Ligand‐Directed Template Assembly for the Construction of Gigantic Molybdenum Blue Wheels

Abstract Template‐mediated synthesis is a powerful approach to build a variety of functional materials and complex supramolecular systems. However, the systematic study of how templates structurally evolve from basic building blocks, and then affect the templated self‐assembly, is critical to understanding and utilizing the underlying mechanism, to work towards designed assembly. Here we describe the templated self‐assembly of a series of gigantic Mo Blue (MB) clusters 1–4 using l‐ornithine as a structure‐directing ligand. We show that by using l‐ornithine as a structure director, we can form new template⊂host assemblies. Based on the structural relationship between encapsulated templates of {Mo8} (1), {Mo17} (2) and {Mo36} (4), a pathway of the structural evolution of templates is proposed. This provides insight into how gigantic Mo Blue cluster rings form and could lead to full control over the designed assembly of gigantic Mo‐blue rings.

Template-mediated synthesis is av ersatile approach to fabricate nanostructured materials and supramolecular architectures. [1] Anion templates are widely investigated in biological systems, [2] but they are much less explored in supramolecular inorganic chemistry,incomparison to cationic and neutral templates. [3] In the field of polyoxometalates (POMs) [4] -a unique class of anionic discrete metal-oxo clusters with aw ide variety of structures and propertiesanion templates are essential for their construction. [5][6][7] Not only do these anions direct the self-assembly of POMs via atemplating effect but also affect the resultant structures and their properties. [8] More recently,n anosized POM clusters have been found as anion templates to direct the selfassembly of larger POMs. [9] One example is the formation of {Mo 36 }u pon acidification of molybdate,w hich allows for the self-assembly of the {Mo 154 }g iant Mo Blue wheel after addition of the reducing agent, revealing the role of the {Mo 36 } anion template. [9a] In general, three types of interactions can be established between the templates and POMs:h ydrogen bonding, [9c] electrostatic interactions [9a] and coordination, [9f] see Scheme 1. [9d] To exploit this,w ec onsidered if multiple interactions could be combined to build aseries of new POM clusters,w hile elucidating the mechanism of templated selfassembly.I na ddition, an understanding of the structural evolution of the template species may be derived by trapping fragments/intermediates in the larger host structures.
Herein we describe the template-based self-assembly of as eries of Mo Blue (MB) wheels showing as tructural evolution 1-4.T hese compounds are formed by using l-ornithine to direct the assembly electrostatically and via hydrogen bonding to stabilize the template&host architecture (Scheme 1), giving new mechanistic insights. Top: representative interactions such as hydrogen bonding, electrostatic interactiona nd coordination bonds found between anion templates and POM hosts. Bottom: l-ornithine as astructure-directing agent during anion-templated self-assemblyo fMoBlue clusters, providing both hydrogen bonding (purple dotted line) and electrostatic interactions.
The{ Mo 17 }u nit is isolated for the first time,a lthough its framework has been seen previously in the {Mo 36 }cluster.All compounds were characterized crystallographically and the formulae were determined using an array of techniques (see SI). Compounds 1-4 can be formulated as: l-ornithine was initially introduced into as tandard version of the lanthanide-doped Mo Blue synthesis system (see SI for details), [10] to establish whether it could assist in trapping an "intrinsic" template that directs the self-assembly process.C rucially,n op reformed anion templates,a se stablished in previous studies,s uch as the Keggin-type {M 12 }o r Dawson-type {M 18 }w ere added, yet crystals of 1 were obtained in two weeks.Single-crystal X-ray structure analysis of 1 reveals ah ost-guest structure 1a in which a-[Mo 8 O 26 ] 4À ({Mo 8 }) is encapsulated by the {Mo 124 Ce 4 }h ost ( Figure 1). [11] {Mo 124 Ce 4 }i sc omposed of 12 {Mo 8 }u nits,8{ Mo 2 }u nits,1 2 {Mo 1 }u nits and 4{ Ce(H 2 O) 5 }u nits functionalized by l-ornithine ligands on the inner surface.T wo Ce 3+ ions are located on the upper rim and separated by one {Mo 2 }u nit while another two are situated on the opposite side and separated by two {Mo 2 }units,giving rise to an elliptical wheel. All the l-ornithine ligands are attached onto {Mo 2 }units with the side chain buried in the pitch of {Mo 124 Ce 4 }. Due to the limited resolution of the X-ray data, only three l-ornithine ligands can be fully located crystallographically.T he {Mo 8 } template located at the centre of {Mo 124 Ce 4 }i sa nchored by as eries of NÀH···O (3.0205(2)-3.7020 (2) )a nd CÀH···O (3.1802(2)-3.7795 (2) )h ydrogen bonds,w hich are formed between the terminal Oa toms of {Mo 8 }a nd the methylene and protonated amine groups on the l-ornithine ligands ( Figure S6). In addition, the protonated l-ornithine also acts as ac harge buffer to reduce the repulsive electrostatic force between the anionic {Mo 8 }template and the {Mo 124 Ce 4 }host. Thecombination of both hydrogen bonding and electrostatic interaction is thus able to stabilize the template&host architecture.
Under more concentrated reaction conditions,decreasing the ratio of Ce 3+ /MoO 4 2À leads to the discovery of compound 2.S ingle crystal X-ray structure analysis of 2 reveals ah ost- isostructural to the {Mo 150 Ce 2 }elliptical ring-shaped structure that is composed of 14 {Mo 8 }u nits,1 2{ Mo 2 }u nits,1 4{ Mo 1 } units and 2{ Ce(H 2 O) 5 }u nits,a nd is functionalized by six protonated l-ornithine ligands on the inner surface. [12] The2 symmetry-related Ce 3+ ions are distributed evenly on the two ends of {Mo 150 Ce 2 }, producing an elliptical wheel with outer and inner ring diameter of % 31 and 12 ,r espectively,a ti ts most elongated points.The six l-ornithine ligands are grafted onto six {Mo 2 }units via carboxylate groups with the side chain buried in the pitch of {Mo 150 Ce 2 } (Figure 2a).
Similar to 1a,t he {Mo 17 }t emplate also resides at the centre of {Mo 150 Ce 2 }a nd is anchored in place by al arge number of NÀH···O (2.9338(1)-3.7231 (1) )a nd CÀH···O (2.9419(1)-3.6863 (1) )hydrogen bonds formed between the terminal Oa toms of {Mo 17 }a nd the l-ornithine ligands  grafted to the inner surface of the wheel ( Figure S7). Moreover, the presence of protonated l-ornithine ligands minimizes the repulsive electrostatic force between the anionic {Mo 17 }template and {Mo 150 Ce 2 }wheel, thus binding the whole structure together.T he {Mo 17 }c luster template is composed of two {Mo 8 }u nits connected by one {Mo 1 }u nit (Figure 2b). Thes tructural motif of {Mo 17 }c ould be related to the {Mo 36 } cluster, [13] which itself can be simplified as two {Mo 17 }j oined by two {Mo 1 }u nits.P reviously,t he {Mo 36 }c luster has been trapped as at ransient template during the self-assembly of {Mo 154 } [9a] and, in this regard, {Mo 17 }m ay therefore be considered as ap ossible precursor in the formation of the {Mo 36 }t emplate.I ts hould be noted that the {Mo 17 }i s disordered equally in two positions within the {Mo 150 Ce 2 } host (Figure 2c and d).
In our previous work, we have shown that both {PMo 12 } and {P 2 W 18 }c an replace the intrinsic {FeMo 6 }t emplate and direct the aggregation of the {Mo 24 Fe 12 }macrocycle. [9d] Given the formation of an intrinsic or "natural" template in situ for 2,w ec onsidered whether using ap reformed template to induce the formation of {Mo 150 Ce 2 }could also be applied here. Thea ddition of preformed {PMo 12 }d uring the synthesis of 2 resulted in the formation of compound 3.C onsistent with 2, the single crystal X-ray structure analysis of 3 reveals ahostguest structure 3a where aK eggin-type anion {PMo 12 Figure S8). Due to the relatively smaller size of {PMo 12 }incomparison to the central cavity of {Mo 150 Ce 2 }, the cluster is disordered over two positions,equally,oneither side of the framework cavity,maximizing electrostatic interactions and hydrogen bonding (NÀH···O (2.8850(2)-3.7598(2) )and C À H···O (3.3208(1)-3.7977(3) ). Notably,{ PMo 12 }a dopts the b-configuration instead of the a-isomer (Figure 3b and 3c), due to the b-form being the preferred species under reduced conditions. [14] Thet emplation effect of encapsulated clusters has been studied in situ by 31 PNMR spectroscopy.F ollowing the synthetic procedure used to produce compound 3,a fter the addition of all starting materials in D 2 O, a 31 PNMR spectrum was recorded every 10 min for 1h.Whenalow concentration of {PMo 12 }( 2.5 mg) was used, much less than the stoichiometry required for the formation of {PMo 12 }&{Mo 150 Ce 2 }, signals resulting from the reduced {PMo 12 }( À5.69 and À6.25 ppm for b-{PMo 12 }a nd a-{PMo 12 }, respectively) could be detected after 10 min and then completely disappeared after 1h ( Figure S11c,d). Note that the signals of fully oxidized a-{PMo 12 }a re located at À3.35 ppm ( Figure S11e). In contrast, the addition of excess {PMo 12 }(8.0 mg) resulted in the presence of peaks of {PMo 12 }t hroughout the reaction process but the signal became much less intense after 1h ( Figure S11a,b). This indicates that {PMo 12 }i satemplate during the self-assembly,w hich is gradually consumed upon the formation of {PMo 12 }&{Mo 150 Ce 2 }( 3a). Once {PMo 12 }i s included as atemplate,its 31 PNMR signal cannot be observed because of the shielding effect of the paramagnetic {Mo 150 Ce 2 }. This is also confirmed by the control experiment where compound 3 showed no 31 PNMR response in solution ( Figure S12). Accordingly,when an inadequate amount of the template was used, the {PMo 12 }iscompletely encaspulated by the host and hence the {PMo 12 }isNMR silent. When an excess of template is used, this results in the presence of signals even after 1h due to the remaining presence of free {PMo 12 } ( Figure S11a).
Thesuccessful encapsulation of {Mo 8 }, {Mo 17 }and {PMo 12 } as templates led us to further explore the potential of using lornithine as as tructure-directing agent to trap different templates,w hich may in turn give us more insight regarding the formation mechanism of the templated self-assembly of Mo Blue clusters.I nt he cases of 1-3,l anthanide-doped Mo Blue (LMB) {Mo 124 Ce 4 }a nd {Mo 150 Ce 2 }p rovide confined environments to enclose templates.I ng eneral, LMB-based wheels exhibit amore curved inner surface and elliptical ring shape,a nd thus as maller size compared with the archetypal {Mo 154 }w heel. [15] With this in mind, we performed the synthesis without adding lanthanides to see how the change of curvature and size of MB will affect the entrapped template.A fter as ystematic optimization of the synthetic conditions we were able to obtain another templated MB cluster 4,see Figure 4.
Single-crystal X-ray structure analysis reveals that 4 crystalizes in space group P " 1a nd consists of two crystallographically independent wheels in the molecular structure, denoted as 4a 1 and 4a 2 ,r espectively (Figure 4a and b).  (Figure 4b).
[15a] The{ Mo 36 }, which is situated in the center of the {Mo 150 }c avity,i ss tructurally equivalent to the well-known and previously described {Mo 36 } cluster, [13] as observed in the previously reported {Mo 36 }& {Mo 150 }. [9a] There are also six l-ornithine ligands attached on six {Mo 2 }u nits in 4a 1 .A mong them, two pairs of l-ornithine are orientated in tail-to-tail mode,w ith the terminal amino groups pointing to each other, while the remaining two are arranged freely. [10] Similar to 1-3,t he positively charged  (2) )a nd CÀH···O (2.7862(1)-3.5290 (2) )h ydrogen bonds with both the terminal and bridging Oa toms on {Mo 36 }, to further stabilize the aggregate ( Figure S9). In addition, several sodium ions are also found between the {Mo 36 }a nd {Mo 150 }. Notably,the {Mo 36 }inthe previously reported {Mo 36 }&{Mo 150 } compound retains ap arallel orientation along with {Mo 150 } and is thus completely encased within the cavity of {Mo 150 } ( Figure S10). [9a] However,t he {Mo 36 }i n4a 1 adopts av ertical orientation that is almost perpendicular to the plane defined by {Mo 150 }, with the two rims stretching out of {Mo 150 } ( Figure 4c). This is caused by the concomitant accommodation of six l-ornithine in 4a 1 and {Mo 36 }, which reduces the available space to fully encompass the {Mo 36 }and thus forces it rotate along its lateral axis,resulting in the {Mo 36 }extending out beyond either side of the {Mo 150 }cavity. 4a 2 adopts the same framework as {Mo 154 }w ith l-ornithine functionalizing the inner surface.D ue to the limited resolution of crystal data, only the carboxylate group of the l-ornithine could be identified. Although the central cavity of 4a 2 is large enough to accommodate {Mo 36 }, no template is found in the cavity of 4a 2 .F rom the space filling modes of 4a 1 , 4a 2 and {Mo 36 }&{Mo 150 }, it can be seen that the {Mo 150 }h ost in 4a 1 and {Mo 36 }&{Mo 150 }e xhibits as lightly elliptical ring due to the symmetric defect of two {Mo 2 }units at the two elongated ends,while 4a 2 adopts aroughly regular ring as exhibited by the archetypal {Mo 154 }( Figure S10). Taking account of the space occupied by l-ornithine and the size and shape of {Mo 36 }(1.4 nm 1.6 nm 2.1 nm), this kind of arrangement means the {Mo 150 }c an either accommodate {Mo 36 }a long its lateral axis (4a 1 )o rl ongitudinal axis ({Mo 36 }&{Mo 150 }).In contrast, the cavity of 4a 2 is too large in comparison to {Mo 36 }a nd thus is unable to capture {Mo 36 } efficiently.T he encapsulation of {Mo 36 }i n4a 1 further confirms that {Mo 36 }i sakey template during the selfassembly of Mo Blue,c onsistent with our previous study. [9a,b] Thef ormation of compounds 1-4 with l-ornithine as as tructure-directing agent allows us to propose ap otential mechanism of templated self-assembly underpinning the formation of the compounds,s ee Scheme 2. Thep rocess could be described as follows:firstly,the basic building blocks of {Mo 8 }, {Mo 2 }a nd {Mo 1 }f orm in solution. Next, the labile {Mo 8 }units could either transform to the isomeric a-{Mo 8 }or dimerize with one {Mo 1 }unit to generate the {Mo 17 }cluster in situ, which then behaves as at emplate to drive the selfassembly of 1 and 2 under the direction of l-ornithine, respectively.I ft he preformed {PMo 12 }i si ntroduced during the synthesis of 2,t hen the formation of {Mo 17 }i sp revented and 3 will be constructed with {PMo 12 }a st he templating species.T he dimerization of {Mo 17 }t of orm {Mo 36 }p resents anew template that can direct the formation of 4a 1 together with l-ornithine.T he templated self-assembly revealed here implies that by rationally controlling the sizes,s hapes and charges of anionic POM templates with the assistance of structure directors,s maller or larger MB clusters that go beyond the current limitation of sizes and nuclearities can be constructed, leading to the discovery of unprecedented template species and MB framework clusters.
In summary,wehave described the anion-templated selfassembly of MB clusters 1-4 featuring template&host architectures with l-ornithine as astructure-directing agent. Upon protonation, l-ornithine not only serves as charge buffer to bind the anionic templates and hosts together,b ut also provides hydrogen bonding sites to strongly interact with templates,which appears to be essential for the formation of 1-4.T he successful construction of 1 unveils the "intrinsic template" of {Mo 8 }formed in situ. Under more concentrated conditions,{Mo 17 }istrapped as atemplate to induce the selfassembly of 2,w hich could be replaced by the preformed {PMo 12 }, leading to the formation of 3.A 31 PNMR study during the synthesis of 3 illustrates the self-assembly is