Formation of a Hexacarbonyl Diiron Complex Having a Naphthalene-1

Nov 27, 2013 - The P–P bond of the cis-1 ligand in (μ-cis-1)[Fe(CO)4]2 (cis-1 = naphthalene-1,8-diphenyldiphosphine) was cleaved by the two iron ce...
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Formation of a Hexacarbonyl Diiron Complex Having a Naphthalene1,8-bis(phenylphosphido) Bridge and the Electrochemical Behavior of Its Derivatives Yuichi Teramoto, Kazuyuki Kubo, Shoko Kume, and Tsutomu Mizuta* Department of Chemistry, Graduate School of Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-hiroshima 739-8526, Japan S Supporting Information *

ABSTRACT: The P−P bond of the cis-1 ligand in (μ-cis1)[Fe(CO)4]2 (cis-1 = naphthalene-1,8-diphenyldiphosphine) was cleaved by the two iron centers after CO dissociation from the iron centers, although the P−P bond of cis-1 was stereochemically stabilized with a robust naphthalene group, unlike the usual diphosphines, which lack such support. The resulting (μ-nabip)[Fe(CO)3]2 (3; nabip = naphthalene-1,8bis(phenylphosphido)) had the diiron core linked by the bisphosphido bridge. Since the trans isomer (μ-trans-1)[Fe(CO)4]2 was stable under ambient conditions, the cis disposition of the two Fe(CO)4 fragments was responsible for the cleavage of the P−P bond. The one or two terminal CO ligands of 3 can be replaced by MeCN and a range of phosphine ligands: i.e., PMe3, PPh3, cis-1, and trans-1. Interestingly, it was found that the diphosphine cis-1 could coordinate the iron center in an unusual κ2 fashion to form a three-membered ring, which was confirmed by NMR spectra as well as X-ray analysis. These diiron complexes can be protonated with the strong acid TfOH in CH2Cl2 to form cationic complexes having a μ-H bridge between the two iron centers. The parent hexacarbonyl complex 3 could act as a proton reduction catalyst at −2.0 V in the presence of TsOH as the proton source in CH2Cl2. When protonated complexes having MeCN or phosphine ligands were used, the proton reduction potentials catalyzed by these complexes were shifted to a more positive range of around −1.77 to −1.37 V, depending on the terminal ligand.

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INTRODUCTION Diphosphines (R2P−PR2) are useful ligands which can bridge two metal centers in close proximity through coordination of metal fragments on the respective phosphorus centers linked directly with a P−P bond.1 The P−P bonds of the diphosphines are generally thermodynamically stable.2 For example, the P−P bond energy of Et2P−PEt2 is reported to be 86 kcal/mol.2e However, it is also well-known that the P−P bonds of diphosphine-bridged binuclear complexes can be readily cleaved to form bisphosphido complexes upon heating.3−6 To increase the thermodynamic stability of the P−P bond of diphosphine, we recently reported a novel diphosphine ligand, 1, in which two phosphorus atoms are locked with a robust naphthalene group, rendering the bond fairly stable.7 In fact, naphthalene-1,8-diphenyldiphosphine (1) works as a more thermally stable bridging ligand for group 6 and 11 metal fragments. As part of our ongoing effort to explore the properties of this ligand, a reaction of the cis isomer cis-1 with the group 8 metal fragment Fe(CO)4 was examined, with the expectation that the diiron complex cis-2 would be formed, as shown in Chart 1. To our surprise, however, the P−P bond of 1 was cleaved at room temperature by the low-valent iron carbonyl fragment to give the bisphosphido-bridged diiron carbonyl complex 3. Here, we report the details of the reaction conditions of this unusual P−P bond activation. © 2013 American Chemical Society

Chart 1

In addition, binuclear iron complexes have been attracting much attention, since they work as [FeFe]-hydrogenase mimics for proton reduction.8 Most of the studies in this field involve bisthiolato-bridged diiron complexes. Bisphosphido-bridged diiron complexes have also been investigated, but much less frequently than the bisthiolato complexes.9 Recently, Tilley et Received: June 26, 2013 Published: November 27, 2013 7014

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al. reported naphthalene-1,8-dithiolato diiron hexacarbonyl complexes, which are more stable than 1,3-propanedithiolato diiron hexacarbonyl complexes when they are electronically reduced.10 Since the naphthalene-1,8-dithiolato bridge has structural resemblance to the present phosphido bridge, the electrocatalytic activity of the present complex is also of interest.

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RESULTS AND DISCUSSION Preparation of Iron Carbonyl Complexes. The diphosphine ligand 1 has two stereogenic centers, and therefore there are two diastereomeric isomers, cis-1 and trans-1. The diphosphine cis-1 is a more useful bridging ligand for the preparation of a binuclear complex than trans-1, because the two phosphorus lone pairs of cis-1 adopt a syn-periplanar disposition, and thus cis-1 can arrange two metal centers in close proximity. However, the free cis-1 is thermodynamically less stable due to the steric congestion of the two Ph groups and therefore isomerizes to trans-1 when it is heated. The cis to trans isomerization proceeds gradually even under an ambient atmosphere. To prepare an iron binuclear complex using cis-1, mild reaction conditions are therefore required to avoid isomerization to trans-1. The diiron nonacarbonyl complex Fe2(CO)9 is known to dissociate an Fe(CO)4 fragment at room temperature, which rapidly binds the incoming ligand. Actually, Zenneck et al. reported that a strained diphosphine derived from a 1,2-diphosphacyclobutane derivative reacts with Fe2(CO)9 without a ring-opening reaction to give a diiron complex bridged by 1,2-diphosphacyclobutane.11 The reaction of cis-1 with 2 equiv of Fe2(CO)9 in THF at room temperature was completed within 1.5 h. A 31P{1H} NMR spectrum of the reaction mixture showed that the signal of free cis-1 disappeared at −19.4 ppm, and two new singlet signals appeared at 83.6 and 111.6 ppm with an intensity ratio of 2:1, suggesting the formation of two species. When a similar reaction was carried out using trans-1 in place of cis-1 (eq 1),

Figure 1. Molecular structure of trans-2 with thermal ellipsoids given at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Fe1−P1 2.2427(6), Fe2−P2 2.2321(7), P1P2 2.2508(8), P1−C1 1.824(2), P1−C11 1.824(2), P2−C8 1.819(2), P2−C17 1.829(2); C1−P1−Fe1 117.41(7), C11−P1−Fe1 117.11(7), C1−P1−P2 92.08(7), C11− P1−P2 102.64(7), Fe1−P1−P2 120.80(3), C8−P2−Fe2 115.56(7), C17−P2−Fe2 116.00(7), C8−P2−P1 93.07(7), C17−P2−P1 100.75(7), Fe2−P2−P1 124.73(3).

only one major signal was observed at 82.5 ppm, with several trace-level signals. In this case, the main product could be isolated and was characterized by 1H and 13C{1H} NMR spectra and determined by X-ray analysis to be the expected binuclear complex (μ-trans-1)[Fe(CO)4]2 (trans-2), as shown in Figure 1. Of the two chemical shifts in the case of the reaction with cis-1, the chemical shift at 83.6 ppm of the major species was almost identical with that of trans-2 at 82.5 ppm, suggesting that this major product was the expected (μ-cis1)[Fe(CO)4]2 (cis-2). While trans-2 was thermodynamically stable under ambient conditions, cis-2 was found to be gradually transformed in the solution to another product having a signal at 111.6 ppm. The latter product could be isolated, and its structure was determined by X-ray analysis, as shown in Figure 2. The structure revealed that the product is the bisphosphidobridged diiron hexacarbonyl complex 3, in which the P−P bond of the starting cis-1 is cleaved. Interestingly, this transformation was greatly accelerated upon irradiation of UV light, as depicted in Scheme 1. When a

Figure 2. Molecular structure of 3 with thermal ellipsoids given at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Fe1−Fe2 2.6366(3), Fe1−P1 2.2106(4), Fe1−P2 2.2133(4), Fe2−P1 2.1960(4), Fe2−P2 2.2071(4), P1−C11 1.8149(15), P1−C1 1.8203(15), P2−C8 1.8153(15), P2−C17 1.8171(15); P1−Fe1−P2 76.462(15), P1− Fe2−P2 76.889(15), Fe2−P1−Fe1 73.498(14), Fe2−P2−Fe1 73.233(13).

THF solution containing cis-2 as the main species in an NMR tube was irradiated with UV light for 1 min, its 31P{1H} NMR spectrum showed that about half the amount of starting cis-2 was consumed, and 3 and the unknown species 4 having a singlet at 44.9 ppm appeared with a 1:1 intensity ratio. After 7015

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identical with the −8.3 ppm of the free trans-1 ligand, while the former was close to that of the initial trans-2, suggesting that this compound is the mononuclear complex Fe(CO)4(trans-1), in which one of the two Fe(CO)4 fragments has dissociated from the starting complex. The observation of a much faster reaction for cis-2 was probably due to the two iron centers in close proximity. After the dissociation of the CO from octacarbonyl diiron complex 2, the transient species 4 starting from cis-2 was stabilized by the formation of the Fe−Fe bond and the change in the coordination mode of the CO group from terminal to bridging, as shown in Scheme 1. On the other hand, such a stabilizing effect was not possible for the transient species formed from trans-2, and thus the free CO was recoordinated. Although the route to the binuclear complex 3 from trans-2 is not clear at present, one plausible route would be a photochemical trans to cis conversion of the trans-1 ligand in the mononuclear complex Fe(CO)4(trans-1), which is formed by the dissociation of the Fe(CO)4 fragment from trans-2. We previously reported that the free trans-1 ligand was converted to cis-1 upon irradiation with UV light.7a If a similar inversion of the configuration takes place at the uncoordinating phosphorus center of Fe(CO)4(trans-1), Fe(CO)4(trans-1) would isomerize to Fe(CO)4(cis-1). Then, the recoordination of the Fe(CO)4 fragment to the cis mononuclear complex Fe(CO)4(cis-1) would give cis-2, which would lead to 3 smoothly. Displacement of CO with L. Binuclear iron carbonyl complexes with chelating dithiolato ligands have been intensively investigated as hydrogenase models. In addition, many of their derivatives in which the terminal carbonyl ligands are replaced with phosphorus ligands have been prepared so far in order to explore the electronic perturbation of the redox properties.13−15,8a,16,17 On the other hand, although a variety of analogues having chelating bisphosphido ligands are also known,9d,18−20 only two reports have described a complex having a phosphine ligand at its terminal position.9a,20a In the present complex 3, because of the rigid naphthalene group of the bridging nabip (nabip = naphthalene-1,8bis(phenylphosphido)), the core (μ-nabip)Fe2 moiety is thermodynamically stabilized, and the terminal CO groups of (μ-nabip)[Fe(CO)3]2 (3) can be replaced with MeCN by treatment with a slight excess of Me3NO in MeCN.14d,21 The 1 H NMR spectrum of the product shows the Me signal of MeCN with a 3H intensity, indicating that the product is the monosubstituted complex (μ-nabip)[Fe2(CO)5(NCMe)] (5MeCN). The 31P{1H} NMR spectrum of 5-MeCN in Figure S1 (Supporting Information) consists of two doublets at 114.1 and 120.5 ppm, which are coupled to each other with a large coupling constant of 203 Hz. This indicates that the two phosphorus centers of the nabip bridge are not equivalent, and the MeCN ligand occupies not an apical position but a basal position of the quasi-square-pyramidal iron moiety, as depicted in Chart 2. The CO signals in the 13C{1H} NMR spectrum shown in Figure S2 (Supporting Information) are also consistent with the basal coordination of MeCN. The two doublet of doublet signals due to the coupling with the two nonequivalent phosphorus centers are observed at 215.5 and 219.4 ppm for the two CO ligands coordinating to the same iron center having the MeCN ligand. The remaining three CO groups coordinating to the other iron center were observed as a broad signal, indicating the twist rotation of the Fe(CO)3 fragment, which is a typical phenomenon for hydrogenasemodel diiron hexacarbonyl complexes.13b,22

Scheme 1

another 4 min of irradiation, the intensity of cis-2 decreased significantly, and that of the product 3 increased until it was the main species, while the intensity of 4 was almost constant. After 15 min of irradiation in all, the signals of both cis-2 and 4 disappeared completely, and only that of 3 was observed. The results indicate that cis-2 was converted to 3 by a photochemical event, and as shown in Scheme 1, 4 was probably an intermediate of the transformation of cis-2 to 3. Since 4 is thought to be formed by photochemical dissociation of the CO ligand/s, a stoichiometric removal of the CO ligand from cis-2 was conducted by using Me3NO, and the reaction again gave 4, as expected. The IR spectrum of the reaction mixture containing 4 showed a band at 1752 cm−1, which was almost comparable to the band at 1754 cm−1 reported for (μ-CO)(μ-dppm)[Fe(CO)3]2, suggesting the presence of a bridging CO ligand in 4.12 The structure of 4 is therefore proposed to be as depicted in Scheme 1, where one CO ligand has been lost from the starting octacarbonyl diiron complex, cis-2, resulting in the formation of an Fe−Fe bond and the migration of one CO group from a terminal position to a bridging position.

Since the P−P bond of the diiron complex cis-2 was cleaved to give 3, we considered that a similar reaction of trans-2 may lead to the same complex 3, after the CO ligand was removed from each iron center of trans-2. As described above, however, the thermodynamically stable trans-2 did not give 3 under ambient conditions. To remove the CO ligand, photochemical conditions were applied for trans-2. Under these conditions, the reaction indeed proceeded but was much slower than that of cis-2. After 1 h of irradiation, the 31P{1H} NMR spectra of the solution revealed that one-third of the initial trans-2 remained in solution and 3 was formed in ca. 30% yield. This is in contrast to the rapid consumption in the case of cis-2. In addition, two doublets were observed at 73.6 and −8.2 ppm, and these were coupled to each other with a large coupling constant of JPP = 275 Hz. The latter chemical shift was almost 7016

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Chart 2

The labile MeCN ligand of 5-MeCN could be readily replaced with phosphine ligands in THF at room temperature or upon mild heating to around 45−65 °C. The 31P{1H} NMR spectrum of the PMe3 complex 5-PMe3 consists of a triplet signal of the PMe3 ligand and only one doublet signal of the bridging nabip ligand, indicating that PMe3 binds to the apical coordination site, as depicted in Chart 2. Similar spectroscopic properties were observed for the PPh3 analogue 5-PPh3, which is consistent with the apical coordination of the PPh3 ligand. The CO signal region in the 13C{1H} NMR spectra of 5-PMe3 and 5-PPh3 comprises a doublet of triplets and a broad multiplet with an intensity ratio of 2:3. The former doublet of triplets is assigned to the two CO ligands coupled with the tertiary phosphine and the two virtually equivalent bridging phosphorus centers, while the latter is assigned to the three CO ligands on the left iron center of 5-PR3 in Chart 2 and is broadened by a rotation of the Fe(CO)3 unit.13b,22,23 In addition to these typical monodentate phosphines, cis- and trans-1 were also allowed to react with 5-MeCN to give the corresponding diiron complexes 5-cis-1 and 5-trans-1. Figure 3 shows the molecular structure of 5-cis-1, in which cis-1 coordinates in a κ1 mode in which only one of the two phosphorus centers coordinates with the iron center. The trans analogue 5-trans-1 has NMR data similar to those of 5-cis-1. Dithiolato-bridged analogues (μ-S−S)[Fe2(CO)5(phosphine)] (S−S = dithiolato bridge) have a similar apical site selectivity for tertiary phosphines.17d,g,h In the present case, the Ph groups of the bridging nabip ligand additionally force the tertiary phosphine ligands to occupy the apical position. The molecular structure of the parent complex 3 shown in Figure 2 indicates that each Ph group on the bridging phosphorus centers is oriented horizontally to avoid steric repulsion with the C−H groups of the vertical naphthalene moiety. As a result, the basal coordination sites of each quasi-square-pyramidal iron moiety are sterically more congested than the apical site. The averaged distance from the ortho hydrogen atoms of these Ph groups to the carbon atom of the nearest basal CO ligand is 2.90 Å, while that to the apical

Figure 3. Molecular structure of 5-cis-1 with thermal ellipsoids given at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Fe1−P1 2.2258(4), Fe1−P2 2.2186(4), Fe1−P3 2.2063(4), Fe1−Fe2 2.6561(3), Fe2−P1 2.2021(4), Fe2−P2 2.2187(4), P1−C1 1.8154(15), P1−C11 1.8193(15), P2−C8 1.8175(15), P2−C17 1.8319(15), P3−P4 2.2569(5), P3−C28 1.8325(15), P3−C38 1.8315(15), P4−C35 1.8217(16), P4−C44 1.8279(16); P2−Fe1−P1 76.178(14), P3− Fe1−P1 107.145(16), P3−Fe1−P2 102.065(15), P1−Fe2−P2 76.658(15), Fe2−P1−Fe1 73.716(13), Fe1−P2−Fe2 73.538(13), Fe1−P3−P4 112.134(19).

CO ligand is 3.47 Å. To avoid steric congestion with the Ph group, ligands that are more sterically demanding than the CO ligand, such as PMe3, PPh3, and diphosphines cis- and trans-1, preferentially occupy the apical position in the monophosphine complexes, while the less sterically demanding MeCN ligand may occupy the basal site. Further replacement of the CO ligand was conducted by a photochemical reaction. Irradiation of 3 in MeCN by UV−vis light for 19 h with bubbling N2 gave the disubstituted (μnabip)[Fe2(CO)4(MeCN)2] (6-MeCN) in 97% yield. The 1H 7017

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NMR of 6-MeCN shows a chemically equivalent Me signal at 0.65 ppm with a 6H intensity. In the 13C{1H} NMR spectrum, the four remaining CO ligands reveal two triplets at 218.7 and 219.7 ppm with an intensity ratio of 3:1, as shown in Figure S2 (Supporting Information), indicating that 6-MeCN adopts the structure depicted in Chart 2, in which both MeCN ligands coordinate to the same iron center at the basal coordination sites. Both MeCN ligands of 6-MeCN could be replaced with a PMe3 ligand to give the bisphosphine complex (μ-nabip)[Fe2(CO)4(PMe3)2] (6-PMe3). Although both MeCN ligands coordinate with one Fe center in the initial 6-MeCN, the small P−P coupling constant between the two PMe3 ligands reveals that the two PMe3 ligands coordinate to each iron center one by one. This can be explained if the steric demand of PMe3 is taken into consideration. In the 31P{1H} NMR spectrum of 6PMe3, neither the two PMe3 ligands nor the two bridging phosphido centers are equivalent. It has been reported that (μpdt)[Fe2(CO)4(PMe3)2] (pdt = 1,3-propanedithiolato) has two 31 P NMR signals at 24.3 and 28.3 ppm due to the PMe3 ligands at the apical and basal positions.17e,24 The present two 31P signals at 8.8 and 22.5 ppm assigned to the PMe3 may result from a similar disposition, with the PMe3 ligands at the apical and basal positions. The reason both PMe3 ligands do not occupy an apical position is not clear at present. However, in the case of the more bulky PPh3 analogue (μ-nabip)[Fe2(CO)4(PPh3)2] (6-PPh3), both PPh3 ligands are bound to the apical sites of the respective iron centers to make the two PPh3 ligands as well as the two bridging phosphidos equivalent. The 31P{1H} NMR spectrum thus becomes simpler than that of 6-PMe3, as shown in Figure S1 (Supporting Information), which indicates that the two triplets at 62.8 and 67.8 ppm are assigned to the PPh3 and μ-nabip ligands, respectively. Since the diphosphine cis-1 has two lone pairs by which cis-1 potentially works as a bidentate ligand, the reaction of cis-1 with 6-MeCN, having two labile MeCN ligands, is of interest. The product shows the 31P{1H} NMR signals at 143.1 and −62.3 ppm, which are assigned to the μ-nabip and κ2-cis-1 ligands, respectively. The significant high-field chemical shift of the κ2cis-1 ligand suggests that cis-1 forms a three-membered ring by a κ2 coordination to one of the two iron centers. Such a highfield shift of a κ2-diphosphine was previously reported for Rh(NacNac)(κ 2 -Ph 2 P−PPh 2 ) (NacNac = HC(CMeN(iPr2C6H2))2),25 in which the 31P{1H} NMR signal of κ2Ph2P−PPh2 was observed at −51.4 ppm. The molecular structure of (μ-nabip)[Fe2(CO)4(κ2-cis-1)] (8-cis-1) was finally confirmed by X-ray analysis as shown in Figure 4, where both phosphorus centers of cis-1 bind to the basal sites of the iron center. Electrochemistry of 3 and Its Derivatives. The redox behavior of the present bisphosphido-bridged diiron complex 3 was examined by cyclic voltammetry measurements, as shown in Figure 5. The CV of 3 consists of a well-defined single redox couple at Epc = −2.14 V and Epa = −1.86 V vs Fc+/0. A relevant bisphosphido-bridged complex, {μ-PhP(CH 2 ) 3 PPh}[Fe(CO)3]2, has been reported to show a two-electron redox wave at a similar potential of −2.08 V,9d while the naphthalene1,8-dithiolato analogue (μ-1,8-S2C10H6)[Fe(CO)3]2 has two well-separated reduction waves at more positive potentials, −1.76 and −2.00 V.10 Since these relevant complexes have one two-electron or two very close one-electron reduction potential(s) for the two-electron reduction, the one reduction wave observed for the present complex 3 is considered to be

Figure 4. Molecular structure of 6-cis-1 with thermal ellipsoids given at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Fe1−P1 2.1417(5), Fe1−P2 2.2390(5), Fe1−Fe2 2.6785(5), Fe2−P1 2.2268(5), P1−C1 1.8249(18), P1−C7 1.825(2), P2−P2 2.1820(9), P2−C16 1.8172(17), P2−C22 1.8151(18); P1−Fe1−P1 79.70(3), P1−Fe1− P2 103.886(18), P1−Fe1−P2 149.82(2), P2−Fe1−P2 58.32(2), P2− Fe1−Fe2 103.944(17), P1−Fe2−P1 76.09(3), Fe1−P1−Fe2 75.605(19), P2−P2−Fe1 60.839(12).

Figure 5. Cyclic voltammograms (100 mV/s) of 3 (1 mM) in CH2Cl2/0.1 M NBu4PF6 with 0−6 equiv of TsOH. All potentials are given with reference to the ferrocene +/0 couple.

the two-electron process which consists of two one-electron processes having potentials very close to each other. The significantly large ΔEp (ca. 280 mV, 100 mV s−1) also supports the two one-electron processes (eq 2).26,27 The finding that the reduction potential for 3 was more negative than that for the dithiolato analogue was probably due to the greater electrondonating ability of the present bisphosphido bridge in comparison to the naphthalene-1,8-dithiolato bridge. 7018

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ppm indicates that the hydride occupies a bridging site between the two iron centers.24,30d In the 13C{1H} NMR spectrum (Figure S5, Supporting Information), the six CO ligands showed two triplets at 202.2 ppm (4C, JCP = 2 Hz) and 203.8 ppm (2C, JCP = 17 Hz), which were assigned to the basal and apical CO ligands, respectively, on the basis of the relative intensities. That finding that the JCP of the apical CO ligands was larger than that of the basal CO ligand was consistent with the previously reported larger coupling constant with the phosphorus center at the cis position. Since the CO ligands of the neutral 3 were observed as a single broad signal at 212.2 ppm due to a rotation of the Fe(CO)3 unit, the presence of the μ-hydride ligand seems to make the rotational barrier of the Fe(CO)3 units greater. Other diiron complexes having one or two terminal ligands, such as MeCN, PMe3, PPh3, and cis-1, could be similarly protonated by adding a slight excess amount of TfOH to give μ-hydride complexes, as indicated by the hydride signals shown in Figure S4 (Supporting Information). In the case of 6-PPh3H+, two species were observed, probably due to the isomers having different dispositions of phosphine ligands. The 31P{1H} NMR spectrum of the major isomer in Figure S3 (Supporting Information) consists of the two nonequivalent PPh3 signals and the two nonequivalent phosphido signals. Interestingly, its hydride signal was observed as an unusual splitting pattern, which could be reproduced by an NMR simulation conducted while taking the exchange process between the two PPh3 signals into account (Figure S6, Supporting Information). This isomer of 6-PPh3H+ was considered to have the PPh3 ligand coordinating with the respective iron centers one by one at the apical and basal positions. The other isomer showed simple 1H and 31P{1H} NMR spectra, indicating the higher molecular symmetry of this isomer. The hydride was observed as a triplet of triplets, and the two PPh3 and two phosphido centers were triplets which were mutually coupled. This second isomer of 6-PPh3H+ was therefore considered to have the PPh3 ligand at the apical positions of each iron center. Two species were also observed for 6-cis-1H+, probably due to the isomers having different dispositions of cis-1, in which the positions of the naphthalene and two Ph groups are switched. The catalytic reduction of protons was examined by using these TfO− salts of the monoprotonated cationic complexes and TsOH as the proton source in CH2Cl2. The potentials observed are summarized in Table 1. The monosubstituted

The reduction of complex 3 by Na in THF successfully formed a diamagnetic species showing a sharp 31P{1H} signal at −32.8 ppm.28 The significant upfield shift of over 140 ppm from 111.6 ppm of the starting 3 suggested a cleavage of the Fe−Fe bond in 3 and the formation of the dianionic complex (μ-nabip)[Fe(CO)3]22−. The νCO band in the IR spectrum of the product was shifted to a lower wavenumber of around 1700−1930 cm−1 in comparison with the shift of the same band of 3 to a wavenumber of around 1972−2053 cm−1, which is consistent with the negative charge of the product. When the THF solution of this product was exposed to air, the starting 3 was almost completely recovered, as confirmed by the 31P{1H} signal at 111.6 ppm of 3. The oxidation of the dianionic species was also possible by the reaction with CF3COOH. In this case, bubbling of gas, probably dihydrogen, was observed owing to the reduction of protons. The chemically reversible oxidation and reduction processes of the diiron complex 3 revealed that complex 3 could potentially function as a redox catalyst for the electrochemical reduction of protons. Actually, as shown in Figure 5, the CV measurement of 3 (1 mM) in the presence of p-toluenesulfonic acid (p-TsOH) in CH2Cl2 revealed that the current of the reduction wave became larger as the amount of p-TsOH increased, when p-TsOH was added to the reaction cell as a proton source. This catalytic process was observed over −2.0 V (vs Fc+/Fc), which is a value more negative than that for the dithiolato analogue (μ-1,8-S2C10H6)[Fe(CO)3]2.10 The greater electron-donating ability of the μ-nabip bridge relative to that of the dithiolato bridge probably made the potential of the catalytic process more negative. However, further accumulation of the electron density at the iron center would increase the basicity of the iron center enough to protonate prior to the reduction. Since such a cationic monoprotonated diiron complex can be reduced at a potential more positive than that for its neutral counterpart, the redox properties of the monoprotonated forms of singly substituted 5 and disubstituted 6 are of importance.17e,24,29,8a,30 First, protonation reactions were examined for the parent complex 3 and its mono- and disubstituted derivatives 5 and 6. The protonation of 3 with 1 equiv of TsOH in CH2Cl2 did not proceed, probably because 3 is not basic enough to receive the proton from TsOH. However, after addition of over 10 equiv of TsOH in CH2Cl2, the 31P{1H} NMR spectrum showed that a new minor signal of the protonated species 3H+ appeared at 107.1 ppm in addition to 110.1 ppm for 3 with an intensity ratio of 1:4. With an excess amount (ca. 4 equiv) of the stronger acid TfOH (TfOH = CF3SO3H), the signal of the starting 3 almost completely disappeared and the signal at 107.1 ppm became the main signal, as shown in Figure S3 (Supporting Information). In the 1H NMR spectrum of this product 3H+, a hydride signal was observed at −16.6 ppm as a triplet that was coupled with the two equivalent bridging phosphorus centers (JHP = 39 Hz) (Figure S4, Supporting Information). The chemical shift at a field higher than −10

Table 1. Reduction Potentials (V vs Fc+/Fc) of Complexes Recorded in CH2Cl2/NBu4PF6 (0.1 M) with a 0.1 V s−1 Sweep Rate complex

Ep/V

complex

Ep/V

3 5-MeCNH+ 5- PMe3H+ 5- PPh3H+

−2.14 −1.36, −1.90 −1.54 −1.34, −1.92

6-MeCNH+ 6- PMe3H+ 6- PPh3H+ 6-cis-1H+

−1.68 −1.77 −1.54 −1.37

complexes showed reduction potentials at −1.34 to −1.54 V, which were greatly shifted in a positive direction in comparison to the reduction potential of −2.0 V for the parent 3. However, 5-MeCNH+ and 5-PPh3H+ in TsOH had a second reduction potential at −1.90 and −1.92 V, respectively. Since the increments of the peak height of these second reduction 7019

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wave are much higher than those of the first peaks, as shown in Figures S10 and S12 (Supporting Information) for 5-MeCNH+ and 5-PPh3H+, respectively, the protonation to the hydride leading to the catalytic cycle is considered to take place, after the second electron is transferred to the neutral monohydride diiron complexes formed by the first reduction step. On the other hand, 5-PMe3H+, which has a more electron donating PMe3 in comparison to MeCN and PPh3 of 5-MeCNH+ and 5PPh3H+, respectively, shows a single reduction step at the more negative potential of −1.54 V, in comparison to −1.36 and −1.34 V of 5-MeCNH+ and 5-PPh3H+, respectively. The results suggest that the hydride ligand in the neutral form of 5PMe3H+ is basic enough to be protonated without further reduction. The disubstituted 6-MeCNH+, 6-PMe3H+, and 5PPh3H+ indicate only one catalytic wave at more negative potentials of −1.54 to −1.77 V in comparison those of monosubstituted complexes, because the disubstituted species 6H+ are more electron rich than the monosubstituted species. It is noteworthy that 6-cis-1H+ worked at −1.37 V, although it is a disubstituted species. The strained coordination mode of the bidentate cis-1 was probably responsible for the relatively positive potential, because the two phosphorus centers could not donate their lone-pair electrons fully.

out using Pyrex-glass-filtered emission from a 400 W mercury arc lamp (Riko-Kagaku Sangyo UVL-400P). The emission lines used and their relative intensities (in parentheses) were 577.0 (69), 546.1 (82), 435.8 (69), 404.7 (42), 365.0 (100), 334.1 (7), 312.6 (38), and 302.2 (9) nm. Cyclic voltammetry experiments were controlled using a HOKUTO DENKO HAB-151 potentiostat. The working electrode was a 3.0 mm o.d. glassy-carbon electrode; a platinum wire served as the auxiliary electrode, and the reference electrode was an Ag+/Ag electrode (a silver wire immersed in 0.1 M Bu4NClO4/0.01 M AgClO4/CH3CN). Measured potentials are reported relative to the ferrocenium(1+)/ ferrocene(0) couple. The solutions were deoxygenated with nitrogen prior to measurement. Preparation of (μ-nabip)[Fe(CO)3]2 (3). A Schlenk tube was charged with cis-1 (909 mg, 2.65 mmol), Fe2(CO)9 (2.00 g, 5.50 mmol), and THF (50 mL). After the solution was stirred for 2.5 h, it was filtered to remove a small amount of precipitate, which was washed with ether (70 mL). The THF filtrate and ether washings were combined, and the mixture in a Schlenk tube was irradiated with a mercury arc lamp for 4.5 h until the reaction was completed. After the solvents were removed in vacuo, the residue was loaded into an Al2O3 column and eluted with CH2Cl2/hexane 1/1. The main yellow band was collected, and the solvents were evaporated in vacuo. The product was recrystallized from hot toluene. After the solution was stored for 2 days in a refrigerator, a precipitate was collected and washed with hexane. The product was dried in vacuo to give an orange powder of 3 (1.621 g, 98%). 1H NMR (300.5 MHz, 297 K, CDCl3): δ 6.91 (dt, 3 JHH = 6.0 Hz, JHP = 12.0 Hz, 2H, 2,7-Naph), 7.26 (t, 3JHH = 6.0 Hz, 2H, 3,6-Naph), 7.62 (br, 6H, p-Ph and m-Ph), 7.86 (br, 6H, o-Ph and 4,5-Naph). 13C{1H} NMR (75.6 MHz, 297 K, CDCl3): δ 125.3 (t, 3JCP = 6 Hz, 3,6-Naph), 128.9 (t, 3JCP = 5 Hz, m-Ph), 129.0 (t, JCP = 8 Hz, Naph), 130.7 (t, 1JCP = 19 Hz, 1,10-Naph), 131.1 (s, p-Ph), 132.0 (s, 4,5-Naph), 132.4 (s, 2,7-Naph), 133.2 (t, JCP = 9 Hz, Naph), 133.8 (t, 1 JCP = 23 Hz, ipso-Ph), 135.5 (t, 2JCP = 4 Hz, o-Ph), 212.2 (br, CO). 31 1 P{ H} NMR (121.7 MHz, 297 K, CDCl3): δ 110.7 (s). IR (THF): νCO (cm−1) 2053, 2015, 1987, 1972. Anal. Calcd for C28H16Fe2O6P2: C, 54.06; H, 2.59. Found: C, 53.93; H, 2.47. Preparation of (μ-trans-1)[Fe(CO)4]2 (trans-2). A Schlenk tube was charged with trans-1 (49 mg, 0.14 mmol), Fe2(CO)9 (108 mg, 0.30 mmol), and THF (10 mL). After the solution was stirred for 1 h, the turbid mixture became a clean solution. After the volatiles were removed in vacuo, the residue was extracted with ether (3 × 10 mL), and then the solvent was removed in vacuo. The residue was loaded into an Al2O3 column and eluted with CH2Cl2/hexane 1/2. The main yellow band was collected, and workup gave a yellow powder of trans2 (81 mg, 83%). 1H NMR (300.5 MHz, 297 K, CDCl3): δ 7.15 (dt, 3 JHP = 13.4 Hz, 3JHH = 6.7 Hz, 4H, o-Ph), 7.32 (dd, 3JHH = 7.3 Hz, 3JHH = 6.6 Hz, 4H, m-Ph), 7.43 (t, 3JHH = 7.3 Hz, 2H, p-Ph), 7.89 (ddm, 3 JHH = 8.1 Hz, 3JHH = 5.9 Hz, 2H, 3,6-Naph), 8.10 (dt, 3JHH = 5.9 Hz, 3 JHP = 11.8 Hz, 2H, 2,7-Naph), 8.25 (d, 3JHH = 8.1 Hz, 2H, 4,5-Naph). 13 C{1H} NMR (75.6 MHz, CDCl3): δ 128.6 (t, JCP = 5 Hz, m-Ph), 128.7 (t, JCP = 6 Hz, 3,6-Naph), 131.3 (s, 4,5-Naph), 131.6 (s, p-Ph), 131.7 (t, JCP = 6 Hz, Naph), 133.1 (t, JCP = 21 Hz, 1,8-Naph), 133.7 (t, JCP = 7 Hz, o-Ph), 133.8 (t, JCP = 6 Hz, 2,7-Naph), 134.9 (t, JCP = 20 Hz, ipso-Ph), 138.0 (t, JCP = 6 Hz, Naph), 212.1 (t, JCP = 7 Hz, CO). 31 1 P{ H} NMR (121.7 MHz, CDCl3): δ 82.8 (s). IR (THF): νCO (cm−1) 2058, 2049, 1977, 1954, 1939. Anal. Calcd for C30H16Fe2O8P2: C, 53.14; H, 2.38. Found: C, 53.13; H, 2.45. Preparation of (μ-cis-1)[Fe(CO)4]2 (cis-2). A Schlenk tube was charged with cis-1 (22 mg, 0.064 mmol), Fe2(CO)9 (51 mg, 0.14 mmol), and THF (5 mL). After the solution was stirred for 1.5 h, volatiles were removed in vacuo. The residue was extracted with ether, and the extracts contains cis-2 and 3 in a ratio of 4:1. The solvent was removed in vacuo. The residue was loaded into an Al2O3 column and eluted with CH2Cl2/hexane 1/2. The main yellow band of cis-2 followed by the first yellow band of 3 was collected, and workup gave a crude yellow powder of cis-2 (81 mg, 83%), which was contaminated with 3. Since cis-2 was found to spontaneously transform to 3, only 31 1 P{ H} NMR and IR (THF) data were recorded. 31P{1H} NMR

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CONCLUSIONS The thermodynamically stabilized P−P bond with a robust naphthalene group was cleaved by the two iron centers, after CO dissociation from the diiron octacarbonyl complex (μ-cis1)[Fe(CO)4]2. The resulting (μ-nabip)[Fe(CO)3]2 (3) had the d ii r o n c o r e l i n k e d b y t h e n a p h t h a l e n e - 1 , 8 - b i s (phenylphosphido) bridge. Since the trans isomer (μ-trans1)[Fe(CO)4]2 was stable under ambient conditions, the cis disposition of the two Fe(CO)4 fragments was responsible for the cleavage of the P−P bond, which was initiated by the loss of CO from each iron center. The terminal CO ligand of 3 can be replaced by MeCN and a range of phosphine ligands: PMe3, PPh3, and cis- and trans-1. Interestingly, it was found that the diphosphine cis-1 could coordinate the iron center in a κ2 fashion to form a threemembered ring. These diiron complexes can be protonated with the strong acid TfOH in CH2Cl2 to form cationic complexes having μ-H between the two iron centers. The hexacarbonyl complex 3 could function as a proton reduction catalyst at −2.0 V in the presence of TsOH as the proton source in CH2Cl2. When the protonated complexes were used, the potential was improved to the more positive range of −1.77 to −1.37 V, depending on the terminal ligand.

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EXPERIMENTAL SECTION

General Comments. All manipulations of the reactions were carried out under an atmosphere of dry nitrogen using Schlenk tube techniques. All solvents were dried and distilled from sodium (for hexane and toluene), sodium/benzophenone (for ether and THF), or P2O5 (for MeCN, CH2Cl2, and CHCl3). These purified solvents were stored under an N2 atmosphere. cis- and trans-17a and Fe2(CO)931 were prepared according to the methods in the literature. Me3NO was dehydrated by an azeotropic distillation from DMF. Other reagents were used as received. NMR spectra were recorded on a JEOL LA-300 spectrometer. 1H and 13C NMR chemical shifts were reported relative to Me4Si and were determined by reference to the residual solvent peaks. 31P NMR chemical shifts were reported relative to H3PO4 (85%) used as an external reference. Elemental analyses were performed with a PerkinElmer 2400CHN elemental analyzer. Photolysis was carried 7020

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(121.7 MHz, THF): δ 83.6 (s). IR (THF): νCO (cm−1) 2065, 2058, 2048, 1981, 1966, 1954, 1944. Photochemical Reaction of cis-2. The crude cis-2 (10 mg) containing 3 was dissolved in THF (2 mL), and the solution was sealed in a 5 mm i.d. NMR tube. The tube was irradiated with a mercury arc lamp, and the 31P{1H} NMR spectra were recorded. The 31 P{ 1H} NMR spectra are given in Figure S7 (Supporting Information). Photochemical Reaction of trans-2. A reaction was carried out in a manner similar to that previously described, but starting from trans-2 (20 mg), and the 31P{1H} NMR spectra of the reaction mixture were monitored, which are given in Figure S8 (Supporting Information). Reaction of cis-2 with Me3NO. The crude cis-2 (10 mg, ca. 0.010 mmol) containing 3 was dissolved in MeCN (5 mL). Me3NO (1.8 mg, 0.016 mmol) was added to the solution, and the 31P{1H} NMR spectra were recorded, which are given in Figure S9 (Supporting Information). Preparation of (μ-nabip)[Fe2(CO)5(MeCN)] (5-MeCN). A Schlenk tube was charged with 3 (603 mg, 0.969 mmol), Me3NO (91 mg, 1.2 mmol), and MeCN (40 mL). After the solution was stirred for 1 h at 60 °C, volatiles were removed in vacuo. The residue was loaded into an Al2O3 column and eluted with MeCN. The main red band was collected, and workup gave a dark red powder of 5-MeCN (610 mg, 99%). 1H NMR (300.5 MHz, 297 K, CDCl3): δ 1.62 (br, 3H, NCMe), 6.70 (m, J = 6.4 Hz, J = 12.4 Hz, 1H), 7.00−7.40 (m, 4H), 7.59 (br, 6H), 7.83 (br, 4H), 8.04 (br, 1H). 13C{1H} NMR (75.6 MHz, 297 K, CDCl3): δ 3.6 (s, CH3), 124.8 (d, 3JCP = 6 Hz, 3,6Naph), 125.0 (d, 3JCP = 6 Hz, 3,6-Naph), 127.8 (d, 3JCP = 10 Hz, mPh), 128.3 (d, 3JCP = 10 Hz, 2C, m-Ph), 129.3 (d, 3JCP = 9 Hz, m-Ph), 129.5 (t, JCP = 13 Hz, Naph), 129.9 (s, p-Ph), 130.2 (d, 4JCP = 2 Hz, pPh), 130.8 (d, 4JCP = 4 Hz, 4,5-Naph), 131.0 (d, 4JCP = 4 Hz, 4,5Naph), 131.1 (d, 2JCP = 4 Hz, 2,7-Naph), 131.2 (d, 2JCP = 2 Hz, 2,7Naph), 132.0 (dd, JCP = 11 Hz, 1JCP = 28 Hz, ipso-Ph), 133.4 (t, JCP = 7 Hz, Naph), 133.5 (dd, JCP = 7 Hz, 1JCP = 22 Hz, ipso-Ph), 135.2 (br, oPh), 135.5 (br, o-Ph), 136.6 (d, 2JCP = 10 Hz, o-Ph), 136.9 (d, 2JCP = 8 Hz o-Ph), 215.0 (t, 2JCP = 4 Hz, 3CO), 215.5 (dd, 2JCP = 10 Hz, 2JCP = 12 Hz, CO), 219.4 (dd, 2JCP = 19 Hz, 2JCP = 26 Hz, CO). 31P{1H} NMR (121.7 MHz, 297 K, CDCl3): δ 114.1 (d, JPP = 200 Hz), 120.5 (d, JPP = 203 Hz). IR (CH2Cl2): νCO (cm−1) 2021, 1979, 1944, 1921. Anal. Calcd for C29H19Fe2NO5P2: C, 54.84 H, 3.02; N, 2.21. Found: C, 54.66; H, 3.21; N, 2.28. Preparation of (μ-nabip)[Fe2(CO)5(PMe3)] (5-PMe3). To a THF (15 mL) solution of 5-MeCN (104 mg, 0.164 mmol) was added a ether solution of PMe3 (0.4 mmol). After the mixture was stirred for 3 h at 45 °C, volatiles were removed in vacuo. The residue was washed with hexane (3 × 10 mL) and dried in vacuo to give 5-PMe3 as a powder almost quantitatively (>99%). 1H NMR (300.5 MHz, 297 K, CDCl3): δ 1.15 (d, 2JPH = 8.6 Hz, 9H, PMe3), 6.90 (ddt, 4JHH = 1.0 Hz, 3 JHH = 7.3 Hz, 3JHP = 7.1 Hz, 2H, 2,7-Naph), 7.19 (t, 3JHH = 7.3 Hz, 3 JHH = 8.1 Hz, 2H, 3,6-Naph), 7.56 (br, 6H, Ph), 7.78 (dd, 2H, 4JHH = 1.4 Hz, 3JHH = 8.1 Hz, 2H, 4,5-Naph), 7.81 (br, 2H, o-Ph), 7.90 (br, 2H, o-Ph). 13C{1H} NMR (75.6 MHz, 297 K, CDCl3): δ 23.1 (dd, 1 JCP = 27 Hz, 3JCP = 2 Hz, PCH3), 125.2 (t, 3JCP = 6 Hz, 3,6-Naph), 128.3 (t, 3JCP = 5 Hz, m-Ph), 129.0 (t, 3JCP = 5 Hz, m-Ph), 129.8 (t, JCP = 12 Hz, Naph), 130.3 (s, p-Ph), 130.8 (s, 4,5-Naph), 131.7 (s, 2,7Naph), 133.1 (t, JCP = 9 Hz, Naph), 133.8 (dt, 3JCP = 2 Hz, 1JCP = 15 Hz, 1,8-Naph), 134.0 (t, 2JCP = 5 Hz, o-Ph), 135.2 (t, 1JCP = 22 Hz, ipso-Ph), 137.1 (br, Ph, o-Ph), 214.1 (m, 3CO), 217.1 (dt, 2JCP = 3 Hz, 2 JCP = 3 Hz, 2CO). 31P{1H} NMR (121.7 MHz, 297 K, CDCl3): δ 19.1 (t, 2JPP = 33 Hz, PMe3), 96.0 (d, 2JPP = 33 Hz, μ-P). IR (CH2Cl2): νCO (cm−1) 2026, 1966, 1940, 1921. Anal. Calcd for C30H25Fe2O5P3: C, 53.77; H, 3.76. Found: C, 53.68; H, 3.91. Preparation of (μ-nabip)[Fe2(CO)5(PPh3)] (5-PPh3). A Schlenk tube was charged with 5-MeCN (148 mg, 0.233 mmol), PPh3 (92 mg, 0.35 mmol), and THF (8 mL). After the solution was stirred for 5 h at 65 °C, volatiles were removed in vacuo. The residue was dissolved in CH2Cl2/hexane 1/1 to give a suspension. The supernatant and the residue were separately loaded into Al2O3 columns, and the former was eluted with CH2Cl2/hexane 1/1, while the latter was eluted with

CH2Cl2. The main yellow band of each column was collected, and they were combined. After workup, 5-PPh3 was obtained as a yellow powder (174 mg, 87%). 1H NMR (300.5 MHz, 297 K, CDCl3): δ 6.73 (ddt, 4JHH = 1.1 Hz, 3JHH = 7.1 Hz, 3JHP = 7.1 Hz, 2H, 2,7-Naph), 7.00 (br, 2H, m-Ph), 7.05−7.24 (m, 19H, PPh3, Ph, 3,6-Naph), 7.43 (t, JHH = 7.5 Hz, 2H, p-Ph), 7.45 (br, 2H, o-Ph), 7.75 (dd, JHH = 8.3 Hz, JHH = 1.5 Hz, 2H, 4,5-Naph), 7.76 (br, 2H, o-Ph). 13C{1H} NMR (75.6 MHz, 297 K, CDCl3): δ 124.8 (t, 3JCP = 5 Hz, 3,6-Naph), 127.8 (br, m-Ph), 127.9 (d, 3JCP = 9 Hz, PPh3(m-)), 128.5 (br, m-Ph), 129.1 (d, 4 JCP = 2 Hz, PPh3(p-)), 129.6 (t, JCP = 12 Hz, Naph), 129.9 (s, p-Ph), 130.7 (s, 4,5-Naph), 132.5 (s, 2,7-Naph), 132.6 (d, 2JCP = 11 Hz, PPh3(o-)), 133.0 (t, JCP = 9 Hz, Naph), 133.5 (br, o-Ph), 135.5 (t, 1JCP = 21 Hz, ipso-Ph), 137.4 (br, o-Ph), 138.5 (dt, 3JCP = 2 Hz, 1JCP = 38 Hz, PPh3(ipso-)), 213.4 (m, 3CO), 217.7 (dt, 2JCP = 3 Hz, 2JCP = 7 Hz, 2CO). 31P{1H} NMR (121.7 MHz, 297 K, CDCl3): δ 65.0 (t, 2JPP = 27 Hz, PPh3), 89.7 (d, 2JPP = 27 Hz, μ-P). IR (CH2Cl2): νCO (cm−1) 2028, 1968, 1942, 1924. Anal. Calcd for C45H31Fe2O5P3: C, 63.12; H, 3.65. Found: C, 62.74; H, 4.00. Preparation of (μ-nabip)[Fe2(CO)5(cis-1)] (5-cis-1). A Schlenk tube was charged with 5-MeCN (211 mg, 0.332 mmol), cis-1 (179 mg, 0.523 mmol), and ether (30 mL). After the solution was stirred for 6 days at room temperature, the precipitate was separated and washed with ether (6 × 10 mL). The product was dried in vacuo to give 5-cis-1 (211 mg, 70%). 1H NMR (300.5 MHz, 297 K, CDCl3): δ 5.94 (bt, J = 7.9 Hz, 2H), 6.58 (dt, J = 1.0 Hz, J = 7.6 Hz, 2H), 6.64 (dm, J = 7.2 Hz, 1H), 6.70 (bt, J = 7.3 Hz, 2H), 6.71−6.86 (m, 4H), 6.91−7.02 (m, 2H), 7.08 (t, J = 7.7 Hz, 1H), 7.19 (dt, J = 2.8 Hz, J = 7.6 Hz, 1H), 7.31 (dt, J = 1.1 Hz, J = 6.7 Hz, 1H), 7.37 (dt, J = 2.6 Hz, J = 7.5 Hz, 2H), 7.53−7.4(m, 5H), 7.58 (dm, J = 8.1 Hz, 2H), 7.63−7.69 (m, 4H), 7.79 (m, 2H), 7.92 (bt, J = 9.0 Hz, 1H). 13C{1H} NMR (75.6 MHz, 297 K, CDCl3): δ 124.9 (d, JCP = 5 Hz), 125.0 (d, JCP = 4 Hz), 126.8 (d, JCP = 9 Hz), 127.1 (d, JCP = 7 Hz), 127.4 (s), 127.5 (d, JCP = 7 Hz), 127.5 (s), 127.5 (d, JCP = 6 Hz), 127.9 (d, JCP = 11 Hz), 128.1 (d, JCP = 3 Hz), 128.1 (s), 128.4 (br), 128.5 (d, JCP = 8 Hz), 128.6 (d, JCP = 5 Hz), 129.4 (t, JCP = 12 Hz), 130.0 (d, JCP = 5 Hz), 130.1 (d, JCP = 4 Hz), 130.7 (s), 130.7 (s), 131.1 (dd, JCP = 6 Hz, JCP = 24 Hz), 131.5 (d, JCP = 9 Hz), 131.7 (s), 131.9 (s), 132.0 (d, JCP = 3 Hz), 132.3 (d, JCP = 2 Hz), 132.4 (d, JCP = 4 Hz), 132.6 (d, JCP = 7 Hz), 132.7 (d, JCP = 3 Hz), 132.9 (t, JCP = 8 Hz), 134.7 (br), 135.0 (d, JCP = 11 Hz), 135.3 (dd, JCP = 10 Hz, JCP = 19 Hz), 135.7 (brd, JCP = 21 Hz), 135.7 (d, JCP = 4 Hz), 136.2 (d, JCP = 8 Hz), 137.1 (br), 137.2 (t, JCP = 12 Hz), 138.7 (d, JCP = 14 Hz), 139.1 (d, JCP = 22 Hz). The signals of the CO ligands could not be observed owing to their low intensities. 31 1 P{ H} NMR (121.7 MHz, 297 K, CDCl3): δ −5.1 (ddd, 1JPP = 265 Hz, 3JPP = 15 Hz, 3JPP = 10 Hz, cis-1), 57.3 (dt, 1JPP = 262 Hz, 2JPP = 29 Hz, cis-1) 91.2 (ddd, 1JPP = 207 Hz, 2JPP = 28 Hz, 3JPP = 10 Hz, μ-P), 96.0 (ddd, 1JPP = 208 Hz, 2JPP = 29 Hz, 3JPP = 13 Hz, μ-P). Anal. Calcd for C49H32Fe2O5P4: C, 62.85; H, 3.44. Found: C, 62.74; H, 3.68. Preparation of (μ-nabip)[Fe2(CO)5(trans-1)] (5-trans-1). A Schlenk tube was charged with 5-MeCN (197 mg, 0.310 mmol), trans-1 (212 mg, 0.619 mmol), and THF (12 mL). After the solution was stirred for 12 days at room temperature, volatiles were removed in vacuo. The residue was loaded into an Al2O3 column and eluted with CH2Cl2/hexane 1/1. The main orange band was collected, and workup gave an orange powder of 5-trans-1 (224 mg, 77%). 1H NMR (300.5 MHz, 297 K, CDCl3): δ 6.38 (ddt, 3JHP = 1.4 Hz, 3JHH = 7.3 Hz, 3JHH = 12.3 Hz, 1H, 3,6-Naph), 6.59 (dt, J = 2.2 Hz, J = 7.5 Hz, 2H, Ph), 6.66−6.78 (m, 3H, Ph), 7.01 (t, 3JHH = 7.7 Hz, 3JHP = 7.7 Hz, 1H, 2,7-Naph), 7.08−7.25 (m, 8H), 7.31−7.56 (m, 10H), 7.69 (br, 1H), 7.70 (dt, 4JHH = 0.9 Hz, 3JHH = 7.1 Hz, 3JHP = 7.1 Hz, 1H, 2,7Naph), 7.79 (dq, 4JHH = 1.6 Hz, 3JHH = 8.0 Hz, 1H, 4,5-Naph), 7.85 (d, 3 JHH = 8.3 Hz, 1H, 4,5-Naph), 7.88 (br, 1H), 7.90 (dd, 4JHH = 1.2 Hz, 3 JHH = 8.0 Hz, 2H, 4,5-Naph). 13C{1H} NMR (75.6 MHz, 297 K, CDCl3): δ 124.2 (s), 125.0 (dd, JCP = 2 Hz, JCP = 9 Hz), 125.3 (dd, JCP = 2 Hz, JCP = 9 Hz), 126.5 (d, JCP = 10 Hz), 127.4 (d, JCP = 8 Hz), 127.5 (d, JCP = 9 Hz), 127.8 (br), 128.0 (br), 128.4 (s), 128.5 (br), 128.8 (d, JCP = 2 Hz), 128.9 (d, JCP = 2 Hz), 129.3 (d, JCP = 9 Hz), 129.5 (d, JCP = 9 Hz), 129.7 (d, JCP = 2 Hz), 129.9 (d, JCP = 2 Hz), 130.0 (d, JCP = 2 Hz), 130.4 (d, JCP = 2 Hz), 130.7 (d, JCP = 3 Hz), 7021

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Hz, PPh3(m-)), 128.6 (s, PPh3(p-)), 128.7 (t, 1JCP = 20 Hz, 1,8-Naph), 128.8 (s, p-Ph), 129.4 (s, 4,5-Naph), 130.9 (s, 2,7-Naph), 132.5 (s), 132.9 (d, 2JCP = 11 Hz, PPh3(o-)), 135.3 (t, 2JCP = 5 Hz, o-Ph), 136.2 (t, 1JCP = 20 Hz, ipso-Ph), 138.9 (d, 1JCP = 35 Hz, PPh3(ipso-)), 218.3 (br, CO). 31P{1H} NMR (121.7 MHz, 297 K, CDCl3): δ 62.8 (t, 2JPP = 27 Hz, PPh3), 67.5 (t, 2JPP = 27 Hz, μ-P)). IR (CH2Cl2): νCO (cm−1) 1985, 1929. Anal. Calcd for C62H46Fe2O4P4·1/4CH2Cl2: C, 67.25; H, 4.22. Found: C, 67.57; H, 4.33. Preparation of (μ-nabip)[Fe2(CO)4(κ2-cis-1)] (6-cis-1). A Schlenk tube was charged with 6-MeCN (119 mg, 0.184 mmol), cis1 (67 mg, 0.20 mmol), and CH2Cl2 (15 mL). After the mixture was stirred for 1 h at room temperature, volatiles were removed in vacuo. The residue was washed with hexane (5 × 15 mL) and dried in vacuo to give 6-cis-1 as a powder (124 mg, 74%). 1H NMR (300.5 MHz, 297 K, CDCl3): δ 7.02 (dt, 4JHH = 1.0 Hz, 3JHH = 6.7 Hz, 3JHP = 13.4 Hz, 2H, 2,7-Naph), 7.08−7.31 (12H), 7.35−7.50 (m, 10H), 7.76 (br, 2H), 7.78 (ddd, 4J = 1.7 Hz, J = 3.2 Hz, J = 8.2 Hz, 2H, 4,5-Naph), 7.88 (d, J = 7.9 Hz, 2H), 8.14 (br, 2H). 31P{1H} NMR (121.7 MHz, 297 K, CDCl3): δ −62.3 (m, κ2-cis-1), 143.1 (m, μ-P)). IR (CH2Cl2): νCO (cm−1) 2007, 1943, 1934, 1899. Anal. Calcd for C48H32Fe2O4P4·1/5CH2Cl2: C, 62.56; H, 3.53. Found: C, 62.81; H, 3.55. Protonation of Diiron Complexes. In a Schlenk tube charged with 3 (57 mg, 0.092 mmol) and CH2Cl2 (5 mL) was added TfOH (25 μL, 0.29 mmol, 3.2 equiv). The solvent was removed in vacuo to give a viscous residue. The NMR data of the residue were recorded without purification, because the product was found to be deprotonated upon washing with solvents. However, the product could be sufficiently characterized with the following NMR data. Other binuclear complexes were protonated in a manner similar to that described for the synthesis of 3H+. Amounts of the acid were varied depending on the complexes as follows: for 5-MeCN, 1 equiv; for 5-PMe3, 1 equiv; for 5-PPh3, 1.5 equiv; for 6-MeCN, 1 equiv; for 6PMe3, 1 equiv; for 6-PPh3, 1 equiv; for 6-cis-1, 1 equiv. CH2Cl2 was used as the solvent for the diiron complexes, except 5-MeCN and 6MeCN, for which MeCN was used. (μ-H)(μ-nabip)[Fe(CO)3]2OTf (3H+). 1H NMR (300.5 MHz, 297 K, CDCl3): δ −16.6 (t, 2JHP =39 Hz, 1H, μ-H), 7.09 (dt, 3JHH = 9.0 Hz, 3JHP = 18.0 Hz, 2H, 2,7-Naph) 7.55 (t, 3JHH = 7.5 Hz, 2H, 3,6Naph), 7.66−7.81 (m, 10H, Ph), 8.25 (d, 3JHH = 9.0 Hz, 2H, 4,5Naph), 12.4 (br, HOTf). 13C{1H} NMR (75.6 MHz, 297 K, CDCl3): δ 118.7 (q, 1JCF = 317 Hz, CF3), 123.2 (t, 1JPC = 23 Hz, 1,8-Naph), 126.9 (t, 3JPC = 7 Hz, 3,6-Naph), 128.1 (t, JPC = 11 Hz, Naph), 128.5 (t, 1JPC = 24 Hz, ipso-Ph), 130.4 (t, 3JPC = 6 Hz, m-Ph), 133.7 (t, JPC = 9 Hz, Naph), 133.8 (s, p-Ph), 134.4 (t, 2JPC = 4 Hz, o-Ph), 136.1 (s, 4,5-Naph), 136.6 (t, 2JPC = 2 Hz, 2,7-Naph), 202.2 (t, 2JPC = 2 Hz, 4CO), 203.8 (t, 2JPC = 17 Hz, 2CO). 31P{1H} NMR (121.7 MHz, 297 K, CDCl3): δ 106.9 (s). (μ-H)(μ-nabip)[Fe2(CO)5(MeCN)]OTf (5-MeCNH+). 1H NMR (300.5 MHz, 297 K, CDCl3): δ −15.0 (t, 2JHP = 42 Hz, μ-H), 1.98 (s, NCMe), 6.83 (dt, 3JHH = 9.0 Hz, 3JHP = 18.0 Hz, 1H, 2,7-Naph), 7.23 (dt, 3JHH = 9.0 Hz, 3JHP = 18.0 Hz, 1H, 2,7-Naph), 7.40 (t, 3JHH = 7.5 Hz, 1H, 3,6-Naph), 7.51 (t, 3JHH = 7.5 Hz, 1H, 3,6-Naph), 7.54 (m, 2H, Ph), 7.67−7.75 (m, 8H, Ph), 8.15 (m, 2H, 4,5-Naph), 14.7 (br, HOTf). 13C{1H} NMR (75.6 MHz, 297 K, CDCl3): δ 3.8 (s, MeCN), 119.3 (q, 1JCF = 317 Hz, CF3), 125.7 (t, 1JCP = 23 Hz, 1,8-Naph), 126.1 (t, 1JCP = 20 Hz, 1,8-Naph), 126.1 (t, 3JCP = 6 Hz, 3,6-Naph), 126.5 (t, 3 JCP = 6 Hz, 3,6-Naph), 126.5 (t, 1JCP = 21 Hz, ipso-Ph), 128.6 (t, JCP = 11 Hz, Naph), 129.7 (t, 3JCP = 6 Hz, m-Ph), 129.9 (br, m-Ph), 130.0 (t, 1 JCP = 23 Hz, ipso-Ph), 130.8 (t, 3JCP = 3 Hz, CN), 132.7 (m, p-Ph), 133.6 (t, JCP = 10 Hz, Naph), 134.2 (t, 2JCP = 4 Hz, o-Ph), 134.5 (s, 4,5-Naph), 134.7 (s, 4,5-Naph), 134.9 (s, 2,7-Naph), 135.3 (br, o-Ph), 135.5 (s, 2,7-Naph), 204.2 (s, COeq), 204.5 (t, JCP = 2 Hz, COeq), 205.8 (t, JCP = 16 Hz, COax), 207.2 (t, JCP = 7 Hz, COeq), 207.5 (t, JCP = 19 Hz, COax). 31P{1H} NMR (121.7 MHz, 297 K, CDCl3): δ 109.1 (s). (μ-H)(μ-nabip)[Fe2(CO)5(PMe3)]OTf (5-PMe3H+). 1H NMR (300.5 MHz, 297 K, CDCl3): δ −16.7 (td, 2JHP = 36.6, 2JHP =9.0 Hz, 1H, μ-H), 1.37 (d, 2JHP = 9.0 Hz, 9H, PMe3), 7.08 (dt, 3JHH = 7.7 Hz, 3JHP = 15.3 Hz, 2H, 2,7-Naph), 7.49 (t, 3JHH = 7.5 Hz, 2H, 3,6-

131.2 (d, JCP = 3 Hz), 131.6 (d, JCP = 3 Hz), 131.8 (dd, JCP = 4 Hz, JCP = 25.6 Hz), 132.0 (dd, JCP = 10 Hz, JCP = 20 Hz), 132.9 (d, JCP = 6 Hz), 132.9 (d, JCP = 3 Hz), 133.3 (d, JCP = 13 Hz), 133.3 (t, JCP = 8 Hz), 134.2 (br), 134.3 (br), 134.6 (s), 134.8 (dd, JCP = 6 Hz, JCP = 12 Hz), 134.9 (s), 135.2 (dd, JCP = 4 Hz, JCP = 13 Hz), 135.3 (d, JCP = 5 Hz), 135.5 (d, JCP = 5 Hz), 137.0 (d, JCP = 6 Hz), 137.3 (d, JCP = 3 Hz), 139.8 (d, JCP = 22 Hz), 140.1 (dd, JCP = 1 Hz, JCP = 8 Hz), 141.2 (m), 141.6 (m), 141.8 (m), 213.2 (br), 214.6 (m), 216.6 (br). 31P{1H} NMR (121.7 MHz, 297 K, CDCl3): δ 9.7 (d, 1JPP = 220 Hz, trans-1), 63.0 (dt, 1JPP = 221 Hz, 2JPP = 28 Hz, trans-1), 90.5 (ddd, 1JPP = 207 Hz, 2JPP = 28 Hz, 3JPP = 5 Hz, μ-P), 93.1 (dd, 1JPP = 207 Hz, 2JPP = 27 Hz, μ-P). Anal. Calcd for C49H32Fe2O5P4·0.5C6H14: C, 63.77; H, 4.01. Found: C, 63.66; H, 4.05. Preparation of (μ-nabip)[Fe2(CO)4(MeCN)2] (6-MeCN). A Schlenk tube was charged with 3 (255 mg, 0.410 mmol) and MeCN (100 mL). The mixture in the Schlenk tube was irradiated with a mercury arc lamp for 19 h with N2 bubbling to purge CO. After the solvents were removed in vacuo, the residue was extracted with ether (6 × 10 mL). After the ether was evaporated under reduced pressure, the product was dried in vacuo to give an orange powder of 6-MeCN (258 mg, 97%). 1H NMR (300.5 MHz, 297 K, C6D6): δ 0.65 (s, 6H, NCCH3), 6.84 (t, J = 7.7 Hz, 2H), 7.15 (t, J = 6.8 Hz, 2H), 7.2−7.3 (6H), 7.41 (ddd, J = 1.8 Hz, J = 3.1 Hz, J = 9.0 Hz, 2H), 7.90 (br, 2H), 8.19 (br, 2H). 13C{1H} NMR (75.6 MHz, 297 K, C6D6): δ 2.46 (s, CH3), 125.1 (t, 3JCP = 5 Hz, 3,6-Naph), 129.1 (s, m-Ph), 129.9 (t, 3JCP = 3 Hz, NCCH3), 130.3 (s), 130.5 (s), 130.8 (t, JCP = 12 Hz, Naph), 134.0 (t, JCP = 9 Hz, Naph), 134.8 (t, 1JCP = 15 Hz, 1,8-Naph), 136.6 (t, 1JCP = 17 Hz, ipso-Ph), 137.0 (br, o-Ph), 218.7 (t, 2JCP = 5 Hz, 3CO), 219.7 (t, 2JCP = 12 Hz, 1CO). 31P{1H} NMR (121.7 MHz, 297 K, C6D6): δ 123.0 (s). IR (CH2Cl2): νCO (cm−1) 1991, 1920, 1890. Elemental analysis did not give satisfactory data owing to the contamination of a trace amount of 5-MeCN. Preparation of (μ-nabip)[Fe2(CO)4(PMe3)2] (6-PMe3). To a CH2Cl2 (10 mL) solution of 6-MeCN (167 mg, 0.258 mmol) was added a ether solution of PMe3 (2.5 equiv). After the mixture was stirred for 3 h at room temperature, volatiles were removed in vacuo. The residue was washed with hexane (5 × 10 mL) and dried in vacuo to give 6-PMe3 as a powder (139 mg, 75%). 1H NMR (300.5 MHz, 297 K, CDCl3): δ 1.11 (d, 2JPH = 7.7 Hz, 9H, PMe3), 1.18 (d, 2JPH = 7.5 Hz, 9H, PMe3), 6.70 (dt, JHH = 7.3 Hz, JHP = 25.4 Hz, 2H), 7.10 (dt, JHH = 6.8 Hz, JHP = 13.6 Hz, 2H), 7.4−7.6 (m, 6H), 7.68 (t, JHH = 7.8 Hz, 2H), 7.85 (m, 2H), 7.96 (dd, JHH = 9.2 Hz, JHH = 16.8 Hz, 2H), 8.06 (t, JHH = 8.3 Hz, 1H). 13C{1H} NMR (75.6 MHz, 297 K, CDCl3): δ 23.4 (d, 1JCP = 25 Hz, Me), 24.6 (d, 1JCP = 24 Hz, Me), 124.7 (t, 3JCP = 9 Hz, 3,6-Naph), 124.8 (t, 3JCP = 10 Hz, 3,6-Naph), 127.1 (d, 3JCP = 9 Hz, m-Ph), 127.5 (d, 3JCP = 11 Hz, m-Ph), 128.8 (d, 3 JCP = 9 Hz, m-Ph), 129.3 (d, 3JCP = 8 Hz, m-Ph), 129.4 (d, 4JCP = 3 Hz, 4,5-Naph), 129.6 (d, 4JCP = 3 Hz, 4,5-Naph), 129.6 (t, 4JCP = 2 Hz, p-Ph), 130.1 (d, 2JCP = 3 Hz, 2,7-Naph), 130.2 (t, JCP = 16 Hz, Naph), 130.6 (s, p-Ph), 130.8 (d, 2JCP = 3 Hz, 2,7-Naph), 133.0 (t, JCP = 8 Hz, Naph), 135.0 (d, 2JCP = 11 Hz, o-Ph), 135.8 (d, 1JCP = 12 Hz, ipso-Ph), 136.2 (d, 2JCP = 11 Hz, o-Ph), 136.4 (d, 2JCP = 8 Hz, o-Ph), 137.6 (d, 2 JCP = 8 Hz, o-Ph), 217.4 (br, CO). 31P{1H} NMR (121.7 MHz, 297 K, CDCl3): δ 8.8 (ddd, 2JPP = 46 Hz, 2JPP = 38 Hz, 4JPP = 9 Hz, PMe3), 22.5 (ddd, 2JPP = 43 Hz, 2JPP = 26 Hz, 4JPP = 9 Hz, PMe3), 95.2 (ddd, 2 JPP = 193 Hz, 2JPP = 43 Hz, 2JPP = 48 Hz, μ-P), 101.9 (ddd, 2JPP = 194 Hz, 2JPP = 37 Hz, 2JPP = 24 Hz, μ-P). IR (CH2Cl2): νCO (cm−1) 1997, 1931, 1891. Anal. Calcd for C32H34Fe2O4P4: C, 53.52; H, 4.77. Found: C, 53.44; H, 4.69. Preparation of (μ-nabip)[Fe2(CO)4(PPh3)2] (6-PPh3). A Schlenk tube was charged with 6-MeCN (133 mg, 0.205 mmol), PPh3 (213 mg, 0.812 mmol), and THF (10 mL). After the mixture was stirred for 35 h at 65 °C, volatiles were removed in vacuo. The residue was washed with ether (5 × 20 mL) and dried in vacuo to give 6-PPh3 as a powder (164 mg, 73%). 1H NMR (300.5 MHz, 297 K, CDCl3): δ 6.45 (dt, 3JHH = 6.6 Hz, 3JHP = 13.2 Hz, 2H, 2,7-Naph), 6.69 (dd, JHH = 5.7 Hz, JHP = 5.8 Hz, 4H, Ph), 6.9−7.3 (38H), 7.72 (dm, 3JHH = 8.0 Hz, 2H, 4,5-Naph). 13C{1H} NMR (75.6 MHz, 297 K, CDCl3): δ 124.7 (t, 3 JCP = 5 Hz, 3,6-Naph), 127.3 (t, 3JCP = 5 Hz, m-Ph), 127.6 (d, 3JCP = 9 7022

dx.doi.org/10.1021/om4006142 | Organometallics 2013, 32, 7014−7024

Organometallics

Article

Naph), 7.64−7.71 (m, 10H, Ph), 8.18 (d, 3JHH = 6.0 Hz, 2H, 4,5Naph). 13C{1H} NMR (75.6 MHz, 297 K, CDCl3): δ 21.4 (dt, 1JCP = 34 Hz, 3JCP = 2 Hz, PMe3), 126.1 (td, 1JCP = 20 Hz, 3JCP = 2 Hz, 1,8Naph), 126.9 (t, 3JCP = 6 Hz, 3,6-Naph), 128.8 (t, JCP = 11 Hz, Naph), 130.0 (t, 3JCP = 5 Hz, m-Ph), 130.1 (t, 1JCP = 23 Hz, ipso-Ph), 132.8 (s, p-Ph), 133.6 (t, JCP = 9 Hz, Naph), 134.8 (s, 4,5-Naph), 135.7 (s, 2,7Naph), 204.2 (m, 2JCP = 2 Hz, 2CO), 207.3 (t, 2JCP = 16 Hz, 1CO), 208.5 (d, 2JCP = 17 Hz, 2CO). 31P{1H} NMR (121.7 MHz, 297 K, CDCl3): δ 18.8 (t, 2JPP = 49 Hz, PMe3), 88.3 (d, 2JPP = 49 Hz, 2P, μP). (μ-H)(μ-nabip)[Fe 2(CO)5(PPh3)]OTf (5-PPh3H+ ). 1 H NMR (300.5 MHz, 297 K, CDCl3): δ −17.45 (td, 2JHP = 36.6 Hz, 2JHP = 3.0 Hz, μ-H), 6.87 (dt, 3JHH = 7.8 Hz, 3JHP = 15.6 Hz, 2H, 2,7-Naph), 7.02 (dd, 3JHH = 7.7 Hz, 3JHP = 11.3 Hz, 6H, PPh3(o-)), 7.19 (br, 2H, o-Ph), 7.22 (dt, 3JHH = 7.8 Hz, 4JHP = 2.2 Hz, 6H, PPh3 (m-)), 7.38 (t, 3 JHH = 7.5 Hz, 2H, 3,6-Naph), 7.41 (t, 3JHH = 7.6 Hz, 3H, PPh3(p-)), 7.47 (m, 4H, m-Ph), 7.61 (t, 3JHH = 7.4 Hz, 2H, p-Ph), 8.13 (d, 3JHH = 9.0, 2H, 4,5-Naph), 13.2 (br, HOTf). 13C{1H} NMR (75.6 MHz, 297 K, CDCl3): δ 119.0 (q, 1JCF = 317 Hz, CF3), 124.8 (dt, 3JCP = 1 Hz, 1 JCP = 22 Hz, 1,8-Naph), 126.4 (t, 3JCP = 6 Hz, 3,6-Naph), 128.7 (t, JCP = 11 Hz, Naph), 129.1 (d, 3JCP = 11 Hz, PPh3(m-)), 129.5 (t, 3JCP = 5 Hz, m-Ph), 130.7 (t, 1JCP = 22 Hz, ipso-Ph), 131.7 (d, 4JCP = 4 Hz, PPh3(p-)), 132.3 (d, 3JCP = 10 Hz, PPh3(o-)), 132.6 (s, p-Ph), 133.3 (t, JCP = 10 Hz, Naph), 134.6 (s, 4,5-Naph), 136.3 (s, 2,7-Naph), 203.3 (d, 2JCP = 2 Hz, 2CO), 207.1 (t, 2JCP = 16 Hz, 1CO), 209.7 (d, 2JCP = 18 Hz, 2CO). 31P{1H} NMR (121.7 MHz, 297 K, CDCl3): δ 56.6 (t, 2 JPP = 41 Hz, PPh3), 82.6 (d, 2JPP = 43 Hz, 2P, μ-P)). (μ-H)(μ-nabip)[Fe2(CO)4(MeCN)2]OTf (6-MeCNH+). 1H NMR (300.5 MHz, 297 K, C6D6): δ −12.2 (t, 2JPH = 44 Hz, 1H, μ-H), 1.74 (s, 6H, NCMe), 6.69 (t, 3JHH = 7.5 Hz, 2H, 3,6-Naph), 6.97 (dt, 3JHH = 6.0 Hz, 3JHP = 12.0 Hz, 2H, 2,7-Naph), 7.20 (br, 3JHH = 7.5 Hz, 6H, Ph), 7.35 (d, 3JHH = 9.0 Hz, 2H, 4,5-Naph), 7.48 (br, 4H, Ph). 31P{1H} NMR (121.7 MHz, 297 K, C6D6): δ 110.8 (s). (μ-H)(μ-nabip)[Fe2(CO)4(PMe3)2]OTf (6-PMe3H+). 1H NMR (300.5 MHz, 297 K, CDCl3): δ −17.6 (tdd, 2JPH = 39.6 Hz, 2JPH = 31.8 Hz, 2JPH = 31.8 Hz, 2JPH = 8.5 Hz, 1H, μ-H), 1.28 (br, 9H, PMe3), 1.42 (br, 9H, PMe3), 6.89 (br, 2H), 7.36 (br, 2H), 7.63 (br, 10H), 8.04 (br, 2H). 31P{1H} NMR (121.7 MHz, 297 K, CDCl3): δ 4.4 (dt, 2JPP = 44 Hz, 2JPP = 39 Hz, 2JPP = 37 Hz, PMe3), 19.6 (dt, 2JPP = 57 Hz, 2JPP = 43 Hz, 2JPP = 39 Hz, PMe3), 83.6 (ddd, 2JPP = 161 Hz, 2JPP = 57 Hz, 2 JPP = 46 Hz, μ-P), 101.9 (ddd, 2JPP = 161 Hz, 2JPP = 43 Hz, 2JPP = 37 Hz, μ-P). (μ-H)(μ-nabip)[Fe2(CO)4(PPh3)2]OTf (6-PPh3H+). Data for the major isomer are as follows. 1H NMR (300.5 MHz, 297 K, CDCl3): δ −17.0 (m, 2JPH = 52.0 Hz, 2JPH = 30.4 Hz, 2JPH = 17.4 Hz, 2JPH = 1.1 Hz, 1H, μ-H), 6.75 (br, 4H), 6.89 (m, 6H, PPh3), 7.10 (m, 6H, PPh3), 7.17−7.55 (28H), 8.06 (br, 2H, Naph). 31P{1H} NMR (121.7 MHz, 297 K, CDCl3): δ 51.7 (dt, 2JPP = 35 Hz, 2JPP = 34 Hz, 2JPP = 4 Hz, PPh3), 56.4 (dt, 2JPP = 44 Hz, 2JPP = 38 Hz, 2JPP = 4 Hz, PPh3), 71.8 (dt, 2JPP = 170 Hz, 2JPP = 38 Hz, 2JPP = 34 Hz, μ-P), 90.7 (dt, 2JPP = 170 Hz, 2JPP = 44 Hz, 2JPP = 35 Hz, μ-P). Data for the minor isomer are as follows. 1H NMR (300.5 MHz, 297 K, CDCl3): δ −18.6 (tt, 2JPH = 36.0 Hz, 2JPH = 2.8 Hz, 1H, μ-H), 6.13 (dt, 3JHH = 6.5 Hz, 3JHP = 6.0 Hz, 4H, o-Ph), 6.47 (dt, 3JHH = 7.4 Hz, 3JHP = 7.1 Hz, 2H, 2,7-Naph), 7.00 (t, 3JHH = 7.2 Hz, 4H, m-Ph), 8.83 (d, 3JHH = 8.2 Hz, 2H, 4,5Naph). 31P{1H} NMR (121.7 MHz, 297 K, CDCl3): δ 55.3 (t, 2JPP = 40 Hz, PPh3), 61.9 (t, 2JPP = 40 Hz, μ-P). (μ-H)(μ-nabip)[Fe2(CO)4(κ2-cis-1)]OTf (6-cis-1H+). 1H NMR (300.5 MHz, 297 K, CDCl3): δ −15.1 (tt, 2JPH = 41.3 Hz, 2JPH = 24.0 Hz, μ-H of the major isomer), −13.8 (tt, 2JPH = 42.9 Hz, 2JPH = 18.7 Hz, μ-H of the minor isomer), 6.78 (dt, J = 6.6 Hz, J = 6.2 Hz), 7.06 (t, J = 7.2 Hz), 7.18 (dt, J = 7.5 Hz, J = 8.0 Hz), 7.20−7.90 (m), 7.89 (t, J = 6.4 Hz), 8.04 (d, J = 7.8 Hz), 8.16 (dd, J = 8.5 Hz, J = 10.1 Hz), 8.30 (d, J = 7.9 Hz). 31P{1H} NMR (121.7 MHz, 297 K, CDCl3): δ −62.8 (m, cis-1 of the major isomer), −63.3 (m, cis-1 of the minor isomer), 132.1 (m, μ-P of the major isomer), 124.0 (m, μ-P of the minor isomer). Reduction of 3 with Na. A Schlenk tube was charged with 3 (10 mg, 0.16 mmol), a small piece of Na (ca. 10 mg), and THF (2.5 mL).

After the mixture was stirred for 4 h at room temperature, a 31P{1H} NMR spectrum was recorded. 31P{1H} NMR (121.7 MHz, 297 K, THF): δ −32.8 (s). Crystallographic Analyses. Measurements were conducted using a Rigaku SCX mini CCD-based diffractometer for trans-2 and a Bruker APEX-II Ultra CCD-based diffractometer for 3, 5-cis-1, and 6-cis-1. The structures were solved by direct methods and expanded using Fourier techniques. Non-hydrogen atoms were refined anisotropically, while hydrogen atoms were located at ideal positions and refined isotropically. All calculations were performed using the SHELXL-97 crystallographic software package.32 The data collection and structure refinement details are provided in Tables S2−S5 (Supporting Information).

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ASSOCIATED CONTENT

S Supporting Information *

Figures giving 1H NMR, 13C{1H} NMR, 31P{1H} NMR, and IR spectra and cyclic voltammograms for new complexes and CIF files and tables giving X-ray crystallographic data for trans-2, 3, 5-cis-1, and 6-cis-1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by Grants-in-Aid for Scientific Research No. 25105741 from the Ministry of Education, Culture, Sports, Science and Technology of Japan. Measurements for the microanalysis was made using a PerkinElmer CHNS 2400II instrument at the Natural Science Center for Basic Research and Development (N-BARD), Hiroshima University.

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