Synthesis and Reactivity of Ruthenium and Platinum Amido and Carbene Complexes: Application Toward Carbon-Nitrogen Bond Forming Reactions

Abstract

Ru carbene complexes have been investigated for potential application toward carbon-nitrogen bond forming and breaking reactions. The reactivity of Grubbs-type olefin metathesis catalysts (Cl)2(L)(L')Ru=CHPh (L = L' = PCy3 or PPh3; L = PCy3 and L' = H2MesNHC) has been studied with cyclic and acyclic imines. The Ru benzylidene complex (Cl)2(PCy3)2Ru=CHPh undergoes a ligand exchange reaction with 1-pyrroline at room temperature, and oligomerizes 1-pyrroline at 90 oC. The low molecular weight poly(1-pyrroline) has been isolated and characterized. The Ru benzylidene complex has no reactivity toward acyclic imine C=N bonds, but reacts with enamine C=C bonds that form through tautomerization of imines.

The Ru amido complexes, (PCP)Ru(CO)(NHPh) and (PCP)Ru(CO)(NHPh)(PMe3) (PCP = 2,6-(CH2P-t-Bu2)C6H3), have been synthesized and characterized. Reaction of (PCP)Ru(CO)(NHPh)(PMe3) with acetonitrile produces the amidinate complex (PCP)Ru(CO)(NHC(Me)NPh). Kinetic studies have demonstrated the formation of the amidinate complex occurs through dissociation of the PMe3 ligand, coordination of MeCN, and intramolecular nucleophilic attack of the amido ligand toward MeCN. The intramolecular nucleophilic attack reaction has been extensively studied with aromatic nitriles, carbodiimides, isocyanates, carboxamides and aldehydes. Reaction of (PCP)Ru(CO)(NHC(Me)NPh) with pentafluorobenzonitrile results in formation of a new amidinate complex (PCP)Ru(CO)(NHC(C6F5)NPh). The complex (PCP)Ru(CO)(NHC(C6F5)NPh) can initiate a C-F bond cleavage reaction of C6F5CN with ortho regioselectivity in the presence of ROH (R = H or Me).

Reaction of five-coordinate complex (PCP)Ru(CO)(OTf) (OTf = OSO2CF3) with NaBAr'4 (Ar' = 3,5-(CF3)2C6H3) under different conditions results in formation of [(PCP)Ru(CO)(ClCH2Cl)][BAr'4], [(PCP)Ru(CO)(N2)][BAr'4] or [(PCP)Ru(CO)(FC6H5)][BAr'4]. Calculations have confirmed that the possible four-coordinate Ru complex with double agostic interactions has a higher ground state energy than these five-coordinate complexes with labile ligands. The coupling reaction of the carbene or vinylidene ligand with the aryl ipso carbon of the PCP ligand has been observed upon reaction of [(PCP)Ru(CO)(ClCH2Cl)][BAr'4] with PhCHN2 or PhCCH. Calculations have supported that the Ru carbene or vinylidene intermediate is formed, and that the coupling reaction is thermodynamically favored.

The Pt(IV) amido systems are studied with potential application toward C-H activation and polymerization sequences. Preliminary studies include isolation of a Pt(IV) amido complex (NCN)Pt(Me)2(NHPh) (NCN = 2,6-(pyrazol-1-yl-CH2)C6H3).