Design, formation and properties of tetrahedral M4L4 and M4L6 supramolecular clusters

J. Am. Chem. Soc. 2001, 123, 8923-8938

8923

Design, Formation and Properties of Tetrahedral M₄L₄and M₄L₆

Supramolecular Clusters¹

Dana L. Caulder, Christian Bru1ckner, Ryan E. Powers, Stefan Ko1nig, Tatjana N. Parac,

Julie A. Leary, and Kenneth N. Raymond*

Contribution from the Department of Chemistry, UniVersity of California, Berkeley, California 94720-1460

ReceiVed February 20, 2001

Abstract: The rigid tris- and bis(catecholamide) ligands H₆A, H₄B and H₄C form tetrahedral clusters of the

type M₄L₄and M₄L₆through self-assembly reactions with tri- and tetravalent metal ions such as Ga^III, Fe^III,

Ti^IVand Sn^IV. General design principles for the synthesis of such clusters are presented with an emphasis on

geometric requirements and kinetic and thermodynamic considerations. The solution and solid-state characteriza-

tion of these complexes is presented, and their dynamic solution behavior is described. The tris-catecholamide

H₆A forms M₄L₄tetrahedra with Ga^III, Ti^IV, and Sn^IV; (Et₃N)₈[Ti₄A₄] crystallizes in R3hc (No. 167), with a )

22.6143(5) Å, c ) 106.038(2) Å. The cluster is a racemic mixture of homoconfigurational tetrahedra (all ∆ or

all Λ at the metal centers within a given cluster). Though the synthetic procedure for synthesis of the cluster

is markedly metal-dependent, extensive electrospray mass spectrometry investigations show that the M₄A₄(M

) Ga^III, Ti^IV, and Sn^IV) clusters are remarkably stable once formed. Two approaches are presented for the

formation of M₄L₆tetrahedral clusters. Of the bis(catecholamide) ligands, H₄B forms an M₄L₆tetrahedron (M

) Ga^III) based on an “edge-on” design, while H₄C forms an M₄L₆tetrahedron (M ) Ga^III, Fe^III) based on a

“face-on” strategy. K₅[Et₄N]₇[Fe₄C₆] crystallizes in I 4h3d (No. 220) with a ) 43.706(8) Å. This M₄L₆tetrahedral

cluster is also a racemic mixture of homoconfigurational tetrahedra and has a cavity large enough to encapsulate

a molecule of Et₄N⁺. This host-guest interaction is maintained in solution as revealed by NMR investigations

of the Ga^IIIcomplex.

Introduction

structure have been cited in reference to the viral protein coats.²

As little as one gene is required to encode the protein that in

turn contains all of the information necessary for multiple copies

of the protein to self-assemble into the complete shell. This

remarkable economy makes the synthesis of artificial shells by

the self-assembly of a large number of small and simple subunits

an attractive goal, particularly in view of the rather involved

syntheses often required for the generation of even small

molecular containers.^5,6Thus, current trends toward the utiliza-

tion of self-assembling identical subunits are not surprising.^7-9

While the individual interactions holding protein subunits

together (e.g., H-bonds, van der Waals interactions, and π-π

interactions) have been recognized, it is less understood what

algorithms direct the synergy of these interactions to form

exclusively the observed clusters. An understanding of the

natural algorithms utilized to induce a single stoichiometry and

symmetry rather than a variety of oligomers and polymers

should lead to the deduction of general design principles for

synthetic self-assembled structures. Attempts have been made

to illustrate mathematically the underlying geometric implica-

tions for a subunit capable of forming such closed-shell

suprastructures.^2,10Viruses have been suggested as models for

new supramolecular architectures without, however, providing

explicit building instructions.¹¹We have suggested the utilization

of a symmetry-based model for the synthesis of supramolecular

Nature provides stunning examples of noncovalently linked

molecular clusters of high symmetry. Studied by electron

microscopy and X-ray diffraction, the high symmetry of viruses

has been recognized since the late 1950s.²The protein coat of

the human rhinovirus, among others, is composed of 60 copies

of each of four protein subunits arranged in icosahedral

symmetry.^2,3Viral protein coats are not the only high-symmetry

natural clusters, however. The Fe-storage protein ferritin is

composed of 24 identical protein subunits arranged in octahedral

symmetry.⁴Interestingly, in most cases these clusters spontane-

ously self-assemble from their identical subunits. Particularly

enticing is the fact that these high-symmetry structures are

utilized as protective shells: The virus coats protect genetic

material, while the ferritin shell stores several thousand iron

atoms as polymeric ferric oxy hydroxide.

Under the assumption that evolutionary pressure drove op-

timization of the shell design, rationales have been put forward

to explain the frequent occurrence of such self-assembled

structures. Subunit economy concomitant with maximization of

the enclosed space and mechanical rigidity of the resulting

(1) Coordination Number Incommensurate Cluster Formation, Part 21.

For the previous paper in this series, see Sun, X.; Johnson, D. W.; Raymond,

K. N.; Wong, E. H., Inorg. Chem. 2001, 40, 4504-4506.

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Published on Web 08/23/2001

8924 J. Am. Chem. Soc., Vol. 123, No. 37, 2001

Caulder et al.

clusters by using metal-ligand interactions to generate the

required symmetries. This building principle, “the coordination

number incommensurate cluster formation model,” provides

guidelines for the rational design of symmetric clusters, the

larger of which have well-defined cavities.¹²The great utility

of such cavities has been shown by the chemistry made possible

in the “inner phase” of calixarenes, carceplexes, and other host

molecules.⁶This is paralleled by the formation of polyoxo-

metalates in the hollow interior of a virus purged of its RNA

genetic material.¹³Future growth of the field of nanotechnology

depends on reliable and predictable container compounds.

The utilization of H-bonds for the construction of molecular

clusters has resulted in spectacular supramolecular struc-

tures,^8,9,14but the use of the stronger, highly directional and

well-studied metal-ligand interaction has also afforded discrete

clusters of a variety of sizes, stoichiometries, and symmetries.

Although early work has focused on the synthesis of helicates,

the wide structural flexibility of coordination chemistry has made

possible the realization of boxes, grids, rings, tetrahedra, cubes,

rotaxanes, and catenates.^9,12,15,16

While there are numerous accounts of the synthesis of M₄L_x-

type (x ) 4,6) tetrahedra, their syntheses have, at least initially,

relied on fortuitous circumstance.¹⁷Few early reports detailed

underlying design principles, despite the fact that supramolecular

chemistry has been described as a kind of molecular information

science,¹⁶which implies that the instructions “programmed” into

each component of an assembly direct the formation of a distinct

cluster topology. If this is indeed the case, then the compilation

of the information required to instruct components to self-

assemble into a predictable cluster should be the starting point

of all synthetic efforts. In practicality, this means that the design

Figure 1. Ligands H₆A, H₄B, and H₄C form tetrahedral clusters with

tri- and tetravalent metal ions.

of components in which the information for the formation of a

cluster is inherent should begin the synthesis of a supramolecular

cluster. Second, the required reaction conditions necessary for

the successful assembly of the cluster should be evaluated. In

a sense this sequence is standard procedure in organic synthesis

when planning a target-oriented synthesis by means of retrosyn-

thetic analysis.¹⁸

It is the goal of this series of papers to develop a symmetry-

based set of principles for the rational design and synthesis of

supramolecular clusters. In this paper, examples for the synthesis

and analysis of M₄L_x-type (x ) 4,6) tetrahedra built using three

different design approaches will be provided. In particular, the

design, synthesis, and solution- and solid-state characterization

of tetrahedral clusters based on ligands H₆A, H₄B, and H₄C

(Figure 1) will be discussed. The design principles delineated

here are conceived to be general and transferable to the design

of metal-ligand clusters of varying symmetries, sizes, and

compositions.

General Principles for the Formation of Metal-Ligand-

Based Clusters. When planning the synthesis of clusters based

on metal-ligand interactions, the general choice of ligand type,

ligand backbone rigidity, metal type, and reaction conditions

have to be considered. The particular geometric requirements

for each cluster’s symmetry and stoichiometry have to be strictly

fulfilled, since it is the metal-coordination geometry and the

orientation of the interaction sites in any given ligand that

provide the inherent information, or blueprint, for the self-

assembly of a designed cluster. We will discuss each point of

consideration separately, starting from more general require-

ments and followed by the design principles for the synthesis

of particular cluster symmetries. Finally, the synthesis, structural

characterization, and solution chemistry of tetrahedral M₄L_x(x

) 4,6) clusters will be detailed.

Choice of a Metal-Ligand Combination. The driving force

for the formation of the cluster is derived from metal-ligand

interactions. Thus, the choice of relatively strong metal-binding

moieties, for example, chelating moieties, is preferred. (Al-

though, a number of large clusters based on the interaction of

monodentate amine- or pyridine-type donors with Pt^IIand Pd^II

are known.¹⁹) The self-assembly product should be the ther-

modynamically favored cluster (for further thermodynamic

considerations, see below); however, kinetic products such as

nonstoichiometric oligomers and polymers will be formed at

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Design of Tetrahedral Supramolecular Clusters

J. Am. Chem. Soc., Vol. 123, No. 37, 2001 8925

Figure 2. When coordinated to a metal (a), the catecholamide binding

unit has a rigidly fixed geometry as a result of the hydrogen bond

between the amide proton and the ortho-oxygen. This hydrogen bonding

motif is preserved in the methyl-protected ligand (b), but reversed in

the unmetalated, free ligand (c).

the onset of the reaction. Consequently, the metal-ligand bonds

must be labile enough to allow the initially formed kinetic

products to rearrange into the thermodynamic product. This

requirement precludes, on one hand, the use of kinetically inert

metals such as Cr^IIIand Co^IIIas building blocks, and, on the

other hand, the use of kinetically disfavored metal chelators such

as rigid tetradentate ligands such as the pyrrolic nitrogens in

porphyrins. This requirement also suggests that the reaction

conditions need to be chosen so as to facilitate the rearrangement

of kinetic products to the thermodynamic product, that is, by

lowering the pH of the solution or using higher reaction

temperatures.

Figure 3. ORTEP diagram of Me₆A illustrating (a) the molecular three-

Choice of a Suitable Ligand-Backbone Combination. The

geometry of a particular ligand and its known interaction with

a metal ion (see below) should contain all of the information

required for the successful self-assembly of the desired cluster.

This mandates a predetermined conformation of the ligand.

Thus, the orientation of multiple binding units within a ligand

must be either rigidly fixed or at least somewhat conforma-

tionally restricted.

With respect to the above considerations, the catecholamide

binding unit (Figure 2) is a good choice for use in cluster

formations because of the high thermodynamic stability, yet

kinetic lability, when coordinated to trivalent metal ions, such

as Fe^III, Ga^III, and Al^III, in an octahedral coordination

environment.^20-23The catecholamide functionality is a rigid

binding unit as a result of a strong hydrogen bond between the

amide hydrogen and the coordinating o-hydroxy oxygen (Figure

2a). This hydrogen bond imposes a coplanar arrangement of

the amide with the catecholate unit, reducing the degrees of

freedom in the ligand and making the conformation of the

coordinated ligand calculable.

Due to the electronically preferred coplanarity of the amide

functionality with aromatic systems, any combination of the

catecholamide moieties with an aromatic backbone (e.g., a

benzene or naphthalene backbone) produces a rigid oligobi-

dentate ligand system that predisposes the coordinating groups

in a highly predictable way. The crystal structure of the methyl-

protected ligand Me₆A illustrates these features. The methyl-

protected catecholamides show the same H-bond pattern as the

metalated catechol amides (Figure 2b). In contrast, the confor-

mation of the catechol moiety of the unprotected ligand is

generally 180° rotated, the hydroxy functionality in the 2-posi-

tion forming one H-bond to the amide oxygen and the hydrogen

fold symmetry of the ligand and (b) its planar configuration.

of the hydroxy functionality in the 3-position being shared

between both hydroxy groups (Figure 2c). The compound Me₆A

crystallizes in the space group P2₁/a with approximate three-

fold symmetry (Figure 3a). The H-bonds between the amide

nitrogen and the catechol oxygen are clearly expressed. Also,

as anticipated, the three catecholamide functionalities are, albeit

to different degrees, predominantly coplanar to the central ring

(Figure 3b).

Thermodynamic Implications of Ligand Rigidity. An

example best illustrates the effect of ligand rigidity on the

thermodynamics controlling complex formation. Consider the

formation of an M₂L₃helicate formed by a ligand (L) with two

(bidentate) binding sites and a metal (M) with three acceptor

sites (i.e., for three bidentate ligands forming a pseudooctahedral

coordination sphere). The only M:L ratio that can satisfy the

binding requirements of both the ligand and metal components

of the cluster is 2M:3L. Now consider an equilibrium between

two triple helicates of the stoichiometry M₂L₃and its dimer, a

tetrahedron of the stoichiometry M₄L₆:

2 M₂L₃) M₄L₆

K_eq) [M₄L₆]/[M₂L₃]²= 1

Assuming a ligand system that has no unfavorable steric

constraints and in which the M-L interactions are the sole

driving forces for cluster formation, both geometries are equally

possible, and the equilibrium constant should be approximately

unity. If the initial concentration of the M₂L₃triple helicate is

2 mM, then the concentration of the M₄L₆tetrahedron will only

be 4 µM! This example illustrates the widely recognized

principle in supramolecular self-assembly: that entropy will

favor the largest number of particles with the smallest possible

stoichiometry. How, then, does one enforce the formation of

assemblies with stoichiometries which are multiples of the

empirical ratio of ligands to metals? Specifically, how does one

expand selectively an M₁L₁complex to an M₄L₄cluster or,

likewise, an M₂L₃helicate to an M₄L₆tetrahedron? The answer

lies in the geometric constraints that must be incorporated into

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8926 J. Am. Chem. Soc., Vol. 123, No. 37, 2001

Caulder et al.

Figure 4. As shown in the enterobactin analogue H₆X, simple addition

of a methylene group between the three-fold benzene scaffold and the

catecholamide binding units gives the ligand enough flexibility to allow

each of the arms to coordinate a single metal ion. This type of ligand

flexibility must be avoided if multimetal clusters are desired.

the ligand design, prohibiting the formation of lower-order

structures. This feat can be accomplished by making the ligand

too rigid for more than one chelation site to simultaneously bind

the same metal.

The ligand H₆A when compared to the structurally similar

enterobactin model compound H₆X exemplifies this effect of

rigidity (Figure 4). H₆X differs from H₆A only by the presence

of the methylene groups between the phenyl backbone and the

three ligand arms. This introduction of flexibility into the ligand

allows all three catecholate moieties to chelate to one metal

simultaneously. Even though the flexible ligand could easily

adopt conformations to accommodate higher-order complexes

of the stoichiometry M_nX_n, because the smallest combination

is favored, only the M₁X₁monomer can be detected. Removal

of the methylene groups results in ligand H₆A. This ligand is

unable to coordinate a single metal ion with more than one

catechol group; as a result, attaining a coordinatively saturated

M₁L₁stoichiometry is impossible. The alternative higher-order

M₄L₄structure that this ligand is forced to form will be described

below.

Definition of Terms. To describe our approach to the rational

design of supramolecular clusters, it is useful to define terms

that more accurately describe the relevant geometric relation-

ships between the metal coordination spheres and the ligands.

The vector that represents the interaction between a ligand and

metal is the Coordinate Vector (Figure 5a). In the case of a

monodentate ligand, this vector is simply the one directed from

the coordinating atom (or electron pair) of the ligand toward

the metal ion. In the case of a bidentate ligand, this vector bisects

the bidentate chelating group and is directed toward the metal

ion.

When using chelating ligands, the plane orthogonal to the

major symmetry axis of a metal complex is the Chelate Plane

(Figure 5b); all of the coordinate vectors of the chelating ligands

lie in the chelate plane. In an octahedral to trigonal prismatic

coordination geometry, three bidentate ligands generate a three-

fold axis at the metal center. While the geometry of such a

complex is often described by the twist angle,²⁴here it is

convenient to describe this geometry with respect to the

orientation of each bidentate ligand.

The Approach Angle is defined as the angle between the axis

determined by the two donor atoms and the three-fold axis of

the (pseudo)octahedron. This angle is expressed by the vector

relationship described in Figure 6. An approach angle of 0°

corresponds to a trigonal prism, while an approach angle of

35.3° corresponds to that of a perfect octahedral complex. Tris-

(catecholate) complexes of Fe^III, for example, tend to be distorted

Figure 5. Illustrations of (a) Coordinate Vectors and (b) Chelate

Planes.

toward trigonal prismatic geometry, having an approach angle

near 23° (40° twist angle).²⁰

Design of the Tetrahedral Clusters. Design Strategy for

a Tetrahedral M₄L₄Cluster. A tetrahedral cluster of the

stoichiometry M₄L₄can be described as four metal ions in a

tetrahedral array connected by four three-fold symmetric

ligands: Each ligand acts as one face of a tetrahedron, while

each metal ion acts as a vertex of the tetrahedron. Since a

tetrahedron has three-fold symmetry through each vertex, the

metal-ligand junction must also be able to achieve three-fold

symmetry. One way this can be accomplished is by using a

metal ion that prefers an (pseudo)octahedral geometry. Since

each of the metal ions has six acceptor sites and the metal-to-

ligand ratio is 1:1, then each ligand must have six donor sites.

With these stoichiometric and geometric considerations in mind,

the problem of designing a tetrahedral supramolecular assembly

of the stoichiometry M₄L₄can be reduced to designing a rigid

three-fold symmetric ligand that positions its three coordinate

vectors correctly. This analysis can best be accomplished by

determining the ideal approach angle for a cluster of this type.

A schematic representation of a tetrahedral M₄L₄cluster in

which the faces of the tetrahedron are made up of strictly planar

three-fold symmetric ligands (represented by the large yellow

faces) and the vertices are made up of (pseudo)octahedrally

coordinated metal atoms (represented by the small red triangular

faces) is depicted in Figure 7. The small red faces actually

represent the previously described chelate planes, which are

perpendicular to the three-fold axis at each metal vertex. In this

design, the coordinate vector of each catecholate unit must lie

simultaneously in both the ligand plane (yellow face) and the

chelate plane (red face) at the metal vertex. The angle between

a three-fold axis and the associated faces in a tetrahedron is

19.4°. If the bound ligands are planar and coordinate as shown,

this angle corresponds to the approach angle. The experimentally

obtained approach angles for mononuclear tris(catecholates) are

∼23°.^25,26This, in turn, implies that the planar ligand to be used

in the formation of a tetrahedral cluster is required to distort

only a few degrees from planarity to comply with the ideal

approach angle at the metal centers.

Design Strategies for the Synthesis of M₄L₆Tetrahedral

Clusters. Similar to the aforementioned M₄L₄tetrahedral cluster,

(24) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M.

AdVanced Inorganic Chemistry, 6th ed.; John Wiley: New York, 1999.

Design of Tetrahedral Supramolecular Clusters

J. Am. Chem. Soc., Vol. 123, No. 37, 2001 8927

Figure 6. When designing high-symmetry clusters it is often useful to consider the Approach Angle of a chelating ligand around a metal center,

rather than the twist angle. The approach angle can more easily be compared to the angles generated by a given cluster.

Figure 8. In this “Edge-On” M₄L₆design, the ligand H₄B will act as

one of the edges of the tetrahedron with the two-fold axis of the

cluster being coplanar with the ligand. In this design, the angle between

the coordinate vectors of the two-fold ligand is the primary consider-

ation.

Figure 7. In this M₄L₄design, the ligand H₆A will act as the face of

the tetrahedron. The angle at which the extended three-fold plane crosses

an adjacent three-fold axis is analogous to the approach angle of the

ligand arm with the metal located at the tetrahedron vertex.

109.4°, the angle between the three-fold axes in a tetrahedron.)

A 60° angle is formed between the chelate vectors for ligand

H₄B; thus, the targeted tetrahedral structure can be achieved

with only slight out-of-plane twisting by each of the chelating

groups. The 60° angle between the coordinate vectors of ligand

H₄B is the geometric implement enforcing the formation of the

M₄L₆assembly, since an M₂L₃helicate would require that the

coordinate vectors be parallel.^29,30

An alternative design strategy for an M₄L₆tetrahedron can

be envisioned in which the two-fold axis of the tetrahedron is

perpendicular to the plane of the ligand. This type of design

strategy we will call the “Face On” approach (Figure 9).³¹If

the ligand conformation is strictly planar, then the approach

angle is 35.3°, identical to that of a perfect octahedral metal

complex (equivalent to a twist angle of 60°). Mononuclear tris-

(catecholamide) complexes of Ga^IIIand Fe^IIIare known to be

slightly distorted toward a trigonal prismatic coordination

geometry (ca. 40° twist angle or 23° approach angle). Thus,

slight out-of-plane twists of the ligands would be required to

ideally accommodate these metals. Slight rotation of the catechol

amide moiety along the backbone-amide axis can be envisioned

to accomplish the required deviation.

a tetrahedral cluster of stoichiometry M₄L₆has four metal ions

at the four vertices of a tetrahedron. In contrast, however, these

four metal ions are linked by six two-fold symmetric ligands,

each ligand acting as one of the six edges of the tetrahedron.

Again, the interaction between a metal ion and the three ligands

converging at each corner of the tetrahedron generates a three-

fold axis through the formation of an (pseudo)octahedral

coordination sphere around the metal. The smallest assembly

that could result from the simultaneous satisfaction of the two-

and three-fold symmetry requirements is an M₂L₃complex. To

avoid the entropically favored smaller cluster, geometric

constraints must be incorporated into the rigid ligand design.

We have described two different approaches to the rational

design of such clusters.^27,28Both approaches employ an ideally

planar C₂-symmetric bis(bidentate) ligand with a rigid backbone,

but the orientation of the C₂axis of the cluster with respect to

the mean plane of the ligand differs.

In the first design strategy, which we call the “Edge On”

approach, the two-fold axis of the tetrahedron is coplanar with

the plane defined by the ligand (Figure 8).²⁷Since the coordinate

vectors must lie within the chelate planes at each of the four

metal vertices, the angle between the coordinate vectors within

a given ligand must be 70.6°. (This angle is the supplement of

The selectivity in the ligand H₄C for the formation of an M₄L₆

cluster versus a M₂L₃helix is achieved using a naphthalene

spacer, which causes the two catechol binding units to be offset

from one another when the ligand is in the conformation

(25) (a) Borgias, B. A.; Cooper, S. R.; Koh, Y. B.; Raymond, K. N.

Inorg. Chem. 1984, 23, 1009-1016. (b) Holmes, R. R.; Shafieezad, S.;

Chandrasekhar, V.; Sau, A. C.; Holmes, J. M.; Day, R. O. J. Am. Chem.

Soc. 1988, 110, 1168-1174.

(26) Albrecht, M.; Kotila, S. Chem. Commun. 1996, 2309-2310.

(27) Beissel, T.; Powers, R. E.; Raymond, K. N. Angew. Chem., Int. Ed.

Engl. 1996, 35, 1084-1086.

(28) Caulder, D. L.; Powers, R. E.; Parac, T.; Raymond, K. N. Angew.

Chem., Int. Ed. 1998, 37, 1840-1843.

(29) Meyer, M.; Kersting, B.; Powers, R. E.; Raymond, K. N. Inorg.

Chem. 1997, 36, 5179-5191.

(30) Powers, R. E. The Rational Design of Supramolecular Assemblies.

Ph.D. Thesis, University of California, Berkeley, December 1997.

(31) Note that the M₄L₄tetrahedron described earlier is also a “Face

On” design.

8928 J. Am. Chem. Soc., Vol. 123, No. 37, 2001

Caulder et al.

Scheme 1. Synthesis of Ligands H₆A, H₄B, and H₄C Used

in This Study

Figure 9. In this “Face-On” M₄L₆design, the two-fold axis of the

cluster is perpendicular to the ligand plane. In this design, the approach

angle is the primary consideration.

Figure 10. Ligand H₄C is deterred from forming an M₂L₃cluster

because of the offset of the chelating units created by the use of the

naphthalene scaffold. Rotation around the arene-N bond puts the ligand

in the conformation required to form an M₄L₆tetrahedral cluster.

required for helicate formation, thus disfavoring the formation

of a helicate (Figure 10).

Molecular Modeling. Prior to the actual ligand synthesis,

the feasibility of the proposed metal-ligand systems is explored

using molecular mechanics calculations.³²Although these

calculations do not guarantee that the proposed structure will

form, they help eliminate structures that are nonviable due to

unfavorable inter- and intraligand steric interactions. When

comparing results of the molecular modeling calculations with

actual crystal structures, it becomes evident that while the gross

structure can be predicted fairly accurately, effects of hosts

inside the clusters (or the absence of guests) cannot be predicted.

Nor can it be predicted that certain clusters require a guest to

be formed.³³Nevertheless, we find molecular modeling to be

an indispensable tool in the design of the clusters presented here.

by reaction with BBr₃to produce, in generally excellent overall

yields, the polycatecholamide ligands H₆A, H₄B, and H₄C,

respectively.

This scheme for the synthesis of catecholamide oligobidentate

ligands is most versatile.^26,35The scheme is limited only by the

availability of the amine backbone and the resistance of the

resulting amide to the deprotection conditions. For acid-sensitive

backbones, however, the use of the benzyl-protected 2,3-

hydroxybenzoic acid chloride, and subsequent removal of the

protecting groups under reductive conditions, has been shown

to be feasible.³⁶

Synthesis of M₄A₄Clusters. Ligand H₆A reacts with

stoichiometric amounts of Ga(acac)₃under basic conditions (3

equiv of KOH) in MeOH to give a precipitate whose ¹H NMR

(300 MHz, D₂O) indicates the formation of one defined and

highly symmetric species (see also below). The mass spectrum

(ESI, cation detection) of the Ga^IIIcomplex, as its K⁺salt, shows

Results and Discussion

Ligand Syntheses. All ligands used in this study belong to

the same class of bis- and tris(catecholamides). They all were

synthesized following established routes (Scheme 1). Polyamines

1, 2, or 3, either commercially available or synthesized by

reduction of the corresponding commercially available nitro

compounds, were reacted with the acid chloride of 2,3-

dimethoxybenzoic acid (4) under standard amide bond-forming

conditions (excess Et₃N in CH₂Cl₂).³⁴The resulting O-methyl-

substituted ligands Me₆A, Me₄B, and Me₄C were deprotected

(34) Weitl, F. L.; Raymond, K. N. J. Am. Chem. Soc. 1980, 102, 2289-

2293.

(35) (a) Enemark, E. J.; Stack, T. D. P. Angew. Chem., Int. Ed. Engl.

1995, 34, 996-998. (b) Enemark, E. J.; Stack, T. D. P. Angew. Chem., Int.

Ed. 1998, 37, 932-935. (c) Albrecht, M. Chem. Eur. J. 1997, 3, 1466-

1471. (d) Albrecht, M.; Riether, C. Synthesis 1997, 957. (e) Albrecht, M.

Synlett 1996, 565.

(32) CAChe MM2 Force Field, V4.0; Oxford Molecular Group, Inc.:

U.S.A., 1997.

(33) For instances in which the cluster formed only in the presence of a

guest see, e.g.: refs 14b and 26; for instances in which the cluster

conformation was affected by a guest, see: Xu, J. D.; Parac, T. N.;

Raymond, K. N. Angew. Chem., Int. Ed. 1999, 38, 2878-2882.

(36) Schuda, P. F.; Botti, C. M.; Venuti, M. C. OPPI Briefs 1984, 16,

119-123.

Design of Tetrahedral Supramolecular Clusters

J. Am. Chem. Soc., Vol. 123, No. 37, 2001 8929

Likewise, the addition of 1 equiv of excess SnCl₄to the cluster

dissolved in DMF does not affect the spectrum (Figure 11d).

These experiments demonstrate that the single set of resonances

observed in the ¹H NMR spectrum are not a result of a rapidly

exchanging polymeric system, but rather they are attributable

to a discrete metal-ligand species. It seems reasonable to predict

that pronounced cooperativity effects prevent fast ligand-

exchange processes in these types of tethered clusters. The

electrospray mass spectrum of the [Sn₄A₄]^8-complex further

provided data to prove the formation of the desired M₄L₄

tetrahedron (see below).

The analogous Ti^IVcomplex [Ti₄A₄](Et₃NH)₈was prepared

by a modified procedure. A solution of Ti(On-Bu)₄in MeOH

was combined with H₆A in MeOH/excess Et₃N. The solution

quickly turned orange and, after 12 h, a gelatinous orange precip-

itate formed. The ¹H NMR spectrum of a DMSO-d₆extract of

this precipitate taken at 25 °C shows the presence of a mixture

of (polymeric) compounds. Heating of the sample to 120 °C

results, over the course of several hours, in a partial simplifica-

tion of the spectrum. This experiment provided the guide for

the next step: dissolution of the centrifuged and washed precip-

itate in DMF under reflux for 12 h. This step facilitated the

rearrangement of the kinetic product mixture into one thermo-

Figure 11. ¹H NMR (400 MHz, 25 °C, DMSO-d₆) of the ligand H₆A

(a) and the complex [Sn₄A₄]^8-(b). Neither excess ligand (c) nor excess

SnCl₄(d) disrupts the formation of the [Sn₄A₄]^8-tetrahedral cluster.

two major peaks at m/z 1463.4 and 988.6. Under these low-

resolution conditions, the first peak can be assigned to the

species {[Ga₄A₄]^12-‚14K⁺}²⁺or {[Ga₂A₂]^6-‚7K⁺}⁺and is,

therefore, not unambiguously indicative of the formation of a

tetrahedron. The peak at m/z 988, however, attributed to the

trication {[Ga₄A₄]^12-‚15K⁺}³⁺provides an indication for the

formation of the desired cluster since it cannot be rationalized

by a metal:ligand stoichiometry lower than 4:4; thus, this

evidence strongly suggests the formation of the tetrahedron.

Despite these encouraging results, we were unsuccessful at

growing crystals suitable for analysis by X-ray diffraction.

Presumably the -12 charge of these tetrahedra and their good

solubility in MeOH, H₂O, DMSO, and DMF forestalled the

formation of high-quality single crystals.

1

dynamic product; H NMR spectra of samples taken from this

solution showed the presence of one major product characterized

by a single set of sharp resonances. Gas-phase diffusion of

MeOH into this DMF solution produced orange crystals of

1

[Ti₄A₄](Et₃NH)₈. The H NMR spectrum of the crystals dis-

solved in DMSO-d₆was very similar in appearance to those

observed for the Ga^IIIand Sn^IVspecies, giving an indication of

successful cluster formation. This finding was also supported

by the electrospray mass spectrum of the complex (see below).

While this procedure produced crystals of analytical and

single-crystal X-ray diffraction quality (see below), it also

highlights the disadvantage of using the much less kinetically

labile ion Ti^IV, as compared to Ga^IIIor Fe^III. The kinetic product

is not the desired cluster and is removed from the metal-ligand

equilibrium by precipitation. A rearrangement step at elevated

temperature and in a polar donor solvent is required to produce

the designed thermodynamic product. The use of bases other

than Et₃N (i.e., NaOH, KOH, CsOH, morpholine, pyridine) did

not produce the clusters cleanly, while the use of Me₄NOH

produced the cluster with greatly reduced solubility in MeOH

and DMSO. These observations provide some rationalization

of the fact that in the solid (crystal structure) and gas phase

(mass spectrum) this octaanionic cluster (and the clusters to be

described below) shows a very strong association with its

triethylammonium countercations. Evidently, these interactions

provide additional driving force for the formation of the desired

clusters in a crystalline state. The reaction of a variety of Ti^IV

sources (TiCl₄or freshly precipitated TiO₂) in hot or cold DMF

did not produce the expected cluster in a clean fashion. Likewise,

the clean formation of the Sn^IV-based cluster was not possible

with the methodology detailed for the Ti^IV-based tetrahedron.

We assume that the differences of the metals and metal sources

with respect to Lewis acidity, rate of hydrolysis, etc. require

individualized reaction conditions. It should be noted, however,

that once formed the respective clusters could be detected by

NMR and mass spectrometry under a wide range of reaction

conditions, attesting to the high thermodynamic stability of the

products; only their isolation and preparation by simple means

required the individualized procedures.

The overall charge of the cluster can be reduced by using

higher-oxidation state metals; for instance, M^IVmetal ions will

reduce the charge from -12 to -8. Pseudooctahedral tris-

(catecholate) complexes of V^IV, Sn^IV, and Ti^IV, for example,

are known. While the formation of the V^IVtris(catecholate)

species is most sensitive to the particular conditions chosen,³⁷

the opposite can be said for the formation of the Sn^IVand Ti^IV

species.²⁵For these reasons we chose to investigate the use of

Sn^IVand Ti^IVin the construction of these self-assembled clusters.

A DMF solution of 1 equiv of H₆A combined with 1 equiv

of SnCl₄and an excess of Et₃N, heated under reflux for 12 h,

produces a clear and colorless solution which, when evaporated

to dryness under vacuum, results in the formation of a pale

pink microcrystalline solid of the composition [Sn₄A₄](Et₃NH)₈‚

xEt₃NHCl (x ) 3-4). Attempts to remove the excess triethyl-

ammonium chloride by recrystallization of the salt mixture from

a variety of solvents failed to produce a triethylammonium-

1

free product. The H NMR (300 MHz, DMSO-d₆) spectrum

indicated the formation of only one defined species in which

the ligand is located in a three-fold symmetric environment (Fig-

ure 11b). The proton signals for the hydroxy groups of the ligand

(Figure 11a) disappeared, and all other signals shifted in a way

consistent with complex formation. In particular, the NH proton

shifted downfield as expected due to the H-bond produced upon

complexation. Addition of an aliquot of excess ligand did not

cause any shift or line-broadening of the peaks of the metal

complex (Figure 11c), even at temperatures up to 100 °C.

Crystal Structure of [Ti₄A₄](Et₃N)₈. Slow gas-phase dif-

fusion of MeOH at room temperature into a DMF solution of

(37) Cooper, S. R.; Koh, Y. B.; Raymond, K. N. J. Am. Chem. Soc.

1982, 104, 5092-5102.

8930 J. Am. Chem. Soc., Vol. 123, No. 37, 2001

Caulder et al.

the ligand used in these studies was flexible and, depending on

the size of the metal used, formed M₁L₁or M₄L₄complexes

demonstrates the importance of incorporating rigidity into the

ligand design. And as detailed above, our design does not allow

the formation of an ML species. [A tetrahedral Tl₄L₄cluster in

which a tetrahedral M-M bond containing tetrahedral arrange-

ment of four Tl⁺ions is capped by four ligands is only related

by stoichiometry and not connectivity.⁴⁰]

Synthesis of M₄B₆Clusters. The synthesis and characteriza-

tion of an M₄L₆tetrahedral cluster based on the “Edge On”

design (see Figure 8) has been previously reported using a bis-

(hydroxamate) ligand.^29,40A catecholate version based on ligand

H₄B is described here. Reaction of 6 equiv of H₄B, 4 equiv of

Ga(acac)₃, and 12 equiv of KOH in methanol led to the

precipitation of a white microcrystalline product after six to 8

h. The solubility of the cluster is much lower in methanol than

that of the analogous helicates,^29,41but both are readily soluble

in water. The ¹H NMR spectrum of the complex in D₂O shows

only one set of peaks, indicating that the ligands in the complex

are in symmetry-related environments and that the C₂axis of

the ligand is maintained. This type of spectrum could be

interpreted as the averaged position of the peaks if the ligands

are in rapid exchange. This possibility can be ruled out by

examining the ¹H NMR spectrum of the cluster prepared in the

presence of excess ligand. Analogous to the previously described

experiments with complexes of H₄A, the spectrum of the mixture

shows two sets of peaks, one for the complex and one for the

excess ligand. In this case, however, these results do not rule

out the presence of an M₂L₃cluster. If lone pair donation of

the amide nitrogens into the aromatic backbone of H₄B is

insufficient to disfavor large deviations from planarity, then the

chelate groups could rotate around the N-arene bond to be

perpendicular with the backbone. This conformation would

allow for the correct arrangement of coordinate vectors and

chelate planes for an M₂L₃cluster.

In a low-resolution positive ion electrospray mass spectrum,

strong signals are seen for the {[Ga₄B₆]‚14K}²⁺doubly charged,

the {[Ga₄B₆]‚15K}³⁺triply charged, and the {[Ga₄B₆]‚16K}⁴⁺

quadruply charged ions. Because the charge states of the peaks

could not be verified, the doubly and quadruply charged ions

might be assigned to the full and half mass peaks of a

corresponding {[Ga₂B₃]‚7K}²⁺cluster. The triply charged ion,

however, cannot be rationalized by the smaller M₂L₃cluster

and appears to be diagnostic in assigning the structure as a

tetrahedron.

Synthesis of M₄C₆Clusters. Reaction of 6 equiv of H₄C, 4

equiv of Ga(acac)₃, 12 equiv of KOH, and 12 equiv of Et₄NCl

in methanol leads to a yellow precipitate which analyzes as

K₅(Et₄N)₇[Ga₄C₆]‚8H₂O that shows unusual features in the ¹H

NMR spectrum (Figure 14).²⁷A ratio of seven Et₄N⁺counte-

rions to six ligands is observed, with the seven counterions being

split into two sets in a ratio of 6:1. The larger set of Et₄N⁺

peaks (δ ) 2.49, q; 0.72, t) is shifted slightly upfield from free

Et₄NCl (δ ) 3.26, q; 1.27, t), while the smaller set is shifted

substantially upfield, showing up at negative ppm (δ ) -0.70,

m; -1.59, t)! The upfield shifts of the exterior Et₄N⁺resonances

are attributed to a strong π-cation naphthalene and catechol

rings of the ligands and the Et₄N⁺counterions (vide infra). On

Figure 12. Based on the X-ray structure coordinates, [Ti₄A₄]^8-in both

a space-filling (a) and wire-frame (b) representation.

Figure 13. In the [Ti₄A₄]^8-cluster, the ligand is significantly distorted

(a) from the planarity observed in the uncoordinated ligand (b).

[Ti₄A₄](Et₃NH)₈produced orange tablets of a quality suitable

for analysis by X-ray diffraction. The single-crystal structure

provided the ultimate proof of the proposed tetrahedral cluster.³⁸

Two views of the cluster in a space-filling and framework

representation, respectively, are shown in Figure 12. The four

three-fold symmetric ligands are required by our design to lie

on the four faces of the [Ti₄A₄]^8-tetrahedron and are linked at

the four tetrahedron vertices by the four metal ions. The

compound crystallizes in space group R3hc with 12 molecules

in the unit cell. The cluster lies on a crystallographic three-fold

axis but has molecular symmetry of the pure rotation group T,

such that all metal ions within a given cluster have the same

chirality (all ∆ or all Λ). Hence, the compound crystallizes as

a racemic mixture of homoconfigurational tetrahedra. The eight

triethylammonium counterions are highly disordered, but the

nitrogen atoms are clearly within hydrogen-bonding distance

to either the carbonyl or the peripheral phenolate oxygens of

the cluster. There is no evidence that the small cavity of the

tetrahedron contains a guest (compare to the larger [Fe₄C₆]^12-

cluster described below). Despite the hydrogen bonds between

the amide proton and the phenolic oxygens, the catechol amides

are more distorted from planarity than required by the design

of the cluster. Figure 13a shows the side view of one ligand as

it appears in the cluster. Compared to the side view of the

uncomplexed ligand (Figure 13b), it becomes evident that this

distortion can be attributed to the compulsion of the system to

minimize the empty volume of the cavity. This may also be

the reason for the larger than expected (19.4°) approach angle

of 28°.

It should be noted that this is not the first tetrahedral cluster

with stoichiometry M₄L₄; a related cluster has appeared in the

literature.³⁹Although the formation of this cluster was reported

as serendipitous, its structure and formation highlight the

building principles exemplified here. In particular, the fact that

(39) Amoroso, A. J.; Jeffery, J. C.; Jones, P. L.; McCleverty, J. A.;

Thornton, P.; Ward, M. D. Angew. Chem., Int. Ed. Engl. 1995, 34, 1443-

1446.

(40) Rheingold, A. L.; Liable-Sands, L. M.; Trofimenco, S. Chem.

Commun. 1997, 1691-1692.

(41) Kersting, B.; Meyer, M.; Powers, R. E.; Raymond, K. N. J. Am.

Chem. Soc. 1996, 118, 7221-7222.

(38) Bru¨ckner, C.; Powers, R. E.; Raymond, K. N. Angew. Chem., Int.

Ed. 1998, 37, 1837-1839.

Design of Tetrahedral Supramolecular Clusters

J. Am. Chem. Soc., Vol. 123, No. 37, 2001 8931

Figure 14. ¹H NMR (D₂O) of K₅(Et₄N)₇[Ga₄C₆] depicting the two sets of Et₄N⁺resonances characteristic of the exterior and encapsulated cations.

Figure 15. Based on the X-ray structure coordinates, Et₄N⁺⊂[Fe₄C₆]^12-in both (a) wire-frame and (b) space-filling representations.

the basis of literature precedent this extreme upfield shift was

taken as evidence of encapsulation of one of the Et₄N⁺

counterions by the tetrahedral cluster.⁴²Another interesting

spectral feature of the encapsulated Et₄N⁺is that the CH₂

resonance is not a simple quartet, but rather it is a complex

multiplet; the methylene protons are rendered diastereotopic

inside the chiral host. This feature is an indication that the cluster

is homoconfigurational (i.e., all ∆ or all Λ coordination at the

metal centers) giving rise to molecular symmetry of the pure

rotation group T (racemic mixture of ∆∆∆∆ and ΛΛΛΛ

clusters). It should be noted that computer modeling of the M₄C₆

cluster correctly predicted that it would have T symmetry (all

metal centers with the same chirality, all ∆ or all Λ) and that

there would be a substantial cavity inside the cluster. Computer

models of the S₄or C₃symmetry isomers of the cluster were

geometrically impossible.

Crystal Structure of K₅(Et₄N)₇[Fe₄C₆]. The Fe^IIIanalogue

was prepared in a manner similar to that for the Ga^IIIcomplex.

NMR studies of the Fe^IIIanalogue were precluded by the

paramagnetism of the Fe^IIIcenter. It was possible, however, to

grow crystals of K₅(Et₄N)₇[Fe₄C₆] suitable for analysis by X-ray

diffraction.²⁸The cluster crystallizes with eight molecules of

water and three molecules of methanol in the cubic space group

I 4h3d (No. 220). One-third of the compound is crystallographi-

cally unique. The [Fe₄C₆]^12-anion lies on a crystallographic

three-fold axis with T molecular symmetry; hence, the crystal

is a racemic mixture of tetrahedra that have homoconfigurational

(all ∆ or all Λ) iron centers. As also observed in solution with

the Ga^IIIcomplex, one of the Et₄N⁺counterions is located inside

the cluster cavity (Figure 15a). The cluster is a tightly closed

“box,” with no aperture through the surface, as demonstrated

by a space-filling model (Figure 15b).

The Fe-Fe distances in the cluster are, on average, 12.8 Å,

bringing the overall size of the cluster just into the nanometer

regime. The naphthalene rings are twisted along the arene-N

bond, so that they bend into the cavity. The hydrogen bonds²³

between the amide proton and the coordinated catechol oxygen

are still maintained, however. The angles between the least-

squares planes calculated for the naphthalene backbone and the

catechol rings are, respectively, 38.0° and 13.6° for one ligand

and 20.7° and 65.3° for the other. We interpret this ligand

distortion as a result of van der Waals interactions between the

cluster and the encapsulated Et₄N⁺. This van der Waals contact

is clearly seen in Figure 16, which depicts a 2 Å thick slice

through the center of the tetrahedral cluster. Solution studies

(vide infra) suggest that this van der Waals contact is a result

of more than just crystal-packing forces. There appears to be a

strong π-cation interaction⁴³between the aromatic rings of the

ligands and the Et₄N⁺counterions.

The crystal packing of the [Fe₄C₆]^12-anion and the exterior

Et₄N⁺cations is such that each dodecaanion is surrounded by

six shared Et₄N⁺cations (Figure 17). Three of these Et₄N⁺

cations are in close contact with the naphthalene rings of the

ligand, and three are located in clefts between the catecholamide

(42) (a) Fujita, M.; Oguro, D.; Miyazawa, M.; Oka, H.; Yamaguchi, K.;

Ogura, K. Nature 1995, 378, 469-471. (b) Jacopozzi, P.; Dalcanale, E.

Angew. Chem., Int. Ed. Engl. 1997, 36, 613-615. (c) Mann, S.; Huttner,

G.; Zsolnai, L.; Heinze, K. Angew. Chem., Int. Ed. Engl. 1996, 35, 2808-

2809. (d) Parac, T.; Caulder, D. L.; Raymond, K. N. J. Am. Chem. Soc.

1998, 120, 8003-8004. (e) Fleming, J. S.; Mann, K. L. V.; Carraz, C.-A.;

Psillakis, E.; Jeffery, J. C.; McCleverty, J. A.; Ward, M. D. Angew. Chem.,

Int. Ed. 1998, 37, 1279-1281. (f) Timmerman, P.; Verboom, W.;

vanVeggel, F. C. J. M.; vanDuynhoven, J. P. M.; Reinhoudt, D. N. Angew.

Chem., Int. Ed. Engl. 1994, 33, 2345-2348. (g) Bryant, J.; Blanda, M. T.;

Vincenti, M.; Cram, D. J. J. Am. Chem. Soc. 1991, 113, 2167-2172.

(43) Ma, J. C.; Dougherty, D. A. Chem. ReV. 1997, 97, 1303-1324.

8932 J. Am. Chem. Soc., Vol. 123, No. 37, 2001

Caulder et al.

tetrahedral cluster and the encapsulated and exterior Et₄N⁺

cations, the titration of K₁₁(Et₄N)[Ga₄C₆] (D₂O, 300 MHz) with

1

Et₄NCl was followed by H NMR spectroscopy (Figure 19).

The catechol resonances show little change, although they are

shifted slightly upfield with increasing equivalents of Et₄N⁺.

The naphthalene resonances are shifted downfield, and protons

of the encapsulated Et₄N⁺cations are shifted upfield. Initially,

a linear change is observed, but a leveling off occurs after six

to eight equivalents of Et₄N⁺per equivalent of [Ga₄C₆]^12-have

1

been added. Similar inspection of the two H resonances for

the exterior Et₄N⁺cations reveals sigmoidal-shaped curves with

inflection points near eight equivalents⁴⁴of Et₄N⁺per equivalent

of [Ga₄C₆]^12-(corresponding to one Et₄N⁺on the interior and

six to seven Et₄N⁺on the exterior of the cluster). Thus, an

apparent equivalence point, corresponding to one Et₄N⁺inside

the cluster cavity and six to seven Et₄N⁺cations on the exterior

of the cluster, is consistently indicated by all of the observed

resonances.

These observations, along with the reproducible isolation of

the cluster as the K₅(Et₄N)₇[M₄C₆] salt (M ) Ga^III, Fe^III) and

the packing of the exterior Et₄N⁺cations around the [Fe₄C₆]^12-

anion in the crystal structure, attest to a well-defined super-

structure of the Et₄N⁺cations and the [M₄C₆]^12-tetrahedral

dodecaanion. The exterior of the tetrahedral cluster is surrounded

by six Et₄N⁺cations, one Et₄N⁺per ligand in the cluster. This

association might be explained in terms of a strong π-cation

interaction between the aromatic rings of the ligands and the

alkylammonium cations. It has been established that electrostatic

interactions play a prominent role in prototypical π-cation

interactions, and fundamental gas-phase studies have established

the π-cation interaction to be among the strongest of nonco-

valent binding forces.⁴³

Figure 16. A 2 Å thick slice through the center of Et₄N⁺⊂[Fe₄C₆]^12-

showing the surface contacts made between the ligands (yellow) and

the encapsulated Et₄N⁺(red).

An inspection of the CH₂resonance of the encapsulated Et₄N⁺

is also revealing (Figure 20). Initially, the CH₂resonance of

the encapsulated Et₄N⁺is a simple quartet. This feature in the

¹H NMR spectrum of K₁₁(Et₄N)[Ga₄C₆] contrasts with that in

the spectrum of K₅(Et₄N)₇[Ga₄C₆], in which the methylene

resonance is a complex multiplet. Addition of Et₄NCl to the

solution of K₁₁(Et₄N)[Ga₄C₆] not only causes the proton

resonances to shift, but it also causes the quartet to gradually

split into a multiplet. Seemingly, in the absence of the adhering

exterior Et₄N⁺cations, the cavity is large enough that the

encapsulated Et₄N⁺does not “see” the chirality of the cluster

host. The walls of the cluster are compressed, however, as the

exterior Et₄N⁺cations adhere to the surface. The cavity then

becomes smaller, and the encapsulated Et₄N⁺begins to experi-

ence the chiral environment of the T symmetry cluster host.

These results imply that the cavity size is affected by the

presence of Et₄N⁺cations on the cluster exterior.

Figure 17. Six crystallographically identical Et₄N⁺cations (blue)

surround the tetrahedral [Fe₄C₆]^12-cluster in the solid state. One Et₄N⁺

cation (yellow) is encapsulated in the cluster interior.

rings. Because these cations are crystallographically identical,

this Et₄N⁺must be surrounded by a naphthalene ring of one

cluster and two catecholate rings of an adjacent cluster (Figure

18). The nitrogen atom of the exterior Et₄N⁺is located 4.0,

4.3, and 4.6 Å from the mean planes calculated for the two

catecholamide moieties and the naphthalene ring, respectively.

These distances are slightly shorter than the distances of the

nitrogen atom of the encapsulated Et₄N⁺to the mean planes

calculated for the two crystallographically independent naph-

thalene rings of the cluster (4.5 and 5.0 Å). This solid-state

packing helps to explain why the cluster always precipitates as

the K₅(Et₄N)₇[M₄C₆] salt (M ) Ga^III, Fe^III) despite the fact that

the reaction solution contains 12 equiv of both K⁺and Et₄N⁺

cations. The upfield shifts of the proton resonances of the

exterior Et₄N⁺cations compared to free Et₄N⁺are consistent

with the idea that these contacts are maintained in solution.

While the six exterior Et₄N⁺counterions prefer to be associated

with the aromatic rings of the ligands, K⁺cations are prefer-

entially coordinated by the catecholate oxygens at the corners

of the tetrahedral cluster or by the carbonyl oxygens of the amide

moieties on the ligands.

1

A variable-temperature H NMR spectroscopy experiment

showed that the encapsulated Et₄N⁺and the exterior Et₄N⁺

cations of K₅(Et₄N)₇[Ga₄C₆] do not coalesce up to 100 °C in

D₂O. In fact, the peaks hardly shift. Inspection of the methylene

resonance of the encapsulated Et₄N⁺reveals that heating does

affect the interconversion of the diastereotopic protons, but even

at 100 °C the protons are not equivalent (Figure 21).

Additional evidence for the solution structure of K₅(Et₄N)₇-

[Ga₄C₆] was obtained from 2D nuclear Overhauser effect

spectroscopy (2D-NOESY).⁴⁵Strong cross-peaks observed

(44) Second derivative plots of the shifts of the proton resonances for

the exterior Et₄N⁺cations are both equal to zero at 8 equiv of Et₄N⁺per

equivalent of [Ga₄C₆]^12-. Note that the conditions for the experiments for

Figures 21 and 22 differ slightly, hence the slight mismatch of overlap.

(45) Friebolin, H. Basic One- and Two-Dimensional NMR Spectroscopy,

2nd ed.; VCH: Weinheim, 1993.

Host-Guest Chemistry Exhibited by K₁₁(Et₄N)[Ga₄C₆].

To further investigate the interactions between the dodecaanionic

Design of Tetrahedral Supramolecular Clusters

J. Am. Chem. Soc., Vol. 123, No. 37, 2001 8933

Figure 18. Stereoview of the environment of the exterior Et₄N⁺cation in the crystal structure of K₅(Et₄N)₇[Fe₄C₆]. The Et₄N⁺cation is surrounded

by the aromatic naphthalene and catecholamide rings.

Figure 19. Shifts of the ¹H NMR resonances for the ligand (catechol

and naphthalene rings) and the encapsulated Et₄N⁺countercation with

increasing equivalents of Et₄NCl.

between the encapsulated Et₄N⁺and only the naphthalene

resonances of the cluster are consistent with the encapsulation

of one molecule of Et₄N⁺(Figure 22). Interestingly, cross-peaks

are also observed between the six Et₄N⁺cations located on the

exterior of the cluster and the naphthalene and catechol protons

on the ligands of the tetrahedral cluster. This evidence indicating

that the exterior Et₄N⁺cations also interact strongly with the

aromatic sections of the cluster backbone is consistent with our

findings from titration studies and the X-ray crystallographic

results (vide supra).

The NOE changes the intensities of the observed signals as

a result of dipole-dipole, or in effect through-space, relaxation

of the nuclear spin. In small molecules with short rotational

correlation times, t_c, one type of relaxation mechanism domi-

nates, which has the effect of amplifying the NOE signal. In

large molecules with long rotational correlation times, a second

type of relaxation mechanism dominates that has the effect of

decreasing the NOE signal.⁴⁵Due to the large size of the cluster

the majority of NOE peaks were found to be negative, indicating

that the cluster is in the slow-tumbling regime. The encapsulated

Et₄N⁺is the only positive NOE peak observed, implying that

it is tumbling inside the cluster cavity in the fast-tumbling regime

independent of the slower-tumbling host cluster.

Cross-peaks were also observed between the encapsulated

and exterior Et₄N⁺cations. This observation suggested that the

inner and outer Et₄N⁺molecules are exchanging on the NMR

Figure 20. The methylene resonance of the encapsulated Et₄N⁺

gradually changes from a quartet to a multiplet as Et₄NCl is added to

a solution of K₁₁[Et₄N][Ga₄C₆].

time scale. While typical line-shape analysis of 1D NMR spectra

is applicable to the study of processes occurring with rate con-

stants between 1 and 10⁴s^-1, the saturation transfer technique

(NOESY and EXSY) is applicable to slower processes with rate

8934 J. Am. Chem. Soc., Vol. 123, No. 37, 2001

Caulder et al.

are two main clusters of peaks corresponding to mono- and

dicationic clusters, respectively. The cluster of peaks is generated

by exchange of protons for (ubiquitously present) Na⁺ions. In

addition, single peaks corresponding to species associated to

triethylammonium ions can be identified. No peak which can

be assigned to a M_xL_yspecies with x,y * 4 can be identified,

attesting to the unique stability of the tetrahedral supramolecular

assembly.

When recording the ESI spectrum of the [Ti₄A₄]^8-cluster in

the negative detection mode, we were pleased to find a

dramatically improved spectrum (Figure 24). The extraordinarily

simple spectrum shows four main peaks, which can be assigned

to the hepta- ({[Ti₄A₄]^8-‚7H⁺}^-at m/z 2300.1), hexa-

({[Ti₄A₄]^8-‚6H⁺}^2-at m/z 1149.6), penta- ({[Ti₄A₄]^8-‚5H⁺}^3-

at m/z 766.1), and tetraprotonated ({[Ti₄A₄]^8-‚4H⁺}^4-at m/z

574.3) anion. This assignment was corroborated by the zoom

scans over the narrow mass range of interest (inserts in Figure

24). The peak separation within the multiply charged clusters

are, as indicated by the charge state of the ion generating the

1

pattern, separated by ¹/₂,

/ and / Da, respectively. The

3 4

presence of up to a heptaprotonated metal-ligand complex

without the observation of any fragmentation may seem unusual.

However, the diprotonated mononuclear Ti^IVtris(catecholate)

species is stable⁴⁹reflecting the high stability of the Ti^IVtris-

(catecholate) Ti-O bonds and the basicity of the resulting

anionic complex. The strong association of the countercations

through H-bonds to the anionic cluster was already described

within the context of the description of the X-ray structure of

this cluster, and we assume that, in addition to the electrostatic

interactions, identical H-bonds are responsible for the adherence

of the protons to the cluster, namely to the catecholate and the

carbonyl oxygens. We surmise that the origin of the protons

were the Et₃NH⁺countercations. Under the drying conditions

following the spraying of the analyte solution into the mass

spectrometer, Et₃N evaporated, leaving the protons with the

anion. Consistent with the absence of solid state and NMR

evidence for the presence of a host-guest complex, no ions in

the mass spectrum were observed that could have indicated an

encapsulated species. Again, fragmentation of the cluster under

the condition of the ESI-MS was not observed.

The ESI spectrum (anion detection) exhibited by the corre-

sponding Sn^IVcluster [Sn₄A₄]^8-, as its triethylammonium salt,

is similar to that of the Ti-analogue in that it shows a rich pattern

of peaks attributable to cluster anions coordinated by x cations,

resulting in various charge states (-8 + x), ranging from

dianionic to tetraanionic, with each group of peaks modulated

by proton-to-triethylammonium exchange processes. It is in-

teresting to note that not all possible peaks attributable to this

exchange process are observed. For instance, for the dicationic

species, only 5 out of the 10 possibilities are observed, and only

one tetraanionic species can be clearly identified.

Figure 21. Heating a solution of K₅[Et₄N]₇[Ga₄C₆] up to 100 °C does

not cause the diasterotopic methylene protons to become equivalent.

constants on the order of 10^-1to 10²s^{-1 46}

To confirm that

.

these cross-peaks were indeed indicative of an exchange process,

2D exchange spectroscopy (EXSY) was employed. In the 2D-

EXSY experiment, which is similar to the 2D-NOESY experi-

ment, the cross-peaks provide a clear picture of the exchange

process.⁴⁶2D-EXSY spectra were recorded at different mixing

times, τ_m. When the mixing time was increased from 0.75 to

1.5 s, the intensity of the cross-peaks increased approximately

two-fold.

Since the crystal structure of K₅(Et₄N)₇[Fe₄C₆] has shown

that the cluster is a tightly closed box lacking holes on the

surface, the guest molecule can presumably only exit the cavity

through openings created by structural fluctuations of the host

cluster. The kinetic lability of Ga^IIIcatecholate complexes²¹

enables ligands to partially dissociate from the metal ions and

creates openings in the host cluster through which the guest

molecules can exit and enter. The ability of the guest molecule

to exit the cluster cavity is supported by competition experi-

ments, in which stepwise replacement of Me₄N⁺for Pr₄N⁺, and

1

Et₄N⁺for Pr₄N⁺, is observed by H NMR spectroscopy.²⁸

Mass Spectrometry of the Tetrahedral Clusters. In recent

years, electrospray ionization mass spectrometry (ESI-MS) has

been proven most useful in the analyses of self-assembled M_xL_y

type clusters and the results obtained by ESI-MS have been

regarded as indicative of the solution behavior of a species rather

than gas-phase behavior.⁴⁷We can confirm this because the

results obtained by ESI-MS correlate very well with all

concurrent observations made by NMR and X-ray diffractom-

etry. Nonetheless, ESI studies of such clusters remain nontrivial.

The optimization of the spectra for the [Ti₄A₄]^8-cluster has

been discussed in detail.⁴⁸

The spectrum shown in Figure 25 was obtained from a cluster

sample prepared with a slight excess of Sn^IV. The minor peaks

at 1348.9 and 1406.4, and 2698.2 and 2813.6 m/z correspond

numerically to the di- and monoanionic clusters with one and

two Sn atoms as countercations, respectively, although the

charge states were not confirmed in a zoom scan experiment.

(47) For examples of ESI analysis of oligonuclear metal complexes see

refs 15a, 46, and (a) Marquis-Rigault, A.; Dupont-Gervais, A.; Baxter, P.

N. W.; Van Dorsselaer, A.; Lehn, J.-M. Inorg. Chem. 1996, 35, 2307-

2310. (b) Blanc, S.; Yakirevitch, P.; Leize, E.; Meyer, M.; Libman, J.; van

Dorsselaer, A.; Albrecht-Gary, A.-M.; Shanzer, A. J. Am. Chem. Soc. 1997,

119, 4934-4944.

(48) Ko¨nig, S.; Bru¨ckner, C.; Raymond, K. N.; Leary, J. A. J. Am. Soc.

Mass Spectrom. 1998, 9, 1099-1103.

(49) Ali, N. J.; Milne, S. J. Br. Ceram. Trans. J. 1987, 86, 113-117.

The spectrum (MeOH:CH₃CN:H₂O, 4:4:1, v/v/v, cation

detection mode) obtained for the [Ti₄A₄]^8-cluster, as its octakis-

triethylammonium salt is shown in Figure 23. Clearly observable

(46) Perrin, C. L.; Dwyer, T. J. Chem. ReV. 1990, 90, 935-967.

Design of Tetrahedral Supramolecular Clusters

J. Am. Chem. Soc., Vol. 123, No. 37, 2001 8935

Figure 22. 2D-NOESY NMR spectrum (500 MHz, D₂O) of K₅[Et₄N]₇[Ga₄C₆]. The horizontal axis shows the ligand resonances of the cluster,

while the vertical axis shows signals corresponding to Et₄N⁺.

Figure 23. ES(+)-MS of [Ti₄A₄][Et₃NH]₈. Each of the peaks in the

figure insert can be assigned to a cluster dication formed from the

Figure 24. ES(-)-MS of [Ti₄A₄][Et₃NH]₈. The inserts show that the

charge states are confirmed for these ions using zoom scans.

exchange of H⁺for K⁺.

Remarkably, even with an excess of metal, the cluster remains

intact and non-M₄A₄fragments are not observed. This is a

particularly powerful demonstration of the cooperative effects

holding the clusters together, resulting in extraordinary stability

of the tetrahedral species, and mirrors the results obtained in

solution and probed by NMR.

To conclude the mass spectral results, we believe that

ESI-MS allows for an unambiguous characterization of the

clusters, although we found that the optimization of the spectra

required the empirical variation of the spray parameters, namely

the solvents and ionization conditions used.⁴⁸Variation in the

drying parameters (evaporation of the solvents) resulted in little

or no changes within the parameters tested (capillary temper-

atures between 125 and 200 °C were probed). While it is

nontrivial to infer a structure from a given composition, our

data are consistent with the notion that the tetrahedral anionic

structures for the complexes observed in the crystal structure

are also preserved in solution under the conditions of electro-

spray mass spectrometry.

Conclusions

We have shown that carefully designed ligands requiring few

synthetic steps can be combined with complementary metal ions

to form tetrahedral tetranuclear clusters in a self-assembly

process. The three design strategies outlined here are generally

valid and will hopefully guide the design and synthesis of new

clusters based on a variety of ligands and metals. Given the

possibility of creating clusters with well-defined cavities, this

seems a desirable goal in light of the rich chemistry anticipated

in the inner-phase of these molecular hosts. We also anticipate

that these rigid, kinetically and thermodynamically stable,

metrically well-defined clusters will find a use as building blocks

in nanotechnological applications.

Experimental Section

General. All NMR spectra were measured with Bruker 300, 400,

or 500 MHz spectrometers. Chemical shifts are reported on the δ scale

in ppm downfield from Na 3-(TMS)-propionate-2,2,3,3-d₄in D₂O, from

8936 J. Am. Chem. Soc., Vol. 123, No. 37, 2001

Caulder et al.

rotary evaporation. Diethyl ether was added to precipitate the product

as tan to white crystalline precipitates, which were collected by filtration

and air-dried.

1,3,5-Tris(2,3-dimethoxybenzamido)benzene (Me₆A): prepared

1

following the general procedure on a 50 mM scale in 88% yield. H

NMR (300 MHz, DMSO-d₆) δ 10.38 (s, 1H), 7.97 (s, 1H), 7.1-7.2

(m, 3H), 3.85 (s, 3H), 3.83 (s, 3H); ¹³C NMR (75 MHz DMSO) δ

165.0, 152.6, 145.9, 139.5, 131.6, 124.2, 120.2, 114.6, 107.1, 61.1,

56.0. Anal. Calcd (Found) for C₃₃H₃₃N₃O₉: C 64.38 (63.80), H 5.40

(5.34), N 6.83 (6.55).

1,3-Bis(2,3-dimethoxybenzamido)benzene (Me₄B): prepared fol-

lowing the general procedure on a 50 mM scale in 91% yield. ¹H NMR

(300 MHz, DMSO-d₆) δ 10.22 (br s, 2H), 8.25 (s, 1H), 7.48 (d, J ) 7

Hz, 2H), 7.2 (m, 1H), 7.0-7.1 (m, 6H), 3.78 (s, 6H), 3.72 (s, 6H); ¹³

C

NMR (75 MHz, DMSO-d₆) δ 165.6, 153.3, 146.6, 140.2, 132.2, 129.6,

124.9, 120.8, 115.9, 115.4, 61.8, 56.7. Anal. Calcd (Found) for

C₂₄H₂₄N₂O₆: C 66.05 (65.75), H 5.54 (5.55), N 6.42 (6.29)

1,5-Bis(2,3-dimethoxybenzamido)naphthalene (Me₄C): prepared

following the general procedure on a 50 mM scale in 82% yield: mp

208-209 °C. ¹H NMR (300 MHz, CDCl₃) δ 10.74 (s, 2H), 8.56 (d, J

) 7.5 Hz, 2 H), 7.88 (m, 4H), 7.60 (t, J ) 8.1 Hz, 2H), 7.26 (t, J )

8.1 Hz, 2H), 7.15 (dd, J ) 1.2, 8.1 Hz, 2H), 4.09 (s, 6H), 3.90 (s, 6H).

¹³C NMR (100 MHz, CDCl₃) δ (C) 163.5, 152.8, 147.4, 134.3, 127.0,

126.9; (CH) 126.7, 125.0, 123.3, 118.9, 116.7, 116.0; (CH₃) 62.1, 56.3.

Anal. Calcd (Found) for C₂₈H₂₆N₂O₆: C 69.12 (68.94), H 5.39 (5.44),

N 5.76 (5.65).

Figure 25. ESI(-)-MS of [Sn₄A₄][Et₃NH]₈. The inserts show that the

charge states are confirmed for these ions using zoom scans.

General Procedure for the Formation of the Ligands H₆A, H₄B,

and H₄C. A four-fold stoichiometric excess per methoxy group of BBr₃

was added at -78 °C via syringe to a solution of the protected ligands

Me₆A, Me₄B, and Me₄C in dry CH₂Cl₂(∼100 mL/5 mmol ligand).

The reaction mixture was allowed to warm to room temperature and

stirred overnight. Workup procedure A: Volatiles were removed under

vacuum, and the remaining residue was suspended in water for 2 h at

100 °C. The white precipitates were collected by filtration and dried

under vacuum at 70 °C. Workup procedure B: Unreacted BBr₃was

quenched by the careful addition of MeOH. The mixture was distilled

while repeatedly adding portions of fresh MeOH until the distillate

were boron-free (flame test: one drop of the distillate when lit on a

cotton swap does not burn green). The products precipitated as

crystalline material from the reduced hot methanolic solution were

filtered off and dried.

TMS in DMSO-d₆, and from the residual protic solvent peak in MeOD

and DMF. Melting points are uncorrected. Elemental analyses were

performed at the UCB Analytical Facilities.

Mass Spectrometry. All electrospray experiments were carried out

on a Finnigan LCQ quadrupole ion trap mass spectrometer (Finnigan

MAT, San Jose, CA) equipped with a microspray ionization source.

Transfer lines (deactivated fused silica tubing, 40 µm and 200 µm inner

diameter, polyimide coated) between the syringe pump and ion source

were purchased from Polymicro (Phoenix, AZ), and all the connection

pieces (Peek or stainless steel with KEL-F fittings) were obtained from

Upchurch Scientific (Oak Harbor, WA). Ionization was performed under

microspray conditions at a flow rate of 0.40 µL‚min^-1and a spray

voltage of 2.8-3.5 kV. For anion detection the spray voltage was set

between -2.8 and -3.2 kV. All spectra were acquired at a capillary

temperature of 175 °C, and all ion guide voltages were tuned to

maximize the total ion current. Approximately 30 scans were averaged

for a low-resolution mass spectrum, while for zoom scans, about 50

scans were averaged. The pressure inside the ion trap was 1.75^-5Torr

or less during the acquisition (uncorrected ion gauge reading). The

calibration of the extended mass range (100-4000 Da) was performed

with bovine insulin (in MeOH and 2% formic acid; c ) 1 pmol/µL)

and NaI (in MeOH:2-propanol 1:1 (v/v), c ) 0.5 mg/mL). The normal

mass range (50-2000 Da) and the zoom scans were calibrated with

the standard calibration procedure and compounds (caffeine, peptide

MFRA, and Ultramark 1621) provided by the instrument manufacturer.

Solutions of the analyte were prepared just prior to their analyses, and

a concentration of 250 pmol/µL was used for all experiments.

NMR Studies. The variable-temperature NMR study was done on

a 500 MHz Bruker DRX spectrometer by heating the sample from room

temperature, cooling the sample back to starting temperature, and

recording the spectra, producing identical results. The accuracy of the

temperature was (0.1 K. For the 2D NOESY experiment, 1 × 10^-6

mol of K₅[Et₄N]₇[Ga₄C₆] was dissolved in 600 µL of D₂O. The spectrum

was measured at 300 K on a 500 MHz Bruker DRX spectrometer using

a NOESY sequence, with d1 ) 2s and d8 ) 100 ms.

General Procedure for the Formation of the Protected Ligands

Me₆A, Me₄B, and Me₄C. The amines of choice (1, 2, or 3) and 1.1

equiv (based on the number of amine functionalities) of 2,3-dimeth-

oxybenzoyl chloride (4) were dissolved in CH₂Cl₂(∼10 mmol amine

per 100 mL). Stoichiometric excess Et₃N was added, and the reaction

mixture was stirred for several hours. The solution was washed with

portions of 1 N HCl, followed by 1 N NaOH and finally water. The

organic layer was isolated, dried over MgSO₄, and concentrated by

1,3,5-Tris(2,3-dihydroxybenzamido)benzene (H₆A): prepared fol-

lowing the general procedure (workup procedure B) on a 50 mM scale

1

in 86% yield: dec >200 °C. H NMR (300 MHz, DMSO-d₆) δ 11.50

(br s, 1H), 10.44 (s, 1H), 9.3 (s, 1H), 7.99 (s, 1H), 7.42 (d, 1H, J ) 7.8

Hz), 6.99 (d, 1H, J ) 7.8), 6.71 (t, 1H, J ) 7.8 Hz); ¹³C NMR (75

MHz DMSO) δ 167.7 (CdO), 148.2, 146.3, 138.7, 119.3, 118.8, 118.6,

117.4, 110.2. Anal. Calcd (Found) for C₂₇H₂₁O₉N₃‚3H₂O: C 55.39

(55.53), H 4.65 (4.51), N 7.18 (7.11).

1,3-Bis(2,3-dihydroxybenzamido)benzene (H₄B): prepared fol-

lowing the general procedure (workup method A or B) on a 25 mM

1

scale in 84% yield: dec >200 °C. H NMR (400 MHz, DMSO-d₆) δ

11.6 (br s, 2H), 10.36 (s, 2H), 9.4 (br s, 2H), 8.08 (s, 1H), 7.4-7.5

(m, 4H), 7.28 (t, J ) 8.1 Hz, 1H), 6.91 (d, J ) 8.1 Hz, 2Η), 6.70 (t,

J ) 8.1 Hz, 2H); ¹³C NMR (125 MHz, DMSO-d₆) δ 167.7, 148.3,

146.3, 138.5, 129.0, 119.2, 118.7, 117.4, 117.3, 114.3. Anal. Calcd

(Found) for C₂₀H₁₆O₆N₂‚H₂O: C 60.16 (60.30), H 4.24 (4.55), N 7.37

(7.03).

1,5-Bis(2,3-dihydroxybenzamido)naphthalene (H₄C): prepared

following the general procedure (workup method A) on a 3 mM scale

in 89% yield: mp ) 298-302 °C (dec). ¹H NMR (400 MHz, DMSO-

d₆) δ 11.93 (s, 2H), 10.91 (s, 2H), 9.52 (s, 2H), 7.92 (d, J ) 8.5 Hz,

2H), 7.87 (d, J ) 7.3 Hz, 2H), 7.64-7.59 (m, 4H), 7.03 (dd, J ) 1.7,

7.9 Hz, 2H), 6.84 (t, J ) 7.9 Hz, 2H). ¹³C NMR (125 MHz, DMSO-

d₆) δ (C) 168.4, 148.7, 146.7, 133.9, 129.5, 117.1; (CH) 126.5, 123.4,

121.0, 119.6, 119.2. FAB(+) MH⁺431. Anal. Calcd (Found) for

C₂₄H₁₈N₂O₆: C 66.97 (66.59), H 4.22 (4.26), N 6.51 (6.39).

(Et₄N)₈[Ti₄A₄]. A freshly prepared solution of 107 mg Ti(On-Bu)₄

(3.1 × 10^-4mol) in MeOH (20 mL) was added to a solution of ligand

H₆A (165 mg, 1 equiv) in a mixture of MeOH (20 mL) and Et₃N (1

mL), and the solution was stirred for 12 h at ambient temperature. The

Design of Tetrahedral Supramolecular Clusters

J. Am. Chem. Soc., Vol. 123, No. 37, 2001 8937

orange gelatinous precipitate was removed by centrifugation and

suspended in DMF (20 mL). The suspension was heated under N₂to

reflux for 12 h. The resulting solution was filtered through Celite and

either evaporated to dryness in vacuo, to produce 190 mg of a

microcrystalline solid, or MeOH vapors were allowed to diffuse into

this solution at room temperature over several weeks to produce 80

0.497 N KOH solution in MeOH (935 µL, 0.464 mmol) was added

via micropipet. The ligand dissolved after ca. 5 min with stirring, after

which a 0.280 M solution of Et₄NCl in MeOH (1.660 mL, 0.465 mmol)

was added via micropipet. Powdered Ga(acac)₃(0.056 g, 0.15 mmol)

was added to the solution. Within seconds the Ga(acac)₃dissolved,

and the solution changed from colorless to yellow. While the reaction

mixture was stirred at room temperature overnight, the product

precipitated as a yellow silky powder. This powder was isolated by

ultracentrifugation and dried under vacuum (0.120 g, 0.029 mmol, 76%).

¹H NMR (300 MHz, D₂O) δ 13.60 (s, 12H, NH), 8.15 (d, J ) 7.8 Hz,

12H, Ar_nH), 7.91 (d, J ) 8.6 Hz, 12H, Ar_nH), 7.35 (dd, J ) 1.5, 8.2

Hz, 12H, Ar_cH), 7.20 (t, J ) 8.2 Hz, 12H, Ar_nH), 6.78 (dd, J ) 1.5,

7.3 Hz, 12H, Ar_cH), 6.63 (t, J ) 7.9 Hz, 12H, Ar_cH), 2.42 (q, J ) 7.2

Hz, 48H, CH₂(out)), 0.68 (t, J ) 7.2 Hz, 72H, CH₃(out)), -0.68 (m,

8H, CH₂(in)), -1.58 (t, J ) 6.9 Hz, 12H, CH₃(in)). ¹³C NMR (125

MHz, D₂O) δ (C) 172.4, 161.4, 157.7, 136.9, 129.4, 117.4; (CH) 129.4,

121.5, 120.3, 118.3, 117.8, 117.7; (CH₂) 54.5, 52.9; (CH₃) 9.1, 7.0.

1

mg (yield 37%, first crop) of orange cuboid crystals. H NMR (300

MHz, DMSO-d₆) δ 11.43 (s, 1.5H, NH), 8.2 (variably br s, 1H, NH),

7.12 (d, J ) 6.5 Hz, 1.5H, ArH), 6.84 (s, 1.5H, ArH), 6.45 (t, 1.5H, J

) 7.8 Hz; ArH), 6.21 (d, 1.5H, J ) 7.8 Hz; ArH), 3.00 (q, 5H, J ) 7.3

Hz; CH₂), 1.14 (t, 9H, J ) 7.3 Hz, CH₃); ¹³C NMR (125 MHz, DMSO-

d₆) δ 158.8 (CdO), 140.3, 138.7, 137.5, 118.5, 116.8, 115.4, 114.5,

112.9, 43.3, 8.3. ES(+)-MS (100% CH₃CN) (Na⁺is ubiquitously

present): ([ ) [Ti₄A₄]^8-) 581 ([ + 9Na⁺+ 4Et₃NH⁺)⁵⁺, 620 ([ +

4H⁺+ 8Na⁺)⁴⁺, 681 ([ + 10Na⁺+ 2Et₃NH⁺)³⁺, 1151 ([ + 10H⁺)²⁺

1162 ([ + 9H⁺+ 1Na⁺)²⁺, 1173 ([ + 8H⁺+ 2Na⁺)²⁺, 1184 ([ +

7H⁺+ 3Na⁺)²⁺, 1195 ([ + 6H⁺+ 4Na⁺)²⁺, 1206 ([ + 5H⁺

5Na⁺)²⁺, 1217 ([ + 4H⁺+ 6Na⁺)²⁺, 1228 ([ + 3H⁺+ 7Na⁺)²⁺

,

+

,

+

ES(-)-MS (100% CH₃OH): ([ ) [Ga₄C₆]^12-) 859.4 [[ + 4Et₄N⁺

+

2302 ([ + 9H⁺)⁺, 2324 ([ + 8H⁺+ Na⁺)⁺, 2346 ([ + 7H⁺

2K⁺+ 2H⁺]^4-, 868.9 [[ + 4Et₄N⁺+ 3K⁺+ 1H⁺]^4-, 882.2 [[ +

5Et₄N⁺+ 1K⁺+ 2H⁺]^4-, 891.8 [[ + 5Et₄N⁺+ 2K⁺+ 1H⁺]^4-, 914.8

[[ + 6Et₄N⁺+ 1K⁺+ 1H⁺]^4-, 924.2 [[ + 6Et₄N⁺+ 2K⁺]^4-, 937.4

[[ + 7Et₄N⁺+ 1H⁺]^4-, 946.9 [[ + 7Et₄N⁺+ 1K⁺]^4-, 1189.7 [[ +

2Na⁺)⁺, 2368 ([ + 6H⁺+ 3Na⁺)⁺, 2390 ([ + 5H⁺+ 4Na⁺)⁺, 2421

([ + 4H⁺+ 5Na⁺)⁺. ES (CH₃CN:MeOH:H₂O 4:4:1 (v/v/v)) 2300.1

([ + 7H⁺)^-, 1149.6 ([ + 6H⁺)^2-, 776.1 ([ + 5H⁺)^3-, 574.3 ([ +

4H⁺)^4-

.

5Et₄N⁺+ 2K⁺+ 2H⁺]^3-, 1202.3 [[ + 5Et₄N⁺+ 3K⁺+ 1H⁺]^3-

,

1219.8 [[ + 6Et₄N⁺+ 1K⁺+ 2H⁺]^3-, 1232.7 [[ + 6Et₄N⁺+ 2K⁺

(Et₄N)₈[Sn₄A₄]. SnCl₄(1.16 g, 4.45 mmol) was dissolved in 80 mL

of DMF. Immediately, a solution of 2.364 g (4.45 mmol) of H₆A in 50

mL of DMF was added slowly, followed by the dropwise addition of

10 mL of Et₃N. The initially formed creamy precipitate dissolved and

the mixture was heated to slow reflux under N₂for 12 h. The cooled

and now light pink solution was, if turbid, filtered through a bed of

Celite and evaporated to dryness under vacuum. The resulting crystalline

solid (quantitative yield) analyzed to be the cluster (Et₄N)₈[Sn₄A₄]

together with up to 4 equiv of Et₃NHCl and, depending on the drying

conditions, varying amounts of DMF. Repetitive recrystallization from

+ 1H⁺]^3-, 1250.2 [[ + 7Et₄N⁺+ 2H⁺]^3-, 1263.0 [[ + 7Et₄N⁺

+

1K⁺+ 1H⁺]^3-, 1275.7 [[ + 7Et₄N⁺+ 2K⁺]^3-, 1293.5 [[ + 8Et₄N⁺

+ 1H⁺]^3-, 1306.0 [[ + 8Et₄N⁺+ 1K⁺]^3-. Anal. Calcd (Found) for

K₅Ga₄C₂₀₀H₂₂₄N₁₉O₃₆‚8H₂O: C 58.75 (58.43), H 5.92 (5.65), N 6.51

(6.19).

K₅(Et₄N)₇[Fe₄C₆]: prepared as in K₅(Et₄N)₇[Ga₄C₆] above using Fe-

(acac)₃. The product was isolated as a dark red powder (79%). Crystals

suitable for analysis by X-ray diffraction grew at room temperature

over two weeks by gas-phase diffusion of acetone into a methanol/

water solution of the complex. ES(-)-MS (100% CH₃OH) [ )

[Fe₄C₆]^12-: m/z ) 613.8 [[ + 2Et₄N⁺+ 1Na⁺+ 4H⁺]^5-, 641.7 [[

1

DMF/MeOH provided a material largely free of Et₃NHCl (as per H

NMR integration) but which still did not provide a clean elemental

+ 2Et₄N⁺+ 3K⁺+ 2Na⁺]^5-, 788.4 [[ + 2Et₄N⁺+ 1K⁺+ 3Na⁺

+

1

analysis. H NMR (300 MHz, DMSO-d₆) δ 11.29 (s, 1H, NH), 7.15

2H⁺]^4-, 1094.5 [[ + 2Et₄N⁺+ 5K⁺+ 2Na⁺]^3-, 1123.4 [[ + 3Et₄N⁺

(d, J ) 7.6 Hz, 1H, ArH), 6.88 (s, 1H, ArH), 6.63 (d, 1H, J ) 7.6 Hz;

ArH), 6.47 (t, 1H, J ) 7.6 Hz; ArH), 2.99 (q, 4H, J ) 7.3 Hz; CH₂),

1.14 (t, J ) 7.3 Hz, CH₃). ¹³C NMR (125 MHz, DMSO-d₆) δ 158.2,

141.3, 139.2, 138.1, 118.9, 116.9, 115.9, 115.1, 113.6, 46.6, 9.2.

ES(+)-MS (100% CH₃OH)sonly clusters of peaks with isotope

distribution patterns indicative of dications could be unambiguously

assigned: ([ ) [Sn₄L₄]^8-) 1799 ([ + 10Et₃NH⁺)²⁺, 1750 ([ +

9Et₃NH⁺+ H⁺)²⁺, 1699 ([ + 8Et₃NH⁺+ 2H⁺)²⁺, 1648 ([ +

7Et₃NH⁺+ 3H⁺)²⁺, 1597 ([ + 6Et₃NH⁺+ 4H⁺)²⁺. ES (100%

CH₃OH) several clusters of peaks with isotope distribution patterns

indicative of multiply charged ions charge could be identified: ([ )

+ 5K⁺+ 1H⁺]^3-, 1183.8 [[ + 5Et₄N⁺+ 3K⁺+ 1H⁺]^3-, 1214.2 [[

+ 6Et₄N⁺+ 2K⁺+ 1H⁺]^3-, 1244.7 [[ + 7Et₄N⁺+ 1K⁺+ 1H⁺]^3-

,

1490.9 [[ + 1Et₄N⁺+ 1K⁺+ 1Na⁺+ 7H⁺]^2-. Anal. Calcd (Found)

for K₅Fe₄C₂₀₀H₂₂₄N₁₉O₃₆‚4H₂O: C 60.65 (60.38), H 5.90 (5.83), N 6.72

(6.44).

K₁₁(Et₄N)[Ga₄C₆]: prepared as in K₅(Et₄N)₇[Ga₄C₆] above using

1

only 1 equiv of Et₄NCl (90%). H NMR (500 MHz, D₂O) δ 13.55 (s,

5H, NH), 8.02 (bs, 12H, Ar_nH), 7.85 (bs, 12H, Ar_nH), 7.32 (d, 12H, J

) 8.1 Hz, Ar_cH), 7.12 (bt, 12H, Ar_nH), 6.78 (d, 12H, J ) 7.3 Hz,

Ar_cH), 6.61 (t, 12H, J ) 7.7 Hz, Ar_cH), -0.62 (q, 8H, J ) 7.0 Hz,

CH₂(in)), -1.54 (t, 12H, J ) 7.0 Hz, CH₂(in)). ¹³C NMR (125 MHz,

D₂O) δ (C) 172.5, 161.1, 157.4, 136.6, 129.0, 117.4; (CH) 129.4, 121.5,

120.4, 118.4, 117.9, 117.6; (CH₂) 53.1; (CH₃) 6.9. Anal. Calcd (Found)

for K₁₁Ga₄C₁₅₂H₁₀₄N₁₃O₃₆‚6H₂O: C 52.08 (51.98), H 3.34 (3.38), N

5.19 (4.79).

Crystallography General. Crystal data for all structures investigated

were collected using a Siemens SMART diffractometer equipped with

a CCD area detector with graphite monochromated Mo KR radiation

(λ ) 0.71073 Å). Frames corresponding to an arbitrary hemisphere of

data were collected using ω scans of 0.3° counted for a total of 30 s

(20 s for Me₆A) per frame at the temperatures noted. SAINT⁵⁰and

XPREP⁵¹were used for data reduction. Data were corrected for Lorentz

and polarization effects. Crystallographic data (excluding structure

factors) for the structures reported in this paper have been deposited

with the Cambridge Crystallographic Data Centre as Supplementary

Publication nos. CCDC-143379 (Me₆A), CCDC-101007 ((Et₄N)₈-

[Ti₄A₄]), and CCDC-100947 (K₅(Et₄N)₇[Fe₄C₆]‚8H₂O‚3CH₃OH), re-

spectively. Copies of the data can be obtained free of charge on

application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax:

(+44)(1223)336-033; e-mail: deposit@ccdc.cam.ac.uk)

[Sn₄A₄]^8-) 1492 ([ + 4Et₃NH⁺+ 2H⁺)^2-, 1442 ([ + 3Et₃NH⁺

+

3H⁺)^2-, 1399 ([ + 2Et₃NH⁺+ 3H⁺+ NH₄

)

^2-, 1391 ([ +

2Et₃NH⁺+ 4H⁺)^2-, 1343 ([ + Et₃NH⁺+ 5H⁺)^2-, 966 ([ +

+

3Et₃NH⁺+ H⁺+ NH₄

)

^3-, 960 ([ + 3Et₃NH⁺+ 2H⁺)^3-, 932 ([ +

+

2Et₃NH⁺+ 2H⁺+ NH₄

)

^3-, 927 ([ + 2Et₃NH⁺+ 3H⁺)^3-, 899 ([

^3-, 894 ([ + Et₃NH⁺+ 4H⁺)^3-, 860 ([ + 5H⁺)^3-

,

+

+ 4H⁺+ NH₄

)

722 ([ + 3Et₃NH⁺+ H⁺)^4-, 645 ([ + 4H)^4-(weak).

K₁₂[Ga₄B₆]. H₄B (100 mg, 0.262 mmol) was dissolved in CH₃OH

(15 mL) containing 1.04 mL of a solution of 0.5 M KOH in CH₃OH

(0.52 mmol). To the light yellow solution, powdered Ga(acac)₃(64

mg, 0.17 mmol) was added and subsequently dissolved. A white powder

started precipitating after 6 h. The mixture was stirred for an additional

12 h and then was reduced to 5 mL. The powder was filtered and

washed with acetone (20 mL) and dried under vacuum to yield 67 mg

(50%). ¹H NMR (300 MHz, D₂O) δ 12.80 (s, 12H, NH), 7.60 (s, 12H,

ArH), 7.09 (dd, J ) 8.4, 1.5 Hz, 12H, ArH), 7.03 (t, J ) 8.1 Hz, 6H,

ArH), 6.34 (dd, J ) 7.8, 1.5 Hz, 12H, ArH) 6.50 (t, J ) 8.1 Hz, 12H,

ArH), 6.04 (dd, J ) 7.8, 1.5 Hz, 12H, ArH). FAB(+)-MS (NBA) [ )

[Ga₄B₄]^12-: m/z ) 1503 [[ + H⁺+ 12K⁺]⁺, 1463 [[ + 2H⁺

+

11K⁺]⁺, 1540 [[ + 13K⁺]⁺. ES(+)-MS (100% CH₃OH) m/e ) 1538

[[ + 14K⁺]²⁺, 1039 [[ + 15K⁺]³⁺, 788 [[ + 16K⁺]⁴⁺. ES(+)-MS

(50) SAINT: SAX Area Detector Integration Program, V4.024; Siemens

Industrial Automation, Inc.: Madison, WI, 1995.

(51) SHELXTL Crystal Structure Determination Software Package;

Siemens Industrial Automation, Inc.: Madison, WI, 1993.

(100% CH₃CN) 1463.4 ([ + 14K⁺)²⁺, 988 ([ + 15K⁺)³⁺

.

K₅(Et₄N)₇[Ga₄C₆]. Ligand H₄C (0.100 g, 0.232 mmol) was sus-

pended, under oxygen-free conditions, in distilled MeOH (30 mL). A

8938 J. Am. Chem. Soc., Vol. 123, No. 37, 2001

Caulder et al.

Crystallography of Me₆A. Clear blades of H₃₃C₃₃N₃O₉having

approximate dimensions of 0.15 × 0.10 × 0.05 mm were grown by

vapor phase diffusion of Et₂O into a CHCl₃solution of Me₆A. T )

-116 °C, monoclinic, P2₁/a (No. 14), a ) 15.406(7) Å, b ) 7.264(5)

Å, c ) 26.48(1) Å, â ) 95.23(5)°, V ) 2951(2) Å³, Z ) 4, µ ) 0.10

mm^-1, F(000) ) 1296, F_calcd) 1.38 Mg m^-3, 2Θ_max) 52.4°. Of the

13319 reflections collected 5648 were unique (R_int) 0.109). The

structure was solved by direct methods (SIR92) and expanded using

Fourier techniques using the teXsan⁵²crystallographic software package.

The O and N atoms were refined anisotropically, while the C atoms

were refined isotropically. Hydrogen atoms were included at calculated

vapor diffusion of acetone into a methanol/water solution of the

complex. T ) -120 °C, cubic, I 4h3d (No. 220), a ) 43.706(8) Å, V )

83488 Å³, Z ) 16, m ) 0.45 mm^-1, F(000) ) 35808, F_calcd) 1.333

Mg m^-3, 2θ_max) 41.67°. Of the 136092 reflections collected 7336

were unique (R_int) 0.214). An empirical absorption correction was

applied using XPREP⁴⁹(ellipsoidal model, R_int) 0.036, T_max) 0.89,

T_min) 0.83). The structure was solved by direct methods and was

refined on F²using SHELXTL.⁵¹The iron atoms, oxygen and nitrogen

atoms of the ligands, the half occupancy potassium, and the nitrogen

and carbon atoms of the full occupancy Et₄N⁺counterion were refined

anisotropically. Hydrogen atoms were included as riding on their

respective carbon and nitrogen atoms for all but the disordered

counterions and solvent. Not all carbon atoms were found for the

disordered Et₄N⁺counterions. The N-C and C-C distances for these

disordered ions were set to target values of 1.4 and 1.5 Å. An

antibumping restraint was applied to carbons of the disordered

Et₄N⁺on the interior of the tetrahedral cluster. Weighting scheme:

positions. Final R ) 0.059 for 1306 I > 3σ(I), 241 parameters, R_w

)

0.060, GOF ) 1.41, min/max residual e^-density ) -0.29/-0.30 e^-/Å³.

Crystallography of (Et₃N)₈[Ti₄A₄]. A red tabular crystal of dimen-

sions 0.30 × 0.22 × 0.07 mm was grown by vapor diffusion of MeOH

into a DMF solution of the complex; -103 ( 2 °C, trigonal, R3hc (No.

167), with a ) 22.6143(5) Å, c ) 106.038(2) Å, V ) 46963 Å³, Z )

12, µ ) 0.279 mm^-1, F(000) ) 19680, F_calcd) 1.32 Mg m^-3, 2Θ_max

) 41.6°. Of the 49921 reflections collected 6019 were unique (R_int

) 0.090). An empirical absorption correction was applied using

SADABS⁵³(T_max) 0.74, T_min) 0.56). The structure was solved by

direct methods (SIR92) and expanded using Fourier techniques using

the teXsan⁵²crystallographic software package. Final R ) 0.109 for

1380 I > 3σ(I), 271 variables, R_w) 0.120, GOF ) 4.21, min/max

residual density ) -0.40/0.51 e^-/Å³. The oxygen and titanium atoms

were refined anisotropically, while other non-hydrogen atoms were

refined isotropically. Hydrogen atoms were included in calculated

idealized positions. All triethylammonium cations are disordered with

respect to the conformation of their ethyl arms and their location in

the crystal but could invariably be located in H-bond distance to the

catecholate oxygens or the carbonyl oxygens; 8/3 Et₃NH⁺cations per

asymmetric unit were distributed unequally over all four possible

catecholate and all four possible carbonyl oxygen positions.

2

{1}/{[σ²(F_o) + (0.1660p)²+ 779.98p]}, where p ) ((F_o,0)_max

+

2F_c²))/3. Final R1 ) 0.0978 for 4672 F_o> 4σ(F_o) (4672 Friedel unique

data, 542 parameters, 14 restraints, 3.75° < 2θ < 34.58°); for all

7336 data, wR₂) 0.3288, GOF ) 1.205; max/min residual density

+0.62/-0.32 e Å^-3, Flack parameter ) 0.03(5).⁵⁴

Acknowledgment. This work was supported by NSF grant

CHE-9709621 and the exchange grants NSF INT-9603212 and

NATO SRG951516. J.A.L. and S.K. gratefully acknowledge

an unrestricted UCB gift fund. We are most appreciative to Dr.

Frederick Hollander (CHEXRAY, UCB) for his assistance with

the structure determinations.

Supporting Information Available: Details of the X-ray

structure solutions for compounds Me₆A, (Et₄N)₈[Ti₄A₄], and

K₅(Et₄N)₇[Fe₄C₆] (CIF). This material is available free of charge

via the Internet at http://pubs.acs.org.

Crystallography of K₅(Et₄N)₇[Fe₄C₆]‚8H₂O‚3CH₃OH. A red blocky

crystal of dimensions 0.25 × 0.23 × 0.10 mm was grown from the

(52) teXsan: Crystal Structure Analysis Package; Molecular Structure

Corp.: The Woodlands, TX, 1992.

JA0104507

(53) Sheldrick, G. SADABS, Siemens Area Detector ABSorption Cor-

rection Program; personal communication, 1996.

(54) Flack, H. D. Acta Crystallogr. A 1993, 39, 876-881.

Article Doi

Design, formation and properties of tetrahedral M4L4 and M4L6 supramolecular clusters

DOI: 10.1021/ja0104507

Source and publish data:

Authors:

Article abstract of DOI:10.1021/ja0104507

Full text of DOI:10.1021/ja0104507

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