Acta Cryst. (2008). E64, m1197 [ doi:10.1107/S1600536808026627 ]
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2N1,N9)niobium(V) hexafluoridophosphateThe title complex, [Nb(C7H12N3)4]PF6, features chelating hpp anions (hpp is 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) that define a distorted dodecahedral coordination geometry based on an N8 donor set. The Nb atom is situated on a site of symmetry 
, and the PF6- anion has crystallographic fourfold symmetry.
The title complex (I) was obtained unintentionally in an attempt to oxidize the paddlewheel complex Nb2(hpp)4 with [Cp2Fe][PF6] in CH2Cl2. X-ray quality crystals were obtained by slow diffusion of hexanes into a CH2Cl2 solution of (I) at room temperature.
The H atoms were geometrically placed (C—H = 0.98 Å) and refined as riding with Uiso(H) = 1.2Ueq(C).
Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT (Bruker, 2001); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEPII (Johnson, 1976); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).
![[Figure 1]](./lx2067fig1thm.gif)
| Fig. 1. Molecular structure of the cation in (I) showing the crystallographic numbering scheme. Displacement ellipsoids are shown at the 35% probability level. The Nb atom is located on a site of symmetry 4. |
| [Nb(C7H12N3)4]PF6 | Z = 2 |
| Mr = 790.66 | F000 = 820 |
| Tetragonal, P4/n | Dx = 1.566 Mg m−3 |
| Hall symbol: -P 4a | Mo Kα radiation λ = 0.71069 Å |
| a = 13.531 (6) Å | Cell parameters from 10356 reflections |
| b = 13.531 (6) Å | θ = 2.1–27.5º |
| c = 9.159 (4) Å | µ = 0.48 mm−1 |
| α = 90º | T = 213 (2) K |
| β = 90º | Block, yellow |
| γ = 90º | 0.20 × 0.15 × 0.10 mm |
| V = 1676.9 (13) Å3 |
| Bruker SMART 1K CCD area-detector diffractometer | 1655 independent reflections |
| Radiation source: fine-focus sealed tube | 1381 reflections with I > 2σ(I) |
| Monochromator: graphite | Rint = 0.025 |
| Detector resolution: 10 pixels mm-1 | θmax = 26.0º |
| T = 213(2) K | θmin = 2.1º |
| ω and φ scans | h = −17→12 |
| Absorption correction: multi-scan (SADABS; Sheldrick, 2004) | k = −17→16 |
| Tmin = 0.910, Tmax = 0.953 | l = −10→11 |
| 10356 measured reflections |
| Refinement on F2 | Secondary atom site location: difference Fourier map |
| Least-squares matrix: full | Hydrogen site location: inferred from neighbouring sites |
| R[F2 > 2σ(F2)] = 0.050 | H-atom parameters constrained |
| wR(F2) = 0.144 | w = 1/[σ2(Fo2) + (0.067P)2 + 3.4248P] where P = (Fo2 + 2Fc2)/3 |
| S = 1.05 | (Δ/σ)max < 0.001 |
| 1655 reflections | Δρmax = 0.69 e Å−3 |
| 111 parameters | Δρmin = −0.39 e Å−3 |
| Primary atom site location: structure-invariant direct methods | Extinction correction: none |
| [Nb(C7H12N3)4]PF6 | γ = 90º |
| Mr = 790.66 | V = 1676.9 (13) Å3 |
| Tetragonal, P4/n | Z = 2 |
| a = 13.531 (6) Å | Mo Kα |
| b = 13.531 (6) Å | µ = 0.48 mm−1 |
| c = 9.159 (4) Å | T = 213 (2) K |
| α = 90º | 0.20 × 0.15 × 0.10 mm |
| β = 90º |
| Bruker SMART 1K CCD area-detector diffractometer | 1655 independent reflections |
| Absorption correction: multi-scan (SADABS; Sheldrick, 2004) | 1381 reflections with I > 2σ(I) |
| Tmin = 0.910, Tmax = 0.953 | Rint = 0.025 |
| 10356 measured reflections |
| R[F2 > 2σ(F2)] = 0.050 | 111 parameters |
| wR(F2) = 0.144 | H-atom parameters constrained |
| S = 1.05 | Δρmax = 0.69 e Å−3 |
| 1655 reflections | Δρmin = −0.39 e Å−3 |
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. |
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > 2sigma(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger. |
| x | y | z | Uiso*/Ueq | ||
| Nb | 0.7500 | 0.2500 | 0.5000 | 0.0388 (2) | |
| N1 | 0.6548 (3) | 0.1964 (3) | 0.3195 (4) | 0.0670 (11) | |
| N2 | 0.6131 (3) | 0.1782 (3) | 0.5468 (4) | 0.0677 (11) | |
| N3 | 0.4885 (3) | 0.1508 (3) | 0.3731 (4) | 0.0598 (9) | |
| C1 | 0.6352 (4) | 0.1994 (4) | 0.1664 (5) | 0.0730 (14) | |
| H1A | 0.6931 | 0.1747 | 0.1137 | 0.088* | |
| H1B | 0.6245 | 0.2682 | 0.1370 | 0.088* | |
| C2 | 0.5483 (5) | 0.1405 (7) | 0.1244 (6) | 0.116 (3) | |
| H2A | 0.5295 | 0.1587 | 0.0247 | 0.139* | |
| H2B | 0.5675 | 0.0706 | 0.1231 | 0.139* | |
| C3 | 0.4618 (4) | 0.1511 (4) | 0.2179 (6) | 0.0783 (15) | |
| H3A | 0.4158 | 0.0967 | 0.1985 | 0.094* | |
| H3B | 0.4281 | 0.2132 | 0.1945 | 0.094* | |
| C4 | 0.4136 (3) | 0.1333 (4) | 0.4827 (6) | 0.0694 (14) | |
| H4A | 0.3720 | 0.0777 | 0.4523 | 0.083* | |
| H4B | 0.3713 | 0.1919 | 0.4909 | 0.083* | |
| C5 | 0.4580 (4) | 0.1111 (4) | 0.6279 (6) | 0.0839 (18) | |
| H5A | 0.4830 | 0.0431 | 0.6279 | 0.101* | |
| H5B | 0.4067 | 0.1159 | 0.7032 | 0.101* | |
| C6 | 0.5406 (4) | 0.1802 (4) | 0.6648 (5) | 0.0745 (14) | |
| H6A | 0.5149 | 0.2474 | 0.6772 | 0.089* | |
| H6B | 0.5718 | 0.1599 | 0.7566 | 0.089* | |
| C7 | 0.5805 (3) | 0.1735 (3) | 0.4103 (5) | 0.0549 (10) | |
| P1 | 0.2500 | 0.2500 | 0.9214 (2) | 0.0475 (5) | |
| F1 | 0.13862 (19) | 0.2132 (2) | 0.9219 (3) | 0.0772 (9) | |
| F2 | 0.2500 | 0.2500 | 0.7468 (5) | 0.0695 (14) | |
| F3 | 0.2500 | 0.2500 | 1.0963 (5) | 0.0562 (11) |
| U11 | U22 | U33 | U12 | U13 | U23 | |
| Nb | 0.0438 (3) | 0.0438 (3) | 0.0288 (4) | 0.000 | 0.000 | 0.000 |
| N1 | 0.056 (2) | 0.107 (3) | 0.0386 (18) | −0.013 (2) | −0.0009 (16) | −0.0025 (19) |
| N2 | 0.061 (2) | 0.095 (3) | 0.0472 (19) | −0.019 (2) | 0.0110 (18) | −0.003 (2) |
| N3 | 0.051 (2) | 0.067 (2) | 0.062 (2) | 0.0073 (17) | −0.0030 (17) | 0.0021 (19) |
| C1 | 0.077 (3) | 0.099 (4) | 0.043 (2) | −0.014 (3) | −0.009 (2) | 0.005 (2) |
| C2 | 0.084 (4) | 0.217 (8) | 0.047 (3) | −0.053 (5) | −0.014 (3) | 0.001 (4) |
| C3 | 0.061 (3) | 0.096 (4) | 0.078 (3) | −0.001 (3) | −0.023 (3) | 0.010 (3) |
| C4 | 0.048 (2) | 0.055 (3) | 0.105 (4) | −0.0021 (19) | 0.012 (3) | −0.006 (3) |
| C5 | 0.090 (4) | 0.072 (3) | 0.089 (4) | −0.004 (3) | 0.054 (3) | 0.000 (3) |
| C6 | 0.076 (3) | 0.101 (4) | 0.047 (2) | −0.013 (3) | 0.018 (2) | 0.000 (3) |
| C7 | 0.055 (2) | 0.065 (3) | 0.045 (2) | −0.005 (2) | 0.0064 (18) | 0.0035 (19) |
| P1 | 0.0530 (7) | 0.0530 (7) | 0.0365 (10) | 0.000 | 0.000 | 0.000 |
| F1 | 0.0582 (15) | 0.112 (2) | 0.0619 (17) | −0.0192 (15) | 0.0028 (13) | −0.0233 (16) |
| F2 | 0.087 (2) | 0.087 (2) | 0.035 (2) | 0.000 | 0.000 | 0.000 |
| F3 | 0.0660 (18) | 0.0660 (18) | 0.036 (2) | 0.000 | 0.000 | 0.000 |
| Nb—N2 | 2.135 (4) | C2—H2B | 0.9800 |
| Nb—N2i | 2.135 (4) | C3—H3A | 0.9800 |
| Nb—N1 | 2.218 (4) | C3—H3B | 0.9800 |
| Nb—N1i | 2.218 (4) | C4—C5 | 1.490 (8) |
| Nb—C7 | 2.648 (4) | C4—H4A | 0.9800 |
| Nb—C7i | 2.648 (4) | C4—H4B | 0.9800 |
| N1—C7 | 1.341 (5) | C5—C6 | 1.496 (8) |
| N1—C1 | 1.428 (5) | C5—H5A | 0.9800 |
| N2—C7 | 1.328 (6) | C5—H5B | 0.9800 |
| N2—C6 | 1.460 (6) | C6—H6A | 0.9800 |
| N3—C7 | 1.326 (5) | C6—H6B | 0.9800 |
| N3—C4 | 1.447 (6) | P1—F1 | 1.587 (3) |
| N3—C3 | 1.466 (6) | P1—F1ii | 1.587 (3) |
| C1—C2 | 1.472 (7) | P1—F1iii | 1.587 (3) |
| C1—H1A | 0.9800 | P1—F1iv | 1.587 (3) |
| C1—H1B | 0.9800 | P1—F2 | 1.600 (5) |
| C2—C3 | 1.457 (8) | P1—F3 | 1.602 (5) |
| C2—H2A | 0.9800 | ||
| N2—Nb—N2i | 156.8 (2) | C7—N1—C1 | 118.4 (4) |
| N2—Nb—N2v | 92.31 (4) | C7—N1—Nb | 92.8 (3) |
| N2i—Nb—N2v | 92.31 (4) | C1—N1—Nb | 146.2 (3) |
| N2—Nb—N2vi | 92.31 (4) | C7—N2—C6 | 118.3 (4) |
| N2i—Nb—N2vi | 92.31 (4) | C7—N2—Nb | 97.0 (3) |
| N2v—Nb—N2vi | 156.8 (2) | C6—N2—Nb | 136.4 (3) |
| N2—Nb—N1 | 59.80 (14) | C7—N3—C4 | 121.1 (4) |
| N2i—Nb—N1 | 143.38 (14) | C7—N3—C3 | 118.7 (4) |
| N2v—Nb—N1 | 80.23 (16) | C4—N3—C3 | 120.0 (4) |
| N2vi—Nb—N1 | 82.54 (17) | N1—C1—C2 | 112.9 (4) |
| N2—Nb—N1v | 82.54 (17) | N1—C1—H1A | 109.0 |
| N2i—Nb—N1v | 80.23 (16) | C2—C1—H1A | 109.0 |
| N2v—Nb—N1v | 59.80 (14) | N1—C1—H1B | 109.0 |
| N2vi—Nb—N1v | 143.38 (14) | C2—C1—H1B | 109.0 |
| N1—Nb—N1v | 123.75 (12) | H1A—C1—H1B | 107.8 |
| N2—Nb—N1vi | 80.23 (16) | C3—C2—C1 | 115.7 (6) |
| N2i—Nb—N1vi | 82.54 (17) | C3—C2—H2A | 108.3 |
| N2v—Nb—N1vi | 143.38 (14) | C1—C2—H2A | 108.3 |
| N2vi—Nb—N1vi | 59.80 (14) | C3—C2—H2B | 108.3 |
| N1—Nb—N1vi | 123.75 (12) | C1—C2—H2B | 108.3 |
| N1v—Nb—N1vi | 83.6 (2) | H2A—C2—H2B | 107.4 |
| N2—Nb—N1i | 143.38 (14) | C2—C3—N3 | 111.8 (4) |
| N2i—Nb—N1i | 59.80 (14) | C2—C3—H3A | 109.3 |
| N2v—Nb—N1i | 82.54 (17) | N3—C3—H3A | 109.3 |
| N2vi—Nb—N1i | 80.23 (16) | C2—C3—H3B | 109.3 |
| N1—Nb—N1i | 83.6 (2) | N3—C3—H3B | 109.3 |
| N1v—Nb—N1i | 123.75 (12) | H3A—C3—H3B | 107.9 |
| N1vi—Nb—N1i | 123.75 (12) | N3—C4—C5 | 111.7 (4) |
| N2—Nb—C7 | 29.86 (14) | N3—C4—H4A | 109.3 |
| N2i—Nb—C7 | 172.73 (14) | C5—C4—H4A | 109.3 |
| N2v—Nb—C7 | 89.57 (16) | N3—C4—H4B | 109.3 |
| N2vi—Nb—C7 | 83.26 (16) | C5—C4—H4B | 109.3 |
| N1—Nb—C7 | 30.39 (13) | H4A—C4—H4B | 107.9 |
| N1v—Nb—C7 | 106.74 (15) | C4—C5—C6 | 112.1 (4) |
| N1vi—Nb—C7 | 100.07 (15) | C4—C5—H5A | 109.2 |
| N1i—Nb—C7 | 113.57 (13) | C6—C5—H5A | 109.2 |
| N2—Nb—C7vi | 89.57 (15) | C4—C5—H5B | 109.2 |
| N2i—Nb—C7vi | 83.26 (16) | C6—C5—H5B | 109.2 |
| N2v—Nb—C7vi | 172.73 (14) | H5A—C5—H5B | 107.9 |
| N2vi—Nb—C7vi | 29.86 (14) | N2—C6—C5 | 108.9 (4) |
| N1—Nb—C7vi | 106.74 (15) | N2—C6—H6A | 109.9 |
| N1v—Nb—C7vi | 113.57 (13) | C5—C6—H6A | 109.9 |
| N1vi—Nb—C7vi | 30.39 (13) | N2—C6—H6B | 109.9 |
| N1i—Nb—C7vi | 100.07 (15) | C5—C6—H6B | 109.9 |
| C7—Nb—C7vi | 95.53 (5) | H6A—C6—H6B | 108.3 |
| N2—Nb—C7v | 83.26 (16) | N3—C7—N2 | 124.4 (4) |
| N2i—Nb—C7v | 89.57 (15) | N3—C7—N1 | 126.7 (4) |
| N2v—Nb—C7v | 29.86 (14) | N2—C7—N1 | 108.8 (4) |
| N2vi—Nb—C7v | 172.73 (14) | N3—C7—Nb | 169.6 (3) |
| N1—Nb—C7v | 100.07 (15) | N2—C7—Nb | 53.2 (2) |
| N1v—Nb—C7v | 30.39 (13) | N1—C7—Nb | 56.8 (2) |
| N1vi—Nb—C7v | 113.57 (13) | F1—P1—F1ii | 90.000 (2) |
| N1i—Nb—C7v | 106.74 (15) | F1—P1—F1iii | 179.7 (2) |
| C7—Nb—C7v | 95.53 (5) | F1ii—P1—F1iii | 90.000 (2) |
| C7vi—Nb—C7v | 143.83 (18) | F1—P1—F1iv | 90.000 (1) |
| N2—Nb—C7i | 172.73 (14) | F1ii—P1—F1iv | 179.7 (2) |
| N2i—Nb—C7i | 29.86 (14) | F1iii—P1—F1iv | 90.000 (2) |
| N2v—Nb—C7i | 83.26 (16) | F1—P1—F2 | 90.16 (12) |
| N2vi—Nb—C7i | 89.57 (16) | F1ii—P1—F2 | 90.16 (12) |
| N1—Nb—C7i | 113.57 (13) | F1iii—P1—F2 | 90.16 (12) |
| N1v—Nb—C7i | 100.07 (15) | F1iv—P1—F2 | 90.16 (12) |
| N1vi—Nb—C7i | 106.74 (15) | F1—P1—F3 | 89.84 (12) |
| N1i—Nb—C7i | 30.39 (13) | F1ii—P1—F3 | 89.84 (12) |
| C7—Nb—C7i | 143.83 (18) | F1iii—P1—F3 | 89.84 (12) |
| C7vi—Nb—C7i | 95.53 (5) | F1iv—P1—F3 | 89.84 (12) |
| C7v—Nb—C7i | 95.53 (5) | F2—P1—F3 | 180.000 (2) |
| N2—Nb—N1—C7 | 7.7 (3) | N3—C4—C5—C6 | 45.0 (6) |
| N2i—Nb—N1—C7 | −173.2 (3) | C7—N2—C6—C5 | 39.2 (7) |
| N2v—Nb—N1—C7 | 106.2 (3) | Nb—N2—C6—C5 | 179.1 (4) |
| N2vi—Nb—N1—C7 | −89.4 (3) | C4—C5—C6—N2 | −54.6 (6) |
| N1v—Nb—N1—C7 | 63.0 (4) | C4—N3—C7—N2 | 3.0 (7) |
| N1vi—Nb—N1—C7 | −43.6 (4) | C3—N3—C7—N2 | 178.2 (5) |
| N1i—Nb—N1—C7 | −170.3 (4) | C4—N3—C7—N1 | −176.2 (5) |
| C7vi—Nb—N1—C7 | −71.7 (3) | C3—N3—C7—N1 | −1.0 (7) |
| C7v—Nb—N1—C7 | 83.7 (2) | C4—N3—C7—Nb | −70.1 (19) |
| C7i—Nb—N1—C7 | −175.65 (18) | C3—N3—C7—Nb | 105.1 (17) |
| N2—Nb—N1—C1 | 165.9 (7) | C6—N2—C7—N3 | −14.1 (7) |
| N2i—Nb—N1—C1 | −15.0 (8) | Nb—N2—C7—N3 | −167.5 (4) |
| N2v—Nb—N1—C1 | −95.6 (7) | C6—N2—C7—N1 | 165.2 (5) |
| N2vi—Nb—N1—C1 | 68.8 (7) | Nb—N2—C7—N1 | 11.8 (4) |
| N1v—Nb—N1—C1 | −138.8 (6) | C6—N2—C7—Nb | 153.4 (5) |
| N1vi—Nb—N1—C1 | 114.6 (6) | C1—N1—C7—N3 | 1.6 (8) |
| N1i—Nb—N1—C1 | −12.1 (6) | Nb—N1—C7—N3 | 168.0 (4) |
| C7—Nb—N1—C1 | 158.2 (9) | C1—N1—C7—N2 | −177.7 (5) |
| C7vi—Nb—N1—C1 | 86.5 (7) | Nb—N1—C7—N2 | −11.3 (4) |
| C7v—Nb—N1—C1 | −118.0 (7) | C1—N1—C7—Nb | −166.4 (5) |
| C7i—Nb—N1—C1 | −17.4 (7) | N2—Nb—C7—N3 | 80.4 (17) |
| N2i—Nb—N2—C7 | 173.5 (3) | N2v—Nb—C7—N3 | 175.8 (17) |
| N2v—Nb—N2—C7 | −85.1 (3) | N2vi—Nb—C7—N3 | −26.3 (17) |
| N2vi—Nb—N2—C7 | 72.2 (3) | N1—Nb—C7—N3 | −113.0 (18) |
| N1—Nb—N2—C7 | −7.8 (3) | N1v—Nb—C7—N3 | 117.6 (17) |
| N1v—Nb—N2—C7 | −144.2 (3) | N1vi—Nb—C7—N3 | 31.3 (18) |
| N1vi—Nb—N2—C7 | 131.0 (3) | N1i—Nb—C7—N3 | −102.5 (17) |
| N1i—Nb—N2—C7 | −4.5 (5) | C7vi—Nb—C7—N3 | 1.0 (17) |
| C7vi—Nb—N2—C7 | 101.9 (3) | C7v—Nb—C7—N3 | 146.5 (18) |
| C7v—Nb—N2—C7 | −113.6 (3) | C7i—Nb—C7—N3 | −106.3 (18) |
| N2i—Nb—N2—C6 | 28.4 (5) | N2v—Nb—C7—N2 | 95.4 (3) |
| N2v—Nb—N2—C6 | 129.7 (6) | N2vi—Nb—C7—N2 | −106.7 (3) |
| N2vi—Nb—N2—C6 | −73.0 (5) | N1—Nb—C7—N2 | 166.6 (5) |
| N1—Nb—N2—C6 | −153.0 (6) | N1v—Nb—C7—N2 | 37.2 (3) |
| N1v—Nb—N2—C6 | 70.6 (5) | N1vi—Nb—C7—N2 | −49.1 (3) |
| N1vi—Nb—N2—C6 | −14.2 (5) | N1i—Nb—C7—N2 | 177.1 (3) |
| N1i—Nb—N2—C6 | −149.7 (5) | C7v—Nb—C7—N2 | 66.1 (3) |
| C7—Nb—N2—C6 | −145.2 (7) | C7i—Nb—C7—N2 | 173.3 (3) |
| C7vi—Nb—N2—C6 | −43.3 (5) | N2—Nb—C7—N1 | −166.6 (5) |
| C7v—Nb—N2—C6 | 101.2 (5) | N2v—Nb—C7—N1 | −71.2 (3) |
| C7—N1—C1—C2 | −22.7 (8) | N2vi—Nb—C7—N1 | 86.7 (3) |
| Nb—N1—C1—C2 | −177.8 (5) | N1v—Nb—C7—N1 | −129.3 (3) |
| N1—C1—C2—C3 | 44.3 (9) | N1vi—Nb—C7—N1 | 144.4 (3) |
| C1—C2—C3—N3 | −43.2 (8) | N1i—Nb—C7—N1 | 10.5 (4) |
| C7—N3—C3—C2 | 21.6 (8) | C7vi—Nb—C7—N1 | 114.0 (3) |
| C4—N3—C3—C2 | −163.1 (5) | C7v—Nb—C7—N1 | −100.5 (3) |
| C7—N3—C4—C5 | −19.0 (6) | C7i—Nb—C7—N1 | 6.8 (3) |
| C3—N3—C4—C5 | 165.8 (5) |
| Symmetry codes: (i) −x+3/2, −y+1/2, z; (ii) −y+1/2, x, z; (iii) −x+1/2, −y+1/2, z; (iv) y, −x+1/2, z; (v) −y+1, x−1/2, −z+1; (vi) y+1/2, −x+1, −z+1. |
The authors thank the Robert A. Welch Foundation and Texas A&M University for financial support.
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The title complex, [Nb(hpp)4][PF6] (I), feaures a [Nb(hpp)4]+ cation, with the Nb atom located on a site of symmetry 4, and a [PF6]- anion, with fourfold symmetry; where hpp is 1,3,4,6,7,8-hexahydro-2H-pyrimido(1,2 - a)pyrimidine. The Nb atom is chelated four hpp ligands and the N8 donor set defines an approximate dodecahedral coordination environment (Fig. 1).
The conformations of the N1- and N-2 containing six-membered rings is twisted chair. Such a binding mode as observed in (I) is uncommon for the hpp ligand, which normally acts as a bridging group in various paddlewheel complexes (Cotton et al., 2005). A related example of hpp acting as a chelating ligand is [Ta(hpp)4][Ta(CO)6] (Cotton et al., 2000). Both complexes were obtained by oxidizing the precursors Nb2(hpp)4 and [Et4N][Ta(CO)6], respectively. The chelating mode of hpp is also found in some Ti complexes (Coles & Hitchcok, 2001).