{2,6-Bis[(2,6-diisopropyl­phosphan­yl)­oxy]-4-fluoro­phenyl-κ3 P,C 1,P′}(1H-pyrazole-κN 2)nickel(II) hexa­fluoro­phosphate

Man-Lung Kwan; Sara J Conry; Charles S Carfagna; Loren P Press; Oleg V Ozerov; Norris W Hoffman; Richard E Sykora

doi:10.1107/S1600536812039207

. 2012 Sep 19;68(Pt 10):m1282–m1283. doi: 10.1107/S1600536812039207

{2,6-Bis[(2,6-diisopropylphosphanyl)oxy]-4-fluorophenyl-κ³ P,C ¹,P′}(1H-pyrazole-κN ²)nickel(II) hexafluorophosphate

Man-Lung Kwan ^a, Sara J Conry ^a, Charles S Carfagna ^a, Loren P Press ^b, Oleg V Ozerov ^b, Norris W Hoffman ^c, Richard E Sykora ^c,^*

PMCID: PMC3470159 PMID: 23125603

Abstract

The title compound, [Ni(C₁₈H₃₀FO₂P₂)(C₃H₄N₂)]PF₆, was prepared by halide abstraction with TlPF₆ in the presence of CH₃CN in CDCl₃ from the respective neutral pincer chlorido analogue followed by addition of pyrazole. The PO—C—OP pincer ligand acts in typical trans-P₂ tridentate fashion to generate a distorted square-planar nickel structure. The Ni—N(pyrazole) distance is 1.925 (2) Å and the plane of the pyrazole ligand is rotated 56.2 (1)° relative to the approximate square plane surrounding the Ni^II center in which the pyrazole is bound to the Ni^II atom through its sp ²-hybridized N atom. This Ni—N distance is similar to bond lengths in the other reported Ni^II pincer-ligand square-planar pyrazole complex structures; however, its dihedral angle is significantly larger than any of those for the latter set of pyrazole complexes.

Related literature

For recent studies on the chemistry of d-block PO—C—OP pincer complexes, see Chen et al. (2012 ▶); Zhang et al. (2012 ▶); Salah & Zargarian (2011 ▶); Hoffman et al. (2009 ▶); Wicker et al. (2011 ▶). For structures of other Ni^II pincer-ligand square-planar pyrazole complexes, see Salem et al. (2007 ▶, 2008 ▶); Peng et al. (2010 ▶). For information regarding the ¹⁹F NMR reference, see: Ji et al. (2005 ▶).

Experimental

Crystal data

[Ni(C₁₈H₃₀FO₂P₂)(C₃H₄N₂)]PF₆
M _r = 631.12
Monoclinic,
a = 9.0380 (9) Å
b = 20.1878 (16) Å
c = 16.1480 (16) Å
β = 98.659 (8)°
V = 2912.7 (5) Å³
Z = 4
Mo Kα radiation
μ = 0.90 mm⁻¹
T = 290 K
0.58 × 0.52 × 0.34 mm

Data collection

Enraf–Nonius CAD-4 diffractometer
Absorption correction: ψ scan (North et al., 1968 ▶) T _min = 0.265, T _max = 0.315
5464 measured reflections
5122 independent reflections
3432 reflections with I > 2σ(I)
R _int = 0.024
3 standard reflections every 120 min intensity decay: none

Refinement

R[F ² > 2σ(F ²)] = 0.037
wR(F ²) = 0.100
S = 1.00
5122 reflections
325 parameters
H-atom parameters constrained
Δρ_max = 0.22 e Å⁻³
Δρ_min = −0.29 e Å⁻³

Data collection: CAD-4-PC (Enraf–Nonius, 1993 ▶); cell refinement: CAD-4-PC; data reduction: XCAD4-PC (Harms & Wocadlo, 1995) ▶; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: OLEX2 (Dolomanov et al., 2009 ▶); software used to prepare material for publication: publCIF (Westrip, 2010 ▶).

Supplementary Material

Crystal structure: contains datablock(s) I, global. DOI: 10.1107/S1600536812039207/hg5249sup1.cif

e-68-m1282-sup1.cif^{(23KB, cif)}

Structure factors: contains datablock(s) I. DOI: 10.1107/S1600536812039207/hg5249Isup2.hkl

e-68-m1282-Isup2.hkl^{(250.9KB, hkl)}

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Acknowledgments

The authors gratefully acknowledge the Department of Chemistry and the Univeristy Committee for Undergraduate Research at the University of South Alabama for their generous support and the Department of Energy and Oak Ridge National Laboratory for the X-ray diffractometer used in these studies. They also appreciate support from the National Science Foundation: grant #CHE-99–09502, REU Supplement with Alan Marshall of Florida State University/National High Magnetic Field Laboratory, Tallahassee, FL, USA and grant CHE-0846680, NSF CAREER grant to RES.

supplementary crystallographic information

Comment

Considerable attention has recently been devoted to nickel PO—C—OP pincer complexes (e.g., Salah & Zargarian, 2011; Chen et al., 2012; Zhang et al., 2012). Our interest in studying relative binding affinities of metal centers for ligands of moderate donor power using ¹⁹F and ³¹P NMR spectroscopy (Hoffman et al., 2009) to monitor ligand-substitution equilibria led us to prepare the title complex (I). The fluoro-pincer ligand precursor was generated by heating 5-fluororesorcinol and diisopropylchlorophosphine in THF in the presence of triethylamine, and then anhydrous NiCl₂ was added to form the (PO—C—OP)NiCl complex. Chloride abstraction with TlPF₆ in the presence of CH₃CN from this species followed by addition of pyrazole afforded an excellent yield of the cationic complex whose structure is shown below in Fig 1. Suitable single crystals were grown via vapor diffusion of methyl tert-butyl ether into a CDCl₃ solution of the highly soluble reaction product at room temperature. Its Ni—N distance, 1.925 (2) Å, fell within the range of such values for the five square-planar Ni^II-pincer unsubstituted-pyrazole complexes found in the Cambridge Structural Database (Salem et al. (2008), Peng et al. (2010), and Salem et al. (2007)). However, its pyrazole-ring/Ni-coordination-plane dihedral angle, 56.2 (1)°, falls significantly outside the range (3–28°) of those for the latter set of pyrazole complexes, in which none of the pendant-ligand arms exert meaningful steric force upon the pyrazole position. The C—F bond length for this complex, 1.355 (4) Å, is identical within experimental error to that, 1.357 (3) Å, of Pd{(3–2,6-[(C₆H₅)₂PO]₂-C₆H₂-4-F}(C₄H₄NO₄S), a Pd(II) acesulfamato complex containing a similar fluoro-pincer ligand (Wicker et al., 2011). Detailed lists of dimensions are available in the archived CIF.

Experimental

The first step entailed generating a neutral nickel(II) halido pincer complex, abbreviated as Pr{Ni_F}Cl. An anhydrous solution of 5-fluororesorcinol (330 mg, 2.58 mmol) in THF was prepared in a 100-ml Kontes Teflon screw-cap flask inside a glovebox. To this solution was added via syringe 1.50 ml NEt₃ (10.77 mmol) followed by 850 µL of ClPPrⁱ₂ (5.34 mmol); immediately a large volume of white precipitate formed. The flask was brought out of the glovebox and heated for 20 minutes at 80 °C; thereafter the flask was returned to the glovebox, where 0.57 grams of anhydrous NiCl₂ (4.40 mmol) was added to afford an orange mixture. After 24 h under heavy stirring at 80 °C, the resulting green-yellow mixture was filtered through a plug of Celite and concentrated under vacuum. This solution was layered with pentane and left overnight in a -30 °C freezer to give dark yellow-brown crystals of Pr{Ni_F}Cl (608 mg, 52% yield).

Complex I, [Ni(C₃H₂N₂)(C₁₂H₁₆FO₂P₂)]PF₆, abbreviated as Pr{Ni_F}(Pz)]PF₆, was prepared as follows. Pr{Ni_F}Cl (32.4 mg, 0.0799 mmol) was stirred at ambient temperature for 24 h with TlPF₆ (Strem Chemicals; 1.15 mol equiv.) in CDCl₃ (Cambridge Isotopes Lab; 1.25 ml) to which 25 µL CH₃CN (Fisher reagent) had been added. The resulting mixture was filtered to remove insoluble TlCl and excess TlPF₆. To the resulting yellow-orange solution of Pr{Ni_F}(CH₃CN)]PF₆ was added a slight excess of solid pyrazole (Aldrich; 5.7 mg), and the reaction solution was stirred one hour. Then it was subjected to vapor diffusion with 30 ml me thyl tert-butyl ether (MTBE; Fisher reagent) at 22 °C for 3 days. The very pale yellow liquid of the resulting mixture was removed from the vial by a small-diameter syringe needle, and the rod-like orange crystals were washed twice with 1.5 ml of MTBE, removed from the vial, and then air-dried overnight in the dark (91% yield). The complex was characterized by NMR at 24 °C in CDCl₃. Unusual features of the spectra are discussed below the data tabulation.

¹H: (relative to internal TMS) δ 11.66 (broad singlet, 0.74 H); δ 8.07 (overlapping doublet of doublets, 1H); δ 7.64 (overlapping doublet of doublets, 1H); δ 6.58 (2H, doublet, ³J_F—H=9.6 Hz) δ 2.34 (4H, septuplet, ³J_H—H=7.1 Hz); δ 1.35 (overlapping inequivalent triplets, 12H); δ 1.08 (overlapping inequivalent triplets, 12H)

¹⁹F: (relative to internal C₆H₅CF₃ at δ -63.00; (Ji et al., 2005)) aryl-F at δ -109.41 (triplet of triplets, ³J_H—F=9.6 Hz, ⁵J_P—F=1.0 Hz) and counterion PF₆^- at δ -72.28 (doublet, ²J_P—F=713 Hz)

³¹P: (relative to external 85% aq. phosphoric acid) pincer-P at δ 190.31 (doublet, ⁵J_P—F=1.0 Hz) and counterion PF₆^- at δ -143.79 (septuplet, ²J_P—F=713 Hz)

¹³C: (relative to internal TMS)

Pyrazole: δ 108.59(s), δ 135.21(s), δ 141.43(s).

Pincer Aryl: ipsoδ 115.49(t of d; ⁴J_F—C=2.9 Hz, ²J_P—C=21 Hz) orthoδ 168.55 (d of t; ³J_F—C=15 Hz, ²J_P—C~7.4 Hz) metaδ 94.89(d of t; ²J_F—C=33 Hz, ⁴J_P—C~6.6 Hz) paraδ 165.16 (d; ¹J_F—C=246 Hz Prⁱ₂P: methyne, δ 27.66(t; ¹J_P—C=11 Hz) methyls δ 16.58(t; ²J_P—C=11 Hz), δ 16.41(s)

Integrals recorded for ¹H signals of Pr{Ni_F}(Pz)]PF₆ match expected values except for the low-field pyrazole N—H resonance in which strong hydrogen-bonding to the hexafluorophosphate counterion is likely. Overlap of ¹H triplets for the inequivalent isopropyl methyl groups created by different rotation rates around the Ni—Pz, P—C, and C—C bonds affords pseudo-quartets centered at δ 1.35 and δ 1.08 (Fig. 2). The same rotational phenomena generate a more unusual ¹³C-NMR methyls pattern (Fig. 3), in which a triplet at δ 16.58 (²J_P—C=11 Hz) and singlet at δ 16.41 (no measurable P—C coupling observed) of equal intensity appear. Recording the ¹³C-NMR spectrum with longer relaxation delay (4 s versus 1 s) or at 44 °C affords no change in this pattern.