Organoruthenium Complexes Containing Phosphinodicarboxamide Ligands

Abstract

Ruthenium complexes of phosphinocarboxamide ligands, and their use to form metallacycles using halide abstraction/deprotonation reactions are reported. Thus, [Ru(p-cym){PPh₂C(=O)NHR}Cl₂; R = ⁱPr (1), Ph (2), p-tol (3)] and [Ru(p-cym){PPh₂C(=O)N(R)C(=O)N(H)R}Cl₂; R = Ph (4), p-tol (5)] were synthesized from [(p-cym)RuCl₂]₂ (p-cym = para-cymene) and phosphinocarboxamides or phosphinodicarboxamides, respectively. Single-crystal X-ray diffraction measurements on 1–5 reveal coordination to ruthenium through the phosphorus donor, with an intramolecular hydrogen bond between the amine-bound proton and a metal-bound chloride. Six-membered metallacycles formed by halide abstraction/deprotonation of complexes 4 and 5 afforded [Ru(p-cym){κ²-P,N-PPh₂C(=O)N(R)C(=O)NR}Cl] [R = Ph (6), p-tol (7)]. These species exist as a mixture of two rotational isomers in solution, as demonstrated by NMR spectroscopy.

1. Introduction

Ligands incorporating both hard and soft donor atoms continue to attract attention [1] and have found applications in coordination chemistry [1,2,3], biomedicine [4,5], enantioselective catalysis [6,7] and supramolecular and self-assembled arrays [8]. Recently, we reported the syntheses of functionalized phosphinocarboxamides (PCAs) and a new family of phosphinodicarboxamides (PDCAs) through the catalytic hydrophosphination of isocyanates (L-1–L-5) [9]. These compounds possess hard (N or O) and soft (P) donor atoms that enable their binding to a wide range of metal centres in diverse coordination modes [10]. Despite this, PCAs have found limited use as ligands [1,2,3,11] and PDCAs have not been investigated in metal complexation reactions. Relevant co-ordination chemistry includes (i) the di-insertion of phenyl isocyanates into an amine bond using lanthanide metal centres to form Cp₂Lnη²:η¹-PyNCON(Ph)CONHPh] (Ln = Yb, Er, Y, Dy, Gd; Py = 2-pyridyl), from which substituted ureas can be prepared [12]; (ii) the use of primary phosphinocarboxamides in the syntheses of cis-[Mo(CO)₄(PH₂C(=O)NH₂)₂] [11] and [Ru(p-cym){PH₂C(=O)N(H)Cy}Cl₂] (Cy = cyclohexyl) [2]; and (iii) the reaction of Fe(η⁵-C₅H₄N(H)C(=O)PPh₂)₂ with PtX₂(PPh₃)₂ (X = Cl, Br) that has allowed for the first selective synthesis of M₄L₆ cage complexes, facilitated by hydrogen bonding interactions between the PCA moiety and the halide ion [13].

Half-sandwich ruthenium complexes have been widely studied as potential catalysts [14,15,16,17,18,19,20,21,22,23], due to their ease of interconversion to other Ru(0) and Ru(II) complexes [17]. In particular, [(η⁶-arene)RuCl₂(PR₃)] (R = aryl or alkyl) [10] complexes are effective precursors for a variety of catalytic and stoichiometric organic transformations [18,24]. Notable examples include the transfer hydrogenation of ketones and benzaldehydes by [Ru(p-cym){OC₆H₄-2-CH₂NHC₆H₄-p-Me}Cl] [15,16,25], the isomerization of olefins such as allylbenzene and 1-octene by [Ru(p-cym)LCl] {L = 4-(phenylazo)resorcinol} [14], and a wide range of heteroatom insertion reactions involving cationic allenylidene and cumulenylidene complexes such as [Ru=(C=C=CPh₂)(η⁵-1,2,3-Me₃C₉H₄)(CO)PPh₃)]⁺ [21]. Thus, the p-cymene-ruthenium(II) fragment is an ideal choice for probing the coordination chemistry of new PDCAs, and for comparisons with the analogous PCA-containing complexes.

Herein, we describe the syntheses of [Ru(p-cym){PPh₂C(=O)NHR}Cl₂; R = ⁱPr (1), Ph (2), p-tol (3)] and [Ru(p-cym){PPh₂C(=O)N(R)C(=O)N(H)R}Cl₂; R = Ph (4), p-tol (5)]. This study includes the first reported examples of PDCAs as monodentate and bidentate ligands, with the latter coordination mode resulting in six-membered metallacycles.

2. Results and Discussion

2.1. Synthesis and Characterization of [Ru(p-cym){PPh₂C(=O)N(H)R}Cl₂] [R = ⁱPr (1), Ph (2), p-tol (3)] and [Ru(p-cym){PPh₂C(=O)N(R)C(=O)N(H)R}Cl₂] ([R = Ph (4), p-tol (5)]

A solution of [Ru(p-cym)Cl₂]₂ and L-1 (PPh₂C(=O)N(H)ⁱPr) in a 1:2 ratio in dichloromethane was stirred at room temperature overnight, which, after removal of solvent and extraction into toluene afforded 1 as a dark red solid (Scheme 1). The use of phosphinocarboxamides with nitrogen-substituted aromatic groups (L-2 and L-3) affords the analogous compounds [Ru(p-cym){Ph₂PC(=O)N(H)R}Cl₂; R = Ph (2) and p-tol (3)] (Scheme 1). Pure samples of 1–3 were isolated in moderate to excellent yields (1, 39%; 2, 53%; 3, 99%). These compounds exist as bright-red crystalline solids and have been characterized by NMR and IR spectroscopies, mass spectrometry, single-crystal X-ray diffraction and elemental analyses (see Supporting Information for full details and the Further Observations section for additional insights).

In parallel, the phosphinodicarboxamides PPh₂C(=O)NPhC(=O)NHR (R = Ph L-4, p-tol L-5) [9] were reacted with [Ru(p-cym)Cl₂]₂ (Scheme 1) under similar conditions, affording [Ru(p-cym){PPh₂C(=O)N(R)C(=O)N(H)R}Cl₂]; {R = Ph (4) and p-tol (5)}. Unlike 1–3, complexes 4 and 5 are sparingly soluble in low-polarity hydrocarbons such as benzene and toluene. Characterization using NMR spectroscopy in CD₂Cl₂ indicate only one species in solution for compounds 1–5 (

Table 1. Selected NMR spectroscopic data δ (ppm) for the free PCA/PDCAs L-1–L-5, and complexes 1–7.

Compound	31P Free PCA/PDCA Ligand a	31P PCA/PDCA Complex b	13C{1H}C=O PCA/PDCA Complex b	1HNH PCA/PDCA Complex b
1	−4.0	29.8 a	167.4 a	8.6 a
2	−0.2	37.2	167.6	10.1
3	−0.9	36.6	167.5	10.0
4	8.3	33.5	178.2/177.7 c	9.2
5	8.0	33.8	177.9/177.5 c	9.2
6	-	52.6	- d	-
7	-	52.3	170.9/162.1c	-

^a Chemical shifts reported in ppm in C₆D₆. ^b Chemical shifts reported in ppm in CD₂Cl₂. ^c Chemical shifts for Ph₂P_(C=O) and N_(C=O) in ppm. ^d Signal not observed.

and Table S1 and Figures S6–S15). These compounds possess bilateral symmetry in the p-cymene ligand as shown by ¹H and ¹³C{¹H} NMR spectroscopic determinations in solution. Thus, in the ¹³C{¹H} NMR spectra of 1–5, the non-quaternary carbons of the p-cymene appear as two doublets, due to scalar coupling with the phosphorus atom of the PCA/PDCA ligand (²J_CP = 4 and 6 Hz; see Figure S8) [26]. While the carbonylic PCA/PDCAs fragments are upfield-shifted with respect to their corresponding free ligand (L-1–L-5) (Table 1) (e.g., δ_C=O = 167 (1) vs. 175 (L-1) ppm), with a downfield shift observed in the ³¹P NMR spectra, upon complexation (e.g., δ_P = 29.8 (1) vs. −4.0 (L-1) ppm) (Figures S6–S15) [9,11,27,28,29]. Similar deshielding for the amide NH signals, is also observed in the ¹H NMR spectra of compounds 1–3 (e.g., δ_H = 8.63 (1) vs. 5.35 (L-1) ppm). This is most likely a consequence of intramolecular hydrogen bonding interactions upon introduction of the [Ru(p-cym)Cl₂] moiety (vide infra).

Crystals of compounds 1–5 suitable for single-crystal X-ray diffraction investigations were obtained from toluene solutions at room temperature (Figure 1 and Figures S1–S5 and see the Supporting Information for additional crystallography details). In the solid-state, 1–5 feature a pseudo-octahedral geometry in a classical piano-stool arrangement, in which the coordination sphere consists of an η⁶-bound p-cymene, two chloride ligands, and the P donor from the PCA/PDCA ligand (Figure 1). In the particular case of the structures of 4 and 5, the core of the PDCA ligands is twisted compared to the free ligand, [9] which most likely arises from π⋯π stacking interactions between an aminic aryl group and the phenyl phosphine fragment, and highlights the conformational flexibility of the PDCA ligand [C17plane ‒ C24plane = 3.545(2) Å (4); C23_plane ‒ C9_plane = 3.540(2) Å (5)]. The Ru–Cl and Ru–P distances for 1–5 (

Table 2. Selected bond lengths (Å) and angles (°) for 1–5.

	1	2	3	4	5
C1plane–Ru1	1.7010 (8)	1.7007 (10)	1.6989 (7)	1.7010 (11)	1.702 (2)
Ru1–Cl1	2.4183 (5)	2.4142 (6)	2.4171 (4)	2.3941 (7)	2.4302 (11)
Ru1–Cl2	2.4115 (5)	2.4151 (6)	2.4078 (4)	2.4296 (9)	2.3974 (10)
Ru1–P1	2.3476 (5)	2.3448 (6)	2.3517 (4)	2.3678 (7)	2.3635 (10)
H1⋯Cl1	2.54 (3) a	2.3455 (6)	2.28 (2)	2.3723 (8)	2.44 (5)
Cl1–Ru1–P1	86.96 (2)	85.17 (2)	90.44 (2)	87.45 (3)	86.49 (4)
Cl2–Ru1–P1	87.35 (2)	86.87 (2)	83.29 (2)	89.33 (2)	89.70 (4)

^a The distances H1⋯Cl1 and H1⋯Cl2 in 1 are very similar in length; H1⋯Cl2 2.53(3) Å.

), are similar to related phosphorus-bound ruthenium compounds [2,15,16,24,25,30] such as [Ru(p-cym){PH₂(CO)NHCy}Cl₂] and [Ru(p-cym){PPh₂C≡CPh}Cl₂] (Ru–Cl; ≈ 2.40 Å, Ru–P; ≈ 2.35 Å) [2,24]. In the case of the crystal structure determinations for 1, 4 and 5, additional interstitial molecules of solvent were found (C₆H₆ and toluene, respectively) and could be sensibly modelled. In addition, complexes 1–5 present intramolecular hydrogen bonding between the amidic proton and one of the metal-bound chlorides in the solid state. Particularly, and due to the interaction between H1 and Cl1, the distances Ru1–Cl1 and Ru1–Cl2 are not equivalent; this is most noticeable in the determinations for 4 and 5 (Table 2).

2.2. Synthesis and Characterization of [Ru(p-cym){κ²-P,N-PPh₂C(=O)N(R)C(=O)NR}Cl] ([R = Ph (6), p-tol (7)]

With the intention to synthesize six-membered metallacycles via intramolecular cyclization, compounds 4 and 5 were reacted with K₂CO₃ and AgPF₆ in CD₂Cl₂ at room temperature (Scheme 2A), affording 6 and 7, respectively (Figures S16–S22). In contrast, compound 2 showed no reaction under the same conditions. It is likely that the formation of the four-membered metallacycle is prohibited due to the higher ring strain resulting from the shorter PCA backbone.

The ³¹P NMR spectroscopic resonances for the cyclized products 6 and 7 are shifted downfield with respect to the corresponding monodenate complexes 4 and 5 (Table 1). This is in line with previously reported six-membered N,P-metallacycles, such as [Ru-κ³-NNP-{HCl(CO)}]; and [NNP = 3-(di-tert-butylphosphino)-N-[(1-methyl-1H-imidazol-2-yl)methyl]propylamine] (Figures S19 and S24) [27,28,29]. Our NMR spectroscopic studies support the structure described in Scheme 2 (Figures S17, S18, S21–S23 and S26–S28); and we propose that upon initial coordination (6–7), reaction with K₂CO₃ can deprotonate the PDCA amide. However, it is not until one of the coordinated halides is removed by AgPF₆, that formal cyclization takes place. This assertion has been corroborated by deprotonation experiments, in the absence of AgPF₆. Further halide replacement by [PF₆]⁻ has been ruled out by means of NMR spectroscopy and MS analyses (ion trap). Although one resonance is observed in the ³¹P NMR spectra (Table 1), both 6 and 7 are observed to exist as a mixture of two distinct isomers (6a–7a/6b–7b), as determined by ¹H NMR spectroscopy (Figures S16–S24 and Table S1). Additionally, ¹H-¹H COSY NMR spectra of the cyclized products (6–7), allowed for the deconvolution of the signals associated with the individual products, with distinctive correlations between the individual ⁱPr fragments (Figures S18 and S22). Integration of the ¹H NMR signals from the p-cymene ring indicate an isomer ratio of 60:40 for 6 and 87:13 for 7. In both cases, the major product shows a complete loss of symmetry (as observed by ¹H and ¹³C{¹H} NMR spectra) on the p-cymene fragment (Figures S18 and S22), with the minor product remaining bilaterally symmetrical (6a–7a (symmetric) and 6b–7b (asymmetric)). The diffusion coefficients (D) of 2, 5 and 7, determined by DOSY NMR experiments (Table S1), allowed for the calculation of the hydrodynamic radii. These values are in good agreement with the values obtained from the crystal structures of 2 and 5, and the geometry optimized structures of 7 (vide infra). An increase in the hydrodynamic radius is observed between 2 and 5 due to the increased length of the ligand, with only a small change in hydrodynamic radius observed upon cyclization (7). Similar to compounds 1–5, scalar spin–spin coupling is observed in the ¹³C{¹H} NMR spectra of 6 and 7 between the p-cymene and the phosphorus of the PDCA ligand [¹³C{¹H} NMR] (Figure S22) [31]. With the signals for the non-quaternary aromatic carbons, in the p-cymene fragment, as four distinctive doublets (²J_CP = 3–6 Hz) (Figure S22).

The existence of two rotational isomers for compounds 6 and 7 can be explained by the restricted rotation of the p-cymene ring. DFT calculations demonstrate the existence of two rotamers, with either the ⁱPr (6a′/7a′) or Me (6b′/7b′) of the p-cymene ring lying above the Cl ligand (Figure 2 and Figures S26–S28). These rotamers are computed to be close in energy (ΔG = −0.4 kcal mol⁻¹ in 6 and −0.2 kcal mol⁻¹ in 7), suggesting minimal thermodynamic preference for either isomer. We propose, therefore, that the product distribution is determined by kinetic control. The strong preference for one isomer in 7 is likely due to a strong conformational preference for complex 5 in solution, which gets “locked in” when the complex cyclises on treatment with a halide abstractor and base. High-temperature NMR measurements on 6 and 7 were hindered as the complexes display poor solubility in CD₃C₆D₅ and decompose in CDCl₃ and CD₃CN. Variable temperature NMR studies (¹H and ³¹P NMR spectroscopy) in CD₂Cl₂ (268–298 K) showed no coalescence, indicating higher temperatures are required for interconversion.

2.3. Ligand Displacement Studies

To test the stability of the Ru–P bond, solutions of 4 and 5 in CD₂Cl₂ were exposed to an atmosphere of dry CO (Scheme 2B). NMR analysis indicated loss of the PDCA ligands and formation of [{p-cymene}RuCl₂(CO)] [32] (Figure S25). Similar behaviour has been observed in [Rh(η³-TMPP)₂][BF₄]₂ [TMPP = tris(2,4,6-trimethoxyphenyl)phosphine] that when exposed to an atmosphere of CO, can reversibly coordinate, a useful feature that has been used for chemosensing applications [33,34]. In contrast, no reaction was observed on treating the metallacycle 6 with CO, suggesting that the chelate complex is more robust to ligand substitution.

3. Materials and Methods

For full details on experimental procedures, see the Supporting Information.

Note: We observed that although compounds 1–3 display some stability, under aerobic conditions, decomposing over the course of a few weeks; samples of 4–7 are susceptible to spontaneous decomposition in solution/solid-state in the glovebox.

L-1–L-5 were prepared according to our previously reported methodologies [9].

Crystals suitable for single-crystal X-ray diffraction for 1–5 were grown from concentrated toluene (layered with hexane) or C₆D₆ extracts at room temperature, respectively.

The phosphine (L-1 12 mg; L-2 6.9 mg; L-3 5.2 mg; L-4 6.9 mg; L-5 7.4 mg, 0.016 mmol) was dissolved in CD₂Cl₂ (0.4 mL) and added dropwise to a solution of [Ru(p-cymene)Cl₂]₂ (5 mg, 8.16 × 10⁻³ mmol) in CD₂Cl₂ (0.4 mL) with stirring, affording an orange solution. Volatiles were removed under vacuum, affording the target compounds 1–5. In the particular case of 2, the reaction was successfully scaled up, employing 50 mg (0.08 mmol) of [Ru(p-cymene)Cl₂]₂, with full characterization described below. Figure 3 shows the general numbering scheme used for the the p-cymene ligand in compounds 1–5.

[Ru(p-cym){PPh₂C(=O)N(H)ⁱPr}Cl₂] (1). ¹H NMR δ/ppm (400 MHz, C₆D₆): 8.63 (d, ³J_HP = 7.4 Hz, 1H, NH), 8.20–8.02 (m, 4H, PPh₂^o), 7.14–6.94 (m, 6H, PPh₂^m+p), 4.75 (d, ³J_HH = 6.1 Hz, 2H, H-5), 4.70 (dd, ³J_HH = 6.1 Hz, ³J_HP = 1.3 Hz, 2H, H-4), 4.05 (m, 1H, NⁱPr^CH), 2.56 (sept, ³J_HH = 7.0 Hz, 1H, H-2), 1.51 (s, 3H, H-7), 1.04 (d, ¹J_HH = 6.6 Hz, 6H, NⁱPr^CH3), 0.82 (d, ³J_HH = 7.0 Hz, 6H, H-1). ¹³C{¹H} δ/ppm (400 MHz, C₆D₆): 167.4 (d, ¹J_CP = 55 Hz, C=O), 134.8 (d, ²J_CP = 9 Hz, PPh₂^o), 133.7 (d, ¹J_CP = 43 Hz, PPh₂ⁱ), 130.7 (d, ⁴J_CP = 2 Hz, PPh₂^p), 128.2 (d, ³J_CP = 10 Hz, PPh₂^m), 109.7 (C-6), 95.2 (C-3), 89.7 (d, ²J_CP = 4 Hz, C-4), 86.0 (d, ²J_CP = 6 Hz, C-5), 43.6 (d, J = 6 Hz, NⁱPr^CH), 30.3 (C-2), 22.2 (C-1), 21.7 (NⁱPr^CH3), 17.3 (C-7). ³¹P NMR δ/ppm (162 MHz, C₆D₆): 29.8 (s, 1P, RuPPh₂). Anal. Calcd for C₂₆H₃₂Cl₂NOPRu: C 54.08, H 5.59, N 2.43; Found C 53.64, H 5.58, N 2.53. HRMS (ESI)⁺ m/z: [M+H]⁺ calculated 578.0715; found 578.0712 formula C₂₆H₃₃Cl₂NOPRu. IR (ATR) 𝜈̅_max/cm⁻¹: 3237 (N-H), 1654 (C=O). Red crystalline solid, 5 mg, 39%.

[Ru(p-cym){PPh₂C(=O)N(H)Ph}Cl₂] (2). ¹H NMR δ/ppm (400 MHz, CD₂Cl₂): 10.11 (s, 1H, NH), 7.88 (ddd, ³J_HH = 10.0 Hz, ³J_HH = 8.2 Hz, ³J_HH = 1.4 Hz, 4H, PPh₂^m), 7.5–7.47 (m, 4H, PPh₂^o), 7.44 (d, ³J_HH = 8.4 Hz, 2H, NPh^o), 7.42–7.38 (m, 2H, PPh₂^p), 7.21 (t, J = 7.6 Hz, 2H, NPh^m), 7.02 (t, ³J_HH = 7.4 Hz, 1H, NPh^p), 5.29 (d, ³J_HH = 6.4 Hz, 2H, H-5), 5.27 (d, ³J_HH = 6.3 Hz, 2H, H-4), 2.55 (sept, ³J_HH = 7.0 Hz, 1H, H-2), 1.84 (s, 3H, H-7), 1.04 (d, ³J_HH = 7.0 Hz, 6H, H-1). ¹³C{¹H} NMR δ/ppm (100 MHz, CD₂Cl₂): 167.8 (d, ¹J_CP = 55 Hz, C=O), 137.0 (NPhⁱ), 134.7(d, ³J_CP = 9 Hz, PPh₂^m), 131.8 (d, ³J_CP = 2 Hz, PPh₂^o), 129.9 (PPh₂ⁱ), 129.3 (NPh^m), 129.1(d, ²J_CP = 7 Hz, PPh₂^p), 124.9 (NPh^p), 120.3 (NPh^o), 111.0 (C-6), 97.3 (C-3), 90.1 (d, ²J_CP = 4 Hz, C-4), 86.9 (d, ²J_CP = 5 Hz, C-5), 30.9 (C-2), 22.1 (C-1), 17.9 (C-7). ³¹P NMR δ/ppm (162 MHz, CD₂Cl₂): 37.2 (s, 1P, RuPPh₂). Anal. Calcd for C₂₉H₃₀Cl₂NOPRu: C 56.96, H 4.95, N 2.29; Found C 55.48, H 5.01, N 2.30. HRMS/ESI⁺ m/z: [M+Na]⁺ calculated 634.0383; found 634.0387 formula C₂₉H₃₀Cl₂NNaOPRu. IR (ATR) 𝜈̅_max/cm⁻¹: 3227 (N-H), 1655 (C=O). Dark red powder, 53 mg, 51%.

[Ru(p-cym){PPh₂C(=O)N(H)p-tol}Cl₂] (3). ¹H NMR δ/ppm (400 MHz, CD₂Cl₂): 10.03 (s, 1H, NH), 7.90–7.84 (m, 4H, PPh₂^o), 7.57–7.47 (m, 6H, PPh₂^m+p), 7.32 (d, ³J_HH = 8.4 Hz, 2H, NPh^m), 7.01 (d, ³J_HH = 8.2 Hz, 2H, NPh^o), 5.28 (d, ³J_HH = 6.9 Hz, 2H, H-5), 5.26 (d, ³J_HH = 6.7 Hz, 2H, H-4), 2.53 (sept, ³J_HH = 6.9 Hz, 1H, H-2), 2.24 (s, 3H, NPh^CH₃), 1.83 (s, 3H, H-7), 1.03 (d, ³J_HH = 7.0 Hz, 6H, H-1). ¹³C{¹H} NMR δ/ppm (100 MHz, CD₂Cl₂): 167.5 (C=O), 136.2 (NPh^CH₃ⁱ), 134.7 (PPh₂^o), 134.6 (NPhⁱ), 131.8 (d, ⁴J_CP = 2 Hz, PPh₂^p), 129.7 (NPh^o), 128.9 (d, ³J_CP = 10 Hz, PPh₂^m), 120.2 (NPh^m), 110.9 (C-6), 97.2 (C-3), 90.1 (d, ²J_CP = 4 Hz, C-4), 86.9 (d, ²J_CP = 5 Hz, C-5), 81.4 (d, ¹J_CP = 70 Hz, PPh₂ⁱ), 30.8 (C-2), 22.1 (C-1), 21.1 (NPh^CH₃), 17.9 (C-7). ³¹P NMR δ/ppm (162 MHz, CD₂Cl₂): 36.6 (s, 1P, RuPPh₂). Anal. Calcd for C₃₀H₃₂Cl₂NOPRu: C 57.60, H 5.16, N 2.24; Found C 56.86, H 5.21, N 2.22. HRMS/ESI⁺ m/z: [M-Cl]⁺ calculated 590.0948, found 590.0961 formula C₃₀H₃₂ClNOPRu. IR (ATR) 𝜈̅_max/cm⁻¹: 3180 (N-H), 1665 (C=O). Dark red powder, Quantitative yield, 11 mg.

[Ru(p-cym){PPh₂C(=O)N(Ph)C(=O)N(H)Ph}Cl₂] (4). ¹H NMR δ/ppm (400 MHz, CD₂Cl₂): 9.25 (s, 1H, NH), 7.71 (t, 3H, J = 9.0 Hz, H-ArP), 7.54–7.49 (dd, J = 11.8, 7.1 Hz, 2H, H-ArN), 7.42–7.37 (m, 3H, H-ArN), 7.33–7.30 (m, 2H, H-ArP), 7.26–7.05 (m, 9H, H-ArN/H-ArP), 6.98 (d, J = 7.4 Hz, 1H, H-ArN), 5.45 (d, ³J_HH = 6.0 Hz, 2H, H-4), 5.15 (d, ³J_HH = 6.0 Hz, 2H, H-5), 2.69 (sept, ³J_HH = 6.7 Hz, 1H, H-2), 1.91 (s, 1H, H-7), 1.11 (d, ³J_HH = 6.9 Hz, 2H, H-1). ¹³C{¹H} NMR δ/ppm (100 MHz, CD₂Cl₂): 178.1 (C=O), 177.7 (C=O), 138.2 (d, ¹J_CP = 9 Hz, PPh₂ⁱ), 134.9 (d, J = 9 Hz, ArP), 131.3 (d, J = 2.5 Hz, ArP), 129.9 (ArN), 129.8 (ArN), 129.1, 128.7 (NPhⁱ x2), 128.6 (ArN), 128.3 (d, J = 10.1 Hz, ArP), 124.7 (ArN), 120.7 (ArN), 120.1 ArN), 110.2 (C-6), 96.9 (C-3), 92.1 (d, ²J_CP = 4 Hz, C-4), 86.5 (d, ²J_CP = 5 Hz, C-5), 30.7 (C-2), 22.3 (C-1), 17.5 (C-7). ³¹P NMR δ/ppm (162 MHz, CD₂Cl₂): 33.50 (s, 1P, RuPPh₂). Anal. Calcd for C₃₆H₃₅Cl₂N₂O₂PRu: C 59.18, H 4.83, N 3.83; Found C 56.48, H 4.73, N 3.67. Despite repeated attempts, a satisfactory elemental analysis for this compound could not be obtained; derived from the aforementioned spontaneous decomposition. HRMS/ESI⁺ m/z: [M-Cl]⁺ calculated 695.1168, found 695.1158; formula C₃₆H₃₅ClN₂O₂PRu. Quantitative conversion as determined by ¹H NMR spectroscopy using TMS as internal standard.

[Ru(p-cym){PPh₂C(=O)N(p-tol)C(=O)N(H)p-tol}Cl₂] (5). ¹H NMR δ/ppm (400 MHz, CD₂Cl₂): 9.20 (s, 1H, NH), 7.68 (t, ³J_HH = 9.2 Hz, 4H, PPh₂^m), 7.34–7.30 (m, 4H, PPh₂^p), 7.25 (d, ³J_HH = 8.5 Hz, 2H, NHPh^m), 7.19 (td, ³J_HH = 7.8, ³J_{HH =} 2.0 Hz, 4H, PPh₂^o), 7.05 (d, ³J_HH = 8.3 Hz, 2H, NHPh^o), 6.88 (d, ³J_HH = 8.2 Hz, 2H, NPh^o), 6.83 (d, ³J_HH = 8.4 Hz, 2H, NPh^m), 5.44 (d, ³J_HH = 5.4 Hz, 2H, H-4), 5.17 (d, ³J_HH = 6.2 Hz, 2H, H-5), 2.70 (sept, ³J_HH = 7.0 Hz, 1H, H-2), 2.29 (s, 3H, NHPh^CH₃), 2.24 (s, 3H, NPh^CH₃), 1.90 (s, 3H, H-7), 1.11 (d, ³J_HH = 7.0 Hz, 6H, H-1). ¹³C{¹H} NMR δ/ppm (100 MHz, CD₂Cl₂):177.9 (C=O), 177.5 (C=O), 151.8 (PPh₂ⁱ), 138.9 (NPhⁱ), 135.7 (NHPh^CH₃ⁱ), 135.2 (NPh^CH₃ⁱ), 135.0 (d, ³J_CP = 9 Hz, PPh₂^m), 134.5 (NHPhⁱ), 131.1 (d, ⁴J_CP = 3 Hz, PPh₂^p ), 130.4 (NPh^o) 129.7 (NHPh^o), 128.6 (NPh^m), 128.2 (d, ²J_CP = 10 Hz, PPh₂^o), 120.0 (NHPh^m), 110.1 (C-6), 96.9 (C-3), 92.1 (d, ²J_CP = 4 Hz, C-4), 86.5 (d, ²J_CP = 6 Hz, C-5), 30.8 (C-2), 22.3 (C-1), 21.3 (NPh^CH₃), 21.1 (NHPh^CH₃), 17.45 (C-7). ³¹P NMR δ/ppm (162 MHz, CD₂Cl₂): 33.76 (s, 1P, RuPPh₂). Anal. Calcd for C₃₈H₃₉Cl₂N₂O₂PRu: C 60.16, H 5.18, N 3.69; Found C 60.12, H 5.43, N 3.20. HRMS/ESI⁺ m/z: [M-Cl]⁺ calculated 723.1476, found 723.1489; formula C₃₈H₃₉ClN₂O₂PRu. Quantitative conversion by NMR using TMS as internal standard. Dark red/orange powder, 15 mg.

Typical procedure for the formation of metallacycles 6 and 7. A solution containing 4 or 5 (0.016 mmol) in CD₂Cl₂ (0.6 mL) was added to a mixture of K₂CO₃ (6.8 mg, 0.048 mmol) and AgPF₆ (5.4 mg, 0.02 mmol) (as solids), and transferred to a J. Young’s tap NMR tube. The heterogeneous mixture was agitated via sonication at room temperature for 10 min, and filtered, affording an orange-red solution. Removal of volatiles in vacuo afforded full conversion to the cyclized products 6 and 7, respectively. Quantitative conversion was determined by ¹H NMR spectroscopy using TMS as the internal standard. Figure 4 shows the general numbering scheme used for the the p-cymene ligand in compounds 6 and 7.

[Ru(p-cym){κ²-P,N-PPh₂C(=O)N(Ph)C(=O)NPh}Cl] (6). Mixture of 2 products, ratio 60:40. (6b as main product) Characterization for 6a and 6b: ¹H NMR δ/ppm (400 MHz, CD₂Cl₂): 7.81–7.71 (m, Ph), 7.66–7.63 (m, 2H, Ph), 7.58–7.53 (m, Ph), 7.51 (d, J = 7.7 Hz, 6H, NPh), 7.47–7.37 (m, Ph), 7.23 (d, J = 7.6 Hz, 4H, NPh), 7.03 (s, 2H, Ph), 5.69–5.62 (m, 4H, 6b-ⁱPr-C₆H₄-Me), 5.64 (d, ³J_HH = 6.1 Hz, 2H, 6a-H5), 5.46 (d, ³J_HH = 6.1 Hz, 2H, 6a-H4), 2.78 (sept, J = 6.8 Hz, 1H, 6a-H2), 2.21 (s, 3H, 6a-H7), 1.85 (sept, J = 6.86 Hz, 1H, 6b-H2), 1.64 (s, 3H, 6b-H5), 1.30 (d, J = 6.9 Hz, 6H, 6a-H1), 0.83 (d, ³J_HH = 6.9 Hz, 3H, 6b-H1), 0.78 (d, ³J_HH = 6.9 Hz, 3H, 6b-H1’). ¹³C{¹H} NMR δ/ppm (100 MHz, CD₂Cl₂): 135.4 (d, J = 11 Hz, Ph), 134.3 (d, J = 36 Hz, Ph), 133.9 (d, J = 2 Hz, Ph), 133.7 (d, J = 10.1 Hz, Ph), 131.7 (d, J = 20 Hz, Ph), 130.9 (d, J = 10 Hz, Ph), 130.1 (NPh), 129.4 (d, J = 11 Hz, Ph), 129.2 (d, J = 11 Hz, Ph), 126.5 (NPh), 108.8 (6b-C4), 102.4 (6a-C6), 97.7 (6a-C3), 97.3 (6b-C3), 90.5 (d, ²J_CP = 3 Hz, 6b- ⁱPr-C₆H₄-Me), 88.3 (d, ²J_CP = 6 Hz, 6b-ⁱPr-C₆H₄-Me), 87.2 (d, ²J_CP = 3 Hz, 6b-ⁱPr-C₆H₄-Me), 85.6 (d, ²J_CP = 4 Hz, 6b-ⁱPr-C₆H₄-Me), 79.4 (6a-C5), 78.6 (6a-C4), 31.9 (6a/6b-C2), 22.2 (6b-C1), 21.4 (6a-C1), 20.9 (6b-C1’), 19.3 (6a-C7), 17.9 (6b-C5). ³¹P NMR δ/ppm (162 MHz, CD₂Cl₂): 52.59 (s, 1P, RuPPh₂). HRMS/ESI⁺ m/z: [M+H]⁺ calculated 695.1168, found 695.1175; formula C₃₆H₃₅ClN₂O₂PRu. IR (ATR) 𝜈̅_max/cm⁻¹: 1616 (C=O), 1584 (C=O). Dark orange powder, 10 mg, 88%.

[Ru(p-cym){κ²-P,N PPh₂C(=O)N(p-tol)C(=O)Np-tol}Cl] (7). Mixture of 2 products, ratio 87:13. (7b as main product). Characterization for 7b: ¹H NMR δ/ppm (400 MHz, CD₂Cl₂): 7.80–7.70 (m, 7H, H-ArP), 7.64 (tq, J = 7.5, 2.04 Hz, 1H, H-ArP), 7.53 (td, J = 7.6, 3.0 Hz, 2H, H-ArP), 7.33 (t, ³J_HH = 8.1 Hz, 5H, H-ArN), 7.10 (d, ³J_HH = 8.3 Hz, 3H, H-ArN), 5.64-5.69 (m, 3H, ⁱPr-C₆H₄-Me), 5.62 (d, ³J_HH = 6.3 Hz, 1H, ⁱPr-C₆H₄-Me), 2.42 (s, 3H, H6), 2.39 (s, 3H, H7), 1.85 (sept, ³J_HH = 6.9 Hz, 1H, H2), 1.65 (s, 3H, H5), 0.84 (d, ³J_HH = 7.0 Hz, 3H, H1), 0.79 (d, ³J_HH = 7.0 Hz, 3H, H1’). ¹³C{¹H} NMR δ/ppm (100 MHz, CD₂Cl₂): 170.9 (d, ¹J_CP = 51 Hz, Ph₂P-C=O), 162.1 (d, ³J_CP = 3 Hz, C=O), 142.5 (quat-N), 139.6 (quat-N), 135.4 (d, J = 11 Hz, ArP), 133.8 (d, J = 3 Hz, ArP), 133.6 (d, J = 10 Hz, ArP), 132.2 (quat-N), 131.4 (ArN), 130.8 (d, J = 10 Hz, ArP), 130.5 (quat-N), 129.4 (d, J = 11 Hz, ArP), 127.1 (d, J = 45 Hz, ipso-P), 126.3 (d, J = 51 Hz, ipso-P), 126.2 (ArN), 108.8 (d, ²J_CP = 2 Hz, C4), 97.1 (C3), 90.6 (d, ²J_CP = 4 Hz, ⁱPr-C₆H₄-Me), 88.1 (d, ²J_CP = 6 Hz, ⁱPr-C₆H₄-Me), 87.1 (d, ²J_CP = 2 Hz, ⁱPr-C₆H₄-Me), 85.6 (d, ²J_CP = 4 Hz, ⁱPr-C₆H₄-Me), 31.1 (C2), 22.1 (C1), 21.5 (C7), 21.4 (C-6), 20.8 (C1’), 17.9 (C5). ³¹P NMR δ/ppm (162 MHz, CD₂Cl₂): 52.34 (s, 1P, RuPPh₂). HRMS/ESI⁺ m/z: [M+H]⁺ calculated 723.1476, found 723.1477; formula C₃₈H₃₉ClN₂O₂PRu. IR (ATR) 𝜈̅_max/cm⁻¹: 1594 (C=O), 1509 (C=O). Dark orange powder, 10 mg, 85%.

Typical procedure for reactivity with CO. An NMR tube containing a solution of 4 (0.016 mmol) in 0.6 mL of CD₂Cl₂, was charged with an atmosphere of CO via three freeze–thaw cycles. The resulting sample was studied via multinuclear NMR spectroscopies (¹H, ¹³C{¹H} and ³¹P NMR); showing formation of the corresponding free phosphine/PDCA (L-4) and [{p-cymene}RuCl₂(CO)], in accordance with the reported literature [9,32].

4. Conclusions

We have described the synthesis of ruthenium(II) complexes coordinated by PCAs and PDCAs, and the first reports of PDCAs as both monodentate and bidentate ligands. In the case of the metallacycles 6 and 7 a mixture of two isomers is obtained, as evidenced by NMR spectroscopy. Whilst the monodentate coordination motif in these complexes can be displaced by CO, chelates 6 and 7 are robust to ligand displacement reactions.