Open-shell organometallics: reactivity at the ligand

Abstract

The purpose of this review is to show that (cooperative) ligand radical reactivity can be effectively employed in synthetic organometallic chemistry and catalysis to achieve selectivity in radical-type transformations. The 'redox non-innocence' of ligands, and the controlled reactivity of 'ligand radicals' -giving rise to new, intriguing substrate transformations -allow unusual and selective radical-type substrate coupling reactions, ligand rearrangements and C-Y bond formations. In this review, several examples of fast and selective ligand-centered radical transformations in the open-shell organometallic chemistry of transition metals are described, focussing on radical-type reactions of olefin-, vinylidene-, vinyl-, alkyne-, allyl-, propargyl-, carbonyl-, and carbene ligands. Intriguing and selective substratesubstrate coupling reactions, (covalent) carbon-metal bond formations and hydrogen atom transfer reactions from and to ligand radicals are summarized. To conclude this chapter, an overview of recently disclosed new insights in the catalytic mechanism of Co II (por) catalysed olefin cyclopropanation is presented, showing that enzyme-like cooperative metal-ligand-radical reactivity is no longer reserved to real enzymes, but is a useful new concept to steer and control radical-type transformations in future bio-inspired organometallic catalysis.

Oxidative Additions to Ti(IV) in [(dadi)4–]TiIV(THF) Involve Carbon–Carbon Bond Formation and Redox-Noninnocent Behavior

“…Carbon–carbon bond formation is often a consequence of redox noninnocence (RNI) that induces radical character in certain ligands. − In many cases, a relatively simple coupling of carbon radicals occurs to form a single new bond, but sometimes the reactivity can be complicated. Figure illustrates some unusual C–C bond forming events discovered in these laboratories. − In A , the Ti­(III) complex, (smif)­{Li­(smif–smif)}­Ti, was shown to contain one C–C bond derived from coupling of smif (smif = 1,3-di­(2-pyridyl)-2-azaallyl) ligands that were reduced during the course of its synthesis.…”

Complexes of [(dadi)Ti(L/X)]m That Reveal Redox Non-Innocence and a Stepwise Carbene Insertion into a Carbon–Carbon Bond

“…The formation of a cyclopropanated backbone of (dadi) n suggested that reversible C–C bond formation could be a means of storing and releasing electrons in a redox noninnocent (RNI) fashion. ,− A possible hindrance to RNI is the transfer of a cyclopropane hydrogen subsequent to ring-opening, which would generate a nacnac fragment in place of the original diimine. The reactivity of {PhC 3 H 3 (-NC 6 H 4 -2-NAr) 2 }­Ti­(THF) ( 3 ) was probed via various potential oxidants and proton acceptors, and while most generated mixtures, the addition of Ph 2 CN 2 yielded a single product (>90%) that was dissymmetric.…”

“…Attempts to generate (dadi)­TiCRR′ have failed, and according to calculations, the generation of a stable alkylidene may not be possible since the RR′C: fragment is not oxidizing enough to pull electrons from (dadi) 4– . As indicated in Figure ., calculations on a benzylidene portray it as a diradical species , best considered (dadi· ↑ ) 3– Ti IV (C· ↓ (H)­Ph) ( 2 -C·HPh). Figure illustrates the calculated geometries of 2 -C·HPh which show the d (CN im ) to be 1.33 Å, and d (CC) = 1.39 Å, values that are between those found for dianionic ( n = −2) and tetraanionic ( n = −4) forms of (dadi) n .…”

Redox Chemistry of Bis(oxazolinyl)cyclopentadienyl and -fluorenyl Rhodium and Iridium Organometallic Compounds

“…Monomeric Rh II and Ir II species are typically supported by multidentate ligands with heteroatom coordination . For example, nitrogen donor ligands such as tetramesitylporphyrin (Chart C) , and bis­(oxazolines) support square-planar Rh II .…”

“…13 Monomeric Rh II and Ir II species are typically supported by multidentate ligands with heteroatom coordination. 14 For example, nitrogen donor ligands such as tetramesitylporphyrin (Chart 1C) 10,11 and bis(oxazolines) support square-planar Rh II . 15 Pincer ligands also stabilize divalent, square-planar rhodium and iridium as in [(PNP)Rh II X] 16 (PNP = (4-Me-2-(iPr 2 P)-C 6 H 3 ) 2 N; X = Cl, OR) and [(PNP)Ir II X] (PNP = (tBu 2 PCHCH) 2 N; X = Cl, N 3 ).…”