title: Probing platinum azido complexes by 14N and 15N NMR spectroscopy.
abstract: 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 (14)N and the lack of scalar (1)H coupling to enhance the sensitivity of the less abundant (15)N by using polarisation transfer. In this work, we report (14)N and (15)N NMR spectroscopic studies of cis,trans,cis-[Pt(N(3))(2)(OH)(2)(NH(3))] (1) and trans,trans,trans-[Pt(N(3))(2)(OH)(2)(X)(Y)], where X=Y=NH(3) (2); X=NH(3), Y=py (3) (py=pyridine); X=Y=py (4); and selected Pt(II) precursors. These studies provide the first (15)N NMR data for azido groups in coordination complexes. We discuss one- and three-bond J((15)N,(195)Pt) couplings for azido and am(m)ine ligands. The (14)N(α) (coordinated azido nitrogen) signal in the Pt(IV) 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 (14)N(α) atom. Through the use of anti-ringing pulse sequences, the (14)N NMR spectra, which show resolution of the broad (14)N(α) peak, were obtained rapidly (e.g., 1.5 h for 10 mM 4). The linewidths of the (14)N(α) signals correlate with the viscosity of the solvent. For (15) N-enriched samples, it is possible to detect azido (15)N resonances directly, which will allow photoreactions to be followed by 1D (15)N NMR spectroscopy. The T(1) relaxation times for 3 and 4 were in the range 5.7-120 s for (15)N, and 0.9-11.3 ms for (14)N. Analysis of the (1)J((15)N,(195)Pt) coupling constants suggests that an azido ligand has a moderately strong trans influence in octahedral Pt(IV) complexes, within the series 2-pic<py<NH(3)<Cl(-)<N(3)(-)<NO(2)(-)<SCN(-) (2-pic=2-methylpyridine). In addition, an axial Cl(-) appears to weaken an equatorial Pt(IV)-NH(3) bond to a greater extent than an axial OH(-) ligand.
authors.affiliation: Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK. N.Farrer@warwick.ac.uk
authors.forename: Nicola J
authors.initials: NJ
authors.lastname: Farrer
authors.forename: Peter
authors.initials: P
authors.lastname: Gierth
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Sadler
mesh_terms: D018360:Crystallography, X-Ray; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D009944:Organoplatinum Compounds / Q000737:chemistry*; D010777:Photochemistry; D010984:Platinum / Q000737:chemistry*; D011355:Prodrugs / Q000737:chemistry*; D012680:Sensitivity and Specificity; D013237:Stereoisomerism
chemical_list: D008024:Ligands; D009944:Organoplatinum Compounds; D011355:Prodrugs; D010984:Platinum