Syntheses, structural characterisation and electronic structures of some simple acyclic amino carbene complexes

Abstract

Reactions of the vinylidene complexes [M(=C=CH₂)(PP)Cp′]PF₆ with NH₃ readily afford the acyclic amino carbene complexes [M{C(Me)NH₂}(PP)Cp′]PF₆ [M(PP)Cp′ = Fe(dppe)Cp, Ru(dppe)Cp, Ru(PPh₃)₂Cp, Ru(dppe)Cp*]. Crystallographic, spectroscopic and computational data support a description of these complexes in terms of a M-{C(Me)=N⁺H₂} mesomeric form, and simple σ-coordination of the singlet acyclic amino carbene.

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Introduction

Heteroatom-stabilised carbene ligands have played an important role in the development of organometallic chemistry. Since the seminal work of Fischer identifying the earliest examples of this class of compound, extensive studies of the synthesis and properties of complexes bearing carbene ligands have seen carbenes rise to such prominence as supporting ligands that they now rival phosphines as the ubiquitous 2-electron σ-donor ligand. Indeed, many of the spectacular advances in the area of metal complexes of carbenes are correlated with the pioneering efforts that have made stable carbenes available as reagents.¹

Stable (i.e. isolable) carbenes have been known since 1988, when the Bertrand group reported the phosphino(silyl)carbene (ⁱPr₂N)₂PCSiMe₃.² Since then, phosphino(phosphonio)-,³ phosphino(amino)-,⁴ acyclic diamino carbenes,⁵ dicarbene-,⁶ mono-phosphino-,⁷ mono-amino-⁸ and a wide range of cyclic heteroatom-stabilised carbenes, including of course N-heterocyclic carbenes (NHCs),⁹ have been explored. However, whilst cyclic heteroatom-stabilised carbenes have proven to be ligands of wide applicability,¹⁰ isolated acyclic carbenes have found less use in the preparation of metal complexes,¹¹ although it should be noted that these are accessible through modular template synthetic routes, and have given rise to a number of useful catalysts.¹² In the case of phosphoranyl carbenes, the relatively poor coordinating ability has been attributed to the wide carbene bond angle associated with the greater stability of the triplet form and the consequent significant reorganisation energy cost of complexation in addition to the higher carbene reactivity.¹³

Relatively wide carbene angles are also observed in acyclic mono- and di-amino carbenes such as ^tBuC=N(ⁱPr)₂ (120.50(12)°)^8a and (ⁱPr)₂NCN(ⁱPr)₂ (121.0(5)°),¹⁴ which can be attributed to the steric bulk of the groups necessary to allow isolation of the carbene. These angles contrast the much narrower angles calculated for the smaller parent singlet amino carbene HCNH₂ (105.3°) but are closer to the structures of the corresponding triplet (124.1°).^13b Whilst metal complexes of these bulky acyclic amino carbenes are known,^8,13 the wide carbene angle decreases the σ-donor properties of the carbene, and steric factors can also destabilise the resulting complexes.¹⁰ The constrained geometries in NHCs such as the prototypical Arduengo system, 1,3-diadamantyl-imidazol-2-ylidene (102.2(2)°),¹⁵ 1,3-dimesitylimidazol-2-ylidene (101.4(2)°)¹⁶ and the saturated analogue 1,3-dimesitylimidazolin-2-ylidene (104.7(3)°)¹⁶ are much closer to the singlet structure of the parent acyclic amino carbene. The propensity of these ligands to form metal complexes is perfectly well established.¹⁰

In contrast to the coordination reactions of isolated heteroatom-stabilised carbenes with metal complexes, the original Fischer synthesis of alkyloxy-stabilised carbenes involved nucleophilic attack of an alkyl group on a metal-bound carbonyl ligand to given an anionic acyl fragment, and subsequent alkylation.¹⁷ As a consequence of the electrophilic nature of the carbene carbon, these alkyloxy carbenes are readily converted to amino-, thio-, alkyl- or aryl-carbenes providing facile ‘on complex’ synthetic routes to a wide range of complexes featuring heteroatom-stabilised carbene ligands.¹⁸ The addition of nucleophiles such as alcohols, thiols or amines to the α-carbon of transition metal vinylidene or allenylidene complexes also affords Fischer-type (heteroatom-stabilised) carbene compounds.^19,20

Here we describe the facile addition of ammonia to the α-carbon of the parent vinylidene ligand in complexes [M(=C=CH₂)(PP)Cp′][PF₆] (M = Fe, PP = dppe, Cp′ = Cp; M = Ru, PP = dppe, Cp′ = Cp; M = Ru, PP = (PPh₃)₂, Cp′ = Cp; M = Ru, PP = dppe, Cp′ = Cp*) to give rare examples of complexes bearing the simple, sterically unencumbered methyl(amino) carbene ligand, [M{C(Me)NH₂}(PP)Cp′]PF₆. These complexes have been spectroscopically and crystallographically characterised, and their geometric and electronic structures, together with those of the free ligand, explored with additional insight from density functional theory based calculations.

Results and discussion

Synthesis and characterisation

The preparation of vinylidene complexes from rearrangement reactions of terminal alkynes within the coordination sphere of metal complexes is very well established.^19,21 For example, in the case of half-sandwich complexes MCl(L₂)Cp′ (L = phosphine or L₂ = chelating bis(phosphine), Cp′ = Cp, Cp* etc.), such reactions with HC≡CR are typically performed in polar solvents such as methanol at reflux and in the presence of a salt, often NH₄PF₆, to enhance the ionisation of the M–Cl bond and provide a large anion to assist isolation of the product cation [M{=C=C(H)R}(L₂)Cp′]⁺ (Scheme 1).^19,22,23 Whilst the ancillary ligands and the R group provide a degree of steric protection for the electrophilic α-carbon, prolonged reaction leads to the conversion of the vinylidene to the corresponding methoxy carbene [M{=C(OMe)CH₂R}(L₂)Cp′]⁺.²⁴ In the case of the formation of terminal vinylidene complexes [M(=C=CH₂)(L₂)Cp*]⁺ from MCl(L₂)Cp*, HC≡CSiMe₃ and NH₄PF₆, methanol remains a suitable solvent;²⁵ however, the less sterically restricted Cp derivatives require the use of either a bulkier alkyl alcohol (such as ^tBuOH)²⁶ or prolonged reaction time in a non-nucleophilic solvent, such as CH₂Cl₂.²⁷

Scheme 1 Representative syntheses of vinylidenes and methoxy carbenes from half-sandwich MCl(L₂)Cp′ precursors.

Interestingly, the prolonged (15 h) reaction of the η⁵-indenyl complex RuCl(dppm)(η⁵-C₉H₇) with HC≡CSiMe₃ and NH₄PF₆ in CH₂Cl₂ at elevated temperatures (90 °C) in a sealed tube gave a methyl(amino) carbene complex, formulated as [Ru{=C(Me)NH₂}(dppm)(η⁵-C₉H₇)]PF₆ (Scheme 2).²⁸ To the best of our knowledge, this is the only example of a complex bearing the parent alkyl(amino) carbene fragment, and whilst spectroscopically well characterised,²⁸ no crystallographic data are available through which to examine the effect of coordination on the key N–C–C_Me bond angle.

Scheme 2 The prolonged reaction of RuCl(dppe)(η⁵-C₉H₇) with HC≡CSiMe₃ and NH₄PF₆.²⁸

Having observed similar products formed as minor by-products in preparations of vinylidene complexes [M(=C=CH₂)(L₂)Cp′]PF₆ from MCl(L₂)Cp′, HC≡CSiMe₃ and NH₄PF₆, we pursued a more direct synthesis to complexes containing this simple acyclic amino carbene. The reactions of the half-sandwich complexes [Fe(=C=CH₂)(dppe)Cp]PF₆ ([1a]PF₆), [Ru(=C=CH₂)(dppe)Cp]PF₆ ([1b]PF₆), [Ru(=C=CH₂)(PPh₃)₂Cp]PF₆ ([1c]PF₆) and [Ru(=C=CH₂)(dppe)Cp*]PF₆ ([1d]PF₆) with NH₃ proceeded smoothly at room temperature in CH₂Cl₂ to give the amino carbene complexes [Fe{C(Me)NH₂}(dppe)Cp]PF₆ ([2a]PF₆), [Ru{C(Me)NH₂}(dppe)Cp]PF₆ ([2b]PF₆), [Ru{C(Me)NH₂}(PPh₃)₂Cp]PF₆ ([2c]PF₆) and [Ru{C(Me)NH₂}(dppe)Cp*]PF₆ ([2d]PF₆) in 77–94% isolated yields (Scheme 3). Whilst reactions were complete in 1–4 h for the Cp complexes, in the case of [2d]PF₆, overnight reaction was required to complete the conversion, no doubt a consequence of the additional steric protection afforded to the vinylidene α-carbon by the Cp* ligand.

Scheme 3 The reaction of [1a–d]⁺ with NH₃ to afford [2a–d]⁺.

The complexes were readily characterised by the usual spectroscopic methods, and single crystal X-ray diffraction studies. In NMR spectra (Table 1), triplets (unresolved in the case of [2c]⁺) in the ¹³C{¹H} NMR spectra between 253–275 ppm confirmed the carbene-nature of the α-carbon, whilst two unresolved low-field doublets in the ¹H NMR spectra, each integrating to a single proton, indicated the restricted rotation about the C(α)–N bond, supporting a C=N⁽⁺⁾H₂ valence description. The iminium description was further enhanced by the observation of the Cp resonances for [2a–c]PF₆ between 4.37–4.80 ppm (cf. FeCl(dppe)Cp 4.76; RuCl(dppe)Cp 4.55;²⁹ RuCl(PPh₃)₂Cp 5.99;²³ [1a]PF₆ 5.25;²⁶ [1b]PF₆ 5.37;²⁶ [1c]PF₆ 5.09²⁷ ppm). A singlet between 1.60–2.90 ppm (integrating to 3H) was assigned to the methyl group of the carbene ligand. In the positive ion electrospray (ES(+)) mass spectra, ions corresponding to the cation were clearly observed. In the IR spectra, bands in the range 1600–1650 cm⁻¹ were assigned to δ(NH₂) with the assistance of DFT based molecular models (vide infra).

Table 1 Selected spectroscopic data for complexes [1a]PF₆,²⁶ [1b]PF₆,^26,27 [1c]PF₆,²⁷ [1d]PF₆,²⁵ and [2a–d]PF₆

	[1a]^+	[2a]^+	[1b]^+	[2b]^+	[1c]^+	[2c]^+	[1d]^+	[2d]^+
a Value as determined here.
δ C(C(α)) (^2JCP)	354.7 (33)	274.1 (22)	343.0 (18)	256.6 (13)	347.2 (15)	253.4 (m)	344.2 (16)	259.6 (13)
δ H(NH2)		7.30, 8.17		7.06, 7.89		6.96, 8.40		7.18, 8.44
δ P(PP)	98.0	106.9	80.8	87.9	37.9^a	48.2	77.3	86.8

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Molecular structures

Single crystal X-ray diffraction studies were carried out on [2a–d]PF₆ to explore the amino carbene ligand geometry and metal–carbene bond lengths. A representative plot of the cation of [2a]⁺ is shown in Fig. 1, and selected bond lengths and angles are summarised in Table 2. The Ru–C(1) bond lengths fall at the longer end of the range spanned by the crystallographically characterised examples of methyl(methoxy) carbene complexes: [Ru{=C(OMe)Me}L_n]⁺ (RuL_n = RuCl{P(CH=CH₂)Ph₂}(η⁶-C₆H₂Me₄) 1.963(7) Å;³⁰ Ru{P(CH=CH₂)₂Ph}₂(η⁵-C₅H₄Me) 1.921(10) Å;³¹ RuCl{P(CH=CH₂)Ph₂}(η⁶-C₆Me₆) 1.964(10) Å; RuCl(PPh₃)(η⁶-C₆Me₆) 1.962(10) Å and RuCl(PMe₃)(η⁶-C₆Me₆) 2.015(8) Å.³² The M–C(1), C(1)–N(1) and C(1)–C(2) distances are consistent with a predominant description in terms of a strongly N-stabilised carbene with a significant degree of C=NH₂ double bond character and a M–C single bond. There is little change in the carbene angles across the series [2a–d]⁺, suggesting π-interaction between the metal and carbene is not significant. The relatively precisely determined M–P bond lengths are sensitive to M–P back-bonding effects, and can be used as a proxy measure of the relative electron density at the metal centre.³³ Here, the Fe–P and Ru–P bond distances in [2a]⁺, [2b]⁺, [2c]⁺ and [2d]⁺ compare with those of the vinylidene cations [Fe(=C=CH₂)(dppe)Cp*]⁺ (2.2219(15), 2.2185(16) Å),³⁴ [Ru(=C=CH₂)(PPh₃)₂Cp*]⁺ (2.362(2), 2.355(2) Å),³⁵ [Ru(=C=CH₂)(dppe)Cp*]⁺ ([2d]⁺ 2.320(3), 2.317(3); 2.318(3), 2.308(3) Å)²⁵ and the methyl(methoxy) carbene complex [Ru{C(OMe)CH₂CO₂Me}(dppe)Cp]⁺ (2.2890(9), 2.3043(10) Å).³⁶ The shorter M–P bond lengths in the amino carbene complexes indicates more electron-density at the metal than in the vinylidenes, which is consistent with the poorer π-accepting character of the Fischer-type carbene.

Fig. 1 Structure of one of the cations of [2a]⁺ with selected hydrogen atoms omitted for clarity. Ellipsoids have been drawn at the 50% probability level.

Table 2 Selected bond lengths (Å), and bond angles (°) for [2a–d]PF₆, together with data from ^tBuC=N(ⁱPr)₂ ^8a and the DFT (B3LYP/LANL2DZ (Ru, Fe), 6-31G**/COSMO (CH₂Cl₂)) optimised geometries of [2a]⁺ and [2b]⁺ and gas phase (B3LYP/6-31G**) geometries of the free singlet carbenes MeC=NH₂ and ^tBuC=N(ⁱPr)₂ (italics)

	MeC=NH2	^tBuC=N(^iPr)2	[2a]PF6	[2b]PF6	[2c]PF6	[2d]PF6
a Methyl group C(2) disordered over two sites.
M–P1			2.1756(4), 2.1829(4) 2.1860	2.2657(7), 2.2733(7) 2.3740	2.326(1)	2.2910(7)
M–P2			2.2032(4), 2.1860(4) 2.1829	2.2566(7), 2.2704(7) 2.3356	2.328(1)	2.2819(7)
M–C1			1.9272(15), 1.9251(15) 1.9250	2.020(3), 2.021(3) 2.0265	2.014(4)	2.016(3)
C1–N1	1.3266	1.2976(17) 1.3214	1.312(2), 1.310(2) 1.3099	1.305(4), 1.307(4) 1.3209	1.309(5)	1.368(4)
C1–C2	1.5071	1.5392(18) 1.5452	1.518(2), 1.519(2) 1.5186	1.518(4), 1.515(4) 1.5157	1.506(6)	1.454(6), 1.477(5)^a
N1–C1–C2	112.30	120.50(12) 122.62	109.4(1), 109.5(1) 109.51	110.0(3), 110.4(2) 111.84	111.9(4)	109.5(4), 108.7(3)^a

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Density functional modelling

To explore the electronic structure of these complexes we turned to DFT calculations (B3LYP/LANL2DZ Ru, Fe; 6-31G** all other atoms/COSMO(CH₂Cl₂)) with gas-phase calculations also performed on the free ligand (B3LYP/6-31G**) for comparison. The free carbene MeC=NH₂ has a singlet ground state (ΔE_S–T = 33.8 kcal mol⁻¹), with an appreciably bent (112.30°) structure, which compares with the corresponding triplet (127.59°). Interestingly, the bulkier analogue ^tBuC=N(ⁱPr)₂, for which a singlet ground state is also calculated (ΔE_S–T = 35.8 kcal mol⁻¹), demonstrates a significant additional steric influence on the ground state structures in both the singlet (122.62°) and triplet (137.40°) configurations.

The compounds [2a]⁺ and [2b]⁺ were chosen as representative examples of the metal complexes of methyl(amino) carbene, allowing exploration of any influence that the 3d vs. 4d metal might play on the electronic structure of the complexes. Key bond lengths and angles from the optimised structures are summarised in Table 2, and the composition of selected frontier orbitals are summarised in Table 3, with supporting plots given in Fig. 2. There is little change between the calculated geometry of the gas-phase carbene MeC=NH₂ and the amino carbene ligand in the complexes. The electronic structures of [2a]⁺ and [2b]⁺ are remarkably similar, with the HOMO−2 to LUMO+1 being well described as the metal d_z², d_x²−y², d_xy, d_yz and d_xz admixed with the N=C π* system. There is no π-type interaction between the metal and the carbene carbon, the N=C π bonding combination lying as low as the HOMO−21 ([2a]⁺) and HOMO−20 ([2b]⁺). The amino carbene moiety therefore serves as a simple σ-donor ligand to the metal centre, with the singlet carbene being stabilised exclusively by π-donation from the N-atom.

Fig. 2 Selected frontier orbitals of [2a]⁺ (contour ± 0.04 (e bohr⁻³)^1/2).

Table 3 Energy (eV) and composition (%) of selected frontier orbital plots of [2a]⁺ and [2b]⁺

	Energy	Fe	C(1)	NH2	Me	dppe	Cp
LUMO+1	−1.29	8	27	10	3	51	1
LUMO	−1.54	37	1	0	0	50	13
HOMO	−6.10	68	2	3	1	9	16
HOMO−1	−6.21	72	2	0	0	9	16
HOMO−-2	−6.90	84	0	3	0	8	4

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	Energy	Ru	C(1)	NH2	Me	dppe	Cp
LUMO+1	−1.28	14	27	8	3	40	7
LUMO	−1.55	28	0	0	0	56	16
HOMO	−5.97	60	2	2	2	16	19
HOMO−1	−6.14	56	4	1	0	12	26
HOMO−2	−6.73	78	1	5	0	11	5

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Conclusion

Reactions of the terminal vinylidene complexes [M(=C=CH₂)(PP)Cp′]PF₆ with NH₃ provide a convenient entry point to complexes bearing the simple acyclic amino carbene ligand. Structural and electronic structure calculations indicate that the ligand acts as a simple singlet carbene, with good σ-donor character but little M(d)–C(p) π-interaction. Gas-phase density functional calculations on the free ligand give a singlet–triplet energy gap of ΔE_S–T = 33.8 kcal mol⁻¹, with an appreciably bent (112.30°) structure. The related, more sterically encumbered acyclic amino carbene ^tBuC=N(ⁱPr)₂ also offers a large singlet–triplet energy gap (ΔE_S–T = 35.8 kcal mol⁻¹), and a more linear structure (N–C–C 122.62°) demonstrating the steric influence on the ground state structure of the singlet. The difficulties in formation and stability of metal complexes of mono(amino) carbenes therefore seems to be due in no small part to the significant steric bulk of the supporting groups necessary to stabilise the singlet carbene. The formation of these ligands through ‘on-complex’ synthetic methods therefore appears a more suitable entry point for further explorations.

Experimental

General conditions

All reactions were carried out under an atmosphere of dry nitrogen using standard Schlenk techniques. Dichloromethane was dried over CaH₂, all other solvents were standard reagent grade and used as received. No special precautions were taken to exclude air or moisture during workup. The compounds [Fe(=C=CH₂)(dppe))Cp]PF₆ ([1a]PF₆),²⁶ [Ru(=C=CH₂)(dppe)Cp]PF₆ ([1b]PF₆),²⁶ [Ru(=C=CH₂)(PPh₃)₂Cp]PF₆ ([1c]PF₆)²⁷ and [Ru(=C=CH₂)(dppe)Cp*]PF₆ ([1d]PF₆)²⁵ were synthesised by literature methods. All other reagents were commercially available and used as received. NMR spectra were recorded in CDCl₃ solutions at 23 °C on Bruker and Varian Mercury-400 (¹H, 399.97 MHz; ³¹P, 161.10 MHz), Bruker Avance 600 (¹H, 600.1 MHz; ¹³C, 150.9 MHz; ³¹P, 242.9 MHz) or Varian VNMRS-700 (¹H, 699.73 MHz, ¹³C, 175.95 MHz; ³¹P, 279.89 MHz) spectrometers. Chemical shifts were determined relative to internal residual solvent signals (¹H, 7.26 ppm; ¹³C, 77.2 ppm) or external H₃PO₄ (³¹P, 0.0 ppm). FT-IR spectra were measured on an Agilent Technologies Cary 660 spectrometer or a Nicolet Avatar 360 spectrometer from solutions in dichloromethane in a thin layer cell fitted with CaF₂ windows. Positive and negative ion electrospray ionization (ES(+), ES(−)) mass spectra were recorded on a Waters LCT Premier XE mass spectrometer from solutions in methanol.

Synthesis of [Fe{C(Me)NH₂}(dppe)Cp]PF₆ [2a]PF₆

The compound [Fe(=C=CH₂)(dppe)Cp]PF₆ (0.10 g, 0.14 mmol) was added to dry degassed CH₂Cl₂ (15 ml) in a dry, degassed Schlenk flask connected to a gas bubbler. To a separate dried, degassed flask, fitted with a Dreschel head connected to a N₂ line, was added 35% NH_3(aq) (25 ml). The output of the Dreschel head was connected by a gas-tight transfer tube in the first flask, fitted below the CH₂Cl₂ solvent level. NH_3(g) was then generated by bubbling N₂ gas through the NH_3(aq) solution, which was then subsequently bubbled through the vinylidene solution for 4 h. The orange solution colour lightened over time. After this period, the 35% NH_3(l) flask is removed and reaction flask flushed with N₂ for at least 10 minutes to remove any excess NH_3(g). The orange solution was concentrated to dryness by rotary evaporation leaving an orange residue. The orange residue was extracted with CH₂Cl₂ and filtered into vigorously stirred hexane, resulting in the instantaneous precipitation of an orange solid. The solid was collected by filtration, washed with hexane (3 × 10 ml) and diethyl ether (3 × 10 ml) then dried under vacuum (0.079 g, 77%). Crystals suitable for single crystal X-ray diffraction were grown from CDCl₃/pentane layer diffusion.

IR (CH₂Cl₂/cm⁻¹): 1651 δ(N–H₂). ¹H NMR (CDCl₃, 700 MHz) δ/ppm: 1.75 (s, 3H, CH₃), 2.39–2.57 (m, 2H, CH₂, dppe), 2.75–2.93 (m, 2H, CH₂, dppe), 4.37 (s, 5H, Cp), 7.17 (t, J = 7 Hz, 4H, H_m, dppe), 7.30 (s, br., 1H, NH₂), 7.38 (t, J = 7 Hz, 4H, H_m, dppe), 7.44 (t, J = 7 Hz, 2H, H_p, dppe), 7.46–7.52 (m, 6H, H_p and H_o, dppe), 7.53–7.60 (m, 4H, H_o, dppe), 8.17 (s, br., 1H, NH₂). ³¹P{¹H} NMR (CDCl₃, 400 MHz) δ/ppm: 106.9 (s, Fe-dppe). ¹³C{¹H} NMR (CDCl₃, 700 MHz) δ/ppm: 26.8 (t, J = 23 Hz, CH₂, dppe), 43.2 (s, CH₃), 83.1 (s, Cp), 128.8 (t, J = 4 Hz, C_m, dppe), 129.5 (t, J = 4 Hz, C_o, dppe), 130.4 (s, C_p, dppe), 131.3 (s, C_p, dppe), 131.8 (t, J = 4 Hz, C_m, dppe), 132.5 (t, J = 4 Hz, C_o, dppe), 133.4–133.7 (m, C_i, dppe), 139.2–139.6 (m, C_i, dppe), 274.1 (t, J = 22 Hz, C_α). ES (+)-MS (m/z): 562 [M − PF₆]⁺. Calculated for C₃₃H₃₄NF₆P₃Fe: C, 56.00; H, 4.85. Found: C, 56.12; H, 4.91.

Synthesis of [Ru{C(Me)NH₂}(dppe)Cp]PF₆ [2b]PF₆

In a manner as described above, NH_3(g) was bubbled through a solution of [Ru(=C=CH₂)(dppe)Cp]PF₆ (0.061 g, 0.082 mmol) in dry degassed CH₂Cl₂ (6 ml) for 4 h. The yellow solution colour lightened over time. Work up as described above gave the product as a yellow powder (0.049 g, 79%). Crystals suitable for single crystal X-ray diffraction were grown from CH₂Cl₂/hexane layer diffusion.

IR (CH₂Cl₂/cm⁻¹): 1605 δ(N–H₂). ¹H NMR (CDCl₃, 400 MHz) δ/ppm: 1.77 (s, 3H, CH₃), 2.40–2.58 (m, 2H, dppe), 2.77–3.00 (m, 2H, dppe), 4.80 (s, 5H, Cp), 7.06 (s, br., 1H, NH₂), 7.15 (t, J = 8 Hz, 4H, H_m, dppe), 7.37 (t, J = 8 Hz, 4H, H_m, dppe), 7.38 (t, J = 8 Hz, 2H, H_p, dppe), 7.46–7.51 (m, 6H, H_o and H_p, dppe), 7.59 (t, J = 8 Hz, 4H, H_o, dppe), 7.89 (s, br., 1H, NH₂). ³¹P{¹H} NMR (CDCl₃, 400 MHz) δ/ppm: 87.9 (s, Ru-dppe). ¹³C{¹H} NMR (CDCl₃, 700 MHz) δ/ppm: 43.9 (s, CH₃), 27.0 (t, J = 23 Hz, CH₂, dppe), 87.2 (s, Cp), 128.7 (t, J = 5 Hz, C_m, dppe), 129.4 (t, J = 5 Hz, C_o, dppe), 130.3 (s, C_p, dppe), 131.1 (t, J = 5 Hz, C_m, dppe), 131.3 (s, C_p, dppe), 132.3–132.5 (m, C_i, dppe), 132.7 (t, J = 5 Hz, C_o, dppe), 139.7–140.1 (m, C_i), 256.6 (t, J = 13 Hz, C_α). ES (+)-MS (m/z): 608 [M − PF₆]⁺. ES (−)-MS (m/z): 145 [PF₆]⁻. Calculated for C₃₃H₃₄NF₆P₃Ru: C, 52.58; H, 4.55. Found: C, 52.54; H, 4.64.

Synthesis of [Ru{C(Me)NH₂}(PPh₃)₂Cp]PF₆ [2c]PF₆

In a manner similar to that described above, NH_3(g) was bubbled through a solution of [Ru(=C=CH₂)(PPh₃)₂Cp]PF₆ (0.10 g, 0.12 mmol) in dry degassed CH₂Cl₂ (6 ml) for 1 h. The orange solution colour lightened over time. Work-up as described above gave the product as an orange powder (0.095 g, 94%). Crystals suitable for single crystal X-ray diffraction were grown from CH₂Cl₂/ethyl acetate layer diffusion.

IR (CH₂Cl₂/cm⁻¹): 1604 δ(N–H₂). ¹H NMR (CDCl₃, 600 MHz) δ/ppm: 2.90 (s, 3H, CH₃), 4.53 (s, 5H, Cp), 6.96 (s, br. 1H, NH₂), 7.05 (t, J = 8 Hz, 12H, H_o, PPh₃), 7.30 (t, J = 8 Hz, 12H, H_m, PPh₃), 7.43 (t, J = 8 Hz, 6H, H_p, PPh₃), 8.40 (s, br., 1H, NH₂). ³¹P NMR (CDCl₃, 400 MHz) δ/ppm: 48.2 (s, Ru-(PPh₃)₂). ¹³C NMR (CDCl₃, 600 MHz) δ/ppm: 44.3 (s, CH₃), 89.1 (s, Cp), 128.7 (t, J = 5 Hz, C_m, PPh₃), 130.5 (s, Cp, PPh₃), 133.6 (t, J = 5 Hz, C_o, PPh₃), 135.9–136.3 (m, C_i, PPh₃), 253.4 (m, C_α). ES (+)-MS (m/z): 734 [M − PF₆]⁺, 472 [M − PPh₃ – PF₆]⁺. ES (−)-MS (m/z): 145 [PF₆]⁻. Calculated for C₄₃H₄₀NF₆P₃Ru: C, 58.65; H, 4.61. Found: C, 58.65; H, 4.61.

Synthesis of [Ru{=C(Me)NH₂}(dppe)Cp*]PF₆ [2d]PF₆

In a manner as described above, NH_3(g) was bubbled through a solution of [Ru(=C=CH₂)(dppe)Cp*]PF₆ (0.11 g, 0.14 mmol) in dry degassed CH₂Cl₂ (15 ml) for 24 h. The solution colour changed from yellow to pale green gradually over time. Work up as described above gave the product as a pale green powder (0.090 g, 78%). Crystals suitable for single crystal X-ray diffraction were grown from CDCl₃/pentane layer diffusion.

IR (CH₂Cl₂/cm⁻¹): 1639 δ(N–H₂). ¹H NMR (CDCl₃, 700 MHz) δ/ppm: 1.49 (s, 15H, Cp*), 1.60 (s, 3H, CH₃), 2.20–2.40 (m, 2H, CH₂, dppe), 2.73–2.91 (m, 2H, CH₂, dppe), 7.07 (t, J = 8 Hz, 4H, H_m, dppe), 7.18 (s, br., 1H, NH₂), 7.36 (t, J = 8 Hz, 4H, H_m, dppe), 7.43 (t, J = 8 Hz, 4H, H_p, dppe), 7.45–7.57 (m, 8H, H_o, dppe), 8.44 (s, br, 1H, NH₂). ³¹P{¹H} NMR (CDCl₃, 400 MHz) δ/ppm: 86.6 (s, Ru-dppe). ¹³C{¹H} NMR (CDCl₃) δ/ppm: 10.5 (s, CH₃, Cp*), 28.1 (t, J = 23 Hz, CH₂, dppe), 41.8 (s, CH₃), 96.9 (s, Cp*), 128.2 (t, J = 5 Hz, C_m, dppe), 129.4 (t, J = 5 Hz, C_o, dppe), 130.5 (s, C_p, dppe), 131.3 (s, C_p, dppe), 132.6 (t, J = 5 Hz, C_o, dppe), 132.9 (t, J = 5 Hz, C_m, dppe), 259.6 (t, J = 13 Hz, C_α). ES (+)-MS (m/z): 678 [M − PF₆]⁺, 677 [M − PF₆ − H]⁺. ES (−)-MS (m/z): 145 [PF₆]⁻. Calculated for C₃₈H₄₄NF₆P₃Ru.CHCl₃: C, 49.72; H, 4.81. Found: C, 50.73; H, 5.14. Compound [2d]PF₆ crystallises as a bis(chloroform) solvate. The analysis suggests one molecule is tenaciously retained.

Crystallography

Diffraction data were collected at 120(2) K (100(2) K for [2b]PF₆) on a Bruker SMART CCD 6000 diffractometer ω-scan, 0.3°/frame) or on an Oxford Diffraction Xcalibur diffractometer ([2b]PF₆) (sealed tubes, graphite monochromators, λMo-Kα, λ = 0.71073 Å). Following absorption corrections and solution by direct methods, the structures were refined against F² with full matrix least-squares using the program SHELXL³⁷ and OLEX2.³⁸ Non-hydrogen atoms were refined with anisotropic displacement parameters with hydrogen atoms added at calculated positions and refined by use of a riding model with isotropic displacement parameters based on those of the parent atoms. Disordered atoms in the structures [2c]PF₆ and [2b]PF₆ were refined in isotropic approximation with fixed site occupation factors.

Crystal data

[2a]PF₆. Formula: C₃₃H₃₄NP₂Fe·PF₆, M = 707.37. Monoclinic, space group P2₁/c, a = 21.8057(6), b = 15.4495(4), c = 18.9135(5) Å, β = 101.4690(10)°, V = 6244.5(3) ų, Z = 8, μ = 0.698 mm⁻¹, D_calc = 1.505 Mg m⁻³, 2θ_max = 60°. Reflections collected = 81 779, unique = 18 220, R_int = 0.0318. Data/restraints/parameters = 18 220/0/811, GooF = 1.036. Final R indices: R₁ = 0.0302 (14 367 > 2σ(I)), wR₂ = 0.0803 (all data). Δρ_max,min = 0.578, −0.302 e Å⁻³. CCDC 1063656.

[2b]PF₆. Formula: C₃₃H₃₄NP₂Ru·PF₆, M = 752.59. Monoclinic, space group P2₁/c, a = 21.9074(3), b = 15.4771(2), c = 19.0631(2) Å, β = 99.9220(10)°, V = 6366.92(14) ų, Z = 8, μ = 0.704 mm⁻¹, D_calc = 1.570 Mg m⁻³, 2θ_max = 55°. Reflections collected = 63 852, unique = 14 611, R_int = 0.0420. Data/restraints/parameters = 14 611/0/809. GooF = 1.043. Final R indices, R₁ = 0.0389 (12 339 > 2σ(I)), wR₂ = 0.0971 (all data). Δρ_max,min = 3.132, −0.869 e Å⁻³. CCDC 1063659.

[2c]PF₆. Formula: C₄₃H₄₀NP₂Ru·PF₆, M = 878.74. Orthorhombic, space group P2₁2₁2₁, a = 14.287(2), b = 21.502(4), c = 37.246(6) Å, V = 11 442(3) ų, Z = 12, μ = 0.600 mm⁻¹, D_calc = 1.530 Mg m⁻³, 2θ_max = 58°. Reflections collected = 158 766, unique = 30 397, R_int = 0.0800. Data/restraints/parameters = 30 397/66/1455. GooF = 1.030. Final R indices, R₁ = 0.0528 (25 573 > 2σ(I)), wR₂ = 0.1265 (all data). Δρ_max,min = 1.666, −1.266 e Å⁻³. CCDC 1063657.

[2d]PF₆. Formula: C₃₈H₄₄NP₂Ru·PF₆·2CHCl₃, M = 1061.46. Triclinic, space group P1̄, a = 10.9534(6), b = 13.4096(8), c = 15.8099(9) Å, α = 83.8710(10), β = 76.3000(10), γ = 81.0140(10)°, V = 2222.5(2) ų, Z = 2, μ = 0.878 mm⁻¹, D_calc = 1.586 Mg m⁻³, 2θ_max = 58°. Reflections collected = 43 327, unique = 11 826, R_int = 0.0228. Data/restraints/parameters = 11 826/44/520. GooF = 1.077. Final R indices, R₁ = 0.0526 (10 787 > 2σ(I)) and wR₂ = 0.1356 (all data). Δρ_max,min = 2.371, −1.956 e Å⁻³. CCDC 1063658.

Computational

All the calculations were performed with the Gaussian 09 program package,³⁹ using the B3LYP functional,⁴⁰ LANL2DZ basis set for Ru or Fe, and 6-31G** for all other atoms.⁴¹ A conductor-like polarization continuum model CPCM of CH₂Cl₂ solvent was applied to all calculations, and results analyzed further with GaussSum.⁴² Structures obtained were confirmed as true minima by the absence of imaginary frequencies.

Acknowledgements

S.G.E. gratefully acknowledges funding from the University of Durham in the form of a Durham Doctoral Scholarship, and the University of Western Australia. P.J.L. holds an Australian Research Council Future Fellowship [FT120100073]. The authors acknowledge the facilities, scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy, Characterisation & Analysis, The University of Western Australia, a facility funded by the University, State and Commonwealth Governments.

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Footnotes

† Dedicated to the memory of Professor Kenneth Wade, FRS, an inspirational teacher, mentor, colleague and friend.

‡ CCDC 1063656–1063659. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt01806d

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