The surprising lability of bis ( 2 , 2 ’ : 6 ’ , 2 ’ ’-terpyridine )-chromium ( III ) complexes †

The complex [Cr(tpy)(O3SCF3)3] (tpy = 2,2’:6’,2’’-terpyridine) is readily made from [Cr(tpy)Cl3] and is a convenient precursor to [Cr(tpy)2][PF6]3 and to [Cr(tpy)(4’-(4-tolyl)tpy)][PF6]3 and [Cr(tpy)(5,5’’-Me2tpy)][PF6]3 (4’-(4-tolyl)tpy = 4’-(4-tolyl)-2,2’:6’,2’’-terpyridine; 5,5’’-Me2tpy = 5,5’’-dimethyl-2,2’:6’,2’’-terpyridine); these are the first examples of heteroleptic bis(tpy) chromium(III) complexes. The single crystal structures of 2{[Cr(tpy)2][PF6]3}·5MeCN, [Cr(tpy)(4’-(4-tolyl)tpy)][PF6]3·3MeCN and [Cr(tpy)(5,5’’-Me2tpy)][PF6]3·3MeCN have been determined. Each cation contains the expected octahedral {Cr(tpy)2} 3+ unit; in all three structures, the need to accommodate three anions per cation and the solvent molecules prevents the formation of a grid-like array of cations that is typical of many lattices containing {M(tpy)2} 2+ motifs. Three reversible electrochemical processes are observed for [Cr(tpy)(4’-(4-tolyl)tpy)][PF6]3 and [Cr(tpy)(5,5’’-Me2tpy)][PF6]3, consistent with those documented for [Cr(tpy)2] . At pH 6.36, aqueous solutions of [Cr(tpy)2][PF6]3 are stable for at least two months. However, contrary to the expectations of the d 3


Introduction
The coordination chemistry of 2,2′:6′,2″-terpyridine (tpy) is a mature area which continues to receive widespread attention with a myriad of potential applications. 1 Significantly, complexes of the group 8, 9 and 10 metals dominate the literature.We are particularly interested in the use of tpy complexes for applications related to energy conversion, where ruthenium(II) compounds are the state-of-the-art sensitizers in Grätzel-type photovoltaic solar cells. 2,3Ruthenium is one of the platinumgroup metals, the total world accessible resources ‡ of which are estimated to be of the order of 10 8 kg. 4 The relative scarceness of the platinum-group metals contrasts with the terres-trial abundances of first row transition metals such as iron, chromium, copper and zinc, world resources ‡ of which are estimated to be between 1.9 × 10 12 (Zn) and >2.3 × 10 14 (Fe) kg. 4 As a part of our efforts towards developing a sustainable materials chemistry for application to energy conversion systems, we are currently focusing attention on applications of earth abundant metals in dye-sensitized solar cells (DSCs) 5 and light-emitting electrochemical cells (LECs) and organic light emitting diodes (OLEDs). 6e and other groups 5 have developed molecular copperbased sensitizers with energy-to-electricity conversion efficiencies approaching 3% in Grätzel-type cells.While the lability of copper(I) can be conveniently utilized to assemble heteroleptic [Cu(L)(L′)] + dyes by ligand exchange reactions on TiO 2 surfaces, 5 the same property may be disadvantageous in terms of ligand dissociation with concomitant dye degradation.We have recently reported that sensitizers incorporating a {Zn(tpy) 2 } 2+ domain can be assembled in a stepwise fashion on TiO 2 ; we adopted this method in response to the observation that ligand exchange at zinc(II) is unexpectedly slow, 7 in contrast to faster exchange at copper(I).On the other hand, once assembled on the surface, the zinc(II) dyes are stable with respect to ligand dissociation under ambient conditions.Chromium(III) (d 3 ) is a text-book example of a kinetically inert metal ion, 8 with the rate of water exchange in the complex [Cr(OH 2 ) 6 ] 3+ being 2.4 × 10 −6 s −1 . 9This low rate of ligand exchange attracted us towards investigating the use of chromium(III) complexes, and in particular those containing its {Cr(tpy) 2 } 3+ cores, as exceptionally stable materials for applications in DSCs and/or LECs.Surprisingly, of all of the tpy complexes of earth abundant first row transition metals, those of chromium(III) are relatively rarely investigated.The series of homoleptic complexes [Cr(tpy) 2 ] n with n = +3, +2, +1, 0, 1 has been known for many years, [10][11][12][13][14][15] and has recently been the subject of in-depth studies. 167][18][19][20][21][22][23][24][25] To the best of our knowledge, no heteroleptic complexes with {Cr(tpy) 2 } 3+ cores have been reported.We now report a convenient route to [Cr(tpy) 2 ] 3+ and heteroleptic derivatives, and the structural characterization of representative complexes.Solution studies reveal that under certain conditions, the chelating tpy ligands in these complexes are surprisingly labile.

Experimental section
General FT IR spectra were recorded using either a Shimadzu FTIR 8400S or a Perkin Elmer Spectrum Two spectrophotometer with solid samples introduced in a Golden Gate ATR or UATR Two, respectively.Electronic absorption spectra were recorded on an Agilent 8453 spectrophotometer. 1 H NMR spectra were recorded on a Bruker Avance-400 spectrometer.
Electrochemical measurements were made on a CH Instruments 900B potentiostat using glassy carbon, platinum wire and a silver wire as the working, counter, and pseudo reference electrodes, respectively.Samples were dissolved in HPLC grade MeCN (10 −4 to 10 −5 mol dm −3 ) containing 0.1 mol dm −3 [ n Bu 4 N][PF 6 ] as supporting electrolyte; all solutions were degassed with argon.Cp 2 Fe was used as internal reference added at the end of experiments.
Reactions were carried out under N 2 .CrCl 3 was obtained from Acros Organics and was used as received.The ligands tpy, 26 4′-(4-tolyl)tpy 27 and 5,5″-Me 2 tpy 26,28 were made by literature routes.The structures of ligands used are shown in Scheme 1.

Crystallography
Data were collected on a Bruker-Nonius APEX2 diffractometer with data reduction, solution and refinement using the programs APEX2, 29 SIR92 30 and CRYSTALS 31 or SHELXL97 or SHELX-13. 32The ORTEP diagrams were prepared and structure analyses carried out using Mercury v. 3.0. 33,34The structure of [Cr(tpy)(4′-(4-tolyl)tpy)][PF 6 ] 3 •3MeCN can be solved in C2/c, but is heavily disordered.The solution in Cc, refined as a twin by inversion (the Flack parameter refines to 0.416 with a su of 0.016), gives an ordered structure where the space that the tolyl group occupies on one side of the complex molecule is taken by an acetonitrile molecule on the other side.This

Syntheses of chromium(III) complexes
The chromium(III) complex [Cr(tpy) 2 ][PF 6 ] 3 has previously been prepared by oxidation of [Cr(tpy) 2 ][PF 6 ] 2 using AgPF 6 . 16We have found that [Cr(tpy) 2 ][PF 6 ] 3 can be conveniently prepared by the strategy shown in Scheme 2, and this method is easily adapted to prepare heteroleptic chromium(III) complexes.The precursor [Cr(tpy)Cl 3 ] was prepared by a modification of the method of Broomhead et al.; 35,36 16 and the appearance of the spectrum of [Cr(tpy)(4′-(4-tolyl)tpy)]-[PF 6 ] 3 between 300 and 380 nm is similar to that reported for [Cr(4′-(4-tolyl)tpy) 2 ] 3+ . 19yclic voltammetry was used to study the electrochemical processes exhibited by the complexes.Each complex shows three redox processes within the solvent accessible window (Table 1) which are fully reversible at a scan rate of 0.1 V s −1 ; the incorporation of the tolyl or methyl substituents has negligible effect on the redox potentials.This is consistent with trends observed between [Cr(tpy)  16 indicate that the redox processes are ligand-based and that chromium remains in oxidation +3 through the series.It has not been confirmed whether this formulation will hold for all complexes with substituted tpy ligands.

Structural characterization
The  3) does not involve a two-dimensional grid-like array arising from face-to-face and edge-to-face interactions between tpy domains of adjacent cations.Such arrangements are typical in salts of [M(tpy) 2 ] 2+ cations. 42owever, we have previously noted that on going from  ] 3 }•5MeCN, the gridlike packing of the cations is lost. 43A similar observation has been made by Kuroda-Sowa and co-workers 44  Instead, the tolyl-unit of one cation is accommodated within the cleft formed by the two pyridine rings containing N1 i and N4 i (symmetry code i = −1/2 + x, 1/2 − y, −1/2 + z) of the adjacent cation, resulting in the propagation of chains which run obliquely through the unit cell (Fig. 5).In Fig. 5, one interaction is represented in space-filling mode, while CH⋯π contacts are shown in red for the next analogous contact along the chain.The closest CH(methyl)⋯π contacts are 3.4 and 3.5 Å.All three anions and MeCN molecules are ordered, and extensive CH⋯F and CH⋯N interactions dominate the crystal packing forces.

Stability of complexes in aqueous solutions
Monitoring the absorption spectra indicates that aqueous solutions (deionized water, pH 6.36) of [Cr(tpy) 2 ][PF 6 ] 3 are stable for at least two months (Fig. 7a).This parallels the behaviour of [Cr(bpy) 3 ][ClO 4 ] 3 in aqueous solutions at pH < 7. 45 Under basic conditions, however, it is known that [Cr(bpy) 3 ] 3+ undergoes base-catalysed ligand loss according to eqn (1).An associative mechanism has been proposed involving a sevencoordinate intermediate [Cr(bpy) 3 (OH 2 )] 3+ which undergoes deprotonation at high pH, followed by loss of bpy; the proposed mechanism invokes a monodentate bpy ligand. 45,46The process under ambient conditions is the same as that of photoaquation, i.e. either the 4 A 2 ground or 2 E excited state associates with H 2 O in the presence of [OH] − resulting in the overall reaction shown in eqn (1). 47,48 Given that chromium(III) (d 3 ) is usually considered 8 to be kinetically inert with respect to ligand exchange, the above observations for the tris(chelate) are somewhat unforeseen.In   [Cr(tpy) 2 ] 3+ , the Cr 3+ ion is bound by two tridentate chelating ligands, and we expected that this coupled with the d 3 configuration would lead to a non-labile system.We were therefore surprised to observe significant changes in the absorption spectrum of [Cr(tpy) 2 ][PF 6 ] 3 under aerobic aqueous alkaline conditions.A titration of aqueous [Cr(tpy) 2 ][PF 6 ] 3 against six equivalents of aqueous NaOH in one equivalent steps was followed by UV-VIS spectroscopy.Fig. 7b shows that the absorption maxima at 363, 347 and 265 nm characteristic of aqueous [Cr(tpy) 2 ][PF 6 ] 3 (compare Fig. 7 with the red curve in Fig. 1) decrease in intensity as new absorptions at λ max = 330 and 283 nm (with shoulders at 315, 301 and 275 nm) grow in.During the titration, the solution turned from yellow to pale green.After the addition of 5 equivalents of NaOH, conversion was essentially complete (Fig. 7b).The final absorption spectrum did not correspond to that of free tpy in aqueous solution at pH 9.5 (for which λ max = 290 and 226 nm), and the data were therefore ambiguous in terms of the presence of uncoordinated ligand.A similar titration was carried out with an aqueous solution of [Cr(tpy)(4′-(4-tolyl)tpy)][PF 6 ] 3 and the changes in the absorption spectra over the addition of 6 equivalents of aqueous NaOH resemble those observed for [Cr-(tpy) 2 ][PF 6 ] 3 (compare Fig. 8 and 7b).Again, a colour change from yellow to pale green was observed.At the end of the titration, the solution was evaporated to dryness and the residue was extracted into chloroform.After filtration, the 1 H NMR spectrum (in CDCl 3 ) of the chloroform-soluble material was recorded.A comparison of the spectrum with those of authentic samples of tpy and 4′-(4-tolyl)tpy (Fig. S1 †) confirmed the presence of both free ligands in an approximate ratio of 1 : 1, providing evidence that [Cr(tpy)(4′-(4-tolyl)tpy)][PF 6 ] 3 is unstable in alkaline solution with respect to displacement of the two chelating ligands.][51] In an additional experiment, [Cr(tpy) 2 ][PF 6 ] 3 was dissolved in CD 3 OD.In the 1 H NMR spectrum, as expected, no signals were observed in the aromatic region of the spectrum.are formed.We are not able to gain evidence for the formation of a simple hydroxo complex akin to the product of the aquation of [Cr(bpy) 3 ] 3+ (eqn (1)), and as far as we are aware, no reports appear in the literature of a mononuclear species such as [Cr(tpy)(OH) 3 ].On the other hand, [Cr(tpy)F 3 ] has been described and structurally characterized, 53 and we therefore hoped that interaction of fluoride ion with [Cr(tpy) 2 ] 3+ might provide further insight into the system.
The addition of [ n Bu 4 N]F to an aqueous solution of [Cr(tpy) 2 ][PF 6 ] 3 resulted in no change in the absorption spectrum of the complex.However, when the reaction was performed in methanol (see Experimental section), the initially pale yellow solution became colourless.A decrease in the amount of hydrogen bonding between F − and solvent molecules on going from water to methanol is expected to render the F − ion a stronger nucleophile in methanol than in water.The absorption spectrum of the final solution (Fig. 9) exhibits a low intensity maximum in the visible region at 570 nm.This maximum agrees with that reported for [Cr(tpy)F 3 ] (λ max = 581 nm in propan-2-ol and 556 in aqueous solution), 53 suggesting that fluoride displaces one of the tpy ligands.To confirm this, the reaction of [ n Bu 4 N]F with [Cr(tpy) 2 ][PF 6 ] 3 was repeated in CD 3 OD in an NMR tube.Fig. S4a † shows that signals assigned to free tpy indeed appear in the 1 H NMR spectrum after [Cr(tpy) 2 ][PF 6 ] 3 is treated with fluoride ion.

Scheme 1
Scheme 1 Ligand structures and abbreviations.
The 1 H NMR spectra before and after adding NaOH are shown in Fig. S3.† Corresponding experiments were carried out with [Cr(tpy)(4′-(4-tolyl)tpy)][PF 6 ] 3 and [Cr(tpy)(5,5″-Me 2 tpy)][PF 6 ] 3 , and in each case, the addition of NaOH to CD 3 OD solutions of the complexes resulted in the appearance of signals for the free ligands (Fig. S2b and S2c †).Note that the NMR spectroscopic data are consistent with both ligands being displaced from the metal centre in the heteroleptic complexes, but do not indicate whether they are both lost from the same or different metal centres.The absorption spectra in Fig. 7b and 8 unambiguously show that [Cr(tpy) 2 ][PF 6 ] 3 and [Cr(tpy)(4′-(4-tolyl)tpy)][PF 6 ] 3 decay in basic aqueous media.The spectra recorded in the presence of 6 equivalents of NaOH are very similar to one another, and we propose that multinuclear chromium(III) aqua/hydroxo species