Inorg. Chem. 2006, 45, 10909−10921
Luminescent Hydrido-Carbonyl Clusters of Rhenium Containing Bridging 1,2-Diazine Ligands Monica Panigati,† Daniela Donghi,† Giuseppe D’Alfonso,*,†,§ Pierluigi Mercandelli,*,‡,§ Angelo Sironi,‡,§ and Laura D’Alfonso# Dipartimento di Chimica Inorganica, Metallorganica e Analitica, UniVersita` di Milano and INSTM UdR di Milano, Via Venezian 21, 20133 Milano, Italy, CIMAINA, Via Celoria 16, 20133 Milano, Italy, Dipartimento di Chimica Strutturale e Stereochimica Inorganica, UniVersita` di Milano, Via Venezian 21, 20133 Milano, Italy, and Dipartimento di Fisica, UniVersita` di Milano-Bicocca, Piazza delle Scienze 6, 20126 Milano, Italy Received August 4, 2006
The reaction of the electronically unsaturated (56 valence electrons, ve) tetrahedral cluster [Re4(µ3-H)4(CO)12] (1) with pyridazine (pydz) gives as the main initial product the tetranuclear cluster [Re4(µ-H)4(µ-pydz)(pydz)2(CO)12] (2a), with 64 ve and four hydrogen-bridged metal−metal interactions, with a spiked-triangle geometry. One of the three pydz ligands bridges, in a cis configuration, the cluster edge opposite to the vertex bearing the spike, as indicated by the X-ray single-crystal analysis. This species slowly decomposes, affording the dinuclear unsaturated (32 ve) complex [Re2(µ-H)2(µ-pydz)(CO)6] (3a) and two isomers of the tetranuclear cluster [Re4(µ-H)4(µ-pydz)2(CO)12] (64 ve), sharing an unusual square cluster geometry and differing in the trans (major, 85%, 4a) or cis (4a′) configuration of the bridging pydz ligands. The structures of 3a and 4a have been ascertained by X-ray analysis, while the characterization of 4a′ was hampered by its instability (slowly transforming into 3a in THF solution). Both the dimer and the square cluster 4a are also formed directly (and quickly) from 1, being present in solution since the beginning of the reaction. Cluster 4a is the main final reaction product. The reaction with phthalazine follows a similar course, with some differences in the relative amount of the final products 3b and 4b. Most of the novel complexes are able to emit light in solution at room temperature, and photophysical measurements were performed in CH2Cl2 solution on the main stable reaction products (i.e., the dinuclear species 3a and 3b and the trans square clusters 4a and 4b). The emission was in the range of 580−645 nm, from MLCT excited states, with lifetimes on the order of a hundred nanoseconds (50−473 ns). The quantum yields were 1 order of magnitude higher for the squares (1.7 and 1.3% for 4a and 4b, respectively, in CH2Cl2) than for the dinuclear complexes (∼0.1%). In the case of 4a, a blue shift and an increase of the emission intensity were observed upon decreasing the solvent polarity.
Introduction A large number of electronically unsaturated hydridocarbonyl clusters of rhenium have been synthesized and characterized in the past,1-8 much larger than for any other transition metal. Many of these unsaturated clusters are of * To whom correspondence should be addressed. E-mail:
[email protected] (G.D.);
[email protected] (P.M.). † Dipartimento di Chimica Inorganica, Metallorganica e Analitica, Universita` degli Studi di Milano. ‡ Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Universita` degli Studi di Milano. § CIMAINA (Centro Interdisciplinare Materiali e Interfacce Nanostrutturati). # Dipartimento di Fisica, Universita ` degli Studi di Milano-Bicocca.
10.1021/ic061467z CCC: $33.50 Published on Web 12/06/2006
© 2006 American Chemical Society
interest because of their peculiar reactivity, arising from the easy addition of nucleophiles. The “ethylene-like” dinuclear [Re2(µ-H)2(CO)8] complex (32 valence electrons, ve), for (1) Among purely hydrido-carbonyl species, we mention the dinuclear [Re2(µ-H)2(CO)8] complex,2 the trinuclear [Re3(µ-H)3(CO)10]2-,3 [Re3(µ-H)4(CO)10]- 4 and [Re3(µ-H)3(µ3-H)(CO)9]- anions (in fact the latter in the solid state has been characterized as a dimeric hexanuclear species,5 but spectroscopic evidence suggests the monomeric trinuclear structure in solution),6 and the tetranuclear [Re4(µ3-H)4(CO)12]7 and [Re4(µ-H)3(µ3-H)2(CO)12]- species.8 (2) (a) Bennet, M. J.; Graham, W. A. G.; Hoyano, J. K.; Hutcheon, W. L. J. Am. Chem. Soc. 1972, 94, 6232-3. (b) Masciocchi, N.; Sironi, A.; D’Alfonso, G. J. Am. Chem. Soc. 1990, 112, 9395-7. (3) Bertolucci, A.; Freni, M.; Romiti, P.; Ciani, G.; Sironi, A.; Albano, V. G. J. Organomet. Chem. 1976, 113, C61-4.
Inorganic Chemistry, Vol. 45, No. 26, 2006
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Panigati et al. Scheme 1
[Re4(µ3-H)4(CO)12] + 4L f [Re3(µ-H)4L(CO)9]- + [ReL3(CO)3]+ (1) In the reaction with CO, a different (neutral) fragmentation has been observed (eq 2),7a while with pyridine, both the products of the ionic and of the neutral path have been identified.12c [Re4(µ3-H)4(CO)12] + 5L f [Re3(µ-H)3L3(CO)9] + [HReL2(CO)3] (2)
instance, provided the first example of addition polymerization of an organometallic complex that closely mimics the polymerization of olefins.9 On the other hand, the monoand bis-unsaturated triangular cluster anions [Re3(µ-H)4(CO)10]- and [Re3(µ-H)3(µ3-H)(CO)9]- (46 and 44 ve, respectively) provided useful platforms for anchoring organic substrates that were successively involved in hydrogentransfer reactions.10 The present paper addresses novel aspects of the reactivity of the tetranuclear complex [Re4(µ3-H)4(CO)12] (1), which possesses four electrons less than that required by the effective atomic number rule (56 instead of 60 ve). In agreement with its high unsaturation, 1 reacts with any (even weakly) donor species, L.6,7a Surprisingly enough, however, 1 usually adds more ligands than required to attain saturation (four instead of two),11 resulting in the formation of an unstable tetra-addition derivative with a spiked-triangle structure, which then undergoes fragmentation to lower nuclearity species, according to the two main pathways depicted in Scheme 1.12 With weak σ-donor ligands, such as acetonitrile, tetrahydrofuran, acetone, or methanol, the reaction has the “ionic” stoichiometry 1 and affords an unsaturated (46 ve) triangular cluster anion and the [ReL3(CO)3]+ cation. (4) Beringhelli, T; Ciani, G.; D’Alfonso, G.; Molinari, H; Sironi, A. Inorg. Chem. 1985, 24, 2666-71. (5) Horng, H. C.; Cheng, C. P.; Yang, C. S.; Lee, G.-H. Organometallics 1996, 15, 2543-47. (6) Beringhelli, T.; D’Alfonso, G. J. Chem. Soc., Chem. Commun. 1994, 2631-2. (7) (a) Saillant, R.; Barcelo, G.; Kaesz, H. J. Am. Chem. Soc. 1970, 92, 5739-41. (b) Wilson, R. D.; Bau, R. J. Am. Chem. Soc. 1976, 98, 4687-9. (c) Masciocchi, N.; D’Alfonso, G.; Kockelmann, W.; Scha¨fer, W; Sironi, A. Chem. Commun. 1997, 1903-4. (8) Beringhelli, T; D’Alfonso, G.; Ciani, G.; Molinari, H. J. Chem. Soc., Chem. Commun. 1987, 486-8. (9) (a) Bergamo, M.; Beringhelli, T.; D’Alfonso, G.; Mercandelli, P.; Moret, M.; Sironi, A. J. Am. Chem. Soc. 1998, 120, 2971-2. (b) D’Alfonso, G. Chem.sEur. J. 2000, 6, 209-15. (10) For example, see: (a) Beringhelli, T.; D’Alfonso, G.; Panigati, M.; Porta, F.; Mercandelli, P.; Moret, M.; Sironi, A. J. Am. Chem. Soc. 1999, 121, 2307-8. (b) Beringhelli, T.; D’Alfonso, G.; Panigati, M.; Mercandelli, P.; Sironi, A. Chem.sEur. J. 2002, 8, 5340-50. (11) Until now, evidence of the formation of a 60 ve species has been obtained only in the reaction of 1 with X- halides. However, also in this case, the initially formed [Re4H4(µ-X)(CO)12]- addition derivatives, spectroscopically characterized, rapidly underwent a 3 + 1 fragmentation. Beringhelli, T; D’Alfonso, G.; Ciani, G.; Molinari, H. J. Chem. Soc., Dalton Trans. 1996, 1771-3. (12) (a) Wang, S. R.; Wang, S.-L.; Cheng, C. P.; Yang, C. S. J. Organomet. Chem. 1992, 431, 215-26. (b) Beringhelli, T.; D’Alfonso, G.; Zarini, M. J. Chem. Soc., Dalton Trans. 1995, 2407-15. (c) Wang, S. R.; Cheng, C. P. J. Organomet. Chem. 1995, 490, 111-6.
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In some cases, the 1H NMR spectra of the reaction mixtures showed (besides the signals of the products of the above-described “3 + 1” fragmentations) another very minor resonance,12b which may be tentatively attributed to a byproduct of formula [Re2(µ-H)2L2(CO)6]. Such product, which is isoelectronic to the unsaturated [Re2(µ-H)2(CO)8] complex, can be assumed to arise from a “symmetric” (i.e., 2 + 2) fragmentation of the starting material 1 or of the first tetra-addition derivative. The reaction would therefore involve the transformation of the 2-fold unsaturated tetranuclear cluster 1 into two monounsaturated dinuclear complexes (eq 3). [Re4(µ3-H)4(CO)12] + 4L f 2[Re2(µ-H)2L2(CO)6] (3) Because of the interest in this kind of unsaturated complex, we wondered if the use of bidentate bridging ligands could favor their formation. Toward this goal, we have investigated the reactivity of 1 with 1,2-diazines because related monodentate nitrogen ligands, such as pyridines, have been found to be very effective reactants toward complex 1.12c Moreover, it has already been proven that 1,2 diazines are able to act as bridging ligands on Re-H-Re interactions in hydridocarbonyl triangular rhenium clusters.13 By using these ligands, we have now found that the desired symmetric cleavage path (eq 3) does occur, but not selectively. Other reaction paths are also effective, affording mainly tetranuclear complexes with an unusual square geometry of the metal cage. Moreover, the novel compounds have been found to possess interesting luminescent properties, and their preliminary photophysical characterization is reported here. Results and Discussion The 1,2-diazines which have been used in this study are pyridazine (pydz) and phthalazine (phal), depicted in Chart 1. The reactivity of [Re4(µ3-H)4(CO)12] (1) with the two ligands was very similar, affording in both cases mixtures whose components have been unambiguously identified by joint spectroscopic (solution) and X-ray single-crystal (solid state) analysis.14 Letters a and b have been used to label the pyridazine and phthalazine derivatives, respectively. (13) Beringhelli, T.; D’Alfonso, G.; Maggioni, D.; Panigati, D.; Mercandelli, P.; Sironi, A. J. Mol. Catal. A 2003, 204-205, 361-9. (14) The spectroscopic (IR and NMR) analysis of isolated crystals confirmed the correspondence between the species observed in solution and those structurally characterized in solid.
Clusters of Re Containing Bridging Diazine Ligands Chart 1
Scheme 2
Figure 1. 1H NMR monitoring of the course of the reaction of 1 with 2 equiv of pyridazine (CD2Cl2, 298 K). In the ordinate, the fraction of the overall hydride intensity for each species is reported. Table 1. 1H NMR Data of the Novel Complexes 2, 3, 4, 4′, and 5 (δ, CD2Cl2, 298 K) pyridazinea
All the reactions have been performed in dichloromethane, because 1 is poorly soluble in hydrocarbons and any donor solvent (including tetrahydrofuran or acetone) causes instantaneous and complete cluster fragmentation (eq 1).6,7 Reaction with Pyridazine (pydz). The reaction of compound 1 with two equivalents of pydz gave an instantaneous color change from red-brown to orange, followed by the slow separation of a yellow precipitate. NMR monitoring has revealed a two-step process: the first NMR spectrum, acquired a few minutes after reagent mixing, indicated that 1 reacts almost instantaneously with pydz, giving four main products, labeled 2a, 3a, 4a, and 5a in Scheme 2, in the relative ratios reported in Figure 1. Then the composition of the reaction mixtures slowly changes because the main kinetic product 2a converts into the thermodynamically favored species 3a and 4a. The species initially present in the highest concentration is the complex [Re4(µ-H)4(µ-pydz)(pydz)2(CO)12], with a tetranuclear spiked-triangular metallic skeleton (2a, Scheme 2), accounting for slightly more than 50% of the overall intensity in the hydride region of the NMR spectra. This compound, with 64 ve and four hydrogen-bridged metalmetal interactions, is analogous to the tetra-addition derivative [Re4(µ-H)4(NCMe)4(CO)12] observed as the initial product in the reaction of 1 with acetonitrile.12b In the present case, 2a contains three ligands only, since one of them acts as four-electron donor, by bridging the cluster edge opposite to the vertex bearing the spike and from the same side of the triangle plane as the spike (cis configuration, see Scheme 2 and Figure S1 in Supporting Information). Its three hydride resonances, in a ratio of 2:1:1 (Table 1) indicate that, in solution, the Re(CO)3(pydz)2 fragment that constitutes the
H3, H6
H4, H5
hydrides
3a 4a
9.71 (dt, 2)t 9.07 (t, 2)b 8.97 (dt, 2)t 9.21 (t, 2) 9.68 (t,4)
7.80 (m, 2)t 7.73 (m, 2)t 7.49 (t, 2)b 7.87 (t, 2) 8.21 (t, 4)
4a′ b
9.02 (t, 4)
7.63 (t, 4)
5a
9.47 (dd,1)t 9.09(t, 2)b 9.00 (dd, 1)t
7.63 (t, 2)b 7.55 (m, 2)t
-10.79 (d, 2) -11.09 (s, 1) -11.54 (t, 1) -3.88 (s, 2) -9.82 (t, 2) -12.54 (t, 2) -10.51 (t, 2) -11.17 (t, 2) -9.85 (s, 2) -11.37 (s, 1)
2a
phthalazinea H3, H8
H4-H7
hydrides
8.16-8.00 (m, 8)
3b 4b
10.09 (s, 2) 9.47 (s, 2) 9.14 (s, 2) 9.68 (s, 2) 9.66 (s, 4)
4b′b
9.37 (s, 4)
5b
9.52 (s, 2)b 9.93 (s, 1)t 9.33 (s, 1)t 9.67 (s, 1)t 9.02 (s, 2)b 8.61 (s, 1)t
-10.59 (d, 2) -10.80 (s, 1) -11.40 (t, 1) -3.95 (s, 2) -9.21 (t, 2) -12.22 (t, 2) -9.98 (t, 2) -10.79 (t, 2) -9.78 (s, 2) -11.16 (s, 1)
2b
5b′
8.21 (m, 4) 8.08 (m, 4) 7.91 (m, 4) 7.86 (m, 4) 7.83 (m, 4) 8.12 (m, 4) 8.03 (m,4) 8.52-8.36 (m, 8)
-8.94 (s, 2) -10.88 (s, 1)
a For compounds 2 and 5, the subscripts b and t indicate protons belonging to the bridging and terminal diazine ligands, respectively. b Data in THF-d8 solution.
spike has a (static or dynamically averaged) conformation which imparts an overall Cs symmetry to the complex. The pattern of the pyridazine resonances (six signals of intensity 2) agrees with this symmetry. The complex 2a has a diazine content higher than the 2:1 experimental reactant ratio. This explains the initial presence Inorganic Chemistry, Vol. 45, No. 26, 2006
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The formulation of 3a was suggested by the chemical shift of its hydride resonance (-3.88 ppm)15,16 and then confirmed by an X-ray single-crystal analysis (Figure S3 in Supporting Information). The other important reaction product is the tetranuclear cluster [Re4(µ-H)4(µ-pydz)2(CO)12] (4a), isoelectronic with 2a, but with a square geometry of the Re(µ-H)Re skeleton (see Scheme 2). It is responsible for two hydride and two pydz resonances, in the ratio of 1:1:2:2, respectively (Table 1), and an X-ray crystal analysis showed that the two pydz molecules are bridging on two opposite edges of the square, in a trans position with respect to the plane of the cluster (Figure S4 in Supporting Information). This complex is poorly soluble in CH2Cl2, so that it was the main constituent of the yellow precipitate that progressively slowly separated from the reaction mixtures (85% from NMR analysis of the precipitate dissolved in THF-d8). The only other species present in the precipitate (representing the remaining 15% of the precipitate, from the NMR analysis) can be reasonably formulated as the isomer of 4a in which the two pyridazine ligands are coordinated in a cis position (4a′, Scheme 2). The formulation is based on the close resemblance of the 1H NMR data of 4a and 4a′, in terms of number and relative intensities of the signals (Table 1). The 1H relaxation times, T1, which are usually reliable
probes of the molecular size, larger molecules having larger correlation times and shorter T1, are also very similar. Indeed the hydrides of 4a′ had T1 values (0.58(0.05) and 0.55(0.06) s) much closer to those of 4a (0.41(0.01) and 0.55(0.01) s) than to that of binuclear 3a (1.08(0.02) s). Further characterization of this very minor species was hampered by its instability, because in solvents in which it is (moderately) soluble, such as THF, it slowly underwent clean cleavage to the binuclear complex 3a. In spite of the insolubility in CD2Cl2 of the square complexes 4, the resonances of 4a were clearly observable in the 1H NMR spectra acquired at the early reaction stages (those of 4a′ were hardly recognizable because of their much smaller concentration). This means that initially the solution was oversaturated with respect to 4a, possibly because of the slowness of the nucleation process. Consequently, the plot of Figure 1 might be misleading with regard to the overall yield of 4a: it depicts the slow decrease in solution because of precipitation, but when the solid phase is also taken into account, the overall amount of 4a actually increased. The precipitation of the two 4 isomers caused a progressive decrease of the overall hydride intensity: at the end of the reaction this “missing” intensity, evaluated by an internal standard, figured up to more than 60%. In agreement with this, when the reaction was performed on a preparative scale, the isolated yields of 4a were ∼50%, confirming that 4a is the main reaction product. Formally, 4a is a dimer of 3a. However, it has not been possible to transform the binuclear complex into the square cluster by refluxing in inert solvents. Irradiation of a suspension of 4a in THF with the pyrex-filtered UV light of a mercury lamp afforded a dark solution in which the main (and the only identified) hydride species was 3a. The reaction is reminiscent of the previously reported photochemical transformation of the square cluster [Re4(µ-H)4(CO)16] into the unsaturated binuclear complex [Re2(µ-H)2(CO)8].17 Since the very beginning of the reaction, the NMR spectra revealed the presence of other minor byproducts. Among these, the most important was the species 5a (∼10%), responsible for two hydride resonances (ratio 2:1), which we formulate18 as the triangular cluster [Re3(µ-H)3(µ-pydz)(pydz)(CO)9], depicted in Scheme 2. This species could be classified as a [Re3(µ-H)3L3(CO)9] derivative, arising from the 3 + 1 neutral fragmentation of 1 (see eq 2). In this case, however, we do not have evidence of the formation of the mononuclear complex HRe(CO)3(pydz)2, which should accompany 5a, according to the stoichiometry of eq 2. We can assume that the bridging nature of the diazine favors the formation of the dinuclear complex 3a rather than the monuclear HRe(CO)3(pydz)2, according to overall stoichiometry 5.
(15) The hydric signal of the isoelectronic complex [Re2(µ-H)2(CO)8] is at -9.0 ppm and according to the empirical rules established for hydrido-carbonyl clusters of rhenium;12b,16 a down-field shift of about 5 ppm is expected upon replacement of two carbonyls by two N-donor ligands. (16) Beringhelli, T.; D’Alfonso, G.; Freni, M.; Ciani, G.; Moret, M.; Sironi, A. J. Chem. Soc., Dalton Trans 1989, 1143-8.
(17) Bergamo, M.; Beringhelli, T.; D’Alfonso, G.; Garavaglia, L.; Mercandelli, P.; Moret, M.; Sironi, A. J. Cluster Sci. 2001, 12, 223242. (18) To confirm this formulation, we have prepared both the pydz and phal derivatives 5 by reaction of [Re3(µ-H)3(µ3-H)(CO)9]- with HBF4 in the presence of 2 equiv of the proper diazine. A full characterization of these species and a study of their properties are in progress.
of some unreacted 1, which rapidly disappeared (see Figure 1), by consumption of the pydz released by the conversion of 2a to species with a lower diazine/rhenium ratio (see below). When 3 equiv of pydz (i.e., the stoichiometric amount required by 2a) was used, the initial relative amount of 2a was higher (∼70%), no unreacted 1 was observed, and significant concentrations of the same byproducts depicted in Figure S2 in the Supporting Information were formed. Interestingly, 2a was the main initial product even when only 1 equiv diazine was used (obviously in this case the amount of unreacted 1 was much higher). Complex 2a is unstable in solution and slowly decomposes (Figures 1 and S2) to give 3a (which remained in solution) and 4a (which progressively precipitated), as discussed below. The species labeled 3a is the dinuclear complex [Re2(µH)2(µ-pydz)(CO)6] (Scheme 2). As shown in Figure 1, it was present in solution since the very beginning of the reaction: actually, it is the unsaturated complex expected from the 2 + 2 fragmentation of the tetranuclear starting compound 1, according to eq 3. The concentration of 3a progressively increased, even after the complete disappearance of 1, showing that it originates also from the spiked-triangular cluster 2a (eq 4). [Re4(µ-H)4(µ-pydz)(pydz)2(CO)12] f 2[Re2(µ-H)2(µ-pydz)(CO)6] + pydz (4)
10912 Inorganic Chemistry, Vol. 45, No. 26, 2006
Clusters of Re Containing Bridging Diazine Ligands
2[Re4(µ3-H)4(CO)12] + 5pydz f 2[Re3(µ-H)3(µ-pydz)(pydz)(CO)9] + [Re2(µ-H)2(µ-pydz)(CO)6] (5) The concentration of 5a remained constant during the decomposition of 2a, showing that the formation of 5a from the spiked-triangle 2a is negligible. Reaction with Phthalazine (phal). The reaction of 1 with phthalazine was performed in the same manner as that with pyridazine. Also in this case, the addition of 2 equiv of ligand to a CH2Cl2 solution of 1 caused a change of the solution color (from red-brown to yellow) and the progressive separation of a yellow precipitate. NMR monitoring (Figure 2) revealed that the reaction processes, although somewhat slower, led to mixtures analogous to those observed for pydz. The main initial product was again the spiked-triangular species (2b) that slowly decomposed to give the binuclear unsaturated complex 3b and the two square clusters 4b and 4b′. Complex 3b was much more insoluble in CH2Cl2 than 3a, so that it was found not only in the solution but also in the precipitate:19 a NMR analysis of the precipitate isolated after .∼20 h, dissolved in THF-d8, showed the following composition: 3b, 48.5; 4b, 45.5; 4b′, 6.0%. Moreover, in this case, two isomers of the triangular cluster (5b and 5b′) have been observed (Table 1), differing in the relative coordination (cis or trans) of the bridging and terminal phal ligands.18 Anyway, their overall hydride intensity remained always very small (
