Synthesis and reactivity of nitrido-rhenium and -osmium complexes with an oxygen tripod ligand

Abstract

Interaction of [ReNCl₃(PPh₃)₂] or [ReOCl₂(PPh₃)₃] with NaL_OEt (L_OEt = [Co(η⁵-C₅H₅){PO(OEt)₂}₃]) afforded [ReL_OEtN(PPh₃)Cl] 1 and [ReL_OEtOCl₂] 2, respectively. Reaction of 1 with AgBF₄ gave the nitridorhenium(VI) complex [ReL_OEtN(PPh₃)Cl]BF₄1·BF₄, which has a μ_eff of 1.8 μ_B. Treatment of 1 with MeOSO₂CF₃, PhCH₂Br or [Ph₃C]BF₄ afforded the respective organoimido species [ReL_OEt(NMe)(PPh₃)Cl][CF₃SO₃] 3, [ReL_OEt(NCH₂Ph)(PPh₃)Cl]Br 4, and [ReL_OEt(NCPh₃)(PPh₃)Cl] 5. Reaction of 1 with [Au(PPh₃)(CF₃SO₃)], [Ru(Et₂dtc)(PPh₃)₂(CO)(CF₃SO₃)], or [ReMeO₃] yielded the bimetallic nitrido complexes [Au(PPh₃){NReL_OEt(PPh₃)Cl}][CF₃SO₃] 6, [Ru(Et₂dtc)(PPh₃)(H₂O)(CO){NReL_OEt(PPh₃)Cl}][CF₃SO₃] 7 or [ReMeO₃{NReL_OEt(PPh₃)Cl}] 8, respectively. Treatment of [NBu₄ⁿ][OsNCl₄] with NaL_OEt gave [OsL_OEtNCl₂] 9. The average Os–O, Os–Cl and Os–N distances in 9 are 2.066, 2.289 and 2.58(1) Å, respectively. Reaction of 9 with PPh₃ afforded the osmium(IV) phosphoran iminate species [OsL_OEt(NPPh₃)Cl₂] 10, which has a μ_eff of 2.0 μ_B. The average Os–O, Os–Cl and Os–N distances in 10 are 2.099, 2.342, 1.893(5) Å, respectively, the Os–N–P angle being 137.5(3)°. The formal potentials of the L_OEt–Re and –Os complexes have been determined by cyclic voltammetry. On the basis of the Re^VI–Re^V formal potential, the π-donor strength was found to decrease in the order N³⁻ > [NAu(PPh₃)]²⁻ > NMe²⁻.

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Introduction

Transition metal nitrido complexes are of interest because of their applications in metal-mediated nitrogen atom transfer reactions.^1–3 While early transition metal nitrides are basic and react with alkyl halides or Lewis acidic metal centres to give imido or μ-nitrido complexes, the late transition metal analogues exhibit electrophilic properties.⁴ In an effort to understand the factors governing reactivity of metal nitrides, isoelectronic nitrido complexes of Re^V and Os^VI were synthesized and their reactivities compared. Of particular interest are complexes of Re and Os with Kläui’s tripod ligand [Co(η⁵-C₅H₅){PO(OEt)₂}₃] or L_OEt, which is known to stabilise metal ions in high oxidation states due to its π-donating capability.⁵ It may be noted that while Ru–L_OEt complexes are well documented,^5,6 there are few examples of the osmium congeners.⁷ In this paper we report on the synthesis and crystal structures of nitrido complexes of Re^V and Os^VI with L_OEt and their reactivity toward electrophiles and PPh₃.

Experimental

NMR Spectra were recorded on a Bruker ALX 300 spectrometer operating at 300 and 121.5 MHz for ¹H and ³¹P, respectively. Chemical shifts (δ, ppm) are reported with reference to Si(CH₃)₄ (¹H) and H₃PO₄ (³¹P). Infrared spectra (Nujol) were recorded on a Perkin-Elmer 16 PC FT-IR spectrophotometer. Cyclic voltammetry was performed with a Princeton Applied Research (PAR) Model 273A potentiostat. The working and reference electrodes were glassy carbon and Ag–AgNO₃ (0.1 M in acetonitrile), respectively. Potentials were reported with reference to the ferrocenium–ferrocene couple (Cp₂Fe^+/0). Elemental analyses were performed by Medac Ltd, Surrey, UK.

Materials

The compounds [ReN(PPh₃)₂Cl₂],⁸ [ReO(PPh₃)₂Cl₃],⁹ [NBu₄ⁿ][OsNCl₄],¹⁰ [Au(PPh₃)(CF₃SO₃)] ¹¹ and [ReMeO₃] ¹² were prepared according to the literature methods. The triflate compound [Ru(Et₂dtc)(PPh₃)₂(CO)(CF₃SO₃)] was synthesized by reaction of [Ru(Et₂dtc)(PPh₃)₂(CO)H] (Et₂dtc = N,N-diethyldithiocarbamate) ¹³ with triflic acid (CF₃SO₃H) as described elsewhere.¹⁴ The compounds MeOSO₂CF₃, PhCH₂Br and [Ph₃C]BF₄ were obtained from Aldrich and used as received.

Preparations

[ReL_OEtN(PPh₃)Cl] 1.. A mixture of [ReN(PPh₃)₂Cl₂] (0.71 g, 0.90 mmol) and NaL_OEt (0.5 g, 0.896 mmol) in tetrahydrofuran (thf )–toluene (50 cm³, 1∶1) was heated at reflux overnight. The volatiles were removed in vacuo and the residue was washed with hexane. Recrystallisation from Et₂O–hexane afforded orange-red crystals, suitable for X-ray analysis (yield: 0.49 g, 54%). NMR (CDCl₃): ¹H, δ 0.77 (t, 3 H, CH₃), 1.02 (t, 3 H, CH₃), 1.18 (t, 3 H, CH₃), 1.30 (t, 3 H, CH₃), 1.36 (t, 3 H, CH₃), 1.37 (t, 3 H, CH₃), 2.71–2.93 (m, 2 H, OCH₂), 3.54–3.69 (m, 2 H, OCH₂), 3.97–4.05 (m, 2 H, OCH₂), 4.29–4.04 (m, 6 H, OCH₂), 4.99 (s, 5 H, C₅H₅) and 7.28–7.83 (m, 15 H, PPh₃); ³¹P, δ 10.83 (s, PPh₃) and 109.1–110.5 (m, P(O)(OEt)₂). IR (cm⁻¹): 946 [ν(Re≡N)]. MS (FAB): m/z 1032 (M⁺) (Found: C, 40.2; H, 4.91; N, 1.32. Calc. for C₃₅H₅₀ClCoNO₉P₄Re: C, 40.7; H, 4.84; N, 1.36%).

[ReL_OEtOCl₂] 2.. A mixture of [ReO(PPh₃)₂Cl₃] (0.3 g, 0.361 mmol) and NaL_OEt (0.15 g, 0.269 mmol) in thf–toluene (50 cm³, 1∶1) was heated at reflux overnight. The volatiles were removed in vacuo and the residue was washed with hexane. Recrystallisation from ether–hexane afforded yellow crystals (yield: 0.10 g, 48%). Despite several attempts, we have not been able to obtain good carbon analysis for complex 2. NMR (CDCl₃): ¹H, δ 1.26 (t, 6 H, CH₃), 1.31 (t, 6 H, CH₃), 1.33 (t, 6 H, CH₃), 4.07–4.30 (m, 12 H, OCH₂) and 5.14 (s, 5 H, C₅H₅); ³¹P-{¹H}, δ 124.2 (m, P(O)(OEt)₂). MS (FAB): m/z 808 (M⁺) (Found: C, 27.1; H, 4.32. Calc. for C₁₇H₃₅Cl₂CoO₁₀P₃Re: C, 25.3; H, 4.34%).

[ReL_OEtN(PPh₃)Cl]BF₄1·BF₄.. To a solution of complex 1 (80 mg, 0.078 mmol) in CH₂Cl₂ (25 cm³) was added AgBF₄ (24 mg, 0.088 mmol). The resulting mixture was stirred for 40 min and filtered. The solvent was pumped off and the residue extracted with CH₂Cl₂. Recrystallisation from CH₂Cl₂–Et₂O–hexane afforded brown crystals (109 mg, 53%). μ_eff = 1.7 μ_B (Found: C, 37.7; H, 4.48; N, 1.19. Calc. for C₃₅H₅₀BClCoF₄NO₉P₄Re: C, 37.5; H, 4.47; N, 1.25%).

[ReL_OEt(NMe)(PPh₃)Cl][CF₃SO₃] 3.. To a solution of complex 1 (80 mg, 0.0784 mmol) in diethyl ether (20 cm³) was added MeOSO₂CF₃ (0.02 ml, 0.14 mmol) at 0 °C under nitrogen. The resulting mixture was allowed to warm to room temperature and stirred for 1 d during which it changed from red to yellow. The solvent was pumped off and the residue extracted with CH₂Cl₂. Recrystallisation from CH₂Cl₂–Et₂O–hexane afforded yellow crystals (yield: 54 mg, 58%). NMR (CDCl₃): ¹H, δ 0.84 (t, 3 H, CH₃), 1.08 (t, 3 H, CH₃), 1.29–1.43 (t, overlapping, 12 H, CH₃), 1.95 (d, ⁴J_HP = 4.92 Hz, 3 H, NCH₃), 3.14–3.26 (m, 2 H, OCH₂), 3.52–3.58 (m, 2 H, OCH₂), 4.09–4.36 (m, 8 H, OCH₂), 5.11 (s, 5 H, C₅H₅) and 7.44–7.59 (m, 15 H, PPh₃); ³¹P, δ −11.38 (s, PPh₃), 110.1, 119.5 and 124.0 (m, P(O)(OEt)₂). MS (FAB): m/z 1047, (M − CF₃SO₃ + 1)⁺ (Found: C, 35.6; H, 4.34; N, 1.12. Calc. for C₃₇H₅₃ClCoF₃N₁₂P₄ReS₂·H₂O: C, 36.0; H, 4.62; N, 1.14%).

[ReL_OEt(NCH₂Ph)(PPh₃)Cl]Br 4.. This was prepared similarly as for complex 3 from 1 (80 mg, 0.0775 mmol) and PhCH₂Br (0.02 cm³, 0.078 mmol) Recrystallisation from CH₂Cl₂–Et₂O–hexane afforded yellow crystals (37 mg, 41%). NMR (CDCl₃): ¹H, δ 0.857 (t, 3 H, CH₃), 1.06 (t, 3 H, CH₃), 1.16 (t, 3 H, CH₃), 1.22 (t, 3 H, CH₃), 1.31 (t, 3 H, CH₃), 1.34 (t, 3 H, CH₃), 3.01–3.40 (m, 4 H, OCH₂), 3.53–3.87 (m, 4 H, OCH₂), 4.04–4.32 (m, 4 H, OCH₂), 5.13 (s, 5 H, C₅H₅) and 6.91–7.58 (m, 20 H, phenyl protons); ³¹P-{¹H}, δ −12.6 (s, PPh₃) and 124.1 (m, P(O)(OEt)₂) (Found: C, 42.1; H, 4.98; N, 1.10. Calc. for C₄₂H₅₇BrClCoN₉P₄Re: C, 41.9; H, 4.74; N, 1.16%).

[ReL_OEt(NCPh₃)(PPh₃)Cl]BF₄5.. This was prepared similarly as for complex 3 from 1 (94 mg, 0.091 mmol) and [CPh₃]BF₄ (30 mg, 0.09 mmol). Recrystallisation from CH₂Cl₂–Et₂O–hexane afforded yellow crystals (74 mg, 60%). NMR (CDCl₃): ¹H, δ 0.80 (t, 3 H, CH₃), 1.05 (t, 3 H, CH₃), 1.18–1.29 (t, overlapping, 9 H, CH₃), 1.34 (t, 3 H, CH₃), 2.35–2.42 (m, 2 H, OCH₂), 3.18–3.55 (m, 4 H, OCH₂), 3.89–4.34 (m, 6 H, OCH₂), 5.21 (s, 5 H, C₅H₅) and 6.92–7.33 (m, 30 H, phenyl protons). ³¹P-{¹H}, δ −10.64 (s, PPh₃) and 119.9–124.5 (m, P(O)(OEt)₂); ¹⁹F, δ −155.0 (BF₄). MS (FAB): m/z 1277, (M − BF₄ + 2)⁺ (Found: C, 47.9; H, 4.86; N, 1.05. Calc. for C₅₄H₆₅BClCoF₄NO₉P₄Re: C, 47.6; H, 4.77; N, 1.03%).

[Au(PPh₃){NReL_OEt(PPh₃)Cl}][CF₃SO₃] 6.. This was prepared as for complex 3 from 1 (130 mg, 0.126 mmol) and [Au(PPh₃)(CF₃SO₃)] (67 mg, 0.126 mmol) Recrystallisation from CH₂Cl₂–Et₂O–hexane afforded greenish yellow crystals (109 mg, 53%). NMR (CDCl₃): ¹H, δ 0.82 (t, 3 H, CH₃), 1.11 (t, 3 H, CH₃), 1.21 (t, overlapping, 6 H, CH₃), 1.29 (t, 3 H, CH₃), 1.34 (t, 3 H, CH₃), 2.90–3.11 (m, 2 H, OCH₂), 3.44–3.63 (m, 6 H, OCH₂), 4.05–4.39 (m, 4 H, OCH₂), 5.05 (s, 5 H, C₅H₅) and 6.91–7.58 (m, 30 H, phenyl protons); ³¹P-{¹H}, δ −12.6 (s, RePPh₃), 30.3 (s, AuPPh₃) and 118.7–120.0 (m, P(O)(OEt)₂). IR (cm⁻¹): 953 [ν(Re[double bond, length half m-dash]N–Au)]. MS (FAB): m/z 1492, (M − BF₄ + 1)⁺ (Found: C, 39.2; H, 4.01; N, 0.84. Calc. for C₅₄H₆₅AuClCoF₃NO₁₂P₅ReS: C, 39.5; H, 3.96; N, 0.85%).

[Ru(Et₂dtc)(PPh₃)(H₂O)(CO){NReL_OEt(PPh₃)Cl}][CF₃SO₃] 7.. This was prepared similarly as for complex 3 from 1 (100 mg, 0.097 mmol) and [Ru(Et₂dtc)(PPh₃)₂(CO)(CF₃SO₃)] (92 mg, 0.097 mmol). Recrystallisation from CH₂Cl₂–hexane afforded brown crystals (yield: 82 mg, 49%). NMR (CDCl₃): ¹H, δ 0.77 (t, 6 H, CH₃), 1.09 (t, 3 H, CH₃), 1.18–1.30 (overlapping t, 9 H, CH₃), 1.40 (t, 3 H, CH₃), 2.61–3.04 (overlapping NCH₂), 3.31–4.39 (m, 12 H, OCH₂), 4.39 (m, 12 H, OCH₂), 5.14 (s, 5 H, C₅H₅) and 7.10–7.48 (m, 30 H, phenyl protons); ³¹P-{¹H}, δ 6.02 (s, RePPh₃), 43.3 (s, RuPPh₃) and 118.3–120.0 (m, PO(OEt)₂). IR (cm⁻¹): 1966 [ν(CO)]. MS (FAB): m/z 1572 (M⁺ − CF₃SO₃) (Found: C, 41.13; H, 4.45; N, 1.55. Calc. for C₇₆H₉₀ClCoF₃N₂O₁₀P₅ReRuS₂: C, 41.01; H, 4.46; N, 1.62%).

[ReMeO₃{NReL_OEt(PPh₃)Cl}] 8.. This was prepared similarly as for complex 3 from 1 (82 mg, 0.08 mmol) and [ReMeO₃] (20 mg, 0.08 mmol). The red product was recrystallised from CH₂Cl₂–hexane (yield: 50 mg, 50%). The complex was found to be unstable and decomposed to 1 and ReMeO₃ in solution, as reflected by NMR spectroscopy (see Results and discussion). Good analytical data were not obtained apparently because of dissociation of ReMeO₃ during recrystallisation. IR (cm⁻¹): 953 [ν(Re[double bond, length half m-dash]N–Re)] and 920 [ν(Re[double bond, length half m-dash]O)] (Found: C, 35.14; H, 4.55; N, 1.10. Calc. for C₃₆H₅₃ClCoNO₉P₄Re₂: C, 33.71; H, 4.14; N, 1.09%).

[OsL_OEtNCl₂] 9.. A mixture of [NBuⁿ₄][OsNCl₄] (60 mg, 0.098 mmol) and NaL_OEt (50 mg, 0.0896 mmol) in acetone (40 cm³) was heated at reflux overnight. The volatiles were removed in vacuo and the residue was washed with hexane. Recrystallisation from CH₂Cl₂–hexane afforded deep red crystals, which are suitable for X-ray analysis (yield: 0.49 g, 54%). NMR (CDCl₃): ¹H, δ 1.21 (t, 6 H, CH₃), 1.33 (t, 6 H, CH₃), 1.35 (t, 6 H, CH₃), 3.99–4.06 (m, 4 H, OCH₂), 4.20–4.37 (m, 8 H, OCH₂) and 5.10 (s, 5 H, C₅H₅); ³¹P-{¹H}, δ 109.9, 122.6–123.7 (m, P(O)(OEt)₂). MS (FAB): m/z 811, (M + 1)⁺ (Found: C, 25.4; H, 4.39; N, 1.73. Calc. for C₁₇H₃₅Cl₂CoNO₉OsP₃: C, 25.2; H, 4.32; N, 1.73%).

[OsL_OEt(NPPh₃)Cl₂] 10.. To a solution of complex 9 (50 mg, 0.062 mmol) in CH₂Cl₂ (25 cm³) was added PPh₃ (20 mg, 0.076 mmol). The resulting mixture was stirred for 12 h, the solvent pumped off and the residue extracted with CH₂Cl₂. Recrystallisation from CH₂Cl₂–Et₂O–hexane afforded brown crystals, suitable for X-ray analysis (yield: 32 mg, 49%). μ_eff = 2.0 μ_B (Found: C, 39.2; H, 4.73; N, 1.29. Calc. for C₃₅H₅₀Cl₂CoN₉OsP₄: C, 39.2; H, 4.66; N, 1.31%).

X-Ray crystallography

A summary of pertinent crystallographic data and experimental details for complexes 1, 9 and 10 is shown in Table 1. All data were collected on a MAR research image-plate diffractometer using Mo-Kα radiation (λ = 0.71073 Å) with a graphite crystal monochromator in the incident beam. The diffracted intensities were corrected for Lorentz-polarisation effects. For 1 the nitride and chloride were found to be twofold disordered. A model with occupancies of 0.7 and 0.3 for the two sites was used for refinement to give a reasonable set of thermal and positional parameters. In complex 9 the phosphorus atoms of the tripod ligand L_OEt exhibit twofold positional disorder. A model with occupancies of 0.5 each gave the best results in terms of both R factor and positional parameters of the ligand. All structures were solved by direct methods and refined on F by a full-matrix least-squares analysis. Non-hydrogen atoms, except the disordered nitride and chloride of complex 1, were refined anisotropically. Calculations were performed on a Silicon-Graphics computer, using the program package TEXSAN.¹⁵ Hydrogen atoms were included and fixed in their idealised positions (C–H 0.95 Å). Selected bond lengths and angles for 1, 9 and 10 are listed in Tables 2–4, respectively.

Table 1 Crystallographic data and experimental details for [ReL_OEtN(PPh₃)Cl] 1, [OsL_OEtNCl₂] 9 and [OsL_OEt(NPPh₃)Cl₂] 10

	1	9	10
Empirical formula	C35H50ClCoNO9P4Re	C17H35Cl2CoNO9OsP3	C35H50Cl2CoNO9OsP4
M	1033.27	810.43	1072.72
Crystal system	Monoclinic	Monoclinic	Monoclinic
Space group	P 21/n (no. 14)	P 21/n (no. 14)	P 21/c (no. 14)
a/Å	14.031(1)	12.326(1)	18.414(2)
b/Å	14.582(1)	18.623(1)	12.214(3)
c/Å	20.993(2)	12.820(1)	19.449(1)
β/°	92.58(1)	94.49(2)	106.40(9)
V/Å^3	4290.8(4)	2933.8(4)	4196.3(6)
Z	4	4	4
D calc/g cm^−3	1.599	1.835	1.513
T/K	298	298	298
μ/mm^−1	3.467	5.277	3.749
No. reflections measured	8244	5513	8165
No. reflections observed	5224	4088	6731
R(F )	0.042	0.056	0.035
R ′	0.045	0.069	0.062

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Table 2 Selected bond lengths (Å) and angles (°) for [ReL_OEtN(PPh₃)Cl] 1

Re(1)–Cl(1)	2.292(3)	Re(1)–Cl(2)	2.216(6)
Re(1)–P(4)	2.372(1)	Re(1)–O(1)	2.251(3)
Re(1)–O(2)	2.179(4)	Re(1)–O(3)	2.115(3)
Re(1)–N(1)	1.795(8)	Re(1)–N(2)	1.97(2)
C(1)–Re(1)–Cl(2)	102.3(2)	Cl(1)–Re(1)–P(4)	89.54(8)
Cl(1)–Re(1)–O(1)	88.5(1)	Cl(1)–Re(1)–O(2)	165.5(1)
Cl(1)–Re(1)–O(3)	89.7(1)	Cl(1)–Re(1)–N(1)	101.8(2)
Cl(1)–Re(1)–N(2)	8.9(7)	Cl(2)–Re(1)–P(4)	99.4(2)
Cl(2)–Re(1)–O(1)	164.7(2)	Cl(2)–Re(1)–O(2)	90.8(2)
Cl(2)–Re(1)–O(3)	87.8(2)	Cl(2)–Re(1)–N(1)	12.6(3)
Cl(2)–Re(1)–N(2)	93.6(7)	P(4)–Re(1)–O(1)	91.32(9)
P(4)–Re(1)–O(2)	94.6(1)	P(4)–Re(1)–O(3)	172.7(1)
P(4)–Re(1)–N(1)	86.9(3)	P(4)–Re(1)–N(2)	89.3(7)
O(1)–Re(1)–O(2)	94.6(1)	O(1)–Re(1)–O(3)	81.4(1)
O(1)–Re(1)–N(1)	169.5(2)	O(1)–Re(1)–N(2)	97.4(7)
O(2)–Re(1)–O(3)	84.4(1)	O(2)–Re(1)–N(1)	92.3(2)
O(2)–Re(1)–N(2)	173.6(7)	O(3)–Re(1)–N(1)	100.4(3)
O(3)–Re(1)–N(2)	91.1(7)	N(1)–Re(1)–N(2)	92.9(7)

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Table 3 Selected bond lengths (Å) and angles (°) for [OsL_OEtNCl₂] 9

Os(1)–Cl(1)	2.300(4)	Os(1)–Cl(2)	2.277(7)
Os(1)–O(1)	2.022(8)	Os(1)–O(2)	2.159(8)
Os(1)–O(3)	2.017(8)	Os(1)–N(1)	1.58(1)
Cl(1)–Os(1)–Cl(4)	87.7(2)	Cl(1)–Os(1)–O(1)	167.7(4)
Cl(1)–Os(1)–O(2)	87.6(3)	Cl(1)–Os(1)–O(3)	89.1(3)
Cl(1)–Os(1)–N(1)	96.8(4)	Cl(2)–Os(1)–O(1)	90.4(3)
Cl(2)–Os(1)–O(2)	86.5(4)	Cl(2)–Os(1)–O(3)	166.0(4)
Cl(2)–Os(1)–N(1)	98.0(5)	O(1)–Os(1)–O(2)	80.1(5)
O(1)–Os(1)–O(3)	89.8(3)	O(1)–Os(1)–N(1)	95.6(6)
O(2)–Os(1)–O(3)	79.7(4)	O(2)–Os(1)–N(1)	173.8(4)
O(3)–Os(1)–N(1)	96.0(5)

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Table 4 Selected bond lengths (Å) and angles (°) for [OsL_OEt(NPPh₃)Cl₂] 10

Os(1)–Cl(1)	2.338(2)	Os(1)–Cl(2)	2.346(2)
Os(1)–O(1)	2.110(4)	Os(1)–O(2)	2.097(4)
Os(1)–O(3)	2.090(4)	Os(1)–N(1)	1.893(5)
P(4)–N(1)	1.575(5)
Cl(1)–Os(1)–Cl(2)	91.63(7)	Cl(1)–Os(1)–O(1)	87.7(1)
Cl(1)–Os(1)–O(2)	174.1(1)	Cl(1)–Os(1)–O(3)	91.9(1)
Cl(1)–Os(1)–N(1)	94.7(2)	Cl(2)–Os(1)–O(1)	89.7(1)
Cl(2)–Os(1)–O(2)	89.1(1)	Cl(2)–Os(1)–O(3)	173.8(1)
Cl(2)–Os(1)–N(1)	94.3(2)	O(1)–Os(1)–O(2)	86.4(2)
O(1)–Os(1)–O(3)	85.4(2)	O(1)–Os(1)–N(1)	175.2(2)
O(2)–Os(1)–O(3)	86.8(2)	O(2)–Os(1)–N(1)	91.1(2)
O(3)–Os(1)–N(1)	90.4(2)	Os(1)–N(1)–P(4)	137.5(3)

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CCDC reference number 186/1741.

Results and discussion

L_OEtRe complexes

The syntheses of L_OEtRe complexes are summarised in Scheme 1. Interaction of [ReNCl₂(PPh₃)₂] and [ReOCl₃(PPh₃)₂] with NaL_OEt afforded air-stable [ReL_OEtN(PPh₃)Cl] 1 and [ReL_OEtOCl₂] 2, respectively. The IR spectrum of 1 shows a peak at 946 cm⁻¹, which is tentatively assigned as ν(Re≡N). The ν(Re[double bond, length half m-dash]O) for 2 was not assigned due to the presence of ligand bands in the region. X-Ray quality crystals of complex 1 were obtained by recrystallisation from CH₂Cl₂–hexane. However, the chloride and nitride in 1 were found to be twofold disordered. A model with site occupancies of 0.3 and 0.7 for the two sites was used for the refinement. Fig. 1 shows a perspective view of the molecule; selected bond lengths and angles are listed in Table 2. The Re–P and average Re–O distances were found to be 2.372(1) and 2.183 Å. Oxidation of 1 with AgBF₄ afforded [ReL_OEtN(PPh₃)Cl]BF₄1·BF₄. The measured μ_eff of 1.8 μ_B is consistent with the formulation of Re^VI. Although the Re^VI–Re^V couple for 2 is reversible (see later section), no well defined products were obtained for oxidation of 2 with silver(I) salts. The nitride in 1 was found to be nucleophilic and react with electrophiles to give imidorhenium(V) complexes. Thus, treatment of 1 with MeOSO₂CF₃ afforded the methylimido complex [ReL_OEt(NMe)(PPh₃)Cl][CF₃SO₃] 3. The N-methyl protons in 3 were found to couple with the phosphorus of PPh₃ and appear as a doublet at δ 1.95 (⁴J_HP = 4.92 Hz). Similarly interaction of 1 with PhCH₂Br or [Ph₃C]BF₄ gave the respective imido species [ReL_OEt(NCH₂Ph)(PPh₃)Cl]Br 4 or [ReL_OEt(NCPh₃)(PPh₃)Cl]BF₄5. These imidorhenium(V) complexes are air stable in both the solid state and solution.

Scheme 1

Fig. 1 Perspective view of [ReL_OEtN(PPh₃)Cl] 1.

Bimetallic nitrido complexes containing L_OEtRe

Reaction of complex 1 with organometallic triflates afforded bimetallic nitrido complexes. Thus, 1 reacted with [Au(PPh₃)(CF₃SO₃)] to give [Au(PPh₃){NReL_OEt(PPh₃)Cl}][CF₃SO₃] 6, isolated as an air-stable yellow solid. No reactions were found between 1 and [Ir(CO)(PPh₃)₂(CF₃SO₃)], which was prepared in situ from trans-[Ir(CO)(PPh₃)₂Cl] and Ag(CF₃SO₃), possibly because of steric reasons. The IR spectrum of 6 shows ν(Re[double bond, length half m-dash]N–Au) at 953 cm⁻¹, which is higher than the ν(Re≡N) for 1. Enhancement of the metal–nitride stretching frequency upon formation of μ-nitride bridges has also been observed for binuclear complexes of Os nitrides, e.g. in [Au(PPh₃)(NOsO₃)].¹⁶ Reaction of 1 with [Ru(Et₂dtc)(PPh₃)₂(CO)(CF₃SO₃)],¹⁴ which was prepared from [Ru(Et₂dtc)(PPh₃)₂(CO)H] ¹³ and triflic acid, afforded [Ru(Et₂dtc)(PPh₃)(H₂O)(CO){NReL_OEt(PPh₃)Cl}][CF₃SO₃] 7. One PPh₃ dissociated from Ru upon formation of 7 possibly due to steric congestion in the bimetallic complex. A preliminary diffraction study of 7 showed that the aqua ligand, presumably derived from moisture in the solvent, is trans to the CO, and the nitride is trans to a sulfur of Et₂dtc as shown below.

Reaction of 1 with [ReMeO₃] afforded the adduct [ReMeO₃{NReL_OEt(PPh₃)Cl}] 8. While complex 8 is stable in the solid state, it decomposed readily to 1 and [ReMeO₃] in solution. The NMR spectrum of 8 in CDCl₃ only shows respective signals due to 1 and [ReMeO₃]. The IR Re[double bond, length half m-dash]O stretching frequency for 8 of 920 cm⁻¹ is lower than that for [ReMeO₃] (950 cm⁻¹),¹⁷ indicating that the Re[double bond, length half m-dash]O bonds are weakened upon adduct formation. Like 6, the Re≡N stretching frequency for 8 (953 cm⁻¹) is higher than that for 1.

L_OEtOs complexes

Interaction of [NBuⁿ₄][OsNCl₄] with NaL_OEt in boiling acetone afforded [OsL_OEtNCl₂] 9, isolated as air-stable deep red crystals. No reactions were found between [PPh₄]₂[OsO₂Cl₄] ¹⁸ and NaL_OEt possibly because of the preference of Os^VI for trans-O[double bond, length half m-dash]Os[double bond, length half m-dash]O geometry. The crystal structure of 9 has been determined and is shown in Fig. 2. Selected bond lengths and angles are listed in Table 3. The geometry around Os is essentially octahedral. The Os–N and average Os–Cl in 9 are 1.58(1) and 2.289 Å, respectively. The Os–O bond that is trans to the nitride (2.159(8) Å) is significantly longer than the other two Os–O bonds (average 2.039 Å), indicative of trans influence of the nitride. The IR Os–N stretching frequency was not assigned due to the presence of ligand bands in the 1000–1100 cm⁻¹ region.

Fig. 2 Perspective view of [OsL_OEtNCl₂] 9.

No alkylation occurred when complex 9 was treated with MeOSO₂CF₃, PhCH₂Br or [Ph₃C]BF₄, indicating that the osmium nitrido complex is less nucleophilic than the rhenium analogue 1. In contrast to electrophilic trans-[OsN(tpy)Cl₂]⁺ (tpy = 2,2′∶6′,2′′-terpyridine) ¹⁹ and [OsTpNCl₂] (Tp = hydridotris(pyrazol-1-yl)borate),²⁰ complex 9 does not react with nucleophiles such as NaN₃, Me₃NO and propylene sulfide. Reaction of 9 with PPh₃ in CH₂Cl₂ afforded the osmium(IV) phosphoraniminato complex [OsL_OEt(NPPh₃)Cl₂] 10, which was characterised by X-ray diffraction. Fig. 3 shows a perspective view of 10, selected bond lengths and angles are given in Table 4. The Os–N distance in 10 of 1.893(5) Å is comparable to that in trans-[Os(tpy)Cl₂(NPPh₃)]⁺ (2.093(5) Å),²¹ consistent with the formulation of a Os–N single bond. The P–N bond distance of 1.575(5) Å is similar to that in trans-[Os(tpy)Cl₂(NPPh₃)]⁺ (1.618(5) Å) ²¹ and is typical for a P[double bond, length half m-dash]N double bond. As expected, the average Os–Cl distance in 10 (2.342 Å) is longer than that in 9. Unlike most osmium(IV) phosphoran iminato complexes that contain linear Os–N[double bond, length half m-dash]PR₃ moieties,²² the Os–N–P linkage in 10 is bent with an angle of 137.5(3)°. The Os–N–P angle is slightly bigger than that in trans-[Os(tpy)Cl₂(NPPh₃)]⁺ (132.5(3)°).²¹ The measured magnetic moment for 10 of ca. 2.0 μ_B is different from the spin–only value for two unpaired electrons (2.83 μ_B) because of spin–orbit coupling.²¹

Fig. 3 Perspective view of [OsL_OEt(NPPh₃)Cl₂] 10.

Electrochemistry

The formal potentials of the L_OEt–Re and –Os complexes have been determined by cyclic voltammetry and are summarised in Table 5. The cyclic voltammogram (CV) of 1 exhibits a reversible couple at −0.296 V vs. Cp₂Fe^+/0, which is assigned as the Re^VI–Re^V couple. Consistent with the assignment, the isolated rhenium(VI) complex 1·BF₄ was reduced at the same potential. The Re^VI–Re^V potential for 2 (0.470 V) is more anodic than that for 1, consistent with the higher π-donor strength of nitride compared with oxide. Oxidation of cationic imido complexes 3–5 is irreversible. The irreversible waves at 0.620, 0.705 and 0.557 V for complexes 3–5 are tentatively assigned as the respective Re^VI–Re^V oxidation. The dimetallic nitrido complexes 6 and 7 exhibit reversible Re^VI–Re^V couples at 0.161 and 0.08 V, respectively, which are higher than that for 1 but lower than those for 3–5. On the basis of the Re^VI–Re^V potential for [ReL_OEt(X)(PPh₃)Cl], the π-donor strength of X is ranked in the order N³⁻ > [N{Au(PPh₃)}]²⁻ > NR²⁻. This trend is consistent with that found for [Os(X)O₃] [X = N, NBu^t or NAu(PPh₃)].¹⁶ Complex 10 exhibits a reversible reduction at −0.718 V, which is tentatively assigned as the Os^IV–Os^III couple. The Os^IV–Os^III potential for 10 is less anodic than that for [Os(tpy)Cl₂(NPPh₃)]⁺ (−0.27 V vs. standard calomel electrode),²¹ indicative of the ability of the L_OEt in stabilising Os in high oxidation states.

Table 5 Formal potentials (E °) for Re– and Os–L_OEt complexes

Complex	E ° ^a/V vs. Cp2Fe^+/0
a  Potential measured in CH2Cl2 with 0.1 mol dm^−3 [NBu^n4]PF6 as supporting electrolyte; scan rate = 100 mV s^−1. b  Irreversible.
[ReLOEtN(PPh3)Cl]	−0.296
[ReLOEtOCl2]	0.470
[ReLOEt(NMe)(PPh3)Cl]^+	0.620 ^b
[ReLOEt(NCPh3)(PPh3)Cl]^+	0.705 ^b
[ReLOEt(NCH2Ph)(PPh3)Cl]^+	0.557 ^b
[Au(PPh3){NReLOEt(PPh3)Cl}]^+	0.161
[Ru(Et2dtc)(PPh3)(H2O)(CO){NReLOEt(PPh3)Cl}]^+	0.080
[OsLOEt(NPPh3)Cl2]	−0.718

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In summary, we have isolated the first nitrido complexes of Re^V and Os^VI with L_OEt. While the rhenium complex is nucleophilic and reacts with electrophiles to give imido or μ-nitrido complexes, the osmium analogue reacts with PPh₃ to give a phosphoraminatoosmium(IV) complex. On the basis of cyclic voltammetry, the π-donor strength for multiply bonded ligands is ranked in the order: N³⁻ > (NML_n)²⁻ > NR²⁻ ≈ O²⁻.

Acknowledgements

The financial support from the Hong Kong Research Grants Council (project no. HKUST6066/98P) is gratefully acknowledged.

References

1. J. T. Groves and T. Takahashi, J. Am. Chem. Soc., 1983, 105, 2073; J. Du Bois, C. S. Tomooka, J. Hong and E. M. Carreira, Acc. Chem. Res., 1997, 30, 364 and refs. therein..
2. L. A. Bottomley and F. L. Neely, J. Am. Chem. Soc., 1989, 111, 5955; C. Tong and L. A. Bottomley, Inorg. Chem., 1996, 35, 5108; L. K. Woo, Chem. Rev., 1993, 93, 1125 and refs. therein. C. J. Chang, D. W. Low and H. B. Gray, Inorg. Chem., 1997, 36, 270.
3. C. E. Laplaza, A. R. Johnson and C. C. Cummins, J. Am. Chem. Soc., 1996, 118, 709.
4. W. A. Nugent and J. M. Mayer , Metal–Ligand Multiple Bonds, Wiley, New York, 1998;; K. Dehnicke and J. Strähle, Angew. Chem., Int. Ed. Engl., 1992, 31, 955.
5. W. Kläui, Angew. Chem., Int. Ed. Engl., 1990, 29, 627.
6. J. M. Power, K. Evertz, L. Henling, R. Marsch, W. P. Schaefer, J. A. Labinger and J. E. Bercaw, Inorg. Chem., 1990, 29, 5058; U. Kölle, G. Flunkert, R. Borissen, M. U. Schmidt and U. Englert, Angew. Chem., Int. Ed. Engl., 1992, 113, 3611; W.-H. Leung, E. Y. Y. Chan and W.-T. Wong, Organometallics, 1997, 16, 3234.
7. A. M. LaPointe and R. R. Schrock, Organometallics, 1993, 12, 3379.
8. B. P. Sullivan, J. C. Brewer, H. B. Gray, D. Linebarrier and J. M. Mayer, Inorg. Synth., 1992, 29, 146.
9. G. Parshall, Inorg. Synth., 1977, 17, 100.
10. W. P. Griffith and D. Paulson, J. Chem. Soc., Dalton Trans., 1973, 1315.
11. P. B. Braunstein, H. Lehner, D. Matt, K. Burgess and M. J. Ohlmeyer, Inorg. Synth., 1990, 27, 218.
12. W. A. Herrmann, F. E. Kühn, W. Fischer, W. R. Thiel and C. C. Romao, Inorg. Chem., 1992, 31, 4431.
13. P. B. Critchow and S. D. Robinson, J. Chem. Soc., Dalton Trans., 1975, 1367.
14. W.-H. Leung , H. G. Zheng , J. L. C. Chim , T. Wong , W. Lai , C.-H. Lam , I. D. Williams and W.-T. Wong , submitted for publication. .
15. TEXSAN Crystal Structure Analysis Package, Molecular Structure Corporation, Houston, TX, 1985 and 1992..
16. W.-H. Leung, J. L. C. Chim and W.-T. Wong, J. Chem. Soc., Dalton Trans., 1997, 3277.
17. W. A. Herrmann, J. G. Kuchler, G. Weichselbaumer, E. Herdtweck and P. Kiprof, J. Organomet. Chem., 1989, 272, 351.
18. P. A. Shapley, H. S. Kim and S. R. Wilson, Organometallics, 1988, 7, 928.
19. K. D. Demadis, E.-S. El-Samanody, T. J. Meyer and P. S. White, Inorg. Chem., 1998, 37, 838; K. D. Demadis, T. J. Meyer and P. S. White, Inorg. Chem., 1998, 37, 3610.
20. T. C. Crevier, S. Lovell, J. M. Mayer, A. L. Rheingold and I. A. Guzei, J. Am. Chem. Soc., 1998, 120, 6607.
21. M. Bakir, P. S. White, A. Dovletoglou and T. J. Meyer, Inorg. Chem., 1991, 30, 2835.
22. J. Strähle and K. Dehnicke, Polyhedron, 1989, 8, 707.

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