High-energy, early transition metal hydrazides for N atom transfer reactions in synthesis and catalysis: scope, mechanism and applications.

Group 6 transition metal hydrazides, (L)M=NNH2, occupy a pivotal position on the pathway for the biological and industrial conversion of N2 to NH3. In nature and in several important model systems this proceeds through a series of electron transfer and protonation steps. Considerable international effort has also been spent on trying to use mid-transition metal hydrazide complexes as reagents for the synthesis of value-added organo-nitrogen products. This holy grail concept of directly using atmospheric N2 as a commercial feedstock would by-pass the high-energy Haber-Bosch (NH3 synthesis) process. However, mid-metal M=NNR2 functional group reactivity is minimal. It is characterised only by transformations involving the NR2 (protonation or insertion/condensation involving N-H groups). No M=N bond reactivity is seen and N-N bond cleavage occurs only under highly forcing conditions using external reductants.Structural data and computational studies explain this: for mid-later metals NNR2 is best viewed as a neutral isodiazene :N=NR2. The more dative nature of the M--NNR2 bond and multiple bond character of N=N reduces the intrinsic reactivity at these two sites. In contrast, our X-ray and DFT results for Group 4 NNR2 systems find them to be reduced hydrazides, [NNR2]2-. Specifically, the N-N bonds are lengthened and weakened as the N-N pi* MOs are occupied; the M=N multiple bond is unsaturated and very reactive, further destabilised by the beta-NR2 lone pair. While sharing a simple formula NNR2 , mid-metal isodiazene and early metal hydrazide ligands are as fundamentally different as Fischer carbenes and Schrock alkylidenes. Just as early metal M=CR2 groups are intrinsically more reactive than later metal M=CR2, so it is for M=NNR2.We have recently developed three methods for making these hitherto undeveloped early transition metal hydrazides with Ti=NNR2 functional groups in very different environments. Preliminary results with a range of organic substrates (terminal and internal alkynes, nitriles, isonitriles, phospha-alkynes, CO2, isocyanates) indicate a wealth of cycloaddition chemistry. Remarkably certain alkynes and nitriles insertion into the N-N bond via a unique single N atom transfer step. We have also found this can be made catalytic so that a 1,2-diaminoalkene is formed catalytically at room temperature from a hydrazine and an alkyne: N-N addition across a C-C triple bond. If the diamination reaction can be extended it could ultimately provide a means of adding any general N-X (X = N, O, P) across any unsaturated C-element bond. This has the potential for a paradigm shift in the synthesis of 1,2-diamine and related compounds.In this project we will therefore develop the virtually unexplored area of early transition metal hydrazide chemistry, capable of delivering high-energy, highly reduced N-NR2 and related functional groups to a range of substrates in a 100% efficient, atom by atom manner. Our very recent synthetic methodology breakthroughs and preliminary reactivity studies provide a perfect and timely platform from which rapid progress can be launched. Working with a leading DFT computational Project Partner, we will deliver a fundamental toolbox for understanding and exploiting this unique reactivity in terms of scope, mechanism and applications in C-N, C-C and general C-heteroelement bond formation.