Synthesis, Characterization, and Reduction of Thorium Pyridinediimine Complexes.

A family of thorium complexes featuring the redox-noninnocent pyridinediimine ligand MesPDIMe was synthesized, including (MesPDIMe)ThCl4 (1-Th), (MesPDIMe)ThCl3(THF) (2-Th), (MesPDIMe)ThCl2(THF)2 (3-Th) and [(MesPDIMe)Th(THF)]2 (5-Th) Full characterization of these species shows that these complexes feature MesPDIMe in four different oxidation states. The electronic structures of these complexes have been explored using 1H NMR and electronic absorption spectroscopies, X-ray crystallography, and SQUID magnetometry where appropriate.

Structural, Spectroscopic, and Computational Analysis of Heterometallic Thorium Phosphinidiide Complexes.

To synthesize complexes with thorium-phosphorus multiple-bond character, reactions of (C5Me5)2Th[P(H)Mes]2 with monovalent alkali-metal bases, MN(SiMe3)2, as well as CuMes, have been investigated. The results with MN(SiMe3)2 are phosphinidiide complexes of the form {(C5Me5)2Th[µ2-P(Mes)][µ2-P(H)Mes]M(L)n}2 (M = Na, n = 0; M = K, L = THF, n = 1; M = Rb, L = THF, n = 1; M = Cs, L = Et2O, n = 1). With CuMes, the product is a Th2Cu3P5 heterometallic structure, {(C5Me5)2Th[(µ2-P(H)Mes)P(Mes)]Cu}2Cu[µ2-P(H)Mes]. All complexes have been characterized using heteronuclear NMR and IR spectroscopy, density functional theory calculations, and their solid-state structure identified by X-ray crystallography. We also report the structure of {(C5Me5)2Th[(µ2-As(H)Mes)As(Mes)]Cu}2Cu[µ2-As(H)Mes] obtained from (C5Me5)2Th[As(H)Mes]2 with CuMes.

Backbonding in Thorium(IV) and Uranium(IV) Diarsenido Complexes with tBuNC and CO.

The coordination of tBuNC and CO with the diarsenido complexes (C5 Me5 )2 An(η2 -As2 Mes2 ), An=Th, U, has been investigated. For the first time, a comparison between isostructural complexes of ThIV and UIV has been possible with CO; density functional calculations indicated an appreciable amount of π backbonding that originates from charge transfer from an actinide-arsenic sigma bond. The calculated CO stretching frequencies in the ThIV and UIV diarsenido complexes are consistent with the experimental measurements, both show large shifts to lower frequency. We demonstrate that the π backbonding is crucial to explaining the red shifts of CO frequency upon AnIV complex formation. Interestingly, this interaction essentially correlates to the parallel orientation of π*(C-O) orbitals relative to the An-As bond.

Systematic Investigation of the Molecular and Electronic Structure of Thorium and Uranium Phosphorus and Arsenic Complexes.

In continuing to examine the interaction of actinide-ligand bonds with soft donor ligands, a comparative investigation with phosphorus and arsenic was conducted. A reaction of (C5Me5)2AnMe2, An = Th, U, with 2 equiv of H2AsMes, Mes = 2,4,6-Me3C6H2, forms the primary bis(arsenido) complexes, (C5Me5)2An[As(H)Mes]2. Both exhibit thermal instability at room temperature, leading to the elimination of H2, and the formation of the diarsenido species, (C5Me5)2An(η2-As2Mes2). The analogous diphosphido complexes, (C5Me5)2An(η2-P2Mes2), could not be synthesized via the same route, even upon heating the bis(phosphido) species to 100 °C in toluene. However, they were accessible via the reaction of dimesityldiphosphane, MesP(H)P(H)Mes, with (C5Me5)2AnMe2 at 70 °C in toluene. When (C5Me5)2AnMe2 is reacted with 1 equiv of H2AsMes, the bridging µ2-arsinidiide complexes [(C5Me5)2An]2(µ2-AsMes)2 are formed. Upon reaction of (C5Me5)2UMe2 with 1 equiv of H2PMes, the phosphinidiide [(C5Me5)2U(µ2-PMes)]2 is isolated. However, the analogous thorium reaction leads to a phosphido and C-H bond activation of the methyl on the mesityl group, forming {(C5Me5)2Th[P(H)(2,4-Me2C6H2-6-CH2)]}2. The reactivity of [(C5Me5)2An(µ2-EMes)]2 was investigated with OPPh3 in an effort to produce terminal phosphinidene or arsinidene complexes. For E = As, An = U, a U(III) cation-anion pair [(C5Me5)2U(η2-As2Mes2)][(C5Me5)2U(OPPh3)2] is isolated. The reaction of [(C5Me5)2Th(µ2-AsMes)]2 with OPPh3 does not result in a terminal arsinidene but, instead, eliminates PPh3 to yield a bridging arsinidiide/oxo complex, [(C5Me5)2Th]2(µ2-AsMes)(µ2-O). Finally, the combination of [(C5Me5)2U(µ2-PMes)]2 and OPPh3 yields a terminal phosphinidene, (C5Me5)2U(═PMes)(OPPh3), featuring a short U-P bond distance of 2.502(2) Å. Electrochemical measurements on the uranium pnictinidiide complexes demonstrate only a 0.04 V difference with phosphorus as a slightly better donor. Magnetic measurements on the uranium complexes show more excited-state mixing and therefore higher magnetic moments with the arsenic-containing compounds but no deviation from uncoupled U(IV) behavior. Finally, a quantum theory of atoms in molecules analysis shows highly polarized actinide-pnictogen bonds with similar bonding characteristics, supporting the electrochemical and magnetic measurements of similar bonding between actinide-phosphorus and actinide-arsenic bonds.

Functionalization of Carbon Monoxide and tert-Butyl Nitrile by Intramolecular Proton Transfer in a Bis(Phosphido) Thorium Complex.

We report intramolecular proton transfer reactions to functionalize carbon monoxide and tert-butyl nitrile from a bis(phosphido) thorium complex. The reaction of (C5 Me5 )2 Th[PH(Mes)]2 , Mes=2,4,6-Me3 C6 H2 , with 1 atm of CO yields (C5 Me5 )2 Th(κ2 -(O,O)-OCH2 PMes-C(O)PMes), in which one CO molecule is inserted into each thorium-phosphorus bond. Concomitant transfer of two protons, formerly coordinated to phosphorus, are now bound to one of the carbon atoms from one of the inserted CO molecules. DFT calculations were employed to determine the lowest energy pathway. With tert-butyl nitrile, t BuCN, only one nitrile inserts into a thorium-phosphorus bond, but the proton is transferred to nitrogen with one phosphido remaining unperturbed affording (C5 Me5 )2 Th[PH(Mes)][κ2 -(P,N)-N(H)C(CMe3 )P(Mes)]. Surprisingly, reaction of this compound with KN(SiMe3 )2 removes the proton bound to nitrogen, not phosphorus.

Influence of Substituents on the Electronic Structure of Mono- and Bis(phosphido) Thorium(IV) Complexes.

A series of metallocene thorium complexes with mono- and bis(phosphido) ligands have been investigated with varying hues: (C5Me5)2Th(Cl)[P(Mes)2] (Mes = mesityl = 2,4,6-(CH3)3C6H2; dark red-purple), (C5Me5)2Th[P(Mes)(CH3)]2 (dark red-purple), (C5Me5)2Th(CH3)[P(Mes)2] (dark red-purple), (C5Me5)2Th(CH3)[P(Mes)(SiMe3)] (orange), (C5Me5)2Th(Cl)[P(Mes)(SiMe3)] (orange), (C5Me5)2Th[P(Mes)(SiMe3)]2 (orange), and (C5Me5)2Th[PH(Mes)]2 (pale yellow). While all of these complexes bear a mesityl group on phosphorus, the electronic structure observed differs depending on the other substituent (mesityl, methyl, trimethylsilyl, or hydrogen). This sparked an investigation of the electronic structure of these complexes using 31P NMR and electronic absorption spectroscopy in concert with time-dependent density functional theory calculations.

Metal-Ligand Multiple Bonding in Thorium Phosphorus and Thorium Arsenic Complexes.

The complexes (C5 Me5 )2 Th(EHTipp)2 , (E=P or As; Tipp=2,4,6-triisopropylphenyl), provide a ligand framework that results in facile access to rare Th-E multiple bonds. The reaction of (C5 Me5 )2 Th(EHTipp)2 with KN(SiMe3 )2 , proceeds cleanly to the desired bridging phosphinidiide or arsinidiide complex, [{(C5 Me5 )2 Th(µ2 -ETipp)(µ2 -EHTipp)}K]2 under ambient conditions. In the absence of a chelating agent, the potassium cation of one monomeric unit interacts with the aryl ring of a second monomer to form a bridged dimer. In the presence of 2,2,2-cryptand, the terminal phosphinidene complex, [(C5 Me5 )2 Th=PTipp(PHTipp)][K(2,2,2-cryptand)] is isolated. Using X-ray crystallographic analysis, we have determined these complexes display the shortest Th-P and Th-As bond lengths reported.

Formation of Methane versus Benzene in the Reactions of (C5 Me5 )2 Th(CH3 )2 with [CH3 PPh3 ]X (X=Cl, Br, I) Yielding Thorium-Carbene or Thorium-Ylide Complexes.

The reaction of (C5 Me5 )2 Th(CH3 )2 with the phosphonium salts [CH3 PPh3 ]X (X=Cl, Br, I) was investigated. When X=Br and I, two equivalents of methane are liberated to afford (C5 Me5 )2 Th[CHPPh3 ]X, rare terminal phosphorano-stabilized carbenes with thorium. These complexes feature the shortest thorium-carbon bonds (≈2.30 Å) reported to date, and electronic structure calculations show some degree of multiple bonding. However, when X=Cl, only one equivalent of methane is lost with concomitant formation of benzene from an unstable phosphorus(V) intermediate, yielding (C5 Me5 )2 Th[κ2 -(C,C')-(CH2 )(CH2 )PPh2 ]Cl. Density functional theory (DFT) investigations of the reaction energy profiles for [CH3 PPh3 ]X, X=Cl and I showed that in the case of iodide, thermodynamics prevents the production of benzene and favors formation of the carbene.

Uranium(iii) and thorium(iv) alkyl complexes as potential starting materials.

The synthesis and characterisation of a rare U(iii) alkyl complex, U[η4-Me2NC(H)C6H5]3, using the dimethylbenzylamine (DMBA) ligand has been accomplished. While attempting to prepare the U(iv) compound, reduction to the U(iii) complex occurred. In the analogous Th(iv) system, C-H bond activation of a methyl group of one dimethylamine was observed yielding Th[η4-Me2NC(H)C6H5]2[η5-(CH2)MeNC(H)C6H5] with a dianionic DMBA ligand. The utility of these complexes as starting materials has been analyzed using a bulky dithiocarboxylate ligand to yield tetravalent actinide species.

Insertion of (t)BuNC into thorium-phosphorus and thorium-arsenic bonds: phosphaazaallene and arsaazaallene moieties in f element chemistry.

The reactivity of thorium-phosphido and thorium-arsenido bonds was probed using tert-butyl isocyanide, (t)BuNC. Reaction of (C5Me5)2Th[E(H)R]2, E = P, As; R = 2,4,6-(i)Pr3C6H2, 2,4,6-Me3C6H2 with (t)BuNC affords the first phosphaazaallene and arsaazaallene moieties with an f-element.

Dithio- and Diselenophosphinate Thorium(IV) and Uranium(IV) Complexes: Molecular and Electronic Structures, Spectroscopy, and Transmetalation Reactivity.

We report a comparison of the molecular and electronic structures of dithio- and diselenophosphinate, (E2PR2)(1-) (E = S, Se; R = (i)Pr, (t)Bu), with thorium(IV) and uranium(IV) complexes. For the thorium dithiophosphinate complexes, reaction of ThCl4(DME)2 with 4 equiv of KS2PR2 (R = (i)Pr, (t)Bu) produced the homoleptic complexes, Th(S2P(i)Pr2)4 (1S-Th-(i)Pr) and Th(S2P(t)Bu2)4 (2S-Th-(t)Bu). The diselenophosphinate complexes were synthesized in a similar manner using KSe2PR2 to produce Th(Se2P(i)Pr2)4 (1Se-Th-(i)Pr) and Th(Se2P(t)Bu2)4 (2Se-Th-(t)Bu). U(S2P(i)Pr2)4, 1S-U-(i)Pr, could be made directly from UCl4 and 4 equiv of KS2P(i)Pr2. With (Se2P(i)Pr2)(1-), using UCl4 and 3 or 4 equiv of KSe2P(i)Pr2 yielded the monochloride product U(Se2P(i)Pr2)3Cl (3Se-U(iPr)-Cl), but using UI4(1,4-dioxane)2 produced the homoleptic U(Se2P(i)Pr2)4 (1Se-U-(i)Pr). Similarly, the reaction of UCl4 with 4 equiv of KS2P(t)Bu2 yielded U(S2P(t)Bu2)4 (2S-U-(t)Bu), whereas the reaction with KSe2P(t)Bu2 resulted in the formation of U(Se2P(t)Bu2)3Cl (4Se-U(tBu)-Cl). Using UI4(1,4-dioxane)2 and 4 equiv of KSe2P(t)Bu2 with UCl4 in acetonitrile yielded U(Se2P(t)Bu2)4 (2Se-U-(t)Bu). Transmetalation reactions were investigated with complex 2Se-U-(t)Bu and various CuX (X = Br, I) salts to yield U(Se2P(t)Bu2)3X (6Se-U(tBu)-Br and 7Se-U(tBu)-I) and 0.25 equiv of [Cu(Se2P(t)Bu2)]4 (8Se-Cu-(t)Bu). Additionally, 2Se-U-(t)Bu underwent transmetalation reactions with Hg2F2 and ZnCl2 to yield U(Se2P(t)Bu2)3F (6) and U(Se2P(t)Bu2)3Cl (4Se-U(tBu)-Cl), respectively. The molecular structures were analyzed using (1)H, (13)C, (31)P, and (77)Se NMR and IR spectroscopy and structurally characterized using X-ray crystallography. Using the QTAIM approach, the electronic structure of all homoleptic complexes was probed, showing slightly more covalent bonding character in actinide-selenium bonds over actinide-sulfur bonds.

Formation of a Bridging Phosphinidene Thorium Complex.

The synthesis and structural determination of the first thorium phosphinidene complex are reported. The reaction of 2 equiv of (C5Me5)2Th(CH3)2 with H2P(2,4,6-(i)Pr3C6H2) at 95 °C produces [(C5Me5)2Th]2(µ2-P[(2,6-CH2CHCH3)2-4-(i)PrC6H2] as well as 4 equiv of methane, 2 equiv from deprotonation of the phosphine and 2 equiv from C-H bond activation of one methyl group of each of the isopropyl groups at the 2- and 6-positions. Transition state calculations indicate that the steps in the mechanism are P-H, C-H, C-H, and then P-H bond activation to form the phosphinidene.

Systematic investigation of thorium(IV)- and uranium(IV)-ligand bonding in dithiophosphonate, thioselenophosphinate, and diselenophosphonate complexes.

Homoleptic soft-donor actinide complexes of the general form An[E2PROR']4 were synthesized from salt metathesis between ThCl4(DME)2 or UI4(1,4-dioxane)2 and M[E2PROR'], M = Na, K, to yield 2 (An = Th, E = S, R = 4-MeOC6H4, R' = Me), 3 (An = Th, E = S, R = 4-MeOC6H4, R' = (t)Bu), 4 (An = U, E = S, R = 4-MeOC6H4, R' = Me), 5 (An = Th, E = Se, R = C6H5, R' = Me), and 6 (An = U, E = Se, R = C6H5, R' = Me). In addition thorium and uranium thioselenophosphinate complexes 7 and 8 were produced from the reaction of ThCl4(DME)2 and UI4(1,4-dioxane)2 and Na[SSePPh2], respectively. All compounds were characterized using elemental analysis, (1)H and (31)P NMR, and IR spectroscopy, and the U(IV) compounds were also examined with UV-vis spectroscopy. The (77)Se NMR spectrum of 5 reveals the first reported resonance with a Th-Se bond. The solid-state structures of 2, 5, 7, and 8 were determined by X-ray crystallography. The actinide-ligand bonding was examined using density functional theory calculations in conjunction with quantum theory of atoms-in-molecules analysis and shows slightly increased covalency in actinide-selenium bonds than actinide-sulfur.