Charge-transfer photochemistry of aromatic .pi.-complexes. Hexamethylbenzene and mercuric trifluoroacetate

Abstract

The «-complex of hexamethylbenzene (HMB) and mercuric trifluoroacetate (HgT2) is converted with high quantum yields to pentamethylbenzyl trifluoroacetate and either mercurous trifluoroacetate or metallic mercury upon irradiation of the charge-transfer absorption band. X-ray crystallography establishes the charge-transfer (CT) excitation to derive from the j)1 2-bonding of the mercuric atom to only two of the aromatic carbons of HMB. The CT photochemistry proceeds via the radical-ion pair [HMB+-,HgT2~•] in accord with Mulliken theory. The high quantum efficiency is ascribed to the rapid ligand dissociation of HgTf• which minimizes the energy wastage due to back electron transfer. The thermal reactions following photoactivation are monitored by following the changes in the CT absorption band and the ESR spectrum of HMB+•. The charge-transfer photochemistry of the [HMB,HgT2] complex shows an unusually high dependency on the solvent-the nature of the mercurycontaining products and the quantum yield for pentamethylbenzyl trifluoroacetate in dichloromethane being distinctively different from that obtained in trifluoroacetic acid. The formation of metallic mercury with quantum yields in trifluoroacetic acid which can be as high as 3 arises from a radical-chain process which sustains itself after the light is turned off. The critical role played by the mercurous trifluoroacetate radical •Hg02CCF3 in the CT photochemistry is described.

Hydrogen/Deuterium Exchange Catalyzed by an Unusually Stable Mercury–Toluene Complex

“…Noteworthy is that these values are either side of the values reported by Lau and Kochi for [Hg 2 (m-O 2 CCF 3 ) 4 (h 2 -C 6 Me 6 ) 2 ] (2.56 and 2.58 [2] ) and significantly shorter than observed for the intramolecular coordination discussed above (ca. 3.2 [10,11] ).…”

“…The band at 305 nm is likely that of the p-p* transition for the coordinated toluene, [21] however, it is also consistent with the tolueneÐHg chargetransfer transitions observed for arene complexes of [Hg 2 -(m-O 2 CCF 3 ) 4 ]. [2] However, two observations differentiate the present system from [Hg 2 (m-O 2 CCF 3 ) 4 (h 2 -arene) 2 ] complexes. First, the absorption for 1 is at significantly lower energy than those observed for [Hg 2 (m-O 2 CCF 3 ) 4 (h 2 -arene) 2 ] (288 ± 315 nm [2] ) or the toluene complexes of HgCl 2 (274 nm [15] (h 2 -arene) 2 ] is transient the absorption for 1 is sustained indefinately.…”

“…[2] However, two observations differentiate the present system from [Hg 2 (m-O 2 CCF 3 ) 4 (h 2 -arene) 2 ] complexes. First, the absorption for 1 is at significantly lower energy than those observed for [Hg 2 (m-O 2 CCF 3 ) 4 (h 2 -arene) 2 ] (288 ± 315 nm [2] ) or the toluene complexes of HgCl 2 (274 nm [15] (h 2 -arene) 2 ] is transient the absorption for 1 is sustained indefinately. Both of these observations suggest a stronger interaction between mercury and the aromatic group than previously observed.…”

“…[1] Extensive studies by Kochi and co-workers showed that the activation of the arene is related to a charge-transfer transition in the p complex. [2,3] As part of this work they also isolated a h 6 complex of C 6 Me 6 and [Hg(O 2 CCF 3 ) 2 ]. [4] Crabtree and coworkers have demonstrated that photolysis of metallic mercury in the presence of aromatic compounds results in a variety of CÀC bond-forming reactions and have proposed a p complex as a key intermediate.…”

Hydrogen/Deuterium Exchange Catalyzed by an Unusually Stable Mercury–Toluene Complex

“…[17] In the 199 Hg NMR spectrum the chemical shift of the mercury center (d À 1970) is downfield to that reported for HgCl 2 (d À 1497 [18] ) and closer to that of Hg II aquated salts (d À 2253 to À 2361 [19] ) than simple Lewis base complexes, for example, [HgCl 2 {P(nBu) 3 } 2 ] (d À 404 [20] ). [2] However, two observations differentiate the present system from [Hg 2 (m-O 2 CCF 3 ) 4 (h 2 -arene) 2 ] complexes. The band at 305 nm is likely that of the p-p* transition for the coordinated toluene, [21] however, it is also consistent with the tolueneÐHg chargetransfer transitions observed for arene complexes of [Hg 2 -(m-O 2 CCF 3 ) 4 ].…”