Electrospray ionization and tandem mass spectrometry characterization of novel heterotrimetallic Ru(η5-C5H5)(dppf) SnX3 complexes and their heterobimetallic Ru(η5-C 5H5)(dppf)X p

Article abstract of DOI:10.1016/j.poly.2005.03.053

Electrospray ionization and tandem mass spectrometry were used to characterize some novel heterotrimetallic complexes Ru(η⁵-C ₅H₅)(dppf)SnX₃ (1) (dppf = diphenylphosphine ferrocene; X = Cl, Br) and their heterob

Full text of DOI:10.1016/j.poly.2005.03.053

Polyhedron 24 (2005) 1153–1159
www.elsevier.com/locate/poly
Electrospray ionization and tandem mass spectrometry
characterization of novel heterotrimetallic Ru(g5-C5H5)(dppf)SnX3
complexes and their heterobimetallic Ru(g5-C5H5)(dppf)X precursors
a
Lilian A. Paim , Daniella V. Augusti , Ilza Dalmazio , Tania M. de A. Alves ,
a
a
b
´
ˆ
a,
a,
*
Rodinei Augusti *, Helmuth G.L. Siebald
a
´
Departamento de Quımica, Universidade Federal de Minas Gerais – UFMG, Avenue Antonio Carlos 6627, Belo Horizonte, MG 31270-901, Brazil
´
Centro de Pesquisas Rene Rachou, Fundac¸a˜o Oswaldo Cruz, Belo Horizonte, MG, 30190-002, Brazil
b
Received 16 December 2004; accepted 3 March 2005
Available online 25 May 2005
Abstract
Electrospray ionization and tandem mass spectrometry were used to characterize some novel heterotrimetallic complexes Ru(g5-
C5H5)(dppf)SnX3 (1) (dppf = diphenylphosphine ferrocene; X = Cl, Br) and their heterobimetallic precursors Ru(g5-C5H5)(dppf)X
(2) (X = Cl, Br, I, N3). The positive-ion ESI mass spectra of 1 and 2 in acetonitrile solution showed the predominant presence of the
[Ru(g5-C5H5)(dppf)]+ cation (3) of m/z 721. In the positive-ion ESI mass spectrum of the complex Ru(g5-C5H5)(dppf)N3 (2d),
another prominent cation of m/z 735, i.e. [Ru(g5-C5H5)(dppf)N]+ (4), was also observed. Its formation was proposed to occur
via the oxidation of the ruthenium atom (RuII ! RuIII) followed by the loss of N2. The positive-ion ESI tandem mass spectra of
ions 3 and 4 showed mainly fragments formed by neutral losses of C5H5, C6H5 and P(C6H5)2. Finally, cation 3 was proposed to
be the catalytic active species in the single-step conversion of methanol to acetic acid.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Keywords: Ru–Sn complexes; Electrospray ionization; Ru–Sn–Fe complexes; Tandem mass spectrometry; Transient ionic species; Catalytic activity
1. Introduction
(ESI-MS2) have been mainly and most successfully ap-
plied to the analysis of biomolecules [5–12], they have
been increasingly used as a powerful structural charac-
terization technique of transient intermediates [5–9], as
well as in the study of intrinsic coordination catalysis
[10,11]. Electrospray ionization can also be used to ob-
serve ionic species in solution, even in very low concen-
trations, which are difficult or impossible to detect by
other techniques, such as electrochemistry and multi
NMR (nuclear magnetic resonance) spectroscopy
[2,12,13].
Electrospray ionization (ESI) is a soft ionization tech-
nique that has revolutionized the way molecules are ion-
ized and transferred to the gas phase [1]. The ability to
transfer an ion from solution to the gas phase makes
ESI an ideal means to study ionic compounds. This
was recognized in a very early stage of the development
of ESI, when numerous inorganic salts were conve-
niently characterized [2–4]. Although electrospray mass
spectrometry (ESI-MS) and tandem mass spectrometry
The purpose of synthesizing heteropolymetallic com-
plexes bearing ferrocene backbones is that the coexis-
*
Corresponding authors. Tel.: +5531 34995734; fax: +5531
tence of two or more metal atoms inside
a
34995700 (H.G.L. Siebald); Tel.: +5531 34995725 (R. Augusti).
E-mail addresses: augusti@ufmg.br (R. Augusti), hsiebald@netuno.
lcc.ufmg.br (H.G.L. Siebald).
coordination compound in principle would enable cre-
ating conditions for delicate and ‘‘tunable’’ mutual
0277-5387/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.03.053

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L.A. Paim et al. / Polyhedron 24 (2005) 1153–1159
cooperative effects of the metal centers in their chemi-
cal interaction with external substrates. Furthermore,
this kind of transition metal derivative possesses
uncommon catalytic properties, as for instance in the
cross-coupling of Grignard reagents of organic bro-
mides and allylic alcohols [14].
Although ESI-MS and ESI-MS2 have been increas-
ingly used as powerful techniques for structural charac-
terization of organometallic and coordination
compounds [2,5,15–24], no similar studies on the charac-
terization of heteropolymetallic complexes, especially
those containing a Ru–Sn bond, have been reported.
Therefore, the main purpose of this paper is to charac-
terize the novel heterotrimetallic RuCp(dppf)SnX3 com-
plexes (1) as well as their heterobimetallic RuCp(dppf)X
precursors (2) by ESI-MS and ESI-MS2.
2. Experimental
Complexes 1 [25] and 2 [26] were synthesized follow-
ing previously described procedures. Acetonitrile solu-
tions of 1 and 2 were prepared in a concentration of
2 · 10ꢀ5 mol Lꢀ1. All experiments were performed using
a commercial LCQ Advantage ion trap mass spectrom-
eter (ThermoElectron, San Jose, CA) equipped with an
ESI source and operated in the positive ion mode. The
spectra shown represent the average of about 50 scans
(0.2 s/scan). Samples were infused into the ESI source
Scheme 1.
2c (not shown). Through ESI, the cationic species
[Ru(C5H5)(dppf)]+ (3) (Scheme 1) is transferred from
solution to the gas phase and detected by mass spec-
trometry. It is likely that the solution contains mainly
neutral 2a, which is in equilibrium with 3 and Clꢀ. Be-
cause ruthenium displays seven abundant isotopes,
with a syringe pump at a flow rate of 3–5 lL minꢀ1
.
Typical ESI conditions were as follows: capillary tem-
perature, 150 ꢁC; sheath gas (N2) flow rate, 20 units
(roughly 0.50 L minꢀ1); spray voltage 4.5 kV; capillary
voltage 25 V; tube lens offset voltage, 25 V. In the MSn
operation mode, the parent ion of interest was first iso-
lated by applying an appropriate waveform across the
end cap electrodes of the ion trap to eject all trapped
ions by resonance, except those with the m/z ratio of
interest. The isolated ions were then subjected to a sup-
plementary ac signal for excitation by resonance and
collision-induced dissociation (CID). The collision en-
ergy was set to a value at which product ions were pro-
duced in measurable amounts, i.e. ca. 10%. The isolation
width used in the MSn experiments was 2 units.
[
104Ru (18.6%), 102Ru (31.6%), 101Ru (17.1%), 100Ru
(12.6%), 99Ru (12.7%), 98Ru (1.86%)], 96Ru (5.54%)]
and iron possesses four isotopes [58Fe (0.28%), 57Fe
(2.1%), 56Fe (91.72%) 54Fe (5.9%)] [27], cation 3 is de-
tected by MS as an isotopomeric cluster of singly
charged ions centered at m/z 721 (for 102Ru and 56Fe),
and with an isotopic pattern that matches well to that
calculated [28] for C39H33 P2RuFe (Fig. 1). In solution,
3 associates with acetonitrile forming [3 + CH3CN]+,
which is transferred to the gas phase by ESI and de-
tected by MS as a solvent adduct ion centered at m/z
762 (Fig. 1).
Fig. 2 shows the positive-ion ESI tandem mass spec-
trum at m/z 721, which is the highest abundance isotope
of structure 3. Upon CID, prominent fragments of m/z
657 ([3 – C5H4]+), 644 ([3 – C6H5]+), 580 ([3 – C6H5–
C5H4]+), 536 ([3 – P(C6H5)2]+) and 305 ([Fe(C5H4)P-
(C6H5)2]+) were observed, which is consistent with the
structure proposed for 3 (Scheme 1).
The spectrum of 2d (Fig. 3) obtained under the same
electrospray conditions displays two major cations: 3
and [Ru(C5H5)(dppf)N]+ (4). The latter was detected
by MS as an isotopomeric cluster of singly charged ions
centered at m/z 735 (for 102Ru and 56Fe), and with an
3. Results and discussion
Compounds 1 [25] and 2 [26] (Scheme 1) have already
been fully characterized both in the solid state and in
solution, which facilitates the comparison of their struc-
tural solution NMR patterns with ESI-MS and ESI-
MS2.
Fig. 1 shows the ESI(+) mass spectrum of complex 2a
in acetonitrile solution. Very similar ESI(+) mass spec-
tra were obtained for the analogous complexes 2b and

L.A. Paim et al. / Polyhedron 24 (2005) 1153–1159
1155
Fig. 1. Positive-ion ESI mass spectrum of the complex [Ru(C5H5)(dppf)]Cl (2a) in acetonitrile solution. Insets show the ion intensities and the m/z
distributions (experimental and calculated [28]) of cation 3 (C39H33P2RuFe).
ruthenium atom of 2d (RuII ! RuIII) followed by the re-
lease of a nitrogen molecule (Scheme 2). We assume that
this initial oxidation contributes to the weakening of the
N–N bond in the azide moiety, which ultimately causes
the formation of cation 4. In this cation, the remaining
nitrogen stays connected to the ruthenium atom, likely
via a double Ru–N bond (Scheme 1). This occurs be-
cause the filled ruthenium dyz orbital is available to
interact with the empty pz nitrogen orbital, thus allow-
ing the formation of a p Ru ! N bond. Moreover, the
extra empty orbital in this N atom and its proximity
with the Fe atom could suggest a Ru–N–Fe interaction,
as represented in Scheme 2. However, additional studies
are required to confirm this hypothesis.
Oxidation processes in electrospray ion sources are
well known and can occur under certain experimental
conditions [29–33]. Similar oxidation processes were
Fig. 2. Positive-ion ESI mass spectrum of mass-selected cation 3 (ionic
cluster centered at m/z 721). For more details on the ‘‘empty and filled
circle’’ notation, see [49].
also verified to take place for the analogous bimetallic
ruthenocenyl ruthenium complex [Ru(C = CHPh)(g5-
C5H5)(dppr)]Cl (dppr = [Ru(g5-C5H4PPh2)2]). In its po-
sitive-ion ESI mass spectrum, beside an intense fragment
related to the singly charged cation [Ru(C = CHPh)(g5-
C5H5)(dppr)]+, a doubly charged cation was also de-
tected at low cone voltages. However, its formation
was attributed to the oxidation of the dppr-ruthenium
moiety, giving rise to a complex with Ru present
in two different oxidation states (RuII and RuIII) [19].
Based on this example, we can also consider the
isotopic pattern that fits well to that calculated for
C39H33P2NRuFe [28]. In the positive-ion ESI tandem
mass spectrum of the mass-selected ion 4 (not shown),
the following most intense fragments are observed: [4
– C5H4] (m/z 671), [4 – C6H5] (m/z 658), [4 – FeC5H4]
(m/z 615) and [4 – (C6H5)3] (m/z 504).
The formation of ion 4 is suggested to occur in the
electrospray ion source via an initial oxidation of the

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L.A. Paim et al. / Polyhedron 24 (2005) 1153–1159
Fig. 3. Positive-ion ESI mass spectrum of the acetonitrile solution of complex [Ru(C5H5)(dppf)]N3 (2d). Insets show the ion intensities and the m/z
distributions (experimental and calculated [28]) of cation 4 (C39H33P2NRuFe).
Scheme 2.
alternative oxidation of the ferrocene centre in 2d
(Fe2+ ! Fe3+), a well known and facile process, to yield
the cation of m/z 735 after the release of N2. Although it
is difficult to distinguish between both oxidation pro-
cesses, i.e. Ru2+ ! Ru3+ or Fe2+ ! Fe3+, by using only
mass spectrometry, electrochemical measurements could
elucidate this issue. In fact, cyclic voltammetry experi-
ments are underway in our laboratory and will be the
subject of a future publication.
Fig. 4 shows the positive-ion ESI mass spectrum of
complex 1a in acetonitrile solution (a similar mass spec-
trum was obtained for the analogous complex 1b). As
previously verified for complex 2a, cation 3 (m/z 721)
is by far the predominant species. Thus, it is likely that
in acetonitrile solution, 1a is in equilibrium with 3 and
its negative counterpart SnCl3ꢀ. This is an unexpected
result since the presence of electronegative substituents
(such as Cl, Br) connected to Sn, which promote the for-
mation of a p RuSn bond and contribute to increase the
order of this bond, should impede, or at least strongly
inhibit, such a dissociation. Other minor clusters cen-
tered at m/z 756, [Ru(C5H5)(dppf)Cl]+ (5), m/z 911,
[Ru(C5H5)(dppf)SnCl2]+ (6) and m/z 946, [Ru(C5H5)-
(dppf)SnCl3]+ (7) are also observed in this spectrum.
The formation of these cations is tentatively explained
in Scheme 3. For instance, cation 5 is likely formed
via the previously described RuII ! RuIII oxidation fol-
lowed by the loss of a neutral SnCl2 molecule. On the

L.A. Paim et al. / Polyhedron 24 (2005) 1153–1159
1157
Fig. 4. Positive-ion ESI mass spectrum of the acetonitrile solution of complex [Ru(C5H5)(dppf)]SnCl3 (1a). Insets show the ion intensities and the m/z
distributions (experimental and calculated [28]) of cation 6 (C39H33P2Cl2 SnRuFe).
other hand, cation 6 is produced via a simple release of
Clꢀ from 1a, whereas cation 7 corresponds to the singly
charged intact complex 1a, likely formed upon
RuII ! RuIII oxidation. The experimental isotopic pat-
terns of these cations fit well to those calculated for
C39H33P2ClRuFe (5), C39H33P2Cl2SnRuFe (6) (Fig. 4)
and C39H33P2Cl3SnRuFe (7) [28]. The mass-selection
and CID of 5 characteristically display the loss of a chlo-
rine atom, giving rise to the fragment of m/z 721 as cat-
ion 3 (spectrum not shown). The positive-ion ESI
tandem mass spectrum of the mass-selected ion 6 (not
shown) furnishes exclusively cation 3 (m/z 721) via the
release of a neutral SnCl2 molecule. Finally, the mass
selection and fragmentation of ion 7 also yields exclu-
Scheme 3.

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L.A. Paim et al. / Polyhedron 24 (2005) 1153–1159
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to acetic acid [38]. The mechanism of this reaction
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understood. Thus, taking into account that ESI is able
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44], and most importantly, the composition of ESI-gen-
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[5,16,17,41,45,46], we hypothesize that cation 3,
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ESI proved to be a suitable and efficient technique for
the characterization of heterotrimetallic Ru(g5-
C5H5)(dppf)SnX3 (1) complexes, containing a Ru–Sn
bond, and their heterobimetallic Ru(g5-C5H5)(dppf)X
(2) precursors. The formation of cation 3, likely the cat-
alytically active species in the single step conversion of
methanol to acetic acid, from 2 and 1 seems to be a gen-
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This work had financial support from FAPEMIG
`
(Fundac¸a˜o de Amparo a Pesquisa do Estado de Minas
Gerais, Brazil) and CNPq (Conselho Nacional de
´ ´
Desenvolvimento Cientıfico e Tecnologico, Brazil).
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