Kinetic Effects in Heterometallic Dinitrogen Cleavage

The rhenium(I) dinitrogen complex (PhMe2P)4ClRe(N2) reacts with [Mo2(S2CNEt2)6](OTf)2 (6) to give the N2-bridged complex [(PhMe2P)4ClRe(μ-N2)Mo(S2CNEt2)3]OTf ([7]OTf). Spectroscopic (νNN = 1818 cm-1) and structural data [d NN = 1.167(6) Å] indicate that the bridging N2 moiety in 7 + is slightly activated relative to free N2 or to the mononuclear Re complex. However, the complex is stable with respect to N2 cleavage. The putative products of such a cleavage, the known (Et2NCS2)3Mo(N) (5) and the newly prepared [(PhMe2P)4ClRe(N)]OTf ([9]OTf), are stable compounds that do not react with each other to give products of nitride coupling. Thus, the failure of 7 + to interconvert with 5 and 9 + is due not to the thermodynamic stability of the N⋮N bond but rather to kinetic factors that disfavor N2 cleavage and nitride coupling. Implications of this result for using polar effects to facilitate N2 cleavage to nitrides as a strategy for nitrogen fixation are discussed.