Allenylidene Iron(II) Complexes and Their Deprotonation, Nucleophilic Addition Reactions, and Cathodic Protonation toward Alkynyl Derivatives: A Chemical and Electrochemical Study

Abstract

The allenylidene complexes trans- [FeBr(dCdCdCRR′) (depe) 2 ][Y] (R ) Me, R′ ) Ph, 1; R ) R′ ) Ph, 2; R ) R′ ) Et, 3; depe ) Et 2 PCH 2 CH 2 PEt 2 ; Y ) BF 4 , BPh 4 ) were obtained by reaction of trans-[FeBr 2 (depe) 2 ] with the appropriate alkynol HCtCCRR′(OH), in MeOH and in the presence of Na[BF 4 ] or Na[BPh 4 ]. Deprotonation of 3 or nucleophilic γ-addition to 2 led to the neutral enynyl and alkynyl complexes trans- [FeBr{-CtCC(dCHMe)Et}-, in acetonitrile solution, upon reaction with NHMe 2 , NH 2 Me, and PMe 3 , respectively. The complexes have been characterized by multinuclear NMR and IR spectroscopy, FAB-MS, and elemental analysis and, in the cases of 5a and 6a, also by X-ray diffraction analysis. Controlled-potential electrolysis of 2 yields the alkynyl species trans-[FeBr{-CtCCPh 2 (H)}(depe) 2 ] (8) via a 2e -/H + process, and the oxidation potential of the complexes, measured by cyclic voltammetry, has allowed us to estimate the electrochemical Pickett (P L ) and Lever (E L ) ligand parameters for the cumulenic ligands. These are then ordered (together with related ligands) according to their net π-electron acceptor minus σ-donor ability as follows: carbynes > aminocarbyne > CO > vinylidenes > aryl allenylidene > alkyl allenylidene > NCR . phosphonium alkynyl > cyanoalkynyl, Br -, NCO -> alkynyl, enynyl, aminoalkynyl.

Transition metals in organic synthesis: Highlights for the year 2005

“…Synthesis and reactions of allenylidene iron complexes with nucleophiles was reported [1608]. Chiral cationic iron carbene complexes participated in cis-and enantioselective cyclopropanations of unactivated alkenes [1609].…”

Characterization of Coordination Compounds by Electrochemical Parameters

“…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 <...>…”

“…The strongest net electron-donors are the following, with P L values below -1.0 V: aryls, alkyls and NO -, [36] OH -, [10] anionic oxycarbenes [=C(O -)Y], [36] hydrogen cyanamide (NCNH -), [49] F -, [54] N 3 -, [10] H -, [10] alkynyls (CϵCR -), [5,36,42,52,53] monodeprotonated cyanoguanidine [NCNC(NH)NH 2 -], [50] acylor aroylcyanamide [NCNC(O)R -], [51] Cl -, [10] Br -, [10] NCO -, [10] I -, [10] dicyanamide (NCNCN -). [50] A modification of a ligand naturally leads to a change of P L .…”

“…Nevertheless, Equation (15) is valid for allenylidenes and alkynyls. [28,36,42] ( 15) E L values are already known for a wide variety of ligands, [13][14][15][16][28][29][30]36,41,57,64,[70][71][72][73][74][75] including an extensive list of recently estimated values by the current author [36] for many carbyne, carbene, vinylidene, allenylidene and alkynyl ligands. Examples are given in Table 1.…”

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

“…However, in the case of the trans-{FeBr-(depe) 2 } + binding center [91], P L could be obtained directly from PickettÕs equation (5) and the derived E L values (according to Eq. (8)) were identical to those estimated by using LeverÕs equation (6), thus corroborating the agreement of both methods and the validity, for allenylidenes, of the relationship (8) between P L and E L .…”