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Functionalized Self-Assembled Monolayers Bearing Diiminate Complexes Immobilized through Covalently Anchored Ligands Elitsour Rozen, Yaron Erlich, Megan E. Reesbeck, Patrick L. Holland, and Chaim N Sukenik Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00984 • Publication Date (Web): 19 Oct 2017 Downloaded from http://pubs.acs.org on October 20, 2017

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Functionalized Self-Assembled Monolayers Bearing Diiminate Complexes Immobilized through Covalently Anchored Ligands Elitsour Rozen1, Yaron Erlich1, Megan E. Reesbeck2, Patrick L. Holland2, Chaim N. Sukenik1* 1

Department of Chemistry and Institute of Nanotechnology and Advanced Materials Bar-Ilan University, Ramat-Gan 52900, Israel

2

Department of Chemistry, Yale University, 225 Prospect Street, New Haven, CT 06520, USA

Abstract The application of synthetic organic chemistry to the surface chemistry of monolayer arrays adds a novel dimension to the power of these systems for surface modification. This paper describes the elaboration of simple functionalized monolayers into dialdimine and dialdiminate ligands tethered to the monolayer surface. These ligands are then used to coordinate metal ions in an effort to form diiminate complexes with control over their environment and orientation. Ligand anchoring is best achieved through either thiol-ene photochemistry or azide-acetylene "click" chemistry. There is an influence of ligand bulk on some surface transformations, and in some cases reactions that have been reported to be effective on simple, homogeneous monolayer surfaces are not applicable to a more complex monolayer environment. The large excess of solution reagents relative to monolayer surface functionality adds another measure of difficulty to the control of interfacial reactions. In instances where the anchoring chain includes functional groups that can directly interact with metal ions, the metalation of ligand-bearing surfaces

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resulted in a higher metal ion content than would have been expected from binding only to the diimine ligands.

INTRODUCTION The challenge of building functional groups into the surface of covalently anchored monolayers has been addressed by a combination of depositing functional group-bearing monolayer forming materials and by their in-situ transformations. The early Sagiv report of the chemical manipulation of an olefin-terminated monolayer surface is a good example of an initial hydrophobic (hydrocarbon) surface being converted in situ into a terminal alcohol and thus providing the basis for covalent multilayer construction1. Variously functionalized monolayers and their in situ conversions have been reported by Whitesides et al,2,3 Ulman,4,5 Cooks,6,7 and by our group.8,9 Installing organometallic complexes on solid supports has been demonstrated for a variety of surfaces and a variety of complexes.10–13 The specific challenge of doing this immobilization on an organized monolayer interface is important both due to the complexity of the surface functionality and due to the possibility that surface immobilization may enhance the activity of some catalysts. Specifically, immobilizing catalysts on an ordered monolayer surface allows for the control of catalyst orientation and can avoid catalyst deactivation from bimetallic reactions.14,15 An example of the need to avoid dimerization is the preparation of complexes for catalytic reduction of the N2 molecule, which in homogeneous systems relies on the binding of N2 through only one of its two lone pairs.16,17 Though multimetallic complexes can give stoichiometric N2 reductions,16–18 the known catalytic N2 reductions by homogeneous complexes proceed through mononuclear M-N2 complexes.19–22 Formation of a bimetallic complex typically


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gives an unreactive M-NN-M unit.23–25 In principle, the formation of bimetallic complexes could be avoided if the supporting ligands are held in an oriented fashion at a monolayer surface. We also anticipated that site isolation could avoid bimetallic decomposition pathways in cobalt catalysts for alkene isomerization26 and hydrosilylation.27 These challenges inspired us to attempt the construction of monolayer surfaces decorated with bulky diiminates and the conversion of these ligands into transition-metal complexes. We specifically targeted the 1,3-dialdiminate complex shown in Figure 1.

Figure 1 Surface anchored metal dialdiminate complex

There are a number of different approaches to attaching complex molecules on a surface. One can pre-assemble a modified version of the ligand which includes an anchoring "tail".28,29 Attaching the molecule to the surface in this case is a one-step process, with no in situ ligand construction. The main advantage of this route is that one can spectroscopically follow the ligand synthesis in solution, while characterizing and purifying the product at each stage. However, the size and complexity of the fully assembled ligand and the interest in using reactive functionalities like trichlorosilanes or trialkoxysilanes for anchoring makes it difficult to assemble and purify the desired anchorable ligand. Also, the self-assembly of such a bulky species into a well-packed


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monolayer on the substrate surface is expected to be more challenging than for monolayers anchored through simple polymethylene chains. Another approach consists of building the molecule directly on the surface. In this case, one initially attaches to the surface a simple molecule bearing a functional group that can be elaborated. Then, by a series of chemical transformations on the surface, the desired molecule is built step by step. In this approach, the uniformity and packing of the initial monolayer provides a well-defined starting point for the in-situ chemistry. The challenges of this approach include the limited tools for monitoring the effectiveness of the in-situ transformations (XPS, FTIR, and UVVis) and the fact that any incomplete reactions would lead to mixtures of partially converted species within the final monolayer array. An example of such an approach to our immobilized ligand monolayer is shown in Scheme 1, which illustrates a series of reactions through which an anchored dialdehyde could be condensed with the requisite aniline to form the desired dialdiminate.


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Scheme 1 Building the ligand by a series of surface transformations

A third approach combines the strengths of each of the above methods by working in parallel on the surface and in solution. By installing a suitably reactive surface group on a wellpacked monolayer, one can construct the ligand in solution and provide it with a point of connection to the functionalized monolayer surface. In parallel, one deposits a monolayer that contains suitable linking functionality (perhaps in masked form). In this way, only a few steps are needed to prepare the target surface, reducing the problems of depositing bulky monomers and of incomplete reactions along the way to the surface immobilized ligand. By starting with a high quality initial SAM and limiting the process to one or two high-yield surface transformations, a high loading of the ligand on a uniformly functionalized surface should be most readily obtained. This approach is illustrated in Scheme 2.


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Scheme 2 Hybrid approach to ligand construction by combining surface transformations and liquid phase reactions. For example, X = Br, Y = Azide, Z = Alkyne, {Y+Z} = triazole.

In this paper, we compare different approaches to the attachment of dialdimines to SiO2/Si wafers and quartz surfaces. The trichlorosilanes used to create the initially deposited monolayer templates are shown in Figure 2. A Si-H functionalized surface, as yet another kind of initial, uniform monolayer, is created by reacting the surface silanols of the quartz or of the SiO2/Si wafer with trimethoxysilane.

Figure 2 Structure of the trichlorosilanes used for building the monolayers

The simple one-step transformations of these SAMs that provided the entry points into our various ligand attachment schemes are shown in Figure 3. All six of these SAMs have been


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previously reported and fully characterized.8,9,30,31 Their transformation into diimines and diiminates is the subject of this report. We also report herein our work towards introducing metal ions into the attached ligands. This effort also revealed that metal incorporation into the surface bound ligands can result in excess metal ions on the surface and that ultimately, the most useful systems will require not only effective metal ion binding but will also have to minimize extraneous metal ions.

8,9,30,31

Figure 3 The six SAMs used for the attachment of the dialdiminate ligands. for creating these SAMs are described in Figure 2

The molecules used

EXPERIMENTAL Materials. Reagents and solvents were obtained from Sigma-Aldrich, Acros Organics, Alfa Aesar and Bio-Lab Ltd. The trichlorosilane thioester,31 thioacetate30 and bromide8 (Figure 2) used to create the SAMs described herein were all prepared using published procedures and their spectral properties were identical to those in the literature. The monolayers they form and the monolayers that result from transformation of the bromide (SAMBr) to an azide (SAMN3),8 of the thioacetate (SAMTA) to a thiol (SAMSH)9 and of the thioester (SAMTE) to a carboxylic acid (SAMCOOH)31 (Figure 3) have all been fully characterized. Water was deionized and then distilled in an all-glass apparatus. Tetrahydrofuran (THF) was dried by distillation under N2 from


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Na and benzophenone. Toluene was dried over Na. DMF was dried in a solvent purification system from the Vacuum Atmospheres Company. Silicon wafers were obtained from Virginia Semiconductor (N type, ‹100›, >1000 Ω·cm). Quartz substrates were obtained from Quarzschmelze Ilmenau. Overview of Monolayer Preparation Silicon wafers (for FTIR-ATR measurements) and quartz wafers (for UV and for XPS) were cleaned, dried and then treated with piranha solution (H2SO4 (98 %):H2O2 (30 %), 70:30 v:v, 80 ˚C) for 20 minutes. WARNING: Piranha solution should be handled with caution. It should not be allowed to contact significant quantities of oxidizable material. Piranha treatment yielded an oxide layer which was totally wetted by water. All the substrates were used within 0.5 h. They were then used to deposit the siloxane-anchored SAMs. The SAMs were characterized by ATR-FTIR on the silicon wafers, and by XPS and UV−Vis spectroscopy on the quartz substrates. These characterization tools were applied both to the directly deposited SAMs and to those produced by in situ chemical transformations. Metal insertion into the monolayers was analyzed using X-ray photoelectron spectroscopy, UV-Vis spectroscopy, and FTIR spectroscopy. FTIR-ATR on SAMs In a usual FTIR-ATR, the internal reflection crystal is part of the device and the compound to be analysed is simply placed on top of the internal reflection element. In this work, double sided polished silicon wafers provided the internal reflection elements. These wafers are used both as the substrate for the SAM depositions and as the ATR crystal for the analysis. Silicon wafers are transparent in the mid-IR region and this allows us to analyse the organic molecules that are present at the surface. In order to obtain total internal reflection in the silicon wafers, the beam should impinge upon the silicon/air interface at less than a 45° angle. To that 8 ACS Paragon Plus Environment

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end, the edges of the wafer are cut at a 45° angle so as to allow the penetration and the exit of the IR beam into and out of the wafer and to maximize the number of reflections inside the wafer. The analysis was then done in transmission mode with a suitable set of mirrors to allow the beam to enter the prism at the desired angle. The IR beam enters the prism and is reflected by the internal surfaces of the wafer multiple times before exiting the wafer (see Figure S10). At each point of reflection of the beam from the surface, there is an evanescent wave that interacts with the monolayer molecules on the wafer surface. These interactions produce the IR spectrum of the organic molecules tethered to the surface. Depositions of alkyl bromide (SAMBr)8 and diluted versions thereof were done as per reference 8. Mixed alkane/alkyl bromide SAMs were made by adding octyltrichlorosilane into the solution with 11-bromoundecanyltricholorosilane (Figure 2) and this mixture of silanes was used to coat the silicon wafer. Azide SAMN3 samples were generated in situ by reacting SAMBr with sodium azide in dry DMF.8 In Situ Generation of Thiol Functionality (SAMSH). Thioacetate-coated wafers (SAMTA) were treated with LiAlH4 according to the published procedure.9 In situ Generation of Carboxylic Acid Functionality (SAMCOOH) Thioester-coated wafers (SAMTE) were oxidized to the corresponding carboxylic acid using Oxone, according to the published procedure.30 Silane-H groups were created on the surface using trimethoxysilane (10 µL) in methanol (10 mL) after activation of the substrate by piranha solution, to form SAMSiH. In situ Reaction of Propargyl Amine with the Carboxylic Acid Bearing Surface (SAMCOOH) to Yield an Amide-Tethered Terminal Acetylene (SAMAcetylene)


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In a dry reaction tube, equipped with a small magnetic stirring bar, were placed 15 µL of propargyl amine, 0.03 g of EDC, 40 mg of DMAP, and 10 mL of dry DMF. SAMCOOH substrates were placed in the tube and the mixture was stirred at room temperature overnight. The substrates were withdrawn from the mixture, rinsed with CHCl3, cleaned in CHCl3 in a sonication bath for 6 minutes, then in warm hexane (~60 °C) for 6 minutes and dried under a filtered nitrogen stream. In situ "Click" Reaction on an Azide-Bearing Substrate32,33 A solution was prepared: 12 mL of water:ethanol, 2:1 v:v, 60 mg of ascorbic acid, 20 mg of CuSO4, and 10 µL of the acetylene-terminated compound (3 or 4). An azide-bearing substrate (SAMN3) was placed in the solution for 2 minutes. The substrates were withdrawn from the solution, rinsed with doubly distilled water and then ethanol, and cleaned in CHCl3 in a sonication bath for 6 minutes, followed by treatment with warm hexane (~60 °C) for 6 minutes and drying under a filtered nitrogen stream. In situ "Click" Reaction on an Acetylene Bearing Substrate A solution was prepared: 12 mL of water:ethanol, 2:1 v:v, of ascorbic acid, 20 mg of CuSO4, and 10 µL of the compound with the azide group (7). An acetylene-bearing substrate (SAMAcetylene) was placed in the solution for 2 minutes. The substrates were withdrawn from the solution and cleaned and dried as described above. Diimine Attachment to a Thiol Decorated Surface34,35 A reaction solution containing 10 mg of an allyl derivatized diimine (5 see SI for synthesis) was put in a dry THF solution (10 mL). A thiol-decorated substrate (SAMSH) was placed in the solution and reacted overnight in a UV chamber (356 nm). The substrate was then rinsed with THF, sonicated with CHCl3 for 6 minutes, cleaned in warm hexane (~60 °C) for another 6 minutes, and dried under a filtered nitrogen stream. 10 ACS Paragon Plus Environment

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Diimine Attachment to a Silane Decorated Surface36,37 A reaction solution containing 10 mg of the allylated diimine (5) was put in a dry THF solution (10 mL). A silane decorated substrate (SAMSiH) was placed in the solution and reacted overnight in a UV chamber (356 nm). The substrate was then rinsed with THF, sonicated with CHCl3 for 6 minutes, cleaned in warm hexane (~60 °C) for another 6 minutes, and dried under a filtered nitrogen stream. Metal Insertion Insertion of palladium(II)38 In a reaction tube, equipped with a small magnetic stirring bar and a reflux condenser, were placed 100 mg of PdCl2 and 10 mL of CH3CN and the suspension was refluxed until a clear orange solution of Pd(CH3CN)2Cl2 was formed. The diimine-bearing substrates (SAMDiimine1) were placed in the tube and the mixture was refluxed for 3 h. The wafer was then cleaned in an acetonitrile reflux. The substrate was withdrawn from the mixture, cleaned using CHCl3 in a sonication bath for 6 minutes and dried under a filtered nitrogen stream. Insertion of iron(II) In a nitrogen filled glovebox, iron was inserted into SAMDiimine1 by adding a solution of 10 mg of Fe[N(SiMe3)2]239 (Fe(NTMS)2) in 10 mL THF, or by treating with n-butyllithium (10 mL at 0.1 M in THF) followed by addition of FeCl2(THF)1.5.40,41 Insertion of Co(II) In a nitrogen filled glovebox, cobalt was inserted into SAMDiimine2 by adding a solution of 10 mg of Co[N(SiMe3)2]239 (Co(NTMS)2) in 10 mL THF, or by treating with n-butyllithium (10 mL at 0.1 M in THF) followed by addition of CoCl2 in THF (10 mg in 10 mL) as described in reference 40.


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RESULTS The objective of this work was to attach a dialdiminate ligand with bulky aryl groups to the surface for future catalytic applications. Attempting to build the ligand entirely in solution to allow for ligand anchoring using a hydrolysable silane attached to a fully assembled diimine required adding a trichloro- or trialkoxy-silane unit on the end of an anchoring chain. This would typically be done by Pt-catalyzed hydrosilylation of a terminal olefin. However, initial experiments showed that the diimine did not survive the hydrosilylation reaction conditions. Thus, this approach was abandoned. We also tried to build the ligand on the surface as illustrated in Scheme 1. To this end, we tried different ways to produce the relevant dialdehyde on the surface. None of these approaches (diester reduction to dialdehyde, diene ozonolysis to dialdehyde) were successful. Details are described in the supplementary information. Successful ligand anchoring was achieved using copper-catalyzed "click" chemistry,42–44 involving the creation of triazole groups by reacting an azide with an acetylene in the presence of a copper catalyst.32,33,45,46 To this end, we needed to synthesize a diimine moiety with either an azide or an acetylene attached to its middle carbon. These modified diimines would react with the complementary azide or acetylene terminated surface. A successful embodiment of this concept is shown in Scheme 3.


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Scheme 3 Attaching an acetylene-bearing dialdimine to a diluted azide decorated SAM using "click" chemistry

In order to functionalize the center carbon of the dialdimine, we first deprotonated it using Schlosser's base.47 This enabled us to attach TMS-protected propargyl bromide. Removing the protecting group with TBAF provided the propargylated dialdimine. This acetylene-bearing diimine was reacted with SAMN3 to bring about Huisgen cycloaddition to a triazole. Since the azide functional group displays a characteristic IR peak near 2100 cm-1, we were able to follow both the installation and the further reaction of the azide group. While the cycloaddition reaction was successful as shown by appearance of the dialdimine peaks at 1603 and 1578 cm-1 in the IR (Figure 4) and at ~313 nm in UV-Vis (Figure S3 in the supplementary information), the azide peak never completely disappeared from the IR spectrum. This indicates that there had been only partial reaction of the alkynylated diimine with the surface. We reasoned that the incomplete reaction was due to the bulky dialdimine groups sterically preventing the reactions of neighbors within the packed monolayer array. In order to completely react the surface azide groups, we "diluted" the initial alkylbromide monolayer (comprised of 11-carbon chains) by co-depositing octyltrichlorosilane. In this way, we were able to provide more "space" for the reaction between 13 ACS Paragon Plus Environment

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the azide groups and the bulky acetylene groups. By using a 2:1 ratio of the C11-alkyl bromide to the C8-alkyl chains, we created a monolayer array wherein the azide peak completely disappeared after the cycloaddition reaction, both on quartz and silicon substrates.

Figure 4 Deposition of SAMDiimine2. (a) SAMBr diluted with octyltrichlorosilane, (b) SAMN3 diluted with octyltrichlorosilane, (c) SAMDiimine2 diluted with octyltrichlorosilane

We also produced the reverse “clickable” system by creating an azide functionalized dialdimine and reacting it with an acetylene-bearing SAM. This would leave no unreacted azide on the surface. The requisite acetylene terminated surface (SAMAcetylene) was created as shown in Scheme 4. NH 2 S

OH Oxone

O

NH

O

O EDC DMAP

10

10

SiO2 /Si

SiO2 /Si

SAMTE

SAMCOOH

10

SiO2/Si SAMAcetylene

Scheme 4 Creation of acetylene decorated SAM


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The acetylene groups were installed on the surface by creating an acid monolayer and reacting them with propargylamine. The amidation reaction could be followed by observing the shift of the carbonyl peak when the amide is formed (Figure 5). Installing the azide functionality on the dialdimine proved to be most effective using a benzene ring as a "spacer" between the azide functionality and the diimine. The synthesis of the azide-bearing diimine shown in Scheme 5 is reported in the SI. The resulting azide-terminated dialdimine was then attached using coppercatalyzed "click" chemistry on the acetylene monolayer to obtain SAMDiimine3 (Scheme 5). We were able to ascertain that the ligand reacted with the surface using IR spectroscopy, by observing the appearance of the diimine peaks at 1603 and 1580 cm-1 (Figure 5).

Figure 5 FTIR spectra of the formation of SAMDiimine3: (a) SAMTE, (b) SAMCOOH, (c) SAMAcetylene, (d) SAMDiimine3


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R

R N

R N R R

R N

N

N

N

R

N

R

NH

NH N3

O 10

Ascorbic acid CuSO 4

SiO2/Si SAMAcetylene

R=iPr

O 10

SiO2 /Si SAMDiimine3

Scheme 5 Attaching an azide terminated dialdimine to SAMAcetylene

Another method of attaching the diimine to the surface was through the addition of either Si-H or S-H across a double bond. To this end, we produced a functionalized dialdimine in which the backbone has a pendant double bond, as shown in Scheme 6.

Scheme 6 Olefin functionalization of the dialdiminate

The terminal olefin (1) was reacted with SAMSiH surfaces that had been produced by reacting trimethoxysilane with surface silanols on silicon or quartz substrates (Scheme 7). The presence of the Si-H groups was ascertained by the appearance of the expected 2280 cm-1 peak in the FTIR spectrum (Figure S2). Alternatively, SAMSH surfaces were made by reducing a


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thioacetate monolayer (SAMTA). Ligand attachment by grafting 1 onto the surface then proceeded using UV activation (Scheme 8).

Scheme 7 SAMDiimine4 as prepared by hydrosilylation

Scheme 8 SAMDiimine5 as prepared by a thiol-ene reaction on SAMSH

Analysis by FTIR-ATR and UV-vis (Figures S1-2 and Figure 6) showed that the reaction with the thiol surface (SAMSH) afforded a higher level of ligand incorporation than the reaction with the silane surface (SAMSiH). The reasons behind this difference are not clear. One possibility is that the SAMTA may present a more uniform and dense array of SH groups on the surface, while the trimethoxysilane may give poor Si-H coverage. Alternatively, the bulkiness of


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the functionalized dialdimine may create more steric interference when the reaction occurs closer to the surface, without the intermediacy of monolayer chains.

Figure 6 UV spectra of SAMDiimines1-5 on quartz. Note the large difference in intensity of the diimine UV signals

To summarize, we successfully attached dialdimine ligands both by azide-acetylene "click" chemistry (SAMDiimines 1, 2, 3) and by thiol-ene (SAMDiimine 5) and silane-ene (SAMDiimine 4) chemistry. The thiol-ene reaction is the most efficient but is somewhat less convenient since it requires UV activation. Click cycloaddition chemistry works well but the preparation of the appropriately functionalized ligands and the corresponding SAM surface template is more challenging. The relative UV intensities of the dialdiminate-functionalized SAMs (Figure 6) indicate that the order of ligand loading on the SAM surface is SAMDiimine5 >> SAMDiimine3 ≈ SAMDiimine 1 ≈ SAMDiimine 2 > SAMDiimine 4. Finally, click chemistry places a triazole group within the anchoring chain. This creates the possibility of the triazole acting as a ligand for the metal ions (as will be addressed below) and may have consequences for the usefulness of the monolayer in the context of a catalytic system.


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Metal complexation in the ligand monolayer To evaluate whether the immobilized ligands can complex to metal ions, we first examined the reaction of SAMDimine1 with palladium(II) chloride in acetonitrile. This complexation is the simplest since there is no need to deprotonate the immobilized diimine. XPS characterization (Figure S6) shows palladium on the surface The binding energy signal at 337 eV indicates that all palladium atoms are present in their oxidized palladium(II) form (metallic palladium would exhibit a binding energy of 335.3 eV). Also, FTIR and UV-visible spectroscopy (Figures 7 and S3) showed a clear difference in the spectra of SAMDiimine1 before and after the insertion of the palladium ion. Importantly, XPS analysis showed that no copper ions remained from the "click" chemistry reaction. However, XPS analysis also showed that the ratio between palladium and nitrogen atoms was on the order of 1:3 instead of the expected 1:5 if the metal ions were only complexed by the immobilized diimine ligand (2 nitrogens from the diimine and 3 from the triazole); i.e. there are excess metal ions in the monolayer. While palladium nanoparticles may have been formed during the insertion process,48,49 the final samples show no evidence of palladium metal (XPS shows only palladium(II) and not palladium(0)). The small amount of excess palladium(II) (1.7 ions per diimine-bearing chain instead of 1 per chain) is also consistent with little or no nanoparticle accumulation. The excess metal ions may be either physically trapped among the alkyl chains of the monolayer, or they may be complexed to the triazole groups.50,51 A control experiment reported in the supplementary information using a triazole containing SAM lacking the diimine, showed that it could complex as much as one metal ion per monolayer chain (Figure S9). Thus, our result of 1.7 ions for a system containing both a diimine and a triazole is reasonable.


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Figure 7 FTIR spectra of building SAMDiimine1 and the insertion of palladium: (a) SAMBr, (b) SAMN3, (c) SAMDiimine1, (d) SAMDiimine1 after Pd insertion.

Insertion of first row transition metals into SAMDiimine1 and SAMDiimine2 was attempted using soluble iron(II) and cobalt(II) bis(trimethylsilyl)amide respectively, and also by adding n-butyllithium followed by anhydrous metal dihalide salts in THF. While the metal ions were taken up by the monolayer in all cases, excess metal ions (as determined by XPS, Figure S7 and S8) remained even after our best efforts to clean the surface. This excess of metal ions on the surface (0.7 extra ions for Pd, 3.6 for Fe and 2.2 for Co) may or may not be of consequence for catalysis applications and would have to be examined for each specific case. Analogous iron and cobalt dialdiminate complexes in solution are air sensitive.52 As cobalt(II) complexes are typically less unstable in air than iron(II) complexes, the cobalt chemistry was pursued to determine if the cobalt was only physically trapped or complexed by the ligand, using UV-visible and FTIR-ATR spectroscopies (Figures 8 and S2). Due to instrument limitations, spectra were taken after the samples were exposed to air, so their results are of limited significance. Nevertheless, comparison between their spectra and the spectra of the


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similarly treated metal free monolayers showed clear differences, suggesting that the metal was likely complexed by the ligand SAM.

Figure 8 Change in the diimine region of the IR spectrum of SAMDiimine2 before (a) and after (b) cobalt insertion

We compared the UV-Vis spectrum of the Co-SAMDiimine2 complex after air exposure with the fully characterized non-immobilized complex. Comparison of the spectra showed a good correlation between the immobilized and the free complex, thus supporting the ability of the monolayer to chelate the metal. However, in this case as well, excess cobalt remained in the system: XPS showed a 1:1.5 Co:N ratio. This is significantly more metal than the 1:5 ratio expected if the cobalt were binding only to the diimine. Therefore, again, the SAM did not bind metal solely at the desired diimine site. Because of air exposure before XPS analysis, both iron and cobalt appear in their oxidized Fe(III) and Co(III) forms. We have previously reported that similar ligands can bind these metals in this more oxidized state.53 CONCLUSION We have explored a number of different approaches to attaching diiminate ligands and their complexes onto silicon and quartz surfaces. Successful approaches used thiol-ene/silane-ene photochemistry or azide-acetylene cycloaddition chemistry as the key steps in the process of


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attaching the ligands to the surface. We note that while azide-acetylene chemistry is a proven, efficient, approach for ligand attachment, the resulting triazole incorporates extra metal in the immobilized ligand monolayers. We also note that in our specific example, the surface incorporation of attached ligand is significantly higher using thiol-ene chemistry than with azideacetylene cycloaddition chemistry. The ultimate optimization of these systems will also have to address problems related to the presence of extraneous metal ions.

ASSOCIATED CONTENT Supporting Information. This material includes details of the preparation and characterization of the organic compounds; additional information about the surface-bound ligands and complexes and FTIR, UV-Vis and XPS spectra and is available free of charge at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected] Notes: The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge the support of the US-Israel Binational Science Foundation and of the Edward and Judith Steinberg Chair in Nanotechnology at Bar Ilan University. References (1) (2)

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