Redox Chemistry Associated with the Complexation of Vanadium(V) and Tungsten(VI) by meso-Octaethylporphyrinogen: Formation and Cleavage of Cyclopropane Units Functioning as Shuttles of Two Electrons

Oxidized forms of meso-octaethylporphyrinogen containing mono- and bis(cyclopropane) units, which function as shuttles of two electrons, form from the reaction of high-valent, early transition metal halides, namely [p-MeC(6)H(4)N=VCl3] (2) and WOCl4, with meso-octaethylporphyrinogen-lithium derivatives, [Et(8)N(4)Li(4)(THF)(4)] (1). The reaction of 2 with 1 occurs in a 2:1 ratio since the intermediate vanadium(V)-porphyrinogen complex undergoes a fast one-electron oxidation by 2 leading to [p-MeC(6)H(4)N=V{Et(8)N(4)(Delta)}] (3), which contains a vanadium(IV)-nitrene fragment bound to the two-electron, oxidized form of the porphyrinogen having a cyclopropane unit. This latter can be reduced using Li metal to a vanadium(IV)-porphyrinogen complex, [p-MeC(6)H(4)N=V(Et(8)N(4))Li-2(THF)(4)}] (5), which was also synthesized from 1 and [(p-MeC(6)H(4)N=VCl2)(2)] (4). In the reaction of WOCl4, we found that the tungsten(VI)-porphyrinogen complex forms first and is able to oxidize the excess of 1, to give [Et(8)N(4)(Delta)Li-2(THF)(2)] (8) or the [Et(8)N(4)(Delta)(2)Li(THF)](+) cation (9B), depending on the stoichiometric ratio. The former forms along with [Et(8)N(4)W=OLi(THF)(4)] (7) when a WOCl4:1 ratio of 2:3 was used. In the case of a 4:5 ratio, 9B was isolated as a countercation of [Et(8)N(4)W=O](-) (9A). The bis(cyclopropane) form of porphyrinogen becomes available by reacting 9 with NaBPh(4), thus forming [Et(8)N(4)(Delta)(2)Li](+)BPk(4)(-) (11). The lithium derivatives, 8 and 11, are the oxidized forms of porphyrinogen available for synthesis. The tungstenyl anion in 7 can be converted into the inorganic ester [Et(8)N(4)WOC(O)Ph] (12) by reacting it with PhCOCl.