This makes direct synthesis problematic because the alkali metals do not react with boron below 700 °C, and thus this must be accomplished in sealed containers with the alkali metal in excess.
ðɪs mˌeɪks dᵻɹˈɛkt sˈɪnθəsˌɪs pɹˌɑːbəlmˈæɾɪk bɪkˈʌz ðɪ ˈælkəlˌaɪ mˈɛɾəlz duːnˌɑːt ɹɪˈækt wɪð bˈoːɹɑːn bᵻlˌoʊ sˈɛvənhˈʌndɹɪd dᵻɡɹˈiːz sˈiː, ænd ðˈʌs ðɪs mˈʌst biː ɐkˈɑːmplɪʃt ɪn sˈiːld kəntˈeɪnɚz wɪððɪ ˈælkəlˌaɪ mˈɛɾəl ɪn ɛksˈɛs.