A multinuclear (1H, 31P, 199Hg) nuclear magnetic resonance study of some complexes of mercury(II) with ditertiary phosphines

Abstract

From Hg(SbF6)2 and an appropriate amount of the ligands Ph2PCHRPPh2 (R = H or Me), 2:2, 1:2, and 1:3 M:L complexes have been prepared in MeNO2 solution and characterized by 1H, 31P, and 199Hg nmr spectroscopy. The 2:2 complexes are ligand-bridged, as are [Hg(Ph2PCH2PPh2)(Ph2PCHMePPh2)Hg]4+ and [Hg(Ph2PCH2PPh2)2Ag]3+ (prepared from Hg(SbF6)2, Ag(AsF6), or Ag(SbF6) and the ligand), examined by nmr for comparison. The complex [Hg(μ-Ph2PCHMePPh2)2Hg]4+ exists as two isomers, which interconvert slowly on the preparative timescale. The 1:2 complexes, and the analogous mixed ligand complex are shown by their nmr spectral properties to be examples of tetrahedral bis(chelate) complexes, which are very unusual for the Ph2PCHRPPh2 ligands. In the 1:3 complexes, and the one of the two possible mixed complexes identified, both η1-and η2-ligands appear to be present and these undergo rapid intramolecular η1-to-η2 exchange. The η1-bonding mode of Ph2PCH2PPh2 also occurs in [(η-Ph2P(CH2)2PPh2)Hg(η1-Ph2PCH2PPh2)2]2+ and [Hg(η1-Ph2PCH2PPh2)4]2+ as evidenced by their reduced temperature 31P nmr spectra in Me2CO—MeNO2.The complex [Hg(η2-Ph2PCH2PPh2)2]2+ has 31P and 199Hg nmr spectral properties significantly different from those of its acyclic analog [Hg(PMePh2)4]2+. To place this anomaly in perspective, the 31P and 199Hg nmr spectra of a much wider series of new bis(chelate) complexes [Hg(LL)(L′L′)]2+ have been obtained, with the ligand combinations LL = L′L′ = Ph2PNPhPPh2; LL = dppm, L′L′ = Ph2PNPhPPh2; LL = L′L′ = Ph2P(CH2)nPPh2(n = 2–4); LL = Ph2P(CH2)mPPh2, L′L′ = Ph2P(CH2)nPPh2 (m ≠ n; m = 1–3, n = 1–5). From the data for the last two series, it is found that only the bis(chelate) complex of Ph2PCH2PPh2 shows a large anomaly in its 199Hg chemical shift (anomalously high shielding), and that in [Hg(LL)(L′L′)]2+ values of 2J(PP), 1J(HgP), and 31P chemical shifts vary systematically with the ligand combination; of particular note, 1J(HgP) to LL decreases as the chelate ring size of L′L′ increases. These observations are discussed in terms of the variation in the intra-and inter-chelate PHgP angles expected in the complexes.