Metal-to-ligand charge-transfer sensitisation of near-infrared emitting lanthanides in trimetallic arrays M2Ln (M=Ru, Re or Os; Ln=Nd, Er or Yb).

The new pro-ligand 4-methyl-4'-(carbonylamino(2-(tert-butoxycarbonylamino)ethyl))-2,2'-bipyridyl (L1) has been prepared and used to synthesise the complex fac-Re(I)Cl(CO)3(L1) 1 and the complex salts [M(II)(bipy)2(L1)](PF6)2 (M=RuII 8 or OsII 15). Deprotection with trifluoroacetic acid affords the amine-functionalised derivatives fac-Re(I)Cl(CO)3(L2) 2, [M(II)(bipy)2(L2)](PF6)2 (M=RuII 9 or OsII 16) which react with the dianhydride of diethylenetriamine pentaacetic acid (DTPA) to give the binuclear complex {fac-Re(I)Cl(CO)3}2(L3) 3 and the complex salts [{M(II)(bipy)2}2(L3)](PF6)4 (M = RuII 10 or OsII 17). The latter react with salts Ln(OTf)3 to afford a series of 12 heterotrimetallic compounds that contain a lanthanide (Ln) ion in the DTPA binding site; {fac-Re(I)Cl(CO)3}2(L3)LnIII (Ln=Nd 4, Er 5, Yb 6 or Y 7) and [{M(II)(bipy)2}2(L3)LnIII](PF6)(OTf)3 (M=RuII, Ln=Nd 11, Er 12, Yb 13 or Y 14; M=OsII, Ln=Nd 18, Er 19, Yb 20 or Y 21). All of these trimetallic species display absorption bands ascribed to metal-to-ligand charge-transfer (MLCT) excitations, and luminescence measurements show that these excited states can be used to sensitise near-infrared emission from LnIII (Ln=Nd, Er or Yb) ions. Single crystal X-ray structures of L1 and [RuII(bipy)2(L2H)](H2PO4)3.(CH3)2CO.0.8H2O were obtained, the latter revealing the presence of H2PO4- counter anions, the source of which is presumed to be hydrolysis of PF6- ions

A dinuclear bis(bipyridine)ruthenium(II) complex, [(bpy)2Ru(II){L2-}Ru-II(bpy)(2)](2+), incorporating an unusual non-innocent bridging ligand containing a p-benzoquinonediimine fragment: synthesis, structure, redox, and UV/VIS/NIR and EPR spectroelectrochemical properties

The reaction of [Ru(bpy)(2)(EtOH)(2)](2+) (bpy = 2,2'-bipyridine) with the symmetrical binucleating phenolatodiimine ligand HOC6H4N=CHC6H4CH=NC6H4OH (H2L1) in ethanol under dinitrogen results in the unexpected formation of the diruthenium complex [(bpy)(2)Ru-II{OC6H4N=C6H3(=NH)O}Ru-II(bpy)(2)](ClO4)(2) 1. In this complex, the bridging ligand (L-sq,L-sq)(2-) contains two inequivalent o-iminophenolate N,O-chelating binding sites, each formally at the semiquinone oxidation level, linked such that there is a p-benzoquinonediimine bridge between the two Ru centres. The crystal structures of H2L1 and of 1 have been determined. Complex 1 is electrochemically active and undergoes two reversible oxidations and two reversible reductions which, on the basis of UV/VIS/NIR and EPR spectroelectrochemical evidence, comparison with related systems and consideration of the redox potentials, we assign as centred on the bridging ligand. The two oxidations (at +0.12 and +0.35 V versus SCE) result in formation of a neutral, fully quinonoidal bridging ligand L-q,L-q; in the 'mixed-valence' mono-oxidised state, a near-IR transition (1570 nm) is ascribed to an intra-ligand charge transfer from the non-oxidised (semiquinone-like) to the oxidised (quinone-like) terminus. The two reductions (at -0.98 and -1.38 V versus SCE) are localised on the central p-benzoquinonediimine unit of the bridging ligand, affording the p-benzosemiquinone radical and then a p-diamide unit. In addition, at more extreme potentials, two oxidations at +1.49 and +1.70 V versus SCE are ascribed to Ru(II)/Ru(III) couples, and the reduction at -1.81 V versus SCE is bpy-based

Dinuclear ruthenium(II) complexes [{(L)ClRuII}(2)(mu-tppz)](2+) (L = an arylazopyridine ligand) incorporating tetrakis(2-pyridyl) pyrazine (tppz) bridging ligand: synthesis, structure and spectroelectrochemical properties

A series of dinuclear complexes [{(L1-4)C1Ru(II)}(2) (mu-tppz)][ClO4](2) {[1](ClO4)(2) to [4](ClO4)(2)} has been prepared, in which two {Ru-II(L1-4)Cl}(+) fragments [L = a 2-arylazopyridine ligand of the type 2-(C5H4N)-N=N-C6H4R; for L-1, R = H; L-2, R = p-Me; L-3, R = p-Cl; L-4, R = m-Me] are linked by the bridging ligand tppz [2,3,5,6-tetrakis(2-pyridyl) pyrazine]. A single isomer forms during the synthesis in each case, and the crystal structure of [4](ClO4)(2) shows it to be a twofold-symmetric isomer with each ligand L arranged such that its pyridine donor is on the long axis of the molecule (trans to the pyrazine ring of tppz) and the azo donor is trans to one of the pyridyl donors of tppz. This allows the peripheral aryl ring attached to the azo unit of each ligand L to be oriented over either face of the bridging ligand giving a three-layer pi-stacked (aryl-pyrazine-aryl) sandwich. Electrochemical studies revealed (i) separations of 190-250 mV (depending on the aryl substituent of L) between the successive Ru(II)/ Ru(III) couples, indicative of a significant inter-metallic electronic coupling, and (ii) several ligand-based reductions of the pi-acceptor pyrazine and arylazopyridine ligands. A UV/Vis/NIR spectroelectrochemical study showed the presence of an IVCT transition at ca. 1900 nm in MeCN for the Ru(II)-Ru(III) mixed-valence states, whose narrowness is indicative of borderline class III behaviour. Several reduced forms of the complexes were also spectroscopically characterised