Synthesis of Carboxylic Acid-Modified [FeFe]-Hydrogenase Model Complexes Amenable to Surface Immobilization

Model complexes of [FeFe]-hydrogenase bearing carboxylic acid functionalities have been designed
for applications toward immobilization of hydrogen production electrocatalysts on amino-functionalized
carbon electrode surfaces. Using carboxylic acid-substituted thiols, complexes incorporating the −COOH
moiety into the thiolate linkers have been synthesized:  (μ-SCH2CH2COOH)2[Fe(CO)3]2 (1) and the
previously known (μ-(SCH2)2CHCOOH)[Fe(CO)3]2 (2). Carboxylic acid units have also been introduced
via ligand substitution with a tricarboxyethyl phosphine to generate (μ-pdt)[Fe(CO)3][Fe(CO)2{P(C2H4COOH)3}] (3). To mimic the linkage of complexes of these types to amino-functionalized monolayers,
it has been demonstrated that 2 can be coupled with aniline in solution to generate (μ-(SCH2)2CHCONHPh)[Fe(CO)3]2 (4), and the stability of this linkage has been addressed. The carboxylic acid-substituted complex 2 undergoes ligand substitution with PMe3 to generate (μ-(SCH2)2CHCOOH)[Fe(CO)2(PMe3)]2 (5). Complex 5 can then be protonated in either methanol or tetrahydrofuran solvent to
generate (μ-(SCH2)2CHCOOMe)(μ-H)[Fe(CO)2(PMe3)]2}PF6 (6) and (μ-(SCH2)2CHCOOH)(μ-H)[Fe(CO)2(PMe3)]2}PF6 (7), respectively, demonstrating that a range of carboxy-functionalized complexes can be
synthesized. Structural characterization and cyclic voltammetry of these complexes indicate that the
carboxylic acid functionality has little effect on the structure and reactivity of the diiron dithiolate core.