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On the structure of zirconium tetrahydroborate and related molecules via vibrational spectroscopy
Journal of Molecular Structure, 33 (1976) 91-95 @Elsevier Scientific Publishing Company, Amsterdam -
Printed in The Netherlands
ON THE STRUCJXJRE OF ZIRCONIUM TETRAHYDROBORATE RELATED MOLECULES VIA VIBRATIONAL SPECTROSCOPY
AND
B. D. JAMES Department of Inorganic and Analytical Chemistry, La Trobe University, Bundoora, Vie 3083 (Australia) B. E. SMITH Inorganic Chemistry Laboratory, South Parks Rd., Oxford OX1 3QR (Gt. Britain) H. F. SHXJRVELL Department of Chemistry, Queen’s University. Kingston, Ontario (Canada) (Received 10 November 1975)
ABSTRACT It is suggested that the vibrational spectra of Zr(BH,), in the vapour and condensed phases may best be interpreted on the basis of a T rather than a Ta symmetrypoint group. The existence of group frequencies, other than skeletal stretching modes, in the 500-650 cm-’ region for triply bridged M(BH,), systems (M = Zr, Hf, U) is confirmed. INTRODUCI’ION
In recent years, the tetrahydroborates M(BH4)4 (M = Zr, Hf, U) have attracted considerable attention from workers utiltiing a wide variety of techniques, including X-ray [ 1, 21 electron [ 33 and neutron [ 41 diffraction methods as well as vibrational [S-lo] NMR [ 10,111 and optical [ 121 spectroscopies. It has generally been held that Zr(BH4)4 conforms to strict Td symmetry. Our current results from vibrational spectroscopy, however, are best interpreted on the basis of the slightly lower 2’ symmetry. For this reason we believe that a discussion of the structural implications of the current data is pertinent at this time. A full vibrational assignment will be presented later, together with force constant calculations (currently being completed). EXPERIMENTAL
Zr(BH4&, and Zr(BD4)4 were prepared and handled by the methods described previously 1131. Infrared spectra were recorded using a Perkin-Elmer 225 spectrometer and Raman spectra with a Cary 82 instrument employing the green (514.5 nm) line of a Model 52A argon laser. The IR and Raman spectra of Zr (B&)4 and Zr(BD& in nujol solution and in the solid phase (at low temperatures) have now been added to the data presented in ref. 7.
DISCUSSION
The vibrational spectra of Zr(BH& have been shown [7] to exhibit fundamentals in the regions: 2560-2580 cm-’ (B-H terminal stretches), 2100-2250 cm-’ (B-H bridge stretches), 1000-1300 cm-’ (modes probably due to some mixture of HBH angle bending and Zr-H stretching), 500-550 cm-’ (ZFB skeletal stretching) and ca. 200 cm-’ (B-Zr-B skeletal bending). We wish to report an additional set of fundamentals characteristic of Zr(BI-&), in the region 500-650 cm-‘. In the vapour phase IR spectrum [7] a very weak shoulder at 560 cm-’ was left unassigned. Marks et al. [S] reported an analogous feature in the Raman spectrum of liquid Zr(BH4)4 which was also left unassigned. We have since observed similar bands in the IR and Raman spectra of this compound in nujol solution and also a number of features in the 500-650 cm-’ region in solid phase IR and Raman spectra (see later) which are well resolved. Clearly these are fundamentals. Force constant calculations utilizing a simple valence force field and a wide range of force constant values have consistently generated frequencies in this region. Keiderling et al. [9] have observed a band at 570 cm-’ in the Raman spectrum of solid phase Hf(BH4), which was tentatively assigned to E and T2 fundamentals. It is probable that the 410 cm- ’ frequency observed in the optical spectrum of U(BD,), in a Hf(BD4)4 host lattice [ 121 is observed from a similar source. It is apparent then, that as well as the modes described in ref. 7, there is an additional set of fundamentals in the region 500-650 cm-’ characteristic of all triply bridged systems M(BH4)4 (M = Zr, Hf, U). Our current results for Zr(BH& are consistent with a structure consisting of triple hydrogen bridges between the zirconium atom and each of the four tetrahedrally oriented boron atoms. However, it is suggested that the molecule as a whole conforms to a T rather than Td symmetry. The observed IR and Raman spectra are relatively simple as expected for a molecule of high symmetry. However, there appear to be more active fundamentals than are predicted by the selection rules of a Ta point group. For this reason we consider the slightly lower symmetry model. Representatives of T and Td structures are shown in Fig. 1. These differ in the relative orientations of the twelve bridging hydrogens. There are two possible structures which conform to T, symmetry: all dihedral angles H,-B-&-B may be equal to O” (Fig. l(a)) or to SO” (Fig. l(b)). For T symmetry all such angles are equal but are neither 0” nor 60” (Fig. l(c)). The overall IR and Raman selection rules pertaining to the 57 degrees of vibrational freedom of the T and T, structural models of Zr(BH4), are: T, Gb = 4A1 (R) + A2 (ia) + 5E(R) + 5T1 (ia) + 9T2 (IR, R) T
Lib = 5A (R) + 5E(R) + 14T (IR, R)
In the following, we compare the specific predictions of the two models in the 500-650 cm-’ and 1000-1300 cm-’ regions. It should be noted that careful
C
Fig. 1. Structures of Zr(BH,), corresponding to Td, (a) and (b) and T, (c) symmetries. In the upper set of diagrams the perspective is down a ZrB axis. The Hb--B-ZrB angles (zw) are: in (a), 0” ; (b), 60” ; (c), 0” < w < 60”. The lower set of diagrams show the view along a B-B axis with the Zr atom positioned below this axis,
a
94
consideration has been given to the question of impurities and it is felt that it is improbable for the features found in addition to those expected for T, symmetry to have originated from such a source. In the 500-650 cm-’ region, the Td point group predicts just two (2T2) IR bands and four (A, + E + 22’,) Raman lines. This region of the vapour phase and solution spectra is dominated by skeletal stretching modes and weaker features are not well resolved. The solid phase spectra, however, show a number of distinct bands and the following are assigned to Zr(1*BHq)4: IR-564,547,581 cm-‘, Raman - 504, 533,543, 581,610 cm-‘. Both IR and Raman observations are incompatible with Td symmetry. The results are, however, explicable in terms of T symmetry. This model, on the basis of the modes described earlier, predicts the observation of five modes in this region, three of which (32’) are IR active and five of which (A + E + 3T) are Raman active. A total of six bands are observed above, however, and it seems reasonable that this slight additional complexity is due to the observation of torsional vibrations. Torsions of BH4 groups about Zr--B axes are of A(R) and T(IR, R) symmetry species which correlate with the A2 (ia) and T1 (ia) species of the Td point group. In the 1000-1300 cm-’ region the T, point group predicts a maximum of three bands in the IR spectrum while T symmetry predicts a maximum of five IR-active modes. The IR spectrum of Zr(BH& in nujol solution exhibits five features in this region: at 1286,1213,1170,1098 and 1056 cm-‘. The vapour phase spectrum shows four distinct bands (1288,1218,1155 and 1034 cm-‘) as does the solid phase IR (1286,1217,1178 and 1058 cm-‘). For this data to be consistent with the Td model one (for vapour and solid phase) or two (for nujol solution) features must be explained as overtones or combinations. Our preference is to accept all bands as fundamentals, compatible with the T structure. It is interesting that in an electron diffraction of 38” was deduced (consistent with study [3] a dihedral angle H, -B-Zr-B T symmetry) though the authors placed little reliance on the accuracy of this value. The IR spectra of Hf(BH4)4 in the vapour phase [ 7,9] and nujol solution [8], together with Raman spectra of a benzene solution [7] neat liquid [S] and single crystal [9] have been interpreted on the basis of Td symmetry. The room temperature solid phase Raman data [9] do not seem to show the resolution nor complexity apparent in the Zr(BH,), spectra [ 141. On the other hand, the vapour phase and solution data that are available do show strong similarities to those of the zirconium system. Reference has been made [9] to a neutron diffraction study of Hf(BHa)a which has been interpreted in terms of a T, molecular symmetry. Very little has been reported on the vibrational spectra of U(BH4)4T Nevertheless it has been clearly demonstrated that there is a considerable structural change on passing from the vapour phase to the solid phase [6] and the structural implications of the solid phase IR data deduced with the benefit of X-ray results [2] were consistent with the conclusions of a neutron
95
diffraction study [4]. The only vapour phase IR spectrum of U(BH4)4 available [6] is of poor quality relative to the zirconium and hafnium spectra (due in part to the lower volatility of the uranium compound) and the subtle distinction between Z’, and T, models cannot be made at this stage. In conclusion, we feel that the evidence relating to Zr(BH,), in the solid phase, vapour and solution is sufficient to indicate a preference for a structure of T symmetry rather than one of T, symmetry. This structural model would also seem worthy of consideration in relation to studies of related M(BH4)4 systems. ACKNOWLEDGEMENTS
This work was supported by the Australian Research Grants Committee. B.E.S. acknowledges receipt of a Commonwealth of Australia Postgraduate Research Award. REFERENCES 1 P. H. Bird and M. R. Churchill, Chem. Commun., (1967) 403. 2 E. R. Bernstein, T. A. Keiderling, S. J. Lippard and J. J. Mayerle, J. Amer. Chem. Sot., 94 (1972) 2552. 3 V. Plato and K. Hedberg, Inorg. Chem., 10 (1971) 590. 4 E. R. Bernstein, W. C. Hamilton, T. A. Keiderling, S. J. La Placa, S. J. Lippard and J. J. Mayerle, Inorg. Chem., 11 (1972) 3009. 5 B. E. Smith and B. D. James, Inorg. Nucl. Chem. Lett., 7 (1971) 857. 6 B. D. James, B. E. Smith and M. G. H. Wallbridge, J. Mol. Struct., 14 (1972) 327. 7 N. Davies, M. G. H. Wallbridge, B. E. Smith and B. D. James, J. Chem. Sot. Dalton, (1973) 162. 8 T. J. Marks, W. J. Kennelly, J. R. Kolb and L. A. Shimp, Inorg. Chem., 11 (1972) 2540. 9 T. A. Keiderhng, W. T. Wozniak, R. S. Gay, D. Jurkowitz, E. R. Bernstein, S. J. Lippard and T. G. Spiro, Inorg. Chem., 14 (1975) 576. 10 T. J. Marks and L. A. Shimp, J. Amer. Chem. Socr, 94 (1972) 1542. 11 M. Ehemann and H. NSth, Z. Anorg. Ailg. Chem., 386 (1971) 87. 12 E. R. Bernstein and T. A. Keiderling, J_ Chem. Phys., 59 (1973) 2105. 13 B. D. James and B. E. Smith, Syn. React. Inorg. Metal-Org. Chem., 4 (1974) 461. 14 B. D. James, B. E. Smith and H. F. Shurvell, unpublished work.
Printed in The Netherlands
ON THE STRUCJXJRE OF ZIRCONIUM TETRAHYDROBORATE RELATED MOLECULES VIA VIBRATIONAL SPECTROSCOPY
AND
B. D. JAMES Department of Inorganic and Analytical Chemistry, La Trobe University, Bundoora, Vie 3083 (Australia) B. E. SMITH Inorganic Chemistry Laboratory, South Parks Rd., Oxford OX1 3QR (Gt. Britain) H. F. SHXJRVELL Department of Chemistry, Queen’s University. Kingston, Ontario (Canada) (Received 10 November 1975)
ABSTRACT It is suggested that the vibrational spectra of Zr(BH,), in the vapour and condensed phases may best be interpreted on the basis of a T rather than a Ta symmetrypoint group. The existence of group frequencies, other than skeletal stretching modes, in the 500-650 cm-’ region for triply bridged M(BH,), systems (M = Zr, Hf, U) is confirmed. INTRODUCI’ION
In recent years, the tetrahydroborates M(BH4)4 (M = Zr, Hf, U) have attracted considerable attention from workers utiltiing a wide variety of techniques, including X-ray [ 1, 21 electron [ 33 and neutron [ 41 diffraction methods as well as vibrational [S-lo] NMR [ 10,111 and optical [ 121 spectroscopies. It has generally been held that Zr(BH4)4 conforms to strict Td symmetry. Our current results from vibrational spectroscopy, however, are best interpreted on the basis of the slightly lower 2’ symmetry. For this reason we believe that a discussion of the structural implications of the current data is pertinent at this time. A full vibrational assignment will be presented later, together with force constant calculations (currently being completed). EXPERIMENTAL
Zr(BH4&, and Zr(BD4)4 were prepared and handled by the methods described previously 1131. Infrared spectra were recorded using a Perkin-Elmer 225 spectrometer and Raman spectra with a Cary 82 instrument employing the green (514.5 nm) line of a Model 52A argon laser. The IR and Raman spectra of Zr (B&)4 and Zr(BD& in nujol solution and in the solid phase (at low temperatures) have now been added to the data presented in ref. 7.
DISCUSSION
The vibrational spectra of Zr(BH& have been shown [7] to exhibit fundamentals in the regions: 2560-2580 cm-’ (B-H terminal stretches), 2100-2250 cm-’ (B-H bridge stretches), 1000-1300 cm-’ (modes probably due to some mixture of HBH angle bending and Zr-H stretching), 500-550 cm-’ (ZFB skeletal stretching) and ca. 200 cm-’ (B-Zr-B skeletal bending). We wish to report an additional set of fundamentals characteristic of Zr(BI-&), in the region 500-650 cm-‘. In the vapour phase IR spectrum [7] a very weak shoulder at 560 cm-’ was left unassigned. Marks et al. [S] reported an analogous feature in the Raman spectrum of liquid Zr(BH4)4 which was also left unassigned. We have since observed similar bands in the IR and Raman spectra of this compound in nujol solution and also a number of features in the 500-650 cm-’ region in solid phase IR and Raman spectra (see later) which are well resolved. Clearly these are fundamentals. Force constant calculations utilizing a simple valence force field and a wide range of force constant values have consistently generated frequencies in this region. Keiderling et al. [9] have observed a band at 570 cm-’ in the Raman spectrum of solid phase Hf(BH4), which was tentatively assigned to E and T2 fundamentals. It is probable that the 410 cm- ’ frequency observed in the optical spectrum of U(BD,), in a Hf(BD4)4 host lattice [ 121 is observed from a similar source. It is apparent then, that as well as the modes described in ref. 7, there is an additional set of fundamentals in the region 500-650 cm-’ characteristic of all triply bridged systems M(BH4)4 (M = Zr, Hf, U). Our current results for Zr(BH& are consistent with a structure consisting of triple hydrogen bridges between the zirconium atom and each of the four tetrahedrally oriented boron atoms. However, it is suggested that the molecule as a whole conforms to a T rather than Td symmetry. The observed IR and Raman spectra are relatively simple as expected for a molecule of high symmetry. However, there appear to be more active fundamentals than are predicted by the selection rules of a Ta point group. For this reason we consider the slightly lower symmetry model. Representatives of T and Td structures are shown in Fig. 1. These differ in the relative orientations of the twelve bridging hydrogens. There are two possible structures which conform to T, symmetry: all dihedral angles H,-B-&-B may be equal to O” (Fig. l(a)) or to SO” (Fig. l(b)). For T symmetry all such angles are equal but are neither 0” nor 60” (Fig. l(c)). The overall IR and Raman selection rules pertaining to the 57 degrees of vibrational freedom of the T and T, structural models of Zr(BH4), are: T, Gb = 4A1 (R) + A2 (ia) + 5E(R) + 5T1 (ia) + 9T2 (IR, R) T
Lib = 5A (R) + 5E(R) + 14T (IR, R)
In the following, we compare the specific predictions of the two models in the 500-650 cm-’ and 1000-1300 cm-’ regions. It should be noted that careful
C
Fig. 1. Structures of Zr(BH,), corresponding to Td, (a) and (b) and T, (c) symmetries. In the upper set of diagrams the perspective is down a ZrB axis. The Hb--B-ZrB angles (zw) are: in (a), 0” ; (b), 60” ; (c), 0” < w < 60”. The lower set of diagrams show the view along a B-B axis with the Zr atom positioned below this axis,
a
94
consideration has been given to the question of impurities and it is felt that it is improbable for the features found in addition to those expected for T, symmetry to have originated from such a source. In the 500-650 cm-’ region, the Td point group predicts just two (2T2) IR bands and four (A, + E + 22’,) Raman lines. This region of the vapour phase and solution spectra is dominated by skeletal stretching modes and weaker features are not well resolved. The solid phase spectra, however, show a number of distinct bands and the following are assigned to Zr(1*BHq)4: IR-564,547,581 cm-‘, Raman - 504, 533,543, 581,610 cm-‘. Both IR and Raman observations are incompatible with Td symmetry. The results are, however, explicable in terms of T symmetry. This model, on the basis of the modes described earlier, predicts the observation of five modes in this region, three of which (32’) are IR active and five of which (A + E + 3T) are Raman active. A total of six bands are observed above, however, and it seems reasonable that this slight additional complexity is due to the observation of torsional vibrations. Torsions of BH4 groups about Zr--B axes are of A(R) and T(IR, R) symmetry species which correlate with the A2 (ia) and T1 (ia) species of the Td point group. In the 1000-1300 cm-’ region the T, point group predicts a maximum of three bands in the IR spectrum while T symmetry predicts a maximum of five IR-active modes. The IR spectrum of Zr(BH& in nujol solution exhibits five features in this region: at 1286,1213,1170,1098 and 1056 cm-‘. The vapour phase spectrum shows four distinct bands (1288,1218,1155 and 1034 cm-‘) as does the solid phase IR (1286,1217,1178 and 1058 cm-‘). For this data to be consistent with the Td model one (for vapour and solid phase) or two (for nujol solution) features must be explained as overtones or combinations. Our preference is to accept all bands as fundamentals, compatible with the T structure. It is interesting that in an electron diffraction of 38” was deduced (consistent with study [3] a dihedral angle H, -B-Zr-B T symmetry) though the authors placed little reliance on the accuracy of this value. The IR spectra of Hf(BH4)4 in the vapour phase [ 7,9] and nujol solution [8], together with Raman spectra of a benzene solution [7] neat liquid [S] and single crystal [9] have been interpreted on the basis of Td symmetry. The room temperature solid phase Raman data [9] do not seem to show the resolution nor complexity apparent in the Zr(BH,), spectra [ 141. On the other hand, the vapour phase and solution data that are available do show strong similarities to those of the zirconium system. Reference has been made [9] to a neutron diffraction study of Hf(BHa)a which has been interpreted in terms of a T, molecular symmetry. Very little has been reported on the vibrational spectra of U(BH4)4T Nevertheless it has been clearly demonstrated that there is a considerable structural change on passing from the vapour phase to the solid phase [6] and the structural implications of the solid phase IR data deduced with the benefit of X-ray results [2] were consistent with the conclusions of a neutron
95
diffraction study [4]. The only vapour phase IR spectrum of U(BH4)4 available [6] is of poor quality relative to the zirconium and hafnium spectra (due in part to the lower volatility of the uranium compound) and the subtle distinction between Z’, and T, models cannot be made at this stage. In conclusion, we feel that the evidence relating to Zr(BH,), in the solid phase, vapour and solution is sufficient to indicate a preference for a structure of T symmetry rather than one of T, symmetry. This structural model would also seem worthy of consideration in relation to studies of related M(BH4)4 systems. ACKNOWLEDGEMENTS
This work was supported by the Australian Research Grants Committee. B.E.S. acknowledges receipt of a Commonwealth of Australia Postgraduate Research Award. REFERENCES 1 P. H. Bird and M. R. Churchill, Chem. Commun., (1967) 403. 2 E. R. Bernstein, T. A. Keiderling, S. J. Lippard and J. J. Mayerle, J. Amer. Chem. Sot., 94 (1972) 2552. 3 V. Plato and K. Hedberg, Inorg. Chem., 10 (1971) 590. 4 E. R. Bernstein, W. C. Hamilton, T. A. Keiderling, S. J. La Placa, S. J. Lippard and J. J. Mayerle, Inorg. Chem., 11 (1972) 3009. 5 B. E. Smith and B. D. James, Inorg. Nucl. Chem. Lett., 7 (1971) 857. 6 B. D. James, B. E. Smith and M. G. H. Wallbridge, J. Mol. Struct., 14 (1972) 327. 7 N. Davies, M. G. H. Wallbridge, B. E. Smith and B. D. James, J. Chem. Sot. Dalton, (1973) 162. 8 T. J. Marks, W. J. Kennelly, J. R. Kolb and L. A. Shimp, Inorg. Chem., 11 (1972) 2540. 9 T. A. Keiderhng, W. T. Wozniak, R. S. Gay, D. Jurkowitz, E. R. Bernstein, S. J. Lippard and T. G. Spiro, Inorg. Chem., 14 (1975) 576. 10 T. J. Marks and L. A. Shimp, J. Amer. Chem. Socr, 94 (1972) 1542. 11 M. Ehemann and H. NSth, Z. Anorg. Ailg. Chem., 386 (1971) 87. 12 E. R. Bernstein and T. A. Keiderling, J_ Chem. Phys., 59 (1973) 2105. 13 B. D. James and B. E. Smith, Syn. React. Inorg. Metal-Org. Chem., 4 (1974) 461. 14 B. D. James, B. E. Smith and H. F. Shurvell, unpublished work.