Transition metal clusters, by virtue of their well-defined structures and unique properties, present themselves as an attractive class of structural and functional building blocks for molecular and supramolecular construction. Summarized in this Account are highlights of our efforts utilizing face-capped octahedral [Re(6)(mu(3)-Se)(8)](2+) clusters as the fundamental building units to create a wide variety of preprogrammed architectures. These include molecular "Tinkertoys", featuring stereospecific cluster units bridged by multitopic ligands and extended arrays of clusters engineered via hydrogen bonding and secondary metal-ligand coordination.

Ligand substitution of [Re(6)(mu(3)-Se)(8)(PEt(3))(5)(CH(3)CN)](SbF(6))(2) (1) with pyridyl-based ligands, 2,4,6-tri-4-pyridyl-1,3,5-triazine (L1) and 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (L2), produced respectively the star-shaped tricluster (T1) and tetracluster (T2) arrays, wherein three (T1) and four (T2) units of the [Re(6)(mu(3)-Se)(8)](2+) core-containing clusters are interconnected by the corresponding bridging ligands. These novel supramolecular assemblies were characterized by a combination of NMR ((1)H and (31)P) spectroscopy, ESI-MS, and microanalysis. The molecular and solid-state structures of T1 have also been established by single-crystal X-ray diffraction.

The reaction of [Re6(mu3-Se)8(PEt3)5(MeCN)](SbF6)2 with an excess of 1,2-bis(4-pyridyl)ethane (L1) and (E)-1,2-bis(4-pyridyl)ethene (L2) produced [Re6(mu3-Se)8(PEt3)5(L1)](SbF6)2 and [Re6(mu3-Se)8(PEt3)5(L2)](SbF6)2, respectively, each bearing an accessible pyridyl N atom capable of further metal coordination. Reacting these cluster complex-based ligands with [Re6(mu3-Se)8(MeCN)6](SbF6)2 afforded two heptacluster metallodendrimers, each featuring a central [Re6(mu3-Se)8]2+ cluster core surrounded by six units of [Re6(mu3-Se)8(PEt3)5]2+ via the bridging interactions of its respective dipyridyl-based ligands. Their identity and stereochemistry have been established, with the most convincing evidence furnished by a unique 77Se NMR spectroscopic study. Electrochemical studies suggest very interesting electronic properties of these novel metallodendrimers.

The reaction between the previously reported site-differentiated cluster solvate [Re(6)(mu(3)-Se)(8)(PEt(3))(5)(MeCN)](SbF(6))(2) (1) with pyridyl-based ditopic ligands 4,4'-trimethylenedipyridine (2), 1,2-bis(4-pyridyl)ethane (3), and (E)-1,2-bis(4-pyridyl)ethene (4) afforded cluster complexes of the general formula [Re(6)(mu(3)-Se)(8)(PEt(3))(5)(L)](SbF(6))(2) (5-7), where L represents one of the pyridyl-based ligands. Reacting these cluster complex-based ligands with the fully solvated cluster complex [Re(6)(mu(3)-Se)(8)(MeCN)(6)](SbF(6))(2) (8) produced dendritic arrays of the general formula {Re(6)(mu(3)-Se)(8)[Re(6)(mu(3)-Se)(8)(PEt(3))(5)(L)](6)}(SbF(6))(14) (9-11), each featuring six circumjacent [Re(6)(mu(3)-Se)(8)(PEt(3))(5)](2+) units bridged to a [Re(6)(mu(3)-Se)(8)](2+) core cluster by the pyridyl-based ligands. Electrochemical studies using a thin-layer electrochemical cell revealed cluster-based redox events in these cluster arrays. For 9 (L = 2), one reversible oxidation event corresponding to the removal of 7 electrons was observed, indicating noninteraction or extremely weak interactions between the clusters. For 10 (L = 3), two poorly resolved oxidation waves were found. For 11 (L = 4), two reversible oxidation events, corresponding respectively to the removal of 1 and 6 electrons, were observed with the 1-electron oxidation event occurring at a potential 150 mV more positive than the 6-electron oxidation. These electrochemical studies suggest intercluster coupling in 11 via through-bond electronic delocalization, which is consistent with electronic spectroscopic studies of this same molecule.

Site-differentiated solvated clusters of the general formula [Re(6)(mu(3)-Se)(8)(PEt(3))(n)(MeCN)(6)(-)(n)](SbF(6))(2) (n = 4, cis and trans; n = 5) undergo ligand substitution reaction with isonicotinamide to afford the corresponding amide derivatives, [Re(6)(mu(3)-Se)(8)(PEt(3))(n)(isonicotinamide)(6)(-)(n)](2+) [1 (n = 5); 2 (n = 4, trans); 3 (n = 4, cis)]. Retention of stereochemistry in each case was confirmed by (1)H and (31)P NMR. The solid-state structures of all three compounds were established crystallographically, which revealed self-complementary hydrogen-bonding interactions between adjacent cluster units. While complex 1 exists as hydrogen-bonded dimers in the solid state, compounds 2 and 3 form one-dimensional chains of clusters bridged by paired hydrogen bonds. It is the rigid stereochemistry of the cluster, combined with the classic crystal engineering motif of complementary N-H.O amide hydrogen bonding, that affords the predictable solid-state structures and dimensionality.