Comment

Our recent report (Larsen, McInnes et al., 2003) of a series of octanuclear {Cr₇M} heterometallic wheel-type complexes with the general formula [NR₂H₂][Cr₇MF₈(O₂CCMe₃)₁₆] was the first report of anti-ferromagnetically coupled cyclic molecules that have a non-diamagnetic ground state. There has been considerable interest in such molecules for applications as diverse as olefin polymerization catalysis (Lassahn et al., 2004), magnetic cooling (Affronte et al., 2004) and quantum computing (Meier et al., 2003). They were synthesized using a secondary ammonium cation, [NR₂H₂]⁺, which is a protonated alkyl-chain secondary amine with R = CH₃(CH₂)_n (e.g. n = 0-7). It acts as a template in the reaction of CrF₃·4H₂O with pivalic acid in the presence of a second divalent metal cation (M²⁺ = Ni, Co, Mn, Fe, Zn or Cd). This procedure gives {Cr₇M} ring compounds in good yield (Larsen, Overgaard et al., 2003; Larsen, McInnes et al., 2003). In each case, the amines are found to be hydrogen-bonded at the centre of the metal ring. The protonated N atoms are not involved in the packing of the molecules in the crystal structure, because they are accommodated completely in the cavity (void) of the wheels. However, it was found that the choice of R can influence the packing of the rings in the crystal structure, regardless of the solvent used for crystallization. For example, both short and very long alkyl chains lead to packing where the octanuclear wheels are coplanar (generally tetragonal or orthorhombic crystals), but for intermediate chain lengths (Et or ⁿPr), monoclinic crystals are found, where the rings pack with an angle of ca 50° between the mean planes of neighbouring rings (Larsen, Overgaard et al., 2003).

The purpose of this paper is to extend further our studies of heterometallic wheels, by the preparation and structural characterization of the new title complex, [N(CH₂CH₂OH)₂H₂][Cr₇NiF₈(O₂CCMe₃)₁₆][C₄H₈O₂]_0.5, (I). Using a protonated secondary amino alcohol as a template, we wished to investigate the influence of the alcohol group on the accommodation of the amine group in the void of the wheel, and also how the -OH group might influence the packing of the anionic rings in the crystal structure. A further goal was to obtain a complex for the synthesis of further polymetallic cage complexes, using the two OH groups of the diethanolamine to bind to further metal centres.

For five of the 16 bridging pivalate groups in (I), two possible positions were located and refined for the tert-butyl methyl groups. The occupation factors for the major component of these five disordered pivalate groups, of which two are on either face, while one is on the edge of the circular ring molecule, are in the range 0.521 (10)-0.592 (9). For all pivalate groups, a restraint was imposed in order to maintain tetrahedral coordination of the CMe₃ groups, and a common C-C bond length was restrained to 1.521 (1) Å.

Compound (I) was crystallized from an ethyl acetate solution and a molecule of ethyl acetate is incorporated in the crystal structure. It is situated near a centre of symmetry and is disordered over two positions, with two of the atom positions coinciding. The protonated ethanolamine molecule appears to be disordered over three orientations in the void of the octagonal ring, with one major [0.444 (5)] and two minor [0.251 (5) and 0.302 (5)] orientations. The amine N atom of the main component sits significantly closer to one side of the ring than the other. The NF distances vary systematically around the ring: N1F1 = 3.104 (3), N1F2 = 3.408 (3), N1F3 = 3.730 (3), N1F4 = 3.829 (3), N1F5 = 3.557 (3), N1F6 = 3.215 (3), N1F7 = 2.878 (3) and N1F8 = 2.855 (3) Å.

In the final cycles of refinement, all Cr/Ni atoms were refined using Cr scattering factors. The metal atoms on the side of the ring with the shorter NF separations tend to have smaller values for their equivalent isotropic displacement parameters: U_eq(Cr1) = 0.0238 (1), U_eq(Cr2) = 0.0281 (1), U_eq(Cr3) = 0.0286 (1), U_eq(Cr4) = 0.0308 (1), U_eq(Cr5) = 0.0234 (1), U_eq(Cr6) = 0.0222 (1), U_eq(Cr7) = 0.0238 (1) and U_eq(Cr8) = 0.0257 (1) Å². Since Ni has four more electrons than Cr, it is conceivable that the Cr atoms with lower U_eq values may have greater Ni occupancy, so there is an indication of an uneven distribution of Ni around the ring.

The O atoms of the ethanolamine molecule are found to have short distances to the same F atoms which are involved in the shortest NF separation [O2AF1 = 2.952 (3) and O2BF8 = 2.933 (3) Å]. The minor disorder components of the ethanolammonium cation show short NF distances to other F atoms than found for the major component and of similar lengths: N1CF1 = 2.721 (3), N1CF2 = 2.922 (3) and N1CF8 = 2.939 (3) Å, and N1EF6 = 2.837 (3), N1EF7 = 2.940 (3) and N1EF5 = 3.084 (3) Å.

Figure 1 A view of (I), showing 50% displacement ellipsoids, with selected atoms labelled. Only one orientation is shown for the disorder components. Colour key: Cr/Ni green, F pink, O red, C grey, N blue. H atoms have been omitted.

Experimental

Compound (I) was synthesized in approximately 65% yield in a similar way to the analogues reported earlier (Larsen, Overgaard et al., 2003), except that the amine used was diethanolamine and the product was crystallized from ethyl acetate. Elemental analysis (dried sample), calculated for C₈₄H₁₅₆Cr₇F₈N₁Ni₁O₃₄: Cr 15.83, Ni 2.55, C 43.89, H 6.84, N 0.61%; found: Cr 15.28, Ni 2.72, C 43.89, H 7.00, N 0.62%. ES-MS (2 THF/MeOH m/z): -2191 [Cr₇NiF₈(O₂CCMe₃)₁₆], 2298 [M^-], 2321 [M+Na]⁺.

Methyl H atoms were located in idealized positions, with C-H = 0.98 Å, and refined as riding, with the constraint U_iso(H) = 1.5U_iso(C). N-bound H atoms were located in idealized positions, with N-H = 0.92 Å, and refined as riding, with the constraint U_iso(H) = 1.2U_iso(N). The highest difference peak is 0.39 Å from H20C and the deepest difference hole is 0.51 Å from C28.

Data collection: APEX-II (Bruker-Nonius, 2004); cell refinement: SAINT-Plus (Bruker-Nonius, 2004); data reduction: SAINT-Plus; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXL97; software used to prepare material for publication: enCIFer (Version 1.1; Allen et al., 2004) and WinGX (Version 1.70.00; Farrugia, 1999).