Tripodal complexes

SiO)3Ti-H and (=SiO)3Ti species react very easily with alcohols to give titanium tris-siloxy mono-alkoxy. Step by step, following the methods described in Scheme 2.10, it is thus possible to obtain well-defined mono-, bi- or tripodal complexes that have been characterized by chemical analysis and by chemical and spectroscopic methods such as IR and solid-state NMR ( H and C). 

The number n can be regulated by methods disclosed earlier in this book. A species linked to Aerosil silica supported by three bonds, (=SiO)3TiOR, is far more active than the one linked by only one bond, =SiOTi(OR)3 (R=H, Me, TBu) (Figure 3.28). In addihon, by comparison to a series of complexes (=Si0) Ti(0 Bu)4 (n = 1, 2 or 3), the monopodal and bipodal complexes exhibited a similar achvity, but far less than that observed for the tripodal complex. The latter displays a similar initial activity when supported on non-porous Aerosil silica, Si02.(soo), or mesopo-rous MCM-41(5oo)- The study has been continued on tripodal species, which are more active and better linked to the support. 

In the literature there has been much debate regarding the role of the lattice or extralattice Ti in Ti silicalite for a variety of oxidation reactions. In order to have a more precise idea of the role of the lattice or surface Ti and more specifically of the role of the coordination sphere of Ti, a series of monopodal and tripodal titanium surface complexes (i. e., =SiOTi(OR)3 and ( SiOIsTiOR) were derived by the reaction of the Ti alkyl (Structure 1) and hydride species with water, oxygen, methanol, and tert-butanol. The resulting complexes were then used in the epoxidation of 1-octene by tert-butyl hydroperoxide. Tripodal complexes, especially (=SiO)3Ti( Bu), were found to be significantly more active and more selective for the epoxidation of 1-octene than their monopodal counterparts [22]. 

Mechanisms and rates of dioxygen binding to copper(I) were determined for a series of tripodal complexes that afford end-on superoxo intermediates. Kinetic parameters for a these complexes proved to be rather similar, with activation enthalpies ranging about 3(MK) k.l/mol for reactions in coordinating solvents (nitriles). Activation entropies were relatively small for these processes both small positive and small negative values have been reported.40 This behavior is indicative of... 

Figure 3 Synthesis of the rf -tripod complex 4 and the reaction of complex 4 with [RhCl2 CO)2]. HOTf= HOSO2CF2...

The catalytic activity and selectivity of 2 was also studied towards the hydroformylation of oct-l-ene. The mole number-time profile of oct-l-ene and reaction products is shown in Figure 5. With this bridging tripod complex the activity is about the same as that of dppm complex 5 (50% conversion after 40 hours) however, the n/iso ratio is much higher, between 8 and 9. The presence of the extra phosphine on the central carbon atom clearly has a marked effect on the selectivity of the reaction. 

With bulky PR3 ligands, dissociation of ligand occurs to give the highly reactive [Ni(PR3)3] and even [Ni(PR3)2]. A range of mixed PR3 and CO complexes [Ni(CO)4 c(PR3) ] (x=l-4) and alkene complexes [Ni(PR3)2(alkene)] have beenreported. The complex [Ni(triphos)(CO)] (21) andrelated tripodal complexes can be readily isolated by reduction of Ni precursors in the presence of CO and triphos the analogous nitrosyl complex [Ni(triphos)(NO)]+ (22) (NO is coordinated as NO+ in this complex) can be prepared by a similar route. 

Ion pairing with hydroxide ion would also lead to curvature if kg. were important. Buckingham [15] has reported that three 3+ tripodal complexes, [Co(tren)(NHj)j (3), s-[Co(Metren(NHj)2] (2) and a/i//-p-[Co(Metren) (NHj)J (1) Fig. 3.11 display such curvature. 

Fig. 3.11 Structures of the tripodal complexes. The p, s, and t nomenclature for [Co(tren)(NH3)X]2+ ions denote primary, secondary and tertiary N centres respectively, trans to the in plane ammine (group being substituted) and anti and syn denote the relationship of the methyl substituent to this amine.

Figure 15.29 Structure of a triscarbene thallium(l) tripodal complex [155],...

In metal-tripod complex based catalytic reactions like hydroformylation, at least one of the ligand(MeC(CH2PPh2)3)-metal bonds dissociate to create free coordination site for substrate binding [89] as shown below -... 

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