Sulfido-Bridged Tetranuclear Titanium−Iridium Complexes with an Unconventional Tetrahedral Iridium Center

A general route to the complexes [CpTi(μ3-S)3M3(diolefin)3] (M = Rh, diolefin = cod, nbd,
tfbb; M = Ir, diolefin = cod) consists of the reactions of the anion [Cp2Ti2(μ-S)2(S)2]2-, obtained
by mono-deprotonation of Cp2Ti(SH)2 with butyllithium in THF, with the appropriate
complexes [{M(μ-Cl)(diolefin)}2]. Replacement of the diolefin by carbon monoxide in [CpTi(μ3-S)3M3(diolefin)3] gives the carbonyl derivatives [CpTi(μ3-S)3M3(CO)6]. Further reactions
of the carbonyliridium complex with tertiary phosphine and phosphite ligands produce the
62-e valence clusters [CpTi(μ3-S)3Ir3(μ-CO)(CO)3(PR3)3] (PR3 = PPh3, PMe3, P(OMe)3,
PMePh2). Remarkable features in the structure of these compounds, as found for [CpTi(μ3-S)3Ir3(μ-CO)(CO)3{P(OMe)3}] by X-ray diffraction studies, are a distorted tetrahedral metal
framework with short Ir−Ir distances and a tetrahedral coordination of the iridium atom
closest to the titanium. A delocalized bonding scheme can be proposed for the iridium triangle
and, at least, an interaction between the tetrahedral iridium and the titanium atom. The
complexes [CpTi(μ3-S)3Ir3(μ-CO)(CO)3(PR3)3] quickly exchange all their carbonyl ligands with
13CO under normal conditions to give the labeled complexes. Indeed, an equilibrium between
the compounds [CpTi(μ3-S)3Ir3(μ-13CO)(13CO)3(PR3)3] and [CpTi(μ3-S)3Ir3(μ-13CO)(13CO)4(PR3)2]
and free phosphine exists under an atmosphere of 13CO. In addition, these compounds are
fluxional, since they exhibit a single carbonyl resonance in the low-temperature 13C{1H}
NMR spectrum, probably due to very fast carbonyl scrambling. Furthermore, the reaction
of [CpTi(μ3-S)3Ir3(μ-CO)(CO)3(PPh3)3] with HBF4 gives the unsymmetric and nonfluxional
hydride of formula [CpTi(μ3-S)3Ir3(μ-CO)(H)(CO)3(PPh3)3]BF4.