Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK.
Chemistry. 2011 Oct 17;17(43):12059-66. doi: 10.1002/chem.201101409. Epub 2011 Sep 16.
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.
金属叠氮配合物因其高能量特性而受到广泛关注，特别是由于其作为光活化抗癌前药的潜力，铂叠氮配合物受到关注。然而，由于(14)N 的四极性质和缺乏标量(1)H 偶合来增强较少丰度的(15)N 的灵敏度，通过极化转移，叠氮配体很难通过 NMR 光谱进行探测。在这项工作中，我们报告了顺式、反式、顺式-Pt(N(3))(2)(OH)(2)(NH(3))和反式、反式、反式-[Pt(N(3))(2)(OH)(2)(X)(Y)]，其中 X=Y=NH(3)(2)；X=NH(3)，Y=py(3)(py=吡啶)；X=Y=py(4)；和选定的 Pt(II)前体的(14)N 和(15)N NMR 光谱研究。这些研究为配位化合物中叠氮基团的首次(15)N NMR 数据提供了依据。我们讨论了叠氮和胺(m)ine 配体的单键和三键 J((15)N,(195)Pt)偶合。与其他金属叠氮配合物相比，Pt(IV)叠氮配合物中(14)N(α)(配位叠氮氮)信号非常宽(W(1/2)≈2124 Hz 用于 4)，这归因于(14)N(α)原子处高度非对称的电场梯度。通过使用反环鸣脉冲序列，可以快速获得(14)N NMR 光谱，其中显示出宽(14)N(α)峰的分辨率(e.g.，对于 10 mM 4，1.5 h)。(14)N(α)信号的线宽与溶剂的粘度相关。对于(15)N 富集的样品，可以直接检测到叠氮(15)N 共振，这将允许通过 1D(15)N NMR 光谱跟踪光反应。3 和 4 的 T(1)弛豫时间范围为(15)N 5.7-120 s，(14)N 0.9-11.3 ms。对(1)J((15)N,(195)Pt)偶合常数的分析表明，在八面体 Pt(IV)配合物中，一个叠氮配体在 2-吡啶<py<NH(3)<Cl(-)<N(3)(-)<NO(2)(-)<SCN(-)(2-吡啶=2-甲基吡啶)系列中具有中等强度的反式影响。此外，轴向 Cl(-)似乎比轴向 OH(-)配体更能削弱配位 Pt(IV)-NH(3)键的轴向 Pt(IV)-NH(3)键。
