The group 7 metal carbonyl complexes from a stable heteroleptic silylene PhC ( N t Bu ) 2 SiNPh 2 †

Two silylene transition metal complexes were prepared by reacting stable N-heterocyclic silylene L {L = PhC(NtBu)2SiNPh2}with Mn2(CO)10 and Re2(CO)10 respectively in a 2 : 1 ratio to yield [L2Mn(CO)4] [Mn(CO)5] − (1) and [L2Re(CO)4] [Re(CO)5] − (2). Both complexes 1 and 2 were characterized by NMR spectroscopy, EI-MS spectrometry and elemental analysis. The molecular structures of complexes 1 and 2 were established by single crystal X-ray analysis.


Introduction
In the past two decades, there has been increasing interest in the chemistry of N-heterocyclic carbenes (NHCs). 1,2The fascinating discovery of stable N-heterocyclic carbenes by Arduengo et al. 3 led to burgeoning research activity.Since then, NHCs have been utilized as strong donor ligands for transition metals.On the one hand, many highly reactive species with low valent and zero valent elements stabilized by NHCs have been documented. 4On the other hand, the congeners of group 14 N-heterocyclic carbenes (N-heterocyclic silylenes, 5,6 N-heterocyclic germylenes, 7 N-heterocyclic stannylenes, 8 and N-heterocyclic plumbylenes 9 ) are less developed as σ-donor ligands for transition metals when compared with those of NHCs.In view of the importance of NHC-transition metal complexes for catalysis, the analogous derivatives of N-heterocyclic silylenes (NHSis) with transition metals 5,6 are still less explored.Currently we are interested in the chemistry of silylenes.5a-f,10,11 A synthetic challenge for the preparation of NHSis involves the use of strong reducing agents such as potassium or potassium graphite.These reducing agents and harsh conditions often lower the yields.Recently, we reported the high yield synthesis of stable N-heterocyclic chlorosilylene PhC(NtBu) 2 SiCl using lithium bis(trimethylsilyl)amide as a dehydrochlorinating agent. 12Further we reported on the facile synthesis of functionalized silylenes by treating monochlorosilylene PhC(NtBu) 2 SiCl with alkali metal amides, phosphide, alkoxide and organo-alkyl reagents. 13In order to gain further insight into the reactivity of N-heterocyclic silylene towards transition metals and to investigate the presence of a bulky substituent on the silicon atom, we carried out the reaction of L {L = PhC(NtBu) 2 SiNPh 2 } with group 7 metal carbonyls Mn 2 (CO) 10 and Re 2 (CO) 10 .The group 7 carbonyl complexes have been utilized for the formation of C-H and C-C bond reactions which have been considered as crucial steps in homogeneously catalyzed conversion of syngas to higher alkanes. 14The latter metal carbonyls were used for the preparation of complexes of composition [L 2 Mn(CO) 4 ] + [Mn(CO) 5 ] − (1) and [L 2 Re(CO) 4 ] + -[Re(CO) 5 ] − (2) with L. The reaction of Re 2 (CO) 10 with PhC-(NtBu) 2 SiNPh 2 shows a different reactivity pattern when compared with that of PhC(NtBu) 2 SiCl, which resulted in the product [(PhC(NtBu) 2 SiCl) 3 Re(CO) 3 ] + [Re(CO) 5 ] − under elimination of two carbonyl groups.5b

Results and discussion
Complexes 1 and 2 were obtained in good yields by the facile reactions of L with Mn 2 (CO) 10 and Re 2 (CO) 10 respectively in a 2 : 1 ratio (Scheme 1).The reaction of L with Re 2 (CO) 10 resulted in the product [L 2 Re(CO) 4 ] + [Re(CO) 5 ] − with elimination of one carbonyl group.The difference in the reactivity when compared with that of PhC(NtBu) 2 SiCl might be due to the presence of the bulky NPh 2 group on the silicon(II) center.
Complexes 1 and 2 are soluble in common organic solvents and are stable both in solution and the solid state for a long Scheme 1 Synthesis of 1 and 2.    † CCDC 886498 (1) and 886499 (2).For crystallographic data in CIF or other electronic format see DOI: 10.1039/c2dt31578e period of time without any decomposition under an inert gas atmosphere.Complexes 1 and 2 were fully characterized by NMR spectroscopy, EI-MS spectrometry, elemental analysis and single crystal X-ray structural analysis.
Complex 1 shows a single resonance at δ 61.11 ppm in its 29 Si NMR spectrum which is downfield shifted compared to L (δ −20.5 ppm).This is similar to the trend observed for other silylene transition metal complexes. 5The tBu protons show a broad singlet at δ 1.10 ppm for complex 1 in its 1 H NMR spectrum.The 13 C NMR spectrum reveals the presence of carbonyl groups which resonate at 218.41 and 239.66 ppm.Moreover 1 displays its fragment ions for [L 2 Mn(CO) 3 ] + , [L 2 Mn(CO)] + and [L 2 Mn] + in its mass spectrum at m/z 993, 937 and 909 respectively.
Complex 1 crystallizes in the monoclinic space group P2 1 /c (Table 1).The molecular structure of 1 is shown in Fig. 1.In 1 both silicon atoms are tetracoordinate and in a distorted tetrahedral geometry comprising three nitrogen atoms (two from the supporting amidinato ligand, one from the NPh 2 moiety) and one manganese atom.Two manganese atoms are present in the molecular structure, one atom as the central atom in the cation and the other in the anion.The manganese atom present in the cationic part is hexacoordinate (derived from two silicon and four carbon atoms of the carbonyl groups) and features a distorted octahedral geometry.The environment of manganese in the cationic part of the trans isomer [L 2 Mn(CO) 4 ] + is depicted in Fig. 1.The silylene ligands are located in trans positions and the Si2-Mn1-Si1 bond angle is 165.80(3)°.The Si-Mn bond distances in 1 are 2.3521(6) and 2.3571(7) Å, which are longer when compared with the average Si-Mn distance of 2.2802(8) Å in [(PhC(NtBu) 2 SiCl) 2 Mn(CO) 4 ] + [Mn(CO) 5 ] − .5b The average manganese carbonyl bond length in the cation is 1.835(3) Å.The manganese atom present in the anion is pentacoordinate with a τ value of 0.48.The structural index τ defines the extent of deviation from trigonal bipyramidal to square pyramidal geometry (τ = 1 for perfect trigonal bipyramidal; τ = 0 for perfect square based pyramidal). 15These Mn-CO bond lengths are quite comparable with those of the literature values. 16The average Si-N (from the NPh 2 moiety) bond length in 1 is 1.7373 (19)    Like 1, complex 2 also crystallizes in the monoclinic space group P2 1 /c (Table 1) and its molecular structure is shown in Fig. 2. The molecular structure of 2 is isostructural to that of 1.The silylene ligands are located in trans positions and the Si2-Re1-Si1 bond angle is 165.95(6)°.The Si-Re bond distances in 2 are 2.4816( 17) and 2.4851(17) Å, which are comparable to the two Si-Re bond distances reported in [(PhC(NtBu) 2 SiCl) 3 Re-(CO) 3 ] + [Re(CO) 5 ] − (shorter one: 2.4384( 18) and 2.4354(17) Å, and longer one: 2.4928(17) Å). 5b The average rhenium carbonyl bond length in the cation is 1.982(7) Å.The rhenium atom in the anionic part is pentacoordinate (τ = 0.34).The data of the Re-CO bond lengths are comparable to the literature values. 17he average Si-N (from the NPh 2 moiety) bond length in 2 is 1.728(6) Å and the bite angles are in both cases 70.9(3)°.

Experimental section
Syntheses were carried out under an inert gas atmosphere of dinitrogen in oven dried glassware using standard Schlenk techniques and other manipulations were accomplished in a dinitrogen filled glove box.Solvents were purified by the MBRAUN solvent purification system MB SPS-800.All chemicals were purchased from Aldrich and used without further purification.L was prepared as reported in the literature.13b 1 H, 13 C 29 Si NMR spectra were recorded with a Bruker Avance DRX 300, or a Bruker Avance DRX 500 spectrometer, using THF-d 8 as a solvent.Chemical shifts δ are given relative to SiMe 4 .The IR spectra were recorded on a Bio-Rad Digilab FTS7 spectrometer in the range of 4000-400 cm −1 as Nujol mulls.EI-MS spectra were obtained using a Finnigan MAT 8230 instrument.Elemental analyses were performed by the Institut für Anorganische Chemie, Universität Göttingen.

Crystal structure determination
Suitable single crystals for X-ray structural analysis of 1 and 2 were obtained by storing their corresponding toluene solutions at −27 °C and the crystals were taken out of the mother liquor under an argon atmosphere using NVH oil.The diffraction data were collected at 100 K on a Bruker three circle diffractometer equipped with a SMART 6000 CCD area detector and a CuKα rotating anode.Due to good crystal quality both datasets were collected to the edge of the Ewald sphere with high completeness and high multiplicity.The raw data were integrated with SAINT 18 and an empirical absorption correction with SADABS 19 was applied.The structures were solved by direct methods (SHELXS-97) and refined against all data by fullmatrix least-squares methods on F 2 (SHELXL-97). 20HELXLE 21 was used as refinement GUI.All non-hydrogenatoms were refined with anisotropic displacement parameters.The rigid body restraint DELU was applied for carbon, nitrogen and oxygen atoms.Hydrogen atoms were constrained using suitable HFIX commands.

Conclusions
Like N-heterocyclic chlorosilylene PhC(NtBu) 2 SiCl, the functionalized silylene L can be also used as a ligand for the synthesis of silylene transition metal complexes.The reaction of L with Mn 2 (CO) 10 and Re 2 (CO) 10 forms complexes of composition [L 2 Mn(CO) 4 ] + [Mn(CO) 5 ] − (1) and [L 2 Re(CO) 4 ] + [Re(CO) 5 ] − (2).Due to the presence of the bulky NPh 2 moiety at the silicon(II) center, the reaction of L shows a different reactivity pattern with Re 2 (CO) 10 compared to that of PhC(NtBu) 2 SiCl.