Cationic Terminal Borylene Complexes: A Synthetic and Mechanistic Investigation of MB Metathesis Chemistry

Abstract

Metathesis reactions constitute a key component of modern synthetic chemistry; olefin metathesis, for example, provides a versatile and widely exploited carbon-carbon bond-forming methodology.[1] Such reactions are typically catalyzed by organometallic complexes that contain M=C bonds.[2] The synthesis of analogous complexes that contain M=Si bonds, for example, has led to an in-depth investigation of their reactivity towards unsaturated substrates. [3] Synthetic approaches that lead to the isolation of related systems with M = B bonds have been developed only recently: [4][5][6][7] for example, halide-abstraction chemistry gives access to cationic terminal borylene complexes, [L n M= BX][7] Consequently, reports of the fundamental chemistry of M=B bonds are somewhat limited (predominantly to metal-metal transfer reactions and addition/substitution reactivity towards nucleophiles). [4,6,7] Thus, the chemistry of [Cp*Fe(CO) 2 (BMes)] + (Mes = mesityl = 2,4,6-Me 3 C 6 H 2 , Cp* = pentamethylcyclopentadienyl), for example, is dominated by the electrophilic character at both the Fe and B centers. [7] In an attempt to tune the reactivity of these highly unsaturated complexes we investigated the synthesis of cationic aminoborylene systems, [L n M = BNR 2 ]+ .[5] Chloride abstraction from [CpFe(CO) 2 {B(NiPr 2 )Cl}] (2) (Cp = cyclopentadienyl) by Na(BAr À . The steric bulk of the amino substituents is a key point: the use of the smaller NMe 2 group results in the formation of a thermally fragile borylene product in the subsequent halide-abstraction step, [7b, 8] whereas [CpFe(CO) 2 {B(tmp)Cl}] (tmp = tetramethylpiperamino) was inaccessible from tmpBCl 2 . Compound 2 is a pale yellow sublimable crystalline solid, which was characterized by multinuclear NMR and IR spectroscopy, mass spectrometry, and X-ray crystallographic analysis (Figure 1).The reaction of 2 with Na(BAr f 4 ) in dichloromethane results in a quantitative conversion (determined by 1 H and 11 B NMR spectroscopy) into 3. The latter product (along with the C 5 H 4 Me analogue) is a colourless oil at (or close to) room temperature, but its formulation can be definitively established from spectroscopic and reactivity data. The measured 11 B chemical shift for 3 (d B = 93.5 ppm) is very close to that reported by Braunschweig et al. for neutral terminal-aminoborylene systems of the type L n M=BN(SiMe 3 ) 2 (d B = 86.6-98.3 ppm).[5] The downfield shift upon chloride abstraction (Dd B = 38.1 between 2 and 3) mirrors that found for and 88.0 ppm, respectively). [7,8] The 1 H and 13 C NMR data are

Electron-Precise Coordination Modes of Boron-Centered Ligands

“…In 2003 the group of Aldridge published the first example of a cationic borylene complex, prepared by halide abstraction from a neutral iron boryl complex. Since this time, both the halide abstraction protocol and the group 8 cationic borylene complexes thus prepared have become particularly prolific (Figure ). – By employing a noncoordinating borate anion (invariably [BAr f 4 ]), one halide is abstracted from the boryl ligand, causing the boron atom to become linear. In terms of first-generation complexes, this family contains the mesityl borylene complex 4.10 , a host of Fe and Ru aminoborylene complexes 4.11a−g , and the dinuclear complex 4.13 .…”

“…The case of borylene complex 4.11b , from which a number of these reaction patterns were observed, is demonstrative of this unpredictability (Figure ). , With excess H 2 O, the usually unreactive B−N double bond was broken along with the MB double bond, resulting in ammonium salt [ i Pr 2 NH 2 ][BAr f 4 ], B(OH) 3 , and [(η 5 -C 5 H 5 )Fe(CO) 2 ] 2 . Addition of phosphine (Ph 3 PS) and arsine (Ph 3 AsO) chalcogenides to 4.11b resulted in mild metathesis of the Fe−B with the E−S or E−O bonds, leading to [(η 5 -C 5 H 5 )Fe(CO) 2 (EPh 3 )][BAr f 4 ] in both cases, with the side products being the cyclic species [ i Pr 2 NBX] n (X = S, n = 2; X = O, n = 3).…”

Boron–Transition-Metal Triple-Bond FB≡MF2 Complexes

“…Since then, various metal–boron-bonding situations have been explored, and a number of transition-metal–borylene complexes with a MB double bond have been characterized . For example, a cationic borylene complex [Me 5 (η 5 -C 5 Me 5 )­FeB-Mes] + (Mes = 2, 4, 6-trimethylphenyl) was synthesized, where the molecule displays a short Fe–B distance with a double bond. − First terminal borylene complexes [(CO) 5 M­{BN­(SiMe 3 ) 2 }] (M = Cr, W) were obtained, which have a metal–boron double bond in terms of strong boron-to-metal σ donation and weaker π back bonding from the metal to boron . The structure and bonding of a TM–ER (E = B, Al, Ga, In, and Tl) bond have been calculated at the DFT level of theory. − Recently, bismuth–borylene complexes (RBiBR) (R = Tbt, etc.)…”

Metal–ligand multiple bonds as frustrated Lewis pairs for C–H functionalization

“…Many transition-metal borylene complexes may also be categorized as M δ− ═E δ+ FLPs, undergoing reactions with heterocumulenes and other polar multiple bonds similar to those reported for silylenes [59]. The [2 + 2]-type reactions with heterocumulenes can lead to insertions or, in some cases, atom transfer [60].…”