Computational Study and Molecular Orbital Analysis of NMR Shielding, Spin–Spin Coupling, and Electric Field Gradients of Azido Platinum Complexes

2012-08-15T00:00:00Z (GMT) by Kiplangat Sutter Jochen Autschbach
195Pt, 14N, and 15N NMR data for five azido (N3) complexes are studied using relativistic density functional theory (DFT). Good agreement with experiment is obtained for Pt and N chemical shifts as well as Pt–N J-coupling constants. Calculated 14N electric field gradients (EFGs) reflect experimentally observed trends for the line broadening of azido 14N NMR signals. A localized molecular orbital analysis of the nitrogen EFGs and chemical shifts is performed to explain some interesting trends seen experimentally and in the first-principles calculations: (i) 14N NMR signals for the Pt-coordinating (Nα) nuclei in the azido ligands are much broader than for the central (Nβ) or terminal (Nγ) atoms. The Nβ signals are particularly narrow; (ii) compared to Nγ, the Nα nuclei are particularly strongly shielded; (iii) Nβ nuclei have much larger chemical shifts than Nα and Nγ ; and (iv) The Pt–Nα J-coupling constants are small in magnitude when considering the formal sp hybridization of Nα . It is found that for Nα a significant shielding reduction due to formation of the dative Nα–Pt bond is counterbalanced by an increased shielding from spin–orbit (SO) coupling originating at Pt. Upon coordination, the strongly delocalized π system of free azide localizes somewhat on Nβ and Nγ . This effect, along with rehybridization at Nα upon bond formation with Pt, is shown to cause a deshielding of Nγ relative to Nα and a strong increase of the EFG at Nα . The large 2p character of the azide σ bonds is responsible for the particularly high Nβ chemical shifts. The nitrogen s-character of the Pt–Nα bond is low, which is the reason for the small J-coupling. Similar bonding situations are likely to be found in azide complexes with other transition metals.