Falguni Basuli

Structural, Spectroscopic, and Theoretical Elucidation of a Redox-Active Pincer-Type Ancillary Applied in Catalysis

Pincer-type ligands are believed to be very robust scaffolds that can support multifarious functionalities as well as highly reactive metal motifs applied in organometallic chemistry, especially in the realm of catalysis. In this paper, we describe the redox and, therefore, noninnocent behavior of a PNP (PNP- = N[2-P(CHMe2)2-4-methylphenyl]2) pincer ancillary bound to nickel. A combination of structural, spectroscopic, and theoretical techniques suggests that this type of framework can house an electron hole when coordinated to Ni(II).

Evidence for the Existence of a Terminal Imidoscandium Compound: Intermolecular CH Activation and Complexation Reactions with the Transient ScNAr Species

Terminal imide ligands in early-transition-metal chemistry have recently undergone a dramatic renaissance, given their potential applications in processes such as group transfer and catalysis. Absent from this extensive list are Group 3 transition-metal imides, an antithesis given the inherent affinity of the highly electropositive metal ions for a hard donor such as nitrogen. To date, complexes of Group 3 transition-metal elements (including the lanthanides) with terminal imido ligands have been neither directly detected nor isolated; their existence during the formation of a narrow list of dinuclear or polynuclear bridging imides has only been speculated. The inability to isolate a terminal imide linkage may be due to the discrepancy in energy between the lanthanide and imide-nitrogen orbitals, rendering this type of bond highly polarized and thus prohibiting the formal M=NR or M NR bond that is prototypical among most early transition metals. As a result, such a mismatch in orbital energies should bestow unprecedented nucleophilic behavior to the imido nitrogen atom when coordinated to an ion such as a lanthanide. Herein, we present credible evidence for the existence of a terminal scandium imido complex by applying a combination of isotopic labeling and reactivity studies of a transient Sc=NR complex, evidenced by the intermolecular C H activation of pyridine and benzene as well as complexation with Al(CH3)3. The fact that we can generate transient, reactive titanium alkylidynes of the type [(PNP)Ti CR] (PNP = N[2-P(CHMe2)2-4-methylphenyl]2, R = Ph, SiMe3, and tBu, among other groups) encouraged the search for an isolobal {(PNP)Sc NR} fragment, owing to the comparable atomic radii between titanium(IV) and scandium(III) when weighed against the other Group 3 congeners. Likewise, the PNP ligand type has been recently demonstrated to be an ideal ancillary support in the preparation of an unprecedented bridging phosphinidene moiety on lutetium(III). For us, assembling the PNP ancillary ligand and Sc to form [(PNP)ScCl2] (1) in 95% yield proved straightforward by treatment of Li(PNP) with [ScCl3(thf)3] in toluene at 70 8C over 48 h. Bright yellow 1 can be readily transmetalated with LiNHAr (Ar = 2,6-iPr2C6H3) to afford [(PNP)Sc(NHAr)Cl] (2) in 76% yield (Scheme 1). To incorporate a

Synthesis, structure and redox properties of some thiosemicarbazone complexes of rhodium

Reaction of salicylaldehyde thiosemicarbazone (H2L1), 2-hydroxyacetophenone thiosemicarbazone (H2L2) and 2-hydroxynaphthaldehyde thiosemicarbazone (H2L3) (general abbreviation H2L, where H2 stands for the two dissociable protons, one phenolic proton and one hydrazinic proton) with [Rh(PPh3)3Cl] afforded a family of rhodium(III) complexes of the type [Rh(PPh3)2(L)Cl]. The crystal structure of [Rh(PPh3)2(L2)Cl] has been determined by X-ray diffraction. The thiosemicarbazone ligands are coordinated, via dissociation of the two protons, as dianionic tridentate O,N,S-donor ligands forming one six-membered and one five-membered chelate rings. The complexes are diamagnetic (low-spin d6, S=0) and their 1H NMR spectra are in excellent agreement with their compositions. All three [Rh(PPh3)2(L)Cl] complexes display intense absorptions in the visible and ultraviolet regions. They also show strong emission in the visible region at ambient temperature. Cyclic voltammetry on all the complexes shows two irreversible oxidations, the first one is observed within 0.77 to 0.85 V vs. SCE and the second one within 1.13 to 1.23 V vs. SCE, and one irreversible reduction within −1.05 to −1.14 V vs. SCE.

Thiosemicarbazone complexes of the platinum metals. A story of variable coordination modes

Salicylaldehyde thiosemicarbazone (H2saltsc) reacts with [M(PPh3)3X2] (M = Ru, Os; X = Cl, Br) to afford complexes of type [M(PPh3)2(Hsaltsc)2], in which the salicylaldehyde thiosemicarbazone ligand is coordinated to the metal as a bidentate N,S-donor forming a four-membered chelate ring. Reaction of benzaldehyde thiosemicarbazones (Hbztsc-R) with [M(PPh3)3X2] also affords complexes of similar type, viz. [M(PPh3)2(bztsc-R)2], in which the benzaldehyde thiosemicarbazones have also been found to coordinate the metal as a bidentate N,S-donor forming a four-membered chelate ring as before. Reaction of the Hbztsc-R ligands has also been carried out with [M(bpy)2X2] (M = Ru, Os; X = Cl, Br), which has afforded complexes of type [M(bpy)2(bztsc-R)]+, which have been isolated as perchlorate salts. Coordination mode of bztsc-R has been found to be the same as before. Structure of the Hbztsc-OMe ligand has been determined and some molecular modelling studies have been carried out determine the reason for the observed mode of coordination. Reaction of acetone thiosemicarbazone (Hactsc) has then been carried out with [M(bpy)2X2] to afford the [M(bpy)2(actsc)]ClO4 complexes, in which the actsc ligand coordinates the metal as a bidentate N,S-donorformingafive-membered chelate ring. Reaction of H2saltsc has been carried out with [Ru(bpy)2Cl2] to prepare the [Ru(bpy)2(Hsaltsc)]ClO4 complex, which has then been reacted with one equivalent of nickel perchlorate to afford an octanuclear complex of type [Ru(bpy)2(saltsc-H)4Ni4](ClO4)4.

Chemistry of ruthenium with some phenolic ligands: synthesis, structure and redox properties

Abstract Reaction of three phenolate ligands, viz. salicylaldehyde (HL 1 ), 2-hydroxyacetophenone (HL 2 ) and 2-hydroxynaphthylaldehyde (HL 3 ), (abbreviated in general as HL, where H stands for the phenolic proton) with [Ru(PPh 3 ) 3 Cl 2 ] in 1:1 mole ratio gives complexes of the type [Ru(PPh 3 ) 2 (L)Cl 2 ]. The structure of the [Ru(PPh 3 ) 2 (L 2 )Cl 2 ] complex has been solved by X-ray crystallography. The coordination sphere around ruthenium is O 2 P 2 Cl 2 with a cis – trans – cis geometry, respectively. The [Ru(PPh 3 ) 2 (L)Cl 2 ] complexes are one-electron paramagnetic (low-spin d 5 , S =1/2) and show rhombic ESR spectra in 1:1 dichloromethane–toluene solution at 77 K. In dichloromethane solution the [Ru(PPh 3 ) 2 (L)Cl 2 ] complexes show several intense LMCT transitions in the visible region. Reaction between the phenolic ligands and [Ru(PPh 3 ) 3 Cl 2 ] in 2:1 mole ratio in the presence of a base affords the [Ru(PPh 3 ) 2 (L) 2 ] complexes in two isomeric forms. 1 H NMR spectra of one isomer shows that it does not have any C 2 symmetry and has the cis – cis – cis disposition of the three sets of donor atoms. 1 H NMR spectra of the other isomer shows that it has C 2 symmetry. The structure of the isomer of the [Ru(PPh 3 ) 2 (L 1 ) 2 ] complex has been solved by X-ray crystallography. The coordination sphere around ruthenium is O 4 P 2 with a cis – trans – cis disposition of the carbonylic oxygens, phenolate oxygens and phosphorus atoms, respectively. The [Ru(PPh 3 ) 2 (L) 2 ] complexes are diamagnetic (low-spin d 6 , S =O) and show intense MLCT transitions in the visible region. Cyclic voltammetry on the [Ru(PPh 3 ) 2 (L)Cl 2 ] complexes shows a ruthenium(III)ruthenium(II) reduction near −0.3 V versus SCE and a ruthenium(III)ruthenium(IV) oxidation in the range 1.08–1.24 V versus SCE. Cyclic voltammetry on both isomers of the [Ru(PPh 3 ) 2 (L) 2 ] complexes shows a ruthenium(II)ruthenium(III) oxidation within 0.09–0.41 V versus SCE, followed by a ruthenium(III)-ruthenium(IV) oxidation within 1.31–1.52 V versus SCE.

P=N bond formation via incomplete N-atom transfer from a ferrous amide precursor.

Incomplete N-atom transfer from Fe to P is observed when the ferrous amide complex (PNP)Fe(dbabh) (PNP-=N[2-P(iPr)2-4-methylphenyl]2, dbabh=2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene), prepared from salt metathesis of (PNP)FeCl and Li(dbabh), is thermolyzed at 70 degrees C over 48 h in C6D6. Several plausible reaction pathways resulting from the transformation of (PNP)Fe(dbabh) are discussed, including the possibility of an Fe(IV) nitride as an intermediate.

Phosphaazaallene and phosphinylimide complexes stemming from a terminal and four-coordinate titanium phosphinidene

Treatment of the terminal titanium–phosphinidene (Nacnac)TiPMes*(CH2tBu) (Nacnac− = [2,6-iPr2C6H3]–NC(CH3)CHC(CH3)N[2,6-iPr2C6H3], Mes*− = 2,4,6-tBu3C6H2) with CNtBu and N2CPh2, in pentane at −35 °C, affords η2-(N,C)-phosphaazaallene and phosphinylimide complexes, respectively.

Chemistry of 2- (arylazo) phenolate complexes of osmium. Synthesis, structure and redox properties

Abstract Reaction of five 2- (arylazo)phenol ligands (abbreviated in general as Hap-R, where H stands for the phenolic proton) with [Os (bpy)2Br2] has afforded complexes of type [OsII (bpy)2 (ap-R)]+, which have been isolated as the perchlorate salts. The complexes are diamagnetic (low-spin d 6, S=0) and in acetonitrile solution shows several MLCT transitions in the visible region. Structure of the [Os (bpy)2 (ap-Me))ClO4 complex has been determined by X-ray crystallography. The 2- (arylazo)phenolate anion is coordinated to osmium as a bidentate N,O-donor forming a five-membered chelate ring and the OsN5O coordination sphere is distorted octahedral. Cyclic voltammetry shows a reversible osmium (II)–osmium (III) oxidation in the range of 0.37–0.51 V vs SCE followed by an irreversible osmium (III)-osmium (IV) oxidation in the range of 1.36–1.50 V vs SCE. These oxidation potentials are sensitive to the electronic nature of the substituent R in the 2- (arylazo)phenolate ligands. Three one-electron reductions of the coordinated bpy ligands are also displayed on the negative side of SCE below −1.0 V. Chemical or electrochemical oxidation of the [OsII (bpy)2 (ap-R)]ClO4 complexes affords brownish-yellow [OsIII (bpy)2 (ap-R)] fn2 species, which have been isolated as the perchlorate salts. These complexes are one-electron paramagnetic (low-spin d 5, S=1/2) and in acetonitrile solution show LMCT transitions in the visible region. Reduction of the brownish-yellow [OsIII (bpy)2 (ap-R)] (ClO4)2 complexes gives back the respective brown [OsII (bpy)2 (ap-R)]ClO4 complexes.

Chemistry of osmium in N2P2Br2 coordination sphere: Synthesis, structure and reactivities

Abstract A family of three mixed-ligand osmium complexes of type [Os(PPh3)2(N-N)Br2], where N-N=2,2′-bipyridine (bpy), 4,4′-dimethyl-2,2′-bipyridine (Me2bpy) and 1,10-phenanthroline (phen), have been synthesized and characterized. The complexes are diamagnetic (low-spin d6, S=0) and in dichloromethane solution they show intense MLCT transitions in the visible region. The two bromide ligands have been replaced from the coordination sphere of [Os(PPh3)2(phen)Br2] under mild conditions by a series of anionic ligands L (where L=quinolin-8-olate (q), picolinate (pic), oxalate (Hox) and 1-nitroso-2-naphtholate (nn)) to afford complexes of type [Os(PPh3)2(phen)(L)]+, which have been isolated and characterized as the perchlorate salt. The structure of the [Os(PPh3)2(phen)(pic)]ClO4 complex has been determined by X-ray crystallography. The PPh3 ligands occupy trans positions and the picolinate anion is coordinated to osmium as a bidentate N,O-donor forming a five-membered chelate ring. The [Os(PPh3)2(phen)(L)]+ complexes are diamagnetic and show multiple MLCT transitions in the visible region. The [Os(PPh3)2(N-N)Br2] complexes show an osmium(II)–osmium(III) oxidation (−0.02 to 0.12 V vs. SCE) followed by an osmium(III)–osmium(IV) oxidation (1.31 to 1.43 V vs. SCE). The [Os(PPh3)2(phen)(L)]+ complexes display the osmium (II)–osmium (III) oxidation (0.26 to 0.84 V vs. SCE) and one reduction of phen (−1.50 to −1.79 V vs. SCE). The osmium (III)–osmium (IV) oxidation has been observed only for the L=q and L=Hox complexes at 1.38 V vs. SCE and 1.42 V vs. SCE respectively. The osmium(III) species, viz. [OsIII(PPh3)2(N-N)Br2]+ and [OsIII(PPh3)2(phen)(L)]2+, have been generated both chemically and electrochemically and characterized in solution by electronic spectroscopy and cyclic voltammetry.

CHEMISTRY OF OSMIUM PHENOLATES. SYNTHESIS, STRUCTURE AND REDOX PROPERTIES

Abstract The reaction of three phenolic ligands ( viz. salicylaldehyde, Hsal; 2-hydroxyacetophenone, Hacp and 2-hydroxynaphthaldehyde, Hnap; generally abbreviated as HL, where H stands for the phenolic proton) with [Os(bpy) 2 Br 2 ] has afforded complexes of type [Os(bpy) 2 (L)] + , which have been isolated as the perchlorate salts. The complexes are diamagnetic (low-spin d 6 , S=O and in acetonitrile solution shows several MLCT transitions in the visible region. Structure of the [Os II (bpy) 2 (sal)]ClO 4 complex has been determined by X-ray crystallography. The salicylaldehyde anion is coordinated to osmium as a bidentate O,O-donor and the OsN 4 O 2 coordination sphere is distorted octahedral. Cyclic voltammetry shows a reversible osmium(II)-osmonium(III) oxidation in the range of 0.17–0.25 V vs SCE followed by an irreversible osmium(III)-osmium(IV) oxidation in the range of 1.38–1.48 V vs SCE. Three one-electron reductions of the coordinated bpy ligands are also observed on the negative side of SCE (−1.49– −2.08 V). Chemical or electrochemical oxidation of the [Os II (bpy) 2 (L)]ClO 4 complex affords green [Os III (bpy) 2 (L)] 2+ species, which have been isolated as the perchlorate salts. These complexes are one-electron paramagnetic (low-spin d 5 , S = 1 2 ) and in acetonitrile solution show LMCT transitions in the visible region. Reduction of the green [Os III (bpy) 2 (L)](ClO 4 ) 2 complexes gives back the respective brown [Os II (bpy) 2 (L)]ClO 4 complexes.