Desymmetrized diiron azadithiolato, carbonyls: A step toward modeling the iron-only hydrogenases

Condensation of Fe-2(SH)(2)(CO)(6), acetaldehyde, and (NH4)(2)CO3 affords the methyl-substituted azadithiolate Fe-2[(SCHMe)(2)NH](CO)(6) (1). The complex exists mainly (similar to 95%) as the meso diastercomer, but the d,l diastereoisomers could be detected. DFT calculations predict that the meso isomer would be 2.5 kcal/mol more stable than the d, 1 isomer due to conventional nonbonding interactions between the methyl groups and the ring hydrogen atoms. Crystallographic analysis of meso-1 confirms that the two methyl groups are equatorial, constraining the diferraazadithiolate bicycle to a conformation that desymmetrizes the diiron center. The lowered symmetry is confirmed by the observation of two C-13 NMR signals in the FeCO region under conditions of fast turnstile rotation at the Fe(CO)(3) groups. The p& value of the amine in 1 is 7.89 (all pK(a)'s determined in MeCN solution), which is similar to a redetermined value for Fe-2[(SCH2)(2)NH](CO)(6) (2, pK(a) = 7.98) and only slightly less basic than the tertiary amine Fe-2[(SCH2)(2)NMe](CO)(6) (pK(a) = 8.14). Substitution of 1 with PMe3 proceeded via the intermediacy of two isomers of Fe-2[(SCHMe)(2)NH](CO)(5)(PMe3), affording Fe-2[(SCHMe)(2)NH](CO)(4)(PMe3)(2) (3). P-31 NMR spectra confirm that the two PMe3 ligands in 3 are nonequivalent, consistent with the desymmetrizing effect of the dithiolate. The pKa value of the amine in 3 was found to be 11.3. Using triphenylphosphine, we prepared Fe-2[(SCHMe)(2)NH](CO)(5)(PPh3) as a single regioisomer.