Characterization and Reactions of [PPh₄]₃[Zr₆Cl₁₈H₅] and Its Deprotonation Products

The octahedral hexazirconium cluster compound [PPh4]3[Zr6Cl18H5] has been structurally characterized by both neutron and X-ray single-crystal diffraction studies. The compound [PPh4]3[Zr6Cl18H5]·3CH2Cl2 crystallizes in the triclinic space group P1̄ with unit cell parameters of a = 15.993(3), b = 22.237(3), and c = 14.670(4) Å, α = 95.31(1), β = 112.07(2), and γ = 82.06(2)°, V = 4784(2) Å3, and Z = 2 at ambient temperature and a = 15.780(6), b = 21.96(3), and c = 14.521(7) Å, α = 94.96(8), β = 111.59(4), and γ = 81.72(5)°, V = 4627(11) Å3, and Z = 2 at T = 15 K. The hydrogen atoms in the cluster anion, [Zr6Cl18H5]3-, were found to be distributed at the centers of the eight triangular faces of the Zr6 octahedron from neutron diffraction data. The occupancy parameters of the sites range from 0.32 to 0.92 with a total of 5.3(1) hydrogen atoms per cluster, close to the value from 1H NMR measurement (5.0). The average Zr−H distance is 1.96(4) Å. A variable temperature 1H NMR study indicated that the cluster hydrogen atoms undergo rapid movement at room temperature. One of the five hydrogen atoms in the cluster [Zr6Cl18H5]3- was readily removed as a proton with primary linear amines with formation of the corresponding ammonium cations, while the cluster anion, [Zr6Cl18H5]3-, was thus converted into a new cluster anion, [Zr6Cl18H4]4-. The feasibility of such a deprotonation reactions is controlled by the size of both the Lewis base and the cavity available on the Zr3 triangular faces of the Zr6 clusters, and also by the basicity of the deprotonating reagents. Two products, [PPh4]4[Zr6Cl18H4]·4CH2Cl2 and [H3NEt]4[Zr6Cl18H4]·4MeCN from the deprotonation reactions were characterized by X-ray crystallography

Study of the N−H···H−B Dihydrogen Bond Including the Crystal Structure of BH₃NH₃ by Neutron Diffraction

Boraneamines tend to have close N−Hδ+···δ-H−B contacts as a result of the intermolecular interaction of the NH proton with the BH bond by a novel type of hydrogen bond (the dihydrogen bond). A CSD structural search provides characteristic metric data for the interaction:  the H···H distance is in the range 1.7−2.2 Å, and the N−H···H group tends to be linear while B−H···H tends to be bent. The reported X-ray structure of BH3NH3 seemed to provide a singular exception in having bent N−H···H and linear B−H···H. Our neutron diffraction structure of BH3NH3 now shows that the B and N atoms must be reversed from the assignment previously published. With the correct assignment we find the expected bent B−H···H and linear N−H···H arrangement in the closest intermolecular N−H···H−B interaction (dHH = 2.02 Å)

False Minima in X-ray Structure Solutions Associated with a “Partial Polar Ambiguity”: Single Crystal X-ray and Neutron Diffraction Studies on the Eight-Coordinate Tungsten Hydride Complexes, W(PMe₃)₄H₂X₂ (X = F, Cl, Br, I) and W(PMe₃)₄H₂F(FHF)

The molecular structures of the eight-coordinate tungsten hydride complexes W(PMe3)4H2X2 (X = F, Cl, Br, I) and W(PMe3)4H2F(FHF) have been determined by single-crystal X-ray diffraction; W(PMe3)4H2Cl2 and W(PMe3)4H2F(FHF) have also been analyzed by single-crystal neutron diffraction, thereby accurately locating the positions of the hydride ligands. The structures of all of these complexes are similar and are based on a trigonal dodecahedron, with a distorted tetrahedral array of PMe3 ligands in which two of the PMe3 ligands are displaced over the halide substituents. However, the initial structures derived for both W(PMe3)4H2Cl2 and W(PMe3)4H2F(FHF) did not exhibit the aforementioned geometry, but were based on an arrangement in which the two transoid-PMe3 ligands are displaced toward the two cis-PMe3 groups, rather than tilted toward the chloride ligands. Interestingly, the unexpected structures for W(PMe3)4H2Cl2 and W(PMe3)4H2F(FHF) were discovered to be the result of an artifact due to the presence of a heavy atom in a polar space group, which allowed the X-ray structure solutions to refine into most deceptive false minima. Specifically, for the structures corresponding to the false minima, the transoid-PMe3 ligands were incorrectly located in positions that are related to their true locations by reflection perpendicular to the polar axis. In effect, the incorrect molecular structures are a composite of the two possible true polar configurations which are related by a reflection perpendicular to the polar axis, i.e. a “partial polar ambiguity”. Of most importance, the solutions corresponding to the false minima are characterized by low R values and well-behaved displacement parameters, so that it is not apparent that the derived structures are incorrect. Thus, for space groups with a polar axis, it is necessary to establish that all of the atoms in the asymmetric unit belong to a single true polar configuration

No Evidence for Proton Transfer along the N−H···O Hydrogen Bond in <i>N</i>-Methylacetamide: Neutron Single Crystal Structure at 250 and 276 K

The crystal structure of N-methylacetamide (C3H7NO), Mr = 73.095, has been determined from single-crystal neutron diffraction data at two temperatures, 250 and 276 K, above and below the previously reported phase transition located at 274 K in this work. Crystal data:  250 K [276 K]:  space group Pn21a [Pn21m], a = 9.671(2) [4.878(1)] Å, b = 6.613(6) [6.567(1)] Å, c = 7.218(1) [7.332(2)] Å; V = 465.7(6) [234.9(5)] Å3; Dn = 1.043 [1.034] g·cm-3, R(F2) = 0.168 [0.134], wR(F2) = 0.062 [0.051], S = 1.18 [1.11]. This new investigation of the structure of N-methylacetamide was undertaken in order to assess a recent suggestion based on inelastic neutron scattering spectroscopy that transfer of the amide proton along the peptide hydrogen bond may be responsible for the vibrational anomalies. While we found no evidence for proton transfer along the N−H···O hydrogen bond (d(NH) = 1.025(17) Å, and d(H···O) = 1.856(14) Å for at T = 250 K) at either temperature evidence for some molecular disorder is present in accord with our previous 13C NMR studies. In addition, we find short intramolecular contacts between the amide hydrogen atom and those on both neighboring methyls, which may well affect the vibrational properties of the respective molecular groups

Absolute Configuration of Chiral Ethanol-1-<i>d</i>: Neutron Diffraction Analysis of the (−)-(1<i>S</i>)-Camphanate Ester of (+)-(<i>R</i>)-Ethanol-1-<i>d</i>

The absolute configuration of (+)-ethanol-1-d has been determined to be R by the single-crystal neutron diffraction analysis of its (−)-camphanate ester. The absolute configuration of the (−)-camphanate group, which served as the chiral reference for the neutron study, was in turn established to be 1S,4R in an X-ray anomalous dispersion study of the complex Cu2(camphanate)4(ethanol)2. These results provide unambiguous confirmation that the optical rotation of (R)-ethanol-1-d (positive) is opposite to that of its higher homologs, (R)-propanol-1-d, (R)-butanol-1-d, and (R)-neopentanol-1-d (all negative), and demonstrates the usefulness of neutron diffraction in determining the absolute configuration of molecules possessing chiral methylene groups (i.e., molecules of the type CHDRR‘). Crystallographic details:  for the neutron analysis of (+)-(R)-ethyl-1-d (−)-(1S)-camphanate:  space group P21212 (orthorhombic), a = 6.422(1) Å, b = 21.004(4) Å, c = 9.275(2) Å, V = 1251.1(7) Å3, Z = 4; R(F2) = 0.083, and wR(F2) = 0.075 for 1114 reflections. For the X-ray analysis of Cu2(camphanate)4(ethanol)2: space group P1 (triclinic), a = 11.086(3), b = 11.244(3), c = 13.293(4) Å, α = 111.59(2), β = 107.71(2), and γ = 105.56(3)°, V = 1397.6(7) Å3, Z = 1; R(F) = 0.054, wR(F) = 0.058 for 3672 reflections

Neutron Diffraction Structure Analysis of a Triply-Bridged Binuclear Cobalt Hydride Complex, [(η⁵-Cp*)Co]₂H₃

Neutron Diffraction Structure Analysis of a Triply-Bridged Binuclear Cobalt Hydride Complex, [(η5-Cp*)Co]2H3</sub

New Mode of Coordination for the Dinitrogen Ligand: Formation, Bonding, and Reactivity of a Tantalum Complex with a Bridging N₂ Unit That Is Both Side-On and End-On

The reaction of a mixture of 1 equiv of PhPH2 and 2 equiv of PhNHSiMe2CH2Cl with 4 equiv of BunLi followed by the addition of THF generates the lithiated ligand precursor [NPN]Li2·(THF)2 (where [NPN] = PhP(CH2SiMe2NPh)2). The reaction of [NPN]Li2·(THF)2 with TaMe3Cl2 produces [NPN]TaMe3, which reacts under H2 to yield the diamagnetic dinuclear Ta(IV) tetrahydride ([NPN]Ta)2(μ-H)4. This hydride reacts with N2 with the loss of H2 to produce ([NPN]Ta(μ-H))2(μ-η1:η2-N2), which was characterized both in solution and in the solid state, and contains strongly activated N2 bound in the unprecedented side-on end-on dinuclear bonding mode. A density functional theory calculation on the model complex [(H3P)(H2N)2Ta(μ-H)]2(μ-η1:η2-N2) provides insight into the molecular orbital interactions involved in the side-on end-on bonding mode of dinitrogen. The reaction of ([NPN]Ta(μ-H))2(μ-η1:η2-N2) with propene generates the end-on bound dinitrogen complex ([NPN]Ta(CH2CH2CH3))2(μ-η1:η1-N2), and the reaction of [NPN]Li2·(THF)2 with NbCl3(DME) generates the end-on bound dinitrogen complex ([NPN]NbCl)2(μ-η1:η1-N2). These two end-on bound dinitrogen complexes provide evidence that the bridging hydride ligands are responsible for the unusual bonding mode of dinitrogen in ([NPN]Ta(μ-H))2(μ-η1:η2-N2). The dinitrogen moiety in the side-on end-on mode is amenable to functionalization; the reaction of ([NPN]Ta(μ-H))2(μ-η1:η2-N2) with PhCH2Br results in C−N bond formation to yield [NPN]Ta(μ-η1:η2-N2CH2Ph)(μ-H)2TaBr[NPN]. Nitrogen-15 NMR spectral data are provided for all the tantalum−dinitrogen complexes and derivatives described

False Minima in X-ray Structure Solutions Associated with a “Partial Polar Ambiguity”: Single Crystal X-ray and Neutron Diffraction Studies on the Eight-Coordinate Tungsten Hydride Complexes, W(PMe₃)₄H₂X₂ (X = F, Cl, Br, I) and W(PMe₃)₄H₂F(FHF)

The molecular structures of the eight-coordinate tungsten hydride complexes W(PMe3)4H2X2 (X = F, Cl, Br, I) and W(PMe3)4H2F(FHF) have been determined by single-crystal X-ray diffraction; W(PMe3)4H2Cl2 and W(PMe3)4H2F(FHF) have also been analyzed by single-crystal neutron diffraction, thereby accurately locating the positions of the hydride ligands. The structures of all of these complexes are similar and are based on a trigonal dodecahedron, with a distorted tetrahedral array of PMe3 ligands in which two of the PMe3 ligands are displaced over the halide substituents. However, the initial structures derived for both W(PMe3)4H2Cl2 and W(PMe3)4H2F(FHF) did not exhibit the aforementioned geometry, but were based on an arrangement in which the two transoid-PMe3 ligands are displaced toward the two cis-PMe3 groups, rather than tilted toward the chloride ligands. Interestingly, the unexpected structures for W(PMe3)4H2Cl2 and W(PMe3)4H2F(FHF) were discovered to be the result of an artifact due to the presence of a heavy atom in a polar space group, which allowed the X-ray structure solutions to refine into most deceptive false minima. Specifically, for the structures corresponding to the false minima, the transoid-PMe3 ligands were incorrectly located in positions that are related to their true locations by reflection perpendicular to the polar axis. In effect, the incorrect molecular structures are a composite of the two possible true polar configurations which are related by a reflection perpendicular to the polar axis, i.e. a “partial polar ambiguity”. Of most importance, the solutions corresponding to the false minima are characterized by low R values and well-behaved displacement parameters, so that it is not apparent that the derived structures are incorrect. Thus, for space groups with a polar axis, it is necessary to establish that all of the atoms in the asymmetric unit belong to a single true polar configuration

Structures of Furanosides: Density Functional Calculations and High-Resolution X-ray and Neutron Diffraction Crystal Structures

Highly accurate and precise crystal structures of methyl α-d-arabinofuranoside, methyl β-d-ribofuranoside, methyl α-d-lyxofuranoside, and methyl α-d-xylofuranoside have been determined at 100 K by X-ray crystallography. The structures of methyl α-d-arabinofuranoside and methyl β-d-ribofuranoside have also been determined at 15 K by neutron diffraction. Equilibrium (re) geometries of the same compounds were computed by means of density functional methods using a variety of exchange-correlation functionals and a sequence of basis sets. The validity of the computed results was assessed by several criteria including agreement between computed and observed bond distances and bond angles, agreement between computed and observed ring conformations, and basis set convergence of the computed geometrical parameters. Particular reference was made to computed internal hydrogen bond parameters, which are especially sensitive to the quality of the theoretical treatment. Because of the intrinsic sensitivity of the conformation of the five-membered ring to bond lengths and bond angles, molecular mechanics and small basis set SCF treatments are wholly inadequate. Local density functional theory also fails because of a tendency to strongly underestimate internal hydrogen bond distances. When the B3LYP exchange-correlation functional is used, bond lengths and bond angles agree with the neutron diffraction values to within their experimental uncertainty and the ring conformation is qualitatively correct, as long as a basis set of at least double-ζ plus polarization quality (such as cc-pVDZ) is used. Further expansion of the basis set leads to more accurate equilibrium bond lengths and bond angles but does not appreciably affect the ring conformation. For methyl α-d-arabinofuranoside, methyl β-d-ribofuranoside, and methyl α-d-xylofuranoside, there is very good correspondence between the best computed and observed ring conformations, even though some intermolecular hydrogen bonds in the crystal give way to internal hydrogen bonds in the predicted gas-phase structures. On the other hand, in the case of methyl α-d-lyxofuranoside, an O2H···O4 internal hydrogen bond between the ring oxygen O4 and the hydroxyl hydrogen of a ring carbon (O2H) in the computed structure leads to a very large change of ring conformation from the northeast corner of the pseudorotation pathway (P = 28°, crystal) to the southeast corner (P = 130°, computed)

New Mode of Coordination for the Dinitrogen Ligand: Formation, Bonding, and Reactivity of a Tantalum Complex with a Bridging N₂ Unit That Is Both Side-On and End-On

The reaction of a mixture of 1 equiv of PhPH2 and 2 equiv of PhNHSiMe2CH2Cl with 4 equiv of BunLi followed by the addition of THF generates the lithiated ligand precursor [NPN]Li2·(THF)2 (where [NPN] = PhP(CH2SiMe2NPh)2). The reaction of [NPN]Li2·(THF)2 with TaMe3Cl2 produces [NPN]TaMe3, which reacts under H2 to yield the diamagnetic dinuclear Ta(IV) tetrahydride ([NPN]Ta)2(μ-H)4. This hydride reacts with N2 with the loss of H2 to produce ([NPN]Ta(μ-H))2(μ-η1:η2-N2), which was characterized both in solution and in the solid state, and contains strongly activated N2 bound in the unprecedented side-on end-on dinuclear bonding mode. A density functional theory calculation on the model complex [(H3P)(H2N)2Ta(μ-H)]2(μ-η1:η2-N2) provides insight into the molecular orbital interactions involved in the side-on end-on bonding mode of dinitrogen. The reaction of ([NPN]Ta(μ-H))2(μ-η1:η2-N2) with propene generates the end-on bound dinitrogen complex ([NPN]Ta(CH2CH2CH3))2(μ-η1:η1-N2), and the reaction of [NPN]Li2·(THF)2 with NbCl3(DME) generates the end-on bound dinitrogen complex ([NPN]NbCl)2(μ-η1:η1-N2). These two end-on bound dinitrogen complexes provide evidence that the bridging hydride ligands are responsible for the unusual bonding mode of dinitrogen in ([NPN]Ta(μ-H))2(μ-η1:η2-N2). The dinitrogen moiety in the side-on end-on mode is amenable to functionalization; the reaction of ([NPN]Ta(μ-H))2(μ-η1:η2-N2) with PhCH2Br results in C−N bond formation to yield [NPN]Ta(μ-η1:η2-N2CH2Ph)(μ-H)2TaBr[NPN]. Nitrogen-15 NMR spectral data are provided for all the tantalum−dinitrogen complexes and derivatives described