Tolerant to air r -alkane complexes by surface modification of single crystalline solid-state molecular organometallics using vapour-phase cationic polymerisation: SMOM@polymer †

Vapour-phase surface-initiated cationic polymerisation of ethylvinylether occurs at single-crystals of the r -alkane complex [Rh(Cy 2 PCH 2 CH 2 PCy 2 )(NBA)][BAr F4 ]. This new surface interface makes these normally very air sensitive materials tolerant to air, while also allowing for onward single-crystal to single-crystal reactivity at metal sites within the lattice. The modification of inorganic materials with covalently anchored polymer chains installs a functional interface at their surface. This allows for properties ( e.g. chemical stability, hydrophobicity and guest exchange rates) of the resulting composite material to be systematically modified for applications in catalysis, medicine, optoelectronics, sensors, coatings and separation. 1,2 One important method for the synthesis of such hybrid materials is surface-initiated polymerisation, 3 where a surface site promotes polymer chain growth on the platform material of choice; e.g. 2-D surfaces, 4,5 nanoparticles, 6 metal organic frameworks (MOFs) 7,8 and supported molecular catalysts. 9 Such grafting-from methodologies are distinct from grafting-to techniques, where a preformed polymer chain is attached to a surface. 4,10 The functionalisation of discrete single-crystalline materials by surface-initiated polymerisation is, however, much less common. Such single-crystal@polymer composites have been reported for MOFs, 7 polyoxometallate frameworks (POMs), 11 benzoic acids Surface-initiated polymerisation to give a SMOM@polymer. SC = single crystal.

The modification of inorganic materials with covalently anchored polymer chains installs a functional interface at their surface. This allows for properties (e.g. chemical stability, hydrophobicity and guest exchange rates) of the resulting composite material to be systematically modified for applications in catalysis, medicine, optoelectronics, sensors, coatings and separation. 1,2 One important method for the synthesis of such hybrid materials is surfaceinitiated polymerisation, 3 where a surface site promotes polymer chain growth on the platform material of choice; e.g. 2-D surfaces, 4,5 nanoparticles, 6 metal organic frameworks (MOFs) 7,8 and supported molecular catalysts. 9 Such grafting-from methodologies are distinct from grafting-to techniques, where a preformed polymer chain is attached to a surface. 4,10 The functionalisation of discrete singlecrystalline materials by surface-initiated polymerisation is, however, much less common. Such single-crystal@polymer composites have been reported for MOFs, 7 polyoxometallate frameworks (POMs), 11 benzoic acids 12 and alkali halides; 13 all of which present chemically robust platforms for surface functionalisation. Closely related work has shown that single-crystals of Co(OR)(salph) (R = Me, Ac) promote ethylene oxide polymerization, with growth occurring at specific loci on the surface of the molecular crystal rather than the whole surface. 14 We (NBA = norbornane), in a solid/gas single-crystal to single-crystal (SC-SC) reaction, Scheme 1A. [16][17][18] The NBA-ligand can be displaced in a further SC-SC solid/gas transformation, by substrates such as propene 17 or isobutene, 18 and this exchange is facilitated by the hydrophobic network of CF 3 -groups in the non-porous lattice. 19 We have termed this overarching concept solid-state molecular organometallic chemistry (SMOM-chem). 17 We reasoned that if this displacement initially occurred at the surface of the crystal 20 use of a volatile monomer would result in a vapour phase 13 cationic polymerisation at surface {Rh(Cy 2 PCH 2 CH 2 PCy 2 )} + initiation sites. In this contribution we report that this is the case, and that by careful control of reaction conditions single-crystallinity can be retained in this process, Scheme 1B. The polymer interface makes these normally extremely air-sensitive crystalline materials tolerant to air, and also allows for solid/gas reactivity to occur at the metal-sites within the molecular crystal, in a SC-SC transformation.
Ethylvinylether (EVE) was chosen as the volatile monomer, due to its low boiling point (33 1C), well-established cationic polymerisation chemistry to form poly(ethylvinylether) using homogeneous 21,22 and heterogeneous catalysis, 23 and use in grafting-from processes on silica surfaces. 24 Two methodologies were developed to use discrete SMOM single-crystals under vapour-phase polymerisation conditions, Fig. 1.
For Method A, a single crystal of precursor [1-NBD][BAr F 4 ] (1.5 mg, 1 Â 1 Â 2 mm) was mounted, using a dab of silicon grease, on the side of a 50 cm 3 flask fitted with a PFTE greaseless stopcock and a glass insert to contain EVE monomer, Fig. 1B After 15 minutes the formation of liquid polymer was observed around the crystal. Over a 48 h period this pooled at the bottom of the flask, removing it from the locus of the solid-catalyst. Analysis of this colourless oil by gel permeation chromatography (GPC) and NMR spectroscopy showed it to be atactic 21,25 poly(ethylvinylether): M n(average) = 21 500 g mol À1 (Ð = 2.5). 26 While the crystal maintained visual integrity, after 15 minutes crystallinity was lost (vide infra). Using the same total mass of precursor but smaller crystals (3 Â 0.5 mg, 0.5 Â 0.5 Â 1 mm) led to shorter polymer chains with a wider distribution (M n(average) = 10 900 g mol À1 , Ð = 3.6), consistent with an increased number of surface initiating sites. 27 No polymerisation was observed using [1-NBD][BAr F 4 ]. While this methodology allowed for bulk polymer to be prepared, a temporal analysis of the catalyst was challenging. This was overcome by a modification of the experimental procedure (Method B), in which 5-6 crystals of precursor [1-NBD][BAr F 4 ] (B1 mg each) were mounted on specially adapted PFTE stopcock fitted with a metal needle, Fig. 1C. This technique allowed for expedient analysis of crystalline SMOM@polymer by scanning electron microscopy (SEM), NMR spectroscopy and single-crystal X-ray diffraction; especially at the early stages of surface polymerisation (0-15 minutes).  16 Additional signals at d 105 and d 80 are tentatively assigned to polymer-bound surface species.
After 2.5 minutes exposure to EVE vapour the polymer layer has increased, as shown by SEM (Fig. 2D), EDX that now reveals no fluorine, and in the 1 H NMR spectrum of the dissolved sample in which the polymer ether groups are now clearly observed: ratio with [BAr   by single-crystal X-ray diffraction provides a very good refinement and solution for [BAr F 4 ]@poly(ethylvinylether) (R = 7.5%) that is essentially isostructural with [1-NBA][BAr F 4 ]. 16 After 2.5 minutes exposure (Fig. 2D) all high-angle data is lost and no structural refinement was possible. 15 minutes exposure to EVE resulted in complete loss of Bragg reflections.
The grafted-from polymer coating on [BAr F 4 ]@poly-(ethylvinylether) makes crystals of this s-alkane complex remarkably tolerant to air. Fig. 3A 16 (Fig. 3C, R = 4.9%). While longer contact times with air resulted in significant decomposition, low-angle Bragg peaks are still evident after 8 hours. Polymer matrices have been previously used to protect electrochemical hydrogen oxidation catalysts towards oxygen damage, 30 or stabilise MOF nanoparticles towards decomposition by air. 8 The polymer coating in [1-NBA][BAr F 4 ]@poly(ethylvinylether) allows for solid/gas SC-SC transformations to occur at the metal centre in the bulk crystal, albeit at an attenuated rate. Exposure to propene (1 bar, 298 K) for 5 days resulted in exchange of the NBA ligand for propene, to form [1-propene][BAr F 4 ]@poly(ethylvinylether), in a SC-SC transformation. This is much slower than for uncoated crystals (2 h 17 ) consistent with the polymer layer, as measured by NMR spectroscopy. The structural refinement (R = 10%) shows a propene ligand bound through alkene and agostic RhÁ Á ÁH 3 C groups, as reported for unfunctionalised [1-propene][BAr F 4 ]. 17 A possible mechanism for the surface-initiated cationic polymerisation of EVE is shown in Fig. 4. [Rh(Cy 2 PCH 2 CH 2 PCy 2 )-(NBA)] + cations close to the surface undergo rapid substitution with EVE to form an intermediate such as A, similar to those proposed as initiating sites for homogeneous cationic polymerisations using transition metal complexes. 22 Chain-propagation then leads to polymer brushes 1 and the resulting SMOM@ polymer. Mechanical stress from growing polymer chains may well lead to detachment from the crystal surface over time, which would also contribute to the observed loss in crystallinity.
In support of this mechanism we have characterised a model complex for intermediate A by using diethyl ether as a saturated analogue of EVE, that mimics initiation but does not propagate. Fig. 5  In summary, vapour phase grafting-from SMOM@polymer method provides a simple method for stabilisation of reactive, crystalline, molecular organometallic species towards air while also allowing for single-crystal structural determinations and retaining bulk SC-SC reactivity. That this methodology results in a s-alkane complex becoming air tolerant in the solid-state further extends the potential use of these fascinating, and reactive, 15 complexes as precursors in synthesis and catalysis. 28 We thank the EPSRC (EP/M024210), Leverhulme Trust (RPG-2015-447) and SCG Chemicals for funding.

Conflicts of interest
There are no conflicts to declare.