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Amidate complexes of the group 4 metals : sythesis, reactivity, and hydroamination catalysis Thomson, Robert Kenneth

Abstract

A series of bidentate amidate ligands with variable groups R' and R" abbreviated by [R"(NO)R'] and adamantyl substituted tetradentate amidate ligands abbreviated by Ad[0₂N₂] were utilized as ancillaries for Ti, Zr, and Hf. Protonolysis routes into homoleptic amidate complexes, tris(amidate) mono(amido), bis(amidate) bis(amido), and bis(amidate) dibenzyl complexes are high yielding when performed with tetrakis(amido) and tetrabenzyl group 4 starting materials. Many of these complexes have been characterized in both the solid-state and in the solution phase, where in the latter case these complexes are fluxional and undergo exchange processes. Multiple geometric isomers are possible with the mixed N,0 chelate provided by the amidate ligands, and geometric isomerization of bis(amidate) bis(amido) complexes has been examined through X-ray crystallographic and density functional theory (DFT) calculations. Isomerization is dictated largely by the steric bulk present at the N of the amidate ligands, and is proposed to proceed through a K²-K¹-K² ligand isomerization mechanism, which is supported by crystallographic evidence of K¹-bound amidate ligands. The amidate ligand system binds to these metals in a largely electrostatic fashion, with poor orbital overlap, generating highly electrophilic metal centers. The bis(amidate) dibenzyl complexes of Zr and Hf are reactive towards insertion, abstraction, and protonolysis. Insertion of isocyanides into the Zr-C bonds of [DMP(NO) tBu]₂Zr(CH₂Ph₂ results in the formation of ƞ₂-iminoacyl complexes, which can either undergo thermally induced C=C coupling to generate an enediamido complex (aryl isocyanides), or rearrange to generate a bis(amidate) bis(vinylamido) complex (alkyl isocyanides). Benzyl abstraction to generate cationic Zr bis(amidate) benzyl complexes is also possible through reaction with [Ph₃C][B(C₆F₅)4] or B(C₆F₅)₃ Terminal imido complexes with novel pyramidal geometries are generated through protonolysis of bis(amidate) bis(amido) Ti and Zr complexes with primary aryl amines. DFT calculations demonstrate the existence of a Zr⁻₌N triple bond for these complexes. Dimeric imido complexes have been characterized in the solid state, but are not maintained in solution. Cycloaddition reactions of the terminal Zr imido complexes with C=0 bonds result in the formation of proposed oxo complexes and organic metathesis products. Catalytic aminoalkene cyclohydroamination has also been realized with these complexes, generating N-heterocyclic products. A series of kinetic and labeling studies support an imido-cycloaddition mechanism for catalytic cyclohydroamination of primary aminoalkenes with neutral bis(amidate) Ti and Zr precatalysts. The intermediate Ti imido complex, K²-[Dipp(NO)tBu-K¹_[DiPP(No) tBu]Ti=NCH₂CPh₂CH₂CH=CH₂(NHMe₂), has been isolated and characterized in the solid-state and in solution. Amine stabilized imido complexes of this type are invoked as the resting state for the catalytic reaction, and solution phase data support a chair-like geometry, where the alkene is coordinated to the metal center. A diastereoselectivity study supports this proposed solution structure. Eyring and Arrhenius parameters, as well as isolation of a 7-coordinate model imido complex, support a seven-coordinate transition state for the rate-determining metallacycle protonolysis reaction. In contrast, secondary aminoalkene hydroamination catalysis with cationic Zr benzyl complexes is proposed to proceed through a σ-bond insertion mechanism. Proton loss from cationic Zr amido complexes to generate imido species is proposed with primary aminoalkenes, and the resultant neutral imido complexes can catalyze the cyclization of these substrates by the aforementioned imido-cycloaddition mechanism. The ability of the amidate ligand system to promote both mechanisms is unique in the field of alkene hydroamination catalysis.

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