Determination of Hydrogen Bond Structure in Water versus Aprotic Environments To Test the Relationship Between Length and Stability.

Hydrogen bonds profoundly influence the architecture and activity of biological macromolecules. Deep appreciation of hydrogen bond contributions to biomolecular function thus requires a detailed understanding of hydrogen bond structure and energetics and the relationship between these properties. Hydrogen bond formation energies (ΔGf) are enormously more favorable in aprotic solvents than in water, and two classes of contributing factors have been proposed to explain this energetic difference, focusing respectively on the isolated and hydrogen-bonded species: (I) water stabilizes the dissociated donor and acceptor groups much better than aprotic solvents, thereby reducing the driving force for hydrogen bond formation; and (II) water lengthens hydrogen bonds compared to aprotic environments, thereby decreasing the potential energy within the hydrogen bond. Each model has been proposed to provide a dominant contribution to ΔGf, but incisive tests that distinguish the importance of these contributions are lacking. Here we directly test the structural basis of model II. Neutron crystallography, NMR spectroscopy, and quantum mechanical calculations demonstrate that O-H···O hydrogen bonds in crystals, chloroform, acetone, and water have nearly identical lengths and very similar potential energy surfaces despite ΔGf differences >8 kcal/mol across these solvents. These results rule out a substantial contribution from solvent-dependent differences in hydrogen bond structure and potential energy after association (model II) and thus support the conclusion that differences in hydrogen bond ΔGf are predominantly determined by solvent interactions with the dissociated groups (model I). These findings advance our understanding of universal hydrogen-bonding interactions and have important implications for biology and engineering.

Revisiting the polyoxometalate-based late-transition-metal-oxo complexes: the "oxo wall" stands.

Terminal oxo complexes of the late transition metals Pt, Pd, and Au have been reported by us in Science and Journal of the American Chemical Society. Despite thoroughness in characterizing these complexes (multiple independent structural methods and up to 17 analytical methods in one case), we have continued to study these structures. Initial work on these systems was motivated by structural data from X-ray crystallography and neutron diffraction and (17)O and (31)P NMR signatures which all indicated differences from all previously published compounds. With significant new data, we now revisit these studies. New X-ray crystal structures of previously reported complexes K(14)[P(2)W(19)O(69)(OH(2))] and "K(10)Na(3)[Pd(IV)(O)(OH)WO(OH(2))(PW(9)O(34))(2)]" and a closer examination of these structures are provided. Also presented are the (17)O NMR spectrum of an (17)O-enriched sample of [PW(11)O(39)](7-) and a careful combined (31)P NMR-titration study of the previously reported "K(7)H(2)[Au(O)(OH(2))P(2)W(20)O(70)(OH(2))(2)]." These and considerable other data collectively indicate that previously assigned terminal Pt-oxo and Au-oxo complexes are in fact cocrystals of the all-tungsten structural analogues with noble metal cations, while the Pd-oxo complex is a disordered Pd(II)-substituted polyoxometalate. The neutron diffraction data have been re-analyzed, and new refinements are fully consistent with the all-tungsten formulations of the Pt-oxo and Au-oxo polyoxometalate species.

Development and loss of ferromagnetism controlled by the interplay of Ge concentration and Mn vacancies in structurally modulated Y4Mn(1-x)Ga(12-y)Ge(y).

The cubic intermetallic phase Y(4)Mn(1-x)Ga(12-y)Ge(y) (x = 0-0.26, y = 0-4.0) has been isolated from a molten gallium flux reaction. It presents a rare example of a system where ferromagnetism can be induced by controlling the vacancies of the magnetic centers. The Y(4)PdGa(12) type crystal structure is made up of a corner-sharing octahedral network of Ga and Ge atoms with Mn atoms at the centers of half the octahedra and Y atoms in the voids. At the highest Ge concentration, y = 4.0, the Mn site is nearly fully occupied, x = 0.05, and the samples are paramagnetic. At a lower Ge concentration, y = 1.0, Mn deficiency develops with x = 0.10. Surprisingly, strong ferromagnetism is observed with T(c) = 223 K. When Ge is excluded, y = 0, Mn is substantially deficient at x = 0.26 and ferromagnetism is maintained with a T(c) of approximately 160 K. In addition, a 6-fold modulated superstructure appears owing to an ordered slab-like segregation of Mn atoms and vacancies. Corresponding bond distortions propagate throughout the octahedral Ga network. Structure-property relationships are examined with X-ray and neutron diffraction, magnetic susceptibility, and electrical resistivity measurements.

Hydrogen bonding effects on the electronic configuration of five-coordinate high-spin iron(II) porphyrinates.

The characterization of a new five-coordinate derivative of (2-methylimidazole)(tetraphenylporphinato)iron(II) provides new and unique information about the effects of forming a hydrogen bond to the coordinated imidazole on the geometric and electronic structure of iron in these species. The complex studied has two crystallographically distinct iron sites; one site has an axial imidazole ligand modified by an external hydrogen bond, and the other site has an axial imidazole ligand with no external interactions. The iron atoms at the two sites have distinct geometric features, as revealed in their molecular structures, and distinct electronic structures, as shown by Mössbauer spectroscopy, although both are high spin (S = 2). The molecule with the external hydrogen bond has longer equatorial Fe-N(p) bonds, a larger displacement of the iron atom out of the porphyrin plane, and a shorter axial bond compared to its counterpart with no hydrogen bonding. The Mössbauer features are distinct for the two sites, with differing quadrupole splitting and isomer shift values and probably differing signs for the quadrupole splitting as shown by variable-temperature measurements in applied magnetic field. These features are consistent with a significant change in the nature of the doubly populated d orbital and are all in the direction of the dichotomy displayed by related imidazole and imidazolate species where deprotonation leads to major differences. The results points out the possible effects of strong hydrogen bonding in heme proteins.

Sigma-borane complexes of iridium: synthesis and structural characterization.

Reaction of NaBH4 with (tBuPOCOP)IrHCl affords the previously reported complex (tBuPOCOP)IrH2(BH3) (1) (tBuPOCOP = kappa(3)-C6H3-1,3-[OP(tBu)2]2). The structure of 1 determined from neutron diffraction data contains a B-H sigma-bond to iridium with an elongated B-H bond distance of 1.45(5) A. Compound 1 crystallizes in the space group P1 (Z = 2) with a = 8.262 (5) A, b = 12.264 (5) A, c = 13.394 (4) A, and V = 1256.2 (1) A(3) (30 K). Complex 1 can also be prepared by reaction of BH3 x THF with (tBuPOCOP)IrH2. Reaction of (tBuPOCOP)IrH2 with pinacol borane gave initially complex 2, which is assigned a structure analogous to that of 1 based on spectroscopic measurements. Complex 2 evolves H2 at room temperature leading to the borane complex 3, which is formed cleanly when 2 is subjected to dynamic vacuum. The structure of 3 has been determined by X-ray diffraction and consists of the (tBuPOCOP)Ir core with a sigma-bound pinacol borane ligand in an approximately square planar complex. Compound 3 crystallizes in the space group C2/c (Z = 4) with a = 41.2238 (2) A, b = 11.1233 (2) A, c = 14.6122 (3) A, and V = 6700.21 (19) A(3) (130 K). Reaction of (tBuPOCOP)IrH2 with 9-borobicyclononane (9-BBN) affords complex 4. Complex 4 displays (1)H NMR resonances analogous to 1 and exists in equilibrium with (tBuPOCOP)IrH2 in THF solutions.

Variable temperature neutron diffraction and X-ray charge density studies of tetraacetylethane.

Single crystal neutron diffraction data have been collected on a sample of enolized 3,4-diacetyl-2,5-hexanedione (tetraacetylethane, TAE) at five temperatures between 20 and 298 K to characterize the temperature-dependent behavior of the short, strong, intramolecular hydrogen bond. Upon decreasing the temperature from 298 K to 20 K, the O2-H1 distance decreases from 1.171(11) to 1.081(2) A and the O1...H1 distance increases from 1.327(10) to 1.416(6) A. The convergence of the C-O bond lengths from inequivalent distances at low temperature to identical values (1.285(4) A) at 298 K is consistent with a resonance-assisted hydrogen bond. However, a rigid bond analysis indicates that the structure at 298 K is disordered. The disorder vanishes at lower temperatures. Short intermolecular C-H...O contacts may be responsible for the ordering at low temperature. The intramolecular O...O distance (2.432 +/- 0.006 A) does not change with temperature. X-ray data at 20 K were measured to analyze the charge density and to gain additional insight into the nature of the strong hydrogen bond. Quantum mechanical calculations demonstrate that periodic boundary conditions provide significant enhancement over gas phase models in that superior agreement with the experimental structure is achieved when applying periodicity. One-dimensional potential energy calculations followed by quantum treatment of the proton reproduce the location of the proton nearer to the O2 site reasonably well, although they overestimate the O-H distance at low temperatures. The choice of the single-point energy calculation strategy for the proton potential is justified by the fact that the proton is preferably located nearer to O2 rather than being equally distant to O1 and O2 or evenly distributed (disordered) between them.

Molecular hydrogen occupancy in binary THF-H2 clathrate hydrates by high resolution neutron diffraction.

We have determined the time-space average filling of hydrogen molecules in a binary tetrahydrofuran (THF)-d(8) + D(2) sII clathrate hydrate using high resolution neutron diffraction. The filling of hydrogen in the lattice of a THF-d(8) clathrate hydrate occurred upon pressurization. The hydrogen molecules were localized in the small dodecahedral cavities at 20 K, with nuclear density from the hydrogen approximately spherically distributed and centered in the small cavity. With a formation pressure of 70 MPa, molecular hydrogen was found to only singly occupy the sII small cavity. This result helps explain discrepancies about the hydrogen occupancy in the THF binary hydrate system.

Direct observation of eta 2-imine formation through beta-H abstraction between amide ligands. Neutron and X-ray diffraction structure of a dihydride imine ditantalum complex.

Reactions of Ta(NMe2)5 with D2SiR'Ph (R' = Me, Ph) were found to give a dideuteride eta 2-imine complex (Me2N)3Ta(mu-D)2(mu-N-eta 2-N,C-CH2NMe)Ta(NMe2)3(1-d2) through C-H activation of an amide ligand via beta-H abstraction, and the structure of 1 was confirmed by single crystal neutron and X-ray diffraction.

Strong N-H⋠⋠⋠O Hydrogen Bonding in a Model Compound of the Catalytic Triad in Serine Proteases.

Low-barrier hydrogen bond (LBHB) involvement in enzyme catalysis is examined by analysis of experimental nuclear and electron densities of a model compound for the catalytic triad in serine proteases (shown schematically), which is based on a cocrystal of betaine, imidazole, and picric acid. The three short, strong N-H⋅⋅⋅O hydrogen bonds in the structure have varying degrees of covalent bonding contributions suggesting a gradual transition to the LBHB situation.

Ln2 Al3 Si2 (Ln=Ho, Er, Tm): New Silicides from Molten Aluminum-Determination of the Al/Si Distribution with Neutron Crystallography and Metamagnetic Transitions.

Highly metallic compounds with a quasi-one-dimensional structure, the new ternary compounds Ln2 Al3 Si2 (Ln=Ho, Er, Tm) are synthesized in molten aluminum from lanthanoid and silicon as reagents. Their structures show a formally [Al3 Si2 ]6- framework that contains infinite Al zigzag chains and Si-Si dimers and accommodates rows of Ln3+ ions in parallel tunnels. The compounds exhibit metamagnetic transitions at high magnetic fields.

Structural, theoretical and spectroscopic studies of the dichloride hexahydrate cube [Cl2(H2O)6]2-.

The structure of the dichloride hexahydrate cube, [Cl(2)(H(2)O)(6)](2-), as a salt with the tris(diisopropylamino)cyclopropenium cation, [C(3)(N(i)Pr(2))(3)](+), has been determined by low-temperature X-ray and neutron-diffraction studies. H atoms not involved in O-H···Cl bonding are disordered over two 0.5 occupancy sites around the O(6) ring. Calculations of the dianionic cube in the gas phase show remarkably good agreement with the solid-state structures with the exception of short O-H bond distances around the O(6) ring that suggests the involvement of a dynamic process. The cluster was also characterised by single-crystal infrared spectroscopy, and vibrational wavenumbers were found to be in good agreement with hydrogen bonding distances. Dibromide and difluoride hexahydrates were also studied theoretically, and O···O distances were found to decrease in the order difluoride > dichloride > dibromide > (H(2)O)(6) and as O···O···O angles increased towards an almost planar ring in (H(2)O)(6). NMR spectra of a chloroform solution of the hydrated salt at -25 °C is consistent with cluster formation.

Synthesis, structure, and properties of BaAl2Si2.

Single crystals of BaAl2Si2 were grown from an Al molten flux and characterized using single-crystal X-ray diffraction at 10 and 90 K and neutron diffraction at room temperature. BaAl2Si2 crystallizes with the alpha-BaCu2S2 structure type (Pnma), is isostructural with alpha-BaAl2Ge2, and is an open 3D framework compound, where Al and Si form a covalent cagelike network with Ba2+ cations residing in the cages. BaAl2Si2 has a unit cell of a=10.070(3) A, b=4.234(1) A, and c=10.866(3) A, as determined by room-temperature single-crystal neutron diffraction (R1=0.0533, wR2=0.1034). The structure as determined by single-crystal neutron and X-ray diffraction (10 and 90 K) indicates that BaAl2Si2 (Pnma) is strictly isostructural to other (alpha)-BaCu2S2-type structures, requiring site specificity for Al and Si. Unlike BaAl2Ge2, no evidence for an alpha to beta (BaZn2P2-type, I4/mmm) phase transition was observed. This compound shows metallic electronic resistivity and Pauli paramagnetic behavior.

X-ray and neutron diffraction studies of water-encapsulated inside potassium aryloxide aggregates: complementary host-guest stabilization of mono- and dihydrated cages.

The controlled hydrolysis of potassium 2-tert-butylphenoxide or 2-isopropylphenoxide leads to the unexpected encapsulation of the water inside K6O6 hexameric drum aggregates. Encapsulation of the neutral molecules is enabled in these instances through the formation of strong hydrogen bonds and dative interactions between the host and guest.

Better understanding of the species with the shortest Re2(6+) bonds and related Re2)(7+) species with tetraguanidinate paddlewheel structures.

A series of compounds has been made containing quadruply bonded Re2(hpp)4X2 species (hpp = the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2a]pyrimidine), where X is CF3SO3 (1), CF3CO2 (2), and F (3). The distances of 2.1562(7), 2.1711(5), and 2.1959(4) A for 1-3 show significant effects of the sigma and pi electron donating ability of the axial ligands on the metal-metal distance. With the weakly coordinating triflate ligand the Re-Re distance is the shortest for any quadruple bonded species known. In addition to examining the effects of axial ligands on the Re2(hpp)42+ core, our study of the Re2(hpp)43+ core is being extended beyond the preliminary results previously reported in only one compound [Re2(hpp)4Cl2]PF6 (Dalton Trans. 2003, 1218). We now report the structural characterization by both X-ray and neutron diffraction of the compound [Re2(hpp)4F](TFPB)2, 4 (TFPB = the anion tetrakis[3,5-bis(trifluoromethyl)phenyl]borate), and a detailed study by EPR spectroscopy of [Re2(hpp)4Cl2]PF6 at 9.5, 34.5, and 95 GHz frequencies, using dilute fluid solutions, frozen glass, and neat powder, show that the unpaired electron in the [Re2(hpp)4Cl2]+ ion is in an MO of predominant metal character with little mixing from the guanidinate ligands.

Syntheses and structures of asymmetric Bis(silyl) niobocene hydrides.

This paper deals with the preparation and structural investigation of asymmetric bis(silyl) niobocene hydrides, Cp2Nb(SiHMe2)(H)(SiXMe2) (2; X = F (a), Cl (b), Br (c), I (d)) and Cp2Nb(SiXMe2)(H)(SiYMe2) (X,Y= F-I; X not equal Y). Complexes 2a-d were prepared by selective electrophilic activation of the Si-H bond in Cp2Nb(SiHMe2)2(H). The Cp2Nb(SiXMe2)(H)(SiYMe2) complexes were prepared by electrophilic activation of the Si-H bond in 2a-d and, in some cases, by electrophilic exchange of the X halides in Cp2Nb(SiXMe2)2(H) (1) for other halides, Y. The structures of complexes 2b and 2c have been studied by X-ray and neutron diffraction (ND). The ND results unequivocally established that the hydride ligand in 2c is shifted toward the SiBrMe2 ligand and that in 2b is positioned symmetrically between two nonequivalent silyl groups, with the H...SiClMe2 distance being shorter because of the shorter Nb-SiClMe2 bond length. Analysis of the X-ray structures of complexes 2a-d and complexes Cp2Nb(SiXMe2)(H)(SiYMe2) shows that the largest structural distortions are observed for the silyl groups substituted by heavy halogen atoms. These trends are rationalized in terms of stronger interligand hypervalent interactions (IHI) Nb-H...Si-X for heavy atoms X from Group 7.

Terminal gold-oxo complexes.

In contradiction to current bonding paradigms, two terminal Au-oxo molecular complexes have been synthesized by reaction of AuCl3 with metal oxide-cluster ligands that model redox-active metal oxide surfaces. Use of K10[alpha2-P2W17O61].20H2O and K2WO4 (forming the [A-PW9O34]9- ligand in situ) produces K15H2[Au(O)(OH2)P2W18O68].25H2O (1); use of K10[P2W20O70(OH2)2].22H2O (3) produces K7H2[Au(O)(OH2)P2W20O70(OH2)2].27H2O (2). Complex 1 crystallizes in orthorhombic Fddd, with a=28.594(4) A, b=31.866(4) A, c=38.241(5) A, V=34844(7) A3, Z=16 (final R=0.0540), and complex 2 crystallizes in hexagonal P6(3)/mmc, with a=16.1730(9) A, b=16.1730(9) A, c=19.7659(15) A, V=4477.4(5) A3, Z=2 (final R=0.0634). The polyanion unit in 1 is disorder-free. Very short (approximately 1.76 A) Au-oxo distances are established by both X-ray and 30 K neutron diffraction studies, and the latter confirms oxo and trans aqua (H2O) ligands on Au. Seven findings clarify that Au and not W is present in the Au-oxo position in 1 and 2. Five lines of evidence are consistent with the presence of d8 Au(III) centers that are stabilized by the flanking polytungstate ligands in both 1 and 2: redox titrations, electrochemical measurements, 17 K optical spectra, Au L2 edge X-ray absorption spectroscopy, and Au-oxo bond distances. Variable-temperature magnetic susceptibility data for crystalline 1 and 2 establish that both solids are diamagnetic, and 31P and 17O NMR spectroscopy confirm that both remain diamagnetic in solution. Both complexes have been further characterized by FT-IR, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and other techniques.

Structural factors influencing linear M-H-M bonding in bis(dialkylphosphino)methane complexes of nickel.

Structural data for four closely related dinuclear nickel hydride complexes have been compared in order to gain insight into the factors governing the Ni-H-Ni geometries. The derivatives [(dippm)2Ni2X2](mu-H) [dippm = 1,2-bis(diisopropylphosphino)methane] were found to contain a linear Ni-H-Ni bridge, whereas the derivatives [(dcpm)2Ni2X2](mu-H) [dcpm = 1,2-bis(dicyclohexylphosphino)methane] were found to contain a bent Ni-H-Ni bridge. The number of internal and interatomic CH-to-halide contacts of the former were much shorter and more numerous than the latter, suggesting an important role of external forces in bridging hydride geometries.