Comparison of Magnesium and Zinc in Cationic π-Arene and Halobenzene Complexes

Abstract

The reaction of (tBuBDI)­ZnEt with [Ph3C+]­[B­(C6F5)4 −] yielded the cation (tBuBDI)­Zn+ (tBuBDI = CH­[C­(tBu)­N-DIPP]2, DIPP = 2,6-diisopropylphenyl). The cation is sterically too shielded to interact with the ion B­(C6F5)4 – but forms complexes with arenes (benzene, toluene, m-xylene) or halobenzenes (PhX: X = F, Cl, Br, I). Crystal structures of these complexes are compared with those of the corresponding Mg complexes. Although Mg2+ and Zn2+ are of equal size, the Zn···arene and Zn···XPh contacts are generally 0.1–0.2 Å shorter than comparable contacts to (tBuBDI)­Mg+. This originates from differences in bond character: bonding to Mg has a more electrostatic nature. A major difference between Mg and Zn is observed for PhF complexation. While the hard Mg2+ cation prefers Mg···FPh bonding, the softer Zn2+ shows a Zn···(π)­PhF interaction. Heavier halobenzenes with softer halogens (Cl, Br, I) show Zn···XPh bonding. DFT calculations on (tBuBDI)­Zn+···XPh (X = F, Cl, Br, I) show decreasing Zn···X–Ph angles from PhF to PhI on account of the increase in the halogen’s σ-hole. Zn···XPh interactions result in C–X bond lengthening, but C–X bond activation is less pronounced than in corresponding Mg···XPh complexes. Weak (tBuBDI)­Zn+···XPh bonding could not be detected in solution but is believed to play a role in the functionalization of organohalides by Zn reagents.

Cationic Aluminium Complexes as Catalysts for Imine Hydrogenation

“…Like in the previously reported structures of ( Me,DIPP BDI)AlMe 2 and ( Me,DIPP BDI)AlH 2 , [36,37] the (BDI)AlR 2 complexes exhibit Al centres À ] in the polar but weakly coordinating solvent chlorobenzene (Scheme 1). Like observed previously in the syntheses of comparable (BDI)Mg + cations [14][15][16][17] or (BDI)Zn + cations, [18] the colour change from dark orange towards pale yellow or colourless indicated completion of the reaction. Purification of the borate salts, however, was challenging and afforded the laborious development of individual procedures for each individual complex.…”

“…[10][11][12][13] Our group has been interested in molecule activation with highly Lewis acidic cationic main group metal complexes. [14][15][16][17][18][19] We recently reported that Jordan's cationic β-diketiminate aluminium complex, ( Me,DIPP BDI)AlMe + , can be used as a highly Lewis acidic component in FLP activation of alkynes or CO 2 . [20] Attempts to convert these stoichiometric reactions to a catalytic protocol failed.…”

Cationic Heterobimetallic Mg(Zn)/Al(Ga) Combinations for Cooperative C–F Bond Cleavage

“…TheZ n/Al pair cleaved the CÀFb ond in fluorobenzene instantaneously at room temperature and ( tBu BDI)ZnPh was isolated in 75 %y ield (crystal structure: Figure S56). Ther ecently reported cation ( tBu BDI)Zn + •(p-C 6 H 5 F), in which fluorobenzene binds Zn via its p-system, [49] showed identical reactivity with ( Me BDI)Al, indicating that the Ph ring in ( tBu BDI)ZnPh originates from fluorobenzene and not from benzene.The onward reaction of ( tBu BDI)ZnPh with I 2 is clean and gave ( tBu BDI)ZnI and PhI (Figure S33-S38). Theother product, ( Me BDI)AlF + ,could not be isolated nor observed which is likely due to its very high reactivity.The Al center in the cation ( Me BDI)AlF + would be even more Lewis acidic and reactive than that in hypothetical alumoxane ( Me BDI)AlO [52] and further fluoride abstraction or bond activation reactivity could be anticipated.…”

“…Forthe cationic fragment we chose the recently reported ( tBu BDI)Mg + or ( tBu BDI)Zn + entities stabilized by al arge tBu BDI ligand with tBu groups in the backbone. [48,49] This bulky ligand prevents contact between the cation and the B(C 6 F 5 ) 4…”

“…Forschungsartikel metal-metal bond of 2.391 is calculated for the cation in 1-Zn/Ga. Since the calculated Zn-Al bond of 2.463 for the cation in 1-Zn/Al is slightly longer, failure to isolate 1-Zn/Al is not related to steric problems.T he considerably shorter bonds to Zn are likely related to ah igher degree of covalency: [49] Zn, Al and Ga have similar electronegativities whereas Mg is significantly less electronegative (Table 1).…”