Mixed-metal alkynyl complexes

Heterobimetallic and polymetallic structures featuring bridging acet-ylides can also be prepared from the respective Cu(I), [or Ag(I)], and Re(I) substmctures described above. Excitation of both the penta- (224, 230) and decanuclear (203) mixed-metal alkynyl complexes in the solid state and in solution at X > 400 nm results in strong red luminescence (680-700 nm) characteristic of predominantly [i Ti(Re) ti (NN)] MLCT excitation mixed with ti(C=C) —> 7t (NN) LLCT perturbed by the Mg or Mg cores of the components. The MLCT state energy in the absorption spectrum for M = Ag(I) is shghtly blue-shifted relative to M = Cu(I) due to the poorer donating ability of the Agg versus Cug core, which causes the Re(I) center of the [Re(CO)g(NN)]" fragment to be less electron rich. 

In view of the rapid development in the synthesis of metal alkynyl complexes and the inherent advantage of the rigidity of such systems, attempts have been made by researchers to synthesize mixed-valence metal alkynyl [210, 211] and mixed-metal alkynyl complexes [64]. The incorporation of different metal centers to the organic spacer, which is usually achieved by the metalloligand approach, may provide a good handle to investigate the electron transfer and communication across the molecule. Isolation of mixed-valence homometallic species have... 

The robustness of the rhenium(i) diimine alkynyl systems and rich photophysical behavior have rendered them suitable as metalloligands for the synthesis of mixed-metal complexes. It is well-known that organometallic alkynes exhibit rich coordination chemistry with Cu(i), Ag(i) and Au(i) [214-218], however, photophysical properties of these r-coordinated compounds are rare. Recent work by Yam and coworkers has shown that luminescent mixed-metal alkynyl complexes could be synthesized by the metalloligand approach using the rhenium(i) diimine alkynyl complexes as the z -ligand. Reaction of the rhenium(i) diimine alkynyl complex [Re(bpy)(CO)3C=CPh] with [M(MeCN)4]PF6 in THF at room temperature in an inert atmosphere afforded mixed-metal Re(i)-Cu(i) or -Ag(i) alkynyl complexes (Scheme 10.31) [89]. Their photophysical properties have also been studied. These luminescent mixed-metal complexes were found to emit from their MLCT[d7i(Re) —> 7i (N N)] manifolds with emission bands blue-shifted relative to their mononuclear precursors (Table 10.5). This has been attributed to the stabilization of the dTi(Re) orbital as a consequence of the weaker t-donating ability of the alkynyl unit upon coordination to the d metal centers. 

Table 10.6. Electronic absorption and emission data of Re-Fe mixed-metal alkynyl complexes in dichloromethane solution. Data adapted from [222].

Table 10.7. Electronic absorption and photophysical data of Re(i)-Pd(ii) mixed-metal alkynyl complexes. Data adapted from [223].

The first and most influential molecular-orbital calculation on metal-alkynyl complexes is that of Kostin and Fenske, who applied the Fenske-Hall method to the complexes FeCp(C=CH)(PH3)2 and FeCp-(C=CH)(C0)2 (11). They concluded that the M-CCH bonds in these complexes are nearly pure a in character. The large energy gap (ca. 15 eV) between the occupied metal orbitals and ir (C=CH) levels severely limits the ir-accepting quality of the latter, with the total electron population for the pair of tt orbitals being 0.22 e for FeCp(C=CH)(PH3)2 and 0.14 e" for FeCp(C=CH)(CO)2. The filled ir(C=CH) orbitals, in contrast, mix extensively with the higher-lying occupied metal orbitals these filled-filled interactions result in the destabilization of the metal-based orbitals. The HOMOs of both complexes possess substantial coefficients at the alkynyl jS-carbon this was noted to be consistent with the alkynyl-localized reactivity of these complexes. 

Syntheses and luminescence studies of mixed-metal gold(I)-copper(l) and -silver (I) alkynyl complexes. The tuming-on of emission upon d metal ion encapsulation. Dalton Transactions, 1830-1835. 

F. Mixed-Metal Clusters Derived from (l-Alkynyl)carbyne Complexes 232... 

Vicente and co-workers have recently synthesized platinum(n) 7 and mixed platinum(II)/ gold(I) a-alkynyl polymers 8.53 The monomers were prepared via dehydrohalogenation between c -[PtCI2( PR jb I and various alkynes. Polymerization occurred via the reaction of the platinum complex with PPN[Au(acac)2], giving polymers with mixed metal backbones. 

Gold(l)-alkynyls have also been employed as T] -metalloligands in the constmction of mixed-metal gold(l)-copper(l) and silver(l) complexes of the form [rj -(R3P)AuC=CC(=CH2)Me2Cu]PF6 (R = Ph, p-Tol) and... 

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