High Yield Synthesis and Characterization of Chromaboranes. Comparison of the Geometric, Electronic, and Chemical Properties of an Electronically Unsaturated (.eta.5-C5Me5)2Cr2B4H8 Cluster with Its Saturated Derivative (.eta.5-C5Me5)2Cr2(CO)2B4H6

Abstract

The reaction of BHa'THF with [Cp*CrCl]2, Cp* = ^-CsMes, leads to the isolation of Cp*2Cr2B4Hg (1) in very good yield. This green-brown, diamagnetic chromaborane cluster exhibits a nido geometry based on a pentagonal pyramidal deltahedron with metal atoms occupying the axial vertices and boron atoms four of the five equatorial vertices. Reaction with CO leads cleanly to brown, diamagnetic Cp*2Cr2<CO)2B4H6 (3) possessing the same cluster core structure as 1. Fenske-Hall molecular orbital calculations show that both 1 and 3 have a significant Cr-Cr bonding interaction and that the cluster geometry is properly described as a bicapped tetrahedron. On this basis, 3, with 6 cluster bonding pairs, is an electronically saturated cluster whereas 1, with only 5 cluster bonding pairs, is unsaturated. The calculations show that it is the high energy of the Cr 3d orbitals that leads to the electronic unsaturation of 1 and that the back-bonding of the CO ligands in 3 selectively stabilizes some of the Cr metal orbitals producing an additional filled skeletal orbital. The reaction of L1BH4 with [Cp*CrCl]2 results in the formation of the green-brown, paramagnetic [Cp*Cr(BHt)]2 borohydride complex, 2, which has been characterized spectroscopically. Chromaborane 2 reacts with stoichiometric amounts of CO at low temperature to yield brown, diamagnetic [Cp*Cr(CO)2(BH4)h (4). Based on the 18 electron rule, 2 is electronically unsaturated whereas 4 is saturated. The intercomparison of these four compounds is used to illustrate the structural expression of an electronically unsaturated metallaborane cluster bonding network and to relate this description to analogous organometallic compounds. Cp*2Cr2B4H8 (1) Cp*2Cr2(CO)2B4H6 (3) _ Distance Cr(l)-Cr(2) 2.870(2) Cr(l)-B(3) 2.01(2) Cr(l)-C(l) 2.20(1) Cr(l)-B(4) 2.11(2) Cr(l)-C(2) 2.20(1) Cr(2)-C(11) 2.209(9) Cr(l)-C(3) 2.18(1) Cr(2)-C(12) 2.17(1) Cr(l)-C(4) 2.19(1) Cr(2)-C(13) 2.18(1) Cr(l)-C(5) 2.225(9) Cr(2)-C(14) 2.16(1) Cr(l)-B(l) 2.13(2) Cr(2)-C(15) 2.15(1) Cr(l)-B(2) 2.01(2) Cr(2)-B(l) 2.15(2) Cr(2)-B(2) 2.02(1) B(l)-B(2) 1.75(3) Cr(2)-B(3) 2.04(2) B(2)-B(3) 1.75(3) Cr(2)-B(4) 2.12(2) B(3)-B(4) 1.61(3) Angle C(l)-Cr(l)-B(l) 157.3(6) C(4)-Cr(l)-B(l) 103.4(6) C(l)-Cr( 1)-B(2) 114.0(5) C(4)-Cr(l)-B(2) 105.1(5) C(l)-Cr(l)-B(3) 90.4(5) C(4)-Cr(l)-B(3) 135.0(6) C(l)-Cr(l)-B(4) 104.8(7) C(4)-Cr(l)-B(4) 165.6(7) C(2)-Cr(l)-B(l) 153.5(6) C(5)-Cr(l)-B(l) 120.6(6) C(2)-Cr(l)-B(2) 151.3(5) C(5)-Cr(l)-B(2) 91.5(5)

Structural, electronic and magnetic properties of some early vs late transition dimetallaborane clusters – A theoretical investigation

“…The same nido pentagonal bipyramidal Cr 2 B 4 cage was confirmed by an X-ray analysis with a CreCr bond length of 2.792(1) Å [14]. A DFT geometry optimization of 10 at the BP86/TZ2P/SC all-electron scalar ZORA level resulted in a structure with a Cr1eCr2 bond length abnormally short of 2.548 Å ( Table 2), suggesting that the chemical formula of 10 reported experimentally might be wrong (hydrogen atoms were not directly located from the X-ray analysis [14]). Due to the large difference with the experimental value (0.322 Å), two hydrogen atoms bridging the Cr1eB4 and Cr2eB5 vectors were added (see 10 in Chart 1).…”

“…With a Cr 2 B 4 core, the compound (Cp*Cr) 2 B 4 H 8 (10) Table 1 Selected bond parameters (Å) for the compounds (CpM) 2 has been characterized instead with a nido pentagonal bipyramidal cage with one missing equatorial vertex and the two Cr atoms occupying the axial positions and separated by 2.870(2) Å (Chart 1) [61]. The reaction of 10 with CO yielded (Cp*Cr) 2 B 4 H 6 (CO) 2 (11, Chart 1) as the major product [14]. The same nido pentagonal bipyramidal Cr 2 B 4 cage was confirmed by an X-ray analysis with a CreCr bond length of 2.792(1) Å [14].…”

“…The reaction of 10 with CO yielded (Cp*Cr) 2 B 4 H 6 (CO) 2 (11, Chart 1) as the major product [14]. The same nido pentagonal bipyramidal Cr 2 B 4 cage was confirmed by an X-ray analysis with a CreCr bond length of 2.792(1) Å [14]. A DFT geometry optimization of 10 at the BP86/TZ2P/SC all-electron scalar ZORA level resulted in a structure with a Cr1eCr2 bond length abnormally short of 2.548 Å ( Table 2), suggesting that the chemical formula of 10 reported experimentally might be wrong (hydrogen atoms were not directly located from the X-ray analysis [14]).…”

Fine-Tuning of Metallaborane Geometries: Chemistry of Iridaboranes Derived from the Reaction of [(Cp*Ir)2HxCl4−x] (x=0-2; Cp*=η5-C5Me5) with LiBH4

“…Indeed, we have demonstrated the reaction of binuclear [Cp*MCl n ] complexes (Cp* h 5 -C 5 Me 5 ) with LiBH 4 is a general route to metallaboranes. Detailed investigations of the Co, [4] Rh, [5,6] Ru, [6] Cr, [7] Mo, [8,9] and W [10,11] systems have been reported to date. Metal identity affects products.…”

Metallaheteroborane clusters of group 5 transition metals derived from dichalcogenide ligands

“…Comparison of the structural parameters and chemical shift of 2 with those of Cr [26], Re [27], Ta [14a] and Mo analogues [13], reveals several contrasting features (Table 2). Although, the MeM distance in 2 is comparable, the average MeB distances are markedly longer.…”