Scorpionate Ligands Based on 2-Mercaptopyridine: A Ligand with a

Article pubs.acs.org/Organometallics

Scorpionate Ligands Based on 2-Mercaptopyridine: A Ligand with a Greater Propensity To Sting? Gavin Dyson, Alexander Zech, Benjamin W. Rawe, Mairi F. Haddow, Alexander Hamilton, and Gareth R. Owen*,‡ The School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K. S Supporting Information *

ABSTRACT: The synthesis and characterization of the first platinum group metal complexes of the recently reported ligand [H2B(mp)2]− (where mp = 2-mercaptopyridyl) are presented herein. Addition of 2 equiv of Na[H2B(mp)2] to [MCl(COD)]2 (where M = Rh, Ir; COD = 1,5-cyclooctadiene) leads to the hydride migration products [Rh{κ 3SSB-BH(mp)2}(η 3-C8H13)] and [Ir(H){κ 3-SSB-BH(mp)2}(η 4-C8H12)], respectively. Structural characterization of the rhodium complex reveals a notably short rhodium−boron distance of 2.054(2) Å. The reactivity observed for the rhodium complex is different from that of all known scorpionate ligands, suggesting a higher propensity for hydride migration within the 2-mercaptopyridine-based ligands. The complex [Ir(Cl){κ 3-SSB-BH(mp)2}(η 4-C8H12)], which is formed via hydride/halide exchange in chloroform, is also structurally characterized. The new complexes provide rare examples of metallaboratrane complexes where one hydrogen substituent remains at the boron center.

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following year.7 The greater flexibility of Tm and Bm allows the boron atom to approach the metal center and form metal-toboron dative (σ-acceptor) interactions (Figure 1, right). This novel class of compound was pioneered by Hill in 1999.8 Since this time there has been a rapid development in the field 9 and examples have been established with ligands containing one (κ 2-LB),10,11 two (κ 3-LBL),12,13 or three (κ 4-LLBL)14,15 tethered (supporting) groups containing donor atoms which hold the boron function in close proximity to the metal center (Chart 1). Some early examples of κ 1-B complexes were

INTRODUCTION The bis-substituted ligand dihydrobis(pyrazolyl)borate (Bp), first reported in 1966, is the simplest version of Trofimenko’s ubiquitous scorpionate ligands.1 Following this publication, there have been a vast array of derivative ligands and their subsequent coordination to a range of metals has been extensively investigated.2 An important feature of these ligands is the presence of BH or BH2 units in which one of the B−H groups can point toward the metal center, forming B−H···metal interactions (Figure 1, left).3 Some of these interactions have

Chart 1. Borane σ-Acceptor Complexes Supported by One, Two, or Three Tethered Groups

Figure 1. Reversible hydride migration between boron and metal centers (N−L represents a three-atom bridging heterocycle; R = H, third heterocycle, aryl substituent).

postulated but later disproved. The κ 3-LBL coordination motif is particularly interesting, since both fac12b,13 and mer12 coordination modes have been observed. Of the σ-acceptor borane complexes reported to date, there is only a single compound where one of the substituents remaining at boron is a hydrogen atom. The complex, [Ir(H){κ 3-SBS-BH(mt)2}-

been found to be particularly strong and exhibit significant metal−hydride character.4 In 1996, Reglinski introduced a new generation of more flexible scorpionates,5 hydrotris(2-mercapto1-methylimidazolyl)borate (Tm),6 which contain an additional atom between the donor (in this case sulfur) and the boron atom. The bis-substituted version of this ligand, dihydrobis(2mercapto-1-methylimidazolyl)borate (Bm), was reported the © 2011 American Chemical Society

Received: July 27, 2011 Published: October 19, 2011 5844

dx.doi.org/10.1021/om200694r | Organometallics 2011, 30, 5844−5850

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(CO)(PPh3)] (1;13f mt = 2-mercapto-1-methylimidazolyl), formed upon reaction of Vaska’s complex with Na[H2B(mt)2], is the sole example of hydride migration involving a BH2-based ligand to date.16 During the course of our investigations we have focused on the various factors which govern hydride migration between boron and metal centers (Figure 1).17 For example, hydride migration has shown a dependence on the transition metal accepting the hydride, on its coligands, and whether there are any substituents at the metal center which can be eliminated together with a hydrogen atom (i.e. a reductive elimination step). A subtle balance between all of these factors appears to determine the resulting reactivity. The substituents at boron are also a governing factor, and we therefore wondered how important the tethered group is itself. Accordingly, we recently reported the synthesis of a new family of scorpionate ligands based on 2-mercaptopyridine units, [HB(mp)3]− and [H2B(mp)2]− (Tmp and Bmp, respectively).18 Their coordination to a series of copper complexes was also reported in addition to the first metallaboratrane complexes supported by this heterocycle, [Ru{κ 4-SSBS-B(mp)3}(CO)(PPh3)] (2) and [Ru{κ 4-SSBS-B(mp)(mt)2}(CO)(PPh3)] (3).14b A comparison of 2 and 3 with related literature examples revealed marked differences in the bond distances of this heterocycle to both metal and boron centers, providing shorter metal−sulfur and longer boron−nitrogen distances.14b,18 These differences were attributed to the pyridine-2-thiolate form of this heterocycle (Figure 2). Furthermore, the 2-thiopyridine unit was found to

Chart 2. Rhodium Complexes Where No Hydride Migration Is Observed (R = Ph, Mes, Naphth, 7-Azaindolyl; R′ = H or a Third Heterocycle)

the ligand (Figure 3). The new product (4) was isolated in good yield following standard workup and fully characterized by spectroscopic and analytical methods (see the Experimental Section). The proton-coupled boron NMR experiment of 4 revealed a doublet signal at 7.9 ppm (1JBH = 137 Hz), confirming the presence of only one hydrogen substituent at the boron center (Figure 3). This value is larger than that found in the free ligand (1JB−H = 101 Hz) and is therefore consistent with a hydroborane (N2BH) function rather than borate (N2BH2).24 This observation suggested the transfer of one hydride from the boron to the metal center. The 1H NMR spectrum showed no signals in the region expected for transition metal hydrides. The location of the former borohydride was confirmed by integration of the remaining signals in the region between 1.20 and 4.52 ppm. The total integration of these signals corresponded to 13 protons relative to the former Bmp ligand (8 protons), revealing that the hydride had been incorporated into the former cyclooctadiene ligand. Of particular note were two signals at 3.77 and 4.52 ppm which integrated for two and one protons, respectively. Three protons in this region of the spectrum were indicative of the formation of a η 3-C8H13 allyl species (Scheme 1). The 13C{1H} NMR supported this postulation, revealing two signals at 67.9 and 101.9 ppm, both showing coupling to rhodium (10.0 and 7.0 Hz, respectively). The infrared spectrum of the product showed a characteristic band at 2422 cm−1 for the terminal B−H stretch. Further evidence came from a 1H{11B} NMR experiment, which revealed an additional signal in comparison to the standard proton experiment at 4.25 ppm. This signal integrated for one proton and was assigned as the one remaining hydrogen substituent at boron. Finally, both mass spectrometry and elemental analytical data were consistent with the molecular composition of the product, [Rh{BH(mp)2}(η 3C8H13)]. In order to explore this reactivity further, an analogous reaction was performed with [IrCl(COD)]2. Two equivalents of Na[Bmp] was added to a THF solution of the metal precursor, and the mixture was stirred for 30 min. An orangebrown solid was isolated following a standard workup. The 11B NMR spectrum of this solid, in C6D6, revealed the formation of one major product, 5, as a doublet signal at 14.1 ppm (1JBH = 94 Hz).25,26 This again confirmed the presence of only one hydrogen substituent at the boron center and therefore suggested that hydride migration had also occurred in this case. The 1H NMR spectrum of the solid, however, showed that the new complex was not the analogous iridium allyl

Figure 2. Tautomerization between the 2-thiopyridone and pyridine2-thiolate forms of Bmp.

provide more electron rich complexes, as determined by measuring the carbonyl stretches of complexes 2 and 3 by infrared spectroscopy. We wondered whether these differences would provide systems with enhanced hydride migration properties. Herein, we report the results of our investigations involving the synthesis of the first rhodium and iridium complexes containing the new ligand Bmp.

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RESULTS AND DISCUSSION Over the past few years, several research groups have reported the coordination chemistry of group nine complexes containing ligands which have the potential to support metal−borane complexes. Chart 2 highlights all of the reported rhodium complexes that contain cyclooctadiene as a coligand. Within the nitrogen-based 7-azaindole complexes shown in Chart 2 (top), hydride migration is not observed in any of the complexes where R is an aryl group or a third 7-azaindole unit under ambient conditions.14d,19,20 In the more closely related sulfurbased ligands containing “mt” or “taz” units (Chart 2, bottom), again no hydride migration is observed in any of the five reported examples.14a,c,21−23 Addition of 2 equiv of Na[Bmp]18 to a THF solution of [RhCl(COD)]2 immediately gave a deep red mixture (Scheme 1). The reaction mixture and was left to stir for 20 min at room temperature after which time a 11B{1H} NMR spectrum was recorded. The spectrum showed the complete disappearance of 5845


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Scheme 1. Synthesis of [Rh{κ 3-SBS-BH(mp)2}(η 3-C8H13)] (4)

Figure 3. 11B NMR spectra for Na[H2B(mp)2] in CDCl3 (left) and 4 in MeCN-d3 (right).

complex. Instead, the NMR spectroscopic data for 5 were consistent with the formation of [Ir(H){κ 3-SBS-BH(mp)2}(η 4C8H12)] (Scheme 2). A characteristic signal, which integrated for one proton, was located at −11.09 ppm, confirming the presence of an iridium hydride species. The remaining signals were consistent with the presence of one BH(mp)2 ligand and a η 4-bound COD ligand. Those signals corresponding to the olefinic protons were particularly broad. These observations are consistent with Connelly’s, who recently reported the synthesis of the closely related complexes [Ir(H){B(mt)2(pz)}(C8H12)] and [Ir(H){B(mt)2(pz3,5‑Me)}(C8H12)].13a The molecular composition of 5 was also confirmed by mass spectroscopy and elemental analysis. Complex 5 underwent a further transformation when placed in chloroform solutions (Scheme 2). The 11B NMR spectrum revealed the complete conversion from 5 to a new complex, 6, after ca. 4 h. The new signal, at 8.2 ppm, also appeared as a doublet signal, confirming that the BH group remained. Crystalline solids were obtained from these solutions, and structural characterization of 6 revealed its identity as [Ir(Cl){κ 3-SBS-BH(mp)2}(η 4-C8H12)], where the chloride ligand is trans to the boron (see below for structural details).27,28 Similar transition metal hydride/halide exchange reactions have been observed in previously reported metallaboratrane complexes.14f Structural Characterization of 4 and 6. The identities of 4 and 6 were confirmed by X-ray single-crystal diffraction studies. Crystals suitable for X-ray diffraction were obtained by allowing a saturated hexane solution of 4 and a chloroform solution of 6 to stand overnight. The crystals containing complex 6 contained one molecule of chloroform for each complex within the structure. The molecular structures of the two complexes are presented in Figures 4 and 5. In 4, the rhodium center adopts a distorted structure based on a squarebased-pyramidal geometry (when the central carbon of the allyl

Figure 4. Ortep representations (two perspectives) of [Rh{κ 3-SBSBH(mp)2}(η 3-C8H13)] (4). Hydrogen atoms, with the exception of H1, have been removed for clarity; ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å) and angles (deg): Rh1−B1 = 2.054(2), B1−H1 = 1.07(2), Rh1−S1 = 2.3513(5), Rh1−S2 = 2.3811(6), Rh1−C18 = 2.152(2), Rh1−C12 = 2.178(2), Rh1−C11 = 2.108(2), B1−N1 = 1.598(3), B1−N2 = 1.593(3), C1−S1 = 1.711(2), C6−S2 = 1.716(2); B1−Rh1−S1 = 87.62(7), B1−Rh1−S2 = 83.20(7), S1−Rh1−S2 = 90.53(2), S1−Rh1−C12 = 102.82(6), S2− Rh1−C18 = 95.07(6), N1−B1−N2 = 107.75(16), N2−B1−Rh1 = 108.50(14), N1−B1−Rh1 = 110.60(14).

species is not considered). The former scorpionate ligand binds to the rhodium center with a facial κ 3-SBS coordination mode. The rhodium−boron distance (B1−Rh1 = 2.054(2) Å) is significantly shorter than the sum of the covalent radii of the two atoms29 (∑ r(B−Rh) = 2.26 Å). The ratio between the M−B distance and the sum of the covalent radii of M and B, r, is commonly used as a guide to the degree of interaction between the metal and boron centers.9b In the case of 4, the r value is 0.91 and is the smallest value reported to date. There is only one previously reported complex, [Pd{κ 4-B(mimBut)3}(PMe3)], featuring a dative σ bond which has a similarly short transition metal−boron distance (cf. 2.050(8) Å).15e In this case the r value (r = 0.92; ∑ r(B−Pd) = 2.23 Å) is also noticeably shorter in comparison to those for all known metallaboratrane complexes. The S1−Rh1−B1 and S2−Rh1− B1 angles are 87.62(7) and 83.20(7)°, respectively, showing that the boron atom is positioned with some distortion from the idealized axial position (as shown in Figure 4). The two five-membered rings formed by the κ 3-SBS coordination motif

Scheme 2. Synthesis of [Ir(H){κ 3-SBS-BH(mp)2}(η 4-C8H12)] (5) and [Ir(Cl){κ 3-SBS-BH(mp)2}(η 4-C8H12)] (6)

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Figure 6. [Ru{HB(CH2Ph)(mt)2}(Cl)(PCy3)]. The blue hydrogen atoms indicate the location of the former BH2 hydrogen atoms.

to be 166.13(4)°. This suggests that a mer -κ 3-SSH coordination mode (i.e. an increase in the S−Ru−S angle) would be required for the B−H unit to approach the metal center and form either a σ-borane (η 2-B−H) complex or potentially for the second hydrogen atom to transfer to the metal center. Postulated Mechanism for the Formation of 4. A number of rhodium complexes featuring the η 3-C8H13 fragment have previously been reported.32 On the basis of these observations and the identity of complexes 5 and 6, a proposed mechanism for the formation of [Rh{κ 3-SBS-BH(mp)2}(η 3C8H13)] is envisaged (Scheme 3). By analogy with the similarly reported examples, the nonmigrated borohydride complex [Rh{κ 3-SSH-BH2(mp)2}(η 4-C8H12)] (A) would initially be formed. One hydrogen atom would then be transferred from boron to the rhodium center, thus forming a transient rhodium hydride species analogous to the iridium hydride complex 5. Unfortunately, no evidence for any rhodium hydride species could be obtained, since the starting material underwent conversion to final product very rapidly. While the η 4 coordination motif for the 1,5-cyclooctadiene ligand is observed in 6, a change in coordination mode from η 4 to η 2 is expected in the case of rhodium to enable migratory insertion of the olefin into the rhodium−hydride bond. A series of productive β-hydride eliminations (β-elim) and migratory insertion (MI) steps will occur where the rhodium migrates around the eightmembered ring until the allyl product [Rh{κ 3-SBS-BH(mp)2}(η 3-C8H13)] is obtained.32 If no other processes were involved in the formation of complex 4, then the hydrogen atom that originates from boron would be found at only one site on the eight-membered ring. In order to test this hypothesis, a deuterium-labeling investigation was undertaken. The anionic ligand [D2B(mp)2]− (Bmp-d2) was prepared as the sodium salt from the precursors Na[BD4] and 2-mercaptopyridine via a methodology similar to that previously reported for Na[Bmp].18 The expected molecular ion peak for the Bmp-d2 anion was observed by mass spectrometry. A 1H{11B} NMR experiment confirmed that the deuterium incorporation was greater than 88% (by measuring the integration of the residual protio signal at 3.64 ppm relative to the integration of the 2-mercaptopyridyl ring protons). The lower than expected isotopic purity of Bmp-d2 is presumably due to deuterium/proton exchange as a result of the hydroscopic nature of the borohydride salts. The deuterium-labeled rhodium complex 4-d2 was targeted via a synthetic methodology analogous to that described for 4 above. The deuterium atoms were expected to be located in two positions: one deuterium should remain on the boron atom and the other at only one position of the eight-membered ring. The NMR spectroscopic data, however, were not consistent with this hypothesis. The 1H NMR spectrum of 4-d2 shows that the deuterium atoms were scrambled over a number of positions on the eight-membered ring. Full assignment of the carbon and hydrogen nuclei in their respective NMR spectra was made on

Figure 5. Ortep representation of [Ir(Cl){κ 3-SBS-BH(mp)2}(η 4C8H12)] (6). Hydrogen atoms, with the exception of H1, and solvent have been removed for clarity; ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å) and angles (deg): Ir1−B1 = 2.198(14), Ir1−Cl1 = 2.561(3), B1−H1 = 1.25(15), Ir1−S1 = 2.347(3), Ir1−C7 = 2.172(9), Ir1−C8 = 2.19(2), Ir1−C11 = 2.21(2), B1−N1 = 1.599(12), C1−S1 = 1.719(9); B1−Ir1−S1 = 83.6(3), S1− Ir1−S1′ = 90.25(18), S1−Ir1−Cl1 = 80.43(8), B1−Ir1−Cl1 = 157.3(4), N1−B1−N1′ = 108.3(10), N1−B1−Ir1 = 109.7(6).

are of interest. While the ring involving the atoms S1 and N1 is essentially planar (N1−B1−Rh1−S1 = 4.48(14)°), the ring involving S2 and N2 is twisted with a N2−B1−Rh1−S2 torsion angle of 31.63(13)°. This large twist allows the boron group to closely approach and form a strong interaction with the metal center. The iridium center in 6 adopts a distorted structure based on an octahedral geometry where the boron-based ligand also binds to the metal center with a facial κ 3-SBS coordination mode (Figure 5). The iridium−boron distance (B1−Ir1 = 2.198(14) Å) is longer than the B(1)−Rh(1) distance in 4, indicating flexibility in the binding properties of the ligand. The key difference between the two complexes is the presence of a chloride ligand in 6, while in 4 there is no ligand trans to the boron atom. The larger distance is also concomitant with a significant distortion of the position of the axial ligands. The S1−Ir1−B1 angle is 83.6(3)° while the S1−Ir1−Cl1 angle is 80.43(8)°, indicating that both atoms are significantly distorted from their idealized axial positions (B1−Ir1−Cl1 = 157.3(4)°). For symmetry reasons the two five-membered rings formed upon coordination of the tridentate ligand are identical. Therefore, both rings involving the atoms S1 and N1 are twisted with a N1−B1−Ir1−S1 angle equal to 14.0(7)°. This is a torsion angle between those angles found in 4, again suggesting some flexibility in coordination of the ligand. The orientation of the B−H group is of interest, since there is the potential to form a B−H σ-bond complex.30 A peak attributable to B−H was observed in the difference electron density maps, providing the location of H1 in 4 and 6, suggesting that the B−H bonds therefore point away from the metal center.31 This is consistent with the spectroscopic data presented above, which show none of the characteristics expected for B−H σ-bond complexes30 in addition to the evidence presented for complex 1.13f The S−M−S angles within 1, 4, and 6 are 91.69(4), 90.53(2), and 90.25(18)°, respectively. In a related compound, [Ru{HB(CH2Ph)(mt)2}(Cl)(PCy3)] (Figure 6),4c where one of the hydrogen atoms was found to be at an intermediate point between the ruthenium and boron centers, the S−Ru−S angle was found 5847


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Scheme 3. Postulated Mechanism for the Formation of 4

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the basis of HMQC, COSY, and NOE experiments (see the Experimental Section and the Supporting Information for further details). Integration of the signals in the 1H NMR confirmed that little or no deuterium incorporation into the positions Ha, Hb, Hc′, and He (Figure 7). However, deuterium

CONCLUSIONS In summary, the flexible scorpionate ligand Bmp appears to show an enhanced propensity for hydride migration in comparison to previously reported ligands and is the first to undergo spontaneous migration within rhodium cyclooctadiene systems. Within this transformation a novel rhodium allyl complex is most likely formed via a “chain walking” mechanism. Deuterium-labeling studies suggest that reversible hydride migration between the boron and metal centers occurs during the formation of 4. Hydride migration was also observed within the corresponding iridium complex. In this case, the hydrogen atom remained at the metal center and did not undergo further reactivity with the cyclooctadiene coligand. The iridium hydride complex was unstable in chloroform solutions and underwent hydride/halide exchange, forming the corresponding iridium chloride complex.

Figure 7. Labeling scheme for the complexes 4 and 4-d2.

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was incorporated into the positions Hd on the ring, as evidenced by a reduction in the integration of these signals. Unfortunately, the signals corresponding to Hc and He′ were overlapping in the spectrum and therefore it was not possible to determine whether deuterium had been incorporated into one or both of these positions. Overall a reduction in the total integration of the signals Hd, Hc, and He′ was observed. The 13 C{1H} NMR spectrum of 5-d2 was also consistent with a mixture of isotopic isomers where the deuterium atoms are bound to the carbon atoms Cc, Cd, and Ce. The corresponding signal in the 11B NMR spectrum appeared as a doublet, suggesting the presence of hydrogen at the boron center. The presence of hydrogen was further confirmed in the 1H{11B} NMR spectrum, which revealed a signal at 4.22 ppm, providing an integration corresponding to 68% hydrogen incorporation (cf. with only 12% in the starting ligand). The deuterium/ hydrogen exchange at boron suggests that any transient rhodium hydride (or rhodium deuteride) species formed during the reaction can undergo reversible migration back to the boron center. Furthermore, the scrambling of deuterium within the eight-membered ring also suggests that reversible hydride migration between boron and metal centers occurs at a faster rate than migratory insertion of the olefin into the rhodium hydride species. The NMR spectroscopic data of the isolated product 4-d2 show that the isotope isomeric ratio remains unchanged over time and low-temperature NMR experiments did not reveal the presence of any rhodium hydride species. This evidence shows that once 4 (or 4-d2) is formed, the reverse reaction is unlikely. Investigations are currently under way to further explore the mechanisms involved in the formation of complex 4.

EXPERIMENTAL SECTION

General Remarks. All manipulations were performed using standard Schlenk techniques. THF and diethyl ether were dried using a Grubbs alumina system and kept in Young ampules under N2 over molecular sieves (4 Å). CDCl3 was degassed by three freeze− thaw cycles and stored in a Young ampule over 4 Å molecular sieves under N2. 1H NMR, 1H{11B} NMR, 11B{1H} NMR, and 11B NMR spectra were recorded on a JEOL Lambda 300 spectrometer operating at 300 MHz (1H). 13C{1H} NMR spectra and correlation experiments were recorded on a Varian VNMR S400 spectrometer operating at 400 MHz (1H). The spectra were referenced internally, to the residual protic solvent (1H) or the signals of the solvent (13C). 11B{1H} NMR and 11B NMR spectra were referenced externally relative to BF3·OEt2. Mass spectra were recorded on a VG Analytic Quattro in ESI + mode. Elemental analyses were performed at the microanalytical laboratory of the School of Chemistry at the University of Bristol. Infrared spectra were recorded on a Perkin-Elmer Spectrum 100 FTIR spectrometer (solid state, neat) from 4000 to 650 cm−1. Synthesis of Rh{κ 3-SBS-BH(mp)2}(η 3-C8H13) (4). A Schlenk was charged with Na[H2B(mp)2]18 (69.5 mg, 2.69 × 10−4 mol) and [RhCl(COD)]233 (66.0 mg, 1.35 × 10−4 mol), THF (25 mL) was added, giving a yellow solution that turned red and was stirred for 20 min. The volatiles were removed under vacuum to give a red-orange solid which was extracted into a 3:1 mixture of diethyl ether and DCM (2 × 15 mL) and filtered and the solvent removed under reduced pressure. The resulting solid was washed with cold hexane (5 mL) and finally dried under reduced pressure to give 4 as an orange solid. Yield: 103 mg, 2.32 × 10−4 mol, 86%. NMR (δ, CDCl3): 1H NMR (see Figure 7 for labeling scheme) 1.20−1.42 (3H, m, He′, 2 × Hc′), 1.51 (4H, m, 4 × Hd), 1.84 (1H, m, He), 2.16 (2H, m, 2 × Hc), 3.77 (2H, dt, 3JHH = 8.4 Hz, 3JHH = 8.4 Hz, Hb), 4.52 (1H, td, 3JHH = 7.3 Hz, 4JHH = 2.0 Hz, Ha), 6.61 (2H, τ, JHH = 6.6 Hz, mpCH ), 7.16 (2H, τ, 3JHH = 7.8 Hz, mpCH), 7.52 (2 overlapping signals, 4H, m, 2 × mpCH); 1 H{11B} NMR 4.25 (1H, s, BH ); 13C{1H} NMR 23.4 (Ce), 28.8 (Cd), 30.2 (Cc), 67.9 (d, 1JRhC = 10.0 Hz, Cb), 101.9 (d, 1JRhC = 7.0 Hz, Ca), 5848


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114.3 (mpC H-5), 129.7 (mpC H-3), 134.6 (mpCH-4), 140.7 (mpCH-6), 179.9 (CS); 11B{1H} NMR 8.0 (s, hhw = 80 Hz); 11B, NMR 8.0 (d, 1 JBH = 132 Hz). MS (ESI): m/z 860 [Rh2{BH(mp)2}2(C8H13)]+ 100%. IR (powder film, cm−1): 2422 w (B−H), 1604 s, 1535 s (pyridine). Anal. Found (calcd) for C18H22N2BRhS2: C, 48.91 (48.67); H, 5.23 (4.99); N, 5.90 (6.31). Synthesis of Ir(H){κ 3-SBS-BH(mp)2}(η 4-C8H12) (5). To a THF (15 mL) solution of [IrCl(COD)]234 (80.6 mg, 1.20 × 10−4 mol) was added a THF (10 mL) solution of Na[H2B(mp)2]18 (61 mg, 2.39 × 10−4 mol), and the mixture was stirred for 30 min. The THF was then removed under vacuum and the resulting orange solid extracted in DCM (10 mL) and filtered through Celite. All volatiles were subsequently removed from the filtrate under vacuum to provide 5 as an orange solid.25,26 The solid was washed with hexane (10 mL) and finally dried under reduced pressure. Yield: 95 mg, 1.78 × 10−4 mol, 74%. NMR (δ, C6D6): 1H NMR −11.09 (1 H, s, IrH ) 2.09 (2H, br m, COD CH2), 2.21 (2H, br m, CODCH2), 2.35 (2H, br m, CODCH2), 2.67 (2H, br m, CODCH2), 4.22 (2H, v. br, CODCH trans to boron), 4.86 (2H, br, CODCH trans to sulfur), 5.78 (2H, td, JHH = 6.6 Hz, 4JHH = 1.3 Hz, mpCH -5), 6.15 (2H, ddd, 3JHH = 8.4 Hz, 3JHH = 6.8 Hz, 4JHH = 1.6 Hz, mpCH -4), 7.23 (2H, d, 3JHH = 8.4 Hz, mpCH -3), 7.61 (2H, d, 3JHH = 6.8 mpCH -6); 1H{11B} NMR 4.14 (1H, s, BH , overlapping with broad signal at 4.22 ppm); 13C{1H} NMR 26.1 (CODCH2), 29.9 (CODCH2), 53.0 (v br signal tentatively assigned as CODCH), 114.8 (mpC H-5), 130.7 (mpCH-3), 134.7 (mpCH-4), 143.9 (mpCH-6), CS not observed; 11B{1H} NMR (C6D6, 96.2 MHz) δ 14.1 (s, hhw = 145 Hz). 11 1 B{ H} NMR (C6D6, 96.2 MHz): δ 14.1 (d, JB−H = 94 Hz). MS (ESI): m/z 533 [Ir(H){BH(mp)2}(C8H12) − H]+ 100%, 425 [5 − COD]+ 60%. IR (powder film, cm−1): 2366 w (B−H), 2160 m (Ir− H), 1602 s, 1533 s (pyridine). IR (THF, cm−1): 2349 m (B−H), 2120 ms (Ir−H), 1608 s, 1541 s (pyridine). Anal. Found (calcd) for C18H22N2BIrS2: C, 40.36 (40.52); H, 4.38 (4.16); N, 5.00 (5.25). Synthesis of Na[D2B(mp)2]. A 200 mL Schlenk flask was charged with NaBD4 (250 mg, 6.00 mmol) and mercaptopyridine (1.13 g, 12.0 mmol). A 3:1 toluene−tetrahydrofuran mixture (50 mL) was added, and this mixture was heated to reflux with stirring overnight. The resulting mixture was filtered, and the solid was washed with toluene (3 × 20 mL) and dried under vacuum to give a yellow powder. Yield: 1.08 g, 4.18 × 10−3 mol, 70%. IR (cm−1, powder film): 1834 m (B− D). 1717 (B−D), 1605 s, 1534 s, 1528 s, 1446 s, 1405 s (CN and CC); weak bands at 2438 w, 2370 w which correspond to B−H were also present in the spectrum. NMR (MeCN-d3): 1H NMR 6.45 (2H, dt, J = 6.8 Hz, 4JHH = 1.7 Hz, mpCH -(5)), 7.03 (2H, ddd, 3JHH = 8.5 and 6.8 Hz, 4JHH = 1.8 Hz, mpCH -(4)), 7.26 (2H, ddd, 3JHH = 8.5 Hz, 4JHH = 1.5 Hz, 5JHH = 0.7 Hz, mpCH -(3)), 8.47 (2H, ddd, 3JHH = 6.8, 4JHH = 1.8 Hz, 5JHH = 0.7 Hz, mpCH-(6)); 1H{11B} NMR 3.64 (residual BH2 approximately 12%); 11B{1H} NMR −1.4 (hhw = 110 Hz). 11B NMR (CD3CN, 96.2 MHz): δ −1.4 (br, no multiplicity observed, hhw = 135 Hz). MS (ESI): m/z = 235 [M]− 100%. Synthesis of Rh{κ 3-SBS-B(H)1−n(D)n(mp)2}{η 3-C8H13−n(D)n} (4d2). The synthesis of 4-d2 was carried out using a methodology analogous to that for complex 4 using the precursors Na[D2B(mp)2] (105 mg, 4.07 × 10−4 mol) and [RhCl(COD)]233 (100 mg, 2.03 × 10−4 mol). Yield: 113 mg, 2.53 × 10−4 mol, 62%. NMR (δ, CD2Cl2): 1 H NMR (see Figure 7 for numbering scheme) 1.23−1.39 (3H, m, H e′, 2 × Hc′, percentage deuterium incorporation measured by integration, 22%), 1.52 (4H, m, 4 × Hd, percentage deuterium 10%), 1.85 (1H, m, He), 2.16 (2H, m, 2 × Hc), 3.71 (2H, dd, 3JHH = 8.6 Hz, 3JHH = 8.2 Hz, Hb), 4.52 (1H, τ, 3JHH = 7.3 Hz, Ha), 6.61 (2H, ddd, 3JHH = 6.4 Hz, 3 JHH = 6.9 Hz, 4JHH = 1.3 Hz, mpCH-5), 7.16 (2H, td, JHH = 6.9 Hz, 4 JHH = 1.7 Hz mpCH -4), 7.42 (2H, m, mpCH-3), 7.53 (br 2H, mpCH6); 1H{11B} NMR 4.22 (1H, s, BH , percentage hydrogen 68%); 13 C{1H} NMR 23.7 (m, Ce), 29.4 (m, Cd), 30.7 (m, Cc), 67.9 (m, Cb), 102.3 (m, Ca), 114.6 (mpCH-5), 129.7 (d, 3JRhC = 2.3 Hz, mpCH-3), 135.2 (mpCH-4), 141.5 (mpC H-6), 177.4 (CS); 11B{1H} NMR 8.0 (s, hhw = 88 Hz); 11B NMR 8.0 (d, 1JBH = 120 Hz). MS (ESI): m/z 469 [Rh{BH(mp)2}(C8H13) + Na]+ 100%. IR (powder film, cm−1): 2912 s (C−H), 2423 m (B−H), 1806 (B−D), 1603 s, 1537 s, 1458, 1411 (CN and CC).

ASSOCIATED CONTENT S Supporting Information * Figures and text giving spectroscopic data and computational details and CIF files and tables giving crystallographic data for 4 and 6. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *[email protected]. Notes ‡ G.R.O. is a Royal Society Dorothy Hodgkin Research Fellow.

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ACKNOWLEDGMENTS We thank the Royal Society (G.R.O.) and Leverhulme Trust (G.D.) and EPSRC (A.H.) for funding and Johnson Matthey for the loan of the group 9 salts. We thank Dr. N. Fey for her helpful discussions regarding this work.

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REFERENCES

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Mercy, M.; Miqueu, K.; Ladeira, S.; Saffon, N.; Maron, L.; Bouhadir, G.; Bourissou, D. Inorg. Chem. 2010, 49, 3983. (12) Examples of the meridional κ 3-LBL (pincer type) coordination mode: (a) Owen, G. R.; Gould, P. H.; Hamilton, A.; Tsoureas, N. Dalton Trans. 2010, 39, 49. (b) Bontemps, S.; Sircoglou, M.; Bouhadir, G.; Puschmann, H.; Howard, J. A. K.; Dyer, P. W.; Bourissou, D. Chem. Eur. J. 2008, 14, 731. (c) Sircoglou, M.; Bontemps, S.; Mercy, M.; Saffon, N.; Takahashi, M.; Bouhadir, G.; Maron, L.; Bourissou, D. Angew. Chem., Int. Ed. 2007, 46, 8583. (13) Selected recent examples of the facial κ 3-LBL coordination mode: (a) López-Gómez, M. J.; Connelly, N. G.; Haddow, M. F.; Hamilton, A.; Lusi, M.; Baisch, U.; Orpen, A. G. Dalton Trans. 2011, 40, 4647. (b) Tsoureas, N.; Kuo, Y.-Y.; Haddow, M. F.; Owen, G. R. Chem. Commun. 2011, 47, 484. (c) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2010, 29, 326. (d) Tsoureas, N.; Haddow, M. F.; Hamilton, A.; Owen, G. R. Chem. Commun. 2009, 2538. (e) Bontemps, S.; Gornitzka, H.; Bouhadir, G.; Miqueu, K.; Bourissou, D. Angew. Chem., Int. Ed. 2006, 45, 1611. (f) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2005, 24, 1062. (g) Conifer, C. M.; Law, D. J.; Sunley, G. J.; White, A. J. P.; Britovsek, G. J. P. Organometallics 2011, 30, 4060. (14) Selected recent examples of the κ 4-LLBL coordination mode where the L groups occupy three sites of a square plane: (a) LópezGómez, M. J.; Connelly, N. G.; Haddow, M. F.; Hamilton, A.; Orpen, A. G. Dalton Trans. 2010, 39, 5221. (b) Owen, G. R.; Gould, H. P.; Charmant, J. P. H.; Hamilton, A.; Saithong, S. Dalton Trans. 2010, 39, 392. (c) Blagg, R. J.; Adams, C. J.; Charmant, J. P. H.; Connelly, N. G.; Haddow, M. F.; Hamilton, A.; Knight, J.; Orpen, A. G.; Ridgway, B. M. Dalton Trans. 2009, 40, 8724. (d) Tsoureas, N.; Bevis, T.; Butts, C. P.; Hamilton, A.; Owen, G. R. Organometallics 2009, 28, 5222. (e) Crossley, I. R.; Hill, A. F. Dalton Trans. 2008, 201. (f) Crossely, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2006, 25, 289. (15) Selected recent examples of the κ 4-LLBL coordination mode where the three L groups occupy the sites of a trigonal plane: (a) Moret, M.-E.; Peters, J. C. Angew. Chem., Int. Ed. 2011, 50, 2063. (b) Nuss, G.; Saischek, G.; Harum, B. N.; Volpe, M.; Gatterer, K.; Belaj, F.; Moesch-Zanetti, N. C. Inorg. Chem. 2011, 50, 1991. (c) Sircoglou, M.; Bontemps, S.; Bouhadir, G.; Saffon, N.; Miqueu, K.; Gu, W.; Mercy, M.; Chen, C.-H.; Foxman, B. M.; Maron, L.; Ozerov, O. V.; Bourissou, D. J. Am. Chem. Soc. 2008, 130, 16729. (d) Pang, K.; Tanski, J. M.; Parkin, G. Chem. Commun. 2008, 1008. (e) Pang, K.; Quan, S. M.; Parkin, G. Chem. Commun. 2006, 5015. (16) We have previously reported a “sting” on Grubbs' catalyst where a hydride is transferred from H2B(mt)2 to the benzylidene unit of Grubbs’ first-generation catalyst; however, this undergoes subsequent rearrangement: Rudolf, G. C.; Hamilton, A.; Orpen, A. G.; Owen, G. R. Chem. Commun. 2009, 553. (17) Interesting examples involving the migration of methyl and phenyl groups from boron to the metal center have been reported by Vedernikov: (a) Khaskin, E.; Zavalij, P. Y.; Vedernikov, A. N. Angew. Chem., Int. Ed. 2007, 46, 6309. (b) Khaskin, E; Zavalij, P. Y.; Vedernikov, A. N. J. Am. Chem. Soc. 2008, 130, 10088. (18) Dyson, G.; Hamilton, A.; Mitchell, B.; Owen, G. R. Dalton Trans. 2009, 6120. (19) (a) Owen, G. R.; Tsoureas, N.; Hamilton, A.; Orpen, A. G. Dalton Trans. 2008, 6039. (b) Owen, G. R.; Tsoureas, N.; Hope, R. F.; Kuo, Y.-Y.; Haddow, M. F. Dalton Trans. 2011, 40, 5906. (20) The utilization of 2,5-norbornadiene in place of 1,5-cyclooctadiene in the rhodium complexes does result in a hydride migration to form a rhodium nortricyclyl species. This was attributed to the greater strain associated with the bicyclic diene.14d (21) (a) Crossley, I. R.; Hill, A. F.; Humphrey, E. R.; Smith, M. K. Organometallics 2006, 25, 2242. (b) Blagg, R. J.; Connelly, N. G.; Haddow, M. F.; Hamilton, A.; Lusi, M.; Orpen, A. G.; Ridgway, B. M. Dalton Trans. 2010, 39, 11616. (22) Hill reported the formation of [Rh{κ 4-SSBS-B(mt)3}(η 4C8H12)]Cl upon reaction of [RhCl(η 4-C8H12)]2 with 2 equiv of Na[HB(mt)3] in DCM for 8 h. The source of chloride in this reaction is unclear, as is whether hydride migration precedes hydride/halide

exchange: Crossley, I. R.; Hill, A. F.; Humphrey, E. R.; Willis, A. C. Organometallics 2005, 24, 4083. See also ref 14a. (23) Connelly also reported an example, involving 2,5-norbornadiene, where hydride migration is likely to have occurred. 21b (24) (a) Heřmánek, S. Chem. Rev. 1992, 92, 325. (b) Bonnier, C.; Piers, W. E.; Parvez, M. Organometallics 2011, 30, 1067. (25) The boron NMR spectrum of the isolated solid revealed two side products that could not be separated from the main product 5. The purity of 5 was estimated to be >90% within the mixture. Complex 5 was found to unstable in solution and slowly precipitated a darker insoluble solid. (26) When the analogous reaction was carried out with Na[Bmp-d2] in THF-d6, the integration of the iridium hydride signal, in the 1H NMR spectrum, suggested an approximate 14% hydrogen atom incorporation at this site. This is consistent with the isotopic purity of the starting material (cf. 12%), suggesting that there are no deuterium/ hydrogen exchange reactions occurring in this complex. Furthermore, there was no significant change in the relative integration of this signal over time. This evidence suggests that the diene moiety does not undergo migratory insertion into the iridium hydride bond in this case. (27) Selected data for 6 are as follows. NMR (CDCl3): 1H NMR 1.87 (4H, m, CODCH2), 2.69 (2H, m, CODCH2), 3.02 (2H, m, COD CH2), 3.78 (2H, m, CODCH), 4.52 (2H, m, CODCH), 6.81 (2H, td, JHH = 6.7 Hz, 4JHH = 1.3 Hz, mpCH-5), 7.24 (overlapping with residual CHCl3 signal, 2H, ddd, 3JHH = 8.4 Hz, 3JHH = 6.9 Hz, 4JHH = 1.5 Hz, mp CH -4), 7.42 (2H, ddd, 3JHH = 8.4 Hz, 4JHH and 5JHH unresolved, mp CH -3), 8.14 (2H, d, 3JHH = 6.3 Hz, mpCH -6); 11B{1H} NMR 8.2 (hhw = 180 Hz); 11B NMR 8.2 (d, 1JBH = 108 Hz). (28) During the conversion of 5 to 6 in CDCl3, a signal corresponding to CHDCl2 also appeared in the 1H NMR spectrum. Signals corresponding to uncoordinated COD were also observed. (29) Cordero, B.; Gómez, V.; Platero-Prats, A. E.; Revés, M.; Echeverria,́ J.; Cremades, E.; Barragán, F.; Alvarez, S. Dalton Trans. 2008, 2832. (30) Selected recent examples and reviews: (a) Dallanegra, R.; Robertson, A. P. M.; Chaplin, A. B.; Manners, I.; Weller, A. S. Chem. Commun. 2011, 47, 3762. (b) Hesp, K. D.; Kannemann, F. O.; Rankin, M. A.; McDonald, R.; Ferguson, M. J.; Stradiotto, M. Inorg. Chem. 2011, 50, 2413. (c) Braunschweig, H.; Kraft, K.; Kupfer, T.; Siedler, E. Z. Anorg. Allg. Chem. 2010, 636, 2565. (d) Esteruelas, M. A.; FernádezAlvarez, F. A.; López, A. M.; Mora, M.; Oňate, E. J. Am. Chem. Soc. 2010, 132, 5600. (e) Alcaraz, G.; Sabo-Etienne, S. Coord. Chem. Rev. 2008, 252, 2395. (f) Perutz, R. N.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2007, 46, 2578. (g) Hartwig, J. F.; Cook, K. S.; Hapke, M.; Incarvito, C. D.; Fan, Y.; Webster, C. E.; Hall, M. B. J. Am. Chem. Soc. 2005, 127, 2538. (31) The locations of H1 were found in the difference map and their positions were allowed to refine freely with thermal parameters limited to 1.2 times that of the boron atom. (32) (a) Downing, S. P.; Pogorzelec, P. J.; Danopoulos, A. A.; ColeHamilton, D. J. Eur. J. Inorg. Chem. 2009, 1816. (b) Alekseev, L. S.; Safronov, A. V.; Dolgushin, F. M.; Korlyukov, A. A.; Godovikov, I. A.; Chizhevsky, I. T. J. Organomet. Chem. 2009, 694, 1727. (c) Alekseev, L. S.; Dolgushin, F. M.; Korlyukov, A. A.; Godovikov, I. A.; Vorontsov, E. V.; Chizhevsky, I. T. Organometallics 2007, 26, 3868. (d) Hodson, B. E.; McGrath, T. D.; Stone, F. G. A. Organometallics 2005, 24, 1638. (e) El Maila, R.; Garralda, M. A.; Hernándeza, R.; Ibarlucea, L.; Pinilla, E.; Torres, M. R. Helv. Chim. Acta 2002, 85, 1485. (f) Teixidor, F.; Flores, M. A.; Viňas, C.; Sillanpäa,̈ R.; Kivekäs, R. J. Am. Chem. Soc. 2000, 122, 1963. (g) Gassner, F.; Dinjus, E.; Görls, H.; Leitner, W. Organometallics 1996, 15, 2078. (33) Herde, J. L.; Lambert, J. C.; Senoff, C. V. Inorg. Synth. 1974, 15, 18. (34) Giordano, G.; Crabtree, R. H. Inorg. Synth. 1990, 28, 88.

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Scorpionate Ligands Based on 2-Mercaptopyridine: A Ligand with a

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