Transition metal clusters oxidative addition reactions

It has been found in the meantime that reaction (1) is generalizable (752), and that oxidative additions of this type occur for such widely differing substrates H2Y as ethylene, benzene 130), cyclic olefins, alkyl and aryl phosphines, aniline 337, 406), and H2S 130), ail of which give the same product structure with a triply-bridging Y ligand. The stability of these third-row transition metal clusters has stiU prevented catalytic reactions of these species, but it is likely that similar ones are involved in olefin and acetylene reactions catalyzed by other metal complexes. 

In the reactions described above, only the AuPR3 fragment of the gold phosphine precursor has added to the cluster. Oxidative addition of Au(PR3)X [where X = Cl (90,188, 191), Br (90, 188), I (179), SCN (104), NCO (231), C2R (92, 97), etc.] has also been observed to occur, yielding clusters in which the transition metal is in a higher oxidation state. This type of reaction was first investigated by Nyholm and coworkers (90, 232), and some examples are given below ... 

This is a special volume of Inorganic Syntheses that focuses on complexes that are likely to be useful as starting materials for the preparations of new transition metal coordination and organometallic compounds. There are chapters on complexes with weakly coordinated and therefore easily displaced ligands, low-valent complexes that undergo oxidative-addition reactions, substituted metal carbonyl complexes, nucleophilic metal carbonyl anions, transition metal clusters, a variety of cyclopentadienyl complexes, lanthanide and actinide complexes, and a range of other useful ligands and complexes. 

Reactions with Soft Bases. Metal-metal bonds are often weaker than those the same metal can form with ligands. Thus the reaction of clusters with nucleophiles frequently leads to cleavage of metal-metal bonds and to cluster breakdown. The normally low oxidation state of metal centers on clusters makes their reaction with soft bases such as CO, PR3, olefins etc. very favorable. Clusters of lighter transition metals undergo mainly fragmentation reactions. Heavier element derivatives however are likely to participate in addition and substitution reactions. 

The 252(]f.pp) niajj spectrum of metal halides and oxides consists of a family cluster ions of these compounds extending to over mIz 10000, produced by the ejection of small domains of the crystal lattice in the region around the fission track. In addition, cluster ions are also observed that do not correlate with the composition of the crystal lattice, indicating that some of the cluster ions are involved in gas phase reactions in the desorption plume. One of the unique applications of 252(]f.pj) jg elucidation of the composition of large transition metal cluster compounds with values approaching 10. ... 

The principal focus of interest has moved toward the study of what may be termed controlled aggregation, that it to say reactions in which transition-metal cluster carbonyls of lower nuclearity are extended by the controlled addition of one or more metal atoms. For example, salts of [Fe5C(CO)i4] will react with /ac-[M(CO)3(NCMe>3] (M=Cr, Mo, W) to form [MFesCCCOh,] -, which is oxidized by iron(ni) to the neutral [MFe4C(CO)xe]. The metal atoms that can be added to the FesC-cluster in this way from different precursors include Cu, Fe, Ir, Ni, Pd, and Rh. The simple addition and subtraction of metal at(xns... 

The reactions of MeOH with some transition metal oxide cluster anions [M O J, where M = Mn, Fe, Co, Ni, Cu n = 1,2 x = 2—4, have been studied (254). The [M03] anions were unreactive toward MeOH, unlike [Nb03]. The addition of the hydrogen molecule to the other cluster anions was the common reaction yielding the following transformations,... 

Beyer and coworkers later extended these reactions to platinum clusters Ptn and have demonstrated that similar reaction sequences for the oxidation of carbon monoxide can occur with larger clusters [70]. In addition, they were able to demonstrate poisoning effects as a function of surface coverage and cluster size. A related sequence for Pt anions was proposed by Shi and Ervin who employed molecular oxygen rather than N2O as the oxidant [71]. Further, the group of Bohme has screened the mononuclear cations of almost the entire transition metal block for this particular kind of oxidation catalysis [72,73]. Another catalytic system has been proposed by Waters et al. in which a dimolybdate anion cluster brings about the oxidation of methanol to formaldehyde with nitromethane, however, a rather unusual terminal oxidant was employed [74]. 

There are three important routes to the formation of the mercury-transition metal bond (a) displacement of halogen or pseudohalogen from mercury(II) salts with carbonyl metallate anions (b) reaction of a halo-phenylmercury compound with a transition metal hydride and (c) oxidative addition of a mercury halide to neutral zero valent metals.1 We report here the syntheses of three compounds containing three-centre, two-electron, mercury-ruthenium bonds utilizing trinuclear cluster anions and mercury(II) halides.2-4... 

Little is known about the oxidative addition of organohaUdes to early transition metal atoms. Nevertheless, macroscale amounts of solvated transition metal halides in lower valency states have been obtained. Re yields interesting clusters in reactions with 1,2-dibromoethane and oxalyl chloride... 

The search for new reactivity and new reactions is an important target in homogeneous catalysis. A declared goal is the selective activation of C-H bonds under mild conditions. Although there are numerous examples of stoichiometric C-H bond oxidative additions to transition metal centers, successful examples regarding catalytic functionalization of C-H bonds have been made only during the last five years. Notable advances have been achieved by Moore and coworkers who described in 1992 the ortAo-acylation of pyridine with olefins and carbon monoxide. The cluster compound triruthenium dodecacarbonyl has been used as catalyst (Scheme 10). 

An investigation of the oxidative addition of ChUand CD4[57a], as well as ethane [57b], to a bare palladium atom has demonstrated that quantum tunneling plays a very important role in the process. The barrier of insertion of different transition metal atoms into a C-C bond has been found to be 14-20 kcal mol higher than the barrier for insertion into a C-H bond [57c], Calculations for the activation of the C-H bond in ethylene by second row transition metal atoms showed that the oxidative addition barrier is lowest for the atoms to the right (for rhodium there is no barrier and for palladium the barrier is almost zero) [57d], The activation energy for B2 insertion into methane has been predicted to be 4.1 kcal mol while this value increases to 16.2 kcal mol for insertion of B [58], Two mechanisms have been considered by the SCF CNDO/S method for the oxidative addition of methane to the palladium cluster Pd2 [59a], In the first possible reaction, the C-H bond oxidatively adds to different palladium atoms ... 

---