Adaptive foraging behaviours in the Horn of Africa during Toba supereruption.

Although modern humans left Africa multiple times over 100,000 years ago, those broadly ancestral to non-Africans dispersed less than 100,000 years ago1. Most models hold that these events occurred through green corridors created during humid periods because arid intervals constrained population movements2. Here we report an archaeological site-Shinfa-Metema 1, in the lowlands of northwest Ethiopia, with Youngest Toba Tuff cryptotephra dated to around 74,000 years ago-that provides early and rare evidence of intensive riverine-based foraging aided by the likely adoption of the bow and arrow. The diet included a wide range of terrestrial and aquatic animals. Stable oxygen isotopes from fossil mammal teeth and ostrich eggshell show that the site was occupied during a period of high seasonal aridity. The unusual abundance of fish suggests that capture occurred in the ever smaller and shallower waterholes of a seasonal river during a long dry season, revealing flexible adaptations to challenging climatic conditions during the Middle Stone Age. Adaptive foraging along dry-season waterholes would have transformed seasonal rivers into 'blue highway' corridors, potentially facilitating an out-of-Africa dispersal and suggesting that the event was not restricted to times of humid climates. The behavioural flexibility required to survive seasonally arid conditions in general, and the apparent short-term effects of the Toba supereruption in particular were probably key to the most recent dispersal and subsequent worldwide expansion of modern humans.

Using Substituted [Fe4N(CO)12]- as a Platform To Probe the Effect of Cation and Lewis Acid Location on Redox Potential.

The impact of cationic and Lewis acidic functional groups installed in the primary or secondary coordination sphere (PCS or SCS) of an (electro)catalyst is known to vary depending on the precise positioning of those groups. However, it is difficult to systematically probe the effect of that position. In this report, we probe the effect of the functional group position and identity on the observed reduction potentials (Ep,c) using substituted iron clusters, [Fe4N(CO)11R]n, where R = NO+, PPh2-CH2CH2-9BBN, (MePTA+)2, (MePTA+)4, and H+ and n = 0, -1, +1, or +3 (9-BBN is 9-borabicyclo(3.3.1)nonane; MePTA+ is 1-methyl-1-azonia-3,5-diaza-7-phosphaadamantane). The cationic NO+ and H+ ligands cause anodic shifts of 700 and 320 mV, respectively, in Ep,c relative to unsubstituted [Fe4N(CO)12]-. Infrared absorption band data, νCO, suggests that some of the 700 mV shift by NO+ results from electronic changes to the cluster core. This contrasts with the effects of cationic MePTA+ and H+ which cause primarily electrostatic effects on Ep,c. Lewis acidic 9-BBN in the SCS had almost no effect on Ep,c.

Secondary Coordination Sphere Design to Modify Transport of Protons and CO2.

An exploration of secondary coordination sphere (SCS) functional groups is presented with a focus on proton transport to a metal hydride active site for H2 formation and transport of CO2 so that formate can be obtained. In MeCN-H2O, pKa(AH) and steric bulk of the SCS groups are discussed along with their influence on each step in the mechanism for CO2 to formate catalysis and along with the influence of the proton source, which is MeCN-H2O or (MeCN)2H2O in MeCN-H2O (95:5) under N2 atmosphere. Under CO2, carbonic acid is also available. Catalysts containing various SCS groups were synthesized from [Fe4N(CO)12]- and have the form [Fe4N(CO)11L]- where L is Ph2P-SCS. Hydride formation rates are distinct under N2 versus CO2, and that variation is dependent on the size of the SCS group. Under CO2, larger SCS groups inhibit access of the MeCN-H2O adducts to the active site and formate formation is observed, whereas smaller SCS groups allow transport of these adducts. This is best illustrated by catalysts containing the small SCS group pyridyl and the large SCS group N,N-dimethylaniline which both have the same pKa(AH) value. The smaller pyridyl group promotes selective H2 evolution, whereas larger N,N-dimethylaniline supports selective formate formation by slowing the transport of large MeCN-H2O adducts, allowing hydride transfer to the smaller substrate CO2.

Renewable Formate from C-H Bond Formation with CO2: Using Iron Carbonyl Clusters as Electrocatalysts.

As a society, we are heavily dependent on nonrenewable petroleum-derived fuels and chemical feedstocks. Rapid depletion of these resources and the increasingly evident negative effects of excess atmospheric CO2 drive our efforts to discover ways of converting excess CO2 into energy dense chemical fuels through selective C-H bond formation and using renewable energy sources to supply electrons. In this way, a carbon-neutral fuel economy might be realized. To develop a molecular or heterogeneous catalyst for C-H bond formation with CO2 requires a fundamental understanding of how to generate metal hydrides that selectively donate H- to CO2, rather than recombining with H+ to liberate H2. Our work with a unique series of water-soluble and -stable, low-valent iron electrocatalysts offers mechanistic and thermochemical insights into formate production from CO2. Of particular interest are the nitride- and carbide-containing clusters: [Fe4N(CO)12]- and its derivatives and [Fe4C(CO)12]2-. In both aqueous and mixed solvent conditions, [Fe4N(CO)12]- forms a reduced hydride intermediate, [H-Fe4N(CO)12]-, through stepwise electron and proton transfers. This hydride selectively reacts with CO2 and generates formate with >95% efficiency. The mechanism for this transformation is supported by crystallographic, cyclic voltammetry, and spectroelectrochemical (SEC) evidence. Furthermore, installation of a proton shuttle onto [Fe4N(CO)12]- facilitates proton transfer to the active site, successfully intercepting the hydride intermediate before it reacts with CO2; only H2 is observed in this case. In contrast, isoelectronic [Fe4C(CO)12]2- features a concerted proton-electron transfer mechanism to form [H-Fe4C(CO)12]2-, which is selective for H2 production even in the presence of CO2, in both aqueous and mixed solvent systems. Higher nuclearity clusters were also studied, and all are proton reduction electrocatalysts, but none promote C-H bond formation. Thermochemical insights into the disparate reactivities of these clusters were achieved through hydricity measurements using SEC. We found that only [H-Fe4N(CO)12]- and its derivative [H-Fe4N(CO)11(PPh3)]- have hydricities modest enough to avoid H2 production but strong enough to make formate. [H-Fe4C(CO)12]2- is a stronger hydride donor, theoretically capable of making formate, but due to an overwhelming thermodynamic driving force and the increased electrostatic attraction between the more negative cluster and H+, only H2 is observed experimentally. This illustrates the fundamental importance of controlling thermochemistry when designing new catalysts selective for C-H bond formation and establishes a hydricity range of 15.5-24.1 or 44-49 kcal mol-1 where C-H bond formation may be favored in water or MeCN, respectively.

Formation of a Stable Complex, RuCl2(S2CPPh3)(PPh3)2, Containing an Unstable Zwitterion from the Reaction of RuCl2(PPh3)3 with Carbon Disulfide.

New insight into the complexity of the reaction of the prominent catalyst RuCl2(PPh3)3 with carbon disulfide has been obtained from a combination of X-ray diffraction and (31)P NMR studies. The red-violet compound originally formulated as a cationic π-CS2 complex, [RuCl(π-CS2)(PPh3)3]Cl, has been identified as a neutral molecule, RuCl2(S2CPPh3)(PPh3)2, which contains the unstable zwitterion S2CPPh3. In the absence of RuCl2(PPh3)3, there is no sign of a reaction between triphenylphosphine and carbon disulfide, although more basic trialkylphosphines form red adducts, S2CPR3. Despite the presence of an unstable ligand, RuCl2(S2CPPh3)(PPh3)2 is remarkably stable. It survives melting at 173-174 °C intact, is stable to air, and undergoes reversible electrochemical oxidation to form a monocation. When the reaction of RuCl2(PPh3)3 with carbon disulfide is conducted in the presence of methanol, crystals of orange [RuCl(S2CPPh3)(CS)(PPh3)2]Cl·2MeOH and yellow RuCl2(CS)(MeOH)(PPh3)2 also form. (31)P NMR studies indicate that the unsymmetrical dinuclear complex (SC)(Ph3P)2Ru(µ-Cl)3Ru(PPh3)2Cl is the initial product of the reaction of RuCl2(PPh3)3 with carbon disulfide. A path connecting the isolated products is presented.

Quantification of the Electrostatic Effect on Redox Potential by Positive Charges in a Catalyst Microenvironment.

Charged functional groups in the secondary coordination sphere (SCS) of a heterogeneous nanoparticle or homogeneous electrocatalyst are of growing interest due to enhancements in reactivity that derive from specific interactions that stabilize substrate binding or charged intermediates. At the same time, accurate benchmarking of electrocatalyst systems most often depends on the development of linear free-energy scaling relationships. However, the thermodynamic axis in those kinetic-thermodynamic correlations is most often obtained by a direct electrochemical measurement of the catalyst redox potential and might be influenced by electrostatic effects of a charged SCS. In this report, we systematically probe positive charges in a SCS and their electrostatic contributions to the electrocatalyst redox potential. A series of 11 iron carbonyl clusters modified with charged and uncharged ligands was probed, and a linear correlation between the νCO absorption band energy and electrochemical redox potentials is observed except where the SCS is positively charged.

A pendant proton shuttle on [Fe4N(CO)12]- alters product selectivity in formate vs. H2 production via the hydride [H-Fe4N(CO)12].

Proton relays are known to increase reaction rates for H2 evolution and lower overpotentials in electrocatalytic reactions. In this report we describe two electrocatalysts, [Fe4N(CO)11(PPh3)]- (1-) which has no proton relay, and hydroxyl-containing [Fe4N(CO)11(Ph2P(CH2)2OH)]- (2-). Solid state structures indicate that these phosphine-substituted clusters are direct analogs of [Fe4N(CO)12]- where one CO ligand has been replaced by a phosphine. We show that the proton relay changes the selectivity of reactions: CO2 is reduced selectively to formate by 1- in the absence of a relay, and protons are reduced to H2 under a CO2 atmosphere by 2-. These results implicate a hydride intermediate in the mechanism of the reactions and demonstrate the importance of controlling proton delivery to control product selectivity. Thermochemical measurements performed using infrared spectroelectrochemistry provided pKa and hydricity values for [HFe4N(CO)11(PPh3)]-, which are 23.7, and 45.5 kcal mol-1, respectively. The pKa of the hydroxyl group in 2- was determined to fall between 29 and 41, and this suggests that the proximity of the proton relay to the active catalytic site plays a significant role in the product selectivity observed, since the acidity alone does not account for the observed results. More generally, this work emphasizes the importance of substrate delivery kinetics in determining the selectivity of CO2 reduction reactions that proceed through metal-hydride intermediates.