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首页> 外文期刊>Organometallics >Theoretical Study of the Oxidative Addition of Ammonia to Various Unsaturated Low-Valent Transition Metal Species
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Theoretical Study of the Oxidative Addition of Ammonia to Various Unsaturated Low-Valent Transition Metal Species

机译：氨向各种不饱和低价过渡金属物种的氧化加成反应的理论研究

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摘要

Reaction profiles for the oxidative addition ofNH3 to a number of unsaturated low-valent transition metal complexes have been computed using gradient-corrected density functional theory. The metal complexes studied are d8 CpM(CO) (M = Rh, Ir)and trans.M(PH3)~ (M = Rh, Ir; X = H, CI) and dlo ML2(M = Pd, Pt; L = PH3, L2 = H2PCH2CH2PH2, dpe). Reactions with the d8 species are characterized by the formation of strongly bound ammine complexes from which computed activation energies for oxidative addition are in excess of 16 kcal mol-l. Computed reaction enthalpies are all exothermic with these complexes. With dlo M(PH3)2 species computed ammine adducts are weak, activation barriers are in excess of 23 kcal mol-l, and the overall reaction is endothermic for both M = Pd and Pt. The introduction of the chelating dpe .ligand results in stronger ammine adducts but only slightly reduced computed activation barriers. Of the dlO species only the reaction with Pt(dpe) is computed to be exothermic. Comparison of the computed reaction profiles for analogous second- and third-row complexes shows the NH3 oxidative addition reaction to be more favorable with the third-row species, which exhibit more strongly bound ammine adducts, lower activation barriers, and more exothermic reactions. Of the species studied the most promising unsaturated fragments for effecting NH3 oxidative addition are Cplr(CO), trans-Ir(PH3)~ (X = H, CI), and Pt(dpe). The more favorable thermodynamics computed with these third- row species arise from higher M-NH2 and M-H homolytic bond strengths in the hydrido- amido products. M-NH2bonds are computed to be between 6 and 13 kcal mol-l and M-H bonds between 5 and 14 kcal mol-l stronger in the third-row complexes compared to their second-row congeners. For complexes exhibiting noN-M :7r-donation M -NH2 bonds are computed to be up to 26 kcalmol-l weaker than M-H bonds.N-M :7r-donation reduces this differential, and in Ir(PH3)2(H)2(NH2) the Ir-NH2 and Ir-H bonds are calculated to have equal homolytic bond strengths. Computed activation energies for NH3 oxidative addition do not appear to be related to the strength of the ammine adduct, and for metal complexes of the same row the computed activation energy is relatively insensitive to the nature of the unsaturated fragment. These findings are discussed in terms of anNH3 reorientation/N -H bond activation model for the oxidative addition reaction. Although strongly Lewis acidic metal fragments usually promote oxidative addition, with NH3 these form strong ammine adducts from which NH3 reorientation is energetically costly. For metal fragments with lower Lewis acidity NH3 reorientation is more facile, but the subsequent oxidative addition remains difficult. These ideas are supported by the accessibility of 1Jl-H and 1J3-H,H,H NH3 adducts formed with Pt(dpe), while with Ir(PH3)2CI only a high-energy '7l-H species was located.
机译：已经使用梯度校正的密度泛函理论计算了将NH 3氧化加成到许多不饱和的低价过渡金属络合物中的反应曲线。研究的金属配合物为d8 CpM（CO）（M = Rh，Ir）和反式M（PH3）〜（M = Rh，Ir; X = H，CI）和dlo ML2（M = Pd，Pt; L = PH3，L2 = H2PCH2CH2PH2，dpe）。与d8物质的反应的特征在于形成强结合的胺配合物，从该配合物中计算出的用于氧化加成的活化能超过16kcal mol-1。计算出的反应焓都与这些配合物放热。使用dlo M（PH3）2物种计算得出的胺加合物较弱，活化势垒超过23 kcal mol-1，并且对于M = Pd和Pt，总体反应都是吸热的。螯合dpe。配体的引入导致更强的胺加合物，但仅稍微降低了计算的活化势垒。在dlO物种中，只有与Pt（dpe）的反应被计算为放热的。比较类似的第二行和第三行络合物的计算反应曲线，表明NH3氧化加成反应对第三行物种更有利，第三行物种显示出结合更牢固的胺加合物，更低的活化能垒和更多的放热反应。在所研究的物种中，最有希望实现NH 3氧化加成的不饱和片段为Cplr（CO），反式Ir（PH3）〜（X = H，CI）和Pt（dpe）。用这些第三排物质计算出的更有利的热力学是由氢化-酰胺基产物中较高的M-NH2和M-H均溶键强度引起的。与第二行同系物相比，第三行复合物中的M-NH2键经计算在6至13 kcal mol-1之间，M-H键在5至14 kcal mol-1之间更强。对于不具有N-M：7r-给体的配合物，M -NH2键的计算强度比MH键弱至26 kcalmol-1.NM：7r-给定减小了该差异，并且在Ir（PH3）2（H）2（ NH2）Ir-NH2和Ir-H键经计算具有相同的均溶键强度。用于NH 3氧化加成的计算活化能似乎与氨加合物的强度无关，对于同一行的金属配合物，计算出的活化能对不饱和片段的性质相对不敏感。这些发现是根据用于氧化加成反应的NH 3重取向/ NH键活化模型来讨论的。尽管强路易斯酸性金属片段通常会促进氧化加成，但与NH3相比，它们会形成强胺加成物，从中氨化的重新取向在能量上很昂贵。对于具有较低路易斯酸度的金属碎片，NH3的重新定向更容易，但是随后的氧化添加仍然很困难。这些想法得到了与Pt（dpe）形成的1J1-H和1J3-H，H，H NH3加合物的可及性的支持，而与Ir（PH3）2CI形成的只是高能'7-1-H物种。

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