Introduction

PCP pincer complexes which feature an aromatic anionic benzene backbone connected to phosphine donors via CH2, O, or NR (R=H, alkyl, aryl) linkers are a very attractive class of compounds [1,2,3]. With respect to group-six metals, PCP complexes are exceedingly rare [1]. In fact, according to our knowledge, there are only three established Mo and W systems, and one Cr system described as shown in Scheme 1 [4,5,6,7]. Such complexes were shown to be promising candidates for the activation and splitting of small molecules such as dinitrogen. Schrock and co-workers discovered [4] that the Mo(III) PCP pincer complex [Mo(POCOP-tBu)(I)2] forms upon reduction with NaHg in the presence of N2 the anionic Mo(IV) nitride complex [Mo(POCOP-tBu)(N)(I)]−.

scheme 1

Here we utilize the oxidative addition of the C–Br bond of the ligand precursor (2-bromo-1,3-phenylene)bis(methylene)bis(diisopropylphosphane) (P(C-Br)PCH2-iPr) (1) to the hexacarbonyl complexes [M(CO)6] (M=Cr, Mo, W) as synthetic entry into group-six metal PCP pincer complexes. This procedure has been successfully applied recently for the synthesis of several Cr, Mo, W, Mn, and Fe PCP complexes bearing NR (R = alkyl) linkers [8, 9]. X-ray structures of the Mo and W complexes are presented.

Results and discussion

The xylene-based PCP ligand 1 was prepared in a two-step process following a literature procedure [10]. 2-Bromo-1,3-bis(bromomethyl)benzene (1a) was treated with one equiv of PiPr2H to give the phosphonium intermediate 1b which was subsequently deprotonated with NaOAc in water to afford the desired ligand (P(C-Br)PCH2-iPr) 1 in 49% isolated yield (Scheme 2).

scheme 2

A suspension of the hexacarbonyl complexes [M(CO)6] (M=Cr, Mo, W) and one equivalent of P(C-Br)PCH2-iPr (1) in CH3CN was heated in a sealed microwave glass tube at 130 °C (Cr), 130 °C (Mo) and 160 °C (W) for 3–12 h. In the case of chromium, a mixture of several complexes was obtained. From this mixture, only the dibromo-Cr(III) complex with the tentative formula [Cr(PCPCH2-iPr)(CH3CN)(Br)2] (2) could be obtained in 40% isolated yield (Scheme 3). According to DFT calculations, the trans-bromide complex is more stable than the cis-isomer by 36.0 kJ/mol, which is in line with the fact that the analogous chloride compound, featuring NEt linkers, adopts a trans-chloride arrangement [7]. Due to the extreme air-sensitivity of this complex characterization was accomplished only by IR spectroscopy and magnetic measurements in solution. A solution magnetic moment of μeff = 3.7 μB (CH2Cl2, Evans method) was determined which is consistent with three unpaired electrons as expected for a d3 configuration. In the IR, the νCN stretching frequency of the acetonitrile ligand was observed at 2194 cm−1. The analogous compound bearing NEt linkers, exhibits the νCN stretching frequency at 2207 cm−1 indicating that the xylene-based PCP pincer ligand is slightly more electron donating [7].

scheme 3

On the other hand, with molybdenum and tungsten the expected seven-coordinate bromo tricarbonyl Mo(II) and W(II) complexes [Mo(PCPCH2-iPr)(CO)3Br] (3) and [W(PCPCH2-iPr)(CO)3Br] (4) were obtained (Scheme 3). These compounds were isolated in 80 and 83% yield and were characterized by 1H, 13C{1H}, and 31P{1H} NMR spectroscopy, IR spectroscopy and HRMS. As typical for seven-coordinate complexes, 3 and 4 are fluxional in solution. Thus, in the 31P{1H} NMR spectra only one resonance is observed at 75.1 and 53.6 ppm, respectively. In the case of 4, the tungsten–phosphorus coupling was observed as a doublet satellite due to 183W (14% natural abundance, I = ½) which is superimposed over the dominant singlet (1JWP = 84 Hz). Characteristic features comprise in the 13C{1H} NMR spectrum of 3 and 4 triplet resonances at 171.3 ppm (JPC = 8.0 Hz) and 168.9 ppm (JPC = 6.8 Hz), respectively, assigned to the ipso carbon atoms of the benzene moiety. Moreover, the CO ligands give rise to one broad and one sharp low-field triplet resonance in a 2:1 ratio in the range of 238–218 ppm. The broad signals arise from a fast interchange process between two CO ligands (vide infra). In the IR spectra, they display the typical three strong νCO bands at 2008, 1923, and 1871 cm−1 for 3 and 2000, 1913, 1859 cm−1 for 4. For comparison, the analogous complexes [Mo(PCPNEt-iPr)(CO)3Cl] and [W(PCPNEt-iPr)(CO)3Cl], featuring NEt linkers, display the three νCO bands at 2007, 1921, and 1899 cm−1 and 2002, 1915, and 1878 cm−1, respectively, again indicating that the xylene-based pincer ligand is slightly more electron donating than the one based on the 1,3-diaminobenzene moiety [7].

The mechanism of the dynamic processes observed for the bromo carbonyl complexes 3 and 4 was investigated by means of DFT calculations (Fig. 1). The free energy profiles of these interconversions are depicted in Fig. 2. All CO ligands are involved in interchange (pseudorotation) processes that occur in single-step paths. The two CO ligands located closer to the PCP plane (CO1 and CO2) exchange positions going through barriers of 53.3 and 57.4 kJ/mol for Mo and W, respectively. This reaction is the faster processes and corresponds to the one that could not be stopped in the NMR experiments even at −50 °C (400 MHz). Exchanging the apical CO (CO3, trans to the bromide ligand) with one of the other two CO ligands is a considerably more difficult rearrangement. The calculated barriers, 96.4 (M = Mo) and 98.9 kJ/mol (M = W) are indicative of a process that should be slow under the experimental conditions. Therefore, the DFT calculations indicate that the apical CO (CO3) corresponds to the single signal in the 13C{1H} NMR spectrum, while the two CO ligands opposite to the PCP ligand and closer to its plane (CO1 and CO2) are engaged in a fast exchange process and give rise to the broad 13C{1H} NMR signals associated with two of those ligands.

Fig. 1
figure 1

Calculated structures of trans- and cis-[Cr(PCPCH2-iPr)(CH3CN)(Br)2]

Full size image
Fig. 2
figure 2

Free-energy profile (kJ/mol) for the “pseudorotations” of the CO ligands in 3 and 4

Full size image

The solid-state structures of [Mo(PCPCH2-iPr)(CO)3Br] (3) and [W(PCPCH2-iPr)(CO)3Br] (4) were determined by X-ray diffraction. Molecular views of 3 and 4 are depicted in Figs. 3 and 4 and selected bond lengths and angles are provided in the captions. Complexes 3 and 4 are seven-coordinate species where the tridentate PCP ligand is bound in the typical meridional coordination mode and three carbonyls ligands and one bromide ligand are filling the remaining four sites.

Fig. 3
figure 3

Structural view of [Mo(PCPCH2-iPr)(CO)3Br] (3) with 50% thermal ellipsoids (H atoms omitted for clarity). Selected bond lengths (Å) and bond angles (°): Mo1-C1 2.286(4), Mo1-C21 1.987(6), Mo1-C22 2.023(6), Mo1-C23 1.952(8), Mo1-Br1 2.697(1), Mo1-P1 2.524(1), Mo1-P2 2.523(1), P1-Mo1-P2 147.84(4)

Full size image
Fig. 4
figure 4

Structural views of [W(PCPCH2-iPr)(CO)3Br] (4) with 50% thermal ellipsoids (H atoms omitted for clarity). Selected bond lengths (Å) and bond angles (°): W1-C1 2.271(3), W1-C21 2.016(4), W1-C22 1.991(3), W1-C23 1.964(5), W1-Br1 2.6871(7), W1-P2 2.5219(7), W1-P1 2.5236(7), P1-W1-P2 148.42(2)

Full size image

Conclusion

In sum, we have prepared xylene-based group-six PCP metal complexes via a solvothermal approach starting from [M(CO)6] (M=Cr, Mo, W) and (2-bromo-1,3-phenylene)bis(methylene)bis(diisopropylphosphane) (P(C–Br)PCH2-iPr). Treatment of [Cr(CO)6] with P(C-Br)PCH2-iPr in CH3CN under solvothermal conditions resulted in the formation of the Cr(III) complex trans-[Cr(PCPCH2-iPr)(CH3CN)(Br)2] together with several unidentified compounds. In contrast, with [M(CO)6] (M=Mo, W) seven-coordinate cationic bromo carbonyl Mo(II) and W(II) complexes of the type [M(PCPCH2-iPr)(CO)3Br] were formed in high isolated yields. These seven-coordinate complexes are fluxional in solution due to CO interconversions. The mechanism of this dynamic behavior was studied by means of DFT calculations. All CO ligands are involved in pseudorotation processes that occur in single-step paths. The structures of the Mo and W complexes could be determined by single-crystal X-ray analysis.

Experimental

All manipulations were performed under an inert atmosphere of argon by using Schlenk techniques or in an MBraun inert-gas glovebox. The solvents were purified according to standard procedures [11]. Deuterated solvents were dried over 4 Å molecular sieves. The synthesis of (2-bromo-1,3-phenylene)bis(methylene)bis(diisopropylphosphane) (P(C–Br)PCH2-iPr) (1) was carried according to the literature [8]. 1H, 13C{1H}, and 31P{1H} NMR spectra were recorded on Bruker AVANCE-250 and 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). Room temperature solution (CH2Cl2) magnetic moments were determined by 1H NMR spectroscopy using the method of Evans [12].

High-resolution accurate mass data mass spectra were recorded on a hybrid Maxis Qq-aoTOF mass spectrometer (Bruker Daltonics, Bremen, Germany) fitted with an ESI source. Measured accurate mass data of the [M]+ ions for confirming calculated elemental compositions were typically within ± 5 ppm accuracy. The mass calibration was done with a commercial mixture of perfluorinated trialkyltriazines (ES Tuning Mix, Agilent Technologies, Santa Clara, CA, USA).

[(Dibromo)[2,6-bis[[bis(1-methylethyl)phosphino-κP]methyl]phenyl-κC](acetonitrile)chromium(III)], [Cr(PCPCH2-iPr)(CH3CN)(Br)2] (2, C22H38Br2CrNP2)

A suspension of 53 mg [Cr(CO)6] (0.24 mmol) and 100 mg 1 (0.24 mmol) in 2.5 cm3 acetonitrile in a sealed microwave tube was heated at 130 °C for 12 h affording an orange solution. After evaporation of the solvent the remaining solid was washed three times with n-pentane (10 cm3). The residue was then taken up in 3 cm3 dichloromethane and precipitated with 10 cm3n-pentane giving 2 as reddish-brown solid. Yield: 45 mg (40%); µeff = 3.7 µB (Evans method, CH2Cl2); IR (ATR): = 2194 (νCN) cm−1.

[(Bromo)[2,6-bis[[bis(1-methylethyl)phosphino-κP]methyl]phenyl-κC](tricarbonyl)molybdenum(II)], [Mo(PCPCH2-iPr)(CO)3Br] (3, C23H35BrMoO3P2)

A suspension of 100 mg 1 (0.24 mmol) and 63.2 mg [Mo(CO)6] (0.24 mmol) in 2.5 cm3 acetonitrile in a sealed microwave vial was heated at 130 °C for 3 h affording a red-brown solution. After cooling to room temperature orange-brown crystals precipitated from the mother liquor. The supernatant solution was decanted and the crystals were washed twice with MeOH (1 cm3) to yield analytically pure 3. Yield: 115 mg (80%) as orange crystals. 1H NMR (400 MHz, CD2Cl2, 20 °C): δ = 7.13 (d, J = 7.4 Hz, 2H, CarH), 6.99 (t, J = 7.4 Hz, 1H, CarH), 4.04 (dt, J = 15.5 Hz, 5.0 Hz, 2H, CH2P), 3.72 (dt, J = 15.5 Hz, 3.9 Hz, 2H, CH2P), 2.91 (m, 2H, PCH(CH3)2), 2.47 (m, 2H, PCH(CH3)2), 1.32 (m, 18H, PCH(CH3)2), 1.10 (m, 6H, PCH(CH3)2) ppm; 13C{1H} NMR (101 MHz, CD2Cl2, 20 °C): δ = 238.1 (br, CO), 224.5 (t, J = 12.0 Hz, CO), 171.3 (t, J = 8.0 Hz, Mo-C), 149.1 (t, J = 7.8 Hz, CarC), 127.4 (CarH), 123.1 (t, J = 8.0 Hz, CarH), 40.6 (vt, CH2P), 27.6 (t, J = 9.5 Hz, PCH(CH3)2), 27.2 (t, J = 10.4 Hz, -PCH(CH3)2), 19.7 (m, PCH(CH3)2) ppm; 31P{1H} NMR (162 MHz, CD2Cl2, 20 °C): δ = 75.1 ppm; IR (ATR): = 2008 (νCO), 1923 (νCO), 1871 (νCO) cm−1; HRMS (ESI+, CH3CN/MeOH + 1% H2O): m/z calcd for C23H35O3BrNaMoP2 ([M + Na]+) 621.0197, found 621.0181.

[(Bromo)[2,6-bis[[bis(1-methylethyl)phosphino-κP]methyl]phenyl-κC](tricarbonyl)tungsten(II)], [W(PCPCH2-iPr)(CO)3Br] (4, C23H35BrO3P2W)

This complex was prepared analogously to 3 with 84.4 mg [W(CO)6] (0.24 mmol) and 100 mg 1 (0.24 mmol) as starting materials at 160 °C. Yield: 136 mg (83%) as yellowish-brown crystals. 1H NMR (400 MHz, CD2Cl2, 20 °C): δ = 7.16 (d, J = 7.5 Hz, 2H, CarH), 7.02 (t, J = 7.5 Hz, 1H, CarH), 4.10 (dt, J = 15.6 Hz, 4.8 Hz, 2H, CH2P), 3.74 (dt, J = 15.5 Hz, 3.9 Hz, 2H, CH2P), 2.95 (m, 2H, PCH(CH3)2), 2.51 (m, 2H, PCH(CH3)2), 1.31 (m, 18H, PCH(CH3)2), 1.11 (m, 6H, PCH(CH3)2) ppm; 13C{1H} NMR (101 MHz, CD2Cl2, 20 °C): δ = 232.2 (br, CO), 217.5 (t, J = 9.15 Hz, JWC = 73 Hz, CO), 168.9 (t, J = 6.80 Hz, W–C), 150.7 (t, J = 7.74 Hz, CarC), 127.6 (CarH), 123.1 (t, J = 7.42 Hz, CarH), 40.8 (vt, CH2P), 27.8 (vt, PCH(CH3)2), 26.8 (vt, PCH(CH3)2),19.8 (m, PCH(CH3)2) ppm; 31P{1H} NMR (162 MHz, CD2Cl2, 20 °C): δ = 53.6 (1JWP = 84 Hz) ppm; IR (ATR): = 2000 (νCO), 1913 (νCO), 1859 (νCO) cm−1; HRMS (ESI+, CH3CN/MeOH + 1% H2O): m/z calcd for C23H35O3BrNaP2W ([M + Na]+) 707.0652, found 707.0620.

X-ray structure determination

X-ray diffraction data of 3 and 4 (CCDC 1897494 and 1897495) were collected at T = 100 K in a dry stream of nitrogen on a Bruker Kappa APEX II diffractometer system using graphite-monochromatized Mo-Kα radiation (λ = 0.71073 Å) and fine-sliced φ- and ω-scans. Data were reduced to intensity values with SAINT and an absorption correction was applied with the multi-scan approach implemented in SADABS [13]. The structure was solved by the dual-space approach implemented in SHELXT [14] and refined against F2 with SHELXL [15]. Non-hydrogen atoms were refined anisotropically. The H atoms were placed in calculated positions and thereafter refined as riding on the parent atoms. The Br and CO ligands were refined as disordered about all four positions. The total occupancy of the Br atom was restrained to one. Molecular graphics were generated with the program MERCURY [16].

Computational details

Calculations were performed using the Gaussian 09 software package [17] with the OPBE and PBE0 functionals without symmetry constraints, the Stuttgart/Dresden ECP (SDD) basis set to describe the electrons of the chromium, molybdenum and tungsten atoms and a standard 6-31G** basis for all other atoms as already described previously [7, 18].