Non‐conventional Behavior of a 2,1‐Benzazaphosphole: Heterodiene or Hidden Phosphinidene?

The titled 2,1‐benzazaphosphole (1) (i. e. ArP, where Ar=2‐(DippN=CH)C6H4, Dipp=2,6‐iPr2C6H3) showed a spectacular reactivity behaving both as a reactive heterodiene in hetero‐Diels‐Alder (DA) reactions or as a hidden phosphinidene in the coordination toward selected transition metals (TMs). Thus, 1 reacts with electron‐deficient alkynes RC≡CR (R=CO2Me, C5F4N) giving 1‐phospha‐1,4‐dihydro‐iminonaphthalenes 2 and 3, that undergo hydrogen migration producing 1‐phosphanaphthalenes 4 and 5. Compound 1 is also able to activate the C=C double bond in selected N‐alkyl/aryl‐maleimides RN(C(O)CH)2 (R=Me, tBu, Ph) resulting in the addition products 7–9 with bridged bicyclic [2.2.1] structures. The binding of the maleimides to 1 is semi‐reversible upon heating. By contrast, when 1 was treated with selected TM complexes, it serves as a 4e donor bridging two TMs thus producing complexes [μ‐ArP(AuCl)2] (10), [(μ‐ArP)4Ag4][X]4 (X=BF4 (11), OTf (12)) and [μ‐ArP(Co2(CO)6)] (13). The structure and electron distribution of the starting material 1 as well as of other compounds were also studied from the theoretical point of view. The dualistic behavior of the titled 2,1‐benzazaphosphole, behaving as a heterodiene versus a hidden phosphinidene, has been theoretically predicted based on an NTR analysis. This spectacular feature has been proven chemically as well. Thus, it is able to participate in hetero‐Diels‐Alder reactions forming expected heterocycles, but it also serves as a 4e ligand, using two phosphorus electron lone pairs, bridging two transition metals (Au, Ag and Co).