Isonitrile Insertion into the Ru−O Bond and Migratory C−C Bond Formation. Novel Organoruthenium Imidic Ester and Acyl Species

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.