Can Serendipity Still Hold Any Surprises in the Coordination Chemistry of Mixed-Donor Macrocyclic ligands? The Case Study of Pyridine-Containing 12-Membered Macrocycles and Platinum Group Metal ions PdII, PtII, and RhIII

This study investigates the coordination chemistry of the tetradentate pyridine-containing 12-membered macrocycles L1-L3 towards Platinum Group metal ions PdII, PtII, and RhIII. The reactions between the chloride salts of these metal ions and the three ligands in MeCN/H2O or MeOH/H2O (1:1 v/v) are shown, and the isolated solid compounds are characterized, where possible, by mass spectroscopy and 1H- and 13C-NMR spectroscopic measurements. Structural characterization of the 1:1 metal-to-ligand complexes [Pd(L1)Cl]2[Pd2Cl6], [Pt(L1)Cl](BF4), [Rh(L1)Cl2](PF6), and [Rh(L3)Cl2](BF4)·MeCN shows the coordinated macrocyclic ligands adopting a folded conformation, and occupying four coordination sites of a distorted square-based pyramidal and octahedral coordination environment for the PdII/PtII, and RhIII complexes, respectively. The remaining coordination site(s) are occupied by chlorido ligands. The reaction of L3 with PtCl2 in MeCN/H2O gave by serendipity the complex [Pt(L3)(μ-1,3-MeCONH)PtCl(MeCN)](BF4)2·H2O, in which two metal centers are bridged by an amidate ligand at a Pt1-Pt2 distance of 2.5798(3) Å and feature one square-planar and one octahedral coordination environment. Density Functional Theory (DFT) calculations, which utilize the broken symmetry approach (DFT-BS), indicate a singlet d8-d8 PtII-PtII ground-state nature for this compound, rather than the alleged d9-d7 PtI-PtIII mixed-valence character reported for related dinuclear Pt-complexes.


Introduction
Macrocyclic chemistry is a very important and active area of chemical science with implications in a wide variety of applications, such as analytical chemistry, separation science, catalysis, and medicinal chemistry [1][2][3][4][5][6], and also in the development of fundamental aspects of supramolecular chemistry, such as molecular recognition, host-guest interactions, design of sensors, and smart artificial molecular devices [7][8][9][10].
Novel macrocyclic chemical structures-differing in molecular shape, architecture, flexibility, arrangement of structural groups, binding sites, and reactive functions-continue to be developed, with the aim of improving performances in the chemical functions of Novel macrocyclic chemical structures-differing in molecular shape, architecture, flexibility, arrangement of structural groups, binding sites, and reactive functions-continue to be developed, with the aim of improving performances in the chemical functions of interest by achieving better control over the strength, selectivity, and dynamics of the binding processes of a variety of cationic, anionic, neutral, organic, and inorganic substrates.
However, the basic aspects of coordination chemistry of macrocyclic ligands towards different substrates, particularly metal ions, continue to be a fascinating area of research in the quest for systems capable of forcing the metal center to adopt unusual coordination geometries and/or oxidation states within stable complexes. For this purpose, the hardsoft nature of donor atoms and their spatial disposition, the cavity size, and flexibility of macrocyclic ligands are the most important parameters that define the coordination properties of these systems in relation to the stereo-electronic requirements of the metal ions of interest [11,12].
In this context, we have been engaged in the development of mixed N/O/S-donor macrocycles featuring rigid heterocyclic moieties, such as pyridine (py) [13][14][15][16][17][18][19][20][21], and 1,10phenanthroline (phen) [22][23][24][25][26][27][28][29][30][31] as integral parts of the macrocyclic structure, which is completed by an aliphatic portion carrying different donor atoms. These systems proved to be highly efficient and selective ionophores in solid-phase extraction, selective transport, preparation of PVC-based ion-selective electrodes, and fluorimetric chemosensors for some transition and heavy metal ions. On the other hand, the conformational constraints on the aliphatic portion of these cyclic systems determined by the rigid heteroaromatic moieties, along with the fact that these heteroaryl frameworks carry one or more borderline N-donor atoms and are excellent π-acceptors groups, can be useful factors in expanding the scope of forcing unusual coordination behaviors, especially on d 8 transition metal ions, such as Pd II and Pt II , having very strict stereo-electronic requirements [32,33].
The present study is strictly related to the previous ones performed on macrocyclic ligands similar to L 1 and L 2 but featuring the phen moiety instead of the py unit [22,32,33,38]. In those cases, the nature of the donor atom sets, the conformational constraints determined by the phen unit on the thioether linkers of the two pentadentate rings, and the locked [4+1] coordination sphere imposed on the Pd II and Pt II ions in their 1:1 complexes were responsible for the stabilization of the corresponding low-valent complexes of Pd I and Pt I [32,33]. The crystal structures of L 1 and L 2 are known [34,39]. In both structures, the aliphatic chain of the rings is tilted over the plane containing the pyridine unit-presumably because of the repulsion between the two sulfur atoms close to the aromatic ring [34]. The other donor atom, independently of its nature (oxygen in the case of L 1 and sulfur in the case of L 2 ), adopts exodentate orientations with the lone pairs of electrons (LPs) pointing out of the ring cavity. Therefore, a conformational change is required for these ligands (and presumably also for L 3 ) to coordinate a metal center with all four donor atoms. A similar situation is observed in the case of the analog of L 1 but featuring a phen moiety instead of the py unit, in the free ligand for which the crystal structure is known [33]. In this case, the explanation given for the conformational behavior observed for L 1 and L 2 cannot be applied, as the S-donors would be too far apart even in a completely planar conformation of the ligand. The tendency of the LPs on the S-donors to occupy exodentate positions pointing out of the ring cavity, with the effect of maximizing the number of gauche placements about the C-S bonds, seems more likely to be responsible for the tilted conformation also observed in the phen analogous of L 1 . In both kinds of macrocyclic ligand, therefore, a conformational change in the aliphatic chains is required upon coordination to bring the lone pair(s) of all donors to adopt endodentate orientations suitable for metal coordination [13,22,[32][33][34][35][36]38,39].

Coordination Chemistry of L 1 towards Pd II , Pt II , Rh III
The reaction of L 1 with one molar equivalent of PdCl 2 in refluxing MeOH/H 2 O (1:1 v/v), followed by reduction of the volume of the reaction mixture under vacuum and slow evaporation in the air of the remaining solvent (water), afforded reddish prismatic crystals. Analytical data (Fast Atom Bombardment (FAB) Mass Spectrum, Figure S1 in the Supplementary Materials (SM), and elemental analysis in the Materials and Methods Section) indicate a Pd/L 1 molar ratio higher than 1:1 in the obtained compound.
The 13 C-NMR spectrum of the complex recorded in CD 3 CN solution at 25 • C shows only three peaks for the aromatic fragment of the macrocyclic ligand (δ C = 122.8, 140.5, 164.1 ppm) and three for the aliphatic chain (δ C = 45.0, 45.9, 65.4 ppm, Figure S2 in SM), thus suggesting that the complex exists in solution in only one form having a C s symmetry with a symmetry plane passing through the N-donor atom of the ligand.
With respect to the free macrocycle, the carbon atoms next to S-donors are deshielded [δ C = 45.0 (36.6) and 45.9 (30.2) ppm for C7/C12 and C8/C11, respectively; see Figure 1 for the numbering scheme adopted, values in parentheses refer to the free macrocycle], whereas those next to the aliphatic O-donor are slightly shielded [δ C = 65.4 (66.7) ppm for C9/C10; see Figure 1]. The 13 C-NMR chemical shifts do not change on changing the temperature in the range allowed by the solvent CD 3 CN. These data are consistent with a coordination sphere imposed in solution by L 1 at the Pd II with possibly the O-donor atom weakly interacting with the metal center.
The doublets at 4.45 and 4.88 ppm are due to an AB spin system (J = 18.5 Hz) for each pair of protons on C7 and C12. This is confirmed by the observation in the HSQC that both doublets correlate with the same 13 C resonance at 45.0 ppm. The same is found for the multiplets mentioned above, whose multiplicity is indicative of an AA'BB' spin system. The four multiplets can be distinguished in two pairs, each correlating with a single 13 C resonance in the HSQC spectrum. These observations indicate the presence of a symmetry plane bisecting the pyridine ring and passing through the metal ion in the complex structure, while the two geminal protons in each of the methylene groups are magnetically inequivalent. This is in agreement with inequivalent dispositions assumed by the protons on C7 and C12 (above and below the plane of the pyridine moiety) as a consequence of the ligand complexation.
The doublets at 4.45 and 4.88 ppm are due to an AB spin system (J = 18.5 Hz) for each pair of protons on C7 and C12. This is confirmed by the observation in the HSQC that both doublets correlate with the same 13 C resonance at 45.0 ppm. The same is found for the multiplets mentioned above, whose multiplicity is indicative of an AA'BB' spin system. The four multiplets can be distinguished in two pairs, each correlating with a single 13 C resonance in the HSQC spectrum. These observations indicate the presence of a symmetry plane bisecting the pyridine ring and passing through the metal ion in the complex structure, while the two geminal protons in each of the methylene groups are magnetically inequivalent. This is in agreement with inequivalent dispositions assumed by the protons on C7 and C12 (above and below the plane of the pyridine moiety) as a consequence of the ligand complexation.
The charge neutrality of the complex is guaranteed by the dinuclear planar [Pd 2 Cl 6 ] 2anion featuring two Pd II metal centers in a square-planar coordination sphere, which is determined by four coordinated chloride anions, two of which bridging the metal ions.
The The charge neutrality of the complex is guaranteed by the dinuclear planar [Pd2Cl6] 2anion featuring two Pd II metal centers in a square-planar coordination sphere, which is determined by four coordinated chloride anions, two of which bridging the metal ions.
Following the same synthetic procedures adopted for the synthesis of [Pd(L 1 )Cl]2[Pd2Cl6], we reacted L 1 with PtCl2 in refluxing MeOH/H2O (1:1 v/v). Yellow crystals were obtained after the addition of excess NH4BF4 to the reaction mixture, evaporation of MeOH under the vacuum, and subsequent crystallization in the air of the remaining aqueous solution by slow evaporation. The FAB mass spectrum of the compound ( Figure S8) exhibits peaks with the correct isotopic distribution for [Pt(L 1 )Cl] + (m/z = 472). These data, together with elemental analysis, confirm the formulation [Pt(L1)Cl](BF4) for the isolated compound. 1 H-and 13 C-NMR spectra ( Figures S9 and S10, respectively) of the complex in CD3CN show features very similar to those observed for [Pd(L 1 )Cl]2[Pd2Cl6], including the evidence of the AB spin system (J = 18.0 Hz) for the doublets at 4.62 and 4.85 ppm for each pair of protons on C7 and C12, respectively, and of the AA'BB' spin system for the other methylene groups resonating at a lower frequency (assignments are made on the basis of 1 H-13 C-NMR HSQC experiments in CD3CN, Figure S11). This strongly suggests a very similar structure for the complexes formed with Pd II and Pt II  An X-ray diffraction analysis was undertaken on the obtained yellow crystals to ascertain the nature of this complex. The crystal structure confirms the formation of the complex cation [Pt(L 1 )Cl] + (Figure 3) balanced by a BF4 -counter-anion.  The coordination environment at the metal center is very similar to that observed in the case of [Pd(L 1 )Cl] + with the macrocyclic ligand adopting the typically folded conformation and imposing a [NS 2 +O] coordination sphere at the Pt II metal ion, which reaches an overall square-based pyramidal geometry thanks to the coordination of a Cl − ligand in the equatorial plane ( Figure 3). The O-donor occupies the apical site of the square-pyramid at a distance of 2.752(4) Å from the metal center, which is slightly longer than the Pd···O distance observed in the complex cation [Pd(L 1 )Cl] + . In the crystal packing, two units of complex cation interact via Pt···S and CH···Cl contacts of 3.625(2) Å and 2.88 Å, respectively, with the relevant equatorial coordination planes facing each other ( Figure 4). Dimers of this kind interact head-to-tail via CH···Cl and CH···O H-bonds of 2.89 and 2.43 Å, respectively, to form zig-zag chains running along the [001] direction. the case of [Pd(L 1 )Cl] + with the macrocyclic ligand adopting the typically folded conformation and imposing a [NS2+O] coordination sphere at the Pt II metal ion, which reaches an overall square-based pyramidal geometry thanks to the coordination of a Clligand in the equatorial plane ( Figure 3). The O-donor occupies the apical site of the square-pyramid at a distance of 2.752(4) Å from the metal center, which is slightly longer than the Pd···O distance observed in the complex cation [Pd(L 1 )Cl] + . In the crystal packing, two units of complex cation interact via Pt···S and CH···Cl contacts of 3.625(2) Å and 2.88 Å, respectively, with the relevant equatorial coordination planes facing each other ( Figure 4). Dimers of this kind interact head-to-tail via CH···Cl and CH···O H-bonds of 2.89 and 2.43 Å, respectively, to form zig-zag chains running along the [001] direction.  Figure S13) of the obtained crystals, which exhibits peaks with the correct isotopic distribution for [Rh(L 1 )Cl 2 ] + (m/z = 414), confirm the formulation [Rh(L 1 )Cl 2 ](PF 6 ) for the isolated compound. Similar to the case of Pd II and Pt II , the 1 H-NMR spectrum reflects an AB spin system for each pair of protons on C7 and C12 (doublets at 5.01 and 5.25 ppm with J = 18.6 Hz, Figures S14-S16) and AA'BB' spin system for the other methylene protons (four multiplets at 4.04-4.07, 3.57-3.62, 3.48-3.51 and 3.36-3.40 ppm). The two doublets showed correlation with a single 13 C resonance at 46.0 ppm in the HSQC spectrum ( Figure S16). The four multiplets can be divided into two pairs, with the two 1 H multiplets at 4.04-4.07 and 3.36-3.40 ppm showing correlation with a single 13 C resonance at 74.1 ppm, while the other two 1 H multiplets at 3.57-3.62 and 3.48-3.51 ppm showed correlation with the same 13 C resonance at 40.5 ppm, in the HSQC spectrum.
These observations strongly indicate a coordination mode of the ligand analogous to that observed in the Pd II and Pt II complexes of L 1 .
An X-ray diffraction analysis was undertaken on the obtained yellow crystals showing the presence of [Rh(L 1 )Cl 2 ] + complex cations counterbalanced by PF 6 − anions in the crystal structure. The complex cations feature a Rh III ion in a distorted octahedral environment defined by the four donor atoms of a macrocycle L 1 and two chlorido ligands ( Figure 5). The structure of the cation is conditioned by the meridional coordination of the 2,6-bis(thiomethyl)pyridine unit, as is observed in the value of the S-Rh-S angle, 170.23(3) • . The O-donor is located perpendicular to the pseudo-plane defined by the metal ion, the pyridine ring, and the two thioether sulfur atoms. The folded conformation, adopted by L 1 as in the cases of [M(L 1 )Cl] + complex cations (M = Pd, Pt), leaves the two coordination sites occupied by two Cl − ligands in a relative cis orientation.
spectrum reflects an AB spin system for each pair of protons on C7 and C12 (doublets at 5.01 and 5.25 ppm with J = 18.6 Hz, Figures S14-S16) and AA'BB' spin system for the other methylene protons (four multiplets at 4.04-4.07, 3.57-3.62, 3.48-3.51 and 3.36-3.40 ppm). The two doublets showed correlation with a single 13 C resonance at 46.0 ppm in the HSQC spectrum ( Figure S16). The four multiplets can be divided into two pairs, with the two 1 H multiplets at 4.04-4.07 and 3.36-3.40 ppm showing correlation with a single 13 C resonance at 74.1 ppm, while the other two 1 H multiplets at 3.57-3.62 and 3.48-3.51 ppm showed correlation with the same 13 C resonance at 40.5 ppm, in the HSQC spectrum. These observations strongly indicate a coordination mode of the ligand analogous to that observed in the Pd II and Pt II complexes of L 1 .
An X-ray diffraction analysis was undertaken on the obtained yellow crystals showing the presence of [Rh(L 1 )Cl2] + complex cations counterbalanced by PF6 -anions in the crystal structure. The complex cations feature a Rh III ion in a distorted octahedral environment defined by the four donor atoms of a macrocycle L 1 and two chlorido ligands ( Figure  5). The structure of the cation is conditioned by the meridional coordination of the 2,6bis(thiomethyl)pyridine unit, as is observed in the value of the S-Rh-S angle, 170.23(3)°. The O-donor is located perpendicular to the pseudo-plane defined by the metal ion, the pyridine ring, and the two thioether sulfur atoms. The folded conformation, adopted by L 1 as in the cases of [M(L 1 )Cl] + complex cations (M = Pd, Pt), leaves the two coordination sites occupied by two Clligands in a relative cis orientation.   (9), S1-Rh1-O1 86.60 (7), S1-Rh1-Cl1 91.94(4), S1-Rh1-Cl2 92.57(4), S1-Rh1-S2 170.23 (3)

Coordination Chemistry of L 2 towards Pd II , Pt II , Rh III
The reaction of L 2 with one molar equivalent of PdCl 2 or PtCl 2 in refluxing MeCN/H 2 O (1:1 v/v) afforded orange and yellow microcrystalline powders, respectively (see Experimental Section). Unfortunately, we were not able to grow crystals suitable for X-ray diffraction analysis. However, mass spectra (Figures S18, S19) and elemental analyses suggest the formation of 1:1 metal-to-ligand complexes having the formulation [M(L 2 )Cl]Cl (M = Pd II and Pt II ). 1  Interestingly, the free macrocycle L 2 is reported to prefer a "chair-like" conformation in which the central S-donor is oriented in the opposite direction with respect to the site perpendicular to the plane containing the remaining NS 2 donor set, and a conformational change is necessary to interact with the d z 2 orbital of the coordinated transition metal ion [39]. No crystals could be grown for the brown solid isolated from the reaction of L 2 with one molar equivalent of RhCl 3 .
Surprisingly, the 1 H-and 13 C-NMR spectra of the Rh III complex ( Figures S25 and S26, respectively) isolated with L 2 recorded in CD 3 CN clearly show two distinct complexes in solution. Two series of homologous resonances can be seen, both for the aromatic and the methylene protons. Homologous resonances differ in position and relative intensity, but share the same fine structure, with very similar J couplings. By the relative intensity of 1.3, we can distinguish one major and one minor species, separated by a ∆G of 0.65 kJ mol −1 . As far as the methylene resonances are concerned, both species are characterized by two doublets around 5 ppm corresponding to an AB spin system and showing scalar correlation with the same 13 C resonance in the HSQC spectrum ( Figure S27). Both the species are characterized by four multiplets between 2.5 and 4.0 ppm corresponding to an AA'BB' spin system and distinguished in couples by showing correlation with the same 13 C resonance in the HSQC spectrum. Definitely, the two complexes appear to be very similar. Since L 1 formed the complex cation [Rh(L 1 )Cl 2 ] + with Rh III , the major and minor species observed for L 2 could be tentatively assigned to the cis and the trans configurations of the two coordinated chloride ions. However, the latter is not compatible with the AB spin system observed for each pair of protons on C7 and C12, which are expected to be equivalent and to appear as a singlet in the 1 H-NMR spectrum. In order to clarify this point further and to characterize the two species in more detail, we acquired NOESY spectra on both the L 1 and L 2 complexes with Rh III . Their analyses (see SI for discussion and Figure  S28 for the molecular model compatible with NMR measurements) clearly point out that in the case of L 2 and Rh III , the dichlorido complex is formed together with another species in which one chloride, at least, is substituted by some other ligands (presumably a solvent molecule, likely MeCN). All attempts to isolate the two complexes by chromatography were unsuccessful. Data available on the solid-state are not conclusive on the presence of the complex featuring only one coordinated chlorido ligand (see above). While the peak at m/z 430 can be unambiguously assigned to the species [Rh(L 2 )Cl 2 ] + , the peak at m/z = 396, which can be assigned to [Rh(L 2 )Cl] + , could either derive from [Rh(L 2 )Cl 2 ] + by loss of one chlorido ligand or from the other complex by the loss of the coordinated solvent molecule.

Coordination Chemistry of L 3 towards Pd II , Pt II , Rh III
The reaction of L 3 with one molar equivalent of PdCl 2 in refluxing MeCN/H 2 O (1:1 v/v), followed by the addition of excess NH 4 PF 6 , the reduction of the solvent volume under vacuum, and the slow evaporation in the air of the remaining solvent (water), afforded a brown microcrystalline powder. The FAB mass spectrum of the compound exhibits a peak at m/z = 381 with the correct isotopic distribution expected for the cation [C 11 H 16 ClN 2 PdS 2 ] + (see Figure S29 in the SM). This, together with elemental analytical data, support the formation of a 1:1 complex having the formulation [Pd(L 3 )Cl](PF 6 ).
Indeed, as already observed for the Pd II complexes with L 1 and L 2 (see above), the 13 C-NMR chemical shifts for the Pd II complex with L 3 in CD 3 CN solution shows only three peaks for the aromatic region and three for the aliphatic chain ( Figure S30), thus suggesting that the complex exists in solution in only one form having a C s symmetry with a symmetry plane passing through the two N-donor atoms of the ligand. With respect to the free macrocycle, the carbon atoms next to S-donors are deshielded [δ C = 45.8 (37.8) and 43.3 (31.7) ppm for C7/C12 and C8/C11, respectively; (values in parentheses refer to the free macrocycle)], whereas those next to the aliphatic N-donor are slightly deshielded [δ C = 48.2 (47.0) ppm for C9/C10]. These features have also been observed in the 13 C-NMR spectra of the 1:1 complexes of Pd II and Pt II with pentadentate macrocyclic ligands similar to L 1 -L 3 , but having a phen unit instead of the py moiety [32,33], which showed a [4 + 1] coordination sphere at the metal centers with the central donor atom in the aliphatic linker occupying the apical site of a distorted square-based pyramid with a long-range interaction to the metal atom, and the ligand adopting a folded conformation. A similar coordination mode of L 3 to the Pd II center can be suggested for the complex cation [Pd(L 3 )Cl] + in which the [4 + 1] pyramidal coordination sphere would be reached thanks to a chloride anion occupying one site in the basal coordination plane (see Pd II and Pt II complexes of L 1 and L 2 above). This hypothesis is supported by the 1 H-NMR spectrum of the complex in D 2 O ( Figure S31) that exhibits four distinct groups of aliphatic protons at 3.03-3.06 (multiplet), 3.54-3.60 (multiplet), 4.61 (doublet), and 5.07 (doublet) ppm integrating for four, four, two, and two protons, respectively. The doublets at 4.61 and 5.07 ppm define an AB spin system for each pair of protons on C7 and C12 ( Figure 1) (J = 18.6 and 18.0 Hz, respectively, assignment of the chemical shift is made for the analogy with the 1 HNMR shits observed for the Pd II , Pt II , and Rh III complexes of L 1 , whose assignment is made via 1 H-13 C-heteronuclear correlation, HSQC, experiments, see below), which agrees with inequivalent dispositions assumed by these protons (above and below the plane of the pyridine moiety) as a consequence of metal complexation.
The  (Figure 7). The maximum deviation from the least-squares plane calculated through the atoms Pt1, Cl1, N3, N4, Pt2 is 0.04 Å for N4. The average coordination plane at the Pt1 atom, which also comprises the two N-donors from the macrocyclic ligand and the two donors from the amidate bridging ligand, is almost perpendicular to the plane containing the pyridine ring and the Pt2, S1, S2, and O1 donors with the interplanar angle being 89.09 • (Figure 7). L 3 adopts the folded conformation already observed in the complex cations [Cu(L 3 )] 2+ and [Zn(L 3 )] 2+ [13] resembling an open book with the spine along the line connecting the S1-Pt2-S2 atoms and the N1-Pt2-N2 hinge angle of 91.81 (13) • . The aliphatic tertiary nitrogen is, therefore, located almost perpendicularly to the pseudo-plane defined by the metal ion, Pt2, the pyridine ring, the S-donors, and the amidate O-donor, in trans-position with respect to the other platinum atom, Pt1 (Figure 7). The four-coordinated platinum atom, Pt1, features the other platinum atom and the chlorido ligand in mutually trans-positions, giving an almost linear Cl-Pt-Pt-N arrangement in the binuclear cation, with the other two trans-positions being occupied by the coordinated acetonitrile molecule and the amidate N-donor. It is interesting to note that L 3 binds metal atoms with almost equivalent M-N1 and M-N2 bond lengths [13]. In contrast, in the complex cation, the bond distance Pt2-N2 = 2.237(3) Å is longer than the Pt2-N1 = 2.006(3) Å due to the higher trans-influence of coordinating Pt1 compared to O1 donor atoms, thus suggesting a donoracceptor nature for Pt-Pt bond, also confirmed by the short Pt1-Pt2 distance [2.5798(3) Å] consistent with a metal-metal bond.

The binuclear [Pt(L 3 )(μ-1,3-MeCONH)PtCl(MeCN)] 2+ complex cations interact through H-bonds involving the BF4 -anions and the water molecule forming head-to-tail chains running along the crystallographic [001] direction (
The structurally characterized diplatinum systems were assigned to the suites of d n -d n and mixed-valence d n -d m complexes (n, n = 6, 7, 8, 9; n, m = 6, 8; 7, 8; and 7, 9), based on the reported formal oxidation states. Figure 9 shows the number of fragments found for the different suites versus the coordination number displayed by the platinum centers.
Indeed, an examination of the data reported in Figure 9 shows that reportedly mixedvalence d n -d m complexes are quite rare, with only five examples known in the literature, among the 505 items found [49][50][51][52][53] beside the complexes [Pt 2 I,III (tfepma) 2 X 4 ] (X = Cl, Br) [46,47]. The category with the higher number of items (212) is that of complexes formally featuring two penta-coordinated Pt II ions, which can be envisaged as two squareplanar complexes interacting through long Pt-Pt bonds ranging from 2.53 to 3.41 Å, with a mean value as long as 2.94 Å and an overall distorted square-based pyramidal environment for both metal ions. (d 8 -d 8 , Pt5-Pt5 green column in Figure 9). Quite numerous (155) are also the binuclear complexes formally featuring two Pt III ions sharing one of the six bonds in a distorted octahedral coordination for both metals, with Pt III -Pt III bond lengths in the range 2.39-3.08 and a mean value of 2.61 Å (d 7 -d 7 , Pt6-Pt6 blue column in Figure 9). For this kind of binuclear system, a significant number of structures (40 items) featuring the two metal ions in a different coordination environment (distorted octahedral/squarebased pyramidal) are reported (d 7 -d 7 , Pt5-Pt6 orange column in Figure 9). These are often described as formal Pt III -Pt III dimers with significant Pt IV and Pt II influences for the octahedral and square pyramidal platinum center, respectively [52]. The Pt-Pt distances again occupy a quite narrow range of 2.50-2.85 Å with a mean value of 2.69 Å. The complex [Pt(L 3 )(μ-1,3-MeCONH)PtCl(MeCN)](BF4)2·H2O belongs to the very unusual category of discrete Pt-dimers, featuring one square-planar and one octahedral platinum center connected by a metal-metal bond (d 9 -d 7 , Pt4-Pt6 red column in Figure 9) and formally considered as mixed-valence Pt I -Pt III systems. It is interesting to note that the only two examples known of binuclear complexes formally sharing a Pt I -Pt III bond, namely, [Pt2 I,III (tfepma)2X4] [X = Cl, Br; tfepma = bis(bis(trifluoroethoxy)-phosphino)methylamine], contain the same trifluoroethyl-imidophosphito ligand bridging the metal centers counterbalanced by halides that complete the platinum coordination spheres [46,47]. Our compound would be the first example supported by a macrocyclic ligand that does not bridge the two metal centers. In these complexes the Pt-Pt distance is quite short [2.6187(7) and 2.6270(9) Å for X = Cl and Br, respectively], and the coordination environment is distorted octahedral for the reportedly Pt III center and square-planar for the Pt I one. This structural feature seems to be peculiar to formally defined d 9 -d 7 Pt2 I,III binuclear complexes, In fact, d 7 -d 7 binuclear Pt2 III,III complexes, also characterized by short metal-metal distances, see above, generally feature both metal centers either in distorted octahedral environments or octahedral/square-based pyramidal coordination spheres. In contrast, binuclear d 8 -d 8 Pt2 II,II complexes are characterized by both metal centers in a distorted square-based pyramidal environment.
Following a synthetic procedure analogous to that adopted for the preparation of  Figure S33 for FAB Mass Spectrum). 13 C-and 1 H-NMR spectra ( Figures S34 and S35, respectively) presented features similar to those observed in the corresponding NMR spectra of [Rh(L 1 )Cl2](PF6), suggesting that the complex of Rh III Discrete dimers formally featuring a Pt I -Pt I bond can be found in complexes featuring distorted square-planar/square-planar, square-planar/square-based pyramidal, and square-based pyramidal/square-based pyramidal coordination environments and ligands able to stabilize low oxidation states, such as phosphine derivatives, carbon monoxide, cyanides, hydrides and carbanions, and comprise metal-metal distances in the quite narrow range 2.53-2.76 Å, with a mean value of 2.62 Å (d 9 -d 9 columns in Figure 9, 67 items). Only two binuclear complexes are known showing hepta-coordinated platinum Pt III -Pt III or Pt IV -Pt IV metal ions (Pt7-Pt7 purple columns in Figure 9) [54,55].
The complex [Pt(L 3 )(µ-1,3-MeCONH)PtCl(MeCN)](BF 4 ) 2 ·H 2 O belongs to the very unusual category of discrete Pt-dimers, featuring one square-planar and one octahedral platinum center connected by a metal-metal bond (d 9 -d 7 , Pt4-Pt6 red column in Figure 9) and formally considered as mixed-valence Pt I -Pt III systems. It is interesting to note that the only two examples known of binuclear complexes formally sharing a Pt I -Pt III bond, namely, [Pt 2 I,III (tfepma) 2 X 4 ] [X = Cl, Br; tfepma = bis(bis(trifluoroethoxy)-phosphino)methylamine], contain the same trifluoroethyl-imidophosphito ligand bridging the metal centers counterbalanced by halides that complete the platinum coordination spheres [46,47]. Our compound would be the first example supported by a macrocyclic ligand that does not bridge the two metal centers. In these complexes the Pt-Pt distance is quite short [2.6187(7) and 2.6270(9) Å for X = Cl and Br, respectively], and the coordination environment is distorted octahedral for the reportedly Pt III center and square-planar for the Pt I one. This structural feature seems to be peculiar to formally defined d 9 -d 7 Pt 2 I,III binuclear complexes, In fact, d 7 -d 7 binuclear Pt 2 III,III complexes, also characterized by short metal-metal distances, see above, generally feature both metal centers either in distorted octahedral environments or octahedral/square-based pyramidal coordination spheres. In contrast, binuclear d 8 -d 8 Pt 2 II,II complexes are characterized by both metal centers in a distorted square-based pyramidal environment.
These combinations correspond to all the possible configurations for both d 8 -d 8 Pt II -Pt II [singlet configuration, I in Figure S36c, and triplet configuration, with the unpaired electrons either on the octahedrally (Pt O ) or square-planar (Pt SP ) coordinated Pt ions, II and III in Figure S36c, respectively] and mixed-valence d 9 -d 7 Pt I -Pt III systems (singlet and triplet configurations, with either Pt O or Pt SP carrying the Q = +1 charge; IV-VII in Figure S36c).
This scheme includes the mixed-valence configuration previously reported for [Pt 2 (tfepma) 2 Cl 4 ], where the charge Q = +3 was assigned to the Pt O center, and the charge Q = +1 to the Pt SP one (VI-VII in Figure S36c). When the optimization of the two model compounds was performed starting from the electron density guess of ground-state configurations I-VII, all calculations converged to two geometries only, corresponding to the closed-shell singlet and triplet ground-states.
According to these results, it appears that the spin densities on the two metal centers in this type of binuclear complexes cannot be separately modeled, probably because of the close proximity of the Pt ions, therefore both should be better described as binuclear d 8 -d 8 Pt II -Pt II complexes. An examination of the optimized geometries in the singlet and triplet ground-states shows that for both complexes the total electronic energy of the geometry in the singlet state is lower than that in the triplet state (by 111.1 and 133.7 kJ mol −1 for [Pt(L 3 )(µ-1,3-MeCONH)PtCl(MeCN)] 2+ and [Pt 2 (tfepma) 2 Cl 4 ], respectively). Accordingly, a better agreement between the optimized geometry and the structural data was found for both complexes for the singlet ground-state. In the case of [Pt(L 3 )(µ-1,3-MeCONH)PtCl(MeCN)] 2+ , the bond lengths and angles of the optimized geometry in the singlet state differ from the experimental ones by less than 0.05 Å and 6 • , respectively, with the sole exception of the Pt2-N2 bond distance, which is overestimated by 0.111 Å (Tables S1-S3, Figure S37). On the other hand, in the optimized geometry in the triplet state, a significant elongation (about 0.3 Å) of the Pt-S bonds within the coordination sphere of the octahedral Pt ion is observed, along with a divergence from the square-planar coordination geometry for Pt1, with a Cl1-Pt1-Pt2-N2 dihedral angle of 89.51 • (Figure S37). In the case of [Pt 2 (tfepma) 2 Cl 4 ], an even further deviation from the experimental geometry was observed for the optimized geometry in the triplet state, featuring both Pt ions pentacoordinated in a trigonal bipyramidal geometry, while a very good agreement was found between the experimental structure and that optimized in the singlet ground-state (Tables S4-S6, Figure S38).

Materials and Methods
All melting points are uncorrected. Elemental analyses were obtained using a Fison EA CHNS-O instrument operating at 1000 °C. FAB mass spectra were measured at the EPSRC National Mass Spectrometry Service at Swansea (UK). 1 H and 13 C NMR experiments were conducted at 25 °C with a Varian VXR400 spectrometer operating at 400 MHz for 1 H and 100.62 MHz for 13 C or with a Bruker Avance III HD spectrometer operating at 600 MHz for 1 H and 150.9 MHz for 13 C, using TMS as an internal standard. Data are reported as chemical shifts (multiplicity, coupling constants where applicable, number of hydrogen atoms, and assignment where possible). Abbreviations are: s (singlet), d (dou-
3.1. General Procedure for the Synthesis of the Pd II , Pt II , Rh III Complexes of L 1 -L 3 The appropriate metal chloride was added to a solution of L, 1 L 2 or L 3 in 1:1 molar ratio. No excess of ligand or starting metal salt was considered, to avoid the formation of coordination compounds with stoichiometries other than 1:1, which is very likely when the macrocyclic ligand is not able to satisfy the stereo-electronic requirements of the metal ion. The reactions were all conducted in MeCN/H 2 O (20 mL, 1:1 v/v) solvent mixture, except in two cases where the MeOH/H 2 O (20 mL, 1:1 v/v) solvent mixture was used for solubility reasons, as specified below. The reaction mixture was refluxed under N 2 for 5 h in all cases. When a pure solid product was not obtainable/isolable from the reaction mixture of the ligands with the chloride of the metal under investigation, a counter-anion metathesis reaction was performed to replace coordinating chlorido ligands with non-coordinating BF 4 − or PF 6 − anions and facilitate the crystallization or formation of solid products. This was necessary in the prepara-  4 )·MeCN for which a ten-fold molar excess NH 4 BF 4 or NH 4 PF 6 was added at room temperature after refluxing of the reaction mixture. This is a well-established synthetic procedure in the field of macrocyclic ligand chemistry [76,77].
[    ] + ). One of the two sets of signals, in both the 1 H-and 13 C-NMR spectra belongs to the species [Rh(L 2 )Cl 2 ](PF 6 ), the other set belongs to a species having only one coordinated chlorido ligand (see discussion above). However, it is not possible to uniquely identify which set of signals corresponds to which complex.
[Pd(L 3 )Cl](PF 6 ). A mixture of L 3 (0.020 g, 0.083 mmol) and PdCl 2 (0.015 g, 0.083 mmol) in MeCN/H 2 O (20 mL, 1:1 v/v) was refluxed for 2 h. A brown solid (0.020 g, yield 46%) was obtained after the addition of excess NH 4 PF 6 to the reaction mixture, reduction of the volume of the reaction mixture under vacuum, and subsequent crystallization in the air from the resulting aqueous solution by slow evaporation. Mp: 220 • C with decomposition. Elem. Anal. found (calc. for C 11
The geometries of all compounds were optimized starting from crystal structure data in their triplet ground-state (2S + 1 = 3, two unpaired electrons), closed-shell singlet state (2S + 1 = 1) after verification of the wavefunction stability (stable = opt), or by means of a broken-symmetry (DFT-BS) approach. The procedure recently developed for bis(1,2dithiolene) metal complexes was followed [70]. In particular, the BS electron density guess was obtained through a fragmented approach (guess = fragment = n, the fragments being the two Pt ions and the various ligands) starting from the geometry optimized at the largest spin multiplicity, by attributing different combinations of charges (Q = +1, +2, +3) and corresponding spin multiplicities (2S + 1 = 1, 2, 3) to the Pt ions, eventually optimizing (opt) the geometry of the complexes for the different combinations and verifying (and in case re-optimizing) the stability of the wavefunctions [70]. Fine numerical integration grids (Integral = ultrafine keyword) were used, and the nature of the minima of each optimized structure was verified by harmonic frequency calculations (freq = raman keyword). A natural population analysis was carried out at the optimized geometries using the natural bonding orbital (NBO) partitioning scheme [80]. The programs GaussView 6.0.16 [81], Molden 6.6 [82], and Chemissian 4.53 [83] were used to investigate the optimized structures and the shapes of Kohn-Sham molecular orbitals.

X-ray Crystallography
A summary of the crystal data and refinement details for the compounds discussed in this paper is given in Tables S7 and S8 (

Conclusions
In this paper, the coordination chemistry of the mixed-donor tetradentate macrocycles L 1 -L 3 featuring a pyridine moiety towards platinum group metal ions Pd II , Pt II , and Rh III has been investigated. In all isolated 1:1 metal-to-ligand complexes, the ligands adopt a folded conformation and impose a [3 + 1] coordination mode at the Pd II and Pt II metal centers within a distorted square-based coordination sphere. In the case of Rh III complexes, the tetradentate ligands occupy four of the six positions of a distorted octahedral geometry with the other two coordination sites in a relative cis orientation occupied by two Cl − ligands. A rare example of a discrete Pt 2 dimer was isolated by serendipity from the reaction of L 3 and PtCl 2 in refluxing MeCN/H 2 O (1:1 v/v), and structurally characterized. This complex, based on data from the literature, could have been formally defined as a d 9 -d 7 Pt 2 I,III mixed-valence binuclear complex featuring a Pt-Pt bond linking a square-planar and an octahedral platinum centers. DFT calculations, following the broken symmetry approach (DFT-BS), identify a singlet ground-state nature (d 8 -d 8 Pt II -Pt II ) both for the isolated compound, as well as for the only other example of a discrete binuclear Pt 2 complex of the same type reported in the literature, despite the different coordination environments of the two metal centers typical for a d 8 Pt II center (square-planar) and a d 7 Pt III center (octahedral). Notwithstanding the theoretical limits inherent to a non-multireference DFT-BS approach, the case of [Pt(L 3 )(µ-1,3-MeCONH)PtCl(MeCN)] + suggests that a more in-depth re-evaluation may be needed for the electronic configurations assigning mixed oxidation states to Pt ions in dinuclear complexes where two directly interacting Pt ions show different coordination geometries and numbers.
The obtained results, especially in the case of L 3 , can be of help in understanding the sensing properties toward metal ions of fluorescent chemosensors featuring this macrocycle as receptor units [13][14][15][16][17][18][19][20][21]. However, as far as the question posed in the title is concerned, based on the results obtained, we can conclude that well-established fields of coordination chemistry, such as that of macrocyclic ligands and Pt II , can still hold some unexpected outcomes. The serendipitous and unexpected isolation of complex [Pt(L 3 )(η-1,3-MeCONH)PtCl(MeCN)](BF 4 ) 2 ·H 2 O stands as a proof of principle for the unexplored synthetic possibilities still available in the coordination chemistry of well-known classes of macrocyclic ligands and Platinum Group metals. We have shown that the dimeric and unique complex cation [Pt(L 3 )(µ-1,3-MeCONH)PtCl(MeCN)] + can exist, despite the fact we were not able to reproduce it or explain its formation. This is still interesting and could open new perspectives in the coordination compounds of Pt II . Furthermore, we strongly believe that it is essential to perform fundamental research even when all the available information suggests that only predictable, trivial results will be obtained.