Heterobimetallic Hydride Complexes

Heterobimetallic complexes have recently attracted considerable attention in light of the promise of enhanced reactivity as a result of the cooperativity between adjacent, but electronically different, metal centers. A large number of these bimetallic compounds have been synthesized by the reactions of organometallic halides with anionic metal carbonyls. Here, we describe an extension of this route to the synthesis of hydride rich. Os—Zr and Os—Rh complexes by the reaction of organometallic halides with a metal poly hydride anion. These preparations demonstrate the synthetic utility of transition metal polyhydride anions. 

Reduction of OsCl3(PMe2Ph)3 with NaBh4 in ethanol, as described in the literature, proceeds smoothly, giving good yields of OsH4(PMe2Ph)3. 

All operations must be performed under an Nj atmosphere with rigorously dried solvents. 

Spectroscopically pure material repeatedly gave low carbon analyses. 

The resulting dark red solution is stirred for 1.5 h, and the THF solvent is evaporated under vacuum. The residue is dissolved in 10 mL of benzene and the solution is Filtered to remove KCl. Benzene is removed under vacuum leaving a burgundy solid. This product can be recrystallized easily by cooling a saturated 10 1 pentane-toluene solution to — 20°C for 1 day. Yield 0.70 g (71%) of dark red crystals, mp under N2 125-130 °C. 

for C34H47 PjOsZr C, 49.20 H, 5.66. Found C, 45.27 H, 5.64. Spectroscopically pure material repeatedly gave low carbon analyses. 

The procedure in Section B is used to prepare a THF solution of K[OsH3(PMe2Ph)3] from 0.750 g (1.20 mmol) of OsH4(PMe2Ph 3, 0.500 g (12.5 mmol) of KH and 30mL of THF. The solution is filtered directly into a 100-mL Schlenk flask containing 0.300 g (0.600 mmol) of [RhCl(l,5-cod)]2.  

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In efforts to mimic the function of heterobimetallic hydrogenases and provide potential molecular catalysts for fuel cell technology the Fe,Ru-heterobimetallic hydride complex [Cp RuH(dppf)], dppf = l,l -bis(diphenylphosphino)ferrocene, has been reported [44b] to catalyze the elementary reaction H2 —> 2H + 2e . It was suggested [44b] that the crucial oxidation involves the ruthenium center and not... 

Hydride complexes discnssed here inclnde both /x-hydride heterobimetallic complexes, and bimetallic complexes with Nb-H or Ta-H ligands. 

Heterobimetallic hydride and alkylidyne complexes are discussed in Sections 3.2.3 and 4.1, respectively. C-H activation of the alkylidyne (see Alkylidyne) complexes are discussed in Section 9.1 below. 

Heterobimetallic zirconocene hydride complexes of the type [M][(CsH4R)2ZrH2] were first characterized by ESR in 1991 (M = Li, Na, K R = Me, Bu, SiMe3).2,43,44 Aluminum-zirconium hydride complexes are involved in a number of catalytic and stoichiometric processes.4 The presence of Zr(m) species in reaction mixtures has been inferred from the color45 and, in the case of CpZr(//-I I)2AI(Mc)(2,4,6-tllu ((, I I2), detected by ESR.46... 

A number of transition metal complexes will catalyze the dehydrogenative coupling of organotin tin hydrides, R SnI I, to give the distannanes, RjSnSnRj.443 These metals include palladium,449 gold,450, hafnium,451 yttrium, and ruthenium.452 The catalyst that is most commonly used is palladium, often as Pd(PPh3>4, and the most active catalysts appear to be the heterobimetallic Fe/Pd complexes, in which both metals are believed to be involved in the catalysis.443... 

This complex is a starting material to make complexes with novel hydride-proton interactions.16 The mer isomer has been used to make heterobimetallic clusters.17... 

The heterobimetallic complexes [N(n-Bu)4] [Os(N)R2(/u.-0)2Cr02] catalyze the selective oxidation of alcohols with molecular oxygen. A mechanism in which alcohol coordinates to the osmium center and is oxidized by B-hydrogen elimination (see -Hydride Elimination) is consistent with the data. The hydroxide adduct of OSO4, [0s(0H)204], with ferric cyanide and other co-oxidants catalyzes the oxidative dehydrogenation of primary aromatic and aliphatic amines to nitriles, the oxidation of primary alcohols to carboxylic acids, and of secondary alcohols to ketones. Osmium derivatives such as OsCb catalyze the effective oxidation of saturated hydrocarbons in acetonitrile through a radical mechanism. ... 

Treatment of Cp2ZrCl2 with LiAlH4 and then HsGaNMes or reaction of the preformed hydride Cp2ZrH2 with H3 AINMes affords the heterobimetallic complex (15). Boron-containing heterobimetallic complexes (16) can be prepared by reaction of metallocene dichlorides with the cyclic hydroborate anion [H2BC5H10] then reduction of MX l bond with LiH. 

The first chiral aluminum catalyst for effecting asymmetric Michael addition reactions was reported by Shibasaki and coworkers in 1986 [82], The catalyst was prepared by addition of two equivalents of (i )-BINOL to lithium aluminum hydride which gave the heterobimetallic complex 394. The structure of 394 was supported by X-ray structure analysis of its complex with cyclohexenone in which it was found that the carbonyl oxygen of the enone is coordinated to the lithium. This catalyst was found to result in excellent induction in the Michael addition of malonic esters to cyclic enones, as indicated in Sch. 51. It had previously been reported that a heterobimetallic catalyst prepared from (i )-BINOL and sodium and lanthanum was also effective in similar Michael additions [83-85]. Although the LaNaBINOL catalyst was faster, the LiAlBINOL catalyst 394 (ALB) led to higher asymmetric induction. 

Ethyl rhenlumpentacarbonyl has been reacted with various metal hydrides in acetonitrile 158). The observed products were heterobimetallic compounds, although a solvated rheniumtetracarbonyl acyl complex was detected [Eq. (47)]. If the metal hydride is in excess, the rate-determining step is formation of the propionyl complex. The reaction was subsequently found to be first order in both the propionyl complex and the metal hydride. The second-order rate constants were measured and were found to be the reverse of the order of the acidities of the transition metal hydrides, which implies that the hydrides react as nucleophiles with the propionyl complex. In a separate experiment, [Re(COEt)(CO)j] was found to react with [Re(H)(CO)j] only after carbonyl dissociation, implying that the metal and not the acyl carbonyl is the site of nucleophilic attack by transition metal hydrides on acyl complexes. 

This mechanism is quite general for this substitution reaction in transition metal hydride-carbonyl complexes [52]. It is also known for intramolecular oxidative addition of a C-H bond [53], heterobimetallic elimination of methane [54], insertion of olefins [55], silylenes [56], and CO [57] into M-H bonds, extmsion of CO from metal-formyl complexes [11] and coenzyme B12- dependent rearrangements [58]. Likewise, the reduction of alkyl halides by metal hydrides often proceeds according to the ATC mechanism with both H-atom and halogen-atom transfer in the propagation steps [4, 53]. 

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