Encapsulation of the 4-Mercaptobenzoate Ligand by Macrocyclic Metal Complexes: Conversion of a Metallocavitand to a Metalloligand.

Complexation of the ambidentate ligand 4-mercaptobenzoate (4-SH-C6H4CO2H, H2mba) by the macrocyclic complex [Ni2L(μ-Cl)]ClO4 (L2- represents a 24-membered macrocyclic hexaazadithiophenolate ligand) has been examined. The monodeprotonated Hmba- ligand reacts with the Ni2 complex in a selective manner by substitution of the bridging chlorido ligand to produce μ1,3-carboxylato-bridged complex [Ni2L(Hmba)]+ (2+), which can be isolated as an air-sensitive perchlorate (2ClO4) or tetraphenylborate (2BPh4) salt. The reactivity of the new mercaptobenzoate complex is reminiscent of that of a "free" thiophenolate ligand. In the presence of air, 2ClO4 dimerizes via a disulfide bond to generate tetranuclear complex [{Ni2L}2(O2CC6H4S)2]2+ (32+). The auration of 2ClO4 with [AuCl(PPh3)], on the other hand, leads to monoaurated complex [Ni2IIL(mba)AuIPPh3]+ (4+). The bridging thiolate functions of the N6S2 macrocycle are deeply buried and are unaffected/unreactive under these conditions. The complexes were fully characterized by electrospray ionization mass spectrometry, IR and UV/vis spectroscopy, density functional theory, cyclic voltammetry, and X-ray crystallography [for 3(BPh4)2 and 4BPh4]. Temperature-dependent magnetization and susceptibility measurements reveal an S = 2 ground state that is attained by ferromagnetic coupling between the spins of the NiII ions in 2ClO4 (J = +22.3 cm-1) and 4BPh4 (J = +20.8 cm-1; H = -2JS1S2). Preliminary contact-angle and X-ray photoelectron spectroscopy measurements indicate that 2ClO4 interacts with gold surfaces. [ABSTRACT FROM AUTHOR]