Organometallics 1Q91,10, 2199-2209
2199
Synthesis, Structure, and Dynamic Behavior of Rhenium Sulfide and Sulfoxide Complexes of the Formula [(q'-C,H,)Re(NO)(L)(XRR')]+X'(X = S, SO) N. Qui&
Mhdez, Atta M. Arif, and J. A. Gladysz'
Department of Chemistty, University of Utah, Salt Lake City, Utah 84 112 Received November 13, 1990
Reactions of (q5-C5H5)Re(NO)(PPh3)(OTf) and sulfides MeSR (R = Me (a), Et (b),i-Pr (c), t-Bu (a)) give sulfide complexes [(q5-C5H5)Re(NO)(PPh3)(S(R)Me)]+TfO(2a-d+TfO-; 9141%). Reactions of [(q5-CsH5)Re(NO)(PPh3)(C1CHzCl)]+BF, with SMezand -Me2 give 2a+BF; (64%) and DMSO complex [ (q6-C5H5)Re(NO)(PPh3)(S(=O)Mez)]+BF~ (4a+BF,; 86%). Reaction of 4a+BF4-and excess SMezgives 2a+BF; (83%). Reactions of (q5-C5H5)Re(NO)(CO)(OTf) with SMe, and S(t-Bu), give carbonyl-substituted (Ga,e+TfO-;99-81% ). Sulfide complexes 2a+TfOsulfide complexes [(q5-C5H5)Re(NO)(CO)(SR,)]+TfOand Ga,e+TfO-exhibit dynamic NMR behavior (AC*(T,) = 9.5-12.9kcal/mol) arising from inversion of configuration at sulfur. Crystal structures of 2a+TfO-.CHZC1, (monoclini_c,E 1 / n , a = 8.254 (2)A, b = 27.429 (8)A, c = 13.782 (7)A, /3 = 96.67 (3)O, 2 = 4), 4a+BF,- (triclinic, P1,a = 10.472 (1)A, b = 14.162 (2)A, c = 9.441 (1)A, a = 101.036(3)O /3 = 90.891(3)O,y = 87.781 (3),2 = 4),and Ga+TfO- (monoclinic, E l / n , a. = 8.346 (2)A, b = 25.387 (10)A, c = 7.287 (2)A, /3 = 96.95 (3)O,2 = 4)are reported. The sulfoxide oxygen in 4a+BF,- is syn to the PPh, ligand (torsion angle P-Re-S-0 = 17'). Transition-metal complexes of dialkyl sulfides are ubiquitous.' Diverse aspects of their physical and chemical properties have attracted the attention of researchers.l+ These include rich conformational and configurational dynamics? reactivity models for catalytic hydrodesulfurization,3 structural and electronic analogues of sulfur-ligated metalloenzymes,4 binding units in macrocyclic ligand complexes,5 and vehicles for the asymmetric oxidation of sulfides to sulfoxides.6 We have had an ongoing interest in the synthesis, structure, and reactivity of adducts of donor ligands and (PPh3)]+ the chiral rhenium fragment [ (q5-C5H5)Re(NO) (I).',* In particular, many reactions have been found in which the rhenium chirality is efficiently transferred to a new ligand-based chiral center? As a prelude to studies involving reactions on sulfur-containing ligands, we sought to define preparative routes to sulfide and sulfoxide com(1) Murray, S. G.; Hartley, F. R. Chem. Reu. 1981,81, 365. (2) (a) Abel, E. W.; Bhargava, S. K.; Orrell, K. 6.h o g . Inorg. Chem. 1984,32,1. (b) Abel, E. W.; Moas, I.; Orrell, K. G.; Sik, V. J. Organomet. Chem. 1987,326, 187. (c) Abel, E. W. Chem. Britain 1990, 148. (3) (a) Angelici, R. J. Acc. Chem. Res. 1988,21, 388. (b) Rakowski, Dubois, M. Chem. Reu. 1989,89,1. (4) (a) Cooper, S. R. Acc. Chem. Res. 1988,21,141. (b) Bemardo, M.
M.; Robandt, P. V.; Schroeder, R. R.; Rorabacher, D. B. J. Am. Chem. SOC. 1989, 111, 1224. (5) Some lead references: (a) Blake, A. J.; SchrGder, M. Adu. Inorg. Chem. 1990.36.1. (b) DesDer. J. M.: Gellman. S. H. J. Am. Chem. SOC. 1990,112,6732.' (c) Yoshida, T.; Adachi, T.; Uda, T.; Tanaka, T.; Gob, F. J. Chem. Soc., Chem. Commun. 1990,342. (6) (a) Pitchen, P.; M a c h , E.; Deshmukh, M. N.; Kagan, H. B. J. Am. Chem. SOC.1984,106,8188. (b) Samuel, 0.;Ronan, B.; Kagan, H. B. J. Organomet. Chem. 1989,379,43. (7) (a) Fernhdez, J. M.; Gladysz, J. A. Organometallics 1989,8, 207. (b) Kowalczyk, J. J.; Agbossou, S. K.; Gladysz, J. A. J. Organomet. Chem. 1990, 397,333.
(8) Recent lead references: (a) Winter, C. H.; Veal, W. R.; Garner, C. M.; Arif, A. M.; Gladysz, J. A,; J.Am. Chem. SOC. 1989, 111,4766. (b) Agbossou, S. K.; Femhdez, J. M.; Gladysz, J. A. Inorg. Chem. 1990,29, 476. (c) Bodner, G. S.; Peng, T.-S.;Arif, A. M.; Gladysz, J. A. Organometallics 1990,9,1191. (d) Agboesou, S. K.; Smith, W. W.; Gladysz, J. A. Chem. Ber. 1990, 123,1293. (e) Dewey, M. A.; Bakke, J. M.; Gladysz, J. A. Organometallics 1990, 9, 1349. (0 Kowalczyk, J. J.; Arif, A. M.; Gladysz, J. A. Organometallics 1991, 10, 1079. (9) (a) Garner, C. M.; Quirb Mhdez, N.; Kowalczyk, J. J.; Femhdez, J. M.; Emerson, K.; Larsen, R. D.; Gladysz, J. A. J.Am. Chem. SOC. 1990, 112, 5146. (b) Peng, T.-S.;Gladysz, J. A. Tetrahedron Lett. 1990, 31, 4417. (c) Dalton, D. M.; Fernhdez, J. M.; Emerson, K.; Larsen, R. D.; Arif, A. M.; Gladysz, J. A. J. Am. Chem. SOC.1990, 112, 9198.
Scheme I. Synthesis of Phosphine-SubstitutedSulfide Complexes 2+TfO-
MeSR
c
R E a,Me b, Et c, i-Pr d, 1-Bu TfO1
2+ TfO'
plexes and fundamental physical properties. In this paper, we report (1)high-yield syntheses of chiral sulfide complexes [ (v5-C5H5)Re(NO)(L)(SRR')]+X(L = PPh,, CO), (2)dynamic NMR studies that establish some of the lowest known sulfur inversion barriers, (3) crystal structures of two dimethyl sulfide complexes, and (4) preliminary studies, including a crystal structure, of analogous sulfoxide complexes. Results 1. Synthesis of Sulfide Complexes. Triflate complex (q5-C5H5)Re(NO)(PPh3)(OTf) (1)'O and sulfides MeSR (R = Me (a), Et (b), i-Pr (c), t-Bu (a); ca. 5 equiv) were reacted in CHzClz (Scheme I). Workup gave sulfide complexes [(q5-C5H5)Re(NO)(PPh3)(S(R)Me)]+TfO(2ad+TfO-). Methyl tert-butyl sulfide complex 2d+TfO- was isolated in somewhat lower yield (51%) than 2a-c+TfO(81-91%). No reaction occurred when 1 and bis(tert-butyl) sulfide were combined under similar conditions, as assayed by IR spectroscopy. Complexes 2a-d+TfO-, and all other new compounds isolated below, were characterized by microanalysis (Experimental Section) and IR and NMR (lH, % ' !{H ' I, 3'PI'H)) spectroscopy (Table I). General features were similar to those previously reported for other adduds of I and neutral heteroatomic Lewis bases, such as alcohol and ether complexes [(q5-C5H5)Re(NO)(PPhS)(ORR')]+X-.sb*d Importantly, the diastereotopic methyl groups in 2a+TfO- gave (10) (a) Merrifield, J. H.; Fernlndez, J. M.; Buhro, W. E.; Gledysz, J. A. Inorg. Chem. 1984,23,4022. (b) TfO- = CF3S03-.
0276-7333/91/2310-2199$02.50/00 1991 American Chemical Society
Quires M6ndez et al.
2200 Organometallics, Vol. 10, No. 7, 1991 Scheme XI. Synthesis and Reaction of Sulfoxide Complex 4a+BF4-
Q Re
ON/
0
&
Me' 'Me
-+
I 'PPh3
C24
ClCH2CI BFi
BFi 4a* B F i
3' B F i
I
MeSMe
A ~
ON/
I 'PPh,
MeSMe
I
s Me/"'Me
BFi 2a* B Fd
only one lH and 1%NMR resonance at room temperature. Also, complexes 2b-d'TfO- contain two stereocenters (rhenium and sulfur) and, hence, can in principle exist as mixtures of diastereomers. However, only one set of NMR resonances was observed. Additional NMR experiments are described below. An alternative route to sulfide complexes 2+X- was briefly probed. The substitution-labile dichloromethane complex [($-C,H,)Re(NO) (PPh3)(C1CH2C1)]+BF4-(3+BF4-) was generated a t -80 "C, as previously described (Scheme II).'* Then dimethyl sulfide was added (3 equiv). Workup gave dimethyl sulfide complex 2a+BF4-in 64% yield. The lH NMR spectrum of 2a+BF4-was identical with that of 2a+TfO-. When the preceding reaction was monitored by 31PNMR spectroscopy, 2a+BF4- was observed to form in quantitative yield over the temperature range -40 "C to 0 "C. 2. Crystal Structure of 2a+TfO-*CH2Cl2. In order to help interpret the dynamic properties suggested by the preceding NMR data, a crystal structure of a sulfide complex was sought. Yellow prisms of a solvate, 2a+TfO--CH2Cl2,were obtained from CH2C12/ether. X-ray data were collected under the conditions summarized in Table 11. Refinement (Experimental Section) gave the structures shown in Figure 1. The sulfide ligand sulfur atom was pyramidal, and a lone-pair (LP) position was calculated. The atomic coordinates of 2a+TfO-*CH2Cl2 and key bond lengths, bond angles, and torsion angles are summarized in Tables I11 and IV. A complete listing of bond lengths and angles is given elsewhere." 3. Synthesis and Properties of Sulfoxide Complexes. Pursuant to projected studies of the oxidation of sulfide complexes 2+X-, data on the physical and chemical properties of analogous sulfoxide complexes were sought. Thus, the dichloromethane complex 3+BF4-was treated with dimethyl sulfoxide (DMSO; Scheme 11). Workup gave the DMSO complex [ ($-C,H,)Re(NO)(PPh,)(S(=O)Me2)]+BF4-(4a+BF4-)in 86% yield. The DMSO complex 4a+BFc exhibited separate lH and 13CNMR resonances for the diastereotopic methyl groups. (11) Quirds M6ndez, N. Ph.D. Thesis, University of Utah, 1991.
Figure 1. Structure of the cation of dimethyl sulfide complex [( T $ C & , ) R ~ ( N O ) ( P P ~ ~ ) ( S M ~ ~ ) ] W O (2aWO--CH2C1& ~CH~C~~ (a) numbering diagram; (b) Newman-type projection with phenyl rings omitted.
C15
Figure 2. Structure of the cation of dimethyl sulfoxide complex [(q5- C5H5)Re (NO)(PPha)(S(=O)Me2)]+BF4- (4a+BF4-): (a) numbering diagram; (b) Newman-type projection with phenyl rings omitted.
It also gave an IR vso absorption (1123 cm-') in a range considered diagnostic of sulfur, as opposed to oxygen, coordination.12 This assignment was verified by a crystal (12) (a) Reynolds, W. L. h o g . Inorg. Chem. 1970,12, 1. (b) White, C.; Thopmson,S. J.; Maitlis, P. M. J . Chem. SOC.,Dalton Trans. 1977, 1654.
Organometallics, Vol. 10, No. 7, 1991 2201
Rhenium Sulfide and Sulfoxide Complexes
Table I. Spectroscopic Characterization of New Sulfide and Sulfoxide Complexes
31Pf1Hl
Q Re
ON/
VNO
NMR; PPmC 11.89 (a)
1716 vs
'H NMR, bo lBC(lH}NMR, ppm* 7.55-7.52 (m, 9 H of 3 C6H5),7.30-7.24 PPh, at 133.0 (d, J = 10.3, o), 132.4 (d, J = (m, 6 H of 3 C6H6),5.66 (e, C&), 55.9, i), 131.6 (8, p ) , 129.3 (d, J = 10.5, 2.55 (a, CHI) m); 92.8 (8, C6H6),32.2 (d, J = 2.6, CHI)
11.69 (8)
1704vs
7.54-7.52 (m, 9 H of 3 C6H6),7.30-7.27 PPh3 at 133.1 (d, J = 11.3, o ) , 132.3 (d, J = (m, 6 H of 3 C&&, 5.65 (8, C&), 55.9, i), 131.6 (d, J = 1.6, p ) , 129.2 (d, J 2.84 (q, J = 7.3, CHJ, 2.40 (e, = 10.3, m); 92.7 (8, C6H6),43.1 (d, J = SCHJ, 1.17 (t,J 7.4, CHpCHS) 1.6, CHJ, 27.7 (8, SCHB), 13.6 (8, CHaCHs)
7.54-7.50 (m, 9 H of 3 C6H6),7.32-7.24 (m, 6 H of 3 C~HS), 5.65 (e, C5H5), 3.11 (ap, J = 6.6, SCH), 2.21 (e, SCHa), 1.40 (d, J 6.6, CHCHJ, 1.30 (d, J = 6.7, CHC'H3)
PPhS at 133.1 (d, J = 11.4, 01, 132.4 (d, J = 55.9, i ) , 131.6 (d, J = 2.6, p ) , 129.2 (d, J = 11.3, m); 92.8 (e, C6H6),50.3 (d, J = 2.6, SCH), 24.0 (8, SCH,), 21.5 (8, CHCHJ, 21.2 (8, CHC'HJ
11.12 (8)
7.54-7.52 (m, 9 H of 3 C6H6),7.34-7.23 (m, 6 H of 3 C&), 5.62 (e, C&,), 2.08 (8, SCHB), 1.38 (8, C(CH3)a)
PPh3 at 133.1 (d, J = 10.2, o), 132.3 (d, i),d 131.5 (d, J = 2.3, p ) , 129.1 (d, J = 11.9, m); 92.4 (e, C6H& 54.6 (d, J = 3.5, C(CHJs), 28.3 (s, C(C&)s), 22.4 (8, SCHJ
11.02 (8)
IR (KBr), cm-l
complex
+
b'PPh3
Me/
'Me
Tto' 20' TfO'
@
@
y ~ 1708 o vs
Tto' 2€+ TIO'
@ de
vNo l7OoVS
+
'eM TO* 2d*W
@
bo1718 vs, vso 1121 m8 7.57-7.42 (m, 3 C6H5),5.67 (s, C6H6), 3.53 (s, CHJ, 3-31 (8, C'HJ'
0
vw 2014 VS,
0
vco 2002 VS, UNO 1746 vs 6.21 (8, CSHS), 1.64
VNO
1738 vs 6.16
(8,
c&), 3.05 (br 8, CH,)
(8,
CHS)
PPh3 at 133.9 (d, J = 11.0, o), 132.8 (d, J = 57.6, i), 132.1 (d, J = 2.2, p ) , 129.5 (d, J 11.3, m); 94.4 (a, CbH,), 56.2 (e, CHJ, 52.8 (e, C'H,)'
9.31
(8)'
193.1 (8, CO), 121.2 (9, Jcp 321.0, CFS), 94.7 (e, C6H6),32.6 (br e, CHI)
195.4 (8, CO), 120.7 (9, JCF 320.1, CF&, 95.2 (8, C&&, 61.6 (8, CCHs), 32.6 (8, CHI)
O A t 300 MHz in CDCla a t ambient probe temperature and referenced to internal Si(CHJ, unless noted. All coupling constants are in Hz. *At 75 MHz in CDCla at ambient probe temperature and referenced to CDC13 (77.0 ppm) unless noted. All coupling are in Hz and to unless noted. Triflate carbon resonances are not observed in all cases. Assignments of phenyl carbon resonances are made as described in footnote c of Table I in: Buhro, W.E.; Georgiou, S.; Fernhdez, J. M.; Patton, A. T.; Strouse, C. E.; Gladyez, J. A. Organometallics 1986,5, 956. eAt 121 MHz in CDClS a t ambient probe temperature and referenced to external 85% HBPOI. dPart of ipso doublet obscured by PPh% T h i s band is a shoulder on a BF,- absorbance. The assignment was confirmed by a spectrum of 4a+TfO- (1123 cm-l). 'Spectrum in CDzCll (ve Si(CHB), at 6 0.00 or CDzClzat 53.8 ppm) due to poor CDC&solubility.
Quir6s MGndez et al.
2202 Organometallics, Vol. 10, No. 7, 1991
Table 11. Summary of Crystallographic Data for Dimethyl Sulfide Complexes [(q6-C,H6)Re(NO)(PPhr)(SMe2)]+"fO-~CHzCl~ (2aUTfO-*CH2C12) and [(q6-C,H6)Re(No)(Co)(SMe2)]+TfO(6a+TfO-) and DMSO Complex [(q6-c6H6)Re(NO)(PPhJ (s (=o)Me2)]+BF4-(4atBFc) 2a+TfO-.CH2C12 4a+BF, 6atTfOmolecular formula CnHmCl2F3NO,PS2Re C,HZ6NO2BPSF4Re CgHllN0,SzF3Re fw 839.7 708.53 520.51 monoclinic triclinic monoclinic cryst system Pi space group E1ln E1/n cell dimens (16 "C) 8.346 (2) 8.254 (2) 10.472 (1) a, A 25.387 (10) 14.162 (2) 27.429 (8) b, A 7.287 (2) 9.441 (1) 13.782 (7) c, A 101.036 (3) a,deg 96.95 (3) 90.891 (3) 96.67 (3) & deg 87.781 (3) 7 , deg 1532.6 (1) 1373.2 (1) 3099.3 (1) v, A3 4 4 4 z 2.26 1.71 1.80 d d d t g/cm3 2.26 (CC14/CBr,) 1.86 (CC14/CBr4) 1.70 (CC14/CHJz) dot., g/cm3 (22 "C) 0.25 X 0.25 X 0.17 0.50 X 0.24 X 0.23 0.35 X 0.26 X 0.13 cryst dimens, mm Mo Ka (0.710 73) Mo Ka (0.71073) radiation (A, A) Mo Ka (0.710 73) 8-26 8-20 8-28 data collcn method 2.5-8.0, variable 3.0 3-10, variable scan speed, deg/min 0,ll; -15,15; -10,lO 0,9; 0,27; -8,8 0,lO; 0,30; -16,16 range/ indices (h,k, I ) 3 I 26 I 48 2 5 28 I 47 3 I 26 5 51 26 scan range, deg 2682 no. of reflcns measd 6516 4153 26(Kal) - 1.0 to 28(Ka2) + 1.0 26(Kal) - 1.3 to 26(Kaz) + 1.6 26(Ka1) - 1.0 to 26(Ka2)+ 1.0 scan range 0.5 0.0 0.5 tot. bkgd/scan time 98 97 98 no. of reflcns between stds 2210 5725 3977 tot. no. of unique data 1659 3594 4406 no. of obsd data, I > 3u(n 0.5382 0.8597 min transm factor 0.7881 0.9943 0.9987 0.9943 mar transm factor 181 370 no. of variables 326 83.5 43.8 46.6 abs coeff ( u ) . cm-' 0.0434 0.030 0.0471 0.037 3.22 1.80 goodness of fit 1.60 unit weight unit weight unit weight weighting factor, w 1.37, Re-NO) previously found in rhenium carbonyl nitrosyl complexes." 5. Dynamic NMR Studies. Variable-temperature 'H and 13C(lHJNMR spectra of dimethyl sulfide complexes 2a+TfO- and 6a+TfO- were recorded in CD2C1,. Those of 2a+TfO- are representative and are illustrated in Figure 4. Additional spectra are shown elsewhere." In both compounds, separate resonances were observed for the (17) For example, Re-CO and Re-NO bond lengths of 2.00 and 1.80
A are found in (11-C6H6)Re(PMe3)2(N0)(CO)(CHS): CFey, C. P.; 0 Connor, J. M.; Jones, W. D.; Haller, K. J. Organometallrcs 1983,2, 535.
Quirds Mgndez et al.
2204 Organometallics, Vol. 10, No. 7, 1991 Table IV. Selected Bond Lengths (A), Bond Angles (deg), and Torsion Angles (deg) in 2a+TfO-m CHIC&, 4a+BF4-,and 6a+TfO0-~** 2a+TfO-CH2C12 Re-S1 2.395 (3) Re-C4 2.29 (1) Re-P 2.370 i2j Re-C5 2.26 (1) 1.81 (1) 1.775 (8) S1-C24 Re-N 1.81 (1) 1.16 (1) Sl-C25 N-0 P-C6 1.83 (1) Re-C1 2.24 (1) P-c12 1.835 (8) 2.27 (1) Re-C2 P-C18 1.849 (9) 2.336 (9) Re43 S1-Re-P S1-Re-N P-Re-N Re-Sl-C24 Re-Sl-C25 C24-Sl-C25 P-Re-S1-LP P-Re-Sl-C24 P-Re-Sl-C25
92.10 (8) 92.4 (3) 90.2 (3) 109.5 (5) 110.4 (5) 99.4 (7) -59.3 (2) 174.7 (5) 66.2 (5)
Cl-C2-C3 c2-c3-c4 C3-C445 c4-c5-c1 C5-Cl-C2 N-Re-S1-LP N-Re-Sl-C24 N-Re-Sl-C25
a1
107 (1) 109 (1)
'C8
111 (1)
102 (1) 111 (1) 30.9 (3) -95.1 (6) 156.5 (6)
4a+BFi Re-S Re-P Re-N N-01 5-02 Re-C1 Re-C2 Re-C3
2.349 (1) 2.407 (1) 1.738 (6) 1.205 (7) 1.462 (4) 2.238 (7) 2.242 (7) 2.287 (6)
Re-C4 Re45 S-C24 S-C25 P-C6
P-c12 P-C18
S-Re-P S-Re-N P-Re-N Re-S-C24 Re-S-C25 c24-s-c25 02-S-C24
90.74 (5) 91.1 (2) 95.1 (2) 109.5 (2) 112.5 (2) 99.7 (4) 106.6 (3)
P-Re-S-02 P-Re-S-C24 P-Re-S-C25
-17.2 (3) -139.7 (3) 110.4 (3)
02-S-C25 Cl-C2-C3 c2-c3-c4 c3-c4-c5 c4-c5-c1 C5-Cl-C2 N-Re-S-02 N-Re-S-C24 N-Re-S-C25
2.320 (6) 2.299 (6) 1.770 (7) 1.783 (7) 1.823 (6) 1.824 (6) 1.827 (5)
2
F i g u r e 3. Structure of the cation of dimethyl sulfide complex [ (tl5-C5H5)Re(NO)(CO)(SMe2)]+TfO- (6a+TfO-): (a) numbering diagram; (b) Newman-type projection.
108.3 (3) 109.0 (7) 105.9 (7) 108.7 (6) 109.1 (6) 107.2 (6) 77.9 (4) -44.6 (4) -154.5 (4)
6a'TfORe-S1 Re-N N-0 Re-C6 C6-01 Re-C1 S1-Re-C6 S1-Re-N N-Re-C6 Re-Sl-C7 Re-Sl-C8 C7-Sl-C8 C6-Re-S1-LP CG-Re-Sl-C7 CG-Re-Sl-C8
2.391 (3) 1.773 (9) 1.20 (1) 1.90 (1) 1.14 (1) 2.274 (9) 93.1 (3) 95.8 (2) 95.4 (3) 107.5 (5) 112.8 (5) 98.2 (8) 130.0 (6) -105.8 (11) 1.4 (10)
Re-C2 Re-C3 Re-C4 Re-C5 S1-C7 si-c8 C1-C2-C3 c2-c3-c4 C3-C445 c4-c5-c1 C5-Cl-C2 N-Re-S1-LP N-Re-Sl-C7 N-Re-Sl-Ca
2.278 (9) 2.260 (9) 2.27 (1) 2.288 (9) 1.73 (2) 1.75 (1)
205
200
112 (1) 107 (1) 104 (1) 109 (1) 107 (1)
-134.3 (5) -10.0 (11) 97.1 (10)
Bond lengths and angles involving the phenyl rings have been omitted. b L P = lone pair. a
diastereotopic methyl groups at low temperatures. The resonances coalesced when the samples were warmed. Thus, some dynamic process can render the two methyl groups equivalent. Application of the coalescence formula**gave AG*(T,) for the process rendering the methyl groups equivalent as 9.6-9.8 kcal/mol for 2a+TfO- and 12.6-12.9 kcal/mol for Ga+TfO-. Data are summarized in Table V. Similarly, low-temperature 13CNMR spectra of bis(tert-butyl) sulfide complex 6e+BF4-showed two tert-butyl C(CH3)3and C(18) Sandstrom, J. Dynamic NMR Spectroscopy; Academic Press: New York, 1982; Chapter 7.
3.0
2.4
,
1.8
36
32
29
ppm
F i g u r e 4. Variable-temperature 'H and 13CIiH)NMR spectra of 2a+TfO- (CD2C12,methyl group region). Table V. Summary of Dynamic 'H and iSC('HJNMR Data" A@( T,),d compd resonance Tc,b K :,6 Hz kcal/mol 2a+TfOSCH3 205 98 9.7 5ch3 213 254 9.6 6a+TfOSCH3 263 49 12.9 5ch3 260 61 12.6 6e+TfOCH3
