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Acta Cryst. (2009). E65, m103-m104    [ doi:10.1107/S1600536808042712 ]

The first oxazoline adduct of Zn(acac)2: bis(acetylacetonato-[kappa]2O,O')(2-phenyl-2-oxazoline-[kappa]N)zinc(II)

I. del Río and R. A. Gossage

Abstract top

The title material, [Zn(C5H7O2)2(C9H9NO)], was synthesized by the treatment of bis(acetylacetonato)zinc(II) monohydrate with 2-phenyl-2-oxazoline. The Zn atom is coordinated by two chelating acetylacetonate groups and one oxazoline ligand in the apical position of a slightly distorted square-pyramidal metal-ligand geometry.

Comment top

In recent years, zinc acetylacetonate (ZAA) complexes have found significant applications in CVD technology (Itoh et al., 1989; Kaeriyama, 1974; Williams, 1989), as Lewis acids in coordination chemistry (Brahma et al., 2008; Fronczek et al., 1990; Hamid et al., 2005) and are used for the formation of inorganic polymers (Qian et al., 2006). In many cases, the ZAA species are paired with N-donor ligands to modify and tune their physical and spectroscopic properties. We recently disclosed the syntheses and structural characterization of a number of Zn coordination compounds containing monodentate 2-oxazoline ligands (Barclay et al., 2003; Decken et al., 2006; Gossage & Jenkins, 2008; Gossage et al., 2009). These materials represent a number of structural motifs around the Zn2+coordination sphere which includes the observation of pseudo-tetrahedral and distorted trigonal bipyramidal geometries. The nature of these coordination motifs are obviously influenced by both the nature of the oxazoline ligand(s) and the structure and donor properties of the various formal anions appending the Zn atom. In this report, we expand these investigations to include ZAA precursors and describe our first results in the coupling of oxazolines to a ZAA unit.

The crystal structure determination of the title compound reveals that the central Zn atom is coordinated by four O-atoms of two chelating (i.e.,κ2O,O') acetylacetonato (acac) fragments in addition to the attachment of the oxazoline ligand via the N-atom. Th Zn–N bond length is similar to that of the distorted tetrahedral (at Zn) complexes (Barclay et al., 2003) [ZnX2(Phox-κ1N)2] (Zn–N = 2.026 (2) and 2.050 (2) Å for X = Cl; Zn–N = 2.025 (3) and 2.053 (3) Å for X = Br; Phox = 2-phenyl-2-oxazoline) and the related (Decken et al., 2006) five-coordinate species [Zn(S2CNEt2-κ2S,S')2(Phox-κ1N)] (Zn–N = 2.082 (4) Å). The Zn–O bond lengths of the formally anionic acac ligands of the title material are all inequivalent but fall within a narrow range (2.01–2.04 Å).

The Zn-phox complex reported by Decken et al. (2006) possesses a coordination motif around Zn that is best described (Addison et al., 1984) as highly distorted trigonal bipyramidal (τ = 0.65) whereas the title complex is strongly disposed towards a structure of idealized square pyramidal (τ = 0.04). The τ parameter is a numerical descriptor defined as unity for pure trigonal bipyramidal structures and zero for true square pyramidal ones (Addison et al., 1984).

This coordination geometry places the N-bound oxazoline in the formal apical position of such a square pyramid. ZAA complexes containing N-donor lignads are often found to be octahedral in nature with an "O4N2" donor atom set (Brahma et al., 2008; Fronczek et al., 1990; Hamid et al., 2005; Qian et al., 2006). The title material therefore represents the more rare "O4N"-type compound.

Our structural studies have so far observed both four- and five-coordinate ZnX2 (X = halide, S2CNRR', acac) oxazoline systems (Barclay et al., 2003; Decken et al., 2006; Gossage & Jenkins, 2008; Gossage et al., 2009). Intriguingly, an octahedral Zn-oxazoline complex has yet to be observed; this suggests that perhaps the use of weakly coordinating anions (e.g., NO3-, ClO4-,etc.) and/or sterically smaller oxazolines may assist us in discovering this structural class of Zn materials. Our future work will involve such investigations.

Related literature top

For general background, see: Addison et al. (1984); Itoh et al. (1989); Kaeriyama (1974); Williams (1989). For related structures, see: Barclay et al. (2003); Brahma et al. (2008); Decken et al. (2006); Fronczek et al. (1990); Gossage & Jenkins (2008); Gossage et al. (2009); Hamid et al. (2005); Qian et al. (2006).

Experimental top

The treatment of a benzene solution of commercially available bis(acetylacetonato-κ2O,O')zinc(II) (in the form of the monohydrate) with an excess of Phox leads to the formation of a clear and colourless solution. The removal of volatile components (vacuo) followed by re-crystallization (CH2Cl2/Et2O) of the resulting off white oily solid leads to the isolation of colourless crystals of the product (65%).

Refinement top

All the hydrogen atom positions were calculated and refined riding on their parent atoms with C—H = 0.96 Å (methyl), 0.97 Å (methylene) or 0.93 Å (aromatic) with Uiso(H) = 1.5Ueq (methyl) or Uiso(H) = 1.2Ueq (other). A racemic twin model has been used in the final refinement with twin ratio 0.65 (3):0.35.

Computing details top

Data collection: COLLECT (Nonius, 1998); cell refinement: SCALEPACK (Otwinowski & Minor, 1997); data reduction: SCALEPACK and DENZO (Otwinowski & Minor, 1997); program(s) used to solve structure: DIRDIF96 (Beurskens et al., 1996); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: EUCLID (Spek, 1982); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures top
[Figure 1] Fig. 1. View of the title compound. Ellipsoids are drawn at the 30% probability level.
bis(acetylacetonato-κ2O,O')(2-phenyl-2-oxazoline- κN)zinc(II) top
Crystal data top
[Zn(C5H7O2)2(C9H9NO)]F(000) = 856
Mr = 410.77Dx = 1.430 Mg m−3
Orthorhombic, P212121Cu Kα radiation, λ = 1.54180 Å
Hall symbol: P 2ac 2abCell parameters from 1999 reflections
a = 9.5009 (3) Åθ = 4.4–69.6°
b = 14.1674 (4) ŵ = 2.03 mm−1
c = 14.2407 (5) ÅT = 200 K
V = 1916.84 (11) Å3Blocks, white
Z = 40.28 × 0.15 × 0.08 mm
Data collection top
Nonius KappaCCD
diffractometer		3600 independent reflections
Radiation source: fine-focus sealed tube3466 reflections with I > 2σ(I)
horizontally mounted graphite crystalRint = 0.035
Detector resolution: 9 pixels mm-1θmax = 69.6°, θmin = 4.4°
φ and ω scansh = −11→11
Absorption correction: part of the refinement model (ΔF)
(Parkin et al., 1995)		k = −17→17
Tmin = 0.542, Tmax = 0.859l = −17→17
8596 measured reflections
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.030Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.080H-atom parameters constrained
S = 1.05 w = 1/[σ2(Fo2) + (0.0434P)2 + 0.5913P]
where P = (Fo2 + 2Fc2)/3
3600 reflections(Δ/σ)max < 0.001
236 parametersΔρmax = 0.24 e Å−3
0 restraintsΔρmin = −0.29 e Å−3
0 constraints
Crystal data top
[Zn(C5H7O2)2(C9H9NO)]V = 1916.84 (11) Å3
Mr = 410.77Z = 4
Orthorhombic, P212121Cu Kα radiation
a = 9.5009 (3) ŵ = 2.03 mm−1
b = 14.1674 (4) ÅT = 200 K
c = 14.2407 (5) Å0.28 × 0.15 × 0.08 mm
Data collection top
Nonius KappaCCD
diffractometer		3600 independent reflections
Absorption correction: part of the refinement model (ΔF)
(Parkin et al., 1995)		3466 reflections with I > 2σ(I)
Tmin = 0.542, Tmax = 0.859Rint = 0.035
8596 measured reflectionsθmax = 69.6°
Refinement top
R[F2 > 2σ(F2)] = 0.030H-atom parameters constrained
wR(F2) = 0.080Δρmax = 0.24 e Å−3
S = 1.05Δρmin = −0.29 e Å−3
3600 reflectionsAbsolute structure: ?
236 parametersFlack parameter: ?
0 restraintsRogers parameter: ?
Special details top

Experimental. Absorption correction: Parkin et al., 1995. Cubic fit to sin(θ)/λ; 24 parameters 1H NMR [300 MHz: CDCl3]: δH (vs. TMS) = 1.87 [s, 12H, –CH3], 4.03 [t, J = 11.7 Hz, 2H, –CH2N], 4.48 [t, 2H, –CH2O], 5.26 [s, 2H, –CH], 7.38 [m, 4H, ArH], 7.85 [d, J = 8.8 Hz,1H, ArH]; 13C{1H}NMR (75 MHz; CDCl3): δC (vs. TMS) = 28.1, 53.7, 68.2, 99.9, 125.9, 128.2, 129.0, 132.0, 165.0(w), 193.1).

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
C10.3061 (3)−0.0086 (3)−0.06798 (17)0.0483 (7)
H1A0.25140.0460−0.08700.058*
H1B0.2575−0.0654−0.08790.058*
C20.4526 (3)−0.0046 (2)−0.10858 (16)0.0433 (6)
H2A0.4650−0.0522−0.15690.052*
H2B0.47200.0570−0.13520.052*
C30.4621 (2)−0.01785 (18)0.04907 (15)0.0316 (5)
C40.5407 (3)−0.02065 (17)0.13820 (16)0.0319 (5)
C50.6862 (3)−0.01280 (19)0.13607 (17)0.0374 (5)
H50.7331−0.00860.07890.045*
C60.7613 (3)−0.0112 (2)0.21951 (18)0.0430 (6)
H60.8589−0.00610.21810.052*
C70.6925 (3)−0.0173 (2)0.30468 (18)0.0406 (6)
H70.7435−0.01490.36040.049*
C80.5487 (3)−0.0269 (2)0.30720 (18)0.0418 (6)
H80.5027−0.03170.36460.050*
C90.4715 (3)−0.02941 (19)0.22414 (18)0.0378 (6)
H90.3743−0.03690.22600.045*
C10−0.1142 (4)−0.2000 (3)−0.0308 (2)0.0620 (9)
H10A−0.1390−0.1528−0.07630.093*
H10B−0.1980−0.2232−0.00100.093*
H10C−0.0667−0.2512−0.06160.093*
C11−0.0186 (3)−0.1573 (2)0.0420 (2)0.0426 (6)
C120.0308 (3)−0.21237 (19)0.1148 (2)0.0485 (7)
H120.0107−0.27660.11190.058*
C130.1077 (3)−0.18128 (18)0.1921 (2)0.0409 (6)
C140.1554 (4)−0.2508 (2)0.2652 (2)0.0602 (8)
H14A0.2077−0.21830.31290.090*
H14B0.2141−0.29770.23640.090*
H14C0.0748−0.28070.29290.090*
C150.0715 (4)0.2718 (2)−0.0305 (2)0.0611 (9)
H15A0.01250.2405−0.07560.092*
H15B0.15450.2951−0.06120.092*
H15C0.02100.3235−0.00300.092*
C160.1131 (3)0.20281 (19)0.0454 (2)0.0430 (7)
C170.1982 (3)0.23396 (18)0.1187 (2)0.0466 (7)
H170.23490.29460.11390.056*
C180.2333 (3)0.18219 (18)0.19881 (19)0.0407 (6)
C190.3242 (4)0.2251 (2)0.2731 (2)0.0591 (9)
H19A0.33840.18020.32280.089*
H19B0.27910.28040.29780.089*
H19C0.41350.24210.24660.089*
N10.32955 (19)−0.00883 (16)0.03537 (13)0.0343 (4)
O10.54378 (17)−0.02335 (15)−0.02807 (12)0.0415 (4)
O20.0063 (2)−0.06991 (14)0.03232 (13)0.0445 (5)
O30.1428 (2)−0.09589 (12)0.20636 (12)0.0397 (4)
O40.0640 (2)0.12044 (14)0.03780 (13)0.0454 (5)
O50.1921 (2)0.09787 (12)0.21440 (13)0.0402 (4)
Zn10.14228 (3)0.00792 (2)0.10978 (2)0.03289 (11)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
C10.0386 (13)0.078 (2)0.0279 (12)−0.0026 (15)−0.0031 (10)0.0011 (15)
C20.0376 (12)0.0633 (17)0.0291 (11)0.0052 (13)−0.0037 (9)0.0008 (15)
C30.0341 (12)0.0299 (13)0.0307 (11)−0.0014 (10)0.0023 (9)−0.0004 (10)
C40.0354 (12)0.0272 (12)0.0331 (11)0.0032 (10)−0.0031 (9)−0.0007 (9)
C50.0336 (12)0.0412 (14)0.0374 (12)0.0001 (11)0.0012 (9)0.0030 (11)
C60.0332 (12)0.0524 (17)0.0435 (14)−0.0003 (13)−0.0040 (10)0.0027 (14)
C70.0400 (13)0.0421 (14)0.0398 (13)0.0048 (12)−0.0103 (10)−0.0032 (12)
C80.0427 (14)0.0507 (17)0.0321 (12)0.0070 (12)−0.0009 (10)0.0006 (11)
C90.0313 (12)0.0457 (16)0.0365 (13)0.0020 (11)0.0007 (10)−0.0022 (11)
C100.068 (2)0.0600 (19)0.0585 (19)−0.0184 (17)−0.0087 (16)−0.0061 (16)
C110.0338 (14)0.0421 (15)0.0519 (16)−0.0045 (12)0.0040 (12)−0.0090 (12)
C120.0532 (17)0.0314 (13)0.0610 (18)−0.0033 (12)−0.0049 (15)−0.0036 (14)
C130.0409 (15)0.0313 (13)0.0505 (15)0.0014 (11)0.0046 (12)0.0035 (11)
C140.067 (2)0.0442 (16)0.069 (2)0.0031 (16)−0.0091 (18)0.0123 (15)
C150.080 (2)0.049 (2)0.0546 (19)0.0087 (17)−0.0094 (18)0.0076 (15)
C160.0487 (17)0.0333 (14)0.0471 (16)0.0081 (12)0.0045 (13)0.0025 (11)
C170.0545 (16)0.0306 (13)0.0548 (18)−0.0056 (12)0.0001 (14)0.0031 (13)
C180.0430 (15)0.0346 (14)0.0446 (15)−0.0032 (12)0.0031 (12)−0.0030 (11)
C190.073 (2)0.0449 (17)0.0589 (19)−0.0137 (16)−0.0123 (16)−0.0033 (14)
N10.0333 (10)0.0416 (11)0.0281 (9)−0.0003 (10)−0.0020 (7)−0.0012 (9)
O10.0335 (8)0.0607 (13)0.0303 (8)0.0036 (9)0.0014 (7)0.0009 (8)
O20.0448 (11)0.0447 (12)0.0439 (10)−0.0115 (9)−0.0053 (8)0.0031 (9)
O30.0431 (10)0.0378 (9)0.0384 (9)−0.0065 (9)0.0006 (9)0.0031 (7)
O40.0506 (12)0.0378 (10)0.0478 (11)0.0042 (9)−0.0092 (9)0.0004 (8)
O50.0517 (12)0.0340 (9)0.0350 (10)−0.0027 (8)0.0014 (8)−0.0017 (7)
Zn10.03214 (17)0.03310 (17)0.03342 (17)−0.00265 (14)0.00149 (12)0.00024 (14)
Geometric parameters (Å, °) top
C1—N11.489 (3)C11—C121.379 (4)
C1—C21.508 (3)C12—C131.393 (4)
C1—H1A0.9700C12—H120.9300
C1—H1B0.9700C13—O31.271 (3)
C2—O11.461 (3)C13—C141.503 (4)
C2—H2A0.9700C14—H14A0.9600
C2—H2B0.9700C14—H14B0.9600
C3—N11.281 (3)C14—H14C0.9600
C3—O11.347 (3)C15—C161.510 (4)
C3—C41.473 (3)C15—H15A0.9600
C4—C51.387 (4)C15—H15B0.9600
C4—C91.395 (4)C15—H15C0.9600
C5—C61.387 (3)C16—O41.261 (4)
C5—H50.9300C16—C171.393 (4)
C6—C71.381 (4)C17—C181.397 (4)
C6—H60.9300C17—H170.9300
C7—C81.373 (4)C18—O51.277 (3)
C7—H70.9300C18—C191.495 (4)
C8—C91.392 (4)C19—H19A0.9600
C8—H80.9300C19—H19B0.9600
C9—H90.9300C19—H19C0.9600
C10—C111.506 (4)N1—Zn12.0844 (19)
C10—H10A0.9600O2—Zn12.0253 (19)
C10—H10B0.9600O3—Zn12.0136 (17)
C10—H10C0.9600O4—Zn12.0359 (19)
C11—O21.268 (4)O5—Zn12.0169 (18)
N1—C1—C2103.94 (19)C12—C13—C14119.9 (3)
N1—C1—H1A111.0C13—C14—H14A109.5
C2—C1—H1A111.0C13—C14—H14B109.5
N1—C1—H1B111.0H14A—C14—H14B109.5
C2—C1—H1B111.0C13—C14—H14C109.5
H1A—C1—H1B109.0H14A—C14—H14C109.5
O1—C2—C1103.86 (18)H14B—C14—H14C109.5
O1—C2—H2A111.0C16—C15—H15A109.5
C1—C2—H2A111.0C16—C15—H15B109.5
O1—C2—H2B111.0H15A—C15—H15B109.5
C1—C2—H2B111.0C16—C15—H15C109.5
H2A—C2—H2B109.0H15A—C15—H15C109.5
N1—C3—O1116.6 (2)H15B—C15—H15C109.5
N1—C3—C4129.2 (2)O4—C16—C17124.9 (3)
O1—C3—C4114.17 (19)O4—C16—C15116.2 (3)
C5—C4—C9119.7 (2)C17—C16—C15118.9 (3)
C5—C4—C3118.9 (2)C16—C17—C18125.8 (2)
C9—C4—C3121.3 (2)C16—C17—H17117.1
C6—C5—C4119.7 (2)C18—C17—H17117.1
C6—C5—H5120.1O5—C18—C17124.1 (3)
C4—C5—H5120.1O5—C18—C19115.8 (3)
C7—C6—C5120.5 (2)C17—C18—C19120.2 (3)
C7—C6—H6119.7C18—C19—H19A109.5
C5—C6—H6119.7C18—C19—H19B109.5
C8—C7—C6120.0 (2)H19A—C19—H19B109.5
C8—C7—H7120.0C18—C19—H19C109.5
C6—C7—H7120.0H19A—C19—H19C109.5
C7—C8—C9120.3 (2)H19B—C19—H19C109.5
C7—C8—H8119.9C3—N1—C1107.34 (19)
C9—C8—H8119.9C3—N1—Zn1140.65 (16)
C8—C9—C4119.7 (2)C1—N1—Zn1112.00 (14)
C8—C9—H9120.2C3—O1—C2106.74 (17)
C4—C9—H9120.2C11—O2—Zn1126.22 (19)
C11—C10—H10A109.5C13—O3—Zn1125.81 (17)
C11—C10—H10B109.5C16—O4—Zn1123.14 (18)
H10A—C10—H10B109.5C18—O5—Zn1122.33 (18)
C11—C10—H10C109.5O3—Zn1—O587.50 (7)
H10A—C10—H10C109.5O3—Zn1—O288.62 (8)
H10B—C10—H10C109.5O5—Zn1—O2153.64 (8)
O2—C11—C12124.8 (3)O3—Zn1—O4156.45 (8)
O2—C11—C10115.4 (3)O5—Zn1—O487.87 (8)
C12—C11—C10119.7 (3)O2—Zn1—O485.36 (8)
C11—C12—C13126.4 (3)O3—Zn1—N1105.19 (8)
C11—C12—H12116.8O5—Zn1—N1104.31 (8)
C13—C12—H12116.8O2—Zn1—N1101.87 (8)
O3—C13—C12124.4 (3)O4—Zn1—N198.33 (8)
O3—C13—C14115.7 (3)
N1—C1—C2—O111.7 (3)C10—C11—O2—Zn1−174.4 (2)
N1—C3—C4—C5−167.8 (3)C12—C13—O3—Zn1−16.7 (4)
O1—C3—C4—C510.4 (4)C14—C13—O3—Zn1163.0 (2)
N1—C3—C4—C911.2 (5)C17—C16—O4—Zn1−17.8 (4)
O1—C3—C4—C9−170.6 (2)C15—C16—O4—Zn1163.8 (2)
C9—C4—C5—C6−1.7 (4)C17—C18—O5—Zn126.4 (4)
C3—C4—C5—C6177.3 (2)C19—C18—O5—Zn1−153.8 (2)
C4—C5—C6—C7−0.1 (5)C13—O3—Zn1—O5174.6 (2)
C5—C6—C7—C81.3 (5)C13—O3—Zn1—O220.7 (2)
C6—C7—C8—C9−0.7 (5)C13—O3—Zn1—O495.8 (3)
C7—C8—C9—C4−1.0 (4)C13—O3—Zn1—N1−81.2 (2)
C5—C4—C9—C82.2 (4)C18—O5—Zn1—O3167.7 (2)
C3—C4—C9—C8−176.7 (2)C18—O5—Zn1—O2−110.5 (2)
O2—C11—C12—C134.3 (5)C18—O5—Zn1—O4−35.4 (2)
C10—C11—C12—C13−173.1 (3)C18—O5—Zn1—N162.7 (2)
C11—C12—C13—O30.4 (5)C11—O2—Zn1—O3−16.6 (2)
C11—C12—C13—C14−179.3 (3)C11—O2—Zn1—O5−98.1 (3)
O4—C16—C17—C18−5.6 (5)C11—O2—Zn1—O4−173.8 (2)
C15—C16—C17—C18172.8 (3)C11—O2—Zn1—N188.7 (2)
C16—C17—C18—O50.8 (5)C16—O4—Zn1—O3110.2 (3)
C16—C17—C18—C19−179.0 (3)C16—O4—Zn1—O531.4 (2)
O1—C3—N1—C10.8 (4)C16—O4—Zn1—O2−174.1 (2)
C4—C3—N1—C1178.9 (3)C16—O4—Zn1—N1−72.7 (2)
O1—C3—N1—Zn1−177.5 (2)C3—N1—Zn1—O3−51.4 (3)
C4—C3—N1—Zn10.6 (5)C1—N1—Zn1—O3130.4 (2)
C2—C1—N1—C3−8.1 (4)C3—N1—Zn1—O539.9 (3)
C2—C1—N1—Zn1170.75 (19)C1—N1—Zn1—O5−138.3 (2)
N1—C3—O1—C27.2 (3)C3—N1—Zn1—O2−143.2 (3)
C4—C3—O1—C2−171.2 (2)C1—N1—Zn1—O238.5 (2)
C1—C2—O1—C3−11.5 (3)C3—N1—Zn1—O4129.8 (3)
C12—C11—O2—Zn18.1 (4)C1—N1—Zn1—O4−48.4 (2)
Table 1
Selected geometric parameters (Å) top
N1—Zn12.0844 (19)O4—Zn12.0359 (19)
O2—Zn12.0253 (19)O5—Zn12.0169 (18)
O3—Zn12.0136 (17)
Acknowledgements top

The authors are grateful for the support of NSERC (Canada), the Atlantic Regional Magnetic Resonance Centre (ARMRC) and for the assistance of the Spanish MEC–MCyT research project BQU2002–2326.

references
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