Reactions of Bare and Ligated Chromium (I) Ions with Gaseous Arenes. Role of a "Spectator" Aromatic Ring in Chelate Complex Formation

The gas-phase reaction of XCr+ (X ) C6H6, NH3) and (NH3)2Cr+ with arenes (L) leads to XCrL+, LCr+, and L2Cr+ complexes. The pattern and the efficiency of the two classes of complex formation processes allow a clear distinction between simple arenes, such as toluene and mesitylene, yielding ultimately sandwich-type L2Cr+ complexes, and alfa,omega-diphenylalkanes, yielding only LCr+ complexes, where a common sandwich-type structure is achieved by simultaneous coordination of the metal cation with the two aryl rings. The critical parameter controlling the extent of chelate coordination is the length of the methylene chain joining the two rings. A different behavior is displayed by ligands such as trans-stilbene and [2.2]paracyclophane, where only one aryl ring is involved in the stepwise NH3 substitution from (NH3)2Cr+. Under low pressure, and hence inefficient collisional deactivation, the Cr+ reaction with the selected arenes is governed by relative binding energies and radiative cooling efficiencies of the ensuing complexes. Insertion of free Cr+ into the cage of [2.2]paracyclophane may not be excluded.