Mechanistic studies on iron phosphine complexes. Part 2. Protonation and substitution reactions of dinitrogen complexes

Abstract

The mechanisms of the reactions of acid (HCl or HBr) or the nucleophiles CO, MeCN, or PhCN with trans-[FeH(N₂)(depe)₂]⁺ and trans-[{FeCl(depe)₂}₂(µ-N₂)]²⁺(depe = Et₂PCH₂CH₂PEt₂) have been investigated in tetrahydrofuran (I= 0.1 mol dm^–3, [NBuⁿ₄][BF₄]; 25.0 °C). In the reactions with acid, rapid phosphine chelate ring opening, from each iron complex, permits protonation of the pendant phosphorus atom, and ultimate loss of the phosphine ligand. In contrast, the mechanisms of the reactions of the nucleophiles L = CO, MeCN, or PhCN depend on the iron complex. Substitution of the bridged-dinitrogen ligand in trans-[{FeCl(depe)₂}₂(µ-N₂)]²⁺ occurs, in general, by two parallel pathways. After initial phosphine chelate ring opening the vacant site thus generated can be attacked either by a molecule of solvent or the nucleophile. Subsequent loss of dinitrogen and phosphine chelate ring closure (and for the solvent route, displacement of the co-ordinated solvent by a molecule of the nucleophile) yields the product trans-[FeCl(L)(depe)₂]⁺. Despite the rapid phosphine chelate ring-opening reaction associated with trans-[FeH(N₂)(depe)₂]⁺, substitution of the dinitrogen ligand by nucleophiles proceeds by a much slower pathway involving initial rate-limiting dissociation of dinitrogen. The reasons for the change in substitution mechanism between the mononuclear and binuclear complexes are discussed.