John S. Anderson

Conversion of Fe-NH2 to Fe-N2 with release of NH3

Tris(phosphine)borane ligated Fe(I) centers featuring N(2)H(4), NH(3), NH(2), and OH ligands are described. Conversion of Fe-NH(2) to Fe-NH(3)(+) by the addition of acid, and subsequent reductive release of NH(3) to generate Fe-N(2), is demonstrated. This sequence models the final steps of proposed Fe-mediated nitrogen fixation pathways. The five-coordinate trigonal bipyramidal complexes described are unusual in that they adopt S = 3/2 ground states and are prepared from a four-coordinate, S = 3/2 trigonal pyramidal precursor.

Synthesis and Characterization of Three-Coordinate Ni(III)-Imide Complexes

A new family of low-coordinate nickel imides supported by 1,2-bis(di-tert-butylphosphino)ethane was synthesized. Oxidation of nickel(II) complexes led to the formation of both aryl- and alkyl-substituted nickel(III)-imides, and examples of both types have been isolated and fully characterized. The aryl substituent that proved most useful in stabilizing the Ni(III)-imide moiety was the bulky 2,6-dimesitylphenyl. The two Ni(III)-imide compounds showed different variable-temperature magnetic properties but analogous EPR spectra at low temperatures. To account for this discrepancy, a low-spin/high-spin equilibrium was proposed to take place for the alkyl-substituted Ni(III)-imide complex. This proposal was supported by DFT calculations. DFT calculations also indicated that the unpaired electron is mostly localized on the imide nitrogen for the Ni(III) complexes. The results of reactions carried out in the presence of hydrogen donors supported the findings from DFT calculations that the adamantyl substituent was a significantly more reactive hydrogen-atom abstractor. Interestingly, the steric properties of the 2,6-dimesitylphenyl substituent are important not only in protecting the Ni═N core but also in favoring one rotamer of the resulting Ni(III)-imide, by locking the phenyl ring in a perpendicular orientation with respect to the NiPP plane.

Reactions of CO2 and CS2 with 1,2-Bis(di-tert-butylphosphino)ethane Complexes of Nickel(0) and Nickel(I)

Reaction of CS(2) with [(dtbpe)Ni](2)(η(2),μ-C(6)H(6)) (1; dtbpe =1,2-bis(di-tert-butylphosphino)ethane) in toluene gives the carbon disulfide complex (dtbpe)Ni(η(2)-CS(2)) (2), characterized by standard spectroscopic methods and X-ray crystallography. Reaction of CS(2) with the Ni(I) complex (dtbpe)Ni(OSO(2)CF(3)) gives the diamagnetic, trimetallic cluster [{(dtbpe)Ni(κ(1),η(2)-CS(2))}(2)(dtbpe)Ni][SO(3)CF(3)](2) (3-OTf). The solid-state structure of 3-OTf reveals that the two CS(2) ligands bind η(2) to two (dtbpe)Ni centers and κ(1) to the third, unique (dtbpe)Ni in the complex dication, and NMR spectroscopic data indicate that this structure is maintained in solution. Oxidation of 2 by ferrocenium hexafluorophosphate affords the identical trimetallic complex dication as the PF(6)(-) salt, [{(dtbpe)Ni(κ(1),η(2)-CS(2))}(2)(dtbpe)Ni][PF(6)](2) (3-PF(6)). These results are consistent with the intermediacy of a Ni(I)-CS(2) complex, [(dtbpe)Ni(CS(2))(+)], that is unstable with respect to disproportionation. Reaction of 1 with one equivalent of CO(2) provides the carbon dioxide adduct (dtbpe)Ni(η(2)-CO(2)) (4), that was also crystallographically characterized. Thermolysis of 4 in benzene solution at 80 °C results in reduction of the CO(2) ligand to CO, trapped as (dtbpe)Ni(CO)(2), and partial oxidation of a dtbpe ligand to give O═P(tert-Bu)(2)CH(2)CH(2)P(tert-Bu)(2).

Development of Picosecond-Resolution Large-Area Time-of-Flight Systems

The measurement of time-of-flight (TOF) of relativistic particles in high-energy colliders with picosecond resolution would qualitatively change the ability to identify underlying parton-level processes at future colliders or upgrades of existing detectors. We have measured the timing properties of three micro-channel plate photo-multiplier tubes (MCPPMTs) from Photonis; one with 1024 anodes and the other two with 64 anodes. The 1024-anode 10-micron pore tube uses a charge-collection scheme at the anode to provide equal arrival time of the signal independent of the hit position of incident light on the face of the tube. The two 64-anode 25-micron pore tubes have a commercially available collection scheme and were used to find a limit on the timing resolution. We have performed these measurements using a newly assembled test-stand based on a Hamamatsu PLP-10 picosecond laser and a commercial CAMAC readout electronics system. We present these results and compare timing properties to earlier versions of the charge collection scheme.

CCDC 1848263: Experimental Crystal Structure Determination

Related Article: McKenna K. Goetz, Ethan A. Hill, Alexander S. Filatov, John S. Anderson|2018|J.Am.Chem.Soc.|140||doi:10.1021/jacs.8b07399

Isolation of a Terminal Co(III)-Oxo Complex

Late transition metal oxo complexes with high d-electron counts have been implicated as intermediates in a wide variety of important catalytic reactions; however, their reactive nature has often significantly limited their study. While some examples of these species have been isolated and characterized, complexes with d-electron counts >4 are exceedingly rare. Here we report that use of a strongly donating tris(imidazol-2-ylidene)borate scaffold enables the isolation of two highly unusual CoIII-oxo complexes which have been thoroughly characterized by a suite of physical techniques including single crystal X-ray diffraction. These complexes display O atom and H atom transfer reactivity and demonstrate that terminal metal oxo complexes with six d-electrons can display strong metal-oxygen bonding and sufficient stability to enable their characterization. The unambiguous assignment of these complexes supports the viability of related species that are frequently invoked, but rarely observed, in the types of catalytic reactions mentioned above. The studies described here change our understanding of the reactivity and bonding in late transition metal oxo complexes and open the door to further study of the properties of this class of elusive and important intermediates.

Redox Activity, Ligand Protonation, and Variable Coordination Modes of Diimino-Pyrrole Complexes of Palladium

Ligand-based functionality is a prominent method of increasing the reactivity or stability of metal centers in coordination chemistry. Some of the most successful catalysts use ligand-based redox activity, pendant protons, or hemilability in order to specifically accelerate catalysis. Here we report the diimino-pyrrole ligand Tol,CyDIPyH (Tol,CyDIPy = 2,5-bis( N-cyclohexyl-1-( p-tolyl)methanimine)pyrrolide), which exhibits all three of these ligand properties. Metalation of Tol,CyDIPy to Pd gives the pseudo-square planar complex (Tol,CyDIPy)PdCl, which upon reduction forms a mixture of products, including a Pd(I)-Pd(I) dimer wherein Tol,CyDIPy bridges the dimeric unit. Upon addition of PMe3, the imine arms of (Tol,CyDIPy)PdCl are displaced to yield (Tol,CyDIPy)Pd(PMe3)2Cl, where the Tol,CyDIPy ligand binds in a monodentate fashion. This complex can be reduced to generate a ligand-based radical, as shown by EPR spectroscopy. Finally, (Tol,CyDIPy)PdCl also can be protonated at the imine arm, exhibiting a total of three different coordination modes across this series of complexes. Taken together, these studies show that Tol,CyDIPy exhibits notable flexibility in its coordination and redox chemistry.

Ligand-Based Storage of Protons and Electrons in Dihydrazonopyrrole Complexes of Nickel

A newly developed dihydrazonopyrrole ligand and corresponding Ni complexes have been synthesized and thoroughly characterized. Electrochemical studies and chemical reactivity tests show that these complexes can reversibly store both electrons and protons, or equivalently H-atoms, via ligand-based events. The stored H-atom equivalent can be transferred to small molecules such as acetonitrile or oxygen. Furthermore, this series of complexes can adopt a variety of different coordination modes. In addition to one e- reactivity, the two e- electrophilic oxidation of phosphines is also demonstrated. Taken together, these results show that dihydrazonopyrrole complexes represent a geometrically and electronically flexible scaffold for controlling the flow of both electrons and protons.

Isolable iodosylarene and iodoxyarene adducts of Co and their O-atom transfer and C–H activation reactivity

We report an unusual series of discrete iodosyl- and iodoxyarene adducts of Co(ii) including detailed studies of their O-transfer reactivity and mechanism.

ULTRA-FAST TIMING AND THE APPLICATION OF HIGH ENERGY PHYSICS TECHNOLOGIES TO BIOMEDICAL IMAGING

We propose to apply the ultra fast Time Of Flight technique (TOR developed for High Energy Physics (HEP) particle detectors to biomedical imaging. The similarity of the problem in the two fields as well as the remarkable opportunities in biomedical imaging to use technologies developed in HEP have the potential to make major advances in the medical world, in particular for Positron Emission Tomography (PET). We will describe and present some preliminary results of the development of a new complete read-out chain able to manage signals from various types of modem photo detectors (MCP, APD, SiPM). This innovative architecture is made of a fast front-end electronics ASIC with novel Digital Signal Processing (DSP) concepts able to reach the Pico-second timing resolution, a time-to-digital converter, a pipelined digital readout and an integrated