[AsCCAs]-Coordinated Rhenium Hydrides and Their Reactivities Toward Unsaturated Hydrocarbons, Heterocumulenes, and CO₂

Abstract

An [AsCCAs] ligand featuring a central alkyne and two flanking arsenic donors was employed for the synthesis of a trihydrido rhenium complex, while the corresponding phosphorus ligand was shown to be less suited. The reactivity of the former trihydride [AsCCAs]ReH 3 (3) was examined in detail, which revealed that two alternative reaction channels may be entered in dependence of the substrate. Upon reaction of 3 with PhC�CPh, ethylene, and CS 2 , monohydrides of the general formula [AsCCAs]Re(L)H with L = η 2 -PhC�CPh (4), η 2 -H 2 C�CH 2 (5), and η 2 -CS 2 (6) were formed along with H 2 . In contrast, insertion products of the type [AsCCAs]Re(X)H 2 (7−9) were obtained upon treatment of 3 with CyN�C�NCy, PhN�C�O, and Ph 2 C�C�O, while CO 2 failed to react with 3 under identical reaction conditions. Given that several productive reactions between CO 2 and hydrido rhenium carbonyls have been reported in the literature, 3 was further derivatized by introducing CO and t BuNC coligands, respectively. This led to the isolation of trans-[AsCCAs]ReH(CO) 2 (trans-10) and trans-[AsCCAs]ReH(CN t Bu) 2 (trans-11), which were shown to thermally isomerize to the corresponding cis-configured products, cis-10 and cis-11. Interestingly, only the cis-complexes were found to react with CO 2 , which was rationalized by evaluating the relative nucleophilicities of the hydrides in cis-10, trans-10, cis-11, and trans-11 via Fukui analysis. The formates cis-[AsCCAs]Re(OCHO)(CO) 2 (12) and cis-[AsCCAs]Re(OCHO)(CN t Bu) 2 (13) were isolated and shown to contain κ 1 -O-coordinated formate moieties. Treatment of 12 with [LutH]Cl/B(C 6 F 5 ) 3 (or with Ph 3 SiCl) led to the liberation of [LutH][OCHO•••B(C 6 F 5 ) 3 ] (or triphenylsilyl formate) with concomitant formation of the expected chloro complex cis-[AsCCAs]ReCl(CO) 2 ( 14). In a closed synthetic cycle, hydride 12 was regenerated from the latter chloride using NaBEt 3 H as a hydride source.

To Split or Not to Split: [AsCCAs]-Coordinated Mo, W, and Re Complexes and Their Reactivity toward Molecular Dinitrogen

“…A comparison of the alkyne units in 2-As, 5, and 3-As revealed that the CC bond lengths steadily decrease in the sequence d CC (2-As) > d CC (5) ≫ d CC . As expected for a 4e − alkyne donor, a rather long CC bond distance of 1.3209 (18) was found in 2-As, which shortens to 1.310(4) upon formal oxidation to 5. In 3-As, a significantly shorter alkyne CC distance of 1.277(3) Å was determined, suggesting that the latter alkyne is best interpreted as a 2e − donor, as also inferred from its 13 C{ 1 H} NMR signal at 134.9 ppm.…”

“…31 Å). 18 These differences between 9 and 11 are expected given that π-donation from the nitride is thought to diminish π-donation from the alkyne (vide supra). In the solid state, an unexceptional Re�N triple bond (∼1.66 Å) with v Re�N = 1050 cm −1 (ATR-IR) was found in 11, which is in line with a dominating metal nitride interaction.…”

“…In the latter target compound, an π 10 δ 4 configuration is to be expected, which is required for N 2 splitting, at least in this particular symmetry. Along these lines, [AsCCAs]­ReCl 3 ( 9 ) was selected as a starting material and reduced under an atmosphere of N 2 using Zn, Na/Pb, or Mashima’s salt-free reductant. In the presence of the latter salt-free reductant (excess), a slow conversion of 9 began, which was accompanied by a color change from red to green.…”

“…In this context, it needs to be noted that 2-P and 3-P were found to be thermally robust, while the free ligand (1-P) is actually prone to cyclization upon heating (see Scheme 1). 18 In view of the thermal stabilities of 2-P and 3-P, numerous attempts to prepare related [PCCP] tungsten and rhenium halides or nitrides have been made over the course of almost two years. Disappointingly, these efforts met with failure (W) and limited success (Re) 18 given that heating is required for the complexation of these kinetically inert 5d-metals, which inevitably leads to the formation of 4 (see Scheme 1).…”

“…18 In view of the thermal stabilities of 2-P and 3-P, numerous attempts to prepare related [PCCP] tungsten and rhenium halides or nitrides have been made over the course of almost two years. Disappointingly, these efforts met with failure (W) and limited success (Re) 18 given that heating is required for the complexation of these kinetically inert 5d-metals, which inevitably leads to the formation of 4 (see Scheme 1). The arsa-derivative of the ligand (1-As), however, was shown to resist thermal cyclization reactions, 19 which prompted us evaluate whether tungsten and rhenium complexes of 1-As may be prepared and exploited for the activation of N 2 .…”