Reactivity of (Triphos)FeBr2(CO) towards sodium borohydrides

Abstract The addition of CO to (Triphos)FeBr2 (Triphos = PhP(CH2CH2PPh2)2) resulted in formation of six-coordinate (Triphos)FeBr2(CO). This coordination compound was found to have cis-bromide ligands and a mer-Triphos ligand by single crystal X-ray diffraction. Once characterized, the reactivity of this compound toward NaEt3BH and NaBH4 was investigated. Adding 1 eq. of NaEt3BH to (Triphos)FeBr2(CO) resulted in formation of (Triphos)FeH(Br)(CO), while the addition of 2.2 eq. afforded previously described (Triphos)Fe(CO)2. In contrast, adding 2.2 eq. of NaBH4 to (Triphos)FeBr2(CO) resulted in carbonyl dissociation and formation of diamagnetic (Triphos)FeH(η2-BH4), which has been structurally characterized. Notably, efforts to prepare (Triphos)FeH(η2-BH4) following 2.2 eq. NaBH4 addition to (Triphos)FeBr2 were unsuccessful. The importance of these observations as they relate to previously reported (Triphos)Fe reactivity and recent developments in Fe catalysis are discussed.


Results and discussion
Having previously studied the reduction of 1-Br 2 , we desired to prepare and investigate the reactivity of a well-defined, six-coordinate dibromide precursor following monodentate ligand addition. upon adding 1 atm of CO to an acetone solution of 1-Br 2 , a diamagnetic compound identified as (Triphos) FeBr 2 (CO) (1-Br 2 (CO), figure 2) was observed by multinuclear NMR spectroscopy. The 31 P NMR spectrum of this compound has two resonances at 134.62 (t) and 56.73 (d) ppm, consistent with a terdentate, and C s -symmetric Triphos ligand. Furthermore, the 13 C NMR spectrum of 1-Br 2 (CO) has a multiplet at 219.76 ppm and infrared spectroscopy revealed a single CO stretch at 1957 cm −1 , further confirming the presence of one CO ligand.
Although the reaction outcomes in figure 2 suggest that dicarbonyl compound formation is favored following bromide ligand removal, efforts were made to prepare a persistent (Triphos)Fe hydride compound following NaBH 4 addition. Fortunately, adding 2.2 eq. of NaBH 4 to 1-Br 2 (CO) rather than NaEt 3 BH did not yield 1-(CO) 2 , but rather a new diamagnetic compound featuring 31 P NMR resonances at 145.08 and 89.87 ppm. The 1 H NMR spectrum of this product was found to possess an upfield-shifted pseudo quartet at −23.95 ppm along with two broad resonances at −12.80 and −14.01 ppm. The pseudo quartet collapsed to a singlet upon 31 P-decoupling (figures S19 and S20 of the Supplementary Information), indicating the presence of an Fe-H moiety. Since a broad resonance at 5.84 ppm that integrates to two hydrogen atoms was also observed, and no CO-derived stretches were detected by IR spectroscopy, this product was formulated to be (Triphos)FeH It is intriguing that B-H σ-bond donation is strong enough to allow for isolation of low-spin 1-H(BH 4 ) and that η 2 -BH 4 coordination is responsible for CO dissociation. To further probe these issues, we attempted to synthesize 1-H(BH 4 ) by adding 2.2 eq. NaBH 4 to 1-Br 2 . Surprisingly, this reaction did not afford 1-H(BH 4 ), suggesting that 1-H(BH 4 ) is formed following salt metathesis between NaBH 4 and 1-H(Br)(CO). This observation also suggests that NaBH 4 addition to 1-Br 2 does not generate a monohydride intermediate that is sufficiently long-lived to yield 1-H(BH 4 ).
The reactivity described herein may prove valuable for future studies which seek to develop thermally stable iron precatalysts for organic transformations. Being an inexpensive and nontoxic base metal [21], the ability of iron compounds to mediate hydrogenation [22] and hydrosilylation [22a, 23] has been widely investigated over the last 20 years. More recently and specifically, researchers have had success in utilizing borane-coordinated (PNP)Fe precatalysts for E-selective alkyne hydrosilylation [24], the reversible hydrogenation of ketones [25], and the hydrogenation of CO 2 to formic acid [26]. It is hoped that this study will enable similar applications to be developed for iron catalysts bearing PPP pincer scaffolds including Triphos.

Conclusion
The synthesis and characterization of six-coordinate (Triphos)FeBr 2 (CO) has been described along with its reactivity toward sodium borohydride reagents. Although attempts to reduce this compound in the presence of excess Na/Hg resulted in (Triphos)Fe(CO) 2 , the use of NaEt 3 BH allowed for the isolation of monohydride intermediate (Triphos)FeH(Br)(CO). In contrast, adding an excess of NaBH 4 to (Triphos) FeBr 2 (CO) afforded (Triphos)FeH(η 2 -BH 4 ). This compound features two B-H σ-bonds which are coordinated to iron, as determined by multinuclear NMR spectroscopy and single crystal X-ray diffraction. It is hoped that the results described herein will guide future efforts in iron-mediated catalysis.

General considerations
unless otherwise stated, all synthetic reactions were performed in an MBraun glovebox under an atmosphere of purified nitrogen. Anhydrous solvents (Sigma-Aldrich) were dried using a Pure Process Technology solvent purification system and 4 Å molecular sieves before use. Chloroform-d, benzene-d 6 , and acetone-d 6 were purchased from Cambridge Isotope Laboratories and dried over molecular sieves prior to use. Bis(2-diphenylphosphinoethyl)phenylphosphine (Triphos) and FeBr 2 were used as received from Strem Chemicals. Carbon monoxide, sodium triethylborohydride (1 M solution in toluene), and acetone were purchased from Sigma-Aldrich. Sodium borohydride was obtained from Sigma-Aldrich, while sodium borodeuteride was purchased from Cambridge Isotope Laboratories. 1-Br 2 was prepared according to the reported procedure [8]. Solution 1 H nuclear magnetic resonance (NMR) spectra were recorded at room temperature on either a Varian MR 400 or 500 MHz instrument. All 1 H and 13 C NMR chemical shifts are reported in ppm relative to SiMe 4 using 1 H (residual) and 13 C chemical shifts of the solvent as secondary standards. 31 P NMR data is reported in ppm relative to H 3 PO 4 . Infrared spectra were recorded on a Thermo Nicolet Nexus 470 FT-IR spectrometer running OMNIC software. Elemental analyses were performed at Robertson Microlit Laboratories Inc. in Ledgewood, NJ.

X-ray crystallography
Single crystals suitable for X-ray diffraction were coated with Paratone oil in the glovebox, transferred to a nylon loop, and finally to the goniometer head of a Bruker APEX II diffractometer (Los Alamos National Laboratory and university of Arizona) equipped with a molybdenum X-ray tube. A hemisphere routine was used for data collection and determination of the lattice constants. The space group was identified and the data were processed using the Bruker SAINT+ program and corrected for absorption using SAdABS. The structures were solved using direct methods (SHELXS), completed by subsequent Fourier synthesis, and refined by full-matrix, least-squares procedures. The solid state structure of 1-Br 2 (CO) features one well-ordered molecule of acetone and half of a disordered molecule that lies on an inversion center. A disordered acetone molecule has also been removed from the difference map using SQuEEZE (void volume 140 Å 3 , 33 electrons). Crystallographic parameters for 1-Br 2 (CO) and 1-H(BH 4 ) are provided in table S1.

Preparation of (Triphos)FeBr 2 (CO) (1-Br 2 (CO))
under an inert atmosphere, a Schlenk tube was charged with 0.197 g (0.263 mmol) of 1-Br 2 in approximately 20 mL dry acetone. The tube was sealed and one atmosphere of CO was added to the frozen solution on a Schlenk line during a freeze-pump-thaw cycle. The solution was warmed to room temperature. The brown suspension turned deep green and was stirred for 30 h at 23 °C. After 30 h, the excess CO was removed on a Schlenk line and the resulting green suspension was heated for 2 h at 50 °C while it turned into an orange suspension (this color change has alternatively been observed over the course of days at ambient temperature). The headspace of the tube was then evacuated to remove any remaining CO. Finally, the orange suspension was concentrated in vacuo and placed in a −35 °C freezer for 12 h. Filtration and drying yielded 0.125 g (0.161 mmol, 61%) of an orange solid identified as 1-Br 2 (CO). Analysis for C 35

Preparation of (Triphos)FeH(Br)(CO) (1-H(Br)(CO))
under an inert atmosphere, a 100 mL round bottom flask was charged with 0.116 g (0.149 mmol) of 1-Br 2 (CO) in approximately 25 mL toluene and placed in a liquid N 2 cooled cold well. A 20 mL scintillation vial containing 0.149 mL (0.149 mmol) of NaEt 3 BH (1 M solution in toluene) in approximately 2 mL toluene was also cooled. After 30 min, the NaEt 3 BH solution was slowly added to the toluene slurry of 1-Br 2 (CO) while stirring. The flask was warmed to room temperature and stirred for 2.5 h, after which time the mixture had turned into an orange solution. It was filtered through Celite and the toluene was removed in vacuo. The orange-yellow film was dissolved in 5 mL toluene and filtered through a Celite column. The filtrate was layered with 1 mL diethyl ether and stored at -35 °C. Brown crystals were isolated (0.035 g, 0.0486 mmol, 32%) and identified as 1-H(Br)(CO). Analysis for C 35

Method A
under an inert atmosphere, a 20 mL vial was charged with 0.101 g (0.129 mmol) of 1-Br 2 (CO) in approximately 15 mL of toluene and cooled. Another vial containing a 3 mL toluene solution of NaEt 3 BH (0.324 mL, 0.324 mmol) was also cooled. After 30 min, the NaEt 3 BH solution was added slowly to the orange 1-Br 2 (CO) slurry. The vial was warmed to room temperature and stirred for 24 h. The resulting yellow solution was filtered through Celite and toluene was removed to obtain a yellow film. The film was scraped with pentane (5 × 4 mL) and dried to isolate 0.038 g (0.0588 mmol, 45%) of a bright yellow solid identified as 1-(CO) 2 [18].

Method B
In the glove box, a 20 mL scintillation vial was charged with 1.08 g (5.33 mmol) of Hg in approximately 5 mL dry THF and 0.007 mg of freshly cut Na metal (0.267 mmol) was added. The resulting amalgam mixture was stirred for 30 min. Then a slurry of 1-Br 2 (CO) (0.0415 g, 0.0533 mmol) in 10 mL THF was added to the amalgam. The reaction mixture was stirred for 4 d at ambient temperature, over which time it turned light red in color. After filtration through Celite and removal of the solvent, 1-(CO) 2 was observed along with unidentified side products by multinuclear NMR spectroscopy. 1

Preparation of (Triphos)FeH(η 2 -BH 4 ) (1-H(BH 4 ))
under an inert atmosphere, a 20 mL scintillation vial was charged with 0.144 g (0.186 mmol) of 1-Br 2 (CO) in approximately 15 mL dry THF. To the orange suspension, 2.2 eq. of NaBH 4 (0.016 g, 0.431 mmol) was added and stirred at 23 °C for 4 d, after which time the orange suspension had turned bright orange. This solution was then filtered through Celite and the tetrahydrofuran was removed. The resulting bright orange film was scraped from the vial twice with 5 mL of diethyl ether and dried under vacuum. The resulting solid was then dissolved in a minimum amount of toluene and layered with diethyl ether and placed in a −35 °C freezer. Recrystallization yielded 0.078 g (0.1287 mmol, 69%) of bright orange crystals upon drying which were identified as 1-H(BH 4 ). Analysis for C 34 H 38 P 3 FeB: Calcd C, 67.37%; H, 6.32%. Found: C, 67.66%; H, 6.06%. 1

Preparation of (Triphos)FeD(η 2 -BD 4 ) (1-D(BD 4 ))
under an inert atmosphere, a 20 mL scintillation vial was charged with 0.030 g (0.039 mmol) of 1-Br 2 (CO) in approximately 15 mL dry THF. To the suspension, 0.024 g (0.578 mmol) of NaBd 4 was added and stirred at room temperature for 4 d. The solution was filtered through Celite and the THF was removed. The resulting orange film was scraped with diethyl ether (2 × 5 mL) and dried to yield 0.018 g (0.030 mmol, yield = 80%) of an orange solid identified as 1-D(BD 4 ). 2

Funding
This work was supported by the Los Alamos National Laboratory [Laboratory directed Research and development Program].