João J. R. Fraústo da Silva

Redox potential and substituent effects at ferrocene derivatives. Estimates of Hammett σp and Taft polar σ substituent constants

Abstract The anodic behaviour of 34 ferrocene derivatives (8 of which have been prepared for the first time) of the general type Fc-CHYZ [Y = R (hydrogen, alkyl or aryl), Fc or Fc+; Z = amino or related functionalized groups, various unsaturated N-moieties (such as cyano, isocyano, imidazoyl, azide or triazene derivative), alkylammonium, hydroxy, alkoxy, thiolate, acyloxy, phosphinyl or thiocarbonyl groups] has been studied by cyclic voltammetry (and, in some cases, also by conrolled-potential electrolysis and differential pulse polarography) at a Pt electrode in an aprotic solvent. The compounds undergo a one-electron reversible or quasi-reversible oxidation centred at each iron centre and the effect of the substituents on the half-wave oxidation potential is discussed in terms of their electronic properties. Linear correlations have been recognized between this potential and the Hammett σp constant or the Taft polar σ* constant for the substituent, and have been used, in the cases they were not known, to estimate the values of these constants for the above-mentioned substituents and of the latter constant for a total of 73 of these substituents or their Z groups. The anomalous behaviours of complexes with an hydroxy, a carbonyl, an acylamido, a thiocarbonyl, or a phosphinyl group are discussed.

Redox potential and substituent effects in ferrocene derivatives: II

Abstract The anodic behavior of 31 ferrocene derivatives (FcX) (four of which have been prepared for the first time) of the types shown below has been investigated by cyclic voltammetry (and, in some cases, also by controlled-potential electrolysis and differential pulse polarography) at a Pt electrode, in an aprotic solvent: FcC(Y)Z (class A; Y  R (hydrogen, alkyl or unsaturated-carbon moiety) or Cl; Z  CRR′, derived functionalized groups or {W(CO) 5 }); FcC(Y)O (class B; Y  R or OH); other types (class C, including azide, other unsaturated N moieties, acyloxy or phosphino); disubstituted ferrocenes (class D, with alkyl, aminoalkyl, phosphino, B(OH) 2 or −HGCl substituents, or the SSS bridge). All these substituted ferrocenes exhibit a one-electron reversible oxidation centered at iron and the effect of the substituents on the half-wave oxidation potential is discussed in terms of electronic properties. Using the accepted linear correlations between the half-wave oxidation potential and the Hammett constant σ p of the substituent, σ p was estimated for the first time for 29 of the substituents in the ferrocenes listed above. The anomalous behavior of complexes with a hydroxy or a carbonyl group is discussed. No correlation was observed between the oxidation potential and either the energy of the electronic absorption bands or the 1 H NMR chemical shift of the unsubstituted cyclopentadienyl ring, although there are rough correlations for class A compounds. This behaviour is compared with those of [Fc-CHYZ]. kw]Iron; Redox potentials; Ferrocenes; Electrochemistry; Substituent effects

Alkanes to carboxylic acids in aqueous medium: metal-free and metal-promoted highly efficient and mild conversions

A convenient and clean route to transform, in aqueous medium, various alkanes to carboxylic acids via single-pot carboxylation with CO and water, under mild conditions, has been achieved, proceeding efficiently and selectively even without any metal catalyst and any acid additive, at low temperatures; the relevant hydroxylating role of H(2)O and radical mechanisms are disclosed by radical-trap, H(2)(18)O and DFT studies.

A new family of luminescent compounds: platinum(II) imidoylamidinates exhibiting pH-dependent room temperature luminescence

The imidoylamidinate platinum(II) compounds [Pt{NH=C(R)NC(Ph)NPh}2] (R = CH2Ph 2, p-ClC6H43, Ph 4) were prepared by the reaction of the appropriate trans-[PtCl2(RCN)2] with 4 equiv of the amidine PhC(NH)NHPh giving 2-4 and 2 equivs of the salt PhC(=NH)NHPh.HCl. We also synthesized, by the double alkylation of 4 with MeOSO2CF3, complex [Pt{NH=C(Ph)N(Me)C(Ph)=NPh}2][CF3SO3]2 (5) which models the bis-protonated form of 4. The complexes were characterized by 1H, 13C NMR, and IR spectroscopies, FAB-MS and by C, H, N elemental analysis. The X-ray crystallography of 4.2CH2Cl2 enables the confirmation of the square planar coordination geometry of the metal center with almost planar imidoylamidine ligands, while in 5.2CHCl3 the planarity of the metallacycles is lost and and the central N atom is sp3-hybridized. The imidoylamidinate complexes represent a new family of Pt(II)-based luminescent complexes and they are emissive at room temperature both in solution and in the solid state, with an emission quantum yield ranging from 3.7 x 10(-4) to 6.2 x 10(-2) in methanol solution; the emission intensity is pH-dependent, being quenched at low pH. UV-visible and luminescence spectroscopies indicate that the lowest excited state of these compounds is 3MLCT or 3IL with significant MLCT character, with emission lifetimes of a few micros. A blue shift of both the absorption and emission with increasing solvent polarity and with decreasing pi-electron withdrawing properties of the ligand substituent was observed.

Syntheses and properties of cyanamide and cyanoguanidine complexes of platinum(II). X-Ray structure of trans-[Pt(CF3)(NCNEt2)(PPh3)2][BF4]

Complexes trans-[PtX(L)(PPh3)2]A [1: X = CF3; A = BF4; L = NCNH2, NCNMe2, NCNEt2, or NCNC(NH2)2. 2: X = Cl; A = BPh4; L = NCNMe2 or NCNEt2] and cis-[PtCl(L)(PPh3)2][BPh4] [3: L = NCNH2 or NCNC(NH2)2], which appear to be the first cyanamide or cyanoguanidine complexes of platinum to be reported, have been prepared by treatment of trans-[PtBr(CF3)(PPh3)2] (in CH2Cl2/acetone and in the presence of Ag[BF4]) or of cis-[PtCl2(PPh3)2] (in THF and in the presence of Na[BPh4]), respectively, with the appropriate substrate. In KBr pellets or in solution 1 (L = NCNMe2 or NCNEt2) undergoes ready replacement of the organocyanamide (under the trans influence of CF3) by bromide to regenerate trans-(PtBr(CF3)(PPh3)2]. The X-ray structure of 1 (X = CF3, A = BF4, L = NCNEt2) is also reported, and shows the presence of two apical intramolecular contacts of the metal with two ortho-hydrogen atoms of the phosphines, whereas the amine N atom of the diethylcyanamide is trigonal planar in the linear NCN framework with a delocalized π system.

Protonation of the nitrite ligand versus protonation of rhenium at cis- or trans-[ReCl(NCC6H4R-4)(Ph2PCH2CH2PPh2)2] (R Cl, F, Me or MeO). A mechanistic study

Abstract Stopped-flow kinetic studies have shown that in tetrahydrofuran the protonation, by HCl, of cis -[ReCl(NCC 6 H 4 R-4)(dppe) 2 ] (R  Cl, F, Me or OMe; dppe  Ph 2 PCH 2 CH 2 PPh 2 ) or trans -[ReCl(NCC 6 H 4 F-4)(dppe) 2 ] to give [ReCl(H)(NCC 6 H 4 R-4) (dppe) 2 ] + involves a rapid β-proton addition to the nitrile ligand forming the corresponding methyleneamido-intermediate [ReCl(NCHC 6 H 4 R-4)(dppe) 2 ] + which, upon rearrangement or further metal-protonation/ligand-deprotonation steps, forms the final hydride product.

Syntheses, properties and Mössbauer studies of cyanamide and cyanoguanidine complexes of iron(II). Crystal structures of trans-[FeH(NCNH2)(Ph2PCH2CH2PPh2)2][BF4] and trans-[Fe(NCNEt2)2(Et2PCH2CH2PEt2)2][BF4]2

Abstract The complexes trans -[FeH(NCR)(dppe) 2 ][BF 4 ] ( 1 ) (R=NH 2 , NMe 2 , NEt 2 or NC(NH 2 ) 2 ; dppe=Ph 2 PCH 2 CH 2 PPh 2 ) and trans -[FeL(NCR)(depe) 2 ]Y n (R=NH 2 , NMe 2 , NEt 2 or NC(NH 2 ) 2 ; depe=Et 2 PCH 2 CH 2 PEt 2 ; Y=BF 4 or BPh 4 ; ( 2 ), L=Br, n =1; ( 3 ), L=NCR, n =2) have been prepared by treatment of trans -[FeHCl(dppe) 2 ] (in THF and in the presence of Tl[BF 4 ]) or trans -[FeBr 2 (depe) 2 ] (in MeOH and in the presence of [NBu 4 ][BF 4 ] or Na[BPh 4 ]), respectively, with the appropriate cyanamide. NMR and Mossbauer spectral, as well as FAB mass spectrometric data are reported. Mossbauer partial isomer shift (PIS) and partial quadrupole splitting (PQS) parameters have been estimated for the cyanamide and dppe ligands and rationalised, with the overall IS and QS, in terms of π- and σ-electronic effects, the cyanamides behaving as more effective σ-donors and weaker π-acceptors than organonitriles. FAB MS fragmentation patterns are also proposed. The crystal structures of 1 (R=NH 2 ) and 3 (R=NEt 2 , Y=BF 4 ) are reported.

Iminoacylation: Part 6: Coupling of chloro- and amidoximes Ar(R)CNOH (R=Cl, NH2) with acetonitrile ligands coordinated to platinum(IV)

Abstract Addition of chloro- and amidoximes RR1CNOH (R/R1=Cl/Ph, Cl/C6H4-p-Me, Cl/C6H4-p-NO2, NH2/Ph) to the acetonitrile ligands in trans-[PtCl4(MeCN)2] proceeds much slower as compared to ketoximes with donor substituents R/R1 [V.Yu. Kukushkin, T.B. Pakhomova, Yu.N. Kukushkin, R. Herrmann, G. Wagner, A.J.L. Pombeiro, Inorg. Chem. 37 (1998) 6511] and leads to the complexes with iminoacylated species [ligating (alkylideneaminooxy)imines] trans-[PtCl4{NHC(Me)ONCRR1}2]. X-ray crystal structure determination of two iminoacylated compounds (R/R1=Cl/C6H4-p-Me, Cl/C6H4-p-NO2) disclosed their overall trans geometry with monodentate organic ligands in E-conformation and indicated NH ⋯ N hydrogen bonding between the imino H atom and the oxime nitrogen.

Interactions of D-ribose with polyatomic anions, and alkaline and alkaline-earth cations: possible clues to environmental synthesis conditions in the pre-RNA world

The interaction of some of the most abundant anions in seawater (borate, sulfate and carbonate/bicarbonate) with D-ribose was studied by 1H, 11B and 13C NMR spectroscopy. The results confirmed that only borate improves the stability of D-ribose and favours significant amounts of the ribofuranose isomer, which is the form occurring in present day living organisms. The effect of cations (Na+, K+, Mg2+, Ca2+ and Sr2+) on ribose–borate-bound species was also studied, and it was found that the divalent cations induce a small increase in the relative amounts of ribopyranose isomers in solution and a corresponding decrease in the abundance of the ribofuranose isomers. It was also found that the stability of ribose–borate-bound species, when compared with free ribose over a wide range of temperature and pH values, is higher for compounds with borate, which are stable even at a relatively low pH (6.6) and a relatively high temperature (60 °C). The stability of ribose under moderate pH and relatively high temperature conditions, in the presence of species that occur in seawater, is important for the viability of the early synthetic steps that led to the first nucleotides, which predated the formation of more complex structures, such as RNA.

Aminocarbyne coupling reactions at M(Ph2PCH2CH2PPh2)2(M = Mo or W) sites. Synthesis and properties of the diaminoacetylene complexes trans-[MX(η2-MeHNCCNHMe)-(Ph2PCH2CH2PPh2)2]A (X = F, Cl or ClO4; A = BF4′PF6′HCl2or ClO4) and of their di(aminocarbyne)-type precursors

Treatment of trans-[M(CNMe)2(dppe)2]1(M = Mo or W, dppe = Ph2PCH2CH2PPh2) in CH2Cl2 with HA (A = BF4′ PF6′ Cl or ClO4) gives the diaminoacetylene complexes trans-[MX(η2-MeHNCCNHMe)(dppe)2]A 2(X = F, Cl or ClO4; A = BF4′ PF6′ HCl2 or ClO4). The crystal structure of trans-[MoF(η2-MeHNCCNHMe)(dppe)2][BF4] has been determined. A key intermediate in the reaction, trans-[M(CNHMe)2(dppe)2]A23(A = HCl2 or ClO4), has also been isolated and shown to be best viewed as a ‘iminomethylenium’ species CNHMe. The acetylenic CC triple bond in 2 undergoes base-induced cleavage (e.g., by LiBun) to form the parent complex 1.