Metallosurfactants Consisting of Amphiphilic Ligands and Transition Metals: Structure, Bonding, Reactivity, and Self-assembling Property

: Metallosurfactants are emerging as a relatively new class of surfactants whose ligand moieties bind to various transition metals. Because transition metal centers are incorporated into the surfactant frameworks, they can form various self-assembled structures with metallic interfaces such as micelles, vesicles, and lyotropic liquid crystals. To reduce the lability of transition metal complexes under aqueous conditions, various amphiphilic ligands have been developed as surfactant frameworks. This review discusses some aspects of the design and chemical structures of amphiphilic ligands, as well as focus on various functions and types of chemical bonds present in metallosurfactants.

erties and applications of MSs 9 11 ; however, it is beyond the scope of the current review to discuss all of them. In the present review, we briefly explore some aspects of the design and chemical structures of amphiphilic ligands, as well as focus on various functions and types of chemical bonds present in MSs.
For this purpose, we further define an MS as an amphiphile in which transition metals are bound to a surfactant framework via coordination or metal-carbon bonds. Therefore, we draw a distinction between the MSs and surfactants in which transition metal ions are bound to a surfactant framework via ionic bonds, such as the studies of ionic-type MSs with double-tailed structures, where two dodecyl sulfates are bound to divalent Ni 2 , Co 2 , Cu 2 , and Mg 2 12, 13 , Lewis acid-surfactant combined catalysts LASCs such as scandium tris dodecyl sulfate , which works both as a Lewis acid to activate the reactants and as a surfactant to form stable colloidal dispersions 14,15 ; Ferrum laurate Fe OOCC 11 H 23 3 with triple-tailed structures, which spontaneously form reverse vesicles in organic media such as CHCl 3 and a mixed solvent of CHCl 3 and CH 3 OH 16 .

Metallosurfactants Having Coordination Bonds
Metallic atoms have ability to act as Lewis acids that form complexes with Lewis bases. Most of the elements in the periodic table are metals, and almost all metals form coordination complexes where a metal center is bound to one or more ligands known as Werner complexes . Organic ligands bind the metal through a heteroatom. Depending on the combination of metals and ligands, MSs with coordination bonds can realize a variety of molecular shapes. To date, nitrogen-based ligands such as amine, pyridine, imidazole, triazole, phenanthroline, bipyridine, and terpyridine have typically been used as ligands for MS synthesis 9, 10 .
Scrimin and Tonellato et al. synthesized amphiphilic pyridine ligands with different hydrophilic/hydrophobic balances 1a-d , as shown in Fig. 1 1, 17,18 . 1a with a methyl group R CH 3 dissolved in water and did not form any self-assembled structures. 1b with short alkyl chains R C 8 H 17 self-assembled to form micelles in water, while 1c R C 12 H 25 and 1d R C 16 H 33 with long alkyl chains formed larger aggregates of vesicles. Although 1b showed complexation with Cu II at the micellar interface, no complexation with Cu II was observed at the vesicular interface of 1c and 1d. These results indicate that the aggregate structure and ligand location significantly influence the ability of the ligands to bind Cu II .
Osakada et al. employed N-alkyl bipyridinium ligands 4,4 -bpy-N-CH 2 10 OC 6 H 3 -3,5-X 2 NO 3 2: X tBu, 3: X OMe , as shown in   ium ligand Pd η 3 -C 3 H 5 OAc L NO 3 L 3 6 was also synthesized 21 . Deposition of the single-and double-tailed MSs 4, 5, and 6 on the mica surface formed organic-inorganic hybrid thin films of bi-and multilayer structures. The single-tailed Pt II -containing MS 7 with a tridentate amine ligand formed metallic micelles above the critical micelle concentration CMC 1.2 mM 22 . It is well-known that the Pd-N coordination bond is labile. The alkylbipyridinium ligand is more labile than the pyridine ligands because of its positive charge. Thus, 4 with bipyridinium ligands showed dynamic behavior in water at ambient temperatures based on the reversible liberation of the ligand. The addition of α-cyclodextrin α-CD to a D 2 O solution of 4 led to form n rotaxanes via a slippage reaction Fig. 3 20, 23 25 . In this reaction, 4 was equilibrated with a mixture of 3 and Pd L D 2 O en NO 3 3 L 3 via the partial dissociation of 3 and its coordination. Once liberated, 3 and α-CD produced pseudorotaxanes 3 α-CD m m 1, 2 , which were end-capped by Pd L D 2 O en NO 3 3 to form the n rotaxanes n 2-5 . Each step in the reaction is reversible, although the high stability of the n rotaxane structure arises from the hydrophobic interaction between the cavity of α-CD and alkylbipyridinium ligands, making the reaction formally irreversible in D 2 O.
A similar reaction of the Pt-containing MS shown in 7 with αand β-CDs in aqueous media produced the corre-sponding 2 rotaxanes with 1:1 stoichiometry 22 . γ-CD and 7 formed a rotaxane with components in a 1:1 or 2:1 molar ratio. The addition of NaBH 4 to the rotaxanes in aqueous media formed Pt nanoparticles with diameters of 1.3-2.8 nm. The Pt nanoparticles formed from the rotaxanes showed higher thermal stability than those obtained from the reduction of the cyclodextrin-free Pt complex of 7.
Phosphines are often used as ligands for homogeneous catalysis. The combination of hydrophilic and hydrophobic substitutes results in amphiphilic phosphine ligands Fig.  4 . Suades et al. reported the amphiphilic phosphine ligands Ph 2 P CH 2 n SO 3 Na 8a: n 2, 8b: n 6, and 8c: n 10 , in which sulfonate was linked to the ligands 26 . The ligands allow the metal atom to be located in a hydrophobic environment. Similar amphiphilic phosphine ligands 9a-f 27 , whose coordination sites were located at the end of the hydrophilic moiety, were synthesized in two steps, starting from the corresponding commercial nonionic surfactants. Other series of amphiphilic phosphine ligands 10a-f were also synthesized 28 . The reaction of amphiphilic phosphine ligands 8a-c with PtCl 2 dmso 2 dmso dimethylsulfoxide gave cis-PtCl 2 L 2 L 8a, 8b, and 8c 29 . The surface properties of the amphiphilic phosphine ligands 8a-c and their respective MSs were studied using the Wilhelmy plate method. The comparison between free phosphines and their respective Pt II -containing MSs demonstrated that CMC values were substantially lower for cis-PtCl 2 L 2 L T. Taira 8a, 8b, and 8c , and γ CMC values were significantly higher. The ratio between the CMC of MSs and their free ligands was almost five for compounds 8a and 8b, and nearly ten for 8c. The authors considered that the PtCl 2 fragment acts as a linker between the two hydrophobic chains, and the MSs cis-PtCl 2 L 2 L 8a-8c can be seen as bolaform surfactants. The study of self-assembled structures using dynamic light scattering spectroscopy and cryo-transmission electron microscopy showed the formation of vesicles in all cases. Pd II -containing MSs were also found to form vesicles in water 28 . Two families of molybdenum carbonyl MSs with single-and double-tailed structures, Mo CO 5 L and Mo CO 4 L 2 , were also synthesized from the same amphiphilic phosphine ligands 26,30 . The authors found that the CMCs of the molybdenum carbonyl MSs were significantly lower than those of the respective amphiphilic phosphine ligands. In all cases, the MSs formed multilamellar vesicles rather than micelles in water.
Van Leewan et al. reported amphiphilic phosphine ligands consisting of rigid Xantphos ligands 31 . Rh-containing MSs were prepared by the reaction of the amphiphilic phosphine ligands with RhH CO PPh 3 3 . The MS formed vesicles with a particle size of 140 nm in water. Incorporation of 1-octene into the vesicle led to the formation of a large aggregate with a particle size of approximately 500-600 nm. Because 1-octene was in close proximity to Rh in the membrane, the hydroformylation reaction proceeded, and the corresponding linear aldehyde was obtained in high yield. The product can be separated with an extraction procedure, and the vesicular Rh catalysis remains in the aqueous phase and can be used repeatedly.
As described above, some transition metals with coordination bonds reversibly dissociate via ligand exchange reactions with water. To reduce lability, macrocyclic ligands, which can encapsulate transition metals within the inner cavity, have been employed Cyclic peptides are also known to act as ionophores, wherein metal ions can be encapsulated within the inner cavity. The cyclic lipopeptide surfactin 15 , which is abundantly produced by microorganisms such as Bacillus subtilis, is regarded as a promising biosurfactant. Owing to two carboxylate groups, 15 can bind divalent transition metals such as Ni 2 , Zn 2 , and Cd 2 and form large aggregates in water 34 . Because the cyclic peptide moiety provides an ideal coordination number and cavity size for Cs encapsulation, 15 selectively encapsulates Cs among the other alkaline metals 35 . High binding affinity and effective removal of Cs from water were achieved through micellarenhanced ultrafiltration.

Metallosurfactants Having Metal-carbon Bonds
Organometallic compounds are chemical compounds that contain at least one direct metal-carbon bond. The metal-carbon bonds in organometallic compounds are generally highly covalent. Thus, metal atoms can be strongly attached to the surfactant frameworks. However, organometallic MSs are still rare, likely due to the difficulty of synthesizing non-symmetric structures, preferentially with distinct amphiphilic character.
Uozumi et al. designed amphiphilic pincer-type ligands and their MSs 16-19 36 40 , in which a chelating agent binds tightly to three adjacent coplanar sites of Pd Fig. 6 . The Pincer ligands permit the labile ligands to be firmly bound, and thus endow their metal complexes with exceptional thermal stability. Owing to oligoethylene glycols and alkyl chains, Pd-containing MSs spontaneously form vesicles in water. The vesicles formed from 16 with concentrated Pd centers within the vesicular interface facilitate the Miyaura-Michael reaction in a highly selective manner. It should be noted that when the same reaction was carried out in an amorphous state, the reactivity decreased significantly. Therefore, the organized membrane structure of vesicles contributes to the expression of excellent catalytic functions. Vesicular reactive environments are applicable to various reactions, such as allylic arylation of allyl acetates with sodium tetraarylborates, cyclization, and the arylation of terminal alkynes.
N-Heterocyclic carbenes NHCs , which are derived from imidazolium salts, have attracted significant interest as ligands for transition metal-catalyzed reactions in water. Glorius et al. reported a combination of an NHC ligand and a structurally simple surfactant 20 with an Au I complex,  T. Taira as shown in Fig. 7 41 . Although 20 has both hydrophilic imidazolium and hydrophobic long alkyl chains, its Au I complex is regarded as totally hydrophobic because of the diminished electronic charge of imidazolium. The authors found that mixing a co-surfactant such as sodium dodecyl sulfate SDS with the Au I -containing MS generated metallic micelles whose interface was covered with Au I atoms. Under these conditions, the hydration of 1,2-dipehnylacetylene was facilitated through the solubilization of the reactants within the metallic micelles. Polarz et al. reported the synthesis of chelating ligands 21a and 21b in which two NHC moieties were attached to a pyridine, resulting in a tridentate amphiphile 42,43 . The CMC of 21a was estimated to be 7.9 10 5 M. Above the CMC, 21a formed spherical micelles. The authors synthesized Pd II -, Fe II , Cu I -, and Ag I -containing MSs, in which one or two NHC ligands 21a were coordinated to the metal centers. The Pd II -containing MS not only exhibited Suzuki-Miyaura cross-coupling reactions, but its amphiphilic design also proved to be advantageous for coupling hydrophobic and hydrophilic compounds, likely because the Pd center at the interface facilitated the reactions between reagents with opposite solubility preferences. Cu I -and Fe II -containing MSs with two NHC ligands can be used for emulsion polymerization under atom transfer radial polymerization. Polymerization of methyl methacrylate yielded  stable poly methyl methacrylate colloids in water . Cu Icontaining MS spontaneously formed vesicular structures in water. Amphiphilic NHC ligands 20 and 21 have NHC moieties at the end of the hydrophilic part. In the case of NHC ligand 22, the NHC moiety was placed at the linkage of hydrophilic and hydrophobic parts, which allowed the transition metals to be located at the air-water interface. The reaction of 22 with Pd OAc 2 in water resulted in Pdcontaining MS 44 . In situ generation of MS reduced the CMC value compared to that of 22 3.9 10 3 M . MS forms micelles in water, with an interface covered with Pd II . When oily substrates such as iodobenzene and styrene were added to the micellar solution, the mixture provided emulsion droplets. The addition of triethylamine as a base facilitated the Mizoroki-Heck reaction to give the crosscoupling product of stilbene. The detailed surface and selfassembly properties were found via synthesis of Pd-containing MS PdBr 2 L NEt 3 25: L 22 with triethylamine as a co-ligand 45 .
Owing to the capability of MSs to bond a broad spectrum of metals, Au I -and Ag I -bonded MSs MBr L 23: M Au I , 24: M Ag I were also synthesized. Compared to the self-assembling behavior of the MSs, the identity of the transition metal had a significant effect on the entire system, setting the self-assembly direction of the NHCbased amphiphile, as shown in Fig. 8 45,46 . The CMCs of the MSs were lower than those of 22, indicating that metal coordination promotes self-assembly in water. Moreover, the CMC of 23 1.3 10 5 M was one order of magnitude lower than that of 24 7.3 10 4 M , even though they consisted of the same NHC and bromide ligands. In addition, the CMC of 25 1.4 10 4 M , bearing triethylamine as an additional hydrophobic ligand, was similar to that of 24. The lowest CMC seen in 23 can be attributed to the distinctive role of Au I , such as Au I -Au I interactions. The γ CMC of 23 44.9 mN/m was higher than those of 24 29.6 mN/m and 25 36.7 mN/m . Therefore, the alignment of the Au I atom at the air/water interface may hamper the disruption of the hydrogen bonds present in water, resulting in the aforementioned difference in γ CMC . In a diluted aqueous solution, 23 and 24 formed spherical micelles with an average diameter estimated to be 5.1 0.9 nm for 24 and 5.7 1.1 nm for 23. On the other hand, 25 formed cylindrical micelles with a maximum dimension of 20 nm.
Interestingly, 23 spontaneously formed not only spherical micelles, but also gold nanoparticles AuNPs in water 47 . The AuNPs spontaneously formed when 23 was dissolved in water above the CMC. Control experiments revealed the role of the surface activity of 23 in AuNP formation. The mixing of β-CD with 23 in water inhibited AuNP formation. The apparent rate of AuNP formation in the presence of β-CD was approximately two times lower than that in its absence. As is the case with 4, β-CD formed pseudorotax-ane with 23, inhibiting the formation of spherical micelles. Moreover, the reaction of 23 in ethanol, whose interfacial tension was lower than that of water, did not produce AuNPs. These results suggest that AuNPs were formed from 23, which was adsorbed at the air/water interface. The formation of n pseudorotaxane and the use of ethanol as a solvent inhibited the adsorption behavior of 23.
The difference in the transition metals also induced geometrical transformation at high concentrations. Although 22 did not display an optically anisotropic phase at any concentration, 23-25 exhibited such phases 46 . In the case of 23, the micellar solution became increasingly viscous with increasing concentration. The phase transition from the isotropic micellar phase to the anisotropic phase occurred above 46 wt , where hexagonal phases were observed. In the case of 24, micellar and hexagonal phases were also observed. These self-assembled structures differed from those of 25, wherein the hexagonal phase was not observed, while sponge and lamellar phases were detected. The formation of the lamellar structure suggests that the curvature of 25, bearing a triethylamine as an additional co-ligand, is considered to be close to zero. In contrast, 23 and 24, which did not contain triethylamine, exhibited positive curvatures, resulting in the formation of hexagonal structures.

Conclusion
In this review, we briefly explore some aspects of the designate amphiphilic ligands and their MSs with coordination and metal-carbon bonds. Owing to the varied electronic configurations and bonding patterns of transition metals, MSs exhibit diverse structures and properties that are typically difficult to realize using commonly used organic surfactants. MSs have the ability to program both physical and chemical functionalities, including catalytic functions. Furthermore, in most cases, metal coordination induces unusual self-assembly, such as the formation of vesicles and liquid crystals in water. Self-assembly via metal coordination is beyond the conventional concept of the critical packing parameter CPP . Therefore, the use of MSs may prove critical in the rational design of nanostructures with various functional interfaces.

Conflicts of Interest
There are no conflicts of interest to declare.