6370 was described as mercury catalyzed. Our present studies indicate that traces of phosphinogermane oxidation products are in fact the catalyst instead of mercury. (26) A referee suggested that the observed catalytic effects in this reaction may be the result of general acid catalysis. This possibility is being ex-
amined currently in our laboratories on model phosphinogermane tems.
sys-
(27) M. Baudler, H. Standeke, M. Borgardt, and H. Stravel, Natufwissenschaften, 52, 345 (1965); M. Baudler, H. Standeke, M. Borgardt, H. Strahel. and J. Dobbers, ibid.,53, 106 (1966).
Structure and Dynamics of Catenated Phosphorus( 111) Dianions P. R. Hoffman and K. G . Caulton* Contribution No. 2731 from the Department of Chemistry, Indiana University, Bloomington, Indiana 47401. Received April 9, 1975
Abstract: 3 ’ P N M R is used to study the structure of MZ(PPh), ( M = Li, N a , K, and Cs; n = 3 and 4) species in tetrahydrofuran. Analysis of phosphorus-phosphorus coupling constants at low temperatures shows that, for n = 4, the sodium and potassium compounds involve chelation of the metal ion by the phosphorus chain (“intimate ion pairing”), while the lithium compound has an acyclic structure. Exchange processes are evident in all three compounds at higher temperatures: for M = N a and K, the coupling constants indicate equilibration to acyclic structures. The compound Kl(PPh)3, which is said to be a salt of the delocalized cyclic dianion (PPh)3*-, is found to exhibit concentration-dependent ) ’ P N M R spectra. This is attributed to “ion pairing” equilibria. Addition of a crown ether dramatically alters the phosphorus chemical shifts of Kl(PPh)3 but leaves the coupling constants unchanged. It is concluded that (PPh)3*- is in reality acyclic, with only two phosphorusphosphorus bonds.
The synthesis and characterization of polyphosphine dianion “salts”, M2(PR),, is due almost exclusively to Issleib and coworkers.’-I0 When R = c 6 H ~ ( P h ) the , preparation involves reaction 1. 5 (PPhh M : ;M2(PPh), (1)
-
+
associated with each chain terminus. The nature of the M-P bond is generally unspecified but “univalent” bonding is an implicit assumption. In the several instances where a magnesium salt was isolated,’ cyclic structures (11) were written, consistent with the “divalency” of magnesium. R
Here M is an alkali metal and n = 1-4. The resultant phosphide compounds are extremely potent nucleophiles. They form cyclic condensation products with both alky14*5-1’,12 R and aryl13 dihalides. With monohalogenated reagents, the I1 expected straight-chain species may be generated. However, if the final phosphorus chain length exceeds two, the We now find that the phosphorus N M R spectra of primary product decomposes to the biphosphine, (PPhR)2, M2(PPh), species are not consistent with simple straightand the cyclopolyphosphine, (PPh)s. Reactions with transichain structures (I). The spectra exhibit complex cation and tion metal complexes are also k n ~ w n . ~ . ’ ~ temperature dependencies. However, the long range couEarlier work3,4~7-10~’3 suggests that the species M z ( P R ) ~ pling which exists in catenated phosphorus compounds serves as a somewhat unique structural probe which proare acyclic. The compound K2(PPh)3 represents the only exception. The 31P N M R singlet observed for this comvides an insight into the solution geometry of M*(PPh), species. pound has been interpreted6 in terms of a cyclic structure with two electrons delocalized in a T orbital. On the other
Experimental Section
Ph
I P
hand, electrochemical experiments have been interpretedI5 as indicating that K2(PPh)3 does not produce a cyclic dianion in solution; the N M R result is attributed to an unspecified “rapid exchange”. These alkali metal compounds are conventionally written as I, implying a truly open chain structure with one metal M\ R R /P+P),
.p
/M -p\R
I Journal of the American Chemical Society
/
97:22
/
Phosphorus N M R spectra (Fourier transform) were recorded at 40.5 M H z as previously described.I6 Chemical shifts are reported with respect to 85% H3P04, downfield chemical shifts being positive. Proton spectra were recorded at 220 M H z . Analyses were performed by Schwarzkopf Microanalytical Laboratory, Woodside, N.Y. Iterative spectral calculations utilized the program L A O C N ~ . ’ ’ For the AA’BB’ spectra, eight possible line assignments were typically made with iteration carried out on each. Typical root mean square errors for the best solution are 0.5 Hz, which is less than the error introduced by digitization. The parameters in Table I are therefore uniquely determined by the data. Materials. Pentaphenylcyclopentaphosphine, (PPh)5, was prepared and characterized as described previously.’* All alkali and alkaline earth metals used were reagent grade and were rinsed well in the reaction solvent prior to use in order to remove any mineral oil. Other compounds were reagent grade and were used without further purification. Tetrahydrofuran (THF) was dried over N a / K alloy and distilled under Nz prior to use. All reactions were carried out under prepurified N2 which had been deoxygenated by activat-
October 29, 1975
637 1 Table I. Calculated "P NMR Parametersa.b for M,(PPh),C
Compound
Temp ("C)
VA
"B
JAA'
JAB
Li,(PPh), K,(PPh), K,(PPh), Na,(PPh), K,(PPh), K,(PPh), + crown ether Cs, (PPh),
-83 30 -50 -59.5 30 -85
-83.4 -70.0 -70.6 -91.4 -50.0 -17.4
-9.1 -21.3 -24.4 -26.6 -46.8 +12.8
+8.4 (1) +82.8 (3) +166.7 (2) +310.6 (7) -
-280.9 (1) -323.3 (3) -337.8 (2) -323.1 (6) 268 (2) 269.9 (1)
30
-42.5
-36.0
-
287.1 (1)
JAB'
+128.4 +30.8 -2.0 -12.3 -
JBB'
(1) (3) (2) (6)
-150.6 -301.8 -315.1 -310.2
-
-
(1) (3) (2) (6)
-
-
QProbableerrors in the last digit shown appear in parentheses. bChemical shifts in ppm; coupling constants in Hz. CIn THF. ed copper and dried by passage through P205. All glassware was flamed out and purged with N2 prior to use. (PPh)5 xM. Reactions were carried out with M = Li ( x = 2.5), N a (x = 2.5), K (x = 2.0, 2.5, 3.0, 3.3, 4.0, 5.0), Cs (x = 2.5), Ca (x = 1.3), and Ba (x = 1.3). Since all preparations were identical, only a general scheme is given. (PPh)5 (0.5 g) was dissolved in 20 ml of T H F , and the required amount of metal was added to the solution. The mixture was stirred a t room temperature for 12 hr. If a t the end of that time any unreacted metal remained, the reaction mixture was refluxed for an additional 12 hr. The mixture was then cooled to room temperature and filtered, and a portion of the filtrate was loaded into an N M R tube which was sealed under vacuum. When it was deemed necessary, the remaining filtrate was concentrated to approximately 5 ml, and a portion of this solution was loaded into an N M R tube and sealed under vacuum. In only one case was any solid ever isolated, that being where M = K and x = 5. This orange compound was washed with T H F and dried under vacuum. Anal. Calcd for KZ(PPh)2-%THF: C, 50.91; H , 4.24; P, 18.79. Found: C, 51.27; H, 4.46; P, 18.69. The above procedure was altered in the case where M = K and x = 3.3. Upon addition of the potassium, the solution was immediately refluxed for 1 hr, cooled, filtered, and then loaded into N M R tubes as above. Complexation of Potassium. A reaction was carried out a s above for M = K and x = 2.5. The mixture was filtered into a flask containing the complexing agent (dibenzo- 1 8-crown-619) present in a 1 : l molar ratio to potassium taken. The resultant mixture was stirred for 10 min, and a portion of it was loaded into an N M R tube which was then sealed under vacuum.
+
Results and Discussion Initially, the K/(PPh)5 mole ratio was varied in order to determine the range of products possible. Of particular interest was the possibility of detecting K2(PPh)s. The reactions of (PPh)S and potassium metal do not appear to proceed in exact stoichiometric ratios, and an excess of potassium is usually required in order to obtain the desired product free from impurities. This may be due to the heterogeneous nature of the reaction. Too much metal may react further producing smaller phosphide chain lengths. Only three phosphides were produced in these preparations, KZ(PPh), where n = 2, 3, or 4. Kz(PPh)4. Reaction of (PPh)S with 2K yields an intensely red solution which in the 3 ' P NMR displays unreacted (PPh)S and an AA'BB' pattern indicative of a four-phosphorus compound, K2(PPh)4. An increase in the K/(PPh)S ratio to 2.5 causes almost complete conversion to K2(PPh)4. The 3 1 PN M R spectrum of the K2(PPh)4 in THF a t 3OoC is shown in the top portion of Figure 1. An approximate solution t o the AA'BB' pattern can be obtained from known relationships.20The spectrum calculated from these parameters was iteratively fit to the experimental data to give the spectral parameters shown in Table I. Since I J A A ~
