narrative focuses on edge- and face-directed self-assembly, however, the application
and extension of direction bonding to novel design strategies and new functions must
not be overlooked [7, 9–16]. The paradigm of two-component assembly, wherein
single types of donor and acceptors are combined, is broken with the realization that
heteroligated metal sites provide a new means to control interactions between
subunits. This technique can be used to effect supramolecule-to-supramolecule
transformations and provide scaffolds for functionalized SCCs via pre- and postself-assembly modifications.
2 Molecular Squares: Establishing a Molecular Library
Among the simplest of the SCCs are square metallacycles. Following the technique
of directional bonding as introduced above, a square is easily reduced to four
vertices and four edges, wherein the angle at each vertex is 90
. Despite this
simplicity, even metallacyclic squares afford some level of versatility regarding
the choice of subunits. A [4+4] assembly represents each edge and vertice as an
individual subunit, demanding a ditopic 90
tecton interaction with a complementary ditopic linear subunit. An early first example of directional bonding, predating
further development of this strategy by almost a decade, is the tetranuclear square
{(CO) 4 M} 4 (P(OCH 2 ) 3 P) 4 (M ¼ W, Cr) prepared by Verkade and coworkers
[17]. The combination of four equivalents of a metal acceptor with an equal amount
of a linear donor was later demonstrated with square planar Group 10 metal ions
and N-heterocyclic donors, two broad categories of building blocks that would later
enjoy use in many new self-assembly reactions. Pd-based squares, of which the
[(en)Pd(4,4
0 -bipy)] 4 (NO 3 ) 8 variant of Fujita and coworkers is a primary example
[18], were quickly joined in the literature by Pt analogues, such as [(en)Pt(UH-N1,
N3)] 4 (NO 3 ) 4 (en ¼ 1,2-diaminoethane, UH ¼ uracil monoanion) [19]. Upon
replacing the amine capping group with phosphine-based ligands, Stang and
coworkers systematically developed a suite of molecular squares, beginning with
[(dppp) 2 M(4,4
0 -bipy)] 4 (OTf) 8 (dppp ¼ diphenylphosphinopropane, M ¼ Pd, Pt)
[20]. This intial [4+4] square established a general synthetic route for the combination of cis-capped Pt and Pd phopshine acceptors with linear, neutral donors to
furnish cationic tetranuclear SCCs [21].
These squares provided the first evidence that such self-assembly reactions were
general for a variety of building blocks; the specific donors and acceptors used
could be changed without affecting the result of a [4+4] square (Fig. 3). The
fledgling directional bonding method was further developed by the recognition
that squares could also be generated in a [2+2] fashion, provided that two of the
requisite 90
angles could be encoded into the donor precursor. This was achieved
first by inbedding an organometallic Pt-aryl corner into a nitrile donor [22, 23] and
later by generating a diypridyl iodonium ligand capable of interacting with the
previously employed cis-capped Pt and Pd acceptors [24, 25]. The use of titanocene
to install a 90
angle for [2+2] squares [26] later inspired a relatively rare
Coordination-Driven Supramolecular Macromolecules via the Directional. . .
233
and extension of direction bonding to novel design strategies and new functions must
not be overlooked [7, 9–16]. The paradigm of two-component assembly, wherein
single types of donor and acceptors are combined, is broken with the realization that
heteroligated metal sites provide a new means to control interactions between
subunits. This technique can be used to effect supramolecule-to-supramolecule
transformations and provide scaffolds for functionalized SCCs via pre- and postself-assembly modifications.
2 Molecular Squares: Establishing a Molecular Library
Among the simplest of the SCCs are square metallacycles. Following the technique
of directional bonding as introduced above, a square is easily reduced to four
vertices and four edges, wherein the angle at each vertex is 90
. Despite this
simplicity, even metallacyclic squares afford some level of versatility regarding
the choice of subunits. A [4+4] assembly represents each edge and vertice as an
individual subunit, demanding a ditopic 90
tecton interaction with a complementary ditopic linear subunit. An early first example of directional bonding, predating
further development of this strategy by almost a decade, is the tetranuclear square
{(CO) 4 M} 4 (P(OCH 2 ) 3 P) 4 (M ¼ W, Cr) prepared by Verkade and coworkers
[17]. The combination of four equivalents of a metal acceptor with an equal amount
of a linear donor was later demonstrated with square planar Group 10 metal ions
and N-heterocyclic donors, two broad categories of building blocks that would later
enjoy use in many new self-assembly reactions. Pd-based squares, of which the
[(en)Pd(4,4
0 -bipy)] 4 (NO 3 ) 8 variant of Fujita and coworkers is a primary example
[18], were quickly joined in the literature by Pt analogues, such as [(en)Pt(UH-N1,
N3)] 4 (NO 3 ) 4 (en ¼ 1,2-diaminoethane, UH ¼ uracil monoanion) [19]. Upon
replacing the amine capping group with phosphine-based ligands, Stang and
coworkers systematically developed a suite of molecular squares, beginning with
[(dppp) 2 M(4,4
0 -bipy)] 4 (OTf) 8 (dppp ¼ diphenylphosphinopropane, M ¼ Pd, Pt)
[20]. This intial [4+4] square established a general synthetic route for the combination of cis-capped Pt and Pd phopshine acceptors with linear, neutral donors to
furnish cationic tetranuclear SCCs [21].
These squares provided the first evidence that such self-assembly reactions were
general for a variety of building blocks; the specific donors and acceptors used
could be changed without affecting the result of a [4+4] square (Fig. 3). The
fledgling directional bonding method was further developed by the recognition
that squares could also be generated in a [2+2] fashion, provided that two of the
requisite 90
angles could be encoded into the donor precursor. This was achieved
first by inbedding an organometallic Pt-aryl corner into a nitrile donor [22, 23] and
later by generating a diypridyl iodonium ligand capable of interacting with the
previously employed cis-capped Pt and Pd acceptors [24, 25]. The use of titanocene
to install a 90
angle for [2+2] squares [26] later inspired a relatively rare
Coordination-Driven Supramolecular Macromolecules via the Directional. . .
233