Bis-chelate complexes

Salicylaldiminato ligands have also been studied and the bis(chelate) complex (101) has been reported to produce high molecular weight PE with an activity of 96gmmol 1 h 1bar 1267... 

Reaction of the bis-chelate complex 149 and various bis(arylalkyl)barium complexes generates heteroleptic barium complexes with one chelate and one reactive arylalkyl ligand 164. The homoleptic and heteroleptic barium complexes both induce living polymerization of styrene to atactic polystyrene in cyclohexane solution. The fact that no stereocontrol is observed during polymerization despite the presence of the chiral carbanionic ligands is... 

Scheme 4 Proposed reaction of Pd-H moiety reacting with excess ligand to form the inactive bis-chelated complex...

In MeOH, Pd - H+ species are unstable and have the tendency to deproto-nate with reduction to less active dimeric Pd(I) and Pd(0) complexes, which may lead to degeneration of the catalyst with formation of inactive palladium metal and free ligands, which in turn may give less active bis-chelate complexes [Pd(P-P)2]2+ [55,61]. Possible deactivation paths have been delineated in [17]. In order to maintain or improve the catalytic activity, the precursor is used together with an oxidant and an excess of acid (usually BQ/Pd = 100 - 200 and acid/Pd = 10 - 20) [15,47]. 

Anionic [ReOBr3(L)] and neutral [ReOBr(L)2] complexes are formed by reactions of [NBu4][ReOBr4] with 2-methyl-3-oxy-4-pyrone or l,2-dimethyl-3-oxy-4-pyridinone which act as monobasic ligands. The monoligand complex can be regarded as an intermediate in the formation of the bis-chelate complex. 

In CH2CI2, where the reaction of the bis-chelate complex (1) with Pd(OAc)2 to give (2) is much faster than in MeOH (Figure 7.5), no significant difference between dppe or dppp Pd" catalysts has been observed in CO/ethene copolymerisation [5e,f]. 

Besides proving the formation of p-chelates [Pd(CH7CH7C(0)Me)(P-P)] at room temperature, the spectra showed the occurrence of chain-transfer by protonolysis with adventitious water to give the p-hydroxo compounds cis/trans [Pd(p-OH)(P-P)]2 as well as the conversion of the latter compounds into cis/trans bis-chelates [Pd(P-P)2] (Chart 7.2) [5f]. Independent experiments with isolated compounds showed that the p-OH and bis-chelate complexes are not dead ends, and can reenter the catalysis cycle to give alternating polyketones. 

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry has contributed remarkably to unravelling the termination and initiation steps of the styrene/CO copolymerisation catalysed by the highly active bis-chelated complex [Pd(bipy)2](Pp5)2 in TFE [40]. Chain-end group analysis of the material produced in the absence of BQ showed that the termination by P-H elimination is accompanied by three different initiators two palladium alkyls from Pd-H formed by reaction of the precursor with CO and water (a and b) and a palladium carboalkoxy species formed by reaction of the precursor with the fluorinated alcohol and CO (c) (Chart 7.4). The suppression of the chain-transfer by alcoholysis was proposed to be responsible for the enhanced stability of the palladium acyl intermediates and hence for the high molecular weight of the copolymers produced. 

If the anion of 2-(2 -hydroxyphenyl)-5(4/l/)-oxazolone 336 is used as a ligand, bis-chelate complexes 337 of copper(II), nickel(II), and zinc(II) have been prepared from the corresponding metal acetates. Alternatively, 336 and 2-(2 -aminophenyl)-5(4//)-oxazolone 340 can act as ligands with metals including palladium(II), platinum(II), ruthenium(II), nickel(II), and copper(II) to produce a variety of structurally diverse complexes 338, 339, and 341 as shown in Schemes 7.109 and 7.110. ° ... 

Patterson and Holm also observed that rigid planar quadridentate or related planar bis-chelate complexes, and complexes differing only in donor atoms, are more readily reduced in the order N4 < N2O2 < N2S2. This observation reflects the relative preference of Cu(II) for these donor atoms in the order N > O > S. Patterson and Holm noted that binuclear complexes were... 

Diaminoethane (en), H2NCH2CH2NH2, forms tris- and bis-chelate complexes. The tris-chelate [Ni(en)3]2+ cation (94) has been known since 1899.658 It is considerably more stable than the six-coordinate complexes formed with monodentate amines as shown in Table 39 where the stability constants of some six-coordinate complexes are reported. 

In general, substituents in 2,9-positions of phen and 6,6 -positions of bipy prevent the formation of tris-chelate complexes due to the steric hindrance of the ligands and consequently the mono- and bis-chelate complexes become preferred. The two complexes [NiX2(N—N)] (102 X = C1, Br) formed by 2,9-dimethyl-l,10-phenanthroline have been found to be square pyramidal dinuclear with both bridging and terminal halides,848 while the iodo derivative [NiI2(N—N)] (103) is monomeric and pseudotetrahedral.847... 

A square planar bis-chelate complex with dithiotropolonate monoanion has been isolated in the solid state (295).2142 This complex does not exhibit the electrochemical properties of the bis(l,2-dithiolene) complexes and undergoes ring alkylation and oxidation reactions (Scheme... 

Transition metal hydroxyoxime complexes have been reviewed very recently.2507 Their use in both analytical chemistry and extraction metallurgy is well known. The square planar structure of the bis chelate complex NiL (347) with the deprotonated 2-hydroxybenzaldoxime (HL) is typical of this series of nickel complexes.2508 Their bis adducts, NiLJ, with bases such as py, substituted pyridines and cyclomethyleneimines, are six-coordinate.2509 The acyl oxime (H2L) complexes are similar to the aforementioned complexes being either square planar bis chelates Ni(HL)2 (348) or octahedral bis adducts, Ni(HL)2B2.2507 When the acyl oxime acts as a dibasic ligand L, the corresponding (NiL) complexes are insoluble and involve extensive polymerization. 

Analogous cyano complexes can be prepared. Treating the bis chelate complex [Pt(dppm)2]2+ with 2 equivalents of NaCN leads to stabilization of the r 1-dppm complex (151) which can be used to prepare heterobimetallic bridging complexes with added M (M= Ag+/I, HgCl2, Rh2Cl2(CO)4) (equation 433).1461 Similar complexes can be formed with dialkyl and diaryl substituents on platinum in place of cyanide.1462... 

A very similar approach can be used to interpret the UPS of bis-chelate complexes of acac, hfa, and related monothio and bisthio analogues (45, 51). Monomeric Ni(acac)2 has been found to possess D2h symmetry in the vapor phase (258), and this geometry is assumed here for the other bis-chelates. In D2d symmetry the rr3, +, and n- MOs of each 0-diketonate ligand combine to afford the symmetry-adapted combinations shown in Fig. 44. Introduction of the metal d orbitals, which transform as ag(d2 ), ag(dx> yz), blg(dxy), bigidxz), and b2g(dyz), completes the scheme. By analogy with the tris-chelates, the strongest interaction anticipated is that between the dxy orbital and the antisymmetric lone pair combination of b g symmetry. 

Complex cations containing 2,2 -bipyridyl or 1,10-phenanthroline as ligands, particularly those of ruthenium, have been encountered in Section 57.3.2.2(iii). However, the bis-chelate complex was generally anchored to a polymer chain by coordination to pendant pyridyl groups although the possibility of electrochemical polymerization of a 4-vinyl-4-methyl-2,2 -bipyridyl was considered.59 In this section, some miscellaneous examples of the role of bipyridyl and phenanthroline complexes are considered. 

One further method to produce tris-chelates requires the use of a bis-chelate complex having two replaceable ligands. This will be considered later. 

Also of interest are the cyclic voltammetric (Box 4.1) properties of the two Cu(I) complexes, [Cu(10.102)2]+ and [Cu(10.104)2]+. The uncatenated bis(chelate) complex exhibits an entirely irreversible reduction at about —1.7 V (versus saturated calomel electrode reference, SCE), consistent with... 

The favourable intra-molecular hydrogen-bonding between the oxime proton and the phenol oxygen atom will reduce or overcome liganddigand repulsion enthalpy terms normally involved in the formation of bis-chelate complexes. The integrity of this head-to-tail macrocyclic assembly is preserved in c/.v-octahcdral complexes which are formed in the presence of a, co-diamines. [66, 67]... 

FIGURE 5.6 Definition of the skew-line convention used to define the absolute configuration of tris- or bis(chelate) complexes and conformation of diamine chelates. 

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