A Simple Decarbonylative Route to Heterodinuclear Alkylborylene

Article pubs.acs.org/Organometallics

A Simple Decarbonylative Route to Heterodinuclear Alkylborylene Complexes Holger Braunschweig,* Rian D. Dewhurst, Thomas Kramer, and Eva Siedler Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany S Supporting Information *

ABSTRACT: A new, photolytic route to heterodinuclear alkylborylene complexes is presented using the terminal manganese borylene complex [(η5-C5H5)(OC)2Mn(BtBu)]. Using this route, three new heterodinuclear alkylborylene complexes were prepared (Mn/Cr and Mn/Co), in addition to a known dimanganese alkylborylene complex usually prepared by salt elimination. The complexes have been characterized by NMR and IR spectroscopy as well as single-crystal X-ray crystallography. The overall picture gained from the spectroscopic and structural data is that the two metals in the Mn/Cr and Mn/Mn complexes appear to share the boron atom more evenly, while the cobalt fragment in the Mn/Co complex is more loosely bound to the boron atom.

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INTRODUCTION Borylene ligands in general show a greater propensity toward bridging than their isoelectronic counterpart CO.1,2 The very first isolated borylene complexes were the bridging dimanganese species [{(η5-C5R5)(OC)2Mn}2(BX)] (R5 = H5, X = NMe2; R5 = H4Me, X = NMe2; R5 = H4Me, X = tBu) reported in 1995,3 with terminal examples arising a few years later.4 These dimanganese bridging borylene complexes were found to be remarkably stable. Computational studies later suggested that this stability may arise from a high level of electronic delocalization across the Mn−B−Mn unit and not, as previously assumed, due to a direct Mn−Mn bond.5 Since the synthesis of the first bridging borylene, the number of bridging borylene complexes has steadily increased, including di- and trinuclear examples.1,2 However, mixed-metal bridging borylene complexes are relatively rare. A number of bridging heterodinuclear borylene complexes have been prepared by the addition of zerovalent group 10 complexes [M′(PCy3)2] (M′ = Pd, Pt) to terminal boryl or borylene complexes, including examples from groups 6 (I; Figure 1), 7 (II), and 8 (III).6 Apart from these complexes, only three other heterodinuclear borylene complexes are known (IV; Figure 1), all products of the decarbonylative addition of [(η5-C5R5)M′(CO)2] (M′ = Co, R = H; M = Rh, R = H; M = Ir, R = Me) to the group 6 aminoborylene complexes [(OC)5M{BN(SiMe3)2}] (M = Cr, W).7 Overall, the synthesis of heterodinuclear borylene complexes is relatively specific and the range of known complexes quite narrow. The first terminal alkylborylene complex, [(η5-C5H5)(OC)2Mn(BtBu)], itself synthesized from a dinuclear complex in 2007,6 shows high reactivity in metathesis reactions with CO bonds8 and has recently been shown to undergo facile borylene−CO coupling triggered by Lewis bases.9 This reactivity led us to attempt the addition of further metal− carbonyl fragments to the MnB bond under photolytic © 2014 American Chemical Society

Figure 1. Reported heterodinuclear borylene complexes, illustrating the original borylene fragments of each in black and added fragments in green.

conditions in order to prepare new heterodinuclear bridging borylene complexes. Our results are presented below. Received: June 5, 2014 Published: July 14, 2014 3877

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RESULTS AND DISCUSSION Solutions of manganese terminal borylene complex 16d and the di- and tricarbonyl complexes [(η6-C6H6)Cr(CO)3] (2), [(η5C5H5)Mn(CO)3] (3), and [(η5-C5H4R)Co(CO)2] (4a, R = H; 4b, R = Me) were subjected to photolysis with UV light for 6− 10 h. Removal of solvent from the mixtures, sublimation, rinsing, and crystallization provided the corresponding dinuclear complexes 5, 6,10 and 7a,b (Figure 2) in 49−56%

Table 1. Spectroscopic and Structural Data for the Complexes Prepared Hereina 5 δH(MnCp) δH(tBu) δB δC(MnCp) IR ν(CO) d(MnB) d(MauxB) d(MnM) ∠(MnBM) ∠(MnBC)

4.40 1.42 173 85.4 1946, 1913, 1882, 1863 2.022(2) 2.100(2) 2.8667(5) 88.12(9) 135.2(2)

610

7a

7b

4.29 1.34 169 85.9 1973, 1932, 1907 2.029(2) 2.031(2) 2.7952(5) 87.03(7) 137.0(1), 136.0(1)

4.17 1.30 165 84.2 1998, 1923, 1874 1.91(6) 2.00(6) 2.66(3) 86(1) 139(5)

4.20 1.32 166 84.1 1994, 1921, 1869 1.95(1) 1.98(1) 2.592(3) 82.5(4) 145.8(9)

a NMR data (δ) expressed in ppm. IR data (ν) expressed in cm−1. Distances (d) expressed in Å. Angles (∠) expressed in deg.

for bridging borylene complexes, the face-capping ligands of which are usually found in an anti geometry (as in the structures of 5 and 610). The Mn−B distance of 7b (1.95(1) Å) is smaller than those of 5 (2.022(2) Å) and 6 (2.029(2), 2.031(2) Å) by a small but statistically significant amount. One could assume a bonding situation in these complexes in which the MnB π bond donates electron density to the other metal center. The shorter Mn−B distance in 7b is thus in agreement with the assumption that the more electron rich metal fragment {Co(CO)(η5C5H5)} requires less electron density from the MnB π bond and thus weakens it less. Similarly, the Mn−B−tBu angle of 7b is larger than those of 5 and 6 (i.e., less perturbed from linearity), similarly suggesting a lesser degree of interaction of the auxiliary metal fragment with the MnB bond in 7b. Overall these data indicate that the boron is more evenly shared between the metals when the auxiliary fragment is {Mn(CO)2(η5-C5H5)} or {Cr(CO)2(η6-C6H6)}, while with the fragment {Co(CO)(η5-C5H5)}, MnB appears to act more as a loosely bound π ligand toward cobalt. As each of the auxiliary fragments of complexes 5, 6, and 7a,b formally possess 16 valence electrons (VE), the distinction is presumably due to either the higher electron density of cobalt or this fragment’s lack of a second π-acidic CO ligand. Alkylborylene complexes have thus far shown no ability to engage in “borylene transfer” reactivity. The borylene transfer reaction, to both metallic and organic substrates, has provided convenient access to a range of otherwise-inaccessible new borirenes and borylene complexes,1 as well as boron-containing organic and heterocyclic ligands.11 Borylene transfer reactions have been demonstrated using amino-,12 aryl-,13 and metalloborylene14 complexes but have without exception failed in the case of alkylborylene complexes, which instead commonly undergo metathesis15 and borylene−CO coupling.9 We have previously shown in reactions of group 6 aminoborylene complexes with zerovalent complexes of group 9 that the intermetallic borylene transfer reaction most likely occurs via formation of dinuclear bridging borylene intermediates.7 It is very interesting to note that, in these intermediate complexes, the boron atom is noticeably skewed toward the incoming metal fragment (Co/Rh/Ir); the intermediates subsequently form new metal−boron double bonds and lose the original group 6 metal fragments. In contrast, the borylene ligands of the Mn/Co complexes 7a,bwhich do not undergo transfer are more strongly associated with the original metal center

Figure 2. Bridging borylene complexes prepared herein.

yield as dark red (5, 6) or black (7a,b) crystals. The 11B NMR signals of 5 (173 ppm), 6 (169 ppm), 7a (165 ppm), and 7b (166 ppm) follow a trend that approximately matches the presumed electron density of the added metal center (see Table 1). Despite this, the 1H and 13C NMR parameters related to the Mn(η5-C5H5) and tBu groups are relatively insensitive to changes in the auxiliary metal fragment. The molecular structures of 5 and 7a,b derived from singlecrystal X-ray crystallography are shown in Figure 3. Unfortunately, the structures of 7a,b suffered from disorder due to the superimposition of the Mn and Co sides of the complex. In addition to this, the crystal of 7a used for data collection was twinned; therefore, bond distances of 7a in particular contain high uncertainty and will not be discussed in detail. It is clear from the structures that this is due to the fact that one CO of the Mn fragment in each complex is found to be semibridging between the two metals (semibridging Co−C (Å), 7a 2.26(2); 7b 2.34(1); terminal Co−C, 7a 1.86(4), 7b 2.04(2)), making the two {M(CO)(η5-C5H4R)] groups spatially very similar. This bridging CO ligand also ensures that the cyclopentadienyl ligands are located on the same side of the MnBCo plane. This is a highly unusual structural motif 3878

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Figure 4. Structural comparison of selected heterodinuclear aminoand alkylborylene complexes and their further reactivity.

CO ligands. The complexes prepared herein, including the addition of transition-metal fragments from groups 6, 7, and 9, show interesting trends that can be related to the electron density of the fragment being added. Notably, it is shown that the interaction of the cobalt fragment in the Mn/Co complex with the boron atom is significantly lower than in the Mn/Cr and Mn/Mn complexes. This effect goes some way toward explaining the dramatic reactivity differences between alkylborylenes and the other known borylene classes (amino-, aryl-, and metalloborylenes), namely the reluctance of the former to undergo borylene transfer reactions.

Figure 3. Molecular structures of 5 and 7a,b in the solid state. Thermal ellipsoids are set to the 50% probability level. Hydrogen atoms and ellipsoids of most carbon atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): for 5, Cr1−B1 2.100(2), Mn1−B1 2.022(2), B1−C31 1.613(3), Cr1−Mn1 2.8667(5); Mn1− B1−Cr1 88.12(9), C31−B1−Mn1 135.2(2), C31−B1−Cr1 136.7(2); for 7a, Co1−B31 2.00(6), Mn2−B31 1.91(6), B31−C31 1.65(3), Co1−Mn2 2.66(3); Mn2−B31−Co1 86(1), C31−B31−Mn2 139(5), C31−B31−Co1 136(5); for 7b, B1−Mn1 1.95(1), B1−Co1 1.98(1), B1−C11 1.62(2), Co1−Mn1 2.592(3); C11−B1−Mn1 145.8(9), C11−B1−Co1 131.0(9), Mn1−B1−Co1 82.5(4).

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EXPERIMENTAL SECTION

General Information. All syntheses were performed under an inert atmosphere of dry argon using standard Schlenk techniques or in a glovebox (MBraun). Hexane, benzene and toluene were dried by distillation over potassium (benzene, toluene) or Na/K alloy (hexane) under argon and stored over activated molecular sieves (4 Å). C6D6 was degassed by several freeze−pump−thaw cycles and stored over molecular sieves (4 Å). The NMR spectra were recorded on a Bruker AV 400 (1H, 400 MHz; 13C, 100 MHz; 11B, 128 MHz) and/or a Bruker Avance 500 FT-NMR spectrometer (1H, 500 MHz; 11B, 160 MHz; 13C{1H}, 126 MHz). Chemical shifts are given in ppm and are referenced to external TMS (1H, 13C), [BF3·OEt2] (11B{1H}), or 85% H3PO4 (31P{1H}). Coupling constants are given in Hz. Elemental analyses were obtained from an Elementar Vario MICRO cube instrument. Infrared spectra were measured on a JASCO FT/IR-6200 spectrometer. The light source for photochemical experiments was a Hg/Xe arc lamp (400−550 W) equipped with IR filters, irradiating at 210−600 nm. Complexes 18b and 216 were prepared according to literature procedures. Preparation of [{(η5-C 5H5)Mn(CO) 2}(μ-BtBu){(η 6-C6H6)Cr(CO)2}] (5). A pale orange solution of [(η5-C5H5)(OC)2MnBtBu] (1; 50.0 mg, 0.20 mmol) and [(η6-C6H6)Cr(CO)3] (2; 43.9 mg, 0.20

(Mn) than the incoming metal (Co). This effect is depicted graphically in Figure 4. This provides a possible rationale for the observed reactivity differences between alkyl- and aminoborylene complexes. That is, manganese alkylborylene complexes appear to be prevented from undergoing transfer by the relatively high affinity that the manganese fragment has for the borylene ligand in the resulting heterodinuclear complexes.

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CONCLUSION The new decarbonylative route to heterodinuclear alkylborylene complexes presented herein is potentially useful for the addition of further metals to the MnBtBu scaffold and could be expanded to include any 18-VE complex with photolabile 3879

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mmol) in benzene (2 mL) was photolyzed at ambient temperature for 8 h. The solvent was removed in vacuo. All impurities were removed by sublimation. The resulting solid was dissolved in toluene (1 mL). After filtration, the solution was layered with hexane (0.5 mL) and cooled to −30 °C, which provided dark red crystals of 5. Yield: 46.3 mg (0.108 mmol, 54%). NMR: 1H (C6D6, 400.1 MHz) δ 4.78 (s, 6H, C6H6), 4.40 (s, 5H, C5H5), 1.42 (s, 9H, tBu); 11B{1H} (C6D6, 128.4 MHz) δ 172.7 (br); 13C{1H} (C6D6, 100.6 MHz) δ 233.4 (s, CO), 95.5 (s, C6H6), 85.4 (s, C5H5), 32.7 (s, C(CH3)3), 28.5 (s, C(CH3)3), a number of carbon nuclei of the CO ligands were not detected due to low intensity. IR (hexane): ν 1946 (s), 1913 (s), 1882 (s), 1863 (s) cm−1. Anal. Calcd for C19H20BCrMnO4: C, 53.06; H, 4.69. Found: C, 53.67; H, 5.05. Crystallographic methods: the crystal data of 5 were collected on a Bruker X8-APEX II diffractometer with a CCD area detector and multilayer mirror monochromated Mo Kα radiation. The structure was solved using direct methods, refined with the Shelx software package, and expanded using Fourier techniques. All nonhydrogen atoms were refined anisotropically. Hydrogen atoms were included in structure factor calculations. All hydrogen atoms were assigned to idealized geometric positions. Crystal data for 5: C19H20BCrMnO4, Mr = 430.10, black block, 0.18 × 0.12 × 0.09 mm3, triclinic space group P1,̅ a = 7.8550(6) Å, b = 8.4231(7) Å, c = 15.5019(15) Å, α = 99.830(3)°, β = 95.706(3)°, γ = 115.083(2)°, V = 898.17(13) Å3, Z = 2, ρcalcd = 1.590 g cm−3, μ = 1.329 mm−1, F(000) = 440, T = 100(2) K, R1 = 0.0430, wR2 = 0.0999, 3801 independent reflections (2θ ≤ 53.48°), and 238 parameters. CCDC 1006309. Preparation of [{(η5-C5H5)Mn(CO)2}2(μ-BtBu)] (6). An orange solution of [(η5-C5H5)(OC)2MnBtBu] (1; 25.0 mg, 0.10 mmol) and cymantrene (3; 20.4 mg, 0.10 mmol) in benzene was photolyzed for 6 h at ambient temperature. After filtration, the solvent of the dark red reaction mixture was removed in vacuo. The resulting red solid was dissolved in hexane (2 mL), and the solution was cooled to −30 °C, thus yielding dark red crystals of 6. NMR spectra of this material were found to be identical with those reported earlier for this compound.10 Yield: 23.5 mg (0.056 mmol, 56%). IR (hexane): ν 1973, 1932, 1907 cm−1. Preparation of [{(η5-C5H5)Mn(CO)2}(μ-BtBu){(η5-C5H5)Co(CO)}] (7a). A dark brown solution of [(η5-C5H5)(OC)2MnBtBu] (1; 50.0 mg, 0.20 mmol) and [(η5-C5H5)Co(CO)2] (4a; 36.0 mg, 0.20 mmol) in benzene (2 mL) was photolyzed for 10 h. The solvent of the black reaction mixture was removed in vacuo. The resulting black oil was dissolved in benzene and cooled to −20 °C. The frozen benzene and all impurities were removed by sublimation. The resulting black solid was dissolved in toluene (2 mL). After filtration, the solution was layered with hexane (0.5 mL) and cooled to −30 °C, yielding black crystals of 7a. Yield: 38.5 mg (0.097 mmol, 49%). NMR: 1H (C6D6, 400.1 MHz) δ 4.52 (s, 5H, C5H5-Co), 4.17 (s, 5H, C5H5-Mn), 1.30 (s, 9H, tBu); 11B{1H} (C6D6, 128.4 MHz) δ 165.3 (br); 13C{1H} (C6D6, 100.6 MHz) δ 87.1 (s, C5H5-Co), 84.2 (s, C5H5-Mn), 31.9 (s, C(CH3)3, 29.7 (s, C(CH3)3), the carbon nuclei of the CO ligands were not detected due to low intensity. IR (hexane): ν = 1998 (s), 1923 (s), 1874 (s) cm−1. Anal. Calcd for C17H19BCoMnO3: C, 51.56; H, 4.84. Found: C, 51.47; H, 4.58. Crystallographic methods: the crystal data of 7a were collected on a Bruker X8-APEX II diffractometer with a CCD area detector and multilayer mirror monochromated Mo Kα radiation. The structure was solved using direct methods, refined with the Shelx software package, and expanded using Fourier techniques. All nonhydrogen atoms were refined anisotropically. Hydrogen atoms were included in structure factor calculations. All hydrogen atoms were assigned to idealized geometric positions. The crystal was a pseudomerohedral twin with domains rotated by 4.4° around a real axis [1.000,0.120,−0.492]. The BASF parameter was refined to 12%. The 239 reported least-squares restraints as shown by the _refine_ls_number_restraints key are attributed to the DELU keyword in the ShelXL input (“rigid bond” restraint for all bonds in the connectivity list; standard values of 0.01 for both parameters s1 and s2 were used). The displacement parameters of all atoms due to total disorder were constrained to the same value. The distances between the atoms of the Cp rings were kept during refinement at a value of 1.55 Å using the DFIX restraint. The distances between atoms

[Mn1, C1, Mn2, C101], [C101, Co2, C1, Co1], [B31 Co1, B131, Co2], and [B31, Mn2, B131, Mn1] were restrained during refinement to the same value with SADI restraint. The Uii displacement parameters of all atoms were restrained with the ISOR keyword to approximate isotropic behavior. Crystal data for 7a: C17H19BCoMnO3, Mr = 396.00, black block, 0.19 × 0.13 × 0.09 mm3, monoclinic space group P21/n, a = 7.3879(6) Å, b = 17.8119(15) Å, c = 12.6099(11) Å, β = 90.048(3)°, V = 1659.4(2) Å3, Z = 4, ρcalcd = 1.585 g cm−3, μ = 1.770 mm−1, F(000) = 808, T = 100(2) K, R1 = 0.0605, wR2 = 0.1368, 3589 independent reflections (2θ ≤ 53.56°), and 285 parameters. CCDC 1006310. Preparation of [{(η5-C5H5)Mn(CO)2}(μ-BtBu){(η5-C5H4Me)Co(CO)}] (7b). A dark brown solution of [(η5-C5H5)(CO)2MnBtBu] (1; 50.0 mg, 0.20 mmol) and [(η5-C5H4Me)Co(CO)2] (4b; 39.8 mg, 0.20 mmol) in benzene (2 mL) was photolyzed for 10 h. The solvent of the black reaction mixture was removed in vacuo. The resulting black oil was dissolved in benzene and cooled to −20 °C. The frozen benzene and all impurities were removed by sublimation. The resulting black solid was dissolved in toluene (2 mL). After filtration, the solution was layered with hexane (0.5 mL) and cooled to −30 °C, yielding black crystals of 7b. Yield: 42.6 mg (0.103 mmol, 52%). NMR: 1H (C6D6, 400.1 MHz) δ 4.71 (m, 2H, C5H4), 4.20 (s, 5H, C5H5), 3.87 (m, 2H, C5H4), 1.85 (m, 2H, C5H4Me), 1.32 (s, 9H, tBu), the carbon nuclei of the CO ligands and Cp′ ring were not detected due to low intensity; 11 1 B{ H} (C6D6, 128.4 MHz) δ 165.5 (br); 13C{1H} (C6D6, 100.6 MHz) δ 90.81, 86.6, 85.7, 82.1 (4 × C5H4), 84.1 (s, C5H5), 31.9 (s, C(CH3)3), 29.7 (s, C(CH3)3), 13.6 (s, CpMe). IR (hexane): ν 1994 (s), 1921 (s), 1869 (s) cm−1. Anal. Calcd for C18H21BCoMnO3: C, 52.72; H, 5.16. Found: C, 52.04; H, 5.39. Crystallographic methods: the crystal data of 7b were collected on a Bruker X8-APEX II diffractometer with a CCD area detector and multilayer mirror monochromated Mo Kα radiation. The structure was solved using direct methods, refined with the Shelx software package, and expanded using Fourier techniques. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included in structure factor calculations. All hydrogen atoms were assigned to idealized geometric positions. The 731 reported least-squares restraints as shown by the _refine_ls_number_restraints key are attributed to the DELU keyword in the ShelXL input (“rigid bond” restraint for all bonds in the connectivity list; standard values of 0.01 for both parameters s1 and s2 were used). The displacement parameters of all atoms due to total disorder were constrained to the same value. The displacement parameters of atoms C21 > C136 were restrained to the same value with the similarity restraint SIMU. The distances between atoms [Co1, C1, Co2, B101], [Mn1, B1, Mn2, B101], [Co1, C1, Co2, C101], and [Mn1, C1, Mn2, C101] were restrained during refinement to the same value with the SADI restraint. The Uii displacement parameters of all atoms were restrained with the ISOR keyword to approximate isotropic behavior. Crystal data for 7b: C18H21BCoMnO3, Mr = 410.03, black block, 0.18 × 0.17 × 0.05 mm3, orthorhombic space group Pna21, a = 18.1160(16) Å, b = 7.4490(6) Å, c = 13.0298(12) Å, V = 1758.3(3) Å3, Z = 4, ρcalcd = 1.549 g cm−3, μ = 1.673 mm−1, F(000) = 840, T = 100(2) K, R1 = 0.0603, wR2 = 0.1245, 3458 independent reflections (2θ ≤ 52.02°), and 328 parameters. CCDC 1006311.

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ASSOCIATED CONTENT

* Supporting Information S

CIF files giving crystallographic data for 5 and 7a,b. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for H.B.: [email protected]. Notes

The authors declare no competing financial interest. 3880

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ACKNOWLEDGMENTS This work was generously supported by a European Research Council Advanced Grant to H.B. REFERENCES

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