Functionalization of Hafnium Oxamidide Complexes Prepared from CO-Induced N₂ Cleavage

Abstract

Functionalization of the nitrogen atoms in the hafnocene oxamidide complexes [Me(2)Si(η(5)-C(5)Me(4))(η(5)-C(5)H(3)-3-(t)Bu)Hf](2)(N(2)C(2)O(2)) and [(η(5)-C(5)Me(4)H)(2)Hf](2)(N(2)C(2)O(2)), prepared from CO-induced N(2) bond cleavage, was explored by cycloaddition and by formal 1,2-addition chemistry. The ansa-hafnocene variant, [Me(2)Si(η(5)-C(5)Me(4))(η(5)-C(5)H(3)-3-(t)Bu)Hf](2)(N(2)C(2)O(2)), undergoes facile cycloaddition with heterocumulenes such as (t)BuNCO and CO(2) to form new N-C and Hf-O bonds. Both products were crystallographically characterized, and the latter reaction demonstrates that an organic ligand can be synthesized from three abundant and often inert small molecules: N(2), CO, and CO(2). Treatment of [Me(2)Si(η(5)-C(5)Me(4))(η(5)-C(5)H(3)-3-(t)Bu)Hf](2)(N(2)C(2)O(2)) with I(2) yielded the monomeric iodohafnocene isocyanate, Me(2)Si(η(5)-C(5)Me(4))(η(5)-C(5)H(3)-3-(t)Bu)Hf(I)(NCO), demonstrating that C-C bond formation is reversible. Alkylation of the oxamidide ligand in [(η(5)-C(5)Me(4)H)(2)Hf](2)(N(2)C(2)O(2)) was explored due to the high symmetry of the complex. A host of sequential 1,2-addition reactions with various alkyl halides was discovered and both N- and N,N'-alkylated products were obtained. Treatment with Brønsted acids such as HCl or ethanol liberates the free oxamides, H(R(1))NC(O)C(O)N(R(2))H, which are useful precursors for N,N'-diamines, N-heterocyclic carbenes, and other heterocycles. Oxamidide functionalization in [(η(5)-C(5)Me(4)H)(2)Hf](2)(N(2)C(2)O(2)) was also accomplished with silanes and terminal alkynes, resulting in additional N-Si and N-H bond formation, respectively.

Photochemically Induced Reductive Elimination as a Route to a Zirconocene Complex with a Strongly Activated N2 Ligand

“…[1] Zirconocene and hafnocene complexes with strongly activated dinitrogen ligands are exemplary offering diverse reactivity ranging from N 2 hydrogenation to ammonia [2] to the synthesis of organic molecules including substituted oxamides, [3] formamide, [4] unsymmetrical ureas, [5] and carbodiimides [5] using CO-induced dinitrogen cleavage as the N À C bond forming strategy. [1] Zirconocene and hafnocene complexes with strongly activated dinitrogen ligands are exemplary offering diverse reactivity ranging from N 2 hydrogenation to ammonia [2] to the synthesis of organic molecules including substituted oxamides, [3] formamide, [4] unsymmetrical ureas, [5] and carbodiimides [5] using CO-induced dinitrogen cleavage as the N À C bond forming strategy.…”

“…[1] Zirconocene and hafnocene complexes with strongly activated dinitrogen ligands are exemplary offering diverse reactivity ranging from N 2 hydrogenation to ammonia [2] to the synthesis of organic molecules including substituted oxamides, [3] formamide, [4] unsymmetrical ureas, [5] and carbodiimides [5] using CO-induced dinitrogen cleavage as the N À C bond forming strategy. Mechanistic studies support the intermediacy of zirconocene arene complexes that in the absence of sufficient dinitrogen promote C À H activation or undergo comproportion to formally Zr III complexes.…”

Insights into the Cascade Reaction of CO and Heteroallenes Mediated by Dinitrogen Hafnocene Complexes: The Indirect Effect of Nitride's Nucleophilicity

“…[2] In particular,c arbonylation of N 2 to form nitrogencarbon bonds is possible, but rare, the first example of which used ansa-hafnocene complexes,a sr eported by Chirik and coworkers. [3] Such complexes can effect the formation of various importantN -containingo rganic ligands,s uch as oxamidides [4] and m-formamides. [5] The formation of an itride intermediate in this type of reaction was also confirmed by kinetics studies reported by the same group.…”

Redox‐Induced N2 Hapticity Switching in Zirconocene Dinitrogen Complexes

“…[1] At the core of this challenge is understanding the interaction of the N 2 molecule with transitionmetal complexes. [8] Although early transition-metal dinitrogen complexes with side-on N 2 coordination are now the most prevalent and dominate the reactivity landscape, this hapticity is by no means a prerequisite for functionalization chemistry. [8] Although early transition-metal dinitrogen complexes with side-on N 2 coordination are now the most prevalent and dominate the reactivity landscape, this hapticity is by no means a prerequisite for functionalization chemistry.…”

“…[16] Attempts to synthesize the neutral dinitrogen complex by reduction with 0.5 % Na(Hg), the typical procedure to prepare such compounds, [8][9][10][11][12]16] produced an intractable mixture of paramagnetic and diamagnetic zirconium products. [16] Attempts to synthesize the neutral dinitrogen complex by reduction with 0.5 % Na(Hg), the typical procedure to prepare such compounds, [8][9][10][11][12]16] produced an intractable mixture of paramagnetic and diamagnetic zirconium products.…”