Redox chemistry in thin layers of organometallic complexes prepared using ion soft landing

Abstract

Soft landing (SL) of mass-selected ions is used to transfer catalytically-active metal complexes complete with organic ligands from the gas phase onto an inert surface. This is part of an effort to prepare materials with defined active sites and thus achieve molecular design of surfaces in a highly controlled way. Solution-phase electrochemical studies have shown that V^IVO(salen) reacts in the presence of acid to form V^VO(salen)⁺ and the deoxygenated V^III(salen)⁺ complex—a key intermediate in the four electron reduction of O₂ by vanadium–salen. In this work, the V^VO(salen)⁺ and [Ni^II(salen) + H]⁺ complexes were generated by electrospray ionization and mass-selected before being deposited onto an inert fluorinated self-assembled monolayer (FSAM) surface on gold. A time dependence study after ion deposition showed loss of O from V^VO(salen)⁺ forming V^III(salen)⁺ over a four-day period, indicating a slow interfacial reduction process. Similar results were obtained when other protonated molecules were co-deposited with V^VO(salen)⁺ on the FSAM surface. In all these experiments oxidation of the V^III(salen)⁺ product occurred upon exposure to oxygen or to air. The cyclic regeneration of V^VO(salen)⁺ upon exposure to molecular oxygen and its subsequent reduction to V^III(salen)⁺ in vacuum completes the catalytic cycle of O₂reduction by the immobilized vanadium–salen species. Moreover, our results represent the first evidence of formation of reactive organometallic complexes on substrates in the absence of solvent. Remarkably, deoxygenation of the oxo-vanadium complex, previously observed only in highly acidic non-aqueous solvents, occurs on the surface in the UHV environment using an acid which is deposited into the inert monolayer. This acid can be a protonated metal complex, e.g. [Ni^II(salen) + H]⁺, or an organic acid such as protonated diaminododecane.

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Introduction

Thin films of organometallic complexes on substrates play important roles in a variety of applications in heterogeneous catalysis and materials science.^1–5 Preparation of catalytic substrates by immobilization of organometallic complexes on surfaces is advantageous because it combines the high selectivity of solution-phase catalysts with the ease of separation of products from catalyst that characterizes heterogeneous catalysts. Furthermore, enhanced catalytic activity observed for a number of organometallic complexes immobilized on surfaces indicates a clear pathway for controlled preparation of highly selective catalysts by proper tuning of molecule–substrate interactions.² Other applications involving immobilization of organometallic complexes include synthesis of hybrid organic–inorganic polymers using molecular layer deposition⁶ and interface synthesis of layers for light-emitting diodes or coatings for biocompatible materials.³

Preparation of organometallic films typically involves chemical vapor deposition or self-assembly from solution.⁷ Here we investigate the use of soft landing (SL) of mass-selected ions, i.e. ions of well-controlled mass, and hence chemical composition to create organometallic films. Ion SL instrumentation and various applications have been reviewed.^8,9 The SL methodology is used to prepare high purity materials with well-defined structures and active sites and thus to achieve molecular design of surfaces in a highly controlled way. The ion deposition approach has advantages in relation to solution-phase techniques, avoiding clumping of materials seen as solvent evaporates and allowing deposition of uniform films while preserving the structure of deposited molecules.^10–15

Our experiments present a first step towards the development of catalytically active organometallic materials using SL and also extend the work of others^16–21 on ion deposition as an approach to catalyst preparation. Metal–salen complexes [salen = N,N′-ethylenebis(salicylideneaminato) ligand] were chosen as model systems in this study because they are effective homogeneous catalysts for the epoxidation of a variety of non-functionalized olefins with high levels of enantioselectivity,²² for ring-opening polymerization of six-membered cyclic carbonates,²³ for the oxygenation of heteroatom containing organic compounds²⁴ and for the hydroxylation of organic substrates.²⁵ In a previous study, we showed that metal–salen complexes display redox activity in the gas phase.²⁶ The protonated mixed complex [Ni^IIV^IVO(salen) + H]⁺, generated by atmospheric pressure thermal desorption ionization, was found to undergo formal oxidation (hydrogen atom loss) to produce Ni(salen) monomer as the molecular radical cation upon collision-induced dissociation (CID). In this paper, we study redox chemistry in thin layers of vanadium–salen complexes initiated by the presence of a proton donor deposited from the gas-phase.

First studies of the reactivity of vanadium–salen complexes in strongly acidic solution were focused on understanding of vanadium bioaccumulation in blood cells of ascidians.²⁷ Bonadies et al. demonstrated that in an acidic solution V^IVO(salen) undergoes a disproportionation reaction shown in eqn (1):²⁸

2V^IVO(salen) + 2H⁺ → V^III(salen)⁺ + V^VO(salen)⁺ + H₂O (1)

The V^III(salen)⁺ species is readily oxidized upon addition of oxygen forming V^VO(salen)⁺. Liu and Anson proposed that V^III(salen)⁺ is a key intermediate in the four-electron reduction of O₂ by vanadium–salen:²⁹

2V^III(salen)⁺ + O₂ → 2V^VO(salen)⁺ + 4e⁻ (2)

Structures and reactivity of vanadium–salen complexes have been extensively studied with an emphasis on understanding their redox properties for applications in electrocatalysis.³⁰ Tsuchida et al. examined the coordination and redox chemistry of the complexes of V(III), V(IV), and V(V) with the salen ligand using electrochemical and spectroscopic measurements.³¹ They proposed the formation of a dinuclear [V^IVOV^IV(salen)]₂ complex as an intermediate in the deoxygenation reaction of V^IVO(salen). Further studies demonstrated an enhanced—i.e. cooperative—reactivity of the dinuclear complex towards O₂ that results in formation of a [(salen)V^IVOV^VO(salen)]⁺ dimeric product.³² Subsequent proton-initiated deoxygenation of this complex completes the catalytic cycle of the four-electron reduction of O₂.

In this study we examine the immobilization and reactivity of V^VO(salen)⁺ deposited on an inert fluorinated self-assembled monolayer surface (FSAM) on gold using SL of mass-selected ions. [We use the term immobilization to indicate that the landed species are bound to the surface strongly enough that desorption from the substrate is very slow.] Ions were generated by electrospray ionization (ESI) and guided to a mass analyzer where species with m/z values corresponding to the desired chemical entities were selected and gently deposited onto an FSAM surface. SAM surfaces are convenient substrates, the chemical properties of which can be easily tailored.³³ It has been demonstrated that FSAM surfaces prevent neutralization of immobilized ions³⁴ including metal–salen complexes that undergo very slow charge reduction through electron transfer to the surface.³⁵ SL also allows convenient co-deposition of protonated ions which serve as proton donors on the surface enabling investigation of acid-mediated reactivity. Secondary ion mass spectrometry (SIMS) was chosen for the analysis of the SL prepared surfaces both before and after exposure to gaseous reagents. We demonstrate facile proton-initiated reduction of V^VO(salen)⁺ to V^III(salen)⁺ in the absence of solvent and regeneration of V^VO(salen)⁺ by exposure to O₂ (Scheme 1). The role of a proton donor is examined by depositing protonated [Ni_II(salen) + H]⁺ complexes or protonated diaminododecane molecules into the SAM containing the V^VO(salen)⁺ species.

Scheme 1

Experimental

The kinetics of transformation and reactivity of metal–salen complexes immobilized by soft-landing were examined using FT-ICR SIMS and TOF-SIMS as described below. The differences in the methods employed for SIMS analysis, viz. FT-ICR and TOF-SIMS discussed in detail in ref. 36, have no effect on the conclusions of this study.³⁶

FT-ICR SIMS experiments

The kinetics of transformation of soft-landed V^VO(salen)⁺ in the presence of the [Ni^II(salen) + H]⁺ was examined using 8 keV Cs⁺ SIMS experiments in an FT-ICR mass spectrometer specially designed for studying ion–surface interactions.^37,38 In these experiments [Ni^II(salen) + H]⁺ and V^VO(salen)⁺ were simultaneously deposited from a mixed ion beam on the FSAM surface positioned at the rear trapping plate of the ICR cell. The experimental approach for SL studies has been described in detail elsewhere.^36–39 Briefly, ions were produced in a high-transmission electrospray source, efficiently thermalized in the collision quadrupole and mass-selected prior to acceleration and collision with the surface. Ion kinetic energy was controlled by varying the voltage difference between the collisional quadrupole of the ion source and the surface and was maintained at 20 eV in these experiments. During ion SL the surface positioned at the rear trapping plate of the ICR cell was exposed to a continuous beam of mass-selected ions. A relatively wide mass window of ca. 15 amu was used for co-deposition of [Ni^II(salen) + H]⁺ and V^VO(salen)⁺. The solution composition was optimized to generate a 1 ∶ 1 mixture of metal–salen complexes in the gas phase. Typical ion current of 7 pA was delivered onto a ca. 3.5 mm diameter spot on the target. The deposition time was 325 min. The maximum coverage of soft-landed ions obtained in these experiments does not exceed 15% of a monolayer, suggesting that the total ion dose is well below the SIMS saturation threshold.³⁷

In situ analysis of surfaces following SL was performed by combining 8 keV Cs⁺ secondary ion mass spectrometry with FT-ICR detection of the sputtered ions (FT-ICR-SIMS) as described elsewhere.^37–39 Static SIMS conditions with a total ion flux of about 2 × 10¹⁰ ions per cm² (current 4 nA, duration 80 μs, spot diameter 4.6 mm, 15 shots per spectrum, 100–200 data points) were used in these experiments. Data acquisition was accomplished using a MIDAS data station.⁴⁰

TOF-SIMS experiments

Ion deposition combined with TOF-SIMS analysis was performed using two different instruments. Initial soft-landing experiments followed by ex situTOF-SIMS characterization were performed using the apparatus described elsewhere⁴¹ while subsequent in situTOF-SIMS experiments were conducted using the ion deposition chamber coupled to a commercial TOF-SIMS instrument (PHI THIFT II, Physical Electronics, Eden Prairie, MN) that enables characterization of surfaces modified by soft-landing without breaking vacuum.³⁶ In both instruments ions were generated and deposited on surfaces as described earlier.

For ex situTOF-SIMS experiments mass-selected ion currents of 300 pA and 200 pA were obtained for V^VO(salen)⁺ and [Ni^II(salen) + H]⁺, respectively. During the deposition (deposition times of 20 min and 30 min for V^VO(salen)⁺ and [Ni^II(salen) + H]⁺, respectively) 2 × 10¹² ions of each species were delivered onto a 5 mm diameter spot on the target. In situTOF-SIMS experiments utilized a different apparatus, in which 10¹² ions of V^VO(salen)⁺ and [Ni^II(salen) + H]⁺ were deposited onto a 3 mm spot. Ion currents of 50 pA and 20 pA were obtained and deposition times of 60 min and 120 min were used for V^VO(salen)⁺ and [Ni^II(salen) + H]⁺, respectively.

In these experiments sequential deposition of metal–salen complexes on the FSAM surface was followed by characterization using 15 keV Ga⁺ SIMS and subsequent reactivity studies. Ex situTOF-SIMS analysis was performed after a brief exposure of surfaces to laboratory air because it took approximately 10 min to remove the sample from the soft-landing apparatus and introduce it into the TOF-SIMS.⁴¹In situTOF-SIMS experiments were performed ca. 5 min after soft-landing—the time required to transfer the surface from the ion deposition apparatus into the TOF-SIMS and position it on the stage. ESI conditions for ion soft-landing experiments were as follows: spray voltage, 3 kV; flow rate, 30 μL h⁻¹; capillary temperature, 160°.

Self-assembled monolayer (SAM) surface

FSAM surfaces were prepared on gold coated silicon wafers. FT-ICR-SIMS experiments utilized a gold coated silicon wafer (5 nm chromium adhesion layer and 100 nm of polycrystalline vapor-deposited gold) from SPI Supplies (Westchester, PA) that was custom laser cut into 4.8 mm diameter substrates by Delaware Diamond Knives (Wilmington, DE). For TOF-SIMS experiments 10 × 10 mm gold substrates (525 μm thick Si, 50 Å Ti adhesion layer, 1000 Å Au layer) were purchased from Platypus Technologies (Madison, WI). 1H, 1H, 2H, 2H-perfluorodecanethiol was purchased from Sigma-Aldrich (St. Louis, MO) and used to form the FSAM surface by exposure of the gold surface to a 1 mM ethanol solution of the thiol for at least 12 h. The surface was removed from the thiol solution, ultrasonically washed in ethanol for 5 min to remove extra layers of the reagent, and dried under nitrogen gas before being introduced into the instrument.

Chemicals

Ni(salen) and diaminododecane were purchased from Sigma-Aldrich (St. Louis, MO); RGDGG was purchased from Peptron. V^VO(salen) was prepared according to the method reported by Choudhary et al.⁴² The metal–salen complexes were dissolved in a MeOH/H₂O spray solvent to a concentration of 10⁻⁴ M. The solutions were analyzed using an HCT-Ultra ion trap (Bruker Daltonics, Billerica). Typical ESI mass spectra are shown in Fig. S1 (ESI†).

Results and discussion

Evidence for V^VO(salen)⁺ reduction to V^III(salen)⁺

Ex situ TOF-SIMS characterization was performed to examine the reactivity of V^VO(salen)⁺ and [Ni^II(salen) + H]⁺ soft landed onto FSAM surfaces. Fig. 1 compares TOF-SIMS spectra obtained in two separate experiments, in which similar amounts of V^VO(salen)⁺ and [Ni^II(salen) + H]⁺ were deposited onto different FSAM surfaces. In this figure the TOF-SIMS spectrum of the FSAM surface prior to ion deposition is shown as a black trace. The TOF-SIMS spectrum obtained following SL of V^VO(salen)⁺ contains an abundant peak of V^VO(salen)⁺ at m/z 333, a peak at m/z 666 corresponding to the [V^IVOV^V(salen)₂]⁺ dimer ion, and a minor peak at m/z 317 assigned as V^III(salen)⁺. In this spectrum the small abundance of V^III(salen)⁺ is most likely produced as a result of gas-phase fragmentation of vibrationally excited V^VO(salen)⁺ species in the SIMS plume. The TOF-SIMS spectrum obtained following SL of [Ni^II(salen) + H]⁺ contains the [Ni^II(salen) + H]⁺ ion at m/z 325, an abundant Ni^III(salen)⁺ ion, the protonated [Ni^II₂(salen)₂ + H]⁺ dimer at m/z 649, and a number of minor features in the trimer region.

Fig. 1 Comparison of ex situTOF-SIMS spectra obtained after the deposition of 2.2 × 10¹² ions of VO(salen)⁺ (blue) and 1.6 × 10¹² ions of [Ni(salen) + H]⁺ (red) on the FSAM surface in two separate experiments; black trace corresponds to the FSAM background.

Sequential deposition of V^VO(salen)⁺ and [Ni^II(salen) + H]⁺ onto the same FSAM surface yields a TOF-SIMS spectrum shown as the red trace in Fig. 2 that closely resembles the superposition of individual V^VO(salen)⁺ and [Ni^II(salen) + H]⁺TOF-SIMS spectra shown in Fig. 1. However, the TOF-SIMS spectrum of the same surface re-examined after 4 days in UHV (blue trace in Fig. 2) shows reactivity of the deposited molecules, while the spectra of the individually deposited V^VO(salen)⁺ and [Ni^II(salen) + H]⁺ do not change a great deal with time (data not shown). This reactivity is manifested by substantial increases in the abundances of V^III(salen)⁺ (m/z 317), [V^IVOV^III(salen)₂]⁺ (m/z 650), [Ni^IIV^III(salen)₂]⁺ (m/z 641), [Ni₂V(salen)₃ + 3H]⁺ (m/z 968), and [Ni₂VO(salen)₃ + 2H]⁺ (m/z 983) ions in the TOF-SIMS spectrum recorded four days after ion deposition. In this paper we will limit the discussion to the monomeric species observed in SIMS spectra. Deposition and analysis of dimer ions will be discussed in a separate publication.

Fig. 2 Ex situ TOF-SIMS characterization of the FSAM surface before the deposition (black), 10 min after the sequential deposition of 2.5 × 10¹² ions of VO(salen)⁺ and 2.5 × 10¹² ions of [Ni(salen) + H]⁺ on the FSAM surface (red), and 4 days later (blue).

The observed increase in the abundance of V^III(salen)⁺ with time and the concomitant decrease in the abundance of V^VO(salen)⁺ could be attributed to a slow interfacial reduction of the V^VO(salen)⁺ on the FSAM surface in UHV. It is remarkable that the large increase in the V^III(salen)⁺ peak is seen only when [Ni^II(salen) + H]⁺ is present on the surface, but such an effect is not observed in its absence.

Kinetics of VO(salen)⁺ reduction to V(salen)⁺

Further evidence for the interfacial reduction reaction was obtained from the kinetics experiment conducted using FT-ICR SIMS.^37,39 In this experiment simultaneous deposition of V^VO(salen)⁺ and [Ni^II(salen) + H]⁺ on FSAM in an ultrahigh vacuum system (<2 × 10⁻⁹ Torr) was examined using 8 keV Cs⁺ SIMS. Fig. 3a shows the composition of the ion beam used for SL experiments. The primary ion beam is dominated by V^VO(salen)⁺ and [Ni^II(salen) + H]⁺ ions with a small amount of Ni^III(salen)⁺ at m/z 324. SIMS spectra shown in Fig. 3(b–d) were recorded in situ during and after ion deposition. The SIMS spectra contain peaks corresponding to the deposited V^VO(salen)⁺ and [Ni^II(salen) + H]⁺ ions at m/z 333 and m/z 325, respectively. In addition, the spectrum contains an abundant V^III(salen)⁺ peak at m/z 317 and the Ni^III(salen)⁺ peak at m/z 324 that is significantly more abundant than the Ni^III(salen)⁺ in the precursor ion beam.

Fig. 3 (a) FT-ICR spectrum of the primary beam of a mixture of V^VO(salen)⁺ and [Ni^II(salen) + H]⁺; FT-ICR-SIMS spectra of the FSAM surface after (b) 170 min and (c) 320 min of soft-landing (at the end of the deposition), (d) 87 h after the end of soft-landing, and (e) after exposure of the sample to laboratory air for 1 h.

The intensity ratio of V^III(salen)⁺ and V^VO(salen)⁺, I(317)/I(333), increases from the value of 0.85 ± 0.15 at the beginning of the deposition to 2.0 ± 0.2 at the end of the deposition and continues to increase after the deposition is finished reaching the value of 3.8 ± 0.2 after 100 h (data not shown). A dramatic decrease in the I(317)/I(333) ratio is observed after exposure of the surface to laboratory air (Fig. 3e); the I(317)/I(333) ratio drops to a value of ca. 0.2.

V^III(salen)⁺ ions observed in the SIMS spectrum may be produced through several processes. First, a small fraction of V^III(salen)⁺ ions is produced as a result of dissociation of V^VO(salen)⁺ in the SIMS analysis. The percentage of V^III(salen)⁺ fragments is expected to remain constant at low surface coverage used in our experiments. The upper limit for the amount of fragmentation in the FT-ICR-SIMS plume of 20% was estimated from the I(317)/I(333) ratio obtained after exposure of the FSAM surface to air. This value is higher than 3.5% fragmentation efficiency observed in the TOF-SIMS experiment (Fig. 1). Lower fragmentation efficiency in TOF-SIMS spectra was previously attributed to the shorter residence time (few microseconds) for ions in the TOF-SIMS as compared to hundreds of milliseconds residence time in FT-ICR SIMS experiments.⁴³

The second process involves dissociation (crash landing)^39,44,45 of V^VO(salen)⁺ during deposition followed by trapping of the V^III(salen)⁺ fragment in the monolayer. Similar to dissociation in SIMS, the fraction of V^III(salen)⁺ ions resulting from crash landing should remain constant during ion deposition and cannot contribute to the observed increase in the yield of V^III(salen)⁺ after the deposition is complete. Finally, V^III(salen)⁺ can be produced through reactions between ions and neutral molecules trapped on the surface. Surface reactivity is the only process that can rationalize the increase in the I(317)/I(333) ratio after the end of ion deposition. From the above discussion it follows that a significant fraction of the V^III(salen)⁺ (m/z 317) is produced as a result of the loss of O from the V^VO(salen)⁺ (m/z 333) deposited on the surface and the I(317)/I(333) ratio is a measure of the extent of the reduction of V^VO(salen)⁺ to V^III(salen)⁺. A significant drop in the I(317)/I(333) ratio upon exposure of the surface to laboratory air (Fig. 3e) indicates a facile oxidation of V^III(salen)⁺ to V^VO(salen)⁺ upon exposure to O₂.

The abundance ratio of Ni^III(salen)⁺ (m/z 324) and [Ni^II(salen) + H]⁺ (m/z 325) also increases with time. It is reasonable to assume that the Ni^III(salen)⁺ ion is produced in SIMS through ionization of the neutral Ni^II(salen). Our previous studies demonstrated that FSAM surfaces are characterized by the highest efficiency for charge retention when protonated molecules are deposited on the substrate.⁴³ We found that both instantaneous proton loss during ion deposition and a slow charge reduction process that continues for many hours after SL are responsible for neutralization of the deposited protonated species.³⁸ It is reasonable to assume that [Ni^II(salen) + H]⁺ undergoes a similar slow charge reduction on the FSAM surface. The resulting neutral Ni^II(salen) complex is subsequently re-ionized in SIMS and observed as Ni^III(salen)⁺ species.

Fig. 4a shows the time dependence of the I(317)/I(333) ratio. In the first 165 min of the kinetics experiment the ratio remains almost constant while an almost linear increase is observed after the deposition of ca. 7% of a monolayer of metal–salen species on the surface (4.5 × 10¹¹ ions). This trend continues after the deposition is finished. Our results can be rationalized assuming that V^VO(salen)⁺ deposited onto the FSAM surface is slowly converted into V^III(salen)⁺. The observed threshold behavior shown in Fig. 4a indicates that slow diffusion-limited interfacial reduction occurs on the inert FSAM surface and confirms that the formation of the reaction product, V^III(salen)⁺, is not associated with the SIMS analysis step.

Fig. 4 Time dependent intensities of (a) V^III(salen)⁺ (m/z 317), (b) [Ni^II(salen) + H]⁺ (m/z 325), and (c) Ni^III(salen)⁺ (m/z 324) in FT-ICR SIMS spectra normalized to the intensity of the V^VO(salen)⁺ (m/z 333). The dashed line shows the end of ion deposition.

Role of proton donor

By analogy with the acid-mediated solution-phase reactivity of V^IVO(salen) we hypothesized that the observed reduction of V^VO(salen)⁺ is facilitated by the presence of a proton donor. To examine whether or not the loss of oxygen from V^VO(salen)⁺ is a proton-mediated process, we conducted several experiments, in which V^VO(salen)⁺ was deposited in sequence with different proton donors on the FSAM surface. Both the time and the order of deposition were varied in these experiments. The results indicated that the initial I(317)/I(333) ratio varies significantly as a function of the deposition time, the type of the proton donor, and the presence of [V^IVO(salen) + H]⁺ that can be produced in the ESI source and co-deposited with V^VO(salen)⁺.

Table 1 lists the I(317)/I(333) ratios observed in TOF-SIMS spectra obtained in situ immediately after sequential deposition of 10¹² ions of V^VO(salen)⁺ and 10¹² ions of a proton donor. The lowest initial yield of V^III (salen)⁺ was obtained when [Ni^II(salen) + H]⁺ was used as a proton donor. The I(317)/I(333) ratio of 0.5–0.7 was obtained in a number of repeated experiments. This ratio increases to 1.0 when V^VO(salen)⁺ is co-deposited with [V^IVO(salen) + H]⁺ which may also serve as a proton donor. Interestingly, the I(317)/I(333) ratio of 2.5–3.2 was obtained when soft-landing of V^VO(salen)⁺/[V^IVO(salen) + H]⁺ was followed by a fairly long deposition (2–3 h) of [Ni^II(salen) + H]⁺. However, a much lower ratio of 1.2 was obtained when the deposition time of [Ni^II(salen) + H]⁺ was reduced to 1.5 hours consistent with the presence of an initial induction period for the reduction reaction.

Table 1 The ratio of intensities of V^III(salen)⁺ and V^VO(salen)⁺, I(317)/I(333), in TOF-SIMS spectra obtained in situ immediately after soft-landing. Ions are deposited sequentially onto the FSAM surface

Deposition 1 (ion)	Time/h	Deposition 2 (ion)	Time/h	I(317)/I(333)
a In these experiments both V^VO(salen)^+ and [V^IVO(salen)+H] were produced in the ESI source and were not separated prior to deposition.
V^VO(salen)^+	0.75	[Ni^II (salen) + H]^+	2	0.5
V^VO(salen)^+	0.75	[Ni^II (salen) + H]^+	3	0.7
V^VO(salen)^+	0.75	[Diaminododecane + H]^+	7	3.1
[Diaminododecane + H]^+	12	V^VO(salen)^+	0.75	1.6
V^VO(salen)^+	1	[RGDGG + H]^+	2	3.7
[RGDGG + H]^+	3	V^VO(salen)^+	0.75	1.3
V^VO(salen)^+/[V^IVO(salen) + H]^+^a	0.75	—	—	1.0
V^VO(salen)^+/[V^IVO(salen) + H]^+^a	1	[Ni^II (salen) + H]^+	2	3.2
V^VO(salen)^+/[V^IVO(salen) + H]^+^a	2	[Ni^II (salen) + H]^+	3	2.5
V^VO(salen)^+/[V^IVO(salen) + H]^+^a	3	[Ni^II (salen) + H]^+	3	2.9
V^VO(salen)^+/[V^IVO(salen) + H]^+^a	3	[Ni^II (salen) + H]^+	1.5	1.2

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Deposition of V^VO(salen)⁺ with protonated diaminododecane resulted in a fairly high initial ratio of 3.1. However, a much lower ratio of 1.6 was obtained when the order of the deposition was reversed and diaminododecane was deposited prior to V^VO(salen)⁺. Similar results were obtained when a small protonated pentapeptide, RGDGG, was used as a proton donor. We attribute the observed dependence on the order of deposition to the shorter residence time of V^VO(salen)⁺ on the surface when it is deposited after the proton donor.

Oxidation of V^III(salen)⁺ and regeneration of V^VO(salen)⁺

The decrease in the I(317)/I(333) ratio observed following exposure of the FSAM surface to laboratory air (Fig. 3e) could be attributed to the oxidation reaction of V^III(salen)⁺ that regenerates V^VO(salen)⁺ species. This process was further investigated using in situTOF-SIMS analysis of surfaces modified by SL in a recently constructed ion deposition apparatus coupled to a TOF-SIMS instrument. Fig. 5 shows TOF-SIMS spectra obtained after co-deposition of V^VO(salen)⁺ and [V^IVO(salen) + H]⁺ that could not be separated in our quadrupole mass filter without a significant loss of the ion current. Therefore, the only protons present on the monolayer are derived from [V^IVO(salen) + H]⁺ as [Ni^II(salen) + H]⁺ was not deposited. As discussed earlier the initial I(317)/I(333) ratio of 0.88 indicates facile formation of V^III(salen)⁺ on the surface during SL. The ratio remains unchanged after four hours (Fig. 5b) in UHV (2 × 10⁻⁹ Torr). Exposure of the surface to 10⁻⁴ Torr of molecular oxygen for 20 min results in a significant depletion of the V^III(salen)⁺ peak and an increase in the V^VO(salen)⁺ peak. The I(317)/I(333) ratio then remains constant during 65 h residence time in UHV. These results indicate that the reduction reaction resulting in formation of V^III(salen)⁺ is complete and all the [V^IVO(salen) + H]⁺ ions that may serve as proton donors are consumed by the end of the ion deposition step and that exposure to oxygen converts a significant fraction of V^III(salen)⁺ into VO(salen)⁺. The fact that no further reduction of V^VO(salen)⁺ occurs on the surface confirms that this process requires the presence of an additional proton donor deposited from the gas phase.

Fig. 5 In situ TOF-SIMS characterization of the FSAM surface (a) after the deposition of 1 × 10¹² ions of VO(salen)⁺, (b) after 4 h in vacuum, (c) after exposure to 10⁻⁴ Torr of O₂ for 20 min, (d) after 65 h in vacuum.

Fig. 6a shows the I(317)/I(333) ratio derived from TOF-SIMS spectra obtained following deposition of V^VO(salen)⁺ and [V^IVO(salen) + H]⁺. As discussed earlier, the ratio changes only upon initial exposure to oxygen and remains constant when the system is kept in vacuum. In contrast, both the reduction of V^VO(salen)⁺ to V^III(salen)⁺ and the oxidation of V^III(salen)⁺ to V^VO(salen)⁺ by exposure to O₂ were observed when V^VO(salen)⁺ was co-deposited with [Ni^II(salen) + H]⁺. The cyclic regeneration of V^VO(salen)⁺ upon repeated exposures to O₂ is evident in the oscillating abundance ratios shown in Fig. 6b. Similar oxidation/reduction cycles were observed when protonated diaminododecane was used as a proton donor. The I(317)/I(333) ratio obtained in this experiment is shown in Fig. 6c. We note that the initial yield of V^III(salen)⁺ obtained after SL was quite high. Exposure to oxygen resulted in a significant decrease in the I(317)/I(333) ratio. Subsequent increase in the I(317)/I(333) ratio indicates that the reduction of V^VO(salen)⁺ to V^III(salen)⁺ on the surface continues even after exposure of the surface to oxygen. This behavior is reproducible and continues for at least three cycles when [Ni^II(salen) + H]⁺ is used as a proton donor. However, the reactivity is clearly suppressed upon multiple exposures of the surface to oxygen. This is likely due to depletion of the population of [Ni^II(salen) + H]⁺ on the surface which results in a smaller number of proton donors, and consequently, a lower overall acidity in the monolayer.

Fig. 6 The intensity ratio of peaks at m/z 317 and m/z 333 observed in situTOF-SIMS spectra showing oxidation and reduction cycles on the FSAM surface for (a) VO(salen)⁺, (b) sequential deposition of VO(salen)⁺ and [Ni(salen) + H]⁺, (c) sequential deposition of the singly protonated diaminododecane and VO(salen)⁺. Individual points were obtained immediately after SL (labeled as End of SL), after one or multiple O₂ exposures (labeled as O₂), and at various times after each processing step, SL or O₂ deposition (labels indicate the time between the corresponding processing step and the analysis). Lines are shown to guide the eye.

Comparison with solution-phase reactivity

It is remarkable that deoxygenation of the oxo-vanadium complex previously observed only in highly acidic non-aqueous solvents occurs on the surface in the UHV (0.5–2 × 10⁻⁹ Torr) environment.^31,32 Bonadies et al. proposed that in a strongly acidic solution V^IVO(salen) undergoes a disproportionation reaction (eqn (1)) forming V^III(salen)⁺ and V^VO(salen)⁺.²⁸ Tsuchida et al. demonstrated that V^III(salen)⁺ is the key intermediate responsible for the electroreduction of O₂ in acidic nonaqueous solvents.⁴⁶ In this process the vanadium–salen complex cycles are between the V^III(salen)⁺ and V^VO(salen)⁺ states. However, the mechanism of the O₂ electroreduction proposed in that study did not involve the disproportionation reaction but rather assumed the formation of doubly and triply charged monomers and dimers of VO(salen) and the reduced species. Because of the high Coulomb barrier for close approach of positively charged ions in the absence of solvent it is unlikely that such multiply charged intermediates are formed on the surface in our experiments.

Liu and Anson re-examined the mechanism of the acid-mediated reactivity of VO(salen) in solution.⁴⁷ They provided further support for the disproportionation reaction and showed that V^III(salen)⁺ in acetonitrile reacts with O₂via the reaction shown in eqn (2). The reaction is fast and the conversion of V^III(salen)⁺ into V^VO(salen)⁺ is quantitative. They also showed that the disproportionation reaction is a multistep process as shown in eqn (3) and (4).

V^IVO(salen) + 2H⁺ → V^IV(salen)²⁺ + H₂O (3)

V^IVO(salen) + V^IV(salen)²⁺ → V^III(salen)⁺ + V^VO(salen)⁺ (4)

These reactions could also be responsible for the proton-mediated reduction of V^VO(salen)⁺ observed in our soft-landing experiments. Because a fraction of the soft-landed V^VO(salen)⁺ ions is neutralized at the time of collision, ion deposition onto the FSAM surface most likely results in immobilization of both V^VO(salen)⁺ and V^IVO(salen),⁴⁷ while neutralization of V^VO(salen)⁺ ions trapped in the monolayer is expected to be very slow.^34,35 In addition, proton loss from [Ni^II(salen) + H]⁺ or any other proton donor acidifies the monolayer, which facilitates reactions (3) and (4). Although we have not observed any doubly charged species in our TOF-SIMS spectra, considerable growth in the intensity of the peak at m/z 336 (Fig. 2) corresponding to V^III(salen)F⁺ could be indicative of the formation of the transient doubly charged V^IV(salen)²⁺ intermediate that is subsequently transformed in the SIMS plume through reaction with the F⁻ anion.

From the above discussion it follows that the slow interfacial reactivity observed in this study could be attributed to reactions between the ionic and neutral species trapped on or in the FSAM. The reaction rate is determined either by the rate of migration of vanadium–salen complexes on the surface or by the rate of the proton loss by the protonated nickel–salen complex. Previous studies demonstrated that both small ions and organometallic clusters can be trapped between adjacent SAM chains.^21,34 Penetration of the soft-landed species into the layer may significantly reduce the rate of migration and slow down the reaction. Alternatively, the reaction rate may be determined by the availability of protons in the layer. Previous work showed that the proton loss from protonated molecules soft-landed onto FSAM surfaces is rather slow.³⁸ Similarly, the decay of the [Ni(salen) + H]⁺ signal observed in FT-ICR SIMS experiments occurs on a timescale of several hours.

Conclusions

This study presents a first step towards immobilization of catalytically active organometallic complexes onto suitable supports using soft-landing of mass-selected ions. The results presented here indicate that V^VO(salen)⁺ ions soft-landed onto inert FSAM surfaces undergo slow proton-mediated reduction forming V^III(salen)⁺ species. The data show that reactivity continues for several days after the deposition, indicating that reaction is interfacial rather than beam-related. In situSIMS data suggest that the reduction reaction proceeds within the deposited monolayer and is characterized by an induction period, during which the ratio of V^III(salen)⁺ to V^VO(salen)⁺ remains constant. The presence of the induction period indicates that the reduction reaction is diffusion-limited. The dependence of the initial yield of V^III(salen)⁺ observed after the deposition on the properties of the proton donor and its residence time on the surface indicates that SL of protonated molecules on inert FSAM surfaces can be used as a tool for controlled preparation of monolayers of varying acidity.

The reverse reaction in which V^III(salen)⁺ is oxidized upon exposure to molecular oxygen regenerating the original V^VO(salen)⁺ complex is fairly efficient and continues for several cycles. This cyclic regeneration of V^VO(salen)⁺ was evident in the oscillating abundance ratios of V^III(salen)⁺ and V^VO(salen)⁺. The yield of the reduction reaction is limited by the amount of protonated molecules co-deposited with V^VO(salen)⁺ and the rate of the proton loss by the proton donor. Our results indicate that in the presence of proton donors soft-landed V^VO(salen)⁺ initiates reduction of molecular oxygen that is similar to the catalytic electrochemical reduction of O₂ observed in strongly acidic solutions. Although this study focused on a specific reduction reaction we believe that the SL methodology can be utilized for preparation of model catalysts of controlled chemical composition and hence, in conjunction with SIMS analysis and other surface characterization techniques, it should enable detailed fundamental studies of the surface reactivity of metal complexes on solid supports and perhaps allow screening and increased understanding of a wider variety of catalysts.⁴⁸

Acknowledgements

This work was supported by grants from the Chemical Sciences Division, Office of Basic Energy Sciences (BES) of the U.S. Department of Energy, DE-FG02-06ER15807 (RGC), National Science Council NSC 96-2112-M-259-010-MY3 (WPP), and the grant from the Chemical Sciences Division of BES (JL). Ivy Fortmeyer acknowledges support from the DOE Science Undergraduate Laboratory Internship (SULI) program at Pacific Northwest National Laboratory (PNNL). The work was performed at the W. R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the U.S. DOE of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle for the U.S. DOE.

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Footnotes

† Electronic supplementary information (ESI) available: Electrospray ionization mass spectra of solutions used in soft-landing experiments. See DOI: 10.1039/c0cp01457e

‡ Undergraduate student from Columbia University

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