Boron cations are elusive and highly electrophilic species that play a key role in the chemistry of boron. Despite early interest in the chemistry of boron cations, until recently they have largely remained chemical curiosities. However, hints at harnessing their potential as potent electrophiles have begun to appear and developments in weakly coordinating anion technology suggest that this is an area of research that is ripe for exploration. It has been nearly 20 years since the last major review on boron cations; herein we summarize the progress in the area since that time.

The ring-opening reactions of a series of sila[1]ferrocenophanes with protic acids of anions with various degrees of noncoordinating character have been explored. Ferrocenyl-substituted silyl triflates FcSiMe2OTf (5 a) and Fc(3)SiOTf (5 b) (Fc=(eta5-C5H4)Fe(eta5-C5H5)) were synthesized by means of HOTf-induced ring-opening protonolysis of strained sila[1]ferrocenophanes fcSiMe2 (3 a) and fcSiFc2 (3 b) (fc=(eta5-C5H4)2Fe). Reaction of 3 a and 3 b with HBF4 yielded fluorosubstituted ferrocenylsilanes FcSiMe2F (6 a) and Fc3SiF (6 b) and suggested the intermediacy of a highly reactive silylium ion capable of abstracting F- from the [BF4]- ion. Generation of the solvated silylium ions [FcSiMe2THF]+ (7a+), [Fc3SiTHF]+ (7b+) and [FcSiiPr2OEt2]+ (7c+) at low temperatures, by reaction of the corresponding sila[1]ferrocenophanes (3 a, 3 b, and fcSiiPr2 (3 c), respectively) with H(OEt2)(S)TFPB (S=Et2O or THF; TFPB=tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) was monitored by using low-temperature 1H, 13C, and 29Si NMR spectroscopy. In situ reaction of 7a+, 7b+, and 7c+ with excess pyridine generated [FcSiMe2py]+ (8a+), [Fc3Sipy]+ (8b+), and [FcSiiPr2py]+ (8c+), respectively, as observed by 1H, 13C, and 29Si NMR spectroscopy. A preparative-scale reaction of 3 b with H(OEt2)(THF)TFPB at -60 degrees C and subsequent addition of excess pyridine gave isolable red crystals of 8b-[TFPB]CHCl3, which were characterized by 1H and 29Si NMR spectroscopy as well as by single-crystal X-ray diffraction.

Cyclopentadienyl (Cp) ligands in moderately strained [1]- and [2]ferrocenophanes [Fe{(eta5-C5H4)2(ERx)y}: Fe{(eta5-C5H4)2SiMe2} (1), Fe{(eta5-C5H4)CH2}2 (10)] and highly strained [2]ruthenocenophanes [Ru{(eta5-C5H4)CR2}2 {R = H (15), Me (16)}] are susceptible to partial substitution by P donors and form mixed-hapticity metallocycles-[M(L2){(eta5-C5H4)(ERx)y(eta1-C5H4)}]: [Fe(dppe){(eta5-C5H4)SiMe2(eta1-C5H4)}] (5), [Fe(dmpe){(eta5-C5H4)SiMe2(eta1-C5H4)}] (6), [Fe(dmpe){(eta5-C5H4)(CH2)2(eta1-C5H4)}] (11), [Ru(dmpe){(eta5-C5H4)(CH2)2(eta1-C5H4)}] (17), [Ru(dmpe){(eta5-C5H4)(CMe2)2(eta1-C5H4)}] (18), and [Ru(PMe3)2{(eta5-C5H4)(CH2)2(eta1-C5H4)}] (19)-through haptotropic reduction of one eta5-, pi-bound Cp to eta1, sigma-coordination. These reactions are strain-controlled, as highly ring-tilted [2]ruthenocenophanes 15 and 16 [tilt angles (alpha) approximately 29-31 degrees ] react without irradiation to form thermodynamically stable products, while moderately strained [n]ferrocenophanes 1 and 10 (alpha approximately 19-22 degrees ) require photoactivation. The iron-containing photoproducts 5 and 11 are metastable and thermally retroconvert to their strained precursors and free phosphines at 70 degrees C. In contrast, the unprecedented ring-opening polymerization (ROP) of the essentially ring-strain-free adduct 6 to afford poly(ferrocenyldimethylsilane) [Fe(eta5-C5H4)2SiMe2]n (Mw approximately 5000 Da) was initiated by the thermal liberation of small amounts of P donor. Unlike reactions with bidentate analogues, monodentate phosphines promoted photolytic ROP of ferrocenophanes 1 and 10. MALDI-TOF analysis suggested a cyclic structure for the soluble poly(ferrocenyldimethylsilane), 8-cyclic, produced from 1 in this manner. While the polymer likewise produced from 10 was insoluble, the initiation step in the ROP process was modeled by isolation of a tris(phosphine)-substituted ring-opened ferrocenophane [Fe(PMe3)3{(eta5-C5H4)(CH2)2(C5H5)}][OCH2CH3] (13[OCH2CH3]) generated by irradiation of 10 and PMe3 in a protic solvent (EtOH). Studies of the cation 13 revealed that the Fe center reacts with a Cp- anion with loss of the phosphines to form [Fe(eta5-C5H5){(eta5-C5H4)(CH2)2(C5H5)}] (14) under conditions identical to those of the ROP experiments, confirming the likelihood of "back-biting" reactions to yield cyclic structures or macrocondensation to produce longer chains.

Kationische Borverbindungen sind instabile und hoch elektrophile Spezies, die eine Schlüsselrolle in der Chemie des Bors einnehmen. Obwohl man sich schon sehr früh für die Chemie dieser Verbindungen interessierte, blieben die meisten bis in die jüngste Zeit hinein chemische Kuriositäten. Allerdings summierten sich auch allmählich die Hinweise auf ihr Potenzial als starke Elektrophile, und die Entwicklungen im Bereich der schwach koordinierenden Anionen ließen es lohnend erscheinen, dieses Teilgebiet der Borchemie genauer zu erforschen. Seit der letzten umfassenden Übersicht über kationische Borverbindungen sind fast 20 Jahre vergangen; die seit damals erzielten Fortschritte werden in diesem Aufsatz zusammengefasst.

Strained rings containing transition-metal elements are of growing interest as a result of their intriguing structures, enhanced reactivity, and ability to function as precursors to high-molecular-weight metallopolymers through ring-opening polymerization. [1][2][3][4] Ring-strained metallocenophanes and their analogues have attracted particular attention in this regard and their ring-opened polymeric derivatives exhibit a variety of interesting properties. [3, 4] The appreciable ring tilting (a) of the planes of the cyclopentadienyl ligands in ferrocenophanes relative to ferrocene, where the ligands are parallel (a = 08), has been shown to comprise the major contribution to ring strain.[5] Silicon-bridged [1]ferrocenophanes such as A (a % 16-218) have been most well investigated in this area. The moderate ring strain present in A has been shown to lead to enhanced reactivity of the bond between the ipso cyclopentadienyl (Cp) carbon and the bridging silicon atom. Stoichiometric reactions of A with protic regents, for example, lead to facile Si À Cp bond cleavage and to ring-opened species B.[6]In contrast, highly strained, boron-bridged [1]ferrocenophanes (a % 31-328) have been shown to undergo unexpected ring-opening chemistry involving an Fe À Cp bond on reaction with, for example, [Fe 2 (CO) 9 ] to yield bimetallic species C.[7]Very recently, Miyoshi and co-workers have shown that UVphotoirradiation of phosphorus-bridged [1]ferrocenophanes in the presence of a phosphine ligand (e.g., P(OMe) 3 ) leads to a haptotropic shift of a Cp ligand (from h 5 to h 1 ) to yield the ring-slipped product D.[8] Herein, we report some of our initial results from comparative studies of the photolytic reactivity of a range of [1]-and [2]metallocenophanes, which demonstrate that either reversible or irreversible haptotropic rearrangements from h 5 -to h 1 -Cp are observed depending on the degree of strain present.UV-photoirradiation of a slight excess of sila[1]ferrocenophane 1 (a = 20.8(5)8) [9] in the presence of 1,2-bis(diphenylphosphanyl)ethane (dppe) in THF at 5 8C led to a haptotropic shift of a Cp ligand from h 5 to h 1 and coordination of the bidentate phosphine ligand to afford [{Fe(h 5 -C 5 H 4 )SiMe 2 (h 1 -C 5 H 4 )}(dppe)] (2) in 90 % yield [Eq.(1)].[10]The 31 P{ 1 H} NMR spectrum of 2 consisted of slightly broad AB doublets at d = 109.4 and 108.5 ppm [11] with the same coupling constant ( 2 J(P,P) = 27.2 Hz); they were assigned to the diastereotopic phosphine atoms coordinated to the iron center. In the 29 Si{ 1 H} NMR spectrum, a single broad resonance signal appeared at d = À8.72 ppm, which is shifted slightly upfield compared to that of 1 (d = À4.6 ppm). Figure 1 shows the molecular structure of 2 determined by X-ray crystallography.[12]