Aluminium

Aluminium alkyls can be prepared by the transmetallation reaction 1.1, or from Grignard reagents (equation1.2). On an industrial scale, the direct reaction of Al with a terminal alkene and H₂ (equation 1.3) is employed.

                                                  (1.1)

                                       (1.2)

                                (1.3)

  Reactions between Al and alkyl halides yield alkyl aluminium halides (equation 1.4); note that 18.7 is in equilibrium with [R₂Al(µ-X)₂AlR₂] and [RXAl(µ-X(₂AlRX] via a redistribution reaction, but the bridge Aluminium compound in below  predominates in the mixture.

                          (1.4)

                                                   (1.5)

  Alkyl aluminium hydrides are obtained by reaction 1.5. These compounds, although unstable to both air and water, are important catalysts for the polymerization of alkenes and other unsaturated organic compounds. Ziegler–Natta catalysts containing trialkyl aluminium compounds are introduced in Box 18.3.

                                                             (1.1)

   Earlier we noted that R₃B compounds are monomeric. Trimethylaluminium (mp 313 K) possesses structure 1.1 and so bonding schemes can be developed in like manner as for B₂H₆. The fact that Al^_C_bridge > Al^_C_terminal is consistent with 3c-2e bonding in the Al^_C^_Al bridges, but with 2c-2e terminal bonds. Equilibria between dimer and monomer exist in solution, with the monomer becoming more favoured as the steric demands of the alkyl group increase. Mixed alkyl halides also dimerize as exemplified in structure 18.7, but with particularly bulky R groups, the monomer (with trigonal planar Al) is favoured, e.g.(2,4,6-^tBu₃C₆H₂)AlCl₂ (Figure 1.1a).

Fig. 1.1 The solid state structures (X-ray diffraction) of (a) (2,4,6-^tBu₃C₆H₂(AlCl₂ [R.J. Wehmschulte et al. (1996) Inorg. Chem., vol. 35, p. 3262], (b) Me₂Al)µ-Ph)₂AlMe₂ [J.F. Malone et al. (1972) J. Chem. Soc., Dalton Trans., p. 2649], and (c) the adduct L_ًAlMe3ق4 where L is the sulfur-containing macrocyclic ligand 1,4,8,11-tetrathiacyclotetradecane [G.H. Robinson et al. (1987) Organometallics, vol. 6, p. 887]. Hydrogen atoms are omitted for clarity; colour code: Al, blue; C, grey; Cl, green; S, yellow.

 Triphenylaluminium also exists as a dimer, but in the mesityl derivative (mesityl=2,4,6-Me₃C₆H₂), the steric demands of the substituents stabilize the monomer. Figure 1.1b shows the structure of Me₂Al)µ-Ph(₂AlMe₂, and the orientations of the bridging phenyl groups are the same as in Ph₂Al(µ-Ph)₂AlPh₂. This orientation is sterically favoured and places each ipso-carbon atom in an approximately tetrahedral environment. The ipso-carbon atom of a phenyl ring is the one to which the substituent is attached; e.g. in PPh₃, the ipso-C of each Ph ring is bonded to P.   In dimers containing RC≡C-bridges, a different type of bonding operates. The structure of Ph₂Al)PhC≡C(₂AlPh₂ (1.2) shows that the alkynyl bridges lean over towards one of the Al centres.  This is interpreted in terms of their behaving as σ ,π-ligands: each forms one Al^_C σ-bond and interacts with the second Al centre by using the C≡C π- bond. Thus, each alkynyl group is able to provide three electrons for bridge bonding in contrast to one electron being supplied by an alkyl or aryl group; the bonding is shown schematically in 1.3.

    (1.2)                                              (1.3)

Trialkylaluminium derivatives behave as Lewis acids, forming a range of adducts, e.g. R₃N.AlR₃, K[AlR₃F], Ph₃P.AlMe₃ and more exotic complexes such as that shown in Figure 18.8c. Each adduct contains a tetrahedrally sited Al atom. Trialkylaluminium compounds are stronger Lewis acids than either R₃B or R₃Ga, and the sequence for group 13 follows the trend R₃B < R₃Al > R₃Ga > R₃In > R₃Tl.

    The first R₂Al^_AlR₂ derivative was reported in 1988, and was prepared by potassium reduction of the sterically hindered {(Me₃Si)₂CH}₂AlCl. The Al^_Al bond distance in {(Me₃Si)₂CH}₄Al₂ is 266pm (compare r_cov = 130 pm) and the Al₂C₄ framework is planar, despite this being a singly bonded compound. A related compound is (2,4,6- ⁱPr₃C₆H₂)₄Al₂ (Al^_Al = 265 pm) and here the Al₂C₄ framework is non-planar (angle between the two AlC₂ planes = 458). One-electron reduction of Al₂R₄ (R = 2,4,6-ⁱPr₃C₆H₂) gives the radical anion [Al₂R₄]^- with a formal Al^_Al bond order of 1.5. Consistent with the presence of a π-contribution, the Al^_Al bond is shortened upon reduction to 253pm for R = (Me₃Si)₂CH, and 247pm for R = 2,4,6-ⁱPr₃C₆H₂; in both anions, the Al₂R₄ frameworks are essentially planar. In theory, a dialane R₂Al^_AlR₂, 1.4, possesses an isomer, 1.5, and such a species is exemplified by (η⁵-C₅Me₅)Al^_Al(C₆F₅)₃. The Al^_Al bond (259 pm) in this compound is shorter than in compounds of type R₂Al^_AlR₂ and this is consistent with the ionic contribution made to the Al^_Al interaction in isomer 1.5.

(1.4)                                (1.5)                                       (1.6)

    The reaction between cyclopentadiene and Al₂Me₆ gives CpAlMe₂ which is a volatile solid. In the gas phase, it is monomeric with an η²-Cp bonding mode (1.6). This effectively partitions the cyclopentadienyl ring into alkene and allyl parts, since only two of the five π-electrons are donated to the metal centre. In the solid state, the molecules interact to form polymeric chains (Figure 1.2a).

Fig. 1.2 The solid state structures (X-ray diffraction) of (a) polymeric CpAlMe₂ [B. Tecle et al. (1982) Inorg. Chem., vol. 21, p. 458], and (b) monomeric(η²-Cp)₂AlMe [J.D. Fisher et al. (1994) Organometallics, vol. 13, p. 3324]. Hydrogen atoms are omitted for clarity; colour code: Al, blue; C, grey.

   The related compound Cp₂AlMe is monomeric with an η²-mode in the solid state (Figure 1.2b). In solution, Cp₂AlMe and CpAlMe₂ are highly fluxional. A low energy difference between the different modes of bonding of the cyclopentadienyl ligand is also observed in the compounds (C₅H₅)₃Al (i.e. Cp₃Al),(1,2,4-Me₃C₅H₂(₃Al and )Me₄C₅H(₃Al. In solution, even at low temperature, these are stereochemically non-rigid, with negligible energy differences between η¹-, η²-, η³- and η⁵-modes of bonding. In the solid state, the structural parameters are consistent solid state, the structural parameters are consistent with the descriptions:

   These examples serve to indicate the non-predictable nature of these systems, and that subtle balances of steric and electronic effects are in operation.

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