Trends in Homolytic Bond Dissociation Energies of Five- and Six-Coordinate Hydrides of Group 9 Transition Metals: Co, Rh, Ir.

The homolytic bond dissociation energies of a series of five- and six-coordinate mono- and dihydride complexes of the type HM(diphosphine)2 and [H2M(diphosphine)2]+ (where M = Co, Rh, and Ir) are calculated and compared with experimental values. This work probes the relationship between the homolytic bond dissociation energies (HMBDEs) of these complexes in these two different coordination environments and formal oxidation states. The results of these calculations and previous experimental observations suggest that for M = Rh the HMBDE of the five-coordinate HM(diphosphine)2 species are 0-2 kcal/mol larger than the HMBDE of the corresponding six-coordinate [H2M(diphosphine)2]+ species. For M = Ir the bond energies of the five- and six-coordinate complexes are nearly the same and for M = Co the six-coordinate species are 1-5 kcal/mol less than the corresponding five-coordinate species. Simplified models of large and complicated ligands seem to capture the essential trends and give very good estimates of these thermodynamic properties compared with experimentally available data that are difficult to obtain.

Comproportionation of cationic and anionic tungsten complexes having an N-heterocyclic carbene ligand to give the isolable 17-electron tungsten radical CpW(CO)2(IMes)(•).

A series consisting of a tungsten anion, radical, and cation, supported by the N-heterocyclic carbene 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) and spanning formal oxidation states W(0), W(I), and W(II), has been synthesized, isolated, and characterized. Reaction of the hydride CpW(CO)(2)(IMes)H with KH and 18-crown-6 gives the tungsten anion [CpW(CO)(2)(IMes)](-)[K(18-crown-6)](+). Electrochemical oxidation of [CpW(CO)(2)(IMes)](-) in MeCN (0.2 M (n)Bu(4)N(+)PF(6)(-)) is fully reversible (E(1/2) = -1.65 V vs Cp(2)Fe(+•/0)) at all scan rates, indicating that CpW(CO)(2)(IMes)(•) is a persistent radical. Hydride transfer from CpW(CO)(2)(IMes)H to Ph(3)C(+)PF(6)(-) in MeCN affords [cis-CpW(CO)(2)(IMes)(MeCN)](+)PF(6)(-). Comproportionation of [CpW(CO)(2)(IMes)](-) with [CpW(CO)(2)(IMes)(MeCN)](+) gives the 17-electron tungsten radical CpW(CO)(2)(IMes)(•). This complex shows paramagnetically shifted resonances in the (1)H NMR spectrum and has been characterized by IR spectroscopy, low-temperature EPR spectroscopy, and X-ray diffraction. CpW(CO)(2)(IMes)(•) is stable with respect to disproportionation and dimerization. NMR studies of degenerate electron transfer between CpW(CO)(2)(IMes)(•) and [CpW(CO)(2)(IMes)](-) are reported. DFT calculations were carried out on CpW(CO)(2)(IMes)H, as well as on related complexes bearing NHC ligands with N,N' substituents Me (CpW(CO)(2)(IMe)H) or H (CpW(CO)(2)(IH)H) to compare to the experimentally studied IMes complexes with mesityl substituents. These calculations reveal that W-H homolytic bond dissociation energies (BDEs) decrease with increasing steric bulk of the NHC ligand, from 67 to 64 to 63 kcal mol(-1) for CpW(CO)(2)(IH)H, CpW(CO)(2)(IMe)H, and CpW(CO)(2)(IMes)H, respectively. The calculated spin density at W for CpW(CO)(2)(IMes)(•) is 0.63. The W radicals CpW(CO)(2)(IMe)(•) and CpW(CO)(2)(IH)(•) are calculated to form weak W-W bonds. The weakly bonded complexes [CpW(CO)(2)(IMe)](2) and [CpW(CO)(2)(IH)](2) are predicted to have W-W BDEs of 6 and 18 kcal mol(-1), respectively, and to dissociate readily to the W-centered radicals CpW(CO)(2)(IMe)(•) and CpW(CO)(2)(IH)(•).

Activation of the S-H group in Fe(mu(2)-SH)Fe clusters: S-H bond strengths and free radical reactivity of the Fe(mu(2)-SH)Fe cluster.

Absolute rate constants were determined for the abstraction of hydrogen atoms from (OC)(3)Fe(mu-SH)(2)Fe(CO)(3) (Fe(2)S(2)H(2)) and (OC)(3)Fe(mu-SCH(3))(mu-SH)Fe(CO)(3) (Fe(2)S(2)MeH) by benzyl radicals in benzene. From the temperature-dependent rate data for Fe(2)S(2)H(2), DeltaH(++) and DeltaS(++) were determined to be 2.03 +/- 0.56 kcal/mol and -19.3 +/- 1.7 cal/(mol K), respectively, giving k(abs) = (1.2 +/- 0.49) x 10(7) M(-1) s(-1) at 25 degrees C. For Fe(2)S(2)MeH, DeltaH(++) and DeltaS(++) were determined to be 1.97 +/- 0.46 kcal/mol and -18.1 +/- 1.5 cal/(mol K), respectively, giving k(abs) = (2.3 +/- 0.23) x 10(7) M(-1) s(-1) at 25 degrees C. Temperature-dependent rate data are also reported for hydrogen atom abstraction by benzyl radical from thiophenol (DeltaH(++) = 3.62 +/- 0.43 kcal/mol, DeltaS(++) = -21.7 +/- 1.3 cal/(mol K)) and H(2)S (DeltaH(++) = 5.13 +/- 0.99 kcal/mol, DeltaS(++) = -24.8 +/- 3.2 cal/(mol K)), giving k(abs) at 25 degrees C of (2.5 +/- 0.33) x 10(5) and (4.2 +/- 0.51) x 10(3) M(-1) s(-1), respectively, both having hydrogen atom abstraction rate constants orders of magnitude slower than those of Fe(2)S(2)H(2) and Fe(2)S(2)MeH. Thus, Fe(2)S(2)MeH is 100-fold faster than thiophenol, known as a fast donor. All rate constants are reported per abstractable hydrogen atom (k(abs)/M(-1) s(-1)/H). DFT calculations predict S-H bond strengths of 73.1 and 73.2 kcal/mol for Fe(2)S(2)H(2) and Fe(2)S(2)MeH, respectively. Free energy and NMR chemical shift calculations confirm the NMR assignments and populations of Fe(2)S(2)H(2) and Fe(2)S(2)MeH isomers. Derived radicals Fe(2)S(2)H(*) and Fe(2)S(2)Me(*) exhibit singly occupied HOMOs with unpaired spin density distributed between the two Fe atoms, a bridging sulfur, and d(sigma)-bonding between Fe centers. The S-H solution bond dissociation free energy (SBDFE) of Fe(2)S(2)MeH was found to be 69.4 +/- 1.7 kcal/mol by determination of its pK(a) (16.0 +/- 0.4) and the potential for the oxidation of the anion, Fe(2)S(2)Me(-), of -0.26 +/- 0.05 V vs ferrocene in acetonitrile (corrected for dimerization of Fe(2)S(2)Me(*)). This SBDFE for Fe(2)S(2)MeH corresponds to a gas-phase bond dissociation enthalpy (BDE) of 74.2 kcal/mol, in satisfactory agreement with the DFT value of 73.2 kcal/mol. Replacement of the Fe-Fe bond in Fe(2)S(2)MeH with bridging mu-S (Fe(2)S(3)MeH) or mu-CO (Fe(2)S(2)(CO)MeH) groups leads to (DFT) BDEs of 72.8 and 66.2 kcal/mol, the latter indicating dramatic effects of the choice of bridge structure on S-H bond strengths. These results provide a model for the reactivity of hydrosulfido sites of low-valent heterogeneous FeS catalysts.

Free energy landscapes for S-H bonds in Cp*2Mo2S4 complexes.

An extensive family of thermochemical data is presented for a series of complexes derived from Cp*Mo(mu-S)(2)(mu-SMe)(mu-SH)MoCp* and Cp*Mo(mu-S)(2)(mu-SH)(2)MoCp*. These data include electrochemical potentials, pK(a) values, homolytic solution bond dissociation free energies (SBDFEs), and hydride donor abilities in acetonitrile. Thermochemical data ranged from +0.6 to -2.0 V vs FeCp(2)(+/o) for electrochemical potentials, 5 to 31 for pK(a) values, 43 to 68 kcal/mol for homolytic SBDFEs, and 44 to 84 kcal/mol for hydride donor abilities. The observed values for these thermodynamic parameters are comparable to those of many transition metal hydrides, which is consistent with the many parallels in the chemistry of these two classes of compounds. The extensive set of thermochemical data is presented in free energy landscapes as a useful approach to visualizing and understanding the relative stabilities of all of the species under varying conditions of pH and H(2) overpressure. In addition to the previously studied homogeneous reactivity and catalysis, Mo(2)S(4) complexes are also models for heterogeneous molybdenum sulfide catalysts, and therefore, the present results demonstrate the dramatic range of S-H bond strengths available in both homogeneous and heterogeneous reaction pathways.

Trends in ground-state entropies for transition metal based hydrogen atom transfer reactions.

Reported herein are thermochemical studies of hydrogen atom transfer (HAT) reactions involving transition metal H-atom donors M(II)LH and oxyl radicals. [Fe(II)(H(2)bip)(3)](2+), [Fe(II)(H(2)bim)(3)](2+), [Co(II)(H(2)bim)(3)](2+), and Ru(II)(acac)(2)(py-imH) [H(2)bip = 2,2'-bi-1,4,5,6-tetrahydropyrimidine, H(2)bim = 2,2'-bi-imidazoline, acac = 2,4-pentandionato, py-imH = 2-(2'-pyridyl)imidazole)] each react with TEMPO (2,2,6,6-tetramethyl-1-piperidinoxyl) or (t)Bu(3)PhO(*) (2,4,6-tri-tert-butylphenoxyl) to give the deprotonated, oxidized metal complex M(III)L and TEMPOH or (t)Bu(3)PhOH. Solution equilibrium measurements for the reaction of [Co(II)(H(2)bim)(3)](2+) with TEMPO show a large, negative ground-state entropy for hydrogen atom transfer, -41 +/- 2 cal mol(-1) K(-1). This is even more negative than the DeltaS(o)(HAT) = -30 +/- 2 cal mol(-1) K(-1) for the two iron complexes and the DeltaS(o)(HAT) for Ru(II)(acac)(2)(py-imH) + TEMPO, 4.9 +/- 1.1 cal mol(-1) K(-1), as reported earlier. Calorimetric measurements quantitatively confirm the enthalpy of reaction for [Fe(II)(H(2)bip)(3)](2+) + TEMPO, thus also confirming DeltaS(o)(HAT). Calorimetry on TEMPOH + (t)Bu(3)PhO(*) gives DeltaH(o)(HAT) = -11.2 +/- 0.5 kcal mol(-1) which matches the enthalpy predicted from the difference in literature solution BDEs. A brief evaluation of the literature thermochemistry of TEMPOH and (t)Bu(3)PhOH supports the common assumption that DeltaS(o)(HAT) approximately 0 for HAT reactions of organic and small gas-phase molecules. However, this assumption does not hold for transition metal based HAT reactions. The trend in magnitude of |DeltaS(o)(HAT)| for reactions with TEMPO, Ru(II)(acac)(2)(py-imH) << [Fe(II)(H(2)bip)(3)](2+) = [Fe(II)(H(2)bim)(3)](2+) < [Co(II)(H(2)bim)(3)](2+), is surprisingly well predicted by the trends for electron transfer half-reaction entropies, DeltaS(o)(ET), in aprotic solvents. This is because both DeltaS(o)(ET) and DeltaS(o)(HAT) have substantial contributions from vibrational entropy, which varies significantly with the metal center involved. The close connection between DeltaS(o)(HAT) and DeltaS(o)(ET) provides an important link between these two fields and provides a starting point from which to predict which HAT systems will have important ground-state entropy effects.

Formation and reactivity of a persistent radical in a dinuclear molybdenum complex.

The reactivity of the S-H bond in Cp*Mo(mu-S) 2(mu-SMe)(mu-SH)MoCp* ( S 4 MeH) has been explored by determination of kinetics of hydrogen atom abstraction to form the radical Cp*Mo(mu-S) 3(mu-SMe)MoCp* ( S 4 Me*), as well as reaction of hydrogen with the radical-dimer equilibrium to reform the S-H complex. From the temperature dependent rate data for the abstraction of hydrogen atom by benzyl radical, Delta H (double dagger) and Delta S (double dagger) were determined to be 1.54 +/- 0.25 kcal/mol and -25.5 +/- 0.8 cal/mol K, respectively, giving k abs = 1.3 x 10 (6) M (-1) s (-1) at 25 degrees C. In steady state abstraction kinetic experiments, the exclusive radical termination product of the Mo 2S 4 core was found to be the benzyl cross-termination product, Cp*Mo(mu-S) 2(mu-SMe)(mu-SBz)MoCp* ( S 4 MeBz), consistent with the Fischer-Ingold persistent radical effect. S 4 Me* was found to reversibly dimerize by formation of a weak bridging disulfide bond to form the tetranuclear complex (Cp*Mo(mu-S) 2(mu-SMe)MoCp*) 2(mu-S 2) ( ( S 4 Me) 2 ). The radical-dimer equilibrium constant has been determined to be 5.7 x 10 (4) +/- 2.1 x 10 (4) M (-1) from EPR data. The rate constant for dissociation of the dimer was found to be 1.1 x 10 (3) s (-1) at 25 degrees C, based on variable temperature (1)H NMR data. The rate constant for dimerization of the radical has been estimated to be 6.5 x 10 (7) M (-1) s (-1) in toluene at room temperature, based on the dimer dissociation rate constant and the equilibrium constant for dimerization. Structures are presented for ( S 4 Me) 2 , S 4 MeBz, and the cationic Cp*Mo(mu-S 2)(mu-S)(mu-SMe)MoCp*(OTf) ( S 4 Me ( + )), a precursor of the radical and the alkylated derivatives. Evidence for a radical addition/elimination pathway at an Mo 2S 4 core is presented.

Activation of the sulfhydryl group by Mo centers: kinetics of reaction of benzyl radical with a binuclear Mo(micro-SH)Mo complex and with arene and alkane thiols.

This paper provides evidence from kinetic experiments and electronic structure calculations of a significantly reduced S-H bond strength in the Mo(micro-SH)Mo function in the homogeneous catalyst model, CpMo(micro-S)(2)(micro-SH)(2)MoCp (1, Cp = eta(5)-cyclopentadienyl). The reactivity of 1 was explored by determination of a rate expression for hydrogen atom abstraction by benzyl radical from 1 (log(k(abs)/M(-)(1) s(-)(1)) = (9.07 +/- 0.38) - (3.62 +/- 0.58)/theta) for comparison with expressions for CH(3)(CH(2))(7)SH, log(k(abs)/M(-)(1) s(-)(1)) = (7.88 +/- 0.35) - (4.64 +/- 0.54)/theta, and for 2-mercaptonaphthalene, log(k(abs)/M(-)(1) s(-)(1)) = (8.21 +/- 0.17) - (4.24 +/- 0.26)/theta (theta = 2.303RT kcal/mol, 2sigma error). The rate constant for hydrogen atom abstraction at 298 K by benzyl radical from 1 is 2 orders of magnitude greater than that from 1-octanethiol, resulting from the predicted (DFT) S-H bond strength of 1 of 73 kcal/mol. The radical CpMo(micro-S)(3)(micro-SH)MoCp, 2, is revealed, from the properties of slow self-reaction, and exclusive cross-combination with reactive benzyl radical, to be a persistent free radical.

Absolute rate constants for reactions of tributylstannyl radicals with bromoalkanes, episulfides, and alpha-halomethyl-episulfides, -cyclopropanes, and -oxiranes: new rate expressions for sulfur and bromine atom abstraction.

Arrhenius rate expressions were determined for the abstraction of bromine atom from 2-phenethyl bromide by tri-n-butylstannyl radical (Bu(3)Sn(*)) in benzene using transient absorption spectroscopy, (log(k(abs,Br)/M(-1) s(-1)) = (9.21 +/- 0.20) - (2.23 +/- 0.28)/theta, theta = 2.3RT kcal/mol, errors are 2sigma) and for the abstraction of sulfur atom from propylene sulfide to form propylene, (log(k(s)/M(-1) s(-1)) = (8.75 +/- 0.91) - (2.35 +/-1.33)/theta). Rate constants for reactions of organic bromides, RBr, with Bu(3)Sn(*) were found to vary as R = benzyl (15.6) > thiiranylmethyl (6.2) > oxiranylmethyl (3.1) > cyclopropylmethyl (1.3) > 2-phenethyl (1.0), with k(abs,Br) = 6.8 x 10(7) M(-1) s(-1) at 353 K for 2-phenethyl bromide. Bromine abstraction from alpha-bromomethylthiirane is about 7-fold faster than sulfur atom abstraction and is comparable to the reactivity of a secondary alkyl bromide. The potential surface for the vinylthiomethyl --> allylthiyl radical rearrangement at UB3LYP/6-31G(d) and UB3LYP/6-311+G(2d,2p) levels of theory suggests that the thiiranylmethyl radical is produced about 9 kcal/mol above the allylthiyl radical on the rearrangement surface, consistent with the observed enhancement of the Br atom abstraction from the thiirane and with synchronous C-S bond scission of the thiirane ring. The selectivities reported in this work for S vs Cl and Br abstraction provide applications for radical-based synthesis and new competition basis rate expressions for trialkylstannyl radicals.

Equilibrium acidities and homolytic bond dissociation energies of acidic C-H bonds in alpha-arylacetophenones and related compounds.

The equilibrium acidities (pK(AH)s) and the oxidation potentials of the congugate anions [E(ox)(A(-))s] were determined in dimethyl sulfoxide (DMSO) for eight ketones of the structure GCOCH(3) and 20 of the structure RCOCH(2)G, (where R = alkyl, phenyl and G = alkyl, aryl). The homolytic bond dissociation energies (BDEs) for the acidic C-H bonds of the ketones were estimated using the equation BDE(AH) = 1.37pK(AH) + 23.1E(ox)(A(-)) + 73.3. While the equilibrium acidities of GCOCH(3) were found to be dependent on the remote substituent G, the BDE values for the C-H bonds remained essentially invariant (93.5 +/- 0.5 kcal/mol). A linear correlation between pK(AH) values and [E(ox)(A(-))s] was found for the ketones. For RCOCH(2)G ketones, both pK(AH) and BDE values for the adjacent C-H bonds are sensitive to the nature of the substituent G. However, the steric bulk of the aryl group tends to exert a leveling effect on BDEs. The BDE of alpha-9-anthracenylacetophenone is higher than that of alpha-2-anthracenylacetophenone by 3 kcal/mol, reflecting significant steric inhibition of resonance in the 9-substituted system. A range of 80.7-84.4 kcal/mol is observed for RCOCH(2)G ketones. The results are discussed in terms of solvation, steric, and resonance effects. Ab initio density functional theory (DFT) calculations are employed to illustrate the effect of steric interactions on radical and anion geometries. The DFT results parallel the trends in the experimental BDEs of alpha-arylacetophenones.

Model compound studies of the beta-O-4 linkage in lignin: absolute rate expressions for beta-scission of phenoxyl radical from 1-phenyl-2-phenoxyethanol-1-yl radical.

Arrhenius rate expressions were determined for beta-scission of phenoxyl radical from 1-phenyl-2-phenoxyethanol-1-yl, PhC*(OH)CH2OPh (V). Ketyl radical V was competitively trapped by thiophenol to yield PhCH(OH)CH2OPh in competition with beta-scission to yield phenoxyl radical and acetophenone. A basis rate expression for hydrogen atom abstraction by sec-phenethyl alcohol, PhC*(OH)CH3, from thiophenol, log(k(abs)/M(-1) s(-1)) = (8.88 +/- 0.24) - (6.07 +/- 0.34)/theta, theta = 2.303RT, was determined by competing hydrogen atom abstraction with radical self-termination. Self-termination rates for PhC*(OH)CH3 were calculated using the Smoluchowski equation employing experimental diffusion coefficients of the parent alcohol, PhCH(OH)CH3, as a model for the radical. The hydrogen abstraction basis reaction was employed to determine the activation barrier for the beta-scission of phenoxyl from 1-phenyl-2-phenoxyethanol-1-yl (V): log(k beta)/s(-1)) = (12.85 +/- 0.22) - (15.06 +/- 0.38)/theta, k beta (298 K) ca. (64.0 s(-1) in benzene), and log(k beta /s(-1)) = (12.50 +/- 0.18) - (14.46 +/- 0.30)/theta, k beta (298 K) = 78.7 s(-1) in benzene containing 0.8 M 2-propanol. B3LYP/cc-PVTZ electronic structure calculations predict that intramolecular hydrogen bonding between the alpha-OH and the -OPh leaving group of ketyl radical (V) stabilizes both ground- and transition-state structures. The computed activation barrier, 14.9 kcal/mol, is in good agreement with the experimental activation barrier.