Conformational effects on high-spin organic molecules

Silverman, Scott; Dougherty, Dennis A.

doi:10.1021/j100152a035

Cited by 48 publications

(46 citation statements)

References 7 publications

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“…[92][93][94][95] However, there are examples in the literature where spin-containing units appear to prefer no coupling (J ) 0) over antiferromagnetic coupling. 96,97 Additional experimentation is necessary to fully evaluate the exchange coupling in the biradical dications presented here, and efforts along these lines are underway.…”

Section: Discussionmentioning

confidence: 99%

Cross-Conjugated Bis(porphryin)s: Synthesis, Electrochemical Behavior, Mixed Valency, and Biradical Dication Formation

et al. 1999

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The synthesis and characterization of seven (Zn II ) 2 bis(porphyrin) molecules are described. The molecular structures of two bis(porphyrin)s (6 and 7) were determined by X-ray diffraction methods. Four of the compounds have their porphyrin moieties attached in a meta-fashion to a substituted benzene ring (1-4), two have porphyrin rings attached in a gem-fashion to a carbon-carbon double bond (6 and 7), and one bis(porphyrin) (5) has a p-phenylene coupler. Bis(porphyrin)s 1, 4, and 5 contain tetraaryl-type porphyrins, while 2, 3, 6, and 7 contain triarylethynylporphyrins. Except for exciton coupling in the unoxidized species, interaction between porphyrins is greater in the triarylethynyl-type bis(porphyrin)s than in the tetraaryl-type bis(porphyrin)s regardless of the number of bonds through which the interaction propagates. The cyclic voltammetry of the bis-(porphyrin)s was examined, and a varying degree of interaction between electrophores within the series was found. Redox splitting was observed for porphyrin ring oxidations of 3, 6, and 7, suggesting interaction between the oxidized porphyrin rings beyond simple electrostatic repulsions. No such redox splitting was observed for any of the tetraaryl-type bis(porphyrin)s (1, 4, and 5). We conclude that two porphyrin radical cations interact best when (1) the interaction pathway is short (∆E 1/2 (6/7) > ∆E 1/2 (2/3)), (2) π-overlap between the electrophore and coupler is maximized (minimal bond torsions: ∆E 1/2 (6/7) > ∆E 1/2 (4/5)), and (3) the electron demand of the coupler matches that of the spin carrier (∆E 1/2 (3) > ∆E 1/2 (2)). One-electron oxidized triarylethynyl-type bis(porphyrin)s exhibit spectral features characteristic of mixed-valent compounds. In two cases (6 •+ and 7 •+ ), near-IR bands near 8300 cm -1 were observed and are tentatively assigned to intervalence transitions. Singly oxidized tetraaryl-type bis(porphyrin)s exhibit no such near-IR transitions, and electronic absorption spectra recorded during electrochemical oxidations are marked by isosbestic points, suggesting negligible interaction between the two halves of the molecule, consistent with cyclic voltammetric results. Two-electron oxidation of 2-7 yields biradical dications whose frozen solution EPR spectra lack fine structure, but exhibit ∆m s ) 2 transitions characteristic of exchange-coupled S ) 1 states. Oxidation of 1 yields a biradical in which both exchange coupling and dipolar interaction between unpaired electrons are presumably very weak; consequently no ∆m s ) 2 is observed. The results of variable-temperature EPR spectroscopy of 2 2•2+ -7 2•2+ suggest either a triplet ground state or a singlet-triplet degeneracy. As a consequence of our results, we hypothesize that the exchange interaction, and therefore the singlet-triplet gap, in a biradical di-ion can be adjusted to favor the triplet state by simple substituent effects.

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Section: Discussionmentioning

confidence: 99%

Cross-Conjugated Bis(porphryin)s: Synthesis, Electrochemical Behavior, Mixed Valency, and Biradical Dication Formation

et al. 1999

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“…The spin polarization mechanism itself provides a key tool for predicting the kind of magnetic interactions in high-spin organic molecules and therefore, designing moleculebased magnets. 13 Thus, in order to get the desired spin alignment between two paramagnetic metal atoms, phenyl derivatives 14 and diazine-type bridging ligands 15 have been widely studied due to their analogy with the organic polyradicals based on m-or pphenylene magnetic couplers. Nevertheless, the application of this strategy to transition metal complexes, which provide a more stable spin source than those based on organic radicals, is more tricky and the spin delocalization effect must also be taken into account.…”

Section: Introductionmentioning

confidence: 99%

Rational design of 1-D metal–organic frameworks based on the novel pyrimidine-4,6-dicarboxylate ligand. New insights into pyrimidine through magnetic interaction

et al. 2007

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Single crystal X-ray analysis of compounds H2pmdc.2H2O (1), KHpmdc (2), and K2pmdc (3) shows that the pyrimidine-4,6-dicarboxylate (pmdc) dianion presents an almost planar geometry which confers a potential capability to act as a bis-bidentate bridging ligand, and therefore, to construct 1-D metal complexes. Based on this assumption, we have designed the first six transition metal complexes based on this ligand of formula {[M(micro-pmdc)(H2O)2].H2O}n [M(II) = Fe (4), Co (5), Ni (6), Zn (7), Cu (8)] and {[Cu(micro-pmdc)(dpa)].4H2O}n (9) (dpa = 2,2'-dipyridylamine). The crystal structure of all of these complexes has been determined by single crystal X-ray measurements, except for compound whose X-ray powder diffraction pattern reveals that it is isostructural to compounds 4-7. The bis-chelating pmdc ligand bridges sequentially octahedrally coordinated M(II) centres leading to polymeric chains. The hexacoordination of the metal centres is completed by two water molecules in compounds 4-8 and by the two endocyclic-N atoms of a terminal dpa ligand in compound . Cryomagnetic susceptibility measurements show the occurrence of antiferromagnetic intrachain interactions for compounds and (J = -2.5 (4), -5.2 (6), -32.7 (8), and -0.9 (9) cm(-1)). Model calculations and analyses of the available experimental data have been used to examine the influence of several factors on the nature and magnitude of the magnetic coupling constants in pyrimidine bridged complexes, showing that metal deviation from the pyrimidine mean plane could lead to ferromagnetic behaviour.

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“…13 Larger non-Kekulé systems have also † IUPAC-preferred names: TMM, 2-methylenepropane-1,3-diyl; TMN ϩ , 2-methylene-2-azoniapropane-1,3-diyl; IA, 2-iminopropane-1,3-diyl; IDMM, 2-iminiopropane-1,3-diyl; 2-oxyallyl, 2-oxopropane-1,3-diyl; 2-hydroxyallyl cation, 2-oxoniumylidenepropane-1, 3-diyl. been the subject of much study, particularly with respect to the development of models allowing the prediction of ground state spin. [14][15][16][17][18][19][20][21][22][23] For instance, the general class of non-Kekulé hydrocarbons can be divided into two subsets, so-called disjoint and non-disjoint diradicals. 14 The former are characterized by frontier MOs that are formed from disjoint sets of atomic orbitals (AOs), and, as a result, violations of Hund's rule can occur.…”

mentioning

confidence: 99%

Monoaza-analogs † of trimethylenemethane. Isoelectronic similarities and differences

Worthington

Cramer³

1998

J. Chem. Soc., Perkin Trans. 2

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Conformational effects on high-spin organic molecules

Cited by 48 publications

References 7 publications

Cross-Conjugated Bis(porphryin)s: Synthesis, Electrochemical Behavior, Mixed Valency, and Biradical Dication Formation

Cross-Conjugated Bis(porphryin)s: Synthesis, Electrochemical Behavior, Mixed Valency, and Biradical Dication Formation

Rational design of 1-D metal–organic frameworks based on the novel pyrimidine-4,6-dicarboxylate ligand. New insights into pyrimidine through magnetic interaction

Monoaza-analogs † of trimethylenemethane. Isoelectronic similarities and differences

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