Desymmetrized Diiron Azadithiolato Carbonyls: A Step Toward Modeling the Iron-Only Hydrogenases

Condensation of Fe2(SH)2(CO)6, acetaldehyde, and (NH4)2CO3 affords the methyl-substituted azadithiolate Fe2[(SCHMe)2NH](CO)6 (1). The complex exists mainly (∼95%) as the meso diastereomer, 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,l 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 13C NMR signals in the FeCO region under conditions of fast turnstile rotation at the Fe(CO)3 groups. The pKa value of the amine in 1 is 7.89 (all pKaʼs determined in MeCN solution), which is similar to a redetermined value for Fe2[(SCH2)2NH](CO)6 (2, pKa = 7.98) and only slightly less basic than the tertiary amine Fe2[(SCH2)2NMe](CO)6 (pKa = 8.14). Substitution of 1 with PMe3 proceeded via the intermediacy of two isomers of Fe2[(SCHMe)2NH](CO)5(PMe3), affording Fe2[(SCHMe)2NH](CO)4(PMe3)2 (3). 31P 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 Fe2[(SCHMe)2NH](CO)5(PPh3) as a single regioisomer.