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Thermochemistry of alkaline-earth phenoxides
J. Chem. Thermodynamics 38 (2006) 296–303 www.elsevier.com/locate/jct
Thermochemistry of alkaline-earth phenoxides Carla Hipo´lito a, Joa˜o Paulo Leal
a,b,*
, Yanzhi Guo c, Matthias Epple
c
a
b
Departamento de Quı´mica, Instituto Tecnolo´gico e Nuclear 2686-953 Sacave´m, Portugal Departamento de Quı´mica e Bioquı´mica, Faculdade de Cieˆncias, Universidade de Lisboa 1749-016 Lisboa, Portugal c Institute of Inorganic Chemistry, University of Duisburg-Essen, D-45117 Essen, Germany Received 14 April 2005; received in revised form 20 May 2005; accepted 25 May 2005 Available online 22 July 2005
Abstract The standard molar enthalpies of formation of some alkaline-earth metal phenoxides were determined by reaction–solution calorimetry. The results obtained at T = 298.15 K were as follows: Df H m ½MðORÞ2 ; cr=kJ mol1 ¼ ð837.9 7.5Þ ½MgðOPhÞ2 ; ð837.4 7.2Þ ½CaðOPhÞ2 ; ð888.1 7.2Þ ½CaðO-2; 6-Me2 PhÞ2 ; ð828.8 7.3Þ ½SrðOPhÞ2 ; ð808.6 7.3Þ ½BaðOPhÞ2 ; ð880.7 7.3Þ ½BaðO-2; 6-Me2 PhÞ2 . Together with an appropriate Born–Haber cycle, these results allow the calculation of lattice energies for the [M(OR)2] compounds. The thermochemical radii of the anions OR were obtained using the Kapustinskii equation and the lattice energies previously determined. A set of lattice energies and standard molar enthalpies of formation for alkaline-earth metal phenoxides [M(OR)2, M = Mg, Ca, Sr, Ba; R = OPh, 2,6-Me2OPh], was presented. Estimates for these properties for unmeasured compounds were made based on a model first applied to aliphatic alkaline metal alkoxides. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Thermochemistry; Enthalpy of formation; Phenoxides; Lattice energy; Thermochemical radii
1. Introduction The interest in the formation and chemistry of alkaline-earth metal compounds has undergone resurgence over the last decade. The discovery of superconducting ceramics containing alkaline-earth ions and the improvements in the respective preparative technique by the use of alkoxide precursors soluble in hydrocarbons were the main reasons for this resurgent interest [1]. Nowadays the chemistry of alkaline-earth metals is no more an undeveloped area of the periodic table [2]. Despite that, thermochemical data for these substances are almost absent from literature. Two values of standard enthalpies of formation of Mg(OMe)2, Mg(OEt)2 and three values for Ca(OMe)2, Ca(OEt)2 and Ca(OBu)2 are the exceptions [3]. Structural studies of alkaline*
Corresponding author. Fax: +351 1 9941455. E-mail addresses: [email protected], [email protected] (J.P. Leal), [email protected] (M. Epple). 0021-9614/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2005.05.016
earth phenoxides are scarce. X-ray diffraction information exists for strontium phenoxide exhibiting an approximate octahedral structure with a metal co-ordination number equal to six [4].
2. Experimental 2.1. General procedures All manipulations were carried out under an oxygen and water free (<5 Æ 106) nitrogen atmosphere inside a glove-box or using standard Schlenk techniques. The THF and toluene were pre-dried over 4A molecular sieves and distilled under sodium. Pentane was pre-dried under calcium sulfate and distilled over P2O5. Phenol (Aldrich, 99%) and 2,6-dimethylphenol (Aldrich, 99%) were sublimed twice. Ethanol (Merck, 99.8%) was pre-dried over calcium sulfate, heated under reflux over activated magnesium and iodine, and finally distilled.
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Calcium hydroxide (May and Baker, Lda, 99%) was used as supplied. Magnesium (turnings, Sigma), calcium (granules, Sigma) and strontium (Aldrich, 99%) were used as supplied and stored in an inert atmosphere glove-box. Barium (Fluka, 99%) was used as small pieces to which the oxidized surface was removed inside a glove-box. All solvents were degassed twice by freeze– thaw cycles before use. The IR spectra were recorded on a Bruker Tensor 27 spectrophotometer as Nujol mulls between CsI plates. The listing of bands observed at 3000 and 1400 cm1 was omitted due to overlapping with the C–H stretching frequencies of Nujol. Elemental analysis (C and H) was performed on a CE Instruments EA1110 CHNS–O automatic analyzer (C and H) or by titration (M). 2.2. Phenoxides synthesis Two of the alkaline-earth phenoxides, Mg(OPh)2, Ca(OPh)2, were synthesized in a mixed ammonia/THF solution; the others were prepared in mixed ammonia/ toluene solution, all based in previous work of Drake et al. [2] 2.2.1. Synthesis of Mg(OPh)2 Magnesium metal granules (0.12 g, 4.8 mmol), preweighed in a glove-box, were placed into a 500 cm3 flask along with phenol (0.93 mg, 9.9 mmol) and THF (100 cm3). Outside the glove-box using Schlenk techniques, ammonia gas (pre-dried under sodium) was bubbled through the pre-cooled (T = 203.15 K) colorless solution. After an induction period of 10 min, the metal granules started to divide into fine particles, the solution became white and the volume increased to 150 cm3 due to ammonia condensation. The reaction was completed after 45 min at a temperature of 203.15 K with the complete dissolution of magnesium metal. The cold bath was taken out and the ammonia gas let to evaporate during the night. The solution was then refluxed during 24 h using the ‘‘toluene reflux’’ method [5]. The solvent was removed and the white solid obtained was washed with pentane and dried in high vacuum (102 to 103) Pa. Yield: 0.45 g = 45%. Found: C, 0.686; H, 0.046. Calc. for C12H10O2Mg: C, 0.685; H, 0.048. i.r. [Nujol, m/(cm1)]: 1594 (vs), 1540 (w), 1494 (s), 1460 (s), 1377 (s), 1259 (vs), 1172, (w), 1154 (w), 1072 (w), 1025 (w), 999 (vw), 880 (s), 847 (vs), 847 (vs), 820 (w), 759 (s), 687 (s), 669 (vw), 595 (vs), 576 (vs), 507 (vs), 418 (vw), 487 (w), 326 (w). 2.2.2. Synthesis of Ca(OPh)2 Calcium metal granules (0.18 g, 4.5 mmol), weighed in a glove-box, were placed into a 500 cm3 flask along with phenol (0.90 mg, 9.6 mmol) and THF (100 cm3). The reaction conditions were as described above for Mg(OPh)2.
297
Yield: 0.80 g = 80%. Found: C, 0.637; H, 0.045. Calc. for C12H10O2Ca: C, 0.636; H, 0.048. i.r. [Nujol, m/(cm1)]: 1594 (vs), 1484 (vs), 1460 (s), 1377 (w), 1279 (vs), 1170 (w), 1072 (w), 1025 (w), 995(w), 878 (w), 819 (s), 760 (s), 695 (s), 557 (w), 524 (vw), 487 (w), 326 (w). 2.2.3. Synthesis of Ca(O-2,6-Me2Ph)2 Calcium metal granules (0.14 g, 3.5 mmol), weighed in a glove-box, were placed into a 500 cm3 flask along with phenol (0.90 mg, 7.4 mmol) and toluene (100 cm3). The reaction conditions were as described above for Mg(OPh)2. Yield: 0.81 g = 81%. Found: C, 0.681; H, 0.064. Calc. for C16H18O2Ca: C, 0.688; H, 0.065. i.r. [Nujol, m/(cm1)]: 1591 (w), 1462 (vs), 1377 (s), 1264 (s), 1226(s), 1195 (s), 1089 (w), 1018 (w), 912 (vw), 844 (w), 800 (w), 755 (s), 684 (w), 670 (w), 562 (w), 517 (w), 452 (vw), 388 (w), 305 (w). 2.2.4. Synthesis of Sr(OPh)2 Strontium metal granules (0.48 g, 5.5 mmol), weighed in a glove-box, were placed into a 500 cm3 flask along with phenol (0.11 mg, 11 mmol) and toluene (100 cm3). The reaction conditions were as described above for Mg(OPh)2. Yield: 1.17 g = 78%. Found: C, 0.526; H, 0.037. Calc. for C12H10O2Sr: C, 0.523; H, 0.042. i.r. [Nujol, m/(cm1)]: 1594 (vs), 1482 (vs), 1464 (s), 1377 (w), 1261 (vs), 1168, (w), 1071 (w), 1020 (w), 993 (w), 876 (w), 818 (s), 759 (s), 695 (s), 551 (w), 523 (vw), 472 (w). 2.2.5. Synthesis of Ba(OPh)2 A piece of barium (0.52 g, 3.8 mmol), weighed in a glove-box, was placed into a 500 cm3 flask along with phenol (0.75 g, 8.0 mmol) and toluene (100 cm3). The reaction conditions were as described above for Mg(OPh)2. Yield: 1.10 g = 89%. Found: C, 0.445; H, 0.034. Calc. for C12H10O2Ba: C, 0.445; H, 0.031. i.r. [Nujol, m/(cm1)]: 1590 (s), 1477 (vs), 1377 (vs), 1261 (s), 1164 (s), 1069 (w), 990 (s), 875 (s), 820 (w), 760 (s), 696 (s), 547 (w), 526 (w), 464 (vw), 397 (vw), 354 (vw) 326 (vw), 302 (vw), 266 (vw). 2.2.6. Synthesis of Ba(O-2,6-Me2Ph)2 A piece of barium (0.48 g, 3.5 mmol), weighed in a glove-box, was placed into a 500 cm3 flask along with 2,6-dimethylphenol (1.0 g, 8.4 mmol) and toluene (100 cm3). The reaction conditions were as described above for Ca(2,6-Me2OPh)2. Yield: 1.15 g = 87%. Found: C, 0.514; H, 0.041. Calc. for C16H18O2Ba: C, 0.506; H, 0.048. i.r. [Nujol, m/(cm1)]: 1458 (vs), 1420 (s), 1376 (s), 1266 (s), 1233 (w), 1091 (s), 1038 (s), 842 (s), 799 (s), 752 (s), 682 (w), 504 (w), 419 (w).
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2.3. EXAFS spectra Extended X-ray absorption fine structure (EXAFS) spectra were measured at beamline E4 at the Hamburger Synchrotronstrahlungslabor (HASYLAB) at Deutsches Elektronen Synchrotron (DESY), Hamburg, Germany. The samples were prepared as a thin layer of powder on adhesive tape using Schlenk techniques, i.e. avoiding the access of air or moisture as far as possible. Experiments were performed at the Ca K-edge (ca. 4038 eV) in transmission mode at liquid nitrogen temperature under high vacuum. The programs SPLINE and XFIT were used for quantitative data evaluation. Theoretical standards were computed with the program FEFF 6.01a [6,7]. EXAFS fit parameters were: k-range 14 to 76 nm1, R-range 0.1 to 0.45 nm, Eo = +8.8 eV, S 2o ¼ 1 (fixed). Variation parameters were N, r2, R (for each shell) and Eo. 2.4. Reaction–solution calorimetry The enthalpies of formation of the prepared phenoxides were determined by measuring the enthalpies of reaction and solution in water, except for the magnesium phenoxide that was measured in 0.1 M HCl aqueous solution due to the low solubility of this compound in water. The calorimeter used was specifically built for the study of oxygen- and water-sensitive compounds, and the experimental procedure was described in a previous paper [8]. The accuracy of the calorimeter was determined by frequent measurement of the hydrolysis
of TRIS in a 0.1 M HCl aqueous solution. Briefly, the calorimeter reaction vessel consisted of a 220 cm3 Dewar flask with two acrylic flanges closed by a lid, which supported a stirred, a quartz crystal thermometer probe, a resistance for electrical calibration, and an ampoulebreaking system. The reaction vessel was immersed in a thermostatic water bath T = (298 ± 103) K controlled by a Tronac PTC-40 unit. In a typical experiment, 12 mm diameter glass ampoules were loaded with (20 to 30) mg of the phenoxide compound inside a glove box, sealed under vacuum and weighed to ±105 g with a Mettler H54AR balance. A mass correction for vacuum was always done. The reaction was started by breaking the glass ampoule in 170 cm3 of the appropriate reaction mixture. This was preceded by an electrical calibration in which a potential difference of ca. 2.5 V was applied to a 48 X resistance during ca. 180 s. The results were averaged using at least four runs. The errors presented are twice the standard deviation of the mean in each case.
3. Results and discussion 3.1. Infrared spectra The alkaline-earth metal phenoxides were spectroscopically studied as Nujol suspensions between CsI windows. Figure 1 presents three of the obtained spectra as an example. The vibrations related to metal–oxygen bond occur at 507 and 551 cm1, 487 and 524 cm1,
FIGURE 1. Typical spectra obtained for: (a) Mg(OPh)2, (b) Ca(OPh)2, and (c) Ba(OPh)2.
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299
o
Mg(OPh)2 (cr) + 2[HCl.552H2O] ∆ r H (1)
∆ r H m (1)
MgCl2 (cr) + 2PhOH (l) + 1104 H2O (l)
∆ sln H(2)
2 ∆ sln H(1)
2∆ sln H(3) 1104 ∆ sln H(4)
MgCl2 (sln) + 2 PhOH (sln) + n (HCl .552H2O)
SCHEME 1.
472 and 523 cm1, 464 and 526 cm1 for Mg(OPh)2, Ca(OPh)2, Sr(OPh)2 and Ba(OPh)2, respectively. 3.2. Calorimetric measurements The standard molar enthalpies of formation, Df H m ½MðORÞ2 ; cr, of alkaline-earth metals phenoxides, [Mg(OPh)2], [Ca(OPh)2], [Ca(O-2,6-Me2Ph)2], [Sr(OPh)2], [Ba(OPh)2], [Ba(O-2,6-Me2Ph)2], were computed according to scheme 1 or scheme 2. The validity of the schemes was checked by dissolving a ‘‘synthetic’’ final solution in a ‘‘real’’ final solution and verifying that it was athermic. Auxiliary data used to derive these quantities from the true experimental ones – the enthalpies of reaction – are presented in table 1. The molar o
M(OR)2 (cr) + 2H2O (l) ∆ r H (2)
∆r Hm(2)
M(OH)2 (cr) + 2ROH (l) ∆ sln H(6)
2 ∆ sln H(5)
2 ∆ sln H(7)
M(OH)2 + ROH (aq) ; R=OPh or 2,6-Me2OPh SCHEME 2.
TABLE 1 Auxillary thermochemical data at T = 298.15 K Compound
Df H m
References 1
kJ Æ mol MgCl2, cr Mg, g Ca(OH)2, cr Ca(OH)2 1 H2O Ca, g Sr(OH)2, cr Sr(OH)2 Æ 800H2O Sr, g Ba(OH)2, cr Ba(OH)2 Æ 500H2O Ba, g PhOH,cr 2,6-Me2OPh, cr H2O, l HCl Æ 552H2O
483.93 ± 0.08 147.7 ± 0.8 986.09 ± 0.08 1002.82 ± 0.08 178.2 ± 0.8 959.0 ± 0.8 1004.6 ± 0.8 164.4 ± 0.8 944.7 ± 0.8 995.4 ± 0.8 180 ± 0.8 165.1 ± 0.7 237 ± 1 285.830 ± 0.040 166.596 ± 0.008
[10] [10] [10] [10] [10] [10] [10] [10] [10] [10] [10] [11] [11] [10] [10]
quantities are based on the 2001 standard atomic masses values [9]. The Dr H m ð1Þ symbolizes the enthalpy change for the reaction of magnesium phenoxide, [Mg(OPh)2], with 0.1 M HCl and Dr H m ð2Þ the enthalpy change for the reaction of M(OR)2 (M = Ca and Ba, R = OPh, 2,6Me2OPh; M = Sr, R = OPh;) with distilled and deionized water and all compounds in their standard state. The DrH(1) and DrH(2) stand for the measured reaction enthalpies under the experimental conditions (see Section 2). The DslnH(1) is the term for the dissolution enthalpy of a 0.1 M HCl solution in the same 0.1 M HCl solution; DslnH(2) is the dissolution enthalpy of stoichiometric amounts of MgCl2 in 0.1 M HCl; DslnH(3) is the dissolution enthalpy of phenol in (0.1 M HCl + MgCl2) solution; DslnH(4) represents the dissolution of water in 0.1 M HCl solution; DslnH(5) is the dissolution enthalpy of water in water; DslnH(6) is the dissolution enthalpy for stoichiometric amounts of M(OH)2 in H2O and DslnH(7) is the dissolution enthalpy of the alcohol in {H2O + M(OH)2}. The DslnH(2) for magnesium chloride was taken as (161 ± 2) kJ Æ mol1. Although this was not measured experimentally, the value proposed is an ‘‘educated guess’’ based on a previous experimental value of the dissolution enthalpy of stoichiometric amounts of CaCl2 in 0.1 M HCl, [3], and by comparison of differences between the enthalpy of formation of aqueous 0.1 M HCl and crystalline magnesium and calcium chloride (table 1). The DslnH(3) was taken as (12.4 ± 1.2) kJ Æ mol1, an experimental value obtained in an earlier study for dissolution enthalpy of phenol in 0.1 M HCl [8]. The DslnH(4), the dissolution of water in the solvent, is insignificant (smaller than the detection limit of the apparatus) and even when multiplied by 1104 (see scheme 1) yields a minor contribution to the final Dr H m ð1Þ value, i.e. smaller than the experimental error, and therefore was not considered. The DslnH(5) and DslnH(1) were obviously zero. The concentrations of alkaline-earth metals hydroxides, M(OH)2 with (M = Ca, Sr, Ba) in the final calorimetric solutions were always very small (typical molar ratios of nMðOHÞ2 : nH2 O 1:107 or less). In these conditions of very dilute solutions, an approximation can be made without significant error and DslnH(6)
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300
can be calculated to yield: (24.51 ± 0.08) kJ Æ mol1 for Ca(OH)2, (44.50 ± 0.08) kJ Æ mol1 for Sr(OH)2, (57.61 ± 0.08) kJ Æ mol1 for Ba(OH)2, all based on the formation enthalpies of crystalline hydroxides and the corresponding enthalpies of formation for very dilute solutions using the data reported in the literature [10]. The DslnH(7) was experimentally determined (see table 2). From schemes 1 and 2, it was possible to calculate the enthalpies of formation of the alkaline-earth metal phenoxides using equations (1) and (2), respectively. The obtained values are presented in table 3. Df H m ½MgðOPhÞ2 ; cr ¼ Df H m ðMgCl2 ; crÞ þ 2Df H m ðPhOH; lÞ 2Df H m ðHCl 552H2 OÞ Dr H ð1Þ þ Dsln H ð2Þ þ 2Dsln H ð3Þ; ð1Þ
Df H m ½MðORÞ2 ; cr ¼ Df H m ½MðOHÞ2 ; cr þ 2Df H m ðROH; lÞ 2Df H m ½H2 O; l Dr H ð2Þ þ Dsln H ð6Þ þ 2Dsln H ð7Þ. ð2Þ Assuming that the bonds in an alkaline-earth metal phenoxide compound are essentially ionic (many metal–organic compounds are not ionic crystals but their bonds still essentially ionic in nature) [12–14], it was possible to define the corresponding lattice energy,
DlatU[M(OR)2], as the internal energy change associated with the following process: MðORÞ2 ðcrÞ ! M2þ ðgÞ þ 2RO ðgÞ ðM ¼ alkaline earth metal; R ¼ phenyl groupÞ: ð3Þ Kapustinskii noticed that, in the formula for the lattice energy in an ionic model, by dividing the Madelung constants by the number of ions in the molecule the new constant obtained is almost independent of the structure of the lattice. He also assumes the repulsive part of the energy is 1/9 of the attractive one and split the inter-nuclear equilibrium distance in a sum of two radii (r+ and r). For a detailed discussion, see e.g. reference [15]. To compute this quantity, scheme 3 can be used. Equation (4), obtained directly from scheme 3, allows one to compute DlatU[M(OR)2], the internal energy change associated with the process described in equation (3) at T = 298.15 K, where R is the gas constant, T is the absolute temperature, Dsub H m ðMÞ and Di H m ðM2þ Þ are the enthalpy of sublimation and the first plus the second enthalpy of ionisation of the metal, respectively, and
M2+(g) + 2OR(g) + 2e– ∆ Ηi (M+) +RT o
∆ Ηi (M) + RT o
TABLE 2 Enthalpies of dissolution of phenols in a water + Ca(OH)2 solution at T = 298.15 K Compound
Mass/kg
e
DT/K
J Æ K1 OPh
2,6-Me2OPh
36.14 51.35 35.74 37.89 29.03 14.17 16.37 36.04 44.88
DslnH(7) kJ Æ mol1
868.2372 792.1251 802.6641 793.4747 793.0613 795.1276 756.5981 737.7007 715.8550
0.0018 0.0033 0.0028 0.0032 0.0011 0.0006 0.0012 0.0013 0.0035
4.0261 4.8068 5.9170 6.2340 5.2 ± 0.5 3.8141 4.2736 6.9452 3.2932 6.7424 5.0 ± 1.1
o
-2∆eaHm (OR) -2RT
M(g) + 2RO(g) M2+(g) + 2OR (g)
∆ sub Hmo (M) M(cr) + 2OR(g) 2 ∆ f Hmo (OR, g)
∆ latU298o + 3RT
M(cr) + O2(g) + xC(cr) + yH2(g)
∆ f Hmo(M(OR)2,cr) M(OR)2 (cr)
SCHEME 3.
TABLE 3 Reaction enthalpies and standard enthalpies of formation of alkaline-earth phenoxides and 2,6-dimethyl phenoxides at T = 298.15 K Metal
DrH(1) or DrH(2) 1
Mg(OPh)2 Ca(OPh)2 Ca(2,6-Me2OPh)2 Sr(OPh)2 Ba(OPh)2 Ba(2,6-Me2OPh)2
Dr H m ð1Þ or Dr H m ð2Þ 1
Df H m ½MðORÞ2 ; cr 1
kJ Æ mol
kJ Æmol
kJ Æmol
114.5 ± 0.2 49.9 ± 0.7 71.3 ± 0.3 60.3 ± 0.8 71.3 ± 0.8 71.3 ± 0.4
34.1 ± 2.3 28.0 ± 0.9 49.6 ± 1.1 9.5 ± 1.6 15.4 ± 1.6 15.6 ± 1.8
837.9 ± 7.5 837.4 ± 7.2 888.1 ± 7.2 828.8 ± 7.3 808.6 ± 7.3 880.7 ± 7.3
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Dea H m ðORÞ is the electron affinity of the OR radical. The electron affinity can be defined as Dea H m ðORÞ ¼ Eea ðORÞ þ 6.197 kJ mol1 , where Eea is the electron affinity of the OR species given in [16]. The addition of the 6.197 kJ Æ mol1 factor to Eea (OR) (thermal ion convention) makes the Dea H m ðORÞ values consistent with the Di H m ðMÞ values taken from reference [10]. In table 4, the DlatU[M(OR)2] values obtained in this work for alkaline-earth metal phenoxides are presented as Dlat U ½MðORÞ2 ¼ Df H m ½MðOHÞ2 ; cr þ 2Df H m ðOR; gÞþ Dsub H m ðMÞ þ Di H m ðM2þ Þ 2Dea H m ðORÞ 3RT . ð4Þ Lattice energy values were analysed using the Kapustinskii approximation represented by equation (5) [15], which gives a reasonable estimation how the electrostatic term varies with ionic size Dlat U 0 ½MðORÞ2 ¼ 1.079 105 ðmZ þ Z Þ=ðrþ þ r Þ.
ð5Þ
Dlat U 0 ½MðORÞ2 1
In this expression, is the lattice energy at T = 0 K in kJ Æ mol , m represents the number of ions per formula unit, and the charge numbers of the cation and the anion and r+ and r the corresponding ion radii in pm. r+ and r calculated through this procedure are called thermochemical radii. Their main importance lies in their capacity to reproduce the approximate val-
TABLE 4 Enthalpies of formation and electron affinities [15] for phenoxide radicals and lattice energies for the substituted phenoxidesa Phenoxide
Df H m ðRO; gÞ
Dea H m ðORÞ
DlatU[M(OR)2]
MORa
kJ Æ mol1b
kJ Æmol1c
kJ Æ mol1
Mg(OPh)2 Ca(OPh)2 Ca(2,6-Me2 OPh)2 Sr(OPh)2 Ba(OPh)2 Ba(2,6-Me2 OPh)2
56.9 ± 2.4 56.9 ± 2.4 22.8 ± 4.8 56.9 ± 2.4 56.9 ± 2.4 22.8 ± 4.8
228.5 ± 9.1 228.5 ± 9.1 208.8 ± 19.3 228.5 ± 9.1 228.5 ± 9.1 208.8 ± 19.3
2838.3 ± 12.0 2415.1 ± 11.9 2345.8 ± 21.2 2271.2 ± 11.9 2120.8 ± 12.0 2072.9 ± 21.2
Dsub H m ðMgÞ ¼ 147.7 0.8 kJ mol1 [10]; Dsub H m ðCaÞ ¼ 78.2 0.8 kJ mol1 [10]; Dsub H m ðSrÞ ¼ 164.4 0.8 kJ mol1 [10]; Dsub H m ðBaÞ ¼ 180.7 0.8 kJ mol1 [10]; Di H m ðMg2þ Þ ¼ 2200.80 kJ mol1 [10], Di H m ðCa2þ Þ ¼ 1747.70 kJ mol1 [10], Di H m ðSr2þ Þ ¼ 1626.14 kJ mol1 [10], Di H m ðBa2þ Þ ¼ 1480.38 kJ mol1 [10] . b Df H m ðRO;gÞ ¼ DðRO HÞ þ Df H m ðROH;gÞ Df H m ðHÞ;DðPhO HÞ ¼ 371.3 2.3 kJ mol1 [17], D(2,6-Me2PhO-H) = 357.3 ± 4.6 kJ Æ mol1 [17], Df H m ðHÞ¼217.9980.006kJmol1 [17], Df H m ðPhOH;gÞ¼ 56.9 2.4 kJ mol1 [17], Df H m ð2; 6-Me2 PhOH; gÞ ¼ 162.1 1.2 kJ mol1 [11]. c Eea(PhO) = 2.304 ± 0.094 eV [18] and Eea(2,6-Me2PhO) = 2.09 ± 0.20 eV. (From the previous value for phenoxide and Eea (2-MePhO) = 2.26 ± 0.11 eV taken also from [18] it was possible to estimate the electron affinity of 2,6-Me2PhO; taken as the Eea of 2-MePhO minus twice the difference between the Eea (PhO) and the Eea(2-MePhO) values, expecting that the two methyl groups stabilize the charge in the same extension.). a
301
ues of lattice energies when introduced in equation (5). It was assumed that for a good approximation, Dlat U 0 ½MðORÞ2 ¼ Dlat U 298 ½MðORÞ2 [16] and so, from data presented in table 4, it was possible to derive the inter-atomic distances M–OR, (r+ + r). The thermochemical radii of the alkaline-earth metal phenoxides cations (r+) were taken from Shannon [19]. The results obtained are presented in table 5. Concerning the estimation of lattice energies, Dlat U 298 , of so far unmeasured phenoxides and from it the standard molar enthalpies of formation, the following procedure was applied. The experimental lattice energies were compared with estimated lattice energies taking as first approximation the average experimental r values taken from table 5 and r+ values taken from Shannon [19]. Using the solver toll of MS Excel 7.0, the differences between them were minimized by least square regression taken as variables the values for r and r+. The generated thermochemical radii were used to estimate a new set of data and by scheme 3, it was possible to calculate the enthalpies of formation of the unmeasured alkaline-earth metal phenoxides as well as to recalculate the enthalpies of formation of measured phenoxides with optimized r+ and r values. Table 6 shows the comparison between experimental and estimated lattice energies and the standard molar enthalpies of formation for the compounds under study. The new set of r+ and r values is consistent and reproduces the experimental Dlat U 298 ½MðORÞ2 data with a maximum relative deviation of 0.02%. These values were subsequently used to estimate the enthalpies of formation of Mg(2,6-Me2OPh)2 and Sr(2,6-Me2OPh)2 compounds using equation (4). The values observed for thermochemical radii of the anions OPh and 2,6-Me2OPh in the alkaline-earth metal phenoxides studied in this work were compared with the values obtained for alkaline-metal phenoxides (see figure 2) [20]. The values of thermochemical radii, OPh, in alkaline-metal phenoxides were recalculations of data using auxiliary values [21]. Values of thermochemical radii, 2,6-Me2OPh for alkaline-metal phenoxides were determined in a previous work [20]. As can be seen in figure 2, the difference between the values for the two anions, OPh and 2,6-Me2OPh, is not significant for alkaline phenoxides but becomes significant for alkaline earth phenoxides.
TABLE 5 Experimental thermochemical radii in picometres obtained for the phenoxide ions in the studied alkaline metal phenoxidesa OR
Mg(OR)2
Ca(OR)2
SrOR
BaOR
OPh 2,6-Me2OPh
142.1 ± 1.4
154.1 ± 1.7 162.0 ± 2.7
153.0 ± 1.8
156.3 ± 2.0 163.3 ± 3.3
a
r+ values used in Kapustinskii equation for calculations: r+(Mg2+) = 86 pm; r+(Ca2+) = 114 pm; r+(Sr2+) = 132 pm; r+(Ba2+) = 149 pm; [19].
302
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TABLE 6 Lattice energies and enthalpies of formation of alkaline metal phenoxides Phenoxide
a
Mg(OPh)2 Mg(2,6-Me2OPh)2 Ca(OPh)2 Ca(2,6-Me2OPh)2 Sr(OPh)2 Sr(2,6-Me2OPh)2 Ba(OPh)2 Ba(2,6-Me2OPh)2
Df H m ðMORÞ=ðkJ mol1 Þ
DlatU(MOR)/(kJ Æ mol1) Experimental
Estimated
Experimentalc
Estimatedd
2838.0 ± 12.0
2838.3 2746.9 2413.8 2347.3 2271.2 2212.3 2122.6 2071.0
837.9 ± 7.5
840.4 869.0 838.0 892.0 831.3 892.3 812.9 881.3
2415.1 ± 11.9 2345.8 ± 21.2 2271.2 ± 11.9 2120.8 ± 12.0 2072.9 ± 21.2
b
837.4 ± 7.2 888.1 ± 7.2 828.8 ± 7.3 808.6 ± 7.3 880.7 ± 7.3
a
Experimental values calculated from equation (4). Estimated values using optimized r+ and r values: r+(Mg2+) = 75.8 pm; r+(Ca2+) = 115.9 pm; r+(Sr2+) = 132.8 pm; r+(Ba2+) = 152.7 pm; r(OPh) = 152.3 pm; r(O-2,6-Me2Ph) = 159.9 pm. c Experimental values calculated from equations (1) and (2). d Estimated values obtained using scheme 3 and estimated lattice energy values. b
Although for alkaline-metal phenoxides the substitution of phenol by 2,6-dimethylphenol had no influence on the values obtained for the thermochemical radii of the anions, this was not observed for alkaline-earth metal phenoxides. In the case of [Ca(OPh)2], [Ca(O-2,6Me2Ph)2] and [Ba(OPh)2], [Ba(O-2,6-Me2Ph)2], the values obtained suggest that the structures of [Ca(O2,6-Me2Ph)2], [Ba(O-2,6-Me2Ph)2] probably were under the steric influence of the methyl groups at positions 2 and 6 because the thermochemical radii of the anions, r(2,6-Me2OPh) = (162.0 ± 2.7) pm for calcium and (163.3 ± 3.3) pm for barium, were substantially larger compared with r(OPh) = (154.1 ± 1.7) and (156.3 ± 2.0) pm for the same metals. The interpretation that steric effects start to be relevant in 2,6-methyl phenoxide alkaline earth metals and
consequently influence the structures of this family of compounds appears clear to us in the base of the structure–energetic relationship proposed in the Kapustinskii equation. As stated before, the principal significance of ‘‘thermochemical radii’’ lies in their capacity to reproduce the lattice energies. In this study, the dCa–O interatomic distance of calcium phenoxide, [Ca(OPh)2], was also measured by X-ray absorption spectroscopy (EXAFS: see Section 2) which allowed us to rescale the (r+ + r) values obtained by Kapustinskii equation for these compounds (see table 7). Figure 3 shows the primary EXAFS oscillations and figure 4 the Fourier transform magnitude of [Ca(OPh)2]. The calcium–oxygen distance obtained from the fit results for the first shell is 0.233 nm, corresponding to the first peak in the Fourier
FIGURE 2. Bar chart to show the thermochemical radii for nine metal alkali and alkaline-earth phenoxides to show the comparison between the OPh (dotted bars) and 2,6-Me2OPh (striped bars).
χ(k)·k →
C. Hipo´lito et al. / J. Chem. Thermodynamics 38 (2006) 296–303
303
6
TABLE 8 Summary of Ca K-edge EXAFS results of [Ca(OPh)2]
4
Shell
R/nm
N
r2 Æ 103/nm2
2
Ca + O Ca + Ca
0.233 0.393
5.9 1.2
0.151 0.083
3
The second shell (Ca + Ca) is merely an assumption. 0
transform magnitude. The fit results for two shells are shown in table 8. The estimated distance obtained, dSr–O = 258±(see table 7) is in agreement with the experimental distances for Sr4(OPh)8(PhOH)2(THF)6 structure [4].
-2
-4
-6
40
20
0
k/
80
60
nm-1
100
Acknowledgements
→
FIGURE 3. Plot showing the primary spectra data from Ca K-edge EXAFS for [Ca(OPh)2]. Solid lines: experimental data, dotted lines: fitted data.
C.H. thanks FCT for a PhD Grant (SRFH/BD3080/ 2000). This work was partially funded by a GRICES/ DAAD protocol grant. References
TABLE 7 Thermochemical radii of alkaline-earth phenoxides, estimateda and experimental interatomic distances dM–O in pm Compound
(r+ + r)/pm
dM–O/pm
Mg(OPh)2 Ca(OPh)2 Ca(2,6-Me2OPh)2 Sr(OPh)2 Ba(OPh)2 Ba(2,6-Me2OPh)2
228.1 ± 1.0 268.1 ± 1.3 276.0 ± 2.5 285.0 ± 1.5 305.0 ± 1.7 312.3 ± 3.2
(174) 233 (245) (258) (288) (298)
a Estimated values in parenthesis obtained from equation: dM–O = 1.4777(r+ + r) 163.15. The slope was taken from reference [21]; it was obtained by plotting d M-OC6 H5 against (r+ + r) data from alkaline phenoxide compounds.
5
3
3
|FT(χ(k)·k )| →
4
2
1 0 0
10
20
30
40
50
60
R / nm-1 → FIGURE 4. Plot showing the Fourier transform spectra from Ca Kedge EXAFS for [Ca(OPh)2]. Solid lines: experimental data, dotted lines: fitted data.
[1] K.G. Caulton, M.H. Chisholm, S.R. Drake, K. Folting, J.C. Huffman, Inorg. Chem. 32 (1993) 816–820. [2] S.R. Drake, D.J. Otway, Polyhedron II-16 (1992) 1995–2007. [3] T. Barreira, J.P. Leal, Eur. J. Inorg. Chem. (2000) 987–991. [4] S.R. Drake, W.E. Streib, M.H. Chisholm, K.G. Caulton, Inorg. Chem. 29 (1990) 2707–2708. [5] C.J. Burns, R.A. Andersen, J. Organomet. Chem. 325 (1987) 31–37. [6] P.J. Ellis, H.C. Freeman, J. Synchrotron Rad. 2 (1995) 190– 195. [7] E.A. Stern, M. Newville, B. Ravel, Y. Yacoby, D. Haskel, Physica B 208–209 (1995) 117–120. [8] J.P. Leal, A. Pires de Matos, J.A. Martinho Simo˜es, J. Organomet. Chem. 403 (1991) 1–10. [9] IUPAC Commission on Atomic Weights and Isotopic Abundances (http://www.chem.qmw.ac.uk/iupac/AtWt/). [10] D.D. Wagman, W.H. Evans, V.B. Parker, R.H. Schumm, I. Halow, S.M. Bailey, K.L. Churney, R.L. Nutall, J. Phys. Chem. Ref. Data 11 (Suppl. 2) (1982). [11] J.B. Pedley, R.D. Naylor, S.P. Kirby, Thermochemical Data of Organic Compounds, second ed., Chapman & Hall, New York, 1986. [12] H. Thoms, M. Epple, H. Viebrock, A. Reller, J. Mater. Chem. 5 (1995) 589–594. [13] E.D. Glendening, D. Feller, J. Am. Chem. Soc. 118 (1996) 6052– 6059. [14] V-C. Arunasalam, I. Baxter, S.R. Drake, M.B. Hursthouse, K.M.A. Malik, D.J. Otway, Inorg. Chem. 34 (1995) 5295–5306. [15] D.A. Johnson, Some Thermodynamic Aspects of Inorganic Chemistry, second ed., Cambridge University Press, Cambridge, 1982. [16] S.G. Lias, J.E. Bartmess, J.F. Liebman, J.L. Holmes, R.D. Levin, W.G. Mallard, J. Phys. Ref. Data 17 (Suppl. 1) (1988). [17] R.M. Borges dos Santos, J.A. Martinho Simo˜es, J. Phys. Chem. Ref. Data 27 (1998) 707–739. [18] NIST Chemistry webbook (http://webbook.nist.gov). [19] R.D. Shannon, Acta Cryst. A 32 (1976) 751–767. [20] C. Hipo´lito, J.P. Leal, Thermochim. Acta 429 (2005) 125–131. [21] P. Nunes, J.P. Leal, V. Cachata, H. Raminhos, M.E. Minas da Piedade, Chemistry – A European Journal 9 (2003) 2095–2101.
JCT 05-94
Thermochemistry of alkaline-earth phenoxides Carla Hipo´lito a, Joa˜o Paulo Leal
a,b,*
, Yanzhi Guo c, Matthias Epple
c
a
b
Departamento de Quı´mica, Instituto Tecnolo´gico e Nuclear 2686-953 Sacave´m, Portugal Departamento de Quı´mica e Bioquı´mica, Faculdade de Cieˆncias, Universidade de Lisboa 1749-016 Lisboa, Portugal c Institute of Inorganic Chemistry, University of Duisburg-Essen, D-45117 Essen, Germany Received 14 April 2005; received in revised form 20 May 2005; accepted 25 May 2005 Available online 22 July 2005
Abstract The standard molar enthalpies of formation of some alkaline-earth metal phenoxides were determined by reaction–solution calorimetry. The results obtained at T = 298.15 K were as follows: Df H m ½MðORÞ2 ; cr=kJ mol1 ¼ ð837.9 7.5Þ ½MgðOPhÞ2 ; ð837.4 7.2Þ ½CaðOPhÞ2 ; ð888.1 7.2Þ ½CaðO-2; 6-Me2 PhÞ2 ; ð828.8 7.3Þ ½SrðOPhÞ2 ; ð808.6 7.3Þ ½BaðOPhÞ2 ; ð880.7 7.3Þ ½BaðO-2; 6-Me2 PhÞ2 . Together with an appropriate Born–Haber cycle, these results allow the calculation of lattice energies for the [M(OR)2] compounds. The thermochemical radii of the anions OR were obtained using the Kapustinskii equation and the lattice energies previously determined. A set of lattice energies and standard molar enthalpies of formation for alkaline-earth metal phenoxides [M(OR)2, M = Mg, Ca, Sr, Ba; R = OPh, 2,6-Me2OPh], was presented. Estimates for these properties for unmeasured compounds were made based on a model first applied to aliphatic alkaline metal alkoxides. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Thermochemistry; Enthalpy of formation; Phenoxides; Lattice energy; Thermochemical radii
1. Introduction The interest in the formation and chemistry of alkaline-earth metal compounds has undergone resurgence over the last decade. The discovery of superconducting ceramics containing alkaline-earth ions and the improvements in the respective preparative technique by the use of alkoxide precursors soluble in hydrocarbons were the main reasons for this resurgent interest [1]. Nowadays the chemistry of alkaline-earth metals is no more an undeveloped area of the periodic table [2]. Despite that, thermochemical data for these substances are almost absent from literature. Two values of standard enthalpies of formation of Mg(OMe)2, Mg(OEt)2 and three values for Ca(OMe)2, Ca(OEt)2 and Ca(OBu)2 are the exceptions [3]. Structural studies of alkaline*
Corresponding author. Fax: +351 1 9941455. E-mail addresses: [email protected], [email protected] (J.P. Leal), [email protected] (M. Epple). 0021-9614/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2005.05.016
earth phenoxides are scarce. X-ray diffraction information exists for strontium phenoxide exhibiting an approximate octahedral structure with a metal co-ordination number equal to six [4].
2. Experimental 2.1. General procedures All manipulations were carried out under an oxygen and water free (<5 Æ 106) nitrogen atmosphere inside a glove-box or using standard Schlenk techniques. The THF and toluene were pre-dried over 4A molecular sieves and distilled under sodium. Pentane was pre-dried under calcium sulfate and distilled over P2O5. Phenol (Aldrich, 99%) and 2,6-dimethylphenol (Aldrich, 99%) were sublimed twice. Ethanol (Merck, 99.8%) was pre-dried over calcium sulfate, heated under reflux over activated magnesium and iodine, and finally distilled.
C. Hipo´lito et al. / J. Chem. Thermodynamics 38 (2006) 296–303
Calcium hydroxide (May and Baker, Lda, 99%) was used as supplied. Magnesium (turnings, Sigma), calcium (granules, Sigma) and strontium (Aldrich, 99%) were used as supplied and stored in an inert atmosphere glove-box. Barium (Fluka, 99%) was used as small pieces to which the oxidized surface was removed inside a glove-box. All solvents were degassed twice by freeze– thaw cycles before use. The IR spectra were recorded on a Bruker Tensor 27 spectrophotometer as Nujol mulls between CsI plates. The listing of bands observed at 3000 and 1400 cm1 was omitted due to overlapping with the C–H stretching frequencies of Nujol. Elemental analysis (C and H) was performed on a CE Instruments EA1110 CHNS–O automatic analyzer (C and H) or by titration (M). 2.2. Phenoxides synthesis Two of the alkaline-earth phenoxides, Mg(OPh)2, Ca(OPh)2, were synthesized in a mixed ammonia/THF solution; the others were prepared in mixed ammonia/ toluene solution, all based in previous work of Drake et al. [2] 2.2.1. Synthesis of Mg(OPh)2 Magnesium metal granules (0.12 g, 4.8 mmol), preweighed in a glove-box, were placed into a 500 cm3 flask along with phenol (0.93 mg, 9.9 mmol) and THF (100 cm3). Outside the glove-box using Schlenk techniques, ammonia gas (pre-dried under sodium) was bubbled through the pre-cooled (T = 203.15 K) colorless solution. After an induction period of 10 min, the metal granules started to divide into fine particles, the solution became white and the volume increased to 150 cm3 due to ammonia condensation. The reaction was completed after 45 min at a temperature of 203.15 K with the complete dissolution of magnesium metal. The cold bath was taken out and the ammonia gas let to evaporate during the night. The solution was then refluxed during 24 h using the ‘‘toluene reflux’’ method [5]. The solvent was removed and the white solid obtained was washed with pentane and dried in high vacuum (102 to 103) Pa. Yield: 0.45 g = 45%. Found: C, 0.686; H, 0.046. Calc. for C12H10O2Mg: C, 0.685; H, 0.048. i.r. [Nujol, m/(cm1)]: 1594 (vs), 1540 (w), 1494 (s), 1460 (s), 1377 (s), 1259 (vs), 1172, (w), 1154 (w), 1072 (w), 1025 (w), 999 (vw), 880 (s), 847 (vs), 847 (vs), 820 (w), 759 (s), 687 (s), 669 (vw), 595 (vs), 576 (vs), 507 (vs), 418 (vw), 487 (w), 326 (w). 2.2.2. Synthesis of Ca(OPh)2 Calcium metal granules (0.18 g, 4.5 mmol), weighed in a glove-box, were placed into a 500 cm3 flask along with phenol (0.90 mg, 9.6 mmol) and THF (100 cm3). The reaction conditions were as described above for Mg(OPh)2.
297
Yield: 0.80 g = 80%. Found: C, 0.637; H, 0.045. Calc. for C12H10O2Ca: C, 0.636; H, 0.048. i.r. [Nujol, m/(cm1)]: 1594 (vs), 1484 (vs), 1460 (s), 1377 (w), 1279 (vs), 1170 (w), 1072 (w), 1025 (w), 995(w), 878 (w), 819 (s), 760 (s), 695 (s), 557 (w), 524 (vw), 487 (w), 326 (w). 2.2.3. Synthesis of Ca(O-2,6-Me2Ph)2 Calcium metal granules (0.14 g, 3.5 mmol), weighed in a glove-box, were placed into a 500 cm3 flask along with phenol (0.90 mg, 7.4 mmol) and toluene (100 cm3). The reaction conditions were as described above for Mg(OPh)2. Yield: 0.81 g = 81%. Found: C, 0.681; H, 0.064. Calc. for C16H18O2Ca: C, 0.688; H, 0.065. i.r. [Nujol, m/(cm1)]: 1591 (w), 1462 (vs), 1377 (s), 1264 (s), 1226(s), 1195 (s), 1089 (w), 1018 (w), 912 (vw), 844 (w), 800 (w), 755 (s), 684 (w), 670 (w), 562 (w), 517 (w), 452 (vw), 388 (w), 305 (w). 2.2.4. Synthesis of Sr(OPh)2 Strontium metal granules (0.48 g, 5.5 mmol), weighed in a glove-box, were placed into a 500 cm3 flask along with phenol (0.11 mg, 11 mmol) and toluene (100 cm3). The reaction conditions were as described above for Mg(OPh)2. Yield: 1.17 g = 78%. Found: C, 0.526; H, 0.037. Calc. for C12H10O2Sr: C, 0.523; H, 0.042. i.r. [Nujol, m/(cm1)]: 1594 (vs), 1482 (vs), 1464 (s), 1377 (w), 1261 (vs), 1168, (w), 1071 (w), 1020 (w), 993 (w), 876 (w), 818 (s), 759 (s), 695 (s), 551 (w), 523 (vw), 472 (w). 2.2.5. Synthesis of Ba(OPh)2 A piece of barium (0.52 g, 3.8 mmol), weighed in a glove-box, was placed into a 500 cm3 flask along with phenol (0.75 g, 8.0 mmol) and toluene (100 cm3). The reaction conditions were as described above for Mg(OPh)2. Yield: 1.10 g = 89%. Found: C, 0.445; H, 0.034. Calc. for C12H10O2Ba: C, 0.445; H, 0.031. i.r. [Nujol, m/(cm1)]: 1590 (s), 1477 (vs), 1377 (vs), 1261 (s), 1164 (s), 1069 (w), 990 (s), 875 (s), 820 (w), 760 (s), 696 (s), 547 (w), 526 (w), 464 (vw), 397 (vw), 354 (vw) 326 (vw), 302 (vw), 266 (vw). 2.2.6. Synthesis of Ba(O-2,6-Me2Ph)2 A piece of barium (0.48 g, 3.5 mmol), weighed in a glove-box, was placed into a 500 cm3 flask along with 2,6-dimethylphenol (1.0 g, 8.4 mmol) and toluene (100 cm3). The reaction conditions were as described above for Ca(2,6-Me2OPh)2. Yield: 1.15 g = 87%. Found: C, 0.514; H, 0.041. Calc. for C16H18O2Ba: C, 0.506; H, 0.048. i.r. [Nujol, m/(cm1)]: 1458 (vs), 1420 (s), 1376 (s), 1266 (s), 1233 (w), 1091 (s), 1038 (s), 842 (s), 799 (s), 752 (s), 682 (w), 504 (w), 419 (w).
298
C. Hipo´lito et al. / J. Chem. Thermodynamics 38 (2006) 296–303
2.3. EXAFS spectra Extended X-ray absorption fine structure (EXAFS) spectra were measured at beamline E4 at the Hamburger Synchrotronstrahlungslabor (HASYLAB) at Deutsches Elektronen Synchrotron (DESY), Hamburg, Germany. The samples were prepared as a thin layer of powder on adhesive tape using Schlenk techniques, i.e. avoiding the access of air or moisture as far as possible. Experiments were performed at the Ca K-edge (ca. 4038 eV) in transmission mode at liquid nitrogen temperature under high vacuum. The programs SPLINE and XFIT were used for quantitative data evaluation. Theoretical standards were computed with the program FEFF 6.01a [6,7]. EXAFS fit parameters were: k-range 14 to 76 nm1, R-range 0.1 to 0.45 nm, Eo = +8.8 eV, S 2o ¼ 1 (fixed). Variation parameters were N, r2, R (for each shell) and Eo. 2.4. Reaction–solution calorimetry The enthalpies of formation of the prepared phenoxides were determined by measuring the enthalpies of reaction and solution in water, except for the magnesium phenoxide that was measured in 0.1 M HCl aqueous solution due to the low solubility of this compound in water. The calorimeter used was specifically built for the study of oxygen- and water-sensitive compounds, and the experimental procedure was described in a previous paper [8]. The accuracy of the calorimeter was determined by frequent measurement of the hydrolysis
of TRIS in a 0.1 M HCl aqueous solution. Briefly, the calorimeter reaction vessel consisted of a 220 cm3 Dewar flask with two acrylic flanges closed by a lid, which supported a stirred, a quartz crystal thermometer probe, a resistance for electrical calibration, and an ampoulebreaking system. The reaction vessel was immersed in a thermostatic water bath T = (298 ± 103) K controlled by a Tronac PTC-40 unit. In a typical experiment, 12 mm diameter glass ampoules were loaded with (20 to 30) mg of the phenoxide compound inside a glove box, sealed under vacuum and weighed to ±105 g with a Mettler H54AR balance. A mass correction for vacuum was always done. The reaction was started by breaking the glass ampoule in 170 cm3 of the appropriate reaction mixture. This was preceded by an electrical calibration in which a potential difference of ca. 2.5 V was applied to a 48 X resistance during ca. 180 s. The results were averaged using at least four runs. The errors presented are twice the standard deviation of the mean in each case.
3. Results and discussion 3.1. Infrared spectra The alkaline-earth metal phenoxides were spectroscopically studied as Nujol suspensions between CsI windows. Figure 1 presents three of the obtained spectra as an example. The vibrations related to metal–oxygen bond occur at 507 and 551 cm1, 487 and 524 cm1,
FIGURE 1. Typical spectra obtained for: (a) Mg(OPh)2, (b) Ca(OPh)2, and (c) Ba(OPh)2.
C. Hipo´lito et al. / J. Chem. Thermodynamics 38 (2006) 296–303
299
o
Mg(OPh)2 (cr) + 2[HCl.552H2O] ∆ r H (1)
∆ r H m (1)
MgCl2 (cr) + 2PhOH (l) + 1104 H2O (l)
∆ sln H(2)
2 ∆ sln H(1)
2∆ sln H(3) 1104 ∆ sln H(4)
MgCl2 (sln) + 2 PhOH (sln) + n (HCl .552H2O)
SCHEME 1.
472 and 523 cm1, 464 and 526 cm1 for Mg(OPh)2, Ca(OPh)2, Sr(OPh)2 and Ba(OPh)2, respectively. 3.2. Calorimetric measurements The standard molar enthalpies of formation, Df H m ½MðORÞ2 ; cr, of alkaline-earth metals phenoxides, [Mg(OPh)2], [Ca(OPh)2], [Ca(O-2,6-Me2Ph)2], [Sr(OPh)2], [Ba(OPh)2], [Ba(O-2,6-Me2Ph)2], were computed according to scheme 1 or scheme 2. The validity of the schemes was checked by dissolving a ‘‘synthetic’’ final solution in a ‘‘real’’ final solution and verifying that it was athermic. Auxiliary data used to derive these quantities from the true experimental ones – the enthalpies of reaction – are presented in table 1. The molar o
M(OR)2 (cr) + 2H2O (l) ∆ r H (2)
∆r Hm(2)
M(OH)2 (cr) + 2ROH (l) ∆ sln H(6)
2 ∆ sln H(5)
2 ∆ sln H(7)
M(OH)2 + ROH (aq) ; R=OPh or 2,6-Me2OPh SCHEME 2.
TABLE 1 Auxillary thermochemical data at T = 298.15 K Compound
Df H m
References 1
kJ Æ mol MgCl2, cr Mg, g Ca(OH)2, cr Ca(OH)2 1 H2O Ca, g Sr(OH)2, cr Sr(OH)2 Æ 800H2O Sr, g Ba(OH)2, cr Ba(OH)2 Æ 500H2O Ba, g PhOH,cr 2,6-Me2OPh, cr H2O, l HCl Æ 552H2O
483.93 ± 0.08 147.7 ± 0.8 986.09 ± 0.08 1002.82 ± 0.08 178.2 ± 0.8 959.0 ± 0.8 1004.6 ± 0.8 164.4 ± 0.8 944.7 ± 0.8 995.4 ± 0.8 180 ± 0.8 165.1 ± 0.7 237 ± 1 285.830 ± 0.040 166.596 ± 0.008
[10] [10] [10] [10] [10] [10] [10] [10] [10] [10] [10] [11] [11] [10] [10]
quantities are based on the 2001 standard atomic masses values [9]. The Dr H m ð1Þ symbolizes the enthalpy change for the reaction of magnesium phenoxide, [Mg(OPh)2], with 0.1 M HCl and Dr H m ð2Þ the enthalpy change for the reaction of M(OR)2 (M = Ca and Ba, R = OPh, 2,6Me2OPh; M = Sr, R = OPh;) with distilled and deionized water and all compounds in their standard state. The DrH(1) and DrH(2) stand for the measured reaction enthalpies under the experimental conditions (see Section 2). The DslnH(1) is the term for the dissolution enthalpy of a 0.1 M HCl solution in the same 0.1 M HCl solution; DslnH(2) is the dissolution enthalpy of stoichiometric amounts of MgCl2 in 0.1 M HCl; DslnH(3) is the dissolution enthalpy of phenol in (0.1 M HCl + MgCl2) solution; DslnH(4) represents the dissolution of water in 0.1 M HCl solution; DslnH(5) is the dissolution enthalpy of water in water; DslnH(6) is the dissolution enthalpy for stoichiometric amounts of M(OH)2 in H2O and DslnH(7) is the dissolution enthalpy of the alcohol in {H2O + M(OH)2}. The DslnH(2) for magnesium chloride was taken as (161 ± 2) kJ Æ mol1. Although this was not measured experimentally, the value proposed is an ‘‘educated guess’’ based on a previous experimental value of the dissolution enthalpy of stoichiometric amounts of CaCl2 in 0.1 M HCl, [3], and by comparison of differences between the enthalpy of formation of aqueous 0.1 M HCl and crystalline magnesium and calcium chloride (table 1). The DslnH(3) was taken as (12.4 ± 1.2) kJ Æ mol1, an experimental value obtained in an earlier study for dissolution enthalpy of phenol in 0.1 M HCl [8]. The DslnH(4), the dissolution of water in the solvent, is insignificant (smaller than the detection limit of the apparatus) and even when multiplied by 1104 (see scheme 1) yields a minor contribution to the final Dr H m ð1Þ value, i.e. smaller than the experimental error, and therefore was not considered. The DslnH(5) and DslnH(1) were obviously zero. The concentrations of alkaline-earth metals hydroxides, M(OH)2 with (M = Ca, Sr, Ba) in the final calorimetric solutions were always very small (typical molar ratios of nMðOHÞ2 : nH2 O 1:107 or less). In these conditions of very dilute solutions, an approximation can be made without significant error and DslnH(6)
C. Hipo´lito et al. / J. Chem. Thermodynamics 38 (2006) 296–303
300
can be calculated to yield: (24.51 ± 0.08) kJ Æ mol1 for Ca(OH)2, (44.50 ± 0.08) kJ Æ mol1 for Sr(OH)2, (57.61 ± 0.08) kJ Æ mol1 for Ba(OH)2, all based on the formation enthalpies of crystalline hydroxides and the corresponding enthalpies of formation for very dilute solutions using the data reported in the literature [10]. The DslnH(7) was experimentally determined (see table 2). From schemes 1 and 2, it was possible to calculate the enthalpies of formation of the alkaline-earth metal phenoxides using equations (1) and (2), respectively. The obtained values are presented in table 3. Df H m ½MgðOPhÞ2 ; cr ¼ Df H m ðMgCl2 ; crÞ þ 2Df H m ðPhOH; lÞ 2Df H m ðHCl 552H2 OÞ Dr H ð1Þ þ Dsln H ð2Þ þ 2Dsln H ð3Þ; ð1Þ
Df H m ½MðORÞ2 ; cr ¼ Df H m ½MðOHÞ2 ; cr þ 2Df H m ðROH; lÞ 2Df H m ½H2 O; l Dr H ð2Þ þ Dsln H ð6Þ þ 2Dsln H ð7Þ. ð2Þ Assuming that the bonds in an alkaline-earth metal phenoxide compound are essentially ionic (many metal–organic compounds are not ionic crystals but their bonds still essentially ionic in nature) [12–14], it was possible to define the corresponding lattice energy,
DlatU[M(OR)2], as the internal energy change associated with the following process: MðORÞ2 ðcrÞ ! M2þ ðgÞ þ 2RO ðgÞ ðM ¼ alkaline earth metal; R ¼ phenyl groupÞ: ð3Þ Kapustinskii noticed that, in the formula for the lattice energy in an ionic model, by dividing the Madelung constants by the number of ions in the molecule the new constant obtained is almost independent of the structure of the lattice. He also assumes the repulsive part of the energy is 1/9 of the attractive one and split the inter-nuclear equilibrium distance in a sum of two radii (r+ and r). For a detailed discussion, see e.g. reference [15]. To compute this quantity, scheme 3 can be used. Equation (4), obtained directly from scheme 3, allows one to compute DlatU[M(OR)2], the internal energy change associated with the process described in equation (3) at T = 298.15 K, where R is the gas constant, T is the absolute temperature, Dsub H m ðMÞ and Di H m ðM2þ Þ are the enthalpy of sublimation and the first plus the second enthalpy of ionisation of the metal, respectively, and
M2+(g) + 2OR(g) + 2e– ∆ Ηi (M+) +RT o
∆ Ηi (M) + RT o
TABLE 2 Enthalpies of dissolution of phenols in a water + Ca(OH)2 solution at T = 298.15 K Compound
Mass/kg
e
DT/K
J Æ K1 OPh
2,6-Me2OPh
36.14 51.35 35.74 37.89 29.03 14.17 16.37 36.04 44.88
DslnH(7) kJ Æ mol1
868.2372 792.1251 802.6641 793.4747 793.0613 795.1276 756.5981 737.7007 715.8550
0.0018 0.0033 0.0028 0.0032 0.0011 0.0006 0.0012 0.0013 0.0035
4.0261 4.8068 5.9170 6.2340 5.2 ± 0.5 3.8141 4.2736 6.9452 3.2932 6.7424 5.0 ± 1.1
o
-2∆eaHm (OR) -2RT
M(g) + 2RO(g) M2+(g) + 2OR (g)
∆ sub Hmo (M) M(cr) + 2OR(g) 2 ∆ f Hmo (OR, g)
∆ latU298o + 3RT
M(cr) + O2(g) + xC(cr) + yH2(g)
∆ f Hmo(M(OR)2,cr) M(OR)2 (cr)
SCHEME 3.
TABLE 3 Reaction enthalpies and standard enthalpies of formation of alkaline-earth phenoxides and 2,6-dimethyl phenoxides at T = 298.15 K Metal
DrH(1) or DrH(2) 1
Mg(OPh)2 Ca(OPh)2 Ca(2,6-Me2OPh)2 Sr(OPh)2 Ba(OPh)2 Ba(2,6-Me2OPh)2
Dr H m ð1Þ or Dr H m ð2Þ 1
Df H m ½MðORÞ2 ; cr 1
kJ Æ mol
kJ Æmol
kJ Æmol
114.5 ± 0.2 49.9 ± 0.7 71.3 ± 0.3 60.3 ± 0.8 71.3 ± 0.8 71.3 ± 0.4
34.1 ± 2.3 28.0 ± 0.9 49.6 ± 1.1 9.5 ± 1.6 15.4 ± 1.6 15.6 ± 1.8
837.9 ± 7.5 837.4 ± 7.2 888.1 ± 7.2 828.8 ± 7.3 808.6 ± 7.3 880.7 ± 7.3
C. Hipo´lito et al. / J. Chem. Thermodynamics 38 (2006) 296–303
Dea H m ðORÞ is the electron affinity of the OR radical. The electron affinity can be defined as Dea H m ðORÞ ¼ Eea ðORÞ þ 6.197 kJ mol1 , where Eea is the electron affinity of the OR species given in [16]. The addition of the 6.197 kJ Æ mol1 factor to Eea (OR) (thermal ion convention) makes the Dea H m ðORÞ values consistent with the Di H m ðMÞ values taken from reference [10]. In table 4, the DlatU[M(OR)2] values obtained in this work for alkaline-earth metal phenoxides are presented as Dlat U ½MðORÞ2 ¼ Df H m ½MðOHÞ2 ; cr þ 2Df H m ðOR; gÞþ Dsub H m ðMÞ þ Di H m ðM2þ Þ 2Dea H m ðORÞ 3RT . ð4Þ Lattice energy values were analysed using the Kapustinskii approximation represented by equation (5) [15], which gives a reasonable estimation how the electrostatic term varies with ionic size Dlat U 0 ½MðORÞ2 ¼ 1.079 105 ðmZ þ Z Þ=ðrþ þ r Þ.
ð5Þ
Dlat U 0 ½MðORÞ2 1
In this expression, is the lattice energy at T = 0 K in kJ Æ mol , m represents the number of ions per formula unit, and the charge numbers of the cation and the anion and r+ and r the corresponding ion radii in pm. r+ and r calculated through this procedure are called thermochemical radii. Their main importance lies in their capacity to reproduce the approximate val-
TABLE 4 Enthalpies of formation and electron affinities [15] for phenoxide radicals and lattice energies for the substituted phenoxidesa Phenoxide
Df H m ðRO; gÞ
Dea H m ðORÞ
DlatU[M(OR)2]
MORa
kJ Æ mol1b
kJ Æmol1c
kJ Æ mol1
Mg(OPh)2 Ca(OPh)2 Ca(2,6-Me2 OPh)2 Sr(OPh)2 Ba(OPh)2 Ba(2,6-Me2 OPh)2
56.9 ± 2.4 56.9 ± 2.4 22.8 ± 4.8 56.9 ± 2.4 56.9 ± 2.4 22.8 ± 4.8
228.5 ± 9.1 228.5 ± 9.1 208.8 ± 19.3 228.5 ± 9.1 228.5 ± 9.1 208.8 ± 19.3
2838.3 ± 12.0 2415.1 ± 11.9 2345.8 ± 21.2 2271.2 ± 11.9 2120.8 ± 12.0 2072.9 ± 21.2
Dsub H m ðMgÞ ¼ 147.7 0.8 kJ mol1 [10]; Dsub H m ðCaÞ ¼ 78.2 0.8 kJ mol1 [10]; Dsub H m ðSrÞ ¼ 164.4 0.8 kJ mol1 [10]; Dsub H m ðBaÞ ¼ 180.7 0.8 kJ mol1 [10]; Di H m ðMg2þ Þ ¼ 2200.80 kJ mol1 [10], Di H m ðCa2þ Þ ¼ 1747.70 kJ mol1 [10], Di H m ðSr2þ Þ ¼ 1626.14 kJ mol1 [10], Di H m ðBa2þ Þ ¼ 1480.38 kJ mol1 [10] . b Df H m ðRO;gÞ ¼ DðRO HÞ þ Df H m ðROH;gÞ Df H m ðHÞ;DðPhO HÞ ¼ 371.3 2.3 kJ mol1 [17], D(2,6-Me2PhO-H) = 357.3 ± 4.6 kJ Æ mol1 [17], Df H m ðHÞ¼217.9980.006kJmol1 [17], Df H m ðPhOH;gÞ¼ 56.9 2.4 kJ mol1 [17], Df H m ð2; 6-Me2 PhOH; gÞ ¼ 162.1 1.2 kJ mol1 [11]. c Eea(PhO) = 2.304 ± 0.094 eV [18] and Eea(2,6-Me2PhO) = 2.09 ± 0.20 eV. (From the previous value for phenoxide and Eea (2-MePhO) = 2.26 ± 0.11 eV taken also from [18] it was possible to estimate the electron affinity of 2,6-Me2PhO; taken as the Eea of 2-MePhO minus twice the difference between the Eea (PhO) and the Eea(2-MePhO) values, expecting that the two methyl groups stabilize the charge in the same extension.). a
301
ues of lattice energies when introduced in equation (5). It was assumed that for a good approximation, Dlat U 0 ½MðORÞ2 ¼ Dlat U 298 ½MðORÞ2 [16] and so, from data presented in table 4, it was possible to derive the inter-atomic distances M–OR, (r+ + r). The thermochemical radii of the alkaline-earth metal phenoxides cations (r+) were taken from Shannon [19]. The results obtained are presented in table 5. Concerning the estimation of lattice energies, Dlat U 298 , of so far unmeasured phenoxides and from it the standard molar enthalpies of formation, the following procedure was applied. The experimental lattice energies were compared with estimated lattice energies taking as first approximation the average experimental r values taken from table 5 and r+ values taken from Shannon [19]. Using the solver toll of MS Excel 7.0, the differences between them were minimized by least square regression taken as variables the values for r and r+. The generated thermochemical radii were used to estimate a new set of data and by scheme 3, it was possible to calculate the enthalpies of formation of the unmeasured alkaline-earth metal phenoxides as well as to recalculate the enthalpies of formation of measured phenoxides with optimized r+ and r values. Table 6 shows the comparison between experimental and estimated lattice energies and the standard molar enthalpies of formation for the compounds under study. The new set of r+ and r values is consistent and reproduces the experimental Dlat U 298 ½MðORÞ2 data with a maximum relative deviation of 0.02%. These values were subsequently used to estimate the enthalpies of formation of Mg(2,6-Me2OPh)2 and Sr(2,6-Me2OPh)2 compounds using equation (4). The values observed for thermochemical radii of the anions OPh and 2,6-Me2OPh in the alkaline-earth metal phenoxides studied in this work were compared with the values obtained for alkaline-metal phenoxides (see figure 2) [20]. The values of thermochemical radii, OPh, in alkaline-metal phenoxides were recalculations of data using auxiliary values [21]. Values of thermochemical radii, 2,6-Me2OPh for alkaline-metal phenoxides were determined in a previous work [20]. As can be seen in figure 2, the difference between the values for the two anions, OPh and 2,6-Me2OPh, is not significant for alkaline phenoxides but becomes significant for alkaline earth phenoxides.
TABLE 5 Experimental thermochemical radii in picometres obtained for the phenoxide ions in the studied alkaline metal phenoxidesa OR
Mg(OR)2
Ca(OR)2
SrOR
BaOR
OPh 2,6-Me2OPh
142.1 ± 1.4
154.1 ± 1.7 162.0 ± 2.7
153.0 ± 1.8
156.3 ± 2.0 163.3 ± 3.3
a
r+ values used in Kapustinskii equation for calculations: r+(Mg2+) = 86 pm; r+(Ca2+) = 114 pm; r+(Sr2+) = 132 pm; r+(Ba2+) = 149 pm; [19].
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TABLE 6 Lattice energies and enthalpies of formation of alkaline metal phenoxides Phenoxide
a
Mg(OPh)2 Mg(2,6-Me2OPh)2 Ca(OPh)2 Ca(2,6-Me2OPh)2 Sr(OPh)2 Sr(2,6-Me2OPh)2 Ba(OPh)2 Ba(2,6-Me2OPh)2
Df H m ðMORÞ=ðkJ mol1 Þ
DlatU(MOR)/(kJ Æ mol1) Experimental
Estimated
Experimentalc
Estimatedd
2838.0 ± 12.0
2838.3 2746.9 2413.8 2347.3 2271.2 2212.3 2122.6 2071.0
837.9 ± 7.5
840.4 869.0 838.0 892.0 831.3 892.3 812.9 881.3
2415.1 ± 11.9 2345.8 ± 21.2 2271.2 ± 11.9 2120.8 ± 12.0 2072.9 ± 21.2
b
837.4 ± 7.2 888.1 ± 7.2 828.8 ± 7.3 808.6 ± 7.3 880.7 ± 7.3
a
Experimental values calculated from equation (4). Estimated values using optimized r+ and r values: r+(Mg2+) = 75.8 pm; r+(Ca2+) = 115.9 pm; r+(Sr2+) = 132.8 pm; r+(Ba2+) = 152.7 pm; r(OPh) = 152.3 pm; r(O-2,6-Me2Ph) = 159.9 pm. c Experimental values calculated from equations (1) and (2). d Estimated values obtained using scheme 3 and estimated lattice energy values. b
Although for alkaline-metal phenoxides the substitution of phenol by 2,6-dimethylphenol had no influence on the values obtained for the thermochemical radii of the anions, this was not observed for alkaline-earth metal phenoxides. In the case of [Ca(OPh)2], [Ca(O-2,6Me2Ph)2] and [Ba(OPh)2], [Ba(O-2,6-Me2Ph)2], the values obtained suggest that the structures of [Ca(O2,6-Me2Ph)2], [Ba(O-2,6-Me2Ph)2] probably were under the steric influence of the methyl groups at positions 2 and 6 because the thermochemical radii of the anions, r(2,6-Me2OPh) = (162.0 ± 2.7) pm for calcium and (163.3 ± 3.3) pm for barium, were substantially larger compared with r(OPh) = (154.1 ± 1.7) and (156.3 ± 2.0) pm for the same metals. The interpretation that steric effects start to be relevant in 2,6-methyl phenoxide alkaline earth metals and
consequently influence the structures of this family of compounds appears clear to us in the base of the structure–energetic relationship proposed in the Kapustinskii equation. As stated before, the principal significance of ‘‘thermochemical radii’’ lies in their capacity to reproduce the lattice energies. In this study, the dCa–O interatomic distance of calcium phenoxide, [Ca(OPh)2], was also measured by X-ray absorption spectroscopy (EXAFS: see Section 2) which allowed us to rescale the (r+ + r) values obtained by Kapustinskii equation for these compounds (see table 7). Figure 3 shows the primary EXAFS oscillations and figure 4 the Fourier transform magnitude of [Ca(OPh)2]. The calcium–oxygen distance obtained from the fit results for the first shell is 0.233 nm, corresponding to the first peak in the Fourier
FIGURE 2. Bar chart to show the thermochemical radii for nine metal alkali and alkaline-earth phenoxides to show the comparison between the OPh (dotted bars) and 2,6-Me2OPh (striped bars).
χ(k)·k →
C. Hipo´lito et al. / J. Chem. Thermodynamics 38 (2006) 296–303
303
6
TABLE 8 Summary of Ca K-edge EXAFS results of [Ca(OPh)2]
4
Shell
R/nm
N
r2 Æ 103/nm2
2
Ca + O Ca + Ca
0.233 0.393
5.9 1.2
0.151 0.083
3
The second shell (Ca + Ca) is merely an assumption. 0
transform magnitude. The fit results for two shells are shown in table 8. The estimated distance obtained, dSr–O = 258±(see table 7) is in agreement with the experimental distances for Sr4(OPh)8(PhOH)2(THF)6 structure [4].
-2
-4
-6
40
20
0
k/
80
60
nm-1
100
Acknowledgements
→
FIGURE 3. Plot showing the primary spectra data from Ca K-edge EXAFS for [Ca(OPh)2]. Solid lines: experimental data, dotted lines: fitted data.
C.H. thanks FCT for a PhD Grant (SRFH/BD3080/ 2000). This work was partially funded by a GRICES/ DAAD protocol grant. References
TABLE 7 Thermochemical radii of alkaline-earth phenoxides, estimateda and experimental interatomic distances dM–O in pm Compound
(r+ + r)/pm
dM–O/pm
Mg(OPh)2 Ca(OPh)2 Ca(2,6-Me2OPh)2 Sr(OPh)2 Ba(OPh)2 Ba(2,6-Me2OPh)2
228.1 ± 1.0 268.1 ± 1.3 276.0 ± 2.5 285.0 ± 1.5 305.0 ± 1.7 312.3 ± 3.2
(174) 233 (245) (258) (288) (298)
a Estimated values in parenthesis obtained from equation: dM–O = 1.4777(r+ + r) 163.15. The slope was taken from reference [21]; it was obtained by plotting d M-OC6 H5 against (r+ + r) data from alkaline phenoxide compounds.
5
3
3
|FT(χ(k)·k )| →
4
2
1 0 0
10
20
30
40
50
60
R / nm-1 → FIGURE 4. Plot showing the Fourier transform spectra from Ca Kedge EXAFS for [Ca(OPh)2]. Solid lines: experimental data, dotted lines: fitted data.
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JCT 05-94