Sandwich ligands, catalytic activity

The bare nitrogen analog of dihydroazaborine, where the R substituent on N is effectively replaced by an electron pair, is the azaboratabenzene anion H4C4BRN. The synthesis of this ligand and a ruthenium sandwich complex Cp Ru(H4C4BPhN) that is catalytically active in the acylation of benzyl alcohol have been reported.151... 

Table II. Catalytic Activity of Sandwich Ligands 1 and 2 in Nucleophilic Substitutions on n-Octyl Methanesulfonate with Alkali Halides (MY) under Solid-Liquid Conditions in Toluene at 50°C...

It was found that substituted cyclopentadienyltitanium trichloride, in a combination with methylaluminoxane, exhibited higher catalytic activity for syndiospecific polymerisation of styrene than CpTiCl3 [52,53]. The efficiency of half-sandwich titanocenes as methylaluminoxane-activated precatalysts for the syndiospecific polymerisation of styrene increases in the following order CpTi(OMe)3 < Me4(Me3Si)CpTi(OMe)3 < Cp Ti(OMe)3. Thus, electron-donating substituents on the cyclopentadienyl ligand lead to increased catalyst activity and stability, stereospecificity and polymer Mw. 

An important advantage of the Cp tethered to NHC ligands compared to the nonlinked systems is that chelation does not consume an extra coordination site, leaving the metal complexes with an additional site for catalysis (Fig. 10.1). As we have already pointed out, the tethered NHC unit could not only increase the stability of the half-sandwich metal-NHC complexes by chelation, but also could enhance the catalytic activity of their metal complexes by the stabilization of intermediate active species. In addition, the hemilabile dynamic behavior of the Cp-NHC ligand may allow to efficiently control the reactivity and stability of the catalytically active center. 

Several studies have been published on neutral and cationic half-sandwich (aiene)Ru(II) derivatives containing differently substituted pyrazoles and pyrazolates [9], and also pyrazole-phosphinite ligands, together with preliminary tests on their catalytic activity in transfer hydrogenation of cyclohexanone by propan-2-ol [10]. 

Figure 4. Comparison of astacin and the catalytic domain of coUagenase. Stereo ribbon diagram [54] of the structure of astacin (green) and the catalytic domain of coUagenase (orange) superimposed using a rigid body fit of the catalytic hehces. The catalytic zinc atom (cyan) and the zinc ligands (blue) are marked. The overall similarity in the open beta sandwich region, N-terminal to the active site helix is apparent.

---