Julien A. Panetier

Catalytic proton reduction with transition metal complexes of the redox-active ligand bpy2PYMe

A new pentadentate, redox-active ligand bpy2PYMe has been synthesized and its corresponding transition metal complexes of Fe2+ (1), Co2+ (2), Ni2+ (3), Cu2+ (4), and Zn2+ (5) have been investigated for electro- and photo-catalytic proton reduction in acetonitrile and water, respectively. Under weak acid conditions, the Co complex displays catalytic onset at potentials similar to those of the ligand centered reductions in the absence of acid. Related Co complexes devoid of ligand redox activity catalyze H2 evolution under similar conditions at significantly higher overpotentials, showcasing the beneficial effect of combining ligand-centered redox activity with a redox-active Co center. Furthermore, turnover numbers as high as 1630 could be obtained under aqueous photocatalytic conditions using [Ru(bpy)3]2+ as a photosensitizer. Under those conditions catalytic hydrogen production was solely limited by photosensitizer stability. Introduction of an electron withdrawing CF3 group into the pyridine moiety of the ligand as in bpy2PYMe-CF3 renders its corresponding Co complex 6 less active for proton reduction in electro- and photocatalytic experiments. This surprising effect of ligand substitution was investigated by means of density functional theory calculations which suggest the importance of electronic communication between Co1+ and the redox-active ligand. Taken together, the results provide a path forward in the design of robust molecular catalysts in aqueous media with minimized overpotential by exploiting the synergy between redox-active metal and ligand components.

Catalytic Hydrodefluorination of Pentafluorobenzene by [Ru(NHC)(PPh3)2(CO)H2]: A Nucleophilic Attack by a Metal-Bound Hydride Ligand Explains an Unusual ortho-Regioselectivity†

The selective synthesis of fluoroarene compounds is a subject of intense current interest, driven by the prominent role such species play in many pharmaceuticals, agrochemicals, and other industrially important products. One attractive route to selectively substituted fluoroarenes involves the activation and functionalization of aromatic C F bonds derived from readily available perfluoroarenes. The simplest example of such a process is the hydrodefluorination reaction (HDF), in which fluorine is substituted for hydrogen. Catalytic HDF of C6F6 and C6F5H has been reported by Milstein et al. [3] and Holland et al. using Rh and Fe catalysts. However, both these systems exhibit practical problems that limit the mechanistic understanding of the HDF cycle. For example the Rh system requires high pressures of H2 as well as a sacrificial amine to remove HF, while with Fe no C F activation is observed in the absence of a reductant. As a consequence, the development of more active Rh or Fe catalysts has not been forthcoming. We recently reported the HDF of C6F6 and C6F5H using the ruthenium N-heterocyclic carbene (NHC) dihydride complex 1 (NHC = IMes, SIMes, IPr, SIPr; see Scheme 1 a) in the presence of trialkylsilanes at 70 8C in THF. Isolation and characterization of 1 allowed detailed kinetic studies to be undertaken, and these supported a mechanism involving initial phosphine dissociation to form 2 followed by HDF of the substrate to give the Ru F species, 3. Isolation of this 16e complex allowed us to demonstrate its reaction with trialkylsilane in the presence of PPh3 to regenerate catalyst 1. The most unusual feature of this system was the high regioselectivity for the formation of 1,2,3,4-C6F4H2 upon HDF of C6F5H, in complete contrast to the Milstein and Holland systems where the 1,2,4,5-isomer dominated. To account for the unusual ortho-regioselectivity we postulated the involvement of a tetrafluorobenzyne intermediate (Scheme 1b). Such species have been reported previously and could be formed here from 2 by successive C H and ortho-C F activation of C6F5H. However, density functional theory (DFT) calculations (with NHC = IMes) have now shown that this species lies more than 200 kJmol 1 above the reactants, effectively ruling it out as a viable intermediate under the conditions used experimentally. Further calculations, however, have now allowed us to define a series of alternative pathways which are based on a novel nucleophilic attack mechanism whereby a hydride ligand reacts directly with C6F5H. [10] These processes produced significantly lower barriers and, moreover, the lowestenergy pathway was found to be consistent with the unusual ortho-regioselectivity observed experimentally. Our calculations have shown that, after initial phosphine loss from 1, nucleophilic attack of hydride at C6F5H can occur through two different pathways (Scheme 2). In the concerted pathway I, the hydride is transfered from the metal onto the arene ring and the displaced fluorine migrates directly onto the metal center. In the alternative stepwise pathway II, an harene adduct, 4, is formed prior to the hydride attack. In this case the different orientation of the arene precludes direct transfer of fluorine onto the metal. Instead an intermediate is formed, 5, from which HF can be lost to form a s-aryl species, 6. Protonolysis by HF with concomitant F transfer to metal then yields 1,2,3,4-C6F4H2 and the M F species 3. The lowest-energy reaction profile for the HDF of C6F5H by 1 to give 1,2,3,4-C6F4H2 is computed to proceed through pathway II, and full details are shown in Figure 1. Initial Scheme 1. a) Catalytic hydrodefluorination (HDF) of C6F5H to 1,2,3,4C6F4H2 by 1; b) postulated tetrafluorobenzyne intermediate.

Mechanism of the electrocatalytic reduction of protons with diaryldithiolene cobalt complexes.

A series of dimeric cobalt-diaryldithiolene complexes [Co(S2C2Ar2)2]2, possessing various aryl para substituents (OMe, F, Cl, and Br), were studied as electrocatalysts for proton reduction in nonaqueous media, in an effort to correlate dithiolene donor strength with catalyst activity. Cyclic voltammetry data acquired for the cobalt-diaryldithiolene dimers guided the isolation of chemically reduced monoanionic ([Co(S2C2Ar2)2](-)) and dianionic ([Co(S2C2Ar2)2](2-)) monomers. The potassium and tetrabutylammonium salts of dianionic cobalt-diaryldithiolene complexes have been characterized by single crystal X-ray crystallography. Treatment of the dianionic species with stoichiometric quantities of a weak acid afforded H2 and the monoanionic cobalt-diaryldithiolene species. Density functional theory (BP86) suggests that hydrogen elimination proceeds through a diprotonated intermediate with a Co-H bond and a protonated S center. A transition state for transfer of the S-H proton to the metal center was located with a computed free energy of 5.9 kcal/mol, in solution (DMF via C-PCM approach).

Computational study of the hydrodefluorination of fluoroarenes at [Ru(NHC)(PR3)2(CO)(H)2]: predicted scope and regioselectivities

Density functional theory calculations have been employed to investigate the scope and selectivity of the hydrodefluorination (HDF) of fluoroarenes, C6F(6-n)H(n) (n = 0-5), at catalysts of the type [Ru(NHC)(PR3)2(CO)(H)2]. Based on our previous study (Angew. Chem., Int. Ed., 2011, 50, 2783) two mechanisms featuring the nucleophilic attack of a hydride ligand at a fluoroarene substrate were considered: (i) a concerted process with Ru-H/C-F exchange occurring in one step; and (ii) a stepwise pathway in which the rate-determining transition state involves formation of HF and a Ru-σ-fluoroaryl complex. The nature of the metal coordination environment and, in particular, the NHC ligand was found to play an important role in both promoting the HDF reaction and determining the regioselectivity of this process. Thus for the reaction of C6F5H, the full experimental system (NHC = IMes, R = Ph) promotes HDF through (i) more facile initial PR3/fluoroarene substitution and (ii) the ability of the NHC N-aryl substituents to stabilise the key C-F bond breaking transition state through F···HC interactions. This latter effect is maximised along the lower energy stepwise pathway when an ortho-H substituent is present and this accounts for the ortho-selectivity seen in the reaction of C6F5H to give 1,2,3,4-C6F4H2. Computed C-F bond dissociation energies (BDEs) for C6F(6-n)H(n) substrates show a general increase with larger n and are most sensitive to the number of ortho-F substituents present. However, HDF is always computed to remain significantly exothermic when a silane such as Me3SiH is included as terminal reductant. Computed barriers to HDF also generally increase with greater n, and for the concerted pathway a good correlation between C-F BDE and barrier height is seen. The two mechanisms were found to have complementary regioselectivities. For the concerted pathway the reaction is directed to sites with two ortho-F substituents, as these have the weakest C-F bonds. In contrast, reaction along the stepwise pathway is directed to sites with only one ortho-F substituent, due to difficulties in accommodating ortho-F substituents in the C-F bond cleavage transition state. Calculations predict that 1,2,3,5-C6F4H2 and 1,2,3,4-C6F4H2 are viable candidates for HDF at [Ru(IMes)(PPh3)2(CO)(H)2] and that this would proceed selectively to give 1,2,4-C6F3H3 and 1,2,3-C6F3H3, respectively.

Computational Characterization of Redox Non-Innocence in Cobalt-Bis(Diaryldithiolene)-Catalyzed Proton Reduction.

Localized orbital bonding analysis (LOBA) was employed to probe the oxidation state in cobalt-bis(diaryldithiolene)-catalyzed proton reduction in nonaqueous media. LOBA calculations provide both the oxidation state and chemically intuitive views of bonding in cobalt-bis(diaryldithiolene) species and therefore allow characterization of the role of the redox non-innocent dithiolene ligand. LOBA results show that the reduction of the monoanion species [1Br](-) is metal-centered and gives a cobalt(II) ion species, [1Br](2-), coordinated to two dianionic ene-1,2-dithiolates. This electronic configuration is in agreement with the solution magnetic moment observed for the analogous salt [1F](2-) (μeff = 2.39 μB). Protonation of [1Br](2-) yields the cobalt(III)-hydride [1Br(CoH)](-) species in which the Co-H bond is computed to be highly covalent (Löwdin populations close to 0.50 on cobalt and hydrogen atoms). Further reduction of [1Br(CoH)](-) forms a more basic cobalt(II)-H intermediate [1Br(CoH)](2-) (S = 0) from which protonation at sulfur gives a S-H bond syn to the Co-H bond. Formation of a cobalt-dihydrogen [1Br(CoH2)](-) intermediate is calculated to occur via a homocoupling (H(•) + H(•) → H2) step with a free energy of activation of 5.9 kcal/mol in solution (via C-PCM approach).

Mechanistic Studies of C X Bond Activation at Transition-Metal Centers

This chapter surveys mechanistic computational studies on the activation of C X single bonds mediated by transition-metal centers. X can be based on any element of groups 13–17 and ‘mechanistic’ indicates that the transition state for C X bond activation has been located. Bond activation itself refers to the cleavage of a C X bond such that both partners may (potentially) act as neutral one-electron donors to the metal center. This definition covers oxidative addition, typically via a concerted and also by S N 2 or radical processes; also included are σ-bond metathesis, as well as C X cleavage promoted by Lewis acidic and Lewis basic species. The reactivity of neutral transition atoms and cations (often modeling gas-phase experimental reactivity) and of molecular complexes (of relevance to solution-phase reactivity and homogeneous catalysis) is discussed.