Fragmentation pathways of peptide radical cations, M ϩ· , with well-defined initial location of the radical site were explored using collision-induced dissociation (CID) experiments. Peptide radical cations were produced by gas-phase fragmentation of Co III (salen)-peptide complexes [salen ϭ N,N'-ethylenebis (salicylideneiminato)]. Subsequent hydrogen abstraction from the ␤-carbon of the side-chain followed by C ␣ -C ␤ bond cleavage results in the loss of a neutral side chain and formation of an ␣-radical cation with the radical site localized on the ␣-carbon of the backbone. Similar CID spectra dominated by radical-driven dissociation products were obtained for a number of arginine-containing ␣-radicals, suggesting that for these systems radical migration precedes fragmentation. In contrast, proton-driven fragmentation dominates CID spectra of ␣-radicals produced via the loss of the arginine side chain. Radicaldriven fragmentation of large M ϩ· peptide radical cations is dominated by side-chain losses, formation of even-electron a-ions and odd-electron x-ions resulting from C ␣ -C bond cleavages, formation of odd-electron z-ions, and loss of the N-terminal residue. In contrast, charge-driven fragmentation produces even-electron y-ions and odd-electron b-ions. (n-1)ϩ· ions produced by capture of low-energy electrons by multiply protonated molecules or by electron-transfer processes [1][2][3][4][5]9], hydrogen deficient radical anions [M Ϫ nH] (n-1)-· generated by electron detachment and photodetachment from multiply deprotonated molecules [10,11], peptides cationized on lithium [12], or transition metals [13][14][15][16], and M ϩ· peptide radical cations. [In this study, we used standard notation for molecular radical cations, M ϩ· , which implies that the molecule has a charge and one unpaired electron, and specified the initial location of the radical site, when possible]. M ϩ· ions can be produced via gas-phase fragmentation of specially designed precursors. For example, radical cations of peptides without additional H atoms are produced by collision-induced dissociation (CID) of ternary metal-ligand-peptide complexes [17][18][19][20][21][22][23][24][25]. In addition, M ϩ· peptide ions have been generated through free radical-initiated reactions [26,27], CID of nitrosopeptides [28], and peptides containing labile serine and homoserine nitrate esters [29], photolysis of peptides containing iodinated tyrosine residues [30,31], and photodissociation of protonated peptides [32,33].Fragmentation of small peptide radical cations has been recently reviewed [7,34]. It is usually initiated by hydrogen abstraction or proton transfer from the initial radical site generated in the ion formation step [35,36]. Reaction barriers for H atom transfer in several small model radicals have been reported by Moran et al. [37]. They demonstrated that in the absence of side chains 1,4 [C↔C] hydrogen transfer along the peptide backbone is associated with substantial barriers of ca. 100 kJ/mol, while 1,5 and 1,6 [C↔N] hydrogen shift...

A new class of readily tunable isocyano rhenium(I) diimine luminophores, cis,cis-[Re(CO)(2)(CNR)(2)(N-N)](+) (R=2,4,6-Cl(3)C(6)H(2), 4-ClC(6)H(4), 4-Br-2,6-(CH(3))(2)C(6)H(2), 2,6-(CH(3))(2)C(6)H(3), 4-[(CH(3))(2)N]C(6)H(4), 4-(C(6)H(5))C(6)H(4), 4-nBuC(6)H(4), tBu), has been synthesized in high yield by a highly selective photochemical substitution reaction. These complexes were characterized by (1)H NMR and IR spectroscopy, mass spectrometry, and elemental analysis. The X-ray crystal structures of one of the complexes and one of the precursor complexes for the photosubstitution reaction were also determined. As the isocyanide ligands are readily tunable, complexes with excellent solubility in benzene or other nonpolar solvents could be designed through slight modification of the isocyanide ligands with a short nBu substituent. With the characteristic strong infrared absorptions of the carbonyl (C≡O) and isocyanide (C≡N) stretches as well as the high solubility of the reactant and product in benzene, which is the solvent for the photoreaction, the photosubstitution reaction of [Re(CO)(3)(nBuC(6)H(4)NC)(2)Br] with 4,4'-di-tert-butyl-2,2'-bipyridine was also studied by in situ IR spectroscopy. The photophysical and electrochemical properties of these complexes were also investigated. Except for the complex with [(CH(3))(2)N]C(6)H(4)NC ligands, all complexes displayed intense luminescence with quantum yields of up to 0.37 in degassed CH(2)Cl(2) solution at room temperature. These emissions were assigned as the phosphorescence derived from the metal-to-ligand charge transfer [dπ(Re)→π*(N-N)] excited state. The emissive excited states of these complexes have also been characterized by transient absorption spectroscopic studies. The capability of tuning the emissive excited-state energy through the modification of the isocyanide ligands could be reflected by the significant shifting of the phosphorescence from 530 to 620 nm with the same phenanthroline ligand.

The electrochemical and photochemical catalytic reductions of CO2 using N,O and N,S-NHC-containing dicarbonyl rhenium(i) bipyridine complexes have been investigated. By replacing the carbonyl ligand in tricarbonyl rhenium(i) complexes with a weaker π-accepting ligand, the characteristic MLCT transitions shifted to lower energy. This makes photocatalysts capable of harvesting low-energy visible light for catalyzing CO2 reduction. A detailed study revealed that these dicarbonyl rhenium(i) complexes are also highly selective for photocatalysis of CO2 to CO with a good quantum efficiency (10%), similar to that of the tricarbonyl rhenium(i) complex analogues. From the electrochemical study, it was observed that the catalysts efficiently produce CO from CO2 with high turnover frequency and good stability over time.

A series of luminescent rhenium­(I) phenanthroline complexes containing benzoxazol-2-ylidene ligands with the general formula {Re­(CO)3(phen)­[CN­(X)­C6H4-2-O]}+ and cis,trans-{Re­(CO)2(phen)­(L)­[CN­(H)­C6H4-2-O]}+ (X = H, methyl; phen = 1,10-phenanthroline; L = PPh3, PPh2Me, P­(OEt)3) have been synthesized and characterized. The X-ray crystal structures of most of the carbene complexes and some of their synthetic precursors have also been determined. A new synthetic methodology for the preparation of dicarbonyl rhenium diimine synthetic precursors with a labile acetonitrile ligand, [Re­(CO)2(phen)­(PR3)­(MeCN)]+, was developed. Photophysical study shows that these carbene complexes display a green to red 3MLLCT [dπ­(Re) → π*­(N–N)] phosphorescence at room temperature. The N-deprotonations of the benzoxazol-2-ylidene ligand in these complexes were investigated.