Cluster Expansion Reactions of Group 6 Metallaboranes. Syntheses, Crystal Structures, and Spectroscopic Characterizations of (Cp*Cr)₂B₅H₉, (Cp*Cr)₂B₄H₈Fe(CO)₃, (Cp*Cr)₂B₄H₇Co(CO)₃, and (Cp*Mo)₂B₅H₉Fe(CO)₃

Abstract

The targeted high-yield syntheses of a number of new clusters by the controlled addition of BH, Fe(CO) 3, or Co(CO) 3 fragments to dinuclear group 6 metallaboranes are reported. The reaction of the chromaborane, (Cp*Cr) 2 B 4 H 8 ( 1 ), Cp* = η 5 -C 5 Me 5, with BHCl 2 ·SMe 2 results in a cluster expansion reaction giving the green-brown diamagnetic species (Cp*Cr) 2 B 5 H 9 ( 2 ) in 43% yield. The six skeletal electron pair (sep) cluster has a structure based on a trigonal bipyramidal Cr 2 B 3 unit with adjacent Cr 2 B faces capped by BH 3 fragments. Reaction of 1 with the dinuclear metal carbonyls Fe 2 (CO) 9 and Co 2 (CO) 8 leads to the isolation of the mixed-metal metallaboranes (Cp*Cr) 2 B 4 H 8 Fe(CO) 3 ( 3 ) and (Cp*Cr) 2 B 4 H 7 Co(CO) 3 ( 4 ) in yields of 76 and 87%, respectively. 3 and 4 can also be viewed as six-sep bicapped trigonal bipyramidal clusters with the metal carbonyl occupying one of the capping positions. Alternatively, they can be viewed as complexes between M(CO) 3 transition metal fragments (M = Fe, Co) and the four-electron (Cp*Cr) 2 B 4 H 8 (M = Fe) or three-electron (Cp*Cr) 2 B 4 H 7 (M = Co) ligands. In this way, by forming two M−Cr bonds and utilizing either one (M = Co) or two (M = Fe) BH→M donor−acceptor bonds, each metal attains an 18-electron configuration. At elevated temperatures (ca. 80 °C), 3 is fluxional, with the Fe(CO) 3 fragment “swinging” between equivalent pairs of BH 2 donor groups. Reaction of the molybdaborane (Cp*Mo) 2 B 5 H 9 ( 5 ) with Fe 2 (CO) 9 gives the orange mixed-metal cluster (Cp*Mo) 2 B 5 H 9 Fe(CO) 3 ( 6 ) in 48% yield. 6 has a bicapped octahedral geometry with the Fe(CO) 3 fragment occupying one of the high-connectivity cluster vertexes, rather than a capping position. That 5 undergoes the expected geometric changes (bicapped trigonal bipyramid → bicapped octahedron) on addition of the two-electron Fe(CO) 3 fragment (giving 6 ) contrasts with the minimal structural perturbation on addition of the same fragment to 1 (yielding 3 ). This in turn provides further evidence for the description of 1 as an “electronically unsaturated” cluster.