Heavier Group 2 Metal Complexes with Flexible Scorpionate Ligand based on 2-Mercaptopyridine

We report the synthesis of novel alkaline earth metal complexes [κ2-SS-(Bmp)2M(THF)n] [M = Ca (2), Sr (3) n = 2; M = Ba (4), n = 3] of a flexible dihydrobis(2-thiopyridone)borate (Bmp) ligand based on 2-mercaptopyridine. Complexes 2–4 were isolated in good yield by the reaction between sodium dihydrobis(2-thiopyridone)borate, [{(Bmp)Na(THF)}2]n (1) and the corresponding alkaline earth metal diiodides in toluene at ambient temperature. The solid-state structures of the strontium and barium complexes, complexes 3 and 4 respectively, were established using single-crystal X-ray diffraction analysis. The solid-state structure of sodium complex 1 was also confirmed using X-ray techniques. The solid-state structures of complexes 3 and 4 revealed that the Bmp ligand coordinates through sulphur atoms to the metal ions in κ2 fashion. The strontium ion is attached symmetrically and the barium ion is asymmetrically linked with the Bmp ligand, manifesting the 2-thiopyridone and pyridine-2-thiolate tautomeric form of the Bmp ligand. The strontium ion in complex 3 adopts a distorted octahedral geometry whereas the geometry around the barium ion can best be described as a distorted pentagonal bipyramidal. Both complexes 3 and 4 also have a short B⋯H⋯M interaction due to the presence of the BH2 group in the ligand. In the solid state, sodium complex 1 is polymeric in nature and in the asymmetric unit each sodium ion is bonded to two sulphur atoms through η1 and μ2 modes. The adjacent BH2 group is also linked with each sodium ion through hydrogen atoms via μ2 and η1.


Introduction
The scorpionate ligands such as [hydrotris(pyrazolyl)borate] [1] and [dihydrobis(pyrazolyl)borate] [2] first reported by Trofimenko were generally thought of as innocent spectator ligands in transition metal chemistry [3,4] to modify the properties of the metal centre without getting directly involved in its reactivity. [5]owever, the more flexible scorpionate ligand, [hydrotris(methylimidazolyl)-borate], (Tm) and [dihydrobis( methylimidazolyl)-borate] (Bm) had two major advantages when compared to original Trofimenko's scorpionate ligands.The Tm and Bm ligands were based on soft sulfur donor atoms and more significantly, incorporate greater flexibility into the ligand by addition of an extra atom between the boron and the donor atom.The enhanced flexibility of these ligand systems provide the greater potential for activation at the boron bridgehead and formation of metal-borane (metallaboratrane) complexes [6] giving rise to reactivity not observed in the analogous Tp compounds. [7]ecently, Owen and co-workers have developed a new family of scorpionate ligands based on boro-mercaptopyridine hydrotris(2thiopyridone)borate (Tmp) and hydrotris(2-thiopyridone)borate dihydrobis(2-thiopyridone)borate (Bmp) to study their properties of coordination to transition metals such as copper, ruthenium, iridium, palladium and platinum. [8,9]However, their work has been restricted within the transition metal chemistry.It has been observed that boro-mercaptopyridine based scorpionate ligands coordinate through sulphur atoms and hydrogen atoms present in BH or BH 2 groups which point towards the metal centre. [10]Some of these B-H...metal interactions have been found to be particularly strong and exhibit significant metal-hydride character 50 and thus these complexes can be used for hydride migration between transition metal and boron centres and this process can be applied to metal-mediated transformations. [11]However, various factors, such as a transition metal's ability to accept a hydride, the nature of ancillary ligands attached to it and 55 favourable substituents at the metal centre govern the hydride migration between boron and metal centre. [12]heme 1. Tautomeric forms of 2-thiopyridone and pyridine-2-thiolate of 60 Bmp.
Another interesting feature of such ligands is that the boromercaptopyridine unit can undergo tautomerisation between 2thiopyridone and pyridine-2-thiolate forms where the electron 65 density moves from the boron atom to the sulphur atom (Scheme 1).It has been reported that although the thione-thiolate tautomeric equilibrium generally favours the thione form, there is a strong dependence on its environment with only a small difference in the energy between the two tautomeric forms. [13] our continuous study of the alkaline earth metal chemistry, our work focuses on the exploration of synthetic methodologies allowing for a facile, clean, high-yield production of the required molecules and determination of their molecular structure and function.We have recently introduced various amidophosphine chalcogenide and borane ligands containing N, E (E = O, S, Se, BH 3 ) and P as donor atoms into alkaline earth metal chemistry in 10 order to study their coordination properties. [14]These unique ligands are potentially capable of coordinating through hard nitrogen and phosphorus donor atoms along with the soft E donor atom.As the Bmp ligand has some resemblance to amidophosphine chalcogenide ligands in terms of hard and soft donor atoms (N, P, S), we have sought to introduce the Bmp ligand into electropositive alkaline earth metal chemistry in order to understand its thermodynamic stability.
Here, we report the syntheses and structural details of 2-  (1).In addition, we also describe the comparative study among the computed structures and experimental structures of strontium and barium complexes.

Sodium complex:
8a] We found this very useful to report the solidstate structure of NaBmp and compare with the solid-state 35 structures of alkaline earth metal complexes.The sodium complex (1) was prepared according to the procedure [8a] described by Owen et al. involving the reaction of NaBH 4 and two equivalents of 2-mercaptopyridine in a mixture of toluene and THF at 80 ºC for 12 hours (Scheme 2).The 1 H, 13 C{ 1 H} and 40 11 B{ 1 H} NMR data for complex 1 are in full agreement with the values in literature.Complex 1 was re-crystallised from concentrated THF solution at 15 ºC and the solid-state structure was confirmed using single-crystal X-ray structure analysis.
Complex 1 crystallises in the triclinic space group P-1, with two molecules in the unit cell.The data collection parameters are set out in Table 1. Figure 1 shows the molecular structure of complex 1, as well as the grown-up structure.The solid-state structure of sodium complex 1 confirmed the κ 2 SS-coordination of the Bmp 50 ligand.Moreover, Na1 and Na2 are bonded to third sulphur atom S4 and S2 respectively (Figure 1b).The other sodium ion Na2 is also bonded with three sulphur atoms (S1, S2 and S3) from another Bmp fragment in the κ 2 mode.However, both the sodium atoms 65 (Na1 and Na2) are connected through a sulphur atom (S1) from the second Bmp ligand through a µ bridging mode, with a distance of 3.953 Å between the sodium atoms.The Na-S bond distances [2.882(14) and 2.923( 14) Å] are similar, which demonstrates the 2-thiopyridone of the ligand.However, the 70 Na1-S1 and Na2-S3 distances [2.815(1) and 2.789(2) Å] indicate a strong attachment between the sodium ions and adjacent Bmp ligands with a symmetry element [x+1, y, z] to grow the polymeric chain.Two eight-membered metallocycles Na1-S3-C14-N3-B1-N4-C16-S4 and Na2-S1-C1-N1-B2-N2-C6-S2 75 are observed due to the coordination of the sulphur atoms of the Bmp ligand to the sodium ions.It is noteworthy that the sulphur atoms are coplanar with their pyridine rings (S1 is coplanar with C1-C2-C3-C5-C6-N1 and S2 is coplanar with C6-C7-C8-C9-C10-N2) and the two planes are almost orthogonal (the dihedral 80 angle is 80.9º), indicating a significant contribution from the 2thiopyridone tautomeric form (Scheme 1).In addition, each sodium atom is coordinated with one THF molecule and the geometry around the sodium ions can be best described as a distorted tetrahedral.Both the BH 2 groups from the Bmp ligands 85 are bonded with the sodium ions through hydrogen atoms via µ (H1B and H2B) and η 1 (H1A and H2A) bonds.The short Na...H-B distances (Na1-H1B 2.488 Å, Na2-H1B 2.544 Å and Na2-H2B 2.503 Å) are in agreement with the linkage between BH 2 and sodium ions. [15]The second sodium atom and one sulphur 90 atom of the Bmp fragment leads the polymeric chain.In the 1 H NMR spectra of 1, only one set of signals in 1 H, 31 C{ 1 H} and 11 B{ 1 H} was obtained due to the fluxional nature of the molecule in the solution.Alkaline earth metal complexes: Determining the structure and reactivity of alkaline earth metal species is an important step in the design and development of efficient catalysts; however, full realisation of the catalytic potential of these elements requires substantial advances in understanding their basic coordination 5 and organometallic chemistry. [16]The heavier alkaline earth metal complexes 2-4 were readily prepared in high yield by the reaction between the sodium salt (1) and corresponding alkaline earth metal diiodides in THF solvent at ambient temperature (Scheme 3).The pure complexes can be obtained from re-10 crystallisation of the crude compound from a concentrated solution of THF.All the complexes are air-and moisturesensitive and sparingly soluble in aromatic solvents such as benzene and toluene.All three compounds were characterised using the spectroscopic/analytic technique and the solid-state The 1 H NMR spectra of complexes 2-4 measured in DMSO-d 6 was particularly broad, revealing four signals (from δ 8.56 to 6.44 ppm for 2, 8.57 to 6.43 ppm for 3 and 8.77 to 6.43 ppm for 25 4), each with an integration of three protons corresponding to the pyridine ring protons.A very broad signal for each complex, centred at δ 3.4 ppm (for 2), 3.59 ppm (for 3) and 3.52 ppm (for 4), with integration of two protons, can be assigned to the two B-H protons present in the Bmp ligand.The broad resonances (δ 30 3.58 and 1.74 ppm for 2, 3.58 and 1.1.74ppm for 3 and 3.59 and 1.71 ppm for 4) are due to the THF protons, which are coordinated to the metal ions.From the integration it can be calculated that there are two coordinated THF molecules in the calcium and strontium complexes, whereas three THF molecules 35 are attached to the barium ion in complex 4. In the 13 C{ 1 H}NMR spectrum, each of the complexes 2-4 displayed five characteristic signals in the low-field region of the spectra.The chemical shift corresponding to the C=S group was found at δ 182.5 (for 2), 182.4 (for 3) and 182.9 (for 4) ppm.9a] In the 11 B{ 1 H} NMR experiment for each of the complexes 2-4, a broad resonance signal at δ 1.11 (for 2), 0.82 (for 3) and 1.73 (for 4) ppm is observed.Although there has been continued interest in alkaline earth organometallics, [17a] particularly in the cyclopentadienyl chemistry of these elements, [17b] complexes 2-4 represent, to the 60 best of our knowledge, the first structurally characterised alkaline earth metal complexes of the Bmp ligand.Therefore, their molecular structures in the solid state were determined using Xray diffraction analysis.X-ray quality single crystals were obtained from the concentrated THF solution of each of the 65 complexes.Complex 2 was found to diffract very weakly under X-ray and data are therefore not publishable.The molecular structures of the strontium and barium complexes are established.Both the complexes 3 and 4 crystallise in the monoclinic space group P2 1 /c, with two and four molecules respectively in the 70 respective unit cell.The data collection parameters are set out in Table 1. Figure 2 and Figure 3 display the molecular structure of strontium complex 3 and barium complex 4 respectively.The solid-state structures of 3 and 4 confirmed a κ 2 -SS coordination mode for Bmp.In complexes 3, Sr1 ion is bonded with four 75 sulphur atoms, S1, S2, S1′ (-x+1, -y, -z) and S2′ (-x+1, -y, -z) from two Bmp ligands.Ba1 is connected to four crystallographically independent sulphur atoms (Sa, S2, S3 and S4) in complex 4. In addition, in complex 3, the strontium ion is coordinated to two THF molecules to adopt a distorted octahedral 80 geometry.In contrast, the barium ion in complex 4 is ligated with three THF molecules to adopt a distorted pentagonal bipyramidal geometry around it.The symmetric Sr-S bond distances [3.0171(8) and 2.9909(10) Å] in complex 3 are in agreement with reported Sr-S bond lengths [2.951(2) Å] in [Sr{S(2,4,6-85 tBu 3 C 6 H 2 )} 2 (THF) 4 ] [18] and [3.025(3) Å] in [Sr 3 (OCSOC 2 H 5 ) 6 (C 2 H 5 OH) 8 ], [19] indicating a major contribution from the 2-thiopyridone form of the Bmp ligands.In the barium complex, the Ba-S distances are not symmetric and a combination of short [Ba1-S1 3.255(1) and Ba1-S4 3.273(1) Å] 90 and long [Ba1-S2 3.346(1) and Ba1-S3 3.449(4) Å] is observed.These sequences of short and long bond distances are in agreement with the Ba-S covalent and coordination bond respectively. [20,21]Thus, it is evident that in complex 4, pyridine-2-thiolate tautomeric form of Bmp is the major contributor (Scheme 1).The variation in ion radii of strontium and barium presumably causes one particular tautomeric form of the 2mercaptopyridine based scorpionate Bmp ligand.Similar observations were reported by Owen et al. [9] The larger Ba-S distances, when compared to Sr-S distances, are due to the larger ion radius of the barium ion (1.35 Å) when compared to the strontium ion (1.12 Å). [22] In complex 3, two eight-membered metallocycles, Sr1-S1-C1-N1-B1-N2-C10-S2 and Sr1′-S1′-C1′-N1′-B1′-N2′-C10′-S2′, (Ba1-S1-C5-N1-B1-N2-C10-S2 10 and Ba2-S3-C11-N3-B2-N4-C20-S4 for complex 4), with a twist-boat configuration are formed by the κ 2 coordination of each Bmp ligand to the strontium ion.Both the sulphur atoms (S1 and S2) are coplanar with the respective pyridine planes (C1-C20C3-C4-C5-N1 and C6-C7-C8-C9-C10-N2) and they 15 bisect each other through B1-N1 and B1-N2 bonds to make a dihedral angle of 76.44º.However, the four sulphur atoms in complex 4 are not coplanar with the pyridine planes and lie slightly above or below the planes (S1 0.143Å above, S2 0.  (Insert Table 1 here.)

Computational analysis:
First-principles calculations were performed to investigate the structure and coordination properties of various metal complexes 50 described in the experimental section of the current study.We were also interested in modelling the elusive calcium complex whose crystals weakly diffracted in X-ray.In addition, we wanted more insight about the structures and bonding of the sodium, strontium and barium complexes by optimising their structure 55 computationally.The chemical nature of bonds between metal and ligands, as well as solvent, was investigated to understand the coordinating behaviour of metal ions.All the model structures were drawn in GaussView [24] and optimised at B3LYP/LANL2DZ level [25] of electronic structure calculation 60 using the Gaussian 09 (Rev B.01) software. [26]The chosen method, combining hybrid density functional and potential for the metals studied here, has been used in earlier studies involving alkaline earth metal complexation. [27]We computed the structures, shown in Figure 4, by following the geometry 65 optimisation procedure.The coordinating environments of metal ions are very similar to the experimentally obtained complex structures for ions other than Ca.A comparison of computed bonds and angles and available experimental data is given in the supporting information.The 70 distances and angles are close to the crystal structure data obtained using X-ray diffraction analysis.For example, calculated Na-S and Na-O distances are 2.99 and 2.27 Å, whereas the experimental values are 2.892 and 2.329 Å, respectively.Table 2 represents the comparison of calculated and 75 experimentally obtained values corresponding to significant coordinating distances around the ions.The symmetric Sr-S bond distances are slightly higher than the experimental values.As observed from earlier discussions, the calculated Ba-S distances in barium complexes are not symmetric and the differences are 80 within 0.2 Å as compared to measured ones.The geometry around calcium shows behaviour similar to that of strontium due to similar ionic radii between the two metal ions.However the Ca-S (3.04-3.10Å) and Ca-O (2.44 Å) distances fall in the range similar to those [2.818(3)Å and 2.495 Å] obtained for [{2,4,6-85 (tBu) 3 C 6 H 2 S}Ca(THF) 4 ] reported by Ruhlandt-Senge et al. [28] The adopted level of calculation can describe the B-H...M interaction satisfactorily.The variation in ionic radii for alkaline earth metal ions plays a major role in the interaction of these metal ions with ligands as well as solvents.The Sr-S distances 90 are symmetric, but only two of the Ba-S distances are the same.Although Ca and Sr show similar interaction towards the solvent THF molecules, the Ba-O distance is the highest among these three.We observed a clear trend of increase in M-S distances as well as M…H(BH 2 ) distances with the increase in ionic radii of 95 metal ions.Though experimental data for the Ca complex is not available, our computational calculations reveal that calcium can make shortest B-H...M contact among the alkaline earth metal ions.The shortest M-H distances are 2.35, 2.67, 2.71, and 2.84 Å for Na, Ca, Sr and Ba, respectively.It is noteworthy that the 100 experimentally obtained value for the Ba complex is between those obtained for the Na and Sr complexes, indicating the stronger M-H interaction between barium and hydrogen atoms compared to other analogues.For a particular metal ion, the hydride interactions with two BH 2 groups are similar, except for Na, where we observed different types of such interactions due to the formation of polymeric forms.and 4) radiation.Crystal data and structure refinement parameters are summarised in Table 1.The structures were solved by direct methods (SIR2004) [29] and refined on F 2 using the full-matrix least-squares method, using SHELXL-97. [30]Non-hydrogen atoms were anisotropically refined.H-atoms were included in the refinement on calculated positions riding on their carrier atoms.

Conclusion
To sum up, we have reported the syntheses of calcium, strontium 20 and barium complexes of the Bmp ligand.In the solid-state structure of the strontium complex 3, a symmetric attachment, κ 2 , of the Bmp ligand to the metal ion is observed.However, an asymmetric κ 2 SS-ligation of Bmp to the barium ion is observed in complex 4.These symmetric and asymmetric attachments are the manifestation of two tautomeric forms, 2-thiopyridone and pyridine-2-thiolate, of the Bmp ligand.We also observed that the sodium complex of the Bmp ligand is polymeric in the solid state.Our computational study also matches well with our experimental observations; in addition, the calculated calcium complex 30 structure is also in agreement with the higher analogues of Sr and Ba complexes.

Figure 4 .X-ray crystallographic studies of complexes 1 , 3 and 4 80 X
Figure 4. Computed structures of complexes are shown in A, B, C and D corresponding to Na, Ca, Sr and Ba, respectively.The coordinating bonds around the metals ions are shown in Å unit.15

Table 2 :
Comparison of computed M-O, M-S and M-H distances with experimental values (where applicable) for the sodium, calcium, strontium and barium complexes.