Organonickel ( II ) Complexes with Anionic Tridentate 1 , 3-Bis ( azolylmethyl ) phenyl Ligands . Synthesis , Structural Characterization and Catalytic Behavior

A reacao de 2-bromo-1, 3-bis(bromometil)benzeno com 3, 5-dimetilpirazol e 1H-indazol produz os ligantes tridentados 2-bromo-1, 3-bis(3, 5-dimetilpirazol-1-ilmetil)benzeno (1) e 2-bromo-1, 3-bis(indazol-2-ilmetil)benzeno (2). Estes compostos reagem com [Ni(cod)2] em tetraidrofurano (thf) para formar os complexos de adicao oxidativa [NiBr{1, 3-bis(azolilmetil)fenil-N, C, N}], azol = 3, 5-dimetilpirazol (3) e indazol (4), os quais foram isolados em bons rendimentos como solidos amarelos estaveis e caracterizados por meio de analise elementar, espectroscopia de absorcao no infravermelho com transformada de Fourier (FTIR), espectrometria de massa e ressonância magnetica nuclear (NMR). Adicionalmente, as estruturas moleculares de 2 e 4 foram determinadas por difratometria de raios X de monocristal. O complexo 4 foi testado como catalisador na reacao de polimerizacao de etileno

][18][19][20][21][22] Vol.22, No. 9, 2011   Recently we reported the synthesis and properties o f t h e p i n c e r-t y p e p a l l a d i u m ( I I ) c o m p l e x e s [PdCl{bis(azolylmethyl)tolyl-N,C,N}] showing that they are active in the polymerization of ethylene. 23Following our interest in these types of compounds, we report in this paper the synthesis and properties of new nickel(II) bromide complexes bearing bis(azolylmethyl)phenyl type ligands, acting as anionic tridentate (N,C,N) donors.The crystal structure of the ligand 2-bromo-1,3-bis(indazol-2-ylmethyl) benzene and the complex {bis(indazol-2-ylmethyl)phenyl} bromonickel(II), determined by single crystal X-ray diffraction analysis, are also reported.In this work the activity of the above mentioned cyclometalated nickel complex for ethylene polymerization is also described.

General remarks
The reactions were carried out under an atmosphere of purified nitrogen.All reagent-grade solvents were dried, distilled and stored under a nitrogen atmosphere.The starting compound 2-bromo-1,3-bis(bromomethyl)benzene was synthesized according to literature procedures. 24,5-Dimethylpyrazole, 1H-indazole and [Ni(cod) 2 ] were purchased from Aldrich and used without further purification.Methylaluminoxane (MAO, 10 wt.% in toluene) was purchased from Witco and used as received.Elemental analyses (C, H, N) were carried out using a Fisons EA 1108 CHNS-O microanalyzer.FTIR spectra were recorded throughout the 4000-250 cm -1 region, with samples prepared as KBr discs on a Bruker Vector-22 spectrophotometer.The NMR spectra were recorded on a Bruker AC-200P and Avance-400 spectrometers and the chemical shifts are reported in ppm relative to SiMe 4 ( 1 H, 13 C).Mass spectra were obtained on a Micromass Quattro LC-Z electrospray mass spectrometer.Melting point was determined using an electrothermal melting point apparatus in open capillary tubes and are uncorrected.

Crystal structure determination for compounds 2 and 4
Crystals of 2 suitable for X-ray diffraction analysis were grown by slow diffusion of diethyl ether into a dichloromethane solution.Crystal data were collected with a Nonius KappaCCD diffractometer.The structure was solved using SHELXS-97 and the refinement used SHELXL-97. 25Crystals of 4 were grown by slow diffusion of diethyl ether into an acetone solution of the complex.Intensity data were collected with a Bruker SMART APEX diffractometer.The structure was solved using XS in SHELXTL by means of direct methods, and completed by Fourier difference synthesis.Refinement until convergence was achieved using XL SHELXTL. 26Crystallographic details are listed in Table 1.

Ethylene polymerization assays
Polymerizations were carried out by charging a 100 mL Parr stainless steel autoclave (in a glove-box under an inert nitrogen atmosphere) with toluene (21 mL), the desired amount of cocatalyst (methylaluminoxane, MAO, 10 wt.% in toluene solution) and complex 4 (4.2 mmol).The autoclave was sealed, brought off the glove-box and connected to a mechanical stirrer, a temperature controller and an ethylene consumption control.The reaction was quenched by the addition of HCl-methanol (15 mL, 20% v/v).The nickel complex was not recovered after reaction, because it decomposes upon addition of the HCl-methanol mixture.The polymer was isolated by filtration, washed with acetone and dried overnight at room temperature.The polymers were characterized by infrared spectroscopy (FTIR), gel permeation chromatography (GPC) and differential scanning calorimetry (DSC).GPC analysis was performed on a Polymer Laboratories high temperature GPC system (model PL 220) equipped with a refractive index detector.Samples were run at 150 °C in spectrophotometric grade 1,2,4-trichlorobenzene (TCB), stabilized with butylated hydroxytoluene, BHT (0.5 g BHT/4L solvent).Molecular weights were calculated by Vol.22, No. 9, 2011 using a universal calibration from narrow polystyrene standards in the molecular weight range of 580 to 7.5 million g mol -1 .Mark-Houwink parameters of a = 0.7 and k = 47.7 were utilized to correct for polyethylene.Polymer melting points (Tm) were determined by differential scanning calorimetry using a Mettler Toledo DSC 821 Calorimetry System Heating Scan at 10 °C min -1 from 25 °C to 200 °C.Reported results are those obtained in the second scan.

Results and Discussion
The ligand 2-bromo-1,3-bis(3,5-dimethylpirazol-1ylmethyl)benzene (1) was prepared by phase-transfer catalyzed reaction of 2-bromo-1,3-bis(bromomethyl) benzene with 3,5-dimethylpyrazole, modifying a previously reported method. 16The ligand 2-bromo-1,3-bis(indazol-2ylmethyl)benzene (2), in its turn, was obtained by reaction of 2-bromo-1,3-bis(bromomethyl)benzene with 1H-indazole in refluxing toluene in the presence of Et 3 N.Interestingly, the reaction with 1H-indazole gives the isomer with the indazolyl groups bonded to the carbon spacer through the N2 atom because the electron pair located at this nitrogen becomes more reactive than the corresponding one to N1 (Scheme 1). 27The structures of the ligands were confirmed on the basis of 1 H and 13 C NMR data and elemental analysis.The 1 H and 13 C chemical shifts were assigned with the aid of 1 H-1 H COSY, DEPT, 1 H- 13 C HMQC, HMBC and NOESY experiments.Moreover, an X-ray diffraction study was undertaken in order to obtain a complete characterization.For 2, crystals suitable for structure determination were grown by slow diffusion of diethyl ether into a dichloromethane solution of the compound.An ORTEP view of the molecule is presented in Figure 3, and selected bond distances and bond angles are listed in the caption.
The benzene and azolyl rings are both planar and the azolyl groups are connected to the methylene spacers through the N2 atoms.All bonds distances and bonds angles fall within the expected ranges.
Compounds 1 and 2 react at low temperature (-78 ºC) in thf with bis(1,5-cyclooctadiene)nickel(0), [Ni(cod) 2 ], to give the oxidative addition complexes of general formula [Ni(N,C,N)Br] ( 3, 4).The complexes are isolated as yellow solids, stable in air at room temperature and soluble in acetone, dichloromethane, acetonitrile and chloroform.They were fully characterized by elemental analysis and FTIR, NMR and mass spectroscopies.The NMR spectra of the complexes are consistent with the proposed structure and exhibited the phenyl and azolyl ring resonances, as well as the methylene spacer in the required intensity ratios (see experimental).
In order to confirm the cyclometalated structure of the nickel compounds, a single crystal X-ray diffraction analysis of complex 4 was undertaken.A perspective view and the atom labeling of the complex are shown in Figure 4, along with a list of selected bond distances and bond angles.In the complex, the nickel atom is four-coordinate with the tridentate ligand bonded to the metal center through the C(ipso) and the two nitrogen atoms in trans positions; a bromide ligand completes the coordination sphere.The complex has an approximately square-planar coordination involving the N,C,N,Br donor set [deviation of the plane: Ni (0.0069 Å), Br (0.0046 Å), C14 (0.0067 Å), N1 (-0.0090Å) and N2 (-0.0091Å).he crystal structure shows that the fused six-membered metallocycles acquired a boat conformation.Interestingly, in this conformation the two hydrogen atoms bonded to the methylene carbons are diastereotopic, giving different signals in the 1 H NMR spectra.In fact, in CD 3 CN at -20 ºC the spectra show two doublets corresponding to the expected AB pattern: complex 3, d 4.9 (d, 2 J(HH) 14 Hz) and 6.3 (d, 2 J(HH) 14 Hz) ppm; complex 4, d 5.7 (d, 2 J(HH) 14 Hz) and 6.6 (d, 2 J(HH) 14 Hz).However,  variable-temperature experiments reveal the existence of an internal dynamic process.Thus, when the sample was brought to room temperature (25 ºC), the spectra showed two broad singlets centered at d 5.0 and 6.3 ppm (complex 3) and d 5.8 and 6.6 ppm (complex 4).These results indicate that the environment of the hydrogen atoms changes at higher temperatures due to an inversion of the boat conformation, which is fast on the NMR time scale.Similar results have been described for some related palladium(II) complexes. 21,28reatment of the nickel complexes with silver methanesulfonate in 1:1 molar ratio in acetone or acetone-water solution results in the formation of the non-soluble silver bromide.However, all the attempts to isolate the corresponding cationic solvated complexes were unsuccessful.In all cases white solids, which were very insoluble in organic solvents, were obtained and not characterized further.

The
In the last few years we have been interested in the synthesis of early and late transition metal complexes containing tridentate pincer ligands, which can activate ethylene to form polyethylene.In recent polymerization studies we found that complexes containing indazolyl groups connected to a central aryl or pyridine ring are more active than the 3,5-pyrazolyl derivatives.For this reason we did not test complex 3 in this reaction. 23,29,30Thus, the behavior of the nickel complex 4 as an initiator for ethylene polymerization under mild conditions was evaluated and compared to the activity of some related palladium complexes bearing the ligands 3,5-bis(azolylmethyl) toluene. 23The catalytic activity of the complex in the presence of MAO and the properties of the polymers obtained in these essays are summarized in Table 2.
As far we know, complex 4 is the first cyclometalated square-planar nickel complex that is active for the polymerization of ethylene to high molecular weight.As expected, the catalytic activity of 4 increases with an increase in the Al MAO /Ni ratio; however, the complex presents lower activity compared with those shown by the related [PdCl{3,5-bis(indazol-1-ylmethyl)tolyl-N,C,N}] complex under similar conditions (41 kg PE (mol Pd) -1 bar -1 h -1 ; Al/Pd = 2500). 23Due to the modest activity found for this complex we evaluated the catalytic activity only at 60 ºC.We suggest that the polymerization reaction catalyzed by 4 occurs by a mechanism similar to those proposed for square-planar cyclopalladate complexes derived from phenylhydrazones (C,N,N-donor), 31 where the first step is the replacement of the halide by a methyl group, increasing the electron density of the metal atom, followed by cleavage of the M-N bond to generate a vacancy at the metal center (Scheme 3).The lower activity of this complex compared with the similar palladium compound is probably due to the shorter metal-nitrogen bond distances expected for a first row transition metal complex compared with the related second row complex. 9Moreover, in the active species the lone pair of the free nitrogen possibly causes an acid-base interaction with MAO, which was previously demonstrated in metal complexes bearing ligands with basic functionalities. 32,33PC, DSC and FTIR techniques were employed to characterize the polymers.GPC analysis shows high molecular weights (M w ) with a monomodal molecular  weight distribution (M w /M n ) that corresponds to a single-site catalyst.As to the thermal properties, DSC analysis showed melting points in the 131-132 ºC range, indicating the absence of branching in the polymer chain, as confirmed by the FTIR spectra, which show the typical linear polyethylene signals [ν(CH), 2918 and 2850 cm -1 ; d (CH), 1473 and 719 cm -1 ). 3

Figure 1 .Figure 2 .
Figure 1.Examples of nickel pincer complexes with five-membered chelate rings.R = Me, Et or i Pr; X = Cl, Br or O 2 CH.