Regioselectivity of Addition to the Azavinylidene Ligand in Tp′W(CO)(η2‑HCCH)(NCHMe): Electrophilic Addition versus Oxidation and Radical Coupling

The Tp′W­(CO)­(HCCH)­(NCHMe) (Tp′ = hydridotris­(3,5-dimethylpyrazolyl)­borate) molecule contains an electron-rich 1-azavinylidene ligand where the preferred site for electrophile addition is the α-nitrogen. As is observed in other cases for electrophile addition to a 1-azavinylidene with a cis-alkyne ligand, this fragment will add small electrophiles such as H+, Me+, and Et+ to the α-nitrogen lone pair, maintaining a WII metal center and forming the coordinated imine products [Tp′W­(CO)­(HCCH)­(NHCHMe)]­[BF4], [Tp′W­(CO)­(HCCH)­(N­(Me)CHMe)]­[OTf], and [Tp′W­(CO)­(HCCH)­(N­(Et)CHMe)]­[OTf]. In contrast, when a one-electron oxidant is employed, net electrophile addition at the β-carbon can be achieved. Changing the nature of the electrophile to [CPh3]­[BF4] leads to a rapid one-electron oxidation and selective radical combination to form the imido product [Tp′W­(CO)­(HCCH)­(NCH­(CH3)­CPh3)]­[BF4]. This net addition of an electrophile at the 1-azavinylidene β-carbon concomitantly oxidizes the metal center to WIV and raises the CO stretching frequency to 2109 cm–1 versus 1946 cm–1 for the WII imine complex. This reactivity is also seen with [CPh2(C6H4OMe)]­[BF4]. This variable reactivity demonstrates the flexibility of the 1-azavinylidene ligand when it is paired with an adjacent alkyne ligand.Tp′W(CO)(HCCH)(N=CHMe)（Tp′=氢化三(3,5-二甲基吡唑基)硼酸盐(hydridotris(3,5-dimethylpyrazolyl)borate)）分子含有一个富电子的1-氮亚乙烯基配体(1-azavinylidene)，该配体发生亲电试剂加成的优先位点为α-氮原子。正如其他带有顺式炔烃配体的1-氮亚乙烯基配体发生亲电加成的情况所示，该片段可与H+、Me+、Et+等小型亲电试剂在α-氮原子的孤对电子处发生加成，保留WII金属中心，并生成配位亚胺类产物[Tp′W(CO)(HCCH)(NH=CHMe)][BF4]、[Tp′W(CO)(HCCH)(N(Me)=CHMe)][OTf]以及[Tp′W(CO)(HCCH)(N(Et)=CHMe)][OTf]。与之形成对照的是，当采用单电子氧化剂时，可实现亲电试剂在β-碳原子处的净加成反应。将亲电试剂更换为[CPh3][BF4]时，会快速发生单电子氧化并发生选择性自由基结合，生成亚氨基产物[Tp′W(CO)(HCCH)(NCH(CH3)CPh3)][BF4]。这种在1-氮亚乙烯基配体β-碳原子处发生的亲电试剂净加成，会同步将金属中心氧化为WIV，并使CO伸缩振动频率从WII亚胺配合物的1946 cm–1提升至2109 cm–1。该反应活性同样可在[CPh2(C6H4OMe)][BF4]中观察到。这种可变的反应活性表明，当1-氮亚乙烯基配体与相邻的炔烃配体配对时，其具有出色的反应灵活性。