10.1021/ja3040762.s001
Kiplangat Sutter
Kiplangat
Sutter
Jochen Autschbach
Jochen
Autschbach
Computational Study and
Molecular Orbital Analysis
of NMR Shielding, Spin–Spin Coupling, and Electric Field Gradients
of Azido Platinum Complexes
American Chemical Society
2012
Molecular Orbital Analysis
azide σ bonds
2 p character
Electric Field Gradients
14 N NMR signals
15 N NMR data
N chemical shifts
azido 14 N NMR signals
EFG
N α nuclei
N β signals
delocalized π system
DFT
chemical shifts
N γ
N β chemical shifts
N β nuclei
Azido Platinum Complexes 195Pt 14 N
N α
2012-08-15 00:00:00
Journal contribution
https://acs.figshare.com/articles/journal_contribution/Computational_Study_and_Molecular_Orbital_Analysis_of_NMR_Shielding_Spin_Spin_Coupling_and_Electric_Field_Gradients_of_Azido_Platinum_Complexes/2496190
<sup>195</sup>Pt, <sup>14</sup>N, and <sup>15</sup>N
NMR data for
five azido (N<sub>3</sub><sup>–</sup>) 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 <i>J</i>-coupling constants.
Calculated <sup>14</sup>N electric field gradients (EFGs) reflect
experimentally observed trends for the line broadening of azido <sup>14</sup>N 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) <sup>14</sup>N NMR signals for the Pt-coordinating
(N<sub>α</sub>) nuclei in the azido ligands are much broader
than for the central (N<sub>β</sub>) or terminal (N<sub>γ</sub>) atoms. The N<sub>β</sub> signals are particularly narrow;
(ii) compared to N<sub>γ</sub>, the N<sub>α</sub> nuclei
are particularly strongly shielded; (iii) N<sub>β</sub> nuclei
have much larger chemical shifts than N<sub>α</sub> and N<sub>γ</sub> ; and (iv) The Pt–N<sub>α</sub> <i>J</i>-coupling constants are small in magnitude when considering
the formal sp hybridization of N<sub>α</sub> . It is found that
for N<sub>α</sub> a significant shielding reduction due to formation
of the dative N<sub>α</sub>–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 N<sub>β</sub> and N<sub>γ</sub> . This effect, along with rehybridization at N<sub>α</sub> upon bond formation with Pt, is shown to cause a deshielding
of N<sub>γ</sub> relative to N<sub>α</sub> and a strong
increase of the EFG at N<sub>α</sub> . The large 2p character
of the azide σ bonds is responsible for the particularly high
N<sub>β</sub> chemical shifts. The nitrogen s-character of the
Pt–N<sub>α</sub> bond is low, which is the reason for
the small <i>J</i>-coupling. Similar bonding situations
are likely to be found in azide complexes with other transition metals.