Dihydrogen acceptor

Fig. 6. PTI spectra of the copper-dihydrogen acceptors that appear in samples that were grown under atmospheres of different hydrogen isotopes, (a) Pure H2, showing the complex spectrum of A(CuH2) (b) a 1 1 mixture of H2 and D2, showing A(CuH2), A(CuHD), and A(CuD2) in a 1 2 1 ratio (c) nearly pure D2, showing A(CuD2) and a trace of A(CuHD).

Interestingly, when benzophenone imine itself is employed as substrate, higher reaction efficiency is observed in the absence of ancillary ligand (Scheme 31). Under identical reaction conditions, a 1.3 1 ratio of the mono and bis ortho arylated benzophenone imine is obtained (256% overall yield with respect to NaBPh4). Under these reaction conditions, the excess of benzophenone imine participates in the reoxidation of the catalyst as a formal dihydrogen acceptor, resulting in the concomitant formation of diphenylmethaneamine. 

Similar dehydrogenation processes can be effected for aminoborane complexes (Scheme 12), although, in both cases, the use of the dihydrogen acceptor ferfbutyl ethene (the) is required to drive the formation of the unsaturated boron-containing l%and, and the process is therefore invariably irreversible. In the case of [(p-cym)Ru(PCy3)(H)(=BN Pr2)] (55, as the [BAr4] salt)—formulated as a (hydrido)ruthenium -aminoborane... 

Scheme 12 Dehydrogenation of aminoborane complexes by the use of tbe as a sacrificial dihydrogen acceptor. Counterions omitted for clarity. ...

Why are transition metals so well suited for catalysis A complete treatment of this critical question lies well beyond the scope of this book, but we can focus on selected aspects of bond activation and reactivity for dihydrogen and alkene bonds as important special cases. Before discussing specific examples that involve formal metal acidity or hypovalency, it is convenient to sketch a more general localized donor-acceptor overview of catalytic interactions in transition-metal complexes involving dihydrogen49 (this section) and alkenes (Section 4.7.4). 

General donor-acceptor motifs in metal-dihydrogen interactions... 

Figure4.59 The geometry (left) and leading cthh hJ, donor-acceptor interaction (right) for molecular-dihydrogen complexes (a) HfH4 H2 and (b) TaHs H2.

Charge-transfer complexes involving one-center n acceptor orbitals (n-n, a-n, and 7t-n Lewis-base-Lewis-acid adducts) have been discussed in Sections 3.2.10 and 3.6. Many CT complexes involving a acceptors have been illustrated for H-bonds (n-a ) and dihydrogen bonds (a-o ), as well as for the Cg H6 -B (7t-a ) example above. In the remainder of this section we shall therefore focus on CT... 

Initial bonds in the proton donor and proton acceptor sites elongate on dihydrogen bonding. 

Commonly, any experimental study of dihydrogen bonds is undertaken to clarify the following aspects (1) establishment of intra- or intermolecular coordination, (2) determination of its stoichiometry, (3) reliable establishment of a proton-donor site and a proton-acceptor center, (4) description of the geometry of dihydrogen-bonded complexes, and (5) correct measurement of bonding energies. In this chapter we demonstrate how to approach these factors using various experimental methods that work in the solid state, the gas phase, and in solution. 

Among various physicochemical methods, IR spectroscopy and NMR are most appropriate tools for the study of dihydrogen bonds in solution. However, it is worth mentioning that these methods are basically different. First, they measure physical properties that change upon complexation bond vibrations and magnetic behavior. Second, equilibrium (4.1) is usually slow on the IR spectroscopy time scale and very fast on the NMR time scale. In other words, proton donors, proton acceptor, and their complexes are detected separately in IR spectra, whereas the NMR parameters of these moieties are usually averaged. 

According to these criteria, a dihydrogen bond is formed when in the presence of a proton acceptor, the v(OH) band intensity of a free proton-donor component decreases and a new low-frequency broad v(OH) band (part of the dihydrogen-bond complex) appears. As in the case of classical hydrogen bonds. 

In the framework of H NMR spectroscopy, the formation of dihydrogen bonds is connected with changes in electron environments of H nnclei that participate in dihydrogen bonding. In other words, dihydrogen bonding can be seen from the H chemical shifts of both proton donors and proton acceptors. 

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