Experimental and Theoretical Investigations of the Contact Ion Pairs Formed by Reaction of the Anions [(EPR 2 ) 2 N] - (R = i Pr, t Bu; E = S, Se) with the Cations [(TePR 2 ) 2 N] + (R = i Pr, t Bu)

The reactions of the sodium salts [(EPR 2 ) 2 N]Na(TMEDA) (R = i Pr, t Bu; E = S, Se) with the iodide salts [(TePR 2 ) 2 N]I (R = i Pr, t Bu) in toluene produce the mixed chalcogen systems [(EPR 2 ) 2 N][(TePR 2 ) 2 N] ( 6b , E = Se, R = t Bu; 6c , E = S, R = t Bu; 7b , E = Se, R = i Pr; 7c , E = S, R = i Pr). Compounds 6b, 6c , 7b and 7c have been characterized in solution by variable temperature multinuclear ( 31 P, 77 Se and 125 Te) NMR spectroscopy and in the solid state by single crystal X-ray crystallography. The structures are comprised of contact ion pairs linked by bonds between tellurium and sulfur or selenium atoms. For the tert-butyl derivatives 6b and 6c the anionic half of the molecule is coordinated in a bidentate (E,E′) fashion to one tellurium atom of the cationic half to give a spirocycle, whereas in the iso -propyl derivatives 7b and 7c the anion acts as a monodentate ligand with only one E-Te bond and the second sulfur or selenium atom pointing away from the cation. Comparison of the chalcogen-chalcogen bond orders in 6b , 6c , and the all-tellurium system 6a (E = Te), as determined from the experimental bond lengths, shows that the Te-Te bond order in the cations decreases as the strength of the E-Te interaction increases. This trend is attributed to increased electron donation from the anion into the LUMO [σ*(Te-Te)] of the cation along the series S < Se < Te. A similar trend is observed for the monodentate contact ion pairs 7b and 7c . Density functional theory calculations provided information about the relative energies of bidentate and monodentate contact ion pair structures and the extent of intramolecular electron transfer in these systems.

dichalcogenidoimidodiphosphinates, have attracted a great deal of attention since their original discovery by Schmidpeter 45 years ago. 1,2 In the case of E = O, S, Se these monoanions are readily accessible by deprotonation of the corresponding imides and a large variety is available as a consequence of varying the chalcogen atoms and/or the organic groups bound to phosphorus. The primary focus has been the coordination chemistry of these bidentate ligands, and several reviews that provide details of the complexes of 1 with a diverse array of main group, lanthanide and transition metals have been published. [3][4][5] A significant recent application of these complexes, particularly in the case of the selenium derivative (1, E = Se, R = i Pr), has been as single-source precursors for the generation of metal selenides in the form of semiconducting thin films [6][7][8][9][10][11][12][13][14][15][16][17] or quantum dots 18 via CVD or solvothermal processes.
The success of this approach to binary metal selenides provided the impetus to develop a synthesis of the tellurium analogues (1, E = Te) with the anticipation that the corresponding metal complexes would serve as single-source precursors to metal tellurides. Unlike the lighter dichalcogenido congeners (1, E = S, Se), the N-protonated derivatives of the ditellurido ligands cannot be prepared by direct oxidation of R2PN(H)PR2 with tellurium. However, deprotonation of the PNP backbone in these P(III)/P(III) systems with sodium hydride prior to oxidation with tellurium affords these ligands as sodium salts (2, E = E' = Te; R = Ph, i Pr; M = Na). 19,20 The coordination chemistry of the iso-propyl derivative has subsequently been studied with a variety of main group, 20, 21 transition 21-24 and f-block 25,26 metals. New structural arrangements have been observed, compared to those observed for the metal complexes of 1 (E = S, Se), especially for the coinage metals, as a result of the propensity of 1 (E = Te) to act as a doubly bridging ligand. 22 Several of these complexes have been successfully employed as single source precursors to binary metal tellurides, e.g. CdTe, 27 In2Te3, 28 Sb2Te3 29 and PbTe. 30 Very recently, the conditions for the optimal syntheses of lithium derivatives of heterodichalcogenidoimidodiphosphinates containing tellurium and either sulfur or selenium (2, E = Te, E' = S, Se; R = i Pr; M = Li) were reported. 31 These novel reagents were subsequently used in metathesis reactions to prepare homoleptic complexes with group 10 metals. 32,33 In addition to coordination complexes, a new aspect of the chemistry of anions of the type 1, viz. redox behavior, has been comprehensively investigated. Two-electron oxidation of these anions with iodine yields a variety of cations of the type 3, which contain a puckered five-membered ring. 34 When a coordinating counter-anion such as I -is present, elongation of the E-E bond in the cation is observed. On the basis of density functional theory (DFT) calculations this elongation has been attributed to donation of electron density from a lone pair of the iodide anion into the LUMO of the cation, which is the  * (E-E) orbital. Such bond elongation is not evident in ion-separated salts containing non-interacting SbF6as the anion. 35 Another intriguing aspect of the redox chemistry of this class of compounds is the formation of dimers via either one-electron oxidation of the anions in alkali-metal salts (2) 36, 37 or one-electron reduction of the cations (3). 38 Such dimeric species have so far been characterized as two distinct structural isomers; the majority of examples exist as dichalcogenide dimers (4) with an elongated E-E interaction. 36,37 For the heterodichalcogenidoimidodiphosphinate (2, E ≠ E′), the resulting dimer is exclusively obtained as a ditelluride with a Te-Te interaction rather than an E-E or E-Te interaction. 38 The second bonding mode is best described as a spirocyclic contact ion pair (5) in which a dichalcogenido anion is chelated to one of the chalcogen atoms of the corresponding cation. 37 To date this structural type has been limited to the all-tellurium system with R = t Bu. However, DFT calculations predict that the difference in the relative energies of the two isomers 4 and 5 is small (< 20 kJ mol -1 ) for the heavier chalcogenido derivatives (E = Se, Te; R = i Pr, t Bu). 37 The above-mentioned one-electron redox processes are limited to the formation of dimers in which the two halves are identical. We were intrigued, therefore, to elucidate

Results and Discussion
Synthesis and NMR spectra of 6b,c and 7b,c.  Table 2). The chemical shifts are compatible with a contact-ion pair structure rather than a dichalcogenide of the type 4. In each case, the tellurium-bound phosphorus atoms display chemical shifts which are downfield-  Table 2) compared to those in the corresponding iodide salts. The S/Se-bound phosphorus atoms of 6b,c and 7b,c also display chemical shifts (and 1 J(P-Se) coupling constants in the case of 6b and 7b) within close proximity to those of the Na-bound  Table 3. The structures of 6b ( Figure 1) and 6c ( Figure 2) confirmed that these chalcogen-rich systems exist as spirocyclic contact ion pairs with a similar molecular arrangement to that previously observed for the all-tellurium containing derivative 6a, i.e. the [EP t Bu2NP t Bu2E] -(E = S, Se) anion is chelated via both chalcogen atoms to one tellurium atom of the [(TeP t Bu2)2N] + cation. 37 The crystal structures of the iso-propyl derivatives 7b (  Table 4. In the monodentate contact ion pairs 7b and 7c the coordinated E(1) atom also forms an approximately linear E-Te-Te structure (7b, 175.28 (2)  The data in Table 4  when monodentate coordination of the anion is enforced.
Finally, there is an obvious disparity between the P-Te bond lengths in these contact ion pairs, with the Te(1)-P(1) bond being longer than the Te(2)-P(2) bond by ca.
0.15 Ǻ for 6b and 6c and 0.06-0.07 Ǻ for 7b and 7c. This observation, which is also apparent in the structure of 6a, 37 is consistent with the difference of 45-80 Hz in the 1 J(P-Te) coupling constants for 7b and 7c at low temperature.
Theoretical Investigations of Structural Isomers of 6 and 7. DFT level geometry optimizations were performed for 6b and 7b as well as for 6c and 7c. Pertinent optimized structural parameters are listed in Table 3; the calculated geometries of 6a and 7a have been reported in an earlier publication. 37 There is a quite good agreement between the theoretical and experimental results, the most notable discrepancies being observed in the bond lengths involving the Te(1) center. In general, DFT calculations overestimate the anion-cation interaction and concomitantly give E-Te(1) and Te(1)-Te(2) bond lengths which are too short and long, respectively. This is most apparent in case of 7b and 7c which coordinate in a monodentate fashion and for which the difference in bond lengths is as much as 0.10 Å. Nevertheless, the trends in optimized structural parameters parallel the data from X-ray structural determinations.
The observed preference of 6b,c and 7b,c to adopt a contact ion pair structure can be explained by the apparent weakness of the E-Te (E = S, Se) bond which would be formed in the dichalcogenide alternative. As noted earlier, the known heterodichalcogenide dimers are exclusively obtained as ditellurides. 38 However, it is not as straightforward to rationalize why 7b and 7c adopt a monodentate coordination mode.
To investigate this further, we performed geometry optimizations for 6a-c and 7b,c using monodentate structures for the former and bidentate for the latter. An energetic comparison between the two isomers shows that the tert-butyl derivatives 6a-c favor the bidentate coordination mode by a clear margin, approximately 35-45 kJ mol -1 .
Interestingly, the bidentate coordination mode is more favorable also for 7b and 7c though the energetic preference is less clear, 10 and 5 kJ mol -1 for the former and latter, respectively. This indicates that crystal packing effects might play a role in stabilizing the monodentate structure. Indeed, the X-ray structure of 7b displays several short intermolecular contacts involving the dangling chalcogen center, whereas none are observed for the tert-butyl analogue 6b.
The charge distributions in 6 and 7 were examined by calculating the molecular electrostatic potentials (ESPs) and plotting the results on molecular van der Waals surfaces, i.e. surfaces for which the total electron density equals 0.001 a.u. The results are shown in Figure 5 for 6c and 7c; the distributions for the corresponding selenium and tellurium systems are qualitatively similar to the given data and deserve no further discussion. As evident from Figure 5, the anionic and cationic halves of the structures can be easily identified and the description of 6a-c and 7b-c as contact ion pairs is clearly warranted. In both isomers, the E(2) atom bears a significant local charge concentration.
This is particularly visible in the monodentate structure in which the E(2) chalcogen atom remains uncoordinated and is surrounded by a negative charge cloud. This is not unexpected considering the shape of the HOMO in the anions, which corresponds to the  * (E-E) LUMO of cation. 34,35 The amount of charge transferred from the anion to the cation can be estimated by summing the atomic partial charges within the two halves of the contact ion pair structure; the calculated values are given in Table 4. As expected, the observed trend parallels the trend seen in Te(1)-Te(2) bond lengths. In both sets 6 and 7, the sum of atomic charges (in absolute sense) decreases in the order S > Se > Te, indicating greatest charge transfer for 6a and, hence, the longest Te(1)-Te(2) bond length. We note that the charges reported in Table 4 were obtained by employing the natural population analysis, but the trend is reproducible using e.g. Mulliken or Hirschfield approaches. However, the absolute values given by each method are expected to differ considerably. Different computational approaches give rise to vastly different values for atomic properties since they use different criteria to divide the total electron density between atoms in a molecule. Hence, it is best not to put too much weight on absolute values, but instead look at the predicted trends.

Conclusions
In an earlier publication 37  parameters were calculated and refined from the full data set. Crystal cell refinement and data reduction were carried out using the Nonius DENZO package. After data reduction, the data were corrected for absorption based on equivalent reflections using SCALEPACK (Nonius, 1998). All structures were solved by Patterson techniques using SHELXS-97, 50 while refinements were carried out on F 2 against all independent reflections by the full-matrix least-squares method by using the SHELXL-97 program. 51 The H atoms were calculated geometrically and were riding on their respective atoms, and all non-H atoms were refined with anisotropic thermal parameters. Crystallographic data are summarized in Table 1.             56.0 a Low temperature data were collected at 185 K for 6b and 6c and at 200 K for 7b and 7c.