Synthetic and Structural Studies of 2-Acylmethyl-6-R-Difunctionalized Pyridine Ligand-Containing Iron Complexes Related to [Fe]-Hydrogenase.

As active site models of [Fe]-hydrogenase, tridentate 2-acylmethyl-6-methoxymethoxy-difunctionalized pyridine-containing complexes η(3)-(2-COCH2-6-MeOCH2OC5H3N)Fe(CO)2(L1) (4, L1 = I; 5, SCN; 6, PhCS2) were prepared via the following multistep reactions: (i) etherification of 2-MeO2C-6-HOC5H3N with ClCH2OMe to give 2-MeO2C-6-MeOCH2OC5H3N (1), (ii) reduction of 1 with NaBH4 to give 2-HOCH2-6-MeOCH2OC5H3N (2), (iii) esterification of 2 with 4-toluenesulfonyl chloride to give 2-TsOCH2-6-MeOCH2OC5H3N (3), (iv) nucleophilic substitution of 3 with Na2Fe(CO)4 followed by treatment of the resulting Fe(0) intermediate Na[(2-CH2-6-MeOCH2OC5H3N)Fe(CO)4] (M1) with I2 to give complex 4, and (v) condensation of 4 with KSCN and PhCS2K to give complexes 5 and 6, respectively. In contrast to the preparation of complexes 4-6, bidentate 2-acylmethyl-6-methoxymethoxy-difunctionalized pyridine-containing model complexes η(2)-(2-COCH2-6-MeOCH2OC5H3N)Fe(CO)2(I)(L2) (7, L2 = PPh3; 8, Cy-C6H11NC) and η(2)-(2-COCH2-6-MeOCH2OC5H3N)Fe(CO)2(L3) (9, L3 = 2-SC5H4N; 10, 8-SC9H6N) were prepared by ligand exchange reactions of 4 with PPh3, Cy-C6H11NC, 2-KSC5H4N, and 8-KSC9H6N, respectively. Particularly interesting is that the tridentate 2,6-bis(acylmethyl)pyridine- and 2-acylmethyl-6-arylthiomethylpyridine-containing model complexes η(3)-[2,6-(COCH2)2C5H3N]Fe(CO)2(L4) (11, L4 = PPh3; 12, CO) and η(3)-2-(COCH2-6-ArSCH2C5H3N)Fe(CO)2(ArS) (13, ArS = PhS; 14, 2-S-5-MeC4H2O) were obtained, unexpectedly, when 2,6-(TsOCH2)2C5H3N reacted with Na2Fe(CO)4 followed by treatment of the resulting mixture with ligands PPh3 and CO or disulfides (PhS)2 and (2-S-5-MeC4H2O)2. Reactions of ligand precursors 3 and 2,6-(TsOCH2)2C5H3N with Na2Fe(CO)4 were monitored by in situ IR spectroscopy, and the possible pathways for producing complexes 4 and 11-14 via intermediates Na[(2-CH2-6-MeOCH2OC5H3N)Fe(CO)4] (M1), Na[(2-CH2-6-TsOCH2C5H3N)Fe(CO)4] (M2), and (2-COCH2-6-CH2C5H3N)Fe(CO)3 (M3) are suggested. New compounds 1-14 were characterized by elemental analysis, spectroscopy, and, for some of them, X-ray crystallography.