Bimetallic hydride

Early and late transition metal combinations were explored by Casey [71] and many others. Reaction of the bis(phosphinomethyl) complex Cp2Zr(CH2PPh2)2 (Cp = cyclopentadienyl) on Rh(H)(PPhj)4 gave the bimetallic hydride Cp2Zr (CH2PPh2)2Rh(H)(PPh3) (30), the crystal structure of which was determined [72],... 

An alternate bimetallic pathway was also suggested, but not favored, by Heck and Breslow (also shown in Scheme 1). The acyl intermediate could react with HCo(CO)4 to undergo intermolecular hydride transfer, followed by reductive elimination of aldehyde to produce the Co-Co bonded dimer Co2(CO)s. A common starting material for HCo(CO)4-catalyzed hydroformylation, Co2(CO)g is well-known to react with H2 under catalysis reaction conditions to form two equivalents of HCo(CO)4. The bimetallic hydride transfer mechanism is operational for stoichiometric hydroformylation with HCo(CO)4 and has been proposed to be a possibility for slower catalytic hydroformylation reactions with internal alkenes.The monometallic pathway involving reaction of the acyl intermediate with H2, however, has been... 

Finally, a last important property of the M-H bond is its role as a ligand to give bimetallic hydrides, given its ability to bridge metals (see Chap. 2). Therefore, a 16-electron species mnst be generated for instance by removing a CO ligand in the second metal complex ... 

This dihydride complex is not the final product, however, since it is converted into [Pt2H3(PEt3)4]HC02, a formate salt of a cationic bimetallic hydride, by subsequent reaction with the CO2 that is also formed in the photolysis reaction. ... 

Bimetallic transition metal hydride complexes. G. L. Soloveichik and B. M. Bulychev, Russ. Chem. Rev. (Engl. Transl), 1983,52, 43-60 (191). 

Hi ly dispersed supported bimetallic catalysts with bimetallic contributions have been prepared from molecular cluster precursors containing preformed bimetallic bond [1-2]. For examples, extremely high dispersion Pt-Ru/y-AUOa could be prepared successfully by adsorption of Pt2Ru4(CO)ison alumina [2]. By similar method, Pt-Ru cluster with carbonyl and hydride ligands, Pt3Ru6(CO)2i(p3-H)(p-H)3 (A) was used in this work to adsorb on MgO support. The ligands were expectedly removable from the metal framework at mild conditions without breaking the cluster metal core. 

Bimetallic Transition Metal-Zirconocene Complexes from Zirconium Hydrides... 

Towards Catalytic Relevance Bimetallic Activation of Acyl Ligands and Transition Organometallic Hydrides as Reducing Agents... 

Bimetallic activation of acetyl and alkoxyacetyl ligands — through formation of cationic P2 acyl complexes — to reaction with nucleophilic hydride donors was established. Cationic transition metal compounds possessing an accessible coordination site bind a neutral T -acyl ligand on another complex as a cationic P2 acyl system. These i2 3icyl systems activate the acyl ligand to reduction analogous to carbocation activation. Several examples of i2-acyl complexation have been reported previously. 

The intent of these experiments was to demonstrate intermediacy of 33 via O-protonation and elimination of the ethylidene salt CpFe(C0)2(CHMe)+, which would rearrange to CpFe (CO) 2 (n2-CH2 CH2)+. We feel however that additional studies are required in order to adequately study the interaction of organometallic hydride reagents and bimetallic y2 acyl complexes. 

Gibson and El-Omrani found that aldehydes were hydrogenated in refluxing THF by the bimetallic Mo hydride [(,u-H)Mo2(CO)10] in the presence of HOAc [37]. These reactions most likely proceed through generation of the mononuclear hydride [HMo(CO)sf, in analogy to the results discussed above for the Cr and W analogues. 

The organometallic products included recovered [(CO)5M(OAc)], along with M(CO)6 and the bimetallic bridging hydride complex [( -H)M2(CO)10]T It was proposed that, under the reaction conditions, [(CO)5MH] and HOAc were produced, and that insertion of the ketone into the M-H bond gave a metal alkox-ide that reacted with HOAc to produce the alcohol. 

Hydride transfer reactions from [Cp2MoH2] were discussed above in studies by Ito et al. [38], where this molybdenum dihydride was used in conjunction with acids for stoichiometric ionic hydrogenations of ketones. Tyler and coworkers have extensively developed the chemistry of related molybdenocene complexes in aqueous solution [52-54]. The dimeric bis-hydroxide bridged dication dissolves in water to produce the monomeric complex shown in Eq. (32) [53]. In D20 solution at 80 °C, this bimetallic complex catalyzes the H/D exchange of the a-protons of alcohols such as benzyl alcohol and ethanol [52, 54]. 

Shvo and coworkers prepared a bimetallic complex in which the two metals are joined by a bridging hydride as well as by an O-H-O hydrogen bond joining the two substituted Cp ligands [70]. Shvo used this versatile catalyst precursor for hydrogenation of C=C and C=0 bonds at 145 °C under 34 bar H2 (Eq. (41))... 

Under these conditions, the bimetallic complex is cleaved into an 18-electron complex that performs the hydrogenations, and an unsaturated 16-electron complex. Addition of H2 to the 16-electron complex produces the 18-electron complex that has an acidic OH and a hydridic RuH (Eq. (42)). 

In D2O, HD was found instead of 0-H2. It is presently assumed that binding of hydrogen to a metal ion in the bimetallic active site weakens the H-H bond sufficiently to enable this reaction. Oxidation of the hydride is expected to be a two-electron process, and hydrogenases should, therefore, contain a redox unit capable of accepting these two electrons simultaneously. I assume here that the bimetallic center plus the conserved proximal Fe-S cluster perform this task. 

Figure 3.5. Continued. The H2-NAD reaction is inhibited neither in air nor in the presence of CO. C,The possible reactions of hydrogen with the Fe-Fe site of active [Fe]-hydrogenases. In the oxidized state, the bimetallic center shows a S = 1/2 EPR signal, presumably due to an Fe -Fe pair (an Fe -Fe pair cannot be excluded). Whether the unpaired spin is localized on iron (Pierik et al. 1998a) or elsewhere (Popescu and Mtlnck 1999) is not known. Hydrogen is presumably reacting at the vacant coordination site on Fe2 (Fig. 3.1C). After the heterolytic splitting, the two reducing equivalents from the hydride are rapidly taken up by the Fe-Fe site (one electron) and the attached proximal cluster (one electron). Subsequently, the electron is transferred from the proximal cluster to the other Fe-S clusters in the enzyme. Under equilibrium conditions, the proximal cluster in the active enzyme appears to be always in the oxidized [4Fe-4S] state (Popescu and Mtlnck 1999). Protons are not shown.

Another example for a bimetallic NHC complex is the combination of a ruthenium hydride fragment with an ytterbium NHC complex. The NHC serves partly as the hydrogen trap [Eq. (39)]. ... 

Heteronuclear Pt-Ru binary carbonyl clusters have been used for the preparation of tailored PtRu bimetallic electrocatalysts. The use of carbonyl complexes such as Ru4Pt2(CO)i8 and closely related carbide and hydride carbonyl-derived clusters, that is, Ru5PtC(CO)i6 and Ru6Pt3(CO)2i( X3-H)( x-H)3, has allowed the preparation of carbon- and y-Al203-supported catalysts in which the presence of RugPts, RusPt and Ru4Pt2 clusters or nanoparticles has been reported [62-65]. 

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