Mixed N‑Heterocyclic Carbene–Bis(oxazolinyl)borato Rhodium and Iridium Complexes in Photochemical and Thermal Oxidative Addition Reactions

In
order to facilitate oxidative addition chemistry of fac-coordinated rhodium­(I) and iridium­(I) compounds, carbene–bis­(oxazolinyl)­phenylborate
proligands have been synthesized and reacted with organometallic precursors.
Two proligands, PhB­(OxMe2)2(ImtBuH) (H­[1]; OxMe2 = 4,4-dimethyl-2-oxazoline;
ImtBuH = 1-tert-butylimidazole)
and PhB­(OxMe2)2(ImMesH) (H­[2]; ImMesH = 1-mesitylimidazole), are deprotonated
with potassium benzyl to generate K­[1] and K­[2], and these potassium compounds serve as reagents for the synthesis
of a series of rhodium and iridium complexes. Cyclooctadiene and dicarbonyl
compounds {PhB­(OxMe2)2ImtBu}­Rh­(η4-C8H12) (3), {PhB­(OxMe2)2ImMes}­Rh­(η4-C8H12) (4), {PhB­(OxMe2)2ImMes}­Rh­(CO)2 (5), {PhB­(OxMe2)2ImMes}­Ir­(η4-C8H12) (6), and {PhB­(OxMe2)2ImMes}­Ir­(CO)2 (7) are synthesized along with ToMM­(η4-C8H12) (M = Rh (8); M
= Ir (9); ToM = tris­(4,4-dimethyl-2-oxazolinyl)­phenylborate).
The spectroscopic and structural properties and reactivity of this
series of compounds show electronic and steric effects of substituents
on the imidazole (tert-butyl vs mesityl), effects
of replacing an oxazoline in ToM with a carbene donor,
and the influence of the donor ligand (CO vs C8H12). The reactions of K­[2] and [M­(μ-Cl)­(η2-C8H14)2]2 (M
= Rh, Ir) provide {κ4-PhB­(OxMe2)­2ImMes′CH2}­Rh­(μ-H)­(μ-Cl)­Rh­(η2-C8H14)2 (10) and {PhB­(OxMe2)2ImMes}­IrH­(η3-C8H13) (11). In the former
compound, a spontaneous oxidative addition of a mesityl ortho-methyl to give a mixed-valent dirhodium species is observed, while
the iridium compound forms a monometallic allyl hydride. Photochemical
reactions of dicarbonyl compounds 5 and 7 result in C–H bond oxidative addition providing the compounds {κ4-PhB­(OxMe2)2ImMes′CH2}­RhH­(CO) (12) and {PhB­(OxMe2)2ImMes}­IrH­(Ph)­CO (13).
In 12, oxidative addition results in cyclometalation
of the mesityl ortho-methyl similar to 10, whereas the iridium compound reacts with the benzene solvent to
give a rare crystallographically characterized cis-[Ir]­(H)­(Ph) complex. Alternatively, the rhodium carbonyl 5 or iridium isocyanide {PhB­(OxMe2)2ImMes}­Ir­(CO)­CNtBu (15)
reacts with PhSiH3 in the dark to form the silyl compound
{PhB­(OxMe2)2ImMes}­RhH­(SiH2Ph)­CO (14) or {PhB­(OxMe2)2ImMes}­IrH­(SiH2Ph)­CNtBu (17). These examples demonstrate the enhanced
thermal reactivity of {PhB­(OxMe2)2ImMes}-supported iridium and rhodium carbonyl compounds in comparison to tris­(oxazolinyl)­borate,
tris­(pyrazolyl)­borate, and cyclopentadienyl-supported compounds.