Philip H. Rieger

ESR Studies of Coordination to the Sixth Position of Vanadyl Acetylacetonate

The electron spin resonance spectra of vanadyl acetylacetonate (VOacac2) and a number of its adducts have been examined in benzene solution at ambient temperatures. It is found that the magnitude of the vanadium nuclear hyperfine splitting constant decreases from 108.19 to 105.3 G as amine adducts are formed, while the g value increases from 1.9695 to 1.9699. The anisotropies of the axially symmetric g and hyperfine tensors, Δg and b, and the rotational correlation time τR have been calculated from the linewidths of VOacac2, VOacac2·pyridine, and VOacac2·piperidine in benzene solution. The correlation time increases from 2.4×10−11 to 3.0×10−11 sec, |b| increases from 2.1×109 to 2.3×109 rad/sec, and |Δg| decreases from 0.034 to 0.029 on coordination of pyridine. Although the g values calculated from the electronic spectra using the theories of Ballhausen and Gray or Kivelson and Lee decrease slightly on adduct formation rather than increase as observed experimentally, the calculated anisotropies Δg follow ...

Electron paramagnetic resonance studies of low-spin d5 transition metal complexes

Abstract A discussion of the connection between EPR parameters and the electronic structures of mono-nuclear low-spin d 5 complexes, including systems with orbitally degenerate ground states which undergo Jahn-Teller distortions. EPR experimental results are critically surveyed for complexes of Ti(—I), V(0), Cr(I), Mn(II), Fe(III), Co(IV), and their heavier congeners. A review with 426 references.

E.S.R. spectra of substituted pyridine and pyrimidine anion radicals

Electron spin resonance spectra were obtained of the 3-nitropyridine, 3,5-dinitropyridine, 2-chloro-5-nitropyridine, 5-nitropyrimidine, 3-acetyl pyridine anion radicals produced electrolytically in dimethyl-sulphoxide solutions. Hyperfine splitting constants were assigned in some cases with the help of molecular orbital calculations. The 3-acetylpyridine anion exists in two isomeric forms, the spectra of which were observed. Ionization of chloride from 2-chloro-5-nitropyridine, 2-chloro-3-nitropyridine, and 2-chloro-5-nitropyrimidine occurred on reduction, yielding the 3-nitropyridine anion from the first two compounds, and the 5-nitropyrimidine anion from the third. Analysis of the data suggests that, for this series of radicals, the σ-π parameter Q cn n is in the range 0 ⩾ Q cn n ⩾ -2·5 gauss.

The substitution chemistry of the tris(3,5-dimethylpyrazolyl)-methanerhodium complex[Rh(CO) 2 {HC(pz') 3 }] +

The complex [Rh(CO)2{HC(pz′)3}][PF6], 1+[PF6]− {HC(pz′)3 = tris(3,5-dimethylpyrazolyl)methane}, prepared by reacting [{Rh(CO)2(μ-Cl)}2] with HC(pz′)3 in the presence of Tl[PF6], has a distorted square pyramidal structure with a κ3-HC(pz′)3 ligand. Carbonyl substitution with Lewis bases gives [Rh(CO)L{HC(pz′)3}][PF6] {L = PPh3, 2+[PF6]−; L = AsPh3, 3+[PF6]−; L = P(o-tolyl)3, 4+[PF6]−}, which have square planar κ2 structures, confirmed by X-ray crystallography for 2+[PF6]−. The cations 2+ and 3+ have the third pyrazolyl ring orientated pseudo-parallel to the square planar metal whereas 4+ more likely has the third ring orientated exo to that plane. One-electron oxidation of 2+ and 3+ gives the Rh(II) dications [Rh(CO)(PPh3){HC(pz′)3}]2+, 22+, and [Rh(CO)(AsPh3){HC(pz′)3}]2+, 32+, characterised by ESR spectroscopy. Complex 1+[PF6]−reacts with PhCCPh to give [Rh(CO)(η2-PhCCPh){HC(pz′)3}][PF6], 5+[PF6]−, in which the two-electron donor alkyne occupies an equatorial position in a trigonal bipyramidal κ3 structure. With MeCCR (R = Me or Et), 1+[PF6]− gives the κ2 square planar complexes [Rh{η4-C4Me2R2C(O)}{HC(pz′)3}][PF6] (R = Me, 6+[PF6]−; R = Et, 7+[PF6]−) in which the cyclopentadienone ligands are coordinated via two Rh–monoalkene bonds; the structurally characterised form of 7+ has the two alkyne units linked head-to-head with the CEt termini bound to the ketonic CO group. With HCCPh or HCCH, 1+ gives the octahedral, κ3 rhodium(III) metallacyclopentadienes [Rh(CO)(η1:η1′-CHCRCHCR){HC(pz′)3}][PF6] (R = Ph, 8+[PF6]−; R = H, 9+[PF6]−) with the two alkynes linked head-to-tail in 8+. The reaction of 1+ with HCCH also gives the cycloheptatrienone (tropone) derivative [Rh{η4-C6H6C(O)}{HC(pz′)3}][PF6], 10+[PF6]−, with a κ3 ligand and the cycloheptatrienone ligand bound to the metal via two Rh–C σ-bonds and one Rh–monoalkene interaction.

Spectrophotometric investigation of vanadyl phosphate equilibria in acid media

Abstract The formation of vanadyl phosphate complexes in acidic LOM NaClO 4 solution was studied by spectrophotometry in the charge-transfer region of the optical spectrum. In the [H + ] range studied, 0.4–13 mM, the principal species appear to be VO(H 2 PO 4 ) + and VO(H 2 PO 4 ) 2 , formed with logarithmic stepwise formation constants of approximately 3.2 and 2.0, respectively. Both species are weakly acidic, ionizing to give VO(HPO 4 ) and VO(H 2 PO 4 )(HPO 4 ) − with pK a values on the order of 3–4. Results obtained for [H + ] = 13 mM are anomalous and are not explained by these equilibrium constants.

Redox routes to arenechromium complexes of two-, three- and four-electron alkynes; structure and bonding in paramagnetic [Cr(CO)L(η-RCCR)(η-arene)]+

X-Ray structural studies on the redox pair [Cr(CO)2(η-PhCCPh)(η-C6Me5H)]z (z = 0 and 1) show that one-electron oxidation of the neutral complex results in a shortening of the Cr–Calkyne bonds and a lengthening of the Cr–C(O) bonds, consistent with depopulation of a HOMO antibonding with respect to the metal–alkyne interaction. Oxidation leads to an increase in the substitutional lability of the Cr–CO bonds so that [Cr(CO)2(η-RCCR)(η-C6Me6)]+ (R = Ph or C6H4OMe-p) reacts with Lewis bases to give [Cr(CO)L(η-RCCR)(η-C6Me6)]+ {L = CNXyl, P(OMe)3 and P(OCH2)3CEt}, X-ray studies on which show a rotation of the alkyne to align with the remaining Cr–CO bond. ESR spectroscopic studies on [Cr(CO)L(η-RCCR)(η-C6Me6)]+ show delocalisation of the unpaired electron onto the alkyne ligand, consistent with its description as a three-electron donor. The cations [Cr(CO)L(η-RCCR)(η-C6Me6)]+ undergo both one-electron reduction and oxidation, and chemical oxidation of [Cr(CO){P(OCH2)3CEt}(η-p-MeOC6H4CCC6H4OMe-p)(η-C6Me6)]+ with AgPF6 gives the dication [Cr(CO){P(OCH2)3CEt}(η-p-MeOC6H4CCC6H4OMe-p)(η-C6Me6)]2+. Thus the two-electron alkyne of [Cr(CO)2(η-RCCR)(η-C6Me6)] is converted into the four-electron alkyne of [Cr(CO)L(η-RCCR)(η-C6Me6)]2+ by an ECE (E = electrochemical, C = chemical) process in which all of the intermediates have been fully characterised.

The decomposition of peroxomonosulfate induced by manganese(II) in acid solution

Abstract The rate law for the manganese(II)-induced decomposition of peroxomonosulfate ion, HOOSO 3 − , is with rate constants k Mn = (5.1±0.3)×10 −3 and (9.7±0.2)×10 −3 M − 1 2 s −1 at 50 °C in H 2 SO 4 and HClO 4 media, respectively. The rate constant is sensitive to the nature and concentration of the anion. The values of Δ H ‡ and Δ S ‡ are 31±2 kJ mol −1 and −194±5 J mol −1 K −1 , respectively, in 1 M H 2 SO 4 . The presence of manganese(III) as an intermediate and as a minor product is confirmed. Peroxodisulfate, a minor product presumably from chain termination, has been used to estimate the chain length. On the basis of all the data, a mechanism quite different from most metal-induced peroxomonosulfate decompositions is postulated and some consequences and complications are elaborated.

Cluster chemistry. Reactions between metal carbonyl clusters and Lewis bases initiated by radical ions: improved syntheses of substituted derivatives of M3and M4 clusters(M = Fe, Ru, Os or Co)

Addition of catalytic amounts of Ph2Co˙–to a mixture of the appropriate cluster with stoicheiometric amounts of Lewis base in tetrahydrofuran gave the substituted derivatives [Fe3(CO)11(L)],[Ru3(CO)12–n(L)n],[Os3(CO)11(L)], and [H4Ru4(CO)12–n(L)n][n= 1–3; L = ButNC,PR3,P(OR)3]in greatly improved yields.

Re-examination of the electron spin resonance spectrum of trigonal-prismatic bis[pentane-2,4-dione benzoylhydrazonato(2–)]vanadium(IV)

The e.s.r. spectrum of the title complex has been reinterpreted in terms of completely anisotropic g and hyperfine tensors. Analysis of these tensors indicates that the unpaired electron occupies an orbital which is ca. 88% metal 3d in character (78%dz2, 10%dx2–y2) and 2% metal 4s. Except for the anisotropy of the 3d contribution, this is virtually identical to the corresponding orbital in [V(mnt)3]2–(mnt = maleonitriledithiolate).

31P magnetic resonance study of phosphate exchange in vanadyl phosphate complexes

The rate of loss of H2PO4− ligand from bis-dihydrogenphosphato oxovanadium(IV) was determined from analysis of broadening of the 31P magnetic resonance line of LOM NaH2PO4 solutions in the presence of vanadyl ion at pH 2.0. The aquation rate constant at 25°C was found to be (2.5 ± 0.3) × 104 sec−1 and the corresponding anation rate constant, estimated from the formation constant of the bis complex, was 2 × 106M−1 sec−1. The activation energy for the aquation process is 52 kJ/mol. The anation rate indicates that coordinated water in VO(H2PO4)+ is much more labile than in the aquo complex. The linewidths of metal-free phosphate solutions at pH 2 were found to be linear in T/η and are thus apparently due to a spin-rotation interaction.