Metal Dependence of Network Dimensionality in 1,2,4‐Diazaphospholide Coordination Polymers

Abstract

Coordination polymers constructed from metals and bridging organic ligands are currently being intensely explored because they offer interesting physical properties or potential uses (e.g. for ion exchange, adsorption, and catalysis).[1] Although various structural architectures are easily synthesized, [2] control in the assembly of the network topology is still a great challenge. Five-membered heteroaromatics with several Group 15 donor atoms have recently been employed with some success because they offer rigid structures that support directional metal coordination, and a tendency to form polynuclear clusters that can serve as defined building blocks. Examples with polydentate N-donor ligands include pyrazolide (pz) and 1,2,4-triazolide (trz) based coordination polymers with 1D to 3D network topologies, [3,4] and pentaphosphaferrocenes [(R 5 C 5 )Fe(P 5 )] have been employed as building blocks to construct P-donor-based 1D and 2D polymeric networks and spherical oligomeric assemblies (Scheme 1).[5]The successful utilization of both P-and N-based homotopic ligands suggested that heterotopic P,N-heterocycles are likewise viable building blocks, in which the presence of two types of donor sites with different ligating power might allow, as an additional advantage, the observation of switchable coordination behavior. In particular, we envisaged that the known preference of the two-coordinate phosphorus to behave as soft donor should allow the ligand to use both Pand N-donor functions to bind to soft metal ions, whereas only N-donor sites should be involved in binding of hard metal ions, thus enabling the construction of coordination polymers with different network topologies from the same ligand. Here, we report on the results of some initial experiments conducted along these lines. The bridging ligand employed, 1,2,4-diazaphospholide (dap), [6] was chosen because its hydrolytic stability offers the prospect of developing the coordination chemistry of low-coordinate phosphorus compounds in aqueous solution, and because 1,2,4-diazaphospholides form molecular complexes with a remarkable variability of bonding modes.[7] Although m-bridging coordination modes have been established in several of these complexes, it seems, however, that the deliberate synthesis of coordination polymers has not been attempted.Experiments aimed at the syntheses of 1,2,4-diazaphospholide salts with distinguishable network topologies were performed by reacting aqueous solutions of 1,2,4-diazaphosphole (dapH) with zinc chloride and silver nitrate, respectively. Zinc(II) is known to form a variety of stable complexes with N-donors but lacks a strong affinity for Pdonors, whereas silver(I) binds easily to both types of ligands and even has a slight preference for P-coordination. The products precipitated upon mixing of the reactants as colorless, microcrystalline solids that were insoluble in common organic solvents and were characterized by elemental analyses, solid-state 31 P and 13 C NMR spectroscopy, and powder X-ray diffraction studies. Th...