Synthesis, Characterization, and Catalytic Reactivity of {CoNO}8 PCP Pincer Complexes

The reaction of coordinatively unsaturated Co(II) PCP pincer complexes with nitric oxide leads to the formation of new, air-stable, diamagnetic mono nitrosyl compounds. The synthesis and characterization of five- and four-coordinate Co(III) and Co(I) nitrosyl pincer complexes based on three different ligand scaffolds is described. Passing NO through a solution of [Co(PCPNMe-iPr)Cl], [Co(PCPO-iPr)Cl] or [Co(PCPCH2-iPr)Br] led to the formation of the low-spin complex [Co(PCP-iPr)(NO)X] with a strongly bent NO ligand. Treatment of the latter species with (X = Cl, Br) AgBF4 led to chloride abstraction and formation of cationic square-planar Co(I) complexes of the type [Co(PCP-iPr)(NO)]+ featuring a linear NO group. This reaction could be viewed as a formal two electron reduction of the metal center by the NO radical from Co(III) to Co(I), if NO is counted as NO+. Hence, these systems can be described as {CoNO}8 according to the Enemark–Feltham convention. X-ray structures, spectroscopic and electrochemical data of all nitrosyl complexes are presented. Preliminary studies show that [Co(PCPNMe-iPr)(NO)]+ catalyzes efficiently the reductive hydroboration of nitriles with pinacolborane (HBpin) forming an intermediate {CoNO}8 hydride species.


■ INTRODUCTION
The investigation of pincer ligands and their transition metal complexes has received significant attention during the past decades. 1 Their adjustable steric and electronic properties enable the stabilization of various chemical systems and thus allow for the investigation of fundamental reactivity and for the design of innovative catalysts. Since their introduction by Moulton and Shaw in 1976, no pincer ligand has retained as much attention as the PCP scaffold based on the phosphinomethylated benzene. 2 Related pincer ligands that feature an aromatic anionic benzene backbone connected to phosphine donors via CH 2 , O, or NR (R = H, alkyl or aryl) linkers provide for a very attractive class of ligands that are simple to prepare and modify. With respect to first-row transition metals, the chemistry of nickel PCP complexes is already quite comprehensive, but studies of cobalt, iron, manganese and chromium PCP pincer complexes are exceedingly rare. This may be attributed to the failure of many simple metal salts to cleave the C−H bonds of the arene moiety of the pincer ligands and/or the thermodynamic instability of the resulting complexes. Another important ancillary ligand in coordination chemistry is the nitrosyl (NO) ligand. 3 Transition metal nitrosyl compounds often play an important role in enzymatic reactions but are relatively unexplored yet in homogeneous catalysis. Indeed, the redox noninnocent character of this ligand is one of its basic and eminent features. It can bind to a metal center in various bonding modes, i.e., the linear and the bent mode, and can be treated as either NO + (nitrosonium), NO • (neutral radical), or NO − (nitroxyl). This could enable the stabilization of intermediates in catalytic cycles by supplying or detracting electron density or generating vacant coordination sites. Since different binding modes can lead to different formal oxidation states, Enemark and Feltham proposed a simple notation, {MNO} n (n = metal d + NO-π* electrons) to describe metal NO complexes unambiguously and ease their theoretical comparison. 4 Surprisingly, as cobalt is concerned, no PCP pincer nitrosyl complexes featuring a direct cobalt−carbon single bond have been reported in the literature. An overview of existing group 9 nitrosyl complexes being supported by pincer type ligands is depicted in Chart 1. 5−7 Recently our group reported a series of chromium, manganese, and iron nitrosyl complexes supported by aromatic PCP and PNP pincer ligands. 8 In this account, we describe a series of well-defined four-and five-coordinate low-spin {CoNO} 8 nitrosyl PCP pincer complexes based on three different ligand scaffolds. X-ray structures of representative complexes are presented.

■ RESULTS AND DISCUSSION
The starting materials for the present study, [Co(PCP NMe -iPr)Cl] (1a) and [Co(PCP O -iPr)Cl] (1b), were obtained by the reaction of anhydrous CoCl 2 and PCP NMe/O -iPr (1a, 1b) in the presence of a strong base as reported previously. 9, 10 The analogous complex [Co(PCP CH2 -iPr)Br] (1c), where the PCP ligand features CH 2 -linkers, was prepared in analogous fashion by treatment of the ligand PCP CH2 -iPr-Br with nBuLi and subsequent transmetalation in 65% isolated yield. 11 In analogy to 1a and 1b, complex 1c is a d 7 low spin complex with a solution magnetic moment μ eff of 2.3(1) μ B (Evans method). The solidstate structures of complexes 1b and 1c were determined by Xray diffraction, and representations of the molecules are presented in Figures 1 and 2.
Selected metrical parameters are provided in the figure captions. The molecular structures of these compounds show the metal in a typical slightly distorted-square planar conformation with the PCP ligands coordinated to the metal center in a tridentate mode. In both complexes the C1−Co−Cl1 and C1−Co1−Br1 angles deviate slightly from linearity being 178.88(5)°and 177.49(4)°, respectively. The P1−M−P2 angles are 162.90(1) and 169.39(2)°, respectively. The Co1−C1 bond distances are 1.9131(9) and 1.955(1), respectively.
Direct nitrosylation of complexes 1a−1c with nitric oxide at ambient pressure afforded the five-coordinate, green colored, diamagnetic compounds 2a−2c in 91−98% isolated yield (Scheme 1). All three complexes were fully characterized by a combination of 1 H, 13 C{ 1 H}, and 31 P{ 1 H} NMR spectroscopy, ATR-IR spectroscopy, and HR-MS analysis. In the proton decoupled 31 P spectrum the compounds exhibit broad resonances at 139. 1, 198.9, and 77.4 ppm, respectively, for the N, O, or CH 2 linked systems. Moreover, one single strong absorption band for the characteristic NO stretching mode is detected at 1643, 1650, and 1639 cm −1 , respectively, in the IR spectrum for 2a−2c. These values are indicative for NO in a bent coordination mode and therefore advocate for a formal interpretation as NO − anion and the assignment of the oxidation state + III for the cobalt center. In addition, they underline the different donor strengths (CH 2 ≈ NR > O) of the ligands in use expressed via NO π-back-bonding, according to the series of Tolman's electronic parameters. 12 In order to unequivocally establish the ligand arrangement around the metal centers, the solid-state structures of complexes 2a and 2b were determined by X-ray diffraction. Suitable single crystals were grown for this purpose from saturated pentane solutions kept at −20°C. A view of the molecular structures is depicted in Figures 3 and 4 with selected bond distances and angles reported in captions. Both complexes adopt a slightly distorted square pyramidal geometry (τ 5 < 0.1) 13     Cyclic voltammetry studies were conducted for compound 2a to gain a deeper insight into the electronic structure and properties. The cyclic voltammogram exhibits a strong redox process at E 1/2 = 0.11 V in dichloromethane, and two consecutive reductions at E 1/2 = −1.21 and −1.64 V (see Supporting Information). The reduction at −1.21 V is an irreversible process and probably metal based. In order to assign the redox processes involved in solution, spectro-electrochemical infrared (SEC-IR, Figure 5) experiments were carried out, and changes in the N−O stretching frequency were monitored.
After applying a voltage of 1.00 V, the absorption at 1654 cm −1 , typical for a bent NO arrangement, decreases with time, and a new peak appears at 1771 cm −1 . The new frequency is characteristic of a linear NO + stretching vibrational mode, therefore suggesting that oxidation took place at the NO moiety rather than at the Co(III) center. DFT (BP86/TZ2P) calculations (see Supporting Information) indicate that the HOMO has mainly d z2 character with both Co−NO σ* and Co−PCP π* antibonding character ( Figure 6 left). The DFToptimized geometry (S = 1/2) of the oxidized species therefore shows only a slight change of the Co−N−O angle from 143.5 (experimentally 140.1°) to 148.2°, despite the more significantly increased ν NO frequency (from 1618 to 1722 cm −1 , calculated). The spin density is predominantly localized on the d z2 orbital of the metal center ( Figure 6, right) suggesting a d 7 low-spin system. According to Hoffmann and co-workers penta-coordinate nitrosyls are in general very likely to bend when the basal ligands are strong σor π-donors. 14 Since the assignment of an oxidation is not trivial owing to the delocalization of the HOMO, [Co(PCP NMe -iPr)(NO)Cl] + is best described as {CoNO} 7 using the Enemark−Feltham notation.
One electron reduction of 2a, on the other side, leads to a loss of the halide ligand (optimized Co−Cl distance 5.41 Å) and formation of a nearly linear NO moiety (calc. Co−N−O angle 176.8°). The Co−NO π bonding and antibonding orbitals are strongly involved in both processes.
Treatment of 2a and 2b with halogen scavengers such as AgBF 4 or TlPF 6 in CH 2 Cl 2 afforded the square planar, diamagnetic cationic complexes 3a and 3b in 75 to 88% yield (Scheme 2). In the IR spectrum both compounds exhibit strong bands at 1806 and 1861 cm −1 , which is a characteristic feature for a linear configuration of the NO moiety. In the 31 P{ 1 H} spectrum the associated resonances at 156.0 and 216.9 ppm are shifted downfield with respect to 2a and 2b.
Single crystals of 3a and 3b were successfully grown by slow diffusion of pentane into a saturated solution in THF and      2)°showing a small deviation from linearity probably as a result of strong σ-donation by the benzene ring. The C ipso −Co bond length is also slightly shortened compared to their fivecoordinate precursors. In this sense, the abstraction of a halide atom leads to formation of a complex with a formal oxidation state of + I and an electronic isomerization between two {CoNO} 8 complexes:  16 An alternative and effective approach to cationic 3a is therefore utilizing a simple and fast ligand exchange reaction starting from [Co I (PCP NMe -iPr)-(CO) 2 ] (4a, vide supra) and NOBF 4 in dichloromethane (Scheme 3). Complex 3a readily reacts with CO in solution to afford the five-coordinate adduct [Co(PCP NMe -iPr)(CO)-(NO)] + (5) similar to the Rh complexes reported by Milstein and co-workers. 5a In the IR spectrum this complex gives rise to strong bands at 2025 (ν CO ) and 1732 (ν NO ) cm −1 . The latter may suggest a linear NO moiety coordinated in the equatorial position trans to the C ipso carbon atom. Compound 5 is inherently unstable and decomposes to 3a instantaneously when exposed to air or a vacuum.
The cyclic voltammogram of cationic 3a exhibits redox processes at 0.68 V and two consecutive reductions at −1.14 V and −1.35 V. The first oxidation wave at 0.68 V can tentatively be assigned to oxidation of the PCP ligand moiety. DFT calculations show that the HOMO of 3a is a delocalized π orbital on the pincer ligand.
Scan rate experiments show that the reduction peak at −1.14 V is an irreversible process, whereas the peak at −1.35 V is associated with a quasi-reversible reduction. DFT calculations reveal that the LUMO and LUMO+1 are both antibonding between Co and NO, and the reduction product has an elongated Co−NO bond. The loss of the NO ligand is thus likely to happen in the first irreversible step, and the second reduction wave might result from a secondary product.
Reaction of 3a with LiCl in MeOH-d 4 leads to formation of 2a in quantitative yield. It must be mentioned that putative 3c can be prepared in the same manner as 3a and 3b, but it proved to be very difficult to afford an analytically pure compound (approximately 75% yield, ν NO = 1823 cm −1 ).
Since various cobalt species including PNN and NNN pincer type complexes were shown to be catalytically active for the reductive hydroboration of ketones and nitriles, we began preliminary studies on the potential of the nitrosyl Co(I) complex 3a for this type of transformation. 17−19 Complex 3a (4 mol %) reacted with both aromatic and aliphatic nitriles and 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (HBpin) in benzene at 25°C (Table 1). After filtration and workup with HCl in dry diethyl ether all amines were isolated as ammonium salts and characterized by 1 H and 13 C{ 1 H} NMR spectroscopy. Monitoring of the reaction by 1 H NMR spectroscopy revealed the formation of a hydride intermediate giving rise to a triplet resonance at −9.3 ppm (J HP = 57 Hz), which may be involved in the catalytic cycle. This species prevails throughout the experiment and may be structurally related to the pyrrolebased hydride complex described by Tonzetich and co-workers. 6

■ CONCLUSION
In sum we have prepared a series of cobalt nitrosyl complexes supported by PCP pincer ligands that are connected via NMe, O, or CH 2 spacers to an aromatic backbone and fully characterized them by a combination of X-ray, NMR, CV, and FT-IR techniques. Five coordinate Co(III) complexes of the type [Co(PCP)(NO)X] (X = Cl or Br) can easily be prepared by direct nitrosylation of the related 15e Co(II) precursors [Co(PCP)X] (1a−1c) using nitric oxide. Treatment of these compounds with halide scavengers (AgBF 4 or TlPF 6 ) leads to the formation of square planar cationic Co(I) complexes of the type [Co(PCP)(NO)] + in good yields. Alternatively, [Co-(PCP NMe -iPr)(NO)] + was also obtained by reacting the   ■ EXPERIMENTAL SECTION General Information. All manipulations were performed under an inert atmosphere of argon by using Schlenk techniques or in an MBraun inert-gas glovebox. The solvents were purified according to standard procedures. 20 The deuterated solvents were purchased from Aldrich and dried over 4 Å molecular sieves. Nitric oxide (NO 2.5) was purchased from MESSER GmbH (Gumpoldskirchen, Austria). The synthesis of 2-bromo-1,3-phenylene)bis(methylene)bis-(diisopropylphosphane) (P(C−Br)P CH2 -iPr), 11 [Co(PCP NMe -iPr)Cl] (1a), 9 [Co(PCP O -iPr)Cl] (1b), 10 and [Co(PCP NMe -iPr)(CO) 2 ] (4a) 16 was carried out according to the literature. 1 H, 13 C{ 1 H}, and 31 P{ 1 H} NMR spectra were recorded on Bruker AVANCE-250 and AVANCE-400 spectrometers. 1 H and 13 C{ 1 H} NMR spectra were referenced internally to residual protiosolvent and solvent resonances, respectively, and are reported relative to tetramethylsilane (δ = 0 ppm). 31 P{ 1 H} NMR spectra were referenced externally to H 3 PO 4 (85%) (δ = 0 ppm). Infrared spectra were recorded in attenuated total reflection (ATR) mode on a PerkinElmer Spectrum Two FT-IR spectrometer.
High resolution-accurate mass spectra were recorded on a hybrid Maxis Qq-aoTOF mass spectrometer (Bruker Daltonics, Bremen, Germany) fitted with an ESI source. Measured accurate mass data of the [M] + ions for confirming calculated elemental compositions were typically within ±5 ppm accuracy. The mass calibration was done with a commercial mixture of perfluorinated trialkyl-triazines (ES Tuning Mix, Agilent Technologies, Santa Clara, CA, USA).
Cyclic voltammograms were recorded in a three-electrode one compartment electrochemical cell with glassy carbon (3 mm diameter, CHI Instruments) and platinum wire as working and counter electrodes, respectively. The reference electrode used was a saturated Calomel electrode (SCE, −0.241 V vs. NHE); however, all potentials are referenced to the Fc + /Fc couple (0.44 V vs SCE). One mM solutions of the complexes in dichloromethane were used and TBAPF 6 (0.2 M) as supporting electrolyte. The working electrode was polished with aluminum oxide, washed thoroughly with Milli-Q water and dried under a nitrogen flux. All experiments were accomplished using an Autolab PGSTAT128N potentiostat (Metrohm GmbH) controlled by NOVA software. SEC-IR experiments were performed using a cell acquired from LabOmak. This cell has two platinum grids as working and counter electrodes and an Ag wire as the pseudoreference electrode. A N 2 saturated MeCN solution with 5 mM of complex and 0.2 M of TBAPF 6 was injected in the SEC-IR cell. A linear voltammogram was performed using a scan rate of 10 mV s −1 to reach the suitable potential (+1.0 vs Ag pseudoreference) after the first oxidation peak. The IR spectra were obtained using a Nicolet Nexus 6700 FTIR spectrophotometer with a resolution of 4 cm −1 with 16 scans between 2200 and 1500 cm −1 . The background spectrum was recorded with the sample at the open circuit potential.
Synthesis. [Co(PCP CH2 -iPr)Br] (1c). To a solution of P(C−Br)P CH2 -iPr (100 mg, 0.24 mmol) in THF (10 mL), n-BuLi (0.32 mL, 1.6 M, 0.48 mmol) was slowly added at −78°C and then stirred for 1 h. After allowing to warm to room temperature a suspension of anhydrous CoBr 2 (55 mg, 0.25 mmol) in THF (5 mL) was added in dropwise fashion. The reaction mixture was stirred for 0.5 h and all volatiles were removed under reduced pressure. The remaining brownish solid was extracted into n-pentane and the yellow extract was filtered through a plug of Celite. The solvent was partially removed under a vacuum and the solution was placed in a freezer at −20°C to obtain a yellow crystalline solid. Yield: 74 mg (65%). μ eff = 2.3(1) μ B (Evans, CH 2 Cl 2 ). HR-MS (ESI + , CH 3 OH) m/z calcd for C 20    [Co(PCP NMe -iPr)(NO)]BF 4 (3a). Method A. To a solution of 2a (80 mg, 0.16 mmol) in CH 2 Cl 2 (6 mL), AgBF 4 (33 mg, 0.16 mmol) was added and the mixture was stirred for 2 h at room temperature. Insoluble materials were removed by filtration through a syringe filter and the solvent was removed under reduced pressure. The remaining red solid was washed twice with pentane (3 mL) and dried under a vacuum. Yield: 75 mg (85%). Method B. To a solution of [Co(PCP NMe -iPr)(CO) 2 ] (4a) (40 mg, 0.08 mmol) in CH 2 Cl 2 (3 mL), NOBF 4 (10 mg, 0.08 mmol) was added and the reaction mixture was stirred for 30 min at room temperature whereupon the color changed from yellow to dark red. All volatiles were removed under reduced pressure; the product was washed with pentane (3 mL) and dried under a vacuum. Yield: 39 mg (88%). 1  [Co(PCP O -iPr)(NO)]PF 6 (3b). This complex was prepared analogously to 3a (method A) with 2b (50 mg, 0.11 mmol) and TlPF 6 (39 mg, 0.11 mmol) as starting materials. Yield: 41 mg (75%). 1  Reaction of 3a with LiCl. Formation of 2a. To a solution of 3a (15 mg, 0.03 mmol) in methanol-d 4 (1 mL), LiCl (in excess) was added and the suspension stirred for 10 min. The mixture was filtrated via syringe filter into an NMR tube and spectra were immediately measured revealing the quantitative formation of 2a. 1 H, 31 P{ 1 H} NMR and ESI-MS spectra were identical to those of 2a.
Reaction of 3a with CO. Formation of [Co(PCP NMe -iPr)(CO)-(NO)] + (5). CO (2 atm) was injected into the headspace of a solution of 3a (15 mg) in CD 2 Cl 2 whereupon the color changed from magenta to brown. The new compound is inherently unstable and decomposes rapidly when the CO atmosphere is removed. Alternatively, the same compound can be prepared by a solid−gas reaction. IR (ATR, cm −1 ) 1732 (ν NO ), 2025 (ν CO ).
General Procedure for the Hydroboration of Nitriles. The nitrile substrates (0.33 mmol, 1 equiv), nitrosyl complex 3a (4 mol %), and pinacolborane (HBpin, 2.2 equiv) were mixed with 1 mL of benzene and stirred at room temperature for 24 h. The reaction mixture was then filtered through a PTFE syringe filter, diluted with Et 2 O (4 mL), and a solution of HCl in Et 2 O was added dropwise until the precipitation of the hydrochloride was complete. The obtained ammonium salts were washed with Et 2 O, dried under vacuum, and analyzed by 1 H NMR spectroscopy.
X-ray Structure Determination. X-ray diffraction data for 1a, 1c, 2a, 2b, 3a, and 3b (CCDC 1953233 (1a), 1959138 (1c), 1953234− 1953237 (2a, 3a, 3b)) were collected at T = 100 or 150 K in a dry stream of nitrogen on a Bruker Kappa APEX II diffractometer system using graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å) and fine sliced φand ω-scans. Data were reduced to intensity values with SAINT and an absorption correction was applied with the multiscan approach implemented in SADABS. 21 The structures were solved by the dual space method implemented in SHELXT 22 and refined against F 2 with SHELXL. 23 Non-hydrogen atoms were refined with anisotropic displacement parameters. The H atoms were placed in calculated positions and thereafter refined as riding on the parent atoms. The Co−N and N−O distances of the minor (3%) orientation of a disordered NO ligand were restrained to 1.74(2) and 1.16(2) Å, respectively. Molecular graphics were generated with the program MERCURY. 24 Computational Details. The Amsterdam Density Functional program (ADF) 25 was used in the Density Functional Theory calculations. 26 Geometries were optimized without symmetry constraints, considering solvent (acetonitrile), with gradient correction, using the Vosko−Wilk−Nusair 27 Local Density Approximation of the correlation energy and the Generalized Gradient Approximation with Becke's exchange 28 and Perdew's 29 correlation functionals. Unrestricted calculations were carried out for open shell complexes. The solvent correction was taken into account using the COSMO approach implemented in ADF. Relativistic effects were treated with the ZORA approximation. 30 Triple ζ Slater-type orbitals (STO) were used to describe all the valence electrons of H, C, B, and N, augmented with a set of two polarization functions (H, single ζ 2s, 2p, C, N, O, and P single ζ, 3d, 4f, and Co single ζ, 4p, 4f) and with a frozen core (1s) for C, O, N, and P, and for Co (1s 2p ■ REFERENCES