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Computational Study and Molecular Orbital Analysis of NMR Shielding, Spin–Spin Coupling, and Electric Field Gradients of Azido Platinum Complexes
journal contribution
posted on 2012-08-15, 00:00 authored by Kiplangat Sutter, Jochen Autschbach195Pt, 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.