Polyamine synthesis and co-ordination: structural characterisation of cobalt(III) complexes from an octadentate ligand preparation

Abstract

The nature of the two principal products of the reaction between pentaerythritol [2,2-bis(hydroxymethyl)propane-1,3-diol] tetrabenzenesulfonate and ethane-1,2-diamine (en) has been confirmed by crystal-structure determinations on their cobalt(III) complexes, the less abundant species, 5,5-bis(4-amino-2-azabutyl)-3,7-diazanonane-1,9-diamine, L¹, being characterised in its bis(quadridentate) form in a green, tetrachloro, binuclear complex, and the more abundant 6,6-bis(4-amino-2-azabutyl)-1,4-diazacycloheptane, L², in a form where the ring nitrogen atoms are uncoordinated, as a green, mononuclear trans dichlorotetramine complex. Compounds L¹ and L² have also been characterised in various forms produced when these dichloro complexes are treated with en. In green [trans-Cl₂CoL¹CoCl₂-trans]CI₂·4H₂O 1, the binuclear cation has quasi-2 symmetry about the central carbon atom of the ligand which embraces the two cobalt atoms each bound equatorially by four N atoms; Co–Cl(trans) 2.234(8)–2.251(7) and Co–N 1.92(2)–1.99(2)Å. In one isomer of the mixed-ligand complexes of L¹ and en, structurally characterised both as the chloride [(en)CoL¹Co(en)]Cl₆·≈ 7H₂O 2a, and as the nitrate [(en)CoL¹Co(en)][NO₃]₆·6H₂O 2b, the cation also has approximate 2 symmetry, broken significantly only by differing conformations of the two en ligands in 2a; Co–N 1.957(7)–1.986(4)Å. Another product of the reaction of 1 with en was the known mononuclear, orange complex [Co(H₂L¹)]Cl₅·≈ 2.3H₂O 3, where the ligand is disposed about the cobalt in a half cage, with the fourth diprotonated arm pendant from the cap; Co–N at the open end 1.97₇Å(mean) nearer the cap 1.96₄Å, the difference being significant. In green trans-[CoCl₂(H₂L²)]Cl₃·≈ 3.45H₂O 4, the bicyclic ligand co-ordinates the cobalt equatorially through four N atoms; Co–Cl(trans) 2.259(1) and 2.263(1)Å, and Co–N (pairwise) 1.946(2), 1.948(3)Å for the ‘inner’ nitrogens and 1.960(2), 1.971(2)Å for the outer/terminal nitrogens. One of the immediate products of the reaction of en with 4 was [Co(en)(H₂L²)]Cl₅·≈ 0.6EtOH 5. In this cation the cobalt is co-ordinated by the en and both arms of ligand L², the seven-membered ring nitrogen atoms being unco-ordinated and protonated after isolation from acidic solution; Co–N 1.95(1)–1.986(5)Å. One of the products of more extended reaction between en and 4 was red [Co(en)(H₂L²)]Cl₅·3H₂O 6, in which the ligand is diprotonated on the two nitrogen atoms of one of the ‘arms’; it co-ordinates through both nitrogen atoms of the seven-membered ring, cis in the co-ordination sphere [Co–N 1.986(4) and 2.012(4)Å], the two nitrogen atoms of the other ligand arm occupying a further cis pair of sites [Co–N 1.966(4) and 1.972(4)Å], with the cis pair of Co–N(en) distances being 1.975(4) and 1.981(4)Å.