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.