Mechanism of H2 Production by Models for the [NiFe]-Hydrogenases: Role of Reduced Hydrides.

The intermediacy of a reduced nickel-iron hydride in hydrogen evolution catalyzed by Ni-Fe complexes was verified experimentally and computationally. In addition to catalyzing hydrogen evolution, the highly basic and bulky (dppv)Ni(µ-pdt)Fe(CO)(dppv) ([1](0); dppv = cis-C2H2(PPh2)2) and its hydride derivatives have yielded to detailed characterization in terms of spectroscopy, bonding, and reactivity. The protonation of [1](0) initially produces unsym-[H1](+), which converts by a first-order pathway to sym-[H1](+). These species have C1 (unsym) and Cs (sym) symmetries, respectively, depending on the stereochemistry of the octahedral Fe site. Both experimental and computational studies show that [H1](+) protonates at sulfur. The S = 1/2 hydride [H1](0) was generated by reduction of [H1](+) with Cp*2Co. Density functional theory (DFT) calculations indicate that [H1](0) is best described as a Ni(I)-Fe(II) derivative with significant spin density on Ni and some delocalization on S and Fe. EPR spectroscopy reveals both kinetic and thermodynamic isomers of [H1](0). Whereas [H1](+) does not evolve H2 upon protonation, treatment of [H1](0) with acids gives H2. The redox state of the "remote" metal (Ni) modulates the hydridic character of the Fe(II)-H center. As supported by DFT calculations, H2 evolution proceeds either directly from [H1](0) and external acid or from protonation of the Fe-H bond in [H1](0) to give a labile dihydrogen complex. Stoichiometric tests indicate that protonation-induced hydrogen evolution from [H1](0) initially produces [1](+), which is reduced by [H1](0). Our results reconcile the required reductive activation of a metal hydride and the resistance of metal hydrides toward reduction. This dichotomy is resolved by reduction of the remote (non-hydride) metal of the bimetallic unit.

N-Substituted Derivatives of the Azadithiolate Cofactor from the [FeFe] Hydrogenases: Stability and Complexation.

Experiments are described that probe the stability of N-substituted derivatives of the azadithiolate cofactor recently confirmed in the [FeFe] hydrogenases (Berggren, G., et al. Nature 2013, 499, 66). Acid-catalyzed hydrolysis of bis(thioester) BnN(CH2SAc)2 gives [BnNCH2SCH2]2 rather than azadithiol BnN(CH2SH)2. Treatment of BnN(CH2SAc)2 with NaO(t)Bu generates BnN(CH2SNa)2, which was trapped with NiCl2(diphos) (diphos = 1,2-C2H4(PR2)2; R = Ph (dppe) and Cy (dcpe)) to give fully characterized complexes Ni[(SCH2)2NBn](diphos). The related N-aryl derivative Ni[(SCH2)2NC6H4Cl](diphos) was prepared analogously from 4-ClC6H4N(CH2SAc)2, NaO(t)Bu, and NiCl2(dppe). Crystallographic analysis confirmed that these rare nonbridging [adt(R)](2-) complexes feature distorted square planar Ni centers. The analogue Pd[(SCH2)2NBn](dppe) was also prepared. (31)P NMR analysis indicates that Ni[(SCH2)2NBn](dppe) has basicity comparable to typical amines. As shown by cyclic voltammetry, the couple [M[(SCH2)2NBn](dppe)](+/0) is reversible near -2.0 V versus Fc(+/0). The wave shifts to -1.78 V upon N-protonation. In the presence of CF3CO2H, Ni[(SCH2)2NBn](dppe) catalyzes hydrogen evolution at rate of 22 s(-1) in the acid-independent regime, at room temperature in CH2Cl2 solution. In contrast to the instability of RN(CH2SH)2 (R = alkyl, aryl), the dithiol of tosylamide TsN(CH2SH)2 proved sufficiently stable to allow full characterization. This dithiol reacts with Fe3(CO)12 and, in the presence of base, NiCl2(dppe) to give Fe2[(SCH2)2NTs](CO)6 and Ni[(SCH2)2NTs](dppe), respectively.

Diiron Azamonothiolates via Scission of Dithiadiazacyclooctanes by Iron Carbonyls.

The reaction of Fe3(CO)12 with the dithiadiazacyclooctanes [SCH2N(R)CH2]2 affords Fe2[SCH2N(Me)CH2](CO)6 (R = Me, Bn). The methyl derivative 1Me was characterized crystallographically (Fe-Fe = 2.5702(5) Å). Its low symmetry is verified by variable temperature 13C NMR spectroscopy which revealed that the turnstile rotation of the S(CH2)Fe(CO)3 and S(NMe)Fe(CO)3 centers are subject to very different energy barriers. Although 1Me resists protonation, it readily undergoes substitution by tertiary phosphines, first at the S(CH2)Fe(CO)3 center, as verified crystallographically for Fe2[SCH2N(Me)CH2](CO)5(PPh3). Substitution by the chelating diphosphine dppe (Ph2PCH2CH2PPh2) gave Fe2[SCH2N(Me)CH2](CO)4(dppe), resulting from substitution at both the S(CH2)Fe(CO)3 and S(NMe)Fe(CO)3 sites.

Iodination of anilines and phenols with 18-crown-6 supported ICl2-.

A highly crystalline iodinating reagent, {[K·18-C-6]ICl(2)}(n), was synthesized in high yield (93%). The trihalide is supported by an 18-crown-6 macrocycle and forms a coordination polymer in the solid state. This reagent iodinates anilines and phenols efficiently under mild conditions. Controlled mono-iodination with anilines was easily achieved while poly-iodination was observed with phenols.