Phosphaamidines and Phosphaguanidines as Ligands: Coordination Chemistry and Catalysis
Hemilabile ligands are useful because the activity at the metal centre can be manipulated and controlled by the labile groups. P,N donors are widely used hemilabile ligands in coordination and organometallic chemistry. Phosphorus is generally considered a soft donor and inert. Alternatively, nitrogen is considered a moderately hard donor and is labile. Late transition metals can complex with either the P or the N as a monodentate ligand or through both the P and the N as a bidentate ligand depending on the hardness of the metal centre and the ligand sterics. Phosphaamidines [RC(PR2)=NR] and phosphaguanidines [R2NC(PR2)=NR] are small bite angle bidentate ligands and promising hemilabile ligands. Having strongly basic sites near the metal centre should promote heterolytic cleavage of H2 or proton transfer that can assist in homogenous catalytic reactions like CO2 hydrogenation. A facile route is developed to prepare acyclic and aprotic phosphaamidine [PhC(PR2)=NR, R=Ph, iPr] and phosphaguanidine [Me2NC(PPh2)=NiPr] ligands that coordinate to Cu(I), Rh(I) and Ru(II) metal centres. Crystal structures are reported for all complexes. Coordination to Cu(I) occurs either through P in η1 mode or by both P or Nimine in a η2 mode. Coordination to Rh(I) occurs through η1or η2 modes while to Ru(II) occurs only through η2 mode. Tests of CO2 hydrogenation using phosphaamidine complexes showed that Ru(II) phosphaamidines are active precatalysts than Rh(I) phosphaamidines. Substitution of phenyl with isopropyl doesn’t have significant impact on the TON or the yield of formic acid in the presence of DBU. However, using Ru(II) phosphaguanidine instead of Ru(II) phosphaamidine increases the yield. A Ru(II) phosphaguanidine complex is also a potential precatalyst for the hydration of benzonitrile. It is also shown that a guanidine [Me2NC(NPh2)=NiPr] and a structurally analogous phosphaguanidine [Me2NC(PPh2)=NiPr] both act as Brønsted bases and have almost same basicity in acetonitrile solution. The proton coordinated to [Me2NC(PPh2)=NHiPr]Cl and therefore the basicity of the neutral form are both retained once the P is coordinated to Cu(I). The retention of basicity upon coordination of this salt to a metal centre is also supported by the observation of π electron delocalization within the amidine unit by X-ray crystallography.