Pincer abnormal carbene

Scheme 9.6 Formation of abnormal carbene pincer complexes 40-42 due to steric reasons.

Within the same triad, iron has the smaller ionic radius compared with ruthenium [20], although electronically and structurally the two elements should form the same complexes. They essentially do, but when a second pincer carbene ligand is coordinated, it coordinates with one NHC moiety in abnormal coordination mode [476] as opposed to ruthenium, where both pincer carbene ligands are coordinated normally [468], In the absence of coordinating anions (BPh instead of bromide) octahedral cationic complexes are formed instead of square pyramidal neutral ones (see Figure 3.158). The additional n-donor ligand compensates the positive charge electronically. 

A similar complex bearing a normal and an abnormal coordinated carbene moiety was reported by Zuo and Braunstein its catalytic activity in the transfer dehydrogenation of cyclooctene was however markedly lower than that of the complexes by Chianese/ Remarkably, CCC pincer complexes of iridium(III) developed by Chianese and by the gyoup of Herrmann and Kuhn " turned out to be active catalysts also for another C—H functionalization reaction, namely theborylation of arenes. Activities were however low compared to iridium(III) complexes with other ligands which were extensively optimized over the years. 

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