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Intermolecular hydrogen bonds with d-electrons of transition metal atoms. H-complexes with metallocenes of the iron subgroup
Journal of Molecular Structure, 301 (1993) l-5 Elsevier Science Publishers B.V., Amsterdam
Intermolecular hydrogen bonds with d-electrons of transition metal atoms. H-complexes with metallocenes of the iron subgroup E.S. Shubina,
A.N. Krylov, A.Z. Kreindlin, M.I. Rybinskaya,
A.N. Nesmeyanov Institute of Organo-element 28, Moscow B-334, Russian Federation
L.M. Epstein*
Compounds of the Russian Academy
of Sciences,
Vavilov str.
(First received 16 October 1992; in final form 12 March 1993) Abstract Hydrogen bonding of phenol (phenol-d) and p-substituted phenols (R = F, NO*) with d-electrons of the metal atom in ruthenocene and osmocene was examined. Two types of the v(bonded OH) bands, u(OH . . T) and v(OH . . M), were observed in the IR spectra. Hydrogen bonding of the OH. . . M type was shown to have the usual spectral characteristics: correlation with basicity, common temperature dependence and isotopic ratio. The peculiarities of the hydrogen-bonding ability of d-electrons in comparison with those of sp-electrons are discussed.
.
.
Introduction
The discovery of the new intermolecular hydrogen bond of the OH...M (M=Ru, OS, Rh) type in the molecules of n-complexes has previously been reported [ 1,2]. As is well known, some of the d-orbitals of transition metal atoms in such complexes are involved in the formation of the bonds with 7r-ligands; the others are filled with d-electron lone pairs. Hence, the latter, similar to the spelectrons of heteroatoms in organic compounds, can take part in hydrogen bonding. We examined regularities in the formation of intramolecular hydrogen bonds with metal atoms in the a-metallocenylcarbinols of the iron subgroup [2-41. It was shown that hydrogen bonding with the Ru and OS atoms is not controlled by the nature of stable conformation. The electrondonating ability of the metal atom increases going down the group, especially on proceeding from the
* Corresponding author.
metal of Period IV to that of Period V. Namely, the relative stability of the cycles can be represented by the ratio Fe : Ru : OS = 0.56 : 1 : 1.24. Hence, the metallocenes of Ru and OS, in contrast to that of Fe [5] and other metals of Period IV [6], appeared to be convenient model compounds for the investigation of the hydrogen bond between proton donors and d-electron lone pairs [1,2]. We now report the results of an IR spectral study of hydrogen-bonded complexes between C6HsOH, C6HSOD, n-FC6H,,0H, nNO$$H40H, n-NO&,H40D and metallocenes (I) or their permethylated analogues (II):
00 M
M = Fe@) M=Ru(Re)
*
00
M=Os(Oc)
*
0 M
M = Ru(Rc’)
0
M = Os(Ocn)
A comparison of spectral and thermodynamic properties between hydrogen-bonded complexes
2
ES. Shubina et a/./J. Mol. Struct. 301 (1993) I-5
of metal d-electrons and those of sp-electrons of common organic compounds was undertaken. Experimental
The compounds studied were prepared by published procedures [7]. The spectra were recorded with IR spectrophotometers ‘Specord M-80’ and ‘Specord M-82’. Measurements were made in Ccl, solutions. The concentrations of bases (Fc, Rc, Oc, Rc’, 0~“) were varied in the range 7 x lo-*-5 x 10e3 mol l-‘, and the concentrations of proton donors n-FC6H40H, n-N02C6H40H) in (C~HSOH, the range 5 x 10V3-1 x 10m3mol I-‘, d = 2cm. Table 1 Spectral characteristics, cc14 Cp2M+CeHsOH(Pi
and the values of (-AH)
Results and discussion
The IR spectra of all systems proton donormetallocene (M = Ru, OS) display the bands of free OH groups and two types of bonded groups (Table 1). The Y(bonded OH) bands in the range 3500-3557 cm-’ are unambiguously assigned to the OH groups bonded with the n-electrons
and Ej of hydrogen-bonded
complexes of phenols and metallocenes Cp2M in
= 1.00)
M
v(free OH) (cm-‘)
v(OH. (cm-‘)
Fe
3613
3550 [5]
RLl OS
The temperature dependence (295-250 K) was investigated in CC& in an argon atmosphere. The enthalpies of the hydrogen bonds, -AH, were evaluated as described previously [3]. The v(OH) values correspond to their centres of gravity (s = 3 cm-‘).
T)
3551 3550
3613 3613
v(OH.. .M) (cm-‘)
Av (cm’)
-AH (kcal mol-t )
Ei
_
_
_
_
3436 3393
177 220
3.6 4.2
0.66 0.80
Cp2M + C6HsOD (Pi = 1.OO) M
RU OS
Ru OS
T)
v(OH...M) (cm-‘)
Au (cm-‘)
v (OH)lplv
(cm-‘)
2668 2668
2636 2632
2550 2518
118 150
1.349 1.350
CprM +p-FCeHsOH M
Y(OH .
v(free OH) (cm-‘)
(OD)
(Pi = 1.04)
v(free OH) (cm’)
Y(OH . . . T)
v(OH . M) 0-m’)
Au (cm-‘)
-AH (kcalmol-‘)
Ej
(cm-‘)
3615 3615
3547 3557
3420 3375
200 230
3.8 4.5
0.67 0.81
u(OH...M) (cm-‘)
Au (cm’)
-AH (kcalmol-‘)
Ei
Cp2M+p-N02CsH50H
(Pi = 1.27)
v(OH . T)
Y(free OH) (cm-‘)
(cm-‘)
Fe
3596
3500
-
_
-
-
Ru OS
3596 3596
3513 3518
3380 3302
220 270
4.4 5.2
0.66 0.82
M
ES. Shubina et al./J. Mol. Strut. 301 (1993) l-5
Fig. 1. IR spectra in the v(OH) range of the hydrogen-bonded complexes of p-nitrophenol (c = 0.0009 mol 1-l) with Fc (1), Rc (2), Oc (3) (c = 0.05-0.07 mol 1-l) in Ccl,, d = 2 cm.
of Cp rings. It is noteworthy that the value of Av(OH) = v(free OH) - v(bonded OH) and the relative intensities of these bands decrease with Cp ring electron density down the group (Fig. 1). The broad band in the 3460-33OOcm-’ range, which is not observed if M = Fe, can be attributed to the stretching vibration of the OH group, bonded with the metal atom (Ru, OS). Such assignment is supported by the sensitivity of the band position to the nature of the metal atom (Table 1). The value of Av(OH.. . OS) is greater than that of AY(OH. e. Ru) for any of the proton donors. Moreover, the relationship between the frequency shifts Av(OH . +. Os)/v(OH . . f Ru)
Fig. 2. The temperature
dependence
remains fixed and is the same as for the abovementioned intramolecular hydrogen bonds (1.24) 12~41. The spectral properties of this new type of hydrogen bonding are similar to those of common organic compounds in many respects. Namely, the isotopic ratio of the frequencies of PhOH and PhOD is approximately 1.35 (Table l), i.e. is in accord with Y(OH . . . B)/ v(OD . . . B) for hydrogen bonds with organic bases (B) [8]. The temperature dependences of the v(OH) bands (e.g. Fig. 2) demonstrate the increase in the number of OH-bonded groups upon cooling
of v(OH) for phenol (c = 0.0009mol 1-l) with Rc (c = 0.07mol 1-l) in Ccl.,, d = 2cn1; 20°C (1), 10°C (2), 0°C (3), -10°C (4), -20°C (5).
ES. Shubinaet al./J. Mol.Struct.301 (1993) I-5
Fig. 3. IR spectra ofp-fluorophenol
(c = 0.0009mo11-‘)
with Rc” (l), Rc (2), CH$N
of the solutions. These changes in the band intensities are reversible. Furthermore, the values of 4v(OH . - - M) grow in parallel with the proton donating ability of phenols (Table l), fulfilling the empirical “rule of factors” [91a. The “factors of basicity” of the metal atom (Ei) in the hydrogenbond remain constant on changing the proton donors. The values of Ej vary in the same order as the metal basicity: Ru < OS (Table 1) [lo]. It is notable that these values are comparable with those of such organic compounds as nitriles, sulphides and aromatic ethers (0.6-0.8) [9]. As can be inferred from Table 1, the values of (-AH) are equal to 3%5.2kcalmol-‘, i.e. characteristic of the medium strength hydrogen bond. The peculiarity of hydrogen bonding with d-electrons appears to be the abnormally low value of the formation constant (Kr). The precise determination of Kr presents severe difficulties, attributable to the existence of the two types of OH-bonded group, but it is possible to make rough estimates of the upper value: Kr < 0.1. Thus, Kr values are much lower than those of organic compound sp-electrons, forming hydrogen-bonded complexes of similar ‘This rule is based on the constant acidic and basic properties in hydrogen-bond formation AX, = AX,, PiEj, where AXI1 is the spectral (Av, M) or thermodynamic (-AH) property of the standard hydrogen-bond complex (phenol-ether) with Pi= Ej= 1 [6].
(3) (c = 0.050.07mol
I-‘, d = 24.
stability (Kf > 1). As can be seen (Fig. 3), the intensity of v(bonded OH) in the system p-FCsH40H - - . NCCHs is much greater than for p-FC6H40H - . . Rc. The number of bonded OH groups is lower for the latter system by a factor of approximately 100. This is also probably the reason why many attempts to observe the hydrogen bond of the HX - - . M type had not met with success (see refs. 2 and 6, and references therein). It is not clear whether the steric hindrance of ligands or the properties of d-orbitals are responsible for such an effect. We tried to solve this problem by examination of the interaction between proton donors and permethylated metallocenes (II). As can be seen from Fig. 3 there is a great difference between the spectra of p-FCbHdOH with Rc” and RC. The intensity of the v(OH . . - r) band is greater while that of v(OH . . . M) is less in the former system. Thus, in spite of the increase of electron-donating ability of the metal atom by the electron releasing CHs groups, the formation of the OH.. . M type hydrogen bond is inhibited by steric hindrance. Conclusion The conditions of hydrogen-bond formation with d-electron lone pairs of transition metal atoms in metallocenes of group VIII of the Periodic Table were determined. It was shown that the spectral characteristics of this new type
5
ES. Shubina et al.lJ. Mol. Struct. 301 (1993) I-5
of hydrogen bond are comparable with those of sp-electrons of organic bases. Properties such as reversible temperature dependence, isotopic ratio, dependence of spectral changes on the proton donating ability of phenols, and basicity of the compounds studied are similar to those of known medium strength hydrogen bonds. The increase in electron-donating ability down the group, the large difference between the metals of Period IV (Fe) and Period V (Ru), and unexpectedly low values of formation constants are the distinctive properties of OH . . . M type hydrogen bonds. The results obtained call for new areas of research. The investigation will be extended considerably to include other metals and ligands, as well as other solvents and a wide variety of temperatures. It is of importance to use stronger proton donors, which make it possible to investigate proton transfer. The above-mentioned problems are under active investigation.
1 L.E.
Vinogradova,
A.Z.
Kreyndlin,
L.A.
Leites,
2 3
4 5
6
7
8 9 10
Chizhevaskii, E.S. Shubina and L.M. I.T. Metalloorg. Khim., 3 (1990) 1192 Epstein, (English translation in Organomet. Chem., USSR, 3 (1990) 618). L.M. Epstein and ES. Shubina, Metalloorg. Khim., 5 (1992) 61. E.S. Shubina, L.M. Epstein, T.V. Timofeeva, Yu.T. Struchkov, A.Z. Kreyndlin, S.S. Fadeeva and M.I. Rybinskaya, J. Organomet. Chem., 401 (1991) 133. E.S. Shubina and L.M. Epstein, J. Mol. Struct., 265 (1992) 367. L.M. Epstein, L.D. Ashkenadse, S.O. Rabicheva and L.A. Kazitzina, Dokl. Akad. Sci. USSR, 190 (1970) 128. V.T. Aleksanyan, Ya.M. Kimmelfeld, R.B. Materikova and E.M. Smimova, Zh. Fiz. Khim., 54 (1980) 663 (English translation in Russ. J. Phys. Chem., 3 (1980)). M.O. Albers, D.C. Liles, D.J. Robinson, A. Shaver, E. Singleton, M.B. Wiege, J.C.A. Boeyens and D.C. Levendis, Organometallics, 5 (1986) 2321. N.D. Sokolov, in The Hydrogen Bond, Nauka, Moscow, 1981, p. 78. A.V. Iogansen, Theor. Exp. Chem., 7 (1971) 302. B.V. Lokshin, A.G. Ginsburg and Ye.B. Nazarova, Usp. Khim., 49 (1986) 223.
Intermolecular hydrogen bonds with d-electrons of transition metal atoms. H-complexes with metallocenes of the iron subgroup E.S. Shubina,
A.N. Krylov, A.Z. Kreindlin, M.I. Rybinskaya,
A.N. Nesmeyanov Institute of Organo-element 28, Moscow B-334, Russian Federation
L.M. Epstein*
Compounds of the Russian Academy
of Sciences,
Vavilov str.
(First received 16 October 1992; in final form 12 March 1993) Abstract Hydrogen bonding of phenol (phenol-d) and p-substituted phenols (R = F, NO*) with d-electrons of the metal atom in ruthenocene and osmocene was examined. Two types of the v(bonded OH) bands, u(OH . . T) and v(OH . . M), were observed in the IR spectra. Hydrogen bonding of the OH. . . M type was shown to have the usual spectral characteristics: correlation with basicity, common temperature dependence and isotopic ratio. The peculiarities of the hydrogen-bonding ability of d-electrons in comparison with those of sp-electrons are discussed.
.
.
Introduction
The discovery of the new intermolecular hydrogen bond of the OH...M (M=Ru, OS, Rh) type in the molecules of n-complexes has previously been reported [ 1,2]. As is well known, some of the d-orbitals of transition metal atoms in such complexes are involved in the formation of the bonds with 7r-ligands; the others are filled with d-electron lone pairs. Hence, the latter, similar to the spelectrons of heteroatoms in organic compounds, can take part in hydrogen bonding. We examined regularities in the formation of intramolecular hydrogen bonds with metal atoms in the a-metallocenylcarbinols of the iron subgroup [2-41. It was shown that hydrogen bonding with the Ru and OS atoms is not controlled by the nature of stable conformation. The electrondonating ability of the metal atom increases going down the group, especially on proceeding from the
* Corresponding author.
metal of Period IV to that of Period V. Namely, the relative stability of the cycles can be represented by the ratio Fe : Ru : OS = 0.56 : 1 : 1.24. Hence, the metallocenes of Ru and OS, in contrast to that of Fe [5] and other metals of Period IV [6], appeared to be convenient model compounds for the investigation of the hydrogen bond between proton donors and d-electron lone pairs [1,2]. We now report the results of an IR spectral study of hydrogen-bonded complexes between C6HsOH, C6HSOD, n-FC6H,,0H, nNO$$H40H, n-NO&,H40D and metallocenes (I) or their permethylated analogues (II):
00 M
M = Fe@) M=Ru(Re)
*
00
M=Os(Oc)
*
0 M
M = Ru(Rc’)
0
M = Os(Ocn)
A comparison of spectral and thermodynamic properties between hydrogen-bonded complexes
2
ES. Shubina et a/./J. Mol. Struct. 301 (1993) I-5
of metal d-electrons and those of sp-electrons of common organic compounds was undertaken. Experimental
The compounds studied were prepared by published procedures [7]. The spectra were recorded with IR spectrophotometers ‘Specord M-80’ and ‘Specord M-82’. Measurements were made in Ccl, solutions. The concentrations of bases (Fc, Rc, Oc, Rc’, 0~“) were varied in the range 7 x lo-*-5 x 10e3 mol l-‘, and the concentrations of proton donors n-FC6H40H, n-N02C6H40H) in (C~HSOH, the range 5 x 10V3-1 x 10m3mol I-‘, d = 2cm. Table 1 Spectral characteristics, cc14 Cp2M+CeHsOH(Pi
and the values of (-AH)
Results and discussion
The IR spectra of all systems proton donormetallocene (M = Ru, OS) display the bands of free OH groups and two types of bonded groups (Table 1). The Y(bonded OH) bands in the range 3500-3557 cm-’ are unambiguously assigned to the OH groups bonded with the n-electrons
and Ej of hydrogen-bonded
complexes of phenols and metallocenes Cp2M in
= 1.00)
M
v(free OH) (cm-‘)
v(OH. (cm-‘)
Fe
3613
3550 [5]
RLl OS
The temperature dependence (295-250 K) was investigated in CC& in an argon atmosphere. The enthalpies of the hydrogen bonds, -AH, were evaluated as described previously [3]. The v(OH) values correspond to their centres of gravity (s = 3 cm-‘).
T)
3551 3550
3613 3613
v(OH.. .M) (cm-‘)
Av (cm’)
-AH (kcal mol-t )
Ei
_
_
_
_
3436 3393
177 220
3.6 4.2
0.66 0.80
Cp2M + C6HsOD (Pi = 1.OO) M
RU OS
Ru OS
T)
v(OH...M) (cm-‘)
Au (cm-‘)
v (OH)lplv
(cm-‘)
2668 2668
2636 2632
2550 2518
118 150
1.349 1.350
CprM +p-FCeHsOH M
Y(OH .
v(free OH) (cm-‘)
(OD)
(Pi = 1.04)
v(free OH) (cm’)
Y(OH . . . T)
v(OH . M) 0-m’)
Au (cm-‘)
-AH (kcalmol-‘)
Ej
(cm-‘)
3615 3615
3547 3557
3420 3375
200 230
3.8 4.5
0.67 0.81
u(OH...M) (cm-‘)
Au (cm’)
-AH (kcalmol-‘)
Ei
Cp2M+p-N02CsH50H
(Pi = 1.27)
v(OH . T)
Y(free OH) (cm-‘)
(cm-‘)
Fe
3596
3500
-
_
-
-
Ru OS
3596 3596
3513 3518
3380 3302
220 270
4.4 5.2
0.66 0.82
M
ES. Shubina et al./J. Mol. Strut. 301 (1993) l-5
Fig. 1. IR spectra in the v(OH) range of the hydrogen-bonded complexes of p-nitrophenol (c = 0.0009 mol 1-l) with Fc (1), Rc (2), Oc (3) (c = 0.05-0.07 mol 1-l) in Ccl,, d = 2 cm.
of Cp rings. It is noteworthy that the value of Av(OH) = v(free OH) - v(bonded OH) and the relative intensities of these bands decrease with Cp ring electron density down the group (Fig. 1). The broad band in the 3460-33OOcm-’ range, which is not observed if M = Fe, can be attributed to the stretching vibration of the OH group, bonded with the metal atom (Ru, OS). Such assignment is supported by the sensitivity of the band position to the nature of the metal atom (Table 1). The value of Av(OH.. . OS) is greater than that of AY(OH. e. Ru) for any of the proton donors. Moreover, the relationship between the frequency shifts Av(OH . +. Os)/v(OH . . f Ru)
Fig. 2. The temperature
dependence
remains fixed and is the same as for the abovementioned intramolecular hydrogen bonds (1.24) 12~41. The spectral properties of this new type of hydrogen bonding are similar to those of common organic compounds in many respects. Namely, the isotopic ratio of the frequencies of PhOH and PhOD is approximately 1.35 (Table l), i.e. is in accord with Y(OH . . . B)/ v(OD . . . B) for hydrogen bonds with organic bases (B) [8]. The temperature dependences of the v(OH) bands (e.g. Fig. 2) demonstrate the increase in the number of OH-bonded groups upon cooling
of v(OH) for phenol (c = 0.0009mol 1-l) with Rc (c = 0.07mol 1-l) in Ccl.,, d = 2cn1; 20°C (1), 10°C (2), 0°C (3), -10°C (4), -20°C (5).
ES. Shubinaet al./J. Mol.Struct.301 (1993) I-5
Fig. 3. IR spectra ofp-fluorophenol
(c = 0.0009mo11-‘)
with Rc” (l), Rc (2), CH$N
of the solutions. These changes in the band intensities are reversible. Furthermore, the values of 4v(OH . - - M) grow in parallel with the proton donating ability of phenols (Table l), fulfilling the empirical “rule of factors” [91a. The “factors of basicity” of the metal atom (Ei) in the hydrogenbond remain constant on changing the proton donors. The values of Ej vary in the same order as the metal basicity: Ru < OS (Table 1) [lo]. It is notable that these values are comparable with those of such organic compounds as nitriles, sulphides and aromatic ethers (0.6-0.8) [9]. As can be inferred from Table 1, the values of (-AH) are equal to 3%5.2kcalmol-‘, i.e. characteristic of the medium strength hydrogen bond. The peculiarity of hydrogen bonding with d-electrons appears to be the abnormally low value of the formation constant (Kr). The precise determination of Kr presents severe difficulties, attributable to the existence of the two types of OH-bonded group, but it is possible to make rough estimates of the upper value: Kr < 0.1. Thus, Kr values are much lower than those of organic compound sp-electrons, forming hydrogen-bonded complexes of similar ‘This rule is based on the constant acidic and basic properties in hydrogen-bond formation AX, = AX,, PiEj, where AXI1 is the spectral (Av, M) or thermodynamic (-AH) property of the standard hydrogen-bond complex (phenol-ether) with Pi= Ej= 1 [6].
(3) (c = 0.050.07mol
I-‘, d = 24.
stability (Kf > 1). As can be seen (Fig. 3), the intensity of v(bonded OH) in the system p-FCsH40H - - . NCCHs is much greater than for p-FC6H40H - . . Rc. The number of bonded OH groups is lower for the latter system by a factor of approximately 100. This is also probably the reason why many attempts to observe the hydrogen bond of the HX - - . M type had not met with success (see refs. 2 and 6, and references therein). It is not clear whether the steric hindrance of ligands or the properties of d-orbitals are responsible for such an effect. We tried to solve this problem by examination of the interaction between proton donors and permethylated metallocenes (II). As can be seen from Fig. 3 there is a great difference between the spectra of p-FCbHdOH with Rc” and RC. The intensity of the v(OH . . - r) band is greater while that of v(OH . . . M) is less in the former system. Thus, in spite of the increase of electron-donating ability of the metal atom by the electron releasing CHs groups, the formation of the OH.. . M type hydrogen bond is inhibited by steric hindrance. Conclusion The conditions of hydrogen-bond formation with d-electron lone pairs of transition metal atoms in metallocenes of group VIII of the Periodic Table were determined. It was shown that the spectral characteristics of this new type
5
ES. Shubina et al.lJ. Mol. Struct. 301 (1993) I-5
of hydrogen bond are comparable with those of sp-electrons of organic bases. Properties such as reversible temperature dependence, isotopic ratio, dependence of spectral changes on the proton donating ability of phenols, and basicity of the compounds studied are similar to those of known medium strength hydrogen bonds. The increase in electron-donating ability down the group, the large difference between the metals of Period IV (Fe) and Period V (Ru), and unexpectedly low values of formation constants are the distinctive properties of OH . . . M type hydrogen bonds. The results obtained call for new areas of research. The investigation will be extended considerably to include other metals and ligands, as well as other solvents and a wide variety of temperatures. It is of importance to use stronger proton donors, which make it possible to investigate proton transfer. The above-mentioned problems are under active investigation.
1 L.E.
Vinogradova,
A.Z.
Kreyndlin,
L.A.
Leites,
2 3
4 5
6
7
8 9 10
Chizhevaskii, E.S. Shubina and L.M. I.T. Metalloorg. Khim., 3 (1990) 1192 Epstein, (English translation in Organomet. Chem., USSR, 3 (1990) 618). L.M. Epstein and ES. Shubina, Metalloorg. Khim., 5 (1992) 61. E.S. Shubina, L.M. Epstein, T.V. Timofeeva, Yu.T. Struchkov, A.Z. Kreyndlin, S.S. Fadeeva and M.I. Rybinskaya, J. Organomet. Chem., 401 (1991) 133. E.S. Shubina and L.M. Epstein, J. Mol. Struct., 265 (1992) 367. L.M. Epstein, L.D. Ashkenadse, S.O. Rabicheva and L.A. Kazitzina, Dokl. Akad. Sci. USSR, 190 (1970) 128. V.T. Aleksanyan, Ya.M. Kimmelfeld, R.B. Materikova and E.M. Smimova, Zh. Fiz. Khim., 54 (1980) 663 (English translation in Russ. J. Phys. Chem., 3 (1980)). M.O. Albers, D.C. Liles, D.J. Robinson, A. Shaver, E. Singleton, M.B. Wiege, J.C.A. Boeyens and D.C. Levendis, Organometallics, 5 (1986) 2321. N.D. Sokolov, in The Hydrogen Bond, Nauka, Moscow, 1981, p. 78. A.V. Iogansen, Theor. Exp. Chem., 7 (1971) 302. B.V. Lokshin, A.G. Ginsburg and Ye.B. Nazarova, Usp. Khim., 49 (1986) 223.