Proton-induced change of the coordination mode of a boron group: Boryl complexes [Mn(CO)4(PR3)(BH2·PMe3) and cationic borane σ complexes [Mn(CO)4(PR3)(η1-BH3 ·PMe3)]

Article abstract of DOI:10.1002/anie.200219992

Mode change on protonation: New borylmanganese complexes 1 undergo protonation to give cationic borane o complexes 2. Heterolytic cleavage of the metal-coordinated B-H bond was observed in the decomposition process of 2.

Full text of DOI:10.1002/anie.200219992

Angewandte
Chemie
Coordination Modes of Boron
Proton-Induced Change of the Coordination
Mode of a Boron Group: Boryl Complexes
[Mn(CO)4(PR3)(BH2·PMe3)] and Cationic
Borane s Complexes [Mn(CO)4(PR3)(h1-
BH3·PMe3)]+**
Takahiro Yasue, Yasuro Kawano,* and
Mamoru Shimoi*
Protonation of hydrido complexes is one of the important
synthetic methods in the chemistry of these interesting
systems.[1–5] However, this methodology has been applied
[*] Dr. Y. Kawano, Prof. M. Shimoi, Dr. T. Yasue
Department of Basic Science
Graduate School of Arts and Sciences
University of Tokyo, Meguro-ku, Tokyo 153-8902 (Japan)
Fax: (+81)3-5454-6567
E-mail: ykawano@jcom.home.ne.jp
cshimoi@mail.ecc.u-tokyo.ac.jp
[**] (PR3 =PMe2Ph, PEt3).
Angew. Chem. Int. Ed. 2003, 42, 1727 – 1730
DOI: 10.1002/anie.200219992
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1727

Communications
sparsely in the preparation of other types of mononuclear s
complexes.[6] We have explored the coordination chemistry of
BH3·PMe3 and reported base-stabilized boryl complexes
(e.g., [Cp*M(CO)3(BH2·PMe3)] (M = Mo, W[7]) and
[Cp*M(CO)2(BH2·PMe3)] (M = Fe, Ru[8])) and borane s
complexes (e.g., [M(CO)5(h1-BH3·PMe3)] (M = Cr, Mo,
W[9]) and [CpMn(CO)2(h1-BH3·PMe3)][10]). Herein, we
report
new
manganese–boryl
complexes,
[Mn(CO)4(PR3)(BH2·PMe3)] (1) and their protonation to
produce cationic borane s complexes, [Mn(CO)4(PR3)(h1-
BH3·PMe3)]+ (2). Heterolytic cleavage of the metal-coordi-
ꢀ
nated B H bond of 2 is also described here.
Photolysis of [MnMe(CO)4(PR3)] with BH3·PMe3 resulted
in the evolution of methane and gave an orange solution, from
which boryl complexes [Mn(CO)4(PR3)(BH2·PMe3)] (1a:
PR3 = PMe2Ph; 1b: PR3 = PEt3) were isolated as pale
yellow crystals in moderate yields (Scheme 1). The
11B NMR spectra of complexes 1a and 1b display a boryl
signal at lower field (d = ꢀ29.4 and ꢀ29.6ppm, respectively)
than that of BH3·PMe3 (d = ꢀ37.0 ppm). The IR spectra of 1
show carbonyl bands shifted to lower energy in comparison to
those of the precursor [MnMe(CO)4(PR3)]. These observa-
Figure 1. Structure of 1a (ORTEP diagram; thermal ellipsoids at the
30% probability level). Selected interatomic distances [Š] and angles
[8]: Mn-B 2.314(2), Mn-P(1) 2.302(1), B-P(2) 1.901(2), B-H(B1) 1.08(2),
B-H(B2) 1.12(2), Mn-C(1) 1.792(2), Mn-C(2) 1.811(1), Mn-C(3)
1.814(1), Mn-C(4) 1.801(1); C(2)-Mn-C(3) 154.62(6), C(4)-Mn-B
171.76(6), Mn-B-P(2) 114.99(9).
ꢀ
ꢀ
tions indicate polarization of the Mn B bond in a Mn(ꢀ)
B(þ) fashion and a resultant increase of electron density on
ꢀ
the metal center. Similar polarization of the M B bond has
been found in phosphane-coordinated primary boryl com-
plexes of Group 6and 8 metals. [7,8] The solid-state structure of
1a (Figure 1)[11] shows that this molecule adopts a highly
distorted octahedral geometry. The phosphane ligand is
located cis to the boryl group. The manganese–boron bond
length (2.314(2) Š) is substantially longer than that in
the catecholboryl complex [Mn(CO)5{B(1,2-O2C6H4)}]
(2.108(6) Š)[12] because of the absence of a vacant p orbital
on the boron center that can be utilized for p interaction with
the metal center. Two of the cis-carbonyl groups significantly
tilt toward the boryl group. The C(2)-Mn-C(3) bond angle is
ꢀ
and the Mn B bond becomes longer. Note that more p-acidic
ligands prefer to be located at equatorial positions in trigonal-
bipyramidal complexes. In the [Mn(CO)4(PR3)] fragment, p-
acidic carbonyl ligands occupy the equatorial positions, and a
less p-acidic phosphane ligand is situated at an apical position,
which is cis to the boryl group in 1.
The boryl complexes were protonated by treating 1 in
[D2]dichloromethane
with
the
Brønsted
acid
ꢀ
154.62(6)8. Owing to the pronounced Mn(ꢀ) B(þ) polar-
[H(OEt2)2](TFPB) (TFPB = [B{3,5-C6H3(CF3)2}4]), which
has a weakly coordinating anion.[13] The resulting pale
yellow solutions showed a broad BH resonance signal
ization mentioned above, the nature of compounds 1 closely
resembles a contact ion pair composed of [Mn(CO)4(PR3)]ꢀ
and [BH2·PMe3]+; the anion-like manganese moiety is iso-
electronic with Fe(CO)5. Consequently, the geometry of the
[Mn(CO)4(PR3)] moiety approaches a trigonal bipyramid,
1
around d = ꢀ4.5 ppm in the H NMR spectra. The 11B NMR
spectrum of the product displayed a doublet of quartets at
higher field (d = ꢀ40.3 to ꢀ40.4 ppm) than that of free
BH3·PMe3. The appearance of these signals clearly
shows the formation of borane
[Mn(CO)4(PR3)(h1-BH3·PMe3)](TFPB)
s
complexes
(2, see
Scheme 1). The value of the chemical shift of the 11B
NMR signal falls in the range of those for s complexes
of BH3·PMe3.[9,10] The 1H NMR signal around d =
ꢀ4.5 ppm is assigned to the BH resonance; the 1H
NMR signals of the metal-coordinated and terminal BH
protons are averaged through fast site-exchange. This
process was not frozen out even at ꢀ808C. Similar
fluxional behavior has been found in other complexes of
phosphaneboranes.[9,10,14,15] Complexes
2 were also
generated by methyl abstraction from [MnMe-
(CO)4(PR3)] using [H(OEt2)2](TFPB), followed by
addition of BH3·PMe3. These complexes have a lifetime
of a few days, and can be observed by spectroscopy;
however, they could not be isolated in pure forms.
Scheme 1. Syntheses of complexes 1 and 2.
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Angewandte
Chemie
Figure 2 shows the DFT-optimized structure of the model
compound [Mn(CO)4(PH3)(h1-BH3·PMe3)]+ (2c).[16,17] It
resembles the structure of [M(CO)5(h1-BH3·PMe3)] (M = Cr,
W) except for the metal-coordinated phosphane ligand. The
Mn···B interatomic distance (2.780 Š) is far longer than that
of 1a, and the Mn-H-B bond angle is 133.448. Thus, the mode
of coordination for the borane ligand is essentially end-on.
atom of free BH3·PMe3 is + 0.002. At the same time, that of
the “BH2·PMe3” group increases from ꢀ0.002 to þ0.311
(Scheme 2). The electron density of the BH3·PMe3 ligand is
thus withdrawn toward the bridging hydrogen by the highly
electrophilic [Mn(CO)4(PR3)]+ ion in the cationic borane
complexes.
ꢀ
ꢀ
The B H(1) bond length is 1.270 Š, and the B H s bond is
elongated by about 6% on coordination.
Scheme 2. NBO charge distribution in free BH3·PMe3 (left) and 2c
(right).
Experimental Section
1a: A mixture of [MnMe(CO)4(PMe2Ph)] (247 mg, 0.75 mmol) and
BH3·PMe3 (203 mg, 2.28 mmol) in hexane (10 mL) was photolyzed at
38C for 90 min using a 450 W medium-pressure Hg arc lamp. The
resulting solution was evaporated and evacuated for 1 h to remove
excess BH3·PMe3. Recrystallization of the solid residue from hexane
at ꢀ808C provided pale yellow crystals of 1a (90 mg, 31%).
Compound 1b was obtained in an analogous manner in 15% yield.
Data for 1a: 1H NMR (500 MHz, [D6]benzene, 238C, TMS): d =
2
2
0.88 (d, J(P,H) = 10.0 Hz, 9H; PMe3), 1.49 (d, J(P,H) = 8.5 Hz, 6H;
PMe2Ph), 7.02, 7.09, 7.36ppm (m, 5H; PMe 2Ph), the BH proton
signals were too broad to be observed; 11B NMR (160.4 MHz,
[D6]benzene, 238C, BF3·OEt2): d = ꢀ29.4 ppm (dt, 1J(B,H) =
105.3 Hz, 1J(B,P) = 73.5 Hz); 31P NMR (202.4 MHz, [D6]benzene,
238C, 85% H3PO4): d = 27.3 (br; PMe2Ph), 0.9 ppm (br; PMe3);
13C NMR (125.7 MHz, [D6]benzene, 238C, TMS): d = 13.0 (d,
1J(C,P) = 36.7 Hz; PMe3), 16.6 (d, 1J(C,P) = 29.3 Hz; PMe2Ph),
129.0, 129.4 (d, 1J(C,P) = 7.3 Hz), 140.7 (d, 1J(C,P) = 38.1 Hz) (Ph),
219.9, 226.8, 227.9 ppm (CO); IR (KBr): n˜ = 1893.8 (vs), 1906.3 (vs),
Figure 2. DFT-optimized structure of 2c. Selected interatomic distan-
ces [Š] and angles [8]: Mn···B 2.780, Mn-H(1) 1.753, B-H(1) 1.270,
B-H(2) 1.200, B-H(3) 1.200, B-P(2) 1.950, Mn-P(1) 2.380, Mn-C(1)
1.840, Mn-C(2) 1.880, Mn-C(3) 1.880, Mn-C(4) 1.840; Mn-H(1)-B
133.44, P(1)-Mn-C(1) 176.80.
1925.6(vs), 2008.5 (s) (C O), 2357 (w) (BH) cmꢀ1; MS (EI, 70 eV):
¼
m/z (%): 394 (12) [M+], 366 (89) [M+ꢀCO], 338 (24) [M+ꢀ2CO], 320
(60) [M+ꢀPMe3], 310 (30) [M+ꢀ3CO], 282 (100) [M+ꢀ4CO];
elemental analysis (%) calcd for C15H22BMnO4P2: C 45.72, H 5.63;
found: C 45.64; H, 5.57.
Borane s complexes 2 are formally the conjugate acids of
1. However, deprotonation from 2 did not occur even when
they were treated with bases such as NaH and diazabicy-
cloundecene. On the other hand, a solution of 2 decomposed
in a few days at room temperature to give a mixture
containing [MnH(CO)4(PR3)] and [BH2·2PMe3]+, although
the decomposition process was not very clean. This suggests
Data for 1b: 1H NMR (500 MHz, [D6]benzene, 238C, TMS): d =
0.91 (dt, 3J(H,H) = 7.5 Hz, 3J(P,H) = 15.5 Hz, 9H; P(CH2CH3)3), 0.92
2
3
2
(d, J(P,H) = 10.5 Hz, 9H; PMe3), 1.59 ppm (dq, J(H,H) ꢁ J(P,H) =
7.5 Hz, 6H; P(CH2CH3)3), the BH proton signals were too broad to be
observed; 11B NMR (160.4 MHz, [D6]benzene, 238C, BF3·OEt2): d =
ꢀ29.6ppm (dt, 1J(B,H) = 106.3 Hz, 1J(B,P) = 71.2 Hz); 31P NMR
(202.4 MHz, [D6]benzene, 238C, 85% H3PO4): d = 43.0 (br; PEt3),
0.7 ppm (br; PMe3); 13C NMR (125.7 MHz, [D6]benzene, 238C,
ꢀ
the coordinated B H s bond of complexes 2 cleaves hetero-
lytically into Hꢀ and “[BH2·PMe3]+”.[18] Recently, Kubas and
co-workers reported similar heterolytic cleavage of H2 and
silanes on the cationic manganese or rhenium fragments
[M(CO)5ꢀn(PR3)n]+ (M = Mn, Re, n = 1, 2).[19] The electron-
deficient metal centers undergo strong s donation from the
H–H, Si–H, as well as B–H s orbitals, but backdonation into
the corresponding s* orbitals hardly occurs. Therefore, the
electron density of these s ligands is significantly reduced and
the metal-coordinated s bond is activated heterolytically. The
natural bond orbital (NBO) analysis based on the aforemen-
tioned DFT calculations on 2c shows that the bridging
hydrogen atom of the borane ligand becomes more hydridic
on coordination to the cationic manganese center.[20] Its
natural charge is ꢀ0.034, whereas that of the B–H hydrogen
1
TMS): d = 7.5 (P(CH2CH3)3), 12.9 (d, J(C,P) = 37.1 Hz; PMe3), 19.0
(d, 1J(C,P) = 24.6Hz; P( CH2CH3)3), 129.0, 129.4 (d, 1J(C,P) = 7.3 Hz),
140.7 (d, 1J(C,P) = 38.1 Hz) (Ph), 220.5, 227.8, 228.0 ppm (br; CO); IR
ꢀ1
¼
(KBr): n˜ = 1889 (vs), 1910 (vs), 2004 (vs) (C O), 2357 (w) (BH) cm
;
MS (EI, 70 eV): m/z (%): 374 (5) [M+], 346(24) [ M+ꢀCO], 318 (5)
[M+ꢀ2CO], 290 (3) [M+ꢀ3CO], 262 (100) [M+ꢀ4CO], 173 (27)
[Mn(PEt3)+], 144 (60) [Mn(BH2·PMe3)+]; elemental analysis (%)
calcd for C13H26BMnO4P2: C 41.74, H 7.01; found: C 41.57, H 6.85.
2a: Compound 1a (41 mg, 0.10 mmol) and [H(OEt2)2](TFPB)
(362 mg, 0.36 mmol) were combined in dichloromethane (10 mL)
under vacuum. After the mixture had been stirred for 3 h, volatiles
were evaporated to dryness. The 1H, 11B, and 31P NMR spectra of the
resulting yellow residue indicated complete consumption of 1a and
Angew. Chem. Int. Ed. 2003, 42, 1727 – 1730
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Communications
displayed new signals assignable to 2a. Alternatively, 2a was cleanly
[14] R. Macías, N. P. Rath, L. Barton, Angew. Chem. 1999, 111, 203;
Angew. Chem. Int. Ed. 1999, 38, 162.
[15] M. Ingleson, N. J. Patmore, G. D. Ruggiero, C. G. Frost, M. F.
Mahon, M. C. Willis, A. S. Weller, Organometallics 2001, 20,
4434.
[16] The stationary point was located at the B3LYP level of theory,
by using density functional theory calculations based on hybrid
functionals. A double z plus polarization valence basis set, 6-
31G(d) was employed for all the atoms except manganese. For
Mn, a basis set with an approximation of effective core
potentials, LANL2DZ was applied. The NBO analysis was
carried out on the resulting structure at the B3LYP level using a
6–311 + G(2d,p) basis set.[20] All calculations were performed
with the Gaussian 98W package of programs.[17]
generated by addition of BH3·PMe3 to a solution of [Mn(CO)4(PMe2-
Ph)(OEt2)]+, which was produced by the reaction of [MnMe(CO)4(P-
Me2Ph)] with [H(OEt2)2](TFPB) in diethyl ether. The PEt3 derivative
2b was prepared by similar methods.
Data for 2a: 1H NMR (500 MHz, [D2]dichloromethane, 238C,
TMS): d = ꢀ4.48 (br, 3H; BH), 1.32 (d, 2J(P,H) = 11.5 Hz, 9H; PMe3),
2
1.98 (d, J(P,H) = 9.0 Hz, 6H; PMe2Ph), 7.57, 7.72 ppm (s, 1H, 2H;
[B{C6H3(CF3)2}4]); 11B NMR (160.4 MHz, [D2]dichloromethane,
238C, BF3·OEt2): d = ꢀ40.4 (dq, 1J(B,H) = 81 Hz, 1J(B,P) = 70 Hz;
BH2·PMe3), ꢀ6.7 ppm (s; TFPB); 31P NMR (202.4 MHz,
[D2]dichloromethane, 238C, 85% H3PO4): d = ꢀ4.0 (br; PMe3),
15.0 ppm (br; PMe2Ph); MS (FAB, sulfolane): m/z (%): 367 (10)
[M+ꢀCO], 193 (100) [Mn(PMe2Ph)+], 139 (43) [PHMe2Ph+].
Data for 2b: 1H NMR (500 MHz, [D2]dichloromethane, 238C,
TMS): d = ꢀ4.43 (br, 3H; BH), 1.10–1.22 (m, 9H; P(CH2CH3)3), 1.35
(d, 2J(P,H) = 11.5 Hz, 9H; PMe3), 1.72–1.80 (m, 6H; P(CH2CH3)3),
7.57, 7.72 ppm (s, 1H, 2H; [B{C6H3(CF3)2}4]); 11B NMR (160.4 MHz,
[D2]dichloromethane, 238C, BF3·OEt2): d = ꢀ40.3 (br), ꢀ6.7 ppm (s;
TFPB); 31P NMR (202.4 MHz, [D2]dichloromethane, 238C, 85%
H3PO4): d = ꢀ5.3 (br; PMe3), 48.7 ppm (br; PEt3); MS (FAB,
sulfolane): m/z (%): 173 (100) [Mn(PEt3)+], 120 (28) [PHEt3+].
[17] Gaussian98 (RevisionA.11), M. J. Frisch, G. W. Trucks, H. B.
Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G.
Zakrzewski, J. A. Montgomery, R. E. Stratmann, J. C. Burant, S.
Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain,
O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B.
Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A.
Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J.
V. Ortiz, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A.
Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M.
Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen, M. W.
Wong, J. L. Andres, M. Head-Gordon, E. S. Replogle, J. A.
Pople, Gaussian, Inc., Pittsburgh, PA, 2001.
Received: August 19, 2002
Revised: December 3, 2002 [Z19992]
Keywords: boranes · boron · coordination modes · manganese ·
.
protonation
[18] Several boryl cations with the formula [BX2·L]+ (L = neutral
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John, G. J. Kubas, Organometallics 2000, 19, 4141.
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ꢀ
[11] Crystal data for 1a: pale yellow crystals, triclinic, space group P1
(no. 2); T= 150 K; a = 10.5746(8), b = 10.6779(9), c =
9.0029(7) Š; a = 109.026(2), b = 96.043(3), g = 88.092(2)8; V=
955.69(13) Š3; Z = 2, R = 0.024, wR2 = 0.060 for 4070 reflections
with j Fo j > 3s(Fo), 296parameters, GoF = 1.110. The boron-
attached hydrogen atoms were found by the difference Fourier
syntheses and their positions were refined. CCDC-190001
contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-
bridge Crystallographic Data Centre, 12, Union Road, Cam-
bridge CB21EZ, UK; fax: (+ 44)1223-336-033; or deposit@
ccdc.cam.ac.uk).
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1730
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Angew. Chem. Int. Ed. 2003, 42, 1727 – 1730