Partially and Fully Reversible Solvation-Controlled Borylene

Note pubs.acs.org/Organometallics

Partially and Fully Reversible Solvation-Controlled Borylene Swapping and Metal-Only Lewis Pair Formation Stefanie Bertsch, Holger Braunschweig,* Rian D. Dewhurst, Krzysztof Radacki, Christian Saalfrank, Benedikt Wennemann, and Qing Ye Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany S Supporting Information *

ABSTRACT: New borylene-containing metal-only Lewis pairs based on group 6 metals and ruthenium are prepared by a partially reversible borylene transfer from group 6 borylene complexes [M{BN(SiMe3)2}(CO)5] (M = Mo, W). The complexes contain strong Ru → Mo/W dative interactions. As in a previous report on related compounds, redissolving these complexes resulted in reversion back to the precursors, enabled by a sacrificial CO donor. It was observed that this reversion process becomes less selective as the group 6 metal becomes heavier, leading to greater amounts of unidentified byproducts as the group is descended.

T

he borylene transfer reaction, discovered in 2003,1 has become a useful tool in expanding the range of known transition-metal borylene complexes.2 However, until recently, the reaction had failed with precursors containing group 8 metals ruthenium and osmium. Consequently, the chemistry of ruthenium borylene complexes was limited to a handful of examples from the groups of Sabo-Etienne and Aldridge,3 only one of which was a terminal, base-free example.3a The state of osmium borylene chemistry was even less advanced, with only one base-stabilized example being known, from the group of Roper.4 Our recent report detailed the first osmium (3d, Figure 1), and second ruthenium (3a) terminal, base-free borylene

bridging dinuclear borylene complexes have been isolated from the process.7 Indeed, bridging motifs appear to be highly favored with the aminoborylene ligand: BN(SiMe3)2; thus, the dinuclear, nonbridging borylene MOLP complexes prepared in this earlier report5 were highly unusual. This MOLP formation was, therefore, presumed to be an offshoot of the conventional borylene transfer mechanism. The MOLP-forming reactions were also found to be reversible,5 driven in the forward direction by the crystallization of the MOLP−borylene complexes 3a and 3d from hexanes, and in the reverse by dissolution in more polar solvents. This unprecedented phenomenon suggested that the energetic profile of the reaction must be relatively flat. In this note, we further explore this reactivity pattern and expand this family of unusual borylene-containing MOLPs by preparing molybdenum and tungsten examples, thus completing the chromium triad. Similarly to the preparation of 3a and 3d,5 hexane solutions containing [Ru(CO)3(PMe3)2] (2a) and a slight excess of group 6 borylene complexes [M{BN(SiMe3)2}(CO)5] (1b: M = Mo; 1c: M = W) in quartz NMR tubes were photolyzed for 15 h using a 550 W Hg/Xe UV lamp (Figure 1). During this time, yellow needles of trans,trans-[(OC)5M←Ru(CO)2(PMe3)2{BN(SiMe3)2}]·(0.5 n-C6H14) (3b: M = Mo; 3c: W) formed, which were used for single-crystal X-ray diffraction studies. Single-crystal X-ray crystallographic study of 3b and 3c (Figure 2) showed the new MOLPs to be isostructural both to each other and to the previously published 3a. Both complexes were found to cocrystallize with half a molecule of hexane in the unit cell. Both compounds possess RuB double bonds (3b: 1.941(3) Å; 3c: 1.949(7) Å), ruthenium−group 6 metal

Figure 1. Synthesis of borylene-containing MOLPs by interrupted intermetallic borylene transfer.

complexes, both prepared by intermetallic borylene transfer from [Cr{BN(SiMe3)2}(CO)5] (1a).5 However, in contrast to all previous borylene transfer processes, the group 6 metal was retained in the complex, leading to metal-only Lewis pairs (MOLPs)6 in which a zerovalent group 8 fragment donates a pair of electrons to the group 6 pentacarbonyl fragment. Although the mechanism of the conventional borylene transfer reaction has not been definitively determined, a number of © 2014 American Chemical Society

Received: March 10, 2014 Published: June 20, 2014 3649

dx.doi.org/10.1021/om5002414 | Organometallics 2014, 33, 3649−3651

Organometallics

Note

than the average drel value (0.98), as surveyed in our recent review on the topic.6 They are additionally some of the highest measured values for MOLP complexes. However, these values are in line with other MOLPs featuring group 6 acceptor fragments, almost all of which have drel values above unity. As this review showed, the nature of the acceptor fragment (coordination number, valence-electron count) has a much larger effect on the drel value of a MOLP than does the nature of the donor fragment. Thus, the long M−M distances found in complexes 3a−3c are presumably a function of the relatively high coordination number (5) and valence-electron count (16) of the {M(CO)5} fragment. The ruthenium-bound CO ligands were found to be pointing toward the group 6 metal (3b: C−Ru−Mo 77.33(7)°, 75.13(7)°; 3c: C−Ru−W 77.5(2)°, 76.0(2)°), while the Ru− C−O axes are slightly bent (3b: 174.8(2)°, 173.4(2)°; 3c: 174.8(6)°, 174.0(5)°). These parameters, as well as the observed low-energy CO bands in the solid IR spectra of the complexes (3b: 1851 cm−1; 3c: 1837 cm−1), indicate that these carbonyl ligands can be designated as semibridging with the group 6 fragments. In contrast, the phosphines point away from the group 6 metal (3b: P−Ru−Mo 95.44(2)°, 95.73(2)°; 3c: P−Ru−W 95.63(4)°, 95.53(4)°) to a lesser degree. Interestingly, the M−C bond found trans to the Ru → M bond in 3b and 3c is significantly shorter than those of the remaining M− CO bonds (by ca. 0.10 and 0.08 Å, respectively), whereas the trans C−O bonds are correspondingly lengthened (by ca. 0.02 Å in both cases) compared to their neighbors, thus providing further evidence for the strong σ donation from the ruthenium atom. Redissolving the crystals of 3b or 3c in C6D6 or toluene-d8 led to an incomplete disproportionation reaction involving a sacrificial CO donor, analogous to that seen in the previous study,5 providing precursors 1b/1c and 2a as determined by 31 P, 11B, and 1H NMR spectroscopy. Thus, we were unable to obtain NMR data to characterize 3b/3c in solution. However, we observed a trend in the selectivity of this disproportionation reaction for 3a−3c. All three complexes 3a−3c decompose immediately and quantitatively upon dissolving, even at low temperatures. However, the selectivity of this decomposition is different in each case. As judged by 31P NMR spectroscopy, 3a undergoes the most selective decomposition, giving 2a as the major product and a small signal at δ 7.4 that we were unable to assign to a compound. With the heavier group 6 metals, the decomposition becomes less selective, with both 3b and 3c giving at least six other signals corresponding to unidentified products. Upon dissolving 3c, the signal for 2a is no longer the major signal in the spectrum. In contrast, the dissolution of the complexes in CO-saturated solutions under 1 atm of CO led, in all cases, to complete reversion back to 2a and the corresponding borylene complex 1 (as ascertained by 1H and 11 B NMR spectroscopy). In this note, we have further explored the reversible borylene transfer between group 6 metals and zerovalent ruthenium, completing the chromium triad by preparing new molybdenum and tungsten borylene-containing MOLPs. We find that the reversion process becomes less selective as the group 6 metal becomes heavier, leading to greater amounts of unidentified byproducts as the group is descended.

Figure 2. Molecular structures of 3b and 3c as derived from X-ray crystallography. Thermal ellipsoids depicted at the 50% probability level. For clarity, half a molecule of cocrystallized hexane, hydrogen atoms, and some carbon ellipsoids have been removed. Selected bond lengths (Å) and angles (deg) for 3b: Ru−Mo 3.1418(3), Ru−B 1.941(3), B−N 1.362(3), Ru−P 2.3622(7), 2.3694(7); B−Ru−Mo 179.35(8). For 3c: W−Ru 3.1410(6), Ru−B 1.949(7), B−N 1.357(8), Ru−P 2.3644(16), 2.3778(16); B−Ru−W 179.6(2).

dative bonds (3b: 3.1418(3) Å; 3c: 3.1410(6) Å) and essentially linear B−Ru−Mo/W axes (3b: 179.35(8)°; 3c: 179.6(2)°) (see Table 1). The Ru−B distances of 3b and 3c are Table 1. Structural Data for Borylene-Containing MOLPs 3ab 3b 3c 3db

d(M′−B)

d(M−M′)

drela

1.949(5) 1.941(3) 1.949(7) 1.967(6)

3.068(1) 3.1418(3) 3.1410(6) 3.0857(9)

1.08 1.05 1.02 1.09

a drel = ratio of measured M−M′ distance to sum of experimental covalent radii.8 bPreviously published complexes.5

statistically identical to that of 3a,5 but much longer than that of the only other known neutral, terminal ruthenium borylene complex (1.780(4) Å), prepared by Sabo-Etienne et al.3a Surprisingly, the Ru−B distances of 3a−3c fit much better with those of the cationic ruthenium aminoborylene complexes (1.960(6), 1.928(4) Å) prepared by Aldridge and co-workers.3c This indicates that the π-donor ability of the borylene substituent (amino/aryl) plays a much larger role in determining the metal−boron bond length than does the charge on the complex or the nature of the metal fragment. While the metal−metal distances in 3b and 3c are significantly longer than that in 3a (by ca. 0.7/0.8 Å), this is in line with the differences in covalent radii of the group 6 metals. In fact, the drel values of the bonds (drel = bond length divided by sum of the covalent radii8) are actually lower in 3b (1.05) and 3c (1.02) than that of 3a (1.08), reflecting slightly shorter bonds when changes in covalent radii are taken into consideration. The drel values of 3a−3c are significantly higher

■

EXPERIMENTAL SECTION

General Information. All syntheses were carried out under an argon atmosphere using standard Schlenk and glovebox techniques 3650

dx.doi.org/10.1021/om5002414 | Organometallics 2014, 33, 3649−3651

Organometallics

■

unless otherwise stated. The complexes [Mo{BN(SiMe3)2}(CO)5],9 [W{BN(SiMe3)2}(CO)5],10 and [Ru(CO)3(PMe3)2]11 were prepared according to published procedures. Hexane was dried by distillation over Na/K alloy under argon and stored over activated 4 Å molecular sieves. C6D6 and toluene-d8 were degassed by three freeze−pump− thaw cycles and stored over activated 4 Å molecular sieves. Solution NMR spectra were recorded at 297 K on a Bruker Avance I 400 NMR spectrometer (1H: 400.1 MHz; 11B: 128.3 MHz; 31P: 162.0 MHz). 1H NMR spectra were referenced to external tetramethylsilane by the residual solvent peak. 11B NMR spectra were referenced to external BF3·Et2O. Chemical shifts (δ) are given in parts per million (ppm). IR spectra were measured using a Jasco FT/IR-6200 spectrometer equipped with a Pike HWG detector connected via an optical fiber to a Pike ATR head in an MBraun glovebox. Elemental analyses were performed on an Elementar vario MICRO cube elemental instrument. A Hg/Xe UV lamp (550 W) was used as the source of radiation in all reactions under photolytic conditions. Preparation of trans,trans-[(OC)5Mo←Ru(CO)2(PMe3)2{BN(SiMe3)2}] (3b). [Ru(CO)3(PMe3)2] (2a, 20.0 mg, 59.5 μmol) and [Mo{BN(SiMe3)2}(CO)5] (1b, 24.2 mg, 59.5 μmol) were dissolved in hexane (0.6 mL) and irradiated in a quartz NMR tube. After 1 h, the formation of yellow needles was observed. To complete the reaction, the irradiation was continued for a further 14 h. The mother liquor was decanted, and the solid was washed with hexane (2 × 1.0 mL). Yield: 8.8 mg (21%) of yellow, crystalline needles. IR (solid): νCO [cm−1]: 2034, 1922, 1901, 1851. Elemental analysis for C19H36BMoNO7P2RuSi2·(C6H14)0.1 [%]: calcd C 32.47, H 5.20, N 1.93; found C 32.23, H 5.28, N 1.79. Preparation of trans,trans-[(OC)5W←Ru(CO)2(PMe3)2{BN(SiMe3)2}] (3c). [Ru(CO)3(PMe3)2] (2a, 20.0 mg, 59.5 μmol) and [W{BN(SiMe3)2}(CO)5] (1c, 29.5 mg, 59.5 μmol) were dissolved in hexane (0.6 mL) and irradiated in an NMR tube. After 2 h, formation of yellow needles began. To complete the reaction, the irradiation was continued for a further 13 h. The mother liquor was decanted, and the solid was washed with hexane (2 × 1.0 mL). Yield: 11.2 mg (23%) of yellow, crystalline needles. IR: νCO [cm−1]: 2032, 1918, 1882, 1837. Elemental analysis for C19H36BNO7P2RuSi2W·(C6H14)0.1 [%]: calcd C 28.96, H 4.64, N 1.72; found C 29.04, H 4.49, N 1.49. Crystallographic Details for 3b. The crystal data of 3b 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 assigned to idealized geometric positions and included in structure factor calculations. Crystal data for 3b: C22H43BMoNO7P2RuSi2, Mr = 759.51, colorless plate, 0.322 × 0.118 × 0.04 mm3, monoclinic space group C2/c, a = 23.1910(12) Å, b = 11.3126(6) Å, c = 25.8255(16) Å, β = 90.258(2)°, V = 6775.3(7) Å3, Z = 8, ρcalcd = 1.489 g·cm−3, μ = 1.017 mm−1, F(000) = 3096, T = 100(2) K, R1 = 0.0375, wR2 = 0.0632, 7229 independent reflections [2θ ≤ 53.58°] and 427 parameters. Crystallographic Details for 3c. The crystal data of 3c were collected on a Bruker APEX diffractometer with a CCD area detector and graphite 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. Crystal data for 3c: C19H36BNO7P2RuSi2W, Mr = 847.42, yellow block, 0.10 × 0.08 × 0.06 mm3, monoclinic space group C2/c, a = 23.279(4) Å, b = 11.3964(17) Å, c = 25.956(4) Å, β = 90.663(2)°, V = 6885.7(17) Å3, Z = 8, ρcalcd = 1.635 g·cm−3, μ = 3.975 mm−1, F(000) = 3352, T = 168(2) K, R1 = 0.0508, wR2 = 0.0982, 7350 independent reflections [2θ ≤ 53.6°] and 361 parameters. Crystallographic data have been deposited with the Cambridge Crystallographic Data Center as supplementary publication nos. CCDC-990181 and -990182. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif.

Note

ASSOCIATED CONTENT

S Supporting Information *

Additional experimental details and NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was supported by the European Research Council through an Advanced Grant to H.B. REFERENCES

(1) Braunschweig, H.; Colling, M.; Hu, C.; Radacki, K. Angew. Chem., Int. Ed. 2003, 42, 205−208. (2) (a) Braunschweig, H.; Dewhurst, R. D.; Gessner, V. H. Chem. Soc. Rev. 2013, 42, 3197−3208. (b) Braunschweig, H.; Dewhurst, R. D.; Schneider, A. Chem. Rev. 2010, 110, 3924−3957. (c) Braunschweig, H.; Kollann, C.; Seeler, F. Struct. Bonding (Berlin, Ger.) 2008, 130, 1− 27. (d) Anderson, C. E.; Braunschweig, H.; Dewhurst, R. D. Organometallics 2008, 27, 6381−6389. (3) (a) Alcaraz, G.; Helmstedt, U.; Clot, E.; Vendier, L.; SaboEtienne, S. J. Am. Chem. Soc. 2008, 130, 12878−12879. (b) Vidovic, D.; Aldridge, S. Angew. Chem., Int. Ed. 2009, 48, 3669−3672. (c) Pierce, G. A.; Vidovic, D.; Kays, D. L.; Coombs, N. D.; Thompson, A. L.; Jemmis, E. D.; De, S.; Aldridge, S. Organometallics 2009, 28, 2947−2960. (4) (a) Irvine, G. J.; Rickard, C. E. F.; Roper, W. R.; Williamson, A.; Wright, L. J. Angew. Chem., Int. Ed. 2000, 39, 948−950. (b) Rickard, C. E. F.; Roper, W. R.; Williamson, A.; Wright, L. J. Organometallics 2002, 21, 4862−4872. (5) Bertsch, S.; Bertermann, R.; Braunschweig, H.; Damme, A.; Dewhurst, R. D.; Phukan, A. K.; Saalfrank, C.; Vargas, A.; Wennemann, B.; Ye, Q. Angew. Chem., Int. Ed. 2014, 53, 4240−4243. (6) Bauer, J.; Braunschweig, H.; Dewhurst, R. D. Chem. Rev. 2012, 112, 4329−4346. (7) Braunschweig, H.; Forster, M.; Radacki, K.; Seeler, F.; Whittell, G. R. Angew. Chem., Int. Ed. 2007, 46, 5212−5214. (8) Cordero, B.; Goḿ ez, V.; Platero-Prats, A. E.; Reveś, M.; Echeverría, J.; Cremades, E.; Barragań, F.; Alvarez, A. Dalton Trans. 2008, 2832−2838. (9) Blank, B.; Braunschweig, H.; Colling-Hendelkens, M.; Kollann, C.; Radacki, K.; Rais, D.; Uttinger, K.; Whittell, G. Chem.Eur. J. 2007, 13, 4770−4781. (10) Braunschweig, H.; Kollann, C.; Englert, U. Angew. Chem., Int. Ed. 1998, 37, 3179−3180. (11) Bellachioma, G.; Cardaci, G.; Macchioni, A.; Madami, A. Inorg. Chem. 1993, 32, 554−560.

3651

dx.doi.org/10.1021/om5002414 | Organometallics 2014, 33, 3649−3651