Conformationally Constrained N‑Heterocyclic Phosphine–Diimine with Dual Functionality

Condensation of octahydro-2,2′-bipyrimidine with P­(NMe2)3 gave a 1,3,2-diazaphospholidine–4,5-diimine 4a in which the “open” (exo/exo) conformation of the diimine unit was enforced by incorporation into a tricyclic molecular backbone. The coordination behavior of this potentially ambident ligand was sampled in reactions with ([(nbd)­W­(CO)4] and [CpCo­(CO)2]) and pnictogen halides ECl3 (E = P, As, Sb). While PCl3 reacted under ring metathesis, all other reactions gave isolable complexes of composition (4a)­MLn (MLn = W­(CO)5, CpCo­(CO), AsCl3, SbCl3); attempted recrystallization of the As-adduct yielded a complex (4a)­(AsCl3)2 which was also accessible from reaction of 4a with 2 equiv of AsCl3. Single-crystal X-ray diffraction studies revealed that the ligand in [(4a)­W­(CO)5] and [(4a)­CpCo­(CO)] binds through its phosphorus lone-pair; [(4a)­SbCl3] and [(4a)­(AsCl3)2] contain a T-shaped ECl3 unit which binds to the chelating diimine moiety, and associate further via chloride bridges to give centrosymmetric dimers. Reactions of 4a with excess metal substrates gave no evidence that formation of bimetallic complexes with μ-bridging 1κ2(N,N′)-2κP-coordination is feasible; the extra AsCl3 moiety in [(4a)­(AsCl3)2] avoids this coordination mode by interacting with the peripheral chlorides of the central core. The observed selectivity suggests that ligand 4a specifically addresses transition metal centers with low positive charge and some back-bonding capacity through the phosphorus lone-pair, and electrophiles that behave essentially as “pure” Lewis acids through the diimine unit. This assumption was confirmed by DFT studies which disclosed further that binding of the first metal center deactivates the opposite binding site and thus strongly inhibits the formation of dinuclear complexes.