Axial Pyridine Research Articles

Coordination Polymers Constructed from Paddle-Wheel Building Units

The two structurally related coordination polymers [Cu(ndc)(pyridine)], CPO-2-Cu, and [Zn(ndc)(3,4-lutidine)], CPO-2-Zn, were obtained by hydrothermal reactions between 2,6-naphthalenedicarboxylic acid (ndc), pyridine and copper(II) nitrate (CPO-2-Cu) and ndc, 3,4-lutidine and zinc(II) nitrate (CPO-2-Zn), respectively. The compounds are based on the binuclear paddle–wheel building unit. In both compounds these building units are connected into 2D sheets by naphthalene rings. In the third dimension there are weaker interactions involving the axial ligands pyridine and 3,4-lutidine. The sheets are stacked so that large 1D channels are formed into which the axial ligands protrude. The crystal structure of CPO-2-Cu was solved from synchrotron powder X-ray data, while the crystal structure of CPO-2-Zn was solved from conventional single-crystal X-ray data. Crystal data for CPO-2-Cu: Monoclinic space group C2/m (No. 12), a=10.2252(2), b=19.0915(4), c=8.0521(2) Å, β=98.824(1)°, V=1553.30(7) Å3 and Z=4. Crystal data for CPO-2-Zn: Triclinic space group P−1 (No. 2), a=7.540(1), b=10.711(1), c=11.196(2) Å, α=66.490(5)°, β=87.265(6)°, χ=88.470(6)°, V=828.2(2) Å3 and Z=2. The thermal properties of both compounds were investigated as well as the magnetic properties of CPO-2-Cu.

The metal–carbon stretching frequencies in methyl complexes of Rh, Ir, Ga and In with porphyrins and a tetradentate pyridine–amide ligand

Interactions of trans-[Rh(bpb)(CH 3)(H 2O)] ( 1) [bpb=1,2-bis(2-pyridinecarboxamido)benzene] with Lewis bases L afford the respective adducts trans-[Rh(bpb)(CH 3)(L)], where L=4-substituted pyridine 4-Xpy (X=H ( 2), t Bu ( 3), NMe 2 ( 4) CN ( 5)), PMe 2Ph ( 6) or benzimidazole ( 7). The structures of complexes 3 and 6 have been established by X-ray crystallography. The Rh–C distance in complex 6 (2.095(6) Å) is longer than that in complex 3 (2.02(1) Å), indicating that PMe 2Ph has a stronger trans influence than 4- t Bupy. The metal–carbon stretching frequencies for trans-[Rh(bpb)(CH 3)(L)] and [M(TTP)(CH 3)] [TTP=5,10,15,20-tetrakis(4-methylphenyl)porphyrin dianion; M=Rh, Ir, Ga, In] have been determined by near IR FT-Raman spectroscopy. Complex 1 exhibits ν(Rh–C) at 562 cm −1, which downshifts to 532 cm −1 upon deuteriation of the axial methyl group. Replacement of the aquo ligand in complex 1 with nitrogen ligands or phosphine resulted in downshift in ν(Rh–C). The ν(Rh–C) for trans-[Rh(bpb)(CH 3)(L)] was found to decrease in the order L: PMe 2Ph≫4-Xpy∼BzIm>H 2O, consistent the order of trans influence of L. For [M(TTP)(CH 3)] (M=Co, Rh, Ir, Ga, or In) the M–C force constant was found to decrease in the orders Ir>Rh>Co and Ga>In, consistent with the trends of metal–carbon bond strength for these metals. For trans-[Rh(bpb)(CH 3)(4-Xpy)] and trans-[Rh(TTP)(CH 3)(4-Xpy)], the ν(Rh–C) were found to be not very sensitive to the nature of X, suggesting that the electronic factors of the axial pyridine ligand do not have a significant effect on the Rh–C bonds for these rhodium alkyl complexes.

High-frequency and field EPR investigation of (8,12-diethyl-2,3,7,13,17,18-hexamethylcorrolato)manganese(III).

High-field and frequency electron paramagnetic resonance (HFEPR) of solid (8,12-diethyl-2,3,7,13,17,18-hexamethylcorrolato)manganese(III), 1, shows that in the solid state it is well described as an S = 2 (high-spin) Mn(III) complex of a trianionic ligand, [Mn(III)C(3)(-)], just as Mn(III) porphyrins are described as [Mn(III)P(2)(-)](+). Comparison among the structural data and spin Hamiltonian parameters reported for 1, Mn(III) porphyrins, and a different Mn(III) corrole, [(tpfc)Mn(OPPh(3))], previously studied by HFEPR (Bendix, J.; Gray, H. B.; Golubkov, G.; Gross, Z. J. Chem. Soc., Chem. Commun. 2000, 1957-1958), shows that despite the molecular asymmetry of the corrole macrocycle, the electronic structure of the Mn(III) ion is roughly axial. However, in corroles, the S = 1 (intermediate-spin) state is much lower in energy than in porphyrins, regardless of axial ligand. HFEPR of 1 measured at 4.2 K in pyridine solution shows that the S = 2 [Mn(III)C(3)(-)] system is maintained, with slight changes in electronic parameters that are likely the consequence of axial pyridine ligand coordination. The present result is the first example of the detection by HFEPR of a Mn(III) complex in solution. Over a period of hours in pyridine solution at ambient temperature, however, the S = 2 Mn(III) spectrum gradually disappears leaving a signal with g = 2 and (55)Mn hyperfine splitting. Analysis of this signal, also observable by conventional EPR, leads to its assignment to a manganese species that could arise from decomposition of the original complex. The low-temperature S = 2 [Mn(III)C(3)(-)] state is in contrast to that at room temperature, which is described as a S = 1 system deriving from antiferromagnetic coupling between an S = (3/2) Mn(II) ion and a corrole-centered radical cation: [Mn(II)C(*)(2-)] (Licoccia, S.; Morgante, E.; Paolesse, R.; Polizio, F.; Senge, M. O.; Tondello, E.; Boschi, T. Inorg. Chem. 1997, 36, 1564-1570). This temperature-dependent valence state isomerization has been observed for other metallotetrapyrroles.

Transition metal complexes with thiosemicarbazide-based ligands. Part XLI. Two crystal structures of cobalt(III) complexes with salicylaldehyde S-methylisothiosemicarbazone and theoretical study on orientations of coordinated pyridines

Two compounds, [Co III(L)(py) 3][Co II(py)Cl 3]·EtOH and [Co III(L)(py) 3]I 3 (H 2L=salicylaldehyde S-methylisothiosemicarbazone, py=pyridine), were synthesized and the crystal structures determined by single-crystal X-ray diffraction. In both structures the geometries of the cation are very similar to a thiosemicarbazide-based ligand coordinated in the mer configuration. The axial pyridines are in mutual perpendicular orientation, the angles between the planes being 85.3(2) and 82.5(2)°. The plane of the equatorial pyridine is tilted with respect to the equatorial plane by about 40°. The orientations of the pyridines were studied in model systems by quantum chemistry calculations. It was shown that the interactions between axial and equatorial pyridines are responsible for the orientation of pyridines in the complex cation; consequently, there are very similar geometries of the complex cation in both crystal structures. The compounds were also characterized by elemental analysis, molar conductivity, magnetic susceptibility and electronic absorption spectra.

Superimposed saddle and ruffled distortions of the porphyrin in iodo(pyridine-N)(5,10,15,20-tetraphenylporphyrinato-kappa(4)N)rhodium(III) toluene solvate.

In the title compound, [RhI(C(44)H(28)N(4))(C(5)H(5)N)].C(7)H(8), the porphyrin ring experiences significant distortion from planarity (a saddle conformation with a superimposed ruffling), as a result of steric interactions with the 2,6-H atoms of the axial pyridine ligand. This also leads to a slight lengthening of the Rh-pyridine bond [Rh-N 2.102 (7) A] relative to those seen in other pyridine adducts of six-coordinate Rh(III). The metric parameters of the porphyrin core are comparable with those of related metalloporphyrin derivatives. No significant intermolecular interactions are observed between the metalloporphyrin and disordered solvate species.

Laser Photolysis of Chromium(III) Porphyrins with Axial Pyridines in Dichloromethane and Toluene Solutions. Novel Effects of a Hydrogen Bond in the Ligand Exchange Reaction

Laser photolysis studies were carried out for (chloro)(pyridine)(5,10,15,20-tetraphenylporphyrinato)chromium(III), [Cr(TPP)(Cl)(Py)], in both dichloromethane and toluene containing water. The five-coordinate [Cr(TPP)(Cl)] produced by the photoinduced dissociation of pyridine from [Cr(TPP)(Cl)(Py)] initially reacts with H2O to give [Cr(TPP)(Cl)(H2O)], which eventually exchanges the axial H2O with Py to regenerate [Cr(TPP)(Cl)(Py)]. The rate for the ligand exchange of [Cr(TPP)(Cl)(H2O)] with exogenous Py is found to exhibit a bell-shaped pyridine-concentration dependence. Kinetic studies revealed that at a high Py concentration, the exogenous Py probably makes a hydrogen bond with the axial H2O of [Cr(TPP)(Cl)(H2O)] to yield [Cr(TPP)(Cl)(HO−H···Py)] as a dead-end complex. A similar structure of the Cr−TPP complex having 2-methylpyridine molecules bound to the coordinated H2O ligand by a hydrogen bond was determined by X-ray structure analysis. The exchange reaction of the axial HO−H···Py in [Cr(TPP)(Cl)(HO−...

Syntheses, structures, and properties of tetrakis(mu-acetato)dirhodium(II) complexes with axial pyridine nitrogen donor ligands with or without assistance of hydrogen bonds.

Eight adducts of Rh2(O2CCH3)4 with axial pyridine derivatives that contain hydrogen-bonding amino and/or steric methyl substituents in the 2- and 6-positions have been prepared and examined by electronic absorption and 1H NMR spectroscopy in solution and by elemental, IR, thermogravimetric, and X-ray diffraction analyses in the solid state. The results indicated that strong hydrogen bonding interactions between Rh2(O2CCH3)4 and axially coordinated pyridine derivatives with a 2- or 6-amino group occur in both solution and the solid state and contribute to the higher thermal stability of the molecular assembly of dirhodium complexes. It was demonstrated that such a combination of coordinate and hydrogen bonds is useful as a building tool in designing and constructing new organic-inorganic hybridized compounds and supramolecular architectures.

Dirhodium(II) carboxylates as building blocks. Synthesis and structures of cis-chelate complexes

Dirhodium tetracarboxylate complexes (C)Rh2(OAc)2 and (C)2Rh2, where C is a chelating dicarboxylate of general form meta-C6H4(OC(CH3)2CO2−)2, were prepared by heating Rh2(OAc)4 and diacids in N,N-dimethylaniline. The structures of six new complexes with one or two chelate rings were obtained, with pyridine ligands (or in one case N,N-dimethylaniline) co-ordinated to rhodium. The geometries of the molecules are similar, with the aromatic part of the chelate ring tilted out of the plane of the Rh–O cage by 57.5 to 66.6°. The monochelate Rh2{C6H4(OC(CH3)2CO2)2-m}(OAc)2·2ButPy (ButPy = 4-tert-butylpyridine) packed in the crystal to generate a hexagonal network of channels of (minimum) diameter 5 Å due to vertical stacking of its axial pyridine ligands. Several other instances of intramolecular CH–π and intermolecular π–π interactions were noted in the packing diagrams. The bischelate Rh2{Ar(OC(CH3)2CO2)2-m}2·PhNMe2, with Ar = 4,6-di-tert-butylphenyl, has a polymeric structure displaying a new mode of arene co-ordination. The aromatic ring of the N-bound aniline co-ordinates to a neighbouring dirhodium complex via the para carbon atom with a Rh–C distance of 2.709 Å.

The reactivity of dibenzotetramethyltetraaza[14]annulene-Mn(II): functionalisation of manganese in a macrocyclic environment

The present report deals with the one electron oxidation and the one- and two-electron reduction of [Mn(tmtaa)L], [L = THF, 1a; L = none, 1b; tmtaa = dibenzotetramethyltetraaza[14]annulene]. The former class of reactions led to the formation of a variety of functionalised Mn(III) species, [(tmtaa)Mn–X] [X = I, 2; X = Cl, 3; X = NCS, 5], including the cationic derivatives [Mn(tmtaa)(L)(L′)]BPh4 [L = L′ = THF, 4a; L = L′ = DME, 4b; L = Py, L′ = none, 4c]. The magnetic properties of the Mn(III) derivatives are strongly dependent on the axial ligand. Reaction of 1a with NO gave [(tmtaa)Mn(NO)], 6, which takes up an axial pyridine leading to the diamagnetic [(tmtaa)Mn(NO)(Py)], 7. Complex 6 displays a peculiar magnetic behavior, which has been interpreted as a S = 0 S = 2 spin equilibrium with a ΔHeff = 4.0 kJ mol−1 and a ΔSeff = 11.3 J mol−1 K−1 associated to the spin transition process. Reaction of 1 with sodium metal in DME leads to [Mn2(μ2-tmtaa)2{Na·(DME)2}2], 8, while in THF [Mn2(*tmtaa2*)Na4·(THF)6], 9, is formed. Extended Huckel calculations have been performed for a better understanding of the magnetic and electronic properties of 6 and 9. The proposed structures have been supported by X-ray analyses of 3, 4b, 6, 8, and 9.

A First-Principles Quantum Chemical Analysis of the Factors Controlling Ruffling Deformations of Porphyrins: Insights from the Molecular Structures and Potential Energy Surfaces of Silicon, Phosphorus, Germanium, and Arsenic Porphyrins and of a Peroxidase Compound I Model

Using nonlocal density functional theory calculations, we have examined several factors influencing ruffling deformations of porphyrin and porphyrazine complexes. Because a ruffling distortion is often a direct result of a small macrocycle core size, which, in turn, is brought about by complexation of a central ion with a small ionic radius, this study focuses on the conformations and potential energy surfaces of porphyrin complexes with small central ions (herafter symbolized M) such as SiIV, PV, GeIV, and AsV. The optimized geometries exhibit ruffling torsion angles ranging from 0° for (P)GeIVF2 and (P)SiIV(C⋮CPh)2 to about 55° for [(P)PVF2]+. For relatively substantial ruffling distortions, a good linear correlation has been found between the ruffling torsion angle and the M−N distance for a wide variety of central ions including transition metals, for sterically unhindered porphyrins, and for a database including both experimental and optimized structures. The threshold between ruffling and planar structures is at M−N bond distances of 2.00−2.02 Å for sterically unhindered porphyrins and at 1.85−1.87 Å for porphyrazines. The calculations confirm an experimental observation that electron-withdrawing axial ligands lead to increased ruffling, especially for phosophorous and silicon porphyrins. The ortho hydrogens of axial phenyl ligands and the 2- and 6- hydrogens of axial pyridine ligands can sterically interfere with the porphyrin and contribute to ruffling. In addition to the M−N distance, a number of other geometrical parameters also vary systematically with the ruffling distortion. Thus, the Cα−Cmeso−Cα angle decreases with ruffling and the CβCβ distance and Cα−N−Cα angle increase with ruffling. These structural variations are reflected in a number of ruffling-sensitive vibrational frequencies.. The ruffled D2d geometries of [(P)PVF2]+ and [(P)PVCl2]+ are stabilized by 9.25 and 5.26 kcal/mol, respectively, relative to planar D4h symmetry-constrained optimized geometries. In contrast, the ruffled D2d geometries of [(Pz)PVF2]+ and of all the silicon complexes studied are more stable than the corresponding D4h symmetry-constrained optimized geometries by less than 0.01 kcal/mol. This underscores the extreme softness of ruffling deformations and shows that even fairly large distortions, where the ruffling torsion angle changes by up to 25°, can occur with almost no expenditure of energy. Finally, through a reinvestigation of a recent study of a peroxidase compound I model, we have uncovered a heretofore unsuspected role of Fe(3dxy)-porphyrin(atu) orbital interactions in macrocycle ruffling. Also seen for certain low-spin ferrihemes, this orbital interaction is not important for manganese porphyrins.

Magnetic Field Dependence of Ultrafast Intersystem-Crossing: A Triplet Mechanism on the Picosecond Time Scale?

The influence of strong magnetic fields up to 9 T on the decay kinetics of Ni2+−bacteriochlorophyll a was studied by femtosecond pump−probe spectroscopy. The appearance of a magnetic field effect depends on the initial electron spin state of the molecule, which can be switched by using suitable solvents. In toluene, where the central Ni2+ is tetracoordinated, the ground state has singlet multiplicity, while in pyridine, where in addition two solvent molecules bind to the Ni2+ ion, it is a paramagnetic triplet state. In toluene, no field dependence of the excited-state decay kinetics is observed, whereas in pyridine the decay is accelerated by strong magnetic fields, on a time scale of 10 ps. The field-dependent process has been identified as a loss of axial pyridine ligands which is accompanied by intersystem crossing to a singlet state. The field dependence is discussed in terms of a triplet mechanism, which is applicable if an electron spin relaxation time of ≥100 ps and a large zero-field splitting of ∼20 cm-1 are assumed.

Bis(isodiazene) and related complexes of molybdenum(VI). Syntheses and structures of [Mo(OTf )2(NNPh2)2(py)2], [MoCl(OTf )(NNPh2){NC5H3(CH2C(O)Ph2)2-2,6}], [{MoCl(NNPh2)2(μ-Cl)(NH2But)}2] and [MoTp′Cl(NNPh2)2] [OTf = O3SCF3, Tp′ = tris(3,5-dimethylpyrazolyl)hydroborate

Possible routes to [Mo(NNPh2)2R2], where R = alkyl or aryl groups, have been investigated including the interaction of [MoCl2(NNPh2)2(dme)] 1 (dme = 1,2-dimethoxyethane) with the standard alkylating agents LiR, RMgX, R2Mg or R2Zn under various conditions of solvent and temperature. There was, however, no evidence to suggest alkylation had taken place. Treatment of 1 with two equivalents of silver trifluoromethanesulfonate, silver triflate (AgOTf), in the presence of pyridine gave the golden-yellow bis(triflate) complex [Mo(OTf)2(NNPh2)2(py)2] 2 in good yield. In the presence of an excess of bipy (2,2′-bipyridine) the known dicationic species [Mo(NNPh2)2(bipy)2]2+ was isolated as its PF6– salt. Attempts to replace triflate by alkyl groups in 2 were unsuccessful. A similar reaction of 1 with silver triflate in the presence of the bulky tridentate alkoxide 2,6-bis(2-hydroxy-2,2-diphenylethyl)pyridine (H2L) led to loss of an NNPh2 group (presumably as hydrazine) and formation of the green mono-isodiazene complex [MoCl(OTf)(NNPh2)(L)] 3. The compound [Mo(CH2CMe2Ph)2(NBut)2] underwent partial imido ligand exchange with an excess of 1,1-diphenylhydrazine hydrochloride to give the brown bis(isodiazene) chloro-bridged dimer [{MoCl(NNPh2)2(µ-Cl)(NH2But)}2] 4. A chloride substitution reaction of 1 with potassium tris(3,5-dimethylpyrazolyl)hydroborate, (KTp′) afforded the bis(isodiazene) complex [MoTp′Cl(NNPh2)2] 5. The crystal and molecular structures of 2–5 have been determined. The complexes all contain linear isodiazene groups with Mo–N distances in the range 1.755(6) to 1.768(7) A. In 2 the molybdenum(VI) centre has a pseudo octahedral geometry with axial pyridines and cis-triflate groups. In six-co-ordinate 3 the pyridinediolate ligand has a meridional donor atom arrangement with an equatorial isodiazene ligand. The binuclear compound 4 has slightly asymmetric chloro bridges coplanar with the terminal isodiazene ligands. Complex 5 is octahedral, having a facially co-ordinated Tp′ ligand and linear NNPh2 ligands.

Axial Ligation Equilibria and Dynamics in a Redox Polyether Hybrid: An Fe Tetraphenylporphyrin Melt

Pyridine axial ligation is investigated in a molten Fe tetraphenylporphyrin fashioned by attaching oligomeric (MW 550) ethylene glycol chains at the p-phenyl ring positions ([Fe(T550PP)Cl]). Dissolution of LiClO4 electrolyte renders the highly viscous, room-temperature melt (0.40 M in porphyrin sites) modestly ionically conductive, so that microelectrode voltammetry can be employed to follow axial pyridine ligation in a thin film of Fe porphyrin melt resting on the electrode assembly. Introduction of pyridine at the gas/melt interface produces distinctive changes in the microelectrode voltammetry, from which relative populations of chloride and pyridine-coordinated Fe(III) porphyrin can be measured and equilibrium constants for ligation of the Fe(III) and Fe(II) oxidation states estimated. Removal of the pyridine at the porphyrin melt/gas interface causes dissociation of the ligating pyridine in an overall process that has a time constant on the order of 5−10 min at 25 °C.

Multitemperature Resonance-Diffraction and Structural Study of the Mixed-Valence Complex [Fe(3)O(OOCC(CH(3))(3))(6)(C(5)H(5)N)(3)

The Fe-O and Fe-N bond lengths at two iron sites of the mixed-valence complex [Fe(3)O(OOCC(CH(3))(3))(6)(C(5)H(5)N)(3)] show a pronounced temperature dependence; the bonds from two of the Fe atoms to the central oxygen atoms vary by more than 0.10 Å on cooling to 10 K whereas the bond from the third iron atom is essentially invariant. The variation is such that the longest Fe-O bonds at ambient temperature are the shorter ones at 10 K, with the crossover occurring at about 90 K. The bonds to the axial pyridine ligand show the opposite dependence. The variation is attributed to an equilibrium between different configurations, which interconvert through vibronic coupling, a process that involves electron transfer between the metal atoms. The position of the absorption edge for each of the iron atoms has been determined by resonance-diffraction experiments at the Fe K edge, performed at four different temperatures. At each temperature, the order of the absorption edges corresponds to that of the experimentally determined bond lengths. The crossover near 90 K is confirmed by the resonance experiments. The absorption-edge positions are related to the formal oxidation state by calibration with reference complexes of known oxidation state. The experiments demonstrate the close relation between the changes in coordination geometry and the oxidation states of the iron atoms.

Effect of a Sterically Hindered Imidazole Ligand on the Molecular Structure and Axial Ligand Substitution Reaction of the Chromium(III) Porphyrin Complex

Abstract A structural, thermodynamic, and kinetic study of a chromium(III) complex of 5,10,15,20-tetraphenylporphyrin, [Cr(tpp)(Cl)(L)], where L represents an imidazole ligand, is presented. The present study is aimed at elucidating the effect of a steric strain induced by the 2-methyl substituent of the imidazole ligand on the molecular structure and the dynamics of an axial ligand-substitution reaction. Crystals of the 1-methylimidazole complex [Cr(tpp)(Cl)(1-Meim)] (1) and 1,2-dimethylimidazole complex [Cr(tpp)(Cl)(1,2-Me2im)] (2) were obtained from a dichloroethane–toluene mixture containing a slight excess of imidazole ligand in the monoclinic space group Cc, Z = 4, a = 18.306(2), b = 13.311(2), c = 21.391(5) Å, β = 120.05(1)°, V = 4512(1) Å3, and in the orthorhombic space group P212121, Z = 4, a = 9.926(1), b = 17.902(3), c = 25.247(4) Å, V = 4486(1) Å3, respectively. The axial Cr–Nim bond lengths for 1 and 2 are 2.103(4) and 2.139(5) Å, respectively. A steric hindrance has been demonstrated by a 0.036 Å increase in the Cr–Nim bond length and a tilting and tipping of the imidazole ligand of 2 from the symmetrical position observed in 1. The substitution reaction of the axial pyridine ligand of [Cr(tpp)(Cl)(py)] by various imidazole ligands was studied spectrophotometrically in dichloromethane or in toluene. A steric effect due to the 2-methyl substituent of the imidazole ligand was observed along with an increase in the ligand-dissociation rate and a decrease in the binding constant of the imidazole ligand to the metalloporphyrin. These results were examined in terms of steric interactions between the axial ligand and the porphyrin core.

Efficient near IR sensitization of nanocrystalline TiO2 films by ruthenium phthalocyanines

Bis(3,4-dicarboxypyridine)(1,4,8,11,15,18,22,25-octamethyl- phthalocyaninato)ruthenium(II) (JM3306) anchored to nanocrystalline TiO2 films through the axial pyridine 3,4-dicarboxylic acid ligands is an efficient near IR sensitizer for photovoltaic injection cells based on nanocrystalline TiO2 films.

Characterization of High-Spin and Low-Spin Iron(III) Quinoxalinotetraphenylporphyrin

The 1H NMR spectra of iron(III) quinoxalinotetraphenylporphyrin ((QTPP)FeIIIXn), iron(III) (methylquinoxalino)tetraphenylporphyrin ((MQTPP)FeIIIXn), and iron(III) pyrazinotetraphenylporphyrin ((PTPP)FeIIIXn) have been studied to elucidate the impact of an aromatic extension of a single pyrrole ring on the electronic structure of the corresponding high- and low-spin iron(III) porphyrins. The 1H NMR spectra of the complexes with the following axial ligands have been reported: chloride, iodide, cyanide, pyridine-d5 (py-d5), 4-aminopyridine (4-NH2py), and imidazole (ImH). Modification of the tetraphenylporphyrin by addition of the quinoxaline (pyrazine) fragment results in stabilization of the rare low-spin iron(III) (dxzdyz)4(dxy)1 electronic ground state in the presence of axial cyanide or pyridine ligands. The more common (dxy)2(dxzdyz)3 electronic ground state has been established for [(QTPP)FeIII(4-NH2py)2]+ and [(QTPP)FeIII(ImH)2]+ species. To account for the substituent contribution, the Hückel linear combination of atomic orbitals (LCAO) method has been used to determine the molecular orbitals involved in the spin density delocalization. The deviation from Curie law observed for [(QTPP)FeIII(CN)2]- suggests Boltzmann equilibrium {(dxz)2(dyz)2(dxy)2(Ψ-1)1 ↔ (dxz)2(dyz)2(dxy)1(Ψ-1)2} ⇌ (dxz)1(dyz)2(dxy)2(Ψ-1)2 ⇌ (dxz)2(dyz)1(dxy)2(Ψ-1)2 where Ψ-1 is related to the a2u orbital of a regular porphyrin. For the first time in the group of low-spin iron(III) tetraarylporphyrins, the sign reversal of the isotropic shift was directly observed for pyrrole-proton resonances. The structure of (QTPP)FeIIICl was determined by X-ray crystallography. (QTPP)FeIIICl crystallizes in the monoclinic space group P21/c with a = 18.016(5) Å, b = 11.399(3) Å, c = 21.996(5) Å, β = 112.22(5)°, and Z = 4. The refinement of 548 parameters and 2696 reflections yields R1 = 0.0654, Rw2 = 0.1717. The (QTPP)FeIIICl presents features of the high-spin five-coordinate iron(III) tetraphenylporphyrin. The quinoxalinotetraphenylporphyrin macrocycle assumes a saddle-shape geometry.

Transition Metal NMR Spectroscopy. One‐bond 103 Rh, 15 N coupling constants of axial and equatorial ligands in rhodoximes

The 103Rh,15N coupling constants and 15N chemical shifts of XRhIII(Hdmg)2L rhodoximes (Hdmg=dimethylglyoximate, L=PPh3 or pyridine, X=halide, alkyl, haloalkyl) were extracted from gradient-selected (1H,15N)-HSQC experiments. Coupling between rhodium and the equatorial oxime nitrogens is large (18–21 Hz) and shows little sensitivity to the nature of the cis-oriented ligands X and L. In contrast, coupling between rhodium and the axial pyridine nitrogen is small (6–9 Hz) for X=alkyl but increases to 16–18 Hz in the halide complexes (‘trans influence’). The structural implications are discussed in conjunction with x-ray data. © 1997 John Wiley & Sons, Ltd.

Structural Diversity in Monomeric Cadmium Phenoxides.

Three monomeric Cd(II) phenoxide complexes have been prepared by reacting Cd[N(SiMe(3))(2)](2) with 2 equiv of 2,6-disubstituted phenols bearing sterically bulky tert-butyl or phenyl groups. The strongly coordinating solvents THF, tetrahydrothiophene (THT), and pyridine used for these reactions were incorporated into the metal's coordination sphere, leading to complexes with a general formulation of Cd(O-2,6-R(2)C(6)H(3))(2)(solv)(2)(-)(3). The Cd complexes obtained have been characterized crystallographically and have been found to adopt differing solid-state geometries. The X-ray crystal structure of Cd(O-2,6-(t)BuC(6)H(3))(2)(THF)(2), 1, previously reported by Buhro, displayed unusual square-planar coordination of the metal center. Complex 2, Cd(O(t)BuC(6)H(3))(2)(THT)(2), has been found to take on the same square-planar geometry, even with the more strongly donating thioether ligand. Alternatively, complex 3, Cd(O-2,6-Ph(2)C(6)H(3))(2)(THF)(2), has been found to adopt distorted-tetrahedral geometry, quite similar to its zinc congener. The O(1)-Cd-O(2) angle between the phenoxide ligands in 3 is 150.1(2) degrees, and the angle between the ether ligands is 83.1(3) degrees. When the strongly basic solvent pyridine was used, a five-coordinate complex 4, Cd(O-2,6-(t)BuC(6)H(3))(2)(py)(3), was isolated. This complex 4 is best described as having trigonal bipyramidal geometry with the phenoxide ligands and one pyridine defining the equatorial plane and two axial pyridine ligands having an angle of 169.7(2) degrees. The angle between the phenoxide ligands in 4 is 156.1(2) degrees. These complexes 1-4 have also been characterized in noncoordinating solvent solutions by (1)H, (13)C, and (113)Cd NMR spectroscopy and have been found to contain labile donor ligands. Preliminary studies indicate that, in a noncoordinating solvent, a rapid equilibrium exists between species with and without coordinated donor solvent ligands.

Multinuclear Paramagnetic NMR Spectra and Solid State X-ray Crystallographic Characterization of Manganese(III) Schiff-Base Complexes

The 1H NMR spectra of symmetrically substituted paramagnetic [(R,R‘-SALOPHEN)MnIII]+ and [(R,R‘-SALOPHEN)MnIII]2(μ-O) complexes (SALOPHEN is 1,3-bis(salicylideneamino)benzene) were obtained. All of the monomers and dimers yielded well-resolved but isotropically shifted 1H NMR spectra. A diamagnetic suppression routine was applied to the 1H NMR spectra that allowed observation of all rapidly relaxing resonances. The binding of axial ligands, acetate, methanol, water, and pyridine was also observable by 1H NMR. Two X-ray crystal structures were obtained: [(3,3‘-17-crown-6-SALOPHEN)MnIII]·(CH3C(O)O)·BaTf2·2H2O, [9b]OAc·BaTf2·2H2O, crystallized in the triclinic space group P1̄; with a = 10.747(2) Å, b = 12.271(8) Å, c = 16.206(6) Å, α = 106.10(4)°, β = 103.07(2)°, γ = 106.06(3), and Z = 2, while [(3,3‘,6,6‘-(CH3)4-SALOPHEN)MnIII]PF6·H2O, [6b]PF6·H2O, crystallized in the monoclinic space group P21/n, with a = 12.465(1) Å, b = 13.366(2) Å, c = 15.240(1) Å, β = 103.051(7)°, and Z = 2. [9b]OAc·BaTf2·2H2O has the Mn(III) center ligated in the axial positions by one oxygen of the acetate group and an oxygen from one SO3CF3-. The second oxygen of the acetate is bound to the Ba2+ residing in the crown ether. The water molecule in [6b]PF6·H2O is O-bound to the Mn(III) center and also hydrogen bonded to one of the fluorine atoms of the PF6 counterion, O−F 3.01 Å. 1H and 19F NMR spectra revealed that the hydrogen bond in [6b]PF6·H2O was preserved in CD3OD.