(Nitrito-κO)(nitro-κN)(nitrosyl-κN)bis­(tri­phenylphosphane-κP)rhodium(III)

Daniel R Albert; Michael Gau; Edward Rajaseelan

doi:10.1107/S241431462500598X

. 2025 Jul 8;10(Pt 7):x250598. doi: 10.1107/S241431462500598X

(Nitrito-κO)(nitro-κN)(nitrosyl-κN)bis(triphenylphosphane-κP)rhodium(III)

Daniel R Albert ^a, Michael Gau ^b, Edward Rajaseelan ^a,^*

Editor: S Bernès^c

PMCID: PMC12326464 PMID: 40777624

The title Rh^III complex is a rare example of a mononuclear compound gathering nitrito, nitro and nitrosyl ligands.

Keywords: crystal structure, rhodium, nitrito, nitrosyl, nitro

Abstract

The structure of the title compound, [Rh(NO)(NO₂)₂(C₁₈H₁₅P)₂] or [Rh(NO)(NO₂)(ONO)(PPh₃)₂], has been determined by single-crystal X-ray diffraction. A previous report of the title compound showed it crystallized in a monoclinic space group [Rajaseelan et al. (1999 ▸). J. PA. Acad. Sci.73, 63–66; refcode SASTOW in the CSD]. However, it was unable to be refined because of severe disorder. In this study, two distinct crystals of the title compound were found showing the compound to be polymorphic. One of the crystals was unable to be resolved but was found to be in a monoclinic space group, as in the previously reported study. The other crystal was able to be refined and crystallizes in the triclinic space group P1. The refined structure consists of two discrete monomeric molecules per unit cell. The molecular geometry around rhodium is that of a distorted square pyramid, with nitrogen of the nitro ligand, oxygen of the nitrito ligand and phosphorus atoms of the phosphane ligands lying in the basal plane and the nitrogen atom of the bent nitrosyl occupying the apical position. The nitrosyl ligand exhibits positional disorder whereas the nitro and nitrito ligands show disorder across coordination sites with the disorder modelled in a 0.91:0.09 ratio. Both intramolecular C—H ⋯O (nitro and nitrito) and intermolecular C—H ⋯N (nitrito) interactions are observed. There are no hydrogen-bonding interactions with the N or O atoms of the nitrosyl ligand.

Structure description

Nitrosyl is a versatile ligand and synthetic, structural, chemical reactivity, and spectroscopic studies of transition metal nitrosyl complexes are of interest due to the role of NO in biochemical systems and in coordination chemistry (Machura, 2005 ▸; Daniel & Gourlaouen, 2019 ▸). Nitrite coordinates to transition metals in different bonding modes. The different MNO₂ isomers are characterized by distinct infrared bands, ¹⁵N-NMR chemical shifts, and crystal structures of these linkages (Feltham, 1989 ▸). Synthesis and structures of many rhodium nitrosyl and nitrite complexes have been reported (English et al., 1987 ▸; Cheung et al., 2007 ▸; Gaviglio et al., 2009 ▸; Singh et al., 2011 ▸; Vorobyeva et al., 2022 ▸). The title compound has previously been reported and characterized with the bonding modes of the NO_x groups confirmed by the infrared spectra, ¹⁵N labelling studies, and multinuclear NMR spectra (Rajaseelan et al., 1999 ▸).

The title complex, [Rh(NO)(NO₂)(ONO)(PPh₃)₂], consists of two well-separated monomeric structural units per unit cell. There is no crystallographically imposed twofold axis on the molecule and the NO_x ligands are disordered and refined with a ratio of 0.91:0.09 as shown in Fig. 1 ▸. The coordination sphere around rhodium, formed by nitrosyl, nitro, nitrito, and two triphenylphosphane ligands, results in a distorted square-pyramidal environment as shown in Fig. 2 ▸. The square pyramidal shape is supported by τ₅ parameters (Addison et al., 1984 ▸) of both the main (τ₅ = 0.078) and disordered (τ₅ = 0.062) structures that are close to zero. The nitrogen atom of the nitrosyl ligand which coordinates in a bent fashion, occupies the apical position. The Rh1—N1 [Rh1—N1*] distance is 1.938 (3) [1.93 (3)] Å and the Rh1—N1—O1 [Rh1—N1*—O1*] bond angle is 126.1 (2) [111 (2)]°. The two phosphane ligands occupy the sterically favoured trans positions in the basal plane. The Rh1—P1 and Rh1—P2 bond lengths are 2.4001 (6) and 2.3978 (6) Å, respectively, and the P2—Rh1—P1 bond angle is 173.609 (19)°. The nitrito (–ONO) ligand is in the endo-conformation with a Rh1—O2 [Rh1—O5*] bond length of 2.1105 (19) [2.09 (2)] Å, and a O2—N2—O3 [O5*—N3*—O4] bond angle of 114.1 (3) [116 (2)]°. The nitrogen atom of the nitro (–NO₂) group occupies the other position on the basal plane and is trans to the oxygen of the nitrito (–ONO) ligand. The Rh1—N3 [Rh1—N2*] bond length is 2.019 (2) [2.03 (3)] Å and the O5—N3—O4 [O2*—N2*—O3*] bond angle is 120.3 (2) [118 (3)]°, as expected for an sp² hybridized nitrogen atom.

Molecular structure of the title compound shown with displacement ellipsoids at the 50% probability level.

A perspective view of the main component (91%) title compound displaying the distorted square-pyramidal coordination around Rh1 with the nitrosyl ligand in the apical position.

It has been proposed and supported by molecular orbital calculations that the nitrosyl group bends in the direction of the strongest π-acceptor ligand coordinating in the basal plane (Hoffmann et al., 1974 ▸; Ibers & Mingos, 1971 ▸; Pierpont & Eisenberg, 1972 ▸). In Rh(NO)Cl₂(PPh₃)₂ the nitrosyl ligand lies in the P—Rh—P plane (Goldberg et al., 1975 ▸). The NO₂⁻ ligand is a better π-acceptor than PPh₃ (Comas-Vilà & Salvador, 2024 ▸) and in the title complex the nitrosyl ligand lies approximately in the O(nitrito)—Rh—N(nitro) plane. This is clearly indicated in Fig. 3 ▸ (major component) and Fig. 4 ▸ (minor disordered component). The dihedral angle formed by the Rh1/N1/O1 and the O2/Rh1/N3 planes is 25.3 (6)°, and similarly, the dihedral angle between the Rh1/N1*/O1* and N2*/Rh/O5* planes is 7 (6)°.

A perspective view of the main component (91%) of the title compound showing the bent nitrosyl ligand oriented along the N—Rh—O plane.

A perspective view of the minor disordered component (9%) of the title compound showing the bent nitrosyl ligand oriented along the N—Rh—O plane.

The packing diagram for the title compound is shown in Fig. 5 ▸. Both intermolecular and intramolecular non-classical hydrogen-bonding interactions are observed. All of the C—H⋯O hydrogen bonding interactions with the phenyl moieties and nitro and nitrito ligands are intramolecular. Whereas all of the C—H⋯N hydrogen-bonding interactions between the phenyl moieties and the nitrito ligand are intermolecular. No hydrogen-bonding interactions are observed with the nitrosyl ligand. All hydrogen-bonding interactions are summarized in Table 1 ▸ and shown as dotted green lines in Fig. 5 ▸.

Crystal packing of the title compound viewed along the b-axis direction. Non-classical hydrogen-bonding interactions are shown as dotted green lines.

Table 1. Hydrogen-bond geometry (Å, °).

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C2—H2⋯O5	0.95	2.27	3.200 (3)	166
C2—H2⋯O5*	0.95	2.40	3.213 (19)	144
C18—H18⋯O4	0.95	2.34	3.130 (3)	140
C21—H21⋯N2ⁱ	0.95	2.44	3.328 (4)	155
C32—H32⋯O2	0.95	2.40	3.118 (3)	132
C32—H32⋯O2*	0.95	2.36	3.24 (2)	154
C33—H33⋯N3*ⁱⁱ	0.95	2.38	3.21 (2)	145

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Symmetry codes: (i) Inline graphic ; (ii) .

Synthesis and crystallization

The tile compound was synthesized by the previously reported procedure (Rajaseelan et al., 1999 ▸). It was crystallized by slow diffusion of pentane into a CH₂Cl₂ solution. The complex was polymorphic with two distinct crystals recovered. Both crystals were dark green with one of them forming irregular shaped blocks and the other forming needle-like crystals. Both types of crystals had identical infrared spectra showing the presence of the nitrosyl, nitro, and nitrito ligands. The irregular shaped block crystal crystallizes in the monoclinic crystal system with large amount of disorder in the NO_x groups, and its crystal structure did not refine in a satisfactory manner. Hence, the structural set-up of the irregular shaped block crystals remains an open question. The disorder in the NO_x groups for the needle-like crystals, which solved in the triclinic P Inline graphic space group, was very minimal and refined well. The structure of the needle-like crystals is reported in this article.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2 ▸. The nitrosyl ligand exhibits positional disorder whereas the nitro and nitrito ligands show disorder across coordination sites with the disorder modelled in a 0.91:0.09 ratio. All nitro and nitrito N—O bond lengths involving disordered parts were restrained to 1.25 (2) Å. In addition, for the minor part of the nitro ligand, the O—N—O bond angle was restrained to ∼120°, with an O⋯O bond distance restrained to 2.15 (4) Å. For the disordered nitrosyl ligand, the N—O bond length in the minor part was restrained to 1.15 (2) Å. The U_ij components of all disordered atoms closer to each other than 2.0 Å were restrained to be similar, within a standard deviation of 0.002 A² (Sheldrick, 2015b ▸).

Table 2. Experimental details.

Crystal data
Chemical formula	[Rh(NO)(NO₂)₂(C₁₈H₁₅P)₂]
M _r	749.48
Crystal system, space group	Triclinic, P
Temperature (K)	100
a, b, c (Å)	10.2732 (2), 10.3317 (2), 18.1514 (4)
α, β, γ (°)	90.118 (2), 105.089 (2), 118.323 (2)
V (Å³)	1619.50 (6)
Z	2
Radiation type	Cu Kα
μ (mm⁻¹)	5.60
Crystal size (mm)	0.17 × 0.1 × 0.03

Data collection
Diffractometer	Rigaku XtaLAB Synergy-S
Absorption correction	Multi-scan (SCALE3 ABSPACK; Oxford Diffraction, 2005 ▸)
T_min, T_max	0.850, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	38997, 6535, 6126
R _int	0.047
(sin θ/λ)_max (Å⁻¹)	0.625

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.030, 0.082, 1.08
No. of reflections	6535
No. of parameters	487
No. of restraints	160
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.50, −1.04

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Computer programs: CrysAlis PRO (Rigaku OD, 2025 ▸), SHELXT (Sheldrick, 2015a ▸), SHELXL2018/3 (Sheldrick, 2015b ▸), OLEX2 (Dolomanov et al., 2009 ▸) and publCIF (Westrip, 2010 ▸).

Supplementary Material

Crystal structure: contains datablock(s) I. DOI: 10.1107/S241431462500598X/bh4097sup1.cif

x-10-x250598-sup1.cif^{(1.1MB, cif)}

Structure factors: contains datablock(s) I. DOI: 10.1107/S241431462500598X/bh4097Isup2.hkl

x-10-x250598-Isup2.hkl^{(519.3KB, hkl)}

CCDC reference: 2469479

Additional supporting information: crystallographic information; 3D view; checkCIF report

full crystallographic data