Metal-oxygen and metal-nitrogen multiply bonded complexes are ubiquitous intermediates in synthetic as well as biological oxidation chemistry. While the effect of π-π* interaction between filled ligand orbital and vacant d-orbitals of early transition metals tend to stabilize the metal-ligand multiply bonded complexes of early transition metals, the same effect destabilizes the metal-ligand multiple bonds in mid-to-late transition metals due to π-π* interaction between filled ligand orbital and filled d-orbitals. These complexes are thus difficult to crystallographically characterize. Synthetic tuning of the ligands can stabilize these reactive complexes for characterization, but it typically renders the resulting complexes unreactive. The highlight of my research so far is demonstrating direct characterization of a reactive Ru2-nitride intermediate without altering its reactivity or stability, using photo-crystallography. The results obtained from photo-crystallographic experiments were in good agreement with the results obtained from EPR and EXAFS experiments, confirming the reliability of this method to characterize reactive molecules.
Figure 1. Photo-crystallographic characterization of reactive Ru2 nitride 2. Photolysis (λ = 365 nm) of a single crystal of Ru2 azide 1 at 95 K results first in a phase transition (P21/c to C2/c) to generate 1linear, which is promoted by photo-induced crystal heating, and subsequently in extrusion of N2 to generate Ru2 nitride complex 2. Thermal ellipsoids are drawn at 50% probability. H- and Cl-atoms are omitted for clarity. Metrics: 1, Ru1−Ru2, 2.3445(8) Å; Ru1−N1, 2.047(6) Å; Ru1−N1−N2, 152.9(5)°; 1linear, Ru1−Ru2, 2.373(2) Å; Ru1−N1, 2.01(1) Å; Ru1−N1−N2, 165.5(1)°; 2, Ru1− Ru2, 2.408(3) Å; Ru1−N1, 1.72(2) Å.
Figure 2. X-band EPR spectra 2 generated in the solid state (powdered crystals, black) and a frozen CH2Cl2 glass (red) demonstrate that nitride 2 generated in the solid state is spectroscopically indistinguishable from that generated in a frozen glass.