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NMR studies of stereochemical non-rigidity in the complexes of tricarbonylrhenium halides with 2,4,6-trithiaheptane
Polyhedron Vol. 3, No. 3, pp. 331-339, Printed in Great Britain.
1984
0
0277-5387184 $3.00 + .oO 1984 Peqamon Pm6 Ltd.
NMR STUDIES OF STEREOCHEMICAL NON-RIGIDITY IN THE COMPLEXES OF TRICARBONYLRHENIUM HALIDES WITH 2,4,6_TRITHIAHEPTANE EDWARD W. ABEL, MOHAMED Department
Z. A. CHOWDHURY, KEITH G. ORRELL* VLADIMIR &K of Chemistry, University of Exeter, Exeter EX4 4QD, England
and
(Received 4 July 1983; accepted 25 July 1983) Abstract-Mononuclear chelate complexesfac-[{ReX(CO),)(MeSCH,SCH,SMe)] X = Cl, Br and I, have been isolated and examined by dynamic NMR methods. Energy data associated with the pyramidal inversion of the sulphur atoms and rotation of the chelated ligands have been obtained.
Metal complexes of sulphur and selenium ligands are known to undergo a wide variety of stereodynamic changes.’ We have shown that sulphur coordinated complexes of [(PtXMe,)d are particularly rich in fluxional motions.2-5 These include pyramidal sulphur inversions, ligand rotations or switchings, and Pt-methyl scramblings. Rewith the chelate ligand cently, complexes 2,4,6_trithiaheptane have been shown to produce highly tractable NMR spectra from which the energies of each of these internal rate processes can be evaluated.6 The isoelectronic nature of Re(1) and Pt(IV) is responsible for the moieties PtXMe, and ReX(CO), yielding isostructural series of complexes with S or Se ligands. We have previously studied the fluxional properties of rhenium(I) complexes with aliphatic, aromatic and olefinic thio-, seleno-, and mixed thio/seleno-ethers, but have identified only pyramidal inversions of the S and/or Se atoms7s8 in these complexes. In order to establish whether any ligand rotations occur in Re(1) complexes as they do in the Pt(IV) complexes, we have prepared the Re(1) comof 2,4,6_trithiaheptane [{ReX(CO),} plexes (MeSCH,SCH,SMe)], X = Cl, Br, I and report herein on their variable temperature NMR properties. EXPERIMENTAL Materials. The pentacarbonylrhenium(1) halides were prepared by previously reported methods.9 The ligand 2,4,6-trithiaheptane, prepared by literature methods,‘O,” was a pale-yellow oil (Yield 7.5 g, *Author to whom correspondence should be addressed.
104_108”C/15 mmHg (Lit. B.p 49%), B.P lOO-103”/3.5 mmHgi3). ‘H 106”C/5 mmHg,*’ NMR data: 6 2.15 (S-Me), 6 3.77 (CH,). The rhenium complexes were prepared as outlined below. An excess of 2,4,6-trithiaheptane (250 mg, 1.62 mmol) was added to a solution of pentacarbonylrhenium(1) halide (1.11 mmol) in chloroform (15 cm’) in a Schlenk tube equipped with a cold finger. The stirred solution was refluxed under dry nitrogen for an expedient length of time (viz. 3 days-chloride, 5 days-bromide, 13 days-iodide). The reaction was judged complete by the appearance of three new CO stretching bands in the IR spectrum and the disappearance of the CO bands due to the starting material. The near-equal intensities of the new bands were attributed to the facial isomers14 of these complexes. The volume of the solution was reduced to 2 cm3 under vacuum and an equal volume of heptane added. Slow cooling of the solution to - 20°C yielded crystals of fat-[chloro-(2,4,6-trithiaheptane)-tricarbonylrhenium(I)] (0.489 g, 96%), fat-[promo-(2,4,6trithiaheptane)-tricarbonylrhenium(I)] (0.509 g, 91%), or fat-[iodo-(2,4,6-trithiaheptane)-tricarbonylrhenium(I)] (0.517 g, 85%). The crystals were washed with petroleum spirit (4060°C) and dried under vacuum. Analytical and IR data of the complexes are collected in Table 1. Spectra. IR spectra were recorded on either a Perkin-Elmer Model 299B or 398 spectrometer using chloroform solutions of the complexes. NMR spectra were recorded on a JEOL PS/PFT-100 spectrometer operating in the Fourier transform mode at 100 MHz for ‘H studies. Variable temperature measurements were carried out as 331
332
E. W. ABEL et al. Table 1. Characterisation of the complexes co1our
Compound
[ReCl(CO),{MeS(CH,S),Me]]
M.P./'C
White
.
Analysisa
%C
%H
18.21
2.18
(18.28)
(2.19)
-lb “WI /cm
117
2043, 1952, 1918
Greyish[REB~(CO),{M~S(CH,S)~M~]]
16.58
1.91
(16.66)
(1.99)
153
2045, 1955, 1920
White
[ReI(CO)3~MeS(~2S)2Me~l
Grey
I
15.21
1.80
(15.24)
(1.82)
167-168
I
2047,
I
I
I
1954, 1921
I
'* Calculated values are given in parentheses b' Infrared spectra were recorded in CHC13 solution; all carbonyl stretching frequencies were very strong.
previously described.’ For the temperature range - 75°C to 50°C complexes were examined in the mixed solvent CD2C12/CS2. For higher temperatures the mixed solvent C6D,N02/C6D6 was employed. RESULTS
The NMR spectra of the ring protons of these Re(1) complexes are very closely related to those of the analogous Pt(IV) complexes which we have recently discussed in detail.‘j We will therefore only summarise the pertinent features of the spectra of the present Re(1) complexes. In the absence of any internal exchange process,
[meso-1]
'.
four diastereoisomers may exist, namely meso - 1, meso- and a degenerate pair of dl isomers (Fig. 1). These were clearly distinguished at low temperatures (ca. - 60°C to - 70°C) by four sharp signals in the methyl region (Fig. 2) and four AB quartets in the ring methylene region. The chemical shifts of all these signals have been collected in Table 2. On warming the complexes to aboveambient temperatures (cc. 40”(Z), the methyl signals coalesced to a singlet and the methylene signals to a single AB quartet. The case of [(ReCl(CO),}(MeSCH,SCH,SMe)] is illustrated in Fig. 3, and the high temperature data of all three complexes are included in Table 2. The changes are
[dl-l]
I _.'
(a) 26.3%
(a) 30.8%
(h) 23.4% (C) 15.3%
(bl 30.9%
6
(c) 30.7",
[dZ-21
(a) 30.8% (h) 30.9':, IC) 30.79,
Fig. 1. Interconversion of the diastereoisomers of [ReX(CO),{MeS(CH,S),Me)]. Percentage populations refer to the complexes (a) X = Cl, (b) X = Br and (c) X = I.
333
NMR studies of stereochemical non-rigidity I XI
CU
dfl me
meso-
Brl
- dll I
dll
1
meso-1 meso-ll
Fig. 2. Methyl proton spectra of [ReX(CO),{MeS(CH,S),SMe}] showing the halogen dependence of invertomer populations.
undoubtedly due to the increasing rate of sulphur pyramidal inversion with temperature,6 and the energy of the process was computed from the line shape changes of the methyl region using standard fitting procedures.’ The methylene region was not used for this purpose because of the added complexities in this region of the spectrum. An essential preliminary to any band shape analysis is the correct assignment of the low temperature spectra. The assignments given in Fig. 2 are based on the fact that the two equal intensity signals must be due to the dl invertomer, while the
+-
Y
Fig. 3. Experimental and computer synthesised spectra of the methyl proton spectra of [ReCl(CO),{MeS(CH,S),SMe}] showing the effects of sulphur pyramidal inversion. remaining two signals are assigned on the basis that the halogen dependence will be such as to favour meso- at the expense of meso-l as the halogen size increases. ‘9’It should be noted that in ReX(CO),L complexes the halogen dependence of invertomer populations is very pronounced.’ With bulky halogens, the E-methyls are forced tram to the halogen atom, methyl-carbonyl interactions being quite weak. In PtXMe,L complexes, the halogen dependence is smaller and the preferred
334
E.W. ABEL
et al.
Table 2. Chemical shifts of ‘H NMR signals of [{ReX(CO),){MeS(CH,S),Me)] in the slow and fast inversion limits X
a(-CH2-)/p.p.m,
Temp./'C
a
GKH3-)/P.P.rn.
5.19, 5.13, 5.03. 5.00
3.80. 3.77. 3.73, 3.68,
2.99, 2.91,
4.95, 4.91, 4.88, 4.81,
3.64, 3.58. 3.56, 3.52,
2.73, 2.67,
-72.1 Cl 40.0
5.10. 4.97
-59.0
3.65, 3.57
2.76
5.27, 5.18, 5.15, 5.12,
3.85, 3.80, 3.77, 3.72,
3.03. 2.94,
5.05, 5.03, 5.00. 4.91,
3.70, 3.64, 3.60, 3.56,
2.75, 2.60,
Br 43.0
5.20, 5.06
-62.1
3.71, 3.64
2.79
5.33, 5.27, 5.21, 5.18,
3.98, 3.94, 3.90, 3.85.
3.05. 2.98,
5.13, 5.11, 5.05, 4.98,
3.81, 3.79, 3.73, 3.67,
2.77, 2.70
3.83, 3.69
2.84
I 36.5
5.28, 5.14
a. Chemical shifts measured relative to SiMe4.
orientations of E-methyls are governed by a balance between halogen-E-methyl and Pt-methyl-Emethyl repulsions.3 In the trithiaheptane complexes of PtXMe, there is the added factor of the 6-membered ring conformation(s) to be considered, and in this case6 the halogen dependence of invertomer populations shows a small, opposite trend to the present ReX(CO), complexes. The conformational nature of the trithiaheptane ring in these Re(1) complexes is not known since cooling the complexes to cu. - 90°C did not reveal any significant changes in the spectra. This implies that either chair-chair interconversion is very fast on the NMR time scale or that one of the conformers is present in overwhelming abundance. The effects of varying rates of pyramidal sulphur inversion on the methyl signals were computed in the usual way. The NMR spin problem for the interconversion of the invertomers may be represented by AA’
L
BC
11k -
11 k
CB k’
DD’
where the labelling of the methyls refers to Fig. 1. Since no methyl-methyl spin couplings were detected the spin system reduces to k
A-B Icky
Ilk
c- -D
where k is the rate constant for [meso-I]+[dl- l/2] and k’ is the rate constant for [df-1/2]+[meso-21.
Band shapes were computed based on the static parameters (chemical shifts, invertomer populations and effective spin-spin relaxation times) listed in Table 3. The “best fit” spectra for [{ReCl(CO),)(MeSCH,SCH,SMe)] are shown in Fig. 3 and the associated rate constants k and k’ in Table 4. The close agreement in band shapes fully justified the assumption of independent inversion of the sulphur atom pairs. Activation parameters based on the Arrhenius and Eyring equations are given in Table 5. The possibility that the signal coalescence was due to some type of dissociation-recombination process was excluded on the basis of the detection of a separate free ligand signal on adding some trithiaheptane to the chloride complex at a temperature considerably higher than its coalescence temperature. When trithiaheptane is chelated to the PtXMe, moiety high temperature fluxions occur causing an effective rotation of the 6-membered ligand ring with respect to the equatorial Pt-Me plane. Such a fluxional movement was therefore sought in these Re(1) complexes. Heating the complexes did indeed cause a significant broadening of the methylene proton AB quartet but coalescence of the signals was not achieved at the highest temperature possible (165°C). The changes, however, were perfectly temperature reversible and it was possible to simulate the spectra quite accurately (Fig. 4). The static parameters on which these theoretical spectra are based are given in Table 6 and the activation energy parameters subsequently deduced are presented in Table 7. We are stongly of the opinion that these spectral changes indicate a 180” “pancake” rotation of the ligand, a process which may precede or may initiate or be simply a consequence
NMR studies of stereochemical Table
3. Static
parameters used in the computation [ReX(CO),(MeS(CH,S)2Me}] for pyramidal
Temp./'C
of sulphtt-methyl inversion studies
dZ-isomers
meso-l isomer X
335
non-rigidity
mew-2
spectra
of
isomer
.
TZf/S v*/nza
p*
b
VB/HZ
vC/Hr
PB + pc
vD/H*
PD
Cl
-72.1
273.3
26.3
266.9
290.8
61.6
299.3
12.1
0.240
Br
-69.2
274.9
23.4
268.3
293.6
61.8
301.5
14.8
0.205
I
-72.5
277.5
15.3
270.7
298.6
61.4
305.4
23.3
0.275
'Solvent was CD,Cl,/CS, mixture, chemical shifts (vi) measured relative to SiMe4.
b% Populations of isomers (*O.S).
Table 4. The “best fit” rate constants used for the synthesis of the spectra shown in Fig. 3 ;pectruma
Temp.l°C
k/s-l
k'/s-'
1
-72.1
0.001
0.001
*
-41.1
0.001
0.60
*
-30.2
0.001
1.40
2
-24.7
1.0
3.5
l
-21.0
2.5
6.0
-16.2
5.0
11.0
-8.7
9.5
21.0
-2.4
18.0
38.0
3.0
33.0
65.0
9.5
62.0
125.0
iumber
15.6
100.0
200.0
21.3
185.0
3M.0
40.0
785.0
1590.3
'See Figure 3. *Spectra not shown in Figure 3.
Table 5. Arrhenius
X
DISCUSSION Pyramidal hue&on. The Arrhenius and Eyring activation parameters in Table 5 are reported for the interconversions [meso-l]+[dl-l/2] and [dl1/2]+[meso-21. In all cases the log,, A and AS * values are in the region of 13 and 0 respectively, indicating purely intramolecular changes with nondissociative transition states. AG * values are of a magnitude expected for sulphur atoms coordinated to transition metals,’ and are ca. 2 kJ mol-’ higher
and Eyring activation parameters [ReX(CO),{MeS(CH,S),Me)l(x
Interconversions
meso-
of scrambling of the groups (CO and X) attached to rhenium. Such a ligand rotation process was clearly identified in the trimethylplatinum(IV) complexes with trithiaheptane,6 where in this case the scrambling of the Pt-methyl environments was also detected by ‘H NMR. In the present Re(1) complexes the carbonyl scrambling fluxion was not identified because of the considerable experimental difficulties in detecting “C carbonyl signals of nuclei attached to the highly quadrupolar rhenium nucleus (I = 5/2). Chemical intuition, however, strongly suggests that carbonyl/halogen scrambling will be occuring at an NMR detectable rate.
-fdl-112
-1 Ea/kJ mol
log&
for pyramidal inversion = Cl, Br and I)
AH9/kJ mol-1
ASt/J K -1
in the complexes
-1 mol
* 4.0
=AC+/kJ mol-'
61.43 f 1.09
13.13 + 0.21
59.16 +_1.09
-1.0
dl-l/2 -+meso-
58.87 + 1.03
12.97 ?.0.19
56.61 f 1.01
-4.15 + 3.73
59.h6 t 0.10 57.85 f 0.09
maso- + d&1/2
62.41 + 1.34
13.29 t 0.25
60.10 + 1.34
1.81 + 4.81
59.56 + 0.09
dl-112 -tmeso-
60.40 + 1.47
13.30 ?;0.27
58.09 f 1.45
2.01 f 5.19
57.49 * 0.10
Cl
Br
wso-l -.dl-l/2
62.54 f.1.49
13.28 + 0.29
60.31 f 1.49
1.90 + 5.58
59.74 + 0.16
dl-l/2 -.meso-
61.11 + 1.54
13.59 + 0.30
58.88 f 1.55
7.95 f 5.77
56.50 ?r0.17
I
%t
298.15 K.
336
.a
_
Fig. 4. Experimental and computer synthesised spectra of the methylene proton [ReCl(CO),{MeS(CH2S)2SMe}l sh owing the effects of ligand rotation.
than in the corresponding trimethylplatinum(IV) complexes. This implies the somewhat greater strength of the Re(I)tS bond compared to Pt(IV)t S bond. No significant halogen dependence of the inversion energies was found in accord with the usually negligible cis influence of halogen. The difference in barrier energies for the inter-
spectra of
conversions [meso--11+1-l/2] and [d/-l/2]+ [meso- can be mainly attributed to the difference in ground state energies of the invertomers. These differences have been calculated (Table 8) and expressed graphically in Fig. 5 where the dGe values have been evaluated from the equation AGe = - RT In Ke where K* is a standard equilibrium constant and represents the ratio
337
NMR studies of stereochemical non-rigidity Table 6. Static parameters used in simulating the methylene region of ‘H NMR spectra of [{ReX(CO)S){MeS(CH,S)2Me}]for ligand rotation studies Temp./Y
",/HZb
V*/HZ
'J,HzC
Tzf/S
[ReC1(C013(MeS(CH2S);Sle)]
99.9
473.5
307.3
13.37
0.175
[ReBt(CO)3(MeS(CH2S)2Me)]
102.5
487.9
328.3
13.43
0.250
94.0
497.9
338.7
13.67
0.320
Complexa
[ReI(CO)$MeS(CH2S)pe~]
'Solvent was C6DSN02/C6D6 mixture.
b
Chemical shifts measured relative to Me3SiOSiMe3.
=2 1 J( H-C-1H).
Table 7. Barrier energies calculated for the ligand rotation in [{ReX(C0),}{MeS(CH2S),Me}1 (x = Cl, Br and I) X
Ea/kJ mol-'
lo%?
-1 AH+/kJ mol
AS+/J K-l mol-'
'AC4/kJ mol-'
Cl
101.69 f 1.65
13.83 f 0.20
96.23 * 1.55
8.72 f 3.67
95.63 f 0.45
Br
98.53 ? 4.17
13.37 + 0.50
94.98 f 4.06
-0.25 f 9.37
95.05 f 1.27
I
97.11 * 4.95
12.90 k 0.61
93.59 + 4.95
-9.19 t11.68
96.33 f 1.47
'Calculated at 298.15 K.
of the population of the appropriate meso invertomer to either dl invertomer. Figure 5 strikingly indicates the cross-over in relative meso populations as the halogen changes from bromine to iodine. Fluxional processes. The activation parameters for the ligand rotation process are listed in Table 7. The values are cu. 35 kJ mol-’ higher than those for pyramidal inversion and are essentially halogen independent. A comparison of the AG * values with those for the corresponding process in the isoelectronic and isostructural trimethylplatinum(W) complexes of the same ligand6 shows that the barrier is cu. 15 kJ mol-’ higher in the rhenium(I) complexes. Indeed, these high AG *
Table 8. The halogen dependence of the relative invertomer populations as expressed by the AG’ parameter (see text) AG' (298.15 K)/kJ mol
values (cu. 95-96 kJ mol-i) represent some of the largest energies barriers which can be reliably measured by the ‘H NMR band coalescence method. The above-ambient temperature spectral changes may be examined in terms of a number of dissociative or non-dissociative mechanisms. One such mechanism involves halogen dissociation-recombination via a highly fluxional
O______,
q----memoI
I
3-
246’ kJ mol-’
-1
X meso-l
mesoj;>
0.39
2.32
Br
0.69
1.83
I
1.73
0.68
Cl
0
Cl
Br
I
Fig. 5. The effects of halogen type on the populations of the meso-isomers relative to the &isomers as expressed by the AC # parameter (see text).
E. W. ABEL et al.
338
Me I
/
+s1’;
/ oc,i,s*;
OC<\S,
oc/~hJ=ocq&!J co
=oc/~\s,J co
\ Me
I Me
Fig. 6. Proposed mechanism for the ligand rotation process via an eight-coordinate
five-coordinate intermediate which would allow the ligand to rotate. In order to test this explanation a halogen exchange experiment was performed. A molar equivalent of potassium iodide added to a solution of [Rewas Cl(CO),{MeS(CH,S),Me}] in C6D5N02/CbD6 solvent mixture at 165”C, a temperature where ligand rotation was rapid. Spectra recorded before and after addition and again at room temperature, showed very little evidence of halogen exchange. Moreover, the existence of such a mechanism would have been reflected in a halogen dependence of the energy barrier. An alternative dissociative mechanism may occur by the breaking and reforming of individual RecS bonds. This possibility was checked by adding some free ligand to a solution of the chloro complex and re-recording the spectra. Separate, sharp signals for the added free ligand observed at 160°C eliminated ligand exchange as a possible explanation of the high temperature spectral changes. Any non-dissociative mechanism for effective ligand rotation requires a twist of the ligand backbone. Such a movement may be a trigonal (Bailar) or a rhombic (Ray-Dutt) twist. When the distance between the two donor atoms in a chelate complex is small and the ring is slightly twisted in the ground state, a direct interaction between the donor atoms may cause the ligand to rotate about the metal atom.” A trigonal twist would not bring about averaging of the different carbonyl environments (i.e. tram S and truns X), whereas a rhombic twist would. Since ligand rotation and platinummethyl scrambling are strongly correlated in the tritrimethylplatinum(IV) complexes of thiaheptane, it would appear very likely that carbonyl/halogen scrambling will be accompanying ligand rotation in these rhenium(I) complexes although no direct evidence for it is avail-
co
\ Me
intermediate.
able (vide supra). Thus, the most probable mechanism would seem to be a twist similar to a rhombic twist in which ligand rotation occurs via a highly fluxional eight-coordinate intermediate analogous to those proposed previously for trimethylplatinum complexes with thio- and seleno- ethers,4 and trithiaheptane.6 During the twist process, the two lone pairs of electrons on the chalcogens are considered to take part in the rehybridisation of the bonding orbitals of the metal (Fig. 6) such that the electronic environments of the three carbonyls as well as the methylene protons become averaged. The energy of this rehybridisation process is clearly considerable (ca. 96 kJ mol-I). We have shown previously4 how a graph diagram in the form of a regular cube can neatly describe the effects of pyramidal inversions and ligand rotations in metal complexes. With mixed chalcogen complexes the eight corners of the cube represent the eight non-superimposable diastereoisomers of the complexes. Interconversion of any isomer to its mirror image form involves pyramidal inversion of both chalcogens and a 180” rotation. In the present rhenium complexes a cube diagram can again be used, but with the difference that there are now only four diastereoisomers, the isomers occupying the back corners of the cube being simply permutation (i.e. superimposable) isomers of the adjoining isomers on the front face. Bearing this difference in mind the cube diagram again presents a unified description of the stereodynamics of these rhenium(I) complexes.
REFERENCES 1. E. W. Abel, S. K. Bhargava and K. G. Orrell, Prog. Inorg. Chem. 1983. 2. E. W. Abel, A. R. Khan, K. Kite, K. G. Orrell and V. Sik, J. Chem. Sot., Dalton Trans. 1980, 1169.
NMR studies of stereochemical 3. E. W. Abel, A. R. Khan, K. Kite, K. G. Orrell and V. Sik, J. Chem. Sot., Dalton Trans. 1980, 1169. 4. E. W. Abel, S. K. Bhargava, K. Kite, K. G. Orrell and V. Sik, J. Chem. Sot., Dalton Trans. 1982, 583. 5. E. W. Abel, K. Kite, K. G. Orrell, V. Sik and B. L. Williams, J. Chem. SOL, Dalton Trans. 1981, 2439, and references therein. 6. E. W. Abel, M. Z. A. Chowdhury, K. G. Orrell and V. Sik, J. Organomet. Chem. accepted for publication. 7. E. W. Abel, S. K. Bhargava, M. M. Bhatti, K. Kite, M. A. Mazid, K. G. Orrell, V. Sik, B. L. Williams, M. B. Hursthouse and K. M. A. Malik, J. Chem. Sot., Dalton Trans. 1982, 2065. 8. E. W. A’oel, S. K. Bhargava, K. Kite, K. G. Orrell, V. Sik and B. L. Williams, J. Chem. Sot., Dalton Trans. accepted for publication.
non-rigidity
339
9. W. Hieber, R. Schuh and H. Fuchs, Z. Anorg. Allg. Chem. 1941, 248, 243. 10. F. G. Bordwell and B. M. Pitt, J. Am. Chem. Sot. 1955, 77, 572. 11. F. Feher and K. Vogelbruch, Chem. Ber. 1958, 91, 1003. 12. M. Ohsaku, Y. Shiro and H. Murata, Bull. Chem. Sot. Japan 1972, 45, ,113. 13. D. Welti and D. Whittaker, J. Chem. Sot. 1962, 4372. 14. E. W. Abel and S. P. Tyfield, Canad. J. Chem. 1969, 47, 4627. 15. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 4th Edn, p. 1223. Wiley, New York (1980).
1984
0
0277-5387184 $3.00 + .oO 1984 Peqamon Pm6 Ltd.
NMR STUDIES OF STEREOCHEMICAL NON-RIGIDITY IN THE COMPLEXES OF TRICARBONYLRHENIUM HALIDES WITH 2,4,6_TRITHIAHEPTANE EDWARD W. ABEL, MOHAMED Department
Z. A. CHOWDHURY, KEITH G. ORRELL* VLADIMIR &K of Chemistry, University of Exeter, Exeter EX4 4QD, England
and
(Received 4 July 1983; accepted 25 July 1983) Abstract-Mononuclear chelate complexesfac-[{ReX(CO),)(MeSCH,SCH,SMe)] X = Cl, Br and I, have been isolated and examined by dynamic NMR methods. Energy data associated with the pyramidal inversion of the sulphur atoms and rotation of the chelated ligands have been obtained.
Metal complexes of sulphur and selenium ligands are known to undergo a wide variety of stereodynamic changes.’ We have shown that sulphur coordinated complexes of [(PtXMe,)d are particularly rich in fluxional motions.2-5 These include pyramidal sulphur inversions, ligand rotations or switchings, and Pt-methyl scramblings. Rewith the chelate ligand cently, complexes 2,4,6_trithiaheptane have been shown to produce highly tractable NMR spectra from which the energies of each of these internal rate processes can be evaluated.6 The isoelectronic nature of Re(1) and Pt(IV) is responsible for the moieties PtXMe, and ReX(CO), yielding isostructural series of complexes with S or Se ligands. We have previously studied the fluxional properties of rhenium(I) complexes with aliphatic, aromatic and olefinic thio-, seleno-, and mixed thio/seleno-ethers, but have identified only pyramidal inversions of the S and/or Se atoms7s8 in these complexes. In order to establish whether any ligand rotations occur in Re(1) complexes as they do in the Pt(IV) complexes, we have prepared the Re(1) comof 2,4,6_trithiaheptane [{ReX(CO),} plexes (MeSCH,SCH,SMe)], X = Cl, Br, I and report herein on their variable temperature NMR properties. EXPERIMENTAL Materials. The pentacarbonylrhenium(1) halides were prepared by previously reported methods.9 The ligand 2,4,6-trithiaheptane, prepared by literature methods,‘O,” was a pale-yellow oil (Yield 7.5 g, *Author to whom correspondence should be addressed.
104_108”C/15 mmHg (Lit. B.p 49%), B.P lOO-103”/3.5 mmHgi3). ‘H 106”C/5 mmHg,*’ NMR data: 6 2.15 (S-Me), 6 3.77 (CH,). The rhenium complexes were prepared as outlined below. An excess of 2,4,6-trithiaheptane (250 mg, 1.62 mmol) was added to a solution of pentacarbonylrhenium(1) halide (1.11 mmol) in chloroform (15 cm’) in a Schlenk tube equipped with a cold finger. The stirred solution was refluxed under dry nitrogen for an expedient length of time (viz. 3 days-chloride, 5 days-bromide, 13 days-iodide). The reaction was judged complete by the appearance of three new CO stretching bands in the IR spectrum and the disappearance of the CO bands due to the starting material. The near-equal intensities of the new bands were attributed to the facial isomers14 of these complexes. The volume of the solution was reduced to 2 cm3 under vacuum and an equal volume of heptane added. Slow cooling of the solution to - 20°C yielded crystals of fat-[chloro-(2,4,6-trithiaheptane)-tricarbonylrhenium(I)] (0.489 g, 96%), fat-[promo-(2,4,6trithiaheptane)-tricarbonylrhenium(I)] (0.509 g, 91%), or fat-[iodo-(2,4,6-trithiaheptane)-tricarbonylrhenium(I)] (0.517 g, 85%). The crystals were washed with petroleum spirit (4060°C) and dried under vacuum. Analytical and IR data of the complexes are collected in Table 1. Spectra. IR spectra were recorded on either a Perkin-Elmer Model 299B or 398 spectrometer using chloroform solutions of the complexes. NMR spectra were recorded on a JEOL PS/PFT-100 spectrometer operating in the Fourier transform mode at 100 MHz for ‘H studies. Variable temperature measurements were carried out as 331
332
E. W. ABEL et al. Table 1. Characterisation of the complexes co1our
Compound
[ReCl(CO),{MeS(CH,S),Me]]
M.P./'C
White
.
Analysisa
%C
%H
18.21
2.18
(18.28)
(2.19)
-lb “WI /cm
117
2043, 1952, 1918
Greyish[REB~(CO),{M~S(CH,S)~M~]]
16.58
1.91
(16.66)
(1.99)
153
2045, 1955, 1920
White
[ReI(CO)3~MeS(~2S)2Me~l
Grey
I
15.21
1.80
(15.24)
(1.82)
167-168
I
2047,
I
I
I
1954, 1921
I
'* Calculated values are given in parentheses b' Infrared spectra were recorded in CHC13 solution; all carbonyl stretching frequencies were very strong.
previously described.’ For the temperature range - 75°C to 50°C complexes were examined in the mixed solvent CD2C12/CS2. For higher temperatures the mixed solvent C6D,N02/C6D6 was employed. RESULTS
The NMR spectra of the ring protons of these Re(1) complexes are very closely related to those of the analogous Pt(IV) complexes which we have recently discussed in detail.‘j We will therefore only summarise the pertinent features of the spectra of the present Re(1) complexes. In the absence of any internal exchange process,
[meso-1]
'.
four diastereoisomers may exist, namely meso - 1, meso- and a degenerate pair of dl isomers (Fig. 1). These were clearly distinguished at low temperatures (ca. - 60°C to - 70°C) by four sharp signals in the methyl region (Fig. 2) and four AB quartets in the ring methylene region. The chemical shifts of all these signals have been collected in Table 2. On warming the complexes to aboveambient temperatures (cc. 40”(Z), the methyl signals coalesced to a singlet and the methylene signals to a single AB quartet. The case of [(ReCl(CO),}(MeSCH,SCH,SMe)] is illustrated in Fig. 3, and the high temperature data of all three complexes are included in Table 2. The changes are
[dl-l]
I _.'
(a) 26.3%
(a) 30.8%
(h) 23.4% (C) 15.3%
(bl 30.9%
6
(c) 30.7",
[dZ-21
(a) 30.8% (h) 30.9':, IC) 30.79,
Fig. 1. Interconversion of the diastereoisomers of [ReX(CO),{MeS(CH,S),Me)]. Percentage populations refer to the complexes (a) X = Cl, (b) X = Br and (c) X = I.
333
NMR studies of stereochemical non-rigidity I XI
CU
dfl me
meso-
Brl
- dll I
dll
1
meso-1 meso-ll
Fig. 2. Methyl proton spectra of [ReX(CO),{MeS(CH,S),SMe}] showing the halogen dependence of invertomer populations.
undoubtedly due to the increasing rate of sulphur pyramidal inversion with temperature,6 and the energy of the process was computed from the line shape changes of the methyl region using standard fitting procedures.’ The methylene region was not used for this purpose because of the added complexities in this region of the spectrum. An essential preliminary to any band shape analysis is the correct assignment of the low temperature spectra. The assignments given in Fig. 2 are based on the fact that the two equal intensity signals must be due to the dl invertomer, while the
+-
Y
Fig. 3. Experimental and computer synthesised spectra of the methyl proton spectra of [ReCl(CO),{MeS(CH,S),SMe}] showing the effects of sulphur pyramidal inversion. remaining two signals are assigned on the basis that the halogen dependence will be such as to favour meso- at the expense of meso-l as the halogen size increases. ‘9’It should be noted that in ReX(CO),L complexes the halogen dependence of invertomer populations is very pronounced.’ With bulky halogens, the E-methyls are forced tram to the halogen atom, methyl-carbonyl interactions being quite weak. In PtXMe,L complexes, the halogen dependence is smaller and the preferred
334
E.W. ABEL
et al.
Table 2. Chemical shifts of ‘H NMR signals of [{ReX(CO),){MeS(CH,S),Me)] in the slow and fast inversion limits X
a(-CH2-)/p.p.m,
Temp./'C
a
GKH3-)/P.P.rn.
5.19, 5.13, 5.03. 5.00
3.80. 3.77. 3.73, 3.68,
2.99, 2.91,
4.95, 4.91, 4.88, 4.81,
3.64, 3.58. 3.56, 3.52,
2.73, 2.67,
-72.1 Cl 40.0
5.10. 4.97
-59.0
3.65, 3.57
2.76
5.27, 5.18, 5.15, 5.12,
3.85, 3.80, 3.77, 3.72,
3.03. 2.94,
5.05, 5.03, 5.00. 4.91,
3.70, 3.64, 3.60, 3.56,
2.75, 2.60,
Br 43.0
5.20, 5.06
-62.1
3.71, 3.64
2.79
5.33, 5.27, 5.21, 5.18,
3.98, 3.94, 3.90, 3.85.
3.05. 2.98,
5.13, 5.11, 5.05, 4.98,
3.81, 3.79, 3.73, 3.67,
2.77, 2.70
3.83, 3.69
2.84
I 36.5
5.28, 5.14
a. Chemical shifts measured relative to SiMe4.
orientations of E-methyls are governed by a balance between halogen-E-methyl and Pt-methyl-Emethyl repulsions.3 In the trithiaheptane complexes of PtXMe, there is the added factor of the 6-membered ring conformation(s) to be considered, and in this case6 the halogen dependence of invertomer populations shows a small, opposite trend to the present ReX(CO), complexes. The conformational nature of the trithiaheptane ring in these Re(1) complexes is not known since cooling the complexes to cu. - 90°C did not reveal any significant changes in the spectra. This implies that either chair-chair interconversion is very fast on the NMR time scale or that one of the conformers is present in overwhelming abundance. The effects of varying rates of pyramidal sulphur inversion on the methyl signals were computed in the usual way. The NMR spin problem for the interconversion of the invertomers may be represented by AA’
L
BC
11k -
11 k
CB k’
DD’
where the labelling of the methyls refers to Fig. 1. Since no methyl-methyl spin couplings were detected the spin system reduces to k
A-B Icky
Ilk
c- -D
where k is the rate constant for [meso-I]+[dl- l/2] and k’ is the rate constant for [df-1/2]+[meso-21.
Band shapes were computed based on the static parameters (chemical shifts, invertomer populations and effective spin-spin relaxation times) listed in Table 3. The “best fit” spectra for [{ReCl(CO),)(MeSCH,SCH,SMe)] are shown in Fig. 3 and the associated rate constants k and k’ in Table 4. The close agreement in band shapes fully justified the assumption of independent inversion of the sulphur atom pairs. Activation parameters based on the Arrhenius and Eyring equations are given in Table 5. The possibility that the signal coalescence was due to some type of dissociation-recombination process was excluded on the basis of the detection of a separate free ligand signal on adding some trithiaheptane to the chloride complex at a temperature considerably higher than its coalescence temperature. When trithiaheptane is chelated to the PtXMe, moiety high temperature fluxions occur causing an effective rotation of the 6-membered ligand ring with respect to the equatorial Pt-Me plane. Such a fluxional movement was therefore sought in these Re(1) complexes. Heating the complexes did indeed cause a significant broadening of the methylene proton AB quartet but coalescence of the signals was not achieved at the highest temperature possible (165°C). The changes, however, were perfectly temperature reversible and it was possible to simulate the spectra quite accurately (Fig. 4). The static parameters on which these theoretical spectra are based are given in Table 6 and the activation energy parameters subsequently deduced are presented in Table 7. We are stongly of the opinion that these spectral changes indicate a 180” “pancake” rotation of the ligand, a process which may precede or may initiate or be simply a consequence
NMR studies of stereochemical Table
3. Static
parameters used in the computation [ReX(CO),(MeS(CH,S)2Me}] for pyramidal
Temp./'C
of sulphtt-methyl inversion studies
dZ-isomers
meso-l isomer X
335
non-rigidity
mew-2
spectra
of
isomer
.
TZf/S v*/nza
p*
b
VB/HZ
vC/Hr
PB + pc
vD/H*
PD
Cl
-72.1
273.3
26.3
266.9
290.8
61.6
299.3
12.1
0.240
Br
-69.2
274.9
23.4
268.3
293.6
61.8
301.5
14.8
0.205
I
-72.5
277.5
15.3
270.7
298.6
61.4
305.4
23.3
0.275
'Solvent was CD,Cl,/CS, mixture, chemical shifts (vi) measured relative to SiMe4.
b% Populations of isomers (*O.S).
Table 4. The “best fit” rate constants used for the synthesis of the spectra shown in Fig. 3 ;pectruma
Temp.l°C
k/s-l
k'/s-'
1
-72.1
0.001
0.001
*
-41.1
0.001
0.60
*
-30.2
0.001
1.40
2
-24.7
1.0
3.5
l
-21.0
2.5
6.0
-16.2
5.0
11.0
-8.7
9.5
21.0
-2.4
18.0
38.0
3.0
33.0
65.0
9.5
62.0
125.0
iumber
15.6
100.0
200.0
21.3
185.0
3M.0
40.0
785.0
1590.3
'See Figure 3. *Spectra not shown in Figure 3.
Table 5. Arrhenius
X
DISCUSSION Pyramidal hue&on. The Arrhenius and Eyring activation parameters in Table 5 are reported for the interconversions [meso-l]+[dl-l/2] and [dl1/2]+[meso-21. In all cases the log,, A and AS * values are in the region of 13 and 0 respectively, indicating purely intramolecular changes with nondissociative transition states. AG * values are of a magnitude expected for sulphur atoms coordinated to transition metals,’ and are ca. 2 kJ mol-’ higher
and Eyring activation parameters [ReX(CO),{MeS(CH,S),Me)l(x
Interconversions
meso-
of scrambling of the groups (CO and X) attached to rhenium. Such a ligand rotation process was clearly identified in the trimethylplatinum(IV) complexes with trithiaheptane,6 where in this case the scrambling of the Pt-methyl environments was also detected by ‘H NMR. In the present Re(1) complexes the carbonyl scrambling fluxion was not identified because of the considerable experimental difficulties in detecting “C carbonyl signals of nuclei attached to the highly quadrupolar rhenium nucleus (I = 5/2). Chemical intuition, however, strongly suggests that carbonyl/halogen scrambling will be occuring at an NMR detectable rate.
-fdl-112
-1 Ea/kJ mol
log&
for pyramidal inversion = Cl, Br and I)
AH9/kJ mol-1
ASt/J K -1
in the complexes
-1 mol
* 4.0
=AC+/kJ mol-'
61.43 f 1.09
13.13 + 0.21
59.16 +_1.09
-1.0
dl-l/2 -+meso-
58.87 + 1.03
12.97 ?.0.19
56.61 f 1.01
-4.15 + 3.73
59.h6 t 0.10 57.85 f 0.09
maso- + d&1/2
62.41 + 1.34
13.29 t 0.25
60.10 + 1.34
1.81 + 4.81
59.56 + 0.09
dl-112 -tmeso-
60.40 + 1.47
13.30 ?;0.27
58.09 f 1.45
2.01 f 5.19
57.49 * 0.10
Cl
Br
wso-l -.dl-l/2
62.54 f.1.49
13.28 + 0.29
60.31 f 1.49
1.90 + 5.58
59.74 + 0.16
dl-l/2 -.meso-
61.11 + 1.54
13.59 + 0.30
58.88 f 1.55
7.95 f 5.77
56.50 ?r0.17
I
%t
298.15 K.
336
.a
_
Fig. 4. Experimental and computer synthesised spectra of the methylene proton [ReCl(CO),{MeS(CH2S)2SMe}l sh owing the effects of ligand rotation.
than in the corresponding trimethylplatinum(IV) complexes. This implies the somewhat greater strength of the Re(I)tS bond compared to Pt(IV)t S bond. No significant halogen dependence of the inversion energies was found in accord with the usually negligible cis influence of halogen. The difference in barrier energies for the inter-
spectra of
conversions [meso--11+1-l/2] and [d/-l/2]+ [meso- can be mainly attributed to the difference in ground state energies of the invertomers. These differences have been calculated (Table 8) and expressed graphically in Fig. 5 where the dGe values have been evaluated from the equation AGe = - RT In Ke where K* is a standard equilibrium constant and represents the ratio
337
NMR studies of stereochemical non-rigidity Table 6. Static parameters used in simulating the methylene region of ‘H NMR spectra of [{ReX(CO)S){MeS(CH,S)2Me}]for ligand rotation studies Temp./Y
",/HZb
V*/HZ
'J,HzC
Tzf/S
[ReC1(C013(MeS(CH2S);Sle)]
99.9
473.5
307.3
13.37
0.175
[ReBt(CO)3(MeS(CH2S)2Me)]
102.5
487.9
328.3
13.43
0.250
94.0
497.9
338.7
13.67
0.320
Complexa
[ReI(CO)$MeS(CH2S)pe~]
'Solvent was C6DSN02/C6D6 mixture.
b
Chemical shifts measured relative to Me3SiOSiMe3.
=2 1 J( H-C-1H).
Table 7. Barrier energies calculated for the ligand rotation in [{ReX(C0),}{MeS(CH2S),Me}1 (x = Cl, Br and I) X
Ea/kJ mol-'
lo%?
-1 AH+/kJ mol
AS+/J K-l mol-'
'AC4/kJ mol-'
Cl
101.69 f 1.65
13.83 f 0.20
96.23 * 1.55
8.72 f 3.67
95.63 f 0.45
Br
98.53 ? 4.17
13.37 + 0.50
94.98 f 4.06
-0.25 f 9.37
95.05 f 1.27
I
97.11 * 4.95
12.90 k 0.61
93.59 + 4.95
-9.19 t11.68
96.33 f 1.47
'Calculated at 298.15 K.
of the population of the appropriate meso invertomer to either dl invertomer. Figure 5 strikingly indicates the cross-over in relative meso populations as the halogen changes from bromine to iodine. Fluxional processes. The activation parameters for the ligand rotation process are listed in Table 7. The values are cu. 35 kJ mol-’ higher than those for pyramidal inversion and are essentially halogen independent. A comparison of the AG * values with those for the corresponding process in the isoelectronic and isostructural trimethylplatinum(W) complexes of the same ligand6 shows that the barrier is cu. 15 kJ mol-’ higher in the rhenium(I) complexes. Indeed, these high AG *
Table 8. The halogen dependence of the relative invertomer populations as expressed by the AG’ parameter (see text) AG' (298.15 K)/kJ mol
values (cu. 95-96 kJ mol-i) represent some of the largest energies barriers which can be reliably measured by the ‘H NMR band coalescence method. The above-ambient temperature spectral changes may be examined in terms of a number of dissociative or non-dissociative mechanisms. One such mechanism involves halogen dissociation-recombination via a highly fluxional
O______,
q----memoI
I
3-
246’ kJ mol-’
-1
X meso-l
mesoj;>
0.39
2.32
Br
0.69
1.83
I
1.73
0.68
Cl
0
Cl
Br
I
Fig. 5. The effects of halogen type on the populations of the meso-isomers relative to the &isomers as expressed by the AC # parameter (see text).
E. W. ABEL et al.
338
Me I
/
+s1’;
/ oc,i,s*;
OC<\S,
oc/~hJ=ocq&!J co
=oc/~\s,J co
\ Me
I Me
Fig. 6. Proposed mechanism for the ligand rotation process via an eight-coordinate
five-coordinate intermediate which would allow the ligand to rotate. In order to test this explanation a halogen exchange experiment was performed. A molar equivalent of potassium iodide added to a solution of [Rewas Cl(CO),{MeS(CH,S),Me}] in C6D5N02/CbD6 solvent mixture at 165”C, a temperature where ligand rotation was rapid. Spectra recorded before and after addition and again at room temperature, showed very little evidence of halogen exchange. Moreover, the existence of such a mechanism would have been reflected in a halogen dependence of the energy barrier. An alternative dissociative mechanism may occur by the breaking and reforming of individual RecS bonds. This possibility was checked by adding some free ligand to a solution of the chloro complex and re-recording the spectra. Separate, sharp signals for the added free ligand observed at 160°C eliminated ligand exchange as a possible explanation of the high temperature spectral changes. Any non-dissociative mechanism for effective ligand rotation requires a twist of the ligand backbone. Such a movement may be a trigonal (Bailar) or a rhombic (Ray-Dutt) twist. When the distance between the two donor atoms in a chelate complex is small and the ring is slightly twisted in the ground state, a direct interaction between the donor atoms may cause the ligand to rotate about the metal atom.” A trigonal twist would not bring about averaging of the different carbonyl environments (i.e. tram S and truns X), whereas a rhombic twist would. Since ligand rotation and platinummethyl scrambling are strongly correlated in the tritrimethylplatinum(IV) complexes of thiaheptane, it would appear very likely that carbonyl/halogen scrambling will be accompanying ligand rotation in these rhenium(I) complexes although no direct evidence for it is avail-
co
\ Me
intermediate.
able (vide supra). Thus, the most probable mechanism would seem to be a twist similar to a rhombic twist in which ligand rotation occurs via a highly fluxional eight-coordinate intermediate analogous to those proposed previously for trimethylplatinum complexes with thio- and seleno- ethers,4 and trithiaheptane.6 During the twist process, the two lone pairs of electrons on the chalcogens are considered to take part in the rehybridisation of the bonding orbitals of the metal (Fig. 6) such that the electronic environments of the three carbonyls as well as the methylene protons become averaged. The energy of this rehybridisation process is clearly considerable (ca. 96 kJ mol-I). We have shown previously4 how a graph diagram in the form of a regular cube can neatly describe the effects of pyramidal inversions and ligand rotations in metal complexes. With mixed chalcogen complexes the eight corners of the cube represent the eight non-superimposable diastereoisomers of the complexes. Interconversion of any isomer to its mirror image form involves pyramidal inversion of both chalcogens and a 180” rotation. In the present rhenium complexes a cube diagram can again be used, but with the difference that there are now only four diastereoisomers, the isomers occupying the back corners of the cube being simply permutation (i.e. superimposable) isomers of the adjoining isomers on the front face. Bearing this difference in mind the cube diagram again presents a unified description of the stereodynamics of these rhenium(I) complexes.
REFERENCES 1. E. W. Abel, S. K. Bhargava and K. G. Orrell, Prog. Inorg. Chem. 1983. 2. E. W. Abel, A. R. Khan, K. Kite, K. G. Orrell and V. Sik, J. Chem. Sot., Dalton Trans. 1980, 1169.
NMR studies of stereochemical 3. E. W. Abel, A. R. Khan, K. Kite, K. G. Orrell and V. Sik, J. Chem. Sot., Dalton Trans. 1980, 1169. 4. E. W. Abel, S. K. Bhargava, K. Kite, K. G. Orrell and V. Sik, J. Chem. Sot., Dalton Trans. 1982, 583. 5. E. W. Abel, K. Kite, K. G. Orrell, V. Sik and B. L. Williams, J. Chem. SOL, Dalton Trans. 1981, 2439, and references therein. 6. E. W. Abel, M. Z. A. Chowdhury, K. G. Orrell and V. Sik, J. Organomet. Chem. accepted for publication. 7. E. W. Abel, S. K. Bhargava, M. M. Bhatti, K. Kite, M. A. Mazid, K. G. Orrell, V. Sik, B. L. Williams, M. B. Hursthouse and K. M. A. Malik, J. Chem. Sot., Dalton Trans. 1982, 2065. 8. E. W. A’oel, S. K. Bhargava, K. Kite, K. G. Orrell, V. Sik and B. L. Williams, J. Chem. Sot., Dalton Trans. accepted for publication.
non-rigidity
339
9. W. Hieber, R. Schuh and H. Fuchs, Z. Anorg. Allg. Chem. 1941, 248, 243. 10. F. G. Bordwell and B. M. Pitt, J. Am. Chem. Sot. 1955, 77, 572. 11. F. Feher and K. Vogelbruch, Chem. Ber. 1958, 91, 1003. 12. M. Ohsaku, Y. Shiro and H. Murata, Bull. Chem. Sot. Japan 1972, 45, ,113. 13. D. Welti and D. Whittaker, J. Chem. Sot. 1962, 4372. 14. E. W. Abel and S. P. Tyfield, Canad. J. Chem. 1969, 47, 4627. 15. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 4th Edn, p. 1223. Wiley, New York (1980).