Detection of an Iridium–Dihydrogen Complex: A Proposed Intermediate in Ionic Hydrogenation

H-atom site exchange in an iridium trans-dihydrogen/hydride complex, trans-[Ir(H)(η2-H2)(iPr)4(POCOP)(DMAP)]. Dalton Trans 2023;52:13858-13863. [PMID: 37743752 DOI: 10.1039/d3dt02366d] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Treatment of trans-[Ir(H)(N)2(iPr)4(POCOP)(DMAP)][BAr4f] (2) with H2 (1 bar) under ambient conditions (298 K) results in the formation of a trans-[Ir(H)(η2-H2)(iPr)4(POCOP)(DMAP)][BAr4f] (3) complex. Complex 3 exhibits H-atom site exchange between the bound H2 and the hydride ligands which are mutually trans to one another. A plausible mechanism of this exchange involves metal-ligand cooperativity as studied by computations.

Ruthenium Complexes of a Triphosphorus-Coordinating Pincer Ligand: Ru-P Ligand-Substituent Exchange Reactions Driven by Large Variations of Bond Energies

The reaction of [(p-cymene)RuCl₂]₂ with the triphosphine ligand bis(2-di-tert-butylphosphinophenyl)phosphine (^tBuP^HPP) results in an unusual exchange reaction in which a chloride ligand and a phosphorus-bound H atom are exchanged ("H-P/Ru-Cl exchange") to give the (chlorophosphine)ruthenium hydride complex (^tBuP^ClPP)RuHCl [1^Cl-HCl; ^tBuP^ClPP = bis(2-di-tert-butylphosphinophenyl)chlorophosphine]. Density functional theory calculations indicate that the presumed initial product of metalation, (^tBuP^HPP)RuCl₂ (1^H-Cl2), undergoes an H-P/Ru-Cl exchange via sequential P-to-Ru α-H migration to give the intermediate (^tBuPPP)RuHCl₂, followed by Ru-to-P α-Cl migration to give the observed product 1^Cl-HCl (crystallographically characterized). Dehydrochlorination of 1^Cl-HCl under a H₂ atmosphere gives (^tBuP^ClPP)RuH₄ (1^Cl-H4), which then can undergo a second dehydrochlorination and addition of H₂ to give (^tBuP^HPP)RuH₄ (1^H-H4). This reaction may proceed via the reverse of the intramolecular exchange by 1^H-Cl2, i.e., loss of H₂ from 1^Cl-H4 to give 1^Cl-H2, which could undergo Cl-P/Ru-H exchange to give (^tBuP^HPP)RuHCl (1^H-HCl). Accordingly, the thermodynamics of Cl-P/Ru-H exchange are found to be highly dependent on the nature of the ancillary anionic ligand (H or Cl), which is not directly involved in the exchange. The origin of this thermodynamic dependence can be explained in terms of the high stability of complexes (^RP^XPP)RuHCl (X = H, Cl; R = Me, ^tBu), in which the hydride is approximately trans to a vacant coordination site and the central phosphine group is approximately trans to the weak-trans-influence chloride ligand. This conclusion has general implications for five-coordinate d⁶ complexes, both pincer- and nonpincer-ligated.

Cp* non-innocence and the implications of (η4-Cp*H)Rh intermediates in the hydrogenation of CO2, NAD+, amino-borane, and the Cp* framework - a computational study

In hydrogenation mediated by half-sandwich complexes of Rh, Cp*Rh(III)-H intermediates are critical hydride-delivery agents. For bipyridine-supported complexes, a unique transformation named 'Cp* non-innocence' leads to the appearance of (Cp*H)Rh(I) intermediates, which are purported to exhibit enhanced hydride-delivery capabilities. In this work, DFT calculations performed to compare the role of these complexes in hydrogenation reveal that (Cp*H)Rh(I) intermediates do not compete with the conventional pathway (involving Cp*Rh(III)-H); instead they can lead to sequential hydrogenation of the Cp* framework, and potentially, catalyst degradation. Thus, caution is warranted when invoking the truly homogeneous nature of hydrogenation catalysis mediated by this popular class of complexes.

Synthesis, stability, and reactivity of mesoionic carbene iridium dihydride complexes

Pyridyl-triazolylidene ligands with variable donor properties were used as tunable ligands at a dihydride iridium (III) center. The straightforward synthesis of this type of ligand allows for an easy incorporation of electron-donating substituents in different positions of the pyridine ring or different functional groups such as esters, alkoxy, or aliphatic chains on the C4 position of the triazole heterocycle. The stability of these hydride metal systems allowed these complexes to be used as models for studying the influence of the ligand modifications on hydride reactivity. Spectroscopic analysis provided unambiguous structural assignment of the dihydride system. Modulation of the electronic properties of the wingtip substituents did not appreciably alter the reactivity of the hydrides. Reactivity studies using acids with a wide range of pK a values indicated a correlation between hydride reactivity and acidity and showed exclusive reactivity towards the less shielded hydride trans to the carbene carbon rather than the more shielded hydride trans to the pyridine ring, suggesting that the trans effect is more relevant in these reactions than the NMR spectroscopically deduced hydridic character.

Chemoselective and efficient catalytic hydrogenation of furfural by iridium and ruthenium half-sandwich complexes

A recyclable homogeneous iridium complex for the selective synthesis of furfuryl alcohol from furfural without additional solvent and hydrogen gas.

Predicted M(H2)12n+ (M = Ac, Th, Pa, U, La and n = 3, 4) complexes with twenty-four hydrogen atoms bound to the metal ion

Herein, we have shown that La(iii), Ac(iii), Th(iii), Th(iv), Pa(iv) and U(iv) can directly bind with a maximum of 24 hydrogen atoms in M(H2)12 in the first sphere of coordination, which would be a new record in any metal-hydrogen complex investigated at the molecular level, where all the hydrogen atoms are directly connected to the central metal ion through M-η2(H2) bonds. Moreover, Ac(H2)n3+ (n = 9-12) systems satisfy the 18-electron rule.