Mono‐ and Dinuclear Heteroleptic Cobalt Complexes with α‐Diimine and Polyarene Ligands

Reactions of the dimeric cobalt complex [(L^?Co)₂] ( 1 , L=[(2,6‐iPr₂C₆H₃)NC(Me)]₂) with polyarenes afforded a series of mononuclear and dinuclear complexes: [LCo(η⁴‐anthracene)] ( 2 ), [LCo(μ‐η⁴:η⁴‐naphthalene)CoL] ( 3 ), and [LCo(μ‐η⁴:η⁴‐phenanthrene)CoL] ( 4 ). The pyrene complexes [{Na₂(Et₂O)₂}{LCo(μ‐η³:η³‐pyrene)CoL}] ( 5 ) and [{Na₂(Et₂O)₃}{LCo(η³‐pyrene)}] ( 6 ) were obtained by treating precursor 1 with pyrene followed by reduction with Na metal. These complexes contain three potential redox active centers: the cobalt metal and both α‐diimine and polyarene ligands. Through a combination of X‐ray crystallography, EPR spectroscopy, magnetic susceptibility measurement, and DFT computations, the electronic configurations of these complexes were studied. It was determined that complexes 2 – 4 have a high‐spin Co^I center coupled with a radical α‐diimine ligand and a neutral polyarene ligand. Whereas, the ligand L in complexes 5 and 6 has been further reduced to the dianion, the cobalt remains in a formal (I) oxidation state, and the pyrene molecule is either neutral or monoanionic.     

Kinetics of Oxidation of L‐Cysteine by trans‐ and cis‐CoIII and FeIII Complexes based on α‐ and γ‐Diimine Schiff Base Ligands

Kinetics of oxidation of L ‐cysteine by Co^III and Fe^III complexes based on α‐ and γ‐diimine Schiff base ligands were studied in aqueous solution. Pairs of trans and cis isomers of the metal complexes were used in the studies. Kinetic measurements were performed at 25 °C and constant pH and ionic strength under pseudo‐first order condition, in which the concentration of cysteine was around two orders of magnitude greater than that of the metal complex. The observed rate constant was obtained by following the change in absorbance of the reaction mixture with time at a predetermined wavelength. The overall rate constant and order of the reaction with respect to cysteine and metal complex were determined. For both metal ions studied, the oxidation rate constant for the trans isomer was higher than that for the cis isomer. This was attributed to the contribution of the steric factor and the trans effect. The effects of substituents and the nature of the metal ion on the reaction rate are discussed.     

Phosphorescent Platinum(II) Complexes with Mesoionic 1H‐1,2,3‐Triazolylidene Ligands

The synthesis and characterization of eight unprecedented phosphorescent C^C* cyclometalated mesoionic aryl‐1,2,3‐triazolylidene platinum(II) complexes with different β‐diketonate ligands are reported. All compounds proved to be strongly emissive at room temperature in poly(methyl methacrylate) films with an emitter concentration of 2 wt %. The observed photoluminescence properties were strongly dependent on the substitution on the aryl system and the β‐diketonate ligand. Compared to acetylacetonate, the β‐diketonates with aromatic substituents (mesityl and duryl) were found to significantly enhance the quantum yield while simultaneously reducing the emission lifetimes. Characterization was carried out by standard techniques, as well as solid‐state structure determination, which confirmed the binding mode of the carbene ligand. DFT calculations, carried out to predict the emission wavelength with maximum intensity, were in excellent agreement with the (later) obtained experimental data.     

Novel Unsymmetric α‐Diimine Nickel(II) Complexes: Suitable Catalysts for Copolymerization Reactions

We established a strategy to synthesize novel unsymmetric 2,3‐diaza‐1,4‐dithiane ligands. Reaction of [Ni(acac)₂] and trityl tetrakis(pentafluorophenyl)borate in the presence of these ligands afforded the corresponding salt‐type complexes. All new compounds were characterized by means of elemental analysis and NMR spectroscopy, and the complexes additionally by mass spectroscopy. NMR spectroscopic experiments on polymers generated by the symmetric ligand/trimethylaluminum catalyst system showed that all products were nearly linear, independent of the polymerization conditions. By contrast, polymers produced by the unsymmetric ligand/trimethylaluminum catalyst system under homopolymerization conditions were branched (15–24 ‰). Additionally, copolymerization experiments with propylene and 1‐hexene afforded copolymers with a branching level of up to 50 ‰.     

Synthesis and Structural Characterization of ZnII α‐Diimine Complexes with Chloride and Thiocyanate Co‐Ligands

The preparation and spectroscopic and structural characterization of three Zn^II complexes with bis[N‐(2,6‐dimethylphenyl)imine]acenaphthene, L¹, and with bis[N‐(2‐ethylphenyl)imine]acenaphthene, L², are decribed herein. Two of the complexes were prepared from ZnCl₂ and the third from Zn(NCS)₂. One‐pot reaction techniques were used, leading to high yields. The complexes were characterized by microanalysis, IR and ¹H NMR spectroscopy, and single‐crystal X‐ray diffraction. The structures of the complexes are significantly different, with the chloride‐containing species forming distorted tetrahedra around the metal, whereas its thiocyanate analog is dimeric, with each metal at the center of a distorted square pyramid, with bridging and terminal [SCN]^– ligands.     

Polymerization of Methyl Methacrylate with Ni(II) α‐Diimine/MAO and Fe(II) and Co(II) Pyridyl Bis(imine)/MAO

Polymerizations of methyl methacrylate with (α‐diimine)nickel(II)/methylaluminoxane (MAO) and (pyridyl bis(imine))iron(II) and (pyridyl bis(imine))cobalt(II)/MAO are reported. Effects of structural variation of the ligand on the activities of catalysts and polymer microstructure are described. The catalyst systems gave syndio‐rich poly(methyl methacrylate). The α‐diimine system showed much higher activity than the pyridyl bis(imine) systems under similar polymerization conditions.

     

Nickel(II) α‐Diimine Catalyst for Grignard Metathesis (GRIM) Polymerization

A nickel α‐diimine catalyst was used for Grignard metathesis (GRIM) polymerization of 2,5‐dibromo 3‐hexylthiophene and 2‐bromo‐5‐iodo‐3‐hexylthiophene monomers. GRIM polymerization of 2‐bromo‐5‐iodo‐3‐hexylthiophene generated regioregular polymers with molecular weights ranging from 3 000 to 12 000 g · mol⁻¹. The nickel α‐diimine catalyst was also successfully used for the GRIM polymerization of a bulky benzodithiophene monomer.

     

Synthesis and X‐ray Crystal Structures of Zinc Dichloride Complexes Supported by a β‐Diimine Ligand

β‐Diimine zinc dichloride complexes [CH₂{C(Me)NAr}₂]ZnCl₂ [Ar = Mes ( 1 ), Dipp ( 2 )] were obtained from the reactions of ZnCl₂ with the corresponding β‐iminoamines [ArN(H)C(Me)CHC(Me)NAr]. Complexes 1 and 2 were characterized by multinuclear NMR (¹H, ¹³C) and IR spectroscopy, elemental analyses as well as by single‐crystal X‐ray diffraction. The energy differences between the enamine‐imine tautomers of the β‐iminoamines were quantified by quantum chemical calculations.     

Trans‐ and cis‐ Cobalt(III), Iron(III), and Chromium(III) Complexes Based on α‐ and γ‐Diimine Schiff Base Ligands: Synthesis and Evaluation of the Complexes as Catalysts for Oxidation of L‐Cysteine

The synthesis of a new series of trans and racemic cis isomers of cobalt(III)‐, iron(III)‐, and chromium(III)‐based complexes with the α‐ and γ‐diimine Schiff base ligands, N,N′‐bis(X)‐2,3‐butandiimine and N,N′‐bis(X)‐1,2‐phenyldiimine (X = cyclohexyl, 2‐isopropylphenyl, 1‐naphthyl) is described. To confirm the identity of the complexes prepared in the present study, a variety of techniques including elemental analysis, magnetic susceptibility, infrared‐, mass‐ (EI), and UV/Vis‐ spectroscopy have been utilized. Some of the isolated complexes have been evaluated as catalysts for the oxidation of L‐cysteine. Preliminary results showed that the metal atoms, geometry of the complexes, auxiliary substituents, and the backbone of the ligand influenced the rate of oxidation reaction.     

ATRP/OMRP/CCT Interplay in Styrene Polymerization Mediated by Iron(II) Complexes: A DFT Study of the α‐Diimine System

A DFT study of various model systems has addressed the interference of catalytic chain transfer (CCT) as a function of the R² substituent in the atom‐transfer radical polymerization (ATRP) of styrene catalyzed by [FeCl₂(R¹N?C(R²)?C(R²)?NR¹)] complexes. All model systems used R¹=CH₃ in place of the experimental Cy and tBu substituents and 1‐phenylethyl in place of the polystyrene (PS) chain. A mechanistic investigation of 1) ATRP activation, 2) radical trapping in organometallic‐mediated radical polymerization (OMRP), and 3) pathways to the hydride CCT intermediate was conducted with a simplified system with R²=H. This study suggests that CCT could occur by direct hydrogen‐atom transfer without any activation barrier. Further analysis of more realistic models with R²=p‐C₆H₄F or p‐C₆H₄NMe₂ suggests that the electronic effect of the aryl para substituents significantly alters the ATRP activation barrier. Conversely, the hydrogen‐atom‐transfer barrier is essentially unaffected. Thus, the greater ATRP catalytic activity of the p‐NMe₂ system makes the background CCT process less significant. The DFT study also compares the [FeCl₂(R¹N?C(R²)?C(R²)?NR¹)] systems with a diaminobis(phenolato) derivative for which the CCT process shows even greater accessibility but has less incidence because of faster ATRP chain growth and interplay with a more efficient OMRP trapping. The difference between the two systems is attributed to destabilization of the Fe^II catalyst by the geometric constraints of the tetradentate diaminobis(phenolato) ligand.     

Highly Robust Palladium(II) α‐Diimine Catalysts for Slow‐Chain‐Walking Polymerization of Ethylene and Copolymerization with Methyl Acrylate

A series of sterically demanding α‐diimine ligands bearing electron‐donating and electron‐withdrawing substituents were synthesized by an improved synthetic procedure in high yield. Subsequently, the corresponding Pd complexes were prepared and isolated by column chromatography. These Pd complexes demonstrated unique properties in ethylene polymerization, including high thermal stability and high activity, thus generating polyethylene with a high molecular weight and very low branching density. Similar properties were observed for ethylene/methyl acrylate copolymerization. Because of the high molecular weight and low branching density, the generated polyethylene and ethylene/methyl acrylate copolymer were semicrystalline solids. The (co)polymers had unique microstructures originating from the unique slow‐chain‐walking activity of these Pd complexes.     

Platinum(II) Chloride Complexes of Linear Alkyl-bridged 2,2′-Bipyridine/Catechol Ligands

Platinum(II) chloride can selectively be coordinated to the 2,2′-bipyridine moiety of the alkyl bridged sequential catechol/2,2′-bipyridine ligand 1 a-H₂ and of the related ligands 1 a/b-Me₂ and 2 . Reaction of ( 1 b-Me₂ )PtCl₂ with BBr₃ produces the platinum(II) complex ( 1 b-H₂ )PtCl₂ while ether cleavage of the uncoordinated ligand 1 b-Me₂ fails. Under basic conditions ( 1 a-H₂ )PtCl₂ forms polymeric/oligomeric species [( 1 a )Pt]_n besides traces of the dinuclear complex [( 1 a )Pt]₂.     

Supramolecular Assembly of Triangulo Pd(II) Diimine Complexes Containing Triply‐Bridging Sulfide Ligands

We report here a substituent effect of diimines on the solid‐state assembly of interesting triangulo Pd(II) complexes, [(Pd(d‐t‐bpy))₃(μJ₃‐S)₂][NO₃]₂ 1 ·[NO₃]₂ and [(Pd(bpy))₃(μ₃‐S)₂][ClO₄]₂ 2 ·[ClO₄]₂ (d‐t‐bpy = 4,4′‐di‐tert‐butyl‐2,2′‐bipyridine, bpy = 2,2′‐bipyridine). 2 ·[ClO₄]₂ shows the intermolecular π···π interactions leading to the formation of one‐dimensional frameworks, whereas 1 ·[NO₃]₂ only shows the discrete structure in the solid state, featuring an interesting herring‐bone arrangement. The variation in structural motifs from 1 ·[NO₃]₂ to 2 ·[ClO₄]₂ is expected to be dominated by the substituent's steric hindrance for the diimine ligand. Thus, the crystal‐engineering approach has proved successful in the solid‐state packing due to a substituent's modification of the diimine ligand.     

Acyl and α-Ketoacyl Complexes of Pt(II) with Weak Donor Ligands

Treatment of trans-Pt(COCOPh)(Cl)(PPh₃)₂ (1a) with AgBF₄in THF led to the formation of a metastatic complex trans-[Pt(COCOPh)(THF)(PPh₃)₂](BF₄) (2) which readily underwent ligand substitution to give a cationic aqua complex trans-[Pt(COCOPh)(OH₂)(PPh₃)₂](BF₄) (5a). Complex 5a has been characterized spectroscopically and crystallographically. Analogous reaction of trans-Pt(COCOOMe)(Cl)(PPh₃)₂ (1b) with Ag(CF₃SO₃) in dried CH₂C1₂ was found first to yield a methoxyoxalyl triflato complextrans-Pt(COCOOMe)(OTf)(PPh₃)₂ (6). Attempts to crystallize the triflato product in CH₂-cl₂hexane under ambient conditions also afforded an aqua complex of the triflate salt f/wu-[Pt(COCOOMe)(OH₂)(PPhj)₂](CF₃SO₃) (5b). Complex 5a in a noncoordinating solvent such as CH₂C1₂ or CHCl₃ suffered spontaneous decarbonylation to form first cis-[Pt(COPh)(CO)(PPh₃)₂l(BF₄) (3a) then the thermodynamically stable isomer trans-[Pt(COPh)(CO)(PPh₃)₂](BF₄) (3b). Crystallization of complex 3b under ambient conditions resulted in an aqua benzoyl complex trans-[Pt(COPh)(OH₂)(PPh₃)₂](BF₄) (7). The replacement of the H_2O ligand in complex 7 by CO was done simply by bubbling CO into the solution of 7. The single crystal structures of 5b and 7 have been determined by X-ray diffraction. The distances of the Pt-O bonds in 5a, 5b, and 7 support that the aqua ligand is a weak donor in such cationic aquaorganoplatinum(lI) complexes, in agreement with their lability to the substitution reactions.     

Insertion Reactions of 1,2‐Disubstituted Olefins with an α‐Diimine Palladium(II) Complex

The migratory insertions of cis or trans olefins CH(X)?CH(Me) (X = Ph, Br, or Et) into the metal–acyl bond of the complex [Pd(Me)(CO)(ⁱPr₂dab)]⁺ [B{3,5‐(CF₃)₂C₆H₃}₄]^? ( 1 ) (ⁱPr₂dab = 1,4‐diisopropyl‐1,4‐diazabuta‐1,3‐diene = N,N′‐(ethane‐1,2‐diylidene)bis[1‐methylethanamine]) are described (Scheme 1). The resulting five‐membered palladacycles were characterized by NMR spectroscopy and X‐ray analysis. Experimental data reveal some important aspects concerning the regio‐ and stereochemistry of the insertion process. In particular, the presence of a Ph or Br substituent at the alkene leads to the formation of highly regiospecific products. Moreover, in all cases, the geometry of the substituents in the formed palladacycle was the same as in the starting olefin, as a consequence of a cis addition of the Pd–acyl fragment to the C?C bond. Reaction with CO and MeOH of the five‐membered complex derived from trans‐β‐methylstyrene (= [(1E)‐prop‐1‐enyl]benzene) insertion, yielded the 2,3‐substituted γ‐keto ester 9 with an (2RS,3SR)‐configuration (Scheme 3).     

Oxidation and β‐Alkylation of Alcohols Catalysed by Iridium(I) Complexes with Functionalised N‐Heterocyclic Carbene Ligands

The borrowing hydrogen methodology allows for the use of alcohols as alkylating agents for C?C bond forming processes offering significant environmental benefits over traditional approaches. Iridium(I)‐cyclooctadiene complexes having a NHC ligand with a O‐ or N‐functionalised wingtip efficiently catalysed the oxidation and β‐alkylation of secondary alcohols with primary alcohols in the presence of a base. The cationic complex [Ir(NCCH₃)(cod)(MeIm(2‐ methoxybenzyl))][BF₄] (cod=1,5‐cyclooctadiene, MeIm=1‐methylimidazolyl) having a rigid O‐functionalised wingtip, shows the best catalyst performance in the dehydrogenation of benzyl alcohol in acetone, with an initial turnover frequency (TOF₀) of 1283 h^?1, and also in the β‐alkylation of 2‐propanol with butan‐1‐ol, which gives a conversion of 94 % in 10 h with a selectivity of 99 % for heptan‐2‐ol. We have investigated the full reaction mechanism including the dehydrogenation, the cross‐aldol condensation and the hydrogenation step by DFT calculations. Interestingly, these studies revealed the participation of the iridium catalyst in the key step leading to the formation of the new C?C bond that involves the reaction of an O‐bound enolate generated in the basic medium with the electrophilic aldehyde.     

Photolysis and Thermolysis of Platinum(IV) 2,2′‐Bipyridine Complexes Lead to Identical Platinum(II)–DNA Adducts

Two Pt^IV and two Pt^II complexes containing a 2,2′‐bipyridine ligand were treated with a short DNA oligonucleotide under light irradiation at 37 °C or in the dark at 37 and 50 °C. Photolysis and thermolysis of the Pt^IV complexes led to spontaneous reduction of the Pt^IV to the corresponding Pt^II complexes and to binding of Pt^II 2,2′‐bipyridine complexes to N7 of guanine. When the reduction product was [Pt(bpy)Cl₂], formation of bis‐oligonucleotide adducts was observed, whereas [Pt(bpy)(MeNH₂)Cl]⁺ gave monoadducts, with chloride ligands substituted in both cases. Neither in the dark nor under light irradiation was the reductive elimination process of these Pt^IV complexes accompanied by oxidative DNA damage. This work raises the question of the stability of photoactivatable Pt^IV complexes toward moderate heating conditions.