CH Activation and CC Double Bond Formation Reactions in Iridium ortho‐Methyl Arylphosphane Complexes

Abstract

The Vaska-type iridium(I) complex [IrCl(CO){PPh(2)(2-MeC(6)H(4))}(2)] (1), characterized by an X-ray diffraction study, was obtained from iridium(III) chloride hydrate and PPh(2)(2,6-MeRC(6)H(3)) with R=H in DMF, whereas for R=Me, activation of two ortho-methyl groups resulted in the biscyclometalated iridium(III) compound [IrCl(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}(2)] (2). Conversely, for R=Me the iridium(I) compound [IrCl(CO){PPh(2)(2,6-Me(2)C(6)H(3))}(2)] (3) can be obtained by treatment of [IrCl(COE)(2)](2) (COE=cyclooctene) with carbon monoxide and the phosphane in acetonitrile. Compound 3 in CH(2)Cl(2) undergoes intramolecular C-H oxidative addition, affording the cyclometalated hydride iridium(III) species [IrHCl(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}{PPh(2)(2,6-Me(2)C(6)H(3))}] (4). Treatment of 2 with Na[BAr(f) (4)] (Ar(f)=3,5-C(6)H(3)(CF(3))(2)) gives the fluxional cationic 16-electron complex [Ir(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}(2)][BAr(f) (4)] (5), which reversibly reacts with dihydrogen to afford the delta-agostic complex [IrH(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}{PPh(2)(2,6-Me(2)C(6)H(3))}][BAr(f)(4)] (6), through cleavage of an Ir-C bond. This species can also be formed by treatment of 4 with Na[BAr(f)(4)] or of 2 with Na[BAr(f)(4)] through C-H oxidative addition of one ortho-methyl group, via a transient 14-electron iridium(I) complex. Heating of the coordinatively unsaturated biscyclometalated species 5 in toluene gives the trans-dihydride iridium(III) complex [IrH(2)(CO){PPh(2)(2,6-MeC(6)H(3)CH=CHC(6)H(3)Me-2,6)PPh(2)}][BAr(f) (4)] (7), containing a trans-stilbene-type terdentate ligand, as result of a dehydrogenative carbon-carbon double bond coupling reaction, possibly through an iridium carbene species.

Reactivity of Cationic Agostic and Carbene Structures Derived from Platinum(II) Metallacycles

“…The new complex is a cationic hydride derivative that contains a tridentate diphosphine–alkene ligand that results from a dehydrogenative C–C coupling reaction between two benzylic groups at the coordinated phosphines in 4 a . Similar metal‐mediated rearrangements of ligands that possess benzylic hydrogen atoms were observed by the groups of Bennett,38 Wilkinson,39 and Baratta 7b,f. Recently, some of us demonstrated the formation of phosphepine ligands by similar intramolecular CC coupling reactions of PMeXyl 2 and related phosphines in metallacyclic complexes of the Ir III (η 5 ‐C 5 Me 5 ) fragment 9a,b…”

Agostic bonds in cyclometalation

“…When a solution of the cationic complex 39 in CD 2 Cl 2 is treated with hydrogen at room temperature, almost quantitative formation of agostic monohydride 40 is observed as inferred from NMR measurements, e.g., the hydride signal, which is trans to the agostic methyl, is shifted to d ¼ À22.35 ppm at À80 C, whereas the other resonances are only slightly affected by the temperature (Eq. (37)) [77].…”

“…The other articles regarding the agostic bonds in cyclometalation with phosphorus-containing compounds have been published on cyclohexylphosphines [75], ferrocenylphosphines [76], triethylphosphites [77] and tri-ortho-tolylphosphines [78], etc.…”

“…The reactions of phosphorus-containing compounds such as 2,6-dimethylphenylphosphines [69e72], iso-butylphosphines [73,74], cyclohexylphosphines [75], ferrocenylphosphines [76], triethylphosphites [77] and tri-ortho-tolylphosphines [78] have been reported. …”

Cobalt, rhodium and iridium

“…Some notable highlights include the synthesis and structural characterisation of the first two-coordinate Co(II) complex with s-bonded aryl ligands, 7 a novel Co carbonyl N-heterocyclic carbene (NHC) salt, 46 the use of a simple organometallic Rh(III) dimer for sensing of nucleotides, 44 impressive anticancer activity of a Rh(III) thiaether complex 63 and cyclometalation reactions in Ir(I) complexes bearing 2-Me substituted phosphines. 80…”