We have long-term interest in the main group chemistry of peri-substituted (i.e. 1,8-disubstituted) naphthalenes and related molecular frameworks. The peri-backbone is best described as a “clamping scaffold” – the rigid backbone forces the two peri-atoms into sub-Van der Waals distances, with overlap of the occupied orbitals. This results in a significantly modified potential energy surface and hence special reactivity and bonding.
Recently, we have recognised ability of certain peri-substitution patterns to provide a fundamentally new and broadly applicable method of stabilising normally fleeting structural motifs. We have coined the term Enforced Proximity Donor for our new strategy. It utilises the peri-backbone to enforce close (i.e. strong) dative interaction of a donor, such as tertiary phosphine group, with another group acting as an acceptor, such as a low-valent centre. This results in exceptionally strong dative bond through enhancing the efficiency of the electron density transfer via enforcing proximity of the two motifs. This in turn locks the decomposition pathways through the (normally labile) dative bond breakage, making range of normally fleeting motifs stable.
Enforced Proximity Donor concept aims at complementing (and surpassing) established stabilisation strategies, such as steric protection, carbene or metal fragment coordination. Stabilisation of unusual motifs unlocks opportunities to explore normally ephemeral species and thus develop new reactivities and fundamentally new chemistries. On a practical level it brings increased synthetic utility of these new species through the ease of their isolation and storage (i.e. bottleability). This enables direct study of their structure, properties, and chemistry, as opposed to an indirect one via trapping reactions. Shifting the boundary of what is possible in terms of bonding and structure is of fundamental interest. Peri-substitution is very rewarding in this sense as unusual features are almost ubiquitous here.
To date, we have used peri-substitution to stabilise a wide range of (normally) fleeting species. For example, phosphine-phosphine donor-acceptor complexes R3P→PRX2, were detected as thermally unstable intermediates as early as the 1950’s. However, isolation of phosphine-phosphine dative species proved difficult as all known examples are redox unstable, i.e. decompose well below room temperature in a complex and not well defined process. Using Enforced Proximity Donor strategy, we have been able to isolate the first room temperature stable phosphine-phosphine DA complex. The unique thermal stability (“bottleability”) of our phosphine-phosphine complex allowed studying reactivity of this class of compounds for the first time and resulted in discoveries of several fundamentally new reactivity patterns. Dative and zwitterionic representations of our phosphine-phosphine complex, along with the crystal structure, are shown above.