Reactivity of TCNE or TCNQ Derivatives of Quinonoid Zwitterions: Platinum-Induced HCN Elimination vs Oxidative-Addition

Abstract

The reaction of the functional, zwitterionic quinonoid molecule (6E)-4-(butylamino)-6-(butyliminio)-3-oxo-2-(1,1,2,2-tetracyanoethyl)cyclohexa-1,4-dien-1-olate, [C6H-2-{C(CN)2C(CN)2H}]-4,6-(···NH n-Bu)2-1,3(···O)2 (2), which has been previously prepared by regioselective insertion of TCNE into the C–H bond adjacent to the C···O bonds of the zwitterionic benzoquinone monoimine (6E)-4-(butylamino)-6-(butyliminio)-3-oxocyclohexa-1,4-dien-1-olate, C6H2-4,6-(···NHn-Bu)2-1,3-(···O)2 (1), with 2 equiv of [Pt(C2H4)(PPh3)2], afforded the Pt(0) complex [Pt(PPh3)2(4)] (6) (4 = 2–HCN; (6E)-4-(butylamino)-6-(butyliminio)-3-oxo-2-(1,2,2-tricyanoethenyl)cyclohexa-1,4-dien-1-olate), in which a tricyanoethenyl moiety is π-bonded to the metal. A metal-induced HCN elimination reaction has thus taken place. The same complex was obtained directly by the reaction of 1 equiv of the Pt(0) complex [Pt(C2H4)(PPh3)2] with the olefinic ligand [C6H-2-{C(CN)C(CN)2}]-4,6-(···NHn-Bu)2-1,3-(···O)2) (4), previously obtained by the reaction of 2 with NEt3 in THF. A similar reactivity pattern was observed between 2 and 2 equiv of the Pd(0) precursor [Pd(dba)2] in the presence of dppe, which led to [Pd(dppe)(4)] (7), which was also directly obtained from 4 and 1 equiv [Pd(dba)2]/dppe. In contrast to the behavior of the TCNE derivative 2, the reaction of the TCNQ derivative (6E)-4-(butylamino)-6-(butyliminio)-2-(dicyano(4-(dicyanomethyl)phenyl)methyl)-3-oxocyclohexa-1,4-dien-1-olate, [C6H-2-{C(CN)2p-C6H4C(CN)2H}]-4,6-(···NHn-Bu)2-1,3-(···O)2) (3), with 2 equiv of [Pt(C2H4)(PPh3)2] led to formal oxidative-addition of the C–H bond of the C(CN)2H moiety to give the Pt(II) hydride complex trans-[PtH(PPh3)2{NCC(CN)p-C6H4C(CN)2-2-[C6H-4,6-(···NHn-Bu)2-1,3-(···O)2)}] (8). The molecular structures of 3, 4, 6·0.5(H2O), and 8·3(CH2Cl2) have been determined by single-crystal X-ray diffraction