Counterion Effects in Pd Polyselenides:  Evolution from Molecular to Three-Dimensional Framework Structures

Several palladium polyselenides were prepared by the reactions of Pd 2+ ions with Se x 2 - (4 < x < 6) ligands in the presence of various cations. {(CH 3 )N(CH 2 CH 2 ) 3 N} 2 [Pd(Se 6 ) 2 ] ( I ) was prepared by the methanothermal reaction of K 2 PdCl 4, K 2 Se 4, and {(CH 3 ) 3 N(CH 2 CH 2 )N(CH 3 ) 3 }I 2 in a 1:3:2 molar ratio at 110 °C. Compound I crystallizes in the space group P 2 1 / c with a = 9.403(3) Å, b = 15.050(5) Å, c = 10.807(4) Å, β = 91.73(3)°, and V = 1528.7(9) Å 3 . (H 3 NCH 2 CH 2 NH 2 ) 2 [Pd(Se 5 ) 2 ] ( II ) was prepared by the methanothermal reaction of PdCl 2, Na 2 Se 4, and (H 3 NCH 2 CH 2 NH 3 )Cl 2 in a 1:3:1 molar ratio at 80 °C. Compound II crystallizes in the space group P 2 1 / n with a = 9.412(3) Å, b = 8.671(3) Å, c = 12.666(3) Å, β = 98.38(2)°, and V = 1023(1) Å 3 . The anions in [Pd(Se 5 ) 2 ] 2 - in II and [Pd(Se 6 ) 2 ] 2 - in I are polymeric and possess layered structures. The organic cations are located between the layers. K 2 (H 3 NCH 2 CH 2 NH 3 ) 2 [Pd(Se 4 ) 2 ·2Se 4 ] ( III ) was prepared by the methanothermal reaction of PdCl 2, K 2 Se 4, and (H 3 NCH 2 CH 2 NH 3 )Cl 2 in a 1:3:1 molar ratio at 80 °C. Compound III crystallizes in the space group I 2 1 2 1 2 1 with a = 12.587(9) Å, b = 15.271(5) Å, c = 15.404(9) Å, and V = 2961(5) Å 3 . The [Pd(Se 4 ) 2 ·2Se 4 ] 6 - anion in III is composed of a [Pd(Se 4 ) 2 ] 2 - three-dimensional adamantane-like framework with large cavities that are filled with the counterions and with uncoordinated Se 4 2 - ligands. K 6 [Pd(Se 5 ) 4 ] ( IV ) was prepared by the methanothermal reaction of K 2 PdCl 4 and K 2 Se 5 in a 1:6 molar ratio at 80 °C. The black polyhedral crystals of IV are slightly soluble in water. Compound IV crystallizes in the space group Pbcn with a = 12.427(4) Å, b = 13.777(9) Å, c = 19.190(9) Å, and V = 3285(5) Å 3 . The [Pd(Se 5 ) 4 ] 6 - anion is molecular and one of the most Se-rich metal polyselenides known. It has a palladium center with four dangling Se 5 2 - ligands around it. Being a molecular fragment, it can be regarded as a building block for extended [Pd(Se n ) 2 ] 2 - frameworks. Optical spectroscopic and thermal gravimetric analysis data for the compounds are reported.