Quadridentate Schiff-base metal complexes as chelating ligands of alkali metals. Synthesis and structure of ammonium and sodium tetra-phenylborate addition complexes with [NN′-ethylenebis(salicylideneiminato)]nickel(II)

Abstract

Transition-metal complexes of the Schiff-base NN′-erhylenebis(salicylaidimine)(H₂salen) co-ordinate ammonium and alkali-metal cations to give addition complexes of different stoicheiometry. The complex [Ni(salen)] with ammonium or potassium tetraphenylborate, M′[BPh₄], gives addition compounds of formula [{Ni(salen)₃-,M′(BPh₄)]·nL [L = solvent, e.g. tetrahydrofuran (thf), acetonitrile, or acetone]. For n= 2 and L = thf the complexes are isomorphous. Reaction with Na [BPh₄] in acetonitrile yields again a compound with Ni : Na 3 : 1, from which [{Ni(salen)}₂Na(NCMe)₂(BPh₄)]·2MeCN is obtained by very slow crystallization. The crystal structures of [{Ni(salen){₃(NH₄)(BPh₄)]·2thf, (I), and [{Ni(salen)}₂Na(NCMe)₂]·2MeCN, (II), have been determined from three-dimensional X-ray data. Crystals of both compounds are triclinic, space group P, Z= 2, with cell parameters: (I), a= 18.93(1), b= 16.94(1), c= 14.84(1)Å, α= 91.6(2), β= 111.4(2), γ= 116.8(2)°; (II), a= 14.64(1), b= 15.56(1), c= 14.91 Å, α= 85.2(1), β= 114.2(1), γ= 98.7(1)°. The structures have been solved by Patterson and Fourier methods and refined by block-diagonal matrix least squares to R 0.11 for (I) and 0.080 for (II). The structure of (I) consists of discrete [BPh₄]^– anions and [{Ni(salen)}₃(NH₄)]⁺ macrocations in which the ammonium ion is surrounded by the six oxygen atoms (mean N H₄⁺⋯ O 2.92 Å) of the three Ni(salen) moieties. The co-ordination polyhedron is intermediate between a trigonal prism and an octahedron. as determined by a balance of steric and electrostatic factors. Similarly the structure of (II) has discrete [BPh₄]^– anions and [{Ni(salen)}₂Na(NCMe)₂]⁺ macrocations with the sodium ion on an approximately two-fold axis, co-ordinated by the four oxygen atoms of the two Ni(salen) molecules and by the nitrogen atoms of the two acetonitrile molecules occupying cis positions. The co-ordination polyhedron approximates to an octahedron with mean Na⁺⋯ O 2.41 and Na⁺⋯ N 2.49 Å.