Molecular nitrogen (N(2)) and carbon monoxide (CO) have the two strongest bonds in chemistry and present significant challenges in developing new transformations that exploit these two abundant feedstocks. At the core of this objective is the discovery of transition-metal compounds that promote the six-electron reductive cleavage of N(2) at ambient temperature and pressure and also promote new nitrogen-element bond formation. Here we show that an organometallic hafnium compound induces N(2) cleavage on the addition of CO, with a simultaneous assembly of new nitrogen-carbon and carbon-carbon bonds. Subsequent addition of a weak acid liberates oxamide, which demonstrates that an important agrochemical can be synthesized directly from N(2) and CO. These studies introduce an alternative paradigm for N(2) cleavage and functionalization in which the six-electron reductive cleavage is promoted by both the transition metal and the incoming ligand, CO, used for the new bond formations.
The scope of CO-induced N(2) cleavage in a series of zirconocene and hafnocene complexes containing activated, side-on bound dinitrogen ligands has been studied. In each case, bridging oxamidide ligands, [N(2)C(2)O(2)](4-), were formed from N-N bond cleavage coupled to N-C and C-C bond assembly. For the zirconium examples, [(eta(5)-C(5)Me(4)H)(2)Zr](2)(mu(2),eta(2),eta(2)-N(2)) and [Me(2)Si(eta(5)-C(5)Me(4))(eta(5)-C(5)H(3)-3-(t)Bu)Zr](2)(mu(2),eta(2),eta(2)-N(2)), dinitrogen loss became competitive with N(2) carbonylation, and significant quantities of the zirconocene dicarbonyl accompanied oxamidide formation. In contrast, the hafnocene complex [(eta(5)-C(5)Me(4)H)(2)Hf](2)(mu(2),eta(2),eta(2)-N(2)) underwent clean carbonylative dinitrogen cleavage with no evidence of N(2) loss. CO-induced N(2) cleavage was also coupled to N-H bond formation by hydrogenation and C-H bond activation, as carbonylation of the zirconocene and hafnocene dinitrogen complexes in the presence of H(2) or phenylacetylene furnished isocyanato metallocene complexes with bridging imido (mu-NH) ligands. In the case of the ansa-hafnocene dinitrogen complex, replacing the dihydrogen atmosphere with various primary silanes yielded an isocyanato hafnocene mu-oxo hydride resulting from cleavage of N(2) and CO, the diatomics with the two strongest bonds in chemistry.
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.
Addition of 2 equiv of carbon dioxide to the ansa-zirconocene dinitrogen complex resulted in selective insertion into each zirconium nitrogen bond, forming a C2 symmetric dicarboxylated diazenido compound. Treatment with excess Me3SiI furnished the ansa-zirconocene diiodide along with the N,N'-dicarboxylated silylated hydrazine. New nitrogen-carbon bonds were also assembled by addition of methyl triflate. Tri- and tetrasubstituted hydrazines could be formed by treatment with water and Me3SiI, respectively. The regiochemistry of the N2 carboxylation is controlled by the ansa-cyclopentadienyl ligand where the sterically demanding tert-butyl substituents and the C2 symmetry of the dimer dictates the stereochemistry of CO2 insertion. These results demonstrate the ability of zirconium dinitrogen compounds to participate in heterocumulene insertion chemistry.
Carbonylation of the hafnocene dinitrogen complex, [Me(2)Si(η(5)-C(5)Me(4))(η(5)-C(5)H(3)-(t)Bu)Hf](2)(μ(2), η(2), η(2)-N(2)), yields the corresponding hafnocene oxamidide compound, arising from N(2) cleavage with concomitant C-C and C-N bond formation. Monitoring the addition of 4 atm of CO by NMR spectroscopy allowed observation of an intermediate hafnocene complex with terminal and bridging isocyanates and a terminal carbonyl. (13)C labeling studies revealed that the carbonyl is the most substitutionally labile ligand in the intermediate and that N-C bond formation in the bridging isocyanate is reversible. No exchange was observed with the terminal isocyanate. Kinetic data established that the conversion of the intermediate to the hafnocene oxamidide was not appreciably inhibited by carbon monoxide and support a pathway involving rate-determining C-C coupling of the isocyanate ligands. Addition of methyl iodide to the intermediate hafnocene resulted in additional carbon-carbon bond formation arising from CO homologation following nitrogen methylation. Similar reactivity with (t)BuNCO was observed where C-C coupling occurred upon cycloaddition of the heterocumulene. By contrast, treatment of the intermediate hafnocene with CO(2) resulted in formation of a μ-oxo hafnocene with two terminal isocyanate ligands.
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