Multi-component orbital interactions during oxyacyl radical addition reactions involving imines and electron-rich olefins

Ab initio and DFT calculations reveal that oxyacyl radicals add to imines and electron-rich olefins through simultaneous SOMO-pi*, SOMO-pi and pi*-HOMO interactions between the radical and the radicalophile. At the BHandHLYP/aug-cc-pVDZ level, energy barriers of 20.3 and 22.0 kJ mol(-1) are calculated for the attack of methoxycarbonyl radical at the carbon and nitrogen ends of methanimine, respectively. In comparison, barriers of 22.0 and 8.6 kJ mol(-1) are calculated at BHandHLYP/aug-cc-pVDZ for reaction of methoxycarbonyl radical at the 1- and 2-positions in aminoethylene, respectively. Natural bond orbital (NBO) analysis at the BHandHLYP/6-311G** level of theory reveals that SOMO-pi*, SOMO-pi and pi*-LP interactions are worth 111, 394 and 55 kJ mol(-1) respectively in the transition state (8) for reaction of oxyacyl radical at the nitrogen end of methanimine; similar interactions are observed for the chemistry involving aminoethylene. These multi-component interactions are responsible for the unusual motion vectors associated with the transition states involved in these reactions.     

Multiorbital interactions during Acyl radical addition reactions involving imines and electron-rich olefins

Ab initio and DFT calculations reveal that acyl radicals add to imines and electron-rich olefins through simultaneous SOMO --> pi*, pi --> SOMO, and HOMO --> pi*C=O interactions between the radical and the radicalophile. At the CCSD(T)/aug-cc-pVDZ//QCISD/cc-pVDZ level, energy barriers of 15.6 and 17.9 kJ mol(-1) are calculated for the attack of the acetyl radical at the carbon and nitrogen ends of methanimine, respectively. These barriers are 17.1 and 20.4 kJ mol(-1) at BHandHLYP/cc-pVDZ. In comparison, barriers of 34.0 and 23.4 kJ mol(-1) are calculated at BHandHLYP/cc-pVDZ for reaction of the acetyl radical at the 1- and 2-positions in aminoethylene, repectively. Natural bond orbital (NBO) analysis at the BHandHLYP/6-311G** level of theory reveals that SOMO --> pi*imine, pi imine--> SOMO, and LPN --> pi*C=O interactions are worth 90, 278, and 138 kJ mol-1, respectively, in the transition state (2) for reaction of acetyl radical at the nitrogen end of methanimine; similar interactions are observed for the chemistry involving aminoethylene. These multiorbital interactions are responsible for the unusual motion vectors associated with the transition states involved in these reactions. NBO analyses for the remaining systems in this study support the hypothesis that the acetyl radical is ambiphilic in nature.     

A computational study of multicomponent orbital interactions during the cyclization of silyl, germyl, and stannyl radicals onto C-N and C-O multiple bonds

BHandHLYP/6-311G** and BHandHLYP/DZP computations of the potential surface of Si-, Ge-, and Sn-radical cyclizations onto the imine double bond reveal that these reactions proceed through simultaneous SOMO --> pi*, LP(N) --> SOMO, and LP(N) --> sigma* interactions. Such multicomponent orbital interactions are responsible for the regioselectivity in these radical cyclizations, where the nucleophilic radical unexpectedly attacks the more electron-rich end of the pi system. Less nucleophilic heteroatoms, for example, the nitrogen atom in nitriles or the oxygen atom in carbonyl compounds, show reduced LP interactions with the radical center in the respective transition states, so that these reactions predominantly occur in the "classical" fashion and with the expected regioselectivities of nucleophilic radicals through SOMO --> pi* interactions. This supports the hypothesis that Si-, Ge- and, to a lesser extent, Sn-radicals are ambiphilic in nature and that the unpaired electron is not necessarily the most reactive site in a radical but can act as an observer of a nucleophilic attack at the radical center.     

H2CCC光谱性质的从头算研究

采用全活化空间自洽场方法(CASSCF)在C2v对称性和ANO-S基组水平下, 研究了H2CCC自由基及其阴阳离子的基态和低激发态性质. 为了进一步考虑动态电子相关效应, 采用多组态二级微扰理论(CASPT2)获得了更加精确的能量值. 计算得到X1A1 → 1A2, X1A1 → 1B1和X1A1 → 21A1在159.0, 216.5和476.4 kJ/mol处的激发可分别归因于π(b2)→π*(b1), n(a1)→π*(b1)和π(b1)→π*(b1)的跃迁, 理论波长与紫外吸收光谱得到的实验数据一致. 计算得到的电子亲和势与实验值也非常接近.     

C6H5N光谱性质的多组态二级微扰理论研究

采用全活化空间自洽场方法(CASSCF)在C2v对称性和6-31g(d,p) 基组水平下, 研究了C6H5N自由基及其阴阳离子的基态和低激发态性质. 为了进一步考察动态电子相关效应, 采用多组态二级微扰理论(CASPT2)获得更加精确的能量值. 计算得到13A2→13A1和13A2→13B2在4303.1和4212.2 kJ/mol处的激发可分别归因于π(b1)→ny和π(a2)→ny的跃迁, 而13A2→13B1和13A2→11B1在2634.9和2700.4 kJ/mol处的激发具有nx→π*(a2)和π(a2)→nx的混合跃迁特征, 理论波长与紫外吸收光谱得到的实验数据一致. 计算得到的电离能与实验值也非常接近.     

Radicals masquerading as electrophiles: a computational study of the intramolecular addition reactions of acyl radicals to imines

Ab initio calculations using 6-311G**, cc-pVDZ, and aug-cc-pVDZ, with (MP2, QCISD, CCSD(T)) and without (UHF) electron correlation, and density functional methods (BHandHLYP and B3LYP) predict that cyclization of the 5-aza-5-hexenoyl and (E)-6-aza-5-hexenoyl radicals proceed to afford the 5-exo products. At the CCSD(T)/cc-pVDZ//BHandHLYP/cc-pVDZ level of theory, energy barriers (deltaE(double dagger)) of 36.1 and 47.0 kJ mol(-1) were calculated for the 5-exo and 6-endo pathways for the cyclization of the 5-aza-5-hexenoyl radical. On the other hand, at the same level of theory, deltaE(double dagger) of 38.9 and 45.4 kJ mol(-1) were obtained for the 5-exo and 6-endo cyclization modes of (E)-6-aza-5-hexenoyl radical, with exothermicities of about 27 and 110 kJ mol(-1) calculated for the exo and endo modes, respectively. Under suitable experimental conditions, the 6-endo cyclization product is likely to dominate. Analysis of the molecular orbitals involved in these ring-closure reactions indicate that both reactions at nitrogen are assisted by dual orbital interactions involving simultaneous SOMO-pi* and LP-pi* overlap in the transitions states. Interestingly, the (Z)-6-aza-5-hexenoyl radical, that cannot benefit from these dual orbital effects is predicted to ring-close exclusively in the 5-exo fashion.     

Acyl radical addition to pyridine: multiorbital interactions

The addition of the acetyl radical at the various positions in both pyridine and the pyridinium ion has been investigated using DFT calculations. Additions at the 2-, 3- and 4-positions in these systems are associated with simultaneous SOMO→π* and π→SOMO interactions, with the former interaction dominating in the case of pyridine, and that latter in the case of pyridinium. Simultaneous SOMO→π*, LP_N→SOMO and LP_N→π*_co interactions are predicted for the addition at the nitrogen atom in pyridine. The energy barrier for attack at the nitrogen atom in pyridine is calculated to be 54 kJ mol⁻¹ at the BHandHLYP/6-311G(d,p) level of theory, some 6 kJ mol⁻¹ lower than for the analogous attack at any other atom in pyridine, or at any position in the pyridinium ion. Multiorbital interactions are responsible this preference, resulting in an unusual motion vector in the transition state for attack at the nitrogen atom in pyridine.     

An ab initio and DFT study of some halogen atom transfer reactions from alkyl groups to acyl radical

Ab initio calculations using the 6-311G**, cc-pVDZ, and (valence) double-zeta pseudopotential (DZP) basis sets, with (MP2, QCISD, CCSD(T)) and without (HF) the inclusion of electron correlation, and density functional (BHandHLYP, B3LYP) calculations predict that the transition states for the reaction of acetyl radical with several alkyl halides adopt an almost collinear arrangement of attacking and leaving radicals at the halogen atom. Energy barriers (DeltaE(double dagger)) for these halogen transfer reactions of between 89.2 (chlorine transfer from methyl group) and 25.3 kJ mol(-1) (iodine transfer from tert-butyl group) are calculated at the BHandHLYP/DZP level of theory. While the difference in forward and reverse energy barriers for iodine transfer to acetyl radical is predicted to be 15.1 kJ mol(-1) for primary alkyl iodide, these values are calculated to be 6.7 and -4.2 kJ mol(-1) for secondary and tertiary alkyl iodide respectively. These data are in good agreement with available experimental data in that atom transfer radical carbonylation reactions are sluggish with primary alkyl iodides, but proceed smoothly with secondary and tertiary alkyl iodides. These calculations also predict that bromine transfer reactions involving acyl radical are also feasible at moderately high temperature.     

Bis(1-(4-(dimethylamino)benzylidene)-4-phenylthiose micarbaznato)palladium(Ⅱ):Synthesis,Crystal Structure and Its Dual Fluorescence Property

The title compound bis(1-(4-(dimethylamino)benzylidene)-4-phenylthiosemicar-bazato)-palladium(Ⅱ)(PdL2) was obtained by reacting 1-(4-(dimethylamino)benzylidene)-4-phenyl-thiosemicarbazide with dichlorobis(benzonitrile)palladium(Ⅱ) in methanol,and its structure was characterized by single-crystal X-ray diffraction.The crystal of PdL2 was obtained in dimethyl-formamide(DMF) solvent with solvent molecules involved in the cell and crystallizes in the monoclinic system,space group C2 with a = 18.485(15),b = 7.090(5),c = 17.595(11) ,β = 121.21(3)o,V = 1972(2) 3,Z = 2,Mr = 847.40,Dc = 1.427 g/cm3,μ = 0.624 mm-1,F(000) = 880,R = 0.0607 and wR = 0.1358.The Pd atom adopts a distorted square planar coordination geometry with two Pd-N and two Pd-S bonds.The ligand loses a proton from its tautomeric thiol form and coordinates to the Pd atom via mercapto sulfur and the imine nitrogen atom,which binds to palladium as bidentate N,S-donors forming five-membered chelate rings.The complex formed hydrogen bonding interaction with solvent DMF molecules from the hydrogen of phenylamine to the oxygen of DMF and several intramolecular hydrogen bonds.Pd(Ⅱ) perturbed ligand π-π* transition and metal-to-ligand charge transfer(MLCT) transition are observed in its electronic absorption spectra.The complex exhibits intraligand 1π-π*(IL) state and MLCT state dual fluorescent emissions in organic solvent at room temperature.     

Theoretical studies on Ru(fppz)2(CO)L (L = N-heterocyclic ligand): Electronic structure, absorption, phosphorescence, and solvatochromism

A series of ruthenium(II) complexes Ru(fppz)₂(CO)L [fppz = 3-trifluoromethyl-5(2-pyridyl)pyrazole; L = pyridine (1), 4-dimethylaminopyridine (2), 4-cyanopyridine (3)] were designed and investigated theoretically to explore their electronic structures, absorption, and emissions as well as the solvatochromism. The singlet ground state and triplet excited state geometries were fully optimized at the B3LYP/LANL2DZ and CIS/LANL2DZ level, respectively. The HOMO of 1–3 is composed of d_yz(Ru) atom and π(fppz). The LUMO of 1 and 2 is dominantly contributed by π*(fppz) orbital, but that of 3 is contribute by π*(L). Absorption and phosphorescence in vacuo, C₆H₁₂, and CH₃CN media were calculated using the TD-DFT level of theory with the PCM model based on the optimized ground and excited state geometries, respectively. The lowest-lying absorption of 1 and 2 at 387 and 391 nm is attributed to {[d_yz(Ru) + π(fppz)] → [π*(fppz)]} transition, but that of 3 at 479 nm is assigned to {[d_yz(Ru) + π(fppz)] → [π*(L)]} transition. The phosphorescence of 1 and 2 at 436 and 438 nm originates from ³{[d_yz(Ru) + π(fppz)] [π*(fppz)]} excited state, while that of 3 at 606 nm is from ³{[d_yz(Ru) + π(fppz)] [π*(L)]} excited state. The calculation results showed that the absorption and emission transition character can be changed from MLCT/ILCT to MLCT/LLCT transition by altering the substituent on the L ligand. The phosphorescence of 1 and 2 does not have solvatochromism, but that of 3 at 606 nm (vacuo), 584 nm (C₆H₁₂), and 541 nm (CH₃CN) is strongly dependent on the solvent polarity, so introducing electron-withdrawing group on ligand L will induce remarkable solvatochromism.     

Theoretical study on the isomerization behavior between alpha,beta-unsaturated acyl radicals and alpha-ketenyl radicals

[reaction: see text] Ab initio calculations using 6-311G**, cc-pVDZ, aug-cc-pVDZ, and a (valence) double-zeta pseudopotential (DZP) basis set, with (QCISD, CCSD(T)) and without (UHF) the inclusion of electron correlation, and density functional methods (BHandHLYP, B3LYP) predict that alpha,beta-unsaturated acyl radicals and alpha-ketenyl radicals exist as isomers. At the CCSD(T)/cc-pVDZ//BHandHLY/cc-pVDZ level of theory, energy barriers of 15.1 and 17.7-21.7 kJ mol(-)(1) are calculated for the isomerization of s-trans-propenoyl and s-trans-crotonoyl radical to ketenylmethyl and 1-ketenylethyl radical, respectively. Similar results are obtained for the reactions of s-trans isomers involving silyl, germyl, and stannyl groups with energy barriers (DeltaE++) of 12.2-12.4, 13.1-13.9, and 12.9-18.2 kJ mol(-)(1) at the CCSD(T)/DZP//BHandHLYP/DZP calculation, respectively. These results suggest that alpha,beta-unsaturated acyl radicals and alpha-ketenyl radicals are not canonical forms but are isomeric species that can rapidly interconvert.     

Emissive Osmium(II) Complexes Supported by N-Heterocyclic Carbene-based C(∧)C(∧)C-Pincer Ligands and Aromatic Diimines

Osmium(II) complexes containing N-heterocyclic carbene (NHC)-based pincer ligand 1,3-bis(1-methylimidazolin-2-ylidene)phenyl anion (C(1)(∧)C(∧)C(1)) or 1,3-bis(3-methylbenzimidazolin-2-ylidene)phenyl anion (C(2)(∧)C(∧)C(2)) and aromatic diimine (2,2'-bipyridine (bpy), 1,10-phenanthroline (phen), or 4,4'-diphenyl-2,2'-bipyridine (Ph(2)bpy)) in the form of [Os(C(∧)C(∧)C)(N(∧)N)(CO)](+) have been prepared. Crystal structures for these complexes show that the Os-C(NHC) bonds are essentially single (Os-C(NHC) distances = 2.079(5)-2.103(7) ?). Spectroscopic comparisons and time-dependent density functional theory (TD-DFT) calculations suggest that the lowest-energy electronic transition associated with these complexes (λ(max) = 493-536 nm, ε(max) = (5-10) × 10(3) dm(3) mol(-1) cm(-1), solvent = CH(3)CN) originate from a d(π)(Os(II)) → π*(N(∧)N) metal-to-ligand charge transfer transition, where the d(π)(Os(II)) and π*(N(∧)N) levels contain significant contribution from the C(∧)C(∧)C ligands. All these complexes are emissive in the red-spectral region (674-731 nm) with quantum yields of 10(-4)-10(-2) and emission lifetimes of around 1-6 μs. Transient absorption spectroscopy and spectroelectrochemical measurements have also been used to probe the nature of the emissive excited-states. Overall, this joint experimental and theoretical investigation reveals that the C(∧)C(∧)C ligands can be used to modulate the photophysical properties of a [Os(N(∧)N)] core via the formation of the hybrid [Os + C(∧)C(∧)C] frontier orbitals.     

π-conjugated phosphasilenes stabilized by fused-ring bulky “Rind” groups

Recent progress in the chemistry of π-conjugated phosphasilenes stabilized by bulky protecting groups based on a fused-ring octa-R-substituted s-hydrindacene skeleton (Rind groups) is reviewed. The phosphasilenes with a variety of aryl substituents at the silicon atom are covered in detail. The X-ray crystallography analysis showed the highly coplanar π-framework reinforced by the perpendicularly-fixed Rind groups. Strong π–π* absorptions have been observed, demonstrating the extension of π-conjugation over the skeleton. The DFT calculations indicated that the LUMO involves significant contribution by the 3pπ*(Si–P)–2pπ*(carbon π-electron system) conjugation. The preparation of the gold(I) complexes of the π-conjugated phosphasilenes is also included.     

Effects of extending the computational model on DNA-protein T-shaped interactions: the case of adenine-histidine dimers

The MP2/6-31G*(0.25) π-π or π(+)-π T-shaped (edge-to-face) interactions between neutral or protonated histidine and adenine were considered using computational models of varying size to determine the effects of the protein and DNA backbones on the preferred dimer structure and binding strength. The overall consequences of the backbones are reasonably subtle for the neutral adenine-histidine T-shaped dimers. Furthermore, the minor changes in the binding strengths of these dimers upon model extension arise from additional (attractive) backbone-π (bb-π) contacts and changes in the preferred π-π orientations, which is verified by the quantum theory of atoms in molecules (QTAIM). Since the binding strength of the extended dimer equals the sum of the individual backbone-π and π-π contributions, the π-π component is not appreciably affected by polarization of the ring upon inclusion of the biological backbone. In contrast, the larger effect of the backbone on the protonated histidine dimers cannot simply be predicted as the sum of changes in the π-π and bb-π components regardless of the dimer type or model. This suggests, and QTAIM qualitatively supports, that the magnitude of the π(+)-π contribution changes, which is likely due to alterations in the electrostatic landscape of the monomer rings upon inclusion of the biological backbone that largely affect T-shaped dimers. These findings differ from those previously reported for (neutral) π-π stacked and (metallic) cation-π interactions, which highlights the distinct properties of each (π-π, π(+)-π, and cation-π) classification of noncovalent interaction. Furthermore, these results emphasize the importance of considering backbone-π interactions when analyzing contacts that appear in experimental crystal structures and cautions the use of truncated models when evaluating the magnitude of the π(+)-π contribution present in large biological complexes.     

2-硫代嘧啶酮和2-硫代吡啶酮在B-吸收带中的动态结构

采用共振拉曼光谱学结合量子化学计算研究了2-硫代嘧啶酮(2TPM)和2-硫代吡啶酮(2TP)在B-带吸收时的动态结构. 在气相时, 2-巯基嘧啶(2MPM, 硫醇式)比2TPM(硫代酮式)更稳定, 能量差约为15.1 kJ·mol^-1, 而在水和乙腈溶液中, 2TPM比2MPM更稳定, 能量差分别为29.3和28.0 kJ·mol^-1. 气相及基电子态时,由B3LYP/6-311++G(d,p)计算水平获得的2TPM和2MPM之间发生质子转移异构化反应的过渡态能垒约为130 kJ·mol^-1. 2TPM三个吸收带分别被指认为π_H→π_L^*, π_H→π_L+1^*和π_H-1→π_L^*跃迁. 基于对2TPM在固体和溶液相傅里叶变换-拉曼(FT-Raman)和傅里叶变换-红外(FT-IR)光谱测量, 以及B3LYP/6-311++G(d,p)计算, 开展了2TPM在水和乙腈溶液中的B-带共振拉曼光谱的振动指认, 由此获得了2TPM的动态结构, 并与2TP的动态结构进行了比较. 2TPM和2TP动态结构的差异反映了ππ^*/πσ^*锥形交叉点结构的差异, 因此, 可被用于洞察光诱导的氢原子脱离-复合机制.     

Quantum chemical characterization of the generation of high-valent oxoruthenium species of Keggin type polyoxometalates: electronic structure and bonding features

High-valent transition-metal-substituted Keggin-type polyoxometalates (POMs) are active and robust oxidation catalyst. The important oxidized intermediates of these POM complexes are very difficult to be characterized by using the experimental method, and thus no detail information is available on such species. In the present paper, density functional theory (DFT) calculations have been carried out to characterize the electronic structures of a series of mono-ruthenium-substituted Keggin-type POMs. We find that the aquaruthenium(II/III/IV) species possess d(xy)(2)d(xz)(2)d(yz)(2), d(xy)(2)d(xz)(2)d(yz)(1), and d(xy)(2)d(xz)(1)d(yz)(1) electronic configuration, respectively, and hydroxyl/oxoruthenium(IV/V/VI) species possess d(xy)(2)d(xz)(1)π*(yz)(1), d(xy)(2)π*(xz)(1)π*(yz)(1), d(xy)(1)π*(xz)(1)π*(yz)(1), and d(xy)(1)π*(xz)(1)π*(yz)(0) electronic configuration, respectively. Mulliken spin population shows that spin density is localized on the ruthenium center in aquaruthenium(II/III/IV) POM complexes, and the RuO(a) unit in hydroxyl/oxoruthenium(IV/V/VI) POM complexes. The O(a) atom has substantial radical character in oxoruthenium(IV/V) species, and the radical character of the O(a) atom are significantly weakened in the oxoruthenium(VI) species. The relevant energy of the important Ru-O(a)π*-antibonding unoccupied orbitals with high RuO(a) compositions of oxoruthenium(IV/V/VI) POM complexes decrease in the order: oxoruthenium(IV) > oxoruthenium(V) > oxoruthenium(VI). The pH-independent multiple reduction energies for Ru(III/II), Ru(V/IV), and Ru(VI/V) couples are calculated, which is in agreement with the experimental data.     

配合物[N,N'-二(亚水杨基)-1,2-乙二胺]Pt(Ⅱ)光谱性质的理论研究

化合物[N,N'-二(亚水杨基)-1,2-乙二胺]Pt(Ⅱ)(1)在OLED材料上具有很大的应用潜力, 我们利用密度泛函(DFT/Lanl2dz)方法计算了它的电子结构和光谱性质. 计算结果与实验值符合得很好. 计算结果表明, 该化合物最低能吸收和三态磷光发射均来自于[L(Phenoxide lone pair)→π*(imine)](LLCT: ligand-to-ligand charge transfer)和[Pt(5d)→π*(Schiff base)](MLCT: metal-to-ligand charge transfer)的混合电荷跃迁. 另外, 计算得到了该配合物在气态中的激发态几何结构. 通过在不同的溶液中计算吸收和发射光谱, 发现该化合物没有明显的溶剂化显色效应, 说明溶液极性对光谱的影响不大.     

N-nitrosation of amines by NO2 and NO: a theoretical study

Gas-phase nitrosation of amines implies a nonionic pathway different from the nitrosonium nitrosation via acidification of nitrite. Electronic structure calculations discussed in this work suggest a free radical mechanism, in which NO2 abstracts a hydrogen atom from the nitrogen in primary and secondary amines to form an intermediate complex of an aminyl radical and nitrous acid. The aminyl radical intermediate is then quenched by nitric oxide, leading to the formation of nitrosamine. High-level calculations (CBS-QB3) show that alkyl substitutions on amines can activate the H-abstraction reaction. Thus, while H-abstraction from NH3 was found to exhibit a reaction barrier (DeltaH) of 106 kJ/mol, similar calculations indicate that the corresponding barriers decrease to 72 and 45 kJ/mol for methylamine and dimethylamine, respectively. Heterocyclic secondary amines have also been investigated in a similar manner. The five-membered-ring (5-m-r) amine appears to be the most reactive: pyrrolidine (DeltaH=30 kJ/mol), azetidine (DeltaH=44 kJ/mol), piperidine (DeltaH=44 kJ/mol), and aziridine (DeltaH=74 kJ/mol). The reaction barrier for 1H-pyrrole, an aromatic 5-m-r secondary amine, was found to be 59 kJ/mol. The origin of the high activity for the 5-m-r alkylamine stems from a hydrogen-bond-like interaction between the aminyl radical and the nascent nitrous acid molecule. This theoretical study suggests that, in the presence of nitrogen oxides, the gas-phase nitrosation of secondary amines is feasible.     

Half-Sandwich Manganese Complex Bearing Fused Oxazoline-NHC Ligand: Conformational Analysis and Evaluation in Asymmetric Ketone Hydrosilylation

The first manganese complex bearing a chiral N-heterocyclic carbene (NHC) ligand was prepared and studied by spectroscopic methods and X-ray diffraction. While IR spectroscopy revealed the existence of two isomers in solution with distinct ν_CO band patterns, DFT calculations indicated that these isomers correspond to rotamers around the Mn−NHC bond and their different spectroscopic properties were rationalized by the occurrence of attractive π(C=C)⋅⋅⋅π*(C≡O) or σ(C−H)⋅⋅⋅π*(C≡O) intramolecular interligand interactions. The evaluation of this complex in catalytic hydrosilylation of acetophenone using Ph₂SiH₂ under UV irradiation led to the formation of the corresponding (R)-alcohol with low enantioselectivity.     

Computational studies on the photophysical properties and NMR fluxionality of dinuclear platinum(II) A-frame alkynyl diphosphine complexes

The structural geometry, electronic structure, photophysical properties, and the fluxional behavior of a series of A-frame diplatinum alkynyl complexes, [Pt(2)(μ-dppm)(2)(μ-C≡CR)(C≡CR)(2)](+) [R = (t)Bu (1), C(6)H(5) (2), C(6)H(4)Ph-p (3), C(6)H(4)Et-p (4), C(6)H(4)OMe-p (5); dppm = bis(diphenylphosphino)methane], have been studied by density functional theory (DFT) and time-dependent TD-DFT associated with conductor-like polarizable continuum model (CPCM) calculations. The results show that the Pt···Pt distance strongly depends on the binding mode of the alkynyl ligands. A significantly shorter Pt···Pt distance is found in the symmetrical form, in which the bridging alkynyl ligand is σ-bound to the two metal centers, than in the unsymmetrical form where the alkynyl ligand is σ-bound to one metal and π-bound to another. For the two structural forms in 1-5, both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels show a dependence on the nature of the substituents attached to the alkynyl ligand. The energies of the HOMO and LUMO are found to increase and decrease, respectively, from R = (t)Bu to R = Ph and to R = C(6)H(4)Ph-p, because of the increase of the π- conjugation of the alkynyl ligand. On the basis of the TDDFT/CPCM calculations, the low-energy absorption band consists of two types of transitions, which are ligand-to-ligand charge-transfer (LLCT) [π(alkynyl) → σ*(dppm)]/metal-centered MC [dσ*(Pt(2)) → pσ(Pt(2))] transitions as well as interligand π → π* transition from the terminal alkynyl ligands to the bridging alkynyl ligand mixed with metal-metal-to-ligand charge transfer MMLCT [dσ*(Pt(2)) → π*(bridging alkynyl)] transition. The latter transition is lower in energy than the former. The calculation also indicates that the emission for the complexes originates from the triplet interligand π(terminal alkynyls) → π*(bridging alkynyl)/MMLCT [dσ*(Pt(2)) → π*(bridging alkynyl)] excited state. In terms of the fluxional behavior, calculations have been performed to study the details of the mechanisms for the three fluxional processes, which are the σ,π-alkynyl exchange, the ring-flipping, and the bridging-to-terminal alkynyl exchange processes.