Isomerisation and controlled condensation in an aqueous medium of allyl alcohol catalysed by new water-soluble rhodium complexes with 1,3,5-triaza-7-phosphaadamantane (PTA)†

New aqua-soluble rhodium(I) [Rh(CO)(PTA)4]Cl (1) (PTA = 1,3,5-triaza-7-phosphaadamantane) and rhodium(III) [RhCl2(PTA)4]Cl (2) complexes have been synthesized via the reaction of [{Rh(CO)2(μ-Cl)}2] or RhCl3·3H2O, respectively, with stoichiometric amounts of PTA in ethanol. Compound 1 is also obtained upon reduction of 2 in an H2/CO atmosphere. They have been characterized by IR, H and P{H} NMR spectroscopies, elemental and single crystal X-ray diffraction analyses. While compound 1 shows distorted square-pyramid geometry (τ5 = 0.09) with a P3C-type basal plane, compound 2 is octahedral with the chloro ligands in the cis position. The hydride rhodium(I) complex [RhH(PTA)4] (3) is formed upon the addition of NaBH4 to an aqueous solution of 1 or 2. Compounds 1–3 (in the case of 2 upon reduction by H2) act as homogeneous catalysts, or catalyst precursors, in the isomerisation and condensation of allyl alcohol at room temperature and in an aqueous medium. The product selectivity is easily controlled by changing the concentration of the base in the reaction mixture, thus resulting in the exclusive formation of either 3-hydroxy-2-methylpentanal (HP) or 2-methyl-2-pentenal (MP) in quantitative yields.


Introduction
The application of the aminophosphine 1,3,5-triaza-7-phosphaadamantane (PTA) 1 as a water-soluble ligand for the synthesis of different metal complexes is a growing research area, since many coordination compounds of PTA show interesting catalytic, 1,2 biological 1,3 or photoluminescent properties. 1,4 In particular, Rh and Ru complexes with PTA and its N-alkylated derivatives have been extensively studied, finding their wide application as efficient and versatile catalysts for the hydrogenation, hydroformylation and isomerisation of unsaturated organic substrates in aqueous media. 5 Besides, in the case of hydrogenation reactions, it was shown that rhodium complexes bearing more than three PTA ligands in the coordination sphere are much less active than those having only two PTA moieties. 6 For example, complex [RhCl(CO)(PTA) 2 ] 7 is a catalyst in water gas shift reaction (WGSR), while [RhCl(PTA-H)-(PTA) 2 ] (PTA-H = N-protonated PTA) 6 catalyzes the hydrogenation of unsaturated aldehydes. Interestingly, in the presence of an excess of PTA, the latter complex is completely inactive due to the formation of the coordinatively saturated Rhcompound [RhCl(PTA-H) 3 (PTA)]Cl 3 . 6 On the other hand, rhodium(I) complexes of the type [RhH(PTA-R) 4 ]I 4 (R = Me, Et) 8 that bear the N-alkylated PTA cationic derivatives (PTA-R) + instead of neutral PTA are efficient catalysts for CvC bond reduction.
Rhodium(I) acetylacetonato complexes with PTA, P(CH 2 CH 2 CN) 3 and P(m-C 6 H 4 SO 3 Na) 3 ligands have also been studied in the hydrogenation of allyl alcohol to n-propanol (Scheme 1). Isomerisation to propanal is also observed, although as a minor product. In the presence of these catalysts, the alcohol undergoes isomerisation to enol before its hydrogenation to n-propanol. 9 Since the rhodium complexes with a higher number of coordinated PTA are less active in the CvC bond reduction reactions, it would be interesting to examine them towards the secondary reaction of alcohol isomerisation to aldehyde. A catalytic system generated in situ from [Rh(COD)(MeCN) 2 ](BF 4 ) and PTA was previously reported 10a,b to catalyse the isomerisation of allylic alcohols into carbonyl compounds in aqueous media, but no aldol condensation products were observed, even in the presence of NaOH. Aldol condensation, which could follow the isomerisation, is one of the important reactions of aldehydes in synthetic organic chemistry, especially because of C-C bond formation. 10c Products of aldol condensation find wide applications in the pharmaceutical field, as plasticizers, detergents, fragrances and cosmetics. 11 One of them, 2-methylpentenal, is an industrially important chemical used as an intermediate for the synthesis of various pharmacologically active compounds. 12 Commercially, condensation reactions are carried out in the presence of strong bases or acids (sodium hydroxide or sulphuric acid) and require high temperatures, thus displaying many disadvantages such as corrosion and environmental problems, and complex workup. 11,12 Therefore, the search for catalytic systems without some of those disadvantages deserves to be explored and water soluble rhodium complexes are promising candidates.
Hence, we report herein the synthesis, structural analysis and catalytic properties of new water-soluble and water-stable rhodium complexes bearing the aminophosphine PTA, [Rh(CO)-(PTA) 4 ]Cl (1) and [RhCl 2 (PTA) 4 ]Cl (2). Besides, formation of the hydride rhodium(I) complex [RhH(PTA) 4 ] (3) upon addition of NaBH 4 to an aqueous solution of 1 or 2 has been monitored by 31 P{ 1 H} NMR. The compounds have been characterized by IR, 1 H and 31 P{ 1 H} spectroscopies, elemental and single crystal X-ray diffraction analyses (for 1 and 2). Complexes 1-3 were found to efficiently catalyze condensation of allyl alcohol under ambient conditions in water as a solvent.

Experimental
All syntheses were performed under an inert atmosphere of dry oxygen-free dinitrogen, using standard Schlenk techniques. Solvents were dried and distilled prior to use.
Catalytic reactions were carried out in an autoclave (150 mL of capacity) (Berghof ), and at atmospheric pressure in glass vessels. An aqueous solution (10 mL) containing one of the complexes 1-3 (0.01 mmol), allyl alcohol (10 mmol) and the appropriate amount of NaOH was stirred at room temperature for 30 min. During this time, the formation of 2-methyl-2pentenal (MP) as a colourless organic phase was observed. A second product, 3-hydroxy-2-methylpentanal (HP), that is partially soluble in water, is formed at a low concentration of NaOH. Therefore, immediately after the reaction, the liquid mixture was separated from the solid residue by the vacuum transfer technique and the organic/aqueous phases were analyzed using gas chromatographs (HP model 5890 and 6890), with FID and MS detectors, with capillary columns HP5 (30 m × 0.32 mm × 0.25 μm) and HP-INNOWax (30 m × 0.5 mm × 0.25 μm). The solid residue, as well as the liquid phase, were additionally analyzed by NMR and/or IR spectroscopy. Blank tests have shown no product formation in the absence of Rh catalysts. The catalyst recycling was performed in the presence of each complex in 0.20 M NaOH as follows: after each run the organic product was removed in a phase separator, followed by the introduction of a new amount of allyl alcohol substrate. Method B. An aqueous solution (10 mL) of 2 (221.0 mg, 0.25 mmol) was transferred into the autoclave and the mixture was stirred at room temperature for 1 h under a CO/H 2 atmosphere [p(CO) = p(H 2 ) = 1.0 MPa]. After depressurization of the mixture, i-propanol (10 mL) was added, and the yellow microcrystalline solid was filtered off, washed with cold i-propanol (2 × 5 mL) and dried in vacuo to give 1 in 70% yield.

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Dalton Transactions resulting mixture was stirred at room temperature for 3 h. The microcrystalline dark yellow product was collected by filtration, washed with cold ethanol (2 × 5 mL) and dried in vacuo to give 2·EtOH.  Refinement details for the X-ray crystal structure analysis of 1-2 Single crystals suitable for X-ray-analyses were grown from the reaction filtrate at 4°C. Intensity data were collected using a Bruker AXS-KAPPA APEX II diffractometer with graphite monochromated Mo-Kα radiation. Data were collected at 150 K using omega scans of 0.5°per frame and a full sphere of data was obtained. Cell parameters were retrieved using Bruker SMART software and refined using Bruker SAINT 14 on all the observed reflections. Absorption corrections were applied using SADABS. 14 Structures were solved by direct methods by using the SHELXS-97 package 15a and refined with SHELXL-97 15a with the WinGX graphical user interface. 15b All hydrogen atoms were inserted in calculated positions. There were disordered molecules present in the structure of complex 2. Since no obvious major site occupations were found for those molecules, it was not possible to model them. PLATON/ SQUEEZE 16 was used to correct the data and a potential volume of 1381.7 Å 3 was found with ca. 209 electrons per unit cell worth of scattering. The electron count suggests the presence of one molecule of ethanol per unit cell, which was confirmed by 1 H NMR (see Experimental). Least square refinements with anisotropic thermal motion parameters for all the non-hydrogen atoms and isotropic for most of the remaining atoms were employed. The selected bond distances (Å) and angles (°) are given in the footnotes of Fig. 1 and 2, and the crystallographic data and refinement parameters are summarized in Table 1. CCDC no. 896276 and 896277 contain the supplementary crystallographic data of 1 and 2 for this paper.   7), P1-Rh1-P3 102.83(9), P2-Rh1-P3 97.18 (18). Symmetry operation to generate equivalent atoms: x, 1/2 − y, z.

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NaBH 4 (Scheme 2). The complexes were isolated as yellow (1-2) or off-white (3) microcrystalline solids in 55-75% yields, and characterized by IR, 1 H and 31 P NMR spectroscopies, elemental and (for 1 and 2) single crystal X-ray diffraction analyses. 1 and 2 are relatively air stable in the solid state and in aqueous solutions, while compound 3 is stable in solid state under inert conditions. The complexes are soluble in polar solvents, such as H 2 O and Me 2 SO, less soluble in medium polarity solvents such as EtOH, n-PrOH, and insoluble in nonpolar ones such as toluene and hexane.

Spectroscopy
The CO (in 1) and H (in 3) ligands are easily identified by their characteristic IR ν(CO) and ν(Rh-H) bands at 1992 and 1942 cm −1 , respectively, which are comparable to those observed in related complexes. 5f,7,8 The IR spectra of 1-3 show related features with typical vibrations due to the PTA ligand, including some characteristic bands (1100-900 cm −1 ) associated with the ν(C-X) (X = N, P) vibrations. Besides, several bands due to ν as and ν s (CH) are also detected in the 2945-2901 cm −1 range. 1 The 1 H NMR spectra of 1-3 in D 2 O show two characteristic types of methylene protons for the coordinated PTA. One of them, assigned to the P-CH 2 -N moiety, occurs as a singlet at δ 4.08-4.17 whereas the other one, corresponding to the N-CH 2 -N group, displays an AB spin system centred at 4.65-5.15 ppm, attributed to the N-CH ax -N and N-CH eq -N protons, as previously reported. 17 Additionally, in the 1 H NMR spectrum of 3, a double set of methylene protons centred at δ 4.65 and 4.59 (for NCH 2 N), as well as δ 4.17 and 4.08 (for PCH 2 N), assigned as two P b and P a,c , were observed due to the nonequivalence of the PTA ligands (see Scheme 2). The multiplicity of the hydride resonance at δ −11.2 confirms the nonequivalence of the coordinated ligands in 3. In contrast to the previously described analogue [RhH(PTA-Me) 4 ]I 4 (PTA-Me = N-methyl-1,3,5-triaza-7-phosphaadamantane cation), the 31 P{ 1 H} and 31 P NMR spectra of 3 reveal the presence of three types of nonequivalent PTA ligands (three resonances at δ −39.5, −53.55 and −53.65 corresponding to the P b , P a and P c ligands, respectively) in the coordination sphere of the rhodium(I) centre. For 1 and 2, in the 31 P{ 1 H} NMR spectra, only a broad singlet is observed at δ −56.6 and −46.0, respectively, due to an average effect of a dynamic process.
In the structure of 2 (Fig. 2) the metal exhibits a rather distorted octahedral environment around the Rh centre, filled by four neutral PTA moieties and two chloride ligands in the cis-configuration. The P2-Rh-P3 angle is 95.59(3)°and that of P1-Rh-P4 is 163.31(3)°. The Rh-P bond distances for the phosphine groups trans to chloride (2.2871 and 2.3203 Å) are

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shorter than those for the mutually trans Rh-P ones (2.3757 and 2.3997 Å) indicating a greater trans effect of the neutral phosphine ligand, relative to the chloride anion. The lengths of the Rh-P bonds in both complexes [in the 2.2872(9)-2.401(5) Å range], the Rh-Cl in 2 [av. 2.4196(9) Å] and the bonding parameters within the PTA cages are comparable to those found in related Rh-PTA derivatives. 6,20
The first step (I) includes the isomerisation of allyl alcohol to n-propanal (P) and does not require the presence of NaOH (Table 2, Fig. 3a-c). At a low concentration of base (0.02-0.05 M) two molecules of n-propanal couple to 3-hydroxy-2-methylpentanal (HP, step II, Scheme 3a). The presence of the rhodium complex is essential for the steps I and III, since none of the products was observed in the absence of the Rh catalyst. Although we detected the formation of small amounts of 2-methyl-2-pentenal (MP) in the absence of 1-3 (Scheme 3b, Fig. 3d) when using n-propanal as the substrate, the presence of rhodium complexes considerably increases the rate of this process, and additionally allows the formation of 3-hydroxy-2-methylpentanal (HP), at an appropriate concentration of the base.
The turnover frequencies (TOFs) achieve values as high as 2000 h −1 for all the complexes 1-3, at a NaOH concentration in the 0.05-0.2 M range. For comparison, the optimized system of [Rh(COD)(MeCN) 2 ](BF 4 ) and PTA catalyses the isomerisation of allylic alcohols into carbonyl compounds with a comparable rate (2400 h −1 ), but at a higher temperature. 10a,b Separately, the catalytic condensation of n-propanal to the 3-hydroxy-2methylpentanal (HP) and 2-methyl-2-pentenal (MP) mixture, with a TOF of 42 mmol of products per g catalyst per h using chitosan as a solid base catalyst at 100°C, was previously reported. 11 It should also be noted that the Rh(III) complex 2 is active only in the presence of H 2 , thus suggesting reduction, by this species, to a catalytically more active Rh(I) form. Expectedly, the presence of H 2 (1.0-2.0 MPa) does not lead to hydrogenation of the allyl alcohol. Since no condensation product was observed in the absence of a base, the involvement of an Scheme 2 Syntheses of 1-3 with a numbering scheme for the PTA ligands in 3 (see Experimental).

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Rh-monohydride complex in the reaction mechanism is suggested. 21 Indeed, 1 H NMR spectra of the complexes separated from the final reaction solution exhibit resonances centred at δ ca. −11.0, when the base was added. Moreover, the loss of CO from the coordination sphere of 1 (in the presence and in the absence of allyl alcohol) was confirmed by IR spectroscopy. Catalysts 1-3 were isolated at the end of the reaction in the presence of NaOH (0.20 M) and tested in two subsequent runs; the same level of activities towards the 2-methyl-2-pentenal (MP) formation was revealed.
Although some examples of isomerisation reactions for allyl and allylic alcohols 10a,22 or condensation of various aldehydes 10-12,23 have been described in the literature, to our knowledge, the complexes 1-3 appear to be the first examples of catalysts which allow not only the selective isomerisation of allyl alcohol to n-propanal (step I), but also (depending on the NaOH concentration) the reaction leading to the selective formation of the condensation products: 3-hydroxy-2-methylpentanal (HP, step II), and after dehydration only 2-methyl-2-pentenal (MP, step III) at a higher concentration of NaOH.
Although the catalytic behaviours of other rhodium complexes with PTA and other water-soluble phosphines 9,23 towards the transformation of allyl and allylic alcohols have been described previously, in such cases the isomerisation products have not been subsequently transformed into any condensation products.

Conclusions
The study reports a suitable way for the syntheses of new water-soluble rhodium complexes of the types [Rh(CO)(PTA) 4 ]-Cl (1), [RhCl 2 (PTA) 4 ]Cl (2) and [RhH(PTA) 4 ] (3), which is based on the reaction of [{Rh(CO) 2 (μ-Cl)} 2 ] or RhCl 3 with 1,3,5-triaza-7-phosphaadamantane (PTA). The solubility and stability in water of the obtained complexes encouraged their applications in aqueous catalysis. In fact, the work shows that they catalyze the isomerisation and controlled condensation of allyl alcohol, under ambient conditions and in aqueous media, thus displaying some advantages over the commercial methods. The selectivity can be controlled simply by changing the concentration of the base in the reaction mixture, thus resulting in the exclusive formation of either 3-hydroxy-2-methylpentanal (HP) or 2-methyl-2-pentenal (MP) in quantitative yields, at lower or higher concentrations of the base, respectively. The activity of the catalyst remains the same at least for the three following runs, thus allowing its efficient recycling. This study opens up the possibility of application of hydro-soluble PTA-Rh complexes in the controlled condensation of allyl alcohol, with an easy base-tuned selectivity in water and under mid conditions, broadening their application in catalysis under green conditions. It deserves to be further explored for other types of condensation catalyses.