The Si2H radical supported by two N-heterocyclic carbenes

A comprehensive experimental and quantum chemical study of the open-shell mixed valent disilicon(0,I) hydride Si2(H)(Idipp)2 (Idipp = C[N(C6H3-2,6-iPr2)CH]2) is reported.


Introduction
Open-shell silicon hydrides are of signicant importance as transient intermediates in the chemical vapor deposition (CVD) of silicon or silicon-containing thin lms, which are extensively used in the semiconductor industry. 1 Fundamental species in the gas phase include the SiH x (x ¼ 1-3) and Si 2 H x (x ¼ 1-5) molecules as well as higher aggregated Si n H m clusters, which are formed from silane (SiH 4 ) or disilane (Si 2 H 6 ) in a complex cascade of reactions. 1 These species, which are also of interest in astrochemistry, 2 are unstable under terrestrial conditions and can only be detected by spectroscopic or mass spectrometric techniques. 3 One scarcely studied species in this context is the Si 2 H molecule, which was so far only detected by vibrationally-resolved photoelectron spectroscopy of Si 2 H À anions. 4 Quantum chemical calculations of Si 2 H suggest two almost isoenergetic, C 2v -symmetric H-bridged structures, in which the unpaired electron occupies either the Si-Si p-bonding orbital ( 2 B 1 state) or a s-type molecular orbital corresponding to the in-phase combination of the Si lone pair orbitals ( 2 A 1 state). 5 Recently, N-heterocyclic carbenes (NHCs) were found to be particularly suitable Lewis bases for the thermodynamic and kinetic stabilization of highly reactive, unsaturated, low-valent Si species, leading to the isolation of a series of novel compounds with intriguing synthetic potential. 6,7 Several CAACstabilized open-shell silicon compounds (CAAC ¼ cyclic alkyl(amino)carbene) were also reported, in which the unpaired electron is mainly located on the CAAC substituent. 8 Trapping of Si 2 H by NHCs appeared therefore an achievable, albeit very challenging goal, given the fact that isolable molecular hydrides of silicon in an oxidation state <2 are very rare 9,10 and open-shell congeners presently unknown. In comparison, three-coordinate Si II hydrides 11 and four-coordinate Si II hydrides of the general formula (LB)SiH(X)(LA) (LB ¼ neutral Lewis base; LA ¼ neutral Lewis acid; X ¼ singly bonded substituent) 12 are meanwhile well documented.  4 ], Idipp ¼ C[N(C 6 H 3 -2,6-iPr 2 )CH] 2 , Ar F ¼ C 6 H 3 -3,5-(CF 3 ) 2 ), which was isolated recently in our group upon protonation of Si 2 (Idipp) 2 (1), 9 appeared to be a suitable starting material to tackle the problem of isolating an NHC-trapped Si 2 H radical. Quantum chemical studies revealed the same sequence of frontier orbitals in 1H + and its isolobal phosphorus counterpart [R 2 P]PR] + , according to which the HOMOÀ1 corresponds to the lone-pair orbital at the two-coordinated E atom (E ¼ Si, P), the HOMO is the E]E p-bonding orbital and the LUMO is the E]E p* orbital. 9 This isolobal interrelationship suggested that 1H + might be also reversibly reducible as the phosphanylphosphenium cation [Mes*(Me)P]PMes*] + (Mes* ¼ C 6 H 2 -2,4,6-tBu 3 ). 13 In fact, cyclic voltammetric (CV) studies of 1H[B(Ar F ) 4 ] in uorobenzene at room temperature revealed a reversible one-electron reduction at a rather low halfwave potential (E 1/2 ) of À1.63 V as well as an irreversible oxidation at +0.67 V versus the [Fe(h 5 -C 5 Me 5 ) 2 ] +1/0 reference electrode ( Fig. 1 and ESI †). 14 9 was found also to undergo a reversible one-electron reduction, albeit at a more negative potential (E 1/2 ¼ À1.85 V) than 1H[B(Ar F ) 4 ]. Notably, reduction of 1H + and 1Me + occurs at much lower potentials than that of the cation [Mes*(Me)P]PMes*] + (E 1/2 ¼ À0.48 V). 13 This marked difference in the redox potentials of the Si-and P-based cations can be rationalized with the large increase of the LUMO energy occurring upon replacement of the two PMes* fragments by the much less electronegative isolobal Si(Idipp) fragments as suggested by quantum chemical calculations. 9 The CV results prompted us to attempt a chemical oneelectron reduction of 1H[B(Ar F ) 4 ]. Indeed, vacuum transfer of THF to a 1 : 1 stoichiometric mixture of 1H[B(Ar F ) 4 ] and KC 8 at À196 C followed by warming to À40 C resulted in a distinct color change of the dark red solution of 1H[B(Ar F ) 4 ] to give an intensely dark green solution, which aer work-up and crystallization from n-hexane at À60 C afforded Si 2 (H)(Idipp) 2 (1H) as a dark green, almost black crystalline solid in 55% yield (Scheme 1) (see ESI †). Compound 1H is extremely air-sensitive and immediately decolorizes upon contact with air, but can be stored under an atmosphere of argon at À30 C without any color change or signs of decomposition in its EPR spectrum. Thermal decomposition of 1H in a vacuum-sealed glass capillary was detected upon melting at 147 C leading to a dark red mass. Analysis of the soluble part of the melting residue in C 6 D 6 by 1 H NMR spectroscopy revealed the presence of Idipp (95%) and 1 (5%).

Results and discussion
Notably, the redox potential of 1H [E 1/2 in C 6 H 5 F ¼ À2.15 V vs. [Fe(h 5 -C 5 H 5 ) 2 ] +1/0 (Fc + /Fc)] 15 lies in-between that of the benzophenone radical anion (E 1/2 in THF ¼ À2.30 V vs. Fc + /Fc) 16 and [Co(h 5 -C 5 Me 5 ) 2 ] (E 1/2 in MeCN ¼ À1.91 V vs. Fc + /Fc), 16 indicating that the radical 1H is a very strong one-electron reducing agent. Consequently, the radical 1H is selectively oxidized back to 1H[B(Ar F ) 4 ] upon treatment with one equivalent of [Fe(h 5 -C 5 Me 5 ) 2 ][B(Ar F ) 4 ] in THF-d 8 (see ESI †). Thereby, the redox pair 1H + /1H provides a very rare example of a chemically reversible Si-based redox system. 7c,17 Compound 1H is well soluble in n-hexane, benzene, diethyl ether or THF affording intensely dark-green solutions, even at low concentrations. The origin of this intense color was analyzed by UV-Vis-NIR spectroscopy of 1H in n-hexane (Fig. 2, le and ESI †), which revealed electronic absorptions in the whole spectral range from 220-1100 nm. Six absorption maxima were located at 254 (9970), 305 (8140), 436 (5170), 608 (7110), 704 (6860) and 958 (1440) nm, of which the intense absorptions at 608 and 704 nm are responsible for the green color of 1H (the values of the molar absorption coefficients 3 l are given in brackets in L mol À1 cm À1 ). The UV-Vis-NIR spectrum was also analyzed by time-dependent density functional theory (TdDFT) calculations (see ESI, Fig. S21 †). 18 Magnetic susceptibility measurements of solid 1H from 300.0-1.9 K suggest the presence of a paramagnetic compound with one unpaired electron following Curie's law. A plot of the reciprocal molar magnetic susceptibility (c m À1 ) against the absolute temperature (T) showed a linear correlation from which the effective magnetic moment m eff was calculated aer linear regression and found to be 1.68 m B (Fig. 2, right and ESI †). This value is slightly lower than the value derived from the spinonly formula for one unpaired electron (m eff ¼ 1.73 m B ). The molecular structure of 1H was determined by single crystal X-ray crystallography. The radical features a crystallographically imposed inversion symmetry (space group: P2 1 /c) in marked contrast to the C 1 -symmetric structure of 1H + in 1H[B(Ar F ) 4 ]. 9 The Si-bonded H atom was located in the difference Fourier map and anisotropically rened with a site occupancy of 1/2 at each Si atom. However, the exact position of this H atom could not be deduced by X-ray crystallography, since structural renements with either a terminal (Si-H) or a bridging (Si-H-Si) position led to identical wR 2 values. 1H features as 1H[B(Ar F ) 4 ] and 1 a trans-bent planar C NHC -Si-Si-C NHC core (Fig. 3). However, distinct structural differences become apparent upon comparing the three structures. For example, the Si-Si bond of  (Table 1), and lies in-between that of a typical Si]Si double bond (2.20Å) 20 and a Si-Si single bond (e.g. 2.352Å in a-Si). 21 In comparison, the Si-C NHC bonds in 1H (1.873(4)Å) are shorter than the Si-C NHC bonds of the dicoordinated Si atoms in 1H[B(Ar F ) 4 ] (1.940(2)Å) 9 and 1 (1.927(1)Å) 19 (Table 1), and similar to that of the trigonal-planar coordinated Si atom in 1H[B(Ar F ) 4 ] (1.882(2)Å). 9 Reduction of 1H + results also in a distinct change of the conformation of the NHC substituents. Thus, both N-heterocyclic rings in 1H are arranged coplanar with the trans-bent C NHC -Si-Si-C NHC core as evidenced by the dihedral angle 4 NHC of 3.3(2) ( Table 1), whereas in 1H + one of the two N-heterocyclic rings (bonded to the two-coordinated Si atom) adopts an almost orthogonal orientation (Table 1). All these structural changes can be rationalized by quantum theory (vide infra). Thus, reduction of 1H + leads to a population of the Si]Si p* orbital with one electron, reducing thereby the formal Si-Si bond order from 2 in 1H + to 1.5 in 1H as nicely reected in the computed Si-Si Wiberg bond indexes (WBI; WBI(Si-Si) of 1H + ¼ 1.70; WBI(Si-Si) of 1H ¼ 1.17) (see ESI, Tables S11 and S12 †). The coplanar arrangement of the N-heterocyclic rings allows for an optimal in-phase interaction (p-conjugation) of the Si]Si p* orbital with p*(CN 2 ) orbitals of the NHC substituents in the SOMO of 1H (Fig. 6), providing a rationale for the shortening of the Si-C NHC bonds and the concomitant elongation of the C NHC -N NHC bonds of 1H versus 1H + (Table 1).
IR spectroscopy proved to be a very useful method to determine unequivocally the position of the Si-bonded H atom. In fact, the ATR FT-IR spectrum of 1H displayed a n(Si-H) absorption band at 2089 cm À1 , which is characteristic for stretching vibrations of terminal Si-H bonds (see ESI, Fig. S4 †). In comparison, the n(Si-H-Si) band of Si 2 H is predicted at signicantly lower wavenumbers (1592 cm À1 ( 2 A 1 state); 1491 cm À1 ( 2 B 1 state)), 4  Further insight into the structure of 1H was provided by continuous wave (cw) EPR spectroscopy at X-band frequencies. Spectra with a nicely resolved hyperne coupling pattern could be obtained from samples of 1H in n-hexane solution at 336 K ( Fig. 4; see also ESI, Fig. S10 † for EPR spectra at different temperatures). Notably, a similar EPR spectrum was obtained in diethyl ether solution at 298 K (see ESI, Fig. S12 †), suggesting that solvent coordination effects are negligible. The EPR spectrum of 1H displays a multiplet at a g iso value of 2.00562, which could be well simulated assuming coupling of the unpaired electron to one 1 H (I ¼ 1/2) nucleus, two different 29 Si (I ¼ 1/2) and two pairs of two magnetically equivalent 14 N (I ¼ 1) nuclei, respectively (Fig. 4). These observations suggest that 1H has  Quantum chemical calculations of 1H were carried out using the B3LYP functional in combination with the 6-311G** basis set for the Si, N, Si-bonded H and NHC ring C atoms and the 6-31G* basis set for the peripheral C and H atoms or the B97-D3 functional in combination with RI-JCOSX approximations and the def2-TZVP basis set for all atoms. 25 The levels of theory are abbreviated in the following with B3LYP/I and B97-D3/II. Remarkably, calculations at the B3LYP/I level of theory yielded one minimum structure (1H calc ), whereas two almost degenerate minimum structures were obtained at the B97-D3/II level of theory (1H calc and 1H 0 calc ) (Fig. 5). All calculated minimum structures display a terminally bonded H atom bound to the Si1 atom. No minimum structure with a bridged H atom was found on the potential energy hypersurface of 1H at both levels of theory. The geometrical parameters of the minimum structure calculated at the B3LYP/I level of theory and the global minimum structure at the B97-D3/II level of theory are almost identical (Table 2 and ESI, Table S8 †). These structures (1H calc ) contain a trigonal-pyramidal coordinated Si1 atom with a sum of angles of 335.51 (B3LYP/I) and 342.58 (B97-D3/II), respectively. Remarkably, the calculated structure of the diphosphanyl radical P 2 (Me)Mes * 2 , which is isolobal to 1H, displays a trigonal pyramidal geometry at the three-coordinated P atom (sum of angles: 337.5 ), 13 as found for 1H calc . In comparison, the second minimum structure obtained at the B97-D3/II level of theory (1H 0 calc ) is only 5.5 kJ mol À1 higher in energy than 1H calc and contains the Si1 atom in a trigonal planar environment (sum of angles: 359.61 ). A comparison of the structural parameters of 1H calc and 1H 0 calc with those obtained by single crystal X-ray diffraction reveals a good agreement of the calculated Si-Si, Si-C NHC and C NHC -N NHC bond lengths of both minimum structures (Table 2 and ESI, Table S8 †). While the experimental results did not allow to clearly distinguish whether a attened trigonal-pyramidal or a trigonal-planar geometry of the Hbound Si atom is present in 1H, the theoretical studies suggest a at energy hypersurface for the planarization of the threecoordinated Si atom.
The calculated quasi-restricted orbitals (QROs) of 1H calc at the B3LYP/I level of theory and of 1H calc and 1H 0 calc at the B97-D3/II level of theory are almost identical ( Fig. 6 and ESI, Fig. S17-S19 †). The SOMO is the Si]Si p* orbital, conrming  19. c 4 NHC denotes the dihedral angles between the C NHC -Si-Si-C NHC least-square plane and the respective N-heterocyclic ring least-square planes.  that reduction of 1H + leads to a population of the empty Si]Si p* orbital of 1H + with one electron (see ESI, Fig. S16 †). The SOMO reveals signicant contributions of p* NHC orbitals due to p-conjugation. The two lower lying doubly occupied molecular orbitals (DOMOs) are the Si]Si p and the n(Si) lone pair orbital, respectively.
Notably, CASSCF(3,3)/def2-TZVP calculations 26 of 1H calc revealed that the overall wave function is described by a major ground state conguration of [2-1-0] of the DOMO, SOMO and LUMO with 96% contribution, suggesting that static correlation can be neglected in the electronic description of 1H (see ESI †).
The calculated spin densities of 1H calc and 1H 0 calc at the B97-D3/II level of theory are depicted in Fig. 7. Mulliken analyses 27 of the spin densities reveal that the highest spin density is located at the dicoordinated Si2 atom (37% in 1H calc , 29% in 1H 0 calc ), whereas the spin density at the Si1 atom is quite small (9% in 1H calc , 6% in 1H 0 calc ), which is in full agreement with the observation of one large and one small a( 29 Si) hfcc in the experimental EPR spectrum of 1H (vide supra) (see ESI, Table  S9 †). 28 Remarkably, a signicant amount of the spin density is delocalized into the C NHC and N NHC atoms of the Si1-bonded (17% in 1H calc , 27% in 1H 0 calc ) and Si2-bonded (29% in 1H calc , 30% in 1H 0 calc ) NHC substituents, which explains the EPRspectroscopic detection of two a( 14 N) hfcc's. The calculated g iso values of 1H calc (2.00483) and 1H 0 calc (2.00454) agree well with the experimentally obtained g iso value (2.00562).
Further insight into the electronic structure of 1H was provided by a natural bond orbital (NBO) analysis at the B3LYP/I level of theory (see ESI, Table S12 †). 25k The Si-Si bond is composed of a Si-Si s bond and a Si]Si p bond with an occupancy of 1.95 and 0.82 electrons, respectively, which indicates indirectly a population of the Si]Si p* orbital with one electron leading thereby to a decrease of the formal Si-Si bond order from 2 in 1H + to 1.5 in 1H (vide supra). The Si2 atom in 1H calc bears a lone pair of high s-character (72%) as similarly found for 1H + calc (75%). Remarkably, both Si-C NHC bonds in 1H calc are composed of one doubly occupied Si-C NHC s NBO and one singly occupied Si]C NHC p NBO, of which the latter is absent in 1H + calc . These additional Si-C NHC p contributions rationalize the shortening and strengthening of the Si-C NHC bonds in 1H, which is also reected in the higher Si-C NHC WBI indexes (1H: WBI(Si-C NHC ) ¼ 1.01 and 0.95; 1H + : WBI(Si-C NHC ) ¼ 0.86 and 0.74).