Hydroxide Bridged Complexes 1 Introduction

Chapter 5

systems as the Fe-O-Fe bridging unit is present in many iron proteins which can function in a similar way to haemoglobin and myoglobin which transport and store molecular oxygen. A bridging-hydroxide ligand was recently found to be present in the mixed-valent [Fe(n)Fe(in)] form of the proteins methane monooxygenase hydroxylase and azidohemerythrin.^®

H

> k ":;k

During investigations into metaloproteins such as hemerythrin and ribonucleotide reductase, Vasilevsky and Stenkamp"^° synthesised a number of interesting bimetallic complexes, one of which contained a bridging- hydroxide ligand, namely c/s-dichloro[p-[bis[p-[[2,6-diacetylpyridine d io x im a to ](2 -)-0 :0 ’]]dih ydroxodiphenyld iborato](3-)]-|i-hydroxodi-iron, [Fe2(C3oH29B2N606)Cl2(p-OH)].H20.2C2H3N. The two non-bonding metal centres are held closely together [Fe-Fe =

2.8708(8)A]

I I C I c

c—c^

I

I

Several examples of iron proteins containing bridging-hydroxide moieties'^^’"^’'^^ are given in the table overleaf, along with their respective

functions.

The reaction of diphenylphosphine with a variety of di-p-methoxo complexes of the form [Rh2Cp*2(p-OM e)2(p-L)]BF4 (L = pz, mpz, dmpz, bmpz) in acetone afforded, among other complexes, the hydroxide species [Rh2Cp*2(ix-OH)()i-L)(}i-PPh2)][BF4]'^'^ and in the case of the bromo-substituted pyrazolate species, the di-p-hydroxo complex [Rh2Cp*2(p-0 H)2(p-PPh2)]BF4 was also formed, suggesting that hydrolysis of the methoxo groups in the starting material had occurred possibly due to the acetone containing traces of water.

There are also examples in the literature of hydroxide ligands bridging scandium,"*® cobalt"*® and zinc"*^ centres (most of which have non-bonding metal-metal interactions), the last of which is a rare example of a di-zinc(II) hydroxide complex with a very stable hydroxide bridge which cannot be substituted by halides.

Protein Occurrence Function

Haemerythrin Several phyla of marine invertebrates.

Stores and transports dioxygen

Ribonucleotide reductase

Animals, bacteria and virus-infected mammalian

cells.

Catalyses formation of deoxyribonucleotide di- or tri­ phosphates (first stage of DNA

synthesis.) Purple acid

phosphatase

Glycoproteins from mammalian, plant and

microbial sources. Catalyses hydrolysis of phosphate esters in pH - 5-6. Methane monooxygenase Methanotropic bacteria. Catalyses oxidation of CH^ to CH3OH. Can also insert O into

C-H bond of large variety of substrates.

Chapter 5

Hydroxide ligands, however, are not always so stabte. There are literature precedents for the interconversion of hydroxide species and oxo- hydride species. The first recorded example where both of these have been isolated is the work of Mayer"^® on a rhenium(I) tris(acetylene) hydroxide complex. Addition of water to the triflate complex [Re(EtCCEt)3 0 Tf] (Tf = C F3SO3) produced the solvated species [Re(OH2)(EtCCEt)3]OTf which on deprotonation formed the hydroxide complex [Re(EtCCEt)3(0 H)]. In benzene, at room temperature, this transformed over a period of two weeks into the 0 x0 - hydride complex [Re(EtCCEt)2(0 )(H)] and free hexyne.

E t 9 " E t 9

R e ^ - E t C - C E t

^ ...

Et E t ^ Et Et E t ^ ^

It appears that the mechanism involved initial hydrogen migration not initial alkyne loss as the presence of 3-hexyne in solution did not affect the rate.

Bubbling molecular oxygen through a xylene solution of the triruthenium cluster [Ru3(C O )8(p-dppm)] afforded the oxo-species [Ru3(p3-0 )(C0 )6(}i- dppm^,"^® containing a pg-oxo ligand on one face of the molecule. It was thought that the bridging oxygen atom may be derived from the attack of dioxygen at the carbon atom in a |i3-(f|®-C,fi^-0 ) type ligand®° with subsequent loss of carbon dioxide and retention of an oxygen atom which becomes triply- bridging.

At low temperature, the cluster [Re4(M.-H)3(|i3-H )2(C O )i2]' coordinated a water molecule from 'wet' solvents to afford the unstable adduct®^ [Re4(p- H)5(H20)(C0)i2]’. On attempting to remove water from the solvents using molecular sieves, the amount of the new adduct present in the final mixture was reduced considerably. The complex was thought to contain a bridging HgO ligand acting as a 4-electron donor. At temperatures above 275K, [Re4(|i- H)3(fi3-H )2(C O )i2]’ decomposed in the presence of water to form the trinuclear hydroxide species [Re3(|i-H)3(pg-0 H )(C0 )g]' possibly due to the decomposition of the HgO adduct [Re3(|i-H)4(H2 0 )(C0 )g]\

H _ H H 8 i ' H

izf'

/ \

5.4.2 Synthesis of Hydroxide Complexes [Fe2(C0)/p-0H)(p-PR2)(p-dppm)],

22 upon Chromatography of Hydride Complexes [Fe2(C0X(p-H)(p-C0)(p-

PR

)(p-dppm)] 1.

On attempting to purify a sample of

1.Ph

22.Ph,

8^ / P h \ (CO)2Feïi|^Fe(CO)2 --- ^ c hr o m a t o g r a p h AI2O3, H2O PhgP^ ^PPhj 0 Hz 1 . P h - C O , - H 2 Cyz / \ (C0)2Fe^^—^Fe(C0)2 I u I PhgP^ ^PPhz 0 Hz 2 2 . P h

Characterisation as a hydroxide complex is made on the basis of the nmr spectrum, in which the hydroxyl-proton resonance occurs at -52.75 as a well resolved doublet of triplets (J = 6.0, 8.4Hz).

Chapter 5

This compares well with the value of -52.86 for the related species [Fe2(CO)6(|i-OH){p-P(C6H4Me-p)2}f^ This species was originally thought to contain a bridging-hydride ligand rather than the hydroxide and the source of the hydroxyl group was a matter of great interest and confusion.

The nmr spectrum for 22.Ph displayed two signals at 5149.4(t, J = 112Hz) and 550.9(d), indicative of a complex containing a plane of symmetry through the methylene group of the dppm ligand, where the two dppm phosphorus nuclei are equivalent. The coupling constant of 112Hz suggested a trans configuration of the two phosphorus ligands which is borne out in the crystal structure data (Figure 5.7). This is of particular interest as the two ligands adopt a c/s geometry in the original hydride complex and so a cis-trans isomérisation of phosphorus containing ligands has occurred. This was also noted upon conversion of the frans-DMAD complex, lO.Ph to the DMAD metalacycle, 11.Ph, by chromatography. It was suggested that the isomérisation required a high energy process and so the free energy activation barrier must be considerably reduced by the alumina support. Sterically, the trans geometry of the phosphorus containing ligands is favoured, the phenyl rings of the diphosphine furthest away from the bulky cyclohexyl groups of the phosphido moiety.

Upon performing column chromatography on a sample of I.C y , two orange bands were observed moving down the column. Separation and subsequent removal of the solvent yielded the bridging-hydroxide species [Fe2(C0)4(p.-0H)(p-PCy2)(|i-dppm)], 22.Cy (41%) and the bridging-chloride species [Fe2(CO)4()i-CI)(p,-PPh2)(|i-dppm)], 23.Cy (38%, discussed in the following section). The ir spectra of both species were similar in appearance as we have come to expect for tetra-carbonyl complexes of this type, and differ by ~10cm’\

CV2 Cyj cyj

/ p \ AUOa / \ / \

(C O ) f e i^ ^ F e ( C O ) : — ( CO) Je^— :Fe(CO)j + ( C O ) J e ^ ^ F e ( C O ) j

I I CH2CI2 1 H I I Cl I

Ph2P ^ g ^ P P h2 chromât. Ph^P-^^^/PPha P h a P -^ ^ ^ P P h j

H2 H2 H2

1.Cy 22.Cy 23.Cy

The hydroxide, as in the case of its diphenylphosphido counterpart, 22.Ph, was characterised by the hydroxyl proton resonance at -52.33 in the nmr spectrum, which also showed familiar signals in the phenyl (57.5-7.2) and cyclohexyl (52.3-0.8) regions as well as two distinct methylene proton resonances at 53.46 and 52.95. The nmr spectroscopic data also matched well with that for 22.Ph, with one signal at 559.9 (d) for the two equivalent diphosphine phosphorus nuclei and a phosphido resonance at 5195.5 (t). The large coupling constant of 107Hz suggested a frans-arrangement of the two ligands.

In order to confirm the formation of a hydroxide ligand and the cis-trans isomérisation of phosphorus-containing ligands, an X-ray crystallographic study was carried out, the results of which are shown in Figure 5.7. The hydroxide ligand lies in-between the two phosphorus ligands adopting a cis configuration to both. It bridges the di-iron centre somewhat asymmetrically [F e(1)-0(5) = 2.224(4)Â and F e(2)-0(5) = 2.288(3)Â] and subtends an angle of 68.7(1 )°. This angle is considerably smaller compared with that in the analogous complex [Fe2(CO)6(|i-O H ){|i-P(C6H4Me-p)2}f^ of 79.1(2)°, this is due to the longer iron- oxygen bonds in 22.Ph compared with an average of 1.972Â in the hexacarbonyl derivative. As the major structural difference between these two complexes is the dppm ligand, this effect is probably due to the extra donor ability of P(1) and P(2) increasing the electron density at the di-iron centre and hence decreasing the donor ability of the hydroxide, therefore lengthening the iron-oxygen bonds.

The phosphido ligand also bridges the iron-centres in an asymmetrical manner [Fe(1)-P(3) = 2.223(2)Â, Fe(2)-P(3) = 2.202(2)Â], and subtends an

C ( 5 3 )