Synthesis and catalytic activity of single-site group V alkoxide complexes for the ring-opening polymerization of ε-caprolactone.

The synthesis, characterization, and ring-opening polymerization (ROP) activity of a family of niobium and tantalum alkoxide catalysts was studied. The final catalysts are made in a two-step synthesis, first by reacting the desired homoleptic metal ethoxide with a phenolketoimine ligand to form a series of synthetic intermediates, followed by reaction with catechol to produce a catalytic platform with a single ethoxide initiator. By using two separate ligands, the electronic properties of the catalyst can be tuned, and the molecular weight of the polymer can be increased. It was found that synthetic intermediates adopted a mer geometry both in solution and in the solid state. This mer geometry was retained for the final catechol derivatives, however in one case, where catechol was substituted for 3-methoxycatechol, the molecule adopted a highly distorted fac geometry. Catalytic ROP activity of the synthetic intermediates and final catechol derivatives with ε-caprolactone was studied through a kinetic analysis. In all seven cases studied the reactions proceeded through the expected coordination-insertion mechanism, following pseudo first-order kinetics and increasing in Mn linearly vs. conversion. The single-initiator catechol derivatives increased the Mn by three times compared to that of the three-initiator synthetic intermediates with little decrease in the overall reaction rate. Both the nature of the ligand and metal were found to impact the rate of reaction in these systems. By switching from an electron donating ligand to an electron withdrawing ligand, the rate was found to nearly double. Tantalum species were faster than their niobium counterparts by ∼3 times in the synthetic intermediates and ∼1.5 times in the catechol derivatives. This observed periodicity supports recent literature findings in this area.

Halogen Bonding Tripodal Metallo-Receptors for Phosphate Recognition and Sensing in Aqueous-Containing Organic Media.

The anion recognition and electrochemical anion-sensing properties of halogen-bonding (XB) tripodal zinc(II) receptors strategically designed and constructed for tetrahedral anion guest binding are described. The XB tris(iodotriazole)-containing hosts exhibit high affinities and selectivities for inorganic phosphate over other more basic, mono-charged oxoanions such as acetate and the halides in a competitive CD3 CN/D2 O (9 : 1 v/v) aqueous solvent mixture. 1 H NMR anion binding and electrochemical voltammetric anion sensing studies with redox-active ferrocene functionalised metallo-tripodal receptor analogues, reveal each of the XB tripods as superior anion complexants when compared to their tris(prototriazole)-containing, hydrogen bonding (HB) counterparts, not only exemplifying the halogen bond as a strong alternative interaction to the traditional hydrogen bond for molecular recognition but also providing rare evidence of the ability of XB receptors to preferentially bind the "harder" phosphate oxoanion over the "softer" and less hydrated halides in aqueous containing media.

Investigating the Ring-Opening Polymerization Activity of Niobium and Tantalum Ethoxides Supported by Phenoxyimine Ligands.

A variety of metal catalysts from around the periodic table have been studied for the ring-opening polymerization (ROP) of cyclic esters. Within this field, group V catalysts have been rarely explored. To better understand the effect the choice of metal and ligand has on ROP activity, a series of 10 niobium and tantalum alkoxide catalysts, supported by a range of phenoxyimine ligands, were synthesized. The electronics and steric bulk of the ligands were varied on the phenoxy group ( t Bu, Cl, and OMe) and the imine group (Ph; 2,6-diMePh; 2,6-di i PrPh; and 2,4,6-tri t BuPh) to probe their effect on the catalyst structure and activity. Catalysts were characterized with 1D, 2D, and variable-temperature NMR techniques to determine their structure in solution. Single crystal X-ray diffraction studies were conducted to establish their solid-state structure. The 10 catalysts are pseudo-octahedral, and each shows ligand coordination through phenoxy-oxygen and imine-nitrogen (O,N). In the case of the o-vanillin ligand set, however, evidence was found for O,O-coordination of the ligand when the steric encumbrance of the imine-nitrogen was increased. Each catalyst was active for the ring-opening polymerization of both rac-lactide (LA) and ε-caprolactone (CL) in the absence of solvent at 140 °C. In the case of CL, the catalysts supported by chloro-containing ligands showed the most polymerization control based on final polymer molecular weight and dispersity. Ligand trends were less clear for the polymerization of LA, though in all cases the catalysts were more controlled than the parent homoleptic alkoxide [M(OEt)5; M = Nb or Ta]. The most promising catalyst in the family was tested for copolymerization activity of LA and CL in one pot. Copolymerization of the two monomers was successful and yielded random poly(caprolactone-co-lactide).

Heterodinuclear Mg(II)M(II) (M=Cr, Mn, Fe, Co, Ni, Cu and Zn) Complexes for the Ring Opening Copolymerization of Carbon Dioxide/Epoxide and Anhydride/Epoxide.

The catalysed ring opening copolymerizations (ROCOP) of carbon dioxide/epoxide or anhydride/epoxide are controlled polymerizations that access useful polycarbonates and polyesters. Here, a systematic investigation of a series of heterodinuclear Mg(II)M(II) complexes reveals which metal combinations are most effective. The complexes combine different first row transition metals (M(II)) from Cr(II) to Zn(II), with Mg(II); all complexes are coordinated by the same macrocyclic ancillary ligand and by two acetate co-ligands. The complex syntheses and characterization data, as well as the polymerization data, for both carbon dioxide/cyclohexene oxide (CHO) and endo-norbornene anhydride (NA)/cyclohexene oxide, are reported. The fastest catalyst for both polymerizations is Mg(II)Co(II) which shows propagation rate constants (kp ) of 34.7 mM-1 s-1 (CO2 ) and 75.3 mM-1 s-1 (NA) (100 °C). The Mg(II)Fe(II) catalyst also shows excellent performances with equivalent rates for CO2 /CHO ROCOP (kp =34.7 mM-1 s-1 ) and may be preferable in terms of metallic abundance, low cost and low toxicity. Polymerization kinetics analyses reveal that the two lead catalysts show overall second order rate laws, with zeroth order dependencies in CO2 or anhydride concentrations and first order dependencies in both catalyst and epoxide concentrations. Compared to the homodinuclear Mg(II)Mg(II) complex, nearly all the transition metal heterodinuclear complexes show synergic rate enhancements whilst maintaining high selectivity and polymerization control. These findings are relevant to the future design and optimization of copolymerization catalysts and should stimulate broader investigations of synergic heterodinuclear main group/transition metal catalysts.

Titanium ONN-(phenolate) Alkoxide Complexes: Unique Reaction Kinetics for Ring-Opening Polymerization of Cyclic Esters.

The synthesis, characterization, and polymerization kinetics of four new titanium ONN-(phenolate) alkoxide catalysts were studied. Each catalyst is fluxional at room temperature, suggesting the ligand amine arm may be labile, but adopts a fac geometry in solution at low temperature (223 K) and in the solid state. All catalysts are active for the ring-opening polymerization of both ε-caprolactone (CL) and rac-lactide (LA). GPC analysis indicates that the well-known coordination-insertion mechanism is being followed. However, whereas the typical first-order dependence on monomer concentration is observed in CL, an unexpected zeroth-order dependence is observed with LA. This suggests that, in the case of LA, catalyst saturation occurs and a Michaelis-Menten model can be used to explain the kinetics. An initial mechanism is discussed within this model that proposes CL polymerization proceeds by a 7-coordinate intermediate, whereas LA polymerization adopts a 6-coordinate intermediate, facilitated by the ligand amine arm. Attempts to isolate catalyst-monomer intermediates are ongoing.

Heterodinuclear Zn(II), Mg(II) or Co(III) with Na(I) Catalysts for Carbon Dioxide and Cyclohexene Oxide Ring Opening Copolymerizations.

A series heterodinuclear catalysts, operating without co-catalyst, show good performances for the ring opening copolymerization (ROCOP) of cyclohexene oxide and carbon dioxide. The complexes feature a macrocyclic ligand designed to coordinate metals such as Zn(II), Mg(II) or Co(III), in a Schiff base 'pocket', and Na(I) in a modified crown-ether binding 'pocket'. The 11 new catalysts are used to explore the influences of the metal combinations and ligand backbones over catalytic activity and selectivity. The highest performance catalyst features the Co(III)Na(I) combination, [N,N'-bis(3,3'-triethylene glycol salicylidene)-1,2-ethylenediamino cobalt(III) di(acetate)]sodium (7), and it shows both excellent activity and selectivity at 1 bar carbon dioxide pressure (TOF=1590 h-1 , >99 % polymer selectivity, 1 : 10: 4000, 100 °C), as well as high activity at higher carbon dioxide pressure (TOF=4343 h-1 , 20 bar, 1 : 10 : 25000). Its rate law shows a first order dependence on both catalyst and cyclohexene oxide concentrations and a zeroth order for carbon dioxide pressure, over the range 10-40 bar. These new catalysts eliminate any need for ionic or Lewis base co-catalyst and instead exploit the coordination of earth-abundant and inexpensive Na(I) adjacent to a second metal to deliver efficient catalysis. They highlight the potential for well-designed ancillary ligands and inexpensive Group 1 metals to deliver high performance heterodinuclear catalysts for carbon dioxide copolymerizations and, in future, these catalysts may also show promise in other alternating copolymerization and carbon dioxide utilizations.

Heterodinuclear complexes featuring Zn(ii) and M = Al(iii), Ga(iii) or In(iii) for cyclohexene oxide and CO2 copolymerisation.

The ring opening copolymerisation of CO2 and epoxides is a useful means to valorise waste emissions and to reduce pollution in polymer manufacturing. Heterodinuclear catalysts, particularly those of Zn(ii)/Mg(ii), have shown better performances than homodinuclear analogues in this reaction. As part of on-going efforts to better understand the catalytic synergy, this work describes a series of heterodinuclear complexes, combining Zn(ii) with a metal from Group 13 (M = Al(iii), Ga(iii) or In(iii)). The complexes are synthesised from a symmetrical macrocyclic ligand in high yields via sequential metalation steps and are the thermodynamic reaction products. The Zn(ii)/Group 13 complexes are effective homogeneous catalysts for the ring opening copolymerisation (ROCOP) of cyclohexene oxide at 1 bar pressure of carbon dioxide, but all show inferior performances compared to the di-zinc analogue. The CO2 uptake into the polymer increases in the order Al(iii) < Ga(iii) < In(iii) which is attributed to lower Lewis acidity heavier Group 13 homologues showing a reduced tendency to form ether linkages. Concurrently, polycarbonate activity increases down the Group 13 series consistent with weaker metal-oxygen bonds which show enhanced lability to insertion reactions.

Groups 1, 2 and Zn(II) Heterodinuclear Catalysts for Epoxide/CO2 Ring-Opening Copolymerization.

A series of heterodinuclear complexes are reported where both Zn(II) and a metal from Group 1 or 2 are chelated by a macrocyclic diphenolate-tetra-amine ligand. The complexes are characterized in the solid state, where relevant by single crystal X-ray crystallography and elemental analysis, and in solution, using NMR spectroscopy and mass spectrometry. The complex synthesis is achieved by reaction of the ligand with diethyl zinc to form the monozinc complex, in situ, followed by subsequent coordination of the second metal; this method enables heterodinuclear conversions >90% as determined by NMR spectroscopy. Alternatively, the same heterodinuclear complexes are accessed by reaction between the two homodinuclear complexes at elevated temperatures for extended periods. These findings suggest that most of the heterodinuclear complexes are the thermodynamic reaction products; the only exception is the Na(I)/Zn(II) complex which is unstable with respect to the homodinuclear counterparts. The catalytic activities and selectivity of the stable heterodinuclear complexes are compared, against each other and the relevant homodinuclear analogues, for the ring-opening copolymerization of CO2 and CHO. Nearly all the heterodinuclear complexes are less active than the dizinc analogues, but the Mg(II)/Zn(II) catalyst is more active. The co-ligand influences the product selectivity, with iodide ligands resulting in cyclic carbonate formation and carboxylate ligands giving a high selectivity for polycarbonate.

New Coordination Modes for Modified Schiff Base Ti(IV) Complexes and Their Control over Lactone Ring-Opening Polymerization Activity.

A series of eight new bis(alkoxy)bis(phenoxy-imine)titanium(IV) catalysts, coordinated by Schiff base ligands derived from o-vanillin (2-hydroxy-3-methoxybenzaldehyde), show good activity and control for the ring-opening polymerization of ε-caprolactone and ω-pentadecalactone. The new complexes are easily prepared in two high-yield steps from commercial reagents. The new ligands can all adopt two different coordination modes, depending on the steric bulk on the imine: a six-membered N-O chelate and/or a five-membered O-O chelate. The complexes show three different structures, depending on the ligand coordination mode: type A (N-O:N-O), type B (N-O:O-O), and type C (O-O:O-O). In all cases, the structures were confirmed in solution using variable temperature NMR spectroscopy and in the solid state using X-ray crystallography. The complex structure influences the polymerization rate, with the catalytic activities decreasing in the order: type C > type B > type A for both monomers. Overall, the work demonstrates potential to use these new ligands to access particular coordination modes, which allows enhancement of catalytic activity.

Indium Catalysts for Low-Pressure CO2/Epoxide Ring-Opening Copolymerization: Evidence for a Mononuclear Mechanism?

The alternating copolymerization of CO2/epoxides is a useful means to incorporate high levels of carbon dioxide into polymers. The reaction is generally proposed to occur by bimetallic or bicomponent pathways. Here, the first indium catalysts are presented, which are proposed to operate by a distinct mononuclear pathway. The most active and selective catalysts are phosphasalen complexes, which feature ligands comprising two iminophosphoranes linked to sterically hindered ortho-phenolates. The catalysts are active at 1 bar pressure of carbon dioxide and are most effective without any cocatalyst. They show low-pressure activity (1 bar pressure) and yield polymer with high carbonate linkage selectivity (>99%) and isoselectivity ( Pm > 70%). Using these complexes, it is also possible to isolate and characterize key catalytic intermediates, including the propagating indium alkoxide and carbonate complexes that are rarely studied. The catalysts are mononuclear under polymerization conditions, and the key intermediates show different coordination geometries: the alkoxide complex is pentacoordinate, while the carbonate is hexacoordinate. Kinetic analyses reveal a first-order dependence on catalyst concentration and are zero-order in carbon dioxide pressure; these findings together with in situ spectroscopic studies underpin the mononuclear pathway. More generally, this research highlights the future opportunity for other homogeneous catalysts, featuring larger ionic radius metals and new ligands, to operate by mononuclear mechanisms.

[MoO(S2)2L]1- (L = picolinate or pyrimidine-2-carboxylate) Complexes as MoSx-Inspired Electrocatalysts for Hydrogen Production in Aqueous Solution.

Crystalline and amorphous molybdenum sulfide (Mo-S) catalysts are leaders as earth-abundant materials for electrocatalytic hydrogen production. The development of a molecular motif inspired by the Mo-S catalytic materials and their active sites is of interest, as molecular species possess a great degree of tunable electronic properties. Furthermore, these molecular mimics may be important for providing mechanistic insights toward the hydrogen evolution reaction (HER) with Mo-S electrocatalysts. Herein is presented two water-soluble Mo-S complexes based around the [MoO(S2)2L2]1- motif. We present 1H NMR spectra that reveal (NEt4)[MoO(S2)2picolinate] (Mo-pic) is stable in a d6-DMSO solution after heating at 100 °C, in air, revealing unprecedented thermal and aerobic stability of the homogeneous electrocatalyst. Both Mo-pic and (NEt4)[MoO(S2)2pyrimidine-2-carboxylate] (Mo-pym) are shown to be homogeneous electrocatalysts for the HER. The TOF of 27-34 s-1 and 42-48 s-1 for Mo-pic and Mo-pym and onset potentials of 240 mV and 175 mV for Mo-pic and Mo-pym, respectively, reveal these complexes as promising electrocatalysts for the HER.

Synthesis, Structure, and Photophysical Properties of Mo2(NN)4 and Mo2(NN)2(T(i)PB)2, Where NN = N,N'-Diphenylphenylpropiolamidinate and T(i)PB = 2,4,6-Triisopropylbenzoate.

Two dimolybdenum compounds featuring amidinate ligands with a C≡C bond, Mo2(NN)4 (I), where NN = N,N'-diphenylphenylpropiolamidinate, and trans-Mo2(NN)2(T(i)PB)2 (II), where T(i)PB = 2,4,6-triisopropylbenzoate, have been prepared and structurally characterized by single-crystal X-ray crystallography. Together with Mo2(DAniF)4 (III), where DAniF = N,N'-bis(p-anisyl)formamidinate, all three compounds have been studied with steady-state UV-vis, IR, and time-resolved spectroscopy methods. I and II display intense metal to ligand charge transfer (MLCT). Singlet state (S1) lifetimes of I-III are determined to be 0.7, 19.1, and 2.0 ps, respectively. All three compounds have long-lived triplet state (T1) lifetimes around 100 µs. In femtosecond time-resolved infrared (fs-TRIR) experiments, one ν(C≡C) band is observed at the S1 state for I but two for II, which indicate different patterns of charge distribution. The electron would have to be localized on one NN ligand in I and partially delocalized over two NN ligands in II to account for the observations. The result is a standard showcase of excited-state mixed valence in coordination compounds.

MM Quadruply Bonded Complexes Supported by Vinylbenzoate Ligands: Synthesis, Characterization, Photophysical Properties and Application as Synthons.

From the reactions between M2(T i PB)4 compounds and meta and para - vinylbenzoic acids (2 equiv) in toluene at room temperature the compounds trans-M2(T i PB)2L2, where L = m-vinylbenzoate 1A (M = Mo) and 1B (M = W) and T i PB = 2,4,6-triisopropylbenzoate, and where L = p-vinylbenzoate 2A (M = Mo) and 2B (M = W) have been isolated. Compounds 1A and 2A have been shown to undergo Heck carbon-carbon coupling reactions with phenyliodide to produce trans-Mo2(T i PB)2(O2CC6H4-m-CH=CH-C6H5)2,3A and trans-Mo2(T i PB)2(O2CC6H4-p-CH=CH-C6H5)2, 4A. The molybdenum compounds 1A and 2A have been structurally characterized by single crystal X-ray crystallography. All the new compounds have been characterized by 1H NMR, IR, UV-Visible absorption and emission spectroscopy, high resolution MALDITOF MS, fs- and ns- transient absorption spectroscopy and fs- time-resolved IR spectroscopy. Electronic structure calculations employing density functional theory, DFT, and time-dependent DFT have been employed to aid in the interpretation of spectral data. All compounds show intense absorptions in the visible region corresponding to M2δ to Lπ* charge transfer transitions. The lifetimes of the 1MLCT state fall in the range of 1 - 10 ps and for the molybdenum complexes the T1 states are 3δδ* with lifetimes ~50 µs while for the tungsten complexes the T1 are 3 MLCT with lifetimes in the range of 3 - 10 ns.

Bismuth-lithium bonding in the ion pairs: LiBiL2, where L = a porphyrin or a salen ligand.

From the reaction between BiCl3 (1 equiv.) and LiN(SiMe3)2 (4 equiv.) and LH2 (2 equiv.), where L = a tetraphenylporphyrin, TPP, an octaethylporphyrin, OEP and phsalen in THF the title compounds have been obtained LiBiTPP2, LiBiOEP2, and LiBi(phsalen)2 and LiBi(phsalen)2·THF. Crystals grown from CH2Cl2-hexanes are colored; (green), (red-purple) and and (red-orange). The molecular structures of compound , and were determined by single-crystal X-ray crystallography and are shown to have short LiBi bonds of distance 2.8 Å involving the LiL(-)BiL(+). Compound shows a slipped structure involving Li to two oxygens and a LiBi distance of 3.1 Å. Compounds and undergo a rapid reversible exchange in toluene-d8 at 90 °C. The MALDI-MS yields weak molecular ions due to LiBiL2(+/-) with more intense ions due to BiL(+) and LiL(-) in the positive and negative modes. The short Li(+) to Bi(3+) distances are comparable to those seen in LiBi compounds, such a LiBiR2, and are comparable to those seen by Pyykkö (P. Pyykkö, J. Phys. Chem. A, 2015, 119, 2326; P. Pyykkö and M. Atsumi, Chem. - Eur. J., 2009, 15, 186; P. Pyykkö and M. Atsumi, Chem. - Eur. J., 2009, 15, 12770) for Li-Bi bonds. These can be seen to be involving Bi6s6p hybrid lone-pairs to Li(+) atoms. The lithium bis(bistrimethylsilyl)amide (2 equiv.) and phsalenH2 in THF gave a compound having Li2L·2THF, . Crystallographically compound contains two Li(+) atoms, one coordinated to five atoms LiO2N2·THF. and the other being coordinated to three atoms, LiO2·THF. By (7)Li and (1)H NMR both lithium atoms share an equivalent environment.

Electronic and Spectroscopic Properties of Avobenzone Derivatives Attached to Mo2 Quadruple Bonds: Suppression of the Photochemical Enol-to-Keto Transformation.

From the reactions between Mo2(T(i)PB)4, where T(i)PB is 2,4,6-triisopropylbenzoate, and 2 equiv of the acids 4-formylbenzoic acid, HBzald; 4-(3-oxo-3-phenylpropanoyl)benzoic acid, HAvo; and 4-(2,2-difluoro-6-phenyl-2H-1λ(3),3,2λ(4)-dioxaborinin-4-yl)benzoic acid, HAvoBF2, the compounds Mo2(T(i)PB)2(Bzald)2, I; Mo2(T(i)PB)2(Avo)2, II; and Mo2(T(i)PB)2(AvoBF2)2, III, have been isolated. Compounds I and II are red, and compound III is blue. The new compounds have been characterized by (1)H NMR, MALDI-TOF MS, steady-state absorption and emission spectroscopies, and femtosecond and nanosecond time-resolved transient absorption and infrared spectroscopies. Electronic structure calculations employing density functional theory and time-dependent density functional theory have been carried out to aid in the interpretation of these data. These compounds have strong metal-to-ligand charge transfer, MLCT, and transitions in the visible region of their spectra, and these comprise the S1 states having lifetimes ∼5-15 ps. The triplet states are Mo2δδ* with lifetimes in the microseconds. The spectroscopic properties of I and II are similar, whereas the planarity of the ligand in III greatly lowers the energy of the MLCT and enhances the intensity of the time-resolved spectra. The Mo2 unit shifts the ground state equilibrium entirely to the enol form and quenches the degradation pathways of the avobenzone moiety.

Steric and Electronic Factors Associated with the Photoinduced Ligand Exchange of Bidentate Ligands Coordinated to Ru(II).

In an effort to create a molecule that can absorb low energy visible or near-infrared light for photochemotherapy (PCT), the new complexes [Ru(biq)2 (dpb)](PF6 )2 (1, biq = 2,2'-biquinoline, dpb = 2,3-bis(2-pyridyl)benzoquinoxaline) and [(biq)2 Ru(dpb)Re(CO)3 Cl](PF6 )2 (2) were synthesized and characterized. Complexes 1 and 2 were compared to [Ru(bpy)2 (dpb)](PF6 )2 (3, bpy = 2,2'-bipyridine) and [Ru(biq)2 (phen)](PF6 )2 (4, phen = 1,10-phenanthroline). Distortions around the metal and biq ligands were used to explain the exchange of one biq ligand in 4 upon irradiation. Complex 1, however, undergoes photoinduced dissociation of the dpb ligand rather than biq under analogous experimental conditions. Complex 3 is not photoactive, providing evidence that the biq ligands are crucial for ligand photodissociation in 1. The crystal structures of 1 and 4 are compared to explain the difference in photochemistry between the complexes. Complex 2 absorbs lower energy light than 1, but is photochemically inert although its crystal structure displays significant distortions. These results indicate that both the excited state electronic structure and steric bulk play key roles in bidentate photoinduced ligand dissociation. The present work also shows that it is possible to stabilize sterically hindered Ru(II) complexes by the addition of another metal, a property that may be useful for other applications.

Isomerization initiated by photoinduced ligand dissociation in Ru(II) complexes with the ligand 2-p-tolylpyridinecarboxaldimine.

The excited state reactivity of Ru(ii) complexes continues to be a topic of intense investigation because of their widespread use in applications related to solar energy conversion, such as in photoswitches and for medicinal chemistry. In an effort to gain further understanding of photoinduced ligand exchange and isomerization in Ru(ii) complexes, various isomers of the formula [Ru(PTPI)2(CH3CN)2](2+) (PTPI = 2-p-tolylpyridinecarboxaldimine) were synthesized and characterized, as well as the tris-heteroleptic complexes cis-[Ru(bpy)(PTPI)(CH3CN)2](2+) (, bpy = 2,2'-bipyridine) and cis-[Ru(bpy)(PAP)(CH3CN)2](2+) (, PAP = 2-(phenylazo)pyridine). All of the complexes containing the PTPI ligand undergo photoinduced ligand exchange of the CH3CN ligands with the coordinating solvent. Each isolated di-substituted PTPI complex, however, also undergoes isomerization of the bidentate PTPI ligands upon irradiation in CH3CN to produce the same ratio of a mixture of 63% ε-[Ru(bpy)(PTPI)(CH3CN)2](2+) and 37% ß-[Ru(bpy)(PTPI)(CH3CN)2](2+). Experiments reveal that the isomerization of PTPI only occurs after the process of ligand dissociation is initiated by light. Evidence of isomerization following photoinduced ligand dissociation was also observed in , but not for the analogous complex . Electronic structure calculations, which included the relative overall energies of the isomers, and emission data were used to explain the results. The work presented herein may be useful in the design of new complexes for solar energy conversion or photoswitching applications.

Unusually efficient pyridine photodissociation from Ru(II) complexes with sterically bulky bidentate ancillary ligands.

The introduction of steric bulk to the bidentate ligand in [Ru(tpy)(bpy)(py)](2+) (1; tpy = 2,2':2',6″-terpyridine; bpy = 2,2'-bipyridine; py = pyridine) to provide [Ru(tpy)(Me2bpy)(py)](2+) (2; Me2bpy = 6,6'-dimethyl-2,2'-bipyridine) and [Ru(tpy)(biq)(py)](2+) (3; biq = 2,2'-biquinoline) facilitates photoinduced dissociation of pyridine with visible light. Upon irradiation of 2 and 3 in CH3CN (λirr = 500 nm), ligand exchange occurs to produce the corresponding [Ru(tpy)(NN)(NCCH3)](2+) (NN = Me2bpy, biq) complex with quantum yields, Φ500, of 0.16(1) and 0.033(1) for 2 and 3, respectively. These values represent an increase in efficiency of the reaction by 2-3 orders of magnitude as compared to that of 1, Φ500 < 0.0001, under similar experimental conditions. The photolysis of 2 and 3 in H2O with low energy light to produce [Ru(tpy)(NN)(OH2)](2+) (NN = Me2bpy, biq) also proceeds rapidly (λirr > 590 nm). Complexes 1-3 are stable in the dark in both CH3CN and H2O under similar experimental conditions. X-ray crystal structures and theoretical calculations highlight significant distortion of the planes of the bidentate ligands in 2 and 3 relative to that of 1. The crystallographic dihedral angles defined by the bidentate ligand, Me2bpy in 2 and biq in 3, and the tpy ligand were determined to be 67.87° and 61.89°, respectively, whereas only a small distortion from the octahedral geometry is observed between bpy and tpy in 1, 83.34°. The steric bulk afforded by Me2bpy and biq also result in major distortions of the pyridine ligand in 2 and 3, respectively, relative to 1, which are believed to weaken its σ-bonding and π-back-bonding to the metal and play a crucial role in the efficiency of the photoinduced ligand exchange. The ability of 2 and 3 to undergo ligand exchange with λirr > 590 nm makes them potential candidates to build photochemotherapeutic agents for the delivery of drugs with pyridine binding groups.

Photophysical properties of cis-Mo2 quadruply bonded complexes and observation of photoinduced electron transfer to titanium dioxide.

The compounds cis-Mo2(DAniF)2(L)2 have been prepared, where DAniF = (N,N')-p-dianisyl formamidinate and L = thienyl-2-carboxylate (Th), 2,2'-bithienyl-5-carboxylate (BTh), and 2,2':5',5″-terthienyl-5-carboxylate (TTh). The compounds have been characterized by proton nuclear magnetic resonance ((1)H NMR), ultraviolet-visible (UV-vis) absorption and emission, differential pulse voltammetry, and time-resolved transient absorption and infrared (IR) spectroscopy. An X-ray crystal structure was obtained for the thienyl complex. The related salt [(n)Bu4N]2[Mo2(DAniF)2(TTh-CO2)2], where TTh-CO2 = 2,2':5',2″-terthienyl-5,5″-dicarboxylate, has also been prepared and employed in the attachment of the complex to TiO2 nanoparticles. The latter have been characterized by ground-state Fourier transform infrared spectroscopy (FTIR) and femtosecond time-resolved IR spectroscopy. The time-resolved data provide evidence for sub-picosecond charge injection from the Mo2 center to the semiconducting oxide particle.

4-Nitrophenyl- and 4'-nitro-1,1'-biphenyl-4-carboxylates attached to Mo2 quadruple bonds: ground versus excited state M2δ-ligand conjugation.

From the reactions between Mo2(T(i)PB)4, where T(i)PB = 2,4,6-triisopropylbenzoate and two equivalents of the carboxylic acid LH (LH = 4-nitrobenzoic acid and 4'-nitro[1,1'-biphenyl]-4-carboxylic acid) the compounds trans-M2(T(i)PB)2L2 have been prepared: I (L = 4-nitrobenzoate and M = Mo), II (L = 4'-nitro-1,1'-biphenylcarboxylate and M = Mo) and III (L = 4-nitrobenzoate and M2 = MoW). The compounds have been characterized by (1)H NMR, UV-Vis and steady state emission spectroscopy, ns and fs transient absorption spectroscopy and cyclic voltammetry. These data are compared with predictions based on electronic structure calculations on model compounds where T(i)PB is substituted for formate. Together these data indicate stronger ground-state coupling of the Mo2δ and ligand π* systems in I relative to II but this order is reversed in the photo excited S1(1)MLCT state. Attempts to prepare the W2 containing analogs were unsuccessful.