Peter W. Lednor

Tris[bis(trimethylsilyl)methyl]tin(III), R3Sn·: an unusually stable stannyl radical, from photolysis of R2Sn

Photolysis of R2Sn [R =(Me3Si)2CH] with visible light in benzene at ambient temperature yields the stable radical R3Sn·, which has an e.s.r. solution spectrum showing coupling with the methine protons and 119Sn and 117Sn nuclei.

Homolysis of metal–carbon and metal–metal bonds: spin-trapping of the resulting carbon- and metal-centred radicals

Homolysis of the C–Mn bond of [RMn(CO)5] or [RCOMn(CO)5] under u.v. irradiation has been studied using nitrosodurene as a spin-trap for both the radicals formed; irradiation of various metal–metal bonded compounds (LM–ML, M = Mn, Re, Fe, Mo, or Co) gives rise to metal-centred radicals which can similarly be trapped.

Mechanistic studies of some oxidative-addition reactions: free-radical pathways in the Pt0–RX, Pt0–PhBr, and PtII–R′SO2X reactions (R = alkyl, R′= aryl, X = halide) and in the related rhodium(I) or iridium(I) systems

Spin-trapping studies, using R′NO (R′= But or C6HMe4-2,3,5,6), have been carried out on: (i) various Pt0–alkyl halide (RX) systems, e.g.[P(PR″3)n](R″= Et or Ph, n= 3 or 4)–Mel; (ii) a number of PtII–sulphonyl or acyl halide reactions, e.g. cis-[PtMe2(PtMe2Ph)2]–p-MeC6H4SO2X; and (iii) several RhI– or IrI–alkyl halide additions. In most cases the appropriate nitroxyl spin adduct R(R′)N·O or R′(p-MeC6H4SO2)N·O [but not R-(R‴CO)N·O] is observed by e.s.r. spectroscopy. In conjunction with appropriate control experiments, this leads to the unequivocal conclusion that free radicals are implicated in systems (i)(X = Cl, Br, or l) and (ii)(X = Cl or Br). By means of the nitrone PhCHN(O) But, a platinum(I) complex has been trapped during the course of the [Pt(C2H4)(PPh3)2]–Etl reaction; its formulation as [Pt{CH(Ph)N·O(But)}l(PPh3)2] is based on e.s.r. data. Trityl chloride adds to [Pt(PPh3)3][but not so rapidly to a rhodium(I) or iridium(I) substrate] to give Ph3Ċ and [PtCl2(PMe2Ph)2]; Ph2CHBr and [Pt(PPh3)3] give (Ph2CH)2 as the principal organic product. Galvinoxyl inhibits the addition of p-MeC6H4SO2Cl to [PtMe2(PMe2Ph)2]. Azobis(isobutyronitrile) under photolysis catalyses the oxidative addition of PhBr to [Pt(PPh3)3]. Whereas the addition of Mel to [Pt(PPh3)3] in benzene leads exclusively to the 1 : 1 adduct, in tetrahydrofuran by far the major product is [Ptl2(PPh3)2]. It is concluded that reactive halides RX add to a platinum(0) substrate via a geminate radical pair [PtILn(X)]+ R˙, whereas with less reactive halides, or in the sulphonyl halide–PtII addition, a radical-chain mechanism is operative.

Photochemical synthesis and electron spin resonance characterisation of stable trivalent metal alkyls (Si, Ge, Sn) and amides (Ge and Sn) of Group IV elements

The reaction of MCl2(M = Ge or Sn) or Si2Cl6 with R1Li or (R22N)Li [R1=(Me3Si)2CH, R2= Me3Si] and subsequent irradiation affords the stable metal-centred radicals R13Si·, R13M·, or (R22N)3M·(e.g. R13Ge· has t½ > 4 months in C6H6 at 20 °C), the solution e.s.r. spectra of which show well defined hyperfine splittings [e.g. for (R22N)3Ge·, a decet of septets, due to coupling with 73Ge (I= 9/2) and 14N (I= 1)].

Alkali metal–naphthalene adducts as reagents for neutralizing oxide surfaces, and the effect of alkali metal treated surfaces in rh-catalysed synthesis gas (CO + H2) conversion

M + C10H8˙– adducts (M = Li, Na, K) are effective reagents for eliminating acidity from the surfaces of siO2, ZrO2 or zeolie y; the treated surfaces [SiO2+ Li +, Na +,Na + from M + C10H8˙–, SiO2+ Na + from NaNo3 or zrO2+ Na+ from Na+C10H8˙–] function as novel supports for heterogeneous Rh [from Rh4(CO)12] catalysts in the conversion of CO + H2 into MeOH with > 90% selectivity at 40–95 bar adn 250–300 °C, which contrasts with the formation of CH4over rh on the untreated oxides.

Subvalent Group 4B metal alkyls and amides. Part 4. An electron spin resonance study of some long-lived photochemically synthesised trisubstituted silyl, germyl, and stannyl radicals

A number of solution-stable species of general formula MR3˙[R = CH(SiMe3)2: M = Si. Ge. or Sn], M(NR′2)3˙ and M (NR′R″)3˙(R′= SiMe3, R″= CMe3, M = Ge or Sn) have been prepared and characterised by e.s.r. spectroscopy. Most of the radicals have been generated by photolysis of the bivalent Group 4 species MR, M(NR′2)2 or M(NR′R″)2 when available; others have been obtained by alternative photochemical experiments. The e.s.r. parameters indicate that the radicals have non-planar structures similar to those of analogous transient species such as MMe3˙. The mechanism of formation of the radicals is discussed : their unusual stability (e.g. SnR3˙ has a halflife of ca. 1 year at 20 °C) is attributed mainly to steric hindrance to dimerisation.

Copolymerization of propene oxide with carbon with carbon dioxide: aselective incorporation of propene oxide into the polycarbonate chains, determined by 100 MHz 13C n.m.r. spectroscopy

13 C N.m.r. spectroscopy has established that propene oxide–carbon dioxide copolymers prepared with diethylzinc-based catalysts can contain significant amounts of inverted propene oxide units, in contrast to the exclusively head-to-tail structured previously.

Photolytic homolysis of the metal–carbon (sp3 or sp2) bond of alkyl or acyl transition-metal complexes: an electron spin resonance study using spin trapping; and a note on aminyl oxides [MLn{N(Ȯ)R}][MLn= Ru(CO)4(SiMe3), Os(CO)4(SiMe3), or Fe(η-C3H5)(CO)3; R = aryl]

Irradiation of the following metal alkyls has been carried out in CH2Cl2(or PhMe) in the presence of nitrosodurene, RNO (R = C6HMe4-2,3,5,6), in the cavity of an e.s.r. spectrometer: [Mn(CO)5R′](R′= CH2Ph or CH2SiMe3), [Fe(η-C5H5)(CO)2R′], [Mo(η-C5H5)(CO)3R′](R′= Me, Et, or CH2Ph), cis-[PtR′2(PMe2Ph)2](R′= CH2SiMe3 or CH2CMe3), [AuR′(PPh3)](R′= Me or CH2SiMe3), and [CoR′L(oep)](H2oep = 2,3,7,8,12,13,17,18-octaethylporphyrin and R′= Me, L = NC5H5; or R′= Et, L = OH2). Similar experiments have been performed on (i) the acylmetal complexes [Mn(Co)5{C(O)R″}](R″= CH2Cl, Me, Et, CH2Ph, or CHPh2) or [Fe(η-C5H5)(CO)2{C(O)R″}](R″= Me or CH2Ph), and (ii) the metal–metal bonded [M2(CO)8(SiMe3)2](M = Ru or Os). Finally, the dark reaction between the stable iron(I) complex [Fe(η-C3H5)(CO)3] and RNO in CH2Cl2 has been investigated. As a consequence, from the alkyls, metallo-aminyl oxides [MLn{N(Ȯ)R}] were observed, except for MLn= a platinum(I), gold(0), or cobalt(II) moiety, but the alkylaminyl oxides RN(Ȯ)R′ were found in every case [although with the molybdenum(II) alkyls as substrates these were not detected at –30 °C but only at 20 °C]; two of these (R′= CH2SiMe3 or CH2CMe3) are new and show remarkably different β-proton hyperfine couplings, attributed in part to a conformational difference allowing for close Si ⋯ O proximity for R′= CH2SiMe3, and also to the greater steric requirements of the neopentyl group. From the acyls, the corresponding metallo-aminyl oxide was invariably detected, but never the spin-trapped acyl radical RN(Ȯ)COR″; however, the corresponding spin-trapped alkyl radical RN(Ȯ)R″ was observed but only for the case of R″= CH2Ph or CHPh2. The remaining experiments led to the e.s.r. characterisation of [MLn{N(Ȯ)R}], MLn= Ru(CO)4(SiMe3), Os(CO)4(SiMe3), or Fe(η-C3H5)(CO)3.

Radical-anion chemistry of carbon monoxide

The ability of CO to from a radical-anion, CO˙–, which can react further with CO or CO˙– with formation of C–C bonds has been suggested by literature data on (i) the reaction of CO with alkali metals, (ii) the electrochemical reduction of CO to squarate dianion, and (iii) adsorption of CO on metal oxides; we show that solutions containing Na and K anions are also effective reductants.

Free-radical displacement reactions at a transition-metal centre: bimolecular homolytic substitution (SH2) at square-planar PtII

The displacement of the free radical R· from the complex cis-PtR2X2(R = Me, Et, Me3SiCH2, or PhCH2; X = PAlk3 or PMe2Ph; Alk = Et, Prn, or Bun) by ButO· or PhS· is demonstrated by e.s.r. spectroscopy (direct detection or spin-trapping) or by initiation of a chain reaction involving R·.