis) isomer in solution. The occurrence of a symmetric isomer for 2 and 3 is confirmed by single-crystal X-ray diffraction analysis. Chlorido abstraction from 2 with AgOTf yields the corresponding cationic complex 2a, which does not exhibit an isomeric equilibrium in solution but adopts the isomeric form predominant for 2 in a given solvent. All complexes were, furthermore, employed in three benchmark oxygen-atom-transfer (OAT) reactions, namely, the reduction of perchlorate, the epoxidation of cyclooctene, and OAT from dimethyl sulfoxide (DMSO) to triphenylphosphane (PPh3), to assess the influence of the isomeric structure on the reactivity in these reactions. In perchlorate reduction, a clear structural influence was observed, where the trans arrangement in 3 led to the complete absence of activity. In the epoxidation reaction, all complexes led to comparable epoxide yields, albeit higher catalytic activity but lower overall stability of the catalysts with a trans arrangement was observed. In OAT from DMSO to PPh3, also a clear structural dependence was observed, where the trans complex 3 led to full phosphane conversion with an excess of oxidant, while the cis compound 1 was completely inactive.

RESUMEN

Preparation of molybdenum dioxido complexes with novel iminophenolate ligands bearing pendant secondary amide functionalities led to unprecedented C-C and C-N coupling reactions of two α-iminoamides upon coordination. The diastereoselective cyclization to asymmetric imidazolidines occurs at the metal center in two consecutive steps via a monocoupled intermediate. A meaningful mechanism is proposed on the basis of full characterization of intermediate and final molybdenum-containing products by spectroscopic means and by single-crystal X-ray diffraction analyses. This process constitutes the first example of a diastereoselective self-cyclization of two α-iminoamides.

RESUMEN

The bidentate phenolate-oxazoline ligands 2-(2'-hydroxyphenyl)-2-oxazoline (1a, Hoz) and 2-(4',4'-dimethyl-3',4'-dihydrooxazol-2'-yl)phenol (1b, Hdmoz) were used to synthesize two sets of oxorhenium(V) complexes, namely, [ReOCl2(L)(PPh3)] [L = oz (2a) and dmoz (2b)] and [ReOX(L)2] [X = Cl, L = oz (3a or 3a'); X = Cl, L = dmoz (3b); X = OMe, L = dmoz (4)]. Complex 3a' is a coordination isomer (N,N-cis isomer) with respect to the orientation of the phenolate-oxazoline ligands of the previously published complex 3a (N,N-trans isomer). The reaction of 3a' with silver triflate in acetonitrile led to the cationic compound [ReO(oz)2(NCCH3)](OTf) ([3a'](OTf)). Compound 4 is a rarely observed isomer with a trans-O═Re-OMe unit. Complexes 3a, 3a', [3a'](OTf), and 4 were tested as catalysts in the reduction of a perchlorate salt with an organic sulfide as the O acceptor and found to be active, in contrast to 2a and 2b. A comparison of the two isomeric complexes 3a and 3a' showed significant differences in activity: 87% 3a vs 16% 3a' sulfoxide yield. When complex [3a'](OTf) was used, the yield was 57%. Density functional theory calculations circumstantiate all of the proposed intermediates with N,N-trans configurations to be lower in energy compared to the respective compounds with N,N-cis configurations. Also, no interconversions between N,N-trans and N,N-cis configurations are predicted, which is in accordance with experimental data. This is interesting because it contradicts previous mechanistic views. Kinetic analyses determined by UV-vis spectroscopy on the rate-determining oxidation steps of 3a, 3a', and [3a'](OTf) proved the N,N-cis complexes 3a' and [3a'](OTf) to be slower by a factor of ∼4.

Asunto(s)

RESUMEN

Oxorhenium(V) complexes of the general formula [ReOCl2(PPh3)(L)] (2a-c) and [ReOCl(L)2] (3a-c) with L being monoanionic, bidentate phenolate-pyrazole ligands 1a-c that bear substituents with various electronic features on the phenol ring (1a Br, 1b NO2, 1c OMe) were prepared. The compounds are stable toward moisture and air, allowing them to be handled in a normal lab atmosphere. All complexes were fully characterized by spectroscopic means and, in the case of 2b, 2c, 3b, and 3c, also by single-crystal X-ray diffraction analyses. Electrochemical investigations by cyclic voltammetry of complexes 3a-c showed a shift to more positive potentials for the Re(V)/Re(VI) redox couple in the order of 3b > 3a > 3c (R = NO2 > Br > OMe), reflecting the higher electrophilic character of the Re atom caused by the ligands 1a-c. Complexes 2a-c and 3a-c display excellent catalytic activity in the epoxidation of cyclooctene, where all six complexes give quantitative conversions to the epoxide within 3 h if tert-butylhydroperoxide (TBHP) is employed as oxidant. Moreover, they represent rare examples of oxorhenium(V) catalysts capable of using the green oxidant hydrogen peroxide, leading to high yields up to 74%. Also, green solvents such as diethylcarbonate can be used successfully in epoxidation reactions, albeit resulting in lower yields (up to 30%).