Reaction Mechanism of the C⋮N Triple Bond Cleavage of <i>β</i>-Ketonitriles on a Molybdenum(0) Center¹

The molybdenum dinitrogen complex trans-[Mo(N2)2(dppe)2] (2) reacted with 2−2.5 equiv of various β-ketonitriles at room temperature to afford the (nitrido)(nitrile-enolato) complexes trans-[Mo(N)(NCCR1COR2)(dppe)2] (4; R1 = H, R2 = 4-MeOCOC6H4, 4-ClC6H4, 4-Tol, 4-MeOC6H4, 2-C4H3O, 2-C4H3S, Pri; R1 = CN, R2 = Me, Ph; dppe = Ph2PCH2CH2PPh2) via the C⋮N triple bond cleavage of the nitriles on the molybdenum center. On the other hand, the reaction of complex 2 with 2 equiv of pivaloylacetonitrile at room temperature led to the isolation of the (alkylideneamido)(nitrile-enolato) complex trans-[Mo(NCHCH2COBut)(NCCHCOBut)(dppe)2] (5k), which further underwent the cleavage of the CN double bond of the alkylideneamido ligand to give the corresponding (nitrido)(nitrile-enolato) complex trans-[Mo(N)(NCCHCOBut)(dppe)2] together with 4,4-dimethyl-1-penten-3-one. Furthermore, treatment of 2 with large excess amounts of 4-chlorobenzoylacetonitrile followed by anion metathesis with [NHEt3][OTf] (OTf = OSO2CF3) resulted in the isolation of the cationic (imido)(nitrile-enolato) complex trans-[Mo{NCH2CH2CO(C6H4Cl-4)}{NCCHCO(C6H4Cl-4)}(dppe)2][OTf] (6c+[OTf]-). The solid-state structures of 4h·1.5C2H4Cl2 (R1 = CN, R2 = Me), 5k·C6H6, and 6c+[OTf]- were determined by single-crystal X-ray analyses. The detailed NMR analysis of the reaction of 2 with aroylacetonitriles revealed that the (alkylideneamido)(nitrile-enolato) complexes trans-[Mo(NCHCH2COR)(NCCHCOR)(dppe)2] (5) act as the key intermediates for the C⋮N triple bond fission, and the rate constants for the conversion of the complexes 5 into the nitrido complexes 4 showed good correlation with the Hammett σp or σa constants for the aroyl substituents, where positive ρ values were obtained (ρp, 1.42; ρa, 0.41). A reaction mechanism for the nitrido complex formation is proposed, which includes (1) the substitution of a dinitrogen ligand in 2 with a β-ketonitrile molecule, (2) the fast protonation of the nitrile ligand by a second β-ketonitrile molecule leading to the formation of complex 5, (3) the relatively slow proton shift from the α-position of the carbonyl group to the amido carbon in the alkylideneamido ligand to form an enolated imido ligand, and (4) the fast elimination of a vinyl ketone from the imido ligand giving the nitrido complex 4 as the final product.二氮合钼配合物反式-[Mo(N₂)₂(dppe)₂]（2）在室温下与2~2.5当量的各类β-酮腈发生反应，经由钼中心上的腈类碳氮三键断裂过程，生成(氮化物)(腈烯醇盐)配合物反式-[Mo(N)(NCCR¹COR²)(dppe)₂]（4；R¹=H，R²=4-MeOCOC₆H₄、4-ClC₆H₄、4-Tol、4-MeOC₆H₄、2-C₄H₃O、2-C₄H₃S、Pri；R¹=CN，R²=Me、Ph；dppe=1,2-双(二苯基膦)乙烷，dppe）。另一方面，配合物2与2当量的特戊酰乙腈在室温下反应，分离得到(亚烷基氨基)(腈烯醇盐)配合物反式-[Mo(NCHCH₂COBut)(NCCHCOBut)(dppe)₂]（5k）；该配合物可进一步发生亚烷基氨基配体的碳氮双键断裂反应，生成对应的(氮化物)(腈烯醇盐)配合物反式-[Mo(N)(NCCHCOBut)(dppe)₂]，并伴随4,4-二甲基-1-戊烯-3-酮生成。进一步地，将配合物2与大大过量的对氯苯甲酰乙腈反应后，再与[NHEt₃][OTf]（OTf=三氟甲磺酰氧基，即OSO₂CF₃）进行阴离子复分解反应，最终分离得到阳离子型(亚胺化物)(腈烯醇盐)配合物反式-[Mo{NCH₂CH₂CO(C₆H₄Cl-4)}{NCCHCO(C₆H₄Cl-4)}(dppe)₂][OTf]（6c⁺[OTf]⁻）。通过单晶X射线衍射分析，确定了4h·1.5C₂H₄Cl₂（R¹=CN，R²=Me）、5k·C₆H₆及6c⁺[OTf]⁻的固态结构。对配合物2与芳酰乙腈的反应进行详细的核磁共振(NMR)分析后发现，(亚烷基氨基)(腈烯醇盐)配合物反式-[Mo(NCHCH₂COR)(NCCHCOR)(dppe)₂]（5）是碳氮三键断裂过程的关键中间体；配合物5转化为氮化物配合物4的速率常数，与芳酰基取代基的哈米特σₚ或σₐ常数呈现良好的相关性，且得到了正的ρ值（ρₚ=1.42；ρₐ=0.41）。本文提出了氮化物配合物的生成反应机理，该机理包含以下步骤：(1) 配合物2中的二氮配体被β-酮腈分子取代；(2) 第二分子β-酮腈对腈配体发生快速质子化，生成配合物5；(3) 羰基α位的质子向亚烷基氨基配体的酰胺碳发生相对缓慢的质子转移，形成烯醇化的亚胺配体；(4) 亚胺配体快速消除一分子乙烯基酮，最终得到氮化物配合物4。