- Email: [email protected]
Unexpected selectivity in electrophilic attack on (PNP)RuN
Inorganica Chimica Acta 363 (2010) 633–636
Contents lists available at ScienceDirect
Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Unexpected selectivity in electrophilic attack on (PNP)RuN q Amy Walstrom, Hongjun Fan, Maren Pink, Kenneth G. Caulton * Department of Chemistry, Indiana University, Bloomington, IN, United States
a r t i c l e
i n f o
Article history: Received 2 September 2008 Accepted 9 November 2008 Available online 18 November 2008 Keywords: Nitride Reactivity Hydrogen bonding Nucleophilicity DFT
a b s t r a c t Protonation of (PNP)RuN, where PNP is (tBu2PCH2SiMe2)2N, with HCl occurs at the amide nitrogen, with coordination of chloride to RuIV, while triflic acid protonates the same nitrogen, but has triflate anion hydrogen-bonded to the proton on the PNP amide nitrogen, not triflate coordinated to the metal. Methyl triflate however alkylates the nitride nitrogen, to give a C2v symmetric product. DFT calculations show that the thermodyamic preference is for proton on amide nitrogen while alkyl favors nitride alkylation, even without the need for a hydrogen bond to reverse the H vs. alkyl preference. Alkylation at the amide nitrogen leads to nearly complete loss of the PN(R)P Ru/N bond in this unobserved isomer. These preferences among nucleophilic sites on (PNP)RuN are rationalized based on the frontier orbitals of this molecule. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
2.1. HCl
Installation of an azide ligand in place of halide in (PNP)Ru–(halide), where PNP is (tBu2PCH2SiMe2)2N, leads to prompt evolution of N2 at 25 °C with formation of (PNP)RuN [1]. This compound of tetravalent ruthenium is (slightly) nonplanar and diamagnetic, with a terminal nitride ligand. We were interested in studying the reactivity of this molecule: reactivity at the nitride nitrogen, or at the metal or, as increasingly seen [2–14], at the amide nitrogen of the PNP ligand. Our initial studies showed that there was no reactivity with the nucleophiles MeCN, PhCN, Et3P (all equimolar in benzene for 12 h at 25 °C), and no reaction with 1 atm CO over 24 h; (PNP)RuN thus shows no significant Lewis acidity towards these diverse Lewis bases. There is also no reaction with 1 atm H2 over 24 h at 25 °C, and no addition of butadiene to the nitrogen, as has been reported for an osmium nitride [15]. We report here studies initiated to attempt electrophilic attack at the nitride nitrogen, although there are clearly other candidates for electrophilic attack on (PNP)RuN: at Ru or at the pincer ligand amide nitrogen.
Upon reaction with 1 equiv. of HCl (2 M in Et2O), in C6D6, there is an immediate color change from the green of (PNP)RuN to yellow, and one product (1) is observed by 31P NMR, having a singlet at 81.5 ppm. The corresponding 1H NMR spectrum shows CS symmetry, with 2 signals for each of the three PNP substituents. One additional peak, assignable to one N–H proton, can be located at 3.94 ppm. This can be interpreted as protonation of either the amide N or the nitride N. As time progresses (4 h), insoluble material precipitates from the C6D6 solution and free protonated ligand, PN(H)P is observed, diagnostic of general decomposition.
2. Results
2.2. HO3SCF3
The reaction of (PNP)RuN with two different Brønsted acids gives surprisingly different results.
In contrast is the reaction of (PNP)RuN with 1 equiv. of HOTf. Upon addition of HOTf, via microliter syringe, to a C6D6 solution of (PNP)RuN, there is an immediate color change to brown (2). 31 P and 19F NMR show complete conversion to one product, with a singlet at 68.3 ppm in 31P and 77.7 ppm in 19F NMR. The 1H NMR spectrum shows C2v symmetry, with only one t-butyl virtual triplet, one SiMe singlet, and one CH2 virtual triplet. A broad resonance at 4.25 ppm, of unit intensity, is also observed. After 15 min, single, dichroic crystals begin to precipitate from solution. These crystals were separated from solution and dissolved in THF-d8 to
q On the occasion of his retirement from ETH, this is dedicated to Paul Pregosin, who has taught us much about organometallic chemistry in his decades of work in catalytic chemistry, in ion pairing, and in innovative applications of new NMR techniques to all of these topics. * Corresponding author. E-mail address: [email protected] (K.G. Caulton).
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2008.11.010
634
A. Walstrom et al. / Inorganica Chimica Acta 363 (2010) 633–636
show that the precipitated product retains C2v symmetry and chemical shifts analogous to those observed in benzene. A crystal was grown by diffusion of benzene into a THF solution containing product 2 and subjected to an X-ray diffraction study. While a satisfactory refinement model could not be found for the diffraction data, the connectivity (see Section 1) is certain, and shows that in the solid state, the proton is on the amide backbone N rather than the nitride N, and that the triflate anion is hydrogen bonded to that proton. The resultant symmetry would be CS (due to the amine proton destroying one mirror plane), not the observed C2v. This oversimplified symmetry is probably the result of rapid proton transfer to triflate, which then delivers that proton to the inverted amide nitrogen, creating time averaged C2v symmetry by 1 H NMR spectroscopy. 2.3. MeO3SCF3 An apparently analogous result was achieved when (PNP)RuN is reacted with MeOTf. Upon mixing of the two reagents, there is an immediate color change to brown (3). Immediate NMR assay showed complete conversion to one product by 31P (69.6 ppm) and 19F NMR (77.6 ppm), and C2v symmetry by 1H NMR. Methylation at amide N to form a tertiary amine nitrogen would impose Cs molecular symmetry, and this suggests that the methyl group has not added there. After approximately 15 min, a viscous oil begins to form on the walls of the reaction vessel. If this oil is redissolved in THF-d8, NMR shows that this is the same product initially formed in C6D6. No solid state structure could be determined for this molecule since it is an oil, but the C2v symmetry suggests that the methyl group has added to the nitride nitrogen. If the PNP nitrogen had been methylated, the observed C2v symmetry would require an unreasonable fast C–N bond cleavage to invert the quaternary amine nitrogen. Perhaps it is hydrogen bonding which reverses the structural preference for binding of H+ vs. CH3 þ . 2.4. Factors influencing selectivity The question remains in all cases, since there are potentially three sites of activity (amide N, nitride N, and Ru): what controls the final location of the electrophile (H+, Me+)? Protonation at the metal would create an unusually high oxidation state, RuVI, which is also reflected in the anticipated low Bronsted basicity of d4 RuIV reagent, and these electrons being in dp orbitals. In the other two structures, the metal oxidation state is the same, RuIV. No hydrides are observed in any case, so protonation at the metal can be excluded. When (PNP)RuN is reacted with HCl etherate, the CS symmetry observed in the 1H NMR spectrum is likely the result of a 5 coordinate Ru, which implies that Cl has bound to the metal (1). This is a typical difference for an acid with a nucleophilic (HCl) vs. a non-nucleophilic (HO3SCF3) conjugate base.
Analysis of the calculated frontier orbitals helps understand the protonation results. Based on the frontier orbitals of (PNP)RuN (Fig. 1), HOMO2 shows that the amide N should have a larger degree of nucleophilicity, based on the amide dominated occupied orbital, than the nitride N, which in the HOMO1 and HOMO2 has negligible contribution. Given the hydrogen bond in the triflate salt of [PN(H)P]RuN+, we thought that this feature, absent in the methylation product, might be the key to the preference (reversed from CH3 þ ) for the N-protonation product. It therefore becomes important to first know the isomer energies intrinsic to the cation, without anion/hydrogen bond effects. Isomer energies were calculated (DFT(B3LYP)) for R = H and CH3. The results (Table 1) show that thermodynamics favors protonation at amide, but alkylation at the nitride. The cause of this reversal of locus of the electrophile might be due to steric effects, with amide N too crowded to accept anything larger than the proton. To test this, the isopropyl analog was calculated (Table 1) and again shows alkylation to be preferred at nitride, and by an amount larger than for the smaller methyl group. This steric effect is seemingly strong enough to lead to an additional structural difference in [PN(R)P]RuN+ for R = Me and i Pr: for R = Me, the Ru to-amine distance is 2.60 A (very long and thus weak), but for iPr it is an essentially nonbonding 3.65 A; for comparison, for R = H, the distance is 2.46 A. [16] Given recent calculations showing the steric interactions within the coordinated PNP ligand are generally less than 3 kcal/mol, [17] it is likely that electronic effects of R+ vs. H+ will modify these preferences attributed to steric effects. The influence of the R group identity in the (PNP)Ru(NR)+ isomer is evident from the DFT geometry optimized structures shown in Table 2. These show that bond lengths within the coordination sphere are very insensitive to changes going from H, past iPr to phenyl. All have transoid angles which are strongly nonplanar, or, more accurately, intermediate between planar and tetrahedral. The angle Ru–N–R increases systematically as the R group gets larger, as do the transoid angles P–Ru–P and N–Ru–N0 . The Ru–NR distances are all short enough to indicate multiple bond character. In all calculated species with R on nitride, the RuNR angle is nonlinear (>126°) and the coordination geometry at Ru is nonplanar. With regard to the fluxionality implied by the C2v symmetry from 1H NMR spectroscopy, the calculated barrier to distort the angle Ru@N–CH3 to linear is only 3.2 kcal/mol, and the barrier to take the nonplanar RuP2N2 geometry to rigorously planar is only 2.0 kcal/mol, fully consistent with rapid fluxionality and timeaveraged C2v symmetry. 2.5. Conclusion Protonation occurs at the PNP amide nitrogen, but larger electrophiles are relegated to the nitride nitrogen in their reaction with
Fig. 1. Frontier orbitals of (PNP)RuN.
635
A. Walstrom et al. / Inorganica Chimica Acta 363 (2010) 633–636 Table 1 Alternate sites for R+ binding to (PNP)RuN, with illustrations for R = iPr. Energy (kcal/mol)
E
R on N5
R on N2
H CH3 CHMe2
7.5 0 0
0 12.6 17.1
3.2. Reaction of (PNP)RuN with 1 equiv. of 1.0 M HCl in Et2O
Table 2 Select bond lengths (in Å) and bond angles (in) for (PNP)Ru(NR)+.
Ru–P Ru–P Ru–NSi2 Ru–NR N–Ru–N R–N–R1u P–Ru–P
H
Me
i
Pr
Ph
2.474 2.486 2.015 1.736 134.3 126.1 148.6
2.505 2.501 2.036 1.724 146.6 147.2 150.0
2.543 2.497 2.037 1.731 141.6 145.7 139.4
2.512 2.492 2.027 1.743 156.8 158.0 157.4
(PNP)RuN. Although triflate hydrogen bonds to the amine proton of coordinated PN(H)P, this is not alone the cause of this reversal of selectivity of electrophilic addition. This hydrogen bonding functionality has been observed earlier [18], indicating that even weakly nucleophilic triflate shows hydrogen bonding ability, especially in nonpolar solvent. The overall conclusion for the reactivity of the diatomic RuIVN ‘‘functionality” is that it is not Lewis acidic, in spite of its valence electron count, and that both nitrogens are targets of electrophilic reagents. [4] 3. Experimental 3.1. General Preparations from literature sources were used to synthesize RuN(SiMe2CH2PtBu2)2Cl and standard Schlenk or glovebox techniques in inert (Argon) atmosphere were used for air sensitive manipulations. All solvents, including deuterated NMR solvents, were dried over and distilled from Na/benzophenone and stored in anaerobic conditions. All other reagents were degassed and/ or used as received from commercial vendors. 1H, 13C{1H} and 31 1 P{ H} NMR spectra were recorded on a Varian Unity I400 (400 MHz 1H, 101 MHz 13C, 162 MHz 31P) instrument, with chemical shifts reported in ppm, referenced to protio impurities in each stated solvent, with the exception of 31P{1H} spectra which were externally referenced to 85% H3PO4 (0 ppm).
To 10.5 mg (0.0171 mmol) of (PNP)RuN in 0.5 mL C6D6, 10.5 lL of 1.0 M HCl in Et2O was added. Upon its addition at 22 °C, there was a rapid, distinct color change of the solution from green to yellow. After solvent and volatiles are quickly removed and the residue redissolved in the same solvent: 1H NMR (400 MHz, C6D6): d 3.94 (bs, 1H, NH), 1.47 (bvt, 18H, JP–H = 6.6 Hz, PCMe3), 1.29 (bvt, 18H, JP–H = 6.6 Hz PCMe3), 0.19 (s, 6H, SiMe2), 0.02 (s, 6H, SiMe2). 31 1 P{ H} (162 MHz, C6D6): d 81.5(s). 3.3. Reaction of (PNP)RuN with 1 equiv. of triflic acid To 10.1 mg (0.0171 mmol) of (PNP)RuN in 0.5 mL C6D6, 1.5 lL of triflic acid was added. Upon its addition at 22 °C, there was a rapid, distinct color change of the solution from green to brown: 1 H NMR (300 MHz, C6D6): d 4.25 (bs, 1H, NH), 1.21 (t, 36H, JP– H = 6.6 Hz, PCMe3), 1.12 (t, 4H, JP–H = 6.6 Hz PCH2Si), 0.49 (s, 12H, SiMe2). 31P{1H} (121 MHz, C6D6): d 68.3 (s). 19F NMR (282 MHz, C6D6): d 77.62 (bs). 13C{1H} (101 MHz, C6D6): d 34.32 (t, JC–P = 4.7 Hz, Si–CH2–P), 29.55 (t, JC–P = 3.6 Hz, PCMe3), 17.37 (s, PCMe3), 6.40 (s, SiMe2). Upon redissolving in THF-d8: 1 H NMR (300 MHz, THF-d8): d 3.62 (s, 1H, NH), 1.43 (t, 36H, JP– H = 6.9 Hz, PCMe3), 1.33 (t, 4H, JP–H = 6.9 Hz PCH2Si), 0.52 (s, 12H, SiMe2). 31P{1H} (121 MHz, THF-d8): d 72.1 (s). 19F NMR (282 MHz, THF-d8): d 78.89 (bs). Crystals were grown by layering from THF/benzene. 3.4. Reaction of (PNP)RuN with 1 equiv. of methyl triflate To 10.1 mg (0.0171 mmol) of (PNP)RuN in 0.5 mL C6D6, 1.9 lL of methyl triflate was added. Upon its addition at 22 °C, there was a rapid, distinct color change of the solution from green to brown: 1 H NMR (300 MHz, C6D6): d 3.03 (s, 3H, NMe), 1.18 (t, 36H, JP– H = 6.6 Hz, PCMe3), 1.12 (t, 4H, JP–H = 6.6 Hz PCH2Si), 0.47 (s, 12H, SiMe2). 31P{1H} (121 MHz, C6D6): d 69.6 (s). 19F NMR (282 MHz, C6D6): d 77.70 (bs).
636
A. Walstrom et al. / Inorganica Chimica Acta 363 (2010) 633–636
Acknowledgment We thank the National Science Foundation (NSF CHE-0544829) for financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2008.11.010. References [1] A. Walstrom, M. Pink, X. Yang, J. Tomaszewski, M.-H. Baik, K.G. Caulton, J. Am. Chem. Soc. 127 (2005) 5330. [2] C. Vogel, F.W. Heinemann, J. Sutter, C. Anthon, K. Meyer, Angew. Chem., Int. Ed. 47 (2008) 2681. [3] F. Akagi, T. Matsuo, H. Kawaguchi, Angew. Chem., Int. Ed. 46 (2007) 8778. [4] A. Walstrom, M. Pink, H. Fan, J. Tomaszewski, G. Caulton Kenneth, Inorg. Chem. 46 (2007) 7704.
[5] L.P. Spencer, B.A. MacKay, B.O. Patrick, M.D. Fryzuk, Proc. Natl. Acad. Sci. USA 103 (2006) 17094. [6] J.S. Figueroa, N.A. Piro, C.R. Clough, C.C. Cummins, J. Am. Chem. Soc. 128 (2006) 940. [7] C.-F. Leung, T.-W. Wong, T.-C. Lau, W.-T. Wong, Eur. J. Inorg. Chem. (2005) 773. [8] T.A. Betley, J.C. Peters, J. Am. Chem. Soc. 126 (2004) 6252. [9] A.G. Maestri, S.D. Taylor, S.M. Schuck, S.N. Brown, Organometallics 23 (2004) 1932. [10] J.K. Brask, V. Dura-Vila, P.L. Diaconescu, C.C. Cummins, Chem. Commun. (2002) 902. [11] M. Nishimura, S. Minakata, T. Takahashi, Y. Oderaotoshi, M. Komatsu, J. Org. Chem. 67 (2002) 2101. [12] W.-H. Leung, J.L.C. Chim, I.D. Williams, W.-T. Wong, Inorg. Chem. 38 (1999) 3000. [13] K.D. Demadis, T.J. Meyer, P.S. White, Inorg. Chem. 37 (1998) 3610. [14] N.M. Doherty, S.C. Critchlow, J. Am. Chem. Soc. 109 (1987) 7906. [15] A.G. Maestri, K.S. Cherry, J.J. Toboni, S.N. Brown, J. Am. Chem. Soc. 123 (2001) 7459. [16] See Supporting Information. [17] X. Yang, A. Walstrom, N. Tsvetkov, M. Pink, G. Caulton Kenneth, Inorg. Chem. 46 (2007) 4612. [18] A. Walstrom, M. Pink, N.P. Tsvetkov, H. Fan, M. Ingleson, K.G. Caulton, J. Am. Chem. Soc. 127 (2005) 16780.
Contents lists available at ScienceDirect
Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Unexpected selectivity in electrophilic attack on (PNP)RuN q Amy Walstrom, Hongjun Fan, Maren Pink, Kenneth G. Caulton * Department of Chemistry, Indiana University, Bloomington, IN, United States
a r t i c l e
i n f o
Article history: Received 2 September 2008 Accepted 9 November 2008 Available online 18 November 2008 Keywords: Nitride Reactivity Hydrogen bonding Nucleophilicity DFT
a b s t r a c t Protonation of (PNP)RuN, where PNP is (tBu2PCH2SiMe2)2N, with HCl occurs at the amide nitrogen, with coordination of chloride to RuIV, while triflic acid protonates the same nitrogen, but has triflate anion hydrogen-bonded to the proton on the PNP amide nitrogen, not triflate coordinated to the metal. Methyl triflate however alkylates the nitride nitrogen, to give a C2v symmetric product. DFT calculations show that the thermodyamic preference is for proton on amide nitrogen while alkyl favors nitride alkylation, even without the need for a hydrogen bond to reverse the H vs. alkyl preference. Alkylation at the amide nitrogen leads to nearly complete loss of the PN(R)P Ru/N bond in this unobserved isomer. These preferences among nucleophilic sites on (PNP)RuN are rationalized based on the frontier orbitals of this molecule. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
2.1. HCl
Installation of an azide ligand in place of halide in (PNP)Ru–(halide), where PNP is (tBu2PCH2SiMe2)2N, leads to prompt evolution of N2 at 25 °C with formation of (PNP)RuN [1]. This compound of tetravalent ruthenium is (slightly) nonplanar and diamagnetic, with a terminal nitride ligand. We were interested in studying the reactivity of this molecule: reactivity at the nitride nitrogen, or at the metal or, as increasingly seen [2–14], at the amide nitrogen of the PNP ligand. Our initial studies showed that there was no reactivity with the nucleophiles MeCN, PhCN, Et3P (all equimolar in benzene for 12 h at 25 °C), and no reaction with 1 atm CO over 24 h; (PNP)RuN thus shows no significant Lewis acidity towards these diverse Lewis bases. There is also no reaction with 1 atm H2 over 24 h at 25 °C, and no addition of butadiene to the nitrogen, as has been reported for an osmium nitride [15]. We report here studies initiated to attempt electrophilic attack at the nitride nitrogen, although there are clearly other candidates for electrophilic attack on (PNP)RuN: at Ru or at the pincer ligand amide nitrogen.
Upon reaction with 1 equiv. of HCl (2 M in Et2O), in C6D6, there is an immediate color change from the green of (PNP)RuN to yellow, and one product (1) is observed by 31P NMR, having a singlet at 81.5 ppm. The corresponding 1H NMR spectrum shows CS symmetry, with 2 signals for each of the three PNP substituents. One additional peak, assignable to one N–H proton, can be located at 3.94 ppm. This can be interpreted as protonation of either the amide N or the nitride N. As time progresses (4 h), insoluble material precipitates from the C6D6 solution and free protonated ligand, PN(H)P is observed, diagnostic of general decomposition.
2. Results
2.2. HO3SCF3
The reaction of (PNP)RuN with two different Brønsted acids gives surprisingly different results.
In contrast is the reaction of (PNP)RuN with 1 equiv. of HOTf. Upon addition of HOTf, via microliter syringe, to a C6D6 solution of (PNP)RuN, there is an immediate color change to brown (2). 31 P and 19F NMR show complete conversion to one product, with a singlet at 68.3 ppm in 31P and 77.7 ppm in 19F NMR. The 1H NMR spectrum shows C2v symmetry, with only one t-butyl virtual triplet, one SiMe singlet, and one CH2 virtual triplet. A broad resonance at 4.25 ppm, of unit intensity, is also observed. After 15 min, single, dichroic crystals begin to precipitate from solution. These crystals were separated from solution and dissolved in THF-d8 to
q On the occasion of his retirement from ETH, this is dedicated to Paul Pregosin, who has taught us much about organometallic chemistry in his decades of work in catalytic chemistry, in ion pairing, and in innovative applications of new NMR techniques to all of these topics. * Corresponding author. E-mail address: [email protected] (K.G. Caulton).
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2008.11.010
634
A. Walstrom et al. / Inorganica Chimica Acta 363 (2010) 633–636
show that the precipitated product retains C2v symmetry and chemical shifts analogous to those observed in benzene. A crystal was grown by diffusion of benzene into a THF solution containing product 2 and subjected to an X-ray diffraction study. While a satisfactory refinement model could not be found for the diffraction data, the connectivity (see Section 1) is certain, and shows that in the solid state, the proton is on the amide backbone N rather than the nitride N, and that the triflate anion is hydrogen bonded to that proton. The resultant symmetry would be CS (due to the amine proton destroying one mirror plane), not the observed C2v. This oversimplified symmetry is probably the result of rapid proton transfer to triflate, which then delivers that proton to the inverted amide nitrogen, creating time averaged C2v symmetry by 1 H NMR spectroscopy. 2.3. MeO3SCF3 An apparently analogous result was achieved when (PNP)RuN is reacted with MeOTf. Upon mixing of the two reagents, there is an immediate color change to brown (3). Immediate NMR assay showed complete conversion to one product by 31P (69.6 ppm) and 19F NMR (77.6 ppm), and C2v symmetry by 1H NMR. Methylation at amide N to form a tertiary amine nitrogen would impose Cs molecular symmetry, and this suggests that the methyl group has not added there. After approximately 15 min, a viscous oil begins to form on the walls of the reaction vessel. If this oil is redissolved in THF-d8, NMR shows that this is the same product initially formed in C6D6. No solid state structure could be determined for this molecule since it is an oil, but the C2v symmetry suggests that the methyl group has added to the nitride nitrogen. If the PNP nitrogen had been methylated, the observed C2v symmetry would require an unreasonable fast C–N bond cleavage to invert the quaternary amine nitrogen. Perhaps it is hydrogen bonding which reverses the structural preference for binding of H+ vs. CH3 þ . 2.4. Factors influencing selectivity The question remains in all cases, since there are potentially three sites of activity (amide N, nitride N, and Ru): what controls the final location of the electrophile (H+, Me+)? Protonation at the metal would create an unusually high oxidation state, RuVI, which is also reflected in the anticipated low Bronsted basicity of d4 RuIV reagent, and these electrons being in dp orbitals. In the other two structures, the metal oxidation state is the same, RuIV. No hydrides are observed in any case, so protonation at the metal can be excluded. When (PNP)RuN is reacted with HCl etherate, the CS symmetry observed in the 1H NMR spectrum is likely the result of a 5 coordinate Ru, which implies that Cl has bound to the metal (1). This is a typical difference for an acid with a nucleophilic (HCl) vs. a non-nucleophilic (HO3SCF3) conjugate base.
Analysis of the calculated frontier orbitals helps understand the protonation results. Based on the frontier orbitals of (PNP)RuN (Fig. 1), HOMO2 shows that the amide N should have a larger degree of nucleophilicity, based on the amide dominated occupied orbital, than the nitride N, which in the HOMO1 and HOMO2 has negligible contribution. Given the hydrogen bond in the triflate salt of [PN(H)P]RuN+, we thought that this feature, absent in the methylation product, might be the key to the preference (reversed from CH3 þ ) for the N-protonation product. It therefore becomes important to first know the isomer energies intrinsic to the cation, without anion/hydrogen bond effects. Isomer energies were calculated (DFT(B3LYP)) for R = H and CH3. The results (Table 1) show that thermodynamics favors protonation at amide, but alkylation at the nitride. The cause of this reversal of locus of the electrophile might be due to steric effects, with amide N too crowded to accept anything larger than the proton. To test this, the isopropyl analog was calculated (Table 1) and again shows alkylation to be preferred at nitride, and by an amount larger than for the smaller methyl group. This steric effect is seemingly strong enough to lead to an additional structural difference in [PN(R)P]RuN+ for R = Me and i Pr: for R = Me, the Ru to-amine distance is 2.60 A (very long and thus weak), but for iPr it is an essentially nonbonding 3.65 A; for comparison, for R = H, the distance is 2.46 A. [16] Given recent calculations showing the steric interactions within the coordinated PNP ligand are generally less than 3 kcal/mol, [17] it is likely that electronic effects of R+ vs. H+ will modify these preferences attributed to steric effects. The influence of the R group identity in the (PNP)Ru(NR)+ isomer is evident from the DFT geometry optimized structures shown in Table 2. These show that bond lengths within the coordination sphere are very insensitive to changes going from H, past iPr to phenyl. All have transoid angles which are strongly nonplanar, or, more accurately, intermediate between planar and tetrahedral. The angle Ru–N–R increases systematically as the R group gets larger, as do the transoid angles P–Ru–P and N–Ru–N0 . The Ru–NR distances are all short enough to indicate multiple bond character. In all calculated species with R on nitride, the RuNR angle is nonlinear (>126°) and the coordination geometry at Ru is nonplanar. With regard to the fluxionality implied by the C2v symmetry from 1H NMR spectroscopy, the calculated barrier to distort the angle Ru@N–CH3 to linear is only 3.2 kcal/mol, and the barrier to take the nonplanar RuP2N2 geometry to rigorously planar is only 2.0 kcal/mol, fully consistent with rapid fluxionality and timeaveraged C2v symmetry. 2.5. Conclusion Protonation occurs at the PNP amide nitrogen, but larger electrophiles are relegated to the nitride nitrogen in their reaction with
Fig. 1. Frontier orbitals of (PNP)RuN.
635
A. Walstrom et al. / Inorganica Chimica Acta 363 (2010) 633–636 Table 1 Alternate sites for R+ binding to (PNP)RuN, with illustrations for R = iPr. Energy (kcal/mol)
E
R on N5
R on N2
H CH3 CHMe2
7.5 0 0
0 12.6 17.1
3.2. Reaction of (PNP)RuN with 1 equiv. of 1.0 M HCl in Et2O
Table 2 Select bond lengths (in Å) and bond angles (in) for (PNP)Ru(NR)+.
Ru–P Ru–P Ru–NSi2 Ru–NR N–Ru–N R–N–R1u P–Ru–P
H
Me
i
Pr
Ph
2.474 2.486 2.015 1.736 134.3 126.1 148.6
2.505 2.501 2.036 1.724 146.6 147.2 150.0
2.543 2.497 2.037 1.731 141.6 145.7 139.4
2.512 2.492 2.027 1.743 156.8 158.0 157.4
(PNP)RuN. Although triflate hydrogen bonds to the amine proton of coordinated PN(H)P, this is not alone the cause of this reversal of selectivity of electrophilic addition. This hydrogen bonding functionality has been observed earlier [18], indicating that even weakly nucleophilic triflate shows hydrogen bonding ability, especially in nonpolar solvent. The overall conclusion for the reactivity of the diatomic RuIVN ‘‘functionality” is that it is not Lewis acidic, in spite of its valence electron count, and that both nitrogens are targets of electrophilic reagents. [4] 3. Experimental 3.1. General Preparations from literature sources were used to synthesize RuN(SiMe2CH2PtBu2)2Cl and standard Schlenk or glovebox techniques in inert (Argon) atmosphere were used for air sensitive manipulations. All solvents, including deuterated NMR solvents, were dried over and distilled from Na/benzophenone and stored in anaerobic conditions. All other reagents were degassed and/ or used as received from commercial vendors. 1H, 13C{1H} and 31 1 P{ H} NMR spectra were recorded on a Varian Unity I400 (400 MHz 1H, 101 MHz 13C, 162 MHz 31P) instrument, with chemical shifts reported in ppm, referenced to protio impurities in each stated solvent, with the exception of 31P{1H} spectra which were externally referenced to 85% H3PO4 (0 ppm).
To 10.5 mg (0.0171 mmol) of (PNP)RuN in 0.5 mL C6D6, 10.5 lL of 1.0 M HCl in Et2O was added. Upon its addition at 22 °C, there was a rapid, distinct color change of the solution from green to yellow. After solvent and volatiles are quickly removed and the residue redissolved in the same solvent: 1H NMR (400 MHz, C6D6): d 3.94 (bs, 1H, NH), 1.47 (bvt, 18H, JP–H = 6.6 Hz, PCMe3), 1.29 (bvt, 18H, JP–H = 6.6 Hz PCMe3), 0.19 (s, 6H, SiMe2), 0.02 (s, 6H, SiMe2). 31 1 P{ H} (162 MHz, C6D6): d 81.5(s). 3.3. Reaction of (PNP)RuN with 1 equiv. of triflic acid To 10.1 mg (0.0171 mmol) of (PNP)RuN in 0.5 mL C6D6, 1.5 lL of triflic acid was added. Upon its addition at 22 °C, there was a rapid, distinct color change of the solution from green to brown: 1 H NMR (300 MHz, C6D6): d 4.25 (bs, 1H, NH), 1.21 (t, 36H, JP– H = 6.6 Hz, PCMe3), 1.12 (t, 4H, JP–H = 6.6 Hz PCH2Si), 0.49 (s, 12H, SiMe2). 31P{1H} (121 MHz, C6D6): d 68.3 (s). 19F NMR (282 MHz, C6D6): d 77.62 (bs). 13C{1H} (101 MHz, C6D6): d 34.32 (t, JC–P = 4.7 Hz, Si–CH2–P), 29.55 (t, JC–P = 3.6 Hz, PCMe3), 17.37 (s, PCMe3), 6.40 (s, SiMe2). Upon redissolving in THF-d8: 1 H NMR (300 MHz, THF-d8): d 3.62 (s, 1H, NH), 1.43 (t, 36H, JP– H = 6.9 Hz, PCMe3), 1.33 (t, 4H, JP–H = 6.9 Hz PCH2Si), 0.52 (s, 12H, SiMe2). 31P{1H} (121 MHz, THF-d8): d 72.1 (s). 19F NMR (282 MHz, THF-d8): d 78.89 (bs). Crystals were grown by layering from THF/benzene. 3.4. Reaction of (PNP)RuN with 1 equiv. of methyl triflate To 10.1 mg (0.0171 mmol) of (PNP)RuN in 0.5 mL C6D6, 1.9 lL of methyl triflate was added. Upon its addition at 22 °C, there was a rapid, distinct color change of the solution from green to brown: 1 H NMR (300 MHz, C6D6): d 3.03 (s, 3H, NMe), 1.18 (t, 36H, JP– H = 6.6 Hz, PCMe3), 1.12 (t, 4H, JP–H = 6.6 Hz PCH2Si), 0.47 (s, 12H, SiMe2). 31P{1H} (121 MHz, C6D6): d 69.6 (s). 19F NMR (282 MHz, C6D6): d 77.70 (bs).
636
A. Walstrom et al. / Inorganica Chimica Acta 363 (2010) 633–636
Acknowledgment We thank the National Science Foundation (NSF CHE-0544829) for financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2008.11.010. References [1] A. Walstrom, M. Pink, X. Yang, J. Tomaszewski, M.-H. Baik, K.G. Caulton, J. Am. Chem. Soc. 127 (2005) 5330. [2] C. Vogel, F.W. Heinemann, J. Sutter, C. Anthon, K. Meyer, Angew. Chem., Int. Ed. 47 (2008) 2681. [3] F. Akagi, T. Matsuo, H. Kawaguchi, Angew. Chem., Int. Ed. 46 (2007) 8778. [4] A. Walstrom, M. Pink, H. Fan, J. Tomaszewski, G. Caulton Kenneth, Inorg. Chem. 46 (2007) 7704.
[5] L.P. Spencer, B.A. MacKay, B.O. Patrick, M.D. Fryzuk, Proc. Natl. Acad. Sci. USA 103 (2006) 17094. [6] J.S. Figueroa, N.A. Piro, C.R. Clough, C.C. Cummins, J. Am. Chem. Soc. 128 (2006) 940. [7] C.-F. Leung, T.-W. Wong, T.-C. Lau, W.-T. Wong, Eur. J. Inorg. Chem. (2005) 773. [8] T.A. Betley, J.C. Peters, J. Am. Chem. Soc. 126 (2004) 6252. [9] A.G. Maestri, S.D. Taylor, S.M. Schuck, S.N. Brown, Organometallics 23 (2004) 1932. [10] J.K. Brask, V. Dura-Vila, P.L. Diaconescu, C.C. Cummins, Chem. Commun. (2002) 902. [11] M. Nishimura, S. Minakata, T. Takahashi, Y. Oderaotoshi, M. Komatsu, J. Org. Chem. 67 (2002) 2101. [12] W.-H. Leung, J.L.C. Chim, I.D. Williams, W.-T. Wong, Inorg. Chem. 38 (1999) 3000. [13] K.D. Demadis, T.J. Meyer, P.S. White, Inorg. Chem. 37 (1998) 3610. [14] N.M. Doherty, S.C. Critchlow, J. Am. Chem. Soc. 109 (1987) 7906. [15] A.G. Maestri, K.S. Cherry, J.J. Toboni, S.N. Brown, J. Am. Chem. Soc. 123 (2001) 7459. [16] See Supporting Information. [17] X. Yang, A. Walstrom, N. Tsvetkov, M. Pink, G. Caulton Kenneth, Inorg. Chem. 46 (2007) 4612. [18] A. Walstrom, M. Pink, N.P. Tsvetkov, H. Fan, M. Ingleson, K.G. Caulton, J. Am. Chem. Soc. 127 (2005) 16780.