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.

Cooperative activation of O-H and S-H bonds across the Co-P bond of an N-heterocyclic phosphido complex.

Metal-ligand cooperation has proven to be a viable concept for σ bond activation and catalysis, however there are few examples involving phosphorus as an active participant in bond cleavage. The reactivity of E-H bonds (E = S, O) across a metal-phosphorus bond of a cobalt(i) center ligated by a tridenate N-heterocyclic phosphido (NHP-) ligand with diphosphine sidearms, (PPP)-, has been explored. Addition of PhOH to (PPP)CoPMe3 (1) cleanly affords (PPOPhP)Co(H)PMe3 (2), in which the O-H bond was heterolytically cleaved across the M-PNHP bond. Addition of PhSH to 1 first generates (PPHP)Co(SPh)PMe3 (3), which undergoes an intermolecular rearrangement to generate (PPSPhP)Co(H)PMe3 (4) as the thermodynamic product. A comparison with a related platinum(ii) system reveals the subtle effects that variations in metal intrinsic properties can have on metal-ligand bifunctional σ bond activation processes.

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.

An Investigation into Rigidity-Activity Relationships in BisQAC Amphiphilic Antiseptics.

Twenty-one mono- and biscationic quaternary ammonium amphiphiles (monoQACs and bisQACs) were rapidly prepared in order to investigate the effects of rigidity of a diamine core structure on antiseptic activity. As anticipated, the bioactivity against a panel of six bacteria including methicillin-resistant Staphylococcus aureus (MRSA) strains was strong for bisQAC structures, and is clearly correlated with the length of non-polar side chains. Modest advantages were noted for amide-containing side chains, as compared with straight-chained alkyl substituents. Surprisingly, antiseptics with more rigidly disposed side chains, such as those in DABCO-12,12, showed the highest level of antimicrobial activity, with single-digit MIC values or better against the entire bacterial panel, including sub-micromolar activity against an MRSA strain.

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.

Multi-electron redox processes at a Zr(iv) center facilitated by an appended redox-active cobalt-containing metalloligand.

The reactivity of a reduced heterobimetallic Co(-I)/Zr(IV) complex, ((t)BuNC)Co((i)Pr2PNMes)3Zr(THF) (), with a series of azido and diazo reagents is explored to demonstrate the feasibility of facilitating two-electron redox processes at a formally d(0) Zr(iv) center using the appended Co fragment exclusively as an electron-reservoir. Addition of mesityl or adamantyl azide to affords the terminal ((t)BuNC)Co((i)Pr2PNMes)3Zr[triple bond, length as m-dash]NMes () and bridging ((t)BuNC)Co((i)Pr2PNMes)2(µ-NAd)Zr((i)Pr2PNMes) () Co(I)/Zr(IV) imido products, respectively. Similarly, diphenyldiazomethane reacts with to afford the terminal Ph2CN2(2-)-bound product ((t)BuNC)Co((i)Pr2PNMes)3Zr[triple bond, length as m-dash]N-N[double bond, length as m-dash]CPh2 () via a two-electron oxidation of the Co center. Thermolysis of results in a structural rearrangement to the diazomethane-bridged isomer ((t)BuNC)Co((i)Pr2PNMes)2(µ-N2CPh2)Zr((i)Pr2PNMes) (). In contrast, treatment of with 0.5 equivalents of the conjugated diazo reagent ethyl diazoacetate affords a tetranuclear Zr(IV)/Co(0) complex, ((t)BuNC)Co((i)Pr2PNMes)3Zr(µ2-κ(1)-O-η(2)-N,N-OC(OEt)CHN2)Zr(MesNP(i)Pr2)3Co(CN(t)Bu) (), bridged through enolate and η(2)-bound diazo functionalities.

N-heterocyclic phosphenium and phosphido nickel complexes supported by a pincer ligand framework.

A chelating diphosphine ligand with a central N-heterocyclic phosphenium cation (NHP(+)) has been used to explore the coordination chemistry of NHPs with nickel. Treatment of the chlorophosphine precursor [PPP]Cl (1) with stoichiometric Ni(COD)2 affords (PPP)NiCl (8), which is best described as a Ni(II)/NHP(-) phosphido complex formed via oxidative addition of the P-Cl bond. In contrast, treating [PPP]Cl (1) with excess Ni(COD)2 results in a mixture of the trimetallic complex (PPP)2Ni3Cl2 (9) and the reduced NHP-bridged dimer [(PPP)Ni]2 (10). Compound 9 is found to be a Ni(II)Ni(II)Ni(0) complex in which the two NHP ligands act as bridging NHP(-) phosphidos, while complex 10 is a Ni(I)Ni(I) complex that is highly delocalized throughout the symmetric Ni2P2 core. In contrast, the reaction of [PPP][PF6] (11) with Ni(COD)2 affords an asymmetrically-bridged dication [(PPP)Ni]2[PF6]2 (12), which is found to contain two bridging NHP(+) cations bridging two Ni(0) centers. Comproportionation of 10 and 12 affords monocationic [(PPP)Ni]2[PF6] (13), completing the redox series. Nickel complexes 8-10 and 12 are largely similar to their Pd and Pt analogues, but a paramagnetic monocation such as 13 was not observed in the Pd and Pt case. Computational studies lend further insight into the electronic structure and bonding in complexes 8-10 and 12-13, and further support the potential redox non-innocent properties of NHP ligands.

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.

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.

A heterobimetallic complex featuring a Ti-Co multiple bond and its application to the reductive coupling of ketones to alkenes.

To explore metal-metal multiple bonds between first row transition metals, Ti/Co complexes supported by two phosphinoamide ligands have been synthesized and characterized. The Ti metalloligand Cl2Ti(XylNPiPr2)2 (1) was treated with CoI2 under reducing conditions, permitting isolation of the Ti/Co complex [(µ-Cl)Ti(XylNPiPr2)2CoI]2 (2). One electron reduction of complex 2 affords ClTi(XylNPiPr2)2CoPMe3 (3), which features a metal-metal triple bond and an unprecedentedly short Ti-Co distance of 2.0236(9) Å. This complex is shown to promote the McMurry coupling reaction of aryl ketones into alkenes, with concomitant formation of the tetranuclear complex [Ti(µ3-O)(NXylPiPr2)2CoI]2 (4). A cooperative mechanism involving bimetallic C[double bond, length as m-dash]O bond activation and a cobalt carbene intermediate is proposed.

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.

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.