Organometallics 1995, 14, 987-991
987
Ab Initio MO Calculations of NMR Spin-Spin Coupling Constants in Methyllithium, tert-Butyllithium,and Methyllithium Oligomers Terutake Koizumi and Osamu Kikuchi" Department of Chemistry, University of Tsukuba, Tsukuba 305, Japan Received April 25, 1994@ Ab initio calculations of NMR spin-spin coupling constants in monomeric methyllithium, tert-butyllithium and methyllithium oligomers were performed by using a self-consistent perturbation theory to examine their molecular structures and the bonding character of the C-Li bond. The calculated 'JcLi values in monomeric methyllithium and tert-butyllithium are largely influenced by solvation, and the 'JCLi value in tert-butyllithium agrees well with the experimental value when tert-butyllithium is coordinated by three ligands. The calculated values in methyllithium oligomers depend on the number of the lithium atoms 'JcLi and 'JCH bonded directly to the carbon atom, and a tetrahedral structure is suggested for the tetramer. Modeling the ionic C-Li bond by using the truncated lithium basis set gives coupling constants which are in good agreement with the experimental ones and suggests the importance of the ionic character of the C-Li bond for alkyllithiums.
Introduction The molecular and electronic structures of organolithium compounds have been studied e~tensively.l-~ Methyllithium, the simplest alkyllithium, was studied by X-ray,5NMR spectroscopy,6and neutron diffraction7 and was found to be a tetrahedral (Td structure) Abstract published in Advance ACS Abstracts, December 15,1994. (1)Brown, T. L. Adu. Organomet. Chem. 1966,3,365. Wardell, J. L. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, England, 1982; Vol. 1, p 43. Young, R. N.;Quirk, R. P.; Fetters, L. J. Advances in Polymer Science; Springer-Verlag: Berlin, 1984;Vol. 56,p 1. Williard, P. G. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Schreiber, S. L., Eds.; Pergamon Press: Oxford, England, 1991;Vol. 1, P 1. (2)(a) O'Brien, D. H. In Comprehensive Carbanion Chemistry; Buncel, E., Durst, T., Eds.; Elsevier: Amsterdam, 1980;Part A, p 271. (b) Fraenkel, G.; Henrichs, M.; Hewitt, J. M.; Su, B. M.; Geckle, M. J. J . Am. Chem. SOC. 1980,102,3345.(c) Seebach, D.; Siegel, H.; Gabriel, J.; Hassig, R. Helu. Chim.Acta 1980,63,2046.(d) Seebach, D.; Hiissig, R.; Gabriel, J. Helv. Chim. Acta lSS3,66,308.(e) Seebach, D.; Gabriel, J.; Hassig, R. Helv. Chim. Acta 1984,67,1083. (0 McGarrity, J. F.; 1985,107,1805. (g) McGarrity, J. F.; Ogle, C. A. J . Am. Chem. SOC. 1985,107,1810. Ogle, C. A.; Brich, Z.; Loosil, H. R. J . Am. Chem. SOC. (h) Thomas, R. D.; Clarke, M. T.; Jensen, R. M.; Young, T. C. Organometallics 1986,5, 1851. (i) Thomas, R. D.; Jensen, R. M.; Young, T. C. Organometallics 1987,6,565.(i)Gunther, H.; Moskau, D.; Bast, P.; Schmalz, D. Angew. Chem., Int. Ed. Engl. 1987,26,1212. Ik) Bauer, W.; Schleyer, P. v. R. In Advances in Carbanion Chemistry; Snieckus, V.; Ed.; JAI Press: Greenwich, CT, 1992;Vol. 1, p 89. (1) 1993,115,10871. (m) Bauer, W.; Griesinger, C. J . Am. Chem. SOC. Bergander, K.; He, R.; Chandrakumer, N.; Eppers, 0.; Giinther, H. Tetrahedron 1994,50, 5861. (3)Setzer, W. N.;Schleyer, P. v. R. Adu. Organomet. Chem. 1985, 24,353.Kottke, T.; Stalke, D.Angew. Chem., Int. Ed. Engl. 1993,32, 580. Weiss, E. Angew. Chem., Int. Ed. Engl. 1993,32,1501. (4)Biihl, M.; van Eikema Hommes, N. J. R.; Schleyer, P. v. R.; Fleischer, U.; Kutzelnigg, W. J . Am. Chem. Soc. 1991, 113, 2459. Hoffmann, D.; Bauer, W.; Hampel, F.; Hommes, N. J . R. v. E.; Schleyer, P. v. R.; Otto, P.; Pieper, U.; Stalke, D.; Wright, D. S.; Snaith, R. J. Am. Chem. SOC. 1994,116,528. (5)(a) Weiss, E.; Henchen, G. J . Organomet. Chem. 1969,21,265. (b) Koster, H.; Thoennes, D.; Weiss, E. J . Organomet. Chem. 1978, 160,1. (6)(a) McKeever, L. D.; Waack, R.; Doran, M. A.; Baker, E. B. J. Am. Chem. Soc. 1968,90,3244.(b) McKeever, L. D.; Waack, R.; Doran, M. A.; Baker, E. B. J . Am. Chem. Soc. 1969,91,1057.(c) McFarlane, W.; Rycroft, D. S. J . Organomet. Chem. 1974,64,303.(d) Eppers, 0.; Gtinther, H. Helu. Chim. Acta 1990, 73,2071. (7)Weiss, E.; Lambertsen, T.; Schubert, B.; Cockcroft, J. K.; Wiedenmann, A. Chem. Ber. 1990,123,79. @
0276-733319512314-O987$O9.O0lO
tetramer. Many theoretical investigations have been carried out for m e t h y l l i t h i ~ mand ~ ~ ~its oligomers.lOJ1 Most of these investigations concerned the structural and energetic relationships obtained from the several levels of molecular orbital calculations.8J0 NMR spectroscopy has primarily been applied for structural analyses of organolithium compounds in solution.2p6J2The one-bond 13C-7Li coupling constant JcLi provides experimental evidence regarding the aggregation state and the nature of the C-Li bond in an organolithium c ~ m p o u n d . ~ ~ - ~'JcLi * ~ J in ~ J the ~ methyllithium tetramer (Td structure) was reported to be 14.5 Hz by McKeever et a1.6a,bAlthough there has been no study of monomeric methyllithium by NMR spectroscopy, Clark et al.9 anticipated that 'JcLi in monomeric methyllithium is over 200 Hz and the C-Li bond is predominantly covalent from their calculations of the coupling constants by using the finite perturba(8)(a) Fitzpatrick, N.J. Inorg. Nucl. Chem. Lett. 1974,10,263.(b) Hinchliffe, A.;Saunders, E. J . Mol. Struct. 1976,31,283. (c) Streitwieser, A., Jr.; Williams, J. E., Jr.; Alexandratos, S.; McKelvey, J. M. J . Am. Chem. SOC.1976,98,4778. (d) Collins, J. B.; Streitwieser, A., Jr. J . Comput. Chem. 1980,1, 81. (e) Graham, G. D.; Marynick, D. S.; Lipscomb, W. N. J . Am. Chem. SOC. 1980,102,4572.(0 Ritchie, J. P.; Bachrach, S. M. J . Am. Chem. SOC. 1987,109,5909. (g) Penotti, F. E. G.; Gerratt, J.; Cooper, D. L.; Raimondi, M. J . Chem. SOC., Faraday Trans. 2 1989,85,151. (h) Wiberg, K.B.; Breneman, C. M. J . Am. Chem. SOC.1990,112,8765. (i)Dixon, R. E.; Streitwieser, A,, Jr.; Laidig, K. E.; Bader, R. F. W.; Harder, S. J . Phys. Chem. 1993, 3728. 97. .. (9)Clark, T.; Chandrasekhar, J.; Schleyer, P. v. R. J. Chem. Soc., Chem. Commun. 1980,672. (10) Guest, M. F.; Hillier, I. H.; Saunders, V. R. J.Organomet. Chem. 1972.44, 59. Baird. N. C.: Barr, R. F.: Datta, R. K. J . Organomet. Chem. 1973,59,65.'Clark,'T.; Schleyer,'P. v. R: J . Chem. Sol., Chem. Commun. 1978,137.Graham, G.; Richtsmeier, S.; Dixon, D. A. J . Am. Chem. Soc. 1980,102,5759. Herzig, L.;Howell, J . M.; Sapse, A. M.; Singman, E.; Snyder, G. J. Chem. Phys. 1982,77,429. Kaufmann, E.; Raghavachari, K.; Reed, A. E.; Schleyer, P. v. R. Organometallics 1988,7,1597. (11)Streitwieser, A.,Jr. J . Organomet. Chem. 1978,156, 1. (12)Bauer, W.; Winchester, W. R.; Schleyer, P. v. R. Organometallics 1987,6, 2371. (13)For the C-Li coupling constant, 'J(l3C-"Li) values are referred to in this paper to compare the calculated values with the previous calculation^.^ The conversion to 1J(13C-6Li) is carried out using the = 2.641('J(13Crelation 'J(13C-'Li) = (y(7Li)/y(6Li))1J(13C-6Li) 6Li)).2c.d,k,12 I
0 1995 American Chemical Society
988 Organometallics, Vol. 14, No. 2, 1995
Koizumi and Kikuchi
Table 1. Symmetry-Restricted Energy-Optimized Structures" and Calculated Coupling Constantsb for Methyllithium Monomers unsolvated
solvated with NH3 (1) solvated with 3NH3 (2) solvated with 3H20 (3) truncated basis set
r(C-Li)
r(C-H)
2.047 2.001 2.009 1.821 2.079 2.021 1.847 2.183 2.184 1.930
1.102 1.094 1.083 1.117 1.103 1.084 1.117 1.111 1.110 1.108
L(HCH)
'J(13C-7Li)
107.3
128.3 115.9 115.9 116.1 74.4 127 118.9 37.0 43.3 44.0
107.1 105.6 105.6 104.6
'JP3C-'H) 131.0 63.6 63.4 57.0 125.0 65.6 60.1 108.5 110.2 101.2
ref this work c,d
c,e cf this work c,e C f
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Bond lengths are in 8, and angles are in degrees. All structures are optimized under the restriction of a C3" symmetry. Values are in Hz. Reference 9; coupling constants were calculated by the FPT-INDO method. Geometries were optimized by the 6-31G* basis set. e Geometries were optimized by the STO-3G basis set. f Geometries were optimized by the MNDO method.
tion theory within the INDO approximation (FPTINDO). On the other hand, Bauer et a1.12reported that ~Jc inL monomeric ~ tert-butyllithium in THF is 31.5 Hz from an NMR study, suggesting that 'JCLi in monomeric methyllithium is much smaller than 200 Hz. Furtheri the orgamore, Bauer et a1.2kJ2reported that l J c ~ in nolithium compounds is not influenced by the structure of the lithiated carbon atonis but depends only on the state of aggregation, that is, the number of lithium atoms bonded directly to the carbon atoms. These results contradict the prediction given by Clark et al.,9 and theoretical efforts are required to evaluate the NMR spin-spin coupling constants correctly for the analysis of the structures of alkyllithiums. In this paper, we provide our results of ab initio calculations of methyllithium and its oligomers and present their structures and NMR spin-spin coupling constants. In addition, we will clarify the relation of i to the molecular structures and to the the l J c ~ values nature of the C-Li bonds.
lithium, Li(31), which includes only the 1s function and corresponds to the lithium cation,14dwas also used to examine the nature of the C-Li bond for methyllithiums. NMR spin-spin coupling constants were calculated by using self-consistent perturbation theory,lgin which only the Fermi contact term was taken into account as the perturbation. The following f o r m ~ l awas ~ ~used ~ ~ to ~ calculate the coupling constant between the nuclei A and B:
Calculation
where CL;ja values are the coefficients of the first-order a-spin molecular orbitals perturbed by the Fermi contact interaction on the nucleus B. values are the zeroth-order coefficients. values were calculated by the procedure given by Ditchfield and Synder.lg The theory was incorporated into the B I N I T 8 8 program written by our group.21 All calculations were carried out on HP-730 workstations.
Ab initio calculations were performed with the MIDI-4 basis set.14 For lithium, the Li(421/1) basis set, which includes the additional p-type polarization functions, was used. The geometries were optimized under the restriction of a given symmetry for each structure. The X-ray structures of benzyllithium depend on the ligands in~1uded.l~Brooks et a1.16 pointed out that the overlap between the vacant lithium 2p orbital and the carbon JC orbitals stabilizes the v3 This idea was applied to the structural analyses of other organolithium comp~unds.'~J'On the other hand, Sygula and Rabideauls showed that the v1 form15cis the most stable when the truncated basis set for lithium is used to model the purely ionic C-Li bond. Taking these results into account, the truncated MIDI-4 basis set for (14)(a) Tatewaki, H.; Huzinaga, S. J . Comput. Chem. 1980,I , 205. (b) Sakai, Y.;Tatewaki, H.; Huzinaga, S. J . Comput. Chem. 1981,2, 100. (c) Sakai, Y.; Tatewaki, H.; Huzinaga, S. J . Comput. Chem. 1981, 2,108.(d) Huzinaga, S.Gaussian Basis Sets for Molecular Calculation; Elsevier: Amsterdam, 1984. (15)(a) Patterman, S.P.; Karle, I. L.; Stucky, G. D. J . Am. Chem. Soc. 1970,92, 1150. (b) Beno, M. A.; Hope, H.; Olmstead, M. M.; Marsch, Power, P. P. Organometallics 1985,4, 2117. (c) Zarges, W.; M.; Harms, K.; Boche, G. Chem. Ber. 1989,122,2303. (16)Brooks, J. J.; Rhine, W.; Stuky, G. D. J . A m . Chem. SOC.1972, 94,7339. (17)Bushby, R. J.; Patterson, A. S. J . Organomet. Chem. 1977,132, 163. Kos, A.J.; Jemmis, E. D.; Schleyer, P. v. R.; Gleiter, R.; Fischbach, U.; Pople, J. A. J . A m . Chem. Soc. 1980,103, 4996. Bushby, R. J.; Tytko, M. P. J . Organomet. Chem. 1984,270,265. Kos, A. J.;Stein, P.; Schleyer, P. v. R. J . Organomet. Chem. 1985,280,C1. Groventein, E.,Jr. In Comprehensive Carbanion Chemistry; Buncel, E., Durst, T., Eds.; Elsevier: Amsterdam, 1987;Part C, p 175. (18)Sygula, A.;Rabideau, P. W. J . Am. Chem. SOC.1992,114,821.
where YA is the magnetogyric ratio of the nucleus A. XA(RA) represents the function value of the atomic orbital 1 evaluated at the nucleus A . & : is the first-order spin-density matrix given by19 nacc
(2)
Results and Discussion Monomeric Methyllithium and tert-Butyllithium. The optimized geometries and calculated coupling constants for the methyllithium monomer are shown in Table 1along with those reported by Clark et al.9 When solvation is not taken into account, the calculated 'JCLi value, 128.3 Hz, is similar to those obtained by the FPTINDO calculations, ca. 116 H Z . ~However, the ' J C L i value is reduced remarkably t o 74.4 Hz when one NH3 molecule is coordinated to the Li atom (1)in order to examine the effect of solvation. This solvation effect was not recognized by the semiempirical FIYT-INDO calculation~.~ X-ray crystal-structure analyses revealed that the geometry about the lithium atom is nearly tetrahedral in monomeric alkyllithiums, such as (bis(trimethylsily1)(19)(a)Blizzard, A.C.; Santry, D. P. J . Chem. Phys. 1971,55,950. (b) Ditchfield, R.; Synder, L. C. J . Chem. Phys. 1972,56,5823. (20)Ostlund, N. S.;Newton, M. D.; McIver, J. W., Jr.; Pople, J. A. J . Magn. Reson. 1969,I , 298. (21)Kikuchi, 0.; Nakano, T.; Morihashi, K. Unpublished.
Coupling Constants in MeLi, tBuLi, and (MeLi),
Table 2. Symmetry-Restricted Energy-Optimized Structures0 and Calculated Coupling Constan& for tertlutyllithium Monomers
Chart 1 H
\
.C-Li-N HH/
\ 1
Organometallics, Vol. 14,No. 2,1995 989
unsolvated solvated with N H 3 (4) solvated with 3NH3 (5) solvated with 3Hz0 (6) truncated basis set
H
2.092 2.111 2.218 2.199 1.964
1.547 1.548 1.548 1.547 1.551
108.3 108.0 107.6 107.8 107.3
exptlC \
\
NH3
OH2
3
2
Chart 2
31.5
Bond lengths are in 8, and angles are in degrees. Geometries for methyl groups were fixed at the structures of those in 2-methylpropane: r(C-H) = 1.096 A, L(HCH) = 108.2°.28 Values are in Hz. Nh4R data for tertbutyllithium monomer in THF at -90 O C . l Z
Table 3. Contributions of the Atomic Orbitals to the ~ J c L ~ Values in Methyllithium Monomen9
H3C .C-Li-N
H3c*.*y
species unsolvated
\
H
CH3 4
5
222.5 85.8 31.8 36.3 43.0
6
xlb
xd
C(1s) C(1s) C(ls) C(2s’) C(1s) C(2s”) C(2s’) C(2s’) C(2s‘) C(2s”) C(2s”) C(2s”) others
solvated with3H20 C(1s) C(1s) C(1S) C(2s’) C(1S) C(2s”) C(2s’) C(2s’) C(2s’) C(2s”) C(2s”) C(2s”) others
x ~ ( R c ) x o ( R d ~e?: 51.7162 0.0369 0.2733 -0.1219 1.2190 -0.1219 0.0014 0.3479 0.0064 0.4123 0.0287 0.3417
51.7162 0.2733 1.2190 0.0014 0.0064 0.0287
0.0118 -0.0557 -0.0041 0.1810 0.1176 -0.1078
(‘JCL)A~~
141.3 -2.5 -11.0 ‘0.1 0.2 0.7 -0.5 128.3 (total) 45.2 -1.1 -0.4 ‘0.1 0.1 -0.2 1.0 44.6 (total)
methy1)lithium-pentamethyldiethyltriaminezz and 2-lithio-2-phenyl-l,3-dithiane-tetramethylethylenediamine-THF.23 Therefore, in our study three NH3 or HzO molecules were included t o complete the tetrahetruncated basis set C(1s) C(1s) 51.7162 0.0091 34.8 dral coordination around the lithium atom (2 or 3). The C(1S) C(2s’) 0.2733 -0.0411 -0.8 calculated ‘JCLi values are 37.0Hz in 2 and 43.3 Hz in C(ls) C(2s”) 1.2190 -0.0142 -1.3 3. These values are very close t o the experimental one C(2s’) C(2s’) 0.0014 0.1105 10.1 C(2s’) C(2s”) 0.0064 0.1322 0.1 in the tert-butyllithium monomer (31.5 Hz).12 It is C(2s”) C(2s”) 0.0287 -0.0211 -0.1 confirmed that ‘JcLi in the methyllithium monomer others 1.o depends largely on the degree of coordination to the 33.8 (total) lithium atom. a In all calculations the geometry for the CH3Li part was fixed at the The structure of the C-Li bond is also influenced by unsolvated state: r(C-Li) = 2.047 A, r(C-H) = 1.102 A, L(HCH) = the solvation; the C-Li bond becomes longer (from 2.05 107.3”. The valence atomic orbitals which are split into inner and outer parts are denoted single and double primes, respectively. The products of to ca. 2.18 A, as shown in Table 1). This change has the function value of the atomic orbital 1 and that of u were evaluated at been shown in the theoretical studies of organolithium the C atom. Spin densities induced by the perturbation of the Fermi contact ~ o m p o u n d s . ~ItJshould ~ ~ ~ ~be~ noted, however, that the interaction on the Li atom. e Values are in Hz. C-Li bond elongation itself is not a key factor for decreasing the l J c value ~ but the coordination of solvent molecules is. We examined the dependence of ~ J C onL that the solvation affects the electronic factor of the C-Li bond, which reduces the ‘JCLi value significantly. the C-Li bond length for the unsolvated species in which the geometry of the methyl group was f ~ e dIt. ~ ~ The calculations were also carried out by using the truncated lithium basis set, which models a purely ionic was found that ‘JCLi is 128.3Hz for r(C-Li) = 2.047A C-Li bond. The calculated l J c ~ ivalue in methyland 169.1Hz for r(C-Li) = 2.184 A. Such a trend has 44.0 Hz, is very close to those calculated for lithium, been reported in previous calculations for the one-bond methyllithium with three solvated ligands. This result in methane26aand in the 13C-lH coupling constant ~JCH strongly suggests that the C-Li bond in methyllithium isopropyl cation;26bsimple elongation of the C-H bond with ligands is ionic, as has been indicated in previous makes the ~ J C value H larger. Therefore, it is suggested studies.8c-h Quite the same calculations were carried out on the (22) Leppert, M. F.; Englehardt, L. M.; Raston, C. L.; White,A. H. tert-butyllithium monomer, in which l J c ~was i experiJ. Chem. SOC.,Chem. Commun. 1982, 1323. (23) Amstutz, R.; Dunitz, J. D.; Seebach, D. Angew. Chem., Znt. Ed. mentally observed in THF.12 The optimized geometries Engl. 1981,20, 465. and calculated ‘JcLi values for the tert-butyllithium (24)Chandrasekhar, J.; Schleyer, P. v. R. J. Chem. SOC., Chem. Commun. 1981,260. Decher, G.;Boche, G. J.Orgummet. Chem. 1983, monomer are shown in Table 2. The trend observed in 259, 31. Lipkowitz, K. B.; Uhegbu, C.; Naylor, A. M.; Vance, R. J. the calculated l J c ~values i is similar t o that for methComput. Chem. 1986, 6, 662. van Eikema Hommes, N. J. R.; Biihl, yllithium (Table 1). When three NH3 or HzO molecules M.; Schleyer, P. v. R.; Wu, Y.-D. J.Organomt. Chem. 1991,409,307. (25)The change in the geometry of the methyl group was not are solvated to the lithium atom (6 or 61, the calculated G important for decreasing the ~ J C value. ~JC values L (31.8Hz for 6 and 36.3Hz for 6) agree well (26)(a) Sergeyev, N. M.; Solkan, V. N. J. Chem. SOC.,Chem. with the experimental one.12 Moreover, modeling the Commun. 1976,12. (b) Maciel, G. E. J.Am. Chem. SOC.1971,93,4375.
990 Organometallics, Vol. 14, No. 2, 1995
Koizumi and Kikuchi
Table 4. Symmetry-RestrictedEnergy-Optimized Structuresnfor Methyllithium Oligomers
dimer (C2h) (7) trimer ( C 3 h ) (8) tetramer (C4h) (9) tetramer ( T d ) (10; staggered) tetramer ( T d ) (10; eclipsed) dimer ( C Z h ) (7) trimer (C3d (8) tetramer (C4h) (9) tetramer ( T d ) (10; staggered) tetramer ( T d ) (10; eclipsed)
r(C-Li)
r(C-H)
2.178 2.192 2.126 2.155 2.102 2.140 2.269 2.269
1.106 1.108 1.105 1.107 1.104 1.106 1.108 1.108
2.094 2.102 2.047 2.062 2.027 2.046 2.203 2.206
1.109 1.110 1.108 1.110 1.108 1.110 1.110 1.110
C-C)
r(Li-Li)
With Li(421/1) Basis Set 3.751 2.199 4.219
2.738
4.240
2.912
3.700 3.695
2.484 2.497
With Li(31) Basis Set 3.573
2.199
4.028
2.717
4.072
2.916
3.572 3.571
2.459 2.470
L(HCH)
L(LiCLi)
L(CLiC)
104.1 106.2 103.9 106.6 104.0 106.9 104.3 103.5
61.7
118.3
79.5
160.5
86.7
176.7
66.4 66.8
109.2 109.0
102.7 104.7 101.9 104.9 101.5 104.9 102.9 102.0
63.2
116.8
82.8
157.2
91.5
178.5
67.8 68.1
108.3 108.1
Bond lengths are in 8, and angles are in degrees. Assumed symmetries for each structure are given in parentheses.
Table 5. Geometric Parameter9 and Calculated Coupling Constantsb for Methyllithium Oligomersc
dimer (CZh)(7) trimer (C3h) (8) tetramer ( C 4 h ) (9) tetramer ( T d ) (10; staggered)
tetramer ( T d ) (10; eclipsed)
dimer (CZh) (7) trimer (C3h) (8) tetramer (c4h) (9) tetramer (Td) (10; staggered) tetramer ( T d ) (10; eclipsed)
2.185 2.126 2.037 2.141 2.121 2.269 2.230 2.199 2.311 2.269 2.250 2.197
With Li(421/1) Basis Set 1.107 104.6 1.090 1.130 1.106 104.8 1.106 105.O 1.108 104.3 1.091 1.136 0.960 1.108 103.5 1.095 1.139
2.098 2.054 2.036 2.203 2.206
With Li(31) Basis Set 1.109 103.4 1.109 102.9 1.109 102.6 1.110 102.9 1.110 102.0
28.9 20.0 25.3 30.9 33.0 16.3 6.9 7.3 7.7 17.2 7.0 7.3
109.9 58.4 56.5 112.3 115.3 107.8 62.1 61.4 51.2 107.8 61.1 59.7
this work this work this work this work d,e dlf d,g this work d,e df
20.9 24.1 24.6 12.5 14.0 14.5
99.7 99.9 99.9 99.3 99.6 98
this work this work this work this work this work exptl"
d,e df
Bond lengths are in 8, and angles are in degrees. Assumed symmetries for each structure are given in parentheses. Values are in Hz. Mean values of the bond lengths, angles, and coupling constants are given for 7, 8, and 9. Coupling constants were calculated by the FPT-INDO m e t h ~ de. Geometries ~ were optimized by the STO-3G basis set. f Geometries were optimized by the MNDO method. g X-ray NMR data for tetramer (Td structure).6a.b
ionic C-Li bond gives a similar 'JcL~ value of 43.0Hz. In order to examine the above-mentioned variations i we analyzed the contribution of each in the l J c ~values, atomic orbital to the 'JCL~ value. The ' J c L i value was divided into atomic orbital pair contribution ('JCLi)Au by allAO
(3) where ( l J ~ ~ i ) is l u each expansion term in eq 1. The calculated contributions of each atomic orbital to 'JCLi in methyllithium are summarized in Table 3. These contributions were obtained with the same geometry for the CHsLi part. As can be seen from Table 3, the contribution from the carbon 1s orbital is predominant value; this is due to the large density of the in the 'JcL~ 1s orbital at the nucleus. The values of the spin density et), which are induced by the perturbation of the Fermi contact interaction on the Li atom, decrease when three H20 molecules are included or the truncated lithium basis set is used in the calculation. Since covalency and ionicity are the concepts for the valence electron behavior and ' J c ~ iis determined mainly by the
core electron behavior, the electronic factor of the C-Li bond may not be derived directly from the calculated ' J c L i values. On the other hand, the fact that the solvation model with three H2O molecules and the truncated basis set model give very similar results for both the core and valence parts of the spin density indicates that the C-Li bond of the methyllithium monomer is ionic in solution and has a small ' J c L i value. Although the experimental one-bond13C- lH coupling constant ~ J C for Hthe lithiated carbon atom in the monomeric alkyllithium has not been reported, Table 1 shows that ~JCH also decreases as the degree of solvation increases or as the ionicity oi the C-Li bond is modeled in the present calculations, in contrast to the previous study.g Methyllithium Oligomers. The ring structures of the dimer (71,trimer (81, tetramer (91, and the tetrahedral structure of the tetramer (10)were calculated. Although trimeric organolithium compounds such as 8 are rare both in the solid state and in solution2' and tetrameric ones such as 9 have not been reported so far, 8 and 9 were calculated to examine the dependence of ' J c ~ ion the ring size. The optimized geometries are
Organometallics, Vol. 14, No. 2, 1995 991
Coupling Constants in MeLi, tBuLi, and (MeLi), Chart 3
H
Hi4
Pi HH
/
H
10
9
values calculated by two different types of basis sets is much smaller than that in monomeric methyllithium. This trend is in accord with the observation that the coupling constants in methyllithium tetramer are independent of so1vent.6a~b The three ring structures ( 7 , 8 , and 9 ) have lJcu values similar t o each other, whereas the tetrahedral structures (10; staggered and eclipsed forms) have L ~ This difference is obviously due smaller ~ J cvalues. to the state of aggregation, not to the degree of aggregation. That is, the value of l J c ~depends i on the number of lithium atoms which are bonded directly to the carbon atom, as the dependence was pointed out by Bauer et a1.2kJ2from the experimental ~ J C values G for several organolithium compounds. The 'JCLi values in the monomer (44.0Hz),dimer (7; 20.9 Hz),and tetrahedral tetramer (lO(staggered form); 12.5 Hz),which were calculated with the truncated lithium basis set, show that l J c ~in i methyllithium varies nearly inversely with the number of the lithium atoms which are bonded directly to the carbon atom. ~ J C values H in methyllithium oligomers also vary according t o the degree and state of aggregation, although the change is small.
shown in Table 4, and the calculated coupling constants are shown in Table 5 along with previous theoreticalg and experimenta16a,bdata. It is seen from Table 5 that the calculated 'JCLi and Conclusion ~JCH values in 10 are in good agreement with the experimental ones,6a>b whereas previous semiempirical Ab initio calculations of NMR coupling constants by FPT-INDO calculationsg give much smaller values. using the self-consistent perturbation theory with the When the truncated lithium basis set is used to model MIDI-4 basis set reproduced well the experimental ~ J C G the ionic C-Li bond, the calculated 'JCLi and VCH value in the tert-butyllithium monomer when the effect values (12.5 and 99.3 Hz in the staggered form and 14.0 of solvation was considered. The calculated 'JCLi and and 99.6 Hz in the eclipsed form, respectively) are in ~JCH values in the tetrahedral structure of methylexcellent agreement with the experimental ones.6asbThe lithium tetramer showed excellent agreement with the sign of the calculated 'JCLi value is also consistent with experimental ones. In methyllithium oligomers, the the experimental one.6c Another interesting point obrelation between l J c and ~ the number of lithium atoms served in Table 5 is that the difference between the ~ J C G directly bonded t o the carbon atom, which was previously found experimentally, was confirmed theoretically. (27) Harder, S.; Boersma, J.; Brandsma, L.; Kanters, J. A.; Bauer, Modeling the ionic C-Li bond using the truncated W.; Schleyer, P. v. R. Organometallics 1989,8, 1696. Harder, S.; lithium basis set reproduced well the experimental ~JCG Boersma, J.; Brandsma, L.; Kanters, J. A.; Duisenberg, A. J. M.; van Lenthe, J. H. Organometallics 1991,10,1623. Harder, S.; Ekhart, P. values in the tert-butyllithium monomer and methylF.; Brandsma, L.; Kanters, J. A,; Duisenberg, A. J. M.; Schleyer, P. v. lithium tetramer, suggesting the importance of the ionic R. Organometallics 1992,11, 2623. character of the C-Li bond in alkyllithiums. (28) The Chemical Society of Japan. Kagaku Binran (Chemistry Handbook), 2nd ed.; Maruzen: Tokyo, 1975; p 11-1387. Lide, D. R., Jr. J . Chem. Phys. 1980,33,1519.
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