Our group has been actively exploring the chemistry and reactivity of the 4,5-diazafluorene (LH) and 4,5-diazafluorenide ligands (L-). Diazafluorene is a bipyridyl ligand with a methylene linker which can be deprotonated to form the diazafluorenide ligand. An interesting feature of the diazafluorenide ligand is that it potentially has two metal-binding sites: the N-donors and the C-donors of central cyclopentadienyl-like ring (see Fig 1).
We explored some of the initial coordination behavior of this L- ligand and its use as a ligand in hydrogenation catalysis (See Organometallics (2008) 27, 3587-3592; European Journal of Inorganic Chemistry (2009) 2083-2089). Unexpectedly we discovered a facile oxidation of the CH2 group into a carbonyl group by air in [RuCl2(LH)(PPh3)2], leaving the two PPh3 ligands intact (See Fig 2. and Dalton Transactions (2008) 5879-5881).
We reported that the zwitterionic complex 1 (see Fig 3.) can reversibly split H2 over a long-range between the Ru(II) center and a carbanion of the diazafuorenide ligand (L-) where the distance between the Ru center and the backbone carbon is ~5.0 Å (See Chemical Communications (2010) 46, 556-558).
Intrigued by this reactivity likely resulting from unquenched basicity on the negatively charged ligand and the Lewis acidity of the positively charged metal center within the zwitterion we decided to explore further reactivity of 1 with other small molecules such as CO2. We discovered a reversible formal insertion of CO2 into the remote C–H bond of the diazafluorenide ligand (L-) complex 1 which occurs at ambient temperature (see Fig 4. and Chemical Communications (2012), DOI: 10.1039/c2cc17933d).
Current efforts are focussed on exploring the reactivity of metal complexes of diazafluorenide-based ligands for small molecule activation and catalysis. We have also been working towards the synthesis of multi-metallic complexes for catalysis utilizing the diazafluorenide ligand scaffold. Our approach has been to functionalize the C9-position with a second coordinating group such as a phosphine (see Fig 5.). We have demonstrated the self-assembly of Rh(I) and Au(I) macrocycles by utilizing ligand exchange of a phosphine functionalized diazafluorenide ligand from Cu(I)-NHC complexes (See Fig 6. and Organometallics (2012), 31, 2184-2192).
Metal-Organic Frameworks (MOFs)
We are also researching metal organic frameworks (MOFs) with a focus on sensing and catalysis. Through our work, we have developed MOFs with sensing capabilities for solvents (Figure 8). We are continuing our research by systematically improving our existing MOF sensors.
Figure 7: Phosphorescence as a result of guest-MOF interactions