Molecular Structure and Cluster Formation of a tert‐Butylborylene Complex

Abstract

In 1995 we reported on the first spectroscopically characterized borylene complexes [m-BX{(h 5 -C 5 H 4 R)Mn(CO) 2 } 2 ] (1: X = NMe 2 , R = H; 2: X = tBu, R = Me) [1] and the molecular structure of 1. During the past decade, intense research efforts were focused on borylene complexes, [2] and many different coordination modes for ligands of the type BÀR were described and include terminal, [3] hetero-dinuclear, [4] and semibridging [5] species, thus experimentally proving the predicted close relationship between borylene ligands and CO. [6,7] More recent studies focused on the reactivity of borylene complexes and revealed some interesting characteristics of these species, for example, i) the potential of terminal borylene ligands to be transferred to metallic [8] and nonmetallic [9] substrates, ii) the strong tendency of some terminal borylene ligands to add metal bases of the type [M(PR 3 ) n ] (M = Pd, Pt), [5,10] and iii) the exceptional stability of the central Mn 2 B framework in 1.[7] This stability allowed for elusive substitution reactions at the borylene center without cleavage of the metal-boron framework.[11] Given the pronounced thermodynamic stability of these compounds with respect to M-B dissociation, [6,7] the latter finding may appear surprising; DFT calculations revealed a build-up of positive charge at the boron center, and hence, the M-B bond is kinetically labile and susceptible to nucleophilic cleavage.

Comparative Analysis of Electron‐Density and Electron‐Localization Function for Dinuclear Manganese Complexes with Bridging Boron‐ and Carbon‐Centered Ligands

“…Its MnÀMn bond length between 2.903 and 3.033 , [32,53,54] depending on the means of determination (see also Table 1), will be the reference value. Obviously, measured and optimized Mn-Mn distances of the bridged compounds 1 to 4 [6,13] are all significantly shorter and range from 2.806 to 2.857 . Together with the 18-valence-electron rule, this has been a main reason for the assumption of a direct metalÀmetal bond in these and related complexes.…”

“…[75] Bridging boron ligands have been rarely studied, and therefore their bonding is of current interest. The charges in Table 4 suggest that a) the significant thermochemical stability of bridged borylene complexes [13] may derive partly from electrostatic contributions (cf. fragment charges of 1 and 2), and b) the appreciably positive charge at boron explains the accessibility towards nucleophilic attack at the bridging ligand.…”

“…Given the prototype nature of 1 as complex with a bridging boron ligand, and the somewhat unexpected results of the experimental QTAIM study (accompanied by computations at the BP86/TZVP level), [9] we provide here a systematic computational bonding comparison for 1 and its homologous amido-substituted complex 2. [6,7,9,12,13] Furthermore, we extend the comparison to the closely related carbonbridged complexes 3 [14] and 4. Complex 3 represents the first methylene-bridged complex ever obtained, [15] and its early electron-density measurement has been interpreted to exhibit a direct Mn À Mn bond [16] Complex 4 in turn represents a related prototypical vinylidene-bridged complex.…”

Heterometallic Triply‐BridgingBis‐Borylene Complexes

“…Based upon the coordination nature of boron and the number of boron-metal bonds, borylene ligands can be classified as terminal, bridging, semibridging, and triply bridging (Chart 1). [7][8][9][10][11][12] Borylenes possess classical 2c-2e bonds, unlike metallaboranes, where M-B bonds are composed of nonclassical multicenter-2e bonds. Apart from their fascinating structural features, the chemistry associated with borylene has been enriching the field of organometallic and organic synthesis.…”

Chemistry of metal – boron double bonds

“…Interestingly, the formation of the heterometallic m 3 -borylene complex Cp 0 Mn(CO) 2 (m 3 -B t Bu)[Pd(PCy 3 )]2 (34) from the related bridged tertbutylborylene [Cp 0 Mn(CO) 2 ] 2 (m-B t Bu)(35) also involves formal extrusion of a terminal borylene moiety from the corresponding bridged system[76].The reactions of 2 and 3 with Pd(PCy 3 ) 2 under thermal conditions generate the heterodinuclear 'semi-bridged' complexes (OC) 4 M[m-BN(SiMe 3 ) 2 ]Pd(PCy 3 ) (M ¼ Cr, 36; W, 37), in effect by incomplete transfer of the borylene ligand (scheme 7) [77]. Structurally authenticated chromium complex 36 has a significantly different (orthogonal) orientation of the bis(trimethylsilyl)amino substituent with respect to Scheme 6.…”