Structural Systematics of Rare Earth Complexes. X (‘Maximally’) Hydrated Rare Earth Acetates

Abstract

Extension/augmentation of preexisting work carried out in respect of room-temperature single-crystal X-ray structural characterization of trivalent rare earth acetates, crystallized as ‘maximal’ hydrates, Ln(ac)₃.x H₂O, from aqueous solution under local ambience, suggests the following array to be prevalent: For Ln = La(-)Pr: triclinic P 1 sesquihydrate, i.e. x = 1½, a ≈ 13·4, b ≈ 10·1, c ≈ 8·6 Å, α ≈ 75·6, β ≈ 103·8, γ ≈ 92·8°, Z = 4 mononuclear f.u., conventional R on |F| for Ln = La, Ce, here, being 0·043, 0·058 for N_o 3199, 4442 independent ‘observed’ (I > 3σ(I)) diffractometer reflections respectively; the complexes have the form of a two-dimensional polymer, in the ac plane, the dominant motif being a chain of lanthanoid atoms of two types linked by acetate bridges along a ... Ln(1)Ln(2)Ln(2)Ln(1)Ln(1)Ln(2) ... with further acetates cross-linking the Ln(1) in the c dimension. For Ln = (Ce(-))Nd: monoclinic P 2₁/c monohydrate, a ≈ 8·4, b ≈ 8·0, c ≈ 15·0 Å, β ≈ 94°, Z = 4 mononuclear f.u., for the present determinations R were 0·024, 0·044 for N_o 2019, 2600, the structure being a one-dimensional polymeric form with acetate bridges. For Ln = Sm(-)Lu, (i.e. implicitly with intermediate Ln): triclinic P1 tetrahydrate, a ≈ 10·4, b ≈ 9·2, c ≈ 8·8 Å, α ≈ 118, β ≈ 114, γ ≈ 92°, Z = 2 mononuclear f.u., R were 0·035, 0·030 for N_o 4583, 4678, the complexes being acetate-bridged dimers. It is of interest that, through the three series, the variation in the degree of hydration is not monotonic.

Determinations are also recorded for a pair of crystalline compounds obtained during the attempted crystallization of europium(III) acetate hydrate from aqueous solution acidified with acetic acid (Hac), supporting their formulation as entailing the formation of mixed water/acetic acid solvates Eu(ac)₃.2H₂O.Hac and Eu(ac)₃.H₂O.2½Hac, i.e. [Eu₂(ac)₆(OH₂)₄].2Hac and [Eu₂(ac)₆(OH₂)₂(Hac)₂].3Hac with common binuclear cores in which a pair of unidentate water molecule ligands in the former is replaced by a pair of unidentate acetic acid ligands in the latter with relatively minor geometrical change. The former array is rhombohedral R3, a 26·865(7), c 10·328(3) Å (hexagonal setting), Z = 9 binuclear units, isomorphous with the previously reported samarium analogue, and the latter triclinic P1, a 14·131(5), b 8·919(4), c 8·582(3) Å, α 65·41(3), β 84·72(3), γ 84·27(3)°, Z = 1 binuclear unit, R 0·046, 0·051 for N_o 1700, 2553.

An interesting double salt, trisodium hexakis(acetato)ytterbate(III) tetrahydrate, Na₃[Yb(ac)₆].4H₂O, is monoclinic, C2/c, a 13·139(3), b 13·936(2), c 26·030(2) Å , β 91·10(1)°, Z = 8, R 0·053 for N_o 3467. The eight-coordinate (YbO₈) environment is comprised of oxygen atoms from a pair of O,O′-chelating and four unidentate acetate moieties.