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Standard enthalpies of formation of tris(β-diketonate)chromium(III) complexes: the mean (CrO) bond-dissociation enthalpies
M-2089 J. Chem.
Thermodynamics
1987, 19, 645-652
Standard enthalpies of formation tris(fi-diketonate)chromium( I I I) complexes: the mean (Cr-0) bond-dissociation enthalpies
of
MANUEL A. V. RIBEIRO DA SILVA and MARIA LUiSA C. C. H. FERRWO Depurtamento de Quimica, Faculdade de CiPncias, Universidade do Porto, 4000 Porto, Portugul (Received 22 October 1986; in final form 14 November 1986) The following standard (p” = 101.325 kPa) molar enthalpies of formation of the crystalline solids were determined, at 298.15 K, by solution-reaction calorimetry, and the enthalpies of sublimation were measured by high-temperature microcalorimetry. From’ the enthalpies of formation of the gaseous complexes, the average molar bond-dissociation enthalpies, (D)(Cr 0) were derived. Cr(III)
complex
ArHC(cr)/(kJ -1185+12 -1977*14 -3486+ -5471*17
Cr(BZAC),
Cr(DPM), CT(TFAC)) CT(HFAC)J
I-phenylbutane-l,3-dionato; trifluoropentane-2,4-dionato:
(BZAC,
‘mol-‘)
A,,&,,/(kJ~
mol-‘)
(D)(Cr-O)/(kJ
186+’ 133*2 117*4 112~4
I7
2.2.6,6-tetramethylheptane-3,5-dionato: l,l.l,S,5.5-hexafluoropentane-2,4-dionato).
DPM. HFAC.
mole ‘)
207kll 209+10 221+11 238111 TFAC.
l,l.l-
1. Introduction The thermochemistry of both p-diketones and metal-gdiketonates has been investigated progressively and extensively over the last two decades. For the chromium(II1) complexes, however, precise thermochemical measurements have been reported only for tris(pentane-2,4-dionato)chromium(III).”’ Here, we report the standard molar enthalpies of formation and of sublimation of four other chromium(II1) P-diketonates to investigate the effect of structural changes in the ligand upon the mean molar (Cr-0) bond-dissociation enthalpy.
2. Experimental 1-Phenylbutane-1,3-dione (HBAC) (Koch-Light Laboratories Ltd.) was purified by repeated crystallization from (ethanol+ water) and the purity checked by microanalysis for C,,H,002, in mass per cent: C. found 73.87, expected 74.06; H. OOZI-9614187:060645+08
$02.00/O
(“ 1987 Academic
Press Inc. (London)
Limited
646
M.
A. V. RIBEIRO
DA
SILVA
AND
M.
L. C. C. H. FERR.60
found 6.26, expected 6.22. 2,2,6,6-Tetramethylheptane-2.4-dione (HDPM) was prepared by Claisen condensation according to the methods of Adams and Hauserc2’ and of Kopecky et ~1.,(~’ was purified by fractional distillation in vacuum. and the purity was checked by g.1.c. using two different columns. l,l,lTrifluoropentane-2,4-dione (HTFAC) (Koch-Light Laboratories Ltd.) was purified by fractional distillation and the purity was checked by g.1.c. using two different columns. 1,l ,1,5,5,5-Hexafluoropentane-2.4-dione (HHFAC) (Koch-Light Laboratories, Ltd.) was dehydrated using the method of Belford et ~1.‘~’ and was purified by fractional distillation. The purity was checked by g.1.c. using two different columns. The purified liquid B-diketones were stored under nitrogen in the dark and were freshly distilled prior to use. Tris(l-phenylbutane-1,3-dionato)chromium(III), Cr(szAc),, was prepared as described by Fernelius and Blanchc5’ and was purified by crystallization from (acetone + hexane). Tris(2,2.6.6-tetramethylheptane-2,4-dionato)chromium(III), Cr(DPM)3, was prepared as described by Hammond et ,1.@’ and was purified by crystallization from light petroleum. Tris( l,l, 1-trifluoropentane-2,4-dionato)chromium(III), Cr(TFAC)3, (R esearch Organic/Inorganic Co.), was purified by crystallization from ethanol. Tris( 1,l .1,.5,5,5-hexafluoropentane-2,4-dionato)chromium(III), Cr(HFAc)3, (Research Organic/Inorganic Co.), was purified by crystallization from tetrachloromethane. The mass-percentage analyses of the complexes were: C Cr(BZAC), Cr(DPM), Cr(TFAC)3 Cr(HFAC),
67.32 65.26 35.31 26.42
H found 5.23 9.50 2.31 0.44
Cr
C
9.78 8.70 10.25 7.83
67.28 65.86 35.24 26.76
H expected 5.08 9.55 2.37 0.45
Cr 9.71 8.64 10.17 7.72
Chromium chloride hexahydrate (Merck, p.a.) was analysed for chromium and shown to be CrCl, .6.OOH,O. 1,4-Dioxan (Carl Erba) and toluene (Merck, p.a.) were purified according to reference 7. Hydrochloric acid (B.D.H., AnalaR) and ethanol (Merck, p.a.) were used. The isoperibol LKB 8700 reaction-and-solution precision calorimeter was used. The operation and calculation methods have been described@*9’ and were used here. The accuracy of the calorimeter was checked by measuring the molar enthalpy of solution of tris(hydroxymethyl)aminomethane (THAM) in aqueous 0.100 mol. dm - 3 HCl at 298.15 K: Aso,Hm = -(29.756&0.024) kJ . mol- ‘, in agreement with the value of Kilday and Prosen: (lo) Aso,Hm = -(29.770f0.032) kJ.mol-‘. The enthalpies of sublimation of the complexes were measured by the “vacuum sublimation” drop-microcalorimetric method. (11) Samples (about 5 mg) of each complex contained in a small thin glass capillary tube sealed at one end, were dropped, at room temperature, into the hot reaction vessel in the Calvet HighTemperature Microcalorimeter held at constant temperature, between 388 and 510 K. and then removed from the hot zone by vacuum sublimation. At these temperatures, the complexes showed no signs of decomposition. and sublimed
A,H~(Cr(III)
j3-diketonates)
647
completely in less than 30 min. The observed standard molar enthalpies of sublimation (H,Z,,(g,T)-HA(cr,298.15 K)} were corrected to 298.15 K using values of {Hi(g,T) - Hk(g,298.15 K)} estimated by group-additivity methods, using data of Stull et al.“” The microcalorimeter was calibrated in situ for these measurements by making use of the known molar enthalpies of sublimation of iodine,‘i3’ naphthalene,‘12’ and benzoic acid.‘14. ’ ” The relative atomic masses used were the recommended by the IUPAC Commission.“” All uncertainty intervals given are twice the standard deviation of the mean. Carbon and hydrogen analyses were carried out by the Microanalytical Service, University of Surrey. Chromium was analysed by the dichromatometric method.“” 3. Results The rates of hydrolyses of the chromium(II1) b-diketonate complexes in aqueous acid were too slow for calorimetric measurement of the enthalpy of reaction; hence other solvents, in which rapid hydrolyses occurred, were used. The thermochemical reaction for determining the standard molar enthalpy of formation of Cr(szAc), and CIfTFAC),
WaS:
CrL,(cr) + 3HCl. 12.21H20(1) = CrCl, ~6.OOH,O(cr) + 30.63H20(1) + 3HtJcr or 1). (1) The initial calorimetric solvent was {0.75( 1,Cdioxan) + 0.25HCl(aq, 4.2 mol. dm- “)} by volume. The same batch of solvent was used for both Cr(szAc), and Cr(TFAc)3. The enthalpies of the thermochemical reactions were determined indirectly, from a series of measurements of the enthalpies of solution of reactants and products in succession in the calorimetric solvent. The difference between the molar enthalpies of solution of the products and reactants, in the correct stoichiometric ratio, gives the enthalpy change A,HA of the thermochemical reaction, provided equilibrium is reached from either side within the period of the experiment. To the calorimetric solvent (100.0 cm3), ampoules containing HCl 12.21 H,O(l) and chromium complex were added consecutively and A,H, were measured. To a second portion of the same solvent (100.0 cm3), ampoules containing water, CrCl, . 6.00H20(cr), and the appropriate ligand, were added consecutively, and the different AiH, were measured. The amounts of reactants in a particular series of experiments were determined by the amount of CrCl, . 6.00H20(cr) (approximately 2.5 x 10e4 mol) in a particular ampoule so that control of the stoichiometry was maintained for each series. With this procedure, the value calculated for A,H; relates to reaction (l), provided that the final solutions resulting from the solution of all reactants and that from all products are identical. To check the validity of this assumption, ampoules of one solution were broken into the other; no enthalpy changes were detected. Table 1 lists the mean values of the molar enthalpies of solution and reaction required for determination of the enthalpies of the thermochemical reactions (1)
648
M. A. V. RIBEIRO TABLE
1.
and
C~(BZAC),
i HCI’
4
H,W
5 6
CrCI,
Solvent
12.21H,O(l)
initial A, A, initial A2 A3 ‘43
Cr(szAc),(cr) Cr(TFAC&(Cr)
.6.OOH,O(cr)
HBZAC(Cr) HTFAc(I)
I
molar
CT(TFAC)~:
Reactant
I 2 3
DA SILVA
AND enthalpies Solution
M. L. C. C. H. FERRAO of reaction
and solution
Wm
Number of experiments
kJ.mol
5 5 5 5 5 5 5
A, B, B2 A2 A3 B, B2
at 29X. I5 K
-21.22kO.07 + 19.11+0.32 +25.70+0.15 -1.025+0.010 -16.53kO.18 + 26.01 k 0.05 -23.08*0.27
used for Cr(szAc), and Cr(rFAc)3. The enthalpies of these thermochemical were derived from the values in table 1 according to A,H~(Cr(szAc),)
= 3Ar H, + A2Hm - 30.63A,H, = -(74.65+0.54) kJ.mol-‘,
A,Hh{Cr(TFAC)3}
= 3A,H,+A3H,-30.63A,H,-ASH,,,-3A7Hm = +(79.21 f0.92) kJ.mol-‘.
and
Cr(DPM)3
Cr(HFAC)3
’
reactions
- A,H, - 3A,H,
were studied using a thermochemical
reaction analogous
to (l), i.e. CrL,(cr) + 3HCl. 10.98H,O(l)
= CrCl, .6.OOH,O(cr)
+ 26.94H,O(l)
+ 3H~(l), (2)
but with an initial calorimetric solvent of composition {0.70toluene+0.30ethanol} by volume. Two different batches of solvent were used for Cr(DPM)3 and Cr(HFAc)J, which made necessary remeasurement of the enthalpies of solution of H,O(l), HCl. 10.98H,O(l), and CrCl, .6.OOH,O(cr). The amounts of reactants in a particular series of experiments were determined by the amount of CrCl, .6.OOH,O(cr) (approximately 2.5 x 10e4 mol) in a particular ampoule. The experimental procedure was analogous to that for the other two chromium complexes. TABLE
2. C~(UPM)~
HCl
enthaipies
Solvent
Solution
10.98H,O(l)
initial C initial C2 C, initial Cd initial C5 G
Cl Dl C* C, D, G D2 C-5 C6 D2
Cr(DPM)3(Cf) H,W HDPM(1)
CrCI, HCI
molar
Reactant
i
8 9 10 11 12 13 14 15 I6 17
and Cr(twAc)a:
.6.OOH,O(cr) 10.98H,O(I)
Cr(HFAc)3(Cr) H,W
CrCI, HHFAC(1)
~6.OOH,O(cr)
of reaction
and solution
at 298.15
K
Number of experiments
kJ,mol-’
5 5 5 5 5 5 5 5 5 5
-r7.11&0.07 +21.24_+0.09 +0.840*0.009 +3.06*0.02 -19.77kO.17 - 16.92kO.17 +45.64+0.66 +0.839&0.009 -20.4510.45 -67.05kO.36
A,HG{Cr(III) TABLE
3. Molar
Number of experiments
Complex
CrmAc)J
3 4 2
Cr(HFAC),
2
C’I’(BZAC), Cr(DPM)3
enthalpies A,,,H,(obs. kJ.mol
T K 507 443 426 388
649
h-diketonatesj of sublimation T)
of metal
complexes
Hm(g,T)-H;(g,298.15
-I
K)
A,\,,&(298.15
328+2 273+2 182k4 164+4
K)
kJ.molF’
kJ.mol-’ 142 140 65 52
186&2 133i2 117*4 112+4
Table 2 lists the mean values of the molar enthalpies of solution and reaction required for determination of the enthalpies of the thermochemical reactions (2) used for Cr(DPrv& and Cr(HFAc)3. The enthalpies of these thermochemical reactions were derived from the values in table 2 according to = 3AsH,+A,H,-26.94A,0H,-3A.,,H,-A,zH, = -(42.13-L-0.38) kJ.mol- ‘,
.&&(Cr(DPM)3}
= 3A,,H,+A,,H,-26.94A,sH,-A,,H,-3A,,H, = +(193.9* 1.5) kJ.mol-‘.
fi,H~(Cr(HFAC)3}
The experimental results for determination of the standard molar enthalpies of sublimation of the complexes are listed in table 3. To derive the standard molar enthalpies of formation of the crystalline chromium(II1) complexes, the following auxiliary data, at 298.15 K. were used: A,H,QH,O,l) = -(285.83 kO.04) kJ ‘mol- *;(“) A,Hh(HCl in 12.21H,O.l) = -(162.26f0.01) kJ.mol-‘;(‘9’ A,Hi(HCl in 10.98H,O,l) = -(161.77+0.01) kJ.mol-1;‘19’ A,H~(CrCl,~6.OOH,O,cr) = -(2455.8f8.4) kJ.mol-‘. The hexahydrate chromium(II1) chloride used in this work, was the isomer [Cr(H,O),Cl]Cl, . H,O, as was shown by conductivity measurements (l- 2 electrolyte). No value has been reported for the standard enthalpy of formation of this isomer, but for other isomers, ArH~[{Cr(H,O),)Cl,,cr] = -2452.7 kJ . mol- 1,(‘9’ AtH~[(Cr(H,0)4Cl,}Cl. 2H,O,cr] = - 2458.9 kJ. mall ’ .(19’ We have adopted the mean of the values for these two isomers, as standard enthalpy of TABLE Ligand HACAC: HBZAC HDPM HTFAC HHFAC
4. Standard
molar A,H;(cr
enthalpies or I)t
kJ7 -425.5* 1.0“ -335.1 +2.gh -587.7+3.gb -1051.0~5.0 -1673.925.0
t In the equilibrium mixture. ’ Reference 20: * Reference 2 1: ” Reference Group Scheme. : HACAC = pentane-2.4-dione.
of formation
and vaporization
of the hgands
A vap,ruJfm
22; d Reference
K
A,H;(enol.g)
kJ,mol-’ +41.8+0.2‘ +83.8+0.4” +59.5_+0.1 +37.2+0.2’ +30.6iO.l’
at 298.15
kJ.mol-’
y
23: ’ Reference
-383.7kl.O -251.3k2.9 -528.2k3.8 ~ 1013.8 + 5.0 f -1643.3k5.0’
24; ‘estimated
by using
a
650
M. A. V. RIBEIRO TABLE
5. Standard
DA SILVA
molar
enthalpies
AND
M.
L. C. C. H. FERRAO
of formation
and sublimation
A,Hm(g) kJ.mol-’
Complex C~(BZAC),
-1185+12
Cr(DPM)J
-1977*14 -3486&
CT(TFAC)J CT(HFAI&
18624 133*4 117+4
17
-5471&17
1: b Reference
-999* 12 -1844+14 -3369f 17 -5359+ 17 - 1441.8k9.4
112*4
-1564.8k8.9”
ct’(ACAC)3
a Reference
at 29X. 15 K
123+3
*
25
formation of the {Cr(H,O),Cl}Cl,. H,O(cr), with an assigned uncertainty of k8.4 kJ.mol-‘. Together with the standard enthalpies of formation of the ligands (table 4) and the above derived molar enthalpies of the thermochemical reactions, table 5 lists the derived standard molar enthalpies of formation of the crystalline chromium(II1) complexes, together with the corresponding value for the tris(pentane-2,4-dionato)chromium(III), recalculated from the published value of Hill and Irving”’ using the most recent auxiliary data. The standard molar enthalpies of sublimation and of formation in the gaseous state. of the complexes, are also listed in table 5.
4. Discussion From the molar enthalpies of the homolytic into metal atoms and ligand radicals:
dissociation
of the gaseous molecules
CrL,(g) = Cr(g) + 3L’(g).
(3)
the mean chromium-oxygen bond-dissociation enthalpies (D)(Cr-0) were derived. Since the oxygen atoms in metal-l)-diketonates are equivalent,‘26’ (D)(Cr-0) may be defined as l/6 of the molar enthalpy of the disruption reaction (3): AdisrHG(3) = AtH,XCr,g) + 3ArHi(HL,g) - 3A,Hi(H,g) where D(O-H,HL,enol) in the parent ligand:
-A,Hk(Crt+g)
is the molar enthalpy of dissociation
+ 3D(O-H,HL,enol), of the enolic hydrogen
H-L(g) = L’(g) + H(g). There are no measured values for the molar enthalpy of dissociation of the enolic hydrogen from l3-diketones. Different values have been estimated, and the situation has been recently reviewed. (27) In this work, for comparison reasons, the value’27’ of D(O-H,HL,enol) = (418 f 20) kJ . mall 1 is used. With A,Hk(H,g) = (218.00_+0.01) kJ .mo11’,(19’ A,H,?,,(Cr,g) = (396.6f4.2) kJ. mol-’ 1(19) the calculated values of A,,,H; and (D)(Cr-0). for the complexes studied here, and also for Cr(AcAc), are given in table 6.
A,H~{Cr(III) TABLE Complex Cr(BZAc), CT(DPM)~ Cr(TFAC)3 Cr(HFAC), Cr(ACAC)3
6.51
B-diketonates]
6. (D)(Cr-0)
A,,& kJ.mol-’ 1243k61 1257f62 1324k65 1427+65 1286k61
at 298.15
K
As noted previous1y’28’ in attempting a rigorous determination of bond enthalpies, estimations of some molar enthalpies of formation have been made, as well as the molar dissociation energy of the enolic hydrogen from the parent pdiketones. The resulting bond enthalpies, therefore, have a large uncertainty attached to them. Within the uncertainties associated with the mean chromiumoxygen bond-dissociation enthalpies, there are no significant differences in (D)(Cr0) for the various chromium(II1) P-diketonates. This shows that there is no effect or only a very small one, of -CH,, -C(CH,),. -C,H,, or -CF, substitution in the ligand, upon (D)(Cr-0). This effect has already been observed for complexes of bdiketones with Be(II),‘28’ A1(III),~28~ and CU(II).‘~~’ Although not all of the molecular structures of these complexes are known, the results seem consistent with the fact that for those chromium(II1) P-diketonates. for which structures have been determined, the Cr-0 bond length is almost constant;‘30S31’ the average Cr-0 bond length is 195.4 pm. Entering this value in the correlation (D)(Cr-0) =fjr(Cr-0)) proposed by Cave11 et u/.,(321 leads to an estimate (D)(Cr-0) = 220 kJ . mall ’ in good agreement with the average value for the chromium(II1) B-diketonates, (D)(Cr-0) = 218 kJ . mol- ’ (table 6) reported here. We thank NATO, Research Grant 1709, for partial financial support and the lnstituto National de Investigacao Cientifica, Lisboa. for support given to a research project (L.5) of Centro de Investigacao em Quimica, Universidade do Porto. Thanks are also due to Junta National de Investigacao Cientifica e Tecnologica, Lisboa, for partial financial support under research contract no. 22080.32. REFERENCES I. 2. 3. 4. 5.
Hill. J. 0.; Irving. R. J. J. Chem. Sot. A 1967, 1413. Adams, J. T.: Hauser. C. R. J. Am. C/rem. Sot. 1944,66, 1220. Kopecky, K. R.; Nonhebcl, D. C.; Morris. G. S. J. Org. Chem. 1962, 27. 1036. Belford, R. L.; Martell. A. E.; Calvin, M. J. Inorg. Nucl. Chem. 1956, 2, 11. Fernelius, W. C.; Blanch, J. E. Inorg. Synthesis 1957. 5, 130. 6. Hammond, G. S.; Nonhebel, D. C.; Wu, C. S. Inorg. C/tern. 1%3, 2. 73. 7. Perrin, D. D.; Armarego, W. L.; Perrin. R. D. PuriJication qflahorarory Chemical.~. Oxford. 1966.
Pergamon Press:
652
M. A. V. RIBEIRO
DA SILVA
AND
M. L. C. C. H. FERRAO
8. Irving, R.J.; Ribeiro da Silva, M. A. V. J. Chem. Sock. Dullon Trans. 1978, 399. 9. Ribeiro da Silva. M. A. V.: Irving. R. J. Bull. Chem. Sot,. Jpn 1977, 50, 734. IO. Kilday. M. V.; Prosen, E. J. J. Res. Nat. Bur. Slund. 1973. A77. 581, 589. Il. Adedeji. F. A.: Brown, D. L. S.: Connor, J. A.; Leung. M.; Paz-Andrade, M. 1.: Skinner. H. A. J. Organomerullic Chem. 1975. 97. 221. 12. Stull, D. R.: Westrum. E. F.: Sinke, G. C. The Chemkul Thermodvnumics of’ Orgunic Compounds. John Wiley: New York. 1969. 13. JANAF Thermochemicul Tables. PB 168 370-2. Stull, D. R.: director. U.S. National Bureau of Standards: Washington. 1968. 14. Morawetz. E. J. Chem. Thermodynamics 1972, 4. 455. 15. Murata. S.; Sakiyama, M.; Seki, S. J. Chem. Thermodynumics 1982, 14, 723. 16. Pure Appl. Chem. 1979, 51. 409. 17. Vogel. A. I. Quantitutive Inorganic Analysis 4th edition. Longmans: London. 1978. 18. CODATA J. Chem. Thermodvnamics 1978, IO, 903. 19. The N.B.S. Tables of Chemical Thermodynamic Properties. J. Phys. Chem. Ref. Data 1982, I I, Supplement no. 2. 20. Hacking, J. M.; Pilcher, G. J. Chem. Thermodynamics 1979, I I, 1015. 21. FerrHo, M. L. C. C. H.: Ribeiro da Silva, M. A. V.: Suradi, S.; Pilcher, G.; Skinner, H. A. J. Chrm. Thermodvnamics 1981, 13, 567. 22. Irving, R. J.; Wads& I. Acra Chem. Stand. 1970, 24, 589. 23. Aihara. A. Bull. Chem. Sot. Jpn 1959, 32, 1242. 24. Irving, R. J.; Ribeiro da Silva, M. A. V. J. Chem. Sot. Dalton Trans. 1975, 798. 25. Naghibi-Bidokhti, H. Ph.D Thesis, University of Surrey, 1977. 26. Lingafelter, E. C.; Braun. R. L. J. Am. Chem. Sot. 1966, 88, 2951. 27. Ribeiro da Silva, M. A. V.: editor. Thermochemistry and 11s Applications to Chemical and Biochemicul Svstems. NATO ASI series. Reidel: Dordrecht. 1984. D. 331. 28. Ribeiro da Silva. M. A. V.; Reis, A. M. M. V. J. Chek. ‘Thermo&namics 1983, 15. 957. 29. Ribeiro da Silva, M. A. V.; Ribeiro da Silva, M. D. M. C.: Carvalho. A. P. S. M.: Akelo. M. J.; Pilcher. G. J. Chem. Thermodynamics 1984, 16. 137. 30. Thomas, B. G.; Morris, M. L.; Hilderbrandt, R. L. Inorg. Chem. 1978, 17. 2901. B. Acru Crysr. 1965, 19. 131. 31. Morosin, 32. Cavell. K. J.; Garner, C. D.; Sirnoes. J. A. M.; Pilcher. G.; Al-Samman, H.: Skinner, H. A.; AlTekhin. G.; Walton, I. B.; Zafarani-Moattar, M. T. J. Chem. Sot. Faraduv Trans. I 1981, 77, 2921.
Thermodynamics
1987, 19, 645-652
Standard enthalpies of formation tris(fi-diketonate)chromium( I I I) complexes: the mean (Cr-0) bond-dissociation enthalpies
of
MANUEL A. V. RIBEIRO DA SILVA and MARIA LUiSA C. C. H. FERRWO Depurtamento de Quimica, Faculdade de CiPncias, Universidade do Porto, 4000 Porto, Portugul (Received 22 October 1986; in final form 14 November 1986) The following standard (p” = 101.325 kPa) molar enthalpies of formation of the crystalline solids were determined, at 298.15 K, by solution-reaction calorimetry, and the enthalpies of sublimation were measured by high-temperature microcalorimetry. From’ the enthalpies of formation of the gaseous complexes, the average molar bond-dissociation enthalpies, (D)(Cr 0) were derived. Cr(III)
complex
ArHC(cr)/(kJ -1185+12 -1977*14 -3486+ -5471*17
Cr(BZAC),
Cr(DPM), CT(TFAC)) CT(HFAC)J
I-phenylbutane-l,3-dionato; trifluoropentane-2,4-dionato:
(BZAC,
‘mol-‘)
A,,&,,/(kJ~
mol-‘)
(D)(Cr-O)/(kJ
186+’ 133*2 117*4 112~4
I7
2.2.6,6-tetramethylheptane-3,5-dionato: l,l.l,S,5.5-hexafluoropentane-2,4-dionato).
DPM. HFAC.
mole ‘)
207kll 209+10 221+11 238111 TFAC.
l,l.l-
1. Introduction The thermochemistry of both p-diketones and metal-gdiketonates has been investigated progressively and extensively over the last two decades. For the chromium(II1) complexes, however, precise thermochemical measurements have been reported only for tris(pentane-2,4-dionato)chromium(III).”’ Here, we report the standard molar enthalpies of formation and of sublimation of four other chromium(II1) P-diketonates to investigate the effect of structural changes in the ligand upon the mean molar (Cr-0) bond-dissociation enthalpy.
2. Experimental 1-Phenylbutane-1,3-dione (HBAC) (Koch-Light Laboratories Ltd.) was purified by repeated crystallization from (ethanol+ water) and the purity checked by microanalysis for C,,H,002, in mass per cent: C. found 73.87, expected 74.06; H. OOZI-9614187:060645+08
$02.00/O
(“ 1987 Academic
Press Inc. (London)
Limited
646
M.
A. V. RIBEIRO
DA
SILVA
AND
M.
L. C. C. H. FERR.60
found 6.26, expected 6.22. 2,2,6,6-Tetramethylheptane-2.4-dione (HDPM) was prepared by Claisen condensation according to the methods of Adams and Hauserc2’ and of Kopecky et ~1.,(~’ was purified by fractional distillation in vacuum. and the purity was checked by g.1.c. using two different columns. l,l,lTrifluoropentane-2,4-dione (HTFAC) (Koch-Light Laboratories Ltd.) was purified by fractional distillation and the purity was checked by g.1.c. using two different columns. 1,l ,1,5,5,5-Hexafluoropentane-2.4-dione (HHFAC) (Koch-Light Laboratories, Ltd.) was dehydrated using the method of Belford et ~1.‘~’ and was purified by fractional distillation. The purity was checked by g.1.c. using two different columns. The purified liquid B-diketones were stored under nitrogen in the dark and were freshly distilled prior to use. Tris(l-phenylbutane-1,3-dionato)chromium(III), Cr(szAc),, was prepared as described by Fernelius and Blanchc5’ and was purified by crystallization from (acetone + hexane). Tris(2,2.6.6-tetramethylheptane-2,4-dionato)chromium(III), Cr(DPM)3, was prepared as described by Hammond et ,1.@’ and was purified by crystallization from light petroleum. Tris( l,l, 1-trifluoropentane-2,4-dionato)chromium(III), Cr(TFAC)3, (R esearch Organic/Inorganic Co.), was purified by crystallization from ethanol. Tris( 1,l .1,.5,5,5-hexafluoropentane-2,4-dionato)chromium(III), Cr(HFAc)3, (Research Organic/Inorganic Co.), was purified by crystallization from tetrachloromethane. The mass-percentage analyses of the complexes were: C Cr(BZAC), Cr(DPM), Cr(TFAC)3 Cr(HFAC),
67.32 65.26 35.31 26.42
H found 5.23 9.50 2.31 0.44
Cr
C
9.78 8.70 10.25 7.83
67.28 65.86 35.24 26.76
H expected 5.08 9.55 2.37 0.45
Cr 9.71 8.64 10.17 7.72
Chromium chloride hexahydrate (Merck, p.a.) was analysed for chromium and shown to be CrCl, .6.OOH,O. 1,4-Dioxan (Carl Erba) and toluene (Merck, p.a.) were purified according to reference 7. Hydrochloric acid (B.D.H., AnalaR) and ethanol (Merck, p.a.) were used. The isoperibol LKB 8700 reaction-and-solution precision calorimeter was used. The operation and calculation methods have been described@*9’ and were used here. The accuracy of the calorimeter was checked by measuring the molar enthalpy of solution of tris(hydroxymethyl)aminomethane (THAM) in aqueous 0.100 mol. dm - 3 HCl at 298.15 K: Aso,Hm = -(29.756&0.024) kJ . mol- ‘, in agreement with the value of Kilday and Prosen: (lo) Aso,Hm = -(29.770f0.032) kJ.mol-‘. The enthalpies of sublimation of the complexes were measured by the “vacuum sublimation” drop-microcalorimetric method. (11) Samples (about 5 mg) of each complex contained in a small thin glass capillary tube sealed at one end, were dropped, at room temperature, into the hot reaction vessel in the Calvet HighTemperature Microcalorimeter held at constant temperature, between 388 and 510 K. and then removed from the hot zone by vacuum sublimation. At these temperatures, the complexes showed no signs of decomposition. and sublimed
A,H~(Cr(III)
j3-diketonates)
647
completely in less than 30 min. The observed standard molar enthalpies of sublimation (H,Z,,(g,T)-HA(cr,298.15 K)} were corrected to 298.15 K using values of {Hi(g,T) - Hk(g,298.15 K)} estimated by group-additivity methods, using data of Stull et al.“” The microcalorimeter was calibrated in situ for these measurements by making use of the known molar enthalpies of sublimation of iodine,‘i3’ naphthalene,‘12’ and benzoic acid.‘14. ’ ” The relative atomic masses used were the recommended by the IUPAC Commission.“” All uncertainty intervals given are twice the standard deviation of the mean. Carbon and hydrogen analyses were carried out by the Microanalytical Service, University of Surrey. Chromium was analysed by the dichromatometric method.“” 3. Results The rates of hydrolyses of the chromium(II1) b-diketonate complexes in aqueous acid were too slow for calorimetric measurement of the enthalpy of reaction; hence other solvents, in which rapid hydrolyses occurred, were used. The thermochemical reaction for determining the standard molar enthalpy of formation of Cr(szAc), and CIfTFAC),
WaS:
CrL,(cr) + 3HCl. 12.21H20(1) = CrCl, ~6.OOH,O(cr) + 30.63H20(1) + 3HtJcr or 1). (1) The initial calorimetric solvent was {0.75( 1,Cdioxan) + 0.25HCl(aq, 4.2 mol. dm- “)} by volume. The same batch of solvent was used for both Cr(szAc), and Cr(TFAc)3. The enthalpies of the thermochemical reactions were determined indirectly, from a series of measurements of the enthalpies of solution of reactants and products in succession in the calorimetric solvent. The difference between the molar enthalpies of solution of the products and reactants, in the correct stoichiometric ratio, gives the enthalpy change A,HA of the thermochemical reaction, provided equilibrium is reached from either side within the period of the experiment. To the calorimetric solvent (100.0 cm3), ampoules containing HCl 12.21 H,O(l) and chromium complex were added consecutively and A,H, were measured. To a second portion of the same solvent (100.0 cm3), ampoules containing water, CrCl, . 6.00H20(cr), and the appropriate ligand, were added consecutively, and the different AiH, were measured. The amounts of reactants in a particular series of experiments were determined by the amount of CrCl, . 6.00H20(cr) (approximately 2.5 x 10e4 mol) in a particular ampoule so that control of the stoichiometry was maintained for each series. With this procedure, the value calculated for A,H; relates to reaction (l), provided that the final solutions resulting from the solution of all reactants and that from all products are identical. To check the validity of this assumption, ampoules of one solution were broken into the other; no enthalpy changes were detected. Table 1 lists the mean values of the molar enthalpies of solution and reaction required for determination of the enthalpies of the thermochemical reactions (1)
648
M. A. V. RIBEIRO TABLE
1.
and
C~(BZAC),
i HCI’
4
H,W
5 6
CrCI,
Solvent
12.21H,O(l)
initial A, A, initial A2 A3 ‘43
Cr(szAc),(cr) Cr(TFAC&(Cr)
.6.OOH,O(cr)
HBZAC(Cr) HTFAc(I)
I
molar
CT(TFAC)~:
Reactant
I 2 3
DA SILVA
AND enthalpies Solution
M. L. C. C. H. FERRAO of reaction
and solution
Wm
Number of experiments
kJ.mol
5 5 5 5 5 5 5
A, B, B2 A2 A3 B, B2
at 29X. I5 K
-21.22kO.07 + 19.11+0.32 +25.70+0.15 -1.025+0.010 -16.53kO.18 + 26.01 k 0.05 -23.08*0.27
used for Cr(szAc), and Cr(rFAc)3. The enthalpies of these thermochemical were derived from the values in table 1 according to A,H~(Cr(szAc),)
= 3Ar H, + A2Hm - 30.63A,H, = -(74.65+0.54) kJ.mol-‘,
A,Hh{Cr(TFAC)3}
= 3A,H,+A3H,-30.63A,H,-ASH,,,-3A7Hm = +(79.21 f0.92) kJ.mol-‘.
and
Cr(DPM)3
Cr(HFAC)3
’
reactions
- A,H, - 3A,H,
were studied using a thermochemical
reaction analogous
to (l), i.e. CrL,(cr) + 3HCl. 10.98H,O(l)
= CrCl, .6.OOH,O(cr)
+ 26.94H,O(l)
+ 3H~(l), (2)
but with an initial calorimetric solvent of composition {0.70toluene+0.30ethanol} by volume. Two different batches of solvent were used for Cr(DPM)3 and Cr(HFAc)J, which made necessary remeasurement of the enthalpies of solution of H,O(l), HCl. 10.98H,O(l), and CrCl, .6.OOH,O(cr). The amounts of reactants in a particular series of experiments were determined by the amount of CrCl, .6.OOH,O(cr) (approximately 2.5 x 10e4 mol) in a particular ampoule. The experimental procedure was analogous to that for the other two chromium complexes. TABLE
2. C~(UPM)~
HCl
enthaipies
Solvent
Solution
10.98H,O(l)
initial C initial C2 C, initial Cd initial C5 G
Cl Dl C* C, D, G D2 C-5 C6 D2
Cr(DPM)3(Cf) H,W HDPM(1)
CrCI, HCI
molar
Reactant
i
8 9 10 11 12 13 14 15 I6 17
and Cr(twAc)a:
.6.OOH,O(cr) 10.98H,O(I)
Cr(HFAc)3(Cr) H,W
CrCI, HHFAC(1)
~6.OOH,O(cr)
of reaction
and solution
at 298.15
K
Number of experiments
kJ,mol-’
5 5 5 5 5 5 5 5 5 5
-r7.11&0.07 +21.24_+0.09 +0.840*0.009 +3.06*0.02 -19.77kO.17 - 16.92kO.17 +45.64+0.66 +0.839&0.009 -20.4510.45 -67.05kO.36
A,HG{Cr(III) TABLE
3. Molar
Number of experiments
Complex
CrmAc)J
3 4 2
Cr(HFAC),
2
C’I’(BZAC), Cr(DPM)3
enthalpies A,,,H,(obs. kJ.mol
T K 507 443 426 388
649
h-diketonatesj of sublimation T)
of metal
complexes
Hm(g,T)-H;(g,298.15
-I
K)
A,\,,&(298.15
328+2 273+2 182k4 164+4
K)
kJ.molF’
kJ.mol-’ 142 140 65 52
186&2 133i2 117*4 112+4
Table 2 lists the mean values of the molar enthalpies of solution and reaction required for determination of the enthalpies of the thermochemical reactions (2) used for Cr(DPrv& and Cr(HFAc)3. The enthalpies of these thermochemical reactions were derived from the values in table 2 according to = 3AsH,+A,H,-26.94A,0H,-3A.,,H,-A,zH, = -(42.13-L-0.38) kJ.mol- ‘,
.&&(Cr(DPM)3}
= 3A,,H,+A,,H,-26.94A,sH,-A,,H,-3A,,H, = +(193.9* 1.5) kJ.mol-‘.
fi,H~(Cr(HFAC)3}
The experimental results for determination of the standard molar enthalpies of sublimation of the complexes are listed in table 3. To derive the standard molar enthalpies of formation of the crystalline chromium(II1) complexes, the following auxiliary data, at 298.15 K. were used: A,H,QH,O,l) = -(285.83 kO.04) kJ ‘mol- *;(“) A,Hh(HCl in 12.21H,O.l) = -(162.26f0.01) kJ.mol-‘;(‘9’ A,Hi(HCl in 10.98H,O,l) = -(161.77+0.01) kJ.mol-1;‘19’ A,H~(CrCl,~6.OOH,O,cr) = -(2455.8f8.4) kJ.mol-‘. The hexahydrate chromium(II1) chloride used in this work, was the isomer [Cr(H,O),Cl]Cl, . H,O, as was shown by conductivity measurements (l- 2 electrolyte). No value has been reported for the standard enthalpy of formation of this isomer, but for other isomers, ArH~[{Cr(H,O),)Cl,,cr] = -2452.7 kJ . mol- 1,(‘9’ AtH~[(Cr(H,0)4Cl,}Cl. 2H,O,cr] = - 2458.9 kJ. mall ’ .(19’ We have adopted the mean of the values for these two isomers, as standard enthalpy of TABLE Ligand HACAC: HBZAC HDPM HTFAC HHFAC
4. Standard
molar A,H;(cr
enthalpies or I)t
kJ7 -425.5* 1.0“ -335.1 +2.gh -587.7+3.gb -1051.0~5.0 -1673.925.0
t In the equilibrium mixture. ’ Reference 20: * Reference 2 1: ” Reference Group Scheme. : HACAC = pentane-2.4-dione.
of formation
and vaporization
of the hgands
A vap,ruJfm
22; d Reference
K
A,H;(enol.g)
kJ,mol-’ +41.8+0.2‘ +83.8+0.4” +59.5_+0.1 +37.2+0.2’ +30.6iO.l’
at 298.15
kJ.mol-’
y
23: ’ Reference
-383.7kl.O -251.3k2.9 -528.2k3.8 ~ 1013.8 + 5.0 f -1643.3k5.0’
24; ‘estimated
by using
a
650
M. A. V. RIBEIRO TABLE
5. Standard
DA SILVA
molar
enthalpies
AND
M.
L. C. C. H. FERRAO
of formation
and sublimation
A,Hm(g) kJ.mol-’
Complex C~(BZAC),
-1185+12
Cr(DPM)J
-1977*14 -3486&
CT(TFAC)J CT(HFAI&
18624 133*4 117+4
17
-5471&17
1: b Reference
-999* 12 -1844+14 -3369f 17 -5359+ 17 - 1441.8k9.4
112*4
-1564.8k8.9”
ct’(ACAC)3
a Reference
at 29X. 15 K
123+3
*
25
formation of the {Cr(H,O),Cl}Cl,. H,O(cr), with an assigned uncertainty of k8.4 kJ.mol-‘. Together with the standard enthalpies of formation of the ligands (table 4) and the above derived molar enthalpies of the thermochemical reactions, table 5 lists the derived standard molar enthalpies of formation of the crystalline chromium(II1) complexes, together with the corresponding value for the tris(pentane-2,4-dionato)chromium(III), recalculated from the published value of Hill and Irving”’ using the most recent auxiliary data. The standard molar enthalpies of sublimation and of formation in the gaseous state. of the complexes, are also listed in table 5.
4. Discussion From the molar enthalpies of the homolytic into metal atoms and ligand radicals:
dissociation
of the gaseous molecules
CrL,(g) = Cr(g) + 3L’(g).
(3)
the mean chromium-oxygen bond-dissociation enthalpies (D)(Cr-0) were derived. Since the oxygen atoms in metal-l)-diketonates are equivalent,‘26’ (D)(Cr-0) may be defined as l/6 of the molar enthalpy of the disruption reaction (3): AdisrHG(3) = AtH,XCr,g) + 3ArHi(HL,g) - 3A,Hi(H,g) where D(O-H,HL,enol) in the parent ligand:
-A,Hk(Crt+g)
is the molar enthalpy of dissociation
+ 3D(O-H,HL,enol), of the enolic hydrogen
H-L(g) = L’(g) + H(g). There are no measured values for the molar enthalpy of dissociation of the enolic hydrogen from l3-diketones. Different values have been estimated, and the situation has been recently reviewed. (27) In this work, for comparison reasons, the value’27’ of D(O-H,HL,enol) = (418 f 20) kJ . mall 1 is used. With A,Hk(H,g) = (218.00_+0.01) kJ .mo11’,(19’ A,H,?,,(Cr,g) = (396.6f4.2) kJ. mol-’ 1(19) the calculated values of A,,,H; and (D)(Cr-0). for the complexes studied here, and also for Cr(AcAc), are given in table 6.
A,H~{Cr(III) TABLE Complex Cr(BZAc), CT(DPM)~ Cr(TFAC)3 Cr(HFAC), Cr(ACAC)3
6.51
B-diketonates]
6. (D)(Cr-0)
A,,& kJ.mol-’ 1243k61 1257f62 1324k65 1427+65 1286k61
at 298.15
K
As noted previous1y’28’ in attempting a rigorous determination of bond enthalpies, estimations of some molar enthalpies of formation have been made, as well as the molar dissociation energy of the enolic hydrogen from the parent pdiketones. The resulting bond enthalpies, therefore, have a large uncertainty attached to them. Within the uncertainties associated with the mean chromiumoxygen bond-dissociation enthalpies, there are no significant differences in (D)(Cr0) for the various chromium(II1) P-diketonates. This shows that there is no effect or only a very small one, of -CH,, -C(CH,),. -C,H,, or -CF, substitution in the ligand, upon (D)(Cr-0). This effect has already been observed for complexes of bdiketones with Be(II),‘28’ A1(III),~28~ and CU(II).‘~~’ Although not all of the molecular structures of these complexes are known, the results seem consistent with the fact that for those chromium(II1) P-diketonates. for which structures have been determined, the Cr-0 bond length is almost constant;‘30S31’ the average Cr-0 bond length is 195.4 pm. Entering this value in the correlation (D)(Cr-0) =fjr(Cr-0)) proposed by Cave11 et u/.,(321 leads to an estimate (D)(Cr-0) = 220 kJ . mall ’ in good agreement with the average value for the chromium(II1) B-diketonates, (D)(Cr-0) = 218 kJ . mol- ’ (table 6) reported here. We thank NATO, Research Grant 1709, for partial financial support and the lnstituto National de Investigacao Cientifica, Lisboa. for support given to a research project (L.5) of Centro de Investigacao em Quimica, Universidade do Porto. Thanks are also due to Junta National de Investigacao Cientifica e Tecnologica, Lisboa, for partial financial support under research contract no. 22080.32. REFERENCES I. 2. 3. 4. 5.
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AND
M. L. C. C. H. FERRAO
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