Jari Konu

New insights into the chemistry of imidodiphosphinates from investigations of tellurium-centered systems.

Dichalcogenido-imidodiphosphinates, [N(PR(2)E)(2)](-) (R = alkyl, aryl), are chelating ligands that readily form cyclic complexes with main group metals, transition metals, lanthanides, and actinides. Since their discovery in the early 1960s, researchers have studied the structural chemistry of the resulting metal complexes (where E = O, S, Se) extensively and identified a variety of potential applications, including as NMR shift reagents, luminescent complexes in photonic devices, or single-source precursors for metal sulfides or selenides. In 2002, a suitable synthesis of the tellurium analogs [N(PR(2)Te)(2)](-) was developed. In this Account, we describe comprehensive investigations of the chemistry of these tellurium-centered anions, and related mixed chalcogen systems, which have revealed unanticipated features of their fundamental structure and reactivity. An exhaustive examination of previously unrecognized redox behavior has uncovered a variety of novel dimeric arrangements of these ligands, as well as an extensive series of cyclic cations. In combination with calculations using density functional theory, these new structural frameworks have provided new insights into the nature of chalcogen-chalcogen bonding. Studies of metal complexes of the ditellurido ligands [N(PR(2)Te)(2)](-) have revealed unprecedented structural and reaction chemistry. The large tellurium donor sites confer greater flexibility, which can lead to unique structures in which the tellurium-centered ligand bridges two metal centers. The relatively weak P-Te bonds facilitate metal-insertion reactions (intramolecular oxidative-addition) to give new metal-tellurium ring systems for some group 11 and 13 metals. Metal tellurides have potential applications as low band gap semiconductor materials in solar cells, thermoelectric devices, and in telecommunications. Practically, some of these telluride ligands could be applied in these industries. For example, certain metal complexes of the isopropyl-substituted anion [N(P(i)Pr(2)Te)(2)](-) serve as suitable single-source precursors for pure metal telluride thin films or novel nanomaterials, for example, CdTe, PbTe, In(2)Te(3), and Sb(2)Te(3).

THE FUTURE OF MAIN GROUP CHEMISTRY

After a brief reflection on the important discoveries in main group chemistry during the past century, this article provides some suggestions concerning areas in which studies of s- and p-block element compounds are likely to have a major impact on the discipline of chemistry and upon society in general in the context of significant advances that have been made in the last decade. These areas range from fundamental to applied aspects of the subject. In the former category, novel aspects of chemical bonding and new reagents for synthetic applications are considered. From a more practical perspective alternative energy sources (especially hydrogen storage), new materials (notably nanomaterials and chemical sensors), catalysis (in the context of green chemistry), and medicinal chemistry (both diagnostic and therapeutic applications) are discussed.

Structural and Spectroscopic Studies of the PCP-Bridged Heavy Chalcogen-Centered Monoanions [HC(PPh2E)(PPh2)]− (E = Se, Te) and [HC(PR2E)2]− (E = Se, Te, R = Ph; E = Se, R = iPr): Homoleptic Group 12 Complexes and One-Electron Oxidation of [HC(PR2Se)2]−

Selenium- and tellurium-containing bis(diphenylphosphinoyl)methane monoanions were prepared by oxidation of the anion [HC(PPh(2))(2)](-) with elemental chalcogens. The selenium-containing isopropyl derivative was synthesized by generating [H(2)C(P(i)Pr(2))(2)] via a reaction between [H(2)C(PCl(2))(2)] and 4 equiv of (i)PrMgCl prior to in situ oxidation with selenium followed by deprotonation with LiN(i)Pr(2). The solid-state structures of the lithium salts of the monochalcogeno anions TMEDA.Li[HC(PPh(2)E)(PPh(2))] (E = Se (Li7a), E = Te (Li7b)) and the dichalcogeno anions TMEDA.Li[HC(PR(2)Se)(2)] (R = Ph (Li8a), (i)Pr (Li8c)) revealed five- and six-membered LiEPCP and LiSePCPSe rings, respectively. The homoleptic group 12 complexes {M[HC(PPh(2)Se)(2)](2)} (M = Zn (9a), Hg (9b)) were prepared from Li8a and MCl(2) and shown to have distorted-tetrahedral structures; the nonplanarity of the carbon center in the PC(H)P unit of the Zn complex 9a is attributed to crystal-packing effects. The complexes Li7a, Li7b, Li8a, TMEDA.Li[HC(PPh(2)Te)(2)] (Li8b), Li8c, 9a, and 9b were characterized in solution by multinuclear ((1)H, (7)Li, (13)C, (31)P, (77)Se, (125)Te, and (199)Hg) NMR spectroscopy. One-electron oxidation of Li8a and Li8c with iodine in a variety of organic solvents produced [H(2)C(PR(2)Se)(2)] (R = (i)Pr, Ph) as the final product, presumably owing to hydrogen abstraction from the solvent. DFT calculations revealed a significant contribution from the p orbital on carbon to the SOMO of the radicals [HC(PR(2)Se)(2)](*) (R = (i)Pr, Ph).

Novel carbon-centred reactivity of [(H)C(PPh2Se)2]− in the formation of structurally diverse Sn(IV), Te(IV) and Hg(II) complexes of the triseleno ligand [(Se)C(PPh2Se)2]2–

Metathetical reactions between TMEDA.Li[(H)C(PPh(2)Se)(2)] and MCl(2) (M = Hg, Sn, Te) in a 2 : 1 molar ratio afforded the homoleptic complexes, {M[(H)C(PPh(2)Se)(2)](2)}, as intermediates which undergo a surprising selenium/hydrogen exchange at the carbon centre to yield the dianionic triseleno ligand in {M(n)[(Se)C(PPh(2)Se)(2)](2)} (n = 1, M = Sn, Te; n = 2, M = Hg).

Synthesis and Redox Behaviour of the Chalcogenocarbonyl Dianions, [(E)C(PPh2S)2]2−: Formation and Structures of Chalcogen−Chalcogen Bonded Dimers and a Novel Selone

The lithium salts of the chalcogenocarbonyl dianions [(E)C(PPh(2)S)(2)](2-) (E=S (4 b), Se (4 c)) were produced through the reactions between Li(2)[C(PPh(2)S)(2)] and elemental chalcogens in the presence of tetramethylethylenediamine (TMEDA). The solid-state structure of {[Li(TMEDA)](2)[(Se)C(PPh(2)S)(2)]}-[{Li(TMEDA)}(2)4 c]-was shown to be bicyclic with the Li(+) cations bis-S,Se-chelated by the dianionic ligand. One-electron oxidation of the dianions 4 b and 4 c with iodine afforded the diamagnetic complexes {[Li(TMEDA)](2)[(SPh(2)P)(2)CEEC(PPh(2)S)(2)]} ([Li(TMEDA)](2)7 b (E=S), [Li(TMEDA)](2)7 c (E=Se)), which are formally dimers of the radical anions [(E)C(PPh(2)S)(2)](-) (.) (E=S (5 b), Se (5 c)) with elongated central E--E bonds. Two-electron oxidation of the selenium-containing dianion 4 c with I(2) yielded the LiI adduct of a neutral selone {[Li(TMEDA)][I(Se)C(PPh(2)S)(2)]}-[{LiI(TMEDA)}6 c]-whereas the analogous reaction with 4 b resulted in the formation of 7 b followed by protonation to give {[Li(TMEDA)][(SPh(2)P)(2)CSS(H)C(PPh(2)S)(2)]}-[Li(TMEDA)]8 b. Attempts to identify the transient radicals 5 b and 5 c by EPR spectroscopy in conjunction with DFT calculations of the electronic structures of these paramagnetic species and their dimers are also described. The crystal structures of [{Li(TMEDA)}(2)4 c], [{LiI(TMEDA)}6 c]⋅C(7)H(8), [Li(TMEDA)](2)7 b⋅(CH(2)Cl(2))(0.33), [Li(THF)(2)](2)7 b, [Li(TMEDA)](2)7 c, [Li(TMEDA)]8 b⋅(CH(2)Cl(2))(2) and [Li([12]crown-4)(2)]8 b were determined and salient structural features are discussed.

Identification of a novel η2-Se2 bonding mode in Cu(I) complexes of the dimeric selenocarbonyl dianions, [(EPh2P)2CSeSeC(PPh2E)2](2-) (E = S, Se).

A metathetical reaction between [Li(TMEDA)][(H)C(PPh2Se)2] and CuCl2 in a 2:1 molar ratio afforded the dimeric Cu(I) complex, {Cu2-η(2):η(2)-[(EPh2P)2CSeSeC(PPh2E)2]} (E = Se), via a selenium-proton exchange and an internal redox process. The analogous sulfur-containing complex (E = S) was obtained by the reactions of the dianions [(Se)C(PPh2S)2](2-) and [(SPh2P)2CSeSeC(PPh2S)2](2-) with Cu(II) and Cu(I) halides, respectively. Structural characterization of the Cu(I) complexes reveals a unique η(2)-Se2 bonding mode for the generic diselenide ligand system RSe-SeR.

Electrochemical and chemical reduction of disulfur dinitride: formation of [S4N4]-*, EPR spectroscopic characterization of the [S2N2H]* radical, and X-ray structure of [Na(15-crown-5)][S3N3].

Voltammetric studies of S(2)N(2) employing both cyclic voltammetry (CV) and rotating disk electrode (RDE) methods on GC electrodes at room temperature (RT) revealed two irreversible reduction processes at about -1.4 V and -2.2 V in CH(3)CN, CH(2)Cl(2), and tetrahydrofuran (vs ferrocene) and no observable oxidation processes up to the solvent limit when the scan is initially anodic. However, after cycling the potential through -1.4 V, two new couples appear near -0.3 V and -1.0 V due to [S(3)N(3)](-/0) and [S(4)N(4)](-/0) respectively. The diffusion coefficient D for S(2)N(2) was determined to be 9.13 x 10(-6) cm(2) s(-1) in CH(2)Cl(2) and 7.65 x 10(-6) cm(2) s(-1) in CH(3)CN. Digital modeling of CVs fits well to a mechanism in which [S(2)N(2)](-*) couples rapidly with S(2)N(2) to form [S(4)N(4)](-*), which then decomposes to [S(3)N(3)](-). In situ electron paramagnetic resonance (EPR) spectroelectrochemical studies of S(2)N(2) in both CH(2)Cl(2) and CH(3)CN resulted in the detection of strong EPR signals from [S(4)N(4)](-*) when electrolysis is conducted at -1.4 V; at more negative voltages, spectra from transient adsorbed radicals are observed. In moist solvent or with added HBF(4), a longer-lived spectrum is obtained due to the neutral radical [S(2)N(2)H](*), identified by simulation of the EPR spectrum and density functional theory (DFT) calculations. The chemical reduction of S(2)N(2) with Na[C(10)H(8)] or Na[Ph(2)CO] produces [Na(15-crown-5)][S(3)N(3)], while reduction with cobaltocene gives [Cp(2)Co][S(3)N(3)]. The X-ray structure of the former reveals a strong interaction (Na...N = 2.388(5) A) between the crown ether-encapsulated Na(+) cation and one of the nitrogen atoms of the essentially planar six-membered cyclic anion [S(3)N(3)](-).

Bond stretching and redox behavior in coinage metal complexes of the dichalcogenide dianions [(SPh2P)2CEEC(PPh2S)2]2- (E=S, Se): diradical character of the dinuclear copper(I) complex (E=S).

The metathetical reactions of a) [Li(tmeda)](2)[(S)C(PPh(2)S)(2)] (Li(2)·3c) with CuCl(2) and b) [Li(tmeda)](2)[(SPh(2)P)(2)CSSC(PPh(2)S)(2)] (Li(2)·4c) with two equivalents of CuCl both afford the binuclear Cu(I) complex {Cu(2)[(SPh(2)P)(2)CSSC(PPh(2)S)(2)]} (5c). The elongated (C)S-S(C) bond (ca. 2.54 and 2.72 Å) of the dianionic ligand observed in the solid-state structure of 5c indicate the presence of diradical character as supported by theoretical analyses. The treatment of [Li(tmeda)](2)[(SPh(2)P)(2)CSeSeC(PPh(2)S)(2)] (Li(2)·4b) and Li(2)·4c with AgOSO(2)CF(3) produce the analogous Ag(I) derivatives, {Ag(2)[(SPh(2)P)(2)CEEC(PPh(2)S)(2)]} (6b, E=Se; 6c, E=S), respectively. The diselenide complex 6b exhibits notably weaker Ag-Se(C) bonds than the corresponding contacts in the Cu(I) congeners, and the (31)P NMR data suggest a possible isomerization in solution. In contrast to the metathesis observed for Cu(I) and Ag(I) reagents, the reactions of Li(2)·4b and Li(2)·4c with Au(CO)Cl involve a redox process in which the dimeric dichalcogenide ligands are reduced to the corresponding monomeric dianions, [(E)C(PPh(2)S)(2)](2-) (3b, E=Se; 3c, E=S), and one of the gold centers is oxidized to generate the mixed-valent Au(I)/Au(III) complexes, {Au[(E)C(PPh(2)S)(2)]}(2) (7b, E=Se; 7c, E=S), with relatively strong aurophilic Au(I)···Au(III) interactions. The new compounds 5c, 6b,c and 7b,c are characterized in solution by NMR spectroscopy and in the solid state by X-ray crystallography (5c, 6b, 7b and 7c) and by Raman spectroscopy (5c and 6c). The UV-visible spectra of coinage metal complexes of the type 5, 6 and 7 are discussed in the light of results from theoretical analyses using time-dependent density functional theory.

Ligand-stabilized chalcogen dications.

Chalcogen-transfer reagents? The bonding in the dicationic rings C(2)N(2)E(2+) (see picture) differs from that in N-heterocyclic carbenes and their isovalent p-block analogues in accommodating a lone pair of electrons with pi symmetry, as well as sigma symmetry, on the chalcogen center. The labile electrophilic chalcogenium dications (E(2+)) are potentially versatile chalcogen-transfer reagents in reactions with a variety of inorganic and organic substrates.

In Search of the [PhB(μ-NtBu)2]2As• Radical: Experimental and Computational Investigations of the Redox Chemistry of Group 15 Bis-boraamidinates

DFT calculations for the group 15 radicals [PhB(mu-N(t)Bu)2]2M. (M = P, As, Sb, Bi) predict a pnictogen-centered SOMO with smaller contributions to the unpaired spin density arising from the nitrogen and boron atoms. The reactions of Li 2[PhB(mu-NR)2] (R = (t)Bu, Dipp) with PCl 3 afforded the unsolvated complex LiP[PhB(mu-N(t)Bu)2] 2 ( 1a) in low yield and ClP[PhB(mu-NDipp)2] (2), both of which were structurally characterized. Efforts to produce the arsenic-centered neutral radical, [PhB(mu-N (t) Bu) 2] 2As., via oxidation of LiAs[PhB(mu-N(t)Bu)2]2 with one-half equivalent of SO 2Cl 2, yielded the Zwitterionic compound [PhB(mu-N (t) Bu) 2As(mu-N(t)Bu)2B(Cl)Ph] (3) containing one four-coordinate boron center with a B-Cl bond. The reaction of 3 with GaCl3 produced the ion-separated salt, [PhB(mu-N(t)Bu)2] 2As (+)GaCl 4 (-) ( 4), which was characterized by X-ray crystallography. The reduction of 3 with sodium naphthalenide occurred by a two-electron process to give the corresponding anion [{PhB(mu-N(t)Bu)2} 2As] (-) as the sodium salt. Voltammetric investigations of 4 and LiAs[PhB(mu-N (t) Bu) 2] 2 ( 1b) revealed irreversible processes. Attempts to generate the neutral radical [PhB(mu-N(t)Bu)2] 2As. from these ionic complexes via in situ electrolysis did not produce an EPR-active species.