Functionalization Reactions Characteristic of a Robust Bicyclic Diphosphane Framework

The 3,4,8,9-tetramethyl-1,6-diphospha-bicyclo-[4.4.0]­deca-3,8-diene
(P2(C6H10)2) framework
containing a P–P bond has allowed for an unprecedented selectivity
toward functionalization of a single phosphorus lone pair with reference
to acyclic diphosphane molecules. Functionalization at the second
phosphorus atom was found to proceed at a significantly slower rate,
thus opening the pathway for obtaining mixed functional groups for
a pair of P–P bonded λ5-phosphorus atoms.
Reactivity with the chalcogen-atom donors MesCNO (Mes = 2,4,6-C6H2Me3) and SSbPh3 has allowed
for the selective synthesis of the diphosphane chalcogenides OP2(C6H10)2 (87%), O2P2(C6H10)2 (94%), SP2(C6H10)2 (56%), and S2P2(C6H10)2 (87%).
Computational studies indicate that the oxygen-atom transfer reactions
involve penta-coordinated phosphorus intermediates that have four-membered
{PONC} cycles. The P–E bond dissociation enthalpies in EP2(C6H10)2 were measured via
calorimetric studies to be 134.7 ± 2.1 kcal/mol for P–O,
and 93 ± 3 kcal/mol for P–S, respectively, in good agreement
with the computed values. Additional reactivity with breaking of the
P–P bond and formation of diphosphinate O3P2(C6H10)2 was only observed
to occur upon heating of dimethylsulfoxide solutions of the precursor.
Reactivity of diphosphane P2(C6H10)2 with azides allowed the isolation of monoiminophosphoranes
(RN)­P2(C6H10)2(R = Mes,
CPh3, SiMe3), and treatment with additional
MesN3 yielded symmetric and unsymmetric diiminodiphosphoranes
(RN)­(MesN)­P2(C6H10)2 (91%
for R = Mes). Metalation reactions with the bulky diiminodiphosphorane
ligand (MesN)2P2(C6H10)2 (nppn) allowed for the isolation and characterization
of (nppn)­Mo­(η3-C3H5)­Cl­(CO)2 (91%), (nppn)­NiCl2 (76%), and [(nppn)­Ni­(η3-2-C3H4Me)]­[OTf] showing that these
ligands provide an attractive preorganized binding pocket for both
late and early transition metals.