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

Abstract

The gas-phase reaction of XCr + (X ) C 6 H 6 , NH 3 ) and (NH 3 ) 2 Cr + with arenes (L) leads to XCrL + , LCr + , and L 2 Cr + 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 L 2 Cr + complexes, and R,ω-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 NH 3 substitution from (NH 3 ) 2 Cr + . 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.

Complexes between Lithium Cation and Diphenylalkanes in the Gas Phase: The Pincer Effect

“…[29,30] The simultaneous coordination of Fe + ions to the two aromatic rings of homologues a,w-diphenylalkanes has also been invoked to explain the highly regioselective intramolecular dihydrogen abstraction, [31] and a similar coordination of Cr + ions to a,w-diphenylalkanes has been deduced from the reactivity of the bare metal ions with these bidentate ligands. [32] Results and Discussion Experimental lithium cation basicities (LCB values): The experimental determination of the LCB values for Ph-CH(Me)-Ph and Ph-(CH 2 ) n -Ph (n = 2, 3, 7) was based on the nine reliable equilibrium measurements for which the resulting DLCB values and inferred LCB values are reported in Table 1. Absolute LCB values may be biased by the choice of the anchor point(s) of the scale, but the differences in LCB values between closely related compounds, within a short range of basicity, are likely to be much more reliable.…”

Binding of Na+, Mg+, and Al+ to the π Faces of Naphthalene and Indole: Ab Initio Mapping Study

“…The recognition of such interactions as an important factor in determining conformations and reactions of metal-containing biologically important systems has sparked interest from a new point of view, 1,2 while the quantitative experimental study of such interactions is advancing rapidly through the exploitation of powerful new gas-phase approaches. [3][4][5][6][7][8][9][10] Especially provocative is the viewpoint, recently advocated by the Dougherty group, taking the electrostatic component of binding as pivotal in understanding variations of cation interactions with different biologically relevant π faces. 2,11,12 Indole is an especially important model molecule for developing understanding of biologically interesting cation/π interactions, inasmuch as it is the side chain of tryptophane, and thus serves to model binding sites widely available on proteins.…”

“…The interaction of metal ions with the π surfaces of aromatic molecules, a central and traditional concern of organometallic chemistry, is receiving a good deal of new attention. The recognition of such interactions as an important factor in determining conformations and reactions of metal-containing biologically important systems has sparked interest from a new point of view, , while the quantitative experimental study of such interactions is advancing rapidly through the exploitation of powerful new gas-phase approaches. − Especially provocative is the viewpoint, recently advocated by the Dougherty group, taking the electrostatic component of binding as pivotal in understanding variations of cation interactions with different biologically relevant π faces. ,, …”

Gaseous Arenium Ions at Atmospheric Pressure: Elementary Reactions and Internal Solvation Effects

“…It may behave as a base leading to interannular hydron transfer, and it may also display chelating properties with respect to metal ions whose coordination demand is not exhausted by the first aryl ring. 34 This type of reactivity was displayed in a comparative study of the reactivity of Cr(I) ions with simple aromatics and with selected R,ω-diphenylalkanes. Kinetic evidence has been obtained by FT-ICR indicating that, for appropriate values of n, a sandwich-type structure is achieved allowing simultaneous coordination of Cr + with the two phenyl rings of Ph(CH 2 ) n Ph.…”