ANTONB. BURG
2226 [ C O N T R I B U l I O N FROM THE
DEPARTMENT O F CNEMISTRY, U N I V E R S I fY CALIFORSIA]
T'ol. 83 OF S O U r H E R N C A I . I F O R N I A ,
1.0s
.\SC>El.ES
7,
Tetramethylbiphosphine: Synthesis, Thermal Condensation, Ethylene Addition, Borine Adducts and Conversion to Phosphinoborine Polymers1 BY ANTOXB. BURG RECEIVED OCTOBER29, 1960 Tetramethylbiphosphine, Pz(CH3)d (m.p. -2"; b.p. est. 140°), has been made by various reactions using ( C H ,)?I" or (CHl)eNP(CH3)2, or both. It decomposes above 300°, forming (CHa)zPH, (CH8)3P,phosphorus-rich sublimates and nonvolatile condensation products of interest from a high-polymer viewpoint. It adds to CzH4 below 3OO", forming mostly (m.p. 0'; b.p. est. 188'). This easily forms an air-stable 2BH3 adduct (sublimable in uuczio a t 150") (CH~)ZPCZH~P(CHB)Z and a more volatile single-BH3adduct. The air-stable adduct PB(CH3)a,2BH:Iforms slightly volatile needles or stabler ui,.l less volatile (1 mm. a t 102.5') crystal-lumps; but the infrared spectra fail t o prove a solid-state stabilization of the rotational isomers. This double-BHp adduct is converted by heat directly to high yields of the trimer and tetramer of ( C H ) 2 PBH2. I t also reacts quantitatively with Pz(CH3)4to form the air-reactive and relatively volatile but poorly charactc-rind units and (CH3)pPH,which supports an open-chain polynier o f P?(CH3)?.BH3. Heat decomposes this to form (CHO)YPBHZ these units by dative bonding. On further heating, this chain polymer is converted almost entirely to (CHJ).PHH?triiiic>r and tetramer.
The recently-new substance P2(CH3), (tetramethylbiphosphine) shows considerable promise as a starting point for a wide variety of new chemistry. I t can be made rather easily from dimethylphosphine and the aminophosphine (CH3)2NP(CH3)23 by several different specific processes, the simplest of which is based upon the somewhat reversible reaction (CH3)2NP(CH& (CH&PH 4 (CH3)ZNH P2(CH3)4. The equilibrium constants of this indicate AFO = -1.79 0 00226T kcal. ,\pparently the net change of bond energy favors the biphosphine; however, i t would be difficult to argue a net increase of purely sigma bonding energy, since H and P have the same electronegativity (on the Pauling scale, based on bond-energies) and the changes of sigma-bond overlap energy should nearly cancel out. Hence the negative AH (- 1.79 kcal.) seems to correlate with two PSp-Pzd pi bonds in the biphosphine, versus only one NzP-Pad pi bond in the aminophosphine. Pure P2(CH3)4 (in contact only with Pyrex glass) withstands temperatures around 300' but decomposes slowly a t 350'. Initially, the nonvolatile products seem to be composed of short
+
+
+
I
(CH3PCH2-)n chains, but further heating leads to cross-linking with loss of volatiles, eventually forming a dark brown or black material much like high-polymeric phosphorus. Typical empirical formulas such as (P3C2H3)xwould suggest that the dark flakes have the P, type of two-layer sheet /P= structure, with many P-P= groupings replaced /P= \p= by HC-P= patterns and with P-CH2-P bridging \p= along the edges or connecting between microflake (1) This research was supported in p a r t by the Office of Xaval Research under Contract Pi6onr-228(13) and also in p a r t by t h e United States Air Force through subcontracts under Contracts A F 33(616)J435 and 6913, monitored by the Materials Laboratory, WrightPatterson Air Force Base, Ohio. Reproduction in whole or in part is permitted for any purpose of the United States Government. (2) (a) A. B. Burg, P. J , Slota, Jr., and W. Mahler, Abstracts of the 134th Xational A.C.S. Meeting, 78P (1958). (b) A. B . Burg, J . I n o r g . Nucleav Chrm., 11, 255 (1959). (c) H. Nbth, Z . h'afuvforsch., 16b, 328 (1960). (d) G. W. Parshall, J . I n o v g . Nuclear Chem., 14, 292 (1960). (3) A . B Burg and P. J. Slota, Jr., J . A m . Chem. SOL.,80, 1107 (1968).
aggregates. A few open CH3-P= corners also are possible. Another kind of hydrocarbon connection occurs when P2(CH3)4 adds to ethylene a t temperatures approaching 300°, to form the slightly volatile liquid 1,2-bis-(dimethylphosphino)-etliane,ICHaj2PCzH4P(CH&. This diphosphine reacts with R5H9 much in the manner of trirnethylaminej4 forming the difficultly volatile single and double borine complexes and becoming incorporated in a boron hydride resin having considerable thermal stability. The addition of P2(CH3)4to ethylene might be regarded as pseudohalogen behavior, with the P-P bond analogous to C1-Cl. This analogy becomes especially appropriate in the case of Pz(CF3)4, for the highly electronegative CF3 groups strongly enhance the halogen character of the P-P bond. Thus it has been found by Louis R. Grant (in these Laboratories) that P2(CF3)4adds to ethylene quite easily a t room temperature. to form mostly the diphosphine (CF3)2PC2H4P(CF3)~ with minor by-products. This work, along with the numerous consequences of adding P?(CF3)d to C2F4 and to acetylene, will be the subject of later publications. The reaction between tetramethylbiphosphine and diborane affords an interesting opportunity to compare the single-borine complex P2(CH3)4.BH3 with the double-borine complex Pz(CH3)4.2BH3 . Each shows two solid forms, but those of the double complex are more recognizably distinct and can be interconverted only a t higher temperatures. The question whether the two bimorphisms have a similar molecular-structural cause has not been decided. Both complexes make (CHs)zPBHz polymers on heating, but only the single-borine cornplex forms also (CH3j2PH,which supports the formation of major proportions of the open-chain high polymer instead of mostly trimer. The singleborine complex is not to be made in pure form by direct addition of diborane to excess P2(CH3)4, for some of the double-borine complex always forms; however, the quantitative reaction Pz(CH3)4.2BH3 P2(CH3)*+- 2Pz(CH3)4.RH3 does
+
(4) A. B. Burg, ibid., 79, 2129 (1957). ( 5 ) A. B. Burg, Angew Chem., 72, 191 (1960).
STUDIES IN TETRAMETHYLBIPHOSPHINES
May 20, 1961
2227
TABLE I Rxpt. no.
--Reactants MenPH
1
10.58
2
39.24
(mmole)MerNPMe?
11.05
a
(1
[L
30.65 a +9.32 3 11.506 12.174 4 6.773 7.385 0 15.091 16.030 Unresolved mixture, reused in a
Time,
Temp.,
hr.
4 62 20 64 19 60 19 67 1 . 14b 26 181 78.5 168 99 supplementary experiments.
show that the P-B bonding is stronger in the singleborine adduct. The weaker attachment of the second BH3 in P2(CH3)4.2BH3 might be attributed partly to steric hindrance but more to the inductive effect of one polar P-B dative bond against the formation of the second such bond. Working against this inductive effect would be the interaction of the B-H bonding electrons with P-3d orbitals,6 partly overcoming the polarity of the P-B dative bonds and attaching the BH3 groups more firmly. Without this effect, P2(CH3)4.2BH3could be expected to dissociate appreciably into P2(CH3),.BH3 1/2B2Ha; instead, i t is easier to cleave the P-P bond to form [ ( C H ~ ) Z P B H ~ ] , ~ . The [(CH&PBHzIn open chain, formed by heating P2(CH3)4.BH3,must be P+B dative bonded from unit to unit, with a supporting (CH&HP+B dative bond a t one end and lone-pair electrons on P a t the other The chain-ending -P(CH3)2 group can close a trimer ring by displacing a dative bond three units down the chain. This happens as the temperature is raised, so that the plastic product is nearly all converted to trimeric (CH&PBH2 and a trace of (CH&PH. Some of this secondary process may occur during the main reaction, accounting for early yields of trimer comparable to the plastic higher polymer. Synthesis and Properties of Tetramethylbiphosphine Methods of Synthesis.-The four different specific methods whereby Pz(CH3)d was made from (CH&NP(CH& and (CH&PH are represented by the equations
+
r
PzMet
OC.
-Components Ma"
6.83 9.22 23.22 33 I47 10,407 5.T60 12.92 Years.
of mixture (mmole) hlezNPMez
a
9.22
1.65 B
.... 10.481 5.626 13.41
MezPH
a
5.10 1.541 1.404 2.719
1.36 (I
... 1.120 0.61 1.820
The one-step process (2) proved to be fairly satisfactory: 5.603 mmoles each of the three reactants, warmed together during 1 hr. from -70 to -40" and then briefly heated to 70°, produced immediately 471 mg. (69% yield) of PZ(CH3)4. After this had been isolated, the remaining components were reheated to bring the final yield of P2( CH3)4 to 557 mg., or 81%. Process (3) was found to be relatively convenient and highly efficient. A 23.74 mmole sample of (CH3)zPH was converted to the hydrochloride, which was heated a t 95' with 25.66 mmoles of (CH3)2NP(CH3)z in a 60 ml. reaction tube attached to the high-vacuum system through a stopcock. The biphosphine was isolated and the remaining volatiles were reheated in the same reaction tube until there was no further yield of biphosphine. The recovered aminophosphine now amounted to 2.13 mmoles; used, 23.53 mmoles. The yield of Pz(CH3)4, 2.783 g. (22.79 mmoles), represented 96.070 of the dimethylphosphine or 96.9% of the consumed aminophosphine. The solid nonvolatile product was extracted by chloroform ( a specific solvent for dimethylammonium chloride) and weighed as 1.8968 g., representing 23.26 mmoles of (CH&NHZCI. The chloride titration gave 23.0 mmoles (calcd., 23.74). Thus the whole experiment was so nearly quantitative as to constitute a proof of the formula Pz(CH3),. The relatively clean and simple process (4)is convenient for large-scale production. The reaction becomes appreciable within a few minutes a t 99" and is 8570 complete a t equilibrium. For fast production of high yields, one may heat the mixture for an hour, remove the resulting biphosphine by fractional condensation a t -40" and reheat the reactants to obtain more biphosphine, finally converting the last traces of the reactants by process (3). At room temperature reaction (4)eventually reaches 89% completion but is so much slower that one must weigh the advantage of a somewhat higher initial yield against the time required for the process.
Equilibria in Process (4) .-The quantitative experiments on process (4) are summarized in Table I. In each case the reaction took place in a 20 ml. sealed tube. The resulting mixture was resolved by high-vacuum fractional condensation, with the (1) (CH3)&"(CH3)2 2HC1 + (CH3)iPCl (CH3)2NH2Cl biphosphine coming out at - 40°, the aminophosphine a t - 78' and the amine-phosphine mixture ~(CH~)ZP + H (CH3)zPHzCl (2) (CH3)zPCl a t -196'. In each case the biphosphine and the Pz(C&)( ( C H ~ ) Z N P ( C H ~ ) HC1 Z (CH3)zPH + aminophosphine were weighed in order to deter(CHs)zNH2Cl P*(CHa)4 mine the extent of the reaction, and in some cases the ( 3 ) (C&)zNP(CH3)2 (CH3)zPHzCl(CH3)zNHzCl Pz(CH3)d result was double-checked (without showing any serious error) by determining (CH3)2NHas the ( 4 ) (CHa)zSP(CHa)i 4-(CH3)iPH + hydrochloride, from which the (CHa)2PH2C1 (vola(cH.3)~" P*(CHs)a tile a t room temperature) had been removed by Process (1) can be made fairly efficient, especially if it sublimation, for separate weighing. is combined with (3) to employ the phosphonium chloride; Most of the experiments of Table I represent however, it requires special efforts for avoiding side reactions in the synthesis of (CH3)zPCl. For this it is im- incomplete approaches toward equilibrium, but portant to employ a slight excess of the aminophosphine*; expts. 3 and 5 are believed to have reached it. and for full efficacy it is important to distil the volatile For the nearly 14-month expt. 3, we can make a components back and forth between two reaction tubes (under high vacuum, using liquid nitrogen) with heating comparison with a smaller and much rougher roomtemperature experiment (not here listed) which of the non-volatile residues. Thus more direct methods of making Pz(C H P )are ~ preferred. reached a similar stage of completion in half the time. Defining K as the product of the amounts ( 6 ) W. A. G . Graham and F. G. A. Stone, J . Xnorg. Nuclznr Chem., of (CH&NH and P2(CH3)4divided bv the product 3 , 164 (1956). of the amounts of (CHs)2PHand (CH3)2NP(CH3)2. (7) R. I. Wagner and F. F. Caserio. J r . , i b i d . , 11, 259 (1959).
+
+
+
+
+
+
+
+ +
+
.INTON €3. BURG
222s
the results of Expis. 3 a:id 3 x c , rcspcct.ively, A-= 63.2 and K = 35.0. leading to the equation log K = 3OI,/T - 0.493 ( S F " = -I.'?!> 0.0031267' kcal.). From this thc va1:ie of K for 78.5'' would be calculated as 4'3.3, only 6.67h :rbovc the Q value (37.8) impliccl by expt. 4. ?'hiis it seeins that the higher-temperature cxipi. 3 xiitst hnv-e reached equilibrium. within srnall esperimenta! error. Disproportionative Methods of Synthesis.-The hydrolysis of (CH3)2NP(CIS3\2 f o r m (CHs)2NH,3 but the parallel product (GH:t'~sPOMis wholly converted to (CH3~2PC)OII and (CIla)zPIZ.8 Thus the treatment of one (C&).LN?(CI&)~ by '?.67&0 gives 0.33 (CH&PH, which c.ati rcnct with t h e reniaininq 0.33 (CH&NP(CH& to fmiil P:(CH3j.! This method has not yet been developed to high efficiency. A siniilar ilispronortiotiation can occur when (CH&NP(CH& is ammonolyzed. The initiallyformed (CH&PNH2 a t once convents to [(CH3)2PI2" and some [(&\H&P]lN,a but longer heating of the mixture leads also to (CHsj2PH a i d presumably (CH&PNHNH2 or its condensation products. Then the (CH3)zPH can attack any of the aminophosphines to niake P2(CIla)4. An experiment by Dr. Peter J. Slota, Jr,, (in these Laboratories) employed 8.M nimolr-.s of NHS and 26.27 mmoles of (C13y)2NP(C€I:i)p in a 100 nil. Pyrex tube, heated at the lower end by a n oil-bath a t 170-200' for 80 iir. The volatile components (in mmoles) nom were R.89NH;, 0 134PH1, 1.0(CH&PJP(CH&, 22.41 (CH:J2NFI and 2.S2P2(CH3)&, the last representing 22%: of the consumed aminophosphine. Another process was discovered by Lotiis R. Grant in these Laboratories. Trying to make [ (CH&PJ3P, he ohscrvcd n vigcrws reaction on warming 6.786 minoles of (CH:!)2PH with 1.143 mrnole of PC13 to 2.5''. The rcsulting volatile phosphine hydrochlorides were treated with (CH3)aN; recovery of (CH.J2P€I.4.128 inmoles; yield of P2(CHa)4, 1.11,4 nimoles or 81%)of the consumed (CH&PII. The I:y--prntitic:. was a non-, volatile orange-colored solid.
+
Physical Properties.--A iiighly purified biphosphine P,(CHJ), rneltrd in Its vapor tensions, shown i n T tion log pmm.= 6.5244 - 0. 2444/T, according to which the and the Trouton constant 20.1 cal./clcg. raoln. weight determinations in the vapor p i mni. (74y0 s:ttd.) and a t SI the respeciirc value? 123.8 122.1.
VOl. 53
Thermal Stability and Decomposition.--A 169.4 mg. sample (1.388 mmoles) of P2( CH3)' was heated in a sealed 45 ml. tube for 25 hr. a t 290' withotit effect on the m.p. However, after a 6 hr. heating from 300 t o 350" the same sample had begun t o decompose: the recovery of Pr (CH& was 138.4 mg., or 81.7%. The volatile product (13.0 mg.; average mol. wt. 67; 0' vapor tensions 175215 mm. in various fractions) was recognized as a mixture of 0.12 niniole of (CH&PH and 0.09 mmole of (CK&lJ. Therefore the non-volatile product ( a colorless oil) could be interpreted as containing 0.09 mmole of CHaP and 0.12 rninole of CHlPCHz as ring-forming or chain-connecting units, with 0.09 mmole of (CH&P as chain end-groups. Tile presence of CHIP units would imply a process analogous t o the very clean formation of (CF&P and (CFaP)* by the pyrolysis of Pt(CFs)d,D but in the present case some P-CH?-P linkages must have been established. Longer periods a t 350' led t o virtually complete conversion, with the formation of methane instead of dimethylphosphine. For the results shown in Table I11 the samples were heated in sealed 50 ml. tubes, in an electric tubefurnace var!.ing f 8' from the average 350'. The CH4 was recognized by its 12 mm. vapor tension at -196" and proved t o be free of hydrogen. The slight ''C2He" fractions mere identified only by the depth of temperature required t o trap them under high vacuum. The (CH&P was recognized by its 160 mm. vapor tension a t 0' and 77.0 mol. wt. (calcd., 76.1). In each case the product delivered under high vacuum at 100' was a mixture of yellow oily droplets and a white sublimate. Acetone extracted nothing from the final residue.
TABLE I11 DECOMPOSITION OF P2(CHa)4AT 350" Rx7t. no.
1
2 3
Volatile at P~(CHi)r(mmole) Time, Volatile p p d u c t s (mmoles) 100' Start End hr. CH, CzHe" (CHi)aP (mg.)
2.193 0.061. 21 0.673 0.013 2.387 29.0 ,027 3.470 70.0 ,054 122 1.715 3.333 ,014 120 1.500 ,021 2.149 3 8 . 1 2.2.52
The 100" product from expt. 3 was analyzed by O r combustion leading t o 1.164 mmoles of &PO, and 0.155 xnrnoie of COe. If it had the same H : C ratio as the final residue, the formula of this residue mould be close t o (PIC?H3),. By similar estimation the final residue of expt. 2 would be written as (PIC~H4)r. The less advanced expt. 1 had a 17: 10 ratio of H t o C in the room-temperature residue, implying more CHa-P and P-CHrP connections /p and fewer €IC--P conncctions than v~ouldhe found in the P ' final residues of espts. 2 and 3.
The Ethylene-connected Diphosphine The Ethylene-addi.tion Reaction.-The condihlolerular tions and results of four experiments on the addition of Pz(CHa)4to C2H4 are shown in Table 117. In expt. 1 the mixture had been heated in a 5 : ) nil. sealed Pyrex tube for 44 hr. a t 232' without effect upon the m.p. of the P2(CHS)4:whereas in Tnrt1.e 11 expt. 2 (also in a -50 ml. tube) a tiny crystal of VAPOR TEXSIOXS OF I ~ ~ r r P,(CFIsh ru iodine (fa. 5 mg.) led to some reaction at 207'. 6 4 . 3 4 1 . 9 4 7 . 7 24 8 30.7 0 00 21.4 1 ("C.) In expt. 1 the desired diphosphine WLSI fractionated 31.6 43.0 18.GO 2XY9 7 . 9 9 9.69 Pmm. (ohsd.) 2.0i out after each heating but not separately weighed; 9 . 7 0 1 3 . 4 3 2 3 . 9 3 J 1 . 7 42.9 2 06 S.01 Pmm. (calcd.) and the -12 to --5' melting range could not be Confirmation of the Formula --.4 middle fractiun ( 8 3 , s improved by more fractional distillation. In nig., or 0.703 mmole) of Pz(C& I V:LS taken by distillation espt. 2 the diphosphine came to no harm through from the \.;eighing tube, in order- t o minimize the possible further heating with the remaining reactants in catalytic effects of grcnsc and mercllrv. This s:iniple the later stages: the pure diphosphine showed slight absorbed 51.7 mg. of ClIJ ifroni a larger sample)--esactly 0.703 mmole, as calkd. This precisely qunot itative forma- melting between - 1.8 and - 1.0' and major melttion of the l : l adduct left :IO possible doubt of thc formula ing in the range - 1.0 to 0.0'. Good samples were PO(CHI)(,which alread:: was strotigly supported h v the obta.ined also from the single-heat expts. 3 a i d 4 stoichiometry of the synthesis, the physical evidence of (100 n:l. Pyrex tubes). uniformity and thc ixolcrdar ~vcigl;l.
STUDIES IN TETRAMETHYLBIPHOSPHINES
May 20, 1961
2229
TABLE IV
ADDITIONOF Pn(CHI),TO ETHYLENE (Quantities in mmoles) Expt.
no.
1
2
Reactants employed
Time,
tu.
OC.
1.125 1.423 (Remainders) (Remainders)
20 72
292 300 303
P:(CH6)4
C2H4
160
Temp., Reactants recovered
4.417 7.616 (Remainders) (Remainders)
118 145 120
207 258 267
Ps(CHI)~
ClH4
...
1.06
0.081
0.77 0,544
...
...
1.39
6.826 2.930 2.043
...
7 -
CdI4
(CHdrP
(CH1)rPH
Diphosphine
... ...
...
.........
...
1.044
0.36 .65 .879
......... 0.666 (My')
... ... ...
0.393 ...
...
.........
...
...
3.027
.306
0.236
......... 2.396(79%)
0.134
1.512 (OGYO)
Nil
3.526 (63%)
11.31
410
280
0.28
6.49
2.31
4.82
.490
4
5.748
13.153
760
260
0.18
6.023
5.57
7.13
1.693
TABLEV VAPORTENSIONS OF ( CH3)2PC2H,P( CH3)2 (0C.l
Pmm. Pmm.
(obsd.) (calcd.)
25.6 1.05
37.1 2.23
1.05
2.21
49.8 4.68 4.69
63.8 9 1 . 2 9.92 35.68 9.95 36.74
114.4
130.2 8 9 . 1 154 0 89.1 154.0
The vapor-tension sample was heated further in the 135 ml. immersible bulb-manometer system, evaporating completely a t 133'. Then the standard volumes of the vapor were determined a t 140.0 and 151.5', leading to mol. wt. values of 154.0 and 152.0, respectively. This approach toward the calculated value (150.1) left no doubt that the compound was (CH3)ZPC2H4P(CH3)2. The Double-borine Adduct of the Diphcsphine,The identity of the diphosphine was further confirmed by the stoichiometry of formation of the adduct (CH&PGH4P(CH3)2.2BH3.A 65.5 mg. sample of the diphosphine smoothly absorbed 0.436 mmole of B2H6 (from a measured larger sample) ; calcd. for the double-borine adduct, 0.4364 mmole. The adduct sublimed slowly a t 150' under high vacuum. It seemed to be unaffected by the open air. It proved to be sparingly soluble in acetone, with a fairly high temperaturecoefficient permitting recrystallization. The crystals appeared as short parallelopipeds, showing red and green stripes on opposite edges when viewed between crossed Polaroid films. An attempt to remove the BH3 groups by heating with (CH3)3N (8 hr. a t 100') was partially successful: some 15% of the BHI groups were taken off in the form of (CH~)&BHQ.
Nil
.7YO 4.686 5.573
...
2.59
+
Products
P:(CHi)d
3
The per cent. yields of (CH&PC2HdP(CH3)2 shown in Table IV are based upon the consumed P2(CH3)4, and it is evident that considerable proportions of this biphosphine went into side reactions which consumed still more ethylene. The persistent formation of trimethylphosphine shows that CH3 groups transferred from one phosphorus atom to another. It would appear that some of the non-volatile products could be explained as polymers involving C ~ H ~ P C Hunits. B Perhaps also some of the missing ethylene was converted to longer-chain material. Physical Properties of the Diphosphine.-The diphosphine sample which melted mostly in the range -1.0 to 0.0' (expt. 2 , Table IV) yielded the vapor-tension data shown in Table V. These values are correlated by the equation log p m m , = 6.72324 - 0.005070T 1.75 log T - 2344/1', according to which the normal b.p. would be 188.1' and the Trouton constant 21.00 cal./deg. mole.
1
Reactants consumed
At this point there was no attempt to make the single-BHS adduct; homver this was later recognized as one of the prodL1ct:; of the reaction of (CH3)2PC2H4P(CH,)2 with IL,H 9.6 Jts presence in a slightly volatile oily fraction was demonstrated by a quantitative experiment on its conversion to the double-BHX adduct by absorption of a measured amount of diborane. Since it can be distilled slowly under high vacuum a t room tcrnperature, it clearly is more vohtile than the double-EH3 adduct, quite as expected for one dative bond per molecule rather than two. Tetramethylbiphosphine with Diborane The Double-borine Adduct.-The formation of the double complex P2(CH3)4.2BHa is fairly straightforward: one condenses a weighed sample of P2(C!H3), thinly upon the walls of the reaction tube and exposes i t to diborane (present in excess) during a slow warming from - $8' to room temperaturc. Sometimes a little of the PP(CH3)4 or P2(CHd),.RH3is trapped in the solid P2(C€13)4.2BH3. In such a case, the tube is heated briefly to loo', cooled, evacuated of diborane, closed and heated a t 100' to sublime the double-RHa adduct. With return of the diborane, the reaction now can be completed. The quantitativc data of three such syntheses are shown in Table VI. TABLE 1'1 FOKMAT~ON OF P2(CH3)4.7BH3 Expt. no.
1 2 3
Re-ictanls (mmo!e) Pr(CH2)c IhHo
1 154 1 191 0 113
1 159 I 40.5 1 076
hliuole I32IIs recovered
Reaction ratio &He/ Pz(CEI3)d
0 0Olj 214 667
0 999 1 000 0 090
In a fourth experiuient, the d0~ble-BI-I~ Adduct was formed directly in an iiniiiersible tensimeter, lo within which it was vacuum-suhlimed for purification. Its vapor tensicins (Table VII) determined the equation log p m n i = 14 211 - 5 3 10 ' T , indicating 1.00 rnm at 105.5'. TABLE VI1 VAPORTENSIONS OF SOLIDP2(CH3)4.2BHI t ("C 1 72 78 85 88 4.3 pmm.(obsd.) 0 05 0 10 0 20 0 28 0 42 pmm.(calcd) 0 056 0 100 0 1'38 0 2G4 0 420
Such low volatility was surprising because an earlier sample of the same material, formed below (10) A B Burg and H.I Srhlesinger, J A m Chrm Soc , 69, 785 (1937)
2230
ANTONB. BURG
room temperature and never warmed above 25O, had undergone major sublimation during 24 hr. under high vacuum a t 22-25'. Such a rate of evaporation would have been quite impossible for a solid substance having only 2 x 10-4 mm. vapor tension a t 25', as calculated from the equation. By microscopic inspection, however, an unheated sample of P z ( C H ~ ) ~ . ~was B Hfound ~ to be a mixture of irregular striated-lump crystals and well formed needles or flat blades. The needles often extended from common centers to form spiny clusters. They sublimed under high vacuum a t 2 5 O , condensing as lumps a t room temperature and as needles just above a trap a t 0 ' . Under static heating, the needles converted entirely to the lump form, and accordingly it was this form which gave the data of Table VII. However, when the lump form was sublimed (very slowly a t 60' under high vacuum) the sublimate consisted of lump-crystals near the warm zone and needles in the cooler regions farther down the tube. Thus it appeared that the two crystal forms interconverted through the vapor phase. Both forms proved to be birefringent, for microscopic observation between crossed polarizers showed delicate blue-green and red fringes on the needles and sometimes the same colors on the lumps, which mostly appeared goldenyellow. Neither with the pure lump-crystals nor with a mixture of the two forms was it possible to reproduce the 106' m.p. given by Noth for P2(CH3)4 2BH3.2C I n fact, no melting occurred when a sealed tube containing the crystal-mixture was immersed suddenly in oil a t 112'; all that happened was that the needles lost their linear shape and became striated, evidently converting to the less volatile form. The lumps finally melted a t 194' (in a sealed 1 ml. tube), but then the evolution of hydrogen indicated enough formation of the (CH&PBHz trimer and tetramer to account for a solution effect. In an effort to recognize a possible chemical reason for the different crystal forms of Pz(CH3)l 2BH3,the molecular weight of a mixed sample was determined. The 25.2 mg. sample lowered the vapor tension of 15.34 mmoles of diethyl ether by 3.59 mm. from the pure-ether value of 328 mm. a t 12.5'. Thus the apparent mol. wt. was 156.2, or 4.4% above the calculated 149.7. Since the solution was nearly saturated, this result is consistent with the absence of any appreciable proportion of material having a higher molecular weight. Infrared Spectrum of the Double Adduct.--A possible reason for the two differently-volatile crystal forms of P2(CH3)4.2BH3would be a stabilization of the rotational isomers (one trans and two GOO-cis, or gaziche, forms of a pattern almost like hexaniethylethane) through interaction of dativebond dipoles in the solid state. Evidence bearing upon this hypothesis was sought through a relatively detailed study of the infrared spectra of two samples. One of these was obtained as a highvacuum sublimate far from the hot region, and under the 160-power microscope it seemed to be about 30% lumps with 70yo needles. The other
Vol. 83
sample was taken as the sublimate nearest to the 100' bath and seemed to be pure lump-form. These air-stable (and acetone-soluble) samples were pulverized in Nujol to make mulls of similar crystal-size (as judged under the microscope) for mounting between salt-windows. ~
Table VI11 describes the spectra of both samples, with mention of the observable differences. Except where the Infracord instrument is mentioned, the bands were recorded a t high resolution by a Perkin-Elmer Model 13 double-beam spectrophotometer, using sodium chloride optics. For purposes of structure confirmation, both spectra are virtually the same, showing only geminal methyl groups (as by the split rocking modes) and BH? groups rather than some other deployment of 2B and 6H. For such results the P atoms cannot be different. Minor differences in bands from the two samples would admit the possibility of rotational isomers; however, the different crystal-field effects in two different crystal structures might alone account for the differences. Hence the stabilization of rotational isomers in the solid state remains an interesting hypothesis, requiring a different basis for decision. TABLEVI11 IXFRARED SPECTRUM OF P2(CH3)4.2BH3a €Ye-
quency (cm. -1)
Intensity and character
2920 Very strong doublet 2880 2220 Medium peak 2345 2315 1400 1305 1297 1286 1150 1125 1101 1068 1057 1049 945 895 849 834 768 7.59 731 722 693 681 a It David data.
Strong doublet
Interpretation
C-H stretching (Infracord) Uncertain; probable COZ effect B-H stretch, affected by COZ
Broad medium peak C-H bending, & e Weak peak Probably a Xujol effect Weak shoulder Uncertain Sharp medium peak C-H bending, 6-a (Present only for lump-form) Broad weak peak Medium peak B-H bending, 6-e Weak shoulder (Present only for lump-form) Medium shoulder Uncertain; more evident for (submerged peak) the needle-form High shoulder on Uncertain; more evident for 1049 peak the needle-form Strong peak B-H bending, 6-a Med. strong peak CHI group rocking (InfraStrong peak cord) Weak peak Uncertain Weak peak Uncertain Could represent the three Weak peak peaks expected for gaucheWeak peak BH3 rocking, but not surely Med.peak Probably a Nujol effect Medium peak Weak peak Probably P-C stretch Weak peak is a pleasure to acknowledge valuable aid by Dr. A. Dows in the recording and interpretation of these
1
The bands in which one would seek differences due to the stabilization of rotational isomers would be those representing B-H bending and BHo-group rocking, for these aspects would be different for trans and gauche placement of the BHa groups. No differences would be expected for the methyl groups, since both trans and gauche relations between methyl groups occur in both kinds of rotational isomers. .4nd in fact, the bands concerning the methyl groups are identical in shape (with only slight differences of intensity of some shoulders), wheresq noticeable qualitative differ-
hlay20, 1961
STUDIES IN TETRAMETHYLBIPHOSPHINES
2231
ences occur in the B-H bending bands. For example, the broken down to the trimer and tetramer. Assuming that the (CH3)2PH was present as such in this non-volatile high shoulder indicating a submerged peak a t 1057 cm.-' is noticeably broader for the pure-lump sample than for material, it would appear that it served as a base t o support the 70yo-needle sample, as though it might not be found a t a chain polymer averaging 22( CH8)zPBHz per molecule, all for pure needles. However, the three peaks a t 768, through successive P-B dative bonds. Soon after this ob759 and 731 are identically related to each other in shape servation, the same principle of base-support of openand intensity for the two samples, only showing somewhat chain phosphinoborine polymerization was studied much less over-all intensity for the needle-crystal sample. One more fully el~ewhere.~ At this point the yield of (CH3)2PBH2units was 89% of may well consider that crystal-field effects might be most important for the groups which are associated with dipoles, the calculated value. The rest of the material could be accounted for by a cream-colored final residue, partly namely, the dative-bonded BH3 groups. soluble in acetone but all dissolved by hot nitric acid, with Decomposition of the Double Adduct.-A pure sample of Pz(CH~)~.~ (from BH~ expt. 2 of Table VI) was unaffected by a negative phosphate test. Such behavior toward nitric acid is characteristic of various highly condensed phosa 5-hr. heating in a sealed tube a t 154' but showed a minor formation of oily material after 11 hr. a t 170'. After 11 phorus-boron-hydride resins which have been made in these Laboratories. hr. longer a t 290' the sample had formed 1.262 mmole of Further Study of the Single-Borine Adduct.-Later Hz, or 6.0% more than required by the equation Pz(CH3)r. 2/n[(CH3)2PBH~],. The yield of (CH8)lPH attempts a t a direct quantitative synthesis of Pz(CHa)r.B.H1 2BH3 + H2 invariablv led to some formation of the double-borine was 0.094 mmole, or 4y0. Also present were 129 mg. of a n d the isolation of the desired single-borine ad]~ [(CH3)2PBH2]3 (m.p. 88"), 25.3 mg. of [ ( C H ~ ) Z P B H Z adduct; (m.p. 160") and 15.5 mg. of a white gum, not volatile under duct by distillation methods was made difficult by the repeated formation of small quantities of (CHa)?PH and a high vacuum a t 100'. Its easy solubility in acetone would suggest that this gum was no very high polymer, or a t least slow disengagement of the last of the excess Pz(CH3)d. not extensively cross-linked. The reaction-balance would For a quantitative synthesis the best procedure was to it with more B H heat ~ give its empirical formula as [(Me2P)6BsH1J., which could form the adduct P z ( C H ~ ) ~ . ~and be interpreted in terms of (PB)3 rings connected together by Pd CHI)(. small boron-hydride units attached to the ring-boron atoms. For this purpose the sample from Expt. 3 of Table VI However, the stoichiometry lacked precision, since almost was heated with 62.5 mg. (0.512 mmole) of Pz(CH3)d for 4 hr. a t 80'. The recovered Pz(CH3)r amounted to 11.3 mg. 90% of the material went to form [(CH3)2PBH2j3,r. The Single-Boron Adduct : Formation and Polymerization. Thus the reaction (with stoichiometry in mmoles) was -The initial experiment on the synthesis of Pz(CH3)(.BH3 Pz(CH3)4*2BH3 Pz(CHa)a --+ ~ P z ( C H ~ ) ~ . B H ~ employed the reactants in the calculated ratio: 0.960 m0.413 0.419 (0.830) mole of P2(CH3)(with 0.480 mniole of B2H6. The biphosphine was condensed as a thin solid layer on the inner wall The stability of theproduct was tested by a series of 4 to of the 50 nil. reaction tube (connected through a stop- 7 hr. heatings a t 80 , each liberating a trace (0.005 to cock t o the high-vacuum system) and warmed with the 0.001 of Pz(CH3)4 but each time less than before. diborane from -60 to -8' during 10 hr. The main prod- I t wasmmole) not quite possible to decide whether any of the uct appeared as a slightly volatile liquid with a little solid. biphosphine came from a reversal of the above reaction. The formation of 0.056 mmole of (CHa)?PH would have Attempts to observe the m.p. and volatility of Pz(CH3)r. been due to a catalytic effect of the stopcock grease (Apie- BH3 were made difficult by its sensitivity to stopcock zon L) upon P2(CH3)*,of which 0.136 mmole (14o/c) was greases and by behavior suggesting more than one solid recovered. The failure to absorb this 14y0 meant that an form. An opaque-crystalline sublimate, taken by highequal part of the initially-formed P?(CH3)4.BH3had reacted vacuum fractional condensation in a U-tube a t O " , developed further to make Pz(CH3)r.2BH3. This secondary reaction less than 0.02 mm. pressure during 20 min. a t 25'; and was not appreciably reversed by heating the product with to sublime away this 0.2 mmole sample under high vacuum the unused biphosphine for 20 min. a t loo", but a later required 5 hr. a t 28". But when the same opaque crystaAz experiment showed that longer heating would have con- were resublimed by briefly heating the closed U-tube a t 50 , verted the mixture almost entirely to Pz(CH3)4.BH3. the more transparent sublimate now could be sublimed Hence the following account of the conversion of the present out in 0.5 hr. a t 28". The opaque form was largely conmixture to polymers must represent almost wholly a reaction verted to the more volatile transparent form during a dayof this single-borine adduct. The entire mixture next was heated in the sealed-off long warming from 10 to 19". A melting range of 41-45' was fairly typical but might correspond to a mixture inreaction tube for 70 hr. a t 174O, producing 0.06 mmole of some of the yet unconverted low-temperature form. H2. The recovery of P2(CH3)(now was only 0.02 mmole cluding Consistent vapor-tension measurements could not be ob(2%). The yield of (CH3)zPH (0.93 mmole) was 97% of tained for the solid P2( CHa)4.BH3, but the roughly useful that calculated from the equation Pz(CHa)(.BH3 + (CH3)z- values shown in Table VI11 could be obtained for the liquid, 1/1[(CH3)2PBH~]n.Only42Y0of the expected (CH3)2- by subtracting PH 0.40 mm. from each measurement, to corPBH2 units (30.0 mg., or 0.405 mmole) appeared as trimer and tetramer; these were separated by vacuum sublimation rect for the (CH3)zPH which was isolated and measured On this basis, log pmm. = 8.37 -2744/T; and identified by their melting points. A slightly higher afterwards. b.p. 254'; Trouton constant 25 cal./deg. mole, a normal j-ield of (CH3)zPBHg units was present in the white residue, result for a fairly polar substance. which remained non-volatile even after brief heating above its melting range (about 180-200"). TABLE IX This residue was heated in the resealed and evacuated ROUGH VAPORTENSIONS OF LIQUIDP2(CH3)4.BH3 tube for 20 hr. a t 330°, yielding 0.07 mmole of Hz, 0.021 mmole of (CH3)2PH, 31.0 mg. of [ ( C H ~ ) ? P B H Z and ] ~ 2.1 t ("C.) 54.0 61.0 68.6 78.2 85.0 mg. of [ ( C H ~ ) Z P B H Z Hence ]~. this residue accounted for 1.44 2.10 3 . 6 3 5.18 pmm.(corr.) 0.99 0.447 mmole (46.5y0) of the expected (CH3)zPBH2units, p,,.(calcd.) 0.96 1.44 2.19 3 . 6 3 5.12 evidently in the form of a higher polymer which could be
+
+
+
