A Series of Dimeric Cobalt Complexes Bridged by N-Heterocyclic Phosphido Ligands.

A tridentate [PPP] ligand has been used to construct a series of dimeric cobalt complexes and explore cooperative multielectron redox processes that are both metal- and ligand-centered. Reduction of (PPClP)CoCl2 (1) with excess magnesium affords the CoICoI N-heterocyclic phosphido (NHP-)-bridged symmetric dimer [(µ-PPP)Co]2 (2). Two-electron oxidation of 2 with FcPF6 generates an asymmetrically bridged dication [(µ-PPP)Co]2[PF6]2 (3) in which the oxidation has occurred in a delocalized fashion throughout the Co2P2 core. In contrast, [(µ-PPP)Co]2+ (5), which can be generated either by one-electron oxidation of 2 with FcPF6 or comportionation of 2 and 3, features an asymmetric geometry and localized mixed valence. Treatment of 1 with the milder reductants CoCp2 and KBEt3H does not lead to formation of 2, 3, or 5 but instead generates dimeric species [(PPP)CoCl]2 (6) and [(PPP)CoH]2 (7). Unlike 2-5, where the phosphine side arms of the tridentate [PPP] ligand span the two Co centers, complex 6 and 7 are connected solely by NHP- ligands that bridge the two (PPP)Co fragments.

O2 Activation by a Heterobimetallic Zr/Co Complex.

Oxygen reduction is a critical half reaction in renewable fuel cell development and a key step in the development of aerobic oxidation reactions. Herein, we report rapid two-electron O2 reduction by a d0 ZrIV center with an appended redox-active Co-I site serving as an electron reservoir. The early/late heterobimetallic Zr/Co complex (THF)Zr(MesNP iPr2)3CoCN tBu (1) reacts readily with O2 and O atom transfer reagents to generate reactive oxygenated species including terminal peroxo and oxo complexes, (O2)Zr(MesNP iPr2)3CoCN tBu (2) and O≡Zr(MesNP iPr2)3CoCN tBu (3). The bimetallic Zr/Co complex provides a new cooperative synthetic pathway to promote multielectron redox processes such as oxygen reduction, with each metal playing a distinct role as a substrate binding site or redox mediator.

Synthesis and Biological Evaluation of 6-[(1 R)-1-Hydroxyethyl]-2,4a( R),6( S),8a( R)-tetrahydropyrano-[3,2- b]-pyran-2-one and Structural Analogues of the Putative Structure of Diplopyrone.

The phytotoxin diplopyrone is considered to be the main phytotoxin in a fungus that is responsible for cork oak decline. A carbohydrate-based synthesis of the enantiomer of the structure proposed for diplopyrone has been developed from a commercially available derivative of d-galactose. Key steps in the synthesis are a highly stereoselective pyranose chain-extension based on methyltitanium, preparation of a vinyl glycoside via Isobe C-alkynylation-rearrangement/reduction, and RCM-based pyranopyran construction. Crystallographic and NMR analysis confirms an earlier report that the structure originally proposed for diplopyrone may require revision. Structural analogues were prepared for biological evaluation, the most promising being a pyranopyran nitrile synthesized from tri- O-acetyl-d-galactal by Ferrier cyanoglycosidation, Wittig chain extension, and lactonization. Biological assays revealed potent antibacterial activity for the nitrile analogue against common bacterial pathogens Edwardsiella ictaluri and Flavobacterium columnare that cause enteric septicemia (ESC) and columnaris disease, respectively, in catfish. The IC50 value of 0.002 against E. ictaluri indicates approximately 100 times greater potency than the antibiotic florfenicol used commercially for this disease. Phytotoxic activity for all three target compounds against duckweed was also observed. The antibiotic and phytotoxic activities of the new pyranopyrans synthesized in this study demonstrate the potential of such compounds as antibiotics and herbicides.

Assessing the Metal-Metal Interactions in a Series of Heterobimetallic Nb/M Complexes (M = Fe, Co, Ni, Cu) and Their Effect on Multielectron Redox Properties.

A one-pot synthetic procedure for a series of bimetallic Nb/M complexes, Cl-Nb( iPrNPPh2)3M-X (M = Fe (2), Ni (4), Cu (5)), is described. A similar procedure aimed at synthesizing a Nb/Co analogue instead affords iPrN═Nb( iPrNPPh2)2(µ-PPh2)Co-I (3) through cleavage of one phosphinoamide P-N bond under reducing conditions. Complexes 4 and 5 are found to have short Nb-M distances, corresponding to unusual metal-metal bonds between Nb and these first row transition metals. For comparison, a series of heterobimetallic O≡Nb( iPrNPPh2)3M-X complexes (M = Fe (7), Co (8), Ni (9), Cu (10)) was synthesized. In these complexes, the NbV center is engaged in sufficient π-bonding to the terminal oxo ligand to remove the driving force for direct metal-metal interactions. A comparison of the cyclic voltammograms of 2 and 4-10 reveals that the presence of a second metal shifts the redox potentials of both Nb and the late metal center anodically, even when direct metal-metal interactions are not present.

Origin of and a Solution for Uneven Efficiency by Cinchona Alkaloid-Derived, Pseudoenantiomeric Catalysts for Asymmetric Reactions.

Cinchona alkaloid-derived chiral catalysts represent one of the most widely applied classes of organocatalysts, which have been successfully utilized in the promotion of a wide variety of asymmetric reactions. Cinchona alkaloids exist in nature as pseudoenantiomers, which allow cinchona alkaloid-catalyzed reactions to provide high enantioselectivities and yields toward both enantiomers of interest in many reactions. On the other hand, the subtle structural difference between pseudoenantiomeric cinchona alkaloids could also lead to uneven efficiency that severely limits the applicability of some cinchona alkaloid-catalyzed reactions. We describe here the elucidation of the origin of and the consequent development of novel modified cinchona alkaloids to address such a problem in asymmetric imine umpolung reactions by cinchonium salts.

Synthesis and Characterization of Neutral Ligand α-Diimine Complexes of Aluminum with Tunable Redox Energetics.

The synthesis and full characterization of a series of neutral ligand α-diimine complexes of aluminum are reported. The compounds [Al(LAr)2Cl2)][AlCl4] [LAr = N, N'-bis(4-R-C6H4)-2,3-dimethyl-1,4-diazabutadiene] are structurally analogous, as determined by multinuclear NMR spectroscopy and solid-state X-ray diffraction, across a range of electron-donating [R = Me (2), tBu (3), OMe (4), and NMe2 (5)] and electron-withdrawing [R = Cl (6), CF3 (7), and NO2 (8)] substituents in the aryl side arm of the ligand. UV-vis absorption spectroscopy and electrochemistry were used to access the optical and electrochemical properties, respectively, of the complexes. Both sets of properties are shown to be dependent on the R substituent. Density functional theory calculations performed on the [Al(LPh)2Cl2)][AlCl4] complex (1) indicate primarily ligand-based frontier orbitals and were used to help support our discussion of both the spectral and electrochemical data. We also report the reaction of the LPh ligand with both AlBr3 and AlI3 and demonstrate a different reactivity profile for the heavier halide relative to the lighter members of the group.

Addition of H2 Across a Cobalt-Phosphorus Bond.

Addition of H2 across the cobalt-phosphorus bond of (PPP)CoPMe3 (3) is demonstrated, where PPP is a monoanionic diphosphine pincer ligand with a central N-heterocyclic phosphido (NHP- ) donor. The chlorophosphine CoII complex (PPCl P)CoCl2 (2) can be generated through coordination of the chlorophosphine ligand (PPCl P, 1) to CoCl2 . Subsequent reduction of 2 with KC8 in the presence of PMe3 generates (PPP)CoPMe3 (3), in which both the phosphorus and cobalt centers have been reduced. The addition of 1 atm of H2 to complex 3 cleanly affords (PPH P)Co(H)PMe3 (4), in which H2 has ultimately been added across the metal-phosphorus bond. Complex 4 was characterized spectroscopically and using computational methods to predict its geometry.

Heterobimetallic Complexes Comprised of Nb and Fe: Isolation of a Coordinatively Unsaturated NbIII/Fe0 Bimetallic Complex Featuring a Nb≡Fe Triple Bond.

Heterometallic multiple bonds between niobium and other transition metals have not been reported to date, likely owing to the highly reactive nature of low-valent niobium centers. Herein, a C3-symmetric tris(phosphinoamide) ligand framework is used to construct a Nb/Fe heterobimetallic complex Cl-Nb(iPrNPPh2)3Fe-Br (2), which features a Fe→Nb dative bond with a metal-metal distance of 2.4269(4) Å. Reduction of 2 in the presence of PMe3 affords Nb(iPrNPPh2)3Fe-PMe3 (6), a compound with an unusual trigonal pyramidal geometry at a NbIII center, a Nb≡Fe triple bond, and the shortest bond distance (2.1446(8) Å) ever reported between Nb and any other transition metal. Complex 6 is thermally unstable and degrades via P-N bond cleavage to form a NbV═NR imide complex, iPrN═Nb(iPrNPPh2)3Fe-PMe3 (9). The heterobimetallic complexes iPrN═Nb(iPrNPPh2)3Fe-Br (8) and 9 are independently synthesized, revealing that the strongly π-bonding imido functionality prevents significant metal-metal interactions. The 57Fe Mössbauer spectra of 2, 6, 8, and 9 show a clear trend in isomer shift (δ), with a decrease in δ as metal-metal interactions become stronger and the Fe center is reduced. The electronic structure and metal-metal bonding of 2, 6, 8, and 9 are explored through computational studies, and cyclic voltammetry is used to better understand the effect of metal-metal interaction in early/late heterobimetallic complexes on the redox properties of the two metals involved.

Cobalt N-Heterocyclic Phosphenium Complexes Stabilized by a Chelating Framework: Synthesis and Redox Properties.

Two cobalt complexes containing coordinated N-heterocyclic phosphenium (NHP+) ligands are synthesized using a bidentate NHP+/phosphine chelating ligand, [PP]+. Treatment of Na[Co(CO)4] with the chlorophosphine precursor [PP]Cl (1) affords [PP]Co(CO)2 (2), which features a planar geometry at the NHP+ phosphorus center and a short Co-P distance [1.9922(4) Å] indicative of a Co═P double bond. The more electron-rich complex [PP]Co(PMe3)2 (3), which is synthesized in a one-pot reduction procedure with 1, CoCl2, PMe3, and KC8, has an even shorter Co-P bond [1.9455(6) Å] owing to stronger metal-to-phosphorus back-donation. The redox properties of 2 and 3 were explored using cyclic voltammetry, and oxidation of 3 was achieved to afford [[PP]Co(PMe3)2]+ (4). The electron paramagnetic resonance spectrum of complex 4 features hyperfine coupling to both 59Co and 31P, suggesting strong delocalization of the unpaired electron density in this complex. Density functional theory calculations are used to further explore the bonding and redox behavior of complexes 2-4, shedding light on the potential for redox noninnocent behavior of NHP+ ligands.

Exploring Trends in Metal-Metal Bonding, Spectroscopic Properties, and Conformational Flexibility in a Series of Heterobimetallic Ti/M and V/M Complexes (M = Fe, Co, Ni, and Cu).

To understand the metal-metal bonding and conformational flexibility of first-row transition metal heterobimetallic complexes, a series of heterobimetallic Ti/M and V/M complexes (M = Fe, Co, Ni, and Cu) have been investigated. The titanium tris(phosphinoamide) precursors ClTi(XylNPiPr2)3 (1) and Ti(XylNPiPr2)3 (2) have been used to synthesize Ti/Fe (3), Ti/Ni (4, 4THF), and Ti/Cu (5) heterobimetallic complexes. A series of V/M (M = Fe (7), Co (8), Ni (9), and Cu (10)) complexes have been generated starting from the vanadium tris(phosphinoamide) precursor V(XylNPiPr2)3 (6). The new heterobimetallic complexes were characterized and studied by NMR spectroscopy, X-ray crystallography, electron paramagnetic resonance, and Mössbauer spectroscopy, where applicable, and computational methods (DFT). Compounds 3, 4THF, 7, and 8 are C3-symmetric with three bridging phosphinoamide ligands, while compounds 9 and 10 adopt an asymmetric geometry with two bridging phosphinoamides and one phosphinoamide ligand bound η2 to vanadium. Compounds 4 and 5, on the other hand, are asymmetric in the solid state but show evidence for fluxional behavior in solution. A correlation is established between conformational flexibility and metal-metal bond order, which has important implications for the future reactivity of these and other heterobimetallic molecules.

Use of a Bidentate Ligand Featuring an N-Heterocyclic Phosphenium Cation (NHP(+)) to Systematically Explore the Bonding of NHP(+) Ligands with Nickel.

A novel bidentate ligand featuring an N-heterocyclic phosphenium cation (NHP(+)) linked to a phosphine side arm is used to explore the coordination chemistry of NHP(+) ligands with nickel. Direct P-Cl bond cleavage from a chlorophosphine precursor [PP]-Cl (1) by Ni(COD)2 affords the asymmetric bimetallic complex [Cl2Ni(µ-PP)2Ni] (2) via a nonoxidative process. Abstraction of the halide with either NaBPh4 or K[B(C6F5)4] prior to metal coordination to form the free phosphenium ligand [PP](+) in situ, followed by coordination to Ni(COD)2, afforded the halide-free Ni(0) complexes [(PP)Ni(COD)] [B(C6F5)4] (4) and [(PP)Ni(COD)][BPh4] (5). Chloride abstraction from 1 is problematic in the presence of a PF6(-) counterion, however, as evident by the formation of [(PP)Ni(PP-F)][PF6] (3). The COD ligand in 5 can be readily displaced with PMe3 or PPh3 to afford [(PP)NiL2][BPh4] (L = PMe3 (6), PPh3 (7)). Complexes 2-7 feature planar geometries about the NHP(+) phosphorus atom and unusually short Ni-P distances, indicative of multiple bonding resulting from both P → Ni σ donation and Ni → P π backbonding. This bonding description is supported by theoretical studies using natural bond orbital analysis.

Heterobimetallic Ti/Co Complexes That Promote Catalytic N-N Bond Cleavage.

Treatment of the tris(phosphinoamide) titanium precursor ClTi(XylNP(i)Pr2)3 (1) with CoI2 leads to the heterobimetallic complex (η(2)-(i)Pr2PNXyl)Ti(XylNP(i)Pr2)2(µ-Cl)CoI (2). One-electron reduction of 2 affords (η(2)-(i)Pr2PNXyl)Ti(XylNP(i)Pr2)2CoI (3), which can be reduced by another electron under dinitrogen to generate the reduced diamagnetic complex (THF)Ti(XylNP(i)Pr2)3CoN2 (4). The removal of the dinitrogen ligand from 4 under vacuum affords (THF)Ti(XylNP(i)Pr2)3Co (5), which features a Ti-Co triple bond. Treatment of 4 with hydrazine or methyl hydrazine results in N-N bond cleavage and affords the new diamagnetic complexes (L)Ti(XylNP(i)Pr2)3CoN2 (L = NH3 (6), MeNH2 (7)). Complexes 4, 5, and 6 have been shown to catalyze the disproportionation of hydrazine into ammonia and dinitrogen gas through a mechanism involving a diazene intermediate.

Formation of heterobimetallic zirconium/cobalt diimido complexes via a four-electron transformation.

The reactivity of the reduced heterobimetallic complex Zr((i)PrNP(i)Pr2)3CoN2 (1) toward aryl azides was examined, revealing a four-electron redox transformation to afford unusual heterobimetallic zirconium/cobalt diimido complexes. In the case of p-tolyl azide, the diamagnetic C3-symmetric bis(terminal imido) complex 3 is formed, but mesityl azide instead leads to asymmetric complex 4 featuring a bridging imido fragment.

One-electron oxidation chemistry and subsequent reactivity of diiron imido complexes.

The chemical oxidation and subsequent group transfer activity of the unusual diiron imido complexes Fe((i)PrNPPh2)3Fe≡NR (R = tert-butyl ((t)Bu), 1; adamantyl, 2) was examined. Bulk chemical oxidation of 1 and 2 with Fc[PF6] (Fc = ferrocene) is accompanied by fluoride ion abstraction from PF6(-) by the iron center trans to the Fe≡NR functionality, forming F-Fe((i)PrNPPh2)3Fe≡NR ((i)Pr = isopropyl) (R = (t)Bu, 3; adamantyl, 4). Axial halide ligation in 3 and 4 significantly disrupts the Fe-Fe interaction in these complexes, as is evident by the >0.3 Å increase in the intermetallic distance in 3 and 4 compared to 1 and 2. Mössbauer spectroscopy suggests that each of the two pseudotetrahedral iron centers in 3 and 4 is best described as Fe(III) and that one-electron oxidation has occurred at the tris(amido)-ligated iron center. The absence of electron delocalization across the Fe-Fe≡NR chain in 3 and 4 allows these complexes to readily react with CO and (t)BuNC to generate the Fe(III)Fe(I) complexes F-Fe((i)PrNPPh2)3Fe(CO)2 (5) and F-Fe((i)PrNPPh2)3Fe((t)BuNC)2 (6), respectively. Computational methods are utilized to better understand the electronic structure and reactivity of oxidized complexes 3 and 4.

Stoichiometric C═O bond oxidative addition of benzophenone by a discrete radical intermediate to form a cobalt(I) carbene.

Single electron transfer from the Zr(III)Co(0) heterobimetallic complex (THF)Zr(MesNP(i)Pr2)3Co-N2 (1) to benzophenone was previously shown to result in the isobenzopinacol product [(Ph2CO)Zr(MesNP(i)Pr2)3Co-N2]2 (2) via coupling of two ketyl radicals. In this work, thermolysis of 2 in an attempt to favor a monomeric ketyl radical species unexpectedly led to cleavage of the C-O bond to generate a Zr/Co µ-oxo species featuring an unusual terminal Co═CPh2 carbene linkage, (η(2)-MesNP(i)Pr2)Zr(µ-O)(MesNP(i)Pr2)2Co═CPh2 (3). This complex was characterized structurally and spectroscopically, and its electronic structure is discussed in the context of density functional theory calculations. Complex 3 was also shown to be active toward carbene group transfer (cyclopropanation), and silane addition to 3 leads to PhSiH2O-Zr(MesNP(i)Pr2)3Co-N2 (5) via a proposed Co-alkyl bond homolysis route.

Noninnocent behavior of bidentate amidophosphido [NP]2- ligands upon coordination to copper.

The synthesis and preliminary coordination chemistry of two new redox-active bidentate ligands containing amido and phosphido donors are described. Treatment of the [(R)NP](2-) (R = Ph, 2,4,6-trimethylphenyl) ligands with CuCl2 and PMe3 results in a dimeric copper(I) P-P coupled product via ligand oxidation. The intermediate of this reaction is proposed to involve a ligand radical generated via oxidation of the [(R)NP](2-) ligand by copper(II), and the existence of such an intermediate is probed using computational methods. Significant radical character on the phosphorus atoms of the alleged [(R)NP](•-)/copper(I) intermediate leads to P-P radical coupling.

Synthesis, structure, and reactivity of an anionic Zr-oxo relevant to CO2 reduction by a Zr/Co heterobimetallic complex.

Oxidative addition of CO2 to the reduced Zr/Co complex (THF)Zr(MesNP(i)Pr2)3Co (1) followed by one-electron reduction leads to formation of an unusual terminal Zr-oxo anion [2][Na(THF)3] in low yield. To facilitate further study of this compound, an alternative high-yielding synthetic route has been devised. First, 1 is treated with CO to form (THF)Zr(MesNP(i)Pr2)3Co(CO) (3); then, addition of H2O to 3 leads to the Zr-hydroxide complex (HO)Zr(MesNP(i)Pr2)3Co(CO) (4). Deprotonation of 4 with Li(N(SiMe3)2) leads to the anionic Zr-oxo species [2][Li(THF)3] or [2][Li(12-c-4)] in the absence or presence of 12-crown-4, respectively. The coordination sphere of the Li(+) countercation is shown to lead to interesting structural differences between these two species. The anionic oxo fragment in complex [2][Li(12-c-4)] reacts with electrophiles such as MeOTf and Me3SiOTf to generate (MeO)Zr(MesNP(i)Pr2)3Co(CO) (5) and (Me3SiO)Zr(MesNP(i)Pr2)3Co(CO) (6), respectively, and addition of acetic anhydride generates (AcO)Zr(MesNP(i)Pr2)3Co(CO) (7). Complex [2][Li(12-c-4)] is also shown to bind CO2 to form a monoanionic Zr-carbonate, [(12-crown-4)Li][(κ(2)-CO3)Zr(MesNP(i)Pr2)3Co(CO)] ([8][Li(12-c-4)]). A more stable version of this compound [8][K(18-c-6)] is formed when a K(+) counteranion and 18-crown-6 are used. Binding of CO2 to [2][Li(12-c-4)] is shown to be reversible using isotopic labeling studies. In an effort to address methods by which these CO2-derived products could be turned over in a catalytic cycle, it is shown that the Zr-OMe bond in 5 can be cleaved using H(+) and the CO ligand can be released from Co under photolytic conditions in the presence of I2.

Multimetallic complexes featuring a bridging N-heterocyclic phosphido/phosphenium ligand: synthesis, structure, and theoretical investigation.

By incorporating an N-heterocyclic phosphenium/phosphide (NHP) ligand into a chelating pincer ligand framework (PPP(+)/PPP(-)), we have elucidated several different and unprecedented binding modes of NHP ligands in homobimetallic, heterobimetallic, and trimetallic metal complexes. One-electron reduction of the previously reported (PPP)(-)/M(II) complexes (PPP)M-Cl (M = Pd (1), Pt (2)) results in clean formation of the symmetric homobimetallic M(I)/M(I) complexes [(µ-PPP)Pd]2 (5) and [(µ-PPP)Pt]2 (6). The tridentate NHP ligand has also been utilized as a bridging linker in the M/Co heterobimetallic compounds (OC)3Co(u-PPP)M(CO) (M = Pd (7), Pt (8)), synthesized via salt elimination from mixtures of 1 and 2 and Na[Co(CO)4]. Furthermore, an NHP-bridged trimetallic complex (PPP)2Pd3Cl2 (9) can be synthesized in a manner similar to precursor 1 (Pd(PPh3)4 + (PPP)Cl) via careful adjustment of reaction stoichiometry. Examination of the interatomic distances and angles in complexes 5-9, in tandem with density functional theory calculations have been used to evaluate and characterize the bonding interactions in these complexes.

Metal-metal bonding in low-coordinate dicobalt complexes supported by phosphinoamide ligands.

Homobimetallic dicobalt complexes featuring metal centers in different coordination environments have been synthesized, and their multielectron redox chemistry has been investigated. Treatment of CoX(2) with MesNKP(i)Pr(2) leads to self-assembly of [(THF)Co(MesNP(i)Pr(2))(2)(µ-X)CoX] [X = Cl (1), I (2)], with one Co center bound to two amide donors and the other bound to two phosphine donors. Upon two-electron reduction, a ligand rearrangement occurs to generate the symmetric species (PMe(3))Co(MesNP(i)Pr(2))(2)Co(PMe(3)) (3), where each Co has an identical mixed P/N donor set. One-electron oxidation of 3 to generate a mixed valence species promotes a ligand reararrangement back to an asymmetric configuration in [(THF)Co(MesNP(i)Pr(2))(2)Co(PMe(3))][PF(6)] (4). Complexes 1-4 have been structurally characterized, and their metal-metal interactions are discussed in the context of computational results.

Metal-metal interactions in C3-symmetric diiron imido complexes linked by phosphinoamide ligands.

The tris(phosphinoamide)-bridged Fe(II)Fe(II) diiron complex Fe(µ-(i)PrNPPh2)3Fe(η(2)-(i)PrNPPh2) (1) can be reduced in the absence or presence of PMe3 to generate the mixed-valence Fe(II)Fe(I) complexes Fe(µ-(i)PrNPPh2)3Fe(PPh2NH(i)Pr) (2) or Fe(µ-(i)PrNPPh2)3Fe(PMe3) (3), respectively. Following a typical oxidative group transfer procedure, treatment of 2 or 3 with organic azides generates the mixed-valent Fe(II)Fe(III) imido complexes Fe((i)PrNPPh2)3Fe≡NR (R = (t)Bu (4), Ad (5), 2,4,6-trimethylphenyl (6)). These complexes represent the first examples of first-row bimetallic complexes featuring both metal-ligand multiple bonds and metal-metal bonds. The reduced complexes 2 and 3 and imido complexes 4-6 have been characterized via X-ray crystallography, Mössbauer spectroscopy, cyclic voltammetry, and SQUID magnetometry, and a theoretical description of the bonding within these diiron complexes has been obtained using computational methods. The effect of the metal-metal interaction on the electronic structure and bonding in diiron imido complexes 4-6 is discussed in the context of similar monometallic iron imido complexes.