A T-shaped platinum(II) boryl complex as the precursor to a platinum compound with a base-stabilized borylene ligand

Tuning N-heterocyclic carbenes in T-shaped Pt(II) complexes for intermolecular C-H bond activation of arenes

Small change matters: T-shaped Pt(II) complexes with less flexible substituents, than, for example, isopropyl or tert-butyl groups, on N-heterocyclic carbene (NHC) ligands allow for C-H bond activation reactions of aromatic compounds (see scheme; BAr(f)(4)(-) =tetrakis[(3,5-trifluoromethyl)phenyl]borate; F yellow, Pt red). NHC substituents that are not highly branched prevent agostic interactions and reduce the barriers to achieve the C-H bond cleavage.

Monohaloboryls (BHX-) as bridging ligands: observable dinuclear σ-(halo)boranyl intermediates in the synthesis of metalloborylenes

Upon treating transition-metal-dihaloboryl complexes of the form [L(n)MBX(2)] with K[(η(5)-C(5)H(5))MnH(CO)(2)], salt elimination occurs along with a migration of the Mn-bound hydride ligand onto the boron atom, thereby forming dinuclear σ-(halo)boranyl complexes of the form [L(n)M(μ-BHX)Mn(CO)(2)(η(5)-C(5)H(5))]. Most of these complexes react further at room temperature to lose HX and provide metalloborylene complexes [L(n)M-B = Mn(CO)(2)(η(5)-C(5)H(5))]; however, when ML(n) = Re(CO)(5) the σ-(halo)boranyl complex decomposes into unidentifiable products. We found through DFT calculations that two electronically and structurally distinct forms of the intermediate σ-(halo)boranyl complexes exist, one of which easily loses HX and one that does not.

Noble reaction features of bromoborane in oxidative addition of B-Br σ-bond to [M(PMe3)2] (M=Pt or Pd): theoretical study

Through detailed calculations by density functional theory and second-order Møller-Plesset perturbation theory (MP2) to fourth-order Møller-Plesset perturbation theory including single, double, and quadruple excitations [MP4(SDQ)] methods, we investigated the oxidative addition of the B-Br bond of dibromo(trimethylsiloxy)borane [Br(2)B(OSiMe(3))] to Pt(0) and Pd(0) complexes [M(PMe(3))(2)] (M = Pt or Pd) directly yielding a trans bromoboryl complex trans-[MBr{BBr(OSiMe(3))}(PMe(3))(2)]. Two reaction pathways are found for this reaction: One is a nucleophilic attack pathway which directly leads to the trans product, and the other is a stepwise reaction pathway which occurs through successive cis oxidative addition of the B-Br bond to [M(PMe(3))(2)] and thermal cis-trans isomerization. In the Pt system, the former course occurs with a much smaller energy barrier (E(a) = 5.8 kcal/mol) than the latter one (E(a) = 20.7 kcal/mol), where the DFT-calculated E(a) value is presented hereafter. In the Pd system, only the latter course is found in which the rate-determining steps is the cis-trans isomerization with the E(a) of 15.1 kcal/mol. Interestingly, the thermal cis-trans isomerization occurs on the singlet potential energy surface against our expectation. This unexpected result is understood in terms of the strong donation ability of the boryl group. Detailed analyses of electronic processes in all these reaction steps as well as remarkable characteristic features of [Br(2)B(OSiMe(3))] are also provided.

Transition metal borylene complexes: boron analogues of classical organometallic systems

In the ten years since the first examples of terminal borylene complexes were reported, rapid advances in the chemistry of this ligand class have been made, and parallels emerged with classical organometallic ligand systems (such as carbenes, vinylidenes and even CO) as well as with other subvalent main group ligand systems. This article reviews key developments in synthetic, structural and reaction chemistry, with particular attention focusing on recent discoveries in the field.

T-shaped platinum boryl complexes: synthesis and structure

A series of cationic T-shaped 14-electron boryl complexes of the type trans-[(Cy3P)2Pt(B(X)X')]+ (X=Br; X'=ortho-tolyl, tBu, NMe2, piperidyl, Br; XX'=(NMe2)2, catecholato) were synthesized by halide abstraction from trans-[(Cy3P)2Pt(Br)(B(X)X')] (Cy=cyclohexyl) with Na[BArf 4] (Arf=3,5-(CF3)2C6H3), K[B(C6F5)4], or Na[BPh4]. X-ray diffraction studies were performed on all compounds, revealing a subtle correlation between the trans-influence of the boryl moiety and the Pt-H and Pt-C separations. However, no notable agostic C-H interaction with the platinum center was detected. trans-[(Cy3P)2Pt(BCat)]+ (Cat=catecholato), the complex with the shortest Pt-H and Pt-C distances, was treated with Lewis bases (L), forming compounds of the type trans-[(Cy3P)2Pt(L)(BCat)]+, thus proving a decisive influence of the degree of trans-influence exerted by the boryl ligands on the chemical reactivity of the title complexes. Another point that was investigated and clarified is the different behavior of trans-[(Cy3P)2Pt(Br)(B(Br)Mes)] (Mes=mesityl) towards K[B(C6F5)4] with formation of the borylene species trans-[(Cy3P)2Pt(Br)(BMes)]+.

Boryllithium: A novel boron nucleophile and its application in the synthesis of borylmetal complexes

The first boryl anion, boryllithium, was synthesized by a reduction of bromoborane precursor using lithium naphthalenide in tetrahydrofuran (THF) solvent at -45 °C. Structural and spectroscopic study revealed an ionic character of B-Li bond. Boryllithium could be utilized as a source of boryl ligand in the transition-metal chemistry. Structural and spectroscopic features of the resulting boryl complexes confirmed the large trans influence of boryl ligand.

Experimental studies on the trans-influence of boryl ligands in square-planar platinum(II) complexes

A series of platinum(II) boryl complexes of general formula trans-[(Cy(3)P)2Pt(Br)(BX2)], including the rare dibromoboryl species trans-[(Cy(3)P)2Pt(Br)(BBr2)], were synthesized by oxidative addition of the B-Br bond of a number of bromoboranes to [Pt(PCy3)2]. X-ray diffraction studies were performed on several such compounds. Comparison of the Pt--Br bond lengths allowed an empirical assessment of the trans-influence of different boryl ligands. A trans-influence scale was thus deduced and the results were compared with those previously computed for compounds of the type trans-[(Me(3)P)2Pt(Cl)(BX2)].

[1]Molybdarenophanes: Strained Metallarenophanes with Aluminum, Gallium, and Silicon in Bridging Positions

The first [1]molybdarenophanes were synthesized and structurally characterized. The aluminum and gallium compounds [(Me2Ntsi)Al(eta6-C6H5)2Mo] (2a) and [(Me2Ntsi)Ga(eta6-C6H5)2Mo] (2b) [Me2Ntsi = C(SiMe3)2(SiMe2NMe2)] were obtained from [Mo(LiC6H5)2].TMEDA and (Me2Ntsi)ECl2 [E = Al, Ga] in analytical pure form with isolated yields of 74% (2a) and 52% (2b). The silicon-bridged species [Ph2Si(eta6-C6H5)2Mo] (2c) was synthesized from [Mo(LiC6H5)2].TMEDA and Ph2SiCl2. Compound 2c was isolated as a crystalline material in an approximately 90% overall purity, from which a single crystal was used for X-ray analysis. The molecular structures of all three [1]molybdarenophanes 2a-c were determined by single-crystal X-ray analysis. The ring-tilt angle alpha was found to be 18.28(17), 21.24(10), and 20.23(29) degrees for 2a, 2b, and 2c, respectively. Variable temperature NMR measurements of 2a and 2b (-80 to 80 degrees C; 500 MHz) showed a dynamic behavior of the gallium species 2b but not of compound 2a. The dynamic behavior of 2b was rationalized by assuming that the Ga-N donor bond breaks, inversion at the nitrogen atom occurs, and a rotation of the Me2Ntsi ligand takes place followed by a re-formation of the Ga-N bond on the other side of the gallium atom. The analysis of the signals of meta and ortho protons of 2b gave approximate values of DeltaG not equal of 59.6 and 59.1 kJ mol-1, respectively. Compound 2b reacted with [Pt(PEt3)3] to give the ring-open product [(eta6-C6H6)Mo{eta6-C6H5[GaPh(Me2Ntsi)]}] (3b). The molecular structure of 3b was deduced from a single-crystal X-ray determination. The formation of the unexpected platinum-free product 3b can be rationalized by assuming that benzene reacted with 2b in a 1:1 ratio. Through a series of 1H NMR experiments with 2b it was shown that small amounts of donor molecules (e.g., THF) in benzene are needed to form 3b; in the absence of a donor molecule, 2b is thermally stable.