Reactivity of a dititanium bis(pentalene) complex toward heteroallenes and main-group element–element bonds

The reactivity of the Ti=Ti double bond in (μ,η 5 :η 5 -Pn † ) 2 Ti 2 (1; Pn † = 1,4-{Si i Pr 3 } 2 C 8 H 4 ) toward isocyanide and heteroallene substrates, and molecules featuring homonuclear bonds between main-group elements (E-E) has bee n explored. Reaction of 1 with methyl isocyanide or 1,3-N,N′-di-p-tolylcarbodiimide resulted in the formation of the 1:1 adducts (μ,η 5 :η 5 -Pn † ) 2 Ti 2 (μ,η 2 -CNMe) (2) and (μ,η 5 :η 5 -Pn † ) 2 Ti 2 (μ-C{N(4-C 6 H 4 CH 3 )} 2 ) (3), respectively, which are thermally stable up to 100 °C in contrast to the analogous adducts formed with CO and CO 2 . Reaction of 1 with phenyl isocyanate afforded a paramagnetic complex, [(η 8 -Pn † )Ti] 2 (μ,κ 2 :κ 2 -O 2 CNPh) (4), in which the "double-sandwich" architecture of 1 has been broken and an unusual phenyl-carbonimidate ligand bridges two formally Ti(III) centers. Reaction of 1 with diphenyl dichalcogenides, Ph 2 E 2 (E = S, Se, Te), led to the series of Ti-Ti single-bonded complexes (μ,η 5 :η 5 -Pn † ) 2 [Ti(EPh)] 2 (E = S (5), Se (6), Te (7)), which can be considered the result of a 2e - redox reaction or a 1,2-addition across the Ti=Ti bond. Treatment of 1 with azobenzene or phenyl azide afforded [(η 8 -Pn † )Ti] 2 (μ-NPh) 2 (8), a bridging imido complex in which the pentalene ligands bind in an η 8 fashion to each formally Ti(IV) center, as the result of a 4e - redox reaction driven by the oxidative cleavage of the Ti=Ti double bond. The new complexes 2-8 were extensively characterized by various techniques including multinuclear NMR spectroscopy and single-crystal X-ray diffraction, and the experimental work was complemented by density functional theory (DFT) studies

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

Bonding in pentalene complexes and their recent applications

Molecular orbital (MO) theory is used to describe the bonding in transition metal pentalene complexes in a variety of its coordination modes. The various MO models account for structural parameters and lead to simple rules for electron counting in pentalene complexes. Recent applications of pentalene complexes are also reviewed, in the areas of small molecule activation and catalysis and as molecular models for conducting organometallic polymers

Bonding in pentalene complexes and their recent applications

Molecular orbital (MO) theory is used to describe the bonding in transition metal pentalene complexes in a variety of its coordination modes. The various MO models account for structural parameters and lead to simple rules for electron counting in pentalene complexes. Recent applications of pentalene complexes are also reviewed, in the areas of small molecule activation and catalysis and as molecular models for conducting organometallic polymers

Carbon dioxide activation by a uranium(III) complex derived from a chelating bis(aryloxide) ligand

The new dianionic ligand C6H4{p-C(CH3)2C6H2Me2O–}2 (=p-Me2bp), featuring two aryloxide donors and a central arene ring, has been synthesized and used to prepare the mixed-ligand U(III) compound [U(Cp*)(p-Me2bp)], which exhibits an η6 interaction with the uranium center. Reductive activation of CO2 was investigated using [U(Cp*)(p-Me2bp)] in supercritical CO2, which gave a dinuclear uranium carbonate complex, {U(Cp*)(p-Me2bp)}2(μ-η1:η2-CO3), cleanly and selectively. Reactivity studies in conventional solvents using lower pressures of CO2 showed the formation of a rare U(IV) oxalate complex, {U(Cp*)(p-Me2bp)}2(μ-η2:η2-C2O2), alongside {U(Cp*)(p-Me2bp)}2(μ-η1:η2-CO3). The relative ratio of the last two products is temperature dependent: at low temperatures (−78 °C) oxalate formation is favored, while at room temperature the carbonate is the dominant product. The U(IV) iodide [U(Cp*)(p-Me2bp)I] was also synthesized and used as part of an electrochemical study, the results of which showed that [U(Cp*)(p-Me2bp)] has a UIV/UIIIredox couple of −2.18 V vs FeCp2+/0 as well as a possible electrochemically accessible UIII/UIIreduction process at −2.56 V vs FeCp2+/0.</p