Approaches to Sigma Complexes via Displacement of Agostic

Jul 20, 2017 - A series of coordinatively unsaturated, 16-electron, cationic ruthenium complexes bearing PNP pincer ligands of the type [RuH(L)(PNRP)]...
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Approaches to Sigma Complexes via Displacement of Agostic Interactions: An Experimental and Theoretical Investigation A. Ramaraj,† K. Hari Krishna Reddy,† Helena Keil,‡ Regine Herbst-Irmer,‡ Dietmar Stalke,‡ Eluvathingal D. Jemmis,*,† and Balaji R. Jagirdar*,† †

Department of Inorganic & Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India Institut für Anorganische Chemie, Universität Göttingen, Tammannstrasse 4, 37077 Göttingen, Germany

‡

S Supporting Information *

ABSTRACT: A series of coordinatively unsaturated, 16-electron, cationic ruthenium complexes bearing PNP pincer ligands of the type [RuH(L)(PNRP)]+ (L = PPh3, R = PhCH2 (3), Ph (4); L = CO, R = PhCH2 (5); PNRP = RN(CH2CH2PPh2)2) has been prepared and characterized. These complexes exhibit agostic interaction in the sixth coordination site. The binding of various X−H (X = H, Si, B, and C) bonds in small molecules such as H2, silanes, tetracoordinate boranes, and CH4 to the ruthenium center via displacement of the relatively weak agostic interaction in these complexes has been studied. Structures of [RuH(L)(PNRP)]+ (L = PPh3, R = PhCH2 (3), Ph (4); L = CO, R = PhCH2 (5); PNRP = RN(CH2CH2PPh2)2) complexes and the sigma-borane complex trans-[RuH(CO)(η1-H-BH2·NMe3)(PNRP)]+ (R = PhCH2 (15)) have been established by X-ray crystallography. The relative binding strengths of the X−H bonds to ruthenium center in these complexes has been studied using computational methods.

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INTRODUCTION Activation of small, relatively inert molecules with strong sigma bonds, X−H (X = H, B, Si, and C), has been an ubiquitous problem in organometallic chemistry since the beginning of its study in the late 1950s. The process of activation is thought to be controlled by a subtle balance between σ-donation from the ligand to the metal and back-donation from the metal d-orbitals to the σ* orbitals of the ligand; under favorable conditions a stable complex with σ-interaction may be observable.1 In the case of polar X−H bonds of silanes (X = Si) and tetracoordinate boranes (X = B), σ-donation from the X−H bond exists to a good extent. Back-donation is reduced in complexes of silane due to steric hindrance of the substituents on silicon,2−4 yet there is finite back-donation in these systems so that the Si−H−M interactions are observable and have been structurally characterized in favorable examples.5−7 However, H2 forms stable σ-complexes despite complete lack of polarity in its σ-bond; absence of steric hindrance and presence of backdonation from the metal to σ* orbitals of H2 help in this. Poor nucleophilicity of the C−H bond due to similar electronegativities of C and H renders the σ-bond in CH4 and/or alkanes as nonpolar. The interaction of C−H bond to a metal center is inherently weak, typically around 7−10 kcal mol−1.8,9 Complexes, wherein X−H (X = H, Si, and B) σ-bond is bound to a metal center, serve as models for the binding of C−H bond to a metal center. Pathways leading to such species (X−H−M) are known and fairly well established.7,10,11 Similar approaches to C−H−M interactions are scarce.12,13 One of them involved the generation of a highly reactive, transient 16-electron metal © XXXX American Chemical Society

carbonyl fragment via photolysis of an 18-electron complex in the presence of molecules with X−H (X = H, Si, B, and C) bonds.7,14−16 Using this route, binding of various X−H bonds on [MnCp(CO)2] (Cp = C5H5) metal fragment has been studied.17−21 Brookhart et al. studied the binding of various X−H bonds on [IrH(POCOP)]+ {POCOP = 2,6-(OP(tBu2)2)} fragment.22−25 One plausible route for X−H (X = H, Si, B, and C) σ-complexes that has been less studied is via displacement of weak interactions, namely, agostic interactions (Chart 1). Agostic interactions, relatively weak intramolecular interactions (ca. 7−10 kcal mol−1) where a C−H fragment of a ligand acts as a two electron donor to stabilize a coordinatively unsaturated metal center, are well-known in organometallic chemistry.26 They could easily be displaced by Lewis bases.27 These interactions protect a vacant site on a metal center in Chart 1. Displacement of Agostic Interaction by σ-Ligand

Received: March 20, 2017

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DOI: 10.1021/acs.organomet.7b00210 Organometallics XXXX, XXX, XXX−XXX

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Organometallics many catalytic reactions before substrate binding.28 Studies on their displacement by X−H fragments, which are deemed stronger than the agostic interactions have been carried out to a limited extent. Reaction of [Mo(CO)(R2PC2H4PR2)2] (R = Et, CH2Ph, and Ph) with H2 and silane led to the displacement of agostic interactions and formation of σ-complexes.29 SaboEtienne et al. studied the reaction of the agostic complexes [RuH(H2)(H−A)(PiPr3)2]+ (A = phenylpyridine and benzoquinoline) with H2.30 Weller et al. synthesized low-valent, cationic complex [Rh(PR3)2]+ (R = tBu, iBu, and iPr) stabilized by agostic interactions from phosphine substituents and explored its reactivity with H2, aryl halides, and B−H bonds.31−34 Whittlesey et al. reported the reactivity of a 14-electron ruthenium fragment [Ru(IPr)2(CO)H]+ {IPr = 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene} supported by agostic interaction of one of the IPr C−H bonds of NHC toward H2, CO, H−BH2·NR2H, and H−Bcat (R = H and Me; cat = C6H4O2).35 Study of the binding of various X−H bonds on the same metal fragment is extremely important to understand the variations of the reactivity with X in these reactions and their potential in small molecule activation. Herein, we report a series of cationic ruthenium pincer36,37 complexes with agostic interactions of the type [RuH(L)(PNRP)]+ (L = PPh3, R = PhCH2 (3), Ph (4); L = CO, R = PhCH2 (5); PNRP = RN(CH2CH2PPh2)2) via abstraction of chloride from [RuHCl(L)(PNRP)] complex using NaBArf4 (BAr4f− = [B{3,5(CF3)2(C6H3)4}]−). The sixth coordination site trans to the hydride ligand is taken up by an agostic interaction involving an ortho C−H bond of the phenyl moiety of the pincer side arm, N−R (R = CH2C6H5 or C6H5). We obtained σ-complexes of H2, silane, and borane from these derivatives via displacement of agostic interactions. The results of these studies from both experimental and theoretical methods are discussed here.

spectra of complexes 3−5 appear at −21.43 ppm (q), − 22.46 ppm (td), and −17.61 ppm (t),38 respectively with H,Pcis coupling ranging from 15 to 30 Hz. A characteristic feature of a species exhibiting agostic interaction involving ortho-protons of a phenyl moiety is an upfield shift of the agostic protons in comparison to the rest of the phenyl protons.30 All of these complexes exhibit a characteristic upfield-shifted (upfield from aromatic region) signal for the agostic C−H of NCH2Ph (5.54 ppm in case of 3 and 6.35 ppm in case of 5) and NPh (6.23 ppm in case of 4) in the 1H NMR spectrum. Upon 31P decoupling, the 1H NMR spectral signals of the agostic protons showed further features. We also established the agostic interactions in these complexes with certainty using 2D and VT NMR spectral study. Complete data of these studies are in the Supporting Information. Among many other complexes, those that have some resemblance to our systems and that exhibit agostic interaction of the phenyl fragment include [Mo(CO)((PhCH2)2PC2H4P(CH2Ph)2)] and [RuH(H2)(H−Ph−Py)(PiPr3)]+ {H−Ph−Py = 2-phenylpyridine}.29,30 The agostic interactions in complexes 3 and 4 could be easily displaced by CH3CN and oxygen leading to the formation of respective nitrile derivatives [RuH(NCCH3)(PPh3)(PNRP)]+ {R = CH2Ph (7), Ph (8)} and dioxygen complexes [RuH(η2-O2)(PPh3)(PNRP)]+ {R = PhCH2 (9), Ph (10)} (eq 3). These derivatives were independently

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RESULTS AND DISCUSSION Synthesis and Characterization of [RuHCl(PPh3)(PNRP)] {R = PhCH2 (1), Ph (2)} Complexes. Treatment of [RuHCl(PPh3)3] with the pincer ligand RN(CH2CH2PPh2)2 (R = PhCH2 and Ph) followed by workup afforded [RuHCl(PPh3)(PNRP)] {R = PhCH2 (1), Ph (2)} complexes in yields of 73 and 53%, respectively (eq 1). The hydride signals for 1

prepared by the reaction of [RuHCl(PPh3)(PNRP)] {R = PhCH2 (1), Ph (2)} with NaBAr4f in air. Disorder in the counteranion BAr4f− precluded a satisfactory solution of the structure of complex 9 (see the Supporting Information). The O−O bond length was found to be 1.438 (9) Å which is in agreement with that of well-characterized dioxygen complexes39 (see the Supporting Information). Also, in order to unequivocally establish the structure formulations of complexes 3−5, we determined their X-ray crystal structures which are described below. Crystal Structures of [RuH(L)(PNRP)]+ {L = PPh3, R = PhCH2 (3), Ph (4); L = CO, R = PhCH2 (5)} Complexes. The ORTEP view of the cationic agostic complexes [RuH(L)(PNRP)]+ {L = PPh3, R = PhCH2 (3), Ph (4); L = CO, R = PhCH2 (5)}

and 2 appear as virtual doublet of triplets at −18.90 and −17.40 ppm with cis phosphorus couplings in the range of 23−30 Hz, respectively, in the 1H NMR spectra. Complex 1 was also structurally characterized (see the Supporting Information) by X-ray crystallography. Preparation of [RuH(L)(PNRP))]+ {L = PPh3, R = PhCH2 (3), Ph (4); L = CO, R = PhCH2 (5)} Complexes. Abstraction of chloride ligand in complexes 1 and 2 and [RuHCl(CO)(PNRP)] {R = PhCH2}38 complex using NaBAr4f resulted in the formation of respective agostic complexes [RuH(L)(PNRP))]+ {L = PPh3, R = PhCH2 (3), Ph (4); L = CO, R = PhCH2 (5)} (eq 2) in reasonable yields. The hydride signal in the 1H NMR B

DOI: 10.1021/acs.organomet.7b00210 Organometallics XXXX, XXX, XXX−XXX

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Figure 1. ORTEP view of agostic complexes [RuH(PPh3)(PNRP))]+ (R = PhCH2 (3)), [RuH(PPh3)(PNRP))]+ (R = Ph (4)), and [RuH(CO)(PNRP)]+ (R = PhCH2 (5)) at the 30% probability level. All hydrogen atoms and solvent molecules except hydride and the agostic hydrogen were omitted for clarity.

by the relatively strongly binding small molecules such as dihydrogen, silane, four-coordinate boranes, and methane. Reaction of complex 3 with H2 at a pressure of 4 bar resulted in the formation of trans-[RuH(η2-H2)(PPh3)(PNRP)]+ (R = PhCH2) complex (11) (eq 4). The hydride and the dihydrogen ligands

(Figure 1) shows that they adopt a distorted octahedral geometry. The pincer ligand is coordinated to the metal in a meridional fashion. The P(1)−Ru−P(2) bond angle in 3−5 are 144.32(3), 161.18(2), and 159.87(2)°, respectively. This significant deviation from linearity is to accommodate the steric demand of the phosphine phenyl substituents. In contrast, the N(1)−Ru−P(3)/C(36) bond angles are 176.42(5), 167.82(4), and 179.48(8)°, respectively, approaching linearity. The Ru− Hago−Cago bond angles in 3−5 are, respectively, 115.13(2), 123.74(2), and 112.86(2)°. These values fall in the range of agostic interactions reported in the literature.26 The sixth site trans to the hydride ligand is occupied by an ortho C−H fragment of the phenyl group of the pincer N−R moiety (L = PPh3, R = PhCH2, Ph; L = CO, R = PhCH2). The orientation of phenyl ring toward the otherwise vacant site at the ruthenium center with or without the methylene spacer suggests that the stabilization provided by the ortho C−H bond is not due to geometrical constraints but due to electronic requirements of the complexes. The distances between the Ru metal center and the agostic hydrogen in complexes 3 and 4 were found to be 2.11(2) and 2.10(2) Å, respectively. The corresponding distances between the agostic carbon and Ru center are 2.713(2) and 2.770(2) Å, respectively in complexes 3 and 4. In contrast, the Ru to C−H agostic hydrogen distance in [RuH(L)(PNRP)]+ {L = CO, R = PhCH2 (5)} complex was found to be 1.99(2) Å and that between Ru and C−H agostic carbon, 2.538(2) Å. That these distances are shorter in complex 5 in comparison to those in complexes 3 and 4 is a manifestation of electron deficiency of the metal center caused by the strong π-acceptor ligand, CO. These parameters obtained by computational methods are in agreement with the experimental values and have been summarized in the Supporting Information. Complexes 3−5 are rare examples of crystallographic snapshots of intramolecular electrophilic C−H bond activation. Agostic interactions in complexes 3−5 are retained both in the solid state and in solution. Reaction of [RuH(L)(PNRP)]+ {L = PPh3, R = PhCH2 (3), Ph (4); L = CO, R = PhCH2 (5)} Complexes With Small Molecules. Complexes 3−5 possess agostic interactions in the sixth coordination site. Since agostic interactions are weak, it is possible to obtain sigma complexes via their direct displacement

are mutually trans to one another. The hydride ligand appears as a virtual doublet of triplet at −9.36 ppm due to coupling with three cis-phosphorus nuclei. The bound H2 ligand appears as a broad singlet at −1.87 ppm (see the Supporting Information). We also carried out VT 1H spin−lattice relaxation time (T1, ms; 400 MHz) measurements of complex 11. Whereas a T1 min could not be found, the short T1 values (Table 1) are Table 1. T1 (ms) Data for trans-[RuH(η2H2)(PPh3)(PNRP)]+ Complex (R = PhCH2 (11)) and trans[RuH(η2-H2)(CO)(PNRP)]+ (R = PhCH2 (12)) Complex T (K)

11

12

203 213 223 233 243 253

2.02 ± 0.41 2.74 ± 0.23 3.03 K ± 0.32 3.61 ± 0.25 4.47 ± 0.41 5.34 ± 0.49

5.77 ± 0.17 5.34 ± 0.25 5.27 ± 0.42

indicative of an intact H−H bond in the bound H2 ligand in this complex.40 Treatment of complex 3 with D2 gas gave the corresponding H−D isotopomer, trans-[RuH(η2-H−D)(PPh3)(PNRP)]+ (11-d1). A mechanism involving isotopic scrambling of the η2-D2 ligand in trans-[RuH(η2-D2)(PPh3)(PNRP)]+ {R = PhCH2 (11-d2)} with adventitious water followed by reprotonation with H2DO+ to give trans-[RuH(η2-HD)(PPh3)(PNRP)]+ C

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attributed to the steric congestion at the sixth site as evidenced in the space-filled models. In contrast, in case of complex 5, the sixth site is quite exposed as revealed in the space filled model; thus, it provides an opportunity for any small molecule to access the metal center via displacement of the agostic interaction relatively easily. Reaction of complex 5 with H2 at room temperature and a pressure of 7 bar resulted in a dihydrogen complex trans[RuH(η2-H2)(CO)(PNRP)]+ (R = PhCH2 (12)) (eq 4). At 293 K, a broad singlet at −17.61 ppm was noted which could be assigned to Ru−H of the starting agostic complex 5. Cooling the sample below 263 K revealed a different spectral pattern. A broad peak at −1.5 ppm along with a triplet at −9.5 ppm with J(H,P) of 20 Hz whose integral ratio is 2:1 were noted at 183 K in the 1H NMR spectrum. A partial VT 1H NMR spectral stack plot of this reaction is shown in Figure 3a. The spectral features in Figure 3a at the low temperature limit indicate a dihydrogen/hydride complex 12. This shows that the binding of H2 is reversible: bound H2 at the low temperature limit (183 K) and agostic complex (and free H2) at the high temperature limit (293 K). Computations at the BP86 level for this reaction indicate that it is, by a small magnitude, endergonic at room temperature. The ΔG became favorable at low temperature; the shift from endergonic to exergonic takes place about the same temperature at which complexation of H2 with complex 5 as noticed by NMR spectroscopy (Figure 3b). The ΔG value determined from 1H NMR spectroscopy is in good agreement with the computed value using DFT (see the Supporting Information). The VT 1H spin−lattice relaxation time (T1, ms; 400 MHz) varies from 5.77 to 5.22 ms over the measured temperature range suggesting the intact nature of the H−H bond in this species (Table 1). No T1 min could be obtained in the temperature range of the measurements. We further obtained the HD isotopomer, trans-[RuH(η2-HD)(CO)(PNRP)]+ (R = PhCH2) (12-d1) by reacting complex 5 with D2(g).41 A 1:1:1 triplet at −1.5 ppm in the 1H NMR spectrum unambiguously established the intact nature of the H−H bond in this complex bound to the metal center. J(H,D) was found to be 32.5 Hz, from which a dHH of 0.87 Å was calculated.

(11-d1) seems plausible in our case. This has been wellestablished in several complexes.41 The 1H NMR spectrum of the HD isotopomer shows a 1:1:1 triplet at −1.80 ppm with a HD coupling of 31.5 Hz (Figure 2). The dHH calculated from

Figure 2. Partial 1H NMR spectrum of the trans-[RuH(η2H−D)(PPh3)(PNRP)]+ (R = PhCH2, 11-d1) complex at 233 K.

J(H,D) is 0.89 Å.42 Previously, Kubas and co-workers reported a reversible displacement of a relatively weak agostic C−H bond by H2 in [Mn(CO)3(PCy3)2]+ to afford [Mn(η2H2)(CO)3(PCy3)2]+.43 In another example, the reaction of a 14-electron, cationic, agostic complex [RuH(IPr)2(CO)]+ with H2 led to the formation of cis-dihydrogen hydride complex cis[RuH(η2-H2)(IPr)2(CO)]+ via displacement of agostic interaction.35 In contrast, the reaction of H2 with a 14-electron, rhodium agostic complex [Rh(PiPr3)3]+ resulted in the dihydride complex via oxidative addition of H2 presumably via an unobserved dihydrogen complex.32 Whereas reaction of complex 3 with O2 was found to be irreversible, binding of H2 to the ruthenium center in complex 11 is reversible. Contrary to the reaction of complex 3 with H2, agostic interaction in complex 4 could not be displaced by H2 even at a pressure of 7 bar suggesting that the hemilability of agostic C−H due to the presence of methylene linker could facilitate the displacement of agostic interaction. There could also be a kinetic contribution from a restricted movement of the agostic C−H in 4 in relation to that in 3. Both complexes 3 and 4 did not afford the sigma complexes when they are allowed to react with Et3Si−H, H−H2B·NMe3, and CH4 (15 bar) molecules in the temperature range 293−183 K. In order to understand this reactivity pattern, space filled models were generated from their respective crystal structures (see the Supporting Information). The lack of reactivity of agostic complexes 3 and 4 could be

Figure 3. (a) Partial 1H NMR stack plot of the reaction of complex [RuH(PNRP)(CO)]+ (R = PhCH2 (5)) with H2 is shown. (b) Change in Gibbs free energy (from DFT calculations at the BP86 level) of H2 binding to the metal complex as a function of temperature. The H2 binding is observed around the temperature range where the Gibbs free energy term goes below zero. D

DOI: 10.1021/acs.organomet.7b00210 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1. Reaction of the Agostic Complex 5 with Sigma Ligands

p-FC6H4; SDmp = 2,6-dimesitylphenyl thiolate}, but the σ−silane complex [(η-H-SiR3)Ru(SDmp)(R3P)]+ was too unstable to be observed experimentally.46 Reactions of [RuH(L)(PNRP)]+ complex {L = CO, R = PhCH2} (5) with tetracoordinate boranes H3B·L (L = NMe3, PPh3) at 293 K led to the formation of σ-borane complexes trans-[RuH(η1-BH3·L)(CO)(PNRP)]+ (R = PhCH2; L = NMe3 (15), PPh3 (16)) (Scheme 1). Unlike the reactions of complex 5 with H2 or silane, reaction with tetracoordinate boranes resulted in stable, isolable products. The 1H NMR spectrum of [RuH(η1-BH3·NMe3)(CO)(PNRP)]+ (R = PhCH2) complex (15) is comprised of a broad signal at −1.27 ppm for the BH3 moiety and a triplet at −13.86 ppm for Ru−H with J(H,Pcis) of 17.3 Hz. From the 1H NMR spectrum, the integral ratio between the peaks of BH3 and Ru−H was found to be 3:1. Even upon cooling the sample to 183 K, the broad signal for BH3 moiety did not undergo any change indicating rapid dynamics between the terminal and the bound protons of BH3. Fluxional behavior of BH3 moiety was studied in case of [M(H)2(PCy3)2(η2-BH3·NMe3)] {M = Ir, Rh} complexes by Weller and co-workers.47 In our case, upon 11B decoupling, the broad peak at −1.27 ppm in the 1H NMR spectrum sharpened (see the Supporting Information). The 11B NMR spectra of complexes 15 and 16 are comprised of a signal at −8.78 (q) ppm and −38.64 (br) ppm, respectively. We were able to obtain single crystals of complex 15 and determine its structure which revealed without any ambiguity the η1-binding mode for H3B·NMe3. We also attempted to prepare a sigma-methane complex via reaction of [RuH(L)(PNRP)]+ complex {L = CO, R = PhCH2} (5) with CH4 at a pressure of 15 bar in the temperature range of 293 to 183 K. Our attempts did not afford the expected trans-[RuH(η2-H-CH3)(CO)(PNRP)]+ {R = PhCH2} complex (17) (Scheme 1). It is quite plausible that the aromatic C−H agostic interaction is stronger to be displaced by H−CH3. This is strongly indicative of the extremely weak binding energy (ca. 7−10 kcal/mol) of CH4 to the metal center. Additionally, reaction of [RuH(PNRP)(L)(CO)]+ {R = Me, L = THF} (18) where the sixth coordination site trans to the hydride is occupied by a weakly bound THF molecule with CH4 at a pressure of 15 bar and 183 K also did not result in a

Treatment of complex 5 with silanes H−SiRR′2 (R = R′ = Et; R = Ph, R′ = Me) at 183 K resulted in the corresponding σ-silane complexes trans-[RuH(η1-H-SiR′R″2)(CO)(PNRP)]+ {R = PhCH2; R′ = R″ = Et (13); R′ = Ph, R″ = Me (14)} (Scheme 1). At 293 K, a broad signal was observed at −17.61 ppm for complexes 13 and 14 which appeared similar to that of the starting agostic complex 5. Upon cooling the sample (complex 13) to 183 K, a sharp singlet at −5.68 ppm along with a triplet at −14.16 ppm with J(H,P) of 16.5 Hz were noted. Careful examination of the singlet at −5.68 ppm revealed satellite peaks which could be ascribed to the silicon satellites with J(H,Si) = 98.4 Hz (Figure 4). In the free silane, J(H,Si) = 178 Hz

Figure 4. Partial 1H NMR spectrum of σ-silane complex [RuH(η1-HSiEt3)(CO)(PNRP)]+ {R = PhCH2 (13)} at 203 K.

suggesting that there is substantial elongation of the H−Si bond in this complex. This coupling constant falls in the range of σ-silane complexes.44,45 The 29Si NMR spectrum of complex 13 shows a signal at 13.3 ppm for the bound silane, roughly a downfield shift of about 13 ppm compared to the free silane, HSiEt3. The binding mode (η1 or η2) of silane with the metal center could not be deciphered from the NMR spectral characteristics. Brookhart and co-workers characterized a rare, cationic end-on σ-silane complex [Ir(H)(η1-Et3SiH)(POCOP)][B(C6F5)4] (POCOP = 2,6-[OP(tBu)2]2C6H3) by X-ray crystallography.23 Recently, Oestreich and co-workers studied the heterolytic cleavage of Si−H bond on a cationic, thiolatestabilized ruthenium complex [Ru(SDmp)(R3P)]+ {R = Et, E

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Mode of Interaction in [RuH(CO)(SiHEt3)(PNRP)]+ and [RuH(CO)(SiHMe2Ph)(PNRP)]+ (R = PhCH2) Complexes. The binding mode in the case of [RuH(CO)(BH3·NMe3)(PNRP)]+ (15) is determined to be η1-H-BH2·NMe3 using X-ray crystallography. The observed Ru(1)−H(2)−B(1) bond angle is 152.20°. Computational studies at BP86 level gave the Ru(1)−H(2)−B(1) bond angle as 156.96°. In the case of L = SiHEt3 and SiHMe2Ph, the DFT studies gave the Ru−H−Si angles as 162.13 and 165.11° respectively. Though the natural atomic orbital (NAO) bond orders between metal and the X atom show weak bonding, in view of the definite nonzero value for the Ru−X interactions, the η2 description is retained for both H−SiEt3 and H−SiMe2Ph molecules interacting with the metal center. Binding Energies and the nature of Metal−Ligand interactions. The binding energies of the ligand−metal complexes are calculated using eq 5.

sigma-methane complex. The crystal structure of complex 18 has been deposited in the Supporting Information. Exposure of the cationic agostic complexes [RuH(L)(PNRP)]+ {L = PPh3, R = PhCH2 (3), Ph (4); L = CO, R = PhCH2 (5)} to CO(g) resulted in the formation of the carbonyl derivatives [RuH(CO)(L)(PNRP)]+ {L = PPh3, R = PhCH2 (19), Ph (20); L = CO, R = PhCH2 (21)} via displacement of agostic interactions. The CO stretching frequencies of complexes 19−21 were measured and summarized in the Supporting Information. Crystal Structure of trans-[RuH(η1-BH3·NMe3)(CO)(PNRP)]+ (R = PhCH2) Complex (15). The ORTEP view of trans-RuH(η1-BH3·NMe3)(CO)(PNRP)]+ (R = PhCH2) complex (15) is shown in Figure 5. It is evident that the agostic

Upon binding to the metal complex, the X−H distance in the ligand increases, and the corresponding NAO bond order decreases as compared to the free ligand. The newly formed M−H and M−X bonds indicate the metal−ligand interaction. The binding energies of the complexes correlate well with the M−H and M−X NAO bond orders. The low value of NAO bond order for M−H and M−X bonds in the case of CH4 metal complex indicates weak binding interaction between metal and the CH4 ligand. The interaction of H2 with the metal complex (eq 5) is exothermic by 6.17 kcal/mol. However, the Gibbs free energy is higher than the reactants by 1.65 kcal/mol indicating that the forward reaction is thermodynamically not favorable. Indeed, experiments at room temperature suggest that H2 does not bind with the metal complex as shown in Figure 3a. H2 interaction with the metal complex is observable at around 250 K and the binding happens in reversible fashion. Only at very low temperatures of 183 K the H2 bound complex is dominantly visible in the NMR spectra. We calculated the Gibbs free energy of the reaction as a function of temperature (Figure 3b). The free energy term becomes negative at around 250 K. This is in agreement with the experimental observations. The binding energies of CH4 are comparable to that of H2 complexes. We did not observe any complex with CH4 binding to the metal in our experiments. To understand this range of reactivity we have analyzed the electronic structure of these complexes. The forward (X−H → M) and backward (M → X−H) interactions are analyzed in detail using fragment molecular analysis using ADF package. The major interaction is the donation of sigma bonding electrons from the X−H bond to the LUMO of the metal fragment. The energies of the HOMOs and LUMOs of different ligands and the metal complex are schematically illustrated in Figure 6. The LUMO of the [RuH(CO)(L)(PNRP)]+ is primarily located on the metal complex. While it is difficult to analyze in detail the extent of stabilization of the X−H bond from the lowering of the corresponding MO due to the contribution of several orbitals, raising of the metal vacant orbital in the complex gives an indirect measure of the strength of this interaction. The relative increase in the LUMO energy level upon complexation with different ligands indicate the strength of the metal complex−ligand interaction. In the case of

Figure 5. ORTEP view of sigma borane complex [RuH(η1-BH3· NMe3)(CO)(PNRP)]+ (15). All hydrogens atoms except BH3 and Ru−H were omitted for clarity. Ru(1)−B(1) = 2.96(3) Å; Ru(1)− H(2)−B(1) = 150.90°.

interaction is displaced by BH3·NMe3. The Ru(1)−B(1) bond distance of 2.96(3) Å is long, and the boron atom is far away from the metal center. The Ru(1)−H(2)−B(1) bond angle is 152.20°. The binding mode of amine borane is of η1-fashion. Other relevant bond parameters are summarized in the Supporting Information. Whittlesey and co-workers characterized end-on binding of amine boranes on a cationic complex, [RuH(xantphos)(PPh3)(OH2)]+ (xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethyl xanthene) wherein the aqua ligand is displaced by B−H moiety.48 Weller and co-workers reported the crystal structure of a cationic σ-borane complex [Rh(H)2 (κ 3P,O,P -xantphos)(η 1-H-BH 2·NMe 3 )] +.49 In addition to these examples, Shimoi and co-workers earlier characterized the prototype neutral, σ-borane complexes [M(CO)5(η1-H-BH2·L)] (M = Cr, Mo, and W; L = NMe3, PMe3, and PPh3).7 Computational Studies. To understand the nature of X−H (X = H, Si, B, and C) bond interactions with the metal complexes we performed DFT calculations. We studied the energy preferences of the agostic complexes in the case of [RuH(L)(PNRP)]+ {L = PPh3, R = PhCH2 (3), Ph (4); L = CO, R = PhCH2 (5)}. The strength of agostic interactions is 8.01 kcal/mol for [RuH(L)(PNRP)]+ {L = PPh3, R = PhCH2 (3)} and 9.82 kcal/mol for [RuH(L)(PNRP)]+ {L = CO, R = PhCH2 (5)}. In the case of [RuH(L)(PNRP)]+ {L = PPh3, R = Ph (4)}, we did not succeed in getting a stationary point corresponding to non-agostic mode. We performed electronic structural analysis on different ligands interacting with [RuH(CO)(PNRP)]+ (R = PhCH2 (5)). The results are tabulated in the Supporting Information. F

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Organometallics

prepared and characterized. The relatively weak agostic interactions in these complexes could be displaced by small molecules such as H2, silanes, and borane-Lewis base adducts to afford the respective sigma-complexes. Efforts to realize sigmamethane complexes via displacement of agostic interactions in these complexes failed however, suggesting a very subtle competition between the strength of agostic interaction and binding of methane. Computational studies using DFT revealed that the agostic stabilization energy is minimum in case of methane. The free energy of formation of sigma-complexes of H2 and methane is endergonic, whereas those of silanes and tetracoordinate boranes are exergonic at room temperature. Strength of sigma ligand interaction with the metal center correlates with the destabilization of the LUMO of the agostic complexes.

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Figure 6. Schematic representation of MO energy levels of the metal complex and different ligands. The energy levels are not to scale.

EXPERIMENTAL SECTION

General Procedures. All reactions were carried out under N2 or Ar atmosphere using Schlenk or glovebox techniques.50 All the glasswares were dried at 403 K. Toluene, n-hexane, petroleum ether, THF, and Et2O were dried over Na-benzophenone. CH2Cl2 and CH3CN were dried over CaH2.51 AgOTf, CH3OTf, and CDCl3 were purchased from Sigma-Aldrich and used as received. CD2Cl2 was purchased from Cambridge Isotope Laboratories. It was dried over CaH2, distilled, and then degassed before use. [RuHCl(PPh3)3],52 [RuHCl(CO)(PNRP] (R = PhCH2 and Me),38,53 NaBAr4f,54 and R−N(CH2CH2PPh2)2 (PNRP, R = PhCH2 and Ph)55,56 were prepared according to literature procedures. NMR spectra were recorded using an Avance Bruker 400 MHz spectrometer. The spectrometer frequencies for 1H, 31P{1H}, 13C{1H}, 11 B, and 19F{1H} nuclei are 400, 162, 100, 128, and 376 MHz, respectively. The 1H NMR chemical shifts were referenced to the residual proton signal of the deuterated solvents. The 13C{1H} NMR signals were referenced to either CDCl3 or CD2Cl2. 31P NMR spectra were referenced relative to 85% H3PO4 aqueous solution (external reference). 19F NMR spectra were referenced to F3B·Et2O. 11B NMR spectra were referenced relative to the external reference H3N·BH3. Variable temperature 1H NMR spin−lattice relaxation time (T1) measurements were performed using the standard inversion recovery method (180°−τ−90° pulse sequence) at 400 MHz.40 FTIR spectra of powder samples were recorded using a Bruker ALPHA-P spectrometer. Mass spectra and elemental analyses were obtained using Micromass Q-TOF (HRMS) spectrometer and ThermoFinnigan Flash EA 1112 CHNS analyzer respectively, in the Department of Organic Chemistry, Indian Institute of Science (IISc), Bangalore. The 1H, 13C, 11 B, and 19F spectral signals for the BAr4f counteranion in the cationic ruthenium compounds remained the same in either CDCl3/CD2Cl2.

SiHEt3 and BH3PH3 the LUMO rises by about 24 kcal/mol. In the case of H2, the LUMO rises up in energy by 22 kcal/mol, and in the case of CH4, the LUMO rises by 10 kcal/mol only. This indicates that CH4 ligand has least interaction with the orbitals of the metal complex. The energy difference between the HOMO and LUMO and their overlap controls the strength of the interaction. We note that the HOMO of H2 and CH4 are farthest in energy from the LUMO of the metal fragment. In the case of SiHEt3 and BH3PH3, the frontier energy levels of the metal complex and the incoming ligands are in close range. This facilitates strong binding interaction. In the case of H2, the absence of steric repulsions allows the ligand to approach the metal center and resulting orbital overlap facilitates stronger binding (Figure 7). In the case of CH4, none of these favorable factors exist. Metal fragments that provide larger free area around the vacant site may allow CH4 to approach even more closely and provide stronger binding. While this may be possible, it is bound to be a formidable task because of the solvent molecules ready to occupy a vacant site than free the position.

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CONCLUSIONS A series of coordinatively unsaturated, 16-electron, cationic ruthenium complexes bearing PNP pincer ligands of the type [RuH(L)(PNRP)]+ (L = PPh3; R = PhCH2 (3), Ph (4); L = CO, R = PhCH2 (5), PNRP = RN(CH2CH2PPh2)2) has been

Figure 7. HOMOs of [RuH(CO)(H2)(PNRP)]+ {R = PhCH2 (12)} and [RuH(CO)(CH4)(PNRP)]+ {R = PhCH2 (17)}. The absence of steric effects in the case H2 makes it possible for the ligand to approach the metal center. G

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R = PhCH2 in all complexes. All NMR spectra were recorded in CD2Cl2 X = H, NMR recorded at 263 K. X = H, NMR recorded at 183 K. X = SiEt3, NMR recorded at 193 K. eX = SiMe2Ph. Spectrum recorded at 198 K; SiMe2Ph signals overlapping in the phenyl region. fX = H2B·NMe3. Spectrum recorded at 293 K. The NMe3 signal was observed at 2.63 ppm almost identical to the chemical shift of free H3B·NMe3. gX = H2B·PPh3. Spectrum recorded at 293 K. The Ru−H−BH2·PPh3 signal overlaps with other phenyl resonance.

51.80 (d)

53.23 (s)

48.43 (d)

The NMR spectral signals of BAr4f in the complexes have been summarized in the Supporting Information For the elemental analysis of the cationic ruthenium complexes, the counterion was changed from BAr4f to BPh4. 1H and 31P NMR spectral data for all complexes have been summarized Tables S1 and S2 (see Supporting Information). The experimental procedure for the synthesis of trans-[RuH(L)(PPh3)(PNRP)]+ {R = PhCH2, L = CH3CN (7), O2 (9) CO (19); R = Ph, L = CH3CN (8), O2 (10), CO (20)}, trans-[RuH(L)(CO)(PNRP)]+ {R = PhCH2, L = CO (21)}, and trans-[RuH(THF)(CO)(PNRP)]+ { R = Me (18)} complexes have been deposited in the Supporting Information Synthesis of [RuHCl(PPh3)(PNRP)] {R = PhCH2 (1), Ph (2)} Complexes. [RuHCl(PPh3)3] (1.01 g, 1.08 mmol) and PhCH2N(CH2CH2PPh2)2 (0.589 g, 1.11 mmol) were refluxed together in THF (50 mL) for 1 h. The reaction mixture was then cooled to room temperature. Then the solution was concentrated to about 10 mL; addition of Et2O gave a yellow precipitate which was washed further with more Et2O to remove PPh3 and was dried in vacuo. Complex 1 was isolated as a yellow solid in a yield of 73% (0.730 g). Characterization details of complex 1 are as follows: HRMS for C53H51NP3ClRu: Found: m/z = 896.2279 {[M − Cl]+, C53H51NP3Ru, calcd: 896.2278}. Elemental analysis for C53H51NP3ClRu·H2O·CH2Cl2: Found: C, 63.19; H, 5.31: N, 1.49. Required: C, 62.70; H, 5.36; N, 1.35. The presence of one molecule of each CH2Cl2 and H2O were confirmed by NMR spectroscopy. Complex 2 was prepared in a manner similar to that of complex 1, except that the starting materials taken are as follows: [RuHCl(PPh3)3] (0.8 g, 0.86 mmol) and PhN-(CH2CH2PPh2)2 (0.448 g, 0.92 mmol). Complex 2 was isolated as a yellow solid in a yield of 53% (0.42 g). Characterization details of complex 2 are as follows: HRMS for C52H49NP3ClRu: Found: m/z = 882.2124 {[M − Cl]+, C52H49NP3Ru, calcd: 882.2124}. Elemental analysis for C52H49NP3ClRu·H2O: Calcd: C, 66.77; H, 5.50; N, 1.50; Found: C, 66.47; H, 5.70; N, 1.81. The presence of one molecule of H2O was confirmed by NMR spectroscopy. Synthesis of [RuH(L)(PNRP)][BAr4f] {L = PPh3, R = PhCH2 (3), Ph (4); L = CO, R = PhCH2 (5)}. [RuHCl(PPh3)(PNRP)] {R = PhCH2} (1) (0.030 g, 0.032 mmol) complex was dissolved in CH2Cl2, then NaBAr4f (0.029 g, 0.033 mmol) was added. The reaction mixture was shaken for 10 min. The clear solution was decanted leaving behind NaCl. The solution was layered with n-hexane and kept at −30 °C to get yellow crystals of complex 3 in a yield of 62.5% (0.040 g). HRMS for C53H51NP3Ru: Found, m/z = 896.2275 [M]+ (Calcd 896.2278 for C53H51NP3Ru). Complex 4 was synthesized in a manner similar to that of complex 3. The starting materials taken were as follows: [RuHCl(PPh3)(PNRP)] {R = Ph} (0.030 g, 0.033 mmol) and NaBAr4f (0.031 g, 0.035 mmol). Complex 4 was isolated as dark yellow crystalline solid in 52% yield (0.030 g). HRMS for C52H49NP3Ru: Found, m/z = 882.2120 [M]+ (Calcd: 882.2121 for C52H49NP3Ru). Complex 5 was synthesized in a manner similar to that of complex 3. The starting materials taken were as follows [RuHCl(CO)(PNRP)] (0.030 g. 0.043 mmol) and NaBAr4f (0.038 g, 0.043 mmol). Complex 5 was isolated as colorless crystals in 60% (0.040 g) yield. In a previous report,38 we characterized [RuH(L)(PNRP))][BAr4f] {L = CO, R = PhCH2 (5)} complex using NMR spectroscopy. Characterization details of complex 5. IR (ϑCO): 1972 cm−1. HRMS for C36H36NOP2Ru: m/z = 662.1318 (Calcd: 662.1316 for C36H36NOP2Ru). Synthesis of trans-[RuH(η2-H2)(L)(PNRP)]+ {L = PPh3, R = PhCH2 (11); L = CO, R = PhCH2 (12)}. [RuH(L)(PNRP)][BAr4f] {L = PPh3, R = PhCH2 (3)} (0.020 g, 0.011 mmol) was dissolved in CD2Cl2 (0.6 mL) in a pressure stable NMR tube. The sample was pressurized with H2(g) (4 bar) at room temperature using a highpressure setup. The sample was shaken for a few minutes, and NMR spectral data of trans-[RuH(η2-H2)(L)(PNRP)]+ {L = PPh3, R = PhCH2 (11)} were acquired. To obtain the H−D isotopomer, trans[RuH(η2-H2)(L)(PNRP)]+ {L = PPh3, R = PhCH2 (11)} sample was pressurized with D2(g), and the reaction progress was monitored. The product was characterized in solution. The trans-[RuH(η2-H2)(L)(PNRP)]+ {L = CO, R = PhCH2 (12)} complex and its H-D isotopomer, trans-[RuH(η2-HD)(CO)(PNRP)]+

d

6.79−7.91 (m)

b

−1.10 (br s)g 10−18 −13.10 (t) [RuH(HBH2·PPh3)(PPh3)(PNRP)]+ (16)

a

2.33−3.48 (m)

3.05−3.11 (m)

3.06−3.48 (m) −1.27 (br s)f −13.86 (t)

R

17.3

17 −14.34 (t)

[RuH(HBH2·NMe3)(CO)(PNRP)]+ (15)

complex

+

[RuH(H-SiMe2Ph)(CO)(PN P)] (14)

−5.01 (s), J(H,Si) = 102 Hz

e

2.44−3.39 (m)

2.69−3.30 (m)

16.5 −14.16 (t) [RuH(H-SiEt3)(CO)(PNRP)]+ (13)

−5.68 (s), J(H,Si) = 98.4 Hzd

15.5 −9.38 (t) [RuH(η2-H2)(CO)(PNRP)]+ (12)

−1.45 (br s)c

CH2 Ru−(H−X)

22

−1.84 (br s)b

J(H,P) (Hz) Ru−H

−9.35 (q) [RuH(η2-H2)(PPh3)(PNRP)]+ (11)

a

H 1

Table 2. 1H and 31P NMR Spectral Data for Complexes 11−16

2.68−2.97 (m)

c

3.48 (s) −1.10(br s) (H2B.PPh3)

7.35−8.00 (m)

6.25−8.40 (m)

3.96 (s) −1.27 (br s) (H2B.NMe3)

3.73 (s) −0.05 (SiMe2Ph)

49.35 (s)

50.42 (s) 7.13−7.85 (m)

7.29−7.94 (m)

3.78 (s)

3.85 (s) −0.02 (m, SiEt3)

J(P,P) (Hz) PPh3

59.75 (t) 59.0 (d)

PNP Ph R

3.88 (s)

X

6.70−7.70 (m)

P 31

26

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H

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(12-d1) were similarly prepared starting from [RuH(CO)(PNRP)][BAr4f] {R = PhCH2} (0.020 g, 0.013 mmol) complex and H2/D2(g) (7 bar), respectively. Synthesis of trans-[RuH(CO)(η1-H-SiR′R″2)(PNRP)]+ Complex (R′ = R″ = Et, R = PhCH2 (13); R′ = Ph, R″ = Me, R = PhCH2 (14)). The [RuH(CO)(PNRP)][BAr4f] {R = PhCH2} (0.020 g, 0.013 mmol) complex was dissolved in CD2Cl2 and excess Et3Si−H (10 μL, 0.065 mmol) or PhMe2Si−H (12 μL, 0.078 mmol) was added. Then, NMR spectra were acquired at 198 K. The product was characterized in solution. Synthesis of trans-[RuH(CO)(η1-H-BH2·L)(PNRP)]+ Complex (L = NMe3, R = PhCH2 (15); L = PPh3, R = PhCH2 (16)). The [RuH(CO)(PNRP)][BAr4f] {R = PhCH2} (0.020 g, 0.013 mmol) complex was dissolved in CH2Cl2 (2 mL) followed by addition of BH3·L {L = NMe3 (0.0044 g, 0.060 mmol); L = PPh3 (0.017 g, 0.060 mmol). The reaction mixture was shaken for a few minutes. Addition of n-hexane gave a pale yellow precipitate, which was subsequently washed with n-hexane to obtain products 15 and 16 in yields of 0.025 g (78%) and 0.028 g (52%) respectively. HRMS for compound 15 (C39H48BN2OP2Ru): Found: 662.1318 [M−BH3· NMe3]+, Calcd: 662.1316 for C36H36NOP2Ru. HRMS for compound 16 (C54H54BNOP3Ru): Found: 662.1317 [M−BH3·PPh3]+, Calcd: 662.1316 for C36H36NOP2Ru. Reaction of [RuH(CO)(PNRP)][BAr4f] {R = PhCH2} with CH4. The [RuH(CO)(PNRP)][BAr4f] {R = PhCH2} (0.020 g, 0.013 mmol) complex was dissolved in CD2Cl2 (0.6 mL) in a pressure-stable NMR tube. The sample was then pressurized with CH4 (15 bar) at room temperature. The NMR tube was then shaken for 10 min, and NMR spectra were acquired. NMR spectroscopy showed no evidence of a reaction between these two species. The expected trans-[RuH(η2CH4)(CO)(PNRP)]+ {R = PhCH2 (17)} complex was not obtained. X-ray Crystal Structure Determination of [RuHCl(PPh3)(PNRP)] {R = PhCH2 (1)}, [RuH(PPh3)(PNRP)] [BAr4f] {R = PhCH2 (3)}, [RuH(PPh3)(PNRP)] [BAr4f] {R = Ph (4)}, [RuH(CO)(PNRP)][BAr4f] {R = PhCH2 (5)}, trans-[RuH(CO)(η1-H-BH2·NMe3)(PNRP)] [BAr4f] {R = PhCH2 (15)}, and [RuH(THF)(CO)(PNRP)] [BAr4f] {R = Me (18)} Complexes. Yellow crystals of complexes 1, 3−5, 15, and 18 were obtained via slow diffusion of hexane into concentrated CH2Cl2 solutions of the respective complexes or by cooling their CD2Cl2 solutions to 243 K. A single crystal of each of the complexes suitable for a diffraction study was chosen after careful examination under a microscope at low temperature employing the X-Temp2 device.57 The crystal was coated with inert perfluorinated polyether oil and mounted at a MiTiGen loop. The unit cell parameters and intensity data were collected using a Bruker SMART APEXII CCD diffractometer equipped with a fine focus Mo Kα X-ray source, except for complexes 3 and 4 for which a Ag Kα IμS was used.58 The SMART software was used for data acquisition and the SAINT program was used for data integration.59 Empirical absorption corrections were made using SADABS program.60 The structures were solved by direct methods (SHELXT)61 and refined on F2 using the full-matrix leastsquares methods of SHELXL.62 All non-hydrogen atoms were located by difference Fourier map and refined anisotropically. The hydride and the agostic hydrogen atoms were located in the respective Fourier maps and refined freely. Other relevant data for structure refinement are summarized in the Supporting Information Computational Methods. All of the DFT calculations were performed using the Gaussian 09 suite of programs.63 Geometries of all the complexes were fully optimized in the gas phase without any symmetry constraints using the BP86 density functional64,65 in combination with the LANL2DZ basis set66−69 for all of the atoms. We added empirical dispersion corrections to the functional using the D3 version of Grimme’s dispersion correction.70 Harmonic vibrational frequency calculations were performed to verify the nature of the stationary points. Zero point energy corrections are added to the final energies. We performed basis set superposition error estimations using counterpoise method of Boys.71,72 The interactions with the X−H and metal complexes are analyzed using Fragment Molecular Orbital approach using the ADF software.73−75 We used BP86D3 functional with the all electron TZP basis set.76

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00210. Experimental details, NMR spectral data, bond parameters of complexes based on computational studies using DFT, and summary of DFT calculations (PDF) HRMS (PDF) Optimized coordinates for complexes (XYZ) Accession Codes

CCDC 1528641−1528644, 1528646, and 1535944 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Eluvathingal D. Jemmis: 0000-0001-8235-3413 Balaji R. Jagirdar: 0000-0002-0048-2252 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from Department of Science & Technology (Science & Engineering Research Board, SERB), of India. A.R. acknowledges the institute fellowship of the Int. Ph.D. program of the IISc.

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REFERENCES

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DOI: 10.1021/acs.organomet.7b00210 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.7b00210 Organometallics XXXX, XXX, XXX−XXX