Anne Brethona, Liliane G. Hubert-Pfalzgraf*a and Jean-Claude Daranb
aUniversité C. Bernard Lyon1, IRC, 2 Avenue A. Einstein, 69 626, Villeurbanne Cedex, France
bLCC, 205 route de Narbonne, 31077, Toulouse, France. E-mail: Hubert@.univ-lyon1.fr
First published on 3rd November 2005
The reactions between Ti(OiPr)4 and Zr2(OiPr)8(HOiPr)2, respectively, and lead 2-ethylhexanoate Pb(O2CC7H15)2 have been investigated at rt and by heating. The initial mixed-metal species, characterized by single-crystal X-Ray diffraction, were adducts namely Pb4Zr4(µ-O2CR′)8(µ-OR)6(µ3-OR)2(OR)8(OHR)21 and Pb2Ti4(µ-O2CR′)4(µ-OR)6(µ3-OR)2(OR)82 (R′ = CHCH(Et)C2H4Me, R = iPr) independently of the stoichiometry used. They are the first Pb–Ti and Pb–Zr non-oxo carboxylatoalkoxides reported. 1 is also the first Pb–Zr species based on an alkoxide-carboxylate ligand set matching the PbZrO3 stoichiometry. Both structures are centrosymmetric with six-coordinate transition metals, as required for the perovskite, and are based on triangular M2Pb cores (M = Zr, Ti). The lead centers display quite high coordination numbers, six and seven. The thermal and hydrolytic condensation reactions of 1 and 2 were investigated. Heat treatment of 2 and elimination of the volatiles under vacuum afforded Pb2Ti3(µ4-O)(µ3-O)(µ-O2CC7H15)2(µ-OiPr)6(OiPr)43 resulting from extrusion of Ti(OiPr)4 and scrambling of carboxylate ligands. Characterization of the various compounds was achieved by elemental analysis, FT-IR, 1H and 207Pb NMR.
We wish to report herein the study of the molecular constitutions of solutions of lead 2-ethylhexanoate and titanium or zirconium isopropoxides. The first Pb–Ti and Pb–Zr mixed-metal carboxylatoalkoxides whose formulae correspond to simple adducts were isolated and structurally characterized as Pb4Zr4(µ-O2CR′)8(µ-OR)6(µ3-OR)2(OR)8(OHR)21 and Pb2Ti4(µ-O2CR′)4(µ-OR)6(µ3-OR)2(OR)82 (R′ = CHCH(Et)C2H4Me, R = iPr). Their thermal and hydrolytic transformations were investigated. An oxo species of unusual stoichiometry Pb2Ti3(µ4-O)(µ3-O)(µ-O2CC7H15)2(µ-OiPr)6(OiPr)43, was isolated. The various compounds were characterized by elemental analysis, FT-IR, multinuclear NMR (1H and 207Pb) and for the powders derived from hydrolysis by TGA and XRD.
4![]() ![]() ![]() | (1) |
2![]() ![]() | (2) |
Their spectroscopic data are collected in Table 1. The FT-IR spectra of 1 show absorption bands at 1562, 1422 and 1410 cm−1 attributed to the νas and νs stretching vibrations of the CO2 moiety, respectively. The νasCO2 stretching vibrations are shifted to higher frequencies with respect to those of lead 2-ethylhexanoate (1519 cm−1). Similar features are observed for 2. The differences ΔνasCO2 − νsCO2, in the range 125–155 cm−1, suggest that the 2-ethylhexanoate ligands are bridging or bridging-chelating for both compounds.13 The νMOR absorptions are observed between 600 and 400 cm−1. An additional feature for the spectra of 1 is the absorption band at 3390 cm−1 suggesting the presence of alcohol in the metal coordination sphere.
1H NMR (CDCl3) | IR (cm−1) | IR | ||||
---|---|---|---|---|---|---|
Compound | T/°C | CH(carb) | OCH(iPr) | CH3 (iPr) | νsCO2, νasCO2 | νMOR |
For 1: νOH 3390 cm−1. | ||||||
1 | 25 | 2.06 (m, 16 H) | 5.15 (sept, J = 6 Hz), 4.59 (sept, J = 6 Hz), 4.28 (sept, J = 6 Hz), 4.05 (br) (4 : 4 : 8 : 2, 18H) | 1.18–1.20 (d, 108 H, J = 6 Hz) | 1562s, 1533sh, 1410s | 644sh, 559s, 462s |
−20 | 2.06 (m, 16 H) | 5.03, 4.54, 4.26, 4.03 (sept, J = 6 Hz) (4 : 4 : 8 : 2, 18H) | 1.18–1.20 (d, 108 H, J = 6 Hz) | |||
2 | 25 | 2.05 (m, 8H) | 5.42, 4.86, 4.45 (sept, J = 6 Hz) (4 : 8 : 4, 16H) | 1.3–1.35 (d, 96 H, J = 6 Hz) | 1562s, 1535sh, 1415s | 594s, 545m, 520m, 476m |
−58 | 2.07 (m, 8H) | 5.72 (sept, J = 6 Hz), 4.85 (sept, J = 6 Hz), 4.68 (sept, J = 6 Hz) (4 : 8 : 4, 16H) | 1.3–1.42 (d, 96H, J = 6 Hz) | |||
3 | 25 | 2.1 m (2H) | 5.38 (sept, J = 6 Hz), 5.05 (sept, J = 6 Hz), 4.86 (sept, J = 6 Hz), 4.75 (sept, J = 6 Hz) (2 : 2 : 4 : 2, 10 H) | 1.25–1.21 (d, 60 H, J = 6 Hz) | 1582vs, 1560sh, 1535sh, 1421s | 603vs, 549s, 500sh, 471m |
The proton NMR spectra of 1 and 2 in CDCl3 confirm the presence of both isopropoxide and carboxylate ligands as well as their relative stoichiometry. The carboxylate resonances are quite broad and mostly uninformative. For 1, the methine groups of the isopropoxides appear at rt as three well resolved septets at 5.15, 4.59, 4.28 ppm and a broad peak at 4.05 ppm, respectively, with a 4 : 4 : 8 : 2 integration ratio. These spectra are independent of the dilution accounting thus for a single molecular species. No additional resonances are observed at low temperature but the broad peak resolved into a septet, suggesting that it could correspond to isopropanol. 207Pb NMR has also been used since it can provide insights into lead coordination chemistry.14,15 The spectrum of 1 in toluene shows two main peaks, a broad one centered at 2735 ppm (Δν1/2 = 1268 Hz) and a sharper one at 2653 ppm (Δν1/2 = 397 Hz) in a 1 : 1 ratio. These chemical shifts suggest quite high coordination numbers for the lead centers. 1H NMR spectra of 2 display peaks with an OR/O2CR′ ratio of four. They resemble to those of 1 and account for at least three types of magnetically non-equivalent isopropoxide ligands with CH signals at 5.42, 4.86 and 4.45 ppm, respectively, in a 4 : 8 : 4 ratio.
Symmetry transformation used to generate equivalent atoms:i −x + 1, −y + 1, −z + 1. | |||
---|---|---|---|
O1–1Zr1 | 2.190(8) | O14–Zr2 | 2.171(7) |
O12–Zr1 | 1.934(8) | O21–Zr2 | 2.206(8) |
O13–Zr1 | 1.920(7) | O22–Zr2 | 1.946(8) |
O24–Zr1 | 2.239(7) | O23–Zr2 | 1.938(8) |
O31–Zr1 | 2.082(7) | O24–Zr2 | 2.230(7) |
O14–Zr1 | 2.189(7) | O32–Zr2 | 2.079(7) |
Zr1–Zr2 | 3.527(1) | ||
Pb3–Zr1 | 3.782(1) | ||
Pb3–Zr2 | 3.771(1) | O41–Pb4 | 2.615(8) |
Pb3–Pb4 | 4.191(1) | O41–Pb4 | 2.597(8) |
Pb4–Pb4i | 4.257 | O43–Pb4 | 2.604(8) |
O31–Pb3 | 2.399(6) | O44–Pb4 | 2.463(8) |
O42–Pb3 | 2.343(8) | O32–Pb3 | 2.386(7) |
O41–Pb3 | 2.882(8) | O45–Pb4 | 2.403(8) |
O24–Pb3 | 2.909(7) | O46–Pb4 | 2.460(8) |
O43–Pb3 | 2.847(7) | O46–Pb4# | 2.679(8) |
O47–Pb4 | 2.78(1) | ||
Zr2–O14–Zr1 | 108.0(3) | ||
Zr2–O24–Zr1 | 104.2(3) | ||
Zr1–O31–Pb3 | 114.9(3) | ||
Zr2–O32–Pb3 | 115.1(3) | ||
O42–Pb3–O32 | 82.2(3) | O42–Pb3–O24 | 127.7(2) |
O42–Pb3–O31 | 92.6(3) | O32–Pb3–O24 | 65.1(2) |
O32–Pb3–O31 | 109.3(2) | O31–Pb3–O24 | 64.3(2) |
O42–Pb3–O41 | 48.0(3) | O41–Pb3–O24 | 158.6(2) |
O32–Pb3–O41 | 126.1(3) | O43–Pb3–O31 | 156.1(3) |
O31–Pb3–O41 | 94.3(2) | ||
O13–Zr1–O12 | 99.0(3) | O31–Zr1–O11 | 167.4(3) |
O13–Zr1–O31 | 98.6(3) | O14–Zr1–O11 | 83.4(3) |
O12–Zr1–O31 | 97.5(3) | O13–Zr1–O24 | 165.5(3) |
O13–Zr1–O14 | 94.9(3) | O12–Zr1–O24 | 95.1(3) |
O12–Zr1–O14 | 163.9(3) | O31–Zr1–O24 | 82.8(3) |
O31–Zr1–O14 | 88.4(3) | O14–Zr1–O24 | 70.7(3) |
O13–Zr1–O11 | 91.7(3) | O11–Zr1–O24 | 85.5(3) |
O12–Zr1–O11 | 88.0(3) | ||
O23–Zr2–O22 | 100.8(4) | O22–Zr2–O21 | 89.9(3) |
O23–Zr2–O32 | 99.9(3) | O32–Zr2–O21 | 167.2(3) |
O22–Zr2–O32 | 97.8(3) | O14–Zr2–O21 | 83.7(3) |
O23–Zr2–O14 | 162.0(3) | O23–Zr2–O24 | 92.1(3) |
O22–Zr2–O14 | 95.4(3) | O22–Zr2–O24 | 166.4(3) |
O32–Zr2–O14 | 85.5(3) | O32–Zr2–O24 | 83.9(3) |
O23–Zr2–O21 | 88.5(3) | O14–Zr2–O24 | 71.2(3) |
O21–Zr2–O24 | 86.2(3) |
Symmetry code to generate equivalent atoms:i x, −y + 1/2, z. | |||
---|---|---|---|
Pb1–O31 | 2.331(5) | ||
Pb1–O33 | 2.366(5) | ||
Pb1–O32 | 2.395(5) | ||
Pb1–O34 | 2.803(6) | ||
Pb1–O34i | 2.909(7) | ||
Pb1–O24 | 2.849(6) | ||
Ti1–O13 | 1.795(5) | ||
Ti1–O12 | 1.804(5) | Ti2–O23 | 1.785(5) |
Ti1–O31 | 1.973(5) | Ti2–O22 | 1.805(4) |
Ti1–O14 | 2.021(5) | Ti2–O32 | 1.963(5) |
Ti1–O11 | 2.081(5) | Ti2–O14 | 2.071(4) |
Ti1–O24 | 2.137(5) | Ti2–O24 | 2.075(5) |
Ti2–O21 | 2.096(5) | ||
Ti1–Ti2 | 3.308(2) | ||
Pb1–Ti2 | 3.658(2) | ||
Pb1–Ti2 | 3.630(2) | ||
Pb1–Pb1i | 4.617(2) | ||
O31–Pb1–O33 | 92.1(2) | O32–Pb1–O34 | 125.4(2) |
O31–Pb1–O32 | 106.5(2) | O31–Pb1–O34 | 109.9(2) |
O33–Pb1–O32 | 85.2(2) | O33–Pb1–O34 | 121.2(2) |
O31–Pb1–O34 | 104.2(2) | O32–Pb1–O34 | 133.1(2) |
O33–Pb1–O34 | 49.5(2) | ||
O13–Ti1–O12 | 98.8(2) | O11–Ti1–O24 | 87.9(2) |
O13–Ti1–O31 | 95.1(2) | ||
O12–Ti1–O31 | 95.7(2) | O23–Ti2–O22 | 97.2(2) |
O13–Ti1–O14 | 163.8(2) | O23–Ti2–O32 | 96.1(2) |
O12–Ti1–O14 | 96.4(2) | O22–Ti2–O32 | 96.9(2) |
O31–Ti1–O14 | 89.2(2) | O23–Ti2–O14 | 95.3(2) |
O13–Ti1–O11 | 87.5(2) | O22–Ti2–O14 | 166.2(2) |
O12–Ti1–O11 | 90.5(2) | O32–Ti2–O14 | 87.3(2) |
O31–Ti1–O11 | 172.7(2) | O23–Ti2–O24 | 166.8(2) |
O14–Ti1–O11 | 86.5(2) | O22–Ti2–O24 | 95.4(2) |
O13–Ti1–O24 | 93.3(2) | O32–Ti2–O24 | 86.0(2) |
O12–Ti1–O24 | 167.8(2) | O14–Ti2–O24 | 71.7(2) |
O31–Ti1–O24 | 85.3(2) | O23–Ti2–O21 | 88.4(2) |
O14–Ti1–O24 | 71.5(2) | O22–Ti2–O21 | 90.1(2) |
O32–Ti2–O21 | 171.1(2) | ||
O14–Ti2–O21 | 84.7(2) | ||
Ti2–O32–Pb1 | 113.8(2) | ||
Ti1–O14–Ti2 | 107.8(2) | ||
Ti2–O24–Ti1 | 103.5(2) | ||
Ti1–O31–Pb1 | 114.8(2) |
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Fig. 1 ORTEP view of 1 with atom labelling for O, Zr and Pb atoms (ellipsoids at 30% probability). C atoms are represented as sphere of arbitrary radius. H atoms as well as the disordered C atoms are omitted for clarity. Symmetry code i: [1 − x, 1 − y, 1 − z]. |
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Fig. 2 ORTEP view of 2 with atom labelling for O, Ti and Pb atoms (ellipsoids at 30% probability). C atoms are represented as sphere of arbitrary radius. H atoms as well as the disordered C atoms are omitted for clarity. Symmetry code i: [x, −y + 1/2, z]. |
The basic heterometallic building block (BB) of 1 is a triangular PbZr2(OR)4(µ-OR)3(µ3-OR)(µ-O2CR′) unit. Two of them are assembled via a Pb2(O2CR′)4(OHR)2 moiety (Pb–Pb distances of 4.22 Å av.) into a centrosymmetric Pb4Zr4 array. All zirconium centers are six-coordinate. The lead atoms belonging to the BB, Pb3, are six-coordinate whereas those who ensure the junctions between the BB via a Pb2O2 ring, Pb4, are seven coordinate as often observed for lead in a carboxylate ligands environment. All lead centers have a highly distorted stereochemistry. The surrounding of Pb3 corresponds to a trigonal prism whereas that of Pb4 corresponds to a pentagonal bipyramid with O43 and O47 in the axial positions. Distortions are due to the various intracyclic angles [O32–Pb3–O31 of 109.3(2) and 68.2(3)° for O46i–Pb4–O46 for instance] as well as to the small bite angles [48.0(3)–53.0(3)°] of the bridging-chelating carboxylates. The lone pairs on lead appear stereochemically inactive with no obvious vacancy in the first coordination sphere. 1 can thus be considered as holodirected.16 Although the hydrogen' could not be located, analysis of the M–OR bond lengths on Zr and Pb centers suggest that isopropanol is linked to Pb4 with a bond distance of 2.78(1) Å. Large variations are observed for the Pb–O bond distances [2.343(8)–2.909(7) Å], the longest one corresponding to the Pb–µ3-OR linkages. Zr–O bond distances spread over the range 1.920(7)–2.230(7) Å. The Zr⋯Zr distance with a value of 3.527(1) Å is slightly longer than for Zr2(OiPr)8(iPrOH)2.17 The Zr–O–C angles of the terminal isopropoxides are large [163.3(1)–171.4(9)°] as common for zirconium alkoxides.18 A short contact [Pb4⋯C45 of 2.81(1) Å] is observed with one of the C of the carboxylate ring.
The structure of 2 is also centrosymmetric and based on triangular Ti2Pb(O2CR′)(OR)8 units similar to those observed for 1. The main difference with 1 lies in the connection between the BB. For 2, the BB are linked by their lead atoms via bridging-chelating µ,η2-carboxylate ligands and its overall stoichiometry is thus that of the BB. This results in a slightly asymmetrical bridge [2.803(6) and 2.909(7) Å] and in six-coordinate lead centers due to Pb–µ3-OR bonds. The Ti–O bond distances range from 1.785(5) to 2.137(5) Å and vary in the order Ti–OR(t) < Ti-µ-OR < Ti–O2CR′ as expected. The M⋯M distance [3.308(1) Å] is shorter than in the case of zirconium. The Pb–O bond lengths vary from 2.331(5) to 2.909(7) Å, the longest ones corresponding to the carboxylate and µ3-OR bridges. The Ti–O–C angles of the terminal OR are smaller than for 1 as usually observed.10
Triangular BB are quite common for heterometallics based on a MM′2 stoichiometry although they are often capped by two triply bridged ligands.19 A large number of heterometallic species involving zirconium are based on the [Zr2(OR)9]− moiety.18,20 An alternative description of the structures of 1 and 2 is based on the [M2(µ-OR)3(µ3-OR)(µ-O2CR′)(OR)4]− moiety trapping Pb2(O2CR′)3(ROH)+ or Pb(O2CR′)+ units, respectively. In contrast with the Pb–M heterometallic species based on acetates,10 the 2-ethylhexanoate ligands display two types of coordination modes, bridging and bridging-chelating. The related Pb–O bond distances are quite long as compared to usual Pb–O(carboxylate) ones. No solid state data are available for lead 2-ethylhexanoate but the lead carboxylates are reported as oligomers due to the assembling behavior of the O2CR′ ligands and as illustrated by lead succinate [Pb(O4C4H4)]∞ for instance.21 Metal alkoxides depolymerize metal acetates giving heterometallic acetatooxoalkoxides.10,22 Depolymerization of lead 2-ethylhexanoate is also achieved here, the difference in the frameworks of 1 and 2 reflecting the lower reactivity of Zr2(OiPr)8(iPrOH)2 as compared to Ti(OiPr)4 for formation of MM′ species.6,18,23 Reactions between anhydrous lead acetate, insoluble, and group 4 alkoxides afforded only oxo heterometallic species even for reactions achieved at rt.10 The acetate ligands usually bridge the two types of metals but the presence of oxo ligands implies more structural reorganization than here, especially in the case of zirconium isopropoxide.10a The solubility of lead 2-ethylhexanoate in non-polar media allows formation of a mixed-metal Pb–Zr species without complete breakdown of the chelating-bridging Pb carboxylate array. Another observation is that for heterometallics involving lead acetate, lead displays stereochemically active lone pairs and lower coordination numbers. It is noteworthy that adducts 1 and 2 were isolated in high yields despite the presence of water (∼0.5%) in commercial lead 2-ethylhexanoate.
Single-crystal data of 2-ethylhexanoate derivatives, often considered as metallic soaps, remain very scarce since the long chain carboxylates favor disorder and the obtaining crystals of poor quality.24 These limitations are valid here, especially for compound 1, but the solid state structures are supported by the 1H and 207Pb NMR spectra. The latter displaying two signals with a 1 : 1 integration ratio for 1 suggest that its solid state structure is retained in solution. 207Pb NMR data on seven-coordinated lead species are scarce. A chemical shift of 1667 ppm has been reported for heptacoordinate lead in Pb6(O2CiPr)12.15 The data obtained for 1 indicate only a difference of 100 ppm for the six- and seven fold-coordinate lead centers. The 1H NMR data of 1 and 2 suggest that the chemical shifts of the µ3-OR and µ-OR(MM) ligands are similar or that the quite long Pb–µ3-OR bonds are broken in solution, reducing the coordination number of the corresponding lead atoms to five in solution. Compounds 1, 2 and 3 (see below) are, to the best of our knowledge, the first examples of mixed-metal species with 2-ethylhexanoate ligands.
The obtaining of homogeneous solutions by hydrolysis allows some 1H NMR monitoring in CDCl3. Hydrolysis was immediate for 1 and 2. The resulting spectra were characterized in the CH region of the OiPr ligands by decrease of the initial peaks, apparition of new sets of resonances as well as apparition (for 2) or increase (for 1) of the peak of isopropanol. The latter shifts to higher frequencies with the extent of hydrolysis suggesting exchange between coordinated and free alcohol molecules. The first hydrolytic steps are likely to be intramolecular as observed for BaZr4(OR)18.25 and to proceed at the electrophilic metals, zirconium or titanium and on their bridging alkoxide ligands.26 This should affect the µ3-OR and transform the [PbM2(µ3-OR)(µ-OR)(µ-OR)2(µ-O2CR′)(OR)4]+ building blocks units into [PbM2(µ3-O)(µ-OR)2(µ-O2CR′)(OR)4]+ ones. This is confirmed by the development of species having two types of OR ligands in a 8 : 4 integration ratio in both cases, namely 4.90, 4.70 and 5.38, 5.68 ppm for 1a (M = Zr) and 2a (M = Ti), respectively. This transformation would leaves the transition metals M five-coordinate, quite unlikely for zirconium with OiPr ligands. However, the chemical shift of isopropanol suggests coordination to the transition metals of some of the alcohol molecules eliminated by hydrolysis. Eqn (3) and (4) summarizes these first hydrolytic steps whereas Scheme 1 represents structures of 1a and 2a based on six-coordinate transition metals in agreement with the 1H NMR data. All attempts to isolate 1a or 2a in crystalline form were unsuccessful.
1 + 2![]() ![]() | (3) |
2 + 2![]() ![]() | (4) |
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Scheme 1 |
1a and 2a were also detected, in small amounts (∼5%) in the reaction medium. Their presence, while no ester was detected, is most likely due to hydrolysis by the traces of water in commercial lead 2-ethylhexanoate.12
Hydrolysis at higher hydrolysis ratio is likely to affect µ-OR(PbM) and terminal OR ligands, the condensation becoming intermolecular and leading to more drastic structural modifications. For instance, hydrolysis at h = 8 of 1 gives spectra displaying the peak of free isopropanol, two main septuplets at 4.68 and 4.28 ppm and numerous smaller broad peaks (up to eight of similar area spreading over the range 6.1–4.20 ppm at −20 °C) belonging to a same species as shown by dilution experiments. These observations account for formation of oligomers due to extensive condensation with a large number of magnetically non-equivalent OR ligands. The last hydrolytic steps involve elimination of some 2-ethylhexanoic acid (FT-IR evidence). Elimination of acid is also confirmed by the obtaining of a powder analyzing as PbZrO(O2CR′)(ROH) by hydrolysis at rt of 1 by an excess of water.
The PbZrO3 (PZ) perovskite is one of least stable and thus difficult to obtain lead based perovskite.6 Hydrolysis of 1, whose stoichiometry matches that of PZ, was thus also achieved with an excess of water (h = 70) in isopropanol. FT-IR data of the amorphous powder obtained show residual carboxylate ligands (νasCO2 1540, νsCO2 1409 cm−1) as expected for differential hydrolysis.7 Its TGA and DTA patterns display several strong exothermic peaks for combustion of the residual organics which occurs in quite mild conditions (250–450 °C). A total weight loss of 37% suggests that the powder has a composition PbZrO(O2CR′)(ROH) (theoretical loss 37.8%). Annealing of the powder under air afforded a crystalline material. Crystallization starts around 450 °C and was analyzed in terms of formation of pyrochlore.6 Crystallization of the PZ perovskite starts around 550 °C and a pure perovskite phase is obtained at 600 °C. Thus the use of a single-source precursor (SSP) allows to avoid segregation and gives access to crystalline PZ at ∼100 °C lower than by using mixtures in which no homogeneity at a molecular level is achieved.6 Hydrolysis of 2 affords PbTi3O7 as well as red lead oxide as observed for the previously reported PbTi2 species.10a
Condensation of the Pb2Ti4 species, 2, is also faster than that of the Pb4Zr4 one 1 since no more evolution of 2 was observed after heating for 6 h at 120 °C. After removal of the volatiles of that reaction medium under vacuum (60 °C/10−3 mm Hg), compound 3 could be crystallized in ∼25% by adding isopropanol to the crude product. Elemental analyses account for a Pb/Ti ratio of 2 : 3. This change in the Pb/Ti stoichiometry is confirmed by isolation of titanium isopropoxide by fractional distillation of the volatiles. The FT-IR spectrum of 3 accounts for an oxo species as illustrated by absorption bands at 837, 784, 704 cm−1. Its 1H NMR spectra indicate a 10 : 2 ratio for the OiPr:O2CR′ ligands and four signals in the methine region at 5.38, 5.05, 4.86 and 4.35 ppm in a 2 : 2 : 4 : 2 ratio. The 207Pb NMR data show two sharp peaks of comparable intensities at 3005 and 2682 ppm.
Despite the unusual M2M′3 stoichiometry, the structure of compound 3, Pb2Ti3(µ4-O)(µ3-O)(µ-O2CC7H15)2(µ-OiPr)6(OiPr)4 appears quite symmetrical (Fig. 3). Selected bond distances and angles are collected in Table 4. 3 can be seen as a Pb2Ti2O2(µ-O2CR′)2(OR)6 species having two types of oxo ligands namely a central µ4 one O2 and a peripheral one O1 which acts as ligand toward a Ti(OiPr)4 moiety leading to a five-coordinate Ti. Such coordination of an alkoxide by a peripheral oxo ligand has been observed for Pb6Nb4O4(OEt)24.27 This µ3-oxo ligand O1 has a quite regular planar trigonal geometry. In contrast, the stereochemistry of the tetragonal oxo ligand is distorted with an increase in the Ti2⋯Ti2 distance up to 3.608 Å (as compared to 2), as well as an opening of the Ti2O2Ti2 angle [up to 136.2(4)°] for accommodation of the two bridging carboxylates. The Ti–O bond distances spread over the range 1.791(7) to 2.112(6) Å with the ranking Ti–OR < Ti-µ3O ≈ Ti-µOR< Ti-µ4-O < Ti-µO2CR′. The Pb–O bond distances are longer [2.161(4)–2.555(6) Å] and vary along Pb-µ3O < Pb-µ4O < Pb–OR but they are shorter than for 1 and 2 due to the lower coordination numbers of lead. The Pb–OR–Ti bridges are quite asymmetrical. In contrast with 1 and 2, the lone pairs of the five-coordinate tetragonal pyramidal lead centers are stereochemically active. Compound 3 can also be seen as the association of two PbO units, Ti(OR)4 and Ti2(O2CR′)2(OR)6.
Ti1–O3 | 1.791(7) | O1–Ti1 | 1.873(7) |
Ti1–O8 | 1.959(6) | O6–Ti2 | 1.805(6) |
O4–Ti2 | 2.093(6) | O9–Ti2 | 2.112(6) |
O5–Ti2 | 1.894(6) | O2–Ti2 | 1.944(3) |
O8–Ti1 | 1.959(6) | O7–Ti2 | 1.896(6) |
O5–Pb1 | 2.506(6) | O2–Pb1 | 2.462(4) |
O8–Pb1 | 2.548(6) | O7–Pb1 | 2.555(6) |
O1–Pb1 | 2.161(4) | ||
Ti2–Ti2 | 3.608(2) | ||
Ti1–Pb1 | 3.498(2) | ||
Ti2–Pb1 | 3.478(2) | ||
Ti2–Pb1 | 3.497(2) | ||
Ti2–O5–Pb1 | 103.6(2) | Ti2–O2–Pb1 | 104.46(9) |
Ti1–O8–Pb1 | 101.0(2) | Ti2–O2–Pb1 | 103.64(9) |
Ti1–O1–Pb1 | 120.1(2) | Ti2–O2–Pb1 | 104.46(9) |
Pb1–O1–Pb1 | 119.8(3) | Pb1–O2–Pb1 | 98.8(2) |
Ti2–O2–Ti2 | 136.2(4) | Ti2–O7–Pb1 | 102.6(2) |
O3–Ti1–O3 | 105.3(4) | O6–Ti2–O2 | 169.6(3) |
O3–Ti1–O1 | 127.4(2) | O5–Ti2–O2 | 87.8(2) |
O3–Ti1–O1 | 127.4(2) | O7–Ti2–O2 | 88.3(2) |
O3–Ti1–O8 | 97.5(3) | O6–Ti2–O4 | 86.0(3) |
O3–Ti1–O8 | 97.1(3) | O5–Ti2–O4 | 172.2(3) |
O1–Ti1–O8 | 77.9(2) | O7–Ti2–O4 | 89.4(3) |
O1–Ti1–O8 | 77.9(2) | O6–Ti2–O9 | 85.7(3) |
O8–Ti1–O8 | 155.8(3) | O5–Ti2–O9 | 89.1(3) |
O6–Ti2–O5 | 98.7(3) | O7–Ti2–O9 | 172.5(3) |
O6–Ti2–O7 | 99.0(3) | O2–Ti2–O9 | 86.3(2) |
O5–Ti2–O7 | 95.9(3) | O4–Ti2–O9 | 85.1(3) |
O1–Pb1–O2 | 70.7(2) | ||
O1–Pb1–O5 | 94.1(2) | ||
O2–Pb1–O5 | 64.8(1) | ||
O1–Pb1–O8 | 60.9(2) | ||
O2–Pb1–O8 | 131.5(2) | ||
O5–Pb1–O8 | 114.5(2) | ||
O1–Pb1–O7 | 93.4(2) | ||
O2–Pb1–O7 | 64.5(1) | ||
O5–Pb1–O7 | 122.2(2) | ||
O8–Pb1–O7 | 118.8(2) |
![]() | ||
Fig. 3 ORTEP view of 3 with atom labelling for O, Ti and Pb atoms (ellipsoids at 30% probability). C atoms are represented as sphere of arbitrary radius. H atoms as well as the disordered C atoms are omitted for clarity. Symmetry code i: [−x, y, −z + 1/2]. |
The formation of 3 results from a drastic reorganization of compound 2 by elimination of Ti(OR)4 as summarized by eqn (5). Spectroscopic data (1H NMR, FT-IR) of the crude product obtained after heating of 2 before elimination of volatiles gave evidence for traces of titanium isopropoxide. However, the passage from a Pb2Ti4 stoichiometry to a Pb2Ti3 one is favored by the treatment under vacuum and the volatility of Ti(OiPr)4. The 2-ethylhexanoate ligands linked to lead appear more labile than those on titanium. This lability and the tendency to form oxo species was also observed by long heating of Pb(O2CR′)2 during its synthesis from lead oxide. 1H and 207Pb NMR spectra account for the retention of the solid state structure of 3 by dissolution in non-polar media.
![]() | (5) |
![]() | (6) |
1 | 2 | 3 | |
---|---|---|---|
a R1 = ∑‖Fo| − |Fc‖/∑|Fo|, wR2 = [∑[w(Fo2 − Fc2)2]/∑[w(Fc2)2]]1/2. | |||
Empirical formula | C108H224O34Pb4Zr4 | C80H168O24Pb2Ti4 | C46H96O16Pb2Ti3 |
Mr | 3260.52 | 2120.03 | 1463.3 |
Space group | P![]() | P![]() | C2/c |
Crystal system | Triclinic | Triclinic | Monoclinic |
a/Å | 11.377(1) | 11.3722(6) | 21.663(2) |
b/Å | 13.118(1) | 12.4435(6) | 14.231(1) |
c/Å | 26.190(3) | 19.303(1) | 22.737(2) |
α/° | 85.559(8) | 91.624(4) | 90 |
β/° | 83.667(7) | 96.393(4) | 116.342(8) |
γ/° | 76.410(8) | 109.728(5) | 90 |
V/Å3 | 3771.0(6) | 2549.0(2) | 6281.7(9) |
Z | 1 | 1 | 4 |
Diffractometer | Oxford X-CALIBUR CCD | Stoe IPDS | Oxford X-CALIBUR CCD |
µ(Mo-Kα)/mm−1 | 4.772 | 3.650 | 5.762. |
No. of unique reflections (Rint) | 10765(0.0674) | 6680 (0.0848) | 4824 (0.0485) |
Absorption correction | Analytical | Semi-empirical from equivalents | Semi-empirical from equivalents |
Data/restraints/parameters | 10765/772/704 | 6680/480/544 | 4824/15/219 |
Goodness-of-fit on F2 | 1.069 | 1.059 | 1.081 |
R1, wR2 [I > 2σ(I)]a | 0.0438, 0.1165 | 0.0554, 0.1484 | 0.0432, 0.1156 |
R1, wR2 (all data) | 0.0568, 0.1225 | 0.0637, 0.1586 | 0.0553, 0.1216 |
CCDC reference numbers 230776–230778 for 2, 3 and 1, respectively.
For crystallographic data in CIF or other electronic format see DOI: 10.1039/b513142c
This journal is © The Royal Society of Chemistry 2006 |