4. ELABORACIÓN DE PLANILLAS
4.1. Encabezados de planillas
Shibasaki and Matsunaga et al. reported a series of homo and heterobimetallic dinuclear PCCs which are postulated to form in situ and promote a variety of asymmetric transformations reactions. These dinuclear PCCs are based on a chiral dinucleating ligand reported by Kozlowski et al., in which NiII and TiIV-GaIII mononuclear complexes promote the asymmetric conjugate addition of dibenzyl malonate to cyclic enones280–282 and asymmetric ring opening of meso epoxides with thiols and selenols, respectively.283,284
These Schiff base ligands share a common ligand structure (Scheme 1.1) and contain two specific cavities for coordination of metal ions. The first is the N2O2 cavity which preferentially coordinates transition metal ions. The second is the O2O2 cavity which can selectively coordinate oxophilic lanthanide ions. This specificity is important as in all the reported protocols the formation of the active species is during the reaction and the PCC catalyst is not characterised independently. The geometry of the common ligand frame brings the two metal ions into proximity (3.5 – 6 Å) which is essential for the promotion of co-operative catalysis.143,285
Various transition metal homometallic dinuclear PCCs supported by these ligands have been reported to form in situ and they promote a variety of asymmetric catalytic transformations (Scheme 1.1). A bimetallic NiII2 catalyst, bearing a 1,1 binaphthyl diamine as a chiral diamine (H2LB), was particularly versatile and promoted Mannich reactions,147,286,287 conjugate additions,286,288,289 amination,290 desymmetization291 and aldol-type reactions.292,293 By substituting NiII ions and the chiral diamine linker, it was possible to tune the catalytic efficacy of this system to specific reactions with CoII and MnIII analogues which promote 1,4 addition reactions.146,294,295
The first example of a heterometallic PCC catalyst of this type was a CuIISmII PCC which promoted a syn-selective asymmetric nitroaldol reaction,296 substituting the metal ions with PdII and LaIII permitted the use of aldehydes as electrophiles and catalysed an anti-syn selective nitroaldol reaction.297 Further examples of heterometallic 3d-4f PCC catalysed reactions will be discussed in the next section.
1.3.5.4 3d-4f PCCs as co-operative catalysts
Though there are few reported 3d-4f PCCs used for catalysis, the most common are dinuclear PCCs (1M2-1), which have been employed for asymmetric transformations. The ligands which have been reported in these systems are shown in Figure 1.18.
Figure 1.18. Schiff base and Schiff base-derived ligands used to form catalytically active 3d-4f
dinuclear PCCs.
The first example of a 3d-4f PCC acting as a co-operative catalyst was reported by Shibasaki for an asymmetric transformation. A bimetallic Lewis acid/Brønsted base CuII-SmIII (1M2-1) PCC which was postulated to form in situ, with an achiral phenol additive, catalysed nitro-Mannich reactions with a high syn diastereoselectivity (>20:1 - 13:1) and enantioselectivity (66 - 99% ee) (Equation 1.20).297+296 It is suggested that the dinucleating Schiff base ligand, H2CL1, supports the PCC with a CuII ion coordinated to the N2O2 cavity and a SmIII ion in the O2O2 pocket to give a dinuclear CuII-SmIII species.
Equation 1.20. A CuII/SmIII catalysed nitro-Mannich reaction.297+296
Interestingly, the substitution of H2CL1 with H2CL2 or H2CL3, under identical conditions, showed no conversion to product. In addition, with the replacement of the CuII(OAc)2 with ZnII(OAc)2 or NiII(OAc)2 no conversion to product was observed, whereas the replacement of the
SmIII salt by other LnIII salts (LaIII, PrIII, NdIII, EuIII, GdIII, DyIII, ErIII, YbIII) resulted in lower yields (63 - 96%) with reduced stereoselectivity (3:1 > 20:1) and enantioselectivity (5 – 80 ee). The co-operative activation of the Sm-OAr moiety as a Brønsted base and the CuII ion as a Lewis acid is thought to be the key to the efficacy of these transformations. Though ESI-MS and optical purity studies indicated a trimer of CuII-SmIII dinuclear PCCs with an achiral phenol additive was the active species in the promotion of this selectivity, many other oligomers including Cu3Sm3, Cu6Sm6, Cu7Sm7, Cu8Sm8 and Cu9Sm9 were detected by ESI-MS. The proposed monomeric and trimeric CuII-SmIII species was never isolated or characterised.
Subsequently, Zhou et al. reported a dinuclear CuII-SmIII PCC supported by H3CL5 which catalysed an in situ anti-selective asymmetric Henry reaction (Equation 1.21).298 Anti-β-nitro- alcohols are obtained in up to 99% conversion, > 30:1 dr and 98% ee. The use of other MII and LnIII ions was not investigated. Furthermore, the reaction with only H3CL5 and Sm(OiPr)3 lead to a reduction in yield to 90% with an ee of 91%. ESI-MS studies indicated the presence of a [CuIISmIIICL5]+ dimer, which was postulated to be the active species, however, a [CuII
2CL5]+ species is also observed with no further explanation. Although the in situ catalysis with H3CL5
and Sm(OiPr)3 gave a higher yield than H3CL5 and Cu(OAc)2, the species in the more active reaction is not studied via ESI-MS.
Equation 1.21. A CuII/SmIII catalysed anti-selective asymmetric Henry reaction.298
These studies were followed by a proposed NiII-LaIII PCC, supported by H2CL4, which catalysed the enantioselective decarboxylation -1,4-addition of malonic acid half thioester to nitroalkenes (Equation 1.22).299 The NiII-LaIII PCC gave products in a 40 - 99% yield with 66 - 94% ee. No further identification or characterisation of the active species was presented.
Equation 1.22. A NiII-LaIII asymmetric decarboxylative 1,4-addition of Malonic acid half thioester.299
More recently, a dinuclear PCC with the general formula [ZnIIYbIII(CL6)(μ1-OAc)2(μ2- OAc)(H2O)] (Figure 1.19) has been reported for the solvent-free copolymerization of cyclohexane
oxide and maleic anhydride (Scheme 1.2).300 The presence of TPP or DMAP as a co-catalyst leads to the formation of polyester and poly (ester-coether) respectively. The ZnII-YbIII PCC can effectively catalyse the co-polymerisation alone, unlike similar salen-based MIII complexes, which may indicate that [OAc]- initiators are working co-operatively with the active ZnII and YbIII centres.301 The ZnII-YbIII structure was elucidated by single-crystal XRD studies and has a similar structure to the 3d-4f PCC catalysts which were proposed in Shikasaki’s work. ESI-MS confirms the [ZnIIYbIII(CL6)]+ core configuration and Thermogravimetric Analysis (TGA) shows an indicative weight loss with the hetero-bimetallic framework stable up to 200 oC.
Scheme 1.2. Solvent-free co-polymerisation of cyclohexane oxide and maleic anhydride by ZnII- YbII.300
Figure 1.19. The molecular structure of ZnII-YbIII. Colour code: YbIII, light blue; ZnII, grey; O, red; C, white; N, blue. Hydrogen ions and counter-ions are omitted for clarity.300
Though all in situ 3d-4f catalytic systems promote enantioselective and diastereoselective transformations, the characterisation of the proposed dinuclear 3d-4f PCCs, which are postulated
to promote the transformations, have been overlooked with only ESI-MS studies and the nature of the dinucleating ligands used as evidence to propose their formation. The characterisation of the active species and understanding of the catalytic mechanism are considered vital aspects for the development of 3d-4f PCC catalysis and will be addressed rigorously throughout the rest of this work.
Higher nuclearity 3d-4f PCCs have been reported as catalysts, however, these are limited to oxidation catalysts and WOCs. Further, these examples report the prior synthesis and characterisation of the PCC and attempt to understand the catalytic mechanism.
A heptanuclear MnIV6CeIV (3M7-1) PCC with TEMPO, was reported to catalyse the oxidation of benzyl alcohol to benzaldehyde with quantitative yields (Equation 1.23).302 The MnIV6CeIV PCC was well characterised in the solid state, which includes single-crystal XRD studies (Figure 1.20). The conversion to the product does not decrease with time and Fourier Transform Infrared Spectroscopy (FT-IR) data of the recovered product is consistent with the starting material, suggesting the PCC remains stable during the reaction. It was highlighted that the high efficiency of transformation could only be achieved when both high oxidation CeIV and MnIV ions were present in the same cluster. In contrast to previous dinuclear examples, the MnIV6CeIV cluster was well characterised and it was suggested that the catalyst remains stable under the reaction conditions.
Equation 1.23. The MnIV6CeIV catalysed oxidation of benzyl alcohol to benzaldehyde.302
More recently, a family of CoII3LnIII cubanes (Figure 1.20) (3M4-1) with the general formula [CoII3LnIII(hmp)4(OAc)]·5H2O (Ln = Ho, Er, Tm, Yb, hmp = 2-(hydroxymethyl)pyridine) were reported as the first 3d-4f WOCs.141,303 Initially synthesised by Wang et al.,303 the series of CoII3LnIII WOC activity is highest in a pH 8 borate/HCl buffer solution with yields of O2 between 90 - 97% and TON vales of 42 - 99. The embedment of the LnIII centres into the cubane was shown to be essential for tunability and performance enhancement, with ErIII and HoIII analogues among leading CoII containing molecular WOCs to date.255,304–306
The stability of the CoII3LnIII cores during the reaction was established with three distinct stages including spectroscopic solution tests and exclusion of nanoparticles, trace metal tests with CoII chelators or ICP-MS analysis and post-catalytic structural integrity checks which encompassed
HPLC analyses as well as XANES/EXAFS spectroscopy. In addition, Density Functional Theory (DFT) studies provides proof that LnIII centres are active catalytic promoters with flexible ligand binding modes. Overall, this contribution has opened up 3d-4f PCCs for photocatalytic applications.
In the wake of the CoII3LnIII cubanes WOC activity, a dinuclear FeIII-CeIV PCC (Figure 1.20) with the general formula [FeIIICeIV(N4Py)(OH2)(NO3)4]+ was reported as a catalyst for water oxidation, which was demonstrated to be a facile and reversible process.307 The position of this equilibrium depends on the number of nitrate ligands on the Ce centre as controlled by the MeCN/H2O ratio in the solvent, which tunes the CeIV/III potential. A variety of characterisation techniques (single- crystal XRD studies, ESI-MS, XAS and TGA) were used to confirm the identity of the FeIIICeIV PCC. Cyclic Voltammetry (CV) confirms the reversible nature of the process and it represents the first example of a reversible inner-sphere electron transfer between CeIII and FeIV= O PCCs.
Figure 1.20. The molecular structure of MnIV6CeIV (upper). The molecular structure of FeIII-CeIV (lower left). The molecular structure of CoII3LnIII (lower right). Colour code: LnIII/IV, light blue; MnIV, purple; FeIII, brown; CoII, pink; O, red; C, white. Hydrogen ions and lattice molecules omitted for clarity.