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 CNBut in benzene solution affording the yellow-colored organoruthenium imidic ester hydrochloride system [Ru(C6H2OCN(But)HCl-2-CHNC6H4R-3-Me-5)(PPh3)2(CO)(CNBut)], 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(CNBut)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.