Synthesis and Interconversion of Some Small Ruthenaboranes: Reaction of a Ruthenium Borohydride with Pentaborane(9) to Form Larger Ruthenaboranes

A number of ruthenaborane clusters have been prepared containing the {(η-C5Me5)Ru(PMe3)} fragment. Reaction of [(η-C5Me5)Ru(PMe3)H3] with 1 equiv of BH3·thf gives [(η-C5Me5)Ru(PMe3)BH4] (1), which reacts further with another equivalent of BH3·thf to give [(η-C5Me5)Ru(PMe3)B2H7] (2) in nearly quantitative yield. Thermolysis of 2 in solution at 60 °C gives a mixture of 1, arachno-2-[(η-C5Me5)Ru(PMe3)B3H8] (3), and arachno-1-[(η-C5Me5)Ru(PMe3)B3H8] (4). Thermolysis of the wing isomer 3 in solution at 100 °C gives 100% conversion to the hinge isomer 4. Thermolysis in a solution of arachno-3-[(η-C5Me5)Ru(PMe3)B4H9] also gives 4 and 1. The borohydride 1 reacts with pentaborane(9) to give four ruthenaborane clusters, compound 3, nido-2-[(η-C5Me5)RuB5H10] (5), arachno-2-[(η-C5Me5)Ru(PMe3)B5H8] (6), and arachno-3-[(η-C5Me5)Ru(PMe3)B4H9] (7). Thermolysis of 5 in solution gives nido-1-[(η-C5Me5)RuB5H10] (8) in good yield. Compound 5 deprotonates cleanly at the unique basal B−H−B bridge on reaction with KH in thf to form nido-2-[(η-C5Me5)RuB5H9 -][K+] (9). Compound 8 deprotonates cleanly at a B−H−B bridge on reaction with KH in thf to form fluxional nido-1-[(η-C5Me5)RuB5H9 -][K+] (10). The X-ray crystal structure of 3 is reported.