Novel main group chemistry of the 1,2-diselenido-1,2-dicarba-closo - dodecaborane(12) dianion

Recent results are summarized on some new aspects of the main group chemistry of the 1,2-diselenido-1,2-dicarba-closo -dodecaborane(12)


Introduction
3][4][5][6] Thus, the dilithiated carborane 1,2-Li 2 -1,2-C 2 B 10 H 10 2 can be prepared in solution (in equilibrium with small amounts of 1 and the monolithiated species), and is a highly convenient precursor for further transformations.][9][10][11][12][13] In contrast, the main group chemistry of the dianions has received much less attention.Although the tellurium derivatives are rarely studied, since the clean access to the anion [1,2-(1,2-C 2 B 10 H 10 )Te 2 ] 2-appears to be difficult, 14 the dianions with sulfur or selenium are readily accessible.The dianion [1,2-(1,2-C 2 B 10 H 10 )Se 2 ] 2-3 is particularly attractive from the NMR point of view 15 ( 77 Se NMR: spin I = ½; natural abundance 7.58 %; about three times more sensitive to NMR experiments than 13 C).We have explored for the first time the reactivity of the dianion 3 in more detail for main group chemistry, with emphasis on the synthesis of five-member heterocycles containing a Group 14 element (carbon, silicon, tin) or phosphorus between the selenium atoms.Furthermore, we were interested in oxidative coupling of the dianion 3 as well as in the reactivity of some of the new heterocycles with respect to oxidative addition.Here, we present a summary of our recent results in this field. 16heme 1
The five-member ring 4 containing the CPh 2 unit was identified in the reaction mixture, and crystalline material suitable for X-ray structural analysis (vide infra) could be isolated.16b In the case of the chlorosilanes, repeated attempts failed to prepare the silaheterocycle 5 with the SiMe 2 unit.In contrast, the reaction of 3 with Ph 2 SiCl 2 gave the expected five-member ring 6 along with traces of a non-cyclic product 7, from the monosubstituted ortho-carborane, and another five-member ring 8. 16b The latter results either from rearrangement of 4 or from changes in the nature of the dianion 3 during its reaction with the silicon chloride, taking into account the somewhat unpredictable behavior of the analogous ditellurido dianion. 14The proposed structure of 8 follows from the 77 Se NMR spectrum (Fig. 1).The NMR parameters (δ 77 Se and 1 J( 77 Se, 77 Se), and 1 J( 77 Se, 29 77 Se satellites (asterisks) corresponding to 1 J( 77 Se, 77 Se) = 300 Hz.The signal at lowest frequency shows also 29 Si satellites (arrows) typical of 1 J( 77 Se, 29 Si) = 132 Hz.
In the cases of the diorganotin chlorides, the expected products could be isolated in reasonable yields (> 70%).Thus, the reaction of 3 with Ph 2 SnCl 2 afforded pure samples of 11, after recrystallization, suitable for X-ray structural analysis (vide infra).The 13 C NMR spectrum of the reaction mixture shows the presence of both 11 and 12 (Fig. 2).Frequently, the 13 C(carborane) NMR signals are not reported in the literature, since there intensity is rather weak and the signals are broadened by partially relaxed 13 C-11 B spin-spin coupling. 3However, the δ 13 C values are diagnostic and coupling constants such as 1 J( 77 Se, 13 C) or 2 J( 117/119 Sn, 13 C) are extremely useful in structural assignments.Similarly to Ph 2 SnCl 2 , the reaction of 3 with Me 2 SnCl 2 gave 9 along with a small amount of 10, although 9 turned out to be fairly instable.16b Decomposition products were the known bis(diselane) 18 16a and dimethyltin selenide (Me 2 SnSe) 3 .
Depending on the orientation of the phenyl group, the phospholane 13 (Scheme 3) can in principle be formed as a mixture of isomers.However, only a single isomer 13 was observed in the reaction solutions and could be isolated as a crystalline solid (X-ray analysis; vide infra).The calculation of the gas phase structures [B3LYP/6-311+G(d,p) level of theory] 17b-f of the parent isomers of 13 gave the lower energy for the structure analogous to 13. 16c As usual for phosphanes, the configuration of 13 was retained upon oxidation with elemental sulfur (X-ray analysis; vide infra) or selenium.16c The formation of the selenide 15 required prolonged times of heating in inert solvents, much longer than for the sulfide 14, and was accompanied by extensive decomposition.The bis(diselane) 18 was identified as the major decomposition product along with Ph 3 PSe and various unidentified phosphorus compounds in low concentration.16c Attempts failed to prepare a phosphonium salt from the reaction of 13 with an excess of methyl iodide.Instead, the selenophosphonic acid 16 was formed, because of partial hydrolysis of 13 owing to traces of water present in the commercial methyl iodide. 31P NMR spectra showed again the formation of the bis(diselane) 18 along with other unknown side products in low concentration.16c In one further attempt to prepare a larger amount of 13, a few crystals of another less soluble phosphorus compound 17 could be isolated, and its molecular structure was determined by X-ray analysis.16e The analogous sulfur compound has been described previously.6b

Oxidative coupling of the 1,2-diselenido-1,2-dicarba-closo-dodecaborane(12) dianion 3
Oxidative coupling of the dianion 3 by its reaction with iodine afforded the bis(diselane) 18 in high yield (Scheme 4).Interestingly, 18 was found also as the major product of other reactions which originally were intended to serve quite different purposes.

Scheme 4
The reaction of 3 with two equivalents of Me 3 SiCl gave the silane derivative 19 in high yield, together with a small amount of the monosubstituted carborane, proving the maximum conversion of 2 into 3.Our current interest in tropylium derivatives [18][19][20] prompted us to study the reaction of 19 with tropylium bromide, C 7 H 7 Br.Surprisingly, the reaction went towards the bis(diselane) 18, and the tropylium derivative could not be observed.In the case of other trimethylsilylselanes Me 3 Si-SeR, this type of reaction gave tropylium selenides C 7 H 7 -Se-R, most of which are fairly stable until room temperature before they decompose. 20Another attempt was made to extend the silane chemistry of 3, aiming at the synthesis of a spirosilane by treatment of SiCl 4 with two equivalents of 3. Instead of the desired silane, again the bis(diselane) 18 could be obtained and could be isolated in reasonably high yield.16a

Oxidative addition of the new cyclic selanes to bis(triphenylphosphane)ethene-platinum(0)
The Se-Se in diselanes 21 and Sn-element bonds in many organotin compounds [22][23][24][25] invite for application in oxidative addition reactions.On the other hand, the phospholane 13 can react with a Pt(0) complex either as a donor via the lone pair of electrons at phosphorus or by oxidative addition of one of the P-Se bonds.Therefore, we have studied the reactivity of 18, 9, 11 and 13 towards bis(triphenylphosphane)ethene-platinum(0) (Scheme 5).
ARKAT USA, Inc.In a clean reaction, oxidative addition of the bis(diselane) 18 to [Pt(PPh 3 ) 2 CH 2 =CH 2 ] afforded the complex 20 with the chelating 1,2-diselenido-1,2-dicarba-closo-dodecaborane(12) ligand and displacement of ethene.16a Complex 20 deserves interest as a potential catalyst for the cis-addition of the Se-Se bond to the C≡C bond in alkynes.21c-d It is fairly stable and well characterized by all spectroscopic data in solution and X-ray structural analysis in the solid state.16a Cleavage of the Pt-Se bonds in 20 was observed, when it was reacted with an excess of methyl iodide, giving in the beginning cis-[Pt(PPh 3 ) 2 I 2 ], and the anionic complexes [Pt(PPh 3 )I 3 ] ⎯ and [PtI 4 ] ‫-2‬ with [P(Me)Ph 3 )] + as the counterion.The complex cis-[Pt(PPh 3 ) 2 I 2 ] slowly rearranged into its more stable trans-isomer.16a Oxidative addition of the 1,3,2-diselenastannacycles 9 and 11 to [Pt(PPh 3 ) 2 CH 2 =CH 2 ] gave at low temperature the bis(triphenylphosphane)platinum(II) complexes 21 and 22, respectively, where in each case the (PPh 3 ) 2 Pt fragment was inserted into one of the Sn-Se bonds.The complexes 21 and 22 decompose at temperatures above -20 °C.The proposed structure is supported by the NMR data (see Fig. 3 for the 31 P NMR spectrum).The complex 20 was found as the major decomposition product, accompanied by (Me 2 SnSe) 3 in the case of 21, and Ph 3 PSe and Se(SnPh 3 ) 2 in the case of 22, and numerous other compounds which could not be identified unambiguously.16b The reaction of the phospholane 13 with [Pt(PPh 3 ) 2 CH 2 =CH 2 ] displaces ethene.The conceivable first reaction products 23 or 24 were not detected by 31 P NMR spectroscopy at -40 °C.However, oxidative addition of one P-Se bond must have occurred, followed immediately by an Arbusov-type rearrangement 25 into the platinum(II) complex 25.In 25, the new chelating ligand is linked to platinum via Pt-Se and Pt-P(Se) bonds, a rare example of a metallophosphane selenide.16c As for 21 and 22, the complex 25 turned out to be fairly instable at temperatures above -20 °C, and the identification of 25 had to rely entirely on NMR data (Figs.4, 5).Prominent decomposition products were the complex 20, 16a Ph 3 PSe, and other unknown species.So far, selenophosphanes comparable with 13 have not been used for oxidative addition reactions of a P(III)-Se bond to Pt(0) or Pd(0) centers.A single report describes the reaction of [M(PEt 3 ) 3 ] (M = Pd, Pt) with P(O)(OPh) 2 SePh, where a P(V)-Se bond is added to the metal. 24Attempts to prepare a Pd(II) complex from the reaction of 13 with [Pd(PPh 3 ) 4 ] were not successful.A large number of broad 31 P NMR signals in the range for coordinated PPh 3 and 13 was observed, indicating an exchange between 13 and PPh 3 at palladium without addition of a P-Se bond.16c

Experimental Section
General Procedures.All syntheses and the handling of the samples required precautions to exclude traces of air and moisture and therefore, carefully dried solvents and oven-dried glassware were used throughout.The complex [(Ph 3 P) 2 PtC 2 H 4 ] 33 , 1,2-dicarba-closododecaborane-1,2-diselenolate 14a,16a and tropylium bromide 34 35 all other NMR spectra were recorded by single pulse methods.The melting points (uncorrected) were determined using a Büchi 510 melting point apparatus.All calculations were carried out using the Gaussian program package.17a The optimized structures were identified as minima on the potential energy surface by the absence of imaginary frequencies in the corresponding calculations.

Reaction of 13 with methyl iodide
To a solution of 13 (0.13 g, 0.32 mmol) in CD 2 Cl 2 (1 mL) at room temperature was added methyl iodide (0.07 g, 0.48 mmol) (without previous purification).The mixture was stirred overnight at room temperature and analysed by NMR spectroscopy.The 31 P, 77 Se, 13 C and 1 H ARKAT USA, Inc.
NMR spectra showed the presence of selenophosphonic acid 16 as the main product (see Scheme 1, the bis(diselane) 18 16a and other unidentified compounds in low yield [δ 31 P = 28.8 1 J( 77 Se, 31

Figure 3 .
Figure 3. 101.3 MHz 31 P{ 1 H} NMR spectrum of the reaction solution in CD 2 Cl 2 (recorded at -20 °C, immediately after mixing the starting materials and warming from -78 to -20 °C).There is still much left of [Pt(PPh 3 ) 2 CH 2 =CH 2 ], and the Pt(II) complex 21 starts to be formed as the result of oxidative addition.However, there are already weak signals belonging to decomposition products (e.g.20 and Ph 3 P=Se). 195Pt and 117/119 Sn satellites are marked by asterisks and arrows, respectively.The assignment of the 2 J( 119 Sn, 31 P) data is confirmed by the 119 Sn NMR spectrum, and the data are typical for a trans-and cis-coupling pathway.

Figure 5 .
Figure 5. 53.5 MHz 195 Pt{ 1 H} NMR spectrum of the platinum complex 25.The splitting into doublets of doublets of doublets corresponds exactly to the 195 Pt-31 P spin-spin coupling observed in the 31 P NMR spectrum (see Figure 4).