A series of uranyl compounds with the redoxactive iminoquinone ligand have been synthesized, and their electronic structures elucidated using multinuclear NMR, EPR, electronic absorption spectroscopies, SQUID magnetometry, and X-ray crystallography. Characterization and analysis of the iminoquinone (iq 0 ) complex, ( dipp iq)UO 2 (OTf) 2 THF (1-iq), the iminosemiquinone (isq 1− ) complex, ( dipp isq) 2 UO 2 THF (2i s q ) , a n d t h e a m i d o p h e n o l a t e ( a p 2 − ) complex, [( dipp ap) 2 UO 2 THF][K(18-crown-6)(THF) 2 ] 2 (3-ap crown) show that reduction events are ligand-based, with the uranium center remaining in the hexavalent state. Reactivity of 2-isq with B-chlorocatecholborane or pivaloyl chloride leads to U−O uranyl bond scission and reduction of U(VI) to U(IV) concomitant with ligand oxidation along with organic byproducts. 18 O isotopic labeling experiments along with IR spectroscopy, mass spectrometry, and multinuclear NMR spectroscopy confirm that the organic byproducts contain oxygen atoms which originate from U−O uranyl bond activation.

A family of neodymium complexes featuring a redox-active ligand in three different oxidation states has been synthesized, including the iminoquinone (L ) derivative, ( iq) NdI (1-iq), the iminosemiquinone (L ) compound, ( isq) NdI(THF) (1-isq), and the amidophenolate (L ) [K(THF) ][( ap) Nd(THF) ] (1-ap) and [K(18-crown-6)][( ap) Nd(THF) ] (1-ap crown) species. Full spectroscopic and structural characterization of each derivative established the +3 neodymium oxidation state with redox chemistry occurring at the ligand rather than the neodymium center. Oxidation with elemental chalcogens showed the reversible nature of the ligand-mediated reduction process, forming the iminosemiquinone metallocycles, [K(18-crown-6)][( isq) Nd(S )] (2-isq crown) and [K(18-crown-6)(THF)][( isq) Nd(Se )] (3-isq crown), which are characterized to contain a 6-membered twist-boat ring.

The continued development of redox-active ligands requires an understanding as to how ligand modifications and related factors affect the locus of redox activity and spin density in metal complexes. Here we describe the synthesis, characterization, and electronic structure of nickel complexes containing triaryl NNNN (1) and SNNS (2) ligands derived from o-phenylenediamine. The tetradentate ligands in 1 and 2 were investigated and compared to those in metal complexes with compositionally similar ligands to determine how ligand-centered redox properties change when redox-active flanking groups are replaced with redox-innocent NMe 2 or SMe. A derivative of 2 in which the phenylene backbone was replaced with ethylene (3) was also prepared to interrogate the importance of o-phenylenediamine for ligand-centered redox activity. Cyclic voltammograms collected for 1 and 2 revealed two fully reversible ligand-centered redox events. Remarkably, several quasireversible ligand-centered redox waves were also observed for 3 despite the absence of the o-phenylenediamine subunit. Oxidizing 1 and 2 with silver salts containing different counteranions (BF 4 − , OTf − , NTf 2 − ) allowed the electrochemically generated complexes to be analyzed as a function of different oxidation states using single-crystal X-ray diffraction (XRD), EPR spectroscopy, and S K-edge X-ray absorption spectroscopy. The experimental data are corroborated by DFT calculations, and together, they reveal how the location of unpaired spin density and electronic structure in singly and doubly oxidized salts of 1 and 2 varies depending on the coordinating ability of the counteranions and exogenous ligands such as pyridine.

Treating a family of uranium benzyl compounds, Tp*2U­(CH2Ph) (1-Bn), Tp*2U­(CH2-para- i PrPh) (1- i Pr), Tp*2­U­(CH2-para- t BuPh) (1- t Bu), or Tp*2­U­(CH2-meta-OMePh) (1-OMe), which are supported by two hydro­tris­(3,5-di­methyl­pyra­zolyl)­borate (Tp*) ligands, with a single equivalent of triphenylphosphine oxide (OPPh3) causes a unique carbon–carbon coupling to occur. The products of this reaction, Tp*2U­[OP­(C6H5)2­(C6H5­CH2­C6H5)] (2-Ph), Tp*2U­[OP­(C6H5)2­(C6H5­CH2-p-iPr­C6H4)] (2- i Pr), Tp*2U­[OP­(C6H5)2­(C6H5­CH2-p-tBu­C6H4)] (2- t Bu), and Tp*2U­[OP­(C6H5)2­(C6H5­CH2-m-OCH3­C6H4)] (2-OMe), are characterized by coupling between the benzyl substituent and the para-carbon of one of the phenyl groups of OPPh3. To probe the scope of this unusual reactivity, 1-Bn was treated with different tris­(aryl)­phos­phine oxides, including tris­(p-tolyl)­phos­phine oxide, which yields Tp*2U­[OP­(p-tolyl)2­(C6H4­(CH3)­CH2­C6H5)] (3-tolyl). All compounds were characterized by multinuclear NMR, vibrational, and electronic absorption spectroscopies. When possible, X-ray diffraction was used to confirm molecular structures.

New uranyl derivatives featuring the amide ligand, -N(SiHMe) Bu, were synthesized and characterized by X-ray crystallography, multinuclear NMR spectroscopy, and absorption spectroscopies. Steric properties of these complexes were also quantified using the computational program Solid-G. The increased basicity of the free ligand -N(SiHMe) Bu was demonstrated by direct comparison to -N(SiMe), a popular supporting ligand for uranyl. Substitutional lability on a uranyl center was also demonstrated by exchange with the -N(SiMe) ligand. The increased basicity of this ligand and diverse characterization handles discussed here will make these compounds useful synthons for future reactivity.