STRUCTURAL CHARACTERIZATION AND REACTIVITY PROPERTIES OF A NEW CLASS OF MO/FE/S DOUBLE CUBANES WITH MO-BOUND S-MU(2)-ETA(1),O-ETA(1) MERCAPTO CARBOXYLATE LIGANDS - NEW CATALYSTS FOR THE REDUCTION OF HYDRAZINE TO AMMONIA AND IMPLICATIONS REGARDING THE FUNCTION OF NITROGENASE

The reaction of (Et(4)N)(2)[MoFe3S4Cl3(Cl-4-cat)(CH3CN)], 1, with mercapto carboxylic acids of the general type HSCH(R)COOH affords in nearly quantitative yields the novel double cubanes of the formulation [MoFe3S4Cl3(SCH(R)COO)](4-)(2) (R = H, mercaptoacetate, 2; R = CH3, thiolactate, 3; R = CH2COOH, thiomalate, 4). Black crystals of (Et(4)N)2 are monoclinic, space group C-2/c, with a = 18.959(9) Angstrom, b = 12.657(7) Angstrom, c = 28.57(1) Angstrom, beta = 98.03(4)degrees, V = 6790(5) Angstrom(3), and Z = 4. Black crystals of (Et(4)N)3 are monoclinic, space group C-2/c, with a = 16.281(5) Angstrom, b = 25.13(1) Angstrom, c = 19.019(7) Angstrom, beta = 112.61(2)degrees, V = 7182(4) Angstrom(3), and Z = 4. The solutions of both structures were accomplished by direct methods. The refinement with full-matrix least-squares methods of 299 parameters based on 5942 unique reflections for (Et(4)N)2 (2 theta(max) = 45 degrees, I > 3 sigma(I)) and of 287 parameters based on 4728 unique reflections (2 theta(max) = 45 degrees, I > 3 sigma(I)) for (Et(4)N)3 resulted in final R (R(W)) values of 6.09% (6.26%) and 7.00% (6.52%), respectively, for 2 and 3. In both structures anisotropic temperature factors were assigned for all the non-hydrogen atoms in the cluster tetraanions. The tetraanions in 2 or 3 are composed of two MoFe3S4 subunits linked by the deprotonated thiol groups. In 2, the mean intracubane Mo-S, Fe-S, and Fe-Cl bond distances are found at 2.356(3), 2.278(4), and 2.219(4) Angstrom, respectively. The mean Mo...Fe and Fe...Fe distances are found at 2.727(2) and 2.728(3) Angstrom, respectively. Similar metric features are found for 3. Each of the two Mo atoms (separated by 3.916(3) Angstrom in 2 and 3.899(3) Angstrom in 3) has a distorted octahedral coordination and in addition to three mu(3)-S2- ligands is coordinated to two mu(2)-thiolato and a eta(1)-carboxylato ligands. The Mo-O(carboxylate) bond in 2 is 2.096(7) Angstrom long (2.10(1) Angstrom for 3), and two types of Mo-S(bridging) bond lengths are present at 2.523(3) and 2.577(3) Angstrom. The closest intercubane S...S distance is 3.266(6) Angstrom. (Et(4)N)(2)[MoFe3S4Cl3(Cl-4-cat)(CH3CN)] also reacts with thiodiglycolic acid to form the [MoFe3S4Cl3(OOCCH2SCH2COO)](2-) cluster, 5, in which the thioether dicarboxylate is coordinated to the Mo atom through its two carboxyl and thioether functionalities. Furthermore 1, reacts with 3,4-dimercaptotoluene and thiosalicylic acid to form respectively compounds 6 and 7 with the bidentate ligands bridging the two MoFe3S4 subunits in a manner similar to that in 2 and 3. EPR spectra are not observed for 2-4 in CH3CN glasses at 5 K. The cyclic voltammetry of 2-4 shows multiple reductions and oxidation waves, and the two reduction waves are separated by 190-210 mV. The electrochemical data and the lack of the characteristic S = 3/2 EPR signal in solutions of 2-4 suggest that these molecules are electronically coupled and retain the double cubane structure in solution. Cluster 5 shows a characteristic S = 3/2 EPR signal and displays a single reduction wave at -745 mV. The reduction of hydrazine to ammonia in the presence of cobaltocene and 2,6-lutidinium chloride was catalyzed by 3. The core of the latter was found to be intact at the end of the catalytic reaction. It is proposed that in the presence of hydrazine, the dimeric structure of 3 collapses to generate single cubanes with Mo-coordinated, terminally bound hydrazine that undergoes subsequent protonation/reduction to ammonia.