1,2-Halosilane vs. 1,2-alkylborane elimination from (boryl)(silyl) complexes of iron: Switching between borylenes and silylenes just by changing the alkyl group

Article abstract of DOI:10.1039/c5cc06425b

Reaction of different combinations of aryl(dihalo)boranes and trialkylsilyl iron metallates, a route previously used to prepare a terminal iron arylborylene complex, is found to lead to three distinct new reaction outcomes, including unselective decomposition, an inert iron(ii) (boryl)(silyl) complex, and a dinuclear bis(μ-silylene) complex. The latter result is to our knowledge the first example of a 1,2-alkylborane elimination, in contrast to the facile and ubiquitous 1,1-alkylborane elimination observed from (alkyl)(boryl) transition metal complexes, and is also a novel route to bridging silylene complexes.

Full text of DOI:10.1039/c5cc06425b

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1,2-Halosilane vs. 1,2-alkylborane elimination
from (boryl)(silyl) complexes of iron: switching
between borylenes and silylenes just by changing
the alkyl group†
Cite this:DOI: 10.1039/c5cc06425b
Received 31st July 2015,
Accepted 28th August 2015
Holger Braunschweig,* Rian D. Dewhurst, Krzysztof Radacki, Benedikt Wennemann
and Qing Ye
DOI: 10.1039/c5cc06425b
www.rsc.org/chemcomm
Reaction of different combinations of aryl(dihalo)boranes and
trialkylsilyl iron metallates, a route previously used to prepare a
terminal iron arylborylene complex, is found to lead to three distinct
new reaction outcomes, including unselective decomposition, an
inert iron(II) (boryl)(silyl) complex, and a dinuclear bis(l-silylene)
complex. The latter result is to our knowledge the first example of
a 1,2-alkylborane elimination, in contrast to the facile and ubiquitous
1,1-alkylborane elimination observed from (alkyl)(boryl) transition metal
complexes, and is also a novel route to bridging silylene complexes.
The 1,2-elimination of halotriorganylsilanes (SiXR3) from 1-halo-2-
silyl-functionalised molecules is often a highly favored reaction,
due to the creation of a thermodynamically-stable silicon–
halide bond (DHSiX = 565 (F), 381 (Cl), 310 (Br) kJ molꢀ1).1 This
technique, often induced thermally, has been applied throughout
organic and main-group chemistry in order to create multiple
bonds in specific positions (Fig. 1A), such as in the formation of
strained alkenes from 1-halo-2-silylalkanes,2 iminophosphines
(RNQPR0) from P-halo-N-silylaminophosphines,3 iminoboranes
(RNRBR0) from B-halo-N-silylaminoboranes,4 as well as the
related syntheses of transition metal iminoboryl,5 oxoboryl,6
and alkylideneboryl complexes.7
Another facile elimination reaction is the reductive elimina-
tion of an alkylborane from a transition metal center (i.e.
1,1-alkylborane elimination; Fig. 1B). This process is the final
step in the catalytic hydroboration, diboration and C–H boryla-
tion protocols,8 and is such a facile process that only one stable
transition metal (alkyl)(boryl) complex has been isolated.9
However, borane elimination from a two-atom system, analogous
to the 1,2-halosilane elimination, is much more rare,10 and to
Fig. 1 Elimination reactions of relevance to this study.
our knowledge no 1,2-alkylborane elimination (Fig. 1C) has yet provided by the creation of a very strong boron-carbon bond
been observed. This is despite the considerable driving force (DHBC = 372 kJ molꢀ1).1
In 2012 we reported the synthesis of a zerovalent iron
¨
¨
¨
Institut fur Anorganische Chemie, Julius-Maximilians Universitat Wurzburg, Am
borylene complex from the combination of a trialkylsilyl iron
metallate and a bulky aryldihaloborane (bottom, Fig. 1A), via a
presumed tandem salt elimination/1,2-halosilane elimination
process.11 However, recent experiments in our laboratories have
shown that minor alterations of the halide and alkyl groups of the
¨
Hubland, 97074 Wurzburg, Germany. E-mail: h.braunschweig@uni-wuerzburg.de;
Web: http://www-anorganik.chemie.uni-wuerzburg.de/Braunschweig/
† Electronic supplementary information (ESI) available: Full experimental and
crystallographic details. CCDC 1061921–1061923. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c5cc06425b
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starting materials in this reaction lead to vastly different outcomes. previously-published12 iron(II) (boryl)(gallyl) complex mer-
While one combination of iron metallate and halosilane leads [Fe(BClDur)(CO)3(GaCl2)(PMe3)] (dB 113; dP 0.6) indicated that
to unselective decomposition, another leads to the isolation of 3a was a trimethylsilyl derivative thereof: mer-[Fe(BClDur)(CO)3-
an iron(II) (boryl)(silyl) complex, and another to a dinuclear (PMe3)(SiMe3)]. A doublet resonance was also observed in the
bis(m-silylene) complex via the aforementioned unprecedented 29Si NMR spectrum of 3a (dSi 16.7, 2JSiP = 12.0) reflecting 29Si–31P
1,2-alkylborane elimination. These results are reported herein. coupling. A single-crystal X-ray diffraction study of 3a (Fig. 3)
The synthesis of the terminal borylene complex [Fe(BDur)- confirmed its connectivity and showed it to be isostructural to mer-
(CO)3(PMe3)] (4b, Fig. 2; Dur = 2,3,5,6-tetramethylphenyl), [Fe(BClDur)(CO)3(GaCl2)(PMe3)] (however, lacking the dimerisation
reported in 2012 by our group,11 was presumed to follow from
an initial salt elimination step where the trialkylsilyl iron
metallate K[Fe(CO)3(PMe3)(SiMe3)] (1a) attacks the borane
BBr2Dur (2b), leading to the presumed intermediate boryl
complex mer-[Fe(BBrDur)(CO)3(PMe3)(SiMe3)] (3b), followed by
conventional bromosilane elimination. In an attempt to detect
or isolate the presumed iron(II) (boryl)(silyl) intermediate,
the dichloroborane BCl2Dur (2a) was instead treated with 1a,
leading to a brown solid (3a). That the reaction had not led to a
terminal borylene complex was evident from the much more
low-frequency 11B and 31P NMR signals of 3a (dB 114.2; dP 2.4)
compared to those of 4b (dB 146; dP 17.6).11 The similarity of the
11B and 31P NMR data of 3a (dB 114.2; dP 2.4) to that of the
Fig. 3 Crystallographically-derived molecular structure of [Fe(BClDur)-
(CO)3(PMe3)(SiMe3)] (3a), [Fe2(CO)6(PMe3)2(m-SiEt2)2] (6c) and [Fe2(CO)4(m-CO)-
(PMe3)2(m-SiEt2)2] (7c). Thermal ellipsoids drawn at the 50% probability level.
Relevant bond lengths [Å] and angles [deg] for 3a: Fe–P 2.2706(5), Fe–Si
2.4527(5), Fe–B 2.036(2), B–Cl 1.816(2), B–C 1.584(2); Si–Fe–B 92.55(5). For
6c: Fe1–Fe2 3.8758(6), Fe1–Si1 2.3908(5), Fe1–Si2 2.4506(6), Fe1–P1 2.2385(5),
Si1–Si2 2.9019(6); C1–Fe1–C2 161.22(8), Si1–Fe1–P1 178.07(2), Fe1–Si1–Fe2
106.36(2). For 7c: Fe1–Fe2 2.6171(4), Fe1–Si1 2.3399(7), Fe1–Si2 2.3281(6),
Fig. 2 Various outcomes of the reactions of trialkylsilyl iron metallates
1a,b with aryldihaloboranes 2a,b.
Fe1–P1 2.2264(7) Fe1–C1 1.955(2), Si1–Si2 3.2593(9); Si1–Fe1–P1 171.64(3),
Fe2–Si1–Fe1 68.54(2), Fe2–Si2–Fe1 68.09(2).
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caused by the dichlorogallyl ligand of the latter complex). The
The observation of small amounts of [Fe(CO)4(PMe3)] and
phosphine, silyl and boryl ligands of 3a adopt a meridional [Fe(CO)3(PMe3)2] suggest the possibility of reductive elimina-
arrangement, with the phosphine and boryl ligands mutually tion of a silylborane from 3c. Although the silylborane was not
trans. It should be noted also that 3a was not observed to convert observed, it cannot be conclusively ruled out. Oxidative addition
further to a terminal borylene (or any other) complex, even at of a B–Si bond (i.e. the reverse reaction) to zerovalent palladium
elevated temperatures.
or platinum complexes has been calculated by Sakaki and
At this point, we turned our attention back to the presum- coworkers16 to be strongly exothermic and proceed with either
ably more-reactive dibromoborane BBr2Dur (2b), combining it a very small or no activation barrier, in marked contrast to the
with K[Fe(CO)3(PMe3)(SiEt3)] (1b) at room temperature. To our difficult oxidative addition of the relatively inert B–C bond.
surprise, 11B and 31P NMR of the reaction mixture showed a Thus the very small amounts – or complete absence – of reductive
mixture of compounds including what was clearly the iron elimination products from complexes 3a–c can be ascribed to the
(boryl)(silyl) complex 3c based on its effectively identical NMR generally disfavored reductive elimination of silylboranes from
data (dB 114.2; dP 2.3) to that of 3a, as well as a number of other transition metal (boryl)(silyl) complexes.
signals. No NMR signals corresponding to a terminal borylene
Under photolytic conditions, the dinuclear complex 6c was
akin to 4b were observed. However, 3c was found to convert further observed to extrude one carbonyl ligand and form the triply-
over time. Upon reducing the volume of the hexane solution, bridged diiron complex 7c (Fig. 2). This complex showed little
yellow crystals precipitated, which showed single 31P NMR (dP 9.6) change in its 31P NMR data (dP 10.1) from precursor 6c,
and 29Si NMR (dSi 29.8) signals but no appreciable 11B NMR but significant complication of the carbonyl region of its IR
resonance. The identity of the compound was ascertained from spectrum. A massively high-frequency-shifted broadened singlet
single-crystal X-ray diffraction, which showed it to be the dinuclear with unresolved coupling was observed in the 29Si NMR spectrum
bis(m-silylene) complex mer,mer-[Fe2(CO)6(PMe3)2(m-SiEt2)2] (6c, of 7c (dSi 190.44), in comparison to that of its precursor 6c (dSi 29.8).
Fig. 2 and 3; yield 25%). The Fe–Fe (3.8758(6) Å) and Si–Si A single-crystal X-ray diffraction study of 7c (Fig. 3) confirmed its
(2.9019(6) Å) distances of 6c are significantly longer than those structure, which contains significantly shorter Fe–Fe (2.6171(4) Å),
of two previously-published iron bis(m-silylene) octacarbonyl and longer Si–Si (3.2593(9) Å), distances than those of 6c. This also
complexes,13 giving the impression of a much more dilated dictates much more acute Fe–Si–Fe angles in 7c (68.54(2), 68.09(2)1)
Fe2Si2 core in 6c. However, the silylene ligands of 6c are much than in 6c (106.36(2)1), which could explain the large difference
less symmetrically bound to the iron atoms (Fe1–Si1 2.3908(5), in the chemical shifts of the 29Si NMR resonances of the two
Fe1–Si2 2.4506(6) Å) than in the literature complexes. The 29Si complexes. The silylene ligands of 7c are now much more
NMR resonance of 6c (dSi 29.8) was also found to be in line with symmetrically bound, as is the bridging carbonyl ligand.
that of the previously-reported bridging bis(dialkylsilylene)
The photolysis of dinuclear bis(m-silylene) complexes of
the form [Fe2(CO)6L2(m-SiR2)2] to release one carbonyl, leading
complex [Fe(m-SiMe2)(CO)4]2 (dSi 17.8).13
Reduction of the mother liquor from which 6c crystallised to triply-bridged [Fe2(m-CO)(CO)4L2(m-SiR2)2] complexes, is also
under vacuum led to a black solid. Sublimation of this solid gave a well-documented in the literature.17 However, 7c is to our
1
white solid containing the borane BBrEtDur (B80 mol% by H knowledge the first bis(m-silylene) complex with a carbonyl
and 11B NMR; dB 81.1), as well as a small amount of the zerovalent ligand bridging the two metals to be structurally authenticated.
iron complexes [Fe(CO)4(PMe3)] (dP B 34) and [Fe(CO)3(PMe3)2] It should also be noted that the combination of the triethylsilyl
(dP B 39).14 All attempts to isolate BBrEtDur were hampered by complex K[Fe(CO)3(PMe3)(SiEt3)] (1b) with the dichloroborane
cocrystallisation or cosublimation of the aforementioned iron(0) BCl2Dur (2a) led only to the formation of many unidentifiable
phosphine complexes. However, its presence was confirmed by 1H products (Fig. 2).
and 11B NMR spectroscopy of the solid obtained by crystallisation,
Unfortunately, attempts to split the bis(m-silylene) into
as well as GCMS, where peaks corresponding to its hydrolysis mononuclear complexes using large excesses of Lewis bases
product BEtDur(OH) were observed. An attempt to independently (DABCO, 4-dimethylaminopyridine or PMe3) were unsuccessful,
synthesise BBrEtDur by addition of ethyl magnesium bromide to in contrast to published reports.13
BBr2Dur was unsuccessful, but addition of ethyllithium to
In conclusion, the generation of the bis(m-silylene) complex 6c,
BBr2Dur gave a mixture with signals corresponding to those of to our knowledge, is first example of an 1,2-alkylborane elimina-
BEt2Dur and the presumed BBrEtDur, thus providing convincing tion, in marked contrast to the vast precedence of 1,1-alkylborane
evidence for the identity of the latter.
eliminations from transition metals (i.e. reductive eliminations),
The identification of the product 3c from this reaction is a and also represents a novel route to silylene complexes. To our
clear indicator that the initial salt elimination step occurs as in surprise, while methyl substituents on the silyl ligands lead to
the reaction to form the isolated complex 3a. From here, however, halosilane elimination and a borylene complex, ethyl substituents
the reaction pathway deviates from those forming 3a and 4b. The appear to promote alkylborane elimination to the complete exclu-
alkylborane BBrEtDur is eliminated instead of the halosilane sion of halosilane elimination. Overall, the four permutations of
SiBrEt3, leading to the mononuclear terminal silylene complex15 the two different iron metallates (1a,b) and two different dihalo-
5c, which dimerises to form the bridging bis(m-silylene) complex durylboranes (2a,b) led to four unique outcomes: unselective
6c. The dimerisation of terminal silylene and related borylene decomposition, an inert iron(II) (boryl)(silyl) complex, a terminal
complexes has precedence in the literature.13
borylene complex,11 and a dinuclear bis(m-silylene) complex.
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162–165; (b) H. Braunschweig, K. Radacki, D. Rais, A. Schneider and
F. Seeler, J. Am. Chem. Soc., 2007, 129, 10350–10351; (c) H. Braunschweig,
T. Kupfer, K. Radacki, A. Schneider, F. Seeler, K. Uttinger and H. Wu,
J. Am. Chem. Soc., 2008, 130, 7974–7983; (d) H. Braunschweig, F. Matz,
K. Radacki and A. Schneider, Organometallics, 2010, 29, 3457–3462;
(e) J. Brand, H. Braunschweig, R. D. Dewhurst, F. Hupp and K. Lang,
Eur. J. Inorg. Chem., 2015, 2592–2595.
6 Transition metal oxoboryl complexes: (a) H. Braunschweig, K. Radacki
and A. Schneider, Science, 2010, 328, 345–347; (b) S. Bertsch, J. Brand,
H. Braunschweig, F. Hupp and K. Radacki, Chem. – Eur. J., 2015, 16,
6278–6285.
7 Transition metal alkylideneboryl complexes: J. Brand, H. Braunschweig,
F. Hupp, A. K. Phukan, K. Radacki and S. S. Sen, Angew. Chem., Int. Ed.,
2014, 53, 2240–2244.
8 (a) T. B. Marder and N. C. Norman, Top. Catal., 1998, 5, 63–73;
(b) Boronic Acids: Preparation and Applications in Organic Synthesis,
Medicine and Materials, ed. D. G. Hall, Wiley-VCH, Weinheim,
Germany, 2006; (c) I. A. I. Mkhalid, J. H. Barnard, T. B. Marder,
J. M. Murphy and J. F. Hartwig, Chem. Rev., 2010, 110, 890–931;
(d) J. F. Hartwig, Acc. Chem. Res., 2012, 45, 864–873.
9 H. Braunschweig, R. Bertermann, P. Brenner, M. Burzler, R. D. Dewhurst,
K. Radacki and F. Seeler, Chem. – Eur. J., 2011, 17, 11828–11837.
10 T. Yoshida, R. M. Williams and E. Negishi, J. Am. Chem. Soc., 1974,
96, 3688–3690.
11 H. Braunschweig, Q. Ye and K. Radacki, Chem. Commun., 2012, 48,
2701–2703.
12 H. Braunschweig, Q. Ye, A. Vargas, R. D. Dewhurst and F. Hupp,
J. Am. Chem. Soc., 2014, 136, 9560–9563.
13 (a) R. S. Simons, K. J. Galat, J. D. Bradshaw, W. J. Youngs,
A small amount of reductive elimination from one (boryl)(silyl)
complex was also inferred from the detection of simple mono-
valent iron carbonyl–phosphine complexes. The bulky duryl
group appears to be integral to this chemistry, by providing
steric shielding of the boron atom while simultaneously denying
it the p electron density needed to quench its electron deficiency,
thus allowing unusual reactions with small groups. Given the
subtle differences between methyl and ethyl groups, the distinct
reactivity difference observed is surprising. While the proposal of
a mechanism would be premature, the results appear to rule out a
radical mechanism, which would presumably favour a methyl
shift over an ethyl shift in accordance with the noted differences
in the relative willingness of methyl and ethyl groups to undergo
radical processes.18
Notes and references
1 J. E. Huheey, E. A. Keiter and R. L. Keiter, Anorganische Chemie–
¨
Prinzipien von Struktur und Reaktivitat, Walter de Gruyter, Berlin,
New York, 2nd edn, 1995.
2 T. H. Chan and D. Massuda, J. Am. Chem. Soc., 1977, 99, 936–937.
3 M. Alajarin, C. Lopez-Leonardo and J. Berna, Sci. Synth., 2007, 31,
1539–1554.
4 Iminoboranes: (a) P. Paetzold, A. Richter, T. Thijssen and
S. Wu¨rtenberg, Chem. Ber., 1979, 112, 3811–3827; (b) P. Paetzold
and C. von Plotho, Chem. Ber., 1982, 115, 2819–2825; (c) P. Paetzold,
Adv. Inorg. Chem., 1987, 31, 123–170; (d) P. Paetzold, Pure Appl.
Chem., 1991, 63, 345–350; (e) P. Paetzold, Phosphorus, Sulfur, Silicon,
´
C. A. Tessier, G. Aullon and S. Alvarez, J. Organomet. Chem., 2001,
628, 241–254; (b) H. Braunschweig, M. Colling, C. Hu and K. Radacki,
Angew. Chem., Int. Ed., 2002, 41, 1359–1361; (c) K. K. Pandey,
H. Braunschweig and R. D. Dewhurst, Eur. J. Inorg. Chem., 2011,
2045–2056; (d) H. Braunschweig, Q. Ye, A. Vargas, R. D. Dewhurst,
K. Radacki and A. Damme, Nat. Chem., 2012, 4, 563–567.
¨
1994, 93–94, 39–50; ( f ) J. Mu¨nster, P. Paetzold, E. Schroder,
H. Schwan and T. von Bennigsen-Mackiewicz, Z. Anorg. Allg. Chem.,
¨
2004, 630, 2641; (g) H. Noth, Angew. Chem., Int. Ed. Engl., 1988, 27,
14 H. Braunschweig, R. D. Dewhurst, F. Hupp, C. Kaufmann,
A. K. Phukan, C. Schneider and Q. Ye, Chem. Sci., 2014, 5, 4099–4104.
15 (a) R. Waterman, P. G. Hayes and T. D. Tilley, Acc. Chem. Res., 2007,
40, 712–719; (b) B. Blom, M. Stoelzel and M. Driess, Chem. – Eur. J.,
2013, 19, 40–62; (c) B. Blom, D. Gallego and M. Driess, Inorg. Chem.
¨
¨
1603–1623; (h) U. Braun, B. Bock, H. Noth, I. Schwab, M. Schwartz,
S. Weber and U. Wietelmann, Eur. J. Inorg. Chem., 2004, 3612–3628;
¨
¨
(i) D. Mannig, H. Noth, I. Schwab, M. Schwartz, S. Weber and
U. Wietelmann, Angew. Chem., Int. Ed., 1985, 24, 998–999;
( j) E. Bulak, G. E. Herberich, I. Manners, H. Mayer and P. Paetzold,
Angew. Chem., Int. Ed., 1988, 27, 958–959; (k) E. Bulak and P. Paetzold,
Z. Anorg. Allg. Chem., 2000, 626, 1277–1278; (l) K. Delpy, D. Schmitz
and P. Paetzold, Chem. Ber., 1983, 116, 2994–2999; (m) P. Paetzold,
K. Delpy, R. P. Hughes and W. A. Herrmann, Chem. Ber., 1985, 118,
1724–1725; (n) P. Paetzold, K. Delpy and R. Boese, Z. Naturforsch., B:
´
´
Front., 2014, 1, 134–148; (d) L. Alvarez-Rodrıguez, J. A. Cabeza,
´
P. Garcia-Alvarez and D. Polo, Coord. Chem. Rev., 2015, 300, 1–28.
16 S. Sakaki, S. Kai and M. Sugimoto, Organometallics, 1999, 18, 4825–4837.
17 (a) G. Schmid and E. Welz, Z. Naturforsch., 1979, 34b, 929–933;
´
(b) F. H. Carre and J. J. E. Moreau, Inorg. Chem., 1982, 21, 3099–3105.
18 (a) L. M. Tolbert, J. Bedlek, M. Terapane and J. Kowalik, J. Am. Chem.
Soc., 1997, 119, 2291–2292; (b) R. Bertermann, H. Braunschweig,
¨
J. Chem. Sci., 1988, 43, 839–845; (o) B. Krockert, K.-H. van Bonn and
P. Paetzold, Z. Anorg. Allg. Chem., 2005, 631, 866–868.
5 Transition metal iminoboryl complexes: (a) H. Braunschweig,
K. Radacki, D. Rais and K. Uttinger, Angew. Chem., Int. Ed., 2006, 45,
¨
R. D. Dewhurst, C. Horl, T. Kramer and I. Krummenacher, Angew.
Chem., Int. Ed., 2014, 53, 5453–5457.
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