Probing Platinum Azido Complexes by 14N and 15N NMR Spectroscopy

摘要

Metal azido complexes are of general interest due to their high energetic properties, and platinum azido complexes in particular because of their potential as photoactivatable anticancer prodrugs. However, azido ligands are difficult to probe by NMR spectroscopy due to the quadrupolar nature of 14N and the lack of scalar 1H coupling to enhance the sensitivity of the less abundant 15N by using polarisation transfer. In this work, we report 14N and 15N NMR spectroscopic studies of cis,trans,cis-[Pt(N₃)₂(OH)₂(NH₃)] (1) and trans,trans,trans-[Pt(N₃)₂(OH)₂(X)(Y)], where X=Y=NH₃ (2); X=NH₃, Y=py (3) (py=pyridine); X=Y=py (4); and selected PtII precursors. These studies provide the first 15N NMR data for azido groups in coordination complexes. We discuss one- and three-bond J(15N,195Pt) couplings for azido and am(m)ine ligands. The 14N (coordinated azido nitrogen) signal in the PtIV azido complexes is extremely broad (W_1/2≈2124 Hz for 4) in comparison to other metal azido complexes, attributable to a highly asymmetrical electric field gradient at the 14N atom. Through the use of anti-ringing pulse sequences, the 14N NMR spectra, which show resolution of the broad 14N peak, were obtained rapidly (e.g., 1.5 h for 10 mM 4). The linewidths of the 14N signals correlate with the viscosity of the solvent. For 15N-enriched samples, it is possible to detect azido 15N resonances directly, which will allow photoreactions to be followed by 1D 15N NMR spectroscopy. The T₁ relaxation times for 3 and 4 were in the range 5.7–120 s for 15N, and 0.9–11.3 ms for 14N. Analysis of the 1J(15N,195Pt) coupling constants suggests that an azido ligand has a moderately strong trans influence in octahedral PtIV complexes, within the series 2-picy33−2−3 bond to a greater extent than an axial OH− ligand.