2.1 Reaction with 2-chloroacetophenone

Our group recently reported the synthesis of new d² cationic phosphane-containing niobocene complexes by nucleophilic substitution of the phosphorus atom in the terminal phosphido ligand of complexes [Nb(η⁵-C₅H₄SiMe₃)₂(L)(PPh₂)], LCO (1) [7] or CNXylyl (2) [8], with alkyl halides. This reactivity seems to be a good synthetic pathway for the isolation of complexes that contain functionalized phosphane ligands by the presence of functional groups in one of the substituents on the phosphorus atom. As a result, we carried out the reaction of complexes 1 and 2 with 2-chloroacetophenone, which incorporates a ketone group in the alkyl chain bonded to phosphorus.

2.1.1 Synthesis of [Nb(η⁵-C₅H₄SiMe₃)₂(P(CH₂CO(C₆H₅))Ph₂)(L)]Cl, LCO (3), CNXylyl (4)

The reaction was performed by the addition of an equimolar amount of 2-chloroacetophenone to a solution of complexes 1 or 2 in hexane at room temperature. After the appropriate work up (see Experimental section), the new d² cationic niobocene complexes 3 and 4 were isolated in good yield (85%), as shown in Scheme 2.

Complexes 3 and 4 were isolated as yellowish-orange and orange air-sensitive solids, respectively.

Complexes 3 and 4 were fully characterized by elemental analysis, IR spectroscopy and, in solution, by NMR spectroscopy. The ionic nature of these compounds was confirmed by measurement of the molar conductivity (Λ_M: 71.9 and 77.4 Ω⁻¹ cm² mol⁻¹ for 3 and 4, respectively) and the values are consistent with 1:1 electrolytes [10].

The most significant bands in the IR spectra appear at 1946 and 2046 cm⁻¹, corresponding to the triple bond of the carbonyl-and isocyanide ancillary ligands for 3 and 4, respectively. Both spectra also contain a strong band at ca. 1677 cm⁻¹ corresponding to the CO double bond of the ketone group.

The presence in the ¹H-NMR spectra of doublet signals, which can be assigned to the CH₂ protons of the alkyl moiety, at δ = 4.85 ppm (²J_HP = 7.7 Hz) for 3 and δ = 5.09 ppm (²J_HP = 7.3 Hz) for 4 confirms that alkylation has occurred at the phosphorus. The two spectra each contain a single signal for the methyl groups of the SiMe₃ and four multiplets corresponding to the cyclopentadienyl ligands, a finding consistent with an ABCD spin system.

In agreement with the IR spectra, the ¹³C{¹H}-NMR spectrum of 4 shows lowfield resonances for the ketone group, δ = 195.4 ppm, and the isocyanide group, δ = 199.0 ppm, as broad signals due to the quadrupolar moment of the niobium atom. The carbon atom of the alkyl moiety, CH₂, is observed as a doublet due to coupling with the phosphorus atom, at δ = 42.6 ppm (¹J_CP = 23.8 Hz) for 3 and δ = 43.1 ppm (¹J_CP = 19.9 Hz) for 4.

In CDCl₃ solution, the ³¹P{¹H}-NMR broad resonances for 3 (δ = 41.0 ppm) and 4 (δ = 41.0 ppm) are shifted downfield with respect to the corresponding signals for the phosphido ligand in complexes 1 and 2. The signals lie in a range typical of d² cationic niobocene complexes, as reported previously by our group [7,8]. This fact shows the effect of alkylation of the phosphorus by comparison with the ³¹P NMR chemical shift of the phosphido terminal ligand in complexes 1 and 2 [7,8].

The spectroscopic data are consistent with a pseudo-tetrahedral arrangement in these niobocene complexes where the phosphorus atom of the terminal phosphane ligand has a functionalized substituent, as shown in Scheme 2.

2.2 Reactions with acyl halides

Functionalization of the phosphanes can also be performed by the reaction of 1 and 2 with acyl halides, RCOX. In this case, direct attachment of the phosphorus atom and the carbonyl group by nucleophilic acyl substitution occurs to give the new cationic complexes 5–8.

2.2.1 Synthesis and characterization of [Nb(η⁵-C₅H₄SiMe₃)₂(P{CO(C₆H₅)}Ph₂)(L)] Cl, LCO (5), CNXylyl (6)

An equimolar amount of benzoyl chloride was added to a solution of 1 or 2 in hexane at room temperature. After the appropriate work up (see Experimental section), the new d² cationic niobocene complexes 5 and 6 were isolated in good yield (ca. 85%), as shown in Scheme 3.

Complexes 5 and 6 were isolated as red air-sensitive solids.

The ionic nature of complexes 5 and 6 was confirmed by measuring the molar conductivity, Λ_M = 78.6 and 76.4 Ω⁻¹ cm² mol⁻¹ for 5 and 6, respectively. These values are consistent with 1:1 electrolytes [10].

Compounds 5 and 6 were also characterized by commonly used spectroscopic techniques. The most significant bands in the IR spectra appear at 1947 cm⁻¹ for 5 and 2054 cm⁻¹ for 6, and these correspond to the carbonyl and isocyanide ligands, respectively. The stretching vibration of the CO double bond of the ketone group also appears in the two spectra as a strong band at ca. 1650 cm⁻¹.

In agreement with the IR spectra, the ¹³C{¹H}-NMR spectra of 5 and 6 show low-field resonances for the carbon atom of the ketone group (ca. δ = 207 ppm), the quaternary carbon of the ancillary ligand carbonyl (δ 248.1 ppm) and isocyanide (δ = 194.7 ppm). These resonances are observed as broad signals, probably due to the quadrupolar moment of the niobium atom.

The ³¹P{¹H}-NMR spectra each contain a broad resonance and these appear at ca. δ = 64.0 ppm (see Experimental section). These values are shifted to lower field in comparison with the ³¹P{¹H}-NMR signals for complexes 3 and 4, and also the cationic phosphane niobocene complexes previously described by our group [7,8]. This change in shift is indicative of the decreasing electronic density at the phosphorus atom of complexes 5 and 6, probably due to the presence of the ketone group bonded directly to the phosphorus atom in the phosphamide moiety resulting from the P–C coupling process. Moreover, these data are consistent with the chemical shift values observed for other phosphamido complexes described previously [11].

The spectroscopic data are consistent with a pseudo-tetrahedral arrangement of these niobocenes containing an acylphosphane ligand, as shown in Scheme 3.

2.2.2 Synthesis of [Nb(η⁵-C₅H₄SiMe₃)₂(P(COCH(C₆H₅)₂)Ph₂)(L)]Cl, LCO (7), CNXylyl (8)

Complexes 7 and 8 were prepared in a similar way to complexes 5 and 6. An equimolar amount of diphenylacetyl chloride was added at room temperature to a solution of 1 or 2 in hexane. After the appropriate work up (see Experimental section), the new d² cationic niobocene complexes 7 and 8 were isolated in good yield (ca. 85%), as shown in Scheme 4.

Complexes 7 and 8 were isolated as orange air-sensitive solids.

Once again the ionic nature of complexes 7 and 8 was established by measuring the molar conductivity, Λ_M = 87.2 and 83.3 Ω⁻¹ cm² mol⁻¹ for 7 and 8, respectively. As commented above, these values are consistent with 1:1 electrolytes [10].

The IR spectra of both complexes show the expected bands at 1950 cm⁻¹ for the carbonyl ligand of 7 and at 2038 cm⁻¹ for the isocyanide ligand of 8 (see Experimental section). Additionally, strong bands appear at 1682 and 1672 cm⁻¹ for 7 and 8, respectively, and these correspond to the stretching vibration of the CO bond of the ketone group.

The ¹H NMR spectra of the two complexes each contain a broad signal and these correspond to the methylenic proton of the diphenylacetylphosphane moiety, PC(O)CHPh₂, at δ = 5.67 ppm for 7 and δ = 5.26 ppm for 8. The chemical shifts of these proton signals indicate their potentially acidic character.

In agreement with the IR spectrum, the ¹³C{¹H}-NMR spectrum of complex 8 shows a broad low-field resonance for the carbon atom of the ketone group, at δ = 193.1 ppm. The methylenic carbon atom of the diphenylacetylphosphane moiety of each complex gives rise to a doublet due to coupling with the phosphorus atom, at δ = 63.0 ppm (²J_CP = 31.4 Hz) for 7 and δ = 63.2 ppm (²J_CP = 29.6 Hz) for 8.

The ³¹P{¹H}-NMR spectra each contain a broad resonance at δ = 74.9 ppm for 7 and δ = 70.9 ppm for 8 (see Experimental section). These values are within the range found for complexes 5 and 6 (as mentioned above) and were shifted to low field in comparison with the ³¹P{¹H}-NMR chemical shifts previously described for cationic phosphane niobocene complexes and for complexes 3 and 4 [7,8]. These chemical shift values are indicative of the direct connection between the ketone group and the phosphorus atom in the phosphamide unit resulting from the P–C coupling process.

The spectroscopic data are consistent with the isolation of new niobocenes containing an acylphosphane ligand. These compounds are formed by S_N2 reaction of the phosphido-niobocene complexes 1 and 2 with diphenylacetyl chloride. According to the behavior of some phosphamido complexes reported in the literature [12], which exhibit keto-enol tautomerism, the presence of the acidic methylenic proton of the acylphosphane ligand may give rise to such a tautomeric equilibrium in which enol and keto niobocene derivatives could be present (See Scheme 5). However, for complexes 7 and 8, this equilibrium was not observed according to the IR and ¹³C{¹H}-NMR data discussed above. Thus, a pseudo-tetrahedral arrangement for these complexes containing a ketone-phosphane moiety is proposed.

4.1 General experimental conditions

All reactions were carried out using Schlenk techniques. Oxygen and water were excluded through the use of vacuum lines supplied with purified N₂. Toluene was distilled from sodium. Hexane was distilled from sodium/potassium alloy. Diethyl ether and THF were distilled from sodium benzophenone. All solvents were deoxygenated prior to use.

4.2 Instrumentation

¹H, ¹³C and ³¹P NMR spectra were recorded on a Varian Innova 500 MHz spectrometer at ambient temperature unless stated otherwise. ¹H, ¹³C and ³¹P NMR chemical shifts (δ values) are given in ppm relative to the solvent signal (¹H, ¹³C) or standard resonances (³¹P, external 85% H₃PO₄). IR spectra were recorded on a Perkin-Elmer 883 spectrophotometer as Nujol mulls on CsI windows. Electrical conductivity was measured on a Metrohm Herisau E587 Conductimeter, with a cell constant of 0.79 cm⁻¹ and 5.10⁻⁴ M concentration of the samples in acetone.

4.3 Reagents

[Nb(η⁵-C₅H₄SiMe₃)₂(PHPh₂)(CO)]Cl (1) [13] and [Nb(η⁵-C₅H₄SiMe₃)₂(PHPh₂)(CNXylyl)]Cl (2) [13] were prepared as described in the literature. Deuterated solvents were dried over 4 Å molecular sieves and degassed prior to use. PhCOCH₂Cl, PhCOCl, Ph₂CHCOCl were used as supplied by Aldrich.

4.4 Synthesis of [Nb(η⁵-C₅H₄SiMe₃)₂(P(CH₂CO(C₆H₅))Ph₂)(L)]Cl, LCO (3), CNXylyl (4)

To a solution of 1 (0.35 g, 0.57 mmol) or 2 (0.40 g, 0.56 mmol) in anhydrous hexane (30 mL) at room temperature was added an equimolar quantity of PhCOCH₂Cl (0.09 g, 0.57 mmol for 1, 0.09 g; 0.56 mmol for 2). A yellowish-orange solid for 3 and an orange solid for 4 precipitated immediately. The mixture was stirred for 1 h and the solids were filtered off and washed with anhydrous hexane (2 × 10 mL). Complexes 3 and 4 are soluble in polar solvents such as acetone, chloroform and dichloromethane, slightly soluble in THF and toluene and insoluble in apolar solvents like diethyl ether, pentane and hexane.

(3): Yield: 0.35 g (85% rel. 1). Electrical conductivity: Λ_M (Ω⁻¹cm²mol⁻¹); 71.9. IR (Nujol/Polyethylene): ν (cm⁻¹); 1946 (C≡O), 1679 (CO). ¹H-NMR (CDCl₃): δ (ppm); 0.24 (s, 18H, SiMe₃), 4.85 (d, 2H, ²J_HP = 7.7 Hz, CH₂), 4.72, 5.44, 5.76, 5.79 (m, 2H, C₅H₄), 7.42–8.13 (m, 15H, Ph). ¹³C{¹H}-NMR (CDCl₃): δ (ppm); 0.1 (SiMe₃), 42.6 (d, ¹J_CP = 23.8 Hz, CH₂), 95.9, 96.7, 100.7, 101.7 (C₅H₄), 128.6–133.5 (Ph). ³¹P{¹H}-NMR (CDCl₃): δ (ppm); 41.0 (s). ³¹P-NMR (CDCl₃): δ (ppm); 41.0 (m). Anal. Found: C, 60.27; H, 5.76. Calcd. (C₃₇H₄₃ClNbO₂PSi₂): C, 60.44; H, 5.89; M, 735.24.

(4): Yield: 0.40 g (85% rel. 2). Electrical conductivity: Λ_M (Ω⁻¹cm²mol⁻¹); 77.4. IR (Nujol/Polyethylene): ν (cm⁻¹); 2046 (C≡N); 1675 (CO). ¹H-NMR (CDCl₃): δ (ppm); 0.17 (s, 18H, SiMe₃), 2.40 (s, 6H, CH₃ Xylyl), 5.09 (d, 2H, ²J_HP = 7.32 Hz, CH₂), 4.50, 5.21, 5.82, 5.89 (m, 2H, C₅H₄), 7.11–8.78 (m, 14H, Ph), 8.19 (d, 4H, ²J_HP = 7.3 Hz, Ph). ¹³C{¹H}-NMR (CDCl₃): δ (ppm); 0.3 (SiMe₃), 19.3 (CH₃ CNXylyl), 43.1 (d, ¹J_CP = 19.9 Hz, CH₂), 91.4, 101.3, 102.5 (C₅H₄), 125.7–134.3 (Ph), 135.9 (d, ¹J_CP = 35.7 Hz, Ph), 137.1 (C_ipso Ph’) 195.8 (CO), 199.0 (CN). ³¹P{¹H}-NMR (CDCl₃): δ (ppm); 38.3 (s). ³¹P-NMR (CDCl₃): δ (ppm); 38.3 (m). Anal. Found: C, 63.36; H, 6.03; N, 1.67. Calcd. (C₄₅H₅₂ClNNbO₂PSi₂): C, 63.26; H, 6.13; N, 1.64; M, 854.40.

4.5 Synthesis of [Nb(η⁵-C₅H₄SiMe₃)₂(P(CO(C₆H₅))Ph₂)(L)]Cl, LCO (5), CNXylyl (6)

Complexes 5 and 6 were prepared using a similar procedure to 3 and 4 by the reaction of complexes 1 and 2 (0.29 g, 0.47 mmol for 1, 0.57 g, 0.79 mmol for 2) in anhydrous hexane (30 mL) with a stoichiometric amount of PhCOCl (ρ = 1.211 g mL⁻¹; 0.07 g, 0.47 mmol for 1, 0.11 g, 0.79 mmol for 2) for 1 h at room temperature. Complexes 5 and 6 were isolated as red solids after drying and they were washed twice anhydrous hexane (2 × 10 mL) at 0 °C. Complexes 5 and 6 are soluble in acetone, chloroform and dichloromethane but are insoluble in diethyl ether, pentane and hexane.

(5): Yield: 0.28 g (83% rel. 1). Electrical conductivity: Λ_M (Ω⁻¹cm²mol⁻¹); 78.6. IR (Nujol/Polyethylene): ν (cm⁻¹); 1947 (C≡O), 1649 (CO). ¹H-NMR (CDCl₃): δ (ppm); −0.06 (s, 18H, SiMe₃), 3.96, 5.12, 5.26, 6.14 (m, 2H, C₅H₄), 6.80–7.39 (m, 15H, Ph). ¹³C{¹H}-NMR (CDCl₃): δ (ppm); 0.1 (SiMe₃), 95.7, 98.4, 100.3, 101.0, 101.3 (C₁) (C₅H₄), 128.5–137.7 (Ph), 208.6 (m, CO), 248.1 (C≡O). ³¹P{¹H}-NMR (CDCl₃): δ (ppm); 64.8 (s). ³¹P-NMR (CDCl₃): δ (ppm); 64.8 (m). Anal. Found: C, 60.09; H, 5.76. Calcd. (C₃₆H₄₁ClNbO₂PSi₂): C, 59.95; H, 5.73; M, 721.21.

(6): Yield: 0.55 g (85% rel. 2). Electrical conductivity: Λ_M (Ω⁻¹cm²mol⁻¹); 76.4. IR (Nujol/Polyethylene): ν (cm⁻¹); 2054 (C≡N), 1649 (CO). ¹H-NMR (CDCl₃): δ (ppm); 0.21 (s, 18H, SiMe₃), 2.40 (s, 6H, CH₃ Xylyl), 4.39, 5.33, 5.36, 5.57 (m, 2H, C₅H₄), 7.01–7.51 (m, 18H, Ph). ¹³C{¹H}-NMR (CDCl₃): δ (ppm); 0.2 (SiMe₃), 19.1 (CH₃ CNXylyl), 90.4, 100.1, 100.6, 101.9 (C₅H₄), 125.6–133.9 (Ph), 194.7 (C≡N), 207.0 (m, CO). ³¹P{¹H}-NMR (CDCl₃): δ (ppm); 64.7 (s). ³¹P-NMR (CDCl₃): δ (ppm); 64.7 (m). Anal. Found: C, 64.24; H, 6.21, N, 1.62. Calcd (C₄₄H₅₀ClNNbOPSi₂): C, 64.11; H, 6.11; N, 1.70; M, 824.38.

4.6 Synthesis of [Nb(η⁵-C₅H₄SiMe₃)₂(P(COCH(C₆H₅)₂)Ph₂)(L)]Cl, LCO (7), CNXylyl (8)

Complexes 7 and 8 were prepared as described previously for complexes 3–6. To a solution of 1 or 2 (0.30 g, 0.49 mmol for 1 and 0.49 g, 0.68 mmol for 2) in anhydrous hexane (30 mL) was added the stoichiometric amount of Ph₂CHCOCl (0.11 g, 0.49 mmol for 1, 0.16 g, 0.68 mmol for 2) at room temperature and the mixture was stirred for 1 hour. Orange solids were isolated after drying these were washed with anhydrous hexane (2 × 10 mL) at 0 °C. The compounds are slightly soluble in hexane, pentane, toluene and diethyl ether and very soluble in polar solvents such as acetone and dichloromethane.

(7): Yield: 0.34 g (86% rel. 1). Electrical conductivity: Λ_M (Ω⁻¹cm²mol⁻¹); 87.2. IR (Nujol/Polyethylene): ν (cm⁻¹); 1950 (C≡O), 1682 (CO). ¹H-NMR (CDCl₃): δ (ppm); 0.18 (s, 18H, SiMe₃), 5.67 (s, 1H, CH), 4.15, 5.18, 5.55, 6.18 (m, 2H, C₅H₄), 7.05 (s, 5H, Ph), 7.24 (m, 10H, Ph), 7.49 (m, 5H, Ph). ¹³C{¹H}-NMR (CDCl₃): δ (ppm); 0.1 (SiMe₃), 63.0 (d, ²J_CP = 31.4 Hz, CH), 95.0, 98.5, 101.1, 103.7 (C₅H₄), 127.9, 129.4, 128.7, 131.6 (Ph), 129.4 (d, ²J_CP = 9.1 Hz, Ph), 134.3 (d, ¹J_CP = 59.3 Hz, C_ipso Ph). ³¹P{¹H}-NMR (CDCl₃): δ (ppm); 74.9 (s). ³¹P-NMR (CDCl₃): δ (ppm); 74.9 (m). Anal. Found: C, 63.57; H, 5.83. Calcd. (C₄₃H₄₇ClNbO₂PSi₂): C, 63.66; H, 5.84; M, 811.34.

(8): Yield: 0.53 (84% rel. 2). Electrical conductivity: Λ_M (Ω⁻¹cm²mol⁻¹); 83.3. IR (Nujol/Polyethylene): ν (cm⁻¹); 2038 (C≡N); 1672 (CO). ¹H-NMR (CDCl₃): δ (ppm); 0.11 (s, 18H, SiMe₃), 2.32 (s, 6H, CH₃ Xylyl), 5.26 (s, 1H, CH), 4.64, 5.02, 5.26, 5.48 (m, 2H, C₅H₄), 6.82–7.78 (m, 23H, Ph). ¹³C{¹H}-NMR (CDCl₃): δ (ppm); –0.1 (SiMe₃), 18.8 (CH₃ CNXylyl), 63.2 (d, ²J_CP = 29.6 Hz, CH), 91.7, 99.7, 100.0, 100.7, 101.4 (C₅H₄), 127.9–135.9 (Ph), 129.2 (d, J_CP = 9.5 Hz, Ph), 133.0 (d, J_CP = 9.9 Hz, Ph), 193.1 (CO), 213.9 (CN). ³¹P{¹H}-NMR (CDCl₃): δ (ppm); 70.9 (s). ³¹P-NMR (CDCl₃): δ (ppm); 70.9 (m). Anal. Found: C, 66.81; H, 6.05; N, 1.49. Calcd. (C₅₁H₅₆ClNNbOPSi₂): C, 66.98; H, 6.17; N, 1.53; M, 914.50.