Cationic complexes, coordinative unsaturation

In the case of cationic complexes with unsaturated macrocycles two molecules of nucleophile, such as ammonia, amines and alkoxides, add to carbon atoms of two inline groups. For example, the reaction of [Ni(Bzo[16]octaeneN4)](C104)2 (Table 106) with sodium methoxide or ethoxide yields the compounds (395),2860 while with secondary amines and diamines complexes of type (396) are obtained.28 1 The reaction of (396) with acetone at room temperature yields complex (397) where the enolate anion of acetone, MeC(0)CH2, replaces the diethylamide group (Scheme 58). 2862 The addition of molecules such as bis(2-hydroxyethyl)methylamine and bis(2-hydroxyethyl) sulfide, HOCH2CH2YCH2CH2OH (Y = NMe, S) results in the formation of derivatives which possess one more coordination site just above the plane of the macrocyclic donors (398).2863... 

The synthesis of unsaturated compounds by C-C bond formation can also be carried out by coupling of alkenes with allenes, intermolecularly or intra-molecularly. Thus, 1,3-dienes were selectively obtained by coupling of allenes and vinyl ketones [28-30]. The reaction was catalyzed by the complex CpRuCl(COD) and with CeCl3 as a cocatalyst (Eq. 19). This cocatalyst is expected to decrease the chloride ion concentration to keep the active cationic ruthenium complex coordinatively unsaturated. 

When the rare earth cation is coordinatively unsaturated by the primary U nds, additional neutral or anionic substrates coordinate onto the metal ion to form a highly coordinated ternary complex. Fig. 5 schematically illustrates a complexation process in which europiiun tris(2,2,6,6-tetramethyl-3,5-heptanedionate) 6(Eu +) binds two pyridine molecules to yield... 

The reaction of the coordinatively unsaturated ruthenium amidinates with [Cp RuCl]4 tetramer or [CpRufMeCNlsJPFg provides access to novel amidinate-bridged dinuclear ruthenium complexes (Scheme 146), which in turn can be transformed into cationic complexes or hydride derivatives. In these complexes, a bridging amidinate ligand perpendicular to the metal-metal axis effectively stabilizes the highly reactive cationic diruthenium species. 

The important feature is the formation of a coordinatively unsaturated site (cus), permitting the reaction to occur in the coordinative sphere of the metal cation. The cus is a metal cationic site that is able to present at least three vacancies permitting, in the DeNOx process, to insert ligands such as NO, CO, H20, and any olefin or CxHyOz species that is able to behave like ligands in its coordinative environment. A cus can be located on kinks, ledges or corners of crystals [16] in such a location, they are unsaturated. This situation is quite comparable to an exchanged cation in a zeolite, as studied by Iizuka and Lundsford [17] or to a transition metal complex in solution, as studied by Hendriksen et al. [18] for NO reduction in the presence of CO. 

The active species of the metallocene/MAO catalyst system have now been established as being three-coordinated cationic alkyl complexes [Cp2MR] + (14-electron species). A number of cationic alkyl metallocene complexes have been synthesized with various anionic components. Some structurally characterized complexes are presented in Table 4 [75,76], These cationic Group 4 complexes are coordinatively unsaturated and often stabilized by weak interactions, such as agostic interactions, as well as by cation-anion interactions. Under polymerization conditions such weak interactions smoothly provide the metal sites for monomers. 

We synthesized cationic y2-acetyl compounds 28,25 by combining iron acetyl complexes CpFe(C0)L(C0CH3) (g7) [L=C0,PPh3] with a coordinatively unsaturated (16-electron) metal carbonyl salt CpM(CO)n+[M=Fe,n=2 M=Mo,n=3], as indicated in Scheme 5. Thus... 

For unsaturated lactones containing an endocyclic double bond also the two previously described mechanisms are presumably involved and the regio-selectivity of the cyclocarbonylation is governed by the presence of bulky substituents on the substrate. Inoue and his group have observed that the catalyst precursor needs to be the cationic complex [Pd(PhCN)2(dppb)]+ and not a neutral Pd(0) or Pd(II) complex [ 148,149]. It is suggested that the mechanism involves a cationic palladium-hydride that coordinates to the triple bond then a hydride transfer occurs through a czs-addition. Alper et al. have shown that addition of dihydrogen to the palladium(O) precursor Pd2(dba)3/dppb affords an active system, in our opinion a palladium-hydride species, that coordinates the alkyne [150]. 

The anion dissociates, and the coordinatively unsaturated metal center then picks up a monomer molecule for subsequent enchainment. This dissociative model has been favored in the past [16, 21-23, 27-28] since it allows a convenient explanation of the observed polymer stereochemistry by considering only the roles of the ligand and the alkyl chain in the cationic metallocene complex. However, anion dissociation opposes the electrostatic attraction between cation and anion and is therefore energetically expensive. So does it operate at all ... 

Tricoordinated boron compounds (boranes) are coordinatively unsaturated and their chemistry is dominated by reactions in which complexes are formed. These complexes are either neutral molecules (borane complexes), anions (borates) or boron cations. Space limitations mean that little or no attention will be paid to complexes containing several boron atoms and to species of the type L-BH3, [BH,]- and [L2BH2]+ (L = neutral ligand), discussed in detail in several books and reviews. Similarly, little attention will be paid to the plethora of metal borates and the cyclic and polymeric amino- and phosphino-boranes. 

One area of silver chemistry that has received much recent attention is the synthesis of multinuclear aggregates.315 Anionic metal dithiolato complexes such as K M(S2C202)2 were found to form adducts with silver(I) phosphines. In these complexes, the inert cations (K+) that accompanied the metallothio anions could be readily substituted by coordinatively unsaturated silver phosphines Ag(PR3)2. For dithiooxalto complexes this led to the isolation of a remarkable range of linkage isomers (34)-(37) 316,317... 

The sole example of a cationic formyl complex was reported recently by Thorn (67). It was obtained by the oxidative addition of formaldehyde to a coordinatively unsaturated iridium cation, as shown in Eq. (14). Characterization included IR and H, l3C, and 3IP NMR. Formyl 49 is stable as a solid to 146°C. 

Tomioka et al. reported the asymmetric Michael addition of lithium thiolates catalyzed by chiral aminoether 31 (Scheme 8D. 18) [39]. Thus, in the presence of catalytic amounts of 31 (10 mol %) and lithium 2-(trimethylsilyl)thiophenolate 32-Li (8 mol %), thiol 32 (3 equiv.) reacted with a,p-unsaturated esters at -78°C in toluene-hexane solvent to give the Michael adduct with up to 97% ee. In the ahsence of 31, the reaction of thiophenol proceeded in only 0.5% yield at room temperature. A monomeric complex consisting of 31 and lithium is proposed as the key reactive species in this asymmetric reaction. The trimethylsilyl group at the ortho-po-sition of the thiol moiety in 32 contributes to the formation of the stereochemically defined monomeric chelated structure, wherein the lithium cation is coordinated with the three heteroatoms of the tridentate ligand 31. The reactions of acyclic /nmv-a,P-unsaturated esters (R1 = Me, Et, Pr, Bu, Bu, PhCH9 R2 = H) proceeds with high enantioselectivity in... 

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