Pt–Mg, Pt–Ca, and Pt–Zn Lantern Complexes and Metal-Only Donor–Acceptor Interactions

Pt-based
heterobimetallic lantern complexes of the form [PtM­(SOCR)4(L)] have been shown previously to form intermolecular metallophilic
interactions and engage in antiferromagnetic coupling between lanterns
having M atoms with open shell configurations. In order to understand
better the influence of the carboxylate bridge and terminal ligand
on the electronic structure, as well as the metal–metal interactions
within each lantern unit, a series of diamagnetic lantern complexes,
[PtMg­(SAc)4(OH2)] (1), [PtMg­(tba)4(OH2)] (2), [PtCa­(tba)4(OH2)] (3), [PtZn­(tba)4(OH2)] (4), and a mononuclear control (Ph4P)2[Pt­(SAc)4] (5) have been synthesized.
Crystallographic data show close Pt–M contacts enforced by
the lantern structure in each dinuclear case. 195Pt-NMR
spectroscopy of 1–4, (Ph4P)2[Pt­(SAc)4] (5), and several
previously reported lanterns revealed a strong chemical shift dependence
on the identity of the second metal (M), mild influence by the thiocarboxylate
ligand (SOCR; R = CH3 (thioacetate, SAc), C6H5 (thiobenzoate, tba)), and modest influence from the
terminal ligand (L). Fluorescence spectroscopy has provided evidence
for a Pt···Zn metallophilic interaction in [PtZn­(SAc)4(OH2)], and computational studies demonstrate significant
dative character. In all of 1–4,
the short Pt–M distances suggest that metal-only Lewis donor
(Pt)–Lewis acceptor (M) interactions could be present. DFT
and NBO calculations, however, show that only the Zn examples have
appreciable covalent character, whereas the Mg and Ca complexes are
much more ionic.