Cristian Andrei Spinu

Céline Pichon

Gabriela Ionita

Teodora Mocanu

Sergiu Calancea

Mihai Raduca

Jean-Pascal Sutter
email: sutter@lcc-toulouse.fr

Mihaela Hillebrand
email: mihaela.hillebrand@gmail.com

Marius Andruh
email: marius.andruh@dnt.ro

Synthesis, crystal structure, magnetic, spectroscopic, and theoretical investigations of two new nitronylnitroxide complexes

Two mononuclear complexes [(Et 3 NH)[M(hfac) 2 L] (M = Ni, 1 Zn, 2) have been synthesized using a nitronyl-nitroxide radical substituted nitrophenol, i.e., 2-(2-Hydroxy-3-methoxy-5nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-3-oxide-1-oxyl, HL, as a proligand. The crystal structures of the two compounds have been solved and indicate an octahedral coordination geometry of the metal ions. The magnetic behavior for compound 1

is characterized by a strong antiferromagnetic metal-radical interaction (J = -351 ± 1 cm -1 ; H = -JS Ni S Rad ). This exchange interaction was rationalized by DFT calculations. The EPR spectra recorded both in solution and solid state at 120 K confirm the S = ½ ground state for compound 1.

INTRODUCTION

A large family of molecular magnetic materials consists of heterospin complexes generated by d or f metal ions and paramagnetic organic radicals acting as ligands [1]. The most popular radicals are the nitronyl-nitroxides (Nit), which are readily obtained, starting from aldehydes, following or adapting Ullman's synthetic approach [2]. The rich diversity of nitronylnitroxide ligands arises from the availability of a very large number of mono-and polyaldehydes, as well as from the possibility to decorate the nitronyl-nitroxide platform with other coordinating groups (preexisting in the staring aldehydes), which play an important role on the nuclearities and spin topologies of the resulting systems [3]. The family of magnetic materials constructed using paramagnetic organic ligands has been recently extended with polynuclear complexes containing two different metal ions, both paramagnetic [4]. Most of these heterotrispin complexes contain 3d and 4f metal ions. The exchange interactions between 2p radicals and paramagnetic metal ions vary between large limits, from strong antiferromagnetic to strong ferromagnetic couplings, [1,3] and usually the 2p-nd couplings are stronger than those involving lanthanides. Magneto-structural studies highlight the factors influencing the nature and strength of the exchange interactions [1]. The geometric parameters, at the level of aminoxyl-metal bond, influence the orientation of the magnetic orbitals, π* (Nit) and d or f, and, consequently, their overlap or orthogonality. Surprisingly, in spite of their simplicity and popularity in coordination chemistry, o-vanillin [5] and its substituted derivatives have been rarely employed as precursors for nitronylnitroxide ligands. To the best of our knowledge, there is only one report on the synthesis of the nitronyl-nitroxide obtained from o-vanillin [6], but this radical has never been employed as a ligand. This might be related to the poor yield of the synthesis of the radical derived from o-vanillin. A more convenient method consists in employment of derivatives that contain a substituent in para position with respect to the phenolic group, preventing the formation of by-products during the oxidation step with sodium periodate. The nitro derivative can be easily synthesized from o-vanillin and nitric acid and is subsequently transformed into the desired radical, HL (Scheme 1) [7]. In this paper we report on the synthesis, crystal structure and physical properties of two new mononuclear compounds, which contains L -as a ligand: Synthesis of 2,3-dimethyl-2,3-dinitrobutane and2-hydroxy-3-methoxy-5-nitrobenzaldehyde were performed according to the reported procedures [8,9]. All other reagents and solvents were commercially purchased and used without any further purification, if not stated otherwise. IR spectra were recorded on a FTIR Bruker Tensor V-37 spectrophotometer (KBr pellets) in the range of 4000-400 cm -1 . UV-Vis diffuse reflectance spectra were recorded on a JASCO V-670 spectrophotometer on undiluted samples in the range 200 -1800 nm.

(Et 3 NH)[M(hfac) 2 L] (M = Ni, Zn).

EXPERIMENTAL PART

General procedures

Elemental analyses (C, H, N) were performed on a EuroEA Elemental Analyzer. All Nuclear Magnetic Resonance ( 1 H and 13 C NMR) measurements were recorded on a Bruker 500 MHz spectrometer, at 25°C in CDCl 3 . The X-ray powder diffraction measurements were carried out on a Proto AXRD Benchtop using Cu-Kα radiation with a wavelength of 1.54059 Å in the range 5-35º (2θ). The metal ratio in the compound [Ni 0.2 Zn 0.8 ] was determined using the element energy dispersive spectroscopy (EDS) system (Smart Insight AMETEK), coupled with a Nova NanoSEM 630 Scanning Electron Microscope (FEI Company, Hillsboro, OR, USA). The EDAX spectrum was acquired at an acceleration voltage of 18 kV, with a working distance of 5 mm and 30000x magnification.

The EPR spectra were recorded with a JEOL FA 100 spectrometer equipped with a VT controller using the following setting: microwave power 1 mW, frequency 100 kHz, sweep field 2000 G, center field 3221.8 G, sweep time 1800 s, modulation width 2 G.

The magnetic field calibration was performed with a DPPH (diphenylpicrylhydrazyl) standard marker, exhibiting a narrow EPR line at g = 2.0036. The EPR spectra of Zn and Ni complexes and their mixture (molar ratio Ni/Zn 2:8) were recorded by filling a glass capillary that was placed in an EPR tube. For solution samples, the complexes or nitronyl-nitroxide radical were dissolved in CH 2 Cl 2 . The oxygen has been removed from solution by bubbling Ar gas. Frozen spectra were recorded at 120 K. The a N values of nitronyl-nitroxide radical were obtained by simulation of experimental spectra using Winsim program.

Magnetic measurements for 1 were carried out with a Quantum Design MPMS 5S SQUID magnetometer in the temperature range 2-300 K. The crystalline powder of the complex was mixed with grease in a gelatin capsule. The temperature dependence of the magnetization was collected in an applied field of 1kOe and the isothermal field dependence of the magnetizations were collected up to 5 T. The molar susceptibility (χ M ) was corrected for sample holder, grease and for the diamagnetic contribution of all the atoms by using Pascal's tables. Possible slow relaxation of the magnetization was examined by AC susceptibility collected in zero field and with applied fields.

X-Ray data collection and crystal structure refinement

Crystallographic data were collected on a STOE IPDS II diffractometer for 1 and on a Rigaku XtaLAB Synergy, single source at offset/far, HyPix diffractometer for 2, using a graphitemonochromated Mo K α radiation source (λ = 0.71073Å). The structures were solved by direct methods and refined by full-matrix least squares techniques based on F 2 . The non-H atoms were refined with anisotropic displacement parameters. Hydrogen atoms were placed in fixed, idealized positions and refined as rigidly bonded to the corresponding atoms.

Calculations were performed using SHELXT and SHELXL-2015 crystallographic software packages [10] A summary of the crystallographic data and the structure refinement is given in Tables 1 andS1 CCDC reference numbers: 2032386 (1); 2032384 (2); 2032385 (Ni 0.2 Zn 0.8 ).

Synthesis of 2,3-bis(hydroxylamino)-2,3-dimethylbutan

The synthesis of this ligand was adapted from reported procedure [11]. A solution of HgCl 2

(1.028 g, 3.78 mmol) in 50 mL of water was added over strips of aluminum foil (1.534 g, 57.82 mmol). The mixture was stirred for 2 minutes after which the liquid was removed with a pipet. The amalgamated aluminum was cooled keeping it in a glassware immersed in an ice bath and then a solution of 2,3-dimethyl-2,3-dinitrobutane (2 g, 11.36 mmol)

Synthesis of 2-(2-Hydroxy-3-methoxy-5-nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-3-oxide-1-oxyl (HL)

The synthesis of this ligand was devised and adapted from reported procedures [2,7]. To a solution of 2,3-bis(hydroxylamino)- S3) The Ni:Zn molar ratio in the resulting alloy was confirmed by EDX measurements.

Computational details

The exchange coupling interaction was calculated by the DFT method using the Gaussian 09

program [START_REF] Frisch | GAUSSIAN 09, Revision C.01[END_REF], considering two spin states, namely a low (S = 1/2 resulting from S Ni = 1 and S Rad = -1/2 spin states) and a high spin HS (S = 3/2 resulting from S Ni = 1 and S Rad = ½ spin states) one. The calculations were performed on the crystal geometry without further optimization. The energy of the low spin state, hereafter labeled as the Broken Symmetry (BS) state, was calculated by the fragment procedure as implemented in the Gaussian09

program and further checked for its stability. The calculations were performed using the basis set, TZVP [START_REF] Weigend | [END_REF], and three functionals, uB3LYP [14], uB3PW91 [15], and uM06 Taking into account the results obtained by these models, the DCM solution spectra were calculated using only the TZVP and the 6-31g* basis sets after the geometry optimization in the solvent.

RESULTS AND DISCUSSION

Description of the crystal structures

The new compounds, (Et Since the two compounds are isostructural, only the crystal structure of 1 will be described in detail.

Its crystal structure consists of anionic complexes, [Ni(hfac) 2 L] -(Figure 1) and organic cations, Et 3 NH + . The nickel ion shows an octahedral geometry, being coordinated by four oxygen atoms arising from the hfac -ligands and two others from L -(one phenoxido and one aminoxyl oxygen). The Ni -O oxygen distances vary between 2.008(4) and 2.072(4) Å. The N-O bond within the aminoxyl group coordinated to Ni II (1.305(5) Å) is longer than the one within the uncoordinated NO group (1.266(5) Å). The complex species are chiral and both enantiomers co-crystallize within the same crystal (Figure 2). The crystal structure of the complex anion in 2 is illustrated in Figure S4. Selected bond distances and angles for compounds 1 and 2 are collected in Table 2. The diffuse reflectance spectra of the ligand and the two complexes are displayed in Figure 3. S1), in agreement with the EDX data. The powder X-Ray diffractogram for the alloy (Figure S3) is identical with those of 1 and 2.

Magnetic properties of (Et 3 NH)[NiL(hfac) 2 ]

The temperature dependence of the molar magnetic susceptibility, χ Μ , for 1 has been investigated in the temperature range 2 -300 K, and the field dependence of the magnetization was recorded at 2 K. The χ M T vs. T and M vs. H plots are presented in Figure 4. The value of χ M T found at 300 K is 0.70 cm 3 mol -1 K, much smaller than the expected 1.375 cm 3 mol -1 K for the uncoupled S = 1 (Ni II ) and S =1/2 (Rad). Moreover, reducing the temperature, χ M T decreases to 0.475 cm 3 mol -1 K at 120 K and below remains constant down to 2 K. Such a behavior is indicative for a strong antiferromagnetic Radical-Ni(II) interaction.

The plateau reached below 150 K is the signature of a spin S = ½, resulting from the antiferromagnetic interaction between the spins S = ½ and 1. The resulting S = ½ ground state is further confirmed by the field dependence of the magnetization recorded at 2 K that tends to 1 µ B at 5 T.

These magnetic data have been analysed using the HDvV Hamiltonain: H = -JS Ni S Rad.. In order to avoid over-parameterization, only a mean g parameter was considered (the local g parameters were accurately determined by EPR spectroscopy, vide infra). The best fit yielded J = -351 ± 1 cm -1 and g = 2.185 ± 0.001. 6)º) suggest that the coupling between the nickel(II) ion and the radical must be antiferromagnetic and quite strong, in line with the experimental value.

DFT calculations undertaken for 1 confirm the strong antiferromagnetic interaction between the paramagnetic centers, the ground state of the complex being the low spin state, i.e. the broken symmetry one. Starting from the spin Hamiltonian previously mentioned, H = -JS Ni S Rad , the J value was obtained using the equation ( 1) [19]:

J = (E BS -E HS )/(2S Ni S Rad +S Rad ) (1)
The results presented in Table 3 are in agreement with the experimental value, J = -351 ± 1

(cm -1 ). It can also be remarked that the three functionals used lead to quite similar results. As can be seen in Figure 5a and b, in both states, BS and HS, the spin densities are localized on the nickel ion and on the NO groups of the nitronyl-nitroxide radical, explaining the EPR spectra, vide infra.

EPR spectra

The EPR spectra of the three paramagnetic compounds, the two complexes 1 and 2 and the nitronyl-nitroxide radical, were recorded both at room temperature, 293 K and at 120 K. The spectrum of HL in CH 2 Cl 2 at room temperature is displayed in Figure S5 and presents the known features of nitronyl-nitroxide radicals with a g factor of 2.0053 and a five line spectrum due to the hyperfine splitting (hfs) of two quasi-equivalent nitrogen atoms; the values obtained by simulation are a N1 = 7.40 G, a N2 = 7.55 G. The EPR spectrum recorded at 120 K indicates a restricted motion (Figure S5c).

The EPR spectra of the two complexes and their mixture, recorded in CH 2 Cl 2 are plotted in Figure 6. At room temperature, the spectrum of the nickel complex, 1 , (Figure 6a), presents only the signal of the organic radical (g = 2.0053); at 120 K, the spectrum consists of two signals, a strong one characterized by g = 2.2751, and a very weak signal at g= 2.0051; the first signal is characteristic to a paramagnetic species with the electron localized on the nickel center [20] while the weak one is due to the organic radical. As expected, at both temperatures the spectrum of complex 2 presents only a single signal belonging to the organic radical the spectrum is unresolved at low temperature due to a strong broadening of the lines , but the g value is not influenced by temperature (293 K, g = 2.0055; 120 K, g = 2.0051). The isodensity spin surface displayed in Figure S6 reflects this behavior, the spin density being localized only on the NO groups. The assignment of the EPR spectra of 1 and 2 is verified by the spectra d and e in Figure 6; where it can be seen that the presence of the nickel ion determines the appearance at 120 K of the very weak signal at g = 2.2761 previously assigned to the spin localization on nickel and a large signal at g = 2.0056 corresponding to complex 2.

The strong signal of nitronyl-nitroxide fragment observed at 120 K is due to the excess of zinc complex in solution. The same behavior is illustrated by the EPR spectra recorded in solid state (Figure 7), for 1, 2

and the molecular alloy [Ni 0.2 Zn 0.8 ]. Although the isostructural zinc derivative, 2, is not a diamagnetic host for 1, and cannot suppress all the dipolar interactions, some information can be extracted by comparing all these spectra. At room temperature compound 1 shows only a broad and weak feature at g = 2.2889, which is due to the S = ½ state. As expected, this signal becomes stronger at 120 K, where only the spin state S = ½ is populated (Figure 7b). The spectra of the zinc derivative do not change with the temperature (Figure 7c,d). The molecular alloy of 1 and 2 shows at room temperature (Figure 7e) only one signal (g = 2.0061) arising from complex 2; at 120 K (Figure 7f) two signals are observed: g = 2.0102 (due to complex 2) and g = 2.2712 (S = ½ ground state of 1).

The hyperfine splitting (hfs) constants for 1 and 2 were firstly calculated using the same model (crystal geometry, B3LYP/TZVP) as for the magnetic measurements. The results listed in Table S2 show a poor agreement with the experimental ones; it is even worse with TVZP/EPR-III. A better agreement is obtained with basis set 6-31*. Therefore, all the further calculations with CH 2 Cl 2 as a solvent were performed using the TZVP and 6-31g* sets (Table 4) It can be seen that for the three paramagnetic species a good agreement is obtained with the 6-31g* basis which have also the advantage of requiring short calculation times. In conclusion, we described dscribed two new metal-radical complxes, using for the first time, as a ligand, a nitronyl-nitroxid radical derived from o-vanillin. The EPR spectra of the nickel complex recorded at two different temperatures clearly show the S = ½ ground state, in line with thee magnetic susceptibility measurements and theoretical calculations.

Scheme 1 .

 1 Scheme 1. Synthesis of the paramagnetic ligand.

  3 NH)[M(hfac) 2 L] (M = Ni 1, Zn 2), have been synthesized by reacting the metal precursors, [M(hfac) 2 (H 2 O) 2 ] with the paramagnetic proligand, HL, in the presence of triethylamine, which was added for the deprotonation of the phenolic group. The PXRD patterns for 1 and 2 confirm the purity of the crystalline phases (Figures S1 and S2).

Figure 1 .

 1 Figure 1. Molecular structure of the complex anion 1, along with the atom numbering scheme. Hydrogen and fluorine atoms have been omitted for clarity.

Compound 1

 1 shows, apart from the bands arising from the organic ligands, two other bands which are due to the d-d transitions: 3 A 2 → 3 T 2 (1122 nm) and 3 A 2 → 3 T 1 (751 nm), assuming the O h point group. One single crystal from the [Ni 0.2 Zn 0.8 ] sample was measured, and the structure was refined with occupation factors 0.2 for Ni and 0.8 for Zn (Table

Figure 2 .

 2 Figure 2. The packing diagram of 1 showing the closest distances between nickel metal ions and delta (Δ) or lambda (Λ) configurations of the metal centers.

Figure 3 .

 3 Figure 3. Diffuse reflectance spectra for HL and complexes 1 and 2.

Figure 4 .

 4 Figure 4. Magnetic behavior for 1: experimental (O) and calculated (-) (a) temperature dependence of the χ M T and (b) magnetization vs. fieldThe nature of the exchange interactions (antiferro-and ferromagnetic) between nickel(II) and the aminoxyl group was subject of several magneto-structural correlations studies[18]. We recall here the most important conclusions, resulting from both experimental and theoretical investigations:(i) the coplanarity between the radical ring and the equatorial plane of the octahedral Ni II ion favors a ferromagnetic coupling (the d x2-y2 and π* radical orbital are orthogonal, while the overlap with the d z2 orbital is symmetry forbidden); (ii) the Ni-O-N-C dihedral angle as well as the Ni-O-N angle play a crucial on the overlap of the magnetic orbitals; (iii) large overlaps between the magnetic orbitals, and, consequently, strong antiferromagnetic interactions are favored by M-O-N-C dihedral angles close to 90º and by Ni-O-N angles close to 120º. These geometrical parameters for complex 1 (Ni1-O1-N1 =

Figure 5 .

 5 Figure 5. Spin density (0.04) isosurfaces (0.04 e -/(a.u.) 3 ) : a) BS state; b) HS state.

Figure 6 .

 6 Figure 6. The EPR spectra in DCM recorded at 293 and 120 K: a) 1 at 293 K; b) 1 at 120 K ; c) 2 at 293 K; d) 2 at 120 K; e) Ni 0.2 Zn 0.8 at 293 K; f) Ni 02 Zn 0.8 at 120 K.

Figure 7 .

 7 Figure 7. Solid state EPR spectra of: a) 1 at 293 K; b) 1 at 120 K; c) 2 at 293 K; d) 2 at 120 K; e) Ni 0.2 Zn 0.8 at 293 K; f) Ni 0.2 Zn 0.8 at 120 K.

Figure S6 .

 S6 Figure S6. The isodensity (0.04 e -/(a.u.) 3 ) surface of complex 2 calculated in DCM at the B3LYP/6-31g* level.

Table 1 .

 1 Crystallographic data, details of data collection and structure refinement parameters for compounds 1 and 2.

	Complex	1	2
	Empirical formula Formula weight Temperature (K) Crystal system Space group a (Å) b (Å) c (Å) α (ᵒ) β (ᵒ) γ (ᵒ) Volume (Å 3 ) Z D c (g/cm 3 ) Absorption coefficient (mm -1 ) F(000) Ɵ range for data collection (ᵒ)	C 30 H 35 N 4 NiO 10 F 12 898.33 293.15 Monoclinic P2 1 /c 20.2955(15) 9.6043(6) 21.5013(15) 90 110.349(5) 90 3929.6(5) 4 1.518 0.607 1836.04 1.93 to 24.999	C 30 H 35 N 4 ZnO 10 F 12 904.99 293.15 Monoclinic P2 1 /c 20.4327(7) 9.5457(2) 21.5624(7) 90 111.461(4) 90 3914.0(2) 4 1.536 0.741 1844.0 2.03 to 30.783
	Index ranges	-24≤h≤24, -11≤k≤11,	-25≤h≤26, -10≤k≤13,
		-25≤l≤25	-26≤l≤27
	Reflections collected Independent reflections [R int ] Completeness to Ɵ full (%) Data/restraints/parameters Goodness of fit on F 2 R 1 , wR 2 [I >2σ I] R 1 , wR 2 (all data) Largest diff. peak/hole (Å 3 )	40888 6936 [R int = 0.1320] 9656 [R int = 0.0337] 39190 100.0 100.0 6936/18/642 9656/25/654 1.039 1.015 0.0596, 0.1108 0.0378, 0.0984 0.1414, 0.1476 0.0680, 0.1111 0.23/-0.34 0.28/-0.24

  in 60 mL of tetrahydrofuran (THF) and 6 mL of H 2 O were added sequentially. The reaction is left to evolve 1 h under strong stirring at low temperature followed by filtration on a pad of celite

which was washed with another 60 mL of THF. The solvents from the filtered solution were removed under vacuum, to obtain a white precipitate. The precipitate was dissolved in 25 mL of CHCl 3 over which 25 mL of diethyl ether were added. The solution was left overnight in the freezer to form the product as a white crystalline powder, 0.555 g, yield 33 %. The product was stored dried in a fridge. Anal. Calcd. for C 6 H 16 N 2 O 2 (%): C, 48.63; H, 10.88; N, 18.90. Found: C, 48.68; H, 10.90; N, 18.96. Selected IR peaks (cm -1 ): 3368 (s), 3287 (s), 3252 (vs), 2988 (vs), 2943 (s), 1479 (m), 1452 (m), 1425 (m), 1404 (s), 1387 (s), 1375 (s), 1358 (m), 1261 (m), 1177 (m), 1146 (s), 1080 (m), 1036 (m), 989 (w), 951 (m), 932 (w), 905 (s), 853 (w), 791 (w), 690 (m), 609 (w), 494 (w), 441(w). 1 H-RMN (500.13 MHz, CDCl 3 , δ ppm): 1.19 (s, 12 H, CH 3 -); 13 C-RMN (125.77 MHz, CDCl 3 , δ ppm): 62.97 (CH 3 -C-CH 3 ), 20.81(CH 3 -).

Synthesis of (Et 3 NH)[NiL(hfac) 2 ], 1

  to obtain the crude blue product which was recrystallized four times from a mixture of CH 2 Cl 2 : diethyl ether, 1 : 1 volumetric ratio, to obtain the pure blue product, 1.391 g, yield 46%. Anal. Calcd. for C 14 H 18 N 3 O 6 (%): C, 51.85; H, 5.59; N, 12.97. Ni(hfac) 2 •2H 2 O (0.060 g, 0.1852 mmol) was dissolved in 15 mL of heptane and refluxed for 30 min. Then, after cooling down the solution , another 15 mL of CH 2 Cl 2 solution containing HL (0.094 g, 0.1852 mmol) and 1 drop of triethylamine was added. The solution is refluxed another 30 min, cooled down and filtered. After allowing the solvent to slowly evaporate for three days, needle shaped violet crystals of the product were obtained, 0.125 g, yield 75%. Anal. Calcd. for C 30 H 35 F 12 N

	Found: C, 52.27; H, 5.41; N, 12.75. Selected IR peaks (cm -1 ): 1581 (w), 1530 (m), 1462 (m),
	1429 (m), 1394 (m), 1373 (w), 1350 (vs), 1258 (m), 1201 (w), 1163 (m), 1138 (w), 1105 (w),
	1069 (w), 923 (w), 885 (w), 739 (m), 457 (m). UV-Vis (nm): 362, 571.
	2.5.
	2,3-dimethylbutan (1.523 g, 10.288 mmol) in 50 mL of
	MeOH, another solution of 2-hydroxy-3-methoxy-5-nitrobenzaldehyde (1.843 g, 9.353
	mmol) in 75 mL of MeOH was added. The mixture was refluxed for 4 h followed by the
	evaporation of the solvent under vacuum. Over the yellow intermediate, 100 mL of CHCl 3
	was added and the solution was ice cooled then, a solution of NaIO 4 (2.002 g, 9.353 mmol) in

100 mL of water was added. The obtained biphasic system was placed under strong stirring and the reaction left to evolve at 0 °C for 15 min and then at 25 °C for 45 min. The organic phase was isolated using a separatory funnel and dried with MgSO 4 . The solvent was evaporated under vacuum 4 O 10 Ni (%): C, 40.11; H, 3.93; N, 6.24; Found: C, 39.61; H, 4.13; N, 5.87. Selected IR peaks (cm -1 ): 1657 (s), 1645 (s), 1600 (w), 1555 (m), 1524 (s), 1502 (s), 1396 (w), 1364 (m), 1310 (s), 1256 (vs), 1234 (m), 1204 (vs) 1148 (vs), 1097 (m), 1070 (w), 870 (w), 793 (m), 673 (m), 586 (w). UV-Vis (nm): 413, 571, 751, 1142.

2.

6. Synthesis of (Et 3 NH)[ZnL(hfac) 2 ], 2

  Zn(hfac) 2 •2H 2 O (0.047 g, 0.0926 mmol) was dissolved in 7 mL of heptane and refluxed for 30 min. Then, after cooling down the solution, another 7 mL of CH 2 Cl 2 solution containing HL (0.030 g, 0.0926 mmol) and 1 drop of triethylamine was added. The solution is refluxed another 30 min, cooled down and filtered. After allowing the solvent to slowly evaporate for three days, needle shaped violet crystals of the product were obtained, 0.046 g, yield 55%.

	Anal. Calcd. for C 30 H 35 F 12 N 4 O 10 Zn (%): C, 39.82; H, 3.90; N, 6.19; Found: C, 40.33; H,
	3.80; N, 6.06. Selected IR peaks (cm -1 ): 1663 (m), 1649 (s), 1601 (m), 1557 (m), 1528 (s),
	1504 (s), 1399 (w), 1366 (m), 1331 (m), 1310 (s), 1286 (m), 1256 (vs), 1234 (m), 1204 (s),
	1144 (vs), 1096 (m), 1071 (w), 869 (w), 795 (m), 765 (w), 746 (w), 669 (m), 583 (w). UV-
	Vis (nm): 424, 584.
	The molecular alloy, [Ni 0.2 Zn 0.8 (L(hfac) 2 ], has been obtained by reacting the zinc and
	nickel salts in 1:4 molar ratio, in the same experimental conditions described above for

compounds 1 and 2. The powder X-Ray diffractogram is similar to those recorded for compounds 1 and 2 (Figure

  [16]. The EPR spectra of the two Ni and Zn complexes were firstly calculated using the same theoretical model and the crystal geometry, but considering literature data[17], we have also used a new basis set recommended for EPR, TZVP/ EPR-III, and a very simple one, 6-31g*.

Table 2 .

 2 Selected bond distances (Å) and angles (º) for compounds 1 and 2.

	1		2
	Distances Ni1-O1	2.072(4)	Distances Zn1-O1	2.204(2)
	Ni1-O3 Ni1-O7 Ni1-O8 Ni1-O9 Ni1-O10 Angles O1-Ni1-O3 O1-Ni1-O7 O1-Ni1-O8 O1-Ni1-O9 O1-Ni1-O10 O3-Ni1-O7 O3-Ni1-O8 O3-Ni1-O9 O3-Ni1-O10 O7-Ni1-O8 O7-Ni1-O9 O7-Ni1-O10 O8-Ni1-O9 O8-Ni1-O10 O9-Ni1-O10	2.035(3) 2.046(4) 2.029(4) 2.008(4) 2.026(4) 89.53(13) 89.67(14) 173.15(16) 82.83(15) 94.70(15) 91.75(14) 97.21(15) 172.16(15) 87.92(15) 88.88(15) 90.00(15) 175.62(15) 90.47(16) 86.82(15) 90.91(15)	Zn1-O3 Zn1-O7 Zn1-O8 Zn1-O9 Zn1-O10 Angles O1-Zn1-O3 O1-Zn1-O7 O1-Zn1-O8 O1-Zn1-O9 O1-Zn1-O10 O3-Zn1-O7 O3-Zn1-O8 O3-Zn1-O9 O3-Zn1-O10 O7-Zn1-O8 O7-Zn1-O9 O7-Zn1-O10 O8-Zn1-O9 O8-Zn1-O10 O9-Zn1-O10	2.035(1) 2.095(2) 2.089(2) 2.051(2) 2.090(2) 85.93(5) 90.65(5) 172.31(6) 79.49(6) 95.85(6) 93.83(6) 101.23(6) 164.47(6) 87.86(6) 86.10(6) 91.67(6) 173.39(6) 93.62(7) 87.30(6) 88.34(6)

Table 3 .

 3 Calculated J values (cm -1 ) for compound 1.

	Functional Eq.(1)
	uB3LYP uB3PW91 -398.23 -400.53 uM06 -408.51

Table 4 .

 4 Calculated isotropic hyperfine splitting constants (G) for the three paramagnetic species optimized in DCM, using the uB3LYP functional and two basis sets, TZVP, and 6-31g*.

		|a N | (exp)	HL TZVP 6-31g* TZVP 6-31g* TZVP 6-31g* 1 2
	a N1 a N2	7.40 7.50	5.33 5.06	7.68 7.59	-4.98 -4.95	-7.22 -7.40	5.12 5.23	7.46 7.79

Table S1 .

 S1 Crystallographic data, details of data collection and structure refinement parameters for [Ni 0.2 Zn 0.8 ].

	Complex Empirical formula Formula weight Temperature (K) Crystal system Space group a (Å) b (Å) c (Å) α (ᵒ) β (ᵒ) γ (ᵒ) Volume (Å 3 ) Z D c (g/cm 3 ) Absorption coefficient (mm -1 ) F(000) Ɵ range for data collection (ᵒ) Index ranges Reflections collected Independent reflections [R int ] Completeness to Ɵ full (%) Data/restraints/parameters Goodness of fit on F 2 R 1 , wR 2 [I >2σ I] R 1 , wR 2 (all data) Largest diff. peak/hole (Å 3 )	[Ni 0.2 Zn 0.8 ] C 30 H 35 N 4 Ni 0.2 Zn 0.8 O 10 F 12 903.66 293.15 Monoclinic P2 1 /c 20.3978(8) 9.5639(2) 21.5481(8) 90 111.215(4) 90 3918.8(2) 4 1.532 0.714 1842.0 2.142 to 30.746 -27≤h≤27, -10≤k≤13, -27≤l≤28 38210 9784 [R int = 0.0305] 99.99 6784/24/657 0.938 0.0402, 0.1237 0.0680, 0.1428 0.26/-0.21

Table S2 .

 S2 Hfs constants for complexes 1 and 2 calculated at the uB3LYP level and three basis sets, TZVP,TZVP/EPR-III, 6-31g* considering the crystal geometry.

		|a N | (exp)	TZVP	1 TZVP, EPR-III	6-31g*	TZVP	2 TZVP, EPR-III	6-31g*
	a N1	7.4	-4.37	-3.23	-5.61	4.61	2.29	6.72
	a N2	7.5	-4.56	-2.89	-6.55	5.18	3.18	7.70

ACKNOWLEDGMENTS

We are thankful to Anamaria Hanganu for NMR spectra recording. Also, we are grateful to Gabriel Crăciun and Adrian Apostol for EDX measurements.

Supporting information

Synthesis, crystal structure, magnetic, spectroscopic, and theoretical investigations of two new nitronyl-nitroxide complexes


Joseph Aneke

Carmelo Ardito

Designing an Intelligent User Interface for Preventing Phishing Attacks

Keywords: Usable Security, Intelligent User Interfaces, Cybersecurity

Most phishing sites are simply copies of real sites with slight features distorted or in some cases masqueraded. This property of phishing sites has made them difficult for humans and various anti-phishing techniques to detect. Also, the attacker community has proved itself able to quickly adapt to antiphishing measures, mainly warning messages to help limit the effectiveness of phishing attacks and protect unsuspecting users. Despite the notable advances made in the last years by the active warning messages for phishing, this attack remains one of the most effective. In this paper we propose an intelligent warning message mechanism, that might limit the effectiveness of phishing attacks and that might increase the user awareness about related risks. It implements an intelligent behavior that, besides warning the users that a phishing attack is occurring, explains why the specific suspect site can be fraudulent.

Introduction

Phishing is a form of social engineering in which an attacker attempts to fraudulently acquire sensitive information from a victim by impersonating a trustworthy third party [START_REF] Jagatic | Social phishing[END_REF]. Today these attacks typically employ generalized "lures." For instance, a common phishing attack is (for a phisher) to obtain a victim's authentication information corresponding to one website that is mimicked by the attacker and then uses this at another site. This is a successful attack given that many users reuse passwordswhether in verbatim or with only slight changes. This attack is typically carried out by e-mail or instant messaging, and often directs users to enter details at a fake website [START_REF]Phishing Attack Trends Report -4Q 2018[END_REF]. A common example is "we need you to confirm your account details or we must shut your account down". The reason why an individual falls prey to this type of trap is that the message, which appears as the victim expects, and therefore legitimate, directs the user to visit fake webpages whose look and feel is similar or identical to the legitimate one.

This phishing modality is also known as context-aware attack and is becoming increasingly common. Fig. 1 shows an example of a phishing attack sent to a user by email.

The email appears genuine from a trusted sender, i.e. "uniba.it" which is the email service provider of the user. However, visualizing the details of the sender's identity reveals that it was masquerading to get the user to fill a form. The effectiveness of phishing techniques, and more in general of cyber-attacks is not only related to the obsolescence of software and hardware. Federal Computer Week reports that almost 59% of security incidents that involve human errors are the result of simple mistakes as opposed to intentional malicious actions [3]. Hosteler found that human error is one of the first cause of cyberattacks (37%) [START_REF]BakerHostetler: Is Your Organization Compromise Ready? 2016[END_REF]. Furthermore, the simplest and fastest way to start an attack is by means of phishing and social engineering attacks, where 91% of all cyberattacks starts with some kind of phishing email that manipulates users to provide sensitive information via various methods of social engineering [START_REF] Gupta | Fighting against phishing attacks: state of the art and future challenges[END_REF].

Because of the risks associated with cyberattacks, it is crucial for Internet users to be aware of when they are being attacked and to be successfully informed on how to combat them. The recent demography results by Anti-Phishing Working Group 4 th quarter report shows that around 45,794 phishing reports have been chronicled [START_REF]Phishing Attack Trends Report -4Q 2018[END_REF]. There is no single way that can prevent all types of phishing. But different methods applied at different stages of a phishing attack can abort the attempt and properly applied technology can significantly reduce the risk of identity theft [START_REF] Emigh | Online identity theft: Phishing technology, chokepoints and countermeasures[END_REF]. Different approaches are already proposed to automatically detect phishing websites [START_REF] Varshney | A survey and classification of web phishing detection schemes[END_REF][START_REF] Abu-Nimeh | A comparison of machine learning techniques for phishing detection[END_REF][START_REF] Almomani | A Survey of Phishing Email Filtering Techniques[END_REF]. These methods and algorithms determine the likelihood that a website can be suspect but without absolute certainty. When the resulting likelihood exceeds a critical threshold, typically the users are informed about the potential risk of phishing attacks. This is done through a visual warning message that should help users in deciding to access or not the suspect website. Despite the significant advances of current warning messages, this attack remains very effective since the users often are not able to make the right decision.

There is a direct need for us to design such a remedy which can address the above problem and stand out from the traditional warning messages available. In this paper, we report on an ongoing work about an intelligent warning message that might limit the effectiveness of phishing attacks and that might increase the user awareness about the related risks. The proposed solution implements an intelligent behavior that explains why the specific suspect site can be fraudulent. It is well-known that explaining the reasons about a fact helps the user being aware of the danger and taking more conscious and adequate decisions [START_REF] Biran | Explanation and justification in machine learning: A survey[END_REF].

Literature Review

Successful security depends on systems, technology and people (including users) collaborating to identify threats, weaknesses, and solutions. However, many initiatives today focus on systems and technology, without addressing well-known user-related issues. In fact, users have been identified as one of the major security weaknesses in today's technologies, as they may be unaware that their behavior while interacting with a system may have security consequences. The user interface is where human users interact with computer systems. It is where the user's intention transforms into the system operation. It is where the semantic gap arises [START_REF] Wu | Fighting phishing at the user interface[END_REF]. And this is the aspect that needs more attention to further limit the effectiveness of cyberattacks. One typical anti-phishing approach is to use visual indicators, for example an informative toolbar, to differentiate legitimate messages from phishing messages [START_REF]FBI Says Web Spoofing Scams Are a Growing Problem[END_REF]. This approach tries to bridge the semantic gap by unveiling to human users the system model and expects them to make a wise decision under phishing attacks. User studies in [START_REF] Wu | Do security toolbars actually prevent phishing attacks?[END_REF] show that the tested anti-phishing toolbars fail to effectively prevent high-quality phishing attacks. Many subjects failed to constantly pay attention to the toolbar's messages; others disregarded the warnings shown in the toolbar if the web page content looked legitimate. The studies also found that many subjects did not understand phishing attacks or realize how sophisticated such attacks can be.

Emigh provided an overview of several studies regarding phishing and usability of browser security clues [START_REF] Herzberg | Why Johnny can't surf (safely)? Attacks and defenses for web users[END_REF]. It emerged that phishing is a threat because users are unable to verify the authenticity of a website asking for their credentials. He questioned the ability of users to avoid phishing sites based on security and identification indicators suggesting that the solutions were still inadequate. Varshney et al. proposed Dynamic Security Skins, a browser extension that allows websites to display a secret image and customizes the browser chrome [START_REF] Dhamija | Why phishing works[END_REF]. Anecdotal evidence suggests that some users may still fall victim of those phishing websites which, in order to cheat about their different appearance (e.g., logo not displayed), claim that the image database is down for maintenance or simply do not provide any explanation about this, since logo absence might not trigger users' attention.

In [START_REF] Martin | Phishing for Answers: Exploring the Factors that Influence a Participant's Ability to Correctly Identify Email[END_REF], the authors sought to determine if user's education was a possible solution to prevent phishing attacks. They explored the impact of both specific users' characteristics (age, gender, education, knowledge about phishing) and of their Internet usage habits on their ability to correctly identify e-mail messages. Quantitative data was collected by showing to participants e-mail messages and quizzing their ability to correctly categorize them. The results show the variables listed above did influence the participant's ability to correctly identify email messages. Although educational efforts are unlikely to solve the phishing problem on its own, vigilant users form an important part of the defensive strategy. Both research efforts and public education campaigns are still not enough.

A study to determine the impact that communicating to users different security policies has on mitigating phishing attacks is discussed in [START_REF] Almomani | A Survey of Phishing Email Filtering Techniques[END_REF]. The research results reveal that a security policy that contains an explanation of the impact of an attack or a statement indicating an evaluation for non-compliance or a statement from a direct authority provides no significant impact on mitigating phishing attacks [START_REF] Mcnealy | Angling for Phishers: Legislative Responses to Deceptive E-Mail[END_REF]. The use of online games to teach users good habits to help them avoid phishing attacks is investigated in [START_REF] Sheng | Who falls for phish?: a demographic analysis of phishing susceptibility and effectiveness of interventions[END_REF]. The authors explore the relationship between demographics and phishing susceptibilities, and the effectiveness of several anti-phishing educational materials. Results suggest that women are more susceptible to phishing than men and participants between the ages of 18 and 25 are more likely to be a victim of a phishing attack than other age groups.

A new anti-phishing approach which uses training intervention for phishing web sites detection is discussed in [START_REF] Kumaraguru | School of phish: a real-world evaluation of anti-phishing training[END_REF]. The results of this work show that technical ability has minimal effect whereas phishing knowledge has a positive effect on phishing web site detection. A system called PhishGuru incorporating an embedded training methodology and learning science principles is proposed in [START_REF] Kumaraguru | Teaching Johnny not to fall for phish[END_REF]. The author evaluates the proposed methodology through laboratory and field studies. Results show that people trained with the proposed system retain knowledge even after 28 days. A major drawback is that the system will need to be trained and updated regularly. Robert W et al [START_REF] Reeder | An Experience Sampling Study of User Reactions to Browser Warnings in the Field[END_REF] found that web browser warnings should help protect people from malware, phishing, and network attacks. Adhering to these warnings keeps people safer online. They further demonstrated that recent improvements in warning designs have raised adherence rates, but they could still be higher. And prior work suggests many people still do not understand them. Thus, two challenges remain: increasing both comprehension and adherence rates. The authors in [START_REF] Reeder | An Experience Sampling Study of User Reactions to Browser Warnings in the Field[END_REF] suggested that further improvements to warnings will require solving a range of smaller contextual misunderstandings.

Most phishing sites are simply copies of real sites with the above-mentioned feature slightly distorted or in some cases masqueraded [START_REF] Afroz | PhishZoo: Detecting Phishing Websites by Looking at Them[END_REF]. This property of phishing sites has made them difficult for humans to detect, but fortunately, easier for computers. However, the attacker community has proved itself able to quickly adapt to anti-phishing measures mainly warning messages. Different warning messages have been already evaluated during controlled experiments [START_REF] Reeder | An Experience Sampling Study of User Reactions to Browser Warnings in the Field[END_REF][START_REF] Egelman | You've been warned: an empirical study of the effectiveness of web browser phishing warnings[END_REF]. Besides evaluating the efficacy of different solutions, these experiments provided useful indications on how to design and evaluate phishing warning messages. Despite the notable advances made in the last years by the active warning messages for phishing [START_REF] Reeder | An Experience Sampling Study of User Reactions to Browser Warnings in the Field[END_REF][START_REF] Egelman | You've been warned: an empirical study of the effectiveness of web browser phishing warnings[END_REF], this attack remains one the most effective. Indeed, algorithms for detecting phishing attacks are only able to determine the likelihood with which a website can be suspect but without absolute certainty. When the likelihood exceeds a critical threshold the warning messages alert the users about a possible risk and the users have to decide to access or not the website. However, current warning messages have large room for improvement, as shown by the high success rate of phishing attacks reported in [START_REF]IBM X-Force Threat Intelligence Index[END_REF]. One of the first problems is the clickthrough effect [START_REF] Felt | Improving SSL Warnings: Comprehension and Adherence[END_REF]: the users tend to skip these alerts because they appear always in the same way, thus pushing most users in neglecting these messages. The second problem is the wrong design of the warning messages in term of colors, words, interaction, as underlined by [START_REF] Reeder | An Experience Sampling Study of User Reactions to Browser Warnings in the Field[END_REF][START_REF] Egelman | You've been warned: an empirical study of the effectiveness of web browser phishing warnings[END_REF]. Lastly, the users are not experts in cybersecurity, they do not know what a phishing attack is and what are the risks they are exposed to [START_REF] Reeder | An Experience Sampling Study of User Reactions to Browser Warnings in the Field[END_REF].

In order to overcome these limitations, in the following section, we propose an intelligent warning message mechanism that might limit the effectiveness of phishing attacks and that might increase the user awareness about related risks. It implements an intelligent behavior that, besides warning the users that a phishing attack is occurring, explains why the specific suspect site can be fraudulent.

A Polymorphic User Interface to Warn Users about Phishing Attacks

An example of polymorphic user interface to warn users about phishing attacks is reported in Fig. 2. In addition to addressing the design guidelines and lesson learned proposed in [START_REF] Reeder | An Experience Sampling Study of User Reactions to Browser Warnings in the Field[END_REF][START_REF] Egelman | You've been warned: an empirical study of the effectiveness of web browser phishing warnings[END_REF], this prototype shows three panels that explain the reasons why the target website can be a fake. In this example, the first panel specifies that the URL of the target website (www.paypaI.com) looks similar to the original one but the l has been replaced by capital I, thus confusing the users. The second panel reports that the suspect website was created three weeks ago, an age typical of phishing websites. The last box reports information about the HTTPS certificate of the suspect website, explaining that even if the users see safe navigation in the browser toolbar, with a self-signed certificate they are not guaranteed that the site behavior is legitimate.

It is worth remarking that the three panels show different information according to the suspect website, thus different reasons would be reported with different phishing websites. Thank to this intelligent warning message, we address three important goals, i.e.:

1. Prevent user habituation: a polymorphic message decreases the clickthrough effect caused by the user habituation [START_REF] Felt | Improving SSL Warnings: Comprehension and Adherence[END_REF]; 2. Provide an explanation about the attack: useful information about the causes of the phishing attacks support the users in deciding if the website is (or not) a phishing attack [START_REF] Adadi | Peeking inside the black-box: A survey on Explainable Artificial Intelligence (XAI)[END_REF]; 3. Educate the users on cyberattacks and related risks: a long-term training of the users on phishing attacks is performed since they understand the reasons for this attack [START_REF] Kumaraguru | School of phish: a real-world evaluation of anti-phishing training[END_REF][START_REF] Jensen | Training to mitigate phishing attacks using mindfulness techniques[END_REF].

Fig. 2.

A prototype of intelligent warning message for phishing attacks.

In our work we are not interested to classify phishing websites [START_REF] Varshney | A survey and classification of web phishing detection schemes[END_REF][START_REF] Abu-Nimeh | A comparison of machine learning techniques for phishing detection[END_REF][START_REF] Almomani | A Survey of Phishing Email Filtering Techniques[END_REF]. We start from the assumption that the browser can detect the phishing website through its internal algorithm, or that we use an API to detect malicious sites 1 . Regardless of which of the two solutions we adopt, when a phishing website is detected, instead of displaying the traditional warning messages implemented in the browser, we show the intelligent UI proposed in this paper (see Fig. 2).

To provide users with information that explain the reasons of the phishing attacks, our approach consists of two main steps, i.e., 1) the computation of a set of indicators that can reveal phishing websites and 2) the use of machine learning approaches to select the most important indicators. The three most important indicators will be shown and explained to the user, as shown in the example above.

According to our goal and a literature review [7-9, 28], we are considering indicators for the suspect web sites like:

 URL: phishing sites typically have URLs containing more than 2/3 number of digits or "-". In addition, they often try to mimic the original URL changing character that looks similar, for example, "l" with "I";  Server location: phishing websites are often hosted by a web server located in countries where there are no strict laws against cyberattacks;

 Alexa or search engine rank: phishing website typically appear after the first 1 million Alexa top results, or in the last positions of search engines like Google Search;  Timelife: this cyber-attack is usually concentrated in a limited time span, thus the suspect website is typically created few days/weeks before the attack;  Top-level domain: attackers typically use free domains to host phishing web sites; one of the most popular is freenom.com, thus domains like ".cf", ".gq", ".ml", ".tk" and ".ga" are common among phishing web sites;  Name length: Attackers may create domains using a specific template, such as random strings of a given length;  Archived domain: a domain archived on the "Wayback Machine" is more likely to be legitimately owned, and vice versa;  Self-signed https certificate: the suspect websites often integrate a self-signed https certificate, i.e., not validated by a certification authority. Including this certificate, attackers confuse users who see safe navigation in the browser toolbar, but without any guarantee about the web site behaviour.

We defined different metrics to calculate each indicator for the suspect website. For example, Alexa rank can be obtained through its API; the Wayback Machine APIs are used to get information about website archiving; SSL certificate is inspected to see if a trustable certification authority signed it. Those indicators, resulting in a numeric value, are normalized in a 0-1 interval using a min-max function, with min and max values obtained calculating each indicator on all the phishing websites available in the PhishTank database and selecting for each indicator the min and max value.

After the computation of the indicators, we use a decision tree model to select the most important indicators. In particular, we adopted the C4.5 algorithm to generate our decision tree. This algorithm was developed by Ross Quinlan [START_REF] Quinlan | C4.5: programs for machine learning[END_REF] and it is an extension of Quinlan's earlier ID3 algorithm. The decision trees generated by C4.5 can be used for classification, and in our case to classify the suspect website. However, we are not interested to understand if it is a phishing site, since we already know it. We only exploit this tree to select those three nodes that positively contribute to determining it as phishing. In other words, we use it to filter the indicators that are more influential in the classification process.

After the selection of the three most important indicators, we dynamically create three panels that are visualized in the warning message and properly adapted if necessary. For example, if a panel has to report the information on the URL, it is customized with the URL of the suspect website and the URL of the Website that is mimicked.

Future work

Based on feedback received during the IFIP WG 13.2 & WG 13.5 Workshop on "Handling Security, Usability, User Experience and Reliability in User-Centered Development Processes" held in conjunction with INTERACT 2019, we are revising our proposal of intelligent warning interface.

A first suggestion we received is related to the graphical aspect of the interface, that should be able to attract user attention and communicate that it contains an alert. Actually, the interface shown at the workshop and reported in Fig. 2 is an early prototype and we are working to change its aspect.

We are also investigating how effective warning messages are constituted in terms of structure, aspect, content, and language. We started from the work in [START_REF] Felt | Improving SSL Warnings: Comprehension and Adherence[END_REF][START_REF] Wogalter | Research-based guidelines for warning design and evaluation[END_REF], whose authors examined various aspects of warning design and how they affect subjective evaluations, memory, comprehension, and behavioural compliance. In fact, there is at least some involvement of perceptual and cognitive processes when making decisions with respect to warnings (e.g., the level of risk involved, whether to comply). In particular, the Communication-Human Information Processing Model (C-HIP) model [START_REF] Wogalter | Research-based guidelines for warning design and evaluation[END_REF] is widely adopted in warning design. It accounts for cognitive-affective processes, such as attitudes and motivation, which are influenced by experience as well as characteristics of the source and the channel. Based on this model, the following requirements must be considered in designing warning messages:

 Salience: the warning must be noticeable. It should be conspicuous enough to draw user's attention to it. In [START_REF] Strawbridge | The Influence of Position, Highlighting, and Imbedding on Warning Effectiveness[END_REF][START_REF] Barlow | Alcohol Beverage Warnings in Print Advertisements[END_REF][START_REF] Strawbridge | The influence of position, highlighting, and imbedding on warning effectiveness[END_REF][START_REF] Barlow | Alcohol beverage warnings in print advertisements[END_REF] the authors showed that salient or conspicuous warnings increase the likelihood of reading, recall, comprehension and compliance.  Wordings: the warning must be formulated according to four components: (1) signal word to attract attention, (2) identification of the hazard, (3) explanation of consequences if exposed to hazard, (4) directives to return to safety/avoiding the hazard.  Layout: it is used to describe the internal characteristics of a warning label. Presenting warning text as bullets in outline form is preferred to continuous flowing text.  Technical Jargon: it should be avoided as much as possible. Except if your audience is technically savvy, it is good practice to avoid technical terms. It may be hard to totally avoid some of these terms entirely, but when you do use them, explain what they mean in the simplest way.  Transparency: when there is a chance of attack, communicate the potential risk, but be transparent about the uncertainty. Thus, use words like "suspect", "potential", "attackers could", etc.

We are designing a user study that will involve around 50 participants, representative of a large community of Internet users, both adult male and female with basic literacy level (i.e., able to use the Internet, read/write emails and perform other simple tasks with a computer). The study aims to evaluate the effectiveness of the proposed approach by comparing our intelligent warning messages to conventional phishing warning messages available in the current web browsers. As the attack has to be plausible. we plan to simulate a spear-phishing attack, which involves personalized emails or emails sent to a specifically targeted group, such as employees of a particular organization. In our case users, we will involve Internet users holding an uniba.it email account, namely people with an institutional account of the University of Bari.

Conclusion

In this paper, we discussed our early work on addressing phishing attack from an HCI perspective. The aim is proposing a mechanism that is able, more the currently available solutions, to attract the users' attention when they are going to fall in the attacker's trap.

Our proposal has also the goal of educating the users to more safe interaction behaviors, making them able to recognize the signs of the threat and aware of the risks. Indeed, we agree with [START_REF] Reeder | An Experience Sampling Study of User Reactions to Browser Warnings in the Field[END_REF] that users need to understand and use systems warnings correctly in order to guarantee the efficacy of any security strategy that has been implemented. AS reported in the future work section, we are currently working at the design of an improved version of the intelligent user interface that will be tested with end users.

Fig. 1 .

 1 Fig. 1. Example of a phishing attack sent by email.

https://safebrowsing.google.com

Acknowledgments. This work is partially supported by the Italian Ministry of University and Research (MIUR) under grant PRIN 2017 "EMPATHY: EMpowering People in deAling with internet of THings ecosYstems".


Morten Hertzum
email: hertzum@hum.ku.dk

Wellbeing at Work: Four Perspectives on What User Experiences with Artifacts May contribute

Keywords: Perspectives on artifact use, User experience, Wellbeing, Work

Most work involves the use of artifacts; thus, user experience (UX) is a factor in how most employees experience their work. This study revisits the tool, media, dialogue-partner, and system perspectives on artifact use to explore how UX may contribute to wellbeing at work.It is found that artifacts foster positive UX when they lend the user expressive power (tool), are transparent (media) or perceptive (dialogue partner).They foster negative UX when they break the user's task focus or make the user a mere system component. These findings are discussed and refined by elaborating the classic concepts of ready to hand and present at hand.

Introduction

Wellbeing at work [START_REF]Wellbeing: A complete reference guide. Volume III: Work and wellbeing[END_REF][START_REF]The Routledge companion to wellbeing at work[END_REF]is a major concern for employees as well as organizations because it is central to employees' mental and physical health and because it influences their productivity. Fisher [START_REF] Fisher | Conceptualizing and measuring wellbeing at work[END_REF]conceptualizes wellbeing at work as consisting of hedonic wellbeing (e.g., job satisfaction and positive affect), eudaimonic wellbeing (e.g., engagement, meaning, and intrinsic motivation), and social wellbeing (e.g., quality connections, satisfaction with coworkers, and social support). This conceptualization makes it apparent that avast array of concrete organizational circumstances enter into shaping wellbeing at work. One of them is theemployees' use of artifacts.

In the research community of human-computer interaction,the experiences associated with the use of artifacts are discussed under the rubric of user experience (UX). While multiple UX definitions have been proposed, they share a focus on the experiences associated with artifact use. For example, Roto et al. [18, p. 6] state that "UX is a subset of experience as a general concept. UX is more specific, since it is related to the experiences of using a system." Some definitions restrict UX to actual system use [START_REF] Colbert | User experience of communication before and during rendezvous: Interim results[END_REF], others include anticipated use [START_REF]ISO 9241: Ergonomics of human-system interaction -Part 210: Human-centred design for interactive systems[END_REF], and still others also include aesthetics [START_REF] Hekkert | Design aesthetics: Principles of pleasure in product design[END_REF].These differences appear, however, minor compared to the shared focus on the experiences associated with artifact use. Well-documented experiences with computer artifacts in work settings include burnout, deskilling, frustration, and helplessness [e.g., [START_REF] Gawande | The upgrade: Why doctors hate their computers[END_REF][START_REF] Lazar | Workplace user frustration with computers: An exploratory investigation of the causes and severity[END_REF]. Countering such negative experiences is central to employee wellbeing. Replacing them with positive experiences would be an even nobler design goal.

In one of the relatively few studies of UX in a work context, Meneweger et al. [START_REF] Meneweger | Factory workers' ordinary user experiences: An overlooked perspective[END_REF]show that ordinary user experiences dominate in factory employees' interactions with technology. The interactions are generally mundane, unremarkable, and shaped by routine activities. At the same time, studies of technology acceptance find that perceived enjoyment, a concept similar to UX, predicts the intention to use an artifact as strongly as do perceived usefulness and perceived ease of use [START_REF] Hornbaek | Technology acceptance and user experience: A review of the experiential component in HCI[END_REF]. While enjoyment contributes to hedonic wellbeing, usefulness contributes to eudaimonic wellbeing; ease of use facilitates both hedonic and eudaimonic wellbeing by reducing the effort that must be expended to obtain them. Thus, replacing ordinary user experiences with positive user experiences stands to improve employees' attitude toward the artifacts they use as well as to improve their wellbeing at work.The relation between the artifact and the user's experience is, however, complex because UX is not determined by the artifact alone. Rather, UX results from the interrelations among the characteristics of the artifact, user, task, and context of use.

While it is a largely trivial observation that UX results from the interrelations among the artifact, user, task, and context of use, it raises the question of whether artifacts as such exert much influence on wellbeing at work. It may well be that wellbeing at work is first and foremost driven byother factors, such as the task content, division of labor, physical work conditions, psychosocial climate, and decision-making influence. These factors are not directly about artifacts and, thereby, not directly about UX.To explore what we might accomplish by designing for good UX at work this study revisits Kammersgaard's [START_REF] Kammersgaard | Four different perspectives on human-computer interaction[END_REF]four perspectives on humancomputer interaction, ponders what constitutes positive and negative UX within each perspective, and discusses the possible contributions of UX to wellbeing at work.

2

Four Perspectives on System Use and UX Kammersgaard [15] outlines four perspectives on human-computer interaction by distinguishing between artifacts for individual and collaborative use and between artifacts for which agency rests with the user and artifacts that split agency between user and artifact. The four perspectives are the tool perspective, the system perspective, the dialogue-partner perspective, and the media perspective, see Table 1.

The tool perspective has its roots in craftwork and emphasizes that in the hands of a skilled user the tool is a seamless extension of the user, who attends to her task rather than to the tool: When hammering the skilled user's attention is on driving the nail, not on the hammer. Conceptually, the tool is said to be ready to hand [START_REF] Heidegger | Being and time[END_REF]. It is only upon breakdowns that the tool becomes the focus of the user's attentionbecomes present at hand. If the hammer is too light for the size of nail or otherwise inadequate for the task then the user's attention shifts from the task to the tool. These shifts are associated with frustration and other negative emotions because the breakdown thwarts progress on the task, at least temporarily. It appears that tools foster good experiences when they are out of mindready to handand poor experiences when they become present at hand. If we take the focus on artifact use in the UX definitions to mean that the user must, in the moment, be conscious that she is using an artifact then the tool perspective rules out positive UX. The positive experiences do not qualify as UX because they are associated with an uninterrupted focus on the task (note that this point will be modified in Section 3). In contrast, the user is conscious of the artifact when it thwarts task progress; thus tools can foster negative UX. If we do not require that the user must, in the moment, be conscious that she is using an artifactand this is probably the more sensible optionthen positive UX is possible within the tool perspective and consists of designing for readiness to hand. The user may however not attribute the positive UX to the tool but, partly or wholly, to other aspects of the use situation.

Table 1. Four perspectives on system use, adapted from Kammersgaard [START_REF] Kammersgaard | Four different perspectives on human-computer interaction[END_REF]. The system perspectivealigns with industrial perceptions of work and promotes a view in which a system consists of components that may be human or automated. Each component is characterized by the input it receives, the activities it performs on those inputs, and the outputs it delivers. The division of the system into components is made by management and defines a division of labor. To perform their work the users need only know the characteristics of the component they embody. Performance is measured by how cheaply, quickly, and consistently the components deliver their outputs. That is, the users' work is measured in the same way as that of the automated components. If the users perform poorer than an automated version of the same component then the users are at risk of being replaced by such an automated component. In this sense the users are measured by their ability to function as automated components.The automation inherent in the system perspective is often associated with deskilling of the users, who become operators of machines that perform more and more components of the work [START_REF] Braverman | Labor and monopoly capital: The degradation of work in the twentieth century[END_REF]. This negative UX results from a primary focus on automation, thereby leaving the users with the components that have not yet been automated. To create positive UX it is necessary to focus on creating meaningful and rewarding human components, for example by automating the parts of work that are monotonous or unhealthy. However, to create meaningful and rewarding human components it may also be necessary to reconsider the separation between a managerial level that defines the components and an operational level that merely performs according to these preset definitions. That is, it may be necessary to challenge the essence of the system perspective.

Individual

The dialogue-partner perspective sees the artifact as an intelligent assistant with which the users can interact in much the same way as they interact with humans.The intelligent assistant empowers the user by serving his or her needs and does so without requiring that the user learns special commands for interacting with the assistant. Unlike the system perspective, which tends to reduce humans to machines, the dialogue-partner perspective seeks to elevate machines to human-like performance.Unlike the tool perspective, which involves the user's moment-tomoment handling of the tool, the intelligent assistant acts autonomously in the user's service.The intelligent assistant may, for example, monitor an architect's work on a building and inform the architect when his current building design violates formal regulations or recognized principles for good design [START_REF] Fischer | Critics: An emerging approach to knowledgebased human-computer interaction[END_REF].The intelligent assistant fosters positive and negative UX in much the same way as a human collaborator. Negative UX ensues if the assistant needs too many instructions,performs poor work, or delivers its work at inopportune moments. Positive UX ensues if the assistant is effective and efficient and, especially, if the assistant also picks up on the tacit conditions for good performance and reacts appropriately to dynamic changes in the environment.Often, intelligent assistants must be supervised by users who need to be ready to take over in situations the assistant cannot handle. This creates poor conditions for positive UX because the user wants to offload the task to the assistant but must, instead, "stay in the loop" to be ready to step in whenever needed.

The media perspectivepositions the artifact as a medium through which the users interact with each other. That is, the medium is merely a channel; agency rests with the users.Rich media [START_REF] Trevino | Media symbolism, media richness, and media choice in organizations: A symbolic interactionist perspective[END_REF]provide for simultaneous interactions in multiple modalities and, thereby, for back-channeling (e.g., nods and raised eyebrows) to occur via some modalities at the same time as the main interaction occupies other modalities (e.g., speech). Thereby, rich media support users in establishing, sustaining, and repairing common ground, which is key to effective collaborative interactions.Conversely, lean media provide few or only a single modality and may be restricted to asynchronous interactions, thereby increasing the risk of breakdowns in common ground. Media provide positive UX when they are transparentsomewhat similar to when a tool is ready to hand. A transparent medium allows the interactions among the users to flow without distortions. Rich media are transparent with respect to more interaction modalities than lean media. In addition to transparency, some media aim to provide positive UX by structuring the interaction, for example by making explicit that an interactional turn is a request and therefore must be answered by accepting, declining or negotiating the request [START_REF] Winograd | Understanding computers and cognition: A new foundation for design[END_REF]. Media foster negative UX when they are insufficiently transparent or enforce a structure that is too rigid. In both cases the medium gets in the way of the interactions among the users.

Discussion

Most work involves the use of artifacts, such as products, systems or services. Thus, UX is a factor inhow most employees experience their work. In the tool and media perspectives,an artifactfosters positive UX by not attracting the user's attention, which instead remains on the task.That is, it is by supporting the user in expressing her skillsas manifested in high-quality work task productsthat tools and media foster positive UX. Seen from these perspectives positive UX is about lending the userexpressive power.In the dialogue-partner perspective, positive UX is as much about how well the artifact engages in the process of its use as it is about the product that results from this process. That is, an artifact fosters positive UX if it is a perceptive and adaptive dialogue partner. In the system perspective, positive UX appears to be secondary to other concerns. That is, to foster positive UX it is necessary to abandon the system perspective or, at least, supplement it with other perspectives.Abandoning the system perspective is a daunting undertaking because this perspective permeates much thinking about how to organize workplaces. For example, physicians are increasingly frustrated that they spend still more of their time documenting their work in electronic patient records and comparatively less time with patients, but the increasing documentation requirements are justified by pointing out that the physician is a component in a much larger system, which needs the documentation for hospital-level quality assurance, national performance indicators, and international healthcare research [START_REF] Gawande | The upgrade: Why doctors hate their computers[END_REF]. This system-perspective thinking is, however, creating frustration and burnout among the physicians because it disregards the personal level from which they experience the electronic patient records.

A less ambitious goal than fostering positive UX in the service of wellbeing at work would be to avoid negative UX. The tool and media perspectives agree that artifacts foster negative UX whenever they attract the user's attention. Thus, users become conscious of their artifact use when they experience problems with the artifacts. The distinction between, on the one hand, positive UX and a task focus and, on the other hand, negative UX and an artifact focus largely stems from the distinction between the concepts of ready to hand and present at hand.Recently, Verbeek [START_REF] Verbeek | What things do: Philosophical reflections on technology, agency, and design[END_REF] has proposed that artifacts need not be either ready to hand or present at hand, but can be both at the same time. He illustrates this possibility by considering the difference between a CD player and a piano [20, p. 194

]:

Someone who plays the piano is directed toward the music and at the same time is substantially involved with the piano itself. When the same piece is played on a CD player, the artifact that mediates between the person and the music is present in an entirely different way. The machinery of a CD player disappears into the background, withdrawing so that people are only engaged with the music and not with its means of production. A piano, however, is never entirely ready-to-hand, but neither is it exclusively present-at-handits machinery is not completely in the background, but not entirely in the foreground either.

In this example the CD player functions as the perfect assistant, to which the task of playing the music can be completely offloaded. In contrast, the piano requires that the human stays in the loop and operates the piano on a moment-to-moment basis.By proposing that pure readiness to hand is only achieved with complete offloading (i.e., with full automation),Verbeek [START_REF] Verbeek | What things do: Philosophical reflections on technology, agency, and design[END_REF]proposes that pure readiness to hand implies that the human is out of the loop. This proposal fundamentally reconceptualizes readiness to hand by dissociating it from skilled human performance. According to Verbeek [START_REF] Verbeek | What things do: Philosophical reflections on technology, agency, and design[END_REF], skilled human performance instead involves that the artifact is simultaneously ready to hand and present at handlike a piano or a hammer.

Verbeek [START_REF] Verbeek | What things do: Philosophical reflections on technology, agency, and design[END_REF]also contends that a purelypresent-to-hand artifact does not necessarily indicate a negatively experienced breakdown in the use of the artifact; it may also indicate that the user is absorbed in cherishing the artifact rather than in using it as a means to an end.This contention accords with many UX studies of users' experiences of the aesthetic and other non-instrumental qualities of their possessions [e.g., [START_REF] Hassenzahl | User experience -A research agenda[END_REF][START_REF] Jordan | Human factors for pleasure in product use[END_REF].Cherishing may to some extent be about possessing the artifact and, thereby, not fully applicable to artifacts that are operated by the user but owned by the workplace. Yet, a sizable number of people have work tools available in a near permanent manner that approaches ownership.

Conclusion

Verbeek's [START_REF] Verbeek | What things do: Philosophical reflections on technology, agency, and design[END_REF]elaboration of the concepts of ready to hand and present at hand offers a way of restructuring and simplifying the insights from the analysis of the four perspectives on how UX may contribute to wellbeing at work, see Table 2. This restructuring leads to three conclusions: First, when artifacts are present at hand the users focus on the artifact rather than their tasks. While this artifact focus may be associated with positive, artifactcherishing experiences, it is often triggered by breakdowns in artifact use. To contribute to wellbeing at work artifacts must be designed so that it is easy to restore their functioning after a breakdown.

Second, when artifacts are simultaneously present at hand and ready to hand the users are conscious that they are using an artifact but their focus is on their task. Following the tool and media perspectives,artifacts that enable a task focus foster positive UX by lending the users expressive power, by being transparent and, possibly, by structuring the interaction.Importantly, Verbeek's [START_REF] Verbeek | What things do: Philosophical reflections on technology, agency, and design[END_REF] reconceptualization does away with the question of whether positive UX is possible within the tool perspective. Third, when artifacts are ready to hand they autonomously produce outputs for the users to consume. This consumption focus is experienced positively when the artifact serves the user as a perceptive and adaptive assistant and negatively when the user must abide the system. Thus, the dialogue-partner perspective may foster positive UX through ready-to-hand artifacts, while the system perspective fosters negative UX by leaving the user out of not just the activity loop but also the decision loop.

It should be noted, in closing, that the four perspectives revisited in this study are exclusively about post-design experiences with artifacts. Neither the four perspectives, nor the concepts of ready to hand and present at hand, concern themselves with how artifacts are designed. Yet, user influence on the design of the artifacts employed in performing work tasks may bean additional UX contribution to wellbeing at work [START_REF] Hertzum | How do users perceive a design-in-use approach to implementation? A healthcare case[END_REF].

Table 2 .

 2 What may UX contribute to wellbeing at work?

	Artifact is…	Focus	UX contribution to wellbeing at work
	Present at hand	Artifact	 Breakdown in use (negative UX)
			 Cherishing the artifact (positive UX)
	Both present at hand	Task	 Expressive power, transparency, and possibly
	and ready to hand		structure (positive UX)
	Ready to hand	Consumption	 Perceptive and adaptive assistant (positive UX)
			 Deskilling and monotony (negative UX)