Cationic mono and dicarbonyl pincer complexes of rhodium and iridium to assess the donor properties of PC_carbeneP ligands

Abstract

The donor properties of five different PCcarbeneP ligands are assessed by evaluation of the CO stretching frequencies in iridium(i) and rhodium(i) carbonyl cations. The ligands feature dialkyl phosphine units (R = (i)Pr or (t)Bu) linked to the central benzylic carbon by either an ortho-phenylene bridge, or a 2,3-benzo[b]thiophene linker; in the former, substituent patterns on the phenyl linker are varied. The carbonyl complexes are synthesized from the (PCcarbeneP)M-Cl starting materials via abstraction of the chlorides in the presence of CO gas. In addition to the expected mono carbonyl cations, products with two carbonyl ligands are produced, and for the rhodium example, a novel product in which the second carbonyl ligand adds reversibly across the Rh[double bond, length as m-dash]C bond to give an η(2) ketene moiety was characterized. The IR data for the complexes shows the 2,3-benzo[b]thiophene linked system to be the poorest overall donor, while the phenyl bridged ligands incorporating electron donating dialkyl amino groups para to the anchoring carbene are very strongly donating pincer arrays.

Tuning iridium (I) PCcarbeneP frameworks for facile cooperative N2O reduction

“…Previously, we established a method for preparing an electron rich, rigidified iridium PC carbene P system 4--Cl. [26] However, the cumbersome 14 step synthesis from 1,3,5--tribromobenzene and unexpectedly low ligand donor strength relative to 3--Cl suggested that replacing the electronegative O linker in the ligand of 4--Cl with a more electron donating fragment like --C(CH 3 ) 2 -- or -Si(CH 3 ) 2 -- would both increase the donor properties of the ligand framework and lower the synthetic pain necessary to obtain workable quantities of ligand. As shown in Scheme 4, both ligands IV and VI can be obtained from the common intermediate I, which was prepared in multigram quantities in two steps starting with 3,5--dibromo--aniline via modified literature procedures [27,28] (see the Supporting Information for further details on the synthesis of I).…”

“…[22] Recently our group ranked a series of iridium PC carbene P complexes (1 to 4--Cl, Scheme 3) according to ligand donor strength by preparing cationic carbonyl and dicarbonyl derivatives and cataloguing the CO stretching frequencies. [26] Among this collection was the tethered system 4--Cl, which exhibited an intermediate ligand donor strength relative to 2--Cl and 3--Cl. However, preliminary experiments showed that the more rigid ligand framework in 4--Cl rendered it immune to C aryl --C anchor cleavage and furthermore that it reacted much more rapidly with N 2 O than 2--Cl (but slower than the more electron rich 3--Cl).…”

Tuning Iridium(I) PCcarbeneP Frameworks for Facile Cooperative N2O Reduction

“…To place these new ligands onto the scale of donicity established previously for ligands of this type, [26] iridium monocarbonyl cationic complexes 5--CO and 6--CO (Scheme 6) were prepared by abstracting the chloride ligands using sodium tetrakis [ , respectively, very similar to that for the less stable unlinked system 3--CO. Dicarbonyl cationic complexes, 5--(CO) 2 and 6--(CO) 2 , were also prepared by the addition of 1 atmosphere of CO to both monocarbonyl complexes. The symmetric and asymmetric frequencies for the dicarbonyl compounds ( Table 1) were quite similar to the other amino functionalized carbenes, 3--(CO) 2 and 4--(CO) 2 .…”

Divergent Reactivity of CO2 and CO and Related Substrates at the Nickel Carbon Double Bond of (PCcarbeneP)Ni(II) Pincer Complexes

“…Previous observations in the chemistry of group 8 complexes of these ligand types with N 2 O indicated that more rigidified pincer frameworks were successful in stabilizing the products of metal-ligand cooperative N 2 O activation. [51][52] Additionally, the rigidification of related PNP ligands using a dimethylacridane core has been shown to decrease deleterious side reactions in PNP Ni systems with CO 2 . 4,9,11 As such, we designed and synthesized a new PC carbene P ligand based on a 10,10-dimethyl-9,10dihydroanthracene core, L 2 (see Scheme S1 for full details of the synthesis of this new ligand).…”