Article pubs.acs.org/Organometallics
Synthesis, Structural Characterization, and Catalytic H2 Production of Ferrocenyl (Fc) Group Containing Complexes [Ni(PFc2NAr2)2](BF4)2 (Ar = Ph, p‑BrC6H4) Li-Cheng Song,* Hao Tan, Fei-Xian Luo, Yong-Xiang Wang, Zhen Ma, and Zheng Niu Department of Chemistry, State Key Laboratory of Elemento-Organic Chemistry, Nankai University, 94 Weijin Road, Tianjin 300071, People’s Republic of China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, People’s Republic of China S Supporting Information *
ABSTRACT: Two new ferrocenyl (Fc) group containing 1,5-diaza-3,7diphosphacyclooctane ligands, PFc2NAr2 (1, Ar = Ph; 2, Ar = p-BrC6H4), were prepared by reaction of FcPH2 with 2 equiv of paraformaldehyde in glycol followed by treatment of the resulting FcP(CH2OH)2 with 1 equiv of aniline or pbromoaniline in 79% and 67% yields, respectively; however, ligands 1 and 2 could be also prepared by treatment of the isolated and purified FcP(CH2OH)2 with 1 equiv of aniline or p-bromoaniline under similar conditions in 80% and 72% yields, respectively. Further treatment of 1 or 2 with the complex salt [Ni(MeCN)6](BF4)2 in MeCN at room temperature resulted in formation of the first ferrocenyl-containing complexes [Ni(PFc2NAr2)2](BF4)2 (3, Ar = Ph; 4, Ar = p-BrC6H4) in 95% and 90% yields, respectively. Compounds 1−4 were structurally characterized by elemental analysis and spectroscopy and particularly for 1, 2, and 4 by X-ray crystallography. When the cyclic voltammetric behavior of 1−4 was investigated, 3 and 4 were found to be electrocatalysts for proton reduction of TFA to give hydrogen under CV conditions.
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INTRODUCTION In recent years, molecular H2 has been considered as a promising energy source to solve the current energy shortage and environmental pollution problems.1 This has led to intensive studies aimed at discovering the highly efficient H2producing catalysts that contain earth-abundant and cheap metals such as iron and nickel present in the active sites of [FeFe]-2 and [NiFe]-hydrogenase3 enzymes (Scheme 1a,b). Inspired by such well-elucidated active site structures, synthetic chemists have so far prepared many homo- and heteronuclear transition-metal complexes as models for the active sites of [FeFe]-4−12 and [NiFe]-hydrogenases,13−24 and some of them have been proved to be catalysts for hydrogen production. Also, inspired by the active site structures of [FeFe]- and [NiFe]hydrogenases, DuBois and co-workers have recently discovered a new type of dicationic mononuclear [Ni(PR2NR′2)2]2+ complex (Scheme 1c) that contains bicyclic diphosphine ligands with pendant amine bases; this type of complex has been found to be a highly effective catalyst for H2 production or H2 oxidation.25−34 To date, a series of experimental and theoretical studies has revealed that the steric and electronic properties of R and R′ substituents on the P and N atoms can influence the hydride acceptor and proton acceptor abilities of this type of complex that tend to promote hydrogen production or hydrogen oxidation. For example, the complexes [Ni(PR2NPh2)2(MeCN)](BF4)2 (R = benzyl, n-butyl, 2-phenylethyl, cyclohexyl, 2,4,4-trimethylpentyl) were found to be © 2014 American Chemical Society
catalysts for H2 production in the presence of the proton source [(DMF)H]+OTf− under electrochemical conditions.28 The H2 oxidation catalyzed by the complex [Ni(PCy2Nt‑Bu2)2](BF4)2 was reported to be accomplished in the presence of base with a turnover rate of 50 s−1 under 1.0 atm of H2 at a potential of −0.77 V versus the ferrocene couple.29 Up to now, although many of these types of complexes have been prepared,25−34 none of them contain the organometallic group on their P and/ or N atoms. Therefore, we decided to prepare such a type of complex with the organometallic ferrocenyl (Fc) groups, in order to see the influence of Fc group on their structures and properties. We chose the Fc group to make such a complex because ferrocene is the most popular organometallic compound and possesses some special steric and electronic effects.35 In this article, we report the synthesis and molecular structures of the two novel Fc group containing complexes [Ni(PFc2NAr2)2](BF4)2 (3, Ar = Ph; 4, Ar = p-BrC6H4) along with their two cyclic diphosphine ligands PFc2NAr2 (1, Ar = Ph; 2, Ar = p-BrC6H4). In addition, the electrochemical properties of 1−4 and particularly the electrocatalytic H2 production catalyzed by complexes 3 and 4 are also described.
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RESULTS AND DISCUSSION Synthesis and Characterization of Ligands PFc2NAr2 (1, Ar = Ph; 2, p-BrC6H4). By use of a previously reported Received: May 28, 2014 Published: September 10, 2014 5246
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Scheme 1. Structures of (a) Active Site of [FeFe]-hydrogenase, (b) Active Site of [NiFe]-hydrogenase, and (c) Dicationic Complexes [Ni(PR2NR′2)2]2+
synthetic method28 with some modifications, the new Fc group containing ligands 1 and 2 used for preparation of their complexes 3 and 4 could be obtained through a sequential reaction of ferrocenylphosphine (FcPH2) with 2 equiv of paraformaldehyde in glycol at 120 °C, followed by treatment of the resulting intermediate bis(hydroxymethyl)ferrocenylphosphine FcP(CH2OH)2 with 1 equiv of aniline or p-bromoaniline in 79% and 67% yields, respectively; however, 1 and 2 could be also prepared by reaction of the isolated and purified bis(hydroxymethyl)ferrocenylphosphine36 with 1 equiv of aniline or p-bromoaniline in glycol at 120 °C in 80% and 72% yields, respectively (Scheme 2).
electron-donating effects stronger than those of common organic groups. The molecular structures of 1 and 2 have been unequivocally established by X-ray diffraction analysis. The ORTEP drawings of 1 and 2 are depicted in Figures 1 and 2, whereas Table 1 gives their selected bond lengths and angles. As shown in Figures 1 and 2, ligands 1 and 2 each contain the eightmembered heterocyclic skeleton of 1,5-diaza-3,7-diphosphacyclooctane that adopts a half-chair/half-chair conformation. While the two phosphorus atoms in the eight-membered ring of 1 or 2 are attached to the two ferrocenyl groups by an equatorial type of bond, the two nitrogen atoms in the eightmembered ring of 1 or 2 are attached to the two phenyl or two p-bromophenyl groups by an axial type of bond. It follows that in 1 and 2 the two ferrocenyl groups are trans to the two phenyl groups or two p-bromophenyl groups, and thus they are the most stable conformers. The P···P distances of 4.092 Å for 1 and 4.358 Å for 2 are considerably longer than that (3.54 Å)37 for the diazadiphosphacyclic ligand with ferrocenyl/ methylbenzyl groups, which is in turn much longer than those (3.128/3.279 Å)38 for the diazadiphosphacyclic ligand with phenyl/methylbenzyl groups. The large increase of the P···P distances of 1 and 2 is apparently due to the strong steric repulsions of the bulky ferrocenyl group with the benzene ring directly bound to phosphorus and nitrogen atoms, respectively. It is interesting to note that, since the ferrocenyl groups are bound to phosphorus atoms in an equatorial type of bond, the lone electron pairs on the phosphorus atoms must reside in axial positions, positions that are suitable for their coordination to the Ni transition-metal atom in a chelating manner. Synthesis and Characterization of Complexes [Ni(PFc2NAr2)2](BF4)2 (3, Ar = Ph; 4, p-BrC6H4). Similar to the case for previously reported complexes,27−29 the ferrocenyl group containing complexes 3 and 4 could be prepared by reaction of 2 equiv of ligands 1 and 2 with the complex salt
Scheme 2
Ligands 1 and 2 are air-stable yellow solids and have been characterized by elemental analysis and various spectroscopic techniques. All of the characterization data are completely consistent with the structures shown in Scheme 2. For example, the 31P{1H} NMR spectra of 1 and 2 exhibited a singlet at ca. −58 ppm for their P atoms. This value is noticeably shifted upfield relative to those from −46 to −53 ppm for the previously reported cyclic [P2N2] ligands without ferrocenyl substituents.27,28 This implies that the ferrocenyl group has
Figure 1. Molecular structure of 1. Hydrogen atoms are omitted for clarity. Ellipsoids are plotted at the 30% probability level. 5247
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Figure 2. Molecular structure of 2. Hydrogen atoms are omitted for clarity. Ellipsoids are plotted at the 30% probability level.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for 1 and 2 Ligand 1 P(1)−C(10) P(1)−C(19) N(1)−C(12) C(10)−P(1)−C(11) C(11)−P(1)−C(19) C(13)−N(1)−C(11) N(1)−C(11)−P(1)
1.817(2) 1.878(2) 1.453(2) 102.7(1) 100.5(1) 120.4(1) 111.0(1)
P(1)−C(10) P(1)−C(12A) N(1)−C(11) C(10)−P(1)−C(11) C(11)−P(1)−C(12A) C(13)−N(1)−C(12) N(1)−C(11)−P(1)
1.808(6) 1.883(6) 1.444(7) 99.0(3) 99.1(3) 120.2(5) 113.6(4)
P(1)−C(11) N(1)−C(13) N(1)−C(11) C(10)−P(1)−C(19) C(13)−N(1)−C(12) C(12)−N(1)−C(11) P(1)−C(10)−Fe(1)
1.871(2) 1.388(2) 1.459(2) 99.8(1) 120.7(1) 118.3(1) 137.2 (2)
P(1)−C(11) N(1)−C(13) N(1)−C(12) C(10)−P(1)−C(12A) C(13)−N(1)−C(11) C(11)−N(1)−C(12) P(1)−C(10)−Fe(1)
1.879(6) 1.381(7) 1.463(7) 95.0(3) 121.5(5) 117.8(5) 131.4 (3)
Ligand 2
Scheme 3
chemical shifts of 3 and 4 are shifted considerably downfield by 56−58 ppm relative to those displayed by their free ligands 1 and 2. Apparently, this is due to ligands 1 and 2 being coordinated with the central Ni2+ dication. The 11B{1H} and 19 1 F{ H} NMR spectra of 3 and 4 showed one singlet at ca. −1.1 and 151 ppm, respectively, for B and F atoms in their BF4− monoanions. Particularly interesting is that the molecular structure of 4 has been unambiguously confirmed by an X-ray crystallographic study. The whole molecular structure of 4 is shown in Figure 3,
[Ni(MeCN)6](BF4)2 in MeCN at room temperature in 95% and 90% yields, respectively (Scheme 3). Complexes 3 and 4 are air-stable purple solids, which have been characterized by various spectroscopic methods and elemental analysis. The elemental analysis, IR, and NMR data are in good agreement with the structures shown in Scheme 3. The 31P{1H} NMR spectra of 3 and 4 displayed singlets at −1.1 and 0.2 ppm, respectively, for their P atoms, which are shifted upfield by ca. 3−10 ppm relative to those exhibited by the complexes [Ni(PPh2NC6H4X2)2](BF4)227 and [Ni(PR2NPh2)2](BF4)2.28 However, in contrast to this, the 31P{1H} NMR 5248
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Figure 3. Full molecular structure of 4. Hydrogen atoms are omitted for clarity. Ellipsoids are plotted at the 10% probability level.
Figure 4. Cationic skeleton of 4 with two BF4− monoanions. Hydrogen atoms are omitted for clarity. Ellipsoids are plotted at the 10% probability level.
in the former pair of six-membered rings by an equatorial and an axial bond, another two p-bromophenyls are bound to N3/ N4 atoms in the latter pair of six-membered rings by an axial and an equatorial bond, respectively. In addition, the four ferrocenyls in the two pairs of six-membered rings are all attached to P1/P4 and P2/P3 atoms by equatorial bonds. It follows that one of the two p-bromophenyls (in each of the two pairs of six-membered metallacycles) is cis to the two Fc groups and the other is trans to the two Fc groups. This means that two molecules of free ligand 2 have undergone a conformational conversion in complex 4. Apparently, such a conformational conversion reduces steric repulsions between the pbromophenyls on phosphorus. The central Ni1 atom adopts a slightly distorted square planar geometry with a dihedral angle of 6.72° between the two planes Ni1P1P4 and Ni1P2P3, which is remarkably less than those corresponding to the previously reported DuBois-type complexes without ferrocenyl substituents, such as [Ni(PPh2NC6H4OMe2)2](BF4)2 (29.07°),27 [Ni(PPh2NC6H4Me2)2](BF4)2 (24.16°),27 and [Ni(PCy2NBz2)2](BF4)2 (34.88°).39 In the anionic part of 4, the two BF4− monoanions are disordered. The nearest distance from the F atom to the Ni center is equal to 5.468 Å, which is far beyond the corresponding distance (2.98 Å) for the complex [Ni-
whereas Figure 4 shows its cationic skeleton along with two BF4− anions. In addition, selected bond lengths and angles are given in Table 2. As shown in Figures 3 and 4, complex 4 Table 2. Selected Bond Lengths (Å) and Angles (deg) for 4 Ni(1)−P(4) Ni(1)−P(1) P(1)−C(9) N(1)−C(1) P(4)−Ni(1)−P(2) P(2)−Ni(1)−P(1) P(2)−Ni(1)−P(3) N(1)−C(1)−P(4) C(1)−N(1)−C(2)
2.249(2) 2.264(2) 1.853(6) 1.438(6) 177.4(1) 97.0(1) 83.3(1) 114.3(3) 111.4(4)
Ni(1)−P(2) Ni(1)−P(3) P(1)−C(2) N(1)−C(2) P(4)−Ni(1)−P(1) P(4)−Ni(1)−P(3) P(1)−Ni(1)−P(3) N(1)−C(2)−P(1) C(2)−P(1)−Ni(1)
2.259(2) 2.272(2) 1.884(5) 1.449(6) 83.4(1) 96.6(1) 174.2(1) 111.7(3) 104.1(2)
indeed comprises one [Ni(PFc2NC6H4Br2)2]2+ dication and two BF4− monoanions. In the cationic part of 4, there exist two molecules of ligand 2 that are coordinated via their four P atoms to the central Ni1 atom to form two pairs of sixmembered metallacycles: the chair Ni1P1C2N1C1P4 fused with the boat Ni1P1C9N2C10P4 and the boat Ni1P2C37N3C38P3 fused with the chair Ni1P2C45N4C46P3. While the two p-bromophenyls are attached to N1/N2 atoms 5249
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(PPh2NC6H4OMe2)2](BF4)2; this implies that cation/anion interactions in the solid state presumably originate from electrostatic/packing effects rather than coordinate interactions.27 Electrochemical Study on Ligands 1/2 and Complexes 3/4. The electrochemical behavior of ligands 1/2 and complexes 3/4 was investigated in CH 2 Cl 2 by cyclic voltammetric techniques. The cyclic voltammograms of 1/2 and 3/4 are shown in Figures 5 and 6, respectively. As shown in
4 displays two quasi-reversible oxidation peaks at +0.49/+0.55 V and two quasi-reversible reduction peaks at −0.71/−1.15 V, respectively. The two oxidation processes displayed by 3 and 4 could be assigned to the oxidations of their pendant amines40 and ferrocenyl groups,37 whereas the two reduction processes might be ascribed to the Ni(II)/Ni(I) and Ni(I)/Ni(0) couples,27,28 respectively. Since the plots of the peak current (ip) versus the square root of the scan rate (v1/2) for the first and second reduction peaks of 3 and 4 are linear, the two reduction processes of 3 and 4 are diffusion-controlled41 (see Figures S1 and S2 in the Supporting Information). It is worth noting that the two reduction potentials of 4 are slightly shifted positively relative to those of 3, which is consistent with complex 4 carrying two electron-withdrawing bromo substituents. Electrocatalytic Study on H2 Production Catalyzed by Complexes 3 and 4. In order to observe whether complexes 3 and 4 could be able to catalyze proton reduction to hydrogen, we further determined their cyclic voltammograms in the presence of TFA (trifluoroacetic acid). The cyclic voltammograms of 3 and 4 with different concentrations of TFA and without TFA (for comparison) are presented in Figures 7 and
Figure 5. Cyclic voltammograms of 1 and 2 (1.0 mM) in 0.1 M nBu4NPF6/CH2Cl2 at a scan rate of 0.1 V s−1.
Figure 7. Cyclic voltammograms of 3 (1.0 mM) with TFA (0−10 mM) in 0.1 M n-Bu4NPF6/CH2Cl2 at a scan rate of 0.1 V s−1.
Figure 6. Cyclic voltammograms of 3 and 4 (1.0 mM) in 0.1 M nBu4NPF6/CH2Cl2 at a scan rate of 0.1 V s−1.
Figure 5, ligand 1 displays one irreversible oxidation peak at +0.07 V and one quasi-reversible oxidation peak at +0.46 V, whereas ligand 2 exhibits one irreversible oxidation peak at +0.09 V and one quasi-reversible oxidation peak at +0.49 V, respectively. The first oxidation processes are one-electron processes (supported by their controlled-potential electrolyses), which could be assigned to the oxidations of their pendant amines.40 However, the second oxidation processes are twoelectron processes (also supported by their controlled potential electrolyses), which might be attributed to the oxidations of their ferrocenyl groups. 37 Different from the case for ligands 1 and 2, Figure 6 shows that complex 3 exhibits two quasireversible oxidation peaks at +0.46/+0.59 V and two quasireversible reduction peaks at −0.78/−1.23 V, whereas complex
Figure 8. Cyclic voltammograms of 4 (1.0 mM) with TFA (0−10 mM) in 0.1 M n-Bu4NPF6/CH2Cl2 at a scan rate of 0.1 V s−1. 5250
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CONCLUSIONS On the basis of preparing the new ferrocenyl group containing 1,3-diaza-5,7-diphosphacyclooctane ligands 1 and 2 by a condensation reaction of FcP(CH2OH)2 with PhNH2 or pBrC6H4NH2, the first ferrocenyl group containing complexes 3 and 4 have been successfully synthesized by reactions of 1 and 2 with [Ni(MeCN)6](BF4)2, respectively. X-ray crystallographic studies have confirmed that (i) 4 consists of a dicationic [Ni(PFc2NC6H4Br2)2]2+ moiety and two BF4− monoanions, (ii) in the dicationic moiety the central Ni atom is coordinated with two molecules of ligands 2 via their four P atoms to give two chair six-membered rings fused respectively with two boat six-membered rings, (iii) while the two pbromophenyls and two ferrocenyls are attached to the two N and two P atoms of the two chair rings by an equatorial type of bond, another two p-bromophenyls and two ferrocenyls are bound to the two N and two P atoms of the two boat rings by an axial type of bond, respectively, (iv) the central Ni atom has a slightly distorted square planar geometry with a dihedral angle of 6.72° between the two planes defined by the Ni atom and the two P atoms of ligand 2, and (v) while one BF4− anion is close to one chair six-membered ring fused with a boat sixmembered ring, another BF4− anion is close to the opposite chair six-membered ring fused with a boat six-membered ring. In addition to the electrochemical study on 1−4, the electrocatalytic study reveals that 3 and 4 are electrocatalysts for TFA proton reduction to H2 with turnover numbers of 4.27 and 3.84 over 0.5 h, respectively. In addition, the yields of H2 produced by bulk electrolyses of 3 and 4 are above 90%.
8. As shown in Figures 7 and 8, when TFA was sequentially added from 2 to 10 mM, the first reduction peaks of complexes 3 and 4 were slightly increased in comparison to their original first peaks at −0.78/−0.71 V, while the second reduction peaks were remarkably increased in comparison to their original second peaks at −1.23/−1.15 V. It is evident that such observations are typical of the electrocatalytic proton reduction processes,27,28,42−46 although the shapes of those catalytic waves shown in Figures 7 and 8 are diffusive (see the section “Some explanations and Figure S3” in the Supporting Information). As we know, the electrocatalytic activity for a catalyst can be evaluated by the ratio between the catalytic current (icat) operating under the steady-state conditions and the peak current (ip) in the absence of added acid.41,47 For catalysts 3 and 4, the relationship between icat/ip and concentrations of TFA is shown in Figure 9, wherein icat/ip
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EXPERIMENTAL SECTION
General Comments. All reactions were carried out using standard Schlenk and vacuum-line techniques under highly prepurified N2. Glycol and acetonitrile were distilled under N2 from CaH2. Ferrocenylphosphine (FcPH2),49 bis(hydroxymethyl)ferrocenylphosphine (FcP(CH2OH)20),36 and Ni(MeCN)6(BF4)250 were prepared according to literature methods with some modifications. Other materials were available commercially and used as received. IR spectra were recorded on a Bruker Tensor 27 FT-IR infrared spectrophotometer. 1H, 13C{1H}, 31P{1H}, 11B{1H}, and 19 1 F{ H} NMR spectra were taken on a Bruker Avance 400 NMR spectrometer. Elemental analyses were performed with an Elementar Vario EL analyzer. Melting points were determined on a SGW X-4 microscopic melting point apparatus and were uncorrected. Preparation of Ligands PFc2NAr2 (1, Ar = Ph; 2, Ar = pBrC6H4). Method i Starting from Ferrocenylphosphine. A 100 mL three-necked flask equipped with a stir bar, a serum cap, and a reflux condenser topped with a N 2 inlet tube was charged with ferrocenylphosphine (0.436 g, 2.0 mmol), paraformaldehyde (0.120 g, 4.0 mmol), and 40 mL of anhydrous glycol. The mixture was stirred at 120 °C for 16 h to give an orange solution containing bis(hydroxymethyl)ferrocenylphosphine. To this solution was added aniline (0.186 g, 2.0 mmol), and then the new mixture was stirred at 120 °C for 12 h to form a yellow suspension. The suspension was filtered to give a light yellow solid, which was washed with anhydrous ethanol (30 mL × 3), recrystallized from CH2Cl2/hexane (1/6 v/v), and dried under vacuum to give 1 (0.530 g, 79%) as a yellow solid, mp 234 °C dec. Anal. Calcd for C36H36Fe2N2P2: C, 64.50; H, 5.41; N, 4.18. Found: C, 64.27; H, 5.46; N, 4.15. 1H NMR (400 MHz, CD2Cl2): 3.78−3.81 (m, 4H, 2PCH2N), 4.38−4.43 (m, 18H, 2C5H5, 2C5H4), 4.46−4.49 (m, 4H, 2PCH2N), 6.74 (t, J = 7.2 Hz, 2H, 2p-H of 2C6H5), 6.88 (d, J = 8.0 Hz, 4H, 4o-H of 2C6H5), 7.29 (t, J = 7.6 Hz, 4H, 4m-H of 2C6H5) ppm. 13C{1H} NMR (100 MHz, CDCl3): 58.2, 58.3 (PCH2N), 69.0, 70.5, 70.7, 70.8 (C5H5, C5H4), 112.6, 116.6, 129.1, 146.1 (C6H5) ppm. 31P{1H} NMR (162 MHz, CDCl3, 85% H3PO4): −57.5 (s) ppm.
Figure 9. Plots of icat/ip versus TFA concentration in the presence of 3 and 4 (1 mM) in 0.1 M n-Bu4NPF6/CH2Cl2 at a scan rate of 0.1 V s−1.
transitions from an initial dependence on the TFA concentration to a region where icat/ip is independent of the TFA concentration.41,47 In the acid-independent region of [TFA] > 0.2 M, the icat/ip values were found to be ca. 45 and 22 for 3 and 4 and the turnover frequencies (TOFs) of 3 and 4 were found to be 386 and 95 s−1,27,48 respectively. It follows that the TOF values for H2 production from TFA catalyzed by 3 and 4 are among those (95−1850 s−1) for H2 production from [(DMF)H] + OTf − catalyzed by the complexes [Ni(PPh2NC6H4X2)2](BF4)2 (X = H, Me, MeO, Br, CF3, CH2P(O)(OEt)2).27 To further confirm the H2 production catalyzed by 3 and 4, we carried out the bulk electrolysis of a CH2Cl2 solution of 3 or 4 (0.5 mM) with excess TFA (15 mM) at about −1.3 V. During 0.5 h of the bulk electrolysis, a total of 9.48 F/ mol of 3 and 8.54 F/mol of 4 passed, which correspond to turnover numbers (TONs) of 4.74 and 4.27, theoretically. In such large-scale electrolytic experiments, H2 evolution was clearly seen and the gas chromatographic analysis indicated that the H2 yields are greater than 90% (this implies a 10% loss of current). Therefore, when this 10% current loss is considered, the practical TON values for 3 and 4 should be 4.27 and 3.84, respectively. 5251
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Similarly, when p-bromoaniline (0.344 g, 2.0 mmol) was used instead of aniline, product 2 (0.555 g, 67%) was obtained as a yellow solid, mp 224 °C dec. Anal. Calcd for C36H34Br2Fe2N2P2: C, 52.21; H, 4.14; N, 3.38. Found: C, 52.19; H, 4.09; N, 3.21. 1H NMR (400 MHz, CDCl3): 3.68−3.73 (m, 4H, 2PCH2N), 4.30−4.34 (m, 18H, 2C5H5, 2C5H4), 4.44 (br.s, 4H, 2PCH2N), 6.71,7.33 (dd, J = 8.2 Hz, 8H, 2C6H4) ppm. 13C{1H} NMR (100 MHz, CDCl3): 58.2, 58.4 (PCH2N), 69.0, 70.4, 70.7, 70.8 (C5H5, C5H4), 112.8, 119.5, 129.7, 144.0 (C6H4) ppm. 31P{1H} NMR (162 MHz, CDCl3, 85% H3PO4): −57.8 (s) ppm. Method ii Starting from Bis(hydroxymethyl)ferrocenylphosphine. The same flask equipped as indicated above was charged with bis(hydroxymethyl)ferrocenylphosphine36 (0.556 g, 2.0 mmol), aniline (0.186 g, 2.0 mmol), and 40 mL of anhydrous glycol. After the mixture was stirred at 120 °C for 12 h, a yellow suspension was formed. The same workup as used in method i afforded 1 (0.536 g, 80%). Similarly, when p-bromoaniline (0.344 g, 2.0 mmol) was utilized in place of aniline, product 2 (0.596 g, 72%) was obtained. Preparation of Complexes [Ni(PFc2NAr2)2](BF4)2 (3, Ar = Ph; 4, Ar = p-BrC6H4). In a 50 mL Schlenk flask were placed 1 (0.134 g, 0.2 mmol), Ni(MeCN)6(BF4)2 (0.048 g, 0.1 mmol), and 30 mL of acetonitrile. The mixture was stirred at room temperature for 16 h to form a purple solution. After the solution was filtered through a plug of Celite and condensed to about 5 mL at reduced pressure, anhydrous diethyl ether (30 mL) was added to give a purple precipitate. The precipitate was filtered out, washed with anhydrous diethyl ether (10 mL × 3), recrystallized from MeCN/Et2O (1/6 v/v), and dried under vacuum to afford 3 (0.157 g, 95%) as a purple solid, mp >300 °C. Anal. Calcd for C72H72B2F8Fe4N4NiP4: C, 54.98; H, 4.61; N, 3.56. Found: C, 54.69; H, 4.41; N, 3.80. 1H NMR (400 MHz, CD3CN): 3.95−4.00 (m, 8H, 4PCH2N), 4.07−4.14 (m, 16H, 4C5H4), 4.29 (s, 20H, 4C5H5), 4.69 (s, 8H, 4PCH2N), 7.00−7.10 (m, 12H, 8o-H of 2C6H5 and 4p-H of 2C6H5), 7.42 (t, J = 7.4 Hz, 8H, 8m-H of 2C6H5) ppm. 13C{1H} NMR (100 MHz, CD3CN): 52.9, 53.1, 53.2, 53.3 (PCH2N), 70.7, 73.1 (C5H5, C5H4), 119.0, 122.7, 130.6, 151.7 (C6H5) ppm. 31P{1H} NMR (162 MHz, CDCl3, 85% H3PO4): −0.6 (s) ppm. 11 1 B{ H} NMR (128 MHz, CD3CN, BF3·Et2O): −1.1 (s) ppm. 19 1 F{ H} NMR (376 MHz, CD3CN, CFCl3): −151.4 (s) ppm. Similarly, when 2 (0.166 g, 0.2 mmol) was utilized in place of 1, product 4 (0.170 g, 90%) was obtained as a purple solid, mp >300 °C. Anal. Calcd for C72H68B2Br4F8Fe4N4NiP4: C, 45.79; H, 3.63; N, 2.97. Found: C, 45.91; H, 3.63; N, 2.97. 1H NMR (400 MHz, CD3CN): 3.87−3.91 (m, 8H, 4PCH2N), 4.03−4.05 (m, 16H, 4C5H4), 4.29 (s, 20H, 4C5H5), 4.68 (s, 8H, 4PCH2N), 7.01, 7.55 (dd, J = 6.6 Hz, 16H, 4C6H4) ppm. 13C{1H} NMR (100 MHz, CD3CN): 52.7, 52.8, 52.9, 53.0, 53.1 (PCH2N), 70.8, 72.9, 73.2 (C5H5, C5H4), 114.5, 121.0, 133.3, 150.8 (C6H4) ppm. 31P{1H} NMR (162 MHz, CDCl3, 85% H3PO4): 0.2 (s) ppm. 11B{1H} NMR (128 MHz, CD3CN, BF3·Et2O): −1.1 (s) ppm. 19F{1H} NMR (376 MHz, CD3CN, CFCl3): −151.6 (s) ppm. X-ray Structure Determinations of 1, 2, and 4. Single crystals suitable for X-ray diffraction analyses were all grown by the slow diffusion of hexane into their CH2Cl2 solutions at −10 °C. All single crystals were mounted on a Rigaku MM-007 (rotating anode) diffractometer equipped with a Saturn 724 CCD. Data were collected at room temperature, using a confocal monochromator with Mo Kα radiation (λ = 0.71073 Å) in the ω−ϕ scanning mode. Data collection, reduction, and absorption correction were performed by the CRYSTALCLEAR program.51 The structures were solved by direct methods using the SHELXS-97 program52 and refined by full-matrix least-squares techniques (SHELXL-97)53 on F2. Hydrogen atoms were located by using the geometric method. Details of crystal data, data collections, and structure refinements are summarized in Table S1 (see the Supporting Information). Electrochemical and Electrocatalytic Experiments. Dichloromethane (HPLC grade) was purchased from Amethyst Chemicals and used as received, whereas n-Bu4NPF6 was prepared from n-Bu4NBr and KPF6, purified by recrystallization from CH2Cl2/Et2O, and dried in an oven at 110 °C for 24 h.54 A solution of 0.1 M n-Bu4NPF6 in CH2Cl2 was used as electrolyte in both the electrochemical and
electrocatalytic experiments. The electrolyte solutions were degassed by bubbling with N2 for at least 10 min before measurements. The measurements were made using a BAS Epsilon potentiostat. All voltammograms were obtained in a three-electrode cell with a 3 mm diameter glassy carbon working electrode, a platinum counter electrode, and an Ag/Ag+ (0.01 M AgNO3/0.1 M n-Bu4NPF6 in MeCN) reference electrode under an atmosphere of nitrogen. The working electrode was polished with 0.05 μm alumina paste and sonicated in water for 10 min. The controlled-potential electrolyses and bulk electrolyses were run with a vitreous carbon rod (A = 2.9 cm2) in a two-compartment, gastight, H-type electrolysis cell containing ca. 25 mL of CH2Cl2. All potentials are quoted against the Fc/Fc+ potential. Gas chromatography was performed with a Shimadzu GC-2014 gas chromatograph under isothermal conditions with nitrogen as a carrier gas and a thermal conductivity detector.
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ASSOCIATED CONTENT
S Supporting Information *
Text, figures, a table, and CIF files giving crystal data, atomic coordinates and thermal parameters, and bond lengths and angles for 1, 2, and 4 and plots of ip versus v1/2 and of scan rate versus icat for 3 and 4. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to the Ministry of Science and Technology of China (973 programs 2011CB935902 and 2014CB845604) and the National Natural Science Foundation of China (21132001, 21272122) for financial support of this work.
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REFERENCES
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