723 research outputs found
The effect of one valence electron: Contrasting (PNP)Ni(CO) with (PNP)Ni(NO) to understand the half-bent NINO unit
No full textReaction of a (PNP)Ni radical with NO finishes in the time of mixing to form a 1:1 adduct with a NO stretching frequency of 1654 cm(-1). NMR data of this diamagnetic product indicate C-2v symmetry, which is contradicted by the X-ray structure, which shows it to be nonplanar at Ni, with a geometry intermediate between planar and tetrahedral; the planar geometry is thus the transition state for fluxionality giving time-averaged C, symmetry. The X-ray structure, together with DFT calculations, reveals that the "half-bent" NiNO unit and the intermediate coordination geometry result from a Ni NO charge transfer, which has a nonintegral value, resulting in a continuum between NO+ (hence Ni-0) and NO- (hence Ni-II). This is related to the nonaxially symmetric character of the Ni -> NO back-donation caused by the (PNP) environment on Ni. Steric effects of Bu-t and even chelate constraints are ruled out as the cause of the unusual electronic and structural features
Lewis Acid Stabilized Methylidene and Oxoscandium Complexes
No full textThe methylidene scandium complex (PNP)Sc(mu(3)-CH(2))(mu(2)-CH(3))(2)[Al(CH(3))(2)](2) (PNP=N[2-P(CHMe(2))2-4-methylpheyl](2)(-)) can be prepared from the reaction of (PNP)Sc(CH(3))(2) and 2 equiv of Al(CH(3))(3). The Lewis acid stabilized methylidenes candium complex has been crystallographically characterized, and its bonding scheme analyzed by DFT. In addition, we report preliminary reactivity studies of the Sc-CH(2) ligand with substrates such as H(2)NAr and OCPh(2). While the former results in an Bronsted acid-base reaction, the latter reagent produces the olefin H(2)C-CPh(2) along with the novel oxoscandium complex (PNP)Sc(mu(3)-O)(mu(2)-CH(3))(2)[Al(CH(3))(2)](2), quantitatively
Intermolecular C-H bond activation reactions promoted by transient titanium alkylidynes. Synthesis, reactivity, kinetic, and theoretical studies of the Ti C linkage
No full textThe neopentylidene-neopentyl complex (PNP)(TiCHBu)-Bu-t((CH2Bu)-Bu-t) (2;PNP- = N[2-P(CHMe2)(2)-4-methylphenyl](2)), prepared from the precursor (PNP)(TiCHBu)-Bu-t(OTf) (1) and (LiCH2Bu)-Bu-t, extrudes neopentane in neat benzene under mild conditions (25 degrees C) to generate the transient titanium alkylidyne, (PNP)(TiCBu)-Bu-t (A), which subsequently undergoes 1,2-CH bond addition of benzene across the TiC linkage to generate (PNP)(TiCHBu)-Bu-t(C6H5) (3). Kinetic, mechanistic, and theoretical studies suggest the C-H activation process to obey pseudo-first-order in titanium, the alpha-hydrogen abstraction to be the rate-determining step (KIE for 2/2-d(3) conversion to 3/3-d(3) = 3.9(5) at 40 degrees C) with activation parameters Delta H = 24(7) kcal/mol and Delta S = -2(3) cal/mol center dot K, and the post-rate-determining step to be C-H bond activation of benzene (primary KIE = 1.03(7) at 25 degrees C for the intermolecular C-H activation reaction in C6H6 vs C6D6). A KIE of 1.33(3) at 25 degrees C arose when the intramolecular C-H activation reaction was monitored with 1,3,5-C6H3D3. For the activation of aromatic C-H bonds, however, the formation of the sigma-complex becomes rate-determining via a hypothetical intermediate (PNP)(TiCBu)-Bu-t(C6H5), and C-H bond rupture is promoted in a heterolytic fashion by applying standard Lewis acid/base chemistry. Thermolysis of (3) in C6D6 at 95 degrees C over 48 h generates 3-d(6), thereby implying that 3 can slowly equilibrate with A under elevated temperatures with k = 1.2(2) x 10(-5) s(-1), and with activation parameters Delta H = 31(16) kcal/mol and Delta S = 3(9) cal/mol.K. At 95 degrees C for one week, the EIE for the 2-3 reaction in 1,3,5-C6H3D3 was found to be 1.36(7). When 1 is alkylated with LiCH2SiMe3 and KCH2Ph, the complexes (PNP)(TiCHBu)-Bu-t(CH2SiMe3) (4) and (PNP)(TiCHBu)-Bu-t(CH2Ph) (6) are formed, respectively, along with their corresponding tautomers (PNP)TiCHSiMe3((CH2Bu)-Bu-t) (5) and (PNP)TiCHPh((CH2Bu)-Bu-t) (7). By means of similar alkylations of (PNP)TiCHSiMe3(OTf) (8), the degenerate complex (PNP)TiCHSiMe3(CH2SiMe3) (9) or the non-degenerate alkylidene-alkyl complex (PNP)TiCHPh(CH2SiMe3) (11) can also be obtained, the latter of which results from a tautomerization process. Compounds 4/5 and 9, or 6/7 and (11), also activate benzene to afford (PNP)TiCHR(C6H5) (R = SiMe3 (10), Ph (12). Substrates such as FC6H5, 1,2-F2C6H4, and 1,4-F2C6H4 react at the aryl C-H bond with intermediate A, in some cases regioselectively, to form the neopentylidene-aryl derivatives (PNP)(TiCHBu)-Bu-t(aryl). Intermediate A can also perform stepwise alkylidene-alkyl metatheses with 1,3,5-Me<INF>3</INF>C<INF>6</INF>H<INF>3</INF>, SiMe<INF>4</INF>, 1,2-bis(trimethylsilyl)alkyne, and bis(trimethylsilyl)ether to afford the titanium alkylidene-alkyls (PNP)TiCHR(R') (R = 3,5-Me<INF>2</INF>C<INF>6</INF>H<INF>2</INF>, R' = CH<INF>2</INF>-3,5-Me<INF>2</INF>C<INF>6</INF>H<INF>2</INF>; R = SiMe<INF></INF>, R' = CH<INF>2</INF>SiMe<INF>3</INF>; R = SiMe<INF>2</INF>CCSiMe<INF>3</INF>, R' = CH<INF>2</INF>SiMe<INF>2</INF>CCSiMe<INF>3</INF>; R = SiMe<INF>2</INF>OSiMe<INF>3</INF>, R' = CH<INF>2</INF>SiMe<INF>2</INF>OSiMe<INF>3</INF>)
1,2-CF bond activation of perfluoroarenes and alkylidene isomers of titanium. DFT analysis of the C–F bond activation pathway and rotation of the titanium alkylidene moiety
No full textIsomeric alkylidene complexes syn- and anti-(PNP)Ti=[C(t)Bu(C(6)F(5))](F) (1) and (PNP)Ti=[C(t)Bu(C(7)F(7))](F) (2) have been generated from C-F bond addition of hexafluorobenzene (C(6)F(6)) and octafluorotoluene (C(7)F(8)) across the alkylidyne ligand of transient (PNP)Ti=C(t)Bu (A) (PNP(-)=N[2-P(CHMe(2))(2)-4-methylphenyl](2)), which was generated from the precursor (PNP)Ti=CH(t)Bu(CH(t)(2)Bu). Two mechanistic scenarios for the activation of the C-F bond by A are considered: 1,2-CF addition and [2 + 2]-cycloaddition/beta-fluoride elimination. Upon formation of the alkylidenes 1 and 2, the kinetic and thermodynamic alkylidene product is the syn isomer, which gradually isomerizes to the corresponding anti isomer to ultimately establish an equilibrium mixture (when using 1, 65/35) if the solution is heated in benzene to 105 degrees C for 1 h. Single crystal X-Ray crystallographic data obtained for the two isomers of 2 (and syn isomer of 1) are in good agreement with computed DFT-optimized models. Our calculations suggest convincingly that the isomerization process proceeds via a concerted rotation involving a heterolytic bond cleavage about the alkylidene bond. The two rotamers are thermodynamically very close in energy and interconvert with an estimated barrier of similar to 26 kcal/mol. The electronic reason for this unexpectedly low barrier is investigated. (C) 2011 Elsevier B.V. All rights reserved
A Co2N2 diamond-core resting state of cobalt(I): A three-coordinate Co-I synthon invoking an unusual pincer-type rearrangement
No full textpi(radical)-pi(radical) bonding interactions generated by halogen oxidation of zirconium(IV) redox-active ligand complexes
No full textThe new complex, [Zr(pda)(2)](n) (1, pda(2-) = N,N(')-bis(neo-pentyl)-ortho-phenylenediamide, n = 1 or 2), prepared by the reaction of 2 equiv of pdaLi(2) with ZrCl(4), reacts rapidly with halogen oxidants to afford the new product ZrX(2)(disq)(2) (3, X = Cl, Br, I; disq(-) = NAr-bis(neo-pentyl)-ortho-diiminosemiquinonate) in which each redox-active ligand has been oxidized by one electron. The oxidation products 3a-c have been structurally characterized and display an unusual parallel stacked arrangement of the disq(-) ligands in the solid state, with a separation of similar to 3 angstrom. Density functional calculations show a bonding-type interaction between the SOMOs of the disq- ligands to form a unique HOMO while the antiboncling linear combination forms a unique LUMO. This orbital configuration leads to a closed-shell-singlet ground-state electron configuration (S = 0). Temperature-dependent magnetism measurements indicate a low-lying triplet excited state at similar to 750 cm(-1). In solution, 3a-c show strong disq(-)-based absorption bands that are invariant across the halide series. Taken together these spectroscopic measurements provide experimental values for the one- and two-electron energies that characterize the pi-stacked bonding interaction between the two disqligands
A Transient Vanadium(III) Neopentylidene Complex. Redox Chemistry and Reactivity of the V=(CHBu)-Bu-t Functionality
No full textThe vanadium(III) bis(neopentyl) complex (PNP)V((CH2Bu)-Bu-t)(2) (PNP = N[4-Me-2-((PPr2)-Pr-i)C6H3](2)(-)), a complex readily prepared from alkylation of (PNP)VCl2 with 2 equiv of LiCH2'Bu, serves as a precursor to the transient vanadium(III) alkylidene complex "(PNP)V=(CHBu)-Bu-t". Two-electron oxidation of the intermediate [(PNP)V=CH'Bu] with chalcogen sources results in formation of the vanadium(V) chalcogenide series (PNP)V=(CHBu)-Bu-t(X) (X = O, S, Se, Te). This family of chalcogenide-alkylidenes has been studied via V-51 NMR spectroscopy in combination with DFr computational methods. The redox chemistry of [(PNP)V=CH'Bu] and the reactivity of the alkylidene ligand are explored with the substrates N3SiMe3, N -(CBu)-Bu-t, N-2, and azobenzene. It was discovered that N=N cleavage of the last substrate can be achieved without oxidation of the metal
Understanding intermolecular C–F bond activation by a transient titanium neopentylidyne: experimental and theoretical studies on the competition between 1,2-CF bond addition and [2 + 2]-cycloadditionβ-fluoride elimination
No full textComplex (PNP)Ti=(CHBu)-Bu-t((CH2Bu)-Bu-t) (PNP- = N[2-P(CHMe2)(2)-4-methylphenyl](2)) eliminates (H3CBu)-Bu-t to form transient (PNP)Ti (CBu)-Bu-t, which activates the C-F bond of ortho-difluoropyridine and ortho-fluoropyridine to form the alkylidene-fluoride complexes, (PNP)Ti=C[Bu-t(NC5H3F)](F) (1) and (PNP)Ti=C[Bu-t(NC5H4)](F) (2), respectively. When (PNP)Ti=(CHBu)-Bu-t((CH2Bu)-Bu-t) is treated with meta-fluoropyridine, the ring-opened product (PNP)Ti(C(Bu-t)CC4H3-3-FNH) (3) is the only recognizable titanium metal complex formed. Theoretical studies reveal that pyridine binding disfavors 1,2-CF bond addition across the alkylidyne ligand in the case of ortho-fluoride pyridines, while sequential [2 + 2]-cycloaddition/beta-fluoride elimination is a lower energy pathway. In the case of meta-fluoropyridine, [2 + 2]-cycloaddition and subsequent ring-opening metathesis is favored as opposed to C-H bond addition or sequential [2 + 2]-cycloaddition/beta-hydride elimination. In all cases, C-H bond addition of ortho-fluoropyridines or meta-fluoropyridine is discouraged because such substrate must bind to titanium via its C-H bond, which is rather weak compared to the titanium-pyridine binding
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