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Use of proton NMR to study low symmetry bonding effects in transition metal complexes
Spectrochimica Acts, Vol. 42A, No. 1I, pp. 1331-1332, 1986.
0584-8539/86 $3.00 + 0.00 © 1986 Pergamon Journals Ltd.
Printed in Great Britain.
RESEARCH NOTE
USE OF PROTON
NMR TO STUDY LOW SYMMETRY BONDING IN TRANSITION METAL COMPLEXES
EFFECTS
(Received 11 April 1986; accepted 5 May 1986) Abstract--The 300 MHz proton NMR chemical shifts of the mixed complex (salicylaldehyde)(acetylacetone)ethylenediimino nickel(II) and the two corresponding non-mixed complexes were studied. The differences in chemical shifts were very small, ~ 0.05 ppm, but by looking at both the mixed and non-mixed species in the same solution relative values could be determined. The differences in chemical shifts were attributed to both a geometry effect and a basicity effect. In a previous paper [1] we reported the anomalous ESR spectrum of bis(salicylaldehyde)ethylenediimino copper(II), (salicylaldehyde)(acetylacetone)ethylenediimino copper(II) and bis(acetylacetone)ethylenediimino copper(II). The copper hyperflne splitting and g value data for the above complexes were not interpretable in terms of a simple average covalency effect of the type that we were able to use in other low symmetry bonding studies [2--4]. In order to examine the electron density distributions in each ligand we have investigated the proton NMR spectra of the corresponding diamagnetic Ni(II)complexes. There are several reports in the literature of characterization proton data for some of these complexes. However, none of the papers contain all the data needed for a bonding effect study.
PROCEDURE
The preparation and characterization of the compounds have been described in a recent paper [5]. Our melting point, chemical analysis and proton NMR spectra are in general agreement with the published data [5]. The proton spectra were obtained at 300 MHz on a Varian HR300. The temperatures and solvents are given in Table 1. The important factor relative to the success of this investigation is that the assignments and the small differences in chemical shifts were checked by running spectra of solutions that contained both the symmetrical and unsymmetrical complexes in relative concentrations of 2/3 and 3/2. To assist in the assignment of the aromatic protons, CNDOS I-6] was used to calculate the chemical shifts of a salicaldoxime type fragment.
RESULTS AND DISCUSSION
For the (acac)(en) part of the complex our assignments are in agreement with the published assignments [5]. For the (sal) part of the complex, we are able to extend their work by being able to assign the aromatic protons due to our use of a 300 MHz spectrometer. For H2(sal)2en a first order spectrum was observed with the expected two triplets (at 6.84 and 7.26 ppm) and two doublets (at 6.95 and 7.18). The accurate values of the two coupling constants determined from the two doublets were used in simultaneous equations to determine which couplings contributed to which triplet. Using the numbering system in Fig. 1, the couplings in H2(sal)2en are Jv.s = 8.25 Hz, J a , 9 = 7.1 and Jg.t0 = 7.25. Although the assignments are consistent, they are not unique. An interchange of numbers 7 with 10 and 8 with 9 also gives a consistent assignment. CNDOS molecular orbital calculations predict a chemical shift order of 8 > 7 > 10 > 9 while our assigned order is 8 > 10 > 7 > 9. Since the extremes agree this does provide some support for our assignment. Additional support is provided by a recent study [7] of a compound which has a CH3 group in position 9. Their ordering is 8 > 10 > 7. That the chemical shift assigned to position 7 is correctly assigned is supported by it having a small solvent dependence [5]. A comparison of the proton chemical shifts in the mixed ligand with the corresponding values for the symmetrical ligands indicates that only the ethylenediamine protons are significantly affected by the formation of the mixed ligand. This suggests a change in geometry possibly due to a change in intermolecular hydrogen bonding.
Table 1. Proton NMR chemical shifts Position No. 1 2 3 4 5 6 7 8 9 10
H2(acae)2en*
H2(sal)2en*
H2(acac)(sai)en*
Ni(acac)2en t 1.65 4.75 1.69 2.86
3.85 8.30 6.95 7.26 6.84 7.18
1.89 4.93 1.97 3.58 3.74 8.30 6.93 7.26 6.85 7.20
1.90 4.98 1.99 3.42
*300 MHz, CDCI3/TMS at 60°C. "t"300MHz, ~bNO2/TMS at 120°C. 1331
Ni(sal)zent
3.41 7.36 6.74 7.01 6.37 6.95
Ni(acac)(sal)en 1.82" 4.88 1.89 3.04 3.33 7.30 6.86 7.07 6.43 6.96
1.69t 4.80 1.72 2.94 3.28 7.31 6.68 6.97 6.34 6.98
1332
Research Note
--0.24
--1.00
H l°
CH6"~N
h -0.42
-0.41 --0.54 }n~r-C~4
/
AC--C~
-0.08
N-~("
/
\/
\
/IN~
AC--H2-0'05
\c=/
Fig. 1. Differences in chemical shifts between Ni(acac)(sal)en and H2(acac)(sal)en (300 MHz, CDCI3/TMS at 60°C).
The effects of replacing the two hydrogens with a Ni(II) are seen in Figs. 1-3. As discussed by FLISZARet al. [8], one can expect that changes in chemical shifts are proportional to changes in total electron density for closely related atoms, i.e. constant hybridization and same nearest neighbors. All of the sal protons show a relatively large increase in shielding especially the sal proton that is para to the phenolic oxygen. This position is also the same number of atoms away from the nitrogen. Since there is not an alternating charge effect, it appears that nickel electron density is being donated through both the oxygen and nitrogen. The acac protons have smaller increases in shielding than the sal protons. In addition, the increase in shielding in the symmetrical acac complex (Fig. 3) is larger than the increase in the mixed complex (Fig. 1). These results are consistent with a model in which the nickel is better able to donate electron density to the weaker base, sal, than to the stronger base, acac. A comparison of the mixed complex with the corresponding symmetrical complex is given in Fig. 4. We would expect that in a mixed system the stronger sigma electron donor, acac, would lose electron density to the less basic sal ligand. The results support this model. However, the larger change in the ethylenediamine protons suggests that a geometry effect may also be present. The ethylenediamine is expected to lie out of the plane of the rest of the complex with the amount of distortion depending on the hybridization of the nitrogens and the necessary bite for the metal ion. From X-ray [9, 10] the N - N i - N angle in Ni(sal)2en was found to be 85 ° while in Ni(acachen it is 88 °. It is possible that the mixed environment allows the sal group to be in a more favorable bonding position relative to the nickel d orbitals. It would be this geometry effect which would result in the anomalous ESR spectrum referred to in the introduction, as our earlier studies
-0,44 CH2~ H
\
-0.47 H
-0.94
/
CH=N
J
\/
/-%_/\ \c__c / / \ H
--0.25
\/ Nl
\/
o/\
C
CH3
-0.10
~H v -0.07
-0.23
C-----N
-0.10 H
008
_0.19H8 /
-0.38
/ HI-O'07
H
-0.21
Fig. 2. Differences in chemical shifts between Ni(salhen and H2(salhen (300 MHz, CDCI3/TMS at 60°C).
Fig. 3. Differences in chemical shifts between Ni(acac)2en and H2(acachen (300 MHz, CDCI3/TMS at 60°C).
-0.13 +0.08 +0.03
_0.05
H
CH---N
\ C--C /
// -0.03 H - - C
%
\ / C~--C / \ H
--0.04
+0.04
C/H2-CH2
\/
N
/Ha C
NI
C--
/\
C--O
\
// O--C
.. 0.05
\ CH3 +0. 03
H
--0.06
Fig. 4. Differences in chemical shifts between Ni(acac)(sal)en and Ni(acac)2en or Ni(sal)gen (300 MHz, ~bNO2/TMS at 120°C).
[2~1] did not have the geometry restricting ethylenediamine bridge between the two chelates. Department of Chemistry, The University of Akron, Akron, Ohio 44325, U.S.A.
H.A. KUSKA E.P. GAUSE
REFERENCES [1] H. A. KUSKA, M. F. FARONA, P. PAPPUS and S. POTTERTON, J. Coord. Chem. 1,259 (1971). [2] M. F. FARONA, D. C. PERRY and H. A. KUSKA, lnorg. Chem. 7, 2415 (1986). [3] H. A. KUSKA, J. Am. chem. Soc. 97, 2289 (1975). [4] H. A. KUSKA and D. H. BEEBE, lnorg. Chem. 17, 198 0978). [5] E. KWIATKOWSKIand M. KWlATKOWSKI,lnorg, chim. Acta 42, 197 (1980). [6] M. RAJZMANNand P. FRANCOIS,QCPE 12, 382 0979). [7] E. KWIATKOWSKIand M. KW1ATKOWSKI,lnorg, chim. Acta 82, 101 (1984). [8] S. FLISZAR,G. CARDINAL and M. BERALDIN,J. Am. chem. Soc. 104, 5287 (1982). [9] L. M. SHKOLN1KOVA,E. M. YUMAL,E. A. SHUGAMand V. A. VOBLIKOVA,J. Struct. Chem. 11,819 (1970). [10] F. CARIATI,F. MORAZZONI,C. BUSETTO, G. DEE PIERO and A. ZAZZETTA,J. chem. Soc. Dalton 342 (1976).
0584-8539/86 $3.00 + 0.00 © 1986 Pergamon Journals Ltd.
Printed in Great Britain.
RESEARCH NOTE
USE OF PROTON
NMR TO STUDY LOW SYMMETRY BONDING IN TRANSITION METAL COMPLEXES
EFFECTS
(Received 11 April 1986; accepted 5 May 1986) Abstract--The 300 MHz proton NMR chemical shifts of the mixed complex (salicylaldehyde)(acetylacetone)ethylenediimino nickel(II) and the two corresponding non-mixed complexes were studied. The differences in chemical shifts were very small, ~ 0.05 ppm, but by looking at both the mixed and non-mixed species in the same solution relative values could be determined. The differences in chemical shifts were attributed to both a geometry effect and a basicity effect. In a previous paper [1] we reported the anomalous ESR spectrum of bis(salicylaldehyde)ethylenediimino copper(II), (salicylaldehyde)(acetylacetone)ethylenediimino copper(II) and bis(acetylacetone)ethylenediimino copper(II). The copper hyperflne splitting and g value data for the above complexes were not interpretable in terms of a simple average covalency effect of the type that we were able to use in other low symmetry bonding studies [2--4]. In order to examine the electron density distributions in each ligand we have investigated the proton NMR spectra of the corresponding diamagnetic Ni(II)complexes. There are several reports in the literature of characterization proton data for some of these complexes. However, none of the papers contain all the data needed for a bonding effect study.
PROCEDURE
The preparation and characterization of the compounds have been described in a recent paper [5]. Our melting point, chemical analysis and proton NMR spectra are in general agreement with the published data [5]. The proton spectra were obtained at 300 MHz on a Varian HR300. The temperatures and solvents are given in Table 1. The important factor relative to the success of this investigation is that the assignments and the small differences in chemical shifts were checked by running spectra of solutions that contained both the symmetrical and unsymmetrical complexes in relative concentrations of 2/3 and 3/2. To assist in the assignment of the aromatic protons, CNDOS I-6] was used to calculate the chemical shifts of a salicaldoxime type fragment.
RESULTS AND DISCUSSION
For the (acac)(en) part of the complex our assignments are in agreement with the published assignments [5]. For the (sal) part of the complex, we are able to extend their work by being able to assign the aromatic protons due to our use of a 300 MHz spectrometer. For H2(sal)2en a first order spectrum was observed with the expected two triplets (at 6.84 and 7.26 ppm) and two doublets (at 6.95 and 7.18). The accurate values of the two coupling constants determined from the two doublets were used in simultaneous equations to determine which couplings contributed to which triplet. Using the numbering system in Fig. 1, the couplings in H2(sal)2en are Jv.s = 8.25 Hz, J a , 9 = 7.1 and Jg.t0 = 7.25. Although the assignments are consistent, they are not unique. An interchange of numbers 7 with 10 and 8 with 9 also gives a consistent assignment. CNDOS molecular orbital calculations predict a chemical shift order of 8 > 7 > 10 > 9 while our assigned order is 8 > 10 > 7 > 9. Since the extremes agree this does provide some support for our assignment. Additional support is provided by a recent study [7] of a compound which has a CH3 group in position 9. Their ordering is 8 > 10 > 7. That the chemical shift assigned to position 7 is correctly assigned is supported by it having a small solvent dependence [5]. A comparison of the proton chemical shifts in the mixed ligand with the corresponding values for the symmetrical ligands indicates that only the ethylenediamine protons are significantly affected by the formation of the mixed ligand. This suggests a change in geometry possibly due to a change in intermolecular hydrogen bonding.
Table 1. Proton NMR chemical shifts Position No. 1 2 3 4 5 6 7 8 9 10
H2(acae)2en*
H2(sal)2en*
H2(acac)(sai)en*
Ni(acac)2en t 1.65 4.75 1.69 2.86
3.85 8.30 6.95 7.26 6.84 7.18
1.89 4.93 1.97 3.58 3.74 8.30 6.93 7.26 6.85 7.20
1.90 4.98 1.99 3.42
*300 MHz, CDCI3/TMS at 60°C. "t"300MHz, ~bNO2/TMS at 120°C. 1331
Ni(sal)zent
3.41 7.36 6.74 7.01 6.37 6.95
Ni(acac)(sal)en 1.82" 4.88 1.89 3.04 3.33 7.30 6.86 7.07 6.43 6.96
1.69t 4.80 1.72 2.94 3.28 7.31 6.68 6.97 6.34 6.98
1332
Research Note
--0.24
--1.00
H l°
CH6"~N
h -0.42
-0.41 --0.54 }n~r-C~4
/
AC--C~
-0.08
N-~("
/
\/
\
/IN~
AC--H2-0'05
\c=/
Fig. 1. Differences in chemical shifts between Ni(acac)(sal)en and H2(acac)(sal)en (300 MHz, CDCI3/TMS at 60°C).
The effects of replacing the two hydrogens with a Ni(II) are seen in Figs. 1-3. As discussed by FLISZARet al. [8], one can expect that changes in chemical shifts are proportional to changes in total electron density for closely related atoms, i.e. constant hybridization and same nearest neighbors. All of the sal protons show a relatively large increase in shielding especially the sal proton that is para to the phenolic oxygen. This position is also the same number of atoms away from the nitrogen. Since there is not an alternating charge effect, it appears that nickel electron density is being donated through both the oxygen and nitrogen. The acac protons have smaller increases in shielding than the sal protons. In addition, the increase in shielding in the symmetrical acac complex (Fig. 3) is larger than the increase in the mixed complex (Fig. 1). These results are consistent with a model in which the nickel is better able to donate electron density to the weaker base, sal, than to the stronger base, acac. A comparison of the mixed complex with the corresponding symmetrical complex is given in Fig. 4. We would expect that in a mixed system the stronger sigma electron donor, acac, would lose electron density to the less basic sal ligand. The results support this model. However, the larger change in the ethylenediamine protons suggests that a geometry effect may also be present. The ethylenediamine is expected to lie out of the plane of the rest of the complex with the amount of distortion depending on the hybridization of the nitrogens and the necessary bite for the metal ion. From X-ray [9, 10] the N - N i - N angle in Ni(sal)2en was found to be 85 ° while in Ni(acachen it is 88 °. It is possible that the mixed environment allows the sal group to be in a more favorable bonding position relative to the nickel d orbitals. It would be this geometry effect which would result in the anomalous ESR spectrum referred to in the introduction, as our earlier studies
-0,44 CH2~ H
\
-0.47 H
-0.94
/
CH=N
J
\/
/-%_/\ \c__c / / \ H
--0.25
\/ Nl
\/
o/\
C
CH3
-0.10
~H v -0.07
-0.23
C-----N
-0.10 H
008
_0.19H8 /
-0.38
/ HI-O'07
H
-0.21
Fig. 2. Differences in chemical shifts between Ni(salhen and H2(salhen (300 MHz, CDCI3/TMS at 60°C).
Fig. 3. Differences in chemical shifts between Ni(acac)2en and H2(acachen (300 MHz, CDCI3/TMS at 60°C).
-0.13 +0.08 +0.03
_0.05
H
CH---N
\ C--C /
// -0.03 H - - C
%
\ / C~--C / \ H
--0.04
+0.04
C/H2-CH2
\/
N
/Ha C
NI
C--
/\
C--O
\
// O--C
.. 0.05
\ CH3 +0. 03
H
--0.06
Fig. 4. Differences in chemical shifts between Ni(acac)(sal)en and Ni(acac)2en or Ni(sal)gen (300 MHz, ~bNO2/TMS at 120°C).
[2~1] did not have the geometry restricting ethylenediamine bridge between the two chelates. Department of Chemistry, The University of Akron, Akron, Ohio 44325, U.S.A.
H.A. KUSKA E.P. GAUSE
REFERENCES [1] H. A. KUSKA, M. F. FARONA, P. PAPPUS and S. POTTERTON, J. Coord. Chem. 1,259 (1971). [2] M. F. FARONA, D. C. PERRY and H. A. KUSKA, lnorg. Chem. 7, 2415 (1986). [3] H. A. KUSKA, J. Am. chem. Soc. 97, 2289 (1975). [4] H. A. KUSKA and D. H. BEEBE, lnorg. Chem. 17, 198 0978). [5] E. KWIATKOWSKIand M. KWlATKOWSKI,lnorg, chim. Acta 42, 197 (1980). [6] M. RAJZMANNand P. FRANCOIS,QCPE 12, 382 0979). [7] E. KWIATKOWSKIand M. KW1ATKOWSKI,lnorg, chim. Acta 82, 101 (1984). [8] S. FLISZAR,G. CARDINAL and M. BERALDIN,J. Am. chem. Soc. 104, 5287 (1982). [9] L. M. SHKOLN1KOVA,E. M. YUMAL,E. A. SHUGAMand V. A. VOBLIKOVA,J. Struct. Chem. 11,819 (1970). [10] F. CARIATI,F. MORAZZONI,C. BUSETTO, G. DEE PIERO and A. ZAZZETTA,J. chem. Soc. Dalton 342 (1976).