Effects of methyl group substitution on metal-coordinated cyclopentadienyl rings. Core and valence ionizations of methylated tricarbonyl(.eta.5-cyclopentadienyl)metal complexes

Abstract

extensive listing of intrinsic proton binding tendencies at N(l)and N(7) of purines. The N(l)/N(7) intrinsic binding ratio for protons is 2 log units greater for the 6-oxopurines than for adenosine.The N(l) to N(7) dienPd2"1" binding ratios in the last column of Table VI have ramifications for the ligand distribution curves.The relatively pronounced favoring of N(l) for IMP and inosine in Figures 9 and 10 results in BM, clearly predominating over M7B" in basic solutions. In contrast the negative logarithm for GMP in Table VI means that the mole fraction of M7B" exceeds that for BM, and no crossover occurs; the N(7) metalated complex predominates at all pH values. Basic solutions of GMP and dienPd2"1" contain a mixture of four species, all present in significant amounts.The intrinsic N(l)/N(7) binding ratios of Table VI show quantitatively that the tendency to favor N(l) over N( 7) is H+ > CH3Hg+ > Pd(II). Indeed for dienPd2"1", the binding constant at N( 7) is often comparable to or even exceeds that at N(l). For inosine the crossover pH, where the M7BH, and BM, mole fractions are equal, occurs at pH 6.1 with dienPd2"1", 1.8 log units higher than with CH3Hg+, which is due to the lesser N(l)/N(7) binding ratio for dienPd2"1".For AMP the values of log (Ki/K•,) and log ( \/ ') are -0.1 and 0.4, respectively, the primes referring to phosphate protonated species. Compared to the monoanionic phosphate, the dianionic phosphate group favors dienPd2+ binding at N(7) over N(l). Thus there are two crossovers between N(7) and N(l) metalated species for AMP in Figure 11. The first crossover appears at pH 2.2 as M7BH,HP gives way to BM,HP when dienPd2"1" successfully competes with H+ for N(l). The maximum role fraction of , occurs near pH 5. Phosphate deprotonation near pH 6 yields a crossover close to pH 7, above which M7B predominates marginally over BM, according to the -0.1 entry in the last column of Table VI.We now compare the order of nucleotide binding strengths of dienPd2"1" with the proton and CH3Hg+, the only other metal for which a series of reliable values exists. As previously, we designate the nucleoside by its capitalized first letter and the nitrogen binding site by the usual numbering scheme.2 The order of decreasing proton binding strengths is then T3 > U3 > G1 >11 » C3 > Al > G7 > 17 > A7. The order of CH3Hg+ is similar, with a promotion of G7 and 17 to greater than Al. These two sites undergo further significant promotion in the series for decreasing dienPd2"1" stability constants T3 > U3 > II > G7 > G1 > 17 » C3 > Al > A7 with Al and A7 now of comparable magnitude.Because of the proton at N(l) in purines and N(3) in pyrimdines the stability order, as opposed to stability constants, is pH dependent. At pH 7 the stability order for dienPd2"1" becomes G7 > 17 > II > G1 > U3 > T3 > C3 > Al > A7. This order agrees with that observed in neutral solutions of nucleotide mixtures.7 Thus N(7) of GMP and IMP offers the strongest effective nucleic base binding site for dienPd2"1" in neutral solutions.

Variable-energy photoelectron spectroscopy of CpM(CO)3 (M = Mn, Re) and CpFe(CO)2I

“…I 4d spectra taken at 106 eV photon energy and 50. 15 widths of 0.52 and 0.54 eV, respectively. The I 4d5/2 binding energy of 55.20 eV is much smaller than that of other small covalent molecules such as HI and I2 (Table 1).…”

Reactions of N-heterocyclic ligands with substitutionally labile organometallic complexes, [(η5-C5R5)Fe(CO)2E]BF4

“…1b), while a singlet corresponding to the equivalent methylene carbons was observed at 81.8 ppm in the 13 C{ 1 H} NMR spectrum of compound 4b indicating a decrease in r acidity of the metal fragment upon changing from Cp to Cp ⁄ which conforms with the expected enhanced p-back donation in Cp ⁄ complexes [54]. The peak corresponding to less deshielded methylene carbons C4, C6, and C10 bridging the uncoordinated nitrogen atoms were observed at 71.1 and 71.2 ppm for compounds 3b and 4b, respectively, implying that the effect of changing the Cp to Cp ⁄ was insignificant on these methylene groups.…”

Magnetic-field dependent optically detected ESR in the photoexcited triplet states of bridged Zr(IV) metallocenes

“…The LUMO is ana l orbital which is predominantly a mixture of the nonbonding d~_,_, and d.~ metal orbitals. The lowest-energy optical transition in the Zr(IV) dichloride metallocenes is therefore a Cp (cyclopentadienyl) ligand-to-metal charge transfer (LMCT) [8,9]. Photoexcitation of the metallocene necessarily selects those molecular orientations with the electric field component of the electromagnetic field of the laser parallel to the transition-dipole moment of the optical transition.…”

“…Subtle changes in the coordination chemistry result in major impacts on the reaction chemistry and photochemical properties of metatlocenes. Significant progress in both theory [4][5][6] and experiments have led to a general understanding of how the halide and Cp ring substituents affect the observed trends of the redox potentials [7], ionization energies [8], and electronic spectra [8][9][10]. The ODMR data in metallocenes can be exploited to study the effect of magnetic field on relaxations amongst the triplet-state sublevels.…”