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Ab initio molecular orbital study of the substituent effect on phosphine–borane complexes
6 November 1998
Chemical Physics Letters 296 Ž1998. 277–282
Ab initio molecular orbital study of the substituent effect on phosphine–borane complexes Hafid Anane a , Abdellah Jarid a , Abderrahim Boutalib Francisco Tomas ´ b a b
a,)
, Ignacio Nebot-Gil b,
Departement de Chimie, UniÕersite´ Cadi Ayyad, Faculte´ des Sciences Semlalia. B.P. S 15 Marrakech, Morocco ´ Departament de Quımica Fısica, UniÕersitat de Valencia, Dr. Moliner, 50, E-46100, Burjassot, Valencia, Spain ´ ´ ` ` Received 8 June 1998; in final form 11 September 1998
Abstract Ab initio molecular orbital calculations have been used to study the substituent effect on H 3 BPH n Me 3yn and Me 3yn H n BPH 3 Ž n s 0–3. phosphine–borane complexes. The ab initio results show that successive methyl substitution on the phosphine favours complex formation, contrary to successive methyl substitution on the borane. The natural bond orbitals partitioning scheme suggests that, in general, there is no correlation between the charge transfer and the complexation energies. It also shows the shortening of the P–H and P–C bond lengths, upon complexation, is due to the increasing ‘s’ character of these bonds. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Understanding the properties of phosphine–borane complexes has been a challenge to chemists for over two decades. Numerous studies have been devoted to these donor–acceptor complexes concerning their structural parameters, the nature of the bonding, their stability and other physical properties w1–8x, where the methods used for analyses differ. Durig and Shen have published a theoretical study at the MP2 level of theory of the H 3 BPH 3 , H 3 BPHF2 , and H 3 BPF3 complexes using various basis sets w7x. They have shown that substituting the hydrogen atoms on the phosphine group by an attractor group such as fluorine has little effect over the stability of the corre-
)
Corresponding author. Fax: q212-4-437408.
sponding complexes. We have obtained similar results for H 3 BPH nCl 3yn Ž n s 0 –3. complexes w9x. This effect is contrary to the introduction of the methyl group in the donor atom, which favours complex formation w8x. For ammonia–borane complexes, Skancke and Skancke have reported that the effect of substituting the hydrogen atoms on boron or nitrogen by fluorine reduces the stability of the F3y n H n BNH 3 and H 3 BNH n F3yn Ž n s 0–3. complexes w10x. More recently, we have shown that methyl substitution of hydrogen atoms on the ammonia group increases the stability of the H 3 BNH n Me 3yn complexes, whereas the introduction of the methyl group in the acceptor atom decreases the stability of the Me 3y n H n BNH 3 Ž n s 0–3. complexes w11x. In this study, we have used the G2ŽMP2. method w12x to investigate the H 3 BPH n Me 3yn and Me 3yn-
0009-2614r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 1 0 3 0 - 6
278
H. Anane et al.r Chemical Physics Letters 296 (1998) 277–282
H n BPH 3 Ž n s 0–3. complexes. To the best of our knowledge no comparative study of these complexes has been carried out. The aim of the present work is to elucidate systematically the changes introduced in the properties of the phosphine–borane complexes by successive methyl substitution on the phosphine and borane separately, to discuss their theoretically predicted structures, and their stability.
2. Computational details All calculations in this work were performed on IBM RSr6000 workstations of the University of Valencia using the Gaussian 94 w13x series of com` puter programs. G2ŽMP2. is a theoretical procedure for the computation of total energies of molecules at their equilibrium geometries. G2ŽMP2. procedure uses the 6-311GŽd, p. basis set and corrections for several basis set extensions at the MP2 level. Treatment of electron correlation is made through Møller–Plesset perturbation theory and quadratic configuration interaction ŽQCISD.. The final total energies obtained using the G2ŽMP2. procedure are effectively at the QCISD ŽT .r6-311 q G Ž3df, 2p .rrMP2 ŽFull.r631GŽd. level, making certain assumptions about the additivity of corrections. The zero-point vibrational energies, ZPE, are obtained from scaled HFr631GŽd. frequencies Žmultiplied by the factor 0.893 w14x.. Finally, a small empirical correction, referred to as the higher-level correction, HLC, is applied to account for the error in the calculated energy of the H 2 molecule and H atom, and it is based on the number of a and b valence electrons. It should be noted that in calculating proton affinities and complexation energies, the empirical correction cancels out and therefore the resulting proton affinities and complexation energies are purely ab initio. The proton affinities of PH n Me 3yn Lewis bases, the electron affinities of BH n Me 3yn and the complexation energies of their complexes have been computed at the G2ŽMP2. level of theory. The investigation of the electronic structure, using the natural bond orbitals partitioning scheme NBO w15x, was carried out at the MP2ŽFull.r6-31GŽd. level.
3. Results and discussion Table 1 lists the most important geometrical parameters of PH n Me 3yn and BH n Me 3yn Ž n s 0–3. moieties and their complexes. It can be seen that very small changes in the P–H and P–C bond distances result from methyl substitution in PH 3 . Similar changes in the B–H and B–C bond distances are obtained for BH n Me 3yn moieties when the number of methyl groups increases. The theoretically calculated donor–acceptor bond length for H 3 BPH 3 and H 3 BPMe 3 at the MP2ŽFull.r6-31GŽd. level is in good agreement with the experimental gas-phase values ŽTable 1.. The calculations show that the B–P bond length for H 3 BPH n Me 3yn Ž n s 0–2. complexes becomes shorter than for H 3 BPH 3 , decreasing from n s 2 to 0. This result is reasonable because the PH n Me 3yn Ž n s 0–2. bases are stronger than PH 3 . Additionally, the successive methylation at the phosphorus atom decreases the B–P distances. The H 3 BPH n Me 3yn complexes exhibit a tetrahedral arrangement around the boron center. The /PBH bond angle is ; 1058. This value is reasonable, because the hybridization changes from sp 2 in BH 3 to sp 3 in complexes. The B–H bond is slightly longer in complexes than in isolated BH 3 . Upon coordination, the MP2 calculation shows a small
Table 1 ˚ .a MP2ŽFull.r6-31GŽd. calculated dŽX–Y. bond lengths Žin A dŽB–P. PH 3 PH 2 Me PHMe 2 PMe 3 BH 3 BH 2 Me BHMe 2 BMe 3 H 3 BPH 3 H 3 BPH 2 Me H 3 BPHMe 2 H 3 BPMe 3 MeH 2 BPH 3 Me 2 HBPH 3 Me 3 BPH 3 a b
dŽP–H. dŽP–C. dŽB–H. dŽB–C. 1.415 1.416 1.419
1.857 1.852 1.849 1.191 1.196 1.202
1.945 Ž1.937. b 1.929 1.919 1.913 Ž1.901. c 1.966 1.991 2.019
1.404 1.406 1.409 1.405 1.406 1.407
1.824 1.822 1.821
1.206 1.207 1.212 1.212 1.212 1.216
1.561 1.567 1.575
1.616 1.617 1.617
˚ .. In parentheses we give the experimental bond lengths Žin A Ref. w3x. c Ref. w16x.
H. Anane et al.r Chemical Physics Letters 296 (1998) 277–282
distortion of the P–C bond length Ž1.78%, 1.62%, and 1.51% for H 3 BPH n Me 3yn Ž n s 2–0., respectively. which decreases with both the increase of the degree of substitution and the decrease of the B–P bond lengths. For the Me 3y n H n BPH 3 Ž n s 0–3. complexes, the calculations show that the B–P bond length becomes slightly longer in going from n s 3 to 0. This prediction is reasonable, because the steric hindrance increases with successive methylation on boron. Furthermore, the distortion of the B–C bond length is more important than for the P–C bond length of H 3 BPH n Me 3yn complexes Ž3.52%, 3.2%, and 2.66% for MeH 2 BPH 3 , Me 2 HBPH 3 , and Me 3 BPH 3 , respectively., which decrease with both the increase in the degree of substitution and the lengthening of the B–P bond lengths of Me 3y nH n BPH 3 Ž n s 3–0. complexes. In general, these distortions do not change in the same order as do the complexation energies and the degree of substitution Žsee below.. On complex formation, the calculated geometrical parameters show a shortening of the P–H and P–C bonds in all complexes investigated here ŽTable 2., which has also been confirmed experimentally and theoretically w5,7,16,17x. To explain this result, we have applied the NBO analysis. MP2ŽFull.r631GŽd.-NBO calculations show that in isolated PH n Me 3yn Ž n s 0–3. moieties the lone pair on P has lower ‘s’ character than in complexes. Therefore, we can deduce from these results that this change
Table 2 MP2ŽFull.r6-31GŽd. calculated P–H and P–C bond lengths Žin ˚ . of H 3 BPH n Me 3yn and Me 3yn H n BPH 3 complexes and their A PH n Me 3yn isolated moieties Ž ns 0–3.. 3s MP2ŽFull.r6-31GŽd.NBO contribution of P atoms in the P–H and P–C bonds Žin %. P–H PH 3 PH 2 Me PHMe 2 PMe 3 H 3 BPH 3 H 3 BPH 2 Me H 3 BPHMe 2 H 3 BPMe 3 MeH 2 BPH 3 Me 2 HBPH 3 Me 3 BPH 3
1.415 1.416 1.419 1.404 1.406 1.409 1.405 1.406 1.407
P–C 1.857 1.852 1.849 1.824 1.822 1.821
3s ŽP – H. 16.22 15.86 15.32 20.44 20.18 19.89 20.60 20.37 20.05
3s ŽP – C. 17.1 16.71 16.22 22.49 22.14 21.74
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Table 3 G2ŽMP2. calculated complexation energies Ec of H 3 BPH n Me 3yn complexes Žin kcalrmol., and proton affinities PA Žin kcalrmol. of PH n Me 3yn moieties Ž ns 0–3. Complexes
Ec a
PAb
H 3 BPH 3 H 3 BPH 2 Me H 3 BPHMe 2 H 3 BPMe 3
y21.10 y27.82 y33.26 y37.15
186.84 203.56 216.70 226.83
a
Ec s EŽH 3 BPH n Me 3yn .yw EŽH 3 B.q EŽPH n Me 3yn .x. ŽThe Ec values include ZPE corrections.. b PAŽPH n Me 3yn . syw EŽwPH nq1 Me 3yn xq .y EŽPH n Me 3yn .x.
alone would imply a shortening of the P–H and P–C bond lengths due to an increase in the ‘s’ character in these bonds. Moreover, Table 2 shows that the 3s atomic orbital ŽAO. contribution of P in the P–H and P–C bond lengths is more important in H 3 BPH n Me 3yn and Me 3yn H n BPH 3 complexes than in isolated PH n Me 3yn Ž n s 0–3. moieties. In Table 3 we give the calculated complexation energies of the H 3 BPH n Me 3yn Ž n s 0–3. complexes and the proton affinities of PH n Me 3yn Ž n s 0–3. moieties. The complexation energies are calculated as the energy differences between the complexes and the respective donor–acceptor moieties. The theoretical proton affinities ŽPA. are taken as the energy difference between the neutral and protonated PH n Me 3yn bases. A higher complexation energy is predicted for the BH 3 complex with the strongest base PMe 3 Žy37.15 kcalrmol.. One can see that successive methyl substitution on phosphine favours the formation of complexes. Moreover, the stability of H 3 BPH n Me 3yn complexes increases as the basicity of PH n Me 3yn Ž n s 0–3. increases Žsee Fig. 1.. The good correlation between the proton affinities of the donor compounds and Ec computed at the G2ŽMP2. level of theory shows that the stability of the complexes depends completely on the type of donor involved. These trends have also been reported elsewhere w18,19x. It is interesting to note that the distortion of the P–C and P–H bonds calculated at the MP2ŽFull.r6-31GŽd., which decreases with the degree of substitution, becomes more important when the stability of the H 3 BPH n Me 3yn complexes reduces. Table 4 shows the G2ŽMP2. calculated complexation energies of Me 3y n H n BPH 3 complexes and elec-
H. Anane et al.r Chemical Physics Letters 296 (1998) 277–282
280
Fig. 1. Linear correlation between G2ŽMP2. calculated proton affinities of the PH n Me 3yn bases and G2ŽMP2. calculated complexation energies of H 3 BPH n Me 3yn complexes Ž ns 0–3..
Fig. 2. Linear correlation between G2ŽMP2. calculated electron affinities of the BH n Me 3yn acids and G2ŽMP2. calculated complexation energies of Me 3y n H n BPH 3 Ž ns 0–3. complexes.
tron affinities ŽEA. of BH n Me 3yn Ž n s 0–3. moieties. We compute the electron affinities as the energy difference between the neutral molecule and its anion. The H 3 BPH 3 complex is calculated to be more strongly bound Žy21.10 kcalrmol. than the Me 3y n H n BPH 3 Ž n s 2–0. complexes Žy13.92, y10.7, and y6.11 kcalrmol for MeH 2 BPH 3 , Me 2 HBPH 3 , and Me 3 BPH 3 , respectively.. It has been found that the introduction of the first methyl group on boron atom destabilizes the complex by ; 7 kcalrmol, the second by ; 10 kcalrmol, and the third by ; 15 kcalrmol. Hence, the calculations predict that successive methyl substitution on boron reduces the complexation energies. Furthermore, the stability of the Me 3y n H n BPH 3 Ž n s 0–3. complexes decreases when the electron affinities of the BH n Me 3yn Ž n s 0–3. acids decrease Žsee Fig. 2.. The correlation between the electron affinities of
acceptor compounds and the complexation energies computed at the G2ŽMP2. level of theory shows that the stability of complexes depends completely on the type of acceptor involved. The theoretically predicted complexation energies of the Me 3y n H n BPH 3 complexes is in agreement with the distortion of the B–H and B–C bonds Žreferred to the isolated acceptor molecule. calculated at the MP2ŽFull.r6-31GŽd. level which decrease with the degree of substitution. By successive methyl substitutions on phosphorus, the highest occupied molecular orbital–lowest unoccupied molecular orbital HOMO–LUMO gap decreases Ž0.448, 0.417, 0.4, and 0.382 au for PH 3 , PH 2 Me, PHMe 2 , and PMe 3 , respectively, obtained at the 6-311 q GŽ3df, 2p. level of theory.. Moreover, the energy of HOMO is lowered from y0.329 to y0.385 au on going from PMe 3 to PH 3 , respectively. This indicates that PMe 3 is a soft base and PH 3 is a hard base. This is in agreement with the calculated hardness h of PH n Me 3yn bases 1 Ž6.1, 5.67, 5.45, and 5.2 eV for PH 3 , PH 2 Me, PHMe 2 , and PMe 3 , respectively.. Referring to the Pearson’s qualitative theory of hardness and softness in chemistry w20,21x and his concept of ‘like prefers like’ our calculated complexation energies are systematically
Table 4 G2ŽMP2. calculated complexation energies Ec Žin kcalrmol. of Me 3y n H n BPH 3 complexes, and electron affinities EA Žin eV. of BH n Me 3yn moieties Ž ns 0–3. H 3 BPH 3 MeH 2 BPH 3 Me 2 HBPH 3 Me 3 BPH 3 a
Ec a
EAb
y21.10 y13.92 y10.70 y6.11
y0.13 y0.52 y0.77 y0.85
Ec s EŽMe 3yn H n BPH 3 .yw EŽH 3 B.q EŽBH n Me 3yn .x. ŽThe Ec value include ZPE corrections.. b EAŽBH n Me 3yn . s EŽwBH n Me 3yn xy .y EŽBH n Me 3yn ..
1
The hardness h was calculated as follow: h syŽ I y A.rŽ2. with I sy ELU MO and Asy EHOMO . The HOMO and LUMO energies were obtained at the MP2r6-311qGŽ3df, 2p.rrMP2r631GŽd. level of theory.
H. Anane et al.r Chemical Physics Letters 296 (1998) 277–282 Table 5 G2ŽMP2. complexation energies Žin kcalrmol. of H 3 BPH n Me 3yn and Me 3yn H n BPH 3 Ž n s 0–3. donor–acceptor complexes. MP2r6-31GŽd.-NBO net charge q ŽP. and q ŽB., and charge transfer from the donor to the acceptor Q T Ec a PH 3 PH 2 Me PHMe 2 PMe 3 BH 3 BH 2 Me BHMe 2 BMe 3 H 3 BPH 3 H 3 BPH 2 Me H 3 BPHMe 2 H 3 BPMe 3 H 3 BPH 3 MeH 2 BPH 3 Me 2 HBPH 3 Me 3 BPH 3 a
q ŽP.
q ŽB.
QT
0.32 0.50 0.70 0.93 y0.65 y0.68 y0.71 y0.72 y0.65 y0.35 y0.07 0.22
y0.63 y0.67 y0.69 y0.72 y0.63 y0.62 y0.61 y0.61
0.05 0.32 0.60 0.87
y21.10 y27.82 y33.26 y37.15 y21.10 y13.92 y10.70 y6.11
0.59 0.88 1.16 1.47 0.59 0.56 0.56 0.54
Ec value include ZPE correction.
reduced from H 3 BPMe 3 to H 3 BPH 3 , as BH 3 is classified as a soft acid. Arguing along the same line for the methyl-substituted BH 3 species, we find that the HOMO–LUMO gap decreases by successive methyl substitutions Ž0.536, 0.511, 0.493, and 0.482 au for BH 3 , BH 2 Me, BHMe 2 , and BMe 3 , respectively, obtained at the 6-311 q GŽ3df, 2p. level of theory.. This is consistent with a corresponding decreases in the hardness h of the BH n Me 3yn acids Ž7.29, 6.96, 6.71, and 6.51 eV for BH 3 , BH 2 Me, BHMe 2 , and BMe 3 , respectively.. According to the above reasoning, we would expect a decrease in complexation energies from H 3 BPH 3 to Me 3 BPH 3 , in agreement with our findings. The NBO results ŽTable 5. show that the charge transfer from PH n Me 3yn donor compounds to BH 3 acceptor increases with the degree of substitution, whereas the charge transfer from PH 3 to BH n Me 3yn is practically identical on going from n s 3 to 0 Žy0.63, y0.62, y0.61, y0.61, respectively.. The charge transfer from PH n Me 3yn to BH 3 varies in the same order as the complexation energy ŽTable 5.. Therefore, the charge transfer is correlated to the complexation energies of H 3 BPH n Me 3yn complexes. For H 3 PBH n Me 3yn complexes, the less stable Me 3 BPH 3 complex Žy6.11 kcalrmol. shows
281
practically the same charge transfer as the more stable H 3 BPH 3 complex Žy21.10 kcalrmol.. In fact, the charge transfer is almost identical in all complexes, whereas the complexation energy varies Žy21.1, y13.92, y10.70, and y6.11 kcalrmol for H 3 BPH 3 , MeH 2 BPH 3 , Me 2 HBPH 3 , and Me 3 BPH 3 complexes, respectively.. From the NBO analysis, it follows that there is no correlation between charge transfer and the complexation energies of Me 3y n H n BPH 3 complexes. 4. Conclusions The substitution effect on phosphine–borane complexes was investigated at the G2ŽMP2. level of theory. The G2ŽMP2. results show that the stability of the H 3 BPH n Me 3yn complexes increases with the degree of the methyl substitution at the phosphorus atom, whereas the introduction of a methyl group on the boron atom reduces the stability of the Me 3y n H n BPH 3 Ž n s 0–3. complexes. Upon complexation, the MP2 theoretical structural parameters of phosphine–borane complexes show a shortening of the P–H and P–C bonds. The analysis of the electronic structure using the NBO partitioning scheme shows that this shortening was related to the increasing of the ‘s’ character in these bonds. It also indicates that there is no correlation between the charge transfer from the donor to the acceptor and the calculated complexation energies of Me 3y nH n BPH 3 Ž n s 0–3. complexes. Acknowledgements HA greatly appreciates the financial support provided by the ‘‘Agencia Espanola de Cooperacion ˜ ´ Iternacional ’’ ŽAECI. and also the financial support from the Spanish DGICYT, Project PB94-0993, are acknowledged.
References w1x R.L. Kuczkowski, D.R. Lide Jr., J. Chem. Phys. 46 Ž1967. 357. w2x J.P. Pasinski, R.L. Kuckowski, J. Chem. Phys. 54 Ž1971. 1903.
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w3x J.R. Durig, Y.S. Li, L.A. Carreira, J.D. Odom, J. Am. Chem. Soc. 95 Ž1973. 2491. w4x H. Umeyama, T. Kudo, S. Nakagawa, Chem. Pharm. Bull. 29 Ž1981. 287. w5x F. Hirota, K. Miyata, S. Shibata, J. Mol. Struct. ŽTheochem. 201 Ž1989. 99. w6x J.D. Carpenter, B.S. Ault, J. Mol. Struct. ŽTheochem. 298 Ž1993. 17. w7x J.R. Durig, Z. Shen, J. Mol. Struct. ŽTheochem. 397 Ž1997. 179. w8x H. Anane, A. Boutalib, I. Nebot-Gil, F. Tomas, ´ Chem. Phys. Lett. 287 Ž1998. 575. w9x H. Anane, A. Jarid, A. Boutalib, I. Nebot-Gil, F. Tomas ´ Žto be published.. w10x A. Skancke, P.N. Skancke, J. Phys. Chem. 100 Ž1996. 15079. w11x H. Anane, A. Jarid, A. Boutalib, I. Nebot-Gil, F. Tomas, ´ J. Mol. Struct. ŽTheochem. Ž1998, in press.. w12x L.A. Curtiss, K. Raghavachari, J.A. Pople, J. Chem. Phys. 98 Ž1993. 1293. w13x M.J. Frisch, G.W. Truks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T. Keith, G.A. Petersson, J.A. Montgomery, K. Raghvachari, M.A. Al-Laham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski,
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B.B. Stefanov, A. Nayakkara, M. Challacombe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Replogle, R. Gomperts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. Defrees, J. Baker, J.P. Stewart, M. Head-Gordon, C. Gonzalez, J.A. Pople, Gaussian 94, Revision B.1, Gaussian, Inc., Pittsburgh, PA, 1995. J.A. Pople, H.B. Schlegel, R. Krishnan, D.J. Defrees, J.S. Binkly, M.J. Frisch, R.A. Whiteside, R.F. Hout, W.J. Hehre, Int. J. Quantum. Chem., Quantum Chem. Symp. 15 Ž1981. 269. A.E. Reed, L.A. Curtiss, F. Weinhold, Chem. Rev. 88 Ž1988. 899. K. Iijima, N. Hakamada, S. Shibata, Bull. Chem. Soc. Jpn. 61 Ž1988. 3033. H. Anane, A. Boutalib, I. Nebot-Gil, F. Tomas, ´ J. Phys. Chem. A 102 Ž1998. 7070. H. Anane, A. Boutalib, F. Tomas, ´ J. Phys. Chem. A 101 Ž1997. 7879. H. Anane, A. Boutalib, I. Nebot-Gil, F. Tomas, ´ J. Mol. Struct. ŽTheochem. Ž1998, in press.. R.G. Pearson, Inorg. Chem. 27 Ž1988. 734, and references cited therein. R.G. Pearson, Hard and Soft Acids and Bases, Dowden, Hutchinson and Ross, Stroudsville, PA, 1973.
Chemical Physics Letters 296 Ž1998. 277–282
Ab initio molecular orbital study of the substituent effect on phosphine–borane complexes Hafid Anane a , Abdellah Jarid a , Abderrahim Boutalib Francisco Tomas ´ b a b
a,)
, Ignacio Nebot-Gil b,
Departement de Chimie, UniÕersite´ Cadi Ayyad, Faculte´ des Sciences Semlalia. B.P. S 15 Marrakech, Morocco ´ Departament de Quımica Fısica, UniÕersitat de Valencia, Dr. Moliner, 50, E-46100, Burjassot, Valencia, Spain ´ ´ ` ` Received 8 June 1998; in final form 11 September 1998
Abstract Ab initio molecular orbital calculations have been used to study the substituent effect on H 3 BPH n Me 3yn and Me 3yn H n BPH 3 Ž n s 0–3. phosphine–borane complexes. The ab initio results show that successive methyl substitution on the phosphine favours complex formation, contrary to successive methyl substitution on the borane. The natural bond orbitals partitioning scheme suggests that, in general, there is no correlation between the charge transfer and the complexation energies. It also shows the shortening of the P–H and P–C bond lengths, upon complexation, is due to the increasing ‘s’ character of these bonds. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Understanding the properties of phosphine–borane complexes has been a challenge to chemists for over two decades. Numerous studies have been devoted to these donor–acceptor complexes concerning their structural parameters, the nature of the bonding, their stability and other physical properties w1–8x, where the methods used for analyses differ. Durig and Shen have published a theoretical study at the MP2 level of theory of the H 3 BPH 3 , H 3 BPHF2 , and H 3 BPF3 complexes using various basis sets w7x. They have shown that substituting the hydrogen atoms on the phosphine group by an attractor group such as fluorine has little effect over the stability of the corre-
)
Corresponding author. Fax: q212-4-437408.
sponding complexes. We have obtained similar results for H 3 BPH nCl 3yn Ž n s 0 –3. complexes w9x. This effect is contrary to the introduction of the methyl group in the donor atom, which favours complex formation w8x. For ammonia–borane complexes, Skancke and Skancke have reported that the effect of substituting the hydrogen atoms on boron or nitrogen by fluorine reduces the stability of the F3y n H n BNH 3 and H 3 BNH n F3yn Ž n s 0–3. complexes w10x. More recently, we have shown that methyl substitution of hydrogen atoms on the ammonia group increases the stability of the H 3 BNH n Me 3yn complexes, whereas the introduction of the methyl group in the acceptor atom decreases the stability of the Me 3y n H n BNH 3 Ž n s 0–3. complexes w11x. In this study, we have used the G2ŽMP2. method w12x to investigate the H 3 BPH n Me 3yn and Me 3yn-
0009-2614r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 1 0 3 0 - 6
278
H. Anane et al.r Chemical Physics Letters 296 (1998) 277–282
H n BPH 3 Ž n s 0–3. complexes. To the best of our knowledge no comparative study of these complexes has been carried out. The aim of the present work is to elucidate systematically the changes introduced in the properties of the phosphine–borane complexes by successive methyl substitution on the phosphine and borane separately, to discuss their theoretically predicted structures, and their stability.
2. Computational details All calculations in this work were performed on IBM RSr6000 workstations of the University of Valencia using the Gaussian 94 w13x series of com` puter programs. G2ŽMP2. is a theoretical procedure for the computation of total energies of molecules at their equilibrium geometries. G2ŽMP2. procedure uses the 6-311GŽd, p. basis set and corrections for several basis set extensions at the MP2 level. Treatment of electron correlation is made through Møller–Plesset perturbation theory and quadratic configuration interaction ŽQCISD.. The final total energies obtained using the G2ŽMP2. procedure are effectively at the QCISD ŽT .r6-311 q G Ž3df, 2p .rrMP2 ŽFull.r631GŽd. level, making certain assumptions about the additivity of corrections. The zero-point vibrational energies, ZPE, are obtained from scaled HFr631GŽd. frequencies Žmultiplied by the factor 0.893 w14x.. Finally, a small empirical correction, referred to as the higher-level correction, HLC, is applied to account for the error in the calculated energy of the H 2 molecule and H atom, and it is based on the number of a and b valence electrons. It should be noted that in calculating proton affinities and complexation energies, the empirical correction cancels out and therefore the resulting proton affinities and complexation energies are purely ab initio. The proton affinities of PH n Me 3yn Lewis bases, the electron affinities of BH n Me 3yn and the complexation energies of their complexes have been computed at the G2ŽMP2. level of theory. The investigation of the electronic structure, using the natural bond orbitals partitioning scheme NBO w15x, was carried out at the MP2ŽFull.r6-31GŽd. level.
3. Results and discussion Table 1 lists the most important geometrical parameters of PH n Me 3yn and BH n Me 3yn Ž n s 0–3. moieties and their complexes. It can be seen that very small changes in the P–H and P–C bond distances result from methyl substitution in PH 3 . Similar changes in the B–H and B–C bond distances are obtained for BH n Me 3yn moieties when the number of methyl groups increases. The theoretically calculated donor–acceptor bond length for H 3 BPH 3 and H 3 BPMe 3 at the MP2ŽFull.r6-31GŽd. level is in good agreement with the experimental gas-phase values ŽTable 1.. The calculations show that the B–P bond length for H 3 BPH n Me 3yn Ž n s 0–2. complexes becomes shorter than for H 3 BPH 3 , decreasing from n s 2 to 0. This result is reasonable because the PH n Me 3yn Ž n s 0–2. bases are stronger than PH 3 . Additionally, the successive methylation at the phosphorus atom decreases the B–P distances. The H 3 BPH n Me 3yn complexes exhibit a tetrahedral arrangement around the boron center. The /PBH bond angle is ; 1058. This value is reasonable, because the hybridization changes from sp 2 in BH 3 to sp 3 in complexes. The B–H bond is slightly longer in complexes than in isolated BH 3 . Upon coordination, the MP2 calculation shows a small
Table 1 ˚ .a MP2ŽFull.r6-31GŽd. calculated dŽX–Y. bond lengths Žin A dŽB–P. PH 3 PH 2 Me PHMe 2 PMe 3 BH 3 BH 2 Me BHMe 2 BMe 3 H 3 BPH 3 H 3 BPH 2 Me H 3 BPHMe 2 H 3 BPMe 3 MeH 2 BPH 3 Me 2 HBPH 3 Me 3 BPH 3 a b
dŽP–H. dŽP–C. dŽB–H. dŽB–C. 1.415 1.416 1.419
1.857 1.852 1.849 1.191 1.196 1.202
1.945 Ž1.937. b 1.929 1.919 1.913 Ž1.901. c 1.966 1.991 2.019
1.404 1.406 1.409 1.405 1.406 1.407
1.824 1.822 1.821
1.206 1.207 1.212 1.212 1.212 1.216
1.561 1.567 1.575
1.616 1.617 1.617
˚ .. In parentheses we give the experimental bond lengths Žin A Ref. w3x. c Ref. w16x.
H. Anane et al.r Chemical Physics Letters 296 (1998) 277–282
distortion of the P–C bond length Ž1.78%, 1.62%, and 1.51% for H 3 BPH n Me 3yn Ž n s 2–0., respectively. which decreases with both the increase of the degree of substitution and the decrease of the B–P bond lengths. For the Me 3y n H n BPH 3 Ž n s 0–3. complexes, the calculations show that the B–P bond length becomes slightly longer in going from n s 3 to 0. This prediction is reasonable, because the steric hindrance increases with successive methylation on boron. Furthermore, the distortion of the B–C bond length is more important than for the P–C bond length of H 3 BPH n Me 3yn complexes Ž3.52%, 3.2%, and 2.66% for MeH 2 BPH 3 , Me 2 HBPH 3 , and Me 3 BPH 3 , respectively., which decrease with both the increase in the degree of substitution and the lengthening of the B–P bond lengths of Me 3y nH n BPH 3 Ž n s 3–0. complexes. In general, these distortions do not change in the same order as do the complexation energies and the degree of substitution Žsee below.. On complex formation, the calculated geometrical parameters show a shortening of the P–H and P–C bonds in all complexes investigated here ŽTable 2., which has also been confirmed experimentally and theoretically w5,7,16,17x. To explain this result, we have applied the NBO analysis. MP2ŽFull.r631GŽd.-NBO calculations show that in isolated PH n Me 3yn Ž n s 0–3. moieties the lone pair on P has lower ‘s’ character than in complexes. Therefore, we can deduce from these results that this change
Table 2 MP2ŽFull.r6-31GŽd. calculated P–H and P–C bond lengths Žin ˚ . of H 3 BPH n Me 3yn and Me 3yn H n BPH 3 complexes and their A PH n Me 3yn isolated moieties Ž ns 0–3.. 3s MP2ŽFull.r6-31GŽd.NBO contribution of P atoms in the P–H and P–C bonds Žin %. P–H PH 3 PH 2 Me PHMe 2 PMe 3 H 3 BPH 3 H 3 BPH 2 Me H 3 BPHMe 2 H 3 BPMe 3 MeH 2 BPH 3 Me 2 HBPH 3 Me 3 BPH 3
1.415 1.416 1.419 1.404 1.406 1.409 1.405 1.406 1.407
P–C 1.857 1.852 1.849 1.824 1.822 1.821
3s ŽP – H. 16.22 15.86 15.32 20.44 20.18 19.89 20.60 20.37 20.05
3s ŽP – C. 17.1 16.71 16.22 22.49 22.14 21.74
279
Table 3 G2ŽMP2. calculated complexation energies Ec of H 3 BPH n Me 3yn complexes Žin kcalrmol., and proton affinities PA Žin kcalrmol. of PH n Me 3yn moieties Ž ns 0–3. Complexes
Ec a
PAb
H 3 BPH 3 H 3 BPH 2 Me H 3 BPHMe 2 H 3 BPMe 3
y21.10 y27.82 y33.26 y37.15
186.84 203.56 216.70 226.83
a
Ec s EŽH 3 BPH n Me 3yn .yw EŽH 3 B.q EŽPH n Me 3yn .x. ŽThe Ec values include ZPE corrections.. b PAŽPH n Me 3yn . syw EŽwPH nq1 Me 3yn xq .y EŽPH n Me 3yn .x.
alone would imply a shortening of the P–H and P–C bond lengths due to an increase in the ‘s’ character in these bonds. Moreover, Table 2 shows that the 3s atomic orbital ŽAO. contribution of P in the P–H and P–C bond lengths is more important in H 3 BPH n Me 3yn and Me 3yn H n BPH 3 complexes than in isolated PH n Me 3yn Ž n s 0–3. moieties. In Table 3 we give the calculated complexation energies of the H 3 BPH n Me 3yn Ž n s 0–3. complexes and the proton affinities of PH n Me 3yn Ž n s 0–3. moieties. The complexation energies are calculated as the energy differences between the complexes and the respective donor–acceptor moieties. The theoretical proton affinities ŽPA. are taken as the energy difference between the neutral and protonated PH n Me 3yn bases. A higher complexation energy is predicted for the BH 3 complex with the strongest base PMe 3 Žy37.15 kcalrmol.. One can see that successive methyl substitution on phosphine favours the formation of complexes. Moreover, the stability of H 3 BPH n Me 3yn complexes increases as the basicity of PH n Me 3yn Ž n s 0–3. increases Žsee Fig. 1.. The good correlation between the proton affinities of the donor compounds and Ec computed at the G2ŽMP2. level of theory shows that the stability of the complexes depends completely on the type of donor involved. These trends have also been reported elsewhere w18,19x. It is interesting to note that the distortion of the P–C and P–H bonds calculated at the MP2ŽFull.r6-31GŽd., which decreases with the degree of substitution, becomes more important when the stability of the H 3 BPH n Me 3yn complexes reduces. Table 4 shows the G2ŽMP2. calculated complexation energies of Me 3y n H n BPH 3 complexes and elec-
H. Anane et al.r Chemical Physics Letters 296 (1998) 277–282
280
Fig. 1. Linear correlation between G2ŽMP2. calculated proton affinities of the PH n Me 3yn bases and G2ŽMP2. calculated complexation energies of H 3 BPH n Me 3yn complexes Ž ns 0–3..
Fig. 2. Linear correlation between G2ŽMP2. calculated electron affinities of the BH n Me 3yn acids and G2ŽMP2. calculated complexation energies of Me 3y n H n BPH 3 Ž ns 0–3. complexes.
tron affinities ŽEA. of BH n Me 3yn Ž n s 0–3. moieties. We compute the electron affinities as the energy difference between the neutral molecule and its anion. The H 3 BPH 3 complex is calculated to be more strongly bound Žy21.10 kcalrmol. than the Me 3y n H n BPH 3 Ž n s 2–0. complexes Žy13.92, y10.7, and y6.11 kcalrmol for MeH 2 BPH 3 , Me 2 HBPH 3 , and Me 3 BPH 3 , respectively.. It has been found that the introduction of the first methyl group on boron atom destabilizes the complex by ; 7 kcalrmol, the second by ; 10 kcalrmol, and the third by ; 15 kcalrmol. Hence, the calculations predict that successive methyl substitution on boron reduces the complexation energies. Furthermore, the stability of the Me 3y n H n BPH 3 Ž n s 0–3. complexes decreases when the electron affinities of the BH n Me 3yn Ž n s 0–3. acids decrease Žsee Fig. 2.. The correlation between the electron affinities of
acceptor compounds and the complexation energies computed at the G2ŽMP2. level of theory shows that the stability of complexes depends completely on the type of acceptor involved. The theoretically predicted complexation energies of the Me 3y n H n BPH 3 complexes is in agreement with the distortion of the B–H and B–C bonds Žreferred to the isolated acceptor molecule. calculated at the MP2ŽFull.r6-31GŽd. level which decrease with the degree of substitution. By successive methyl substitutions on phosphorus, the highest occupied molecular orbital–lowest unoccupied molecular orbital HOMO–LUMO gap decreases Ž0.448, 0.417, 0.4, and 0.382 au for PH 3 , PH 2 Me, PHMe 2 , and PMe 3 , respectively, obtained at the 6-311 q GŽ3df, 2p. level of theory.. Moreover, the energy of HOMO is lowered from y0.329 to y0.385 au on going from PMe 3 to PH 3 , respectively. This indicates that PMe 3 is a soft base and PH 3 is a hard base. This is in agreement with the calculated hardness h of PH n Me 3yn bases 1 Ž6.1, 5.67, 5.45, and 5.2 eV for PH 3 , PH 2 Me, PHMe 2 , and PMe 3 , respectively.. Referring to the Pearson’s qualitative theory of hardness and softness in chemistry w20,21x and his concept of ‘like prefers like’ our calculated complexation energies are systematically
Table 4 G2ŽMP2. calculated complexation energies Ec Žin kcalrmol. of Me 3y n H n BPH 3 complexes, and electron affinities EA Žin eV. of BH n Me 3yn moieties Ž ns 0–3. H 3 BPH 3 MeH 2 BPH 3 Me 2 HBPH 3 Me 3 BPH 3 a
Ec a
EAb
y21.10 y13.92 y10.70 y6.11
y0.13 y0.52 y0.77 y0.85
Ec s EŽMe 3yn H n BPH 3 .yw EŽH 3 B.q EŽBH n Me 3yn .x. ŽThe Ec value include ZPE corrections.. b EAŽBH n Me 3yn . s EŽwBH n Me 3yn xy .y EŽBH n Me 3yn ..
1
The hardness h was calculated as follow: h syŽ I y A.rŽ2. with I sy ELU MO and Asy EHOMO . The HOMO and LUMO energies were obtained at the MP2r6-311qGŽ3df, 2p.rrMP2r631GŽd. level of theory.
H. Anane et al.r Chemical Physics Letters 296 (1998) 277–282 Table 5 G2ŽMP2. complexation energies Žin kcalrmol. of H 3 BPH n Me 3yn and Me 3yn H n BPH 3 Ž n s 0–3. donor–acceptor complexes. MP2r6-31GŽd.-NBO net charge q ŽP. and q ŽB., and charge transfer from the donor to the acceptor Q T Ec a PH 3 PH 2 Me PHMe 2 PMe 3 BH 3 BH 2 Me BHMe 2 BMe 3 H 3 BPH 3 H 3 BPH 2 Me H 3 BPHMe 2 H 3 BPMe 3 H 3 BPH 3 MeH 2 BPH 3 Me 2 HBPH 3 Me 3 BPH 3 a
q ŽP.
q ŽB.
QT
0.32 0.50 0.70 0.93 y0.65 y0.68 y0.71 y0.72 y0.65 y0.35 y0.07 0.22
y0.63 y0.67 y0.69 y0.72 y0.63 y0.62 y0.61 y0.61
0.05 0.32 0.60 0.87
y21.10 y27.82 y33.26 y37.15 y21.10 y13.92 y10.70 y6.11
0.59 0.88 1.16 1.47 0.59 0.56 0.56 0.54
Ec value include ZPE correction.
reduced from H 3 BPMe 3 to H 3 BPH 3 , as BH 3 is classified as a soft acid. Arguing along the same line for the methyl-substituted BH 3 species, we find that the HOMO–LUMO gap decreases by successive methyl substitutions Ž0.536, 0.511, 0.493, and 0.482 au for BH 3 , BH 2 Me, BHMe 2 , and BMe 3 , respectively, obtained at the 6-311 q GŽ3df, 2p. level of theory.. This is consistent with a corresponding decreases in the hardness h of the BH n Me 3yn acids Ž7.29, 6.96, 6.71, and 6.51 eV for BH 3 , BH 2 Me, BHMe 2 , and BMe 3 , respectively.. According to the above reasoning, we would expect a decrease in complexation energies from H 3 BPH 3 to Me 3 BPH 3 , in agreement with our findings. The NBO results ŽTable 5. show that the charge transfer from PH n Me 3yn donor compounds to BH 3 acceptor increases with the degree of substitution, whereas the charge transfer from PH 3 to BH n Me 3yn is practically identical on going from n s 3 to 0 Žy0.63, y0.62, y0.61, y0.61, respectively.. The charge transfer from PH n Me 3yn to BH 3 varies in the same order as the complexation energy ŽTable 5.. Therefore, the charge transfer is correlated to the complexation energies of H 3 BPH n Me 3yn complexes. For H 3 PBH n Me 3yn complexes, the less stable Me 3 BPH 3 complex Žy6.11 kcalrmol. shows
281
practically the same charge transfer as the more stable H 3 BPH 3 complex Žy21.10 kcalrmol.. In fact, the charge transfer is almost identical in all complexes, whereas the complexation energy varies Žy21.1, y13.92, y10.70, and y6.11 kcalrmol for H 3 BPH 3 , MeH 2 BPH 3 , Me 2 HBPH 3 , and Me 3 BPH 3 complexes, respectively.. From the NBO analysis, it follows that there is no correlation between charge transfer and the complexation energies of Me 3y n H n BPH 3 complexes. 4. Conclusions The substitution effect on phosphine–borane complexes was investigated at the G2ŽMP2. level of theory. The G2ŽMP2. results show that the stability of the H 3 BPH n Me 3yn complexes increases with the degree of the methyl substitution at the phosphorus atom, whereas the introduction of a methyl group on the boron atom reduces the stability of the Me 3y n H n BPH 3 Ž n s 0–3. complexes. Upon complexation, the MP2 theoretical structural parameters of phosphine–borane complexes show a shortening of the P–H and P–C bonds. The analysis of the electronic structure using the NBO partitioning scheme shows that this shortening was related to the increasing of the ‘s’ character in these bonds. It also indicates that there is no correlation between the charge transfer from the donor to the acceptor and the calculated complexation energies of Me 3y nH n BPH 3 Ž n s 0–3. complexes. Acknowledgements HA greatly appreciates the financial support provided by the ‘‘Agencia Espanola de Cooperacion ˜ ´ Iternacional ’’ ŽAECI. and also the financial support from the Spanish DGICYT, Project PB94-0993, are acknowledged.
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