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Cite This: Organometallics XXXX, XXX, XXX−XXX
Aldehyde Decarbonylation by a Cobalt(I) Pincer Complex Hussah Alawisi, Kathlyn F. Al-Afyouni, Hadi D. Arman, and Zachary J. Tonzetich* Department of Chemistry, University of Texas at San Antonio (UTSA), San Antonio, Texas 78249, United States
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S Supporting Information *
ABSTRACT: The cobalt(I) pincer complex, [Co(N2)(CyPNP)] (CyPNP = anion of 2,5-bis((dicyclohexylphosphino)methyl)pyrrole), reacts with aromatic, vinylic, and aliphatic aldehydes to produce the corresponding hydrocarbon products and [Co(CO)(CyPNP)]. The pathway for aldehyde decarbonylation is found to involve initial coordination of the aldehyde to Co(I), followed by oxidative addition of the C−H bond to produce a cobalt(III) acyl hydride. The acyl hydride species then undergoes CO deinsertion, followed by reductive elimination to afford the decarbonylated product and [Co(CO)(CyPNP)]. Reactions of [Co(N2)(CyPNP)] with other carbonyl containing groups such as carboxylic acids and amides also proceed via oxidative addition to give Co(III) intermediates arising from activation of the X−H (X = O or NH) bond. In these cases, however, the Co(III) species extrude molecular hydrogen to produce Co(II) species of the form [Co(X{O}CR)(CyPNP)] (X = O or NH). The ability of [Co(N2)(CyPNP)] to undergo facile oxidative addition is discussed in the context of potential bond activation processes mediated by well-defined Co species.
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INTRODUCTION Decarbonylation offers a desirable synthetic strategy for the construction of complex organic architectures given the ubiquity of CO units in commercially available starting materials and the facility of carbonyl functional group transformations.1 The coupling of C−C and C−X bond formation events to decarbonylative processes further extends the utility of these reactions.2−5 Among homogeneous systems employed for aldehyde and ketone decarbonylation, precious metals have found the greatest application dating back to early studies employing Wilkinson’s catalyst.6−10 Since then, increasing efforts with different second and third row transition metals such as Rh, Ru, Ir, and Pd have produced a relatively robust class of catalytic decarbonylation methodologies.11−22 However, the reaction normally requires somewhat forceful conditions (>150 °C), mitigating its usefulness in certain applications.23−25 The majority of mechanistic proposals for the decarbonylation process rely on C−H (aldehyde) or C−X (ester, acid chloride, and others) oxidative addition events.26−29 Consequently, precious metal systems have excelled due to their propensity for two-electron redox processes. Surprisingly little contemporary work has examined the use of nonprecious metals in decarbonylation protocols.30−35 Ding and co-workers published the first general method for direct nickel-catalyzed decarbonylation of aromatic aldehydes in 2017.36 More recently, Wei has demonstrated that nickel can also be employed for the catalytic decarbonylation of aryl ketones.37 In addition to these examples of direct decarbonylation, Rueping has exploited nickel catalysts for a variety of decarbonylative cross-coupling strategies.38 Despite this progress, the use of earth-abundant transition metal catalyst systems in decarbonylation remains relatively scarce. More© XXXX American Chemical Society
over, mechanistic details surrounding this reaction are lacking. Such mechanistic information is of great interest given that many base metals operate via single-electron processes.39 In this vein, pincer complexes of earth-abundant transition metals are a promising platform for the development of catalytic reactions involving C−H bond functionalization.40 Their robust nature and modularity permits detailed study of mechanism and isolation of catalytic intermediates that are often challenging with simple binary systems.41,42 With these themes in mind, we chose to examine the ability of a well-defined Co(I) pincer complex to effect the decarbonylation of aldehydes. Cobalt pincers have already proven successful in a variety of reductive functionalization protocols including hydrogenation, hydrosilylation, and hydroboration.43−51 In many of the aforementioned processes, oxidative addition involving the Co(I)/Co(III) redox couple is invoked as a key pathway in bond activation.52 We therefore surmised that this couple might prove equally effective in promoting the C−H activation required for aldehyde decarbonylation.53−55 In this contribution, we describe precisely this reaction, mediated by a Co(I) pincer complex. In many cases, the reaction is found to proceed at relatively mild temperatures, and the stability afforded by the pincer framework has resulted in the observation of several key intermediates.
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RESULTS AND DISCUSSION The Co(I) pincer complex, [Co(N2)(CyPNP)] (1, CyPNP = anion of 2,5-bis((dicyclohexylphosphino)methyl)pyrrole), can be prepared by treatment of [CoCl(CyPNP)] with a variety of Received: September 10, 2018
A
DOI: 10.1021/acs.organomet.8b00668 Organometallics XXXX, XXX, XXX−XXX
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Organometallics reducing agents as reported previously.56,57 In order to test the suitability of 1 toward oxidative addition processes, we first examined its reactivity with several molecules containing polarized C−X bonds. We observed that compound 1 reacted cleanly with octyl bromide to generate [CoBr(CyPNP)] (2, Scheme 1) and the products of alkyl radical decomposition Scheme 1. One-Electron Reactivity of 1
Figure 1. Thermal ellipsoid rendering (50%) of the solid-state structure of 4. Hydrogen atoms omitted for clarity. Selected bond distances (Å) and angles (deg): Co(1)−N(1) = 1.876(4); Co(1)− C(31) = 1.697(5); Co(1)−Pavg = 2.1850(13); C(31)−O(1) = 1.172(5); P(1)−Co(1)−P(2) = 165.39(6).
spectroscopy demonstrated the appearance of several new species immediately after addition of benzaldehyde to 1. The most prominent resonance was a broad peak at 42 ppm (see Supporting Information). This chemical shift value is significantly upfield of that for both 1 and 4, and its broad nature suggests a compound in dynamic equilibrium. We assign this resonance to the benzaldehyde adduct, [Co(η2OCHPh)(CyPNP)] (5a, eq 2, R = Ph). This assignment is
(hexadecane and octene were detected by GC−MS), consistent with facile halogen atom abstraction. Similar treatment with bromobenzene afforded a mixture of 2 and [CoPh(CyPNP)] (3) as judged by 1H NMR spectroscopy, further implicating the propensity of 1 to react via single electron processes (Scheme 1). The identities of 2 and 3 were each confirmed by independent syntheses. Both compounds are paramagnetic and display highly shifted and broadened 1H NMR features consistent with low-spin (S = 1/2) Co(II).56 The structures of 2 and 3 were also determined by X-ray diffraction and can be found in the Supporting Information. Reasoning that engaging the Co(I)/Co(III) couple would require less polarized C−X sigma bonds, we proceeded to investigate reactions of 1 with aldehydes. Accordingly, treatment of 1 with benzaldehyde at room temperature was found to generate benzene and the Co(I) carbonyl species, [Co(CO)(CyPNP)] (4, eq 1) after several hours in benzene-d6
based upon the logical intermediacy of 5a in route to 4 and the NMR spectra of related aldehyde adducts (vide infra). In addition to 5a, the initial 31P NMR spectrum of the reaction in eq 1 shows the presence of 1, 4, and a fourth Co species that appears as a doublet resonance (J ≈ 40 Hz) at 93.7 ppm. This coupling constant is somewhat lower than those observed for isolable Ni(II) and Fe(II) hydride complexes of CyPNP,59,60 but still consistent with a putative Co(III) hydride. The assignment as a hydride is further bolstered by the observation of a triplet resonance in the 1H NMR spectrum (J = 39 Hz) at −10.10 ppm. A survey of other aromatic aldehydes (a−g, Table 1) gave similar results to those in eq 1, with the corresponding arenes being produced concomitantly with 4. Depending upon the nature of the aldehyde, the reaction proceeded with different facility, requiring heat in the case of very bulky substrates (Table 1). In addition, the aldehyde adducts (5) were observed to form in varying ratios relative to 1. In the case of more electron-donating aldehydes such as anisaldehyde (b), the adducts were not detected at all by 31P NMR, and only signals for 1 and 4 were apparent during the reaction. These results are consistent with an equilibrium process for binding of aldehydes to 1 (eq 2). Aromatic aldehydes with more electron-withdrawing substituents have lower energy C−O π* orbitals and are therefore better π-acid ligands. Consequently, such substrates permit observation of the aldehyde adducts, 5,
at room temperature. Like 1, compound 4 is diamagnetic and demonstrates a strong infrared absorption for the CO ligand at 1894 cm−1, consistent with related Co(I) pincer complexes.58 The carbonyl compound could also be independently synthesized from 1 by treatment with an atmosphere of carbon monoxide. The solid-state structure of 4 was determined by X-ray diffraction and is displayed in Figure 1. The bond metrics about Co are unremarkable and similar to those of 1. The propensity for aldehyde decarbonylation by 1 prompted us to delve deeper into the scope and mechanism of the transformation. Following the reaction in eq 1 by 31P NMR B
DOI: 10.1021/acs.organomet.8b00668 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Table 1. Decarbonylation of Aldehydes by 1a
the spectrum. The lack of observable vinylic protons by 1H NMR suggests the possibility that 5j involves olefin as opposed to aldehyde coordination to cobalt. However, the room temperature 13C NMR spectrum of 5j reveals a resonance for the carbonyl carbon atom (identified by HSQC) that demonstrates coupling to two phosphorus centers (JCP = 3 Hz). The presence of 31P coupling to the carbonyl carbon atom argues in favor of the structure featuring CO coordination to cobalt. Solution IR spectra of 5j also demonstrated a 60 cm−1 shift in νCO versus free acrolein (see Supporting Information). Based on these spectroscopic features we favor the structure for 5j shown in eq 2 containing a side-on bound aldehyde. The η2 coordination is supported by the asymmetry of the low-temperature spectra of 5j and the πbasic nature of Co(I). Furthermore, treatment of 1 with DMF, a stronger C−O σ-donor ligand, failed to produce any new species as judged by NMR. The observation of a putative C O π-complex in the cobalt PNP system is notable as all previous mechanistic schemes involving rhodium have favored a σ-bound carbonyl intermediate.26,28 Compiling the observations described above regarding the intermediates in the decarbonylation reaction, we propose the mechanistic pathway shown in Scheme 2. Compound 1 Scheme 2. Proposed Mechanism for Aldehyde Decarbonylation by 1
a
Reactions conducted in benzene-d6 for 20 h at ambient temperature unless noted. In all cases, complete conversion of 1 to 4 was confirmed by 31P NMR spectroscopy.
due to a larger equilibrium binding constant. For example, treatment of 1 with p-trifluoromethylbenzaldehyde resulted in immediate, complete formation of the adduct, 5d. Aliphatic and vinylic aldehydes were also examined as substrates for the decarbonylation reaction mediated by 1. In both cases, formation of the corresponding alkanes and alkenes was observed; although for both classes of substrate, heat was required (Table 1). When acrolein (j) was employed, the aldehyde adduct, 5j, was found to form exclusively and remain stable to decarbonylation unless heated. The 1H and 31P NMR spectra of 5j were therefore examined at lower temperature in toluene-d8 to better understand the nature of the adduct and those like it in eq 2. At −20 °C, the 31P NMR spectrum of 5j displayed an AB pattern (2JPP = 200 Hz) for the two phosphorus atoms consistent with a species lacking a C2 axis (see Supporting Information). Moreover, the 1H NMR spectrum at this temperature also showed inequivalent resonances for the pyrrolic CH atoms further confirming the lack of 2-fold symmetry. The coordinated aldehyde proton appeared as a broadened signal at 8.45 ppm, but the vinylic protons of the coordinated acrolein could be not identified in
undergoes ligand exchange in an equilibrium fashion with aldehyde to generate the η2-bound adduct 5. The aldehyde adduct then undergoes C−H oxidative addition to generate a Co(III) acyl hydride (A) followed by subsequent COdeinsertion to produce the six-coordinate Co(III) carbonyl (B). Reductive elimination from B extrudes the hydrocarbon moiety resulting in compound 4. As to the nature of the hydride intermediate detected in reactions with benzaldehyde (vide supra), we favor a species akin to A based upon the reaction with benzoyl chloride described below. In total, the mechanistic cycle in Scheme 1 is very much in line with those described previously for precious metal catalysts.28,61 The most unique feature of the CyPNPCo system appears to be the propensity for formation of π-adducts (5). We also attempted to realize a closed chemical cycle for aldehyde decarbonylation, by transforming 4 back into 1. C
DOI: 10.1021/acs.organomet.8b00668 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
case, these hydride species were found to evolve into the Co(II) carboxylate complexes (6) after 18 h. Spectra recorded after these longer time periods showed the presence of molecular hydrogen, consistent with loss of H atoms and/or bimolecular coupling of the hydride to generate 6 (Scheme 3). The benzoate complex, 6a, was isolated from the reaction of 1 with benzoic acid and its solid-state structure determined by X-ray diffraction. The compound crystallized with four molecules in the asymmetric unit displaying a mix of distorted trigonal-bipyramidal and square-pyramidal coordination geometries (Figure 2 and see Supporting Information). In each
Unfortunately, exposure of 4 to repeated vacuum/N2 cycles failed to generate 1 as did treatment with trimethylamine-Noxide. Thus, prospects for developing a catalytic decarbonylation reaction with 1 are unlikely at present due to strong product inhibition by CO. Indeed, catalytic trials employing an excess of benzaldehyde, high temperature reflux in diglyme, and/or N2 purge failed to proceed beyond one turnover. In order to gain more information about the oxidative addition process proposed in Scheme 2, we next examined the reaction of 1 with benzoyl chloride. We anticipated that C−Cl oxidative addition would proceed in a straightforward manner akin to that for the C−H bonds of aldehydes, but that formation of 4 would be hindered by sluggish C−Cl reductive elimination.62 Upon addition of benzoyl chloride to 1, signals for a new diamagnetic CyPNPCo species were evident by both 1 H and 31P NMR (see Supporting Information). The 31P NMR spectrum of the new species displayed a single sharp peak at 52.1 ppm, whereas the 1H NMR spectrum evinced several peaks for the methylene arms of the PNP ligand denoting a lack of C2 symmetry. The similarity in 31P chemical shift (∼50 ppm) to the aldehyde adducts (5) suggests the possibility that the new species is a π-complex of benzoyl chloride. However, the presence of single sharp 31P resonance requires a complex with mirror symmetry, which is incompatible with a structure akin to 5. Therefore, the reaction product of 1 with benzoyl chloride is most consistent with either a five- or six-coordinate Co(III) chloride complex analogous to intermediates A and B, respectively, in Scheme 2. Examination of the reaction solution by IR spectroscopy revealed no νCO attributable to a Co(III) carbonyl species. We therefore assign the product of reaction of benzoyl chloride with 1 to the Co(III) acyl chloride complex, [CoCl(C{O}Ph)(CyPNP)]. By extension, we also favor intermediate A in Scheme 2 as the observable Co(III) hydride species in decarbonylation reactions with aldehydes. Results with aldehyde substrates prompted us to next examine similar reactions with other molecules containing carbonyl functionalities. Esters and ketones failed to react cleanly with 1 at or above ambient temperature, but carboxylic acids and amides, RC(O)X (X = OH and NH2), proved more pliant. Treatment of 1 with benzoic acid ultimately resulted in formation of paramagnetic [Co(O2 CPh)( CyPNP)] (6a, Scheme 3). 1H NMR spectra of the reaction immediately after addition of benzoic acid, however, displayed a new diamagnetic species with a triplet Co−H resonance at −30.6 ppm (JHP = 61 Hz). When other carboxylic acids such as diphenylacetic acid and pivalic acid were used in place of benzoic acid, analogous hydride species were observed in even greater concentration (see Supporting Information). In each
Figure 2. Thermal ellipsoid rendering (50%) of the solid-state structure of 6a. Only one of the molecules of 6a from the asymmetric unit is shown. Hydrogen atoms and cocrystallized solvent molecule omitted for clarity. See Supporting Information for bond metrics.
case, the benzoate ligand is bound in a κ2-fashion with inequivalent Co−O distances. The structure of 6a therefore stands in contrast to that of the recently reported [Co(κ1O2CH)(tBuPNP)], which features κ1-coordination of the formate ligand albeit with bulkier tert-butyl substituents on the phosphine donors.57 The remaining structural aspects of 6a with respect to the PNP ligand are similar to those of the chloride complex, [CoCl(CyPNP)].56 Reaction of 1 with benzamide proceeded in similar fashion as that with benzoic acid producing the paramagnetic Co(II) carbamate complex, [Co(NH{O}CPh)(CyPNP)] (7a, eq 3). In
Scheme 3. Reaction of 1 with Carboxylic Acids
the case of the amide, however, no intermediate hydride species were detected by NMR spectroscopy. Although we have been unable to obtain crystals of 7a suitable for X-ray crystallography, the compound displays an infrared absorption for the CO unit at 1678 cm−1, consistent with κ1-N binding of the benzamido ligand.63 D
DOI: 10.1021/acs.organomet.8b00668 Organometallics XXXX, XXX, XXX−XXX
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min) by 1H and 31P NMR spectroscopy. Based on the extent of reaction at this time as judged by the formation of 4, heating was either commenced (see Table 1) or the solution was allowed to react at room temperature for an additional 20 h. [CoBr(CyPNP)], 2. A flask was charged with 0.300 g (0.61 mmol) of Li(CyPNP) and 15 mL of THF. To the resulting solution was added 0.133 g (0.61 mmol) of anhydrous CoBr2. The mixture was stirred for 12 h at room temperature. After this time, all volatiles were removed in vacuo leaving a brown residue. The residue was extracted into 30 mL of toluene and filtered through a pad of Celite. Evaporation of the toluene afforded 0.213 g (56%) of the desired material as a brown solid. Crystals suitable for X-ray diffraction were grown by slow diffusion of pentane into a concentrated benzene solution at room temperature. μeff (Evans): 1.8(2) μB. NMR (C6D6): 1H (300 MHz) δ 25.2, 22.3, 16.1, 8.1, 5.5, 3.6, 2.9, 1.4, 1.3, 0.8, −0.1, −58.7. Anal. Calcd for C30H50CoBrNP2: C, 57.60; H, 8.06; N, 2.24. Found: C, 57.91; H, 8.10; N, 2.68. [CoPh(CyPNP)], 3. A vial was charged with 0.223 g (0.38 mmol) of [CoCl(CyPNP)] and 5 mL of THF. While stirring, 0.38 mmol of PhMgCl was added as a solution in THF. The mixture was stirred for 3 h at room temperature. No visible color change occurred. All volatiles were removed in vacuo, and the remaining residue was extracted into toluene and filtered through a plug of Celite. Compound 3 precipitated from a saturated toluene solution upon standing at −30 °C yielding 0.172 g (70%) of brown crystals. Melting point: 204−207 °C. μeff (Evans): 2.0(2) μB. NMR (C6D6): 1H (500 MHz) δ 42.5, 35.7, 25.0, 12.5, 11.3, 6.5, 5.9, 4.0, 2.7, 1.5, 0.2, −6.5, −16.3, −41.4. Anal. Calcd for C36H55CoNP2: C, 69.44; H, 8.90; N, 2.25. Found: C, 69.02; H, 8.83; N, 2.14. [Co(CO)(CyPNP)], 4. A flask was charged with 0.113 g (0.20 mmol) of [Co(N2)(CyPNP)] and 10 mL of toluene. The flask was capped with a septum, and 1 atm of CO was introduced. The mixture was stirred for 12 h at room temperature during which time the color changed from brown to red. The mixture was filtered through a pad of Celite, and all volatiles were evaporated to dryness leaving a dark pink residue. The residue was washed with 5 mL of pentane and dried in vacuo to afford 0.096 g (85%) of a dark pink solid. Crystals suitable for X-ray diffraction were grown by slow diffusion of pentane into a concentrated benzene solution at room temperature. NMR (C6D6): 1 H (500 MHz) δ 6.45 (s, 2 pyr−CH), 2.98 (app t, JHP = 4.2 Hz), 2.23 (app d, 4 Cy), 1.99 (m, 4 Cy), 1.71 (app t, 8 Cy), 1.60 (app d, 4 Cy), 1.55 (m, 8 Cy), 1.40 (app q, 4 Cy), 1.17 (app q, 4 Cy), 1.07 (app q, 8 Cy). 13C: δ 138.80, 105.37, 35.57 (t, JCP = 10 Hz), 29.78, 29.33, 27.55 (t, JCP = 6 Hz), 27.44 (t, JCP = 4 Hz), 26.92, 25.01 (t, JCP = 7.5 Hz). 31 P: δ 81.85. IR (KBr): cm−1 1894 (νCO). Anal. Calcd for C31H50CoNOP2: C, 64.91; H, 8.79; N, 2.44. Found: C, 64.69; H, 8.75; N, 2.27. [Co(O2CPh)(CyPNP)], 6a. Method A. A flask was charged with 0.045 g (0.078 mmol) of [Co(N2)(CyPNP)] and 5 mL of toluene. To the resulting solution was added 0.0105 g (0.086 mmol) of benzoic acid. The mixture was stirred for 12 h at room temperature. After this time, the mixture was filtered through a pad of Celite to give a dark brown solution. All volatiles were removed in vacuo, and the resulting residue was redissolved in 5 mL of pentane , then evaporated to dryness to afford 0.035 g (67%) of a brown solid. Method B. A flask was charged with 0.250 g (0.43 mmol) of [CoCl(CyPNP)] and 15 mL of THF. To the resulting solution was added 0.124 g (0.86 mmol) of sodium benzoate. The mixture was stirred for 12 h at 60 °C. After this time, the mixture was filtered through a pad of Celite to generate a dark brown solution. All volatiles were removed in vacuo, leaving a dark brown residue. The product was redissolved in ether and evaporated to dryness to afford 0.119 g (42%) of a brown solid. Crystals suitable for X-ray diffraction were grown by slow cooling of a concentrated pentane solution at −30 °C. μeff (Evans): 1.8(2) μB. NMR (C6D6): 1H (300 MHz) δ 17.5, 11.3, 6.7, 6.5, 3.7, 2.6, −0.4, −43.2. IR (KBr): cm−1 1626, 1617, 1572 (νCO). Anal. Calcd for C37H56CoNO2P2: C, 66.55; H, 8.45; N, 2.10. Found: C, 65.93; H, 8.10; N, 1.71. [Co(NH{O}CPh)(CyPNP)], 7a. A flask was charged with 0.211 g (0.37 mmol) of [Co(N2)(CyPNP)] and 10 mL of toluene. To the
CONCLUSIONS In this contribution we have demonstrated that a Co(I) pincer complex is capable of mediating the decarbonylation of a variety of aromatic and aliphatic aldehydes. For many substrates, the decarbonylation process occurs at room temperature, and several intermediates can be detected by NMR spectroscopy, consistent with a mechanism involving C−H oxidative addition. Most notably, aldehyde π-complexes as opposed to σ-adducts appear to be the initial products of N2 displacement from 1. The ultimate product of aldehyde decarbonylation, the carbonyl complex 4, has thus far proved intransigent to further substitution precluding realization of a catalytic cycle. Other carbonyl functionalities such as carboxylic acids and amides likewise react with 1 via oxidative addition pathways. However, in these instances, O−H and N− H activation occur preferentially over C−X oxidative addition. In total, these investigations demonstrate that the PNP Co(I) fragment is active toward reductive bond activations. Future work will examine the efficacy of 1 in catalytic transformations that exploit the Co(I)/Co(III) redox couple.
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EXPERIMENTAL SECTION
General Comments. Manipulations of air- and moisture-sensitive materials were performed under an atmosphere of purified nitrogen gas in an Vacuum Atmospheres glovebox. Tetrahydrofuran, diethyl ether, pentane, and toluene were purified by sparging with argon and passage through two columns packed with 4 Å molecular sieves (all solvents) and alumina (THF and diethyl ether). Benzene-d6 and toluene-d8 were sparged with argon and stored over 4 Å molecular sieves prior to use. 1H NMR spectra were recorded on spectrometers operating at 300 (Varian) or 500 MHz (Bruker) and referenced to the residual protium signal of the solvent. 31P NMR spectra were recorded at 202 MHz and referenced using the 2H lock frequency. FT-IR spectra were recorded with a ThermoNicolet iS 10 spectrophotometer running the OMNIC software; solid samples were pressed into KBr disks, and solution samples were observed using an airtight liquid transmission cell (Specac OMNI) with KBr windows (path length = 0.05 mm). Solution magnetic susceptibility measurements were determined by the Evans method without a solvent correction using reported diamagnetic corrections.64 Elemental analyses were performed by the CENTC facility at the University of Rochester. Crystallography. Crystals suitable for X-ray diffraction were mounted, using Paratone oil, onto a nylon loop. Data were collected at 98(2) K using a Rigaku AFC12/Saturn 724 CCD fitted with MoKα radiation (λ = 0.71075 Å). Low-temperature data collection was accomplished with a nitrogen cold stream maintained by an X-Stream low-temperature apparatus. Data collection and unit cell refinement were performed using CrystalClear software.65 Data processing and absorption correction, giving minimum and maximum transmission factors, were accomplished with CrysAlisPro66 and SCALE3 ABSPACK,67 respectively. The structure was solved with the ShelXT68 structure solution program within Olex269 using direct methods and refined (on F2) with the ShelXL package70 using fullmatrix, least-squares techniques. All non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atom positions were determined by geometry and refined by a riding model. Materials. [Co(N2)(CyPNP)], [CoCl(CyPNP)], and Li(CyPNP) were prepared according to the literature procedures or slight modifications thereof.56,71 All other reagents were obtained from commercial suppliers and used as received. Carbon monoxide (99.99%) was purchased from Matheson and delivered to reaction vessels via needle/septa. General Procedure for Reaction of 1 with Aldehydes. In a typical protocol, ca. 25 μmol of 1 was dissolved in 0.5 mL of benzene-d6 (∼50 mM) and transferred to an NMR tube. An equimolar quantity of aldehyde was added, and the mixture was assayed immediately (
