Complexes of TaOCl 3 and TaSCl 3 with Neutral N- and O-donor ligands synthesis, properties and comparison with the niobium analogues.

The white complexes, [TaOCl 3 (OPPh 3 ) 2 ], [TaOCl 3 (L-L)] (L-L = 1,10-phenanthroline, 2,2’-bipyridyl, Ph 2 P(O)CH 2 P(O)Ph 2 , Ph 2 P(O)CH 2 CH 2 P(O)Ph 2 and o -C 6 H 4 (P(O)Ph 2 ) 2 ), have been prepared from TaCl 5 , O(SiMe 3 ) 2 and the ligands in anhydrous CH 2 Cl 2 solution. The corresponding yellow [TaSCl 3 (OPPh 3 ) 2 ] and [TaSCl 3 (L-L)] were made similarly using S(SiMe 3 ) 2 . The complexes have been characterised by microanalysis, IR and NMR ( 1 H, 31 P{ 1 H}) spectroscopy. X-ray crystal structures have been obtained for [TaOCl 3 (1,10-phen)], [TaSCl 3 (1,10-phen)], [TaOCl 3 { o -C 6 H 4 (P(O)Ph 2 ) 2 }], [TaSCl 3 (OPPh 3 ) 2 ], [TaSCl 3 {Ph 2 P(O)CH 2 CH 2 P(O)Ph 2 }] and [TaSCl 3 (MeCN) 2 ], which all contain mer -chlorines and with the neutral ligands trans to O/Cl or S/Cl. The structure of the Ta(V) dimer [Cl 2 S(1,10-phen)Ta(µ-O)Ta(1,10-phen)SCl 2 ], formed by trace hydrolysis, is also reported. Comparisons between the complexes of TaOCl 3 and TaSCl 3 and their niobium analogues are discussed. 1 added dropwise to the solution and the reaction mixture was stirred for a further 2 h. No visible change was observed. The mixture was filtered giving a light brown solution that was concentrated in vacuo yielding a light brown solid. This was collected by filtration and dried in vacuo . Yield: 0.251 g, 60%. Yellow crystals were obtained by layering a dichloromethane solution of the product with n-hexane. Anal: Required for C 12 H 8 Cl 3 N 2 STa (499.35): C, 28.84; H, 1.61; N, 5.61. Found: C, 28.76; H, 1.59; N, 5.51 [H]), [H]), [2H]), [H]), [H]), (m, [H]), spectrum (Nujol)/cm

Recent detailed studies by Marchetti et. al. [15,16,17,18]  It is also notable that attempts to prepare neutral ligand adducts of TaOF3 have failed, whereas niobium analogues are well established [10].
Here we report the synthesis, spectroscopic and structural analyses of TaOCl3 and TaSCl3 with phosphine oxide and diimine ligands and compare these to their Nb analogues.

Complexes of TaOCl3
Direct reaction of (insoluble) polymeric TaOCl3 with neutral ligands is not a viable route to the desired complexes, and in this work the reaction of TaCl5 with O(SiMe3)2 and the appropriate ligand in MeCN or CH2Cl2 solution was used. After a variety of trial and error variations of the reaction conditions, the best routes were identified and are described in Section 3. Obtaining pure samples depends upon careful control of the reaction conditions, including the ratio of the reactants and maintaining rigorously anhydrous conditions. The diimine complexes [TaOCl3(1,10-phen)] and [TaOCl3(2,2'-bipy)] are very poorly soluble in weak donor solvents like chlorocarbons and MeCN, a property common to complexes of these ligands with early d-block halides in high/medium oxidation states [12,26,27,28,29,30]. They are readily hydrolysed in solution with formation of protonated diimine, and generally seem less robust than the niobium analogues. Colourless crystals of [TaOCl3(1,10-phen)] were grown by slow evaporation of a CH2Cl2 solution of the complex in the glove box. The structure (Figure 1) confirms the complex as a monomer with a terminal Ta=O and shows a six-coordinate tantalum centre with the diimine trans to O/Cl. In contrast to many complexes of this type [27 and below], the structure is free of O/Cl disorder. The tantalum coordination sphere is distorted by the short chelate bite of the rigid 1,10-phenanthroline, and the axial Cl-Ta-Cl unit is bent away from the Ta=O group (Cl3-Ta1-Cl1 = 161.7°). The d(Ta-Cl)trans-N is shorter than the d(Ta-Cl)transCl and d(Ta=O) is 1.7268(13) Å. As indicated in the Introduction, although a range of complexes of TaOCl3 has been reported, a search of the CCDC (accessed January 2019) shows the [TaOCl3(1,10-phen)] is the first crystallographically authenticated example of a mononuclear complex of TaOCl3. The IR spectra of [TaOCl3(1,10-phen)] and [TaOCl3(2,2'-bipy)] show the single ν(Ta=O) at 941 and 938 cm -1 , respectively, consistent with terminal Ta=O double bonds, and two ν(Ta-Cl) bands in the region 315-350 cm -1 (theory: three bands, 2A1 + E, although often only two bands are resolved in practice [12,27]). The poor solubility made obtaining NMR spectra difficult, but the complexity of the 1 H NMR spectra is consistent with inequivalent aromatic rings as found in the crystal structure; weaker resonances due to some protonated diimine formed by hydrolysis were also typically present.  [12,31]. In the tantalum case the O/Cl trans to the phosphine oxides were disordered (as they are in the niobium complexes), and in view of the mediocre crystal quality and apparent disorder, the structure is not reported in detail here (see Supplementary Information).
Several sets of data were collected on crystals grown from [TaOCl3{o-C6H4(P(O)Ph2)2}], all of which produced the same basic structure on refinement. The data showed a well-defined local environment about tantalum, but had disorder in two of the aromatic rings. The disorder problem is discussed in the Supplementary Information. Because of this disorder, the metrical data needs to be viewed with care, but it is certainly good enough to identify the structure as shown in Figure 2, which reveals the second example of a mononuclear TaOCl3 complex.

Complexes of TaSCl3
Previous syntheses of TaSCl3 complexes have used pre-isolated TaSCl3, which was subsequently reacted with the ligands in an appropriate solvent [21,24]. In the present work, TaSCl3 was generated in situ by reaction of TaCl5 and S(SiMe3)2 in CH2Cl2, followed by addition of the appropriate ligand. The complexes, [TaSCl3(L-L)] (L-L = 2,2'-bipy, 1,10-phen, dppmO2, dppeO2 and PPO2) and [TaSCl3(OPPh3)2] were obtained as yellow, moisture sensitive powders in 50 -80% yield (Scheme 2). The reaction of TaCl5 and S(SiMe3)2 in MeCN generated the known yellow complex, [TaSCl3(MeCN)2] [21,24]. The spectroscopic data were in good agreement with the literature and are not discussed, but the X-ray structure was also determined and is shown in Figure 3. The X-ray structure of [TaSCl3(1,10-phen)] is shown in Figure 4 and is similar to that of [TaOCl3(1,10-phen)] discussed above, although in the thiochloride complex the S/Cl in plane are disordered. As discussed elsewhere [20,22,23], disorder is a particular problem in thiochloride complexes since the similar scattering power of S and Cl makes it very difficult/impossible to establish that some degree of disorder is not present.
If disorder is absent (or only slight), one would expect d(Ta-Cl) and d(Ta=S) to differ by ~ 0.15 Å and the d(Ta-N) should also differ due to the different trans influence of S and Cl [22,24].

Experimental
Syntheses were performed by using standard Schlenk and glove-box techniques under a dry N2 atmosphere.
Infrared spectra were recorded on a Perkin-Elmer Spectrum 100 spectrometer in the range 4000-200 cm −1 , with samples prepared as Nujol mulls between two CsI plates. 1 H NMR spectra were recorded using a Bruker AV 400 spectrometer and referenced to the residual protio-resonance of the solvent. 31 P{ 1 H} NMR spectra were obtained from CD2Cl2 solutions using a Bruker AV 400 spectrometer and referenced external 85% H3PO4.
Microanalyses on new compounds were undertaken by London Metropolitan University.

X-ray experimental:
Data collections used a Rigaku AFC12 goniometer equipped with an enhanced sensitivity (HG) Saturn724+ detector mounted at the window of an FR-E+ SuperBright molybdenum (λ = 0.71073) rotating anode generator with VHF Varimax optics (70 micron focus) with the crystal held at 100 K (N2 cryostream).
Crystallographic parameters are in the (Table 1). Structure solution and refinement were performed using SHELX(S/L)97, SHELX-2014/7 [33], H atoms were added and refined with a riding model. In [TaOCl3(PPO2)] discordant thermal displacement parameters were interpreted as disorder in two of the rings and refined as such using geometrical and thermal parameter restraints and constraints (see SI).

[TaSCl3(dppeO2)]
TaCl5 (0.150 g, 0.419 mmol) was dissolved in dichloromethane (10 mL) at 50 ⁰C and then cooled to room temperature. To the clear solution was added S(SiMe3)2 (0.075 g, 0.419 mmol) in dichloromethane. A solution of dppeO2 (0.180 g, 0.419 mmol) in dichloromethane (5 mL) was then added dropwise. After 1 h., the reaction mixture was filtered giving a bright yellow solution. The solution was concentrated in vacuo and a yellow precipitate was formed through the addition of hexane (2 mL

[TaSCl3(PPO2)]
TaCl5 (0.150 g, 0.419 mmol) was dissolved in dichloromethane (10 mL) at 50 ⁰C and then cooled to room temperature. To the solution was added S(SiMe3)2 (0.075 g, 0.419 mmol) in dichloromethane (3mL) forming a brown precipitate. A solution of PPO2 (0.200 g, 0.419 mmol) in dichloromethane (5 mL) was added dropwise and the precipitate immediately dissolved. After 10 min. a straw coloured precipitate formed. The reaction was stirred for 1 h., then filtered, giving a dark yellow solution. This was concentrated and a yellow precipitate formed, which was filtered off and dried in vacuo. Yield: 0.220 g, 66 %. Anal: Required for C30H24Cl3O2P2STa

[TaSCl3(MeCN)2]
TaCl5 (0.30 g, 0.837 mmol) was stirred in acetonitrile (5 mL) and then S(Me3Si)2 (0.149 g, 0.837 mmol) dissolved in acetonitrile added the mixture and stirred for 16 h., when it turned from yellow to orange to a very dark brown. The solution was concentrated, and hexane (1 mL) was added, forming some yellow crystals suitable for X-ray structure determination. The complex has been reported previously [24] and the spectroscopic data are in excellent agreement. 1

Conclusion
Two series of complexes derived from TaOCl3 and TaSCl3 with neutral N-and O-donor ligands have been synthesised from TaCl5 and O(SiMe3)2 or S(SiMe3)2, respectively and characterised spectroscopically and in seven cases by X-ray crystallography. The X-ray structures have confirmed rare octahedral monomer structures derived from TaECl3. The tantalum complexes appear significantly less robust than their niobium analogues [10], being both more difficult to obtain in a pure form and more readily hydrolysed in solution, with loss of the neutral ligand. Although the thiochloride complexes appear less stable than the oxochloride analogues, suggesting TaSCl3 is the weaker Lewis acid, comparison of the spectroscopic data on the phosphine oxide complexes, specifically ν(P=O) and δ( 31 P), reveals only small and irregular differences between comparable complexes.