Cross-Coupling Polymerization at Iodophenyl Thin Films Prepared By
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1 Cross-coupling polymerization at 2 iodophenyl thin films prepared by 3 spontaneous grafting of a diazonium salt 1 2 4 Nicholas Marshall and Andres Rodriguez 1 5 Department of Chemistry and Physics, University of South Carolina Aiken, 471 6 University Parkway, Aiken, SC 29801, USA 2 7 College of Pharmacy, Medical University of South Carolina, 280 Calhoun Street, 8 Charleston, SC 29425, USA 9 Corresponding author: 1 10 Nicholas Marshall 11 Email address: [email protected] 12 ABSTRACT 13 Cross-coupling at aryl halide thin films has been well-established as a technique for the surface- 14 confinement of the Kumada catalyst transfer polymerization (KCTP) reaction. The spontaneous grafting of 15 4-iodobenzenediazonium tetrafluoroborate on gold substrates creates a durable thin film which is effective 16 as a substrate for cross-coupling reactions including the surface-initiated KCTP reaction. Using cyclic 17 voltammetry of a surface-coupled ferrocene derivative, we have measured initiator surface coverage 18 produced by oxidative addition of Pd(t-Bu3P)2 and used the resulting initiator to prepare thick, well-defined 19 polythiophene thin films. 20 INTRODUCTION 21 Since the development of the first polyacetylenes by Shirakawa and MacDiarmid in the 1970s, (Chiang 22 et al. (1977, 1978)) applications of pi-conjugated polymers (CPs) have proliferated. A number of refined 23 synthetic approaches for forming these polymers have been developed. CPs based on arene repeat units 24 are the single largest category of CP with current practical applications, due to the inherent stability 25 of the aromatic system vs. oxidation. To prepare polyarenes, oxidative polymerization approaches are 26 common. (Kaloni et al. (2017); Niemi et al. (1992)) Oxidative polymerization is difficult to control 27 and applicable only to electron-rich arene monomers. More recently, cross-coupling strategies have 28 been developed, (Yamamoto (2002); Heeger (2001)) most commonly coupling of a aryl dihalide with 29 an arene bearing two nucleophilic groups, e.g., an “AA/BB” strategy. Stille and Suzuki coupling are 30 commonly employed in these syntheses. (Bao et al. (1999); Argun et al. (2004),Yiu et al. (2012)) The 31 single greatest refinement in the cross-coupling synthesis of polyarene CPs came in the mid-2000s from 32 the Yokozawa and McCullough groups, (Yokoyama and Yokozawa (2007); Sheina et al. (2004)) in 33 which “AB” monomers were prepared by monometallation of an aryl dihalide, most commonly through 34 magnesium-halogen exchange to prepare an arylmagnesium halide monomer. This modification yielded a 35 dramatic improvement in polydispersity of the prepared polymers, especially polythiophenes and poly(p- 36 phenylenes). The improvement is attributed to a change in mechanism, from a typical cross-coupling 37 catalytic cycle to the so-called “catalyst-transfer” (CT) mechanism. (Miyakoshi et al. (2005)) In this 38 mechanism, the zerovalent, coordinatively unsaturated transition metal does not dissociate from the 39 growing polymer chain, but remains complexed to the pi-system. (Figure 1) Due to this complexation, 40 the zerovalent center reacts next with the halide endgroup of the same polymer chain, and chain-growth, 41 pseudoliving polymerization ensues. (Bryan and McNeil (2013)) 42 The existence of this CT effect has been borne out by a number of theoretical studies, (He et al. 43 (2018)) as well as exploits in which CTP is used to prepare block copolymers and end-functionalized 44 polymers. (Zhang et al. (2018); Yokozawa and Yokoyama (2009); Aplan and Gomez (2017)) Many of the Figure 1. The Kumada catalyst-transfer polymerization (CTP) of a magnesiated dihaloarene proceeds in a chain-growth fashion, yielding a polyarene chain bound to the initiator. 45 most common applications of CPs involve their deposition in the form of thin films, including uses in 46 photovoltaics, electrochromics, and sensors. (Marshall et al. (2011)) 47 In the late 2000s, we and others used CTP to prepare conjugated polymer films grafted from a surface 48 (SI-CTP). (Sontag et al. (2009)) Typically, this feat is accomplished by formation of a self-assembled 49 monolayer (SAM) bearing an aryl halide, followed by reaction of the halide film surface with a zerovalent 50 metal precatalyst to form a surface-bound metal complex. Thin films prepared from silanes are most 51 commonly used in the preparation of the aryl halide film. This approach is reasonably effective and 52 many interesting surface structures have been prepared using it. However, limitations of silane-based 53 initiators exist; purification of these materials is often problematic, the surface produced is highly variable 54 based on difficult-to-control factors such as moisture content, and the most effective coupling agents 55 for SAM formation contain a long central alkyl chain which limits electronic coupling between the 56 surface and the endgroup, an undesirable property for electronic applications. So, we sought to develop 57 a more convenient initiator system for a SI-CTP reaction, specifically the surface-initiated Kumada 58 polymerization (SI-KCTP). 59 Reductive electrografting of aryl diazonium salts is well-established as a surface modification protocol. 60 (Mahouche-Chergui et al. (2011); Assresahegn et al. (2015)) While careful controls such as use of 61 antioxidant additives (Anariba et al. (2003); Menanteau et al. (2013)) or incorporation of a bulky protecting 62 group (Combellas et al. (2008)) are necessary to ensure formation of a well-defined monolayer, this method 63 is known to form highly functionalized and durable thin films composed of a conductive arene multilayer. 64 (Lee et al. (2012)) While deposition of thin films from aryldiazonium salts using electrochemical reduction 65 of the diazonium salt is a well-known technique for surface functionalization, spontaneous reaction of 66 aryldiazonium salts with a surface is less common. However, a number of groups have explored this 67 approach in recent years. (Stewart et al. (2004); Podvorica et al. (2009); Mesnage et al. (2012)) Terminal 68 functional groups (as the para substituent) which have been deposited using a spontaneous method include 69 NO2, (Laurentius et al. (2011); Cullen et al. (2012)) COOH, (Polsky et al. (2008)) n-alkyl, (Combellas 70 et al. (2005a)) perfluoroalkyl (Combellas et al. (2005b)) diazonium, (Marshall et al. (2018)) and amine 71 (Kesavan and Abraham John (2014)). In particular, halide-functionalized thin films formed by spontaneous 72 diazonium grafting from organic solutions have not been reported, and only a few (2,4,6-trichlorophenyl 73 and 4-bromophenyl) (Mesnage et al. (2012)) from aqueous solution. The Tour group has prepared a 74 great variety of conjugated linkers deposited spontaneously from organic solvents, particularly on silicon 75 surfaces. (Kosynkin and Tour (2001)) 76 Most work in this area uses aqueous solutions of the diazonium salt, exploiting the inherent instability 77 of arenediazonium salts in water due to diazonium hydrolysis to diazohydroxide compounds. (Lewis 78 and Johnson (1960)) Diazonium salt hydrolysis reliably forms thin films on metallic, oxide, and carbon 79 surfaces. (Combellas et al. (2005a); Podvorica et al. (2009); Lehr et al. (2010); Berisha et al. (2016)) 80 These films contain a substantial fraction of azo R-N=N-R’ linkages, and XPS evidence indicates that the 81 aryl film is sometimes linked to metal surfaces through a nitrogen-metal bond. (Combellas et al. (2005a)) 82 However, evidence is beginning to emerge that spontaneous deposition in acetonitrile (and likely other + 83 polar aprotic solvents) proceeds by a different mechanism, possibly by direct Au catalysis of C-N2 bond 2/11 84 cleavage yielding C-Au bonds. (Mesnage et al. (2012)) Bolstering this hypothesis, we have found in this 85 work and past studies that arenediazonium-based films formed spontaneously from acetonitrile generally 86 do not contain any nitrogen at all, in sharp contrast to aqueous-based films driven by diazohydroxide 87 deposition. 88 As a component of this work, we needed to demonstrate the spontaneous grafting of the aryldiazonium 89 halide 4-iodobenzenediazonium tetrafluoroborate, and demonstrate that the resulting layer reacts to form 90 a surface-bound initiator for SI-KCTP. We found that 4-iodobenzenediazonium salt spontaneously forms 91 a thin film at a clean gold surface, and that the resulting aryl iodide layer is convenient and effective for 92 cross-coupling. (Scheme 2) In particular, this iodoarene layer yields a high density of surface-bound 93 Pd(II) sites active in the cross-coupling reaction as measured electrochemically using a well-known 94 ferrocenyl probe, ferrocenyl 2-(5-chloromagnesiothienyl)methane (FcCH2ThMgCl). This surface-bound 95 cross-coupling initiator also reacts effectively with a thiophene AB monomer to yield polythiophene 96 brushes. Figure 2. The aryl diazonium salt 4-iodobenzenediazonium tetrafluoroborate reacts spontaneously with freshly cleaned Au surfaces to yield an aryl iodide-functionalized surface. Figure 3. Reaction of an aryl iodide layer on gold with Pd(0) followed by Grignard reagents yields a ferrocene-functionalized layer with a Fc Grignard reagent, and a thick polythiophene film with a metallated A-B monomer in the SI-KCTP reaction. 97 In this work, we report a useful instance of spontaneous aryl diazonium salt grafting to a gold surface 98 to prepare a functionalized surface which can serve as an initiator platform for the Pd-catalyzed SI-KCTP 99 reaction. XPS survey scans of the functionalized surface revealed no nitrogen in the film, supporting 100 the hypothesis that gold can directly catalyze the dissociation of the diazonium salt to give dinitrogen. 101 The resulting surface has a high density of reactive groups as measured under standard conditions for 102 evaluation of SI-KCTP initiator surfaces, and a remarkably thick, brushlike polythiophene film is formed 103 on the surface when used in the SI-KCTP reaction. The convenience of this preparation method, and the 104 thickness of the films formed, are promising for this particular method’s potential for device construction.