Tuning the Reactivity and Bonding Properties of Metal Square-Planar Complexes by the Substitution(s) on the Trans-Coordinated Pyridine Ring

The kinetics of the hydration reaction
on trans-[Pt­(NH3)2(pyrX)­Cl]+ (pyr = pyridine)
complexes (X = OH–, Cl–, F–, Br–, NO2–, NH2, SH–, CH3, CCH,
and DMA) was studied by density functional theory calculations in
the gas phase and in water solution described by the implicit polarizable
continuum model method. All possible positions ortho, meta, and para
of the substituent X in the pyridine ring were considered. The substitution
of the pyr ligand by electron-donating X’s led to the strengthening
of the Pt–N1­(pyrX) (Pt–NpyrX) bond and the
weakening of the trans Pt–Cl or Pt–Ow bonds.
The electron-withdrawing X’s have exactly the opposite effect.
The strengths of these bonds can be predicted from the basicity of
sigma electrons on the NpyrX atom determined on the isolated
pyrX ligand. As the pyrX ring was oriented perpendicularly with respect
to the plane of the complex, the nature of the X···Cl
electrostatic interaction was the decisive factor for the transition-state
(TS) stabilization which resulted in the highest selectivity of ortho-substituted
systems with respect to the reaction rate. Because of a smaller size
of X’s, the steric effects influenced less importantly the
values of activation Gibbs energies ΔG⧧ but caused geometry changes such as the elongation
of the Pt–NpyrX bonds. Substitution in the meta
position led to the highest ΔG⧧ values for most of the X’s. The changes of ΔG⧧ because of electronic effects were
the same in the gas phase and the water solvent. However, as the water
solvent dampened electrostatic interactions, 2200 and 150 times differences
in the reaction rate were observed between the most and the least
reactive mono-substituted complexes in the gas phase and the water
solvent, respectively. An additional NO2 substitution of
the pyrNO2 ligand further decelerated the rate of the hydration
reaction, but on the other hand, the poly-NH2 complexes
were no more reactive than the fastest o-NH2 system. In the gas phase, the poly-X complexes showed the additivity
of the substituent effects with respect to the Pt–ligand bond
strengths and the ligand charges.

本研究采用密度泛函理论（density functional theory, DFT）计算方法，分别在气相及基于隐式可极化连续介质模型（implicit polarizable continuum model）的水溶液体系中，对反式-[Pt(NH₃)₂(pyrX)Cl]⁺（pyr=吡啶（pyridine））配合物的水合反应动力学展开了系统研究，所考察的取代基X包括OH⁻、Cl⁻、F⁻、Br⁻、NO₂⁻、NH₂、SH⁻、CH₃、C≡CH以及DMA，研究中考虑了吡啶环上邻位、间位、对位所有可能的取代位置。给吡啶配体引入给电子取代基X后，会强化Pt–N₁(pyrX)（即Pt–NpyrX）键，同时削弱反位的Pt–Cl或Pt–Ow键；而吸电子取代基X则会产生完全相反的调控效应，上述Pt-配体键的强度可通过分离得到的pyrX配体上NpyrX原子的σ电子碱性进行预测。当pyrX环与配合物平面呈垂直取向时，X···Cl静电相互作用的性质是过渡态（transition-state, TS）稳定化的决定性因素，这使得邻位取代体系在反应速率层面表现出最优的选择性。由于取代基X的空间尺寸较小，空间效应对活化吉布斯自由能ΔG‡的影响相对有限，但会引发Pt–NpyrX键伸长等几何结构变化。对于绝大多数取代基X而言，间位取代的配合物拥有最高的活化吉布斯自由能ΔG‡值。电子效应引起的ΔG‡变化在气相和水溶液体系中保持一致，但由于水溶液会削弱静电相互作用，在气相和水溶液中，活性最高与活性最低的单取代配合物的反应速率分别相差2200倍和150倍。额外在pyrNO₂配体上引入NO₂取代基会进一步减慢水合反应速率；而多氨基（poly-NH₂）配合物的反应活性并未超过活性最高的邻位-NH₂体系。在气相中，多取代配合物的取代基效应对Pt-配体键强度及配体电荷的影响具有严格的加和性。