Contents VIl 3 Bio-organometallic Approaches to Nitrogen Fixation Chemistry 81 lonas C.Peters and Mark P.Mehn ntroduction-The N2 Fixation Challenge 81 32 Biological na Reduction 83 3.2.1 General Comments 83 3.2.2 Structural Data 84 3.23 Assigning the FeMoco Oxidation States 85 3.3 Biomimetic Systems that Model Structure and Function 86 331 General Comments 86 232 Mon clear molybde um Systems of Biomimetic Interest 86 2201 The Originally Proposed"Cha Cycle"87 3.3.22 An Ele ocatalytic A M()- med: ng Low-valent Tungsten 89 A Cp Me (Na)Model System (M Mo,W) 9 3.32 Bimetallic Mol m Systems that Cleave N2 93 3.32.6 Sulfur-supported Mo-N2 Complexes 95 3.3.3 Considering Mechanisms Involving Multiple and Single Iron Sites for N2 Reduction 96 3.3.3.1 General Comments 96 3.3.32 Theoretical Studies that Invoke Iron-mediated Mechanisms 96 3.3.3.2.1 3.333 Shncoculple ro Sle1 3334 Nitrogenase-related Transformations at Cluster Models 104 3339 Considering N.Fixation Involving a Scheme single iron Site 102 3.3.3.6 Model Studies that May be Relevant to N,Fixation Involving a Single Iron Site 108 3.3.3.61 Fe(0)-N2Co and NH,versus NaH Production 108 3.33.6.2 109 3.4 ordinate Iron Model Systems ences A The Activation of Dihydrogen Jesse W.Tye and Michael B.Hall 4.1 ntroduction 12 4.1.1 Why Activate H,?121 4.1.2 Why is it so Difficult to Activate H2?122 4.2 Structure and Bonding of Metal-bound H-Atoms 124 4.2.1 Why can Metal Centers React Directly with H2. yhile most Nonmetals Cannot?124 4)) Seminal Work:The Discovery of Metal-bound H2 Complexes 125 4.2.3 What are the possible co gely Uns 126 424 o 13g 4.25 of Activation esof the H Interaction and Degree3 Bio-organometallic Approaches to Nitrogen Fixation Chemistry 81 Jonas C. Peters and Mark P. Mehn 3.1 Introduction – The N2 Fixation Challenge 81 3.2 Biological N2 Reduction 83 3.2.1 General Comments 83 3.2.2 Structural Data 84 3.2.3 Assigning the FeMoco Oxidation States 85 3.3 Biomimetic Systems that Model Structure and Function 86 3.3.1 General Comments 86 3.3.2 Mononuclear Molybdenum Systems of Biomimetic Interest 86 3.3.2.1 The Originally Proposed “Chatt Cycle” 87 3.3.2.2 An Electrocatalytic Reduction Cycle using Low-valent Tungsten 89 3.3.2.3 A Mo(III)-mediated Catalytic N2 Reduction System 90 3.3.2.4 A Cp*MMe3(N2) Model System (M = Mo, W) 92 3.3.2.5 Bimetallic Molybdenum Systems that Cleave N2 93 3.3.2.6 Sulfur-supported Mo-N2 Complexes 95 3.3.3 Considering Mechanisms Involving Multiple and Single Iron Sites for N2 Reduction 96 3.3.3.1 General Comments 96 3.3.3.2 Theoretical Studies that Invoke Iron-mediated Mechanisms 96 3.3.3.2.1 Comparing Several Proposed Mechanisms 97 3.3.3.3 Synthetic Efforts to Model N2 Reduction by Multiple Iron Sites 103 3.3.3.4 Nitrogenase-related Transformations at Cluster Models 104 3.3.3.5 Considering N2 Fixation Involving a Scheme Single Iron Site 107 3.3.3.6 Model Studies that May be Relevant to N2 Fixation Involving a Single Iron Site 108 3.3.3.6.1 Fe(0)-N2 Complexes and NH3 versus N2H4 Production 108 3.3.3.6.2 Low-coordinate Iron Model Systems 109 3.4 Concluding Remarks 115 References 116 4 The Activation of Dihydrogen 121 Jesse W. Tye and Michael B. Hall 4.1 Introduction 121 4.1.1 Why Activate H2? 121 4.1.2 Why is it so Difficult to Activate H2? 122 4.2 Structure and Bonding of Metal-bound H-Atoms 124 4.2.1 Why can Metal Centers React Directly with H2, while most Nonmetals Cannot? 124 4.2.2 Seminal Work: The Discovery of Metal-bound H2 Complexes 125 4.2.3 What are the Possible Consequences when H2 Approaches a Coordinatively Unsaturated Transition Metal Center? 126 4.2.4 Elongated
2 -H2 Complexes 128 4.2.5 Experimental Gauges of the H–H Interaction and Degree of Activation 129 Contents VII