Monometallic Precursors Research Articles

To Bond or Not to Bond: Metal-Metal Interaction in Heterobimetallic Rare-Earth Metal-Silver Complexes.

The reaction of 2,4-tBu2-6-(PPh2)PhOH (HOArP) with silver(I) triflate in a 3:1 molar ratio gave the mononuclear coinage metal complex (HOArP-κP)3AgIOTf (1). Treatment of HOArP with LnIII[N(SiMe3)2]3 (Ln = La, Sm, Y, Yb) in a 3:1 molar ratio yielded the mononuclear rare-earth metal complexes LnIII(OArP-κ2O,P)3 (2-Ln). The heterobimetallic rare-earth metal-silver complexes LnIII(OTf)(μ-OArP-1κ1O,2κ1P)3AgI (3-Ln) were prepared from monometallic precursors by reactions of equimolar amounts of 1 with LnIII[N(SiMe3)2]3 or 2-Ln with silver(I) triflate, respectively. The compounds were characterized by NMR, ultraviolet-visible (UV-vis), and infrared (IR) spectroscopy, single-crystal X-ray diffraction, elemental analysis, and the effective magnetic moments of the paramagnetic complexes were determined via the Evans NMR method. Computational studies were conducted on 3-La and 3-Y.

Homo and heterometallic ruthenium and platinum complexes with multiple targets for therapeutic applications: a review

Abstract We are now well-positioned to comprehend carcinogenesis at a molecular level in greater detail due to significant technological advancements. Additionally, we are now able to rationally design and develop drug molecules with the ability to either selectively enhance or disrupt important biological processes, maximizing their therapeutic potential. This has heralded a new era in drug design. The heterometallic ruthenium–platinum complexes can be used as anticancer, photodynamic therapy, diabetes treatment, and molecular sensors for thiol-containing peptides due to their multifunctional interactions with nuclear DNA, mitochondrial DNA, RNA, and proteins. Compared to cisplatin and its Ru-based monometallic precursors, a significant number of reported ruthenium–platinum complexes exhibit enhanced cytotoxicity and tumor selectivity. Due to the covalent binding of the cis-PtIICl2 moiety to DNA, photoactive Ru(II)–Pt(II) complexes were designed to prelocalize a photodynamic therapy agent at the site of action. The development of ruthenium–platinum-based heterometallic complexes has recently advanced, opening up new avenues for the development of drugs that are more efficient. Metal complexes’ potential as important cancer therapeutic agents will be the primary focus of this review. The development of ruthenium and platinum-based mono and mixed-metal complexes with therapeutic and biomedical applications are discussed in detail in this article.

Reversible Hydride Migration from C5Me5 to RhI Revealed by a Cooperative Bimetallic Approach.

The use of cyclopentadienyl ligands in organometallic chemistry and catalysis is ubiquitous, mostly due to their robust spectator role. Nonetheless, increasing examples of non‐innocent behaviour are being documented. Here, we provide evidence for reversible intramolecular C−H activation at one methyl terminus of C5Me5 in [(η‐C5Me5)Rh(PMe3)2] to form a new Rh−H bond, a process so far restricted to early transition metals. Experimental evidence was acquired from bimetallic rhodium/gold structures in which the gold center binds either to the rhodium atom or to the activated Cp* ring. Reversibility of the C−H activation event regenerates the RhI and AuI monometallic precursors, whose cooperative reactivity towards polar E−H bonds (E=O, N), including the N−H bonds in ammonia, can be understood in terms of bimetallic frustration.

Reversible Hydride Migration from C5Me5 to RhI Revealed by a Cooperative Bimetallic Approach

AbstractThe use of cyclopentadienyl ligands in organometallic chemistry and catalysis is ubiquitous, mostly due to their robust spectator role. Nonetheless, increasing examples of non‐innocent behaviour are being documented. Here, we provide evidence for reversible intramolecular C−H activation at one methyl terminus of C5Me5 in [(η‐C5Me5)Rh(PMe3)2] to form a new Rh−H bond, a process so far restricted to early transition metals. Experimental evidence was acquired from bimetallic rhodium/gold structures in which the gold center binds either to the rhodium atom or to the activated Cp* ring. Reversibility of the C−H activation event regenerates the RhI and AuI monometallic precursors, whose cooperative reactivity towards polar E−H bonds (E=O, N), including the N−H bonds in ammonia, can be understood in terms of bimetallic frustration.

Cyclophanes as Platforms for Reactive Multimetallic Complexes.

Multimetallic cofactors supported by weak-field donors frequently function as reaction centers in metalloproteins, and many of these cofactors catalyze small molecule activation (e.g., N2, O2, CO2) with prominent roles in geochemical element cycles or detoxification. Notable examples include the iron-molybdenum cofactor of the molybdenum-dependent nitrogenases, which catalyze N2 fixation, and the NiFe4S4 cluster and the Mo(O)SCu site in various carbon monoxide dehydrogenases. The prevailing proposed reaction mechanisms for these multimetallic cofactors relies on a cooperative pathway, in which the oxidation state changes are distributed over the aggregate coupled with orbital overlap between the substrate and more than one metal ion within the cluster. Such cooperativity has also been proposed for chemical transformations at the surfaces of heterogeneous catalysts. However, the design details that afford cooperative effects and allow such reactivity to be harnessed effectively in homogeneous synthetic systems remain unclear. Relatedly, hydride donors ligated to these metal cluster cofactors are suggested as precursors to the state that reacts with substrates; here too, however, the reactivity of hydride-decorated clusters supported by weak-field ligands is underexplored. Inspired by the reactivity potential of multimetallic assemblies evidenced in biological systems, approaches to design, synthesize, and evaluate reactivity of polynuclear metal compounds have been actively explored. In a similar vein to the templating function afforded by enzyme active sites, a carefully engineered organic ligand can be employed to control metal nuclearity of the complex and the local coordination environment of each metal center. This Account presents our efforts within this field, beginning with ligand design considerations followed by a survey of observed small molecule activation by trimetallic cyclophanates. We highlight the distinct reactivity outcomes accessed by multimetallic compounds as compared to aggregates that assemble in reaction mixtures from monometallic precursors. Contributing to the opportunity for programmed cooperativity in these designed multimetallic compounds, the cyclophane also dictates the orientation of substrate binding and metal-substrate interactions, which has a prominent influence on reactivity. For example, the dinitrogen-tricopper(I) cyclophanate reacts with dioxygen with markedly different results as compared to monocopper compounds. As an unexpected outcome, one series of tricopper compounds were discovered to be competent catalysts for carbon dioxide reduction to oxalate-a formally one-electron process-hinting at an inherently broader reaction scope for weak-field clusters at lowering the barrier for one-electron pathways as well as multielectron redox transformations. Further reflecting the role of the ligand in tuning reactivity, the trimetallic trihydride cluster compounds, [M3(μ-H)3]3+ (M = FeII, CoII, ZnII), demonstrate substrate specificity for CO2 over various other unsaturated molecules and surprising stability toward water. This series reflects the role of the local environment of a shallow ligand pocket to control substrate access. Summed together, the systems described here evidence the anticipated cooperative reactivity accessed in designed multimetallic species vs self-assembled monometallic systems (e.g., O2 activation and O atom transfer) as well as control of substrate access by seemingly subtle structural effects. Indeed, future efforts aim to interrogate the limits of cooperativity in these systems as well as the role of ligand dynamics and sterics on reactivity.

Bottom-Up Assembly of a Highly Efficient Metal-Organic Framework for Cooperative Catalysis.

In this study, we demonstrate a bottom-up assembly of a monomeric copper complex and a two-dimensional (2-D) heterometallic metal-organic framework (MOF) from a carboxylate-functionalized tridentate Schiff base ligand and metal ions. The obtained 2-D MOF features a unique bimetallic copper center which is different from its monometallic precursor and acts as an efficient heterogeneous catalyst for the Friedel-Crafts reaction and Henry reaction. The MOF catalyst shows a remarkably superior activity compared to its homogeneous counterparts in a wide range of substrates. It is presumably ascribed to the dual activation of the substrates by the active bimetallic copper center confined in the MOF network, which is supported by the significant changes in catalytic activity at low catalyst/substrates ratios when using the 2-D MOF and its precursor as catalysts, respectively. Moreover, the MOF catalyst also shows an excellent stability and recyclability. Our work, therefore, provides a stepwise strategy to design a heterogeneous cooperative catalyst, by taking advantage of the modulated structure of MOF and tunable functionality of the tridentate Schiff base, with high performance in a variety of organic synthesis.

Effect of metal-metal and metal-support interaction on activity and stability of Pd-Rh/alumina in CO oxidation

A series of Pd-Rh/alumina catalysts with total precious metals loading of 0.2wt.% was prepared by means of mechanical mixing of monometallic precursors and incipient wetness impregnation of the support with dual complex salt. Gamma and delta alumina supports were obtained from commercial aluminum hydroxide by calcinations at 720 and 1000°C respectively. Supports were examined by electron paramagnetic resonance spectroscopy, low-temperature nitrogen adsorption, X-ray diffraction analysis and photoluminescence spectroscopy. It was shown that concentration of donor sites of the supports differs according to distinctions between values of their specific surface areas. Finally, the strength of metal-support interaction is getting much weaker in the case of δ-Al2O3. Bimetallic Pd-Rh species with strong metal-metal interaction supported on γ-Al2O3 were found to be characterized by superior high-temperature stability. Pd/Rh ratios of 3:2 and 7:3 were shown to be the most appropriate for this purpose.

Synthesis and characterization of triply-bonded titanium-iron complexes supported by 2-(diphenylphosphino)pyrrolide ligands

The synthesis of several titanium-iron multiply bonded complexes supported by a 2-(diphenylphosphino)pyrrolide (NP) ligand is reported. Treatment of the homoleptic, 8-coordinate Ti(NP)4 monometallic precursors with FeCl2 and KC8 yielded the tetragonally-symmetric heterobimetallic complex Ti(μ2-NP)4Fe, 2. X-ray crystallographic analysis reveals a short metal-metal distance (2.021(2) and 2.028(2)Å for two independent molecules in the asymmetric unit) consistent with a Fe-Ti triple bond. Theoretical calculations also indicate 3 bonding interactions. Further reduction of 2 by 2 electrons with KC8 results in the isolation of the dianion, [K2][(κ1-NP)Ti(μ2-NP)3Fe], 3, where one phosphine arm has decoordinated from Fe, and the Fe-Ti triple bond has shortened to 1.9474(7)Å. This shortening results from the local ligand field change about Fe from tetragonal to trigonal, which allows for significantly better orbital overlap with Ti.

Hollow Ni–Co–B amorphous alloy nanospheres: facile fabrication via vesicle-assisted chemical reduction and their enhanced catalytic performances

In this paper, we develop a simple vesicle-assisted chemical reduction approach for synthesizing hollow Ni–Co–B nanospheres. With various characterization techniques, the resulting Ni–Co–B nanospheres are identified as amorphous alloys with a hollow chamber. Coexistence of NiII and CoII species plays a significant role in fabricating hollow nanospheric structures, because only solid nanoparticles can be obtained in the presence of a mono-metallic precursor. During liquid-phase hydrogenation of 2-ethyl-2-hexenaldehyde, hollow Ni–Co–B catalyst displays significant bi-site catalysis from bimetals and delivers much greater activity as well as better selectivity than associated with the dense Ni–Co–B catalyst. Additionally, this catalyst is also easily handled in liquid-phase reactions due to its lower density and magnetic property. The material design concept presented in this work opens a new avenue for the development of hollow non-noble metallic nanospheres and will draw more attention in the foreseeable future.

Light harvesting and directional energy transfer in long-lived homo- and heterotrimetallic complexes of Fe(II), Ru(II), and Os(II).

A new family of trimetallic complexes of the form [(bpy)2 M(phen-Hbzim-tpy)M'(tpy-Hbzim-phen)M(bpy)2](6+) (M=Ru(II), Os; M'=Fe(II), Ru(II), Os; bpy=2,2'-bipyridine) derived from heteroditopic phenanthroline-terpyridine bridge 2-{4-[2,6-di(pyridin-2-yl) pyridine-4-yl]phenyl}-1H-imidazole[4,5-f][1,10]phenanthroline (phen-Hbzim-tpy) were prepared and fully characterized. Zn(2+) was used to prepare mixed-metal trimetallic complexes in situ by coordinating with the free tpy site of the monometallic precursors. The complexes show intense absorptions throughout the UV/Vis region and also exhibit luminescence at room temperature. The redox behavior of the compounds is characterized by several metal-centered reversible oxidation and ligand-centered reduction processes. Steady-state and time-resolved luminescence data show that the potentially luminescent Ru(II)- and Os(II)-based triplet metal-to-ligand charge-transfer ((3)MLCT) excited states in the triads are quantitatively quenched, most likely by intercomponent energy transfer to the lower lying (3)MLCT (for Ru and Os) or triplet metal-centered ((3)MC) excited states of the Fe(II) subunit (nonluminescent). Interestingly, iron did not adversely affect the photophysics of the respective systems. This suggests that the multicomponent molecular-wire-like complexes investigated here can behave as efficient light-harvesting antennas, because all the light absorbed by the various subunits is efficiently channeled to the subunit(s) in which the lowest-energy excited states are located.

Versatile Supramolecular Coordination Behaviour of a Bis(bidentate) Tetraphosphane

AbstractThe bis(bidentate) phosphane cis,trans,cis‐1,2,3,4‐tetrakis(diphenylphosphanyl)cyclobutane (dppcb) has been used for the synthesis of supramolecular complexes, so‐called dyads and triads. Depending on the monometallic precursor compound [Ru(bpy)2(dppcb)](PF6)2 or [Ru(bpy)2(cis‐dppcbO2)](PF6)2 [bpy = 2,2′‐bipyridine, cis‐dppcbO2 = cis,trans,cis‐1,2‐bis(diphenylphosphanoyl)‐3,4‐bis(diphenylphosphanyl)cyclobutane], [Ru(bpy)2(dppcb)NiBr2](PF6)2 (1) or ΔΛ/ΛΔ‐[{Ru(bpy)2(cis‐dppcbO2)}2NiBr](PF6)5 (2) is exclusively formed in good yield by reaction with [NiBr2(DME)] (DME = dimethoxyethane). The versatile coordination behaviour of dppcb compared with that of cis‐dppcbO2 is confirmed by cis,trans,cis‐2,3‐bis(diphenylphosphanoyl)‐1,4‐bis(diphenylphosphanyl)cyclobutane (2,3‐trans‐dppcbO2, 4). Although two dppcb ligands coordinated simultaneously to a PdII centre cannot produce a square‐planar PdP4 core, the reaction of two equivalents of 4 with [Pd(CH3CN)4](BF4)2 exclusively leads to meso‐(MMMP/MPPP)‐[Pd(2,3‐trans‐dppcbO2‐P,P′)2](BF4)2 (3). This means that trans configurations of coordinating 2,3‐trans‐dppcbO2 with respect to the cyclobutane rings allow the formation of this PdP4 moiety. However, in the case of 4 it is also possible to obtain [PdCl2(2,3‐trans‐dppcbO2‐P,P′)] (5) in excellent yield by using [PdCl2(COD)] (COD = cyclooctadiene). In 5, only one ligand 4 is attached to the PdII centre, and a trans configuration of 4 within the five‐membered chelate ring can again be seen. All solid‐state structures 1–5 have been determined by single‐crystal X‐ray structure analysis. The corresponding solution structures are in agreement with these solid‐state structures and have been authenticated by multinuclear NMR spectroscopy, FAB and MALDI‐TOF‐MS measurements, UV/Vis spectroscopy and cyclic voltammetry. In the case of 2, the emission and lifetime properties of the excited state indicate that triads such as 2 could be suitable catalysts for the reductive part of water splitting.

Heterobimetallic dioxomolybdenum(VI) complexes derived from bis(2-hydroxy-1-naphthaldehyde)malonoyldihydrazone: Synthesis and characterization

The heterobimetallic complexes [MMoO2(L)(H2O)2] (where M = Zn2+ (1), Cu2+ (2), and Co2+ (4)) and [{MMoO3(H2L)(H2O)2}2] (where M = Ni2+ (3) and Mn2+ (5)) are synthesized from bis(2-hydroxy-1-naphthaldehyde)malonoyldihydrazone (H4L) using the monometallic precursor complex [MoO2(H2L)]·H2O in ethanol. The composition of the complexes is established based on the data obtained from the elemental analysis and molecular weight determinations. The structure of the complexes is discussed in the light of data obtained from molar conductance, magnetic moment, electronic, EPR and IR spectroscopic studies.

Electronic and interfacial behavior of gemini metallosurfactants with copper(ii)/pseudohalide cascade cores

In this paper we discuss the newly synthesized binuclear species [Cu2(L(PY18))2(μ1,1-N3)2(N3)2] (1) and [Cu2(L(PY18))2(μ1,3-SCN)2(NCS)2] (2), as obtained from the monometallic precursor [Cu(L(PY18))Br2]. These gemini metallosurfactants incorporate metal/anion cascade cores and are investigated by experimental and theoretical methods. Diagnostic IR stretches support the presence of μ1,1-bridged (end-on, 2075 cm(-1)) azide groups in 1 and μ1,3-bridged (end-to-end, 2117 cm(-1)) thiocyanate groups in 2. Anion-to-copper LMCT electronic processes at 390 and 440 nm for 1 and at 415 nm for 2 reinforce the nature of the metal/anion cascade cores. Both species are redox-active, magnetically uncoupled due to poor orbital overlap, and robust in the presence of strongly coordinating solvents. At the air-water interface, 1 and 2 yield Langmuir films with high collapse pressures of ca. 60 mN m(-1). Domain formation is considerably less extensive than that observed for the related monometallic precursor and the average molecular areas are in good agreement with their modeled molecular size. The resulting Langmuir-Blodgett films are isolated on silica substrates and investigated using IR-reflectance/absorbance spectroscopy.

Combining photosensitizers: The case of [Cl2Pt(bpym)Re(CO)3Cl] and its dithiolate analogs

The bimetallic complex [Cl2Pt(bpym)Re(CO)3Cl] (4) (where bpym denotes 2,2′-bipyrimidine) has been prepared along with the corresponding monometallic precursors and the dithiolate derivatives with bdt2− (1,2-benzene-dithiolate) and mnt2− (1,2-maleonitrile-dithiolate). The compounds have been characterized using elemental analysis, NMR, FTIR, UV–Vis and cyclic voltammetry methods. DFT and TD-DFT calculations on all the complexes under study allow us to correlate geometries and electronic structures. The theoretical studies of the dithiolate bimetallic complexes assigned the main band in the UV–Vis spectrum as a mixed metal ligand to ligand charge transfer transition (MMLL′CT), analogous to [M(diimine)(dithiolate)] complexes, with the former red shifted due to the presence of the Re(CO)3Cl moiety. Moreover coordination of the dithiolate ligand to the bimetallic complex 4 influences not only the energy, but also the nature of HOMO orbital altering it from Re(CO)3Cl to Pt/bdt character.

Characterization of heterobimetallic and mixed-valence complexes of molybdenum(V) derived from bis(2-hydroxy-1-naphthaldehyde) malonoyldihydrazone

Heterobimetallic complexes [UO2MoV(CH2L)(hzd)(H2O)2] n , [ZnMoV(CH2L)(hzd)(H2O)2] n and mixedvalence complexes [MoVIO2MoV(CH2L)(hzd)(H2O)2] n (where hzdH3 = inhH3, n = 1; slhH3, n = 2) are synthesized from bis(2-hydroxy-1-naphthaldehyde)malonoyldihydrazone (CH2LH4) and monometallic precursor complexes [Mo(CH2LH2)(hzd)]·nH2O (n = 0, 1) in ethanol. The composition of the complexes is established based on the data obtained from the elemental analysis. The structure of the complexes is discussed in the light of data obtained from molar conductance, magnetic moment, electronic, EPR, and IR spectroscopic studies. All complexes have μB values in the range 1.59–1.64 B.M., slightly lower than that required for one unpaired electron. The heterobimetallic complexes show two bands, while mixed-valence complexes show only one band in the visible region assigned to the d-d transition. The g-values decrease in going from uranyl-to-molybdenyl-to-zinc complexes containing the isonicotinoyldiazenido (inh) group, however, no such regular trend is observed in the case of complexes containing the salicyloyldiazenido (slh) group in the coordination sphere. In all complexes, the principal dihydrazone ligand is present in the enol form as a bridging hexadentate ligand in the anti-cis configuration where hydrazide ligands are coordinated to the metal centre as a trinegative bidentate ligand in the diazenido form.

Redox, Spectroscopic, and Photophysical Properties of Ru−Pt Mixed-Metal Complexes Incorporating 4,7-Diphenyl-1,10-phenanthroline as Efficient DNA Binding and Photocleaving Agents

The redox, spectroscopic, and photophysical properties as well as DNA interactions of the new bimetallic complexes [(Ph2phen)2Ru(BL)PtCl2](2+) (Ph2phen = 4,7-diphenyl-1,10-phenanthroline, and BL (bridging ligand) = dpp = 2,3-bis(2-pyridyl)pyrazine, or dpq = 2,3-bis(2-pyridyl)quinoxaline) were investigated. These Ru-polyazine chromophores with Ph2phen TLs (terminal ligands) and polyazine BLs are efficient light absorbers. The [(Ph2phen)2Ru(BL)PtCl2](2+) complexes display reversible Ru(II/III) oxidations at 1.57 (dpp) and 1.58 (dpq) V vs SCE (saturated calomel electrode) with an irreversible Pt(II/IV) oxidation occurring prior at 1.47 V vs SCE. Four, reversible ligand reductions occur at -0.50 dpp(0/-), -1.06 dpp(-/2-), -1.37 Ph2phen(0/-), and -1.56 V vs SCE Ph2phen(0/-). For the [(Ph2phen)2Ru(dpq)PtCl2](2+) complex, the first two reductions shift to more positive potentials at -0.23 and -0.96 V vs SCE. The electronic absorption spectroscopy is dominated in the UV region by π → π* ligand transitions and in the visible region by metal-to-ligand charge transfer (MLCT) transitions at 517 nm for [(Ph2phen)2Ru(dpp)PtCl2](2+) and 600 nm for [(Ph2phen)2Ru(dpq)PtCl2](2+). Emission spectroscopy shows that upon attaching Pt to the Ru monometallic precursor the λmax(em) shifts from 664 nm for [(Ph2phen)2Ru(dpp)](2+) to 740 nm for [(Ph2phen)2Ru(dpp)PtCl2](2+). The cis-Pt(II)Cl2 bioactive site offers the potential of targeting DNA by covalently binding the mixed-metal complex to DNA bases. The multifunctional interactions with DNA were assayed using both linear and circular plasmid pUC18 DNA gel shift assays. Both title complexes can bind to and photocleave DNA with dramatically enhanced efficiency relative to previously reported systems. The impact of the Ph2phen TL on photophysics and bioreactivity is somewhat surprising given the Ru → BL charge transfer (CT) nature of the photoreactive state in the complexes.

Design of Pt–Sn catalysts on mesoporous titania films for microreactor application

A new generation of nanostructured Pt–Sn/TiO 2 catalytic thin films has been developed by deposition of Pt–Sn mixed-metal precursors from organic solvents on mesoporous TiO 2/Ti films with a thickness of 200–300 nm. The titania sol was obtained by templating a TiO 2 precursor with Pluronic F127 surfactant. The films were prepared on Ti substrates by spin-coating. The influence of the F127/Ti ratio in the range between 0.006 and 0.050, the pH of the titania sol between 1.5 and 2.0, and the aging time between 8 and 240 h on the morphology and porous structure of titania films was investigated. A TiO 2 film with the highest degree of the long-order structure was obtained at a surfactant/Ti molar ratio of 0.009, a pH of 1.5, and an aging time of 24 h. This film has a hexagonal pore structure with a mean pore size of 3.5 nm and a porosity of 25%. A powder titania support with a similar chemical composition and morphology was also produced and used for optimization of an active component deposition. The Pt–Sn carbonyl [Pt 3(CO) 3(SnCl 3) 2(SnCl 2·H 2O)] n −2 n clusters were synthesized separately from monometallic precursors. They were loaded onto the TiO 2 supports by impregnation or adsorption. The adsorption of the Pt–Sn precursor for 24 h from an ethanol solution with concentrations of Pt and Sn of 2.0 and 1.2 mg/ml, respectively, followed by a vacuum treatment at 463 K, resulted in Pt–Sn nanoparticles embedded in the mesoporous titania network. An average size of bimetallic nanoparticles was 1.5–2 nm with a narrow particle size distribution. A reaction rate in terms of TOF between 0.2 and 3.3 min −1 was observed in the hydrogenation of citral over the Pt–Sn/TiO 2 catalysts. The selectivity to the unsaturated alcohols was as high as 90% at a citral conversion above 95%.

Syntheses and Properties of Heterobimetallic Ligand-Bridged Ruthenium(II)/Rhenium(I) Complexes and Their Monometallic Congeners

We have prepared three new heterobimetallic complexes containing electron-donating trans-{RuIICl(pdma)2}+ [pdma = 1,2-phenylenebis(dimethylarsine)] centers linked to electron-deficient fac-{ReI(biq)(CO)3}+ (biq = 2,2′-biquinolinyl) units. The bridging units are 4,4′-bipyridyl (4,4′-bpy), E-1,2-bis(4-pyridyl)ethylene (bpe), or 1,4-bis[E-2-(4-pyridyl)ethenyl]benzene (bpvb) ligands. A number of new monometallic precursor complexes have also been synthesized and fully characterized, primarily for purposes of comparison. The electronic absorption spectra of the bimetallic species are dominated by intense, visible d(RuII) → π*(4,4′-bpy/bpe/bpvb) metal-to-ligand charge-transfer (MLCT) bands and d(ReI) → π*(biq) absorptions in the near-UV region. Cyclic voltammetric studies reveal both RuIII/II oxidation and ligand-based reduction processes and show no evidence for significant electronic communication between the two metal centers. Stark spectroscopic studies on the visible MLCT bands show that extending the conjugation leads to increases in the dipole moment change and the transition dipole moment, and these changes combine to afford increased static first hyperpolarizabilities, β0, estimated by using the two-state model. Comparisons with monometallic RuII complexes reveal that methylation of the free pyridyl nitrogen leads to larger β0 responses than does coordination of the fac-{ReI(biq)(CO)3}+ center. Single-crystal X-ray structures have been determined for solvated adducts of the bimetallic complex salts trans,fac-[RuIICl(pdma)2(μ-L−L)ReI(CO)3(biq)](PF6)2 (L−L = 4,4′-bpy or bpe) and also for the monometallic compounds fac-[ReI(biq)(CO)3(L−L)]PF6 (L−L = 4,4′-bpy or bpe) and trans-[RuIICl(pdma)2(L−L)]PF6 [L−L = 2,7-diazapyrene or (E,E)-1,4-bis(4-pyridyl)-1,3-butadiene].

Proton-linked bi- and tri-metallic gold cyanide complexes observed by ESI-MS spectrometry

Electrospray ionization spectra of potential cyanide-containing gold-drug metabolites revealed additional, weak, unanticipated peaks at approximately twice the mass of the gold(I) and gold(III) cyanide complexes. The exact masses correspond to proton-linked bimetallic complexes, [H{Au(CN) m } 2] −, ( m = 2,4). Further investigation revealed a total of 12 examples, including trimetallic complexes, [H 2{Au(CN) m } 3] −; mixed species with two complexes, [H{Au(CN) 2}{Au(CN) 4}] −; and thiolato species, [H{(RS)Au(CN) 3} 2] −. trans-[AuX 2(CN) 2Cl 2] − and trans-[AuX 2(CN) 2Br 2] − generated 35Cl/ 37Cl and 79Br/ 81Br isotopic patterns for the protonated bi- and tri-metallic analogues which were in good agreement with the presence of four or six halide ligands, respectively. Concentration-dependent studies demonstrated that the signals are independent of the solution concentrations of mono-metallic precursors, suggesting formation in the gas phase during or following droplet desolvation.

Syntheses and Properties of Bimetallic Chromophore-Quencher Assemblies Containing Ruthenium(II) and Rhenium(I) Centers

We have prepared a family of six new bimetallic complexes containing electron-donating trans-{RuIICl(pdma)2}+ [pdma = 1,2-phenylenebis(dimethylarsine)] centers linked to trans-[RuIICl(pdma)(2,2‘-bpy)(NO)] (bpy = bipyridyl) or fac-{ReI(biq)(CO)3}+-based (biq = 2,2‘-biquinolinyl) electron acceptors. The extended bridging units are based on 4,4‘-bpy or bpe [E-1,2-bis(4-pyridyl)ethylene] ligands, and these assemblies are designed to display long-lived photoinduced charge-separated states. A number of new monometallic precursor complexes have also been synthesized and fully characterized. The electronic absorption spectra of the bimetallic species are dominated by intense, visible d(RuII) → π*(4,4‘-bpy/bpe) metal-to-ligand charge-transfer (MLCT) bands and d(ReI) → π*(biq) absorptions in the near-UV region. Cyclic voltammetric studies show both metal-based oxidation and ligand-based reduction processes and provide evidence that these assemblies contain thermodynamic driving forces for charge separation. Single-crystal X-ray structures have been determined for the monometallic complex salts fac-[ReI(biq)(CO)3(MeCN)]CF3SO3·CHCl3, fac-[ReI(biq)(CO)3(4-BrCH2C5H4N)]PF6, fac-[ReI(biq)(CO)3(4-HCO2CH2C5H4N)]PF6·Me2CO, fac-[ReI(biq)(CO)3(ISNE)]CF3SO3 (ISNE = ethylisonicotinate), and fac-[ReI(biq)(CO)3(NC5H4CO2)]2·HPF6·3MeCN. Ultrafast time-resolved infrared and Raman spectroscopic measurements will be employed to investigate the photoexcitation properties of the bimetallic species and of monometallic reference complexes.