Evidence for a dinuclear mechanism in alkyne hydrogenations catalyzed by pyrazolate-bridged diiridium complexes

The products obtained from the sequential reaction of [Ir2(mu-H)(mu-Pz)2H3(NCCH3)(PiPr3)2] (1) with diphenylacetylene and their subsequent reactions with hydrogen have been investigated in order to deduce the mechanisms operating in the hydrogenation reactions catalyzed by 1. The reaction of 1 with an excess of diphenylacetylene gives cis-stilbene and [Ir2(mu-H)(mu-Pz)2-[eta1-C6H4-2-[eta1-(Z)-C=CHPh]]((Z)-C(Ph) =CHPh](NCCH3)(PiPr3)2] (2), the structure of which has been determined by X-ray diffraction. The formation of 2 involves the intermediate species [Ir2(mu-H)(mu-Pz)2H2((Z)-C(Ph)=CHPh](NCCH3)-(PiPr3)2](3),[Ir2(mu-H)(mu-Pz)2H[(Z)-C(Ph)=CHPh]2(NCCH3)(PiPr3)2] (4), and [Ir2(mu-H)(mu-Pz)2H[eta1-C6H4-2-[eta1-(Z)-C=CHPh](NCCH3)(PiPr3)2] (5), which have been isolated and characterized. These three complexes react with hydrogen to give cis-stilbene and 1 and are possible intermediates of the diphenylacetylene hydrogenation under catalytic conditions. Nevertheless, the rate of formation of 5 is very slow compared with the rate of catalytic hydrogenation, which excludes its participation during catalysis. Compound 2 also reacts with hydrogen in benzene, but in this case the hydrogenation gives 1,2-diphenylethane as the sole organic product. The course of this reaction in acetone has been investigated, and deuteration experiments were carried out. The formation of [Ir2(mu-H)(mu-Pz)2H[eta1-C6H4-2-[eta1-(Z)-C=CHPh]](OC(CD3)2)(PiPr3)2] (6) and [Ir2(mu-H)(mu-Pz)2H[eta1-C6H4-2-[eta1-(Z)-C-CHPh]](NCCH3)(PiPr3)2] (7) was observed under these conditions. The experimental evidence obtained supports two alternative mechanisms for the alkyne hydrogenation catalyzed by 1, one of them being dinuclear and the other mononuclear. The experimental data suggest that the former is favored.

Kinetic mechanism of the molecular forms of chicken liver mitochondrial malate dehydrogenase.

1. The reaction kinetic mechanism (pH 7.4) of the molecular forms of chicken liver m-MDH is of the order bi-bi ternary complex type with the existence of the E-oxaloacetate, E-L-malate, E-NAD+-oxaloacetate, E-NADH-L-malate, E-NAD+-NADH, E-NAD+-NAD+, E-NADH-NAD+ and E-NAD-NADH abortive complexes. 2. The saturating concentration values of the substrates are notably modified, in certain cases, in the presence of the reaction products.

[Kinetics of chicken liver mitochondrial and cytoplasmic glutamate oxalacetate transaminase (author's transl)]. / Cinética de la glutamato oxalacetato transaminasa soluble y mitocondrial de hígado de pollo.

Electrophoresis at pH 7.4, on cellulose polyacetate strips, and specific staining, show the occurrence of two molecular forms of the mitochondrial and soluble isoenzymes from chicken liver aspartate aminotransferase. The optimum pH of the cytoplasmic enzyme with L-aspartate and alpha-ketoglutarate as substrats is approximately 7, while the mitochondrial one is practically unaffected in the interval 6-8. The kinetic reactional mechanism is of ping-pong bi-bi type for both enzymes, as confirmed by the method of Garces-Cleland, and their inhibitions by excess of the substrates L-aspartate and alpha-Ketoglutarate are competitive, in accordance with the proposed mechanism.

Intramitochondrial location of the molecular forms of chicken liver mitochondrial malate dehydrogenase.

1. The two molecular forms of mitochondrial malate dehydrogenase are partly bound to the mitochondrial membranes. 2. The A form is located on the outer surface of the inner mitochondrial membrane and also in the intermembrane space. 3. The B form of the enzyme appears in the matrix and bound in part, probably, to the inner surface of the inner mitochondrial membrane. 4. Glutamate dehydrogenase, glutamate oxaloacetate transaminase, fumarase and lactate dehydrogenase are bound, to a greater or lesser extent, to the mitochondrial membranes, the fumarase having the highest degree of binding.

Transmission of trans effects in dinuclear complexes.

The substitution of a terminal hydride ligand in the complexes [Ir(2)(mu-H)(mu-Pz)(2)H(3)(L)P(i)Pr(3))(2)] (L = NCCH(3) (1) or pyrazole (3)) by chloride provokes a significant change in the lability of the L ligand, despite the fact that the substituted hydride and the L ligand lie in opposite extremes of the diiridium(III) complexes. Detailed structural studies of complex 3 and its chloro-trihydride analogue [Ir(2)(mu-H)(mu-Pz)(2)H(2)Cl(HPz)(P(i)Pr(3))(2)] (4) have shown that this behavior is a consequence of the transmission of ligand trans effects from one extreme of the molecule to the other, with the participation of the bridging hydride. Extended Hückel calculations on model diiridium complexes have suggested that such trans effect transmissions are due to the formation of molecular orbitals of sigma symmetry extended along the backbones of the complexes. This is also an expected feature for metal-metal bonded complexes. The feasibility of the transmission of ligand trans effects and trans influences through metal-metal bonds and its relevance to the understanding of both the reactivity and structures of metal-metal bonded dinuclear compounds have been substantiated through structural studies and selected reactions of the diiridium(II) complexes [Ir(2)(mu-1,8-(NH)(2)naphth)I(CH(3))(CO)(2)(P(i)Pr(3))(2)] (isomers 6 and 7) and their cationic derivatives [Ir(2)(mu-1,8-(NH)(2)naphth)(CH(3))(CO)(2)(P(i)Pr(3))(2)](CF(3)SO(3)) (isomers 8 and 9).