The effect of steric bulk on electron delocalization in 4-arylpyridines has been studied by computational methods. Ab initio (HF, UHF, ROHF, MP2, UMP2, and ROMP2) as well as density functional theory (USVWN and UB-LYP) approaches were applied to a series of molecules and their corresponding anions. These molecules are put forth as models for the ground and MLCT excited states of three polypyridyl ligands that were the subject of a recent report on the effects of sterics and delocalization on the photophysics of several Ru II complexes (Damrauer, et al. J. Am. Chem. Soc. 1997, 119, 8253). The present study finds that, in the series 4-phenylpyridine, 4-(o-tolyl)pyridine, and 4-(2,6-dimethylphenyl)pyridine, the steric effect of the ortho-methyl groups serves to increase the dihedral angle between the pyridyl and phenyl rings of the neutral compounds from ca. 45°in the case of 4-phenylpyridine to ca. 65°and 90°in the mono-and dimethylated compounds, respectively. These results are generally consistent with the single-crystal X-ray structures of the three corresponding bipyridines, also reported herein. Upon one-electron reduction, calculations on all three model ligands reveal a preference for a coplanar structure, with the optimized geometries reflecting a balance between an energetic stabilization gained via conjugation in the planar form and unfavorable steric interactions between the methyl group(s) of the 4-aryl substituent and the pyridyl protons ortho to the central C-C bond. Calculated dihedral angles were 0°, ∼25°, and ∼45°for 4-phenyl-, 4-(o-tolyl)-, and 4-(2,6-dimethyl)pyridine, respectively. Finally, a simulation of the Franck-Condon state evolution of MLCT states of molecules containing the bipyridyl analogues of the three models was carried out by computing singlepoint energies of each compound as its monoanion in the optimized neutral geometry. Comparison of these energies with those of the fully optimized anions revealed effective reorganization energies of 4-7 kcal/mol for 4-phenylpyridine, 4-7 kcal/mol for 4-(o-tolyl)pyridine, and ca. 6 kcal/mol for 4-(2,6-dimethylphenyl)-pyridine. The implications of these results as they pertain to ultrafast spectroscopic studies of MLCT excitedstate evolution in the corresponding Ru II bipyridyl complexes are discussed.

The great importance of heterogeneous hydrodesulfurization (HDS)"] has spawned numerous studies with homogeneous model systems.I2] Of particular interest has been the activation of derivatives of thiophene (T), benzothiophene (BT), and dibenzothiophene (DBT). While all three of these sulfur-containing molecules are relevant to HDS, the latter two are especially so because they are more difficult to desulfurize and thus are much more abundant in fossil fuels. The key step in HDS is cleavage of the S-C bonds, and a reasonable way for this to occur is by initial coordination to a metal followed by insertion into the S-C bond. In particular, precoordination of the metal to the sulfur atom of the thiophene unit is thought to be crucial to S-C bondIn the case of BT ( I ) , several examples of the insertion of metal fragments ML, into the have been reported ; however, there have been no reports of S-C(aryl) bond cleavage to give 3. The X-ray structurer4] of [(BT)Re(CO),Cp*] (Cp* = C,Me,), in which the rhenium S-C(viny1) bond to give 2 (M = Fe, Ru, Rh, Ir, Pt13b.5361 ) 1 2 3atom is q'-bonded to the sulfur atom. suggests that ql-S coordination selectively weakens the S-C(viny1) bond but not the S-C(ary1) bond, thus providing an explanation16e1 for why 2, and not 3, is formed. Herein we demonstrate that the insertion of a metal atom into the S-C(ary1) bond of BT to give complexes of type 3 can, in fact, be readily accomplished.

The synthesis, structure, and physical properties of a Heisenberg exchange-coupled cluster containing naphthalene groups are described. [Fe2(O)(O2CCH2C10H7)2(TACN-Me3)2]2+ (3) crystallizes in space group P1 with unit cell parameters a = 12.94(2) A, b = 14.84(2) A, c = 15.23(2) A, alpha = 101.12(7) degrees, beta = 90.8(1) degrees, gamma = 114.14(7) degrees, V = 2605(6) A3, and Z = 2 with R = 0.0425 and wR2 = 0.1182. Variable-temperature magnetic susceptibility data indicate that the two high-spin FeIII centers are antiferromagnetically coupled with J = -105 cm-1 (H = -2 JS1.S2), which is typical for this class of compounds. The room-temperature static emission spectrum of the compound in deoxygenated CH3CN solution is centered near 335 nm and has features reminiscent of both methyl-2-naphthylacetate (1) and [Zn2(OH)(O2CCH2C10H7)2(TACN-Me3)2]+ (2) with the following two caveats: (1) the overall emission intensity is roughly a factor of 10 less than that of the free ester (1, phi = 0.13) or the ZnII analogue (2, phi = 0.14), and (2) there is significant broadening of the low-energy shoulder of the emission envelope. Time-correlated single photon counting data revealed biphasic emission for 3 with tau 1 = 4.6 +/- 1 ns and tau 2 = 47 +/- 1 ns. The latter compares favorably with that found for 2 (tau = 47 +/- 1 ns) and is assigned as the S0-S1 fluorescence of naphthalene. Emission anisotropy, time-gated emission spectra, and nanosecond time-resolved absorption measurements all support the assignment of the 4.6 ns component as being due to a singlet excimer that forms between the two naphthylacetate groups of 3, a process that is likely mediated by the structural constraints of the oxo-bis-carboxylato diiron core. No direct evidence for intramolecular electron and/or energy transfer from the photoexcited naphthyl group to the iron-oxo core was obtained, suggesting that the short-lived excimer may contribute to circumventing such pathways in this type of system.

Bimolecular quenching between photosensitizers and exchange-coupled transition metal complexes has been studied in an effort to experimentally establish a link between Heisenberg spin exchange and chemical reactivity. The acceptors are members of the oxo/hydroxo-biscarboxylato class of dinuclear Fe(III) compounds, where protonation of the oxo bridge provides a means for modulating the magnitude of spin exchange within the cluster. Photoexcitation of solutions containing Ru(II) polypyridyl sensitizers and the Fe(III) complexes results in quenching of emission from the (3)MLCT excited state of the Ru(II) chromophores; nanosecond time-resolved absorption measurements demonstrate that quenching occurs, in part, by electron transfer. Decoupling electron transfer driving force (DeltaG(0)(ET)) from changes in the magnitude of spin exchange was achieved by varying the bridging carboxylate to afford a series of complexes of the form [Fe(2)O(H)(O(2)CR)(2)(Tp)(2)](n)(+) (n = 0, 1, 2). Electrochemical measurements reveal a greater than 500 mV shift in cluster reduction potential across the series (i.e., R = CH(3) to CF(3)), whereas variable-temperature magnetic susceptibility measurements demonstrate a corresponding invariance in spin exchange between the metal centers (J(oxo) = -119 +/- 4 cm(-1) and J(hydroxo) = -18 +/- 2 cm(-1) for H = -2JS(1).S(2)). Structural analyses suggest that reorganization energies (lambda) associated with electron transfer should be identical for all molecules within a given series (i.e., oxo or hydroxo bridged); likewise Deltalambda between the series is expected to be small. A comparison of quenching rates for the two extended series firmly establishes that neither reorganization energy nor electron transfer driving force considerations can account for differences in reactivity between oxo-bridged (large spin exchange) and hydroxo-bridged (small spin exchange) quenchers. Upon consideration of energy transfer contributions, it is determined that reactivity differences between the oxo- and hydroxo-bridged quenchers must lie in the relative rates of Dexter energy transfer and/or electron transfer, with the origin of the latter linked to something other than DeltaG(0)(ET) or lambda. Finally, the extent to which spin exchange within the dinuclear Fe(III) quenchers can be identified as the key variable influencing these reactivity patterns is discussed.