127 Journal of Organometallic Chemistry, 476 (1994) 127-132 Applications of HPLC-MS to carbonyl clusters of the iron triad. The behaviour of dinuclear “flyover-bridged” iron compounds Maria Careri, Alessandro Mangia, Paola Manini and Giovanni Predieri Istitutodi Chimica Generale e Inorganica, Universitci di Parma, Wale delle Scienze, 43100 Parma (Italy) Enrico Sappa Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universitci di Torino, Ha P. Giuria 7, 10125 Torino (Italy) (Received August 4, 1993) Abstract High-performance liquid chromatography coupled with mass spectrometry detection was used to separate and characterize some dinuclear iron derivatives of general formula [Fe,(CO)s(PhzPXEPh,-p)(Et,C,(CO)C,Et,)] (X = CK, or CH,CH,, E = P or As). Reversed-phase conditions and a particle-beam (PB) interface were used. The chromatographic behaviour of these compounds depends strongly on the substituents in the ligands. The fragmentation pathways both in electron impact and in chemical ionization mode mass spectrometry were studied. Key words: Iron; Carbonyls; High-performance liquid chromatography; 1. Introduction The use of high-performance liquid chromatography (HPLC) for preparative and analytical separations of mono- and poly-nuclear organometallic compounds is now a well consolidated technique. It offers superior efficiency, high speed, and light and dioxygen exclusion compared with traditional column and thin-layer chromatography [l]. Recent papers in this field have described the behaviour of neutral and cationic dinuclear and cluster compounds [2] and the separation of a number of enantiomeric complexes with metal chiral centres [31. The variable-wavelength spectrophotometric detector is the most widely used for organometallic compounds, owing to the high absorbance electronic bands of these systems. Nevertheless, the need for more sensitive and specific detectors has generated considerable interest in on-line liquid chromatography-mass spectrometry (LC-MS) which allows the simultaneous Correspondence to: Prof. G. Predieri. 0022-328X/94/$7.00 SSDI 0022-328X(94)24371-B Mass spectrometry separation and identification of non-volatile compounds and provides structural information for each component, even of complex mixtures. Although LCMS has been widely applied to the analysis of biomolecules and drugs, it has been rarely applied in the field of organometallic chemistry. There has been significant recent progress towards interfacing the two techniques. The possibility of obtaining electron impact (EI) mass spectra, rich in structurally significant fragmentations, has been provided by the MAGIC [4] (Monodisperse Aerosol Generation for Introduction of liquid Chromatographic effluents) and PB [5] (Particle Beam) interfaces; chemical ionization (CI) mass spectra are also available for these interface devices. In a systematic study on the influence of the structure of polynuclear organometallic compounds on their chromatographic behaviour, we have applied PB HPLC-MS to a small group of iron-carbonyl dinuclear complexes, with the aim of assessing the capabilities of this combined technique. The compounds were obtained while studying the reactivity of alkynes towards metal carbonyls: in the presence of Me,NO, the “fer0 1994 - Elsevier Science S.A. All rights reserved 128 X=CH,, M. Careri et al. / Dim&ear E=P (I) X = CH,CH,, E = P (II) X = CH,CH,, E = As (III) Scheme 1. role” complex [Fe,(CO),(C,Et,),] reacts with Ph,PXEPh, (E = P, X = CH,, dppm; E = P, X = CH,CH,, dppe; E = As, X= CH,CH,, dppae), affording as major products the “flyover-bridged” derivatives [Fe,(CO),(Ph,PXEPh,-p){Et,C,(CO)C,Et,)l (see Scheme 11, apparently produced by CO insertion into the central C-C single bond of the original butadiendiyl [6]. This paper deals with the separation and the mass spectral characterization of these dinuclear iron derivatives by on-line reversed-phase HPLC mass spectrometry. Mass spectra of the eluates were obtained using both EI and CI sources. In a preceding paper [71 we have described the HPLC behaviour and the fragmentation patterns of trinuclear alkyne(carbonyl)ruthenium derivatives. 2. Experimental section The three dinuclear substituted “flyover-bridged” complexes were obtained by the method described in ref. 6 for the dppm derivative I. A derivative similar to compound II was obtained by photochemical substitution of one CO by dppe in the diphenylacetylene-flyover complex [81. Compounds II (dppe) and III (dppae) are new compounds (yields 25% and 30% respectively); they gave satisfactory elemental (C, H) analyses and were inferred to have the same structural framework of I on the basis of spectroscopic evidence. Compound II: IR (heptane), &CO, carbonyl) 2048vs, 1996vs, 1986~s 1946s cm- ‘, v(C0, ketonic) 1650s cm-‘; 13C NMR (chloroform-d,, TMS), in the carbonyl region, S 201.1s (ketonic), 208.1s, 211.2s, 211.9s, 211.4d, 212.9d ppm; 31P NMR (choroform-d,, H,PO,), S 48Sd, - 13.4d ppm (J(P, P) 26 Hz). Found: C, 62.2; H, 5.4. C,,H,,Fe,O,P, requires: C, 62.7; H, 5.3%. Compound III: IR (heptane), v(C0, carbonyl) 205Ovs, 1997vs, ‘flyover-bridged” Fe compounds 1988vs, 1946s cm-‘, Y(CO, ketonic) 1642s cm-‘; 13C NMR (chloroform-d,, TMS), in the carbonyl region, 6 201.5s (ketonic), 208.0s, 210.2s, 211.7s, 211.ld, 212.5d ppm; 31P NMR (chloroform-d,, H,PO,), 6 50.1s ppm (P coordination). Found: C, 59.3; H, 4.9; C,H,,AsFe,O,P requires: C, 59.6; H 5.0%. For the chromatographic separations a HewlettPackard Model HP1090 chromatograph equipped with a Rheodyne 7125 injector was used. A stainless steel column (25 cm X 0.4 cm i.d.1 filled with 5 pm LiChrosorb RP-18 (Merck) was used. In the case of HPLC-UV, a Perkin Elmer LC-75 variable-wavelength UV-visible detector was used, monitoring the eluates at 300 nm; dichloromethane or acetonitrile solutions of the compounds (20 ~1) were injected. An acetonitrilemethanol (80/20) mixture was used for the isocratic elution of the dinuclear iron compounds. The flow rates were 0.8 and 1 ml min-’ for the LC-MS and LC-UV, respectively. The solvents used were HPLC grade (C. Erba). The interface was a Hewlett-Packard model HP 59980A particle beam LC/MS device. As nebulizing gas, high purity helium at 50 psi (1 psi = 6894.76 Pa) was used, the desolvation chamber temperature being maintained at 55°C. The mass spectrometer was a Hewlett Packard Model HP5989A, the HP MS apparatus was equipped with dual electron impact (EI)/chemical ionization (CI) ion source, a hyperbolic quadrupole mass analyzer, a continuous dynode electron multiplier detector and a differentially pumped vacuum system with diffusion pumps. An HP MS 59940A ChemStation (HP-UX series) was used as analytical workstation, Both EI and CI sources were utilized; source temperature was held at 260°C the optimum value. Using the electron impact source, mass spectra were obtained under these conditions: electron energy 30 eV, electron multiplier voltage 2300 V. When operating under CI conditions, a CH,-NH, 95/5 mixture was used as the reagent gas: the ionization energy was 230 eV and the voltage applied to the electron multiplier was 2100 V. In this case, both positive and negative ions were monitored. The quadrupole temperature was maintained at 100°C; for scan acquisition, the system was scanned from 150 to 900 amu. 3. Results and discussion The three dinuclear “flyover-bridged” iron complexes were separated under reversed-phase conditions with an acetonitrile-methanol (80/20) mixture. The flow rate used was higher (0.8 ml min- ‘1 than those normally adopted (0.4-0.5 ml min-‘1 for the PB interface nebulizer, because at this flow-rate an efficient M. Careri et al. / Dinuclear “flyover-bridged” 129 Fe cotnp~unds TABLE 1. Main fragments observed in the electron impact (EI) mass spectra of the examined dinuclear “flyover” compounds (a) 4 0 8 m/z 183 185 192 199 262 276 289 307 321 365 343 370 414 384 398 442 440 454 498 min. 80000 (bl 70000 60000: g 50000~ 23 40000: a 30000~ $ ad ai Ia II a III a [PPh*-2H]+ [PPhJ+ KC,EQ,Col+ [PPh,CH,]+ [PPhJ+ 18 5 5 25 100 loo 49 9 2 42 28 65 3 loo 49 9 _ 85 7 69 5 14 67 [Fe,(C,Et,)J+ [PPh,CH,CH]+ [dppm-Ph]+ [dppe-Ph]+ [dppae-Phlf [Fe2(PPh2CH2PH)]+ [PPh,l: [PhZPAsPh2]+ [dppml+ 1 1 5 _ 37 24 45 1 _ 3 [dppel + [dppael+ [Fe(dppmllf [Fe(dppe)l+ [Fe(dppae)l+ - - 1 a Percentage in compounds I, II, and III. z OD ti 20000~ 10000~ 01-4 Jl ,$$=o ,G Ion d&w 5’ 6 7 8 9 IO II Time (min) Fig. 1. Reversed-phase HPLC separation of compounds I-III, under the conditions described in the Experimental section: (a) UV detection (A = 300 nm); (b) MS detection (EI, total ion signal). separation was obtained and only a slight reduction in sensitivity was observed. Figure 1 compares the chromatograms obtained with UV detection at 300 nm (Fig. l(a)) with the total-ion signals from L&MS (Fig. l(b)), which shows excellent chromatographic fidelity. The total ion chromatogram was obtained by using the electron-impact source and monitoring positive ion signals of the compounds. As expected, upon monitoring an abundant single ion (m/z = 262, [PPh,]+) present in the fragmentation patterns of all the compounds, the peaks due to the free diphosphines dppm and dppe (present in unpurified samples of the complexes) were considerably enhanced. The chromatographic behaviour of the three dinuclear compounds apparently depends on two factors: the length of the alkyl chain and the nature of the donor atoms. Compound II is eluted after I because the alkyl chains are different, whereas III is more strongly adsorbed than II because of the presence of the less electronegative uncoordinated arsenic atom. These effects previously observed for ruthenium carbony1 clusters [93 are particularly marked in this case, probably because the alkyl chain and one donor atom are dangling and able to interact more strongly with the stationary phase. However, these two factors do not operate in the same way for the free donors, as dppe is eluted before dppm, as shown in Fig. l(b). Tables l-3 report the most significant ionic fragments found in the EI, PICI and NICI mass spectra of the three compounds respectively, while Figs. 2-4 show the mass spectra of compound III obtained in the three modes: EI (Fig. 2), PICI (Fig. 3) and NICI (Fig. 4). The TABLE 2. Main fragments observed in the positive-ion chemicalionization (PICI) mass spectra of the examined dinuclear “flyover” compounds m/z Ion Ia 187 193 201 210 229 322 333 385 399 443 401 415 459 440 454 747 803 IPPh,H,l+ [H+iC,k,),CO]+ [HPPh,CH,]+ [PPh,CCH]+ [AsPhJ+ [OAsPhJ+ [H+ Fe2((C,Et&WoO)I+ D-I+ dppml+ [H+dppel+ [H + dppae]+ [H+O+dppm]+ [H+O+dppe]+ [H + 0 + dppae]+ [Fe(dppm)l+ [Fe(dppe)l+ [H + Fe2((C2Etz)z(COIXdppae)l+ [H + Fe,((C2Et,),(COIXCO)2(dppaellf 4 3 3 loo loo 100 4 1 1 13 14 6 19 7 11 7 27 95 96 94 6 11 11 1 1 1 1 a Percentage in compounds I, II, and III. II a III a 132 M. Careri et al. / Dim&ear ‘f[yover-bridged” Fe compounds tion of analytical techniques. Furthermore, valuable structural information for the identification of unknown species can be obtained, particularly from the NICI spectra, which give rise to a richer fragmentation pattern, including a greater number of metal-containing fragments. 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