Ring size effects in cyclic fluorophosphites: ligands that span the bonding space between phosphites and PF3

The 5- to 8-membered cyclic fluorophosphites L5-8 have been prepared from the corresponding chlorophosphites which are derived from dihydroxyarenes or bis(trimethylsiloxy)arenes. Ligand L5 is very sensitive to hydrolysis but L6-8 are much more kinetically robust. The coordination chemistry of L5-8 has been explored with Mo(0), Pt(0) and Rh(i) and it is shown that the π-acceptor properties of L5-8 increase with decreasing ring size. The IR spectra and X-ray crystal structures of the [Mo(CO)4L2] complexes show that L5-8 lie between PF3 and P(OAr)3 in terms of their σ/π-bonding properties. The [PtL4] complexes are readily prepared from [Pt(nbe)3] and 4 equiv. of L5-8 whereas equilibrium mixtures of PtLx(nbe)y species form when 2 equiv. of L5-8 are added to [Pt(nbe)3]. The CO substitution reactions of [Rh2Cl2(CO)4] with L5-8 to give [Rh2Cl2L4] are evidence of the PF3-like ligand properties of L5-8. The trends in the properties of L5-8 are analysed in terms of their proximity to PF3 or P(OPh)3.

Metal-free dehydropolymerisation of phosphine-boranes using cyclic (alkyl)(amino)carbenes as hydrogen acceptors

The divalent carbene carbon centre in cyclic (alkyl)(amino)carbenes (CAACs) is known to exhibit transition-metal-like insertion into E-H σ-bonds (E = H, N, Si, B, P, C, O) with formation of new, strong C-E and C-H bonds. Although subsequent transformations of the products represent an attractive strategy for metal-free synthesis, few examples have been reported. Herein we describe the dehydrogenation of phosphine-boranes, RR'PH·BH3, using a CAAC, which behaves as a stoichiometric hydrogen acceptor to release monomeric phosphinoboranes, [RR'PBH2], under mild conditions. The latter species are transient intermediates that either polymerise to the corresponding polyphosphinoboranes, [RR'PBH2]n (R = Ph; R' = H, Ph or Et), or are trapped in the form of CAAC-phosphinoborane adducts, CAAC·H2BPRR' (R = R' = tBu; R = R' = Mes). In contrast to previously established methods such as transition metal-catalysed dehydrocoupling, which only yield P-monosubstituted polymers, [RHPBH2]n, the CAAC-mediated route also provides access to P-disubstituted polymers, [RR'PBH2]n (R = Ph; R' = Ph or Et).

The surprisingly facile formation of Pd(i)-phosphido complexes from ortho-biphenylphosphines and palladium acetate

The widely-used ortho-biphenylphosphine ligands SPhos and RuPhos not only undergo facile orthometallation with palladium acetate, yielding strained, four-membered dimeric palladacycles but more surprisingly, in the presence of alcoholic solvents, along with the less encumbered analogue MePhos, yield unusual dinuclear Pd(i) complexes, in which the Pd-centers are bridged by both a phosphide ligand and by the arene of a coordinated phosphine donor.

Inorganic Triphenylphosphine

A completely inorganic version of one of the most famous organophosphorus compounds, triphenylphosphine, has been prepared. A comparison of the crystal structures of inorganic triphenylphosphine, PBaz₃ (where Baz=B₃ H₂ N₃ H₃ ) and PPh₃ shows that they have superficial similarities and furthermore, the Lewis basicities of the two compounds are remarkably similar. However, their oxygenation and hydrolysis reactions are starkly different. PBaz₃ reacts quantitatively with water to give PH₃ and with the oxidizing agent ONMe₃ to give the triply-O-inserted product P(OBaz)₃ , an inorganic version of triphenyl phosphite; a corresponding transformation with PPh₃ is inconceivable. Thermodynamically, what drives these striking differences in the chemistry of PBaz₃ and PPh₃ is the great strength of the B-O bond.

Iron Precatalysts with Bulky Tri(tert-butyl)cyclopentadienyl Ligands for the Dehydrocoupling of Dimethylamine-Borane

In an attempt to prepare new Fe catalysts for the dehydrocoupling of amine-boranes and to provide mechanistic insight, the paramagnetic Fe^II dimeric complex [Cp'FeI]₂ (1) (Cp'=η⁵ -((1,2,4-tBu)₃ C₅ H₂ )) was used as a precursor to a series of cyclopentadienyl Fe^II and Fe^III mononuclear species. The complexes prepared were [Cp'Fe(η⁶ -Tol)][Cp'FeI₂ ] (2) (Tol=C₆ H₅ Me), [Cp'Fe(η⁶ -Tol)][BAr^F₄ ] (3) (BAr^F₄ =[B(C₆ H₃ (m-CF₃ )₂ )₄ ]^- ), [N(nBu)₄ ][Cp'FeI₂ ] (4), Cp'FeI₂ (5), and [Cp'Fe(MeCN)₃ ][BAr^F₄ ] (6). The electronic structure of the [Cp'FeI₂ ]^- anion in 2 and 4 was investigated by SQUID magnetometry, EPR spectroscopy and ab initio Complete Active Space Self Consistent Field-Spin Orbit (CASSCF-SO) calculations, and the studies revealed a strongly anisotropic S=2 ground state. Complexes 1-6 were investigated as catalysts for the dehydrocoupling of Me₂ NH⋅BH₃ (I) in THF at 20 °C to yield the cyclodiborazane product [Me₂ N-BH₂ ]₂ (IV). Complexes 1-4 and 6 were active dehydrocoupling catalysts towards I (5 mol % loading), however 5 was inactive, and ultra-violet (UV) irradiation was required for the reaction mediated by 3. Complex 6 was found to be the most active precatalyst, reaching 80 % conversion to IV after 19 h at 22 °C. Dehydrocoupling of I by 1-4 proceeded via formation of the aminoborane Me₂ N=BH₂ (II) as the major intermediate, whereas for 6 the linear diborazane Me₂ NH-BH₂ -NMe₂ -BH₃ (III) could be detected, together with trace amounts of II. Reactions of 1 and 6 with Me₃ N⋅BH₃ were investigated in an attempt to identify Fe-based intermediates in the catalytic reactions. The σ-complex [Cp'Fe(MeCN)(κ² -H₂ BH⋅NMe₂ H][BAr^F₄ ] was proposed to initially form in dehydrocoupling reactions involving 6 based on ESI-MS (ESI=Electrospray Ionisation Mass Spectroscopy) and NMR spectroscopic evidence. The latter also suggests that these complexes function as precursors to iron hydrides which may be the true catalytic species.

Preparation and reactivity of rhodium and iridium complexes containing a methylborohydride based unit supported by two 7-azaindolyl heterocycles

The synthesis and characterisation of a new anionic flexible scorpionate ligand, methyl(bis-7-azaindolyl)borohydride [MeBai]- is reported herein. The ligand was coordinated to a series of group nine transition metal centres forming the complexes, [Ir(MeBai)(COD)] (1), [Rh(MeBai)(COD)] (2), [Rh(MeBai)(CODMe)] (2-Me) and [Rh(MeBai)(NBD)] (3), where COD = 1,5-cyclooctadiene, CODMe = 3-methyl-1,5-cyclooctadiene and NBD = 2,5-norbornadiene. In all cases, the boron based ligand was found to bind to the metal centres via a κ3-N,N,H coordination mode. The ligand and complexes were fully characterised by spectroscopic and analytical methods. The structures of the ligand and three of the complexes were confirmed by X-ray crystallography. The potential for migration of the "hydride" or "methyl" units from boron to the metal centre was also explored. During these studies an unusual transformation, involving the oxidation of the rhodium centre, was observed in complex 2. In this case, the η4-COD unit transformed into a η1,η3-C8H12 unit where the ring was bound via one sigma bond and one allyl unit. This is the first time such a transformation has been observed at a rhodium centre.

Anion Recognition by a Bioactive Diureidodecalin Anionophore: Solid-State, Solution, and Computational Studies

Recent work has identified a bis-(p-nitrophenyl)ureidodecalin anion carrier as a promising candidate for biomedical applications, showing good activity for chloride transport in cells yet almost no cytotoxicity. To underpin further development of this and related compounds, a detailed structural and binding investigation is reported. Crystal structures of the transporter as five solvates confirm the diaxial positioning of urea groups while revealing a degree of conformational flexibility. Structures of complexes with Cl- , Br- , NO3- , SO42- and AcO- , supported by computational studies, show how the binding site can adapt to accommodate these anions. 1 H NMR binding studies revealed exceptionally high affinities for anions in DMSO, decreasing in the order SO42- >H2 PO4- ≈HCO3- ≈AcO- ≫HSO4- >Cl- >Br- >NO3- >I- . Analysis of the binding results suggests that selectivity is determined mainly by the H-bond acceptor strength of different anions, but is also modulated by receptor geometry.

Lamotrigine ethanol monosolvate

Lamotrigine is an active pharmaceutical ingredient used as a treatment for epilepsy and psychiatric disorders. Single crystals of an ethano-late solvate, C9H7Cl2N5·C2H5OH, were produced by slow evaporation of a saturated solution from anhydrous ethanol. Within the crystal structure, the lamotrigine mol-ecules form dimers through N-H⋯N hydrogen bonds involving the amine N atoms in the ortho position of the triazine group. These dimers are linked into a tape motif through hydrogen bonds involving the amine N atoms in the para position. The ethanol and lamotrigine are present in a 1:1 ratio in the lattice with the ethyl group of the ethanol mol-ecule exhibiting disorder with an occupancy ratio of 0.516 (14):0.484 (14).

Bis(μ-chloro) bridged 1D CuI and CuII coordination polymer complex and mononuclear CuII complex: Synthesis, crystal structure and biological properties

A novel one-dimensional coordination polymer containing Cu(I)Cu(II) core with chloro bridge on Cu(I) and ligand bridge on Cu(II) ions (1) and a mononuclear Cu(II) complex (2) have been synthesized from the reactions of 3- and 4-methoxy-3-quinolin-3-ylimino-methyl-2-phenol with [CuCl₂(PPh₃)₂]. The ligands and the complexes have been characterized by spectral and analytical methods. In addition, the structures of both the ligands and the copper complexes were confirmed by single crystal X-ray diffraction studies. In both complexes, the phenolic oxygen and azomethine nitrogen atom of the ligand coordinate to the copper ions in a monobasic bidentate manner resulting in an approximately square planar geometry around the copper ion. In the polymeric complex, the N atom of the quinoline ring is coordinated to Cu(I) in addition to the phenolic oxygen and azomethine nitrogen atom coordinating to Cu(II) ion, thus bridging Cu(I) and Cu(II) ions in the complex. The interactions of the compounds with calf thymus DNA (CT-DNA) have been followed by absorption and emission titration methods, which revealed that the compounds interact with CT-DNA through intercalation. Further, the interactions of the compounds with bovine serum albumin (BSA) were also investigated using UV-visible, fluorescence spectroscopic methods. The results indicated that complex 1 exhibited a stronger binding to CT-DNA and BSA than the free ligands and complex 2. In addition, the in vitro cytotoxicity experiment showed that complexes 1 and 2 exhibit potent cytotoxic properties against PANC-1and Hela cells. Moreover, while complex 1 showed prominent cytotoxic activity against both PANC-1 and Hela cells with IC₅₀ of 17.91 and 11.67 μM, complex 2 showed moderate cytotoxic activities with IC₅₀ of 25.13 and 16.41 μM in PANC-1 and Hela cells. Further, apoptosis was confirmed by fluorescence image using EB/AO reagent.

2-Pyridyl substituents enhance the activity of palladium-phospha-adamantane catalysts for the methoxycarbonylation of phenylacetylene

The synthesis of a series of CgPAr ligands is reported, where CgP is the 6-phospha-2,4,8-trioxa-1,3,5,7-tetramethyladamant-6-yl moiety and Ar = 2-pyridyl (L2), 3-pyridyl (L3), 2-pyrimidyl (L4), 4-R-2-pyridyl [R = Me (L5a), CF₃ (L6a), SiMe₃ (L7a)] or 6-R-2-pyridyl [R = Me (L5b), CF₃ (L6b), SiMe₃ (L7b). Testing of these ligands in the Pd-catalysed methoxycarbonylation of phenylacetylene reveals that the activity and branched selectivity of the catalysts derived from these ligands varies as a function of the N-heterocycle, with the catalyst derived from L5b being the most active of those tested. This, together with the poor performance of catalysts derived from L3 supports the hypothesis that the catalysis proceeds by a "proton shuttling" mechanism, an idea that previously had only been applied to arylphosphines. Reaction of [PtCl₂(cod)] with L where L = L2 or L4-7 yields a rac/meso mixture of the trans-[PtCl₂(L)₂] (1a-h) complexes, three of which are structurally characterised. ³¹P NMR spectroscopy shows that reaction of L3 with [PtCl₂(cod)] gives a mixture of mononuclear and binuclear metal complexes in solution. The complex trans-[PdCl₂(L2)₂] (4) reacts with AgBF₄ to give the [PdCl(κ¹-L2)(κ²-L2)]BF₄ (5) with spectroscopic and structural characterisation confirming the presence of a P,N-chelate. ¹H and ³¹P NMR evidence supports the assignment of a pyridyl-protonated species being formed upon treatment of 4 with TsOH·H₂O in CD₂Cl₂; both the protonated species and chelate 5 are observed when the reaction is carried out in MeOH.

A new strategy for the synthesis of diverse benzo[a]carbazoles via a divergent catalytic Michael reaction

A new type of divergent catalytic Michael addition reaction of α,α-dicyanomethylidene carbazoles afforded multifunctional benzo[a]carbazoles in good yields.

Syntheses, crystal structures, magnetic properties and ESI-MS studies of a series of trinuclear CuIIMIICuII compounds (M = Cu, Ni, Co, Fe, Mn, Zn)

Six homo/heterotrinuclear Cu^IIM^IICu^II (M = Mn–Zn) compounds derived from salicylaldehyde-trans-1,2-diaminocyclohexane, which is a rarely utilized ligand, are reported.

[Fe(abpt)2(NCSe)2] polymorph A: structural studies into the spin crossover behaviour

Crystallographic and UV-Vis analysis of the thermal spin-transition in [Fe(abpt)₂(NCSe)₂], polymorph A, is presented alongside LIESST and high pressure structures.

(−)-Cytisine: Access to a stereochemically defined and functionally flexible piperidine scaffold

Cytisine undergoes ready fragmentation to provide a highly flexible (and “privileged”) piperidine scaffold capable of exploring a diversity of chemical space.