Article pubs.acs.org/Organometallics
Utilizing the 8‑Methoxycyclooct-4-en-1-ide Unit As a Hydrogen Atom Acceptor en Route to “Metal−Borane Pincers” Alexander Zech, Mairi F. Haddow, Hafiizah Othman, 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 palladium and platinum complexes containing the neutral ligand HB(mp)2 (where mp = 2-mercaptopyridyl) are presented. Addition of 2 equiv of Na[H2B(mp)2] to [M(Cl)(COEOMe)]2 (where M = Pt, Pd; COEOMe = 8-methoxycyclooct-4-en-1-ide) in the presence of 2 equiv of PPh3 leads to the formation of the metal−borane pincer complexes [Pt{κ3SBS-HB(mp)2}(PPh3)] and [Pd{κ3SBS-HB(mp)2}(PPh3)]. In these reactions, a hydrogen migration reaction occurs from the borohydride ligand to the metal center, eventually leading to the elimination of the COEOMe unit from the metal center. X-ray crystallographic characterization of the two isostructural complexes reveals a rare mer-κ3S,B,S coordination mode with short platinum− and palladium−boron distances: 2.098(4) and 2.091(3) Å, respectively (the shorter distances of two independent complexes in the unit cells of both structures). The complexes [Pd{κ3S,B,S-HB(mp)2}(PPh3)] and [Pt{κ3S,B,S-HB(mp)2}(PPh3)] are the first examples of metal−borane complexes featuring a pincer-type coordination where one hydrogen substituent remains at the boron center.
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hydrogen atom can remain at the metal center as a “hydride” species, in which case there is a potential for it to be transferred back. Alternatively, an irreversible transformation can occur where the hydrogen atom is somehow “used up”. This provides a driving force for the hydride migration in some instances. Indeed, several strategies have been employed utilizing “hydrogen atom acceptor groups” which react with the former borohydride species and remove the hydrogen.1a In an attempt to explore new synthetic routes to metal−borane complexes, we investigated the 8-methoxycyclooct-4-en-1-ide unit as a potential hydrogen atom acceptor. The results of our investigations are outlined below. Within the many examples of metal−borane complexes, there are surprisingly only two reports of hydrogen migration from a BH2 unit. In 2005, Hill reported the first iridium-based metal−borane complexes.4 The reported complex, [Ir(H){κ3S,B,S-BH(mt)2}(CO)(PPh3)] (1; where mt = 2-mercapto-1methylimidazolyl), was formed upon reaction of Vaska’s complex with Na[H2B(mt)2]. More recently, we reported the synthesis of [Rh{κ3S,B,S-BH(mp)2}(η3-C8H13)] (2) and [Ir(H){κ3S,B,S-BH(mp)2}(η4-C8H12)] (3; mp = 2-mercaptopyridyl) as the only other examples of metal−borane complexes derived from a bis-substituted borohydride ligand.5,6 In each of the group 9 complexes 1−3, the κ3S,B,S ligand coordinates to the metal center with a facial coordination mode (Chart 1). There are no examples of related group 10 complexes to date
INTRODUCTION Transition-metal−borane complexes, those which feature Zclass (σ-acceptor) interactions between the metal and borane functions, have generated considerable interest over the past decade or so.1−3 They are often termed “metallaboratranes”, where the borane group has been tethered to the metal center by three supporting groups. A wide range of structural motifs has been developed by several research groups.1 While the strength of the metal−borane interaction is expected to be weak (there are no authenticated examples of κ1B complexes), the borane coordination certainly has a significant impact on the properties of the metal center. For some time now, we have been interested in the hydrogen atom storage properties of borane functional groups and reversible transformations between borane and borohydride functionalities (Scheme 1). There are many examples where migration of hydrogen from boron to the metal center has been observed. In fact, this is one of the most common synthetic routes to metal−borane complexes. Upon transfer, the Scheme 1. Concept of Reversible Hydride Migration between Boron and Metal Centersa
a
NL represents a three-atom bridging heterocycle. R = H, third heterocycle, or other carbon-based substituent. © 2012 American Chemical Society
Received: May 31, 2012 Published: September 21, 2012 6753
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experiment did not reveal any 1JBH coupling. The new product (4) was isolated in excellent yield following standard workup and fully characterized by spectroscopic and other analytical methods (see the Experimental Section). The 1H NMR spectrum of 4 showed no signals in the region expected for transition-metal hydrides, and all signals corresponding to the COEOMe unit were absent. The spectrum confirmed the presence of the former Bmp ligand and triphenylphosphine in a 1:1 ratio. The signal corresponding to the BH group was not observable in the standard proton experiment. It was, however, possible to observe this signal in the 1H{11B} experiment as a broad resonance at 6.68 ppm. This signal also exhibited platinum satellites with a 1JPtH coupling constant of 117 Hz. To the best of our knowledge, there are no known examples of a nitrogen-substituted borane species (either primary or secondary) featuring B−H···Pt interactions. Furthermore, there are no spectroscopic data available (i.e., chemical shift of hydrogen and Pt−H coupling constant) for any borane species adjacent to a platinum center.14 The broadness of this signal makes its location in the 1H NMR spectrum very challenging, even upon decoupling of the quadrupolar nucleus.15 Nevertheless, the small coupling constant observed and downfield chemical shift of this signal suggests that the hydrogen substituent is directed away from the platinum center (vide infra). The coordination of the phosphorus ligand to the platinum center was confirmed by 31 1 P{ H} NMR spectroscopy, which showed a single resonance at 30.3 ppm with platinum satellites (1JPtP = 1540 Hz). The infrared spectrum of 4 showed a characteristic band at 2318 cm−1 in the region expected for a terminal B−H stretch. Finally, both mass spectrometry and elemental analytical data were consistent with the molecular composition of the product, [Pt{κ3S,B,S-BH(mp)2}(PPh3)]. The same methodology was utilized with the metal precursor [PdCl(COEOMe)]2,13b,16 and the analogous complex [Pd{κ3S,B,S-BH(mp)2}(PPh3)] (5) was obtained. In this case, the signal in the 11B{1H} NMR was significantly sharper than that found for 4 and the signal at 12.5 ppm also revealed coupling to the phosphorus ligand trans to the boron atom (d, 2 JPB = 59 Hz, hhw = 180 Hz). While further coupling was apparent in the corresponding proton-coupled experiment, the signal was unfortunately too broad to be resolved. A small quantity of another product (generally 95% relative to any other eight-membered-ring product) was clearly identified as
disappearance of the ligand precursor and the appearance of a new broad signal at 6.7 ppm with platinum satellites (1JPtB = 507 Hz). The conversion of A to complex 4 then took place over extended periods of time (up to 48 h). This is in contrast to the standard reaction, under higher dilution, which went to completion within 1 h. The NMR-scale reaction provided a fairly resolved 1H NMR spectrum in which the major species, complex A, was apparent.17 The spectrum revealed five broad signals in the aromatic region integrating for a total of eight protons (in a ratio of 2:2:2:1:1) corresponding to the 2mercaptopyridine protons of the Bmp ligand in addition to signals corresponding to triphenylphosphine. Confirmation that the triphenylphosphine remained uncoordinated was obtained from the corresponding 31P{1H} NMR experiment. The presence of a new set of signals corresponding to COEOMe was also apparent in the spectrum (by comparison with the precursor and other related complexes).13 No signals were observed in the high-field region of the spectrum, confirming that no platinum−hydride species are formed and that the two hydrogen substituents on the Bmp ligand remain at boron. On the basis of this evidence, species A was tentatively assigned as [Pt{κ3S,S,H-H2B(mp)2}{COEOMe}].19−21 In the reaction to form metal−borane complexes 4 and 5, the COEOMe unit acts as a hydrogen atom acceptor and is eliminated from the coordination sphere of the complex. There are a number of ways in which this can be achieved, either from a species such as A in the presence of triphenylphosphine or from phosphine complex such as B (Scheme 3). In all cases, transfer of one of the hydrogen substituents from the scorpionate ligand to the metal center to form an intermediate metal−hydride species (and new metal−borane interaction) is expected, as indicated in Scheme 1. It was originally envisaged that the COEOMe unit would act directly as a hydrogen atom acceptor to form the eliminated organic species 8-methoxycyclooct-4-ene. This would proceed via pathway a and would involve a C−H reductive elimination step of the newly formed metal−hydride and the alkyl unit of the COEOMe ligand. However, two alternative pathways (b and c) are also possible. It has previously been reported that [M(COE OMe )Cl]2 complexes (M = Pt, Pd) also serve as convenient routes to metal−hydride complexes. Goel and Clarke showed that adding 6755
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Figure 1. Crystal structures of isostructural complexes [Pt{κ3S,B,S-BH(mp)2}(PPh3)] (4) and [Pd{κ3S,B,S-BH(mp)2}(PPh3)] (5) (left and middle) and [Pd{κ4S,B,S,S-B(mp)3}(PPh3)] (6) (right). Hydrogen atoms, with the exception of H(1b) hydrogen atoms (which were located in the difference map), and solvents of crystallization have been removed for clarity. Selected bond distances (Å) and angles (deg) for 6: Pd(1)−B(1) = 2.065(3), Pd(1)−S(1) = 2.4196(7), Pd(1)−S(2) = 2.4389(7), Pd(1)−S(3) = 2.4516(8), Pd(1)−P(1) = 2.4119(7), B(1)−N(1) = 1.566(4), B(1)−N(2) = 1.567(4), B(1)−N(3) = 1.600(3), C(1)−S(1) = 1.710(3), C(6)−S(2) = 1.705(3), C(11)−S(3) = 1.701(3); B(1)−Pd(1)−S(1) = 79.69(9), B(1)− Pd(1)−S(2) = 78.52(9), B(1)−Pd(1)−S(3) = 79.88(9), S(1)−Pd(1)−P(1) = 103.20(3), S(2)−Pd(1)−P(1) = 102.41(2), S(1)−Pd(1)−S(2) = 96.03(3), S(1)−Pd(1)−S(2) = 119.34(3), S(2)−Pd(1)−S(3) = 112.77(3), S(3)−Pd(1)−S(1) = 117.89(3), B(1)−Pd(1)−P(1) = 175.81(9), N(1)−B(1)−N(2) = 108.7(2), N(2)−B(1)−N(3) = 106.1(2), N(3)−B(1)−N(1) = 109.1(2); N(1)−B(1)−Pd(1)−S(1) = 36.2(2), N(2)−B(1)− Pd(1)−S(2) = 37.9(2), N(3)−B(1)−Pd(1)−S(3) = 36.2(2).
recently reported similar findings where the transformation of a “BH(mt)2” unit into “B(mt)3” has been observed.30 In their article, they suggest a plausible mechanism involving exchange of substituents between two boron centers. This is certainly possible, given the reported disproportionation reactions involving substituent exchange of various differently substituted boranes.31 In contrast, no conversion of 4 to the corresponding complex [Pt{B(mp)3}(PPh3)] was observed after prolonged periods of time in solution or in the presence of 2mercaptopyridine. Structural Characterization of 4−6. The identities of 4− 6 were confirmed by X-ray single-crystal diffraction studies. Crystals suitable for X-ray diffraction were obtained by layering saturated DCM solutions of 4 and 5 with hexane. Single crystals of 6 were obtained by leaving solutions of 5 to stand for periods longer than 48 h. The crystalline materials of complexes 4 and 5 both contained two independent molecules of the complex and partial occupancy of DCM in the asymmetric unit. The molecular structures of the three complexes are presented in Figure 1. Complexes 4 and 5 are isostructural (and their crystals are isomorphous) (Table 1). The metal centers adopt highly distorted square planar geometries with cis inter ligand angles in the range 79.83(12)−100.37(3)° for 4 and 79.03(8)− 101.67(2)° for 5. The S−M−B1 angles range between 79.03(8) and 85.02(12)°, indicating that the sulfur atoms are significantly distorted from their idealized square-planar positions. The angles S(1)−Pt(1)−S(2) and S(1)−Pd(1)−S(2) are 161.14(3) and 158.88(2)°, respectively. While these angles confirm a significant distortion from the idealized geometry, the corresponding B(1)−Pt(1)−P(1) and B(1)−Pd(1)−P(1) angles are closer to the expected values (cf. 176.75(12)° for 4 and 177.17(8)° for 5). The boron centers are approximately tetrahedral with non-hydrogen angles in the range 109.0(2)−
1,5-cyclooctadiene (2.38 (8H) and 5.59 ppm (4H)), and small quantities of MeOH were also observed in the spectrum. This confirmed that for platinum the reaction proceeds via pathway c, where the methoxide anion acts as the hydrogen atom acceptor. In order to support these findings, we carried out further investigations on these complexes utilizing the labeled ligand Na[D2B(mp)2].5 In these reactions we found no evidence of deuterium incorporation into the “cyclooctyl” unit. Furthermore, reactions were performed in sealed Young NMR tubes and no indications of the formation of HD (or H2) were apparent by 1H NMR spectroscopy. The corresponding palladium reactions follow a similar pathway in which 1,5cyclooctadiene and methanol are the major organic species found in the reaction mixture.27 Transformation of [Pd{BH(mp)2}(PPh3)] into [Pd{B(mp)3}(PPh3)]. Complex 5 underwent a further transformation when left in DCM for extended periods of time. Small quantities of a dark crystalline solid were obtained from various solutions over time. Structural characterization of the material revealed its identity as [Pd{κ4S,B,S,S-B(mp)3}(PPh3)] (6), where the hydrogen substituent on boron had been replaced by a third 2-mercaptopyridyl unit (see below for structural details). We initially thought that 6 could be formed in the presence of small quantities of 2-mercaptopyridine, suggesting a dehydrocoupling reaction between the B−H group and the NH group of 2-mercaptopyridyl.28 Indeed, upon addition of 1 equiv of this heterocycle to the mixture, the signal corresponding to 5 was significantly reduced in the 11B NMR spectrum. While a major species at 17.5 ppm (which we have tentatively assigned as 6) was apparent, the mixture was found to contain several other species by 11B and 31P NMR spectroscopy. Unfortunately, we were unable to separate complex 6 from its contaminants, apart from small quantities of crystalline material.29 Crossley has very 6756
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M−B bond (the torsion angle S(1)−M(1)−B(1)−N(1) is −3.4(3)° in 4 and 0.73(17)° in 5), the rings involving S2 and N2 are highly twisted, forming an envelope motif, with S(2)− M(1)−B(1)−N(2) angles of 40.1(2)° (4) and −40.95(16)° (5). This morphology, where one ring is coplanar and the other is twisted, is typical of metal−borane complexes where the supporting group is a six-membered heterocycle (see below for details). The Pt(1)−P(1) and Pd(1)−P(1) distances in 4 and 5 are 2.3838(9) and 2.4424(7) Å, respectively. There is a significant difference between the palladium and platinum distances, even though their respective covalent radii are similar (cf. 1.39 and 1.36 Å).34 Nevertheless, all of these distances are significantly longer than those for the majority of the compounds deposited in the Cambridge Structural Database (CSD).35 While the Pt(1)−P(1) distance in 4 is particularly long, it is still shorter than that of the related platinum complex [Pt{κ4S,B,S,S-B(mt)3}(PPh3)H]Cl (8).8a In this complex, the corresponding distance is significantly larger (2.4626(9) Å) and is the second largest in the CSD to date. The Pt−PPh3 distance in 4 is more comparable to that found in [Pt{κ3S,B,SB(mt)2(mp)}(PPh3)] (9), one of only two previously reported pincer-type metal−borane complexes reported to date.8c,33 This complex features a hybrid ligand containing both “mt” and “mp” rings, one ring of each coordinates to the platinum center. The Pt−PPh3 distance in 9 is 2.3797(5) Å. Complexes 4 and 5 share a number of structural features similar to those of 9. For example, the S−Pt−B−N torsion angles found in 9 are 35.7(1) and −8.3(1)° involving the “mp” and “mt” rings, respectively. Furthermore, the Pt−P distances found in 4 and 9, in comparison to that in 8, highlight the different binding properties of the phosphorus ligand between the two oxidation states of platinum. Intuitively, the higher oxidation state complex would be expected to bind the phosphorus ligand more strongly; however, the reverse is true.36 The significant trans influence of the borane unit is further exemplified in the palladium complex 5. A survey of the CSD reveals that, out of the 1051 Pd−PPh3 distances reported in the database, there is only one example which is larger than that found in this complex.37 The palladium center in 6 adopts a distorted structure based on a trigonal-bipyramidal geometry where the boron-based ligand binds to the metal center with a κ4S,S,B,S coordination mode (Figure 1). In this five-coordinate complex, the Pd(1)− B(1) and Pd(1)−P(1) distances are 2.065(3) and 2.4119(7) Å, respectively. Both are shorter than those found in 5 (cf. 2.091(3) and 2.4424(7) Å). Again, this observation is perhaps counterintuitive, since the addition of a further donor ligand to the palladium center (i.e. transformation from a four-coordinate to a five-coordinate complex) would be expected to weaken the bond strength of the ligands already coordinated to the metal center. However, while the two ligand distances (Pd−B and Pd−P) on the axial sites of the newly formed trigonalbipyramidal complex decrease, the three Pd−S distances become significantly longer relative to those found in 5 (cf. Pd(1)−S(1) = 2.4196(7) Å, Pd(1)−S(2) = 2.4389(7) Å, and Pd(1)−S(3) = 2.4516(8) Å in 6 with Pd(1)−S(1) = 2.2924(7) Å and Pd(1)−S(2) = 2.3303(7) Å in 5). The three sulfur atoms of the boron-based ligand occupy the equatorial positions with S−Pd−S angles of 119.34(3), 117.89(3), and 112.77(3)°, confirming some distortion from the idealized geometry. The structures of metallaboratrane complexes based on trigonal-pyramidal and trigonal-bipyramidal geometries have interesting motifs, and several different parameters have been
Table 1. Comparison of Structural Data for Complexes 4 and 5 bond distance (Å)/angle (deg)a,b M(1)−B(1) M(1)−S(1) M(1)−S(2) M(1)−P(1) B(1)−N(1) B(1)−N(2) C(1)−S(1) C(6)−S(2) B(1)−M(1)−S(1) B(1)-M(1)-S(2) S(1)−M(1)−P(1) S(2)−M(1)−P(1) S(1)−M(1)−S(2) B(1)−M(1)−P(1) N(1)−B(1)−N(2) N(1)−B(1)−M(1) N(2)−B(1)−M(1) ∑ of angles at borond S(1)−M(1)−B(1)− N(1) S(2)−M(1)−B(1)− N(2)
4b,c
5b
2.098(4)/2.104(4) 2.2751(9)/ 2.274(4)c 2.3040(9)/ 2.3001(10) 2.3838(9)/ 2.3812(10) 1.579(5)/1.580(5) 1.596(5)/1.593(5) 1.730(4)/ 1.723(5)c 1.733(4)/1.734(4) 85.02(12)/ 85.01(14)c 79.83(12)/ 80.44(11) 95.87(3)/ 95.91(12)c 99.88(3)/ 100.37(3) 161.14(3)/ 156.8(4)c 176.75(12)/ 174.32(12) 111.6(3)/112.1(3) 113.7(3)/113.5(2) 109.0(2)/109.2(2) 334.3/334.8 −3.4(3)/2.8(5)c
2.091(3)/2.094(3) 2.2924(7)/2.2893(7)
111.2(2)/111.5(2) 114.23(18)/114.84(17) 109.35(17)/109.45(17) 334.78/335.79 0.73(17)/−4.03(18)
40.1(2)/38.7(2)
−40.95(16)/−39.09(16)
2.3303(7)/2.3293(7) 2.4424(7)/2.4373(7) 1.576(3)/1.573(3) 1.587(4)/1.586(4) 1.721(3)/1.719(3) 1.723(3)/1.728(3) 84.30(8)/83.91(8) 79.03(8)/79.98(8) 96.61(2)/96.26(2) 100.67(2)/101.67(2) 158.88(2)/155.38(3) 177.17(8)/174.07(9)
a
M = Pt (4), Pd (5). bThere are two independent molecules in the asymmetric unit of both structures 4 and 5. The equivalent bond distances and angles are also provided in the table as the second values. c There is disorder in the position of S(3), and the values for S(3a) are given in the table. The corresponding values for S(3b) are as follows: Pd(2)−S(3b) = 2.294(10) Å, S(3b)−C(30) = 1.757(14) Å, B(2)− Pt(2)−S(3b) = 84.4(4)°, S(3b)−Pt(2)−P(2) = 95.2(3)°, S(3b)− Pt(2)−S(4) = 164.2(3)°, S(3b)−Pt(2)−B(2)−N(3) = −11.1(15)°. d Involving non-hydrogen substituents.
113.7(3)° for 4 and 109.35(17)−114.84(17)° for 5. The former scorpionate ligand binds to the platinum and palladium centers with a κ3S,B,S-pincer type coordination mode. Pincertype complexes have been of great interest for many years now. However, those featuring S,B,S motifs are still very rare.8c,32,33 The platinum−boron distances in 4, 2.098(4) and 2.104(4) Å, are shorter than those of previously reported platinum−borane complexes8,9 and shorter than the sum of the covalent radii of the two elements (∑r(B−Pt) = 2.20 Å).34 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.1c In the case of 4, the r value is 0.95. The palladium−boron distances in 5 are 2.091(3) and 2.094(3) Å (∑r(B−Pd) = 2.23 Å), and the r value is 0.94. A previously reported complex, [Pd{κ4-B(mimBut)3}(PMe3)] (7) (mimBut = 2-mercapto-1-tert-butylimidazolyl), has a shorter distance (cf. 2.050(8) Å)7 (see also discussion on complex 6 below). The two five-membered rings formed by the κ3S,B,S coordination motif are of particular interest. While the rings involving the atoms S1 and N1 are essentially coplanar with the 6757
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for B−H σ bonds, and the structural evidence is consistent with no such interaction in the case of 4 and 5.40
used to describe them. The molecular structure of 6 reveals a pseudo-3-fold symmetry where the supporting groups (“mp” heterocycles) are twisted by 36.8° (defined by the average torsion angle S−Pd−B−N).38 In the related sulfur-based complex [Pd{κ4-B(mimBut)3}(PMe3)] (7), there is little twisting of the heterocycles and the average torsion angle is only 7.5°. Meanwhile, in the phosphorus complex [Pt{κ4B(cp)3}] system (the supporting unit cp is shown in Figure 2,
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CONCLUSIONS In summary, the first group 10 complexes to contain the flexible scorpionate ligand Bmp have been reported, providing rare examples of both “metal-borane pincers” and Z-class borane complexes where one hydrogen remains at the boron center. Their syntheses have been achieved via a new methodology utilizing the 8-methoxycyclooct-4-en-1-ide unit as a hydrogen atom acceptor. While the product 8-methoxycyclooct-4-ene was initially envisaged, it appears that the methoxide is transferred back to the metal center and it is this moiety which accepts the hydrogen atom. The palladium complex 5 undergoes further reactivity at the B−H site, while the platinum complex 4 does not. The fact that the B(mt)2(mp) ligand adopts a κ3S,B,S coordination mode in 9 rather than κ3S,B,S,S may be an important factor. The implications of the different reactivities between the palladium and platinum centers and the potential reactivity of the B(H)−metal function is currently under investigation.
Figure 2. Comparison of steric properties of supporting units based on five- and six-membered heterocycles (the other substituents at boron at boron have been omitted for clarity).
middle) the twisting is 28.3°.9b Similar observations are found in other transition-metal complexes where cycles based on sixmembered heterocycles have significantly larger torsion angles than those based on five-membered heterocycles. A further parameter of interest is the degree of pyramidalization of the boron center.1c In 6, the sum of the three N−B−N angles at boron equals 323.9°, showing significant pyramidalization, and is consistent with the short Pd−B distance of 2.065(3) Å (cf. the corresponding values 321.1° and 2.050(8) Å in 7).7 These distances are also noticeably shorter in comparison to those for all known metal−borane complexes. A larger twist would be expected to provide the shorter palladium−boron distances; however, this is not the case. It appears that the palladium− boron distances are essentially the same in both complexes (6 and 7); the most significant difference in the structures is the position of the palladium center relative to the equatorial plane (defined by the three sulfur atoms). In 6, the metal center is located approximately 0.450 Å below the plane, whereas in 7, the palladium center is 0.159 Å below the plane defined by the sulfur atoms. Clearly there are several factors that will determine the structure of these compounds. Note that 6 and 7 contain phosphine ligands with significantly different steric and electronic properties. This and other comparisons nevertheless suggest that the ring size of the supporting group and element (i.e. N or C) attached to boron is important in controlling the structure of the complex and nature of the metal−boron interaction. This, of course, will become increasingly important when fine-tuning and targeting specific properties and reactivity of this important class of compounds. The steric factors of the supporting heterocycles also appear to be particularly important in the bis-substituted ligand systems. The neutral BH(mp)2 ligand coordinates to the metal centers via a mer-κ3S,B,S (pincer) coordination mode in the group 10 complexes reported herein. In contrast, the same ligand adopts a facial coordination mode in the previously reported group 9 complexes.5 This fine balance between the two ligand conformations (fac and mer) has previously been shown by Bourissou, Dyer, and Miqueu in the diphosphine− borane ligands featuring the “cp” unit.9a It has previously been postulated that the orientation of the B−H group might change significantly upon increasing the S−M−S angle of BH(mp)2 and perhaps form a B−H σ bond complex.39 The spectroscopic data presented above show none of the characteristics expected
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EXPERIMENTAL SECTION
General Remarks. All manipulations were performed using standard Schlenk techniques. DCM and hexane 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). In the assignment of the proton NMR spectra the symbol τ is used to represent an apparent triplet in cases where a doublet of doublets signal is expected. 13C{1H} NMR spectra and correlation experiments were recorded on a Varian VNMR S400 instrument operating at 400 MHz (1H). The spectra were referenced internally, to the residual protic solvent (1H) or the signals of the solvent (13C). 11 1 B{ H} NMR and 11B NMR spectra were referenced externally relative to BF3·OEt2. Mass spectra were recorded on a VG Analytic Quattro instrument 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 Pt{κ3S,B,S-BH(mp)2}(PPh3) (4). A Schlenk flask was charged with [PtCl(COEOMe)]213 (100.0 mg, 1.35 × 10−4 mol) and DCM (30 mL). The resulting solution was cooled to 0 °C, after which PPh3 (70.9 mg, 2.70 × 10−4 mol) was added. To this mixture was subsequently added Na[H2B(mp)2]10 (69.3 mg, 2.70 × 10−4 mol). The mixture was stirred for 1 h and warmed to room temperature. The mixture was filtered, the volume of solvent was reduced to 5 mL, and hexane (15 mL) was added. The resulting solid was washed with cold hexane (5 mL) and finally dried under reduced pressure to give 4 as an orange solid. Yield: 155 mg, 2.24 × 10−4 mol, 83%. NMR (δ, CDCl3): 1 H, 6.75 (τd, JHH = 6.6 Hz, 4JHH = 1.4 Hz, 2H, mpCH), 7.31 (overlapping with next signal, τd, JHH = 8.5 Hz, 4JHH = 1.4 Hz, 2H, mp CH), 7.34−7.42 (m, 9H, m/p-PC6H5), 7.49 (ddd, 3JHH = 8.5 Hz, 4 JHH = unresolved, 5JHH = unresolved, 2H, mpCH), 7.52−7.61 (m, 6H, o-PC6H5), 7.96 (br. d, 3JHH = 6.1 Hz, 2H, mpCH); 1H{11B}, 6.68 (br. s, BH, 1JPtH = 117 Hz); 13C{1H}, 114.9 (s, mpCH), 128.1 (d, 3JCP = 9.2 Hz, m-PC6H5), 129.4 (s, p-PC6H5), 129.8 (s, mpCH), 134.0 (d, 2JCP = 13.8 Hz, o-PC6H5), 134.9 (d, 1JCP = 31.2 Hz, i-PC6H5), 135.8 (s, mp CH), 142.2 (s, mpCH), 178.2 (d, 3JCP = 19.6 Hz, CS); 11B{1H}, 15.2 (s, 1JPtB unresolved, hhw = 450 Hz); 11B, 15.2 (observed as a singlet, hhw = 480 Hz); 31P{1H}, 30.3 (1JPtP = 1540 Hz, hhw = 160 Hz). MS (ESI)+: m/z 678.08 [4 − BH]+ 100%, 646.11 [Pt(PPh3)(mp)(mp-S)]+ 80%; no peaks corresponding to the molecular ion 6758
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Organometallics were observed. IR (powder film, cm−1): 3054 w, 2318 w (B−H), 1602 s, 1541 s (pyridine). Anal. Found (calcd) for C28H24BN2PPtS2: C, 48.60 (48.77); H, 3.51 (3.51); N, 4.32 (4.06). Synthesis of Pd{κ3S,B,S-BH(mp)2}(PPh3) (5). A Schlenk flask was charged with [PdCl(COEOMe)]216 (100.0 mg, 1.78 × 10−4 mol) and DCM (15 mL). The resulting solution was cooled to 0 °C, after which PPh3 (93.4 mg, 3.54 × 10−4 mol) was added. To this mixture was subsequently added Na[H2B(mp)2]10 (91.2 mg, 3.54 × 10−4 mol). The mixture was stirred for 15 min and warmed to room temperature. The mixture was filtered, the volume of solvent was reduced to 5 mL, and hexane (15 mL) was added. The resulting solid was washed with cold hexane (5 mL) and finally dried under reduced pressure to give 5 as an orange solid. Yield: 121 mg, 2.01 × 10−4 mol, 57%. NMR (δ, CD2Cl2): 6.80 (τd, JHH = 6.6 Hz, 4JHH = 1.1 Hz, 2H, mpCH), 7.37 (overlapping with next signal, τd, JHH = 8.3 Hz, 4JHH = unresolved, 2H, mp CH), 7.38−7.42 (m, 9H, m/p-PC6H5), 7.47−7.53 (two overlapping signals, 2H, mpCH and 6H, o-PC6H5), 7.75 (br d, 3JHH = 6.1 Hz, 2H, mp CH); 1H{11B}, 5.82 (1H, br. s, BH); 13C{1H}, 115.2 (s, mpCH), 128.9 (d, 3JCP = 8.6 Hz, m-PC6H5), 129.9 (s, p-PC6H5), 130.7 (s, mp CH), 134.5 (d, 2JCP = 14.8 Hz, o-PC6H5), 135.7 (d, 1JCP = 19.5 Hz, iPC6H5), 136.7 (s, mpCH), 141.8 (s, mpCH), 179.3 (tentatively assigned, unresolved d, CS); 11B{1H}, 12.5 (d, 2JPB = 59 Hz, hhw = 180 Hz); 11 B, 12.5 (dd unresolved, hhw = 280 Hz); 31P{1H}, 11.2 (hhw = 160 Hz). MS (ESI)+: m/z 589.02 [5 − BH]+ 100%, 557.18 [Pd(PPh3)(mp)(mp-S)]+ 90%; no peaks corresponding to the molecular ion were observed. IR (powder film, cm−1): 3054 w, 2353 w (B−H), 1603 s, 1543 s (pyridine). Anal. Found (calcd) for C28H24BN2PPdS2: C, 55.29 (55.97); H, 4.43 (4.03); N, 4.84 (4.66). Crystallography. X-ray diffraction experiments on 4−6 were carried out at 100 K on a Bruker APEX II diffractometer using Mo Kα radiation (λ = 0.710 73 Å). Data collections were performed using a CCD area detector from a single crystal mounted on a glass fiber. Intensities were integrated41 from several series of exposures measuring 0.5° in ω or ϕ. Absorption corrections were based on equivalent reflections using SADABS.42 The structures were solved using SHELXS and refined against all Fo2 data with hydrogen atoms riding in calculated positions using SHELXL,43 except for those attached to boron (H(1b) or H(2b) in complexes 4 and 5), which were found in the difference map and their positions allowed to refine freely with thermal parameters limited to 1.2 times that of the boron atom. Crystal structure and refinement data are given in Table S1 in the Supporting Information. One of the complexes in the asymmetric unit of 4 displayed disorder in one of the phenyl rings of the triphenylphosphine ligand and one of the sulfur atoms of the BH(mp)2 ligand. The atoms C40−C45 (phenyl) were modeled as sitting over two positions with the occupancy ratio 0.58:0.42. The two phenyl groups were constrained to adopt an ideal six-membered-ring geometry. The atom S3 also displayed disorder and was modeled sitting over two positions with the occupancy ratio 0.77:0.23. Two complexes per asymmetric unit were observed in both structures 4 and 5 along with partial occupancy of DCM solvent (20% in 4 and 14% in 5).
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ACKNOWLEDGMENTS
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REFERENCES
We thank the Royal Society (G.R.O.) for funding and Johnson Matthey for the loan of the group 10 salts.
(1) (a) Owen, G. R. Chem. Soc. Rev. 2012, 41, 3535. (b) Braunschweig, H.; Dewhurst, R. D. Dalton Trans. 2011, 40, 549. (c) Amgoune, A.; Bourissou, D. Chem. Commun. 2011, 47, 859. (d) Bouhadir, G.; Amgoune, A.; Bourissou, D. Adv. Organomet. Chem. 2010, 58, 1. (e) Owen, G. R. Transition Met. Chem. 2010, 35, 221. (f) van der Vulgt, J. I. Angew. Chem., Int. Ed. 2010, 49, 252. (g) Braunschweig, H.; Dewhurst, R. D.; Schneider, A. Chem. Rev. 2010, 110, 3924. (h) Kuzu, I.; Krummenacher, I.; Meyer, J; Armbruster, F.; Breher, F. Dalton Trans. 2008, 5836. (i) Fontaine, F.-G.; Boudreau, J.; Thibault, M.-H. Eur. J. Inorg. Chem. 2008, 5439. (2) (a) Hill, A. F. Organometallics 2006, 25, 4741. (b) Parkin, G. Organometallics 2006, 25, 4744. (3) Hill, A. F.; Owen, G. R.; White, A. J. P.; Williams, D. J. Angew. Chem., Int. Ed. 1999, 38, 2759. (4) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2005, 24, 1062. (5) Dyson, G; Zech, A.; Rawe, B. W.; Haddow, M. F.; Hamilton, A.; Owen, G. R. Organometallics 2011, 30, 5844. (6) 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. (7) Pang, K.; Quan, S. M.; Parkin, G. Chem. Commun. 2006, 5015. (8) (a) Crossely, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2008, 27, 312. (b) Crossely, I. R.; Hill, A. F. Organometallics 2004, 23, 5656. (c) Owen, G. R.; Gould, P. H.; Hamilton, A.; Tsoureas, N. Dalton Trans. 2010, 39, 49. (9) (a) Bontemps, S.; Sircoglou, M.; Bouhadir, G.; Puschmann, H.; Howard, J. A. K.; Dyer, P. W.; Bourissou, D. Chem. Eur. J. 2008, 14, 731. (b) 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. (c) Bontemps, S.; Bouhadir, G.; Gu, W.; Mercy, M.; Chen, C.-H.; Foxman, B. M.; Maron, L.; Ozerov, O. V.; Bourissou, D. Angew. Chem., Int. Ed. 2008, 47, 1481. (10) Dyson, G.; Hamilton, A.; Mitchell, B.; Owen, G. R. Dalton Trans. 2009, 6120. (11) Owen, G. R.; Gould, H. P.; Charmant, J. P. H.; Hamilton, A.; Saithong, S. Dalton Trans. 2010, 39, 392. (12) A number of related complexes which undergo dehydrochlorination have previously been reported, many of which form stable compounds.8b,c The presence of two hydrogen substituents at boron appears to reduce the stability of the resulting complexes significantly (see also ref 30 for details). (13) (a) Chatt, J.; Vallarino, L. M.; Venanzi, L. M. J. Chem. Soc. 1957, 2496. (b) O, W. W. N.; Lough, A. J.; Morris, R. H. Organometallics 2010, 29, 570. (c) Boyer, J. L.; Cundari, T. R.; DeYonker, N. J.; Rauchfuss, T. B.; Wilson, S. R. Inorg. Chem. 2009, 48, 638. (d) Ghavale, N.; Dey, S.; Jain, V. K.; Tewari, R. Bull. Mater. Sci. 2009, 32, 15. (14) The most closely related examples come from a series of carborane compounds reported by Stone: (a) Franken, A.; McGrath, T. D.; Stone, F. G. A. Organometallics 2010, 29, 4790. (b) Carr, N.; Mullica, D. F.; Sappenfield, E. L.; Stone, F. G. A.; Went, M. J. Organometallics 1993, 12, 4350. (15) Braunschweig, H.; Matz, F.; Radacki, K.; Schneider, A. Organometallics 2010, 29, 3457. (16) (a) Chatt, J.; Vallarino, L. M.; Venanzi, L. N. J. Chem. Soc. 1957, 3413. (b) Bailey, C. T.; Lisensky, G. C. J. Chem. Educ. 1985, 62, 896. (c) Bianchini, C.; Meli, A.; Oberhauser, W.; Segarra, A. M.; Passaglia, E.; Lamač, M.; Štěpnička, P. Eur. J. Inorg. Chem. 2008, 441. (d) Hoel, G. R.; Stockland, R. A., Jr.; Anderson, G. K.; Ladipo, F. T.; Braddock-
ASSOCIATED CONTENT
S Supporting Information *
CIF files and a table giving crystallographic data for 4−6, including collection parameters and refinement details. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. † GRO is a Royal Society Dorothy Hodgkin Research Fellow. 6759
dx.doi.org/10.1021/om300482m | Organometallics 2012, 31, 6753−6760
Organometallics
Article
Wilking, J.; Rath, N. P.; Mareque-Rivas, J. C. Organometallics 1998, 17, 1155. (17) Selected data for mixture containing species A + uncoordinated PPh3: NMR (δ, CD2Cl2): 1H, 1.83 (m, 2H, COEOMe), 2.14 (m, 3H, COEOMe), 2.15 (m, 1H, 1JPtH = 73 Hz, PtCHCH(OMe)), 2.26 (m, 1H, COEOMe), 3.01 (m, 1H, COEOMe), 3.17 (br. s, 3H, OMe), 3.31 (m, 1H, COEOMe), 3.58 (m, 1H, COEOMe), 5.54 [m, 1H, C(H) C(H)], 5.77 (m, 1H, C(H)C(H)), 6.67 (m, 2H, mpCH), 7.21 (m, 2H, mpCH), 7.33 (m, 15H, PPh3), 7.49 (m, 2H, mpCH), 7.99 (m, 1H, mp CH), 8.26 (m, 1H, mpCH); 11B{1H}, 6.7 (s, 1JPtB = 507 Hz); 31 1 P{ H}, −5.1 (uncoordinated PPh3). (18) In the case of [PdCl(COEOMe)]2, when the reaction was carried out in the absence of PPh3, the mixture appeared to decompose almost immediately following the addition of Na[Bmp]. (19) The observations that the reactions are concentration dependent and the fact that no Pt−H coupling is apparent for the alkene protons of the COEOMe ligand in A mean that dinuclear species which bridge via the sulfur donors cannot be ruled out. (20) It was also found that the order in which the reactants were added to the mixture had no bearing on the final product. Addition of 2 equiv of triphenylphosphine to [PtCl(COEOMe)]2 led to the formation of 2 equiv of [PtCl(COEOMe)(PPh3)], as determined by 31 1 P{ H} NMR spectroscopy (δ 24.6 ppm, 1JPtP = 4326 Hz). Addition of Na[H2B(mp)2] to the mixture led to the elimination of the phosphorus ligand from the complex and the rapid formation of species A. (21) Signals corresponding to 1,5-cyclooctadiene began to appear in the 1H NMR spectrum of A over time, suggesting that the β-methoxy elimination pathway also occurs from complex A. However, since the resulting species is unstable and decomposes, it was not possible to confirm this. (22) (a) Goel, A. B.; Goel, S. Inorg. Chim. Acta 1980, 45, L85. (b) Clarke, H. C.; Goel, A. B.; Goel, S. J. Organomet. Chem. 1981, 216, C25. (c) Li, H.; Grasa, G. A.; Colacot, T. J. Org. Lett. 2010, 12, 3332. (23) (a) Soulie, J.; Chottard, J.-C.; Mansuy, D. J. Organomet. Chem. 1979, 171, 113. (b) Goel, A. B.; Goel, S. Inorg. Chem. Acta 1983, 77, L5. The complex [PtCl(COEOMe)(PPh3)] does not undergo βhydride elimination in solution; see footnote 20. (24) (a) Solé, D.; Serrano, O. Angew. Chem., Int. Ed. 2007, 46, 7270. (b) Scarborough, C. C.; Stahl, S. S. Org. Lett. 2006, 8, 3251. (c) Brice, J. L.; Meerdink, J. E.; Stahl, S. S. Org. Lett. 2004, 6, 1845. (d) Vitagliano, A.; Paiaro, G. J. Organomet. Chem. 1973, 49, C49. (25) (a) Miura, T.; Sasaki, T.; Harumashi, T.; Murakami, M. J. Am. Chem. Soc. 2006, 128, 2516. (b) Miura, T.; Shimada, M.; Murakami, M. J. Am. Chem. Soc. 2005, 127, 1094. (c) Zhang, H.; Fu, X.; Chen, J.; Wang, E.; Liu, Y.; Li, Y. J. Org. Chem. 2009, 74, 9351. (26) Foreman, M. R. St.-J.; Hill, A. F.; Owen, G. R.; White, A. J. P.; Williams, D. J. Organometallics 2003, 22, 4446. (27) While the reactions to form the platinum complex 4 proceeded slowly in concentrated solutions (25 mM), the reactions to form the corresponding palladium complex 5 went to completion within 1 h at similar concentrations. (28) For recent reviews concerning dehydrocoupling reactivity of borane (BH) and amine (NH) functions see: (a) Less, R. J.; Melen, R. L.; Wright, D. S. RSC Adv. 2012, 2191. (b) Sewell, L. J.; Lloyd-Jones, G. C.; Weller, A. S. J. Am. Chem. Soc. 2012, 134, 3598. (c) Staubitz, A.; Robertson, A. P. M.; Manners, I. Chem. Rev. 2010, 110, 4079. (29) Elemental analysis of crystals of complex 6 is as follows. Anal. Found (calcd) for C33H27BN3PPdS3: C, 55.46 (55.83); H, 3.98 (3.83); N, 5.98 (5.92); S, 13.46 (13.55). (30) Crossley, I. R.; Hayes, J. J. Organomet. Chem. 2012, 716, 285. (31) (a) Fuller, A.-M.; Hughes, D. L.; Lancaster, S. J.; White, C. M. Organometallics 2010, 29, 2194. (b) Frohn, H.-J.; Franke, H.; Fritzen, P.; Bardin, V. V. J. Organomet. Chem. 2000, 598, 127. (c) Piers, W. Adv. Organomet. Chem. 2004, 52, 1. (32) For general references concerning pincer complexes see: (a) Morales-Morales, D. Jensen, C. M. The Chemistry of Pincer Compounds; Elsevier: Amsterdam, 2007. (b) Selander, N.; Szabó, K. J. Chem. Rev. 2011, 111, 2048. For the only other structurally
characterized examples of an S,B,S pincer complex see: (c) Spokoyny, A. M.; Reuter, M. G.; Stern, C. L.; Ratner, M. A.; Seideman, T.; Mirkin, C. A. J. Am. Chem. Soc. 2009, 131, 9482. (33) A square-planar gold(I) complex featuring a κ3P,B,P pincer ligand has previously been reported; see: Sircoglou, M.; Bontemps, S.; Mercy, M.; Saffon, N.; Takahashi, M.; Bouhadir, G.; Maron, L.; Bourissou, D. Angew. Chem., Int. Ed. 2007, 46, 8583. (34) Cordero, B.; Gómez, V.; Platero-Prats, A. E.; Revés, M.; Echeverrı ́a, J.; Cremades, E.; Barragán, F.; Alvarez, S. Dalton Trans. 2008, 2832. (35) A search of the Cambridge Structural Database revealed 1341 examples of Pt−PPh3 bonds. There were 19 examples where the bond distances were greater than those found in 4. This includes the bond distance found in [Pt{κ4S,B,S,S-B(mt)3}(PPh3)H]Cl, where the phosphorus ligand is also trans to boron.8a The distances in 14 structurally characterized examples where a PPh3 ligand is trans to a Pt−boryl group range from 2.3370 to 2.3950 Å. (36) A possible explanation for this observation can be made in terms of the stabilization of certain geometries at the metal center.1e (37) Mizuta, T.; Tanaka, N.; Iwakuni, Y.; Kubo, K.; Miyoshi, K. Organometallics 2009, 28, 2808. (38) Foreman, M. R. St.-J.; Hill, A. F.; White, A. J. P.; Williams, D. J. Organometallics 2003, 22, 3831. (39) Selected recent examples and reviews: (a) Dallanegra, R.; Robertson, A. P. M.; Chaplin, A. B.; Manners, I.; Weller, A. S. Chem. Commun. 2011, 47, 3763. (b) Hesp, K. D.; Kannemann, F. O.; Rankin, M. A.; McDonald, R.; Ferguson, M. J.; Stradiotto, M Inorg. Chem. 2011, 50, 2431. (c) Braunschweig, H.; Kraft, K.; Kupfer, T.; Siedler, E. Z. Anorg. Allg. Chem. 2010, 636, 2565. (d) Esteruelas, M. A.; Fernández-Alvarez, 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. (40) A peak attributable to B−H was observed in the difference electron density maps indicating the location of H1b, in 4 and 5, suggesting that the B−H bonds therefore point away from the metal centers. 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. (41) Bruker-AXS SAINT, Madison, WI. (42) Sheldrick, G. M. SADABS V2008/1; University of Göttingen, Göttingen, Germany. (43) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.
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