Hindered ethers represent an underexplored area of chemical space due to the difficulty and inoperability associated with conventional reactions, despite the high-value of such structural motifs in a variety of societal applications 1-2. For example, such motifs are highly coveted in medicinal chemistry, as extensive substitution about the ether bond prevents unwanted metabolic processes that can lead to rapid in vivo degradation. Demonstrated herein is an exceptionally simple solution to this problem that leverages the power of electrochemical oxidation to liberate *

In a single step, from [Cp*RuCl 2 ] 2 (Cp* ) η 5 -C 5 Me 5 ) and Li [BH 4 ], nido-1,2-(Cp*Ru) 2 (µ-H) 2 B 3 H 7 , 1, is produced in high yield. Addition of BH 3 ‚THF to 1 results in conversion to nido-1,2-(Cp*Ru) 2 (µ-H)B 4 H 9 , 2. Reaction of BH 3 ‚THF directly with [Cp*RuCl 2 ] 2 yields a mixture of 1 and 2. In two steps, a rhodium analogue, nido-2,3-(Cp*Rh) 2 B 3 H 7 , 9, is accessible by the reaction of [Cp*RhCl 2 ] 2 and Li [BH 4 ] to exclusively produce (Cp*Rh) 2 B 2 H 6 , 8, which adds BH 3 ‚THF to give 9 as the major product in a mixture. Reaction of BH 3 ‚THF directly with [Cp*RhCl 2 ] 2 yields the chloro derivative of 9, nido-1-Cl-2,3-(Cp*Rh) 2 B 3 H 6 , 11, in high yield via the intermediate positional isomer, nido-3-Cl-1,2-(Cp*Rh) 2 B 3 H 6 , 10. With high concentrations of Co 2 (CO) 8 , 1 reacts with Co 2 (CO) 8 to give nido-1-(Cp*Ru)-2-(Cp*RuCO)-3-Co(CO) 2 (µ 3 -CO)B 3 H 6 , 3, whereas low concentrations permit competitive degradation of 1 to yield arachno-(Cp*Ru)(CO)(µ-H)B 3 H 7 , 4. On the other hand, reaction of 11 with Co 2 (CO) 8 gives closo-1-Cl-6-{Co(CO) 2 }-2,3-(Cp*Rh) 2 (µ 3 -CO)B 3 H 3 , 12. Mild thermolysis of 3 results in loss of hydrogen and the formation of closo-6-Co(CO) 2 -2,3-(Cp*Ru) 2 (µ-CO)(µ 3 -CO)B 3 H 4 , 5, whereas thermolysis of 2 results in loss of hydrogen and formation of pileo-2,3-(Cp*Ru) 2 B 4 H 8 , 6, with a BH-capped square pyramidal structure. Finally, 6 reacts with Fe 2 (CO) 9 to yield pileo-6-Fe(CO) 3 -2,3-(Cp*Ru) 2 (µ 3 -CO)B 4 H 4 , 7, with a BH-capped octahedral cluster structure. The overall isolated yield of 7, formed in four steps from [Cp*RuCl 2 ] 2 , is ≈50% and evidences good control of reactivity.

The reaction of (CpReH(2))(2)B(4)H(4) with monoborane leads to the sequential formation of (CpRe)(2)B(n)()H(n)() (n = 7-10, 1-4). These species adopt closed deltahedra with the same total connectivities as the closo-borane anions [B(n)()H(n)()](2)(-), n = 9-12, but with flattened geometries rather than spherical shapes. These rhenaborane clusters are characterized by high metal coordination numbers, Re-Re cross-cluster distances within the Re-Re single bond range, and formal cluster electron counts three skeletal electron pairs short of that required for a canonical closo-structure of the same nuclearity. An open cluster, (CpReH)(2)B(7)H(9) (5), is isolated that bears the same structural relationship to arachno-B(9)H(15) as 1-4 bear to the closo-borane anions. Chloroborane permits the isolation of (CpReH)(2)B(5)Cl(5) (6), an isoelectronic chloro-analogue of known open (CpWH(2))(2)B(5)H(5) and (CpRe)(2)B(6)H(4)Cl(2) (7), a triple-decker complex containing a planar, six-membered 1,2-B(6)H(4)Cl(2) ring. Both are putative five- and six-boron intermediates in the formation of 1. Electronic structure calculations (extended Hückel and density functional theory) yield geometries in agreement with the structure determinations, large HOMO-LUMO gaps in accord with the high stabilities, and (11)B chemical shifts accurately reflecting the observed shifts. Analyses of the bonding in 1-4 reveal that the CpRe.CpRe interaction generates fragment orbitals that are able to contribute the "missing" three skeletal electron pairs required for skeletal bonding. The necessity of a Re.Re interaction for strong cluster bonding requires a borane fragment shape change to accommodate it, thereby explaining the noncanonical geometries. Application of the debor principle of borane chemistry to the shapes of 1-4 readily rationalizes the observed geometries of 5 and 6. This evidence of the scope of transition metal fragment control of borane geometry suggests the existence of a large class of metallaboranes with structures not found in known borane or metal clusters.