The synthesis and characterization of isocyanide complexes of (porphyrinato)iron(III) species, [(Porph)-Fe(t-BuNC) 2 ]ClO 4 , Porph ) OEP, TPP, are reported. The crystal structures of [(TPP)Fe(t-BuNC) 2 ]ClO 4 and [(OEP)-Fe(t-BuNC) 2 ]ClO 4 have been determined. Consistent with the expected effect from the strong π-acceptor character of the axial tert-butyl isocyanide ligands, the X-ray structure of the complex shows that the porphyrinate ring is strongly ruffled. The spectroscopic properties of these complexes suggest the possibility of "blurring" of the definitions of the electron configurations of low-spin Fe(III) macrocycles having (d xy ) 1 electronic ground states, with the extreme possibilities being low-spin Fe(III)-(macrocycle) 2-, with the unpaired electron localized in the d xy orbital of the metal, and low-spin Fe(II)-(macrocycle) 1-• , with the unpaired electron localized on the macrocycle. EPR spectroscopy of the TPP and OEP complexes shows that the g-values (g ⊥ ) 2.20-2.28, g | ) 1.94-1.83) are consistent with an electron configuration that is (d xz ,d yz ) 4 (d xy ) 1 , the purest (d xy ) 1 ground state system with the most complete quenching of orbital angular momentum discovered thus far (∑g 2 as small as 13.5). Proton NMR spectra of [OEPFe(t-BuNC) 2 ]-ClO 4 in CD 2 Cl 2 , recorded over the temperature range -100 to +37°C, also support the (d xy ) 1 ground state, where ruffling of the porphyrinate ring makes it possible for unpaired electron spin delocalization to the 3a 2u (π) orbital of the porphyrinate ring. This orbital has very large electron density coefficients at the meso positions and hence explains the very large negative contact shift of the meso-H; its size indicates considerable (∼19%) spin delocalization from low-spin Fe(III) to the 3a 2u (π) orbital by porphyrin f Fe π donation. Mössbauer and IR spectral data are also consistent with the (d xy ) 1 ground state.

The structure of (porphinato)nickel(II) [Ni(P)] has been determined by X-ray diffraction and inferred from a combination of single-crystal and solution resonance Raman measurements. The crystal structure reveals a planar porphyrin macrocycle with a π-π dimer packing configuration exhibiting a small lateral shift. This group S crystallographic packing arrangement has been suggested to give strong π-π interactions between the porphyrin rings on the basis of the small interplanar spacing (3.355 Å) and lateral shift (1.528 Å) between the porphine planes. Strong π-π interactions are usually associated with geometrically inequivalent structural parameters such as different metal-nitrogen bond distances, but this is not observed for Ni(P). The average nickel-nitrogen bond distance is 1.951 Å, consistent with planar nickel porphyrins. The root-mean-square out-of-plane displacement from the mean plane of the macrocyclic atoms is 0.019 Å, consonant with the observed very slight ruffling of the macrocycle. A salient feature of the resonance Raman spectra of Ni(P) in solution is the apparent sidebands of some structure-sensitive lines. This observation was interpreted previously as resulting from an equilibrium between planar and nonplanar conformers in solution. However, the similarities of the resonance Raman spectra of Ni(P) in the single crystal and in solution suggest that Ni(P) exists only in the planar conformation. This conclusion is corroborated by solution resonance Raman spectra of the four-coordinate (porphinato)copper(II) and (porphinato)cobalt(II), which are more likely than Ni(P) to be planar because of their larger central metals, yet they also show the sidebands of the same structure-sensitive lines. Crystal data: [C 20 H 12 N 4 ]Ni; a ) 10.1066(7) Å, b ) 11.945-(9) Å, c ) 12.229(2) Å, β ) 101.56(3)°, Z ) 4, V ) 1446.4(11) Å 3 , space group P2 1 /c; 3084 unique observed data; refinement converged to final values of R 1 ) 0.039, wR 2 ) 0.090; all measurements at 127(2) K.

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The coexistence in solution of at least two conformers of (meso-tetraphenylporphinato)nickel(II) [Ni(TPP)] is inferred from solution and single-crystal resonance Raman spectra obtained at different temperatures (170 − 297 K) and excitation wavelengths (413.1 and 457.9 nm). The shapes of the structure-sensitive Raman lines ν8 and ν2 are clearly asymmetric and change with temperature. These broad lines can be decomposed into at least two sublines, a low-frequency (LF) and a high-frequency (HF) component. In contrast, the corresponding single-crystal Raman lines of the nonplanar structure of Ni(TPP) in the crystal are narrow and symmetric. For the line ν2, the broad LF subline results from nonplanar conformers and the narrow HF subline arises from a more planar conformer. This assignment is consistent with the observation that the LF subline of ν2 is more enhanced upon changing the excitation wavelength from 413.1 to 457.9 nm. The selective resonance enhancement is caused by the red shifts of the UV−visible absorption bands and Raman excitation profiles of the nonplanar form. The frequency assignment for the sublines of ν8 is reversed from that of ν2 (i.e., the HF subline of ν8 arises from nonplanar conformers and the LF subline results from a more planar macrocycle). This assignment is based on subline broadness and enhancement behavior using an excitation wavelength located on the red side of the B band. The assignments of the sublines to the nonplanar conformer are also in agreement with the Raman spectra of single crystals in which Ni(TPP) is known from X-ray crystallography to have a predominantly ruffled nonplanar conformation. Specifically, the frequencies of the sublines of ν8 and ν2 that are assigned to the nonplanar form in solution closely match the frequencies of Ni(TPP) in the single crystal. We propose two thermodynamic models for the interpretation of the temperature dependence of the intensity ratios of the sublines. A two-state model assumes one planar and one nonplanar conformer in solution. From the slopes in the van't Hoff plots, the nonplanar conformer is energetically favored by about 1.8 kJ mol-1 in this model. Since molecular mechanics calculations predict three conformers of Ni(TPP) in solution, we also consider a three-state model. The three structures are two nonplanar structures of purely ruffled (ruf) and purely saddled (sad) macrocyclic distortions and a planar conformer. In the calculations, the ruf conformation is the lowest-energy structure with the planar and sad conformers having almost equal higher energies. Assuming this relationship between the energies, but allowing the actual energy separation to vary, the three-state analysis gives the similar result that the ruf conformation is stabilized by about 2.3 kJ mol-1 with respect to the planar and sad conformers.

The preparation and characterization of two crystalline forms of [Fe(TMP)(5-MeHIm)2]ClO4 with distinctly different molecular structures are reported. Crystal structure analysis shows that paral-[Fe(TMP)(5-MeHIm)2]ClO4 has the axial imidazole ligands arranged in a relative parallel orientation (over a slightly S 4-ruffled porphyrin core) and perp-[Fe(TMP)(5-MeHIm)2]ClO4 has the axial imidazole ligands arranged in a relative perpendicular orientation (over a considerably S 4-ruffled porphyrin core). The two species have different Mössbauer and solid-state EPR spectra. The small quadrupole splitting (ΔE q = 1.78(1) mm/s, 120 K) and a single observable EPR g max value (3.43 at 4.2 K) for perp-[Fe(TMP)(5-MeHIm)2]ClO4 are indicative of the relative perpendicular arrangement of the axial ligands. The larger quadrupole splitting (ΔE q = 2.557(3) mm/s, 120 K) and rhombic g-tensor (g 1 = 2.69, g 2 = 2.34−2.43, and g 3 = 1.75) in the solid state and in frozen DMF−acetonitrile 3:1 (g 1 = 2.64, g 2 = 2.30, and g 3 =1.80) at 4.2 K for paral-[Fe(TMP)(5-MeHIm)2]ClO4 are indicative of a relative parallel axial ligand orientation. The actual axial ligand dihedral angles are Δφ = 76° and Δφ = 26 or 30° for perp- and paral-[Fe(TMP)(5-MeHIm)2]ClO4, respectively, and thus the dihedral angle at which the EPR spectral type changes from large g max to rhombic must be 30 < Δφ < 76°. Because the porphyrin and axial ligands are similar for both crystalline forms of [Fe(TMP)(5-MeHIm)2]ClO4, a more direct correlation between molecular and electronic structure has been established. Molecular mechanics calculations indicate that nonbonded interactions between the axial ligands and meso-mesityl groups of [Fe(TMP)(5-MeHIm)2]+ destabilize a relative parallel orientation for the axial ligands, yet the parallel orientation is observed in all frozen solution samples as confirmed by EPR investigations. This is believed to be due to the competing stabilization of the electronic state of the rhombically distorted parallel complex with an energy stabilization of 2.8−3.7 kcal/mol, as compared to the energy destabilization of 2.6 kcal/mol obtained from MM calculations.

The structural changes in a crystal of 9-methylanthracene (1) during the [4 + 4] photodimerization were monitored by means of X-ray diffraction. This is the first example in the literature of such a study of a [4 + 4] photodimerization. The results obtained were compared with data for the [2 + 2] photodimerization. The shape of the product molecules and their preferred packing can explain the crystal disintegration. This was the reason that the reaction was monitored only to 28% completion. As far as could be determined the reaction proceeds with a constant rate. The cell volume increases at the beginning of the transformation and decreases afterwards. The product molecules do not assume a fixed position in the crystal during the photo-reaction, but move in a smooth way that includes a rotational component. The movements of the reactant are much smaller. Movements of molecules characterized by a rotational component were also observed in the case of the [2 + 2] photodimerization of 5-benzylidene-2-benzylcyclopentanone and 5-benzylidene-2-(4-chlorobenzyl)-cyclopentanone. The distance between the reacting atoms of the adjacent monomer molecules of (1) decreases with the degree of reaction completion, but more slowly than in the case of the [2 + 2] photodimerizations cited above. The orientation of the neighbouring monomer molecules changes during the phototransformation so that the monomer pair resembles the dimer product.

Structural changes taking place in a crystal during an intramolecular photochemical reaction [the Yang photocyclization of the alpha-methylbenzylamine salt with 1-(4-carboxybenzoyl)-1-methyladamantane] were monitored step-by-step using X-ray structure analysis. This is the first example of such a study carried out for an intramolecular photochemical reaction. During the photoreaction, both the reactant and product molecules change their orientation, but the reactant changes more rapidly after the reaction is about 80% complete. The distance between directly reacting atoms in the reactant molecule is almost constant until about 80% reaction progress and afterwards decreases. The torsion angle defined by the reactant atoms that form the cyclobutane ring also changes in the final stages of the photoreaction. These phenomena are explained in terms of the influence of many product molecules upon a small number of reacting molecules. The adamantane portion shifts more than the remaining part of the anionic reactant species during the reaction, which is explained in terms of hydrogen bonding. The structural changes are accompanied by changes in the cell constants. The results obtained in the present study are compared with analogous results published for intermolecular reactions.

In the 1980s and 1990s, x‐ray studies of the photochemical reaction course in crystals dealt with the analysis of changes in cell constants or movements of atom groups inside molecules. This review presents the results of crystallographic studies on the monitoring of the behaviour of whole molecules in organic crystals during photochemical reactions. Papers on this subject started to appear only a few years ago. The studies showed quantitatively that reactant and product molecules do not take a fixed position in a crystal during the reaction. The product molecules move smoothly to a position assumed in the pure product crystal and the reactant molecules move from a position occupied in the pure reactant crystal. Moreover, with the reaction progress the adjacent reactant molecules gradually come closer and change their mutual orientation to resemble the product. The analysis of the photoreaction kinetics in crystals is also presented. Copyright © 2004 John Wiley & Sons, Ltd.