Aryl Phosphine Research Articles

Multi-Nuclear NMR Investigation of Nickel(II), Palladium(II), Platinum(II) and Ruthenium(II) Complexes of an Asymmetrical Ditertiary Phosphine

Complexes synthesized by reacting alkyl and aryl phosphines with different transition metals are of great inter- est due to their catalytic properties. Many of the phosphine complexes are soluble in polar solvents as a result they find appli- cations in homogeneous catalysis. In our present work we report, four transition metal complexes of Ni(II), Pd(II), Pt(II) and Ru(II) with an asymmetrical ditertiaryphosphine ligand. The synthesized ligand bears a less electronegative substituent such as methyl group on the aromatic nucleus hence makes it a strong σ-donor to form stable complexes and thus could effectively used in catalytic reactions. The complexes have been completely characterized by elemental analyses, FTIR, 1 HNMR, 31 PNMR and FAB Mass Spectrometry methods. Based on the spectroscopic evidences it has been confirmed that Ni(II), Pd(II) and Pt(II) complexes with the ditertiaryphosphine ligand showed cis whereas the Ru(II) complex showed trans geometry in their molec- ular structure.

ChemInform Abstract: Pd‐Catalyzed P—C Cross‐Coupling Reactions for Versatile Triarylphosphine Synthesis.

AbstractA series of aryl(diphenyl)phosphine sulfides (III) is readily prepared by a Pd‐catalyzed P—C cross‐coupling reaction of (hydroxymethyl)diphenylphosphine sulfide (I) with aryl iodides.

Cyclometalated Iridium Complexes of Bis(Aryl) Phosphine Ligands: Catalytic C–H/C–D Exchanges and C–C Coupling Reactions

This work details the synthesis and structural identification of a series of complexes of the (η(5)-C5Me5)Ir(III) unit coordinated to cyclometalated bis(aryl)phosphine ligands, PR'(Ar)2, for R' = Me and Ar = 2,4,6-Me3C6H2, 1b; 2,6-Me2-4-OMe-C6H2, 1c; 2,6-Me2-4-F-C6H2, 1d; R' = Et, Ar = 2,6-Me2C6H3, 1e. Both chloride- and hydride-containing compounds, 2b-2e and 3b-3e, respectively, are described. Reactions of chlorides 2 with NaBArF (BArF = B(3,5-C6H3(CF3)2)4) in the presence of CO form cationic carbonyl complexes, 4(+), with ν(CO) values in the narrow interval 2030-2040 cm(-1), indicating similar π-basicity of the Ir(III) center of these complexes. In the absence of CO, NaBArF forces κ(4)-P,C,C',C″ coordination of the metalated arm (studied for the selected complexes 5b, 5d, and 5e), a binding mode so far encountered only when the phosphine contains two benzylic groups. A base-catalyzed intramolecular, dehydrogenative, C-C coupling reaction converts the κ(4) species 5d and 5e into the corresponding hydrido phosphepine complexes 6d and 6e. Using CD3OD as the source of deuterium, the chlorides 2 undergo deuteration of their 11 benzylic positions whereas hydrides 3 experience only D incorporation into the Ir-H and Ir-CH2 sites. Mechanistic schemes that explain this diversity have come to light thanks to experimental and theoretical DFT studies that are also reported.

A general synthesis of phosphaalkenes at zirconium with liberation of phosphaformamides

A general, atom-economical method for the synthesis of phosphaalkenes is reported via the net coupling of primary alkyl or aryl phosphines with aryl or alkyl isocyanides at zirconium. The phosphorus-containing ligand can be liberated as the phosphaformamide from zirconium by reaction with an organic electrophile.

π-Stacking motifs in the crystal structures of bis(phosphine) copper(I) η2-tetrahydroborate complexes

The crystal and molecular structures of three bis(phosphine) copper(I) tetrahydroborate complexes have been determined at 125K. Bis(tricyclohexylphosphine) copper(I) tetrahydroborate, (Cy3P)2Cu(BH4) (1), bis(tri-p-tolylphosphine) copper(I) tetrahydroborate, (p-tolyl3P)2Cu(BH4) (2) and bis(tri-p-methoxyphenylphosphine) copper(I) tetrahydroborate, (p-MeO-Ph3P)2Cu(BH4) (3) all exhibit a distorted tetrahedral 4-coordinate copper(I) ion. In each structure, the tetrahydroborate ligand is bound to copper through two hydrogen atoms in a bidentate fashion. In the structure of (Cy3P)2Cu(BH4) (1) the P–Cu–P bond angle of 132.45(2)° is significantly larger than the corresponding angle in the structures with aryl phosphine ligands, with a P–Cu–P bond angles of 119.91(2)° in (p-tolyl3P)2Cu(BH4) (2) and 121.51(5)° in (p-MeO-Ph3P)2Cu(BH4) (3). The arylphosphine analogs (p-tolyl3P)2Cu(BH4) (2) and (p-MeO-Ph3P)2Cu(BH4) (3) display intramolecular face-to-face π-stacking interactions between an aryl group from each coordinated phosphine, at centroid-to-centroid distances of 3.768(1) and 3.581(3)Å, respectively, while (p-MeO-Ph3P)2Cu(BH4) (3) also exhibits intermolecular edge-to-face π-stacking interactions.

Cationic Ir(III) Alkylidenes Are Key Intermediates in C–H Bond Activation and C–C Bond-Forming Reactions

This work describes the chemical reactivity of a cationic (η(5)-C(5)Me(5))Ir(III) complex that contains a bis(aryl) phosphine ligand, whose metalation determines its unusual coordination in a κ(4)-P,C,C',C" fashion. The complex (1(+) in this paper) undergoes very facile intramolecular C-H bond activation of all benzylic sites, in all likelihood through an Ir(V) hydride intermediate. But most importantly, it transforms into a hydride phosphepine species 4(+) by means of an also facile, base-catalyzed, intramolecular dehydrogenative C-C coupling reaction. Mechanistic studies demonstrate the participation as a key intermediate of an electrophilic cationic Ir(III) alkylidene, which has been characterized by low-temperature NMR spectroscopy and by isolation of its trimethylphosphonium ylide. DFT calculations provide theoretical support for these results.

A Guide to Sonogashira Cross-Coupling Reactions: The Influence of Substituents in Aryl Bromides, Acetylenes, and Phosphines

The conversion-time data for 168 different Pd/Cu-catalyzed Sonogashira cross-coupling reactions of five arylacetylenes (phenylacetylene; 1-ethynyl-2-ethylbenzene; 1-ethynyl-2,4,6-R(3)-benzene (R = Me, Et, i-Pr)) and Me(3)SiCCH with seven aryl bromides (three 2-R-bromobenzenes (R = Me, Et, i-Pr); 2,6-Me(2)-bromobenzene and three 2,4,6-R(3)-bromobenzenes (R = Me, Et, i-Pr)) with four different phosphines (P-t-Bu(3), t-Bu(2)PCy, t-BuPCy(2), PCy(3)) were determined using quantitative gas chromatography. The stereoelectronic properties of the substituents in the aryl bromides, acetylenes, and phosphines were correlated with the performance in Sonogashira reactions. It was found that the nature of the most active Pd/PR(3) complex for a Sonogashira transformation is primarily determined by the steric bulk of the acetylene; ideal catalysts are: Pd/P-t-Bu(3) or Pd/t-Bu(2)PCy for sterically undemanding phenylacetylene, Pd/t-BuPCy(2) for 2- and 2,6-substituted arylacetylenes or Me(3)SiCCH and Pd/PCy(3) for extremely bulky acetylenes and aryl bromides. Electron-rich and sterically demanding aryl bromides with substituents in the 2- or the 2,6-position require larger amounts of catalyst than 4-substituted aryl bromides. The synthesis of tolanes with bulky groups at one of the two aryl rings is best done by placing the steric bulk at the arylacetylene, which is also the best place for electron-withdrawing substituents.

Monochromic Red-Emitting Nonconjugated Copolymers Containing Double-Carrier-Trapping Phosphine Oxide Eu3+ Segments: Toward Bright and Efficient Electroluminescence

Two metallopolymers P1 and P2 were prepared by using a novel polymerizable Eu3+ complex Eu(TTA)3VBADPO (EuMA) (TTA = 2-thenoyltrifluoroacetonate) and vinylcarbazole as the monomers in the ratios of 1:99 and 3:97, respectively, in which VBADPO is 2-(diphenylphosphoryl)-N-(2-(diphenylphosphoryl)-4-methoxyphenyl)-4-methoxy-N-(4-vinylbenzyl)aniline as a polymerizable aryl phosphine oxide ligand with a bipolar structure. The copolymers exhibit the excellent optical properties with photoluminescence quantum yield more than 60% in film. Both Gaussian simulation and electrochemical analysis indicated that the Eu3+-complexed segments form double-carrier traps (Eu-trap) in the copolymers with depth of 0.1 eV for the hole and 0.7 eV for the electron. The single-layer spin-coated devices of P1 and P2 realized the pure red emissions from Eu3+ ions. The biggest luminance of 149.1 cd m–2 was achieved, which is the highest among those of electroluminescent (EL) Eu3+-containing copolymers reported so far. The unusual effi...

Copolymerization of Ethylene with Norbornene by Neutral Aryl Phosphine Sulfonate Palladium Catalyst

Complex [Pd(κ2-P,O-{2-(2-MeOC6H4)2P}C6H4SO3)Me(dmso)] (1) was investigated as a single component catalyst for the copolymerization of ethylene with norbornene affording P(E-co-N) in excellent yields and molar masses significantly higher than those of polyethylene. Copolymer molar masses increased with N concentration. Determination of microstructure and reactivity ratios revealed a strong inherent tendency to form alternating copolymers. The ability of palladium complex 1 to effectively copolymerize ethylene with functionalized norbornene derivatives was also demonstrated.

ChemInform Abstract: A New Method for the Synthesis of Dichlorophosphines.

AbstractReaction of alkyl and aryl phosphines with PCl5 provides a simple synthesis of the target compounds.

Synthesis and reactivity of half-sandwich (η5-C5Me5)Ir(iii) complexes of a cyclometallated aryl phosphine ligand

Reaction of the Ir(III) dimer [(η5-C5Me5)IrCl2]2 with PMeXyl2 (Xyl = 2,6-C6H3Me2), in the presence of the poorly coordinating base 2,2,6,6-tetramethyl piperidine, gives a chloride complex 1-Cl, resulting from hydrogen chloride elimination involving one of the phosphine benzylic hydrogen atoms and concomitant cyclometallation. Related compounds containing other halide or pseudohalide ligands, 1-Br, 1-Cl, 1-SCN, can be made, the latter featuring S-coordination of the ambidentate thiocyanate to the soft Ir(III) Lewis acid centre, as suggested by IR data and demonstrated by X-ray crystallography. Hydride 2-H, and alkyl derivatives 3 (Me) and 4 (CH2SiMe3) can also be prepared from 1-Cl and appropriate hydride and alkylating reagents. An interesting H/D exchange chemistry that occurs in the presence of CD3OD has been disclosed for 1-Cl, 1-Br and 2-H. For the halide derivatives, deuterium incorporation takes place into the methylene and methyl sites of their cyclometallated ligand, whereas for 2-H only the hydride and methylene (Ir–CH2) protons participate in the exchange, which is strikingly accelerated by catalytic amounts of acids.

Photophysical and electroluminescent properties of a Series of Monochromatic red-emitting europium-complexed nonconjugated copolymers based on diphenylphosphine oxide modified polyvinylcarbazole

Two novel copolymers P1 and P2 were prepared by using the polymerizable aryl phosphine oxide based Eu(TTA) 3VBCzDPO ( EuM) ( VBCzDPO = 3,6-bis(diphenyl-phosphoryl)-9-(4- vinylbenzyl)-9H-carbazole) and vinylcarbazole as the monomers with the compositions of 1:99 and 1:33, respectively. It is showed that the stronger coordinate ability of bidentate APO ligands facilitates the stability of the complex monomer during polymerization, and their beetling coordinate sites and adjustable structure efficiently reduce the steric effect of bulky EuM. Both of photoluminescent (PL) spectra in solution and solid of the copolymers exhibited improved emission from Eu 3+ ion. The high PL quantum yield (PL QY) in solid of 60% is realized. Further investigation indicates that for the intra-chain energy transfer VBCzDPO serves as the intermediate and bridge between PVK and Eu(TTA) 3, which ensures the high efficiency of whole intra-chain energy transfer. The pure-red emission from the devices of P1 and P2 was demonstrated.

Influence of bidentate structure of an aryl phosphine oxide ligand on photophysical properties of its Eu III complex

The bidentate phosphine oxide ligand 1,8-bis(diphenylphosphino) naphthalene oxide (NAPO) and its Eu III complex 1 Eu(TTA) 3(NAPO) (TTA=2-thenoyltrifluoroacetonate) were chosen to study the effect of bidentate phosphine oxide ligand on the photophysical properties of the corresponding complex. The intramolecular energy transfer processes of 1 were studied. The investigation showed that with bidentate structure NAPO could suppress solvent-induced quenching by enforcing the ligand-ligand interaction and the rigidity of the complex. Contrasted with the monodentate triphenylphosphine oxide (TPPO), NAPO had a higher first triplet excited energy level (T 1) and a lower first singlet excited energy level (S 1), which fitted to the corresponding excited energy levels of anion ligand TTA much more and supported more efficient singlet and triplet energy transfer in its Eu III complex.

Synthesis, photophysical and electroluminescent properties of a novel bright light-emitting Eu 3+ complex based on a fluorene-containing bidentate aryl phosphine oxide

A novel bidentate aryl phosphine oxide (APO) ligand with fluorene as the chromophore (9,9-diethyl-9H-fluorene-2,7-diyl)bis(diphenylphosphine)oxide (EFDPO), as well as the corresponding Eu 3+ complex 1 were designed and synthesized. The strong absorption antennae effect of the functional APO ligands was proved. The effect of EFDPO in the intramolecular singlet and triplet energy transfer was also investigated. It is showed that the rigid structure and the appropriate energy levels of EFDPO not only reduce the energy loss induced by the structure relaxation, but also restrain the solvent quenching and facilitate the energy transfer from EFDPO to Eu 3+. The thermal analysis was also performed to prove the improved thermal stability and phase stability of the complex. The complex exhibited good electroluminescent (EL) performance, such as maximum brightness more than 600 cd m −2, E.Q.E. around 2% and stable monochromic red emission at 616 nm.

Single-Step Synthesis of Secondary Phosphine Oxides

We report that in the presence of trifluoroacetic acid primary phosphines undergo efficient addition to aldehydes to form the corresponding secondary phosphine oxides in 47−97% yield. This transformation is compatible with aryl and alkyl phosphines, as well as a broad range of aldehydes, including formaldehyde. By using 1,5-dialdehydes as reaction partners, the addition provides a straightforward route to bis(phosphine oxides), which are difficult to prepare by alternative methods. In the presence of boron trifluoride diethyl etherate as reagent, benzophenone was shown to couple to phenylphosphine and cyclohexylphosphine in 92% and 72% yield, respectively. Twenty-three examples are presented.

ChemInform Abstract: Supercritical Carbon Dioxide Extraction of Metals from Aqueous Solutions: A Review

Organometallic chemistry, chemistry of compounds containing metal–carbon bonds or compounds in which an organic molecule (sometimes with a net negative charge) is bonded to a metal atom through an oxygen or nitrogen atom, is one of the most rapidly growing areas of chemical research. Organometallic compounds are being extensively utilized as reagents in preparation and processing of advanced inorganic materials, as catalysts in production of a wide variety of chemicals and as chemotherapy drugs. Supercritical fluid (SCF) science and technology is another rapidly growing field due to the interesting and desirable properties of SCFs as solvents. The combination of organometallic chemistry and SCFs is a relatively new research area with significant potential. Some applications include (1) use of organotransition metal complexes as homogeneous catalysts for reactions in SCFs [D.A. Morgenstern, R.M. LeLacheur, D.K. Morita, S.L. Borkowsky, S. Feng, G.H. Brown, L. Luan, M.F. Gross, M.J. Burk, W. Tumas, Supercritical carbon dioxide as a substitute solvent for chemical synthesis and catalysis, in: P.T. Anastas, T.C. Williamson (Eds.), Green Chemistry: Designing Chemistry for the Environment, American Chemical Society, Washington, DC, 1996, p. 132 and G.P. Jessop, T. Ikariya, R. Noyori, Homogeneous catalysis in supercritical fluids, Science 269 (1995) 1065], (2) impregnation of polymers with various organometallic complexes from SCF solutions for property enhancement or for subsequent in-situ chemical transformations within such matrices [J.J. Watkins, T.J. McCarthy, Polymer/metal nanocomposite synthesis in supercritical CO2, Chem. Mater. 7 (1995) 1991, and A.I. Cooper, S.G. Kazarian, M. Poliakoff, Supercritical fluid impregnation of polyethylene films, a new approach to studying equilibria in matrices; the hydrogen bonding of fluoroalcohols to (η5-C5Me5)Ir(CO) and the effect on CH activation, Chem. Phys. Lett. 206 (1993) 175], (3) decomposition of organometallic complexes in SCFs for formation of inorganic powders with controlled size distribution [M. Barj, J.F. Bocquet, K. Chhor, C. Pommier, Submicronic MgAl2O4 powder synthesis in supercritical ethanol, J. Mater. Sci. 27 (1992) 2187], (4) SCF extraction of heavy metals from various matrices by formation of organometallic complexes [K.E. Laintz, C.M. Wai, C.R. Yonker, R.D. Smith, Extraction of metal ions from liquid and solid materials by supercritical carbon dioxide, Anal. Chem. 64 (1992) 2875]. At the University of Connecticut, our research efforts are concentrated on evaluation of technical and economical feasibility of some of these applications. The three primary research thrusts in our group have been the utilization of supercritical carbon dioxide (scCO2) as a solvent in rhodium catalyzed homogeneous hydroformylation reactions [D.R. Palo, C. Erkey, Homogeneous catalytic hydroformylation of 1-octene in supercritical carbon dioxide using a novel rhodium catalyst with fluorinated aryl phosphine ligands, Ind. Eng. Chem. Res. 37 (1998) 4203], impregnation of polyurethane foams with organometallic oxidants from scCO2 solutions and subsequent vapor phase polymerization in these foams for production of electrically conductive composite foams [Y. Fu, D.R. Palo, C. Erkey, R.A. Weiss, Synthesis of conductive polypyrrole/polyurethane foams via a supercritical fluid process, Macromolecules 30 (1997) 7611], and investigation of extraction of heavy metals from aqueous solutions by compound formation using scCO2 [J. Murphy, C. Erkey, Copper(II) removal from aqueous solutions by chelation in supercritical carbon dioxide using fluorinated β-diketones, Ind. Eng. Chem. Res. 36 (1997) 5371]. Advances in these areas greatly depend on our understanding the interactions of SCFs and organometallic complexes and how these interactions affect a particular application. The subject matter of this review is extraction of heavy metals from aqueous solutions in the presence of SCFs. Since solvent extraction of heavy metals is utilized on a commercial scale, the replacement of organic solvents by SCFs has been the major driving force behind our research efforts. Therefore, this review was prepared to highlight the areas important for commercial scale application of the technology. In Section 1, an introduction to solvent extraction of metals is given. A brief introduction to the possible advantages of using SCFs is also presented in the same section. The fundamentals of extraction with different types of extractants (cation exchangers, solvating extractants and ion-pair extractants) are given in Sections 2, 3 and 4, together with the studies in the literature on metal extraction using SCFs for each type of extractant. Thermodynamics of extraction is particularly emphasized due to its governing role in the economical feasibility of a large scale process. The experimental methods that are utilized in evaluation of thermodynamic behavior of such systems are provided in Section 5. The current methods to recycle the extractants are presented in Section 6. The kinetics of extraction is described in Section 7 where no studies using SCFs have been reported to date and a brief conclusion is provided in Section 8.

Novel Light-Emitting Ternary Eu3+ Complexes Based on Multifunctional Bidentate Aryl Phosphine Oxide Derivatives: Tuning Photophysical and Electrochemical Properties toward Bright Electroluminescence

Three functional bidentate aryl phosphine oxide (APO) derivatives characterized by two diphenylphosphine oxide moieties [2-(diphenylphosphoryl)-N-(2-(diphenylphosphoryl)-4-methoxyphenyl)-4-methoxy-N-(4-methoxyphenyl)aniline (TMOADPO), 3,6-bis(diphenylphosphoryl)-9-ethyl-9H-carbazole (EtCzDPO), and 3,6-bis(diphenylphosphoryl)-9-phenyl-9H-carbazole (PhCzDPO)] bridged with a hole-transporting arylamine, as well as their tertiary complexes [Eu(TTA)3(TMOADPO)2 1, Eu(TTA)3(EtCzDPO)2 2, and Eu(TTA)3(PhCzDPO)2 3 (TTA = 2-thenoyltrifluoroacetonate)], were designed and synthesized. The strong absorption antennae effect of the functional APO ligands was proved. It is shown that their more rigid structure and chelate coordinate mode impart a decreased degree of freedom and form much more compact complex structures, which not only reduces the energy loss caused by the structure relaxation but also restrains the solvent quenching and facilitates the energy transfer from the APO ligands to Eu3+. Thermal analysis was als...

Recent Advances in the Mitsunobu Reaction: Modifications and Applications to Biologically Active Molecules

The Mitsunobu reaction involves the “redox” condensation of an acidic pronucleophile with a primary or secondary alcohol promoted by a reactive phosphonium salt derived from an alkyl or aryl phosphine and an azo compound. The synthetic utility of the reaction is demonstrated by the wide range of acidic pronucleophiles that are tolerated including carboxylic acids, phenols, imides, hydroxamates, thiols, thioamides, fluorinated alcohols, oximes, pyridinium and imidazolium salts, pyrimidine bases, β-ketoesters and trimethylmethane tricarboxylate (TMET) thereby generating a variety of functionally diverse molecules. The focus of this review is to highlight the recent development of new or alternative reagents and purification strategies for the Mitsunobu reaction with a discussion of their suitability for modern drug design supported by selected examples from the literature.

On the Origin of Reversible Hydrogen Activation by Phosphine–Boranes

Mechanistic insights into the factors responsible for the reversible hydrogen-activation ability exhibited by an aryl phosphine-borane system ((CH(3))(2)P-C(6)F(4)-B(CF(3))(2)) are presented. A detailed evaluation of the energies of various intermediates, generated by the addition of molecular hydrogen, and their interconverting barriers have been carried out using ab initio and DFT methods. Several rearrangement possibilities of the H(2)-phosphino-borane adduct have been investigated so as to unravel the lower energy pathways that convert the initial adduct to a series of other intermediates. The initial adduct formed by the heterolytic addition of a molecular hydrogen across the C-B bond is identified to undergo a series of rearrangement reactions until it terminates at the C-P end of the molecule. Among the possible 1,n-migrations (for which n=1-5), 1,2-proton migrations are found to possess lower energy transition states, whereas 1,2-hydride (in a zwitterionic intermediate) and 1,4-proton-coupled electron transfers exhibited much higher energy transition states. The minimum energy pathway for the transfer of a proton and hydride from the C-B bond to the C-P bond is found to involve a cascade of 1,2-proton transfers followed by a 1,2-hydride migration and finally a 1,4-proton-coupled electron transfer. The higher energy pathways identified for the hydride transfer suggest the possibility of a cascade of reversible proton migrations from a thermodynamically stable intermediate (M(a)). Possible uptake of two hydrogen molecules by the phosphine-borane system is additionally considered in the present study, in which relatively higher barriers than those with one molecule of hydrogen are observed. The computed thermodynamic parameters are found to be in accordance with the experimental observations, in which the uptake and storage of molecular hydrogen are carried out at lower temperatures whereas the liberation demands elevated temperatures.

Bisphosphine monoxides with o-phenylene backbones in Pt, Pd and Fe complexes

A new route to aromatic bisphosphine monoxides has been explored through ortholithiation of triphenylphosphine oxide and subsequent reaction of the lithiated intermediate with a range of alkyl and aryl phosphine chlorides. Routes to the known bisphosphine monoxide (BMPO) ( o-C 6H 4){P(O)Ph 2}PPh 2 ( 2aO) and a range of new BPMOs of the type Ph 2P(O)( o-C 6H 4)PR 2 where R 2 = iPr 2, Cy 2, Et 2 are described. Reaction of 2aO with MCl 2(cod) (M = Pd, Pt; cod = cyclooctadiene) gives products of the form [MCl( κ 1- P- 2aO)( κ 2- P, O -2aO)] + Cl − and MCl 2( κ 2- P,O- 2aO); the former exhibits fluxional behaviour which has been analysed by 195Pt NMR and 31P variable-temperature NMR spectroscopy. The bidentate complex is not fluxional for either the Pd or the Pt example; the Pt complex PtCl 2( κ 2- P,O- 2aO) has been characterised by X-ray crystallography. By comparison of the product distribution seen by 31P NMR spectroscopy and ESI-MS it was established that the different coordination modes of 2aO result in quite different behaviour of the complexes when studied by ESI-MS; when the O is formally coordinated to the metal its ionisation efficiency is very low. Synthesis of Fe(CO) 4( 2aO) confirmed the ability of the 2aO ligand to render a neutral complex with no alternative pathways for ionisation to be readily detected by ESI-MS.