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Single crystal magnetic studies of cobalt(II) schiff-base chelate compounds
Volume
22, number
CHEMICAL
2
PHYSICS
SINGLE CRYSTAL OF COBALT(I1)
Department
MAGNETIC
SCHIFF-BASE
1 Octob&
LETTERS
1973
STUDIES
CHELATE
COMPOUNDS
KS. MURRAY and R.M. SHEAHAN ofChemistry, Monash University, Clayton, Victoria, Australia 3168
Received
19 July 1973
Single crystal susceptibility studies on the oxygen-inactive compounds Co salen and Co salen (py) have been made over the temperature range 320-100°K. Their molecular susceptibility ellipsoids are similar with K, pointing along the axial-bond direction, and K,, KY between the in-plane donor atoms. The unusual temperature dependence of Ki arises due to mixing of excited quartet states into the ground doublet state.
1. Introduction There is much current interest in the structure and reactivity of cobalt(H) complexes with tetradentate chelating ligands such as Schiff-bases [l-4] , phthalocyanines [S, 61, porphyrins [7, 81, etc. The main reason for this is because these compounds, under certain conditions, can reversibly coordinate molecular oxygen and they, therefore, serve as potential model systems for biological oxygen-carriers [9] . We are particularly interested in finding the relation, if any, between the electronic structure of these essentially lowspin d7 systems and this novel chemical reactivity. We are, therefore, studying the magnetic and spectral properties of single crystal samples of some of these Schiff-base chelates. Some of the compounds are inactive to oxygen binding in the solid state whilst others are active. Most of the compounds are active to some extent in solution [lo] . We report here variable temperature susceptibility studies on single-crystals of two oxygen-inactive compounds, Co salen (salen = N, N’ ethylenebis(salicylaldiminato) oxygen) and Co salen (py) (py = pyridine). Co salen is monoclinic and has a binuclear structure with square-pyramidal S-coordination around each cobalt atom [ 11, 121. Co salen is orthorhombic, mononuclear, and also has square-pyramidal coordination around cobalt [ 131 (fig. 1). The magnetic anisotropy measurements have given accurate values and directions of the susceptibility tensors for the 406
Co salen /
/
Co salen (py)
Fig. 1. Molecular (Coo0 distances
structure of Co salen and Co salen in A units).
cobalt(I1) ion in these compounds, which together with their temperature dependence, allows rigorous
Volume
22, number
2
CHEMICAL
PHYSICS
1 October
LETTERS
1973
comparison with theory. A number of ESR studies have recently been carried out on these and other cobalt(I1) chelates in powder form or in frozen solvents [2-7, 14, 151. The g tensor anisotropy has been obtained to varying degrees of accuracy, with some disagreements, and then related to the ground state d-orbital configuration. The present single-crystal study yields additional information on the ordering of the other d-orbitals.
2. Experimental Large crystals of Co salen were obtained from dimethylformamide solution using a thermal gradient technique in an evacuated glass-tube. Co salen crystals were similarly obtained from pyridine solution. Crystal axes were determined by X-ray goniometry. Magnetic anisotropy measurements were made on several crystals of each compound, using a null-deflection torsion technique similar to that originated by Stout and Griffel [ 161. The quartz suspension tibres were calibrated with the accurately known susceptibility values of CuSO4 . 5H20. Measurements were made in the liquid nitrogen range using a cryostat based on a design by Figgis and Nyholm [ 171. In the case of monoclinic Co salen the crystals were mounted along the unique b axis, and along X2, the direction of which was determined using the wheel device of Gerloch and Quested [ 181. The orthorhombic Co salen crystals were mounted along the a, b and c crystal axes, the third direction serving as a convenient check on the other two. Difficulties were encountered in the case of Co salen with static electricity which caused the sample to stick against the cryostat inner wall. These problems were overcome by evaporating a small volume of acetone in the sample chamber just befoie taking measurements.
3. Results and discussion The principal molecular susceptibility ellipsoids (Kj) for Co salen and Co salen are shown in fig. 2. These are obtained from the measured crystal susceptibilities (Xi) by tensor transformation as described by Lonsdale and Krishnan [ 191. In the case of Co salen,
Co salen
(inactive)
Fig. 2. Molecular susceptibility ellipsoids salen (Ki in lo6 cm3 mole-‘).
Co salen
(py)
for Co salen and Co
which is monoclinic, the modified procedure of Gerloch and Quested [ 181 was employed; a computer program was devised to find the best set of direction cosines which fitted the Ki data over the whole temperature range. Though an axial assumption gave a good fit over a small temperature range, the rhombic assumption gave an excellent fit at all temperatures. As shown in fig. 2, the smallest susceptibility, K,, points along the Co-O (bridging) direction whilst K, and KY are directed midway between the donor atoms. In the case of Co salen( which has higher molecular symmetry, the orientation of the magnetic ellipsoid was easier to determine. It is also rhombic, with K, pointing along the Co-N (pyridine) direction. The anisotropy in the metal-ligand plane is smaller than it is in Co salen. In both cases, K, is the largest susceptibility. These results show that the orbital energy levels are in the order d,, < dvZ < d,z__,,z, d,z < dxY, with the relative positions of the dxzPv2 and d,z orbitals uncertain at this stage. Further calculations, under C2, symmetry, are proceeding to resolve this problem. The g tensor components for Co salen in pyridine solution at 77°K have been measured previously [4, 20, 211 and are given for comparison in table 1. The rhombic nature has been resolved in one study and the results correlate well with the present /_+values. The g directions were assumed to be the same as those directly determined here for the pi values. It is not possible to make a meaningful comparison of the present results for Co salen (inactive) with published g values, since those recently reported [20-221 for Co 407
1 October 1973
CHEMICAL PHYSICS LETTERS
Volume 22, number 2
Table 1 Co salen (inactive)
Co salen
pi (BM)
300°K
100°K
hi (BM)
300°K
100°K
4 pY
2.54 2.08 1.81
2.12 1.70 1.51
flX
flY
2.50 2.32 2.05
1.99 1.84 1.69
PZ
flZ
Co salen/pyridine gi
refs. [ 3, 20)
gx gY gz
2.354 2.27 2.028
2.26 2.26 2.03
b
Fig. 3. Reciprocal susceptibility versus temperature curves for (a) Co salen and (b) Co salen(
408
(77°K)
ref. [4]
Volume 22, number 2
CHEMICAL PHYSICS LETTERS
salen doped into Ni salen powders and crystals differ from each other. Furthermore, the binuclear structures of Co salen and Ni salen are markedly different [ 11, 12,231 and the nature of the Co salen species present is not clear. The temperature variation of the molecular susceptibilities for the two compounds is displayed in fig. 3.
1 October 1973
Plots of KIY1 ver&s T(“K) for each are similar; broad maxima are exhibited at ca. 300’K for Co salen and ca. 270°K for Co salen( The corresponding graphs of magnetic moment values (fig. 4) show a pronounced increase at higher temperatures. The magnetic moments correspond to essentially low-spin Co(U) (derived from *E, term in 0, symmetry). However,
)1x
_I a
1.4 -
2.6
1.6 1
s 100
140
160
220
1 260
1 300
T” Fig. 4. Magnetic moment curves for (a) Co salen and (b) Co salen( 409
Volume 22, number 2
CHEMICAL PHYSICS LETTERS
the unusual temperature dependence of the susceptibility cannot be ascribed solely to this state and derives, we suggest, from the influence of excited quartet states (from 4T,, in 0, symmetry). This explains the rapidly increasing pi values at high temperatures. The extent of the quartet state interaction appears to be dependent on the nature of the axial group, an effect also observed in iron-porphyrin systems [24] . Further single-crystal studies are in progress on related oxygen-active Co(I1) chelates.
Acknowledgement The help of Mr. J.E. Davies with computer programming is gratefully acknowledged. This work was carried out during the tenure of an Australian Research Grant Committee award.
References 111 A. Earnshaw, P.C. Hewlett, E.A. King and L.F. Larkworthy, J. Chem. Sot. A (1968) 241.
121 B.M. Hofmann, D. Diemente and F. Basolo, J. Am. Chem. Sot. 92 (1970) 61. 13: C. Busetto. F. Cariati, A. Fusi, M. Gullotti, F. Morazzoni, A. Pasini, R. Ugo and V. Valenti, J. Chem. Sot. Dalton (1973) 754. 141 Ei-lchiro Ochiai, Chem. Commun. (1972) 489; J. Inorg. Nucl. Chem. 35 (1973) 1727. I51 R.L. Martin and S. Mitra, Chem. Phys. Letters 3 (1969) 183. [61 L.M. Enpelhardt and M. Green, J. Chem. Sot. Dalton (1972) 724.
410
1 October 1973
[71 G.N. la Mar and F.A. Walker, J. Am. Chem. Sot. 95
(1973) 1790. 181 H.C. Stynes and J.A. Ibers, J. Am. Chem. Sot. 94 (1972)
1559. [91 E. Bayer and P. Schretzmann,
Structure Bonding 2 (1967) 181. [lOI A.E. Martell and M. Calvin, Chemistry of the metal chelate compounds (Prentice-Hall, Englewood Cliffs, 1952) ch. 8. 1111 S. Bruckner, M. Calligaris, G. Nardin and L. Randaccio, Acta Cryst. B25 (1969) 1671. (121 R. de Iasi, B. Post and S.L. Holt, Inorg. Chem. 10 (1971) 1498. [I31 M. Calligaris, D. Minichelli, G. Nardin and L. Randaccio, J. Chem. Sot. A (1970) 2411. [I41 C. Busetto, F. Cariati, P. Fantucci, D. Galizzioli, F. Morazzoni and V. Valenti, Gazz. Chim. Ital. 102 (1972) 1040. [I51 J.R. Pilbrow and M.E. Winfield, Mol. Phys. 25 (1973), to be published. I161 J.W. Stout and M. Griffel, J. Chem. Phys. 18 (1950) 1449. [I71 B.N. Figgis and R.S. Nyholm, J. Chem. Sot. (1959) 331. [I81 M. Gerloch and P.N. Quested, J. Chem. Sot. A (1971) 2307. 1191 K.S. Krishnan and K. Lonsdale, Proc. Roy. Sot. Al56 (1936) 597. [201 L.M. Engelhardt, J.D. Duncan and M. Green, Inorg. Nucl. Chem. Letters 8 (1972) 725. [211 C. Busetto, F. Cariati, P.C. Fantucci, D. Gallizioli and F. Morazzoni, Inorg. Nucl. Chem. Letters 9 (1973) 313. (221 A. von Zelewsky and H. Fierz, Helv. Chim. Acta 56 (1973) 977. (231 L.M. Shkolnikova, E.M. Yumal, E.A. Shugam and V.A. Voblikova, Zh. Strukt. Khim. 11 (1970) 886 [English transl. J. Struct. Chem. 11 (1970) 8191. 1241 D.W. Smith and R.J.P. Williams, Structure Bonding 7 (1970) 1.
22, number
CHEMICAL
2
PHYSICS
SINGLE CRYSTAL OF COBALT(I1)
Department
MAGNETIC
SCHIFF-BASE
1 Octob&
LETTERS
1973
STUDIES
CHELATE
COMPOUNDS
KS. MURRAY and R.M. SHEAHAN ofChemistry, Monash University, Clayton, Victoria, Australia 3168
Received
19 July 1973
Single crystal susceptibility studies on the oxygen-inactive compounds Co salen and Co salen (py) have been made over the temperature range 320-100°K. Their molecular susceptibility ellipsoids are similar with K, pointing along the axial-bond direction, and K,, KY between the in-plane donor atoms. The unusual temperature dependence of Ki arises due to mixing of excited quartet states into the ground doublet state.
1. Introduction There is much current interest in the structure and reactivity of cobalt(H) complexes with tetradentate chelating ligands such as Schiff-bases [l-4] , phthalocyanines [S, 61, porphyrins [7, 81, etc. The main reason for this is because these compounds, under certain conditions, can reversibly coordinate molecular oxygen and they, therefore, serve as potential model systems for biological oxygen-carriers [9] . We are particularly interested in finding the relation, if any, between the electronic structure of these essentially lowspin d7 systems and this novel chemical reactivity. We are, therefore, studying the magnetic and spectral properties of single crystal samples of some of these Schiff-base chelates. Some of the compounds are inactive to oxygen binding in the solid state whilst others are active. Most of the compounds are active to some extent in solution [lo] . We report here variable temperature susceptibility studies on single-crystals of two oxygen-inactive compounds, Co salen (salen = N, N’ ethylenebis(salicylaldiminato) oxygen) and Co salen (py) (py = pyridine). Co salen is monoclinic and has a binuclear structure with square-pyramidal S-coordination around each cobalt atom [ 11, 121. Co salen is orthorhombic, mononuclear, and also has square-pyramidal coordination around cobalt [ 131 (fig. 1). The magnetic anisotropy measurements have given accurate values and directions of the susceptibility tensors for the 406
Co salen /
/
Co salen (py)
Fig. 1. Molecular (Coo0 distances
structure of Co salen and Co salen in A units).
cobalt(I1) ion in these compounds, which together with their temperature dependence, allows rigorous
Volume
22, number
2
CHEMICAL
PHYSICS
1 October
LETTERS
1973
comparison with theory. A number of ESR studies have recently been carried out on these and other cobalt(I1) chelates in powder form or in frozen solvents [2-7, 14, 151. The g tensor anisotropy has been obtained to varying degrees of accuracy, with some disagreements, and then related to the ground state d-orbital configuration. The present single-crystal study yields additional information on the ordering of the other d-orbitals.
2. Experimental Large crystals of Co salen were obtained from dimethylformamide solution using a thermal gradient technique in an evacuated glass-tube. Co salen crystals were similarly obtained from pyridine solution. Crystal axes were determined by X-ray goniometry. Magnetic anisotropy measurements were made on several crystals of each compound, using a null-deflection torsion technique similar to that originated by Stout and Griffel [ 161. The quartz suspension tibres were calibrated with the accurately known susceptibility values of CuSO4 . 5H20. Measurements were made in the liquid nitrogen range using a cryostat based on a design by Figgis and Nyholm [ 171. In the case of monoclinic Co salen the crystals were mounted along the unique b axis, and along X2, the direction of which was determined using the wheel device of Gerloch and Quested [ 181. The orthorhombic Co salen crystals were mounted along the a, b and c crystal axes, the third direction serving as a convenient check on the other two. Difficulties were encountered in the case of Co salen with static electricity which caused the sample to stick against the cryostat inner wall. These problems were overcome by evaporating a small volume of acetone in the sample chamber just befoie taking measurements.
3. Results and discussion The principal molecular susceptibility ellipsoids (Kj) for Co salen and Co salen are shown in fig. 2. These are obtained from the measured crystal susceptibilities (Xi) by tensor transformation as described by Lonsdale and Krishnan [ 191. In the case of Co salen,
Co salen
(inactive)
Fig. 2. Molecular susceptibility ellipsoids salen (Ki in lo6 cm3 mole-‘).
Co salen
(py)
for Co salen and Co
which is monoclinic, the modified procedure of Gerloch and Quested [ 181 was employed; a computer program was devised to find the best set of direction cosines which fitted the Ki data over the whole temperature range. Though an axial assumption gave a good fit over a small temperature range, the rhombic assumption gave an excellent fit at all temperatures. As shown in fig. 2, the smallest susceptibility, K,, points along the Co-O (bridging) direction whilst K, and KY are directed midway between the donor atoms. In the case of Co salen( which has higher molecular symmetry, the orientation of the magnetic ellipsoid was easier to determine. It is also rhombic, with K, pointing along the Co-N (pyridine) direction. The anisotropy in the metal-ligand plane is smaller than it is in Co salen. In both cases, K, is the largest susceptibility. These results show that the orbital energy levels are in the order d,, < dvZ < d,z__,,z, d,z < dxY, with the relative positions of the dxzPv2 and d,z orbitals uncertain at this stage. Further calculations, under C2, symmetry, are proceeding to resolve this problem. The g tensor components for Co salen in pyridine solution at 77°K have been measured previously [4, 20, 211 and are given for comparison in table 1. The rhombic nature has been resolved in one study and the results correlate well with the present /_+values. The g directions were assumed to be the same as those directly determined here for the pi values. It is not possible to make a meaningful comparison of the present results for Co salen (inactive) with published g values, since those recently reported [20-221 for Co 407
1 October 1973
CHEMICAL PHYSICS LETTERS
Volume 22, number 2
Table 1 Co salen (inactive)
Co salen
pi (BM)
300°K
100°K
hi (BM)
300°K
100°K
4 pY
2.54 2.08 1.81
2.12 1.70 1.51
flX
flY
2.50 2.32 2.05
1.99 1.84 1.69
PZ
flZ
Co salen/pyridine gi
refs. [ 3, 20)
gx gY gz
2.354 2.27 2.028
2.26 2.26 2.03
b
Fig. 3. Reciprocal susceptibility versus temperature curves for (a) Co salen and (b) Co salen(
408
(77°K)
ref. [4]
Volume 22, number 2
CHEMICAL PHYSICS LETTERS
salen doped into Ni salen powders and crystals differ from each other. Furthermore, the binuclear structures of Co salen and Ni salen are markedly different [ 11, 12,231 and the nature of the Co salen species present is not clear. The temperature variation of the molecular susceptibilities for the two compounds is displayed in fig. 3.
1 October 1973
Plots of KIY1 ver&s T(“K) for each are similar; broad maxima are exhibited at ca. 300’K for Co salen and ca. 270°K for Co salen( The corresponding graphs of magnetic moment values (fig. 4) show a pronounced increase at higher temperatures. The magnetic moments correspond to essentially low-spin Co(U) (derived from *E, term in 0, symmetry). However,
)1x
_I a
1.4 -
2.6
1.6 1
s 100
140
160
220
1 260
1 300
T” Fig. 4. Magnetic moment curves for (a) Co salen and (b) Co salen( 409
Volume 22, number 2
CHEMICAL PHYSICS LETTERS
the unusual temperature dependence of the susceptibility cannot be ascribed solely to this state and derives, we suggest, from the influence of excited quartet states (from 4T,, in 0, symmetry). This explains the rapidly increasing pi values at high temperatures. The extent of the quartet state interaction appears to be dependent on the nature of the axial group, an effect also observed in iron-porphyrin systems [24] . Further single-crystal studies are in progress on related oxygen-active Co(I1) chelates.
Acknowledgement The help of Mr. J.E. Davies with computer programming is gratefully acknowledged. This work was carried out during the tenure of an Australian Research Grant Committee award.
References 111 A. Earnshaw, P.C. Hewlett, E.A. King and L.F. Larkworthy, J. Chem. Sot. A (1968) 241.
121 B.M. Hofmann, D. Diemente and F. Basolo, J. Am. Chem. Sot. 92 (1970) 61. 13: C. Busetto. F. Cariati, A. Fusi, M. Gullotti, F. Morazzoni, A. Pasini, R. Ugo and V. Valenti, J. Chem. Sot. Dalton (1973) 754. 141 Ei-lchiro Ochiai, Chem. Commun. (1972) 489; J. Inorg. Nucl. Chem. 35 (1973) 1727. I51 R.L. Martin and S. Mitra, Chem. Phys. Letters 3 (1969) 183. [61 L.M. Enpelhardt and M. Green, J. Chem. Sot. Dalton (1972) 724.
410
1 October 1973
[71 G.N. la Mar and F.A. Walker, J. Am. Chem. Sot. 95
(1973) 1790. 181 H.C. Stynes and J.A. Ibers, J. Am. Chem. Sot. 94 (1972)
1559. [91 E. Bayer and P. Schretzmann,
Structure Bonding 2 (1967) 181. [lOI A.E. Martell and M. Calvin, Chemistry of the metal chelate compounds (Prentice-Hall, Englewood Cliffs, 1952) ch. 8. 1111 S. Bruckner, M. Calligaris, G. Nardin and L. Randaccio, Acta Cryst. B25 (1969) 1671. (121 R. de Iasi, B. Post and S.L. Holt, Inorg. Chem. 10 (1971) 1498. [I31 M. Calligaris, D. Minichelli, G. Nardin and L. Randaccio, J. Chem. Sot. A (1970) 2411. [I41 C. Busetto, F. Cariati, P. Fantucci, D. Galizzioli, F. Morazzoni and V. Valenti, Gazz. Chim. Ital. 102 (1972) 1040. [I51 J.R. Pilbrow and M.E. Winfield, Mol. Phys. 25 (1973), to be published. I161 J.W. Stout and M. Griffel, J. Chem. Phys. 18 (1950) 1449. [I71 B.N. Figgis and R.S. Nyholm, J. Chem. Sot. (1959) 331. [I81 M. Gerloch and P.N. Quested, J. Chem. Sot. A (1971) 2307. 1191 K.S. Krishnan and K. Lonsdale, Proc. Roy. Sot. Al56 (1936) 597. [201 L.M. Engelhardt, J.D. Duncan and M. Green, Inorg. Nucl. Chem. Letters 8 (1972) 725. [211 C. Busetto, F. Cariati, P.C. Fantucci, D. Gallizioli and F. Morazzoni, Inorg. Nucl. Chem. Letters 9 (1973) 313. (221 A. von Zelewsky and H. Fierz, Helv. Chim. Acta 56 (1973) 977. (231 L.M. Shkolnikova, E.M. Yumal, E.A. Shugam and V.A. Voblikova, Zh. Strukt. Khim. 11 (1970) 886 [English transl. J. Struct. Chem. 11 (1970) 8191. 1241 D.W. Smith and R.J.P. Williams, Structure Bonding 7 (1970) 1.