Oxygen Reduction with a Bifunctional Iridium Dihydride Complex

The iridium dihydride [Ir(H)₂(HPNP)]⁺ (PNP=N(CH₂CH₂PtBu₂)₂) reacts with O₂ to give the unusual, square‐planar iridium(III) hydroxide [Ir(OH)(PNP)]⁺ and water. Regeneration of the dihydride with H₂ closes a quasi‐catalytic synthetic oxygen‐reduction reaction (ORR) cycle that can be run several times. Experimental and computational examinations are in agreement with an oxygenation mechanism via rate‐limiting O₂ coordination followed by H‐transfer at a single metal site, facilitated by the cooperating pincer ligand. Hence, the four electrons required for the ORR are stored within the two covalent M? H bonds of a mononuclear metal complex.     

Interconversion of Phosphinyl Radical and Phosphinidene Complexes by Proton Coupled Electron Transfer

The isolable complex [Os(PHMes*)H(PNP)] (Mes*=2,4,6‐^tBu₃C₆H₃; PNP=N{CHCHP^tBu₂}₂) exhibits high phosphinyl radical character. This compound offers access to the phosphinidene complex [Os(PMes*)H(PNP)] by P?H proton coupled electron transfer (PCET). The P?H bond dissociation energy (BDE) was determined by isothermal titration calorimetry and supporting DFT computations. The phosphinidene product exhibits electrophilic reactivity as demonstrated by intramolecular C?H activation.     

Novel P‐Stereogenic PCP Pincer‐Aryl Ruthenium(II) Complexes and Their Use in the Asymmetric Hydrogen Transfer Reaction of Acetophenone

Achiral P‐donor pincer‐aryl ruthenium complexes ([RuCl(PCP)(PPh₃)]) 4c , d were synthesized via transcyclometalation reactions by mixing equivalent amounts of [1,3‐phenylenebis(methylene)]bis[diisopropylphosphine] ( 2c ) or [1,3‐phenylenebis(methylene)]bis[diphenylphosphine] ( 2d ) and the N‐donor pincer‐aryl complex [RuCl{2,6‐(Me₂NCH₂)₂C₆H₃}(PPh₃)], ( 3 ; Scheme 2). The same synthetic procedure was successfully applied for the preparation of novel chiral P‐donor pincer‐aryl ruthenium complexes [RuCl(P*CP*)(PPh₃)] 4a , b by reacting P‐stereogenic pincer‐arenes (S,S)‐[1,3‐phenylenebis(methylene)]bis[(alkyl)(phenyl)phosphines] 2a , b (alkyl=ⁱPr or ^tBu, P*CHP*) and the complex [RuCl{2,6‐(Me₂NCH₂)₂C₆H₃}(PPh₃)], ( 3 ; Scheme 3). The crystal structures of achiral [RuCl(equation/tex2gif-sup-3.gifPCP)(PPh₃)] 4c and of chiral (S,S)‐[RuCl(equation/tex2gif-sup-6.gifPCP)(PPh₃)] 4a were determined by X‐ray diffraction (Fig. 3). Achiral [RuCl(PCP)(PPh₃)] complexes and chiral [RuCl(P*CP*)(PPh₃)] complexes were tested as catalyst in the H‐transfer reduction of acetophenone with propan‐2‐ol. With the chiral complexes, a modest enantioselectivity was obtained.     

A Cyclic Hydroxylaminato‐aminoxide Complex of Gallium

The reaction of HON(tBu)CH₂CH₂N(tBu)OH with tri‐tert‐butyl gallium affords a hydroxylaminato complex of the formula [tBu₂Ga{ON(tBu)CH₂CH₂N(H)(tBu)O}], which contains a monoanionic bishydroxylaminato ligand with one anionic and one neutral, but tautomeric aminoxide end, both linked to gallium by their oxygen atoms leading to a seven‐membered ring. The compound was characterised by elemental analysis, ¹H and ¹³C NMR and determination of its crystal structure.     

P,C,P-Pincer complexes of ruthenium based on ruthenocene and pentamethylruthenocene

The first ruthenocene- and pentamethylruthenocene-based ruthenium pincer complexes, RuCl(CO)[{2,5-(Bu^t ₂PCH₂)₂C₅H₂}Ru(C₅H₅)] and RuCl(CO)[{2,5-(Bu^t ₂PCH₂)₂C₅H₂}Ru-(C₅Me₅)], were synthesized by cyclometallation of {1,3-(Bu^t ₂PCH₂)₂C₅H₂}Ru(C₅H₅) and {1,3-(Bu^t ₂PCH₂)₂C₅H₂}Ru(C₅Me₅), respectively, with RuCl₂(DMSO)₄ in 2-methoxyethanol and characterized by ¹H and ³¹P{¹H} NMR spectroscopy, and X-ray diffraction.     

Characterization of a Palladium Dihydrogen Complex

The preparation and isolation of the first palladium dihydrogen complex is described. NMR spectroscopy reveals a very short H? H bond length, but the hydrogen molecule is activated toward heterolytic cleavage. An X‐ray crystal structure suggests that proton transfer to the ^tBuPCP (κ³‐2,6‐(^tBu₂PCH₂)₂C₆H₃) pincer ligand is possible. The basicity of the ipso‐carbon atom of the pincer ligand was investigated in a related complex.     

Phosphorus(III) Cations Supported by a PNP Pincer Ligand and Sub‐Stoichiometric Generation of P4 from Thermolysis of a Nickel Insertion Product

Neutral, mono‐, and dicationic phosphorus(III) compounds are accessible with a supporting PNP pincer ligand (PNP=[4‐Me‐2‐iPr₂P‐C₆H₃)₂N]). Reaction of (PNP)H with PCl₃ and nBu₃N furnished (PNP)PCl₂ ( 1 ), which displays a highly temperature‐dependent structure in solution. Synthesis and characterization by NMR spectroscopy and X‐ray crystallography of Cl/Br‐scrambled derivatives, a monocationic derivative [(PNP)PCl][HCB₁₁H₁₁] ( 4 ), and the dicationic derivatives [(PNP)P][OTf]₂ ( 5 ), [(PNP)P][B(C₆F₅)₄]₂ ( 6 ), [(PNP)P][B₁₂Cl₁₂] ( 7 ) established that 1 not only undergoes several fluxional processes in solution but also possesses a temperature‐dependent ground state structure. Reaction of 1 with a Ni⁰ source initially leads to a phosphine–phosphinidene complex, followed by thermal generation of P₄.     

New ruthenocene-based ruthenium pincer complex bearing the C<Subscript>5</Subscript>Me<Subscript>4</Subscript>CF<Subscript>3</Subscript> ligand

The first (trifluoromethyl)tetramethylruthenocene-based ruthenium pincer complex RuCl(CO)[{2,5-(Bu ₂ ^t PCH₂)₂C₅H₂}Ru(C₅Me₄CF₃)] was synthesized by cyclometallation of the bisphosphine ligand {1,3-(Bu ₂ ^t PCH₂)₂C₅H₂}Ru(C₅Me₄CF₃) with RuCl₂(DMSO)₄ in 2-methoxyethanol in the presence of NEt₃. The new complex was fully characterized by ¹H, ¹⁹F, ³¹P{¹H}, ¹³C{¹H} NMR and IR spectroscopy.     

Transfer hydrogenation of ketones catalyzed by nickel complexes bearing an NHC [CNN] pincer ligand

Four NHC [CNN] pincer nickel (II) complexes, [^iPrCNN (CH₂)₄‐Ni‐Br] ( 5a ), [^nBuCNN (CH₂)₄‐Ni‐Br] ( 5b ), [^iPrCNN (Me)₂‐Ni‐Br] ( 6a ) and [^nBuCNN (Me)₂‐Ni‐Br] ( 6b ), bearing unsymmetrical [C (carbene)N (amino)N (amine)] ligands were synthesized by the reactions of [CNN] pincer ligand precursors 4 with Ni (DME)Cl₂ in the presence of Et₃N. Complexes 5a and 5b are new and were completely characterized. The transfer hydrogenation of ketones catalyzed by the four pincer nickel complexes were explored. Complexes 5a and 6a have better catalytic activity than 5b and 6b . With a combination of NaO^tBu/ⁱPrOH/80 °C and 2% catalyst loading of 5a , 77–98% yields of aromatic alcohols could be obtained.     

Palladium(II) aryl-amido complexes of diphosphinoazines in unsymmetrical PNP’ pincer-type configuration

Square planar palladium(II) aryl-amido complexes of diphosphinoazines in monoanionic unsymmetrical PNP’ pincer-type coordination were prepared by reactions of phenyl-, o-tolyl-, or 2,6-dimethylphenyllithium with previously described chloro-amido complexes of diphosphinoazines having isopropyl, cyclohexyl and tert-butyl substituents on phosphorus atoms. The compounds were characterized by NMR showing free rotation around metal-aryl bond in the complexes; the presence of C_ipso-Pd bond was detected by two-dimensional experiments. In addition to that, crystal and molecular structure of one phenyl-amido complex, [Pd(C₆H₅){P(C₆H₁₁)₂CHC(Bu^t)NNC(Bu^t)CH₂P(C₆H₁₁)₂}], was determined by X-ray diffraction together with the structure of a chloro-amido complex [PdCl{PBu^t₂CHC(Bu^t)NNC(Bu^t)CH₂PBu^t₂}]. In both structures the ligand trans to the amide nitrogen is well surrounded by substituents on phosphorus atoms, the former complex showing significant interactions between two cyclohexyl hydrogen atoms and the π-system of the phenyl ring. The values od Pd-C and Pd-N bond distances in this complex are the same as those in a monodentate analog [Pd(PMe₃)₂(C₆H₅)(NHC₆H₅)] which contrasts with the different values in a similar PNP symmetrical pincer complex reported in the literature.     

Tuning main group redox chemistry through steric loading: subvalent Group 13 metal complexes of carbazolyl ligands

The ability of substituted carbazol‐9‐yl systems to ligate in σ fashion through the amido N‐donor, or to adopt alternative coordination modes through the π system of the central five‐membered ring, can be tuned by systematic variation in the steric demands of substituents in the 1‐ and 8‐positions. The differing affinities of the two modes of coordination for hard and soft metal centres can be shown to influence not only cation selectivity, but also the redox properties of the metal centre. Thus, the highly sterically sterically demanding 1,3,6,8‐tetra‐tert‐butylcarbazolyl ligand can be used to generate the structurally characterised amido‐indium(I) complex, [{(tBu₄carb)In}_n], (together with its isostructural thallium counterpart) in which the metal centre interacts with the central pyrrolyl ring in η³ fashion [d(In? N)=2.679(3) Å; d(In? C)=2.819(3), 2.899(3) Å]. By contrast, the smaller 3,6‐di‐tert‐butylcarbazolyl system is less able to restrict the metal centre from binding at the anionic nitrogen donor in the plane of the carbazolyl ligand (i.e. in σ fashion). Analogous chemistry with In^I precursors therefore leads to disproportionation to the much harder In^II [and In⁰], and the formation of the mixed‐valence product, [In₂{In₂(tBu₂carb)₆}], a homoleptic molecular [In₄(NR₂)₆] system. This chemistry reveals a flexibility of ligation for carbazolyl systems that contrasts markedly with that of the similarly sterically encumbered terphenyl ligand family.     

Cyclization of N-alkyldithiocarbamates in alkaline media,a counter example of well-known chemistry – an experimental and theoretical study

In strong alkaline media, the reaction of 2-(tert-butylamino)ethanol (3: R?=?Bu^t) with CS₂ at 0°C produced a cyclic dithiocarbamate, 3-tert-butylthiazolidine-2-thione (1: R?=?Bu^t), rather than alkaline metal or ammonium salts of [S₂CN(Bu^t)CH₂CH₂OH]^?. This is in contrast to isolation of stable alkaline metal or ammonium salts of [S₂CN(R)CH₂CH₂OH]^? (R?=?Me, Et, Pr, or CH₂CH₂OH) obtained in analogous reactions. The use of Ni(OAc)₂, both as a source of Ni(II) and a weaker base, in a one-pot reaction with (3: R?=?Bu^t) and CS₂, successfully gave the first reported metal complex of [S₂CN(Bu^t)CH₂CH₂OH]^?, namely [Ni{S₂CN(Bu^t)CH₂CH₂OH}₂] (2: R?=?Bu^t). Compounds 1 and 2 have been fully characterized by infrared and NMR spectroscopies, and by X-ray crystallography. DFT calculations on the cyclization and stabilities of [S₂CN(R)CH₂CH₂OH]^? (R?=?Pr and Bu^t) have been carried out.     

Neutral and Anionic Monomeric Zirconium Imides Prepared via Selective C=N Bond Cleavage of a Multidentate and Sterically Demanding β‐Diketiminato Ligand

A sterically encumbering multidentate β‐diketiminato ligand, ^tBuL2 (^tBuL2=[ArNC(tBu)CHC(tBu)NCH₂CH₂N(Me)CH₂CH₂NMe₂]^?, Ar=2,6‐iPr₂C₆H₃), is reported in this study along with its coordination chemistry to zirconium(IV). Using the lithio salt of this ligand, Li(^tBuL2) ( 4 ), the zirconium(IV) precursor (^tBuL2)ZrCl₃ ( 6 ) could be readily prepared in 85 % yield and structurally characterized. Reduction of 6 with 2 equiv of KC₈ resulted in formation of the terminal and mononuclear zirconium imide‐chloride [C(tBu)CHC(tBu)NCH₂CH₂N(Me)CH₂CH₂NMe₂]Zr(=NAr)(Cl) ( 7 ) as the result of reductive C=N cleavage of the imino fragment in the multidentate ligand ^tBuL2 by an elusive Zr^II species (^tBuL2)ZrCl ( A ). The azabutadienyl ligand in 7 can be further reduced by 2 e^? with KC₈ to afford the anionic imide [K(THF)₂]{[CH(tBu)CHC(tBu)NCH₂CH₂N(Me)CH₂CH₂N(Me)CH₂]Zr=NAr} ( 8‐2THF ) in 42 % isolated yield. Complex 8‐2THF results from the oxidative addition of an amine C?H bond followed by migration to the vinylic group of the formal [C(tBu)CHC(tBu)NCH₂CH₂N(Me)CH₂CH₂NMe₂]^? ligand in 7 . All halides in 6 can be replaced with azides to afford (^tBuL2)Zr(N₃)₃ ( 9 ) which was structurally characterized, and reduction with two equiv of KC₈ also results in C=N bond cleavage of ^tBuL2 to form [C(tBu)CHC(tBu)NCH₂CH₂N(Me)CH₂CH₂NMe₂]Zr(=NAr)(N₃) ( 10 ), instead of the expected azide disproportionation to N^3? and N₂. Solid‐state single crystal structural studies confirm the formation of mononuclear and terminal zirconium imido groups in 7 , 8‐Et₂O , and 10 with Zr=NAr distances being 1.8776(10), 1.9505(15), and 1.881(3) Å, respectively.     

[2,6‐Bis(isopropylthiomethyl)phenyl‐κ3S,C1,S′]bromopalladium(II)

The title compound, [PdBr(C₁₄H₂₁S₂)] or [PdBr{C₆H₃(CH₂SⁱPr)₂‐2,6}], exhibits square‐planar geometry at the Pd centre, with three atoms of the square plane provided by the rigid thio­pincer ligand, i.e. 1,3‐bis­(thio­methyl)­benzene.     

Lanthanide(II) Complexes of the Dihydrotriazinido Ligand [N{C(Ph)=N}2C(tBu)Ph]−

The potassium dihydrotriazinide K(L^Ph,tBu) ( 1 ) was obtained by a metal exchange route from [Li(L^Ph,tBu)(THF)₃] and KOtBu (L^Ph,tBu = [N{C(Ph)=N}₂C(tBu)Ph]). Reaction of 1 with 1 or 0.5 equivalents of SmI₂(thf)₂ yielded the monosubstituted Sm^II complex [Sm(L^Ph,tBu)I(THF)₄] ( 2 ) or the disubstituted [Sm(L^Ph,tBu)₂(THF)₂] ( 3 ), respectively. Attempted synthesis of a heteroleptic Sm^II amido‐alkyl complex by the reaction of 2 with KCH₂Ph produced compound 3 due to ligand redistribution. The Yb^II bis(dihydrotriazinide) [Yb(L^Ph,tBu)₂(THF)₂] ( 4 ) was isolated from the 1:1 reaction of YbI₂(THF)₂ and 1 . Molecular structures of the crystalline compounds 2 , 3· 2C₆H₆ and 4· PhMe were determined by X‐ray crystallography.     

A Germylene Supported by Two 2-Pyrrolylphosphane Groups as Precursor to PGeP Pincer Square-Planar Group 10 Metal(II) and T-Shaped Gold(I) Complexes

An efficient synthesis of 2-di-tert-butylphosphanylmethylpyrrole (HpyrmPtBu₂), by treating 2-dimethylaminomethylpyrrole (HpyrmNMe₂) with tBu₂PH at 135 °C in the absence of any solvent, has allowed the preparation of the new PGeP germylene Ge(pyrmPtBu₂)₂ ( 1 ), by treating [GeCl₂(dioxane)] with LipyrmPtBu₂, in which the Ge atom is stabilized by intramolecular interactions with one (solid state) or both (solution) of its phosphane groups. Reactions of germylene 1 with Group 10 metal dichlorido complexes containing easily displaceable ligands have led to [MCl{κ³P,Ge,P-GeCl(pyrmPtBu₂)₂}] [M=Ni ( 2 ), Pd ( 3 ), Pt ( 4 )], which have an unflawed square-planar metal environment. Treatment of germylene 1 with [AuCl(tht)] (tht=tetrahydrothiophene) rendered [Au{κ³P,Ge,P-GeCl(pyrmPtBu₂)₂}] ( 5 ), which is a rare case of a T-shaped gold(I) complex. The hydrolysis of 5 gave the linear gold(I) derivative [Au(κP-HpyrmPtBu₂)₂]Cl ( 6 ). Complexes 2 – 5 contain a PGeP pincer chloridogermyl ligand that arises from the insertion of the Ge atom of germylene 1 into a M−Cl bond of the corresponding metal reagent. The bonding in these molecules has been studied by DFT/NBO/QTAIM calculations. These results demonstrate that the great flexibility of germylene 1 makes it a better precursor to PGeP pincer complexes than the previously known germylenes of this type.     

Ruthenium(II) Complexes Containing Lutidine‐Derived Pincer CNC Ligands: Synthesis,Structure, and Catalytic Hydrogenation of CN bonds

A series of Ru complexes containing lutidine‐derived pincer CNC ligands have been prepared by transmetalation with the corresponding silver‐carbene derivatives. Characterization of these derivatives shows both mer and fac coordination of the CNC ligands depending on the wingtips of the N‐heterocyclic carbene fragments. In the presence of tBuOK, the Ru‐CNC complexes are active in the hydrogenation of a series of imines. In addition, these complexes catalyze the reversible hydrogenation of phenantridine. Detailed NMR spectroscopic studies have shown the capability of the CNC ligand to be deprotonated and get involved in ligand‐assisted activation of dihydrogen. More interestingly, upon deprotonation, the Ru‐CNC complex 5 e (BF₄) is able to add aldimines to the metal–ligand framework to yield an amido complex. Finally, investigation of the mechanism of the hydrogenation of imines has been carried out by means of DFT calculations. The calculated mechanism involves outer‐sphere stepwise hydrogen transfer to the C?N bond assisted either by the pincer ligand or a second coordinated H₂ molecule.     

Alcohol oxidation reactions catalyzed by ruthenium–carbonyl complexes of thioarylazoimidazoles

Alcohols are oxidized by N‐methylmorpholine‐N‐oxide (NMO), Bu^tOOH and H₂O₂ to the corresponding aldehydes or ketones in the presence of catalyst, [RuH(CO)(PPh₃)₂(SRaaiNR′)]PF₆ ( 2 ) and [RuCl(CO)(PPh₃)(S_κRaaiNR′)]PF₆ ( 3 ) (SRaaiNR′ ( 1 ) = 1‐alkyl‐2‐{(o‐thioalkyl)phenylazo}imidazole, a bidentate N(imidazolyl) (N), N(azo) (N′) chelator and S_κRaaiNR′ is a tridentate N(imidazolyl) (N), N(azo) (N′), S_κ‐R is tridentate chelator; R and R′ are Me and Et). The single‐crystal X‐ray structures of [RuH(CO)(PPh₃)₂(SMeaaiNMe)]PF₆ ( 2a ) (SMeaaiNMe = 1‐methyl‐2‐{(o‐thioethyl)phenylazo}imidazole) and [RuH(CO)(PPh₃)₂(SEtaaiNEt)]PF₆ ( 2b ) (SEtaaiNEt = 1‐ethyl‐2‐{(o‐thioethyl)phenylazo}imidazole) show bidentate N,N′ chelation, while in [RuCl(CO)(PPh₃)(S_κEtaaiNEt)]PF₆ ( 3b ) the ligand S_κEtaaiNEt serves as tridentate N,N′,S chelator. The cyclic voltammogram shows Ru^III/Ru^II (~1.1 V) and Ru^IV/Ru^III (~1.7 V) couples of the complexes 2 while Ru^III/Ru^II (1.26 V) couple is observed only in 3 along with azo reductions in the potential window +2.0 to ?2.0 V. DFT computation has been used to explain the spectra and redox properties of the complexes. In the oxidation reaction NMO acts as best oxidant and [RuCl(CO)(PPh₃)(S_κRaaiNR′)](PF₆) ( 3 ) is the best catalyst. The formation of high‐valent Ru^IV=O species as a catalytic intermediate is proposed for the oxidation process. Copyright © 2014 John Wiley & Sons, Ltd.     

Unexpected Silaazetidine Formation in the Thermolysis of thf‐Supported Silanimine Et2Si=N–SitBu3·thf

The silyl amide Et₂SiCl‐NLi‐SitBu₃ can be cleanly prepared from precursor silylamine Et₂SiCl‐NH‐SitBu₃ and Li[nBu]. The CF₃SO₃SiMe₃ induced LiCl elimination of Et₂SiCl‐NLi‐SitBu₃ in thf afforded a 2‐silaazetidine derivative by [2+2] cycloaddition of Et₂Si=N–SitBu₃ with Et₂Si(OCH=CH₂)–NH–SitBu₃. X‐ray quality crystals of this 2‐silaazetidine derivative (triclinic, space group P$\bar{1}$ ) were grown from benzene at room temperature. The starting material for this approach, Et₂SiCl–NH–SitBu₃, is water‐sensitive. Hydrolysis of Et₂SiCl‐NH‐SitBu₃ gave [tBu₃SiNH₃]Cl along with (Et₂SiO)_n oligomers. The hydro chloride [tBu₃SiNH₃]Cl could be isolated and was characterized by X‐ray crystallography (trigonal, space group P$\bar{3}$ ).