- Email: [email protected]
Contact shift studies of some sulfoxide and ketone complexes of cobalt(II) and nickel(II)
J inorte nuci. ('hem
1977 Vo] 39, pp 11173 1078. Pergamon Press.
Printed in Great Britain
CONTACT SHIFT STUDIES OF SOME SULFOXIDE AND KETONE COMPLEXES OF COBALT(II) AND NICKEL(II) KAROL JACKOWSKI and ZBIGNIEW K[~CKI Laboratory of Intermolecular Interactions, Institute of Basic Problems of Chemistry, Warsaw University, ul. Pasteura 1.02-093 Warszawa, Poland (Received 28 May 1976)
Abstract--The PMR isotropic shifts of the Co(lI) and Ni(II) octahedral complexes of dimethylsulfoxide. ethylmethysulfoxide, acetone and ethylmethylketone are reported. From a consideration of the magnitude and the temperature dependence of these shifts it is concluded that the main effect, which determines the shifts, is the o--delocalizationof e, unpaired electrons in the Ni(II) and also Co(II) complexes. In the case of the ketone complexes the o--~- polarization mechanism is also necessary in order to explain the upfield shifts of the methyl and methylene groups which are directly bonded to the carbonyl group. performed using JEOL, JES-VT-2 equipment with an accuracy of -+I°C. Tetramethylsilane (TMS) and hexamethylsiloxane (HMS) were used as the internal reference standards.
INTRODUCTION
NMR contact shifts arise from the contact coupling of unpaired electrons and nuclear magnetic moments. The contact shift studied are widely and successfully applied in the investigations of paramagnetic metal complexes [1]. The octahedral complexes especially afford an excellent opportunity for the elucidation of the metal-ligand spin delocalization because of the strict separation of o-- and 7r-bonding systems[2, 3]. The hexacoordinated sulfoxide and ketone complexes of transition metal ions were extensively studied by IR and NMR methods[4-14] but the mechanism of spin delocalization was really investigated only for the diphenylsulfoxide (DPSO) complexes of iron(II), cobalt(II) and nickel(II)[10]. In this study we applied the PMR contact shift method to investigate the dimethylsulfoxide (DMSO), ethylmethylsulfoxide (EMSO), acetone and ethylmethylketone complexes of cobalt(II) and nickel(II). We hoped to learn the mechanism of spin delocalization in these complexes.
RESULTS AND DISCUSSION
Sulfoxide complexes The PMR isotropic shifts were determined for dimethylsulfoxide (DMSO) and ethylmethylsulfoxide (EMSO) complexes of cobalt(If) and nickel(II) in the appropriate sulfoxide itself as a solvent and in nitromethane solutions. In the solution of the complex in the corresponding sulfoxide the coordinated ligands rapidly exchange with the molecules in the bulk and only the averaged shift for each proton group of sulfoxide is observed (6~b~), which is a function of the molar fractions (PL, P~) and the shifts (~', 6~') of the coordinated and bulk molecules, respectively. 6,',b~= PL6r. i + PB6~ i.
EXPERIMENTAL
Materials Sulfoxide complexes. Dimethylsulfoxide (V/0Sojuzchimexport, Moscow) was dried over molecular sieves (3 A) and distilled at low pressure (at 60°C). Ethylmethylsulfoxide was prepared and purified in the Institute of Basic Problems of Chemistry (Warsaw University). Nitromethane (Xenon, L6d~) was dried with anhydrous CaSO4 and distilled. The hydrated perchlorates were obtained from appropriate chlorides and perchloric acid. The complexes of dimethylsulfoxide were prepared using the method of Selbin et al. [4] whereas the complexes of ethylmethylsulfoxide were synthesized by the method described by Currier and Weber[l 1]. Ketone complexes. Co(If), Ni(II) and Zn(II) perchlorates were obtained from the dry chlorides and AgCIO4directly in the ketone solutions. AgCIO, was dried at a temperature of 130°Cfor several days. Co[CH3.CO.H]6[InCI,]~ complex was synthesized in nitromethane solution as described in Ref. [15]. All the reagents were carefully purified and dehydrated. All preparations and all handling operations were carried out in a PzOs-dried-glovebox. The analyses of the metals were performed by complexometric titration [16]. PMR spectra. All PMR spectra were recorded with JEOL, JNM-3H-60 spectrometer. Shifts were measured with the conventional sideband technique. Temperature measurements were
(1)
The isotropic shifts (&~) could be evaluated as the values of PL, Ps and 6d were known. On the other hand, the shifts 6L~ could be observed without averaging effect, when the sulfoxide complexes were dissolved in nitromethane. The values measured for the nitromethane solutions were found to be independent of complex concentration. The isotropic shifts measured relative to the diamagnetic sulfoxide complexes of zinc(lI) are listed in Table 1. It is seen in Table 1 that the shifts measured in corresponding sulfoxides as solvents and in nitromethane solutions are convergent and the differences, which are within the range of 10%, are probably due to intermolecular interactions between the complex and solvent bulk molecules. The above comparison provides a control of the influence of solvent on the observed PMR isotropic shifts. Of importance is the fact that all the isotropic shifls are attenuating negative (downfield). Moreover, the appropriate shifts for both nickel(I1) and cobalt(II) complexes are quite similar. It suggests that the isotropic shifts are the contact shifts, without any significant pseudocontact contribution even for the cobalt(II) complexes and that the same spin delocalization mechanism is
1073
K. JACKOWSKI and Z. Kl$KI
1074
Table 1. PMR isotropic shifts for sulfoxide complexes 0018
Signalt
In sulfoxide itself
In nitromethane
Extrapolated to T-’ = 0
CH, -CH, -CH,‘M’ -CH,1 -CH,‘E’ -CH,‘“’ -CH,( -CH,‘E’
-9.OkO.3 -6.9k0.3 - 6.9 2 0.3 -23.4? 1.0 l.lkO.1 -5.OkO.3 $ -2.2kO.2
-9.720.1 -7.720.1 -6.220.1 -20.1 kO.2 -1.OkO.l -4.9kO.l -17.OkO.2 -1.8+0.1
- 2.04 +0.47 -3.80 t 13.7 -5.5 -0.63 t 12.0 0
Confplex Co(DMSO),(ClOJ, Ni(DMSO),(ClO,), Co(EMSO),(ClOJ,
Ni(EMSO)&lO&
Isotropic shift [ppm]
tFor EMS0 molecule: CH,‘“‘.SO.CH,.CH,‘“‘. SThe signal was not observed because of paramagnetic broadening. operative in the nickel(H) and cobalt(I1) complexes [2,3].
In order to verify the last assumption we investigated the temperature dependence of the isotropic shifts. The results are shown in Figs. l-3. As it is seen in Fig. 1 the entire temperature effect is a paramagnetic one; the DMSO shift of the diamagnetic zinc(I1) complex is completely independent of temperature. Each of the paramagnetic shifts is a linear function of the inverse temperature. The linearity of this function can be explained in terms of theory given by McConnell[l7]:
c
-CHF
ww +1) (yiv/2?r)3kT -140
where (AH/H) is the NMR contact shift, g is the averaged g value for the complex, p is the absolute value of the Bohr magneton, yN is the nuclear magnetogyric ratio, S is the total spin number of the electrons which are delocalized onto ligands and A, (in Hz) is the coupling constant which is proportional to the density of unpaired electrons occupying s-orbitals. However, the extrapolation of the temperature functions in Figs. l-3 does
1 -150 i -160 ' -170.
Fig. 2. The temperature dependence of the PMR isotropic shifts for Ni(EMSO)&IO,), complex in nitromethane. The shifts were measured relative to Zn(EMS0)6(CIO~)2complex.
Fig. 1. The temperature dependence of the PMR isotropic shifts for complexes: Co(DMSO),(ClO.&, Ni(DMSO)&lO& and Zn(DMSO),(ClO& in nitromethane. The shifts were measured relative to hexamethylsiloxane (HMS) as the internal reference standard.
not provide a zero value of the isotropic shifts when the temperature rises to infinity (see Table 1). These disagreements are significant for both the nickel(I1) and cobalt(I1) complexes. But since the ground state of the octahedral cobalt(H) complexes is ?f,, the disagreement with the eqn (2) is more considerable in this case. The disagreements cannot be explained on the grounds of the simple theory and more advanced treatment must be used for this purpose [ 181. Spin delocalization in sulfoxide complexes. Nickel(I1) has two e, unpaired electrons. In the octahedral complex the e, orbitals are no longer the pure atomic orbitals but they contribute to the u* anti-bonding molecular orbitals of the complex. The positive spin density may be partly found on the ligands. The spin density rapidly decreases for aliphatic protons when the distance, which is measured by the number of u-bonds from the metal, increases. Such mechanism, which is called the udelocalization mechanism, was found also in octahedral alkylamine complexes [3]. It is seen from Table 1 that the same mechanism occurs for the studied sulfoxide complexes of nickel(I1). All the PMR shifts are negative
Contact shift studies of sulfoxide and ketone complexes T Ixl03 0
300
280
IO
.
520
~
E
) - ' '3
~0 o - - - - - < ) - - - < : ~ _ . < ~ ~ -CH3 ~MJ
~40 oO~ --150 [60 4
\
- ~70 i
i i 180 4 ~90
i J
200 q
Fig. 3. The temperature dependence of the PMR isotropic shifts for Co(EMSO)dCIO4h complex in nitromethane. The shifts were measured relative to Zn(EMSO)6(C10~)2complex. (which means that there are the positive spin densities on appropriate protons) and decreasing in the alkyl chain for EMSO coordinated molecules. The case of the cobalt(lI) complexes is more complicated. In general, the delocalization mechanism can be understood if we consider the main effects which can contribute to the observed PMR shifts of the octahedral cobalt(lI) complexes. Cobalt(If) has three unpaired electrons: two e, and one G electron. The e, electrons are responsible for the same mechanism of spin delocalization as it occurs in the nickel(II) complexes (tr-delocalization). The G electron can be transferred onto ligands only in the case when the ~--bonds are formed between the metal ion and the ligands (Trdelocalization). The ~r-delocalization is significant only for planar ligands and usually dominates, when it is present in the cobalt(II) complexes. In such case the observed contact shifts for the cobalt(II) and nickel(II) complexes are quite different as it has been found for the acetonitrile complexes[19]. Usually, the pseudocontact contribution is not dominant in the octahedral cobalt(II) complexes. Therefore, the mechanism of spin delocalization is still manifested in the PMR isotropic shifts. If the ~r-delocalization from the metal ion does not occur and the pseudocontact contribution is small, then the ~-delocalization mainly determines the PMR isotropic shifts of cobalt(II) complexes. In our results, indeed, we can observe a very similar pattern of isotropic shifts in both cobalt(II) and nickel(II) complexes, which reflects the same spin delocalization. Finally, it is worth while to notice that Wicholas[10] obtained different results for the PMR isotropic shifts of the cobalt(II) and nickel(II) octahedral complexes with diphenylsulfoxide and he suggested that the pseudocontact contribution was significant for the cobalt(II) complex in an ion-paired species.
Ketone complexes Ketones belong to very weakly coordinating ligands
1(175
and special conditions must be undertaken in order to obtain their hexacoordinated complexes of transition metal ions. In the general procedure developed by Driessen and Groneveld[12] the complexes were synthesized from anhydrous metal chlorides and antimony pentachloride in the ketone solution: ketone
MCI, + 2SbCL
~ M (ketone)6(SbCl6)>
(3)
The competition between the ligands and the anions to coordinate the metal ions did not occur because the anions were large enough. It was also shown that the regular octahedral species M(O)62÷ w e r e present in these complexes. In this PMR study we used the anhydrous ketone solutions of cobalt(II), nickel(II) and zinc(II) perchlorates. It was shown that in acetone the perchlorate anions are not coordinated to the transition metal cations[13]. Therefore we can expect that M(ketone),, ~~ species will be mainly created in the perchlorate solutions. The perchlorates were used because of their excellent solubility in ketones (M(ketone)6(SbC16)2 complexes are rather poorly soluble even in the ketones themselves). We examined the P MR shifts of the acetone and ethylmethylketone molecules coordinated to the paramagnetic cobalt(II) and nickel(II) cations. If the only species present in the ketone solutions be M(ketone)6 2' (and of course CIO4 ) then the 6-coordinated ligands rapidly exchange with the bulk molecules and only the weighted average shifts of the ketone are observed (~,b~) (see eqn (1)). If we can measure the 3~b~and 6/ shifts relative to the shift of the bulk molecules (G' = 0), then:
A3~b~= P~A6L'
t4)
the observed shift (A6obs) linearly depends on the concentration of metal ions and the PMR shifts of coordinated molecules (ASL') can be easily determined from eqn (4). In Figs. 4-6 we present the results which were obtained for acetone and ethylmethylketone solutions of cobalt(H) and nickel(II) perchlorates. A3~,b~values express the only paramagnetic effects as the shifts were measured relative to the analogous solutions of zinc(II). It is seen from Fig. 4 that upfield shifts are found for the acetone molecules coordinated to both the cobalt(II) and nickel(II) cations. In the case of the ethylmethylketone
Ni(II) ro
CO(I)
()5
o;r
o'2
o;~
Fig. 4. PMR isotropic shifts of acetone in the acetone solutions of nickel(II) and cobalt(H) perchlorates (PL is the fraction of
molecules coordinated to the cations).
K. JACKOWSK[ and Z. KI~CKI
1076
a) 2.0
OI
0.2
0.3pL
0.4
0.5
0.6
1.5 -0.2
b)
) "~ 0.5
O,I /
-CH 2
0
~
0.2
0.3
Q5
0.6
_
'<] -0.2 [_
-CH3
Fig. 7. PMR shifts of acetone (a) and ethylmethylketone (b) in the solutions of zinc(II) perchlorate (PL is the fraction of molecules coordinated to the cation).
-0.5
Fig. 5. PMR isotropic shifts of ethylmethylketone in the ethylmethylketone solution of nickel(II) perchlorate.
Table 2. PMR isotropic shifts for ketone complexes Complex
Signal
(ASL~)isotropic shift [ppm]
- C H ~ M)
1.0
Co(CH3COCH3)62+ Ni(CH3COCH3)62+ Co(CH3(M)COCH2CH3tE))6 2+
'•Q.0.5
0.4
o
o
"CH3
[ /
--ca3
-CH3(~) -CH2-
-CH3(~) -CH~~M) Ni(CH~(M)COCH2CH3~E))62+ -CH2-CH¢ ~)
0
-0,2 - C H 2-
Fig. 6. PMR isotropic shifts of ethylmethylketone in the ethylmethylketone solution of cobalt(lI) perchlorate. (CH3(M).CO.CH2.CH3 (z)) solutions (Figs. 5 and 6) an upfield shift is observed for the -CH/M) group and a downfield shift for the --CH/~) group, this is also true for both the cobalt(II) and nickel(II) solutions. The shift of the methylene group (-CH2-) is upfield for the nickel(II) solution but downfield for the cobalt(II) solution. It is interesting to compare these results with the results obtained for the diamagnetic zinc(II) perchlorate, Fig. 7. The shifts due to the diamagnetic species are fairly small and less complicated: all the shifts are downfield. The diamagnetic shifts express the descreening effect of the molecules which are coordinated to the zinc(II) cation through the oxygen atom. The results placed in Figs. 4--6 enabled the isotropic shifts of coordinated molecules (ASL~) in the octahedral species to be determined. These values are listed in Table 2. Spin delocalization in ketone complexes. We become convinced that the PMR isotropic shifts obtained for the acetone and ethylmethylketone complexes can be fully explained in terms of the spin delocalization which occurs in the octahedral complexes. Moreover, it will be shown that the spin delocalization is quite similar to that found in
+ 2.95 -+0.30 + 3.60 -+0.30 +7.70_+0.50 - 1.70-+0.20 - 0.40 _+0. I0 +7.00_+0.50 + 1.10_+0.20 - 0.90 _+0.10
sulfoxide complexes of cobalt(II) and nickel(II) and in other octahedral complexes [1, 3]. If we assume that the pseudocontact contribution to the isotropic shifts [20] is small and can be neglected then the contact shift is the only contribution to the observed shifts and arises from the delocalization of unpaired electrons within the octahedral complexes. The downfield and upfield PMR shifts are related respectively to the positive and negative spin densities on the protons. The significant result of this study can be summarized (Table 2): (i) The upfield shifts (negative spin densities) were observed only for the methyl or methylene protons next to the carbonyl group; (ii) all the other shifts were downfield (positive spin densities); (iii) the shift of the cobalt(II) and nicket(II) complexes were similar (except for the shift of the methylene group in ethylmethylketone). This result can be explained in terms of the spin density delocalization within the octahedral complexes as follows: (1) The primary effect is the delocalization of two eg unpaired electrons (positive spin density) in the trbonding system. It is well-known as the tr-delocalization mechanism in the octahedral cobalt(II) and nickel(II) complexes; (2) the ~r-delocalization causes the spin polarization of the carbonyl ~r-orbital by inducing a positive spin density at the oxygen atom and a negative spin density at the carbon atom as shown in Fig. 8; (3) the negative spin density is transferred directly from the ~r-orbital to the Is hydrogen orbitals of neighbouring alkyl groups by hyperconjugation;
1077
Contact shift studies os sulfoxide and ketone complexes (4) the isotropic shifts of the nickel(II) and cobalt(II) complexes are similar because in the cobalt(II) complexes only the ee electrons are delocalized onto the ligands and the delocalization of the G electron does not occur. The 1-4 mechanism gives the proper spin densities on the ketone protons due to the characteristic distribution of spin density within the carbonyl group, Fig. 8. It is seen that the delocalization of the G electron (~--delocalization) in the cobalt(II) complexes cannot take place because it would give a significant difference in the delocalization mechanism of both the cobalt(II) and nickel(II) complexes. The ~--delocalization would transfer the positive spin density to the neighbouring alkyl protons via the 7r-orbital. Such effect is not observed for the cobalt(II) complexes. The only different shifts of the methylene protons in the cobalt(II) and nickel(II) complexes (Table 2) arise from the competition between the positive spin density (¢-delocalization) and the negative spin density (Tr-polarization plus hyperconjugation) at this site. A small pseudocontact contribution is also possible in the cobalt(II) complexes as the distribution of the cobalt(II) unpaired electrons (t~G 2) is spatially asymmetric. It was reported by Driessen and Groeneveld[15,21] that esters and aldehydes also form octahedral complexes with divalent metal ions and that the coordination of ketones, esters and aldehydes occurs in the same manner. We took this opportunity to verify the (1-4) mechanism of spin delocalization. For this purpose we estimated the relative spin densities in the cobalt(II) and nickel(II) complexes of: di-n-propylketone, t-butylmethylketone, methyl acetate, ethyl acetate and acetaldehyde. The results are listed in Table 3. It is seen from Table 3 that the negative spin densities can be found on the methyl and methylene protons next to the 7r-orbital of the carbonyl group. The attenuating positive spin densities are clearly observed for the sites where the hyperconjugation is not possible but o-delocalization still operates. The large positive spin density is obtained for the aldehyde proton as a result of o--delocalization effect. The significant down-field shift (positive spin density) of the aldehyde proton was also observed by Kluiber and Kopycinski[14] for the series of aromatic aldehydes coordinated to bis (2,4-pentanedionato) nickel(Ill and cobalt(II) complexes. All the results are consistent with the proposed (1-4) mechanism of spin delocalization within the octahedral Ni(II) and CoOl) complexes of ketones. It means that the same mechanism operates for the complexes of esters and aldehydes and that all the carbonyl molecules are coordinated to the metal ions in the same way, i.e. by the o--bond between the metal ion and the oxygen atom of the /
',.
Table 3. The relative spin densities evaluated directly from the appropriate PMR isotropic shifts. A positive spin density implies a downfield PMR shift and a negative spin density implies an upfield shift. The largest value was normalised to -+1.00. Diamagnetic and pseudocontact contributions were neglected Coordinated Cation molecule
Co(Ill
Ni(ll)
CH_,
-0.10
- 1.00
CH2
+ 1,00
+ 0.50
CH~
+ 0.22
+ 0.2S
CH3
-
1.00
1,00
C(CH~)3
+0.31
- (I.30
CH~
-0.43
1.00
+ 1.00
0. I0
1.00
- 1.00
CH2
+ 0.90
+ 0.20
CH~
+0.30
+0.i5
CH3
- 0.03
C~H7
I
O=C
I I
O=C
I I
O=C
I
OCH~ CH~
I
O=C
L
O
L I
L I
H
+ 1.00
carbonyl group. And finally, it seems that the 7r-carbonyl orbital is partly extended onto the neighbouring hydrogen atoms of alkyl groups (hyperconjugation). This feature is intrinsic property of the investigated molecules (e.g. ketones, esters and aldehydes) and is no doubt related to the reactivity of carbonyl compounds. Acknowledgements--The authors express their sincere thanks to Dr. J. Nowacki from the institute of Basic Problems of Chemistry (Warsaw University) for synthesizing ethylmethylsulfoxide and to Miss G. Stanney (SheffieldUniversity) for help with preparing the text. REFERENCES
" t
I
') ~.-'CV-~./
Ni 2+
',
t
Fli
cr-delocalization
/
~
~
spin polarization
Fig. 8. The spin density distribution within the carbonyl group. JINCVol 39No 6 K
1. G. N. La Mar, W. Dew Horrocks, Jr. and R. H. Holm, NMR of Paramagnelic Molecules. Academic Press, New York (1973). 2. J. A. Happe and R. L. Ward, J. Chem. Phys. 39, 1211 (1963). 3. R. Fitzgerald and R. S. Drago, .L Am. Chem. Soc. 90, 2523 (1968). 4. J. Selbin, W E. Bull and L. H. Holmes, ,L lnorg. Nucl. Chem. 16, 219 (1961). 5. S. Thomas and L. Reynolds, J. Chem. Phys. 46, 4164 (1967). 6. S. Blackstaffe and R. A. Dwek, Mol. Phys. 15, 279 (1968). 7. N. S. Angerman and R. B. Jordan, lnor. Chem. 8, 2579 (1969). 8. G. S. Vigee and P. Ng, J. Inorg. Nucl. Chem. 33, 2477 (1971).
1078
K. JACKOWSKI and Z. KI~CKI
9. K. Jackowski and Z. K~cki, Advances in Molecular Relaxation Processes 5, 269 (1973). 10. M. Wicholas, Jr. Inorg. Nucl. Chem. 32, 873 (1970). 11. W. F. Currier and J. H. Weber, Inorg. Chem. 6, 1539 (1967). 12. W. L. Driessen and W. L. Groeneveld, Rec. Tray. Chim. 88, 977 (1969). 13. J. Bukowska and Z. K~cki, Roczniki Chem. 49, 11 (1975). 14. R. W. Kluiber and S. Kopycinski, J. lnorg. Nucl. Chem. 30, 1891 (1968). 15. W. L. Driessen and W. L. Groeneveld, Rec. Tray. Chim. 90, 87 (1971).
16. G. Schwartzenbach and H. Flaschka, Die komplexometrische Titration, Vedag, Stuttgart (1965). 17. H. M. McConnell and D. B. Chesnut, J. Chem. Phys. 28, 107 (1958). 18. J. P. Jesson, J. Chem. Phys. 47, 582 (1967). 19. N. A. Matwiyoff and S. V. Hooker, lnorg. Chem. 6, 1127 (1967). 20. H. M. McConnell and R. E. Robertson, J. Chem. Phys. 29, 1361 (1958). 21. W. L. Driessen, W. L. Groenveld and F. W. Van der Wey, Rec. Tray. Chim. 89, 353 (1970).
1977 Vo] 39, pp 11173 1078. Pergamon Press.
Printed in Great Britain
CONTACT SHIFT STUDIES OF SOME SULFOXIDE AND KETONE COMPLEXES OF COBALT(II) AND NICKEL(II) KAROL JACKOWSKI and ZBIGNIEW K[~CKI Laboratory of Intermolecular Interactions, Institute of Basic Problems of Chemistry, Warsaw University, ul. Pasteura 1.02-093 Warszawa, Poland (Received 28 May 1976)
Abstract--The PMR isotropic shifts of the Co(lI) and Ni(II) octahedral complexes of dimethylsulfoxide. ethylmethysulfoxide, acetone and ethylmethylketone are reported. From a consideration of the magnitude and the temperature dependence of these shifts it is concluded that the main effect, which determines the shifts, is the o--delocalizationof e, unpaired electrons in the Ni(II) and also Co(II) complexes. In the case of the ketone complexes the o--~- polarization mechanism is also necessary in order to explain the upfield shifts of the methyl and methylene groups which are directly bonded to the carbonyl group. performed using JEOL, JES-VT-2 equipment with an accuracy of -+I°C. Tetramethylsilane (TMS) and hexamethylsiloxane (HMS) were used as the internal reference standards.
INTRODUCTION
NMR contact shifts arise from the contact coupling of unpaired electrons and nuclear magnetic moments. The contact shift studied are widely and successfully applied in the investigations of paramagnetic metal complexes [1]. The octahedral complexes especially afford an excellent opportunity for the elucidation of the metal-ligand spin delocalization because of the strict separation of o-- and 7r-bonding systems[2, 3]. The hexacoordinated sulfoxide and ketone complexes of transition metal ions were extensively studied by IR and NMR methods[4-14] but the mechanism of spin delocalization was really investigated only for the diphenylsulfoxide (DPSO) complexes of iron(II), cobalt(II) and nickel(II)[10]. In this study we applied the PMR contact shift method to investigate the dimethylsulfoxide (DMSO), ethylmethylsulfoxide (EMSO), acetone and ethylmethylketone complexes of cobalt(II) and nickel(II). We hoped to learn the mechanism of spin delocalization in these complexes.
RESULTS AND DISCUSSION
Sulfoxide complexes The PMR isotropic shifts were determined for dimethylsulfoxide (DMSO) and ethylmethylsulfoxide (EMSO) complexes of cobalt(If) and nickel(II) in the appropriate sulfoxide itself as a solvent and in nitromethane solutions. In the solution of the complex in the corresponding sulfoxide the coordinated ligands rapidly exchange with the molecules in the bulk and only the averaged shift for each proton group of sulfoxide is observed (6~b~), which is a function of the molar fractions (PL, P~) and the shifts (~', 6~') of the coordinated and bulk molecules, respectively. 6,',b~= PL6r. i + PB6~ i.
EXPERIMENTAL
Materials Sulfoxide complexes. Dimethylsulfoxide (V/0Sojuzchimexport, Moscow) was dried over molecular sieves (3 A) and distilled at low pressure (at 60°C). Ethylmethylsulfoxide was prepared and purified in the Institute of Basic Problems of Chemistry (Warsaw University). Nitromethane (Xenon, L6d~) was dried with anhydrous CaSO4 and distilled. The hydrated perchlorates were obtained from appropriate chlorides and perchloric acid. The complexes of dimethylsulfoxide were prepared using the method of Selbin et al. [4] whereas the complexes of ethylmethylsulfoxide were synthesized by the method described by Currier and Weber[l 1]. Ketone complexes. Co(If), Ni(II) and Zn(II) perchlorates were obtained from the dry chlorides and AgCIO4directly in the ketone solutions. AgCIO, was dried at a temperature of 130°Cfor several days. Co[CH3.CO.H]6[InCI,]~ complex was synthesized in nitromethane solution as described in Ref. [15]. All the reagents were carefully purified and dehydrated. All preparations and all handling operations were carried out in a PzOs-dried-glovebox. The analyses of the metals were performed by complexometric titration [16]. PMR spectra. All PMR spectra were recorded with JEOL, JNM-3H-60 spectrometer. Shifts were measured with the conventional sideband technique. Temperature measurements were
(1)
The isotropic shifts (&~) could be evaluated as the values of PL, Ps and 6d were known. On the other hand, the shifts 6L~ could be observed without averaging effect, when the sulfoxide complexes were dissolved in nitromethane. The values measured for the nitromethane solutions were found to be independent of complex concentration. The isotropic shifts measured relative to the diamagnetic sulfoxide complexes of zinc(lI) are listed in Table 1. It is seen in Table 1 that the shifts measured in corresponding sulfoxides as solvents and in nitromethane solutions are convergent and the differences, which are within the range of 10%, are probably due to intermolecular interactions between the complex and solvent bulk molecules. The above comparison provides a control of the influence of solvent on the observed PMR isotropic shifts. Of importance is the fact that all the isotropic shifls are attenuating negative (downfield). Moreover, the appropriate shifts for both nickel(I1) and cobalt(II) complexes are quite similar. It suggests that the isotropic shifts are the contact shifts, without any significant pseudocontact contribution even for the cobalt(II) complexes and that the same spin delocalization mechanism is
1073
K. JACKOWSKI and Z. Kl$KI
1074
Table 1. PMR isotropic shifts for sulfoxide complexes 0018
Signalt
In sulfoxide itself
In nitromethane
Extrapolated to T-’ = 0
CH, -CH, -CH,‘M’ -CH,1 -CH,‘E’ -CH,‘“’ -CH,( -CH,‘E’
-9.OkO.3 -6.9k0.3 - 6.9 2 0.3 -23.4? 1.0 l.lkO.1 -5.OkO.3 $ -2.2kO.2
-9.720.1 -7.720.1 -6.220.1 -20.1 kO.2 -1.OkO.l -4.9kO.l -17.OkO.2 -1.8+0.1
- 2.04 +0.47 -3.80 t 13.7 -5.5 -0.63 t 12.0 0
Confplex Co(DMSO),(ClOJ, Ni(DMSO),(ClO,), Co(EMSO),(ClOJ,
Ni(EMSO)&lO&
Isotropic shift [ppm]
tFor EMS0 molecule: CH,‘“‘.SO.CH,.CH,‘“‘. SThe signal was not observed because of paramagnetic broadening. operative in the nickel(H) and cobalt(I1) complexes [2,3].
In order to verify the last assumption we investigated the temperature dependence of the isotropic shifts. The results are shown in Figs. l-3. As it is seen in Fig. 1 the entire temperature effect is a paramagnetic one; the DMSO shift of the diamagnetic zinc(I1) complex is completely independent of temperature. Each of the paramagnetic shifts is a linear function of the inverse temperature. The linearity of this function can be explained in terms of theory given by McConnell[l7]:
c
-CHF
ww +1) (yiv/2?r)3kT -140
where (AH/H) is the NMR contact shift, g is the averaged g value for the complex, p is the absolute value of the Bohr magneton, yN is the nuclear magnetogyric ratio, S is the total spin number of the electrons which are delocalized onto ligands and A, (in Hz) is the coupling constant which is proportional to the density of unpaired electrons occupying s-orbitals. However, the extrapolation of the temperature functions in Figs. l-3 does
1 -150 i -160 ' -170.
Fig. 2. The temperature dependence of the PMR isotropic shifts for Ni(EMSO)&IO,), complex in nitromethane. The shifts were measured relative to Zn(EMS0)6(CIO~)2complex.
Fig. 1. The temperature dependence of the PMR isotropic shifts for complexes: Co(DMSO),(ClO.&, Ni(DMSO)&lO& and Zn(DMSO),(ClO& in nitromethane. The shifts were measured relative to hexamethylsiloxane (HMS) as the internal reference standard.
not provide a zero value of the isotropic shifts when the temperature rises to infinity (see Table 1). These disagreements are significant for both the nickel(I1) and cobalt(I1) complexes. But since the ground state of the octahedral cobalt(H) complexes is ?f,, the disagreement with the eqn (2) is more considerable in this case. The disagreements cannot be explained on the grounds of the simple theory and more advanced treatment must be used for this purpose [ 181. Spin delocalization in sulfoxide complexes. Nickel(I1) has two e, unpaired electrons. In the octahedral complex the e, orbitals are no longer the pure atomic orbitals but they contribute to the u* anti-bonding molecular orbitals of the complex. The positive spin density may be partly found on the ligands. The spin density rapidly decreases for aliphatic protons when the distance, which is measured by the number of u-bonds from the metal, increases. Such mechanism, which is called the udelocalization mechanism, was found also in octahedral alkylamine complexes [3]. It is seen from Table 1 that the same mechanism occurs for the studied sulfoxide complexes of nickel(I1). All the PMR shifts are negative
Contact shift studies of sulfoxide and ketone complexes T Ixl03 0
300
280
IO
.
520
~
E
) - ' '3
~0 o - - - - - < ) - - - < : ~ _ . < ~ ~ -CH3 ~MJ
~40 oO~ --150 [60 4
\
- ~70 i
i i 180 4 ~90
i J
200 q
Fig. 3. The temperature dependence of the PMR isotropic shifts for Co(EMSO)dCIO4h complex in nitromethane. The shifts were measured relative to Zn(EMSO)6(C10~)2complex. (which means that there are the positive spin densities on appropriate protons) and decreasing in the alkyl chain for EMSO coordinated molecules. The case of the cobalt(lI) complexes is more complicated. In general, the delocalization mechanism can be understood if we consider the main effects which can contribute to the observed PMR shifts of the octahedral cobalt(lI) complexes. Cobalt(If) has three unpaired electrons: two e, and one G electron. The e, electrons are responsible for the same mechanism of spin delocalization as it occurs in the nickel(II) complexes (tr-delocalization). The G electron can be transferred onto ligands only in the case when the ~--bonds are formed between the metal ion and the ligands (Trdelocalization). The ~r-delocalization is significant only for planar ligands and usually dominates, when it is present in the cobalt(II) complexes. In such case the observed contact shifts for the cobalt(II) and nickel(II) complexes are quite different as it has been found for the acetonitrile complexes[19]. Usually, the pseudocontact contribution is not dominant in the octahedral cobalt(II) complexes. Therefore, the mechanism of spin delocalization is still manifested in the PMR isotropic shifts. If the ~r-delocalization from the metal ion does not occur and the pseudocontact contribution is small, then the ~-delocalization mainly determines the PMR isotropic shifts of cobalt(II) complexes. In our results, indeed, we can observe a very similar pattern of isotropic shifts in both cobalt(II) and nickel(II) complexes, which reflects the same spin delocalization. Finally, it is worth while to notice that Wicholas[10] obtained different results for the PMR isotropic shifts of the cobalt(II) and nickel(II) octahedral complexes with diphenylsulfoxide and he suggested that the pseudocontact contribution was significant for the cobalt(II) complex in an ion-paired species.
Ketone complexes Ketones belong to very weakly coordinating ligands
1(175
and special conditions must be undertaken in order to obtain their hexacoordinated complexes of transition metal ions. In the general procedure developed by Driessen and Groneveld[12] the complexes were synthesized from anhydrous metal chlorides and antimony pentachloride in the ketone solution: ketone
MCI, + 2SbCL
~ M (ketone)6(SbCl6)>
(3)
The competition between the ligands and the anions to coordinate the metal ions did not occur because the anions were large enough. It was also shown that the regular octahedral species M(O)62÷ w e r e present in these complexes. In this PMR study we used the anhydrous ketone solutions of cobalt(II), nickel(II) and zinc(II) perchlorates. It was shown that in acetone the perchlorate anions are not coordinated to the transition metal cations[13]. Therefore we can expect that M(ketone),, ~~ species will be mainly created in the perchlorate solutions. The perchlorates were used because of their excellent solubility in ketones (M(ketone)6(SbC16)2 complexes are rather poorly soluble even in the ketones themselves). We examined the P MR shifts of the acetone and ethylmethylketone molecules coordinated to the paramagnetic cobalt(II) and nickel(II) cations. If the only species present in the ketone solutions be M(ketone)6 2' (and of course CIO4 ) then the 6-coordinated ligands rapidly exchange with the bulk molecules and only the weighted average shifts of the ketone are observed (~,b~) (see eqn (1)). If we can measure the 3~b~and 6/ shifts relative to the shift of the bulk molecules (G' = 0), then:
A3~b~= P~A6L'
t4)
the observed shift (A6obs) linearly depends on the concentration of metal ions and the PMR shifts of coordinated molecules (ASL') can be easily determined from eqn (4). In Figs. 4-6 we present the results which were obtained for acetone and ethylmethylketone solutions of cobalt(H) and nickel(II) perchlorates. A3~,b~values express the only paramagnetic effects as the shifts were measured relative to the analogous solutions of zinc(II). It is seen from Fig. 4 that upfield shifts are found for the acetone molecules coordinated to both the cobalt(II) and nickel(II) cations. In the case of the ethylmethylketone
Ni(II) ro
CO(I)
()5
o;r
o'2
o;~
Fig. 4. PMR isotropic shifts of acetone in the acetone solutions of nickel(II) and cobalt(H) perchlorates (PL is the fraction of
molecules coordinated to the cations).
K. JACKOWSK[ and Z. KI~CKI
1076
a) 2.0
OI
0.2
0.3pL
0.4
0.5
0.6
1.5 -0.2
b)
) "~ 0.5
O,I /
-CH 2
0
~
0.2
0.3
Q5
0.6
_
'<] -0.2 [_
-CH3
Fig. 7. PMR shifts of acetone (a) and ethylmethylketone (b) in the solutions of zinc(II) perchlorate (PL is the fraction of molecules coordinated to the cation).
-0.5
Fig. 5. PMR isotropic shifts of ethylmethylketone in the ethylmethylketone solution of nickel(II) perchlorate.
Table 2. PMR isotropic shifts for ketone complexes Complex
Signal
(ASL~)isotropic shift [ppm]
- C H ~ M)
1.0
Co(CH3COCH3)62+ Ni(CH3COCH3)62+ Co(CH3(M)COCH2CH3tE))6 2+
'•Q.0.5
0.4
o
o
"CH3
[ /
--ca3
-CH3(~) -CH2-
-CH3(~) -CH~~M) Ni(CH~(M)COCH2CH3~E))62+ -CH2-CH¢ ~)
0
-0,2 - C H 2-
Fig. 6. PMR isotropic shifts of ethylmethylketone in the ethylmethylketone solution of cobalt(lI) perchlorate. (CH3(M).CO.CH2.CH3 (z)) solutions (Figs. 5 and 6) an upfield shift is observed for the -CH/M) group and a downfield shift for the --CH/~) group, this is also true for both the cobalt(II) and nickel(II) solutions. The shift of the methylene group (-CH2-) is upfield for the nickel(II) solution but downfield for the cobalt(II) solution. It is interesting to compare these results with the results obtained for the diamagnetic zinc(II) perchlorate, Fig. 7. The shifts due to the diamagnetic species are fairly small and less complicated: all the shifts are downfield. The diamagnetic shifts express the descreening effect of the molecules which are coordinated to the zinc(II) cation through the oxygen atom. The results placed in Figs. 4--6 enabled the isotropic shifts of coordinated molecules (ASL~) in the octahedral species to be determined. These values are listed in Table 2. Spin delocalization in ketone complexes. We become convinced that the PMR isotropic shifts obtained for the acetone and ethylmethylketone complexes can be fully explained in terms of the spin delocalization which occurs in the octahedral complexes. Moreover, it will be shown that the spin delocalization is quite similar to that found in
+ 2.95 -+0.30 + 3.60 -+0.30 +7.70_+0.50 - 1.70-+0.20 - 0.40 _+0. I0 +7.00_+0.50 + 1.10_+0.20 - 0.90 _+0.10
sulfoxide complexes of cobalt(II) and nickel(II) and in other octahedral complexes [1, 3]. If we assume that the pseudocontact contribution to the isotropic shifts [20] is small and can be neglected then the contact shift is the only contribution to the observed shifts and arises from the delocalization of unpaired electrons within the octahedral complexes. The downfield and upfield PMR shifts are related respectively to the positive and negative spin densities on the protons. The significant result of this study can be summarized (Table 2): (i) The upfield shifts (negative spin densities) were observed only for the methyl or methylene protons next to the carbonyl group; (ii) all the other shifts were downfield (positive spin densities); (iii) the shift of the cobalt(II) and nicket(II) complexes were similar (except for the shift of the methylene group in ethylmethylketone). This result can be explained in terms of the spin density delocalization within the octahedral complexes as follows: (1) The primary effect is the delocalization of two eg unpaired electrons (positive spin density) in the trbonding system. It is well-known as the tr-delocalization mechanism in the octahedral cobalt(II) and nickel(II) complexes; (2) the ~r-delocalization causes the spin polarization of the carbonyl ~r-orbital by inducing a positive spin density at the oxygen atom and a negative spin density at the carbon atom as shown in Fig. 8; (3) the negative spin density is transferred directly from the ~r-orbital to the Is hydrogen orbitals of neighbouring alkyl groups by hyperconjugation;
1077
Contact shift studies os sulfoxide and ketone complexes (4) the isotropic shifts of the nickel(II) and cobalt(II) complexes are similar because in the cobalt(II) complexes only the ee electrons are delocalized onto the ligands and the delocalization of the G electron does not occur. The 1-4 mechanism gives the proper spin densities on the ketone protons due to the characteristic distribution of spin density within the carbonyl group, Fig. 8. It is seen that the delocalization of the G electron (~--delocalization) in the cobalt(II) complexes cannot take place because it would give a significant difference in the delocalization mechanism of both the cobalt(II) and nickel(II) complexes. The ~--delocalization would transfer the positive spin density to the neighbouring alkyl protons via the 7r-orbital. Such effect is not observed for the cobalt(II) complexes. The only different shifts of the methylene protons in the cobalt(II) and nickel(II) complexes (Table 2) arise from the competition between the positive spin density (¢-delocalization) and the negative spin density (Tr-polarization plus hyperconjugation) at this site. A small pseudocontact contribution is also possible in the cobalt(II) complexes as the distribution of the cobalt(II) unpaired electrons (t~G 2) is spatially asymmetric. It was reported by Driessen and Groeneveld[15,21] that esters and aldehydes also form octahedral complexes with divalent metal ions and that the coordination of ketones, esters and aldehydes occurs in the same manner. We took this opportunity to verify the (1-4) mechanism of spin delocalization. For this purpose we estimated the relative spin densities in the cobalt(II) and nickel(II) complexes of: di-n-propylketone, t-butylmethylketone, methyl acetate, ethyl acetate and acetaldehyde. The results are listed in Table 3. It is seen from Table 3 that the negative spin densities can be found on the methyl and methylene protons next to the 7r-orbital of the carbonyl group. The attenuating positive spin densities are clearly observed for the sites where the hyperconjugation is not possible but o-delocalization still operates. The large positive spin density is obtained for the aldehyde proton as a result of o--delocalization effect. The significant down-field shift (positive spin density) of the aldehyde proton was also observed by Kluiber and Kopycinski[14] for the series of aromatic aldehydes coordinated to bis (2,4-pentanedionato) nickel(Ill and cobalt(II) complexes. All the results are consistent with the proposed (1-4) mechanism of spin delocalization within the octahedral Ni(II) and CoOl) complexes of ketones. It means that the same mechanism operates for the complexes of esters and aldehydes and that all the carbonyl molecules are coordinated to the metal ions in the same way, i.e. by the o--bond between the metal ion and the oxygen atom of the /
',.
Table 3. The relative spin densities evaluated directly from the appropriate PMR isotropic shifts. A positive spin density implies a downfield PMR shift and a negative spin density implies an upfield shift. The largest value was normalised to -+1.00. Diamagnetic and pseudocontact contributions were neglected Coordinated Cation molecule
Co(Ill
Ni(ll)
CH_,
-0.10
- 1.00
CH2
+ 1,00
+ 0.50
CH~
+ 0.22
+ 0.2S
CH3
-
1.00
1,00
C(CH~)3
+0.31
- (I.30
CH~
-0.43
1.00
+ 1.00
0. I0
1.00
- 1.00
CH2
+ 0.90
+ 0.20
CH~
+0.30
+0.i5
CH3
- 0.03
C~H7
I
O=C
I I
O=C
I I
O=C
I
OCH~ CH~
I
O=C
L
O
L I
L I
H
+ 1.00
carbonyl group. And finally, it seems that the 7r-carbonyl orbital is partly extended onto the neighbouring hydrogen atoms of alkyl groups (hyperconjugation). This feature is intrinsic property of the investigated molecules (e.g. ketones, esters and aldehydes) and is no doubt related to the reactivity of carbonyl compounds. Acknowledgements--The authors express their sincere thanks to Dr. J. Nowacki from the institute of Basic Problems of Chemistry (Warsaw University) for synthesizing ethylmethylsulfoxide and to Miss G. Stanney (SheffieldUniversity) for help with preparing the text. REFERENCES
" t
I
') ~.-'CV-~./
Ni 2+
',
t
Fli
cr-delocalization
/
~
~
spin polarization
Fig. 8. The spin density distribution within the carbonyl group. JINCVol 39No 6 K
1. G. N. La Mar, W. Dew Horrocks, Jr. and R. H. Holm, NMR of Paramagnelic Molecules. Academic Press, New York (1973). 2. J. A. Happe and R. L. Ward, J. Chem. Phys. 39, 1211 (1963). 3. R. Fitzgerald and R. S. Drago, .L Am. Chem. Soc. 90, 2523 (1968). 4. J. Selbin, W E. Bull and L. H. Holmes, ,L lnorg. Nucl. Chem. 16, 219 (1961). 5. S. Thomas and L. Reynolds, J. Chem. Phys. 46, 4164 (1967). 6. S. Blackstaffe and R. A. Dwek, Mol. Phys. 15, 279 (1968). 7. N. S. Angerman and R. B. Jordan, lnor. Chem. 8, 2579 (1969). 8. G. S. Vigee and P. Ng, J. Inorg. Nucl. Chem. 33, 2477 (1971).
1078
K. JACKOWSKI and Z. KI~CKI
9. K. Jackowski and Z. K~cki, Advances in Molecular Relaxation Processes 5, 269 (1973). 10. M. Wicholas, Jr. Inorg. Nucl. Chem. 32, 873 (1970). 11. W. F. Currier and J. H. Weber, Inorg. Chem. 6, 1539 (1967). 12. W. L. Driessen and W. L. Groeneveld, Rec. Tray. Chim. 88, 977 (1969). 13. J. Bukowska and Z. K~cki, Roczniki Chem. 49, 11 (1975). 14. R. W. Kluiber and S. Kopycinski, J. lnorg. Nucl. Chem. 30, 1891 (1968). 15. W. L. Driessen and W. L. Groeneveld, Rec. Tray. Chim. 90, 87 (1971).
16. G. Schwartzenbach and H. Flaschka, Die komplexometrische Titration, Vedag, Stuttgart (1965). 17. H. M. McConnell and D. B. Chesnut, J. Chem. Phys. 28, 107 (1958). 18. J. P. Jesson, J. Chem. Phys. 47, 582 (1967). 19. N. A. Matwiyoff and S. V. Hooker, lnorg. Chem. 6, 1127 (1967). 20. H. M. McConnell and R. E. Robertson, J. Chem. Phys. 29, 1361 (1958). 21. W. L. Driessen, W. L. Groenveld and F. W. Van der Wey, Rec. Tray. Chim. 89, 353 (1970).