NHC‐Containing Manganese(I) Electrocatalysts for the Two‐Electron Reduction of CO2

The synthesis and characterization of the first catalytic manganese N‐heterocyclic carbene complexes are reported: MnBr(N‐methyl‐N′‐2‐pyridylbenzimidazol‐2‐ylidine)(CO)₃ and MnBr(N‐methyl‐N′‐2‐pyridylimidazol‐2‐ylidine)(CO)₃. Both new species mediate the reduction of CO₂ to CO following two‐electron reduction of the Mn^I center, as observed with preparative scale electrolysis and verified with ¹³CO₂. The two‐electron reduction of these species occurs at a single potential, rather than in two sequential steps separated by hundreds of millivolts, as is the case for previously reported MnBr(2,2′‐bipyridine)(CO)₃. Catalytic current enhancement is observed at voltages similar to MnBr(2,2′‐bipyridine)(CO)₃.     

Formation of the Salt‐like Complexes [Co{S2CC(PPh3)2}3][Co(CO)4]3 and [(CO)4Mn{S2CC(PPh3)2}][Mn(CO)5] from the Reaction of the Related Carbonyl Compounds with S2CC(PPh3)2

The betain‐like compound S₂CC(PPh₃)₂ ( 1 ), which is obtained from CS₂ and the double ylide C(PPh₃)₂, reacts with [Co₂(CO)₈] and [Mn₂(CO)₁₀] in THF to afford the salt‐like complexes [Co{S₂CC(PPh₃)₂}₃][Co(CO)₄]₃ ( 2 ) and [(CO)₄Mn{S₂CC(PPh₃)₂}][Mn(CO)₅] ( 3 ), respectively, in good yields. At both d⁶ cations 1 acts as a chelating ligand. Disproportionation reactions from formal Co⁰ into Co^III and Co^?I and from Mn⁰ into Mn^I and Mn^?I occurred with the removal of four or one carbonyl groups, respectively. The crystal structures of 2· 5.5THF and 3· 2THF are reported, which show a shortening of the C–C bond in the ligand upon complex formation. The compounds are further characterized by ³¹P NMR and IR spectroscopy.     

Mapping the Transformation [{RuII(CO)3Cl2}2]→[RuI2(CO)4]2+: Implications in Binuclear Water–Gas Shift Chemistry

The complete sequence of reactions in the base‐promoted reduction of [{Ru^II(CO)₃Cl₂}₂] to [Ru^I₂(CO)₄]²⁺ has been unraveled. Several μ‐OH, μ:κ²‐CO₂H‐bridged diruthenium(II) complexes have been synthesized; they are the direct results of the nucleophilic activation of metal‐coordinated carbonyls by hydroxides. The isolated compounds are [Ru₂(CO)₄(μ:κ²‐C,O‐CO₂H)₂(μ‐OH)(NP^F‐Am)₂][PF₆] ( 1 ; NP^F‐Am=2‐amino‐5,7‐trifluoromethyl‐1,8‐naphthyridine) and [Ru₂(CO)₄(μ:κ²‐C,O‐CO₂H)(μ‐OH)(NP‐Me₂)₂][BF₄]₂ ( 2 ), secured by the applications of naphthyridine derivatives. In the absence of any capping ligand, a tetranuclear complex [Ru₄(CO)₈(H₂O)₂(μ³‐OH)₂(μ:κ²‐C,O‐CO₂H)₄][CF₃SO₃]₂ ( 3 ) is isolated. The bridging hydroxido ligand in 1 is readily replaced by a π‐donor chlorido ligand, which results in [Ru₂(CO)₄(μ:κ²‐C,O‐CO₂H)₂(μ‐Cl)(NP‐PhOMe)₂][BF₄] ( 4 ). The production of [Ru₂(CO)₄]²⁺ has been attributed to the thermally induced decarboxylation of a bis(hydroxycarbonyl)–diruthenium(II) complex to a dihydrido–diruthenium(II) species, followed by dinuclear reductive elimination of molecular hydrogen with the concomitant formation of the Ru^I? Ru^I single bond. This work was originally instituted to find a reliable synthetic protocol for the [Ru₂(CO)₄(CH₃CN)₆]²⁺ precursor. It is herein prescribed that at least four equivalents of base, complete removal of chlorido ligands by Tl^I salts, and heating at reflux in acetonitrile for a period of four hours are the conditions for the optimal conversion. Premature quenching of the reaction resulted in the isolation of a trinuclear Ru^I₂Ru^II complex [{Ru(NP‐Am)₂(CO)}{Ru₂(NP‐Am)₂(CO)₂(μ‐CO)₂}(μ₃:κ³‐C,O,O′‐CO₂)][BF₄]₂ ( 6 ). These unprecedented diruthenium compounds are the dinuclear congeners of the water–gas shift (WGS) intermediates. The possibility of a dinuclear pathway eliminates the inherent contradiction of pH demands in the WGS catalytic cycle in an alkaline medium. A cooperative binuclear elimination could be a viable route for hydrogen production in WGS chemistry.     

Electrochemical Investigations Give Some Insights into the Coordination Chemistry of New Stable Iridium(+1), Iridium(0), and Iridium(−1) Complexes

The redox chemistry of the stable tetracoordinated 16 valence electron d⁸‐[Ir^+I(tropp^Ph)₂]⁺(PF₆)^? and pentacoordinated 18 valence d⁸‐[Ir^+I(tropp^Ph)₂Cl] complexes was investigated by cyclic voltammetry (tropp^Ph=dibenzotropylidenyl phosphine). The experiments were performed using a platinum microelectrode varying scan rates (100 mV/s–10 V/s) and temperatures (? 40 to 20 °C) in tetrahydrofuran, THF, or acetonitrile, ACN, as solvents. In THF, the overall two‐electron reduction of the 16 valence electron d⁸‐[Ir^+I(tropp^Ph)₂]⁺(PF₆)^? proceeds in two well separated slow heterogeneous electron transfer steps according to: d⁸‐[Ir^+I (tropp^Ph)₂]⁺+e^?→d⁹‐[Ir⁰(tropp^Ph)₂]+e^?→d¹⁰‐[Ir^?I(tropp^Ph)₂]^?, [k^s₁=2.2×10^?3 cm/s for d⁸‐Ir^+I/d⁹‐Ir⁰ and k^s₂=2.0×10^?3 cm/s for d⁹‐Ir⁰/d¹⁰‐Ir^?I]. In ACN, the two redox waves merge into one “two‐electron” wave [k^s_1,2=7.76×10^?4 cm/s for d⁸‐Ir^+I/d⁹‐Ir⁰ and d⁹‐Ir⁰/d¹⁰‐Ir^?I] most likely because the neutral [Ir⁰(tropp^Ph)₂] complex is destabilized. At low temperatures (ca. ? 40 °C) and at high scan rates (ca. 10 V/s), the two‐electon redox process is kinetically resolved. In equilibrium with the tetracoordianted complex [Ir^+I(tropp^Ph)₂]⁺ are the pentacoordinated 18 valence [Ir^+I(tropp^Ph)₂L]⁺ complexes (L=THF, ACN, Cl^?) and their electrochemical behavior was also investigated. They are irreversibly reduced at rather high negative potentials (? 1.8 to ? 2.4 V) according to an ECE mechanism 1) [Ir^+I(tropp^Ph)₂(L)]+e^?→[Ir⁰(tropp^Ph)₂(L)]; 2) [Ir⁰(tropp^Ph)₂(L)]→[Ir(tropp^Ph)₂]+L, iii) [Ir⁰(tropp^Ph)₂]+e^?→[Ir^?I(tropp^Ph)₂]^?. Since all electroactive species were isolated and structurally characterized, our measurements allow for the first time a detailed insight into some fundamental aspects of the coordination chemistry of iridium complexes in unusually low formal oxidation states.     

Metal–Organic Frameworks (MOFs) of a Cubic Metal Cluster with Multicentered MnIMnI Bonds

MOFs with both multicentered metal–metal bonds and low‐oxidation‐state (LOS) metal ions have been underexplored hitherto. Here we report the first cubic [Mn^I₈] cluster‐based MOF ( 1 ) with multicentered Mn^I? Mn^I bonds and +1 oxidation state of manganese (Mn^I or Mn(I)), as is supported by single‐crystal structure determination, XPS analyses, and quantum chemical studies. Compound 1 possesses the shortest Mn^I? Mn^I bond of 2.372 Å. Theoretical studies with density functional theory (DFT) reveal extensive electron delocalization over the [Mn^I₈] cube. The 48 electrons in the [Mn^I₈] cube fully occupy half of the 3d‐based and the lowest 4s‐based bonding orbitals, with six electrons lying at the nonbonding 3d‐orbitals. This bonding feature renders so‐called cubic aromaticity. Magnetic properties measurements show that 1 is an antiferromagnet. This work is expected to inspire further investigation of cubic metal–metal bonding, MOF materials with LOS metals, and metalloaromatic theory.     

Rate coefficients for the reaction of the acetyl radical,CH3CO,with Cl2 between 253 and 384 K

Rate coefficients, k, for the gas‐phase reaction CH₃CO + Cl₂ → products (2) were measured between 253 and 384 K at 55–200 Torr (He). Rate coefficients were measured under pseudo‐first‐order conditions in CH₃CO with CH₃CO produced by the 248‐nm pulsed‐laser photolysis of acetone, CH₃C(O)CH₃, or 2,3‐butadione, CH₃C(O)C(O)CH₃. The loss of CH₃CO was monitored by cavity ring‐down spectroscopy (CRDS) at 532 nm. Rate coefficients were determined by first‐order kinetic analysis of the CH₃CO temporal profiles for [Cl₂] < 1 × 10¹⁴ molecule cm^?3 and the analysis of the CRDS profiles by the simultaneous kinetics and ring‐down method for experiments performed with [Cl₂] > 1 × 10¹⁴ molecule cm^?3. k₂(T) was found to be independent of pressure, with k₂(296 K) = (3.0 ± 0.5) × 10^?11 cm³ molecule^?1 s^?1. k₂(T) showed a weak negative temperature dependence that is well reproduced by the Arrhenius expression k₂(T) = (2.2 ± 0.8) × 10^?11 exp[(85 ± 120)/T] cm³ molecule^?1 s^?1. The quoted uncertainties in k₂(T) are at the 2σ level (95% confidence interval) and include estimated systematic errors. A comparison of the present work with previously reported rate coefficients for the CH₃CO + Cl₂ reaction is presented. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 543–553, 2009     

Surface and Structural Investigation of a MnOx Birnessite‐Type Water Oxidation Catalyst Formed under Photocatalytic Conditions

Catalytically active MnO_x species have been reported to form in situ from various Mn‐complexes during electrocatalytic and solution‐based water oxidation when employing cerium(IV) ammonium ammonium nitrate (CAN) oxidant as a sacrificial reagent. The full structural characterization of these oxides may be complicated by the presence of support material and lack of a pure bulk phase. For the first time, we show that highly active MnO_x catalysts form without supports in situ under photocatalytic conditions. Our most active ⁴MnO_x catalyst (～0.84 mmol O₂ mol Mn^?1 s^?1) forms from a Mn₄O₄ bearing a metal–organic framework. ⁴MnO_x is characterized by pair distribution function analysis (PDF), Raman spectroscopy, and HR‐TEM as a disordered, layered Mn‐oxide with high surface area (216 m²g^?1) and small regions of crystallinity and layer flexibility. In contrast, the ^SMnO_x formed from Mn²⁺ salt gives an amorphous species of lower surface area (80 m²g^?1) and lower activity (～0.15 mmol O₂ mol Mn^?1 s^?1). We compare these catalysts to crystalline hexagonal birnessite, which activates under the same conditions. Full deconvolution of the XPS Mn2p_3/2 core levels detects enriched Mn³⁺ and Mn²⁺ content on the surfaces, which indicates possible disproportionation/comproportionation surface equilibria.     

Novel Trinuclear MnII/MnII/MnII Complexes – Crystal Structures and Catalytic Properties

The trinuclear manganese(II) complexes [Mn₃(L¹)₂(μ‐OAc)₄]·2Et₂O {HL¹ = (1‐hydroxy‐4‐nitrobenzyl)((2‐pyridyl)methyl)((1‐methylimidazol‐2‐yl)methyl)amine} ( 1·2EtOH ), [Mn₃(L²)₂(μ‐OAc)₄] {HL² = ((1‐methylimidazol‐2‐yl)methyl)(1‐hydroxybenzyl)amine} ( 2 ) and [Mn₃(L³)₂(μ‐OAc)₆] {L³ = bis(1‐methylimidazol‐2‐yl)methanone} ( 3 ) were synthesized. The compounds were characterized by X‐ray crystallography, mass spectrometry, IR, UV‐vis spectroscopy, and elemental analysis. The manganese atoms in 1 and 2 are bridged by phenol moieties of the ligands and acetates. In 3 they are only bridged by acetates in a mono‐ and bi‐dentate way. The resulting Mn···Mn distances are 3.233(1) Å ( 1 ), 3.364(1) Å ( 2 ) and 3.643(7) Å ( 3 ). In the present compounds different limiting cases for the phenomenon of the carboxylate shift are realized. Besides symmetric mono‐ and bi‐dentate bridging an unusual intermediate is also observed. 1·2EtOH is the first example of a trinuclear model for the OEC that shows catalase activity. Furthermore it was characterized by temperature dependent magnetic susceptibility measurements and a total spin ground state of S_t = 5/2 was found. The results for 1 reveals antiferromagnetic coupling between the central and the terminal manganese ions, with J = ?1.2 cm^?1, g = 2.00 (fixed), χ_TIP = 150×10^?6 cm³mol^?1.     

A Systematic Investigation of Coinage Metal Carbonyl Complexes Stabilized by Fluorinated Alkoxy Aluminates

The reaction of Cu^I, Ag^I, and Au^I salts with carbon monoxide in the presence of weakly coordinating anions led to known and structurally unknown non‐classical coinage metal carbonyl complexes [M(CO)_n][A] (A=fluorinated alkoxy aluminates). The coinage metal carbonyl complexes [Cu(CO)_n(CH₂Cl₂)_m]⁺[A]^? (n=1, 3; m=4?n), [Au₂(CO)₂Cl]⁺[A]^?, [(OC)_nM(A)] (M=Cu: n=2; Ag: n=1, 2) as well as [(OC)₃Cu???ClAl(OR^F)₃] and [(OC)Au???ClAl(OR^F)₃] were analyzed with X‐ray diffraction and partially IR and Raman spectroscopy. In addition to these structures, crystallographic and spectroscopic evidence for the existence of the tetracarbonyl complex [Cu(CO)₄]⁺[Al(OR^F)₄]^? (R^F=C(CF₃)₃) is presented; its formation was analyzed with the help of theoretical investigations and Born–Fajans–Haber cycles. We discuss the limits of structure determinations by routine X‐ray diffraction methods with respect to the C? O bond lengths and apply the experimental CO stretching frequencies for the prediction of bond lengths within the carbonyl ligand based on a correlation with calculated data. Moreover, we provide a simple explanation for the reported, partly confusing and scattered CO stretching frequencies of [Cu^I(CO)_n] units.     

Catalytic Reduction of CN−, CO,and CO2 by Nitrogenase Cofactors in Lanthanide‐Driven Reactions

Nitrogenase cofactors can be extracted into an organic solvent to catalyze the reduction of cyanide (CN^?), carbon monoxide (CO), and carbon dioxide (CO₂) without using adenosine triphosphate (ATP), when samarium(II) iodide (SmI₂) and 2,6‐lutidinium triflate (Lut‐H) are employed as a reductant and a proton source, respectively. Driven by SmI₂, the cofactors catalytically reduce CN^? or CO to C₁–C₄ hydrocarbons, and CO₂ to CO and C₁–C₃ hydrocarbons. The C? C coupling from CO₂ indicates a unique Fischer–Tropsch‐like reaction with an atypical carbonaceous substrate, whereas the catalytic turnover of CN^?, CO, and CO₂ by isolated cofactors suggests the possibility to develop nitrogenase‐based electrocatalysts for the production of hydrocarbons from these carbon‐containing compounds.     

Kinetic and Mechanisms of the Homogeneous,Unimolecular Elimination of Phenyl Chloroformate and p‐Tolyl Chloroformate in the Gas Phase

The gas‐phase elimination of phenyl chloroformate gives chlorobenzene, 2‐chlorophenol, CO₂, and CO, whereasp‐tolyl chloroformate produces p‐chlorotoluene and 2‐chloro‐4‐methylphenol CO₂ and CO. The kinetic determination of phenyl chloroformate (440–480^oC, 60–110 Torr) and p‐tolyl chloroformate (430–480°C, 60–137 Torr) carried out in a deactivated static vessel, with the free radical inhibitor toluene always present, is homogeneous, unimolecular and follows a first‐order rate law. The rate coefficient is expressed by the following Arrhenius equations: Phenyl chloroformate: Formation of chlorobenzene, log k_I = (14.85 ± 0.38) – (260.4 ± 5.4) kJ mol^?1 (2.303RT)^?1; r = 0.9993 Formation of 2‐chlorophenol, log k_II = (12.76 ± 0.40) – (237.4 ± 5.6) kJ mol^?1(2.303RT)^?1; r = 0.9993 p‐Tolyl chloroformate: Formation of p‐chlorotoluene: log k_I = (14.35 ± 0.28) – (252.0 ± 1.5) kJ mol^–1 (2.303RT)^?1; r = 0.9993 Formation of 2‐chloro‐4‐methylphenol, log k_II = (12.81 ± 0.16) – (222.2 ± 0.9) kJ mol^?1(2.303RT)^–1; r = 0.9995 The estimation of the k_I values, which is the decarboxylation process in both substrates, suggests a mechanism involving an intramolecular nucleophilic displacement of the chlorine atom through a semipolar, concerted four‐membered cyclic transition state structure; whereas the k_II values, the decarbonylation in both substrates, imply an unusual migration of the chlorine atom to the aromatic ring through a semipolar, concerted five‐membered cyclic transition state type of mechanism. The bond polarization of the C–Cl, in the sense C^{δ+ …} Cl^δ?, appears to be the rate‐determining step of these elimination reactions.     

Chelating and Tellurolate Ligand‐Transfer Studies of the Complex fac‐[Fe(CO)3(TePh)3]−: Crystal Structures of Heterodinuclear (CO)3Mn(μ‐TePh)3Fe(CO)3 and CpNi(TePh)(PPh3)

Complex fac‐[Fe(CO)₃(TePh)₃]^? was employed as a “metallo chelating” ligand to synthesize the neutral (CO)₃Mn(μ‐TePh)₃Fe(CO)₃ obtained in a one‐step synthesis by treating fac‐[Fe(CO)₃(TePh)₃]^? with fac‐[Mn‐(CO)₃(CH₃CN)₃]⁺. It seems reasonable to conclude that the d⁶ Fe(II) [(CO)₃Fe(TePh)₃]^? fragment is isolobal with the d⁶ Mn(I) [(CO)₃Mn(TePh)₃]^2? fragment in complex (CO)₃Mn(μ‐TePh)₃Fe(CO)₃. Addition of fac‐[Fe(CO)₃(TePh)₃]^? to the CpNi(I)(PPh₃) in THF resulted in formation of the neutral CpNi(TePh)(PPh₃) also obtained from reaction of CpNi(I)(PPh₃) and [Na][TePh] in MeOH. This investigation shows that fac‐[Fe(CO)₃(TePh)₃]^? serves as a tridentate metallo ligand and tellurolate ligand‐transfer reagent. The study also indicated that the fac‐[Fe(CO)₃(SePh)₃]^? may serve as a better tridentate metallo ligand and chalcogenolate ligand‐transfer reagent than fac‐[Fe(CO)₃(TePh)₃]^? in the syntheses of heterometallic chalcogenolate complexes.     

Structure,Magnetic Properties and Catalase-like Activity of Asymmetric Dimanganese(III,III) Carboxylate Complexes with Salicylaldimine Ligands

The crystal structures, magnetic properties, and catalase-like activities of assymmetric dinuclear manganese(III, III) complexes, [Mn₂^{III, III}(spa)₂(μ-Me₃CCO₂)(Me₃CCO₂)(CH₃OH)] ( 1 ) and [Mn₂^{III, III}(vpa)₂(μ-Me₃CCO₂)(Me₃CCO₂)(CH₃OH)] ( 2 ), (H₂spa = 3-salicyclideneamino-1-propanol, H₂vpa = O-vanillin), were reported. The crystal structures of complexes 1 and 2 consist of the same discrete asymmetric coordination environment of dinuclear clusters, where the two manganese atoms are bridged by two alkoxo oxygens of the spa or vpa ligands and one bidentate carboxylate ion, whereas an additional oxygen atom of monodentate carboxylate coordinated to the first metal ion, and the second metal ion was coordinated by one oxygen atom of the solvent CH₃OH. Magnetic investigations (2–300 K) reveal an intramolecular antiferromagnetic spin exchange interaction with axial-field splittings: J = ?12.3 cm^?1 (D = ?0.10 cm^?1) and J = ?13.3 cm^?1 (D = ?0.15 cm^?1) for complexes 1 and 2 , respectively. The complexes should show catalase-like activity for H₂O₂ disproportionation in CH₃OH solvent at 25° with rate constants of k = 6.35 dm³moI^?1s^?1 and 6.20 dm³mol^?1s^?1 for complexes 1 and 2 , respectively.     

How the [NiFe4S4] Cluster of CO Dehydrogenase Activates CO2 and NCO−

Ni,Fe‐containing CO dehydrogenases (CODHs) use a [NiFe₄S₄] cluster, termed cluster C, to reversibly reduce CO₂ to CO with high turnover number. Binding to Ni and Fe activates CO₂, but current crystal structures have insufficient resolution to analyze the geometry of bound CO₂ and reveal the extent and nature of its activation. The crystal structures of CODH in complex with CO₂ and the isoelectronic inhibitor NCO^? are reported at true atomic resolution (d_min≤1.1 Å). Like CO₂, NCO^? is a μ₂,η² ligand of the cluster and acts as a mechanism‐based inhibitor. While bound CO₂ has the geometry of a carboxylate group, NCO^? is transformed into a carbamoyl group, thus indicating that both molecules undergo a formal two‐electron reduction after binding and are stabilized by substantial π backbonding. The structures reveal the combination of stable μ₂,η² coordination by Ni and Fe2 with reductive activation as the basis for both the turnover of CO₂ and inhibition by NCO^?.     

Double Carbonyl Substitution in [BrMn(CO)5]: Reaction with Benzoylhydrazine and Synthesis of fac‐[Mn(Br)(CO)3(BHD)]·2THF (BHD = OC(Ph)—NH—NH2)

[BrMn(CO)₅] reacts with benzoylhydrazine in THF occurring substitution of two CO groups by a Metal‐ligand ring to give fac‐[Mn(Br)(CO)₃(BHD)]·2THF (BHD = C₆H₅CONHNH₂). The novel compound shows a distorted octahedral arrangement at the manganese atom, with three nearly linear carbonyl ligands in a fac arrangement, illustrating another example that the CO group in position trans to the bromine ligand in [BrMn(CO)₅] presents the most intensive metal‐CO backbonding effect of all the CO groups of the parent complex, leading to the formation of a facial (and not meridional) isomer, even in the presence of a bidentate ligand like benzydrazide. X‐ray measures of yellow crystals showed that the title complex belong to space group P2₁/c, with the asymmetric unit containing one crystallographically independent [Mn(Br)(CO)₃(BHD)] complex and two tetrahydrofurane solvate molecules. The new compound represents heretofore the unique occurrence of the complexing single bidentate ligand ‐O=C(Ph)‐N(H)‐N(H)₂‐ with an octahedral coordination at the Mn^I atom supported chiefly by carbonyl groups.     

Hydrogenation of CO2 to Formic Acid with a Highly Active Ruthenium Acriphos Complex in DMSO and DMSO/Water

The novel [Ru(Acriphos)(PPh₃)(Cl)(PhCO₂)] [ 1 ; Acriphos=4,5‐bis(diphenylphosphino)acridine] is an excellent precatalyst for the hydrogenation of CO₂ to give formic acid in dimethyl sulfoxide (DMSO) and DMSO/H₂O without the need for amine bases as co‐reagents. Turnover numbers (TONs) of up to 4200 and turnover frequencies (TOFs) of up to 260 h^?1 were achieved, thus rendering 1 one of the most active catalysts for CO₂ hydrogenations under additive‐free conditions reported to date. The thermodynamic stabilization of the reaction product by the reaction medium, through hydrogen bonds between formic acid and clusters of solvent or water, were rationalized by DFT calculations. The relatively low final concentration of formic acid obtained experimentally under catalytic conditions (0.33 mol L^?1) was shown to be limited by product‐dependent catalyst inhibition rather than thermodynamic limits, and could be overcome by addition of small amounts of acetate buffer, thus leading to a maximum concentration of free formic acid of 1.27 mol L^?1, which corresponds to optimized values of TON=16×10³ and TOF_avg≈10³ h^?1.     

Rate constant of the gas-phase reaction between Fe atoms and CO2

The rate constant of the gas-phase reaction Fe(a ⁵ D ₄) + CO₂ at 1180–2380 K and a total gas density of (7.0–10.0) × 10^?6 mol/cm³ behind incident shock waves is k(Fe + CO₂) = 1.4 × 10^{14.0 ± 0.3}exp[?(14590 ± 1100)/T] cm³ mol^?1 s^?1, as determined by resonance atomic absorption photometry. Using thermochemical data available from the literature, the rate constant of the reverse reaction was calculated to be k(Fe + CO) = 9.2 × 10^{11.0 ± 0.3} (T/1000)^0.57exp[?(490 ± 1100)/T] cm³ mol^?1 s^?1. The results are compared with data reported earlier.     

Synergistic Porosity and Charge Effects in a Supramolecular Porphyrin Cage Promote Efficient Photocatalytic CO2 Reduction**

We present a supramolecular approach to catalyzing photochemical CO₂ reduction through second-sphere porosity and charge effects. An iron porphyrin box ( PB ) bearing 24 cationic groups, FePB-2(P) , was made via post-synthetic modification of an alkyne-functionalized supramolecular synthon. FePB-2(P) promotes the photochemical CO₂ reduction reaction (CO₂RR) with 97 % selectivity for CO product, achieving turnover numbers (TON) exceeding 7000 and initial turnover frequencies (TOF_max) reaching 1400 min⁻¹. The cooperativity between porosity and charge results in a 41-fold increase in activity relative to the parent Fe tetraphenylporphyrin ( FeTPP ) catalyst, which is far greater than analogs that augment catalysis through porosity ( FePB-3(N ), 4-fold increase) or charge (Fe p-tetramethylanilinium porphyrin ( Fe-p-TMA ), 6-fold increase) alone. This work establishes that synergistic pendants in the secondary coordination sphere can be leveraged as a design element to augment catalysis at primary active sites within confined spaces.     

Synthesis and structures of photoactive manganese–carbonyl complexes derived from 2‐(pyridin‐2‐yl)‐1,3‐benzothiazole and 2‐(quinolin‐2‐yl)‐1,3‐benzothiazole

PhotoCORMs (photo‐active CO‐releasing molecules) have emerged as a class of CO donors where the CO release process can be triggered upon illumination with light of appropriate wavelength. We have recently reported an Mn‐based photoCORM, namely [MnBr(pbt)(CO)₃] [pbt is 2‐(pyridin‐2‐yl)‐1,3‐benzothiazole], where the CO release event can be tracked within cellular milieu by virtue of the emergence of strong blue fluorescence. In pursuit of developing more such trackable photoCORMs, we report herein the syntheses and structural characterization of two Mn^I–carbonyl complexes, namely fac‐tricarbonylchlorido[2‐(pyridin‐2‐yl)‐1,3‐benzothiazole‐κ²N ,N ′]manganese(I), [MnCl(C₁₂H₈N₂S)(CO)₃], (1), and fac‐tricarbonylchlorido[2‐(quinolin‐2‐yl)‐1,3‐benzothiazole‐κ²N ,N ′]manganese(I), [MnCl(C₁₆H₁₀N₂S)(CO)₃], (2). In both complexes, the Mn^I center resides in a distorted octahedral coordination environment. Weak intermolecular C—H…Cl contacts in complex (1) and Cl…S contacts in complex (2) consolidate their extended structures. These complexes also exhibit CO release upon exposure to low‐power broadband visible light. The apparent CO release rates for the two complexes have been measured to compare their CO donating capacity. The fluorogenic 2‐(pyridin‐2‐yl)‐1,3‐benzothiazole and 2‐(quinolin‐2‐yl)‐1,3‐benzothiazole ligands provide a convenient way to track the CO release event through the `turn‐ON' fluorescence which results upon de‐ligation of the ligands from their respective metal centers following CO photorelease.     

Insight into the structures of [M(C5H4I)(CO)3] and [M2(C12H8)(CO)6] (M = Mn and Re) containing strong I...O and π(CO)–π(CO) interactions

The compounds tricarbonyl(η⁵‐1‐iodocyclopentadienyl)manganese(I), [Mn(C₅H₄I)(CO)₃], (I), and tricarbonyl(η⁵‐1‐iodocyclopentadienyl)rhenium(I), [Re(C₅H₄I)(CO)₃], (III), are isostructural and isomorphous. The compounds [μ‐1,2(η⁵)‐acetylenedicyclopentadienyl]bis[tricarbonylmanganese(I)] or bis(cymantrenyl)acetylene, [Mn₂(C₁₂H₈)(CO)₆], (II), and [μ‐1,2(η⁵)‐acetylenedicyclopentadienyl]bis[tricarbonylrhenium(I)], [Re₂(C₁₂H₈)(CO)₆], (IV), are isostructural and isomorphous, and their molecules display inversion symmetry about the mid‐point of the ligand C[triple‐bond]C bond, with the (CO)₃M(C₅H₄) (M = Mn and Re) moieties adopting a transoid conformation. The molecules in all four compounds form zigzag chains due to the formation of strong attractive I...O [in (I) and (III)] or π(CO)–π(CO) [in (I) and (IV)] interactions along the crystallographic b axis. The zigzag chains are bound to each other by weak intermolecular C—H...O hydrogen bonds for (I) and (III), while for (II) and (IV) the chains are bound to each other by a combination of weak C—H...O hydrogen bonds and π(Csp²)–π(Csp²) stacking interactions between pairs of molecules. The π(CO)–π(CO) contacts in (II) and (IV) between carbonyl groups of neighboring molecules, forming pairwise interactions in a sheared antiparallel dimer motif, are encountered in only 35% of all carbonyl interactions for transition metal–carbonyl compounds.