Abstract
The interaction of free-base triarylcorroles with Re2(CO)10 in 1,2-dichlorobenzene in the presence of 2,6-lutidine at 180 °C under strict anerobic conditions afforded approximately 10% yields of rhenium corrole dimers. The compounds exhibited diamagnetic 1H NMR spectra consistent with a metal–metal quadruple bond with a σ2π4δ2 orbital occupancy. One of the compounds proved amenable to single-crystal X-ray structure determination, yielding a metal–metal distance of ∼2.24 Å, essentially identical to that in triple-bonded osmium corrole dimers. On the other hand, the electrochemical properties of Re and Os corrole dimers proved to be radically different. Thus, the reduction potentials of the Re corrole dimers are some 800 mV upshifted relative to those of their Os counterparts. Stated differently, the Re corrole dimers are dramatically easier to reduce, reflecting electron addition to δ* versus π* molecular orbitals for Re and Os corrole dimers, respectively. The data also imply electrochemical HOMO-LUMO gaps of only 1.0–1.1 V for rhenium corrole dimers, compared with values of 1.85–1.90 V for their Os counterparts. These HOMO–LUMO gaps rank among the first such values reported for quadruple-bonded transition-metal dimers for any type of supporting ligand, porphyrin-type or not.
Highlights
The interpretation of the very short Re−Re distance in the [Re2Cl8]2− dianion[1,2] as indicative of a metal−metal quadruple bond by Cotton in 1965 stands as a landmark in the history of chemical bonding.[3−7] The novel feature of such a bond is a δorbital interaction, in addition to a σ and two π interactions
Studies of molybdenum and tungsten porphyrin dimers provided some of the first estimates of the strength of the δ interaction.[10−12] Likewise, resonance Raman studies of molybdenum, rhenium, and osmium porphyrin dimers provided some of the first insights into the vibrational characteristics of metal−metal multiple bonds.[13]
Attempted derivatization of an ReCl2 Viking helmet corrole with PhMgBr failed to yield the expected RePh2 product; mass spectrometric (MS) analysis of the products instead showed the presence of small quantities of what appeared to be a rhenium corrole dimer (Scheme 1)
Summary
The interpretation of the very short Re−Re distance in the [Re2Cl8]2− dianion[1,2] as indicative of a metal−metal quadruple bond by Cotton in 1965 stands as a landmark in the history of chemical bonding.[3−7] The novel feature of such a bond is a δorbital interaction, in addition to a σ and two π interactions. It became clear that the δ interaction makes only a small contribution to the metal−metal interaction energy and has next to no effect on the metal−metal distance.[8] the δ interaction has major implications for many physicochemical properties and especially for redox chemistry. (a) How much are typical δ−δ* transition energies, especially as a function of different metals? (b) How much are typical singlet−triplet gaps? In spite of sustained attention over decades, significant questions remain relative to the energetics of δ bonds. (a) How much are typical δ−δ* transition energies, especially as a function of different metals? (b) How much are typical singlet−triplet gaps? (c) What about the electrochemical
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