Johanna M. Blacquiere

Synthesis and Reactivity of Tripodal Complexes Containing Pendant Bases

The synthesis of a new tripodal ligand family that contains tertiary amine groups in the second-coordination sphere is reported. The ligands are tris(amido)amine derivatives, with the pendant amines attached via a peptide coupling strategy. They were designed to function as new molecular catalysts for the oxygen reduction reaction (ORR), in which the pendant acid/base group could improve the catalyst performance. Two members of the ligand family were each metalated with cobalt(II) and zinc(II) to afford trigonal-monopyramidal complexes. The reaction of the cobalt complexes [Co(L)](-) with dioxygen reversibly generates a small amount of a cobalt(III) superoxo species, which was characterized by electron paramagnetic resonance (EPR) spectroscopy. Protonation of the zinc complex Zn[N{CH2CH2NC(O)CH2N(CH2Ph)2}3)](-) ([Zn(TN(Bn))](-)) with 1 equiv of acid occurs at a primary-coordination-sphere amide moiety rather than at a pendant basic site. The addition of excess acid to any of the complexes [M(L)](-) results in complete proteolysis and formation of the ligands H3L. These undesired reactions limit the use of these complexes as catalysts for the ORR. An alternative ligand with two pyridyl arms was also prepared but could not be metalated. These studies highlight the importance of the stability of the primary-coordination sphere of ORR electrocatalysts to both oxidative and acidic conditions.

Synthesis, characterization and antifungal testing of 3,4-dihydropyrimidin-2(1H)-(thio)ones containing boronic acids and boronate esters

The addition of formylphenylboronic acid derivatives to thiourea and ethyl acetoacetate proceeds in the presence of an additional Lewis acid catalyst to give the corresponding 3,4-dihydropyrimidin-2(1H)-(thio)ones (Biginelli products) in moderate yield. Compounds were tested for antifungal activity against pure cultures of Aspergillus niger, Aspergillus flavus, Candida albicans and Saccharomyces cerevisiae but, unfortunately, none showed any appreciable activity.

Unusually Strong Binding of Dinitrogen to a Ruthenium Center

Metal–dinitrogen complexes are of great interest for their potential to reduce energy costs in ammonia production, and to facilitate access to nitrogen-containing organic compounds. Many examples have been discovered since the 1965 report of [Ru(NH3)5(N2)] . Despite the prominence of iron systems in catalyzing the reduction of dinitrogen in industrial and biological contexts, 3, 5,6] and important recent advances in dinitrogen fixation by iron complexes, earlyand mid-transition metal complexes have shown greatest promise to date in cleaving the N N bond. 8–11] A benchmark was set by Yandulov and Schrock, who reported molybdenum triamidoamine derivatives, the first welldefined catalysts capable of selectively reducing N2 to ammonia. Late-metal complexes tend to be limited by the lower energy of their d orbitals (which impedes back-donation into the high-energy N N antibonding orbitals), 14] and high N2 lability. The latter has been identified as a key barrier to the development of late-metal catalysts for N2 activation. [2a] Herein we present an experimental and computational study that addresses this barrier. Our interest in the broad catalytic utility of hydridoruthenium complexes of N-heterocyclic carbene (NHC) ligands led us to a report from Morris and co-workers describing synthesis of the activated IMes complex 2 (Scheme 1a; IMes = N,N’-bis(mesityl)imidazol-2-ylidene) through the thermolysis of 1 with excess IMes under argon. We observed a very different reaction chemistry under an atmosphere of dinitrogen: P{H} NMR analysis indicated the formation of less than 10 % of 2 (59.35, 58.72 ppm; ABq, JPP = 13 Hz, THF), but considerable free PPh3 (Scheme 1b). The absence of any phosphine ligands in the major product 3 is evident from its null P{H} NMR spectrum, and from the singlet multiplicity of its hydride signal. This species could be obtained free of 2 by carrying out the reaction at room temperature, and was isolated as an orange powder in 75% yield by precipitation from hexanes. Single-crystal X-ray analysis of 3 indicated a mononuclear structure containing two mutually trans, unactivated IMes ligands (cf. the activated IMes group present in 2). While disorder impeded the initial assignment of the remaining ligands, we identified 3 as [RuHCl(IMes)2(N2)] by detailed NMR, IR, and MALDI-TOF mass spectrometric analysis. Refinement of the X-ray data with an appropriate disorder model yields a satisfactory solution. For both complexes in the unit cell, the N2 and Cl sites are disordered over two positions (as found for other structures containing Cl trans to N2); [18] in one, the hydride is also disordered over two positions. The excellent agreement between the model and the observed data provides unambiguous confirmation of connectivity, although the presence of the disorder limits the discussion of the metrical parameters. The upfield location of the H NMR singlet for the hydride ligand in 3 ( 28.03 ppm; C6D6) is clear evidence for a square pyramidal complex with apical hydride. Integration confirms the presence of two IMes ligands. Rotation of Ru CNHC and N Mes bonds is rapid on the NMR timescale at 22 8C, as deduced from the equivalence of the mesityl orthoCH3 groups (these resolve into four singlets at 0 8C). The retention of chloride is confirmed by charge-transfer MALDI-TOF mass spectrometry, which shows an excellent match between the simulated and observed isotope patterns for both [RuHCl(IMes)2(N2)+H] + (minor signal) and [RuCl(IMes)2 H]C (major signal). We initially ruled out the possibility that an N2 ligand occupied the fifth coordination site on the basis of reactions with CO described below, but revised this conclusion in light of the quantitative formation Scheme 1. Reaction of 1 with IMes under (a) Ar (12 h, THF, reflux); (b) N2 (20 h, reflux or RT). The ORTEP diagram for 3 [35] shows nonhydrogen atoms as Gaussian ellipsoids set at the 50% probability level.

CapturePhos – A phosphorus-rich polymer as a homogeneous catalyst scavenger

A soft polymer network prepared through a phosphane–ene reaction successfully sequestered Rh and Ru from hydrogenation and ring closing metathesis reactions, respectively. Scavenging effectively quenches catalytic activity and ultimately removes >98% of the metal.

Pincer-plus-one ligands in self-assembly with palladium(II): a molecular square and a molecular tetrahedron

The combination of a palladium(ii) precursor with a diimine-phenol ligand and an oxidant (H2O2 or O2) under different conditions has, serendipitously, given both a molecular square and a molecular tetrahedron by self-assembly of building blocks comprising palladium(ii) centres coordinated to the oxidised forms of the ligand.

Catalyst Pendent-Base Effects on Cyclization of Alkynyl Amines

A family of [CpRu(PP)(MeCN)]PF6 complexes (2 a–e and 4) were prepared in which the bis‐phosphine ligand contains a pendent tertiary amine in the second‐coordination sphere. 2 a–e contain PPh2NR′2 ligands with two amine groups as the pendent base. Complex 4 has the PPh2NPh1 ligand with only one pendent amine. The catalytic performance of 2 a–e and 4 were assessed in the cyclization of 2‐ethynyl aniline and 2‐ethynylbenzyl alcohol. It was revealed that the positioning of the pendent amine near the metal active site is essential for high catalyst performance. A comparison of PPh2NR′2 catalysts (2 a–e) showed minimal difference in performance as a function of pendent amine basicity. Rather, only a threshold basicity – in which the pendent amine was more basic than the substrate – was required for high performance.

A Mass-Producible and Versatile Sensing System: Localized Surface Plasmon Resonance Excited by Individual Waveguide Modes

A plasmonic sensing system that allows the excitation of localized surface plasmon resonance (LSPR) by individual waveguide modes is presented conceptually and experimentally. Any change in the local environment of the gold nanoparticles (AuNPs) alters the degree of coupling between LSPR and a polymer slab waveguide, which then modulates the transmission-output signal. In comparison to conventional LSPR sensors, this system is less susceptible to optical noise and positional variation of signals. Moreover, it enables more freedom in the exploitation of plasmonic hot spots with both transverse electric (TE) and transverse magnetic (TM) modes. Through real-time measurement, it is demonstrated that the current sensing system is more sensitive than comparable optical fiber plasmonic sensors. The highest normalized bulk sensitivity (7.744 RIU-1) is found in the TM1 mode. Biosensing with the biotin-streptavidin system shows that the detection limit is on the order of 10-14 M of streptavidin. With further optimization, this sensing system can easily be mass-produced and incorporated into high throughput screening devices, detecting a variety of chemical and biological analytes via immobilization of the appropriate recognition sites.