Syntheses, crystal structures, and Raman and resonance Raman spectroscopy of platinum(II), platinum(IV), and halogen-bridged chain platinum(II,IV) complexes involving the terdentate ligand 3-methyl-3-azapentane-1,5-diamine(4Me-dien)

Abstract

The effect of introducing a methyl substituent onto the central nitrogen atom of the ligand dien(dien = diethylenetriamine or 3-azapentane-1,5-diamine) on the ease of formation of mixed-valence chain complexes of platinum has been investigated. The chain complex prepared by partial oxidation of [Pt(4Me-dien)I]I, viz. [Pt(4Me-dien) I][Pt(4Me-dien)I₃]I₂(4)(4Me-dien = 3-methyl-3-azapentane-1,5-diamine), breaks down on recrystallisation to yield separate crystals of [Pt(4Me-dien)I]X [X = I₃^–(1) or I^–(2)] and [Pt(4Me-dien)I₃]I(3). X-Ray structural investigation of (1)–(3) and of similar discrete complexes of platinum reported previously indicate that dien co-ordinates in the planar (pseudo-mer) conformation in four-co-ordinate platinum(II) but in the bent (fac) conformation in six-co-ordinate platinum(IV) complexes, However, in mixed-valence chain complexes, dien is forced to co-ordinate to the platinum(IV) centre in the mer conformation. In the case of 4Me-dien the mer conformation around platinum(IV) is severely hindered; hence unless this species is trapped quickly between two platinum(II) entities it isomerises from mer to fac, thereby preventing the assembly of the mixed-valence chain complex. Complex (4) has a very intense Raman spectrum at resonance with the intervalence band, the principal observed progression being v₁ν₁[ν₁(116.5 cm^–1) is the symmetric stretching mode of the chain, ν_sym(I–Pt^IV–I)]. The wavenumber of ν₁ found for this complex is lower than that of the analogous species with unsubstituted dien in accord with a lengthening of the Pt^IV–I linkage on N-methyl substitution of the ligand. Crystallographic data: (1), monoclinic, space group P2₁/_n, a= 15.433(3), b= 7.856(5), c= 13.327(4)Å, β= 102.24(2)°, and Z= 4; (2), monoclinic, space group P2₁/c, a= 20.434(7), b= 8.267(2), c= 15.571(1)Å, β= 110.67(2)°, and Z= 8; (3), monoclinic, space group P2₁/m, a= 8.960(6), b= 10.561(4), c= 7.651(4)Å, β= 103.10(2)°, and Z= 2.