N,N,N ‘,N ‘-Tetrakis(2-pyridylmethyl)ethylenediamine and Its Analogues as Hypodentate Ligands. Synthesis and Characterization of the Rhodium(III), Ruthenium(II), and Palladium(II) Complexes

New complexes of Rh(III), Ru(II), and Pd(II) with N,N,N ‘,N ‘-tetrakis(2-pyridylmethyl)ethylenediamine (tpen) and its analogues have been prepared. The reaction of RhCl3·nH2O with tpen is slow and allows one to isolate the products of three consecutive substitution steps: Rh2Cl6(tpen) (1), cis-[RhCl2(η4-tpen)]+ (2), and [RhCl(η5-tpen)]2+ (3). In acetonitrile the reaction stops at the step of the formation of cis-[RhCl2(η4-tpen)]+, whereas [RhCl(η5-tpen)]2+ is the final product of the further reaction in ethanol. Fully chelated [Rh(tpen)]3+ could not be obtained. Bis(acetylacetonato)palladium(II), Pd(acac)2, reacts with tpen and its analogues, N,N,N ‘,N ‘-tetrakis(2-pyridylmethyl)-1,3-propanediamine (tptn) and N,N,N ‘,N ‘-tetrakis(2-pyridylmethyl)-(R)-1,2-propylenediamine (R-tppn), to give [Pd(η4-tpen)]2+ (4), [Pd(η4-tppn)]2+ (5), and [Pd(η4-tptn)]2+ (6), respectively. Two pyridyl arms remain uncoordinated in these cases. The formation of unstable Pd(III) complexes from these Pd(II) complexes in solution was suggested on the basis of electrochemical measurements. Ruthenium(III) trichloride, RuCl3·nH2O, is reduced to give a Ru(II) complex with fully coordinated tpen, [Ru(tpen)]2+ (7). The same product was obtained in a more straightforward reaction of RuIICl2(dimethyl sulfoxide)4 with tpen. Electrochemical studies showed a quasi-reversible [Ru(tpen)]2+/3+ couple for [7](ClO4)2 (E 1/2 = 1.05 V vs Ag/AgCl). Crystal structures of [2](PF6)·2CH3CN, [3](PF6)2·CH3CN, [6](ClO4)2, and [7](ClO4)2·0.5H2O were determined. Crystal data: [2](PF6)·2CH3CN, monoclinic, C2, a = 16.974(4) Å, b = 8.064(3) Å, c = 13.247(3) Å, β = 106.37(2)°, V = 1739.9(8) Å3, Z = 2; [3](PF6)2·CH3CN, triclinic, P1̄, a = 11.430(1) Å, b = 19.234(3) Å, c = 8.101(1) Å, α = 99.43(1)°, β = 93.89(1)°, γ = 80.10(1)°, V = 1729.3(4) Å3, Z = 2; [6](ClO4)2, orthorhombic, Pnna, a = 8.147(1) Å, b = 25.57(1) Å, c = 14.770(4) Å, V = 3076(3) Å3, Z = 4; [7](ClO4)2·0.5H2O, monoclinic, P21/c, a = 10.046(7) Å, b = 19.049(2) Å, c = 15.696(3) Å, β = 101.46(3)°, V = 2943(2) Å3, Z = 4.