Synthetic methods

   Homoleptic ruthenium(II) diimine complexes

   The synthesis of the homoleptic [Ru(bpy)3]Cl2, first reported in 1936, involved prolonged heating of a mixture of RuCl3 · x H2O and excess of 2,2'- bipyridine, according to [8]:

   A more versatile method involving reduction of RuCl3 · x H2O with sodium phosphinate or phosphinic acid in the presence of a diimine ligand was subsequently used to prepare [Ru(diimine)3]2 + complexes incorporating an array of substituted 2,2'-bipyridines and 1,10-phenanthrolines. A later preparation involves reaction of the versatile [Ru(CO)2Cl2]n with 2,2'-bipyridine and trimethylamine-N-oxide in 2-methoxyethanol [8].

The method, which can be applied to heteroleptic complexes, is capable of generalisation.

Another method involving conversion of [Ru(C6H6)Cl2]2 might well yield homoleptic complexes [8]:

   Heteroleptic ruthenium(II) diimine complexes [Ru(L)2(L1)]2 +

   The development of a synthetic methodology to heteroleptic tris(diimine) complexes of type [Ru(L)2(L1)]2 + was a major advance providing access to an array of complexes. One route involved the reaction of a ruthenium(III) complex, [Ru(L1)Cl3], with an excess of a different diimine, L [8].

A facile, widely used, route involves reaction of [Ru(L)2Cl2] with a second diimine in an appropriate medium [8].

   Syntheses of [Ru(L)2Cl2] required for this reaction were initially unsatisfactory and included thermal decomposition of [Ru(bpy)3]Cl2 and (LH)[Ru(L)Cl4] (LH =protonated diimine) and a photochemical process involving irradiation of [Ru(bpy)3]X2 or [Ru(bpy)3]Cl2. A most convenient synthesis of such complexes entails the direct reaction of RuCl3 ·x H2O with slightly more than two molar equivalents of a diimine ligand in DMF in which the solvent acts as a reductant, converting Ru(III) to Ru(II). A minor amount of the monocarbonyl complex [Ru(bpy)2(CO)Cl]+ may also be formed, presumably by the thermal decarbonylation of the solvent, but in the context of [Ru(L)2(L1)]2 + syntheses such an impurity is not problematic. Further reaction with another diimine ligand then produces the desired [Ru(L)2(L1)]2 + complex [8].

   The dimethyl sulfoxide complex [Ru(DMSO)4Cl2], formed by reacting RuCl3 · x H2O with DMSO, has also proved a convenient synthon for the preparation of heteroleptic complexes with two diimine ligands. Addition of a bidentate ligand, L, to [Ru(DMSO)4Cl2] in a non-donor solvent replaces two DMSO ligands and forms [Ru(L)(DMSO)2Cl2]. The lability of the DMSO and chloride ligands allows their replacement by a bidentate ligand, L1, yielding [Ru(L)(L1)2]2+ [8].

   Synthesis of [Ru(H2dcbpy)2(NCS)2]

   The complex associated with the break-through discovery of DSSCs was [Ru(H2dcbpy)2(NCS)2],(commonly called N3), where H2dcbpy =4,4-dicarboxy-2,2-bipyridine. The synthesis of this complex follows the typical path for bis(diimine)Ruthenium(II) complexes, i.e., reaction of the ligand with RuCl3 · x H2O at high temperatures in DMF produces [Ru(H2dcbpy)2(Cl)2], which is treated with an excess of a thiocyanate salt also in DMF to yield the product. The product contained small amounts of the tris(diimine) complex, [Ru(H2dcbpy)3]Cl2, and the S-bound thiocyanate linkage isomers, [Ru(H2dcbpy)2(NCS)(SCN)] and [Ru(H2dcbpy)2(SCN)2]. Purification was achieved by slowly precipitating the dye by acidifying an alkaline solution to pH 3.3. Moreover, procedures have been developed which allow the isolation of two tetra-butylammonium salts, (Bu4N)4[Ru(dcbpy)2(NCS)2] and (Bu4N)2[Ru(Hdcbpy)2(NCS)2] and these led to improvements in cell performance over N3 [8].

   Synthesis of the "black dye" [Ru(H3tctpy)NCS3]-

   Another efficient dye, termed the ‘black dye’ because of its excellent light absorption over most of the visible region, incorporates a terpyridine tridentate ligand, 4,4',4''-tricarboxy-2,2',2''-terpyridine (H3tctpy). To facilitate purification the ligand was first converted into the trimethyl ester and then refluxed with RuCl3 · x H2O under argon in an EtOH/CH2Cl2 mixture to form [Ru(tri(methoxycarbonyl)terpy)Cl3]. Further reaction with an excess of ammonium thiocyanate, at reflux in DMF, followed by hydrolysis with triethylamine produces the dye which was isolated as {(C2H5)3NH}[Ru(H3tctpy) (NCS)3] [8].

   The product was found to be predominantly a mixture of linkage isomers: 60% N-bonded isomer (preferred for DSSCs), 20% N- and S-bonded isomers and 10% S-bonded isomer. Purification of the product was achieved by a chromatographic method [8].

   Synthetic strategies to heteroleptic tris(diimine) ruthenium(II) complexes, [Ru(L)(L1)(L2)]2+

   Syntheses of heteroleptic tris(diimine) Ru(II) complexes of type [Ru(L)(L1)(L2)]2+, where L, L1 and L2 are dissimilar bidentate ligands have been reported involving use of [Ru(CO)2Cl2]n as initial synthon. The dicarbonyldichlorideruthenium(II) polymer, [Ru(CO)2Cl2]n is readily prepared from commercial hydrated RuCl3, as shown in figure 1 and figure 2 [8]. Two routes to [Ru(L)(L1)(L2)]2+ complexes based on [Ru(CO)2Cl2]n are described in the two figures and both initially involve the conversion of [Ru(CO)2Cl2]n into [Ru(L)(CO)2Cl2]. From this common product, the two routes diverge with one following the path of exchange of the chloro ligands by triflate followed by substitution of this relatively labile anion by a diimine ligand while in the other photodecarbonylation of [Ru(L)(CO)2Cl2] yields a dinuclear product which is subjected to cleavage by a strongly binding ligand. For both routes, the final step involved chemical decarbonylation with Me3NO in the presence of a third diimine ligand [8].

Fig. 1. Triflate route to heteroleptic
[Ru(L)(L1)(L2)]2+ complexes.
Fig. 2. Photochemical decarbonylation route
to heteroleptic [Ru(L)(L1)(L2)]2+ complexes.

    Reaction of [Ru(DMSO)4Cl2] complex with a diimine ligand, L, in a low boiling point solvent, CHCl3, produce [Ru(L)(DMSO)2Cl2]. Protic solvents or high boiling point aprotic solvents result in a mixture of mono- and bis- complexes and their use has to be avoided. Subsequently, reaction of [Ru(L)(DMSO)2Cl2] with a second diimine ligand, L1, in DMF at high temperature successfully substituted the remaining DMSO ligands and produced the heteroleptic complex, [Ru(L)(L1)Cl2]. Further reaction of [Ru(L)(L1)Cl2] with a third diimine L2 in an appropriate medium gives the product [Ru(L)(L1)(L2)]2+ [8].