Crystal structure of catena-[(bis(O,O′-diethyl dithiophosphato-S,S′)-μ₂-1,2-bis(3-pyridylmethylene)hydrazine-N,N′)zinc(II)], {C₂₀H₃₀N₄O₄P₂S₄Zn}_n

Source of material

The Zn[S₂P(OEt)₂]₂ precursor was prepared in high yield from the in situ reaction of Zn(NO₃)₂ ⋅ 6 H₂O (Alfa Aesar; 14.87 g, 0.05 mol), EtOH (Merck; 12.25 mL, 0.21 mol), P₂S₅ (Sigma-Aldrich; 11.11 g, 0.05 mol) and 50% w/w NaOH solution (Merck; 8.80 mL, 0.11 mol). 1,2-Bis(3-pyridylmethylene)aldazine was prepared in high yield from reaction of 3-picolylamine (Sigma-Aldrich; 2.03 mL, 0.02 mol) and hydrazinium hydroxide (Merck; 0.49 mL, 0.01 mol) in ratio 2:1 in ethanol solution (Merck; 5 mL) under reflux for 1 h. The title compound was obtained by mixing a suspension of Zn[S₂P(OEt)₂]₂ (0.50 g, 1.15 mmol) and 1,2-bis(3-pyridylmethylene)hydrazine (0.25 g, 1.19 mmol) in dimethylformamide (Merck; 5 mL), followed by stirring for 30 min at 373 K. The solution was filtered and the filtrate was collected in a sample vial containing acetonitrile (Merck; 1 mL). Colourless prisms formed after one day. Yield: 0.49 g, (66.0%, based on Zn[S₂P(OEt)₂]₂). M.pt (Stuart SMP 30 Melting point apparatus): 387.6–388.6 K. IR (Bruker Vertex 70 V equipped with Platinum ATR from 400 to 80 cm⁻¹): 1059(w) ν(C—O); 1015(s) ν(P—O); 651(s) ν(P—S)asym; 522(w) ν(P—S)sym, 287(m) ν(Zn—S); 379(w) ν(Zn—N).

Experimental details

The C-bound H atoms were geometrically placed (C—H = 0.95–0.99 Å) and refined as riding with U_iso(H) = 1.2–1.5U_eq(C). The maximum and minimum residual electron density peaks of 1.63 and 1.24 eÅ⁻³, respectively, were located 1.15 and 0.75 Å from the H5a and S4 atoms, respectively, belonging to one of the two symmetry-independent diethyl dithiophosphate anions. There is some evidence of disorder in this ligand, which could not be modelled satisfactorily.

Comment

The isomeric, potentially bridging molecules, 1,2-bis(n-pyridylmethylene)hydrazine, n-NC₅H₄C(H)=N—N=C(H)C₅H₄N-n, often referred to as the n-pyridylaldazines (n-PyAld), have revealed interesting monodentate modes of coordination in their adducts with zinc-triad 1,1-dithiolates [5]. For example, when the metal node is zinc complexed to dithiocarbamate (⁻S₂CN(R)R′) and the ligand is 4-PyAld, monodentate coordination of 4-PyAld is observed in mononuclear Zn[S₂CN(iPr)CH₂CH₂OH]₂(4-PyAld) with five-coordinate zinc(II) [6]; the non-coordinating pyridyl-nitrogen atom engages in hydroxy-O—H⋯N(pyridyl) hydrogen bonding. When 3-PyAld is employed and the 1,1-dithiolate ligand is dithiophosphate [⁻S₂P(OR)₂], bidentate bridging is found in {Zn[S₂P(O-iPr)₂]₂(3-PyAld)}_n, (I), which is a one-dimensional coordination polymer with a step-ladder topology [7]. In the present report, the crystal and molecular structures of the ethyl analogue of the latter is described as it is well documented in the structural chemistry of the zinc-triad 1,1-dithiolates that changes in R groups can have profound implications on the ultimate structural motif adopted in the solid-state [5], [8].

The asymmetric unit of (I) comprises Zn[S₂P(OEt)₂]₂ and two-half 3-PyAld molecules as each is disposed about a centre of inversion, as indicated in the figure (70% probability displacement ellipsoids; the unlabelled atoms of the N1-3-PyAld molecule are related by the symmetry operation (i) 2 − x, 1 − y, 1 − z and those of the N3-3-PyAld molecule by (ii) 1 − x, − y, 2 − z). The zinc(II) centre is tetrahedrally coordinated by two sulphur atoms derived from two monodentate dithiophosphate anions as well as two nitrogen atoms derived from two different 3-PyAld molecules. The dithiophosphate ligands have different modes of coordination. The S1-dithiophosphate coordinates via the S1 atom [Zn—S1 = 2.2896(8) Å] and is orientated so the O1 atom [Zn⋯O1 = 3.286(2) Å], rather than the S2 atom, is directed towards the zinc atom. By contrast, the S3-dithiophosphate ligand coordinates via the S3 atom [Zn—S3 = 2.3243(9) Å] with the S4 atom [Zn⋯S4 = 3.4460(10) Å] directed towards the zinc atom. As anticipated, the P—S bond lengths reflect the different environments of the S1–S4 atoms in that the P1—S1 [2.0208(11) Å] and P2—S3 [2.0021(13) Å] bond lengths, involving the coordinating sulphur atoms are longer than those not involved in coordination [P1—S2 = 1.9418(11) Å and P2—S4 = 1.9265(14) Å]. The Zn—N1 [2.050(3) Å] and Zn—N3 [2.067(3) Å] bond lengths are experimentally equivalent. The range of tetrahedral angles subtended by the N₂S₂ donor set is a narrow 96.82(8)°, for S3—Zn—N3, to a wide 121.49(4)°, for S1—Zn—S3. Small twists are noted in the 3-PyAld bridges as seen in the C9—C10—C14—N2 [170.5(3)°] and C17—C16—C20—N4 [176.2(3)°] torsion angles.

As seen from the lower view of the figure, the application of symmetry gives rise to a coordination polymer. The topology of the chain is twisted which contrasts the step-ladder topology noted for the R = i-Pr analogue [7]. The chain is aligned along [1 1 −1]. The atom-to-atom connections between chains that sustain the three-dimensional architecture are methylene-C—H⋯N(aldazine) [C1—H1b⋯N4ⁱⁱⁱ: H1b⋯N4ⁱⁱⁱ = 2.62 Å, C1⋯N4ⁱⁱⁱ = 3.520(5) Å with angle at H1b = 151° for (iii) x, 1 + y, z] and pyridyl-C—H⋯S(thiolate) [C18—H18⋯S1^iv: H18⋯S1^iv = 2.84 Å, C18⋯S1^iv = 3.675(3) Å with angle at H18 = 147° for (iv) 1 + x, y, z] interactions.