Some years ago, we reported the tridentate [PNP] ligand, which incorporates a hard amido unit flanked by two soft phosphine donors. While much new chemistry was discovered with this system, one problem encountered with the [PNP] ligand is that early transition metal complexes were prone to phosphine dissociation. In an effort to prevent this, a macrocyclic version was developed. This ligand, which is designated as [P2N2], combines two amido and two phosphine donors mutually trans disposed. The coordination chemistry of the [P2N2] ligand is incredibly rich including complexes of group 3 [Y(III)], group 4 [Zr(IV), Hf(IV)], and group 5 [Nb(III), Ta(V)] elements, as well as the lanthanides. One of the most attractive features of this ligand is that due to its small cavity size, larger metal ions are forced to sit above the "plane" of the donor atoms exposing more of the metal’s coordination sphere. Also, by altering the R substituent of the phosphine donors, its steric and electronic properties can easily be modified.
Another combination within this group of donor types is the [NPN] ligand set. An advantage of this system is that it offers the opportunity for coordinative unsaturation, at least compared to the [P2N2] ligand. In addition, the R substituents of the phosphine as well as both amide donors can be modified so that desired ligand properties can be obtained. Although most of the work on this system has focused on the group 5 elements, in particular Ta(V), this ligand is well suited for a wide variety of transition and lanthanide metal complexes in different oxidation states.
We have also initiated new tridentate ligands that incorporate N-heterocyclic carbene units as the central donors in the chelating array. These ligands are being investigated for their potential as ancillary ligands in activating small molecules such as H2, O2 and alkanes.