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.

A Low‐Valent Molybdenum Nitride Complex: Reduction Promotes Carbonylation Chemistry

“…The 1 HNMR spectrum, consistent with aC s symmetric solution structure,displays two central arene resonances (5.93 and 4.43 ppm) suggestive of an h 2 metal-arene interaction. Using at erminally 15 N-enriched (50 %) source of azide afforded ap roduct with three resonances in the 15 NN MR spectrum at 833.09, 137.23, and 62.23 ppm, consistent with at erminal nitride motif [7f, 13] and abound azide ligand, [14] respectively (2,Scheme 1).…”

Prediction of 195Pt NMR of photoactivable diazido‐ and azine‐Pt(IV) anticancer agents by DFT computational protocols

“…The lack of experimental data for the 195 Pt NMR chemical shifts of most of the photoactivable diazido‐Pt IV complexes studied so far prompt us to assess the performance of the previously developed simple non‐relativistic (NR) GIAO–PBE0/SARC–ZORA(Pt)∪BS(E)/PCM (BS = 6‐31+G(d) or 6‐31G(d,p)) computational protocols in the calculation of the 195 Pt NMR chemical shifts for a series of photoactivable diazido‐Pt IV complexes (Table ). In Table , we also included the δ SO 195 Pt chemical shifts obtained by all electron relativistic ZORA in its SO two‐component form augmented with the conductor‐like screening solvation model, as well as the experimental data available for a few of the cytotoxic diazido‐Pt IV complexes . The σ iso 195 Pt isotropic magnetic shielding tensor elements; their principal components σ XX , σ YY and σ ZZ ; shielding anisotropy (SA); and span of the tensor for the series of photoactivable diazido‐Pt IV complexes under study calculated by GIAO–PBE0/SARC–ZORA(Pt)∪6‐31+G(d)(E)/PCM (E = main group element) computational protocol in aqueous solution are given in the electronic supporting information (Table S1).…”

“…Although various physical methods have been used to elucidate the electronic structure of the diazido‐Pt(IV) complexes, NMR presented particular hurdles to probe owing to the low NMR receptivity of the nuclei and low solubility of many of the complexes of interest. Farrer et al using the anti‐ringing pulse sequences provided the first 14 N and 15 N NMR data along with 195 Pt chemical shifts and one‐bond and three‐bond J ( 15 N, 195 Pt) couplings for azido and am(m)ine ligands of the diazido‐Pt(IV) complexes. Later, Sutter and Autschbach studied the 195 Pt, 14 N and 15 N NMR for five diazido‐Pt(IV) complexes employing relativistic density functional theory (DFT) calculations.…”

Insights into the Acid–Base Properties of PtIV–Diazidodiam(m)inedihyroxido Complexes from Multinuclear NMR Spectroscopy

“…Typical acquisition parameters for 1D 14 N{ 1 H} NMR spectra ( 14 N: 43.35 MHz): 32k transients, spectral width 24 kHz, 16k data points and a delay time of 0.20 s. Due to acoustic resonance, the anti-ringing proton-decoupled ARINDEC pulse sequence was used 38. Data were processed using an exponential line-broadening of 5 Hz and a resolution of 1.5 Hz.…”