Reactivity of Boryl Complexes: Synthesis and Structure of New Neutral

Organometallics , 2012, 31 (5), pp 1897–1907. DOI: 10.1021/ ... Publication Date (Web): February 8, 2012. Copyright © 2012 American Chemical Societ...
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Reactivity of Boryl Complexes: Synthesis and Structure of New Neutral and Cationic Platinum Boryls and Borylenes Nicole Arnold, Holger Braunschweig,* Peter Brenner, J. Oscar C. Jimenez-Halla, Thomas Kupfer, and Krzysztof Radacki Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany S Supporting Information *

ABSTRACT: A reactivity study on a series of platinum boryl complexes was performed. The first stable base adducts of cationic haloboryl complexes of the form trans-[Pt{B(Br)(NMe2)}(NCMe)(PCy3)2]+ were isolated and fully characterized. The dianion [B12Cl12]2− was introduced as a weakly coordinating anion to complex chemistry forming a A2X salt. Through the reaction of trans-[Pt{B(Br)(tBu)}Br(PCy3)2] with BBr2tBu, the first highly soluble dinuclear platinum boryl complex, [Pt{B(Br)(tBu)}(μ-Br)(PCy3)]2, could be synthesized with concomitant buildup of the corresponding phosphine-borane adduct. In contrast to this observation, reaction of trans-[Pt{B(Br)(Mes)}Br(PCy3)2] with BBr3 leads to the formation of the cationic borylene complex trans[Pt(BMes)Br(PCy3)2]+ by abstraction of the bromo ligand bound mutually trans to the boryl ligand in the precursor and concomitant buildup of [BBr4]−. Reaction of [Pt(PCy3)2] with BCl3 and subsequent abstraction of the platinum-bound chloro ligand enabled the structural characterization of trans-[Pt(BCl2)(PCy3)2]+, which is isoelectronic with the metal-only Lewis pair trans-[Pt(BeCl2)(PCy3)2]. The bonding situation in both systems was investigated in detail using quantum chemical calculations. A T-shaped cationic complex, trans-[Pt{B(Br)(Fc)}(PiPr3)2]+, and its precursor trans-[Pt{B(Br)(Fc)}Br(PiPr3)2], both with reduced steric bulk at the phosphine ligands compared with their PCy3 derivatives, were fully characterized.

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INTRODUCTION Ever since the Nobel Prize in Chemistry in 1976 for Lipscomb’s “studies on the structure of boranes illuminating problems of chemical bonding” and three years later for H. C. Brown for “the development of boron-containing compounds”, the chemistry of boron has been a matter of particular interest in both modern organic and organometallic chemical research.1 Recently this has been reaffirmed by the award of the 2010 Nobel Prize in Chemistry partly for the development of palladium-catalyzed cross-coupling reactions with unsaturated boranes, now known as the Suzuki−Miyaura protocol.2 For a chemist new to the field, even a perfunctory glance over the organometallic literature of the last 20 years will betray an immense research interest in the isolation of postulated intermediates in functionalization reactions of organic substrates via late transition metals. In particular, boryl complexes3 have been investigated in great detail, as these act as intermediates in hydroboration,4 diboration,5 and C−H activation reactions.6 Platinum-centered complexes are of particular interest, as these play a role as key intermediates especially in the former functionalization reactions. In our laboratories recent studies on platinum−boron chemistry have enabled the isolation and structural characterization of a number of new bonding motifs. Through the oxidative addition of B−Hal bonds (Hal = F, Cl, Br, I) to [Pt(PCy3)2] (1) (Cy = © 2012 American Chemical Society

C6H11), we were able to isolate a number of complexes with the general formula trans-[Pt(BRR′)Hal(PCy3)2] with several halide and R, R′ combinations.7 X-ray crystal structure analysis of the bromide systems and comparison of Pt−Br and Pt−B separations were used to measure the boryl ligands’ transinfluence, confirming theoretical predictions that postulated that electron donor substituents at the boron atom intensify the ligand’s trans-influence.7c,8 The low-coordinate Pt(0) complex 1 plays a particularly important role as a substrate therein, as this fragment has enabled the stabilization of unprecedented boron-centered ligand systems, e.g., cationic T-shaped boryls,8c,9 cationic and heterodinuclear borylenes,10 neutral and cationic base-stabilized borylenes,8a,b,11 iminoboryls,12 oxoboryls,13 σ-borolyls,14 diboran(4)yls,15 and very recently also (alkyl)(boryls).16 It should be mentioned also that E−Hal (E = Ga, Hal = Br, I; E = Bi, Hal = Cl) 17 bonds oxidatively add to the aforementioned platinum bis(phosphine) system, whereas Lewis-acidic substrates such as BeCl2 (s-block),18 AlHal3 (pblock),19 and ZrCl4 (d-block)20 form T-shaped metal-only Lewis pairs of the form trans-[Pt(EHaln)(PCy3)2]. Received: December 9, 2011 Published: February 8, 2012 1897

dx.doi.org/10.1021/om2012248 | Organometallics 2012, 31, 1897−1907

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Scheme 1. Reaction of trans-[Pt{B(Br)(NMe2)}(PCy3)2][BArf4] (2) in Dichloromethane: (a) MeCN Attacks the Platinum Center; (b) 4-Picoline Results in the Formation of a Base-Stabilized Borylene

the case of related T-shaped cationic complexes in which at least one halide is bound to the boron atom, the base 4-picoline instead attacks the boron atom. This results in a formal halide shift from the boron to the platinum center and the formation of a so-called “base-stabilized borylene” of the general formula trans-[Pt{BR(4-Me-C5H4N)}Br(PCy3)2]+, typically displaying a 31P NMR signal 20 ppm upfield of that of their cationic starting materials, and a concomitant decrease in the 31P−195Pt coupling constant of about 500 Hz.8c,11b,23 In contrast, addition of MeCN to T-shaped cationic platinum haloboryls usually leads to decomposition, as reported for trans-[Pt{B(Br)(Fc)}(PCy3)2]+.8c (i). Adduct Formation of a T-Shaped Cationic Haloboryl Complex. Contrasting these findings, we report here the equimolar reaction of acetonitrile with the T-shaped cationic amino(halo)boryl platinum complex trans-[Pt{B(Br)(NMe2)}(PCy3)2][BArf4] (2) (Arf = 3,5-C6H3(CF3)2). Addition of dissolved MeCN in CD2Cl2 to a yellowish solution of 2 led to the formation of a colorless solution. Monitoring the reaction via multinuclear NMR spectroscopy revealed a new 31 P NMR signal at 32.9 ppm with platinum satellites (1JP−Pt = 2786 Hz). In the case of this reaction, the new resonance showed a shift of 10 ppm to higher field, but only a small (ca. 20 Hz) decrease in the 31P−195Pt coupling constant compared to the starting material (δ = 42.8, 1JP−Pt = 2803 Hz). This can be attributed to the fact that the smaller base MeCN does not attack the boron atom but occupies the vacant coordination site at the platinum center due to steric reasons. While no 11B NMR signal could be detected for the T-shaped cationic precursor,9 the spectrum of the new compound exhibits a very weak signal at 25 ppm, which is comparable to that of trans-[Pt{B(Br)(NMe2)}Br(PCy3)2] (30 ppm).7c The 1H NMR signal of the methyl group could be detected at 2.17 ppm (trans-[Pt(BO)(NCMe)(PCy3)2][BArf4] (4) = 2.46 ppm) and the 13C{1H} signal at 2.9 ppm (4 = 4.0 ppm). Layering the reaction mixture with hexane and cooling to −35 °C led to the formation of colorless single crystals of trans[Pt{B(Br)(NMe2)}(NCMe)(PCy3)2][BArf4] (3) suitable for X-ray diffraction analysis (for crystallographic details see Supporting Information). The compound crystallizes in the triclinic space group P1̅. The molecular structure (Figure 1) confirms the formation of a square-planar cationic platinum complex, in which MeCN is bound to the platinum center. The B−Pt−N1 and the P1−Pt−P2 angles of 173.7(2)° and 170.5(1)°, respectively, lie in the typical range of slightly distorted square-planar Pt complexes. Compared to its precursor, the Pt−B bond is slightly elongated (201.7(5) pm in 3 and 198.7(4) pm in 2)9 and comparable to the isoelectronic, neutral parent compound trans-[Pt{B(Br)(NMe2)}Br(PCy3)2] (Pt−B = 200.9(3) pm).7c The Pt−N1 distance (217.7(4) pm) is distinctly longer than that of trans-[Pt(BO)(NCMe)(PCy3)2][BArf4] (4) (209.4(3) pm),13c indicative of the stronger trans-influence of the more

In this work, we present several examples that show a more general applicability of our results concerning the choice of the phosphine substituents that stabilize unsaturated cationic Tshaped platinum boryl complexes, increasing the usability and practicality of anions and the ease of abstraction of the bromo ligand situated trans to the boryl ligand. We present here (i) the first base adduct of a haloboryl complex, in which a Lewis base (here, MeCN) does not attack the Lewis-acidic boron center and (ii) the first organometallic complex salt composed of two identical cationic complex fragments with one dianionic dodecaborate [B 12 Cl 12 ] 2− counterion. We show different reactivities of bis(phosphine) platinum boryl complexes toward Lewis-acidic boranes that form (iii) a new dinuclear platinum boryl complex through the abstraction of a phosphine ligand and (iv) the synthesis of the cationic platinum borylene trans-[Pt(BMes)Br(PCy3)2]+ obtained through the abstraction of a bromide ligand using a borane as the abstracting agent. In part (v) we report the synthesis of the T-shaped cationic complex trans-[Pt(BCl2)(PCy3)2]+, which is isoelectronic to trans-[Pt(BeCl2)(PCy3)2]. The bonding situation in both systems was analyzed using quantum chemical methods, and the results are discussed. Finally in (vi) we account for the isopropylphosphine derivative of the first reported T-shaped cationic boryl complex, trans[Pt{B(Br)(Fc)}(PCy3)2]+.

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RESULTS AND DISCUSSION T-shaped cationic complexes of the general formula trans[Pt(L)(PCy3)2]+ (L = BR2) are extraordinarily stable in comparison to those in which the ligand L is not a boroncentered ligand.8c,9 Due to the highly unsaturated, Lewis-acidic nature of the metal center in cationic 14-electron d8 [ML3] complexes, the reactivity of these species toward even weakly coordinating nucleophiles is greatly enhanced, leading to interactions with solvent molecules, weakly coordinating anions,21 or agostic C−H bonds.22 In the Pt boryl complexes investigated in our laboratories, trans-[Pt(BRR′)X(PCy3)2] (R = e.g., NMe2, tBu, Me, Fc; RR′ = Cat′, X = Br; R = R′ = X = I, Br; R = R′ = F, X = Br, Cl, BF4), the boryl ligand’s transinfluence generally prevents such interactions with the vacant site, an effect that can be moderated by altering the electronreleasing ability of the boryl substituents. This phenomenon validates the well-established exceptions in which the boron atom bears electronegative substituents, namely, trans-[Pt(BCat)(PCy3)2][BArf4] (Cat = catecholato, C6H4O2) and the cationic oxoboryl complex trans-[Pt(BO)(PCy3)2][pftb] (pftb = OC(CF3)3). The former complex can alternatively form an adduct with 4-picoline (= 4-Me-C5H4N) or interact with solvent molecules (CD2Cl2) (as seen by low-temperature NMR experiments).9 The latter complex forms an adduct with acetonitrile (MeCN) upon halide abstraction, as an alternative to the formation of a BO-cyclodimerization product in the absence of donor solvents.13a It should be mentioned that, in 1898

dx.doi.org/10.1021/om2012248 | Organometallics 2012, 31, 1897−1907

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Scheme 2. Synthesis of Derivatives of 3 Using Alternative Counterionsa

Figure 1. Molecular structure of trans-[Pt{B(Br)(NMe2)}(NCMe)(PCy3)2][BArf4] (3) in the crystal. The thermal ellipsoids of the phosphine substituents, counterion ([BArf4]), cocrystallized dichloromethane, and hydrogen atoms were omitted for clarity. Thermal ellipsoids are displayed at the 50% probability level.

electron-releasing boryl moiety {B(Br)(NMe2)}. Still, the Pt− N1 distance in trans-[PtH(NCMe)(PCy3)2][BPh4] (5) is shorter (Pt−N 207.9(6) pm),24 indicating the strong transinfluence of the boryl moiety. The N1−C1 triple bond (114.1(5) pm) in contrast does not show a distinct elongation compared to 4 (112.9(5) pm) and 5 (112(1) pm). (ii). Synthesis of Platinum Boryl Complexes by Variation of the Weakly Coordinating Anion. Attempts to vary the counterion using several main-group salts such as Na2SO4 or Na2SiF6 in the T-shaped cationic systems were unsuccessful and led to either no reaction or decomposition. Exclusively weakly coordinating anions, i.e., sodium or silver salts of tetraarylborates (e.g., [BArf4]−, [B(C6F5)4]−, [BPh4]−), hexabromocarbaborate (e.g., [CB11Br6H6]−), or tetraalkoxides of aluminum ([Al(pftb)4]−; pftb = OC(CF3)3) were suitable alternatives to the routinely employed Na[BArf4] (Arf = 3,5C6H3(CF3)2) for the halide abstraction. In a typical reaction, addition of equimolar amounts of the salt of the weakly coordinating anion to trans-[Pt{B(Br)(NMe2)}Br(PCy3)2] readily provided the corresponding T-shaped platinum boryl complexes. It should be mentioned that the closo-dodecaperchloroborate salt Na2[B12Cl12],25 which shows very low solubility in polar organic solvents such as CH2Cl2 or C6H5F, could also be successfully used as a suitable abstracting reagent after ultrasonic trituration of a 2:1 reaction mixture in these solvents for two hours. The resulting A2X salt shows very good solubility and yielded amorphous solids after filtering off the sedimented NaBr salt and evaporation of the solvent. In order to enhance the crystallization properties of the synthesized compounds, we attempted to saturate the 14electron complex trans-[Pt{B(Br)(NMe2)}(PCy3)2]2[B12Cl12] (6) both electronically and coordinatively through the addition of an excess of MeCN to a light yellow solution of the compound. Decoloration of the solution, monitoring of the reaction via multinuclear spectroscopy (δ 31P{1H} 6 = 28.8, 1 JP−Pt = 2778 Hz; δ 11B{1H} = 24.4), and comparison of the spectroscopic data with those of trans-[Pt{B(Br)(NMe2)}(NCMe)(PCy3)2][BArf4] (3) indicated the formation of similar platinum-phosphine environments. Layering the reaction mixture with hexanes and slow evaporation of the solvents yielded colorless crystals suitable for X-ray diffraction (Figure 2). The compound crystallizes in the monoclinic space group P21/n, and the observed bond distances are unobtrusive and very

a

WCA = weakly coordinating anion.

Figure 2. Molecular structure of trans-[Pt{B(Br)(NMe2)}(NCMe)(PCy3)2]2[B12Cl12] (6) in the crystal. The cations of the A2X salt are crystallographically identical. The thermal ellipsoids of the phosphine substituents, cocrystallized dichloromethane, toluene, and hydrogen atoms were omitted for clarity. Thermal ellipsoids are displayed at the 50% probability level.

similar to those of 3 and need no further discussion (Table 1), except that the MeCN ligand is no longer linearly bound to the Table 1. Bond Distances (pm) and Angles (deg) in trans-[Pt{B(Br)(NMe2)}(NCMe)(PCy3)2][BArf4] (3) and trans-[Pt{B(Br)(NMe2)}(NCMe)(PCy3)2]2[B12Cl12] (6) N1−C1 Pt−B Pt−N1 average Pt−P P1−Pt−P2 B−Pt−N1 Pt−N1−C1 N1−C1−C2 1899

3

6

114.1(5) 201.7(5) 217.7(4) 233.5 170.5(1) 173.7(2) 170.9(4) 179.1(5)

114.5(5) 200.5(4) 216.8(3) 234.3 168.0(1) 164.5(1) 149.5(3) 175.1(4)

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and crystallization from toluene led to the formation of colorless crystals of 8, which were found to be suitable for X-ray diffraction (Figure 3).

platinum (Pt−N1−C1 = 149.5(3)°). This might be explained by packing effects. However, the geometrical arrangement of the counterion in the molecular structure shows no deviation from perfect icosahedral symmetry, and therefore only a very weak interaction between the two ions is suggested. The weak interaction is also accompanied in solution by slight differences in the positions of the 13C{1H} and 1H NMR signals (δ 13 C{1H} = 4.2; δ 1H = 2.46) compared with 3 (δ 13C{1H} = 2.9; δ 1H = 2.17)., Reactivity of Platinum Boryl Complexes toward LewisAcidic Boranes. The platinum boryls investigated in our laboratories display a specific reactivity toward Lewis-acidic boranes. As we reported earlier, reaction of trans-[Pt(BI2)I(PCy3)2] with BI3 or trans-[Pt{B(Br)(Fc)}Br(PCy3)2] with BBr2Fc (Fc = ferrocenyl) led to the abstraction of one phosphine ligand with concomitant buildup of the corresponding phosphine-borane adduct and formation of halide-bridged dinuclear complexes [Pt{B(X)(R)}(μ-X)(PCy3)]2 (X = Br, I).7e,11a (iii). Phosphine Abstraction from a Platinum Bis(phosphine) Boryl Complex via a Lewis-Acidic Borane. In a similar reaction of [Pt(PCy3)2] with two equivalents of BBr2tBu in benzene, the formation of the known complex trans[Pt{B(Br)(tBu)}Br(PCy3)2] (7), characterized by its characteristic 31P{1H} NMR shift (23.7 ppm, 1JP−Pt = 2962 Hz),7c the phosphine-borane adduct Cy3P−BBr2tBu (δ 31P{1H} = 0.0, δ 11 B = 3.1), and a new platinum-containing compound (δ 31 1 P{ H} = 28.2, 1JP−Pt = 4740 Hz) was observed, of which the latter data were assigned to the dinuclear complex [Pt{B(Br)(tBu)}(μ-Br)(PCy3)]2 (8) (cf. [Pt{B(Br)(Fc)}(μ-Br)(PCy3)]2 (9), δ = 23.6, 1JP−Pt = 4739 Hz).11a

Figure 3. Molecular structure of [Pt{B(Br)(tBu)}(μ-Br)(PCy3)]2 (8) in the crystal. Cocrystallized toluene, hydrogen atoms, and thermal ellipsoids of the phosphine substituents were omitted for clarity. Thermal ellipsoids are displayed at the 50% probability level.

The molecular structure of 8 displays a virtually centrosymmetric dinuclear moiety in which two platinum centers are bridged by two bromo ligands. Only one platinum environment will be discussed here. The metal center shows a slightly distorted square-planar coordination sphere (P1−Pt1−Br1′ = 171.7(1)°; B1−Pt1−Br1 = 164.3(1)°), and the Pt2Br2 fragment adopts the geometric arrangement of a slight butterfly (torsion angle Br1−Pt−Br1′−Pt′ = 17.0(1)°). The Pt−Br1 bond trans to the boryl moiety (269.2(1) pm) is about 1% longer than that in 9 (267.9(6) pm) and 8% longer than the Pt−Br1′ bond trans to the phosphine (249.4(1) pm), indicative of the strong transinfluence of the boryl ligand. The {BBrtBu} group, with its strong σ-donating substituent, was found to exert the strongest trans-influence of all boryl groups examined since 2007.7c The Pt−B distance (197.3(4) pm) is in the expected range for these compounds (Table 2).

Scheme 3. Synthesis of [Pt{B(Br)(tBu)}(μ-Br)(PCy3)]2 (8) via Phosphine Abstraction

Table 2. Bond Distances (pm) and Angles (deg) in [Pt{B(Br)(tBu)}(μ-Br)(PCy3)]2 (8) and [Pt{B(Br)(Fc)}(μ-Br)(PCy3)]2 (9) Pt−Br1 Pt−Br1′ Pt−B Pt−P P−Pt−Br1′ B−Pt−Br1 Br1−Pt−Br1′−Pt′

8

9

269.2(1) 249.4(1) 197.3(4) 225.0(1) 171.7(1) 164.3(1) 17.0(1)

267.9(1) 249.2(1) 198.3(1) 224.2(1) 170.1(1) 167.3(1) 34.1(1)

It should be emphasized that in contrast to known monophosphine platinum boryl complexes,7e,11a which show poor solubility even in polar solvents such as dichloromethane or fluorobenzene, the new compound 8 shows very good solubility even in hexanes, a feature that might be useful for

The mixture was heated to 50 °C for five hours, after which the consumption of 7 was complete. Evaporation of the solvent 1900

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investigating the subsequent reactivity of this class of compounds. (iv). Formation of a Cationic Platinum Borylene Complex by Halide Abstraction via a Strongly LewisAcidic Borane. Contrasting these observations, the addition of an excess of BBr2Mes to a solution of trans-[Pt{B(Br)(Mes)}Br(PCy3)2] (10) did not lead to a reaction, perhaps due to the steric hindrance of the mesityl substituent at the boron and the reduced Lewis acidity of the parent borane, due to the π−p overlap from the aromatic substituent to the vacant orbital of boron. However, addition of the more Lewis acidic BBr3 to a solution of 10 in benzene led to the precipitation of a colorless solid. Whereas the investigation of the solution via multinuclear NMR spectroscopy did not give information about the reaction products, dissolution of the solid in dichloromethane enabled the detection of new signals. A high-field-shifted (with respect to its precursor 10) singlet at −24.5 ppm could be detected in the 11B NMR spectrum, which suggests the formation of tetrabromoborate.26 In analogy, the formation of the unusual dinuclear cationic complex [Pt2(BCl2)2(PMe3)4(μ-Cl)][BCl4] linked by a single bridging chloro ligand was observed by the formal reaction of two equivalents of trans-[Pt(BCl2)Cl(PMe 3 ) 2 ] with one equivalent of BCl 3 . 27 A doublet corresponding to a phosphine-borane adduct could not be detected. The 31P{1H} NMR spectrum shows a singlet at 45 ppm with platinum satellites (1JP−Pt = 2073 Hz). Comparison with the 31P{1H} NMR signal of trans-[Pt(BMes)Br(PCy3)2][B(C6F5)4]10b (11) suggested the formation of the cationic platinum borylene complex fragment with concomitant buildup of [BBr4]−.

Figure 4. Molecular structure of trans-[Pt(BMes)Br(PCy3)2][BBr4] (12) in the crystal. Cocrystallized dichloromethane, hydrogen atoms, and thermal ellipsoids of the phosphine substituents were omitted for clarity. Thermal ellipsoids are displayed at the 50% probability level.

Table 3. Bond Distances (pm) and Angles (deg) in trans[Pt(BMes)Br(PCy3)2][B(C6F5)4] (11) and trans[Pt(BMes)Br(PCy3)2][BBr4] (12) Pt−Br1 Pt−B average Pt−P P−Pt−P B−Pt−Br

Scheme 4. Reactivity of 10 toward Lewis-Acidic Boranes

11

12

254.1(1) 185.9(3) 236.0 171.7(1) 178.1(1)

251.9(1) 186.3(4) 236.5 168.7(1) 167.3(1)

equimolar reaction of BCl3 with 1 we could isolate trans[Pt(BCl2)Cl(PCy3)2] (15) as a colorless, moderately air- and moisture-stable solid, which appears to be a suitable precursor for the formation of 13. The 31P{1H} NMR spectrum shows a singlet at 24.2 ppm with 195Pt satellites (1JP−Pt = 2675 Hz) that lie in the range of other derivatives of this type, e.g., trans[Pt{B(Br)(NMe2)}Br(PCy3)2] (26.8 ppm, 1JP−Pt = 2845 Hz).7d Due to the unresolved coupling to the 31P and 195Pt nuclei, the 11 B NMR shows a very broad singlet at 59.4 ppm (ω1/2 ≈ 1500 Hz). The isolation of 15 completes the series of dihaloboryl complexes of the general formula trans-[Pt(BHal2)X(PCy3)2] (Hal = X = I, Br, Cl; Hal = F, X = Br, Cl, BF4).7a,c,e X-ray diffraction experiments performed on suitable single crystals (Figure 5) show a platinum center in a slightly distorted squareplanar coordination sphere (P−Pt−P = 174.11(3)° and B−Pt− Cl = 178.7(1)°). The Pt−Cl bond (244.1(2) pm) is about 10% longer than those in trans-[PtCl2(PCy3)2] (231.7(2) pm),29 but lies in the range of other chloroboryl complexes of the general formula trans-[Pt(BCl 2 )Cl(PR 3 ) 2 ] (R 3 = Ph 3 , MePh 2 , Me2Ph),27 which can be assigned to the strong trans-influence of the boryl moiety. Addition of equimolar amounts of Na[BArf4] to a colorless solution of 15 in CD2Cl2 led to the formation of a light yellow solution and the precipitation of a colorless solid. NMR experiments performed on the reaction mixture revealed a

X-ray diffraction experiments confirm the formation of the suggested complex salt trans-[Pt(BMes)Br(PCy3)2][BBr4] (12) (Figure 4). The molecular structure of 12 reveals a completely separated pair of ions. The cation is essentially identical to that of the related borylene 11 and needs no further discussion (Table 3). (v). Synthesis of trans-[Pt(BCl2)(PCy3)2]+: A Cationic Boryl Complex Isoelectronic to the Metal-Only Lewis Pair trans-[Pt(BeCl2)(PCy3)2]. Due to the fact that the late transition metal base [Pt(PCy3)2] (1) forms dative bonds to several s-, p-, and d-block elements,18−20,28 we were especially interested in the structural characterization of the cationic system trans-[Pt(BCl2)(PCy3)2]+ (13), which is isoelectronic to the metal-only Lewis pair trans-[Pt(BeCl2)(PCy3)2)]18 (14). By 1901

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shorter than in the precursor 15, which lies in the range of other cationic platinum boryl complexes in which one bromo ligand has been abstracted.8c,9,14 The solid-state molecular structures of the isoelectronic complexes 13 and 14 appear very similar, although the bond distances around the metal center and the element atom differ (Table 4). This is most likely due Table 4. Bond Distances (pm) and Angles (deg) in trans[Pt(BCl2)(PCy3)2]+ (13) and trans-[Pt(BeCl2)(PCy3)2] (14) Pt−E average Pt−P average E−Cl closest Pt−C P−Pt−P Pt−P−C

Figure 5. Molecular structure of trans-[Pt(BCl2)Cl(PCy3)2] (15) in the solid state. Hydrogen atoms and thermal ellipsoids of the phosphine substituents were omitted for clarity. Thermal ellipsoids are displayed at the 50% probability level.

E = B (13)

E = Be (14)

194.2(3) 232.5 175.3 302.4(2) 170.1(1) 102.1(1)

216.8(4) 228.7 192.2 324.2(3) 172.6(1) 107.6(1)

to the different nature of the bonding situations prevailing in these systems. Frenking et al.28 showed via DFT calculations that the metal fragment in 14 acts as both a σ and a π donor, while it was demonstrated that in transition metal boryl complexes the boryl group acts as a σ donor,7b,8a−c and the π back-donation into the vacant orbital at the boron atom is weak to negligible.8d−r Also the bond distances of neutral and cationic compounds cannot be compared directly because of the ionic character of 13 and crystal-packing effects. In order to gain deeper insights into the nature of Pt−B and Pt−Be bonds in the isoelectronic systems, we performed quantum chemical calculations at the ZORA BP86/TZ2P level. By structural optimization of both complexes restricted with a Cs symmetry plane (the maximum symmetry allowed within the particular substitution pattern in these species), we were able to separate σ and π orbital contributions by an energy decomposition analysis (EDA, Table 5), which is discussed

31

P{1H} NMR signal at 43.5 ppm (1JP−Pt = 2657 Hz) that is low field shifted with respect to that of the precursor. Scheme 5. Synthesis of trans-[Pt(BCl2)Cl(PCy3)2] (15) and trans-[Pt(BCl2)(PCy3)2]+ (13)

Filtering off the precipitate and slow evaporation of the solvent enabled the isolation of yellow, rhombic single crystals. The molecular structure of [13][BArf4] (Figure 6) shows a T-shaped platinum center, with the phosphine ligands situated

Table 5. EDA Results for the Bonding Analysis in Both Species Calculated at the ZORA BP86/TZ2P Levela symmetry ΔEint ΔEPauli ΔVelstatb ΔEoib ΔEσc ΔEπc ΔEprep ΔE (= −De) r(Pt−E)d

13

14

Cs −153.73 155.24 −118.69 (38.4%) −190.27 (61.6%) −160.67 (84.4%) −29.61 (15.6%) 58.60 −95.13 196.2

Cs −40.63 80.31 −66.75 (55.2%) −54.18 (44.8%) −44.42 (82.0%) −9.76 (18.0%) 21.64 −18.99 220.7

Energy values are in kcal mol−1. bThe percentage values in parentheses give the contribution to the total attractive interactions ΔEelstat + ΔEoi. cThe percentage values in parentheses give the contribution to the total orbital interactions, ΔEoi. dCalculated length of the analyzed bond in pm (E = B, Be). a

Figure 6. Molecular structure of trans-[Pt(BCl2)(PCy3)2]+ (13) in the solid state. The counterion ([BArf4]), cocrystallized dichloromethane, hydrogen atoms, and the thermal ellipsoids of the phosphine substituents were omitted for clarity. Thermal ellipsoids are displayed at the 50% probability level.

below (see also the Computational Methodology section in the Supporting Information). However, it should be indicated that the true minima are 12.0 kcal/mol (13) and 12.2 kcal/mol (14) lower in energy and possess C1 symmetry. In all cases, the optimized molecular structures are in good agreement with experimental ones (Table 5). The values obtained for trans[Pt(BeCl2)(PCy3)2] are very close to those reported by

mutually trans. This discrete mononuclear arrangement strongly contrasts the aforementioned dinuclear cationic complex trans-[Pt 2 (BCl 2 ) 2 (PMe 3 ) 4 (μ-Cl)][BCl 4 ]. 27 The {BCl2} fragment is staggered by 72.5(2)° with respect to the P−Pt−P axis, and the Pt−B bond (194.2(3) pm) is about 1.5% 1902

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Frenking et al., who previously performed an analysis on a simplified model.28 According to our calculations, the Pt−B bond dissociation energy of 13 is five times that of the electron-neutral trans[Pt(BeCl2)(PCy3)2]. The different strength of the stabilization of the Pt−B bond in 13 is substantiated by orbital interactions (ΔEoi), which show a value of 190.3 kcal per mole, representing nearly 62% of the total attractive interactions in the system. In contrast, the Pt−Be interaction has a much smaller value of 54.2 kcal per mole, and this represents only 45% of the total stabilizing interactions in 14. Moreover, σ interactions in trans[Pt(BCl2)(PCy3)2]+ contribute more to bond stabilization (84%) than in trans-[Pt(BeCl2)(PCy3 )2] (82%). Steric repulsions display the same scaled trend in both complexes. Further (de)stabilization results from the deformation energies (ΔEprep), whereas the {Pt(PCy3)2} fragment in 13 is more distorted than that in 14. Furthermore, we computed the degree of donation and backdonation in these compounds as well as atomic charges derived from our analysis (Table 6). As expected, a large electron

molecular plane for maximizing the number of these interactions. Moreover, we located a curved bond path connecting the Pt center and one proximal hydrogen in both complexes. More specifically, there are two interactions between the metal center and the phosphine substituents: one between the dxz orbital of Pt and the π* orbital of one vicinal CH group and one between the σCH* orbital and the dx2−y2 orbital of Pt. As mentioned above, this agostic interaction was not observed experimentally. (vi). Stabilization of a T-Shaped Cationic Complex with Reduced Steric Bulk at the Phosphine Substituents. The platinum boryl systems investigated in our laboratories comprise the sterically extremely demanding PCy3 ligand, which kinetically stabilizes the boryl complexes. In contrast to d8 ML3 systems in which agostic interactions were found to activate ligand C−H bonds (e.g., [PtMe(PiPr3)2][1-H-closo-CB11Me11]: Pt−C = 285.9 pm),22g such an interaction could be excluded in [Pt{B(Br)(Fc)}(PCy3)2][BArf4] (16) via the structural characterization, low-temperature NMR experiments, and ELF calculations.8c In order to investigate whether trialkylphosphines other than PCy3 have the potential to stabilize T-shaped cationic boryl complexes, we attempted to prepare the PiPr3 derivative of trans-[Pt{B(Br)(Fc)}(PCy3)2][BArf4] (16). The compound trans-[Pt{B(Br)(Fc)}Br(PiPr3)2] (17), which is the iPr3P derivative of trans[Pt{B(Br)(Fc)}Br(PCy3)2] (18),7b appeared to be a suitable precursor for trans-[Pt{B(Br)(Fc)}(PiPr3)2][BArf4] (19). Because the tris(phosphine) precursor [Pt(PiPr3)3] 20 is known to dissociate to form a bis(phosphine) complex and one equivalent of free PiPr3 when dissolved in organic solvents,30 the formation of phosphine-borane Lewis adducts in the course of the reaction was assumed. For this reason two equivalents of FcBBr2 were added to a yellowish solution of 20 in C6D6, which led to the formation of a light red solution and concomitant precipitation of a red solid. Monitoring the reaction via multinuclear NMR spectroscopy revealed a new signal in the 31P{1H} NMR spectrum at 30.7 ppm with 195Pt satellites (1JP−Pt = 2936 Hz), which is shifted to high field with respect to its precursor [Pt(PiPr3)2] (21) (δ 31P = 72.8, 1JP−Pt = 4202 Hz) and a broad singlet at −1 ppm, which was assigned to the phosphine-borane adduct iPr3P−B(Fc)Br2.30 A broad singlet was detected in the 11B NMR spectrum at 72.9 ppm (w1/2 ≈ 1680 Hz), which is in good agreement with that of 18 (δ 11B = 82, ω1/2 ≈ 1980 Hz) and a broad singlet at −4.4 ppm, which was assigned to the phosphine-borane adduct. Filtering off the precipitate, evaporation of the solvent, and recrystallization from toluene led to the formation of single crystals suitable for X-ray diffraction. Using freshly prepared 21 from trans-[PtCl2(PiPr3)2] and Na[C10H8] in tetrahydrofuran, in situ reaction with BBr2Fc improved the yield of 17 to 71%. The molecular structure of 17 (Figure 7) shows a platinum center in a slightly distorted square-planar coordination geometry. The Pt−B bond (197.4(3) pm) is about 1% shorter than that in 18 (199.6(4) pm), which is most likely due to the difference in the Tolman cone angle (PiPr3 = 160 ± 10°; PCy3 = 179 ± 10°)31 and the decreased steric demand at the periphery of the PiPr3 ligand compared to PCy3, thus relieving the interaction of the ligands with the Fc group.31 The other bond lengths and angles are unremarkable and very similar to those in 18 and require no further discussion (Table 7). Addition of Na[BArf4] to a solution of 17 in CD2Cl2 intensified the red color of the solution and furthermore led

Table 6. Results of Structural Analyses in trans[Pt(ECl2)(PCy3)2]n (13, E = B, n = +; 14, E = Be, n = 0) Calculated at the ZORA BP86/TZ2P Level 13

14

donation (d) 1.12 0.41 back-donation (b) 0.30 0.18 ratio d/b 3.73 2.28 VDD atomic charges (in au) and dipole moment (in D) Q(Pt) −0.086 −0.281 Q(E) 0.009 0.043 Q(P) 0.127 0.100 Q(Cl) −0.014 −0.220 μ 2.024 6.069

donation from the metal center toward boron can be observed. As a result, the platinum fragment in the cationic compound 13 displays an electron donation nearly 1.5 times that of the platinum fragment to the BeCl2 fragment in 14. The ratios of donation/back-donation in the particular systems suggest that this effect is more important for the stabilization in 13 rather than in 14, which is in good agreement with the aforementioned EDA results. Voronoi-like atomic charges show that the Pt center in the cationic boron complex displays only a small negative value and the B and Cl atoms are nearly electron neutral. Compared to these values, larger negative values for both chlorine and platinum atoms and a larger positive value for the Be atom in 14 are calculated. Also, the P atoms in 13 show positive values larger than those in 14 due to the strong donation from both phosphines to the metal center. This indicates a compensation of the global positive charge in 13 by electron density flows from Pt and Cl atoms to the boron atom (increasing donation from phosphorus to Pt as well) and back-donation from the boron to the Pt center, which strengthens the Pt−B bond in this complex. The dipole moment value in 14 is three times that of complex 13, which is in line with the results of the EDA. Analysis of the topology of the electron density from the fully relaxed geometry optimizations results in expected bond and ring critical points along with bond paths between chlorines and hydrogens of cyclohexyl groups (Figure S1 in Supporting Information). Therefore, {ECl2} fragments deviate from the Cs 1903

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Figure 7. Molecular structure of trans-[Pt{B(Br)(Fc)}Br(PiPr3)2] (17) in the crystal. Cocrystallized toluene, disorder of one phosphine group, and thermal ellipsoids of the phosphine substituents were omitted for clarity. Thermal ellipsoids are displayed at the 50% probability level.

Figure 8. Solid-state molecular structure of trans-[Pt{B(Br)(Fc)}(PiPr3)2][BArf4] (19). Counterion ([BArf4]−), hydrogen atoms, and the thermal ellipsoids of the phosphine substituents are omitted for clarity. Thermal ellipsoids are displayed at the 50% probability level.

Table 7. Bond Distances (pm) and Angles (deg) in trans[Pt{B(Br)(Fc)}Br(PiPr3)2] (17) and trans[Pt{B(Br)(Fc)}Br(PCy3)2] (18)

other cationic 14-electron platinum compounds, for which agostic interactions were reported.22 The disorder of the particular propyl group might render an agostic interaction unlikely, as such an interaction would fix the geometric arrangement of the isopropyl group. The bond distances of 16 and 19 are similar and need no further discussion (Table 8).

Pt−Br1 Pt−B average P−Pt P−Pt−P Br−Pt−B

17

18

261.1(1) 197.4(3) 233.0 172.4(1) 169.1(1)

161.4(1) 199.6(3) 234.2 170.0(1) 172.7(1)

Table 8. Bond Distances and Angles in trans-[Pt{B(Br)(Fc)}(PiPr3)2][BArf4] (19) and trans-[Pt{B(Br)(Fc)}(PCy3)2][BArf4]

to precipitation of a colorless solid within minutes. A 31P{1H} NMR spectrum performed on the reaction mixture shows a singlet at 56.7 ppm (1JP−Pt = 2936 Hz), which is low-field shifted by about 26 ppm with respect to that of the starting material 17. Besides the sharp 11B NMR resonance at −7.6 ppm for the borate anion, a broad resonance at 43.3 ppm (ω1/2 ≈ 1180 Hz) could be observed. This value is high-field shifted by about 30 ppm compared with that of its precursor, which lies in the expected range for T-shaped cationic boryl complexes,9 and constitutes the first detectable 11B NMR shift of a cationic platinum ferrocenylboryl complex.8c

Pt−B average Pt−P P−Pt−P

19

16

195.6(2) 231.8 164.76(2)

196.6(4) 232.0 163.0(1)

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CONCLUSION The reactivity studies performed on a series of platinum boryl complexes reported in this article include the full characterization of the first base adducts of cationic haloboryl complexes in which the base attacks the metal center and not the boronbased ligand. The first highly soluble tBu-substituted dinuclear platinum boryl complex was obtained by abstraction of one phosphine ligand from the initially formed mononuclear boryl complex by reaction of the low-valent platinum precursor bis(tricyclohexylphosphine)platinum(0) with two equivalents of borane. In contrast, the mesityl-substituted boryl complex reacts with a Lewis-acidic borane with formation of the cationic borylene complex by abstraction of the bromo ligand and concomitant buildup of the corresponding borate. The bonding situation in structurally optimized full geometries restricted to Cs symmetry was analyzed via DFT calculations of the fully characterized T-shaped cationic dichloro boryl complex reported herein and the isoelectronic beryllium dichloride adduct of the 14-electron platinum precursor. We could show that the dissociation energy of the cationic boryl complex is five times that of the metal-only Lewis pair and also the metal−element bond has a more covalent character than in the beryllium dichloride adduct. Although agostic interactions could not be observed experimentally, weak orbital interactions between the metal center and one proximal CH group of a phosphine substituent were found in both

Scheme 6. Synthesis of the T-Shaped Cationic Boryl Complex trans-[Pt{B(Br)(Fc)}(PiPr3)2][BArf4] (19)

Slow evaporation of the solvent yielded single crystals of trans-[Pt{B(Br)(Fc)}(PiPr3)2][BArf4] (19), from which an Xray diffraction study confirmed that the Pt-bound bromide is abstracted in the course of the reaction (Figure 8). The compound crystallizes in the orthorhombic space group P2(1)2(1)2(1) and shows the appearance of the expected, slightly distorted, T-shaped cationic three-coordinate platinum center (P−Pt−P 164.8(1)°; 16 163.0(1)°) in which one propyl group is disordered. Also, the shortest Pt−C distance is that to a methine-carbon of the disordered propyl group (320.2 and 338.0 pm) and to the corresponding disordered methyl group (324.5 and 326.0 pm), which is considerably longer than in 1904

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days, colorless crystals were formed, which were washed gently with a small amount of cold hexane. Crystallization from toluene yielded trans-[Pt{B(Br)(tBu)}(μ-Br)(PCy3)]2 (126 mg, 89.8 μmol, 73%). 1 H NMR (500.13 MHz, CD2Cl2, 297.6 K): δ 2.14−2.07 (m, 6H, Cy), 1.85−1.82 (m, 6H, Cy), 1.72−1.65 (m, 9H, Cy), 1.34−1.24 (m, 12H, Cy), 1.21 (s, 9H, tBu). 11B{1H} NMR (160.46 MHz, CD2Cl2, 298.1 K): δ 68.7 (ω ≈ 1700 Hz). 13C{1H} NMR (100.61 MHz, CD2Cl2, 296.2 K): δ 35.9 (d, 1JC−P = 31 Hz, C1 Cy); 30.3 (br s, Ci tBu, C3,5 Cy), 29.6 (d, 3JC−P = 11 Hz, C3,5 Cy), 26.6 (s, C4 Cy). 31P{1H} NMR (202.46 MHz, CD2Cl2, 298.0 K): δ 28.2 (1JP−Pt = 4740 Hz). Anal. Calcd (%) for C44H84B2Br2P2Pt2: C 37.57, H, 6.02. Found: C 38.15, H 5.98. Synthesis of trans-[Pt(BMes)Br(PCy3)2][BBr4] (12). A solution of freshly prepared trans-[Pt(BBrMes)Br(PCy3)2] (27.7 mg, 26.1 μmol) was mixed with a solution of BBr3 in C6D6 (0.3 mL, 0.1 M, 30.0 μmol). Within a few minutes, colorless crystals of trans-[Pt(BMes)Br(PCy3)2][BBr4] were formed (22.0 mg, 17.0 μmol, 65%). 1 H NMR (400.13 MHz, CD2Cl2, 296.1 K): δ 7.05 (s, 2H, Mes), 2.79 (m, 6H, Cy), 2.74 (s, 6H, Meo, Mes), 2.39 (s, 3H, Mep, Mes), 2.05−2.00 (m, 12H, Cy), 1.79−1.61 (m, 30H, Cy), 1.10−1.07 (m, 18H, Cy). 11B{1H} NMR (128.39 MHz, CD2Cl2, 296.1 K): δ −24.5 (s, BBr4). 13C{1H} NMR (100.61 MHz, CD2Cl2, 296.5 K): δ 151.0 (s, Cp, Mes), 147.1 (s, Co, Mes), 132.0 (br s, Ci, Mes), 130.2 (m, Cm, Mes), 37.5 (vt, N = |1JP−C + 3JP−C| = 28 Hz, C1, Cy); 30.6 (m, C3,5, Cy), 27.8 (vt, N = |1JP−C + 3JP−C| = 11 Hz, C2,6, Cy), 26.5 (s, C4, Cy), 23.3 (s, Meo, Mes), 22.7 (s, Mep, Mes). 31P{1H} NMR (161.98 MHz, CD2Cl2, 296.5 K): δ 45.0 (1JP−Pt = 2073 Hz). Anal. Calcd (%) for C45H77B2Br5P2Pt: C 41.70, H 5.99. Found: C 41.10, H 6.21. Synthesis of trans-[Pt(BCl2)Cl(PCy3)2] (15). Solid [Pt(PCy3)2] (201 mg, 265 μmol) was dissolved in 2 mL of toluene and cooled to −78 °C. Addition of a BCl3 solution in toluene (2 M, 0.14 mL, 280 μmol) led to the formation of a colorless suspension. The mixture was heated until all solids were dissolved. Slow cooling to ambient temperature yields trans-[Pt(BCl2)Cl(PCy3)2] as a colorless solid (168 mg, 192 μmol, 72%). 1 H NMR (400.13 MHz, CD2Cl2, 297.2 K): δ 2.49 (m, 6H, Cy), 2.03 (m, 12H, Cy), 1.81 (m, 12H, Cy), 1.70 (m, 18H, Cy), 1.25 (m, 18H, Cy). 11B{1H} NMR (160.46 MHz, CD2Cl2, 297.2 K): δ 59.0 (br s, ω1/2 ≈ 1500 Hz). 13C{1H} NMR (100.61 MHz, CD2Cl2, 297.2 K): δ 35.4 (m, C1, Cy), 30.4 (m,C3,5, Cy), 28.0 (m, C2,6, Cy), 26.9 (m, C4, Cy). 31P{1H} NMR (161.98 MHz, CD2Cl2, 297.2 K): δ 24.2 (s, 1JP−Pt = 2675 Hz). Anal. Calcd (%) for C36H66BCl3P2Pt: C 49.52, H 7.62. Found: C 49.42, H 7.64. Synthesis of trans-[Pt(BCl2)(PCy3)2][BArf4] (13). Solid trans[Pt(BCl2)Cl(PCy3)2] (15) (50.0 mg, 0.057 mmol) and Na[BArf4] (50.8 mg, 57.3 μmol) are mixed in CD2Cl2. The reaction mixture immediately turns yellow, and a colorless solid precipitates, which is filtered off. Layering the solution with hexane and cooling to −35 °C yields colorless crystals (52.0 mg, 30.6 μmol, 53%). 1 H NMR (500.13 MHz, CD2Cl2, 297.2 K): δ 7.73 (m, 8H, BArf4), 7.57 (br s, 4H, BArf4), 2.34 (m, 6H, Cy), 2.91 (m, 24H, Cy), 1.80 (m, 6H, Cy), 1.61 (m, 12H, Cy), 1.33 (m, 18H, Cy). 11B{1H} NMR (160.46 MHz, CD2Cl2, 297.2 K): δ −7.6 (s, BArf4). 13C{1H} NMR (125.76 MHz, CD2Cl2, 297.2 K): δ 162.2 (q, 1JC−B = 49.9 Hz, Ci, BArf4), 135.2 (s, Co, BArf4), 129.3 (qq, 2JC−F = 31.4 Hz, 3JC−B = 3.17 Hz, Cm, BArf4), 125.0 (q, 1JC−F = 272 Hz, CF3, BArf4), 117.9 (sep, 3 JC−F = 4.2 Hz, Cp, BArf4), 35.1 (m, C1, Cy), 30.4 (s, C3,5, Cy), 27.4 (m, C2,6, Cy), 26.1 (s, C4, Cy). 31P{1H} NMR (202.46 MHz, CD2Cl2, 297.2 K): δ 43.5 (br s, 1JP−Pt = 2657 Hz). Anal. Calcd (%) for C68H78B2Cl2F24P2Pt: C 48.02, H 4.62. Found: C 48.21, H 4.79. Synthesis of trans-[Pt{B(Br)(Fc)}Br(PiPr3)2] (17). Addition of a solution of BBr2Fc (280 mg, 767 μmol) in toluene (2 mL) to a freshly prepared solution of [Pt(PiPr3)2] (396 mg, 767 μmol) in toluene (3 mL) results in a light red solution. Cooling of the reaction mixture to −65 °C overnight led to the formation of a red solid, which was separated and washed gently with 2 mL of cold hexane (474 mg, 545 μmol, 71%). Single crystals were obtained by crystallization from dichloromethane at −35 °C. 1 H NMR (500.13 MHz, C6D6, 297.2 K): δ 4.74 (m, 2H, Cp), 4.26 (m, 2H, Cp), 4.09 (s, 5H, Cp), 2.94 (m, 6H, CH), 1.35 (m, 18H, CH3,

systems by analysis of the topology of the electron density in the fully relaxed geometry optimizations. A T-shaped cationic complex with reduced steric bulk at the phosphine substituents, compared to its tricyclohexylphosphine derivative, was fully characterized.

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EXPERIMENTAL SECTION

All manipulations were conducted either under an atmosphere of dry argon or in vacuo using standard Schlenk line or glovebox techniques. [Pt(PCy3)2],32 [Pt(PiPr3)3],33 Na[BArf4],34 BBr2tBu,35 BBr2Mes,36 and BBr2Fc37 were prepared according to published procedures. BCl3 was purchased from GHC Gehrling, Holz + Co and used as received. BBr3 was purchased from Acros Organics and stirred over Hg prior to use. Solvents were dried according to standard procedures, degassed, and stored under argon over activated molecular sieves. C6D6 and CD2Cl2 were degassed by three freeze−pump−thaw cycles and stored over molecular sieves. Reagents were dried and purified by standard procedures. NMR spectra were acquired on a Bruker AMX 400 or a Bruker Avance 500 NMR spectrometer. Chemical shifts are given in ppm and are referenced against external Me4Si (1H, 13C), BF3·Et2O (11B), and H3PO4 (85%, 31P). Assignments were made from the analysis of 1H,13C-HMQC and 1H,13C-HMBC NMR spectroscopic experiments. Microanalyses (C, H, N) were performed on a Leco Instruments elemental analyzer, type CHNS 932. Synthesis of trans-[Pt{B(Br)(NMe2)}(NCMe)(PCy3)2][BArf4] (3). Addition of MeCN (1.0 mg, 24.4 μmol) to a light yellow solution of trans-[Pt{B(Br)(NMe2)}(PCy3)2][BArf4] (30.0 mg, 17.1 μmol) in CD2Cl2 (0.5 mL) led to the formation of a nearly colorless solution of trans-[Pt{B(Br)(NMe2)}(NCMe)(PCy3)2][BArf4]. Layering the solution with a few drops of hexane and cooling to −35 °C yielded colorless crystals (25.0 mg, 13.9 μmol, 82%). 1 H NMR (400.13 MHz, CD2Cl2, 296.3 K): δ 7.73 (m, 8H, BArf4), 7.56 (br s, 4H, BArf4), 3.18 (s, 3H, NMe2), 2.89 (s, 3H, NMe2), 2.33− 2.27 (m, 6H, Cy), 2.17 (s, 3H, NCMe), 2.02−1.99 (m, 6H Cy), 1.90− 1.85 (m, 18H, Cy), 1.78−1.75 (m, 6H, Cy), 1.67−1.51 (m, 12H, Cy), 1.31−1.21 (m, 20H, Cy). 11B{1H} NMR (128.38 MHz, CD2Cl2, 296.4 K): δ 25.0 (br s), −6.7. 13C{1H} NMR (100.61 MHz, CD2Cl2, 196.2 K): δ 162.1 (q, Ci, 1JC−B = 50 Hz), 135.2 (s, Co, BArf4), 129.2 (br q, 2 JC−F = 32 Hz Cm, BArf4), 125.0 (q, 1JC−F = 272 Hz, CF3, BArf4), 117.7 (br s, Cp, BArf4). 47.2 (s, NMe2), 41.1 (s, NMe2), 35.5 (vt, N = |1JP−C + 3JP−C| = 27 Hz, C1, Cy), 30.8 (m, C3,5, Cy), 28.0 (m, C2,6, Cy), 26.5 (s, C4, Cy), 2.9 (s, MeCN). 19F{1H} NMR (376.50 MHz, CD2Cl2, 296.3 K): δ −62.9. 31P{1H} NMR (161.98 MHz, CD2Cl2, 296.1 K): δ 32.9 (s, 1JP−Pt = 2786 Hz). Anal. Calcd (%) for C72H87B2BrF24N2P2Pt: C 48.18, H 4.98, N 1.56. Found: C 48.68, H 4.93, N 1.51. Synthesis of trans-[Pt{B(Br)(NMe2)}(NCMe)(PCy3)2]2[B12Cl12] (6). Addition of Na2[B12Cl12] (31.0 mg, 51.6 μmol) to a solution of trans-[Pt{B(Br)(NMe2)}Br(PCy3)2] (100 mg, 103 μmol) in (0.5 mL) CD2Cl2 and one drop of MeCN, sonication, and filtering off the precipitated colorless solid led to the formation of a nearly colorless solution of trans-[Pt{B(Br)(NMe2)}(NCMe)(PCy3)2]2[B12Cl12]. Layering the solution with a few drops of hexane and cooling to −35 °C led to the formation of colorless crystals (88.0 mg, 72.8 μmol, 73%). 1 H NMR (400.13 MHz, CD2Cl2, 296.3 K): δ 3.17 (s, 3H, NMe2), 2.90 (s, 3H, NMe2), 2.44 (s, 3H, NCMe), 2.32−2.26 (m, 6H, Cy), 2.00−1.78 (m, 30H, Cy), 1.70−1.62 (m, 6H, Cy), 1.57−1.49 (m, 6H, Cy), 1.34−1.21 (m, 18H, Cy). 11B{1H} NMR (128.38 MHz, CD2Cl2, 296.2 K): δ 24.4 (br s), −12.9 (s, B12Cl12). 13C{1H} NMR (100.61 MHz, CD2Cl2, 296.2 K): 124.1 (s, MeCN), 47.5 (s, NMe2), 41.2 (s, NMe2), 35.5 (vt, N = |1JP−C + 3JP−C| = 27 Hz, C1, Cy), 30.9 (m, C3,5, Cy), 28.1 (m, C2,6, Cy), 26.7 (s, C4, Cy), 4.2 (s, MeCN). 31P{1H} NMR (161.98 MHz, CD2Cl2, 296.1 K): δ 28.8 (br s, 1JP−Pt = 2778 Hz). Anal. Calcd (%) for C80H150B14Br2Cl12N4P4Pt2: C 39.73, H 6.25, N 2.32. Found: C 40.19, H 6.18, N 2.27. Synthesis of trans-[Pt{B(Br)(tBu)}(μ-Br)(PCy3)]2 (8). A yellowish mixture of BBr2tBu (140 mg, 615 μmol) and [Pt(PCy3)2] (186 mg, 246 μmol) in C6D6 (0.5 mL) was heated to 50 °C for 5 h. After 11 1905

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iPr), 1.28 (m, 18H, CH3, iPr). 11B{1H} NMR (160.46 MHz, C6D6, 297.2 K): δ 72.9 (br s; ω1/2 ≈ 1680 Hz). 13C{1H} NMR (125.76 MHz, C6D6, 297.2 K): δ 77.2 (br s, C5H5FeC5H4B), 71.8 (br s, C5H5FeC5H4B), 69.6 (br s, C5H5FeC5H4B), 26.2 (m, CH, iPr), 20.8 (br s, CH3, iPr), 20.6 (br s, CH3, iPr). 31P{1H} NMR (202.46 MHz, C6D6, 297.2 K): δ 30.7 (br s, 1JP−Pt = 2936 Hz). Anal. Calcd (%) for C28H51BBr2FeP2Pt: C 38.60, H 5.90. Found: C 39.52, H 5.99. Synthesis of trans-[Pt{B(Br)(Fc)}(PiPr3)2][BArf4] (20). Solid trans-[Pt(BBrFc)Br(PiPr3)2] (50.0 mg, 57.4 μmol) and Na[BArf4] (50.8 mg, 57.3 μmol) were mixed in CD2Cl2 (2 mL), and a colorless solid precipitated. The mixture was layered with hexane and cooled to −35 °C, which led to the formation of a deep red solid (47.7 mg, 28.7 μmol, 50%). Dark red single crystals were obtained by crystallization from dichloromethane at −35 °C. 1 H NMR (500.13 MHz, C6D6, 296.7 K): δ 7.73 (m, 8H, BArf4), 7.57 (br s, 4H, BArf4), 4.67 (m, 2H, Cp), 4.52 (m, 2H, Cp), 4.22 (s, 5H, Cp), 2.44 (m, 6H, CH), 1.37 (m, 18H, CH3, iPr), 1.34 (m, 18H, CH3, iPr). 13C{1H} NMR (125.76 MHz, C6D6, 279.2 K): δ 162.2 (q, 1 JC−B = 49.7 Hz, Ci, BArf4), 135.2 (s, Co, BArf4), 129.6 (qq, 2JC−F = 30.9 Hz, 3JC−B = 3.02 Hz, Cm, BArf4), 125.0 (q, 1JC−F = 273 Hz, CF3, BArf4), 117.9 (sep, 3JC−F = 4.4 Hz, Cp, BArf4), 76.1 (br s, C 5 H 5 FeC 5 H 4 B), 73.9 (br s, C 5 H 5 FeC 5 H 4 B), 70.6 (br s, C5H5FeC5H4B), 25.0 (m, Ci, iPr), 20.2 (br s, CH3, iPr), 20.0 (br s, CH3, iPr). 11B{1H} NMR (160.46 MHz, C6D6, 297.2 K): δ 43.3 (br s, w1/2 ≈ 1180 Hz), −7.6 (s, BArf4). 31P{1H} NMR (202.46 MHz, C6D6, 297.2 K): δ 56.7 (s, 1JP−Pt = 2969 Hz). Anal. Calcd (%) for C60H63B2BrF24FeP2Pt: C 43.56, H 3.84. Found: C 43.85, H 3.75.

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ASSOCIATED CONTENT

S Supporting Information *

Details on the X-ray diffraction and theoretical studies are provided. CCDC-857112 (3), 857113 (6), 857114 (8), 857115 (12), 857116 (15), 857117 (13), 857118 (17), and 857119 (19) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac. uk/data_request/cif. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: (+49) 931-31-85260. Fax: (+49) 931-31-84623. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Dr. Carsten Knapp and his co-workers for the donation of Na2[B12Cl12] and the Deutsche Forschungsgemeinschaft (DFG) for financial support.

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REFERENCES

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