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 N(2)-bridged complex [(PhMe2P)4ClRe(mu-N2)Mo(S2CNEt2)3]OTf ([7]OTf). Spectroscopic (nu(NN) = 1818 cm(-1)) and structural data [d(NN) = 1.167(6) A] indicate that the bridging N(2) 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)(4)ClRe(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 NN 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.