Rotaxane PtII-complexes: mechanical bonding for chemically robust luminophores and stimuli responsive behaviour†

We report an approach to rotaxanes in which the metal ion of a cyclometallated PtII luminophore is embedded in the space created by the mechanical bond. Our results show that the interlocked ligand environment stabilises a normally labile PtII–triazole bond against displacement by competing ligands and that the crowded environment of the mechanical bond retards oxidation of the PtII centre, without perturbing the photophysical properties of the complex. When an additional pyridyl binding site is included in the axle, the luminescence of the PtII centre is quenched, an effect that can be selectively reversed by the binding of AgI. Our results suggest that readily available interlocked metal-based phosphors can be designed to be stimuli responsive and have advantages as stabilised triplet harvesting dopants for device applications.


Introduction
Many mechanically chelated 1 metal complexes, in which a metal ion bridges mechanically bonded covalent subcomponents, 2 have been synthesised since Sauvage's rst report of the Cu I templated synthesis of catenanes. 3 These complexes are typically reported as intermediates in the synthesis of interlocked molecules, 4 as opposed to being objects of study in their own right. 5,6 This is despite the potential benets that embedding the metal ion within the crowded environment of the mechanical bond might bring in terms of the kinetic stabilisation and electronic, magnetic and catalytic properties. Indeed, Sauvage and co-workers highlighted the kinetic stabilisation of a bound Cu I ion due to a "catenand effect". 3,7 Recently, we demonstrated that the mechanical bond allows access to complexes of which the non-interlocked equivalents are inaccessible, 8 including examples with highly distorted coordination geometries reminiscent of the entatic state of enzyme active sites. 9,10 These reports suggest that interlocked metal complexes in which the mechanical bond alters the stability or properties of the ion are an untapped resource in the development of coordination complexes for a range of applications, including catalysis, metallo-pharmaceuticals and light harvesting.
Metal-based luminophores have been at the forefront of many recent developments in light-emitting organic electronic materials for various applications. 10,11 Iridium(III)-based phosphors are now widely incorporated into commercial organic light-emitting devices (OLEDs) to allow the harvesting of triplet excitons that would otherwise be wasted. 12 The strong spinorbit coupling associated with the heavy metal ion promotes the T 1 / S 0 phosphorescence process, which is strongly forbidden in purely organic materials. 13 Complexes of other 2 nd and 3 rd row metal ions are also widely investigated in this context, particularly those of platinum(II). 14 To date, only a limited number of mechanically chelated heavy-metal complexes have been reported. This is perhaps not surprising as most metal-directed passive template syntheses rely on early transition metals, 5 due to their more favourable ligand exchange kinetics. Furthermore, the small number of passive templates based on Pd II , 15 Ru II , 16 and Au I (ref.17) reported are not luminescent. 18,19 Thus, the limited examples of interlocked molecules containing metal-based luminophores incorporate the metal complex as a substituent of the interlocked scaffold, or as a structural unit. In 2012, Terao and coworkers demonstrated that emissive Pt II -acetylide units embedded in the axle of a rotaxane were insulated from the local environment, leading to no change in emission between solution and solid state. 20 More recently, Ma and co-workers demonstrated that the emission of a Pt II -porphyrin complex appended to a rotaxane could be modulated by mechanical motion. 21 Here we describe the synthesis and properties of rotaxanebased Pt II complexes in which the metal ion is embedded within the mechanical bond. We show that the mechanical bond does not signicantly perturb the photophysical properties of the metal ion but, pleasingly, the chemical stability of the complexes is greatly enhanced relative to non-interlocked analogues. Furthermore, while studying this stabilising inuence, we serendipitously identied a metallorotaxane in which the phosphorescence shows a dramatic and selective enhancement in the presence of Ag I , through the inhibition of a quenching pathway arising from a pyridyl unit incorporated in the structure.

Synthesis of mechanically chelated Pt II complexes
Our study was in part inspired by a cyclometallated catenanebased Pd II complex reported in early work by Sauvage and coworkers. 22 27 Using these more forcing conditions, it was also possible to obtain this complex directly from 1.H and K 2 PtCl 4 in reasonable yield. The formation of the corresponding noninterlocked complex [Pt(2)(3)]BF 4 required a two-step procedure involving initial formation of Pt(2)Cl followed by Ag Imediated substitution of the chloride ligand by the triazole (Scheme 1b). 28 All novel compounds and intermediates were characterised by NMR spectroscopy and MS (see ESI †  (Fig. 1) were further characterised in the solid-state by single-crystal X-ray diffraction (SCXRD). The solid-state structure of [Pt(1)]BF 4 clearly demonstrates the mechanically chelating environment of the metal ion which bridges N 3 of the triazole axle and the N^N^C chelate provided by the macrocycle. As is typically observed with small, sterically crowded interlocked molecules, 24 several short intercomponent contacts are present in addition to metal-ligand interactions (Fig. 1a), including a CH/Pt contact. Viewing the solid-state structure in space-lling representation (Fig. 1b) underlines the sterically crowded nature of the mechanical bond; the metal ion is embedded in the cavity of the ring and largely isolated from the local environment.

Photophysical properties of metallorotaxane [Pt(1)]BF 4 and [Pt(2)(3)]BF 4
The photophysical properties of the interlocked Pt II complex [Pt(1)]BF 4 and its non-interlocked analogue, [Pt (2)(3)]BF 4 , are remarkably similar (Table 1 and   mechanical bond does not signicantly perturb the pertinent electronic excited states. Both complexes display UV-visible absorption properties typical of cyclometallated Pt II complexes of arylpyridine ligands, 29 with intense intra-ligand bands in the far-UV and a somewhat less intense band at lower energy (350-380 nm). Bands in the latter region are typically associated with charge-transfer transitions of mixed d Pt |p Ar /p * N^N orbital parentage, that are introduced upon cyclometallation. Notably, there is no such band for the non-cyclometallated Excitation of both cyclometallated complexes at 360 nm in degassed solution leads to an emission band centred around 575 nm. The corresponding luminescence lifetimes of around 2 ms in deoxygenated solution are indicative of a formally spinforbidden phosphorescence process, promoted by the spinorbit coupling associated with the metal ion. Such phosphorescence is typical of Pt(II) complexes of 6-phenylbipyridinebased ligands, of the form Pt(N^N^C)X or [Pt(N^N^C)L] + , and assigned to a 3 ½d Pt |p Ar /p * N^N state. 30 The measured quantum yields of around 2-3% under these conditions are also quite typical of such complexes. The phosphorescence is modestly quenched by dissolved molecular oxygen, as evident from the shorter lifetimes observed in air-equilibrated solution. At 77 K, vibrational structure becomes clearly resolved, with a progression of around 1300 cm À1 , quite typical of complexes featuring aromatic ligands. The complexes are also luminescent in the solid state, with similar quantum yields and lifetimes (  (2)(3)]BF 4 with t BuO 2 H led to rapid decomposition to a complex mixture of products (Scheme 2). The same experiment with [Pt(1)]BF 4 led to a slow reaction to give an oxidised Pt IV complex as the major product, which was identied by ESI-MS as [Pt(1)Cl 2 ]BF 4 . The Cl ligands in this compound are presumably provided through oxidative decomposition of CH 2 Cl 2 over the extended reaction time (Scheme 2).
These results suggest that the mechanical bond signicantly retards displacement of the weakly coordinating triazole ligand, presumably due to steric repulsion of the incoming Cl nucleophile and destabilisation of the expected 5-coordinate intermediate of the presumed associative pathway. The same steric hindrance would also thermodynamically destabilise the product of the reaction with [Pt(1)]BF 4 as the macrocycle cannot readily accommodate both a Pt-Cl moiety and the uncoordinated axle. The mechanical bond also appears to kinetically disfavour oxidation of the Pt II centre, presumably for similar reasons, as the change from Pt II to Pt IV requires the inclusion of two additional ligands within the metal primary coordination sphere.
To examine if the latter effect could be overcome by providing an additional donor atom in the rotaxane framework, we synthesised complex [Pt(k 4 -4)]BF 4 (see ESI †) and examined  S56 †) in which the species assigned as [Pt IV (k 5 -4)(OH)](BF 4 ) 2 was the only signicant product. This assignment was supported by SCXRD analysis of crystals grown from the crude product mixture which, although of low quality, conrmed the expected connectivity (Table S5 †). Serendipitously (vide infra), when [Pt(k 4 -4)]BF 4 was treated with AgSbF 6 without protection from light (Scheme 2c), higher quality crystals were produced and found to correspond to [Pt(k 5 -4)(OH)](SbF 6 ) 2 in which the same coordination environment was also observed (Fig. 3b). The structures of [Pt(k 4 -4)]BF 4 and [Pt(k 5 -4)(OH)](SbF 6 ) 2 (Fig. 3) clearly demonstrate the change from a square-planar Pt II unit to a pseudo-octahedral Pt IV complex in which the rotaxane framework provides ve of the six donor atoms required to complete the Pt IV coordination sphere (three from the macrocycle and now two from the axle).
The corresponding non-interlocked complex, [Pt (2)(5)] BF 4 , also reacted rapidly with t BuO 2 H to give a mixture of products (Fig. S57 †), including uncoordinated axle 5, and a species that was assigned as Pt IV complex by ESI-MS analysis (m/z ¼ 551.2 corresponding to M 2+ ). Replacing t BuO 2 H with H 2 O 2 simplied the product mixture at short reaction times (Fig. S58 †). However, whereas [Pt IV (k 5 -4)(OH)](BF 4 ) 2 proved stable over extended reaction times, treatment of [Pt(2)(5)]BF 4 with H 2 O 2 for 24 h led to decomposition to produce signicant quantities of non-interlocked axle (Fig. S58 †), suggesting that even in the Pt IV oxidation state, the Pt-triazole bond remains labile.

Photophysical properties of [Pt II (k 4 -4)]BF 4response to Ag I
The pyridine appended rotaxane complex [Pt(k 4 -4)]BF 4 displays a similar phosphorescence spectrum to that of [Pt(1)]BF 4 . However, its emission quantum yield in solution is dramatically reduced by around 20-fold, and the phosphorescence lifetime is an order of magnitude shorter (  (2)(3)]BF 4 . Estimation of the radiative and non-radiative rate constants, k r and Sk nr respectively, from the quantum yields and lifetimes suggests that the effect is due primarily to the introduction of an additional non-radiative decay pathway that increases k nr by an order of magnitude when the additional pyridine ring is present. Interestingly, the quenching effect is apparently eliminated in the solid state (Table S9 †), under which conditions the quantum yields and lifetimes of all of the complexes are similar to one another, suggesting that the quenching process in the pyridine-functionalised structures requires some degree of intramolecular reorganisation that is inhibited in the rigid environment of a solid matrix. 34 Examining the solid state structure of [Pt(k 4 -4)]BF 4 , a striking feature is that the lone pair of the axle pyridine moiety is projected near to the Pt centre (3.39Å) in the ground state. Thus, (2)(5)]BF 4 . Ar ¼ 3,5-di-t Bu-C 6 H 3 . one explanation for the quenching of the emission is through exciplex formation, where a conformational rearrangement takes place to bring the pyridine lone pair closer to the Pt II centre, which becomes more electrophilic in the 3 MLCT excited state. Indeed, it has been noted that Pt II complexes with "dangling" nucleophilic groups are oen non-emissive (or only weakly so), ostensibly due to such axial interactions, whilst similar reasoning has been invoked to explain the frequently lower quantum yields of Pt II complexes in solvents incorporating potential donor atoms like DMF. 35 McMillin and coworkers have highlighted the correlation between the Gutmann donor number 36 and the rate constant for intermolecular quenching of the luminescence of Pt II complexes by donors such as pyridine. 37 To probe the role of the axle pyridine in [Pt(k 4 -4)]BF 4 , we attempted to protonate it, in order to determine if its elimination as a potential donor group would enhance the luminescence. However, no change was observed by 1 H NMR spectroscopy (Fig. S61 †) when [Pt(k 4 -4)]BF 4 was treated with triuoroacetic acid. The apparent lack of basic behaviour may be due to charge-charge repulsion between the formally cationic Pt II centre and the protonated pyridine moiety that destablises the protonated state. We therefore examined instead the binding of the so, monovalent cations Cu I , Ag I , and Au I in the hope that this charge-charge repulsion might be compensated for by attractive metal-metal interactions. Pleasingly, when [Pt(k 4 -4)]BF 4 was titrated with AgSbF 6 or [Cu(MeCN) 4 ]PF 6 , signicant changes were observed by 1 H NMR (Fig. 4a), suggesting that Ag I or Cu I may bind to the pyridine ring of the axle. No signicant changes were observed upon addition of [AuCl(SMe 2 )]. Similarly, divalent cations Zn II and Cd II failed to elicit any observable change by 1 H NMR (Fig. S61 †).

Scheme 2 Reactions of (a) [Pt(1)]BF 4 , (b) [Pt(2)(3)]BF 4 , (c) [Pt(k 4 -4)]BF 4 and (d) [Pt
Titration of [Pt(k 4 -4)]BF 4 with Ag I and Cu I monitored by UVvis or luminescence spectroscopy revealed signicant changes with incremental addition of metal salt (Fig. S61-S70 †). Most strikingly, a switch-on of the emission was observed as a result of Ag I binding but not in the case of Cu I ; in the presence of 3 equiv. of Ag I , the emission intensity increased by $25-fold at l max upon excitation at 400 nm (Fig. 4b). The quantum yield and lifetime of the emission increased by two orders of magnitude, and the corresponding estimated rate constants (Table 2) suggest that the dramatic effect of silver is due to the combined effect of suppressed non-radiative decay and an increase in k r . The consequent increase in luminescence is strikingly visible to the naked eye (Fig. 4b inset). SCXRD analysis of crystals obtained from a solution of [Pt(k 4 -4)]BF 4 and AgSbF 6 in the dark conrmed that the isolated product incorporates a Ag I ion coordinated to the pyridine ring of the axle (Fig. 4c). This heterodinuclear complex appears to be stabilised both by a metal-metal interaction between the Pt II and Ag I centres (2.83Å) and by an h 2 p-interaction between the Ag I centre and the bipyridine unit (2.54Å), although it should be noted that in crystals grown under different conditions, only the Ag-Pt contact is maintained (Table S7 †). Thus, we tentatively assign the observed increase in the luminescence quantum yield of ]BF 4 in the presence of Ag I to inhibition of the proposed quenching mechanism involving the pyridine-N/Pt interaction in the excited state, thus decreasing Sk nr . The increase in k r , albeit a modest one, may reect more efficient spin-orbit coupling pathways in the dinuclear system, in line with other recent reports of hetero-dinuclear 38 and polynuclear 39 complexes incorporating Pt II , although these pathways remain poorly understood.
Titration of [Pt(2)(5)]BF 4 with Ag I revealed an increase in luminescence efficiency, albeit with a lower switch-on effect values in parenthesis refer to air-equilibrated solution. c Radiative k r and non-radiative Sk nr rate constants estimated using the approximation that the emissive state is formed with unitary efficiency and thus k r ¼ F/s and Sk nr ¼ (1 À F)/s.
($12 fold). The difference in behaviour between ]BF 4 and the corresponding non-interlocked complex may reect the lower rigidity of the non-interlocked system; the pyridine moiety is sterically constrained in [Pt(k 4 -4)]BF 4 to project towards the Pt II centre, whereas the non-interlocked complex has more conformational freedom which may entropically disfavour the Pt II /N interaction and thus lower the efficiency of the proposed quenching mechanism.

Conclusions
In conclusion, we have demonstrated that interlocked cyclometallated Pt II complexeswhich are readily synthesised using an active template strategyretain the photophysical properties of the parent non-interlocked complexes. The mechanical bond sterically stabilises the metal centre towards ligand displacement andwhen no additional donors are present in the structuretowards oxidation, suggesting that this approach may have signicant benets for the construction of more stable emitters (e.g., as required in applications such as OLEDs, where stability and longevity are crucial). In particular, it allows one or more weakly coordinating ligands to be considered for the optimisation of the luminescence properties, since the chemical integrity of the luminophore is maintained by the mechanical bond rather than relying on the intrinsic strength of the metal-ligand interaction. 33 In addition, we serendipitously identied a luminescent sensory system for Ag + with >20 fold switch-on, in which the preorganisation provided by the mechanical bond appears to lead to enhanced performance compared with the non-interlocked analogue.
More generally, we have demonstrated that interlocked, mechanically chelated late transition metal complexes can be stabilised by taking advantage of the catenand effect, something that has previously, to our knowledge, only been observed in the case of Cu I . 7 Just taking the example of platinum alone, such complexes are not only of interest for their photophysical properties but also in cancer chemotherapywhere both Pt II and Pt IV species are known to be effective 40,41and in catalysis. 42 Our results indicate that it may be possible to use the mechanical bond to augment the stability of key reaction intermediates or even divert the system down an alternative pathway 43 in such chemical applications.
Finally, it is also noteworthy that all of the interlocked Pt complexes reported here are examples of mechanically planar chiral rotaxanessystems in which the mechanical bond acts as a stereogenic unit. 44 Indeed, 1 H NMR analysis of [Pt(1)]BF 4 in the presence of the chiral anion "trisphat" revealed the appearance of two sets of signals corresponding to the diastereomeric ion pairs of R mp -[Pt(1)]D-trisphat and S mp -[Pt(1)]D-trisphat (Fig. S75 †). [Pt(k 5 -4)(OH)](BF 4 ) 2 also contains a stereogenic Pt IV centre, the conguration of which is dictated by the mechanical stereochemistry of the precursor. Although all of these compounds were formed as racemic mixtures in the present study, we have recently developed methodology to stereoselectively access mechanically planar chiral rotaxanes 45 and related topologically chiral catenanes. 46 These results add further perspectives to the potential applications of interlocked heavy metal complexes in, for example, asymmetric catalysis 47 and circularly polarised luminescence. 48

Conflicts of interest
There are no conicts to declare.

Notes and references
1 By this we mean complexes in which a metal ion bridges donor atoms provided by two or more interlocked covalent