Multinuclear Complexes Crystallized from a Mixture of Diphosphine Digold(I) Metalloligands with Cysteinate or Penicillaminate

概要

The self-assembly of metal ions and bridging ligands in solution has been one of the universal methods for creating metal complexes. However, the self-assembly of more than two kinds of components often results in the formation of two or more products. For example, when a mixture of two kinds of ligands (LA and LB) is reacted with a metal ion (M), a pair of coordination isomers, homoleptic complexes [M(LA)2] and [M(LB)2] and a heteroleptic complex [M(LA)(LB)], are formed as the 1:2 complex. In some cases, the selective formation of one species from a mixture has been observed. Such a phenomenon is represented by the term ‘self-sorting’ and is classified into (i) homoleptic self-sorting and (ii) heteroleptic self-sorting, which show the selective formation of homoleptic and heteroleptic products, respectively. As a similar concept, chiral self-sorting phenomenon, showing the selective formation of homochiral (e.g., [M(R-L)2] and [M(S-L)2]) or heterochiral isomer (e.g., [M(R-L)(S-L)]) from a pair of enantiomers of ligand (e.g., R-L and S-L), is also established and referred as the homochiral and heterochiral self-sorting, respectively. Usually, the self-sorting events are directed by ligand-ligand or ligand-metal repulsive or attractive interactions. Therefore, the preference of self-sorting is pre-determined by ligand structures. Thus, coordination systems that can switch the homoleptic vs heteroleptic and homochiral vs heterochiral isomers have rarely been reported. Recently, our laboratory has reported that a series of homoleptic digold(I) metalloligands with penicillaminate (pen) and diphosphine (P^P), [Au2(pen)2(P^P)]2–, can function as a metalloligand toward octahedral metal ions to form cyclic multinuclear complexes, such as [MII{AuI 2(D-pen)2(dppm)}] (M = Ni, Zn).

The studies presented in this thesis focused on the coordination behavior of a mixture of two or more kinds of digold(I) metalloligands [Au2(L)2(P^P)]2 (L = cysteinate (cys), pen; P^P = dppm, dppe) toward octahedral M2+ ions (M = Ni, Zn). Furthermore, the effects on the self-sorting behavior due to chemical modification and reaction condition were investigated.

Firstly, the coordination behavior of a mixture of two kinds of metalloligands, [Au2(L-cys)2(P^P)]2– and [Au2(D-pen)2(P^P)]2–, toward M2+ ion (M = Ni, Zn) was investigated (Scheme 1). The homoleptic AuI 2MII trinuclear complexes, [M{Au2(L-cys)2(dppm)}][M{Au2(D-pen)2(dppm)}] ([1M]) were obtained for dppm system. On the other hand, the corresponding dppe system gave the heteroleptic AuI 2MII trinuclear complex, [M{Au2(L-cys)(D-pen)(dppe)}] ([2M]). The molecular modeling study indicated that the matching of the directions of the P Au bonds in the [Au2(P^P)]2+ unit and the lone pairs on the sulfur atoms of the [MII(L-cys or D-pen)2]2– unit is a key factor for the coordination isomerism. Therefore, the ancillary P^P ligand governs the coordination isomerism around the MII center in [1M] and [2M], leading to the selective homochiral and heterochiral self-sorting in this system.

In order to study the effect of solvent on the self-sorting, the coordination behavior of a mixture of four kinds of metalloligands, [Au2(D-cys)2(P^P)]2–, [Au2(L-cys)2(P^P)]2–, [Au2(D-pen)2(P^P)]2–, and [Au2(L-pen)2(P^P)]2–, toward M2+ ion (M = Ni, Zn) in various solvents (EtOH/H2O, MeOH/H2O, and MeOH) was examined (Scheme 2). In the dppm system, the reaction in EtOH/H2O, which is a moderate polar solvent, formed the homochiral/heteroleptic AuI 2MII trinuclear complexes, [M{Au2(D-cys)(D-pen)(dppm)}][M{Au2(L-cys) (L-pen)(dppm)}] ([3M]). A similar reaction using MeOH and MeOH/H2O, which is a less polar solvent and highly polar solvent, produced the heterochiral/homoleptic AuI 2MII trinuclear complexes, [M{Au2(D-cys)(L-cys)(dppm)}] ([4M]), and the homochiral/homoleptic AuI 2MII trinuclear complexes, [M{Au2(D-pen)2 (dppm)}][M{Au2(L-pen)2(dppm)}] ([5M]), respectively. This trend is reasonable because the relatively polar cys complex may prefer to dissolve in the highest polar solvent. On the other hand, the dppe system showed the opposite trend; the heterochiral/homoleptic AuI 2MII trinuclear complexes, [M{Au2(D-cys)(L-cys)(dppe)}] ([6M]), were yielded in EtOH/H2O that has the moderate polarity, whereas the homochiral/homoleptic AuI 4MII2 hexanuclear complexes, [M2{Au2(D-pen)2 (dppe)}2][M2{Au2(L-pen)2(dppe)}2] ([7M]), were formed from less polar MeOH. The preferable formation of hexanuclear complex in MeOH was explained by the non-polar D2 symmetric structure of [7M].

Furthermore, the coordination behavior of an enantiomeric mixture of the metalloligands, [Au2(D-cys)2(P^P)]2– and [Au2(L-cys)2(P^P)]2–, toward Ni2+ ion was investigated. The reaction without the additive gave the heterochiral AuI 2NiII complexes, [Ni{Au2(D-cys)(L-cys)(P^P)}] (P^P = dppm ([4Ni]), dppe ([6Ni])), indicating the heterochiral self-sorting phenomenon. Interestingly, the co-presence of [Au2(D-pen)2(dppm)]2– with the mixture of [Au2(D-cys)2(dppm)]2– and [Au2(L-cys)2(dppm)]2– gave the homochiral AuI 2NiII complex, [Ni{Au2(D-cys)2(dppm)}][Ni{Au2(L-cys)2(dppm)}] ([8Ni]) (Scheme 3). Thus, the formation of [8Ni] is a kind of an additive-controlled crystallization. It is likely that the ratio of homochiral and heterochiral species in the reaction solution was changed by the additive, leading to the switching of the chiral self-sorting behavior. Such an additive-controlled formation was not observed in the dppe system, probably due to the high stability of the heterochiral structure.

Finally, this study demonstrated that the self-sorting behavior of multi-component metal complexes could be controlled by the subtle changes of ancillary ligands or reaction solvents. Indeed, eight of ten possible coordination isomers of the [M{Au2(L)2(P^P)}]-type complexes (L = D-cys, L-cys, D-pen, or L-pen) were successfully isolated in a controlled manner in the present study. Such a systematic preparation of coordination isomers of a multi-component coordination system has never been achieved so far. The chemical library obtained in this work should be a good platform for studying the relationship between the physical and chemical properties and the coordination isomerism in the future study.