James R. Gardinier

Chemical switching behaviour of tricarbonylrhenium(I) complexes of a new redox active 'pincer' ligand.

The structures and optoelectronic properties of tricarbonylrhenium(I) complexes of di(2-pyrazolyl-p-tolyl)amine in its neutral and deprotonated (uninegative amido) form were investigated. Reactions of the complexes with Brønsted acids or bases result in distinctive changes of colour and electrochemical activity owing to the non-innocent nature of the ligand.

Multitopic third generation tris(pyrazolyl)methane ligands built on alkyne structural scaffolding: first preparation of mixed tris(pyrazolyl)methane/tris(pyrazolyl)borate ligands

A series of new multitopic ligands with rigid linear geometry are formed by joining tris(pyrazolyl)methane and tris(pyrazolyl)borate units with arenyl and alkynyl linkers using Sonogashira and related alkynyl coupling reactions. These ligands are new examples of “third generation” poly(pyrazolyl)borate and poly(pyrazolyl)methane ligands, ligands functionalized at the non-coordinating “back” positions of either the boron or central carbon atoms. The reaction of Na[OCH2C(pz)3] with propargyl bromide yields HC2CH2OCH2C(pz)3 (2) and homocoupling of this alkyne yields [-C2CH2OCH2C(pz)3]2. The reaction of Na[OCH2C(pz)3] with 3,5-(BrCH2)2C6H3I yields 3,5-[(pz)3CCH2OCH2]2C6H3I (4), which can be converted to 3,5-[(pz)3CCH2OCH2]2C6H3(C2H) (6) by reaction with HC2SiMe3 followed by removal of the SiMe3 group. Compounds 4 and 6 can be combined to form {3,3′,5,5′-[(pz)3CCH2OCH2]4(1,1′-C6H3C2C6H3) (7) and 6 homocoupled to form {3,5-[(pz)3CCH2OCH2]2C6H3C2-}2. Compound 6 reacts with p-I2C6H4 to produce 3,3′,5,5′-[(pz)3CCH2OCH2]4[p-(1,1′-C6H3C2)2C6H4], which can also be formed by the reaction of 4 with bis(ethynyl)benzene. The reaction of 2 with Fe[(p-IC6H4)B(pz)3]2 yields the bitopic, metalloligand Fe[(pz)3CCH2OCH2-C2-C6H4B(κ3-N,N′,N″-pz)3]2 (10) and a similar reaction with 6 yields the tetratopic metalloligand Fe[{3,5-[(pz)3CCH2OCH2]2C6H3C2}C6H4B(κ3-N,N′,N″-pz)3]2. The molecular structures of 2, 4, 7, and 10·4CH2Cl2 are reported and their supramolecular structures, organized by a series of CH⋯I and CH–π interactions, are detailed.

Using sterics to promote reactivity in fac-Re(CO)3 complexes of some ‘non-innocent’ NNN-pincer ligands

Two new redox active ligands based on di(2-(3-organopyrazolyl)-p-tolyl)amine have been prepared in order to investigate potential effects of steric bulk on the structures, electronic properties, or reactivity of tricarbonylrhenium(I) complexes. Replacing the hydrogens at the 3-pyrazolyl positions with alkyl groups causes significant distortion to the ligand framework due to potential interactions between these groups when bound to a fac-Re(CO)(3) moiety. The distortions effectively increase the nucleophilic character of the central amino nitrogen and ligand-centered reactivity of the metal complexes.

Proton-coupled electron transfer at a [Co-OHx]z unit in aqueous media: evidence for a concerted mechanism

The proton-coupled electron transfer (PCET) chemistry associated with the [Co-OH]2+/[Co-OH2]2+ redox couple for [Co(PY5)(OH2)]2+ (1; PY5 = 2,6-(bis(bis-2-pyridyl)-methoxymethane)-pyridine) and [CoII(pz4depy)(OH2)]2+ (2; pz4depy = 2,6-bis(1,1-di(1H-pyrazol-1-yl)ethyl)pyridine) is reported. It is found that the couple is acutely sensitive to the geometry of the axially ligated group in addition to the electronic-donating/-withdrawing character of the ligand. Interrogation of the electron-transfer kinetics by electrochemical methods also shows for the first time that the interconversion of [CoIII-OH]2+ and [CoII-OH2]2+ proceeds through a concerted pathway in favour of energetically unfavourable stepwise electron-transfer or proton-transfer reaction steps.

A ‘metallic tape’ stabilized by an unprecedented (μ5-κ2,κ2,κ2,κ1,κ1-) scorpionate binding mode

The Janus scorpionate ligand, tris(mercaptothiadiazolyl)borate, exhibits extraordinary coordination capacity and versatility, binding from two up to five metal cations as demonstrated by its thallium(i) salt, a compound that serves as a model for metal-surface binding.

Homoleptic nickel(II) complexes of redox-tunable pincer-type ligands

Different synthetic methods have been developed to prepare eight new redox-active pincer-type ligands, H(X,Y), that have pyrazol-1-yl flanking donors attached to an ortho-position of each ring of a diarylamine anchor and that have different groups, X and Y, at the para-aryl positions. Together with four previously known H(X,Y) ligands, a series of 12 Ni(X,Y)2 complexes were prepared in high yields by a simple one-pot reaction. Six of the 12 derivatives were characterized by single-crystal X-ray diffraction, which showed tetragonally distorted hexacoordinate nickel(II) centers. The nickel(II) complexes exhibit two quasi-reversible one-electron oxidation waves in their cyclic voltammograms, with half-wave potentials that varied over a remarkable 700 mV range with the average of the Hammett σ(p) parameters of the para-aryl X, Y groups. The one- and two-electron oxidized derivatives [Ni(Me,Me)2](BF4)n (n = 1, 2) were prepared synthetically, were characterized by X-band EPR, electronic spectroscopy, and single-crystal X-ray diffraction (for n = 2), and were studied computationally by DFT methods. The dioxidized complex, [Ni(Me,Me)2](BF4)2, is an S = 2 species, with nickel(II) bound to two ligand radicals. The mono-oxidized complex [Ni(Me,Me)2](BF4), prepared by comproportionation, is best described as nickel(II) with one ligand centered radical. Neither the mono- nor the dioxidized derivative shows any substantial electronic coupling between the metal and their bound ligand radicals because of the orthogonal nature of their magnetic orbitals. On the other hand, weak electronic communication occurs between ligands in the mono-oxidized complex as evident from the intervalence charge transfer (IVCT) transition found in the near-IR absorption spectrum. Band shape analysis of the IVCT transition allowed comparisons of the strength of the electronic interaction with that in the related, previously known, Robin-Day class II mixed valence complex, [Ga(Me,Me)2](2+).

The 4-(dipyridylamino)benzoylpyridine ligand as a supramolecular synthon. Solid state organization of a bis(bipyridyl)ruthenium(II) complex

Abstract The bis(2,2′-bipyridyl)ruthenium(II) derivative [Ru(bpy)2(p-py2N(C6H4)C(O)py)][PF6]2 was prepared by reaction between ‘[Ru(bpy)2(EtOH)2][PF6]2’ and an excess of p-py2N(C6H4)C(O)(py). X-ray analysis shows that the ruthenium is coordinated via the N,O ligand set rather than the via the N,N′-dipyridylamino fragment. The cations are arranged in dimeric units as a result of (C)H⋯N weak hydrogen bonding interactions. The dimeric units are further organized into a supramolecular chain structure of cations by π–π stacking interactions involving bipyridyl ligands and by weak CH⋯O interactions. The complicated 1H and 13C NMR spectra were completely assigned by using two-dimensional methods. The optical spectra in CH3CN show two high-energy ligand-based absorption maxima that show a small bathochromic shift when compared to the spectral data of the free arylpyridylmethanone ligand and two long-wavelength absorption maxima centered near 440 and 574 nm that are assigned to Ru(dπ)–π*(py) and Ru(dπ)–π*[ArC(O)(py)] MLCT transitions, respectively. The cyclic voltammogram shows an irreversible oxidation wave for the Ru(II)/(III) couple at +1.31 V versus SCE.

Electronic Communication Across Diamagnetic Metal Bridges: A Homoleptic Gallium(III) Complex of a Redox-Active Diarylamido-Based Ligand and Its Oxidized Derivatives

Complexes with cations of the type [Ga(L)(2)](n+) where L = bis(4-methyl-2-(1H-pyrazol-1-yl)phenyl)amido and n = 1, 2, 3 have been prepared and structurally characterized. The electronic properties of each were probed by electrochemical and spectroscopic means and were interpreted with the aid of density functional theory (DFT) calculations. The dication, best described as [Ga(L(-))(L(0))](2+), is a Robin-Day class II mixed-valence species. As such, a broad, weak, solvent-dependent intervalence charge transfer (IVCT) band was found in the NIR spectrum in the range 6390-6925 cm(-1), depending on the solvent. Band shape analyses and the use of Hush and Marcus relations revealed a modest electronic coupling, H(ab) of about 200 cm(-1), and a large rate constant for electron transfer, k(et), on the order of 10(10) s(-1) between redox active ligands. The dioxidized complex [Ga(L(0))(2)](3+) shows a half-field ΔM(s) = 2 transition in its solid-state X-band electron paramagnetic resonance (EPR) spectrum at 5 K, which indicates that the triplet state is thermally populated. DFT calculations (M06/Def2-SV(P)) suggest that the singlet state is 21.7 cm(-1) lower in energy than the triplet state.

Comment on “Synthesis, Characterization, and Structures of Persistent Aniline Radical Cation”: It Is a Protonated Aniline and Not an Aniline Radical Cation

The same, but different: The reaction of tri-tert-butylaniline (TBA) with AgSbF6 in CH2 Cl2 produces a green-colored intermediate which undergoes decomposition to form a protonated aniline (TBAH(+) SbF6 (-) ). Crystals of the protonated aniline salt were analyzed by X-ray diffraction and found to have the same crystal characteristics as the crystals of the supposed cation radical first identified in 2012.

Reaction Chemistry of Silver(I) Trifluoromethanesulfonate Complexes of Nitrogen-Confused C-Scorpionates

Two new C-scorpionate ligands with a bis(3,5-dimethylpyrazol-1-yl)methyl group bound to the 3 position of either an N-tosyl (TsL*) or an N-H pyrazole (HL*) ring have been prepared. The silver(I) complexes of these new ligands and the two previously reported analogous ligands with unsubstituted bis(pyrazol-1-yl)methyl groups (TsL and HL) in both 1:1 and 2:1 ligand/metal ratios were investigated to explore the effects of ligand sterics on their physical and chemical properties. The structurally characterized derivatives of the type [Ag(L)2](OTf) are four-coordinate, where the confused pyrazolyl is not bound to the metal. On the other hand, three 1:1 complexes [Ag(L)](OTf) had all pyrazolyls bound, while the μ-κ1,κ1-TsL derivative had an unbound confused pyrazolyl. The molecularity of the latter four ranged from polymeric to dimeric to monomeric in the solid with increasing steric bulk of the ligand. The utility of these complexes in stoichiometric ligand-transfer reactions and in styrene aziridination was demonstrated. Thus, tricarbonylmanganese(I) complexes were prepared as kinetically inert models for comparative solution diffusion NMR studies. Also, [Fe(HL)2](OTf)2 was prepared for similar reasons and to compare the effects of anion on spin-crossover properties.