J . Am. Chem. SOC. 1991, 113, 135-144
135
Rhenium( VII) Monoimido Alkylidyne Complexes. The Importance of Face Selectivity in the Metathesis of Acetylenes via Rhenacyclobutadiene Intermediates I. A. Weinstock, R. R. Schrock,* and William M. Davis Contribution from the Department of Chemistry 6-331, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139. Received April 30, 1990
Abstract: Re(NAr),(CH-t-Bu)CI (Ar = 2,6-C&,-i-Prz) reacts with 3 equiv of HCI in diethyl ether to give [ArNH,] [Re(C-t-Bu)(NHAr)CI,] (la) as an insoluble orange powder. [NEt,] [Re(C-t-Bu)(NHAr)CI,] (lb) can be prepared from l a by cation exchange. Addition of ZnCI, to l b yields [ R ~ ( C - ~ - B U ) ( N H A ~ ) C(Za), I , ] ~an X-ray study of which shows it to be a dimer in which each Re is a square pyramid with the neopentylidyne a-carbon atom in the apical position and a weakly bound bridging chloride ligand trans to it (space group P2,/c, a = 9.953 (4) A), b = 12.398 (9) A, c = 19.720 (6) A, /3 = 93.08 (3)O, V = 2430 (2) AS,Z = 4, p = 1.473 g/cm3, R = 0.050, R, = 0.084). Adducts of the type Re(C-t-Bu)(NHAr)CI,(L) ( L = THF, py) can be prepared from 2a. Addition of DBU to Re(C-t-Bu)(NHAr)CI,(py) in the presence of pyridine gives R ~ ( C - ~ - B U ) ( N A ~ ) C I (4a). ,(~YA ) ~more useful analogue, Re(C-t-Bu)(NAr)CI,(DME) (4b), can be prepared by adding ZnC12 to l b followed by NEt, in DME. Complexes of the type Re(C-t-Bu)(NAr)(OR), (OR = 0-t-Bu (5a), OCMe2(CF3) (Jb), 0-2,6-C6H3-i-Pr2(5c), OC(CF3)2Me(5d), or OC(CF3),H (as a diethyl ether adduct, 5e)) can be prepared from l b or 4b. Reactions of 5d and 5e with symmetric internal acetylenes give fluxional rhenacyclobutadiene complexes, Re(NAr)(C3R3)[OC(CF3)2Me]2and Re(NAr)(C3R3)[OC(CF3)2H]2 for R = Et, n-Pr, i-Bu, and i-Pr. Crystals of Re(NAr)(C,Et,)[OC(CF,),Me] belong to the space group P2,/a with a = 19.553 (8) A, b = 9.225 (4) A, c = 20.246 (8) A, p = 1 17.36 (3)O, V = 3243 (2) Z = 4, paid = 1.73 g/cm3, cc = 40.70 cm-l. The core structure is roughly a trigonal bipyramid containing axial carbon and oxygen atoms. The rhenacyclobutadiene ring is bent and asymmetric with Re-C, bond lengths of I .88 ( I ) and 2.09 ( I ) A and C,-C, bond lengths of 1.33 (2) and 1.46 (1) A. All but one of the rhenacycles have analogous structures (type 1) according to NMR studies and are stable toward loss of an acetylene or further reactions with an internal acetylene for hours at 25 OC. The exception is Re(NAr)(C3-i-Pr3)[OC(CF,),Me],, a significantly different rhenacycle (type 2) that readily loses i-PrCEC-i-Pr to give Re(C-i-Pr)(NAr)[OC(CF,),Me],. It is proposed that rhenacycles of type 2 are the ones that are active for metathesis of internal acetylenes, that rhenacycles of type 2 form when an acetylene attacks a C/O/O face in Re(CR’)(NAr)(OR),, and that inactive rhenacycles of type 1 form when an acetylene attacks a C / N / O face in Re(CR’)(NAr)(OR)2. Only acetylenes containing bulky groups are metathesized and only complexes that contain OC(CF,),Me ligands are active. The ultimate fate of Re(VI1) metallacyclobutadiene complexes over a period of hours to days is reduction to yield Re(V) complexes.
k3,
Introduction Metathesis of acetylenes by ”do” molybdenum and tungsten alkylidyne complexes of the type M(CR’)(OR), (R’ is alkyl, R is alkyl, fluoroalkyl, or aryl)] and of olefins (mostly cyclic2) by tungsten3 and molybdenum4 alkylidene complexes of the type M(CHR’)(NAr)(OR), (Ar = 2,6-C6H3-i-Pr2) has been established in the last few years. In contrast, although classical rhenium metathesis catalysts have been known for more than two decades,5 all are heterogeneous and the oxidation state and coordination geometry of the metal are unknown. Several years ago we began to explore the organometallic chemistry of Re(VI1) with the intent of preparing well-characterized metathesis catalysts.6 More ( I ) (a) Schrock, R. R. Arc. Chem. Res. 1986,19, 342. (b) Murdzek, J. S.; Schrock, R. R. In Carbyne Complexes; Verlag Chemie: Weinheim, New
York, 1988. (2) (a) Schrock, R. R. Arc. Chem. Res. 1990, 24, 158. (b) Bazan, G.; Khosravi, E.; Schrock, R. R.; Feast, W. J.; Gibson, V. C. Polym. Commun. 1989,30, 258. (c) Swager, T. M.; Dougherty, D. A.; Grubbs, R. H. J. Am. Chem. SOC.1988, 110, 2973. (d) Klavetter, F. L.; Grubbs, R. H. J. Am.
Chem. SOC.1988, 110, 7807. (3) (a) Feldman, J.; DePue, R. T.; Schaverien, C. J.; Davis, W. M.; Schrock, R. R. In Advances in Metal Carbene Chemistry; Schubert, U.,Ed.;
Kluwer Academic Publishers: Boston, 1989. (b) Schrock, R. R.; DePue, R.
T.;Feldman, J.; Yap, K. B.; Yang, D. C.; Davis, W. M.; Park, L.; DiMare, M.; Schofield, M.; Anhaus, J.; Walborsky, E.; Evitt, E.; Kriiger, C.; Betz, P.
Organometallics 1990, 9, 2262. (c) Schrock, R. R.; Krouse, S.A.; Knoll, K.; Feldman, J.; Murdzek, J. S.; Yang, D. C. J . Mol. Catal. 1988, 46, 243. (d) Schrock, R. R.; DePue, R.; Feldman, J.; Schaverien, C. J.; Dewan, J. C.; Liu, A. H. J. Am. Chem. SOC.1988,110, 1423. (e) Johnson, L. K.; Virgil, S. C.; Grubbs, R. H. J. Am. Chem. Sor. 1990, 112, 5384. (4) (a) Murdzek, J. S.;Schrock, R. R. Organometallics 1987,6, 1373. (b) Schrock, R. R.; Murdzek, J. S.; Bazan, G.; Robbins, J.; DiMare, M.; O’Regan, M. J. Am. Chem. SOC.1990, 112, 3875. (5) (a) Ivin, K. J. Olefin Metathesis; Academic Press: London, 1983. (b) Grubbs, R. H. In Comprehensive Organometallic Chemistry; Wilkinson, G . ,
Stone, F. G. A., Abel, E. W., Eds.; Pergamon: New York, 1982; Vol. 8. (c) Dragutan, V.; Balaban, A. T.; Dimonie, M. OleJn Metathesis and Ringopening Polymerization of Cyclo-Olefins, 2nd ed.; Wiley-Interscience: New York, 1985.
0002-7863/91 / I 513-135$02.50/0
recently we have developed an entry into Re(VI1) organometallic chemistry that employs 2,6-disubstituted arylimido ligands as protecting and stabilizing groups for the Re(VI1) complexes7 and are now in a position to explore in a systematic fashion the synthesis of Re(VI1) alkylidyne and alkylidene complexes and their potential as catalysts for metathesis reactions. In a preliminary communication we reported several monoimido neopentylidyne complexes and the metathesis of internal acetylenes by one of the bisalkoxide derivatives.* Here we report the full version of these and related studies, including elucidation of the chemistry that limits the longevity of acetylene metathesis. We have found that acetylene metathesis is limited by formation of a relatively stable rhenacyclobutadiene complex, the one that we believed originally to be part of the catalytic cycle, and have been able to identify the type of metallacycle that is active for metathesis of internal acetylenes, but which is not observable under most circumstances.
Results Preparation of Amido Neopentylidyne Complexes. Re(NAr)z(CH2-t-Bu)C12can be prepared in high yield in three steps from R e 2 0 7and can be dehydrohalogenated to give Re(NAr)2(CH-t-Bu)CL7 Addition of 3 equiv of HCI to Re(NAr),(CHt-Bu)Cl gives la quantitatively as an insoluble orange powder (eq I ) . A more tractable tetraethylammonium salt (lb) can be
la
(6) Edwards, D. S.; Biondi, L. V.; Ziller, J. W.; Churchill, M. R.; Schrock, R. R. Organometallics 1983, 2, 1505. (7) Horton, A. D.; Schrock, R. R. Polyhedron 1988, 7, 1841. (8) Schrock, R. R.; Weinstock, 1. A.; Horton, A. D.; Liu, A. H.; Schofield, M. H. J. Am. Chem. Soc. 1988, 110, 2686.
0 1991 American Chemical Society
Weinsrock er al.
136 J . Am. Chem. Soc.. Vol. 113. No. I . 1991 Tmbk 1. Selected Bond Distances (A) and Angles (deg) i n [Rc(C-r-Bu)(NHAr)CI,]* (h) Re-CKI) 2.407 ( 3 ) RtCl(2) 2.429 (3) RtN 1.901 (9) Rc-CKIY 2.712 ( 3 ) RtCI(3) 2.298 (4) R t C ( I ) 1.75 (I)
CI(I)-Rc-CI~I)' CI(I)-RrC1(2) CI(I)-RtC1(3) CI(3)kRtN C1(3l-Re-C(II N-Rc-C(I) Re-CI( I)-Rc Rc-N-C(I2) Re-C(l)-C(Z)
80.5 (1) 80.0 (I) 158.9 (I) 101.5 (3) 99.5 (6) 95.2 ( 5 ) 99.5 (I) 130.7 (8) 165 11)
CI(I)-RtN CI(I)-RtC(I) CI(IY-RtCI(2) CI(I)'-RtC1(3) CI(1)'-Re-N CI(I)-Re-C(I) C1(2)-RtCI(3) CI(2kR-N CI(Z)-RtCIIl
89.8 (3)
97.2 (6)
87.1 (I) 83.0 (I) 83.5 (3)
177.4 ( 6 ) 86.0 (I) 167.2
93.8 141
prepared by cation exchange. I n the 'H NMR spectrum of I b in CD,CI, the A r N H resonance isobserved at 16.93 ppm and the neormtvlidvne C- resonance is found at 315.9 oom. The cis . r r ~ structure shown for the anion in eq I is the most plausible one, since the I bonds in the nmpentylidyne ligand and the likely dative I bond in the amido ligand thereby do not compete with one another. I n theory two isomers would be possible. a syn isomer in which the A r group points toward the neopentylidyne ligand, and an anti isomer in which the A r group points away from the neopentylidyne ligand: only one isomer is observed. The process of converting Rc(NAr),(CH-r-Bu)CI into l a is believed to be related to the reaction between Re(N-r-Bu),(CHr-Bu)(CH,-r-Bu) and 3 equiv of H C I to give [Re(C-r-Bu)(CH~ - B U ) ( N H , - ~ - B U ) C Ithe ~ ] ~key . ~ feature in that case being initial protonation of the imido ligand followed by transfer of the neopentylidene n proton to an imido or amido nitrogen atom. One of several plausible sequences for forming I n is shown in eq 2.
. .
~~
+%-
(2.). F i w e 1. A view of the structure of [Rc(C-r-Bu)(NHAr)CI,l,.~.
~~~~~~~
to it. (The C( I)-ReLkl angle are all in the range 94-W while the Cl(l)'-Re-Lbmi angles are all in the range 8G87O.) The R e C ( I ) distance is typical of M = C bonds in high oxidation state alkylidyne complexes' and should be compared with R e bond lengths of 1.742(9) A in Re(C-r-Bu)(CH-r-Bu)(py),l~and 1.755 ( 6 )A in Re(s,-C,Me,)(C-r-Bu)Br,.l' The R e ( I ) - C ( 2 ) angle (165 (I)") should be compared to the R e C - C angles of 1 7 5 O and 119' in the above two compounds (respectively). The phenyl ring o f the amido ligand i s tipped up toward C ( I ) (the 'syn" conformation) and the ten-butyl group o f the nmpentylidyne ligand is tip d toward CI(2). The relatively short Re-N bond ( I ,901 (9) is consistent with a significant amount o f Re-N T bonding. Since C(I2) lies in the N/Re/C(I) plane N must be spz hybridized and pseudo doubly bound to rhenium with the d,y orbital in the basal plane. Pyridine (h) and T H F (3b) adducts of Re(C-1-Bu)(NHAr)CI, can be prepared as shown in eqs 4 and 5. Resonances in the proton N M R spectrum of 3a are broad, and addition ofpyridine results
5
Re(C-c-Bu)(NHAr)CI,(py)
CI
I t is not necessary that the neopentylidene proton migrates to the imido nitrogen atom intramolecularly. For example, further protonation of the amido ligand to give the aniline (perhaps in a cationic complex) followed by loss of H, in the neopentylidene ligand to some extramolecular base is equally plausible. Addition of ZnCI, to I b in methylene chloride yields red Za (eq 3). Proton NMR spectra of Za in CD,CI, suggest that it i s in equilibrium with a closely related species (Zb). but at a rate
slower than the NMR time scale. A t 220 K in CD,CI, only resonances for 2. are observed. while at room temperature both 2. and Zb are observed. Resonances for 2n are broad; those for Zb are sharp. A plausible scenario is that 2b is a dimer that is relatively stable to cleavage while 2. breaks UD readilv and raoidlv a t room temperature to-form a dichloromcihane adduct." An example of uhat 2b might be is a species u i t h a different relative arrangement of metal corm a t each end (e.g.. cisoid alkylidyne ligands) or one in n hich the amido ligand i s a different rotamer. C r ) d l i n c samples do not contain dichloromethane. according to elemental analjsis. An X-ray study of 2. shows i t to have the structure shown in eq 3 (Figure I: important bond distances and angles can be found in~~b~~I), rhenium is best d c w r l as ~ a square pyramid with a ncopentylidyne n-carbon atom in the apical position and a ucakl) chloride (RtCI( I,,= 2,71 (, A) tranP (9) \t*bound. T D:Colsman. M R ; Hdlo. M M.:Wullskrg. G P . Andcnan. 0 P . Straua. S H J Am Chrm Sor 1989. 1 1 1 . I162 1101 H u m " . H A ; Fel8rkrecr. J K , Anuando. R.. Herdlueck. E : Kwror. P. Rude. J Or~onomrralbrr1990. 9. 1414
(4)
3. Z&ll
THF
[NEt,] [Re(C-r-Bu)(NHAr)CI,]
Re(C-r-Bu)(NHAr)CI,(THF) (5) 3b in an averaged pyridine resonance. Proton NMR spectra at 253 K are consistent with a structure having mer chloride ligands in which rotation about the N-C, bond of the amido ligand i s slow or a structure having lac chloride ligands in which rotation about the N-C, bond i s still rapid at 253 K. At low temperatures in CD,CI1 two isomers of 3b are observed in roughly a I:Iratio. and a trace o f Zb is observed. A t room temperature the two isomers of 3b interconvert readily, but they do not interwnvert readily with Zb. The isomers of 3b probably are analogous to those proposed for 3s. Preparation of Imido Neopntylidyne Complexes. D B U (1.8diazabicyclol5.4.0lundec-7-ene) reacts with Re(C-f-Bu)(YHAr)CI,(py) in ktrahydrofuran at - 4 0 T in the presenccof pyridine to give dark blue Rc(C-r-Bu)(hAr)Cl,(py)2 (41: eq 6) C
CI , - B J C ~I ,py RI ArY*
CI
I \py
t.BuC*
m I
, a
RC
A ~ N + I \py
CI
n
4. M W l
4. nuno,
(61
as a mixture of isomers in which added pyridine exchanges rapidly with coordinated p)ridine. NMR spectra are consistent with the structures shown. In the minor isomer a pyridine could bc trans to the imido ligand instead of the neopentylidyne ligand. Note that the electron count in Re(C-f-Bu)(UAr)Cl,(py), IS 18 ifthe eleclrOn p31f on !he imido ligand I~nor included, Synthcw of a more versatile precursor to imido alkylidyne complexes. an analogue of M(NAr)(CH.r-Bu)CI,(DME) ( M =
J . A m . Chem. SOC.,Vol. 113, No. I , 1991 137
Rhenium( VII) Monoimido Alkylidyne Complexes
8,
ArN
Table 11. Chemical Shifts (ppm) of C, Resonances in Monoimido
Neopentylidyne Complexes . ~. [NEt,] [Re(C-r-Bu)(NHAr)CI,] ( l b ) Re(C-r-Bu)(NArlCl,(dme) (4b) R ~ ( C - ~ - B U ) ( N A ~ ) C(4a) I;(~~)~
3 1 5.96 312.1' 318.7' 29 1.O' 297.5' 304.4b
R~(C-~-BU)(NA~)(O-~-BU)~ (Sa) Re(C-r-Bu)(NAr)[OC(CF,)Me2I2 (5b) Re(c-r-B~)(NAr)(oAr)~ (5c)
OR = 'X(ff3hH
I
CP
Re(C-r-Bu)(NAr)[OC(CF,),Mel2 (5d) 304.6" R~(C-I-BU)(NA~)[OC(CF,)~H]~(THF) (5e) 308.5' C6D6. *CD2CI2.
Mo or W) complexes that have proven so useful for the preparation can be of complexes of the type M(CH-~-BU)(NA~)(OR)~,~-~ achieved as shown in eq 7. 4b is soluble and stable in pentane, diethyl ether, methylene chloride, and toluene. It is very sensitive to water. Proton N M R studies show that added D M E exchanges with coordinated DME. At 253 K N M R spectra are consistent with a molecule having the structure shown in eq 7. (Mo(CHt-Bu)(NAr)(triflate),(DME) has a structure in which D M E is bound trans to the neopentylidene and imido ligands.4b) Note that the total electron count again is 18 if the imido electron pair is not counted. + ZnClz - [NEt4][ZnC13]
-
[NEf~l[Re(C-t-Bu)(N)C141
DME
C1 Me ""~".deP'~
+ NEt3 - NEt3HCI * "Re(C-r-Bu)(MIAr)C13(DME)"
(7)
ArNH I ko, CI Me 4b
DME
Bisalkoxide complexes can be prepared as shown in eq 8. Complexes 5a-d also may be prepared from 4b. Addition of 3 equiv of LiOC(CF3),H to 4b a t -40 OC in diethyl ether gave red crystalline [Li( DME)] { Re( C-t-Bu)(NAr) [OC( CF3),H] 3) instead of Re(C-t-Bu)(NAr) [OC(CF3),HI2. However, a T H F derivative of this compound could be prepared by removing an alkoxide with zinc chloride as shown in eq 9. NAr + 3 LiOR in CH2Cl2
[NE~][Rc(C-~-BU)(N)C~~] -NEt4C1 - 3 LiCl - ROL
RON
(8)
Row %c-~-B~
5 OR = 0-t-Bu @a), OC(CF3)Me2 (5b). 0-2,6-C6H3-i-Pr2 (DlPP) (Sc), OC(CF&Me (5d)
ZnCI,
[L~(DME)]{R~(C-~-BU)(NA~)(OR)~} Re(C-t-Bu)(NAr)(OR),(THF) (5e) (9) OR = OC(CF3)2H
N MR spectra of neopentylidyne alkoxide complexes are straightforward. The chemical shift of the neopentylidyne acarbon atom (Table l l ) correlates with the electron-withdrawing power of the alkoxide, although the entire range of chemical shifts is only -20 ppm. Reactions of Imido Neopentylidyne Complexes with Symmetrical Internal Acetylenes. Since Re(C-t-Bu)(NAr)(OR), complexes are Re(VI1) alkylidyne analogues of olefin metathesis catalysts of the type W(CH-t-Bu)(NAr)(OR),, one might expect them to react with acetylenes. Complexes containing 0-t-Bu, DIPP, or OC(CF3)Me2groups do not react with internal acetylenes at room temperature over a period of hours, but 5d and 5e do react to give rhenacyclobutadiene complexes. Examples are shown in eqs I O and 1 1. 2,2-Dimethyl-3-hexyne must be lost rapidly from the
c2EtGCEt
56
"
II
- f-BuC&Et
RO ,,,( Ro/Re$-Et
OR =OC(CF,),Me
6a
Re(C-r-Bu)(NAr)(OR)z
+ 2 EtCiCEt
Re(C-r-Bu)(NAr)(OR),(THF)
(10)
Et AI N
11
Et
- 1-BuCaCEf - THF 5e
OR = OC(CF&H
7a
Et
r60'C
T
o
Figure 2. Variable-temperature I3C NMR spectrum of Re(NAr)(C3-iPrd[OC(CF,)2Hl2 ( 7 4 . Table 111. Chemical Shifts (ppm) of C,, C,', and C, in
Rhenacvclobutadiene Comulexes' C, C,' 199.7 Re(NAr)(C3Et3)[OC(CF3)2Me]2 (6a) 277.9 Re(NAr)(C3-n-Pr,)[OC(CF3)2Me]2 (6b) 276.6 199.5 R~(NA~)(C,-~-BU,)[OC(CF,)~M~]~ (6c) 282.1 201.7 Re(NAr)(C7-i-Pr7)[OC(CF3)zMe]z (6d) 257.5 255.7 288.0 204.6 Re(NAr)[C(t-BujCHC(i-BuI1compd
C,
T, K
141.9 142.0 150.0 107 120.4
293 193 203 187
295
IOC(CFhH1, .-(7e) .
Re~NAr)(C3Et3)[OC(CF3)2H]2 (7a) Re(NAr)(C3-i-Pr,)[OC(CFS)zHI2 (7d)
282.6 198.1 141 183 287.4 194.7 140.6 193 'Solvent = toluene-d8. ')C NMR studies of Re(NAr)(C,-n-Pr,)[OC(CF,),HI2 (7b; observed in situ) were not completed. The spectrum of R~(NA~)(C,-~-BU,)[OC(CF,)~H]~ (7c) was still broad at 273 K. Table IV. Selected Bond Distances (A) and Angles (deg) in Re(NArl(C,Et,)lOC(CF,),Me~, (6a)
Re-C(41) Re-C(43) Re-N
Re-O(2) O(2)-Re-N O(3)-Re-N N-Re-C(43)
N-Re-C(41) Re-N-C(11) C(43)-Re-0(3)
1.88 ( I ) 2.09 ( I ) 1.72 ( I ) 2.034 (7)
109.6 (4) 119.1 (4) 119.8 (4) 98.6 (4) 173.1 (8) 121.1 (4)
Re-O(3)
1.958 (7)
C(41)-C(42) C(42)-C(43)
1.46 (1) 1.33 (2)
0(2)-Re-C(4 1) 141.8 (4) 0(2)-Re-0(3) 83.2 (3) O( 3)-Re-C( 4 I ) 105.5 (4) C(43)-Re-C(4 1) 65.3 (4) C941)-C(411)-C(412) 1 1 5 ( I )
initially formed a-tert-butyl-substituted metallacycles in each case since 1 equiv of 3-hexyne consumes only 0.5 equiv of 5d or 5e. Although 6a decomposes slowly in solution, 7a is unchanged after 24 h. Variable-temperature N M R studies show that in the static structures for 6a and 7a (Figure 2) the a-carbon resonances are significantly different (Table 111). Typically only a single broad a-carbon resonance is observed near room temperature. The primary difference between the low-temperature N M R behavior of 6a and 7a is that slightly lower temperatures are required to freeze out the structure of 7a. An X-ray study showed that 6a (Figure 3) can be described roughly as a trigonal bipyramid in which N, 0 ( 3 ) , and C(43) are located in equatorial positions with N-Re-0(3), N-Re-C(43), and C(43)-Re-0(3) angles of 119.1 (4)O, 119.8 (4)O, and 121.1 ( 4 ) O , respectively. (Important bond distances and angles can be found in Table IV.) The large 0(2)-Re-C(41) angle (141.8 (4)') suggests that a square-pyramidal description (with N at the apex) also may be appropriate, but we shall discuss the structure as a TBP. The ReC3 ring is bent (the dihedral angle between the C(41)/Re/C(43) and C(41)/C(42)/C(43) planes is 34') and unsymmetrically bound between "axial" (C(41)) and "equatorial" (C(43)) ositions. The two Re-C, bond lengths (Re-C(41) = 1.88 ( I ) Re-C(43) = 2.09 A) differ by an amount well beyond 3u, as do the C,-C, bond lengths (C(42)-C(43) = 1.33 (2) A; C(41)-C(42) = 1.46 ( 1 ) A), Le., the double bonds in the ReC3
1;
Weinstock et ai.
138 J . Am. Chem. SOC., Vol. 113, No. I, I991 ring are localized as shown. These values should be compared with those for a bent ring in a tungstacyclobutadiene complex (W-C, = 1.943 (5) and 2.132 (5) A, C,-C, = 1.382 (8) and 1.4485 (7) A, dihedral angle = 58O)." The short Re-C, bond distance (Re-C(41) = 1.88 ( 1 ) A) is almost identical with that of the formal rhenium-carbon double bond in Re(C-t-Bu)(CHt-Bu)12(py) (Re-C, = 1.873 (9) A).6 The imido ligand is almost linear (Re-N(1)-C(I 1 ) = 173.1 (8)') and the Re-N bond distance (1.72 (1) A) is relatively short. The large (-80 ppm) difference in chemical shift between C, and C,' in the low-temperature I3C N M R spectrum of 6a is consistent with the different bond orders for the two rhenium-carbon bonds, the lower field resonance presumably being that associated with the axial carbon that is formally doubly bound to Re. This type of rhenacycle will be called type I . lnterconversion of C, and C,' (and the two alkoxide ligands) in rhenacycles of type I can be explained by a pseudorotation about the equatorial nitrogen atom as shown in eq 12. If the intermediate symmetrical square pyramid does not build up to any significant concentration, then the chemical shift for C, should change little during equilibration of Caxialand Cequatorial, consistent with what is observed.
A
I
'OR
RO
-
ArN
ii
RO-Re
