J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2611-2616 261 1 Spectroscopic Investigations on lminophosphanes and Methylenephosphanes G. David, V. von der Gonna and E. Niecke" Anorganisch-Chemisches lnstitut der Universitat Bonn, Gerhard-DomagkStr. I, 53l21-BonnI Germany T. Busch and W. W. Schoeller* Fakultat fur Chemie der Universitat Bielefeld, Postfach 10 01 3 I, 33615 Bielefeld, Germany P. Rademacher lnstitut fur Organische Chemie der Universitat Essen HGS, Universitatsstrasse 57,4514 I Essen I, Germany According to qualitative theoretical considerations, P'" double-bonded systems possess two energetically closely spaced frontier orbitals. This concept, which explains the reactivity of these species, has been investi- gated experimentally via a systematic study of the photoelectron spectra of a selected variety of substituted iminophosphanes.The assignment of the bands is assisted by UV spectroscopic investigations and quantum chemical calculations at MNDO and a6 initio levels of theory. The latter are of double-[ quality and were per- formed at SCF and MCSCF levels of sophistication. Since the pioneering studies of Dimroth and Hoffmann on phosphamethane cyanines,' PIr1 double-bonded systems (Scheme 1) have been investigated in detail as documented in a pertinent review.2 While the preparative aspects of this new class of compounds are now becoming well understood, reports on spectroscopic details are rare.3 Qualitative theo- retical arguments reveal two energetically closely spaced fron- tier orbitals in the PI1' double-bonded compounds.This is depicted schematically in Scheme 2 for the case of the parent iminophosphane (Scheme 1, X = NH). For this system the highest occupied molecular orbital (HOMO4) is a orbital constituted mainly by the lone pair at the phosphorus. The molecular orbital lower in energy is the A orbital of the PN double bond. In accordance with qualitative theory' such an orbital pattern dictates carbenic behaviour to the imin-ophosphanes. The predictions are supported by a variety of experiment a1 investigations. 3a The fact that a simple frontier orbital theory on the reacti- vity of PIr1double-bonded systems exists while the factors that influence the levels of the two closely spaced frontier orbitals are only poorly understood prompted us to investi- \ /P=X x=C ,N\ \ Scheme 1 H @/ /P=NH x.x* x* -4-b-Scheme 2 gate this aspect in more detail by various spectroscopic tech- niques. In the present work we report photoelectron spectroscopic and UV spectroscopic investigations on a selected variety of imino- and methylene-phosphanes. In particular, we will be concerned with a detailed investigation of the substituent effects on the spectroscopic properties of these compounds. The experimental findings will be supplemented by quantum- chemical calculations at ab initio and semiempirical levels of sophistication. On this basis, a more clearcut interpretation of the spectroscopic properties is possible. Theoretical Procedures The ab initio calculations were carried out at double-[ level.The corresponding basis sets were constructed from Huzin- aga primitive functions6 At times they were augmented with polarization functions at the atoms. For the ab initio calcu-lations the following basis sets were used. Basis I: P, Si (1 Is, 7p) in the contraction [5, 6 x 1/4, 3 x 13 + Id ([d = 0.5; 0.4); N, C (8s, 4p) in the contraction [5, 3 x 1/3, 13 + Id (rd = 0.95; 0.8); H (4s) [3, 13. Basis 11: P (lls, 7p) [5, 6 x 1/4, 3 X 13 4-2d ([d(l, 2) = 0.96; 0.32); N (9s, 5p) [5, 4 X 1/3, 2 X 13 + 2d ([d(l, 2) = 1.654; 0.469); c (9S, 5p) [5, 4 X 1/3, 2 x 13 + 2d ([d(l, 2) = 1.097; 0.318); H (4s) in the contraction [3, 13 + 2p([d(1, 2) = 1.20;0.30). The SCF calculations were performed with the Karlsruhe version of the Columbus set of programs.' The energy opti- mizations of the structures were directed with the Murtagh- Sargent algorithm.8 All energy minima were obtained with an accuracy of 0.1 pm (0.05') and subjected to vibrational analysis within the harmonic approximation.On this basis it was ascertained that the closed species are true energy minima on the electronic hypersurface. At the SCF geome- tries, additional correlation calculations were performed including all single and double excitations from the ground state (SDCI). MCSCF'" calculations were carried out at the parent compounds methylene- and imino-phosphane. Com- plete active space (CAS-SCF) was included in the energy optimizations at the MCSCF level.Finally, the results were refined by multi-reference CI calculations (MRC19b*'). All correlation calculations were performed with the program system MOLPRO.'? For the larger molecules ab initio calcu-lations of this accuracy were not possible. For these cases, i.e. the heavily substituted n-systems, the assignment of bands in the photoelectron spectroscopic investigations was assisted with energy-optimized MNDO calculations. lo Experimental All of the various substituted methylene-and imino-phosphanes were synthesized according to literature pro- cedures.2*'' UV Spectra The UV spectra were recorded at room temperature with a Kontron UVIKON spectrometer. The compounds dissolved in heptane were placed in quartz cuvettes (d =0.1 cm) and their spectra were taken at intervals and standardized against that of pure solvent.The concentration of the samples varied from lop3 to mol 1-'. The samples were prepared under an atmosphere of nitrogen, since the compounds are sensitive to moisture and oxidation. The solvents were, in addition, dried by refluxing over lithium aluminium hydride for several days. After purification, the solvents were stored over molecular sieves (4 A). Before use, the solvents were degassed via the frozen pump technique. The maxima of the bands were determined with an accu- racy of &1 nm. The error in the molar absorption coefficients is within &15%. Photoelectron Spectra The photoelectron spectra were recorded on a Leybold-Heraeus UPG 200 spectrometer with He I excitation (21.21 eV).Calibration of the spectra was performed before and after each measurement with an Ar-Xe gas mixture. The determined vertical ionization energies are the average of several measurements. The accuracy of determination was f50 mV. Results and Discussion Vertical Ionization in Iminophosphanes At the beginning of our study we report theoretical aspects of the ionization energies of the parent compounds imino- and methylene-phosphane. For this purpose we employed high- quality ab initio calculations. From a quantum-chemical viewpoint, ionization energies can be computed at various levels of sophistication: (a) on the basis of Koopmans' theorem12 and (b) by separate evaluation of the energies of the ground states and the corresponding radical cations; after correction for electron corrleation, the energy differences then refer to the vertical ionization energies of the molecules under consideration.Procedure (b) is, in general, more accurate for .the quantum-chemical evaluation of ionization energies. We have probed in detail the assignment of bands for the parent compounds with both procedures. The energy-optimized structures obtained at the MCSCF level (basis 11, CAS-SCF) for the parent compounds methylene- and imino-phosphane (cis and trans) are sum- marized in Fig. 1. The geometrical parameters of these compounds have alreddy been determined by previous investigators at the SCF J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 119.2 H 169.3 &/l09.8 109.6 H 159.7 &/103.0 P=N 142.3/4 H 99.0 158.7 P=N 144.0/# %\102.6 H 105.6 117.1 H Fig. 1 Ab initio MCSCF geometries of methylene- and imino- phosphane; bond lengths (angles) in pm (degrees) level,13 and their values agree with ours. However, as expected, the n bonds at the MCSCF level are slightly longer than those obtained at the SCF level. Based on the energy- optimized geometries, ionization energies were computed at various levels of sophistication. The results are collected in Table 1. The photoelectron spectrum of the parent methy-lenephosphane has been reported. l4 In the experiment, the first two ionization energies (lowest in energy) appear at 10.30 and 10.70 eV.This is in good accord with our best results at the MCSCF/MRCI level (see Table 1). Our calculations place the 7c orbital slightly below the c orbital in energy. In con- trast, we predict for the parent iminophosphane a sizeable energy between the two lowest ionization energies. In con- trast to methylenephosphane, the first ionization is due to electron loss from the c orbital and the second to that from the 7c-orbital. This is in accord with previous qualitative pre- dictions.' Ionization Energies in Substituted Imino- and Methylene- phosphanes Irninophosphanes,R'P=NR A representative photoelectron spectrum of a substituted iminophosphane is shown in Fig. 2. The first two lowest- energy ionization bands are strongly separated [appearing at 7.88 eV (Ei, and 9.58 eV (Ei,2)], which agrees with the results of the quantum-chemical calculations.A variety of substituted iminophosphanes have been measured and the resulting ionization energies are collected in Table 2. Quantum-chemical theory predicts ionization either from the 71 orbital (of the PN double bond) or from the c orbital (n) at phosphorus. As will be shown (vide infra)assignment of the orbitals is possible by utilization of a linear relationship between UV and photoelectron (PE) data. The UV spectra of the iminophosphanes show two well separated bands in the short-wave region, one of high inten- Table 1 Computed vertical ionization energies (in eV) of the parent methylene- and imino-phosphanes compound Ei(a) Ed.) method HPNH (trans) 9.184 (10a') 11.386 (2a") SDCI" 8.835 10.726 8.562 10.472 HPNH (cis) 9.407 10.753 MCSCFb MCSCF/MRCIb MCSCFb MCSCF/M RCI MCSCF~ MCSCF/MRCIb ~ ~ ~ ____~~ 9.687 11.071 -kMOLPRO-89 is a package of ab initio programs written by H. J.HPCH, 9.427 9.262 'tyerner aiid P. J. Knowles, with contributions from J. Almlof, R. 10.193 9.935 .knos, S. Elbert, W. Meyer, E. A. Reinsch, K. Pitzer and A. Stone.' 3.E .arevim of the various methods see ref. 9(d). 'Basis I level; Basis I1 level. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Vertical ionization energies (in eV) of iminophosphanes, RP-NR' ~ CEt, But 7.88 (n) 9.58 (n) this work But CEt , 7.94 (n) 9.59 (n) 10.00 this work But Bu 8.10 (n) 9.70 (n) 3(4N(SiMe,), SiMe, 8.10 (n) 8.75 (n) 3(4N(SiMe,), But 8.05 (n) 8.48 (n) 3(4 TmP SiMe, 7.85 (n) 8.25 (n) this work N(Bu')SiMe, Bu' 7.89 (n) 8.06 (n) 9.76 (n) 3(4NPri Bu' 7.90 (n) 7.99 (n) 3(4Bu' TmP 7.23 (n) 7.65 (n) this work But NMe, 7.66 (n) 8.32 (n) this work TmP N(SiMe,), 7.32 (n) 8.08 (n) 8.89 (n) this work CP* But 7.44 (na) 8.13 9.33 this work CP* NMe, 7.03 7.68 8.10 9.25 this work CP* N(SiMe,), 7.11 7.96 9.07 this work R :Tmp, tetramethylpiperidyl ;Cp*, pentamethylcyclopentadienyl. >C-P=N-Cf >C-P=N-Cf 0 >C*N-PP= NC~ >C-P=N-NC2f .,--,A1O-1 ' ' ' ' ' I ' ' -1.0 -0.5 0.0 0.5 1.0 1.5 -Ei(n)l/eV I ,8 10 12 14 16 Fig.3 Relationship between ionization energy differences and the EJeV corresponding electron excitations in the iminophosphanes, RP=NRFig.2 Photoelectron spectrum of the iminophosphane, Et,C-P-N-Bu' sets of quantities are linearly related.I5 A corresponding plot sity and the other of low intensity (Table 3), referring to n-n*, for the case of the iminophosphanes is shown in Fig. 3. Hence o-n* transitions, respectively. The latter has a low molar on the basis of the linear correlation, the first two bands in absorption coefficient. It is clear that the the UV data refer to the PE spectrum can be assigned unequivocally; the corre- energy differences between ground and excited states of the sponding results are collected in Table 2. neutral molecule, while the photoelectron spectroscopic mea- The trend in the ionization energies is obvious.An amino surements refer to energy differences between ground states of substituent affects the n orbital by conjugation, lowering the the corresponding ionized forms. It has been shown that both Table 3 UV data of iminophosphanes, RP-NR' 4naJnm (E,,,,J~o-~1 mol-' cm-') R R' n-n* n-n* ref. CEt, Bu' 230 (1 1.7) 440 (0.25) this work But CEt, 235 (10.8) 448 (0.21) this work B u' Bu' 233 (12.0) 431 (0.25) 3(4N(SiMe,) SiMe, 262 (6.4) 354 (0.27) 3(4N(SiMe,), But 267 (5.6) 348 (0.20) 3(4 '4 TmP SiMe, 276 (5.4) 354 (0.26) this work N(Bu')SiMe, Bu' 278 (4.9) 345 (0.21) 3(4NPri Bu' 315 (4.6) 340 (s) 3(9 But ButBut TMP 318 (3.0) 340 (0.39) this work \ \ Pr;N \ (Me3si)p\But NMe, 310 (4.7) 345 (0.50) this work P= N P= N P=N P= N TmP N(SiMe,), 304 (6.0) 398 (0.72) this work \ \ \ \ CP* NMe, 264 (2.7) 346 (0.50) this work me2 But But But CP* N(SiMe,), 271 (2.6) 341 (0.50) this work Scheme 3 ionization, which is stronger in magnitude at nitrogen than at phosphorus.This, in fact, is due to the larger coefficient of the n orbital at N compared with that at P. Hence conjugation with an attached amino group exerts stronger destabilization (energy lowering of the n orbital) at N than at P. Note the different action of the disilylamino group compared with the dialkylamino substituent; this is illustrated in Scheme 3. The Si,N substituent is a less effective n donor than the C,N sub- stituent, owing to the tendency of a silyl group to delocalize the lone-pair orbital at the amino nitrogen atom.As a conse- quence, the Si,N group is less effective for n donation into the PN double bond. In order to confirm these assignments of the PE bands we computed the electronic properties of various model substi- tuted iminophosphanes R’P = NR2 (basis I level), in their trans conformations and with a conjugating (planar) amino group. The geometrical parameters obtained are collected in Table 4. Again we have determined the two vertical ioniza- tion energies from separate computations of the various ionized states [method (b)].The results of these investigations are listed in Table 5. The calculated data confirm the experi- mental assignments. Interestingly, for the case of dial-kylamino (disilylamino) groups at nitrogen, the theory always predicts the a orbital to be below the n orbital in energy, which is in accord with the stronger n-donation ability of an amino group at nitrogen than at phosphorus.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Pentamethylcyclopentadienyl exhibits a characteristic PE spectrum. The first ionization band of RPNR (R= CP*, R’ = Bu‘) appears at almost the same energy as that of cyclo- pentadiene, Ei, = 7.48 (n,),Ei,, = 9.54 eV (n1).l6Hence it is assigned to ionization of the n2 orbital of the diene unit. According to MNDO calculations, the n orbitals of the cyclopentadiene system mix strongly with the central x bond in all of the pertinent Cp compounds.Consequently, further assignment of this band to local contributions is not possible. Methylenephosphanes RP=CR’R” We have also recorded PE spectra of a variety of methylene- phosphanes. The results are listed in Table 6. The assignment of bands was again performed via a linear correlation of PE and UV data. The amino compound exhibits a markedly lowered first ionization energy. Again silyl groups tend to lower the n ionization energies relative to those of the parent amino group. Iminophosphanes, RP=NMes* For these cases (see Table 7) the first ionization band in the photoelectron spectrum appears at almost the same energy, affected by the substituents only to a minor extent, except for the amino compounds which have a slightly lowered ioniza- tion energy.Similarly, the positions of the second ionization bands are almost constant at 8.1-8.2 eV; we assign these two bands (Ei.,) Ei,and as ionizations from the aryl group, which is confirmed by the MNDO calculations on these Table 4 Geometrical parameters of planar amino-substituted imino- systems. According to the calculations, the third ionization phosphanes; bond lengths (angles) in pm (degrees), at SCF level band refers to the n orbital mainly located at phosphorus. R’ R2 <R’PN <R2NP PN NN’ Note that a-electron-withdrawing substituents, such as the NR, or OR groups tend to lower the n-orbital energy, a con- H H 99.9 112.3 154.8 sequence of the increasing s character of the lone-pair orbital NH2 H 104.7 113.6 153.6 at phosphorus with increasing electronegativity of the neigh- N(CH3)2 105.7 113.0 154.0 bouring substituents.As for the iminophosphanes discussed N(SiH3), H 102.8 116.6 153.5 H NH2 96.0 123.9 159.2 129.7 H N(CH3)2 95.3 125.1 160.3 128.9 H N(SiH,), 96.6 124.1 157.6 135.9 Table 7 Vertical ionization energies (in eV) of iminophosphanes, R-P-N-Mes* R Table 5 Theoretically derived ionization energies (in eV) of model substituted iminophosphanes (trans conformation) But Pri 7.32 7.25 8.27 8.17 8.62 8.79 9.40 9.40 CEt, 7.22 7.20 8.44 9.27 NH2 H 8.860 (13a’) 9.325 (3a”) CH( SiMe ,) PBu; 7.58 7.20 8.14 8.09 8.21 8.59 9.57 9.45 N(CH3)2 N(SiH,), H H H H NH, N(CH,), N(SiH,), 8.534 9.120 8.918 8.457 8.702 (19a’) (25a’) (13a’) (19a’) (25a’) 8.294 9.661 8.121 7.366 8.491 (5a”) (7a”) (3a”) (5a”) (7a”) N(SiMe,), N( Bu‘)SiMe, OBu‘ Ph Mes* 6.95 6.95 7.26 7.22 7.20 8.25 8.10 8.12 8.17 8.20 8.76 8.71 9.02 9.03 9.64 9.55 9.48 9.39 9.70 For R’ = R2 = H see Table 1.Mes*, 2,4,7-tri-tert-butylphenyl. Table 6 Vertical ionization energies (in eV) of methylenephosphanes, RP-CR‘R” R R‘ R“ Ei, 2 Ei, 3 ref. But SiMe, SiMe, 8.10 (n) 8.85 (n) 9.63 3(b) TmP H SiMe, 7.48 (n) 8.82 (n) 9.26 (n) this work TmP SiMe, SiMe, 7.26 (n) 8.56 (n) 8.97 (n) 3(b)c1 SiMe, Ph 8.60 (n) 9.27 (n) 9.98 this work Cl SiMe, SiMe, 9.22 (n) 9.94 3(b)SiMe, OSiMe, Pr 7.88 (n) 8.22 (n) 9.80 this work But NMe, H 7.17 (n) 8.62 (n) 10.09 (A) this work But But H 8.74 (n) 8.95 (n) 3(4 J. CHEM. SOC.FARADAY TRANS., 1994, VOL. 90 earlier, Ei, appears at the same level as the PN double bond. Hence, tentatively this band may be assigned to ionization of the PN n bond. Methylenephosphanes, Mes*-P=CR'R" In the case of the compounds Mes*-P=CR'R", assignment of the bands could no longer be achieved by comparison with the corresponding UV data. The UV spectra of these com- pounds are extremely complex, indicating only minor changes due to substituent effects. An assignment of bands in the PE spectra could be achieved only with the aid of MNDO calculations. The results obtained are listed in Table 8. The first band is assigned to ionization from the aromatic ring system, as has already been observed for the correspond- ing iminophosphane compounds. The second band refers to ionization from the PC n bond.This is also in accord with the above discussed results on other methylenephosphanes. Finally, Ei, is assigned to ionization from the n orbital. As for the iminophosphanes, halogens increase the ionization energy Ei, 3 (n). 31P NMR Shifts and Electron Excitation energies of RP=NMes* We now discuss the relationship between 31PNMR shifts in substituted iminophosphanes of the type RP=NMes* and corresponding UV data. Within this class of compounds, a large variation in the n-n* transitions is' observed, e.g. that for Bu:P-P-NMes* appears at 580 nm. This indicates that the LUMO in these compounds is strongly affected by sub- stituents. The same tendency is also observed for the 31P NMR shifts (Table 9).A plot of both quantities (Fig. 4) reveals a satisfactory linear relationship; a regression analysis yields the following Table 8 Vertical ionization enephosphanes, Mes*P= CR'R" energies (in eV) of methyl- R' R" H H 8.17 8.81 9.33 H F 8.24 8.50 9.64 H c1 8.20 8.45 9.47 H H SiMe, NMe, 7.83 6.83" 8.50 7.76" 8.90 8.09 Br Br 8.09 8.37 9.42 Br c1 8.12 8.38 9.42 c1 c1 8.19 8.44 9.52 Ph Si Me 7.67 8.12 8.90 Table 9 31P NMR shifts and UV n -,n* transitions for imino- phosphanes,R-P=NMes* R d(31P) A(n-n*)/nm PBu', 580 570 CEt, 520 544 Pr 49 1 533 Bu 490 525 CH(SiMe,), 476 532 Ph 415 576 Mes* 396 568 N(SiMe,), 327 416 N(SiMe3)Bu' 313 40 1 OBu' 179 340 OMe 156 335 2615 600-500- --.* n I *5 400- I a/ 3001 /O-0- I I I I 100 200 300 400 500 600 700 S(3'P) Fig.4 Relationship between 31PNMR chemical shifts and electron excitations (n-n*) in the iminophosphanes, RP=NMes* equation c~(~IP)= 1.6lnP,p-359.94 (1) with a correlation coefficient r2 = 0.99. The result in Fig. 3 is, therefore, not surprising, since it indicates that the paramag- netic contribution to the NMR chemical shifts is dominant. Conclusion In the present publication we report detailed investigations on the spectroscopic properties (UV and PE data) of a selec- ted variety of imino- and rnethylene-phosphanes. Our find- ings can be summarized as follows. (1) According to quantum-chemical calculations, Prrr double-bonded systems possess two closely spaced frontier orbitals, a n and a c orbital.The latter is constituted by a combination of lone-pair orbitals at phosphorus and nitro- gen. In the case of the parent iminophosphane, the o orbital in the HOMO is well separated from the n orbital (HOMO1). For the parent compounds the vertical ionization energies were calculated at an ab initio MCSCF level of soph-istication. (2) Experimentally, the assignment of ionization bands in the PE spectra is performed by using the linear relationship between PE and UV data. The effects of substituents are in agreement with expected trends; a n-donating amino group lowers the ionization energy of the PN double bond, in other words, the orbital energy of the PN double bond is decreased.Dialkylamino groups are more effective 7t donors than disilylamino groups, owing to the competing accepting ability of the silyl substituent. In addition, n donors are more effective at nitrogen than at phosphorus. This tendency is even more pronounced for an analogous substitution pattern (P us. C substitution on the resulting n-orbital energies) in methylenephosphanes. (3) The 31P NMR parameters of the investigated imin- ophosphanes are strongly influenced by the UV properties. In fact, a linear correlation is observed which emphasizes the major role of the paramagnetic term in the.NMR properties of this new class of Prrrdouble-bonded systems. This work has been supported by the Fonds der Chemischen Industrie and the Deutsche Forschungsgemeinschaft.The quantum-chemical calculations were performed on the Convex C240 at the University of Bielefeld and on the Cray- YMP at KFA, Jiilich. References 1 (a) K. Dimroth and P. Hoffmann, Angew. Chem., 1964, 76, 433; Angew. Chem., Int. Ed. Engl., 1964, 5, 846; (b) G. Markl, Angew. Chem., 1966,78,907; Angew. Chem., Znt. Ed. Engl., 1966,5,846. 2616 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2 3 4 5 6 7 M. Regitz and 0.J. Scherer, Multiple Bonds and Low Coordi- nation In Phosphorus Chemistry, Thieme Verlag, Stuttgart, 1990. (a) E. Niecke, D. Gudat, W. W. Schoeller and P. Rademacher, J. Chem. SOC., Chem. Commun., 1985, 1050; (b) D. Gudat, E. Niecke, W. Sachs and P.Rademacher, Z. Anorg. Allg. Chem., 1987, 545, 7. K. Fukui, Theory of Orientation and Stereoselection, Springer-Verlag, Berlin, 1975. W. W. Schoeller and E. Niecke, J. Chem. SOC., Chem. Commun., 1982,569. S. 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