Correlating Mechanical Sensitivity with Spin Transition in the Explosive Spin Crossover Complex [Fe(Htrz)3]n[ClO4]2n.

Spin crossover complexes are known to undergo bond length, volume, and enthalpy changes during spin transition. In an explosive spin crossover complex, these changes could affect the mechanical and initiation sensitivity of the explosive and lead to the development of a new class of sensitivity switchable materials. To explore this relationship, the well-known spin crossover compound [Fe(Htrz)3]n[ClO4]2n (1) was re-evaluated for its explosive properties, and its mechanical impact sensitivity was correlated to spin transition. A variable temperature impact test was developed and used to evaluate the impact sensitivity of 1 in the low spin (LS, S = 0), thermally accessed high spin (HS, S = 2), and mixed LS and HS states. For comparison, the structurally similar Ni compound, [Ni(Htrz)3]n[ClO4]2n (2), which does not undergo a spin transition at accessible temperatures, was synthesized and characterized, and its explosive properties and variable temperature impact sensitivity measured. These results reveal a correlation between impact sensitivity and spin transition, where 1 exhibits lower impact sensitivity in the LS state and increases in sensitivity upon transition to the HS state. Density functional theory was used to predict structural changes that occur upon spin transition that correlate to the change in sensitivity. This demonstrates, for the first time, an explosive spin crossover compound (ExSCO) that exhibits switchable impact sensitivity with a fully reversible internal switching mechanism.

Laser Initiation of Fe(II) Complexes of 4-Nitro-pyrazolyl Substituted Tetrazine Ligands.

The synthesis and characterization of new 1,2,4-triazolyl and 4-nitro-pyrazolyl substituted tetrazine ligands has been achieved. The strongly electron deficient 1,2,4-triazolyl substituted ligands did not coordinate Fe(II) metal centers, while the mildly electron deficient 4-nitro-pyrazolyl substituted ligands did coordinate Fe(II) metal centers in a 2:1 ratio of ligand to metal. The thermal stability and mechanical sensitivity characteristics of the complexes are similar to the conventional explosive pentaerythritol tetranitrate. The complexes had strong absorption in the visible region of the spectrum that extended into the near-infrared. In spite of having improved oxygen balances, increased mechanical sensitivity, and similar absorption of NIR light to recently reported Fe(II) tetrazine complexes, these newly synthesized explosives were more difficult to initiate with Nd:YAG pulsed laser light. Specifically, the complexes required lower densities (0.9 g/cm3) to initiate at the same threshold utilized to initiate previous materials at higher densities (1.05 g/cm3).

Energetic Chromophores: Low-Energy Laser Initiation in Explosive Fe(II) Tetrazine Complexes.

The synthesis and characterization of air stable Fe(II) coordination complexes with tetrazine and triazolo-tetrazine ligands and perchlorate counteranions have been achieved. Time-dependent density functional theory (TD-DFT) was used to model the structural, electrochemical, and optical properties of these materials. These compounds are secondary explosives that can be initiated with Nd:YAG laser light at lower energy thresholds than those of PETN. Furthermore, these Fe(II) tetrazine complexes have significantly lower sensitivity than PETN toward mechanical stimuli such as impact and friction. The lower threshold for laser initiation was achieved by altering the electronic properties of the ligand scaffold to tune the metal ligand charge transfer (MLCT) bands of these materials from the visible into the near-infrared region of the electromagnetic spectrum. Unprecedented decrease in both the laser initiation threshold and the mechanical sensitivity makes these materials the first explosives that are both safer to handle and easier to initiate than PETN with NIR lasers.

Synthesis and Electrochemical Behavior of Electron-Rich s-Tetrazine and Triazolo-tetrazine Nitrate Esters.

We have prepared energetic nitrate ester derivatives of 1,2,4,5-tetrazine and 1,2,4-triazolo[4,3-b]-[1,2,4,5]-tetrazine ring systems as model compounds to study the electrochemical behavior of tetrazines in the presence of explosive groups. The model compounds showed lower thermal stabilities relative to PETN (pentaerythritol tetranitrate), but slightly improved mechanical sensitivities. The presence of electron-rich amine donors leads to a cathodic shift of the tetrazine redox potentials relative to those of previously reported tetrazine explosives. At these potentials, electron-rich tetrazines with either covalently bound or co-dissolved nitrate ester groups are irreversibly reduced. Effectively, changes in the electronic structure of tetrazines affect their electrochemical response to the presence of nitrate ester groups. Thus, it may be possible to develop tetrazine-based electrochemical sensors for the detection of specific explosives and electrocatalysts for their disposal.

Two-Photon Absorption in Conjugated Energetic Molecules.

Time-dependent density functional theory (TD-DFT) was used to investigate the relationship between molecular structure and the one- and two-photon absorption (OPA and TPA, respectively) properties of novel and recently synthesized conjugated energetic molecules (CEMs). The molecular structures of CEMs can be strategically altered to influence the heat of formation and oxygen balance, two factors that can contribute to the sensitivity and strength of an explosive material. OPA and TPA are sensitive to changes in molecular structure as well, influencing the optical range of excitation. We found calculated vertical excitation energies to be in good agreement with experiment for most molecules. Peak TPA intensities were found to be significant and on the order of 10(2) GM. Natural transition orbitals for essential electronic states defining TPA peaks of relatively large intensity were used to examine the character of relevant transitions. Modification of molecular substituents, such as additional oxygen or other functional groups, produces significant changes in electronic structure, OPA, and TPA and improves oxygen balance. The results show that certain molecules are apt to undergo nonlinear absorption, opening the possibility for controlled, direct optical initiation of CEMs through photochemical pathways.

Independent Control of Optical and Explosive Properties: Pyrazole-Tetrazine Complexes of First Row Transition Metals.

Complexes of 3-amino-6-(3,5-dimethylpyrazole)tetrazine) (NH2TzDMP, 1) and 3-(3,3'-dinitroazetidine)-6-(3,5-dimethylpyrazole)tetrazine) (DNAZTzDMP, 2) with first row transition metal centers were synthesized. Reactions of Fe(II)(H2O)6(BF4)2 and Fe(NO3)3·9H2O with 1 and 2 both led to complexes of the form [(RTzDMP)3Fe]X2 (X = BF4, R = NH2 (3), DNAZ (4); X = NO3, R = NH2 (5), DNAZ (6)), which showed intense MLCT bands in the visible region of the spectrum. Ligands 1 and 2 also reacted with Cu(II)(NO3)2·5/2H2O to form [(RTzDMP)2Cu(NO3)][NO3] (R = NH2 (7), DNAZ (8)) in addition to reacting with Cu(I)(CH3CN)4(PF6) to form [(RTzDMP)2Cu][PF6] (R = NH2 (9), DNAZ (10)). Lastly reactions of 1 and 2 with Co(NO3)2·6H2O and Ni(NO3)2·6H2O led to [(NH2TzDMP)2Co(H2O) (NO3)][NO3] (11), [(DNAZTzDMP)2Co(H2O)2][NO3]2 (12), [(NH2TzDMP)3Ni][NO3]2 (13), and [(DNAZTzDMP)2Ni(H2O)2][NO3]2 (14). The complexes display rich electrochemical and photophysical properties that are unaffected by derivation with explosive groups.

Synthesis of square-planar aluminum(III) complexes.

The synthesis of two four-coordinate and square planar (SP) complexes of aluminum(III) is presented. Reaction of a phenyl-substituted bis(imino)pyridine ligand that is reduced by two electrons, Na2((Ph)I2P(2-)), with AlCl3 afforded five-coordinate [((Ph)I2P(2-))Al(THF)Cl] (1). Square-planar [((Ph)I2P(2-))AlCl] (2) was obtained by performing the same reaction in diethyl ether followed by lyphilization of 2 from benzene. The four-coordinate geometry index for 2, τ4, is 0.22, where 0 would be a perfectly square-planar molecule. The analogous aluminum hydride complex, [((Ph)I2P(2-))AlH] (3), is also square-planar, and was characterized crystallographically and has τ4=0.13. Both 2 and 3 are Lewis acidic and bind 2,6-lutidine.

High-specificity single-tube multiplex genotyping using Ribo-PAP PCR, tag primers, alkali cleavage of RNA/DNA chimeras and MALDI-TOF MS.

Here, we describe a high-throughput, single-tube, allele-specific ribonucleotide analog pyrophosphorolysis-activated polymerization (ribo-PAP) PCR multiplex genotyping and resequencing method. An RNA/DNA chimeric PCR product is generated using genomic DNA as starting template, a panel of allele-selective 5'-tagged primers, a reverse primer, one nucleotide in the ribo-form (90-100%), the other nucleotides in the deoxy-form, a DNA polymerase capable of incorporating ribonucleotides, a suitable buffer and thermal cycling. The RNA/DNA chimeric PCR products are fragmented by treatment with alkali and analyzed by mass spectrometry. All allele-selective primers have a 5' repetitive motif where each repeat unit has a unique, distinct mass upon reverse copying and alkali fragmentation. The mass of the complement repeat fragment or flag identifies the primer or primers that were recruited in the ribo-PAP PCR. The method readily identifies homozygous and heterozygous positions in simplex or duplex ribo-PAP PCR. Many different tags can be analyzed simultaneously. The assay can genotype several SNPs in a single tube. It thus constitutes the simplest genotyping protocol with multiplex analysis. This novel genotyping and resequencing protocol was applied to different genomic loci: NOS1 and H19 in 30 individuals in simplex ribo-PAP PCR and at two SLCO1B1 loci in 95 individuals in duplex ribo-PAP PCR.

Aluminum-ligand cooperative N-H bond activation and an example of dehydrogenative coupling.

Activation of N-H bonds by a molecular aluminum complex via metal-ligand cooperation is described. ((Ph)I2P(2-))AlH (1b), in which (Ph)I2P(2-) is a tridentate bis(imino)pyridine ligand, reacts with anilines to give the N-H-activated products ((Ph)HI2P(-))AlH(NHAr) (2). When heated, 2 releases H2 and affords ((Ph)I2P(-))Al(NHAr) (3). Complex 1b catalyzes the dehydrogenative coupling of benzylamine to afford H2, NH3, and N-(phenylmethylene)benzenemethanamine.

Ribo-polymerase chain reaction--a facile method for the preparation of chimeric RNA/DNA applied to DNA sequencing.

We describe ribo-polymerase chain reaction (PCR), a method for the preparation of chimeric RNA/DNA. The RNA/DNA chimeric nucleic acids are generated directly from genomic DNA starting templates with two locus-specific primers, three nucleotides in their deoxy form and the fourth in its ribo form, a DNA polymerase capable of incorporating ribo bases, a suitable buffer, and thermal cycling. We have applied ribo-PCR to resequence DNA by directly fragmenting the RNA/DNA chimeras with alkali and analyzing the fragments by mass spectrometry (MS). Mass fingerprint is used to identify deviations from the reference sequence. This method readily detects homozygous sequence deviations as well as heterozygous positions directly from genomic DNA samples. With the high-throughput capability of MS, this facile method is well suited for screening DNA sequences of limited regions of the genome in a large number of individuals. It can also be used to sequence multiple distant genomic loci in a single reaction. This novel ribo-PCR resequencing protocol was applied to different genomic loci involving nitric oxide synthase 1 (NOS1) and H19 in 30 individuals and SLCO1B1 in 95 individuals.

(IP)2Ga(III) complexes facilitate net two-electron redox transformations (IP = α-iminopyridine).

Reaction of M(+)[(IP(2-))(2)Ga](-) (IP = iminopyridine, M = Bu(4)N, 1a; (DME)(3)Na, 1b) with pyridine N-oxide affords two-electron-oxidized (IP(-))(2)Ga(OH) (2) in reactions where the product outcome is independent of the cation identity, M(+). In a second example of net two-electron chemistry, outer sphere oxidation of M(+)[(IP(2-))(2)Ga](-) using either 1 or 2 equiv of the one-electron oxidant ferrocenium afforded [(IP(-))(2)Ga](+) (3) in either 44 or 87% yield, respectively. Reaction with 1 equiv of TEMPO, a one-electron oxidant, afforded the two-electron-oxidized product (IP(-))(2)Ga(TEMPO) (4). Reduction of 2IP by 3Na and subsequent reaction with GaCl(3) yielded a 1:1 mixture of (IP(-))(2)GaCl and 1. Most remarkably, all of these reactions are overall two-electron processes and only the (IP(-))(2)GaX and [(IP(2-))(2)Ga](-) oxidation states are thermodynamically accessible to us. Analogous aluminum chemistry previously afforded either one-electron or two-electron reactions and mixed-valent states. The thermodynamic accessibility of the mixed-valent states of (IP(2-))(IP(-))E, where E = Al or Ga, can be compared using cyclic voltammetry measurements. These measurements indicated that the mixed-valent state [(IP(2-))(IP(-))Ga](+) is not significantly stabilized with respect to disproportionation on the time scale of the electrochemistry experiment. The electrochemically observed differences in thermodynamic stability of the mixed-valent state [(IP(2-))(IP(-))E](+) can be rationalized by the observation that the dihedral angle between the ligand planes containing the π-system of IP is roughly 5° larger in all gallium complexes compared with aluminum analogs. Presumably, a larger dihedral angle provides weaker electronic coupling between the π-systems of IP via the E-X σ* orbital. Alternatively, the observed difference may be a result of the "inert pair effect": a contracted Ga component in the E-X σ* orbital would also afford weaker electronic coupling.

A sterically demanding iminopyridine ligand affords redox-active complexes of aluminum(III) and gallium(III).

The combination of an electrophilic metal center with a redox active ligand set has the potential to provide reactivity unique from transition metal redox chemistry. In this report, substituted iminopyridine complexes containing monoanionic and dianionic (Me)IP(Mes) ligands have been characterized structurally and electronically. Green ((Me)IP(Mes)(-))AlCl(2) (1), ((Me)IP(Mes)(-))AlMe(2) (2), and ((Me)IP(Mes)(-))GaCl(2) (5) have a doublet spin state which results from the anion radical form of (Me)IP(Mes). Purple ((Me)IP(Mes)(2-))AlCl(OEt(2)) (3), ((Me)IP(Mes)(2-))AlMe(OEt(2)) (4), and ((Me)IP(Mes)(2-))GaCl(OEt(2)) (6) are each diamagnetic. We have also investigated the solvent dependence of the decomposition of the (Me)IP(Mes) anion radical. Complexes 1 and 2 can be obtained from benzene and hexanes whereas the use of ether solvents results in the formation of undesirable ((CH2)IP(Mes)(-))AlCl(2) (1a) and ((CH2)IP(Mes)(-))AlCl(2) (2a) formed by loss of a hydrogen atom from the (Me)IP(Mes)(-) ligand. Electrochemical measurements indicate that 1, 2, and 5 are redox active.

Redox routes to substitution of aluminum(III): synthesis and characterization of (IP-)2AlX (IP = α-iminopyridine, X = Cl, Me, SMe, S2CNMe2, C≡CPh, N3, SPh, NHPh).

Redox active ligands are shown to facilitate a variety of group transfer reactions at redox inert aluminum(III). Disulfides can be used as a two-electron group transfer reagent, and we show that (IP(-))(2)AlSR can be formed by reaction of [(THF)(6)Na][(IP(2-))(2)Al] (1c) with disulfides RSSR (where X = C(S)NMe(2), 4; SMe, 5). In a more general redox route to substitution of aluminum bis(iminopyridine) complexes, we report zinc(II) salts as a group transfer reagent. Reaction of [((R)IP(2-))(2)Al](-) (R = H, 1c; Me, 1d) with ZnX(2) affords ((R)IP(-))(2)AlX (where IP = iminopyridine, R = H, and X = Cl, 2; CCPh, 6; N(3), 7; SPh, 8; or R = Me and X = NHPh, 9). Single crystal X-ray diffraction analysis of the complexes reveal that each of the five coordinate complexes reported here has a trigonal bipyramidal geometry with τ = 0.668 - 0.858. We observed a correlation between the greatest deviations from ideal trigonal bipyramidal symmetry (lowest τ values), the bond lengths consistent with smallest degree of ligand reduction, and the least polarizable X ligand in (IP(-))(2)AlX. Complex 4 is six-coordinate and is best described as distorted octahedral. Variable temperature magnetic susceptibility measurements indicate that each of the complexes 3-9 has a biradical electronic structure similar to previously reported 2. Magnetic exchange coupling constants in the range J = -94 to -212 cm(-1) were fit to the data for 2-9 to describe the energy of antiferromagnetic interaction between ligand radicals assuming a spin Hamiltonian of the form H = -2JS(L(1))·S(L(2)). The strongest coupling occurs when the angle between the ligand planes is smallest, presumably to afford good overlap with the Al-X σ* orbital. Electrochemical properties of the complexes were probed using cyclic voltammetry and each of 3-9 displayed a reversible two-electron reduction and two quasi-reversible one-electron oxidation processes. The energy of the ligand based redox processes for 2-9 differ by about 150 mV over all complexes and show a correlation with the degree of IP(-) reduction observed crystallographically; more reduced IP(-) ligands require higher potentials for further reduction. Comproportionation constants that describe the equilibrium for the reaction (IP(-))(2)AlX + (IP)(2)AlX ↔ (IP(-))(IP)AlX fall in the range of K(c) = 10(5.7) to 10(7.9) for 3-9.

Countercations direct one- or two-electron oxidation of an Al(III) complex and Al(III)-oxo intermediates activate C-H bonds.

Hydrogen abstraction by aluminum(III)-oxo intermediates via reaction pathways reminiscent of late transition metal chemistry has been observed. Oxidation of M(+)[(IP(2-))(2)Al](-) (IP = iminopyridine, M = Na, Bu(4)N) yielded [Na(THF)(DME)][(IP(-))(IP(2-))Al(OH)] (3) or [(IP(-))(2)Al(OH)] (4), via O-atom transfer and subsequent C-H activation or proton abstraction, respectively.

A redox series of aluminum complexes: characterization of four oxidation states including a ligand biradical state stabilized via exchange coupling.

Electrophilic activation and subsequent reduction of substrates is in general not possible because highly Lewis acidic metals lack access to multiple redox states. Herein, we demonstrate that transition metal-like redox processes and electronic structure and magnetic properties can be imparted to aluminum(III). Bis(iminopyridine) complexes containing neutral, monoanionic, and dianionic iminopyridine ligands (IP) have been characterized structurally and electronically; yellow (IP)AlCl(3) (1), deep green (IP(-))(2)AlCl (2) and (IP(-))(2)Al(CF(3)SO(3)) (3), and deep purple [(IP(2-))Al](-) (5) are presented. The mixed-valent, monoradical complex (IP(-))(IP(2-))Al is unstable toward C-C coupling, and [(IP(2-))Al](2-)(µ-IP-IP)(2-) (4) has been isolated. Variable-temperature magnetic susceptibility and EPR spectroscopy measurements indicate that the biradical character of the ligand-based triplet in 2 is stabilized by strong antiferromagnetic exchange coupling mediated by aluminum(III): J = -230 cm(-1) for H = -2J(S(L(1))·S(L(2))). Coordination geometry-dependent (IP(-))-(IP(-)) communication through aluminum(III) is observed electrochemically. The cyclic voltammogram of trigonal bipyramidal 2 displays successive ligand-based oxidation events for the two IP(1-/0) processes, at -0.86 and -1.20 V vs SCE. The 0.34 V spacing between redox couples corresponds to a conproportionation constant of K(c) = 10(5.8) for the process (IP(-))(2)AlCl + (IP)(2)AlCl → 2(IP(-))(IP)AlCl consistent with Robin and Day Class II mixed-valent behavior. Tetrahedral 5 displays localized, Class I behavior as indicated by closely spaced redox couples. Furthermore, CV's of 2 and 5 indicate that changes in the coordination environment of the aluminum center shift the potentials for the IP(1-/0) and IP(2-/1-) redox couples by up to 0.9 V.

DNA sequencing by MALDI-TOF MS using alkali cleavage of RNA/DNA chimeras.

Approaches developed for sequencing DNA with detection by mass spectrometry use strategies that deviate from the Sanger-type methods. Procedures demonstrated so far used the sequence specificity of RNA endonucleases, as unfortunately equivalent enzymes for DNA do not exist and therefore require transcription of DNA into RNA prior to fragmentation. We have developed a novel, rapid and accurate concept for DNA sequencing using mass spectrometry and RNA/DNA chimeras and applied it to sequence mitochondrial DNA. Our method is based on the preparation of a chimeric RNA/DNA with a DNA polymerase that also incorporates ribonucleotides. Sequencing is carried out with one ribonucleotide (ATP, CTP or GTP) and the other three nucleotides in their deoxyribo-form. The product is treated with alkali, which cleaves 3' of all ribonucleotides to form a terminal 3' phosphate. Conditions have been streamlined so that molecular, biological and alkali cleavage conditions are compatible with matrix-assisted laser desorption/ionization time-of-flight (MALDI) mass spectrometric analysis. Fragment analysis by MALDI MS provides a sequence-specific fingerprint, which allows the identification of differences between a reference and another sequence. Due to the mass profile, the position and kind of the mutation can be assigned. These differences between signatures are indicative of known, unidentified, rare and private mutations. This novel DNA sequencing protocol was applied to sequence the hypervariable region 1 (HV1) of mitochondrial DNA in 22 individuals.

Examining the chemical and structural properties that influence the sensitivity of energetic nitrate esters.

The sensitivity of explosives is controlled by factors that span from intrinsic chemical reactivity and chemical intramolecular effects to mesoscale structure and defects, and has been a topic of extensive study for over 50 years. Due to these complex competing chemical and physical elements, a unifying relationship between molecular framework, crystal structure, and sensitivity has yet to be developed. In order to move towards this goal, ideally experimental studies should be performed on systems with small, systematic structural modifications, with modeling utilized to interpret experimental results. Pentaerythritol tetranitrate (PETN) is a common nitrate ester explosive that has been widely studied due to its use in military and commercial explosives. We have synthesized PETN derivatives with modified sensitivity characteristics by substituting one -CCH2ONO2 moiety with other substituents, including -CH, -CNH2, -CNH3X, -CCH3, and -PO. We relate the handling sensitivity properties of each PETN derivative to its structural properties, and discuss the potential roles of thermodynamic properties such as heat capacity and heat of formation, thermal stability, crystal structure, compressibility, and inter- and intramolecular hydrogen bonding on impact sensitivity. Reactive molecular dynamics (MD) simulations of the C/H/N/O-based PETN-derivatives have been performed under cook-off conditions that mimic those accessed in impact tests. These simulations infer how changes in chemistry affect the subsequent decomposition pathways.

Synthesis and characterization of bis(imino)pyridine complexes of divalent Mg and Zn.

Phenyl-bis(imino)pyridine (PhI2P) complexes, (PhI2P)ZnCl2 (1), (PhI2P−)ZnCl (2) and (PhI2P−)Zn(py)Cl (3) were obtained with the I2P ligand in both the neutral and the one-electron reduced state. In all examples, the metal ion is Zn(II). Metrical parameters obtained from solid state structures of 2 and 3 indicate that the PhI2P− ligand exists as a radical which is supported at the carbon atom of the imino donor, and this electronic state is also apparent in the analogous one-electron reduced ligand Al(III) complex, (PhI2P−)AlCl2 (4), that we prepared for comparison. We were unable to obtain PhI2P Mg complexes, and so the more electron rich methyl-substituted bis(imino)pyridine ligand, MeI2P, was investigated. Reaction of two-electron reduced MeI2P with MgCl2 and Mg(OTf)2 did afford the two-electron reduced ligand complexes [(MeI2P2−)Mg(THF)]2(µ-MgCl2) (5) and (MeI2P2−)Mg(THF)2 (6), respectively (MeI2P = 2,6-bis(1-methylethyl)-N-(2-pyridinylmethylene)phenylamine). Complex 5 crystallizes as a trinuclear Mg complex consisting of two (MeI2P2−)Mg moieties bridged by MgCl2 and the (MeI2P2−) ligand is symmetric across the pyridine ring, but is not planar. In contrast, the (MeI2P2−) ligand in 6 is asymmetric across the pyridine ring and all atoms in the ligand are coplanar. Cyclic voltammetry measurements reveal that in complexes, 1, 4, 5, 6, the I2P0, I2P−, and I2P2− ligand charge states are accessible electrochemically.

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.