Complexes anionic metal hydrides

Unsaturated organic compounds are reduced by various neutral and anionic metal hydrides with or without transition metal catalysts. Certain chiral transition metal complexes, when used as catalysts, exhibit unique regio- and stereoselectivity. 

Reaction of Hydrides. The donor semi-bridging interaction of the carbonyls in 1 might render them more susceptible to nucleophilic attack by such reagents as hydrides. We reasoned that the formation of n2-formyl species, e.g., IV, or alternatively, anionic metal hydrides, e.g., V, might result. The actual reaction of l with Et3BH is consistently more complex than anticipated (-3.31. 

Table 5.3 Anions of complex transition metal hydrides (from Ref [208]).

Although Hg hydrides are claimed, most studies relate to Zn and Cd, where polymeric binary compounds, (MHj), are isolated as well as anionic metal hydrides, e.g., LiZnHj, LijZnH, Li3ZnHj. In addition, borohydride complexes such as Cd(BH )2 THE are reported, some having more than one metal atom in the anion. 

M(C0)5X , and H2 in the presence of a general base to provide anionic metal hydrides. This process was shown to be first-order in both metal complex and dihydrogen and was not inhibited by addition of carbon monoxide. Consistent with the rds in catalysis being formation of the metal hydride intermediate, the metal catalyzed reaction of RX/CO2/H2 to provide HCOOR is not inhibited by CO. The well-established formation of metalloformate, M(C0)s02CH", from M(C0)5H and CO2 is followed by a less facile process involving the reaction of the metalloformate with RX. This latter reaction is first-order in both metal complex and alkyl halide and is inhibited by carbon monoxide. 

Stoichiometric Hydride Tranter Reactivity of Anionic Metal Hydride Complexes 57... 

The characteristic part of a complex transition metal hydride is the anionic homoleptic transition metal (M) hydride complex [MmH/,]""", which is balanced by the cation A+ or A + (A = Li-Cs, Mg-Ba, Eu, Yb). The formation of an extended solid is a consequence of the attractive electrostatic Coulomb interaction between cations and complex anions, whereas within the complex, hydrogen... 

According to a detailed mechanistic study, the first step is the abstraction of the relatively acidic hydrazone proton (93- 97). This is followed by hydride attack on the trigonal carbon of the C=N bond, mainly from the a-side at C-3, together with the concomitant loss of the tosylate anion (97 -> 98). Expulsion of nitrogen from the resulting intermediate (98) yields a fairly insoluble anion-metal complex (99) which upon decomposition with water provides the methylene derivative (100). 

The fact that both the thermal and the photochemical insertion reactions yield the same products via formation of charge-transfer complexes leads to the conclusion that the reactive ion-radical pair in equation (52) is the common intermediate for both activation processes. Such a conclusion is further verified by the direct observation of anion-radical intermediates from the thermal reaction of TCNE and DDQ with various metal hydrides.188... 

The reactivity of these metal hydride-metal carbonyl reactions can be correlated with the nature of the reactants in a manner consistent with the proposed mechanism nucleophilic attack by hydride on coordinated CO. Thus reactions involving the highly nucleophilic group IV hydride, Cp gZrHg, are much faster than those of group V metal hydrides. On the other hand, the relatively electrophilic neutral binary metal carbonyls all react with Cp2NbH under mild conditions (20-50° C), whereas more electron-rich complexes such as cyclopentadienylmetal carbonyls (Cp2NbH(C0), CpV(CO) ) or anionic carbonyls (V(CO)g ) show no reaction under these conditions. 

Several anionic metal carbonyl hydrides stoichiometrically convert acyl chlorides to aldehydes. The anionic vanadium complex [Cp(CO)3VH] reacts quickly with acyl chlorides, converting them to aldehydes [44]. Although no further reduction of the aldehyde to alcohol was observed, the aldehydes reacted further under the reaction conditions in some cases, so a general procedure for isolation of the aldehydes was not developed. 

A mechanism similar to Scheme 10 was proposed, involving CO addition, followed by H20 addition (in lieu of hydroxide anion) to form a metallocarboxylic acid complex. Then, decomposition to C02 and a metal hydride was proposed, followed by hydride elimination. Table 15 provides data from reaction testing in the temperature range 140 to 180 °C. In later testing, they compared Rh and Ir complexes for the reduction of benzalacetone under water-gas shift conditions. 

A number of complex metal hydrides such as lithium aluminium hydride (LiAlH4, abbreviated to LAH) and sodium borohydride (NaBHj) are able to deliver hydride in such a manner that it appears to act as a nucleophile. We shall look at the nature of these reagents later under the reactions of carbonyl compounds (see Section 7.5), where we shall see that the complex metal hydride never actually produces hydride as a nucleophile, but the aluminium hydride anion has the ability to effect transfer of hydride. Hydride itself, e.g. from sodium hydride, never acts as a nucleophile owing to its small size and high charge density it always acts as a base. Nevertheless, for the purposes of understanding the transformations. 

Whilst the complex metal hydride is conveniently regarded as a source of hydride, it never actually produces hydride as a nucleophile, and it is the aluminium hydride anion that is responsible for... 

Acknowledgments. Like editors of previous volumes of this series, I am indebted to many colleagues for contributions to this work. First there are Wolfgang Beck in Munich, Ekkehard Lindner in Tubingen, and John R. Shapley in Urbana, who assisted me in the selections and solicitations for three special collections transition metal complexes containing weakly bonded anions (Chapter 3), metalocyclic complexes (Chapter 4), and polynuclear transition metal complexes (Chapter 5), respectively. Each has contributed a preface for the chapter he helped to form, following the pattern set in Volume XII when Alan G. MacDiarmid invited E. C. Ashby and myself to form a chapter on metal hydrides. As a consequence of such efforts, more than two-thirds of this volume consists of invited preparations. 

---