Iminophosphorano‐Substituted Bispyridinylidenes: Redox Potentials and Substituent Constants from Tolman Electronic Parameters

Bispyridinylidenes (BPYs) have emerged as an important class of neutral organic electron donors, with redox potentials that vary widely with choice of substituent. Methods to predict the effect of substitution on the redox potential are therefore highly desirable. Here we show that the redox potential of BPYs featuring iminophosphorano substituents (R3P=N‐), which represent the most reducing class of BPYs, can be predicted based on the well‐known Tolman electronic parameter (TEP) for the respective phosphine fragment (R3P). Moreover, building on earlier work relating redox potentials to Hammett‐type substituent constants, it is now possible to quantitatively predict σp+ values for iminophosphorano substituents from TEP values. These results provide a path for precisely tailoring redox potentials of iminophosphorano‐substituted BPYs, but also give quantitative descriptors for how these highly versatile iminophosphorano substituents can impact the properties of any molecular scaffold. TEP shows the way: The Tolman electronic parameter (TEP) of the phosphine unit is found to be directly related to the redox potential of the corresponding iminophosphorano‐substituted bispyridinylidene (BPY). With the earlier established relationship between the E1/2 of BPYs and the related substituent constants (σp+), TEP can now be used to design BPYs with specific redox potentials and to predict generally applicable σp+ for R3PN‐ substituents.