Cyclometalated Ruthenium Complexes Research Articles

Synthesis of a new class of cyclometallated ruthenium(II) complexes and their application in dye-sensitized solar cells

Abstract We synthesized a new class of cyclometallated ruthenium(II) complexes, Ru(tctpy)(C^N)(NCS) ( 1 , 2 ), where C^N is a bidentate cyclometallating ligand such as 2-phenylpyridinato or 2-(4-(2-phenylethynyl)phenyl)pyridinato. Although these complexes exist as stereoisomers, the microwave synthetic technique yielded only one isomer. These compounds act as light sensitizers and have excellent light-harvesting properties, especially in the near-IR region. Therefore, they can be used in dye-sensitized solar cells (DSCs). A DSC sensitized with 2 shows a 10% incident photon-to-current conversion efficiency at 900 nm.

“Living” radical polymerization of styrene catalyzed by cyclometalated ruthenium(II) complexes bearing nonlabile ligands

AbstractCationic substitutionally inert cyclometalated ruthenium (II) and osmium (II) complexes, ([Mt(o‐C6H4‐2‐py)(LL)2]PF6), where LL‐1,10‐phenanthroline (phen) or 2,2′‐bipyridine (bipy), were used for radical polymerization of styrene. Gradual modification of the complexes within the series allowed comparison of the catalytic activity and the redox properties. There was no correlation between the reducing powers of the complexes and their catalytic activities. The osmium compound of the lowest reduction potential was not active. All the ruthenium complexes catalyzed the polymerization of styrene in a controlled manner; but the level of control and the catalytic activity were different under the same polymerization conditions. [Ru(o‐C6H4‐2‐py)(phen)2]PF6 demonstrated the best catalytic performance though its redox potential was the highest. It catalyzed the “living” polymerization with a reasonable rate at a catalyst‐to‐initiator ratio of 0.1. 1 equiv. of Al(OiPr)3 accelerated the polymerization and improved the control, but higher amount of Al(OiPr)3 did not speed up the polymerization and moved the process into the uncontrollable regime. Under the most optimal conditions, the controlled polymerization occurs fast without any additive and the catalyst degradation. Added free ligands inhibited the polymerization suggesting that the catalytically active ruthenium intermediates are generated via the reversible dechelation of bidentate phen or bipy ligands. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3814–3828, 2009

Cyclometallated ruthenium(III) complexes: Synthesis, structure and properties

Abstract Reactions of Schiff bases (H 2 apahR) derived from acetophenone and acid hydrazides, triethylamine and [Ru(PPh 3 ) 3 Cl 2 ] (1:2:1 mole ratio) in methanol provide cyclometallated ruthenium(III) complexes of formula trans -[Ru(apahR)(PPh 3 ) 2 Cl] in 74–81% yields. The complexes have been characterized by elemental analysis, magnetic susceptibility, spectroscopic (infrared, electronic and EPR) and electrochemical measurements. X-ray crystal structures of two representative complexes have been determined. In each complex, the metal centre is in distorted octahedral CNOClP 2 coordination sphere assembled by the C,N,O-donor meridionally spanning apahR 2− , the chloride and the two mutually trans -oriented PPh 3 molecules. All the complexes are one-electron paramagnetic ( μ eff. = 1.85–1.98 μ B ) and display rhombic EPR spectra in frozen (120 K) dichloromethane-toluene (1:1) solution. Electronic spectra of the complexes display several absorptions within 470–270 nm due to ligand-to-metal charge transfer and ligand centred transitions. The complexes are redox active and display a Ru(III) → Ru(II) reduction and a Ru(III) → Ru(IV) oxidation in the potential ranges −0.66 to −0.70 V and 0.75 to 0.80 V (vs. Ag/AgCl), respectively.

Electronic Effects in PCP-Pincer Ru(II)-Based Hydrogen Transfer Catalysis

The synthesis and characterization of novel cyclometalated ruthenium(II) complexes [RuCl(PCPOMe)(PPh3)] and [RuCl(PCPCF3)(PPh3)] containing monoanionic, tridentate coordinating PCP-pincer ligands [C6H3{CH2P(p-MeOC6H4)2}2-2,6]- (PCPOMe) and [C6H3{CH2P(p-CF3C6H4)2}2-2,6]- (PCPCF3) are reported. These compounds have been tested as catalyst precursors in the hydrogen transfer reaction of cyclohexanone to cyclohexanol in 2-propanol using NaOH as a base. The initial rate of the hydrogen transfer reaction appeared to depend on the electronic character of the Ar2P− groups of the PCP-pincer ligand. Among the catalyst precursors studied, the complex [RuCl(PCPCF3)(PPh3)] was found to exhibit the highest activity and the initial TOF exceeded that observed for the Ph2P analogue [RuCl(PCPPh)(PPh3)]. Most importantly, catalysis performed with [RuCl(PCPCF3)(PPh3)] does not require pretreatment of the precursor in the absence of substrate. Conversely, a different catalytic profile and a low activity were observed when either the electron-poor [RuCl{C6H3(CH2P(C6F5)2)2-2,6}(PPh3)] ([RuCl(PCPF20)(PPh3)]) complex or its triflate analogue was used as catalyst precursors. NMR studies and ESI-MS measurements provided information concerning the catalytically active species formed during the pretreatment of the precursor complexes. The results indicate that during the pretreatment period a monoanionic, monohydride ruthenium(II) species, Na[Ru(H)(PCPiPr)(OiPr)(PPh3)], is selectively formed. The latter hydride complex was also obtained via an independent synthetic route. On the basis of both the present results and those previously reported in literature, a mechanism is proposed.

Cyclometalated ruthenium complexes for sensitizing nanocrystalline TiO2 solar cells

Cyclometalated ruthenium complexes of [Ru(C--arrow--N) (N--N--N)] configuration are a promising new class of molecular sensitizers for dye-sensitized solar cells, as a result of their broad and red-shifted visible absorption in comparison to the analogous [Ru(N--N--N)2] type coordinative complexes.

Facile Synthesis of Cyclometalated Ruthenium Complexes with Substituted Phenylpyridines

AbstractWe have developed a new strategy that uses the Kröhnke synthesis for the preparation of various substituted phenylpyridines in excellent yields (up to 88 %). Starting with the appropriate commercially available acetophenone, a variety of phenylpyridines substituted by either electron‐donating (i.e. methyl, methoxy) or ‐withdrawing groups (i.e. bromide, nitro) on the phenyl ring are obtained in a two‐step synthesis. The corresponding functionalized cyclometalated ruthenium complexes can be prepared with unusually high yields by using methanol as reaction solvent. The electrochemical data of the complexes demonstrate the strong σ‐donating character of the anionic phenylpyridine ligand. X‐ray analyses of four complexes show a shortening of the Ru–C bond associated with the elongation of only one of the five Ru–N bonds (trans effect).(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Rapid energy transfer in bichromophoric tris-bipyridyl/cyclometallated ruthenium(ii) complexes

The synthesis, characterization, and photophysical properties of the N6-N5C bichromophoric [(bpy)2Ru(I)Ru(ttpy)][PF6]3 (bpy is 2,2'-bipyridine and ttpy is 4'-p-tolyl-2,2':6',2''-terpyridine) and [(bpy)2Ru(II)Ru(ttpy)][PF6]3 (I and II are bpy-dipyridylbenzene ditopic ligands bridged by an ethynyl and phenyl unit, respectively) complexes are reported together with the model mononuclear complexes [(bpy)2Ru(I)][PF6]2, [(bpy)2Ru(II)][PF6]2, [Ru(VI)(ttpy)][PF6] (VI is 3,5-di(2-pyridyl)-biphenyl) and [Ru(dpb)(ttpy)][PF(6)] (Hdpb is 1,3-di(2-pyridyl)-benzene). The electrochemical data show that there is little ground state electronic communication between the metal centers in the bimetallic complexes. Selective excitation of the N(5)C unit in the bichromophoric systems leads to luminescence typical for a bis-tridentate cyclometallated ruthenium complex and is similar to the [Ru(VI)(ttpy)][PF6] model complex. In contrast, the luminescence from the tris-bidentate N6 unit is efficiently quenched by energy transfer to the N5C unit. The energy transfer rate has been determined by femtosecond pump-probe measurements to 0.7 ps in the ethynyl-linked [(bpy)2Ru(I)Ru(ttpy)][PF6]3 and to 1.5 ps in the phenyl-linked [(bpy)2Ru(II)Ru(ttpy)][PF6]3 (in acetonitrile solution at 298 K), and is inferred to occur via a Dexter mechanism.

Switching on a Signaling Pathway with an Organoruthenium Complex

Altering cellular function with small synthetic molecules is a general approach for the design of drugs (medicinal chemistry) and molecular probes (chemical genetics). Medicinal chemistry and chemical genetics are focused predominately on the design of organic molecules, whereas inorganic compounds find applications mainly for their reactivity (e.g., cisplatin as DNA-reactive therapeutic) or imaging properties (e.g., gadolinium complexes as MRI diagnostics). 2] We are exploring a new direction that aims at utilizing the unique structural opportunities of metal complexes for the design of “organiclike” small-molecule probes and drugs. 4] We recently described an organoruthenium compound to be a low-nanomolar inhibitor for the protein kinase glycogen synthase kinase 3 (GSK-3). Here we disclose our success in developing this compound into a molecular probe for cellular signal transduction pathways involving GSK-3. Our strategy for the design of metal complexes as protein kinase inhibitors uses the class of indolocarbazole alkaloids (e.g., staurosporine) as a lead structure. Accordingly, these metal complexes bear a bidentate ligand which retains the structural features of the indolocarbazole heterocycle. This targets the metal complexes to the ATP-binding site by enabling two hydrogen bonds to the backbone of the hinge between the N-terminal and C-terminal kinase domains, analogous to ATP and conventional organic indolocarbazole inhibitors (Scheme 1). The remaining ligand sphere of the ruthenium atom gives the opportunity to create interactions with other parts of the ATP-binding site. Following this strategy, we recently described ruthenium compound 1-H as a selective low-nanomolar and ATP-competitive inhibitor of GSK-3. The key component of the design was the novel pyridocarbazole ligand 2-H, derived from arcyriaflavin A by just replacing one indole moiety with a pyridine residue. Here we introduce the ruthenium complex 1-OH (Scheme 1), derived from 1-H by adding a single hydroxy group to the indole moiety. Molecular modeling suggests that this OH group can occupy a small hydrophilic cavity within the ATP-binding site of GSK-3. Indeed, this modification not only increases the affinity for GSK-3 by an order of magnitude, but at the same time increases water solubility considerably, and thus makes it a promising molecular probe for GSK-3. GSK-3 is a component and negative regulator of the wnt signaling pathway, and we demonstrate in the following that 1-OH can switch on the wnt signaling pathway by inhibiting GSK-3 inside living cells and in Xenopus embryos. We devised a short and economic synthetic route to this class of metalated pyridocarbazoles (Scheme 2). Starting with aryl hydrazine hydrochloride 3, Fischer indole synthesis and protecting-group exchange lead to pyridoindole 4, which is deprotonated and treated with the TBS-protected dibromomaleimide 5 to yield monosubstitution product 6. The key step of the synthetic scheme is the following smooth anaerobic photocyclization to the TBS-protected pyridocarbazole 7 in 78% yield. Compound 7 can be deprotected with TBAF to give 2-OH or treated with [Ru(Cp)(CO)(CH3CN)2]PF6 in the presence of K2CO3 to yield cyclometalated ruthenium complex 8. Subsequent TBS deprotection with TBAF gives 1-OH. Complex 1-OH is stable in aqueous solution, in cell culture medium, under air, and can even withstand a 5 mm solution of 2-mercaptoethanol for 12 h without any sign of decomposition, as determined by 1H NMR spectroscopy. Furthermore, 1-OH also has favorable solubility in water. For example, it can be dissolved in 3% DMSO/water at a concentration of 1 mm. We next examined the potency of 1-OH by determining the concentration required for 50% inhibition (IC50) of GSK3a (the a isoform) and GSK-3b (the b isoform). The IC50 of 1OH is 300 pm and 500 pm for GSK-3a and GSK-3b, respectively, approximately an order of magnitude lower than that [*] D. S. Williams, G. E. Atilla, H. Bregman, Prof. Dr. E. Meggers Department of Chemistry University of Pennsylvania 231 South 34th Street, Philadelphia, PA 19104 (USA) Fax: (+1)215-746-0348 E-mail: meggers@sas.upenn.edu

Dibenzo[a:c](dipyrido[2,3-h:2′,3′-j])phenazine (dbdpzH 2) mono and dinuclear cyclometallated ruthenium complexes

The mono and dimetallic cyclometallated ruthenium complexes of dibenzo[a:c](dipyrido[2,3-h:2′,3′-j])phenazine (dbdpzH 2) have been prepared. The monocyclometallation of this potentially dicyclometallating bridging ligand in the dimetallic complex: [(tpy)Ru(dbdpzH)Ru(H 2O)(tpy)] 3+ has been attributed to the distortion of the ligand upon binding to the first metal ion, which increases the size of the second binding site, inhibiting the second cyclometallation. The mono and dimetallic cyclometallated ruthenium complexes of dibenzo[a:c](dipyrido[2,3-h:2′,3′-j]) phenazine (dbdpzH 2) have been prepared. The monocyclometallation of this potentially dicyclometallating bridging ligand in the dimetallic complex has been attributed to distortion of the second chelating site by the first complexation which inhibits the second cyclometallation.

A new redox site as an alternative to ferrocene to study electron transfer in self-assembled monolayers.

The cyclometallated ruthenium complex [Ru(bpy)2(pp)]+ (bpy: 2,2'-bipyridine; pp: 2-(2'-ylphenyl)pyridine) was easily grafted to a omega-alkanethiol and the resulting compound was coadsorbed with 11-hydroxyundecanethiol on gold yielding a Self-Assembled Monolayer (SAM) in an analogous manner as for a ferrocene derivative, as shown by impedance spectroscopy; the kinetics of the heterogeneous electron transfer were shown to be very fast, compared to ferrocene, which makes this new redox site a promising candidate for further studies about molecular wires.

Electrophilic functionalization of a cyclometallated ruthenium complex, an easy entry to new organometallic synthons

With the regiospecific halogenation of the complex [Ru- (bpy)2L1]+ (bpy = 2,2′-bipyridine, HL1 = 2-phenylpyridine) new organometallic starting materials are now available.

Paramagnetic ruthenium(III) cyclometallated complex. Synthesis, spectroscopic studies and electron-transfer properties

The reaction of Ru II(PPh 3) 3X 2 (X = Cl, Br) with o-(OH)C 6H 4C(H)=N-CH 2C 6H 5 (HL) under aerobic conditions affords Ru II(L) 2(PPh 3) 2, 1, in which both the ligands (L) are bound to the metal center at the phenolic oxygen (deprotonated) and azomethine nitrogen and Ru III(L 1)(L 2)(PPh 3), 2, in which one L is in bidentate N,O form like in complex 1 and the other ligand is in tridentate C,N,O mode where cyclometallation takes place from the ortho carbon atom (deprotonated) of the benzyl amine fragment. The complex 1 is unstable in solution, and undergoes spontaneous oxidative internal transformation to complex 2. In solid state upon heating, 1 initially converts to 2 quantitatively and further heating causes the rearrangement of complex 2 to the stable RuL 3 complex. The presence of symmetry in the diamagnetic, electrically neutral complex 1 is confirmed by 1H and 31P NMR spectroscopy. It exhibits an Ru II → L, MLCT transition at 460 nm and a ligand based transition at 340 nm. The complex 1 undergoes quasi-reversible ruthenium(II)—ruthenium(III) oxidation at 1.27V vs. SCE. The one-electron paramagnetic cyclometallated ruthenium(III) complex 2 displays an L → Ru III, LMCT transition at 658 nm. The ligand based transition is observed to take place at 343 nm. The complex 2 shows reversible ruthenium(III)—ruthenium(IV) oxidation at 0.875V and irreversible ruthenium(III)—ruthenium(II) reduction at −0.68V vs. SCE. It exhibits a rhombic EPR spectrum, that has been analysed to furnish values of axial (6560 cm −1) and rhombic (5630 cm −1) distortion parameters as well as the energies of the two expected ligand field transitions (3877 cm −1 and 9540 cm −1) within the t 2 shell. One of the transitions has been experimentally observed in the predicted region (9090 cm −1). The first order rate constants at different temperatures and the activation parameter Δ H #/Δ S # values of the conversion process of 1 → 2 have been determined spectrophotometrically in chloroform solution.

A cyclometallated ruthenium(ii) complex with a sterically hindered ligand displaying a long-lived MLCT excited state

Two cyclometallated ruthenium(II) complexes incorporating mono- or di-substituted phenanthroline moieties are synthesized and characterized by single-crystal X-ray diffraction; the complex with a sterically hindered ligand displays a long-lived MLCT excited state.

Synthesis and molecular structure of di- and mono-nuclear Schiff-base phenolate complexes: facile formation of cyclometallated ruthenium complexes

Condensation of 2,6-diformyl-4-methylphenol with aliphatic diamines, NH 2 (CH 2 ) n NH 2 (n = 2–4) in the presence of [Ru(dmso) 4 Cl 2 ] (dmso = dimethyl sulfoxide) resulted in acyclic dinuclear six-co-ordinate ruthenium(II) complexes [Ru 2 L(dmso) 4 Cl 4 ]. The crystal structure of a representative complex was determined. However these reactions in the presence of PPh 3 gave four-membered cyclometallated dinuclear complexes [Ru 2 L′(PPh 3 ) 4 (CO) 2 Cl 2 ]. Compounds formed by condensation of 2,6-diformyl-4-methylphenol with aromatic monoamines reacted with [Ru(dmso) 4 Cl 2 ]–PPh 3 to give cyclometallated mononuclear ruthenium(II) complexes. The structure of a representative complex was determined. The facile formation of cyclometallated ruthenium complexes is rationalised.

Multinucleating ligands from multiple palladium(0)-catalysed cross-coupling reactions — synthesis and characterisation of a trinuclear cyclometallated ruthenium(II) complex and a hexanuclear EPR-active molybdenum complex

Two multinucleating ligands have been prepared from 1,3,5-tris(3,5-dibromophenyl)benzene by multiple Pd(0)-catalysed cross-coupling reactions. 1,3,5-Tris[3,5-bis(4-pyridylethenyl)phenyl]benzene ( L 1 ) has six remote pyridyl moieties, each of which can coordinate a 17 valence-electron Mo(tp ∗)(NO)Cl fragment ( tp ∗ = hydrotris(3,5- dimethylpyrazolyl)borate ), affording the hexanuclear complex [Cl(NO)(tp ∗)Mo 6( L 1 ) ( 1). 1,3,5-Tris[3,5-bis(2-pyridyl)phenyl]benzene ( L 2 ) incorporates three potentially terdentate, cyclometallating N, C, N-donor sets, and can coordinate three Ru(tpy) 2+ fragments (tpy = 2,2′:6′,2″-terpyridine) giving the trinuclear complex [(tpy)Ru 3( L 2 )][PF 6] 3 ( 2). Complex 1 is EPR active, with nearest-neighbour pairs of molybdenum centres displaying magnetic exchange interactions. Electrochemical studies of the two complexes suggest that there is little ground-state interaction between the metal centres in either case.

A room temperature luminescent cyclometallated ruthenium(II) complex of 6-phenyl-2,2′-bipyridine

The cyclometallated complex Ru(tt)(phbp) + (tt=4′-tolyl-2,2′:6′2″-terpyridine, phbp=6-phenyl-2,2′-bipyridine) with a (N,N,N)(C,N,N) coordination, has been synthesized and characterized by 1H NMR, UV, FAB-MS spectral techniques and by elemental analysis. We have compared its electrochemical and photophysical properties with those of the non-orthometallated analogues Ru(tt) 2 2+ and Ru(terpy) 2 2+ (terpy=2,2′:6′,2″-terpyridine). The most remarkable feature of Ru(tt)(phbp) + is its ability to luminesce at room temperature in alcoholic and nitrile solvents. The lifetime of its 3MLCT excited state is 60 ns in CH 3CN.