Iridium heterobimetallic complexes

Scheme 35 Boron-iridium heterobimetallic complex (32)-catalyzed A alkylation reaction of ammonia and primary, secondary, and cyclic amines with alcohols under air...

This section will focus on the reactivity and catalytic applications of homobimetallic iridiiun complexes containing the two metal centers in close proximity, purposely excluding examples of heterobimetallic systems that contain an iridium center, since their rich chemistry makes it impossible to deliver a comprehensive description within the scope of this chapter. However, for the interested reader, catalytic cooperativity in heterobimetallic complexes has been recently reviewed [69]. 

The combination of hard oxophilic early transition metals and soft nucleophilic late transition metals with opposite functionalities, provided they do not inhibit one another, is a priori ideal for promoting cooperative effect. A proof of concept can be found in the stoichiometric reactivity of early—late heterobimetallic complexes featuring a metal-metal bond [76]. It has been shown that such complexes are good candidates to realize the heterolytic cleavage of a bond in polar and apolar substrates. An illustrative example by Bergman et al. is the reaction of the Zr-lr complex 20 with carbon dioxide which leads to the rupture of the metal—metal bond (Scheme 18) [77]. The CO2 insertion occurs in the expected fashion with the CO2 bridging the two metals, the carbon atom coordinated to iridium, and the oxygen atom on the zirconium center. 

In order to vary the electronic situation at the carbene carbon atom a number of carbo- and heterocycle-annulated imidazolin-2-ylidenes like the benzobis(imida-zolin-2-ylidenes) [58-60] and the singly or doubly pyrido-annulated A -heterocyclic carbenes [61-63] have been prepared and studied. Additional carbenes derived from a five-membered heterocycle like triazolin-5-ylidenes 10 [36], which reveals properties similar to the imidazolin-2-ylidenes 5 and thiazolin-2-ylidene 11 [37] exhibiting characteristic properties comparable to the saturated imidazolidin-2ylidenes 7 have also been prepared. Bertrand reported the 1,2,4-triazolium dication 12 [64]. Although all attempts to isolate the free dicarbene species from this dication have failed so far, silver complexes [65] as well as homo- and heterobimetallic iridium and rhodium complexes of the triazolin-3,5-diylidene have been prepared [66]. The 1,2,4-triazolium salts and the thiazolium salts have been used successfully as precatalysts for inter- [67] and intramolecular benzoin condensations [68]. 

Polymetallic alkynes, with platinum heterobimetallic derivatives, 8, 647 homobimetallic complexes, 8, 645 polymetallic derivatives, 8, 648 Polymetallic cyclopentadienyl iridium complexes, preparation and characteristics, 7, 370... 

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