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.