- Email: [email protected]
Ir complexes?
3 July 1998
Chemical Physics Letters 290 Ž1998. 535–542
Can Si5O bonds be stabilized by RhrIr complexes? A density functional theory study Olivier Uzan, Jan M.L. Martin
)
Department of Organic Chemistry, Kimmelman Building, Room 262, Weizmann Institute of Science, 76100 RehoÕot, Israel Received 19 March 1998; in final form 24 April 1998
Abstract In an attempt to design a transition-metal complex capable of stabilizing an Si5O bond, the interaction of various RhrIr complexes with silanones was investigated using the B3LYP density functional method and relativistic effective core potentials. Ir complexes are systematically more strongly bound than their Rh counterparts. Contrary to PdrPt complexes, phosphine chelate ligands do not improve bonding, while a strong trans effect exists. IrŽtrans-ŽPH 3 . 2 .ClŽŽCF3 . 2 SiO. is calculated to exhibit strong three-center M–Si–O binding, while a CO ligand trans to the oxygen leads to an almost linear two-center M–Si5O binding, which is calculated to be strongest in IrŽtrans-ŽPH 3 . 2 .ŽCO.ŽiPr2 SiO.. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Generation of stabilized silicon–oxygen multiple bond compounds is a current topic of study in both experiment and theory w1–4x. The chemistry of silicon compounds is fundamentally different from its isovalent carbon counterpart, since the p-bonding interaction involving silicon does not lead to as efficient an overlap as for carbon. Problems were encountered in preparing derivatives with silicon– silicon or silicon–oxygen double bonds w5,6x. No silanones Ži.e. ketone analogs. have been isolated so far; they were only produced as short-lived intermediates, in gas-phase reactions of ozone with silane w7,8x or in an argon matrix w9x, requiring elaborate procedures for preparation and decomposition of
)
Corresponding author.
suitable precursors w5x. The unstable silanones show a strong tendency to polymerize into cyclic siloxanes w2,10x. Recently, Milstein et al. w1x reported strong evidence of silanone generation through a homogeneous catalytic pathway of transition metal complexes, but isolation of the monomeric form was not achieved. Finding a suitable organometallic system may enable the trapping of the silanones by forming a kind of metal-protected silicon–oxygen double bond, and assist their further inclusion into other molecules. In a previous publication w11x — which we will denote ‘Paper I’ henceforth — we reported a density functional theory study on the modeling of the stabilization of the Si5O bond by palladium and platinum complexes; the interaction was shown to be mostly three-center, consisting of electron donation from the silanone oxygen to the metal and back-donation of the metal towards the silicon.
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 5 2 7 - 2
536
O. Uzan, J.M.L. Martinr Chemical Physics Letters 290 (1998) 535–542
As a result of the d 8 ground state configuration of Rh and Irq and their tendency to form trigonal bipyramid pentacoordinate complexes Žas opposed to the planar d10 complexes of PdŽ0. and Pt Ž0. . the behavior of Rhq and Irq complexes in this regard is expected to be significantly different from that of their Pd and Pt counterparts. We will demonstrate in the present study that this is indeed so. In particular, we shall show that in the complex q
the nature of the trans ligand LX dramatically affects the structure of the complex and the nature of the metal–silanone binding. In addition, we will see that the strongly stabilizing effect of phosphochelate ligands on the Pd and Pt complexes is wholly absent here.
2. Methods All ab initio calculations have been carried out using a combination of the molecular mechanics, semi-empirical and graphical front-end modules in SPARTAN w12x and the density functional modules in GAUSSIAN 94 w13x, running on DEC Alpha 500 and SGI Origin 2000 computers at the Weizmann Institute of Science. Starting geometries were generated using the SYBYL w14x molecular mechanics force field as implemented in SPARTAN, followed by the PM3Žtm. semi-empirical method for transition metals w15x. Like in our previous work ŽPaper I. full geometry optimizations were carried out using the B3LYP w16,17x density functional method ŽDFT. w18x, in conjunction with the Hay and Wadt w19x relativistic effective core potential ŽECP. basis set combination customarily referred to as LANL2DZ. This basis set is of double-zeta quality in the valence and ‘ valence-1’ shells, while the ECP contains Darwin and mass-velocity contributions. Since this basis set was found insufficient for reliable thermochemistry, single-point energy calculations were subse-
quently carried out using a larger basis set denoted LANL2DZ q P here, which consists of the LANL2DZ basis set augmented with a single f function w20,21x on Rh and Ir and the standard Dunning’s cc-pVDZ Žcorrelation consistent polarized valence double zeta. basis set w22x on first- and second-row atoms. For technical reasons, geometry optimizations using analytical gradients w23x could not be carried out in this basis set, while energy only optimizations would have been prohibitive in cost. Equivalent levels of theory were previously used to good effect by the Morokuma group w24,25x on several reaction mechanisms involving Pd and Pt complexes, as well as in Paper I.
3. Results and discussion All relevant results concerning the complexes have been collected in Table 1. 3.1. Comparison of the basis sets Comparison of the LANL2DZ and LANL2DZq P interaction energies De reveals, for almost all complexes, a lowering of De ranging between 1 and 6 kcalrmol. Exceptions to this rule are observed for F2 SiO and ŽCF3 . 2 SiO silanone complexes, for which the interaction energies are severely overestimated with the smaller basis set. For example, the comp u te d L A N L 2 D Z D e o f IrŽtra n s ŽPH 3 . 2 .HŽŽCF3 . 2 SiO. is 74.1 kcalrmol, while it drops to 63.2 kcalrmol upon addition of polarization functions. These findings roughly parallel those in Paper I, in which that effect was ascribed to the inadequacy of the LANL2DZ basis set for fluorinecontaining compounds. 3.2. Comparison of rhodium and iridium For all the organometallic systems for which calculations were performed with both metals, iridium systematically provided stronger coordination of the silanones than rhodium. The energy gain fluctuates around 11 kcalrmol for the LANL2DZ basis set as well as for its polarized counterpart. As for the case of platinum compared to palladium ŽPaper I., the
O. Uzan, J.M.L. Martinr Chemical Physics Letters 290 (1998) 535–542
higher stability of iridium complexes is related to the fact that their 5d orbitals are more diffuse than the 4d orbitals of rhodium and that they are, therefore, more adaptable to three-center bonding of silanone. As a matter of fact, the silicon–oxygen bonds are ˚ for three-center systematically shortened by 0.02 A rhodium complexes with respect to their iridium counterparts, while the metal–oxygen and metal– silicon bond lengths practically do not change for neutral complexes. The variation in the silicon– oxygen bond length thus appears to be correlated with the average 11 kcalrmol gain in silanone trapping ability for iridium systems relative to their rhodium counterparts. It should be emphasized that neutral tetravalent complexes of Pd and Pt with the same kind of ligands Žcf. Paper I. exhibit smaller energy variations and greater changes in metal– oxygen and metal–silicon bond lengths. 3.3. Analysis of the bond lengths Except for a few complexes Žsee Section 3.4., the calculated silicon–oxygen bond lengths are similar to almost single bonds and pure single bonds for rhodium and iridium complexes, respectively. ŽThe corresponding Si–O bond lengths for H 2 SiO and H 3 SiOH at the same level of theory are 1.587 and ˚ respectively.. It was found for palladium 1.703 A, and platinum metal complexes Žsee Paper I. that silanone coordination towards the metal center consists of a three-center interaction between the metal and R 2 Siq–Oy; the computed data obtained in this work reveal that this bonding mode also applies for rhodium and iridium. 3.3.1. Metal–silicon bond The metal–silicon bond length increases when the metal is electron deficient. This effect can be partially compensated by improving the electron-donating character of the ligands. For instance the Si–Ir ˚ in IrŽtransbond length goes up from 2.391 A ˚ in IrŽtransŽPH 3 . 2 .ClŽH 2 SiO . to 2.526 A ˚ ŽPH 3 . 2 .ŽCO.ŽH 2 SiO.q, but is found to be 2.454 A q Ž . Ž . for Ir PH 3 3 H 2 SiO . The charge on the silicon atom is obviously also influencing the metal–silicon bond length; considering that the metal gives electron density toward the silicon, the more positively charged the latter is Ži.e. the more electron-withdraw-
537
ing are the silanone substituents., the shorter will be the bond and the stronger will be the coordination. For example, for the IrŽPH 3 .q 3 complexes of iPr 2 SiO, Me 2 SiO, H 2 SiO, ŽCF3 . 2 SiO, F2 SiO, the computed Si–Ir bond lengths are 2.532, 2.498, 2.454, 2.372, ˚ and the LANL2DZq P dissociation ener2.355 A, gies are 51.8, 54.2, 54.0, 55.2, 56.4 kcalrmol, respectively. Although the same tendency was observed for Pd and Pt complexes, the amplitude of the effects described here was found to be only of the ˚ for Pd–Si and Pt–Si bonds Žsee order of 0.03 A Paper I, Table 2.. This statement also applies to the metal–oxygen bond length variations. 3.3.2. Metal–oxygen bond The metal–oxygen bond is found to be dependent on the electron density of the metal and on the charge of the silanone oxygen. Positively charged metal centers show a better ability to form strong metal–oxygen bonds. The calculated Ir–O bond ˚ for IrŽtranslength is, for instance, 2.335 A ˚ are ŽPH 3 . 2 .HŽH2SiO., while 2.123 and 2.147 A observed for IrŽtrans-ŽPH 3 . 2 .ŽCO.ŽH 2 SiO.q and IrŽPH 3 . 3 ŽH 2 SiO.q, respectively. For neutral systems, shorter bonds are also obtained in the presence of electron withdrawing groups on the metal; an ˚ is found for IrŽtransIr–O bond length of 2.178 A ŽPH 3 . 2 .ClŽH 2 SiO.. In the absence of electron-withdrawing groups on the metal Ži.e. for ‘pure’ neutral metal centers., the ligand arrangement influences the interaction: placing a phosphine group trans to the silanone Žmore precisely trans to its oxygen. reduces the length of both the metal–silicon and the metal–oxygen bonds — the latter shortening being dramatic — and increases the silicon–oxygen bond distance. This may be explained by the fact that phosphines are strongly electron donating groups; since the metal is not electron deficient, the excess of electron density is transferred in the trans position to the silicon–oxygen antibonding p ) orbital, resulting in an elongation of the Si–O and Ir–O bonds. It should be emphasized that having a positive charge on the metal or an electron deficiency due to withdrawing ligands cancels the interaction with the p ) orbital: this may be interpreted as a suggestion that the metal center ‘prefers’ to compensate its electron deficiency prior to ceding electrons to the silanone.
538
LANL2DZ
LANL2DZqP a
r ŽSi–O.
r ŽM–Si.
r ŽM–O.
LANL2DZ
LANL2DZqP a
r ŽSi–O.
r ŽM–Si.
r ŽM–O.
Ligands_M
De ŽIr.
De ŽIr.
ŽIr.
ŽIr.
ŽIr.
De ŽRh.
De ŽRh.
ŽRh.
ŽRh.
ŽRh.
trans-ŽPH 3 . 2 ClqH 2 SiO cis-ŽPH 3 . 2 ClqH 2 SiO trans-ŽPH 3 . 2 ClqiPr2 SiO trans-ŽPH 3 . 2 ClqŽCF3 . 2 SiO trans-ŽPH 3 . 2 ClqŽMeO.SiO trans-ŽPH 3 . 2 COqH 2 SiO trans-ŽPH 3 . 2 COqH 2 SiO b cis-ŽPH 3 . 2 COqH 2 SiO c trans-ŽPH 3 . 2 COqiPr2 SiO b,c cis-ŽPH 3 . 2 COqiPr2 SiO b cis-ŽPH 3 . 2 COqiPr2 SiO trans-ŽPH 3 . 2 COqŽCF3 . 2 SiO trans-ŽPH 3 . 2 COqŽCF3 . 2 SiO b trans-ŽPH 3 . 2 COqŽMeO. 2 SiO b trans-ŽPMe 3 . 2 COqH 2 SiO b trans-ŽPMe 3 . 2 COqH 2 SiO ŽPH 3 .q 3 qH 2 SiO ŽPH 3 .q 3 qF2 SiO ŽPH 3 .q 3 qMe 2 SiO
y68.7 y55.7 y59.8 y84.1 y67.6 y54.8 y54.7 y55.5 y65.4 y65.8 y59.0 y51.5 y37.5 y60.0 y46.3 y54.9 y52.4 y63.8 y56.9
y66.5 y48.9 y58.6 y75.8 y64.4 y54.0 y50.7 y53.1 y62.2 y60.1 y56.7 y53.2 y41.0 y56.8 y41.7 y53.7 y53.9 y56.4 y54.2
1.701 1.694 1.702 1.684 1.685 1.681 1.586 1.684 1.604 1.613 1.685 1.674 1.570 1.580 1.587 1.691 1.695 1.674 1.695
2.391 2.411 2.419 2.336 2.358 2.526 3.639 2.530 3.628 3.519 2.660 2.431 3.646 3.592 3.597 2.458 2.454 2.355 2.498
2.178 2.157 2.177 2.228 2.211 2.123 2.062 2.131 2.057 2.061 2.111 2.172 2.076 2.087 2.068 2.139 2.147 2.193 2.137
y54.2 y44.1 y45.5 y67.4 y52.4 y42.2 y47.0 y42.2 y56.7 y55.7 y55.4 y37.5 y31.1 y52.2 y38.9 y42.6 y45.0 y47.6 y43.4
y52.3 y39.1 y44.4 y60.0 y49.7 y41.0 y43.3 y40.4 y53.4 y51.6 y51.0 y39.4 y34.6 y49.1 y35.0 y41.3 y41.5 y42.3 y42.1
1.683 1.678 1.684 1.666 1.667 1.657 1.585 1.661 1.603 1.611 1.612 1.654 1.570 1.580 1.585 1.670 1.675 1.656 1.672
2.391 2.419 2.430 2.331 2.366 2.593 3.662 2.591 3.640 3.544 3.553 2.454 3.667 3.604 3.640 2.478 2.481 2.367 2.546
2.168 2.154 2.163 2.226 2.199 2.110 2.078 2.122 2.066 2.080 2.078 2.159 2.098 2.098 2.087 2.129 2.128 2.179 2.121
O. Uzan, J.M.L. Martinr Chemical Physics Letters 290 (1998) 535–542
Table 1 ˚ . in various organometallic complexes using the B3LYP density functional method Computed binding energies of silanones Žkcalrmol. and bond distances ŽA
a
y55.0 y60.7 y61.9 y49.1 y52.5 y65.8 y74.1 y22.9 y84.6 y52.9 y17.0 y66.4 y56.7 y19.0 y66.5 y17.3 y52.5 y67.2 y55.6 y53.3
y51.8 y55.2 y56.1 y45.1 y50.8 y59.9 y63.2 y12.6 y70.7 y49.5 y16.3 y59.2 y55.6 y14.2 y61.6 y10.8 y45.8 y54.2 y47.2 y46.1
Calculated at the LANL2DZ geometry. Linearly bound silanone. c Sole energy minimum obtained for this complex. d dhpes1,2-bisŽdihydrophosphine. ethane. e dhpps1,3-bisŽdihydrophosphine. propane. b
1.696 1.682 1.685 1.696 1.677 1.699 1.651 1.602 1.682 1.661 1.596 1.687 1.683 1.609 1.702 1.627 1.691 1.673 1.679 1.693
2.532 2.372 2.430 2.448 2.404 2.378 2.334 3.527 2.300 2.366 3.530 2.346 2.373 3.529 2.361 3.661 2.410 2.332 2.376 2.406
2.132 2.204 2.157 2.180 2.335 2.166 2.535 2.058 2.242 2.394 2.190 2.187 2.366 2.081 2.218 2.068 2.167 2.257 2.189 2.179
y42.0 y45.3 y47.5 y37.6 y42.7 y54.0 y63.6 y19.5 y72.1 y42.6 y13.7 y52.5 y y y y y41.6 y56.3 y43.6 y42.7
y39.8 y41.8 y43.3 y34.3 y40.8 y49.1 y52.2 y9.8 y59.4 y39.0 y14.5 y46.8 y y y y y36.7 y45.1 y37.3 y37.4
1.673 1.663 1.663 1.676 1.662 1.685 1.633 1.596 1.666 1.645 1.587 1.672 y y y y 1.673 1.658 1.662 1.676
2.598 2.382 2.468 2.473 2.400 2.371 2.335 3.568 2.292 2.362 3.539 2.341 y y y y 2.417 2.338 2.380 2.415
2.117 2.194 2.142 2.167 2.340 2.161 2.641 2.070 2.240 2.429 2.222 2.187 y y y y 2.161 2.251 2.194 2.174
O. Uzan, J.M.L. Martinr Chemical Physics Letters 290 (1998) 535–542
ŽPH 3 .q 3 qiPr2 SiO ŽPH 3 .q Ž . 3 q CF3 2 SiO ŽPH 3 .q Ž . 2 SiO q MeO 3 ŽPMe 3 .q 3 qMe 2 SiO trans-ŽPH 3 . 2 HqH 2 SiO cis-ŽPH 3 . 2 HqH 2 SiO trans-ŽPH 3 . 2 HqŽCF3 . 2 SiO trans-ŽPH 3 . 2 HqŽCF3 . 2 SiO b cis-ŽPH 3 . 2 HqŽCF3 . 2 SiO trans-ŽPH 3 . 2 HqŽMeO. 2 SiO trans-ŽPH 3 . 2 HqŽMeO. 2 SiO b cis-ŽPH 3 . 2 HqŽMeO. 2 SiO trans-ŽPMe 3 . 2 HqH 2 SiO trans-ŽPMe 3 . 2 HqH 2 SiO b cis-ŽPMe 3 . 2 HqH 2 SiO trans-ŽPMe 3 . 2 HqiPr2 SiO Ždhpe.ClqH 2 SiO d Ždhpe.ClqŽCF3 . 2 SiO d Ždhpe.ClqŽMeO. 2 SiO d Ždhpp.ClqH 2 SiO e
539
540
O. Uzan, J.M.L. Martinr Chemical Physics Letters 290 (1998) 535–542
The metal–oxygen bond distance is also dependent on the substituents of the silanone. It appears that the metal center cannot accommodate too large an amount of electron density from the silanone oxygen; favoring the R 2 Siq–Oy over the R 2 Si5O resonance structure Žwith strongly electronwithdrawing substituents on Si. increases the electronic density on the oxygen and thus leads to an elongation of the Ir–O bond. The latter can be huge for neutral compounds; Ir–O bond lengths are found ˚ for Ir Ž transto be 2.335 and 2.535 A ŽP H 3 . 2 .H ŽH 2 S iO . and IrŽtra n s ŽPH 3 . 2 .HŽŽCF3 . 2 SiO., respectively. For positively charged metal complexes, the effect is more modest; in Ir Ž PH 3 . 3 Ž M e 2 SiO . q , Ir Ž PH 3 . 3 Ž H 2 SiO . q , IrŽPH 3 . 3 ŽŽMeO. 2 SiO.q and IrŽPH 3 . 3 ŽF2 SiO.q, the calculated Ir–O bond distances are 2.137, 2.147, ˚ respectively. Note that there is 2.157 and 2.193 A, no clear correspondence between these changes and those in the computed binding energies. 3.4. Linear coordination By favoring the stabilization of the metal–oxygen bond an entirely different bonding situation was observed. Upon increasing the electron donating character of the silanone substituents and the electron deficiency of the metal center, binding of the silanone by coordination of the oxygen only occurs. This alternative type of bonding, which we call ‘linear’ Žsince the angle metal–oxygen–silicon is 179 or 1568 depending on the geometry of the non-bonding electron pairs of the oxygen with respect to the d orbitals of the metal., exhibits silicon–oxygen bond lengths close to those of calcu˚ for the IrŽtranslated free silanones Že.g. 1.604 A ŽPH 3 . 2 .ŽCO.ŽiPr2 SiO. complex, compared to 1.595 ˚ at the same level of theory w11x for free iPr2 SiO.. A When substituents like isopropyls were used on the silicon and when a carbonyl group was located trans to the silanone Žfor back-bonding effects. on a positively charged complex, relatively high stabilization energies were obtained. Conversely, forcing convergence of the geometry optimization towards linear structures for neutral complexes with electronwithdrawing substituents on the silanone leads to extremely low interaction energies. Interestingly, this two-center silanone coordina-
tion presents differences with respect to the metal used. As it was suggested previously, three-center silanone coordination requires better spreading of the d-orbitals and is consequently more favored with iridium. In the case of linear silanone binding, rhodium complexes are also less stable than their iridium analog, but they are among the most stable ones with respect to other three center rhodium complexes. Indeed, diffuse d orbitals are not relevant in the case of linear coordination, since the latter involves two-center rather than three-center binding. The existence of ‘linear’ bonding appears to be specific to RhrIr complexes: no reasonably stable PdrPt compounds of this sort were found. 3.5. Effect of chelating phosphines Chelating phosphines ligands were considered in order to investigate the influence of the angle between the ligands on the silanone coordination. This angle was found to play a key role for tetracoordinated palladium and platinum complexes ŽPaper I.. In contrast, calculations performed in this work with dhpe Ž1,2-bisŽdihydrophosphino. ethane. or dhpp Ž1,3-bisŽdihydrophosphine. propane. ligands did not show any influence on the interaction energy between the silanone and the metal complex. In fact chelates do not appreciably change the geometry of the pentavalent rhodium and iridium complexes, compared to what occurs for palladium and platinum species. The data obtained even show that the interaction energy of the silanone with respect to the metal complex slowly decreases upon increasing the chelate chain length, presumably because of steric interactions. As an illustration, the LANL2DZq P interaction energies calculated for Ir Ž cisŽ PH 3 . 2 . Cl Ž H 2 SiO . , Ir Ž dhpe . Cl Ž H 2 SiO . and IrŽdhpp.ClŽH 2 SiO. are y48.9, y45.8 and y46.1 kcalrmol, respectively.
4. Conclusions The silanone trapping ability of late transition metal complexes was studied using the B3LYP hybrid density functional method and relativistic effective core potentials. By considering the variation of such parameters as the central metal atom, the lig-
O. Uzan, J.M.L. Martinr Chemical Physics Letters 290 (1998) 535–542
ands and the silanone substituents, a clear model of silanone coordination to rhodium and iridium complexes was obtained. It was found that, thanks to its more diffuse 5d-orbitals, iridium gives higher interaction energies than rhodium for three-center bonding. As for palladium and platinum metal complexes ŽPaper I., the coordination of the silanone can be described as electron donation from the oxygen of R 2 Siq–Oy to the metal and back-donation of electron density from the metal to the silicon. Thus, the influence on silanone coordination of donor or attractor groups, both on the metal and on the silicon, can be explained: positive charges on the metal center allow better acceptance of silanone oxygen electrons, while donor ligands provides better bonding to the silicon. Favoring one of the situations on the metal can be compensated for by adjusting the ratio between the p and p ) character of the silicon–oxygen bond. As an extreme case, a possible linear or pseudo-linear coordination of the silanone to the metal is observed, showing a high Si5O character. According to our calculations Ir Ž transŽPH 3 . 2 .ClŽŽCF3 . 2 SiO.
541
may also be an interesting target, since it might allow for observation of an actual silicon–oxygen double-bond, as opposed to one protected by a three-center interaction.
Acknowledgements OU is a doctoral fellow of the Feinberg Graduate School, Weizmann Institute of Science, and participates in the Franco-Israeli Scientific Cooperation Program sponsored by the French Embassy in Israel. JM is a Yigal Allon Fellow, the incumbent of the Helen and Milton A. Kimmelman Career Development Chair and an Honorary Research Associate Ž‘Onderzoeksleider in eremandaat’. of the National Science Foundation of Belgium ŽNFWOrFNRS.. The authors thank David Milstein for helpful and enlightening discussions and for critical reading of the manuscript prior to submission. This research was partially supported by the Minerva Foundation, Munich, Germany. The DEC Alpha workstation at ¨ the Weizmann Institute was purchased with USAID ŽUnited States Agency for International Development. funds.
References appears to be the most stable silanone complex and it should therefore be considered as a synthetic target. Although it has a lower interaction energy, the linear c o o rd in a te d sila n o n e c o m p le x Ir Ž tra n sŽPH 3 . 2 .ŽCO.ŽiPr2 SiO.
w1x R. Goikhman, M. Aizenberg, L.J.W. Shimon, D. Milstein, J. Am. Chem. Soc. 118 Ž1996. 10894. w2x T. Kudo, F. Hashimoto, M.S. Gordon, J. Comput. Chem. 17 Ž1996. 1163.
542
O. Uzan, J.M.L. Martinr Chemical Physics Letters 290 (1998) 535–542
w3x Y. Apeloig, in: S. Patai, Z. Rappoport ŽEds.., The Chemistry of Organic Silicon Compounds ŽWiley, New York, 1989. p. 57. w4x K.K. Baldridge, J.A. Boatz, S. Koseki, M.S. Gordon, Ann. Rev. Phys. Chem. 38 Ž1987. 211. w5x G. Raabe, J. Michl, Chem. Rev. 85 Ž1985. 419. w6x C.E. Zybill, C. Liu, Synlett Ž1995. 687. w7x S. Bailleux, M. Bogey, C. Demuynck, J.L. Destombes, A. Walters, J. Chem. Phys. 101 Ž1994. 2729. w8x M. Bogey, B. Delcroix, A. Walters, J.C. Guillemin, J. Mol. Spectrosc. 175 Ž1996. 421. w9x R. Withnall, L. Andrews, J. Phys. Chem. 89 Ž1985. 3261. w10x T. Kudo, K. Nagase, J. Am. Chem. Soc. 107 Ž1985. 1186. w11x O. Uzan, Y. Gozin, J.M.L. Martin, Chem. Phys. Lett. 288 Ž1998. 356. w12x Spartan User’s Guide — Version 4.1, Wavefunction, Irvine, CA, 1995. w13x M.J. Frisch, G.W. Trucks, 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. Raghavachari, M.A. Al-Laham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Re-
w14x w15x w16x w17x w18x w19x w20x
w21x
w22x w23x w24x w25x
plogle, 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, Pittsburgh, PA. 1995. M. Clark, R.D. Cramer III, N. Van Opdenbosch, J. Comput. Chem. 10 Ž1989. 982. W. Huang, W.J. Hehre, manuscript to be published. A.D. Becke, J. Chem. Phys. 98 Ž1993. 5648. C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 Ž1988. 785. R.G. Parr, W. Yang, Density Functional Theory of Atoms and Molecules ŽOxford University Press, Oxford, 1989.. P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 Ž1985. 270. A. Hollwarth, M. Bohme, S. Dapprich, A.W. Ehlers, A. ¨ ¨ Gobbi, V. Jonas, K.F. Kohler, R. Stegmann, A. Veldkamp, ¨ G. Frenking, Chem. Phys. Lett. 208 Ž1993. 237. A.W. Ehlers, M. Bohme, S. Dapprich, A. Gobbi, A. ¨ Hollwarth, V. Jonas, K.F. Kohler, R. Stegmann, A. Veld¨ ¨ kamp, G. Frenking, Chem. Phys. Lett. 208 Ž1993. 111. T.H. Dunning Jr., J. Chem. Phys. 90 Ž1989. 1007. B.G. Johnson, M.J. Frisch, J. Chem. Phys. 100 Ž1994. 7429. Q. Cui, D.G. Musaev, K. Morokuma, Organometallics 16 Ž1997. 1355. M. Svensson, T. Matsubara, K. Morokuma, Organometallics 15 Ž1996. 5568.
Chemical Physics Letters 290 Ž1998. 535–542
Can Si5O bonds be stabilized by RhrIr complexes? A density functional theory study Olivier Uzan, Jan M.L. Martin
)
Department of Organic Chemistry, Kimmelman Building, Room 262, Weizmann Institute of Science, 76100 RehoÕot, Israel Received 19 March 1998; in final form 24 April 1998
Abstract In an attempt to design a transition-metal complex capable of stabilizing an Si5O bond, the interaction of various RhrIr complexes with silanones was investigated using the B3LYP density functional method and relativistic effective core potentials. Ir complexes are systematically more strongly bound than their Rh counterparts. Contrary to PdrPt complexes, phosphine chelate ligands do not improve bonding, while a strong trans effect exists. IrŽtrans-ŽPH 3 . 2 .ClŽŽCF3 . 2 SiO. is calculated to exhibit strong three-center M–Si–O binding, while a CO ligand trans to the oxygen leads to an almost linear two-center M–Si5O binding, which is calculated to be strongest in IrŽtrans-ŽPH 3 . 2 .ŽCO.ŽiPr2 SiO.. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Generation of stabilized silicon–oxygen multiple bond compounds is a current topic of study in both experiment and theory w1–4x. The chemistry of silicon compounds is fundamentally different from its isovalent carbon counterpart, since the p-bonding interaction involving silicon does not lead to as efficient an overlap as for carbon. Problems were encountered in preparing derivatives with silicon– silicon or silicon–oxygen double bonds w5,6x. No silanones Ži.e. ketone analogs. have been isolated so far; they were only produced as short-lived intermediates, in gas-phase reactions of ozone with silane w7,8x or in an argon matrix w9x, requiring elaborate procedures for preparation and decomposition of
)
Corresponding author.
suitable precursors w5x. The unstable silanones show a strong tendency to polymerize into cyclic siloxanes w2,10x. Recently, Milstein et al. w1x reported strong evidence of silanone generation through a homogeneous catalytic pathway of transition metal complexes, but isolation of the monomeric form was not achieved. Finding a suitable organometallic system may enable the trapping of the silanones by forming a kind of metal-protected silicon–oxygen double bond, and assist their further inclusion into other molecules. In a previous publication w11x — which we will denote ‘Paper I’ henceforth — we reported a density functional theory study on the modeling of the stabilization of the Si5O bond by palladium and platinum complexes; the interaction was shown to be mostly three-center, consisting of electron donation from the silanone oxygen to the metal and back-donation of the metal towards the silicon.
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 5 2 7 - 2
536
O. Uzan, J.M.L. Martinr Chemical Physics Letters 290 (1998) 535–542
As a result of the d 8 ground state configuration of Rh and Irq and their tendency to form trigonal bipyramid pentacoordinate complexes Žas opposed to the planar d10 complexes of PdŽ0. and Pt Ž0. . the behavior of Rhq and Irq complexes in this regard is expected to be significantly different from that of their Pd and Pt counterparts. We will demonstrate in the present study that this is indeed so. In particular, we shall show that in the complex q
the nature of the trans ligand LX dramatically affects the structure of the complex and the nature of the metal–silanone binding. In addition, we will see that the strongly stabilizing effect of phosphochelate ligands on the Pd and Pt complexes is wholly absent here.
2. Methods All ab initio calculations have been carried out using a combination of the molecular mechanics, semi-empirical and graphical front-end modules in SPARTAN w12x and the density functional modules in GAUSSIAN 94 w13x, running on DEC Alpha 500 and SGI Origin 2000 computers at the Weizmann Institute of Science. Starting geometries were generated using the SYBYL w14x molecular mechanics force field as implemented in SPARTAN, followed by the PM3Žtm. semi-empirical method for transition metals w15x. Like in our previous work ŽPaper I. full geometry optimizations were carried out using the B3LYP w16,17x density functional method ŽDFT. w18x, in conjunction with the Hay and Wadt w19x relativistic effective core potential ŽECP. basis set combination customarily referred to as LANL2DZ. This basis set is of double-zeta quality in the valence and ‘ valence-1’ shells, while the ECP contains Darwin and mass-velocity contributions. Since this basis set was found insufficient for reliable thermochemistry, single-point energy calculations were subse-
quently carried out using a larger basis set denoted LANL2DZ q P here, which consists of the LANL2DZ basis set augmented with a single f function w20,21x on Rh and Ir and the standard Dunning’s cc-pVDZ Žcorrelation consistent polarized valence double zeta. basis set w22x on first- and second-row atoms. For technical reasons, geometry optimizations using analytical gradients w23x could not be carried out in this basis set, while energy only optimizations would have been prohibitive in cost. Equivalent levels of theory were previously used to good effect by the Morokuma group w24,25x on several reaction mechanisms involving Pd and Pt complexes, as well as in Paper I.
3. Results and discussion All relevant results concerning the complexes have been collected in Table 1. 3.1. Comparison of the basis sets Comparison of the LANL2DZ and LANL2DZq P interaction energies De reveals, for almost all complexes, a lowering of De ranging between 1 and 6 kcalrmol. Exceptions to this rule are observed for F2 SiO and ŽCF3 . 2 SiO silanone complexes, for which the interaction energies are severely overestimated with the smaller basis set. For example, the comp u te d L A N L 2 D Z D e o f IrŽtra n s ŽPH 3 . 2 .HŽŽCF3 . 2 SiO. is 74.1 kcalrmol, while it drops to 63.2 kcalrmol upon addition of polarization functions. These findings roughly parallel those in Paper I, in which that effect was ascribed to the inadequacy of the LANL2DZ basis set for fluorinecontaining compounds. 3.2. Comparison of rhodium and iridium For all the organometallic systems for which calculations were performed with both metals, iridium systematically provided stronger coordination of the silanones than rhodium. The energy gain fluctuates around 11 kcalrmol for the LANL2DZ basis set as well as for its polarized counterpart. As for the case of platinum compared to palladium ŽPaper I., the
O. Uzan, J.M.L. Martinr Chemical Physics Letters 290 (1998) 535–542
higher stability of iridium complexes is related to the fact that their 5d orbitals are more diffuse than the 4d orbitals of rhodium and that they are, therefore, more adaptable to three-center bonding of silanone. As a matter of fact, the silicon–oxygen bonds are ˚ for three-center systematically shortened by 0.02 A rhodium complexes with respect to their iridium counterparts, while the metal–oxygen and metal– silicon bond lengths practically do not change for neutral complexes. The variation in the silicon– oxygen bond length thus appears to be correlated with the average 11 kcalrmol gain in silanone trapping ability for iridium systems relative to their rhodium counterparts. It should be emphasized that neutral tetravalent complexes of Pd and Pt with the same kind of ligands Žcf. Paper I. exhibit smaller energy variations and greater changes in metal– oxygen and metal–silicon bond lengths. 3.3. Analysis of the bond lengths Except for a few complexes Žsee Section 3.4., the calculated silicon–oxygen bond lengths are similar to almost single bonds and pure single bonds for rhodium and iridium complexes, respectively. ŽThe corresponding Si–O bond lengths for H 2 SiO and H 3 SiOH at the same level of theory are 1.587 and ˚ respectively.. It was found for palladium 1.703 A, and platinum metal complexes Žsee Paper I. that silanone coordination towards the metal center consists of a three-center interaction between the metal and R 2 Siq–Oy; the computed data obtained in this work reveal that this bonding mode also applies for rhodium and iridium. 3.3.1. Metal–silicon bond The metal–silicon bond length increases when the metal is electron deficient. This effect can be partially compensated by improving the electron-donating character of the ligands. For instance the Si–Ir ˚ in IrŽtransbond length goes up from 2.391 A ˚ in IrŽtransŽPH 3 . 2 .ClŽH 2 SiO . to 2.526 A ˚ ŽPH 3 . 2 .ŽCO.ŽH 2 SiO.q, but is found to be 2.454 A q Ž . Ž . for Ir PH 3 3 H 2 SiO . The charge on the silicon atom is obviously also influencing the metal–silicon bond length; considering that the metal gives electron density toward the silicon, the more positively charged the latter is Ži.e. the more electron-withdraw-
537
ing are the silanone substituents., the shorter will be the bond and the stronger will be the coordination. For example, for the IrŽPH 3 .q 3 complexes of iPr 2 SiO, Me 2 SiO, H 2 SiO, ŽCF3 . 2 SiO, F2 SiO, the computed Si–Ir bond lengths are 2.532, 2.498, 2.454, 2.372, ˚ and the LANL2DZq P dissociation ener2.355 A, gies are 51.8, 54.2, 54.0, 55.2, 56.4 kcalrmol, respectively. Although the same tendency was observed for Pd and Pt complexes, the amplitude of the effects described here was found to be only of the ˚ for Pd–Si and Pt–Si bonds Žsee order of 0.03 A Paper I, Table 2.. This statement also applies to the metal–oxygen bond length variations. 3.3.2. Metal–oxygen bond The metal–oxygen bond is found to be dependent on the electron density of the metal and on the charge of the silanone oxygen. Positively charged metal centers show a better ability to form strong metal–oxygen bonds. The calculated Ir–O bond ˚ for IrŽtranslength is, for instance, 2.335 A ˚ are ŽPH 3 . 2 .HŽH2SiO., while 2.123 and 2.147 A observed for IrŽtrans-ŽPH 3 . 2 .ŽCO.ŽH 2 SiO.q and IrŽPH 3 . 3 ŽH 2 SiO.q, respectively. For neutral systems, shorter bonds are also obtained in the presence of electron withdrawing groups on the metal; an ˚ is found for IrŽtransIr–O bond length of 2.178 A ŽPH 3 . 2 .ClŽH 2 SiO.. In the absence of electron-withdrawing groups on the metal Ži.e. for ‘pure’ neutral metal centers., the ligand arrangement influences the interaction: placing a phosphine group trans to the silanone Žmore precisely trans to its oxygen. reduces the length of both the metal–silicon and the metal–oxygen bonds — the latter shortening being dramatic — and increases the silicon–oxygen bond distance. This may be explained by the fact that phosphines are strongly electron donating groups; since the metal is not electron deficient, the excess of electron density is transferred in the trans position to the silicon–oxygen antibonding p ) orbital, resulting in an elongation of the Si–O and Ir–O bonds. It should be emphasized that having a positive charge on the metal or an electron deficiency due to withdrawing ligands cancels the interaction with the p ) orbital: this may be interpreted as a suggestion that the metal center ‘prefers’ to compensate its electron deficiency prior to ceding electrons to the silanone.
538
LANL2DZ
LANL2DZqP a
r ŽSi–O.
r ŽM–Si.
r ŽM–O.
LANL2DZ
LANL2DZqP a
r ŽSi–O.
r ŽM–Si.
r ŽM–O.
Ligands_M
De ŽIr.
De ŽIr.
ŽIr.
ŽIr.
ŽIr.
De ŽRh.
De ŽRh.
ŽRh.
ŽRh.
ŽRh.
trans-ŽPH 3 . 2 ClqH 2 SiO cis-ŽPH 3 . 2 ClqH 2 SiO trans-ŽPH 3 . 2 ClqiPr2 SiO trans-ŽPH 3 . 2 ClqŽCF3 . 2 SiO trans-ŽPH 3 . 2 ClqŽMeO.SiO trans-ŽPH 3 . 2 COqH 2 SiO trans-ŽPH 3 . 2 COqH 2 SiO b cis-ŽPH 3 . 2 COqH 2 SiO c trans-ŽPH 3 . 2 COqiPr2 SiO b,c cis-ŽPH 3 . 2 COqiPr2 SiO b cis-ŽPH 3 . 2 COqiPr2 SiO trans-ŽPH 3 . 2 COqŽCF3 . 2 SiO trans-ŽPH 3 . 2 COqŽCF3 . 2 SiO b trans-ŽPH 3 . 2 COqŽMeO. 2 SiO b trans-ŽPMe 3 . 2 COqH 2 SiO b trans-ŽPMe 3 . 2 COqH 2 SiO ŽPH 3 .q 3 qH 2 SiO ŽPH 3 .q 3 qF2 SiO ŽPH 3 .q 3 qMe 2 SiO
y68.7 y55.7 y59.8 y84.1 y67.6 y54.8 y54.7 y55.5 y65.4 y65.8 y59.0 y51.5 y37.5 y60.0 y46.3 y54.9 y52.4 y63.8 y56.9
y66.5 y48.9 y58.6 y75.8 y64.4 y54.0 y50.7 y53.1 y62.2 y60.1 y56.7 y53.2 y41.0 y56.8 y41.7 y53.7 y53.9 y56.4 y54.2
1.701 1.694 1.702 1.684 1.685 1.681 1.586 1.684 1.604 1.613 1.685 1.674 1.570 1.580 1.587 1.691 1.695 1.674 1.695
2.391 2.411 2.419 2.336 2.358 2.526 3.639 2.530 3.628 3.519 2.660 2.431 3.646 3.592 3.597 2.458 2.454 2.355 2.498
2.178 2.157 2.177 2.228 2.211 2.123 2.062 2.131 2.057 2.061 2.111 2.172 2.076 2.087 2.068 2.139 2.147 2.193 2.137
y54.2 y44.1 y45.5 y67.4 y52.4 y42.2 y47.0 y42.2 y56.7 y55.7 y55.4 y37.5 y31.1 y52.2 y38.9 y42.6 y45.0 y47.6 y43.4
y52.3 y39.1 y44.4 y60.0 y49.7 y41.0 y43.3 y40.4 y53.4 y51.6 y51.0 y39.4 y34.6 y49.1 y35.0 y41.3 y41.5 y42.3 y42.1
1.683 1.678 1.684 1.666 1.667 1.657 1.585 1.661 1.603 1.611 1.612 1.654 1.570 1.580 1.585 1.670 1.675 1.656 1.672
2.391 2.419 2.430 2.331 2.366 2.593 3.662 2.591 3.640 3.544 3.553 2.454 3.667 3.604 3.640 2.478 2.481 2.367 2.546
2.168 2.154 2.163 2.226 2.199 2.110 2.078 2.122 2.066 2.080 2.078 2.159 2.098 2.098 2.087 2.129 2.128 2.179 2.121
O. Uzan, J.M.L. Martinr Chemical Physics Letters 290 (1998) 535–542
Table 1 ˚ . in various organometallic complexes using the B3LYP density functional method Computed binding energies of silanones Žkcalrmol. and bond distances ŽA
a
y55.0 y60.7 y61.9 y49.1 y52.5 y65.8 y74.1 y22.9 y84.6 y52.9 y17.0 y66.4 y56.7 y19.0 y66.5 y17.3 y52.5 y67.2 y55.6 y53.3
y51.8 y55.2 y56.1 y45.1 y50.8 y59.9 y63.2 y12.6 y70.7 y49.5 y16.3 y59.2 y55.6 y14.2 y61.6 y10.8 y45.8 y54.2 y47.2 y46.1
Calculated at the LANL2DZ geometry. Linearly bound silanone. c Sole energy minimum obtained for this complex. d dhpes1,2-bisŽdihydrophosphine. ethane. e dhpps1,3-bisŽdihydrophosphine. propane. b
1.696 1.682 1.685 1.696 1.677 1.699 1.651 1.602 1.682 1.661 1.596 1.687 1.683 1.609 1.702 1.627 1.691 1.673 1.679 1.693
2.532 2.372 2.430 2.448 2.404 2.378 2.334 3.527 2.300 2.366 3.530 2.346 2.373 3.529 2.361 3.661 2.410 2.332 2.376 2.406
2.132 2.204 2.157 2.180 2.335 2.166 2.535 2.058 2.242 2.394 2.190 2.187 2.366 2.081 2.218 2.068 2.167 2.257 2.189 2.179
y42.0 y45.3 y47.5 y37.6 y42.7 y54.0 y63.6 y19.5 y72.1 y42.6 y13.7 y52.5 y y y y y41.6 y56.3 y43.6 y42.7
y39.8 y41.8 y43.3 y34.3 y40.8 y49.1 y52.2 y9.8 y59.4 y39.0 y14.5 y46.8 y y y y y36.7 y45.1 y37.3 y37.4
1.673 1.663 1.663 1.676 1.662 1.685 1.633 1.596 1.666 1.645 1.587 1.672 y y y y 1.673 1.658 1.662 1.676
2.598 2.382 2.468 2.473 2.400 2.371 2.335 3.568 2.292 2.362 3.539 2.341 y y y y 2.417 2.338 2.380 2.415
2.117 2.194 2.142 2.167 2.340 2.161 2.641 2.070 2.240 2.429 2.222 2.187 y y y y 2.161 2.251 2.194 2.174
O. Uzan, J.M.L. Martinr Chemical Physics Letters 290 (1998) 535–542
ŽPH 3 .q 3 qiPr2 SiO ŽPH 3 .q Ž . 3 q CF3 2 SiO ŽPH 3 .q Ž . 2 SiO q MeO 3 ŽPMe 3 .q 3 qMe 2 SiO trans-ŽPH 3 . 2 HqH 2 SiO cis-ŽPH 3 . 2 HqH 2 SiO trans-ŽPH 3 . 2 HqŽCF3 . 2 SiO trans-ŽPH 3 . 2 HqŽCF3 . 2 SiO b cis-ŽPH 3 . 2 HqŽCF3 . 2 SiO trans-ŽPH 3 . 2 HqŽMeO. 2 SiO trans-ŽPH 3 . 2 HqŽMeO. 2 SiO b cis-ŽPH 3 . 2 HqŽMeO. 2 SiO trans-ŽPMe 3 . 2 HqH 2 SiO trans-ŽPMe 3 . 2 HqH 2 SiO b cis-ŽPMe 3 . 2 HqH 2 SiO trans-ŽPMe 3 . 2 HqiPr2 SiO Ždhpe.ClqH 2 SiO d Ždhpe.ClqŽCF3 . 2 SiO d Ždhpe.ClqŽMeO. 2 SiO d Ždhpp.ClqH 2 SiO e
539
540
O. Uzan, J.M.L. Martinr Chemical Physics Letters 290 (1998) 535–542
The metal–oxygen bond distance is also dependent on the substituents of the silanone. It appears that the metal center cannot accommodate too large an amount of electron density from the silanone oxygen; favoring the R 2 Siq–Oy over the R 2 Si5O resonance structure Žwith strongly electronwithdrawing substituents on Si. increases the electronic density on the oxygen and thus leads to an elongation of the Ir–O bond. The latter can be huge for neutral compounds; Ir–O bond lengths are found ˚ for Ir Ž transto be 2.335 and 2.535 A ŽP H 3 . 2 .H ŽH 2 S iO . and IrŽtra n s ŽPH 3 . 2 .HŽŽCF3 . 2 SiO., respectively. For positively charged metal complexes, the effect is more modest; in Ir Ž PH 3 . 3 Ž M e 2 SiO . q , Ir Ž PH 3 . 3 Ž H 2 SiO . q , IrŽPH 3 . 3 ŽŽMeO. 2 SiO.q and IrŽPH 3 . 3 ŽF2 SiO.q, the calculated Ir–O bond distances are 2.137, 2.147, ˚ respectively. Note that there is 2.157 and 2.193 A, no clear correspondence between these changes and those in the computed binding energies. 3.4. Linear coordination By favoring the stabilization of the metal–oxygen bond an entirely different bonding situation was observed. Upon increasing the electron donating character of the silanone substituents and the electron deficiency of the metal center, binding of the silanone by coordination of the oxygen only occurs. This alternative type of bonding, which we call ‘linear’ Žsince the angle metal–oxygen–silicon is 179 or 1568 depending on the geometry of the non-bonding electron pairs of the oxygen with respect to the d orbitals of the metal., exhibits silicon–oxygen bond lengths close to those of calcu˚ for the IrŽtranslated free silanones Že.g. 1.604 A ŽPH 3 . 2 .ŽCO.ŽiPr2 SiO. complex, compared to 1.595 ˚ at the same level of theory w11x for free iPr2 SiO.. A When substituents like isopropyls were used on the silicon and when a carbonyl group was located trans to the silanone Žfor back-bonding effects. on a positively charged complex, relatively high stabilization energies were obtained. Conversely, forcing convergence of the geometry optimization towards linear structures for neutral complexes with electronwithdrawing substituents on the silanone leads to extremely low interaction energies. Interestingly, this two-center silanone coordina-
tion presents differences with respect to the metal used. As it was suggested previously, three-center silanone coordination requires better spreading of the d-orbitals and is consequently more favored with iridium. In the case of linear silanone binding, rhodium complexes are also less stable than their iridium analog, but they are among the most stable ones with respect to other three center rhodium complexes. Indeed, diffuse d orbitals are not relevant in the case of linear coordination, since the latter involves two-center rather than three-center binding. The existence of ‘linear’ bonding appears to be specific to RhrIr complexes: no reasonably stable PdrPt compounds of this sort were found. 3.5. Effect of chelating phosphines Chelating phosphines ligands were considered in order to investigate the influence of the angle between the ligands on the silanone coordination. This angle was found to play a key role for tetracoordinated palladium and platinum complexes ŽPaper I.. In contrast, calculations performed in this work with dhpe Ž1,2-bisŽdihydrophosphino. ethane. or dhpp Ž1,3-bisŽdihydrophosphine. propane. ligands did not show any influence on the interaction energy between the silanone and the metal complex. In fact chelates do not appreciably change the geometry of the pentavalent rhodium and iridium complexes, compared to what occurs for palladium and platinum species. The data obtained even show that the interaction energy of the silanone with respect to the metal complex slowly decreases upon increasing the chelate chain length, presumably because of steric interactions. As an illustration, the LANL2DZq P interaction energies calculated for Ir Ž cisŽ PH 3 . 2 . Cl Ž H 2 SiO . , Ir Ž dhpe . Cl Ž H 2 SiO . and IrŽdhpp.ClŽH 2 SiO. are y48.9, y45.8 and y46.1 kcalrmol, respectively.
4. Conclusions The silanone trapping ability of late transition metal complexes was studied using the B3LYP hybrid density functional method and relativistic effective core potentials. By considering the variation of such parameters as the central metal atom, the lig-
O. Uzan, J.M.L. Martinr Chemical Physics Letters 290 (1998) 535–542
ands and the silanone substituents, a clear model of silanone coordination to rhodium and iridium complexes was obtained. It was found that, thanks to its more diffuse 5d-orbitals, iridium gives higher interaction energies than rhodium for three-center bonding. As for palladium and platinum metal complexes ŽPaper I., the coordination of the silanone can be described as electron donation from the oxygen of R 2 Siq–Oy to the metal and back-donation of electron density from the metal to the silicon. Thus, the influence on silanone coordination of donor or attractor groups, both on the metal and on the silicon, can be explained: positive charges on the metal center allow better acceptance of silanone oxygen electrons, while donor ligands provides better bonding to the silicon. Favoring one of the situations on the metal can be compensated for by adjusting the ratio between the p and p ) character of the silicon–oxygen bond. As an extreme case, a possible linear or pseudo-linear coordination of the silanone to the metal is observed, showing a high Si5O character. According to our calculations Ir Ž transŽPH 3 . 2 .ClŽŽCF3 . 2 SiO.
541
may also be an interesting target, since it might allow for observation of an actual silicon–oxygen double-bond, as opposed to one protected by a three-center interaction.
Acknowledgements OU is a doctoral fellow of the Feinberg Graduate School, Weizmann Institute of Science, and participates in the Franco-Israeli Scientific Cooperation Program sponsored by the French Embassy in Israel. JM is a Yigal Allon Fellow, the incumbent of the Helen and Milton A. Kimmelman Career Development Chair and an Honorary Research Associate Ž‘Onderzoeksleider in eremandaat’. of the National Science Foundation of Belgium ŽNFWOrFNRS.. The authors thank David Milstein for helpful and enlightening discussions and for critical reading of the manuscript prior to submission. This research was partially supported by the Minerva Foundation, Munich, Germany. The DEC Alpha workstation at ¨ the Weizmann Institute was purchased with USAID ŽUnited States Agency for International Development. funds.
References appears to be the most stable silanone complex and it should therefore be considered as a synthetic target. Although it has a lower interaction energy, the linear c o o rd in a te d sila n o n e c o m p le x Ir Ž tra n sŽPH 3 . 2 .ŽCO.ŽiPr2 SiO.
w1x R. Goikhman, M. Aizenberg, L.J.W. Shimon, D. Milstein, J. Am. Chem. Soc. 118 Ž1996. 10894. w2x T. Kudo, F. Hashimoto, M.S. Gordon, J. Comput. Chem. 17 Ž1996. 1163.
542
O. Uzan, J.M.L. Martinr Chemical Physics Letters 290 (1998) 535–542
w3x Y. Apeloig, in: S. Patai, Z. Rappoport ŽEds.., The Chemistry of Organic Silicon Compounds ŽWiley, New York, 1989. p. 57. w4x K.K. Baldridge, J.A. Boatz, S. Koseki, M.S. Gordon, Ann. Rev. Phys. Chem. 38 Ž1987. 211. w5x G. Raabe, J. Michl, Chem. Rev. 85 Ž1985. 419. w6x C.E. Zybill, C. Liu, Synlett Ž1995. 687. w7x S. Bailleux, M. Bogey, C. Demuynck, J.L. Destombes, A. Walters, J. Chem. Phys. 101 Ž1994. 2729. w8x M. Bogey, B. Delcroix, A. Walters, J.C. Guillemin, J. Mol. Spectrosc. 175 Ž1996. 421. w9x R. Withnall, L. Andrews, J. Phys. Chem. 89 Ž1985. 3261. w10x T. Kudo, K. Nagase, J. Am. Chem. Soc. 107 Ž1985. 1186. w11x O. Uzan, Y. Gozin, J.M.L. Martin, Chem. Phys. Lett. 288 Ž1998. 356. w12x Spartan User’s Guide — Version 4.1, Wavefunction, Irvine, CA, 1995. w13x M.J. Frisch, G.W. Trucks, 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. Raghavachari, M.A. Al-Laham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Re-
w14x w15x w16x w17x w18x w19x w20x
w21x
w22x w23x w24x w25x
plogle, 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, Pittsburgh, PA. 1995. M. Clark, R.D. Cramer III, N. Van Opdenbosch, J. Comput. Chem. 10 Ž1989. 982. W. Huang, W.J. Hehre, manuscript to be published. A.D. Becke, J. Chem. Phys. 98 Ž1993. 5648. C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 Ž1988. 785. R.G. Parr, W. Yang, Density Functional Theory of Atoms and Molecules ŽOxford University Press, Oxford, 1989.. P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 Ž1985. 270. A. Hollwarth, M. Bohme, S. Dapprich, A.W. Ehlers, A. ¨ ¨ Gobbi, V. Jonas, K.F. Kohler, R. Stegmann, A. Veldkamp, ¨ G. Frenking, Chem. Phys. Lett. 208 Ž1993. 237. A.W. Ehlers, M. Bohme, S. Dapprich, A. Gobbi, A. ¨ Hollwarth, V. Jonas, K.F. Kohler, R. Stegmann, A. Veld¨ ¨ kamp, G. Frenking, Chem. Phys. Lett. 208 Ž1993. 111. T.H. Dunning Jr., J. Chem. Phys. 90 Ž1989. 1007. B.G. Johnson, M.J. Frisch, J. Chem. Phys. 100 Ž1994. 7429. Q. Cui, D.G. Musaev, K. Morokuma, Organometallics 16 Ž1997. 1355. M. Svensson, T. Matsubara, K. Morokuma, Organometallics 15 Ž1996. 5568.