Synthesis, structures and catalytic activity of cyclometalated rhenium complexes

Abstract

Thermal reactions of aryl-substituted phosphines or phosphinites with Re₂(CO)₁₀ in chlorobenzene resulted in the corresponding five-membered cyclometalated rhenium complexes (1–5) via an intramolecular activation of the C(sp²)–H or C(sp³)–H bond. The only exception occurred in the case of (1-naphthyl)diisopropylphosphinite, which gave a diphosphinite-substituted dinuclear rhenium complex 6. Competition reaction indicated that the aromatic C(sp²)–H bond is more likely to be activated than the C(sp³)–H bond under the same conditions. Photolysis of 1 or 2 in CHX₃ led to the cleavage of the Re–C σ bond to yield corresponding phosphine-substituted tetracarbonyl rhenium halides 7–10. Complex 1 reacted with CF₃COOH in CH₂Cl₂ to give addition product 11. Photolysis of cyclorhenated complexes 1–3 with a series of aryl halides in benzene resulted in the stoichiometric formation of biphenyl, together with corresponding phosphine-substituted tetracarbonyl rhenium halides (7, 9, 10, 12, and 13). When base was introduced into the above reaction, a catalytic system was established. Under optimized conditions, complex 1 provided moderate yield of biphenyl in a couple of hours at a [Re] : substrate ratio of 1 : 200. Molecular structures of complexes 1, 6, 9, and 11 were determined by X-ray diffraction.

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Introduction

Cyclometalation has always been one of the most widely investigated reactions in the field of organometallic chemistry since it was first discovered in the early 1960s.¹ The reason that cyclometalation has received such considerable attention is mainly due to the following three aspects: (i) it represents probably the mildest route for the intramolecular activation of C–H bonds, and forms a cyclometalated complex containing a new M–C σ bond. (ii) Cyclometalated complexes have wide applications in many fields, such as photosensitizers for solar cells² and OLED materials.³ (iii) As catalysts for a wide range of reactions, including metathesis reactions,⁴ cross-coupling reactions,⁵ and transfer hydrogenation reactions.⁶ Of all transition metals, cyclometalated rhenium complexes have recently attracted more and more attention due to their luminescent properties, which may have potential as liquid crystals,⁷ and in particular, their indispensable role in many catalytic cycles, including the rhenium-catalyzed annulation reaction of aromatic imines with alkynes,⁸ alkenes,⁹ or aldehyde,¹⁰ and alkylation of 2-phenyl, or alkenyl-substituted pyridine.¹¹ Generally, the most widely used precursors for cyclorhenation are Re(X)(CO)₅ (X = H, Me, Cl, or Br) and Re(Cp)L₃ (L = CO, PR₃, or a mixture thereof). In this paper, we report the synthesis of a series of cyclometalated rhenium complexes starting from the precursor Re₂(CO)₁₀, as well as their catalytic activity in direct arylation of arenes with aryl halides.

Results and discussion

Reaction of Re₂(CO)₁₀ with (1-naphthyl)diphenyl phosphine or diisopropyl(1-naphthyl) phosphine

A solution of Re₂(CO)₁₀ and 2 equiv. of (1-naphthyl)diphenyl phosphine or diisopropyl(1-naphthyl) phosphine in chlorobenzene was refluxed overnight to result in corresponding five-membered cyclorhenated complex 1 or 2 in good yields (Scheme 1). Obviously, the formation of complexes 1 and 2 goes through an intramolecular activation of C(sp²)–H bond on the adjacent α position of naphthyl ring. The ¹H NMR spectrum of 1 showed six peaks centered at δ 8.21 (1H), 7.95 (1H), 7.68 (1H), 7.60 (1H), 7.55–7.49 (1H, overlapping with peaks for protons on two phenyl rings), 7.35 (1H) corresponding to six protons of the metalated naphthyl group. More distinctly, the ¹H NMR spectrum of 2 exhibited five well-separated multiplets at δ 8.16 (1H), 7.85 (1H), 7.58 (2H), 7.43 (1H), 7.28 (1H) for resonance of six protons on the metalated naphthyl group. The IR spectra of 1 and 2 (KBr film) indicated the expected strong ν(CO) absorptions in the range 2084–1925 cm⁻¹ for terminal carbonyls. The structure of 1 was further confirmed by single-crystal diffraction analysis, which is shown in Fig. 1, with selected bond lengths and angles. It reveals an octahedral geometry with the rhenium(I) center coordinated by four carbonyl groups, one phosphorus atom P(1), and one metalated carbon atom C(3), in which P(1) and C(3) are mutually adjacent. Re–P bond length is 2.4413(9) Å, a litter longer than that (2.428(3) Å) in four-membered cyclorhenated analogue Re(CO)₄(PPh₂CH₂CH₂).¹²

Scheme 1 Reactions of 1-naphthyl-substituted phosphines with Re₂(CO)₁₀.

Fig. 1 Thermal ellipsoid drawing of 1 showing the labeling scheme and 50% probability ellipsoids; hydrogens are omitted for clarity. Selected bond lengths [Å] and angles [°] are Re(1)–P(1) 2.4413(9), P(1)–C(1) 1.812(3), Re(1)–C(23) 1.987(4), Re(1)–C(24) 1.960(4), Re(1)–C(25) 1.969(4), Re(1)–C(26) 2.003(4), ∠C(3)–Re(1)–C(25) 173.87(14), ∠C(24)–Re(1)–P(1) 170.15(12), ∠C(23)–Re(1)–C(26) 171.80(16), ∠C(3)–Re(1)–P(1) 79.05(9), ∠Re(1)–P(1)–C(1) 103.52(12).

Reaction of Re₂(CO)₁₀ with diisopropyl(o-methylphenyl) phosphine

Having demonstrated that phosphine ligand could be employed as a donor group for metalation of a naphthyl C(sp²)–H bond in complexes 1 and 2, we decided to test whether it could also promote activation of a C(sp³)–H bond. The corresponding reaction of Re₂(CO)₁₀ with diisopropyl(o-methylphenyl) phosphine was carried out, which afforded a product 3 in 50% yield (Scheme 2). The ¹H NMR spectrum of 3 showed three multiplets at δ 7.38 (1H), 7.24 (2H), 7.04 (1H) duo to four protons on the phenyl ring, one multiplet at δ 2.49 (2H) for methenyl signal, and two doublets of doublets at δ 1.20 (6H) and 1.05 (6H) for signals of four methyls on two isopropyl groups. However, there was no signal for the methyl on the phenyl ring; instead one multiple at δ 2.50, integrating to 2H, assigned to a metal-bound CH₂. These resonances confirm the expected C(sp³)–H bond activation.

Scheme 2 Reaction of diisopropyl(o-methylphenyl) phosphine with Re₂(CO)₁₀.

Reactions of Re₂(CO)₁₀ with diisopropyl(2-methylnaphthyl) phosphine

Since both C(sp²)–H bond and C(sp³)–H bond could be activated in cyclometalation of Re₂(CO)₁₀ with phosphines, we were curious about the relative activity between them. Therefore, diisopropyl(2-methylnaphthyl) phosphine was chosen as the ligand, which provides both C(sp²)–H and C(sp³)–H bonds at the appropriate positions, to test the competition reaction. Therefore, reaction of Re₂(CO)₁₀ with diisopropyl(2-methylnaphthyl) phosphine under the above-mentioned conditions was performed, which gave a product 4 (Scheme 3). The ¹H NMR spectrum of 4 showed four multiplets in the aromatic region, all integrating to 5H due to the naphthyl ring, and a doublet at δ 2.71, assigned to signal for the methyl on the naphthyl ring. These resonances indicated that the aromatic C(sp²)–H bond is selectively activated in the reaction to form the five-membered cyclometalated complex 4. The reason that the aromatic C(sp²)–H bond activation is favored over C(sp³)–H bond activation is probably owing to the higher thermal stability of aryl hydrogen activated product when compared to alkyl hydrogen activated product.

Scheme 3 Reaction of diisopropyl(2-methylnaphthyl) phosphine with Re₂(CO)₁₀.

Reactions of Re₂(CO)₁₀ with phosphinites

In order to explore applicable scope of this type of cyclometalation, ligand was extended to phosphinite. Therefore reaction of Re₂(CO)₁₀ with o-methylphenyl diisopropylphosphinite was examined, which produced the corresponding five-membered cyclometalated complex 5 (Scheme 4). The ¹H NMR spectrum of 5 showed three multiplets (δ 7.55, 6.84, and 6.71) in the aromatic region, each integrating to 1H due to the phenyl ring, and a singlet at δ 2.25, assigned to signal for the methyl on the phenyl ring. These ¹H NMR information confirms an ortho aromatic C(sp²)–H bond activation.

Scheme 4 Reaction of o-methylphenyl diisopropylphosphinite with Re₂(CO)₁₀.

However, when Re₂(CO)₁₀ reacted with another phosphinite, 1-naphthyl diisopropylphosphinite under the above-mentioned conditions, the reaction did not produce the similar five-membered cyclometalated complex, instead a dinuclear complex 6 was obtained (Scheme 5). The ¹H NMR spectrum of 6 showed six multiplets in the aromatic region, all integrating to 7H corresponding to protons of the naphthyl ring. These resonances indicate the absence of aromatic C(sp²)–H bond activation. The structure of 6 was further confirmed by single-crystal diffraction analysis, which is shown in Fig. 2, with selected bond lengths and angles. It consists of a pair of square pyramidal Re(CO)₄(P) units linked by a Re–Re bond. Each phosphinite ligand is located at the top of the pyramid. Two sets of carbonyl groups bounded to two rhenium atoms occur as the staggered conformation. Re–Re bond length is 3.0606(5) Å, a litter longer than that (3.04 Å) in Re₂(CO)₁₀.¹³

Scheme 5 Reaction of 1-naphthyl diisopropylphosphinite with Re₂(CO)₁₀.

Fig. 2 Thermal ellipsoid drawing of 6 showing the labeling scheme and 50% probability ellipsoids; hydrogens are omitted for clarity. Selected bond lengths [Å] and angles [°] are Re(1)–Re(1A) 3.0606(5), Re(1)–P(1) 2.3419(15), Re(1)–C(1) 1.982(10), Re(1)–C(2) 1.977(7), Re(1)–C(3) 1.977(10), Re(1)–C(4) 1.958(8), ∠P(1)–Re(1)–Re(1A) 177.56(4), ∠C(1)–Re(1)–C(3) 173.0(3), ∠C(2)–Re(1)–C(4) 168.1(3).

Cleavage of the cyclometalated Re–C bond of 1 or 2

Lu has reported the chemical behavior of the similar N-contained cyclorhenated complexes, including the electrophilic addition reactions of cyclorhenated complexes with HBr, RCO₂H, or I₂, which caused the opening of the metallacyclic ring to generate the corresponding rhenium products.¹⁴ For comparison of reactivity, we similarly treated the P-contained cyclorhenated complex 1 with excess I₂ in CH₂Cl₂ at room temperature for 24 h, however, no reaction occurred. This result may indicate the higher stability of 1 when compared with N-contained analogues. Therefore we decided to try the decyclometalation of 1 or 2 under UV light conditions. A solution of complex 1 or 2 in CHCl₃ or CHBr₃ was irradiated by UV lamp to give corresponding phosphine-substituted tetracarbonyl rhenium halide 7–10 (Scheme 6). Obviously, the Re–C bond of the five-membered ring in 1 or 2 is cleavaged readily during the photolysis. Complexes 7–10 were fully characterized by spectroscopic analysis, as well as crystallographic analysis for 9. The molecular structure of 9 was shown in Fig. 3, with selected bond lengths and angles. Pseudooctahedral geometry is observed around the Re atom. The bromine and the (1-naphthyl)diphenyl phosphine groups are cis to each other. The Re(1)–P(1) bond length is 2.5016(12) Å, longer than that (2.4413(9) Å) in five-membered cyclorhenated structure of 1, which could be attributed to nonbonding interactions between the phosphine and Re(CO)₄Br units.

Scheme 6 Photolysis of 1 or 2 in CHX₃ (X = Cl, Br).

Fig. 3 Thermal ellipsoid drawing of 9 showing the labeling scheme and 50% probability ellipsoids; hydrogens are omitted for clarity. Selected bond lengths [Å] and angles [°] are Re(1)–P(1) 2.5016(12), Re(1)–Br(1) 2.620(2), Re(1)–C(23) 2.016(6), Re(1)–C(24) 1.968(5), Re(1)–C(25) 1.997(6), Re(1)–C(27) 1.952(8), ∠P(1)–Re(1)–C(24) 176.40(16), ∠C(23)–Re(1)–C(25) 178.0(2), ∠Br(1)–Re(1)–C(27) 169.1(6).

Besides, for further comparison of reactivity, we also performed the photolysis of 1 with CF₃COOH. After a solution of 1 with CF₃COOH in CH₂Cl₂ was irradiated for 8 h, a white product 11 was obtained in moderate yield (Scheme 7). Complexes 11 was fully characterized by spectroscopic and crystallographic analysis. The molecular structure of 11 was shown in Fig. 4. Similarly to 9, the pseudooctahedral geometry is observed around the Re atom. The carboxylate (CF₃CO₂) and the (1-naphthyl)diphenyl phosphine groups are cis to each other. The Re(1)–P(1) bond length is 2.494(2) Å, close to (2.5016(12) Å) that in 9, but also longer than that (2.4413(9) Å) in five-membered ring structure of 1. The formation of 11 indicates that the reactivity of 1 is similar to that of N-contained analogues when reacted with CF₃CO₂H.¹⁴

Scheme 7 Photolysis of 1 with CF₃COOH in CH₂Cl₂.

Fig. 4 Thermal ellipsoid drawing of 11 showing the labeling scheme and 50% probability ellipsoids; hydrogens are omitted for clarity. Selected bond lengths [Å] and angles [°] are Re(1)–P(1) 2.494(2), Re(1)–O(1) 2.145(5), Re(1)–C(23) 1.971(9), Re(1)–C(24) 2.007(8), Re(1)–C(25) 1.915(8), Re(1)–C(26) 2.003(8), ∠P(1)–Re(1)–C(23) 174.4(2), ∠C(24)–Re(1)–C(26) 176.4(3), ∠O(1)–Re(1)–C(25) 176.0(3).

Photochemical reactions of 1–3 with aryl halides in benzene

We have reported the photochemical reactions of cyclopentadienylrheniumtricarbonyls [(η⁵-C₅R₅)Re(CO)₃] (R = Me or H), or doubly-bridged biscyclopentadienyl dirhenium carbonyls with aryl halides in benzene, which generate biphenyl almost stoichiometrically, as well as the corresponding rhenium dicarbonyl products.¹⁵ In order to expand the reaction scope of this type, we used cyclometalated rhenium complexes 1–3 as starting materials and tried the similar photochemical reactions with aryl halides in benzene.

Photolysis of 1–3 with 5 equiv. of bromobenzene in benzene under N₂ for 1–4.5 h produced a yellow solution. After removal of the solvent under vacuum, the residue was carefully separated in air by column chromatography on silica gel, and afforded the corresponding phosphine-substituted tetracarbonyl rhenium bromides 9, 10, and 12 (Scheme 8). Besides, the reactions also stoichiometrically produced biphenyl (92–98%, entries 1–3 in Table 1). Further activation of C–I bond and the relatively inert C–Cl bond by 1 was also conducted by photolysis of 1 with iodobenzene and chlorobenzene in benzene, which similarly provided the corresponding phosphine-substituted tetracarbonyl rhenium iodide (13) and chloride (7), as well as the biphenyl (Scheme 8) (entries 4–5, Table 1). To expand the scope of the substrates, photolysis of several para-substituted aryl halides with 1 were examined, and the reactions gave similarly the rhenium halides 7 or 9, and the para-substituted biphenyl, no matter when the substituent is an electron-donating (MeO) or electron-withdrawing (CF₃, CH₃CO) group (entries 6–10, Table 1). Next, the source of the biphenyl was investigated. Photolysis of 1 with p-bromoanisole in benzene-d6 generated the 4-methoxybiphenyl-d5 (p-CH₃O–C₆H₄–C₆D₅), completely no biphenyl or 4,4′-dimethoxybiphenyl were observed. It is revealed that the p-CH₃O–C₆H₄ and C₆D₅ moieties in the resulting substituted biphenyl come from p-bromoanisole and solvent benzene-d6, respectively.

Scheme 8 Photochemical reactions of 1–3 with aryl halides in benzene.

Table 1 Photochemical reactions of 1–3 with aryl halides in benzene^a

Entry	Re	R′	X	t [h]	Product^b	Yield^c [%]	Re prod.
a Reaction conditions: irradiation of 1–3 (0.10 mmol) and Ar–X (0.50 mmol) in 10 mL benzene solvent with a 500 W high-pressure Hg lamp (λmax = 365 nm) in the ice-water bath.b Biphenyl products were isolated by routine silica gel column chromatography, and identified by comparing their respective ^1H NMR spectra with authentic samples.c Isolated yield (based on unrecovered 1–3).
1	1	H	Br	1	Biphenyl	>98	9
2	2	H	Br	1	Biphenyl	95	10
3	3	H	Br	1	Biphenyl	92	12
4	1	H	Cl	4	Biphenyl	85	7
5	1	H	I	1	Biphenyl	92	13
6	1	p-MeO	Br	4	p-MeO–biphenyl	98	9
7	1	p-CF3	Br	1	p-CF3–biphenyl	97	9
8	1	p-CH3CO	Br	1	p-CH3CO–biphenyl	80	9
9	1	p-MeO	Cl	4.5	p-MeO–biphenyl	90	7
10	1	p-CF3	Cl	1.5	p-CF3–biphenyl	87	7

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Rhenium-catalyzed direct arylation of arenes with aryl halides

On the basis of the photochemical reactions described above, we considered if the products, the phosphine-substituted tetracarbonyl rhenium halides, could revert to the original cyclometalated rhenium complexes 1–3, then a catalytic cycle would be created. Generally, cyclometalation of the ligand-substituted rhenium halides could be realized in the presence of base. Lu has reported a cyclometalation reaction of ReBr(CO)₄[C(NHR)(NHPh)] (R = ⁱPr, or Pr) with Et₂NH in CH₂Cl₂ to give Re(CO)₄[η²-C(NHR)(NHPh)].¹⁴ For optimization of reaction conditions, the cross-coupling of 4-bromoanisole with benzene was selected as the model reaction. The reactions were carried out with 0.5 mol% of cyclometalated rhenium complex 1 in benzene and different bases were used. Only NaOH as the base afforded the biphenyl product in moderate yield (59%, entry 1 of Table 2), whereas other tested bases such as K₂CO₃, KOAc, ^tBuOK, and pyrrolidine gave rise to lower yields (26–50%, entries 2–5). Other two cyclometalated rhenium complexes 2 and 3 as catalysts were also screened under the optimized conditions, complex 2 afforded slightly higher yield (68%, entry 6) than 1, while complex 3 gave much lower yield (26%, entry 7). To expand the scope of the substrates, other aryl bromides, including bromobenzene, and 4-bromobenzotrifluoride were examined, and the reactions afforded the corresponding biphenyl in yields of 65% and 60% (entries 8 and 9). Besides, the substrates chlorobenzene and iodobenzene were also tested using the catalyst 1, which resulted in lower yields (26% and 35%, entries 10 and 11).

Table 2 Rhenium-catalyzed direct arylation of arenes with aryl halides^a

Entry	Cat	Base	R′	X	t [h]	Product^b	Yield^b^,^c [%]
a Reaction conditions: irradiation of 1–3 (0.01 mmol), Ar–X (2.0 mmol), and base (2.0 mmol) in 20 mL benzene solvent with a 500 W high-pressure Hg lamp (λmax = 365 nm) in the ice-water bath.b Biphenyl products were isolated by routine silica gel column chromatography, and identified by comparing their respective ^1H NMR spectra with authentic samples.c Isolated yield (based on unrecovered 1–3).
1	1	NaOH	p-MeO	Br	4	p-MeO–biphenyl	59
2	1	K2CO3	p-MeO	Br	4	p-MeO–biphenyl	30
3	1	KOAc	p-MeO	Br	4	p-MeO–biphenyl	26
4	1	^tBuOK	p-MeO	Br	4	p-MeO–biphenyl	50
5	1	Pyrrolidine	p-MeO	Br	6	p-MeO–biphenyl	29
6	2	NaOH	p-MeO	Br	5	p-MeO–biphenyl	68
7	3	NaOH	p-MeO	Br	5	p-MeO–biphenyl	26
8	1	NaOH	H	Br	4	Biphenyl	65
9	1	NaOH	p-CF3	Br	3	p-CF3–biphenyl	60
10	1	NaOH	H	Cl	8	Biphenyl	26
11	1	NaOH	H	I	8	Biphenyl	35

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Unfortunately, rhenium catalysts 1–3 did not show higher activity to direct arylation of arenes with aryl halide under our current conditions. When we performed the photolysis of phosphine-substituted tetracarbonyl rhenium bromide 9 in benzene in the presence of NaOH, the expected cyclometalated rhenium complex 1 was observed, at the same time, we also obtained some free ligand, (1-naphthyl)diphenyl phosphine. Obviously, complex 9 was decomposed partially under UV irradiation, which may explain the moderate catalytic performance of 1. Based on the results described above, a plausible catalytic mechanism is tentatively proposed: first, a CO is dissociated from cyclometalated rhodium complexes 1–3 under UV irradiation to give a 16e⁻ unsaturated species; this is followed by oxidative addition of a C–X bond on aryl halides (Ph–X) to form the Re(III) intermediate. In the next step, a radical aromatic substitution that involves the aromatic radical with the solvent benzene might occur to give biphenyl and the corresponding cyclometalated rhenium hydride halide complexes, which undergo reductive elimination and recoordination of CO to afford the ring-opening complexes (7, 9, 10, 12, and 13). In the final step, cyclometalation of ring-opening complexes is achieved in the presence of base to generate the catalytic cycle. When we tried to synthesize the Re(III) intermediate to support the mechanism, unfortunately, we did not obtained the expected product either from photolysis of 1–3 in pure Ph–X or photolysis of 1–3 and Ph–X in cyclohexane.

Summary

A series of five-membered cyclometalated rhenium complexes (1–5) were synthesized by thermal treatment of aryl-substituted phosphines or phosphinites with Re₂(CO)₁₀ in chlorobenzene. The formation of cyclorhenated complexes went through an intramolecular activation of C(sp²)–H or C(sp³)–H bond. Competition reaction indicated that the aromatic C(sp²)–H bond is more likely to be activated than C(sp³)–H bond under the same conditions. The chemical behavior of the cyclorhenated complexes 1, 2 or 3 has been investigated, including photolysis of 1 or 2 in CHX₃, reaction of 1 with CF₃COOH, and photolysis of 1–3 with aryl halides in benzene, all of them caused the cleavage of Re–C σ bond and then the opening of metallacycle. Some reaction indicated the lower reactivity of the cyclometalated rhenium complexes in this work than that of N-contained cyclorhenated complexes. Besides, complexes 1–3 exhibited moderate catalytic activity on direct arylation of arenes with aryl halides under UV irradiation at a [Re] : substrate ratio of 1 : 200.

Experimental

General considerations

All reactions were carried out under nitrogen using standard Schlenk and vacuum line techniques; however the workup was carried out in air unless stated otherwise. All solvents were distilled from appropriate drying agents under nitrogen prior to use. ¹H (400 MHz), ¹³C{¹H} (100 MHz), and ³¹P{¹H} (162 MHz) NMR spectra were recorded on a Bruker AV400 instrument at room temperature with CDCl₃ as solvent. Chemical shifts were recorded in ppm, referenced to residual ¹H and ¹³C signals of the non-deuterated CDCl₃ (δ 7.26 and 77.16) as internal standards or to the ³¹P signal of PPh₃ (δ −5.65) as an external standard. Elemental analyses were performed on a Perkin-Elmer 240C analyzer. IR spectra were recorded as KBr disks on a Nicolet 560 ESP FTIR spectrometer. Re₂(CO)₁₀ was purchased from Strem without further purification; the ligands diisopropyl(1-naphthyl) phosphine,¹⁶ (1-naphthyl)diphenyl phosphine,¹⁶ diisopropyl(o-methylphenyl) phosphine,¹⁷ diisopropyl(2-methylnaphthyl) phosphine,¹⁷ o-methylphenyl diisopropylphosphinite,¹⁸ and 1-naphthyl diisopropylphosphinite¹⁸ were prepared by literature methods.

General procedure for the reactions of Re₂(CO)₁₀ with phosphines or phosphinites in chlorobenzene

A mixture of Re₂(CO)₁₀ (0.05 mmol) and phosphine or phosphinite (2.0 equiv.) in chlorobenzene (10 mL) was refluxed for 10 h and then concentrated under vacuum, the residue was chromatographed on a silica gel column with a mixture of petrol ether/CH₂Cl₂ as the eluent. The products were recrystallized from n-hexane/CH₂Cl₂ (1 : 1) at −10 °C to afford 1–6 as colorless crystals in yields of 45–85%.

1, (82% yield). ¹H NMR: δ 8.21 (d, J = 6.0 Hz, 1H), 7.95 (d, J = 8.0 Hz, 1H), 7.68 (d, J = 8.4 Hz, 1H), 7.60 (m, 1H), 7.55–7.49 (m, 5H), 7.44 (m, 6H), 7.35 (m, 1H). ¹³C{¹H} NMR: δ 189.1, 189.0, 152.4, 149.7, 144.3, 138.2, 134.7, 134.3, 132.8, 132.6, 132.5, 131.6, 130.8, 129.0, 128.9, 127.9, 124.6, 123.9. ³¹P{¹H} NMR: δ 26.4. IR (ν_CO): 2084 (m), 1998 (s), 1976 (s), 1955 (s), 1929 (s) cm⁻¹. Anal. calcd for C₂₆H₁₆O₄PRe: C, 51.23; H, 2.65. Found: C, 51.37; H, 2.42.

2, (85% yield). ¹H NMR: δ 8.16 (m, 1H), 7.85 (m, 1H), 7.58 (m, 2H), 7.43 (m, 1H), 7.28 (m, 1H), 2.58 (m, 2H, CH), 1.33 (dd, 6H, CH₃), 1.04 (dd, 6H, CH₃). ¹³C{¹H} NMR: δ 190.2, 189.8, 189.7, 189.4, 152.5, 149.7, 143.9, 138.8, 134.5, 132.2, 128.1, 127.5, 124.1, 123.8, 29.7, 29.5, 20.3, 20.1. ³¹P{¹H} NMR: δ 49.2. IR (ν_CO): 2076 (w), 1970 (s), 1925 (s) cm⁻¹. Anal. calcd for C₂₀H₂₀O₄PRe: C, 44.36; H, 3.72. Found: C, 44.49; H, 3.48.

3, (56% yield). ¹H NMR: δ 7.38 (m, 1H), 7.27–7.21 (m, 2H), 7.04 (m, 1H), 2.50 (m, 2H, CH₂), 2.49 (m, 2H, CH), 1.20 (dd, 6H, CH₃), 1.05 (dd, 6H, CH₃). ¹³C{¹H} NMR: δ 191.1, 190.3, 189.2, 162.8, 134.3, 131.4, 130.2, 129.5, 123.9, 77.4, 28.1, 27.8, 19.3, 18.9. ³¹P{¹H} NMR: δ 48.8. IR (ν_CO): 2073 (s), 1986 (s), 1931 (s) cm⁻¹. Anal. calcd for C₁₇H₂₀O₄PRe: C, 40.39; H, 3.99. Found: C, 40.74; H, 3.75.

4, (63% yield). ¹H NMR: δ 8.18 (m, 1H), 7.74 (m, 1H), 7.54 (m, 1H), 7.21 (m, 2H), 2.91 (m, 2H, CH), 2.71 (d, 3H, CH₃), 1.49 (dd, 6H, CH₃), 0.83 (dd, 6H, CH₃). ³¹P{¹H} NMR: δ 64.0. IR (ν_CO): 2074 (s), 1980 (s), 1927 (s) cm⁻¹. Anal. calcd for C₂₁H₂₂O₄PRe: C, 45.40; H, 3.99. Found: C, 45.27; H, 3.83.

5, (45% yield). ¹H NMR: δ 7.55 (d, J = 7.2 Hz, 1H), 6.84 (d, J = 7.2 Hz, 1H), 6.71 (t, J = 7.2 Hz, 1H), 2.53 (m, 2H, CH), 2.25 (s, 3H, CH₃), 1.30 (dd, 6H, CH₃), 1.21 (dd, 6H, CH₃). ³¹P{¹H} NMR: δ 158.3. IR (ν_CO): 2080 (s), 1982 (s), 1955 (s) cm⁻¹. Anal. calcd for C₁₇H₂₀O₅PRe: C, 39.15; H, 3.87. Found: C, 39.43; H, 3.81.

6, (66% yield). ¹H NMR: δ 8.20 (m, 2H), 7.80 (m, 2H), 7.58 (m, 2H), 7.48 (m, 4H), 7.34 (m, 2H), 7.15 (m, 2H), 2.85 (m, 4H, CH), 1.43–1.31 (m, 24H, CH₃). ¹³C{¹H} NMR: δ 200.2, 135.2, 128.0, 127.3, 126.6, 125.9, 125.4, 123.7, 122.2, 120.2, 115.8, 36.9, 32.8, 18.8, 18.5. ³¹P{¹H} NMR: δ 159.0. IR (ν_CO): 1965 (s), 1933 (s), 1897 (s) cm⁻¹. Anal. calcd for C₄₀H₄₂O₁₀P₂Re₂: C, 43.01; H, 3.79. Found: C, 42.85; H, 3.62.

General procedure for photolysis of 1 or 2 in CHX₃ (X = Cl, Br)

A solution of 1 or 2 (0.05 mmol) in CHX₃ (5 mL) was irradiated by UV light for 8 h, then the solvent was removed under vacuum, the residue was chromatographed on a silica gel column with a mixture of petrol ether/CH₂Cl₂ as the eluent. The products were recrystallized from n-hexane/CH₂Cl₂ (1 : 1) at −10 °C to afford 7–10 as colorless crystals in yields of 54–81%.

7, (62% yield). ¹H NMR: δ 8.01 (m, 1H), 7.96 (m, 2H), 7.78–7.73 (m, 4H), 7.54 (m, 1H), 7.50–7.39 (m, 8H), 7.36 (m, 1H). ³¹P{¹H} NMR: δ 3.1. IR (ν_CO): 2105 (m), 1997 (s), 1943 (s), 1896 (m) cm⁻¹. Anal. calcd for C₂₆H₁₇ClO₄PRe: C, 48.34; H, 2.65. Found: C, 48.57; H, 2.49.

8, (54% yield). ¹H NMR: δ 8.42 (d, J = 8.4 Hz, 1H), 7.94 (d, J = 8.0 Hz, 2H), 7.73 (m, qH), 7.65 (m, 1H), 7.60–7.52 (m, 2H), 3.39 (m, 2H, CH), 1.58 (dd, 6H, CH₃), 1.38 (dd, 6H, CH₃). ³¹P{¹H} NMR: δ 12.4. IR (ν_CO): 2105 (m), 2011 (s), 1922 (s) cm⁻¹. Anal. calcd for C₂₀H₂₁ClO₄PRe: C, 41.56; H, 3.66. Found: C, 41.35; H, 3.71.

9, (81% yield). ¹H NMR: δ 8.00 (m, 1H), 7.95 (m, 2H), 7.76 (m, 4H), 7.53 (m, 1H), 7.48–7.38 (m, 8H), 7.35 (m, 1H). ³¹P{¹H} NMR: δ −0.3. IR (ν_CO): 2105 (s), 2012 (s), 1947 (s), 1921 (s) cm⁻¹. Anal. calcd for C₂₆H₁₇BrO₄PRe: C, 45.23; H, 2.48. Found: C, 45.18; H, 2.36.

10, (78% yield). ¹H NMR: δ 8.38 (m, 1H), 7.94 (m, 2H), 7.73 (m, 1H), 7.65 (m, 1H), 7.60–7.53 (m, 2H), 3.44 (m, 2H, CH), 1.58 (dd, 6H, CH₃), 1.38 (dd, 6H, CH₃). ³¹P{¹H} NMR: δ 7.3. IR (ν_CO): 2106 (s), 2010 (s), 1919 (s) cm⁻¹. Anal. calcd for C₂₀H₂₁BrO₄PRe: C, 38.59; H, 3.40. Found: C, 38.43; H, 3.42.

Reaction of 1 with CF₃COOH

A solution of 1 (0.05 mmol) in CH₂Cl₂ (5 mL) was treated with excess CF₃COOH (0.3 mmol). The mixture was stirred for 24 h at room temperature. The solvent was removed under vacuum, and the residue was chromatographed on a silica gel column with a mixture of petrol ether/CH₂Cl₂ as the eluent. The product was recrystallized from n-hexane/CH₂Cl₂ (1 : 1) at −10 °C to afford 11 as colorless crystals in 48% yield.

11 ¹H NMR: δ 8.03 (d, J = 8.4 Hz, 1H), 7.97 (d, J = 8.0 Hz, 1H), 7.92 (d, J = 8.8 Hz, 1H), 7.56–7.41 (m, 12H), 7.38 (d, J = 8.8 Hz, 1H), 7.34 (m, 1H). ³¹P{¹H} NMR: δ 9.8. IR (ν_CO): 2106 (s), 1998 (s), 1960 (s), 1691 (s) cm⁻¹. Anal. calcd for C₂₈H₁₇F₃O₆PRe: C, 46.48; H, 2.37. Found: C, 46.63; H, 2.45.

General procedure for photolysis of 1–3 with aryl halides in C₆H₆

A solution of 1–3 (0.05 mmol) and aryl halides (0.25 mmol) in dry benzene (10 mL) in nitrogen flushed quartz tube was additionally degassed by bubbling nitrogen thru for 5 min. After irradiation for 1–4.5 h in the ice-water bath, the solvent was removed under the reduced pressure. The residue was chromatographed in air on silica gel using petroleum ether–CH₂Cl₂ mixture as eluent. The first band (colorless) afforded the corresponding biphenyl as white solids (80–98%). The second band (colorless) gave corresponding phosphine-substituted tetracarbonyl rhenium halides (7, 9, 10, 12, and 13).

12, (32% yield). ¹H NMR: δ 7.46 (m, 1H), 7.32 (m, 3H), 3.27 (m, 2H, CH), 2.60 (s, 3H, CH₃), 1.58 (dd, 6H, CH₃), 1.37 (dd, 6H, CH₃). ³¹P{¹H} NMR: δ 9.8. IR (ν_CO): 2102 (s), 2018 (s), 1980 (s), 1938 (s) cm⁻¹. Anal. calcd for C₁₇H₂₁BrO₄PRe: C, 34.82; H, 3.61. Found: C, 35.13; H, 3.55.

13, (53% yield). ¹H NMR: δ 8.01–7.92 (m, 3H), 7.79–7.74 (m, 4H), 7.53 (m, 1H), 7.49–7.34 (m, 9H). ³¹P{¹H} NMR: δ −6.2. IR (ν_CO): 2099 (s), 1992 (br), 1942 (s) cm⁻¹. Anal. calcd for C₂₆H₁₇IO₄PRe: C, 42.34; H, 2.32. Found: C, 42.47; H, 2.15.

General procedure for catalytic reactions of 1–3 with aryl halides in C₆H₆

A solution of 1–3 (0.01 mmol), aryl halides (2.0 mmol) and base (2.0 mmol) in dry benzene (20 mL) in nitrogen flushed quartz tube was additionally degassed by bubbling nitrogen thru for 5 min. After irradiation for 3–8 h in the ice-water bath, the solvent was removed under the reduced pressure. The residue was chromatographed in air on silica gel using petroleum ether as eluent. The corresponding biphenyl products were obtained as colorless solids in yields of 26–68%.

Crystallography

Single crystals of complexes 1, 6, 9, and 11 (CCDC 1520723–1520726) suitable for X-ray diffraction were obtained by crystallization from n-hexane/CH₂Cl₂ (1 : 1). Data collection was performed on a Bruker SMART 1000, using graphite-monochromated Mo Kα radiation (ω–2θ scans, λ = 0.71073 Å). Semiempirical absorption corrections were applied for all complexes. The structures were solved by direct methods and refined by full-matrix least squares. All calculations were using the SHELXTL-97 program system. The crystal data and summary of X-ray data collection are presented in Table 3.

Table 3 Crystal data and summary of X-ray data collection for 1, 6, 9, and 11

	1	6	9	11
Formula	C26H16O4PRe	C20H21O5PRe	C26H17BrO4PRe	C28H17F3O6PRe
fw	609.56	558.54	690.47	723.58
T (K)	173.00(10)	296(2)	173.00(10)	154.5(9)
Cryst syst	Triclinic	Orthorhombic	Triclinic	Monoclinic
Space group	P1̄	Fdd2	P1̄	P21/c
a (Å)	11.9989(3)	24.564(3)	9.9390(5)	24.3119(9)
b (Å)	14.4984(3)	42.547(5)	11.0789(5)	11.3315(3)
c (Å)	14.8134(4)	7.7934(8)	11.2433(6)	19.6695(6)
α (deg)	64.239(2)	90	94.119(4)	90
β (deg)	84.8034(19)	90	97.332(4)	98.995(3)
γ (deg)	74.8834(19)	90	99.066(4)	90
V (Å^3)	2239.82(10)	8145.1(15)	1207.22(10)	5352.1(3)
Z	4	16	2	8
Dcalc (g cm^−3)	1.808	1.822	1.900	1.796
μ (mm^−1)	11.544	6.073	6.784	4.662
F(000)	1176.0	4336	660.0	2800.0
Cryst size (mm)	0.20 × 0.18 × 0.15	0.22 × 0.21 × 0.20	0.25 × 0.15 × 0.14	0.25 × 0.24 × 0.2
Max. 2θ (deg)	148.728	53.414	50.018	50.02
No. of reflns collected	16 330	13 167	7610	45 685
No. of indep reflns/Rint	8839/0.0273	4177/0.0346	4240/0.0407	9416/0.0564
No. of params	577	254	320	703
Goodness-of-fit on F^2	1.061	1.001	0.958	1.087
R1, wR2 [I > 2σ(I)]	0.0294, 0.0764	0.0245, 0.0435	0.0329, 0.0571	0.0487, 0.1086
R1, wR2 (all data)	0.0313, 0.0780	0.0295, 0.0453	0.0394, 0.0608	0.0563, 0.1129
Largest diff peak and hole (e Å^−3)	1.88, −0.99	0.578, −0.298	1.16, −1.42	1.92, −1.43

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Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 21002069 and 21572160), the Natural Science Foundation of Tianjin (No. 14JCYBJC20300), and the Program for Innovative Research Team in University of Tianjin (TD12-5038).

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Footnote

† CCDC 1520723–1520726. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra28263f

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