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Solvent effects on the electronic spectra of vanadyl(IV) ß-diketonate derivatives
J, inor~ nm I Chem. Vol. 41, pp. 59-61 ~: Pergamon Press Itd 1979. Printed in Great Britain
0022-1902[7910101~0059{$02.C010
SOLVENT EFFECTS ON THE ELECTRONIC SPECTRA OF VANADYL(IV) /3-DIKETONATE DERIVATIVES CRAIG E. MANNIX and ARDEN P. ZIPP* Chemistry Department, State University College, Cortland, NY 13045, U.S.A.
(Received 21 April 1978) Abstract--The visible spectra of VO(acac-Cl): and VO(acac-Br)2 have been measured in several solvents with a wide range of donor properties. Spectra in the non-coordinatingsolvent benzene show little difference from those of other vanadyl ~3-diketonates. Results for other solvents show a linear relationship between the energy differences of the first two visible bands, D~.2and the solvent Donor Numbers. The smaller sensitivity of D,,z for these species to Donor Number (relative to that for VO(acac),,) has been interpreted in terms of a decreased tendency of the substituted species to undergo reaction with donor molecules.
INTRODUCTION The electronic spectra of oxovanadium(IV) or vanadyl acetylacetonate, VO(acac)2, and related compounds have been examined in a variety of solvents during the last several years [1-10]. The goals of these studies have been not only to establish the relative energies of the vanadium orbitals[l-9], but also to estimate the donor properties of the solvents used to dissolve them[2, 3, 9, I0]. Both of these goals have been reached for VO(acac)2 itself and the spectrum of this square pyramidal molecule has been discussed in terms of a number of orbital energy level schemes[5, 8, 11], while the energy difference between the first two visible bands in different solvents has been found to give a linear correlation with the Donor Number of the solvent[12]. Similar studies have also been carried out for VO(bza)2 (bza= benzoylacetone), VO(dbm)2 (dbm --- dibenzoylmethane), and VO(tfa)2 (tfa = trifluoroacetylacetone), and their behavior has been interpreted in terms of the electronic and steric effects of the substituents in the diketone ligands. Despite this extensive activity, no studies have been made of the spectral behavior of VO(acac)~ derivatives in which the methine hydrogen is replaced with other substituents.'We were interested in the effect that such substitutions might have on the energies of the vanadium d-orbitals, as well as on the interaction of Lewis bases with the open site on vanadium. To this end we have prepared VO(acac-C1)2 and VO(acac-Br)2 and examined their spectra in several solvents with different donor abilities and report the results of this study here.
RESULTS AND DISCUSSION Although both VO(acac-C1)2 and VO(acac-Br)2 are known compounds, their IR spectra have not been discussed previously. The most striking contrast between the spectra of these solid compounds (as Nujol mulls) and VO(acac)2 is the appearance of the strong absorbance band due to the V=O bond at 906 and 907 cm-', respectively, rather than at 997 cm-' as it is in VO(acac)2 itself[15]. Although this absorbance is also found near 1000cm-' in VO(bza)2, (997cm-'), VO(dbm)z, (995cm-') and VO(dpm)2, (1005cm-'), it appears at a lower frequency in VO(ffa)2, (932 cm-')[16]. This lowering of the frequency has been attributed to the presence of a polymeric structure in the solid involving coordination of the vanadyl oxygen of one VO(tfa)2 unit to the vanadium atom of an adjacent one[17]. That this also occurs in VO(acac-Cl)2 and VO(acac-Br)2 is suggested by the appearance of the V=O bands at 1005 crn-' when they are dissolved in CHzCiz where intermolecular association is no longer likely, but at 965cm -t in the coordinating solvent pyridine, in which the V=O band of VO(acach is shifted to 958 cm-' [18]. It is striking that the frequency shift of the V=O band, due to replacing the methine hydrogen in the present study, is greater than that caused by replacing the hydrogens in the methyl group in VO(ffa)2 and implies a stronger association between individual vanadyl units in the solid. This may be due to the enhanced withdrawal of electrons from vanadium by the chlorine or bromine atoms attached to the "pseudoaromatic" VO(acac)2 rings [19], leading to an increase in the Lewis acid character of the vanadium atom, and a stronger association. A number of other differences between the spectra of VO(acac)2 and VO(acac-X)2 were also observed which are similar to changes reported by other investigators who have studied the ring substitution of metal acetylacetonates. Specifically, the band at 1190 cm -1 in VO(acac)2, which has been attributed to the methine C-H bending vibration[20], is absent in the spectra of VO(acac-Cl)2 and VO(acac-Br)2 and only a single band is found between 1500 and 1600cm-' (at 1575cm ~for VO(acacC1)2 and 1560cm -~ for VO(acac-Br)2), rather than the two peaks, 1520 and 1570cm-', in VO(acac)2 itself[21]. Visible spectra. The absorbance maxima for VO(acacC1)2 and VO(acac-Br)2 in the six solvents investigated are listed in Table 1. Although both compounds exhibit three bands in all solvents the band maxima are frequently
IR
EXPERIMENTAl, VO(acac)2 was prepared by neutralizingan aqueous solution of VOSO4 and 2,4-pentanedione as described in the literature[13]. The derivatives VO(acac-CI)2 and VO(acac-Br)2 were synthesized by reacting VO(acac)2 (2.2gm, 8.3 mmole) in CHCI3 ~25ml) with 20mmole of N-chlorosuccinimide (2.7g) or Nbromosuccinimide (3.5g), respectively. The solutions were cooled to 0°C, and filtered to collect the yellow-green crystals[14]. All solvents were reagent or spectroquality grade and were used without further purification. Visible spectra were obtained at 25.0-+0. I° on 0.01 M solutions (prepared immediately before making measurements) in l cm cells using a Cary 14 spectrophotometer. IR spectra were determined as Nujol mulls from 500 to 4000cm ~ between NaCI plates with a Beckman IR-12 spectrophotomeler. *Author for correspondence. 59
spectra:
60
CRAIG E. MANNIX and ARDEN P. ZIPP
Table 1. Visible spectra of VO(acac-Clh and VO(acac-Br)z in various solvents Solvent CH3OH
CzH~OH
(CH3)_,SO
(CH3)2C0
CHCI3
C6H6
VO(acac-Cl)2 kK DL2 vl 12.85 v,. 16.55 v323.0
VO(acac-Br)2 Donor kK D,.2 Number
3.70
vl 12.87 v2 16.34 v323.1
vl 12.77 1,2 16:39 3.62 v322.7 vl 12.66 v2 16.13 3.47 v323.3 vl 13.59 v2 16.23 2.64 v322.7 vl 14.97 v2 16.78 1.81 v323.3
vj 12.63 v2 16.08 v323.3 vt 12,58 v2 15.90 v323.3 vt 13.74 v2 16.39 v323.3 vt 14.99 v2 17.06 v~23.5
vl 15.34 v2 16.84 v323.2
vl 15.11 v2 16.67 v323.3
1.50
33.8 3.46 30.4 3.45 29.8 3.32 17.0 2.65 10.9 2.07 3.4 1.56
difficult to establish due to their low intensities and, in the case of the highest energy band, proximity to the strong charge-transfer band near 25,000cm -t. By comparing the results obtained for these compounds in benzene with those which have been found for other vanadyl diketonates in this same solvent (where solvent effects should be minimized); VO(acac)z--15.44, 16.88, 25.81; VO(bza)z--15.38, 16.67, 21.74; VO(dbm)r--14.93, 16.95, 20.83 [9], it can be seen that the visible spectrum is influenced very little by the introduction of the chloro- or bromo- substituents. In fact, the average energies of the two lower energy bands are calculated to be 15.2-+ 0.2 kK and 16.8-+0.1 kK although the third band shows a much greater range of values, 23-+ 2 kK. The variability in the position of this third band could be due to changes in the energies of the vanadium d orbitals caused by variations in molecular structure or to the uncertainty in establishing the maximum for this band which frequently appears as a shoulder to a much stronger absorbance. Assuming that the differences in the position of the third band are, in fact, real (i.e., due to differences in orbital energy levels), it should be possible to identify the orbital or orbitals responsible. According to the molecular orbital scheme for VO(acac)2 which assumes C4v symmetry for this species [l l], the lowest energy absorbance, vt can be attributed to the transition b2(dxy)~ e(d=, dxy), v2 (which is equivalent to 10 Do) to b2~b~(d~z_y2), and v3 to bz-->a~(dz2). For the more representative C2v symmetry, the assignment is similar, differing only in the interchange of the positions of the dx2-x2 and dxy orbitals [22]. Since v3 is the only transition which changes with the nature of the diketone, and the d~2 orbital is unique to this transition, it can be concluded that it is this orbital which is influenced by substitution, with its energy varying in the order VO(acac)2 > VO(acac-C1)2 = VO(acac-Br)2 > VO(bza)2 > VO(dbm)2. At the present time there is no obvious explanation for the origin of this substitution effect, its apparent specific influence on the d~2 orbital, or the relative impact of different substituents. When VO(acac-Cl)2 and VO(acac-Br)2 are dissolved in solvents which are better coordinating agents than ben-
zene, the values of vl and vz are shifted to lower energies, with v, being shifted somewhat more than v2, as shown in Table I. The greater effect which solvation has on Vl causes the difference between v~ and rE, OI,2, to change as well, as shown by the data in Table 1. When the DL2 values for VO(acac-Cl)2 and VO(acac-Br)2 are plotted against the solvent Donor Number, the results shown in Fig. I are obtained. Both of these plots are linear, although the data are somewhat more scattered than are those for VO(acac)2[12], which are plotted in the same figure for comparison. From this figure it can also be seen that the slopes of the plots for VO(acac-Cl)2 and VO(acac-Br)2 are greater than is that for VO(acac)2. A linear least squares computer program yields values of 12.4, 12.6 and 10.0 for these three slopes, while similar calculations on data from the literature yield slopes of 9.5, 10.2 and 22.8 for VO(bza)2[9], VO(dbm)2[9], and VO(ffa)2[10], respectively. These results indicate that the Di.2 values for VO(acac-C1)2, VO(acac-Br)2 and VO(tfa)2 are less sensitive to the donor ability of the solvent than are those for VO(acac)2, VOOaza)2 and VO(dbm)2. Although early workers in this field have commented on the lack of theoretical significance for the parameter DL2, recent studies [8, 9] have shown that its value is expected to increase with increasing distortion of the VO(acac)2 square pyramid toward octahedral geometry due to the substitution of the donor ligand. Thus, the present results seem to suggest that VO(acac-Cl)2, VO(acac-Br)2 and VO(tfa)2 undergo distortion from square pyramid to octahedron (and therefore substitution?) less readily than does VO(acac)2 itself. Recently other workers[10] have concluded that VO(tfa)2 coordinates more readily than VO(acac)2 based on the smaller temperature dependence of D~.2 for the former compound. Their results for VO(ffa)2 at 25°C indicate a smaller dependence of Dr.2 on the donor molecule for VO(tfa)2 than for VO(acac)2, however, in agreement with the present study. By way of reconciling this discrepancy, it seems that the dependence of DI.2 on the nature of the solvent should provide a better measure of coordinating ability than its dependence on temperature, since the latter is a second order effect.
40
3O
E ~_2o o a
to
J
I I
i
I 2
i
I
3
l
I
4
Ol,2, k K
Fig. 1. Plot of Donor Number vs DI.2 for VO(acach (@), VO(acac-C1)2(0), and VO(acac-Br)2(~) in various solvents.
Solvent effects on the electronic spectra of vanadyl(IV) fl-diketonate derivatives The conclusion, based on the D~.2-Donor Number relationship, that VO(acac-Cl)2, VO(acac-Br)2 and VO(tfa)2 are poorer acceptors than VO(acac)2 is, however, quite surprising since the greater electronegativities of fluorine, chlorine, and bromine (relative to H) would seem to make these species better acceptors by withdrawing electrons from vanadium. To date, only a single formation constant (Kt) has been measured for any of these acceptors (between VO(tfa)2 and 4-methyl pyridine N-oxide) and this is greater than the Kj for VO(acac)2 with the same donor (500_+ 150 vs 76 + - 3)[17]. It may be, however, that using the slopes of D,.2-Donor Number plots as we have done, to draw conclusions about the coordinating ability of different vanadyl /3diketones is invalid because of differences in some other factor such as the geometry of the adduct formed. In this regard, it is notable that the geometry of the adduct varies with the nature of the acceptor as well as with that of the donor. For example, 4-methyl pyridine N-oxide bonds to VO(tfah cis to the vanadyl oxygen but to VO(acac)z predominantly in the trans position[17] and substituted pyridines form either cis or trans isomers with VO(acac)2 although with no apparent relationship to the electronic or steric properties of the donor[23, 24]. Clearly the resolution of this question must await further studies on the geometries and formation constants of adducts of VO(acac-C1)2, VO(acac-Br)2, and VO(tfa)2 with a variety of donor species similar to those which have been carried out on VO(acac)2 itself [25] Acknowledgement--The authors wish to thank Dr. Charles H. Spink for helpful discussions.
REFERENCES
1. J. Selbin, T. R. Ortolano and F. J. Smith, Inorg. Chem. 2, 1315 (1963).
61
2. I. Bernal and P. H. Rieger, Inorg. Chem. 2, 256 (19631. 3. J. Selbin and T. R. Ortolano, J. lnorg. NucL Chem. 26, 37 (1964). 4. G. Basu, W. A. Yeranos and R. L. Belford, Inorg. ('hem. 3, 929 (1964). 5. T. R. Ortolano, J. Selbin and S. P. Glynn, J. Chem. Phy~. 41, 262 (1964). 6. J. Selbin, G. Maus and D. L. Johnson, J. Inorg. Nucl. Chem. 29, 1735 (1967). 7. D. Ogden and J. Selbin, J. Inorg. Nucl. Chem. 30, 1227(Irk68). 8. J. R. Wasson and H. J. Stoklosa, J. Inorg. Nuci Chem. 36, 227 (1974). 9. H. J. Stoklosa and J. R. Wasson, J. Inorg Nut!i Chem. 38, 677 (1976). 10. N. Adachi, Y. Fukuda and K. Sone, Bull. Chem. So~' Jaoan 50, 401 (1977). II. C. J. Ballhausen and H. B. Gray, Inorg. Chem. 1, I I l I I9621. 12. V. Guttmann, Coordination Chemist~ in N~m.Aqueous Solution. Springer-Verlag, Wien (1963). 13. R. Rowe and M. M. Jones, Inorg. Synth. 5, 115 (1957). 14. P. R, Singh and R. Sahai, Aust. J. Chem. 20, 639 (1967). 15. J. Selbin, H. R. Manning and G. Cessac, .I. Inor~. Nuct. Chem. 25, 1252 (1963). 16. R. G. Garvey and R. O. Ragsdale, Inorg. Chim. 4eta 2, 191 (1968). 17. N. S. AI-Niami, A. R. AI-Karaghouli, S. M. Aliwi and M G. Jalhoom, J. Inorg. Nucl. Chem. 36, 283 (1974). 18. R. Larsson, Acta Chem. Scand. 26, 549 (1972). 19. J. P. Collman, Transition Metal Chemisto' (Edited by R. L. Carlin), Vol. 2, p. 35. Marcel Dekker, New York (1966) 20. C. Djordjevis, J. Lewis and R. S. Nyholm, Chem, Ind. (London) 141 (1962). 21. R. P. Dryden and A. Winston, J. Phys. Chem. 62, 635 (I958). 22. M. H. Valek, W. A. Yeranos, G. Basu, P. K. Hon and R. L. Belford, J. Mol. Spect. 37, 228 (1971). 23. M. R. Caira, J. M. Haigh and I. R. Nassimbendi, J. lnor~. Nucl. Chem. 34, 3171 (1972). 24. J. J. R. Frausto da Silva and R. Wooton, Chem, Commun. 421 (1969). 25. E. K. Jaife and A. P. Zipp, J. lnorg. Nucl. Chem. 40,839 (1978); and references therein.
0022-1902[7910101~0059{$02.C010
SOLVENT EFFECTS ON THE ELECTRONIC SPECTRA OF VANADYL(IV) /3-DIKETONATE DERIVATIVES CRAIG E. MANNIX and ARDEN P. ZIPP* Chemistry Department, State University College, Cortland, NY 13045, U.S.A.
(Received 21 April 1978) Abstract--The visible spectra of VO(acac-Cl): and VO(acac-Br)2 have been measured in several solvents with a wide range of donor properties. Spectra in the non-coordinatingsolvent benzene show little difference from those of other vanadyl ~3-diketonates. Results for other solvents show a linear relationship between the energy differences of the first two visible bands, D~.2and the solvent Donor Numbers. The smaller sensitivity of D,,z for these species to Donor Number (relative to that for VO(acac),,) has been interpreted in terms of a decreased tendency of the substituted species to undergo reaction with donor molecules.
INTRODUCTION The electronic spectra of oxovanadium(IV) or vanadyl acetylacetonate, VO(acac)2, and related compounds have been examined in a variety of solvents during the last several years [1-10]. The goals of these studies have been not only to establish the relative energies of the vanadium orbitals[l-9], but also to estimate the donor properties of the solvents used to dissolve them[2, 3, 9, I0]. Both of these goals have been reached for VO(acac)2 itself and the spectrum of this square pyramidal molecule has been discussed in terms of a number of orbital energy level schemes[5, 8, 11], while the energy difference between the first two visible bands in different solvents has been found to give a linear correlation with the Donor Number of the solvent[12]. Similar studies have also been carried out for VO(bza)2 (bza= benzoylacetone), VO(dbm)2 (dbm --- dibenzoylmethane), and VO(tfa)2 (tfa = trifluoroacetylacetone), and their behavior has been interpreted in terms of the electronic and steric effects of the substituents in the diketone ligands. Despite this extensive activity, no studies have been made of the spectral behavior of VO(acac)~ derivatives in which the methine hydrogen is replaced with other substituents.'We were interested in the effect that such substitutions might have on the energies of the vanadium d-orbitals, as well as on the interaction of Lewis bases with the open site on vanadium. To this end we have prepared VO(acac-C1)2 and VO(acac-Br)2 and examined their spectra in several solvents with different donor abilities and report the results of this study here.
RESULTS AND DISCUSSION Although both VO(acac-C1)2 and VO(acac-Br)2 are known compounds, their IR spectra have not been discussed previously. The most striking contrast between the spectra of these solid compounds (as Nujol mulls) and VO(acac)2 is the appearance of the strong absorbance band due to the V=O bond at 906 and 907 cm-', respectively, rather than at 997 cm-' as it is in VO(acac)2 itself[15]. Although this absorbance is also found near 1000cm-' in VO(bza)2, (997cm-'), VO(dbm)z, (995cm-') and VO(dpm)2, (1005cm-'), it appears at a lower frequency in VO(ffa)2, (932 cm-')[16]. This lowering of the frequency has been attributed to the presence of a polymeric structure in the solid involving coordination of the vanadyl oxygen of one VO(tfa)2 unit to the vanadium atom of an adjacent one[17]. That this also occurs in VO(acac-Cl)2 and VO(acac-Br)2 is suggested by the appearance of the V=O bands at 1005 crn-' when they are dissolved in CHzCiz where intermolecular association is no longer likely, but at 965cm -t in the coordinating solvent pyridine, in which the V=O band of VO(acach is shifted to 958 cm-' [18]. It is striking that the frequency shift of the V=O band, due to replacing the methine hydrogen in the present study, is greater than that caused by replacing the hydrogens in the methyl group in VO(ffa)2 and implies a stronger association between individual vanadyl units in the solid. This may be due to the enhanced withdrawal of electrons from vanadium by the chlorine or bromine atoms attached to the "pseudoaromatic" VO(acac)2 rings [19], leading to an increase in the Lewis acid character of the vanadium atom, and a stronger association. A number of other differences between the spectra of VO(acac)2 and VO(acac-X)2 were also observed which are similar to changes reported by other investigators who have studied the ring substitution of metal acetylacetonates. Specifically, the band at 1190 cm -1 in VO(acac)2, which has been attributed to the methine C-H bending vibration[20], is absent in the spectra of VO(acac-Cl)2 and VO(acac-Br)2 and only a single band is found between 1500 and 1600cm-' (at 1575cm ~for VO(acacC1)2 and 1560cm -~ for VO(acac-Br)2), rather than the two peaks, 1520 and 1570cm-', in VO(acac)2 itself[21]. Visible spectra. The absorbance maxima for VO(acacC1)2 and VO(acac-Br)2 in the six solvents investigated are listed in Table 1. Although both compounds exhibit three bands in all solvents the band maxima are frequently
IR
EXPERIMENTAl, VO(acac)2 was prepared by neutralizingan aqueous solution of VOSO4 and 2,4-pentanedione as described in the literature[13]. The derivatives VO(acac-CI)2 and VO(acac-Br)2 were synthesized by reacting VO(acac)2 (2.2gm, 8.3 mmole) in CHCI3 ~25ml) with 20mmole of N-chlorosuccinimide (2.7g) or Nbromosuccinimide (3.5g), respectively. The solutions were cooled to 0°C, and filtered to collect the yellow-green crystals[14]. All solvents were reagent or spectroquality grade and were used without further purification. Visible spectra were obtained at 25.0-+0. I° on 0.01 M solutions (prepared immediately before making measurements) in l cm cells using a Cary 14 spectrophotometer. IR spectra were determined as Nujol mulls from 500 to 4000cm ~ between NaCI plates with a Beckman IR-12 spectrophotomeler. *Author for correspondence. 59
spectra:
60
CRAIG E. MANNIX and ARDEN P. ZIPP
Table 1. Visible spectra of VO(acac-Clh and VO(acac-Br)z in various solvents Solvent CH3OH
CzH~OH
(CH3)_,SO
(CH3)2C0
CHCI3
C6H6
VO(acac-Cl)2 kK DL2 vl 12.85 v,. 16.55 v323.0
VO(acac-Br)2 Donor kK D,.2 Number
3.70
vl 12.87 v2 16.34 v323.1
vl 12.77 1,2 16:39 3.62 v322.7 vl 12.66 v2 16.13 3.47 v323.3 vl 13.59 v2 16.23 2.64 v322.7 vl 14.97 v2 16.78 1.81 v323.3
vj 12.63 v2 16.08 v323.3 vt 12,58 v2 15.90 v323.3 vt 13.74 v2 16.39 v323.3 vt 14.99 v2 17.06 v~23.5
vl 15.34 v2 16.84 v323.2
vl 15.11 v2 16.67 v323.3
1.50
33.8 3.46 30.4 3.45 29.8 3.32 17.0 2.65 10.9 2.07 3.4 1.56
difficult to establish due to their low intensities and, in the case of the highest energy band, proximity to the strong charge-transfer band near 25,000cm -t. By comparing the results obtained for these compounds in benzene with those which have been found for other vanadyl diketonates in this same solvent (where solvent effects should be minimized); VO(acac)z--15.44, 16.88, 25.81; VO(bza)z--15.38, 16.67, 21.74; VO(dbm)r--14.93, 16.95, 20.83 [9], it can be seen that the visible spectrum is influenced very little by the introduction of the chloro- or bromo- substituents. In fact, the average energies of the two lower energy bands are calculated to be 15.2-+ 0.2 kK and 16.8-+0.1 kK although the third band shows a much greater range of values, 23-+ 2 kK. The variability in the position of this third band could be due to changes in the energies of the vanadium d orbitals caused by variations in molecular structure or to the uncertainty in establishing the maximum for this band which frequently appears as a shoulder to a much stronger absorbance. Assuming that the differences in the position of the third band are, in fact, real (i.e., due to differences in orbital energy levels), it should be possible to identify the orbital or orbitals responsible. According to the molecular orbital scheme for VO(acac)2 which assumes C4v symmetry for this species [l l], the lowest energy absorbance, vt can be attributed to the transition b2(dxy)~ e(d=, dxy), v2 (which is equivalent to 10 Do) to b2~b~(d~z_y2), and v3 to bz-->a~(dz2). For the more representative C2v symmetry, the assignment is similar, differing only in the interchange of the positions of the dx2-x2 and dxy orbitals [22]. Since v3 is the only transition which changes with the nature of the diketone, and the d~2 orbital is unique to this transition, it can be concluded that it is this orbital which is influenced by substitution, with its energy varying in the order VO(acac)2 > VO(acac-C1)2 = VO(acac-Br)2 > VO(bza)2 > VO(dbm)2. At the present time there is no obvious explanation for the origin of this substitution effect, its apparent specific influence on the d~2 orbital, or the relative impact of different substituents. When VO(acac-Cl)2 and VO(acac-Br)2 are dissolved in solvents which are better coordinating agents than ben-
zene, the values of vl and vz are shifted to lower energies, with v, being shifted somewhat more than v2, as shown in Table I. The greater effect which solvation has on Vl causes the difference between v~ and rE, OI,2, to change as well, as shown by the data in Table 1. When the DL2 values for VO(acac-Cl)2 and VO(acac-Br)2 are plotted against the solvent Donor Number, the results shown in Fig. I are obtained. Both of these plots are linear, although the data are somewhat more scattered than are those for VO(acac)2[12], which are plotted in the same figure for comparison. From this figure it can also be seen that the slopes of the plots for VO(acac-Cl)2 and VO(acac-Br)2 are greater than is that for VO(acac)2. A linear least squares computer program yields values of 12.4, 12.6 and 10.0 for these three slopes, while similar calculations on data from the literature yield slopes of 9.5, 10.2 and 22.8 for VO(bza)2[9], VO(dbm)2[9], and VO(ffa)2[10], respectively. These results indicate that the Di.2 values for VO(acac-C1)2, VO(acac-Br)2 and VO(tfa)2 are less sensitive to the donor ability of the solvent than are those for VO(acac)2, VOOaza)2 and VO(dbm)2. Although early workers in this field have commented on the lack of theoretical significance for the parameter DL2, recent studies [8, 9] have shown that its value is expected to increase with increasing distortion of the VO(acac)2 square pyramid toward octahedral geometry due to the substitution of the donor ligand. Thus, the present results seem to suggest that VO(acac-Cl)2, VO(acac-Br)2 and VO(tfa)2 undergo distortion from square pyramid to octahedron (and therefore substitution?) less readily than does VO(acac)2 itself. Recently other workers[10] have concluded that VO(tfa)2 coordinates more readily than VO(acac)2 based on the smaller temperature dependence of D~.2 for the former compound. Their results for VO(ffa)2 at 25°C indicate a smaller dependence of Dr.2 on the donor molecule for VO(tfa)2 than for VO(acac)2, however, in agreement with the present study. By way of reconciling this discrepancy, it seems that the dependence of DI.2 on the nature of the solvent should provide a better measure of coordinating ability than its dependence on temperature, since the latter is a second order effect.
40
3O
E ~_2o o a
to
J
I I
i
I 2
i
I
3
l
I
4
Ol,2, k K
Fig. 1. Plot of Donor Number vs DI.2 for VO(acach (@), VO(acac-C1)2(0), and VO(acac-Br)2(~) in various solvents.
Solvent effects on the electronic spectra of vanadyl(IV) fl-diketonate derivatives The conclusion, based on the D~.2-Donor Number relationship, that VO(acac-Cl)2, VO(acac-Br)2 and VO(tfa)2 are poorer acceptors than VO(acac)2 is, however, quite surprising since the greater electronegativities of fluorine, chlorine, and bromine (relative to H) would seem to make these species better acceptors by withdrawing electrons from vanadium. To date, only a single formation constant (Kt) has been measured for any of these acceptors (between VO(tfa)2 and 4-methyl pyridine N-oxide) and this is greater than the Kj for VO(acac)2 with the same donor (500_+ 150 vs 76 + - 3)[17]. It may be, however, that using the slopes of D,.2-Donor Number plots as we have done, to draw conclusions about the coordinating ability of different vanadyl /3diketones is invalid because of differences in some other factor such as the geometry of the adduct formed. In this regard, it is notable that the geometry of the adduct varies with the nature of the acceptor as well as with that of the donor. For example, 4-methyl pyridine N-oxide bonds to VO(tfah cis to the vanadyl oxygen but to VO(acac)z predominantly in the trans position[17] and substituted pyridines form either cis or trans isomers with VO(acac)2 although with no apparent relationship to the electronic or steric properties of the donor[23, 24]. Clearly the resolution of this question must await further studies on the geometries and formation constants of adducts of VO(acac-C1)2, VO(acac-Br)2, and VO(tfa)2 with a variety of donor species similar to those which have been carried out on VO(acac)2 itself [25] Acknowledgement--The authors wish to thank Dr. Charles H. Spink for helpful discussions.
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