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Molybdenum-centred carboncarbon bond formation reactions
Polyhe+on Vol. 5, No. l/Z, pp. 427433, Printed in Great Britain
1986 0
MOLYBDENUM-CENTRED FORMATION
CARBON-CARBON REACTIONS
MICHAEL
0277-5387/X6 $3.00 + .OO 1986 Pergamon Pm% Ltd
BOND
GREEN
Department of Inorganic Chemistry, School of Chemistry, University of Bristol, Bristol BS8 lTS, U.K. (Received 19 June 1985) Abstract-The chemistry of the bisalkyne molybdenum complexes [Mo(L)(q2-R&R)&C,H, or q5-C,H,)][BF,] (L = CO or MeCN) is described. The reaction of the carbonyl cation with o-diphenylphosphinostyrene affords an alkyne-alkene complex which is transformed into a 1,3diene complex in refluxing MeCN via C-C bond formation and a 43-H shift, whereas the acetonitrile species with cycle-octatetraene and cycle-heptatriene undergo (6 + 2)~ addition reactions with the co-ordinated alkyne. Addition of one electron to the acetonitrile cation leads to novel alkyne-linking reactions at two molybdenum centres.
Recently l we have been exploring the chemistry of an interesting group of cationic alkyne molybdenum complexes in which the alkyne functions as either a three- or a four-electron donor. The starting point for our investigation was the molecule
[Mo~(CO)&-C~H~)~] with AgBF, in the presence of but-Zyne. An alternative synthesis which can be extended to alkynes carrying reactive functional groups such as SiMe,,2 makes use of the lability of the cation cis-[Mo(NCMe),(CO),(q-C,HS)I[BF,], and has the advantage that it avoids the use of CMo(CO)(?2-MeC2Me)2(tl-C5H5)1CBF,I expensive silver salts. The methodology can be (Scheme l), which was readily available from the readily extended to allow easy access to the more redox cleavage of the MO-MO bond present in reactive corresponding q5-indenyl complexes.
67\
““-J
Lo
Mci,-co
-
a
(ii)
-
oc:’
0
oc
0
(9
I
yo
-CH3 \
co
I
Scheme 1.(i)AgBF,, CH&,
Me&Me; (ii) Na-Hg, Me1 ; (iii) HBF, * Et,O, CH,Cl,, - 78”C, MeCN; (iv) MeC,Me, CH,Cl,. 427
M. GREEN
428
a
a
BFi
a
BFi
BF;
(MeO)$
(i)
I
a
a
BFi
XeFf+-q _
I
(iii)
N
Bh
Et,PEt,P
Me
MO+ 6
Me
Mce
Scheme 2. (i) Reflux MeCN; (ii) P(OMe),, CH,Cl,; (iii) PEt,, CH,Cl,.
These monocarbonylbis(alkyne) complexes are reactive towards donor ligands. In refluxing acetonitrile in the presence of but-Zyne the remaining carbon monoxide is readily displaced by acetonitrile forming a member of a potentially interesting group of molecules,
(Scheme 2). Trimethylphosphite also reacts rapidly with this cation at room temperature, but this leads instead to displacement of both carbon monoxide and one of the co-ordinated alkynes : generalization of this reaction provides access to the cations CMo{P(OMe),)2(?2-RC2R)(?-C5H5 or $-C9H7)][BF4] where the alkyne is a fourelectron donor to the molybdenum centre. The corresponding bisphosphine-substituted monoalkyne cations can only be synthesized by reacting the acetonitrile complexes with a phosphine, the reaction of
with PR, affording the asymmetric cations
When this type of reaction was extended to o-diphenylphosphinostyrene an interesting cation was obtained which was thought from examination of the NMR spectra to contain a co-ordinated alkene and a four-electron alkyne3 (Scheme 3). This was confirmed by X-ray crystallography which showed that the molybdenum atom is bonded to an q-CSH5, an q2-bonded but-Zyne, and a chelating odiphenylphosphinostyrene ligand, where the C-C vectors of the co-ordinated alkyne and alkene lie
essentially parallel to the MO-P axis. This is a potentially interesting molecule since it has been suggestedG6 that oxidative coupling of coordinated alkynes and alkenes to form a metallacyclopenta-2-ene is a key step in the metalcatalysed cyclocotrimerization of alkynes and alkenes. When this cation was heated under reflux in acetonitrile solution an orange crystalline complex was formed, which was identified by X-ray crystallography as the illustrated 1,3diene complex (Scheme 3). Examination of the solution NMR spectra showed the existence of two conformational isomers differing only in the orientation of the 1,3-diene relative to the Mo(NCMe)(P)(q-CSH5) fragment. Thus, co-ordination of acetonitrile appears to promote oxidative coupling of the coordinated alkyne and alkene : however, this is then followed by an apparent 1,3-hydrogen shift affording the 1,3-diene cation (Scheme 4). It is interesting to note that some years ago Pauson and Khand’ reported that alkenes carrying strong electronwithdrawing groups (CN and CO,Et) react with the transversely bonded alkyne complex [Co2(pPhCZH)(CO)6] to give conjugated 1,3-dienes, a reaction which resembles in some respects the processes which occur at a molybdenum centre. In view of these observations we then examined the reaction of the labile bisalkyne acetonitrile molybdenum cations with cycle-octatetraene and cycloheptatriene in the belief that we might see analogous behaviour to that found with the odiphenylphosphinostyrene system. However, when the cations [Mo(NCMe)(q2-PhC,Ph),(?-C,H, or q5-C,H7)][BF,J are reacted with an excess of cyclooctatetraene in refluxing acetonitrile an organic compound is formed which was identified as a
Molybdenum-centred
carbtikarbon
bond formation reactions
429
(i)
(ii) I
Me
Scheme 3. (i) o-Diphenylphosphinostyrene,
substituted
bicyclo[4.2.2]decatetraene.8
Both the q-
cyclopentadienyl and the corresponding $-indenyl cations are catalysts for the transformation of diphenylacetylene and cycle-octatetraene into the decatetraene. The catalytic reaction could also be extended to the synthesis using phenylprop-Zyne, and, significantly, an analogous reaction occurred
-
(ii) reflux MeCN.
between
CMo(NCMe)(rl’-PhC,Me),(rl-C~H~)l CBhI and cycloheptatriene affording a bicyclo[4.2.l]nona2,4,7-triene (Scheme 5). Thus, these observations suggest that a formally forbidden 67r8+ 27r8reaction is occurring at a molybdenum centre, and it is
(i)
Scheme 4. (i) Reflux MeCN.
M. GREEN
430
R= Me.
R = R’ = Ph
R’= Ph
R=Me.R’=Ph
Scheme 5. (i) Alkyne + CBHO,reflux MeCN ; (ii) alkyne + C,Hs, reflux MeCN.
possible that this involves a stepwise rather than a concerted addition of the alkyne to co-ordinated CsH, or C,H,. Recently, it has been reported9 that norbornadiene(cycloheptatriene)ruthenium(O) reacts with ethyne in a stoichiometric reaction to afford norbornadiene(bicyclo[4.2.1]nona-2,4,7triene)ruthenium(O). In contrast, when the but-Zyne complex
CMo(NCMe)(t12-MeC2Me)2(‘1-C~H~)l CBFJ was reacted in an analogous manner with CsHs in MeCN a neutral air-stable crystalline molybdenum complex was formed. An X-ray crystal structure determination revealed that a complex C,, ligand had been formed by formal (6 + 2)x addition ofbut-Z yne to co-ordinated C,H,, but, instead of a 7,8disubstituted bicyclo[4.2.2]decatetraene being displaced, as happens with the corresponding reactions of PhCzPh and PhC,Me, hydrogen migration occurs via the metal, followed by proton loss to give a complex containing q4-1,3-diene and $-ally1 ligands (Scheme 6). This type of molecule had not been obtained before and it was obviously interesting to examine the reactivity towards protons. Addition of CF,SOJH in CH,Cl, to the $-indenyl species gave dark brown crystals of
the $-ally1 moiety into an alkene. The resultant unsaturation at the molybdenum centre is then relieved by an agostic en&-C-H interaction.” This interaction models the initial stage (Bin Scheme 8) of a metal-assisted 1,3-hydrogen shift.” In this context it is interesting to note the shortening of the allylic C-C bond [to 1.471(8) A] prior to C-H bond cleavage, and the non-planar geometry of the four-centre Ma-C,-C,H system [dihedral angle 27(3)“]. This latter feature is in marked contrast to other /I-agostic interactions, where M-C,-C,-H angles are < 17”, but rather close to that in [IrCl(PPh,),($C3H4Ph)(H)],12 a model for C in Scheme 8. It is interesting that in solution at room temperature a dynamic process occurs, which involves reversible transfer of the agostic hydrogen to an end carbon of the 1,3-diene system (see Scheme 7). When the cation [Mo(NCMe)(q2-MeC,Me)2(q-C5H5)][BF4]1 is reacted with Na or Mg amalgam, or Na[Fe(CO),(q-C,H,)] in tetrahydrofuran (THF) an apparent one-electron reduction occurs, resulting in the formation of an octamethyloctatrienediylidenedimolybdenum complex
where the Cs chain begins and ends with u-bonds to one molybdenum. 13*i4This reaction is interesting in the context of the Reppe synthesis” of [S] annulene, cycle-octatetraene, where a nickel salt catalyses the cyclotetramerization of ethyne. Remarkably, despite the advances which have been made in the last 40 years in organometallic chemistry, we still do not
CMo(C1,H,,)(r15-C9H,)lCCF,SOJI showing a high-field ‘H NMR signal (-80°C CD2C12) at -7.83 ppm. A low-temperature X-ray structural analysis showed the cation geometry illustrated in Scheme 7. Protonation has occurred at one of the terminal ally1 carbons transforming
Me -
(i)
I
Me H Go
Scheme 6. (i)+ CsHB,reflux MeCN.
Molybdenum-centred carbon-carbon bond formation reactions
431
Scheme 7. (i) CF,SO,H, - 78”C, CH,Cl, ; (ii) base; (iii) room temperature.
M
M-H
M-H
H
w I
A
C
B Scheme 8. Metal-mediated
the mechanism16 of Reppe’s reaction. Recently attention has focused on reaction pathways which involve the linking of ethyne molecules at two adjacent metal centres. 1‘*I8 Hence the interest in the reaction illustrated in Scheme 9. When a THF solution of the indenyl analogue understand
CMoOVCMe)(?2-MeC,Me)z(‘15-CsH7)1 IX41 was reacted with a suspension of Na or Mg amalgam a green crystalline air-sensitive material was formed, which was identified by NMR and X-ray crystallography as a three-alkyne “fly-over” complexlg (Scheme 10) with C,-symmetry. In contrast,
alkene isomerization.
with the molecule [Mo,{~~~‘,r13,12,13,111_CgMe8))(tlCSH5)J, the three-alkyne “fly-over” complex is reactive towards carbon monoxide, forming a kinetically controlled isomer (D) (Scheme 11) at room temperature and the thermodynamic product (E) in refluxing hexane. These isomeric dicarbonyl complexes were identified by NMR and X-ray crystallography, and it isinteresting that the reaction with carbon monoxide results in a lengthening of the Mo-MO bond from a triple [MO(~)-MO(~) 2.306(2) A] to a single [Mo( l)-MO(~) 2.933( 1) A in (D)] bond with a consequent uncoiling of the Cs “fly-over” array.
Scheme 9. (i) NaFe(C0)&C5H5)-THF
+
le
or Mg-Hg, THF.
*
Scheme 10. (i) Mg-Hg, THF, room temperature.
432
M. GREEN Me
@+Z@ Me
Me
-;;-I
Mo,%
Me
I
Me
410,
.
I\-__
Me
Me
@%:s Me
,y
Me
--;,
Mo+
‘,
(ii)
Me
‘MO
Me
?__
tie
\,
he
E
Scheme 11. (i) + CO, room temperature ; (ii) -I-CO, reflux hexane.
Scheme 12. q-&H, ligands omitted for clarity,(i) + RC2R.
From studies on the thermal reaction of alkynes with complexes of the type
the C6 “fly-over” complex was indeed a precursor of
CMo,(C1-alkyne)(CO),(tl-C,H,),I
CMo2{~-(?‘,r13,r12,13,~1-CgMe,))(?-C,H,),1,
then the n’-indenyl complex should rapidly react with but-Zyne, and it would be expected that this reaction would be facilitated from slippage (r$ to q3) of the indenyl ligand. However, the three-alkyne ‘Vlyover” proved to be unreactive towards but-Zyne. Thus suggests that, in the case of the methylCMo,{~~~‘,r13,r12,~3,~1-CgRs))(~-C~H5)21 substituted systems, three-alkyne “fly-over” com(R = H, Ph or C02Me) (Scheme 12).18 It was plexes and octatrienediylidenedimolybdenum therefore important to examine the reactivity of the species are not formed sequentially, but instead arise structurally characterized three-alkyne “fly-over” from competitive reaction pathways which are complex available to some earlier intermediates in the reaction sequence. it has been suggested that alkynes can sequentially link at two metal centres, and in particular it was proposed that three-alkyne “fly-over”complexes are the immediate precursors of the dinuclear C, species.
CMo2(~-(r11,‘t3,rt3,tl’-C6Me6))(r15-CgH,)zl
towards but-Zyne. Since the octamethyl dimolybdenum complex
Acknowledgements--I gratefully acknowledge all the intellectual and practical contributions of my co-workers whose names are given in the references. The work was CMoz(~-(r11,r13,12,13,01-C,Me,)}(rl-C~H,),1 made possible by support from the SERC and the donors is formed rapidly at room temperature then it might of the Petroleum Research Fund administered by the be expected that, if the cyclopentadienyl analogue of American Chemical Society.
Molybdenum-centred
carbon
REFERENCES
1. S. R. Allen, P. K. Baker, S. G. Barnes, M. Green, L. Trollope, L. M. Muir and K. W. Muir, J. Chem. Sot., Dalton Trans. 1981,873. 2. S. R. Allen, M. Green, A. G. Orpen and I. D. Williams, J. Chem. Sot., Chem. Commun. 1982,826.
3. S. R. Allen, M. Green, G. Moran, A. G. Orpen and G. E. Taylor, J. Chem. Sot., Dalton Trans. 1984,441. 4. Y. Wakatsuki, K. Aoki and H. Yamazaki, J. Am. Chem. Sot. 1979,101,1123.
5. P. Caddy, M. Green, E. O’Brien, L. E. Smart and P. Woodward, J. Chem. Sot., Dalton Trans. 1980,962. 6. L. D. Brown, K. Itoh, H. Susuki, K. Hirai and J. A. Ibers, J. Am. Chem. Sot. 1978,100,8232. 7. P. L. Pauson and I. U. Khand, Ann. N.Y Ad. Sci. 1977,295, 1. 8. D. G. Bourner, L. Brammer, M. Green, G. Moran, A. G. Orpen, C. Reeve and C. J. Schaverien, J. Chem. Sot., Chem. Commun. 1985,1409. 9. H. Nagashima, H. Matsuda and K. Itoh, J. Organomet. Chem. 1983,258, C15. 10. M. Brookhart and M. L. H. Green, J. Organomet. Chem. 1983,250,395. 11. G. F. Emerson and R. Pettit, J. Am. Chem. Sot. 1962,
bond formation reactions
433
84,459l; C. P. Casey and C. R. Cyr, J. Am. Chem. Sot. 1973,95,2248 ; G. M. Whitesides and J. P. Neilen, J. Am. Chem. Sot. 1976,98,63; J. W. Byrne, H. U. Blaser and J. A. Osborn, J. Am. Chem. Sot. 1975,97,3871; M. Green and R. P. Hughes, J. Chem. Sot., Dalton Trans. 1976,1907 ; D. B. Jacobson and B. J. Freiser, J. Am. Chem. Sot. 1985,107,72. 12. T. H.Tulip and J. A. Ibers, J. Am. Chem. Sot. 1979,101,
4201. 13. M. Green, N. C. Norman and A. G. Orpen, J. Am.
Chem. Sot. 1981,103,1269. 14. M. Green, N. C. Norman, A. G. Orpen and C. J. Schaverien, J. Chem. Sot., Dalton Trans. 1984,2455. 15. W. Reppe, 0. Schlichting, K. Klager and T. Toepel, Liebigs Ann. Chem. 1948,560, 1. 16. R. E. Colborn and K. P. C. Vollhardt, J. Am. Chem. Sot. 1981,103,6259 (and references therein). 17. G. Wilke, Pure Appl. Chem. 1978,50,677. 18. S. A. R. Knox,R. F. D. Stansfield,F. G. A. Stone, M. J. Winter and P. Woodward, J. Chem. Sot., Dalton Trans. 1982, 173.
19. L. Brammer, M. Green, A. G. Orpen, K. E. Paddick and D. R. Saunders, J. Chem. Sot., Dalton Trans. (in press).
1986 0
MOLYBDENUM-CENTRED FORMATION
CARBON-CARBON REACTIONS
MICHAEL
0277-5387/X6 $3.00 + .OO 1986 Pergamon Pm% Ltd
BOND
GREEN
Department of Inorganic Chemistry, School of Chemistry, University of Bristol, Bristol BS8 lTS, U.K. (Received 19 June 1985) Abstract-The chemistry of the bisalkyne molybdenum complexes [Mo(L)(q2-R&R)&C,H, or q5-C,H,)][BF,] (L = CO or MeCN) is described. The reaction of the carbonyl cation with o-diphenylphosphinostyrene affords an alkyne-alkene complex which is transformed into a 1,3diene complex in refluxing MeCN via C-C bond formation and a 43-H shift, whereas the acetonitrile species with cycle-octatetraene and cycle-heptatriene undergo (6 + 2)~ addition reactions with the co-ordinated alkyne. Addition of one electron to the acetonitrile cation leads to novel alkyne-linking reactions at two molybdenum centres.
Recently l we have been exploring the chemistry of an interesting group of cationic alkyne molybdenum complexes in which the alkyne functions as either a three- or a four-electron donor. The starting point for our investigation was the molecule
[Mo~(CO)&-C~H~)~] with AgBF, in the presence of but-Zyne. An alternative synthesis which can be extended to alkynes carrying reactive functional groups such as SiMe,,2 makes use of the lability of the cation cis-[Mo(NCMe),(CO),(q-C,HS)I[BF,], and has the advantage that it avoids the use of CMo(CO)(?2-MeC2Me)2(tl-C5H5)1CBF,I expensive silver salts. The methodology can be (Scheme l), which was readily available from the readily extended to allow easy access to the more redox cleavage of the MO-MO bond present in reactive corresponding q5-indenyl complexes.
67\
““-J
Lo
Mci,-co
-
a
(ii)
-
oc:’
0
oc
0
(9
I
yo
-CH3 \
co
I
Scheme 1.(i)AgBF,, CH&,
Me&Me; (ii) Na-Hg, Me1 ; (iii) HBF, * Et,O, CH,Cl,, - 78”C, MeCN; (iv) MeC,Me, CH,Cl,. 427
M. GREEN
428
a
a
BFi
a
BFi
BF;
(MeO)$
(i)
I
a
a
BFi
XeFf+-q _
I
(iii)
N
Bh
Et,PEt,P
Me
MO+ 6
Me
Mce
Scheme 2. (i) Reflux MeCN; (ii) P(OMe),, CH,Cl,; (iii) PEt,, CH,Cl,.
These monocarbonylbis(alkyne) complexes are reactive towards donor ligands. In refluxing acetonitrile in the presence of but-Zyne the remaining carbon monoxide is readily displaced by acetonitrile forming a member of a potentially interesting group of molecules,
(Scheme 2). Trimethylphosphite also reacts rapidly with this cation at room temperature, but this leads instead to displacement of both carbon monoxide and one of the co-ordinated alkynes : generalization of this reaction provides access to the cations CMo{P(OMe),)2(?2-RC2R)(?-C5H5 or $-C9H7)][BF4] where the alkyne is a fourelectron donor to the molybdenum centre. The corresponding bisphosphine-substituted monoalkyne cations can only be synthesized by reacting the acetonitrile complexes with a phosphine, the reaction of
with PR, affording the asymmetric cations
When this type of reaction was extended to o-diphenylphosphinostyrene an interesting cation was obtained which was thought from examination of the NMR spectra to contain a co-ordinated alkene and a four-electron alkyne3 (Scheme 3). This was confirmed by X-ray crystallography which showed that the molybdenum atom is bonded to an q-CSH5, an q2-bonded but-Zyne, and a chelating odiphenylphosphinostyrene ligand, where the C-C vectors of the co-ordinated alkyne and alkene lie
essentially parallel to the MO-P axis. This is a potentially interesting molecule since it has been suggestedG6 that oxidative coupling of coordinated alkynes and alkenes to form a metallacyclopenta-2-ene is a key step in the metalcatalysed cyclocotrimerization of alkynes and alkenes. When this cation was heated under reflux in acetonitrile solution an orange crystalline complex was formed, which was identified by X-ray crystallography as the illustrated 1,3diene complex (Scheme 3). Examination of the solution NMR spectra showed the existence of two conformational isomers differing only in the orientation of the 1,3-diene relative to the Mo(NCMe)(P)(q-CSH5) fragment. Thus, co-ordination of acetonitrile appears to promote oxidative coupling of the coordinated alkyne and alkene : however, this is then followed by an apparent 1,3-hydrogen shift affording the 1,3-diene cation (Scheme 4). It is interesting to note that some years ago Pauson and Khand’ reported that alkenes carrying strong electronwithdrawing groups (CN and CO,Et) react with the transversely bonded alkyne complex [Co2(pPhCZH)(CO)6] to give conjugated 1,3-dienes, a reaction which resembles in some respects the processes which occur at a molybdenum centre. In view of these observations we then examined the reaction of the labile bisalkyne acetonitrile molybdenum cations with cycle-octatetraene and cycloheptatriene in the belief that we might see analogous behaviour to that found with the odiphenylphosphinostyrene system. However, when the cations [Mo(NCMe)(q2-PhC,Ph),(?-C,H, or q5-C,H7)][BF,J are reacted with an excess of cyclooctatetraene in refluxing acetonitrile an organic compound is formed which was identified as a
Molybdenum-centred
carbtikarbon
bond formation reactions
429
(i)
(ii) I
Me
Scheme 3. (i) o-Diphenylphosphinostyrene,
substituted
bicyclo[4.2.2]decatetraene.8
Both the q-
cyclopentadienyl and the corresponding $-indenyl cations are catalysts for the transformation of diphenylacetylene and cycle-octatetraene into the decatetraene. The catalytic reaction could also be extended to the synthesis using phenylprop-Zyne, and, significantly, an analogous reaction occurred
-
(ii) reflux MeCN.
between
CMo(NCMe)(rl’-PhC,Me),(rl-C~H~)l CBhI and cycloheptatriene affording a bicyclo[4.2.l]nona2,4,7-triene (Scheme 5). Thus, these observations suggest that a formally forbidden 67r8+ 27r8reaction is occurring at a molybdenum centre, and it is
(i)
Scheme 4. (i) Reflux MeCN.
M. GREEN
430
R= Me.
R = R’ = Ph
R’= Ph
R=Me.R’=Ph
Scheme 5. (i) Alkyne + CBHO,reflux MeCN ; (ii) alkyne + C,Hs, reflux MeCN.
possible that this involves a stepwise rather than a concerted addition of the alkyne to co-ordinated CsH, or C,H,. Recently, it has been reported9 that norbornadiene(cycloheptatriene)ruthenium(O) reacts with ethyne in a stoichiometric reaction to afford norbornadiene(bicyclo[4.2.1]nona-2,4,7triene)ruthenium(O). In contrast, when the but-Zyne complex
CMo(NCMe)(t12-MeC2Me)2(‘1-C~H~)l CBFJ was reacted in an analogous manner with CsHs in MeCN a neutral air-stable crystalline molybdenum complex was formed. An X-ray crystal structure determination revealed that a complex C,, ligand had been formed by formal (6 + 2)x addition ofbut-Z yne to co-ordinated C,H,, but, instead of a 7,8disubstituted bicyclo[4.2.2]decatetraene being displaced, as happens with the corresponding reactions of PhCzPh and PhC,Me, hydrogen migration occurs via the metal, followed by proton loss to give a complex containing q4-1,3-diene and $-ally1 ligands (Scheme 6). This type of molecule had not been obtained before and it was obviously interesting to examine the reactivity towards protons. Addition of CF,SOJH in CH,Cl, to the $-indenyl species gave dark brown crystals of
the $-ally1 moiety into an alkene. The resultant unsaturation at the molybdenum centre is then relieved by an agostic en&-C-H interaction.” This interaction models the initial stage (Bin Scheme 8) of a metal-assisted 1,3-hydrogen shift.” In this context it is interesting to note the shortening of the allylic C-C bond [to 1.471(8) A] prior to C-H bond cleavage, and the non-planar geometry of the four-centre Ma-C,-C,H system [dihedral angle 27(3)“]. This latter feature is in marked contrast to other /I-agostic interactions, where M-C,-C,-H angles are < 17”, but rather close to that in [IrCl(PPh,),($C3H4Ph)(H)],12 a model for C in Scheme 8. It is interesting that in solution at room temperature a dynamic process occurs, which involves reversible transfer of the agostic hydrogen to an end carbon of the 1,3-diene system (see Scheme 7). When the cation [Mo(NCMe)(q2-MeC,Me)2(q-C5H5)][BF4]1 is reacted with Na or Mg amalgam, or Na[Fe(CO),(q-C,H,)] in tetrahydrofuran (THF) an apparent one-electron reduction occurs, resulting in the formation of an octamethyloctatrienediylidenedimolybdenum complex
where the Cs chain begins and ends with u-bonds to one molybdenum. 13*i4This reaction is interesting in the context of the Reppe synthesis” of [S] annulene, cycle-octatetraene, where a nickel salt catalyses the cyclotetramerization of ethyne. Remarkably, despite the advances which have been made in the last 40 years in organometallic chemistry, we still do not
CMo(C1,H,,)(r15-C9H,)lCCF,SOJI showing a high-field ‘H NMR signal (-80°C CD2C12) at -7.83 ppm. A low-temperature X-ray structural analysis showed the cation geometry illustrated in Scheme 7. Protonation has occurred at one of the terminal ally1 carbons transforming
Me -
(i)
I
Me H Go
Scheme 6. (i)+ CsHB,reflux MeCN.
Molybdenum-centred carbon-carbon bond formation reactions
431
Scheme 7. (i) CF,SO,H, - 78”C, CH,Cl, ; (ii) base; (iii) room temperature.
M
M-H
M-H
H
w I
A
C
B Scheme 8. Metal-mediated
the mechanism16 of Reppe’s reaction. Recently attention has focused on reaction pathways which involve the linking of ethyne molecules at two adjacent metal centres. 1‘*I8 Hence the interest in the reaction illustrated in Scheme 9. When a THF solution of the indenyl analogue understand
CMoOVCMe)(?2-MeC,Me)z(‘15-CsH7)1 IX41 was reacted with a suspension of Na or Mg amalgam a green crystalline air-sensitive material was formed, which was identified by NMR and X-ray crystallography as a three-alkyne “fly-over” complexlg (Scheme 10) with C,-symmetry. In contrast,
alkene isomerization.
with the molecule [Mo,{~~~‘,r13,12,13,111_CgMe8))(tlCSH5)J, the three-alkyne “fly-over” complex is reactive towards carbon monoxide, forming a kinetically controlled isomer (D) (Scheme 11) at room temperature and the thermodynamic product (E) in refluxing hexane. These isomeric dicarbonyl complexes were identified by NMR and X-ray crystallography, and it isinteresting that the reaction with carbon monoxide results in a lengthening of the Mo-MO bond from a triple [MO(~)-MO(~) 2.306(2) A] to a single [Mo( l)-MO(~) 2.933( 1) A in (D)] bond with a consequent uncoiling of the Cs “fly-over” array.
Scheme 9. (i) NaFe(C0)&C5H5)-THF
+
le
or Mg-Hg, THF.
*
Scheme 10. (i) Mg-Hg, THF, room temperature.
432
M. GREEN Me
@+Z@ Me
Me
-;;-I
Mo,%
Me
I
Me
410,
.
I\-__
Me
Me
@%:s Me
,y
Me
--;,
Mo+
‘,
(ii)
Me
‘MO
Me
?__
tie
\,
he
E
Scheme 11. (i) + CO, room temperature ; (ii) -I-CO, reflux hexane.
Scheme 12. q-&H, ligands omitted for clarity,(i) + RC2R.
From studies on the thermal reaction of alkynes with complexes of the type
the C6 “fly-over” complex was indeed a precursor of
CMo,(C1-alkyne)(CO),(tl-C,H,),I
CMo2{~-(?‘,r13,r12,13,~1-CgMe,))(?-C,H,),1,
then the n’-indenyl complex should rapidly react with but-Zyne, and it would be expected that this reaction would be facilitated from slippage (r$ to q3) of the indenyl ligand. However, the three-alkyne ‘Vlyover” proved to be unreactive towards but-Zyne. Thus suggests that, in the case of the methylCMo,{~~~‘,r13,r12,~3,~1-CgRs))(~-C~H5)21 substituted systems, three-alkyne “fly-over” com(R = H, Ph or C02Me) (Scheme 12).18 It was plexes and octatrienediylidenedimolybdenum therefore important to examine the reactivity of the species are not formed sequentially, but instead arise structurally characterized three-alkyne “fly-over” from competitive reaction pathways which are complex available to some earlier intermediates in the reaction sequence. it has been suggested that alkynes can sequentially link at two metal centres, and in particular it was proposed that three-alkyne “fly-over”complexes are the immediate precursors of the dinuclear C, species.
CMo2(~-(r11,‘t3,rt3,tl’-C6Me6))(r15-CgH,)zl
towards but-Zyne. Since the octamethyl dimolybdenum complex
Acknowledgements--I gratefully acknowledge all the intellectual and practical contributions of my co-workers whose names are given in the references. The work was CMoz(~-(r11,r13,12,13,01-C,Me,)}(rl-C~H,),1 made possible by support from the SERC and the donors is formed rapidly at room temperature then it might of the Petroleum Research Fund administered by the be expected that, if the cyclopentadienyl analogue of American Chemical Society.
Molybdenum-centred
carbon
REFERENCES
1. S. R. Allen, P. K. Baker, S. G. Barnes, M. Green, L. Trollope, L. M. Muir and K. W. Muir, J. Chem. Sot., Dalton Trans. 1981,873. 2. S. R. Allen, M. Green, A. G. Orpen and I. D. Williams, J. Chem. Sot., Chem. Commun. 1982,826.
3. S. R. Allen, M. Green, G. Moran, A. G. Orpen and G. E. Taylor, J. Chem. Sot., Dalton Trans. 1984,441. 4. Y. Wakatsuki, K. Aoki and H. Yamazaki, J. Am. Chem. Sot. 1979,101,1123.
5. P. Caddy, M. Green, E. O’Brien, L. E. Smart and P. Woodward, J. Chem. Sot., Dalton Trans. 1980,962. 6. L. D. Brown, K. Itoh, H. Susuki, K. Hirai and J. A. Ibers, J. Am. Chem. Sot. 1978,100,8232. 7. P. L. Pauson and I. U. Khand, Ann. N.Y Ad. Sci. 1977,295, 1. 8. D. G. Bourner, L. Brammer, M. Green, G. Moran, A. G. Orpen, C. Reeve and C. J. Schaverien, J. Chem. Sot., Chem. Commun. 1985,1409. 9. H. Nagashima, H. Matsuda and K. Itoh, J. Organomet. Chem. 1983,258, C15. 10. M. Brookhart and M. L. H. Green, J. Organomet. Chem. 1983,250,395. 11. G. F. Emerson and R. Pettit, J. Am. Chem. Sot. 1962,
bond formation reactions
433
84,459l; C. P. Casey and C. R. Cyr, J. Am. Chem. Sot. 1973,95,2248 ; G. M. Whitesides and J. P. Neilen, J. Am. Chem. Sot. 1976,98,63; J. W. Byrne, H. U. Blaser and J. A. Osborn, J. Am. Chem. Sot. 1975,97,3871; M. Green and R. P. Hughes, J. Chem. Sot., Dalton Trans. 1976,1907 ; D. B. Jacobson and B. J. Freiser, J. Am. Chem. Sot. 1985,107,72. 12. T. H.Tulip and J. A. Ibers, J. Am. Chem. Sot. 1979,101,
4201. 13. M. Green, N. C. Norman and A. G. Orpen, J. Am.
Chem. Sot. 1981,103,1269. 14. M. Green, N. C. Norman, A. G. Orpen and C. J. Schaverien, J. Chem. Sot., Dalton Trans. 1984,2455. 15. W. Reppe, 0. Schlichting, K. Klager and T. Toepel, Liebigs Ann. Chem. 1948,560, 1. 16. R. E. Colborn and K. P. C. Vollhardt, J. Am. Chem. Sot. 1981,103,6259 (and references therein). 17. G. Wilke, Pure Appl. Chem. 1978,50,677. 18. S. A. R. Knox,R. F. D. Stansfield,F. G. A. Stone, M. J. Winter and P. Woodward, J. Chem. Sot., Dalton Trans. 1982, 173.
19. L. Brammer, M. Green, A. G. Orpen, K. E. Paddick and D. R. Saunders, J. Chem. Sot., Dalton Trans. (in press).