Synthesis, Reactivity, and Fluxional Behaviour of [Ir2Rh2(CO)12], and Crystal Structure of [Ir2Rh2(CO)8(norbornadiene)2]

Merbach

et al. 1994

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The reaction of [Ir2Rhz(CO)lz] with 1 mol-equiv. of PPh; yields [Ir,Rh,(CO),,PPh,] (1) as a mixture of two isomers with the phosphine ligand axially bound either to one basal Rh-atom in the kinetically preferred isomer 1R or to one basal Ir-atom in the thermodynamically preferred isomer 11. Both isomers are fluxional on the "C-NMR time scale at low temperature due to CO scrambling. Around room temperature, a new type of fluxional process starts to operate which is responsible for the isomerisation 1R F=? 11, i.r. the intramolecular migration of the reputedly inert PPh, ligand from one metal centre to another. The activation volumes ofconversions 1R + 11 and 11 + 1R are both positive, indicating that the migration of PPh, is dissociative in character. This article reports the first application of variable pressure 3'P-NMR to mechanistic studies. We report here on a new type of intramolecular rearrangement, distinct from CO scrambling, which was identified by a variable-temperature and variable-pressure "P-NMR study of the cluster compound [Ir2Rh,(CO),,PPh,] (1).

mentioning

confidence: 74%

Variable‐Temperature and ‐Pressure ³¹P‐NMR Study of the Intramolecular PPh₃ Migration in the Cluster Compound [Ir₂Rh₂(CO)₁₁PPh₃]

Laurenczy

Merbach

et al. 1994

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“…Substitution of CO by monodentate ligands gave [lr,Rh(CO)&,-CO),L] (L = Br -, 2; I -, 3; bicyclo 4;PPh,,5). Clusters 2-5 have C, symmetry with the ligand L bound to the basal Rh-atom in axial position.…”

Section: Synthesis Of [Irrh(co)] and Fluxional Behaviour Of Some Omentioning

confidence: 99%

“…~ The tetrahedral carbonyl cluster compounds of mixed d ' metals known to date are [Co,Rh,-,(CO),,] (x = 2, 3) [l] [2], [Co,Ir,(CO),,] [l] [3], and [Ir, Rh,-,(CO),,] (x = 1, 2) [l] [4]. [CoRh,(CO),,] and [CoIr,(CO),,] are thermally unstable [l] [5], and [Ir, Rh(CO),,] could not be obtained by standard redox condensation of two monomeric complexes of Ir-' and Rh"' or by the reaction of [Rh(CO)J with [1r4(CO),,].…”

Section: Synthesis Of [Irrh(co)] and Fluxional Behaviour Of Some Omentioning

confidence: 99%

“…These results, as well as those concerning 2 and 3, show the necessity of referring to 2D-EXSY data to distinguish parallel, localised processes in cluster compounds from scrambling processes involving all metal atoms. The merry-go-round of the six basal CO's in [Ir4(CO)6@2-CO)3( 1,3,5-trithiane)] and [Ir,Rh2(CO),@,-C0),(1,3,5trithiane)] have a AG* (at 298 K) of 38.0 f 0.5 [14] and > 68 kJ mol-' [4], respectively. Therefore, substitution of Ir by Rh increases the activation energy of the merry-go-round.…”

Section: IImentioning

confidence: 99%

“…The same trend is also observed when comparing the A c t ' s of CO rotation in 6 (49. [4]). This suggests that some electron density in the Ir(apica1)-M(basa1) bonds is shifted towards the basal plane when M = Rh.…”

Section: IImentioning

confidence: 99%

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Synthesis of [Ir₃Rh(CO)₁₂] and Fluxional Behaviour of Some of Its Substituted Derivatives

Laurenczy

Ros

et al. 1994

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The redox condensation of [Ir(C0)4] -, [Ir(cod)(THF),] +, and [Rh(cod)(THF),] + (cod = cycloocta-1,5-diene) followed by saturation with CO (1 atm) in THF afforded the first synthetic route to pure [Ir,Rh(CO),,] (1). Substitution of CO bymonodentate ligands gave [lr,Rh(CO)&,-CO),L] (L = Br -, 2; I -, 3; bicyclo[2.2. Ilhept-2ene, 4; PPh,, 5). Clusters 2-5 have C, symmetry with the ligand L bound to the basal Rh-atom in axial position. They are fluxional in solution at the NMR time scale due to two CO scrambling processes: the merry-go-round of basal CO's and changes of basal face. An additional process takes place in 5 above room temperature: the intramolecular migration of PPh, from the Rh-to a basal Ir-atom. Substitution of CO by polydentate ligands gave [Ir3Rh(C0)7-,&2-C0)3(q4-L).J ( L = bicyclo[2.2.l]hepta-2,5-diene (= norbornadiene; nbd), x = 1, 6 ; L = nbd, x = 2, 13; L =cod, x = I, 7; L = cod .x = 2, 15), [Ir,Rh(C0)7&2-CO)j(q2-diars)] (diars = 1,2phenylenebis-(dimethylarsine); 8), [Ir3Rh(CO),&,-CO),(q4-L)] (L = methylenebis(diphenylphosphine), bonded to 2 basal Ir-atom (9a) or one Ir-and one Rh-atom (9b)), [1r,Rh(CO)&~~-CO)~(q~-nbd)PPh,] (121, and [Ir,Rh(CO),(uz-CO),(uci-L)] (L = 1,3,5-trithiane, 10; L = CH (PPh2)?, 11). Complexes M , 9 a , 10, and 11 have C; symmetry, the others C , symmetry. They are fluxional in solution due to CO scrambling processes involving I, 3, or 4 metal centres as deduced from 2D-EXSY spectra. Comparison of the activation energies of these processes with those of the isostructural Ir, and Ir,Rh, compounds showed that substitution of Ir by Rh in the basal face of an Ir, compound slows the processes involving 3 or 4 metal centres (merry-go-round and change of basal face), but increases the rate of carbonyl rotation about an Ir-atom.

Intramolecular Dynamics of Tetranuclear Iridium Carbonyl Cluster Compounds. Part V. Polysubstituted derivatives of [Ir₄(CO)₁₂]

Strawczynski

Hall

et al. 1994

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The dynamic behaviour of twelve polysubstituted derivatives of [Ir4(CO) has been investigated in solution, using 2D-EXSY, and VT-,'P-and I3C-NMR. [Ir,(C0)6(u2-CO),(r14-diarSine)PPh,] and [Ir4(CO)6(u2-CO)3(q4-norbornadiene)(PMePh2)] exhibit two isomeric forms in solution, which interconvert through an intramolecular change of basal Face. The related cluster [Ir4(CO)60(2-CO),(~4-norbornadiene)PPh,] exists as a single isomer in solution. It displays rotation of CO ligands about the apical Ir-atom, followed by two consecutive changes of basal face. The tetrasubstituted clusters with two chelating ligands [Ir4(CO)sb ,-CO),(~4-diolefin),] also exhibit rotation of apical CO's, the activation energy increases with greater steric hindrance of the radial ligands. A quantitative analysis of the 31P-and I3C-2D-EXSY spectra followed by simulation of the corresponding VT-NMR spectra of [Ir4(CO)s(u ,-CO),(u *-L)*] (L = bis(dipheny1phosphino)methane and 1,3-bis(diphenyIphosphino)propane) revealed a pairwise averaging of the P-atoms, caused by two parallel changes of basal face averaging all CO ligands. In addition, the restricted rotation of ligands about the apical Ir-atom was identified at higher temperatures. The remaining clusters are either rigid on the NMR time scale, or display CO-scrambling about a single Ir-atom.