Guanidine-Functionalized Rhenium Cyclopentadienyl Carbonyl

Jul 30, 2014 - Complexes: Synthesis and Cooperative Activation of H−H and O−H ... The latter are employed as platforms to study heterolytic H−H ...
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Guanidine-Functionalized Rhenium Cyclopentadienyl Carbonyl Complexes: Synthesis and Cooperative Activation of H−H and O−H Bonds Thomas S. Teets, Jay A. Labinger,* and John E. Bercaw* Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena, California, United States S Supporting Information *

ABSTRACT: Catalytic reactions utilizing carbon monoxide as a substrate are numerous, and they typically involve selective functionalization of a metal-bound CO. We have developed group 7 carbonyl complexes where secondary coordination sphere, Lewis acidic functionalities can assist in the activation of substrate molecules, mainly in the context of syngas conversion. This work describes a new class of cyclopentadienyl (Cp) rhenium carbonyl compounds of the type [Re(η5C5H4DMEG)(CO)3−n(NO)n]n (DMEG = dimethylethyleneguanidine, n = 0, 1), where a tethered guanidine base is appended to the Cp ring to participate in cooperative substrate activation with the electrophilic carbonyl. A reliable synthetic route for these complexes is presented, with crystallographic characterization of the free-base and protonated forms for both the carbonyl and mixed carbonyl-nitrosyl complexes. The latter are employed as platforms to study heterolytic H−H and O−H bond cleavage reactions that result in nucleophilic CO functionalization. The corresponding formyl complex is prepared by hydride transfer, and by measuring its hydricity (ΔG°H−) and pKa of the protonated base, the free energy of H2 cleavage is found to be +3.3(6) kcal/mol. The activation of methanol to form methoxycarbonyl complexes is found to be more favorable, with ΔG° ≈ 0 for the intramolecular addition of methanol to the guanidine-appended carbonyl complex. A detailed thermodynamic study is described for both the intramolecular methanol activation reaction and related intermolecular reactions with external bases. The results highlight some tangible thermodynamic benefits of tethering the base in the secondary coordination sphere.

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INTRODUCTION The nucleophilic activation of carbon monoxide plays an important role in numerous homogeneous catalytic processes, with both kinetic and thermodynamic advantages for adduct formation realized when the CO is bound to a transition metal center.1 Metal carbonyl adducts with a number of oxygen-, carbon-, and nitrogen-based nucleophiles have been isolated and characterized, which give rise to hydroxycarbonyl, alkoxycarbonyl, acyl, and carbamoyl complexes. Attack by the simplest nucleophile, hydride (H−), produces metal formyl complexes, which are frequently unstable with respect to decarbonylation but nonetheless have been isolated and characterized for several transition metals.2 The above-mentioned nucleophile adducts can be conveniently accessed using strong anionic nucleophilic reagents such as alkoxide salts, Grignard reagents, and borohydrides. However, for applications in catalysis where these species are often proposed as intermediates, the nucleophile is typically derived from a much less reactive precursor, often by cooperative activation of a small-molecule substrate. A representative example is found in the area of syngas conversion, where CO and H2 are cooperatively activated along the way to forming value-added hydrocarbon3 or oxygenated products.4 Whereas most known catalysts for © 2014 American Chemical Society

syngas conversion are heterogeneous, in which CO and H2 activation occur via surface-bound intermediates, homogeneous platforms for upgrading syngas offer the promise of improved product selectivity and a more detailed mechanistic understanding.5 A metal formyl is frequently invoked as the first intermediate in homogeneous syngas conversion, and its formation in this context is challenging. As mentioned above, metal formyl complexes are typically unstable with respect to CO loss, such that, with rare exception,6 direct insertion of CO into a metal−hydrogen bond is not thermodynamically feasible. Furthermore, it was recently demonstrated via experimental and computational analyses that the direct hydrogenation of a metal-bound CO is thermodynamically unfavorable.7 As such, strategies for the cooperative activation of H2 that deliver a hydride nucleophile to a metal carbonyl are required. Recent efforts have resulted in some key fundamental advances in homogeneous syngas conversion. Our group has demonstrated that pendant Lewis acidic boranes, in concert with a strong base, promote the activation of two equivalents of H2, forming C−H and ultimately C−C bonds,8 and that milder Lewis acids can promote C−C bond formation via intraReceived: June 18, 2014 Published: July 30, 2014 4107

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molecular attack on CO via methyl migration.9 In other groups, recent accomplishments include main group systems capable of stoichiometric hydrogenation of CO10 and intramolecular, reductive coupling of CO at an iron center.11 In none of these examples has catalytic syngas conversion been achieved. More recently, we outlined a new strategy for the cooperative activation of H2 at a metal carbonyl, utilizing tethered Brønsted bases in the secondary coordination sphere of phosphineligated rhenium pentacarbonyl complexes of the type [Re(P∼B:-κ1-P)(CO)5]n+ (n = 0, 1), where P∼B: is a phosphine ligand with a tethered neutral or anionic base.12 The target reaction in this work was heterolytic activation of H2 to form a rhenium formyl, with the tethered base accepting the H+ equivalent and the Lewis acidic carbonyl accepting the H− equivalent. A thermodynamic cycle, inspired by work from DuBois and co-workers,13 shows that the overall free energy of H2 cleavage, ΔG°H2, depends on the formyl’s hydricity (ΔG°H−) and the pKa of the protonated base. We ultimately determined that H2 cleavage was thermodynamically unfavorable in all cases, the least unfavorable being by 8(2) kcal/mol when a guanidine-tethered base was in place. We noted a strong attenuation in base strength upon coordination of the tetheredbase ligand to rhenium12 and also that ΔG°H− for these rhenium-phosphine complexes was no higher than 45(2) kcal/ mol, toward the low end for rhenium formyls.13 In thinking of potential modifications that would lead to more favorable free energy for H2 cleavage, one can envision either tethering stronger bases with larger pKa’s or utilizing more electrophilic carbonyl complexes, with larger ΔG°H− values. In this report, we disclose a new class of rhenium−carbonyl complexes, where a guanidine base is tethered to a cyclopentadienyl ligand. Complexes of the type [Re(η5-C5R5)(CO)2(NO)]+ give rise to formyl complexes that show exceptional thermal stability2,14 and have the highest recorded ΔG°H− values (R = H: 55.0 kcal/mol; R = CH3: 52.6 kcal/ mol).13 We have developed a synthetic route to install a guanidine base onto rhenium cyclopentadienyl complexes, and by sequential H− and H+ delivery we can observe the protonated formyl species that would result from heterolytic H2 cleavage. A quantitative thermodynamic assessment shows a significantly improved ΔG°H2 value relative to our initial foray, albeit still unfavorable. In addition to H2 cleavage, we also examine the cooperative activation of the O−H bond of methanol, which reacts reversibly with the guanidine-appended complex to furnish a methoxycarbonyl complex protonated at the guanidine base; a detailed thermodynamic analysis of this transformation is also undertaken. Alkoxycarbonyl complexes are common in the middle to late transition metal series,1,15 with a few structurally characterized examples from group 7 metals.16 To our knowledge, only one example exists of such a complex formed by intramolecular activation of methanol,15c and this lone example lacks any thermodynamic or kinetic analysis. We compare our results to the intermolecular activation of methanol with the known complexes [Re(η 5 -C 5 R 5 )(CO)2(NO)](BF4) (R = H, CH3), utilizing 2,6-lutidine as an external base and furnishing the respective neutral methoxycarbonyl complexes. All told, this work provides rigorous thermodynamic analyses of two classes of nucleophilic carbonyl activation, highlighting the potential for tethered bases to assist in these transformations and enumerating the thermodynamic advantages of having the base tethered to the metal complex.

Article

RESULTS Synthesis of Guanidine-Appended Carbonyl Complexes. The synthesis of the guanidine-appended carbonyl complexes described herein is summarized in Scheme 1. The Scheme 1. Synthesis of Guanidine-Substituted Rhenium Cyclopentadienyl Complexes

known complex Re(η5-C5H4NH2)(CO)317 was treated first with 2-chloro-1,3-dimethylimidazolinium chloride and triethylamine, followed by KOCMe3, which affords the neutral complex Re(η5-C5H4DMEG)(CO)3 (1, DMEG = dimethylethyleneguanidine) in 77% isolated yield. The 1H NMR spectrum shows the expected resonances attributed to the guanidine functionality, in addition to two triplets for the C5H4 protons; these resonances are consistent with an AA′BB′ spin system with JAB = JAB′, which gives the appearance of a simple A2B2 system and is typical of singly substituted cyclopentadienyl ligands. In the IR spectrum for complex 1 two strong ν̃CO stretching bands are resolved at 2012 and 1912 cm−1, slightly lower in energy than those of the parent amine complex.17 To substitute a nitrosyl ligand onto the guanidine-appended complex, we first opted to protect the basic nitrogen, to avoid potential deleterious reactions with nitrosonium. Protonation of neutral complex 1 proceeds in high yield using HBF4·Et2O in CH2Cl2. By this route the complex [Re(η5-C5H4DMEGH+)(CO)3](BF4) (2) was isolated in 80% yield in pure form. The 1 H NMR features attributed to the protonated C5H4DMEGH+ ligand in 2 are shifted downfield by 0.2−0.4 ppm relative to the corresponding resonances in 1, and a distinct singlet appears at 7.69 ppm, which is attributed to the NH proton. The carbonyl IR absorptions for protonated complex 2 are noticeably shifted relative to those of 1, with ν̃CO = 2024 and 1928 cm−1. Treatment of complex 2 with nitrosonium tetrafluoroborate results in clean substitution of one CO ligand for NO, in 4108

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Figure 1. X-ray crystal structures of 1−4, with ellipsoids drawn at the 50% probability level. Carbon-bound hydrogen atoms and outer-sphere counterions are omitted.

by equilibration with suitable bases. Complex 2 equilibrates with 2,6-lutidine (pKa = 14.13) and 4-dimethylaminopyridine (DMAP, pKa = 17.95),19 which in each case results in an equilibrium mixture of 1 and 2, along with the conjugate acid and base forms of the added base. With these two separate trials an average pKa of 16.0(2) was obtained (reported error for pKa values is two standard deviations). Similarly, addition of one equivalent of pyridine to a solution of 3 gives an equilibrium mixture consisting of 3 and 4 along with pyridinium and pyridine. Over two self-consistent trials the pKa of complex 3 was determined to be 10.9(3), demonstrating that the basicity of the tethered guanidine is substantially attenuated by substituting one of the carbonyls in complex 1 with a nitrosyl in complex 4. Synthesis of Formyl Complexes and Evaluation of ΔG° H2. As depicted in Scheme 2, complex 4 reacts

analogy to the route used for the parent unsubstituted cyclopentadienyl complex.18 The dicationic complex [Re(η5C5H4DMEGH+)(CO)2(NO)](BF4)2 (3) was isolated in 92% yield. The two chemically distinct groups of C5H4 protons are coincident in the 1H NMR spectrum (CD3CN), giving rise to a singlet at 6.11 ppm, with the NH proton shifting downfield to 8.27 ppm, as compared to tricarbonyl complex 2. The IR spectrum of complex 3 shows two distinct ν̃CO bands at 2109 and 2055 cm−1, occurring at higher energy than those of 2, with a band at 1816 cm−1 consistent with the presence of a NO ligand. The final step in the synthesis of the target complex [Re(η5C5H4DMEG)(CO)2(NO)](BF4) (4) involves deprotonation of 3. We found that the use of polymer-supported 1,3,4,6,7,8hexahydro-2H-pyrimido[1,2-a]pyrimidine gave good isolated yields (76%) of 4, minimizing decomposition and allowing for easy separation of the protonated base. Complex 4 lacks an NH peak in the 1H NMR and shows signatures that are qualitatively similar to tricarbonyl complex 2, albeit shifted downfield and with a larger frequency difference between the two C5H4 resonances. The IR spectrum of 4, much like the protonated precursor 3, shows two CO absorptions (2089 and 2031 cm−1) and one NO absorption (1783 cm−1), which shift to significantly lower energy upon deprotonation. Carbonyl complexes 1−4 were all crystallographically characterized. Crystallographic data and refinement parameters for 1−4 are summarized in Tables S1 and S2, with the structures depicted in Figure 1. The complexes are all geometrically similar, with η5-ligation of the C5H4DMEG ligand apparent in all cases. The distance between the Re center and the centroid of the substituted cyclopentadienyl ring is similar for complexes 1−4, ranging from 1.937(32) Å (one independent molecule of 3) to 1.9617(7) Å (1), with seemingly no systematic dependence on the protonation state of the guanidine or the charge at rhenium. In all cases the cyclopentadienyl and guanidine rings are twisted relative to one another, with dihedral angles ranging between 41.87(45)° (one independent molecule of 3) and 53.55(8)° (2). This twisted conformation does not appear to persist in solution, as the 1H NMR spectra of all complexes show single, sharp signals for the guanidine methylene and methyl groups. Also depicted in Figure 1, protonated complexes 2 and 3 show solid-state hydrogen-bonding interactions between the acidic N−H proton and a BF4− counterion; the structure of 3 shows a second counterion that is outer-sphere, and the single counterion in 4 likewise shows no interaction with the cationic complex. Quantitative Acid/Base Properties of 1−4. The acetonitrile pKa values for complexes 2 and 3 were determined

Scheme 2. Synthesis of Formyl Complex 5

quantitatively with the strong hydride donor [PPN][cisW(H)(P(OMe)3)(CO)4] (W−H, PPN = bis(triphenylphosphine)iminium)20 to instantly form the neutral formyl complex Re(η5-C5H4DMEG)(CO)(NO)(CHO) (5). Complex 5 was not isolated, but as we found previously,12 the tungsten byproduct cis-W(P(OMe)3)(NCMe)(CO)4 (W0) and the salt product (PPN)(BF4) do not interact with the formyl product or interfere with subsequent studies. Once formed, complex 5 shows good stability at room temperature, with minimal decomposition observed during the first 24 h. Complex 5 possesses several distinguishing spectroscopic features. The chirality at the rhenium center is evident from both the 1H and 13C{1H} NMR spectra. In the 1H NMR spectrum there are four distinct multiplets for the C5H4 protons in the cyclopentadienyl region, spanning 4.67−5.87 ppm, and the 13C{1H} NMR spectrum displays five distinct signals for the cyclopentadienyl carbons, with four clustered together between 4109

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by 1H NMR integration. An average ΔG°H− of 51.7(6) kcal/ mol was determined for complex 5 over three independent trials. The pKa of the protonated formyl complex 6 was determined by equilibrating neutral formyl 5 with 2,6lutidinium tetrafluoroborate, with an average pKa of 15.35(1) obtained. Using a thermodynamic cycle,12 the ΔG°H2 for eq 1 was determined using eq 2:

78.7 and 87.2 ppm and the ipso carbon appearing much further downfield at 143.2 ppm. The presence of the formyl group is also clearly evident, from both NMR and IR spectroscopies. The downfield 1H (14.80 ppm) and 13C{1H} (254.3 ppm) NMR signals are diagnostic of a formyl ligand,2 with the 13 C{1H} spectrum also showing a peak at 205.8 ppm for the unaltered CO ligand. The IR spectrum shows three distinct bands at 1980, 1703, and 1605 cm−1 for the carbonyl, nitrosyl, and formyl ligands, respectively. As shown in Scheme 3, neutral formyl complex 5 can be protonated in thawing acetonitrile solution with pyridinium

ΔGo H2 = (76.0 − 1.37 × pK a − ΔGo H −) kcal/mol

(2)

With the above-mentioned values, a ΔG°H2 of +3.3(6) kcal/ mol is calculated for the cleavage of H2 by complex 4. Synthesis of Methoxycarbonyl Complexes. The neutral methoxycarbonyl complexes Re(η5-C5H4DMEG)(CO)(NO)(C(O)OCH3) (7), Re(η5-Cp*)(CO)(NO)(C(O)OCH3) (8), and Re(η5-Cp)(CO)(NO)(C(O)OCH3) (9) are prepared by the general method summarized in Scheme 4, whereby the

Scheme 3. Synthesis of Protonated Formyl Complex 6

Scheme 4. Synthesis of Methoxycarbonyl Complexes 7−9

tetrafluoroborate, generating the protonated formyl complex [Re(η5-C5H4DMEGH+)(CO)(NO)(CHO)][BF4] (6), which forms nearly quantitatively. Complex 6 shows poor thermal stability, particularly in concentrated solutions, which hindered characterization by 13C{ 1H} NMR. Prior to complete decomposition, a 13C{1H} NMR signal for the formyl carbon is observed at 247.2 ppm, considerably upfield of that for neutral formyl 5. The 1H NMR spectrum of 6 is diagnostic: the four distinct cyclopentadienyl signals, occurring significantly downfield of those in 5, again indicate metal-based chirality, and the formyl resonance at 16.01 ppm shifts substantially downfield in the protonated complex. The formyl band in the IR is obscured by ligand-based absorptions that grow in upon protonation, but the carbonyl stretching frequency (2012 cm−1) and nitrosyl stretching frequency (1738 cm−1) both shift to higher energy, consistent with protonation of the C5H4DMEG ligand. Complex 6 is the desired product of heterolytic H2 cleavage by carbonyl complex 4, as shown in eq 1:

respective cationic carbonyl precursors are treated with one equivalent of methanolic KOMe in thawing acetonitrile. The products are extracted into Et2O to separate the KBF4 byproduct, allowing complexes 7−9 to be isolated in good yields and high purities. The 1H NMR spectra of complexes 7−9 all show a singlet near 3.5 ppm that is characteristic of the OCH3 protons. In addition, the 1H NMR spectrum of guanidine-substituted complex 7, much like those of the formyl complexes 5 and 6, shows four distinct multiplets in the cyclopentadienyl region (5.11−5.57 ppm), characteristic of a chiral metal center. The 13 C{1H} NMR spectra of 7−9 show two distinct downfield resonances for the carbonyl and methoxycarbonyl carbons. The furthest downfield peak, occurring at 207.6 (7), 209.1 (8), and 203.9 (9) ppm, is assigned to the unmodified CO ligand. As confirmed by an HMBC correlation to the OCH3 protons for complex 8 and by analogy for 7 and 9 (Figure S1 in the Supporting Information), the more upfield carbonyl resonance, occurring at 186.4 (7), 188.8 (8), and 182.9 (9) ppm, is attributed to the metal-bound carbon of the methoxycarbonyl group. Much like formyl complex 5, the IR spectra of 7−9 show three distinct stretching frequencies for the carbonyl, nitrosyl, and methoxycarbonyl groups, with the latter occurring at 1622 (7), 1626 (8), and 1630 (9) cm−1, in the range expected for a CO double bond. Guanidine-substituted complex 7 was analyzed by singlecrystal X-ray diffraction, with the structure depicted in Figure 2. The distance from the rhenium atom to the cyclopentadienyl centroid is observed to be 1.9649(12) Å, slightly longer than those of carbonyl complexes 1−4. The dihedral angle between the cyclopentadienyl and guanidine rings is 37.08(20)°, slightly smaller than any of the corresponding angles for 1−4. The

We subjected complex 4 to H2, at pressures of up to 3 atm, and over prolonged time periods saw no evidence for the formation of 6 or any other product. Evaluation of the free energy of the reaction shown in eq 1, which we abbreviate as ΔG°H2, requires the hydricity (ΔG°H−) of the formyl complex 5 and the pKa of the product 6.12 The hydricity of 5 was established by transformylation, using known formyl complexes Re(η5-Cp)(CO)(NO)(CHO) (Cp = cyclopentadienyl, ΔG°H− = 55.0 kcal/mol) and Re(η5-Cp*)(CO)(NO)(CHO) (Cp* = pentamethylcyclopentadienyl, ΔG°H− = 52.6 kcal/mol)13 in combination with carbonyl complex 4 and measuring equilibrium populations of the formyl and carbonyl species 4110

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The equilibrium constant for the methanol activation reaction in eq 3, which is given by Keq = [10]/[4][CH3OH], was determined by subjecting complex 4 to varying concentrations in methanol in CD3CN. The ratio [10]/[4] was determined by NMR integration and plotted against [CH3OH]. As shown in Figure S2 of the Supporting Information, the resulting plot is linear over a concentration range of 0.13 to 2.1 M, and the slope gives a value of 1.25(7) for Keq. The same analysis was carried out using CH3OD, with the data also shown in Figure S2. A significantly smaller value of 0.99(8) was determined for the Keq of CH3OD addition, giving KH/KD = 1.3(1). These Keq values correspond to ΔG° of −0.13(3) and 0.00(5) kcal/mol for CH3OH and CH3OD addition, respectively. The methanol-addition equilibrium in eq 3 was evaluated as a function of temperature, and inspection of the variabletemperature NMR spectra (Figure S3) shows that the equilibrium becomes more favorable at lower temperatures. The resulting van’t Hoff plots, where ln(Keq) is plotted against 1/T, are shown in Figure 3. Good linearity is observed between

Figure 2. X-ray crystal structure of 7. Hydrogen atoms are omitted for clarity.

C(2)−O(3) internuclear distance is 1.214(2) Å, consistent with a carbon−oxygen double bond and longer than the C(1)−O(1) carbonyl distance of 1.146(3) Å. The bond angles about C(2) are 119.2(2)° (O(3)−C(2)−O(4)), 127.4(2)° (O(3)−C(2)− Re(1)), and 113.43(18)° (O(4)−C(2)−Re(1)), consistent with sp2 hybridization. Intramolecular Methanol Activation by Complex 4. When complex 4 is treated with an excess of methanol in CD3CN, the complex [Re(η5-DMEGH+)(CO)(NO)(C(O)(OCH3)](BF4) (10) forms rapidly (eq 3), with the yield

Figure 3. van’t Hoff plots for the formation of 10 by the addition of CH3OH (●, blue) and CH3OD (■, red) to complex 4 in CD3CN.

−15 and +65 °C. For the addition of CH3OH, the linear fit gives ΔH° = −9.0(1) kcal/mol and ΔS° = −29.7(4) eu. When CH 3 OD is substituted, minimally different van’t Hoff parameters are obtained, with ΔH° = −8.8(1) kcal/mol and ΔS° = −29.2(5) eu. The ΔG° values determined from these van’t Hoff parameters, −0.1(2) kcal/mol, are within experimental error of the free energy determined from the equilibrium constant determination described above. All thermodynamic parameters for the activation of methanol by complex 4 are summarized in Table 1.

dependent on the amount of added methanol. Complex 10 is the protonated analogue of methoxycarbonyl complex 7 and shows analogous but shifted NMR features. The 1H NMR features of 10 are all shifted downfield by ca. 0.2−0.3 ppm compared to those of 7, with the characteristic OCH3 singlet observed at 3.67 ppm.21 At very high methanol concentrations the N−H proton resonance is not observed, but when [CH3OH] is lower than ∼2 M, a broad resonance at 9.47 ppm is consistent with the presence of a protonated guanidine group. The key 13C NMR resonances also are shifted in 10; the CO resonance shifts upfield from 207.6 ppm in 7 to 201.0 ppm in 10, with the methoxycarbonyl resonance shifting downfield from 186.4 ppm in 7 to 193.2 ppm. In addition, whereas the CO stretching frequency occurs at slightly lower energy in 10 (1606 cm−1 compared to 1622 cm−1), the hypsochromic shifts of the ν̃CO (1979 to 2007 cm−1) and ν̃NO (1709 to 1745 cm−1) bands mirror the shifts observed for carbonyl complexes 1−4 when comparing free-base and protonated congeners. The pKa of the protonated methoxycarbonyl complex 10 was evaluated by treating a sample of free-base complex 7 with one equivalent of 2,6-lutidinium tetrafluoroborate. In this solution, with no added methanol present, the major species at equilibrium is carbonyl complex 4, formed by loss of methanol. However, ∼6% of the sample remains as methoxycarbonyl complexes, with the 1H NMR chemical shifts indicating a mixture of free-base complex 7 and protonated complex 10. By evaluating this equilibrium relative to the pKa of 2,6-lutidinium, a pKa of 16.2(1) was determined for 10.

Table 1. Thermodynamic Parameters for Methanol Activation by Complex 4 CH3OH CH3OD a

Keq

ΔG°a,b

ΔH°a,c

ΔS°c,d

ΔG°a,c

1.25(7) 0.99(8)

−0.13(3) 0.00(5)

−9.0(1) −8.8(1)

−29.7(4) −29.2(5)

−0.1(2) −0.1(2)

In kcal/mol. bFrom Keq determination. cFrom van’t Hoff plot. dIn eu.

Intermolecular Activation of Methanol. We also investigated the formation of methoxycarbonyl complexes by intermolecular activation of methanol, using an external, untethered base. The Cp*-ligated methoxycarbonyl complex 8 is formed reversibly when [Re(η5-Cp*)(CO)2(NO)](BF4) is treated with 2,6-lutidine and methanol, as summarized in Scheme 5. Keq for this reaction, measured in an analogous fashion to the intramolecular equilibrium (Figure S4), was found to be 0.85(2), giving ΔG° = 0.10(1) kcal/mol. 4111

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Scheme 5. Intermolecular Activation of Methanol to Form 8 and 9

Figure 4. van’t Hoff plots for the formation of 8 (●, blue) and 9 (■, red) from the respective [Re(η5-C5R5)(CO)2(NO)](BF4) complex, CH3OH, and 2,6-lutidine.

are observed for the formation of 8 and 9 using 2,6-lutidine as the base. For the Cp* analogue 8, the van’t Hoff analysis gives ΔH° = −8.6(1) kcal/mol, with ΔS° = −29.0(4) eu. For the formation of Cp-ligated methoxycarbonyl complex 9, the plot in Figure 4 gives ΔH° = −10.4(1) kcal/mol and ΔS° = −26.6(4) eu. The corresponding ΔG° values, extracted from the van’t Hoff parameters, are 0.0(2) kcal/mol (8) and −2.5(2) kcal/mol (9), in good agreement with the ΔG° values obtained from measurement of Keq at 298 K over a range of methanol concentrations. The thermodynamic parameters for intermolecular methanol activation are summarized in Table 2. Table 2. Thermodynamic Parameters for Intermolecular Methanol Activation to Form 8 and 9 8 9 a

ΔG°a,b

ΔH°a,c

ΔS°c,d

ΔG°a,c

0.85(2) 71(1)

0.10(1) −2.52(1)

−8.6(1) −10.4(1)

−29.0(4) −26.6(4)

0.0(2) −2.5(2)

DISCUSSION

The development of homogeneous catalysts for the conversion of synthesis gas requires metal platforms that can simultaneously activate CO and H2. We recently outlined a new strategy for the heterolytic cleavage of H2, where an electrophilic metal carbonyl and a tethered strong base cooperatively cleave H2, with the carbonyl accepting a hydride equivalent and the base accepting a proton.12 In this previous work, we noted that the free energy for this H2 cleavage step, ΔG°H2, was unfavorable in all cases. To achieve more favorable ΔG°H2 values, next-generation platforms required either stronger tethered bases (i.e., larger pKa for the conjugate acid) or more electrophilic carbonyl ligands (i.e., larger ΔG°H−). Our previous work also suggested that neutral bases were the smarter strategy, as our lone attempt with an anionic phenoxide base led to a large attenuation of ΔG°H− that more than compensated the gain in base strength. In addition, while there are several neutral nitrogenous bases available with larger pKa values19 than anything we employed, synthetic inroads for tethering these bases to rhenium carbonyl complexes did not immediately present themselves. As such, we honed in on the alternative strategy, focusing on developing platforms where Brønsted bases are appended to cationic cyclopentadienyl rhenium carbonyl complexes, which have the largest known ΔG°H− values.13 To synthesize the guanidine-appended rhenium carbonyl complexes utilized in this study, we adapted methodology (see Scheme 1) used for installing guanidine groups onto ferrocene.23 The cyclic guanidine dimethylethyleneguanidine was chosen, owing to the commercial availability of the 2chloroimidazolium salt that is coupled with the amine group of Re(η5-C5H4NH2)(CO)3. Complex 1 is thus obtained, and replacement of a carbonyl in 1 with nitrosyl first requires protection of the basic nitrogen by protonation, necessitating the isolation of protonated complexes 2 and 3 on the way to finally obtaining target complex 4. The suite of carbonyl complexes outlined in Scheme 1 provide an opportunity to assess how the electronic structure at rhenium influences the basicity of the tethered guanidine. Substitution of a carbonyl ligand in 1/2 with a nitrosyl to generate 3/4 maintains an 18-electron count at rhenium, but increases the charge by 1 and increases the formal valency.24 This change has a large effect on the acid/base properties: the pKa of tricarbonyl complex 2 is 16.0(2), decreasing dramatically to 10.9(3) in nitrosyl-substituted 3. Thus, it is evident that the basic nitrogen experiences the decrease in electron density brought on by substituting a carbonyl for nitrosyl, and although there is not a readily accessible “free-ligand” pKa to compare to, these pKa values are both substantially lower than those of simple aryl guanidine compounds, which typically fall in the range of ∼18−20.25 We can also assess the inverse relationship, the effect of protonation on the electronic environment of the rhenium center, by comparing IR bands for protonated and free-base analogues. In both instances, we see evidence for a decrease in electron density upon protonation. In comparing tricarbonyl complexes 1 and 2, protonation induces a 12 and 16 cm−1 hypsochromic shift in the two CO stretches. Similarly, protonation of 4 to give 3 shifts the CO stretches by 20 and 24 cm−1 and the NO stretch by 33 cm−1 to higher energy. This observation stands in contrast to what we saw before with pentacarbonyl rhenium phosphine complexes, where the largest such shift upon protonation was 10 cm−1, with ∼5 cm−1 more

To investigate the effect of altering the electrophilicity of the carbonyl, the unsubstituted complex [Re(η5-Cp)(CO)2(NO)](BF4) was investigated under identical conditions (Scheme 5). The unsubstituted complex, on the basis of IR spectra18,22 and hydricity measurements,13 is expected to be substantially more electrophilic than the methylated analogue. The combination of this complex with CH3OH and 2,6-lutidine also forms the methoxycarbonyl complex, 9, as the exclusive product. The equilibrium constant for this reaction was determined to be 71(1), which corresponds to a ΔG° of −2.52(1) kcal/mol (Figure S5). The equilibrium constants for the reactions described in Scheme 5 were also evaluated over a range of temperatures. Partial variable-temperature 1H NMR spectra are shown in Figures S6 and S7, with the overlaid van’t Hoff plots displayed in Figure 4. Significant differences in the ΔH° and ΔS° values

Keq

Article

In kcal/mol. bFrom Keq determination. cFrom van’t Hoff plot. dIn eu. 4112

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Organometallics

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common.12 This difference may simply be due to the different symmetries and disparate number of CO/NO ligands between the two classes of complexes, but it seems reasonable to suppose that in the complexes presented here, where the basic nitrogen is bonded directly to the rhenium-bound cyclopentadienyl ligand, there is greater coupling between the protonation state of the guanidine and the electron density at the metal center. Our initially targeted application for complex 4 is heterolytic H2 cleavage, which would generate protonated formyl complex 6. To evaluate the thermodynamics of this transformation (eq 1), we need both the hydricity of the neutral formyl complex 5 and the pKa of 6. By transformylation with known Cp- and Cp*-ligated formyl complexes, we measure a ΔG°H− of 51.7(6) kcal/mol for 5, which is smaller than those of both the Cp (55.0 kcal/mol) and Cp* analogues (52.6 kcal/mol). This observation is another example of how the presence of the electron-rich guanidine influences the rhenium center’s electronic structure, where the presence of a single guanidine is observed to give a more electron-rich (less electrophilic) complex than the presence of five methyl groups as in Cp*. The pKa of formyl complex 6 is 15.35(1), which in combination with the aforementioned hydricity gives ΔG°H2 = +3.3(6) kcal/ mol. Thus, H2 cleavage is still in a thermodynamically unfavorable regime, though less so than in the best case (ΔG°H2 = 8(2) kcal/mol) we observed in our first-generation phosphine complexes.12 This improvement is largely tied to the enhancement of ΔG°H−, which is 7(2) kcal/mol higher for complex 5 than for our previous phosphine-terminated species. The pKa of 6 (15.35(1)) is actually slightly lower than that of the phosphine-linked guanidine complex studied previously (16.6(3)), though this difference, which corresponds to 1.7(4) kcal/mol in driving force, is more than compensated by the difference in hydricity. In this study we also present a thermodynamic analysis of the activation of methanol by complex 4, which produces the protonated methoxycarbonyl complex 10. This equilibrium (described in eq 3) is established rapidly and the product is indefinitely stable, facilitating an in-depth study of its thermodynamic profile. The Keq for methanol addition is found to be 1.25(7) at 298 K and is only slightly smaller (0.99(8)) with CH3OD, giving a small but statistically significant KH/KD of 1.3(1). A van’t Hoff analysis was carried out, and we find that the activation of methanol by complex 1 is enthalpically favored, with ΔH° = −9.0(1) kcal/mol, though this enthalpy is almost perfectly offset by a large, negative entropy of −29.7(4) eu.; only minimal differences were observed with CH3OD, consistent with the small KH/KD measured independently. Of greater interest to us are differences between the intramolecular activation of methanol by 4, where the base is tethered directly to the carbonyl complex, and an intermolecular activation using an external base. The driving force for such a reaction depends on the strength of the base used, and we first focused on a system for which the free energy change would be close to 0, similar to the intramolecular reaction described above. By using 2,6-lutidine as the base in concert with [Re(η5-Cp*)(CO)2(NO)](BF4), intermolecular methanol activation to form methoxycarbonyl complex 8 occurs with an observed ΔG° of 0.10(1) kcal/mol, very near that of the intramolecular activation by 4. The van’t Hoff analysis of this intermolecular case gives ΔH° and ΔS° values that are virtually indistinguishable from the intramolecular activation, suggesting

that at parity of driving force there is no enthalpic or entropic benefit or detriment to tethering the base. We also show that the more electrophilic rhenium carbonyl complex [Re(η5Cp)(CO)2(NO)](BF4), combined with the same 2,6-lutidine base, forms the resulting methoxycarbonyl complex 9 with ΔG° = −2.52(1) kcal/mol, which is 2.62(1) kcal/mol more favorable than the formation of 8. Most of this difference in driving force comes from the difference in enthalpy, which is 1.8(1) kcal/mol more negative for the formation of 8. The above-mentioned analysis of methanol activation allows for the construction of a relative scale of methoxide affinity, by considering a thermodynamic cycle for base-assisted methanol activation that is analogous to the sequence used to evaluate base-assisted H2 cleavage.12,13 Figure 5 summarizes the relative

Figure 5. Relative anion affinities for rhenium cyclopentadienyl complexes. The numbers represent ΔΔG° values for hydride and methoxide attack on the rhenium carbonyl center.

anion affinities of the three variants, which track in the same direction for hydride and methoxide attack. Placing the electron-rich guanidine onto the Cp ring gives rise to the least electrophilic complex with the smallest anion affinity, and whereas the hydride and methoxide affinities of the Cp and Cp* complexes differ by approximately the same amount (2.4 and 2.6 kcal/mol), there is a much larger difference in the relative methoxide affinity for the guanidine complex (2.6 kcal/ mol smaller than the Cp* analogue) as compared to the relative hydride affinity (0.9 kcal/mol). Determination of absolute methoxide affinities, i.e., ΔG°CH3O−, requires a value for the pKa of methanol in acetonitrile, which we can estimate to be 40.26 With this pKa estimate in hand, we can determine absolute values for methoxide affinities that are ∼18 kcal/mol smaller in magnitude than the corresponding hydride affinities, ΔG°H−. This difference is in line with the difference between gas-phase hydride and methoxide affinities for Fe(CO)5 (12.5 kcal/mol), determined previously.27 The results presented here beg the obvious question: is there a thermodynamic benefit of tethering the base to the metal center for cooperative, nucleophilic activation of metal-bound carbonyls? To answer this question, consider the hypothetical reaction shown in eq 4, where complex 4 participates in the bimolecular activation of H2 to form protonated carbonyl complex 3 and neutral formyl complex 5. Using eq 2, along with the ΔG°H− of 5 and the pKa of 3, ΔG°H2 is calculated to be +9.4(7) kcal/mol, which is 6.1(9) kcal/mol less favorable than the intramolecular activation. In other words, given the hypothetical reaction in eq 4, there would be 6 kcal/mol of driving force for complexes 5 and 3 to 4113

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Organometallics

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bases to electrophilic carbonyl complexes that may lead to enhanced reactivity toward substrates of interest.

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EXPERIMENTAL SECTION

Materials. All reactions were executed in a glovebox filled with argon or on a Schlenk line under argon. Solvents were dried by passing through an alumina column using the method of Grubbs,28 and deuterated NMR solvents were passed through a short column of activated alumina prior to use. CD3CN was further dried by storing over molecular sieves. The starting material Re(η5-C5H4NH2)(CO)317 was prepared by a modified literature procedure; we found that the crude material isolated from this procedure could be carried forward without recrystallization or other purification. The reagents (NO)(BF4), 2-chloro-1,3-dimethylimidazolinium chloride, KOCMe3, polymer-bound 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine, 4dimethylaminopyridine, and HBF4·Et2O were obtained commercially and used without purification. Commercially available bases 2,6lutidine and pyridine were purified by standard methods.29 The tetrafluoroborate salts of 2,6-lutidine and pyridine were obtained by treating a solution of the base in Et2O with one equivalent of HBF4· Et2O; the salt precipitated as a white solid, which was deemed pure by 1 H NMR. The hydride reagent (PPN)[cis-W(H)(P(OMe)3)(CO)4]20 and the rhenium complexes [Re(η5-Cp)(CO)2(NO)](BF4)18 and [Re(η5-Cp*)(CO)2(NO)](BF4)22 were prepared as previously described. Physical Methods. 1H NMR spectra were recorded on a Varian Mercury 300 spectrometer, operating at 300 MHz. 13C{1H} NMR were recorded on a Varian Inova 500 spectrometer, operating at 125.7 MHz. 1H and 13C{1H} spectra were referenced to solvent resonances and acquired at 25 °C. For 13C{1H} NMR spectra of formyl complexes, the default decoupling frequency resulted in incomplete decoupling for the formyl resonance. Hence for all formyl complexes additional scans were recorded with the 1H decoupling frequency centered at 15 ppm. IR spectra were recorded on a Nicolet 6700 spectrometer. Samples were housed in a solution cell with KBr windows and a 0.1 mm Teflon spacer. Elemental analyses were performed by Midwest Microlab LLC or Robertson Microlit Laboratories. Preparation of Re(η5-C5H4DMEG)(CO)3 (1). A 100 mL Schlenk flask was charged with Re(η5-C5H4NH2)(CO)3 (500 mg, 1.43 mmol) and NEt3 (0.20 mL, 1.4 mmol, 1.0 equiv) dissolved in 20 mL of CH2Cl2. A solution of 2-chloro-1,3-dimethylimidazolinium chloride (482 mg, 2.85 mmol, 1.99 equiv) in 20 mL of CH2Cl2 was added via cannula, giving a yellow solution. The solution was stirred under argon for 20 h at room temperature. At this point, solid KOCMe3 (∼800 mg, 7.1 mmol, 5.0 equiv) was added under argon counterflow, and the mixture was stirred for 15 min. The volatiles were removed in vacuo, and the remaining tan solid was taken into the glovebox and washed with 75 mL of hexane. The crude product was extracted three times with 80 mL aliquots of Et2O, and each extract was filtered through Celite and concentrated in vacuo. The off-white product was dissolved in 10 mL of CH2Cl2 and filtered again through Celite. The solution was concentrated to ∼2 mL, causing some white solid to precipitate, and addition of 15 mL of hexane further separated the product. The brown supernatant was decanted, and the white microcrystalline product was dried in vacuo overnight. Yield: 490 mg (76.9%). 1H NMR (300 MHz, CD3CN) δ: 5.20 (t, J = 2.2 Hz, 2H, C5H4), 5.10 (t, J = 2.2 Hz, 2H, C5H4), 3.28 (s, 4H, CH2), 2.67 (s, 6H, CH3). 13C{1H} NMR (126 MHz, CD2Cl2) δ: 196.6 (CO), 160.2 (CN), 140.1 (C5H4ipso), 80.0 (C5H4), 73.2 (C5H4), 48.7 (CH2), 35.4 (CH3). IR (CH3CN): ν̃CO = 2012 (s), 1912 (s) cm−1. Anal. Calcd for C13H14N3O3Re: C, 34.97; H, 3.16; N, 9.41. Found: C, 34.84; H, 2.98; N, 9.35. Preparation of [Re(η5-C5H4DMEGH+)(CO)3](BF4) (2). A sample of 1 (442 mg, 0.990 mmol) was dissolved in 4 mL of CH2Cl2 and filtered through a plug of glass microfiber into a scintillation vial. To the stirring solution was added HBF4·Et2O (140 μL, 1.04 mmol, 1.05 equiv), initially giving no change in appearance. The solution was stirred for 30 min at room temperature, during which time some white

react to furnish protonated formyl complex 6 and the starting carbonyl species 4. This difference in driving force between the bimolecular and unimolecular activation is governed by the ΔpKa between the formyl or methoxycarbonyl complex and the starting carbonyl complex. The same analysis for methanol activation reveals that the intramolecular activation of methanol to form complex 10 is 7.3(4) kcal/mol more favorable than intermolecular activation by the same complex. The precise origin of this stabilization is likely a combination of factors, namely, Coulombic stabilization provided by placing the proton and anion equivalents in close proximity and perhaps (though we have no direct evidence to support this) intramolecular hydrogen bonding between the protonated tethered base and the formyl/methoxycarbonyl oxygen; these proposed hydrogen-bonding interactions are shown for the structures of 6 and 10 throughout. The cyclopentadienyl complexes described here provide greater stabilization for the intramolecular adducts as compared to our previously studied rhenium-phosphine complexes. For H2 activation on the phosphine complexes with neutral nitrogenous bases, the intramolecular adducts showed stabilizations of 1.9(3) and 4.8(8) kcal/mol. It is also important to note that tethering the base, as we have seen here and previously,12 attenuates the basicity by a substantial amount and, in the case described here, also attenuates the carbonyl’s electrophilicity. As such, the analysis presented above, which points to the thermodynamic benefit of tethering the base versus using an external base, only holds at parity of pKa and carbonyl electrophilicity and does not imply that using an external base of the same type, i.e., a substituted guanidine in this case, would be thermodynamically disadvantageous. The work described here shows that there are thermodynamic benefits of tethering a Brønsted base to an electrophilic carbonyl complex for the cooperative activation of substrates. We have demonstrated an improvement in ΔG°H2 by incorporating a guanidine base into a more electrophilic rhenium carbonyl complex, but have still fallen short of bringing this reaction into a thermodynamically favorable regime. Complementary studies on methanol activation, which is close to thermoneutral for complex 4, provide a platform for in-depth thermodynamic interrogation, allowing for a detailed comparison between intramolecular and intermolecular activation. Moving forward, we are interested in incorporating these motifs into catalysts, both for upgrading syngas and for other transformations involving CO as a substrate. From a fundamental standpoint we are also interested in the potential kinetic benefits of a tethered base; the methanol activation reactions described here typically reach equilibrium quite rapidly (at least at NMR concentrations), which is not conducive to a thorough study of reaction rates. We are targeting alternative platforms where rates and activation parameters for cooperative bond cleavage can be accessed and exploring alternative architectures for tethering strong 4114

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indicated good yield of formyl complex 6. 1H NMR (500 MHz, CD3CN) δ: 16.01 (s, 1H, CHO, 1JCH = 160 Hz), 9.97 (br s, 1H, NH), 6.02 (m, 1H, C5H4), 5.98 (m, 1H, C5H4), 5.80 (m, 1H, C5H4), 5.62 (m, 1H, C5H4), 3.71 (m, 4H, CH2), 2.95 (s, 6H, CH3). This complex showed poor stability at high concentrations, complicating analysis by 13 C NMR. Prior to decomposition, and with the 1H decoupling frequency centered at 15 ppm, a singlet at δ = 247.2 ppm was observed, consistent with the formyl. IR (CH3CN): ν̃CO = 2012 (s) cm−1; ν̃NO = 1738 (s) cm−1; three broad, poorly resolved peaks were observed in the ν̃CO region, precluding accurate assignment of a formyl stretch. Preparation of Re(η5-C5H4DMEG)(CO)(NO)(C(O)OCH3) (7). An aliquot of methanolic KOMe (24.5 wt %, 38 μL, 0.13 mmol, 1.0 equiv) was added to a thawing solution of 4 (68 mg, 0.13 mmol) in 4 mL of CH3CN. The solution was allowed to warm to room temperature with stirring, with a yellow-brown color resulting. After stirring for 20 min, the mixture was filtered through a plug of glass microfiber and concentrated in vacuo. The crude product was extracted with 4 × 8 mL of Et2O, and the combined ether extracts were concentrated and dried in vacuo, leaving a yellow-orange powder. Yield: 45 mg (74%). 1 H NMR (500 MHz, CD3CN) δ: 5.57 (m, 1H, C5H4), 5.44 (m, 1H, C5H4), 5.37 (m, 1H, C5H4), 5.12 (m, 1H, C5H4), 3.47 (s, 3H, OCH3), 3.35 (s, 4H, CH2), 2.69 (s, 6H, NCH3). 13C{1H} NMR (126 MHz, CD3CN) δ: 207.6 (CO), 186.4 (C(O)OCH3), 162.2 (CN), 147.1 (C5H4ipso), 90.0 (C5H4), 87.3 (C5H4), 80.6 (C5H4), 80.0 (C5H4), 50.0 (OCH3), 48.8 (CH2), 35.3 (NCH3). IR (CH3CN): ν̃CO = 1979 (s) cm−1; ν̃NO = 1709 (s) cm−1; ν̃CO (methoxycarbonyl) = 1622 (m) cm−1. Anal. Calcd for C13H17N4O4Re: C, 32.56; H, 3.57; N, 11.68. Found: C, 33.38; H, 3.72; N, 11.71. Preparation of Re(η5-Cp*)(CO)(NO)(C(O)OCH3) (8). A solution of [Re(η5-Cp*)(CO)2(NO)](BF4)22 (82 mg, 0.16 mmol) in 4 mL of CH3CN was frozen. The solution was removed, and upon thawing, a KOMe solution (24.5 wt %, 50 μL, 0.16 mmol, 1.0 equiv) was added via syringe. The color darkened from yellow to orange as the solution was allowed to warm to room temperature and stirred for 20 min. At this time the mixture was filtered and the volatiles were removed in vacuo. The crude product was extracted with 4 mL of Et2O, filtered, and concentrated in vacuo, leaving an orange solid. Yield: 66 mg (90%). 1H NMR (300 MHz, CD3CN) δ: 3.50 (s, 3H, OCH3), 2.11 (s, 15H, C5(CH3)5). 13C{1H} NMR (126 MHz, CD3CN) δ: 209.1 (CO), 188.8 (C(O)OCH 3), 105.9 (C 5(CH3 )5 ), 50.7 (OCH3 ), 10.4 (C5(CH3)5). IR (CH3CN): ν̃CO = 1974 (s) cm−1; ν̃NO = 1709 (s) cm−1; ν̃CO (methoxycarbonyl) = 1626 cm−1 (m). Anal. Calcd for C13H18NO4Re: C, 35.61; H, 4.14; N, 3.19. Found: C, 35.83; H, 3.97; N, 3.31. Preparation of Re(η5-Cp)(CO)(NO)(C(O)OCH3) (9). To a thawing solution of [Re(η5-Cp)(CO)2(NO)](BF4) (70 mg, 0.16 mmol) in 4 mL of CH3CN was added KOMe in methanol (24.5 wt %, 50 μL, 0.16 mmol, 1.0 equiv). The resulting yellow-orange solution was allowed to warm to room temperature with stirring. After 20 min, the solution was filtered through a plug of glass microfiber and concentrated in vacuo. The product was extracted with 6 mL of Et2O and filtered. The Et2O was removed in vacuo, leaving the product as a yellow-orange solid. Yield: 54 mg (88%). 1H NMR (300 MHz, CD3CN) δ: 5.86 (s, 5H, C5H5), 3.49 (s, 3H, OCH3). 13C{1H} NMR (126 MHz, CD3CN): ν̃ = 203.9 (CO), 182.9 (C(O)OCH3), 94.7 (C5H5), 50.5 (OCH3). IR (CH3CN): ν̃CO = 1993 (s) cm−1; ν̃NO = 1729 (s) cm−1; ν̃CO (methoxycarbonyl) = 1630 cm−1 (m). Anal. Calcd for C8H8NO4Re: C, 26.09; H, 2.19; N, 3.80. Found: C, 26.11; H, 2.05; N, 3.89. Preparation of [Re(η5-C5H4DMEGH+)(CO)(NO)(C(O)OCH3)](BF4) (10). The title complex forms reversibly from the reaction of complex 4 and methanol. In a representative example, where 10 was observed to be the major species (∼80%), complex 4 (15 mg, 0.028 mmol) was dissolved in 0.4 mL of CD3CN, and 150 μL of methanol was added. The color of the solution lightens slightly upon formation of methoxycarbonyl complex 10. Attempts to crystallize 10 for analysis by X-ray diffraction were unsuccessful. 1H NMR (300 MHz, CD3CN) δ: 6.09 (m, 2H, C5H4), 5.78 (m, 1H, C5H4), 5.56 (m, 1H, C5H4), 3.68 (s, 4H, CH2), 3.65 (s, 3H, OCH3), 2.90 (s, 6H, NCH3). At this

solid precipitated from solution. Addition of 15 mL of Et2O further precipitated the off-white product, which was decanted from the mother liquor, washed with 2 × 4 mL of Et2O, and dried in vacuo. Yield: 426 mg (80.5%). 1H NMR (300 MHz, CD3CN) δ: 7.69 (s, 1H, NH), 5.60 (t, J = 2.4 Hz, 2H, C5H4), 5.42 (t, J = 2.3 Hz, 2H, C5H4), 3.69 (s, 4H, CH2), 2.87 (s, 6H, CH3). 13C{1H} NMR (126 MHz, CD3CN) δ: 194.9 (CO), 158.0 (CN), 115.0 (C5H4ipso), 83.4 (C5H4), 78.5 (C5H4), 49.8 (CH2), 35.3 (CH3). IR (CH3CN): ν̃CO = 2024 (s), 1928 (s) cm−1. Anal. Calcd for C13H15BF4N3O3Re: C, 29.22; H, 2.83; N, 7.86. Found: C, 29.21; H, 2.90; N, 7.79. Preparation of [Re(η5-C5H4DMEGH+)(CO)2(NO)](BF4)2 (3). Complex 2 (331 mg, 0.620 mmol) was suspended in 20 mL of CH2Cl2 in a 50 mL Schlenk flask and cooled in an ice−water bath under argon. (NO)(BF4) (145 mg, 1.24 mmol, 2.00 equiv), which had been previously washed with 2 × 2 mL of CH2Cl2 and dried in vacuo, was added as a solid under argon counterflow. The mixture was allowed to warm to room temperature with continuous stirring. A yellow solid formed during the course of the reaction, which was allowed to proceed for a total of 19 h. The volatiles were removed in vacuo, and in the glovebox the yellow solid was resuspended in 15 mL of CH2Cl2, collected on a fritted disk, washed with 2 × 20 mL of THF, and dried in vacuo. Yield: 355 mg (92.0%). 1H NMR (300 MHz, CD3CN) δ: 8.27 (s, 1H, NH), 6.11 (s, 4H, C5H4), 3.80 (s, 4H, CH2), 2.92 (s, 6H, CH3). 13C{1H} NMR (126 MHz, CD3CN) δ: 182.8 (CO), 157.1 (CN), 130.6 (C5H4ipso), 91.5 (C5H4), 81.1 (C5H4), 49.9 (CH2), 35.5 (CH3). IR (CH3CN): ν̃CO = 2109 (s), 2055 (s) cm−1; ν̃NO = 1816 (s) cm−1. Anal. Calcd for C12H15B2F8N4O3Re: C, 23.13; H, 2.43; N, 8.99. Found: C, 22.88; H, 2.44; N, 8.77. Preparation of [Re(η5-C5H4DMEG)(CO)2(NO)](BF4) (4). A solution of complex 3 (315 mg, 0.506 mmol) in 6 mL of CH3CN was added to solid polymer-bound 1,3,4,6,7,8-hexahydro-2Hpyrimido[1,2-a]pyrimidine (2.6 mmol/g, 233 mg, 0.607 mmol, 1.20 equiv). The mixture was stirred for 1 h at room temperature. The polymer-bound base was removed by filtration through a plug of glass microfiber, and the resulting red-orange solution was concentrated in vacuo. The residue was dissolved in 6 mL of CH2Cl2 and filtered. The solution was concentrated to one-half its original volume, and 15 mL of Et2O was added with stirring to separate an ochre solid, which was triturated, decanted, and dried in vacuo. Yield: 207 mg (76.4%). The product as isolated was pure by 1H NMR, 13C{1H} NMR, and IR and could be used in subsequent transformations. Material suitable for elemental analysis required recrystallization of the product from CH2Cl2/Et2O. 1H NMR (300 MHz, CD3CN) δ: 5.98 (t, J = 2.4 Hz, 2H, C5H4), 5.22 (t, J = 2.4 Hz, 2H, C5H4), 3.54 (s, 4H, CH2), 2.74 (s, 6H, CH3). 13C{1H} NMR (126 MHz, CD3CN) δ: 187.3 (CO), 165.1 (CN), 161.1 (C5H4ipso), 88.4 (C5H4), 72.4 (C5H4), 48.4 (CH2), 34.4 (CH3). IR (CH3CN): ν̃CO = 2089 (s), 2031 (s) cm−1; ν̃NO = 1783 (s) cm−1. Anal. Calcd for C12H14BF4N4O3Re: C, 26.93; H, 2.64; N, 10.47. Found: C, 27.10; H, 2.68; N, 10.51. Preparation of Re(η5-C5H4DMEG)(CO)(NO)(CHO) (5). Complex 4 (25 mg, 0.047 mmol) and W−H (49 mg, 0.052 mmol, 1.1 equiv) were each dissolved in ca. 0.35 mL of CD3CN. The solution of W−H was added via pipet to the solution of 5, giving an orange solution. The NMR spectra showed features for the cis-W(P(OMe)3)(NCMe)(CO)4 (W0) and bis(triphenylphosphine)iminium (PPN+) cation byproducts12 and also indicated quantitative formation of formyl complex 5. 1H NMR (500 MHz, CD3CN) δ: 14.80 (s, 1H, CHO, 1JCH = 152 Hz), 5.87 (m, 1H, C5H4), 5.47 (m, 1H, C5H4), 5.18 (m, 1H, C5H4), 4.67 (m, 1H, C5H4), 3.35 (s, 4H, CH2), 2.75 (s, 6H, CH3). 13 C{1H} NMR (126 MHz, CD3CN) δ: 254.3 (CHO), 205.8 (CO), 161.9 (CN), 143.2 (C5H4ipso), 87.2 (C5H4), 83.4 (C5H4), 80.0 (C5H4), 78.7 (C5H4), 48.8 (CH2), 35.5 (CH3). IR (CH3CN): ν̃CO = 1980 (s) cm−1; ν̃NO = 1703 (s) cm−1; ν̃CO (formyl) = 1605 (m) cm−1. Preparation of [Re(η5-C5H4DMEGH+)(CO)(NO)(CHO)](BF4) (6). A solution of complex 5 was prepared as described above. The solution was frozen in the glovebox coldwell. The vial was removed, and upon thawing, a solution of pyridinium tetrafluoroborate (8.5 mg, 0.051 mmol, 1.1 equiv) in 0.2 mL of CD3CN was added. The NMR spectra showed features for the W0, pyridine, and PPN+ byproducts12 and also 4115

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intermolecular methanol activation, 5 mg of either [Re(η5-Cp*)(CO)2(NO)](BF4) or [Re(η5-Cp)(CO)2(NO)](BF4) was dissolved in a CD3CN solution containing 0.79 M methanol and 0.026 M (approximately equimolar) 2,6-lutidine. The respective solution was transferred to a J. Young NMR tube, which was sealed. 1H NMR spectra of the resulting equilibrium mixtures were recorded in 10 °C temperature increments between −15 and 65 °C. The temperatures were calibrated with an external standard of either neat methanol (−15 to 25 °C) or ethylene glycol (T > 25 °C) using the “tempcal” function within the VNMRJ software. Spectra at each temperature were recorded multiple times in increments of at least 5 min until no change was observed, to ensure that equilibrium was established. For most samples equilibrium was reached on the time scale of setting the probe temperature and locking/shimming, though for the low-temperature (−15 °C) reaction of 4 with CH3OH/CH3OD additional time (∼1 h) was required to reach equilibrium. X-ray Crystallographic Procedures. All crystallizations were carried out at room temperature. Crystals of 1 and 7 deposited from THF solution by vapor diffusion of pentane. Crystals of 2 and 3 were grown from CD3CN solution by vapor diffusion of Et2O, and crystals of 4 were obtained by diffusion of pentane into a CH2Cl2/THF solution. Crystals were mounted on either a Bruker APEXII four-circle diffractometer or a Bruker three-circle diffractometer with a SMART 1K CCD detector using Mo radiation from a sealed-tube 3 kW X-ray generator. The data were collected at 100(2) K and were processed and refined using the program SAINT supplied by Siemens Industrial Automation. Structures were solved by direct methods in SHELXS and refined by standard difference Fourier techniques in the SHELXTL program suite (6.10 v., Sheldrick, G. M., Siemens Industrial Automation, 2000). Hydrogen atoms bonded to carbon were placed in calculated positions using the standard riding model and refined isotropically; all non-hydrogen atoms were refined anisotropically. Nitrogen-bound hydrogen atoms in 2 and 3 were located in the difference map, restrained to a distance of 0.88 Å, and refined isotropically with the isotropic displacement parameter constrained to be 1.2 times greater than that of the nitrogen atom. In the structure of 3 one of the BF4− counterions and one of the crystallographically independent guanidine rings were found to be disordered about two positions. The structure of 4 refined best with the nitrosyl disordered with one of the carbonyls. For the disordered parts, bond distances and angles were restrained to be similar using the “SADI” command, and the rigid bond restraints “SIMU” and “DELU” were also employed. A summary of crystallographic details for all structures is provided in Tables S1 and S2 in the Supporting Information.

methanol concentration the OH resonance was not observed. At lower methanol concentrations (