Delving into the ambiguity in lattice site assignment of aquo-H atom in the orthorhombic PPh3O.hemihydrate: A comparative account of three PPh3O.hemihydrate polymorphs

The structural parameters of the lattice water (∠H-O-H=141.79° and dO-H=0.896 Å) in the orthorhombic Fdd2 polymorph of PPh3O.hemihydrate as revealed by the Baures’s report more than three decades ago, considerably deviate from the standard bond parameters of a water molecule at equilibrium geometry. In particular, the anomalous value of ∠H-O-H in lattice water has unfortunately turned the H-bond network in the Fdd2 lattice completely unrealistic from chemical viewpoint. We have successfully synthesized the Fdd2 polymorph of TPPO.hemihydrate through oxygen atom transfer from [ReO]3+ moiety to the metal-bound oxophilic PPh3 group in presence of α-benzilmonoxime ligand. Successful X-ray structure reassessment of the Fdd2 polymorph has resolved the dire issue related to lattice water and the H-bond network engendered from our orthorhombic Fdd2 structure has now become chemically valid as well as meaningful. We have rationally attempted a comparative analysis of the salient structural features and existential non-covalent interactions for all the three polymorphs of PPh3O.hemihydrate crystallised in the Fdd2, C2/c and Cc space groups. The orthorhombic crystal polymorph has exhibited particularly the O–H…O and C–H…π intermolecular non-covalent interactions that result in the generation of characteristic seven-member rings in the lattice. An interplay of the O–H…O, C–H…O and C–H…π interactions has been noticed in the monoclinic C2/c and Cc space groups. Such interactions have triggered the formation of several exquisite ring patterns, viz., R32(7), R22(12), R43(16) and R42(22) in the monoclinic C2/c space group, unlike the monoclinic Cc space group where no such definitive rings have been isolated due to intergrowth of crystals during twinning. The quantum-chemical topological descriptors arising from QTAIM analysis have acquainted us with various aspects of chemical bonding including mild to moderate H-clashes. We have also performed the SAPT analysis that has revealed the average strength of the intermolecular non-covalent interactions: C–H…OPPh3O (∼ -6.7 kcal/mol) > C–H…π (∼ -3.0 kcal/mol) > C–H…Owater (∼ -1.0 kcal/mol).