Mechanism of Photodenitrogenation of Salen Azido-Metal Complexes

J. Phys. Chem. B 2000, 104, 8361-8365

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Mechanism of Photodenitrogenation of Salen Azido-Metal Complexes within the Cavities of Zeolite Y Pilar Formentı´n,† Jose´ V. Folgado,‡ Vicente Forne´ s,† Hermenegildo Garcı´a,*,† Francisco Ma´ rquez,† and Marı´a J. Sabater*,† Instituto de Tecnologı´a Quı´mica UPV-CSIC, UniVersidad Polite´ cnica de Valencia, AV. Los Naranjos s/n, 46022 Valencia, Spain, and Institut de Ciencia dels Materials de la UniVersitat de Valencia, Dr. Moliner, 50, 46100-Burjassot, Valencia, Spain ReceiVed: NoVember 30, 1999; In Final Form: June 14, 2000

Salen nitridomanganese(V) and nitridochromium(V) complexes inside zeolite Y have been obtained by photodenitrogenation of encapsulated Mn(III) and Cr(III) azide complexes. The azide ligand was introduced in the preformed metallosalen complex using TMSN3. Supported Mn(III) and Cr(III) azides exhibit roomtemperature emission centered at 600 and 575 nm, respectively, which is identical to the emission observed for nitridomanganese(V) and nitridochromium(V) salen. Such emission is not observed in solution. It is concluded that in the zeolite the photodenitrogenation proceeds through adiabatic crossing from the azide excited state surface to the nitrido-excited surface with simultaneous dinitrogen loss.

Introduction Recent efforts to develop general olefin amination strategies have led to impressive advances in the chemistry of nitrido complexes containing transition metal-nitrogen multiple bonds (MtN).1-5 Complexes based on salen ligands [salen ) 1,2bis(salicylidenimino)cyclohexane] and transition metals such as manganese(V) or chromium(V) demonstrated efficient transfer of the nitrogen atom to alkenes.6,7 The remarkable stability of this metal-nitrogen triple bond, contrasting sharply with that of its isoelectronic manganese-oxygen MdO analogues, has given an impetus to the synthesis and characterization of novel nitridotransition metal complexes. Among the general synthetic procedures to prepare nitridometal, the photochemical denitrogenation of azides has the widest applicability. However, the preparative yields are typically only moderate due to subsequent oligomerization of the metal nitride under the irradiation conditions.8 Incorporation into rigid inorganic matrices such as zeolites is an increasingly used methodology to enhance the stability of the metal complexes and their activity in catalytic reactions.9,10 Similarly, immobilization of photoactive guests inside zeolites can influence their photochemical and photophysical properties.11-13 The remarkable ability of zeolites to stabilize reactants, intermediates, or even excited states due to a confinement effect has prompted us to carry out the photochemical preparation of nitridomanganese and nitridochromium complexes from the corresponding azides inside the supercages of zeolite Y. Encapsulation of the azidometal complexes into zeolite Y has allowed us to establish in the photodenitrogenation mechanism an adiabatic crossing from the excited azidometal complex to the potential energy surface of the excited nitridometal complex. The assignment of the denitrogenation mechanism was based on the emission spectra of the zeolite encapsulated azido-metal complexes that are absent in solution. This observation illustrates the potential of zeolites as a constrained medium to study photochemical reactions. * Corresponding authors. Communications to H.G. via fax: 963877809; e-mail: hgarcı´[email protected]. † Universidad Polite ´ cnica de Valencia. ‡ Institut de Ciencia dels Materials de la Universitat de Valencia.

Figure 1. (a) FT-IR spectrum of the unsupported (salen)MnN3 complex on KBr disk, prepared as reported in the literature.17 (b) FT-IR spectrum of zeolite Y sample after the ship-in-a-bottle synthesis of (salen)MnN3 as indicated in Scheme 1. Spectrum b was recorded at room temperature after outgassing at 100 °C under 10-2 Pa for 1 h.

Results and Discussion Preparation of the Azidometal Complexes inside the Zeolite Y Supercages. Manganese salen complex (1) was synthesized within the cavities of zeolite Y in a stepwise manner by reacting one diamine molecule (1,2-cyclohexanediamine) and two molecules of salicylaldehyde in the presence of Mn2+preexchanged zeolite Y (0.48% Mn2+, 1 Mn2+ every 5 supercages) (see Scheme 1). The sample was Soxhlet extracted with dichloromethane to remove the excess of ligand and reactants from the inorganic solid; oxidation to the Mn(III)salen complex was accomplished by air bubbling as previously reported.9,14 Subsequent treatment of the solid with trimethylsilyl azide (TMSN3) at ambient temperature for 12 h afforded the heterogeneous complex 1-Y. Formation of 1-Y was confirmed by comparison of its spectroscopic properties with those of an authentic sample of the unsupported complex 1 prepared following literature procedures.15,16 Particularly important for the identification are several sharp IR absorbances at 2037 cm-1 attributable to the azide ligand, the imine stretching vibration around 1615 cm-1, and the typical band of metallosalen complexes at 1535 cm-1 (Figure 1).15,16 On the other hand, recent quantitative EPR studies on the intrazeolitic oxidation process to obtain Mn(III)salen have shown that only about 20%

10.1021/jp9942163 CCC: $19.00 © 2000 American Chemical Society Published on Web 08/04/2000

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SCHEME 1. Preparation of Nitridomanganese(V) and Nitridochromium(V) Schiff Base Complexes in the Supercages of Zeolite Y by Photodenitrogenation of the Corresponding Azidometal

CHART 1

of initial Mn(II) cations (EPR active) are oxidized to Mn(III) (EPR silent) by molecular dioxygen under reaction conditions identical to those employed here.14 Our own EPR measurements show a similar population of azidomanganese(III) complexes in the cavities of zeolite Y after the above stepwise synthesis (around 20% of the initial Mn2+ ions). Despite this, the quantity and purity of encapsulated salen-derived azidomanganese(III) complexes is sufficiently high to characterize these molecules unequivocally by comparison with authentic specimens of the complex prepared by reported procedures. Specifically, results of UV-vis and IR spectroscopic studies rule out the presence of significant amounts of Mn(II)salen that could interfere in subsequent photochemical studies of these samples. A likely possibility that would reconcile all of the available data is that the fraction of non-oxidizable Mn(II) ions are those placed in the inaccessible hexagonal prisms (sites I) and sodalite cages (site type II) (Chart 1). Similarly, treatment of the Mn2+-preexchanged zeolite Y (0.48% Mn2+) with the previously synthesized ligand 1,2-bis(salicylideniminoethylene) followed by Soxhlet extraction, oxidation, and additional treatment with TMSN3 led to the formation of the heterogeneous Mn(III)(1,2-bis(salicylideniminoethylene)N3 complex 2-Y (Scheme 1). In this case, the ligand is not as big and rigid as that of complex 1, and, being flexible, it is able to diffuse into the micropores of Y zeolite. After complexation with Mn2+, the resulting rigid complex is larger than the cavity windows and remains immobilized inside the supercages. The encapsulated complex 2-Y was characterized

by diffuse reflectance UV-vis (DR) and IR spectroscopies, and these spectra compared with those obtained for a pure Mn(III)(1,2-bis(salicylideniminoethylene)]N3 (2) sample synthesized according to previously described methods.17 Analogously, synthesis of Cr(III) (1,2-bis(salicylideniminoethylene)]N3 inside the cages of zeolite Y (3-Y) was accomplished by a procedure similar to that described for complex 2 but in the presence of Cr3+-preexchanged zeolite (0.38% Cr3+; 1 Cr3+ every 5 supercages). No oxidation step is needed in the preparation of 3-Y (Scheme 1). The encapsulated complex 3-Y was fully characterized by IR and DR spectroscopies. The spectra obtained coincides with those obtained for a pure sample of Cr(III)(1,2-bis(salicylideniminoethylene)]N3 (3) synthesized according to reported methods.17 Concerning axial azide complexation, all of the attempts to form the encapsulated complexes 1-Y, 2-Y, and 3-Y using NaN3 were unsuccessful as proven by the absence of the most characteristic 2037 cm-1 band in the IR. Given that the crystal lattice of zeolites is negatively charged, this fact can be attributed to electrostatic repulsion of the framework on any negative ion. Thus, the diffusion of N3- through the interior of the zeolite pores would be impeded. The use of covalently bonded TMSN3 circumvents this problem, while acting as a convenient precursor of N3-. The formal TMS+ counterion is desorbed from the system probably as TMSOH or disiloxane after nucleophilic trapping by H2O. One of the main advantages of zeolite-supported complexes over their unsupported analogues is that, obviously, the compartmentalized structure of zeolite Y microporous makes impossible the dimerization and aggregation of individual complexes. This is a major drawback in solution that lowers the yield on the photochemical generation of metallonitrides from azides. The rigid zeolite Y supercage can accommodate only a single complex, the two metal centers being separated by a large distance estimated as that of two centers of neighbor cavities (∼14 Å). In contrast, it has been reported in the literature for numerous Cr(III) and Mn(III) azide complexes, such as 1, 2, and 3, their tendency in the solid state to aggregate in a chain-type structure that renders these compounds rather insoluble in noncoordinating solvents.18-20 In fact, from the FAB mass spectrum of 1, it is confirmed that Mn(III)salen solid exists as a dimer through the apical N3- ligand by the detection of a peak at the corresponding m/z ratio (m/z ) 792). This dimer cannot be accommodated, however, within the supercages of zeolite Y. Photochemical Denitrogenation of Encapsulated Azidometal Complexes. IR and Raman spectroscopic studies were

Photodenitrogenation of Salen Azido-Metal Complexes

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Figure 3. (a) UV-vis absorption spectrum of a 10-3 M solution of 2 in dichloromethane and (b) DR spectrum (plotted as the inverse of the reflectivity, 1/R) of the corresponding heterogeneous supported azidomanganese(III) complex 2-Y. The inset shows the detail of the 600 nm band of the emission spectra upon 400 nm excitation recorded at room temperature for the (a) homogeneous and (b) solid azide 2-Y.

Figure 2. Part of the FT-IR (A) and Raman (B) spectra of the (salen)MnN3 encapsulated within Y zeolite upon photolysis. The spectra were recorded at (a) 0 min, (b) 60 min, (c) 120 min, and (d) 240 min of irradiation time. The sample was irradiated in a sealed cell under vacuum through CaF2 windows.

found very informative to follow the photochemical transformation of the supported complexes 1-Y, 2-Y, and 3-Y to the corresponding nitrides. In principle, the photochemical azide-tonitride conversion of pure complex 1 should produce the disappearance of the typical intense azide stretching vibration at 2037 cm-1 with the concomitant increase of a new absorption in the 1000-1100 cm-1 region, characteristic of a MntN triple bond. For the nonencapsulated complexes, these two changes could be simultaneously recorded by IR spectroscopy. However, for the complexes embedded within the zeolite, the very intense absorption of the Si-O bonds of the zeolite framework near 1010 cm-1 makes impossible to monitor below 1300 cm-1. Therefore only the disappearance of the N3- upon the irradiation time can be recorded. Fortunately, the information gained by IR spectroscopy can be combined with that obtained from Raman spectroscopy. In the latter spectroscopy, neither N3- nor SiO give any band, but the metal-nitride triple bond can be observed at 1050 cm-1.4 Figure 2 shows parallel IR and Raman spectra during the photolytic decomposition of zeolite embedded complex 1-Y recorded at different time intervals. Note that in Figure 2, with the disappearance of the characteristic IR azide stretching band, there is a concomitant increase of a Raman absorption at 1050 cm-1 due to the manganese(V)-nitride triple bond that is being progressively formed during the course of the irradiation. Finally, DR of Y zeolites containing metalnitride complexes showed slight blue-shifted bands with respect to the corresponding UV-vis spectra of the same complexes in solution (Figure 3). This shift can be interpreted as a reflection of the interaction of the substrate and the zeolite due to the steric restrictions imposed by the crystal structure of the host. Molecular modeling predicts that there is a tight fit of the optimized geometry for the nitridomanganese(V) complex inside the supercavities of zeolite Y (Figure 4).

Figure 4. Molecular modeling of optimized geometry of salen-derived nitridomanganese(V) complex inside the cavities of zeolite Y.

Interestingly, pure manganese(III) azide complexes 1 and 2 do not fluoresce in solution. In contrast, when the same complexes are encapsulated within the zeolite Y a characteristic emission dominated by a weak and broad band around 600 nm is observed (see Figure 3) whose excitation spectra are compatible with the DR absorption of the azides. On the other hand, emission spectra of the corresponding nitrido manganese(V) species showed a common broad emission centered at 600 nm both in solution and embedded within the zeolite. Chromium-salen complexes exhibit a parallel behavior to that commented above for manganese complexes: (i) no emission for the unsupported azido complex in solution, (ii) broad emission for the embedded azido complex (λmax ) 575 nm), and (iii) emission for the unsupported and supported nitridochromium complexes that coincides with that of encapsulated azido complex. The possibility that the emission would arise from the direct light absorption of some nitrido complexes generated photochemically during the fluorescence measurement was ruled out based on the following observations: (i) excitation spectra matches the UV-vis absorption spectra of the azide, (ii) the same metal-azido complexes in solution do not undergo transformation to the nitridometal complexes even after much longer exposures (1 day) to the fluorimeter excitation source,

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Figure 6. Square-pyramidal coordination around the metal ion in the salen-derived nitrido complexes and the corresponding ligand field energy diagram.

Figure 5. Potential energy diagram of the adiabatic phototransformation of azides to the corresponding nitrido complexes. Diagram shows the overlap of vibrational levels between the singlet excited states of the azide (S1) and nitride (S1′). Decay from S1′ gives the fluorescence that is observed both for the encapsulated azides and nitrides.

and (iii) the intensity of the emission bands for the embedded metal-azide complexes does not vary upon prolonged exposures to the fluorimeter excitation light for periods 2 orders of magnitude longer than those required to record the luminescence spectra. These observations agree with the very low intensity of the excitation lamps employed on fluorimeters.21 Even more, it is a general phenomenon that, due to strong light scattering, photochemical reactions in opaque solids are considerably much slower compared to the same process in solution. What we propose is that the luminescence from the metalazido complexes embedded within zeolites arises from an adiabatic crossing of the potential energy surface of excited metalazide to that of excited metal-nitride complex. This process, not observed in solution, involves loss of nitrogen and concomitant oxidation of the metal while the excited state of the azide complex is converted into the excited state of metal-nitride. Clearly, the absorbing species is the azide complex, but the emitting one would be the nitride (a schematic illustration of this process is depicted in Figure 5). Related precedents for adiabatic photochemical reactions in the photodehydroxylation of polycyclic arenes leading to aromatic cations have been reported.22 Concerning the different photochemical behavior of the metal-azide complexes in solution or when adsorbed on zeolites, it is now well established that immobilization of a bulky guest inside the micropores of zeolites increases the efficiency of emission, disfavoring other competitive radiationless deactivating pathways. Nonemissive decays generally arise from conformational freedom and collision with the solvent that efficiently remove the excess energy, returning the azide to the ground state. We have already provided several examples of a non-emitting molecule in solution that become emissive upon incorporation within zeolites.23,24 On the other hand, assignment of the nitrido-emitting state is possible based on recent structural studies.25 As revealed by X-ray diffraction, the structure of a salen-derived nitridomanganese(V) complex has a distorted square-pyramidal geometry with a short MntN distance (1.52 Å) and the Mn raised around 0.492 Å above the base (Figure 6). According to these results, the electronic structure of nitridomanganese(V) complexes can be readily described by the ligand-field theory where the inequivalent π interactions between the MtN chromophore and the salen ligands would remove the degeneracy of the dxz and dyz orbitals in the unoccupied dπ* level leading to the ligandfield splitting diagram shown in Figure 6.26 A similar electronic

structure can be applied for nitridochromium(V) complexes. As can be seen in Figure 6, in the 1A(a′(x2-y2)]2 ground state, the formally nonbonding a′(x2-y2) level is filled; then the lowest excited states from which emission could be observed should be 1A′(a′(x2-y2)a′(yz)] and 1A′(a′(x2-y2)a′(xz)]. Moreover, spin-orbit coupling constants for 1d [Cr(salen)tN] complexes predict also low spin-orbit coupling values for 3MntN complexes, thus intersystem crossing from the singlet state to the triplet state is expected to be inefficient. This could facilitate the observation of fluorescence from the first singlet excited state.27 Conclusion The photolytic denitrogenation of manganese(III) and chromium(III) azide complexes embedded inside the cavities of zeolite Y clearly furnishes nitridomanganese(V) and nitridochromium(V) complexes. This transformation can be followed by the disappearance in IR of the typical azide vibration and the growth in Raman spectroscopy of the band characteristic of the nitrido group. Nitridomanganese(V) and nitridochromium(V) complexes exhibit fluorescence either in solution or incorporated within zeolite Y as a result of a spin-allowed a′(x2-y2) r dπ* transition facilitated by low spin-orbit coupling constants for these 3MtN complexes. The fact that the same emission is observed upon excitation of azidometal complexes within zeolites indicates that the mechanism of photodenitrogenation takes place through an adiabatic crossing from the excited state azidometal surface to the excited state of the nitridometal. This insight into the reaction mechanism has been possible due to the effect of the zeolite host enhancing the emission efficiency of immobilized hosts. This examplifies the potential of zeolites as a compartmentalized, rigid matrix to perform preparative photochemistry of transition metal complexes. Experimental Section Irradiation Procedure. The irradiation of unsupported complexes in pressed KBr pellets was carried out at room temperature using Pyrex filtered light (λg300 nm) from a mediumpressure 125 W mercury lamp equipped with an outer jacket refrigerating system. Similarly, irradiation of zeolite-embedded samples was carried out with self-supported compressed pellets at room temperature through Pyrex (λ g 300 nm) using the same photochemical reactor as for the supported complexes. EPR Spectroscopy. Room-temperature EPR spectra were recorded on a Bruker ER200D spectrometer, working at X band (9.65GHz) and using DPPH (g ) 2.0036) as standard reference. IR Spectroscopy. FT-IR spectra of complexes in zeolites were recorded at room temperature using a greaseless CaF2 cell in a Nicolet 710 FT spectrophotometer. Self-supported wafers (∼10 mg) were prepared by pressing the zeolite powder at 1 Ton × cm-2. The samples were outgassed at 100 °C under 10-2 Pa for 1 h before recording the spectra.

Photodenitrogenation of Salen Azido-Metal Complexes Raman Spectroscopy. The FT-Raman spectra were recorded on a Bio-Rad FT-Raman II spectrophotometer. The 1.064 µm line of a Nd:YAG laser was used for excitation along with a germanium detector cooled at liquid nitrogen temperature. The Raman spectra of powder samples were examined in the 180° scattering configuration using high-quality quartz tubes as cells. The laser power at the samples was ∼100 mW. The Raman spectra were corrected for instrumental response using a white light reference spectrum. Emission Spectroscopy. The emission spectra of pure complexes and powder samples were recorded with a FS900 Edinburgh spectrofluorimeter with a Czerny-Turner monochromator in the 200-800 nm range exciting at wavelengths of the CT maxima. The spectra were recorded under nitrogen. The emission spectra of nonsupported complexes were recorded in dichloromethane 10-2 M under nitrogen. Molecular Modeling. Molecular modeling was performed at the semiempirical level using the Insight II program from Biosym and a Silicon Graphics workstation. Preparation of 1-Y, 2-Y and 3-Y Complexes. A mixture of calcined and dehydrated Mn2+- preexchanged zeolite Y (1 g, 0.48% Mn2+, determined by atomic absorption spectroscopy) and salicylaldehyde (0.37 g, 3.03 mmol) was stirred under reflux in dichloromethane (10 mL) for 6 h. The mixture was filtered and the amount of adsorbed salicylaldehyde was calculated from the difference between the weight of the residue and the initial amount of salicylaldehyde; then a stoichiometric amount of the cyclohexanediamine in dichloromethane (10 mL) was added. The resulting yellow slurry was refluxed for 12 h under nitrogen. Addition of equimolar amounts of trimethylsilyl azide was followed by stirring at room temperature with air bubbling for 12 h. The resulting encapsulated complex 1-Y was filtered and Soxhlet extracted in dichloromethane for 8 h to remove excess of complex and reactants from the inorganic solid. Sample 2-Y was prepared by refluxing calcined Mn2+ preexchanged zeolite Y (1 g) and 1,2-bis(salicylideniminoethylene (0.150 g, 0.56 mmol) of) in dichloromethane (10 mL) for 6 h. The mixture was filtered and the residue was dichloromethane washed, dried, and treated with a solution of trimethylsilyl azide (0.02 g, 0.173 mmol). The suspension was stirred at room temperature for 12 h while bubbling air and filtered. The heterogeneous complex 2-Y thus obtained was purified by Soxhlet extraction with dichloromethane for 8 h to remove excess complex and starting materials. A similar procedure was adopted for the preparation of the heterogeneous complex 3-Y using Cr3+-preexchanged zeolite (0.38% Cr3+, determined by atomic absorption spectroscopy). The heterogeneous complex 3-Y was afforded without bubbling air.

J. Phys. Chem. B, Vol. 104, No. 35, 2000 8365 Acknowledgment. Financial support by the Spanish DGICYT (H.G., MAT97-1016-CO2) is gratefully acknowledged. M.J.S. thanks the DGICYT for a reincorporation fellowship. We thank Mr. Emilio Palomares for technical assistance. References and Notes (1) Denicke, K.; Stra¨hle, J. Angew. Chem., Int. Ed. Engl. 1981, 20, 413. (2) Buchler, J. W.; Dreher, C.; Lay, K.-L.; Lee, J. A.; Scheidt, W. R. Inorg. Chem. 1983, 22, 888. (3) Che, C.-M.; Ma, J.-X.; Wong, W.-T.; Lai, Z.-F.; Poon, C.-K. Inorg. Chem. 1988, 27, 2547. (4) Griffith, W. P. Coord. Chem. ReV. 1972, 8, 372. (5) Denicke, K.; Stra¨hle, J. Angew. Chem., Int. Ed. Engl. 1992, 31, 955. (6) (a) Du Bois, J.; Tomooka, C. S.; Hong, J. H.; Carreira, E. M. Acc. Chem. Res. 1997, 30, 364. (b) Du Bois, J.; Hong, J.; Carreira, E. M.; Day, M. W. J. Am. Chem. Soc. 1996, 118, 915. (c) Du Bois, J.; Tomooka, C. S.; Hong, J.; Carreira, E. M. J. Am. Chem. Soc. 1997, 119, 3179. (7) Groves, J. T.; Takahashi, T. J. Am. Chem. Soc. 1983, 105, 2073. (8) Groves, J. T.; Takahashi, T.; Butler, W. M. Inorg. Chem. 1983, 22, 884. (9) Sabater, M. J.; Corma, A.; Dome´nech, A.; Forne´s, V.; Garcı´a, H. Chem. Commun. 1997, 1285. (10) Ogunwumi, S. B.; Bein, T. Chem. Commun. 1997, 901. (11) Corma, A.; Garcı´a, H.; Miranda, M. A.; Primo, J.; Sabater, M. J. J. Org. Chem. 1993, 58, 6892. (12) Sabater, M. J.; Garcı´a, S.; Alvaro, M.; Garcı´a, H.; Scaiano, J. C. J. Am. Chem. Soc. 1998, 120, 8521. (13) Alvaro, M.; Forne´s, V.; Garcı´a, S.; Garcı´a, H.; Scaiano, J. C. J. Phys. Chem. 1998, 102, 8744. (14) Sabater, M. J.; Corma, A.; Folgado, J. V.; Garcı´a, H. J. Phys. Org. Chem. 2000, 13, 57. (15) Li, H.; Zhong, Z. J.; Duan, C.-Y.; You, X.-Z.; Mak, T. C. W.; Wu, B. Inorg. Chim. Acta 1998, 271, 99. (16) Prasad, D. R.; Ramasami, T.; Ramaswamy, D.; Santappa, N. Synth. React. Inorg. Met.-Org. Chem. 1981, 11, 431. (17) Leighton, J. L.; Jacobsen, E. N. J. Org. Chem. 1996, 61, 389. (18) Stults, B. R.; Marianelli, R. S.; Day, V. W. Inorg. Chem. 1975, 14, 722. (19) Li, H.; Zhong, Z. J.; Duan, C.-Y.; You, X.-Z.; Mak, T. C. W.; Wu, B. Inorg. Chim. Acta 1998, 271, 996. (20) Hansen, K. B.; Leighton, J. L.; Jacobsen, E. N. J. Am. Chem. Soc. 1996, 118, 10924. (21) Attempts to measure the irradiance of the fluorimeter lamp at 350 nm using a ferrioxilate actinometer for 5 h were unsuccessful due to the low intensity of the source. (22) (a) Wan, P.; Krogh, E. J. Am. Chem. Soc. 1989, 111, 4887. (b) Wan, P.; Yates, K.; Boyd, M. K. J. Org. Chem. 1985, 50, 2881. (23) Alvaro, M.; Garcı´a, H.; Garcı´a, S.; Ma´rquez, F.; Scaiano, J. C. J. Phys. Chem. B 1997, 101, 3043. (24) Cano, M. L.; Cozens, F. L.; Forne´s, V.; Garcı´a, H.; Scaiano J. C. J. Phys. Chem. 1996, 100, 18145. (25) Jepsen, A. S.; Robertson, M.; Hazell, R. G.; Jorgensen, K. A. Chem. Commun. 1998, 1599. (26) Chang, C. J.; Connick, W. B.; Low, D. W.; Day, M. D.; Gray, H. B. Inorg. Chem. 1998, 37, 3107. (27) Azuma, N.; Imori, Y.; Yoshida, H.; Tajima, K.; Li, Y.; Yamauchi, J. Inorg. Chim. Acta 1997, 266, 29.