Ruthenium complexes neutral

Imidazole is characterized mainly by the T) (N) coordination mode, where N is the nitrogen atom of the pyridine type. The rare coordination modes are T) - (jt-) realized in the ruthenium complexes, I-ti (C,N)- in organoruthenium and organoosmium chemistry. Imidazolium salts and stable 1,3-disubsti-tuted imidazol-2-ylidenes give a vast group of mono-, bis-, and tris-carbene complexes characterized by stability and prominent catalytic activity. Benzimidazole follows the same trends. Biimidazoles and bibenzimidazoles are ligands as the neutral molecules, mono- and dianions. A variety of the coordination situations is, therefore, broad, but there are practically no deviations from the expected classical trends for the mono-, di-, and polynuclear A -complexes. 

This type of hydrodehalogenation has been performed generally in the presence of organic or inorganic bases to neutralize the hydrogen halides formed. Among published results, the use of rhodium complexes as catalysts dominates, but palladium and ruthenium complexes have also been applied on a frequent basis. 

The reactivity of neutral square-planar d butatrienylidene complex 11 (Scheme 3.8) strongly deviates from that of cationic d ruthenium complexes. The deviation is readily understood when considering the orbital contributions of the metal and the carbon atoms of the chain to the LUMO. In d and d complexes the LUMO is predominantly localized at the metal, at Cl and C3. However, the relative contribution of the metal in d and d complexes is significantly different. In d complexes the metal contributes considerably less than Cl and C3, in d complexes its contribution is approximately equal to that of Cl and C3. 

Allenylidene-ruthenium complex Ib readily promotes the ROMP of norbornene, much faster than the precursor RuCl2(PCy3)(p-cymene) [39] (Table 8.1, entry 1). The ROMP of cyclooctene requires heating at 80 °C (5 min), however a pre-activation of the catalyst allows the polymerization to take place at room temperature. The activation consists, for example, in a preliminary heating at 80 °C or UV irradiation of the catalyst before addition of the cyclic aikene, conditions under which rearrangement into indenylidene and arene displacement take place [39] (Table 8.1, entries 2,3). The arene-free allenylidene complexes, the neutral RuCl2(=C=C=CPh2)... 

Paetzold and Backvall [27] have reported the DKR of a variety of primary amines using an analog of the ruthenium complex 1 as the racemization catalyst and isopropyl acetate as the acyl donor, in the presence of sodium carbonate at 90 °C (Fig. 9.17). Apparently, the function of the latter was to neutralize traces of acid, e.g. originating from the acyl donor, which would deactivate the ruthenium catalyst. 

Proton abstraction of the polar C-H bond with base is a well-established heteroly-tic C-H bond cleavage to obtain carbanion. Ruthenium complexes can act as a base in nonpolar media to provide highly selective catalyses, as in the Murahashi aldol and Michael reactions. These reactions are highly chemoselective under neutral and mild conditions, where cyanoesters preferentially react over 2,4-pentanedione with nucleophiles (Scheme 14.12) [26]. The mechanistic basis of this reaction is described in Section 14.2.2. 

This section surveys the use of various di-, tri-, and polynuclear ruthenium complexes as precursors for the homogeneous hydroformylation of alkenes. Several arbitrary assumptions have been made so as to include dinuclear starting complexes which are strictly not cluster compounds. Moreover, no distinction is made between neutral and anionic precursors. Also, in several cases, particularly in the patents, information is lacking concerning the intermediate species involved in the catalytic cycles. Interestingly, half of the described systems come from patents, and there are few fundamental studies which clearly establish the implication of cluster species during the catalysis. 

Treatment of 203 with PMe3 affords [ Ru(H)(pz)(cod) 2(PMe3)], 204, in which the neutral pyrazole ligand is replaced by trimethylphosphine (151). Further examples of ruthenium complexes containing bridging pyrazolate and hydride ligands were prepared (152, 153) from the ruthenium(II) precursor [Ru(H )(cod)(NH2NMe2)3](PF6). 

Neutral and cationic ruthenium complexes containing monodentate pyrazo-late groups were obtained by using the /3-diketonato complex [Ru(/>-cy-mene)(acac)Cl], or the dinuclear complex [ Ru(p-cy mcnc)Cl 2(/i-Cl)J as the starting material (7) (Scheme 11). 

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