PROGRAMA DE ESCUELAS TALLER DE LA COOPERACIÓN ESPAÑOLA

Surprisingly, there are few reports of bimetallic organometallic complexes containing bridging-halide ligands (in particular, those with a single halide bridge.) The related hexacarbonyl species [Fe2(CO)g(p-CI)(|i-PPh2)] has been synthesised along with the bromo- and iodo-bridged complexes.®^ Bubbling dry HOI or HBr gas through a benzene solution of [Fe2(C0)g(p- PPh2){ii-CHC(Ph)NHEt}] afforded the chloro- and bromo-bridged complexes respectively; the iodo complex was formed from the reaction of the zwitterionic complex with aqueous HI.

H

E t— N+ Ph

_ i ^ H

/ \ HX / \

(CO)3Fe— ^ F e ( C 0 ) 3 ►--- (C0)3Fe--- — Fe(C0)3

Ph2 Phz

X = I, Br, Cl

The X-ray structures of the two bridging-chloride complexes, [lrCl2Cp*2(|i-H)(|i-CI)]and [lrCl2Cp*2(|i-CI)2] were described by Churchill and Julis.®® In both complexes, the iridium-chloride(bridging) bond lengths, [2.451 (4), 2.456(3) and 2.449(3)Â respectively] are significantly longer than the iridium-chloride(terminal) lengths, [2.397(4) and 2.387(4)A]. Also of interest is the small deviation from a ’normal’ metal-metal bond length in the Ir-lr distance in the hydride,chloro-species [2.903(1 )A], compared with the much larger deviation for the dichloro-species [3.769(1 )A]. Clearly, in the latter no bonding

Chapter 5

is present between the two metals and the chlorine atoms have a dominant role in determining the M-M distance, in comparison to the former in which a considerable degree of bonding is present (see Chapter 1). This difference in M-M distance has a huge effect on the angle the halide atom subtends to the two metals : the shorter the M-M bond, the more acute the angle [M-GI-M = 72.65(1)° for the hydride, 100.45(12)° for the dichloro complex].

Complexes containing metal-metal multiple bonds are known to react with hydrogen halides and other such sources of X" (X = F, Cl, Br, I) to form bridging halide species. It was Curtis and Klingler®'^ who first reported the reactions of [M0gCp2(C0)J with HCI and HI to afford bridging-chloride and iodide species respectively. The latter exists as an equilibrium of two isomers in solution with the iodide ligands terminal or bridging. Initially, in this reaction the bridging hydrido-iodo complex is formed which reacts with excess HI to afford the di-iodo species. The entire sequence can be reversed using a source of hydride such as LiAIH(0'Bu)3 and the metal-metal triple bond can be recovered. The two di-halide complexes can also be formed by reaction of Ig and CIg (or PhlClg). The di-chloride species was found to contain one bridging- chloride and one terminal, and formally contains one metal in the +1, and the other +3, oxidation state. Similarly, the di-tantalum species, [Tag(PMeg)4Cl6] reacts with HCI and CIg to afford bridging-hydrido, chloro and di-chloro species, again with the reduction of the metal-metal bond order (from 2 to 1).

H C l C p ( C O ) 2 M o = M o ( C O ) 2 C p - t i Ç L * M o c , / H I C p (C O )2 M o ^ M o (C O )2 C p — — ► ^ M o \ | / \ , / i t \ M o = M o

\.

Reaction of FeCI^ with SnClg in THF at room temperature afforded the salt [Fe2(p-CI)g(THF)g][SnCl5(THF)]. The cation contains two iron metals bridged by three chloride ligands with no metal-metal bond [Fe-Fe = 3.086(2)Â]. On refluxing a TH F solution of this salt, the tetramer [Fe4(|i3-CI)2(fi- CI)4Cl2(THF)g] was formed along with the tin complex [SnCl4(TH F)2] which reacted with moisture to afford the bridging-hydroxide species [Sn2(p- O H )2Clg(THF)2].2 THF, in which two TH F molecules are hydrogen bonded to the OH-bridges.

The mononuclear species [fVloBr2(CO)2(PPhg)2] reacted with excess NaBp4 in H2O /C H2CI2 at room temperature to afford a number of dinuclear species containing bridging-fluoride, bromide and chloride ligands, [Mo2(CO)4(^-F)2(|i-X)(PPh3)4][BF4]"' (X = F, Br, Cl). Reaction with AgBF4 affords only the trifluoride species.®®

P P h , C C

\ ^ y \ I

XSNaBF4 ^ PhaPi.A / ...PPha

O C - / ° - B r

PPha C C

O O

X = F, Br, Cl

It is thought that formation of the coordinated H2O species [N/loBr(CO)2(H2 0 )(PPhg)2]^ occurs, which abstracts F” from BF4" to afford [MoBrF(CO)2(H2 0 )(PPh3)2r and after dimérisation forms the bridging-bromo, difluoro complex. The bromide is bound weakly to the metal centres due to a double frans-effect from two of the carbonyl ligands, and so is readily replaced by F‘ or Cl". The bridging-chloride species was only formed when the reaction was allowed to proceed for a longer time, suggesting that the origin of the C r anions was the solvent i.e. dichloromethane. Formation of the hydroxide species (where X = OH) also occurred. Hughes et al.®^ reported the related ditungsten complex [W2(CO)4(|i-F)3(PM e2Ph)4][BF4] from the protonation of the polyhydride species [WHg(PMe2Ph)3] with HBF4.Et2 0 under a carbon monoxide atmosphere. The central [W2(ji-F)3] core of this complex

Chapter 5

is comparable to the dimolybdenum species [{MoH2(PM ePh2)3}2(|i-F)3]®® synthesised by the reaction of [IVIoH4(PIVIePh2)J with HBF^. All these reactions occur by F* abstraction from B F /.

This prompted the author to investigate the possibility of forming bridging-halide complexes of the form [Fe2(CO)4(|i-X)(ji-PCy2)(ii-dppm)].

5.5.2 Synthesis of Bridging-Halide Complexes [Fe2(C0)4(p-X)(p-PCy2)(p- dppm)] (R = 01, Br, I, F) 23-26.Cy

As mentioned previously, chromatography of the hydride complex 1.Cy afforded two products, the hydroxide species 22.Cy and the bridging-chloride complex [Fe2(CO)4(|i-CI)(|i-PCy2)()J.-dppm)]. The latter was formed in approximately 38% yield. In order to investigate this complex more fully a reaction with a higher yield of the chloride species was sought. Thus, reactions of I.C y with a number of reagents containing chlorine were carried out.

Bubbling chlorine through a toluene solution of 1.Cy for 2 hours afforded a red solution. Removal of the solvent and chromatography on alumina afforded the bridging-chloride species 23.Cy (62%). Similarly, bubbling HCI gas through a toluene solution of 1 .Cy with stirring, afforded 23.Cy in high yield (70%), as did the addition of 40% conc. hydrochloric acid to a toluene solution of the hydride (61%), although the latter reaction needed a longer reaction time and the assistance of heat (50°C).

Initially, after the chromatography reaction (Section 5.4.2), the chloride species proved difficult to characterise as the source of the chloride ion was not clear. The ^^P NMR spectrum showed a resonance at Ô217.0 (t, J 99Hz) corresponding with the dicyclohexylphosphido moiety and a single resonance at 560.7 (d) corresponding with equivalent phosphorus nuclei of the diphosphine ligand indicating a symmetrical species with a further ligand symmetrically bridging the di-iron centre. The nmr spectrum showed familiar resonances in the phenyl and cyclohexyl regions and the methylene proton resonances appeared as two pseudo-quartets at 53.51 and 53.00. However, on integration, no evidence for another source of hydrogen was observed

indicating that the unidentified bridging ligand did not contain hydrogen. Chemical analysis identified the ligand as being chlorine.

Under similar conditions, 40% conc. hydrobromic and hydrofluoric acids were added drop wise to separate toluene solutions of I.C y and the reaction mixtures warmed to 50°C. In both cases darkening of the orange solution to a deep red colour was noted. Subsequent removal of the solvent and chromatography with non-chlorinated solvents (to avoid halide substitution) afforded the bridging-bromo (69%) and fluoro (64%) species [Fe2(CO)4(|i-X)(|i- PCyz)(p-dppm)] (X = Br, 24.Cy; X = F, 26.Cy).

/ ? \ HX / \ (CO)2Fe^^)-%Fe(CO)2 --- ► (0 0 ) ^ 6 ^ — ^ F e (C 0 )2 \ ^ \ - CO, -H2 ' ^PPh2 P ^ 2P \^^P P h 2 H2 H2 1 .Cy X = Cl, Br, F 23, 24, 26.Cy

As HI is not readily available, being unstable under normal conditions, the analogous bridging-iodide complex, 25.Cy was prepared by reaction of iodine with a toluene solution of I.C y. Again, subsequent chromatography yielded the halide species in high yield (60%).

All three of these new complexes display very similar spectroscopic data to the chloride species. The table below shows that the ir absorptions vary little between each halide, clearly the increased electronegativity going from F>CI>Br>l does not affect the strength of the terminal 0 - 0 bonds. Considering such a small structural change has occurred, this is not surprising, although one would possibly expect some change in the frequency for the carbonyls trans \o the halide [0(2) and 0 (4)] as the electronegativity increases on going from I to Br to 01, and hence the Fe-X bond shortens, increasing the Fe-O(O) bond due to the effect of trans influence.

Neither does there seem to be any correlation between the nmr chemical shifts and the halide present in the molecule. All four complexes give rise to similar triplet and doublet patterns for phosphido and diphosphine

Chapter 5

ligands respectively, indicating symmetrical structures. The spectra of the chloride and bromide complexes were also recorded in benzene in case halide substitution occurred in the latter in chlorinated solvents to form the bridging- chloride species. However, different resonances for the two halide species indicated that no such substitution had occurred. Of possible note, is the fact that the iodide species gives rise to a phosphido signal at higher field than the other halides with a larger coupling between phosphorus nuclei. This may be due to steric reasons, the larger iodide ligand forcing the two phosphorus ligands further apart (producing a wider angle between the ligands) which would increase the coupling as explained in Chapter 1.

ir (cm''') nmr in CDCI3 (5) nmr in CgDg (5) 23.Cy 1980.5(m) 1950.5(s) 1915.7(m) 217.0 (t, J 99Hz) 60.7 (d) 216.1 (t, J 99Hz) 62.5 (d) 24.Cy 1980.1 (m) 1950.1 (s) 1915.8(m) 220.1 (t, J 95Hz) 62.7 (d) 219.0 (t, J 95Hz) 62.3 (d) 25.Cy 1979.0(m) 1949.5(s) 1915.7(m) 195.4 (t, J 108Hz) 60.2 (d) - 26.Cy 1978.2(m) 1948.0(s) 1916.4(m) 217.0 (t, J 99Hz) 62.5 (d) -

To further investigate the effect of the size of the halide on the geometry of the molecule, separate X-ray diffraction studies were carried out on single crystals of 23.Cy, 24.Cy and 25.Cy (Figures 5.8 - 5.10 respectively). Crystals of 26.Cy were obtained of suitable quality for X-ray diffraction, however, it was not possible to solve the data due to a disorder in the crystal

Fe(1)-Fe(2) 2.546 (1) Fe(1)-P(1) 2.242 (2) Fe(1)-P(3) 2.222 (1) Fe(1)-CI(1) 2.311 (1) Fe(1)-C(2) 1.733 (6) Fe(2)-P(2) 2.242 (2) Fe(2)-P(3) 2.226 (2) Fe(2)-CI(1) 2.291 (2) Fe(2)-C(4) 1.733 (7) Fe(1)-P(3)-Fe(2) 69.8 (1) Fe(1)-CI(1)-Fe(2) 67.2 (1) P(1)-Fe(1)-P(3) 152.0 (1) P(2)-Fe(2)-P(3) 151.1 (1) P(1)-Fe(1)-CI(1) 88.8 (1) P(2)-Fe(2)-CI(1) 86.2 (1) P(3)-Fe(1)-CI(1) 80.7 (1) P(3)-Fe(2)-CI(1) 80.8 (1) C(13) C(46) C (45) C(44) C I34)

C(64)