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 merc apto carboxylic acids of the general type HSCH(R)COOH affords in nearl y quantitative yields the novel double cubanes of the formulation [MoF e3S4Cl3(SCH(R)COO)](4-)(2) (R = H, mercaptoacetate, 2; R = CH3, thiola ctate, 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 = 12.65 7(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) Angstro m, c = 19.019(7) Angstrom, beta = 112.61(2)degrees, V = 7182(4) Angstr om(3), and Z = 4. The solutions of both structures were accomplished b y direct methods. The refinement with full-matrix least-squares method s 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 o n 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 anisotropi c 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 m ean intracubane Mo-S, Fe-S, and Fe-Cl bond distances are found at 2.35 6(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, res pectively. 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)-carbo xylato ligands. The Mo-O(carboxylate) bond in 2 is 2.096(7) Angstrom l ong (2.10(1) Angstrom for 3), and two types of Mo-S(bridging) bond len gths are present at 2.523(3) and 2.577(3) Angstrom. The closest interc ubane 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 [MoFe3S 4Cl3(OOCCH2SCH2COO)](2-) cluster, 5, in which the thioether dicarboxyl ate is coordinated to the Mo atom through its two carboxyl and thioeth er functionalities. Furthermore 1, reacts with 3,4-dimercaptotoluene a nd thiosalicylic acid to form respectively compounds 6 and 7 with the bidentate ligands bridging the two MoFe3S4 subunits in a manner simila r to that in 2 and 3. EPR spectra are not observed for 2-4 in CH3CN gl asses 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 solut ion. Cluster 5 shows a characteristic S = 3/2 EPR signal and displays a single reduction wave at -745 mV. The reduction of hydrazine to ammo nia in the presence of cobaltocene and 2,6-lutidinium chloride was cat alyzed 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 hydr azine, the dimeric structure of 3 collapses to generate single cubanes with Mo-coordinated, terminally bound hydrazine that undergoes subseq uent protonation/reduction to ammonia.