Metal/Metal Redox Isomerism Governed by Configuration

Abstract A pair of diastereomeric dinuclear complexes, [Tp′(CO)BrW{μ‐η2‐C,C′‐κ2‐S,P‐C2(PPh2)S}Ru(η5‐C5H5)(PPh3)], in which W and Ru are bridged by a phosphinyl(thiolato)alkyne in a side‐on carbon P,S‐chelate coordination mode, were synthesized, separated and fully characterized. Even though the isomers are similar in their spectroscopic properties and redox potentials, the like‐isomer is oxidized at W while the unlike‐isomer is oxidized at Ru, which is proven by IR, NIR and EPR‐spectroscopy supported by spectro‐electrochemistry and computational methods. The second oxidation of the complexes was shown to take place at the metal left unaffected in the first redox step. Finally, the tipping point could be realized in the unlike isomer of the electronically tuned thiophenolate congener [Tp′(CO)(PhS)W{μ‐η2‐C,C′‐κ2‐S,P‐C2(PPh2)S}Ru(η5‐C5H5)‐(PPh3)], in which valence trapped WIII/RuII and WII/RuIII cationic species are at equilibrium.


Introduction
In the last decades valence tautomerism in metal complexes has emerged as av ital research topic in modernc oordination chemistry. [1] The term denotes the coexistence of two different isomeric species,w hich are interrelated by an intramolecular electron transfer.T he barrier between both states must be sufficiently high to allow for the spectroscopicc haracterization of the individual redox forms. The synonymic but more specific term electromerism [2] for the phenomenon takes account of the fact that tautomers as fast interchanging isomers should have different distance matrices,w hich usually does not apply. The majority of systems, for which the phenomenon is reported so far,i nvolvearedox-active ligand and ad irectly coordinated metal. Since the discoveryo fs uch metal/ligand valence tautomerism in ortho-benzoquinone complexeso fc obalt by Pierpont, [3] new systemsw ith redox-activel igands based on ortho-quinone, [4] their imino derivatives [5,6] or phenolate inclos-ing Schiff base ligands [7] have been developed. The electronic and magnetic behaviour of their complexesw ith av ariety of transition metals [8] as well as lanthanides [5] has been thoroughly investigated. [9] In particular, switching of magnetic states by externals timuli,l ike light or heat, has attracted much interest due to potential applications in spintronics and sensing. [4,10] However,t he limited structurald ifferences of the electromers in bond lengths and angles at the redoxcentrescause low barriers for the intramolecular electront ransfer. The resulting rapid interconversion prevents separation into true redox isomers.O nlyr ecently,H immel and co-workersp resented ad inuclear coppert etrakis(guanidine) complex, which could be obtainedi nt wo different redox-isomericf orms by intentional choiceo fs olvent. [11] Polynuclearc omplexes,s howing metal/metalv alence tautomerism,a re considerably less well established. [12] Respective dinuclearc omplexes,i nw hich identicalm etals in mixedv alent states areb ridged by symmetric ligands, have been an essential tool fort he developmento fc lassical electron transfer theory. [13] Theseb asic investigations lead progressivelyi ntot he topical fieldo fm olecular electronics. [14] Relateds ystems with different metals or alternativelyanasymmetric bridging ligand can generally exista sm etal/metalv alence tautomers, if ther edox potentialsa re sufficientlyc lose.S electede xampleso ft hist ypec ompriseo rganometallicc ompounds with carbon-based phenylene and/or ethynylenemoietiesaswellaspolynuclear complexesrelyingo nb ridgingc yanide. [15][16][17][18] Oner ecently presentedF e(CN)-Co(NC)Fe complexc an be reversibly switched betweenF e III -h.s.-Co II -Fe III andF e III -l.s.-Co III -Fe II (h.s.h ighs pin, l.s. lows pin) by energy-selective irradiationa tl ow temperature. [18] Fundamentally,v alencet automers cannot be separateda nd isolated in substance.I nt hisc ontributionw ep resent dinuclearW /Ruc omplexes,i nwhich twos tereogenic metalc entresa re bridgedb ya phosphinyl(thiolato)alkyne ligand.T he separatedd iastereomeric complexesformmetal/metal redoxisomers upon oxidation.

Results and Discussion
The synthetic scheme starts with the W II alkyne complex [Tp'W(CO) 2 {h 2 -C 2 H(SBn)}]PF 6 , 1-PF 6 (Tp' = hydridotris{3,5-di-methylpyrazolyl}borate, Scheme 1). After conversion of the complexc ation into an eutral one by substitution of aC O ligand by bromide, ap hosphinyl group was straightforwardly introduced by deprotonation with nBuLi and addition of PPh 2 Cl. According to NMR spectroscopice videncet he alkyne complex 2 exists in two isomeric forms with respectt ot he alkyne rotationa tt ungsten. The rotamer mixture shows one distinctive, non-broadened CO band at 1919 cm À1 ,adifference in the 31 Pc hemical shift of less than 1ppm and as inglereversible W II /W III oxidation wave at À0.01 Vv s. Fc/Fc + in cyclic voltammetry.I somerization can be observed on at imescale of weeks by NMR monitoringb ut separation of the two compounds wasn ot pursued, since the mixture proved adequate for the generation of dinuclear compounds. Reaction of complex ligand 2 with [(h 5 -C 5 H 5 )Ru(PPh 3 )(MeCN) 2 ]PF 6 and subsequent reductive removal of the benzyl group led to the neutral air-and water-stable dinuclear complex [Tp'(CO)BrW{m-h 2 -C,C'k 2 -S,P-C 2 (PPh 2 )S}Ru(h 5 -C 5 H 5 )(PPh 3 )] 3.
As both metal centres exhibit chirality in their pseudo-octahedral (W)a nd pseudo-tetrahedral (Ru) coordination spheres, two sets of diastereomers were formed, which could be separated by column chromatography and subsequentc rystallization. The molecular structures determined by XRD analysis prove the identity as P,S-chelate complexes with ad iastereomeric relationship (Figure 1). The isomer characterized by the Br-liganda tWand the PPh 3 at Ru being oriented towards the same side of the bridging plane shows the same stereodescriptors on both metal centres (like)a nd is therefore denoted as 3 l. Conversely,t he second isomer shows an orientation of the Br-liganda tWand the PPh 3 at Ru in opposite directions of the plane, leading to different stereodescriptors at the metals (unlike)a nd thereby an otation of 3 u (see Supporting Information). [19] The pure compounds can be isomerized in refluxing toluene, giving access to increased amounts of the u-isomer.
Conveniently,t he latter is kinetically less favouredb ut thermodynamically more stable allowing target-oriented synthesis of ap articulari somer.W ith the dinuclearc ompounds 3 l and 3 u in hand, we investigated their spectroscopic and chemical properties, especially with regards to configuration-based differences between the diastereomers.
The bonding parameters in the moleculars tructures of 3 l and 3 u are very similar.E xclusively the distances W1-N1 (trans to alkyne), W1-N5 (trans to CO) and Ru1-S1 are longer in the l-isomera ccording to the 3s significance criterion. The most strikingg eometricald ifference applies to the bendo ft he C 2 PS bridging moiety,w hichi sm ore pronounced in the l-isomer (see SupportingI nformation). Naturally,t his bend in 3 l leads to as horter W-Ru distance of 508.5 pm when compared to 515.1 pm in 3 u.
However,s toichiometric oxidation of both complexes in CH 2 Cl 2 produced bewildering results. Addition of acetyl ferrocenium tetrafluoroborate ( Ac Fc + BF 4 À , E 1/2 =+ 0.27 Vv s. Fc/Fc + ) [20] to 3 l caused both ad istinctive colourc hange from red to green and ad rastic shift of Dn = 164 cm À1 for the CO stretching frequency.T his value is typical for an W II /W III oxidation due to the decrease of p-back-bonding ability of tungsten. The mononuclear complex 2 showsachange by 168 cm À1 upon oxidation, supporting the assignment of at ungsten based oxidation in 3 l to 3 l + .R ather unexpectedly,i somer 3 u shows as imilar colourc hange indeedb ut am uch smaller shift of merely Dn = 28 cm À1 in the IR spectrum, indicating al ocalization of the electron transfer 3 u to 3 u + at ruthenium.
Spectro-electrochemicali nvestigations (SEC) provided the proof of complementarity of the oxidation steps ( Figure 2). Observation of the short-lived dicationic species 3 l 2 + + and 3 u 2 + + at 2103 and 2107 cm À1 ,r espectively,i ndicated now the small change for 3 l + /2 + {oxidation Ru II to Ru III }a nd the larger shift for 3 u + /2 + {oxidation W II to W III }.
To confirm the regioselectivity of the oxidation, we recorded X-band EPR spectra of the mono-cations ( Figure 3). Both W III and Ru III are S = 1 / 2 ions, which should clearly show different g values owing to the differing d-electronsc ount (d 3 and d 5 ,r espectively).
In frozen CH 2 Cl 2 solution both cationss how rhombic spectra with only partly resolved hyperfine coupling. However,t he redox isomersc an even already be differentiated by the g values. For the W-oxidized isomer 3 l + ,t he main g components at 1.921, 1.965 and 1.999 are all smaller than the g value of the free electron, while the respective values for the Ru-oxidized 3 u + amount to 2.011, 2.077 and 2.130, matching well with comparable complexes in the literature. [15,17,21] The comparatively small g value anisotropy in both cases points to substantial delocalization to either bromine or sulfur.H ence, hyperfine coupling to bromine (S = 3 / 2 for 79 Br and 81 Br,c ombined natural abundance 100 %) is resolved in the spectrumo f3 l + .I nc ontrast, simulation of the spectrumo fc ation 3 u + + reveals hyperfine coupling to two 31 Pn uclei of the coordinated phosphine groups.
The visible absorption spectra of the neutral complexes 3 l/ 3 u and their corresponding cations all exhibit one dominating band, which is characterizedb yabathochromic shift of 3790 cm À1 for 3 l/3 l + and4 290 cm À1 for 3 u/3 u + upon oxidation (see SupportingI nformation). Interestingly,e ven though the oxidation is localized at different metal centres, the neutral complexes 3 l/3 u show al arger difference of the absorption maxima( 680 cm À1 )c ompared with the redox isomers 3 l + /3 u + (180 cm À1 ). Most importantly,b oth cations show intense intervalence absorptions bands in the near IR region ( Figure S4). Electron transfer from Ru II to W III in 3 l + is caused by absorption at 2270 nm maximum (4400 cm À1 ,F igure 4) and from W II to Ru III in 3 u + by % 2850nme xcitation ( % 3500 cm À1 ).
The clear regioselectivity of the oxidation prompted us to tune the dinuclears ystem to the tippingp oint by substitution of co-ligands. As part of as ystematic study,w es ucceeded in the synthesis of the thiophenolate complex 7 u (Scheme2). Since direct substitution experiments at the assembled dinuclear complexes 3 l/3 u failed, an alternative reaction sequence had to be developed. The iodide complex 4 could be converted into thiophenolate derivative 5 via an intermediate triflate using consecutively AgOTf and NaSPh. Subsequently,t he phosphine function was introduced as describedb efore to yield  alkyne complex 6.T reatingc omplex ligand 6 againw ith [(h 5 -C 5 H 5 )Ru(PPh 3 )(MeCN) 2 ]PF 6 and subsequently KC 8 led to the dinuclear complex 7.A fter chromatography,o nly as ingle isomer was isolated from this reactionsequence. XRD analysisofsuitable crystalsu ncovered an unlike configuration, in which thiophenolate and PPh 3 are directeda td ifferent sides of the planar bridging alkyne ligand ( Figure 5). In contrastt o3 l/3 u complex 7 u did not show any isomerization at high temperature.
Advantageously,isomer 7 u is of particular interest for our investigations, because the diastereomer being characterizedb y Ru-oxidation in 3 u is now substituted at tungsten. Replacement of bromide by the stronger donating thiophenolate shouldc ause am ore electron-rich Wc omplex centre, promoting oxidation at this site. [22] Indeed, the substitution is reflected in most bondingp arameters aroundt ungsten. The shorter W1ÀS2 bond of 238.46 (19) pm in 7 u (compared with 258.42(3)pmf or W1ÀBr1 in 3 u)i sa ccompanied by an elongation of all three WÀNb onds, which is most pronounced for W1ÀN5 in trans-position. In addition, the alkyne is coordinated less symmetrically to tungsten. Surprisingly,t he CO vibration in the IR spectrum (being primarily indicative of the electronic situation at W) is not really influenced (Table 1);h owevert he generali ncreaseo fe lectron density within the dinuclearc omplex is reflected by the redox potential. According to cyclic voltammetry,t he E 1/2 value has changed about 100 mV from À0.04 Vf or 3 u to À0.14 Vf or 7 u ( Figure 4). As imilars hift was observed for the second oxidation at + 0.47 V( 3 u)a nd + 0.35 V(7 u), respectively.
Upon stoichiometric oxidation of complex 7 u,w en ow observed two distinct new CO absorption bands in the IR spec-trum. One stronger band at 1941 cm À1 shows as malls hift of Dn = 35 cm À1 being indicative of aR u-centredo xidation. Furthermore as econd absorption appearsa t2 025 cm À1 ,r epresenting aW -centred oxidation due to ac hange by 119cm À1 . The latter is confirmed by as imilars hift of Dn = 128 cm À1 for the oxidation of mononuclearS Ph-complex 6 to its cationic form (see Supporting Information). Thes maller change in relation to the bromide complexes can be attributedt ot he charge compensation by the stronger donating thiophenolate. Again, SEC allowed us to observe the dicationic compound 7 u 2 + at 2058 cm À1 (Figure 6). Just as with the bromide complexes, this value is close to the sum of the shifts caused by oxidation at each metal.
For the Br-complexes, the remarkable dependence of the redox state on the type of the diastereomer is most likely caused by steric interactions of the ligand spheres and ac orresponding feedback with the specific complex geometry at the metals.V ariation of the anion by using Ac Fc + [B{C 6 H 3 (CF 3 ) 2 } 4 ]i nstead of Ac Fc + BF 4 À as oxidation agent did not result in substantial change of the redox behaviour.H ence, intrinsic structural reasonsc ame into the fore and prompted DFT calculations at the b3lyp/def2-TZVP/ECP(W,Ru) level of theory. Comparison of the optimized molecular structures of the pairs 3 l/3 l + and 3 u/ 3 u + ,r espectively,a llowed for conclusions on the impact of structuralc hanges at one metal on the overall structure. The calculated metricp arameters at the metal centre unaffected by the oxidation match those of the experimentally determined ones of the neutral complex forms in both cases (see Supporting Information). An inherent support for the validity of the calculated cationic speciesi sd elivered by TD-DFT calculations which confirmt he assignment of the strong NIRb ands to intervalence transfer (see Supporting Information). The calculated ande xperimental values are in reasonable accordance (3 l + : calc. 1685, exp. 2270 nm; 3 u + :c alc. 2992, exp. 2860 nm).
The frontier orbitals of the neutrali somers 3 l/3 u exhibit high similarity showing tungstenc entred HOMOs. In contrast, the differing distribution of the Mulliken spin densities of the cations 3 l + /3 u + reflect the experimental findings ( Figure 8).
Therefore, specific factors seem to force the Ru-based oxidation of 3 u whereas the like-isomer behaves at least qualitatively according to Koopmans' theorem. Higher calculated reorganization energy after oxidation of 3 l (0.25 vs. 0.20 eV for 3 u) is accompanied by significant structuralc hanges at tungsten. Essentially,t he whole Tp'-ligand changes its position with respect to the Ru-complex moiety (B-W-Ru angle:1 34.2 8 in 3 l; 121.7 8 in 3 l + ). In the unlike-isomer,t wo of the phenyl rings of different phosphinesw hich are interrelated by p-stacking are located in one pyrazole-pocket of the Tp'-ligand. Accordingly, geometry relaxation after oxidation should be significantly hindered in that isomer,leaving tungsten unable to accommodate the introduced charge. In view of the smaller energy gap between the W-based HOMO and Ru-based HOMO-1 in 3 u compared with 3 l (0.24 vs. 0.38 eV), the system can evade this pressure by adapting aR u III state. Oxidation at ruthenium is particularly characterized by shortening of the RuÀSb ondb y % 0.13 ,w hich is supported by an experimentally determined value of 0.15 for the redox pair [(h 5 -C 5 H 5 )Ru(dippe)(SPh)] 0/ + {dippe = bis(diisopropylphosphinyl-ethan)}. [23] This major change at ruthenium owing to the oxidation shouldi nterfere much lessw ith the resto ft he molecule (see Supporting Informationf or amore detailedd iscussion).

Conclusion
In summary,o nt he basis of coordination chemistry with a bridging phosphinyl thiolato alkyne ligand,af irst example of Figure 6. IR-SEC measurement in 1,2-dichloroethane(0.26 m nBu 4 PF 6 ): redox pairs 7 u/7 u + (left) and 7 u + /7 u 2 + (right). Figure 7. Te mperature-dependent IR-spectra (left);experimental (black) and simulated (red) X-band EPR spectrum of 7 u + collected at 100 Ki nfrozen CH 2 Cl 2 /T HF mixture( right). Chem. Eur.J.2020, 26,16811-16817 www.chemeurj.org 2020 The Authors. Published by Wiley-VCH GmbH diastereomers has been developed, for which the localization of as ingle redox step belongs to the differences in physical properties. Complex [Tp'(CO)BrW{m-h 2 -C,C'-k 2 -P,S-C 2 (PPh 2 )S}-Ru(h 2 -C 5 H 5 )(PPh 3 )] 3 could be isolated in two stable diastereomeric forms like (3 l)a nd unlike (3 u), which show similar spectroscopic properties and redoxp otentials. However,t he first oxidation in the l-isomer is assigned to W II /W III while aR u II /Ru III redox step is observed in the u-isomer,b oth processes being reversible. Spectroscopic evidence for this exceptional behaviour was provided by IR, NIR, EPR and spectroelectrochemistry (SEC). Thus,c onfigurationi somerism of ad inuclear complex based on two stereogenic metal centres resultsi nr edox regioselectivity.C onsistently,t he separatedo xidation products can be denoted metal/metal redox isomersw ithin as tringent definition. In addition, tuning of the complex system by substitutiono ft he W-coordinated bromide by thiophenolate allowed the isolation of unlike-[Tp'(CO)(SPh)W{m-h 2 -C,C'-k 2 -P,S-C 2 (PPh 2 )S}Ru(h 5 -C 5 H 5 )(PPh 3 )] 7 u. This related compound exhibited electromerism,b ecause two valence-trapped redox forms W III Ru II andW II Ru III were shown to co-existi ne quilibrium. Ongoing efforts are directed at resolution of the pure enantiomers in order to investigate the redox-dependence of chiroptic properties. [24] In addition, our findings encourage comprehensive investigations with regard to fine-tuning by variation of the co-ligands in the W/Ru complex and to new homoleptic complexes with the P,S-alkynec omplex ligand and av ariety of metals.

Experimental Section
Crystallographic data Deposition numbers 1980458, 1980457, and 1080456 (3 l, 3 u,a nd 7 u)c ontain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.
Full experimental details including spectroscopic ( 1 H, 13 C, 31 PN MR, IR vis/NIR), crystallographic data and details of DFT calculations are given in the Supporting Information.