Anionic clusters, protonation

Hydroxide induced disproportionation has been applied to osmium clusters whose stability again prevents total decomposition. In most cases one osmium atom is removed leaving behind an anionic cluster (176, 177). Thus, Osg(CO)22 converts into Os CO), Os7(CO)2l goes to Os6(CO) g, and Os6(CO),8 changes into Os5(CO)jj. All these species can be protonated to the corresponding hydrido clusters. 

The reaction of a carbonylmetalate with a neutral metal carbonyl has been labeled a redox condensation by Chini et al. (40, 41) and has been as widely used as a pyrolysis reaction for synthesizing mixed-metal clusters. Carbonylmetalates usually react rapidly with most neutral carbonyls, even under very mild conditions. A large number of mixed-metal hydride clusters have been formed via this type of reaction, primarily because the initial products are anionic clusters that in many cases may be protonated to yield the neutral hydride derivative. 

Likewise, we have prepared the anionic clusters [CoFeRu2(CO)13] and [CoFe2Ru(CO)13T, but all attempts to isolate the corresponding neutral hydride clusters after protonation have failed, apparently because of their instability (147). 

Larger clusters show an increasing intensity of fluorescence centered at about 25000 cm-1, which is characteristic of the naphtholate anion, with proton transfer occurring for n > 30. However, this emission is not the fluorescence of the pure naphtholate anion, but can be seen as the superpositions of both naphthol and naphtholate emission. This behavior is different that observed in the liquid water where only naphtholate emission is observed, and also from that observed in ice where only naphthol emission is observed. 

In many cases, however, protonation reactions, especially of the anionic clusters, result in oxidation and/or concomitant fragmentation and reorganization of the cluster. The pathways are complicated and not understood. In the protonation of [E Fe(CO)4 4]3 (E = Sb, Bi) there appear to be... 

One should not draw structural/stability conclusions from the absence of MB 5 and M5B clusters in the first series. Quite possibly it is simply a question of finding the appropriate synthesis. On the other hand, there is reason to believe that there are problems with both structures. The two bridging H atoms of the known compounds are associated with M-M edges, and the MB5 cluster with a single metal atom possesses no M-M edge for protonation. An anionic cluster would not have this problem. At the other compositional extreme, the M5B system may not follow the main-group cluster scenario because a more stable alternative is accessible. For... 

Addition of protons to anionic clusters to generate hydrides leaves the cluster electron count unaffected, yet the process is sometimes accompanied by structural changes. For example, both [Os6(CO)i8] (87) and [Os6(CO)igH] (88) have octahedral arrangements of metal atoms as predicted by Wade s rules. In the dihydride Osg(CO)i8H2, however, the metal atoms describe a capped square-based pyramid (87). 

The endo-endo conformation of cryptands can be internally protonated to form proton cryptates. With the small cryptands, e.g. [1.1.1]- and [2.1.1]-cryptand (15a and 15b), the two internal protons are so efficiently shielded from H2O and OH that deprotonation only very slowly occurs even in strong base (8UA6044). Alkali cation cryptates are able to stabilize unusual species as their counterions. Dye and coworkers have isolated several alkali metal anions by this method. The sodium species (Na [2.2.2]cryptand Na ) was obtained as gold metallic crystals and gave a Na NMR with a broad Na -cryptate resonance and a narrow, upheld Na resonance. The other alkali metals show similar behavior and an electride salt (Na [2.2.2]cryptand e l has even been isolated (B-79MI52105). Crystalline anionic clusters of the heavy post-transition metals (such as Sb7 , Pbs , Sng ) were first obtained with alkali metal cryptates as the counterions (75JA6267). 

The pores of friendly nanomaterials could be used to store strong adds, even super acids, in some cases. Likewise, weak bases or strong bases could be stored for use as needed in killing or destroying advanced enemy toxins. In addition, the nanomaterial itself could be produced with acidic sites (metal ions and/or certain proton donors) built into the pore walls and crystal faces. For example, titanium or zirconium ions can serve as acid sites if adjacent to sulfate species. Likewise, the proton forms of some transition-metal oxygen-anion clusters (polyoxometalates or POMs ), like some metal oxides, are effective superacids in commercial processes. Polyoxometalates could be physically held within the pores or could be grafted onto the pore walls or onto the outer nanocrystal faces. Basic sites can also be built into the nanostructure, such as oxide anions near a metal cation vacancy. There are many other possibilities, such as sulfide substitution for oxide anions on the surface of the nanocrystals. 

Many neutral and all anionic clusters are Lewis bases. Dependent upon the nucleophilic character of the metal atoms and ligands, addition of a proton may occur either to the metal or to the ligand. Usually, M —H bonds are formed first. There are known cases of addition of two protons to neutral electron-rich clusters, and the formation of anionic clusters possessing charges ranging from -1 to -6, for example, [Ni3sPt6(CO)4s] -. ... 

The structurally characterised anion COs (p-NO) (CO) has a lower activation barrier for CO scrambling than related hydrldo-bridged clusters.Protonations of complexes COs-(CO), (MeCM) 3 (n =1,2) occur, and the structure of [Os (p-H) (CO) j q(M6CN) 2] is reported. 

Protonation reactions are often complicated by further chemical transformations of the unstable hydride derivatives. Thus in many instances the reaction of anionic clusters with acids leads to oxidation products with elimination of molecular hydrogen. That occurs for example in the reaction of the anion [Ir6(CO)i5] with acetic acid from which, depending of the reaction conditions, two isomers of the metal cluster Ir6(CO)i6 can be isolated. 

The complexity of subsequent and secondary reactions in the protonation of anionic clusters may be observed in the following equations that illustrate the protonation of the tetraanion [Rh6(CO)i4] under different reaction conditions. 

Protonation of clusters is another simple reaction that yields metal hydride clusters in which the hydride ligands are bridging two metals. For instance, the binary anionic cluster isoelectronic to [OssfCO) )] is [ResfCO) )] . This trianion can undergo three successive protonation reactions ultimately yielding a neutral triply bridging trihydride cluster. 

Justus et al. (2008a) published Af,Al,Al-trialkylammonioundecahydro-do5 0-dodecaborates(l-) as anions for ILs, which were obtained by alkylation of deprotonated [B,2H NH3] with cations such as lithium, potassium, and protons (Figure 29.14). This anion structure allows to vary to a large extent the properties of the anion, which is not readily achieved with the other anionic cluster compounds. 

The higher is the exothermicity of proton solvation, the less multisolvation of that proton is required. Thus, more neutrals typically solvate protons in positive clusters (i.e., [M H], n higher than 2) than anions solvate protons in negative clusters (i.e., [2(A - H) ,H ] ). 

B5H9 also acts as a weak Brpnsted acid and, from proton competition reactions with other boranes and borane anions, it has been established that acidity increases with increasing size of the borane cluster and that arachno-boranes are more acidic than nido-horancs ... 

The diagrams also indicate why neutral c/oio-boranes BnHn4.2 are unknown since the 2 anionic charges are effectively located in the low-lying inwardly directed orbital which has no overlap with protons outside the cluster (e.g. above the edges or faces of the Bg oct edron). Replacement of the 6 Ht by 6 further builds up the basic three-dimensional network of hexaborides MB6 (p. 150) just as replacement of the 4 H in CH4 begins to build up the diamond lattice. 

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