Crystal structure of bis(tetrapropylammonium) nonaselenidotetrastannate(IV), (Pr₄N)₂[Sn₄Se₉]

Source of material

The starting materials [CH₃COCHC(O)CH₃]₂SnCl₂ (0.020 g, 0.052 mmol), K₂Se (0.016 g, 0.10 mmol) and Pr₄NBr (0.028 g, 0.10 mmol) were charged to a Pyrex tube with diameter of 9 mm under an argon atmosphere and about 0.5 mL 1:2 (v/v) H₂O/MeOH mixture was added as a solvent. While the solvent was being frozen, the Pyrex tube was evacuated under vacuum and sealed with the use of a flame. The sealed tube was placed in an oven and heated at 110 °C for a day, then cooled to room temperature. Orange red rectangular chunky crystals were isolated by filtration and washed with MeOH and diethyl ether several times. Crystals of (Pr₄N)₂[Sn₄Se₉] were obtained in 65% yield, based on the Sn metal used.

Experimental details

H atoms were positioned geometrically and treated as riding, with C–H = 0.97 (CH₂) and 0.96 (CH₃) Å with U_iso(H) = 1.2 (1.5 for methyl) U_eq(C). H atoms of the CH₃ were positioned to be staggered with respect to the shortest other bond to the atom to which the CH₃ is attached.

Comment

The title compound, (Pr₄N)₂[Sn₄Se₉], which has been prepared by the solvothermal reaction of [CH₃COCHC(O)CH₃]₂SnCl₂, K₂Se and Pr₄NBr with 1:2 (v/v) H₂O/MeOH mixture as a solvent, is composed of a 2D polymeric, layered [Sn₄Se₉]²⁻ anion and charge-balancing Pr₄N⁺ cations (see upper part of the figure). (Pr₄N)₂[Sn₄Se₉] features to be a new member of A₂Sn₄Se₉ (A = Rb, Cs, NH₄, {Fe(bipy)₃}_0.5, (Bmmim)_0.5{Ni(1,2-pda)₃}_0.25, (Bmmim)_0.375(dienH)_0.125{Ni(dien)₂}_0.25) family and the Se analogue of (Pr₄N)₂[Sn₄S₉] [5], [6], [7], [8], [9], [10], [11]. In the A₂Sn₄Se₉ family, there exist three different structural types of the [Sn₄Se₉]²⁻ anion. Small cations such as Rb⁺, Cs⁺, NH 4 + stabilize a 2D layered structure of [Sn₄Se₉]²⁻ containing two 5-coordinate trigonal bipyramidal Sn and two 4-coordinate tetrahedral Sn [5, 6]. In the case including large transition metal complex cations such as {Fe(bipy)₃}_0.5, (Bmmim)_0.5{Ni(1,2-pda)₃}_0.25, (Bmmim)_0.375(dienH)_0.125{Ni(dien)₂}_0.25, a 3D framework structure of [Sn₄Se₉]²⁻ with all four Sn of 5-coordinate trigonal bipyramidal geometry have been stabilized [7], [8], [9]. In the new member, (Pr₄N)₂[Sn₄Se₉], a 2D layered structure of [Sn₄Se₉]²⁻ with three 5-coordinate trigonal bipyramidal Sn and one 4-coordinate tetrahedral Sn has been stabilized.

(Pr₄N)₂[Sn₄Se₉] crystallizes in the space group P2₁/n, which is the same space group as the sulfur analogue, (Pr₄N)₂[Sn₄S₉] crystallizes in [10, 11]. As it can be seen from the top-view of the [Sn₄Se₉]²⁻ layer of (Pr₄N)₂[Sn₄Se₉] (see the upper part of the figure), building units of the [Sn₄Se₉]²⁻ layer are edge-shared two Sn₃Se₄ broken cubes and SnSe₄ tetrahedra. The SnSe₄ tetrahedra connect the edge-shared two Sn₃Se₄ broken cubes, forming large elliptical 32-membered rings. A closer inspection of the edge-shared two Sn₃Se₄ cubes reveals that the edge-sharing of two Sn₃Se₄ cubes is accomplished through μ₂–Se (which is Se(9)) bridges. An asymmetric unit of Sn₄Se₉ is therefore composed of a Sn₃Se₄ broken cube, a Se bridge and a SnSe₄ tetrahedron. There are three SnSe₅ trigonal bipyramids in the edge-shared Sn₃Se₄ cube, with equatorial Sn–Se bond distances ranging from 2.5030(6) to 2.5687(6) Å and axial Sn–Se bond distances ranging from 2.6600(6) to 2.8822(6) Å. Sn–Se bond distances in the SnSe₄ tetrahedron range from 2.4753(6) to 2.5410(5) Å. Another feature of the [Sn₄Se₉]²⁻ layers in (Pr₄N)₂[Sn₄Se₉] is that they are quite corrugated running along the crystallographic b axis, as it can be seen from the side view of the layers (lower part of the figure).