Chapter 2
2.1 Introduction
Mononuclear transition metal carbonyl complexes react with secondary phosphines at moderate temperatures to afford simple substitution products where the carbonyls are replaced by phosphine ligands/ At higher temperatures phosphorus-hydrogen bond cleavage in the phosphine, and dimérisation of the mononuclear species can occur to afford binuclear complexes bridged by phosphido ligands. This latter reaction can also be effected using ultra-violet irradiation.
-C O - C O . - H ,
+ --- ► M(C0)4(PR2H) ► (C0)3M---M(C0)3
o L j o d i m é r i s a t i on
r nr<2
Bimetallic transition metal carbonyl complexes react with secondary phosphines to afford bridging-hydrido and phosphido species in which oxidative-addition of the phosphorus-hydrogen bond across the two metals has occurred. For example, the dimolybdenum species [Mo2Cp2(C O )j reacts with diphenylphosphine in refluxing toluene to afford the [Mo2Cp2(CO )4(p-H)(p- PPhg)],^ a complex previously synthesised by Treichel from the reaction of [MoCpBr(CO)2(PPh2H)] with LPBu.^
Similar hydride species are also formed from the oxidative-addition of phosphines PPh2H and PMen2H (Men = menthyl) to [Mn2(CO)io]'^ and [Fe2Cp2(CO)J^ respectively. The latter reaction is thought to proceed via initial substitution of a carbonyl for the phosphine to afford the intermediate, [Fe2Cp2(CO)3(PMen2H)].
OC Cp o c \ p
- CO
C P ,
V è — F e (
The synthesis of a number of complexes of the general formula [Fe2(CO)6()i-CO)(|i-diphosphine)] have previously been reported,® the first example being, [Fe2(CO)6(fi-CO)(|i-dppm)] (Figure 2.1), reported by Cotton and Troup.^ Simple addition of the diphosphine to a THF solution of di-iron nonacarbonyl afforded the disubstituted complex in 56% yield.
C ( 7 ) 0 ( 7 ) 0 (6) F e ( 2 ) C ( 2 ) 0(2) 0 ( 4 ) I 0(1) 0(1) 0 ( 5 ) 0 (3 ) 0 ( 5 ) 0 ( 3 )
Figure 2.1 X-ray structure of [Fe2(C0)g(|i-C0)(|i-dppm)]
It has recently been found that addition of an excess (-1cm®) of iron pentacarbonyl increases the yield considerably. A number of minor by products are obtained upon performing column chromatography and characterisation of these and comparison with other observations have led to the following suggested reaction scheme (overleaf).^
Chapter 2
M
(CO)3Fe—-^ F e(C O) 3 C o THF (CO)Æe Fe(CO)4 T t PhzP^ ,PPhg - CO Fe(CO)5 + Fe(C0)4.THF dppm (C0 )4Fe (CO)3Fe. T ^ PhzP. .PPhz PhzP. /P P h z (CO)3Fe PhzP^ .PPhz
[Fe2(CO)6(|x-CO)(p-dppm)] has previously been found to be relatively unreactive in the absence of uv radiation, however, upon photolysis carbonyl loss occurs readily and the complex reacts with a variety of organic molecules including alkynes® and phosphines.®’"'®
2.2 Synthesis of Hydride Complexes [Fe2(CO)4(p-CO)(p-H)(p-PR2)(M- dppm)] 1
Ultraviolet irradiation of a toluene solution of [Fe2(CO)6(|i-CO)(|i-dppm)] and a small excess of diphenylphosphine was found to generate a new dimetallic complex containing bridging-hydride and phosphido ligands, namely [Fe2{CO)4()x-CO)(|i-H)(|i-PPh2)(|i-dppm)], 1.Ph, as a brown, air-sensitive solid in 77% yield.
Formation of the product was observed by systematically measuring the solution ir spectrum at hourly intervals. This was possible due to the fact that the spectra of the two species concerned were considerably different. For
[Fe2(CO)6(M.-CO)(|i-dppm)], each of the seven carbonyl signals can clearly be distinguished, the most characteristic being the bridging carbonyl at 1755cm '\ Although 1 .Ph also contains a bridging carbonyl, this appeared to considerably lower wavenumbers at 1711cm '\ and hence the relative amounts of each species could be monitored. The terminal-carbonyl absorptions for the new complex occur at 1973(s), 1938(s) and 1890(m) cm '\
The related dicyclohexylphosphido complex, [Fe2(CO)4(|i-CO)(p.-H)()i- PCy2)(p.-dppm)], 1.Cy, was synthesised in 60% yield in an analogous fashion using dicyclohexylphosphine. The product is an orange solid which is moderately stable in air even in solution. The ir spectrum of the new hydride complex was comparable to its diphenylphosphido derivative, with a similar pattern of bridging- and terminal-carbonyl absorptions. The latter are shifted to higher wavenumbers than the same absorptions for 1.Ph, occurring at 1993(m), 1970(s) and 1931(s) c m '\ Experience suggests that the |i-PCy2 ligand is a stronger electron donor than |i-PPh2 which would lead to more electron density on the metals. This in turn would give rise to more back bonding between the metal and any carbonyl carbon atoms and thus induce a weaker C-O interaction. This clearly cannot be the case here as this would mean that the terminal carbonyl absorptions for 1 .Cy would be shifted to lower wavenumber relative to those for 1.Ph.
y 95 :C 2- ^ : 0 22 scans. 4cm- I, Gain = l
20 oc 1550 1900 1850 1800 1750 1700
wavenumser (cm- l )
Figure 2.2 Infra-red spectrum of [Fe2(CO)^(p-CO)()i-H)(u-PCy2)((i-dppm)] in DCM
Chapter 2
2.3 Characterisation of [F0 2(CO)4(n-C O )(|j-H )(|j-PR2)(|J-dppm)] 1
The two complexes were confirmed as hydrides by nmr spectroscopy, each showing a hydride signal which characteristically appears at high f i e l d . B o t h appeared as doublets of triplets a t -89.50 (J 51.0, 26.5Hz) for 1.Ph and-810.14(J 44.8, 24.9Hz) for 1.Cy (Figure 2.3). Hydride signals are believed to appear at such high field due to a paramagnetic deshielding effect, a consequence of the closeness of the hydride ligands to the electrons on the metals which have a high paramagnetic shift contribution. As previously mentioned (Chapter 1 ), coupling constants between nuclei can give valuable information about the stereochemistry of the complexes under observation. The larger phosphorus-hydrogen coupling constant in each of the ^H nmr spectra (51.0Hz, 1.Ph; 44.8Hz, I.C y ) occur as doublets corresponding with the coupling to the phosphido-bridge, suggesting that in both complexes the phosphido ligand lies trans to the hydride ligand. The diphosphine gives rise to a triplet in both spectra with relatively small coupling constants, suggesting a c/s-configuration of hydride and diphosphine.