Structure and H(2)-Loss Energies of OsHX(H(2))(CO)L(2) Complexes (L = P(t-Bu)(2)Me, P(i-Pr)(3); X = Cl, I, H): Attempted Correlation of (1)J(H-D), T(1min), and DeltaG()

1J(H-D), T(1min) and k(1) for H(2) dissociation from OsHX(H(2))(CO)L(2) have been measured for X = Cl, I, H (L = P(t-Bu)(2)Me or P(i-Pr)(3)), as well as for OsCl(2)(H(2))(CO)(P(i-Pr)(3))(2). For comparison, new data (including previously unobserved coupling constants) have been reported for W(HD)(CO)(3)(P(i-Pr)(3))(2). A comprehensive consideration of T(1min) data for over 20 dihydrogen complexes containing only 1-2 phosphines cis to H(2), together with a consideration of the shortest "conceivable" H-H distance for H(2) bound to a d(4) or d(6) metal, is used to argue that the "fast spinning" model is not appropriate for determining r(H-H) in such complexes. Regarding OsHX(H(2))(CO)L(2), the stronger electron-donor (lighter) halide, when cis to H(2), facilitates loss of H(2). The complete absence of pi-donor ability when X = H renders H(2) loss most difficult. However, a pi-donor trans to H(2) also makes H(2) loss unobservable. Within the series of isoelectronic, structurally analogous Os complexes, a longer H-H bond shows a larger DeltaG() for H(2) loss. However, this correlation does not continue to W(H(2))(CO)(3)(P(i-Pr)(3))(2), which has r(H-H) comparable to that of OsH(halide)(H(2))(CO)(P(i-Pr)(3))(2), but a significantly higher DeltaG(). This may originate from lack of a pi-donor ligand to compensate as H(2) leaves W.     

Structural, electronic, and vibrational characterization of Fe-HNO porphyrinates by density functional theory

A recent report of the structural and vibrational properties of heme-bound HNO in myoglobin, MbHNO, revealed a long Fe-N(HNO) bond with the hydrogen atom bonded to the coordinated N atom. The Fe-N(H)-O moiety was reported to exhibit an unusually high Fe-N(HNO) stretching frequency relative to those of the corresponding [FeNO]6 and [FeNO]7 porphyrinates, despite the Fe-N(HNO) bond being longer than either of its Fe-N(NO) counterparts. Herein, we present results from density functional theory calculations of an active site model of MbHNO that support the previous assignment and clarify this seemingly contradictory result. The results are consistent with the experimental evidence for a ground-state Fe-N(H)-O structure having a long Fe-N(HNO) bond and a uniquely high nu(Fe)(-)(N(HNO)) frequency. This high frequency is the result of the correspondingly low reduced mass of the normal mode, which is largely attributable to significant motion of the N-bound hydrogen atom. Additionally, the calculations show the Fe-N(H)O bonding in this complex to be remarkably insensitive to whether the HNO and ImH ligand planes are parallel or perpendicular. This is attributed to insensitivities of the Fe-L(axial) characters of molecular orbitals to the relative HNO and ImH orientation in both the parallel and perpendicular conformers.     

Data requirements for protein identification using chemically-assisted fragmentation and tandem mass spectrometry

Many laboratories identify proteins by searching tandem mass spectrometry data against genomic or protein sequence databases. These database searches typically use the measured peptide masses or the derived peptide sequence and, in this paper, we focus on the latter. We study the minimum peptide sequence data requirements for definitive protein identification from protein sequence databases. Accurate mass measurements are not needed for definitive protein identification, even when a limited amount of sequence data is available for searching. This information has implications for the mass spectrometry performance (and cost), data base search strategies and proteomics research.     

Comparison of thioethers and sulfoxides as axial ligands for N-acetylmicroperoxidase-8: implications for oxidation of methionine-80 in cytochrome c

Methionine-80 (Met-80) in mitochondrial cytochrome c (cyt c) can be oxidized to the corresponding sulfoxide by reactive oxygen species, a reaction of potential biological significance. As an approach to investigating how oxidation of Met-80 would influence its interactions with heme iron, we have examined binding of 2-(methylthio)ethanol (MTE) and dimethyl sulfoxide (DMSO), models for the side chains of Met and Met(SO), respectively, to ferrous and ferric N-acetylmicroperoxidase-8 (AcMP8). We find that DMSO coordinates 1.2 kcal/mol less strongly to Fe(III)-AcMP8 than does MTE, although both ligands form low-spin complexes. Comparison of spectroscopic data for the DMSO complex of Fe(III)-AcMP8 with published data for the Met(SO)-80 form of ferric cyt c allows us to conclude that Met(SO)-80 does not coordinate to iron in the latter. DMSO coordinates to Fe(II)-AcMP8 1.3 kcal/mol more strongly than does MTE, whereas Met-80 and Met(SO)-80 are reported to have approximately equal affinity for Fe(II) in cyt c. This result suggests that the steric environment near the heme iron in cyt c discriminates against coordination of Met(SO)-80. Vacuum quantum chemical density functional theory calculations confirm the greater affinity of the sulfoxide and show that coordination via oxygen is strongly favored. Resonance Raman spectroscopic data indicate that the preference for coordination via oxygen is maintained in solution. The computational data further indicate that the DMSO complex derives significant enthalpic stabilization from pi back-bonding but that iron to sulfur pi back-bonding does not make a significant contribution to bonding in the thioether complex.     

On the mechanism of (PCP)Ir-catalyzed acceptorless dehydrogenation of alkanes: a combined computational and experimental study

Pincer complexes of the type ((R)PCP)IrH(2), where ((R)PCP)Ir is [eta(3)-2,6-(R(2)PCH(2))(2)C(6)H(3)]Ir, are the most effective catalysts reported to date for the "acceptorless" dehydrogenation of alkanes to yield alkenes and free H(2). We calculate (DFT/B3LYP) that associative (A) reactions of ((Me)PCP)IrH(2) with model linear (propane, n-PrH) and cyclic (cyclohexane, CyH) alkanes may proceed via classical Ir(V) and nonclassical Ir(III)(eta(2)-H(2)) intermediates. A dissociative (D) pathway proceeds via initial loss of H(2), followed by C-H addition to ((Me)PCP)Ir. Although a slightly higher energy barrier (DeltaE(+ +)) is computed for the D pathway, the calculated free-energy barrier (DeltaG(+ +)) for the D pathway is significantly lower than that of the A pathway. Under standard thermodynamic conditions (STP), C-H addition via the D pathway has DeltaG(o)(+ +) = 36.3 kcal/mol for CyH (35.1 kcal/mol for n-PrH). However, acceptorless dehydrogenation of alkanes is thermodynamically impossible at STP. At conditions under which acceptorless dehydrogenation is thermodynamically possible (for example, T = 150 degrees C and P(H)2 = 1.0 x 10(-7) atm), DeltaG(+ +) for C-H addition to ((Me)PCP)Ir (plus a molecule of free H(2)) is very low (17.5 kcal/mol for CyH, 16.7 kcal/mol for n-PrH). Under these conditions, the rate-determining step for the D pathway is the loss of H(2) from ((Me)PCP)IrH(2) with DeltaG(D)(+ +) approximately DeltaH(D)(+ +) = 27.2 kcal/mol. For CyH, the calculated DeltaG(o)(+ +) for C-H addition to ((Me)PCP)IrH(2) on the A pathway is 35.2 kcal/mol (32.7 kcal/mol for n-PrH). At catalytic conditions, the calculated free energies of C-H addition are 31.3 and 33.7 kcal/mol for CyH and n-PrH addition, respectively. Elimination of H(2) from the resulting "seven-coordinate" Ir-species must proceed with an activation enthalpy at least as large as the enthalpy change of the elimination step itself (DeltaH approximately 11-13 kcal/mol), and with a small entropy of activation. The free energy of activation for H(2) elimination (DeltaG(A)(+ +)) is hence found to be greater than ca. 36 kcal/mol for both CyH and n-PrH under catalytic conditions. The overall free-energy barrier of the A pathway is calculated to be higher than that of the D pathway by ca. 9 kcal/mol. Reversible C-H(D) addition to ((R)PCP)IrH(2) is predicted to lead to H/D exchange, because the barriers for hydride scrambling are extremely low in the "seven-coordinate" polyhydrides. In agreement with calculation, H/D exchange is observed experimentally for several deuteriohydrocarbons with the following order of rates: C(6)D(6) > mesitylene-d(12) > n-decane-d(22) > cyclohexane-d(12). Because H/D exchange in cyclohexane-d(12) solution is not observed even after 1 week at 180 degrees C, we estimate that the experimental barrier to cyclohexane C-D addition is greater than 36.4 kcal/mol. This value is considerably greater than the experimental barrier for the full catalytic dehydrogenation cycle for cycloalkanes (ca. 31 kcal/mol). Thus, the experimental evidence, in agreement with calculation, strongly indicates that the A pathway is not kinetically viable as a segment of the "acceptorless" dehydrogenation cycle.     

Toward rational control of metal stoichiometry in heterobimetallic coordination complexes: synthesis and characterization of Pb(Hsal)2(Cu(salen*))2, [Pb(NO3)(Cu(salen*))2](NO3), Pb(OAc)2(Cu(salen*)), and [Pb(OAc)(Ni(salen*))2](OAc)

The ability of the transition metal complex M(salen)* (M = Ni, Cu) to form Lewis acid-base adducts with lead(II) salts has been explored. The new complexes Pb(Hsal)(2)(Cu(salen*))(2) (1), [Pb(NO(3))(Cu(salen*))(2)](NO(3)) (2), Pb(OAc)(2)(Cu(salen*)) (3), and [Pb(OAc)(Ni(salen*)(2)](OAc) (4) (Hsal = O(2)CC(6)H(4)-2-OH, salen* = bis(3-methoxy)salicylideneimine) have been synthesized and characterized spectroscopically and by single-crystal X-ray diffraction. The coordination environment of the lead in the heterobimetallic complex is sensitive both to the initial lead salt and to the transition metal salen* complex that is employed in the synthesis. As a result, we have been able to access both 2:1 and 1:1 adducts by varying either the lead salt or the transition metal in the heterobimetallic coordination complex. In all cases, the salen* complex is associated with the lead center via dative interactions of the phenolic oxygen atoms. The relationship between the coordination requirements of the lead and the chemical nature of the anion is examined. In compound 1, the Pb(2+) ion is chelated by two Cu(salen*) moieties, and both salicylate ligands remain attached to the lead center and bridge to the Cu(2+) ions. The two Cu(salen*) groups are roughly parallel and opposed to each other as required by crystallographic inversion symmetry at lead. In contrast, the two Cu(salen*) groups present in 2 and 4 attached to the lead ion show considerable overlap. Furthermore, only one nitrate ion in 2 and one acetate ion in 4 remain bonded to the lead center. Compound 3 is unique in that only one Cu(salen*) group can bind to lead. Here, both acetate ligands remain attached, although one is chelating bidentate and the other is monodentate.     

Adducts of thianthrene- and phenoxathiin cation radical salts with symmetrical alkynes. Structure and formation of cumulenes on alumina leading to alpha-diketones, alpha-hydroxyalkynes, and alpha-acetamidoalkynes

[reaction: see text] Thianthrene cation radical tetrafluoroborate (Th*+ BF4-) added to 2-butyne, 3-hexyne, 4-octyne, and 5-decyne in MeCN to form trans bisadducts R(Th+)C=C(Th+)R, where R = Me, Et, Pr, Bu (7a-d). Phenoxathiin cation radical tetrafluoroborate (PO*+ BF4-) added similarly to the last three alkynes to form adducts R(PO+)C=C(PO+)R, 8b-d. Cyclic monoadducts were not found. The trans structures of 7 and 8 were deduced with X-ray crystallography (7c) and NMR spectroscopy. When solutions of adducts in CHCl3 and MeCN were deposited on activated alumina, elimination of thianthrene (Th) and phenoxathiin (PO) occurred almost quantitatively. Detailed studies with (7b-d) indicated that a cumulene (15) was formed by the elimination of Th and that 15 was subsequently converted into small amounts of other products. In CHCl3, these products were the respective alkyne, thianthrene 5-oxide, an alpha-diketone (11), an alpha-hydroxyalkyne (12), and hydrogen. The same products were formed in MeCN along with an alpha-acetamidoalkyne (13). The formation of 15 and products derived from it is explained and was confirmed by preparation and reactions of 2,3,4-hexatriene.     

Fe-N-O structure and bonding in six-coordinate {FeNO}6 porphyrinates containing imidazole: implications for reactivity of coordinated NO

We report density functional theory calculations on six-coordinate ferric-NO ({FeNO}6) porphyrinates that contain either imidazole or imidazolate as the trans axial ligand. Our results show that the sensitivities of the Fe-NO and N-O stretching frequencies to cis and trans influences are directly correlated. In other words, as one decreases so does the other for both the imidazole and the imidazolate complexes. This correlation is opposite that of the isoelectronic ferrous-CO systems, whose Fe-CO and C-O frequencies are well-known to be inversely correlated. Based on the results of our calculations, the molecular origin of the direct correlation in {FeNO}6 porphyrinates can be explained by trends in the electron density distributions within the HOMO or HOMO-1, which exhibits Fe-NO and N-O pi-antibonding character. Variability in the Fe-N-O pi-antibonding character of the HOMO or the HOMO-1 modulates the angleFeNO as well as the Fe-NO and N-O bond strengths in concert. Orbital interactions in the six-coordinate FeIIINO porphyrin complexes are compared and contrasted with those of the isoelectronic FeIICO analogues, and an overall view of {FeNO}6 bonding in these complexes is set forth.