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 [NiII2L­(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.