Second Sphere Coordination Adducts of Phosphane‐Transition Metal Complexes with β‐Cyclodextrin and its Methylated Derivative

Abstract

The second sphere coordination ['] of transition metal complexes bearing hydrophobic ligands by cyclodextrins is a relatively unexplored field.[21 Only two such supramolecular adducts have been ~haracterized'~] structurally in the solid state by X-ray crystallography. In both cases, the hydrophobic ligand of the complex penetrates the cavity of the cyclodextrin via the wider aperture of the receptor associated with the face bearing the secondary hydroxy groups. 'H-NMR spectroscopy reveals that the hydrophobic ligand also enters the cavity in solution, though unequivocal structural information regarding the preferred face[4J at which adduct formation occurs has not been obtained. A large number of transition metal complexes bearing hydrophobic trialkylphosphane ligands find applications in catalysis.[51 In addition, some are employed in the treatment of rheumatoid arthritis.[61 Our interest"' in the second sphere coordination of transition metal complexes has led us to investigate adduct formation between trialkylphosphaneplatinum complexes and cyclodextrins and their methylated derivatives in aqueous solution. We also describe the first example of a transition metal complex adduct with a cyclodextrin in which a crystalline adduct is formed as a result of occupation of the narrower primary hydroxy-group-bearing face of the receptor by the substrate. I+] Present address:metal complex, an analogous shift to higher frequency (0.104 ppm) was observed for the protons of the PMe3 ligand in the complex 1 in the presence of 1 mol equivalent of PCD. These results indicate that, in solution, the PMe3 HO A OH Fig. 1. Top: Structure of the adduct formed between @CD and 1 in the crystal viewed looking into the PCD face carrying the secondary hydroxy groups. Only the PMe3 ligand lies in the cavity of the CD above the lower rim of primary hydroxy groups. The unresolved component of the adduct which lies within a plane parallel to the secondary hydroxy-group-bearing face, and also the peripheral water molecules, have been omitted for clarity. Bottom: In the side-on view of the adduct formed between PCD and 1 in the crystal, the degree of the penetration of the PMe, ligand of 1 into the cavity of PCD is easily discernible.-Crystal data: orthorhombic, a = 19.431 (7) ligand, in common with other hydrophobic first sphere ligands, enters the cavity of the cyclodextrin. Furthermore, the associated chemical shift differences vary with the concentration of the 1 : 1 adduct in a manner that verifies the 1 : 1 stoichiometry. Quantitative treatmentl7I of the concentration dependence in D 2 0 at 22°C afforded an average value[*] for the association constant (K,) for adduct formation of 2950 mol-' kg, corresponding to a value for the free energy of adduct formation (-AGO) of 19.6 kJ mol -I. The K , value is ten times higher than those so far found13' for adduct formation between a C D and other transition metal complexes.[91 Interestingly, however, the absence of 'H-NMR chemical shift changes in the spectrum of a 1 : 1 molar mixture of 1 an...

Selective isolation of gold facilitated by second-sphere coordination with α-cyclodextrin

“…1), with [C−H···Br−Au] contacts (Supplementary Table S3) of 2.92−3.19 Å. The [C−H···Br−Au] hydrogen bonds13495051 favour an equal distribution of orientations of the [AuBr 4 ] − anions around the c -axis. Accordingly, the dimers are stacked along the c -axis with [AuBr 4 ] − anions acting as linkers through multiple [C−H···Br−Au] hydrogen bonds with the α-CDs in the a − b plane and two [O−H···Br−Au] hydrogen bonds with the [K(OH 2 ) 6 ] + ions oriented in the c -axial direction.…”

“…Some of the most common hosts are the cyclodextrins (CDs), which are cyclic oligosaccharides with hydrophobic cavities that can house nonpolar guests. They often lead to the formation of one-dimensional supramolecular inclusion polymers456789, typically pseudopolyrotaxanes, with a wide variety of organic101112, organometallic13141516171819 and inorganic20212223 guests. The structural compatibility between the CDs and the guests is the key preorganization parameter to consider when designing CD inclusion complexes, while hydrophobic interactions24, in addition to second-sphere coordination252627, provide major contributions to the stabilities of these complexes.…”

Whither Second-Sphere Coordination?

“…Following our initial work on using synthetic molecular receptors to bind coordination complexes, there was a surge in the study of second-sphere coordination. Numerous macrocyclic and polycyclic crown ethers, [39][40][41][42][43][44][45][46][47][48][49][50] cyclodextrins (Figures 3a and 3b), [51][52][53][54][55][56][57][58][59][60][61][62][63][64][65] calixarenes (Figure 3e), [66][67][68][69][70][71] , resorcinarenes (Figures 3d) 72 , pillararenes (Figures 3f) 73 , cucurbiturils (Figures 3c) 74 and cyclophanes [75][76][77][78][79] all proved to be efficient second-sphere ligands and even, in some examples, acted simultaneously as both first-and second-sphere coordination ligands (Figure 2i). [80][81][82]…”

Second-Sphere Coordination Revisited

“…The X-ray crystal structure (Fig. 2) of the 1:1 adduct [{trans-Pt(PMe 3 )Cl 2 (NH 3 )}•β-CD] indicates, [17] for instance, that the PMe Pt complexes with two or more NH 3 ligands should have even more opportunities to hydrogen bond with second-sphere ligands and this outcome is indeed the one observed. While [trans-Pt(NH 3 ) 2 Cl 2 ] and 18C6 form a highly insoluble polymeric superstructure with a 1:1 stoichiometry, the isomeric [cis-Pt(NH 3 ) 2 Cl 2 ] -the antitumor drug Cisplatin -generates [18] a sol- [19] a 1:1 adduct [{Pt(NH 3 ) 2 (CBDCA)}•α-CD] with α-CD in aqueous solution as well as in the solid state.…”