Isolation of an intermediate in the insertion of a carbodiimide into a boron-aryl bond

Article abstract of DOI:10.1039/b511080g

A 1 : 1 Lewis acid-base complex between CyN=C=NCy and PhBCl₂ has been isolated and structurally characterized, heating of which in refluxing toluene results in the amidinate, [PhC{NCy}₂]BCl₂; the overall reaction has been

Full text of DOI:10.1039/b511080g

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Isolation of an intermediate in the insertion of a carbodiimide into a
boron-aryl bond{
Nicholas J. Hill, Jennifer A. Moore, Michael Findlater and Alan H. Cowley*
Received (in Columbia, MO, USA) 3rd August 2005, Accepted 13th September 2005
First published as an Advance Article on the web 3rd October 2005
DOI: 10.1039/b511080g
of the carbodiimide, N(1), forms a donor–acceptor bond of length
˚
1.583(3) A to the boron atom of PhBCl2. As a consequence of the
A 1 : 1 Lewis acid–base complex between CyNLCLNCy and
PhBCl2 has been isolated and structurally characterized,
heating of which in refluxing toluene results in the amidinate,
[PhC{NCy}2]BCl2; the overall reaction has been modeled by
DFT calculations.
donor action on the part of the carbodiimide, the C(13)–N(1)–C(7)
angle becomes more acute than the corresponding angle at N(2)
˚
and the C(13)–N(1) bond distance is y 0.12 A longer than that of
the C(13)–N(2) bond. Although there is some departure from the
ideal angles, the geometry at N(1) is trigonal planar (sum of
angles 5359.9(2)u). The donor function is also evident from the fact
that the sum of bond angles at B(1) is 334.0(2)u. In the case of
B(C6F5)3 as a reference Lewis acid, the departure of the sum of
bond angles at boron from the ideal value of 360u has been taken
as a measure of donor strength of the Lewis base.12,13 Another
significant structural feature is the close proximity of the ipso
carbon of the phenyl ring to the carbodiimide carbon, a conforma-
tion that favours the migration of the phenyl group from B(1) to
Despite the fact that aluminium and gallium amidinates can serve
as olefin polymerization catalysts1 and as precursors for thin film
deposition,2 very little information is available regarding the
congeneric amidinates of boron.3 We have therefore embarked
on synthetic, structural and reactivity studies of this class of
compound.4 One of the potential routes to the desired compounds
features the insertion of a carbodiimide into a boron–carbon or
boron–heteroatom bond of a boron(III) derivative. In fact,
insertion reactions of this type represent part of a broader tapestry
of heterocumulene insertion processes that includes commercially
and environmentally important CO2. In an early mechanistic study
of the insertion of a variety of heterocumulenes into Zr–carbon
bonds, Gambarotta et al.5 proposed that the prerequisite initial
step involves coordination of one of the terminal heteroatoms to
the Lewis acidic zirconium centre. A similar conclusion has since
been drawn by others.6 Additional support for this proposal has
also been forthcoming from theoretical studies of the insertion
of CO2 into the Rh(III)–H bond7 and of carbodiimide insertions
into Al–Me and Al–NMe2 bonds.8 Interestingly, and in contrast
to Al2(NMe2)6 and aluminium alkyls, our attempts to insert
CyNLCLNCy (Cy 5 cyclohexyl) into B(NMe2)3 and BEt3 were
not successful. However, we have now succeeded in the isolation
and structural characterization of an example of the putative initial
Lewis acid–base complex (1) from the reaction of PhBCl2 with
CyNLCLNCy, subsequent heating of which in toluene furnishes the
amidinate complex, [PhC{NCy}2]BCl2 (2). The experimental work
is supported by a DFT study of the overall reaction mechanism.
To the best of our knowledge, the present work represents (a) the
first example of the preparation of a boron amidinate by means of
carbodiimide insertion,9 and (b) the first example of such a process
occurring at a p-block element/aryl bond.
…
C(13) (the C(1) C(13) distance is 2.796(7) u and the C(1)–B(1)–
N(1)–C(13) torsion angle is 7.5u). Upon migration of the phenyl
group, formation of amidinate 210 is accomplished by means of a
rotation of the N(1)–C(13) bond followed by ring closure. Despite
making several attempts, we were unable to isolate or detect the
second intermediate by 11B NMR. Compound 2 was prepared
independently via the salt metathesis reaction of [Ph{NCy}2Li]
with BCl3 and characterized by X-ray crystallography.11
Treatment of PhBCl2 with an equimolar quantity of
CyNLCLNCy in hexane solution at ambient temperature results,
after isolation and crystallization, in a 96% yield of 1.10 The X-ray
crystal structure of 1 (Fig. 1) reveals that one of the nitrogen atoms
Fig. 1 View of 1 showing the atom numbering scheme and thermal
ellipsoids at 40% probability with hydrogen atoms omitted for clarity.
˚
Selected bond distances (A) and angles (u): N(1)–B(1) 1.583(3), B(1)–C(1)
Department of Chemistry and Biochemistry, The University of Texas at
Austin, 1 University Station A5300, Austin, Texas, 78712, USA.
E-mail: cowley@mail.utexas.edu; Fax: (+1) 512-471-6822;
Tel: (+1) 512-471-7484
{ Electronic supplementary information (ESI) available: details of DFT
calculations. See DOI: 10.1039/b511080g
1.589(3), B(1)–Cl(1) 1.887(3), B(1)–Cl(2) 1.874(3) N(1)–C(13) 1.282(3),
N(1)–C(7) 1.501(3), N(2)–C(13) 1.166(3), N(2)–C(14) 1.468(3), C(7)–N(1)–
C(13) 116.91(17), C(13)–N(1)–B(1) 119.68(18), C(7)–N(1)–B(1) 123.34(17),
C(13)–N(2)–C(14) 142.8(2), C(1)–B(1)–Cl(1) 112.99(17), C(1)–B(1)–Cl(2)
113.21(17), Cl(1)–B(1)–Cl(2) 107.78(13).
5462 | Chem. Commun., 2005, 5462–5464
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Fig. 2 (a) Structures and energies (kcal mol21) of intermediates IM1 and IM2, transition states TS1 and TS2, and amidinate 3 as calculated by DFT. (b)
Graphical representation of the calculated energy profile of the reaction between PhBCl2 and MeNLCLNMe.
To gain additional mechanistic insights, the overall process was
modeled by DFT calculations.14–16 In the interest of computa-
tional efficiency, the carbodiimide MeNLCLNMe was used instead
of CyNLCLNCy. The profile for the reaction of MeNLCLNMe
with PhBCl2 features two intermediates (IM1 and IM2) and two
transition states (TS1 and TS2) leading to product 3 as depicted in
Fig. 2. The overall reaction is exergic by 245.5 kcal mol21. The
first intermediate, IM1, is a donor–acceptor complex between
MeNLCLNMe and PhBCl2. Comparison of the metrical para-
meters for 1 with those of the model complex, IM1, reveals that
they are in good agreement (,2%) with the exception of the NAB
bond distance which is overestimated by 3.7%. The first transition
state, TS1, features a four-centre interaction between boron,
nitrogen, the carbodiimide carbon and the ipso carbon of the
general features of this mechanism are similar to those discussed
for the insertion of heterocumulenes into Rh–H,7 Al–C8 and
Al–N8 bonds.
In summary, we have isolated and structurally characterized a
1 : 1 Lewis acid–base complex of CyNLCLNCy and PhBCl2.
Complexes of this general type have long been postulated as
representing the first step in a variety of heteroallene insertion
reactions. In the present case the acid–base complex 1 is readily
converted to the boron amidinate 2 upon heating in toluene
solution. The overall mechanism has been modeled satisfactorily
by DFT calculations.
We are grateful to the National Science Foundation (Grant
CHE-0240008) and the Robert A. Welch Foundation (Grant
F-135) for support.
…
˚
phenyl ring (C C 5 1.871 A). The phenyl migration is completed
by cleavage of the Ph–B bond in concert with mutation of the
NAB dative bond into an N–B s-bond. Transition state TS2,
which is lower in energy than TS1, involves rotation around a
C–N bond such that the two nitrogen atoms attain a conformation
that will permit chelation. Finally, chelation of both nitrogen
atoms to the BCl2 moiety completes the formation of the boron
amidinate, [PhC{NMe}2]BCl2, (3). We also computed the 11B
chemical shifts for IM1, IM2, and 3. The calculated (GAIO) values
of d 10.5 and 8.5 for IM1 and 3, respectively, are in good
agreement with those obtained experimentally for 1 and 2, while
the calculated value of d 33.8 for IM2 falls within the range
observed for similar three-coordinate boron compounds.17 The
Notes and references
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4 N. J. Hill, M. Findlater and A. H. Cowley, Dalton Trans., 2005, 3329.
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6 (a) P. Braunstein and D. Nobel, Chem. Rev., 1989, 89, 1927; (b)
J. A. Tunge, C. J. Czerwinski, D. A. Gately and J. R. Norton,
Organometallics, 2001, 20, 254.
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 5462–5464 | 5463

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7 Y. Musashi and S. Sakaki, J. Chem. Soc., Dalton Trans., 1998, 577.
8 C. N. Rowley, G. N. Dilabio and S. T. Berry, Inorg. Chem., 2005, 44,
1983.
9 We have recently prepared [FcC{NCy}2]BBr2 by this method
(Fc 5 (C5H5)2Fe) (ref. 4).
carbodiimide, in agreement with the observed lack of reaction between
these two compounds, even in refluxing toluene solution.
14 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, J. A. Montgomery, Jr, T. Vreven, K. N. Kudin,
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B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson,
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X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi,
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Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M W. Gill,
B. Johnson, W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople,
GAUSSIAN 03, (Revision B.04), Gaussian, Inc., Pittsburgh PA,
2003.
15 All calculations were performed at the RB3LYP level of theory
and 6-311++G(d,p) basis set using the Gaussian 03 suite of programs.
Points on the potential energy surface were confirmed by the calculation
of the vibrational frequencies, with no imaginary frequencies for
IM1, IM2 and 3 and one imaginary frequency for TS1 and TS2.
Graphical representations of the calculated molecular orbitals were
produced using the Molden program16 and the POV-ray windows
program.
10 Spectroscopic data for 1 (1.71 g, 97% yield): 1H NMR (C6D6): d 7.39–
7.19 (s, 5H); 3.13 (br s, 2H); 2.17 (br s, 4H); 1.68–0.95 (br m, 16H); 13C
NMR (C6D6): d 154.34; 136.28; 133.93; 133.11; 58.52; 55.68; 33.95;
32.57; 31.26; 26.51; 26.37; 25.38; 25.04; 24.40; 11B NMR (C6D6): d 8.17
(s). MS (CI+, CH4): m/z 364 (M+), 207 (100%, CyNLCLNCy + H+);
HRMS (CI, CH4) calcd for C19H27N2BCl2, 364.1644; found 364.1671.
Spectroscopic data for 2 (1.03 g, 88% yield): 1H NMR (C6D6): d 3.49 (s,
2H); 2.09 (s, 4H); 1.67–1.45 (m, 10H); 1.14–1.05 (m, 6H); 13C{1H}
NMR (C6D6): d 176.17; 132.55; 129.79; 127.99; 54.95; 33.70; 27.23;
25.94; 11B NMR (C6D6): d 6.25 (s). MS (CI+, CH4): m/z 364 (M+), 329
(100%, M–Cl); HRMS (CI, CH4) calcd for C19H27N2BCl2, 364.1644;
found 364.1642.
11 Crystal data for 1: C19H27BCl2N2, monoclinic, space group C2/c,
˚
a 5 37.418(5), b 5 6.561(5), c 5 15.964(5) A, b 5 100.318(5)u, V 5
3856(3) A , Z 5 4, Dc 5 1.258 g cm23, m (Mo–Ka) 5 0.340 mm21
,
R1 5 0.0480, wR2 5 0.1018, GOF 5 1.031. For 2: C19H27BCl2N2,
3
˚
¯
˚
triclinic, space group P1, a 5 10.355(5), b 5 11.914(5), c 5 16.592(5) A,
3
˚
a 5 73.933(5), b 5 83.657(5), c 5 80.431(5)u, V 5 1935.1(14) A , Z 5 2,
Dc 5 1.250 g cm23, m (Mo–Ka) 5 0.338 mm21, R1 5 0.0562, wR2 5
0.1362, GOF 5 0.979. Both data sets were collected at 153(2) K on a
Nonius-Kappa CCD diffractometer. There are two independent
molecules in the asymmetric unit of 2. CCDC 278056 (1) and 278734
(2). For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b511080g.
16 G. Shaftenaar, MOLDEN 3.4; CAOS/CAMM Center Nijmegen:
Toernooiveld, Nijmegen, The Netherlands, 1991.
17 H. No¨th and B. Wrackmeyer in Nuclear Magnetic Resonance
Spectroscopy of Boron Compounds, Springer Verlag, Berlin, 1978, ch. 7,
pp. 74–101.
12 H. Jacobsen, H. Berke, S. Do¨ring, G. Kehr, G. Erker, H. Fro¨hlich and
O. Meyer, Organometallics, 1999, 18, 1724.
13 Preliminary DFT modeling of the B(NMe2)3/CyNLCLNCy system
suggests that there is no interaction between the amidoborane and the
5464 | Chem. Commun., 2005, 5462–5464
This journal is ß The Royal Society of Chemistry 2005