While there are several reported 2,2'-bis(diphenylphosphino)biphenyl (BIPHEP)-based ligands bearing meta-substituted -P(3,5-R₂-Ph)₂ and para-substituted -P(4-R-Ph)₂ groups, there are currently no literature articles reports of BIPHEPs bearing ortho-substituted -P(2-R-Ph)₂ groups (2-R-BIPHEP). This is a promising design space to explore because, once coordinated to a transition metal center, the ortho-substituents on the ligand can significantly impact both the steric and electronic profile immediately surrounding the metal center. Furthermore, proton donor or acceptors ortho-substituents may allow for metal-ligand cooperative effects of these transition-metal complexes in proton-transfer reactions such as catalytic transfer hydrogenation or acceptorless dehydrogenation. This presentation will focus on our group’s ongoing synthetic efforts in preparing 2-R-BIPHEP ligands and 6,6'-dimethoxy-2,2'-bis(di(-2-R-phenyl)phosphino)-1,1’-biphenyl analogs (OMe-2-R-BIPHEP), where R = ethyl and methyl. Successes and challenges targeting these ligands with several synthetic routes will be presented. Some routes involve a final transition-metal-catalyzed C-C biaryl coupling of two triarylphosphine units. Other routes begin with either the biphenyl or 6,6'-dimethoxy-biphenyl backbone, generate 2,2'-bis(diethylphosphonate) biphenyl compounds via transition-metal-catalyzed P-C coupling, and yield the target BIPHEP-derivatives through iterative nucleophilic substitutions and phosphine oxide reduction. The synthesis of a number of Ru(II)-phosphine, Ru(II)-(2-R-BIPHEP), and Ru(II)-(OMe-2-R-BIPHEP) complexes will be shared, as well as their characterization by NMR and single-crystal XRD. Preliminary evaluation of some of these complexes as catalysts in proton-transfer reactions will also be presented.

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