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J. Am. Chem. Soc. 1993,115, 8788-8792
Structure and Bonding in Diphosphanylcarbenes. An ab Initio Investigation Oliver Treutler,t Reinhart Ahlrichs,'tt and Michele Soleilhavoupt Contributionfrom the Lehrstuhl f i r Theoretische Chemie, Institut f i r Physikalische Chemie und Elektrochemie, Universitat Karlsruhe, D- 7500 Karlsruhe, Germany, and Laboratoire de Chimie de Coordination du CNRS, 31077 Toulouse Ckdex, France Received February 22, 1993
Abstract: Self consistent field calculations, supplemented by M0ller-Plesset second-order estimates of correlation effects, are presented for the compounds R~PCPRZ, R = NHz and R = N(iPr)2, as well as for species resulting from protonation or addition of H2 and HF. The relative stability of possible isomers is computed and discussed. Diphosphanylcarbenes always prefer an asymmetric PC multiple-bond structure such as >P-Ce=PeIP-c-P l X
X
H
P-c-P R
H P-c-P < R R I R X
Ea: X
= H & : X = F
4 is of special interest, since phosphonio and silyl groups are isoelectronic and isovalent. In the present work we report and discuss the results of a theoretical study undertaken to answer the just mentioned problems. In the first part we deal with compounds 1 4 (Chart I); the second part is devoted to dihydrogen and HF derivatives 5-8 (Chart 11) of the parent compound 1. We treat especially two structures of 1 in symmetry C, (la) and C1 (lb) as well as two conformations of 3, open (3a) and closed (3b). The computations have been performed for R = NH2 throughout. Since 4 has been isolated for R = N(iPr)Z (iPr = isopropyl CsH7) 0 1993 American Chemical Society
Structure and Bonding in Diphosphanylcarbenes Table I. Computed Structural Constants, in pm and degrees, at the SCF Level (If Not Stated Differently), Shared Electron Numbers (SEN), and Atomic Net Charges of Compounds 1-4 la lb
2
3a
3b
4
4
4
structure SEN charge structure
PC 171.9, PN 170.2/172.3, PCP 131.4 PC 1.55, PN 1.20 e . 5 , p+0.6 P(1)C 176.5, P(2)C 153.3, P(1)N 171.4/174.1, P(2)N 167.5/167.8, P(l)CP(2) 160.5 SEN P(1)C 1.40, P(2)C 2.30, PN 1.1-1.2 charge P . 8 , P(1)+0.6, P(2)+'JJ structure P(1)C 185.6, P(2)C 171.5, P(1)N 169.7/171.5, P(2)N(2) 176.4, CN(1) 133.7, P(l)CP(2) 119.6, P(l)CN(1)115.3, P(Z)CN(l) 124.8 SEN P(1)C 1.25, P(2)C 1.77, P(1)N 1.21, P(2)N(2) 1.18, CN(1) 1.64 charge ~-0.1, P(I)+o.~, P(~)+o.o, ~ ( i p . 2~,( 2 1 4 . 5 structure P(1)C 185.8, P(2)C 160.6, P(1)N 167.7/173.3, P(2)N 161.2/161.9, P(l)CP(2) 127.0, P(l)CH(l) 117.2,P(Z)CH(l) 115.4 SEN P(1)C 1.23, P(2)C 1.96, P(1)N 1.17/1.28, P(2)N 1.41/1.43 charge ( 3 5 , P( 1)+0.6,P(2)+'J structure P(1)C 188.0, P(2)C 180.7, P(1)N 168.6/169.5, P(2)N(1) 193.3, P(2)N(2) 164.3, CN(1)148.0, P(l)CP(2) 118.0, P(2)N(I)C62.3, N(l)CP(2) 71.2, CP(2)N(l) 46.5 SEN P(1)C 1.19, P(2)C 1.35, CN(1) 1.19, P(I)N 1.24/ 1.25, P(2)N(1) 0.75, P(2)N(2) 1.46 P(1)+o.6,P(2)+0.7,N(1)4,2, N(2)4.4, N(i)4.5 charge structure P(1)C 167.4, P(2)C 153.1, P(1)N 164.9/164.9, P(2)N 162.6/162.9, P(l)CP(2) 162.0 SEN P(1)C 1.49, P(2)C 2.19, PN 1.2-1.3 charge e . 0 , p(i)+l.2, ~(2)+1,3 with R = NH2 on the MP2 level structure P(1)C 169.2, P(2)C 157.9, P(1)N 165.9/166.0, P(2)N 163.9/164.8, P(l)CP(2) 135.7 SEN P(1)C 1.46, P(2)C 2.10, P N 1.2-1.3 charge P9, P(l)+l.', P(2)+*,' with R = N(iPr)2 on the SCF level structure P(1)C 169.8, P(2)C 155.7, P(I)N 166.2/166.6, P(2)N 165.0/165.8, P(l)CP(2) 151.2 SEN P(1)C 1.46, P(2)C 2.18, PN 1.2-1.3 charge P(l)+l.l, P(2)+l.l
with triflate CF3SO3- as counterion, we also considered R = N(iPr)z in this case. Only singlet carbenes are taken into consideration, since spectroscopic data of 4 (with R = N(iPr)z) point at a singlet ground state. Furthermore, calculations of related compoundsb6 and tentative investigations by ourselvespredict singlet carbenes to be more stable than triplet carbenes.
XI. Methods SCF and MP2 calculations were performed with the program system TuRBoMoLE70nIBM 6000/32H workstations. Basis sets of split valence plus polarization type (SVP) were employed* for P, C, F, and N. For H we used a basis of DZ (double 0 type (or SV (split valence), which is identical in this case), Le. polarization functions were considered to be unnecessary. A molecule-optimized basis was employed for iPr groups, where only the a! carbon atom has an SV basis (for all basis set data see ref 8) and all other atoms are described by minimal basis sets of three Gaussians for the 1s atomic orbital (AO) of H and four Gaussians for the Is, two Gaussians for the 2s, and three Gaussians for the 2p AOs of C. This procedure has proven very economic and reliable.8 All molecules were treated as closed shell systems. Equilibrium geometries were determined by a geometry relaxation procedure based on analytical gradient^.^ These geometries were obtained on the SCF level for all compounds and additionally for 4 by a Meller-Plesset second-order (MP2) ( 5 ) Nguyen, M. T.;McGinn, M.A.; Hegarty, A. F.Inorg. Chem. 1986, 25. 2185. (6) Hoffmann, M. R.; Kuhler, K. J . Chem. Phys. 1991, 94, 8029. (7) Ahlrichs, R.;Blr, M.; Hlser, M.; Horn, H.; Kblmel, C. Chem. Phys. Lett. 1989, 162, 165. (8) Schlfer, A.; Horn, H.; Ahlrichs, R. J . Chem. Phys. 1992, 97, 2571.
(9) Geometry optimization terminated after the total gradient norm was smaller than l t 3for Cartesian coordinates (in atomic units) and for internal coordinates (atomic units and radians).
J . Am. Chem. Soc., Vol.115, No. 19, 1993 8789 estimate of correlation effects.1° Second-order analytical gradients were computed to confirm that a minimum was found. This was not feasible for the iPr analogue of 4 (Le. -NH2 -N(iPr)2). Atomic charges and shared electron numbers (SEN) were obtained by population analyses based on occupation numbers." The discussion of electronic structures (such as A l , A2, and A3) will be based mainly on computed structure parameters (Le., PC bond lengths of 186,165, and 152 pm will be associated with single, double, or triple bonds) and the resultsofpopulationanalyses: SENof 1.31,2.16,and 3.10corresponding to covalent single, double, and triple PC bonds.I2 Because of bond polarities the SEN have to be considered with caution. Bond distances are certainly a more reliable indication of bond strength.
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III. Results and Discussion (a) Diphosphacarbene and Its Protonated Derivatives ( 1 4 ) . The most instructive results of the present investigations concerning the compounds 1 4 are given in Table I (structural properties) and Table I1 (energies), always for R = NHz, if not stated differently. The optimized geometries are depicted in Figure 1. All energy differences quoted below refer to SCF data (MP2 in parentheses, as computed at the SCF structure). The C,structure l a shows two very strong PC single bonds (171.9 pm, SEN 1.5) and a PCP bond angle of 131.4 degrees, which is in line with the idea of a singlet carbene (B3). However, this structure represents a saddle point of the energy hypersurface. Removing the symmetry restrictions leads to the CIminimum lb, which is more stable than l a by 24 kJ/mol(62 kJ/mol) and possesses an extremely short and strong P(2)C bond (153.3 pm, SEN 2.3) as well as a significantly longer and weaker P(1)C bond (176.5 pm, SEN 1.4) in combination with a large PCP bond angle of 1609'. In agreement with atomic charges P(1)+o.6, C( l)-O.*,and P(2)+I,O,the short bond can be described as a strong covalent double bond with additional Coulomb attractions (resulting in an effective triple bond) as represented by structure B2 with admixtures of B1. No indication is detected that B3
plays a role in lb. The best description of bonding is probably given by B4, since an inspection of the MOs reveals a slight delocalization of the carbon lone pair into low-lying orbitals of the two phosphorus atoms. The symmetry lowering C,- C1 can be considered as a typical second-order Jahn-Teller effect:I3a CI (configurationinteraction) includingthe SCF function and all singleexcitations (in C,) yields an excitation energy 1 'A' 2 'A' of only 1.6 eV (with the a' HOMO-LUMO replacement as leading term with a CI coefficient of 0.89). We now turn to 2, which, for R = N(iPr)z, is obtained by depr~tonationl~ of 4 or by photolysis15of >P-C(Nz)-P
