pubs.acs.org/IC Published on Web 11/08/2010 r 2010 American Chemical Society 10756 Inorg. Chem. 2010, 49, 10756–10758 DOI: 10.1021/ic1018532 A Hybrid Lithium Oxalate-Phosphinate Salt Andrew R. Shaffer, Nihal Deligonul, Daniel A. Scherson, and John D. Protasiewicz* Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106-7708, United States Received September 9, 2010 The novel organophosphorus-containing lithium salt Li(THF)- [(C 2 O 4 )B(O 2 PPh 2 ) 2 ](1; THF = tetrahydrofuran) was synthesized and characterized using a variety of spectroscopic techniques. An X-ray structural analysis on crystals of 1 grown from THF reveals a dimeric structure [Li(THF)(C 2 O 4 )B(O 2 PPh 2 ) 2 ] 2 3 THF, whereby the two units of 1 are bridged via P-O 333 Li interactions. Compound 1 displays high air and water stability and is also thermally robust, properties needed of electrolytes for their possible use as electro- lytes and/or additives in lithium-ion battery applications. Since their commercial inception about 2 decades ago, lithium-ion batteries have become the power sources of choice for mobile comunication devices, portable computers, and power tools. 1 Attention has most recently been focused on their potential use for vehicular applications, spurned by the ever-rising demand for clean transportation energy. 2,3 However, safety concerns and cycle life continue to delay widespread introduction of this promising technology into this fast-growing industrial segment. Among the major hurdles to be overcome is the danger associated with the flammability of the organic solvents. Not surprisingly, efforts are being devoted toward the development of flame-retar- dant additives aimed at protecting systems and consumers during catastrophic failure or abuse. 4 We have thus initiated efforts to rationally design new flame-retardant ions (FRIONs) that retain and/or enhance the operational needs of a lithium-ion salt in terms of optimal charge storage transport and solid electrolyte interface (SEI) attributes. While LiPF 6 has been one of the most employed salts for lithium-ion batteries, one of the most intriguing new salts contains boron-carboxylic acid fragments. Initially disclosed in a German patent, 5 lithium bis(oxalato)borate (LiBOB) has garnered much attention as a lithium salt for electrolytes. 6 Recently, Lucht and co-workers reported a hybrid lithium tetrafluorooxalatophosphate derivative that has displayed characteristics similar to that of the parent LiPF 6 compound. 7 One component of our design is similarly built around the lithium oxalato subunit. The other design compo- nent centers on organophosphorus compounds, containing PdO bonds, which have been shown to be excellent flame retardants in the polymer industry. 8 Furthermore, phosphorus compounds have been shown to be more environmentally benign compared to their halogenated analogues. 9 In this Communication, we report the synthesis and characterization of a possible FRION, Li(THF)[(C 2 O 4 )- B(O 2 PPh 2 ) 2 ](1; THF = tetrahydrofuran). Compound 1 ex- hibits excellent water and air stability as well as stability to a wide range of solvents including propylene carbonate at elevated temperatures for several days. The initial synthesis of 1 involved the solid-state reaction of diphenylphosphinic acid, oxalic acid, and boric acid in the presence of lithium hydroxide (Scheme 1, route A). While conversions of up to 50% to 1 could be achieved (as estimated by 31 P NMR spectroscopy, δ 20.9 ppm), much diphenylphosphinic acid, as well as an additional compound Li(THF)(Ph 2 PO 2 H)- (O 2 PPh 2 )(2) in ca. 10% (δ 20.3 ppm), remained. Compound 2 was predominantly formed when an additional 3 equiv of lithium hydroxide was added to the reaction mixture. Com- pound 2 was isolated as the sole product (79%) after a 2:1:1:2 mixture of diphenylphosphinic acid, boric acid, oxalic acid, and excess lithium hydroxide in toluene was refluxed and water was removed using a Dean-Stark trap apparatus. The identity of 2 was rigorously assigned by 1 H, 31 P, and 13 C NMR, elemental analysis, and the results of a single-crystal *To whom correspondence should be addressed. E-mail: protasiewicz@ case.edu. (1) Terada, N.; Yanagi, T.; Arai, S.; Yoshikawa, M.; Ohta, K.; Nakajima, N.; Yanai, A.; Arai, N. J. Power Sources 2001, 100, 80. (2) (a) Ritchie, A. G. J. Power Sources 2001, 96, 1. 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