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.