Homoleptic complexes palladium

Note Herrmann et al. observed an equilibrium between the mixed NHC/phosphane palladiumill) complex and the two homoleptic complexes [Pd(PR )JJ and [Pd(NHC)JJ. The ratio is given as 7 3. This would mean that three different palladium(II) complexes are potentially present in a catalytic reaction. 

Reaction of the same functionalised imidazolium salt with PdCl produced interesting results [123], In the presence of pyridine as base, the simple palladium(ll) carbene adduct was formed (see Figure 4.42), but when K COj was used as base, activation of the amide functional groups produced the corresponding homoleptic complexes, as a mixture of the cis and trans isomers. 

In 1998, Kempe and coworkers [34] reported the first aminopyridinato rare-earth metal complexes. 4-Methyl-2-[(trimethylsilyl)amino]pyridine(HLl) was utilized in this complex. The reaction of lithiated LI and YCI3 in ether and pyridine led to the ate complex [Y(Ll)4(LiPy)] (Py = pyridine) (1). The complex 1 catalytically mediated a ligand transfer reaction to form [Pd(Ll)2] and [Y(Ll)3(py)] (2) from [Pd(cod)Cl2] (cod = cyclooctadiene). The LI ligand transfer from yttrium to palladium and the regeneration of 1 are significant in the efficient synthesis of the very strained amido palladium complexes (Scheme 2). Lithiated LI underwent a salt metathesis reaction with ScCb, at low temperature in THF, to yield the homoleptic complex [Sc(L1)3] (3) (Scheme 2). 3 is the first reported scandium aminopyridinato complex [35]. 

Atypical synthesis of naked NHC-containing complexes has been reported by Cloke and coworkers. As shown in equation (7), the co-condensation of nickel, palladium, and platinum vapor with the free NHC, PBu, provided two-coordinated homoleptic complexes of Ni(0), Pd(0), and Pt(0) (43). Unfortunately, in addition to the poor yield of the isolated product, this original methodology required a metal vapor synthesis apparatus. 

Elements of this group, like metals of groups 8 and 9, form mononuclear and cluster homoleptic complexes in which metals adopt a zero oxidation state. The following homoleptic complexes of these elements are known [M(CNR)4] (M = Ni, Pd, Pt), [Ni(CNR)4], [Ni4(CNR),], [Ni4(CNR)4] , [NigiCNR). ], [Ni4(CNR)i4](C104)s, and [Ni3(CNR),i](C104)6. Linear structures were assigned for the last two compounds.Monomeric platinum and palladium compounds... 

By way of example, we can focus on the cyano complexes. Palladium and platinum can form homoleptic complexes with two, four, or six cyanides, depending on the metal oxidation state zero, two, and four, respectively. Rhenium forms heptacyano complexes in its - -3 and -t-4 oxidation states but forms an octacyano complex in its -t-5 state. Molybdenum and tungsten also appear commonly in octacyano complexes for +4 and - -5 oxidation states, and heptacyano analogs for the - -2 and - -3 cases, with only one occurrence of [Mo(CN)6]" anions in two oxidation states (n = 3, 4). Similar situations can be found with phosphine or carbonyl complexes. For instance, the group... 

Trimethylsiloxyphenyl isocyanide enters the cyclization reaction with [MCl2(NCPh)2] (M = Pt, Pd) to yield the homoleptic tetracarbenes 77 (M=Pt, Pd) (97JOM(541)51). Complex 77 (M = Pd) enters an interesting reaction with ammonia to yield the species 78 where two of this benzoxazol-2-ylidene ligands are deprotonated and become C-coordinated benzoxazole moieties, while the other two remain intact. Palladium(II) iodide in these conditions behaves differently yielding the di-Mo-cyanide complex, which in the presence of tetra- -butyl ammonium fluoride gives the dicarbene 79. 

Potentially coordinatively unsaturated dithiolene-metal complexes are rare,298-306 and 1 1 dithiolene-transition-metal complexes with no other ligands are, to our knowledge, unprecedented.307 The neutral complex [PdS2C2(COOMe)2]6,308 is homoleptic containing one dithiolene unit for each palladium atom and no other ligands. Electrochemical reduction of the compound depicted in Figue 21 proceeds in four reversible steps. 

In contrast to nickel, simple homoleptic Tpx or Bpx, and heteroleptic pyrazolylborate-halide complexes of palladium and platinum are largely unexplored. Indeed, while for 3d metals these are "standard" targets for each successive generation of poly(pyrazolyl)borate ligand, for the... 

Homoleptic carbonyl ligands, in palladium complexes, 8, 197 Homoleptic chromium alkyl compounds, preparation,... 

Palladium forms with both silylenes 83 and 84 silylene-bridged dinuclear complexes. The dinuclear Pd(0) complex 110 (R = Ph) was found to be active in Suzuki and Stille cross-coupling reactions <2001CC2372, 2002JOM141>. NMR investigations indicate that also homoleptic silylene complexes of Pd(0) such as Pd(83)(, [Pd(83)2]2, and Pd(84)4 exist these complexes are, however, very labile <2002JOM141>. 

Figure 4.42 Synthesis of homoleptic palladium(ll) carbene complexes using a carboxylic acid amide functionalised NHC ligand.

A homoleptic square-planar palladium(II) carbonyl complex [Pd(CO)4](Sb2Fii)2 that displays v(CO) at 2259 cm (the value for free CO is 2143 cm ) has been recently prepared. This is an exceptional complex, dubbed by the authors as superelectrophilic metal carbonyl . The very high value of v(CO) shows that the carbonyl hgands behave as a-donors (see Carbonyl Complexes of the Transition Metals). Additional data (e g. v(M )) and stability data support that this ligand is very weakly coordinated as only a a-donor. ... 

Palladium(O) complexes bearing NHC ligands have been used as active precatalysts in the Suzuki-Miyaura reaction. Homoleptic (NHC)2Pd complexes were prepared by phosphine displacement from Pd(P(o-tol)3)2 with free carbene... 

The chemistry of palladium-carbonyl complexes has experienced extensive recent developments due to an increased interest in the role of palladium in surface catalysis especially in automobile exhaust catalysts. Palladium-carbonyl complexes are nevertheless stiU relatively rare, which is probably due to their relative instability in comparision with that of Ni complexes. The homoleptic Pd(CO)4 only exists at low temperatures (<80 K) in noble gas or CO matrices, in sharp contrast with isoleptic Ni(CO)4, which is stable at ambient temperature. Table 1 compiles some of the known and representative carbonyl complexes of palladium. 

This design idea, however, has not led to clear improvement yet. Indeed, if this principle were operative, then a hybrid complex would perform superior to the respective homoleptic phosphine complex or a mixture of a simple palladium source and a phosphine. In fact, this design idea implies that during the preactivation stage the hybrid complex 199 loses palladacychc ligand while keeping phosphine to furnish monophosphine palladium species... 

---