Elemental Carbon Chain Bridging Two Iron Centers:  Syntheses and Spectroscopic Properties of Donor−Acceptor [Fe]−C₄−[Fe] Complexes Isolated in Two Different Oxidation States. X-ray Crystal Structure of [Cp*(dppe)Fe−C₄−Fe(CO)₂Cp*]

Abstract

The complexes Fe(η5-C5R5)(CO)2(C⋮CC⋮CSiMe3) (R = Ph, 2a; R = Me, 2b) were obtained from the reaction of (η5-C5Ph5)Fe(CO)2Br (1a) or (Cp*)Fe(CO)2I (Cp* = η5-C5Me5, 1b) with Me3SiC⋮CC⋮CLi (1.05 equiv) in THF at −80 °C (70−75%). The butadiynyl complex (η5-C5Me5)Fe(dppe)(C⋮CC⋮CSiMe3) (2c) was prepared upon photolysis of 2b in a toluene/acetonitrile mixture (95/5 v/v) in the presence of dppe (1,2-bis(diphenylphosphino)ethane) (85%). Treatment of complexes 2a,2b with 1.1 equiv of potassium fluoride in a 50/50 CH3OH/THF mixture at 20 °C afforded the terminal butadiyne complex (η5-C5R5)Fe(CO)2(C⋮CC⋮CH) (R = Ph, 3a; R = Me, 3b), isolated in 85 and 95% yields respectively. Complex 3c was obtained by reaction of 2c with 0.2 equiv of Bu n 4NF in THF (95%). The mononuclear complexes 2a − c and 3a − c were characterized by cyclic voltammetry and IR, 1H, 13C, and 31P NMR, and Mössbauer spectroscopy. The binuclear complexes 5 a,b were obtained (59−69%) in a one-step procedure by treatment of (η5-C5R5)Fe(CO)2(C⋮CC⋮CH) (3 a,b) with 1 equiv of Cp*Fe(dppe)Cl (4) in the presence of KPF6 and KOBu t in methanol. The lower IR frequencies of the carbonyl groups in 5 a,b relative to the isolated acceptor group in 2 a,b and 3 a,b are indicative of the electronic communication between the metal centers through the −C4− spacer. In the complexes 5 a,b, the polarization of the spacer was shown by the 13C NMR chemical shifts of the carbon of the −C4− chain. The Mössbauer spectrum of 5b establishes that the electron density at the two iron atoms is quite different. The structure of 5b has been determined by X-ray diffraction. Cyclic voltammograms of complexes 5 a,b from −0.8 to −1.4 V/SCE displayed one fully reversible wave and a second one almost reversible, showing that the binuclear systems undergo two successive one-electron oxidations at the electrode. Comparison of both redox potentials and current ratio with those of the corresponding monomers 2a − c demonstrates that a strong electronic communication between the metal centers takes place across the all-carbon bridge. Treatment of the neutral complexes with 1 equiv of ferrocenium allowed isolation of the salts [5 a,b ][PF 6 ] in 85−90% yield. Mössbauer spectroscopy showed that both salts were trapped Fe(II)-Fe(III) mixed-valence compounds. The g tensors and couplings with the 31P nuclei are very close for both Fe(III) low-spin radicals. Intense absorption bands were observed at 810 and 829 nm for [5a][PF 6 ] and [5b][PF 6 ], respectively. A second absorption band was observed in the near-infrared for [5b][PF 6 ] (1600 nm) and ascribed to an ICT band. It allowed determination of the electronic coupling between the electron-poor and the electron-rich iron centers through the all-carbon spacer (V ab = 0.021 eV).

Synthesis and structure of the diynyl complexes [Mo(CCCCR)L2(η-C7H7)], L2=chelate N- or P-donor ligand, R=SiMe3 or H: X-ray crystal structures of [Mo(CCCCSiMe3)(2,2′-bipyridine)(η-C7H7)] and [Mo(CCCCH)(Ph2PCH2CH2PPh2)(η-C7H7)]

“…Complexes 2 and 3 each exhibit two bands in the region 2156-2113 cm À1 but the bipy and dppe derivatives are less straightforward with an additional third band below 2000 cm À1 . This phenomenon has been reported previously for [M(C"CC"CSiMe 3 )L 2 (g-C 5 R 5 )] (L 2 = dppe, R = Me, M = Fe or Ru; L 2 = 2PPh 3 , R = H, M = Ru) and is ascribed to Fermi coupling of one of the m(C"C) modes with another oscillator [3,27]. An additional feature of 4, not previously reported in infra red data for metal diynyl complexes, is the observation of two shoulders (2105, 2063 cm À1 ) on the main band at 2090 cm À1 ; these features were also present in a sample of 4 prepared by photolysis of [Mo(C"CC"CSiMe 3 )(CO) 2 (g-C 7 H 7 )] with dppe.…”

“…The chemical shift of C a is strongly dependent on electron density at the metal centre with electron-rich metal centres inducing a low field shift. Thus the bipy and dppe derivatives 1 and 4 both exhibit dC a around 142 ppm, almost identical to the values observed for [Fe(C"CC"CSiMe 3 )(dppe)Cp * ] and the alkynyl complex [Mo(C"CPh)(dppe)(g-C 7 H 7 )] [3,20]. A high field shift in C a is observed for the less electron-rich centres of 2, 3, the dicarbonyl [Mo(C"CC"CSiMe 3 )-(CO) 2 (g-C 7 H 7 )] and the ruthenium analogue [Ru(C" CC"CSiMe 3 )(dppe)Cp * ].…”

“…These studies are driven by potential applications as precursors to one-dimensional molecular wires and metal containing polymers. An important feature of many recent advances has been the incorporation of redox-active, electron-rich metal centres into metal diynyl complexes and subsequent use as precursors to all-carbon bridged, diyndiyl complexes M-C"CC"C-M; leading metal end-caps M for this purpose are the d 6 centres Fe(dppe)Cp * [3,4], Ru(dppe)Cp * [5][6][7] and Re(NO)(PPh 3 )Cp * [8,9] (dppe = Ph 2 PCH 2 CH 2 PPh 2 , Cp * = C 5 Me 5 ). It is well established that the identity of the metal end-cap group, M, plays a key role in the properties and electronic structure of all-carbon bridged systems [M-C"CC"C-M] z+ , but this notwithstanding, the range of electron-rich metal end-cap groups is still relatively limited with none to rival the strongly electron donating capacity of the Fe(dppe)Cp * system.…”

“…Investigations into metal-diynyl and all-carbon-bridged diyndiyl complexes of the group VI metals are well developed for d 4 systems based upon Mo(CO)(dppe)Cp [16,17] and W(CO) 3 (g-C 5 R 5 ) (R = H, Me) [18,19] as supporting fragments whilst we have previously reported the syntheses of butadiynyl derivatives of dicarbonyl cycloheptatrienyl molybdenum complexes [Mo(C"CC"CR)(CO) 2 (g-C 7 H 7 )] (R = SiMe 3 or H) [20]. However, where the redox chemistry of these examples has been investigated [19] it is clear that the presence of carbonyl acceptor groups results in poor accessibility of higher oxidation states and therefore limited application as end-caps in all-carbon bridged organometallics.…”

Ligand Redox Non‐Innocence in Transition‐Metal σ‐Alkynyl and Related Complexes

“…[20] Similar observations have been made for the series of complexes [Fe(CϵCR)(dppe)Cp*]; the CϵCH acetylide complex dimerises on low-temperature oxidation [112] whereas the tert-butyl, phenyl and a range of other substituted arylalkynyl complexes form stable, isolable radical cations. [11,13,14,16,17,[115][116][117][118][119] Very recently this work has been extended by Paul gands in the assembly of larger, redox-active polymetallic complexes. [120] The reactions of analogous complexes [M(CϵCR)-(PP)Cp] with potential oxidising agents such as tropylium [M = Fe, Ru, Os; R = Ph, PP = (PPh 3 ) 2 , dppm, dppe; not all combinations], [121,122] diazobenzene [M = Ru, R = Ph, PP = (PPh 3 ) 2 , dppe] [121] or halogens [R = Ph, PP = (PPh 3 ) 2 ] [123,124] gave products resulting from the electrophilic addition to C β rather than one-electron oxidation.…”