A complete series of halocarbonyl molybdenum PNP pincer complexes – Unexpected differences between NH and NMe spacers

Abstract

In the present study a complete series of seven-coordinate neutral halocarbonyl Mo(II) complexes of the type [Mo(PNP^Me-Ph)(CO)₂X₂] (X = I, Br, Cl, F), featuring the new PNP pincer ligand N,N′-bis(diphenylphosphino)-N,N′-methyl-2,6-diaminopyridine (PNP^Me-Ph), were prepared and fully characterized. The synthesis of these complexes was accomplished by different methodologies depending on the halide ligands. For X = I and Br, [Mo(PNP^Me-Ph)(CO)₂I₂] and [Mo(PNP^Me-Ph)(CO)₂Br₂] were obtained by reacting [Mo(PNP^Me-Ph)(CO)₃] with stoichiometric amounts of I₂ and Br₂, respectively. Alternatively, these complexes were obtained upon treatment of [MoX₂(CO)₃(CH₃CN)₂] (X = I, Br) with 1 equiv. of PNP^Me-Ph. On the other hand, in the case of X = Cl, [Mo(PNP^Me-Ph)(CO)₂Cl₂] was afforded by the reaction of [Mo(CO)₄(μ-Cl)Cl]₂ with 1 equiv. of PNP^Me-Ph. The equivalent procedure also worked for X = Br. Finally, addition of 1 equiv. of 1-fluoro-2,4,6-trimethylpyridinium tetrafluoroborate to [Mo(PNP^Me-Ph)(CO)₃] yielded the analogous fluorine complex [Mo(PNP^Me-Ph)(CO)₂F₂]. The modification of the ligand scaffold by introducing a Me group instead of H changed the properties of the PNP-Ph ligand significantly. While in the present case exclusively neutral seven-coordinate complexes of the type [Mo(PNP^Me-Ph)(CO)₂X₂] were obtained, with the parent PNP-Ph ligand, i.e., featuring NH spacers, cationic seven-coordinate complexes of the type [Mo(PNP-Ph)(CO)₃X]X were afforded. DFT calculations indicated that the reactions are under thermodynamic control. The structures of representative complexes were determined by X-ray single crystal analyses.

Graphical abstract

A complete series of seven-coordinate neutral halocarbonyl Mo(II) complexes of the type [Mo(PNP^Me-Ph)(CO)₂X₂] (X = I, Br, Cl, F) were prepared by three different methodologies and fully characterized.

Highlights

• •A complete series of seven-coordinate halocarbonyl complexes are described.
• •Modification of the PNP ligand scaffold changes properties significantly.
• •Fragmentation of complexes is investigated by means of ESI-MS.
• •Structures of complexes were determined by X-ray single crystal analyses.

1. Introduction

One of the ways of modifying and controlling the properties of transition metal complexes is the use of appropriate ligand systems such as pincer ligands. Usually consisting of a central aromatic backbone tethered to two two-electron donor groups by different spacers, this class of tridentate ligands has found numerous applications in various areas of chemistry, including catalysis, due to their combination of stability, activity and variability [1]. We are currently focusing on the synthesis and reactivity of transition metal PNP pincer complexes where the pincer ligands contain amine (NH and NR) spacers between the aromatic pyridine ring and the phosphine moieties [2]. It has to be noted that these type PNP ligands are readily available via condensation reactions between 2,6-diaminopyridines and electrophilic chlorophosphines R₂PCl.

In a recent study we have begun to investigate the chemistry of cationic seven-coordinate halocarbonyl molybdenum complexes of the type [Mo(PNP)(CO)₃X]⁺ (X = I, Br) with PNP pincer ligands where the central pyridine ring contains NHPR₂(R = Ph, iPr) substituents in the two ortho positions [3]. These complexes were obtained either by direct oxidation of the Mo(0) complexes [Mo(PNP)(CO)₃] with X₂ (X = I, Br) or by treatment of the dimeric Mo(II) complex[Mo(CO)₄(μ-X)X]₂ (X = Cl, Br) with stoichiometric amounts of a PNP ligand (Scheme 1). It has to be noted that the formation of halocarbonyl Mo(II) and W(II) complexes of the type [ML(CO)₃X] (M = Mo, W; X = Cl, Br, I; L = neutral or anionic tridentate ligands such as Cp, Cp*, HCpz₃, HBpz₃) via oxidative addition of X₂ to M(0) complexes [ML(CO)₃] is a well-established reaction [4]. As PNP pincer complexes are concerned, most recently Templeton and coworkers described the synthesis of a series of halocarbonyl tungsten pincer complexes featuring the silazane-based PNP pincer-type ligand HN(SiMe₂CH₂PPh₂)₂ [5].

Scheme 1.

Formation of cationic seven-coordinate halocarbonyl molybdenum complexes of the type [Mo(PNP)(CO)₃X]⁺ (X = Cl, Br, I).

Here we report on the synthesis, characterization and reactivity of a complete series of new halocarbonyl molybdenum(II) PNP pincer complexes of the type [Mo(PNP^Me-Ph)(CO)₂X₂] (X = I, Br, Cl, F) where the PPh₂ moieties of the PNP ligand are connected to the pyridine ring via a NMe spacer. It has to be mentioned that the number of molybdenum (and tungsten) carbonyl fluoride derivatives known is small [5], [6], [7], [8]. The modification of the ligand scaffold by introducing a Me group instead of H changes the properties of the PNP-Ph ligand significantly. A rationale for this behavior will be provided by exploratory DFT calculations [9]. X-ray structures of representative complexes will be given.

2. Results and discussion

We have recently reported the synthesis of molybdenum tricarbonyl complexes of the type [Mo(PNP)(CO)₃] by reacting [Mo(CO)₃(CH₃CN)₃] with a series of PNP ligands in high isolated yields [3]. The same procedure was followed here with the new N-methylated PNP ligand N,N′-bis(diphenylphosphino)-N,N′-methyl-2,6-diaminopyridine (PNP^Me-Ph) (1) affording [Mo(PNP^Me-Ph)(CO)₃] (2) in 92% yield. The new PNP ligand 1 was prepared in a modified two-step procedure as reported elsewhere [3a]. In addition to the spectroscopic characterization, the solid-state structure of 2 was determined by single-crystal X-ray diffraction. A structural view is depicted in Fig. 1 with selected bond distances given in the caption. The coordination geometry around the molybdenum center corresponds to a distorted octahedron with a P–Mo–P bond angle of 155.48(1)°. The carbonyl-Mo-carbonyl angles of the CO ligands trans to one another typically vary strongly with the bulkiness of the PR₂ moiety (PNP-Ph < PNP^Me-Ph > PNP-iPr₂ < PNP^Me-iPr < PNP-tBu₂) and decrease from 171.1(8)° in [Mo(PNP-Ph)(CO)₃] [10] to 166.15(5)° in [Mo(PNP^Me-Ph)(CO)₃] (2) to 166.03(5)° in [Mo(PNP-iPr)(CO)₃] [3a] to 162.93(7)° in [Mo(PNP^Me-iPr)(CO)₃] [3b], and finally to 156.53(4)° in [Mo(PNP-tBu)(CO)₃] [3a]. Accordingly, the new ligand 2 resembles sterically the PNP-iPr ligand.

Fig. 1.

Structural view of [Mo(PNP^Me-Ph)(CO)₃]·CH₂Cl₂ (2·CH₂Cl₂) showing 50% thermal ellipsoids (hydrogen atoms and solvent omitted for clarity). Selected bond lengths (Å) and bond angles (°): Mo1-C32 2.020(1), Mo1-C331.957(1), Mo1-C34 2.030(1), Mo1-P1 2.3816(4), Mo1-P2 2.3890(4), Mo-N1 2.246(1), P1-Mo1-P2 155.48(1), N1-Mo1-C33176.22(5), C32-Mo1-C34166.15(5).

Treatment of 2 with stoichiometric amounts of X₂ (X = I, Br) in CH₃CN yields the neutral seven-coordinate complexes [Mo(PNP^Me-Ph)(CO)₂I₂] (3a) and [Mo(PNP^Me-Ph)(CO)₂Br₂] (3b) in 81% and 84% isolated yield (Scheme 2). Alternatively, these complexes were also obtained by reacting [MoX₂(CO)₃(CH₃CN)₂] (X = I, Br) with 1 equiv. of PNP^Me-Ph. The analogous chlorine complex [Mo(PNP^Me-Ph)(CO)₂Cl₂] (3c) was afforded by treatment of [Mo(CO)₄(μ-Cl)Cl]₂ in CH₃CN with 1 equiv. of PNP^Me-Ph in 81% yield. Noteworthy, this methodology also works very well for the synthesis of 3b. Finally, addition of 1 equiv. of 1-fluoro-2,4,6-trimethylpyridinium tetrafluoroborate, a reagent acting as net source of “F⁺”, to a solution of [Mo(PNP^Me-Ph)(CO)₃] in CH₂Cl₂ yielded the analogous fluorine complex [Mo(PNP^Me-Ph)(CO)₂F₂] (3d) in 93% isolated yield (Scheme 3). The long reaction time required (3 days) and the high yield of this reaction clearly suggest that the BF₄⁻ counterion acts as an additional fluoride source which has indeed precedence in molybdenum and tungsten chemistry [11].

Scheme 2.

Formation of neutral seven-coordinate halocarbonyl molybdenum complexes of the type [Mo(PNP)(CO)₂X₂] (X = F, Cl, Br, I).

Scheme 3.

Synthesis of the fluoride complex [Mo(PNP^Me–Ph)(CO)₂F₂] (3d).

All complexes are thermally robust red to yellow solids which are air stable in the solid state but slowly decompose in solution. Characterization was accomplished by elemental analysis and by ¹H and ³¹P{¹H} NMR, and IR spectroscopy. Due the poor solubility and thus the low concentration of the complexes, quaternary carbons could not be detected or was completely precluded (3d). Thus solid-state ¹³C NMR spectra were recorded giving rise to two characteristic low-field resonances at 228.5 and 218.2 ppm for 3a, 244.8 and 225.7 ppm for 3b, 257.6 and 215.1 ppm for 3c, and 250.4 and 214.3 ppm for 3d assignable to the carbonyl carbon atoms trans and cis to the pyridine nitrogen, respectively. The ³¹P{¹H} NMR spectrum of complexes 3a–c gave rise to singlets at 141.3, 145.2, and 147.2 ppm, respectively, while for 3d a triplet resonance centered at 155.7 ppm with a ²J_PF coupling constant of 41.7 Hz was observed. These chemical shifts are obviously directly related to the increasing electronegativity of the halide ligands. Complexes 3a–d exhibited two bands in the range of 1946–1858 cm⁻¹ in the IR spectrum (cf 2143 cm⁻¹ in free CO) for the mutually cis CO ligands assignable to the symmetric and asymmetric CO stretching frequencies, respectively. The appearance of two sets resonances in the IR spectrum of 3c, which are 26 and 16 cm⁻¹ apart, may be due to intermolecular interactions, e.g., C O⋯Cl–Mo bonds, in the solid state [12].

Moreover, all complexes were also investigated by means of ESI-MS (Table 1). A solution of complexes 3a–d in MeOH in the presence of the corresponding sodium salts NaX (X = I, Br, Cl, F) was subjected to ESI-MS analysis in the positive-ion mode. Under so called “soft ionization” conditions, the most abundant signals observed at m/z are 901.88, 805.91, and 718.01, respectively, which correspond to the sodiated complexes [Mo(PNP^Me-Ph)(CO)X₂] ([M + Na − CO]⁺) where one CO ligand is dissociated suggesting that one CO ligand is labile (vide infra). Further abundant fragments are [Mo(PNP^Me-Ph)(CO)X]⁺ ([M-(CO+NaX)]⁺) and [Mo(PNP^Me-Ph)X]⁺ ([M-(2CO + NaX)]⁺) as shown in Scheme 4. Corresponding positive-ion ESI full scan mass and MS/MS (low energy CID) spectra of 3b are depicted in Fig. 2. For the fluoride complex 3d a different behavior was found. This complex forms after loss of CO and NaF exclusively the cationic fragment [Mo(PNP^Me-Ph)(CO)F]₂²⁺ ([M-(CO + NaF)]₂²⁺) which is apparently a doubly charged dimer based on isotope spacings (0.5 Da). Higher molecular mass fragments were not observed in this particular case.

Table 1.

Elemental compositions and calculated mass values of intact compounds 1, 2, 3a–d and the two most abundant ions related to the sodiated molecules ([M + Na − CO]⁺, [M − (CO + NaX)]⁺ where X = F, Cl, Br, I).

Compound	Elemental composition/Molecular weighta	Elemental composition	Elemental composition
1	C31H29N3P2 505.18	[C31H30N3P2]+ [M + H]+ = 506.19
2	C34H29Mo1N3O3P2 681.08	[C34H29Mo1N3O3P2Na1]+ [M + Na]+ = 704.07 [C33H29Mo1N3O2P2Na1]+ [M + Na − CO]+ = 676.07
3a	C33H29I2Mo1N3O2P2 906.88	[C32H29I2Mo1N3O1P2Na1]+ [M + Na − CO]+ = 901.88	[C32H29I1Mo1N3O1P2]+ [M − (CO + NaX)]+ = 751.99
3b	C33H29Br2Mo1N3O2P2 810.92	[C32H29Br2Mo1N3O1P2Na1]+ [M + Na − CO]+ = 805.91	[C32H29Br1Mo1N3O1P2]+ [M − (CO + NaX)]+ = 704.00
3c	C33H29Cl2Mo1N3O2P2 723.02	[C32H29Cl2Mo1N3O1P2Na1]+ [M + Na − CO]+ = 718.01	[C32H29Cl1Mo1N3O1P2]+ [M − (CO + NaX)]+ = 660.05
3d	C33H29F2Mo1N3O2P2 691.08	[C64H58F4Mo2N6O2P4Na2]2+ [M + Na − CO]+ = 686.07b	[C64H58F2Mo2N6O2P4]2+ [M − (CO + NaX)]+ = 644.08

^aMass calculations are based on the lowest mass molybdenum isotope (⁹²Mo) and monoisotopic values.

^bNot detected.

Scheme 4.

Fragmentation pattern of [Mo(PNP^Me–Ph)(CO)₂X₂] complexes observed in the ESI-MS experiments and the DFT calculated energy balance for X = Br.

Fig. 2.

Positive-ion ESI full scan mass spectrum of [Mo(PNP^Me-Ph)(CO)₂Br₂](3b) (A) and corresponding MS/MS (low energy CID)-spectrum of in-source-generated [M + Na − CO]⁺ precursor ions (B). Inset shows the calculated and measured isotopic pattern of [M + Na − CO]⁺. In both spectra only signals containing the Mo-isotope of highest abundance (⁹⁸Mo) are annotated.

In addition to the spectroscopic characterization, the solid-state structures of 3a, 3b, and 3c were determined by single-crystal X-ray diffraction. Structural diagrams are depicted in Fig. 3, Fig. 4, Fig. 5 with selected bond distances given in the captions. The coordination geometry around the molybdenum center may be described as a trigonal monocapped antiprism with C32-O1 as capping ligand (Fig. 4b).

Fig. 3.

(a) Structural view of [Mo(PNP^Me-Ph)(CO)₂l₂]·CD₂Cl₂ (3a·CD₂Cl₂) showing 50% thermal ellipsoids (hydrogen atoms and solvent omitted for clarity). Selected bond lengths (Å) and bond angles (°): Mo1-l1 2.8888(5), Mo1-l22.8911(5), Mo1-P1 2.4364(7), Mo1-P2 2.4181(6), Mo-N1 2.276(2), Mo-C32 1.937(2), Mo-C33 1.991(2); P1-Mo1-P2 111.84(2), N1-Mo1-l1 94.11(4), N1-Mo1-l2 88.10(4), N1-Mo1-C32 122.64(7), N1-Mo1-C33 124.49(7), l1-Mo1-l2 82.27(1), C32-Mo1-C33 73.60(8).(b) Side view of 3a (hydrogen atoms, most phenyl carbon atoms, and solvent omitted for clarity).

Fig. 4.

(a) Structural view of [Mo(PNP^Me-Ph)(CO)₂Br₂]·3CDCl₃ (3b·3CDCl₃) showing 50% thermal ellipsoids (hydrogen atoms and solvent molecules omitted for clarity). Selected bond lengths (Å) and bond angles (°): Mo1-Br1 2.6887(6), Mo1-Br22.6537(5), Mo1-P1 2.429(1), Mo1-P2 2.422(1), Mo-N1 2.249(3), Mo-C32 1.929(4), Mo-C33 2.016(4); P1-Mo1-P2 118.46(3), N1-Mo1-Br1 92.01(8), N1-Mo1-Br2 87.46(7), N1-Mo1-C32 120.5(2), N1-Mo1-C33 165.2(2), Br1-Mo1-Br2 80.56(2), C32-Mo1-C33 74.3(2).(b) Structural view of the inner coordination sphere of 3b emphasizing the trigonal monocapped antiprism with the C32-O1 ligand as capping ligand.

Fig. 5.

Structural view of [Mo(PNP^Me-Ph)(CO)₂Cl₂]·1.5CD₂Cl₂ (3c·1.5CD₂Cl₂) showing 50% thermal ellipsoids (hydrogen atoms and solvent molecules omitted for clarity). Selected bond lengths (Å) and bond angles (°): Mo1-Cl1 2.5424(7), Mo1-Cl22.5031(8), Mo1-P1 2.4253(9), Mo1-P2 2.4259(8), Mo-N1 2.248(1), Mo-C32 1.950(2), Mo-C331.990(2); P1-Mo1-P2 120.18(2), N1-Mo1-Cl192.20(4), N1-Mo1-Cl2 86.96(3), N1-Mo1-C33117.00(6), N1-Mo1-C32169.96(6), Cl1-Mo1-Cl281.07(2), C32-Mo1-C3372.92(7).

The crystal structure shows the tridentate PNP ligand bound meridionally with two carbonyl and two halide ligands filling the remaining four sites. The metal-CO bond lengths in the three complexes average to 1.97 Å (1.93–2.01 Å). The P1-Mo1-P2 angles in 3a–c increase from 111.84(2) to 118.46(3) to 120.18(2)°, respectively, while the corresponding X1-Mo1-X2 and C32-Mo1-C33 angles are essentially independent of the nature of the halide being 82.27(1), 80.56(2) and 81.07(2)°, and 73.60(8), 74.3(2) and 72.92(7)°, respectively. In all complexes the molybdenum center is significantly bent out of the least squares plane defined by the atoms of the pyridine ring (N1, C1–C5) by 0.687(3), 0.788(5), and 0.632(3) Å.

The different reactivity toward halogen (Br₂) shown by the tricarbonyl complexes, [Mo(PNP)(CO)₃], with different PNP ligands was investigated by means of DFT calculations (see Computational details). When the spacer in those ligands is NH (ligand PNP-Ph) the outcome of the reaction is a cationic complex with one bromine as counter ion that is, thus, expelled from the metal coordination sphere. On the other hand, in the case of PNP^Me-Ph, that has NMe as spacer in both arms of the pincer ligand, the reaction product is the neutral complex with both bromine ligands coordinated to the metal and with loss of one of the carbonyl ligands of the parent species. The energy balances calculated for the reactions are presented in Scheme 5 for the complexes with the two types of PNP ligands, PNP-Ph and PNP^Me-Ph. Formation of the cationic complexes [Mo(PNP^R-Ph)(CO)₃Br]Br is presented on the left side, while the reaction that forms the neutral species, [Mo(PNP^R-Ph)(CO)₂Br₂], with loss of CO is shown on the right side of Scheme 5 (R = H or Me).

Scheme 5.

Energy balance for the formation of complexes [Mo(PNP–Ph)(CO)₃Br]Br and [Mo(PNP–Ph)(CO)₂Br₂].

The energy balances in Scheme 5 show that the most stable product is obtained in each case. For the complex with PNP-Ph formation of the cationic complex [Mo(PNP-Ph)(CO)₃Br]Br is more favorable than formation of the neutral species [Mo(PNP-Ph)(CO)₂Br₂] by 16 kcal/mol. Conversely, in the case of the complex with PNP^Me-Ph the neutral complex 3b is the most favorable product by 9 kcal/mol.

The results above indicate that the reactions are under thermodynamic control and this conclusion is further supported by a closer look into the bonding of the relevant species. In fact, for the cationic complexes, the corresponding reaction (left side of Scheme 5) is more favorable by 21 kcal/mol in the case of the PNP-Ph ligand (R = H) compared to the system with PNP^Me-Ph (R = Me). The main structural difference between the two products is the existence of a strong H-bond, NH⋯Br (d_H-Br = 2.01 Å), in [Mo(PNP-Ph)(CO)₃Br]Br that, naturally, is absent in the corresponding product with R = Me. In the case of [Mo(PNP^Me-Ph)(CO)₃Br]Br, the bromide ion approaches one C–H of the pyridine ring (d_H-Br = 2.28 Å) in the ion pair. The NH⋯Br H-bond stabilizes the product with NH spacers on the PNP ligand, and hints that that interaction may play a role during the course of Br₂ addition to the initial tricarbonyl complex, [Mo(PNP-Ph)(CO)₃], assisting the loss of Br⁻ from the metal coordination sphere, and leading to the observed product.

For the system with PNP^Me-Ph (R = Me), formation of the neutral species is the most favorable process. In fact, that reaction is 4 kcal/mol more favorable than the equivalent reaction in the case of the PNP-Ph species (R = H). This stability difference, although rather small, can still be traced to the bonding in the corresponding [Mo(PNP^R-Ph)(CO)₂Br₂] complexes (R = H or Me). In fact, the coordination of the PNP^Me-Ph ligand in 3b is stronger than the one observed for the PNP ligand with R = H in the analogous complex. This is most clearly seen in the Mo–N(py) bonds, with distances of 2.28 and 2.32 Å in the heptacoordinated complexes with R = Me (3b) and R = H, respectively. The corresponding Wiberg indices (WI) [13] confirm this result indicating a stronger Mo–N(py) bond in complex 3b (WI = 0.29), when compared to the equivalent bond in [Mo(PNP-Ph)(CO)₂Br₂] (R = H, WI = 0.27). The charge distribution (NPA, see Computational details) calculated for the two complexes corroborates that result. The PNP^Me-Ph ligand in 3b is slightly more positive (C_PNP = 1.16) than the PNP-Ph ligand the corresponding complex (C_PNP = 1.12), while the opposite happens with the metal atom, that is, the Mo is electron richer in 3b (C_Mo = −0.60) than the metal atom in [Mo(PNP-Ph)(CO)₂Br₂] (C_Mo = −0.57). This indicates that there is a stronger electron donation from the pincer ligand to the metal in the case of PNP^Me-Ph (R = Me) and, hence, a stronger coordination and a more stable molecule in this case.

The fragmentation pattern observed in the MS experiments was also investigated using DFT through the calculation of the energy balances for the fragmentation steps involving the most abundant fragments observed. The values shown in Scheme 4 were obtained for the bromide system (X = Br) without considering the sodium ion. Most interestingly, the second step, i.e., loss of one bromide ligand from the bis-bromo mono-carbonyl complex [Mo(PNP^Me-Ph)(CO)Br₂] is by far the least favorable of all three corroborating, thus, the high abundance observed for that particular fragment.

3. Conclusion

In the present study a complete series of seven-coordinate neutral halocarbonyl Mo(II) complexes of the type [Mo(PNP^Me-Ph)(CO)₂X₂] (X = I, Br, Cl, F) were prepared and fully characterized. The synthesis of these complexes was accomplished by different methodologies depending on the halide ligands. The modification of the ligand scaffold by introducing a Me group instead of H changes the properties of the PNP ligand significantly. In fact, the PNP^Me-Ph ligand is sterically more demanding than the PNP-Ph ligand which is apparent from the carbonyl-Mo-carbonyl angles of the CO ligands trans to one another being 171.1(8)° in [Mo(PNP-Ph)(CO)₃] and166.15(5)° in [Mo(PNP^Me-Ph)(CO)₃]. While in the present case exclusively neutral seven-coordinate complexes of the type [Mo(PNP^Me-Ph)(CO)₂X₂] were obtained, with the parent PNP-Ph ligand, i.e., featuring NH spacers, cationic seven-coordinate complexes of the type [Mo(PNP-Ph)(CO)₃X]X were afforded. DFT calculations indicate that the most stable product is obtained in each case and, thus, the reaction is under thermodynamic control. The structures of representative complexes were determined by X-ray single crystal analyses.

4. Experimental

4.1. General

All manipulations were performed under an inert atmosphere of argon by using Schlenk techniques or in a MBraun inert-gas glovebox. The solvents were purified according to standard procedures [14]. The deuterated solvents were purchased from Aldrich and dried over 4 Å molecular sieves. The precursor complexes [Mo(CO)₄(μ-X)X]₂ (X = Cl, Br) and [MoX₂(CO)₃(CH₃CN)₂] (X = l, Br) were prepared according to the literature [15], [16].

4.2. Characterization techniques

¹H, ¹³C{¹H}, and ³¹P{¹H} NMR spectra were recorded on Bruker AVANCE-250, AVANCE-300 DPX, and DRX 400 spectrometers. ¹H and ¹³C{¹H} NMR spectra were referenced internally to residual protio-solvent and solvent resonances, respectively, and are reported relative to tetramethylsilane (δ = 0 ppm). ³¹P{¹H} NMR spectra were referenced externally to H₃PO₄ (85%) (δ = 0 ppm).

All solid-state ¹³C NMR spectra were recorded on a Bruker Avance-300 spectrometer (standard bore), equipped with a 4 mm broad-band MAS probe-head and ZrO₂ rotors. The rotational speed for all experiments was 12 kHz.

All mass spectrometric measurements were performed on an Esquire 3000^plus 3D-quadrupole ion trap mass spectrometer (Bruker Daltonics, Bremen, Germany) in positive-ion mode electrospray ionization (ESI-MS). Mass calibration was done with a commercial mixture of perfluorinated trialkyl-triazines (ES Tuning Mix, Agilent Technologies, Santa Clara, CA, USA). All analytes were dissolved in methanol hypergrade for LC–MS Lichrosolv (Merck, Darmstadt, Germany) to a concentration of roughly 1 mg/mL and doped with sodium fluoride, sodium chloride, sodium bromide or sodium iodide (Merck, Darmstadt, Germany) to promote [M + Na]⁺-adduct ion formation of the neutral molybdenum complexes as previously described for titanium and zirconium complexes [17], [18]. Direct infusion experiments were carried out using a Cole Parmer model 74900 syringe pump (Cole Parmer Instruments, Vernon Hills, IL, USA) at a flow rate of 2 μL/min. Full scan and MS²-scans were measured in the mass range m/z 100–1100 with the target mass set to m/z 1000. Further experimental conditions include: dry gas temperature: 150 C; capillary voltage: −4 kV; skimmer voltage: 40 V; octapole and lens voltages: according to the target mass set. Helium was used as buffer gas for full scans and as collision gas for MS²-scans in the low energy CID mode. The activation and fragmentation width for tandem mass spectrometric (MS/MS) experiments was set to 14 Da to cover the entire isotope cluster for fragmentation. The corresponding fragmentation amplitude ranged from 0.4 to 0.6 V in order to keep a low abundant precursor ion intensity in the resulting spectrum. As precursor ions for tandem mass spectrometric experiments only the ions [M + Na − CO]⁺ and [M − (CO + NaX)]⁺, where X = F, Cl, Br or I, could be selected as precursor ions. Ions containing two carbonyls coordinated with molybdenum could not be detected. All mass calculations are based on the lowest mass molybdenum isotope (⁹²Mo-isotope). It should also be mentioned here that the sodium halides had to correspond to the halide content of the molybdenum complex as an easy exchange between different halides in the analyte solution was observed obscuring correct mass assignments of the analytes. Mass spectra and tandem mass spectra were averaged during data acquisition time of 1–2 min and one analytical scan consisted of five successive microscans resulting in 50 and 100 analytical scans, respectively, for the final mass spectrum.

4.3. Syntheses

4.3.1. N,N′-bis(diphenylphosphino)-N,N′-methyl-2,6-diaminopyridine (PNP^Me-Ph) (1)

A solution of 2,6-N,N′-dimethyldiaminopyridine (2.0 g, 14.58 mmol) in toluene (125 mL) was cooled down to −20 °C and n-BuLi (2.5 M solution in hexane, 6.10 mg, 15.31 mmol) was added. The mixture was stirred at room temperature for 2 h. After this period, the mixture was cooled down to −60 °C and then PPh₂Cl (2.6 mL, 14.58 mmol) was added. The mixture was stirred for 2 h at room temperature and then refluxed overnight at 80 °C. The mixture was allowed to cool down to room temperature and 8 mL of a saturated solution of NaHCO₃ was added. The two phases were separated and anhydrous Na₂SO₄ was added to the organic phase. The mixture was filtered and the solvent was evaporated leading to a yellow oil. The oil was dissolved in 125 mL of toluene and cooled down to −20 °C and then n-BuLi (2.5 M solution in hexane, 6.10 mg, 15.31 mmol) was added. The mixture was stirred at room temperature for 2 h. After this period, the mixture was cooled down to −60 °C and then again PPh₂Cl (2.6 mL, 14.58 mmol) was added. The mixture was stirred for 2 h at room temperature and then refluxed overnight at 80 °C. After this period a saturated solution of NaHCO₃ was added to the mixture at room temperature. The two phases were separated and anhydrous Na₂SO₄ was added to the organic phase. The mixture was filtered and the solvent was removed under vacuum yielding a yellow oil together with a white precipitate. In order to purify the crude product filtration with hot CH₃CN was carried out. The resulting white compound was dried under vacuum. Yield: 3.99 g (54%). Anal. Calcd. for C₃₁H₂₉N₃P₂ (505.54): C, 73.65; H, 5.78; N, 8.31%. Found: C, 73.79; H, 5.50; N, 8.23%. ¹H NMR (δ, CDCl₃, 20 °C): 7.38–7.44 (m, 21H, Ph, py⁴), 6.87 (d, J = 7.4 Hz, 2H, py^3,5), 2.89 (s, 6H, NCH₃). ¹³C{¹H} NMR (δ, CDCl₃, 20 °C): 159.6 (d, J = 27.4 Hz, 2C, py^2,6), 138.4 (t, J = 3.0 Hz, py⁴), 137.5 (d, J = 16.0 Hz, 4C, Ph¹), 132.2 (d, J = 20.8 Hz, 8C, Ph^2,6), 129.2 (s, 4C, Ph⁴), 128.5 (d, J = 5.9 Hz, 8C, Ph^3,5), 99.9 (d, J = 21.2 Hz, 2C, py^3,5), 34.1 (d, J = 8.6 Hz, 2C, NCH₃). ³¹P{¹H} NMR (δ, CDCl₃, 20 °C): 61.4. ESI-MS (m/z, CH₃OH, HCOOH) positive ion: 506.19 [M + H]⁺.

4.3.2. [Mo(PNP^Me-Ph)(CO)₃] (2)

A suspension of [Mo(CO)₆] (264 mg, 1.0 mmol) in CH₃CN (20 mL) was refluxed for 3 h under an argon atmosphere. The resulting yellow solution was cooled down to room temperature and 1 equiv. of ligand (505 mg, 1.0 mmol) in toluene (8 mL) was added and the solution was refluxed for 4 days. After this period the solvent was evaporated under vacuum and the compound was washed with CH₃CN and n-pentane. The compound was dried under vacuum affording a yellow powder. Yield: 633 mg (92%). Anal. Calcd. for C₃₄H₂₉N₃O₃P₂Mo (685.51): C, 59.57; H, 4.26; N, 6.13%. Found: C, 59.74; H, 4.19; N, 6.23%. ¹H NMR (δ, CD₂Cl₂, 20 °C): 7.44–7.60 (m, 21H, Ph, py⁴), 6.34 (d, J = 8.2 Hz, 2H, py^3,5), 2.99 (s, 6H, NCH₃). ¹³C{¹H} NMR (δ, CD₂Cl₂, 20 °C): 227.8 (t, J = 4.9 Hz), 211.9 (t, J = 9.9 Hz), 161.0–161.1 (m, py^2,6), 143.9 (s, py⁴), 136.3 (t, J = 5.8 Hz, py^2,6), 134.9 (d, J = 45.5 Hz, Ph¹), 131.8 (s, Ph⁴), 130.6 (t, J = 5.2 Hz, Ph^3,5), 130.5 (s, Ph⁴), 129.0 (d, J = 58.8 Hz, Ph¹), 128.4 (t, J = 4.9 Hz, Ph^3,5), 127.9 (t, J = 5.4 Hz, Ph^2,6), 100.0 (s, py^3,5), 35.7 (s, NCH₃). ³¹P{¹H} NMR (δ, CD₂Cl₂, 20 °C): 143.0. IR (ATR, 25 °C): 1956 (ν_{C O}), 1911 (ν_{C O}), 1850 (ν_{C O}). ESI-MS (m/z, CH₃OH, NaCl) positive ion: 704.07 [M + Na]⁺.

4.3.3. [Mo(PNP^Me-Ph)(CO)₂I₂] (3a)

Method A: A solution of 2 (200 mg, 0.292 mmol) in CH₃CN (10 mL) was cooled down to −78 °C and 1 equiv. of I₂ (74.1 mg, 0.292 mmol) was added. The solution was slowly warmed to room temperature and stirred for 18 h. After this period the solution was filtered, solvent was removed under vacuum, and the red solid was washed twice with CH₃CN and dried under vacuum. Method B: A solution of [MoI₂(CO)₃(CH₃CN)₂] (200 mg, 0.883 mmol) in CH₃CN (10 mL) was treated with 1 equiv. of 1 (196.0 mg, 0.388 mmol). The solution was stirred for 18 h and then filtrated. The product was washed twice with CH₃CN and n-pentane and then dried under vacuum to yield a red powder. Yield: 217 mg (81%). Anal. Calcd. for C₃₃H₂₉I₂N₃O₂P₂Mo (911.31): C, 43.49; H, 3.21; N, 4.61%. Found: C, 43.30; H, 3.18; N, 4.73%. ¹H NMR (δ, CD₂Cl₂, 20 °C): 7.86 (t, J = 8.2 Hz, 1H, py⁴), 7.63 (t, J = 8.2 Hz, 4H, Ph^2,6), 7.52 (t, J = 7.0 Hz, 2H, Ph⁴), 7.41 (t, J = 6.9 Hz, 4H, Ph^2,6), 7.07 (t, J = 7.2 Hz, 2H, Ph⁴), 6.85 (t, J = 7.1 Hz, 4H, Ph^3,5), 6.60 (t, J = 7.3 Hz, 4H, Ph^3,5), 6.36 (d, J = 8.2 Hz, 2H, py^3,5), 3.14 (s, 6H, NCH₃). ¹³C{¹H} NMR (δ, CD₂Cl₂, 20 °C): 162.1–163.1 (m, py^2,6), 144.8 (s, py⁴), 138.7 (t, J = 5.7 Hz, py^2,6), 132.6 (s, Ph⁴),130.6 (t, J = 5.7 Hz, Ph^3,5), 130.5 (s, Ph⁴), 129.0 (t, J = 4.9 Hz, Ph^3,5), 128.3 (t, J = 5.5 Hz, Ph^2,6), 100.3 (s, py^3,5), 36.1 (s, NCH₃). The quaternary Ph and CO carbon atoms could not be detected. ¹³C solid-state NMR (δ, 12 kHz, 25 °C): 228.5 (CO), 218.2 (CO). ³¹P{¹H} NMR (δ, CD₂Cl₂, 20 °C): 141.3. IR (ATR, 25 °C): 1946 (ν_{C O}), 1870 (ν_{C O}). ESI-MS (m/z, CH₃OH, NaI) positive ion: 901.88 [M + Na − CO]⁺, 751.99 [M − (CO + NaI)]⁺.

4.3.4. [Mo(PNP^Me-Ph)(CO)₂Br₂] (3b)

Method A: This complex was prepared analogously to 3a (method A) with 2 (200 mg, 0.292 mmol) and (Br₂) (14.96 μL, 0.292 mmol) as starting materials. Method B: This complex was prepared analogously to 3a (method B) with [MoBr₂(CO)₃(CH₃CN)₂] (200 mg, 0.474 mmol) and 1 (239.6 mg, 0.474 mmol) as starting materials. Method C: A solution of [Mo(CO)₄(μ-Br)Br]₂ (200 mg, 0.272 mmol) in CH₃CN (10 mL) was treated with 1 equiv. of 1 (137.5 mg, 0.272 mmol) and the solution was stirred for 18 h. After this period the solution was filtered, the solvent was removed under reduced pressure, and the remaining yellow solid was washed twice with CH₃CN and n-pentane. Yield: 186.5 mg (84%). Anal. Calcd. for C₃₃H₂₉Br₂N₃O₂P₂Mo (817.31): C, 48.50; H, 3.58; N, 5.14%. Found: C, 48.42; H, 3.63; N, 5.06%. ¹H NMR (δ, CD₂Cl₂, 20 °C): 7.84 (t, J = 7.8 Hz, 1H, py⁴), 7.63 (t, J = 7.4 Hz, 4H, Ph^2,6), 7.53 (t, J = 7.2 Hz, 2H, Ph⁴), 7.44 (t, J = 7.1 Hz, 4H, Ph^2,6), 7.16 (t, J = 7.2 Hz, 2H, Ph⁴), 6.93 (t, J = 7.2 Hz, 4H, Ph^3,5), 6.64 (t, J = 8.0 Hz, 4H, Ph^3,5), 6.39 (d, J = 8.0 Hz, 2H, py^3,5), 3.11 (s, 6H, NCH₃). ¹³C{¹H} NMR (δ, CD₂Cl₂, 20 °C): 161.4–161.6 (m, py^2,6), 144.6 (s, py⁴), 137.6 (t, J = 6.0 Hz, py^2,6), 135.8 (d, J = 45.9 Hz, Ph¹), 132.2 (s, Ph⁴), 130.3 (t, J = 4.9 Hz, Ph^3,5), 130.2 (s, Ph⁴), 129.3 (d, J = 56.2 Hz, Ph¹), 128.5 (t, J = 4.8 Hz, Ph^3,5), 128.2 (t, J = 5.5 Hz, Ph^2,6), 100.1 (s, py^3,5), 35.9 (s, NCH₃). The quaternary CO carbon atoms could not be detected. ¹³C solid-state NMR (δ, 12 kHz, 25 °C): 244.8 (CO), 225.7 (CO). ³¹P{¹H} NMR (δ, CD₂Cl₂, 20 °C): 145.2. IR (ATR, 25 °C): 1979 (ν_{C O}), 1858 (ν_{C O}). ESI-MS (m/z, CH₃OH, NaBr) positive ion: 805.91[M + Na − CO]⁺, 704.00 [M − (CO + NaBr)]⁺.

4.3.5. [Mo(PNP^Me-Ph)(CO)₂Cl₂] (3c)

This complex was prepared analogously to 3b (Method C) with [Mo(CO)₄(μ-Cl)Cl]₂ (200 mg, 0.359 mmol) and 1 (181.3 mg, 0.359 mmol) as starting materials. Yield: 211.7 mg (81%). Anal. Calcd. for C₃₃H₂₉Cl₂N₃O₂P₂Mo (728.41): C, 54.22; H, 4.01; N, 5.77%. Found: C, 54.10; H, 3.91; N, 5.81%. ¹H NMR (δ, CD₂Cl₂, 20 °C): 7.83 (t, J = 7.6 Hz, 1H, py⁴), 7.67 (t, J = 8.9 Hz, 4H, Ph^2,6), 7.54 (t, J = 7.4 Hz, 2H, Ph⁴), 7.45 (t, J = 6.9 Hz, 4H, Ph^2,6), 7.24 (t, J = 7.3 Hz, 2H, Ph⁴), 7.00 (t, J = 7.3 Hz, 4H, Ph^3,5), 6.69 (t, J = 7.6 Hz, 4H, ph^3,5), 6.40 (d, J = 7.3 Hz, 2H, py^3,5), 3.07 (s, 6H, NCH₃).¹³C{¹H} NMR (δ, CD₂Cl₂, 20 °C): 161.4–161.7 (m, py^2,6), 144.3 (s, py⁴), 136.7 (t, J = 5.9 Hz, py^2,6), 132.3 (s, Ph⁴), 131.12 (t, J = 5.1 Hz, Ph^3,5), 130.9 (s, Ph⁴), 128.9 (t, J = 4.9 Hz, Ph^3,5), 128.4 (t, J = 5.4 Hz, Ph^2,6), 100.5 (s, py^3,5), 36.2 (s, NCH₃). The quaternary Ph and CO carbon atoms could not be detected. ¹³C solid-state NMR (δ, 12 kHz, 25 °C): 257.6 (CO), 215.1 (CO). ³¹P{¹H} NMR (δ, CD₂Cl₂, 20 °C): 147.2. IR (ATR, 25 °C): 2000 (ν_{C O}), 1974 (ν_{C O}), 1862 (ν_{C O}), 1846 (ν_{C O}). ESI-MS (m/z, CH₃OH, NaCl) positive ion: 718.01 [M + Na − CO]⁺, 660.05 [M − (CO + NaCl)]⁺.

4.3.6. [Mo(PNP^Me-Ph)(CO)₂F₂] (3d)

To a solution of 2 (200 mg, 0.292 mmol) of CH₂Cl₂ (10 mL) 1 equiv. of 1-fluoro-2,4,6-trimethylpyridinium tetrafluoroborate (66.28 mg, 0.292 mmol) was added. The solution was stirred for 3 days. After this period the solution was filtered, the solvent was then removed under vacuum, and the remaining orange solid was washed with twice with CH₂Cl₂ and dried under vacuum. Yield: 189 mg (93%). Anal. Calcd. for C₃₃H₂₉F₂N₃O₂P₂Mo (695.50): C, 56.99; H, 4.20; N, 6.04%. Found: C, 57.15; H, 4.26; N, 6.11%. ¹H NMR (δ, CD₃CN, 20 °C): 7.98 (t, J = 8.1 Hz, 1H, py⁴), 7.58 (dd, J = 14.2, 6.9 Hz, 4H, Ph^2,6), 7.11–7.49 (m, 8H, Ph⁴, Ph^2,6), 7.01 (t, J = 7.2 Hz, 4H, Ph^3,5), 6.66 (d, J = 8.3 Hz, 2H, py^3,5), 6.37 (dd, J = 10.8, 7.7 Hz, 4H, Ph^3,5), 2.54 (s, 6H, NCH₃). ³¹P{¹H} NMR (δ, CD₃CN, 20 °C): 155.7 (t, ²J_PF = 41.7 Hz). ¹³C solid-state NMR (δ, 12 kHz, 25 °C): 250.4 (CO), 214.3 (CO). IR (ATR, 25  C): 1977 (ν_{C O}), 1880 (ν_{C O}). ESI-MS (m/z, CH₃OH, NaF) positive ion: 644.08 [M − (CO + NaF)]⁺.

4.4. X-ray structure determinations

Single crystals for X-ray diffraction were obtained as follows: [Mo(PNP^Me-Ph)(CO)₃]·CH₂Cl₂ (2·CH₂Cl₂) by slow diffusion of n-pentane into a CH₂Cl₂ solution, [Mo(PNP^Me-Ph)(CO)₂l₂]·CD₂Cl₂ (3a·CD₂Cl₂) and [Mo(PNP^Me-Ph)(CO)₂Br₂]·3CDCl₃ (3b·3CDCl₃) and [Mo(PNP^Me-Ph)(CO)₂Cl₂]·1.5CD₂Cl₂ (3c·1.5CD₂Cl₂) by slow evaporation of CD₂Cl₂ and CDCl₃ solutions, respectively. The color of the crystals varied from yellow to orange. X-ray diffraction data were collected at T = 100 K in a dry stream of nitrogen on Bruker Smart APEX CCD (3a·CD₂Cl₂) and Bruker Kappa APEX-2 CCD (3b·3CDCl₃ and 3c·1.5CD₂Cl₂) diffractometer systems using graphite-monochromatised Mo-Kα radiation (λ = 0.71073 Å) and fine sliced φ- and ω-scans (3a·CD₂Cl₂: only ω-scans) covering complete spheres of the reciprocal space. After data integration with the program SAINT corrections for absorption and detector effects were applied with the program SADABS [19]. The structures were solved by charge-flipping implemented in SUPERFLIP and refined against F values with JANA2006 [20]. Non-hydrogen atoms were refined anisotropically. The H atoms were placed in calculated positions and thereafter refined as riding on the parent atoms. Molecular graphics was generated with the program MERCURY [21]. Crystal data and experimental details are given in Table 2.

Table 2.

Details for the crystal structure determinations of the solvates 2·CH₂Cl₂, 3a·CD₂Cl₂, 3b·3CDCl₃, and 3c·1.5CD₂Cl₂.

	2·CH2Cl2	3a·CD2Cl2	3b·3CDCl3	3c·1.5CD2Cl2
Formula	C35H31Cl2MoN3O3P2	C34H29Cl2D2I2MoN3O2P2	C36H29Br2Cl9D3MoN3O2P2	C34.5H29Cl5D3MoN3O2P2
Fw	770.4	998.3	653.23	858.80
Cryst. size, mm	0.71 × 0.34 × 0.05	0.64 × 0.62 × 0.46	0.54 × 0.22 × 0.03	0.72 × 0.35 × 0.30
Color, shape	Yellow rod	Orange block	Yellow rod	Yellow block
Crystal system	Monoclinic	Monoclinic	Monoclinic	Triclinic
Space group	P21/n (no. 14)	P21/c (no. 14)	P21/c (no. 14)	$\mathit{P}\overline{\mathit{1}}$ (no. 2)
a, Å	9.7002(5)	9.4602(8)	16.1514(9)	10.9919(15)
b, Å	23.0768(11)	17.8964(16)	11.1407(6)	11.9429(16)
c, Å	15.6657(7)	21.2970(19)	25.4476(12)	15.499(2)
α, deg	90	90	90	111.189(3)
β, deg	100.1630(9)	90.001(2)	104.289(2)	95.185(3)
γ, deg	90	90	90	105.903(3)
V, Å3	3451.7(3)	3605.7(5)	4437.3(4)	1783.3(4)
T, K	100	100	100	100
Z	4	4	4	2
ρcalc, g cm−3	1.4821	1.8383	1.7634	1.5989
μ, mm−1 (Mo-Kα)	0.667	2.349	2.748	0.869
F(000)	1568	1936	2320	866
Absorption corrections, Tmin − Tmax	Multi-scan,0.76–0.98	Multi-scan, 0.24–0.34	Multi-scan, 0.49–0.92	Multi-scan, 0.70–0.77
θ range, deg	2.20–30.01	2.15–32.54	1.30–27.54	1.89–32.68
No. of rflns measd	27,241	31,658	77,590	62,193
Rint	0.0167	0.0185	0.0983	0.0543
No. of rflns unique	10,026	12,333	10,223	13,007
No. of rflns I > 3σ(I)	8629	11,346	6237	11,319
No. of params/restraints	432/0	415/0	509/0	442/0
R1 (I > 3σ(I))a	0.0259	0.0254	0.0378	0.0323
R1 (all data)	0.0328	0.0288	0.0908	0.0393
wR2 (I > 3σ(I))	0.0314	0.0343	0.0339	0.0453
wR2 (all data)	0.0333	0.0355	0.0365	0.0462
GooF	1.86	1.87	1.15	1.79
Diff. four. peaks min/max, eÅ−3	−0.51/0.62	−1.35/1.60	−0.96/0.88	−0.97, 0.95

^a$R_1=\sum \Vert F_{\text{o}}|-|F_{\text{c}}\Vert /\sum \left|F_{\text{o}}\right|,w{R}_2={\left\{\sum \left[w{\left(F_{\text{o}}^2-F_{\text{c}}^2\right)}^2\right]/\sum \left[w{\left(F_{\text{o}}^2\right)}^2\right]\right\}}^{1/2},\text{GooF}={\left\{\sum \left[w{\left(F_{\text{o}}^2-F_{\text{c}}^2\right)}^2\right]/\left(n-p\right)\right\}}^{1/2}$.

4.5. Computational details

Calculations were performed using the Gaussian 09 software package [22], and the PBE0 functional, without symmetry constraints. That functional uses a hybrid generalized gradient approximation (GGA), including 25% mixture of Hartree–Fock [23] exchange with DFT [9] exchange-correlation, given by Perdew, Burke and Ernzerhof functional (PBE) [24]. The optimized geometries were obtained with the Stuttgart Effective Core Potentials and associated basis set [25] augmented with a f-polarization function [26] for Mo, and a standard 6-31G(d,p) [27] for the remaining elements (basis b1). A Natural Population Analysis (NPA) [28] and the resulting Wiberg indices [13] were used to study the electronic structure and bonding of the optimized species. The NPA analysis was performed with the NBO 5.0 program [29]. The energy values referred along the text were calculated with the same functional and a 6-311++G(d,p) basis set [30], using the geometries obtained at the PBE0/b1 level.

Appendix A. Supplementary material

The following is the supplementary data related to this article: