Cationic Magnesium π–Arene Complexes

Abstract

Reaction of the trityl cation in [Ph3C+]­[B­(C6F5)4 –] with the n-butyl anion in (BDI)­MgnBu led to ß-hydride abstraction and formation of Ph3CH, 1-butene, and [(BDI)­Mg+]­[B­(C6F5)4 –] (1) (BDI = CH­[C­(CH3)­N-Dipp]2; Dipp = 2,6-diisopropylphenyl). The “naked” Mg center in 1 is weakly bound to B­(C6F5)4 – through two Mg···F interactions. Addition of arenes to 1 gave strongly bound cationic magnesium π–arene complexes (BDI)­Mg+·arene in good yields arene = benzene (94%), toluene (74%), m-xylene (82%), and mesitylene (63%). 1,2,4,5-Tetramethylbenzene is too bulky to give a coordination complex. Crystal structures of these π-arene complexes show η3–arene–Mg interactions for the smaller arenes (benzene, toluene and m-xylene). In each case, the coordination sphere was filled by an additional Mg···F interaction. For mesitylene, η6-coordination was found, leaving no space at the metal for supplementary Mg···F interaction. Dissolved in C6D5Br, all arene complexes are in association–dissociation equilibrium: (BDI)­Mg+ + arene ⇄ (BDI)­Mg+·arene. For the most strongly bound mesitylene ligand, a decoalescence of the 1H NMR resonances was reached at −20 °C, each species giving separate signals. From the temperature dependency of this equilibrium, the following thermodynamic parameters have been deduced: ΔH 0 = −6.9 kcal·mol–1 and ΔS 0 = −28.2 cal·mol–1·K–1. DFT calculations reveal that the Mg···arene bonding is mainly electrostatic of nature with only little charge transfer from arene to Mg2+ (ca. 0.05 e) and a slight polarization of π-electron density toward the metal. Substitution of benzene in (BDI)­Mg+·C6H6 for mesitylene is exothermic by −11.7 kcal/mol; however, including the B­(C6F5)4 – counterion in the calculation gave an energy gain of −2.2 kcal/mol. This clearly demonstrates that weakly coordinating anions can affect these Mg···arene interactions substantially.

Complexation and Versatile Reactivity of a Highly Lewis Acidic Cationic Mg Complex with Alkynes and Phosphines

“…Reactions of the in situ generated “naked” cationic complex 1 with the terminal alkynes PhC≡CH and Me 3 SiC≡CH led to alkyne deprotonation by the BDI ligand (Scheme ). As in the previously reported reaction of 1 with water, the anionic BDI ligand was converted into a neutral bis‐imine ligand, and the Mg‐alkynide complexes [(BDI‐H)Mg + C≡CPh] 2 ⋅ 2 [B(C 6 F 5 ) 4 − ] ( 2 ) and [(BDI‐H)Mg + C≡CSiMe 3 ] 2 ⋅ 2 [B(C 6 F 5 ) 4 − ] ( 3 ) were isolated in yields of 70 % and 63 %, respectively. This cooperative acid–base transformation may be described as intramolecular FLP reactivity, and is analogous to (alkyne)C−H bond cleavage by the FLP Cp 2 Zr(OAr) + /PCy 3 .…”

Facile Benzene Reduction by a Ca2+/AlI Lewis Acid/Base Combination

“…[1][2][3] In contrast to previous work on (BDI)Ae + · (pyridine) 3 complexes, [4] our (BDI)Ae + complexes are completely free of additional Lewis bases.T hese naked cations show an extraordinary Lewis acidity that exceeds that of the benchmark Lewis acid B(C 6 F 5 ) 3 .T his acidity has been demonstrated by the syntheses of as et of robust (BDI)Ae + · (arene) complexes (II) [1,3] and the isolation of the first unsupported Mg-alkyne complex (III). [1][2][3] In contrast to previous work on (BDI)Ae + · (pyridine) 3 complexes, [4] our (BDI)Ae + complexes are completely free of additional Lewis bases.T hese naked cations show an extraordinary Lewis acidity that exceeds that of the benchmark Lewis acid B(C 6 F 5 ) 3 .T his acidity has been demonstrated by the syntheses of as et of robust (BDI)Ae + · (arene) complexes (II) [1,3] and the isolation of the first unsupported Mg-alkyne complex (III).…”

“…[1,3] Thes lightly polar bromobenzene was found to be as uitable solvent for this compound class.H owever,i n light of the recent use of (BDI)Al I to activate strong C À F bonds, [8] bromobenzene would not be ag ood solvent choice. [1,3] Thes lightly polar bromobenzene was found to be as uitable solvent for this compound class.H owever,i n light of the recent use of (BDI)Al I to activate strong C À F bonds, [8] bromobenzene would not be ag ood solvent choice.…”

“…This exchange indicates that the formation of the heterobimetallic Ca-Al complex is ar eversible equilibrium that strongly lies at the product side:( BDI)Ca + ·(C 6 H 6 ) + (BDI)Al I Q (BDI)Ca + (C 6 H 6 )Al III (BDI). [1][2][3] We earlier showed that calculations on charged species are challenging:t he effect of the weakly coordinating anion on complexation energies can be substantial. [12] Thef acile benzene dearomatization by (BDI)Ca + and (BDI)Al I under very mild reaction conditions (20 8 8C, 1h)i s remarkable.T his reaction formally equals ao ne-step Birch reduction of benzene,asynthetic pathway that given the very negative reduction potential of benzene (< À3V) [10] generally makes use of strongly reducing agents like Li, Na, or K. [18] As (BDI)Ca + ·(C 6 H 6 )i si ne quilibrium with its components,there are three possible pathways for formation of the heterobimetallic product (Scheme 2).…”

“…As imilar observation was made earlier for the complex VII. [1][2][3] Size restrictions,h owever, did not allow af ull calculation that also includes the loosely bound B(C 6 F 5 ) 4 À counteranion. Complex (BDI)Al I reacts either with (BDI)Ca + ·(C 6 H 6 )( pathway 1), with (BDI)Ca + (pathway 2), or with benzene (pathway 3).…”