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Kenneth G Caulton

Publications and source records attributed to Kenneth G Caulton.

At least 19 recordsLinked to original sources
PubMed2005

[(tBu2PCH2SiMe2)2N]RuCH3: the origin of extremely facile, double H-C(sp3) activation generating a "hydrido-carbene" complex.

The four-coordinate compound [(tBu2PCH2SiMe2)2N]RuCH3 undergoes rapid double H-C(sp3) activation at -78 degrees C to generate a "hydrido-carbene" complex. DFT calculations suggest that the origin of the low barrier to methane elimination is an alpha-agostic interaction in the low-lying singlet state of the highly unsaturated (PNP)RuMe. The hydrido-carbene complex can be viewed as a "masked" resting state of the four-coordinate cyclometalated alkyl complex, [(tBu2PCH2SiMe2)N(Me2SiCH2P(tBu)(C(CH3)2CH2)]Ru, where hydride migration from metal to carbon occurs before any subsequent reactivity.

Journal Article↗
PubMed2005

Exceptionally facile CO addition to a saturated ruthenium complex.

Synthesis and characterization of Cp*Ru[eta3-HC(PPh2NPh)2], 1, reveals it to have a "piano stool" structure with the ligand bound to Ru(II) via two N and the unique, sp3 hybridized carbon. While the analogous (cymene) Ru[eta3-HC(PPh2NPh)2]+ does not react with CO, under the same conditions, 1 adds one CO rapidly (25 degrees C, 1 atm CO). Characterization, including an X-ray structure determination, shows that CO has displaced one chelate ligand nitrogen, which then hangs off the molecule, free of Ru. DFT calculations reveal a possible mechanism via a remarkably low energy (+9.3 kcal/mol) intermediate, pendant N, but with one phenyl on phosphorus stabilizing Ru via donation from a C(ipso)=C(ortho) bond. DFT calculations show that the electronic energy change for binding CO is over 20 kcal/mol less favorable for cymene than for C5Me5- as ligand; the reactivity difference is thus thermodynamic in origin.

Carbon Monoxide↗
PubMed2005

A facile approach to a d4 RuN: moiety.

Replacement of chloride in (PNP)RuCl, PNP = (tBu2PCH2SiMe2)2N, by Me3SiN3 gives a pre-redox adduct that, already at -30 degrees C, releases N2 to produce the mononuclear nonplanar Ru(IV) nitride (PNP)RuN, characterized by spectroscopic and X-ray methods. DFT calculations show the planar structure to be only 1.6 kcal/mol less stable, which explains the time-averaged simplicity of the 1H NMR spectrum, as well as the large vibrational amplitude of the nitride ligand.

Journal Article↗
PubMed2004

Is the allylpalladium structure altered between solid and solutions?

Recent EXAFS measurements on [(Ph(2)PCH(2)CH(2)PPh(2))Pd(H(2)CCHCMe(2))]O(3)SCF(3) (Tromp et al. J. Am. Chem. Soc. 2002, 124, 14814) were interpreted as evidence that, when the complex is dissolved in THF, the allyl ligand adopts an eta(2) structure with a dangling allyl CH(2) substituent. DFT calculations of the Pd complex using H(2)P-CH(2)CH(2)-PH(2) as a model for Ph(2)P-CH(2)CH(2)-PPh(2) (dppe), in the absence or the presence of the triflate counteranion, and modeling the THF solvent by explicit Me(2)O molecules or by a continuum model give always a conventional eta(3)-H(2)CCHCMe(2) structure with equal Pd-C bonds to the terminal carbon centers of the allyl. QM/MM calculations using the dppe ligand also fail to support an eta(2)-allyl structure as a global minimum. The EXAFS parameter space is shown to have multiple minima. These have very similar overall EXAFS, but have very different structural parameters. The minimum that was the basis for the previous structural conclusion gives a slightly better fit but has unrealistic Debye-Waller factors and threshold energies.

Alkenes↗
PubMed2004

Hydrocarbyl ligand "tuning" of the PtII/IV redox potential.

The potentially tridentate macrocycle [2.1.1]-(2,6)-pyridinophane (L) enables the transient LPt(II)(CH(3))(+) to cleave the C-H bond of two molecules of C(6)F(5)H. The resulting product has two aryl groups on Pt but, in contrast to nonfluorinated analogue, varies in its location of the cleaved H, as is evident from the two products (HL(+))Pt(II)R(2) and (eta(3)-L)Pt(IV)H(R)(2)(+). At equilibrium, the related example where R = CH(3) is purely the Pt(IV) redox isomer, which with R = C(6)H(5) shows detectable populations of both isomers, and with R = C(6)F(5) is purely the pyridine-protonated (HL(+))Pt(II) redox isomer. All species show a hydrogen bond from the pyridinium proton to Pt(II). Consistent with the idea that electron-withdrawing R makes platinum(II) more resistant to oxidation (i.e., a proton on Pt), and thus less Brønsted basic, the (1)J(PtH) coupling constant falls in the series R = Me (90 Hz), R = C(6)H(5) (86 Hz), and R = C(6)F(5) (63 Hz).

Journal Article↗
PubMed2004

Transformation of acyclic alkenes to hydrido carbynes by (PNP(R))Re complexes.

Synthesis of (PNP(R))ReOCl(2) (PNP(R) = (R(2)PCH(2)SiMe(2))(2)N, R = (i)()Pr, Cy, and (t)()Bu) from (Me(2)S)(2)ReOCl(3) and (PNP(R))MgCl is described. Magnesium and H(2) convert (PNP(R))ReOCl(2) first to (PNP(R))ReO(H)(2) and then to (PNP(R))Re(H)(4), the last being an operationally unsaturated species which can bind PMe(3) or p-toluidine. Acyclic alkenes react with (PNP(R))Re(H)(4) at 22 degrees C to give first (PNP(R))Re(H)(2)(olefin) and then (PNP(R))ReH(carbyne), in equilibrium with its eta(2)-olefin adduct. Re can also migrate to the terminal carbon of internal olefins to form a carbyne complex. Allylic C-SiMe(3) or C-NH(2) bonds are not broken, but OEt, OPh, and F vinyl substituents (X) are ultimately cleaved from carbon to give the ReC-CH(3) complex and liberate HX. DFT calculations, together with detection of intermediates for certain olefins, help to define a mechanism for these conversions.

Journal Article↗
PubMed2004

Multifunctional behavior by a Bis-(phosphinimino)methanide ligand: eta 2- vs eta 3-coordination vs Bronsted basicity.

The reaction of [(Cymene)RuCl2]2 with the chelate LiHC(PPh2NPh)2 occurs to remove both chloride ligands, to furnish a cationic Ru(II) complex with the monoanionic ligand bound eta3, through two N and an sp3 carbon. This cation is also produced from the conjugate acid of the ligand H2C(PPh2NPh)2 because this molecule can serve as a Brønsted base, to deprotonate the acidic carbon of another molecule of H2C(PPh2NPh)2. DFT calculations show an energy surface where (Cymene)RuHC(PPh2NPh)2L is more stable with a Ru-CH(PPh2NPh)2 bond and with L = Cl- or MeCN not coordinated to Ru, than to an eta2-HC(PPh2NPh)2 structure with coordinated L; this is tested experimentally. The greater tendency for this ligand to be coordinated eta3 vs analogous diketiminates is discussed. The nucleophilicity of Cgamma in structure 1, vs that of donors L = Cl- or MeCN, is evaluated to understand the preference of the bis(phosphinimino)methanide to be bidentate or tridentate.

Journal Article↗
PubMed2004

Aromatic vs aliphatic C-H cleavage of alkyl-substituted pyridines by (PNPiPr)Re compounds.

Both (PNP)Re(H)(4) and (PNP)ReH(cyclooctyne) (PNP(i)(Pr) = ((i)Pr(2)PCH(2)SiMe(2))(2)N) react with alkylpyridines NC(5)H(4)R to give first (PNP)ReH(2)(eta(2)-pyridyl) and cyclooctene and then, when not sterically blocked, (PNP)Re(eta(2)-pyridyl)(2) and cyclooctane. The latter are shown by NMR, X-ray diffraction, and DFT calculations to have several energetically competitive isomeric structures and pyridyl N donation in preference to PNP amide pi-donation. DFT studies support NMR solution evidence that the most stable bis pyridyl structure is one that is doubly eta(2)- with the pyridyl N donating to the metal center. When both ortho positions carry methyl substituents, cyclooctane and the carbyne complex (PNP)ReH(tbd1;C-pyridyl) are produced. Excess 2-vinyl pyridine reacts with (PNP)Re(H)(4) preferentially at the vinyl group, to give 2-ethyl pyridine and the sigma-vinyl complex (PNP)ReH[eta(2)-CH=CH(2-py)]. The DFT and X-ray structures show, by various comparisons, the ability of the PNP amide nitrogen to pi-donate to an otherwise unsaturated d(4) Re(III) center, showing short Re-N distances consistent with the presence of pi-donation.

Journal Article↗
PubMed2004

Reactivity of the hydrido/nitrosyl radical MHCl(NO)(CO)(P(i)Pr(3))(2), M = Ru, Os.

The reaction of equimolar NO with the 16 electron molecule RuHCl(CO)L(2) (L = P(i)Pr(3)) proceeds, via a radical adduct RuHCl(CO)(NO) L(2), onward to form RuCl(NO)(CO)L(2) (X-ray structure determination) and RuHCl(HNO)(CO)L(2), in a 1:1 mole ratio. The HNO ligand, bound by N and trans to hydride, is rapidly degraded by excess NO. The osmium complex behaves analogously, but the adduct has a higher formation constant, permitting determination of its IR spectrum; both MHCl(CO)(NO)L(2) radicals are characterized by EPR spectroscopy, and DFT calculations on the Ru system show it to have a "half-bent" Ru-N-O unit with the spin density mainly on nitrogen. DFT (PBE) energies rule out certain possible mechanistic steps for forming the two products. A survey of the literature leads to the hypothesis that NO should generally be considered as a (neutral) Lewis base (2-electron donor) when it binds to a 16 electron complex which is resistant to oxidation or reduction, and that the resulting N-centered radical has a M-N-O angle of approximately 140 degrees, which distinguishes it from NO(-) (bent at <140 degrees ) and from NO(+) (>170 degrees ).

Journal Article↗
PubMed2003

A pi-basic rhenium center that effects cyclohexene isomerization to a beta-agostic carbene ligand.

The molecule (PNPR)Re(H)4 (PNPR = (R2PCH2SiMe2)2N, R = iPr or cyclohexyl) reacts at 20 degrees C with 2 mol of cyclohexene to form equimolar cyclohexane and (PNPR)Re(H)2[=C(CH2)5]. This product is characterized by 1H, 13C, and 31P NMR and by X-ray diffraction as having one CH2 hydrogen (from a carbon located beta to Re) donating to the metal ("agostic CH"). This interaction occurs in preference to PNPR amide nitrogen pi-donation. DFT calculations confirm this agostic interaction, and show that the (PNPR)Re(H)2 fragment indeed reverses the greater stability of free olefin vs free carbene.

Journal Article↗
PubMed2003

Angular ligand constraint yields an improved olefin aziridination catalyst.

[reaction: see text] The use of a pyridinophane, a macrocycle composed of three pyridines linked, via all ortho positions through CH(2) or CH(2)CH(2) groups, bound to copper, gives good performance (rate and yield) catalyzing the conversion of substituted aliphatic olefins and PhINTs to aziridines. Advantages also derive from using CH(2)Cl(2) solvent and the weakly coordinating anions BAr(4)(-) (Ar = C(6)H(5) or 3,5-C(6)H(3)(CF(3))(2)). Reactions are complete in minutes at 20 degrees C, and yields are almost quantitative for olefins not bearing secondary allylic CH bonds; however, cis-cyclooctene gives only the aziridine despite the allylic hydrogens.

Journal Article↗
PubMed2003

Four-coordinate, planar RuII. A triplet state as a response to a 14-valence electron configuration.

The ligand (tBu2PCH2SiMe2)2N1- (PNP) in [PNP]RuCl leads to an intermediate spin ground state, S = 1, which has been characterized by NMR and X-ray diffraction as having a planar structure. This spin state is attributed in part to N --> Ru pi donation. DFT calculations confirm that the singlet state lies higher in energy and is nonplanar. The molecule is converted to a diamagnetic product by addition of 2 mol of PhCN. The half-filled orbitals of the S = 1 state are suggested to be the reason agostic interactions do not compensate for the 14-valence electron count.

Journal Article↗
PubMed2003

Design and synthesis of tridentate facially chelating ligands of the [2.n.1]-(2,6)-pyridinophane family.

Syntheses are reported for tripyridine macrocycles 2 and 3 and some of their alkyl derivatives. The macrocycles are designed to stabilize to various extents coordinated d(8) metal precursors and d(6) alkane oxidative addition products (Pt(IV)), therefore allowing favorable kinetics and thermodynamics of (e.g., Pt(II)) the cleavage of substrate H-C(sp(3)) bonds. Both the Chichibabin protocol and oxidative coupling of carbanions by copper(I) iodide were used for the macrocyclization step. Crystal structures of singly and doubly protonated 2 establish atom connectivity in the macrocycle, and reveal structural features which are obscured in solution NMR by rapid proton migration.

Journal Article↗
PubMed2003

N-PtIV-H/N-H...PtII intramolecular redox equilibrium in a product of H-C(sp2) cleavage and unusual alkane/arene C-H bond selectivity of ([2.1.1]pyridinophane)PtII(CH3)+.

T-shaped 14 valence electron (eta2-L)PtMe+ (based on DFT geometry optimization, L = [2.1.1]-2,6-pyridinophane) reacts with benzene to give (eta3-L) PtIV(Ph)2H+ and methane; the latter cation is in thermal equilibrium with the N-protonated PtII tautomer (eta2-L-H)Pt(Ph)2+, and these complexes react with ethane or cyclopentane to produce benzene and (L)PtH(olefin)+.

Journal Article↗
PubMed2002

[N.1.1]-(2,6)-Pyridinophanes: a new ligand type imposing unusual metal coordination geometries.

A series of new ligands with three pyridines linked into a macrocycle by various CH(2), CMe(2), and CH(2)CH(2) groups at all sites ortho to the pyridine nitrogen have been synthesized and attached to PdCl(2) or PtMe(2). The ligands bind to them through only two nitrogens, and the third pyridine is constrained in close proximity to the planar complex with a filled d(z)2 orbital. Rapid reversible migration of PdCl(2) or PtMe(2) to the unused pyridine nitrogen is observed at 20 degrees C in the case of the CH(2)-bridged macrocycle and does not occur in the case of the CMe(2)-bridged analogue, and the mechanism of this fluxionality has been established by NMR and computational techniques.

Journal Article↗
PubMed2002

Vinyl C-F cleavage by Os(H)3Cl(P(i)Pr3)2.

Os(H)(3)ClL(2) (L = P(i)Pr(3)) reacts at 20 degrees C with vinyl fluoride in the time of mixing to produce OsHFCl([triple bond]CCH(3))L(2) and H(2). In a competitive reaction, the liberated H(2) converts vinyl fluoride to C(2)H(4) and HF in a reaction catalyzed by Os(H)(3)ClL(2). A variable-temperature NMR study reveals these reactions proceed through the common intermediate OsHCl(H(2))(H(2)C=CHF)L(2), via OsClF(=CHMe)L(2) and OsHCl(H(2))(C(2)H(4))L(2), all of which are detected. DFT(B3PW91) calculations of the potential energy and free energy at 298 K of possible intermediates show the importance of entropy to account for their thermodynamic accessibility. Calculations of unimolecular C-F cleavage of coordinated C(2)H(3)F confirms the high activation energy of this process. Catalysis by HF is thus suggested to account for the fast observed reactions, and scavenging of HF with NEt(3) changes the product to exclusively Os(H)(2)Cl(CCH(3))L(2). The analogous reaction of Os(H)(3)ClL(2) with H(2)C=CF(2) produces exclusively OsHFCl(=CCH(3))L(2) and HF, and the latter is again suggested to catalyze C-F scission via the observed intermediates Os(H)(2)Cl(CF(2)CH(3))L(2) and OsHCl(=CFMe)L(2).

Journal Article↗
PubMed2002

Coordination chemistry of tripyridinedimethane.

The ligand tripyridinedimethane (tpdm), consisting of three pyridine residues linked at their ortho carbons by two CH(2) groups, is shown to be a sterically flexible ligand capable of binding in a meridional arrangement in trigonal bipyramidal (tpdm) Cu(II)Cl(2) but binding in a facial arrangement in tetrahedral (tpdm) Cu(I)Cl. Nucleophilic substitution of chloride by (t)BuO(-) and PhC[triple bond]C(-) is possible, and deprotonation of the acidic benzylic protons does not take place because the resulting carbanion cannot achieve coplanarity with the aryl rings. RhCl(3) forms, with tpdm in boiling methanol, a 1:1 kinetic mixture of fac- and mer-isomers RhCl(3)(tpdm). The former isomerizes slowly at RT (room temperature) in DMSO solution into the latter with Rh-N bond dissociation as the rate-determining step.

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