Heterometallic Aluminum–Chromium Phenazine and Thiophenazine Complexes. Formation of a Tetranuclear Chromium(I) Sandwich Complex

Treatment of a suspension of phenazine and CrCl₂(THF)₂ in toluene with 4 equiv of triethylaluminum (Scheme ) affords the pentanuclear, heterobimetallic, and divalent [AlEt₂(THF)­(μ₃-η¹:η¹-C₁₂H₈N₂)­(μ-κ¹-AlEt₂Cl)]₂(μ-Cr) (1). In this species the two nitrogen atoms of each phenazine ligand are connected to three metals. A similar reaction with thiophenazine afforded trimetallic [(η¹:η¹-C₁₂H₈NS)­(μ-κ¹-AlEt₂Cl)]₂(μ-Cr) (2), where the sulfur atoms remained uncoordinated. When the preparation of 1 was carried out in the presence of excess Et₃Al, the reaction took a different pathway, affording [AlEt₂]₂(μ-η¹:η¹-η⁶:η⁶-C₁₂H₈N₂Cr)₂ (3). The aluminate residues hold two parallel and eclipsed phenazine molecules, forming a rigid cage where the two chromium atoms are encapsulated. The ambiguity about the metal oxidation state has been solved by DFT calculations, revealing the formal oxidation state of chromium as being better described as monovalent.