Minimum Energy Crossing Point Research Articles

Reaction of (N4Py)Fe with H2O2 and the relevance of its Fe(IV)=O species during and after H2O2 disproportionation

AbstractThe catalytic disproportionation of by non‐heme Fe(II) complexes of H2O2 the ligand N4Py (1,1‐bis(pyridin‐2‐yl)‐N,N‐bis(pyridin‐2‐ylmethyl)methanamine) and the formation and reactivity of Fe(III)‐OOH and Fe(IV)=O species is studied by UV/Vis absorption, NIR luminescence, (resonance) Raman and headspace Raman spectroscopy, 1O2 trapping and DFT methods. Earlier DFT studies indicated that disproportionation of H2O2 catalysed by Fe(II)‐N4Py complexes produce only 3O2, however, only the low‐spin state pathway was considered. In the present study, DFT calculations predict two pathways for the reaction between Fe(III)‐OOH and H2O2, both of which yield 3O2/H2O2 and involve either the S=1/2 or the S=3/2 spin state, with the latter being spin forbidden. The driving force for both pathways are similar, however, a minimal energy crossing point (MECP) provides a route for the formally spin forbidden reaction. The energy gap between the reaction intermediate and the MECP is lower than the barrier across the non‐adiabatic channel. The formation of 3O2 only is confirmed experimentally in the present study through 1O2 trapping and NIR luminescence spectroscopy. However, attempts to use the 1O2 probe ( ‐terpinene) resulted in initiation of auto‐oxidation rather than formation of the expected endoperoxide, which indicated formation of OH radicals from Fe(III)‐OOH, e. g., through O−O bond homolysis together with saturation of methanol with 3O2. Microkinetic modelling of spectroscopic data using rate constants determined earlier, reveal that there is another pathway for Fe(III)‐OOH decomposition in addition to competition between the reaction of Fe(III)‐OOH with H2O2 and homolysis to form Fe(IV)=O and hydroxyl radical. Notably, after all H2O2 is consumed the decay of the Fe(III)‐OOH species is predominantly through a second order self reaction (with Fe(III)‐OOH). The conclusion reached is that the rate of O−O bond homolysis in the Fe(III)‐OOH species to form Fe(IV)=O and an hydroxyl radical is too low to be responsible for the observed oxidation of organic substrates.

Photoisomerisation of Exo-Imine Complexes and the Role of MECP in the Reverse Thermal Equilibrium: An Experimental and DFT Computational Investigation.

Complexes [MCl2Cp*]2 (M= Ir, Rh), [RuCl2(p-cymene)]2 and [Ir(C^N)2Cl]2 (HC^N = a, phenylpyridine ; b, phenylpyrazole,) react with imine ligands derived from o-aminophenol to yield complexes with an exocyclic C=N bond which has a cis or trans configuration. The trans isomer is favoured except for sterically crowded complexes Cp*M (M = Ir, Rh) when the imine has a mesityl substituent, for which the cis isomer is favoured. The complexes undergo photoisomerization in visible light but revert back to the original isomer over time or when heated. The rate of the thermal reverse isomerisation depends on the imine substituent and the metal fragment. DFT calculations correctly reproduce the favoured isomer and suggest that the reverse isomerisation occurs by a rehybridisation at the N atom as found in organic imines. In addition, a triplet state, thermally accessible by a Minimum Energy Crossing Point (MECP) provides a low energy pathway for reverse isomerisation in the case of the half-sandwich complexes.

Theoretical Study on the Rate Constant of Spin‐forbidden Reaction SO2+O(3P) (+M)→SO3 (+M)

AbstractTo improve the understanding of the oxidation of SO2 to SO3, the non‐adiabatic transition state theory (NA‐TST) coupling with the master equation (ME) method was adopted to study the kinetic characteristics of the spin‐forbidden recombination reaction SO2+O(3P) (+M)→SO3 (+M). By determining the minimum energy crossing point (MECP) of the triplet‐singlet potential energy surfaces, the high‐pressure limit (HPL), k∞=3.81×108 T0.998 e−1214/T cm3 mol−1 s−1 and the low‐pressure limit (LPL), k0=4.78×1036 T−6.38 e−4610/T cm6 mol−2 s−1 were obtained, respectively. The LPL rate constants were compared with the reported experimental data and good consistency was found. Besides, the hopping probability near the MECP was also determined. In addition, this work shows the potential application of NA‐TST/ME method to investigate the kinetics of similar spin‐forbidden reactions.

Unraveling the Pivotal Roles of Various Metal Ion Centers in the Catalysis of Quercetin 2,4-Dioxygenases.

Quercetin 2,4-dioxygenase (QueD) with various transition metal ion co-factors shows great differences, but the internal reasons have not been illustrated in detail. In order to explore the effects of metal ion centers on the catalytic reactivity of QueD, we calculated and compared the minimum energy crossing point (MECP) of dioxygen from the relatively stable triplet state to the active singlet state under different conditions by using the DFT method. It was found that the metal ions play a more important role in the activation of dioxygen compared with the substrate and the protein environment. Simultaneously, the catalytic reactions of the bacterial QueDs containing six different transition metal ions were studied by the QM/MM approach, and we finally obtained the reactivity sequence of metal ions, Ni2+ > Co2+ > Zn2+ > Mn2+ > Fe2+ > Cu2+, which is basically consistent with the previous experimental results. Our calculation results indicate that metal ions act as Lewis acids in the reaction to stabilize the substrate anion and the subsequent superoxo and peroxo species in the reaction, and promote the proton coupled electron transfer (PCET) process. Furthermore, the coordination tendencies of transition metal ion centers also have important effects on the catalytic cycle. These findings have general implications on metalloenzymes, which can expand our understanding on how various metal ions play their key role in modulating catalytic reactivity.

Finding Excited-State Minimum Energy Crossing Points on a Budget: Non-Self-Consistent Tight-Binding Methods.

The automated exploration and identification of minimum energy conical intersections (MECIs) is a valuable computational strategy for the study of photochemical processes. Due to the immense computational effort involved in calculating non-adiabatic derivative coupling vectors, simplifications have been introduced focusing instead on minimum energy crossing points (MECPs), where promising attempts were made with semiempirical quantum mechanical methods. A simplified treatment for describing crossing points between almost arbitrary diabatic states based on a non-self-consistent extended tight-binding method, GFN0-xTB, is presented. Involving only a single diagonalization of the Hamiltonian, the method can provide energies and gradients for multiple electronic states, which can be used in a derivative coupling-vector-free scheme to calculate MECPs. By comparison with high-lying MECIs of benchmark systems, it is demonstrated that the identified geometries are good starting points for further MECI refinement with ab initio methods.

Predicting Kinetics and Dynamics of Spin-Dependent Processes

ConspectusPredicting mechanisms and rates of nonadiabatic spin-dependent processes including photoinduced intersystem crossings, thermally activated spin-forbidden reactions, and spin crossovers in metal centers is a very active field of research. These processes play critical roles in transition-metal-based and metalloenzymatic catalysis, molecular magnets, light-harvesting materials, organic light-emitting diodes, photosensitizers for photodynamic therapy, and many other applications. Therefore, accurate modeling of spin-dependent processes in complex systems and on different time scales is important for many problems in chemistry, biochemistry, and materials sciences.Nonadiabatic statistical theory (NAST) and nonadiabatic molecular dynamics (NAMD) are two complementary approaches to modeling the kinetics and dynamics of spin-dependent processes. NAST predicts the probabilities and rate constants of nonradiative transitions between electronic states with different spin multiplicities using molecular properties at only few critical points on the potential energy surfaces (PESs), including the reactant minimum and the minimum energy crossing point (MECP) between two spin states. This makes it possible to obtain molecular properties for NAST calculations using accurate but often computationally expensive electronic structure methods, which is critical for predicting the rate constants of spin-dependent processes. Alternatively, NAST can be used to study spin-dependent processes in very large complex molecular systems using less computationally expensive electronic structure methods. The nuclear quantum effects, such as zero-point vibrational energy, tunneling, and interference between reaction paths can be easily incorporated. However, the statistical and local nature of NAST makes it more suitable for large systems and slow kinetics. In contrast, NAMD explores entire PESs of interacting electronic states, making it ideal for modeling fast barrierless spin-dependent processes. Because the knowledge of large portions of PESs is often needed, the simulations require a very large number of electronic structure calculations, which limits the NAMD applicability to relatively small molecular systems and ultrafast kinetics.In this Account, we discuss our contribution to the development of the NAST and NAMD approaches for predicting the rates and mechanism of spin-dependent processes. First, we briefly describe our NAST and NAMD implementations. The NAST implementation is an extension of the transition state theory to the processes involving two crossing potential energy surfaces of different spin multiplicities. The NAMD approach includes the trajectory surface hopping (TSH) and ab initio multiple spawning (AIMS) methods. Second, we discuss several applications of NAST and NAMD to model spin-dependent processes in different systems. The NAST applicability to large complex systems is demonstrated by the studies of the spin-forbidden isomerization of the active sites of metal-sulfur proteins. Our implementation of the MECP search algorithm within the fully ab initio fragment molecular orbital method allows applying NAST to systems with thousands of atoms, such as the solvated protein rubredoxin. Applications of NAMD to ultrafast spin-dependent processes are represented by the generalized AIMS simulations utilizing the fast GPU-based TeraChem electronic structure program to gain insight into the complex photoexcited state relaxation in 2-cyclopentenone.

Sulfur Monoxide Release and Capture: A Computational Study

AbstractThermolysis of thiirane oxide leads to production of highly reactive sulfur monoxide. The liberated SO can in turn be trapped with a diene scavenger forming dihydrothiophene oxide. Since the intermediate diatomic possesses a triplet ground state, the SO transfer can proceed on two spin‐state surfaces. Here, we study the competition between singlet concerted and stepwise triplet diradical mechanisms utilizing the M06‐2X density functional as well as CCSD(T) and MRCI+Q wavefunction theories. We find that the decomposition of thiirane oxide prefers to pass through a triplet diradical intermediate that becomes accessible from a nearby minimum energy crossing point (MECP). Hence, the thermolysis of thiirane oxide is expected to predominantly release triplet ground state sulfur monoxide in agreement with previous experimental reports. The addition of 3SO to 1,3‐butadiene initially generates an allylic diradical, that ring‐closes to a thiirane oxide through another MECP, and a subsequent rearrangement gives access to the final product.

Dynamic-dependent selectivity in a bisphosphine iron spin crossover C-H insertion/π-coordination reaction.

Reaction pathway selectivity is generally controlled by competitive transition states. Organometallic reactions are complicated by the possibility that electronic spin state changes rather than transition states can control the relative rates of pathways, which can be modeled as minimum energy crossing points (MECPs). Here we show that in the reaction between bisphosphine Fe and ethylene involving spin state crossover (singlet and triplet spin states) that neither transition states nor MECPs model pathway selectivity consistent with experiment. Instead, single spin state and mixed spin state quasiclassical trajectories demonstrate nonstatistical intermediates and that C-H insertion versus π-coordination pathway selectivity is determined by the dynamic motion during reactive collisions. This example of dynamic-dependent product outcome provides a new selectivity model for organometallic reactions with spin crossover.

Study on the activation mechanism of protactinium and NH3 by density functional theory

The interaction of Protactinium with NH3 was analyzed by Density Functional Theory. We obtained the dehydrogenation paths with the lowest energy and plotted the potential energy surfaces (PESs). At the same time, the intersection of the PES is discussed by minimum energy crossing point (MECP). Combined with topological analysis, the bonding properties were systematically explored, and the 5f orbital properties and charge changes were explained by the nature bond orbital (NBO). The results shown that 5f electrons are not conducive to the reaction progress. Finally, the reaction rate of the transition state was discussed in detail by using the variational transition state theory (VTST).

Evidence of Gas-Phase Attachment of Molecular Oxygen to Deprotonated Hydroquinone During Ion-Mobility Mass Spectrometry.

Gas-phase addition of dioxygen to certain ions is a well-known phenomenon in mass spectrometry. For this reaction to occur, the presence of a distonic radical site on the precursor ion is thought to be a prerequisite. Herein, we report that oxygen adduct formation can take place also with deprotonated hydroquinone, which in fact is an even-electron species without a radical site. When the product-ion spectrum of the m/z 109 ion, generated by electrospray ionization from a solution of hydroquinone in acetonitrile, was recorded under ion-mobility conditions, a new peak was observed at m/z 141. However, an analogous peak was not visible in the spectrum acquired under nonmobility conditions (i.e., without any gas introduced to the mobility cell). Presumably, traces of oxygen present in the collision gas instigate an ion-molecule reaction to produce an adduct of m/z 141, which upon activation results in CO and H2O loss to form a product ion of m/z 95. Isotope-labeling studies confirmed that one of the hydrogen atoms from the hydroxy group and another from the aromatic ring contribute to the water loss instigated from the m/z 141 adduct. Furthermore, computational methods indicated the three-dimensional structure of the ground-state deprotonated hydroquinone to be distinctly different from those of its 1,2- and 1,3-isomers. Calculations predicted that all atoms in the two m/z 109 ions generated from catechol and resorcinol lie on one plane. In contrast, the structure of the m/z 109 ion from hydroquinone was significantly different. Computations predicted that the hydrogen atom on the intact hydroxyl group of deprotonated hydroquinone protrudes out of plane from rest of the atoms. Consequently, the exposed OH group can interact with an incoming dioxygen molecule. Computations conducted at the CAM-B3LYP/6-311++g(2d,2p) level of theory detected a minimum energy crossing point (MECP) at -4.3 kJ mol-1 below the separated O2 + deprotonated hydroquinone triplet threshold. In contrast, similar calculations conducted for catechol and resorcinol yielded MECPs of +116.9 and +69.1 kJ mol-1, respectively, above the associated triplet thresholds. These results indicated that the curve crossing required to form singlet products upon reaction with triplet O2 is favorable in the case of hydroquinone and unfavorable in the cases of catechol and resorcinol. In practical terms, the selective oxygen addition appears to be a diagnostically useful reaction to differentiate hydroquinone from its ring isomers.

Fast Screening of Minimum Energy Crossing Points with Semiempirical Tight-Binding Methods.

The investigation of photochemical processes is a highly active field in computational chemistry. One research direction is the automated exploration and identification of minimum energy conical intersection (MECI) geometries. However, due to the immense technical effort required to calculate nonadiabatic potential energy landscapes, the routine application of such computational protocols is severely limited. In this study, we will discuss the prospect of combining adiabatic potential energy surfaces from semiempirical quantum mechanical calculations with specialized confinement potential and metadynamics simulations to identify S0/T1 minimum energy crossing point (MECP) geometries. It is shown that MECPs calculated at the GFN2-xTB level can provide suitable approximations to high-level S0/S1ab initio conical intersection geometries at a fraction of the computational cost. Reference MECIs of benzene are studied to illustrate the basic concept. An example application of the presented protocol is demonstrated for a set of photoswitch molecules.

QM/MM study on the O2 activation reaction of 4-hydroxylphenyl pyruvate dioxygenase reveals a common mechanism for α-ketoglutarate dependent dioxygenase

The dioxygen activation catalyzed by 4-hydorxylphenyl pyruvate dioxygenase (HPPD) were reinvestigated by using hybrid quantum mechanics/molecular mechanics (QM/MM) approaches at the B3LYP/6-311++G(d,p):AMBER level. These studies showed that this reaction consisted of two steps including the dioxygen addition/decarboxylation and hetero OO bond cleavage, where the first step was found to be rate-determining. The former step initially runs on a septet potential energy surface (PES), then switches to a quintet PES after crossing a septet/quintet minimum energy crossing point (MECP) 5-7M2, whereas the rest step runs on the quintet PES. The reliability of our theoretical predictions is supported by the excellent agreement of the calculated free-energy barrier value of 16.9 kcal/mol with available experimental value of 16–17 kcal/mol. The present study challenges the widely accepted view which holds that the O2 activation catalyzed by α-keto glutamate (α-KG) dioxygenase mainly runs on the quintet PES and provides new insight into the catalytic mechanism of α-KG dioxygenase and/or other related Fe(II)-dependent oxygenase.

Visualization of electron density changes along chemical reaction pathways

We propose a simple procedure for visualising the electron density changes (EDC) during a chemical reaction, which is based on a mapping of rectangular grid points for a stationary structure into (distorted) positions around atoms of another stationary structure. Specifically, during a small step along the minimum energy pathway (MEP), the displacement of each grid point is obtained as a linear combination of the motion of all atoms, with the contribution from each atom scaled by the corresponding Hirshfeld weight. For several reactions (identity S N 2, Claisen rearrangement, Diels-Alder reaction, [ 3 + 2 ] cycloaddition, and phenylethyl mercaptan attack on pericosine A), our EDC plots showed an expected reduction of electron densities around severed bonds (or those with the bond-order lowered), with the opposite observed for newly-formed or enhanced chemical bonds. The EDC plots were also shown for copper triflate catalyzed N 2 O fragmentation, where the N–O bond weakening initially occurred on a singlet surface, but continued on a triplet surface after reaching the minimum-energy crossing point (MECP) between the two potential energy surfaces.

Insight into Isomeric Effect on the Photoluminescence and Mechanoluminescence of Cyanostilbene Derivatives.

Molecular structures, packings, and intermolecular interactions significantly affect the photophysical properties of organic luminogens. In this work, the photoluminescence (PL) and mechanoluminescence (ML) of two pairs of isomers, 1/2 and 3/4, were systematically explored. The fluorescence of crystals 1c and 4c is much brighter than that of their isomers 2c and 3c, respectively. Only 1c is ML-active among all four molecules. Single-crystal structural analysis revealed that isomerization of a substituent group affected their molecular packing and intermolecular interactions. Stronger intermolecular interaction and intact three-dimensional hydrogen-bonded networks were formed only in crystal 1c, which were essential for preventing slippage of molecular layers and generating ML; the other molecules were either lacking π-π interactions or C-H···π interactions. Theoretical calculation suggested that the energy barrier between the Franck-Condon (FC) structure and minimum energy crossing point (MECP) structure of 2/3 was much lower than that of 1/4. Nonradiative decay channels of molecules 2 and 3 were thus more easily activated, which led to their lower quantum yield.

NAST: Nonadiabatic Statistical Theory Package for Predicting Kinetics of Spin-Dependent Processes.

We present a nonadiabatic statistical theory (NAST) package for predicting kinetics of spin-dependent processes, such as intersystem crossings, spin-forbidden unimolecular reactions, and spin crossovers. The NAST package can calculate the probabilities and rates of transitions between the electronic states of different spin multiplicities. Both the microcanonical (energy-dependent) and canonical (temperature-dependent) rate constants can be obtained. Quantum effects, including tunneling, zero-point vibrational energy, and reaction path interference, can be accounted for. In the limit of an adiabatic unimolecular reaction proceeding on a single electronic state, NAST reduces to the traditional transition state theory. Because NAST requires molecular properties at only a few points on potential energy surfaces, it can be applied to large molecular systems, used with accurate high-level electronic structure methods, and employed to study slow nonadiabatic processes. The essential NAST input data include the nuclear Hessian at the reactant minimum, as well as the nuclear Hessians, energy gradients, and spin-orbit coupling at the minimum energy crossing point (MECP) between two states. The additional computational tools included in the NAST package can be used to extract the required input data from the output files of electronic structure packages, calculate the effective Hessian at the MECP, and fit the reaction coordinate for more advanced NAST calculations. We describe the theory, its implementation, and three examples of application to different molecular systems.

A computational scheme for evaluating the phosphorescence quantum efficiency: applied to blue-emitting tetradentate Pt(II) complexes.

Phosphorescent organic light-emitting diodes (PhOLEDs) are leading candidates for displays or lighting technologies. Recently, blue phosphorescent tetradentate Pt(II) complexes have been attracting extensive attention due to their high phosphorescence quantum efficiency and numerous chemical structures on account of flexible ligand frames and modifications. Using quantum chemistry coupled with our thermal vibration correlation function (TVCF) formalism, we investigated the triplet excited state energy surface and the decay processes involving both direct vibrational relaxation and minimum energy crossing point (MECP) via the transition state (3TS) to the ground state (S0) for 16 recently experimentally reported blue-emitting tetradentate Pt(II) emitters containing fused 5/6/6 metallocycles. We found that (i) in most cases, the direct vibrational relaxation deactivations dominated the triplet non-radiative decay because either the 3TS is too high or the MECP is not reachable. Hence, results from the TVCF formalism agreed well with the experiments for the phosphorescence quantum efficiency; (ii) only when both 3TS and MECP are low, for instance, for PtON1-oMe, deactivations via MECP dominated the triplet non-radiative decay.

Photostability of the deprotonated forms of the UV filters homosalate and octyl salicylate: molecular dissociation versus electron detachment following UV excitation.

While common molecular anions show a strong propensity to undergo electron detachment upon UV excitation, this process often occurs in competition with molecular ion dissociation. The factors that affect the balance between these two major possible decay pathways have not been well understood to date. Laser photodissociation spectroscopy of the deprotonated forms of the UV filter molecules, Homosalate (HS) and Octyl Salicylate (OS), i.e. [HS − H]− and [OS − H]−, was used to acquire gas-phase UV absorption spectra for [HS − H]− and [OS − H]−via photodepletion from 3.0–5.8 eV. No photofragmentation (i.e. dissociation of the ionic molecular framework) was observed for either [HS − H]− and [OS − H]− following photoexcitation, revealing that electron loss entirely dominates the electronic decay pathways for these systems. High-level quantum chemical calculations were used to map out the excited states associated with [HS − H]− and [OS − H]−, revealing that the minimum-energy crossing points (MECPs) between the S1 and S0 states are located in elevated regions of the potential energy surface, making internal conversion unlikely. These results are consistent with our experimental observation that electron detachment out-competes hot ground state molecular fragmentation. More generally, our results reveal that the competition between molecular dissociation and electron detachment following anion photoexcitation can be determined by the magnitude of the energy gap between the excitation energy and the MECPs, rather than being a simple function of whether the excitation energy lies above the anion's vertical detachment energy.

Radical Mechanism of Ir III /Ni II -Metallaphotoredox-Catalyzed C(sp 3 )–H Functionalization Triggered by Proton-Coupled Electron Transfer: Theoretical Insight

Open AccessCCS ChemistryRESEARCH ARTICLE1 Apr 2022Radical Mechanism of IrIII/NiII-Metallaphotoredox-Catalyzed C(sp3)–H Functionalization Triggered by Proton-Coupled Electron Transfer: Theoretical Insight Yu-Jiao Dong†, Bo Zhu†, Yun Geng, Zhi-Wen Zhao, Zhong-Min Su and Wei Guan Yu-Jiao Dong† Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun 130024 †Y.-J. Dong and B. Zhu contributed equally to this work.Google Scholar More articles by this author , Bo Zhu† Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun 130024 †Y.-J. Dong and B. Zhu contributed equally to this work.Google Scholar More articles by this author , Yun Geng Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun 130024 Google Scholar More articles by this author , Zhi-Wen Zhao Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun 130024 Google Scholar More articles by this author , Zhong-Min Su *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun 130024 College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author and Wei Guan *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun 130024 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202100802 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Photoredox catalysis can be induced to activate organic substrates or to modulate the oxidation state of transition-metal catalysts via unique single-electron transfer processes, so as to achieve challenging C(sp3)–H functionalization under mild conditions. However, the specific reaction mechanism and relevant electron transfer process still need to be clarified. Here, a highly regioselective IrIII/NiII-metallaphotoredox-catalyzed hydroalkylation of asymmetrical internal alkyne with an ether α-hetero C(sp3)–H bond has been investigated by density functional theory (DFT) calculations. A novel radical mechanism was predicted to merge oxidative quenching (IrIII–*IrIII–IrIV–IrIII) and nickel catalytic cycles (NiII–NiIII–NiI–NiIII–NiII) for this C(sp3)–H functionalization to construct C(sp3)–C(sp2) bonds. It consists of seven major steps: the single-electron transfer involved in the photoredox cycle for generating active Ni(I)–chloride complexes, proton-coupled electron transfer process to provide α-carbon-centered tetrahydrofuran (THF) radicals, radical capture by Ni(II), reductive elimination to obtain 2-chlorotetrahydrofuran, alkyne oxidative hydrometallation, inner-sphere electron transfer, and σ-bond metathesis to yield the desired alkyne hydroalkylation product. Importantly, both the thermodynamic performance for redox potentials and the kinetic exploration for energy barriers and electron-transfer rates have also been evaluated for the corresponding electron transfer processes. In addition, the steric effects play a major role in determining the regioselectivity of alkyne oxidative hydrometallation. Download figure Download PowerPoint Introduction The functionalization of C–H bonds to construct C–C and C–X (X = N, O, S, etc.) bonds has been recognized as an active field in current organic research.1–3 It is not only due to the readily available starting materials to satisfy the requirement of organic transformations, but also the natural features of C–H bonds broadly existing in organic molecules, such as accessibility, activity, and selectivity.4 Over the past decades, activation of C–H bonds with a wide variety of directing groups has become an economically attractive synthetic strategy, which plays an important role in determining proximal C–H reactivity.5,6 Also, transition-metal catalysts, by Pd,7–9 Rh,10,11 Ru,12,13 Ir,14,15 and Ni16–18 complexes, are crucial for the functionalization of C–H bonds as a versatile and efficient strategy to construct the C–C and C–X bonds in organic synthesis.19–21 These reactions have been realized using the directing groups strategy,22,23 radical addition,24 the concerted metalation–deprotonation,25 and single-electron transfer (SET),26 respectively. It is still a significant challenge to realize regioselective remote C–H functionalization in the absence of directing groups.4 Therefore, it is highly desirable that a novel strategy can remove basic additives and then broaden the scope of substrates under mild conditions. The visible-light photocatalysis,27–29 because of the natural advantages of the visible light, is attractive for developing important and accessible chemical conversions. Specifically, photoredox catalysis developed simultaneously by Nicewicz and MacMillan,30 Yoon et al.,31 and Stephenson et al.32 were applied to the direct asymmetric alkylation of aldehydes, efficient [2 + 2] enone cycloadditions, and SET instructional tin-free reductive dehalogenation reaction, respectively. In comparison with traditional C–H functionalization, the photocatalyst using its redox properties can directly produce reactive radical species from C–H bond substrates under extremely mild conditions.33 Furthermore, the synergistic catalysis of photocatalyst and transition-metal catalyst has been widely reported by Molander, Gutierrez, Doyle, et al.,34–39 and can be traced back to Pd/Ru cooperative catalytic system. It can convert some substrates to reactive radicals as nucleophilic coupling partners that are subsequently involved in the transition-metal-catalyzed cross-coupling reactions. This electron-transfer-mediated catalysis can break through the inherent limitations of the two-electron pattern and then as an innovative catalytic platform expand the scope of the building various C–C/C–X bonds. Based on the interest in theoretical mechanistic studies of iridium/nickel metallaphotoredox-catalyzed C–O, C–S, and C–N cross-couplings,40–42 we further questioned whether the previously proposed oxidation state modulation mechanism merging oxidative quenching and nickel catalytic cycles or an alternative radical mechanism is applicable to C–H functionalization to achieve C–C cross-couplings. Recently, a photoredox-mediated IrIII/NiII dual-catalyzed hydroalkylation of ether α-hetero C(sp3)–H bond has been achieved to construct C(sp3)–C(sp2) bonds, as shown in Scheme 1.43 Compared with transition-metal catalysis, this dual catalysis requires no basic additive and has wide substrate scope and an excellent regioselectivity of alkyne insertion. Based on the radical trapping experiments, a hypothetical radical mechanism consisting of reductive quenching and nickel catalytic cycles had been proposed to understand the dual IrIII/NiII-catalyzed C–H functionalization. Furthermore, a chlorine radical was speculated to be generated by the visible-light excitation of high-oxidation-state nickel(III) chloride, as shown in Scheme 2 (mechanism A). Next the chlorine radical may abstract an α-hydrogen atom of tetrahydrofuran (THF) to generate a key α-carbon-center radical. On the other hand, a unique photocatalytic strategy in which an excited photocatalyst is employed to directly activate substrates, such as R–H (R = C, Si, and S) bonds, by proton-coupled ET (PCET) process, has been proposed in previous studies.44–46 Thus, it is likely that the PCET process of photoexcited species *IrIII and THF may generate α-carbon-centered radical and the ground-state IrII species (mechanism B in Scheme 2). Subsequently, single-electron reduction of NiII catalyst by IrII generates active NiI and IrIII species. Another plausible mechanism is the oxidative quenching cycle (IrIII–*IrIII–IrIV–IrIII) where an active NiI species is generated via oxidative quenching of photoexcited *IrIII by NiII. Next, the oxidized IrIVCl with THF triggers a PCET process to afford α-carbon-centered THF radical (mechanism C in Scheme 2). Indeed, the generated chlorine radical as an electrophilic species can abstract hydrogen from the electron-rich α-C(sp3)–H bond of ethers via direct hydrogen atom transfer (HAT) using the polar effects.18,47,48 However, the chlorine photoelimination from a charge-transfer excited state of mononuclear nickel(III) complex49 may inevitably trigger a multistep prereaction process. Considering the successive HAT and other steps involved in the C–H functionalization reaction, a high energy barrier might need to be overcome to construct the desired C–C bond. Therefore, how the radical is generated in this radical mechanism of the C–H functionalization by such a dual catalysis still remains ambiguous but a quite crucial issue. In this regard, theoretical calculations complemented with experimental observations can predict enough short-lived reactive intermediates and transition states, and thus disclose the rational reaction mechanism in a more advantageous approach.5,28,50–68 Here, we want to investigate the existing photoredox catalysis, clarify reasonable mechanism and propose the meaningful C–H activation modes for constructing C–C bond. More importantly, we will solve some problems as follows: (1) How do we understand the PCET process induced by the quenching of the excited photocatalyst? (2) What is the origin of regioselectivity in this asymmetrical hydroalkylation reaction? Scheme 1 | Photoredox-mediated IrIII/NiII dual-catalyzed asymmetrical hydroalkylation of ether α-hetero C(sp3)–H bond to achieve C–C cross-coupling. Download figure Download PowerPoint Scheme 2 | Three possible mechanisms regarding α-C(sp3)–H functionalization of THF. Download figure Download PowerPoint Methods All calculations were carried out with the Gaussian 09 package69 using density functional theory (DFT) functional (U)M06.70 Geometry optimizations, together with frequency analyses, were used to verify whether the stationary points are minima without imaginary frequencies or a transition state with only one imaginary frequency conducted in the gas phase to acquire the thermodynamic properties at 333.15 K and 1 atm. Intrinsic reaction coordinate (IRC)71,72 analyses were performed to identify the transition states to connect the correct reactants and products. Here, a mixed basis set approach was employed with LanL2DZ73 for Ir and Ni, 6-31++G(d,p)74 for the α-hydrogen atom of THF, and 6-31G(d)75 for the other main-group elements, respectively. The single-point energies of all stationary points were performed at the (U)M06/[6-311++G(d,p)/SDD76(Ir and Ni)] level. The solvent effect of THF in single-point energies was considered by the SMD77 solvation model. In addition, the translational entropy was corrected with the method developed by Whitesides et al.78 (see Supporting Information for computational details). In this work, all of the DFT calculations employ the default pruned numerical integration grid. To avoid numerical errors in both SCF and frequency calculations, two kinds of pruned numerical integration grids, fine (75, 302) and ultrafine (99, 590), have been used to evaluate the energy barrier of rate-determining step. The result shows that the integral grid has little influence on the energy barriers. In the present calculations, the photoredox-mediated IrIII/NiII dual-catalyzed C–C cross-coupling of THF and unsymmetrical alkynes (3,3-dimethylbut-1-yn-1-yl) benzene ( AL) were selected as the model reactions, where IrIII[dF(CF3)ppy]2(dtbbpy)PF6 was adopted as photocatalyst and abbreviated as Ir III (Scheme 1). Considering the computational cost, the realistic transition-metal catalyst NiII(dtbbpy)Cl2 was simplified to NiII(bpy)Cl2 ( Ni II). In addition, the absorption spectra of Ir III and Ni II were simulated by the time-dependent DFT (TDDFT) calculation at the SMD(THF)/M06/[6-31G(d)/LanL2DZ(Ir and Ni)] level to validate the computational rationality ( Supporting Information Figures S1 and S2). Note that, when the realistic ligand dtbbpy was employed instead of the simplified ligand bpy, the Gibbs activation energy of the rate-determining step does not differ very much, indicating the rationality of simplification ( Supporting Information Table S2). Considering the diversity of oxidation state and spin state of nickel, such as the planar singlet (SNiII) and tetrahedral triplet (TNiII) states of Ni(II) and the doublet (DNiI/III) and quartet (QNiI/III) states of Ni(I) and Ni(III), all species were discussed at the lowest energy spin state in the nickel catalytic cycles ( Supporting Information Tables S3 and S4 and Figure S4). In addition, to evaluate the effects of gas- and solvent-phase optimizations on the ion migration involved in the SET process, we have compared the Gibbs free energies of the key SET processes at two levels ( Supporting Information Figure S5). The calculated results show that the solvent effect of THF has little influence on their gas-phase optimized geometries. Results and Discussion The photomediated α-C(sp3)–H functionalization of THF Generation of α-carbon-centered radical Radical trapping experiments indicate that α-carbon-centered THF radicals participate in the reaction process.43 However, the generation pathway of α-carbon-centered radical needs to be clarified. As described by the above strategies, three generation pathways of α-carbon-centered radical have been examined here (Figure 1). In mechanism A, although T Ni II may be oxidized by photoexcited * Ir III according to the redox potential comparisons ( Supporting Information Table S5), the reductive quenching process of * Ir III with T Ni II to afford Ir II and D Ni III species is predicated to be thermodynamically unfavorable with the Gibbs free energy change (ΔG°) of 13.5 kcal/mol. This reversal is speculated to be due to arduous PF6− migration between iridium and nickel to remain electrically neutral. Such inert photoredox-mediated excited-state SET process has been difficult enough to trigger the next step,31,79 and the photolysis of resultant oxidative product D Ni III and following HAT require ΔG° values of −4.5 and 7.9 kcal/mol, respectively, to deliver the α-carbon-centered THF radical. Thus, such high endothermic reactions induced by the above SET and HAT may not be the most favorable mechanism to produce the carbon-centered radical. Figure 1 | The Gibbs free energy change (ΔG°333.15 in kcal/mol) of selected key steps for generating α-carbon-centered radical in mechanisms A, B, and C. Spin densities for the main intermediates and transition states are given in grey italic font. Download figure Download PowerPoint In comparison with mechanism A, * Ir III could be reductively quenched by THF instead of T Ni II to provide an α-carbon-centered radical and Ir II in mechanism B. However, the whole PCET process is still highly endoergic at 32.0 kcal/mol, and mechanism B can be ruled out. It is noteworthy that the excited photocatalyst may simultaneously serve as a strong 1e-oxidant and -reductant, so the present photoredox catalysis could adopt an oxidative quenching mechanism, in addition to the above two reductive quenching mechanisms. In mechanism C, photoexcited * Ir III is oxidatively quenched by T Ni II via the SET process accompanied by the facile Cl− migration to generate the ground-state Ir IVCl and D Ni I with a ΔG° value of −5.6 kcal/mol, although the difference between E1/2red[IrIV/*IrIII] and E1/2red[NiII/NiI] is very small ( Supporting Information Table S5). Next, a stepwise PCET process between THF and Ir IVCl can release the desired α-carbon-centered THF radical (rather than β-carbon-centered THF radical, Supporting Information Figure S6) and regenerate Ir III to restart the photocatalytic cycle with ΔG° and ΔG°‡ values of −0.6 and 4.8 kcal/mol. Note that the dissociation of HCl from the THF•⋯HCl complex requires a ΔG° value of 4.1 kcal/mol and the HCl is more likely to interact with the α-C center rather than the O of THF• ( Supporting Information Figure S6c). In this stepwise PCET process, the proton transfer will occur prior to the ET through the redox potential comparisons ( Supporting Information Tables S5 and S6). Therefore, both ET steps are thermodynamically accessible for modulating the oxidation state of nickel catalyst and affording the α-carbon-centered radical. Kinetic evaluation for SET processes To further verify the rationality of the mechanism C, the kinetic exploration of ET processes involved in the above oxidative quenching cycle has been performed. The energy profiles of these processes are intuitive in Figure 2. Optimized structures for selected important stationary points in the energy profiles are shown in Supporting Information Scheme S1. First, the calculated absorption spectrum of Ir III is in good agreement with the experimental one and supports the robustness of our theoretical methods ( Supporting Information Figure S1). The strong absorption bands calculated at 292, 300, 341, and 372 nm mainly involve the charge-transfer transitions from the d orbital of iridium center to the ligands (denoted as S0 → SMLCT), localized π–π* transitions within the same phenylpyridine or bipyridine ligand, and the charge-transfer transitions from phenylpyridine ligands to bipyridine ligand ( Supporting Information Figure S3 and Table S1). Thereby, upon excitation by light, Ir III was initially excited to the Frank–Condon region mainly characterized as metal-to-ligand charge transfer (MLCT) and LCT states, as illustrated in Figure 2a. Subsequently, photoexcited * Ir III rapidly relaxes to the minimum state (S1) through internal conversion according to Kasha’s rule. Then the intersystem crossing between a singlet and a triplet electronic state occurs through the singlet–triplet crossing (STC) and the resultant TMLCT relaxes to its minimum state (T1). It is worth mentioning that there is competition between phosphorescence emission and SET from this T1 state. Comparing the rates of phosphorescent radiation (red line, kp = 2.35 × 104 s−1) and SET oxidative quenching (blue line, kSET = 2.22 × 1013 s−1) based on the value of reorganization energy 1, the SET process occurs much more quickly than the emission route; see Supporting Information for computational details of rate calculations. Based on this discovery, the ET processes are expected to play a crucial role in modulating the oxidation state of nickel and activating α-hetero C(sp3)–H bond of THF. Starting from the T1 state of Ir III, the SET oxidative quenching is instantaneously triggered by interaction with the ground state of T Ni II. Next, the excited-state charge-transfer * Ir III/ Ni II complex A (exciplex) is formed. The dimer configurations for * Ir III/ Ni II were initially screened out from the conformers with ≥3% probability in the Boltzmann distribution80 ( Supporting Information Scheme S2), and then the electron coupling values were calculated based on these conformers. To guarantee the ET from * Ir III to T Ni II, a relatively large electron coupling value of 0.058 eV was chosen to discuss the corresponding SET process, as shown in Figure 2b. A shorter π–π stacking distance between phenylpyridine and bipyridine ligands (3.47 Å) can be observed in such optimized dimer complex, which also effectively promotes the SET through stronger electronic coupling. Besides, the small reorganization energies with 0.13 and 0.20 eV could promote the rate of SET process. With structural relaxation and charge redistribution, the exciplex overcomes a very low energy barrier of 0.9 kcal/mol to afford Ir IVCl and D Ni I complexes (blue line in Figure 2a). Note that the energy transfer between T Ni II and * Ir III should not occur because of no overlap between the emission spectrum of Ir III and UV-absorption spectrum of T Ni II.41 Similarly, the PCET process between the resultant Ir IVCl and newcomer THF occurs with an energy barrier of 4.8 kcal/mol, the reorganization energies of 0.14 and 0.42 eV, and the electron-transfer rate of 2.64 × 1010 s−1, respectively. The isomers of THF/ Ir IVCl dimer complex have been also examined ( Supporting Information Scheme S2). Finally, the α-hetero C(sp3)–H bond of THF can be successfully cleaved to afford the desired α-carbon-centered THF radical, and the ground-state Ir III is regenerated. In addition, two nucleophilic substrates N,N′-dimethylacetamide (DMA) and dioxane employed in the experiments were chosen to confirm the feasibility of the mechanism C ( Supporting Information Figure S7). In brief, the ET processes involved in the oxidative quenching cycle have been supported by the kinetic exploration for energy barriers and electron-transfer rates. At present, mechanism C seems to be the only acceptable one. Figure 2 | (a) Energy profiles of ET processes in the oxidative quenching cycle. The kinetic calculating of reorganization energy λ1 and λ2 (eV), electronic free energy change ΔE (kcal/mol), Gibbs activation energy ΔG°⧧ (kcal/mol) and the charge-transfer rate k (s−1). The key characteristic transition points are schematically shown with their orbitals and key distances in Å. (b) The isomers of *Ir III/Ni II complex (exciplex) with the absolute value of their electronic coupling |V (eV)|. Download figure Download PowerPoint The most favorable catalytic cycle The whole nickel catalytic cycle In addition to the iridium photoredox cycle, the possible nickel catalytic cycles must be evaluated. In view of the previous studies about alkyne insertion reactions,81,82 three probable nickel catalytic cycles (Scheme 3) have been put forward to understand the present C–C cross-coupling reaction of THF and asymmetrical alkynes, which were evaluated through DFT calculations in Figures 3 and 4 ( Supporting Information Table S7 and Figure S10). Scheme 3 | Three proposed mechanisms regarding photoredox-mediated Ir III/Ni II dual-catalyzed hydroalkylation to achieve C–C cross-coupling. Download figure Download PowerPoint Figure 3 | Energy profiles (ΔG°333.15 in kcal/mol) of the most favorable nickel catalytic cycle. Selected bond distances and spin densities are given in Å and grey italic font, respectively. Download figure Download PowerPoint Figure 4 | Energy profiles (ΔG°333.15 in kcal/mol) of the two possible competitive paths (2 and 3). Selected bond distances and spin densities are given in Å and grey italic font, respectively. Download figure Download PowerPoint As illustrated in Figure 3, the α-carbon-centered electrophilic THF radical would be rapidly captured by the triplet Ni(II) complex 1 to generate a stable doublet NiIII species 2 with a ΔG° value of −7.5 kcal/mol ( Supporting Information Figure S9). Then, the C–Cl bond formation via reductive elimination occurs from 2 to afford the 2-chlorotetrahydrofuran (2-Cl–THF) through the three-membered-ring transition-state TS1 with a small ΔG°⧧ value of 8.2 kcal/mol. The formation of 2-Cl–THF aligns with the experimental proposal.41 Next, the oxidative hydrometallation between AL and 3 occurs via the concerned five-membered-ring transition-state TS2 to yield the alkyne hydrometallation intermediate 5. In TS2, the H–Cl distance is elongated to 1.63 Å from 1.33 Å in 4, while the Ni–Cl, Ni–C2, and C3–H bonds are shortened to 2.85, 2.04, and 1.37 Å, respectively, indicating the H–Cl bond is cleaved and the Ni–Cl, Ni–C2, and C3–H bonds are formed simultaneously. The ΔG°⧧ and ΔG° values of this oxidative hydrometallation step are 17.5 and −3.5 kcal/mol, respectively. Note that oxidative hydrometallation is more favorable than the alternative stepwise mechanism consisting of the oxidative addition of Ni(I) to HCl and the alkyne insertion to Ni–H bond ( Supporting Information Figure S8). Based on deuterium-labeling experiments,43 the hydrogen source of the targeted alkyne hydroalkylation product is expected to be originated from both the THF C(sp3)–H bonds and the following proton exchange with trace water in the reaction mixtures. In the present calculations, HCl is generated along with the formation of α-carbon-centered THF radical. It can be expected that H(Cl) in THF•⋯HCl complex or intermediates containing HCl (like 2, 3, and 4) could exchange with H(OH) to contribute the following hydrogen in the addition to alkyne. In the subsequent comproportionation reaction, a stable mixed valence complex 6 is initially produced with a ΔG° value of −4.6 kcal/mol, in which NiIII and NiI are bridged by a chloride ion. From the homobimetallic dimer 6, a chloride radical migration from Ni(III) to Ni(II) occurs via the transition-state TS3 to afford the Ni(II)-intermediate 7 and regenerate an initial Ni(II) catalyst 1. Such comproportionation reaction can be also regarded as inner-sphere ET. The ΔG°⧧ and ΔG° values of this inner-sphere ET are 4.6 and −12.2 kcal/mol relative to 5, respectively. In the final σ-bond metathesis, spin inversion between the singlet and triplet energy profiles effectively decreases the activation barrier and switches the ground-state profile from the singlet state to the triplet state via a minimum energy crossing point (MECP).83–86 In the favorable pathway, the σ-bond metathesis between Ni–C2 and Cl–C1(THF) bonds occurs via the MECP and the triplet transition-state TS4 to afford the alkyne hydroalkylation product P and regenerate the other triplet Ni(II) catalyst 1. The singlet transition state of σ-bond metathesis has also been examined to be unfavorable than the present triplet one ( Supporting Information Figure S11). In the four-membered Ni–C2–C1–Cl ring of TS4, the Ni–C2 and C1–Cl bonds are elongated to 2.05 and 3.04 Å from 1.88 Å in 7 and 1.86 Å in 2-Cl–THF, whereas the C1–C2 and Ni–Cl distances are shortened 3.12 and 2.42 Å, respectively, indicating the Cl–C1(THF) bond cleavage and the C1–C2 bond formation occur simultaneously. The ΔG°⧧ and ΔG° values of the rate-determining σ-bond metathesis process are 23.7 and −40.1 kcal/mol, respectively, which can be overcome under the experimental conditions (60 °C). In addition, inspired by the energetically favorable SET from photoexcited * Ir III to T Ni II forming D Ni I (Figure 1), an alternative oxidative quenching of * Ir III by 7 has also been evaluated to be unfavorable to provide Ir IVCl and Ni(I)-vinyl species. Because 1 ( T Ni II) produced simultaneously with 7 is much easier and faster to quench the *Ir III than 7 by comparing the energy barrier and redox potentials ( Supporting Information Figures S12 and S13), although the subsequent oxidative addition of 2-Cl–THF to Ni(I)-vinyl species and reductive elimination from nickel(III) intermediate just require moderate energy barriers. As the competitive paths of the above mechanism (path 1), the direct radical capture by D Ni I and the alkyne coordination with D Ni I could lead to possible inner-sphere path 2 and outer-sphere path 3, as shown in Figure 4 ( Supporting Information Figure S16). Note that the following results are discussed based on the Gibbs energy calculated on the triplet energy profiles, because the singlet profiles lie higher than the triplet ones ( Supporting Information Figure S15). Starting from the active D Ni I, the α-carbon-centered THF radical could be captured by D Ni I to generate a relatively stable Ni(II) species 8 (path 2) or coordinate with AL to from a four-coordinate Ni(I) intermediate 10 (path 3) with the ΔG° values of −7.6 and 1.1 kcal/mol, respectively. In path 2, AL is inserted into the Ni–C(THF) bond of 8 through a four-membered-ring (Ni–C3–C2–C1) transition-state TS5 to afford an intermediate 9. The ΔG°⧧ and ΔG° values of this alkyne insertion are 26.6 and −22.2 kcal/mol, respectively. In path 3, the outer-sphere electrophilic THF radical attack to the coordinated AL moiety of 10 could occur via the transition-state TS6 to afford 9 ( Supporting Information Figure S14). This alkyne bifunctionalization step requires the ΔG°⧧ and ΔG° values of 10.1 and −30.9 kcal/mol, respectively. At last, a σ-bond metathesis between 9 and HCl could yield the alkyne hydroalkylation product P and regenerate the nickel(II) chloride complex 1. However, the regioselectivity of alkyne bifunctionalization in path 3 was examined to be contrary to the experimental results (Scheme 4). Overall, paths 2 and 3 are less favorable than path 1 because they lie higher than path 1 based on the Gibbs energy calculated on the potential energy surface. Scheme 4 | The regioselect

DFT computational insight into the mechanism of the monomer–trimer isomerism of Ni(II) bis-acetylacetonate

The possible reaction paths for the spin-forbidden dissociation of paramagnetic nickel(II) bis-acetylacetonate trimer (Ni(acac)2) have been studied based on the DFT UB3LYP/6-311++G(d,p) calculations. Two energy preferred reaction routes involve three consecutive steps resulting in spin state changes of the system: (S = 3) → (S = 2) → (S = 1) → (S = 0). The highest theoretically-estimated limits for energy barriers necessary to overcome along these routes, as estimated by the relative energies of the minimum energy crossing points (MECPs) on the seams of the intersecting potential energy surfaces of different multiplicity, do not exceed the value of 33 kcal/mol. The results obtained are in a good agreement with available experimental data. The considered dissociation – association rearrangement represents an example of a thermally induced multi-step Coordination-Induced Spin-State Switching phenomenon.

Revealing the marked differences of phosphorescence efficiencies on C˄N˄N‐coordinated Pt(II) complexes: A theoretical study

In this study, green phosphorescent Pt(II) complexes with N,N‐diphenyl‐6‐(1H‐pyrazol‐1‐yl)pyridin‐2‐amine (Ndpp) coordinated ligands, [Pt (Ndpp)Cl] 2a, [Pt (Ndpp)Pb, Pb = (prop‐1‐ynyl)benzene] 2b, and [Pt (Ndpp)CN] 2aCN were theoretically investigated by means of density functional theory and time‐dependent density functional theory calculations to reveal their marked distinct phosphorescence quantum yields. These complexes exhibit evident absorption bands in the 200–450 nm region but emit strong green light with marked differences of phosphorescence quantum yields. Compared with the complex 2a, the complex 2b possesses large oscillator strengths of absorption spectra, strong spin‐orbit coupling, and transition electric dipole moment, as well as small singlet‐triplet splitting energies, which conduces to enhancing its radiative decay. To illustrate the nonradiative decay process, the transition state (TS) between the triplet metal‐centered (3MC) state and the excited state (T1) was optimized. The 3MC state is found to be the minimum energy crossing point (MECP) between the T1 state and the S0 state. Compared with the complex 2a, the complex 2b possesses a much larger energy barrier to the MECP state from the T1 state, so it is strongly emissive in the green region. Besides, the introduction of CN substitutions on 2a is useful for enhancing the energy barrier to the thermal deactivation pathway of 3MLCT → TS → MECP. These results demonstrate that the modification of metal–ligand conjugation is an effective way to develop high‐performance phosphorescent materials.