Complexes NCN de Ni(II) et Ni(III) : synthèse, caractérisation et rôle dans le mécanisme de couplage C-O, C-N et C-halogènes
This thesis describes the synthesis, characterization and reactivity of NCN type Ni(II) and Ni(III) pincer complexes. The thesis is divided in three parts. The first discusses the synthesis of a new family of Ni(II) complexes based on the 1,3-bis(pyrazole),5-R-C6H3 ligand (R = H, OMe). Ligand synthesis is executed by Ullman coupling between pyrazole and 1,3-diiodobenzene or 1,3-dibromo,5-methoxy-benzene. Refluxing ligands with nickel precursor {[NiBr2(iPrCN)]}n in the presence of triethylamine leads to the formation of pincer complexes (NCNpz)Ni(II)Br and (MeO-NCNpz)Ni(II)Br by C-H activation (pz = pyrazole). The alkoxide-type Ni(II) complex (NCNpz)Ni(BHT) (BHT = 2,6-t-Bu2-4-Me-OC6H2) is isolated from the reaction between (NCNpz)Ni(II)Br and NaBHT. Oxidation attempts did not lead to the characterization of an authentic Ni(III) species. Reacting bromo complexes with H2O2 leads to the formation of Br-NC(OH)Npz and NC(OH)Npz when starting from (NCNpz)Ni(II)Br and MeO-NC(OH)Npz when starting from (MeO-NCNpz)Ni(II)Br. The aerobic reaction of (NCNpz)Ni(II)Br with different alcohols and amines leads to the functionalization products of the ligand NC(OR)Npz and NC(NR2)Npz.  The second theme is the functionalization of the Cispo-Ni(III) bond in (NCN)Ni(III)Br2. The reaction of (NCN)Ni(III)Br2 with H2O, various alcohols, amines and strong acids leads to the formation of C-O, C-N and C-halogen bonds. A kinetic study shows that Ni(III) displays first order behavior excluding the possibility of a disproportionation reaction between 2 Ni(III) which would lead to a Ni(IV) species. Measurement of an inverse kinetic isotopic effect of 0.47 for the reaction with MeOH/CD3OD indicates a proton transfer arising from one or several pre-equilibria occuring before the rate determining step. The observation of yields of less than 50% for the functionalization reaction indicates a comproportionation reaction between the Ni(I) resulting from the reductive elimination and the starting Ni(III). These observations allow proposition of a mechanism for C-O, C-N and C-halogen bond formation from the (NCN)Ni(III)Br2 complex.  The third part reports the synthesis of new cationic and dicationic NiI(III)complexes starting from (NCN)Ni(III)Br2. In the presence of AgSbF6 and acetonitrile, (NCN)Ni(III)Br2 reacts to give dicationic complex [(NCN)Ni(III)(MeCN)3]2+ in 83% yield. The reaction between (NCN)Ni(III)Br2 and [(NCN)Ni(III)(MeCN)3]2+ leads to the formation of the cationic [(NCN)Ni(III)(Br)(MeCN)]+ with a yield of 84%. These complexes are characterized by X-ray diffraction and electron paramagnetic resonance. Reacting these complexes with MeOH leads to the formation of the functionalized product NC(OMe)N and a cationic divalent nickel complex suggesting a comproportionation between [(NCN)Ni(III)(Br)(MeCN)]+ or [(NCN )Ni(III)(MeCN)3]2+ with the NiI generated by the formation of the C-O bond. Reacting [(NCN)Ni(III)(Br)(MeCN)]+ or [(NCN)Ni(III)(MeCN)3]2+ with MeNH2 leads to the formation of the functionalized product NC(NHMe)N with typical yields of 30%. These low yields are explained by the ability of the ionized Ni(III) complexes to oxidize MeNH2. A comprehensive DFT study highlights the reaction mechanism steps for C-O and C-N bond formation. The first step is the coordination of the substrate with the trivalent metal center. Then deprotonation of MeOH is provided by the pincer’s amine moiety while deprotonation of MeNH2 arises from an outer sphere process. Reductive elimination was found to be the limiting step for C-O and C-N bond formation. The value of the isotopic kinetic effect of 0.62 is also rationalized by a DFT vibrational analysis.
Ni(II) Ni(III) Ni(IV) NCN Nickel Couplage C-O Couplage C-N Comproportionation Disproportionation Pincer complexes High-valent NCN pincer complexes Functionalization C-O coupling C-N coupling C-halogen coupling Reductive elimination C-H activation DFT