Benzyl Ligand Research Articles

Tetravalent Cerium Alkyl and Benzyl Complexes.

High-valent cerium complexes of alkyl and benzyl ligands are unprecedented due to the incompatibility of the typically highly oxidizing Ce4+ ion and the reducing alkyl or benzyl ligand. Herein we report the synthesis and isolation of the first tetravalent cerium alkyl and benzyl complexes supported by the tri-tert-butyl imidophosphorane ligand, [NP(tBu)3]1-. The Ce4+ monoiodide complex, [Ce4+I(NP(tert-butyl)3)3] (1-CeI), serves as a precursor to the alkyl and benzyl complexes, [Ce4+(Npt)(NP(tert-butyl)3)3] (2-CeNpt) (Npt = neopentyl, CH2C(CH3)3) and [Ce4+(Bn)(NP(tert-butyl)3)3] (2-CeBn) (Bn = benzyl, CH2Ph). The bonding and structure of these complexes are characterized by single-crystal XRD, NMR and UV-vis-NIR spectroscopy, cyclic voltammetry, and DFT studies.

Alkali Metal Complexes of Phosphine–Borane-Substituted Benzyl Ligands and Their Application in the Synthesis of B–H···Sn Stabilized Dialkylstannylenes

Alkali Metal Complexes of Phosphine–Borane-Substituted Benzyl Ligands and Their Application in the Synthesis of B–H···Sn Stabilized Dialkylstannylenes

Niobium Complexes Supported by Chalcogen-Bridged [OEO]-Type Bis(phenolate) Ligands (E = S, Se): Synthesis, Characterization, and Phenylacetylene Polymerization.

Trichloro niobium(V) complexes 3 and 4 with the sulfur- or selenium-bridged [OEO]-type bis(phenolate) ligands (E = S, Se) were synthesized and fully characterized on the basis of their NMR spectroscopic data and X-ray crystallographic analysis. In the crystalline state of 4, the [OSeO]-core of the ligand was coordinated to the niobium center in a fac-fashion. The corresponding tribenzyl niobium(V) complexes 5 and 6 were also prepared by the reactions of 3 and 4 with 3 equivalents of PhCH2MgCl in toluene. The X-ray diffraction analysis of 6 revealed that the distorted six-coordinated niobium center incorporated in the [OSeO]-type ligand took a mer-fashion, and one benzyl ligand was coordinated to the niobium center by η2-fashion. Complexes 5 and 6 were tested for the phenylacetylene polymerization that produced poly(phenylacetylene)s (PPAs), oligomers, and triphenylbenzenes (TPBs) depending on the chalcogen atom in the [OEO]-type ligand.

Synthesis of Pd-NNP Phosphoryl Mononuclear and Phosphinous Acid-Phosphoryl-Bridged Dinuclear Complexes and Ambient Light-Promoted Oxygenation of Benzyl Ligands

Synthesis of Pd-NNP Phosphoryl Mononuclear and Phosphinous Acid-Phosphoryl-Bridged Dinuclear Complexes and Ambient Light-Promoted Oxygenation of Benzyl Ligands

Cationic barium benzyl and hydride complexes that contain an eighteen-membered N,N,N,N,N,N-type macrocycle

Using the eighteen-membered polyamine macrocycle Me6HACO (Me6HACO = 1,4,7,10,13,16-hexamethyl-1,4,7,10,13,16-hexaazacyclooctadecane, Me6[18]aneN6), the synthesis of a hydride cation [(Me6HACO)BaH)]+ was attempted by hydrogenolysis of the benzyl cation. Protonolysis of [Ba(CH2Ph)2] with the conjugate acid of the macrocyclic ligand [(Me6HACO)H][B(C6H3-3,5-Me2)4] afforded the benzyl cation [(Me6HACO)Ba(CH2Ph)][B(C6H3-3,5-Me2)4], which was recrystallized from THF/n-pentane at −40 °C. Single crystal X-ray diffraction analysis showed a seven-coordinate metal center with an η1-coordinated benzyl ligand, subtending a long Ba-C distance of 2.85 Å. The macrocyclic ligand is folded with two opposite methyl groups pointing away from the metal center, while the other four methyl groups are oriented towards the metal center. Hydrogenolysis of the benzyl cation indicated the formation of toluene and a hydride species with a 1H NMR signal at δ = 8.82 ppm but the product could not be isolated.

Synthesis of Triarylmethanes via Palladium-Catalyzed Suzuki-Miyaura Reactions of Diarylmethyl Esters.

The synthesis of triarylmethanes via Pd-catalyzed Suzuki-Miyaura reactions between diarylmethyl 2,3,4,5,6-pentafluorobenzoates and aryl boronic acids is described. The system operates at mild conditions and has a broad substrate scope, including the coupling of diphenylmethanol derivatives that do not contain extended aromatic substituents. This is significant as these substrates, which result in the types of triarylmethane products that are prevalent in pharmaceuticals, have not previously been compatible with systems for diarylmethyl ester coupling. Further, the reaction can be performed stereospecifically to generate stereo-inverted products. On the basis of DFT calculations, it is proposed that the oxidative addition of the diarylmethyl 2,3,4,5,6-pentafluorobenzoate substrate occurs via an SN2 pathway, which results in the inverted products. Mechanistic studies indicate that oxidative addition of the diarylmethyl 2,3,4,5,6-pentafluorobenzoate substrates to (IPr)Pd(0) results in the selective cleavage of the O-C(benzyl) bond in part because of a stabilizing η3-interaction between the benzyl ligand and Pd. This is in contrast to previously described Pd-catalyzed Suzuki-Miyaura reactions involving phenyl esters, which involve selective cleavage of the C(acyl)-O bond, because there is no stabilizing η3-interaction. It is anticipated that this fundamental knowledge will aid the development of new catalytic systems, which use esters as electrophiles in cross-coupling reactions.

Ligand-Mediated C-Br Oxidative Addition to Cycloplatinated(II) Complexes and Benzyl-Me C-C Bond Reductive Elimination from a Cycloplatinated(IV) Complex.

Reaction of the Pt(II) complexes [PtMe2(pbt)], 1a, (pbt = 2-(2-pyridyl)benzothiazole) and [PtMe(C^N)(PPh2Me)] [C^N = deprotonated 2-phenylpyridine (ppy), 1b, or deprotonated benzo[h]quinoline (bhq), 1c] with benzyl bromide, PhCH2Br, is studied. The reaction of 1a with PhCH2Br gave the Pt(IV) product complex [PtBr(CH2Ph)Me2(pbt)]. The major trans isomer is formed in a trans oxidative addition (2a), while the minor cis products (2a′ and 2a″) resulted from an isomerization process. A solution of Pt(II) complex 1a in the presence of benzyl bromide in toluene at 70 °C after 7 days gradually gave the dibromo Pt(IV) complex [Pt(Br)2Me2(pbt)], 4a, as determined by NMR spectroscopy and single-crystal XRD. The reaction of complexes 1b and 1c with PhCH2Br gave the Pt(IV) complexes [PtMeBr(CH2Ph)(C^N)(PPh2Me)] (C^N = ppy; 2b; C^N = bhq, 2c), in which the phosphine and benzyl ligands are trans. Multinuclear NMR spectroscopy ruled out other isomers. Attempts to grow crystals of the cycloplatinated(IV) complex 2b yielded a previously reported Pt(II) complex [PtBr(ppy)(PPh2Me)], 3b, presumably from reductive elimination of ethylbenzene. UV–vis spectroscopy was used to study the kinetics of reaction of Pt(II) complexes 1a–1c with benzyl bromide. The data are consistent with a second-order SN2 mechanism and the first order in both the Pt complex and PhCH2Br. The rate of reaction decreases along the series 1a ≫ 1c > 1b. Density functional theory calculations were carried out to support experimental findings and understand the formation of isomers.

Secondary phosphine oxide-triggered selective oxygenation of a benzyl ligand on palladium.

The oxygenation of a benzyl ligand in [PdBnCl(cod)] was dramatically accelerated by using secondary phosphine oxides (SPOs), selectively affording either BnOOH or BnOH, depending on the concentration of O2. The SPOs coordinate to palladium in the form of phosphinous acids, operating as Brønsted acids to facilitate further reaction with O2.

Synthesis and characterization of a sterically encumbered homoleptic tetraalkyliron(III) ferrate complex

Homoleptic iron-alkyl complexes have been implicated as key intermediates in iron-catalyzed cross-coupling with simple iron salts. Tetraalkyliron(III) ferrate species have been shown to be accessible with either methyl or benzyl ligands, where the former complex is S = 3/2 and distorted square planar while the latter is a S = 5/2 distorted tetrahedral species. In the current study, a new tetraalkyliron(III) complex is synthesized containing modified methylene substituents that incorporate large trimethylsilyl (TMS) groups to further probe steric and electronic ligand effects in tetraalkyliron(III) complexes by increasing the electron-donating ability of the ligand while retaining steric bulk. Detailed structural and DFT studies provide insight into electronic structure and bonding of the four-coordinate trimethylsilylmethyl iron(III) complex compared to the previously reported analogs containing methyl and benzyl ligands.

Organometallic Zirconium Compounds in an Oxygen-Rich Coordination Environment: Synthesis and Structural Characterization of Tris(oxoimidazolyl)hydroboratozirconium Compounds.

A series of tris(oxoimidazolyl)hydroborato ligands, which serve as L2X [O3] donors, have been employed to obtain organometallic zirconium compounds in an uncommon oxygen-rich coordination environment. For example, Cp[ToMeBenz]ZrCl2 has been synthesized via the reaction of [ToMeBenz]Na with CpZrCl3 and bears a structural resemblance to the bent metallocene dichloride derivative Cp2ZrCl2. In addition, the half-sandwich counterparts [ToMeBenz]ZrCl3 and [ToBut]ZrCl3 have been obtained by metathesis of ZrCl4 with [ToMeBenz]Na and [ToBut]Na, respectively. The structurally related zirconium benzyl compounds [ToRBenz]Zr(CH2Ph)3 (R = Me, But, 1-Ad) have also been synthesized via the reactions of [ToRBenz]Tl with Zr(CH2Ph)4, and X-ray diffraction studies demonstrate that the benzyl ligands in these compounds are conformationally flexible and exhibit a large range of Zr-CH2-Ph bond angles (94.7-131.7°). Protolytic cleavage of one of the benzyl ligands of [ToRBenz]Zr(CH2Ph)3 (R = But, 1-Ad) may be achieved by treatment with [PhNHMe2][B(C6F5)4] to generate {[ToRBenz]Zr(CH2Ph)2}[B(C6F5)4], which are catalysts for the polymerization of ethylene. The molecular structure of the ether adduct, {[ToButBenz]Zr(CH2Ph)2(OEt2)}[B(C6F5)4], has been determined by X-ray diffraction. In addition to the use of tris(oxoimidazolyl)hydroborato ligands, bis(oxoimidazolyl)hydroborato ligands have also been used to obtain zirconium benzyl compounds in oxygen-rich environments, namely, [BoMeBenz]2Zr(CH2Ph)2 and [BoAdBenz]2Zr(CH2Ph)2.

Crystal structure of tri-benzyl-bis-(tetra-hydro-furan-κO)lutetium(III).

In the title compound, [Lu(C7H7)3(C4H8O)2] (1), the Lu ion is coordinated by three benzyl and two tetra-hydro-furan ligands. Two of the benzyl groups are bonded in a classical η1-fashion through the methyl-ene via the ipso-carbon atom of the benzyl ligand in addition to bonding through the methyl-ene C atom, resulting in a modified trigonal-bipyramidal coordination geometry about the Lu center.

An Icosidodecahedral Supramolecule Based on Pentaphosphaferrocene: From a Disordered Average Structure to Individual Isomers.

Pentaphosphaferrocenes [CpRFe(η5‐P5)] (1) and CuI halides are excellent building blocks for the formation of discrete supramolecules. Herein, we demonstrate the potential of Cu(CF3SO3) for the construction of the novel 2D polymer [{Cp*Fe(μ4,η5:1:1:1‐P5)}{Cu(CF3SO3)}]n (2) and the unprecedented nanosphere (CH2Cl2)1.4@[{CpBnFe(η5‐P5)}12{Cu(CF3SO3)}19.6] (3). The supramolecule 3 has a unique scaffold beyond the fullerene topology, with 20 copper atoms statistically distributed over the 30 vertices of an icosidodecahedron. Combinatorics was used to interpret the average disordered structure of the supramolecules. In this case, only two pairs of enantiomers with D5 and D2 symmetry are possible for bidentate bridging coordination of the triflate ligands. DFT calculations showed that differences in the energies of the isomers are negligible. The benzyl ligands enhance the solubility of 3, enabling NMR‐spectroscopic and mass‐spectrometric investigations.

Structural and Chemical Effects of the PtBu2 Bridge at Unsaturated Dimolybdenum Complexes Having Hydride and Hydrocarbyl Ligands.

A high-yield synthetic route for the preparation of the unsaturated anion [Mo2Cp2(μ-PtBu2)(μ-CO)2]- (2) was implemented, via two-electron reduction of the chloride complex [Mo2Cp2(μ-Cl)(μ-PtBu2)(CO)2] (1). Reaction of 2 with [NH4][PF6] led to the formation of the 30-electron complex [Mo2Cp2(H)(μ-PtBu2)(CO)2] (3), in which the hydride ligand adopts an uncommon terminal disposition. DFT analysis of the electronic structure of 3 gave support to the presence of a M≡M triple bond in this complex following from a σ2δ2δ2 configuration, a view also supported by the high electron density accumulated at the corresponding Mo-Mo bond critical point. In contrast, reactions of 2 with IMe or ClCH2Ph gave the alkyl-bridged complexes [Mo2Cp2(μ-κ1:η2-CH2R)(μ-PtBu2)(CO)2] (R = H (4a), Ph (4b)), which in solution display agostic Mo-H-C interactions. Decarbonylation of 4a took place rapidly under photochemical conditions to give the 30-electron complex [Mo2Cp2(μ-κ1:η2-CH3)(μ-PtBu2)(μ-CO)] (7), with a stronger agostic coordination of its methyl ligand. In contrast, irradiation of 4b led to the formation of the benzylidyne derivative [Mo2Cp2(μ-CPh)(μ-PtBu2)(μ-CO)] (9), following from fast decarbonylation and dehydrogenation of the bridging benzyl ligand. Low-temperature photochemistry allowed for the NMR characterization of an intermediate preceding the hydrogen elimination, identified as the carbene hydride [Mo2Cp2(H)(μ-CHPh)(μ-PtBu2)(CO)] (10), a product which evolves slowly by H2 elimination to the benzylidyne derivative. Analogous dehydrogenation of the methyl ligand in 7 could be accomplished upon moderate heating, to yield the corresponding methylidyne derivative [Mo2Cp2(μ-CH)(μ-PtBu2)(μ-CO)] (9). A complete reaction mechanism accounting for these photochemical reactions was elaborated, based on the reaction intermediates identified experimentally and on extensive DFT calculations. Surprisingly, for both systems the C-H bond activation steps are relatively easy thermal processes occurring with modest activation energies after photochemical ejection of CO, with a rate-determining step involving the formation of agostic carbenes requiring also a strong structural reorganization of the central Mo2PC rings of these molecules.

Homoleptic organolanthanide compounds supported by the bis(dimethylsilyl)benzyl ligand.

A β-SiH functionalized benzyl anion [C(SiHMe2)2Ph]- is obtained by deprotonation of HC(SiHMe2)2Ph with KCH2Ph or by reaction of KOtBu and (Me2HSi)3CPh; LnI3(THF)n and three equivalents of this carbanion combine to provide homoleptic tris(alkyl)lanthanide compounds Ln{C(SiHMe2)2Ph}3 (Ln = La, Ce, Pr, Nd) containing secondary metal-ligand interactions.

Organotin(IV) complexes derived from proteinogenic amino acid: synthesis, structure and evaluation of larvicidal efficacy on Anopheles stephensi mosquito larvae

Five new organotin(IV) complexes of composition [Bz2SnL1]n (1), [Bz3SnL1H⋅H2O] (2), [Me2SnL2⋅H2O] (3), [Me2SnL3] (4) and [Bz3SnL3H]n (5) (where L1 = (2S)‐2‐{[(E)‐(4‐hydroxypentan‐2‐ylidene)]amino}‐4‐methylpentanoate, L2 = (rac)‐2‐{[(E)‐1‐(2‐hydroxyphenyl)methylidene]amino}‐4‐methylpentanoate and L3 = (2S)‐ or (rac)‐2‐{[(E)‐1‐(2‐hydroxyphenyl)ethylidene]amino}‐4‐methylpentanoate) were synthesized and characterized using 1H NMR, 13C NMR, 119Sn NMR and infrared spectroscopic techniques. The crystal structure of 2 reveals a distorted trigonal‐bipyramidal geometry around the tin atom where the oxygen atoms of the carboxylate ligand and a water ligand occupy the axial positions, while the three benzyl ligands are located at the equatorial positions. On the other hand, the analogous derivative of enantiopure L3H (5) consists of polymeric chains, in which the ligand‐bridged tin atoms adopt the same trans‐Bz3SnO2 trigonal‐bipyramidal configuration and are now coordinated to a phenolic oxygen atom instead of H2O. In 2, the OH hydrogen of the ketoimine substituent has moved to the nearby nitrogen atom while in the salicylidene derivative 5, the OH is located almost midway between the phenolic oxygen atom and the nitrogen atom of the CN group. For the dibenzyltin derivative 1, a polymeric chain structure is observed as a result of a long intermolecular Sn⋅⋅⋅O bond involving the exocyclic carbonyl oxygen atom from the tridentate ligand of a neighbouring tin‐complex unit. The tin atom in this complex has distorted octahedral coordination geometry. In contrast, the racemic dimethyltin(IV) complexes 3 and 4 display discrete monomeric structures with a distorted octahedral‐ and trigonal‐bipyramidal geometry, respectively. The structures show that the coordination mode of the Schiff base ligand depends primarily on the number of bulky benzyl ligands (R) at the tin atom, as indeed found in the structures of related complexes where R = phenyl. With three bulky R groups, the tridentate chelating O,N,O coordination mode is preferred, whereas with fewer or less bulky R ligands, only the carboxylate and hydroxy groups are involved, which leads to polymers. Larvicidal efficacies of two of the new tribenzyltin(IV) complexes (2 and 5) were assessed on the second larval instar of Anopheles stephensi mosquito larvae and compared with two triphenyltin(IV) analogues, [Ph3SnL1H]n and [Ph3SnL3H]n. The results demonstrate that the compounds containing Sn–Ph ligands are more effective than those with Sn–Bz ligands. Copyright © 2016 John Wiley & Sons, Ltd.

Oxygenation of a benzyl ligand in SNS-palladium complexes with O2: acceleration by anions or Brønsted acids.

A cationic SNS-benzylpalladium complex was reacted with O2 under several conditions, affording oxygenated compounds derived from the benzyl ligand. The addition of n-Bu4NX remarkably accelerated the oxygenation, furnishing benzaldehyde as a main organic product and dinuclear complexes; in contrast, HX also promoted the oxygenation, but mainly produced benzyl hydroperoxide and mononuclear complexes.

Unusual Benzyl Migration Reactivity in NHC-Bearing Group 4 Metal Chelates: Synthesis, Characterization, and Mechanistic Investigations

The reaction of 1 equiv of [M(CH2Ph)4] (M = Zr, Hf) and 1,3-bis(3,5-di-tert-butyl-2-hydroxyphenyl)imidazolinium chloride [tBu(OCO)H3, 1] cleanly yielded the corresponding M-NHC chloro benzyl derivatives [[tBu(OCO)]M(Cl)(CH2Ph)] (2Zr and 2Hf) along with 3 equiv of toluene. For both metal complexes, the effective formation of a (κ3-OCO) metal chelate and the coordination of a benzyl ligand onto the M(IV) metal center were established by NMR and elemental analysis. In contrast, under identical conditions, the reaction of Ti(CH2Ph)4 with the imidazolinium proligand 1 yielded the unexpected rearranged dimer product 3Ti, arising from the migration of the Ti-Bn group from the metal center to the Ccarbene atom. The molecular structure of 3Ti was established by analogy with the X-ray-determined Zr analogue 3Zr. Compound 3Zr quantitatively formed upon heating a benzene solution of 2Zr at 60 °C. In the solid state, compound 3Zr consists of two seven-coordinate mononuclear Zr fragments that are associated by two brid...

Reversible 1,2-Alkyl Migration to Carbene and Ammonia Activation in an N-Heterocyclic Carbene–Zirconium Complex

Addition of trimethylphosphine to a bis(phenolate)benzylimidazolylidene(dibenzyl)zirconium complex induces migration of a benzyl ligand from the metal center to the C(carbine) atom. This process may be reversed, resulting in Csp(3)-Csp(3) activation, by abstraction of the phosphine, an example of regulated, reversible alkyl migration. Addition of ammonia to the dibenzyl complex results in migration of one benzyl group and protonolysis of the other to generate a bis(NH2)-bridged dimer via an NMR-observable intermediate NH3 adduct.

Hydrogenolysis of Tantalum Hydrocarbyl Complexes: Intermediates on the Road to a Dinuclear Tantalum Tetrahydride Derivative

The synthesis, characterization, and reactivity with H2 of a series of tantalum hydrocarbyl complexes are reported. The reaction of [NPN*]TaMe3 (4, where NPN* = PhP(2-(N-mesityl)-5-Me-C6H3)2) with dihydrogen (H2, 4 atm) results in the formation of the dinuclear tetrahydride ([NPN*]Ta)2(μ-H)4 (5), without the observation of intermediates. The preparations of two alkyne benzyl complexes of the formula [NPN*]Ta(alkyne)(CH2Ph) (where alkyne = BTA = bis(trimethylsilyl)acetylene (6), 3-hexyne (7)) are reported starting from the respective chloroalkyne complexes [NPN*]Ta(alkyne)Cl, by addition of benzylpotassium. Hydrogenation of these two alkyne benzyl complexes ultimately results in the formation of the same dinuclear tetrahydride 5; however, using lower pressures of H2 and shorter reaction times results in the isolation of an intermediate in each case. Hydrogenation of 6 generates the alkene hydride complex [NPN*]Ta(trans-1,2-C2H2(SiMe3)2)H (8); addition of H2 to 7 gives [NPN*]Ta(1-hexene)H (9), in which the 3-hexyne moiety has been partially hydrogenated and isomerized to the 1-hexene regioisomer. Both of these alkene hydride complexes can be converted to the dinuclear tetrahydride complex 5 by addition of H2. A mechanism is proposed for the formation of the intermediates that involves hydrogenolysis of the alkyne moiety prior to the benzyl ligand; the formation of the trans-alkene units is suggested to be a result of a zwitterionic alkylidene intermediate that allows free rotation of a C–C single bond.

Dibenzylstrontium and -barium: Simple Molecules with Complex Structures

The reactions between two equivalents of PhCH2K and AeI2 (Ae = Ca, Sr, Ba) in THF give the alkaline earth metal benzyls [(PhCH2)2Ca(THF)4] (3), [(PhCH2)4Sr2(THF)3] (4), and [(PhCH2)6Ba3(THF)4] (5), as orange, crystalline solids in good yield. While 3 crystallizes as a molecular species, compound 4 crystallizes as a ribbon polymer and 5 crystallizes as a 2-dimensional sheet in which the metal ions are coordinated by either benzyl and THF molecules or benzyl ligands alone.