We first describe these two mechanisms, and then show that the re mechanism is operative.
According to the re mechanism, when turnover is carried out under N2/D2, D2 can react with E4(N2/N2H2), replacing the N2 and undergoing oxidative addition to generate E4 2D).
A test of the mechanism involving turnover under N2/D2/C2H2, as in Figure 15, validated the re mechanism, and in so doing confirmed the stoichiometry of nitrogen fixation, eq 1, as requiring eight [e /H+].
Subsequent to formulation of the re mechanism for the activation of FeMo-cofactor to reduce N2, we noted that addition of C2H2 to a N2/D2 reaction mixture should offer a rigorous test of the mechanism.
Figure 12, above, presents a formal integration of the reaction pathway for nitrogen fixation (intermediates E4 E8) with the LT kinetic scheme, the key to the resulting mechanism being N2 binding and H2 release through the re mechanism, Figure 13, lower.
We tested the predictions based on the re mechanism of an unprecedented involvement of gaseous D2 in substrate reduction by use of C2H2 reduction under N2/D2/C2H2 gas mixtures to intercept the E4 2D) and E2(D) states.
In contrast, in the reverse of the re mechanism, shown in Figure 14, lower, D2 binding and N2 release would generate E4 2D), the E4 isotopomer in which both atoms of D2 exist as deuteride bridges.
The re mechanism further involves D2 binding to a state at the "diazene level" of reduction, as required by the constraint of eq 3 and Chart 1, line iii.
Although diatomic D2 does not reduce nitrogenase C2H2 in the absence of N2, the re mechanism successfully predicted that turnover under C2H2/D2/N2 would breach the separation of gaseous D2 from solvent protons by generating both C2H3D and C2H2D2.
Otherwise, in a process analogous to that for N2H2 formation in the re mechanism, C2H2 might in principle displace D2 formed by reductive elimination of the E4 2D) deuterides, leading to direct formation of C2H4 without D incorporation, Figure 16C.
