The reaction of N,N-bis(diphenylphosphanyl)naphthylamine C10H7-1-N(PPh2)2 with (C5H10NH)2Cr(CO)4 (1 : 1 molar ratio) in dichloromethane afforded cis-[Cr(CO)4C10H7-1-N(PPh2)2] (1). This complex was crystallized in the monoclinic space group P21/n. The structure was solved by direct methods and refined by full-matrix least squares techniques to an factor of 0.0313 for 6488 observed reflections. The Cr-metal is coordinated by four terminal CO molecules and a P,P′-bidentate N,N-bis(diphenylphosphanyl)naphthylamine ligand in a distorted octahedral array. The N-atom adopts a planar geometry with the two P-atoms and C-atom attached to it. The four-membered metallacycle ring P2CrN is nearly planar.

1. Introduction

As a continuation of our work on the synthesis and solid-state structures of phosphorus(III) ligands containing direct P–N bonds and derivatives [14], as these have shown a broad spectrum of anticancer, herbicidal, neuroactive, and antimicrobial activities [59], thereof, herein we report the synthesis and crystal structure of cis-[Cr(CO)4{C10H7-1-N(PPh2)2}] (1).

2. Materials and Methods

2.1. Chemistry

All experiments were carried out under purified dry nitrogen using standard Schlenk and vacuum line techniques. Solvents were dried and freshly distilled under nitrogen [10]. The chemicals Cr(CO)6 were used as purchased. C10H7-1-N(PPh2)2 [1] and (C5H10NH)2Cr(CO)4 [11] were prepared according to the literature methods. Infrared spectra were recorded on a Shimadzu FTIR-8400S spectrometer between 4000 and 400 cm−1 using KBr disks. The NMR spectra were recorded at 25°C on a Bruker-Avance-DRX-400 MHz NMR spectrometer operating at 400.17 (1H) and 100.63 (13C) using tetramethylsilane as external standard. Melting point was carried out on a Gallenkamp apparatus with open capillaries.

2.2. Preparation of cis-Tetracarbonyl[N-(diphenylphosphino-kP)-naphthalen-1-yl-P,P-diphenylphosphinous amide-kP]chromium(0) (1)

A solution of N,N-bis(diphenylphosphanyl)naphthylamine (0.26 g, 0.51 mmol) and (C5H10NH)2Cr(CO)4 (0.17 g, 0.51 mmol) in 20 mL of CH2Cl2 was refluxed for 2 h. The orange solution was concentrated to ca. 5 mL under reduced pressure, and n-hexane (5 mL) was added. Cooling this solution to 0°C gave 3 as yellow crystals in 85% yield. Mp 180–183°C. 1H NMR (CDCl3, /ppm): 6.40–7.78 (m, 27 H, C10H7 and 4C6H5). 13C NMR (CDCl3, /ppm): 124.8, 125.2, 125.6, 125.9, 127.1, 127.9, 128.6, 129.6, 130.5, 131.3, 132.6, 134.1, 135.4, 138.8 (C10H6 and 4C6H5), 221.2 (), 228.2 (). 31P NMR (CDCl3, /ppm): 117.41 (s, 2P). IR (selected bands, KBr, cm−1): (s, br), 1920 (s), 2008 (s) ().

2.3. Data Collection and Structure Determination

Crystallographic data are given in Table 1. Single-crystal X-ray diffraction data were collected using an Oxford Diffraction Supernova dual-source diffractometer equipped with a 135 mm Atlas CCD area detector. Crystals were selected under Paratone-N oil, mounted on micromount loops, and quench-cooled using an Oxford Cryosystems open flow N2 cooling device [12]. Data were collected at 150 K using mirror monochromated radiation ( = 1.5418 Å) and processed using the CrysAlisPro package, including unit cell parameter refinement and interframe scaling (which was carried out using SCALE3 ABSPACK within CrysAlisPro) [13]. Equivalent reflections were merged and diffraction patterns were processed with the CrysAlisPro. The structure was subsequently solved using direct methods and refined on F2 using the SHELXL 97-2 package [1416]. All nonhydrogen atoms were refined with anisotropic displacement parameters. All H-atoms bonded to carbon atoms were placed in geometrically optimized positions and refined with an isotropic displacement parameter relative to the attached atoms. Crystallographic data (excluding structure factors) for the structure in this paper has been deposited with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge, UK. Copies of the data can be obtained free of charge on quoting the depository number CCDC-973175 for 1 (Fax: +44-1223-336-033; E-Mail: [email protected], http://www.ccdc.cam.ac.uk/).

3. Results and Discussion

3.1. Synthesis

Complex 1 was previously prepared by the reaction of Cr(CO)6 with C10H7-1-N(PPh2)2 in refluxing toluene for 36 hours [1]. Here, an alternative synthetic methodology was devised using (C5H10NH)2Cr(CO)4 instead of Cr(CO)6 in refluxing CH2Cl2 to avoid prolonged refluxing time and further purification of the desired product. This methodology gave 1 in high yield and without the need for further purification. The NMR data are in agreement with those reported previously in the literature [1].

3.2. Molecular Structure

Yellow colored crystals of 1 were obtained as described in the Experimental Section. 1 crystallizes in the monoclinic space group P21/n. Selected interatomic distances and angles are collected in Table 2. The molecular structure is depicted in Figure 1.

The crystal structure of 1 shows a distorted octahedral environment around the Cr-metal surrounded by four terminal CO ligands and two phosphorus centers (Figure 1).

The ability of the N,N-bis(diphenylphosphanyl)naphthylamine ligand to act as bidentate P,P′-chelating ligand to the Cr-metal results in the formation of a four-membered metallacycle, that is, P–Cr–P–N, that is approximately planar with a torsion angle P–Cr–P–N of −9.66(5)°, with a smaller P–Cr–P bite angle [68.74(3)°] and larger P–N–P bond angle [100.70(7)°] (Table 2). The nitrogen atom is displaced out of the plane (Cr1, P1, P2) by 0.2873(14) .

To the best of our knowledge, there are other three structurally characterized monocyclic four-membered ring complexes of bidentate P,P′-chelating bis(phosphino)amine ligand, namely, cis-[Cr(CO)4{((o-MeOC6H4)2P)2NCH3}] (2) [17], cis-[Cr(CO)4{(Ph2P)2Pr}] (3) [18], and cis-[Cr(CO)4{Ph2P)2NH}] (4) [19].

A comparison of the structural data of the P–Cr–P and P–N–P bond angles in 1 (Table 2) with those of the four-membered rings in similar cis-chelated tetracarbonylchromium(0) 24 shows that the P–Cr–P bite angle in 1 is larger than those in 2 [67.54(2)°], 3 [67.82(4)°], and 4 [68.58(2)°]. The P–N–P bond angle in 1 is smaller than those in 2 [101.24(7)°] and 4 [103.24(9)°] and larger than those in 3 [99.86(11)°].

The P–N–P [100.70(7)°] bond angle is significantly smaller than those in the free diphosphinoamine ligands [19, 20] due to the formation of a strained four-membered chelate ring.

The C-napthyl skeleton in 1 is almost planar and virtually perpendicular to the P–Cr–P–N plan. A planar environment would be expected for the three-coordinate nitrogen atoms in 1 and the sum of bond angles is indeed close to 360° (Table 2). The P–Cr–C trans angles of 1 [166.57(7) and 168.99(6)°] differ significantly from 180°.

The average P–N bond distance in 1 [av. 1.721 Å] is slightly longer than those in 2 [av. 1.699 Å], 3 [av. 1.713 Å], and 4 [av. 1.692 Å] and significantly shorter than the sum of Pauling covalent radii (1.77 Å) as expected due to P–N -bonding. Consistent with this, the nitrogen atom is planar as evidenced by the sum of angles about nitrogen being 359.91(9)° for 1. Also, the P–N bond distances in 1 are slightly shorter than those in the free diphosphinoamine ligands [19, 20] which clearly indicate an enhancement of -bonding in the P–N unit.

The two Cr–P bond distances [2.347(1) and 2.347(1) Å] in 1 are equal. The average Cr–P bond distance in 1 [av. 2.347 Å] is slightly shorter than those in cis-chelated tetracarbonylchromium(0) 2 [av. 2.364 Å], 3 [av. 2.350 Å], and 4 [av. 2.354 Å]. The atoms P1, P2, Cr1, C12, and C13 are essentially coplanar with a maximum deviation from the mean plane of 0.0246(7)  for C13.

The Cr–C bond distances are 1.859(2)–1.883(2) Å for 1. The two Cr–C bonds mutually trans are longer (weaker) than those trans to Cr–P bonds (Table 2). This result reflects the difference in the strength of the metal-to-ligand -bonding [21]. The aromatic rings in 1 as expected have usual bond lengths and angles.

4. Conclusion

In conclusion, we have shown the synthesis and molecular structure of cis-chelated tetracarbonylchromium(0) complex 1. The Cr-atom has a distorted octahedral arrangement with four CO ligands and two P-centers. The two Cr–C bonds mutually trans are longer (weaker) than those trans to Cr–P bonds due to the various strengths of the metal-to-ligand -bonding.

Conflict of Interests

The author declares that there is no conflict of interests regarding the publication of this paper.