On the Nature of Ni Ni Interaction in Model Dimeric Ni Complex

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1 On the Nature of Ni Ni Interaction in Model Dimeric Ni Complex Radosław Kamiński, a* Beata Herbaczyńska, b Monika Srebro, c Antoni Pietrzykowski, b Artur Michalak, c Lucjan B. Jerzykiewicz, d Krzysztof Woźniak a* a Department of Chemistry, University of Warsaw, Pasteura 1, Warszawa, Poland b Department of Chemistry, Warsaw University of Technology, Noakowskiego 3, Warszawa, Poland c Department of Chemistry, Jagiellonian University, Ingardena 3, Kraków, Poland d Department of Chemistry, Wrocław University, Joliot-Curie 14, Wrocław, Poland * Corresponding authors: Radosław Kamiński (rkaminski@chem.uw.edu.pl), Krzysztof Woźniak (kwozniak@chem.uw.edu.pl) Keywords: nickel complexes, charge density studies, metal-metal bonding, DFT calculations, orbital analysis Abstract: A new dinuclear complex (NiC 5 H 4 SiMe 2 CHCH 2 ) 2 (2) was prepared by reacting nickelocene derivative [(C 5 H 4 SiMe 2 CH=CH 2 ) 2 Ni] (1) with methyllithium (MeLi). Good quality crystals were subjected to a high-resolution X-ray measurement. Subsequent multipole refinement yielded accurate description of electron density distribution. Detailed inspection of experimental electron density in Ni Ni contact revealed that the nickel atoms are bonded and significant deformation of the metal valence shell is related to different populations of the d- orbitals. The existence of the Ni Ni bond path explains the lack of unpaired electrons in the complex due to a possible exchange channel.

2 1. Synthesis of starting materials 1.1. General Information. All reactions and manipulations were carried out under an atmosphere of dry argon. Solvents were dried with potassium and distilled prior to use. Chloro(dimethyl)vinylsilane (Sigma-Aldrich) was distilled under an argon atmosphere prior to use. Solutions of n-butyllithium in heptane and of methyllithium in diethylether (Sigma- Aldrich) were used as purchased. Cyclopentadiene was prepared by distillation of dicyclopentadiene (Fluka) (through retro-diels-alder reaction). NiBr 2 2DME was prepared by the reaction of nickel powder with bromine in the presence of dimethyl ether (DME). 1 H NMR (400 MHz) spectra were recorded on a Mercury-400BB spectrometer in benzene-d 6 at ambient temperature. Mass spectra (EI, 70 ev) were recorded on an AMD-604 mass spectrometer. Reactions are presented in Scheme 1S. H H BuLi Me _ Li + Me Me Me Si 1) BuLi 2) NiBr 2. 2 DME 1 Cl Si Scheme 1S. Synthesis of the compound One-pot synthesis of 1,1 -bis[dimethyl(vinyl)silyl]nickelocene (1). A solution of cm 3 BuLi (2.3 M in heptane, 25.0 mmol) was added within 1 min with a syringe to a solution of 2.05 cm 3 (25.0 mmol) of freshly distilled cyclopentadiene in 60 cm 3 THF at 0 C. After stirring for 1 h at room temperature, the solution of cyclopentadienyllithium was cooled to 0 C and 3.45 cm 3 (25.0 mmol) of chloro(dimethyl)vinylsilane was added within 1 min. The orange-yellow solution of dimethyl(vinyl)silylcyclopentadiene was stirred for 1 h at room temperature. The reaction mixture was then cooled again to 0 C and the second portion of BuLi solution (10.85 cm 3, 25.0 mmol) was added drop by drop. After 1 h at room temperature, the reaction mixture was cooled to 0 C and transferred slowly (within 10 min) to a suspension of 5.00 g (12.5 mmol) NiBr 2 2DME in 40 cm 3 THF. The color of the solution changed immediately to green. After stirring for 4 h at room temperature the solvent was removed under reduced pressure. Residue was dissolved in 100 cm 3 diethyl ether and 60 cm 3 of water was added. After stirring the mixture for 30 min, the organic layer was separated, solvents were evaporated and the residue was dried under reduced pressure. 50 cm 3 of hexane was added to the green oil and the solution was filtered through a bed of Celite. Hexane was

3 distilled off and the residue was dried under reduced pressure g (10.9 mmol) of 1,1 - bis[dimethyl(vinyl)silyl]nickelocene was obtained as a green oil (yield: 87%). M.p. = ca. 65 C. 1 H NMR (benzene-d 6, MHz): from 200 to 200 ppm no signals. EI-MS m/z: 356 (M +, 70%), 300 (C 18 H 26 Si 2 + 2H, 12%), 298 (C 18 H 26 Si 2, 15%), 262 (10%), 244 (10%), 207 (C 9 H 13 SiNi, 12%). EI-HRMS: (C 18 H 26 Si 58 2 Ni) calculated , found Synthesis of bis[dimethyl(vinyl)silylcyclopentadienylnickel] (2). The solution of 1.00 g (2.8 mmol) of 1 in 20 cm 3 of THF and 40 cm 3 of diethyl ether was cooled to 75 C cm 3 of methyllithium (1.2 M in diethyl ether, 3.7 mmol) was then added drop by drop within 3 min. After stirring for 1 h the solution was slowly warmed up to room temperature and stirred for additional 3 h. Next, 50 cm 3 of water was added and the mixture was stirred for 20 min. the organic layer was separated, solvents were evaporated and the residue was dried under reduced pressure. The obtained green solid was re-dissolved in 20 cm 3 of heptane and filtered through a bed of Celite. Dark green crystals of 2 were obtained from this solution at 30 C (0.48 g, 1.1 mmol, yield 82 %). 1 H NMR (benzene-d 6, 400 MHz) δ / ppm: 5.64 (s, 1H, C 5 H 4 ), 5.51 (s, 1H, C 5 H 4 ), 5.22 (s, 1H, C 5 H 4 ), 5.04 (d, J H H = 15.7 Hz, 1H, trans-ch=ch 2 ), 3.52 (s, 1H, C 5 H 4 ), 3.44 (d, J H H = 11.5 Hz, 1H, cis-ch=ch 2 ), 2.46 (dd, J H-H =15.7 and 11.5 Hz, 1H, Si CH=CH 2 ), 0.78 (s, 3H, Si CH 3 ), 0.28 (s, 3H, Si CH 3 ). 13 C NMR (benzene-d 6, MHz) δ / ppm: 93.97, 93.82, 92.55, 91.05, 49.29, 45.59, 2.08, EI-MS m/z: 414 (M +, 5%), 356 (M + Ni, 100%), 300 (C 18 H 26 Si 2 + 2H, 18%), 298 (C 18 H 26 Si 2, 16%), 262 (12%), 244 (12%), 207 (C 9 H 13 SiNi, 10%). Crystals suitable for high-resolution X-ray diffraction experiment were obtained by slow re-crystallization from heptane at 0 C.

4 2. X-ray data collection and refinement 2.1. General. A single crystal high-resolution X-ray data collection for 2 was performed on a Bruker Kappa APEX II Ultra diffractometer equipped with a TXS rotating anode (MoK α radiation, λ = Å), multi-layer optics and an Oxford Cryosystems nitrogen gas-flow apparatus. A single crystal of suitable size was attached to a cactus spine using Paratone N oil, mounted on a goniometer head 50 (first 12 runs low angle data) and 40 mm (high angle data) from the APEX II CCD camera and maintained at a temperature of 90 K. Details of the data collection are given in Table 1S. The data collection strategy was optimized and monitored using the appropriate algorithms implemented by the APEX2 program. [1] Only the ω scans were taken into account, using 0.3 intervals with a counting time of 10 s and 90 s for low and high angle data respectively, resulting in a total of 8832 frames. Determination of the unit cell parameters and integration of the raw images was performed with the APEX2 suite of programs (integration was done by SAINT [1] ). The data set was corrected for Lorentz and polarization effects. The multi-scan absorption correction, scaling and merging of reflections were done with SORTAV. [2] 2.2. IAM refinement. Structure was solved by direct methods using SHELXS-97 [3] and refined using SHELXL-97 [3] using the IAM approximation. The refinement was based on F 2 for all reflections except those with very negative F 2. Weighted R factors (wr) and all goodness-of-fit (GooF) values are based on F 2. Conventional R factors are based on F with F set to zero for negative F 2. The F 2 o > 2σ(F 2 o ) criterion was used only for calculating R factors and is not relevant to the choice of reflections for the refinement. The R factors based on F 2 are about twice as large as those based on F. Scattering factors were taken from the International Tables for Crystallography. [4] All non-hydrogen atoms were refined anisotropically. Hydrogen atoms of C H bonds were placed in idealized positions(all these hydrogen atoms were visible on difference density maps). The lattice parameters, including the final R indices obtained by spherical refinement, are presented in Table 1S Multipole refinement. Multipole refinement of 2 was performed with the XDLSM module of the XD program suite, [5] using the Hansen-Coppens formalism. [6] In this formalism, the total atomic electron density (of the k-th atom) is a sum of three components: ρ k l max 3 3 () r = ρ ( r) + P κ ρ ( κ r) + P κ' R ( κ' r) d ( r / r) ck vk k vk k l l= 0 m= l lmk lk lk lk lmk

5 where ρ c and ρ k are spherical core and valence densities, respectively. The third term contains the sum of the angular functions (d lm ) that take into account aspherical deformations. The angular functions d lm are real spherical harmonic functions, which are normalized for the electron density. The coefficients P v and P lm are multipole populations for the valence and deformation density multipoles, respectively. κ and κ are scaling parameters which control the expansion or contraction of the valence and deformation densities, respectively. In the Hansen-Coppens formalism, P v, P lm, κ and κ are refinable parameters together with the atomic coordinates and thermal motion coefficients. Here the P 00 parameter was not refined as it is highly correlated with P v. The least-squares multipole refinement was based on F 2, with only those reflections with I > 3σ(I). Atomic coordinates x, y, and z and anisotropic displacement parameters (U ij ) for each atom were taken from the spherical refinement stage and freely refined. Each atom was assigned core and spherical-valence scattering factors derived from atomic Volkov and Macchi wave functions [5] A single-ζ Slater-type radial function multiplied by densitynormalized spherical harmonics was used for describing the valence deformation terms. The multipole expansion was truncated at the hexadecapole (l max = 4) and quadrupole (l max = 2) levels for all non-hydrogen and hydrogen atoms, respectively. The valence-deformation radial fits were described by the use of their expansion-contraction parameters κ and κ. The κ values were refined for non-hydrogen atoms and constrained to 1.20 for hydrogen atoms. Identical values of the κ parameter was used for all l > 0 multipoles for all other nonhydrogen atoms and kept unrefined at values of 1.20 for hydrogen atoms. No symmetry constraints were applied. The parameters refined at each stage of the refinement strategy were as follows: (1) only the scale factor (which was also refined in other stages of the procedure); (2) the coordinates together with thermal parameters for non-hydrogen atoms were refined against the high-angle data (sinθ/λ > 0.8 Å 1 ); (3) coordinates together with isotropic thermal parameters for hydrogen atoms against the low-angle data (sinθ/λ < 0.6 Å 1 ); (5) the hydrogen atom positions were shifted along the bond directions found to the standardized average neutron values (1.083 Å, Å and Å for C Ar H, C Me H and C=C H bond distances, respectively [7] ; recent paper of Allen and Bruno [8] corrects some of those values using updated database with entries, however, we did not observed significant differences, and thus the original model is presented here) and the isotropic thermal parameters for the nonhydrogen atoms at the high-angle data (sinθ/λ > 0.8 Å 1 ); (6) estimation of anisotropic thermal parameters for hydrogen atoms was accomplished using the SHADE2 server [9] (see Figure 5S; such a procedure have been recently shown to be the best approach, at least within the

6 Hansen-Coppens approximation, for hydrogen atoms treatment [10] ); (7) κ parameters for the non-hydrogen atoms; (8) multipole parameters refined in a stepwise manner; (9) coordinates and thermal parameters together with all multipole populations; (10) κ and κ parameters; (11) coordinates and thermal parameters together with all multipole populations. Proper deconvolution of thermal motion from the density features was tested by using the Hirshfeld rigid-bond test. [11] The differences of mean-squares displacement amplitudes (DMSDA) were higher than the Å 2 limit for the Ni C bonds (this was analyzed previously in the literature for Fe Cp [12] and this phenomenon is due to the liability of Cp rings). For the rest of the bonds the highest DMSDA values are oberved for Si C Me bonds (probably because of the mass difference). According to the above general refinement strategy, several models with different scattering factors were tested but there were no significant changes in the final electron density results obtained. The maximum and minimum of residual density were equal to e Å 3 and e Å 3, respectively. The residual density maps show some deviations near the nickel atoms (see Figures 1S). The presence of anharmonic vibrations was excluded, as their refinement did not improve the model in any way. All R-factors and other parameters characterizing the refinement, residual density maps, static deformation and laplacian maps are presented in Table 1S and in Figures 1S-5S.

7 Table 1S. Crystal and refinement data for compound 2. Formula C 18 H 26 Ni 2 Si 2 Molecular mass a.u. Measurement temperature (T) 90(2) K Crystal system monoclinic Space group C2/c Unit cell parameters: a (7) Å b (2) Å c (7) Å α 90 β (10) γ 90 Volume (V) (12) Å 3 Z 4 Calculated density g cm 3 F(000) 872 Crystal size mm 3 θ range for data collection Absorption coefficient (μ abs ) mm 1 (sinθ/λ) max 1.09 Å 1 Index ranges 39 < h < < k < < l < 37 No. of reflections collected / unique / 9930 Completeness > 99% R int 3.28 % Absorption correction multi-scan IAM refinement No. reflections / restrains / parameters 9930 / 0 / 100 R(F) / wr(f 2 ) [for I > 2σ(I)] 2.55% / 6.85% R(F) / wr(f 2 ) [for all data] 2.99% / 7.12% GooF Largest residual density peak and hole e Å 3 / e Å 3 Multipole refinement No. of reflections [for I > 3σ(I)] / parameters 8383 / 492 = R(F] / wr(f 2 ) [for I > 3σ(I)] 1.23% / 1.83% R(F 2 ] / wr(f 2 ) [for I > 3σ(I)] 1.73% / 3.43% R(F] / R(F 2 ) [for all data] 1.76% / 1.78% GooF [for I > 3σ(I)] Largest residual density peak and hole e Å 3 / e Å 3

8 3. Computational details 3.1. ADF calculations. All the results were obtained from the DFT calculations based on the Becke-Perdew exchange-correlation functional, [13] using the Amsterdam Density Functional (ADF) program, version [14] A standard double-ζ STO basis with one set of polarization functions was used for main-group elements, H, C and Si, while a standard triple-ζ STO basis set was employed for a transition metal, Ni. The 1s electrons of C, as well as the 1s-2p electrons of Si and Ni were treated as frozen core. Auxiliary s, p, d, f and g STO functions, centered on all nuclei, were used to fit the electron density and obtain an accurate Coulomb potential in each SCF cycle. Relativistic effects were considered using the firstorder scalar relativistic correction. [15] 3.2. Energy decomposition scheme. In the Ziegler-Rauk bond energy decomposition analysis [16] total interaction energy of distorted fragments in a molecule is divided into following components: Δ E tot = ΔE steric + ΔE orb = ( ΔEelstat + ΔEPauli ) + ΔEorb The first contribution, Δ Esteric, corresponds to the steric interaction between the fragment considered. It comprises two terms, namely (i) the classical electrostatic interaction between the promoted fragments, Δ Eelstat, and (ii) the Pauli repulsion between the occupied orbitals on the two fragments, Δ EPauli. The second component is the orbital interaction term, Eorb Δ, representing the interactions between the occupied molecular orbitals on one fragment with the unoccupied molecular orbitals of the other fragment as well as mixing of occupied and virtual orbitals within the same fragment (intra-fragment polarization). This latter term may be directly linked to the electronic bonding effect coming from the formation of a chemical bond. Natural Orbitals for Chemical Valence (NOCV) are obtained by diagonalization of the deformational density matrix. [17] They can be grouped in pairs ( ϕ k, ϕ k ) characterized by the eigenvalues of the opposite sign and the same absolute value, v k. Thus, the NOCV pairs allow for a decomposition of the differential Δρ, into NOCV contributions, Δρ k : [17] Δρ n / 2 n / 2 [ k k ] = Δ k () r 2 2 () r = ϕ () r + ϕ () r ν k ρ k = 1 k = 1

9 The picture of the bonding obtained from the deformational density NOCV contributions, is further enriched by providing the energetic estimations, Δ ( k) E orb, for each Δρ k within the ETS-NOCV method. [17-18] In such a combined scheme the orbital interaction term is expressed in terms of NOCVs eigenvalues as: ΔE orb = n / 2 k = 1 ΔE orb n / 2 TS TS ( k) = νk [ F k, k + Fk, k ] k = 1 where TS F i,i are the diagonal Kohn-Sham matrix elements defined over NOCV with respect to the transition state (TS) density at the midpoint between density of the molecule and the sum of fragment densities. This term gives the energetic measure of Δρ k that may be related to the importance of a particular electron flow channel contributing to the bonding between fragments considered Topological analysis. Bader s Quantum Theory of Atoms In Molecules [19] (QTAIM) was applied to perform topological analysis of experimental and theoretical electron density distributions.. In the framework of this approach Critical Points (CPs) together with the Bond Paths (BPs) were found as well as valence shell charge concentrations (VSCCs). In order to determine the nature and relative strength of the bonds, electron density and its Laplacian were evaluated at the BCPs. Atomic charges, dipoles and sources were calculated by integrating the respective quantities within the corresponding atomic basins. For theoretically obtained structures all analyses were carried out with DGRID program, [20] whereas for experimental data topological analyses were done with XDPROP and TOPXD modules from the XD package.

10 4. Supporting maps, plots and figures (a) (b) (c) Figure 1S. Residual density maps for 2 after multipole refinement: (a) Ni(1)Ni(1)#C(3)# plane; (b) Ni(1)C(10)#C(11)# plane; (c) C(3)C(5)C(6) plane (# 2-fold symmetry transformation present in C2/c space group). Contours at ±0.05 n e Å 3 (n = 1, 2, ). Color coding: blue solid lines positive values, red dashed lines negative values, black dotted line zero contour.

11 (a) (b) Figure 2S. Static deformation density maps for 2 after multipole refinement: (a) Ni(1)C(10)#C(11)# plane (# 2-fold symmetry transformation, contours at ±0.3 n e Å 3, n = 1, 2, ); (b) C(3)C(5)C(6) plane (contours at ±0.1 n e Å 3, n = 1, 2, ) Color coding: blue solid lines positive values, red dashed lines negative values, black dotted line zero contour. Figure 3S. Normal probability plot for 2 after final multipole refinement.

12 Figure 4S. Scale plot for 2 after final multipole refinement. Figure 5S. SHADE-estimated ADPs for hydrogen atoms (only the asymmetric part is shown).

13 (a) (b) Figure 6S. Isosurface representations of negative electron density Laplacian distributions in the vicinity of nickel atoms: (a) theoretical data for model system (1566 e Å 5 ); (b) experimental data for complex 2 (1807 e Å 5 ).

14 Figure 7S. Local source function profiles along Ni Ni bond. The reference point is taken as the BCP of the nickel nickel interaction: (a) IAM (red curve) and (b) multipole model (blue curve). The respective integrated source function contributions for the Ni atom are equal to about e Å 3 and e Å 3 for the IAM and multipole model, respectively. The contribution for IAM is clearly much smaller compared with the multipole model (as it could be concluded from the local source profiles). This might suggest a weaker charge depletion from the 3 rd atomic shell of the Ni atom (i.e. from the region around at 0.45 Å from the nucleus), as it is anticipated in the IAM, and may explain rather small positive charge on the metal atom (ca e). Also it suggests that the atomic core is less flexible (in terms of deformations) for the Ni I complexes in comparison with Ni II compounds. The detailed analysis is beyond the scope of this paper and will be published elsewhere.

15 5. Supporting tables Table 2S. Maxima and minima of residual density (RMS = e Å 3 ). Highest peaks Height / e Å 3 Remarks Random position Å from Ni(1) Random position Å from Ni(1) Å from Si(2) Random position Random position Å from Ni(1) Random position Random position Deepest holes Height / e Å 3 Remarks Å from Ni(1) Å from Ni(1) Å from Si(2) Random position Å from Si(2) Å from C(10) Random position Random position Å from C(8) Random position Table 3S. Selected geometrical and topological parameters in BCPs for 2 (a) and optimized model system (b) (d bond length, ρ electron density, # 2-fold symmetry axis transformation). (a) (b) Bond d / Å ρ / e Å 3 2 ρ / e Å 5 d / Å ρ / e Å 3 2 ρ / e Å 5 Ni(1) Ni(1)# (1) 0.235(1) 1.746(1) Ni(1) C(3) (3) 0.51(1) 5.40(2) Ni(1) C(4) (3) Ni(1) C(5) (4) 0.48(1) 5.38(2) Ni(1) C(6) (4) 0.47(1) 5.43(2) Ni(1) C(7) (4) Ni(1) C(10)# (3) 0.66(2) 6.29(3) Ni(1) C(11)# (4) 0.61(2) 6.54(4) Si(2) C(3) (3) 0.87(2) 0.64(7) Si(2) C(8) (4) 0.85(2) 0.29(7) Si(2) C(9) (4) 0.86(2) 2.62(7) Si(2) C(10) (3) 1.00(3) 2.12(8) C(3) C(4) (5) 1.96(3) 16.1(1) C(4) C(5) (6) 2.03(4) 14.1(1) C(5) C(6) (6) 2.04(3) 18.1(1) C(6) C(7) (5) 2.06(3) 15.4(1) C(7) C(3) (5) 1.84(3) 12.8(1) C(10) C(11) (5) 1.97(3) 12.9(1) C(4) H (4A) (7) 19.6(3) C(5) H (5A) (7) 17.1(3) C(6) H (6A) (7) 18.4(3)

16 C(7) H (7A) (7) 20.6 (3) C(8) H (8A) (8) 11.3(3) C(8) H (8B) (8) 9.7(3) C(8) H (8C) (8) 10.6(3) C(9) H (9A) (8) 13.9(3) C(9) H (9B) (8) 9.5 (3) C(9) H (9C) (7) 10.5(3) C(10) H (10A) (7) 19.2(3) C(11) H (11A) (7) 21.4 (3) C(11) H (11B) (7) 18.6(3) C(3) H (3A) C(10) H(10B) Table 4S. Cartesian coordinates of model optimized structure illustrated in Figure Additional references Atom x / Å y / Å z / Å C C C C C Ni C C H H H H H H H H H Ni C C C C C C C H H H H H H H H H [1] APEX2 package, Bruker AXS, Madison, WI, USA, [2] a) R. H. Blessing, J. Appl. Cryst. 1989, 22, 396; b) R. H. Blessing, Acta Cryst. 1995, A51, 33; c)r. H. Blessing, J. Appl. Cryst. 1997, 30, 421. [3] G. M. Sheldrick, Acta Cryst. 2008, A64, 112. [4] H. Fuess, International Union of Crystallography, Chester, 2006.

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