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Christopher R Clough

Publications and source records attributed to Christopher R Clough.

6 recordsLinked to original sources
PubMed2006

A nitridoniobium(V) reagent that effects acid chloride to organic nitrile conversion: synthesis via heterodinuclear (Nb/Mo) dinitrogen cleavage, mechanistic insights, and recycling.

The transformation of acid chlorides (RC(O)Cl) to organic nitriles (RC[triple bond]N) by the terminal niobium nitride anion [N[triple bond]Nb(N[Np]Ar)3]- ([1a-N]-, where Np = neopentyl and Ar = 3,5-Me2C6H3) via isovalent N for O(Cl) metathetical exchange is presented. Nitrido anion [1a-N]- is obtained in a heterodinuclear N2 scission reaction employing the molybdenum trisamide system, Mo(N[R]Ar)3 (R = t-Bu, 2a; R = Np, 2b), as a reaction partner. Reductive scission of the heterodinuclear bridging N2 complexes, (Ar[R]N)3Mo-(mu-N2)Nb(N[Np]Ar)3 (R = t-Bu, 3b; R = Np, 3c) with sodium amalgam provides 1 equiv each of the salt Na[1a-N] and neutral N[triple bond]Mo(N[R]Ar)3 (R = t-Bu, 2a-N; R = Np, 2b-N). Separation of 2-N from Na[1a-N] is readily achieved. Treatment of salt Na[1a-N] with acid chloride substrates in tetrahydrofuran (THF) furnishes the corresponding organic nitriles concomitant with the formation of NaCl and the oxo niobium complex O[triple bond]Nb(N[Np]Ar)3 (1a-O). Utilization of 15N-labeled 15N2 gas in this chemistry affords a series of 15N-labeled organic nitriles establishing the utility of anion [1a-N]- as a reagent for the 15N-labeling of organic molecules. Synthetic and computational studies on model niobium systems provide evidence for the intermediacy of both a linear acylimido and niobacyclobutene species along the pathway to organic nitrile formation. High-yield recycling of oxo 1a-O to a niobium triflate complex appropriate for heterodinuclear N2 scission has been developed. Specifically, addition of triflic anhydride (Tf2O, where Tf = SO2CF3) to an Et2O solution of 1a-O provides the bistriflate complex, Nb(OTf)2(N[Np]Ar)3 (1a-(OTf)2), in near quantitative yield. One-electron reduction of 1a-(OTf)2 with either cobaltocene (Cp2Co) or Mg(THF)3(anthracene) provided the monotriflato complex, Nb(OTf)(N[Np]Ar)3 (1a-(OTf)), which efficiently regenerates complexes 3b and 3c when treated with the molybdenum dinitrogen anions [N2Mo(N[t-Bu]Ar)3]- ([2a-N2]-) or [N2Mo(N[Np]Ar)3]- ([2b-N2]-), respectively.

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PubMed2005

Solution calorimetric and stopped-flow kinetic studies of the reaction of *Cr(CO)3C5Me5 with PhSe-SePh and PhTe-TePh. Experimental and theoretical estimates of the Se-Se, Te-Te, H-Se, and H-Te bond strengths.

The kinetics of the oxidative addition of PhSeSePh and PhTeTePh to the stable 17-electron complex *Cr(CO)3C5Me5 have been studied utilizing stopped-flow techniques. The rates of reaction are first-order in each reactant, and the enthalpy of activation decreases in going from Se (deltaH(double dagger) = 7.0 +/- 0.5 kcal/mol, deltaS(double dagger) = -22 +/- 3 eu) to Te (deltaH(double dagger) = 4.0 +/- 0.5 kcal/mol, deltaS(double dagger) = -26 +/- 3 eu). The kinetics of the oxidative addition of PhSeH and *Cr(CO)3C5Me5 show a change in mechanism in going from low (overall third-order) to high (overall second-order) temperatures. The enthalpies of the oxidative addition of PhE-EPh to *Cr(CO)3C5Me5 in toluene solution have been measured and found to be -29.6, -30.8, and -28.9 kcal/mol for S, Se, and Te, respectively. These data are combined with enthalpies of activation from kinetic studies to yield estimates for the solution-phase PhE-EPh bond strengths of 46, 41, and 33 kcal/mol for E = S, Se, and Te, respectively. The corresponding Cr-EPh bond strengths are 38, 36, and 31 kcal/mol. Two methods have been used to determine the enthalpy of hydrogenation of PhSeSePh in toluene on the basis of reactions of HSPh and HSePh with either *Cr(CO)3C5Me5 or 2-pyridine thione. These data lead to a thermochemical estimate of 72 kcal/mol for the PhSe-H bond strength in toluene solution, which is in good agreement with kinetic studies of H atom transfer from HSePh at higher temperatures. The reaction of H-Cr(CO)3C5Me5 with PhSe-SePh is accelerated by the addition of a Cr radical and occurs via a rapid radical chain reaction. In contrast, the reaction of PhTe-TePh and H-Cr(CO)3C5Me5 does not occur at any appreciable rate at room temperature, even in the presence of added Cr radicals. This is in keeping with a low PhTe-H bond strength blocking the chain and implies that H-TePh < or = 63 kcal/mol. Structural data are reported for PhSe-Cr(CO)3C5Me5 and PhS-Cr(CO)3C5Me5. The two isostructural complexes do not show signs of an increase in steric strain in terms of metal-ligand bonds or angles as the Cr-EPh bond is shortened in going from Se to S. Bond strength estimates of the PhE-H and PhE-EPh derived from density functional theory calculations are in reasonable agreement with experimental data for E = Se but not for E = Te. The nature of the singly occupied molecular orbital of the *EPh radicals is calculated to show increasing localization on the chalcogenide atom in going from S to Se to Te.

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PubMed2004

Organic nitriles from acid chlorides: an isovalent N for (O)CL exchange reaction mediated by a tungsten nitride complex.

Nitride NW(N[i-Pr]Ar)3 (1, Ar = 3,5-C6H3Me2) was synthesized in two steps from known NW(O-t-Bu)3 (41% overall yield). Complex 1 is the tungsten congener of NMo(N[i-Pr]Ar)3, a known molecule that has been synthesized using N2 as the nitrido nitrogen source, but which undergoes no reaction with pivaloyl chloride. Compound 1 undergoes metathesis with pivaloyl chloride at 25 degrees C to form the corresponding nitrile in 97% yield. Another substrate examined in this work was the labeled acid chloride 1-Ad13C(O)Cl (Ad = adamantyl). The "(O)Cl" moiety is transferred to tungsten forming an oxo-chloride, (Ar[i-Pr]N)3W(O)Cl (3), as the final tungsten product; both 1 and 3 were characterized structurally by X-ray diffraction. An intermediate observed in the nitrile-forming reaction was characterized spectroscopically to be a tungsten acylimido complex. The latter assignment was substantiated by the synthesis and structural characterization of the compound (Ar[i-Pr]N)3W(NC(O)CF3)(O2CCF3) (2m). In addition, density functional theory calculations performed using ADF lent insight into the thermochemistry of the overall process.

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PubMed2002

Interactions of aziridines with nickel complexes: oxidative-addition and reductive-elimination reactions that break and make C-N bonds.

Reaction of the N-tosylaziridines (p-CH(3)C(6)H(4)SO(2))NCH(2)CHR (1a, R = H; 1b, R = Me; 1c, R = n-Bu; 1d, R = i-Pr) with (bpy)Ni(cod) (2; bpy = 2,2'-bipyridine; cod = 1,5-cyclooctadiene) or (bpy)NiEt(2) (3) results in elimination of cod or butane from 2 and 3, respectively, and oxidative addition of an aziridine C-N bond to give the azametallacyclobutane complexes (bpy)Ni(NTosCHRCH(2)) (4a, R = H; 4b, R = Me; 4c, R = n-Bu; 4d, R = i-Pr) as maroon solids in 50-70% isolated yields. The structure of 4b exhibits a puckered four-membered azametallacycle containing a pyramidal nitrogen and with Ni-N(1) = 1.911(5) A; the tosyl group on N and the methyl substituent on the adjacent C are disposed in an anti conformation. The monodeuterated aziridine syn-(p-CH(3)C(6)H(4)SO(2))NCHDCH-n-Bu (1e) reacts with either 2 or 3 to give (bpy)Ni[NTosCH(n-Bu)CHD] (4e) in 60-65% yield, having an anti arrangement of the methine and methylene protons in the azametallacycle, and indicates that >95% inversion of stereochemistry has occurred at the methylene carbon during the oxidative-addition reaction. When the azametallacyclobutane complexes 4a-e are exposed to oxygen, oxidatively induced reductive elimination ensues, giving the free aziridines in 30-60% isolated yields. In the oxidation of 4e, the product aziridine is spectroscopically identical to its parent, 1e, indicating the elimination that forms the C-N bond also proceeds with inversion of stereochemistry (approximately 92% by (1)H NMR) at the methylene carbon.

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