Isonitrile insertion into the Ru-O bond and migratory C-C bond formation. Novel organoruthenium imidic ester and acyl species

Abstract

Organometallics of type [Ru(C₆H₂O-2-CHNHC₆H₄R-3-Me-5)(PPh₃)₂(CO)(Cl)], 3 (R = H, Me, OMe, Cl), incorporating a four-membered C,O chelate ring smoothly react with CNBu^t in benzene solution affording the yellow-colored organoruthenium imidic ester hydrochloride system [Ru(C₆H₂OCN(Bu^t)HCl-2-CHNC₆H₄R-3-Me-5)(PPh₃)₂(CO)(CNBu^t)], 4, in which a five-membered C,C chelate ring is present. While stable in the solid state, 4 is spontaneously reactive in solution, furnishing the red-colored chelate [Ru(C₆H₂(CO-1)O-2-CHNHC₆H₄R-3-Me-5)(PPh₃)₂(CNBut)(Cl)], 5, incorporating five-membered acyl-phenolato C,O chelation. Chloride elimination from 5 by excess CNBut furnishes [Ru(C₆H₂(CO-1)O-2-CHNHC₆H₄R-3-Me-5)(PPh₃)₂(CNBu^t)₂]Cl, [6]Cl. Characteristic spectral features of the families are reported. A notable electrochemical feature is that the Ru(III)/Ru(II) reduction potential increases with increasing number of carbon atoms in the coordination sphere: 5 < 6⁺ < 4. The X-ray structures of 4(Me), 5(H), and [6(Me)]Cl·2CH₂Cl₂ are reported. The equatorial coordination planes are RuC₄, RuC₂OCl, and RuC₃O, respectively, the axis being uniformly defined by the trans RuP₂ fragment. Both 5(H) and 6(Me)⁺ display iminium-phenolato N-H···O hydrogen bonding, while in 4(Me), N-H···Cl and C-H···O hydrogen bondings are present. In the transformation 3 → 4, one CNBut molecule displaces the chloride ligand and another inserts into the Ru-O bond, generating a chelated imidic ester function. In 4(Me) the Ru-CO bond (1.891(8) Å) lying trans to the imidic ester carbon is 0.1 Å longer than that in the precursor complex 3(Me). The Ru-C(aryl) distance is also longer in 4(Me) by 0.04 Å. Thus both the bonds are activated by isonitrile insertion and the transformation 4 → 5 occurs spontaneously with C-C bond formation via aryl migration attended with isonitrile elimination, the reaction following first-order kinetics. The activation parameters determined for the 4(Cl) → 5(Cl) reaction are ΔH, 18.72(1.00) kcal mol^-1, and ΔS,-17.89(3.20) eu, implying that the transition state is ordered but with an excess of net bond breaking over net bond formation. Logarithmic rate constants are found to vary linearly with the Hammett constant of the R substituent, consistent with nucleophilic aryl migration.