Synthesis and crystal structure of rubidium hydrogen oximate complexes with 18-crown-6: is it possible to reach a perfect fit of the rubidium atom inside the crown ether cavity?

Complexes of composition Rb(18-crown-6) {H(X)(2)}. nH(2)O (X = isonitrosocyanoacetamide, n = 3(1) and isonitrosobenzoylacetonitrile, n = 0 (2)) have been prepared and studied by means of X-ray diffraction (1: triclinic, space group P (1) over bar, with a = 10.143(2), b = 11.912(2), c = 13.518(2) Angstrom; alpha = 114.40(1), beta = 91.61(1), gamma = 103.92(1)degrees, V = 1428.6(4) Angstrom(3), Z = 2; final R1 = 0.032 based on 3660 independent reflections with I>2 sigma(I); 2: monoclinic, space group P2(1)/n, with a=9.522(2), b=16.469(3), c=10.794(2) Angstrom; beta=95.13(3)degrees, V=1685.9(6) Angstrom(3), Z = 2; final R1 = 0.038 based on 1547 independent reflections with I>2 sigma(I)). Both the structures consist of Rb(18-crown-6)(+) cations and complex hydrogen oximate anions {H(X)(2)}(-), formed via strong hydrogen bonding between the oxygen atoms of nitroso groups (O ... O ca. 2.455(6)-2.473(3) Angstrom). In 1 the rubidium atom is situated 1.012(1) Angstrom above the mean plane of macrocyclic oxygen atoms (Rb-O 2.921(2)-3.129(2) Angstrom) and adopts a typical 'sunrise coordination', while the structure of complex 2 demonstrates an unprecedented example of a perfect fit of the rubidium atom inside the crown ether cavity. Metal atom occupies a center of inversion and resides exactly in the center of the crown ether (Rb-O 2.822(5)-2.872(4) Angstrom). (C) 1998 Elsevier Science S.A.