DOI: 10.1002/ejic.201900817 Full Paper Main Group Hydrides Thermal Decomposition of Donor-Stabilized Phosphinoborane PH 2 BH 2 NMe 3 : A Tensimetry Study Alina V. Butlak, [a] Igor V. Kazakov, [a] Andreas Stauber, [b] Oliver Hegen, [b] Manfred Scheer, [b] Anna V. Pomogaeva, [a] and Alexey Y. Timoshkin* [a] Abstract: The thermal decomposition of PH 2 BH 2 ·NMe 3 (1) in closed system was studied by static tensimetry with membrane null-manometer. Compound 1 is unstable already at room tem- perature and partially decomposes with the release of gaseous trimethylamine and nonvolatile polymeric material. Storage of 1 at 311 K for 53 days results in evolution of small amounts of Introduction Inorganic analogs of organic polymers, in particularly poly(phosphinoboranes) [R 1 R 2 PBH 2 ] n , [1,2] attract attention due to the broad range of their applications, such as in lithographic techniques, as precursors for ceramic materials, as flame-retard- ant materials with tunable degradation temperatures. [3–8] There are several synthetic approaches to poly(phosphinoboranes) (Scheme 1). In early studies, reactions between phosphine and diborane (pathway A) or the dehydrocoupling of the corre- sponding R 1 R 2 PHBH 3 complexes (pathway B) have been em- ployed. [9,10] The reaction between PH 3 and B 2 H 6 at 65 °C for 19 days results in H 2 evolution and the solid residue PB 1.15 H 3.91 , which the authors interpret as [PBH 3.75 ] n with some BH polymer, probably resulting from the decomposition of unreacted B 2 H 6 . [9] Thus, 0.125 mol of H 2 was eliminated from the mono- meric unit of [PH 2 BH 2 ] n at these conditions. The resulting refrac- tory solid [PBH 3.75 ] n is insoluble in ether, benzene, methanol, water and aqueous HCl under mild conditions. The authors at- tribute it to the cross-linking nature of the polymer. [9] The parent complex PH 3 BH 3 is unstable towards hydrogen evolution even at 0 °C. [11] Heating of PH 3 BH 3 at 200 °C for 15 minutes results in formation of hydrogen gas, a mixture of diborane and phosphine and solid residue PBH 3.36 . [11] These re- sults indicate, that the formed [PH 2 BH 2 ] n is unstable at these conditions and loses 0.32 mol of hydrogen gas per monomeric PH 2 BH 2 unit. [a] Institute of Chemistry, St. Petersburg State University, 199034 Universitetskaya emb. 7/9, St. Petersburg, Russia E-mail: a.y.timoshkin@spbu.ru [b] Institut für Anorganische Chemie, Universität Regensburg, 93040 Regensburg, Germany E-mail: manfred.scheer@ur.de Supporting information and ORCID(s) from the author(s) for this article are available on the WWW under https://doi.org/10.1002/ejic.201900817. Eur. J. Inorg. Chem. 2019, 3885–3891 © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 3885 hydrogen and phosphine. At temperatures up to 329 K, PH 3 evolution and BH 3 NMe 3 adduct formation is observed after 3 days, and above 387 K, considerable amount of hydrogen gas is released. The data obtained indicate that the pyrolysis of 1 in the closed system may result in the enrichment of the con- densed phase with boron due to phosphine loss. Scheme 1. Synthetic approaches to poly(phosphinoboranes). For the substituted derivatives, the bulkiness of the R 1 ,R 2 sub- stituents influences the degree of oligomerization within poly(phosphinoboranes). Subsequent introduction of the sub- stituents on the P center results in a decrease of the oligomeri- zation degree. [9] Thus, the parent [PBH 3.75 ] n polymer is highly cross-linked and insoluble, the monomethyl derivative [MePHBH 2 ] n is a mostly non-branched linear polymer, while the dimethyl derivative forms trimers and tetramers [Me 2 PBH 2 ] 3,4 with traces of high mass oligomers. [9] [Me 2 PBH 2 ] 3,4 can also be obtained by the reaction of tetra- methyldiphosphine with diborane (pathway C, Scheme 1). [12] The structures of several cyclic oligomers, such as trimers [Me 2 PBH 2 ] 3 , [13] [Ph 2 PBH 2 ] 3 , [14] [(Me 3 Si) 2 PBH 2 ] 3 , [15] and tetramers [Me 2 PBH 2 ] 4 , [16] [Ph 2 PBH 2 ] 4 , [17,18] [(p-CF 3 C 6 H 4 ) 2 PBH 2 ] 4 [17,18] have been determined by X-ray structural analysis. Other studied oligomers also include [(CF 3 ) 2 PBH 2 ] 3,4 , [19] and [Ph 2 PBH 2 ] 3 . [20] Heating of MePH 2 BH 3 for 20 h at 100 °C yields one mol of H 2 and leads to the formation of mostly linear “a fairly high polymer” [MeHPBH 2 ] n which occurs as viscous non-volatile oil which cannot be distilled. [7] On heating by free flame, this oil evolved additional 0.95 mol of H 2 and changed to a brown non- volatile solid, which was not further characterized. [9] The thermolysis of PhPH 2 BH 3 at 150 °C for 3 h liberated one mol of hydrogen gas with the formation of a solid, brittle, white [PhPHBH 2 ] n polymer with a molecular weight of 2150, [10] partly