Rhodium and Iridium Complexes of Anionic Thione and Selone Ligands Derived from Anionic N‐Heterocyclic Carbenes

Abstract The lithium salts of anionic N‐heterocyclic thiones and selones [{(WCA‐IDipp)E}Li(toluene)] (1: E=S; 2: E=Se; WCA=B(C6F5)3, IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene), which contain a weakly coordinating anionic (WCA) borate moiety in the imidazole backbone were reacted with Me3SiCl, to furnish the silylated adducts (WCA‐IDipp)ESiMe3 (3: E=S; 4: E=Se). The reaction of the latter with [(η 5‐C5Me5)MCl2]2 (M=Rh, Ir) afforded the rhodium(III) and iridium(III) half‐sandwich complexes [{(WCA‐IDipp)E}MCl(η 5‐C5Me5)] (5–8). The direct reaction of the lithium salts 1 and 2 with a half or a full equivalent of [M(COD)Cl]2 (M=Rh, Ir) afforded the monometallic complexes [{(WCA‐IDipp)E}M(COD)] (9–12) or the bimetallic complexes [μ 2‐{(WCA‐IDipp)E}M2(COD)2(μ 2‐Cl)] (13–16), respectively. The bonding situation in these complexes has been investigated by means of density functional theory (DFT) calculations, revealing thiolate or selenolate ligand character with negligible metal‐chalcogen π‐interaction.


Introduction
Immediately after the isolation and structural characterization of the first stable and "bottleable" N-heterocyclic carbene (NHC), [1] the synthesis of NHC main group element adducts has become an area of extensive research. [2] NHC-phosphinidene species (NHC)PR have been among the earliest examples, [3] and their versatile chemistry has again attracted great interest in recent years, [4] not least because of their use as ligands in transition metal chemistry. [5] In this context, the reaction of the trimethylsilyl-substituted phosphinidene derivative (IDipp)PSiMe 3 (I, IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene, Figure 1) with transition metal halides proved to be particularly useful to enable the transfer of the monoanionic NHC-phosphinidenide moiety II into the coordination sphere of transition metals via MÀ P bond formation accompanied by trimethylsilyl chloride elimination. [6] Following this approach, half-sandwich complexes such as [{(IDipp)P} MCl(η 6 -p-cymene)] (M = Ru, Os) and [{(IDipp)P}MCl(η 5 -C 5 Me 5 )] (M = Rh, Ir) as well as related cationic species were prepared, [6,7] which have structural and spectroscopic properties very similar to prototypic nucleophilic phosphinidene complexes of the type [RP=ML n ], for example, low-field 31 P NMR resonances and short metal-phosphorus double bonds. [8] These characteristics can be ascribed to a strong polarization of the (IDipp)P À ligand upon metal complexation as described by the mesomeric forms IIA and IIB, with the latter revealing its ability to act as a 2σ,2πelectron donor. Compound I has also been used to synthesize NHC-stabilized element-phosphorus species (EP = PP, AsP, GeP, SnP), [9] while several other phosphinidene and also arsinidene derivatives such as (IMes)PSiMe 3 , (IDipp)AsSiMe 3 and (IMes)AsSiMe 3 have recently become available for future application in transition metal and main group element chemistry (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2ylidene). [10] Replacement of the pnictogen element in (NHC)P À and (NHC)As À systems by the neighboring chalcogen atoms affords neutral (valence) isoelectronic imidazolin-2-thiones and -selones (NHC)E (E = S, Se), which are well-established and important ligands in transition metal chemistry, [5] including the species (IDipp)S and (IDipp)Se analogous to II. [11] (NHC)Se species have also found considerable interest, since their 77 Se NMR chemical shifts are frequently used as an indicator for the π-accepting properties of the corresponding NHCs. [11d,12] Anionic N-heterocyclic carbenes represent another important variation of classical NHC ligands, [13] and our group introduced carbenes that contain a weakly coordinating anionic (WCA) fluoroborate moiety, for example, B(C 6 F 5 ) 3 , at the C4 position of the imidazole ring. [14] These carbenes were termed WCA-NHCs and successfully introduced as ancillary ligands in transition metal chemistry and homogenous catalysis. [14,15] More recently, these carbenes have also found application in main group element chemistry, [9c,16] including the preparation of the complete series of lithium chalcogenides [{(WCA-NHC)E}Li(solv.)] (E = O, S, Se, Te). [17] Furthermore, the 77 Se NMR spectra of the selenides [{(WCA-NHC)Se}Li(toluene)] were studied to determine the πaccepting properties of the WCA-NHCs, revealing that the counter cation does not exert any significant influence, if it is fully separated from the selenium atom by solvation, for example, in THF-d 8 . [18] In this context, we envisaged that the lithium salts [{(WCA-IDipp)E}Li(toluene)] (1: E = S; 2: E = Se) or, similarly to I, the trimethylsilyl derivatives (WCA-IDipp)ESiMe 3 (3: E = S; 4: E = Se) might serve as suitable reagents for the transfer of the anionic (WCA-IDipp)E À ligand (III, Figure 1). The resulting metal complexes of the type [{(WCA-IDipp)E}ML n ] (E = S, Se) will have an identical total charge as the corresponding NHC-phosphinidenide complexes [{(IDipp)P}ML n ] and therefore allow the comparison of the phosphorus-metal and chalcogen-metal bonds, while preserving the overall structural and electronic properties. Since previous studies with the ligand II included rhodium and iridium complexes, [6,7] we aimed at the synthesis and investigation of related systems with the ligands III, which are reported in this contribution.

Synthesis and characterization of rhodium(III) and iridium(III) complexes
The lithium salts 1 and 2 were prepared as toluene solvates following the published procedures. [17,18] Their reactions with trimethylsilyl chloride (Me 3 SiCl) in toluene afforded the silanes (WCA-IDipp)ESiMe 3 (3: E = S; 4: E = Se) as off-white solids in 57 % and 34 % yield, respectively, after separation of LiCl by filtration and recrystallization from toluene/n-hexane solution (Scheme 1). The 1 H NMR spectra of 3 and 4 (in C 6 D 6 ) indicate a loss of symmetry due to the presence of the B(C 6 F 5 ) 3 moiety in the backbone, and a splitting of the signals corresponding to the Dipp groups into two sets is observed. The characteristic signals for the hydrogen atom in the imidazole backbone are observed at 6.67 ppm (3) and 6.73 ppm (4). In the 13 C NMR spectra, low-field resonances at 141.7 and 136.6 ppm can be assigned to the CS and CSe carbon atoms, respectively. The 11 B resonances for both compounds are found at ca. À 15 ppm, and the 19 F NMR spectra show three signals for the ortho-, metaand para-fluorine atoms. For the selenium compound 4, the 77 Se NMR chemical shift is 79 ppm, which is at higher field compared to 114 ppm reported for [{(WCA-IDipp)Se}Li(toluene)] (2) [18] and 147 ppm reported for (WCA-IDipp)SeH (both in THFd 8 ); [17] similar chemical shifts have been reported for trimethylsilyl aryl selenides such as 1,4-C 6 H 4 (SeSiMe 3 ) 2 (78 ppm in C 6 D 6 ). [19] NMR data of all compounds are assembled in Table 1.  [a] ref. [17].
Single crystals were obtained from toluene/n-hexane solutions of 3 and 4 at À 30°C and subjected to X-ray diffraction analysis. The molecular structure of 4 is presented in Figure 2, whereas the molecular structure of 3 is shown in the Supporting Information ( Figure S1). Pertinent structural data are summarized in Table 2. As expected, the C1À S1 bond length of 1.7358(11) Å as well as the C1À Se1 bond length of 1.890(2) Å are elongated, when compared to the distances found in the lithiated species 1 (1.685(2) Å) and 2 (1.845(2) Å). [17,18] The N1À C1À N2 angles increase from 105.23(13)°in 1 to 107.22(9)°i n 3 and from 106.03(17)°in 2 to 107.55 (19)°in 4, respectively, in agreement with an increase of imidazolium character. Overall, these structural parameters are similar to those established for the protonated species (WCA-IDipp)EH (E = S, Se). [17] The EÀ Si bond lengths are 2.2116(5) Å (E = S) and 2.3333(9) Å (E = Se), which is slightly longer compared to trimethylsilyl aryl sulfides [20] and selenides, [19] whereas the C1À EÀ Si angles of 107.97°(3) and 110.20°(4) correspond to the C1À PÀ Si and C1À AsÀ Si angles of 110.71(4)°/106.40(4)°and 108.68(5)°/ 104.10(5)°established each for two independent molecules of the phosphorus and arsenic analogues (IDipp)E'SiMe 3 (E' = P, As). [6,10b] It is noteworthy that recrystallization of the selenium compound 4 from THF afforded crystals of the trimethylsilyl ether (WCA-IDipp)Se(CH 2 ) 4 OSiMe 3 , which must have formed by activation and insertion of ring-opened THF into the SeÀ Si bond; see the Supporting Information for analytical and crystallographic data. This reactivity is reminiscent of frustrated trimethylsilyl-NHC adducts [21] and suggests an intrinsic lability of the chalcogen-silicon bonds in 3 and 4, which could be exploited for the transfer of the (WCA-IDipp)E upon reaction with transition metal halides. To investigate this reactivity, 3 and 4 were each reacted with half an equivalent of the binuclear rhodium(III) and iridium(III) complexes [(η 5 -C 5 Me 5 )MCl 2 ] 2 (M = Rh, Ir) in toluene. After stirring for 15 min at room temperature, the solutions were evaporated, and the 16 valence electron complexes [{(WCA-IDipp)E}MCl(η 5 -C 5 Me 5 )] (5-8) were isolated as dark brown (5/7) or dark orange (6/8) solids in moderate yields after washing with diethyl ether and extraction with THF. The 77 Se NMR spectra for the selones 7 and 8 were recorded in THF-d 8 and reveal a high-field shift of the selenium signal to 117 ppm (7) and 122 ppm (8), compared to 79 ppm in the silylated adduct 4.
Single crystals suitable for X-ray diffraction analysis were obtained by layering C 6 D 6 solutions of 5 and 7 or CH 2 Cl 2 Figure 2. Molecular structure of 4 in 4 · n-hexane with thermal displacement parameters drawn at 50 % probability; hydrogen atoms and solvent molecules are omitted for clarity; pertinent structural data for the compounds 1-16 are assembled in Table 2.  (17) 86.40(4) 86.58 (5) 107.8 (5) [a] From ref. [17].
[c] Two independent molecules in the asymmetric unit.

Synthesis and characterization of rhodium(I) and iridium(I) complexes
We have previously studied the reactions of the N-heterocyclic carbene-trimethylsilyl phosphinidene adduct (IDipp)PSiMe 3 (I) with the binuclear rhodium(I) and iridium(I) 1,5-cyclooctadiene (COD) complexes [M(COD)Cl] 2 (M = Rh, Ir), which independently of the stoichiometry afforded a bimetallic rhodium complex [μ 2 -{(IDipp)P}Rh 2 (COD) 2 (μ 2 -Cl)]. [6] In contrast, no elimination of Me 3 SiCl was observed with iridium, but instead the monometallic complex [{(IDipp)PSiMe 3 }Ir(COD)Cl] was isolated. [25] Therefore, the reactions of the silylated sulfur congener 3 with [M(COD)Cl] 2 were initially studied by NMR spectroscopy, which, depending on the stoichiometry, resulted in a clean formation of the corresponding mono-and bimetallic complexes 9 and 13. However, since the preparation of the silylated educts 3 and 4 proceeded via the lithium species 1 and 2 with a significant loss of yield, they were also directly employed for the synthesis of the corresponding rhodium and iridium COD complexes 9-12 The 1 H and 13 C NMR spectra of complexes 9-12 indicate the formation of C ssymmetric complexes, e. g., by observation of four well separated doublets and two septets for the isopropyl methyl and methine hydrogen atoms, respectively. In addition, two different sets of CH and CH 2 signals are found for the COD ligand. The 77 Se NMR spectra (in THF-d 8 ) of the two selenides 11 and 12 exhibit signals at 38 ppm (11) and 53 ppm (12), respectively, indicating less deshielded selenium nuclei compared to the corresponding rhodium(III) and iridium(III) complexes 7 (117 ppm) and 8 (122 ppm, see above). The molecular structures of 9-12 were determined by single-crystal X-ray diffraction analysis; see Figure 4 for 12 and the Supporting Information for 9-11 (Figures S8-S10). Pertinent structural parameters are assembled in Table 2. In all four complexes, the metal atoms reside in square-planar environments. In addition to binding of the η 4 -COD ligand and the selenium atom, the coordination sphere is completed by an arene-metal interaction (π-face donation) [26] that largely involves the ipso-carbon atom of the Dipp substituent facing away from the borate moiety. Significantly longer, albeit distinctly different metal-carbon distances are found for the adjacent ortho-carbon atoms, which indicates a situation intermediate between η 1 -and η 2 -coordination modes. [27] Surprisingly, the RhÀ C ipso distances of 2.579(2) Å (9) and 2.587(2) Å (11) are significant longer than the IrÀ C ipso distances of 2.381(3) Å (10) and 2.3990(19) Å (12); in view of similar Rh and Ir atomic radii, [28] this finding indicates a significantly stronger arene-iridium interaction. It is also surprising that π-face donation in complexes 9-12 does not involve the borateflanking Dipp substituent, which was consistently found in WCA-NHC complexes, including related rhodium(I) and iridium(I) complexes of the type [(WCA-NHC)M(COD)] (M = Rh, Ir). [15a,b,g,h] Theoretical calculations performed for the iridium complexes 10 and 12 (see details below, Table 3) indicate similar energies of the isomeric complexes in which the metal atom is bound to either one of the Dipp substituents, with a slight preference for the experimentally observed anti-isomer versus the syn-isomer (10: ΔH 298K = À 1.5 kcal mol À 1 ; 12: ΔH 298K = À 2.5 kcal mol À 1 ). Moreover, the intramolecular π-face donation in 9-12 enforces a coplanar orientation of the imidazole ring towards the MÀ EÀ C1 plane with significantly smaller MÀ EÀ C1 angles of 99°-104°compared to the perpendicular arrangement found in the rhodium(III) and iridium(III) half-sandwich complexes 5-8. Perpendicular orientations are also found in related Rh(I) and Ir(I) imidazolin-2-thione complexes such as neutral [{( Me IMe)S}Rh(COD)Cl], [29] and cationic [{( Me IiPr)S} 2 M(COD)] + (M = Rh, Ir; [30] for NHC acronyms see ref. [5] ). These complexes exhibit longer metal-sulfur bonds compared to 2.3073(2) Å in 9 and 2.3165(9)/2.3148(9) Å in 10 (for two independent molecules). As observed for the Rh/Ir pair 9/10, the metal-selenium bonds in 11/12 (2.4328(4)/2.4323(4) Å) are almost identical; to the best of our knowledge, however, no related imidazolin-2-selone complexes have been reported for comparison.
The The molecular structures of all four bimetallic complexes could be established by single-crystal X-ray diffraction analysis; see Figure 5 for the representation of 16 and Figures S12-S14 in the Supporting Information for the molecular structures of 13-15. Overall, the structures resemble that of the bridged NHC-phosphinidenide complex [μ 2 -{(IDipp)P}Rh 2 (COD) 2 (μ 2 -Cl)], [6] with the bridging chalcogen atoms residing in trigonal-  Table 2.  (Table 2).

Computational studies
In order to assess the bonding situation in the thione and selone complexes described above, the structures of the iridium derivatives 6, 8, 10, and 12 were optimized using the density functional theory (DFT) method B97-D, followed by natural bond orbital (NBO) analysis. The computational details are summarized in the Supporting Information, and contour plots of selected NBOs are presented in Tables S16-S19. The calculated structural parameters are in good agreement with the solid-state structures, with the optimized gas-phase geometries consistently showing slightly longer bond lengths (Tables 2 and  3). The Wiberg bond index (WBI) associated with the iridiumchalcogen bonds in 6 (0.88) and 8 (0.90) clearly reveal the presence of single bonds in contrast to the situation established for the related NHC-phosphinidenide complex [{(IDipp)P}IrCl(η 5 -C 5 Me 5 )] (1.32) [7a] and related cationic systems (1.35-1.40) [7b] with a significant degree of phosphorus-metal π-interaction. The negligible degree of chalcogen-metal π-bonding results in significantly smaller charge transfer towards the metal atom as indicated by comparatively small NBO charges q(Ir). Accordingly, two lone pairs can be assigned each to the sulfur and selenium atoms, which show the expected pure p or high s orbital character, respectively. This bonding situation can be best described by polarization of the (WCA-IDipp)E À ligands III (E = S, Se) upon metal coordination as illustrated by the mesomeric form IIIB in Figure 1, in line with WBI(EÀ C NHC ) values close to 1. For complexes 10 and 12, the geometries of both isomers with syn-or anti-orientations of the borate moiety with respect to the iridium atom were optimized, and as mentioned above, the experimentally observed anti-isomer is energetically slightly favored (Table 3). Somewhat weaker iridium-chalcogen bonds can be assigned to these iridium(I) complexes based on the WBI values of 0.75 (10) and 0.79 (12) compared to the iridium(III) species 6 and 8. Again, two lone pairs are located at the chalcogen atoms with high s and pure p orbital character; the latter is perpendicular to the imidazole ring, which is coplanar with the IrÀ EÀ C1 plane to enable π-face donation through the ipso-carbon atom. This bonding scheme agrees with the NBO analyses of the anionic thione and selone ligands III (Figure 1), which reveal three lone pairs located at the chalcogen atoms, one with high s orbital character, while the other two pure p orbitals have similar energies and are available for coordination to the metal atom in either 6/8 or 10/12 (see the Supporting Information). Interestingly, the monometallic COD species 9-12 display a πface donation of one Dipp substituent towards the Rh or Ir atoms in order to saturate their coordination sphere. The bonding schemes of the ligands III were further assessed by computational studies, revealing in all cases distinct metalchalcogen single bonds and two lone pairs located at the chalcogen atoms. This contrasts with the related NHC-phosphinidenide complexes, where substantial metal-phosphorus π bond character was found.

Conclusion
With the lithium complexes 1/2 and the silyl derivatives 3/4, suitable reagents are available for the transfer of the anionic sulfur and selenium ligands III, which we regard as important new additions to the large family of chalcogenolate ligands. [31] The coordination chemistry of these systems with main group elements and transition metals has been extensively studied, [32] with considerable implications for the preparation of complexes that mimic the active sites in metalloproteins. [33] We envisage that the anionic charge will significantly expand the scope of N- heterocyclic thione and selone ligands in coordination chemistry and homogeneous catalysis, [5] which follows the same concept developed for neutral NHCs with the introduction of their anionic WCA-NHC congeners. [14,15a] Experimental Section Materials and Methods: Unless otherwise indicated, all starting materials were obtained from commercial sources (Sigma-Aldrich, Alfa-Aesar, Roth, TCI, VWR or Fisher Chemical) and were used without further purification. Elemental analyses were carried out on a Vario Micro Cube System. All operations with air-and moisturesensitive compounds were performed in a glove box under a dry argon atmosphere (MBraun 200B) or on a high vacuum line using Schlenk techniques. The 1 H, 11 B, 13 C, 19 F and 77 Se NMR spectra were recorded on Bruker AVII300 (300 MHz), Bruker AVIIHD400 (400 MHz), Bruker AVIIHD500 (500 MHz) and Bruker AVII600 (600 MHz). All spectra were recorded at 298 K. The chemical shifts are expressed in parts per million (ppm) with the residual solvent signal as internal standard for 1 H and 13 C NMR spectra. All other spectra were calibrated using external references ( 11 B: BF 3 · OEt 2 ; 19 F: CFCl 3 ; 77 Se: Me 2 Se). Coupling constants (J) are reported in Hertz (Hz) and splitting patterns are indicated as s (singlet), d (doublet), t (triplet), sept (septet), m (multiplet) and br (broad). 11 B, 13 C, 19 F and 77 Se NMR spectra were measured broadband proton decoupled. Signal assignments were performed based on 2D NMR analysis. Presentations of all NMR spectra can be found in the Supporting Information. n-hexane, tetrahydrofuran (THF), diethyl ether (Et 2 O), and toluene were purified by distillation over sodium/benzophenone. Chlorobenzene was purified by distillation over CaH 2 . Deuterated solvents were purified by stirring the degassed solvents with Na/K alloy overnight. Subsequently, the solvents were filtered and then distilled under reduced pressure. All solvents were stored over molecular sieves (4 Å) in argon atmosphere prior to use. All yields were calculated based on the WCA-IDipp containing substrate. [Rh(COD)Cl] 2 , [34] [Ir(COD)Cl] 2 , [35] [(η 5 -C 5 Me 5 )RhCl 2 ] 2 , [36] [(η 5 -C 5 Me 5 )IrCl 2 ] 2 , [37] [{(WCA-IDipp)S}Li(toluene)] [17] and [{(WCA-IDipp)Se} Li(toluene)] [18] were prepared according to literature procedures.
For full crystallographic details, see the Supporting Information. Deposition Number(s) 2122410-2122424 contain(s) the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures.

Synthesis of [{(WCA-IDipp)S}IrCl(η 5 -C 5 Me 5 )] (6):
(WCA-IDipp)SSiMe 3 (20 mg, 0.020 mmol, 1 equiv.) and [(η 5 -C 5 Me 5 )IrCl 2 ] 2 (7.93 mg, 0.010 mmol, 0.5 equiv.) were mixed and dissolved in 2 mL toluene. The mixture turns dark brown within 2 min and was stirred for a total of 15 min at room temperature. The solvent was removed in vacuo and the resulting dark orange-brown solid was washed with 2 × 1 mL Et 2 O and subsequently extracted with 0.2 mL THF, giving a dark brown clear solution. The solvent was removed in vacuo and the residual THF was removed co-evaporation with n-hexane, yielding the product as a dark orange solid (17 mg

Synthesis of [{(WCA-IDipp)Se}RhCl(η 5 -C 5 Me 5 )] (7):
(WCA-IDipp)SeSiMe 3 (20 mg, 0.0199 mmol, 1 equiv.) and [(η 5 -C 5 Me 5 )RhCl 2 ] 2 (5.9 mg, 0.0095 mmol, 0.5 equiv.) were mixed and dissolved in 2 mL toluene. The mixture turns dark brown within 2 min and was stirred for a total of 15 min at room temperature. The solvent was removed in vacuo and the resulting dark brown solid was washed with 2 × 1 mL Et 2 O and subsequently extracted with 0.2 mL THF, giving a dark brown clear solution. The solvent was removed in vacuo and the residual THF was removed co-evaporation with nhexane, yielding the product as a dark brown solid (9 mg, 0.0072 mmol, 36 %  (WCA-IDipp)SeSiMe 3 (20 mg, 0.0199 mmol, 1 equiv.) and [(η 5 -C 5 Me 5 )IrCl 2 ] 2 (7.55 mg, 0.0095 mmol, 0.5 equiv.) were mixed and dissolved in 2 mL toluene. The mixture turns dark brown within 2 min and was stirred for a total of 15 min at room temperature. The solvent was removed in vacuo and the resulting dark orange-brown solid was washed with 2 × 1 mL Et 2 O and subsequently extracted with 0.2 mL THF, giving a dark brown clear solution. The solvent was removed in vacuo and the residual THF was removed co-evaporation with nhexane, yielding the product as a dark orange-brown solid (