Cluster tetrahedrane

Molecules such as P4 and the polyanionic clusters such as Si4- or As2- that are discussed in Section 13.2 are representatives of electron precise closo clusters. Organic cage molecules like tetrahedrane (C4R4), prismane (C6H6), cubane (C8H8), and dodecahedrane (C20H20) also belong to this kind of cluster. 

Similarly, the classical tetrahedrane D with six 2c2e bonds may be classified as a nido-cluster having 2n + 4 SE. It is derived from the doso-structure (trigonal-bipyramid), with one apical vertex missing. 

Example IV The thermal and oxidative isomerization of tetraalkyl-substituted tetrahedrane clusters (CR)4 to cyclobu-tadienederivatives. 

Example IV The Thermal and Oxidative Isomerization of Tetraalkyl Substituted Tetrahedrane Clusters to Cyclobutadiene Derivatives. Other remarkable structural changes during redox reactions, i.e. charge redistributions enforced by the respective energy differences, are observed for cluster compounds (1). On... 

Tetrahedrane (CR)4 with bulky tert. butyl substituents R has been synthesized by G. Maier and coworkers in 1978 (28). In their already classic investigations, an interesting valence isomerization has been observed on melting, the C4 cluster rearranges to the thermodynamically more stable cyclobutadiene derivative, which on photochemical energy uptake forms again the tetrahedrane (28) ... 

Fig. 28. Substitution of isolobal and isoelectronic CH and M(CO)3 (M = Co or Ir) fragments in tetrahedral clusters. It is interesting that the final member of the series C4H4 (tetrahedrane) has not been isolated.

The analogies are far from perfect. For example, CH dimerises to give acetylene HC CH but the isolobal Co(CO)3 does not dimerise. However, Co(CO)3 does behave as a trivalent radical. The cluster carbonyl Co4(CO)12 (Fig. 8.8) can be seen as analogous to tetrahedrane C4H4 (note, however, that three of the CO groups are bridging). The compounds Co (CO)3 (CR)4 n = 1,2,3) have been prepared, all containing tetrahedral Co C4 clusters. 

It reduces to ct + e. Again, the ordering is different, but the similarity between their electronic structure is obvious. A series of molecules and their similarities are illustrated in Figure 7-30. The first molecule is tetrahedrane and the last one is a cluster with metal-metal bonds which can be considered as being the inorganic analog of tetrahedrane. 

Vahrenkamp985 first synthesized the chiral tetrahedrane type clusters (270), which have been resolved for M = Mo via the diastereoisomeric phosphane (271),985 and a directed synthesis987 has achieved the construction of the 3-S-bridged tetranuclear CoFeMoW cluster (272). 

The studies of the tetrahedrane-Mi, clusters, Table VI, show quite generally that the importance of dissociative processes within the excited state lifetime is low. Clean, but low quantum yield, photoreactions are detectable in some cases. The resistance of the complexes to declusterification may be exploited to study and utilize bimoleoular processes, and in the case of the Hi(Rei,(C0) 12 the reactions and non-chemical, non-radiative decay are sufficiently low that the excited state lifetime allows... 

Finally, in analogy with organic chemistry, "polymerization" of metal-metal multiple bonds may lead to clusters as illustrated in eqs. 12 and 13. To date, the only definitely characterized oligomerization reaction of this type has been reported by McCarley et al. (8j(eq. 14), although Chisholm et al. ( ) have observed that Mo2(0Et)6 dimerizes to a tetranuclear complex of unknown structure and that the 0-H bond of isopropanol oxidatively adds to the WeW bond of W2(i-Pr0)6 to give a tetranuclear complex with an "open" as opposed to a closed, or cluster, structure (10). Also, some evidence has been presented that Cp2Mo2(C0)it may form unstable tetrahedrane intermediates (eq. 12) (. 

Additional tetrahedrane clusters are formed when vertices in Co4(CO)i2 are substituted with isolobal (see Isolobal Analogy) (or nonisolobal) metal fragments. In this category are the mixed Co-Rh and Co-Ir carbonyl complexes, which can be formed from the reaction of M4(CO)i2 (M = Rh, Ir) and Co2(CO)g. For example, Co3Rh(CO)i2, which has been used as a hydroformylation catalyst precursor, is prepared in this way. Furthermore, since the Co(CO)3 group is isolobal... 

Tetrahedrane (11) is the ruthenium analog of the much-studied tricobaltnonacarbonyl clusters Co3(CO)9CR see Cobalt Organometallic Chemistty). The substitution chemistry of (11) has been studied. A starting material is prepared from (11) by reaction with BX3 (equation 2), which gives the chloro and bromo compounds (12). In addition, (11) can also be treated directly with compounds such as diynes to yield interesting substitution products. For example, when (11) is refluxed in THF with diphenylbutadiyne, cis- and trans-alkene isomers of two alkyne insertion regioisomers are formed (equation 3). The product seems to arise from dehydrogenation of one end of the diyne to yield cis and trans enynes and an imsaturated monohydride cluster intermediate, which then reacts with the enynes to yield the allylic derivative products... 

The neutral chloride B4CLt, with four skeletal bond pairs, and tetrahedrane C4H4 or tetraphosphorus P4 and related molecules, with six skeletal bond pairs, are systems with regular tetrahedral structures. Tetrahedral molecules held together by six skeletal bond pairs can of course be accommodated in the carborane cluster systematics as nido clusters with n atoms and n -1- 2 skeletal bond pairs, if a low connectivity (axial) vertex of the parent trigonal bipyramid is left vacant, instead of the expected high-connectivity (equatorial) vertex. 

AlkyM Cobalt Carbonyl Alkynes are protected as the tetrahedrane-like clusters 14.7. In this case, deprotection is carried out oxidatively with a reagent such as FeCIa, or Et3NO as we saw in Section 4.3, oxidation often... 

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