Linear End-On Coordination Modes of CO2.

The second case of linear end-on and evidence for an unprecedented bridging end-on coordination mode of CO2 have been discovered for vanadium aryloxide complexes of the tetradentate ligand system (ONNO)2- (ONNO=2,4-Me2 -2-(OH) C6 H2 CH2 ]2 N(CH2 )2 NMe2 ). The reaction of divalent (ONNO)VII (TMEDA) with CO2 and under the appropriate reaction conditions affords the trivalent (ONNO)VIII (OH)(η1 -CO2 ) resulting from an intermediate CO2 deoxygenation pathway followed by H-atom abstraction from the aromatic solvent, and CO2 fixation. In contrast, the reduction of trivalent (ONNO)VIII Cl(THF) with K, followed by exposure to CO2 in ethereal solvent, afforded the dinuclear [(ONNO)VII ]2 (µ, η1 -CO2 ).

Reaction of CO2 with a Vanadium(II) Aryl Oxide: Synergistic Activation of CO2 /Oxo Groups towards H-Atom Radical Abstraction.

Treatment of divalent (ONNO)V(TMEDA) (1; ONNO=[2,4-Me2 -2-(OH)C6 H2 CH2 ]2 N(CH2 )2 NMe2 ) with CO2 afforded [(ONNO)V]2 (µ-OH)(µ-formate) (2). Whereas the bridging hydroxo and formate groups both originated from CO2 , the H atoms present on the two residues were obtained through H-atom radical abstraction from the solvent. DFT calculations revealed an initially linear CO2 bonding mode, followed by deoxygenation, and highlighted a synergistic effect between the so-formed oxo group and an additional bridging CO2 residue in promoting radical behavior.

Radical Behavior of CO2 versus its Deoxygenation Promoted by Vanadium Aryloxide Complexes: How the Geometry of Intermediate CO2 -Adducts Determines the Reactivity.

The reactivity of carbon dioxide with vanadium(III) aryloxo complexes has been investigated. The formation of either carbon monoxide or incorporation into the ligand system with the ultimate formation of organic ester was observed depending on the overall electron donor ability of the ligand field. DFT calculations were carried out to investigate the proposed mechanism for carbon dioxide coordination and reduction.

Revisiting the behaviour of BiVO4 as a carbon dioxide reduction photo-catalyst.

Bismuth vanadate is a widely known photocatalyst for the hydro-reduction of CO2. In spite of the great appeal of such a catalytic system, problems arise due to deactivation of the catalyst with consequent low reaction yield. We have investigated the catalyst behavior during methanol production and have found that the catalyst irreversibly loses vanadium from the structure whilst depositing bismuth oxides on the surface of the catalyst. While catalyst activity can be restored upon heating, leaching of vanadium, leading in long term to catalyst decomposition, is unavoidable and irreversible.

Chromium-chromium interaction in a binuclear mixed-valent Cr(I)-Cr(II) complex.

A mixed-valent Cr(I)-Cr(II) binuclear complex, {κ(1),κ(2),κ(3)-N,P,P-cyclo[(Ph)PCH2N(CH2Ph)CH2]}2(CrCl2)[Cr(µ-Cl)(AlClMe2)]·4toluene (1), of a P2N2 cyclic ligand was obtained upon treatment of the chromium precursor with alkylaluminum. Complex 1 was accessible from either its trivalent or divalent precursors, and density functional theory calculations revealed the presence of only σ- and π-orbital interactions in the Cr-Cr bond.

Isolation of a hexanuclear chromium cluster with a tetrahedral hydridic core and its catalytic behavior for ethylene oligomerization.

A chromium complex [2-(NHCH2PPh2)C5H4N]CrCl3·THF2 (1) of the ligand PyNHCH2PPh2 has been synthesized, characterized, and examined for its catalytic behavior toward ethylene oligomerization. When complex 1 was treated with (i-Bu)3Al, an unprecedented divalent polyhydride chromium cluster µ,κ(1),κ(2),κ(3)-N,N,P-{[2-(NCH2PPh2)C5H4N]Cr(µ-H)}4[(µ-Cl)Cr(µ-Cl)Al(i-Bu)2Cl]2 (2) was obtained. The complex contains a Cr4H4 core, which is expected to be diamagnetic, and which remains coordinated to two additional divalent high-spin Cr atoms via bridging interactions. Two aluminate residues remain bonded to the peripheral chromium atoms. The structure, magnetism, and electronic configuration are herein discussed.

Preparation and characterization of a reduced chromium complex via vinyl oxidative coupling: formation of a self-activating catalyst for selective ethylene trimerization.

Reaction of the divalent [(t-Bu)NP(Ph)(2)N(t-Bu)]CrCl(2)Li(THF)(2) (1) with 1 equiv of vinyl Grignard (CH(2)=CH)MgCl reproducibly afforded the triangulo {π-[(t-Bu)N-P(Ph)(2)-N(t-Bu)]Cr}(2)(µ,µ',η(4),η(4)'-C(4)H(4)){σ-[(t-Bu)N-P(Ph)(2)-N(t-Bu)]Cr} (2) containing a σ-/π-bonded butadiene-diyl unit. The diene-diyl moiety was generated by an oxidative coupling and deprotonation of two vinyl anions. The crystal structure revealed that of the three chromium atoms, each bearing one NPN ligand, two are perpendicularly bonded to the two sides of the π-system of the butadiene-diyl residue in a sort of inverted sandwich type of structure. The third is instead coplanar with the doubly deprotonated C(4) unit and σ-bonded to the two terminal carbon atoms. Despite the appearance as a Cr(II)/Cr(I) mixed valence species, DFT calculations have revealed that the structure of 2 consists of three divalent chromium atoms, while the additional electron resides on the π-system of the bridging organic residue. Complex 2 behaves as a single component selective catalyst for ethylene trimerization.

Vinyl oxidative coupling as a synthetic route to catalytically active monovalent chromium.

Reaction of the deprotonated form of cis-{(t-Bu)N(H)P[µ-N(t-Bu)](2)PN(H)(t-Bu)} with CrCl(3)(THF)(3) afforded the trivalent cis-{(t-Bu)NP[µ-N(t-Bu)](2)PN(t-Bu)}[Li (THF)])CrCl(2) (1). Subsequent reaction with 2 equiv of vinyl Grignard (CH(2)=CH)Mg Cl gave the butadiene derivative (cis-{(t-Bu)NP[µ-N(t-Bu)](2)PN(t-Bu)}[Li(THF)])Cr(cis-η(4)-butadiene) (3) formally containing the metal in its monovalent state. The presence of the monovalent state was thereafter confirmed by DFT calculations. The coordination of the butadiene unit appears to be rather robust since reaction with Me(3)P afforded cleavage of the dimeric ligand core but not its displacement. The reaction formed the new butadiene complex [(t-Bu)N-P-N(t-Bu)]Cr(cis-η(4)-butadiene)PMe(3) (4) containing a regular NPN monoanion. In agreement with the presence of monovalent chromium, complexes 3 and 4 act as single-component self-activating catalysts for selective ethylene trimerization and dimerization, respectively.

Redox-active ligands and organic radical chemistry.

Knowledge about bonding in diiminepyridine (L) halide, alkyl, and dinitrogen complexes of the metals iron, cobalt, and nickel is summarized, and two new examples are added to the set: L(1)Ni(Me) and L(1)Ni(N(2)). Reactivity of these types of complexes is discussed in terms of organic radical chemistry. New C-C couplings with L(2)CoAr complexes are described and proposed to involve halide abstraction and radical coupling. Calculations support the high tendency of the diiminepyridine ligand to accept an electron coming from a metal-carbon bond and so facilitate loss of a radical.

Samarium complexes of a sigma-/pi-pyrrolide/arene based macrocyclic ligand.

A bis-pyrrolide macrocyclic ligand [L = trans-calix[2]benzene[2]pyrrole(H)] containing two aromatic phenyl rings in the macrocycle backbone was reacted with SmCl(3)(THF)(3) to afford the corresponding [LSm(III)Cl] (1) complex. Its crystal structure showed the ligand adopting the sigma-bonding mode with the pyrrolide moieties and the pi-bonding with the two aromatic rings. Reaction of 1 with MeLi in THF gave a mixture of two compounds. The major was a C-H activated complex [LSm(III)(THF)] (2a) where the bonding mode of the pyrrolide rings was switched from sigma- to pi- as a result of the deprotonation and metallation of one of the two aromatic rings. The minor component was an unusual [(L)Sm(III)(HL')] (2b) complex containing both a regular ligand and an "N-confused" macrocyclic ligand. The two ligands wrapped the Sm center with the regular ligand adopting a bonding mode similar to 1. The second ligand instead acted as a simple sigma-bonded monodentate ligand, only using one nitrogen atom of one pyrrolide ring. However, this particular pyrrolide moiety has been isomerized by shifting the ring attachment to the macrocycle (N-confused system). In addition, the second pyrrolide ring has been protonated at the nitrogen atom. Complex 2b was obtained as major compound and in analytically pure form by reacting 1 with NaH.

Ligand metalation in the reactivity of a tetravalent uranium amides.

The reactions of organolithium reagents with the tetravalent UCl(2)[N(SiMe(3))(2)](2) and its DME solvate have been examined. Treatment of both compounds with methyl-lithium in diethyl ether resulted in one electron reduction of the metal center and gamma-deprotonation of one of the ligands. The dimeric {U[(mu-CH(2)-SiMe(2))N(SiMe(3))](2)[mu-Li(DME)]}(2) (1) was isolated from the reaction mixture regardless of the amount of MeLi employed. The employment of LiCH(2)SiMe(3) in DME led instead to multiple gamma-deprotonation events at the same carbon atom with formation of the trimetallic {U[(mu-CH(2)-SiMe(2))N(SiMe(3))][N(SiMe(3))(2)]}(2){U[(mu(3)-C-SiMe(2))N(SiMe(3))][N(SiMe(3))(2)]}{mu-OMe} (2) cluster centered on a fully deprotonated carbon atom. Crystallographic analysis revealed the presence of mu-OCH(3) units in the cluster as generated by DME solvent cleavage. A similar reaction carried out in the absence of DME led to the isolation of a closely related trimetallic {U[mu-(CH(2)-SiMe(2))N(SiMe(3))][N(SiMe(3))(2)]}(2){U[(mu(3)-C-SiMe(2))N(SiMe(3))][(mu-CH(2)-SiMe(2))N(SiMe(3))]} (3). One additional gamma-deprotonated fragment replacing the bridging methoxy group of 2 was present in this case. The presence of a fully deprotonated carbon atom bridging three metal centers and of one silicon atom was confirmed by both X-ray structures and NMR data. An attempt to reduce UCl(2)[N(SiMe(3))(2)](2) with KC(8) in a coordinating solvent resulted in ligand scrambling with the formation of two products. The first is a trimeric U(III) cluster formulated as {U-mu-Cl[N(SiMe(3))(2)][DME]}(2){U-mu-Cl[N(SiMe(3))(2)](2)}{mu(3)-Cl}(2) (4). The second was U[N(SiMe(3))(2)](3). A similar reduction reaction carried out in noncoordinating toluene resulted instead in an attack on the ligand affording the dimeric {U-mu-Cl[N(SiMe(3))(2)](2)[ horizontal lineN(SiMe(3))]}(2) (5). Alkylation of UCl(2)[N(SiMe(3))(2)](2) with n-butyl-lithium in hexane surprisingly yielded the pentavalent U[(mu-CH(2)-SiMe(2))N(SiMe(3))](2)[N(SiMe(3))(2)] (6). The acquisition of one additional ligand during the reaction hinted at the presence of other products in the reaction mixture.

Reduced uranium complexes: synthetic and DFT study of the role of pi ligation in the stabilization of uranium species in a formal low-valent state.

Reaction of UCl(4)(THF)(4) with 1,3-[2,5-(i-Pr)(2)PhNC( horizontal lineCH(2))](2)C(6)H(4)Li(2) produced a complex formulated as [{1,3-[2,5-(i-Pr)(2)PhNC( horizontal lineCH(2))](2)C(6)H(4)}UCl(3)][Li(THF)(4)] (1) that exhibits a nonagostic interaction between one of the carbon atoms of the central phenyl ring and the U metal center. This interaction leads to significant weakening of the corresponding C-H bond, thereby facilitating proton removal in consecutive transformations. Attempts to form trivalent uranium derivatives were carried out by reacting the same ligand dianion with in situ-prepared "UCl(3)". The reaction indeed afforded a trivalent species formulated as {1,3-[2,5-(i-Pr)(2)PhNC( horizontal lineCH(2))](2)C(6)H(4)}U(mu-Cl)(3)[Li(THF)(2)](2) (2). The configuration of the ligand system in this complex is similar to that in 1, with the same type of arrangement of the central phenyl ring. Further reduction chemistry with a variety of reagents and conditions was examined. Reaction of 1 with 1 equiv of lithium naphthalenide at 0 degrees C did not afford 2 but instead gave a closely related U(III) complex formulated as {1,3-[2,5-(i-Pr)(2)PhNC( horizontal lineCH(2))](2)C(6)H(4)}U(THF)(mu-Cl)(2)[Li(Et(2)O)(2)] (3). Both of the trivalent complexes 2 and 3 reacted thermally in boiling THF, undergoing oxidation of the metal center to afford a new tetravalent compound {1,3-[2,5-(i-Pr)(2)PhNC( horizontal lineCH(2))](2)C(6)H(3)}U(THF)(mu-Cl)(2)[Li(THF)(2)] (4) in which the oxidation of the trivalent center occurred at the expense of the central phenyl ring C-H bond. Reaction of 1 with 3 equiv of lithium naphthalenide at room temperature afforded {{1,3-[2,5-(i-Pr)(2)PhNC( horizontal lineCH(2))](2)C(6)H(3)}U(mu-Cl)(mu-[O(CH(2))(3)CH(2)])[Li(DME)]}[Li(DME)(3)] (5). In this species, the tetravalent metal center forms a six-membered metallacycle ring with a moiety arising from THF ring opening. Reaction in DME afforded reductive cleavage of the solvent accompanied by reoxidation of U to the tetravalent state. Reduction of 1 in DME with 2 equiv of potassium naphthalenide at room temperature gave a mixture of two compounds having very similar structures. The two different species [{1,3-[2,5-(i-Pr)(2)PhNC( horizontal lineCH(2))](2)C(6)H(3)}UCl(OCH(3))][Li(DME)(3)] (6a) and [{1,3-[2,5-(i-Pr)(2)PhNC( horizontal lineCH(2))](2)C(6)H(3)}UCl(2)][Li(DME)(3)] (6b) cocrystallized in a ratio very close to 1:1 within the same unit cell. The methoxide group was generated from cleavage of the DME solvent. We also attempted the reduction of 1 with a different reducing agent such as NaH in DME. After a slow reaction, a new species formulated as {1,3-[2,5-(i-Pr)(2)PhNC( horizontal lineCH(2))](2)C(6)H(3)}U(mu-OCH(3))(3)(mu,eta(6)-Na)[eta(3)-Na(DME)] (7) was isolated in significant yield. Once again, the crystal structure revealed the presence of several methoxy groups coordinated to the U center in addition to the metalation of the ligand phenyl ring. To minimize solvent cleavage, reduction of 1 was also carried out at low temperature (-35 degrees C) and with a larger amount (4 equiv) of lithium naphthalenide. After suitable workup, the new species {[{1,3-[2,5-(i-Pr)(2)PhNC( horizontal lineCH(2))](2)C(6)H(3)}U{1,3-[2,5-(i-Pr)(2)PhN horizontal lineC(CH(3))](2)C(6)H(4)}][Li(DME)(THF)]}.Et(2)O (8) was isolated in significant yield. Even in this case, the uranium atom is surrounded by the expected trianionic, ring-metalated ligand. However, a second ligand unit surrounds the metal center, being bonded through a part of the pi system. Reaction of 1 with excess NaH in toluene proceeded slowly at room temperature, affording a significant yield of {[{1,3-[2,5-(i-Pr)(2)PhNC( horizontal lineCH(2))](2)C(6)H(3)}U{1,3-[2,5-(i-Pr)(2)PhN horizontal lineC(CH(3))](2)C(6)H(4)}{Na(DME)(2)}][Na(DME)(3)]}.(1)/(2)C(7)H(8) (9) after crystallization from DME/toluene. Similar to 8, the complex still contains one ring-metalated trianionic ligand and one intact ligand that has regained the H atoms and restored the two imine functions. Although according to their connectivities, complexes 8 and 9 could be assigned with the formal oxidation states +2 and +1, respectively, density functional theory calculations clearly indicated that these species contain additional spin density on the ligand system with the metal center in its more standard trivalent state.