Modeling the platinum-catalyzed intermolecular hydroamination of ethylene: The nucleophilic addition of HNEt 2 to coordinated ethylene in trans-PtBr 2(C 2H 4)(HNEt 2)

Abstract

Compound trans-PtBr 2(C 2H 4)(NHEt 2) ( 1) has been synthesized by Et 2NH addition to K[PtBr 3(C 2H 4)] and structurally characterized. Its isomer cis-PtBr 2(C 2H 4)(NHEt 2) ( 3) has been obtained from 1 by photolytic dissociation of ethylene, generating the dinuclear trans-[PtBr 2(NHEt 2)] 2 intermediate ( 2), followed by thermal re-addition of C 2H 4, but only in low yields. The addition of further Et 2NH to 1 in either dichloromethane or acetone yields the zwitterionic complex trans-Pt (−)Br 2(NHEt 2)(CH 2CH 2N (+)HEt 2) ( 4) within the time of mixing in an equilibrated process, which shifts toward the product at lower temperatures (Δ H° = −6.8 ± 0.5 kcal/mol, Δ S° = 14.0 ± 2.0 e.u., from a variable temperature IR study). 1H NMR shows that free Et 2NH exchanges rapidly with H-bonded amine in a 4·NHEt 2 adduct, slowly with the coordinated Et 2NH in 1, and not at all (on the NMR time scale) with Pt-NHEt 2 or –CH 2CH 2N (+)HEt 2 in 4. No evidence was obtained for deprotonation of 4 to yield an aminoethyl derivative trans-[PtBr 2(NHEt 2)(CH 2CH 2NEt 2)] − ( 5), except as an intermediate in the averaging of the diasteretopic methylene protons of the CH 2CH 2N (+)HEt 2 ligand of 4 in the higher polarity acetone solvent. Computational work by DFT attributes this phenomenon to more facile ion pair dissociation of 5·Et 2NH 2 +, obtained from 4·Et 2NH, facilitating inversion at the N atom. Complex 4 is the sole observable product initially but slow decomposition occurs in both solvents, though in different ways, without observable generation of NEt 3. Addition of TfOH to equilibrated solutions of 4, 1 and excess Et 2NH leads to partial protonolysis to yield NEt 3 but also regenerates 1 through a shift of the equilibrium via protonation of free Et 2NH. The DFT calculations reveal also a more favourable coordination (stronger Pt–N bond) of Et 2NH relative to PhNH 2 to the Pt II center, but the barriers of the nucleophilic additions of Et 2NH to the C 2H 4 ligand in 1 and of PhNH 2 to trans-PtBr 2(C 2H 4)(PhNH 2) ( 1a) are predicted to be essentially identical for the two systems.