Aluminum dimers/trimers

The sum of the covalent radii for the aluminum-sulfur pair is 2.23 A and the observed interatomic distances in the cited compounds fall in the range 2.35-2.40 A (see the review [113]). In monomeric Al[S(2,4,6-Bu 3QH2)]3, in which bulky organic groups prevent self-assembly, the Al-S interatomic distances are in the range 2.177-2.191 A [114], suggesting a small amount of r-delocalization. Supra-molecular self-assembly persists in solution, and the dimer-trimer equilibria have been investigated by NMR spectroscopy. 

The NMR spectrum of the pure aluminosiloxane (n = 2) in CH2CI2 (Fig. 69) is composed of an intense narrow band at 3.5 ppm assigned to octahedrally coordinated aluminum, a broad band at 64 ppm assigned to tetrahedrally coordinated aluminum, and a rather weak, broad band at 38.9 ppm assigned to pentacoordinate aluminum. Based on the structures of aluminum alkoxides [181], Pouxviel et al. [175] proposed that the aluminosiloxane solution is a mixture of the dimer, trimer, and tetramer ... 

Iron polycations are not as well known as chromium or aluminum polycations because of the lability of ferric complexes. Only a few polycations (dimers, trimers) have been characterized in acidic solutions (pH < 1.5) [39]. [Fe2(OH)2] and [Fc20] dimers are present in organic complexes such as L3(H20)Fe(OH>2-Fc(OH2)L3 and LjFeOFeLs, where the L3 ligand is a tridentate picolinate and L5 a tridentate amine [16,40,41]. Other polydentate ligands, such as proteins, are able to stabilize many polynuclear iron complexes [42-45]. The existence of lire aquo complexes [(H20)4Fe2(0H)2(0H2)4] " and [(H20)5Fe20(0H2)s] is very probable in spite of the lack of structural data. 

Other organometallic compounds of aluminum include the alkyl hydrides, R2A1H. Molecular association of these compounds leads to cyclic tetramers. When the dimeric and trimeric compounds are dissolved in a basic aprotic solvent, the aggregates separate as a result of formation of bonds between A1 and the unshared pair of electrons on the solvent molecule. Toward Lewis bases such as trimeth-ylamine, aluminum alkyls are strong Lewis acids (as are aluminum halides). 

Monomeric dialkylaluminum hydrazides with terminal hydrazido groups (A, Scheme 2) were observed only in those cases in which donor ligands such as ether molecules (28 to 30) or hydride ions (26) saturate the aluminum atoms to yield coordination numbers of four. In the absence of Lewis bases dimers are formed of the monoaluminum and monogallium monohydrazido derivatives, a trimer has never been detected by crystal structure determinations. Three different types of structures were reported ... 

A further point of interest is that in both the dimeric and trimeric species shown, the beryllium atom still has a vacant orbital available which may be used in adduct formation without disruption of the electron-deficient bond. This type of behavior leads to the formation of dimers with four-coordinate beryllium atoms, e.g., structure XX (86). This structure has been determined in the solid state and shows that the phenylethynyl-bridging group is tipped to the side, but to a much smaller extent than observed in the aluminum derivative (112). One cannot be certain whether the distortion in this case is associated with a it - metal interaction or is simply a result of steric crowding, crystal packing, or the formation of the coordination complexes. Certainly some differences must have occurred since both the Be—Be distance and Be—C—Be angle are substantially increased in this compound relative to those observed in the polymer chain. 

Fig. 2 Examples of photosensitizers presently approved for clinical applications or in clinical studies. mTHPC, tetra (meso-hydroxy) phenyl chlorin BPD-MA, benzoporphyrin derivative Photofrin is a mixture of several compounds where dimers or trimers of the indicated structure are assumed to be of major importance PpIX, protoporphyrin IX - accumulating upon treatment with 5-aminolevulinic acid or its ester derivatives, NPe6, HPPH, Hexyl pyropheophorbide TPPS2a, disulfonated (adjacent) tetraphenylporphin AlPcS2a, disulfonated (adjacent) aluminum phthalocyanine. Areas with ionic side groups are indicated in shadow...

When propylene oxide is polymerized by trimethyl aluminum at 0 °C, cyclic tetramer together with some cyclic dimer and trimer is formed in the first stages of the polymerization (5). 

The dimers have a four-membered A12N2 ring (6-XV) which may have isomers if the groups are different. The aluminum imides [RA1(NR )] usually have cubane or larger cage structures (6-XVI) trimers (e.g., the alumazene 6-XVII) and heptamers have also been found, depending on the size of R and R and the reaction conditions.29... 

The alkoxides of aluminum are polymeric with aluminum achieving four or six coordination. The adopted structure depends greatly on the bulkiness of the alkyl groups. For sterically large groups, for example, tertiary butyl, the preferred structure is a dimer (25). The isopropoxide, on the other hand, is trimeric (26) at elevated temperatures but tetrameric (27) at room temperature. ... 

Dialuminoxanes obtained in this reaction have been determined to have dimeric or trimeric structures depending on the substituent (Bu or Et) on aluminum. 

One of the most interesting reactions involves trimethylaluminum with 2,6-diisopropylaniline. The initial product is an aluminum-nitrogen dimer, however, upon further heating additional alkane elimination occurs resulting in the Al-N trimer (Equation (10)) ... 

Surprisingly, aluminum cluster reactivity with O2 depends sensitively on the cluster size. The atom and dimer are very reactive, but a sharp decrease in reactivity is observed for the trimer. Above the trimer a nearly montonic increase in reactivity occurs as the number of aluminum atoms increases. By n = 25-30 the clusters are once again nearly as reactive as the dimer. The dominant product peak is AI3O2, but it is not produced directly through chemisorption of O2 onto an AI3 cluster. The species appears to be the product... 

In 1974, Jeffery et al. reported the first use of well-defined dimeric or trimeric aluminum enolates in the aldol reaction. - Reaction of dimethyl-Z(0)-4,4-dimethylpent-2-en-2-oxyalane (123) with acetaldehyde or benzaldehyde provides the anti aldol products (124) as chelated dimers involving five-coordinate aluminum atoms, as shown in Scheme 51. However, the corresponding (0)-enolate (125) gives dimeric products of syn configuration which undergo isomerization, if not hydrolyzed immediately, to provide the same anti products. This syn-anti isomerization is much more facile with benzaldehyde, and is in sharp contrast to aldol reactions mediated by boron. 

Cryoscopic and NMR investigation (14) indicates that starting from the pure dimer, aluminum isopropoxide undergoes a very fast equilibrium shift to a mixture containing mainly ftie trimeric form followed by a quite slow conversion to the tetrameric form. Conversely, starting from the purely tetrameric form, an equilibrium mixture is slowly obtained consisting of all three forms. 

Kleinschmidt (14) postulated an alternative trimeric form containing two 4-coordinate A1 centers and one penta-coordinate aluminum as shown in Figure 3. He proposed a tetramer to dimer conversion mechanism (Scheme 1) and a trimer to hexamer conversion mechanism (Scheme II). Both of these proposed mechanisms involve species containing 5-coordinate A1 centers. 

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