Antibacterial Activity of Amidodithiophosphonato Nickel(II) Complexes: An Experimental and Theoretical Approach

The reactions of 2,4-bis(4-methoxyphenyl)-1,3-dithio-2,4-diphosphetane-2,4-disulfide (Lawesson’s Reagent, LR) with benzylamine (BzNH2) and 4-phenylbutylamine (PhBuNH2) yield benzylammonium P-(4-methoxyphenyl)-N-benzyl-amidodithiophosphonate (BzNH3)(BzNH-adtp) and 4-phenylbutylammonium P-(4-methoxyphenyl)-N-(4-phenylbutyl)-amidodithiophosphonate (PhBuNH3)(PhBuNH-adtp). The relevant nickel complexes [Ni(BzNH-adtp)2] and [Ni(PhBuNH-adtp)2] and the corresponding hydrolysed derivatives (BzNH3)2[Ni(dtp)2] and (PhBuNH3)2[Ni(dtp)2] were prepared and fully characterized. The antimicrobial activity of the aforementioned amidodithiophosphonates against a set of Gram-positive and Gram-negative pathogen bacteria was evaluated, and [Ni(BzNH-adtp)2] and [Ni(PhBuNH-adtp)2] showed antiproliferative activity towards Staphylococcus aureus and Staphylococcus haemolyticus strains. density functional theory (DFT) calculations were performed to shed some light on the activity of reported compounds related to their tendency towards P–N bond cleavage.


Introduction
Phosphorus-1,1-dithiolates such as dithiophosphates, dithiophosphinates, dithiophosphonates, and amidodithiophosphonates (I, II, III and IV, respectively, see Scheme 1), are important classes of sulfur-donor anionic ligands that display a multiplicity of coordination patterns with transition metal ions and main group elements [1][2][3]. The PS 2 − moiety can coordinate in monodentate, bidentate (with either symmetric or asymmetric bonding), and polydentate modes. A huge variety of both discrete and polymeric structures are prevalent in the literature [4]. Since the early 1960s, dithiophosphates and dithiophosphinates gained increasing importance due to their applications as pesticides and extracting agents in mineral ores, and a large amount of information describing the reactivity of compounds I and II was reported [5]. On the contrary, due to synthetic difficulties, the chemistry of dithiophosphonate The tendency to undergo cleavage of the P-N bond to give the corresponding dithiophosphonic acid may explain the low occurrence of structurally characterized amidodithiophosphonates ( Figure  1). The majority of known structures are in an anionic form, with the released protonated amine acting as a counterion [7][8][9][10]. Some examples are also reported where the complete hydrolysis of P-N and P─S bonds in amidodithiophosphonates yields phosphonates, with a concurrent loss of the amine and hydrogen sulphide [11,12].
Recently, the antiproliferative and antibacterial activity of this class of compounds was reported to be related to the slow release of H2S [11,12]. This aspect is of particular interest as antimicrobial resistance is becoming one of the principal public health problems of the 21st century [13][14][15]. In the search for novel antimicrobial agents, coordination compounds containing transition metal ions represent a promising avenue for drug development [16,17]. Complexes of metals such as Au, Ir, Co, and Cu have demonstrated an excellent activity against aerobic Gram-positive pathogenic bacteria, such as Staphylococcus spp. [17][18][19][20]. This bacterial group, in particular S. aureus, can cause many forms The tendency to undergo cleavage of the P-N bond to give the corresponding dithiophosphonic acid may explain the low occurrence of structurally characterized amidodithiophosphonates ( Figure 1). The majority of known structures are in an anionic form, with the released protonated amine acting as a counterion [7][8][9][10].
Molecules 2020, 25, x 2 of 18 compounds I and II was reported [5]. On the contrary, due to synthetic difficulties, the chemistry of dithiophosphonate complexes (III) became relevant only after the turn of the new century, with the development of a novel synthetic route starting from 1,3-dithiadiphosphetane-2,4-disulfides (such as Lawesson's Reagent) [3,6]. A similar synthetic route was used to prepare amidodithiophosphonates (IV in Scheme 1), a class of phosphorus-1,1-dithiolates, featuring a P-N bond, that still remains largely unexplored.
The tendency to undergo cleavage of the P-N bond to give the corresponding dithiophosphonic acid may explain the low occurrence of structurally characterized amidodithiophosphonates ( Figure  1). The majority of known structures are in an anionic form, with the released protonated amine acting as a counterion [7][8][9][10]. Some examples are also reported where the complete hydrolysis of P-N and P─S bonds in amidodithiophosphonates yields phosphonates, with a concurrent loss of the amine and hydrogen sulphide [11,12].
Recently, the antiproliferative and antibacterial activity of this class of compounds was reported to be related to the slow release of H2S [11,12]. This aspect is of particular interest as antimicrobial resistance is becoming one of the principal public health problems of the 21st century [13][14][15]. In the search for novel antimicrobial agents, coordination compounds containing transition metal ions represent a promising avenue for drug development [16,17]. Complexes of metals such as Au, Ir, Co, and Cu have demonstrated an excellent activity against aerobic Gram-positive pathogenic bacteria, such as Staphylococcus spp. [17][18][19][20]. This bacterial group, in particular S. aureus, can cause many forms Some examples are also reported where the complete hydrolysis of P-N and P-S bonds in amidodithiophosphonates yields phosphonates, with a concurrent loss of the amine and hydrogen sulphide [11,12].
Recently, the antiproliferative and antibacterial activity of this class of compounds was reported to be related to the slow release of H 2 S [11,12]. This aspect is of particular interest as antimicrobial resistance is becoming one of the principal public health problems of the 21st century [13][14][15]. In the search for novel antimicrobial agents, coordination compounds containing transition metal ions represent a promising avenue for drug development [16,17]. Complexes of metals such as Au, Ir, Co, and Cu have demonstrated an excellent activity against aerobic Gram-positive pathogenic bacteria, such as Staphylococcus spp. [17][18][19][20]. This bacterial group, in particular S. aureus, can cause many forms of infections in different organs and is one of the major causes of nosocomial infections of surgical wounds and in indwelling medical devices [21,22]. Methicillin-resistant S. aureus (MRSA) is solely responsible for many life-threatening nosocomial infections in humans, causing an increase both in the treatment duration and medical costs [23]. The problem of resistance is amplified by the ability of S. aureus to form biofilms on biotic and abiotic surfaces and is of particular concern with several implanted medical devices [24][25][26]. Bacteria in these biofilms are stubbornly difficult to treat because such microbial aggregates are traditionally considered impervious to drug diffusion [27,28].
Given the scarcity of data reported on square-planar complexes of d 8 metal ions with potential antimycotic and antimicrobial activity [3,11,12,18,29,30] and the different hydrolytic products described [7][8][9][10][11][12], we report here the synthesis, characterization and activity (against a set of Gram-positive and Gram-negative pathogenic bacteria), of the novel benzylammonium P-(4methoxyphenyl)-N-benzyl-amidodithiophosphonate (BzNH 3 )(BzNH-adtp), 4-phenylbutylammonium P-(4-methoxyphenyl)-N-(4-phenylbutyl)-amidodithiophosphonate ( of infections in different organs and is one of the major causes of nosocomial infections of surgical wounds and in indwelling medical devices [21,22]. Methicillin-resistant S. aureus (MRSA) is solely responsible for many life-threatening nosocomial infections in humans, causing an increase both in the treatment duration and medical costs [23]. The problem of resistance is amplified by the ability of S. aureus to form biofilms on biotic and abiotic surfaces and is of particular concern with several implanted medical devices [24][25][26]. Bacteria in these biofilms are stubbornly difficult to treat because such microbial aggregates are traditionally considered impervious to drug diffusion [27,28]. Given the scarcity of data reported on square-planar complexes of d 8 metal ions with potential antimycotic and antimicrobial activity [3,11,12,18,29,30] and the different hydrolytic products described [7][8][9][10][11][12], we report here the synthesis, characterization and activity (against a set of Grampositive and Gram-negative pathogenic bacteria), of the novel benzylammonium P-(4-
The reaction of the amidodithiophosphonate salts with NiCl 2 ·6H 2 O afforded the corresponding nickel(II) complexes [Ni( i PrNH-adtp) 2 ], [Ni(BzNH-adtp) 2 ] and [Ni(PhBuNH-adtp) 2 ] (Scheme 2), as purple solids. The obtained compounds were fully characterized by elemental analysis, m.p. determination, FT-IR and 1 H and 31 P{ 1 H} NMR spectroscopies, confirming their nature of amidodithiophosphonato nickel complexes. The FT-IR spectra of the compounds (Figures S12 and S15) show the N-H stretching vibration as a strong single peak falling at around 3250 cm −1 , and the bands for asymmetric and symmetric P-S stretching modes are found, as expected, around 660 and 560 cm −1 , respectively [29]. Due to the peak broadening encountered in DMSO-d 6 Figure S4) shows the signal that can be assigned to CH 2 protons overlapped with the water residue; the signals at 2.62, and 3.17-2.99 ppm can be attributed to the protons of the aliphatic chain of the amine, and the broad signal at around 2.91 ppm can be assigned to the NH proton, similar to that found for (BzNH 3 )(BzNH-adtp) and [Ni(BzNH-adtp) 2 ]; the singlet at 3.86 ppm is attributed to the protons of the OCH 3 group. The protons of the aromatic portion display signals at 7.90 and 6.97 ppm, assigned to the methoxyphenyl P-substituent, and signals between 7.3-7.1 ppm can be assigned to the aromatic protons of the amine.  2 ], mainly regarding the melting points and the IR and NMR signals relative to the ammonium -NH 3 + groups (Figures S9, S10 and S13-S16).
[Ni(BzNH-adtp)2]; the singlet at 3.86 ppm is attributed to the protons of the OCH3 group. The protons of the aromatic portion display signals at 7.90 and 6.97 ppm, assigned to the methoxyphenyl Psubstituent, and signals between 7.3-7.1 ppm can be assigned to the aromatic protons of the amine.   The asymmetric unit of the (BzNH 3 ) 2 [Ni(dtp) 2 ]·2H 2 O comprises half a molecule, with a Ni II ion lying about a crystallographic inversion center, one dithiophosphonato ligand (ArPOS 2 ) − , one benzylammonium cation, and one water molecule. The metal center is tetracoordinated in a square-planar geometry by four sulphur atoms belonging to two isobidentate ligands with Ni-S bond lengths of 2.2255(4) and 2.2232(4) Å, respectively, and an S-Ni-S angle of 87.92(2) • . The P-S1 and P-S2 bond lengths show very similar values [2.0409(5) and 2.0387(5) Å respectively], indicating an electron delocalization over the whole PS 2 − fragment and an S-P-S angle of 98.39(2) • ; the P-O bond exhibits a length of 1.5094(10) Å (Table 1).
The structure of (PhBuNH 3 ) 2 [Ni(dtp) 2 ] ( Figure 3) displays four nickel metal centers in the asymmetric unit, each sitting on a special position with its occupancy necessarily set to 0.5. One half of each metal complex is crystallographically unique, with the other chelated ligand being generated through the symmetry of the space group. There are also four crystallographically unique protonated organic amine counter ions, three of which are disordered ( Figure S6). The nickel coordination closely resembles that found in (BzNH 3 ) 2 [Ni(dtp) 2 ]·2H 2 O (see above) with similar bond lengths and angles (Table 1)  It is interesting to note that the bond lengths and angles of the ─POS2 moiety are comparable with those found in similar neutral and anionic dithiophosphonato nickel complexes described as bearing either a P=O or P─OH bond [7][8][9][10][33][34][35][36][37][38][39][40]. It is therefore very difficult (from comparison of the crystallographic P─O and/or P─S bond lengths alone) [7][8][9][10][33][34][35][36][37][38][39][40], to discriminate between purely single and double P─O bonds in this class of compounds, or to confidently assign the negative charge on either the oxygen or on the sulphur atoms. Additionally, the [ArPOS2] -fragments are often engaged in strong H-bonds with counterions, which affects the bond lengths between the atoms involved. A better understanding of the nature of the P─O bond (and its charge distribution) may be gleaned if the bond lengths involved in the ─POS2 fragment are considered together. The correlation reported in Figure 4 suggests that purely double P=O bonds may only be found on P(O)S2 fragments not directly bearing a negative charge (empty green circles, CCDC ref-codes IDUNEC, NEYLUA, NEYMAH, and YABQEZ) [7][8][9][10][33][34][35][36][37][38][39][40]. Similarly, pure single P─O bonds are detected in neutral Oalkyl-dithiophosphonates (III in Scheme 1, yellow circles in Figure 4). All the fragments bearing a negative charge fall in the same area, notwithstanding the attributions, as single P─OH (blue squares) or double P=O bonds (full green circles) reported for the deposited structures. Compounds (BzNH3)2[Ni(dtp)2]·2H2O (black triangle in Figure 4) and (PhBuNH3)2[Ni(dtp)2] (red triangle in Figure  4) lie in the same region as the anionic fragments. It is worth noting the three blue squares lying in the same region as the yellow dots: these data refer to structures (IKOSUX and LIFGAJ) [10,[33][34][35][36][37][38][39][40] containing single P─OH bonds in neutral fragments, thus confirming the proposed correlation. It is interesting to note that the bond lengths and angles of the -POS 2 moiety are comparable with those found in similar neutral and anionic dithiophosphonato nickel complexes described as bearing either a P=O or P-OH bond [7][8][9][10][33][34][35][36][37][38][39][40]. It is therefore very difficult (from comparison of the crystallographic P-O and/or P-S bond lengths alone) [7][8][9][10][33][34][35][36][37][38][39][40], to discriminate between purely single and double P-O bonds in this class of compounds, or to confidently assign the negative charge on either the oxygen or on the sulphur atoms. Additionally, the [ArPOS 2 ] − fragments are often engaged in strong H-bonds with counterions, which affects the bond lengths between the atoms involved. A better understanding of the nature of the P-O bond (and its charge distribution) may be gleaned if the bond lengths involved in the -POS 2 fragment are considered together. The correlation reported in Figure 4 suggests that purely double P=O bonds may only be found on P(O)S 2 fragments not directly bearing a negative charge (empty green circles, CCDC ref-codes IDUNEC, NEYLUA, NEYMAH, and YABQEZ) [7][8][9][10][33][34][35][36][37][38][39][40]. Similarly, pure single P-O bonds are detected in neutral O-alkyl-dithiophosphonates (III in Scheme 1, yellow circles in Figure 4). All the fragments bearing a negative charge fall in the same area, notwithstanding the attributions, as single P-OH (blue squares) or double P=O bonds (full green circles) reported for the deposited structures. Figure 4) and (PhBuNH 3 ) 2 [Ni(dtp) 2 ] (red triangle in Figure 4) lie in the same region as the anionic fragments. It is worth noting the three blue squares lying in the same region as the yellow dots: these data refer to structures (IKOSUX and LIFGAJ) [10,[33][34][35][36][37][38][39][40] Table 1, Tables S1 and S4) containing the bis(4-methoxyphenyl)tetrathiodiphosphonate anion [(ArPS 2 ) 2 O] 2− counterbalanced by two 4-phenylbutylammonium cations. The structure was solved in the space group P-1, and the main structural and refinement parameters are reported as Supporting Information (Tables S1 and S4, Figure S7). There is a large extent of disorder in the crystal. The disordered atoms were modelled and refined over two or four positions using a combination of thermal and geometric parameter restraints and/or constraints where necessary (see Experimental).

Antibacterial Activity
A set of different tests were performed to evaluate the antimicrobial activity of amidodithiophosphonate salts   In this context, microbial species described as being commensals or pathogens in humans were assayed, namely Staphylococcus aureus, Staphylococcus haemolyticus, Escherichia coli, and two strains of Pseudomonas aeruginosa, PA-01 and PA-02, that showed a different susceptibility pattern to disinfectants [42]. In addition, three different clinical isolates of Candida spp. were assayed, namely Candida albicans, Candida kruseii, and Candida glabrata. The antimicrobial activity of the ligand salts and nickel complexes was measured by the Agar diffusion method against the mentioned strains. These tests revealed that, while none of the tested Gram-negative bacteria or fungi were sensitive to any of the compounds examined, complexes [Ni(BzNH-adtp) 2 ] and [Ni(PhBuNH-adtp) 2 ] are active against both Staphylococcus spp. (Figure S8 in Supplementary Material). In particular, a growth inhibition (Ø) of 12 and 8 mm was exerted on S. aureus and of 17 and 15 mm against S. haemolyticus by [Ni(BzNH-adtp) 2 ] and [Ni(PhBuNH-adtp) 2 ], respectively ( Figure S8 in Supplementary Material). The fact that there was activity against Gram-positive bacteria and not against Gram-negative may be related to the increased difficulty of these compounds to penetrate the cell wall of the Gram-negatives [43][44][45]. Notably, no inhibitory activity towards S. aureus and S. haemolyticus was observed for the complexes' ligand precursors (BzNH 3 )(BzNH-adtp), (PhBuNH 3 )(PhBuNH-adtp), and NiCl 2 ·6H 2 O, showing that the coordination compounds are responsible for the antimicrobial activity. In contrast, the inability of [Ni( i PrNH-adtp) 2 ] to inhibit bacterial growth suggests that the tendency towards hydrolysis of the complexes [Ni(BzNH-adtp) 2 ] and [Ni(PhBuNH-adtp) 2 ] could play an important role in their antimicrobial activity. The tendency to hydrolysis could be tentatively ascribed to the different nature of the alkyl/aryl amine substituents, as evidenced by the slight elongation of the P-N bond on passing from [Ni( i PrNH-adtp) 2 ] to [Ni(PhEtNH-adtp) 2 ] (1.619(5) and 1.641(4) Å, respectively) [3,31,32].

Minimum inhibitory concentration (MIC) represents the lowest concentration of an antimicrobial that inhibits the visible growth of a microorganism after an appropriate incubation time. Evaluating the MIC confirmed the activity against Staphylococci.
Both [Ni(BzNH-adtp) 2 ] and [Ni(PhBuNH-adtp) 2 ] were observed to inhibit the growth of S. aureus up to just a 2-fold dilution of the stock solution (200 µg/mL; 5.95·10 −4 and 5.00·10 −4 M respectively). S. haemolyticus showed a MIC up to a 32-fold dilution in the case of [Ni(BzNH-adtp) 2 ], corresponding to 6.25 µg/mL (1.56·10 −5 M). In contrast, [Ni(PhBuNH-adtp) 2 ] lost the ability to inhibit the bacterial growth after dilution (MIC > 100 µg/mL). Moreover, the bactericidal activity was assessed by evaluating the minimum bactericidal concentration (MBC), i.e., the lowest concentration of an antimicrobial required to kill a particular bacterium life in suspension (planktonic status). This approach is established when the substance under investigation can inactivate bacterial contamination in a fluid, such as water, saliva and urine. Neither [Ni(BzNH-adtp) 2 ] nor [Ni(PhBuNH-adtp) 2 ] showed any bactericidal activity (MBC > 100 µg/mL; Table 2) against these strains. Microorganisms living within a structured biofilm cause many human infections. Such a sessile structure is generally more resistant to various antimicrobial treatments [28]. For this reason, we measured the influence of complexes [Ni(BzNH-adtp) 2 ] and [Ni(PhBuNH-adtp) 2 ] on biofilm formation by evaluating the minimum concentration required to inhibit the formation of the biofilm in vitro, i.e., the minimum biofilm inhibitory concentration (MBIC). Both complexes showed the ability to inhibit the biofilm growth, however they required relatively high concentrations in the case of S. aureus (MBIC = 100 µg/mL).  (Tables S6 and S7).

DFT Calculations
Following recent studies on different complexes featuring chalcogen donors [18,[46][47][48][49], the electronic structures of salts ( i PrNH 3 )( i PrNH-adtp), (BzNH 3 )(BzNH-adtp), and (PhBuNH 3 )(PhBuNH-adtp), and the corresponding Ni II complexes were investigated by theoretical calculations carried out at the density functional theory (DFT) [50] in order to theorize why complexes [Ni(BzNH-adtp) 2 ] and [Ni(PhBuNH-adtp) 2 ] undergo hydrolysis and try to explain the different antimicrobial activity determined between these complexes and the analogous [Ni( i PrNH-adtp) 2 ] and the free amidodithiophosphonate salts. DFT calculations were carried out on the starting amines i PrNH 2 , BzNH 2 , PhBuNH 2 , the corresponding ammonium cations, the relevant amidodithiophosphonate anions  Tables 1 and 2). This hypothesis would also be consistent with the lack of antibacterial activity determined for [Ni( i PrNH-adtp) 2 ], which has proved experimentally to be less prone to hydrolysis, as confirmed by the stability in solution and isolation in the solid state [3]. It is interesting to note that no virtual MOs (antibonding with respect to the P-S bonds) can be found at energies close to, or lower than, those of the aforementioned antibonding P-N MOs, either in the free R-adtp − anions or in the corresponding [Ni(R-adtp) 2 ] complexes. This indicates that hydrolysis of the compounds should be expected to occur through P-N bond breaking and dithiophosphonate anion formation, causing some doubt regarding the previously hypothesized emission of dihydrogen sulfide as the first step of the hydrolysis [51]. The hydrolysis of P-N was also confirmed by the isolation of a few crystals of the 4-phenylbutylammonium salt of bis(4-methoxyphenyl)-tetrathiodiphosphonate (PhBuNH 3 ) 2 [(ArPS 2 ) 2 O] ( Figure 5) during an attempt at crystallizing (PhBuNH 3 )(PhBuNH-adtp). A similar salt was hypothesized as the intermediate in the in situ formation of a mixed cymene-ferrocenylphosphonodithiolate ruthenium complex, obtained by the hydrolysis of 2,4-diferrocenyl-1,3-dithiadiphosphetane 2,4-disulfide in the presence of ammonium hydroxide [35].

Materials and Methods
Starting materials and solvents were purchased from commercial sources TCI (Tokio, Japan) and Aldrich (Darmstadt, Germany) and, when necessary, the solvents have been distilled and dried according to the standard literature techniques. Melting point measurements were determined in capillaries, using melting point apparatus BUCHI M-560 (30-240 • C, Flawil, Svizzera). Elemental analyses were performed with an EA1108 CHNS-O Fisons instrument (Thermo Fisons, Okehampton, EX20 1UB, UK). 1 H and 31 P NMR measurements were carried out at 25 • C using a Bruker Avance 300 MHz (7.05 T, Billerica, MA, USA) spectrometer at operating frequencies of 300.13 and 121.49 MHz, respectively. Chemical shifts for 1 H-NMR are reported in parts per million (ppm), calibrated to the residual solvent peak set, with coupling constants J reported in Hertz (Hz). Chemical shifts for 31 P NMR are reported in parts per million (ppm), calibrated to the external reference TPP 48.5 mM in acetone-d 6 . Infrared (IR) spectra were recorded on a Thermo Nicolet 5700 FT-IR spectrophotometer (Waltham, MA, USA) using KBr pellets and reported in wavenumbers (cm −1 ).
Single-crystal X-ray diffraction data were collected at 100 K on a Rigaku FRE+ equipped with VHF Varimax confocal mirrors and an AFC12 goniometer (Tokio, Japan) and HyPix 6000 detector diffractometer (Tokio, Japan) [52]. The structures were solved with the ShelXT [53] structure solution program using the Intrinsic Phasing solution method, using Olex2 [54] as the graphical interface. The model was refined with version 2018/3 of ShelXL [55] using Least Squares minimization. All hydrogen atoms were added in calculated positions and refined in riding positions relative to the parent atom. CCDC deposition numbers: 1944063-1944065.

Conclusions
The reaction between Lawesson's Reagent (LR) and isopropylamine ( i PrNH 2 ), benzylamine (BzNH 2 ), and 4-phenylbutylamine (PhBuNH 2 ) in toluene gave rise to the corresponding amidodithiophosphonate ammonium salts ( i PrNH 3 )( i PrNH-adtp), (BzNH 3 )(BzNH-adtp), and (PhBuNH 3 )(PhBuNH-adtp) that were reacted with nickel chloride hexahydrate, yielding the corresponding amidodithiophosphonato complexes [Ni( i PrNH-adtp) 2 ], [Ni(BzNH-adtp) 2 ] and [Ni(PhBuNH-adtp) 2 ]. All the compounds were tested against a library of bacteria and fungi of clinical importance belonging to the genera Staphylococcus, Escherichia, and Pseudomonas, and Candida, but only the complexes [Ni(BzNH-adtp) 2 ] and [Ni(PhBuNH-adtp) 2 ] demonstrated some antimicrobial activity that was tentatively ascribed to their tendency towards hydrolysis. Theoretical and experimental results evidenced that [Ni(BzNH-adtp) 2 ] and [Ni(PhBuNH-adtp) 2 ] undergo hydrolysis and that during the hydrolytic process a cleavage of the polarized P-N bond occurs with consequent formation of a P-O bond and retaining of the two P-S bonds in the amidodithiophosphonate moiety. Even if hydrolysis was proven to occur both in the amidodithiophosphonate salts and in the corresponding nickel complexes, an increased polarization of the P-N bond was calculated for the latter, suggesting a higher tendency to undergo hydrolysis. The antibacterial inactivity of the salts can be tentatively explained by taking into account their high hydrophilicity associated with their ionic nature, which circumvents the penetration of the cellular membrane. On the contrary, the neutral complexes [Ni(BzNH-adtp) 2 ] and [Ni(PhBuNH-adtp) 2 ] can pass the cellular membrane and thus exploit their activity. The inactivity of the analogous [Ni( i PrNH-adtp) 2 ], can be explained, taking into account its higher resistance to hydrolysis, demonstrated by its higher stability both in solution and in the solid state, also confirmed by a P-N bond that is slightly shorter than those determined for analogous phenyl-alkyl-amidodithiophosphonato complexes. Further studies are ongoing in order to better understand the role of the alkyl-aryl substituents of the amines in the final amidodithiophosphonato complexes.