Square planar Pd complexes

The active species in the catalytic cycles are square-planar Pd complexes of the formula [Pd (X)(S)(L-L)]Y where L-L is a chelating ligand with the same or different donor atoms among P, N, O and S X is the growing polyketone chain or hydride S may be a solvent molecule, a co-monomer, or a keto group from the chain. Finally, Y is a counter-anion of weak nucleophilicity in order to avoid competition with the co-monomer for coordination to palladium (Scheme 7.1). 

Figure 3.10 Examples of a CO insertion and b CH3 migration, in a square-planar Pd complex with a P—N chelating ligand. Note that the migration mechanism retains the cis configuration between the CO group and the chelate nitrogen atom.

Reductive elimination (X-M-Y — M + X-Y) is the microscopic reverse of oxidative addition. This reaction is usually most facile when the X-Y bond is strong (e.g., H-Ti-Bu —> Bu-H + Ti). Not as much is known about the mechanism of reductive elimination as is known about oxidative addition. It is known that the two groups must be adjacent to each other in the metal s coordination sphere. In square planar Pd complexes ((R3P)2PdR2), if the PR3 groups are forced to be trans... 

Scheme 12.18 outlines the general catalytic cycle for cross-coupling, the key steps of which are (a) oxidative addition of R -X to Pd(0), (b) cis-trans isomerization of a square planar Pd complex (if necessary), (c) transmetalation of R-M to give R -Pd-R, (d) trans-cis isomerization (if necessary), and (e) reductive elimination of R -R and regeneration of Pd(0). 

Dehydrative condensation of transition metal hydroxo complexes is a versatile synthetic method, when the conjugated acid of the incoming ligand (H-A) is a protic compound. There are reports of the synthesis of two types of square-planar hydroxopalladium complexes, mononuclear [TplPr2 x(L)Pd-OH] (X = H, Br) and dinuclear complexes [TplPr2Pd(H20)( -0H)2 (H20)PdTplPr2] (TplPr2 = hydrotris(3,5-diisopropylpyrazolyl)borate), and some dehydrative condensations of them.278... 

Among the numerous nickel dithiophosphates, the chiral complexes deserve to be cited.41 Unusual trinuclear Pd3(p3-E)2[S2P(OR)2]2(PPh3)2 with E = S, Se, R = Et, Pr, Pr, Bu, have been prepared and Pd3(p3-S)2[S2P(OEt)2]2(PPh3)2 revealed a structure containing three distorted square-planar Pd atoms and sharing two p3-S ligands.42... 

The most active and selective catalysts for both the copolymerisation process and for the apparently simpler ethene carbonylation to monocarbonylated products MP or DEK are cationic square planar Pd(II) complexes in which the metal centre is czs-coordinated by a bidentate P - P ligand, by a Ugand involved in the initial step of the catalysis or in the process of forming the product and with the fourth vacant site coordinated by CO or ethene or a keto group of the growing chain or MeOH (or H2O, always present in the solvent even when not added on purpose) or even by a weakly coordinating anion. 

Deeth et al. used density functional theory to model water exchange on square-planar [Pd(H20)4] and [Pt(H20)4] (90). The calculations strongly support that water exchange proceeds via an a-activation mode on these complexes and the trigonal bipyramidal structures calculated for [Pd(H20)5] and [Pt(H20)5] " were very similar. There is a good agreement between experimental and calculated activation enthalpies for an la mechanism, whereas for an I mechanism, a difference of more than 100 kJ mol is observed. 

Square-planar Pd (23) and octahedral Rh (24) hydroperoxide complexes containing hydrotris(3,5-diisopropylpyrazolyl)borate ligand have been prepared as well and characterized in the solid state. Strong hydrogen-bond interaction between —OOH moiety and ligands has been detected in these complexes. 

In other instances, irradiation of the d-d transition leads to no observable reaction. Examples of this behavior are found for complexes having a variety of d electron configurations and coordinative geometries square planar Ni(II) (3d)3 in Ni(CN)42 124 and mww-Ni(gIy)2 124 square planar Pd(II) in Pd(CN)42-,124 and tra -Pd(gly)2 square planar Pt(II) in Pt(CN)42" (5d)3 124 octahedral Co(III) (3d)6 in a variety of complexes (cf. Sect. III-C and III-D). A striking example of this type of behavior is afforded by the nonreversible photoisomerization of cis-Pt(gly)2 (5d)8 to trans-Pt(g y)2 [reaction (2)].124 It has been proposed that irradiation of either of these square planar complexes leads to the same tetrahedral intermediate which decays exclusively to mwj-Pt(gly)2. This behavior may be contrasted with the reversible photoisomerization shown in reaction (3).3... 

The square planar palladium complexes which give values of m of ca. 0.4 (Table 1) are known to react via a mainly associative mechanism so that the values of m are taken to indicate that Pd—Cl bond cleavage and leaving Cl solvation were both important in determining the reactivity trend for these complexes, i.e. there is a greater degree of M—Cl bond breaking in the transition state of palladium compared with cobalt. 

The former contains octahedral complex molecules with six-coordinate Ni(II). The latter contains square planar Pd(NH3) 4+ cations the bromide ions complete an ionic lattice but are not part of the coordination sphere of Pd(II). 

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