Axial Base Ligand Research Articles

Exchange of axial base ligand during the crystallization of rhodol linked chlorocobaloxime

Synthesis of a novel chlorocobaloxime with rhodol dye as an axial ligand which is linked to the cobalt metal through the base pyridine has been reported. X-ray quality single crystal of 3-(diethylamino)-9-(1,3-dihydro-1-oxoisobenzofuran-3-yl)-9H-xanthen-6-yl isonicotinate was obtained during the crystallization of this rhodol linked chlorocobaloxime. This compound crystallizes in the triclinic crystal system and the space group of P 1. Crystal packing diagram of this structure reveals a 3-D network due to extensive intermolecular hydrogen bonding. This rhodol linked pyridine base was further characterized by 1H and 13C NMR. UV-vis absorption spectrum of this compound revealed an absorption peak in the visible region of 515.5 nm. This compound is 25 nm red shifted compared to the precursor rhodol chromophore. Upon excitation at 515 nm, this molecule reveals a strong fluorescence emission at 545 nm.

Heme-Cu Binucleating Ligand Supports Heme/O2 and FeII-CuI/O2 Reactivity Providing High- and Low-Spin FeIII-Peroxo-CuII Complexes.

The focus of this study is in the description of synthetic heme/copper/O2 chemistry employing a heme-containing binucleating ligand which provides a tridentate chelate for copper ion binding. The addition of O2 (-80 °C, tetrahydrofuran (THF) solvent) to the reduced heme compound (PImH)FeII (1), gives the oxy-heme adduct, formally a heme-superoxide complex FeIII-(O2•-) (2) (resonance Raman spectroscopy (rR): νO-O, 1171 cm-1 (Δ18O2, -61 cm-1); νFe-O, 575 cm-1 (Δ18O2, -24 cm-1)). Simple warming of 2 to room temperature regenerates reduced complex 1; this reaction is reversible, as followed by UV-vis spectroscopy. Complex 2 is electron paramagnetic resonance (EPR)-silent and exhibits upfield-shifted pyrrole resonances (δ 9.12 ppm) in 2H NMR spectroscopy, indicative of a six-coordinate low-spin heme. The coordination of the tethered imidazolyl arm to the heme-superoxide complex as an axial base ligand is suggested. We also report the new fully reduced heme-copper complex [(PImH)FeIICuI]+ (3), where the copper ion is bound to the tethered tridentate portion of PImH. This reacts with O2 to give a distinctive low-temperature-stable, high-spin (S = 2, overall) peroxo-bridged complex [(PImH)FeIII-(O22-)-CuII]+ (3a): λmax, 420 (Soret), 545, 565 nm; δpyrr, 93 ppm; νO-O, 799 cm-1 (Δ18O2, -48 cm-1); νFe-O, 524 cm-1 (Δ18O2, -23 cm-1). To 3a, the addition of dicyclohexylimidazole (DCHIm), which serves as a heme axial base, leads to low-spin (S = 0 overall) species complex [(DCHIm)(PImH)FeIII-(O22-)-CuII]+ (3b): λmax, 425 (Soret), 538 nm; δpyrr, 10.2 ppm; νO-O, 817 cm-1 (Δ18O2, -55 cm-1); νFe-O, 610 cm-1 (Δ18O2, -26 cm-1). These investigations into the characterization of the O2-adducts from (PImH)FeII (1) with/without additional copper chelation advance our understanding of the dioxygen reactivity of heme-only and heme/Cu-ligand heterobinuclear system, thus potentially relevant to O2 reduction in heme-copper oxidases or fuel-cell chemistry.

Isocyanide or nitrosyl complexation to hemes with varying tethered axial base ligand donors: synthesis and characterization.

A series of ferrous-heme 2,6-dimethylphenyl isocyanide (DIMPI) and ferrous-heme mononitrosyl complexes have been synthesized and characterized. The heme portion of the complexes studied is varied with respect to the nature of the axial ligand, including complexes, where it is covalently tethered to the porphyrinate periphery. Reduced heme complexes, [(F8)Fe(II)], [(P(Py))Fe(II)], [(P(Im))Fe(II)], and [(P(ImH))Fe(II)], where F8=tetrakis(2,6-difluorophenyl)-porphyrinate and P(Py), P(Im), and P(ImH) are partially fluorinated tetraaryl porphyrinates with covalently appended axial base pyridyl/imidazolyl or histamine moieties, were employed; P(ImH) is a new construct. Room temperature addition of DIMPI to these iron(II) complexes affords the bis-isocyanide species [(F8)Fe(II)-(DIMPI)2] in the case of [(F8)Fe(II)], while for the other hemes, mono-DIMPI compounds are obtained, [(P(Py))Fe(II)-(DIMPI)] [(2)-DIMPI], [(P(Im))Fe(II)-(DIMPI)] [(3)-DIMPI], and [(P(ImH))Fe(II)-(DIMPI)] [(4)-DIMPI]. The structures of complexes (3)-DIMPI and (4)-DIMPI have been determined by single crystal X-ray crystallography, where interesting H…F(porphryinate aryl group) interactions are observed. (19)F-NMR spectra determined for these complexes suggest that H…F(porphyrinate aryl groups) attractions also occur in solution, the H atom coming either from the DIMPI methyl groups or from a porphyinate axial base imidazole or porphyrinate pyrrole. Similarly, we have used nitrogen monoxide to generate ferrous-nitrosyl complexes, a five-coordinate species for F8, [(F8)Fe(II)-(NO)], or low-spin six-coordinate compounds [(P(Py))Fe(II)-(NO)], [(P(Im))Fe(II)-(NO)], and [(P(ImH))Fe(II)-(NO)]. The DIMPI and mononitrosyl complexes have also been characterized using UV-Vis, IR, (1)H-NMR, and EPR spectroscopies.

ChemInform Abstract: Dynamic Motion and Various Reaction Paths of Cobaloxime Complexes in Crystalline‐State Photoreaction

AbstractReview: 153 refs.

Synthesis, Characterization, and Reactivity of (Fluoroalkyl)- and (Fluorocycloalkyl)cobaloximes: Molecular Structure of a (2-Fluorocyclohexyl)cobaloxime Complex and Hindered Rotation of 2-Fluorocycloalkyl Ligands

Reaction of Ph 3 P-[Co] - ([Co] = Co(dmgH) 2 ; dmgH 2 = dimethylglyoxime), prepared by reduction of Ph 3 P-[Co]-Cl with NaBH 4 in methanolic NaOH, with BrCH 2 CH 2 CH 2 F resulted in formation of Ph 3 P-[Co]-CH 2 CH 2 CH 2 F (3). Inneutral methanolic solutions py*-[Co]-H/py*-[Co] (py* = py, 4-(t-Bu)py, 3-Fpy; 4-(t-Bu)py = 4-tert-butylpyridine, 3-Fpy = 3-fluoropyridine) were found to react with BrCH 2 CHF 2 , yielding the 2,2-difluoroethyl complexes py*-[Col-CH 2 CHF 2 (py* = py (4a), 4-(t-Bu)py (4b), 3-Fpy (4c)). Reactions of XCH 2 CH 2 F (X = Br, TfO; TfO = triflate) with reduced cobaloximes in alkaline and neutral methanolic solutions resulted in formation of the 2-methoxyethyl complexes py*-[Co]-CH 2 -CH 2 OMe (py* = py (5a), 4-(t-Bu)py (5b), 3-Fpy (5c)) with ethylene as side product. py*-[Co]-H/py*-[Co] - (py* = py, 3-Fpy) was found to react with TfCCH 2 CMe 2 F, yielding py*-[Co]-CH=CMe 2 (py* = py (6a), 3-Fpy (6b)) and H 2 C=CMe 2 . All these reactions indicate the formation of the (unseen) intermediate py*-[Col-CH 2 CR 2 F (R = H, Me), which decomposes via nucleophilic substitution (F → OMe), heterolytic fragmentation, yielding olefins, and HF elimination, yielding vinyl complexes, respectively. Analogous reactions of reduced cobaloximes with trans-1-bromo-2-fluorocyclohexane and trans-1-bromo-2-fluorocyclopentane resulted in the formation of (2-fluorocyclohexyl)- and (2-fluorocyclopentyl)-cobaloximes, py*-[Co]-C 6 H 1 0 F (py* = py (7a), 4-(t-Bu)py (7b)) and py*-[Co]-C 5 H 8 F (py* = py (10a), 4-(t-Bu)py (10b)). All these complexes were fully characterized by 1 H, 1 3 C, and 1 9 F NMR spectroscopic investigations. Molecular structures of the cobaloximes 3, 4a, 7b, and 7b.CH 2 CL were obtained by single-crystal X-ray diffraction analyses, exhibiting complexes with an equatorial pseudomacrocyclic (dmgH) 2 ligand as well as axial base (PPh 3 , py*) and organo ligand in mutually trans positions. The cycloalkyl complexes are the ae isomers, having the sterically demanding Co(dmgH) 2 moieties as equatorial substituents. The axially oriented fluoro substituents give rise to hindered rotation of 2-fluorocycloalkyl ligands, as indicated by two distinct sets of signals for dmgH ligands in the 1 H and 1 3 C NMR spectra. Further proof for this came from DFT calculations of py-[Co]-C 6 H 1 0 F (11) and, for comparison, of the cyclohexyl complex py-[Co]-C 6 H 1 1 (12). The conformational energy diagrams of 11 and 12 are discussed.

Low-temperature UV-visible and NMR spectroscopic investigations of O(2) binding to ((6)L)Fe(II), a ferrous heme bearing covalently tethered axial pyridine ligands.

In this report, we describe the reversible dioxygen reactivity of ((6)L)Fe(II) (1) [(6)L = partially fluorinated tetraphenylporphyrin with covalently appended TMPA moiety; TMPA = tris(2-pyridylmethyl)amine] using a combination of low-temperature UV-vis and multinuclear ((1)H and (2)H) NMR spectroscopies. Complex 1, or its pyrrole-deuterated analogue ((6)L-d(8))Fe(II) (1-d(8)), exhibits downfield shifted pyrrole resonances (delta 28-60 ppm) in all solvents utilized [CH(2)Cl(2), (CH(3))(2)C(O), CH(3)CN, THF], indicative of a five-coordinate high-spin ferrous heme, even when there is no exogenous axial solvent ligand present (i.e., in methylene chloride). Furthermore, ((6)L)Fe(II) (1) exhibits non-pyrrolic upfield and downfield shifted peaks in CH(2)Cl(2), (CH(3))(2)C(O), and CH(3)CN solvents, which we ascribed to resonances arising from the intra- or intermolecular binding of a TMPA-pyridyl arm to the ferrous heme. Upon exposure to dioxygen at 193 K in methylene chloride, ((6)L)Fe(II) (1) [UV-vis: lambda(max) = 433 (Soret), 529 (sh), 559 nm] reversibly forms a dioxygen adduct [UV-vis: lambda(max) = 422 (Soret), 542 nm], formulated as the six-coordinate low-spin [delta(pyrrole) 9.3 ppm, 193 K] heme-superoxo complex ((6)L)Fe(III)-(O(2)(-)) (2). The coordination of the tethered pyridyl arm to the heme-superoxo complex as axial base ligand is suggested. In coordinating solvents such as THF, reversible oxygenation (193 K) of ((6)L)Fe(II) (1) [UV-vis: lambda(max) = 424 (Soret), 542 nm] also occurs to give a similar adduct ((6)L)Fe(III)-(O(2)(-)) (2) [UV-vis: lambda(max) = 418 (Soret), 537 nm. (2)H NMR: delta(pyrrole) 8.9 ppm, 193 K]. Here, we are unable to distinguish between a bound solvent ligand or tethered pyridyl arm as axial base ligand. In all solvents, the dioxygen adducts decompose (thermally) to the ferric-hydroxy complex ((6)L)Fe(III)-OH (3) [UV-vis: lambda(max) = 412-414 (Soret), 566-575 nm; approximately delta(pyrrole) 120 ppm at 193 K]. This study on the O(2)-binding chemistry of the heme-only homonuclear ((6)L)Fe(II) (1) system lays the foundation for a more complete understanding of the dioxygen reactivity of heterobinuclear heme-Cu complexes, such as [((6)L)Fe(II)Cu(I)](+), which are models for cytochrome c oxidase.

Modified photoreactivity due to mixed crystal formation. I. Three mixed crystals between isostructural cobaloxime complexes.

Three crystals of 2-cyanoethyl cobaloxime complexes with 3-chloropyridine, 3-bromopyridine and 3-methylpyridine as axial base ligands are isostructural to one another. Three mixed crystals were formed between the pairs: (3-bromopyridine)(2-cyanoethyl)cobaloxime-(2-cyanoethyl)(3-methylpyridine)cobaloxime(0.45/0.55);(3-chloropyridine)(2-cyanoethyl)cobaloxime-(2-cyanoethyl)(3bromopyridine)cobaloxime (0.61/0.39); (3-chloropyridine)(2-cyanoethyl)cobaloxime-(2-cyanoethyl)(3-methylpyridine)cobaloxime (0.44/0.56). The X-ray crystal structure analysis revealed that the mixed compounds are also isostructural to the component crystals. It was found from the change in IR spectra that the 2-cyanoethyl groups in the three mixed crystals isomerized to 1-cyanoethyl groups on exposure to a xenon lamp, as observed for the 2-cyanoethyl groups in the component crystals. Rate constants for the three mixed and three component crystals, measured under the same conditions, are approximately the same, as the reaction cavities for the 2-cyanoethyl groups in the six crystals have almost the same size. For the mixed crystal between 3-chloropyridine and 3-methylpyridine complexes, the isomerization proceeded with retention of the single-crystal form. The conformation and configuration of the 1-cyanoethyl group that was produced were well explained by the shape of the reaction cavity before irradiation.

Synthesis and characterisation of dipyridinated magnesium phthalocyaninato(2-) complex

A new complex of dipyridinated magnesium phthalocyanine has been synthesised in crystalline form by dissolving magnesium phthalocyanine in pyridine and heating at 160 °C. The crystals are formed during the slow cooling process. MgPc(py) 2 crystallises in the monoclinic system, space group P2 1/ c with two molecules in a unit cell. The structure was solved by direct and Fourier difference methods and refined by full-matrix least-squares techniques to R=0.0462, wR=0.1210 and S=1.012 for 3856 independent reflections and 242 refined parameters. The magnesium cation coordinates four isoindole nitrogen atoms of the almost planar phthalocyaninato(2-) ligand and axially two nitrogen atoms of pyridine molecules. The coordination polyhedron around the magnesium atom approximates to a tetragonal bypyramid with relatively long bonds between the Mg atom and the axial base ligands. The thermal analysis shows that the crystals of MgPc(py) 2 are stable up to ∼180 °C, at higher temperature the release of pyridine ligands is rapid. The UV–Vis and FT–IR and Raman spectra of MgPc(py) 2 are discussed in relation to its structure and compared with the spectral data of magnesium phthalocyanine.

The Role of Hydrogen Bond and Crystal Solvent in the Solid-state (β-α) Photoisomerization of Novel 2-(N-phenylcarbamoyl)ethyl Cobaloxime Complexes [1

The 2-(phenylcarbamoyl)ethyl group bonded to the cobalt atom in cobaloxime complexes was isomerized on exposure to visible light in the solid-state. The three complexes with different amines as axial base ligands were prepared and their crystal structures were determined by X-ray analysis. The reaction rates in the solid-state were mainly affected by the hydrogen bonds of the reactive group with the neighboring molecules and the occupation of crystal solvent molecules around the reactive group.

Asymmetric Induction in Cobaloxime Complex Crystals Due to Chiral Crystal Environment

The achiral 2-cyanoethyl group of cobaloxime complexes was isomerized to the chiral 1-cyanoethyl group on exposure to a xenon lamp in the solid state. From the crystal structures and the enantiomer or diastereomer excesses of the seven cobaloxime complexes with different axial base ligands, it was found that the chirality of the produced 1-cyanoethyl group depends on the asymmetric shape of the reaction cavity for the 2-cyanoethyl group before irradiation.

Solid State Photoisomerization of But-3-en-1-yl Group in Cobalt Complexes

Abstract The but-3-en-1-yl group bonded to the cobalt atom in some cobaloxime complexes is isomerized to the but-2-en-1-yl group in the solid state on exposure to a xenon lamp. In order to make clear the relationship between the reaction rate and the crystal structure, six complexes with different axial base ligands were prepared; piperidine, 1, 1-vinylimidazole, 2, pyrrolidine, 3, 1-methylimidazole, 4, 4-t-butylpyridine, 5, and triphenylphosphine, 6. The crystal structures were determined by X-ray analyses and the reaction rates were estimated from the change of the 1H NMR spectra observed in several intermediate stages. One of the photo products, which has 1-vinylimidazole as an axial base ligand, 7, was prepared separately and its crystal structure was also analyzed. A pair of the but-3-en-1-yl groups are in contact with each other around an inversion center in the crystal structures of 2, 5, and 6, whereas the but- 3-en-1-yl groups in 1, 3, and 4 are isolated from the other but-3-en-1-yl groups of the neighboring molecules in their crystal structures. The reaction rates of the former complexes are significantly greater than those of the latter complexes, except 2. This suggests that the cooperative motion of the but-3-en-1-yl groups plays an important role in the process of the isomerization. A pair of the but-3-en-1-yl groups around an inversion center in 2 are too closely packed for the isomerization to occur even if the cooperative motion is taken into account.

X-ray Analysis of Successive Reaction in Crystalline State Photoisomerization of Cobaloxime Complexes

The γ-cyanopropyl group bonded to the cobalt atom in some cobaloxime complexes was isomerized to the α-cyanopropyl group through β-cyanopropyl group on exposure to visible light in the solid state. For the complex with 1-phenylethylamine as an axial base ligand almost chiral α-cyanopropyl group was produced with retention of the single crystal form. The mechanism is discussed on the basis of the structural change.

Direct Observation of Correlation between Crystalline-State Deuterium Transfer and Racemization of 1-Cyanoethylcobaloxime Complex by Neutron Diffraction

Abstract The [(R)-1-cyanoethyl-dα] group, which has a deuterium atom attached to the chiral carbon and is bonded to the cobalt atom, in a cobaloxime complex with pyridine as an axial base ligand is inverted to the opposite configuration on exposure to visible light with retention of the single crystal form. The C-D bond is not cleaved in the process of the inversion.

β–α Photoisomerization of Cobaloxime Complexes in the Solid State. 5. Reaction Rate Reduced by a Hydrogen Bond

Abstract The crystal structures of three cobaloxime complexes with different axial base ligands have been analyzed by X-ray analysis at room temperature. I: (2-Cyanoethyl)bis(dimethylglyoximato)((S)-phenylalaninol)cobalt(III); the crystal is orthorhombic, the space group being P212121, Z = 4 with a = 9.416(3), b = 27.184(3), c = 9.184(3) Å. II: (2-Cyanoethyl)bis(dimethylglyoximato)((S)-phenyl-alanine methyl ester)cobalt(III); the crystal is monoclinic, the space group being P21, Z = 4 with a = 14.376(2), b = 11.988(2), c = 14.874(1) Å, and β = 101.05(1)°. III: (2-Cyanoethyl)bis(dimethylglyoximato)(methyldiphenylphosphine)cobalt(III); the crystal is monoclinic, the space group being P21/n, Z = 4 with a = 11.558(4), b = 15.542(3), c = 14.393(5) Å, and β = 91.02(4)°. The structures were refined by the full-matrix least-squares method to final R values of 0.041, 0.052, and 0.044 for 2340, 3287, and 4400 observed reflections, respectively. The photoisomerization rates of I, II, and III in the solid state were obtained from the changes in the IR spectra. The rate of I was insignificantly small, but the rates of II and III were calculated to be 0.1 × 10−4 and 1.9 × 10−4 s−1, respectively, assuming first-order kinetics. The 2-cyanoethyl groups in the three complex crystals take perpendicular conformations to their cobaloxime planes. The 2-cyanoethyl group of I makes a hydrogen bond with the N–H group of the neighboring molecule. The non-reactivity of I may be brought about by the hydrogen bond.

Quantitative comparison of photoisomerization of three cobaloxime complexes in the solid and crystalline states

The β-cyanoethyl group bonded to the cobalt atom in cobaloxime complexes with 3-methyl-, 3-chloro- and 3-bromopyridines as axial base ligands was found, on exposure to a xenon lamp, to be isomerized to the α-cyanoethyl group with retention of the single-crystal form. The reaction rate of each compound was estimated from the change of unit-cell dimensions on a diffractometer (crystalline state) and from the change of the IR spectra of the powdered sample (the solid state). The reaction rates thus obtained were compared with the volumes of the reaction cavities for the cyanoethyl groups in the crystal structures of the three cobaloxime complexes. A quantitative relationship was observed between structure and reactivity.

Quantitative comparison of photoisomerization of three cobaloxime complexes in the solid and crystalline states

The β-cyanoethyl group bonded to the cobalt atom in cobaloxime complexes with 3-methyl-, 3-chloro- and 3-bromopyridines as axial base ligands was found, on exposure to a xenon lamp, to be isomerized to the α-cyanoethyl group with retention of the single-crystal form. The reaction rate of each compound was estimated from the change of unit-cell dimensions on a diffractometer (crystalline state) and from the change of the IR spectra of the powdered sample (the solid state). The reaction rates thus obtained were compared with the volumes of the reaction cavities for the cyanoethyl groups in the crystal structures of the three cobaloxime complexes. A quantitative relationship was observed between structure and reactivity.

Dioxygen adducts of cobalt(II) complexes of 6,6′-bis(p-substituted benzoylamino)-2,2′-bipyridines and their catalytic activities in oxygenation of 2,6-di-tert-butylphenol

The oxygen affinities and properties of oxygen adducts of complexes [CoL][H2L = 6,6′-bis(benzoyl-amino)-2,2′-bipyridine and its 4-methyl-, -tert-butyl or -chloro-benzoylamino analogues] has been studied by cyclic voltammetry and IR, electronic and ESR spectroscopy. The complexes exhibited oxygen-binding ability in the presence of an appropriate axial base ligand (B), and the oxygen adducts were of the end-on bonded CoIII(O2–) type [CoL(B)(O2)]. More complete electron transfer from the metal centre to the oxygen was indicated compared to that with [Co(salen)][H2salen =N,N′-ethylenebis(sali-cylideneimine)]. Electron-donating substituents gave higher concentrations of the oxygen adducts. The superior catalytic activity of these complexes, especially the Me- and But-substituted derivatives, in the oxygenation of 2,6-di-tert-butylphenol to the corresponding quinone is interpreted in terms of the reactivity of the co-ordinated oxygen and the oxygen affinity of the complexes.

Multi-Stage Crystalline State Racemization of a Bulky Chiral Group

Abstract A chiral dimethoxycarbonylethyl group in a cobaloxime complex with pyridine as an axial base ligand revealed crystalline-state racemization by X-ray exposure or visible light, whereas those of complexes with (S)- and (R)-phenylethylamines and propylamine showed no change. Such differences can be explained by the size of the reaction cavity for the dimethoqcarbonylethyl group and the packing mode of the groups in each crystal.

Crystalline-state reaction of cobaloxime complexes by X-ray exposure. 15. Different reactivity between two crystallographically independent molecules

Introduction A new type of crystalline-state reaction has been found in [(R)-l-cyanoethyl]bis(dimethylglyoximato)(piperidine)cobalt(III) (dimethylglyoximato = 2,3butanedione dioximato). Crystal data at 293 K are as follows: [Co(C3H4N)(C4H7NzOz)2(CsH l iN)], M,=4284, a=11-733(2) , b=31-026(5 ) , c = 11.311(2)/k, V 4117(1)A 3, P2~2~2~, Z = 8 , Dx= 1.383 g cm3, F(000) = 2024, A(Mo Ka) = 0.79069/k, Iz = 8.06 c m ' . The final R value was 0.062 for 3573 reflections. The cell dimensions changed gradually on exposure to X-rays at 333 K, although the crystalline-state racemization was not observed at 293 K. Three-dimensional intensity data were collected in the early stages at 333 K (stage I). The cell dimensions before data collection were a = 11.788 (2), b = 31.146 (7), c = 11.359 (2)/k, V= 4170(1) A3; and after data collection were a11.779(2), b=31 .085(7 ) , c=11 .368(2 ) A, V= 4163 (1)/k 3. The average dimensions were used for the structure determination: a = 11.784(2), b = 31-112(7), c=11 .364 (2 )~ , , V = 4167 (1) ,~, 3. The structure analyses revealed that one cyanoethyl group of the two crystallographically independent molecules is disordered about the C o C bond. After 92 h X-ray exposure at 333 K three-dimensional intensity data were collected (stage II) since the changes in cell dimensions became sufficiently small. Average cell dimensions for stage II are a = 11.774(2), b = 31.043 (5), c = 11.379 (2)/k, V = 4159 (1)A3. With regard to the disordered cyanoethyl group, 24% was inverted, whereas another crystallographically independent cyanoethyl group remained unaltered. The difference in reactivity between the two groups is explained by the different cavity sizes for the two reactive groups. 0108-7681/91/050702-06503.00 It has been found that the chiral l-cyanoethyl (ce) group in crystals of some bis(dimethylglyoximato)cobalt (cobaloxime) complexes is racemized by X-ray exposure without degradation of the crystal (Ohashi & Sasada, 1977). The racemization processes observed in the related cobaloxime crystals are classified into three modes. In mode 1, the crystal has one molecule in the asymmetric unit and the disordered racemates appear after racemization (Ohashi, Yanagi, Kurihara, Sasada & Ohgo, 1981; Ohashi, Sasada & Ohgo, 1978a,b; Tamura, 1987). The crystals of modes II and III have two crystallographically independent molecules which a re related by a pseudoinversion center that becomes a crystallographic center after racemization. In this case the space group changes from chiral to centrosymmetric. In mode II the configuration of one of two crystallographically independent ce groups is fully inverted and the ordered chiral crystal is transformed to the ordered racemic one (Ohashi, Uchida, Sasada & Ohgo, 1983; Ohashi, Yanagi, Kurihara, Sasada & Ohgo, 1982; Uchida, Ohashi, Sasada, Ohgo & Baba, 1984). On the other hand, both of the crystallographically independent ce groups are converted to the disordered racemates in mode III (Tomotake, Uchida, Ohashi, Sasada, Ohgo & Baba, 1985). In the previous paper, we reported a new type of racemization which belonged to none of the above three classes (Ohashi, Tomotake, Uchida & Sasada, 1986). The crystal, in which the ligand 3-methylpyridine occupies an axial base position, R-ce-3mepy, contains two molecules, A and B, in the P212~2, cell. At an early stage both of the ce groups were inverted to give the disordered racemates, but the ce group of the A molecule, the A ce group, returned to the © 1991 International Union of Crystallography YASUKO T. OSANO et al. 703 original configuration after about 400 h exposure. Finally, the B ce group was fully inverted to the opposite configuration and the A ce group was transformed to the original one. The somewhat large size of the reaction cavity of the A c e group and the chiral space group after racemization may cause such differences in the racemization process. Recently, we prepared another crystal which had two independent molecules, A and B (space group P21212~), using piperidine as an axial base ligand (R-ce-pip). The racemization process, however, is somewhat different from that of R-ce-3mepy. The present paper reports the new type of racemization.

β-α Photoisomerization of Cobaloxime Complexes in the Solid State. III. Accelerated Reaction Rate by Hydrogen Bond

Abstract The crystal structures of two cobaloxime complexes with different axial base ligands have been determined by X-ray analysis at room temperature. I: (3-Aminopyridine)(2-cyanoethyl)bis(dimethylglyoximato)cobalt(III); the cystal is monoclinic, the space group being C2⁄c, Z=8 with a=28.849(8), b=7.746(2), c=18.429(5) Å, and β=105.71(3)°. II: (Aniline)(2-cyanoethyl)bis(dimethylglyoximato)cobalt(III) aniline solvate; the crystal is monoclinic, the space group being P21⁄c, Z=4 with a=14.228(7), b=11.426(4), c=14.178(3) Å, and β=91.92(3)°. Both structures were refined by a full-matrix least-squares method to final R values of 0.070 and 0.074 for 3886 and 3311 observed reflections, respectively. Their photoisomerization rates in the solid state obtained from changes in the IR spectra, 3.2 and 4.2×10−4 s−1, respectively, are significantly greater than those of the related compounds. The hydrogen bonds between the reactive 2-cyanoethyl groups and the atoms of the neighboring molecules in the two crystals should be responsible for the greater reaction rates. Taking into account the role of the hydrogen bonds, a probable isomerization mechanism is proposed.