Concepción
López
*a,
Asensio
González
b,
Ramon
Bosque
a,
Pradipta K.
Basu
ab,
Mercè
Font-Bardía
c and
Teresa
Calvet
d
aDepartament de Química Inorgànica, Facultat de Química, Universitat de Barcelona, Martí i Franquès 1-11. E-08028-Barcelona, SPAIN
bLaboratori de Química Orgánica, Facultat de Farmacia, Universitat de Barcelona, Pl. Pius XII s/n. E-08028-Barcelona, SPAIN
cUnitat de Difracció de Raigs-X. Centre Científic i Tecnològic de la Universitat de Barcelona. Universitat de Barcelona, Solé i Sabaris 1-3, 08028-Barcelona, SPAIN
dDepartament de Cristal·lografia Mineralogía i Dipòsits Minerals. Facultat de Geologia. Universitat de Barcelona, Martí i Franquès s/n. E-08028-Barcelona, SPAIN
First published on 10th January 2012
The synthesis and characterization of the novel pyrazole derivative [1-(Fc–CH2)-3,5-Ph2–(C3HN2)] (2) {Fc = (η5–C5H5)Fe(η5–C5H4)–} with a ferrocenylmethyl substituent on position 1 of the heterocycle is described. The study of the reactivity of 2 with cis-[MCl2L2] (M = Pt and L = dmso or M = Pd and L = dmso or CH3CN), Pd(AcO)2 or Na2[PdCl4] under different experimental conditions, has allowed us to isolate and characterize a wide variety of platinum(II) or palladium(II) complexes: trans-[Pt{1-(Fc–CH2)-3,5-Ph2-(C3HN2)}Cl2(dmso)] (3), the cis- isomers of [M{1-(Fc–CH2)-3,5-Ph2–(C3HN2)}Cl2(dmso)] {M = Pt (4) or Pd (7)}, trans-[Pd{1-(Fc–CH2)-3,5-Ph2–(C3HN2)}2Cl2] (8), the cyclometallated compounds [M{1-(Fc–CH2)-(3–C6H4)-5-Ph–(C3HN2)}Cl(L)] {with M = Pt and L = dmso (5) or PPh3 (6) or M = Pd and L = PPh3 (9)} and the palladium(II) complex [Pd{1-[(η5–C5H4)Fe{(η5–C5H4)–CH2]-3,5-Ph2–(C3HN2)}Cl(PPh3)] (10) that arises from a transannulation process. The crystal structures of the free ligand 2 and compounds 4, 7, 9 and 10 are also reported and confirm the cis- disposition of the Cl− ligands in 4 and 7, the trans- arrangement of the phosphorous and the nitrogen atoms in 9 and 10, the mode of binding of the ligand in 4, 7, 9 and 10 and the nature of the metallated carbon atom {C(sp2, phenyl) in 9 or the C(sp2, ferrocenyl) of the C5H5 ring in 10}. In order to rationalize the different nature of the products isolated in the reactions of 2 with Pd(AcO)2 or Na2[PdCl4] and NaAcO density functional theory (DFT) calculations of the complexes have also been carried out.
On the other hand, the investigation of palladium(II) and platinum(II) coordination complexes with pyrazoles as ligands is one of the research areas that has undergone rapid development during recent years.5–8 Compounds of this kind exhibit greater antitumoral activity and lower toxicity than cis-[PtCl2(NH3)2] or antibacterial activity have been reported.6c Furthermore, some examples of their utility in macromolecular chemistry,7a or in homogeneous catalysis7b have also been published.
In addition, cyclometallated Pd(II) and Pt(II) complexes of N– donor ligands are particularly relevant due to their photophysical properties, biological activity and applications in homogeneous catalysis, or as building blocs in supra- and macromolecular chemistry.9–15
Despite of: a) the increasing interest in new ferrocene derivatives containing pyrazole units,16 b) their use as ligands in front of transition metals,4b,4c,7 c) the study of heterodi-, tri- or in general polymetallic complexes with ferrocenyl units,1,3,4 or d) the relevance of palladium(II) and platinum(II) in synthesis,9,10a,b,14 only a few palladium(II) complexes containing hybrid ferrocene-pyrazole units have been reported so far,4b,4c and as far as we know, platinum(II) derivatives are unknown.
We have recently described the syntheses of [3,5-Ph2-4-(Fc–CH2)–(C3HN2)] (1) {Fc = (η5–C5H5)Fe(η5–C5H4)–} (Fig. 1) and a few palladium(II) and platinum(II) complexes where 1 behaves as an (N) or [C(phenyl), N(pyrazol)]− ligand.4b In view of the results obtained in these studies, and in order to elucidate whether the position of the Fc–CH2– unit on the heterocycle could affect: a) the reactivity of this sort of compound, b) the nature of the palladium(II) and platinum(II) complexes formed or c) their properties, we decided to prepare and characterize the new ligand [1-(Fc–CH2)-3,5-Ph2–(C3HN2)] (2) (Fig. 1).
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Fig. 1 Chemical formulae of 1-methyl-3,5-diphenyl-4-ferrocenylene-pyrazole (1) studied before4c and of the new ligand [1-(Fc–CH2)-3,5-Ph2–(C3HN2)] (2) prepared and used in this work. |
In compound 2, the presence of the Fc–CH2– unit on position 1 of the heterocycle has an additional interest due to the existence of different types of carbon atoms [C(phenyl) and C(ferrocenyl)] susceptible to metallation. The activation of the σ[C(phenyl)–H] bond would produce five-membered metallacycles (Fig. 2, A); while the metallation of the C5H4 ring of the ferrocenyl unit, that induces planar chirality, would give chiral compounds (Rp or Sp isomers) containing six-membered rings (Fig. 2, B). Moreover, since a few uncommon examples of palladacycles arising from a transannulation process have been recently reported,4a,18 the formation of palladium(II) or platinum(II) complexes arising from the activation of the σ[C(ferrocene)–H] bond of the unsubstituted C5H5 ring of the Fc moiety (Fig. 2, C) or complexes containing [C,N,C']2− pincer ligands (Fig. 2, D and E) formed by the activation of two σ(C−H) bonds cannot be ruled out.19
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Fig. 2 Schematic view of the different types of pallada- and platinacycles that could be formed through: a) activation of a σ(Cphenyl–H) bond (A) or one of the two σ[C(ferrocenyl)–H] bonds on the ortho site of the C5H4 ring (B) {in this case two enantiomers: (Sp) and (Rp) could be formed}, b) from a transannulation process (C) or c) from a more complex reaction involving a double cyclometallation (D) or a transannulation and a cyclometallation reaction simultaneously (E). |
In this paper we present the synthesis of ligand 2 and the study of its coordination chemistry in front of palladium(II) and platinum(II) in view of the potential applications of the complexes in different fields.
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Scheme 1 i) in CH2Cl2, treatment with aqueous HBF4, 1 h at room temperature. |
Elemental analyses (experimental section) of 2 were consistent with the proposed formula, its mass spectra showed a peak at m/z = 419.1, that agrees with the value expected for the {[M] + H}+ cation and the infrared spectrum of 2 exhibited the typical bands due to monosubstituted ferrocene derivatives.22
The X-ray crystal structure of 2 is depicted in Fig. 3. Average values of bond lengths [Fe–Cring: 2.07(4) Å and Cring–Cring: 1.43(3) Å] and angles of the Fc unit fall in the range reported for most monosubstituted ferrocene derivatives.23 The two pentagonal rings are planar, nearly parallel (tilt angle = 1.69°) and they deviate by ca. 18° from the ideal eclipsed conformation.
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Fig. 3 ORTEP plot of [1-(Fc–CH2)-3,5-Ph2–(C3HN2)] (2), 50% probability displacement ellipsoids are shown and hydrogen atoms have been omitted for clarity. Selected bond lengths (in Å) and angles (in deg): C(10)–C(11), 1.513(5); C(11)–N(2), 1.449(4); N(1)–C(12), 1.318(4); C(12)–C(13), 1.390(5); C(13)–C(14), 1.393(4); C(14)–N(2), 1.340(4); N(1)–N(2), 1.367(4); C(14)–C(15), 1.514(4); 1.480(4); C(14)–C(21), 1.480(4); Fe–C (average value), 2.07(4) (C–C) of Fc (average value), 1.43(3); C(6)–C(10)–C(9), 105.4(3); N(2)–C(11)–C(10), 111.2(3), C(6)–C(10)–C(11), 125.0(3); C(9)–C(10)–C(11), 129.3(3); N(1)–C(12)–C(13), 109.2(3); N(1)–C(12)–C(15), 120.8(3); N(2)–C(14)–C(13), 103.7(8); N(2)–C(14)–C(11), 123.3(3); C(12)–C(13)–C(14), 108.1(3) and C(13)–C(14)–C(21), 133.0(3). |
The pyrazole ring is planar, its main plane forms an angle of 80.9° with the C5H4 ring of the ferrocene and the N(1)–N(2) bond is located on the same direction as the Fe(η5–C5H5) moiety. As a consequence of this arrangement of groups, the N(1) atom is quite close to one hydrogen atom of the –CH2– {separation N(1)⋯H(11A) = 2.50 Å} and proximal to the H(1) and H(2) atoms of the (η5–C5H5) unit, {the distances N(1)⋯H(1) and N(1)⋯H(2) = 2.90 Å and 3.02 Å}. The two phenyl rings defined by the sets of atoms [C(15)–C(20)] and [C(21)–C(26)] are planar and their mean planes form angles of 2.6° and 63.6° with the pyrazole ring respectively.
In the crystal, the relative orientation of two neighbouring molecules with a head-to-tail disposition is such that the separation between N(1) atom of one unit and the H(20) atom of the other one (and vice versa) is 2.540 Å. This suggests the existence of two C–H(20)⋯N(1) intermolecular interactions that results in the formation of a dimer which is connected to proximal ones by C–H⋯π interactions,‡as shown in Fig. 4, b.
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Fig. 4 a) Schematic view of the assembly of a molecule of compound 2, at (x, y, z) with a close one located at (1 − x, 1 − y, − z) through C–H⋯N interactions {atoms marked with an asterisk belong to the molecule at (1 − x, 1 − y, −z)}} and b) the connectivity of these dimeric units by different sorts of C–H⋯π intermolecular interactions. |
Compound 2 was also characterized in solution by mono- [1H and 13C{1H}] and two dimensional [{1H–1H} NOESY and COSY and {1H–13C}–HSQC and HMBC] NMR experiments. The most relevant features observed in the 1H–NMR spectrum was the presence of a group of two singlets of relative intensities 2:
7 in the range 4.00–4.20 ppm, the less intense one is due to the pair of protons H3 and H4 of the C5H4 ring, while the other one was assigned to the remaining protons of the Fc group. The two additional singlets at δ = 6.56 and 5.09 ppm (of relative intensities 1
:
2) were ascribed to the proton on position 4 of the pyrazolyl ring and to those of the –CH2– unit.
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Scheme 2
i) cis-[PtCl2(dmso)2] {in a molar ratio 2:Pt(II) = 1} in refluxing methanol. ii) SiO2 column chromatography. iii) cis-[PtCl2(dmso)2] and NaAcO (in a molar ratio: 2![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
Entry | Reagents (molar ratios) | Solvent | T | t | Products (molar ratios) |
---|---|---|---|---|---|
a A (5![]() ![]() ![]() ![]() |
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I | 2 and cis-[PtCl2(dmso)2] | MeOH | reflux | 1.0 | 3 and 4 |
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II | 2 and cis-[PtCl2(dmso)2] | MeOH | reflux | 3.5 | 3 and 4 |
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(0.21![]() ![]() |
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III | 2, cis-[PtCl2(dmso)2] and NaAcO | Toluene/MeOHa | reflux | 3.5 | 3 b, 4 and 5 |
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IV | 2, cis-[PtCl2(dmso)2] and NaAcO | Toluene/MeOHa | reflux | 3.5 | 4 and 5 |
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(0.36![]() ![]() |
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IV | 2, cis-[PtCl2(dmso)2] and NaAcO | Toluene/MeOHa | reflux | 7.0 | 5 d |
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V | 5 and PPh3 | CH2Cl2 | 298 K | 1.0 | 6 |
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The treatment of equimolar amounts of 2 and cis-[PtCl2(dmso)2] in refluxing methanol for 1.0 h followed by the work up of a SiO2 column chromatography afforded the trans- and cis- isomers of [Pt{1-(Fc–CH2)-3,5-Ph2–C3HN2}Cl2(dmso)] (3) and (4) respectively, in a molar ratio 3:
4 =1.00
:
0.11 (Table 1, entry I). Longer refluxing periods (t = 3.5 h, Table 1, entry II)} also produced a mixture of the two isomers but with the molar ratio 3
:
4 inverted (0.21
:
1.00). Thus indicating that longer reaction periods induced the preferential formation of the cis- isomer (4). This finding, is consistent with previous work on the reactivity of cis-[PtCl2(dmso)2] with related N–donor ligands4a,4c,25 and has allowed us to optimize the synthesis of 4.
Compounds 3 and 4 were characterized by elemental analyses, mass spectrometry, IR and mono and two-dimensional NMR studies and product 4 was also characterized by X-ray diffraction (see below). Elemental analyses were consistent with the proposed formulae and the IR spectra of 3 and 4 showed the typical bands due to the coordinated dmso ligand.26 The assignment of the signals observed in the 1H NMR spectra of 3 and 4 was achieved with the aid of {1H–1H} NOESY and COSY experiments. 195Pt{1H} NMR spectra of 3 and 4 showed a singlet (at δ = −2998 and −2850 ppm, respectively). The variation of the chemical shift Δδ(195Pt) = δ(for 4) − δ(for 3) = 148 ppm agrees with the values reported for the trans- and cis- isomers of the related [Pt(N–donor ligand)Cl2(dmso)] complexes.4b,4c,25,27,28
The crystal structure of 4 consists of discrete molecules of [Pt{1-(Fc–CH2)-3,5-Ph2–(C3HN2)}Cl2(dmso)] (Fig. 5) in which the platinum(II) is bound to the nitrogen N(1) of the pyrazolyl unit, the sulphur, S(1), of the dmso ligand and two chloride ligands {Cl(1) and Cl(2)} in a cis- arrangement {Cl(1)–Pt–Cl(2) bond angle = 90.61(5)°}. Bond lengths as well as bond angles around the platinum(II) are consistent with those reported for related cis-[Pt(N–donor ligand)Cl2(dmso)] complexes.23,25,27,29
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Fig. 5 ORTEP plot of cis-[Pt{1-(Fc−CH2)-3,5-Ph2–(C3HN2)}Cl2(dmso)] (4) showing 50% probability displacement ellipsoids. Hydrogen atoms have been omitted for clarity. |
The three rings of the 3,5-Ph2–C3HN2 moiety are planar and the two phenyl rings, defined by the sets of atoms [C(15)–C(20)] and [C(21)–C(26)] form angles of 47.5° and 49.6°, respectively with the main plane of the heterocycle. The N(1)–N(2) bond of the pyrazole ring is directed in the opposite direction to the Fe(η5–C5H5) unit. As expected, the bond lengths and angles of the ferrocenyl unit are consistent with those of monosubstituted ferrocene derivatives,23 the two pentagonal rings are parallel (tilt angle = 0.41°) and their relative orientation deviates by ca. 8.8(8)° from the ideal eclipsed conformation.
Due to the increasing interest on platina- and palladacycles containing planar N–donor ligands due to their applications in different areas,11–17 and since it is well-known that the formation of platina- (or palladacycles) with bidentate [C(phenyl), N]− or [C(ferrocenyl), N]− ligands commonly requires the presence of a base (i.e.NaOAc or the ligand itself),4b,4c,10a,10b,25b,30 we also explored the reactivity of ligand 2 toward Pt(II) in the presence of sodium acetate.
When compound 2 was treated with equimolar amounts of cis-[PtCl2(dmso)2]24 and NaAcO in a mixture of toluene:
methanol (5
:
1) under reflux for 3.5 h (Table 1, entry III) the formation of a platinum mirror on the walls of the Erlenmeyer flask was observed and a deep-brown solution was obtained. Concentration of the solution, followed by silica gel column chromatography, gave traces of 3, compound 4 and a new platinum(II) complex, 5. Characterization data of 5 (see the experimental section) were consistent with those expected for [Pt{3-(C6H4)-5-Ph-1-(Fc−CH2)–(C3HN2)}Cl(dmso)] (5), in which ligand 2 behaves as a monoanionic bidentate [C(phenyl), N(pyrazole)]− ligand (type B in Fig. 2) and the dmso is in a cis-arrangement to the metallated carbon atom in agreement with the transphobia effect.31
Several additional experiments were carried out in order to increase the yield of 5. Shorter refluxing periods (t) (1.0 h ≤ t < 3.5 h) produced a significant decrease of the molar ratios 5:
4. and for longer reaction times (4 h ≤ t ≤ 24 h) the amount of metallic platinum formed increased, but the yield of 5 did not improve significantly.§ Better results were obtained when the reaction was performed using a two-fold excess of sodium acetate (molar ratio NaAcO:Pt(II) = 2, Table 1, entry IV and Scheme 2, step B) and long refluxing periods (t = 7.0 h), in this case 5 was the major product. Furthermore, it should be noted that the 1H–NMR spectra of the crude materials isolated in any of these reactions did not provide evidence of the presence of any other type of platinacycle suggesting that under the experimental conditions used, the activation of the σ[C(phenyl)–H] bond of 2 is preferred over that of the σ[C(ferrocenyl)–H] bond.
The results presented in this section suggest that the formation of the platinacycle 5 is a multistep process that involves the replacement of one dmso ligand of the starting material by methanol to give cis-[PtCl2(MeOH)(dmso)] which may isomerize to the trans- form. Further coordination of the N–donor ligand 2 would produce the cis- and trans- isomers of [Pt{1-(Fc–CH2)-3,5-Ph2–(C3HN2)}Cl2(dmso)] respectively. Finally, the activation of the σ[C(sp2, phenyl)–H] bond of 3 would give the platinacycle 5. This in good agreement with the mechanism postulated for the cycloplatination of aryloximes and related N–donor ligands and the results obtained recently from a DFT based theoretical study of the cycloplatination process that points out that the trans- isomers of [Pt(L)Cl2(dmso)] are the key intermediates of the process.30b However, it should be noted that ligand 2 has different σ(C–H) bonds which are susceptible to metallation (Fig. 1) and although ferrocene derivatives are more prone to electrophillic attacks than their phenyl analogues,1 the formation of 5 should be ascribed to the closer orientation between the platinum(II) bound to heterocyclic nitrogen and the σ[C(phenyl)–H] rather than the σ[C(ferrocenyl)–H] bonds.
The use of molecular models for the cis- (3) and trans- (4) isomers of [Pt{1-(Fc–CH2)-3,5-Ph2–(C3HN2)}Cl2(dmso)] reveals that the “PtCl2Sdmso” moiety should be nearly orthogonal to the pyrazole ring to minimize steric hindrance between the phenyl ring and the Cl or dmso ligands. This is the typical orientation found in the crystal structures of cis- or trans-[Pt(planar N–donor ligand)Cl2(dmso)].23,24,25,29 The formation of the metallacycle 5 from 3 requires the release of one of the ligands in a cis- arrangement to the σ(Pt–N) bond and the rotation of the phenyl ring (on position 3 of the pyrazole ring) to achieve a proper orientation between the σ[C(phenyl)–H] bond and the platinum(II).
Further treatment of the cycloplatinated complex 5 with the equimolar amount of PPh3 in CH2Cl2 produced [Pt{3-(C6H4)-5-Ph-1-(Fc−CH2)–(C3HN2)}Cl(PPh3)] (6) (Scheme 2, step C and Table 1, entry V) in a fairly good yield. Elemental analyses of 6 were consistent with the proposed formula. The IR spectrum suggested the binding of the PPh3 ligand to the platinum(II) and 1H, 31P{1H} and 195Pt{1H} NMR spectra indicated the presence of a [C(phenyl), N(pyrazole)]−group and a PPh3 ligand in a cis- arrangement to the metallated carbon, in agreement with the transphobia effect.31
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Scheme 3
i) Equimolar amount of [PdCl2(dmso)2], in refluxing MeOH at 298 K for 0.5 h. ii) [PdCl2(CH3CN)2] or Na2[PdCl4] {molar ratio 2:Pd(II) = 1 or 2 (see text)} in MeOH at 298 K, 24h. iii) Equimolar amount of Pd(OAc)2 in toluene under reflux for 3.5 h followed by the addition of PPh3 in CH2Cl2 under stirring at 298 K for 1.5 h, subsequent treatment with LiCl in acetone for 2.5 h. iv) SiO2 column chromatography. v) Reaction with Na2[PdCl4] and Na(OAc)·3H2O (in a 1![]() ![]() ![]() ![]() |
Entry | Reagents (molar ratios) | Solvent | T | t | Products |
---|---|---|---|---|---|
a See text.
b Yields of 8 were 35% or 89% for molar ratios 2![]() ![]() |
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I | 2 and cis-[PdCl2(dmso)2] | MeOH | reflux | 0.5 | 7 |
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II | 2 and cis-[PdCl2(CH3CN)2] | MeOH | 298 K | 24.0 | 8 |
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III | 2 and Na2[PdCl4]b | MeOH | 298 K | 24.0 | 8 |
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IV | 2 and Pd(AcO)2c | Toluene | reflux | 3.5 | 9 d |
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V | 2, Na2[PdCl4] and NaAcO·3H2Oe | MeOH | 298 Kf | 24.0 | 10 d |
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The reaction between equimolar amounts of 2 and cis-[PdCl2(dmso)2] in refluxing methanol for 0.5 h (Scheme 3, A and Table 2, entry I) produced a yellow solid that was identified according to its elemental analyses, mass spectra and NMR studies (see experimental section) as cis-[Pd{1-(Fc-CH2)-3,5-Ph2–(C3HN2)}2Cl2] (7). Compound 7 is similar to complex 4, except for the nature of the metal atom {M = Pt (in 4) or Pd (in 6)} and in fact X-ray studies reveal that they are isostructural.
The crystal structure of 7 consists of discrete molecules of [Pd{1-(Fc–CH2)-3,5-Ph2–(C3HN2)}Cl2(dmso)] (Fig. 6). In each of these molecules the palladium(II) atom is in a slightly distorted square-planar environment and it is bound to the nitrogen N(1) of the pyrazolyl unit, two chloride ligands {Cl(1) and Cl(2)} in a cis- arrangement {Cl(1)-Pd–Cl(2) bond angle = 90.74(7)°} and the sulphur, S(1), of the dmso ligand. Bond lengths and angles around the palladium(II) (Table 3) are similar to those of related cis-[Pd(N–donor ligand)Cl2(dmso)] complexes.23,32 The two phenyl rings, defined by the sets of atoms [C(15)–C20)] and [C(21)–C(26)] are planar and form angles of 47.0° and 48.9°, respectively with the main plane of the heterocycle.
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Fig. 6 ORTEP plot of cis-[Pd{1-(Fc–CH2)-3,5-Ph2–(C3HN2)}Cl2(dmso)] (7), 50% probability displacement ellipsoids are shown and hydrogen atoms have been omitted for clarity. |
M = Pt (4) | M = Pd (7) | |
---|---|---|
a Average values for the Fc moiety. | ||
Bond lengths | ||
M–N(1) | 2.021(3) | 2.010(5) |
M–Cl(1) | 2.2813(12) | 2.2630(17) |
M–Cl(2) | 2.3166(10) | 2.3151(18) |
M–S(1) | 2.2044(10) | 2.1965(19) |
N(1)–N(2) | 1.362(3) | 1.355(9) |
N(2)–C(11) | 1.450(4) | 1.444(11) |
N(2)–C(12) | 1.363(4) | 1.382(10) |
C(10)–C(11) | 1.515(5) | 1.471(13) |
C(12)–C(19) | 1.367(5) | 1.400(8) |
Fe–Ca | 2.039(7) | 2.04(4) |
C–Ca | 1.41(1) | 1.41(2) |
Bond angles | ||
Cl(1)–M–Cl(2) | 90.61(5) | 90.74(7) |
Cl(1)–M–S(1) | 89.94(5) | 90.04(7) |
N(1)–M–Cl(2) | 88.84(8) | 88.35(19) |
N(1)–M–S(1) | 90.68(8) | 90.92(19) |
M–N(1)–N(2) | 123.29(19) | 121.8(5) |
C(10)–C(11)–N(2) | 112.4(3) | 115.1(7) |
C(12)–N(2)–N(1) | 108.9(2) | 106.0(6) |
C(11)–N(2)–N(1) | 121.1(2) | 124.8(7) |
N(2)–C(12)–C(13) | 122.4(3) | 124.7(7) |
As expected, the bond lengths and angles of the ferrocenyl unit are consistent with those of monosubstituted ferrocene derivatives.23 The two pentagonal rings are parallel (tilt angle = 0.41°) and their relative orientation deviates by 8.8(8)° from the ideal eclipsed one. In this compound the orientation of the pyrazolyl ring and the Fe(η5–C5H5) unit it is similar to that of 4, suggesting that the formation of 4 or 7 requires rotation around the N–CH2– or the Fc–CH2– bonds to reduce steric effects.
When the reaction was carried out using equimolar amounts of ligand 2 and cis-[PdCl2(CH3CN)2] in methanol for 24 h (Table 2, entry II and Scheme 3, step B), a yellow solid was obtained and its characterization data (see experimental section) was consistent with those expected for the trimetallic complex trans-[Pd{1-(Fc–CH2)-3,5-Ph2–(C3HN2)}2Cl2] (8). The yield of this synthesis was low but it improved considerably when a two fold excess of 2 was used. Compound 8 was also isolated by treatment of the ligand and Na2[PdCl4] {in molar ratios (1:
1) or (2
:
1)} in methanol at room temperature (Table 2, entry III).
It should be noted that no evidence of the presence of any other type of palladium(II) complex was detected by 1H–NMR studies of the crude (as well as of the mother liquors) of any of the experiments presented in Table 2, entries I–III.
In view of these results and in a further attempt to achieve cyclopalladated complexes, we decided to use more severe experimental conditions based on the reaction of the ligand 2 and Pd(AcO)2 (in a 1:
1 molar ratio) in refluxing toluene for 3.5 h (Scheme 3, C), subsequent treatment of the crude with a slight excess of PPh3 (in CH2Cl2) followed by LiCl (in acetone) (Scheme 3, step C). Evaporation of the solvent produced a brownish residue, which was later on purified by silica gel column chromatography. The use of CH2Cl2 as eluant released a band that was collected and concentrated to dryness giving a bright yellow solid 9, whose elemental analyses (see the experimental section) were consistent with those expected for a palladacycle of the type: [Pd(C,N)Cl(PPh3)]. Mono- and two dimensional NMR spectra revealed that 9 contains: a) a five-membered palladacycle in which compound 2 acts as a [C(phenyl), N(pyrazolyl)]− ligand (Fig. 2, type A), and b) the PPh3 ligand occupies the adjacent position to the metallated carbon atom in the coordination sphere of the palladium(II). All these findings suggested that 9 was [Pd{3–(C6H4)–5-Ph-1-(Fc-CH2)–(C3HN2)}Cl(PPh3)] (9), which is similar to complex 6, except for the nature of the metal atom.
X-ray diffraction studies revealed that the crystals contained a 1:
1 array of molecules of CH2Cl2 and [Pd{3–(C6H4)-5-Ph-1-(Fc–CH2)–(C3HN2)}Cl(PPh3)] (9). In each one of these heterodimetallic units (Fig. 7 and Table 4), the palladium(II) is in a slightly distorted square-planar environment, where it is bound to the heterocyclic nitrogen of the pyrazole ring and the C(26) atom, thus confirming that metallation occurred on the phenyl ring on position 3 of the heterocycle. The two remaining coordination sites are occupied by a chloride and the phosphine ligand. The value of the P–Pd–C(26) bond angle {93.18(8)°} indicates that the PPh3 ligand is in a cis position in relation to the Pd–C bond, in agreement with the transphobia effect.31 Bond lengths and angles around the palladium(II) (Table 4) are similar to those reported for related palladacycles of the type [Pd{C(sp2, phenyl),N}Cl(PPh3)].23
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Fig. 7 ORTEP plot of compound [Pd{[3-(C6H4)–5-Ph-1-{(η5–C5H5)Fe(η5–C5H4)–CH2}–(C3HN2)]}Cl(PPh3)] (9), 50% probability displacement ellipsoids are shown and hydrogen atoms have been omitted for clarity. |
9 | 10 | |
---|---|---|
a This refers to the C(26) atom of complex 9 or to the C(1) atom of 10. b Average value for the Fc unit. | ||
Bond lengths | ||
Pd–P | 2.2492(12) | 2.2314(14) |
Pd–Cl | 2.4007(14) | 2.3907(14) |
Pd–N(1) | 2.122(2) | 2.128(3) |
Pd–Ca | 2.043(3) | 2.017(4) |
N(1)–N(2) | 1.365(2) | 1.339(4) |
N(2)–C(11) | 1.477(3) | 1.371(5) |
C(11)–C(10) | 1.500(4) | 1.521(6) |
Fe–Cb | 2.037(7) | 2.04(2) |
C–Cb | 1.41(2) | 1.43(1) |
Bond angles | ||
N(1)–Pd–Ca | 80.46(10) | 91.53(15) |
N(1)–Pd–Cl | 97.13(7) | 84.63(10) |
P–Pd–Cl | 91.05(5) | 94.20(5) |
P–Pd–Ca | 93.18(8) | 91.08(12) |
In the crystal, two molecules of [Pd{3–(C6H4)–5-Ph-1-(Fc–CH2)–(C3HN2)}Cl(PPh3)] are assembled in pairs by C–H⋯π interactions involving the hydrogen atom H(29) of a molecule and the ring defined by the set of atoms [C(39)–C(44)] [the distance H(29)⋯centroid of this ring is 3.491 Å] of the other one and vice versa. In addition, in each one of these units, the separation between the Cl(1) atom and the H(45A) atom of the CH2Cl2 molecule (2.837 Å) suggests a weak C–H⋯Cl interaction.
For ligand 2, the activation of any of the two ortho σ[C(ferrocene)–H] bonds of the Fc unit would give the six-membered metallacycles with a σ[Pd–C(ferrocene)] bond (B-type in Fig. 1) but none of the reactions studied here allowed us to isolate this sort of product. In view of this, and since it is well-known that one of the most common methods used to obtain palladacycles with [C(ferrocene),N]− ligands consists of the treatment of the ferrocenyl ligand, M2[PdCl4] (M = Li+ or Na+) and NaAcO·3H2O, in methanol at room temperature,17b,33,34 we also decided to explore whether for ligand 2 this procedure could allow us to isolate metallacycles with a σ[Pd–C(ferrocene)] bond.
Reaction of 2 with Na2[PdCl4] and NaAcO·3H2O in methanol at room temperature for 24 h produced a brown solid (Scheme 3, D). Further treatment with PPh3 in dichloromethane and the subsequent purification of the crude by column chromatography on silica gel gave an orange solid 10. Its elemental analyses were consistent with those expected for a palladacycle of the type [Pd(C,N)Cl(PPh3)], but its 1H and 31P{1H}–NMR spectra were markedly different from those of 9. The position of the singlet detected in the 31P{1H} NMR spectrum (δ = 38 ppm) was similar to those reported for related compounds with N–donor ferrocenyl ligands and a σ[Pd–C(sp2,ferrocene)] bond (35 ppm < δ < 40 ppm),33 and was up-field shifted when compared with 9 (δ = 46 ppm). Its 1H–NMR spectrum was more complex than expected and showed six singlets with relative intensities (1:
1
:
2
:
1
:
1
:
1) in the range 3.0 < δ < 4.2 ppm. This pattern, which is markedly different from those of compounds 3–9 or of five-membered palladacycles formed by metallation of the (η5–C5H4) ring of the Fc group,33 suggests the presence of a σ(Pd–C) bond generated by a transannulation process.
The X-ray crystal structure of 10 (Fig. 8) revealed that the palladium(II) was bound to the nitrogen atom N(1) and the C(1) atom of the unsubstituted (η5–C5H5) ring of 2. Two remaining coordination sites are occupied by a chloride, and the phosphorous atoms of the PPh3 ligand, which is in the adjacent position to the Pd–C bond in agreement with the transphobia effect.31
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Fig. 8 ORTEP plot of compound [Pd{1-[(η5–C5H4)Fe(η5–C5H4)–CH2]-3,5-Ph2-1-(C3HN2)}Cl(PPh3)] (10, 50% probability displacement ellipsoids are shown and hydrogen atoms have been omitted for clarity. |
As a consequence of the different mode of binding, the separation between the two metal centres (3.563 Å) is smaller than in 9 (5.358 Å) or in the coordination complexes 5 and 7, but it clearly exceeds the sum of the van der Waals radii of these two atoms.35 This precludes the existence of any direct interaction between iron and palladium.
The average values of the Fe–C and C–C bond lengths of the Fc moiety are similar to those reported for related 1,1′ disubstituted ferrocene derivatives.23 Moreover, the two pentagonal rings of this unit are planar, parallel (tilt angle of 1.44°) and they deviate by ca. 9° from the ideal eclipsed conformation. The pyrazole ring is planar and it is nearly orthogonal to the rings of the Fc unit (angles between their mean planes = 89.0° and 90.0°). This arrangement of substituents is very similar to that of the free ligand¶ and differs from those found in 7 and 9 in which the pyrazole and the Fe(η5–C5H5) unit are on opposite sides of the coordination plane of the metal.
According to the mechanism accepted for the cyclopalladation and cycloplatination of N– donor ligands, the reaction proceeds through coordination of the metal(II) ion to the nitrogen followed by the subsequent electrophillic attack at the carbon atom.36 Once the M–N bond is formed the relative orientation between the metal centre and the σ(C–H) bonds susceptible to metallation appears to be particularly relevant in determining the regioselectivity of the process.
It should be noted that since: a) previous theoretical studies revealed that six-membered palladacycles are expected to be more stable than those containing five-membered ring37 and b) compound 10 was isolated under milder experimental conditions (room temperature) than 9 (under reflux and using longer reaction periods), we tentatively propose that 10 arises from kinetic control while 9 is formed under thermodynamic control. In order to confirm this hypothesis, we decided to investigate the reaction between equimolar amounts of ligand 2, Na2[PdCl4] and NaAcO·3H2O in methanol for 24 h, followed by the treatment of the raw material with PPh3. When the reaction was carried out at room temperature the 1H–NMR spectrum of the crude revealed the presence of 10 and 9 in a molar proportion 6.0:
0.8 and when the reaction was performed under reflux during the same period of time the molar ratio 10:9 was 2.3
:
1.0. This suggests that the proclivity of 2 to undergo the transannulation process to give 10 decreases upon heating when compared with metallation of the phenyl ring to produce 9.
It should be noted that despite this, it is well-known that: a) ferrocene derivatives are more prone to electrophillic attacks than their benzene analogues1 and b) pallada- and platinacycles derived from the metallation of the substituted (η5–C5H4) ring of the Fc moiety are known,4,33,34 and in the case of ligand 2 the presence of this type of six-membered palladacycles were not detected in any of the reactions investigated.
For these six model compounds, the geometries were optimized before the calculations and their total energy was determined. The results (Table 5) obtained showed that the total energy of type A molecules was smaller than those of their isomers arising from the metallation of any of the two rings of the Fc unit (types B or C). For compounds with L = PH3 the total energy of the molecule increased according to the sequence A < C < B and an identical trend was found for their analogues with dmso. Consequently, from a theoretical point of view, the formation of A-type palladacycles with a σ[Pd–C(sp2, phenyl)] bond appears to be strongly favoured in the gas phase.
Since the polarity of the solvents used was different in the preparation of 9 and 10, we carried out additional computational studies in order to elucidate whether the solvent (toluene or methanol) could affect the relative stability of the three models. As can be easily seen in Table 5, in both solvents the minimum energy was obtained for A-type palladacycles and, for an identical neutral ligand, the energy of model B was again higher than that of its C-type analogue. When toluene is replaced by methanol the relative total energy of models B and C decreased, thus indicating that the polarity of the solvent may play a key role.
In a further step and in order to modelize the isolated products, we also calculated the total energies of complexes containing PPh3. This family of products shows the same stability trend (Table 5) as the simpler models with L = PH3 or dmso, and again the replacement of toluene by methanol reduces the differences ΔEr = [ET (for B or C types) − ET (for A-model)]. Thus, according to these computational studies the cyclometallation of the phenyl ring is strongly preferred over the transannulation process or the metallation of the (η5–C5H4) unit.
In view of this, we focused our attention in other factors that could affect these metallation processes. First, structural studies have shown that the orientation of the pyrazole ring in the free ligand 2 and in complex 10 is similar.¶ This is in agreement with the results obtained for 4-ferrocenyl-1,3-oxazolidine (11) and its palladium(II) complex formed by metallation of the (η5–C5H5) ring.4a Second, in 9 (as well as in the coordination compounds 4 and 7, which were also isolated under refluxing conditions) the heterocycle is in the opposite direction to Fe(η5–C5H5). This requires a change of the conformation of the ligand before or after the formation of the N–metal bond (Pd or Pt). Consequently, we decided perform molecular mechanics calculations to evaluate the variation of the total energy of ligand 2 for different orientations of the (η5–C5H4) ring of the Fc moiety and the heterocycle using the MM3 method.39 Such arrangements of substituents were generated by modifying the torsion angles defined by the atoms C2Fc–C1Fc–C–N2 and C1Fc–C–N2–N1 {hereinafter referred to as Φ(1) and Φ(2), respectively} in the range: [−180°, +180°].
The conformational map shows that if the free ligand is to achieve the same orientation as in 9 (or products 4 or 7), prior to the binding of the metal, this requires the change of Φ(1) and Φ(2) and an energy input. This would be more likely to occur if the reactions are performed under reflux (as happens in the preparation of 4, 7 and 9).
On the other hand, 9 and 10 were isolated using different starting reagents {Na2[PdCl4] or Pd(AcO)2}, and then the first step of the process consists of the coordination of the PdCl moiety (for 10) or of Pd(AcO)
(for 9) to the nitrogen of ligand 2. In the former case, where the reaction is performed in methanol at 298 K: a) according to the calculations, the difference between the relative total energy of the A and C–types decreases and b) the change of the conformation of the ligand requires an energy input and appears to be less likely to occur; consequently, once the Pd–N bond is formed the σ(C–H) bond of the C5H5 ring is extremely close to the metal centre and has the proper orientation as to undergo activation, this would lead to the C–type metallacycles.
In contrast with these findings, the first step of the formation of 9 consists of the binding of the Pd(AcO) unit, (with a greater effective bulk than the PdCl
unit) and in order to minimize steric effects, the conformation of 2 should change. This may be promoted at high temperatures (under reflux). A similar argument can be used to explain the variation observed in the arrangement of the ferrocene and pyrazole units in cis-[M[1-{(η5–C5H5)Fe(η5–C5H4)–CH2}-3,5-Ph2–C3HN2]Cl2(dmso)] {M = Pt (4) or Pd (7)} when compared with those of 2 or 9.
For this arrangement of groups the σ[C(phenyl)–H] bond on the ortho site is closer to the M(II) centre and has a better orientation than any of the σ[C(sp2, ferrocene)–H] bonds of the (η5–C5H5). Thus the cyclometallation of the phenyl ring is strongly preferred over the transannulation process.
Furthermore, we have demonstrated that the proper selection of the initial Pd(II) starting material {cis-[PdCl2L2] (L =dmso or CH3CN), Na2[PdCl4] or Pd(AcO)2} and the experimental conditions {temperature, solvents (methanol or mixtures of toluene/methanol); the presence or absence of additional NaAcO} allows the control of: a) the type of final product {heterodi- (in 7, 9 and 10) or trimetallic (in 8)}, b) the mode of binding of the ligand {monodentate (in 7 and 8) or bidentate (C,N)− (in 9 and 10)} and c) the regioselectivity of the cyclometallation process and the type of σ(Pd–C) bond formed {σ[Pd–C(sp2, phenyl)] (in 9) or σ[Pd–C(sp2,ferrocene)] (in 10)}.
The formation of compound 10 bond takes place under milder experimental conditions than that of the metallacycle 9 with a five-membered ring and a σ[Pd–C(sp2,phenyl)] bond. The computational studies undertaken for the simplified models of the palladacycles (A–C) indicate that complexes formed by metallation of the phenyl ring (A-type) are more stable than type C (transannulation products) and B models (with a 1,2-disubstituted ferrocene unit) that are the less stable products.
In addition, the studies summarized here provide conclusive evidences of the crucial role of the solvent, the temperature and the steric effects induced by the palladium(II) or platinum(II) salt or complex used as starting material in determining the nature of the C–H bond to be activated. Both theoretical and experimental data suggest that: a) the formation of the metallacycles with a σ{M–C(sp2, phenyl)} bond (5, 6, and 9 with A-types cores) is strongly favoured at high temperatures (refluxing toluene or methanol or mixtures of both) which may provide the energy input required to change the orientation of the ligand and when the starting reagents are bulky {[MCl2(dmso)2] or Pd(AcO)2}; b) the activation of the C–H bond of the unsubstituted (η5–C5H5) ring, to form 10 (C–type) is favoured at low temperature (methanol) and c) metallacycles with a 1,2-(η5–C5H3) ring (B-type cores) are the less stable ones in both solvents and have not been obtained in any experiment.
Finally, since it is well-known that palladium(II) and platinum(II) complexes containing bidentate (C,N)− ligands have a variety of applications in several fields (i.e. organometallic synthesis and homogeneous catalysis) and are attractive in view of their potential photo-optical properties or biological activities, the new products presented here appear to be excellent candidates to be studied in these fields.
2 | 4 | 7 | 9 | 10 | |
---|---|---|---|---|---|
Empirical formula | C26H22FeN2 | C28H28Cl2FeN2OPtS | C28H28Cl2FeN2OPdS | C44H36ClFeN2PPt·CH2Cl2 | C44H36ClFeN2PPd |
Formula weight | 418.31 | 762.42 | 673.74 | 906.34 | 821.42 |
Crystal size/mm× mm× mm | 0.2 × 0.09 × 0.09 | 0.2 × 0.1 × 0.1 | 0.2 × 0.1 × 0.1 | 0.2 × 0.1 × 0.1 | 0.1 × 0.09 × 0.09 |
Crystal system | Triclinic | Orthorhombic | Orthorhombic | Triclinic | Triclinic |
Space group |
P![]() |
Pbca | Pbca |
P![]() |
P![]() |
a/Å | 10.345(7) | 15.865(4) | 15.938(5) | 9,817(6) | 10.625(6) |
b/Å | 10.589(6) | 13.213(6) | 13.226(5) | 11.219(5) | 12.789(5) |
c/Å | 11.221(4) | 26.348(10) | 26.424(4) | 19.794(9) | 14.469(6) |
α (°). | 67.47(4) | 90.0 | 90.0 | 76.47(3) | 93.58(2) |
β (°). | 67.79(4) | 90.0 | 90.0 | 82.46(3) | 105.398(2) |
γ (°). | 82.84(5) | 90.0 | 90.0 | 69.94(3) | 107.09(2) |
T/K | 293(2) | 173(2) | 293(2) | 293(2) | 233(2) |
λ/Å | 0.71073 | 0.71073 | 0.71073 | 0.71073 | 0.71073 |
V/Å3 | 1050.8(10) | 5523(4) | 5570(3) | 1987.9(18) | 1790.71(14) |
Z | 2 | 8 | 8 | 2 | 2 |
D calc./Mg × m-3 | 1.322 | 1.834 | 1.607 | 1.514 | 1.523 |
μ/mm−1 | 0.731 | 5.879 | 1.458 | 1.094 | 1.062 |
F(000) | 436 | 2976 | 2720 | 920 | 836 |
Θ range for data collection/deg. | from 2.66 to 32.98 | from 2.57 to 32.53 | from 2.98 to 32.39 | from 2.59 to 32.58 | from 2.87 to 32.12 |
N. of collected reflections | 10793 | 38751 | 40442 | 18781 | 16450 |
N. of unique reflections {R(int)} | 5687{0.0606} | 7641{0.0833} | 7038{0.0542} | 10074{0.0481} | 8927{0.0502} |
N. of parameters | 263 | 327 | 326 | 478 | 452 |
Completeness to Θ = 25° | 93.0% | 96.3% | 90.2% | 92.5% | 92.4% |
Absorption correction | Empirical | Empirical | Empirical | Empirical | Empirical |
Max. and min transmission | 0.94 and 0.92 | 0.55 and 0.49 | 0.86 and 0.83 | 0.89 and 0.88 | 0.91 and 0.90 |
Goodness of fit on F2 | 1.130 | 1.083 | 1.082 | 1.078 | 1.089 |
R indices {I > 2σ(I)} | R 1 = 0.0568, | R 1 = 0.0414, | R 1 = 0.0675, | R 1 = 0.0401 | R 1 = 0.0624 |
wR2 = 0.1842 | wR2 = 0.1174 | wR2 = 0.2041 | wR2 = 0.0995 | wR2 = 0.1884 | |
R indices (all data) | R 1 = 0.0827, | R 1 = 0.0440, | R 1 = 0.0761, | R 1 = 0.0502 | R 1 = 0.0763 |
wR2 = 0.1949 | wR2 = 0.1148 | wR2 = 0.2098 | wR2 = 0.1034 | wR2 = 0.1198 |
Crystal structures were solved by direct methods (for 2, 7 and 9) or by Patterson synthesis (for 4 and 10), using SHELXS computer program42 and refined by full-matrix least-squares method with the SHELX97 computer program.43 Lorentz–polarization corrections were made in all cases and absorption corrections were also carried out for 2,4 and 10.
For the five compounds, all hydrogen atoms were computed and refined using a riding model with an isotropic temperature factor equal to 1.2 times the equivalent temperature factor of the atom to which is linked. The final R(on F) and wR(on │F│2) factors were 0.057 and 0.194, respectively (for 2); 0.041 and 0.115 (for 4); 0.067 and 0.204 (for 7); 0.040 and 0.099 (for 9) and goodnesses of fit = 1.130, 1.083, 1.082, 1.078 and 1.088 (for 2, 4,7, 9 and 10, respectively) for all the observed reflections. The number of parameters and other relevant data concerning the resolution and refinement of these structures is presented in Table 6.
The crystallographic information files of the crystal structures of compounds 2, 4, 7, 9 and 10 have been deposited at the Cambridge Crystallographic Data Centre (deposition numbers: CCDC 826854–826858).
Footnotes |
† Electronic supplementary information (ESI) available: Conformational map showing the variation of the total energy of the ligand as a function of the torsion angles Φ(1) and Φ(2) (Fig. S1) and tables containing final atomic coordinates of the optimized geometries of the three types of complexes (A–C, shown in the lower part of Table S5) with L = PH3, dmso or PPh3 used as models in the DFT study together with their calculated total energy in vacuum, in toluene and in methanol (Tables S1-S9). CCDC reference numbers 826854–826858. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1ra01080h |
‡ The separation between the centroid of the ring defined by the set of atoms [C(6)–C(10] of a molecule at (x, y, z) and the H(18) atom of a close one located at (−x, −y, 1 − z) is 2.997 Å and the distance between the center of the ring [C(6)–C(10)] of the unit at (x, y, z) and the H(24) of another one at (2 − x, 1 − y, −z) is 3.036 Å. The separation between the H(16) atom and the phenyl ring [C(21)–C(26)] of another unit is 3.963 Å. |
§ For illustrative purposes, the yields of 5 for different reaction periods (t) are given: 67.8, 69.1 and 70.2% for t = 3.5, 12 and 24 h, respectively. |
¶ In 2, the main plane of the heterocycle forms angles of 80.9° and 80.3° with the two pentagonal rings of the Fc unit. |
|| The success of all preparations described in this section is strongly dependent on the quality of the methanol used. The presence of small amounts of water reduces significantly the yield of the process. Thus the use of high quality (HPLC-grade) methanol is strongly recommended. In all cases the reaction flask was protected from the light with aluminium foil. |
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