Cationic complexes, bimetallic

Bimetallic catalysts can be prepared by a direct redox method when a cationic complex of a metal of higher electrochemical potential is reduced by another metal of lower electrochemical potential that has been deposited and reduced first187 (see Table 4.10) PdAu/SiO 88 and PdAu/C189 have been made in this way, gold being deposited after the palladium. Small amounts of the metals were found in filtrates, and XRD and temperature-programmed decomposition of palladium hydride indicated a substantial... 

Preference for neutral metallocene dimethyl coordination over anion coordination to form /(-Me di-nuclear cationic complexes (e.g., 43) is observed for those truly weakly coordinating anions such as B(C6F5)4 and MePBB . Stabilization of highly reactive and unstable metallocenium cations by /(-Me coordination allows isolation and characterization of such species in the pure state, yet affords excellent polymerization activity in solution, presumably via dissociation to a more reactive monomeric form, as indicated by NMR studies.For more coordinating anions such as CH3B(C6F5)3, /(-Me bimetallic cationic complexes are not detected, except when an excess of neutral metallocene dimethyl is employed (Scheme 25) 143,315 equilibrium can be utilized to stabilize... 

Recently, another crown-strapped porphyrin 219 similar to 217 has been prepared by the condensation of 225b (see Scheme 67) with the bis-amino crown ether 218 (Scheme 65) . The corresponding metalloporphyrin is a potential host for both anionic and cationic species. Bimetallic complexes were prepared and, for paramagnetic guest cations e.g. Cu , Fe , fluorescence quenching was observed. Studies with the perchlorate salts of diamines indicated attachment of the perchlorate salt at the porphyrin metal site and complexation of the ammonium species at the crown ether. 

The neutral complexes can be converted into the cationic Ind complexes of the type [NiInd(PR 3)L] (197 L = PR 3, PR 3, MeCN, CNBu ) by halide abstraction, protonation of organic ligands, or simple displacement of the triflate moiety. This strategy has been employed for the preparation of an extensive series of cationic complexes, including the monometallie species [Ni(2-Me-Ind)(dippe)]BPh4, 198, " and the bimetallic Ni complexes bearing... 

Harrowfield et al. [37-39] have described the structures of several dimethyl sulfoxide adducts of homo bimetallic complexes of rare earth metal cations with p-/e rt-butyl calix[8]arene and i /i-ferrocene derivatives of bridged calix[4]arenes. Ludwing et al. [40] described the solvent extraction behavior of three calixarene-type cyclophanes toward trivalent lanthanides La (Ln = La, Nd, Eu, Er, and Yb). By using p-tert-huty ca-lix[6Jarene hexacarboxylic acid, the lanthanides were extracted from the aqueous phase at pH 2-3.5. The ex-tractability is Nb, Eu > La > Er > Yb. 

Bimetallic activation of acetyl and alkoxyacetyl ligands — through formation of cationic P2 acyl complexes — to reaction with nucleophilic hydride donors was established. Cationic transition metal compounds possessing an accessible coordination site bind a neutral T -acyl ligand on another complex as a cationic P2 acyl system. These i2 3icyl systems activate the acyl ligand to reduction analogous to carbocation activation. Several examples of i2-acyl complexation have been reported previously. 

A two-component bimetallic catalytic system has been developed for the allylic etherification of aliphatic alcohols, where an Ir(i) catalyst acts on allylic carbonates to generate electrophiles, while the aliphatic alcohols are independently activated by Zn(n) coordination to function as nucleophiles (Equation (48)).194 A cationic iridium complex, [Ir(COD)2]BF4,195 and an Ru(n)-bipyridine complex196 have also been reported to effectively catalyze the O-allylation of aliphatic alcohols, although allyl acetate and MeOH, respectively, are employed in excess in these examples. 

The thermal reaction of 1,3-dienes with Co2(CO)8 gives the corresponding bimetallic dimers [(diene)Co(CO)2]2 132 as orange red solids in good yields (Scheme 27)28,133. Oxidation of the dimeric complexes 132 with ferricinium tetrafluoroborate or triphenyl-carbenium tetrafluoroborate gives the monomeric (diene)Co(CO)3+ cations 133 in modest... 

The formation of the bimetallic complexes 118-123 involves combination of di-cationic transition metal fragments with the dianion of compound 14. A further possibility exists, scarcely studied for 12-vertex 0/050-2,l-ReCBjo species but more extensively exploited in the analogous 11-vertex c/o5o-l,2-MCBg system (M = Mn, Re) discussed in Section IV. Treatment of the dianion of 14 with two... 

Although stable and neutral bimetallic complexes were obtained from the dicat-ionic fragments and 129 or 130 there was the possibility that reactions with two molar equivalents of a metal-ligand monocation should give rise to trimetallic species. However, reactions of 130 with sources of the cations M (CO)3 (M = Mn, Re), namely [Mn(NCMe)3(CO)3][PF6] and [ReBr(THF)2(CO)3], yielded as products [N(PPh3)2][l,3,6- M (CO)3 -3,6-(p-H)2-l,l,l-(CO)3-2-Ph-c/ow-l,2-ReCBgHT] [M = Mn (156), Re (157)], respectively (Chart 27). ° In these molecules, the exo-polyhedral M (CO)3 moiety is almost certainly bonded to the cluster via two non-equivalent B-H M interactions with an Re-M bond completing... 

Other Compounds. Adsorption of MeNC into a cobalt(ii) zeolite at — 196"C was followed by e.s.r. spectroscopy, and the presence of low-spin [Co-(CNMe)j] and [Co(CNMe) ] complex cations demonstrated.This work represents one of the few successful attempts to produce well-characterized transition-metal complexes in a zeolite framework. Addition of NaCp to CoClj and [(BgC2H,Q)CoCp], previously reduced with sodium naphthalide gives a new bimetallic complex, for which structure (94) is proposed. ... 

Oxidation of d V(V) complexes must occur at the ligands. Although many oxidations are irreversible, some reversible processes are known. For example, the square-pyramidal bimetallic complex shown in Fig. 23 has two reversible oxidations Ef" = —0.05 and 0.37 V versus Cp2Fe/MeCN), the first of which results in a cationic ligand radical species, on the basis of its Ef value, its EPR spectrum, and UV-vis data [111]. The complex also shows two closely spaced reductions (Ef =—1.2(> and —1.36 V... 

---