Thiacrown PtII complexes with group 15 donor ligands: pentacoordination in Pt(ii) complexes

We report the synthesis and full characterization for a series of thiacrown complexes of Pt(II) incorporating the fluxional trithiacrown ligand 1,4,7-trithiacyclononane ([9]aneS3) and several group 15 donors ligands. Reaction of [Pt([9]aneS3)Cl2] with a full stoichiometric equivalent of the group 15 donor (L = 2 x AsPh3, SbPh3 or 1,2-bis(diphenylarsenio) ethane (dpae) followed by metathesis with NH4PF6 yields [Pt([9]aneS3)L](PF6)2. We also report the analogous Pd(II) complex with dpae. Similar reactions of the starting Pt complex with one equivalent of XPh3 (X = As or Sb) result in complexes of the formula [Pt([9]aneS3)(XPh3)(Cl)](PF6). All six new complexes have been fully characterized by multinuclear NMR, IR, and UV-Vis spectroscopies in addition to elemental analysis and single crystal structural determinations. The X-ray structures of each complex indicate an axial M-S interaction formed by the endodentate conformation of the [9]aneS3 ligand. The axial M-S distance is highly dependent upon the ancillary donor set. The axial M-S distance shortens with the identity of the group 15 donor ligand according to the trend, Sb < As < P, due to their increasingly poorer donor qualities. The two bis pnictogen complexes, [Pt([9]aneS3)(AsPh3)2](PF6)2 and [Pt([9]aneS3)(SbPh3)2](PF6)2 form unusual five-coordinate distorted trigonal bipyramids in contrast to the pseudo-five coordinate, elongated square pyramidal structures typically observed in Pt(II) complexes of [9]aneS3. The distortion arises from intramolecular pi-pi interactions between the phenyl rings on the two different triphenyl ligands. Chemical shifts in the 195Pt NMR also show similar periodic relationships which trend progressively upfield as the donor atom becomes larger. As expected, the coordinated [9]aneS3 ligand shows fluxional behavior in its NMR spectra, resulting in a single 13C NMR resonance, despite the asymmetric coordination environment found in both chloro complexes. The line width for the carbon NMR resonance as well as for the 195Pt NMR peak is highly sensitive to the nature of the group 15 donor, with poorer donors such as SbPh3 showing significant line broadening. Measurements from the electronic spectra support that the ligand field strength of the pnictogen donor decreases with its increasing size.