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Preparation and Reactivity of Molybdenum Complexes Bearing Pyrrole-Based PNP-Type Pincer Ligand

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Preparation and Reactivity of Molybdenum Complexes Bearing Pyrrole-Based PNP-Type Pincer Ligand ChemComm Preparation and reactivity of molybdenum complexes bearing pyrrole-based PNP-type pincer ligand Journal: ChemComm Manuscript ID CC-COM-04-2020-002852.R2 Article Type: Communication Page 1 of 5 Please doChemComm not adjust margins COMMUNICATION Preparation and reactivity of molybdenum complexes bearing Received 00th April 2020, pyrrole-based PNP-type pincer ligand Accepted 00th April 2020 Yoshiaki Tanabe,a Yoshiya Sekiguchi,a Hiromasa Tanaka,b Asuka Konomi,b Kazunari Yoshizawa,b DOI: 10.1039/x0xx00000x Shogo Kuriyamaa and Yoshiaki Nishibayashi*a Molybdenum complexes bearing an anionic pyrrole-based PNP- TOF values reported for the bacterial nitrogen fixation.15 In type pincer ligand have been prepared and have been found to this catalytic system using SmI2/H2O, efficient reduction and work as catalysts for the conversion of N2 into NH3 under ambient protonation are proposed to proceed via the proton-coupled 16 conditions. electron-transfer (PCET) process. The SmI2/H2O pair can be also applicable to classical molybdenum dinitrogen complexes Artificial nitrogen fixation to convert N2 into NH3 by using bearing monodentate or bidentate phosphines,17 where stepwise transition metal catalysts under ambient conditions has attracted six-electron reduction and sextuple protonation are proposed to attention as a candidate to develop small-scale reactor systems occur onto the coordinated dinitrogen to afford 2 equiv of NH3 1,2 for convenient NH3 production. Entering the 21st century, per catalytic cycle (classical Chatt cycle),18 but the catalytic catalytic conversion of N2 into NH3 has been reported for activity is better for molybdenum complexes with N- several homogeneous catalysts containing transition metals heterocyclic carbene-based PCP-type or pyridine-based PNP- such as Ti,3 V,4 Mo,5–7 Re,8 Fe,9,10 Ru,11 Os,11 and Co.12 type pincer ligands, where direct cleavage of NN triple bond However, applicable pairs of reducing reagents with proton is proposed to proceed first in dinitrogen-bridged sources have been limited to metallocenes or KC8 with the dimolybdenum complexes to afford the nitride species,7,19–21 F F highly acidic Brookhart’s acid ([H(OEt2)2]BAr 4, (BAr 4 = followed by presumable three-electron reduction and triple tetrakis(3,5-bis(trifluoromethyl)phenyl)borate)) or conjugate protonation to give 1 equiv of NH3 per nitride species. acids of pyridine derivatives, amines, or a phosphine, In order to investigate the electronic and steric effects of the responsible in part for the requirement of low temperatures pincer ligands on the catalytic reactivity and reaction pathways (such as –78 °C) to inhibit the formation of side products such for the conversion of N2 into NH3, we have come to the idea to as H2 from the direct reaction of reducing reagents with proton prepare and examine molybdenum complexes bearing an sources. anionic pyrrole-based PNP-type pincer ligand.22,23 Indeed, we 13 Very recently, our group has confirmed that SmI2 can be have already achieved catalytic conversion of N2 into NH3 for used as the most effective reducing reagent so far to convert N2 pyrrole-based PNP-type pincer complexes of V,4 Fe,10 and into NH3 in combination with ethylene glycol or H2O as a Co.12 Molybdenum complexes with azaferrocene-based PNP- proton source, thus H2O has become available for use in the type pincer ligands, where the anionic pyrrolide is coordinated 14 5 artificial catalytic nitrogen fixation under ambient conditions. to both Mo and FeCp* (Cp* = η -C5Me5) moieties, have been Here, the turnover number (TON) and the initial turnover already prepared, but yielding only a stoichiometric amount of 24 frequency (TOF) for the formation of NH3 using SmI2, H2O, NH3 even in catalytic conditions. and a molybdenum complex bearing a carbene-based PCP-type Introduction of an anionic pyrrole-based PNP-type pincer pincer ligand as a reducing reagent, a proton source, and a ligand (PNP = 2,5-bis(di-tert-butylphosphinomethyl)pyrrolide) catalyst, respectively, were found to be 4,350 equiv Mo–1 and onto the molybdenum atom was achieved when the lithiated 112.9 equiv min–1 Mo–1, respectively, with both giving the best ligand LiPNP was treated with Mo(III) triiodo precursor TON and TOF values among the artificial nitrogen fixation [MoI3(thf)3] in THF at room temperature to afford the Mo(III) under mild reaction conditions, and the latter reaching to the diiodo complex [MoI2(PNP)] (1) in 81% yield (Scheme 1). The effective magnetic susceptibility μeff determined by Evans method was found to be 1.4 μB for 1, interpretable as the low- 3 a Department of Systems Innovation, School of Engineering, The University of spin d electronic configuration with an S = 1/2 spin state. The Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. E-mail: [email protected] molecular structure of 1 was unambiguously determined by an tokyo.ac.jp b Institute for Materials Chemistry and Engineering, Kyushu University, Nishi-ku, X-ray analysis (Fig. 1a), which demonstrates that 1 has a Fukuoka 819-0395, Japan. E-mail: [email protected] distorted square pyramidal structure with a 5-coordinate †Electronic Supplementary Information (ESI) available. CCDC 1992844, 1992845, 25 1995258, and 2000401. For ESI and crystallographic data in CIF or other geometry index value at τ5 = 0.19. electronic format see DOI: 10.1039/x0xx00000x. Please do not adjust margins Please doChemComm not adjust margins Page 2 of 5 COMMUNICATION Journal Name Next, we examined the reduction of 1 under an N2 moieties {MoI(N(PNP)} are linked with two cationic Li atoms atmosphere to obtain the corresponding dinitrogen complexes. bound to two μ3-nitrido N atoms bridging one Mo and two Li Indeed, treatment of 1 with KC8 in THF under 1 atm of N2 gave atoms for each. One of the two Li atoms in 3C4H10O is –1 brown solid with an IR absorption band at 2060 cm , additionally coordinated to an O atom of Et2O to display a attributable to the formation of a dinitrogen- and pyrrolide- pseudo rotational C2 symmetry around the Li···Li–O axis (Fig. bridged Mo(0)–Mo(I) mixed-valence coordination polymer 1b), whereas each of the Li atoms in 3(C4H8O)2 is additionally [(K(N2){Mo(N2)(PNP)}2)n] (2), which could not be isolated coordinated to a THF O atom to display a pseudo rotational C2 sufficiently pure (Scheme S2, ESI†). On the other hand, symmetry around the vertical axis that passes through the reduction of 1 with metallic lithium in toluene at –50 °C under center of the bridging Li2N2 core (Fig. 1c). Similar dimeric 1 atm of N2 afforded the dimeric 5-coordinate Mo(IV) nitrido structure was previously reported for the 5-coordinate Mo(VI) complex [{Li(PhMe)MoI(N)(PNP)}2] as diamagnetic brown nitrido complex bearing an anionic OCO-type pincer ligand 26 solid (Scheme 1). (3(C7H8)2) exhibited a new strong IR bridged by Na(dmf) units. The nitrido ligands in 3C4H10O or absorption band assignable to the coordinated MoN nitrido 3(C4H8O)2 possess the apical positions of distorted square –1 vibration at 1063 cm , comparable to the values reported for pyramidal structures with an averaged τ5 value at = 0.12±0.05 other 5-coordinate Mo nitrido complexes (948–1109 cm–1) or 0.09±0.03, and with an averaged MoN bond length at † 31 1 (Table S10, ESI ). In solution, 3(C7H8)2 gave a P{ H} NMR 1.677(4) or 1.678(7) Å, respectively, comparable to the values resonance at δ 95.7. found for 5-coordinate Mo(IV), Mo(V), or Mo(VI) nitrido t P Bu2 complexes bearing pincer ligands with τ5 values at 0.001–0.26 and MoN bond lengths at 1.63–1.73 Å (Table S9, ESI†). N Li PhMe I t Further DFT calculations gave the estimation that the split of t t tBu Bu2 P Bu I Bu2 2 Li 2 P 2 P P Li the dimer 3(C H O) to the corresponding monomeric nitrido (LiPNP) N2 (1 atm) N Mo 4 8 2 [MoI3(thf)3] N Mo N N THF, rt I PhMe Mo N complex [Li(thf)MoI(N)(PNP)] (A) at 298 K is endergonic (ΔG Li P 20 h P –50 °C, 2 h P t –1 t t Bu2 Bu2 Bu I = +20.2 kcal mol ), demonstrating that that the dimeric – LiI then rt, 20 h 2 PhMe – LiI structure is kept even in solution (Scheme S5, ESI†). 1, 81% 3(C7H8)2, 42% PhMe t O t I Bu2 t t Scheme 1 Preparation of molybdenum diiodo and nitrido complexes bearing a Bu2 P Bu2 Bu2 P Li P Li P N Mo N pyrrole-based PNP-type pincer ligand with the cleavage of N2. N N N N N Mo N Mo Mo Li P Et2O, –20 °C Li P t P P t Bu2 under N2 t t Bu2 I Bu I I Bu PhMe 2 2 3C H O 3(C7H8)2 4 10 N , 66% THF–pentane 2 from 3(C7H8)2 rt, under N2 – N2 Et O, rt, 6 h O t 2 t I Bu2 under Ar Bu2 P P Li N N Mo Mo N N LiMoI(PNP) P – N2 P Li t t Bu2 4, 92% from 3(C H ) Bu2 I O 7 8 2 via 3C4H10O 3(C4H8O)2 Scheme 2 Release of N2 from the nitrido complex. It must be noted that the dimeric nitrido complex 3Ln, rather stable in non-polar solvents such as toluene or benzene, was found to be labile in O-donor solvents such as diethyl ether and Fig.
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