Structure and Solution Speciation of U(IV) Linked Phosphomolybdate (Mo(V)) Clusters.

Crystals of [NaU(Mo6P4O31H7)2]·5Na·(H2O)n (NaUMo6) have been synthesized by slow evaporation of an aqueous mixture containing uranyl nitrate, sodium molybdate, phosphoric acid, and sodium dithionate. Single crystal diffraction of NaUMo6 reveals the assembly of {Mo6P4} clusters linked into one-dimensional chains with alternating Na(+) and U(4+) cations. To our knowledge, NaUMo6 is a unique example of Mo(5+) based polyoxometalate associated with actinides. With the use of similar synthesis conditions but without uranium in the aqueous solution, [Na(Mo6P4O31H10)2]·5Na·(H2PO4)·(H2O)n (NaMo6) is obtained. NaMo6 is a sandwich type cluster which is built on the assemblage of two {Mo6P4} units linked by one sodium cation. Using small-angle X-ray scattering techniques and aqueous electrolyte based dissolution strategies, we can accurately observe chains of [UNa(Mo6P4O31H7)2]n(5n), where n = 7 is the dominant soluble specie. Likewise, the dimeric form of [Na(Mo6P4O31H10)2](5-) dominates the aqueous solution, revealing the structural units observed in the crystal structure are also stable in solution, under appropriate dissolution conditions.

Electrophilic alkylation of pseudotetrahedral nickel(II) arylthiolate complexes.

A kinetic study is reported for reactions of pseudotetrahedral nickel(II) arylthiolate complexes [(Tp(R,Me))Ni-SAr] (Tp(R,Me) = hydrotris{3-R-5-methyl-1-pyrazolyl}borate, R = Me, Ph, and Ar = C6H5, C6H4-4-Cl, C6H4-4-Me, C6H4-4-OMe, 2,4,6-Me3C6H2, 2,4,6-(i)Pr3C6H2) with organic electrophiles R'X (i.e., MeI, EtI, BzBr) in low-polarity organic solvents (toluene, THF, chloroform, dichloromethane, or 1,2-dichloroethane), yielding a pseudotetrahedral halide complex [(Tp(R,Me))Ni-X] (X = Cl, Br, I) and the corresponding organosulfide R'SAr. Competitive reactions with halogenated solvents and adventitious air were also examined. Akin to reactions of analogous and biomimetic zinc complexes, a pertinent mechanistic question is the nature of the reactive nucleophile, either an intact thiolate complex or a free arylthiolate resulting from a dissociative pre-equilibrium. The observed kinetics conformed to a second-order rate law, first order with respect to the complex and electrophile, and no intermediate complexes were observed. In the absence of a mechanistically diagnostic rate law, a variety of mechanistic probes were examined, including kinetic effects of varying the metal, solvent, electrophile, and temperature, as well as the 3-pyrazolyl and arylthiolate substituents. Compared to zinc analogues, the effect of Ni-SAr covalency is also of interest herein. The results are broadly interpreted with respect to the disparate mechanistic pathways.

Steric and electronic effects on arylthiolate coordination in the pseudotetrahedral complexes [(Tp(Ph,Me))Ni-SAr] (Tp(Ph,Me) = hydrotris{3-phenyl-5-methyl-1-pyrazolyl}borate).

Synthesis and characterization of several new pseudotetrahedral arylthiolate complexes [(Tp(Ph,Me))Ni-SAr] (Tp(Ph,Me) = hydrotris{3-phenyl-5-methyl-1-pyrazolyl}borate; Ar = Ph, 2,4,6-(i)Pr3C6H2, C6H4-4-Cl, C6H4-4-Me, C6H4-4-OMe) are reported, including X-ray crystal structures of the first two complexes. With prior results, two series of complexes are spanned, [(Tp(Ph,Me))Ni-S-2,4,6-RC6H2] (R'' = H, Me, (i)Pr) plus the xylyl analogue [(Tp(Ph,Me))Ni-S-2,6-Me2C6H3], as well as [(Tp(Ph,Me))Ni-S-C6H4-4-Y] (Y = Cl, H, Me, OMe), intended to elucidate steric and/or electronic effects on arylthiolate coordination. In contrast to [(Tp(Me,Me))Ni-SAr] analogues that adopt a sawhorse conformation, the ortho-disubstituted complexes show enhanced trigonal and Ni-S-Ar bending, reflecting the size of the 3-pyrazole substituents. Moreover, weakened scorpionate ligation is implied by spectroscopic data. Little spectroscopic effect is observed in the series of para-substituted complexes, suggesting the observed effects are primarily steric in origin. The relatively electron-rich and encumbered complex [(Tp(Ph,Me))Ni-S-2,4,6-(i)Pr3C6H2] behaves uniquely when dissolved in CH3CN, forming a square planar solvent adduct with a bidentate scorpionate ligand, [(κ(2)-Tp(Ph,Me))Ni(NCMe)(S-2,4,6-(i)Pr3C6H2)]. This adduct was isolated and characterized by X-ray crystallography. Single-point DFT and TD-DFT calculations on a simplified [(κ(2)-Tp)Ni(NCMe)(SPh)] model were used to clarify the electronic spectrum of the adduct, and to elucidate differences between Ni-SAr bonding and spectroscopy between pseudotetrahedral and square planar geometries.

Elucidating self-assembly mechanisms of uranyl-peroxide capsules from monomers.

Self-assembly of uranyl peroxide polyoxometalates (POMs) in alkaline peroxide solutions has been known for almost a decade, but in these dynamic solutions that contain high concentrations of base and peroxide the reaction pathway could never be discerned, mixed species are obtained, and reproducibility is sometimes a challenge. Here we elucidate the reaction mechanisms utilizing self-assembly of the U24 cluster, [UO2(O2)(OH)]24(24-), from monomers as a model system. Using Raman as our main spectroscopic probe, we learned that the monomeric species is persistent in water at room temperature indefinitely. However, if a redox-active transition metal catalyst (copper (Cu(2+)) or cobalt (Co(2+))) is added, self-assembly is accelerated in a significant manner, forming U24 peroxide clusters in several hours, which is a good time scale for studying reaction mechanisms. From semiquantitative treatment of the spectroscopic data, we elucidate reaction mechanisms that are consistent with prior structural and computational studies that suggest uranyl peroxide rings templated by alkalis are the building units of clusters. By understanding aqueous speciation and processes, we are moving toward assuming control over cluster self-assembly that has been mastered for decades now in the analogous transition-metal POM systems.

Structural and spectroscopic trends in a series of half-sandwich scorpionate complexes.

Fifteen half-sandwich scorpionate complexes [(L)M(NCMe)(3)](BF(4))(n) (L = tris(3,5-dimethylpyrazol-1-yl)methane, Tpm(Me,Me), n = 2, 1(M), M = Mn, Fe, Co, Ni; L = tris(3-phenylpyrazol-1-yl)methane, Tpm(Ph), n = 2, 2(M), M = Mn, Fe, Co, Ni; L = hydrotris(3,5-dimethylpyrazol-1-yl)borate, [Tp(Me,Me)](-), n = 1, 3(M), M = Fe, Co, Ni; L = hydrotris(3-phenyl-5-methylpyrazol-1-yl)borate, [Tp(Ph,Me)](-), n = 1, 4(M), M = Mn, Fe, Co, Ni) were prepared by addition of the tripodal ligands to solvated [M(NCMe)(x)](2+) (M = Mn, x = 4; M = Fe, Co, Ni, x = 6) precursor complexes. The product complexes were characterized by (1)H NMR (except M = Mn), UV-vis-NIR, and FTIR spectroscopy. The structures of 2(Mn), 2(Ni), 3(Fe), 3(Co), and 4(Fe) were determined by X-ray crystallography. The data were consistent with complexes of high-spin divalent metal ions in idealized piano-stool geometries in all cases. Consequent lability of the acetonitrile ligands will enable use of these complexes as synthetic precursors and as catalysts. Comparison to previously reported structures of 1(Fe), 1(Co), 2(Fe), and 2(Co), the triflate salt analogues of 4(Co) and 4(Ni), as well as related sandwich complexes (e.g., [(Tp(Me,Me))(2)M]) and solvated metal dications [M(NCMe)(6)](2+) reveals numerous trends in M-N bond lengths. Primary among these are the Irving-Williams series, with significant structural effects also arising from ligand charge and sterics. Systematic trends in spectroscopic data were also observed which further elucidate these issues.

Aerobic and hydrolytic decomposition of pseudotetrahedral nickel phenolate complexes.

Pseudotetrahedral nickel(II) phenolate complexes Tp(R,Me)Ni-OAr (Tp(R,Me) = hydrotris(3-R-5-methylpyrazol-1-yl)borate; R = Ph {1a}, Me {1b}; OAr = O-2,6-(i)Pr(2)C(6)H(3)) were synthesized as models for nickel-substituted copper amine oxidase apoenzyme, which utilizes an N(3)O (i.e., His(3)Tyr) donor set to activate O(2) within its active site for oxidative modification of the tyrosine residue. The bioinspired synthetic complexes 1a,b are stable in dilute CH(2)Cl(2) solutions under dry anaerobic conditions, but they decompose readily upon exposure to O(2) and H(2)O. Aerobic decomposition of 1a yields a range of organic products consistent with formation of phenoxyl radical, including 2,6-diisopropyl-1,4-benzoquinone, 3,5,3',5'-tetraisopropyl-4,4'-diphenodihydroquinone, and 3,5,3',5'-tetraisopropyl-4,4'-diphenoquinone, which requires concurrent O(2) reduction. The dimeric product complex di[hydro{bis(3-phenyl-5-methylpyrazol-1-yl)(3-ortho-phenolato-5-methylpyrazol-1-yl)borato}nickel(II)] (2) was obtained by ortho C-H bond hydroxylation of a 3-phenyl ligand substituent on 1a. In contrast, aerobic decomposition of 1b yields a dimeric complex [Tp(Me,Me)Ni](2)(µ-CO(3)) (3) with unmodified ligands. However, a unique organic product was recovered, assigned as 3,4-dihydro-3,4-dihydroxy-2,6-diisopropylcyclohex-5-enone on the basis of (1)H NMR spectroscopy, which is consistent with dihydroxylation (i.e., addition of H(2)O(2)) across the meta and para positions of the phenol ring. Initial hydrolysis of 1b yields free phenol and the known complex [Tp(Me,Me)Ni(µ-OH)](2), while hydrolysis of 1a yields an uncharacterized intermediate, which subsequently rearranges to the new sandwich complex [(Tp(Ph,Me))(2)Ni] (4). Autoxidation of the released phenol under O(2) was observed, but the reaction was slow and incomplete. However, both 4 and the in situ hydrolysis intermediate derived from 1a react with added H(2)O(2) to form 2. A mechanistic scheme is proposed to account for the observed product formation by convergent oxygenation and hydrolytic autoxidation pathways, and hypothetical complex intermediates along the former were modeled by DFT calculations. All new complexes (i.e., 1a,b and 2-4) were fully characterized by FTIR, (1)H NMR, and UV-vis-NIR spectroscopy and by X-ray crystallography.

Steric titration of arylthiolate coordination modes at pseudotetrahedral nickel(II) centers.

Several derivatives of the pseudotetrahedral phenylthiolate complex Tp(Me,Me)Ni-SPh (1), Tp(Me,Me-) = hydrotris(3,5-dimethyl-1-pyrazolyl)borate, were prepared incorporating substituted arylthiolates, including a series of ortho-substituted ligands Tp(Me,Me)Ni-SR (R = 2,6-Me(2)C(6)H(3), 2; 2,4,6-Me(3)C(6)H(2), 3; 2,4,6- (i)Pr(3)C(6)H(2), 4; and 2,6-Ph(2)C(6)H(3), 5) and a series of para-substituted complexes (R = C(6)H(4)-4-OMe, 6; C(6)H(4)-4-Me, 7; and C(6)H(4)-4-Cl, 8). The products were characterized by (1)H NMR and UV-vis spectroscopy. Spectra of 6-8 were consistent with retention of a common structure across the para-substituted series with modest perturbation of the spectral features of 1 assisting their assignment. In contrast, spectra of 2-5 were indicative of a significant change in configuration across the ortho-disubstituted series. The structure of complex 5 was determined by X-ray crystallography and a distinctive arylthiolate ligation mode was found, in which the N(3)S ligand field was significantly distorted toward a sawhorse, compared to a more common trigonal pyramidal shape (e.g., 1). Moreover, the arylthiolate substituent rotated from a vertical orientation co-directional with the pyrazole rings and disposed between two of them in 1, to a horizontal orientation perpendicular to and over a single pyrazole ring in 5. This reorientation is necessary to accommodate the large ortho substituents of the latter complex. The divergent Ni-S coordination modes result in distinct (1)H NMR and electronic spectra that were rationalized by density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. These results demonstrate rich coordination chemistry for arylthiolates that can be elicited by steric manipulation at the periphery of pseudotetrahedral ligand fields.

Arylthiolate coordination and reactivity at pseudotetrahedral nickel(II) centers: modulation by noncovalent interactions.

Five new pseudotetrahedral nickel(II) arylthiolate complexes Tp (R,Me)Ni-SR' [(Tp (R,Me)) (-) = 2,2,2-kappa (3)-hydridotris(3-R,5-methylpyrazolyl)borate; R = Me, R' = C 6H 5 (Ph), 2,4,6-C 6H 2(CH 3) 3 (Mes); R = Ph, R' = C 6H 5 (Ph), 2,4,6-C 6H 2(CH 3) 3 (Mes), and 2,6-C 6H 3(CH 3) 2 (Xyl)] were prepared by metathesis reactions of known chloride complexes with sodium arylthiolate salts in THF. The new products were fully characterized. The effect of increasing bulk of substituents at the proximal 3-pyrazolyl and ortho-thiolate positions represented in this series was evident in spectroscopic studies (UV-vis-NIR, (1)H NMR) of the product complexes. Increased steric contact induced red-shifting of nickel-thiolate ligand to metal charge transfer (LMCT) bands and enhanced contact shifts of arylthiolate protons with the paramagnetic ( S = 1) nickel(II) ion. These spectroscopic effects arise from structural distortion of the nickel(II)-thiolate bond revealed by X-ray crystal structure determinations of the structural extremes of the series, Tp (Me,Me)Ni-SPh and Tp (Ph,Me)Ni-SXyl. The distortion consists of a significantly increased tilting of the Ni-S bond from an ideal trigonal axis and increased linearity of the Ni-S-R angle that alters covalency of the Ni-S coordinate bond. Reactivity of the nickel-thiolate linkage toward electrophilic alkylation with MeI is also significantly affected, showing enhanced rates according to two distinct competing mechanisms, direct bimolecular alkylation of intact complex and rate-limiting unimolecular dissociation of free thiolate. Possible biochemical relevance of these observations to tetrahedral nickel(II) centers in metalloenzymes is considered.