Dihydrogen and dihydride complexes

Thus it is clear that the electronegativity of the subsituents has a critical role to play in dietating whether the phosphine oxide or phosphinous acid form will prevail. Other routes to trapping the phosphinous acid form have been demonstrated via complexation to transition-metal atoms [64], work that has attracted attention from the field of catalysis. 

Orbital symmetries and electron donation/back-donation in transition-metal dihydrogen complexes. 

Upon moving from a tt acceptor (left) to a strong a donor (middle) to a weak a donor (right) the H—H hond length 

Classification of dihydrogen complexes based on the interatomic separation of H2. Adapted with permission from [66]. Copyright 2007 American Chemical Society. 

It isn t always possible to get single crystals of metal hydride complexes suitable for single-crystal neutron diffraction smdies, nor do all metal hydrides give useful NMR spectra. What factors limit the apphcation of these methods In such circumstances, what else could you do to find out about the coordination of hydrogen ligands to the metal atoms  

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Similarly, recent years have brought new insights into the way dihydrogen can be bound at a transition metal site. Kubas and others(69.70.711 have shown that H2 Itself can form simple complexes with a variety of transition metal sites In which the H-H bond Is largely maintained. This finding contrasts with the classical situation in which H2 interacts with a transition metal site by oxidative addition to form a dihydride complex(22). In certain cases the dihydrogen and dihydride complexes exist in simple equilibrium (71 )... 

Dihydrogen complexes are a unique class of hydride complex in which the H-H bond is retained. Gomplexes of the formula [GpFeH2L2] can exist in the pure dihydrogen (H2) form, or as mixtures of the dihydrogen with the trans- and dihydride complexes produced since 1993 are listed in Table 2. 

Transition-metal catalyzed photochemical reactions for hydrogen generation from water have recently been investigated in detail. The reaction system is composed of three major components such as a photosensitizer (PS), a water reduction catalyst (WRC), and a sacrificial reagent (SR). Although noble-metal complexes as WRC have been used [214—230], examples for iron complexes are quite rare. It is well known that a hydride as well as a dihydrogen (or dihydride) complex plays important roles in this reaction. 

Scheme 20.4 shows a set of dynamics involving the binding and splitting of H2 that essentially represent the reaction coordinate for homolytic H-H bond cleavage or in the reverse process, the formation of H2 from a dihydride species. Remarkably these can be equilibrium processes in certain cases. Solutions of the complex W(CO)3(PTr3)2(H2) were observed by NMR spectroscopy to contain about a 4 1 ratio of dihydrogen to dihydride complex, proving that side-on bonded H2 complexes were the first step in formation of hydride complexes (Eq. (20.1) P = PiPrj) [lb, 2]. 

We and others have found that CH2CI2 is an excellent solvent for acidic dihydrogen and dihydride compounds. Methylene chloride has a low dielectric constant so that monocationic complexes will usually form 1 1 ion pairs below 0.01 M and higher aggregates above this concentration. Several pK/ of metal hydride complexes in CH2CI2 anchored to the pKa fl of phosphonium salts or other acids have been reported (e.g. see Table 1.2 from our work) they fall in the range from about -5 to 12 [23,30,49-52]. These are actually ion-pair pK values because they have not been corrected for the effects of ion-pairing. 

A catalytic cycle is proposed in eq 50 displaces the ketone complex to give a dihydrogen or dihydride complex, this dihydrogen complex protonates the ketone, H is transferred to the carbon of the substrate, and the 16-electron M complex is coordinated by ketone to complete the cycle. Although MoH2 /Mo(ti -H2) complexes have not been directly observed, some WH2 AV(ti -H2) complexes have been structurally characterised [44] and are known to be acidic [46]. 

In solution, an equilibrium is established for the Kubas complex between the dihydrogen and dihydride forms, confirming that the molecular hydrogen complex is an intermediate in the oxidative addition reaction of H2. 

The infrared stretching frequency has also been used to distinguish dihydrogen from dihydride complexes. The Raman frequency of free H is 4300 cm and typical v j vibrations are found between 1700 and 2300 cm" . The dihydrogen ligand vibrates at a frequency between that of free H and those of metal hydrides. " Typical values for the H-H stretch in dihydrogen complexes lie between 2400 and 3100 cm" . For example, Kubas s compoimd contained a band at 2690 cm" (see Equation 2.19) for the H-H stretch. Although observed in some cases, the H-H stretch is weak and has not been identified in the majority of H complexes. 

The direct reductive amination (DRA) is a useful method for the synthesis of amino derivatives from carbonyl compounds, amines, and H2. Precious-metal (Ru [130-132], Rh [133-137], Ir [138-142], Pd [143]) catalyzed reactions are well known to date. The first Fe-catalyzed DRA reaction was reported by Bhanage and coworkers in 2008 (Scheme 42) [144]. Although the reaction conditions are not mild (high temperature, moderate H2 pressure), the hydrogenation of imines and/or enam-ines, which are generated by reaction of organic carbonyl compounds with amines, produces various substituted aryl and/or alkyl amines. A dihydrogen or dihydride iron complex was proposed as a reactive intermediate within the catalytic cycle. 

Cobalt(III) sepulchrate (l)8 and tetrazamacrocyclic complexes of cobalt(II) (2)9 and nickel(II) (3) (6)9-11 catalyze the electroreduction of water to dihydrogen, at potentials ranging from - 0.7 V (complex (1)) to — 1.5 V (complexes (4)-(6)) vs. SCE in aqueous electrolytes, with current efficiencies as high as 95% for complex (4).9 It is noteworthy that the binuclear nickel biscyclam complex (6) is 10 times more active (at pH 7) than the mononuclear nickel cyclam complex (5). This behavior tends to indicate that some cooperativity between the two metal centers occurs in complex (6), as depicted in the possible reaction (Scheme 3) involving a dihydride intermediate.11... 

Dative and synergistic complexation of dihydrogen Direct observation of complexation of molecular H2 at metal centers is a relatively recent phenomenon. Virtually all such complexes exhibit relatively minor elongation of the H—H bond, indicating that description of these species as molecular-H2 complexes (rather than, e.g., metal dihydrides or some intermediary resonance mixture) is well justified. Experimentally observed molecular H2 complexes of the synergistic type are common, but those of simple dative type are not. 

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