Contents 1. Introduction and Historical Perspective 4152 2. Types and Synthesis of H 2 Complexes 4154 2.1. Stable H 2 Complexes 4154 2.1.1. Complexes Synthesized by Addition of H 2 Gas to an Unsaturated Precursor 4156 2.1.2. Complexes with the Most Weak, Reversible H 2 Binding and the Shortest H−H Distances 4156 2.1.3. Complexes Prepared from H 2 Gas by Ligand Displacement or Reduction 4157 2.1.4. Protonation of a Hydride Complex 4158 2.1.5. Other Methods of Preparation 4158 2.2. H 2 Complexes Unstable at Room Temperature 4158 2.2.1. Organometallic Complexes Observed at Low Temperature in Rare Gas or Other Media 4158 2.2.2. Binding of H 2 to Bare Metal Atoms, Ions, and Surfaces 4160 3. Structure and Bonding of H 2 Complexes 4160 3.1. Theoretical Analysis of Nonclassical Bonding of H 2 4160 3.2. M f H 2 Backdonation and Influence of CO Ligands on Activation of H 2 4161 4. Properties and Spectroscopic Diagnostics for H 2 Complexes 4163 4.1. Properties of H 2 Complexes 4163 4.2. Spectroscopic and Other Diagnostics for H 2 Complexes 4164 5. Vibrational Spectroscopy of H 2 Complexes 4164 6. Dynamics of H 2 and Hydride Complexes 4165 7. Thermodynamics, Kinetics, and Isotope Effects for H 2 Binding 4168 8. Biological Activation of H 2 in Hydrogenase Enzymes 4169 8.1. Introduction and Structure and Function of Hydrogenases 4169 8.2. Dihydrogen Coordination and Organometallic Chemistry Relevant to H 2 ases 4171 8.2.1. Introduction 4171 8.2.2. Formation of H 2 Ligands by Protonation and Factors That Control H 2 Binding and Activation in H 2 ases 4173 8.2.3. Heterolytic Cleavage and Acidity of H 2 Coordinated to Metal Complexes 4174 8.2.4. Intermolecular Heterolytic Cleavage of Coordinated H 2 4175 8.2.5. Intramolecular Heterolytic Cleavage of H 2 4176 8.2.6. Proton Transfer to Anions 4180 8.2.7. Strength of Binding of H 2 Compared to Water and N 2 . Importance of Entropy Effects 4180 8.2.8. Isotopic Exchange and Other Intramolecular Hydrogen Exchange Reactions 4181 8.2.9. The Need for a Low-Spin State in H 2 ases and the Possible Role of Cyanide Ligands 4183 8.2.10. Why Do Enzymes Such as H 2 ases Have Polymetallic Active Sites with Metal−Metal Bonds? 4185 8.2.11. Mechanism of Hydrogen Activation in Hydrogenases 4185 8.2.12. Summary of the above Relationships 4188 9. Hydrogen Activation in Nitrogenases 4189 10. Biomimetic Hydrogen Production 4190 11. H 2 Coordination Chemistry Relevant to Hydrogen Storage 4191 11.1. Introduction 4191 11.2. H 2 Binding to Naked Metal Ions 4192 11.3. Interaction of H 2 with Metal Surfaces, Metal Oxides and Hydrides, and Non-transition-Metal Compounds 4193 11.4. Inelastic Neutron Scattering (INS) Studies of H 2 Coordination and Rotation 4195 11.5. Binding of H 2 to Highly Porous Solids and INS Studies 4197 12. Acknowledgments 4198 13. References 4198 4152
was found between 48 and 66 ppm relative to external 1,4-dioxane.6,7 In natural abundance studies the resonance of the ring oxygen appeared considerably broader than that of the other lines7 as a result of the more restricted rotational freedom of 0-5. From another work the surprisingly low-field oxygen NMR signal at 86.0 ppm of 7-oxanorbornane was also avaialble.8 This prompted us to label the anomeric oxygen atom5b of la (H2170 10% enrichment, in boiling dry 1,4-dioxane) and after acetylation to treat lc with sodium azide. The 48.8-MFIz 170 NMR spectrum of the resulting purified 1,4-anhydro sugar 3b, shown in Figure 2, indicates a very broad signal at 85.9 ppm in perfect agreement with that reported for 7-oxanorbornane8 and strongly deshielded with respect to the ring oxygen atom of monosaccharides.6,7 This result afforded evidence for pathway b in the formation of the 1,4-anhydro sugar 3b.However, neither the 170 chemical shift of the model 2-oxanorbornane nor the signal position of 0-5 of monosaccharides in conformations other than chair was available to us. Thus, it appeared advisable to prove that the nO NMR consistency of 3b with 7-oxanorbornane was not fortuitous.Therefore, labeling of the anomeric oxygen atom of la was also carried out with 50% enriched H2lsO. After acetylation and sodium azide treatment of Id, the resulting 1,4-anhydro sugar 3c was examined by 100.62-MHz 13C NMR spectroscopy. After Gaussian resolution enhancement,9 oxygen-18-induced upfield isotopic shifts5a,5b of 0.025 ppm were detected at the two directly attached carbon sites (99.1 and 81.9 ppm in CDC13), assigned unambiguously by specific proton decoupling1 to C-l and to C-4, respectively. These resonances appeared as a pair of signals corresponding to the 13C-160 and 13C-180 species. This experiment confirmed the C-l oxy anion mechanism of pathway b in the formation of the 1,4-anhydro sugar 3c.
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