Rhenium calix[4]arenes: Precursors to novel imaging and cancer therapy agents

Interaction of [ReOCl3(PPh3)2] with tert-butylcalix[4]areneH4, Cax(OH)4, in the presence of MOtBu (M = Na, K), or MH, under anaerobic conditions affords the isostructural rhenium(V) oxo complexes [Re(O)(PPh3)Cax(O)4M(NCMe)2]?·?4MeCN [M = Na, (1); K, (2)] containing an alkali-metal cation within an elliptical ligand conformation. In the presence of air, the same reaction affords dioxo rhenium(VII) complexes of the form [Re(O)2Cax(O)4M(NCMe)2] [M = Na, ·?ca. 2.8MeCN (3); K, ·?3MeCN (4)]. Prolonged heating also led to the isolation of the first examples of rhenium–rhenium bonding supported by calixarene ligands, viz. [ReCax(O)4M(NCMe)2(µ-O)]2?·?4MeCN [M = Na, (5); K, (6)], and in one case to {ReCax(O)4K(NCMe)2[µ-OK(NCMe)2]}2Cax(O)2(OH)2?·?7MeCN (7). The use of 2 as starting material led to the isolation of the complex {[Re(O)Cax(O)4K(NCMe)2]2(µ-O)}?·?2MeCN (8). Given the diversity and complexity of the products arising from these “oxo” reactions, attention was switched to the isoelectronic organoimido [NR] group, and given its synthetic utility, the tert-butylimido group [NtBu] was chosen as our entry point. Reaction of [Re(NtBu)2Cl3] with Cax(OH)4 in a 1:1 ratio afforded the orange [Re(NtBu)2ClCax(O)2(OH)2]·?MeCN (9), containing both a linear and a bent imido group. Increasing the amount of Cax(OH)4 (2:3 ratio) afforded both 9 together with the dark red complex [Re(NtBu)Cax(O)4Cax(O)(OH)3]·?6MeCN (10), which contains both mono- and tetra-dentate calixarene ligands. In the presence of adventitious oxygen, the complex {[Re(NtBu)Cax(O)4]2(µ-O)}?·?3MeCN (11) was formed. To favour monomeric complex formation, the lithium salt of the monomethoxycalix[4]arene, namely Cax(OLi)3(OMe), was treated with [Re(NtBu)2Cl3] affording the mono- and bisacetonitrile-ligated species [Re(NtBu)2Cax(O)4Li(NCMe)2][Re(NtBu)2Cax(O)4Li(NCMe)]?·?MeCN (12). To avoid the lithium incorporation of 12, Cax(OH)3(OMe) was treated with [Re(NtBu)3(OSiMe3)] affording [Re(NtBu)2Cax(O)3(OMe)] (13), for which the methoxy ether group protudes into the calixarene cavity. The compounds 1–13 have been structurally characterised by single-crystal X-ray diffraction (synchrotron radiation was used for 4, 6, 7, 9, 11–13).