Yunho Lee

Silylation of Iron-Bound Carbon Monoxide Affords a Terminal Fe Carbyne

A series of monocarbonyl iron complexes in the formal oxidation states 0, +1, and +2 are accessible when supported by a tetradentate tris(phosphino)silyl ligand (SiP(iPr)(3) = [Si(o-C(6)H(4)PiPr(2))(3)](-)). X-ray diffraction (XRD) studies of these carbonyl complexes establish little geometrical change about the iron center as a function of oxidation state. It is possible to functionalize the terminal CO ligand of the most reduced carbonyl adduct by addition of SiMe(3)(+) to afford a well-defined iron carbyne species, (SiP(iPr)(3))Fe≡C-OSiMe(3). Single-crystal XRD data of this iron carbyne derivative reveal an unusually short Fe≡C-OSiMe(3) bond distance (1.671(2) Å) and a substantially elongated C-O distance (1.278(3) Å), consistent with Fe-C carbyne character. The overall trigonal bipyramidal geometry of (SiP(iPr)(3))Fe≡C-OSiMe(3) compares well with that of the corresponding carbonyls, (SiP(iPr)(3))Fe(CO)(-), (SiP(iPr)(3))Fe(CO), and (SiP(iPr)(3))Fe(CO)(+). Details regarding the electronic structure of the carbyne complex have been explored via the collection of comparative Mössbauer data for all of the complexes featured and also via DFT calculations. In sum, these data point to a strongly π-accepting Fischer-type carbyne ligand that confers stability to a low-valent iron(0) rather than high-valent iron(IV) center.

Transmethylation of a four-coordinate nickel(I) monocarbonyl species with methyl iodide

Three distinct oxidation states of a nickel carbonyl species, formally Ni(II), +1 and 0, (compounds 1, 2 and 3 respectively) have been realized using a (PNP)Ni scaffold (PNP− = N[2-PiPr2-4-Me-C6H3]2−). X-ray diffraction (XRD) studies of these carbonyl complexes show a geometrical change about the nickel center from square planar (1) to pyramidal (2) and pseudotetrahedral (3). Interestingly, the Ni–C bond distance of 2 is longer than that of 1 and 3 due to the electron population of the antibonding dx2−y2 orbital. A difference in the reactivity of these nickel carbonyl species was clearly observed. Reaction of the monovalent nickel carbonyl species (2) with CH3I revealed the formation of (PNP)NiCOCH3 (4) via C–C bond coupling while the zerovalent congener (3) underwent an oxidative ligand substitution reaction.

Phosphinite-Ni(0) Mediated Formation of a Phosphide-Ni(II)-OCOOMe Species via Uncommon Metal–Ligand Cooperation

Reversible transformations are observed between a phosphide-nickel(II) alkoxide and a phosphinite-nickel(0) species via a P-O bond formation coupled with a 2-e(-) redox change at the nickel center. In the forward reaction, the nickel(0) dinitrogen species (PP(OMe)P)Ni(N2) (2) and {(PP(OMe)P)Ni}2(μ-N2) (3) were formed from the reaction of (PPP)NiCl (1) with a methoxy anion. In the backward reaction, a (PPP)Ni(II) moiety was regenerated from the CO2 reaction of 3 with the concomitant formation of a methyl carbonate ligand in (PPP)Ni(OCOOMe) (7). Thus, unanticipated metal-ligand cooperation involving a phosphide based ligand is reported.

Direct CO2 Addition to a Ni(0)–CO Species Allows the Selective Generation of a Nickel(II) Carboxylate with Expulsion of CO

Addition of CO2 to a low-valent nickel species has been explored with a newly designed acriPNP pincer ligand (acriPNP- = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide). This is a crucial step in understanding biological CO2 conversion to CO found in carbon monoxide dehydrogenase (CODH). A four-coordinate nickel(0) state was reliably accessed in the presence of a CO ligand, which can be prepared from a stepwise reduction of a cationic {(acriPNP)Ni(II)-CO}+ species. All three Ni(II), Ni(I), and Ni(0) monocarbonyl species were cleanly isolated and spectroscopically characterized. Addition of electrons to the nickel(II) species significantly alters its geometry from square planar toward tetrahedral because of the filling of the dx2-y2 orbital. Accordingly, the CO ligand position changes from equatorial to axial, ∠N-Ni-C of 176.2(2)° to 129.1(4)°, allowing opening of a CO2 binding site. Upon addition of CO2 to a nickel(0)-CO species, a nickel(II) carboxylate species with a Ni(η1-CO2-κC) moiety was formed and isolated (75%). This reaction occurs with the concomitant expulsion of CO(g). This is a unique result markedly different from our previous report involving the flexible analogous PNP ligand, which revealed the formation of multiple products including a tetrameric cluster from the reaction with CO2. Finally, the carbon dioxide conversion to CO at a single nickel center is modeled by the successful isolation of all relevant intermediates, such as Ni-CO2, Ni-COOH, and Ni-CO.

Selective Transformation of CO2 to CO at a Single Nickel Center

Carbon dioxide conversion mediated by transition metal complexes continues to attract much attention because of its future potential utilization as a nontoxic and inexpensive C1 source for the chemical industry. Given the presence of nickel in natural systems that allow for extremely efficient catalysis, albeit in an Fe cluster arrangement, studies that focus on selective CO2 conversion with synthetic nickel species are currently of considerable interest in our group. In this Account, the selective conversion of CO2 to carbon monoxide occurring at a single nickel center is discussed. The chemistry is based on a series of related nickel pincer complexes with attention to the uniqueness of the coordination geometry, which is crucial in allowing for particular reactivity toward CO2. Our research is inspired by the efficient enzymatic CO2 catalysis occurring at the active site of carbon monoxide dehydrogenase. Since the binding and reactivity toward CO2 are controlled in part by the geometry of a L3Ni scaffold, we have explored the chemistry of low-valent nickel supported by PPMeP and PNP ligands, in which a pseudotetrahedral or square-planar geometry is accommodated. Two isolated nickel-CO2 adducts, (PPMeP)Ni(η2-CO2-κ C) (2) and {Na(12-C-4)2}{(PNP)Ni(η1-CO2-κ C)} (7), clearly demonstrate that the geometry of the nickel ion is crucial in the binding of CO2 and its level of activation. In the case of a square-planar nickel center supported by a PNP ligand, a series of bimetallic metallacarboxylate Ni-μ-CO2-κ C, O-M species (M = H, Na, Ni, Fe) were synthesized, and their structural features and reactivity were studied. Protonation cleaves the C-O bond, resulting in the formation of a nickel(II) monocarbonyl complex. By sequential reduction, the corresponding mono- and zero-valent Ni-CO species were produced. The reactivities of three nickel carbonyl species toward various iodoalkanes and CO2 were explored to address whether their corresponding reactivities could be controlled by the number of valence d electrons. In particular, a (PNP)Ni(0)-CO species (13) shows immediate reactivity toward CO2 but displays multiple product formation. By incorporation of a -CMe2- bridging unit, a structurally rigidified acriPNP ligand was newly designed and produced. This ligand modification was successful in preparing the T-shaped nickel(I) metalloradical species 9 exhibiting open-shell reactivity due to the sterically exposed nickel center possessing a half-filled d x2- y2 orbital. More importantly, the selective addition of CO2 to a nickel(0)-CO species was enabled to afford a nickel(II)-carboxylate species (22) with the expulsion of CO(g). Finally, the (acriPNP)Ni system provides a synthetic cycle in the study of the selective conversion of CO2 to CO that involves two-electron reduction of Ni-CO followed by the direct addition of CO2 to release the coordinated CO ligand.

A nonclassical dihydrogen adduct of S = ½ Fe(I).

We have exploited the capacity of the ‘(SiPiPr3)Fe(I)’ scaffold to accommodate additional axial ligands and have characterized the mononuclear, S = ½ H2-adduct complex (SiPiPr3)FeI(H2). EPR and ENDOR data, in the context of X-ray structural results, reveal that this complex provides a highly unusual example of an open-shell metal complex that binds dihydrogen as a ligand. The H2 ligand at 2 K dynamically reorients within the ligand-binding pocket, tunneling among the energy minima created by strong interactions with the three Fe-P bonds.

Characterization of a Non-classical Dihydrogen Adduct of an S = ½ Fe(I) Center

We have exploited the capacity of the ‘(SiPiPr3)Fe(I)’ scaffold to accommodate additional axial ligands and have characterized the mononuclear, S = ½ H2-adduct complex (SiPiPr3)FeI(H2). EPR and ENDOR data, in the context of X-ray structural results, reveal that this complex provides a highly unusual example of an open-shell metal complex that binds dihydrogen as a ligand. The H2 ligand at 2 K dynamically reorients within the ligand-binding pocket, tunneling among the energy minima created by strong interactions with the three Fe-P bonds.