Computational and Experimental Test of Steric Influence on Agostic Interactions:  A Homologous Series for Ir(III)

Abstract

Chloride abstraction (using NaBAr‘4, Ar‘ = 3,5-(CF3)2C6H3) from Ir(H)2Cl(PtBu2Ph)2 gives cis,trans-Ir(H)2(PtBu2Ph)2 +, which has two agostic interactions with methyl C−H groups on different tBu groups. The molecule exists as diastereomers, due to stereochemistry at P. Chloride can be similarly abstracted from ortho-metalated IrH(η2-C6H4PtBu2)Cl(PtBu2Ph) to give square-pyramidal IrH(η2-C6H4PtBu2)(PtBu2Ph)+, which has only one agostic interaction, involving a tBuC−H bond; steric constraints on each phosphine leave no more C−H bonds available to donate to the remaining empty Ir(III) orbital. The smaller ligand PCy2Ph yields only the tris-phosphine complex Ir(H)2(PCy2Ph)3 +, and this is shown to have a square-pyramidal structure with one agostic cyclohexyl group and large Pax−Ir−P angles (104−106°). The analogous Ir(H)2(PiPr2Ph)3 + has similar inter-phosphorus angles, but no agostic interaction. Geometrical optimization of IrH2L3 + (PCy2Ph, PiPr2Ph) with the hybrid quantum mechanics/molecular mechanics (QM/MM) method (IMOMM) at the IMOMM (B3LYP:MM3) and IMOMM (MP2:MM3) levels permits a more detailed understanding of the influence of steric factors on the occurrence of an agostic bond. The MP2/MM3 method gives the results in closer agreement with experiment. Steric factors place the agostic bond in the vicinity of the metal center but at a distance that is too long to be considered as bonding. The electron-donating ability of the C−H bond and the electron accepting capacity of the metal center, which are introduced only at the QM level, bring the two partners in a bonding situation.

Rhodium and Iridium Aminoborane Complexes: Coordination Chemistry of BN Alkene Analogues

“…[16] The metal-catalyzed dehydrogenation of ammonia borane and related amine derivatives has been the subject of intense recent study, both with a view to unlocking the potential of such systems as hydrogen storage media, and with the aim of developing new Group 13/Group 15 materials by dehydrocoupling methodologies. [22] We have therefore investigated the reactivity of [M(IMes) 2 (H) 2 Cl] (M = Rh (1 a), Ir (1 b); IMes = N,N-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) [23] towards the potent halide abstraction agent Na-[BAr F 4 ] (Ar F = 3,5-C 6 H 3 (CF 3 ) 2 ) with a view to generating reactive cationic bis(NHC) systems capable of coordinating aminoborane molecules. [18,19] The monomeric nature of R 2 NBH 2 (R = iPr, Cy), [18a, 20] on the other hand, led us to consider the in situ rhodium-or iridiumcatalyzed dehydrogenation of iPr 2 HN .…”

“…[22] We have therefore investigated the reactivity of [M(IMes) 2 (H) 2 Cl] (M = Rh (1 a), Ir (1 b); IMes = N,N-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) [23] towards the potent halide abstraction agent Na-[BAr F 4 ] (Ar F = 3,5-C 6 H 3 (CF 3 ) 2 ) with a view to generating reactive cationic bis(NHC) systems capable of coordinating aminoborane molecules. [22] We have therefore investigated the reactivity of [M(IMes) 2 (H) 2 Cl] (M = Rh (1 a), Ir (1 b); IMes = N,N-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) [23] towards the potent halide abstraction agent Na-[BAr F 4 ] (Ar F = 3,5-C 6 H 3 (CF 3 ) 2 ) with a view to generating reactive cationic bis(NHC) systems capable of coordinating aminoborane molecules.…”

Alkyl dehydrogenation in a Rh(i) complex via an isolated agostic intermediate

“…Complex 1 spontaneously, 6,16 but slowly (t 1/2 B 1.5 h), undergoes dehydrogenation of one of the isopropyl substituents in solution at room temperature (C 6 15 Due to this process and the structural similarity of 4, isolation of 1 by crystallisation at low temperature occurs concomitant with small amounts of 4 (1:4 B 4:1 on the basis of integrals in the 1 H NMR spectrum and X-ray diffraction). The dehydrogenation/hydrogenation equilibrium in solution is shifted towards 4 by successive removal of hydrogen through freezevacuum-thaw cycles, while addition of the hydrogen acceptor t-butylethene (tbe) results in the clean formation of 4 (86% isolated yield).…”

“…Although this is contrary to what is observed in the solid-state structure our model does not take into account the effects of ligand bulk which can play a significant role in determining the nature of an agostic interaction. 15 In addition solid-state packing effects may play a role in favouring one agostic form over the other. 1 0 b and 1 0 c are linked via 1 0 an (E = 2.9 kcal mol À1 ), the highest point along this pathway being loss of the b-CH agostic via a TS at 5.9 kcal mol À1 .…”

Agostic bonds in cyclometalation

“…The molecular structure of the product 42 reveals two Rh/HeC agostic interactions trans to the hydride ligands, Rh/C ¼ 2.90 Å, 2.89 Å, and is closely related to a "14-electron" iridium bis-phosphine and an N-heterocyclic carbene hydride reported by Caulton (Ir/C ¼ 2.754 Å)[81] and Nolan (Ir/C ¼ 2.653 Å)[82] as shown in Eq. (39)[74].…”