configurations of phosphorus-containing ligands. This is borne out in the following chapters where their reactivity differs in the products formed. These types of complex are known to be highly reactive towards many organic molecules and the next three chapters deal with this reactivity and the formation of new bridging ligands.
2.6 References
1. A. J. M. Caffyn, M. J. Mays and P. R. Raithby, 7. Chem. Soc., D a lto n T rans., 1991, 2349.
2. K. Henrick, M . McPartlin, A. D. Horton and M . J. Mays, J. Chem. Soc., D a lto n T rans., 1988, 1083.
3. P. M . Treichel, W. K. Dean and W. M . Douglas, J. O rg a n o m e t. C hem ., 1972, 42, 145.
4. J. A. Iggo, M . J. Mays and P. R. Raithby, J. Chem. Soc., D a lto n T ra n s., 1983, 205.
5. H. Brunner and M. Rotzer, J. O rg a n o m e t. C hem ., 1992, 425, 119. 6. G. Hogarth, University o f Bristol, PhD Thesis, 1986.
7. F. A. Cotton and J. M. Troup, J. A m . Chem. Soc., 1974, 96, 4422.
8. G. Hogarth, F. Kayser, S. A. R. Knox, D. A. V. Morton, A. G. Orpen and M. L. Turner, J. Chem. Soc., Chem. C o m m u n ., 1988, 358.
9. N. M. Doherty, G. Hogarth, S. A. R. Knox, K. A. MacPherson, F. Melchior and A. G. Orpen, J. Chem. Soc., Chem. C o m m u n ., 1986, 540.
10. G. Hogarth, J. O rg a n o m e t. C hem ., 1991, 407, 91.
11. R. K. Harris "Nuclear Magnetic Resonance Spectroscopy" 1986, Longman Group UK.
12. P. E. Garrou, Chem. Rev., 1985, 85, 171. 13. P. E. Garrou, Chem. Rev., 1981, 81, 229.
14. E. Skordalakes and G. Hogarth, Unpublished Results. 15. A. J. Carty, A d v Chem Ser., 1982, 163.
16. J. L. Petersen, L. F. Dahl and J. M. W illiam s, J. Am . Chem. Soc., 1974, 96, 6610. 17. J. P. Olsen, T. F. Koetzle, S. W. Kirtley, M . Andrews, D. L. Tipton and R. Bau, J. Am . Chem. Soc., 1974, 96, 6621.
18. R. A. Lore, H. B. Chin, T. F. Koetzle, S. W. Kirtley, B. R. Whittlesey and R. Bau, J. A m . Chem. Soc. ,1976, 98, 4491.
Chapter 2
20. H. B. Abrahamson, M. C. Palazzotto, C. L. Reichel and M. S. Wrighton, J. A m . Chem. Soc., 1979, 101, 4123.
Chapter 3
3.1 Introduction
Aikynes have been shown to give rise to an incredibly rich and diverse chemistry. Reactions with transition metal complexes in particular are known to originate a huge variety of coordinated ligands, such as 7 1-alkynes, vinylidenes, alkynyls, vinyls, carbenes, carbynes, carbides and metalacycles for example. Transformation from one type of ligand to another is also common allowing the synthetic chemist the freedom to change ligands to suit his requirements.
The simplest alkyne complex consists of a transition metal (with ancillary ligands) and an alkyne molecule which occupies a position similar to that of ethylene in Zeise’s salt.^ These are known as
v^-
or Ti-alkynes, as the unsaturated organic molecule is bound to the metal via Tc-donation from the alkyne. Complexes of this type may be synthesised by the direct reaction of aikynes with transition metal complexes.^H ^ n *
U n M - i e g. Ru cM e
H
Generally, it is this type of product which is formed first in reactions with aikynes with transition metal complexes. These can be precursors to other types of product and, in the case of primary aikynes are often converted into hydride-acetylide or vinylidene derivatives via hydrogen migration reactions.
Rhodium (I) complexes of the type [(NP3)Rh]^ and [(PP3)Rh]^ {P3 = (CH2CH2PPh2)3}, react with primary aikynes at room temperature to afford
cis
hydride-alkynyl complexes of rhodium (III) isolated as BPh^" salts, [(N P3)R h(H )(C=C H)][B PhJ and [(PP3)Rh(H)(C =C H )][BPhJ. At low temperatures (-
40
°C) for most primary aikynes, the same products are obtained, however with propiolic acid (HCSCCO2H), the initial product formed with [(NP3)Rh]" is one of 71-coordination with no hydride signal in the nmrspectrum (see scheme below.) The ir spectrum showed a band at
1800
cm'^ assigned to a E-bonded acetylene group.^ Addition of ethanol formed two products namely the hydrido-acetylide complex (formed via C-H bond activation) and a hydrido-carboxylate species formed by0
-H oxidative- addition. H C CI
COgH P ;Rh: I p H COgH Rh' \ H H Rh: \ \ C— C = C HThere is considerable debate as to whether hydride-alkynyl complexes are precursors in the formation of vinylidene com plexes/ or whether they are formed via a direct
1
,2
-hydrogen shift from the7
c-alkyne derivatives.^®LpMX + H C = C R + LnM’ H C C R + L n M = C = C . R vinylidene
Experimental evidence for the latter was recently reported by Akita et al.^ who described the structure and fluxional behaviour of the di-iron p-alkynyl cations [Fe2(CO)4Cp*2()i-f|\-fi^-C=CR)][BFJ (R = H, Ph). This was the first direct experimental evidence for the intramolecular
1
,2
-H shift rearrangement within a metal coordination sphere.®’®''®Chapter 3 R
I
M ^ f M = C = C C ______ d ir e c t______________ H slow V. fastThe diagram above shows the difficulty in distinguishing the pathway between 7i-aikynes and vinylidenes. Since the conversion of hydride-alkynyl to vinylidene is a very fast process, detection of the former is very difficult and hence the mechanism may appear to proceed via the direct route when in fact an in term ediate is form ed. Recently, the alkyne com plex [RuCp(PM e3)2(HC=CM e)][PF6f was synthesised by treatment of [RuCp(PMe3)2CI] with propyne, in the presence of NH^PFg. It readily converts to the vinylidene complex [RuCp(PMe3)2(=C=CHMe)][PFg] in acetonitrile with no intermediate products detected.
Reaction of the Tc-alkyne complex [RhCI(HC=CH)(P'Pr3)2] with NaCp afforded the vinylidene species [RhCp(=C=CH2)(P'Pr3)].^^ The phenylethyne complex [RhCI(PhC=CH)(P'Pr3)2] afforded
7
t-alkyne, hydride-alkynyl and vinylidene complexes with no apparent interconversion between the products. It was also found that warming hexane solutions of the complexes [RhCI(RC=CH)(P'Pr3^ [R = H, Me, Ph, COgMe] afforded the vinylidene species [Rh CI(=C=CHR)(P'Pr3)2]. This seems to be further evidence for suggesting that hydride-alkynyl species are intermediates for vinylidene f o r m a t i o n . T h i s behaviour has previously been noted for related iridium complexes.The reactivity of hydride complexes can be dramatically affected by the other ligands attached to the metal(s)."'® For instance,
2
-butyne readily inserted into the hydride [RuHI(CO)(P'Bu2Me)2] but the reaction does not proceed when the halide ligand present is chloride. Phenylethyne does insert into the metal- hydride bond of the chloride complex to afford a a-vinyl complex, however, the alkyne reacts further with the iodide species to eliminate styrene. Reactionwith the fluoride, hydride species on the other hand eliminates hydrogen fluoride to afford a a-alkyne, hydride complex.
In some cases more than one primary alkyne can be incorporated^^ for example, the reaction of [Cr(CO)5(OEt2)] with an excess of methyl acetylenecarboxylate did not afford a simple vinylidene complex,- rather two a l k y n e m o l e c u l e s w e r e c o m b i n e d to f or m t h e s p e c i e s [Cr(C0)5{=C=C(C02M e)CH=CH(C02M e)}]. H (I COgMe (OC)5Cr(OEt2) + III --- ► ( O C ) g C r = C = C ^ H CI \ = = C ^ C02Me H C02Me
Vinylidenes are an important class of molecules as they are known intermediates in such organic reactions as a-elimination from vinyl halides. Unfortunately they are difficult to study as isolated molecules, the simplest, vinylidene itself, has not been observed experimentally (it has a lifetime of approximately
10
'^^ seconds) as it undergoes fast1
,2
-hydrogen shift to give acetylene. To reverse the transformation^® requires temperatures in excess of500
°C.Hx
c = c : ► H— C = C — H
h'^
However, as already shown, it has proved possible to stabilise them as ligands in transition metal complexes, the first reported example of which was from [Fe
2
(C0
)g] and diphenylketene.^® The product contained a bridging diphenylvinylidene ligand. Such complexes have been found to be very important in the metabolism of some chlorinated hydrocarbons (DDT) using iron porphyrin-based enzymes.^®The variety of different types of coordination of primary aikynes to metal centres is much greater for binuclear complexes in which the coordinated
Chapter 3
organic can act as a one, two, three or four-electron donor;
H R
g
V
H R
g
I
. H C = C RI
II
T
/ \ I \ / \ M— M IVI— M M— M M---- M M— MY
Y
(A)
(B)
(C)
<D)
(E>
1 e-1
2e-'i ---
►
M M--- M--- M M--- M M---M X (F) (G) (H ) (I)3
e-i --- ►4
e-i --- ►Coordination types A, E, F and G can all in principle be obtained from oxidative addition of primary aikynes across the metal-metal
(the E type structure is most commonly found in the complexes of copper and main group elements).^® Isolation of complexes with coordination type A is difficult in most cases because of strong driving forces for formation of the other three bonding types, as well as further reaction with additional alkyne.^® One such complex, however, was isolated by Brown et.
a\}^
namely [Re2(CO)7(p,-H)(fi^-G=CPh)(|i-dmpm)].To obtain coordination of type F in the product requires a specific type of dinuclear transition metal complex, namely one containing a hydride ligand. The mechanism involves insertion of the unsaturated organic molecule into the metal-hydrogen bond and thus the hydride hydrogen atom becomes attached to a carbon of the alkyne. This is not the only product obtained from such reactions and often binding of the alkyne to the metal centres is dependent on
the particular alkyne used. For instance, in reactions of the dihydride complex [fV1n2(|i-H)2(C0)g(p-dppm)] with primary aikynes, acetylene or phenylacetylene lead to vinyl complexes [Mn2(|i-H)(|i2,f|\T^^-RC=CH2)(CO)6(|i-dppm)](R=H,Ph), whereas tert-butylacetylene affords vinylidene and alkynyl complexes [Mn2
(ti2,f|\f|^-C=CH'Bu)(C0)g(|i-dppm)] and [Mn2(|i-H)(}i2,fi\fi^-C2Bu)(CO)6(|i- dppm)] resulting from loss of hydrogen.^® Interconversion is possible between the latter two. Thus taking a sample of the hydride-alkynyl and treating with Li[AIHJ in TH F followed by HBF^ afforded the vinylidene complex. The vinylidene group is linearly bonded to the Mn atom (Mn-C-C angle is -
179
°) with a bond order of2
[Mn-C =1
.77
(2
)Â].H