[Triangulo-Hg3(μ-dppm)3](O3SCF3)4: first complete NMR analysis of a homoleptic [M3(μ-dppm)3] type system and study of the conformation dynamics in solution by use of its dpam/mdppm derivatives

Abstract

The ³¹P and ¹⁹⁹Hg spectra of the new subvalent [triangulo-Hg₃]⁴⁺ based clusters [Hg₃(μ-mdppm)₃](O₃SCF₃)₄ (1a; mdppm = Ph₂PCH(Me)PPh₂), which is formed stereoselectively with a syn–anti–anti orientation of the methyl groups, and [Hg₃(μ-dppa)₃](O₃SCF₃)₄ (4; dppa = Ph₂PNHPPh₂) are successfully simulated with the program WIN-DAISY. By use of the parameters derived from 1a and 4 the spectra of the parent system [Hg₃(μ-dppm)₃](O₃SCF₃)₄, which show a very complex pattern, are for the first time completely analysed. The simplification of the spectral pattern in 1a or 4 compared to [Hg₃(μ-dppm)₃](O₃SCF₃)₄, respectively, is a result of either symmetry reduction (1a) or quasi selective variation of one Hg–P coupling constant (4). For δ(³¹P), δ(¹⁹⁹Hg), J(HgHg) and the different classes of P–P and Hg–P couplings specific values are found, some of which are powerful tools for the identification of [triangulo-Hg₃]⁴⁺ compounds. The ¹H NMR parameters of 1a, anti- and syn-[Hg₃(μ-dpam)(μ-mdppm)₂](O₃SCF₃)₄ (2a, 2b; dpam = Ph₂AsCH₂AsPh₂), [Hg₃(μ-dpam)₂(μ-mdppm)](O₃SCF₃)₄ (3) and [Hg₃(μ-dpam)₃](O₃SCF₃)₄ lead to a rational interpretation of conformation dynamics in these systems: The three five-membered rings formed by the edges of the [triangulo-Hg₃]⁴⁺ cluster and the three bridging ligands adopt C-envelope conformations in a way that two of the flaps are oriented above and one below the Hg₃ plane, and vice versa. The mdppm ligands feature a rigid conformation with an equatorial methyl group and an axial hydrogen atom at the flap carbon atom. The methylene carbon atom of the dpam ligands is able to flip between two positions above and below the Hg₃ plane, if the relative orientations of the three flaps mentioned above are retained. The overall found rigid conformation of the bridging mdppm ligand allows a qualitative estimation of relative thermodynamic preferences of isomers resulting from syn or anti orientation of mdppm methyl groups.