Enhanced Dihydrogen Activation by Mononuclear Iridium(II) Compounds: A Mechanistic Study

Abstract

The organometallic chemistry of 4d and 5d transition metals has been vastly dominated by closedshell states. The reactivity of their metalloradical species is though remarkable, albeit yet poorly understood and with limited mechanistic investigations available. In this work we report the synthesis and characterization of two mononuclear Ir II species, including the first dinitrogen adduct. These compounds activate dihydrogen at a dissimilar rate, in the latter case several orders of magnitude faster than its Ir I precursor. A combined experimental/computational investigation to ascertain the mechanism of this transformation in Ir II compounds is reported.

An Open-Shell IrII/IrIV Redox Couple Outperforms the Conventional IrI/IrIII Pair for the Catalytic Isomerization of Olefins

“…To explore the catalytic potential of Ir II complexes compared to isostructural Ir I analogues, we have synthesized a small family of mononuclear Ir II metalloradicals bearing a pincer POCOP ligand (POCOP = C6H3-2,6-(OP( t Bu)2)2). We already demonstrated (15) that oxidation of the Ir I precursor [Ir(POCOP)N2] (1) with AgOTf or Ag[BAr F ] leads to the corresponding Ir II compounds, Ir(POCOP)OTf (3•OTf) and [Ir(POCOP)N2][BAr F ] (1•BAr F ), respectively (Scheme 1; OTf -= trifluoromethanesulfonate, [BAr F ] -= tetrakis [3,5-bis(trifluoromethyl)phenyl]borate). While the triflate anion replaces the dinitrogen ligand to access an overall neutral structure, the even lower coordinating capacity of the [BAr F ] − anion results in an ionic pair with the dinitrogen molecule retained.…”

“…The addition of the commercially available silver salts AgBF4, AgNTf2 or AgOTf to compounds 1 or 2 causes the release of N2 or C2H4, respectively, and the oxidation of the metal center in compounds 3 (Figure 2), evinced by a rapid color change from yellow to green, in line with their corresponding UV-vis spectra (Figures S14-17). An important exception to this reactivity arises from the reaction with Ag[BAr F ], leading to either the dinitrogen complex 1•BAr F (15) or a rather uncommon Ir II -C2H4 adduct (2•BAr F ), from precursor 1 or 2, respectively. At variance with the only other reported example of an Ir II -C2H4 complex, (10b) 2•BAr F maintains its original square planar geometry, as in other Ir II compounds reported herein (Figure 3).…”

“…The Ir-P bond distances of all Ir II complexes are elongated (1•BAr F 2.313 (2) and 2.316(2); 2•BAr F 2.327(1) and 2.330(2); 3•NTf2 2.310 (5) and 2.297(5); 3•BF4 2.299(1) and 2.300(1); 3•OTf 2.2932(8) and 2.3092(8); 3•Cl 2.2902(7) and 2.2913( 7)) in comparison to those in the Ir I precursor 1 (2.276(1) and 2.278(1)) and 2 (2.2771(8) and 2.2793( 7)) ( 18), which may be attributed to electrostatic and Pauli repulsion effects. (15,19) At the same time the Ir-Cipso distances in complexes 1•BAr F (1.979( 5)), 2•BAr F (2.026( 4)) and 3 (3•NTf2 1.961(2); 3•BF4 1.955(4); 3•OTf 1.968(4); 3•Cl 1.989(3)) are shortened compared to 1 (2.017(4)) and 2 (2.033(3)), attributed to the stronger σ-donation to the more electrophilic Ir II . ( 18) The shorter C-C distance exhibited by the coordinated ethylene in complex 2•BAr F (1.248(1)) versus 2 (1.390( 5)) reflects the lesser extent of backdonation ability of the oxidized metal center.…”

“…Recently, we reported an Ir II metalloradical surpassing the dihydrogen activation capacity of its conventional Ir I counterpart by three orders of magnitude. (15) This discovery prompted us to investigate the catalytic potential of monometallic Ir II complexes, posing a fundamental question: is it possible to develop a more efficient catalytic cycle revolving around the Ir II /Ir IV redox couple, diverging from the tenet of Ir I /Ir III or Ir III /Ir V pathways (Figure 1)? To address this question, herein we focus on the isomerization of olefins, a pivotal process in alkane metathesis, ( 16) notorious for its demanding conditions when employing traditional Ir I complexes.…”