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Isopolyoxorhenates observed by positive ion electrospray mass spectrometry
Inorganic Chemistry Communications 5 (2002) 23–27 www.elsevier.com/locate/inoche
Isopolyoxorhenates observed by positive ion electrospray mass spectrometry Frans Sahureka, Robert C. Burns *, Ellak I. von Nagy-Felsobuki School of Biological and Chemical Sciences, Department of Chemistry, The University of Newcastle, Callaghan 2308, Australia Received 29 May 2001; accepted 22 October 2001
Abstract Unknown positive ion isopolyoxorhenates have been observed using electrospray ionization mass spectrometry (ESI+). The ESI+ studies of ammonium and alkali metal (Naþ and Kþ ) perrhenate salts in aqueous solution at pH 4.5 show the existence of the þ þ series ½Axþ1 ReVII (where x ¼ 1–5 and A ¼ NHþ and Kþ ). In the potassium perrhenate system, the series x O4x 4 , Na þ V VII ½Kxþ2 Re Rex O4xþ3 (x ¼ 0–4) has also been characterised. All of these four series have fAReO4 g as the aggregation unit. In the ammonium perrhenate system, the monomeric Re(VII)-containing species, ½ðNH4 Þ2 ðH2 ReO5 Þþ , ½ðNH4 Þ3 ðHReO5 Þþ and ½ðNH4 Þ4 ðReO5 Þþ were also detected. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrospray mass spectrometry; Isopolyoxorhenates; Polyoxometalates; Rhenium(VII)
1. Introduction Electrospray ionization mass spectrometry (ESI-MS) has been used to characterise isopoly- and heteropolyoxometalates of molybdenum, tungsten, vanadium and rhenium in both aqueous and non-aqueous solutions [1– 7]. In addition to known ions of the isopolyoxometalates that appear in the bulk, in some of these studies fragment ions as well as polymeric ions that had not been previously observed were identified in the gas phase. Thus, there is direct evidence that this technique has generated new species as a result of the evaporation/ fission process [3–6]. The formation of polymeric ions, both positively and negatively charged, has also provided some insight into how the oxometalates assemble into aggregates, and the nature of the polymerization units used in these assembly processes. There have been a sparse number of studies of perrhenate salts by mass spectrometry [7–9]. Secondaryion mass spectrometric studies of ammonium and alkali metal salts supported on graphite showed the existence of monomeric ions of the type ½A2 ReVII O4 þ in positive *
Corresponding author. Tel.: +61-249-215-479; fax: +61-249-215472. E-mail address: [email protected] (R.C. Burns).
ion studies, and both ½ReVII O4 and ½ReV O3 in negative ion studies, as well as ½H4 ReIII O4 when a NðC2 H4 OHÞ3 matrix was used [8]. Negative ion electrospray time-of-flight mass spectrometry was used to study ammonium perrhenate, as well as tungstate and vanadate, in aqueous solution under both acidic (pH 3) and basic (pH 10) conditions [7]. In that study only the ½ReVII O4 ion was observed [7]. More recently, negative ion electrospray quadrupole mass þ spectrometry was used to study three [NHþ 4 , Na and þ K ] perrhenate salts [9]. This study investigated the percentage relative abundance (%RA) of ½ReVII O4 , ½ReV O3 and ½ReIII O2 as a function of the cone voltage [9]. The essential requirements for the formation of both isopoly- and heteropoly-oxoanions have been discussed by Pope [10], and include the cation size (i.e. cation radius) and ability to act as a good acceptor of oxygen pp electrons. While Re(VII) has an ionic radius that falls within the accepted range for polyoxometalate formation, it was concluded that the high oxidation state of Re(VII) would preclude the formation of structures similar to those of known polyoxo-molybdates and -tungstates, and that structures based on corner- or edge-shared ReVII O6 octahedra would be positively charged. Moreover, while anion structures
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F. Sahureka et al. / Inorganic Chemistry Communications 5 (2002) 23–27
featuring fac-trioxo rhenium(VII) can be envisaged, they would most likely be unstable with respect to dissociation into monomeric ½ReO4 units [10]. Species containing lower oxidation states such as Re(VI) and Re(V) were considered possible, although such polynuclear species are very likely to disproportionate into ½ReO4 and other species containing Re(IV), a more stable oxidation state. As an extension of our previous electrospray mass spectrometric studies of the vanadate, molybdate and tungstate systems [3–6], we wish to report on the positive ion mode electrospray (ESI+) þ of the NHþ and Kþ perrhenate systems. This 4 , Na study will establish a number of previously unknown polyoxorhenate cationic species containing Re(VII) and Re(VII)/Re(V).
2. Experimental Ammoniun perrhenate (Aldrich, 99+%), sodium perrhenate (Aldrich, 99.99%) and potassium perrhenate (Aldrich, 99.0%) were used as received. Solutions of these compounds (10 3 M) were made up in Millipore Milli-Q deionised water and adjusted to a pH of 4.5 using AR grade acetic acid (0.1%). The ESI+ spectra were obtained using a VG Platform II single quadrupole mass spectrometer (Micromass, UK) coupled to a HPLC binary pump system. For all spectral acquisitions the capillary tip was set at a potential of +3.5 kV relative to ground. The cone voltage (CV) was set to either +20, +40 or +60 V relative to the skimmer, which was set to ground. The source temperature was at 353 K. A Rheodyne injector fitted with a 10 ll loop was used to inject the sample solution into the flow of the mobile phase (10 ll/min), the pH of which had been adjusted to a value of 4.5 using 0.1% acetic acid. All spectra were acquired between 50 and 1500 m=z, at a scan speed of 3.6 s/scan. Calibration of the m=z scale was accomplished using a 3.0 lg=ll solution of NaI in a 50:50 mixture of water and acetonitrile as the mobile phase. In the positive ion mode the spectra yielded NaI clusters of the form ½Naxþ1 Ix þ . All data were acquired and analysed using the Micromass Masslynx system. Tabulated values are based on the most abundant isotope of rhenium, i.e. 187 Re, and on peaks above 4%RA.
3. Results and discussion The ESI+ mass spectra of the perrhenate salts of þ þ NHþ 4 , Na and K at pH 4.5 and a CV of +20 V are shown in Fig. 1. The assignments of the peaks above 4%RA are given in Table 1. The ESI+ spectra measured at CV values of +40 and +60 V showed no significant variations in the %RA values.
The spectra show a series of singly charged ions for all three systems, with the most abundant peak (100%RA) in each case attributable to the monomeric þ þ species ½A2 ðReVII O4 Þþ , with A ¼ NHþ 4 , Na and K . þ VII For each of the systems the series ½Axþ1 Rex O4x (with x ¼ 1–5) was observed. In the potassium perrhenate system a second series was also present, which has the þ form ½Kxþ2 ReV ReVII x O4xþ3 (with x ¼ 0–4). The members of this latter series contained one Re(V) in addition to Re(VII) (the latter for x > 0). In general this series had much lower %RA values than those of the totally Re(VII)-containing series in this system. For all of the above series, the aggregation (or polymerization) unit is þ þ fAReO4 g, where A ¼ NHþ 4 , Na and K . This is the first time that isopolyoxorhenates have been observed either in bulk solution studies, or in gas phase studies under electrospray conditions. The appearance of the lower oxidation state [i.e. Re(V)] component in the þ ½Kxþ2 ReV ReVII series appears initially surprisx O4xþ3 ing. On closer examination of the ESI+ spectra of the isopolyoxovanadates, the presence of V(IV) in addition to V(V) was also found in the KVO3 system. This þ V yielded species of the type ½Kxþ3 ðVIV 2 O5 ÞðV O3 Þx (where x ¼ 1–6) [6]. No lower oxidation states were observed in any of the positive-ion series for the other alkali metal cations (Liþ and Naþ ), or in corresponding studies of the alkali metal salts of ½MoO4 2 and ½WO4 2 [3,5]. It is known that simple metal salts, such as NaI (used for the mass scale calibration) and both NaðCF3 COOÞ and NaðCH3 COOÞ [11] will generate both positive and negative ion series by aggregation as a result of the concentration effect, and this may also have occurred with the perrhenate salts used in the present study. However, such non-specific aggregation is usually observed with higher initial concentrations (e.g. NaI, 2 10 2 M) than was used for the perrhenate salts, and generally employs a mixed water–acetonitrile solvent system. More importantly, however, no negative ion þ series of the type ½Ax 1 ðReO4 Þx (A ¼ NHþ 4 , Na and þ K ) were observed under identical concentration and corresponding cone voltage conditions for any of the same three perrhenate salts, with only ½ReVII O4 , ½ReV O3 and ½ReIII O2 observable depending on the cone voltage [9]. Thus it would seem unlikely that the positively charged series found in the present study were generated as a result of non-specific aggregation processes. Although the oxo chemistry of Re(VII) is dominated by tetrahedrally coordinated ½ReO4 , six coordination is observed for Re(VII) in, for example, the structure of solid Re2 O7 [12] and in anhydrous perrhenic acid, ½O3 ReOReO3 ðH2 OÞ2 [13]. Thus the formation of an expanded coordination number for Re(VII) is well known, and would allow the formation of charged polymeric species containing Re(VII) with oxo bridging ligands.
F. Sahureka et al. / Inorganic Chemistry Communications 5 (2002) 23–27
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þ þ Fig. 1. ESI-MS spectra of the positive-ion species detected in the AReO4 systems (A ¼ NHþ 4 , Na and K ) at a pH of 4.5 and a CV value of +20 V: þ (a) NH4 ReO4 , (b) NaReO4 , and (c) KReO4 . The representation ½Knþ1 Ren O4n 1 þ is a contracted form for the ½Kxþ2 ReV ReVII x O4xþ3 series, where x ¼ n 1, and does not differentiate between Re(V) and Re(VII).
þ For all three ½Axþ1 ReVII series, where x O4x þ þ þ A ¼ NH4 , Na and K , plots of log(%RA) against the number of perrhenate aggregation units provide linear þ þ 2 relationships (NHþ 4 , R ¼ 0:950; Na , 0.949; K , 0.873). Previously, the linearity of such plots has been shown to be consistent with an addition polymerization mechanism [3]. In the present case the observed linear relationships again show that these three series are formed by a similar mechanism from the additive fAReO4 g þ þ moiety (where A ¼ NHþ 4 , Na and K ). The seeds for þ VII the ½Axþ1 Rex O4x series are the monomeric ½A2 ReVII O4 þ ions, while in the case of the ½Kxþ2 ðReV O3 ÞðReVII O4 Þx þ series it is the ½K2 ReV O3 þ ion. The polyoxorhenate frameworks of the polymeric þ ions observed in all three ½Axþ1 ReVII x O4x series can be represented by the structural formula ½ðOe Þi Te (where i ¼ 0–4, and O and T are octahedral and tetrahedral units, respectively). The ‘e’ refers to edge sharing of the
polyhedra, with the Aþ cations located in edge sharing or face capping positions on the ReO6 octahedra. For example, ½ðOe Þ3 Te represents a chain of three edgebridged ReO6 octahedra with a terminating tetrahedral unit, and represents the ion ½A5 ðReO4 Þ4 þ . Similarly, the series ½Kxþ2 ðReV O3 ÞðReVII O4 Þx þ can be described by Be ½ðOe Þi Te (where i ¼ 1–3). Here the symbols have the above meanings except that the symbol B represents a trigonal bipyramidal unit [which contains the singular Re(V)]. Again the cations, in this case Kþ , would be located in either edge bridging or face capping positions. It is likely that the cationic species observed in this study are Coulombically stabilized by the presence and spatial arrangement of the ammonium and alkali metal cations around the polyoxorhenate frameworks. There are several peaks not accounted for by the þ ½ðNH4 Þxþ1 ReVII x O4x (x ¼ 1–5) series in the ðNH4 ÞReO4 system. These include a group of three peaks at A, and
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F. Sahureka et al. / Inorganic Chemistry Communications 5 (2002) 23–27
Table 1 þ þ Positive ion ESI-MS data for AReO4 , where A ¼ NHþ 4 , Na and K ðCV ¼ þ20 VÞ Species
m=zcalc
m=zexpt
%RA
(a) ðNH4 ÞReO4 system ½ðNH4 Þ2 ðReO4 Þþ ½ðNH4 Þ3 ðReO4 Þ2 þ ½ðNH4 Þ4 ðReO4 Þ3 þ ½ðNH4 Þ5 ðReO4 Þ4 þ ½ðNH4 Þ6 ðReO4 Þ5 þ ½ðNH4 Þ2 ðH2 ReO5 Þþ ½ðNH4 Þ3 ðHReO5 Þþ ½ðNH4 Þ4 ðReO5 Þþ ½ðHÞ3 ðNH4 Þ2 ðReO4 Þ4 þ ½ðHÞ6 ðReO3 Þ2 ðReO4 Þ3 þ ½ðHÞ6 ðNH4 ÞðReO3 Þ2 ðReO4 Þ4 þ
287.0 556.0 822.9 1091.9 1360.9 305.0 321.0 338.0 1040.8 1226.7 1493.7
287.4 554.4 823.4 1092.3 1361.5 304.4 321.4 338.3 1040.2 1227.4 1494.9
100.0 60.2 49.1 31.4 13.4 31.7 40.8 18.3 9.6 6.7 4.1
(b) NaReO4 system ½Na2 ðReO4 Þþ ½Na3 ðReO4 Þ2 þ ½Na4 ðReO4 Þ3 þ ½Na5 ðReO4 Þ4 þ ½Na6 ðReO4 Þ5 þ
296.9 570.8 842.8 1116.7 1388.6
297.2 570.8 843.1 1116.7 1389.4
100.0 38.8 19.9 12.6 9.7
(c) KReO4 system ½K2 ðReO4 Þþ ½K3 ðReO4 Þ2 þ ½K4 ðReO4 Þ3 þ ½K5 ðReO4 Þ4 þ ½K6 ðReO4 Þ5 þ ½K2 ðReO3 Þþ ½K3 ðReO3 ÞðReO4 Þþ ½K4 ðReO3 ÞðReO4 Þ2 þ ½K5 ðReO3 ÞðReO4 Þ3 þ ½K6 ðReO3 ÞðReO4 Þ4 þ
328.9 618.8 906.7 1196.6 1486.5 312.9 602.8 890.7 1180.6 1468.5
329.2 618.6 906.7 1195.8 1487.5 313.1 601.2 889.8 1181.7 1468.6
100.0 25.5 32.6 9.0 7.6 12.8 12.7 7.6 10.2 11.6
the peaks labeled as B, C and D in Fig. 1(a). The group of three peaks labeled as A belong to the monomeric species ½ðNH4 Þ2 ðH2 ReO5 Þþ , ½ðNH4 Þ3 ðHReO5 Þþ and ½ðNH4 Þ4 ðReO5 Þþ . In concentrated HCl solution, ½ReO4 is known to generate the anion fac½ReVII O3 Cl3 2 [14]. The Re(VII)-containing units of the above three ions are thus assumed to have structures with a trigonal bipyramidal geometry, with three mutually arranged cis-oxo groups (as in fac½ReVII O3 Cl3 2 ), together with an axial and an equatorial group which may be oxo or protonated oxo (i.e. hydroxo) ligands depending on the cation. Of the three remaining peaks B, C and D, the first may be assigned as the tetrameric species ½ðHÞ3 ðNH4 Þ2 ðReVII O4 Þ4 þ . The two very low %RA peaks labeled as C and D cannot þ have the same type of combination of ðNHþ 4 =H Þ ca VII tion(s) to ½Re O4 anion composition based on their m=z values as does B. It is suspected that these species contain Re(V), as observed in the ESI+ spectra of the KReO4 system. Assuming the presence of two ½ReV O3 components in the composition of these ions, the two peaks could correspond to polymeric species such as ½ðHÞ6 ðReV O3 Þ2 ðReVII O4 Þ3 þ and ½ðHÞ6 ðNH4 ÞðReV O3 Þ2 ðReVII O4 Þ4 þ , although such an assignment without additional evidence must remain speculative at present.
Acknowledgements The acquisition of the electrospray mass spectrometer was made possible because of the support of the Australian Research Council, Research Infrastructure Equipment Facility Grant. Mr. Frans Sahureka wishes to acknowledge support from AUS aid for a University of Newcastle postgraduate scholarship and from Cenderwasih University for granting him leave to study abroad. We wish to thank Mr. Brian Mason of the AMSU for his helpful advice in obtaining the ESI+ spectra.
References [1] R. Colton, J.C. Traeger, Inorg. Chim. Acta 201 (1992) 153. [2] M.J. Deery, O.W. Howarth, K.R. Jennings, J. Chem. Soc., Dalton Trans. (1997) 4783. [3] D.K. Walanda, R.C. Burns, G.A. Lawrance, E.I. von NagyFelsobuki, J. Chem. Soc., Dalton Trans. (1999) 311. [4] D.K. Walanda, R.C. Burns, G.A. Lawrance, E.I. von NagyFelsobuki, Inorg. Chem. Commun. 2 (1999) 487. [5] D.K. Walanda, R.C. Burns, G.A. Lawrance, E.I. von NagyFelsobuki, J. Cluster Sci. (2000) 5. [6] D.K. Walanda, R.C. Burns, G.A. Lawrance, E.I. von NagyFelsobuki, Inorg. Chim. Acta 305 (2000) 118.
F. Sahureka et al. / Inorganic Chemistry Communications 5 (2002) 23–27 [7] C.S. Truebenbach, M. Houalla, D.M. Hercules, J. Mass Spectrom. 35 (2000) 1121. [8] O.W. Hand, S.M. Scheifers, R.G. Cooks, Int. J. Mass Spectrom. Ion Processes 78 (1987) 131. [9] F. Sahureka, R.C. Burns, E.I. von Nagy-Felsobuki, J. Mass Spectrom. 12 (2001) 1136. [10] M.T. Pope, Heteropoly and Isopoly Oxometalates, Springer, Berlin, 1983 (Chapter 8).
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[11] M. Moini, B.L. Jones, R.M. Rogers, L. Jiang, J. Am. Soc. Mass Spectrom. 9 (1998) 977. [12] B. Krebs, A. M€ uller, H.H. Beyer, Inorg. Chem. 8 (1969) 436. [13] H. Beyer, O. Glemser, B. Krebs, G. Wagner, Z. Anorg. Allg. Chem. 376 (1970) 87. [14] T. Lis, Acta Crystallogr. C 39 (1983) 961.
Isopolyoxorhenates observed by positive ion electrospray mass spectrometry Frans Sahureka, Robert C. Burns *, Ellak I. von Nagy-Felsobuki School of Biological and Chemical Sciences, Department of Chemistry, The University of Newcastle, Callaghan 2308, Australia Received 29 May 2001; accepted 22 October 2001
Abstract Unknown positive ion isopolyoxorhenates have been observed using electrospray ionization mass spectrometry (ESI+). The ESI+ studies of ammonium and alkali metal (Naþ and Kþ ) perrhenate salts in aqueous solution at pH 4.5 show the existence of the þ þ series ½Axþ1 ReVII (where x ¼ 1–5 and A ¼ NHþ and Kþ ). In the potassium perrhenate system, the series x O4x 4 , Na þ V VII ½Kxþ2 Re Rex O4xþ3 (x ¼ 0–4) has also been characterised. All of these four series have fAReO4 g as the aggregation unit. In the ammonium perrhenate system, the monomeric Re(VII)-containing species, ½ðNH4 Þ2 ðH2 ReO5 Þþ , ½ðNH4 Þ3 ðHReO5 Þþ and ½ðNH4 Þ4 ðReO5 Þþ were also detected. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrospray mass spectrometry; Isopolyoxorhenates; Polyoxometalates; Rhenium(VII)
1. Introduction Electrospray ionization mass spectrometry (ESI-MS) has been used to characterise isopoly- and heteropolyoxometalates of molybdenum, tungsten, vanadium and rhenium in both aqueous and non-aqueous solutions [1– 7]. In addition to known ions of the isopolyoxometalates that appear in the bulk, in some of these studies fragment ions as well as polymeric ions that had not been previously observed were identified in the gas phase. Thus, there is direct evidence that this technique has generated new species as a result of the evaporation/ fission process [3–6]. The formation of polymeric ions, both positively and negatively charged, has also provided some insight into how the oxometalates assemble into aggregates, and the nature of the polymerization units used in these assembly processes. There have been a sparse number of studies of perrhenate salts by mass spectrometry [7–9]. Secondaryion mass spectrometric studies of ammonium and alkali metal salts supported on graphite showed the existence of monomeric ions of the type ½A2 ReVII O4 þ in positive *
Corresponding author. Tel.: +61-249-215-479; fax: +61-249-215472. E-mail address: [email protected] (R.C. Burns).
ion studies, and both ½ReVII O4 and ½ReV O3 in negative ion studies, as well as ½H4 ReIII O4 when a NðC2 H4 OHÞ3 matrix was used [8]. Negative ion electrospray time-of-flight mass spectrometry was used to study ammonium perrhenate, as well as tungstate and vanadate, in aqueous solution under both acidic (pH 3) and basic (pH 10) conditions [7]. In that study only the ½ReVII O4 ion was observed [7]. More recently, negative ion electrospray quadrupole mass þ spectrometry was used to study three [NHþ 4 , Na and þ K ] perrhenate salts [9]. This study investigated the percentage relative abundance (%RA) of ½ReVII O4 , ½ReV O3 and ½ReIII O2 as a function of the cone voltage [9]. The essential requirements for the formation of both isopoly- and heteropoly-oxoanions have been discussed by Pope [10], and include the cation size (i.e. cation radius) and ability to act as a good acceptor of oxygen pp electrons. While Re(VII) has an ionic radius that falls within the accepted range for polyoxometalate formation, it was concluded that the high oxidation state of Re(VII) would preclude the formation of structures similar to those of known polyoxo-molybdates and -tungstates, and that structures based on corner- or edge-shared ReVII O6 octahedra would be positively charged. Moreover, while anion structures
1387-7003/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 7 0 0 3 ( 0 1 ) 0 0 3 3 5 - 5
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F. Sahureka et al. / Inorganic Chemistry Communications 5 (2002) 23–27
featuring fac-trioxo rhenium(VII) can be envisaged, they would most likely be unstable with respect to dissociation into monomeric ½ReO4 units [10]. Species containing lower oxidation states such as Re(VI) and Re(V) were considered possible, although such polynuclear species are very likely to disproportionate into ½ReO4 and other species containing Re(IV), a more stable oxidation state. As an extension of our previous electrospray mass spectrometric studies of the vanadate, molybdate and tungstate systems [3–6], we wish to report on the positive ion mode electrospray (ESI+) þ of the NHþ and Kþ perrhenate systems. This 4 , Na study will establish a number of previously unknown polyoxorhenate cationic species containing Re(VII) and Re(VII)/Re(V).
2. Experimental Ammoniun perrhenate (Aldrich, 99+%), sodium perrhenate (Aldrich, 99.99%) and potassium perrhenate (Aldrich, 99.0%) were used as received. Solutions of these compounds (10 3 M) were made up in Millipore Milli-Q deionised water and adjusted to a pH of 4.5 using AR grade acetic acid (0.1%). The ESI+ spectra were obtained using a VG Platform II single quadrupole mass spectrometer (Micromass, UK) coupled to a HPLC binary pump system. For all spectral acquisitions the capillary tip was set at a potential of +3.5 kV relative to ground. The cone voltage (CV) was set to either +20, +40 or +60 V relative to the skimmer, which was set to ground. The source temperature was at 353 K. A Rheodyne injector fitted with a 10 ll loop was used to inject the sample solution into the flow of the mobile phase (10 ll/min), the pH of which had been adjusted to a value of 4.5 using 0.1% acetic acid. All spectra were acquired between 50 and 1500 m=z, at a scan speed of 3.6 s/scan. Calibration of the m=z scale was accomplished using a 3.0 lg=ll solution of NaI in a 50:50 mixture of water and acetonitrile as the mobile phase. In the positive ion mode the spectra yielded NaI clusters of the form ½Naxþ1 Ix þ . All data were acquired and analysed using the Micromass Masslynx system. Tabulated values are based on the most abundant isotope of rhenium, i.e. 187 Re, and on peaks above 4%RA.
3. Results and discussion The ESI+ mass spectra of the perrhenate salts of þ þ NHþ 4 , Na and K at pH 4.5 and a CV of +20 V are shown in Fig. 1. The assignments of the peaks above 4%RA are given in Table 1. The ESI+ spectra measured at CV values of +40 and +60 V showed no significant variations in the %RA values.
The spectra show a series of singly charged ions for all three systems, with the most abundant peak (100%RA) in each case attributable to the monomeric þ þ species ½A2 ðReVII O4 Þþ , with A ¼ NHþ 4 , Na and K . þ VII For each of the systems the series ½Axþ1 Rex O4x (with x ¼ 1–5) was observed. In the potassium perrhenate system a second series was also present, which has the þ form ½Kxþ2 ReV ReVII x O4xþ3 (with x ¼ 0–4). The members of this latter series contained one Re(V) in addition to Re(VII) (the latter for x > 0). In general this series had much lower %RA values than those of the totally Re(VII)-containing series in this system. For all of the above series, the aggregation (or polymerization) unit is þ þ fAReO4 g, where A ¼ NHþ 4 , Na and K . This is the first time that isopolyoxorhenates have been observed either in bulk solution studies, or in gas phase studies under electrospray conditions. The appearance of the lower oxidation state [i.e. Re(V)] component in the þ ½Kxþ2 ReV ReVII series appears initially surprisx O4xþ3 ing. On closer examination of the ESI+ spectra of the isopolyoxovanadates, the presence of V(IV) in addition to V(V) was also found in the KVO3 system. This þ V yielded species of the type ½Kxþ3 ðVIV 2 O5 ÞðV O3 Þx (where x ¼ 1–6) [6]. No lower oxidation states were observed in any of the positive-ion series for the other alkali metal cations (Liþ and Naþ ), or in corresponding studies of the alkali metal salts of ½MoO4 2 and ½WO4 2 [3,5]. It is known that simple metal salts, such as NaI (used for the mass scale calibration) and both NaðCF3 COOÞ and NaðCH3 COOÞ [11] will generate both positive and negative ion series by aggregation as a result of the concentration effect, and this may also have occurred with the perrhenate salts used in the present study. However, such non-specific aggregation is usually observed with higher initial concentrations (e.g. NaI, 2 10 2 M) than was used for the perrhenate salts, and generally employs a mixed water–acetonitrile solvent system. More importantly, however, no negative ion þ series of the type ½Ax 1 ðReO4 Þx (A ¼ NHþ 4 , Na and þ K ) were observed under identical concentration and corresponding cone voltage conditions for any of the same three perrhenate salts, with only ½ReVII O4 , ½ReV O3 and ½ReIII O2 observable depending on the cone voltage [9]. Thus it would seem unlikely that the positively charged series found in the present study were generated as a result of non-specific aggregation processes. Although the oxo chemistry of Re(VII) is dominated by tetrahedrally coordinated ½ReO4 , six coordination is observed for Re(VII) in, for example, the structure of solid Re2 O7 [12] and in anhydrous perrhenic acid, ½O3 ReOReO3 ðH2 OÞ2 [13]. Thus the formation of an expanded coordination number for Re(VII) is well known, and would allow the formation of charged polymeric species containing Re(VII) with oxo bridging ligands.
F. Sahureka et al. / Inorganic Chemistry Communications 5 (2002) 23–27
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þ þ Fig. 1. ESI-MS spectra of the positive-ion species detected in the AReO4 systems (A ¼ NHþ 4 , Na and K ) at a pH of 4.5 and a CV value of +20 V: þ (a) NH4 ReO4 , (b) NaReO4 , and (c) KReO4 . The representation ½Knþ1 Ren O4n 1 þ is a contracted form for the ½Kxþ2 ReV ReVII x O4xþ3 series, where x ¼ n 1, and does not differentiate between Re(V) and Re(VII).
þ For all three ½Axþ1 ReVII series, where x O4x þ þ þ A ¼ NH4 , Na and K , plots of log(%RA) against the number of perrhenate aggregation units provide linear þ þ 2 relationships (NHþ 4 , R ¼ 0:950; Na , 0.949; K , 0.873). Previously, the linearity of such plots has been shown to be consistent with an addition polymerization mechanism [3]. In the present case the observed linear relationships again show that these three series are formed by a similar mechanism from the additive fAReO4 g þ þ moiety (where A ¼ NHþ 4 , Na and K ). The seeds for þ VII the ½Axþ1 Rex O4x series are the monomeric ½A2 ReVII O4 þ ions, while in the case of the ½Kxþ2 ðReV O3 ÞðReVII O4 Þx þ series it is the ½K2 ReV O3 þ ion. The polyoxorhenate frameworks of the polymeric þ ions observed in all three ½Axþ1 ReVII x O4x series can be represented by the structural formula ½ðOe Þi Te (where i ¼ 0–4, and O and T are octahedral and tetrahedral units, respectively). The ‘e’ refers to edge sharing of the
polyhedra, with the Aþ cations located in edge sharing or face capping positions on the ReO6 octahedra. For example, ½ðOe Þ3 Te represents a chain of three edgebridged ReO6 octahedra with a terminating tetrahedral unit, and represents the ion ½A5 ðReO4 Þ4 þ . Similarly, the series ½Kxþ2 ðReV O3 ÞðReVII O4 Þx þ can be described by Be ½ðOe Þi Te (where i ¼ 1–3). Here the symbols have the above meanings except that the symbol B represents a trigonal bipyramidal unit [which contains the singular Re(V)]. Again the cations, in this case Kþ , would be located in either edge bridging or face capping positions. It is likely that the cationic species observed in this study are Coulombically stabilized by the presence and spatial arrangement of the ammonium and alkali metal cations around the polyoxorhenate frameworks. There are several peaks not accounted for by the þ ½ðNH4 Þxþ1 ReVII x O4x (x ¼ 1–5) series in the ðNH4 ÞReO4 system. These include a group of three peaks at A, and
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F. Sahureka et al. / Inorganic Chemistry Communications 5 (2002) 23–27
Table 1 þ þ Positive ion ESI-MS data for AReO4 , where A ¼ NHþ 4 , Na and K ðCV ¼ þ20 VÞ Species
m=zcalc
m=zexpt
%RA
(a) ðNH4 ÞReO4 system ½ðNH4 Þ2 ðReO4 Þþ ½ðNH4 Þ3 ðReO4 Þ2 þ ½ðNH4 Þ4 ðReO4 Þ3 þ ½ðNH4 Þ5 ðReO4 Þ4 þ ½ðNH4 Þ6 ðReO4 Þ5 þ ½ðNH4 Þ2 ðH2 ReO5 Þþ ½ðNH4 Þ3 ðHReO5 Þþ ½ðNH4 Þ4 ðReO5 Þþ ½ðHÞ3 ðNH4 Þ2 ðReO4 Þ4 þ ½ðHÞ6 ðReO3 Þ2 ðReO4 Þ3 þ ½ðHÞ6 ðNH4 ÞðReO3 Þ2 ðReO4 Þ4 þ
287.0 556.0 822.9 1091.9 1360.9 305.0 321.0 338.0 1040.8 1226.7 1493.7
287.4 554.4 823.4 1092.3 1361.5 304.4 321.4 338.3 1040.2 1227.4 1494.9
100.0 60.2 49.1 31.4 13.4 31.7 40.8 18.3 9.6 6.7 4.1
(b) NaReO4 system ½Na2 ðReO4 Þþ ½Na3 ðReO4 Þ2 þ ½Na4 ðReO4 Þ3 þ ½Na5 ðReO4 Þ4 þ ½Na6 ðReO4 Þ5 þ
296.9 570.8 842.8 1116.7 1388.6
297.2 570.8 843.1 1116.7 1389.4
100.0 38.8 19.9 12.6 9.7
(c) KReO4 system ½K2 ðReO4 Þþ ½K3 ðReO4 Þ2 þ ½K4 ðReO4 Þ3 þ ½K5 ðReO4 Þ4 þ ½K6 ðReO4 Þ5 þ ½K2 ðReO3 Þþ ½K3 ðReO3 ÞðReO4 Þþ ½K4 ðReO3 ÞðReO4 Þ2 þ ½K5 ðReO3 ÞðReO4 Þ3 þ ½K6 ðReO3 ÞðReO4 Þ4 þ
328.9 618.8 906.7 1196.6 1486.5 312.9 602.8 890.7 1180.6 1468.5
329.2 618.6 906.7 1195.8 1487.5 313.1 601.2 889.8 1181.7 1468.6
100.0 25.5 32.6 9.0 7.6 12.8 12.7 7.6 10.2 11.6
the peaks labeled as B, C and D in Fig. 1(a). The group of three peaks labeled as A belong to the monomeric species ½ðNH4 Þ2 ðH2 ReO5 Þþ , ½ðNH4 Þ3 ðHReO5 Þþ and ½ðNH4 Þ4 ðReO5 Þþ . In concentrated HCl solution, ½ReO4 is known to generate the anion fac½ReVII O3 Cl3 2 [14]. The Re(VII)-containing units of the above three ions are thus assumed to have structures with a trigonal bipyramidal geometry, with three mutually arranged cis-oxo groups (as in fac½ReVII O3 Cl3 2 ), together with an axial and an equatorial group which may be oxo or protonated oxo (i.e. hydroxo) ligands depending on the cation. Of the three remaining peaks B, C and D, the first may be assigned as the tetrameric species ½ðHÞ3 ðNH4 Þ2 ðReVII O4 Þ4 þ . The two very low %RA peaks labeled as C and D cannot þ have the same type of combination of ðNHþ 4 =H Þ ca VII tion(s) to ½Re O4 anion composition based on their m=z values as does B. It is suspected that these species contain Re(V), as observed in the ESI+ spectra of the KReO4 system. Assuming the presence of two ½ReV O3 components in the composition of these ions, the two peaks could correspond to polymeric species such as ½ðHÞ6 ðReV O3 Þ2 ðReVII O4 Þ3 þ and ½ðHÞ6 ðNH4 ÞðReV O3 Þ2 ðReVII O4 Þ4 þ , although such an assignment without additional evidence must remain speculative at present.
Acknowledgements The acquisition of the electrospray mass spectrometer was made possible because of the support of the Australian Research Council, Research Infrastructure Equipment Facility Grant. Mr. Frans Sahureka wishes to acknowledge support from AUS aid for a University of Newcastle postgraduate scholarship and from Cenderwasih University for granting him leave to study abroad. We wish to thank Mr. Brian Mason of the AMSU for his helpful advice in obtaining the ESI+ spectra.
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