Timothy J. Schmeier

Secondary Coordination Sphere Interactions Facilitate the Insertion Step in an Iridium(III) CO2 Reduction Catalyst

There is considerable interest in both catalysts for CO(2) conversion and understanding how CO(2) reacts with transition metal complexes. Here we develop a simple model for predicting the thermodynamic favorability of CO(2) insertion into Ir(III) hydrides. In general this reaction is unfavorable; however, we demonstrate that with a hydrogen bond donor in the secondary coordination sphere it is possible to isolate a formate product from this reaction. Furthermore, our CO(2) inserted product is one of the most active water-soluble catalysts reported to date for CO(2) hydrogenation.

Synthesis and Structure of Six-Coordinate Iron Borohydride Complexes Supported by PNP Ligands

The preparation of a number of iron complexes supported by ligands of the type HN{CH2CH2(PR2)}2 [R = isopropyl (((i)Pr)PNP) or cyclohexyl ((Cy)PNP)] is reported. This is the first time this important bifunctional ligand has been coordinated to iron. The iron(II) complexes (((i)Pr)PNP)FeCl2(CO) (1a) and ((Cy)PNP)FeCl2(CO) (1b) were synthesized through the reaction of the appropriate free ligand and FeCl2 in the presence of CO. The iron(0) complex (((i)Pr)PNP)Fe(CO)2 (2a) was prepared through the reaction of Fe(CO)5 with ((i)Pr)PNP, while irradiating with UV light. Compound 2a is unstable in CH2Cl2 and is oxidized to 1a via the intermediate iron(II) complex [(((i)Pr)PNP)FeCl(CO)2]Cl (3a). The reaction of 2a with HCl generated the related complex [(((i)Pr)PNP)FeH(CO)2]Cl (4a), while the neutral iron hydrides (((i)Pr)PNP)FeHCl(CO) (5a) and ((Cy)PNP)FeHCl(CO) (5b) were synthesized through the reaction of 1a or 1b with 1 equiv of (n)Bu4NBH4. The related reaction between 1a and excess NaBH4 generated the unusual η(1)-HBH3 complex (((i)Pr)PNP)FeH(η(1)-HBH3)(CO) (6a). This complex features a bifurcated intramolecular dihydrogen bond between two of the hydrogen atoms associated with the η(1)-HBH3 ligand and the N-H proton of the pincer ligand, as well as intermolecular dihydrogen bonding. The protonation of 6a with 2,6-lutidinium tetraphenylborate resulted in the formation of the dimeric complex [{(((i)Pr)PNP)FeH(CO)}2(μ2,η(1):η(1)-H2BH2)][BPh4] (7a), which features a rare example of a μ2,η(1):η(1)-H2BH2 ligand. Unlike all previous examples of complexes with a μ2,η(1):η(1)-H2BH2 ligand, there is no metal-metal bond and additional bridging ligand supporting the borohydride ligand in 7a; however, it is proposed that two dihydrogen-bonding interactions stabilize the complex. Complexes 1a, 2a, 3a, 4a, 5a, 6a, and 7a were characterized by X-ray crystallography.

Organometallic Ni Pincer Complexes for the Electrocatalytic Production of Hydrogen

Nonplatinum metals are needed to perform cost-effective water reduction electrocatalysis to enable technological implementation of a proposed hydrogen economy. We describe electrocatalytic proton reduction and H(2) production by two organometallic nickel complexes with tridentate pincer ligands. The kinetics of H(2) production from voltammetry is consistent with an overall third order rate law: the reaction is second order in acid and first order in catalyst. Hydrogen production with 90-95% Faradaic yields was confirmed by gas analysis, and UV-vis spectroscopy suggests that the ligand remains bound to the catalyst over the course of the reaction. A computational study provides mechanistic insights into the proposed catalytic cycle. Furthermore, two proposed intermediates in the proton reduction cycle were isolated in a representative system and show a catalytic response akin to the parent compound.

Synthesis of PCP-supported nickel complexes and their reactivity with carbon dioxide.

The Ni amide and hydroxide complexes [(PCP)Ni(NH(2))] (2; PCP=bis-2,6-di-tert-butylphosphinomethylbenzene) and [(PCP)Ni(OH)] (3) were prepared by treatment of [(PCP)NiCl] (1) with NaNH(2) or NaOH, respectively. The conditions for the formation of 3 from 1 and NaOH were harsh (2 weeks in THF at reflux) and a more facile synthetic route involved protonation of 2 with H(2)O, to generate 3 and ammonia. Similarly the basic amide in 2 was protonated with a variety of other weak acids to form the complexes [(PCP)Ni(2-Me-imidazole)] (4), [(PCP)Ni(dimethylmalonate)] (5), [(PCP)Ni(oxazole)] (6), and [(PCP)Ni(CCPh)] (7), respectively. The hydroxide compound 3, could also be used as a Ni precursor and treatment of 3 with TMSCN (TMS=trimethylsilyl) or TMSN(3) generated [(PCP)Ni(CN)] (8) or [(PCP)Ni(N(3))] (9), respectively. Compounds 3-7, and 9 were characterized by X-ray crystallography. Although 3, 4, 6, 7, and 9 are all four-coordinate complexes with a square-planar geometry around Ni, 5 is a pseudo-five-coordinate complex, with the dimethylmalonate ligand coordinated in an X-type fashion through one oxygen atom, and weakly as an L-type ligand through another oxygen atom. Complexes 2-9 were all reacted with carbon dioxide. Compounds 2-4 underwent facile reaction at low temperature to form the κ(1)-O carboxylate products [(PCP)Ni{OC(O)NH(2)}] (10), [(PCP)Ni{OC(O)OH}] (11), and [(PCP)Ni{OC(O)-2-Me-imidazole}] (12), respectively. Compounds 10 and 11 were characterized by X-ray crystallography. No reaction was observed between 5-9 and carbon dioxide, even at elevated temperatures. DFT calculations were performed to model the thermodynamics for the insertion of carbon dioxide into 2-9 to form a κ(1)-O carboxylate product and understand the pathways for carbon dioxide insertion into 2, 3, 6, and 7. The computed free energies indicate that carbon dioxide insertion into 2 and 3 is thermodynamically favorable, insertion into 8 and 9 is significantly uphill, insertion into 5 and 7 is slightly uphill, and insertion into 4 and 6 is close to thermoneutral. The pathway for insertion into 2 and 3 has a low barrier and involves nucleophilic attack of the nitrogen or oxygen lone pair on electrophilic carbon dioxide. A related stepwise pathway is calculated for 7, but in this case the carbon of the alkyne is significantly less nucleophilic and as a result, the barrier for carbon dioxide insertion is high. In contrast, carbon dioxide insertion into 6 involves a single concerted step that has a high barrier.

Flexible Binding of PNP Pincer Ligands to Monomeric Iron Complexes

Transition metal complexes supported by pincer ligands have many important applications. Here, the syntheses of five-coordinate PNP pincer-supported Fe complexes of the type (PNP)FeCl2 (PNP = HN{CH2CH2(PR2)}2, R = iPr ((iPr)PNP), tBu ((tBu)PNP), or cyclohexyl ((Cy)PNP)) are reported. In the solid state, ((iPr)PNP)FeCl2 was characterized in two different geometries by X-ray crystallography. In one form, the (iPr)PNP ligand binds to the Fe center in the typical meridional geometry for a pincer ligand, whereas in the other form, the (iPr)PNP ligand binds in a facial geometry. The electronic structures and geometries of all of the (PNP)FeCl2 complexes were further explored using (57)Fe Mössbauer and magnetic circular dichroism spectroscopy. These measurements show that in some cases two isomers of the (PNP)FeCl2 complexes are present in solution and conclusively demonstrate that binding of the PNP ligand is flexible, which may have implications for the reactivity of this important class of compounds.

An Unusual Example of Hypervalent Silicon: A Five‐Coordinate Silyl Group Bridging Two Palladium or Nickel Centers through a Nonsymmetrical Four‐Center Two‐Electron Bond

Pd and Ni dimers supported by PSiP ligands in which two hypervalent five-coordinate Si atoms bridge the two metal centers are reported. Crystallographic characterization revealed a rare square-pyramidal geometry at Si and an unusual asymmetric M2 Si2 core (M=Pd or Ni). DFT calculations showed that the unusual structure of the core is also found in a model in which the phosphine and Si centers are not part of a pincer group, thus indicating that the observed geometry is not imposed by the PSiP ligand. NBO analysis showed that an asymmetric four-center two-electron (4c-2e) bond stabilizes the hypervalent Si atoms in the M2 Si2 core.