Palladium hydride complexes

Abstract

The stable colourless crystalline complex, (Et,P),Pd(H)CI is obtained by the reduction of (Et,P),PdCI, with trimethylgermane at 40". Reduction of (Et,P),PdBr, is slow unless palladium black is added. The analogous reaction with (Et,P),NiBr, gives a highly unstable nickel hydride complex. The reactions are considered to follow a radical chain mechanism.The reactions of (Et,P),Pd(H)CI with the following are reported : KI, HCI-Et,O, NH,-H,O, CCI,, MeSH-KCN, and MeOH-KCN. Unsaturated compounds (CH,:CHCN, PhCiCH, C, F, , C, H, ) also react with it to give a complexity of products.OF the metals, nickel, palladium, and platinum only the latter has been shown t o form a well defined series of hydrido-complexes. These are mostly uninuclear compounds of the type trans-L,Pt (H)X, although two cisuninuclear complexes have been described, cis- (P h,PCH2) ,Pt (H) GePh, and cis-(Ph,P) ,P t (H) C1 .l Preparative methods usually involve the reduction of a bis(tertiary phosphine)platinum dihalide by use of reagents which result in either a strongly basic environment (EtOH + KOH, LiAlH,, N,H,) or an acidic medium (H, which gives HX, HCO,H)., The reason why these methods are unsatisfactory for palladium analogues is that hydridopalladium complexes decompose rapidly in acidic or basic media. " Neutral " reducing agents, which have been applied to the formation of platinum h y d r i d e ~, ~-~ are illustrated in reactions (1)-(3); these reactions are believed to proceed through octahedral plat iniim intermediates.

Nickel Hydride Complexes

“…In the 1970s and 1980s, the chemistry of nickel hydride complexes lagged way behind that of many other metal hydride complexes, largely due to the limited availability of well-defined systems. Early attempts to isolate nickel hydride complexes using the protocols well established for palladium and platinum analogs often failed, which was attributed to the low thermal stability for the nickel hydrides. , The exploration of new ligand scaffolds and polydentate ones in particular has later on expanded the inventory of nickel hydrides and improved the landscape of this research field. Different synthetic strategies, including the failed ones, are detailed in the following subsections.…”

“…For Group 14 elements, trimethylgermane (Me 3 GeH) has been used to prepare palladium hydrides from palladium chloride complexes. 21 The same method applied to (Et 3 P) 2 NiBr 2 produces a hydride species based on the infrared spectrum (a band at 1937 cm −1 ) and its ability to reduce CCl 4 95 (α-Diimine)NiBr 2 is reactive toward NaH, producing a dinickel bridging hydride complex with the αdiimine ligand being reduced (Scheme 21). 97 Further reaction with Na or K results in three different nickel hydride species, all depending on the equivalent of alkali metal used.…”

Ex situ generation of stoichiometric HCN and its application in the Pd-catalysed cyanation of aryl bromides: evidence for a transmetallation step between two oxidative addition Pd-complexes

“…This result could be explained by the slow transmetallation between two Pd-hydride species 32 leading to the formation of (P( t Bu) 3 ) 2 Pd(H) 2 and (P( t Bu) 3 ) 2 Pd( 13 CN) 2 . 35 , 38 Whereas, the former can reductively eliminate generating Pd(0) and dihydrogen, we speculate that (P( t Bu) 3 ) 2 Pd( 13 CN) 2 could abstract cyanide from two Pd-hydride complexes 32 ultimately leading to the formation of the Pd 3 -complex 33 .…”

Intermolecular Heck Reaction: Scope, Mechanism, and Other Fundamental Aspects of the Intermolecular Heck Reaction

“…Finally, in order to close the catalytic cycle, a base is needed to regenerate the active Pd(0) species from the hydridopalladium complex. [52]- [54] The discrete steps and related processes of importance to the synthetic chemist, such as catalyst generation, decomposition of oxidative addition complexes, and the key aspects of regiocontrol with regard to insertion in particular, but also double bond migration and elimination, will be discussed in some detail below.…”