Synthesis and structures of dinuclear RhIII and IrIII complexes supported by a tetraphosphine, meso- or rac-bis{[(diphenylphosphinomethyl)phenyl]phosphino]}methane

Journal of Organometallic Chemistry 797 (2015) 37e45

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Synthesis and structures of dinuclear RhIII and IrIII complexes supported by a tetraphosphine, meso- or rac-bis {[(diphenylphosphinomethyl)phenyl]phosphino]}methane Tomoaki Tanase*, Akiko Yoshii, Risa Otaki, Kanako Nakamae, Yumina Mikita, Bunsho Kure, Takayuki Nakajima Department of Chemistry, Faculty of Science, Nara Women's University, Kitauoya-nishi-machi, Nara 630-8506, Japan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 June 2015 Received in revised form 7 July 2015 Accepted 20 July 2015 Available online 29 July 2015

Reactions of [Cp*MCl2]2 with meso- or rac-dpmppm in the presence of NH4PF6 afforded mixtures of stereoisomers formulated as [(Cp*MCl)2(meso- or rac-dpmppm)](PF6)2, from which the major isomers, rac-M2-[(Cp*MCl)2(meso-dpmppm)](PF6)2 (M ¼ Rh (1a), Ir (1b)) and rac-M2-[(Cp*MCl)2(racdpmppm)](PF6)2 (M ¼ Rh (2a), Ir (2b)), were isolated and characterized by IR, UVevis, ESI mass, and 1H and 31P{1H} NMR spectroscopy and X-ray crystallography, where dpmppm is bis{[(diphenylphosphinomethyl)phenyl]phosphino}methane and Cp* is h5-pentamethylcyclopentadienyl. The isomeric tetraphosphine incorporates two {Cp*MCl} fragments with four-membered chelation through a pair of outer and inner phosphine units, and fixes them ca. 6.9 Å apart from each other. The configurations around the metal centers in 2 were determined to avoid repulsive interaction between the phenyl group on the inner P atom and the M-bound chloride anion, while those in 1 would be determined by minimizing not only the Ph/Cl repulsion but also repulsion between the two metal fragments. In the dichloromethane solutions, the major isomers 1 and 2 were not converted to any other minor isomers even at high temperature, but in dmf and dmso, only 1a was readily transformed into the minor isomer, meso-M2[(Cp*RhCl)2(meso-dpmppm)](PF6)2. Cyclic voltammograms of 1a,b and 2a,b demonstrated two irreversible 2e reduction waves corresponding to MIIIMIII / MIMIII / MIMI, in which the mixed-valence states with meso-dpmppm could be stabilized to a larger extent than with rac-dpmppm. Complex 1a heated in acetonitrile with AgOTf/NH4OTf afforded a Rh2Ag2 mixed-metal complex, [(Cp*Rh(CH3CN))2Ag2(OTf)2(meso-dpmppm)2](OTf)4 (5). © 2015 Elsevier B.V. All rights reserved.

Keywords: Tetraphosphine ligand Rhodium Iridium Dinuclear complex Stereoisomer

1. Introduction Multidentate phosphines have widely been used to arrange diand polynuclear metal centers in proximity to create reactivity and property that are induced by cooperative effects of multinuclear metal centers [1e4], and in particular, bis(diphenylphosphino) methane (dppm) and bis(diphenylphosphinomethyl)phenylphosphine (dpmp) have been exploited to create metalemetal bonded homo- and hetero-multinuclear structures [5e17]. Tetraphosphines, such as bis{[(2-diphenylphosphino)ethyl]phenylphosphino}methane (tetraphos-2,1,2), 1,2-bis{[(2diphenylphosphino)ethyl]phenylphosphino}ethane (tetraphos-

* Corresponding author. E-mail address: [email protected] (T. Tanase). http://dx.doi.org/10.1016/j.jorganchem.2015.07.033 0022-328X/© 2015 Elsevier B.V. All rights reserved.

2,2,2), 1,3-bis{[(2-diphenyl-phosphino)ethyl]phenylphosphino} propane (tetraphos-2,3,2), and their derivatives have also been known to form mono- and dinuclear systems [18e33]. Especially, Stanley et al. have disclosed that the dinuclear rhodium complex supported by Et,Ph-tetraphos-2,1,2 (bis{[(diethylphosphinoethyl) phenyl]phosphino}methane) was remarkably effective for catalytic hydroformylation of alkenes under mild conditions, suggesting crucial importance of the central single methylene bridge of the tetraphosphine [2,24]. In this regard, we have synthesized a methylene-bridged linear tetraphosphine ligand, bis{[(diphenylphosphinomethyl)phenyl] phosphino}methane (dpmppm, tetraphos-1,1,1) [33e43], which consists of two stereoisomers, meso- and rac-forms, and has proven very effective to assemble linearly ordered tetrametallic chains of group 11 metal ions, [M4(m-dpmppm)2]2þ, and heterometallic octanuclear rings of {[Au2MCuCl2(m-meso-dpmppm)2]2}4þ (M ¼ Au,

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Ag, Cu) [33e37]. The flexible coordination behavior of mesodpmppm further organized stepwise assemblies of heterotrinuclear metal ions of [PdCl2(Cp*M0 Cl2)(Cp*MCl2)(m-dpmppmk2,k1,k1)] and [PdCl(m-Cl)(Cp*M0 Cl)(Cp*MCl2)(m-dpmppm-k2,k1,k1)]þ (M, M0 ¼ Rh, Ir, Cp* ¼ h5-pentamethylcyclopentadienyl) [38]. In addition, we have recently been successful in establishing lowvalent octapalladium chains supported by meso-dpmppm, as a coordinatively unsaturated Pd8 rod, [Pd8(m-dpmppm)4](BF4)4, and the terminal-capped [Pd8(m-dpmppm)4(L)2](BF4)4 (L ¼ 2,6-xylyl isocyanide (XylNC), CH3CN, N,N-dimethylformamide (dmf)) [39,40]. These Pd8 chains exhibited interesting temperaturedependent dynamic behaviors in the solution states. In contrast to the higher nuclearity systems, the dpmppm also provides a scaffold for hydride-bridged dicopper(I) center, [Cu2(m-H)(m-mesodpmppm)2]X (X ¼ BF4, PF6), which undergo insertion of CO2 under mild conditions to yield a formate-bridged dicopper complex, [Cu2(m-HCO2)(m-meso-dpmppm)2]X [41]. In the present study, we have prepared dinuclear RhIII and IrIII complexes supported by meso- or rac-dpmppm, [(Cp*MCl)2(m-meso or rac-dpmppm)](PF6)2, and characterized their configurational structures around the metal centers, which may be useful information in elucidating correlation between property and reactivity of two Cp*M centers and their configurations. 2. Results and discussion 2.1. Synthesis and characterization of dinuclear RhIII and IrIII complexes with meso- and rac-dpmppm Reactions of [Cp*MCl2]2 with meso-dpmppm and NH4PF6 in CH2Cl2/acetone mixed solvents at room temperature afforded dinuclear RhIII and IrIII complexes, [(Cp*MCl)2(mesodpmppm)](PF6)2 (M ¼ Rh (1a), 67%; Ir (1b), 52%) as orange and yellow crystals in good yields. The isolated compounds were characterized by elemental analyses, IR, UVevis, 1H and 31P{1H} NMR, and ESI mass spectra and X-ray crystallographic analyses to reveal that the dinuclear complexes existed in a racemic configuration with respect to the two tetrahedral metal centers, abbreviated as meso-P2/rac-M2 form in Scheme 1. The 1H NMR spectra of the reaction mixtures showed the presence of two stereoisomers in ratios of 1 to 0.3e0.6, which were confirmed with a set of two triplets (4JPH ¼ 4 Hz) of the Cp* methyl protons around d 1.65e1.80 ppm for a major species and a broad triplet around 1.58e1.59 ppm for a minor one, and consequently, the major

species were crystallized as 1a and 1b. The 1H NMR spectrum of 1a in CD2Cl2 exhibited the diagnostic methyl signals as two triplets at d 1.65 and 1.75 ppm (4JPH ¼ 4 Hz) together with a residual broad peak at 1.58 ppm for the minor isomer (Fig. 1a). In the 31P{1H} NMR spectrum of 1a, the resonances for the outer phosphine units were observed as two dd signals at 7.1 ppm (1JRhP ¼ 115 Hz, 2 JPP0 ¼ 99 Hz) and 0.5 ppm (1JRhP ¼ 115 Hz, 2JPP0 ¼ 94 Hz), and those for the inner P atoms appeared in complicated patterns around 12 to 14 ppm (Fig. S4a). These spectra were unchanged at higher temperature to indicate that the asymmetric major compound is not equilibrated to the minor isomer. The 1H NMR spectrum of 1b in CD2Cl2 showed similar two triplets at d 1.67 and 1.80 ppm (4JPH ¼ 3 Hz) with a small peak of the minor isomer at 1.59 ppm (Fig. 1c). The 31P{1H} NMR spectrum of 1b was rather simple with two dd peaks at 39.6 ppm (2JPP0 ¼ 60 Hz, 4JPP0 ¼ 4 Hz) and 30.9 ppm (2JPP0 ¼ 57 Hz, 4JPP0 ¼ 4 Hz) for the outer P atoms and two ddd peaks at 53.9 ppm (2JPP0 ¼ 60 Hz, 55 Hz; 4JPP0 ¼ 4 Hz) and 52.8 ppm (2JPP0 ¼ 57 Hz, 53 Hz; 4JPP0 ¼ 4 Hz) for the inner P atoms (Fig. S4b). The ESI mass spectra of 1a and 1b in dichloromethane showed monovalent cation peaks at m/z ¼ 1318.999 (1a) and 1499.173 (1b) corresponding to {[(Cp*MCl)2(dpmppm)]PF6}þ (1319.124 (M ¼ Rh), 1499.238 (M ¼ Ir)) (Fig. S9). From reactions similar to those of 1a and 1b by using racdpmppm, orange and yellow crystals of [(Cp*MCl)2(racdpmppm)](PF6)2 (M ¼ Rh (2a), Ir (2b)) were isolated in 53 and 18% yields, respectively (Scheme 1). In the 1H NMR spectra of 2a and 2b in CD2Cl2, the Cp* methyl protons were observed as a triplet at 1.60 ppm with 4JHP ¼ 4 Hz (Fig. 2), and the methylene protons of dpmppm appeared with two multiplets at d 3.12 and 4.16 (2JHH ¼ 16 Hz) and a triplet at d 4.03 (2JHP ¼ 6 Hz) for 2a and two multiplets at d 3.22 and 5.56 (2JHH ¼ 16 Hz) and a triplet at d 4.10 (2JHP ¼ 7 Hz) for 2b in 1:1:1 ratio. The 31P{1H} NMR spectra of 2a and 2b consist of two multiplets at d 13.3 and 2.0 ppm (1JRhP ¼ 115 Hz) for 2a and at d 55.1 and 34.2 ppm for 2b in 1:1 ratio (Fig. S5). These NMR spectral features clearly indicated a symmetrical structure of 2 which was determined by X-ray diffraction analysis as a C2 symmetrical rac-P2/rac-M2 form (Scheme 1). The 1H NMR spectrum of the reaction mixtures with Rh suggested the presence of two stereoisomers in a ratio of 1:0.6 with a triplet at 1.60 ppm for the major species, which crystallized as 2a, and a pair of triplets at 1.67 and 1.85 ppm for minor one. The ESI mass spectra of 2a and 2b in CH2Cl2 (Fig. S10) showed monovalent cation peaks at m/z ¼ 1319.097 (2a) and 1499.217 (2b) and dication peaks at m/z ¼ 587.038 (2a) and 677.115 (2b), which are assignable

Scheme 1. Preparations of dinuclear RhIII and IrIII complexes, [(Cp*MCl)2(meso-dpmppm)](PF6)2 (M ¼ Rh (1a), Ir (1b)) and [(Cp*MCl)2(rac-dpmppm)](PF6)2 (M ¼ Rh (2a), Ir (2b)).

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39

Scheme 2. Possible stereoisomers of dinuclear RhIII and IrIII complexes with (a) meso-dpmppm and (b) rac-dpmppm, [(Cp*MCl)2(meso- or rac-dpmppm)](PF6)2 (M ¼ Rh, Ir). The observed major and minor isomers are indicated in d and // boxes.

to {[(Cp*MCl)2(dpmppm)]PF6}þ (1319.124 (M ¼ Rh), 1499.238 (M ¼ Ir)) and [(Cp*MCl)2(dpmppm)]2þ (587.080 (M ¼ Rh), 677.137 (M ¼ Ir)), respectively. The piano stool mononuclear complexes, [Cp*MCl(dppm)]PF6 (M ¼ Rh (3a), Ir (3b)) [44e46], were prepared as reference compounds, and showed a characteristic triplet peak of Cp* at 1.77 ppm (4JPH ¼ 4 Hz) (3a) and 1.79 ppm (4JPH ¼ 4 Hz) (3b) in the 1H NMR spectra (Fig. S6), and a doublet of P atoms at 5.2 (1JRhP ¼ 114 Hz) (3a) and a singlet at 38.6 ppm (3b) in the 31P{1H} NMR spectra (Fig. S7), which are characteristic for the four-membered chelation of dppm, and basically similar to the spectral features of 1a,b and 2a,b. 2.2. X-ray crystal structures of 1a,b and 2a,b The structures of 1a,b and 2a,b were determined by X-ray crystallography, and ORTEP diagrams for the complex cations are illustrated in Figs. 3, S1, and S2. The crystal structures of 1a and 1b are isomorphous to each other, and comprise two {Cp*MCl} units bridged by the two inner P atoms of meso-dpmppm with each

chelated by a pair of the outer and inner P atoms; the MeP and MeC bond distances are in the normal range of 2.3017(13)e 2.3328(11) Å (av. 2.313 Å) and 2.162(5)e2.238(5) Å (av. 2.219 Å) for 1a and 2.2944(15)e2.3135(12) Å (av. 2.302 Å) and 2.173(5)e 2.268(5) Å (av. 2.232 Å) for 1b. The two Ph2PCH2PPh moieties of meso-dpmppm coordinate to the metal centers to form strained four-membered chelate rings with P1eMeP2 ¼ 72.49(4) (1a), 72.17(4) (1b) and P3eM2eP3 ¼ 72.58(4) (1a), 72.31(5) (1b), which are usual values for the methylene-bridged diphosphine complexes [47]. Two chloride anions bind to the respective metals to complete two chiral piano stool structures (M1eCl1 ¼ 2.4053(14) Å (1a), 2.4105(15) Å (1b), and M2eCl2 ¼ 2.3899(13) Å (1a), 2.3951(13) Å (1b)), where the configurations around the two metal centers are same, namely racemic two M centers are coupled with meso-form of the configurations for the inner chiral P atoms, providing two different diastereomeric environments around the metal centers abbreviated as rac-M2/ meso-P2 in Scheme 2. In CIP notation, the meso-P2/rac-M2 form is described as RMSPRPRM or SMRPSPSM, where RM and SM indicate

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Fig. 1. 1H NMR spectra for the Cp* methyl protons of 1a in (a) CD2Cl2 and (b) dmso-d6, and 1b (c) CD2Cl2 and (d) dmso-d6 at room temperature. The peaks with (red) correspond to the major species 1a and 1b, and those with (blue) to minor isomers as indicated in Scheme 2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2. 1H NMR spectra for the Cp* methyl protons of (a) 2a and (b) 2b in CD2Cl2 at room temperature. The peaks with (red) correspond to the major species 2a and 2b, and those with (blue) to minor isomers as indicated in Scheme 2. *H2O as impurity. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

absolute configuration around the metal and RP and SP around the P atom. The M/M separations are almost identical as 6.9720(5) Å (1a) and 6.9357(2) Å (1b) and indicate the absence of any metalemetal interactions. The crystal structure of the complex cation 2a (Fig. 3b) possesses a crystallographically imposed C2 symmetry and contains two {Cp*RhCl} units supported by rac-dpmppm in a similar manner to 1a, with Rh1eCl1 ¼ 2.3911(6) Å, Rh1eP ¼ 2.3091(7)e2.3187(7) Å (av. 2.314 Å), Rh1eC ¼ 2.183(2)e2.247(3) Å (av. 2.213 Å), and P1eRh1eP2 ¼ 71.62(2) (Table S4). The structure of 2b is isomorphous to that of 2a with structural parameters of Ir1eCl1 ¼ 2.4047(8) Å, Ir1eP ¼ 2.3018(9)e2.3115(10) Å (av. 2.307 Å), Ir1eC ¼ 2.197(3)e2.268(3) Å (av. 2.227 Å), and P1eIr1eP2 ¼ 71.34(3) (Fig. S2, Table S4). The racemic two M centers are combined with rac-form of dpmppm, leading to a single stereo environment around the metal centers abbreviated as racP2/rac-M2 in Scheme 2 (RMSPSPRM/SMRPRPSM). The M/M separation of 6.9143(5) Å (2a) and 6.9288(2) Å (2a) are almost identical to those of 1a,b despite the distinct isomeric structure. The configuration around the metal centers in 2a,b is assumed to result from avoiding repulsive interaction between the phenyl group on the inner P2 atom and the M-bound Cl1 anion, the Ph

group and Cl ligand being mutually in opposite side with respect to the Rh1eP2 bond (anti-arrangement), which is corresponding to the configurations of RMSP or SMRP. In contrast, in 1a,b, the M1 center takes the same anti-arrangement (RMSP/SMRP) as in 2a,b, and the M2 metal sits in a different disposition where the phenyl group on the inner P atom and the Cl ligand are in the same side with respect to Rh2eP3 bond (syn-arrangement, RMRP/SMSP). In our previous studies on dinuclear RhIII and IrIII complexes with a series of NPPN ligands, 2-PyCH2(Ph)P(CH2)nP(Ph)CH2-2-Py (Py ¼ pyridyl, n ¼ 2e4), all the isolated complexes of [(Cp*MCl)2(meso- or rac-(2PyCH2(Ph)P(CH2)nP(Ph)CH2-2-Py))](BF4)2 exhibited the Ph/Cl antiarrangement which may be more stable when the two metal centers are apart enough with the central (CH2)n chains (n ¼ 2e4) [48]. Consequently, the stereo isomeric form of rac-M2/meso-P2 of 1a,b was determined by minimizing not only the Ph/Cl repulsion but also repulsion between the two metal fragments. 2.3. Stereoisomers in the solutions of 1 and 2 As described above, from the mixtures of the stereoisomers [(Cp*MCl)2(meso- or rac-dpmppm)](PF6)2 generated by reactions of [Cp*MCl2]2 with meso- or rac-dpmppm, complexes, the major

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41

Scheme 3. Structure of 4.

coordination ability of dmso solvent with displacing phosphine and/or chloride ligands, which is evidenced in part by the formation of 4. The reasons for the failure to isolate the minor isomer are not cleared yet, but it may be ascribable in part to thermodynamic instability of the minor species. 2.4. Reaction of 1a with AgOTf

Fig. 3. ORTEP diagrams for the complex cation of [(Cp*RhCl)2(meso-dpmppm)](PF6)2 (1a) and [(Cp*RhCl)2(rac-dpmppm)](PF6)2 (2a) with the atomic numbering schemes. The thermal ellipsoids are drawn at the 40% probability level, and the hydrogen atoms are omitted for clarity.

isomers were crystallized as 1a,b and 2a,b with meso-P2/rac-M2 and rac-P2/rac-M2 configurations, respectively. While the 1H NMR spectra of 2a,b in CD2Cl2 comprised exclusive single triplet peak for the symmetric rac-M2/rac-P2 major isomer (Fig. 2), those of 1a,b indicated an appreciable involvement of the minor isomer with a broad triplet around 1.58 ppm in a ratio of 0.1 vs the asymmetric rac-M2/meso-P2 major isomers (Fig. 1a,c). On the basis of the structure of 2a in rac-M2/rac-P2 configuration with the Ph/Cl antiarrangement, the symmetric minor isomer of 1a,b is estimated to be the meso-P2/meso-M2 form with the Ph/Cl anti-arrangement (Scheme 2a, top). The peak intensity of the minor species in 1H NMR spectra would not increase even upon heating the solutions, suggesting no transformation between the isomers occurred in the CD2Cl2 solution. It should be noted, however, that only the rhodium complex 1a in dmso-d6 underwent rapid racemization of the metal centers to afford a mixture of meso-P2/rac-M2 and meso-P2/mesoM2 in a ratio of 1:1. Some attempts to isolate the minor meso-P2/ meso-M2 isomer just result in obtaining the crystals of the major meso-P2/rac-M2 isomer and unidentified decomposed materials from the mother liquor, the latter involving a trace amount of crystals of a trinuclear rhodium(III) complex, [(Cp*RhCl2)2(Cp*RhCl)(meso-dpmppm)]PF6 (4) (Scheme 3) which was characterized by X-ray diffraction as shown in Fig. S3. The similar combination of {Cp*RhCl2} and {Cp*RhCl} units was reported in [(Cp*MCl2)(Cp*MCl)(dpmp)]þ (M ¼ Rh, Ir) [49e51]. It is estimated that the racemization of 1a is accelerated by strong

When 1a was heated in acetonitrile in the presence of AgOTf, a RhIII2AgI2 mixed-metal complex, [(Cp*Rh(CH3CN))2Ag2(OTf)2(mesodpmppm)2](OTf)4 (5) was obtained in 33% yield, and characterized by elemental analysis, IR, UVevis, 1H and 31P{1H} NMR, and ESI mass spectra, and an X-ray crystallographic analysis (Scheme 4). The complex cation involves two {Cp*Rh(CH3CN)} and two Ag(OTf) unites supported by two meso-dpmppm ligands in a pseudo Ci symmetric fashion (Fig. 4, Table S6). The two AgI ions are bridged by two pairs of the outer and inner P atoms of mesodpmppm, to form an eight-membered [AgPCPAgPCP] macrocyclic ring with two PeAgeP linear components (AgeP ¼ 2.389(2)e 2.406(2) Å (av. 2.398 Å), PeAgeP ¼ 160.27(6)e164.53(6) (av. 162.41 )). A triflate anion weakly coordinates to each Ag ion in the opposite site of the macrocyclic core (Ag1eO1 ¼ 2.552(6) Å, Ag2eO4 ¼ 2.569(6) Å). The Ag/Ag distance is 3.1087(6) Å which indicates the presence of d10 closed-shell metallophilic interaction [52,53]. The other pair of the outer and inner P atoms of the tetraphosphine traps the {Cp*Rh(CH3CN)} fragment via the fourmembered [RhPCP] chelation, in which the chloride ligand of 1a is replaced with an acetonitrile to complete a piano stool geometry (RheP ¼ 2.320(2)e2.327(2) Å (av. 2.323 Å), RheC ¼ 2.146(8)e 2.238(7) Å (av. 2.202 Å), RheN ¼ 2.068(6)e2.082(6) Å (av. 2.075 Å), and PeRheP ¼ 71.17(6)e71.19(6) (av. 71.18 )). The overall configurational structure is described with a notation of meso-M2/mesoP2 with Ph/CH3CN syn-arrangement. The analogous RhIII2AgI2 structure with a triphosphine dpmp was reported in [(Cp*RhCl2)2Ag2(dpmp)2](TfO)2 which involves monophosphinetrapped Cp*RhCl2 pendants [49,50]. 2.5. Electrochemical properties of 1a,b and 2a,b In order to elucidate the electrochemical behaviors of the

Scheme 4. Preparation of 5.

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Fig. 4. ORTEP diagram for the complex cation of [(Cp*Rh(CH3CN))2Ag2(OTf)2(mesodpmppm)2](OTf)4 (5) with the atomic numbering schemes. The thermal ellipsoids are drawn at the 40% probability level, and the hydrogen atoms are omitted for clarity.

dinuclear RhIII and IrIII complexes with meso- and rac-dpmppm ligands, cyclic voltammograms of 1a,b and 2a,b together with 3a,b as references were measured in acetonitrile containing 0.1 M [nBu4N] [PF6] with a glassy carbon working electrode (Fig. 5). The CVs of the mononuclear complexes 3a (M ¼ Rh) and 3b (M ¼ Ir) showed an irreversible reduction wave at 1.19 V and 1.50 V (vs Fc/FcPF6), respectively, which was assigned by coulometric analysis to twoelectron reduction of MIII / MI (Fig. 5a,d). The CV of 1a (M ¼ Rh, meso-dpmppm) was comprised of two irreversible reduction waves at 1.11 V (E1pc) and 1.32 V (E2pc) (Fig. 5b), which were corresponding to two successive two-electron reductions, RhIIIRhIII / RhIRhIII / RhIRhI, in the light of the CV of 3a as well as coulometric analyses at 1.20 V and 1.50 V. Although the twoelectron-reduced species of 1a was presumed to include two diastereomers with respect to the Ph/Cl arrangement, {MIMIII}anti and {MIMIII}syn, as shown in Scheme S1a, the CV of 1a notably featured the two successive overall responses. The CV of 2a (M ¼ Rh, racdpmppm) also showed reduction waves similar to those of 1a at 1.08 V (E1pc) and 1.20 (E2pc) (Fig. 5c). The E2pc of 2a was positively shifted by 0.12 V from that of 1a, resulting in a smaller DEpc (¼ E1pc  E2pc) of 0.12 V compared with 0.21 V for 1a. These results may suggest that stability of the mixed-valence RhIRhIII state from 1a with meso-dpmppm is appreciably higher than that from 2a with rac-dpmppm, implying importance of stereo configurations on electronic property of the dinuclear metal centers, or long-range communication between the two Rh centers. A similar propensity was observed in CVs of the IrIII complexes (Fig. 5e,f); the CV of 1b

Fig. 5. Cyclic voltammograms of (a) [(Cp*RhCl)(dppm)]PF6 (3a), (b) [(Cp*RhCl)2(meso-dpmppm)](PF6)2 (1a), (c) [(Cp*RhCl)2(rac-dpmppm)](PF6)2 (2a), (d) [(Cp*IrCl)(dppm)]PF6 (3b), (e) [(Cp*IrCl)2(meso-dpmppm)](PF6)2 (1b), and (f) [(Cp*IrCl)2(rac-dpmppm)](PF6)2 (2b), measured in CH3CN containing 0.1 M [nBu4N][PF6] with a glassy carbon working electrode at a scan rate of 100 mV/s.

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(meso-dpmppm) showed three irreversible reduction responses at 1.37 V (E1pc), 1.48 V (E2pc), and 1.71 V (E3pc), which may be derived from two diastereomeric IrIIrIII intermediates, {MIMIII}anti and {MIMIII}syn (Scheme S1a), generated in the successive reductions of IrIIIIrIII / IrIIrIII / IrIIrI. In contrast, the CV of 2b (racdpmppm) showed two irreversible reduction waves at 1.36 V (E1pc) and 1.51 V (E2pc). The E3pc potential of 1b (1.71 V) was significantly shifted to negative side to result in a very large DEpc value of 0.34 V (E1pc  E3pc) in comparison with that of 0.15 V (E1pc  E2pc) for 2b, implying a stronger long-range communication arose in the diiridium centers with meso-dpmppm, partially attributable to the greater spatial extent of the Ir 5d orbitals.

3. Conclusion In the present study, dinuclear RhIII and IrIII complexes, [(Cp*MCl)2(meso-dpmppm)](PF6)2 (M ¼ Rh (1a), Ir (1b)) and [(Cp*MCl)2(rac-dpmppm)](PF6)2 (M ¼ Rh (2a), Ir (2b)), were prepared and their configurations around the two M and the two inner P centers were assigned to rac-M2/meso-P2 for 1 and rac-M2/rac-P2 for 2. The configurational structures of the dimetal centers are retained in the solution state and display interesting long-range electronic communication between the metal centers, which was demonstrated by their CVs showing two successive 2e reductions of MIII2 / MIMIII / MI2 and was appreciably influenced by the configurations of meso- and rac-dpmppm. Complex 1a (M ¼ Rh) reacted with AgOTf in acetonitrile to give the AgI2RhIII2 mixed metal complex of [(Cp*Rh(CH3CN))2Ag2(OTf)2(meso-dpmppm)2](OTf)4 (5). These results could provide useful information in synthesizing sterically strained dinuclear complexes and elucidating correlation between the property and reactivity of two metal centers and their configurational arrangements.

4. Experimental 4.1. General All preparative procedures were carried out under nitrogen atmosphere using standard Schlenk techniques. Reagent grade solvents were dried by the standard procedures and were freshly distilled prior to use. Compounds meso- and rac-dpmppm [33,36] and [Cp*MCl2]2 (M ¼ Rh, Ir; Cp* ¼ pentamethylcyclopentadienyl) [54] were prepared by the methods described in the literature. 1H and 1H{31P} NMR spectra were recorded on Bruker AV-300N or Varian Gemini2000 instruments (300 MHz) and the frequencies were referenced to TMS as an external reference. 31P{1H} NMR spectra were recorded on the same instrument at 121 MHz with chemical shifts being calibrated to 85% H3PO4 as an external reference. Electronic absorption spectra were recorded on Shimadzu UV-3100 and Jasco UV600 spectrophotometers. IR spectra of solid compounds as KBr disks were recorded on a Jasco FT/IR-410 spectrophotometer at ambient temperature. ESI-TOF mass spectra were recorded on a JEOL JMS-T100LC in a positive detection mode in the range of m/z 100e3000, equipped with an ion spray interface. Electrochemical measurements were performed with a HokutoDenko HZ-3000 system. [nBu4N][PF6] was used as supporting electrolyte, which was recrystallized from ethanol before in use. Cyclic voltammetry experiments were carried out with 1 mM acetonitrile solutions of the samples containing 0.1 M [nBu4N][PF6], by using a standard three-electrode cell consisting of a Ag/AgPF6 reference electrode, platinum wire as counter-electrode, and glassy carbon electrode as working electrode.

43

4.2. Preparation of [(Cp*MCl)2(meso-dpmppm)](PF6)2 (M ¼ Rh (1a), Ir (1b)) To a dichloromethane/acetone (5 mL/5 mL) solution containing [Cp*RhCl2]2 (50.6 mg, 0.082 mmol) and NH4PF6 (33.7 mg, 0.207 mmol) was added meso-dpmppm (50.4 mg, 0.080 mmol) and the mixture was stirred overnight at room temperature. The solvent was removed under reduced pressure and the residue was extracted with dichloromethane (10 mL). The extract was passed through a membrane filter and concentrated to ca. 3 mL, to which Et2O (0.6 mL) was slowly added. The resultant solution was allowed to stand in refrigerator for a couple of days to afford orange crystals of [(Cp*RhCl)2(meso-dpmppm)](PF6)2, which were filtered off, washed with Et2O, and dried under vacuum. Yield 67% (79 mg). Anal. Calcd for C59.5H67Cl3F12P6Rh2 (1a$0.5CH2Cl2): C, 47.38; H, 4.48. Found: C, 47.76; H, 4.64. IR (KBr): n 1437 (s), 1102 (m), 841 (s), 739 (s), 692 (m), 557 (s), 526 (m), 501 (m) cm1. 1H NMR (CD2Cl2): d 1.65 (t, 15H, Cp*, 4JPH ¼ 4 Hz), 1.75 (t, 15H, Cp*, 4JPH ¼ 4 Hz), 3.4e5.2 (m, 6H, CH2), 6.8e7.8 (m, 30H, Ar). 31P{1H} NMR (CD2Cl2): d 12.0 to 16.6 (m, 2P), 7.1 (dd, 1P, 1JPRh ¼ 115 Hz, 2JPP ¼ 99 Hz), 0.5 (dd, P, 1 JPRh ¼ 115 Hz, 2JPP ¼ 94 Hz), 144.2 (sep, 2P, 1JPF ¼ 714 Hz, PF6). UVevis (CH2Cl2, r.t.): lmax (log 3 ) 362 (4.25), 453sh (3.49) nm. ESIMS (in CH2Cl2): m/z 1318.999 (z ¼ 1, {[(Cp*RhCl)2(dpmppm)]PF6}þ (1319.124)). Recrystallization of 1a from CH2Cl2/Et2O mixed solvents yielded orange crystals of 1a$3CH2Cl2, which were suitable for X-ray crystallography. To a dichloromethane/acetone (20 mL/20 mL) solution containing [Cp*IrCl2]2 (301 mg, 0.38 mmol) and NH4PF6 (153 mg, 0.94 mmol) was added meso-dpmppm (241 mg, 0.38 mmol) and the mixture was stirred overnight at room temperature. The solvent was removed under reduced pressure and the residue was extracted with dichloromethane (20 mL). The extract was passed through a membrane filter and concentrated to ca. 4 mL, to which hexane (0.8 mL) was slowly added. The resultant solution was allowed to stand at room temperature to afford colorless crystals of [(Cp*IrCl)2(meso-dpmppm)](PF6)2, which were filtered off, washed with Et2O, and dried under vacuum. Yield 52% (333 mg). Anal. Calcd for C59.5H67Cl3F12P6Ir2 (1b$0.5CH2Cl2): C, 42.37; H, 4.00. Found: C, 42.39; H, 4.17. IR (KBr): n 1438 (s), 1104 (m), 841 (s), 740 (s), 692 (m), 558 (s), 541 (m), 485 (m) cm1. 1H NMR (CD2Cl2): d 1.67 (t, 15H, Cp*, 4 JPH ¼ 3 Hz), 1.80 (t, 15H, Cp*, 4JPH ¼ 3 Hz), 3.3e6.6 (m, 6H, CH2), 6.8e7.8 (m, 30H, Ar). 31P{1H} NMR (CD2Cl2): d 53.9 (ddd, 1P, 2 JPP ¼ 60, 55 Hz, 4JPP ¼ 4 Hz), 52.8 (ddd, 1P, 2JPP ¼ 57, 53 Hz, 4 JPP ¼ 4 Hz), 39.6 (dd, 1P, 2JPP ¼ 60 Hz, 4JPP ¼ 4 Hz), 30.9 (dd, 1P, 2 JPP ¼ 57 Hz, 4JPP ¼ 4 Hz), 143.8 (sep, 2P, 1JPF ¼ 712 Hz, PF6). UVevis (CH2Cl2, r.t.): lmax (log 3 ) 299 (4.04), 338sh (3.87), 407sh (3.19) nm. ESI-MS (in CH2Cl2): m/z 1499.173 (z ¼ 1, {[(Cp*IrCl)2(dpmppm)] PF6}þ (1499.238)). Recrystallization of 1b from a CH2Cl2/Et2O, mixed solvent gave colorless crystals of 1b$3CH2Cl2 which were suitable for X-ray analysis. 4.3. Preparation of [(Cp*MCl)2(rac-dpmppm)](PF6)2 (M ¼ Rh (2a), Ir (2b)) To a dichloromethane/acetone (10 mL/10 mL) solution containing rac-dpmppm (39.8 mg, 0.063 mmol) were added [Cp*RhCl2]2 (39.8 mg, 0.064 mmol) and NH4PF6 (31.8 mg, 0.195 mmol) and the mixture was stirred overnight at room temperature. The solvent was removed under reduced pressure and the residue was extracted with dichloromethane (10 mL). The extract was passed through a membrane filter and concentrated to ca. 3 mL, to which Et2O (0.5 mL) was slowly added. The solution was allowed to stand in refrigerator to afford orange crystals of [(Cp*RhCl)2(rac-dpmppm)](PF6)2. Yield 53% (50 mg). Anal. Calcd for C59H66Cl2F12P6Rh2 (2a): C, 48.35; H, 4.54. Found: C, 48.17; H, 4.38.

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T. Tanase et al. / Journal of Organometallic Chemistry 797 (2015) 37e45

IR (KBr): n 1438 (s), 1100 (m), 840 (s), 782 (m), 737 (m), 711 (m), 692 (m), 557 (w) cm1. 1H NMR (CD2Cl2): d 1.60 (t, 30H, Cp*, 4JPH ¼ 4 Hz), 3.05e3.19 (m, 2H, CH2), 4.03 (t, 2H, 2JPH ¼ 5 Hz, CH2), 4.13e4.21 (m, 2H, CH2), 6.9e7.8 (m, 30H, Ar). 31P{1H} NMR (CD2Cl2): d 13.3 (m, 2P), 2.0 (m, 2P), 144.6 (sep, 2P, 1JPF ¼ 708 Hz, PF6). UVevis (CH2Cl2, r.t.): lmax (log 3 ) 365 (4.48), 455sh (3.81) nm. ESI-MS (in CH2Cl2): m/z 1319.097 (z ¼ 1, {[(Cp*RhCl)2(dpmppm)]PF6}þ (1319.124)), 587.038 (z ¼ 2, [(Cp*RhCl)2(dpmppm)]2þ (587.080)). Recrystallization of 2a from CH2Cl2/Et2O yielded orange crystals of 2a$2.5CH2Cl suitable for X-ray diffraction. By a procedure similar to that for 2a, yellow crystals of [(Cp*IrCl)2(rac-dpmppm)](PF6)2 (2b) were obtained from [Cp*IrCl2]2 (62.1 mg, 0.078 mmol), rac-dpmppm (241 mg, 0.38 mmol), and NH4PF6 (39.1 mg, 0.24 mmol). Yield 52% (333 mg). Anal. Calcd for C59H66Cl2F12P6Ir2 (2b): C, 43.10; H, 4.05. Found: C, 42.70; H, 3.92. IR (KBr): n 1438 (s), 1103 (m), 839 (s), 738 (s), 714 (w), 692 (w), 557 (m), 425 (w), 484 (w) cm1. 1H NMR (CD2Cl2): d 1.60 (br s, 30H, Cp*), 3.14e3.29 (m, 2H, CH2), 4.10 (t, 2H, 2 JPH ¼ 7 Hz, CH2), 5.50e5.62 (m, 2H, CH2), 6.9e7.8 (m, 30H, Ar). 31P {1H} NMR (CD2Cl2): d 55.1 (m, 2P), 34.2 (m, 2P), 144.6 (sep, 2P, 1 JPF ¼ 709 Hz, PF6). UVevis (CH2Cl2, r.t.): lmax (log 3 ) 297 (4.14), 339sh (4.00), 403sh (3.56) nm. ESI-MS (in CH2Cl2): m/z 1499.173 (z ¼ 1, {[(Cp*IrCl)2(dpmppm)]PF6}þ (1499.238)), 677.115 (z ¼ 2, [(Cp*IrCl)2(dpmppm)]2þ (677.137)). Recrystallization of 2b from CH2Cl2/Et2O gave yellow block-shaped crystals of 2b$2CH2Cl2. 4.4. Preparation of [Cp*MCl(dppm)]PF6 (M ¼ Rh (3a), Ir (3b)) Portions of [Cp*RhCl2]2 (31.2 mg, 0.050 mmol), dppm (41.4 mg, 0.11 mmol), and NH4PF6 (19.9 mg, 0.12 mmol) were dissolved in a dichloromethane/acetone (10 mL/10 mL) mixed solvent and were stirred overnight at room temperature. The solvent was removed under reduced pressure and the residue was extracted with dichloromethane (10 mL). The extract was passed through a membrane filter and concentrated to ca. 3 mL, to which Et2O (3 mL) was slowly added. The resultant solution was allowed to stand in refrigerator to afford yellow needle crystals of [Cp*RhCl(dppm)]PF6, which were filtered off, washed with Et2O, and dried under vacuum. Yield 80% (64 mg). Anal. Calcd for C35H37ClF6P3Rh (3a): C, 52.36; H, 4.64. Found: C, 52.10; H, 4.63. IR (KBr): n 1438 (s), 1103 (m), 839 (s), 742 (s), 730 (m), 718 (m), 693 (m) cm1. 1H NMR (CD2Cl2): d 1.77 (t, 15H, Cp*, 4JPH ¼ 4 Hz), 4.4e4.8 (m, 2H, CH2), 7.1e7.7 (m, 20H, Ar). 31P{1H} NMR (CD2Cl2): d 5.2 (d, 2P, 1 JPRh ¼ 114 Hz), 144.6 (sep, P, 1JPF ¼ 709 Hz, PF6). UVevis (CH2Cl2, r.t.): lmax (log 3 ) 360 (3.80), 460sh (2.91) nm. ESI-MS (in CH2Cl2): m/ z 657.254 (z ¼ 1, [Cp*RhCl(dppm)]þ (657.111)). By a procedure similar to that for 3a, colorless crystals of [Cp*IrCl(dppm)]PF6 (3b) were obtained from [Cp*IrCl2]2 (51.7 mg, 0.065 mmol), dppm (50.3 mg, 0.13 mmol), and NH4PF6 (44.9 mg, 0.28 mmol). Yield 18% (21 mg). Anal. Calcd for C35H37ClF6P3Ir (3b): C, 47.11; H, 4.18. Found: C, 46.77; H, 3.97. IR (KBr): n 1438 (s), 1103 (m), 839 (s), 742 (s), 731 (m), 703 (m), 693 (m) cm1. 1H NMR (CD2Cl2): d 1.79 (t, 15H, Cp*, 4JPH ¼ 3 Hz), 4.5e4.8 (m, 1H, CH2), 6.0e6.2 (m, 1H, CH2), 7.2e7.7 (m, 20H, Ar). 31P{1H} NMR (CD2Cl2): d 38.6 (s, 2P), 144.6 (sep, P, 1JPF ¼ 709 Hz, PF6). UVevis (CH2Cl2, r.t.): lmax (log 3 ) 291sh (3.7), 328sh (3.5), 408sh (2.7) nm. ESI-MS (in CH2Cl2): m/z 747.194 (z ¼ 1, [Cp*IrCl(dppm)]þ (747.168)). 4.5. Preparation of [(Cp*RhCl2)2(Cp*RhCl)(meso-dpmppm)]PF6 (4) To a dichloromethane/acetone (10 mL/10 mL) solution containing meso-dpmppm (39.8 mg, 0.063 mmol) were added [Cp*RhCl2]2 (51.3 mg, 0.083 mmol) and NH4PF6 (39.2 mg, 0.24 mmol) and the mixture was stirred overnight at room temperature. The solvent was removed under reduced pressure and the

residue was extracted with dichloromethane (10 mL). The extract was passed through a membrane filter and concentrated to dryness to give an orange powder of 1a, which was then dissolved in DMSO or DMF (3 mL) and stirred overnight. The solvent was removed under reduced pressure and the residue was extracted acetone. The extract was concentrated to ca. 3 mL and addition of hexane (1 mL) gave an orange powder of 1a. To the mother liquor was added Et2O (0.2 mL), which was allow to stand in refrigerator to deposit a small amount of red needle crystals of 4$2.5(CH3)2CO together with orange needle crystals of 1a. The formula of 4$2.5(CH3)2CO was determined by X-ray crystallography. 1H NMR (CD2Cl2): d 1.24 (d, 30H, Cp*, 4JPH ¼ 4 Hz), 1.96 (t, 15H, Cp*, 4JPH ¼ 4 Hz). 4.6. Preparation of [(Cp*Rh(CH3CN))2Ag2(OTf)2(mesodpmppm)](OTf)4 (5) To an acetonitrile solution (10 mL) containing 1a (86.0 mg, 0.058 mmol) was added AgOTf (46.7 mg, 0.18 mmol) in 15 mL of acetonitrile. The resultant mixture was heated at reflux for 25 h and then stirred at room temperature overnight. The solvent was removed under reduced pressure and the residue was extracted with dichloromethane (15 mL). The extract was passed through a membrane filter and concentrated to ca. 7 mL, to which NH4OTf (48.6 mg, 0.29 mmol) in acetone (10 mL) was added. The resultant mixture was stirred at room temperature. After a similar work-up, the acetone solution was concentrated to ca. 3 mL, and after addition of Et2O (3 mL), the solution was kept in refrigerator to afford yellow crystals of 5$4CH2Cl2, which were filtered off, washed with Et2O, and dried under vacuum. Yield 33% (31 mg). Anal. Calcd for C112H116Ag2Cl8F18N2O18P8Rh2S6 (5$4CH2Cl2): C, 41.19; H, 3.58; N, 0.86. Found: C, 41.14; H, 3.49; N, 0.90. IR (KBr): n 1436 (s), 1258 (m), 1159 (m), 1030 (s), 844 (s), 741 (s), 693 (m), 638 (s), 517 (m) cm1. 1H NMR (CD3CN): d 1.64 (t, 30H, Cp*, 4JPH ¼ 5 Hz), 1.24 (br, CH3CN), 3.2e4.3 (m, 12H, PCH2P), 6.5e7.7 (m, 60H, Ar). 31P{1H} NMR (CD3CN): d 15.0 to 2.0 (m, 4P), 13.2 (m, 2P, 1JRhP ¼ 198 Hz), 17.1 (m, 2P, 1JRhP ¼ 196 Hz). UVevis (CH2Cl2, r.t.): lmax (log 3 ) 362 (4.25), 453sh (3.49) nm. ESI-MS (in CH2Cl2): m/z 2736.078 (z ¼ 1, {[(Cp*2Rh2(CH3CN)Ag2(OTf)2(dpmppm)](OTf)3}þ (2735.999)), 2695.041 (z ¼ 1, {[(Cp*2Rh2Ag2(OTf)2(dpmppm)](OTf)3}þ (2694.972)). UVevis (CH2Cl2, r.t.): lmax (log 3 ) 336 (4.36) nm. Recrystallization of 5 from a CH2Cl2/Et2O solution afforded yellow crystals of 5$4.5CH2Cl2 suitable for X-ray crystallography. 4.7. X-ray crystallographic analysis The crystals of 1a$3CH2Cl2, 1b$3CH2Cl2, 2a$2.5CH2Cl2, 2b$2CH2Cl2, 4$2.5(CH3)2CO, and 5$4.5CH2Cl2 were quickly coated with Paratone N oil and mounted on top of a loop fiber at room temperature. The crystal and experimental data are summarized in Tables S1eS3 (see Supporting Information). All data were collected at 120  C (1a, 1b, 2a, 2b), 110  C (4), and 100  C (5) on a Rigaku VariMax Mercury CCD diffractometer equipped with graphitemonochromated Mo Ka radiation using a rotating-anode X-ray generator (50 kV, 200 mA). A total of 2160 oscillation images, covering a whole sphere of 6 < 2q < 55 , were collected by the uscan method. The crystal-to-detector (70  70 mm) distance was set at 60 mm. The data were processed using the Crystal Clear 1.3.5 program (Rigaku/MSC) [55] and corrected for Lorentz-polarization and absorption effects [56]. The structures were solved by direct methods with SHELXS-97 [57] (1a, 1b, 2a) and SIR-92 [58,59] (4, 5), and were refined on F2 with full-matrix least-squares techniques with SHELXL-97 [57] using the Crystal Structure 4.0 package [60]. All non-hydrogen atoms were refined with anisotropic thermal parameters, except the disordered PF6 anion of 5, and the CeH hydrogen atoms were calculated at ideal positions and refined with

T. Tanase et al. / Journal of Organometallic Chemistry 797 (2015) 37e45

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