Anne L. Rieger

Synthesis of the 17-electron cations [FeL(L′)(NO)2]+(L, L′= PPh3, OPPh3): structure and bonding in four-co-ordinate metal dinitrosyls, and implications for the identity of paramagnetic iron dinitrosyl complex catalysts

The complex [FeL2(NO)2](L = PEt31a, L = PPh31b or L2= dppe 1c) prepared from [{Fe(µ-I)(NO)2}2] and PPh3 or Ph2PCH2CH2PPh2(dppe){in the presence and absence of [Co(cp)2](cp =η5-C5H5) respectively} undergo one-electron oxidation at a platinum electrode in CH2Cl2. The complex [{Fe(µ-dppm)(NO)2}2]2, prepared from [{Fe(µ-I)(NO)2}2] and Ph2PCH2PPh2(dppm) in the presence of [Co(cp)2], undergoes two sequential one-electron oxidations. Complex 1b with [Fe(cp)2]+ gave 1b+, X-ray studies of which show a distorted tetrahedral geometry with near C2v symmetry. Oxidation of 1b leads to substantial lengthening of the Fe–P bonds and changes in the P–Fe–P and N–Fe–N angles. These changes are consistent with significant Fe–P π-bonding character in the singly occupied molecular orbital of 1b+. Cation 1b+ reacts with halide ions, giving [FeX(PPh3)(NO)2](X = Cl or I) and then [FeX2(NO)2]–, and with OPPh3 to give [Fe(OPPh3)(PPh3)(NO)2]+3. X-Ray studies on the last, as its [PF6]– salt, show a distorted tetrahedral geometry; the co-ordination angles at iron approach trigonal bipyramidal with the PPh3 ligand in one apical site and the other apical site vacant. The complex [Fe(OPPh3)2(NO)2]+4+ resulted from the reaction between [{Fe(µ-I)(NO)2}2] and OPPh3 in the presence of TlPF6. An analysis of the ESR spectra of the paramagnetic cations 1b+, 3+ and 4+, together with extended-Huckel MO calculations on models of 1b+ and 3+, suggest that the complex catalysts formed from [{Fe(µ-Cl)(NO)2}2] and Ag+ or Tl+ are also four-co-ordinate 17-electron radicals. A crystallographic database study of four-co-ordinate dinitrosyl complexes of iron and other metals confirms that the N–Fe–N and O ⋯ Fe ⋯ O angles are linearly related. Consideration of these geometric effects, and those resulting from oxidation of 1b, in the light of a model proposed by Summerville and Hoffmann provides insight into the bonding in these and related species.

Intramolecular electron transfer in linear trinuclear complexes of copper(I), silver(I) and gold(I) bound to redox-active cyanomanganese ligands

The reaction of [Cu(NCMe)4][PF6] with 2 equivalents of [Mn(CN)Lx]{Lx=(CO)(dppm)2, cis-or trans-(CO)2[P(OR)3](dppm)(R = Ph or Et, dppm = Ph2PCH2PPh2)} in CH2Cl2 gave [Cu{µ-NC)MnLx}2][PF6]. With 2 equivalents of [Mn(CN)Lx] in toluene, AgPF6 gave [Ag{(µ-NC)MnLx}2]+{Lx=cis- or trans-(CO)2[P(OR)3](dppm)(R = Ph or Et)} but in CH2Cl2cis-[Mn(CN)(CO)2(PEt3)(dppe)](dppe = Ph2PCH2CH2PPh2) or trans-[Mn(CN)(CO)(dppm)2] and AgX (X = BF4–, PF6– or SbF6–) gave the tricationic manganese(II) complexes [Ag{(µ-NC)Mn(CO)(dppm)2}2][PF6]3 and [Ag{(µ-NC)MnLx}2]X3{Lx=trans-(CO)2(PEt3)(dppe)]; the complexes [Ag{(µ-NC)MnLx}2][PF6]3{Lx=trans-(CO)2(P(OR)3](dppm)(R = Ph or Et)} were prepared directly from Ag[PF6] and trans-[Mn(CN)(CO)2{P(OR)3}(dppm)][PF6](R = Ph or Et) in CH2Cl2. Treatment of [AuCl(tht)](tht = tetrahydrothiophene) with [Mn(CN)Lx] in CH2Cl2 in the presence of Tl[PF6] yielded [Au{(µ-NC)MnLx}2][PF6]{Lx=(CO)(dppm)2, cis- or trans-(CO)2[P(OR)3](dppm)(R = Ph or Et)}. X-Ray structural studies on [Ag{(µ-NC)MnLx}2][PF6]{Lx=trans-(CO)2[P(OPh)3](dppm)}, [Au{(µ-NC)MnLx}2][PF6]{Lx=trans-(CO)2[P(OEt)3](dppm)}, and [Ag{(µ-NC)MnLx}2][PF6]3[Lx=(CO)(dppm)2] showed, in each case, near linear Mn–CN–M′–NC–Mn skeletons (M′= Ag or Au); the Mn–P and P–substituent bond lengths are consistent with octahedral MnI and MnII centres in the monocations and trication respectively. Each of the complexes [M′{(µ-NC)MnLx}2][PF6]{M′= Cu or Au, Lx=(CO)(dppm)2; M′= Cu or Ag, Lx=trans-(CO)2[P(OR)3](dppm)(R = Ph or Et)} showed one reversible two-electron oxidation wave at a platinum electrode in CH2Cl2; the trication [Cu{(µ-NC)Mn(CO)(dppm)2}2]3+ was generated in solution by controlled potential electrolysis of [Cu{(µ-NC)Mn(CO)(dppm)2}2]+, and [Au{(µ-NC)Mn(CO)(dppm)2}2][PF6]3 was prepared by chemical oxidation of [Au{(µ-NC)Mn(CO)(dppm)2}2][PF6] with [Fe(cp)2][PF6](cp =η-C5H5) in CH2Cl2. Magnetic and ESR spectroscopic studies provided further evidence for the presence of two isolated low-spin MnII centres in the trications [Ag{(µ-NC)MnLx}2]3+{Lx=(CO)(dppm)2, trans-(CO)2[P(OR)3](dppm)(R = Ph or Et) or trans-(CO)2(PEt3)(dppe)}. By contrast, [Au{(µ-NC)MnLx}2]+{Lx=trans-(CO)2[P(OR)3(dppm)(R = Et or Ph)} showed two reversible one-electron oxidation waves corresponding to the stepwise formation of di- and tri-cations. Electrolytic oxidation of [Au{(µ-NC)MnLx}2]+ in tetrahydrofuran, or chemical oxidation with [N(C6H4Br-p)3]+ or [Fe(η-C5H4COMe)(cp)]+ in CH2Cl2, gave solutions of [Au{(µ-NC)MnLx}2]2+{Lx=trans-(CO)2[P(OEt)3](dppm)}, IR spectroscopic and voltammetric studies on which are compatible with weak interaction between the two manganese centres in the mixed-valence dication.

ESR spectroscopic characterisation of a coordinated alkyne radical, and redox-induced vinylidene–alkyne complex isomerisation

One-electron oxidation of [Cr(CO)2(RC2R)(η-C6Me6)]1 gives paramagnetic [Cr(CO)2(RC2R)(η-C6Me6)]+1+ in which the unpaired electron in highly delocalised onto the alkyne; the radical cation (1+; R = SiMe3) is an intermediate in the oxidative isomerisation of the vinylidene complex [Cr(CO)2{CC(SiMe3)2}(η-C6Me6)]2 to [Cr(CO)2(Me3SiC2SiMe3)(η-C6Me6)](1; R = SiMe3).

Dramatic tensor-axis non-coincidence effects in the electron spin resonance spectra of some low-spin manganese(II) complexes

ESR spectra are reported for frozen CH2Cl2/ClCH2CH2Cl solutions of 14 low-spin manganese(II) cations: [(η5-C5H5)Mn(CO)L2]+, L = 1/2 Ph2PCH2CH2PPh2(dppe), 1/2 Me2PCH2CH2PMe2(dmpe), 1/2 Ph2PCH2PPh2(dppm), PMe3, PPh3; [(η5-MeC5H4)Mn(CO)dppe]+; [(η5-6-exo-PhC6H6)Mn(CO)L2]+, L = 1/2 dppe, 1/2 dmpe, PMe3; [(η5-6-exo-PhC6HMe5)Mn(CO)dppe]+; [(η5-6-exo-PhC7H8)Mn(CO)dppe]+; [(η5-C5H5-Mn(CO)2PPh3]+; and [(η5-6-exo-PhC6H6)Mn(CO)2L]+, L = PMe3, PnBu3. The spectra show anisotropic 55Mn hyperfine coupling and nearly isotropic 31P hyperfine coupling. There are dramatic departures from first-order line spacings which result from non-coincidence of the X and Z principal axes of the g and the 55Mn hyperfine tensors. In most cases, the ESR parameters can be interpreted in terms of a semi-occupied molecular orbital (SOMO) primarily dx2–y2 in character, but rotated about the y axis to avoid an antibonding interaction with the dienyl ring. The spectrum of [(η5-6-exo-PhC6H6)Mn(CO)(PMe3)2]+ shows non-equivalent 31P couplings, suggesting an unsymmetrical dienyl ring conformation. This spectrum, and those of the monophosphine cations, are best explained in terms of a SOMO primarily dyz in character.

Electron paramagnetic resonance study of the 19-electron Co(CO)3L2 and Co(CO)2L2PPh3 complexes [L2= 2,3-bis(diphenylphosphino)maleic anhydride]

Isotropic electron paramagnetic resonance spectra of square pyramidal Co(CO)3L2 and trigonal bipyramidal Co(CO)2PPh3L2 in toluene, dichloromethane and THF solutions show small 59Co hyperfine couplings and g-values close to the free-electron value, suggesting that the complexes are best described as CoI coordinated by an L2 radical anion. The frozen solution spectrum of Co(CO)3L2 in toluene or THF shows anisotropic 59Co coupling corresponding to a 3d spin density of 0.016. In both cases, the 59Co and 31P hyperfine couplings are strongly temperature dependent. Complex linewidth effects in the spectra of the PPh3 derivative suggest a pseudorotation process which interconverts trigonal bipyramidal and square pyramidal conformations, averaging the couplings of the 31P nuclei of the L2 ligand. Analysis of spectra of CH2Cl2 solutions leads to activation parameters for pseudorotation: ΔH‡= 14 kJ mol–1, ΔS‡=–66 J mol–1 K–1.

17-Electron alkynyl complexes of cyclopentadienyliron(III)

The complexes [Fe(CCR)(L–L)(η-C5R′5)][R = But, CO2Me, CO2Et, SiMe3, Ph or CH2OMe, L–L = Ph2PCH2PPh2(dppm), R′= H; R = But or Ph, L–L = Ph2PCH2CH2PPh2(dppe), R′= Me] undergo reversible one-electron oxidation at a platinum electrode in CH2Cl2. Chemical oxidation with [Fe(η-C5H5)2][PF6] gave the isolable salts [Fe(CCR)(dppe)(η-C5Me5)][PF6](R = But or Ph) which Mossbauer spectroscopy suggests to be complexes of FeIII. The ESR spectra of these salts, and of the cations [Fe(CCR)(dppm)(η-C5H5)]+(R = But, CO2Me, CO2Et or Ph), generated by in situ oxidation with [Fe(η-C5H5)2][PF6], are similar to those of low-spin d5 complexes of CrI, MnII and FeIII.

Paramagnetic tetrahedral dinitrosyliron complexes containing redox-active cyanomanganese ligands

The redox-active cyanomanganese ligands trans-[Mn(CN)(CO)2{P(OEt)3}(dppm)](dppm = Ph2PCH2PPh2), cis-[Mn(CN)(CO)2(PEt3)(dppe)](dppe = Ph2PCH2CH2PPh2) and trans-[Mn(CN)(CO)(dppm)2] reacted with [{Fe(µ-I)(NO)2}2] in CH2Cl2 to give the heterobinuclear complexes [FeI{(µ-NC)MnLx}(NO)2]{Lx=trans-(CO)2[P(OEt)3](dppm)1, cis-(CO)2(PEt3)(dppe)2 and trans-(CO)(dppm)23}; the molecular structure of 3 is consistent with a tetrahedral iron(–I) centre bound to octahedral manganese(I) by a near linear Mn–CN–Fe bridge. The ESR spectra of complexes 1–3 are very similar to those of the tetrahedral Fe–1 complex [FeI2(NO)2]–. Complexes 1–3 reacted with PPh3 in the presence of TIPF6 to give [Fe(PPh3){(µ-NC)MnLx}(NO)2][PF6]{Lx=trans-(CO)2[P(OEt)3](dppm)5+, cis-(CO)2(PEt3)(dppe)6+ and trans-(CO)(dppm)27+} which were also prepared from [Fe(PPh3)2(NO)2][PF6] and the appropriate cyanomanganese ligand in a 1 : 1 ratio; the related cation [Fe(PPh3){(µ-NC)MnLx}(NO)2]+{Lx=trans-(CO)2[P(OPh)3](dppm)8+} was generated in solution. The ESR spectra of complexes 6+–8+ showed hyperfine coupling to N(O), P, N(C) and Mn, suggesting a structure similar to that of [Fe(PPh3)(OPPh3)(NO)2]+ with some delocalisation through the cyanide bridge to manganese. Treatment of 2 equivalents of trans-[Mn(CN)(CO)2{P(OEt)3}(dppm)] or cis-[Mn(CN)(CO)2(PEt3)(dppe)] with [Fe(PPh3)2(NO)2][PF6] gave the heterotrinuclear complexes [Fe{(µ-NC)MnLx}2(NO)2][PF6]{Lx=trans-(CO)2[P(OEt)3](dppm)9+ and cis-(CO)2(PEt3)(dppe)10+} which may also be prepared from the reaction of 1 with trans-[Mn(CN)(CO)2{P(OEt)3(dppm)] or 2 with cis-[Mn(CN)(CO)2(PEt3)(dppe)] in the presence of TIPF6. Complexes 1–10+, which contain diamagnetic MnI and paramagnetic Fe–I centres, undergo oxidation and reduction at a platinum electrode in CH2Cl2. The MnII derivatives 3+ and 72+ and the Fe–II complex 7 have been generated in solution by ferrocenium ion oxidation or cobaltocene reduction respectively.

PHOTOOXIDATION OF MAGNESIUM TETRAPHENYLPORPHYRIN IN THE PRESENCE OF METHYL VIOLOGEN

Abstract— A study of the photooxidation of magnesium tetraphenylporphyrin by oxygen in methanol with methyl viologen present has been carried out. The reaction was normally followed by the decrease of Soret band absorbance with time. The influences of air, methyl viologen, light wavelength and intensity, presence of ß‐carotene, pH, and MgTPP concentration were investigated. From bulk photolysis experiments, three products have been separated and partially characterized. A mechanism based on superoxide as the dominant oxidant is proposed.

Structure and bonding in low-spin octahedral manganese(II) carbonyls : ligand-set control of spin delocalisation

X-Ray structural studies on the redox pair trans-[Mn(CN)(CO)(dppm)2](dppm = Ph2PCH2PPh2) and trans-[Mn(CN)(CO)(dppm)2][PF6]·CH2Cl2 showed that one-electron oxidation results in changes consistent with depopulation of an orbital involved in Mn–P π-back bonding. The ESR spectra of trans-[Mn(CN)(CO)(dppm)2]+, [Mn(CO)(CNCH2CHCH2)(dppm)2]2+, trans-[Mn(CN)(CO)2(PEt3)(dppe)]+(dppe = Ph2CH2CH2PPh2) and trans-[MnBr(CO)2(PEt3)(dppe)]+ in frozen dichloromethane–dichloroethane (1:1) solutions at 90 K, and extended-Huckel molecular-orbital calculations on the model compounds [Mn(CN)(CO)(H2PCH2PH2)2]+, [Mn(CO)(CNMe)(H2PCH2PH2)2]2+, cis- and trans-[Mn(CN)(CO)2(PH3)3]+ and trans-[MnBr(CO)2(PH3)3]+, showed that the semi-occupied molecular orbital of these low-spin octahedral cyanomanganese(II) carbonyls is always primarily manganese dπ in character and in the plane of the phosphorus ligands, either aligned along the Mn(CN) axis as in trans-[Mn(CO)2(PEt3)(dppe)]+ or perpendicular to this axis as in trans-[Mn(CN)(CO)(dppm)2]. The relative arrangement of the cyanide and carbonyl ligands is shown to control the extent of spin delocalisation onto the cyanide ligand.

Electron spin resonance study of the frozen-solution equilibrium [Co{S2C2(CF3)2}2L]+ L ⇌[Co{S2C2(CF3)2}2L2][L = P(OR)3 or PPh(OMe)2; R = alkyl]

The ESR spectra have been recorded for a series of cobalt dithiolene complexes [Co(S2C2R2)2L][R = CF3; L = P(OMe)3, P(OEt)3, P(OBun)3 or PPh(OMe)2]. In toluene, tetrahydrofuran or CH2Cl2–1,2-C2H4Cl2 solutions containing a small excess of L, reversible spectral changes are observed between 165 and 130 K which correspond to the addition of a second ligand to form a six-co-ordinate complex. For L = P(OMe)3 in toluene, equilibrium constants were estimated from the spectra which lead to ΔH°=–10 kJ mol–1, ΔS°=–68 J K–1 mol–1 for the five-/six-co-ordinate equilibrium. Ligand addition is not observed when CF3 is replaced by Ph, 4-MeC6H4 or 4-MeOC6H4 or L = PPh3, PEt3, P(OPri)3, P(OCH2CF3)3 or P(OPh)3. Complexes with L =(Ph2P)2CH2, (Ph2PCH2)2 or [(MeO)2PCH2]2 are five-co-ordinate with no evidence for chelation. Thus both steric and electronic effects are critical to the ability of the five-co-ordinate complex to add a sixth ligand. Unlike the five-co-ordinate complexes which are significantly distorted from ideal C2v symmetry, the six-co-ordinate complexes are highly symmetrical with greater delocalization of the unpaired electron onto the phosphite ligands.