638 J.C.S. DaltonMultinuclear Magnetic Resonance Studies. Part 4.' Tetra-alkyldi-phosphines with Bulky Substituents tBy Ahmed A. M. Ali, Gabriele Bocelli, and Robin K. Harris, School of Chemical Sciences, University ofManfred Fild, Lehrstuhl B fur Anorganische Chemie der Technischen Universitat, 33 Braunschweig, Pockels-East Anglia, Norwich NR4 7TJstrasse 4, GermanyPhosphorus-31 and carbon-13 n.m.r. spectra have been obtained for three new tetra-alkyldiphosphines withbulky substituents: ( B U ~ P ~ ' P ) ~ ( l ) , (ButEtP), (2), and (Pr'EtP), (3). The spectra are assigned and the n.rn.r. par-ameters are compared to those for anologous compounds with lighter alkyl substituents. Molecular conformationsare deduced and the effects of inversion at phosphorus discussed.DIPHOSPHINES are of interest to the structural chemistfrom a number of aspects.They have all the con-formational possibilities of ethanes, and interconversionbetween the various forms can occur by internal rotationabout the P-P bond. In discussions of their structure,the lone pairs may be treated as pseudo-substituents, butinversion at phosphorus is an additional feature thatneeds to be considered. Usually it is assumed that, aswith ethane derivatives, the stable conformations arestaggered. A Raman study2 of P2Me4 shows it to beessentially trans in the solid, but a mixture of trans andgauche forms in the liquid. However, a recent X-raystudy of tetracyclohexyldiphosphine indicates that themolecule is close to the semi-eclipsed form in the solid,and microwave studies show a dihedral angle of 74" forgaseous P,H4.Photoelectron spectroscopy 5*6 and elec-tron diffraction' have also been used to study thestructure of diphosphines without electronegative sub-stituents, and recently n.m.r. spectroscopy has beenable to provide detailed information of the solutionstate. Thus low-temperature 13C and lH n.m.r. spectrahave shown that P2But4 is not trans nor symmetricallyeclipsed, since two types of But group are observed.These observations were interpreted in terms of thegauche conformation (I) although even large deviationstowards the semi-eclipsed form (11) cannot be excluded.Bu /t \&t /I But ButOf course there is no reason to expect that the projectionof the CPC angle on to a plane perpendicular to the P-Pbond should be 120"; in the case of solid P2(C6Hl1), it isreported 3 to be 1OSo, although presumably it is likely tobe higher for P2But4.Measurements of N(PC) =IIJ(PC) + 2J(PPC)I have indicated lo that N(PC) ischaracteristically near zero for alkyl groups gauche to thelone pair on the remote phosphorus, but substantialt Systematic names for the compounds studied are 1,2-di-isopropyl-l,2-di-t-butyldiphosphane ( l ) , 1,2-diethyl-1,2-di-t-butyldiphosphane (2), and 1,2-diethyl- l12-di-isopropyldiphos-phane (3).(15-50 Hz) for trans groups, subject to a reasonableassumption regarding the preferred conformation ofcompounds of the type (R1R2P),. The possible influenceof trans conformers on discussions of N(PC) is, however,problematical.The n.m.r.parameters for diphosphines are themselvesof considerable interest [for instance the variation ofN(PC) referred to above, and the large variation ofJ(PP) and 6(P) with the nature of the substituents 12913].A considerable body of data has appeared in theliterature 12*13 for diphosphines of the types P2R,,(R1R2P),, R1,PPR2,, and R1R2PPR3, (R,R1, R2, andR3 = Me, Et, Pri, or But) although several of the com-pounds with the more bulky ligands have never beeninvestigated by n.m.r. Since some of the results 1 2 9 1 3 forP2But4 and Me2PPBut2 appeared somewhat anomalous,it was decided to prepare and examine (ButPriP), (l),(ButEtP), (2), and (Pr'EtP), (3), which have not, to ourknowledge, been studied previously.We report then.m.r. data herein.There are two special features of such compounds thatdeserve mention. First, they may exist in either racemicor meso forms; the relevant ideal staggered conform-ations (excluding mirror images) are shown in structures(111) and (IV). Secondly, the isopropyl groups in(PriEtP)2 and (ButPriP)2 are in a prochiral situation;the two methyl groups of a given isopropyl are chemicallynon-equivalent and give anisochronous 13C (or lH)resonances when 31P inversion is slow on the n.m.r.time scale.mesointerconversion and interchange of the two anisochronousmethyl groups for a given isopropyl group. Such 31Pinversion has been shown12 to be slow on the n.m.r.time scale at ambient probe temperature for P2Pri4 andfor (PriMeP),, but in the former case the separate 13Cmethyl-group n.m.r.signals coalesced above ca. 75 "C.However, internal rotation about the P-P bond isthought12 to be rapid on the n.m.r. time scale aboveca. - 100 "C for all tetra-alkyldiphosphines so far studiedexcept P2But4.Proton-decoupled 13C and 31P n.m.r. spectra of thesimple symmetrical tetra-alkyldiphosphines are usuallyeasy to assign and interpret.12 However, the protonspectra are not so readily analysed because of theRapid 31P inversion causes both racemi1980€Bdd2;.b-1--2-3-6392-'-0-0-/' Usymmetry; 14*15 therefore the information to be obtainedmay be rather limited. In fact, the 13C spectra are inprinciple l2 second order, forming the X parts of ABXspin systems with vA z vB, but in practice deceptivelysimple triplets are normally observed, which yield only.' R, R2fiR' R2RZ R2 R' R'V gaucheracemic isomer\ * \ *R' jq R2 R;l&R2R R*I =a) ( X b 1gauche vmeso isomertranstranschemical shifts and the sum of the two relevant (P,C)coupling constants, N(PC).RESULTSThe 31P-{ lH} spectra of the three compounds showimmediately that (RutPri P)2 and (ButEtP), exist pre-dominantly in one isomeric form, presumed to be racemic,since only one signal can be found.The alternative hypo-thesis, that 31P inversion is rapid, was discounted byvariable-temperature experiments, and [in the case of ( 1) Jby observation of methyl anisochronicity in the 13C spectrum.The conclusion is conistent with previous work l2 on(ButMeP),, which was also found to exist essentially onlyThe 31P chemical shifts for (1)-(3) are given in Table 1.These shifts were found to vary substantially with tem-perature, in a roughly linear fashion over the range studied,the rate of change is given in Table 1.We have alsomeasured the effect of temperature on the 31P chemicalshifts of other tetra-alkyldiphosphines (see Figure 1 andTable 1). These variable-temperature experiments werecarried out using an internal ,H lock, b u t checks using anexternal l9F lock showed little difference in AS(P)/AT sothat the temperature variation is genuinely that of S(P)and not that of S(H) for the lock.The 31P shift for P,Me, is riot very sensitive to dilution inbenzene: we have measured (using 2H internal lock)3r-4LFIGURE 1 Effect of temperature on 8(P) for compounds of thetype P2R4: (0).P2Me4; ( x 1, P2Et4; (U). P2Pri4; and ( A )P,But4. For convenience the shifts have been referenced t othe following values of G(P)/p.p.m.: -58.0 (P2Me,), -32.0(P2Et4), - 10.0 (P2PrI4), and $40.0 (P2But,)S(P) = -57.74 and -57.77 p.p.m. for ca. 5 and 50% v/vsolutions respectively. The values of S(P) for the twoisomers of (Pr'EtP),, measured using 19F external lock, areunchanged (to within a few Hz) in going froin the neatliquid to a solution in benzene.TABLE 1Phosphorus-3 1 chemical shifts for tetra-a1 kyldiphosphines ( R1R2P)R1R2 Me2 rac-MeEt nteso-MeEt rac-MePri meso-MePri rac-MeButw ) / P .P . m . - 57.86 a -46.43 ' - 44.59 ' -38.35 ' -31.31 -30.20'1 02[ A8 (P)] (A T)-l /p. p. in. I<-1 0.6 2.6 2.4 3.7 3.2 3.2W) /P. P. m . -31.92 ' -21.40 -17.12 -6.03 ' - 10.64 ' 15.12 40.96 '1 O2 [ A8 (P) ] (A T)-' / p . p . m . K-' 4.7 6.3 5.0 3.9 8.9 6.6 5.2a In toluene + C,D, a t 36 "C (refs. 12 and S. Aime and R. K. Harris, J . Magn. Reson., 1974, 13, 236 give -57.58 p.p.in.).R'H2 Et2 rac-EtPri nzeso-EtPri rac-EtBut Pri , rac-Pri But But,' AtThe measurements were taken for the same samples used in the earlier study l2 and the data are consistent with the earlier 40 O C .(36 "C) results. " A t 36 "C.in the racenzic forni. In contrast, two 31P-(1H) signals werefound for (Pri EtP), with an intensity ratio of ca. 2 : 1 (frommeasurement of peak heights).I t is concluded that bothstereoisomers are present, the racemic being favoured, asfound previously l2 for (EtMeP), and (Pr'MeP),. Itappears then, that the meso isomer of (R1R2P), (R1,R2 =Me, Et, Pri, or But,) is only too low in population to be de-tected by n.m.r. when one of the alkyl groups is But. Thetwo ,1P signals for (Pr'EtP), broaden as the temperatureis raised and 31P inversion becomes more rapid. At thehighest temperature we used (161 "C), the spectrum wasjust short of coalescence, implying AG: N" 86 kJ niol-l.The 13C spectra are, of course, somewhat more complicatedthan the 31P spectra, but in practice they can be readilyinterpreted. Analysis is simplified by the fact that, for thetetra-alkyldiphosphines of type ( R1R2P), (R1,R2 = Me,Et, Pri, or Rut) the 13C chemical-shift ranges of the differentalkyl groups do not overlap.They occur as shown below.E t (U- Pri (a- But ( U -Group Me and p-C) and p-C) and p-C)The quaternary and CH, carbon resonances may be readilydistinguished from each other and from the CH and CH,G(C)/p.p.m. 2-11 11-20 21-26 2 8-3 J.C.S. Daltonresonances by low-power noise decoupling. Under suchconditions, the CH and CH, resonances are broadened andappear with poor signal-to-noise ratio. The quaternarycarbon signals remain sharp and have the same intensity asunder high-power noise decoupling because only small(long-range) (C,H) couplings have to be removed. TheCH, resonances also appear sharp but with only ca.theintensity obtained under high-power decoupling. TheCH, effect, which is very marked, is shown in Figure 2. Itdoes not appear to have been extensively used in otherpublications, although it has been noted for correspondingoff-resonance experiments.ls I t arises from the fact thatfor the CH, protons (unless they are markedly non-equivalent) the singlet state (in composite particle notation)acts as if it were non-magnetic. Thus, 13C signals from suchCH, groups (a of the total) do not require proton decouplingexcept to remove long-range (C,H) coupling effects.The 13C signals obtained under proton-noise-decouplingconditions are all triplets due to deceptively simplecoupling l2 to the two phosphorus nuclei (although in somecases the relevant splitting is very small).The splittingsyield values for N(PC), i . e . IIJ(PC) + ,J(PPC)I for the acarbons and 12J(PCC) + ,J(PPCC) I for the p carbons. Thevalues of N(PC) (a-C) were used to determine the assign-ment of the stereoisomers as described previously.1°*12 InFIGURE 2 25 MHz l3C-i1H) spectra of (Pr' EtP), at ambient probetemperature. The lower trace shows the effect of high-powernoise decoupling, and gives the peak assignments (Y = racemicisomer, m = meso isomer). The upper spectrum was recordedunder conditions of low-power noise decoupling ; only the CH,peaks remain sharp (see the text). For the lower and upperspectra, 500 and 750 transients, respectively, were co-addedbefore Fourier transformationthe case of compounds (1) and (2) such measurementsshowed that only the racemic isomer is present, while for(3) they indicated that the favoured isomer was racemic(intensity considerations then allowed the p-C peaks to beassigned unambiguously to the racemic and meso forms).The 13C spectrum of compound (3), with the resonancesassigned, is shown in Figure 2, and the 13C chemical shifts,together with data on N(PC), for the three compoundsstudied are given in Table 2.For compound (3) the p-Catoms of the isopropyl groups (for each stereoisomer) areanisochronous a t ambient probe temperature, as mentionedabove. However, for rac-(ButPri P), the temperature mustbe lowered to observe two sharp triplets for the isopropylp carbons: the signals were found to coalesce at ca.40 "C(leading to AGT M 65 kJ mol-l) and a single sharp triplet isobserved above 80 "C. The barrier may be compared tothat for (Pr'EtP), (86 kJ mol-l, see above) and for P,Pri,(75 kJ mol-l, derived from 13C work 12). It is clear thatincreasing the bulk of the alkyl groups attached to phos-phorus in diphosphines lowers the barrier to phosphorusinversion (although this was not obvious in earlier work 17)since the other factors of importance (substituent electro-negativity and n-bonding ability) do not vary appreciablyfor the tetra-alkyldiphosphines. Presumably substituentbulk destabilises the ground state and increases the CPPand CPC angles.DISCUSSIONThe data of Tables 1 and 2 will be discussed in termsof the conformations (1IIa)-(IIIc) and (1Va)-(IVc),together with the corresponding forms for R1 = R2.However, these are idealised forms, subject to severaltypes of distortion (e.g.towards semi-eclipsed forms) asTABLE 2Carbon- 13 n.m.r. parameters for tetra-alkyldiphosphines *a-carbons P-carbons-7 7 - 7 W)/ N(PC)/ w/ N(PC)/Molecule Group p.p.m. Hz p.p.m. Hz25.92 26.89.7 (ButPr'P), Pri 23.56 34.5 (24.31But 31.34 4.3 31.78 22.0(But EtP) , Et 12.26 31.1 14.93 26.51 . 1 29.69 21.3 But 29.13ruc-(Pr* EtP), Et 14.34 22.0 13.60 22.2Pri 24.57 < 2 {i;::; 21.1 21.2meso-(PriEtP), Et 15.75 9.4 13.44 24.5Pri 24.11 11.1 {21.43 22.35 23.2 17.0* Data given are for ambient probe temperature (cu. 36 "C),mentioned in the introduction. We shall assume,following McFarlane and McFarlane,ll that if R2 is morebulky than R1, form (IIIb) is preferred over form (IIIa),but there is as yet no definitive proof on this point.Phosphorus-31 Chemical Shifts.-The pattern of shiftsfor compounds of the type (R1R2P), is displayed inFigure 3 as a function of the total number, n, of methylgroups @ to either phosphorus. For compounds withoutBut there is a linear relationship (line A on Figure 3)between 6(P) and n, as might be expected if there are noeffects from large geometry distortions or changes inconformer populations.The slope of this graph (5.92p.p.m.) is, in that case, 4(@ + y), where p and y are theeffects of replacement of H by Me two and three bondsremoved from the probe nucleus re~pective1y.l~ How-ever, compounds containing But show departures fromline A, as has been noted previously l3 for P2But4 (and,in a corresponding situation,13 for Me2PPBut2).Mattersare complicated because meso stereoisomers of ( RButP),have not been detected. I t would be anticipated thatmeso forms would behave similarly to compounds P2R,because the gauche rotamers (TVa) and (IVb) areequivalent, whereas conformational bias for racemicexcept for (ButPriP),, at T = 1 "C1980E2 8 -$ 6 - Y4cCI n$4- zs N6412-forms should lead to deviations. Some justification forthis situation is evident in Figure 3. Moreover thevalue of A = S(P)(meso) - S(P)(rac) would be expectedto increase as the difference in bulk between R1 and R2increases. The data for (EtMeP), ( A = 1.8 p.p.m.),(Pr'EtP), ( A = 4.3 p.p.m.), and (Pr'MeP), (A = 7.0p.p.m.) bear this out.Thus a large value of A isexpected for (ButMeP),, and it may be that the value&[S(P)(meso) + S(P)(rac)] or, perhaps more likely,*/ B100 r ,0' /PA' O t n /y20t n /sI I 1 1 I0 2 4 6 8 10 12nRFIGURE 3 Plot of chemical shifts (with respect to the valuefor P,Me,) for tetra-alkyldiphosphines as a function of np, thetotal number of methyl groups B to phosphorus. (O), P2R4and meso-(RIRaP),; (m), rac-(RButP),; and ( x ) , otherrac-(R1R2P), compounds. The lines A and B are referred to inthe textB(P)(meso) itself for this compound may fall on the line Ain Figure 3. The point for (ButEtP), does fall on line A,but this is probably fortuitous since only the racemiccompound is known.The point for rac-(ButPr'P), fallsoff line A, as already also noted for P2But4. In factpoints for compounds of the type rac-(RButP), plusP2But4 fall on a second line, B, in Figure 3, of greaterslope than line A. Presumably the meso isomers of(RButP), plus P2But4 would fall on a third line of slopeintermediate between those of A and B. Clearly thedeviations of the t-butyl compounds from line A are notdue to a difference in y effect for the final Me substituentarising from a different orientation l8 (yrrans, say, differ-ing from ygallche), since in that case a constant deviationwould be expected, rather than a change in slope.Therefore, we believe that the t-butyl compounds( R B u ~ P ) ~ show a progressive geometric distortion as thesize of R increases. However, the normal p-deshieldingeffect, which operates for the smaller substituents, hasbeen attributed l9 to changes in hybridisation (increase ins character of the P-C bonds as the substituent bulkincreases) consequent upon changes in the CPC bondangles.Figure 4 plots the rate of change of B(P) with tem-perature against np.It can be seen that although thereis a relatively wide scatter of points (possibly arising in*A previous paper i 3 estimated 8(P) (meso) for this compoundto be - 14.6 p.p.m., on the assumption that &[6(P)(meso) + 8(P)-(rac)] should lie on a line analogous to A. We now believe this isnot necessarily so, and that 8(P) (meso) may be nearer to -19p,p.m.part from concentration variations), there is a generaltendency for AS(P)/AT to increase as the substituentsbecome larger, but the point for P2But4 is anomalouslylow, again possibly indicating geometrical distortion.Values for the other t-butyl compounds are also low.Temperature variation of S(P) may partly arise fromintermolecular interactions and partly from conform-ational averaging.From the latter point of view it isinteresting to note that AS(P)/AT is less for the mesoisomers than for the racemic forms. According toFigure 3 this implies that G(P)(rac) will approachS(P) (meso) at high temperatures, suggesting that thepopulations of (IIIa) and (IIIb) are becoming closer.The large value of AS(P)/AT for P2Pri4 implies that athigh temperatures the point for this compound wouldfall closer to line A of Figure 3 than currently displayed.Carbon-13 Chemical Shifts.-The values of S(C) forsymmetrical tetra-alkyldiphosphines (R1R2P), areplotted in Figure 5. Several points of interest emerge.(i) For a-carbon atoms S(C)(racemic) is always less thanS(C)(nzeso) for the lighter of the two substituents R1 andR2. This means that the plots for a given carbon in themeso and racemic forms cross at the point for the relevantcompound of the type P2R4.Such an observationimplies that groups gauche to a lone pair are less shieldedthan those in a trans position. However, there is nodirect evidence on this point for diastereoisomeric t-butylcompounds, and a contrary situation (Le. apauehe > crtrans)is found a at low temperatures for P2But4 (an assignmentof the a-carbon signals may be deduced from refs.8 and0(0" 0D0D 0 2 4 6 8 1 0 1 2"PFIGURE 4 Temperature variation of 6(P) for tetra-alkyldiphos-phines as a function of nB, the total number of methyl groups pto phosphorus. For meanings of symbols see Figure 310). Again, therefore, the bulky t-butyl groups appearto be causing departures from ' normal ' behaviour.(ii) The difference IS(C)(meso) - S(C)(racemic)l for thea-carbons increases as the bulk of the substituents R1and R2 differs increasingly. This would be expected,either on the basis of variations in conformationalbiassing or of geometry distortions. The smooth cross-over discussed under (i) above tends to suggest thatconformational changes are the more likely origin of theobservations, although this is not entirely in accord withthe results for N(PC) discussed below.(iii) For methyl carbons in (RMeP), compounds642Pri Group observed Pr* Pr' PriR1R2 MePri EtPri Pri Pr' But34.5 f trans to lone pair egauche to lone pair < 1.0 <2Average 9.3 11.1 12.0 bJ.C.S.DaltonBut But But ButEtBut PriBut But,0.8 1.1 h 4.3 f < 1 gMeBut45.5 g24S(C) decreases smoothly as the bulk of R increases.Such a change represents a y + 6 effect of substitutionof H by Me provided the situation is not affected byconformational variations. It is best to compare meso36 r A1612m0 2 4 6 8 1 0 1 2nPFIGURE 5 Carbon-13 chemical shifts for tetra-alkyldiphosphinesas a function of n g , the total number of methyl groups p tophosphorus. Racemic ( Y ) and meso (m) results are distinguishedonly for a carbons. The two anisochronous !3 methyls forisopropyl groups are not distinguished. Solid lines link resultsfor a carbons of meso-(R1R2P), and P2R4, and also p carbons.(O), a-Et; (a), ct-Pr*; (A), a-Rut,; ( x ) , Me; ( O ) , P-Et;(m), @-Pri ; (A), @-Butcompounds with the symmetrical P,Me, since the con-formational situation is similar, and these give onaverage y + 6 = -2.4 p.p.m., presumably because ofthe usual y-shielding effect.12 (Variations in thepopulation of trans rotamers may however, complicateas the bulk of R increases, another example of theanomalous behaviour of the compounds with bulkysubstituents. The racemic forms show larger values ofIy + 61 than the meso isomers for Me, Et, and Pri groups,omitting rac-(ButPriP),, so the reversal of the slope inthe rac-(RButP), series is all the more anomalous.(iv) The 8-carbon (CH,) of the ethyl groups in(REtP), has a steadily increasing value of S(C) as thebulk of R increases.This represents the effect at a 6position (via one phosphorus atom) of progressive sub-stitution of H by Me, and it is ca. 1.1 p.p.m. per replace-ment. The shift difference between racemic and mesoforms appears to be small. Similar replacement effectsmay be noted on the p-carbon chemical shifts for Priand But groups, and the more bulky groups do notappear to introduce any substantial anomaly.If asimilar 6 effect (but via two phosphorus atoms) occursfor the a-carbons, then the normal y effect of the Me forH replacement, mentioned in (iii) above, must beca. -3.5 p.p.m.on P2But4 showed that there is a relatively small differ-ence (1.36 p.p.m.) between 6(C) for P-carbons in Butgroups gauche and trans to the lone pair.(2)) The anisochronicity between the P-methyl carbonsof an isopropyl group decreases from meso-(Pri MeP), tomeso-(Pr'EtP), to P2Pri4 [A6(C) = 1.39, 0.92, and 0.64p.p.m. respectively], as might be expected. For theracemic isomers of (RPr'P), the values are A6(C) = 0.53,0.06, and 1.61 p.p.m. for R = Me, Et, or But respectively;the large value for (ButPr'), may be related to thepreferred position of Pri trans to a lone pair in thiscompound (in contrast to the other two).(P,C) Coupling Constants.-In general, values ofN ( P C ) for a-carbons follow the pattern suggestedearlier,l**l2 assuming reasonable conformational bias-sing.l0?l1 Thus, for the three compounds (1)-(3) in theracemic form, the lighter alkyl substituent has a largevalue of N(PC), corresponding to a position trans to theThe low-temperature measurement1980Group observedRIRaracemic isomermeso isomer or P,R, d643Pr' Pri Pr* PriMePr' EtPri Pri, Pr'But21.1 26.8 8{ f::: p;::9.71820(2224:;: 21.2Table 3 sets out all the data for tetra-alkyldiphosphines.It is apparent that N(PC) for R1 in the trans position of(R1R2P), increases as the bulk of R2 increases and as thebulk of R1 itself increases (except for the comparison ofR1 = Me with R1 = Et).The ' average ' value ofN(PC) obtained from P,R, and meso forms of (R1R2P),varies similarly but to a lesser extent. These changesmay be associated with varying amounts of the transrotamers (IIIc) and (IVc), or with geometry changescaused by substituent bulk. In so far as Ramanspectroscopic evidence shows that liquid P,Me, containsa considerable proportion of trans rotamer (in contrast tothe deductions of McFarlane and McFarlane 11), whereaswhen the statistical advantage of the former is dis-counted. )The p-carbon nuclei do not, in most cases, show anysubstantial difference in N(PC) for the meso and racemicforms (see Table 4). For the ethyl group, IN(PC)I liesbetween 20 and 27 Hz, and for the t-butyl group therange is 19-22 Hz.However, the isopropyl groupshows some oddities, associated with the non-equivalenceof the p methyls. Thus, although nine of the relevantvalues of IN(PC)I lie between 18 and 27 Hz and are thus' normal ', the meso forms of (Pr'MeP), and (Pr'EtP),each show one low value (15.8 and 17.0 Hz respectively),and the racemic form of (ButPriP), has one very lown.m.r. spectra clearly show P2Bue4 is predominantlygauche, it is likely that the effect on N(PC) of conform-ational variations throughout the series of tetra-alkyl-diphosphines may be considerable. Indeed, this hypo-thesis predicts lower values for N(PC) ('trans ') forP,Me, than for P,But,, as observed, since in the formercase there will be a contribution from a gauche orientationof a carbon nucleus with respect to a lone pair arisingfrom the trans conformer.The evidence for trends inN(PC) for the gauche position is less clear. Variable-temperature experiments in the range 0-130 "C for thea-carbon of But in rac-(ButPriP), show that N(PC)increases steadily wit11 t t,inperature, whereas for rac-(ButEtP), the opposite behaviour appears to occur in theregion 7-61 "C, suggesting the signs of N(PC) (gauche)may be opposite in the two compounds. HoweverN(PC) for the Pri a-carbon in rac-(ButPriP), does notappear to vary substantially with temperature. Again,the temperature variations of N(PC) may be associatedwith conformational changes, or possibly with inter-molecular interactions.It is also possible, although un-likely, that for compounds (1) and (2) racemic === mesoexchange is fast with respect to the 13C time scale at thetemperatures studied and that there is a contribution tothe observed N(PC) from a small population (varyingwith temperature) of the meso form. It may be notedthat for P,Me, in toluene IN(PC)I varies from 7.3 to8.4 Hz in the temperature range -63 to 104 "C. Thisobservation is consistent with an increasing amount ofgauche conformer. (Raman evidence suggests thisform has a slightly higher free energy than the trans formvalue (9.7 Hz). Although the explanation presumablylies in the details of molecular conformation, we areunable to present any satisfactory reason for theseoddities.However, the low-temperature data forP,But, show there is a substantial dependence of N(PC)for p-carbons on orientation with respect to the lonepair, since values of 21.2 and 15.6 Hz are reported, thelower value being due to the But group trans to the lonepair.EXPERIMENTALCompounds (1)-(3) do not appear to have been reportedfollowing standard reaction sequence. Alkyldichlorophos-phines PRlCl, (R1 := Et or But) were treated with theGrignard reagent MgR2Cl (R2 = Pri or But) in a 1 : 1 molarratio to yield the asymmetric phosphinous chloridesPEtButC1, PEtPr' C1, and PPr' BuU, respectively. Thediphosphines were then prepared from the chlorides bydehalogenation with sodium in dioxan, and purified byrepeated distillation. All operations were carried out underan atmosphere of dry oxygen-free nitrogen, since the di-phosphines are readily oxidised by air.(ButEtP),, b.p.72 "C (0.5 mmHg) * (Found: P, 26.15. C,2H2sP2 requiresP, 26.45%); (EtPr'P),, b.p. 57 "C (0.5 mmHg) (Found: P,30.2. C1,,HwP2 requires P, 30.05%) ; (PriRutP),, b.p.116 "C (4 mmHg) (Found: P, 23.1. C1,H,,P2 requires P,23.6%).The n.m.r. samples of the diphosphines were prepared bydissolution in benzene, with C,D, to provide a lock signal,using a dry-box under a nitrogen atmosphere. The con-centrations were ca. 20, 25, and 30% by volume for ( l ) ,* Throughout this paper: 1 mmHg c 13.6 x 9.8 PaJ.C.S. Dalton(2), and (3) respectively. The solutions were placed inn.m.r. tubes (outside diameter 12 mm). The samples werethen degassed by the freeze-pump-thaw method usingliquid nitrogen.The samples were stored a t -4 "C whenthe spectra were not being obtained.The spectra were recorded using a Varian XL 100 spectro-meter, in the Fourier-transform mode for carbon (25.14MHz) and phosphorus (40.51 MHz). Both types of spectrawere normally obtained under conditions of proton-noisedecoupling, with a 2H field-frequency lock. For certainvariable-temperature experiments other medium conditions(e.g. neat liquid) were used, together with the lgF externallock facility of the XL 100.The chemical shifts axe quoted with the positive fre-quency 21 i.e. a positive sign implies thesample resonates to high frequency of the reference (SiMe,and 85% H,PO, for 13C and 31P respectively).The shiftswere measured by the absolute frequency indirect methoddescribed previously,22 using Sp (85% H,PO,) = 40 480 720Hz and Ec (SiMe,) = 2 5 145 004 Hz. The standardisedlock frequency was SD (C,D,) = 15 350 721 Hz. The valueof S p given above was used for consistency with previouslyreported data for diphosphines, so that all relative chemicalshifts would be accurate. However, McFarlane 23 hasreported Ep (85% H,PO,) as 40 480 740 Hz, and we our-selves have measured this parameter to be 40 480 737 Hz.I t should be appreciated that no fundamental importanceattaches to the precise value of E for the reference com-pound, and our data can be adjusted, if required, to fit anyfuture internationally agreed value for this parameter.The spectrometer conditions were varied as required.Spectral widths (SW) were in the range 250-500 Hz, withacquisition times (TaJ 4.0-8.0 s and pulse angles 35-45"for carbon and 50-65" for phosphorus.The number oftransients co-added in the computer ranged from 250 to 500for carbon and 10 to 50 for phosphorus.The temperature was directly measured by a thermo-meter, placed in a n.m.r. tube containing methanol for lowtemperatures (< 37 "C) and ethylene glycol for high tem-peratures ( > 37 "C) . The quoted temperatures are probablyaccurate t o + 2 "C. Ambient probe temperature is ca. 36-37 "C.One of us (G. B.) thanks N.A.T.O. and C.N.R. (Rome) forthe award of a research fellowship. Another of us(A. A. M. A.) thanks the University of Kuwait for a post-graduate studentship.[9/224 Received, 12th February, 19791REFERENCESDalton, 1978, 9.Part 3, R. K. Harris, E. M. McVicker, and G. Hagele, J.C.SJ . R. Durig and J. S. DiYorio, Inovg. Chem., 1969, 8, 2796.R. Richter, J. Kaiser, J. Sieler, H. Hartung, and C. Peter,J . R. Durig, L. A. Carreira, and J. D. Odom, J . Amer. Chem.A. H. Cowley, M. J. S. Dewar, D. W. Woodman, and M. C.D. L. Ames and D. W. Turner, J.C.S. Chem. Comm., 1975,A. McAdam, B. Beagley, and T. G. Hewitt, Trans. FaradayS. Aime, R. K. Harris, E. M. McVicker, and M. Fild, J.C.S.@ J . A. Brunelle, C. H. Bushweller, and A. D. English, J . Amer.lo R. K. Harris, E. M. McVicker, and M. Fild, J.C.S. Chem.l1 H. C. E. McFarlane and W. McFarlane, J.C.S. Chem. Comm.,l2 S. Aime, R. K. Harris, E. M. McVicker, and M. Fild, J.C.S.l3 R. K. Harris, E. M. McVicker, and M. Fild, J.C.S. Dalton,l4 R. 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