Cody Williams

Michel Ferreira

Eric Monflier

Selwyn F Mapolie

Gregory S Smith
email: .gregory.smith@uct.ac.za

Synthesis and hydroformylation evaluation of Fréchet-type organometallic dendrons with N , O -salicylaldimine Rh

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

In the current era, the demand for highly specific materials is increasing at an alarming rate. These materials should be easily functionalised to address an array of utilitarian applications. Dendrimers form one class of these macromolecules which addresses these challenges. The inception of dendrimers in the 1970's has stimulated interest into the study and synthesis of these versatile structures. 1 The introduction of metals into dendrimers allows for a plethora of applications, which are predominantly used in catalytic, sensing and biomedical spheres. 2 This metal-dendrimer relationship offers a niche of new materials, which emanates from the synergistic effect between a metal and dendrimer. These effects are often attributed to their cooperativity, steric control, recyclability and site isolation. Since the seminal work by Balzani and Newkome on metallodendrimers in the 1990's, a myriad of metallodendrimers were discovered, thus opening new vistas in chemistry. 3 In the context of this work, core-functionalised organometallic dendrimers are of particular importance, as these structures are often less explored due to drawbacks associated with the synthesis and purification of these dendrons. 4 More importantly, there are few examples of dendrons with transition metals at the focal point (Figure 1). Examples include Parrot's high generation polyester dendrons containing a rhenium bis(pyridyl)amine moiety at the core used as precursors for radiolabelling studies, de la Mata's watersoluble carbosilane dendrons containing ferrocene at the focal point used as anti-bacterial agents and Gómez-Sal's Fréchettype palladium-dendrimers containing bis(pyrazolyl) methane ligands at the core used as Heck cross-coupling catalyst precursors, 5 to name a few.

[insert Figure 1 here]

Figure 1

Selected examples of dendrons bearing transition metals at the focal point, namely, Gómez-Sal's Fréchet-type palladium-dendrimers (A), Parrot's rhenium bis(pyridyl)amine dendron (B) and de la Mata's water-soluble carbosilane dendrons containing ferrocene at the focal point (C). 5 The paucity of literature on the synthesis of organometallic dendrons containing discrete rhodium(I) entities at the focal point has motivated the study of these complexes. More importantly, there are no reports of dendrons with metals at the core being used in the hydroformylation reaction. The hydroformylation reaction, also known as the "Oxo process", is one of the most important industrial processes as it is estimated to produce 10 million tonnes of aldehydes annually. 6 This reaction is a transition metal-catalysed addition of carbon monoxide and hydrogen gas to olefins resulting in the formation of aldehydes. 7 The homogeneity of the reaction allows for superior activity and selectivity in comparison to the heterogeneous counterpart. However, the separation of the catalyst and product is much more facile in the latter process. 7- 8 One means of overcoming this drawback is to immobilise the complex onto a dendritic support to facilitate this separation. 9 Once anchored to dendrimers, these can be effectively separated by ultrafiltration or membrane technology. 10 Dendrimers can aid in the stabilisation of the metal complex, by virtue of the microenvironment imparted by the dendritic architecture. 9c, 11 Most scaffolds utilised in the hydroformylation reaction are based on DAB-PPI, PAMAM or carbosilane scaffolds, with the metals located on the periphery or interior cavities of these dendritic architectures. [START_REF] Astruc | [END_REF] The majority of dendrimers used for hydroformylation reactions are anchored to insoluble supports, thus the activity and selectivity of the catalysts are hampered due to the heterogeneous nature of the catalysts. 9c, 13 Selected examples include Arya and Alper's 9c, 13b dendritic arms anchored to amide-resin beads, Cole-Hamilton's 13d, 14 polyhedral silsesquioxane dendrimers, Alper's 13a PAMAM dendrons containing silica-gel particles at the core and Smith's 9a water-soluble DAB-PPI-N,O-salicylaldimine dendrimer. More importantly, Cole-Hamilton, Alper and Smith formed the discrete metallodendrimers in situ. 9a, 9c, 13a, 13c, 13d More importantly, Ropartz, Bourque and Hager formed the discrete metallodendrimers in situ. 9a, 9c, 13a, 13c, 13d As an extension to our previous studies, this work is aimed at evaluating the effect of the dendron size on the hydroformylation reaction, using 1-octene as the model substrate. 9a, 15 To the best of our knowledge, no work has been reported using Fréchet-type dendritic scaffolds with rhodium at the focal point in the hydroformylation reaction. Herein, we report on the catalytic evaluation of mononuclear Rh(I) organometallic dendrons as catalyst precursors in the hydroformylation of olefins.

Results and discussion

Synthesis and characterisation of Fréchet dendrons with N,Osalicylaldimine ligands at the focal point.

4-Hydroxybenzylidene-2-hydroxyaniline was coupled to the bromide-Fréchet dendrons via a Williamson-ether reaction (Scheme 1). The Schiff-base dendron products were isolated as stable yellow powders in high yields. The 1 H NMR spectra of the ligands 1-3 display one broad phenolic signal (X, Scheme 1) in comparison to the two phenolic signals (X and Y, Scheme 1) observed for 4-hydroxybenzylidene-2-hydroxyaniline. This signal integrates in a 1:1 ratio with the diagnostic imine proton singlet observed between 8.49 -8.53 ppm in the 1 H NMR spectrum. This suggests that the Williamson-ether reaction proceeds with 100% regioselectivity at position X. In the 1 H NMR spectrum of 1-3, the aromatic protons are observed between 6.30 -8.05 ppm, methylene and methoxy proton signals are observed between 3.83 -5.07 ppm. The IR spectrum displays the imine as an intense absorption band at 1617 cm -1 . EI-MS, ESI-MS and MALDI-TOF-MS were performed on ligands 1, 2 and 3 respectively. The mass spectra show peaks at m/z = 361 [M] + , 632 [M + H] + and 1172 [M + H] + for ligands 1, 2 and 3 respectively. To further confirm the regioselectivity, ligand 1 was also characterised using single-crystal X-ray diffraction. The crystals were obtained by slow diffusion of pentane into a concentrated solution of ligand 1 dissolved in dichloromethane. The ORTEP illustration with atom labelling is displayed in Fig. 2. Ligand 1 crystallizes in the orthorhombic space group, Pna21, with six molecules present per unit cell. The torsion angle about the imine bond is observed to be 175°, thus confirming the trans-configuration of the imine in the solid state. This observation is consistent for structurally similar compounds obtained by Smith and co-workers. 15b, 15d More importantly, an intramolecular O1-H1

•••N1 hydrogen bond (H1•••N1 distance = 1.78 Å, O1-H1•••N1 angle = 147°
) is observed for ligand 1. 15b, 16 These observations are consistent for structurally similar Schiffbase compounds. 15b, 16 [insert Scheme 1 here]

Scheme 1

Synthesis of the Fréchet dendrons with N,Osalicylaldimine ligands (1-3) or Fréchet organometallic dendrons with Rh(I) N,O-salicylaldimine complexes at the focal point (4-9).

[insert Figure 2 here]

Figure 2

Molecular structure of 1 as elucidated by single crystal X-ray diffraction. Hydrogen atoms are omitted for clarity.

Synthesis and characterisation of Fréchet dendrons with N,Osalicylaldimine Rh(I)-COD complexes at the focal point.

The Fréchet dendrons with N,O-salicylaldimine ligands at the focal point (1-3) were dissolved in a minimum amount of dichloromethane. The deprotonation of the phenolic proton was accomplished using excess equivalents of NEt3. The dimeric metal precursor [RhCl(η 2 :η 2 -COD)]2 was reacted with the ligands to afford complexes 4-6 as depicted in Scheme 1. The complexes were isolated as stable yellow powders in high yields. The key evidence of successful complexation in 1 H NMR is the upfield shift of the imine proton resonance from 8.49 -8.53 ppm in the free ligand to 7.96 -7.97 ppm in the complex. The upfield shift is attributed to the increased shielding of the proton due to the back-donation of Rh-metal via the imine nitrogen. Furthermore, the imine signal is observed as a doublet, indicative of coupling to the spin active 103 Rh centre. In comparison to the ligands (1-3) no phenolic proton signal between 13.25 -13.26 ppm was observed for the resultant complexes 4-6, which confirms the deprotonation and subsequent bidentate co-ordination mode of the salicylaldimine ligand to the Rh(I) metal. The presence of 4 new signals are observed. These are attributed to the olefinic and methylene proton signals of the COD moiety. A splitting of the olefinic signals is observed, which is attributed to the trans effect induced by the chelating N,O-salicylaldimine moiety. This phenomenon is consistent with structurally similar N,Ochelating Rh(I) complexes. 15a, 17 In the 13 C{ 1 H} NMR spectra, the number of carbon signals correlate with the proposed complexes. The IR spectra show a characteristic imine absorption band between 1605 -cm -1 . The shift to lower wavenumber from the metal-free ligand is expected due to the synergic effect and further corroborates the coordination to the imine nitrogen. EI-MS, ESI-MS and MALDI-TOF-MS confirmed the integrity of the proposed structures, owing to the observation of [M] + , [M-Rh-COD] + and [M + H] + fragments for 4, 5 and 6 respectively. Single crystal Xray diffraction further confirmed the molecular structure of complex 4. Single crystals were obtained for 4 via a slow diffusion of petroleum ether into a concentrated dichloromethane solution of the complex. The compound crystallizes in the monoclinic P21/c space group. Four molecules per unit cell were observed. Furthermore the imine bonds were found to be anti to the aryl moieties, as indicated by the torsion angles around the imine bond (176°). This is comparable to structurally similar structures. 15d

The molecular structure of complex 4 shows a square-planar geometry at the rhodium metal centre. The rhodium metal is co-ordinated to the COD moiety in an (η 2 :η 2 ) manner and to the N,O-chelating ligand in a bidentate fashion (Figure 3). The bond angles between the metal centre and the N,O-salicylaldimine moiety are between 81 -94°. This is similar to structurally analogous compounds reported and further confirms the square-planar geometry at the metal centre. 15d, 17 The formation of a six-membered chelate ring imposes distortion around the rhodium atom. The distortion is evident as the O1-Rh1-C27 angle (161°) is smaller than the expected value of 180°.

[insert Figure 3 here] Figure 3 ORTEP representaton of the molecular structure of 4, as elucidated by single crystal X-ray diffraction. Hydrogen atoms are omitted for clarity.

Synthesis and characterisation of Fréchet dendrons with N,Osalicylaldimine Rh(I)-(CO)2 complexes at the focal point

Complexes 4-6 were dissolved in DCM and CO was bubbled through the solution to displace the COD moiety. Complexes 7, 8 and 9 were isolated in high yields as an orange, red or yellow powders respectively. The 1 H NMR spectra for complexes 7-9 displays a downfield shift for the imine proton resonance from 7.96 -7.97 to 8.16 -8.17 ppm, which suggests that CO is a better π-acceptor than the N,O-salicylaldimine moiety. The imine proton signal is observed as a doublet, similar to complexes 4-6. The absence of the COD signal indicates successful displacement by CO. In 13 C{ 1 H} NMR, the CO signals are observed as two sets of doublets, with a coupling constant ( 1 JRh-C) which ranges between 68 -72 Hz for the complexes 7-9. This is consistent with structurally similar rhodium carbonyl complexes. 15e, 18 The IR spectroscopic analysis shows two terminal carbonyl absorption bands between 2000 -2007 cm -1 . HR-ESI-MS shows [M] + and [M-(CO)2] + peaks for complexes 7-9, the latter fragment being attributed to the loss of the carbonyl ligands. Single crystal X-ray diffraction further confirmed the molecular structure of complex 7. Single crystals were obtained for 7 via a slow evaporation of a concentrated dichloromethane solution of the complex. Like complex 4, complex 7 crystallizes in the monoclinic P21/c space group. The molecular structure of complex 7 shows a square-planar geometry at the rhodium metal centre. This is confirmed by the bond angles about the metal centre which are between 87 -94°. The bond angle between O1A-Rh1A-C24A is 175°, which is lower than the expected 180° due to the distortion caused by the six-membered chelate ring. A comparison of the crystallographic information for complex 1 and 4 revealed similar bond lengths and angles about the salicylaldimine moiety. A slight shortening of the imine bond is observe from 1.303 Å to 1.298 Å, which supports the conclusion from the IR spectroscopic analysis that CO is a better π-acceptor than the N,O-salicylaldimine ligand. A small change was observed for the chelation angle (N-Rh-O) which suggests the chelate is unaffected by the change in ancillary ligand.

[insert Figure 4 here] Table 1 Selected crystallographic data for the molecular structures of 1, 4 and 7.

Preliminary screening

Catalyst precursor 7 was selected as a model catalyst as it has the simplest and most representative structure of all the catalysts synthesized. Conditions were optimised with respect to temperature and pressure (Table 2, vide infra). At fixed temperature (either 75°C or 95°C), an increase in pressure results in a general increase in conversion. It is understood that the increased CO pressure results in increased CO concentration in solution. The vacant site of the HRh(CO)L2 (where L denotes the appropriate ligand) species is occupied by an additional CO, which limits isomerisation and accelerates the CO migration step. This process results in much faster conversion of olefin to aldehydes and was subsequently observed for catalyst precursor 7 in this study. Similarly at fixed pressure, an increase in conversion is observed with an increase in temperature. This indicates that more energy is required for the activation of the alkene and subsequent hydroformylation. The catalyst precursor 7 displays moderate regioselectivity and excellent chemoselectivity at higher pressures and temperatures. The regioselectivity for catalyst 7 at 55 °C and 75 °C are comparable which suggests the active species is the same across the conditions tested. When comparing temperatures at 75 °C and 95 °C, it is noted that the percentage of branched aldehydes increases, which ultimately lowers the n:iso ratio. One postulation is that the formation of the tetracarbonyl rhodium species [Rh(CO)4] + at higher pressures, could influence the selectivity of the catalyst. 15b Hence based on this observation, the pressure and temperature which gives quantitative conversion and the best n:iso ratio was 30 bar and 75 °C. This result correlates with the optimised hydroformylation conditions for similar complexes. 9a, 15b With respect to the chemoselectivity, an increased percentage of iso-octenes are observed at lower pressures and temperatures. The isomerisation behaviour may be ascribed to either lower CO concentration in solution or the formation of a different catalytic species. One form is attributed to the formation of rhodium nanoparticles which are known to catalyse the isomerisation of olefins. 15d Mercury drop experiments have been performed and are discussed later.

Table 2 Hydroformylation of 1-octene using catalyst precursors 4 -9.

[insert Table 2 here] Feedstock Variation Having identified the optimised conditions as 75°C and 30 bar, the catalyst precursor 7 was evaluated in the hydroformylation reaction against various substrates. These include 1-octene, styrene, 7-tetradecene, methyl oleate, triolein, D-limonene and R-citronellal. Catalyst precursor 7 displays excellent conversion (86-100%) and chemoselectivity towards aldehydes for 1-octene, methyl oleate, 7-tetradecene and D-limonene. For D-limonene, no hydroformylation was observed for the cyclic olefin which demonstrates the catalysts' high selectivity towards terminal olefins. Selected internal olefins were tested, these include 7-tetradecene, methyl oleate and triolein. Moderate to good conversions were observed for these substrates (51-77%). The regioselectivity of the major branched product ranged between 85-98%. This result indicates the isomerisation is less favoured under the conditions tested, which bodes well for potential tandem-catalytic reactions.

Table 3 Hydroformylation of various substrates using catalyst precursor 7.

[insert Table 3 here] Effect of the ancillary ligands. The Rh(CO)2 precursors (7-9) display superior turnover frequencies in comparison to the analogous Rh(COD) precursors (4-6). In the hydroformylation reaction, it is thought that the COD ancillary ligand is displaced under hydroformylation conditions, resulting as either dicarbonyl or carbonyl hydride species. In the dicarbonyl case (4-6), this process is absent, which theoretically results in a shorter time to form the active species. This postulation is in accordance with the observed results. However further studies are required to understand the N,O-salicylaldimine catalyst precursors under hydroformylation conditions. The chemoselectivity and regioselectivity for complexes 4-6 vs 7-9 are within the standard deviation for the experiments as expected due to the structural similarity of the catalyst precursors.

Effect of the dendron size. The catalyst performances across the dendron series are comparable (Figure 5). This was expected as no significant chemical shift changes are observed for the aromatic proton signals of the Schiff-base moiety when comparing the spectroscopic and analytical results for the G0 (4 and 7), G1 (5 and 8) and G2 (6 and 9) analogues. Furthermore the dendron is not a conjugated system, which does not significantly affect the electronics of the catalyst precursor. A general trend across the dendron series is a slight increase in the chemoselectivity to form aldehydes. This may be attributed to the bulkier nature (steric-effect) of the dendrons, which limits the isomerisation from terminal to internal alkenes. Furthermore, the electronics are similar across the dendron series, which further attests to the similar chemoselectivity observed. In both complexes 5 and 8 (G1), a drop in the activity is observed. However, this drop in the 1 st generation species is often observed for dendrimers. These are described in literature for similar dendritic catalysts applied in the hydroformylation reaction. 13b, 13c A general trend observed across the dendron series (G0 -G2) is that the regioselectivity is consistent. One reason is that the dendron is presumed to be too far from the catalytic centre to impact on the regioselectivity of the hydroformylation reaction.

[insert Figure 5 here]

Selectivity as a function of time.

The chemoselectivity and regioselectivity of the Rh(I) complexes (4 and 7) were evaluated at various times (Figure 6). This was conducted using the optimised conditions 75 °C and 30 bar. The precursors were evaluated at 1 hour, 2 hours, 4 hours and 8 hours respectively. A general trend is observed after 1 hour, indicating the chemoselectivity towards aldehydes ranging between 52-53% for complexes 4 and 7. An increase in the chemoselectivity towards aldehydes is observed which ranged between 76-79% for the complexes (4 and 7) after 2 hours, 78-87% after 4 hours and reached 100% (complete aldehyde formation) after 8 hours. Hence the chemoselectivity towards aldehydes increases as a function of time for catalysts 4 and 7. From this result, the isomerisation products (iso-octenes) decrease as a function of time, which is due to the hydroformylation of iso-octenes to branched aldehydes. Further evidence of this is obtained by comparing the n:iso ratio at the various times tested. A notable decrease is observed with a function of time, which provides further evidence of the hydroformylation of iso-octenes to branced aldehydes at longer reaction times.

[insert Figure 6 here]

Mercury drop experiments.

A useful method for differentiating between homogeneous and heterogeneous catalysts is performing mercury drop experiments. 8, 15d, 19 If nanoparticles are formed in solution, the introduction of mercury results in the formation of an amalgam between mercury and the metalnanoparticles. 15a This amalgam is not active in the hydroformylation reaction, thus poisoning of the catalytic system results in a drop in the catalytic activity if nanoparticles are present. 15a The reactions were performed using the temperature (75 °C), time (2 hours) and pressure (30 bar) in the presence of mercury (Table 2, entries 15 and 16). A substantial drop in conversion is observed for the G0 compounds 4 and 7 in the presence of mercury. This suggests that the conversion is attributed to a combination of heterogeneous and homogeneous catalysis. The formation and accumulation of colloidal particles are often associated with the increase in the rate of isomerisation; this is consistent with results for catalyst precursors 4 and 7.

Experimental General Details

All reagents were purchased from commercial suppliers (KIMIX, Sigma-Aldrich, Merck and Acros Organics) and used without further purification unless otherwise stated. Rhodium(III) trichloride trihydrate was purchased from Heraeus SA. The rhodium chloro-1,5-cyclooctadiene dimer 20 [Rh(μ-Cl)(η 2 :η 2 -COD)]2, Fréchet dendrons 21 and 4-hydroxybenzylidene-2-hydroxyaniline 16 were prepared by means of published methods. Nuclear magnetic resonance (NMR) spectra were recorded on either a Bruker Topsin GmbH 400 plus ( 1 H: 400.22 MHz; 13 C{ 1 H}: 100.65 MHz; 31 P{ 1 H}: 162.01 MHz) or a Varian Mercury 300 ( 1 H: 300.08 MHz; 13 C{ 1 H}: 75.46 MHz; 31 P{ 1 H}: 121.47 MHz) spectrometer. These were equipped with a Bruker Biospin GmbH casing and sample injector at 30 °C. Chemical shifts for 1 H and 13 C{ 1 H} NMR were reported using tetramethylsilane (TMS) as the internal standard. Infrared (IR) absorptions were measured on a Perkin-Elmer Spectrum 100 FT-IR spectrometer using Attenuated Total Reflectance Infrared spectroscopy (ATR-IR). Electron impact mass spectrometry (EI-MS) was conducted using a JEOL GCMateII mass spectrometer. Low resolution and high resolution (HR) electrospray ionisation mass spectrometry (ESI-MS) was carried out on a Waters API Quattro mass spectrometer, with scans being conducted in the positive mode. Matrix assisted laser desorption time-of-flight (MALDI-TOF) mass spectra were carried out on a Bruker Daltronics Ultraflex 2 equipped with a nitrogen laser and operated at an accelerating voltage of 25 kV. The matrix utilised was 2,5-dihydroxybenzoic acid (6.5 mM). Elemental analysis (C, H, N) were performed using a Fissions EA 110 CHNS apparatus or an Elementar Vario EL Cube Analyser. Some of the data obtained are outside acceptable limits, this can be ascribed to the presence of solvent molecules due to the encapsulation. This is a common observation for dendritic molecules. 22 Melting Points were determined using a Büchi melting point apparatus B-540 and are uncorrected.

Fréchet dendrons with N,O-salicylaldimine ligand precursors at the focal point (1 -3)

General Method 4-Hydroxybenzylidene-2-hydroxyaniline (1.0 -2.0 eq.), potassium carbonate (1.0 -2 eq.) and 18-crown-ether (0.2 eq.) was dissolved in anhydrous acetone (30 mL) and refluxed for 1 hour. To this, a solution of the appropriate bromide-Fréchet dendron (1.0 eq.) in dry acetone (20 mL) was added dropwise. The resultant mixture was further refluxed for an additional 23 hours. The mixture was cooled to room temperature and evaporated to dryness under reduced pressure. The residue was partitioned between water (10 mL) and dichloromethane (10 mL), and the aqueous layer was extracted with dichloromethane (3 x 10 mL). The organic fractions were combined and collected. The combined organic fractions were dried using anhydrous magnesium sulfate. After removal of the drying agent by filtration, the solvent was removed to yield a brown residue. The crude product was purified as outlined in the following text for the respective ligands 1 -3.

Synthesis and characterisation of the G0-COOMe-N,Osalicylaldimine dendron (1).

4-Hydroxybenzylidene-2-hydroxyaniline (60.1 mg, 0.281 mmol, 1.00 eq.), potassium carbonate (39.2 mg, 0.281 mmol, 1.00 eq.), 18-crown-ether (15.2 mg, 0.0561 mmol, 0.200 eq.) and the G0bromido-Fréchet dendron (65.0 mg, 0.281 mmol, 1.00 eq.) were reacted in acetone. The residue was dissolved in a minimum amount of chloroform and was kept at 5 o C for 24 hours. The resultant crystals were filtered and washed with cold chloroform. The product (1) was identified as a yellow crystalline solid. Yield: 84.3 mg (84%). Mp: 159.9 -162.6 °C. IR (ATR, cm -1 ): v = 1720 (sharp, strong, C=O), 1617 (sharp, medium, C=N). 1 H NMR (300 MHz, CDCl3): δ 13.25 (brs, 1H, OH), 8.53 (s, 1H, CHimine), 7.99 (d,3 JHH = 8.3 Hz,2H,Ar),7.43 (d,3 JHH = 8.3 Hz,2H,Ar),2H,Ar),7.20 (d,3 JHH = 9.3 Hz,2H,Ar),6.94 (m,3H,Ar),6.85 (td,3 JHH = 7.4 Hz, 4 JHH = 1.0 Hz, 1H, Ar), 5.07 (s, 2H, CH2), 3.85 (s, 3H, CH3). 13 C{ 1 H} NMR (101 MHz, CDCl3): δ 166.90 (C=O), 161.37 (Ar), 161.06 (N=Cimine), 158. 00, 142.42, 142.23, 132.95, 132.17, 130.22, 130.12, 127.14, 122.52, 119.67, 119.14, 117.44, 116.03 (Ar), 70.08 (CH2), 52.15 (CH3). Elemental Analysis for C22H19NO4 (361.1314): C, 73.12; H, 5.30; N, 3.88; Found C, 72.94; H, 5.27; N, 3.60%. EI-MS (m/z): 361.1263 [M] + , 212.0207 [C13H10NO2] + .

Synthesis and characterisation of the G1-COOMe-N,Osalicylaldimine dendron (2).

4-Hydroxybenzylidene-2-hydroxyaniline (200.0 mg, 0.938 mmol, 2.00 eq.), potassium carbonate (97.2 mg, 0.703 mmol, 1.10 eq.), 18-crown-ether (37.0 mg, 0.141 mmol, 0.300 eq.) and the G1-bromido-Fréchet dendron (234 mg, 0.469 mmol, 1.00 eq.) were reacted in anhydrous acetone. The crude product was suspended in methanol, filtered and washed with diethyl ether to yield the product (2) as a pale yellow solid. Yield: 257 mg (86%). Mp: 131.7 -134.7 °C. IR (ATR, cm -1 ): v = 1720 (sharp, strong, C=O), 1617 (sharp, medium, C=N). 1 H NMR (300 MHz, CDCl3): δ 13.26 (brs, 1H, OH), 8.51 (s, 1H, Himine), 7.96 (d,3 JHH = 8.4 Hz,4H,Ar),7.39 (d,3 JHH = 8.4 Hz,4H,Ar),2H,Ar),7.16 (d,3 JHH = 8.8 Hz,2H,Ar),4H,Ar),6.59 (d,4 JHH = 2.2 Hz, 2H, Ar), 6.46 (t, 4 JHH = 2.2 Hz, 1H, Ar), 5.02 (s, 4H, CH2), 4.93 (s, 2H, CH2), 3.83 (s, 6H, CH3). 13 C{ 1 H} NMR (101 MHz, CDCl3): δ 165.77 (C=O), 160.01 (Ar), 159.66 (Cimine), 158.92, 156.77, 140.88, 140.72, 138.52, 131.74, 130.99, 128.88, 128.78, 125.96, 121.30, 118.36, 117.98, 116.16, 114.61, 105.45, 100.71 (Ar), 69.04 (CH2), 68.46 (CH2), 51.12 (CH3). Elemental Analysis for C38H33NO8• H2O (631.2206): C, 70.25; H, 5.43; N, 2.16; Found: C, 70.45; H, 5.

Synthesis and characterisation of the G2-COOMe-N,Osalicylaldimine dendron (3).

4-Hydroxybenzylidene-2-hydroxyaniline (40.7 mg, 0.191 mmol, 2.00 eq.), potassium carbonate (20.0 mg, 0.143 mmol, 1.50 eq.), 18-crown-ether (6.31 mg, 0.0239 mmol, 0.250 eq.) and the G2bromido-Fréchet dendron (100 mg, 0.0954 mmol, 1.00 eq.) were reacted in anhydrous acetone. The crude product was triturated with methanol, filtered and washed with diethyl ether to yield the product (3) as a pale yellow powder. Yield: 95 mg (86%). Mp: 130.3 -133.5 °C. IR (ATR, cm -1 ): v = 1716 (sharp, strong, C=O), 1617 (sharp, medium, C=N). 1 ), 160.14, 160.02, 157.96, 142.01, 141.78, 139.61, 139.46, 132.84, 132.11, 129.99, 129.88, 127.08, 122.42, 119.46, 119.10, 117.27, 115.73, 106.62, 106.47, 101.77, 101.74 (Ar), 70.25 (CH2)

Rhodium(I) 1,5-Cyclooctadiene N,O-Salicylaldimine Organometallic dendrons (4 -6)

General Method

A solution of triethylamine (1 -1.2 eq.) in dichloromethane (10 mL) was added dropwise to a stirring solution of compound 1, 2 or 3 (1 eq.) in dichloromethane (20 mL). The solution was stirred for 1 hour at 25 °C. [Rh(μ-Cl)(η 2 :η 2 -COD)]2 (0.50 -0.55 eq.), dissolved in dichloromethane (5 mL), was subsequently added and the reaction mixture was stirred for an additional 3 hours. The solution was then washed with water (3 x 10 mL) in a separating funnel to remove the ammonium chloride formed. The aqueous layer was extracted with dichloromethane (3 x 10 mL). The combined organic extracts were dried using magnesium sulfate. The drying agent was removed by filtration and the filtrate was collected and evaporated to dryness. The residue was taken up in a minimum amount of chloroform, to which petroleum ether was added to precipitate the desired complex.

(CH2-COD). Elemental Analysis for C78H72NO16Rh (1381.3906): C, 67.77; H, 5.25; N, 1.10; Found: C, 68.02; H, 4.88; N, 0.72

Rhodium(I) Dicarbonyl N,O-Salicylaldimine Organometallic dendrons (7 -9)

General Method

The rhodium COD precursor (4 -6) was dissolved in 15 mL of dichloromethane. Carbon monoxide gas (1 bar) was bubbled through the solution for 0.5 hours. The solvent was removed under reduced pressure, and the product was dried in vacuo. The crude product was dissolved in a minimum amount of DCM, to which petroleum ether was added to precipitate the crude product. The crude product was triturated with pentane (to remove the displaced 1,5-cyclooctadiene) to yield the desired complex.

Synthesis and characterisation of the G0-N,O-COOMe Rh(CO)2 organometallic dendron (7).

Complex 4 (50.0 mg, 0.0880 mmol) was reacted with carbon monoxide to afford complex 7 as an orange crystalline solid. Yield: 45 mg (99 %). Mp: Decomposes with melting, onset occurs at 198.9 °C. IR (ATR, cm -1 ): v = 2067 (sharp, strong, C≡O), 1996 (sharp, strong, C≡O), 1714 (sharp, strong, C=O), 1609 (sharp, strong, C=N). 1 H NMR (300 MHz, CDCl3): δ 8.17 164.65, 157.31, 152.06, 141.87, 136.07, 135.45, 130.06, 130.00, 127.13, 124.45, 121.85, 118.38, 116.36, 115.15 (Ar) Complex 5 (132 mg, 0.157 mmol) was reacted with carbon monoxide to afford complex 8 as a red powder. Yield: 117 mg, (95%). Mp: Decomposes with melting, onset occurs at 214 °C. IR (ATR, cm -1 ): v = 2071 (sharp, strong, C≡O), 2011 (sharp, strong, C≡O), 1714 (sharp, strong, C=O), 1611 (sharp, strong, C=N). 1 H NMR (400 MHz, CDCl3): δ 8.18 (d, 3 JRhH = 1.7 Hz, 1H, CHimine), 8.08 (d,3 JHH = 8.3 Hz, 4H, Ar), 7.51 (d,3 JHH = 8.3 Hz, 4H, Ar), 7.49 -7.43 (m, 1H, Ar), 7.32 -7.21 (m, 3H, Ar), 7.13 (appd, 3 JHH = 8.6 Hz, 1H, Ar), 6.98 (d,3 JHH = 8.8 Hz, 2H, Ar), 6.78 -6.68 (m, 3H, Ar), 6.58 (t, 4 JHH = 2.2 Hz, 1H, Ar), 5.13 (s, 4H, CH2), 5.06 (s, 2H, CH2), 3.94 (s, 6H, CH3). 13 C{ 1 H} NMR (101 MHz, CDCl3): δ 184.25 (d, 2 JRhC = 69.9 Hz, C≡O), 183.56 (d,2 JRhC = 68.8 Hz,C≡O),166.78 (Ar),165.58 (Cimine),164.53,159.97,157.33,151.82,141.89,139.34,135.95,135.34,129.92,129.82,126.99,124.28,121.73,118.28,116.25,115.00,106.51,101.76 (Ar),70.14 (CH2) Synthesis and characterisation of the G2-N,O-COOMe Rh(CO)2 organometallic dendron (9). Complex 6 (65.0 mg, 0.0470 mmol) was reacted with carbon monoxide to afford complex 9 as a pale yellow powder. Yield: 37 mg (79%). Mp: Decomposes with melting, onset occurs at 198.0 °C. IR (ATR, cm -1 ): v = 2071 (sharp, strong, C≡O), 2000 (sharp, strong, C≡O), 1716 (sharp, strong, C=O), 1607 (sharp, medium, C=N). 1 H NMR (400 MHz, CDCl3): δ 8.16 (d, 3 JRhH = 1.9 Hz, 1H, CHimine), 8.03 (d,3 JHH = 8.5 Hz,8H,Ar),7.46 (d,3 JHH = 8.6 Hz,8H,Ar),7.20 (s,4H,Ar),7.10 (m,1H,Ar),6.97 (d,3 JHH = 7.4,2H,Ar),6.71 (ddd, 3 JHH = 7.9, 3 JHH = 6.9, 4 JHH = 1.0 Hz, 1H, Ar), 6.66 (m, 6H, Ar), 6.53 (m, 3H, Ar), 5.10 (s, 8H, CH2), 5.02 (s, 2H, CH2), 4.98 (s, 4H, CH2), 3.91 (s, 12H, CH3). 13 ), 164.77, 160.11, 157.33, 151.97, 151.95, 142.05, 139.66, 139.30, 136.10, 135.43, 130.06, 129.99, 127.13, 127.28, 121.93, 118.42, 116.38, 115.20, 106.73, 106.59, 101.88, 101.87 (Ar)

Single X-ray crystallography methods

Single-crystal X-ray diffraction data (compounds 1, 4, and 7) were collected using a Bruker Kappa APEX II DUO diffractometer equipped with a graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The temperature was controlled by an Oxford Cryostream cooling system (Oxford Cryostat). Cell refinement and data reduction were achieved using the program SAINT. 23 The data were scaled, and absorption correction was accomplished using SADABS. 24 The structure was solved by direct methods by using SHELXS-9733 and refined by full-matrix least-squares methods based on F 2 using SHELXL-9733. 25 All non-hydrogen atoms were refined anisotropically. All hydrogen atoms (except H1 for compound 1) were placed in idealised positions and refined in riding models with Uiso assigned 1.2 or 1.5 times Ueq of their parent atoms and the C-H bond distances were constrained from 0.95 to 0.99 Å. With respect to compound 1, the hydroxyl hydrogen H1 was located in the difference electron density map and refined independently. Flack x = 0.182(908) by classical fit to all intensities, 0.673(189) from 1821 selected quotients (Parsons' method). The programs X-Seed, Mercury and POV-Ray were used to produce ORTEP images of compounds 1, 4 and 7. 24

General method for the hydroformylation reaction

The hydroformylation reactions were performed in a 90 mL stainless steel pipe reactor equipped with a Teflon-coated magnetic stirrer bar. The reactor was charged with toluene (5 mL), substrate (1-octene and styrene (7.175 mmol), 7tetradecene, methyl oleate, triolein, D-citronellal and R-Limonene (0.574 mmol)), internal standard n-decane (1.26 mmol for 1-octene and styrene only) and Rh-metal loading (2.87 x 10 -3 mmol). The pipe reactor was purged with nitrogen three times, followed by purging with syngas (1:1, CO: H2) three times. The reactor was pressurised to the desired pressure and consequently heated to the desired temperature. Samples were collected at the beginning and at the end of each reaction. All reactions were performed in duplicate and are recorded as an average of two identical experiments.

Conclusions

A series of Fréchet metallodendrons with Schiff-base rhodium(I) complexes at the focal point were successfully synthesised and characterised using an array of spectroscopic and analytical techniques. The reaction of the Fréchet-type dendron and N,Osalicylaldimine proceeded with 100% regioselectivity. This was confirmed in the solid state for compound 1 using single crystal X-ray diffraction. Complexation with the appropriate dimer and subsequent reaction with CO yielded a new class of Fréchet organometallic dendrons with Rh(I) complexes at the focal point. The complexes displayed excellent moisture and air stability at room temperature. The Rh(I) organometallic dendrons were evaluated as catalyst precursors in the hydroformylation of 1-octene. All of the catalyst precursors tested were active in the hydroformylation of 1-octene. The catalyst precursors displayed excellent conversion, turnover frequencies and chemoselectivity towards aldehydes after 4 hours. The simplest and most representative catalyst precursor (7) was evaluated against an array of substrates, these include styrene, 7-tetradecene, methyl oleate, triolein, D-limonene and R-citronellal. The catalyst precursor 7 was displayed hydroformylation activity in all of the tested substrates, the most promising results were observed for 7-tetradecene, methyl oleate and D-limonene whereby excellent conversions were observed. With respect to the dendron size, an increase in generation number generally results in an increase in the chemoselectivity towards aldehydes, which suggests that the larger dendron limits isomerisation towards iso-octenes and therefore favours the hydroformylation towards aldehydes. The mercury drop experiments were performed on the G0-catalyst precursors. Notably a suppression of the activity was observed, which further corroborates the fact that the hydroformylation activity for this class of compounds is attributed to a combination of homogeneous catalysis and catalysis mediated by nanoparticle in solution. The reactor was loaded with toluene (5 mL), 1-octene (0.805 g, 7.175 mmol), internal standard n-decane (0.204 mg, 1.435 mmol) and Rh-metal loading (2.87 x 10 -3 mmol). The reactor was purged with nitrogen three times, followed by purging thrice with syngas. TOF = (mmol of aldehydes/mmol of Rh)/time. Catalyst to substrate ratio utilised was (1:2500). The data was analysed using GC-FID. Reactions were conducted for 4 hours. Entries 15 and 16 were conducted in the presence of mercury. The reactor was loaded with toluene (5 mL), substrate (0.574 mmol) and Rh-metal loading (2.87 x 10 -3 mmol for entries 1 and 2, 35.88 x10 -3 for entries 3-7). The reactor was purged with nitrogen three times, followed by purging thrice with syngas. The data was analysed using GC-FID and/or 1 H NMR spectroscopy. Conversion is referred to as olefins (includes isoolefins) to aldehydes exclusively. Reactions were conducted for 4 hours. l and b denotes linear and branched respectively. The reactor was loaded with toluene (5 mL), 1-octene (0.805 g, 7.175 mmol), Rh-metal loading (2.87 x 10 -3 mmol). The reactor was purged with nitrogen three times, followed by purging thrice with syngas. TOF = (mmol of aldehydes/mmol of Rh)/time. The data was analysed using GC-FID.

Reactions were performed for 1, 2, 4 and 8 hours.

Figure 4

 4 Figure 4 Molecular structure of 7 as elucidated by single crystal X-ray diffraction.

Figure 5

 5 Figure 5 Chemoselectivity across the dendritic series in the hydroformylation of 1-octene.

Figure 6

 6 Figure 6 Chemoselectivity as a function of time for catalyst precursors 4 and 7.

  ; %. MALDI-TOF-MS (+ve mode, m/z): 1382.5970 [M+H] + , 1278.5550 [M-COD] + , 1172.5660 [M-Rh-COD] + .

  (d, 3 JRHH = 1.6 Hz, 1H, CHimine), 8.09 (d, 3 JHH = 8.2 Hz, 2H, Ar), 7.53 (d, 3 JHH = 8.2 Hz, 2H, Ar), 7.51 -7.39 (m, 1H, Ar), 7.27 (m, 3H, Ar), 7.12 (appd, 3 JHH = 8.6 Hz, 1H, Ar), 7.01 (d, 3 JHH = 8.8 Hz, 2H, Ar), 6.73 (appt, 3 JHH = 7.3 Hz, 1H, Ar), 5.18 (s, 2H, CH2), 3.95 (s, 3H, CH3). 13 C{ 1 H} NMR (101 MHz, CDCl3): δ 184.35 (d, 2 JRhC = 65.5 Hz, C≡O), 183.66 (d, 2 JRhC = 64.3 Hz, C≡O), 166.89 (C=O), 165.70 (Cimine),

  , 69.85 (CH2), 52.27 (CH3). Elemental Analysis for C24H18NO6Rh•2H2O (519.0189): C, 51.91; H, 3.99; N, 2.52; Found: C, 52.27; H, 3.51; N, 2.22%. HR-ESI-MS (m/z): 520.0323 [M] + , 464.0416 [M-(CO)2] + , 362.1433 [M-Rh-(CO)2] + . Synthesis and characterisation of the G1-N,O-COOMe Rh(CO)2 organometallic dendron (8).

  C{ 1 H} NMR (101 MHz, CDCl3): δ 184.76 (d, 2 JRhC = 70.3 Hz, C≡O), 183.35 (d, 2 JRhC = 69.1 Hz, C≡O), 166.91 (C=O), 165.72 (Cimine

Figure 2 ORTEP

 2 Figure 2 ORTEP representation of 1 as elucidated by single crystal X-ray diffraction. Hydrogen atoms are omitted for clarity.

Figure 3 ORTEP representaton of 4 ,

 34 Figure 3ORTEP representaton of 4, as elucidated by single crystal X-ray diffraction. Hydrogen atoms are omitted for clarity.

Figure 4

 4 Figure 4Molecular structure of 7 as elucidated by single crystal X-ray diffraction.

Figure 5 Figure 6

 56 Figure 5 Chemoselectivity across the dendritic series in the hydroformylation of 1-octene.

  06; N, 1.80%. ESI-MS (m/z, +ve mode): 632.2280 [M+H] + .

Table 1

 1 Selected crystallographic data for the molecular structures of 1, 4 and 7.

Table 2

 2 Hydroformylation of 1-octene using catalysts 4 -9.

	Entry	Catalyst Precursor	Temperature ( °C)	Pressure (bar)	Conversion (%)	Aldehydes (%)	iso-Octenes (%)	n:iso	TOF (h -1 )
	1	4	75	30	99.27	88.63	11.37	1.12	549.93
	2	5	75	30	98.98	87.12	12.88	1.39	538.94
	3	6	75	30	99.30	92.79	51.83	1.08	575.91
	4	7	55	20	10.77	63.95	36.05	2.14	43.04
	5	7	55	30	20.97	77.67	22.33	1.67	98.71
	6	7	55	40	7.85	60.70	39.30	1.58	30.21
	7	7	75	20	50.38	62.13	37.87	2.54	199.98
	8	7	75	30	99.19	86.44	13.56	1.25	535.90
	9	7	75	40	99.64	98.61	1.39	1.20	591.05
	10	7	95	20	96.82	84.98	15.02	0.61	512.43
	11	7	95	30	99.65	91.99	8.01	0.88	573.02
	12	7	95	40	100.00	100.00	0.00	0.77	625.00
	13	8	75	30	78.04	66.09	33.91	2.06	322.36
	14	9	75	30	99.25	95.46	4.54	1.16	592.13
	15	4 + Hg	75	30	46.89	46.22	53.78	-	-
	16	7 + Hg	75	30	45.20	46.03	53.97	-	-

Table 3

 3 Hydroformylation of various feedstocks using catalyst precursor 7.

		Ligand 1	Complex 4	Complex 7
	Empirical Formula	C22H19NO4	C30H30NO4Rh	C24H18NO6Rh
	Formula weight (g•mol -1 )	361.38	571.48	519.30
	Crystal system	Orthorhombic	Monoclinic	Monoclinic
	Space group	Pna21	P21 /c	P21 /c
	Crystal colour and shape	Colourless block	Yellow block	Red block
	Crystal size	0.48 x 0.52 x 0.65	0.18 x 0.19 x 0.21	0.09 x 0.14 x 0.19
	a / Å	6.0825(9)	11.1388(15)	17.7573(17)
	b / Å	7.0656(11)	18.327(2)	10.5493(10)
	c / Å	40.788(6)	12.3562(17)	24.617(2)
	α / °	90	90	90
	β / °	90	102.026(3)	109.528(2)
	γ / °	90	90	90
	V / Å 3	1752.93	2467.05	4346.17
	Z	4	4	8
	T / K	173(2)	173(2)	173(2)
	Dc / g•cm -1	1.369	1.539	1.587
	μ / mm -1	0.095	0.730	0.827
	Reflections used [ I> 2s(I)]	4250	6182	8892
	Rint	36694, 4250, 0.027	55561, 6182, 0.084	56349, 8892, 0.051
	Final R indices [I > 2s(I)]	4045	5006	6624
	R indices (all data)	0.0350	0.0484	0.0647
	Goodness-of-fit	1.042	1.046	1.023
	Max, Min Δρ/e Λ -3	-0.20, 0.24	-0.52, 0.49	-0.74, 1.94

Acknowledgements

Financial support from the University of Cape Town, the National Research foundation and Harry Crossley foundation, NRF-DSR Centre of Excellence in Catalysis -c*change is gratefully acknowledged.

Conflicts of interest

To the best of our knowledge there are no conflicts of interests to declare.