Heptacoordinated W(IV) Cyanido Supramolecular Complex Trapped by Photolysis of a [W(CN)₆(bpy)]^2–/Zn²⁺ System

Abstract

The synthesis and single-crystal X-ray structure of the supramolecular complex {[Zn] 1.5 [Zn(H 2 O)][W(CN) 5 (OH) 2 ][W(CN) 5 (OH)(H 2 O)]} (1) are reported. We found that photolysis of PPh 4 [W(CN) 6 (bpy)] in the presence of Zn(NO 3 ) 2 aq results in formation of a porous supramolecular network (channels with a volume of 44.1% of the unit cell volume) with all cyanido ligands involved in W−CN−Zn bridges. Complex 1 crystallizes in the Pm3̅ n space group with a cell volume of 20560.7 Å 3 with three Zn and two W distinct coordination spheres. All water coordinated to Zn is directed into holes of 8.27 Å in diameter in porous 1. The presence of Zn cations during the photolysis results in isolation and structural characterization of unique seven-coordinate complex ions, [W(CN) 5 (OH) 2 ] 3− and [W(CN) 5 (OH)(H 2 O)] 2− , which were earlier only postulated as the photolysis intermediates. Quantum chemical calculations for isolated [W(CN) 5 (OH) 2 ] 3− and [W(CN) 5 (OH)(H 2 O)] 2− ions show that their ideal pentagonal bipyramid geometries around W(IV) atoms are stable only in the crystal phase due to interactions with Zn 2+ cations. The energy difference between the most stable distorted pentagonal bipyramid and that found in the crystal structure is predicted to be only 4 or 11 kcal/mol for each ion, respectively. The tungsten and zinc sites present in the crystal lattice of 1 pose different acidic properties for their aqua ligands with theoretically estimated pK a values of ca. 7 and 8, respectively.

Benchmarks for transition metal spin-state energetics: why and how to employ experimental reference data?

“…The good agreement of the COSMO geometry with the crystal one is not accidental, 177 but it is clear that the accuracy of this approach may vary with the nature of the metal-ligand bond and in some cases strong interactions with the lattice may critically influence the coordination geometry. 187…”