Ligand exchange, pincer complexes

Figure 3.152 The redox and ligand exchange behaviour of a cobaltfII) pincer carbene complex.

A similar route was used by us to synthesize pincer complexes with the monoanionic CNC ligand 5 (bimca, l,8-bis(imidazolin-2-yliden-l-yl)carbazolide). Three equivalents of the base (lithium diisopropylamide (LDA), MeLi) are necessary to obtain the in situ formed Li(bimca) complex 23 (Scheme 9.2). Salt metathesis and ligand exchange with [Rh(CO)2(p-Cl)]2 afforded the Rh(CO)(bimca) complex 24 in good yields. 

As a ligand for trapping unstable or transient species, carbon monoxide worked well for Goldman and coworkers [33]. They reacted the pentacoordinated (PCP)Ir(III) pincer complex (75) with CO in benzene at 6 °C affording compound (76) quantitatively (Scheme 2.35). Unlike (75), carbonyl compound (76) does not undergo arene exchange and is stereochemically rigid at ambient temperature. 

Steinke et al. synthesized a new type of PCP pincer ligand where the central carbon donor is a saturated NHC (Figure 4.11) [45]. These structures showed a considerable twisting of the ligand framework above and below with respect to the square plane of the metal complex and the PCP donors. In solution, this twisting is not fixed, as the two possible chiral conformations exchange first on the NMR time scale. 

---