Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270106026060/sq3030sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270106026060/sq3030IIsup2.hkl |
CCDC reference: 618609
Adventitious hydrolysis of O═NbCl3 in the presence of pyridine yielded (II) (Quantities?). Crystals of (II) were obtained from a pyridine–hexane solvent system (Solvent ratio?).
All H atoms were placed in idealized locations and refined as riding, with C—H = 0.95 Å and N—H = 0.88 Å [Please check added text], and with Uiso(H) = 1.2Ueq(C,N).
Data collection: SMART (Bruker, 2003); cell refinement: SAINT (Bruker, 2003); data reduction: SAINT; program(s) used to solve structure: SHELXTL (Bruker, 2003); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL.
(C5H6N)2Cl[NbCl4O(C5H5N)] | F(000) = 1048 |
Mr = 525.48 | Dx = 1.675 Mg m−3 |
Monoclinic, P21/c | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -P 2ybc | Cell parameters from 12954 reflections |
a = 7.6122 (9) Å | θ = 2.5–26.4° |
b = 20.091 (2) Å | µ = 1.23 mm−1 |
c = 13.8061 (16) Å | T = 100 K |
β = 99.328 (2)° | Block, yellow |
V = 2083.5 (4) Å3 | 0.42 × 0.34 × 0.33 mm |
Z = 4 |
Bruker SMART1000 CCD area-detector diffractometer | 4257 independent reflections |
Radiation source: fine-focus sealed tube | 4097 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.025 |
0.30° ω scans | θmax = 26.4°, θmin = 2.7° |
Absorption correction: multi-scan SADABS; Bruker, 2003) | h = −9→9 |
Tmin = 0.60, Tmax = 0.66 | k = −25→25 |
16884 measured reflections | l = −17→17 |
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.030 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.079 | H-atom parameters constrained |
S = 1.11 | w = 1/[σ2(Fo2) + (0.0377P)2 + 2.8236P] where P = (Fo2 + 2Fc2)/3 |
4257 reflections | (Δ/σ)max = 0.003 |
226 parameters | Δρmax = 1.44 e Å−3 |
0 restraints | Δρmin = −0.96 e Å−3 |
(C5H6N)2Cl[NbCl4O(C5H5N)] | V = 2083.5 (4) Å3 |
Mr = 525.48 | Z = 4 |
Monoclinic, P21/c | Mo Kα radiation |
a = 7.6122 (9) Å | µ = 1.23 mm−1 |
b = 20.091 (2) Å | T = 100 K |
c = 13.8061 (16) Å | 0.42 × 0.34 × 0.33 mm |
β = 99.328 (2)° |
Bruker SMART1000 CCD area-detector diffractometer | 4257 independent reflections |
Absorption correction: multi-scan SADABS; Bruker, 2003) | 4097 reflections with I > 2σ(I) |
Tmin = 0.60, Tmax = 0.66 | Rint = 0.025 |
16884 measured reflections |
R[F2 > 2σ(F2)] = 0.030 | 0 restraints |
wR(F2) = 0.079 | H-atom parameters constrained |
S = 1.11 | Δρmax = 1.44 e Å−3 |
4257 reflections | Δρmin = −0.96 e Å−3 |
226 parameters |
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > σ(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 | ||
Nb1 | 0.70819 (3) | 0.675338 (10) | 1.049357 (15) | 0.01892 (8) | |
Cl1 | 0.43043 (8) | 0.73264 (3) | 0.99715 (4) | 0.02288 (13) | |
Cl2 | 0.85624 (9) | 0.75775 (3) | 0.96552 (5) | 0.02930 (15) | |
Cl3 | 0.96676 (8) | 0.60520 (3) | 1.06840 (4) | 0.02684 (14) | |
Cl4 | 0.53663 (8) | 0.58115 (3) | 1.08599 (4) | 0.02295 (13) | |
Cl5 | 0.21261 (7) | 0.45823 (3) | 0.53357 (4) | 0.02222 (13) | |
O1 | 0.7502 (2) | 0.71086 (9) | 1.16772 (12) | 0.0221 (4) | |
N1 | 0.6515 (3) | 0.62402 (10) | 0.88127 (14) | 0.0190 (4) | |
N2 | 0.0902 (3) | 0.55976 (11) | 0.67884 (16) | 0.0261 (5) | |
H2N | 0.0844 | 0.5355 | 0.6254 | 0.031* | |
N3 | 0.5352 (3) | 0.37994 (11) | 0.62502 (16) | 0.0247 (5) | |
H3N | 0.4293 | 0.3945 | 0.5985 | 0.030* | |
C1 | 0.5816 (3) | 0.65964 (13) | 0.80195 (18) | 0.0232 (5) | |
H1 | 0.5503 | 0.7047 | 0.8111 | 0.028* | |
C2 | 0.5526 (4) | 0.63385 (14) | 0.70714 (19) | 0.0268 (5) | |
H2 | 0.5028 | 0.6606 | 0.6529 | 0.032* | |
C3 | 0.5983 (4) | 0.56806 (14) | 0.69388 (18) | 0.0265 (5) | |
H3 | 0.5809 | 0.5489 | 0.6301 | 0.032* | |
C4 | 0.6697 (3) | 0.53062 (13) | 0.77506 (19) | 0.0246 (5) | |
H4 | 0.7016 | 0.4854 | 0.7678 | 0.030* | |
C5 | 0.6940 (3) | 0.56017 (12) | 0.86714 (17) | 0.0205 (5) | |
H5 | 0.7426 | 0.5342 | 0.9225 | 0.025* | |
C6 | 0.1761 (3) | 0.53518 (14) | 0.7633 (2) | 0.0281 (6) | |
H6 | 0.2291 | 0.4923 | 0.7650 | 0.034* | |
C7 | 0.1876 (4) | 0.57206 (16) | 0.8474 (2) | 0.0323 (6) | |
H7 | 0.2483 | 0.5551 | 0.9080 | 0.039* | |
C8 | 0.1095 (4) | 0.63456 (15) | 0.8431 (2) | 0.0320 (6) | |
H8 | 0.1167 | 0.6609 | 0.9007 | 0.038* | |
C9 | 0.0210 (4) | 0.65829 (13) | 0.7545 (2) | 0.0308 (6) | |
H9 | −0.0334 | 0.7010 | 0.7509 | 0.037* | |
C10 | 0.0122 (4) | 0.61986 (14) | 0.6719 (2) | 0.0291 (6) | |
H10 | −0.0483 | 0.6355 | 0.6105 | 0.035* | |
C11 | 0.6743 (4) | 0.39724 (13) | 0.58428 (19) | 0.0279 (6) | |
H11 | 0.6583 | 0.4250 | 0.5278 | 0.034* | |
C12 | 0.8410 (4) | 0.37542 (14) | 0.6231 (2) | 0.0309 (6) | |
H12 | 0.9412 | 0.3873 | 0.5938 | 0.037* | |
C13 | 0.8606 (4) | 0.33608 (14) | 0.7051 (2) | 0.0325 (6) | |
H13 | 0.9755 | 0.3206 | 0.7333 | 0.039* | |
C14 | 0.7140 (4) | 0.31876 (13) | 0.7473 (2) | 0.0330 (6) | |
H14 | 0.7272 | 0.2918 | 0.8046 | 0.040* | |
C15 | 0.5483 (4) | 0.34132 (14) | 0.7046 (2) | 0.0282 (6) | |
H15 | 0.4451 | 0.3296 | 0.7313 | 0.034* |
U11 | U22 | U33 | U12 | U13 | U23 | |
Nb1 | 0.01934 (12) | 0.01774 (12) | 0.01947 (12) | −0.00143 (8) | 0.00254 (9) | 0.00043 (8) |
Cl1 | 0.0240 (3) | 0.0214 (3) | 0.0223 (3) | 0.0031 (2) | 0.0008 (2) | 0.0011 (2) |
Cl2 | 0.0342 (3) | 0.0240 (3) | 0.0320 (3) | −0.0098 (3) | 0.0124 (3) | −0.0020 (2) |
Cl3 | 0.0227 (3) | 0.0324 (3) | 0.0239 (3) | 0.0055 (2) | −0.0006 (2) | −0.0051 (2) |
Cl4 | 0.0281 (3) | 0.0188 (3) | 0.0222 (3) | −0.0041 (2) | 0.0047 (2) | 0.0022 (2) |
Cl5 | 0.0208 (3) | 0.0232 (3) | 0.0223 (3) | 0.0046 (2) | 0.0025 (2) | −0.0004 (2) |
O1 | 0.0178 (8) | 0.0269 (9) | 0.0206 (8) | 0.0003 (7) | 0.0007 (7) | 0.0023 (7) |
N1 | 0.0221 (10) | 0.0195 (10) | 0.0152 (9) | −0.0024 (8) | 0.0021 (8) | −0.0013 (8) |
N2 | 0.0320 (12) | 0.0237 (11) | 0.0236 (11) | −0.0044 (9) | 0.0071 (9) | −0.0051 (9) |
N3 | 0.0214 (10) | 0.0239 (10) | 0.0267 (11) | 0.0049 (8) | −0.0031 (9) | −0.0081 (9) |
C1 | 0.0264 (12) | 0.0218 (12) | 0.0207 (12) | 0.0009 (10) | 0.0020 (10) | 0.0005 (10) |
C2 | 0.0290 (13) | 0.0319 (14) | 0.0188 (12) | −0.0004 (11) | 0.0016 (10) | 0.0038 (10) |
C3 | 0.0302 (13) | 0.0311 (14) | 0.0186 (12) | −0.0055 (11) | 0.0047 (10) | −0.0044 (10) |
C4 | 0.0292 (13) | 0.0205 (12) | 0.0250 (13) | −0.0051 (10) | 0.0070 (10) | −0.0040 (10) |
C5 | 0.0243 (12) | 0.0183 (11) | 0.0188 (11) | −0.0035 (9) | 0.0031 (9) | 0.0002 (9) |
C6 | 0.0251 (12) | 0.0258 (13) | 0.0347 (14) | 0.0058 (10) | 0.0087 (11) | 0.0058 (11) |
C7 | 0.0244 (13) | 0.0485 (17) | 0.0233 (13) | −0.0029 (12) | 0.0016 (10) | 0.0068 (12) |
C8 | 0.0261 (13) | 0.0402 (16) | 0.0318 (14) | −0.0139 (12) | 0.0115 (11) | −0.0152 (12) |
C9 | 0.0242 (13) | 0.0198 (12) | 0.0483 (17) | −0.0016 (10) | 0.0052 (12) | −0.0043 (12) |
C10 | 0.0272 (13) | 0.0277 (13) | 0.0298 (14) | −0.0045 (11) | −0.0032 (11) | 0.0055 (11) |
C11 | 0.0410 (15) | 0.0203 (12) | 0.0225 (12) | −0.0014 (11) | 0.0052 (11) | −0.0013 (10) |
C12 | 0.0260 (13) | 0.0311 (14) | 0.0375 (15) | −0.0065 (11) | 0.0104 (11) | −0.0113 (12) |
C13 | 0.0265 (13) | 0.0303 (14) | 0.0356 (15) | 0.0067 (11) | −0.0102 (11) | −0.0136 (12) |
C14 | 0.0554 (19) | 0.0227 (13) | 0.0177 (12) | −0.0016 (12) | −0.0035 (12) | 0.0009 (10) |
C15 | 0.0323 (14) | 0.0279 (13) | 0.0272 (13) | −0.0078 (11) | 0.0131 (11) | −0.0107 (11) |
Nb1—O1 | 1.7643 (17) | C4—H4 | 0.9500 |
Nb1—Cl4 | 2.3992 (6) | C5—H5 | 0.9500 |
Nb1—Cl3 | 2.4002 (7) | C6—C7 | 1.368 (4) |
Nb1—Cl2 | 2.4029 (7) | C6—H6 | 0.9500 |
Nb1—Cl1 | 2.4127 (6) | C7—C8 | 1.386 (4) |
Nb1—N1 | 2.5119 (19) | C7—H7 | 0.9500 |
N1—C1 | 1.344 (3) | C8—C9 | 1.382 (4) |
N1—C5 | 1.345 (3) | C8—H8 | 0.9500 |
N2—C6 | 1.336 (4) | C9—C10 | 1.370 (4) |
N2—C10 | 1.342 (4) | C9—H9 | 0.9500 |
N2—H2N | 0.8800 | C10—H10 | 0.9500 |
N3—C11 | 1.324 (4) | C11—C12 | 1.367 (4) |
N3—C15 | 1.335 (4) | C11—H11 | 0.9500 |
N3—H3N | 0.8800 | C12—C13 | 1.369 (4) |
C1—C2 | 1.392 (4) | C12—H12 | 0.9500 |
C1—H1 | 0.9500 | C13—C14 | 1.385 (5) |
C2—C3 | 1.386 (4) | C13—H13 | 0.9500 |
C2—H2 | 0.9500 | C14—C15 | 1.380 (4) |
C3—C4 | 1.386 (4) | C14—H14 | 0.9500 |
C3—H3 | 0.9500 | C15—H15 | 0.9500 |
C4—C5 | 1.388 (3) | ||
O1—Nb1—Cl4 | 98.45 (6) | C5—C4—H4 | 120.5 |
O1—Nb1—Cl3 | 96.52 (6) | N1—C5—C4 | 122.8 (2) |
Cl4—Nb1—Cl3 | 88.77 (2) | N1—C5—H5 | 118.6 |
O1—Nb1—Cl2 | 97.97 (6) | C4—C5—H5 | 118.6 |
Cl4—Nb1—Cl2 | 163.50 (2) | N2—C6—C7 | 119.7 (3) |
Cl3—Nb1—Cl2 | 90.97 (3) | N2—C6—H6 | 120.1 |
O1—Nb1—Cl1 | 95.87 (6) | C7—C6—H6 | 120.1 |
Cl4—Nb1—Cl1 | 87.63 (2) | C6—C7—C8 | 119.1 (3) |
Cl3—Nb1—Cl1 | 167.47 (2) | C6—C7—H7 | 120.5 |
Cl2—Nb1—Cl1 | 89.09 (2) | C8—C7—H7 | 120.5 |
O1—Nb1—N1 | 179.35 (8) | C9—C8—C7 | 119.7 (3) |
Cl4—Nb1—N1 | 81.56 (5) | C9—C8—H8 | 120.2 |
Cl3—Nb1—N1 | 82.83 (5) | C7—C8—H8 | 120.2 |
Cl2—Nb1—N1 | 82.04 (5) | C10—C9—C8 | 119.6 (3) |
Cl1—Nb1—N1 | 84.78 (5) | C10—C9—H9 | 120.2 |
C1—N1—C5 | 117.5 (2) | C8—C9—H9 | 120.2 |
C1—N1—Nb1 | 121.35 (16) | N2—C10—C9 | 119.1 (3) |
C5—N1—Nb1 | 121.08 (15) | N2—C10—H10 | 120.5 |
C6—N2—C10 | 122.9 (2) | C9—C10—H10 | 120.5 |
C6—N2—H2N | 118.5 | N3—C11—C12 | 120.3 (3) |
C10—N2—H2N | 118.5 | N3—C11—H11 | 119.8 |
C11—N3—C15 | 123.0 (2) | C12—C11—H11 | 119.8 |
C11—N3—H3N | 118.5 | C11—C12—C13 | 118.6 (3) |
C15—N3—H3N | 118.5 | C11—C12—H12 | 120.7 |
N1—C1—C2 | 123.3 (2) | C13—C12—H12 | 120.7 |
N1—C1—H1 | 118.3 | C12—C13—C14 | 120.4 (3) |
C2—C1—H1 | 118.3 | C12—C13—H13 | 119.8 |
C3—C2—C1 | 118.3 (2) | C14—C13—H13 | 119.8 |
C3—C2—H2 | 120.8 | C15—C14—C13 | 118.7 (3) |
C1—C2—H2 | 120.8 | C15—C14—H14 | 120.6 |
C4—C3—C2 | 119.0 (2) | C13—C14—H14 | 120.6 |
C4—C3—H3 | 120.5 | N3—C15—C14 | 118.9 (3) |
C2—C3—H3 | 120.5 | N3—C15—H15 | 120.5 |
C3—C4—C5 | 119.0 (2) | C14—C15—H15 | 120.5 |
C3—C4—H4 | 120.5 | ||
Cl4—Nb1—N1—C1 | 127.21 (19) | Nb1—N1—C5—C4 | −178.00 (18) |
Cl3—Nb1—N1—C1 | −142.97 (19) | C3—C4—C5—N1 | −0.2 (4) |
Cl2—Nb1—N1—C1 | −50.97 (18) | C10—N2—C6—C7 | −0.3 (4) |
Cl1—Nb1—N1—C1 | 38.83 (18) | N2—C6—C7—C8 | −0.1 (4) |
Cl4—Nb1—N1—C5 | −54.31 (17) | C6—C7—C8—C9 | 0.3 (4) |
Cl3—Nb1—N1—C5 | 35.51 (17) | C7—C8—C9—C10 | −0.3 (4) |
Cl2—Nb1—N1—C5 | 127.51 (18) | C6—N2—C10—C9 | 0.3 (4) |
Cl1—Nb1—N1—C5 | −142.68 (18) | C8—C9—C10—N2 | −0.1 (4) |
C5—N1—C1—C2 | −0.4 (4) | C15—N3—C11—C12 | 0.2 (4) |
Nb1—N1—C1—C2 | 178.12 (19) | N3—C11—C12—C13 | −0.6 (4) |
N1—C1—C2—C3 | 0.0 (4) | C11—C12—C13—C14 | 0.2 (4) |
C1—C2—C3—C4 | 0.3 (4) | C12—C13—C14—C15 | 0.6 (4) |
C2—C3—C4—C5 | −0.2 (4) | C11—N3—C15—C14 | 0.7 (4) |
C1—N1—C5—C4 | 0.5 (4) | C13—C14—C15—N3 | −1.1 (4) |
D—H···A | D—H | H···A | D···A | D—H···A |
N2—H2N···Cl5 | 0.88 | 2.32 | 3.108 (2) | 150 |
N3—H3N···Cl5 | 0.88 | 2.16 | 3.015 (2) | 163 |
Experimental details
Crystal data | |
Chemical formula | (C5H6N)2Cl[NbCl4O(C5H5N)] |
Mr | 525.48 |
Crystal system, space group | Monoclinic, P21/c |
Temperature (K) | 100 |
a, b, c (Å) | 7.6122 (9), 20.091 (2), 13.8061 (16) |
β (°) | 99.328 (2) |
V (Å3) | 2083.5 (4) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 1.23 |
Crystal size (mm) | 0.42 × 0.34 × 0.33 |
Data collection | |
Diffractometer | Bruker SMART1000 CCD area-detector diffractometer |
Absorption correction | Multi-scan SADABS; Bruker, 2003) |
Tmin, Tmax | 0.60, 0.66 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 16884, 4257, 4097 |
Rint | 0.025 |
(sin θ/λ)max (Å−1) | 0.625 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.030, 0.079, 1.11 |
No. of reflections | 4257 |
No. of parameters | 226 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 1.44, −0.96 |
Computer programs: SMART (Bruker, 2003), SAINT (Bruker, 2003), SAINT, SHELXTL (Bruker, 2003), SHELXTL.
Nb1—O1 | 1.7643 (17) | Nb1—Cl2 | 2.4029 (7) |
Nb1—Cl4 | 2.3992 (6) | Nb1—Cl1 | 2.4127 (6) |
Nb1—Cl3 | 2.4002 (7) | Nb1—N1 | 2.5119 (19) |
O1—Nb1—Cl4 | 98.45 (6) | Cl3—Nb1—Cl1 | 167.47 (2) |
O1—Nb1—Cl3 | 96.52 (6) | Cl2—Nb1—Cl1 | 89.09 (2) |
Cl4—Nb1—Cl3 | 88.77 (2) | O1—Nb1—N1 | 179.35 (8) |
O1—Nb1—Cl2 | 97.97 (6) | Cl4—Nb1—N1 | 81.56 (5) |
Cl4—Nb1—Cl2 | 163.50 (2) | Cl3—Nb1—N1 | 82.83 (5) |
Cl3—Nb1—Cl2 | 90.97 (3) | Cl2—Nb1—N1 | 82.04 (5) |
O1—Nb1—Cl1 | 95.87 (6) | Cl1—Nb1—N1 | 84.78 (5) |
Cl4—Nb1—Cl1 | 87.63 (2) |
D—H···A | D—H | H···A | D···A | D—H···A |
N2—H2N···Cl5 | 0.88 | 2.32 | 3.108 (2) | 150 |
N3—H3N···Cl5 | 0.88 | 2.16 | 3.015 (2) | 163 |
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Recently, we reported a niobium(V) complex, (pyH)[O═NbCl4(py)].py (py is pyridine), (I) (see scheme) (Guzei et al., 2002), which imposed an interesting crystallographic problem. The assignment of the space group was problematic due to pseudosymmetry, and a careful differentiation between space groups Pnc2 and Pmna was necessary. We have since isolated the title compound, (pyH)2[O═NbCl4(py)]Cl, (II), which presents a different issue, in that the Nb═O double bond is considerably longer than that in (I), and we report here a discussion of this fact based on the results of crystallographic data mining and theoretical computations.
The asymmetric unit of (II) contains four discrete ions: the niobium(V) anion [O═NbCl4(py)]−, (III), two pyridinium cations and a Cl− anion (Fig. 1). The pyridinium cations form two unequal hydrogen-bonding interactions with the Cl− counterion. The N2—H2N···Cl5 interaction is somewhat weaker than the N3—H3N···Cl5 bond, as revealed by the donor–acceptor distances and N—H···Cl angles (Table 2). The charge-assisted hydrogen bonds are medium-strong and agree well with the average N···Cl separation [3.05 (5) Å] calculated for 18 hydrogen-bonding contacts between pyridinium and Cl− in 13 relevant structures reported to the Cambridge Structural Database (CSD, Version 5.27, update of May 2006; Allen, 2002). The N—H···Cl angle in these complexes averaged 164 (8)°.
Compound (II) is a congener of complexes (I) and (H2aza-aza)[O═ NbCl4Haza)]·0.5CH2Cl2 (Haza is 7-azaindole; Poitras & Beauchamp, 1994). We have also optimized the geometry of the C2v symmetrical [O═NbCl4(py)]− anion at the pbe1pbe/SDD level of theory using GAUSSIAN03 (Frisch et al., 2004) and will refer to it as (III-DFT) (DFT is density functional theory).
In compounds (I), (II), (III-DFT) and (IV), the Nb metal center is in a distorted octahedral environment, with the equatorial plane formed by four Cl atoms. The Nb—Cl distances average 2.400 (4), 2.404 (6), 2.50 and 2.387 (8) Å, respectively. The experimental values are typical and in accord with each other. However, the theoretical value is ~4% larger. The Nb atom is displaced from the equatorial plane toward the oxo ligand by 0.303 (2), 0.3014 (4), 0.37 and 0.261 (2) Å, respectively. The Nb—N bond lengths are 2.486 (4), 2.5119 (19), 2.52 and 2.534 (11) Å, respectively. The experimental bonds are all statistically different, and the theoretical value is within the range of the observed bond lengths. The somewhat longer Nb—N distance in (IV) is probably a result of intramolecular hydrogen-bonding interactions between the coordinated aza ligand and two equatorial Cl atoms (see scheme).
Of primary interest are the differences in the Nb═O bond lengths, which are 1.706 (3), 1.7643 (17), 1.73 and 1.691 (9) Å, respectively. Theoretical DFT bond distances are typically longer than the observed ones by a few percent, which is the case for (I) (1.4%) and (IV) (2.3%). However, in the case of (III), the theoretical distance is shorter by 1.9%, or 0.034 Å. The Nb═O bond lengths in the chemically identical anions in (I) and (II) differ by 0.058 Å, a value exceeding the precision of a typical crystallographic experiment by a factor ~15. This 0.058 Å difference in the Nb═O bond lengths is in sharp contrast with the data observed for several related compounds. The corresponding difference for the [O═NbCl4(NCMe)]− monoanion in [PPh3Me][O═NbCl4(NCMe)] (Hiller et al., 1984) and Na[O═NbCl4(NCMe)].2(10-crown-5)2 (Ruhlandt-Senge & Müller, 1991) is 0.008 Å, for [O═NbCl4(THF)]− (THF is tetrahydrofuran) in [chloro-(η5-cyclopentadienyl)tetrakis(tert-butylisocyanide)niobium][O═NbCl4(THF)]·THF (Aspinall et al., 1984) and [hexakis(µ2-acetato-O,O')-bis(µ3-oxo)-tris(tetrahydrofuran)-triniobium] [O═NbCl4(THF)]·THF (Cotton et al., 1988) it is 0.017 Å, and for [O═NbCl4(H2O)]− in (4,5-diazoniafluoren-9-one)2[O═ NbCl4(H2O)]Cl (Balagopalakrishna et al., 1996), [1,2,3-tris(dimethylamino)cyclopropenylium][O═NbCl4(H2O)] (Schäfer et al., 1991) and [PPh4][O═NbCl4(H2O)] (Klingelhofer & Muller, 1984), the range is 0.038 Å.
It has been suggested that the standard uncertainites for atomic coordinates reported for individual crystallographic experiments are underestimated by a factor of 1.4–1.45 (Taylor & Kennard, 1986), but even if the s.u.s for the Nb═O bond distances in (I) and (II) are tripled, the bonds still remain statistically different. While both distances fall in the expected range for Nb═O double bonds of 1.66—1.77 Å (Nugent & Mayer, 1988), we sought to formulate an explanation for the length of the Nb═O double bond in (II).
Firstly, we observe that the Nb—Cl, C–C and C–N distances in (I) and (II) are statistically indistinguishable. Thus, a systematic error is unlikely. Secondly, we observe that both Nb═O and Nb—N bond distances in (II) are longer than those in (I). This cannot be attributed to a trans correlation, since the elongation of both bonds is simultaneous. An examination of intermolecular contacts in the crystal structure does not reveal the presence of weak C—H···O or C—H···Cl hydrogen-bonding interactions that could result in such elongations.
Our next step was an evaluation of energies attributable to the conformational changes of (III-DFT). We performed minimizations for (III-DFT) to discover that its staggered conformation (torsion angle Cl—Nb—N—C = 45°) is 1.5 kcal mol−1 (1 kcal mol−1 = 4.184 kJ mol−1) more stable than the eclipsed conformation (torsion angle Cl—Nb—N—C = 0°). These dihedral angles are 38.29 (10)° in (I) and 35.51 (17) in (II), and in both cases the molecular symmetry is approximately C2 rather than C2v.
A relaxed potential-energy surface scan of (III-DFT), in which the Nb═O distance was varied between 1.65 and 1.80 Å in 0.03 Å increments and the Nb—N distances were varied between 2.45 and 2.54 Å in 0.03 Å steps, demonstrated that the energy difference among all the optimized geometries did not exceed 4.3 kcal mol−1. In particular, the optimized geometry of (III-DFT) with Nb═O and Nb—N distances constrained to be identical to those in (I) (1.706 and 2.486 Å, respectively) is 0.36 kcal more stable than the optimized geometry of (III-DFT) with the Nb═O and Nb—N distances constrained to equal those in (II) (1.7643 and 2.5119 Å, respectively). The strength of the N—H···Cl hydrogen bonds present in the lattice of (II) is estimated to be between 10 and 24 kcal mol−1. Since the geometry of the [O═NbCl4(py)]− anion in (I) and (II) differs from the idealized C2v symmetric staggered conformation in both bond distances and torsion angles, it is proposed that the lattice energy and crystal forces that cause the deformation of the ideal geometry of [O═NbCl4(py)]− also result in the extended axial bond distances about the Nb metal center. From the frequentist point of view, the probability that the two Nb═O bonds in question are equal is less than 0.01%. On the other hand, in view of the fact that the [O═NbCl4(py)]− anions in (I) and (II) are chemically equivalent and the energy difference between them is small, the Bayesian approach is to conclude that this difference between two chemically equivalent Nb═O double bonds is inconsequential. Nonetheless, the fact that the Nb═O bond in (II) is longer than that in (III-DFT) and substantially longer than that in (I) is worrisome. Unfortunately, acquiring enough statistics for anions (III) by collecting several datasets on crystals of (I) and (II) was not feasible, due to the instability of the crystals and their scarce number; complex (II) formed adventitiously and there is no established synthetic procedure for its preparation.
To date, the Nb═O distance in (II) at 1.7643 (17) Å is the longest among those reported for ten six-coordinate monoanionic oxo NbV complexes in the CSD.