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The core of the matter – arene substitution determines the coordination and catalytic behaviour of tris(1-phosphanyl-1  -ferrocenylene)arene gold( I ) complexes Depending on the substitution of the central core, tris(1-phosphanyl-1  -ferrocenylene)arenes bind gold( I ) in diff erent coordination modes. The resulting complexes are investigated by variable-temperature NMR spectroscopy, DFT calculations, electrochemical methods, and show stepwise activity changes in redox-switchable gold( I Changing the aromatic core of C 3 -symmetric tris(ferrocenyl)arene-based tris-phosphanes has profound e ﬀ ects on their coordination behaviour towards gold( I ). Depending on the arene ( s -triazine, benzene, or tri ﬂ uorobenzene), four di ﬀ erent coordination modes can be distinguished and their preference has been rationalised using computational methods. The corresponding 1 : 1 ligand-to-metal complexes, studied by variable-temperature NMR spectroscopy, revealed ﬂ uctional behaviour in solution. Given the presence of up to three or six ferrocenylene spacers per complex, their electrochemistry was investigated. The redox-responsive nature of the complexes can be advantageously exploited in the catalytic ring-closing isomerisation of N -(2-propyn-1-yl)benzamide, where the benzene-based 2 : 3 ligand-to-metal complex has been shown to display multiple activity states depending on the degree of (reversible) oxidation in a preliminary trial.

Even though the formation of this CP was not intended and its preparation suffers from poor reproducibility, redox-active ferrocene-derived CPs have recently attained great interest for the insight into molecular architecture and self-assembly [75][76][77] they provide as well as for their potential applications in catalysis, 78,79 non-linear optics, and molecular magnetism. 77 Following the classification of Mochida, {[(1c) 2 (Au) 3 ](OTf ) 3 } n is a one-dimensional main-chain CP derived from a neutral ligand. 77 As such it is, to the best of our knowledge, the first CP to feature a ferrocene-derived tris-phosphane. Phosphanebased CPs are, in general, more elusive than analogues built from nitrogen-or sulfur-containing ligands, 80 and often constructed around gold(I) with its tendency for simple coordination geometries and formative aurophilic interactions. [81][82][83] Last but not least, the 2 : 3 ligand-to-metal stoichiometry of {[(1c) 2 (Au) 3 ](OTf ) 3 } n is also uncommon among CPs, a cadmium(II)-based CP by Tian and co-workers being the only other example. 84 In light of 1a-c showing three different coordination modes for gold(I), this unexpected behaviour was studied by an energy decomposition analysis using density-functional theory (DFT) level calculations (for computational details, s. ESI, section 1.1 †). Geometry optimisations and energy calculations for both the closed tricoordinate, The total coordination energy E coord , the energy gain upon combining the (native) tris-phosphane and a gold(I) ion, can be split into two contributions, namely the preparation energy E prep (required to form the geometry of the coordinated ligand from the native tris-phosphane, representing steric contributions) and the interaction energy E inter (gained from the interaction between the isolated gold(I) ion and the pre-formed ligand, representing electronic contributions). Comparing the obtained values for all six complexes (Fig. 3, Tables 1 and S4 †), the decisive role of the preparation energy becomes clear. Obviously, steric repulsion between the constituting arene fragments E (N|, C-H, C-F) and the ortho hydrogen atoms of the C-bound cyclopentadienyl rings governs the coordination geometry. The close-to co-planar arrangement of the three cyclopentadienyl rings and the arene core, a feature of the C 3 -symmetric closed geometry, is therefore less feasible, and open structures become more attractive. Correspondingly, the fluorine atoms, largest in this series, lead to the highest arenecyclopentadienyl torsion φ both in the calculated and in the experimentally determined structures of 1c (Table 1). In terms of interaction energies E inter , the closed form is more favourable in all cases, likely owing to a higher coordination number (3 vs. 2) and attractive gold-π(arene) interactions increasing in strength from benzene over 1,3,5-trifluorobenzene to s-triazine. These findings might explain the formation of coordination polymer {[(1c) 2 (Au) 3 ](OTf ) 3 } n from [1c(Au)]OTf. As the experimentally determined φ values are the highest in the series of the three solid-state structures (Table 1), the μ 3 :κ 1 P,κ 1 P′,κ 1 P″ coordination mode of 1c apparently accommodates the clash-   . While this conformation was found to be significantly less stable than the allsyn conformer by about 50 kJ mol −1 , the initial geometry of 1b was shown to be irrelevant for the outcome of the analysis in that the experimentally found all-syn structure of [1b(Au)]OTf was the most stable one. Intrigued by the possibility of modifying the coordination properties of tris(ferrocenylene)arene-based tris-phosphanes via the substitution pattern of the arene, we next introduced fluorine in the diphenylphosphanyl groups of 1a in form of the bis( pentafluorophenyl)phosphanyl moiety [P(Ph F ) 2 ] (Chart 3). With regard to (redox-switchable) gold(I) catalysis, the P(Ph F ) 2 moiety is of considerable interest due to its very low σ-donor capability. 85,86 The resulting electronpoor gold(I) centres show great potential in gold(I)-catalysed multiple bond activation [87][88][89][90][91] and could also be of interest in gold(I)-catalysed oxidative addition reactions. 92 Besides catalyst design, the P(Ph F ) moiety has also found applications in the preparation of stimuli-responsive fluorescent metal complexes 93 and organic materials with intriguing optoelectronic properties. 94,95 To date, ferrocene-based derivatives are rare, and only the 1,1′-diphenylphosphanylferrocene (dppf ) analogue VII 96-98 and a FcPHOX derivative VIII (Chart 3) 99 have been prepared and used for complexation of palladium(II). Tris-phosphane 1a F can be prepared from 2,4,6-tris(1-bromo-1′-ferrocenylene)-1,3,5-triazine 4a, albeit in a low yield of only 15%, still highlighting the modular preparation of tris-phosphanes 1. The NMR chemical shifts of 1a F , an air-and moisture-stable, deep red microcrystalline solid, in 19 F{ 1 H} and 31 P{ 1 H} NMR spectroscopy match those of VII very well. 96,100 Reacting 1a F with [Au(η 2 -nbe) 3 ]OTf in CH 2 Cl 2 resulted in a less clean conversion than anticipated, and only after several recrystallisation steps could a few crystals suitable for XRD be obtained. [1a F (Au)]OTf is the first crystallographically characterised example of a ferrocene-derived ligand featuring any P(Ph F ) group and, among the few reported gold(I) complexes of this donor, the only example of a bidentate phosphane. [101][102][103] The solid-state structure of [1a F (Au)]OTf (Fig. 4, top) differs only slightly from that of [1b(Au)]OTf. Both complexes share the C s -symmetric, P,P′-dicoordinate geometry with a pendant phosphane group. Apparently, the steric strain imposed by the H-to-F substitution on the P-bound phenyl rings outweighs the energy gain by the attractive gold(I)-π(C 3 N 3 ) interaction. 85  This points to free (A) vs. partly hindered rotation (B) about the P-C(Ph F ) bonds of the two P-bound pentafluorophenyl groups involved in coordination (Fig. 5, top left). The assignment of the resonances to either ring has been established through a 19 F, 19 F COSY NMR experiment, but the relative orientation of the Ph F rings is not clear. Warming the same solution to 45°C does not result in apparent C 3v but in a clearer C s molecular symmetry for both nuclei.
The loss of symmetry is also apparent from the corresponding 1 H NMR spectra. At 25°C, the 1 H NMR spectrum of [1a F (Au)]OTf is difficult to assign due to significant broadening of the signals. At −60°C, 11 distinct signals with matching integrals for the 12 protons of a C s -symmetric species can be identified (the signal at 4.45 ppm contributing with a relative integral of 2). The mononuclear composition in solution is also confirmed by HR-ESI mass spectrometry. Since we were unable to obtain satisfactorily pure samples and sufficient amounts of [1a F (Au)]OTf for further analyses or applications, we turned our attention to the reaction of 1a F with silver(I) triflate. In the corresponding complex of 1a, the silver(I)-π(C 3 N 3 ) interaction had been found stronger than for the gold(I) analogue. 35 While the formation and isolation of [1a F (Ag)]OTf proved much more facile, the solid-state molecular structure (Fig. 4, bottom) is analogous to that of the gold(I) congener. As previously noted for isostructural pairs of silver(I) and gold(I) complexes, the M-P bonds ( Table 2) are longer for the silver(I) complex. 35,71,105 The P(1)-Ag(1)-P(2) bond angle deviates more strongly from the ideal 180°, paralleled by a shorter Ag(1)⋯N(1) distance of 3.89(1) Å ([1a F (Au)]OTf: 4.14(1) Å). Moreover, a significantly closer contact between the triflate anion and the silver(I) cation (d(Ag(1)⋯O(1)) = 2.57(1) Å) can be discerned. While this distance is still well above the sum of the covalent radii (2.11 Å), 104 its deviation from the sum of the van der Waals radii (4.03 Å) is far greater than in [1a F (Au)]OTf.
[1a F (Ag)]OTf is, to the best of our knowledge, the first silver complex of a pentafluorophenylphosphane (one example of a corresponding phosphanido complex, formed by C 6 F 5 group transfer from AgC 6 F 5 to a PCl 2 moiety, has been reported by Schulz and co-workers). 106 Despite their similar structures, the silver(I) complex deviates from the gold(I) analogue in its (VT) NMR-spectroscopic features (Fig. 5, right, and Fig. S48-S50 †).
Above 5°C, [1a F (Ag)]OTf appears C 3v -symmetric in CD 2 Cl 2 solution in the corresponding 1 H and 31 P{ 1 H} NMR spectra (Fig. 5, top right, and Fig. S48, S49 †); coalescence in the 19 F{ 1 H} NMR spectra occurs only at higher temperatures. Upon cooling, the 31 P{ 1 H} resonance decoalesces into a 2 : 1 pattern indicative of the P,P′-dicoordinate bonding mode, and the signal for the silver-bound phosphane further splits into a doublet due to unresolved 1 J coupling with both 107/109 Ag isotopes. The close silver-triflate contact of the molecular structure is also observed in the 19 F{ 1 H} NMR spectra below −30°C due to a strong splitting of the triflate-CF 3 resonance (+), clearly setting [1a F (Ag)]OTf apart from [1a F (Au)]OTf. Furthermore, the low-temperature 1 H and 19 F{ 1 H} NMR spectra of [1a F (Ag)]OTf are more complex, and no unambiguous assignments could be made.
The molecular structures of [(1a) 2 (Au) 3 ](OTf ) 3 and [(1b) 2 (Au) 3 ](OTf ) 3 (Fig. 6 top and centre) can be understood as two mononuclear, P,P′-dicoordinate complexes linked by a third gold(I) ion coordinated by the remaining phosphane. Thus, even though the ligand-to-metal stoichiometry of 2 : 3 is formally identical to that of {[(1c) 2 (Au) 3 ](OTf ) 3 } n , the discrete trinuclear complexes contain 1a and 1b in a different, μ 2 :κ 1 P,κ 2 P′,P″ coordination mode. In general, trinuclear gold(I) complexes formed from two tris-phosphanes (or other P-donors) are relatively scarce in the literature and mostly contain linear Au 3 chains. [107][108][109][110][111] The particular coordination mode observed here has not yet been reported for solid-state structures deposited in the Cambridge Structural Database. 112 As a result of the packing, which does not involve aurophilic interactions or close contacts between the individual tricationic molecules, small and large channels are formed along the crystallographic a axis (Fig. 6, bottom). The smaller channels (approx. diameter of 5.3 Å) are occupied by the severely disordered triflate anions. Only a single one in the structure

Dalton Transactions Paper
This journal is © The Royal Society of Chemistry 2020  3 } n , particularly regarding the P-Au-P bond angles which, for the central gold(I) ion, are closer to the ideal 180°than for the outer gold(I) ions (Tables 3 and S3 †). [(1a) 2 (Au) 3 ](OTf ) 3 and [(1b) 2 (Au) 3 ](OTf ) 3 maintain their structure in CD 2 Cl 2 solution as evident from HR-ESI mass spectrometry through signals for the tricationic species, and from 1 H and 31 P{ 1 H} NMR spectroscopy (Fig. S15ff †). The two distinct 31 P{ 1 H} resonances also demonstrate the influence of the P-Au-P bond angle on the exact chemical shiftmore acute angles result in a slight deshielding.
These results corroborate the decisive role of the arene core constituent E and, as demonstrated for 1a F , the role of the P-bound substituents in determining the coordination mode of the tris-phosphanes towards gold(I) as schematically summarised in Fig. 7.

Electrochemistry and redox-switchable catalysis
As trinuclear chloridogold(I) complexes of 1a-c have shown high potential for multi-state redox-switchable catalysis due to their stepwise, well-separated, and reversible trifold oxidation, 36 the electrochemical and, following, catalytic characterisation of the previously described gold(I) complexes were also of interest. The electrochemistry of tris-phosphanes 1a-c and [1a(Au)]OTf has already been studied, 35 (2) 164.05 (8) 162.44 (7) 171.4(1) Au (3) 162.92 (8) 164.85 (6) 172.72(5) Fig. 7 Schematic depiction of the different coordination modes observed for tris-phosphanes 1a-c and 1a F depending on the arene core constituent E and on the phosphanyl moiety PR 2 . For clarity, charges and triflate anions have been omitted from the depicted structures. The dashed arrow represents the indirect access to the coordination polymer from 1c. free phosphorus-centred lone pairs of electrons in ferrocene oxidation, [113][114][115][116] the oxidation of [1b(Au)]OTf (orange trace) in the BF 4 -based SE (Fig. 8, left) was found quasireversible (cf. section 5.1 of the ESI †). 117,118 Assuming that the fluctional coordination behaviour observed in the VT NMR experiments renders all three ferrocenylene moieties equivalent on the cyclic voltammetry timescale and hence not bearing lone pairs of electrons available for follow-up chemistry, all three ferrocenylene groups are most likely oxidised at the same potential. Similar behaviour has been reported for 1,3,5-tris(ferrocenyl)benzene 37,119 and its 1,1′-substituted derivatives, yet not for free 1b itself. 36 Further support comes from hexa-ferrocene derivatives [(1b) 2 (Au) 3 ](OTf ) 3 (red trace) and [(1a) 2 (Au) 3 ](OTf ) 3 ( purple trace) which also yield one quasireversible redox event under the same conditions, as all phosphanes are permanently involved in gold(I) coordination. Twice-as-high currents i a/c suggest the simultaneous oxidation of all six ferrocenylene groups. The decisive role of the high mobility and of the potential to form tight ion pairs of the BF 4 − anions becomes apparent when the SE is changed to the much more weakly coordinating BAr F 4 − anions (Fig. 8, right). 120 The oxidations of all three complexes follow the same pattern of a low-current, irreversible first oxidation (*) followed by a much stronger second, also not fully reversible, oxidation event (for corresponding potentials, cf. Table 4). On reversing the scan direction, two close-spaced reductions take place, less reversible for the triazine than for the benzene core. In general and in line with our expectations, the less electron-rich triazine leads to appreciably higher oxidation/reduction potentials for its gold(I) complex (with the notable exception for the weak first oxidation (*)), while the similar coordination modes of all complexes result in overall very similar electrochemical characteristics. The electrochemistry of 1a F and the two mononuclear complexes [1a F (Ag)]OTf (Fig. 9, Fig. S66 and S67 †) and [1a F (Au)]OTf (Fig. S68, † measured at a lower concentration due to a limited amount of available material; all potentials in Table S6 †) is, in contrast, very different from that of their non-fluorinated counterparts. Free 1a F (light green) can be reversibly oxidised in three well-separated steps in the BAr F 4 -based SE (1a-c only feature a first reversible oxidation and need to be protected by BH 3 groups to see the same pattern), 36 in line with reports for dppf derivative VII (Chart 3) by the Gusev group. 98 Gusev and co-workers speculated that this reversibility originated from steric protection of potential P-centred radicals from dimerisation. While the steric demand of fluorine is indeed greater than that of hydrogen, 85 we believe the lowered energy of the lone pair of electrons at the P atoms to be a more likely explanation. 113,114,116,121,122 Accordingly, the Mulliken spin densities of monocations [1a] + and [1a F ] + calculated at the DFT level are distributed differently, with [1a F ] + showing a four times greater spin density (0.012) summed up over all three P atoms than [1a] + (0.003). In the BF 4 -based SE, the oxidations are not reversible anymore, and much like for 1a, the first oxidation induced a delayed reduction.
In the BF 4 -based SE, neither the two complexes nor free 1a F are reversibly oxidisable, and all compounds show oxidationinduced delayed reductions (and re-oxidations; Fig. 8, *). This Table 4 Redox potentials a E 0 , oxidation E ox , and reduction E red peak potentials of the gold(I) complexes in two different supporting electrolytes determined by cyclic voltammetry (nBu 4 N)BF 4 (nBu 4 N)BAr F  . Traces have been recorded at 100 mV s −1 (working electrode: glassy carbon, counter electrode: Pt wire) at room temperature. The second of three cycles are shown and asterisks (*) mark oxidation/reduction events which depend on prior oxidation in the first cycle (cf. Fig. S66 and S67 †).

Dalton Transactions Paper
This journal is © The Royal Society of Chemistry 2020 behaviour has previously been observed for 1a and its coinage metal complexes as well. A higher degree of reversibility is observed in the BAr F 4 -based SE; yet, similar to [1b(Au)]OTf, the oxidations are not completely reversible and, in the case of [1a F (Ag)]OTf, delayed reductions are observed again. Given their complex redox chemistry and unsatisfactory yields and purity, [1a F (Ag)]OTf and [1a F (Au)]OTf were not considered for the following redox-switchable catalysis (RSC) investigations. As a suitable model reaction for testing the previously presented mono-and trinuclear complexes in RSC, the ringclosing isomerisation of propargylic amide 2 to oxazoline 3a (Scheme 1) 18,123-126 was studied by time-resolved 1 H NMR spectroscopy vs. an internal standard (1,3,5-trimethoxybenzene) in CD 2 Cl 2 . While trinuclear chloridogold(I) complexes of 1a-c exclusively yielded 3a with an exocyclic methylene group, 36 other (redox-switchable) gold(I/III) catalysts are known to also catalyse the aromatisation of 3a to oxazole 3b in a subsequent step. 51,127 [1a(Au)]OTfwhich had not yet been tested in catalysisand [1b(Au)]OTf, at a concentration of 3 mol% gold(I) with respect to 2, were found to be catalytically inactive (Fig. 10, phase i). Most likely, steric hindrance arising from the κ 3 P,P′,P″ coordination mode in [1a(Au)]OTf prevents the substrate from approaching the cationic gold(I) centre which otherwise would be expected to show some, albeit low, activity (vide infra). Given the fluctional coordination behaviour of [1a(Au)]OTf and the resulting apparent C 3v -symmetric tricoordinate geometry on the NMR time scale, the substrate might be unable to approach the gold(I) ion on the catalytic time scale, too. Attempting to rationalise this assumption, the buried volume %V bur , a measure for the steric bulk of a ligand at and around the coordinated metal originally developed for N-heterocyclic carbenes, 128,129 was calculated for both coordination modes (bi-and tridentate). Based on the available solid-state structures and using the SambVca web application by Cavallo and co-workers, 130 the C 3 -symmetric coordination of the three phosphanes results in a %V bur of almost 90%, while the bidentate bonding mode in [1b(Au)]OTf corresponds to a significantly smaller %V bur of about 67% (cf. ESI, section 7.2 †). Due to the fast coordination-dissociation equilibrium of [1b(Au)]OTf, the effective buried volume in solution is likely closer to the 90% of [1a(Au)]OTf.
The fate of [1a(Au)]OTf upon oxidation had already been studied in some detail and was revealed to involve electron transfer from P to Fe III after an initial iron(II)-centred oxidation, leading to the formation of a phosphane oxide species due to adventitious traces of water. 35 Oxidation-induced reactivity, potentially involving substrate 2 and generating new, catalytically active species, is thus quite likely in this case, too.
The stepwise oxidation of [1b(Au)]OTf with 5a in CD 2 Cl 2 was followed by a multinuclear NMR experiment, including the attempted reduction with decamethylferrocene (6) (Fig. S92-S94 †). The appearance of several 31 P{ 1 H} signals centred at about 43 ppm with small but noticeable changes between the addition of one and two equivalents of 5a indicates the generation of P,P′-dicoordinate species. Regeneration Scheme 1 Gold(I)-catalysed ring-closing isomerisation of N-(2-propyn-1-yl)benzamide (2) to oxazoline 3a and potential subsequent aromatisation to oxazole 3b, including oxidants 5a and 5b and reductant 6 used in redox switching experiments. of [1b(Au)]OTf by addition of 6 was not observed in neither the 1 H nor the 31 P{ 1 H} NMR spectra, even though crystalline [6](TEF) was isolated from the reaction mixture after work-up. Furthermore, the 19 F{ 1 H} NMR signal of the triflate anion is strongly broadened suggesting its involvement in the follow-up chemistry. Spectroelectrochemical (SEC) measurements in the BAr F 4 -based SE at 25°C, −50°C, and −80°C (Fig. S69 and S70 †), contrasting the cyclic voltammetry experiments in the same SE, showed good reversibility of the likely Fe II -centred first oxidation and a less reversible second oxidation at 25°C which became more reversible at −50°C. All taken together, these partly conflicting and complex observations disqualify [1a(Au)]OTf and [1b(Au)]OTf for further applications in RSC.
We thus focused our attention on the tri-gold, hexaferrocene complexes [(1a) 2 (Au) 3 ](OTf ) 3 and [(1b) 2 (Au) 3 ](OTf ) 3 , expecting them to show a more predictable and rewarding catalytic profile. The buried volumes %Vol bur for the central (73%) and peripheral (66%, cf. section 7.2 of the ESI †) gold(I) centres are similar to that of [1b(Au)]OTf and likely profit from the fixed coordination geometry (i.e., no pendant phosphane group). Indeed, both complexes displayed low but discernible catalytic activity in their native states (Fig. 11, top, hollow symbols). At this low activity level ( phase iii), the TOF of both catalysts are similar and, over long time periods (native species), virtually identical (cf. Table S7 †).
To gain more insight into the redox-switching behaviour, SEC measurements of [(1a) 2 (Au) 3 ](OTf ) 3 and [(1b) 2 (Au) 3 ](OTf ) 3 in the BAr F 4 -based SE were conducted ( Fig. 12 and S71, S72 †), the BAr F 4 − anion being a suitable substitute for the teflonate anion concerning its low ion-pairing properties and inertness. 132 In some contrast to the results from the CV (vide supra), the oxidation of [(1b) 2 (Au) 3 ](OTf ) 3 (Fig. 12, left) was found to be fully reversible under these conditions, even though the reduction potential had to be applied for an extended time. The UV/Vis signature relates to an iron-centred oxidation, [135][136][137] and the broad band centred at about 820 nm likely relates to a ligand-to-metal charge transfer between the cyclopentadienyl rings and iron(III). 37 When [(1a) 2 (Au) 3 ](OTf ) 3 is oxidised at potentials greater than 1 V (vs. FcH/[FcH] + , Fig. 12 top right), the intensely purple species formed cannot  be reduced anymore, while oxidation below 1 V leads to a slightly different UV/Vis trace and allows for a reduction, however requiring a cathodic potential of −1 V. In both cases, the UV/Vis spectrum obtained after oxidation strongly resembles that obtained from oxidising [1a(Au)]OTf in the same SE, a process which was found to generate very reactive species from intramolecular electron transfer after an initial iron-centred oxidation. 35 Following the oxidation of [(1a) 2 (Au) 3 ](OTf ) 3 with two equivalents of 5a by multinuclear NMR spectroscopy (Fig. S95 †), good yet delayed reducibility by 2.2 equivalents of 6 was observed. Using BF 4 -based oxidant 5b in the same stoichiometric ratio (Fig. S96 †), a different spectral fingerprint was observed, and addition of 6 did not regenerate the initial spectral features. Treating [(1b) 2 (Au) 3 ](OTf ) 3 with two equivalents of 5b (Fig. S97 †), full reversibility upon addition of 6 was found. Studying 5b as a substitute for 5aanions are known to play important roles in gold(I) catalysis 89,138,139 was, however, complicated by its poor solubility in CH 2 Cl 2 /CD 2 Cl 2 , as exact dosing of the required small amounts was found to be impossible. Owing to its more attractive redox features, especially as observed in SEC, a series of catalytic runs in which [(1b) 2 (Au) 3 ](OTf ) 3 was oxidised by one to six equivalents of 5a was conducted (Fig. 13). When left oxidised, the doubly oxidised species (light green hollow circles) performs with a TOF of 5.4 ± 0.1 h −1 (32.6-41.0 h), similar to the TOF of 4.5 ± 0.1 h −1 observed after re-oxidation in the previous experi-ment (Fig. 11). Using only one equivalent of 5a, an approximately halved TOF for the same conversion interval (30-75%) of 2.4 ± 0.1 h −1 results. When three or more equivalents of 5a are added to [(1b) 2 (Au) 3 ](OTf ) 3 , the increment in TOF is by far greater than expected from the addition of one and two equivalents. When the reaction profiles obtained from adding four and six equivalents of 5a to [(1b) 2 (Au) 3 ](OTf ) 3 are followed more closely by using smaller time intervals for the measurements (Fig. 13, top right), close-to-identical TOF for both cases result (shaded bars in Fig. 13, bottom). Relating these findings to the steric characterisation of the gold(I) centres by their respective buried volume (vide supra), it thus seems possible that the first two oxidations might be localised at the central ferrocenylene moieties (Fe(1) and Fe(4)), in turn activating the more buried central gold(I) ion Au(1). In the third and fourth oxidation, the chelated outer gold(I) centres Au(2) and Au (3), characterised by a higher substrate approachability, might followingly be activated, resulting in a sharp activity increase. In the absence of substrate, stepwise addition of up to six equivalents of 5a to [(1b) 2 (Au) 3 ](OTf ) 3 resulted in a broadening and subsequent disappearance of the ferrocenylene and benzenecore signals in the corresponding 1 H NMR spectra, while the 31 P{ 1 H} NMR signals, next to being strongly broadened, became successively shifted to higher field ( Fig. S98 and S99 †). Upon addition of 6.6 eq. of 6, the original spectral features of [(1b) 2 (Au) 3 ](OTf ) 3 were almost fully recovered with some delay. Since no significant changes between the addition of four and six equivalents of 5a are observed in the corresponding 31 P{ 1 H} NMR spectra, it is possible that the fourfold oxidation is the limit of the oxidation potential of 5a under these conditions. Thus, 4.4 eq. of 6 were added to a final run of [(1b) 2 (Au) 3 ](OTf ) 3 oxidised by 4 eq. of 5a ( purple hollow diamonds in Fig. 13, top right), and a decrease of activity to 0.27 ± 0.08 h −1 ( phase xi) results after prolonged waiting (>6 h after addition of 6), mirroring the observations from SEC and NMR spectroscopy. Immediately after the addition of 6 ( phase x), the TOF is reduced to about a third (14 ± 1 h −1 ) of the previous value (41 ± 1 h −1 ).
While all catalytic activities are far from record-breaking, 123 the highly oxidised forms of both [(1a) 2 (Au) 3 ](OTf ) 3 and [(1b) 2 (Au) 3 ](OTf ) 3 convert 2 faster than tris(chloridogold(I)) complexes of 1a-c at the same gold(I) concentration. 36 Their intriguing behaviour warrants a more detailed study on the corresponding oxidation/reduction mechanisms in RSC, and we are currently working on understanding the switching behaviour as well as investigating the use of 1a-c for different catalytic transformations.

Conclusions
C 3 -Symmetric tris-phosphanes 1a-c and a perfluorophenyl derivative of 1a, 1a F , have been studied with respect to their coordination chemistry towards gold(I). Owing to steric strain arising from the clashing ortho substituents of the arene and Fig. 13 Top left: Reaction profiles for ring-closing isomerisation of 2 to 3a by [(1b) 2 (Au) 3 ](OTf ) 3 (3 mol% Au, [2] 0 = 60 mmol L −1 , CD 2 Cl 2 , 25°C) oxidised by 1-6 eq. of 5a at the indicated time (arrow). The insert shows a magnified time period, and lines connecting the symbols are for visual guidance only. Top right: Reaction profiles for ring-closing isomerisation of 2 to 3a by [(1b) 2 (Au) 3 ](OTf ) 3 (conditions as above), oxidised by four (orange/grape) and six (mustard) eq. of 5a directly before the start of the 1 H NMR monitoring (viii). To one run (grape), 4.4 eq. of reductant 6 were added at the indicated time (arrow). Reaction phases ix-xi correspond to that run only. For TOF regression plots, s. Fig. S81-S89 † referring to reaction phases vii and viii-xi. the cyclopentadienyl rings, the tricoordinate bonding mode is reserved for the triazine derivative [1a(Au)]OTf, while 1 : 1 complexes of 1b and 1c prefer a fluctional dicoordinate bonding mode. Steric strain can further be mitigated by adopting a μ 3 :κ 1 P,κ 1 P′,κ 1 P″ coordination mode, observed for 1c forming a one-dimensional coordination polymer with gold(I) in a 2 : 3 ligand-to-metal ratio. This very ratio can also be used to form discrete 2 : 3 complexes of 1a and 1b which, in contrast to the 1 : 1 complexes, show promising properties in the redoxswitchable ring-closing isomerisation of propargylic amide 2 including multi-state activity behaviour.

Conflicts of interest
There are no conflicts to declare.