/Ve /Va- q45
AN NCR INVESTIGATION OF ARYL
MERCURY COMPOUNDS
THESIS
Presented to the Graduate Council of the
North Texas State University in Partial
Fulfillment of the Requirements
For the Degree of
MASTER OF SCIENCE
By
Keith E. Rowland, B. S.
Denton, Texas
May, 1987 Rowland, Keith E., An NMR Investigation of Aryl Mercury
Com pounds. Master of Science (Chemistry), May, 1987, 39 pp.,
8 tables, 3 illustrations, bibliography, 35 titles.
A variable temperature C and 19'Hg NMR study has been conducted for diphenyl-, bis(o-tolyl)-, bis(m-tolyl)-, and bis(2, 6-xylyl)mercury in dimethyl sulfoxide and 1,1,2,2 tetrachloroethane; 13C Ti relaxation times are reported as a function of temperature. Barriers to rotation of the aryl rings are obtained. Chemical shifts and couplings in CDCI3 are given for bis(p-tolyl)-, bis(2, 5-xylyl)-, bis(mesityl)-, phenyl(o-tolyl)-, phenyl(fn-tolyl)mercury, and the compounds listed above. The steric interactions of these aryl mercury compounds are discussed. TABLE OF CONTENTS
page LIST OF TABLES . .- "-- --. -- .r.. ." . . . . .0 iv
LIST OF ILLUSTRATIONS - ...... V
Chapter
I. r INTRODUCTION . ."...... ".r.. . . 1
II. EXPERIMENTAL r-...... 6
Synthesis of Diaryl Mercury Compounds Preparation of NMR Spectra Assignments of Chemical Shifts T Relaxation Times NME Measurement
III. RESULTS AND DISCUSSION .. 0 . . . . 13
Chemical Shifts and Couplings Introduction to Molecular Dynamics Molecular Dynamics Data Conclusions
IV. BIBLIOGRAPHY .37. - . . .00..0*..0 .. 37 LIST OF TABLES
Tables Page
I. 13C Data for Diaryl Mercury Compounds r . 14
II. 3C Chemical Shift Substituent Parameters for Diaryl Mercury Compounds ...... 15
III. 9Hg Chemical Shifts for Diaryl
Mercury Compounds ...... * . 17
IV. 13C T Data for Diaryl Mercury Compounds . . 24
V. HOE Data for Diaryl Mercury Compounds. . . . 25
VI. Energies of Activation for Diaryl
Mercury Compounds ...... 26
VII. Tumbling Ratios of Diaryl Mercury Compounds . . 31
VIII. Barriers to Methyl Rotation . . . .0. . .32
iv LIST OF ILLUSTRATIONS
Figure Page
1. Arrhenius Plots for Rotation About the Preferred Axis ...... 27 13 2. C T Data for Phenyl- (o-tolyl)mercury . . 0. &. 0. 9 . 0. 0.". 0. 29
3. NOE Data for the methyl carbons of Bis(2, 5-xylyl )mercury...... 33
1y CHAPTER 1
INTRODUCTION
Knowledge of the nature of carbon-metal bonds is im- portant in understanding many areas of chemistry. Steric constraints in a molecule play an important role in deter- mining its reactivity. Barriers to rotation have been one of the more important tools in studying these constraints by evaluating both double bond character and intramolecular interactions in organic molecules (1). There has, however, been little information on the rotation and dynamics of carbon-metal sigma bonds.
Traditionally, barriers to rotation have been measured by many different methods. Line shape analysis and co- alescence allow the determination of barriers in the range of 4.7-27 kcal/mol (2). Below this range the intermolecular processes occur too rapidly on the NMR timescale. Microwave and infrared methods have been utilized to explore this lower region and racemization or epimerization have been used to explore the region above that of coalescence (2).
T relaxation times can be accurately measured by NMR
(3). Since these relaxations are dependent on molecular motions, NMR can be used to explore above and below the aforementioned ranges. Relaxation experiments can extend the
I 2
range from near 0 to 30 kcal/mol. This broader range, how- ever, has not been fully utilized, and experimentation has been limited primarily to the rotation in methyl groups
(4, 5).
This study will explore the applicability of 13C re- laxation times to measuring the rotation about carbon-metal bonds. The questions to be answered are: 1) are the barriers to rotation about these bonds in a range that can be mea- sured by these techniques and 2) are the barriers sensitive enough to the small steric changes within the molecule to be an effective tool in the determination of the molecular interactions.
A molecule for this study would need to have a pro- tonated carbon in which the carbon-hydrogen vector is in a defined position in reference to the rest of the molecule.
It would also require a protonated carbon whose relaxation is independent of the internal rotation and is only effected by the tumbling of the entire molecule. Phenyl rings fit these criteria very well. The ortho and meta carbon-hydrogen bonds have a specific geometry with respect to the preferred axis and the pars carbon-hydrogen bond lies along this axis, unaffected by the internal rotation of the ring.
Aryl mercury compounds are the choice for these studies. They are known to be linear in the solid state (6).
They are stable in air, heat, and most organic solvents (7). 3
They are relatively easily synthesized and different functionalities can be placed on the phenyl rings. 199Hg NMR is now routine, and 199Hg-13C coupling constants can easily be seen.
The method to be employed for measuring the energy of activation in phenyl rings has been previously developed by
Kimber and Harris (8). For diphenylmercury a value of 4.0
0.2 kcal/mol (9) was determined for the energy of acti- vation of the rotation of the molecule about the preferred axis. The activation energy for the para carbon was found to be 3.1 0.1 kcal/mol which compares well with the value for the mercury atom of 3.18 0.02 (9); these two similar values indicate that the same motional parameters govern the relaxation of the mercury and para carbons, which is con- sistent with a linear molecule, in solution.
NMR T relaxation and NOE data for a series of bis- methyl-substituted aryl mercury compounds in coordinating, dimethyl sulfoxide (d6), and non-coordinating,' 1, 1, 2, 2 tetra- chloroethane (d2), solvents will be presented. This data is part of a study of the steric interactions in aryl mercury compounds.
Many of the chemical shift studies to date have cen- tered on establishing a relationship between electronic sub- stituent effects and variations in i9Hg and 13C chemical
shifts, thus purposely avoiding derivatives with ortho- 4 substituents. There are only limited data for ortho-substi- tuted derivatives (10,11) and almost no data for bis-ortho- substituted derivatives (12). As part of the study of steric effects in organometallic compounds, I now report 13C and
1 9 9 Hg data for the series of methyl-substituted aryl mercury compounds in deuteriochloroform. Although partial data for phenyl mercury and for the bistolyl compounds have been reported earlier, these compounds are included here for comparison in a common solvent. CHAPTER BIBLIOGRAPHY
1. Jackman, L. M., Cotton, F. A., Ed., Dynamic Nuclear Magnetic Resonance Spectroscopy, New York, Academic Press, 1975.
2. Lambert, J. B., Nienhuis, R. J., and Keepers, J. W., Angew. Chem. I nt. Ed. Engl., 20 (1981), 487.
3. Levi, G. C., Ed., T pics in Carbon-13 NMR Spectroscopy, v. 1, New York, John Wiley and Sons, Inc., 1974.
4. Woessner, D. E., J. Chem. Phys., 36 (1962), 1.
5. Woessner, D. E., Snowdon, B. J., and Meyer, G. H. , J. Chem. Phys., 50 (1969), 719.
6. a) Liptak, D., Ilsley, W. H., Glick, M. D., and Oliver, J. P., J. Orqanometal. Chem., 191 (1980), 339. b) Pakhomov, V. I., Kitaizonskii, A. I. Zh. Struct. Khim.. 7 (1966), 860. c) Kuncher, N. R., Mathew, M., Chem. Commun., 3 (1966), 71.
7. Coates, G. E., and Wade, K., The Main GrowpElements, Volume 1 of OrganometallicCompounds, Coates, Green, and Wade, London, Methuen, 1967.
8. Harris, R. K., and Kimber, B. J., Advan. MoI. Relaxation Processes, 8 (1976), 23.
9. Gillies, D. G., Blaauw, L. P., Hays, G. R., Elis, R., and Clague, A. D. H., J. Magn. Reson., 42(3) (1981), 420
10. Grishin, Yu. K., Strelenko, Yu. A., ?argulis, L. A., Ustynyuk, Yu. A., Golochenko, L. S., Peregudov, A. S., and Kravtdov, D. N., Dok. Akad. Nauk SSSR, 249 (1979), 892.
11. Grishin, Yu. K., Roznyatovskii, V. A., Vanchikov, A. N., and Ustynyuk, Yu. A.,Vestn. Mosk. Univ., 22 (1981), 374.
12. Huffman, J. C., Nugent, W. A., and Kochi, 5. K., Inorq. Chem., 19 (1980), 2749.
5 CHAPTER II
EXPERIMENTAL
Synthesis of Diaryl Mercury Compounds
The diarylmercury compounds were synthesized, using a method compiled by Nesmeyanov (1). These compounds were prepared by first creating a Grignard reagent and then reacting it with mercuric chloride in tetrahydrofuran (THF).
The resulting aryl mercuric halide is then reduced using sodium stannite to form the symmetrical compound (2).
Since all symmetrical compounds were synthesized using the same methods, only the synthesis of bis(o-tolyl)mercury will be described in detail. This synthesis was begun by placing 7.17g (0.29 mole) of magnesium (Mg) turnings in a
three-necked flask fitted with a dropping funnel, mechanical stirrer, and condenser. The Mg and the entire apparatus was
heated while purging the system with argon to remove all
traces of water. Approximately 200 ml of THE was decanted
into the flask and then a few ml of 1,2-dibromoethane and 2-
bromotoluene were added to help initiate the reaction. After
the reaction began, the remaining portion of the 2-bromo-
toluene was diluted with THE and added dropwise until a
total of 49.7g (0.29 moles) was used. To ensure the reaction
6 7
was complete the mixture was stirred for two hours while cooling to room temperature.
The results of the Grignard reaction were decanted into a duplicate of the apparatus just described. In 110 ml of
THF, 41.45g (0.15 mole) of mercuric chloride (HgCl2 ) was dissolved and this solution was placed in a dropping funnel.
The mixture was brought to ref lux and the HgCl 2 solution was added dropwise, this reaction was allowed to reflux overnight.
After refluxing, the results were allowed to cool and
water was used to quench any remaining Grignard; this also
dissolved any magnesium salts. The THF and water layers were
separated and the water layer was extracted with three 50 ml
portions of THF which were combined with the THF layer.
To begin the symmetrization reaction, the THF solution
was poured into a three-necked flask and cooled in an ice
bath. The flask was fitted with a mechanical stirrer and
jacketed dropping funnel. In two separate beakers, 10g of
NaOH and 36g of stannous chloride were dissolved in 10 and
150 ml of water respectively. These two solutions were
cooled to 0 C in an ice bath; they were then mixed to form
sodium stannite. Care was taken to keep all solutions below
10 C to inhibit decomposition. The jacket of the dropping
funnel was filled with ice and the sodium stannite solution
was poured into the funnel, then added to the reaction flask 8 very slowly to prevent the exothermic reaction from raising the temperature. This solution was stirred for. two hours after the addition of sodium stannite.
The results of the symmetrization reaction were ex- tracted with chloroform, dried with magnesium sulfate, and the chloroform was removed by distillation. The resulting solid was recrystallized from ethanol. A total of 19.88g, a
36 per cent yield based on o-bromotoluene, of white needle- like crystals were recovered which had a melting point of
106-109*C, which compares well with the value of 107C for bis(o-tolyl)mercury listed below.
The uncorrected melting point for each compound (with literature values (3) in parentheses) are as follows:
(C H )2Hg, 123-124C (120C); (o-CH3C6H4)2Hg, 106-109C
(107 C); (m-CH3 C6H)2Hg, 98-99C (102C); (p-CH3 H4 )2 Hg, 241-
242C (238C); C2, 5-(CH 3 )2C 6 H 3 J2 Hg, 125C (123*); 12,4,6-
(CH3 )3C6H22Hg, 240WC (236C); [2, 6-(CH ) C6H3 2Hg, 180*C.
Analysis: calculated for C1 6 H1 8 Hg, C 46.76, H4.42; found, C
46.64, H 4.46 per cent. Elemental analysis was performed by
Schwartzkopf Microanalytical Laboratory, Woodside, NY, USA.
The synthesis of the unsymmetrical compound was carried out using the same apparatus described above (4). The two unsymmetrical compounds were synthesized in the same manner, therefore only the synthesis of phenyl(o-tolyl)mercury will be described. 9
First, tolyl Grignard, from reacting 2.60g (0.015 mole) of tolyl halide with 8.Og Mg turnings in THF, was reacted with 162.Og (0.45 mole) HgBr2. A total of 19.Og (0.05 mole) of the resulting tolyl mercuric halide was then reacted with an excess of 0.075 mole phenyl Grignard, from the reaction of 10.1g (0.1 mole) of bromobenzene with 4.Og (0.15 mole) magnesium, at -10 *C to form phenyl tolyl mercury. The ad- ditional Grignard was quenched with water. The results were extracted with ether. The result was 14.5g of pale yellow crystals, a yield of approximately 50 per cent. The melting points for the o-tolyl and m-tolyl derivatives were 64-66*C and 63-73 C, respectively. The literature value for phenyl-
(o-tolyl)mercury is 65 C and for phenyl(m-tolyl)mercury is
65-70*C. Solutions of these compounds contained significant amounts of the corresponding bis-compounds, which were only partially removed by recrystallization. The NMR evidence for these compounds is discussed below.
Preparation of NMR Samples
The samples for 13C NMR measurement were placed in 10 ml NMR tubes with concentrations ranging from 0.2 to 0.7 M, depending on the solubility of the diarylmercury compound in a given solvent. All samples were degassed either by three freeze-pump-thaw cycles followed by sealing on a vacuum line, or by bubbling argon through the samples for no less than 20 minutes. The samples degassed with argon were 10
covered with parafilm and therefore had to be continually
of degassed every few days during an experiment. All samples diphenyl-, bis(o-tolyl)-, and bis(m-tolyl)mercury were de- gassed on the vacuum line, the unsymmetrical compounds and xylyl-compounds were degassed with argon.
Determination of NHMR Spectra
The 13C and 199Hg spectra were acquired on a JEOL FX90Q spectrometer at 22.5 and 16.0 MHz respectively with a sweep width of 2500 Hz for 13C and 5000 Hz for 99Hg. The temper- ature was controlled when necessary with a JEOL JMN-VT-3B temperature controller. The sample temperature was deter- mined by placing a 10mm NMR tube of solvent, fitted with a thermometer, into the probe, letting the temperature stabilize for 10 minutes, then removing the thermometer and immediately recording the temperature. This procedure was carried out before and after each run to help ensure a constant temperature was maintained. By utilizing this method the temperature of the sample remained constant to within one degree Celcius.
Assignment of Chemical Shifts
The 13C chemical shifts were assigned relative to
internal solvent peaks as follows: CDC1 3 , 77.0 ppm; dimethyl
sulfoxide(d6), 39.5 ppm; carbon tetrachloride (CC1 4 ), 96.7
ppm (6). When measuring 199Hg chemical shifts a capillary of 11
diethylmercury, -289 ppm (7), was placed in the center of the sample tube as a reference.
T Relaxation Times
The T relaxation times were measured using the standard inversion recovery sequence and calculated from peak intensities with a semi-logrithmic plot provided by the
JEOL software. No less than six tau values were used for each experiment. The pulse delay was maintained greater than five times the T relaxation time. The temperature was kept at a constant level as described earlier.
NOE Measurement
The nuclear Overhauser enhancement (NOE) was measured by taking the ratio of the integration of peaks obtained from a spectrum with complete decoupling, full NOE, with those obtained from a spectrum in which the the decoupling pulse is only gated on during acquisition. Since NOE re- quires time to build up to maximum, this latter spectrum has no NOE and still maintains decoupling of the protons. During
NOE measurement the pulse delay was kept greater than ten times the T relaxation time to ensure the magnetization was completely relaxed. CHAPTER BIBLIOGRAPHY
1. Makarova, L. G., and Nesmeyanov, A. N., The Organic of Compounds of Mercury, Vol. 4 of Series: Methods Elemento-Oranic Chemistry, edited by A. N. Nesmeyanov and K. A. Kocheshkov, Amsterdam, North Holland Publishing Company (1967).
2. Nesmeyanov, A. N., Borisov, A. E., and Novikova, N. V., Izv. Akad. Nauk SSSR. Otdel. Khim. Nauk (Engl. transl.), (1959) 1174.
3. Whitmore, F. C., -Organic compounds of Mercury. ACS Mono- graph Series. Chemical Catalog Co., New York (1921).
4. Unsymmetrical compounds were synthesized by Jose E. Cortez.
5. Kharasch, M. S. , and Flenner, A. L. , J. Am. Chem. Soc. , 54 (1932), 674 b). Kharasch, M. S., and Marker, R., J. Am. Chem. Soc., 48 (1926), 3130.
6. Stothers, J. B., Carbon-13 NMR Spectroscopy, Academic Press, New York (1972).
7. Sens, M. A., Wilson, N. K., Ellis, P. D., and Odom, J. D. ,J.Magn=. Reson., 19 (1975), 323.
12 CHAPTER III
RESULTS AND DISCUSSION
Chemical Shifts and Couplings
The 13C chemical shifts and 13C-199Hg coupling con- stants for seven bisarylmercury compounds are reported in
Table 1 (1). Quaternary carbons were assigned based on their low relative intensities. Coupling constants were used to identify the number of bonds between the mercury atom and a carbon atom based on the relationship J1 > J3 > J2 > J4 (2).
The only assignments that remained ambiguous were for -C3 and
C6 for bis(2, 5-xylyl)mercury. The similarity of the coupling constants for these carbons compared to those of bis(o- tolyl)mercury, and the additivity relationships presented below were the basis for these assignments. With only a minor variation in the '3C-1 9Hg coupling constants, the chemical shifts and coupling constants compare favorably with values previously reported for diphenylmercury and bis(o-tolyl)mercury in chloroform (3). The assignments are also consistent with chemical shift data reported for diphenylmercury and bis(p-tolyl)mercury in other solvents
(4).
In Table II (1) are listed the changes in 13C chemical shifts on replacement of a ring carbon by an -HgR group.
13 14
TABLE I
1 C DATA FOR DIARYL MERCURY COMPOUNDS
13C a!(J)b
R Group Cl C2 C3 C4 CS C6 o-CH3 m-CH3 p-CH3
Phenylc 170.5 137.6 128.5 128.2 128.5 137.6 ...... d (85.7) (100.0) (8.8) (100.0) (85.7) .. .. « .. o-Tolylc 172.0 1144.6)e 129.9 127.9 125.3 136.8 25.0 ..i .. (1124.8) (64.7) (70.8) (13.4) (98.9) (83.0) (101.3) .. .. m-Tolylc 170.2 138.2 [137.9] 129.0 128.4 134.4 .. 21.7 (1162.1) (85.9) (99.6) (17.6) (103.5) (85.0) .. d
0 p-To1yl 167.3 137.4 129.4 [137.9] 129.4 137.3 .. .. 21.4 d (87.9) (102.5) d (102.5) (87.9) .. .. d
2, 5-Xylylc 172.0 (141.22 129.7 128.6 (134.43 137.6 24.6 21.2 .4 d (64.5) (74.2) (14.6) (98.6) (84.0) (102.6) d ..
2,6-Xylylc 173.0 (144.23 127.1 128.0 127.1 [144.23 24.4 .. . d (60.6) (69.3) (7.8) (69.3) (60.6) (108.4) ".. ..
Mesitylc 169.9 f1144.3] 128.1 [137.8] 128.1 (144.3) 24.3 .. 20.9 d (62.5) (71.3) (8.8) (71.3) (62.5) (107.4) .. (7.8) Phenyl(m-tolyl)fff Phenyl ring 170.9 137.6 128.6' 128.2 128.6 137.6 .. .. m-Tolyl ring 170.0 138.2 ' 137.8] 129.0 128.4 134.4E .i. 21.7 .
Phenyl o-tolyl)f Phenyl ring 170.5 137.4 128.6 128.2 128.6 137.4 .. .. . (85.5) (85.5) ...... o-Tolyl ring 172.3 (144.43 129.9' 127.9 125.4 137.1 25.4 .. . . (83.0) .. .. ."
a1 3 C chemical shift relative to chloroform (dl) at 77.0 ppm ( 0.1 ppm). bMercury-carbon coupling constant in Hz ( 0.5 Hz). cBis aryl mercury compound, R-Hg-R. Coupling not observed. Brackets denote methyl substituted carbon. Not resolved from the corresponding bisarylmercury compound present in the sample. gUnsymmetrical aryl mercury compound, R-Hg-R'. 15
Chemical shifts of the parent hydrocarbon were taken from the literature (5). The nearly constant downfield chemical shift change of 42.3 0.6 ppm for C-I has proved useful in differentiating between bisarylmercury compounds and aryl- mercury halides. The carbon chemical shift change for C1 of arylmercury halides is more variable, but is typically in the 20-30 ppm range (6). The 13C-199Hg coupling for C1 can also be used to differentiate between these two compounds, but is more difficult to observe due to the poor sensitivity of the mercury substituted ring carbon.
TABLE II
13C CHEMICAL SHIFT SUBTITUENT PARAMETERS FOR DIARYL MERCURY COMPOUNDS
R Group C1 C2 C3 C4 CS C6 o-CH3 -CH3 p-CH3
Phenyl 41.8 8.9 -0.2 -0.5 -0.2 8.9 ...... o-Tolyl 42.7 (6.73 0.6 -0.6 -0.3 8.3 3.7 .. .. u-Tolyl 41.7 8.9 (0.13 -0.3 -0.1 8.8 .. 0.4 p-Tolyl 41.7 8.9 0.1 [0.13 0.1 8.9 .. .. 0.1
2,5-Xylyl 42.9 (6.73 0.6 -0.5 (-0.1] 8.5 3.7 0.3
2,6-Xylyl 42.9 (6.73 0.7 -0.3 0.7 (6.7] 3.1 .. ..
Mesityl 42.5 (6.7] 0.7 (0.23 0.7 (6.7] 3.1 .. -0.3
R 2 Hg RH' Brackets denote methyl-substituted carbons. 16
The apparently similar effects on a methyl-substituted ring carbon of either an ortho-methyl group or an ortho- mercury atom are interesting. The downfield shift change of 6.7 ppm for a methyl-substituted ring carbon ortho to the mercury atom can be viewed as the sum of the 8.9 ppm shift change (normal shift change for a ring carbon ortho to the mercury atom) plus a -2.2 ppm correction. This ad- ditional upfield shift is nearly identical with the -2.4 ppm steric correction required for a methyl-substituted aromatic carbon ortho to a CH3 or CH2R group (7).
NMR data are also presented in Table I for two unsym- metrically substituted mercury compounds, phenyl(m-tolyl)- and phenyl(o-tolyl)mercury. The two ipso-carbons of phenyl-
(m-tolyl)mercury were resolved from the resonances for diphenyl- and bis(m-tolyl)mercury, both of which were present in the sample. By comparing the relative magnitudes of the peaks in the 13C spectrum to those in the 199Hg spectrum and by comparing the chemical shift to spectra of the pure bis-compounds, the peaks at 170.9 and 170.0 ppm are assigned to the ipso-carbons of the unsymmetrical compound, with tentative assignments as shown.
For phenyl(o-tolyl)mercury, separate resonances could be resolved from the bis-compounds for the ortho- and methyl- carbons and for one of the ipso-carbons. The bis resonances were assigned by monitoring the change in peak intensity on 17
the addition of additional bis(o-tolyl)mercury, and by comparison with the chemical shifts of the bis-compounds.
Given in Table III are the 199Hg chemical shifts for the nine mercury compounds. The spectra for the unsym- metrical compounds also included peaks for the corresponding bis compounds, which were contained in the samples as con- taminants. The chemical shifts of the bis-compounds con- tained in the samples were identical, within one ppm, with the values obtained for the pure bis-compounds.
TABLE III
199Hg NMR DATA FOR DIARYL MERCURY COMPOUNDS
R Group 199Hg 6 R group 199Hg 6
Phenyl -750 2,6-Xylyl -503
o-Tolyl -638 Mesityl -470
m-Tolyl -739 Phenyl(m-Tolyl) -744
p-Tolyl -708 Phenyl(o-Tolyl) -694
2, 5-Xylyl -629
*l 9 9 Hg chemical shift relative to external dimethylmercury at -289 ppm ( 1 ppm).
The 199Hg chemical shifts for diphenylmercury and
bis(p-tolyl)mercury are identical with those reported
earlier in dilute ( <= 0.5 M ) chloroform solution(8). The 18
19 9 Hg chemical shift for 1 M diphenylmercury in methylene chloride is -742 ppm, (4) which is 8 ppm downfield from our value of -750 ppm in CDCI3. Such a relative shift for the chloroform and methylene chloride solutions is opposite to that expected based on the solvent-dependent shifts of di- methylmercury (4). However, a more recent report of di- phenyl mercury in dilute methylene chloride solution (0.2
M ) gives a value of -756 ppm (8).
In every case methyl substitution leads to a de- shielding of the mercury atom, which is consistent with previous observations for electron-releasing substituents in the meta and pare positions (4,8). The mercury chemical shift changes per methyl group of 56, 6, and 21 ppm for substitution at the ortho, meta, and para positions, respec- tively predict successfully the shifts for the remaining compounds, except for bis(2,6-xylyl)- and bis(mesityl)- mercury. These di-ortho-substituted aryl derivatives show additional downfield shifts of 23 and 14 ppm, respectively.
Although consistent with earlier observations, the source of the downfield chemical shifts with electron- releasing groups on the aryl ring is not totally clear.
However, one can make some qualitative observations. The paramagnetic shielding term is likely to be the predominant factor in the 199Hg chemical shift changes (9,10). Therefore an increase in the electron density at the mercury atom 19 should lead to an increase in shielding (upfield shift), if the increase in electron density leads to an increase in the size of the valence p orbitals (11). However, any change in the energy of the aryl groups which leads to an increase in the population of the mercury p orbitals would result in a downfield shift. In fact, the change in p orbital contri- bution to the valence molecular orbitals has recently been shown to be the dominant factor in the chemical shifts of cadmium and zinc complexes (10).
The bonding in diarylmercury compounds is not well understood, but recent studies on dimethylmercury have sug- gested that as much as 80% of the mercury bonding is through the 6p orbital (12), as opposed to sp hybridization often attributed to mercury in such compounds. This implies a delocalized three-center, two electron bond for this molecule. The mercury 6p orbital is significantly higher in energy and contributes only slightly to the highest occupied molecular orbital. Assuming a similar bonding scheme for bisarylmercury compounds, increasing the energy of the ligand by adding electron-releasing groups would increase overlap with the 6p orbital, resulting in a downfield shift.
Although this argument may help to explain the additive downfield chemical shift changes observed with methyl sub- stitution on aryl rings, it does not explain the unusually large downfield shifts for the di-ortho-substituted com- 20
pounds. We note, however, that the additional downfield shifts for these compounds are comparable to the additional downfield shifts of 27 ppm observed for diphenylmercury in going from chloroform to a non-coordinating solvent such as cyclohexane (4,8). The additional shifts for the di-ortho- substituted compounds may merely reflect the unavailability of the chloroform molecule to interact with the mercury atom
when it is surrounded by four ortho-methyl groups.
Introduction to Molecular Dynamics
Barriers to rotation have, in the past, been measured
successfully primarily by lineshape decoalescence. There is
a limit, however, to this method in that decoalescence has
never been observed in exchanging systems with barriers
below ca. 4.5 kcal/mol (13).
The Woessner method (14,15) using dipolar relaxation as
a probe to molecular motions has been an enhancement to pre-
vious methods. For protonated carbons, dipole-dipole relaxa-
tion usually dominates the 13C spin-lattice relaxation
times. Smaller molecules and methyl groups may have signifi-
cant spin rotational contributions to the relaxation, and at
high magnetic fields chemical shift anisotropy may aid in
relaxation.
The dipole-dipole component is related to the total
relaxation time by the expression (1) dd + 1/T sr + 1/T csa + 1/T other 1/T 1 obs = 1/T1 1" 11 21
dd sr osa in which the T , T , and T are the dipole-dipole, spin rotation and chemical shift anisotropy relaxation times, respectively.
The fact that Nuclear Overhauser Enhancement (NOE) is dependent on the dipole-dipole mechanism allows the deter- mination of T dd from the observed T obs and n, NOE. The relationship of r and T dd is given by
T1 dd= T1 (max)/n = T1 1.988/n (2) 131 where n (max) for 13C is 1.988.
Molecular motions provide the mechanism for dipole- dipole relaxation from the time dependent reorientation of magnetic dipoles. An overall correlation time, tc, the time required for the molecule to rotate one radian, is used to describe these molecular motions. In the extreme narrowing limit, t >= 11w, T 1 is related to the correlation time by the equation
dd 2 2M2 -6 1/T = nyHYC rCH c in which n is the number of protons attached to the carbon nucleus undergoing relaxation, H and yC are the gyro- magnetic ratio of the nuclei involved, and rCH is the length of the C-H bond. If the molecular motion is isotropic then
1/6D (4) c = in which D is the isotropic diffusion constant. 22
For a molecule undergoing anisotropic motion, the most complex case can be described using three separate axes of rotation in which D D2#D3. The problem will simplify if one can assume that there is a preferred axis of rotation and two of the rotations occur at the same rate, i.e., D1 #D 2 =D3 .
Then Tc is then given by (13)
-c = A/6D2 + B/(D +5D2) + C/(4D +2D2) J (5)
A, B, and C are geometric factors dependent on the angle, d, between the bond connecting the two dipoles and the pre- ferred axis of rotation
A = 1/4(3cos2 d-1)2 (6)
B = 3/4sin22d (7)
C = 3/4sin4d (8)
To be consistent with Harris and Kimber (16), we assume that D =D2 3, but there is a group undergoing rapid internal rotation. The internal diffusion constant along the preferred axis and the isotropic diffusion constant are Rd and D, respectively.
The motions of the diarylmercury molecule effect the 13 C T 's differently. Assuming a preferred axis along the
Hg-CI-C4 axis the para carbon has its C-H bond aligned along the preferred axis, whereas the ortho and meta C-H bonds are aligned at an angle of 60 degrees with this axis. This difference allows us the opportunity to separate the Rd component from that of D. Since the ortho and meta C-H bond 23
are geometrically equivalent with respect to the overall dd are averaged. The anisotropy can be motion, their T 1 a evaluated by taking the ratio, x, of the averaged ortho-meta
T1, Tiom, and the para T1 , T1 p. Thus
X=Tlom /T (9)
Using equations 3, 4 and 5 and simplifying, we obtain
x = 64/[(1 + 36/(1+r) + 27/(1+4r)) (10)
where
r = Rd/6D (11)
for the description of an internally rotating aryl group
undergoing overall isotropic reorientation.
If we obtain Rd values at different temperatures the
Arrhenius equation can be used to calculate barriers to
rotation about the preferred axis by
Rd = e-Ea/RT (12)
Molecular Dynamics Data
Reported in Table IV are the T data for diphenyl-,
bis(o-tolyl)-, bis(m-tolyl)mercury, and bis(2, 5xylyl)mercury
in DMSO (d6) and a 50/50 mixture of 1,1,2,2 tetrachloro-
ethane (EtCl ) (d2) and carbon tetrachloride at a series of
different temperatures. 24
TABLE IV
TI DATA FOR DIARYL MERCURY COMPOUNDS
R Group Solvent Temperature T x r D Rd
2 3 4 5 6 phenyl EtCl b 305 2.7 2.3 1.2 2.1 2.7 2.1 39.1 4.4 16.8 315 3.1 3.0 1.4 3.0 3.1 2.2 33.2 5.0 21.3 321 3.4 3.2 1.5 3.2 3.4 2.2 30.6 5.4 22.5 327 3.3 3.4 1.6 3.4 3.3 2.1 29.4 5.7 21.7 338 3.9 3.4 1.6 3.4 3.9 2.3 29.3 5.7 26.7 346 5.5 6.2 2.7 6.2 5.5 2.2 17.2 9.7 40.5 phenyl DS01c 304 1.9 1.8 0.7 1.8 1.9 3.3 84.6 2.0 17.8 315 2.2 2.2 0.7 2.2 2.2 3.1 84.6 2.5 20.5 323 2.5 2.5 0.7 2.5 2.5 3.6 67.4 2.5 25.3 329 3.2 3.0 0.8 3.0 3.2 3.6 54.7 3.0 31.6 340 3.4 3.1 0.9 3.1 3.4 3.5 50.6 3.3 32.4 349 3.6 3.6 1.2 3.6 3.6 3.2 40.4 4.1 34.0
1.8 1.8 46.1 3.6 9.7 o-tolyl EtC14 309 .. d 1.8 1.0 1.8 323 .. 2.1 1.0 2.1 2.1 2.1 46.1 3.6 13.4 330 .. 2.2 1.1 2.4 2.2 2.0 42.3 3.9 14.3 339 .. 2.9 1.4 2.9 2.6 1.9 32.5 5.1 16.4
o-tolyl D0SO 305 .. 1.2 0.6 1.3 1.4 2.2 80.2 2.1 9.0 313 .. 1.7 0.7 1.5 1.6 2.1 62.8 2.7 10.5 323 .. 1.8 0.8 1.8 1.7 2.7 57.4 2.9 11.9 327 .. 2.0 0.8 1.8 1.7 2.2 57.6 2.9 13.1 333 .. 2.1 1.1 2.3 2.3 2.0 42.3 3.9 14.4 341 .. 2.7 1.2 2.7 2.6 2.4 36.4 4.6 17.3
a-tolyl EtCI 306 1.8 .. 0.7 1.6 1.2 2.3 68.4 2.4 11.1 318 2.0 .. 0.7 2.1 1.9 3.0 70.5 2.4 18.2 328 2.5 .. 0.9 1.9 2.5 2.5 50.6 3.3 18.2 340 2.3 .. 1.0 3.1 2.2 2.5 45.6 3.7 20.4
m-tolyl D0SO 310 1.2 .. 0.5 1.1 1.0 2.3 96.8 1.7 9.5 315 2.0 .. 0.7 1.8 2.1 2.9 68.3 2.4 12.7 321 2.2 .. 0.7 2.0 2.0 3.0 66.9 2.5 13.7 327 2.7 .. 1.0 2.3 2.3 2.3 43.5 3.8 12.9 332 2.7 .. 1.1 2.5 2.8 2.5 44.2 3.8 15.3 335 2.9 .. 1.2 2.6 2.7 2.3 38.6 4.2 14.8 340 2.9 .. 1.1 2.7 2.8 2.6 43.0 3.8 16.9
2.0 .. 2.0 46.5 3.6 11.7 2,6-xylyl EtC 4 305 .. 2.0 1.0 311 .. 1.5 0.9 1.5 .. 1.6 51.7 3.2 6.4 323 .. 2.2 1.3 2.2 .. 1.7 36.6 4.6 10.6 333 .. 1.9 1.3 1.9 .. 1.5 35.2 4.7 6.6 341 .. 1.9 1.1 1.9 .. 1.7 40.5 4.1 9.2
aAll compounds are bis aryl mercury compounds, R-Hg-R. b50 / 50 mixture of 1,1,2,2 terachloroethane (d2) and carbontetrachloride. CDimethyl sulfoxide (d6). Data reported for protonated ring carbons only. t 1 Units: 7 (sec), t (psec), D (nsec Rd (naec-1).-), 25
From a simple observation of the T 's it is apparent that there is a preferred axis through the para carbons. In all cases the para T s are significantly shorter than the ortho and meta, indicating a longer correlation time for the para carbon. All T a increase with temperature which is consistent with the expected increase in molecular motions.
The ortho and meta T values are, within experimental error, identical within each compound.
NOE values for bis(m-tolyl)- and bis(o-tolyl)mercury are shown in Table V. Note that the NOE values are for protonated ring carbons only.
TABLE V
NOE DATA FOR BIS(O-TOLYL)MERCURY
AND BIS(M-TOLYL)MERCURY
R Group Solvent Temperature 2 3 4 5 6
o-tolyl DSOb 317 .. c 3.33 3.00 3.00 2.75 319 .. 2.80 2.95 3.00 2.83 328 .. 2.95 3.20 2.60 3.00
EtCl4 330 .. 2.80 2.80 2.98 2.93
m-tolyl CDCJ3d ambient 3.06 .. 2.81 2.88 2.98
bAll compounds are bis aryl mercury compounds. cDimethyl sulfoxide (d6). dData reported for protonated ring carbons only. Chloroform (dl). 26
In all cases the protonated ring carbons have essen- tially full NOE, within the experimental error, which indi- cates that T dd is the major contribution to the relaxation.
This is shown to be true even at 55 C where molecular mo- tions are faster. Based on this data, it is assumed that all protonated in carbons of the compounds in this study are dd relaxed by T .
Table VI lists the energies of activation calculated from the data in Table IV. These are interpreted as barriers for the rotation about the preferred axis.
TABLE VI
ENERGIES OF ACTIVATION FOR DIARYL MERCURY COMPOUNDS
Solvent
R Group EtCI b DMSOc
(Phenyl ) 2Hg 3.8 0. 8 d 3.3 0.5
(m-Tolyl) 2 Hg 3.5 1.2 3.2 0.6
(o-Tolyl) 2 Hg 3.5 0.4 3.6 0.2
aAll compounds are bisarylmercury compounds, R-Hg-R. 50/50 mixture of 1, 1,2,2-tetrachloroethane (d2) and chloroform (dl). cDimethyl Sulfoxide (d6). All energies of activation in kcal/mol. 27
The data indicate that the change in the barrier to rotation when adding a methyl group to the ring, or in changing from coordinating to noncoordinating solvent, is too small to measure with our techniques; support for this small difference in energy of activation for the phenyl and o-tolyl rings will be presented below. Note that the value of 3.8 0.8 kcal/mol is the same, within the experimental error, as the value of 4.0 0.1 kcal/mol for diphenyl- mercury reported by Gillies, et. al. (17). The Arrhenius plots for this data are shown in Figure 1.
-2.00 -
0 -2.40 - 0 0
1 0 -2.80 - 0
C -3.20 -
QODMSO 3.2 kcal/mol
-3.60 0 EtCI4 3.5 kcal/mol
-4.00 2.80 2.90 3.00 3.10 3.20 3.30 3.40 1000/T (K)
Bis (m-tolyl)mercury
Fig. 1--Arrhenius plots of diaryl mercury compounds. 28
-2.00
-2.40
-2.80 N.r
C -3.20 O DMSO 3.6 kcal/rnol
-3.60 * EtC14 3.5 kcal/mol
-4.00 2.80 2.90 3.00 3.10 3.20 3.30 3.40 1000/T (K)
Bis (o-tolyl) mercury
-2.00 0oDMS ; 3.3 kcal/mal 4 EtC14 4 3.8 kcal/rnol -2.40
-2.80 ti
-3.20 0 07 -3.60 0
-4.00 2.80 2.9 0 3.00 3.10 3.20 3.30 3.40 1000/T (K)
Diphenylmercury
Fig. I (continued) 29
The same T analysis was also attempted for as a function of temperature for bis(2,6-xylyl)mercury. However, this preliminary data showed much greater scatter in the corresponding Arrhenius plot than found in the data pre-
sented above. Although no conclusion can be drawn, it is sig-
nificant that this symmetrical compound has the same type of
scatter. Since the preferred axis of rotation should be well
defined in this molecule, we conclude that the scatter in
all these experiments is primarily due to poor temperature
control, poor temperature calibration, or some other exper-
imental factor and not due to an axis of rotation other than
C4-Hg-C4'.
It was of interest to know whether the preferred rota-
tional motion about the C4-Hg-C4' axis is due to rotation of
the entire molecule, or is the rotation of the phenyl ring
about the carbon-mercury bond. Figure 2 shows T data for
phenyl(o-tolyl)mercury in CDCl3 at O C.
CH 3 2.3
1.1 Hg1.0
1.8
Fig. 2--13C T Data (in seconds) for Phenyl(o-tolyl)mercury 30
Although there is significant overlap of peaks with the symmetrical compounds present in solution, one meta and the para resonance could be seen for each ring. The ratio of T to T (X)is 2.3 for the phenyl ring but only 1.6 for the o-
tolyl ring. These correspond to tumbling ratios, rates of at
least rotation about the preferred axis vs. the other axes,
of approximately 6 and 3 respectively (18). These results
indicates that the two rings are rotating independently and
that the rotation about the preferred axis is rotation about
the C-Hg bond of at least one of the rings, and not tumbling
of the entire molecule. Noteworthy is the identical values
of the para carbons. This indicates that these nuclei are
reorienting at the same rate; this is consistent with a
linear molecule having the preferred axis of rotation de-
fined by C4-Hg-C4'.
Since the rotation rate is related to the energy of
activation by the Arrhenius equation (eq. 12), knowing the
relative rates of the rotation of the rings will allow us to
compare their energies of activation. Applying this to the
unsymmetrical compound, a comparison of the tumbling ratios
of 3 and 6 for the o-tolyl and phenyl rings, respectively,
yields a difference of 0.4 kcal/mole for the calculated
difference in their energies of activation. A value of this
magnitude would be undetectable within the experimental
error. 31
The tumbling ratios for the bis-compounds reported in
Table IV have also been determined (18) and are shown in
Table VII. All ratios are the same except for diphenylmer- cury in DMSO which has a ratio twice as large as the others.
If this is an accurate value no explanation can be offered at this time.
TABLE VII
TUMBLING RATES OF DIARYL MERCURY COMPOUNDS
R Group Solvent Xa rhob
(Phenyl) 2 Hg DMSOc 3.4 11 EtC1 d 2.2 5
(o-Tolyl) 2 Hg DMSO 2.1 5 EtCl4 2.0 5
(m-Tolyl)2 Hg DMSO 2.6 6 EtCl4 2.6 6
(2, 5-xylyl) 2 Hg EtCl4 1.7 5
Average of x values. Approximate tumbling ratio. cdimethyl sulfoxide (d6). 50/50 mixture of 1,1,2,2-tetrachloroethane (d2) and chloroform (dl).
A T study of methyl rotation was carried out in chloroform and the results are reported in TABLE VIII. Ener- gies of activation were obtained by methods previously
utilized by Platzer, et.al. (19). In this method the Rd 32
value of the methyl group is compared to the Rd0 value for an unhindered methyl group undergoing free rotation. Values of 0.23 kcal/mol was obtained for bis(m-tolyl)mercury and
1.06 kcal/mol for bis(o-tolyl)mercury. This seems to indi- cate steric hindrance to a methyl group in the ortho position, however there is question as to the accuracy of the experiments.
TABLE VIII
BARRIERS TO METHYL ROTATION
R Group Temperature CH3 -T1 HOE T dd Ea o-Tolyl -11 *C 3.83 2.4 5. 41(CH3) 1.23 2.23(M) m-Tolyl -18 C 3.81 2.5 5.03(CH3) 0.23 2.55(M) 3
*Ea in kcal/mol. **R Groups are for bis compounds
Evidence for steric interaction of the ortho methyl group is reported in Figure 3 for bis(2,5-xylyl)mercury in
EtCI /CCI4. The HOE values for the methyl carbons are 2.95 and 2.16 for the ortho and meta methyl groups, respective- ly. For the methyl group in the meta position, the less than maximum NOE suggests that there is a significant spin ro- tation contribution for the relaxation. Since the ortho 33
methyl group has full NOE, it has no spin rotational contri- bution, suggesting hindered rotation of this ortho methyl group in non-coordinating solvent, and some steric inter- action taking place in the vicinity of the mercury atom.
2 9 5 CH3 CH3 QHgQ
2.1 6CH3 CH3
Fig. 3--Methyl NOE data for the methyl carbons of bis(2,5-xylyl)mercury
Conclusions
Evidence has been presented here that suggests that the bisarylmercury compounds studied are linear, that the rota- tion about the preferred axis is due to rotation of the carbon-mercury bond, and that there is steric hindrance of the ortho methyl groups. However, the differences in ener- gies of activation for the rotations of the phenyl rings are too small to be studied by these methods. This study has shown NHR to be a useful tool in the study of molecular dynamics in solution, and the bisarylmercury compounds use- 34
sensi- ful in the study of carbon-mercury bonds, but their for tivity to molecular interactions may not be great enough them to be useful in rotational studies with NMR. It may be for neccessary to add more steric bulk, tert-butyl groups example, in the ortho position, or add electronegative
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