cyclo-Tetra-μ-oxido-tetrakis[(acetylacetonato-κ2 O,O′)bis(ethanolato-κO)niobium(V)]

The asymmetric unit of the title tetranuclear niobium(V) compound, [Nb4(C2H5O)8(C5H7O2)4O4], contains two NbV atoms, two bridging O atoms, two acetylacetonate and four ethanolate ligands. Each NbV atom is six-coordinated by the bridging O atoms, two ethanolate and one chelating acetylacetonate ligands. The Nb—O distances vary between 1.817 (2) and 2.201 (2) Å and the O—Nb—O angles vary between 78.88 (8) and 102.78 (9)°, illustrating the significant distortion from ideal ocahedral geometry. The rest of the tetranuclear unit is generated through an inversion centre. The C atoms of two of the ethanolate molecules are disordered over two sites [occupancy ratio 0.601 (12):0.399 (12)].

In the title tetranuclear Niobium(V) compound, [Nb(CH 3 CH 2 O) 2 (C 5 H 7 O 2 )(µ 2 -O)] 4 , the asymmetric unit contains two niobium atoms, separated by a bridging oxygen atom, two acetylacetonato bidentate ligands, four ethanolate ligands and another bridging oxygen atom coordinated to Nb1. The rest of the title compound is generated through an inversion centre (see Figure 1).
The four niobium atoms and the four bridging oxygen atoms form a slightly distorted square with Nb-Nb distances of 3.8339 (13) and 3.8229 (9) ° respectively and O-Nb-O angles of 93.526 (14) and 97.123 (13) Å (see Figure 2). The planarity of this square arrangement is indicated by the small distances that the Nb and O atoms are protruding from a plane generated through Nb1, Nb2, O1 and O5; the largest distance from the plane being 0.575 (14) Å, obtained for O1.
Two of the carbon atoms of one of the ethanolate ligands are disordered over two positions (53% to 47%) while the methyl carbon of another ethanolate ligand displays a vibrational disorder of 72%. Two of the ethanolate molecules are disordered over two positions.

Experimental
The reaction was performed under modified Schlenk conditions under an argon atmosphere. Nb(OEt) 5 (1.16 mmol, 0.291 ml) and acetylacetone (1.16 mmol, 0.119 ml) were added together and stirred for 30 min. Absolute methanol (5 ml) was added to the reaction mixture and allowed to stir for another 30 min at room temperature. The colourless solution was left to stand at 252 K for a few days after which white crystals, suitable for X-ray diffraction were obtained.

Refinement
The methine and methylene H atoms were placed in geometrically idealized positions at C-H = 0.93 and 0.97 Å, respectively and constrained to ride on their parent atoms, with U iso (H) = 1.2U eq (C). The highest peak is located 0.81 Å from NB2 and the deepest hole is situated 0.67 Å from Nb2.
A larger than usual U(eq) range for the disordered methyl atoms is observed and were refined using the DELU and SIMU instructions.
A few reflections were influenced by the beamstop and therefore omitted to obtain a better refinement. Fig. 1. Molecular structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms ommitted for clarity. Symmetry code: (i) -x + 2, -y + 1, -z + 1.

Special details
Experimental. The intensity data were collected on a Bruker X8 ApexII 4 K Kappa CCD diffractometer using an exposure time of 40 s/frame. A total of 1709 frames were collected with a frame width of 0.5° covering up to θ = 28.39° with 99.9% completeness accomplished.
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ.