<Introduction>

Multicomponent self-assembly from a number of different metal ions and ligands has receivedconsiderable attention in these years as an excellent strategy to construct higher-order, functionalizedmolecular architectures. To achieve this, each component is required to contain structural and unctionalinformation for self-assembly. Basically, the reversible nature of coordination bonding would allowsite-selective metal-ligand exchange on a certain metal center by controlling reaction condition such asacid-base balance. Such dynamic behaviors on metal centers lead to switching functions of the resultingentities. Furthermore, the use of "hard and soft natures of metal ions and ligands" is pretty commonlyknown for multiple metal-centered self-assembly directed towards multifunctional supermolecules. So far,a number of excellent examples of dynamic supramolecules have been reported mainly using soft metalions, but the dynamic chemistry on a hard metal ion has only just begun due to its relatively labile nature.Along this line, this work aimed at developing a hard metal-centered dynamic chemistry which wouldprovide a platform for heteronuclear self-assembled systems.

In this study, a novel heteroleptic Ti(IV) complex, [Ticat2(acac)]-, (cat = catecholate, acac =acetylacetonate) was synthesized and its dynamic properties have been established. Detailed solutionanalyses revealed that this Ti(IV) complex is interconvertible with a well-known [Ticat3]2- complex bycontrolling acid-base balance, and that its acac ligand can be site-selectively replaced by a tropolonate(trop) ligand (Scheme 1).Furthermore, a new Pd(II)-Ti(IV)multicomponent self-assembledsystem has been established usinga ditopic pyridyl catechol ligand,H21, which was designed based onthe HSAB principle.

<A Novel [Ti12(acac)]- Complex which Allows Ti(V)-Centered Dynamic Interconversionbetween Pd(II)-Ti(IV) Ring and Cage Complexes>

Based on the Ti(IV)-centeredsite-selective ligand exchangeinduced by acid-base control(Scheme 1), a Ti(IV)-centeredstructural switching system betweenmultinuclear Pd(II)-Ti(IV) ring andcage complexes has beenestablished using a ditopic ligandH21 (Figure 1).

Towards the construction ofhigher-order heteronuclearcomplexes, a ditopic ligand H21bearing a harder catechol and asofter pyridyl moieties has beendesigned and synthesized, and thenits complexation with Ti(IV) inDMF-d7 solution has been firstlyexamined. Upon addition of TiO(acac)2 to amixture of H21 and n-Bu4NOH in DMF-d7,the 1H NMR spectrum immediately aftermixing showed the formation of two distinctspecies (Figure 2b). However, after 20 h, aset of signals for one species completelydisappeared (Figure 2d). From a time-courseanalysis of ESI-TOF mass spectra of themixture, it appears that the final product andan intermediate firstly formed can beassigned to [Ti13]2- and [Ti12(acac)]-,respectively. Since the intermediate specieswas also detected as [Ti12(acac) + 2H]+ in itsESI-TOF mass measurement, theintermediate species was thought to be stableunder a weakly basic condition. To confirmthis, the effect of a weaker base,N-methylmorpholine, instead of n-Bu4NOH,was examined. As a result, the 1H NMRspectrum of the mixture was identical withthat of the intermediate [Ti12(acac)]- (Figure2e). A series of amine bases were thenexamined to clarify the relationship between their degree of basicity and the molar fraction of the twospecies, [Ti13]2- and [Ti12(acac)]-. As shown in Figure 3, the molar fraction of generated kineticintermediate is potentially correlated with the pKa values of the conjugated acids. Moreover, from various1H NMR titration studies, it was also found that [Ti12(acac)]- is partially protonated as TiH12(acac)whereas the remaining complex [Ti12(acac)]- pairs with protonated N-methylmorpoline in solution. Thus, the stability of [Ti12(acac)]- depends on the degree of the protonation(hereafter referred to as [Ti12(acac)]- for clarify). In addition, the bindingof acac to [Ti12(acac)]- was also confirmed by IR spectra of a solidsample. Its UV-vis absorption spectrum in DMF showed a characteristicLMCT band which is red-shifted from that of [Ti13]2-.

These two distinct Ti(IV) complexes were found to beinterconvertible, and the component fraction depends on the acid-basebalance. When TiO(acac)2 and TFA were added to a solution of [Ti13]2-,[Ti12(acac)]-was produced. On the other hand, when H21 andn-Bu4NOH were added to the solution of [Ti12(acac)]-, [Ti13]2- wasquantitatively regenerated. In this reversible process, the pyridyl group ofligand 1 was coordinatively free.

In the next step, the complexation behavior of the pyridyl groups ofthe Ti(IV) complexes was examined using softer Pd(II) ions. From a 1:1mixture of PdCl2(CH3CN)2 and [Ti12(acac)]-, [Pd2Ti214(acac)2Cl4]2- ringcomplex was formed selectively, and similarly the complexation of [Ti13]2- with PdCl2(CH3CN)2 provided[Pd3Ti216Cl6]4- cage complex. The formation of these complexes was characterized by 1H NMR (Figure 2),ESI-TOF mass, and UV-vis absorption measurements. The interconversion between the[Pd2Ti214(acac)2Cl4]2- ring and [Pd3Ti216Cl6]4- cage was also achieved by the Ti(IV)-centered ligandexchange similarly to the interconversion between [Ti12(acac)]- and [Ti13]2- complexes.

<Efficient Construction of Self-Assembled Pd(II)-Ti(IV) Complexes with a DitopicTropolonate Bridging Ligand Based on Site-Selective Ligand Exchange Reactions on theHeteroleptic [Ti12(acac)]- Complex>

Site-selective ligand exchange on a heteroleptic complex is an effective tool for introducingadditional functionality and thereby constructing higher-order self-assembled structures with greaterfunctions. In this regard, a heteroleptic [Ti12(acac)]- complex appeared to show promise for site-selectiveligand exchange because it contains both relatively labile acac and relatively inert catecholate ligands.

As a result of exploring bidentate ligands which can replace acac in [Ti12(acac)]-, it was found that amixture of H21, Ti(OiPr)4, N-methylmorpholine, and various bidentate ligands in DMF provide [Ti12X]-(HX = benzoylacetone,dibenzoylmethane,3-phenyl-2,4-pentanedione, -benzoyl-Nphenylhydroxylamine,quinolinol, maltol, andtropolone). Furthermore,from competitiveexperimentsto evaluate the relativestability of a series ofthese complexes itwas proven that[Ti12(trop)]- complexis much more stablethan [Ti12(acac)]- complex. Actually, when an equimolaramount of Htrop (= tropolone) was addedto a solution of [Ti12(acac)]- in DMF, 70% of [Ti12(trop)]- was formed as aresult of site-selective ligand exchange ofacac with trop as monitored by 1H NMRspectroscopy (Figures 5a-b). Thisreaction took place similarly in the[Pd2Ti214(acac)2Cl4]2- ring complex. 1HNMR spectrum of a 2:1 mixture of Htropand [Pd2Ti214(acac)2Cl4]2- ring complexshowed that [Pd2Ti214(trop)2Cl4]2- ringcomplex was quantitatively formed in away that the precursory ring frameworkwas preserved (Figures 5c-d). A modelstudy with [Ti{Pd(dien)1}2(acac)]- (dien= diethylenetriamine) suggested that thisligand exchange reaction could bepromoted by the binding of the pyridylgroups of [Ti12(acac)]- to Pd(II) ions.

Based on the above-mentionedsite-selective ligand exchange reactionon the Pd(II)-Ti(IV) ring complex, synthetic bridging ligands with two terminal tropolone ligands, flexibleH22 and rigid H23, have been examined to exploit higher-order self-assembled constructs. Upon theaddition of an equimolar amount of H22 to a solution of [Pd2Ti214(acac)2Cl4]2- ring in DMF-d7, two acacmoieties were intramolecularly replaced by ligand 2 and quantitatively formed a tetranuclear[Pd2Ti2142Cl4]2- cage-shaped complex as determined by 1H NMR (Figure 5e) and ESI-TOF massmeasurements. On the other hand, a 1:1 mixture of H23 and [Pd2Ti214(acac)2Cl4]2- ring resulted in thequantitative formation of an intermolecularly connected complex, an octanuclear [Pd4Ti41832Cl8]4-complex (Figure 5f).

To firmly establish the efficacy of such a stepwise procedure, one-pot synthesis was conducted in away that all components were mixed at a time. In the result, tetranuclear [Pd2Ti2142Cl4]2- and octanuclear[Pd4Ti41832Cl8]4- were generated in 69% and 38% yields, respectively. This lower efficiency is due to theformation of an insoluble material kinetically formed from Ti(IV) and bridging ligands as revealed byvarious NMR titration studies. Thus, the stepwise procedure employed is an excellent tool for constructingthe higher-order heteronuclear self-assembled complexes.

<Conclusion>

In this study, a synthetic method of a novel heteroleptic Ti(IV) complex has been established, and itwas further extended to a structural switching system ([Ticat2(acac)]- and [Ticat3]2-) which is controllableby acid-base balance. The hetroleptic [Ticat2(acac)]- complex also serves as a platform for constructinghigher-order constructs through site-selective ligand exchange reactions on more labile acac sites. By usinga ditopic pyridyl catechol ligand instead of catechol, a stepwise synthetic method was accomplished forconstructing heteronuclear Pd(II)-Ti(IV) complexes. Such a synthetic strategy would be useful insupramolecular approaches to heterogeneous, multicomponent self-assembled systems which possessmolecular functions depending on metal-centered dynamic chemistry.

Scheme 1. Ti(IV)-centered dynamic chemistry developed in this study.

Figure 1. Formation of Ti(IV) and heterologous multinuclearPd(II)-Ti(IV) complexes.

Figure 2. Partial 1H NMR spectra (500 MHz, DMF-d7, 293K, [H21] = 15 mM); (a) H21; (b)-(d) a 1:0.43:0.67 mixture ofH21, TiO(acac)2, and n-Bu4NOH. (b) 5 min; (c) 1 h; (d) 20 h([Ti13]2-); (e) a 4:4:3 mixture of H21, TiO(acac)2, andN-methylmorpholine ([Ti12(acac)]-); (f) Pd(II)-Ti(IV) ring([Pd2Ti214(acac)2Cl4]2-); (g) Pd(II)-Ti(IV) cage([Pd3Ti216Cl6]4-). Signals with a + mark in (f) are assignedto those of a cyclic 3:3 complex [Pd3Ti316(acac)3Cl6]3-.

Figure 3. Plot of the molar fraction of [Ti12(acac)]- in the mixture against pKe value of the conjugated acids.

Figure 4. Schematic representation of construction of higher-order multicomponentassembled complexes by site-selective ligand exchange reactions on thePd(II)-Ti(IV) ring complex.

Figure 5. Partial 1H NMR spectra (500 MHz, DMF-d7, 293 K,[H21] = 12 mM) of site-selective ligand exchange of acac withtrop on [Ti12(acac)]- (a-b) and [Pd2Ti214(acac)2Cl4]2- ringcomplex (c-f). a) [Ti12(acac)]-, b) a 1:1 mixture of [Ti12(acac)]-and Htrop (20 h). The signals for [Ti12(acac)]- complex aremarked by open circles. c) Pd(II)-Ti(IV) ring, d) a 1:2 mixture ofPd(II)-Ti(IV) ring and Htrop (20 h), e) a 1:1 mixture ofPd(II)-Ti(IV) ring and H22 (20 h), f) a 1:1 mixture of Pd(II)-Ti(IV)ring and H23 (20 h).

複数種の金属イオンと配位子から成る多成分系の超分子金属錯体の構築法は、より高次な分子構造と機能を創出するストラテジーの一つとして、近年注目を集めている。複数種の金属イオンと配位子から目的とする構造体を一義的に構築するためには、これらの構成要素を精密に配置する方法論が必要となる。例えば、金属イオンと配位子のハード・ソフト親和性は、複数種の金属イオンを位置選択的に配列する際に有用な特性である。また、金属イオンの配位数や配位方向は外部刺激に応答して変化するため、その特性は超分子金属錯体の動的機能の要となりうる。特に、異種金属イオンが集積した金属錯体内で、特定の金属イオン上の配位構造変化を位置選択的に引き起こすことができれば、従来の単一種の金属イオンから成る系では発現できなかった新たな機能の創出が可能になる。本研究では、異種多核錯体系においても展開可能な汎用性の高いハードな金属イオンを中心とした動的化学の開拓を目的とした。ハードなTi(IV)イオンとカテコールなどのキレート型配位子との錯体形成挙動を詳細に追跡した結果、Ti(IV)中心における特異な配位子交換反応が開発された。さらに、比較的ハードなカテコール部位と比較的ソフトなピリジル部位を組み込んだ配位子H21を新たに設計・合成し、Ti(IV)イオン上における位置選択的な配位子交換反応と連動したPd(II)-Ti(IV)多核錯体系の動的構造変化の制御法を確立した。

本論文は全4章から成り、第1章では、本研究の目的、背景が詳述されている。

第2章では、Ti(IV)イオンを中心とした新たな動的構造変換システムの創製と、これを利用した二種類の異種多核錯体間の構造スイッチング制御について述べられている。強塩基であるn-Bu4NOH存在下における配位子H21とTiO(acac)2との錯体形成挙動を詳細に追跡した結果、熱力学的に安定な[Ti13]2-錯体を形成する過程で、Ti(IV)イオンに二分子のカテコラトと一分子のアセチルアセトナトが選択的に配位した新規[Ti12(acac)]-錯体が速度論的な中間体として生成することが見出された。この新規Ti(IV)錯体は、より弱い塩基であるN-メチルモルホリン存在下において、熱力学的に安定な種として得られることも明らかにされた。これら二種類のTi(IV)単核錯体の形成は用いた塩基の種類や配位子と金属イオンの比に依存することから、これらの要素を厳密に制御することで[Ti13]2-錯体と[Ti12(acac)]-錯体が定量的に可逆的構造変換可能であることも示された。さらにこのシステムを異種多核錯体系へと応用するため、得られた二種のTi(IV)錯体とPdCl2(CH3CN)2との錯体形成挙動を追跡した結果、[Ti13]2-錯体からはPd(II)-Ti(IV)かご型錯体が、[Ti12(acac)]-錯体からはPd(II)-Ti(IV)環状錯体がそれぞれ選択的に得られることが明らかにされた。これらの三次元構造の違いはTi(IV)イオン周りの配位構造の違いに起因することから、Ti(IV)単核錯体間の相互変換を行った際と同様な化学的な刺激を与えることにより、かご型錯体および環状錯体間の構造スイッチングが達成された。

第3章においては、新規Ti(IV)錯体における位置選択的な配位子交換反応の開発および高次多成分系自己集合体の効率的な形成への展開について述べられている。新規[Ti12(acac)]-錯体のacac部位の多様性について調べたところ、種々のキレート型配位子(HX)を用いても類似の構造を持つ[Ti12X]-錯体がそれぞれ選択的に形成できることが見出された。続いてこれら錯体間の相対的な熱力学的安定性の違いを競合実験によって調べた結果、トロポロナト(trop)が配位した[Ti12(trop)]-錯体は[Ti12(acac)]-錯体に比べて十分に安定であるという知見が得られた。この結果をもとに、種々のTi(IV)錯体内のacac部位とtropとの配位子交換を検討するといずれの場合も高収率でtrop置換体が得られた。また、この反応はPd(II)-ピリジン結合の解離を伴うことなく独立に進行することも明らかにされた。これを利用し、Pd(II)-Ti(IV)環状錯体を構成ユニットとした高次構造の構築をビストロポロン型の配位子を用いて行ったところ、Pd(II)-Ti(IV)四核錯体および八核錯体をそれぞれ定量的に得ることに成功した。これらの高次構造体は従来の自己集合法である、ワンポット合成によっては定量的には得られなかったことから、Ti(IV)イオン上における位置選択的配位子交換を利用した段階的構築法が有用な手法であることが示された。

第4章では、本論文の総括と、今後の展望が述べられている。

以上のように、本博士論文では、ハードなTi(IV)イオンを中心とした新たな動的化学を開拓することに成功した。また、独自に設計・合成した新規配位子H21およびビストロポロン配位子と組み合わせにより、異種多核金属錯体の動的機能を創出することができた。本研究成果は、高度な金属錯体型自己集合システムの構築のための有用な方法論を与えるものであり、理学の発展に大いに貢献するものである。よって、博士(理学)取得を目的とする学術研究として十分な意義を有する。なお、本論文における各章の研究は他の複数の研究者との共同研究によるものであるが、論文提出者が主体となって実験、解析および考察を行ったものであり、論文提出者の寄与が十分であると判断する。

したがって、博士(理学)の学位を受けるのに十分な資格を有するものと認める。