Metal complexes with varying intramolecular hydrogen bonding networks

Synthesis and Characterization of a Masked Terminal Nickel-Oxide Complex

In exploring terminal nickel-oxo complexes, postulated to be the active oxidant in natural and non-natural oxidation reactions, we report the synthesis of the pseudo-trigonal bipyramidal NiII complexes (K)[NiII (LPh )(DMF)] (1[DMF]) and (NMe4 )2 [NiII (LPh )(OAc)] (1[OAc]) (LPh =2,2',2''-nitrilo-tris-(N-phenylacetamide); DMF=N,N-dimethylformamide; - OAc=acetate). Both complexes were characterized using NMR, FTIR, ESI-MS, and X-ray crystallography, showing the LPh ligand to bind in a tetradentate fashion, together with an ancillary donor. The reaction of 1[OAc] with peroxyphenyl acetic acid (PPAA) resulted in the formation of [(LPh )NiIII -O-H⋅⋅⋅OAc]2- , 2, that displays many of the characteristics of a terminal Ni=O species. 2 was characterized by UV-Vis, EPR, and XAS spectroscopies and ESI-MS. 2 decayed to yield a NiII -phenolate complex 3 (through aromatic electrophilic substitution) that was characterized by NMR, FTIR, ESI-MS, and X-ray crystallography. 2 was capable of hydroxylation of hydrocarbons and epoxidation of olefins, as well as oxygen atom transfer oxidation of phosphines at exceptional rates. While the oxo-wall remains standing, this complex represents an excellent example of a masked metal-oxide that displays all of the properties expected of the ever elusive terminal M=O beyond the oxo-wall.

The unexpected reactivity of 9-iodo-nido-carborane: from nucleophilic substitution reactions to the synthesis of tricobalt tris(dicarbollide) Na[4,4',4''-(MeOCH2CH2O)3-3,3',3''-Co3(μ3-O)(μ3-S)(1,2-C2B9H10)3]

An unusual reactivity of 9-iodo-nido-carborane [9-I-7,8-C₂B₉H₁₁]^- towards nucleophiles under strong basic conditions was revealed. The nucleophilic substitution of iodine with O- and N-nucleophiles results in [9-RO-7,8-C₂B₉H₁₁]^- (R = H, CH₂CH₂OMe) and [9-L-7,8-C₂B₉H₁₁] (L = Py, NEt₃, Me₂NCH₂CH₂NMe₂), respectively. Reaction of [9-I-7,8-C₂B₉H₁₁]^- with CoCl₂ in 1,2-dimethoxyethane in the presence of t-BuOK, depending on the order of addition of the reagents, leads either to a diastereomeric mixture of diiodo derivatives cobalt bis(dicarbollide) rac-[4,4'-I₂-3,3'-Co(1,2-C₂B₉H₁₀)₂]^- and meso-[4,7'-I₂-3,3'-Co(1,2-C₂B₉H₁₀)₂]^- or to the corresponding mixture of 2-methoxyethoxy derivatives rac-[4,4'-(MeOCH₂CH₂O)₂-3,3'-Co(1,2-C₂B₉H₁₀)₂]^- and meso-[4,7'-(MeOCH₂CH₂O)₂-3,3'-Co(1,2-C₂B₉H₁₀)₂]^-. In the presence of accidental admixture of sodium thiosulfate, the reactions of 9-iodo-nido-carborane and 9-(2'-methoxyethoxy)-nido-carborane with CoCl₂ in 1,2-dimethoxyethane were found to produce additionally unprecedented tricobalt tris(dicarbollide) cluster Na[4,4',4''-(MeOCH₂CH₂O)₃-3,3',3''-Co₃(μ³-O)(μ³-S)(1,2-C₂B₉H₁₀)₃], the central fragment of which is a trigonal bipyramid with apical oxygen and sulfur atoms, and the base is formed by the Co₃ triangle flanked by three dicarbollide ligands. In addition, the 2-methoxyethoxy substituents of the dicarbollide ligands chelate the sodium cation in such a way that they form a helix whose rotation direction depends on the enantiomer of the parent ligand. Thus, in this case, induction of the helical chirality of the complex occurs due to the point chirality of the initial inorganic ligand. It is worth noting that in the case of symmetrically substituted 2-methoxyethoxy derivative of nido-carborane [10-MeOCH₂CH₂O-7,8-C₂B₉H₁₁]^- only formation of the corresponding cobalt bis(dicarbollide) complex [8,8'-(MeOCH₂CH₂O)₂-3,3'-Co(1,2-C₂B₉H₁₀)₂]^- was observed.

Manganese-Hydroxido Complexes Supported by a Urea/Phosphinic Amide Tripodal Ligand

Hydrogen bonds (H-bonds) within the secondary coordination sphere are often invoked as essential noncovalent interactions that lead to productive chemistry in metalloproteins. Incorporating these types of effects within synthetic systems has proven a challenge in molecular design that often requires the use of rigid organic scaffolds to support H-bond donors or acceptors. We describe the preparation and characterization of a new hybrid tripodal ligand ([H₂pout]^3-) that contains two monodeprotonated urea groups and one phosphinic amide. The urea groups serve as H-bond donors, while the phosphinic amide group serves as a single H-bond acceptor. The [H₂pout]^3- ligand was utilized to stabilize a series of Mn-hydroxido complexes in which the oxidation state of the metal center ranges from 2+ to 4+. The molecular structure of the Mn^III-OH complex demonstrates that three intramolecular H-bonds involving the hydroxido ligand are formed. Additional evidence for the formation of intramolecular H-bonds was provided by vibrational spectroscopy in which the energy of the O-H vibration supports its assignment as an H-bond donor. The stepwise oxidation of [Mn^IIH₂pout(OH)]^2- to its higher oxidized analogs was further substantiated by electrochemical measurements and results from electronic absorbance and electron paramagnetic resonance spectroscopies. Our findings illustrate the utility of controlling both the primary and secondary coordination spheres to achieve structurally similar Mn-OH complexes with varying oxidation states.

Thermodynamics of Proton and Electron Transfer in Tetranuclear Clusters with Mn-OH2/OH Motifs Relevant to H2O Activation by the Oxygen Evolving Complex in Photosystem II

We report the synthesis of site-differentiated heterometallic clusters with three Fe centers and a single Mn site that binds water and hydroxide in multiple cluster oxidation states. Deprotonation of Fe^III/II₃Mn^II-OH₂ clusters leads to internal reorganization resulting in formal oxidation at Mn to generate Fe^III/II₃Mn^III-OH. ⁵⁷Fe Mössbauer spectroscopy reveals that oxidation state changes (three for Fe^III/II₃Mn-OH₂ and four for Fe^III/II₃Mn-OH clusters) occur exclusively at the Fe centers; the Mn center is formally Mn^II when water is bound and Mn^III when hydroxide is bound. Experimentally determined p K_a (17.4) of the [Fe^III₂Fe^IIMn^II-OH₂] cluster and the reduction potentials of the [Fe₃Mn-OH₂] and [Fe₃Mn-OH] clusters were used to analyze the O-H bond dissociation enthalpies (BDE_O-H) for multiple cluster oxidation states. BDE_O-H increases from 69 to 78 and 85 kcal/mol for the [Fe^IIIFe^II₂Mn^II-OH₂], [Fe^III₂Fe^IIMn^II-OH₂], and [Fe^III₃Mn^II-OH₂] clusters, respectively. Further insight of the proton and electron transfer thermodynamics of the [Fe₃Mn-OH _x] system was obtained by constructing a potential-p K_a diagram; the shift in reduction potentials of the [Fe₃Mn-OH _x] clusters in the presence of different bases supports the BDE_O-H values reported for the [Fe₃Mn-OH₂] clusters. A lower limit of the p K_a for the hydroxide ligand of the [Fe₃Mn-OH] clusters was estimated for two oxidation states. These data suggest BDE_O-H values for the [Fe^III₂Fe^IIMn^III-OH] and [Fe^III₃Mn^III-OH] clusters are greater than 93 and 103 kcal/mol, which hints to the high reactivity expected of the resulting [Fe₃Mn═O] in this and related multinuclear systems.

Lessons from Nature: A Bio-Inspired Approach to Molecular Design

Metalloproteins contain actives sites with intricate structures that perform specific functions with high selectivity and efficiency. The complexity of these systems complicates the study of their function and the understanding of the properties that give rise to their reactivity. One approach that has contributed to the current level of understanding of their biological function is the study of synthetic constructs that mimic one or more aspects of the native metalloproteins. These systems allow individual contributions to the structure and function to be analyzed and also permit spectroscopic characterization of the metal cofactors without complications from the protein environment. This Current Topic is a review of synthetic constructs as probes for understanding the biological activation of small molecules. These topics are developed from the perspective of seminal molecular design breakthroughs from the past that provide the foundation for the systems used today.

Redox activity and π bonding in a tripodal seven-coordinate molybdenum(vi) tris(amidophenolate)

A tripodal seven-coordinate tris(amidophenolato)molybdenum(vi) complex shows strong ligand-to-metal π donation (40 kcal mol⁻¹ per π bond) and undergoes facile ligand-centered oxidation.

Thioether-ligated iron(II) and iron(III)-hydroperoxo/alkylperoxo complexes with an H-bond donor in the second coordination sphere

The non-heme iron complexes, [Fe(II)(N3PySR)(CH3CN)](BF4)2 () and [Fe(II)(N3Py(amide)SR)](BF4)2 (), afford rare examples of metastable Fe(iii)-OOH and Fe(iii)-OOtBu complexes containing equatorial thioether ligands and a single H-bond donor in the second coordination sphere. These peroxo complexes were characterized by a range of spectroscopic methods and density functional theory studies. The influence of a thioether ligand and of one H-bond donor on the stability and spectroscopic properties of these complexes was investigated.

Dramatically accelerated selective oxygen-atom transfer by a nonheme iron(IV)-oxo complex: tuning of the first and second coordination spheres

The new ligand N3Py^amideSR and its Fe^II complex [Fe^II(N3Py^amideSR)](BF₄)₂ (1) are described. Reaction of 1 with PhIO at −40 °C gives metastable [Fe^IV(O)(N3Py^amideSR)]²⁺ (2), containing a sulfide ligand and a single amide H-bond donor in proximity to the terminal oxo group. Direct evidence for H-bonding is seen in a structural analogue, [Fe^II(Cl)(N3Py^amideSR)](BF₄)₂ (3). Complex 2 exhibits rapid O-atom transfer (OAT) toward external sulfide substrates, but no intramolecular OAT. However, direct S-oxygenation does occur in the reaction of 1 with mCPBA, yielding sulfoxide-ligated [Fe^II(N3Py^amideS(O)R)](BF₄)₂ (4). Catalytic OAT with 1 was also observed.