T-Shaped Palladium and Platinum {MNO}10 Nitrosyl Complexes.

The synthesis and characterization of a homologous series of T-shaped {MNO}10 nitrosyl complexes of the form [M(PR3)2(NO)]+ (M = Pd, Pt; R = tBu, Ad) are reported. These diamagnetic nitrosyls are obtained from monovalent or zerovalent precursors by treatment with NO and NO+, respectively, and are notable for distinctly bent M-NO angles of ∼123° in the solid state. Adoption of this coordination mode in solution is also supported by the analysis of isotopically enriched samples by 15N NMR spectroscopy. Effective oxidation states of M0/NO+ are calculated, and metal-nitrosyl bonding has been interrogated using DFT-based energy decomposition analysis techniques. While a linear nitrosyl coordination mode was found to be electronically preferred, the M-NO and P-M-P angles are inversely correlated to the extent that binding in this manner is prevented by steric repulsion between the bulky ancillary phosphine ligands.

Ethene Activation and Catalytic Hydrogenation by a Low-Valent Uranium Pentalene Complex.

The reaction of the uranium(III) complex [U(η8-Pn††)(η5-Cp*)] (1) (Pn†† = C8H4(1,4-SiiPr3)2, Cp* = C5Me5) with ethene at atmospheric pressure produces the ethene-bridged diuranium complex [{(η8-Pn††)(η5-Cp*)U}2(µ-η2:η2-C2H4)] (2). A computational analysis of 2 revealed that coordination of ethene to uranium reduces the carbon-carbon bond order from 2 to a value consistent with a single bond, with a concomitant change in the formal uranium oxidation state from +3 in 1 to +4 in 2. Furthermore, the uranium-ethene bonding in 2 is of the δ type, with the dominant uranium contribution being from f-d hybrid orbitals. Complex 2 reacts with hydrogen to produce ethane and reform 1, leading to the discovery that complex 1 also catalyzes the hydrogenation of ethene under ambient conditions.

The experimental determination of Th(iv)/Th(iii) redox potentials in organometallic thorium complexes.

The first ThIV/ThIII redox couple values have been determined experimentally using cyclic voltammetry (CV), which has been facilitated by the use of [nBu4N][BPh4] as a supporting electrolyte in THF. Th(iv) and Th(iii) metallocene compounds have been studied and their redox couple values are in the range of -2.96 V to -3.32 V vs FeCp2+/0.

Bis(pentalene)dititanium chemistry: C-H, C-X and H-H bond activation.

The reaction of the bis(pentalene)dititanium complex Ti2(µ:η5,η5-Pn†)2 (Pn† = C8H4(1,4-SiiPr3)2) (1) with the N-heterocyclic carbene 1,3,4,5-tetramethylimidazol-2-ylidene results in intramolecular C-H activation of an isopropyl substituent to form a tucked-in hydride (3). Whilst pyridine will also effect this cyclometallation reaction to form (5), the pyridine analogue of (3), the bases 1,2,4,5-tetramethyl-imidazole, 2,6-lutidine, DABCO or trimethylphosphine are ineffective. The reaction of (1) with 2,6-dichloro-pyridine affords crystallographically characterised (6) which is the product of oxidative addition of one of the C-Cl bonds in 2,6-dichloro-pyridine across the Ti-Ti double bond in (1). The tucked-in hydride (3) reacts with hydrogen to afford a dihydride complex (4) in which the tuck-in process has been reversed; detailed experimental and computational studies on this reaction using D2, HD or H2/D2 support a mechanism for the formation of (4) which does not involve σ-bond metathesis of H2 with the tucked-in C-H bond in (3). The reaction of (3) with tBuCCH yields the corresponding acetylide hydrido complex (7), where deuteration studies show that again the reaction does not proceed via σ-bond metathesis. Finally, treatment of (3) with HCl affords the chloro-derivative (9) [(NHC)Ti(µ-H)Ti{(µ,η5:η5)Pn†}2Cl], whereas protonation with [NEt3H]BPh4 yielded a cationic hydride (10) featuring an agostic interaction between a Ti centre and an iPr Me group.

Activation of carbon suboxide (C3O2) by U(iii) to form a cyclobutane-1,3-dione ring.

The activation of C3O2 by the U(iii) complex [U(η5-Cp')3] (Cp' = C5H4SiMe3) is described. The reaction results in the reductive coupling of three C3O2 units to form a tetranuclear complex with a central cyclobutane-1,3-dione ring, with concomitant loss of CO. Careful control of reaction conditions has allowed the trapping of an intermediate, a dimeric bridging ketene complex, which undergoes insertion of C3O2 to form the final product.

Trimerisation of carbon suboxide at a di-titanium centre to form a pyrone ring system.

The reaction of the syn-bimetallic bis(pentalene)dititanium complex Ti2(µ:η5,η5-Pn†)2 (Pn† = C8H4(1,4-SiiPr3)2) 1 with carbon suboxide (O[double bond, length as m-dash]C[double bond, length as m-dash]C[double bond, length as m-dash]C[double bond, length as m-dash]O, C3O2) results in trimerisation of the latter and formation of the structurally characterised complex [{Ti2(µ:η5,η5-Pn†)2}{µ-C9O6}]. The trimeric bridging C9O6 unit in the latter contains a 4-pyrone core, a key feature of both the hexamer and octamer of carbon suboxide which are formed in the body from trace amounts of C3O2 and are, for example, potent inhibitors of Na+/K+-ATP-ase. The mechanism of this reaction has been studied in detail by DFT computational studies, which also suggest that the reaction proceeds via the initial formation of a mono-adduct of 1 with C3O2. Indeed, the carefully controlled reaction of 1 with C3O2 affords [Ti2(µ:η5,η5-Pn†)2 (η2-C3O2)], as the first structurally authenticated complex of carbon suboxide.

Single-molecule magnet properties of a monometallic dysprosium pentalene complex.

The pentalene-ligated dysprosium complex [(η8-Pn†)Dy(Cp*)] (1Dy) (Pn† = [1,4-(iPr3Si)2C8H4]2-) and its magnetically dilute analogue are single-molecule magnets, with energy barriers of 245 cm-1. Whilst the [Cp*]- ligand in 1Dy provides a strong axial crystal field, the overall axiality of this system is attenuated by the unusual folded structure of the [Pn†]2- ligand.

Correction: C-H and H-H activation at a di-titanium centre.

Correction for 'C-H and H-H activation at a di-titanium centre' by Nikolaos Tsoureas et al., Chem. Commun., 2017, 53, 13117-13120.

C-H and H-H activation at a di-titanium centre.

The reaction of the bis(pentalene)dititanium complex Ti2(µ:η5,η5-Pn†)2 (Pn† = C8H4(1,4-SiiPr3)2) with the N-heterocyclic carbene 1,3,4,5-tetramethylimidazol-2-ylidene results in intramolecular C-H activation of one of the iPr methyl groups of a Pn† ligand and formation of a "tucked-in" bridging hydride complex. The "tuck-in" process is reversed by the addition of hydrogen, which yields a dihydride featuring terminal and bridging hydrides.

A base-free synthetic route to anti-bimetallic lanthanide pentalene complexes.

We report the synthesis and structural characterisation of three homobimetallic complexes featuring divalent lanthanide metals (Ln = Yb, Eu and Sm) bridged by the silylated pentalene ligand [1,4-{SiiPr3}2C8H4]2- (= Pn†). Magnetic measurements and cyclic voltammetry have been used to investigate the extent of intermetallic communication in these systems, in the context of molecular models for organolanthanide based conducting materials.

Steric control of redox events in organo-uranium chemistry: synthesis and characterisation of U(v) oxo and nitrido complexes.

The synthesis and molecular structures of a U(v) neutral terminal oxo complex and a U(v) sodium uranium nitride contact ion pair are described. The synthesis of the former is achieved by the use of t BuNCO as a mild oxygen transfer reagent, whilst that of the latter is via the reduction of NaN3. Both mono-uranium complexes are stabilised by the presence of bulky silyl substituents on the ligand framework that facilitate a 2e- oxidation of a single U(iii) centre. In contrast, when steric hindrance around the metal centre is reduced by the use of less bulky silyl groups, the products are di-uranium, U(iv) bridging oxo and (anionic) nitride complexes, resulting from 1e- oxidations of two U(iii) centres. SQUID magnetometry supports the formal oxidation states of the reported complexes. Electrochemical studies show that the U(v) terminal oxo complex can be reduced and the [U(iv)O]- anion was accessed via reduction with K/Hg, and structurally characterised. Both the nitride complexes display complex electrochemical behaviour but each exhibits a quasi-reversible oxidation at ca. -1.6 V vs. Fc+/0.

The Reductive Activation of CO2 Across a Ti=Ti Double Bond: Synthetic, Structural, and Mechanistic Studies.

The reactivity of the bis(pentalene)dititanium double-sandwich compound Ti2Pn†2 (1) (Pn† = 1,4-{SiiPr3}2C8H4) with CO2 is investigated in detail using spectroscopic, X-ray crystallographic, and computational studies. When the CO2 reaction is performed at -78 °C, the 1:1 adduct 4 is formed, and low-temperature spectroscopic measurements are consistent with a CO2 molecule bound symmetrically to the two Ti centers in a µ:η2,η2 binding mode, a structure also indicated by theory. Upon warming to room temperature the coordinated CO2 is quantitatively reduced over a period of minutes to give the bis(oxo)-bridged dimer 2 and the dicarbonyl complex 3. In situ NMR studies indicated that this decomposition proceeds in a stepwise process via monooxo (5) and monocarbonyl (7) double-sandwich complexes, which have been independently synthesized and structurally characterized. 5 is thermally unstable with respect to a µ-O dimer in which the Ti-Ti bond has been cleaved and one pentalene ligand binds in an η8 fashion to each of the formally TiIII centers. The molecular structure of 7 shows a "side-on" bound carbonyl ligand. Bonding of the double-sandwich species Ti2Pn2 (Pn = C8H6) to other fragments has been investigated by density functional theory calculations and fragment analysis, providing insight into the CO2 reaction pathway consistent with the experimentally observed intermediates. A key step in the proposed mechanism is disproportionation of a mono(oxo) di-TiIII species to yield di-TiII and di-TiIV products. 1 forms a structurally characterized, thermally stable CS2 adduct 8 that shows symmetrical binding to the Ti2 unit and supports the formulation of 4. The reaction of 1 with COS forms a thermally unstable complex 9 that undergoes scission to give mono(µ-S) mono(CO) species 10. Ph3PS is an effective sulfur transfer agent for 1, enabling the synthesis of mono(µ-S) complex 11 with a double-sandwich structure and bis(µ-S) dimer 12 in which the Ti-Ti bond has been cleaved.

Bonding in Complexes of Bis(pentalene)dititanium, Ti2(C8H6)2.

Bonding in the bis(pentalene)dititanium "double-sandwich" species Ti2Pn2 (Pn = C8H6) and its interaction with other fragments have been investigated by density functional calculations and fragment analysis. Ti2Pn2 with C2v symmetry has two metal-metal bonds and a low-lying metal-based empty orbital, all three frontier orbitals having a1 symmetry. The latter may be regarded as being derived by symmetric combinations of the classic three frontier orbitals of two bent bis(cyclopentadienyl) metal fragments. Electrochemical studies on Ti2Pn†2 (Pn† = 1,4-{SiiPr3}2C8H4) revealed a one-electron oxidation, and the formally mixed-valence Ti(II)-Ti(III) cationic complex [Ti2Pn†2][B(C6F5)4] has been structurally characterized. Theory indicates an S = 1/2 ground-state electronic configuration for the latter, which was confirmed by EPR spectroscopy and SQUID magnetometry. Carbon dioxide binds symmetrically to Ti2Pn2, preserving the C2v symmetry, as does carbon disulfide. The dominant interaction in Ti2Pn2CO2 is σ donation into the LUMO of bent CO2, and donation from the O atoms to Ti2Pn2 is minimal, whereas in Ti2Pn2CS2 there is significant interaction with the S atoms. The bridging O atom in the mono(oxo) species Ti2Pn2O, however, employs all three O 2p orbitals in binding and competes strongly with Pn, leading to weaker binding of the carbocyclic ligand, and the sulfur analogue Ti2Pn2S behaves similarly. Ti2Pn2 is also capable of binding one, two, or three molecules of carbon monoxide. The bonding demands of a single CO molecule are incompatible with symmetric binding, and an asymmetric structure is found. The dicarbonyl adduct Ti2Pn2(CO)2 has Cs symmetry with the Ti2Pn2 unit acting as two MCp2 fragments. Synthetic studies showed that in the presence of excess CO the tricarbonyl complex Ti2Pn†2(CO)3 is formed, which optimizes to an asymmetric structure with one semibridging and two terminal CO ligands. Low-temperature 13C NMR spectroscopy revealed a rapid dynamic exchange between the two bound CO sites and free CO.

Group 4 metal compounds incorporating the amide ligand, [N(SiMe2{C6H4-2-OMe})2](-).

The anisole-substituted silyl-amide anion, [N(SiMe2{C6H4-2-OMe})2](-) (L), has been used as a pincer-type ligand in coordination chemistry. X-ray diffraction data for the lithium salt shows a trimetallic structure consisting of two equivalents of Li(L) that sequester a molecule of LiCl. The potassium salt K(L) is dimeric in the solid-state with bridging amide ligands. Each structure shows chelation of both O-donor groups to the electropositive metal. In contrast, the titanium compound Ti(L)Cl3 is four-coordinate with a monodentate amide. The zirconium compound Zr(L)2Cl2 is monometallic with a six-coordinate metal and two N,O-bidentate amides.

Synthesis of an [(NHC)2 Pd(SiMe3 )2 ] Complex and Catalytic cis-Bis(silyl)ations of Alkynes with Unactivated Disilanes.

The novel complex cis-[(ITMe)2 Pd(SiMe3 )2 (ITMe=1,3,4,5-tetramethylimidazol-2-ylidene) has been synthesized by mild oxidative cleavage of Me3 SiSiMe3 using [(ITMe)2 Pd(0) ]. The use of this complex as precatalyst for the cis-bis(silyl)ation of alkynes using unactivated disilanes is reported.

Mixed sandwich thorium complexes incorporating bis(tri-isopropylsilyl)cyclooctatetraenyl and pentamethylcyclopentadienyl ligands: synthesis, structure and reactivity.

The Th(iv) mixed-sandwich halide complexes Th(COT(TIPS2))Cp*X (where COT(TIPS2) = 1,4-{Si(i)Pr(3)}(2)C(8)H(6), X = Cl, I) have been synthesised, and structurally characterised. When Th(COT(TIPS2))Cp*I is reduced in situ in the presence of CO(2), a mixture of dimeric carboxylate and oxalate complexes {Th(COT(TIPS2))Cp*}(2)(µ-κ(1):κ(2)-CO(3)) and {Th(COT(TIPS2))Cp*}(2)(µ-κ(2):κ(2)-C(2)O(4)) are formed, possibly via a transient Th(iii) species. Th(COT(TIPS2))Cp*Cl is readily alkylated to yield the benzyl complex Th(COT(TIPS2))Cp*CH(2)Ph, which reacts with CO(2) to form a carboxylate and with H(2) to form a hydride; the latter inserts CO(2), giving the bridging formate complex {Th(COT(TIPS2))Cp*(µ-κ(1):κ(1)-O(2)CH)}(2).

Formation of cyanates in low-valent uranium chemistry: a synergistic experimental/theoretical study.

Computational studies on the reductive activation of a mixture of CO and NO by the U(iii) complex [U(η-C8H6{Si(i)Pr3-1,4}2)(η-Cp*)], which affords a mixture of [U(η-C8H6{Si(i)Pr3-1,4}2)(η-Cp*)]2(µ-OCN)21 and [U(η-C8H6{Si(i)Pr3-1,4}2)(η-Cp*)]2(µ-O) 2, show that the reaction proceeds via an initial attack of CO on a µ-η(2):η(2) coordinated NO, side-on bridged between two uranium centres. This leads to the formation of the bridging oxo complex 2 and the cyanate radical; coordination of the latter to the starting complex and dimerisation affords 1. The DFT studies also predict the existence of the monocyanate-bridged, mixed valence species [U(η-C8H6{Si(i)Pr3-1,4}2)(η-Cp*)]2(µ-OCN) 3, which has now been experimentally observed.

Reductive deoxygenation of CO2 by a bimetallic titanium bis(pentalene) complex.

The bimetallic bis(pentalene) complex (µ:η(5),η(5)-Pn(†))2Ti2 reductively splits CO2 to form a bis(oxo) bridged dimer [(η(8)-Pn(†))Ti(µ-O)]2, in which the Ti-Ti bond has been cleaved, and the dicarbonyl complex (µ:η(5),η(5)-Pn(†))2[Ti(CO)]2.

Bis(pentalene)di-titanium: a bent double-sandwich complex with a very short Ti-Ti bond.

The novel bimetallic bis(pentalene) complex Ti2(µ:η(5),η(5)-Pn(†))2 (Pn(†) = C8H4{Si(i)Pr3-1,4}2) has been synthesised and structurally characterised. Structural data show a Ti-Ti distance of 2.399(2) Å, consistent with a strong metal-metal interaction, which DFT calculations best describe as a double bond with σ and π components.

Computational insight into the reductive oligomerisation of CO at uranium(III) mixed-sandwich complexes.

Calculations reveal a multistep pathway towards formation of linear [U](2)-(µ-η(1):η(1)-C(2)O(2)); [U] = U(η-C(8)H(6){SiH(3)-1,4}(2))(η-Cp). However formation of deltate-bridged [U](2)-(µ-η(1):η(2)-C(3)O(3)) requires an alternative mechanism, involving a side-on [U](2)-(µ-η(2):η(2)-CO) complex and whereby the bridging units of [U](2)-(µ-η(2):η(2)-C(n)O(n)) intermediates (n = 1, 2) react directly with free CO.