Organometallics 1983, 2, 135-140 (1)' for the phosphorus-bridged and the values of 19.2 and 16.6' for the silicon- and germanium-bridged analogues, respectively. The C(1)-C(6) distance and the Cp-Fe-Cp angle in 9 are also intermediate between the corresponding values observed for the phosphorus and the silicon/germanium derivatives (see Table XII). The cyclopentadienyl and phenyl rings in all four compounds are planar to within fO.O1 A although seven of the eleven rings exhibit statistically significant deviations from planarity (x2values for the C(1) and C(6) cyclopentadienyl rings are 11.7 and 2.3 for 3, 16.6 and 3.9 for 4, 38.0 and 26.0 for 7, and 0.4 and 13.6 for 9; x2 = 4.3, 15.0, and 11.5, respectively, for the phenyl rings in 3,7, and 9). The mean displacements of the P or As atoms from the mean planes of the cyclopentadienyl rings are 0.988 (l), 1.004 (4), 1.015 (6), and 1.075 (4) A for 3, 4, 7,and 9, respectively. The mean l i b r a t i o n - c o r r e ~ t e ddistance ~ ~ , ~ ~ in the cyclopentadienyl rings of 1.428 A is in excellent agreement with the accepted value of 1.429 A.31 The cyclopentadienyl rings all display substituent-induced geometrical distortions, the mean bond length involving the P- or As-substituted carbon atom being 1.445 (9) A, adjacent C-C bonds averaging 1.419 (8) A, and the unique bonds (across the ring from the substituted atom) averaging 1.414 (9) A. The pattern of C-C bond length variation in the 1,2-disubstituted cyclopentadienyl rings differs somewhat (see Table X), the longest distances (1.464 (3) A in 7,and 1.445 (2) A in 9) being between the substituted carbon atoms. Ring bond angles at the substituted carbon atoms are (29)Schomaker, V.;Trueblood, K. N. Acta Crystallogr., Sect. E 1968, B24, 63. (30)Cruickshank, D.W. J. Acta Crystallogr. 1956,9,747,754;1961, 14, 896. (31)Churchill, M.R.; Karla, K. L. Inorg. Chem. 1973,12,1650.
135
contracted from the "ideal" 108.0' to 104.9 (4)-106.3 (2)' for monosubstituted 106.4 (2)-107.4 (2)' for disubstituted rings with corresponding increases in the angles at the unsubstituted carbons. Substituent-induced ring distortions for the P- and As-substituted phenyl rings are as expected.32 The mean bond angles at phosphorus are 98.3,102.9, and 99.2" in 3, 4, and 7, respectively, and the mean angle a t As in 9 is 96.8', all of these values being substantially smaller than the tetrahedral angle (vide supra). The mean angles at nitrogen (113.2' for 7, 112.5' for 9), in contrast, are larger than the tetrahedral angle, indicating some delocalization of the lone pair.
Acknowledgment. Computing funds for the structure analyses of 4,7, and 9 were provided by grants from the Xerox Corp. of Canada and the U.B.C. Computing Centre. Financial assistance in the form of operating grants from NSERC, Canada, to W.R.C. and F.W.B.E. is gratefully acknowledged. Registry No. 3, 72954-06-4;4, 83547-83-5;5, 72954-08-6;6, 83560-65-0; 6a, 31904-34-4; 6b, 83547-84-6;7, 83547-85-7;7a, 83601-98-3;7b, 83560-64-9;8,83547-86-8;9, 83547-87-9;TMED, 110-18-9; tert-butyldichlorophosphine,25979-07-1; As,As-diiodophenylarsine, 6380-34-3; P,P-dichlorophenylphosphine, 644-97-3. Supplementary Material Available: Tables of anisotropic thermal parameters, measured and calculated thermal parameters, bond distances and angles involving hydrogen atoms and torsion angles, and structure factor amplitudes (Tables VI-IX and XIII-XXIII) (77 pages). Ordering information is given on any current masthead page. (32)Domenicano, A,; Vaciago, A.; Coulson, C. A. Acta Crystallogr., Sect. E 1975,B31, 221, 234.
An v2-Arene Complex of Rhenium: Synthesis, Characterization, and Activation of the Aromatic Carbon-Hydrogen Bonds James R. Sweet and William A. G. Graham* Department of Chemisty, University of Alberta, Edmonton, Alberta, Canada T6G 2G2 Received July 2, 1982
Investigations of a cationic q2-arenecomplex of rhenium having activated aromatic carbon-hydrogen [PF,] (2.PF6)forms as yellow microcrystals bonds are described. [ (I~-C~H~)R~(NO)(CO)(~,~-~~-C~H~CHP~~)] from the reaction of Ph3CPF6with (q-CSH5)Re(NO)(CO)Hin CH2C12at -78 "C; the solid is stable at room temperature for limited periods, but solutions begin to decompose at a significant rate above -40 "C. The [PF,]. In CHzClz triphenylmethane ligand is readily displaced by PPh, to form [(q-C5H5)F&(NO)(CO)(PPh3)] at -78 'C, %PF6is deprotonated by Et3N affording para and meta isomers of (q-C5H5)Re(NO)(CO)(q1C6H4CHPh2).'H NMR spectra indicate that 2 is stereochemically nonrigid over the range studied (-40 to -70 'C). Migration of the rhenium group about the coordinated ring is proposed to occur via intermediate 7'-arenium structures, a model related to that of electrophilic aromatic substitution. Formation of these intermediates in solution accounts for activation of the carbon-hydrogen bonds toward deprotonation. Arene molecules coordinated in v2 fashion to a transition metal have often been suggested as intermediates in the activation of carbon-hydrogen bonds or other catalytic processes.' The $-arene bonding mode is well established ' metals such as Cu+ and Ag+,2and there are several for dO (1)References to the original literature are cited in the following reviews: (a) Parshall, G. W. Acc. Chem. Res. 1970,3,139.(b) Ibid. 1975, 8, 113. (c) "Homogeneous Catalysis"; Wiley: New York, 1980;Chapter 7. (d) Muetterties, E.L.; Bleeke, J. R. Acc. Chem. Res. 1979, 12,325.
examples involving transition metals with perfluoro- or perfluoroalkyl-substituted derivative^;^ the latter cannot be regarded as typical arenes, however. The crystal structure of a (1,2-q2-anthracene)nickel(0) complex4 ap(2)Silverthorne, W. E. Adu. Organomet. Chem. 1975, 13, 47. (3) (a) Browning, J.; Green, M.; Penfold, B. R.; Spencer, J. L.; Stone, F. G. A. J.Chem. SOC., Chem. Commun. 1973,31.(b) Cobbledick, R. E.; Einstein, F. W. B. Acta Crystallogr., Sect. B. 1978,E34,1&19. (c) Cook, D. J.; Green, M.; Mayne, N.; Stone, F. G. A. J . Chem. SOC.A 1968,1771.
0276-7333/83/2302-0l35$01.50/0 0 1983 American Chemical Society
136 Organometallics, Vol. 2, No. 1, 1983 pears to provide the only direct evidence for q2-coordination of a representative aromatic hydrocarbon to a transition metal. Thus, (q2-arene)metalcomplexes of d" configuration ( n < 10) are a little known but credible species about which information is required, as was recently pointed out by Muetterties and Bleeke.Id We have prepared and characterized a cationic triphenylmethane complex in which one of the phenyl rings is q2 coordinated to rhenium(1). The compound is prepared by addition of (q-C5H,)Re(NO)(CO)H5,6 (1) to 1 mol of Ph3C+PF6-in dichloromethane at low temperature' and is formulated on the basis of chemical and spectroscopic studies as [ (q-C5H5)Re(NO)(CO)(3,4-q2-C6H5CHPh2)][PF6](2-PF6). Although cation 2 is stable in solution (CH2C1,) only a t temperatures below -40 "C, the pale yellow microcrystalline hexafluorophosphate salt can be isolated in analytical purity and survives for several hours at room temperature without noticeable decomposition. Previous work in this and other laboratories has shown the ability of the rhenium group, (+2,H5)Re(N0)(CO), to form complexes of exceptional stability with organic functional groups and small molecule^;^^^ the present results provide a further striking example. An important aspect of 2 is that aromatic carbon-hydrogen bonds of the V2-coordinatedring have been activated to such an extent that hydrogen can be removed as a proton by triethylamine. The products of deprotonation are isomers of the neutral substituted phenyl derivative (q-C5H5)Re(NO)(CO)(C6H4CHPh2).Moreover, low-temperature 'H NMR spectroscopy has established that 2 is nonrigid. The intermediate in this dynamic process, as well as in the deprotonation reactions, is proposed to have the 11'-cyclohexadienyl cation structure of the type established in electrophilic aromatic substitution. Results and Discussion In recent studies of (q-C,H,)Re(NO)(CO)H (l),we were unable to demonstrate any Lowry-Bronsted acidity, but found that it reacted readily with the strong hydride acceptor cations tropylium (C7H7+)and trityl (Ph3C+). Reaction with C7H7+(eq la) gave a l,2-q2-C7H8cation in which cycloheptatriene (produced by hydride abstraction) formed the ligand.8b Reaction with Ph3C+(eq lb) in donor solvents led to rhenium cations having solvent molecules as ligands.5bIn both reactions, the hypothetical 16-electron cation [ (q-C,H,)Re(NO)(CO)]+,which would be the formal product of abstraction of the hydride ligand, is converted to the observed coordinatively saturated product by ligands which are present. We were intrigued with the possibility of generating and perhaps isolating the reactive species [ (q-C5H5)Re(NO)(CO)]+and supposed that the reaction of eq lb, carried out in the generally non-coordinating solvent dichloro-
Sweet and Graham
Re
(L
=
CH3CN,
methane, would provide a route to this potentially useful intermediate. In view of the work of Beck and Schloterg on the reaction of (q-C5H,)M(C0)3H(M = Mo, W) with Ph3C+X- (X- = BF4-, PFL), it seemed possible that the species we sought would emerge as a "lightly stabilized" complex with coordinated CH2C12or counterion. However, the work described here shows that the course of events in the rhenium system is both unanticipated and unprecedented. Reaction of (q-C,H,)Re(NO)(CO)H (1) w i t h Ph3CPF6. Although the reaction at 25 "C was unpromising,1° slow addition of solid 1 to an equimolar amount of Ph3CPF6in CH2C12at -78 "C produced a yellow, microcrystalline precipitate after a short interval. The color, solubility, and infrared spectra (vco 2026 cm-l, vN0 1765 cm-') of this air-stable solid were consistent with a carbonylnitrosylrhenium cation; reproducible analyses for C, H, and N led to its formation as [(q-C5H5)Re(NO)(CO)(Ph3CH)][PF,] (2.PF6).11 The reaction may be written as in eq 2, in which the product incorporates 1 mol of triphenylmethane. ( v - G H ~ ) R ~ ( N O ) ( C O+) H Ph,CPF,
CH,Cl,
1
[(q-C5H,)Re(NO)(CO)(Ph3CH)l [PF61 (2)
2.PF6 Complex %PF6decomposes rapidly above -40 "C in dichloromethane to give unidentified q-C5H5products and 1 mol of Ph3CH; the latter confirms the formulation. As a solid %PF6exhibits much higher thermal stability, with fairly slow decomposition at room temperature.12 Keeping in mind the 18-electron formalism, two plausible structures 2a and 2b may be considered for the tri-
e
e Re
Re
+
0 C'/\'CPh3 N H
'
0
0 2a
(4) Brauer, D. J.; Kruger, C. Inorg. Chem. 1977, 16, 884. (5) (a) Stewart, R. P.; Okamoto, N.; Graham, W. A. G. J. Organomet. Chem. 1972,42, C32. (b) Sweet, J. R.; Graham, W. A. G. organometallics, 1982, 1, 982. (6) The prefix q used alone implies that all the atoms in a ring or chain, or all the multiply bonded ligand atoms, are bound to the central atom: International Union of Pure and Applied Chemistry, 'Nomenclature of Inorganic Chemistry", 2nd ed. (Definitive Rules 1970); Butterworths: London, 1971. Cf. also: Pure Appl. Chem. 1971, 28, 1. (7) The conditions are critical in this reaction. Under other conditions (room temperature, reverse addition, 0.5 mol of Ph,Ct) the product is [(~-CSH,)zRez(NO)2C0)2H] [PF,]: Sweet, J. R.; Graham, W. A. G., manuscript in preparation. (8) (a) Sweet, J. R.; Graham, W. A. G. J. Organomet. Chem. 1979,173, C9. (b) Ibid. 1981,217, C37. (c) J. Am. Chem. SOC. 1982,104, 2811. (d) Casey, C. P.; Andrews, M. A.; McAlister, D. R.; Rmz, J. E. Ibid. 1980,102, 1927. (e) Tam, W.; Lin, G.-Y.; Wong, W.-K.; Kiel, W. A,; Wong, V. K.; Gladysz, J. A. Ibid. 1982, 104, 141.
0
THF,acetone)
2b
phenylmethane cation. Both are consistent with properties so far described for %PF6.The hydridoalkyl structure 2a could form by direct attack of trityl cation at the metal center.13 The v2-arene structure 2b could arise as does (9) Beck, W.; Schloter, K. 2.Nuturforsch., B.: Anorg. Chem., Org. Chem. 1978,33B, 1214. (10) A black solution formed from which no carbonyl-containing products were isolated; 'H NMR monitoring showed the presence of Ph,CH and several unidentified 7-cyclopentadienyl products. (11) In particular, the high carbon and hydrogen content excludes such or (q-C,H,)a priori possibilities as [(q-C6HS)Re(NO)(CO)CH2C12]PF, Re(NO)(CO)PF,. (12) Solid 2.PF6 darkens noticeably after a few hours under nitrogen at room temperature, but excellent microanalytical results are obtained 2-3 h after a freshly prepared sample has reached room temperature.
Organometallics, Vol. 2, No. I , 1983 137
An q2-Arene Complex of Rhenium
the q2-cycloheptatriene complex of eq la: initial hydride abstraction from 1 followed by coordination of the 16electron rhenium moiety to one double bond of Ph3CH. In the following sections we discuss the evidence bearing on the structure of cation 2, first examining its chemical reactions and then the 'H NMR data. Reaction of 2.PF6 with Ph3P. One equivalent of Ph3P reacts with a CHzClzsolution of 2.PF6 at low temperature forming the new complex [ (q-C,H,)Re(NO)(CO)(PPh3)][PF6](3) in 96% yield as stable yellow crystals which have been fully characterized.14 The reaction also forms 1equiv of Ph&H and may therefore be written as eq 3. -C5H5
CHDCI,
This facile reaction might reasonably be expected for either of the structures proposed for 2. For 2a, the process would be visualized as a ligand-assisted reductive elimination, while displacement of the q2-arene ligand would be expected to occur readily, although there is of course no exact precedent. Reaction of 2.PF6 with Et3N. The reaction with ESN was expected to be more informative. A cation having structure 2a might lose a proton to this base forming the neutral complex (q-C5H5)Re(NO)(CO)(q'-CPh3) or the products of its decomposition. It was less clear whether deprotonation of 2b would occur, but q2-coordination to the metal cation might activate the carbon-hydrogen bonds of the arene sufficiently for this to occur; the product would then be a neutral (ql-ary1)rhenium derivative. Reaction of %PF6with Et3N occurred readily at -78 "C in CH2C12affording a red, air-stable crystalline product. The color, solubility, and IR (hexane, vco 1982 cm-l, vN0 1731 cm-') indicated a neutral (pC,H,)Re(NO)(CO) derivative. Mass spectroscopy and analysis indicated the composition (C5H5)Re(NO)(CO)(C19H15). The 400-MHz 'H NMR spectrum was more complex than expected if the C19H15 residue had been a CPh, group; in particular, two (q-C5H5)resonances in 55:45 ratio were observed and it became evident that two isomers had been formed. After separation by multiple fractional crystallization,these were characterized as the para (4) and meta (5) forms of (7-
CHPh2 4
5
C5H5)Re(NO)(CO)(C6H4CHPh2). The 'H NMR spectra with assignments are shown in Figure 1 and justified in the Experimental Section. It is noteworthy that protons ortho to the rhenium group are shifted to lower field than the other aromatic protons. The deprotonation products from the Et3N reaction are consistent with the q2-arenestructure 2b. It is significant that no ortho isomer was formed in the reaction; the isolated yield of unseparated para and meta isomers was 89%, and the para:meta ratio in the initial product mixture was (13) A related but neutral complex which definitely has the hydriFischer, E. 0.; doalkyl structure is (?-CSHS)Re(CO),(H)(?l-CH,Ph): Frank, A. Chem. Ber. 1978,111, 3740. (14) Spectroscopic parameters of 3 are similar to those of [(?-C6H6)Re(NO)(CO)(CH,CN)][PF,]?b Preparation of the cation of 3 as ita BF4salt has very recently been reported.&
H 2 v H 6 CHPh2
I I I
Figure 1. 'H NMR spectra (400MHz, CD2C12)of para (4 upper spectrum) and meta (5, lower spectrum) isomers of (v-C5H5)Re-
(NO)(CO)(C6H4CHPh2).
5545. The products imply that it is the para and meta C-H bonds of the $-aryl ligand that are being activated. This would suggest that in 2, the (q-C5H5)Re(NO)(CO)+ group is attached to C3 and C4 of the coordinated phenyl group. On this argument, 2b could be more precisely formulated as [ (q-C5H5)~(NO)(CO)(3,4q2-C,H5~HPh2)]+. We shall consider this point in more detail below. Deprotonation of the triphenylmethane cation 2 is reversible. Reaction of the 5545 mixture of 4 and 5 with HBF4.0Et2 at -78 "C afforded [ (q-C,H,)Re(NO)(CO)(Ph3CH)][BF4]in 82% yield, and the 'H NMR of cation 2 formed in this way was the same as that of 2 formed in eq 2. Protonation of pure para (4) and meta ( 5 ) isomers separately gave the same result. Reaction of %PF6with PMe2Ph. In the reaction of 2 with Ph3P, triphenylmethane displacement occurred (eq 3). With the more basic Et3N, deprotonation of the triphenylmethane ligand was the exclusive pathway. Both of these reaction modes are exhibited in the reaction of 2.PF6 with phenyldimethylphosphine. In CH2C12at -78 "C, the products consisted of an approximately 1:1:2 mixture of 4,5, and the new phosphine cation [(q-C,H,)Re(NO)(CO)(PMe2Ph)][PI?,] (6).15 Stable, yellow 6 was isolated in good yield and fully characterized. A noteable feature of its 'H NMR is the nonequivalence of the (diastereotopic) methyl groups, a consequence of the chirality of the metal center. 'H NMR Studies of 2.PF6. The foregoing chemical investigations are suggestive of the q2-C6H,CHPh2(2b) structure and more specifically the 3,4-q2-C6H,CHPh2 isomer. As we shall see, the results of 'H NMR studies are generally in agreement with this picture but permit an elaboration of considerable interest. (15) The exact product ratio in this reaction is somewhat temperature dependent. At -78 OC,the yield of 6 is about 60%, dropping at higher temperatures to about 50%. The ratio of 4 to 5 remains constant and equal to that obtained on deprotonation with Et3N.
138 OrganometaZZics, Val. 2, No. I, I983
Sweet and Graham
CHDCli
I
The evidence discussed so far for the structure of 2 points to the coordination of one phenyl ring of triphenylmethane. The 18-electron rule suggests that the coordination is of the $-type, while deprotonation to para and meta products suggests that C3 and C4 are bonded to rhenium. The structure most consistent with these observations is 2c.
Figure 2. 'H NMR spectrum (400 MHz, CD,CI,, -50 " C ) of I(n-C,H~)Re~NOJ(COI(n*~C~HbCHPh,)li~F~l.
Efforts to obtain 'H NMR data of high quality were hampered by the low thermal stability of 2 and by its low solubility in CD2Cb. No other suitahle solvent was found. Very long acquisition times were required. It was advantageousto react 1 with Ph3CPF6in the NMR tube, thereby generating a supersaturated solution which only slowly precipitated 2.PF,. Spectra of these solutions were identical with those of samples prepared by dissolving solid 2.PF6 at low temperature. The 400-MHz 'H NMR spectrum of 2-PF6is shown in Figure 2. The spectrum was invariant over the -70 to -40 "C range, above which decomposition became quite rapid. Major featnres are the signal at 6 5.90 (5 H), a value typical of cations in this system, a series of complex multiplets in the aromatic region (15 H), and a singlet at 6 5.65 (1 H). The latter singlet enables a clear choice to be made between possibilities 2a and 2b for the structure of the cation. For 2a, the resonance of the hydridic hydrogen would be expected at very much higher field. The observed position at 6 5.65 is more consistent with the aliphatic hydrogen of the v2-C6H5CHPhzgroup; it is only slightly shifted to lower field from its position in free Ph3CH (6 5.56 in CD,Cld. Moreover, at higher amplitude, satellites with 'J(I3C-H) = the 6 5.65 singlet exhibits 128 Hz, a normal value for a C(sp3)-H bond. In the aromatic region, again consistent with the structure 2b, several small multiplets appear shifted upfield from the phenyl multiplets. These are assigned to the unique phenyl ring which is bonded to the metal center. Coordination to rhenium would be expected to shift the resonances of these protons to higher fields. Further evidence for the structure of cation 2 and for the mechanism by which it is formed comes from studies with the deuteridesb (n-C,H,)Re(NO)(CO)D (7). The reaction product of 7 with Ph3CPF6has a 'H NMR spectrum that is the same as that of 2 (Figure 2) except for the singlet at 6 5.65 which is absent in the deuterated complex. Finally, deprotonation of deuterated 2 p r e p a d in this way forms deuterated 4 and 5, with deuterium in the aliphatic position as shown in eq 4 (where the (q-C,H,)Re(NO)(CO)
Re
2c
Stereochemical Aspects a n d t h e Nonrigidity of 2 in Solution. As reasonable as the arguments to this point may appear, the single static structure 2c does not fully explain the 'H NMR spectrum of 2. It is well-known that coordination of a metal to an unsymmetrical olefm imparts chirality to the system,'6 and when the metal center is chiral as is the case with the (q-C5H5)Re(NO)(CO)group, two diastereomers will result. We have demonstratedab that the diastereomers of the olefin cation [(q-C,H,)Re(NO)(C0)(1,2-2-C,Hd]+ are readily distinguished by their 'H NMR spectra, and the same would be anticipated for 2. The diastereomers expected for the triphenylmethane cation 2 are shown as 2d and 2e (only one enantiomer is shown for each). Yet the single q-C5H, signal of Figure
e e 2d
2e
2 does not indicate the presence of diastereomers nor do the signals of the coordinated phenyl ring. It is possible that by coincidence these signals differ so little from one diastereomer to the other as to be unresolvable; it is also possible that only one diastereomer has been formed in an asymmetric synthesis. We think it more likely that the diastereomers are interconverted by a process which is rapid on the NMR time ~cale.'~' The observed 'H NMR s p e d " of 2 would thus reflect the average environment of each of the protons in the two distinct structures. An observation to be mentioned later enabled us to conclude that reversible dissociation of the nZ-Ph3CHligand is not occurring rapidly on NMR time scale, so other interconversion pathways must be considered. If we exclude inversion at rhenium on the grounds that the activation energy is likely to be high,'7b the simplest intramolecular process that would interconvert 2d and 2e is migration of the rhenium group from the 3,4-$ to the adjacent 4 3 - 2 position on the m e face of the phenyl ring. We now direct attention to the question of possible in-
dPh2
group is represented for simplicity by Re). These observations directly c o n f m that the proton is removed from an sp2carhon atom, demonstrating the extent to which the coordinated arene ring has been activated in the complex 2.
(16) Paiaro, G.; Panunzi, A. J. Am. Chem. Sac. 1964,86,5148. Panunzi, A.; Paiaro, G. Ibid. 1966,88, 4843. (17) (8) On the assumption that the vC& 'H NMR signals of the diastereomers of 2 would be separated by 16 Hz as they m e in [(nC~H,)Re(NO)(CO)(l,Z-~z.C,Hs)l',Bb the rate of interconversion at -10 "C muat exceed 35.5 8',leading to m upper limit on AGam of 10.3 k d . (b) Inversion clearly does not occur at ambient temperature in the olefin cation 6(n-C~H,)Re(NO)(CO)(l,Z.~z.C~H~~]+ since diastereomers me o b served' nor in the Me2PPh cation 6 where diastereotopic methyls are observed. In (,-C,HS)Re(NO)(CO)(,'-C,H,),the rhenium centre is eonfigurationally atable on the NMR time scale to at least 130 T.Bb
Organometallics, Vol. 2, No. I , 1983 139
A n q2-Arene Complex of Rhenium
termediates in this migration process. This is an important point because it may be the intermediate, not the ground state of the q2-Ph3CHcation, that actually undergoes deprotonation. We shall examine two models. The Hydridoaryl Intermediate. This possibility derives from the early suggestion of Chatt and Davidson18 of the tautomeric equilibrium shown in eq 5 (dmpe =
8
9
Me2PCH2CH2PMe2).Only the hydridoaryl form 9 was observed in solution, and it was also found in the solid state.lg The q2-naphthaleneform 8 was proposed to account for the chemical reactions, especially the release of naphthalene on pyrolysis. Application of an equilibrium of this sort to the interconversion of diastereomers of 2 is shown in Figure 3. Although deprotonation of 11 or 13 should lead to the correct products, careful interpretation of the arene resonances in the 'H NMR spectrum enables hydridoaryl intermediates to be excluded. The 'H NMR spectrum of 2 (Figure 2) has small multiplets at 6 6.83 (d, l H), 6.95 (m, 2 H), and 7.01 (t, l H). Decoupling established that these protons were on the same ring (clearly the coordinated phenyl) and that the 6 6.95 multiplet involved two different protons. The signal due to the fifth proton of this ring must be at another position where it is obscured. The conclusion that there are five separate signals for the coordinated ring requires that the process which interconverts diastereomers does not interchange any of the protons of this ring. However, in the hydridoaryl intermediate 11,free rotation about the rhenium-phenyl bond would interchange two pairs of protons, and three signals at most would be observed. Thus, rapid interconversion of the structures shown in Figure 3 does not properly account for the observed spectrum of 2.20 The Arenium Intermediate.21 A second type of intermediate through which diastereomers of 2 could interconvert is the arenium ion 2f, and this pathway is shown
Re
2f
in Figure 4. The q2 to q1 conversions proposed here are reminiscent of the T complex to arenium ion sequence of electrophilic aromatic substitution22(eq 6), the last two (18) Chatt, J.; Davidson, J. M. J. Chem. SOC. 1965,843. This seminal paper appears to contain the first suggestion of an aromatic system as a two-electron donor, "after the manner of a mono-olefin", to a transition metal not involving a d'" configuration: an q2-arenecomplex in present terminology. Inherent in it was the notion that activation of an aromatic C-H bond involved an intermediate V2-arene complex. (19) Gregory, U. A.; Ibekwe, S. D.; Kilbourn, B. T.; Russell, D. R. J. Chem. SOC. A, 1971, 1118. (20) The same observation also excludes interconversion of diastereomers on the NMR time scale by reversible dissociation of the triphenylmethane ligand. (21) We utilize the nomenclature which has been proposed for carbocations of the cyclohexadienyl type: Olah, G. A. J.Am. Chem. SOC.1972, 94, 808. (22) March, J. "Advanced Organic Chemistry"; McGraw-Hill New York, 1979; p 453.
steps of which provide a model for the deprotonation of 2. E+
E
The sequences of Figure 4 are consistent with the lH NMR spectrum of 2. In every structure the five protons of the coordinated ring are nonequivalent, and in particular in the intermediate 2f, proton pairs on opposite sides of the ring are diastereotopic. Thus five different signals, each a weighted average, would be expected in the absence of accidental degeneracies. While one cannot predict the appearance of the averaged spectrum in an exact way, contributions from the arenium structures would be expected to shift the hydrogens of the rhenium-bonded ring to considerably higher field than in free triphenylmethane. The overall similarity of the lH NMR spectrum of 2 to that of free Ph3CH (only small upfield shifts of protons in the coordinated ring) suggests that the q2-bonded form is dominant. This conclusion is in agreement with the chemical shift of the q-C5H5group and the carbonyl and nitrosyl stretching frequencies, both of which we have found to be sensitive indicators of positive charge localized on the rhenium atom.23 If significant quantities of the arenium structures had been present in solution, their IR bands would have been clearly observable. Deprotonation of 2 would presumably occur from the arenium intermediates. The para and meta arenium ions of Figure 4 would form 4 and 5 , respectively. The lack of an ortho phenyl product from deprotonation may reflect an absence of the appropriate ortho arenium precursor; this would be reasonable on steric grounds. We note that Figure 4 could be extended to include 2 , 3 - ~ ~ - P h , c H structures, although this is not required by the experimental evidence. Conclusions. From its chemical reactions and spectroscopic properties, we conclude that 2 contains an q2coordinated phenyl group. Furthermore, the probe provided by the chiral rhenium center shows that the complex is stereochemicallynonrigid, and a pathway for migration of the rhenium group is proposed which involves an arenium ion intermediate. The proposed intermediate accounts for the activation of the aromatic hydrogens in the complex to the extent that they can be removed as protons by triethylamine. The concepts of electrophilic aromatic substitution provide a model for this and vice versa. The thermal stability of 2 is much less than that of an v2 complex of a nonaromatic system, for which [(qC5H5)Re(NO)(CO)(1,2-q2-C7H8)]+ serves as an example.8b The q2-Ph3CHis also much more readily displaced as shown by the low temperature reaction with Ph3P.24 These observations indicate a substantial difference in the energetics of q2-olefinand q2-arenecoordination; one likely rationale for this is the loss of aromatic stabilization when the arene coordinates. (23) Values of vc0 and vN0 for 2 (2026,1765 cm-') fall in the range of cationic complexes of more traditional ligands: [(&H,)Re(NO)(CO)(PPhJ]+ (6), 2023, 1767 cm-'; [(s-C5H5)Re(NO)(CO)(CH3CN)]+, 2030, 1769 [(~-C5H5)Re(NO)(CO)(q2-C~H*)]+, 2050, 1774 cm-l.ab Frequencies in the arenium structures where positive charge is mainly localized in the ring would be expected to fall nearer the range of such derivatives as (?-C5H,)Re(NO)(CO)CH,58." (1963, 1696 cm-') or 4 and 5 (1976, 1715 cm-'). T o facilitate comparison with the ionic derivatives, values listed here for the neutral complexes were also measured in CH2C12;in the latter solvent they are shifted to lower frequency by 6-19 cm-' from their values in hexane. (24) [(q-C5H5)Re(NO)(CO)(1,2-q2-C,H,)][BF,] does not react with Ph3P in CH& at temperatures up to 25 "C.
Sweet a n d G r a h a m
140 Organometallics, Vol. 2, No. 1, 1983
12
,jf'
Figure 3. A possible mechanism for interconversion of diastereomers 10 and 12 via hydridoaryl intermediates 11 and 13. Two, equivalent orientations of 13 are shown. The chiral (q-C,H,)Re(NO)(CO)group is represented by Re. This pathway 1s inconsistent with observed 'H NMR spectra (see text). Re
10
12
!t
J.
RwRe mH Re
R
H
R
=
Ph2HC
Figure 4. Interconversion of diastereomers 10 and 12 via the arenium intermediate 2f. The para and meta arenium ions shown would form 4 and 5 upon deprotonation. The chiral (q-C5H5)Re(NO)(CO)group is represented by Re.
The characteristics of the triphenylmethane cation 2 (low solubility, low stability, complex 'H NMR) present problems in the determination of its structure. Simpler arene derivatives of this system should be advantageous, and we are seeking to prepare other [(q-C,H,)Re(NO)(CO)(arene)]+complexes to gain further insight into the nature of this important class of compounds.
Experimental Section All reactions were carried out under an atmosphere of nitrogen. Solvents were dried by using standard procedures and distilled immediately before use. Rhenium carbonyl was purchased from Strem Chemicals, Inc., and triphenylcarbenium hexafluorophosphate from Aldrich; (&5H5)Re(NO)(CO)H (1) was prepared as previously described." Infrared spectra were measwed by using a Nicolet MX-1 fourier transform instrument and 'H NMR spectra by using a Bruker WH-200 or WH-400 Fourier transform spectrometers. Preparation of [(q-C5H5)Re(NO)( C O )(Ph,CH)][PFe] (2. PFs). Triphenylcarbenium hexafluorophosphate (0.125 g, 0.32 mmol) was dissolved in 10 mL of CH2Cl2and cooled to -78 "C. was added in four equal portions over Solid 1 (0.100 g, 0.32 "01) about 15 min. The solution turned reddish yellow and within 0.5 h a yellow precipitate appeared. The solution was filtered a t -78 "C and the solid washed with cold CHzClz(2 X 5 mL) to afford 2.PFs as yellow microcrystals (0.21 g, 93%): IR (cm-'; CH2C1z) vco 2026 (s), vN0 1765 ( 8 ) ; 'H NMR see Results and Discussion and Figure 2. Anal. Calcd for C25H21N02RePFs:C, 42.98; H, 3.12; N, 2.00. Found: C, 42.96; H, 3.02; N, 2.11. Reaction of %PF6with Triphenylphosphine. A solution of Ph3CPF6 (0.125 g, 0.32 mmol) in 10 mL of CHzClzwas cooled to -78 "C, and solid 1 (0.100 g, 0.32 m o l ) was added in four equal portions over 15 min. Triphenylphosphine (0.085 g, 0.32 mmol) was added, and the resulting yellow solution was stirred at -78
"C for 1 h before being warmed to room temperature. Addition of diethyl ether afforded a precipitate which was collected and washed with ether (3 X 10 mL) to give [(s-C5H5)Re(NO)(CO)(PPh,)][PF,] (3) as yellow microcrystals (0.22 g, 96%): IR (cm-'; CH2Clz) vco 2023 (s), VNO 1767 (5); 'H NMR (CDzC12) 6 5.83 (5 H), 7.5 (m, 15 H). Anal. Calcd for C24HzoN0zP2F,Re:C, 40.23; H, 2.81; N, 1.95. Found: C, 40.29; H, 2.80; N, 1.92. Reaction of %PF6with Triethylamine. Triethylamine (0.90 mL, 6.47 mmol) was added to a solution of &PFs (1.61 mmol, prepared as above) in 10 mL of CHzClzmaintained at -78 "C. After being stirred at -78 "C for 1 h, the solution was warmed to room temperature and solvent removed at reduced pressure, leaving a red solid. This solid was extracted into hexane, and the extracts were filtered and cooled to -78 "C, affording pink crystals of 4 and 5 (totalyield 0.79 g, 89%). Isomers 4 and 5 were separated by repeated fractional crystallizationsfrom CHZCl2-hexane.Para isomer (4): IR (cm-'; hexane) vco 1982 (s), vN0 1731 (s): mass spectrum, (24 eV, 120 "C, m / e ) ,M+, (M - CO)+,(M - CO - NO)'. Anal. Calcd for Cz5H&J02Re:C, 54.33; H, 3.65; N, 2.53. Found: C, 54.04; H, 3.65; N, 2.57. Meta Isomer (5): IR (cm-l; hexane) vco 1982 (s), vNO 1731 (5): MS (20 eV, 120 "C, m / e ) M+, (M CO)', (M - CO - NO)'. Anal. Calcd for CZ5H2,O2NRe:C, 54.33; H, 3.65; N, 2.53. Found: C, 54.09; H, 3.64; N, 2.59. 'H NMR Spectrum of 4. Singlets at 6 5.67 (5 H) and 5.46 (1 H) are assigned to the 9-C5H5ring and the aliphatic CHPhz proton (Figure 1). A series of multiplets centered ca. 6 7.2 (10 H) are assigned to the monosubstituted phenyls. Bracketing these multiplets are doublets at 6 6.82 (2 H) and 7.43 (2 H); decoupling at 6 6.82 caused collapse of the 6 7.43 doublet. This AA'XX' pattern is only consistent with para disubstitution. The 6 6.82 and 7.43 signals are assigned to Hz,6 and respectively (numbering as in Fig l),based on the fact that irradiation at 6 5.46 eliminated a small coupling on the 6 6.82 doublet. A NOE experiment complemented this result by confirming the close proximity of the CHPhz proton to the protons at 6 6.82. 'H NMR Spectrum of 5. Singlets at 6 5.60 ( 5 H) and 5.44 (1H) me assigned to 9-C5H5and CHPh2. In the aromatic region, decoupling the triplet at 6 6.96 (1 H) collapsed the doublets at both 6 6.71 (1H) and 7.40 (1H). This coupling pattern implies meta substitution and leads to the assignment of the 6 6.96 triplet to H5 with J(H4-H5) E J(H5,H6) = 7.3 Hz. The 6 6.71 and 7.40 doublets are assigned to H6 and H4, respectively;each component of these doublets is a triplet (J = 1.5 Hz) due to meta coupling. H2 is obscured by the CsH5 multiplets at ca. 6 7.2 (10 H). Reaction of %PF6with Phenyldimethylphosphine. Phenyldimethylphosphine (0.25 mL, 1.69 mmol) was added to a solution of %PF6 (1.61 mmol, prepared as above) in 10 mL of CH2Clzmaintained at -78 "C. The resulting orange solution was stirred for 1 h at -78 "C and then warmed to room temperature. Addition of ether gave a precipitate which was washed with ether (3 X 20 mL) affording [(q-C5H5)Re(NO)(CO)(PMe2Ph)][PFs] (6) as yellow microcrystals (0.55 g, 58%, see Results and Discussion): IR (cm-') (CHZCl2)vco 2022 (s), vNo 1771 (5): 'H NMR (CDzC12) 6 5.83 (5 H), 7.68 (m, 5 H), 2.285 (d, 3 H), 2.291 (d, 3 H); J [(CH,),-P] E J[(C&)b-P] = 10.7 Hz. Anal. Calcd for Cl,Hl6F,NO2P2Re: C, 28.38; H, 2.72; N, 2.36. Found: C, 28.34; H, 2.69; N, 2.43. Protonation of 4 and 5. A mixture of isomers 4 and 5 (0.100 g, 0.18 mmol, prepared in 55:45 molar ratio by reaction of %PF6 with EbN as described above)was dissolved in CHZClzand cooled to -78 "C. Addition of excess HBF4.Et20 (Aldrich) formed a yellow-orange solution from which a yellow solid slowly precipitated. The solid was collected by filtration at -78 "C and washed with cold CHZCl2(2 X 5 mL); IR and 'H NMR of the solid identified it as 2.BF4 (0.116 g, 82%).
Acknowledgment. We thank the Natural Sciences and Engineering Research Council of Canada for financial support and a scholarship to J.R.S. and T. Nakashima and G. Bigham for their cooperation in measuring NMR spectra. Registry No. 1, 38814-46-9;2+ PFe-, 83585-32-4; 2+ BF,-, 83585-33-5; 3, 79919-50-9; 4, 83572-98-9; 5 , 83572-99-0; 6, 83573-01-7;triphenylcarbenium hexafluorophosphate, 437-17-2; triethylamine, 121-44-8.
