The thioester Ph 2 PC 6 H 4 -2-C(O)SPh reacts with Fe 2 (CO) 9 to give [Ph 2 PC 6 H 4 C(O)]Fe(SPh)(CO) 3 , a model for the CO-inhibited active site of the enzyme Hmd. This species, which reversibly decarbonylates to give a diiron derivative, reacts with cyanide to give [[Ph 2 PC 6 H 4 C(O)]Fe-(SPh)(CN)(CO) 2 ] -.

New derivatives of 2-hydroxypyridine (2-hpH) and Cp*Ir(III) are described. Under conditions for catalytic dehydrogenation of 1-phenylethanol catalyzed by Cp*IrCl(κ 2 -2-hp) (1), the main species observed are [Cp* 2 Ir 2 H 2 (2-hp)]Cl ([2]Cl) and Cp*IrHCl(κ 1 -2-hpH) (3). Crystallographic analysis confirms that the cation in [2]PF 6 consists of a Cp* 2 Ir 2 (μ-H) x 2þ core complemented by a pyridonate ligand that bridges via O and N centers. Although [2]Cl is catalytically highly active, the related salt [2]PF 6 is not. Addition of chloride sources reactivates [2]PF 6 . Collectively, our experiments indicate that [2]Cl is a resting state that reverts to a more active species, which we propose is 1 itself. In situ NMR observations and PPh 3 trapping experiments show that under catalytically relevant conditions 1 rapidly converts to 3, which can be observed spectroscopically. Compound 3 was independently generated by transfer hydrogenation of 1. In other experiments, 1 was found to ring-open upon treatment with PPh 3 to give Cp*IrCl(κ 1 -2-hp)(PPh 3 ), which in turn was found to react with AgPF 6 to give [Cp*Ir(κ 2 -2-hp)(PPh 3 )]PF 6 . Both PPh 3 derivatives proved catalytically inactive for dehydrogenation. Cp*IrCl(κ 2 -2-hp-6-Me) was also prepared but could not be converted to κ 1 -2-hpH-6-Me derivatives. The complex Cp*IrCl(C 5 H 3 O 2 NH), nominally derived from the conjugate base of 2,6dihydroxypyridine, features the novel ligand η 3 -C 3 H 3 (CO) 2 NH.

Phosphine-modified thioester derivatives are shown to serve as efficient precursors to phosphine-stabilized ferrous acyl thiolato carbonyls via the reaction of phosphine thioesters and sources of Fe(0). The reaction generates both Fe(SPh)(Ph2PC6H4CO)(CO)3 (1) and the diferrous diacyl Fe2(SPh)2(CO)3(Ph2PC6H4CO)2, which carbonylates to give 1. For the extremely bulky arylthioester Ph2PC6H4C(O)SC6H4-2,6-(2,4,6-trimethylphenyl)2, oxidative addition is arrested and the Fe(0) adduct of the phosphine is obtained. Complex 1 reacts with cyanide to give Et4N[Fe(SPh)(Ph2PC6H4CO)(CN)(CO)2] (Et4N[2]). 13C and 31P NMR spectra indicate that substitution is stereospecific and cis to P. The IR spectrum of [2]− in CH2Cl2 solution very closely matches that for HmdCN. XANES and EXAFS measurements also indicate close structural and electronic similarity of Et4N[2] to the active site of wild-type Hmd. Complex 1 also stereospecifically forms a derivative with TsCH2NC, but the adduct is more labile than Et4N[2]. Tricarbonyl 1 was found to reversibly protonate to give a thermally labile derivative, IR measurements of which indicate that the acyl and thiolate ligands are probably not protonated in Hmd.

Cp(*)M(2+) complexes (M = Rh, Ir; Cp(*) = C(5)Me(5)) are described for 6-(carboxymethyl)-4-methyl-2-hydroxypyridine (cmhpH(2)), an analogue of the guanylylpyridone cofactor in the hydrogenase Hmd. Three findings indicate that Cp(*)M(Hcmhp)(+) stabilizes the binding of hydrogen-bond acceptors to the sixth coordination site: (i) water binds in preference to Cl-, (ii) the adduct Cp(*)Rh(cmhp)(2-hydroxypyridine) exhibits a very short intramolecular hydrogen bond (r(o-o) = 2.38 A; (1)H NMR delta(H) 17.2), and (iii) Cp(*)Ir(cmhpH)Cl efficiently catalyzes the dehydrogenation of PhCH(OH)Me to PhC(O)Me.

Reaction of Fe(bda)(CO)3 (bda = benzylideneacetone) and Ph2P-2-C6H4CHO (PCHO) affords the bis­phosphine bisalkoxide complex Fe[(Ph2PC6H4)2C2H2O2](CO)2 (1) arising from the head-to-head coupling of two formyl groups concomitant with oxidation of Fe(0) to Fe(II). Crystallographic studies show that 1 features cis alkoxide ligands that are trans to CO; the two phosphine groups are mutually trans with a P–Fe–P angle of 167.44(4)°. The pathway leading to 1 was examined, starting with the adduct Fe(PCHO)(CO)4 (2), which was obtained by addition of PCHO to Fe2(CO)9. Compound 2 decarbonylates to give tricarbonyl Fe(κ1,η2-PCHO)(CO)3 (3), which features a π-bonded aldehyde. Photolysis of 2 gives a mixture of 3 and isomeric hydride HFe(κ2-PCO)(CO)3. Complex 3 reacts with an additional equivalent of PCHO to afford 1, whereas treatment with PPh3 afforded the substituted product Fe(κ1,η2-PCHO)(PPh3)(CO)2 (4). In 4, the phosphine ligands are trans and the aldehyde is π-bonded. The geometry around Fe is pseudo trigonal bipyramidal. To gain insights into the mechanism and scope of the C–C coupling reaction, complexes were prepared with the imine Ph2PC6H4CHNC6H4Cl (abbreviated as PCHNAr), derived by condensation of 4-chloroaniline and PCHO. PCHNAr reacts with Fe2(CO)9 and with Fe(bda)(CO)3 to afford the tetra- and tricarbonyl compounds Fe(PCHNAr)(CO)4 (5) and Fe(PCHNAr)(CO)3 (6), respectively. Treatment of 6 with PCHO gave the unsymmetrical C–C coupling complex Fe[(Ph2PC6H4)2CH(O)CH(NAr)](CO)2 (7). Compound 7 was also prepared by the reaction of 3 and PCHNAr. The solid-state structure of 7, as established by X-ray crystallography, is similar to that of 1 but with an amido group in place of one alkoxide. The deuterium-labeled phosphine aldehyde PCDO was prepared by the reaction of ortho-lithiated phosphine Ph2PC6H4-2-Li with DMF-d 7. Reaction of 6 with PCDO gave 7-d 1 with no scrambling of the deuterium label. Attempted oxidation of 1 with FcBF4 (Fc+ = ferrocenium) gave the adduct Fe[(Ph2PC6H4)2C2H2O2(BF3)2](CO)2 (8). The structures of 1 and 8 are almost identical. Compound 8 was independently synthesized by treating 1 with BF3OEt2 via the intermediacy of the 1:1 adduct, which was detected spectroscopically. Qualitative tests showed that 1 also reversibly protonates with HOSO2CF3 and binds TiCl4.