The Yamashita Group@NU
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Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
Department of Molecular and Macromolecular Chemistry,
Graduate School of Engineering, Nagoya University
access to Higashiyama campus, campus map
#1 Building, room 1029 (10th floor, entrance is the 2nd floor)

(2b) Exploring new elementary reaction such as bond cleavage by using organometallic complexes

As shown in Research topic (1), we succeeded to develop the method for the preparation of transition metal complexes through a nucleophilic borylation. On the other hand, it has been known that some paticular boryl complexes could borylate C-H bond of alkanes and arenes. Therefore, one can expect controlling the reactivity of boryl complexes to achieve catalytic reactions other than borylation reaction. However, boryl ligand easily comes off from the metal center due to its high reactivity as an anionic monodentate ligand. In this context, we designed a novel boron-containing pincer ligand having phosphorus tethers to aim for new catalytic functionalization of hydrocarbons. Our achievement is summarized below.

(2b-1) Hydroborane derivative as a precursor for PBP pincer ligand was synthesized and applied to complexation with Ir. From the structure of six-coordinate PBP-Ir complex in comparison with the corresponding PCP-pincer derivative, we could summarize that the boryl ligand in our PBP-pincer ligand could have strong trans influence as other literatures reported. It was also found that tBu-, Cy-, Ph-substituted PBP-Ir complexes did not have any activity, while we recently found that the iPr-substituted PBP-Ir complexes had catalytic activity for transfer dehydrogenation of alkanes.

(2b-2) We also succeeded to synthesize PBP-Rh(H)(Cl) and PBP-Rh(H)(OTf) complexes by the same method for Ir complexes. Treatment of the latter complex with strong base led to a formation of T-shape, 14-electron Rh complexes as isolable solids. In the crystal, this complex has intermolecular C-H---Rh interaction to be stabilized with linear and polymeric structure. This T-shape complex rapidly reacted with phenol to give PBP-Rh(H)(OPh) complex through an oxidative addition of O-H bond. In the reaction of the T-shape complex with aliphatic 1° alcohols, PBP-Rh(CO) complex and linear alkanes were liberated. On the other hand, it was also found that the T-shape complex could react with cyclobutenones to cleave C-C single bond through an oxidative addition under mild condition. This is stark contrast to that the reaction of ClRh(PPh3)3complex required higher temperature and afforded a mixture of isomeric products.

(2b-3) By a similar method, we also prepare PBP-Pt(Cl) complex and it could be converted to PBP-Pt(H) complex exhibited unusual 1H NMR chemical shift of the hydride ligand and Pt-H vibration in IR spectrum. Furtheremore, we also found that the former complex showed catalytic activity for hydrosilylation of alkenes and PBP-pincer ligand lost the central boron atom upon treatment with water or ethenol.

(2b-4) Reaction of hydroborane precursor with Ru(CO)3(cod) furnished PBP-Ru(H)(CO)2. Treatment of this complex with NMO lead to a formation of boronato complex, in which B-O bond of 1.329(6) Å was very short to have a double bond character. By using other Ru sources, PBP-Ru(Cl)(CO), PBP-Ru(CO)(η2-BH4), PBP(μ-H)2-Ru(OAc-κ2O), and PBP(μ-H)2-Ru(η2-BH4) complexes were obtained. Some of them were shown to have catalytic activity for hydrogenation of aldehydes by using hydrogen gas.

(2b-5) Synthesis of neopentyl-substituted PNP-pincer Ir complexes and elucidation of their reactivity
Iridium complexes having bulky group substituted pincer ligand are attractive because they could cleave C-H, N-H, O-H bonds. However, examples to utilize these bond cleavage reactions with pincer-ligated complexes are rare probably due to the steric crowding around the metal center. We recently developed a new pincer ligand system having neopentyl substituents on the phosphorus atoms to keep sufficient space around the metal center. Complexation of neopentyl-substituted PNP ligand with iridum easily gave an iridacycle complex via an CH activation at one of the neopentyl groups. This complex could react with arenes possessing a directing group and could catalyze a deaminative dimerization of alkylamines to afford dialkylamines with very high activity.

1) Segawa, Y.; Yamashita, M.; Nozaki, K. J. Am. Chem. Soc. 2009, 131, 9201-9203. doi.
2) Segawa, Y.; Yamashita, M.; Nozaki K. Organometallics 2009, 28, 6234-6242. doi
3) Hasegawa, M.; Segawa, Y.; Yamashita, M.; Nozaki, K. Angew. Chem. Int. Ed. 2012, 51, 6956-6960.   doi
4) Ogawa, H.; Yamashita, M. Dalton Trans. 2013, 45, 625-629   doi
5) Masuda, Y.; Hasegawa, M.; Yamashita, M.; Nozaki, K.; Ishida, N.; Murakami, M. J. Am. Chem. Soc. 2013, 135, 7142–7145.    doi
6) Miyada, T.; Yamashita, M. Organometallics 2013, 32, 5281–5284.    doi
7) Ogawa, H.; Yamashita, M. Chem. Lett. 2014, 43, 664-666.    doi
8) Miyada, T.; Kwan, E. H.; Yamashita, M. Organometallics 2014, 33, 6760-6770.    doi
9) Tanoue, K.; Yamashita, M. Organometallics 2015, 34, 4011-4017.    doi
10) Yano, T.; Moroe, Y.; Yamashita, M.; Nozaki, K. Chem. Lett. 2008, 37, 1300-1301. doi
11) Yamashita, M.; Moroe, Y.; Yano, T.; Nozaki, K. Inorg. Chim. Acta 2011, 363, 15-18. doi