This research report summarizes recent results in the chemistry of lanthanide disiloxanediolates and metallasilsesquioxanes. Both classes of compound can be regarded as realistic model compounds for silica-supported lanthanide catalysts.
Heterobimetallic siloxanediolates of the lanthanidesThe chemistry of metallasiloxanes derived from silanediols, disiloxanediols and related Si-OH species continues to be an area of vigorous research activities [1][2][3][4][5][6][7]
Lanthanide(III) disiloxanediolates a) Lanthanide(III) mono(disiloxanediolates)By now, the chemistry of lanthanide bis(disiloxanediolates) is well established, and some lanthanide tris(disiloxanediolates) have also been prepared and structurally characterized (vide infra) [9]. Only the synthesis of mono-substituted rare earth metal complexes derived from difunctional siloxanediolate ligands had not been achieved until recently. Thus far only one compound of this type has been synthesized and structurally 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59
Scheme 1An X-ray study confirmed the presence of the first mono(trisiloxanediolate) complex of a lanthanide element. The molecule contains a puckered eight-membered for hafnium and uranium [6,7,9]. In these cases, however, the trisiloxanediolate ligand is formed by ring-enlargement reactions starting from disiloxanediolate precursors. A prominent structural feature of compound 5 is the presence of a so-called "ate" complex resulting from retention of lithium chloride. "Ate" complexes are a common phenomenon in organolanthanide chemistry. Their formation involves retention of alkali halides formed during the course of a reaction, thereby reflecting the strong tendency of the large lanthanide ions to achieve high coordination numbers [11]. In compound 5 the situation is such that a complex of the type "[Ph 2 Si(OSiPh 2 O) 2 ]YbCl" is sterically highly unsaturated (coordination number 3 at Yb). In this case stabilitzation is achieved not only by coordination of one THF ligand but also through "ate" complex formation with retention of two equivalents of LiCl. One lithium chloride is connected to the central ytterbium via two bridging chloride ligands. The second LiCl is coordinated in a rather peculiar way by additional coordination of a siloxide oxygen atom to Li. In both cases, the coordination sphere around lithium is supplemented by two THF ligands (Fig. 1) [10].
Page 4 of 53Wiley-VCH ZAAC 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 Accordingly these complexes have been termed "metallacrown" derivatives of scandium and yttrium [9]. In contrast, large Ln 3+ ions such as Pr 3+ , Nd 3+ or Sm 3+ do no fit into the center of the twelve-membered ...