Electrochemical Behaviour of trans-[FeH(CN)(dppe)2] Adducts

Abstract

Electrochemical behaviour of various metallo cyano adducts of trans-[FeH(CN)(dppe)2], viz. the dinuclear complexes [FeH(dppe)2(μ-CN)PdCl2(PPh3)], [FeH(dppe)2(μ-CN)NiCl2(PCy3)], and trinuclear [{FeH(dppe)2(μ-CN)}2(ReOCl3)], [{FeH(dppe)2(μ-CN)}2PtCl(Ph)] and [{FeX(dppe)2(μ-CN)}2WCl3(OH)] (X = Cl or OH, dppe = 1,2-diphenylphosphinoethane, PCy3 = P(C6H11)3), as well as the benzoylisocyanide mononuclear adduct trans-[FeH(CNCOPh)- (dppe)2], is reported. All of them exhibit FeII/FeIII-based oxidations (which are reversible, except for trans-[FeH(CNCOPh)(dppe)2]). The metallocyanide bridges C≡N-Re-N≡C and C≡N-W-N≡C in [{FeH(dppe)2(μ-CN)}2(ReOCl3)] and [{FeX(dppe)2(μ-CN)}2WCl3(OH)], respectively, allow electronic communication between the iron centres, with possible generation of mixed-valence FeII/FeIII complexes whose comproportionation constant could be estimated in the former case. From the values of the measured oxidation potentials, the electrochemical PL and EL ligand parameters have been estimated for the metallocyanide ligands that were shown to behave as stronger net electron donors than organoisocyanides, although weaker than cyanide itself. Ligand-centred reduction processes were also observed to lead, in the cases of complexes trans-[FeH(CNCOPh)(dppe)2] (CNCOPh-based reduction) and [FeH(dppe)2(μ-CN)PdCl2(PPh3)] (PdII-based reduction in the CNPdCl2(PPh3)- metallocyanide ligand), to the dissociation of the adduct, with regeneration of the parent cyano complex trans-[FeH(CN)(dppe)2], thus reflecting the reductive decrease of the electrophilic (or Lewis acidic) character of the benzoyl and {PdCl2(PPh3)} groups.

Characterization of Coordination Compounds by Electrochemical Parameters

“…Hence, for such ligands, Equation (15) has to be used cautiously for the estimate of the value of one of the parameters from that of the other one. Recognized examples of failure of Equation (15) [30,40] ]. Nevertheless, Equation (15) is valid for allenylidenes and alkynyls.…”

“…[24,25,34,43] Metallo-cyanide adducts and other cyano adducts are quite sensitive to the Lewis acid but are always stronger net electron acceptors than cyanide: CNBF 3 - - [40] Ͼ CN -. [10,30] Carbenes and related ligands are spread over an even wider window of P L values (from 0 to -1.7 V): vinylidenes C=CHR [29,36] Ն allenylidenes C=C=CR 2 [36,42] Ͼ bithiophene-carbenes [44] [5,36] ferrocenyl-oxocarbenes [36] Ն phosphoylide-aminocarbenes [41] Ͼ ferrocenyl-aminocarbenes [36] Ͼ anionic oxycarbenes C(O -)Y.…”

“…1.2 [36] Benzoyl isocyanide (CϵNCOPh) 0.20 [30] 0.16 [30] PF 3 0.14 [32] --Aminocarbyne (=CNH 2 + ) 0.09 [31] ca. 1.1 [36] CO 0 [10] 0.99 [13] η 1 -Phosphaalkyne (η 1 -PϵCtBu) -0.04 [33] --N 2 -0.07 [10] --Isocyanides (bent) (CϵNR) [b] -0.07 to -0.18 [24,25,34,35] --Hydrazinium (NH 2 NH 3 + ) --0.80 [57] Ethylene (CH 2 =CH 2 ) --0.76 [13] N-Pyrrolylphosphanes --0.69 to 0.53 [28] PPh n (NC 4 H 4 ) 3-n (n = 0-3) Vinylidenes (=C=CHR) 0 to -0.6 [29,36] 0.7 to 0.2 [36] P(OPh) 3 -0.18 [10] 0.58 [13] η 2 -Allene (η 2 -CH 2 =C=CHPh) -0.21 [31] 0.56 [36] Metallo-dinitriles (NϵC-X-CϵN-ML n-1 ) -0.18 to -0.40 [37] 0.58 to 0.39 [36] Ferricinium isocyanides -0.22 to -0.28 [38,39] 0.55 to 0.50 [36] Nitriles (NϵCR) -0.23 to -0.58 [10,27] 0.49 to 0.33 [13] CϵN-BF 3 --0.24 [30] 0.20 [30] Metallo-cyanides (CϵN-ML n-1 -) -0.25 to -0.61 [30,40] 0.50 to 0.21 [36] Phosphonium isocyanides -0.28 to -0.36 [41] 0.50 to 0.43 [41] Allenylidenes (=C=C=CR 2 ) 0 t o -0.8 [36,42] 0.8 to 0 [36] Me 2 SO (S-coordinated) -0.19 <...>…”

Electron-donor/acceptor properties of carbynes, carbenes, vinylidenes, allenylidenes and alkynyls as measured by electrochemical ligand parameters

“…Other indirect methods for the estimate of E s , b and/or P L have been used for certain types of 18-electron complexes, such as [M s LL 0 ] [31][32][33] and [M s L n ] [5,13] Values of the P L and E L parameters for selected ligands are shown in Table 2 [14,18,21,27,[29][30][31][32][33][34]36,[41][42][43][44][45][46][47][48][49][50][51][52][53][54], whereas E s , b and S M , I M parameters are listed in Tables 3 [5,14,27,30,[32][33][34][35]42,[44][45][46][47]49,55,56] and 4 [18,19,…”