Chromium(III) complexes with terdentate 2,6-bis(azolylmethyl)pyridine ligands: Synthesis, structures and ethylene polymerization behavior

Reactions of 2,6-bis(bromomethyl)pyridine with 3,5-dimethylpyrazole and 1H-indazole yield the terdentate ligands 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine (5) and 2,6-bis(indazol-2-ylmethyl)pyridine (6). The molecular structure of the new compound 6 was determined by single-crystal X-ray diffraction. These ligands react with the CrCl₃(THF)₃ complex in THF to form neutral complexes of general formula [CrCl₃{2,6-bis(azolylmethyl)pyridine-N,N,N}] (7, 8) which are isolated in high yields as stable green solids and characterized by means of elemental analysis, magnetic moments, IR, and mass spectroscopy. Theoretical calculations predict that the thermodynamically preferred structure of the complexes is the fac configuration. After reaction with methylaluminoxane (MAO) the chromium(III) complexes are active in the polymerization of ethylene.     

Aldimine 2,6-bis(imino)pyridine iron(II) and cobalt(II)/methyl aluminoxane catalyst systems for polymerization of <Emphasis Type="Italic">tert-</Emphasis>butylacrylate

Aldimine 2,6-bis[(imino)methyl]pyridine iron(II) (1, 4, and 6) and cobalt(II) (3 and 5) complexes bearing bulky cycloaliphatic (bornyl and myrtanyl) or aromatic (naphthyl) terminal groups have been applied successfully, after activation with methyl aluminoxane (MAO), as catalysts for the polymerization of tert-butylacrylate. For comparison reasons, complex 2 that contains the ketimine ligand, 2,6-bis[(−)-cis-myrtanylimino)ethyl]pyridine (BMEP), has also been utilized. All studied complexes showed moderate polymerization activities, and they produced high molar mass syndiorich-atactic polymers. Surprisingly, the aldimine-based catalyst systems showed comparable activities compared with the corresponding ketimine complex (2), and they produced high molar mass polymers. In addition, complexes with bulky terminal cycloaliphatic substituents on the tridentate aldimine ligands showed higher polymerization activity compared with the aromatic ones (6). Polymerization activity and polymer molar masses are dependent on the ligand framework.     

Producing highly linear polyethylenes by using t‐butyl‐functionalized 2,6‐bis(imino)pyridylchromium(III) chlorides

A new family of t‐butyl substituted chromium(III) chloride complexes ( Cr1 – Cr6 ), bearing 2‐(1‐(2,6‐dibenzhydryl‐4‐t‐butylphenylimino)ethyl)‐6‐(1‐(arylimino)ethyl)pyridine (aryl = 2,6‐Me₂C₆H₃ Cr1 , 2,6‐Et₂C₆H₃ Cr2 , 2,6‐i‐Pr₂C₆H₃ Cr3 , 2,4,6‐Me₃C₆H₂ Cr4 and 2,6‐Et₂‐4‐MeC₆H₂ Cr5 ) or 2,6‐bis(1‐(2,6‐dibenzhydryl‐4‐t‐butylphenylimino)ethyl)pyridine ( Cr6 ), has been synthesized by the reaction of CrCl₃·6H₂O in good yield with the corresponding ligands ( L1 – L6 ), respectively. The molecular structures of Cr2 and Cr6 were characterized by X‐ray diffraction highlighted a distorted octahedral geometry with the coordinated N,N,N ligand and three bonded chlorides around the metal center. On activation with modified methylaluminoxane or triisobutyl aluminum, most of the chromium precatalysts exhibit good activities toward ethylene polymerization and produce linear polyethylenes with high‐molecular weight. In addition, an in‐depth catalytic evaluation of Cr2 was conducted to investigate how cocatalyst type and amount, reaction temperature, and run time affect the catalytic activities and polymer properties. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 1049–1058     

Tricoordinate Organochromium(III) Complexes Supported by a Bulky Silylamido Ligand Produce Ultra‐High‐Molecular Weight Polyethylene in the Absence of Activators

Low‐coordinate organoCr(III) complexes supported by the silylamido ligand –N(SiMe₃)DIPP (DIPP = 2,6‐diisopropylphenyl) are ethylene polymerization catalyst precursors without the need of additional cocatalyst. The reaction of CrCl₃(THF)₃ with 3 or 2 equiv. of LiN(SiMe₃)DIPP yields either a four‐membered cyclometalated Cr complex or Cr[N(SiMe₃)DIPP]₂Cl, respectively, with no trace of Cr[N(SiMe₃)DIPP]₃. Addition of 1 equiv. of LiN(SiMe₃)DIPP to Cr[N(SiMe₃)DIPP]₂Cl also leads to the four‐membered metallacycle, which upon heating transforms to a new six‐membered Cr metallacycle, likely via a σ‐bond metathesis step. Cr[N(SiMe₃)DIPP]₂Cl can be readily converted to bis(amido)Cr(III) vinyl and alkyl complexes Cr[N(SiMe₃)DIPP]₂R (R = vinyl, Bn, and Me). All of these structurally characterized low‐coordinate Cr(III) complexes with a Cr–C bond initiate the polymerization of ethylene in the absence of activators or cocatalysts, producing ultra‐high‐molecular weight polyethylene.     

Models for Copper-Dioxygen Complexes: The chemistry of copper(II) with some planar tridentate nitrogen ligands

The solution chemistry of Cu(II) with a series of five planar tridentate nitrogen ligands, 2,6-bis(benzimidazol-2-yl)pyridine (bzimpy, 1 ), 2,6-bis(l-methylbenzimidazol-2-yl)pyridine (mbzimpy, 2 ) 2,6-bis(benzothiazol-2-yl) pyridine (bzthpy, 3 ), 2,6-bis(benzoxazol-2-yl)pyridine (bzoxpy, 4 ), and 2,2′, 6′, 2″-terpyridyl (terpy, 5 ) is reported. Electronic and EPR spectra are consistent with the complexes [CuL]²⁺ having essentially tetragonal structure in solution, with the fourth coordination site in the plane of the ligand occupied by solvent. bzthpy and bzoxpy show smaller ligand-field splittings than bzimpy, mbzimpy, and terpy, and are easily decomplexed from the copper. Substitution of the coordinated solvent molecule in the plane of the ligand is observed with Cl^? and OH^? (provided that the ligand has no acidic protons) for all ligands except terpy. The reaction between [Cu(mbzimpy)]²⁺ and imidazole has been studied by potentiometric titration in MeCN/H₂O 1:1 and shows strong binding of the imidazole in the plane (log K = 4.5 at 25°), and also the formation of an imidazolate-bridged dinuclear species.     

The chloro-substituent enhances performance of 2,4-bis (imino)pyridylchromium catalysts yielding highly linear polyethylene

The five unsymmetrical 2-[1-(2,4-dibenzhydryl-6-chlorophenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridine compounds (aryl: 2,6-Me₂Ph L1 , 2,6-Et₂Ph L2 , 2,6-ⁱPr₂Ph L3 , 2,4,6-Me₃Ph L4 and 2,6-Et₂–4-MePh L5 ) were prepared and characterized with FT-IR and ¹H/¹³C NMR spectroscopy as well as elemental analysis. The treatment of L1 – L5 with CrCl₃·3THF affords the corresponding chromium chloride complexes ( Cr1 – Cr5 ) in excellent yields. The molecular structures of Cr2 and Cr3 characterized by X-ray diffraction show a distorted octahedral geometry with three nitrogen atoms and three chlorine atoms around the metal center. On activation with either MAO or MMAO, Cr1 – Cr5 collectively display high activity (up to 14.96 × 10⁶ g (PE) mol⁻¹ (Cr) h⁻¹ at 60 °C) affording highly linear polyethylene with low molecular weight distribution (M_w/M_n) ranging from 1.06 to 2.81. An in-depth catalytic evaluation of Cr1 was conducted in order to investigate how the cocatalyst type and its amount, reaction temperature and polymerization time affect the catalytic activities and polymer properties.     

Binuclear and polynuclear transition metal complexes with macrocyclic ligands. 5. Novel complexes of asymmetric polydentate macrocyclic Schiff bases. Step-by-step synthesis

Reactions of 2,6-bis(3-aminopropylaminocarbonyl)pyridine (1) with 4-tert-butyl-2,6-diformylphenol and 2,5-diformylpyrrole in the presence of Ba(ClO₄)₂ in EtOH afford barium complexes with asymmetric macrocyclic Schiff bases as soft and hard ligands. The reaction of compound 1 with Cu(OCOCMe₃)₂ involves closure of a tetrahydropyrimidine ring to give a mononuclear complex, which was structurally characterized by X-ray diffraction analysis.     

Synthesis,crystal structure and catalytic performance of bis (imino)pyridyl nickel complexes

Two new Ni(II) complexes of 2,6-bis[1-(2,6-diethylphenylimino)ethyl]pyridine (L¹), 2,6-bis[1-(4-methylphenylimino)ethyl]pyridine (L² ) have been synthesized and structurally characterized. Complex Ni(L¹)Cl₂?·?CH₃CN (1), exhibits a distorted trigonal bipyramidal geometry, whereas complex Ni(L¹)(CH₃CN)Cl₂ (2), is six-coordinate with a geometry that can best be described as distorted octahedral. The catalytic activities of complexes 1, 2, Ni{2,6-bis[1-(2,6-diisopropyl-phenylimino)ethyl]pyridine} Cl₂?·?CH₃CN (3), and Ni{2,6-bis[1-(2,6-dimethylphenylimino) ethyl]pyridine}Cl₂?·?CH₃CN (4), for ethylene polymerization were studied under activation with MAO.     

Unifying Molecular Weights of Highly Linear Polyethylene Waxes through Unsymmetrical 2,4-Bis(imino)pyridylchromium Chlorides

By dealing CrCl₃∙3THF with the corresponding ligands (L1–L5), an array of fluoro-substituted chromium (III) chlorides (Cr1–Cr5) bearing 2-[1-(2,4-dibenzhydryl-6-fluoro- phenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridine (aryl = 2,6-Me₂Ph Cr1, 2,6-Et₂Ph Cr2, 2,6-iPr₂Ph Cr3, 2,4,6-Me₃Ph Cr4, 2,6-Et₂-4-MePh Cr5) was synthesized in good yield and validated via Fourier Transform Infrared (FT-IR) spectroscopy and elemental analysis. Besides the routine characterizations, the single-crystal X-ray diffraction study revealed the solid-state structures of complexes Cr2 and Cr4 as the distorted-octahedral geometry around the chromium center. Activated by either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all the chromium catalysts exhibited high activities toward ethylene polymerization with the MMAO-promoted polymerizations far more productive than with MAO (20.14 × 10⁶ g (PE) mol⁻¹ (Cr) h⁻¹ vs. 10.03 × 10⁶ g (PE) mol⁻¹ (Cr) h⁻¹). In both cases, the resultant polyethylenes were found as highly linear polyethylene waxes with low molecular weights around 1–2 kg mol⁻¹ and narrow molecular weight distribution (MWD range: 1.68–2.25). In general, both the catalytic performance of the ortho-fluorinated chromium complexes and polymer properties have been the subject of a detailed investigation and proved to be highly dependent on the polymerization reaction parameters (including cocatalyst type and amount, reaction temperature, ethylene pressure and run time).     

Palladium(II) complexes of (t-butyl salicylidene) diphenyl disulfide diamine: synthesis,structure, spectral characterization and catalytic properties

The hexadentate N₂S₂O₂ donor ligand N,N’-bis(3,5-tert-butylsalicylidene) diphenyl disulfide-2,2’-diamine was synthesised by the condensation of 2-aminophenyl disulfide and 3,5-di-tert-butyl-2-hydroxybenzaldehyde and its molecular structure was confirmed by X-ray studies. One of the tert-butyl groups in the Schiff base has rotational disorder around the C–C bond with ratio 0.56:0.44. The palladium complexes were prepared by the direct reaction of PdCl₂(CH₃CN)₂ and Schiff base ligands N,N’-bis (5-tert-butylsalicylidene) diphenyl disulfide-2,2’-diamine and N,N’-bis(3,5-tert-butylsalicylidene) diphenyl disulfide-2,2’-diamine, respectively. The structure of the metal complexes was characterized by physico-chemical and spectroscopic methods. Palladium is in square-planar geometry bonded to imine nitrogen and phenolic O in both the complexes. The catalytic efficiency of the palladium complexes was evaluated in the cross-coupling reactions; Heck-Mizoroki reaction of iodobenzene and methyl acrylate and the Suzuki-Miyaura reaction of phenylboronic acid and iodobenzene, which gave low to moderate yields. Higher conversions were obtained for 2a as catalyst due to the increase in the number of bulky tertiary butyl groups in the structure.

     

An Improved Method for the Preparation of Tetra-Coordinate Platinum(II) Chloro-Complexes Containing Bidentate or Terdentate Ligands Starting from cis/trans-[PtCl2(SMe2)2]

An improved method for the preparation of differently charged chelate Pt(II) chloro-complexes is reported. All the complexes have been obtained rapidly and in high yield, by simply reacting equimolar amounts of cis/trans- dichlorobis(dimethylsulphide)platinum(II) with the chelate ligand in an appropriate solvent (CH₂Cl₂, MeOH or H₂O). The ligands chosen were: 1,2-bis(diphenylphosphino)ethane (P—P), pyridine–2-carboxylate (N—O⁻), 2-[(methylthio)methyl]pyridine (N—S), 1,10-phenanthroline (N—N), bis(2-pyridylmethyl)sulphide (N—S—N), di-(2-picolyl)amine (N—N—N), pyridine-2,6-dicarboxylate (O—N—O²⁻) and 2,6-bis(methylthiomethyl)pyridine (S—N—S).     

Stereospecific Formation of Pentaamine Complexes of Co(III) with Terdentate 2,6-Bis(aminomethyl)pyridine

Some pentaamine complexes of Co(III) with 2,6-bis(aminomethyl)pyridine (bamp), a diamine ligand (or two ammonia ligands) and one unidentate ligand have been prepared (Table 1). In all these species, bamp remains coordinated meridionally under a variety of conditions as shown by ¹H- and ¹³C-NMR. spectroscopy and correlations by stereoretentive reaction cycles. The rates of amine proton exchange and of spontaneous aquation, Hg²⁺-induced aquation and base hydrolysis of some chloropentaamine complexes have been determined. They essentially follow the patterns observed for complexes with purely aliphatic amine ligands; the presence of a pyridine donor in these complexes does not suggest deviations from the mechanistic schemes usually proposed for the solvolytic reactions investigated.     

Iron(II) Complexes of Chiral Tridentate Nitrogen Donors and their Application in Catalytic Hydrosilylation Reactions

Enantiomerically pure, C₂-symmetric 2,6-bis(pyrazol-3-yl) pyridine ligands were obtained by treatment of diethyl-2,6-pyridinedicarbonate with (1R,4R)-(+)-camphor in the presence of NaH followed by ring closure with hydrazine. After twofold N-alkylation at the pyrazole rings, the addition of iron(II) chloride led to the according pentacoordinate dichloridoiron(II) complexes. All intermediates of the ligand synthesis, the ligands bearing NCH₃ and NCH₂C₆H₅ groups and the derived iron(II) complexes were structurally characterized by means of X-ray structure analysis. In-situ reaction with iron(II) carboxylates resulted in the formation of iron(II) carboxylate complexes, which turned out to be highly active in the hydrosilylation of acetophenone. However, even at room temperature, the enantiomeric excess of the product 1-phenylethanol is poor. ⁵⁷Fe Mössbauer spectroscopy gave an insight into the species formed during catalysis.     

Multidentate bis(pyrazolylmethyl)pyridine ligands: coordination chemistry and binding properties with zinc(II) and cadmium(II) cations

The coordination chemistry and cationic binding properties of 2,6-bis(pyrazol-1-ylmethyl)pyridine (L1), 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine (L2), and 2,6-bis(3,5-ditertbutylpyrazol-1-ylmethyl)pyridine (L3) with zinc(II) and cadmium(II) have been investigated. Reactions of L2 with zinc(II) and cadmium(II) nitrate or chloride salts produced monometallic complexes [Zn(NO₃)₂(L2)] (1), [ZnCl₂(L2)] (2), [Cd(NO₃)₂(L2)] (3), and [CdCl₂(L2)] (4). Solid state structures of 1 and 3 confirmed that L2 binds in a tridentate mode. While the nitrates in the zinc complex (1) adopt monodentate binding fashion, in cadmium complex (3), they exhibit bidentate mode. L1–L3 show binding efficiencies of 99% for zinc(II), 60% for lead(II), and 30% for cadmium(II) cations from aqueous solutions of the metal ions. Theoretical studies using Density Functional Theory were consistent with the observed extraction results.     

Synthesis and X-ray crystal structures of N,N,N′,N′-tetraalkylpyridine-2,6-dithiocarboxamides (S-dapt) complexes of cobalt(II) and nickel(II)

Reactions of anhydrous CoX₂ (X?=?Br^?, SCN^?) and Ni(ClO₄)₂ with N,N,N′,N′-tetraisobutylpyridine-2,6-dithiocarboxamides (S-dbpt), N,N,N′,N′-tetraisopropyl pyridine-2,6-dithiocarboxamides (S-dppt), and N,N,N′,N′-tetraethylpyridine-2,6-dithiocarboxamides (S-dept) lead to the formation of [Co(S-dbpt)Br₂] (1), [Co(S-dppt)(SCN)₂] (2), and [Ni(S-dept)₂]·(ClO₄)₂·H₂O (3), respectively. The X-ray crystal structures of the three S-dapt ligands and three complexes along with spectroscopic analyzes are presented. The molecular structure investigations of the S-dapt ligands show that the thiamide planes are twisted with respect to the pyridine ring, which is more in the case of phenyl groups. The structures of the Co(II) complexes reveal that an increase in steric crowding on the amide side arms of the ligands has no substantial effect on the geometry adopted by the corresponding complexes. The Co(II) gives only 1?:?1 five-coordinate, ion-paired complexes with a distorted square pyramidal geometry. Ni(II), on the other hand, prefers an octahedral geometry with 1?:?2 metal–ligand ratio. The coordination behavior of S-dapt has been compared to the analogous oxo(O-daap) ligands. Lesser propensity of S atom to get involved in H-bonding interactions ensures an S-N-S type of tridentate coordination by S-dapt.     

Tripodale Bis(2,6‐iminophosphoranyl)pyridin‐Liganden: Eisen‐ und Cobalt‐Komplexe mit Potential in der Ethen‐Polymerisation

Tripodal Bis(2,6‐iminophosphoranyl)pyridine Ligands: Iron and Cobalt Complexes with a Potential in Ethene Polymerisation By Staudinger Reaction of bis‐2,6‐diphenylphosphanyl‐pyridine with aryl‐, alkyl‐ and silylazides tripodal ligands L = 2,6‐(Ph₂P=NR)₂C₅H₃N (R = Ph 1 a , Mes 1 b , Ad  1 c , SiMe₃ 1 d ) are synthesized. The reaction of ligand 1 b  with equimolar amounts of [CoCl₂(THF)₂] and [FeCl₂(THF)_1.5] in THF does not lead to the expected neutral complexes [(k³‐L)MCl₂] but to coordination compounds of the composition L₂(CoCl₂)₃ ( 2 a ) und L(FeCl₂)₂ ( 3 ). By using acetonitrile as solvent or by crystallisation of 2 a from hot acetonitrile the cationic complex [(k³‐L)CoCl(MeCN)]Cl ( 2 b ) is formed as a second product. The molecular structure 2 b has been characterized by an X‐ray single crystal structure analysis (triclinic, P1, Z = 2, a = 1299.8(1), b = 1488.8(2), c = 1674.2(2) pm, α = 82.911(13)°, β = 76.715(12)°, γ = 72.758(11)°). A preliminary test with 3 shows, that coordination compounds of the ligand system introduced here have potential as catalysts in methyl alumoxane mediated ethene polymerisation.     

Template Assembling of the Pentadentate Redox-Active Ligand in the Coordination Sphere of Tin(IV)

Heptacoordinated tin complexes with pentadentate redox-active ligands containing the diiminopyridine fragment combined with two sterically hindered phenolate coordination centers, LSn-Cl₂ and L'SnCl₂ (L and L' are dianions of deprotonated 2,6-bis[2,4-di-tert-butyl-6-(methylidenylamino) phenol]pyridine and 2,6-bis[2,4-di-tert-butyl-6-(ethylidenylamino)phenol]pyridine, respectively), are synthesized. The molecular and electronic structures of the synthesized compounds were studied by X-ray diffraction analysis (for complex I, CIF file CCDC no. 1557838), a set of spectral methods, and quantum-chemical calculations. The redox properties of the obtained complexes are characterized by cyclic voltammetry.     

Crown ether complexes of sodium and potassium 2-(benzotriazol-2-yl)phenolates: Synthesis and catalysis towards the ring-opening polymerization of rac-lactide

Crown ether complexes of sodium and potassium 2-(benzotriazol-2-yl)phenolates were synthesized and characterized. In the presence of BnOH these complexes are highly active catalysts for the controlled ring-opening polymerization of rac-lactide. The polymerizations are iso-selective and the P_m of polylactide reached 0.77 when the polymerization was performed in toluene at ?60°C; whereas heterorich-polylactide was obtained when the polymerization was carried out in CH₂Cl₂ or THF.     

Aminophosphine ligands: synthesis,coordination chemistry,and activity of their palladium(II) complexes in Heck and Suzuki cross-coupling reactions

The reaction of 4-aminodiphenylamine or 2-aminofluorene with two equivalents of PPh₂Cl in the presence of Et₃N gives new bis(diphenylphosphino)amines N,N-bis(diphenylphosphino)-4-aminodiphenylamine 1 and N,N-bis(diphenylphosphino)-2-aminofluorene 2 in good yields. Oxidation of 1 or 2 with hydrogen peroxide, elemental sulfur or gray selenium affords the corresponding chalcogen derivatives. The palladium and platinum complexes of these P–N–P donor ligands were prepared by the reaction of the bis(phosphino)amines with MCl₂(cod) (M = Pd or Pt, cod = cycloocta-1,5-diene). All the new compounds have been characterized by analytical and spectroscopic methods, including ¹H-³¹P NMR, ¹H-¹³C HETCOR, or ¹H-¹H COSY correlation experiments. The Pd(II) complexes were investigated as catalysts in the Suzuki and Heck reactions; both showed good catalytic activity affording high yields of the desired products.     

M(CO)4 (M = Mo or W) complexes of 2,6-bis(methylthiomethyl)pyridine (L) and 2,6-bis(p-tolylthiomethyl)pyridine (L): a dynamic NMR study

The hybrid S/N/S donor ligands 2,6-bis(methylthiomethyl)pyridine (L¹) and 2,6-bis(p-tolylthiomethyl)pyridine (L²) react with the [M(CO)₅(THF)] (M = Mo or W) compounds to form complexes of general formula [M(CO)₄L] (M = Mo, L = L²; M = W, L = L¹ or L²), where both L¹ and L² act in a S/N bidentate chelate fashion. In solution, these complexes undergo three fluxional processes, viz. inversion at the coordinated S atom, S¹–S² switching, and combined inversion and S¹–S² switching, leading to an interconversion of the four possible permutational isomers. Energy barriers for all three processes have been evaluated by standard one-dimensional band-shape analysis techniques. The mechanism of the S¹–S² switch is discussed.