CO reductive oligomerization by a divalent thulium complex and CO2-induced functionalization.

The divalent thulium complex [Tm(Cpttt)2] (Cpttt = 1,2,4-tris(tert-butyl)cyclopentadienyl) reacts with CO to afford selective CO reductive dimerization and trimerization into ethynediolate (C2) and ketenecarboxylate (C3) complexes, respectively. DFT calculations were performed to shed light on the elementary steps of CO homologation and support a stepwise chain growth. The attempted decoordination of the ethynediolate fragment by treatment with Me3SiI led to dimerization and rearrangement into a 3,4-dihydroxyfuran-2-one complex. Investigation of the reactivity of the C2 and C3 complexes towards other electrophiles led to unusual functionalization reactions: while the reaction of the ketenecarboxylate C3 complex with electrophiles yielded new multicarbon oxygenated complexes, the addition of CO2 to the ethynediolate C2 complex resulted in the formation of a very reactive intermediate, allowing C-H activation of aromatic solvents. This original intermolecular reactivity corresponds to an unprecedented functionalization of CO-derived ligands, which is induced by CO2.

Does Reduction-Induced Isomerization of a Uranium(III) Aryl Complex Proceed via C-H Oxidative Addition and Reductive Elimination across the Uranium(II/IV) Redox Couple?

Reaction of the uranium(III) bis(amidinate) aryl complex {TerphC(NiPr)2}2U(Terph) (2, where Terph = 4,4″-di-tert-butyl-m-terphenyl-2'-yl) with a strong reductant enabled isolation of isomeric uranium(III) bis(amidinate) aryl product {TerphC(NiPr)2}2U(Terph*) (3, where Terph* = 4,4″-di-tert-butyl-m-terphenyl-4'-yl). In terms of connectivity, 3 differs from 2 only in the positions of the U-C and C-H bonds on the central aryl ring of the m-terphenyl-based ligand. A deuterium labeling study ruled out mechanisms for this isomerization involving intermolecular abstraction or deprotonation of the ligand C-H bonds activated during the reaction. Due to the complexity of this rapid, heterogeneous reaction, experimental studies could not further distinguish between two different intramolecular C-H activation mechanisms. However, high-level computational studies were consistent with a mechanism that included two sets of unimolecular, mononuclear C-H oxidative addition and reductive elimination steps involving uranium(II/IV).

A Combined Experimental and Theoretical Investigation of Arene-Supported Actinide and Ytterbium Tetraphenolate Complexes.

Modular tetraphenolate ligands tethered with a protective arene platform (para-phenyl or para-terphenyl) are used to support mononuclear An(IV) (An = Th, U) complexes with an exceptionally large and open axial coordination site at the metal. The base-free complexes and a series of neutral donor adducts were synthesized and characterized by spectroscopies and single-crystal X-ray diffraction. Anionic Th(IV) -ate complexes with an additional axial aryloxide ligand were also synthesized and characterized. The para-phenyl-tethered mononuclear complexes exhibit rare An(IV)-arene interactions, and the An(IV)-arene distance broadly increases with axial donor strength. The para-terphenyl-tethered complexes have almost no interaction with the arene base, isolating the central metal cation. Computational analysis of the mononuclear complexes and their reduced analogues, and Yb(III) congeners, as well as the effect of additional donor ligand binding, seek to elucidate the electronic structure of the metal-arene interactions and establish whether they, or their reduced or oxidized counterparts, could function as molecular qubits.

Deaggregation of Zinc Dihydride by Lewis Acids Including Carbon Dioxide in the Presence of Nitrogen Donors.

Thermally sensitive polymeric zinc dihydride [ZnH2]n can conveniently be prepared by the reaction of ZnEt2 with [AlH3(NEt3)]. When reacted with CO2 (1 bar) in the presence of chelating N-donor ligands Ln = N,N,N',N'-tetramethylethylenediamine (TMEDA), N,N,N',N'-tetramethyl-1,3-propanediamine (TMPDA), N,N,N',N″,N''-pentamethyldiethylenetriamine (PMDTA), and 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane (Me4TACD), insertion into the Zn-H bond readily occurred. Depending on the denticity n, formates [(Ln)Zn(OCHO)2] were isolated and structurally characterized, either as a molecule (Ln = TMEDA, TMPDA, PMDTA) or a charge-separated ion pair [(Ln)Zn(OCHO)][OCHO] (Ln = Me4TACD). The reaction of [ZnH2]n with the mild Lewis acid BPh3 in the presence of chelating N-donor ligands Ln gave a series of hydridotriphenylborates, either as a contact ion pair [(L2)Zn(H)(HBPh3)] (L2 = TMEDA, TMPDA) or a separated ion pair [(Ln)Zn(H)][HBPh3] (Ln = PMDTA, Me4TACD). In the crystal, the contact ion pair [(TMEDA)Zn(H)(HBPh3)] showed a bent Zn-H-B bridge indicative of a delocalized Zn-H-B interaction. In contrast, a linear Zn-H-B bridge for [(TMPDA)Zn(H)(HBPh3)] was observed, suggesting a contact ion pair. In THF solution, both complexes show an exchange with free BPh3 as well as [HBPh3]-. DFT calculations suggest the presence of [HBPh3]- anion with a highly polarized B-H bond that interacts with the Lewis acidic zinc hydride cation [(L2)Zn(H)]+. The hydridotriphenylborates [(Ln)Zn(H)(HBPh3)] underwent CO2 insertion to give (formato)zinc (formoxy)triphenylborate complexes [(Ln)Zn(OCHO)][(OCHO)BPh3] (Ln = TMPDA, PMDTA, Me4TACD). For Ln = TMEDA, a dinuclear complex [(Ln)2Zn2(µ-OCHO)3][(OCHO)BPh3] was isolated. Hydridotriphenylborates [(Ln)Zn(H)(HBPh3)] catalyzed the hydrosilylation of CO2 (1 bar) by nBuMe2SiH in THF at 70 °C to give formoxysilane and (methoxy)silane.

Cationic Zinc Hydride Catalyzed Carbon Dioxide Reduction to Formate: Deciphering Elementary Reactions, Isolation of Intermediates, and Computational Investigations.

Zinc has been an element of choice for carbon dioxide reduction in recent years. Zinc compounds have been showcased as catalysts for carbon dioxide hydrosilylation and hydroboration. The extent of carbon dioxide reduction can depend on various factors, including electrophilicity at the zinc center and the denticity of the ancillary ligands. In a few cases, the addition of Lewis acids to zinc hydride catalysts markedly influences carbon dioxide reduction. These factors have been investigated by exploring elementary reactions of carbon dioxide hydrosilylation and hydroboration by using cationic zinc hydrides bearing tetradentate tris[2-(dimethylamino)ethyl]amine and tridentate N,N,N',N'',N''-pentamethyldiethylenetriamine in the presence of triphenylborane and tris(pentafluorophenyl)borane.

A transient lanthanum phosphinidene complex.

Deprotonation of the terminal phosphido complex (PN)2La(PHMes) (1) results in the C-H-activation of one of the PN ligands, formally retaining the PHMes group. The reaction mechanism and the possible involvement of the transient phosphinidene complex 2 are investigated by theoretical and chemical means including a deuteration experiment employing (PN)2La(PDMes) (1-d). Furthermore, the thermal stability of product [K(2.2.2-cryptand)][(PN)(PNcyclo)La(PHMes)] (3b) is examined, giving the diphosphido complex [K(2.2.2-cryptand)][(PN)2La(P2Mes2)] (6).

Synthesis and versatile reactivity of scandium phosphinophosphinidene complexes.

M=E/M≡E multiple bonds (M = transition metal, E = main group element) are of significant fundamental scientific importance and have widespread applications. Expanding the ranges of M and E represents grand challenges for synthetic chemists and will bring new horizons for the chemistry. There have been reports of M=E/M≡E multiple bonds for the majority of the transition metals, and even some actinide metals. In stark contrast, as the largest subgroup in the periodic table, rare-earth metals (Ln) were scarcely involved in Ln=E/Ln≡E multiple bonds. Until recently, there were a few examples of rare-earth monometallic alkylidene, imido and oxo complexes, featuring Ln=C/N/O bonds. What are in absence are rare-earth monometallic phosphinidene complexes with Ln=P bonds. Herein, we report synthesis and structure of rare-earth monometallic phosphinidene complexes, namely scandium phosphinophosphinidene complexes. Reactivity of scandium phosphinophosphinidene complexes is also mapped out, and appears to be easily tuned by the supporting ligand.

Uranium Metallocene Azides, Isocyanates, and Their Borane-Capped Lewis Adducts.

Uranium(IV) metallocene complexes (CpiPr4)2U(N3)2 (1-N3), (CpiPr)2U(NCO)2 (1-NCO), and (CpiPr4)2U(OTf)2 (1-OTf) containing the bulky CpiPr4 ligand (CpiPr4 = tetra(isopropyl)cyclopentadienyl) were prepared directly from reactions between (CpiPr4)2UI2 or (CpiPr4)2UI and corresponding pseudohalide salts. The mixed-ligand complex (CpiPr4)2U(N3)(OTf) (1-N3-OTf) was isolated after heating a 1:1 mixture of 1-N3 and 1-OTf. The coordination of 1 equiv B(C6F5)3 to 1-N3 produced the borane-capped azide (CpiPr4)2U(N3)[(µ-η1:η1-N3)B(C6F5)3] (2-N3), while the reaction of 1 equiv B(C6F5)3 with 1-NCO yielded (CpiPr4)2U(NCO)[(µ-η1:η1-OCN)B(C6F5)3] (2-NCO) in which the borane-capped cyanate ligand had rearranged to become O-bound to uranium. The reaction of (CpiPr4)2UI and NaOCN led to the isolation of the uranium(III) cyanate-bridged "molecular square" [(CpiPr4)2U(µ-η1:η1-OCN)]4 (3-OCN). Cyclic voltammetry and UV-vis spectroscopy revealed small differences in the electronic properties between azide and isocyanate complexes, while X-ray crystallography showed nearly identical solid-state structures, with the most notable difference being the geometry of borane coordination to the azide in 2-N3 versus the cyanate in 2-NCO. Reactivity studies comparing 3-OCN to the azide analogue [(CpiPr4)2U(µ-η1:η1-N3)]4 (3-N3) demonstrated significant differences in the chemistry of cyanates and azides with trivalent uranium. A computational analysis of 1-NCO, 1-N3, 2-NCO, and 2-N3 has provided a basis for understanding the energetic preference for specific linkage isomers and the effect of the B(C6F5)3 coordination on the bonding between uranium, azide, and isocyanate ligands.

Construction of heterometallic clusters with multiple uranium-metal bonds by using dianionic nitrogen-phosphorus ligands.

Compared with the prevalent metal-metal bond in transition metals, examples of the actinide-metal bond in heterometallic clusters are rare. Herein, a series of heterometallic clusters with multiple uranium-metal bonds has been prepared based on two newly synthesized nitrogen-phosphorus ligands L1 {O[(CH2)2NHP(iPr)2]2} and L2 {[CH2O(CH2)2NHP(iPr)2]2}. Different P-P distances, 6.069 and 4.464 Å, are observed in the corresponding uranium complexes 1 {O[(CH2)2NP(iPr)2]2UCl2} and 2 {[CH2O(CH2)2NP(iPr)2]2UCl2}, respectively, and lead to the different coordination modes with transition metals. The reactions of zero-valent group 10 metal compounds with complex 1 generate heterometallic clusters (3-U2Ni2 and 4-U2Pd2) featuring four uranium-metal bonds; whereas reactions with 2 afford one-dimensional metal-chain 5-(UNi)n, bimetallic species 6-UPd, and a tri-platinum bridged diuranium molecular cluster 7-U2Pt3. Complex 5-(UNi)n represents the first infinite chain containing the U-M bond and 7-U2Pt3 is the first species with multiple U-Pt bonds. This study further highlights the important role of ligands in the construction of multiple uranium-metal bonds and may allow the synthesis of novel d-f heterometallic clusters and the investigation of their applications in catalysis and small-molecule activation.