For a class of pi-complexes as energy donors, with the use of highly emissive d(0) metallocenes Cp(2)MCl(2) where M = Zr and Hf, it is shown that radiationless triplet energy transfer to unsaturated hydrocarbons in a rigid nonpolar methylcyclohexane (MCH) matrix (where molecular diffusion is absent) obeys the Perrin equation, occurs at radii R(0), being close to the diameter of interacting molecules, and is well described by an electron-exchange resonant (Dexter) mechanism of interaction. Principle correlation between values of the critical radius and a number of C atoms in a linear alpha-olefin is for the first time reported, viz. R(0) (Angstrom) = -2.47 + 1.35n (obtained for systems [Cp(2)HfCl(2) + CnH(2)n + MCH] at 77 K). Pronounced blue shifts (up to 1000 cm(-1)) of low-temperature emission spectra of Cp(2)ZrCl(2) in MCH in the presence of dienes, possessing nonconjugated C=C bonds, are observed, thus suggesting inner-sphere diene coordination. In view of the experimental data, inner- and outer-sphere coordinative interaction between the components of catalytic systems for polymerization (d(0) metallocene precatalysts and alkenes and dienes) is rationalized.
One of the most promising approaches to an understanding of the nature and properties of frontier orbitals of organometallic π complexes of Group IVB metalsmajor components of catalysts of a large number of organic syntheses-involves generation and study of electronically excited and ground redox states of molecules [1-6]. However, fundamental processes, such as energy and electron transfer, and properties of electronically excited states of organometallic compounds have been little studied partly due to the complexity of their synthesis and of manipulations with solutions of these compounds (especially with catalytic concentrations), which are extremely sensitive to moisture and oxygen traces and to minor impurities [7].In this paper, we report on direct photophysical experiments that demonstrate for the first time the triplet nature of long-lived ligand-to-metal charge transfer (LMCT) excited states based on Group IVB metallocenes. A strong specific interaction and competitive coordination of substrates of Ziegler-Natta catalystsunsaturated hydrocarbons (UHCs)-with metallocene precursors of active centers have been observed. Our approach is based on the method of radiationless T − T energy transfer from organometallic complexes of d 0 metals to UHCs and on the shift of optical spectra.It should be noted that solutions of metallocenes Cp 2 M ( IV ) Cl 2 (M = Zr, Hf; C = 10 -5 -10 -4 mol L -1 ) in unsaturated and saturated hydrocarbons were prepared and the hydrocarbons used additionally purified from trace impurities and moisture immediately prior to photophysical experiments. Energy transfer between UHC molecules and aromatic or organometallic donors at low concentrations of the donor ( c D ≤ 10 -4 M) and acceptor ( c Ä ~ 0.1-0.5 M) and cooperative effects were studied in glassy solutions of individual UHCs and in binary glassy solutions (UHC acceptor-MCH (methylcyclohexane), 1 : 1 vol/vol) at 77 K.Group IVB metal ions in most compounds have a closed electronic configuration d 0 and act as rather hard acids. However, as follows from the photoproperties of the representative triad of π complexes Cp 2 M ( IV ) Cl 2 , their radiative lifetimes decrease manyfold in the series Ti 4+ → Zr 4+ → Hf 4+ (table). It is worth noting that this is a consequence of a considerable increase in spinorbit coupling typical for transition metal ions with open electronic subshells, i.e., for soft acids.Previously [2,4,5], we assumed that these longlived states (their experimental lifetimes are among the largest known for metal-containing molecules) are triplet. In this case, we should observe their luminescence quenching in the presence of a triplet quencher of UHCs with large S -T splitting energies. Note that the properties of excited, especially triplet, states of UHCs have not hitherto been systematically studied; therefore, they represent a fundamental independent problem [8,9]. Under the conditions of the series (in MCH glasses), the lowest triplet level of UHCs with one or several unconjugated C=C bonds (3,3-dimethyl-1-butene...
The fundamental processes, such as energy and electron transfers and the optical and redox properties of coordination organometallic molecules have not been studied systematically [1, 2]. Studies of the energy transfer between organic molecules and transition metal compounds have been started only recently. The incomplete quenching of luminescence of complexes by aromatic molecules [3] is observed even when the triplet state of aromatic quencher is higher than the luminescing level of the metal complex. The outersphere complexation between the organometallic donor molecule and the aromatic quencher was found to influence the luminescence quenching [3].In this work, we describe for the first time the direct photophysical approach to the estimation of the energy and orbital nature of electron-excited states of organometallic compounds of early transition metals by the method of nonradiative triplet-triplet ( í -í ) energy transfer from organometallic d 0 metal complexes to unsaturated hydrocarbons with a higher energy of S -T splitting [4,5]. In the first approximation, the nonradiative energy transfer should be determined only by the energy of triplet levels of a donor ( D ) and acceptor ( A ). Energy acceptors were components of polymerization catalytic systems: unsaturated hydrocarbons ( UHCs ) of the ethylene (3,3-dimethylbut-1-ene, hex-1-ene, oct-1-ene, cyclohexene, etc.) and diene (1,7-octadiene, cyclopentadiene, and R(+)-limonene) series. The triplet levels were estimated for some hydrocarbons [6].The energy transfer between unsaturated hydrocarbon molecules and aromatic molecules, as well as organometallic donors (Group IVB metallocenes), and cooperative effects of components of the systems were studied in glasses of individual unsaturated hydrocarbons and glasses of two-component systems (unsaturated hydrocarbon acceptor + methylcyclohexane ( MCH ) 50 + 50 vol %) at 77 K. Note that the properties of excited states of unsaturated hydrocarbons, especially triplets, were not systematically studied, remaining a separate fundamental problem [5,7].Under conditions of the experimental series (in MCH glasses), the lowest triplet level ( E T ) of UHCs with ordinary or nonconjugated C=C bonds ranges within 24700-25740 cm -1 , whereas that of cyclopentadiene is below 24700 cm -1 (UHCs quenched the phosphorescence of diphenylamine and did not quench the phosphosphorescence of triphenylamine, while cyclopentadiene quenched the phosphorescence of both di-and triphenylamines). We have previously measured the luminescence lifetimes of Group IVB metallocenes Cp 2 å IV Cl 2 , where Cp is cyclopentadienyl, and M is Ti, Zr, or Hf [8]. They range from tenths to several milliseconds at 77 K, being among the longest lifetimes for the known metal-containing molecules [9]. We assumed that these long-lived states are triplet [8,10] and, hence, their luminescence should be quenched in the presence of a triplet unsaturated hydrocarbon quencher.Indeed, we observed the quenching of metallocene Cp 2 å IV Cl 2 luminescence by UHCs. In pa...
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