Highly basic alkyl-substituted bis(benzhydryl) CaII and YbII complexes with β-CH–M agostic interactions

Article abstract of DOI:10.1016/j.mencom.2021.04.017

The reaction between potassium derivative {[(4-Bu^tC₆H₄)₂CH]^?K⁺}_n and alkyl iodides gives new 1,1-bis(4-tert-butylphenyl)alkanes which can be deprotonated with BuⁿNa to produce species [(4-Bu^tC₆H₄)₂CR]^?Na⁺ (R = Me, Buⁱ). The salt metathesis of the latter with base-free CaI₂ or YbI₂ affords novel bis(hydrocarbyl) complexes [(4-Bu^tC₆H₄)₂CR]₂M(TMEDA) (M = Ca, R = Me; M = Yb, R = Buⁱ). The hyperconjugation of the central carbanion with alkyl groups pronouncedly enhances the basic properties of the compounds which would perform CH bond activation in toluene and thiophene, in distinction to the related ‘nonalkylated’ analogues with (4-Bu^tC₆H₄)₂CH^? ligand.

Full text of DOI:10.1016/j.mencom.2021.04.017

Mendeleev
Communications
Mendeleev Commun., 2021, 31, 334–336
Highly basic alkyl-substituted bis(benzhydryl) Caii andYbii complexes
with b-CH–M agostic interactions
Alexander N. Selikhov,a,b Anton V. Cherkasov,a Yulia V. Nelyubinab and Alexander A. Trifonov*a,b
a G. A. Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences,
603950 Nizhnii Novgorod, Russian Federation. E-mail: trif@iomc.ras.ru
b A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
119991 Moscow, Russian Federation
DOI: 10.1016/j.mencom.2021.05.017
The
reaction
between
potassium
derivative
But
{[(4-ButC6H4)2CH]–K+}n and alkyl iodides gives new
1,1-bis(4-tert-butylphenyl)alkaneswhichcanbedeprotonated
with BunNa to produce species [(4-ButC6H4)2CR]–Na+
(R = Me, Bui). The salt metathesis of the latter with base-free
CaI2 or YbI2 affords novel bis(hydrocarbyl) complexes
[(4-ButC6H4)2CR]2M(TMEDA) (M = Ca, R = Me; M = Yb,
R = Bui). The hyperconjugation of the central carbanion
with alkyl groups pronouncedly enhances the basic properties
of the compounds which would perform CH bond activation
in toluene and thiophene, in distinction to the related ‘non-
alkylated’ analogues with (4-ButC6H4)2CH– ligand.
R
Me
Me
–
N
Highly-basic bis(hydrocarbyl)
complexes
N
M
But
Me Me
But
M = YbII, CaII
R = Me, Bui
R
–
But
Keywords: rare-earth metals, alkaline earth metals, calcium complexes, ytterbium complexes, alkyl ligands, agostic interactions, CH
bond activation.
Highly reactive hydrocarbyl derivatives of large and
electropositive Mii rare- and heavy alkaline-earth metals
despite their powerful potential in numerous catalytic
applications1 still remain poorly explored.2 The presence of
highly reactive M–C bonds which readily undergo insertions
into multiple C=C and C=E bonds,3 σ-bond metathesis4 and
enable activation of normally inert CH-bonds5 provides unique
reactivity of these complexes and makes them exciting objects
of research. However, the development of this field is slowed
down by the extremely high air-sensitivity of the complexes as
well as by the synthetic problems associated with the difficulties
in stabilization of the coordination sphere of large metal ions6
prone to ligand redistribution and b-hydrogen elimination or
b-hydrogen abstraction of hydrocarbyl ligands.7 Rational
selection of the sterically demanding silyl-8 and aryl-substituted
methanide ligands9 lacking b-hydrogen atoms allowed one to
expand significantly the family of Mii alkyl complexes.2
Recently the synthesis of extremely reactive methyl
ytterbium(ii)10 and calcium(ii)11 complexes was reported.
Scarce Caii species with hydrocarbyl fragments containing
hydrogen atoms1(e),12 in b-position to metallocentre were
obtained by addition of calcium hydrides at C=C bonds.12
However, no Ybii analogues are known so far. Successful
application of sterically demanding benzhydryl anion
[(4-ButC6H4)2CH]– in Lnii, Lniii and Caii chemistry made it
possible to obtain isolable thermally stable hydrocarbyl
complexes.13 Cationic benzhydryl Lniii complexes proved to be
highly efficient, regio- and chemoselective catalysts for
hydroarylation and hydrobenzylation of C=C bonds of a variety
of substrates with substituted pyridines.14 Herein, we report on
the synthesis, structures and reactivity of Ybii and Caii
complexes with highly basic carbanions of type
[(4-ButC6H4)2C–R]2M2+(TMEDA) (M = Ca, Yb).
1,1-Bis(4-tert-butylphenyl)ethane 1a and 1,1-bis(4-tert-
butylphenyl)-4-methylbutane 1b were synthesized by nucleo-
philic substitution reaction of {[(4-ButC6H4)2CH]–K}n with the
appropriate alkyl iodides (Scheme 1). After flash chromato-
graphy on silica gel, hydrocarbons 1a,b were isolated in 82–90%
yields. Deprotonation of these diarylmethanes 1a,b with BunNa15
(24 h for 1a and 100 h for 1b) and subsequent salt metathesis
of the resulting [(4-ButC6H4)2CR]–Na+ with base-free MI2
(2:1 molar ratio, TMEDA–PhH) allowed for the
preparation of [(4-ButC6H4)2CMe]2Ca(TMEDA) 2a and
[(4-ButC6H4)2CBui]2Yb(TMEDA) 2b as orange and bloody-red
But
But
But
Me
R
Me
N
N
i, ii
iii, iv
M
R
But
Me Me
But
R
But
But
But
1a,b
2a,b
a R = Me, 82%
b R = Bui, 90%
a M = Ca, R = Me, 53%
b M = Yb, R = Bui, 85%
Scheme 1 Reagents and conditions: i, BunLi, ButOK, hexane, 0 ® 20 °C;
ii, RI, THF, 0 °C; iii, BunNa, TMEDA, hexane, room temperature, 24–100 h;
iv, 1/2 MI2, TMEDA, PhH, room temperature.
© 2021 Mendeleev Communications. Published by ELSEVIER B.V.
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.
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Mendeleev Commun., 2021, 31, 334–336
crystals, respectively (see Scheme 1). The moderate yield of 2a
(53%) can be explained by its high solubility in benzene from
which its crystallization was performed. Complexes 2a,b are
non-pyrophoric, highly air- and moisture-sensitive compounds.
Their solutions in benzene and aliphatic hydrocarbons are stable
at room temperature.
Cꢀ2ꢃꢁ
Cꢀ2ꢄꢁ
Cꢀꢆꢃꢁ
Cꢀꢅꢃꢁ
Nꢀ2ꢁ
Nꢀ1ꢁ
ꢂbꢀ1ꢁ
In the 13C{1H} NMR spectrum of 2a, the signal at 60.9 ppm
corresponds to the carbon atom bonded to Caii. It is significantly
shifted upfield in comparison to those in the related Caii
Cꢀ1ꢆꢁ
Cꢀ12ꢁ
Cꢀ1ꢁ
Cꢀ2ꢁ
Cꢀ22ꢁ
complexes [dipp-NacNac]Ca(CPh2Me)(THF) (88.5 ppm)12(a)
i
and [(TpAd,Pr )Ca{CPh2Me}] (72.01 ppm).12(c) The chemical
shift of the central carbon in the 13C{1H} NMR spectrum of 2b
(71.2 ppm), surprisingly, completely coincides with that for the
bis(benzhydryl) complex [(4-ButC6H4)2CH]2Yb(TMEDA).13(a)
The 2D 171Yb−1H NMR long-range correlation of 2b revealed
that the 171Yb chemical shift extracted from the 2D spectrum
(329 ppm) proved to be noticeably shifted to upfield region,
compared to the values previously published for Ybii alkyl
complexes.8(a),(b),(g),13 It is noteworthy that intensive cross-peaks
are observed for the CH2 protons and the coordinated TMEDA
ligand. Due to the parameters of the experiment used, this should
mean that s-orbitals are significantly involved into the
coordination bond formation, or the mechanism of spin–spin
interaction is other than Fermi-contact in this case. These
interactions can be regarded as b-agostic,16,17 however no cross-
peaks of ytterbium with protons for the CHMe2 groups
corresponding to g- and δ-CH–Yb agostic interactions were
detected.
Figure 1 Molecular structure of complex 2b. Thermal ellipsoids are given
at 30% probability level. Methyl and methylene groups of tert-butyl
substituents and TMEDA, and all hydrogen atoms are omitted for clarity.
bond length in the methylene group involved into the interaction
with Ybii is 0.88(4) Å and the Yb∙∙∙H distance is 2.63(3) Å. Both
ligands in 2b are strongly distorted: the dihedral angles between
the benzene planes are 41.4(2) and 46.9(2)°. The Yb−N bonds
[2.570(2), 2.596(3) Å] are comparable with those in four-
coordinate Ybii complexes [(4-ButC6H4)2CH]2Yb(TMEDA)13(a)
and (Bu4t Carb)Yb[(4-ButC6H4)2CH](TMEDA)13(b) [2.519(2)–
2.617(3) Å].
The single crystal X-ray diffraction data obtained for 2a
provide confidence in the molecular and crystal structure of this
compound (Figure 2), but do not allow one to discuss in details
its geometric parameters.
The molecular structures of complexes 2a,b (Figures 1, 2)
were determined by X-ray diffraction analysis.† X-ray data
revealed that the coordination environment of the Ybii ion in 2b
consists of two isobutyl-substituted benzhydryl ligands and a
TMEDA molecule (Figure 1).
Complexes 2a,b demonstrate high thermal stability: no
decomposition was observed after heating at 70 °C for 48 h
(C6D6, H NMR-control). However, raising the temperature to
1
100 °C leads to decomposition with a half-life of 14 and 12 h,
The benzhydryl ligands in 2b are bound to the metal atom via
central carbon atom; however, their coordination modes are
somewhat different. In contrast to the previously published
Cꢀ2ꢂꢁ
Cꢀ2ꢃꢁ
Cꢀꢂꢂꢁ
Nꢀ2ꢁ
complex
[(4-ButC6H4)2CH]2Yb(TMEDA)13(a)
featuring
Nꢀ1ꢁ
η3-interaction of both benzhydryl ligands with the metal ion, in
compound 2b the increased steric demand of the ligands results
in realization of η3-mode only for one ligand [Yb–C(Bui):
2.638(3) Å; Yb–Cipso: 2.845(3) Å; Yb–Cortho: 2.817(3) Å]. The
second one is η2-coordinated and demonstrates a short contact
only with one ipso aryl carbon atom [Yb–C(Bui): 2.593(3) Å;
Yb–Cipso: 2.884(3) Å]. The Yb–C bond distances in 2b and
[(4-ButC6H4)2CH]2Yb(TMEDA) have similar values. The C–H
Caꢀ1ꢁ
Cꢀ1ꢃꢁ
Cꢀ12ꢁ
Cꢀ2ꢁ
Cꢀ22ꢁ
Cꢀ1ꢁ
Figure 2 Molecular structure of complex 2a. Thermal ellipsoids are given
at 30% probability level. Methyl and methylene groups of tert-butyl
substituents and TMEDA, and all hydrogen atoms are omitted for clarity.
†
Crystal data for 2a. C50H74CaN2 ∙1/4C6H6, M = 762.71, space group
Bruker D8 Quest diffractometer (ω-scans technique, MoKa-radiation,
l = 0.71073 Å, μ = 1.803 mm–1, 2qmax = 61.01°). After merging of
equivalents and absorption correction, 16071 independent reflections
(Rint = 0.0316) were used for the structure solution and refinement. Final
R factors: R1 = 0.0220 [15093 reflections with I > 2s(I)], wR2 = 0.0518
(all reflections), S = 1.047, Flack parameter = 0.012(2), largest diff. peak
and hole are 1.53 and –0.80 e Å–3, respectively.
The intensity data for 2a,b were integrated by APEX318 software
package. SADABS19 was used to absorption corrections. The structures
were solved via intrinsic phasing algorithm and refined by full-matrix
least squares on F2 using SHELX20 and Olex2.21 All non-hydrogen atoms
were found from Fourier syntheses of electron density and were refined
anisotropically. The H atoms were placed in the calculated positions and
refined in the ‘riding’ model with U(H)iso = 1.2Ueq of their parent atoms
[U(H)iso = 1.5Ueq for methyl groups] except for H(3A), H(7A), H(13A),
H(17A), H(28A), H(32A), H(38A) and H(42A) atoms in 2a located from
the difference Fourier synthesis of the electron density. All hydrogen
atoms were refined isotropically.
–
P1, T = 120 K, a = 17.737(6), b = 18.065(5) and c = 31.270(10) Å,
a = 99.577(9)°, b = 101.372(7)°, g = 91.142(8)°, V = 9671(5) Å3, Z = 8,
dcalc = 1.048 g cm–3, F000 = 3348. Colourless needle-shaped single
crystal with dimensions 0.15×0.01×0.01 mm was selected and
intensities of 110551 reflections were measured using Bruker APEXII
DUO CCD diffractometer (ω-scans technique, MoKa-radiation,
l = 0.71073 Å, μ = 0.163 mm–1, 2qmax = 50.06°). After merging of
equivalents and absorption correction, 33746 independent reflections
(Rint = 0.8047) were used for the structure solution and refinement.
Final
R factors: R1 = 0.1741 [6442 reflections with I > 2s(I)],
wR2 = 0.2677 (all reflections), S = 0.905, largest diff. peak and hole are
0.52 and –0.44 e Å–3, respectively. High R-factors result from the best
available crystals being twinned, small and, hence, of an extremely low
reflective power; however, the quality of the obtained dataset was still
enough to unambiguously establish the crystal and molecular structure
of 2a.
Crystal data for 2b. C56H86N2Yb, M = 960.30, space group Fdd2,
T = 100 K, a = 34.9243(15), b = 50.360(2) and c = 12.0265(5) Å,
V = 21152.1(15) Å3, Z = 16, dcalc = 1.206 g cm–3, F000 = 8096. Brown
prism-shaped single crystal with dimensions 0.65 × 0.42 × 0.28 mm
was selected and intensities of 82179 reflections were measured using
CCDC 2055806 (2a) and 2055470 (2b) contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
http://www.ccdc.cam.ac.uk.
– 335 –

Mendeleev Commun., 2021, 31, 334–336
7 (a) A. L. Wayda and W. J. Evans, J. Am. Chem. Soc., 1978, 100, 7119;
respectively. In the contrary to the prevailing opinion about the
tendency of alkyl derivatives of lanthanides to undergo
intramolecular b-elimination reactions,7 no products with
multiple bonds were detected in the reaction mixture. Instead,
the parent hydrocarbons 1a,b formed in quantitative yields.
Highly likely, due to the high basicity of anionsAr2CR– (R = Me,
Bui) the abstraction of a proton from the solvent or TMEDA
takes place. To support this hypothesis, the reactions of complex
2b with a 5-fold molar excess of toluene and thiophene (pKa 43
and 42) which are prone to CH bond activation in benzylic and
a-hetaryl positions were performed. The reactions lead to the
rapid release of alkane, however all our trials to isolate and
characterize the metal-containing products failed due to their
low solubility.
(b) P. L. Watson and D. C. Roe, J. Am. Chem. Soc., 1982, 104, 6471;
(c) M. R. MacDonald, R. R. Langeslay, J. W. Ziller and W. J. Evans,
J. Am. Chem. Soc., 2015, 137, 14716.
8 (a) C. Eaborn, P. B. Hitchcock, K. Izod and J. D. Smith, J. Am. Chem.
Soc., 1994, 116, 12071; (b) P. B. Hitchcock, S. A. Holmes, M. F. Lappert
and S. Tian, J. Chem. Soc., Chem. Commun., 1994, 2691; (c) C. Eaborn,
P. B. Hitchcock, K. Izod, Z. R. Lu and J. D. Smith, Organometallics,
1996, 15, 4783; (d) W. Clegg, C. Eaborn, K. Izod, P. O’Shaughnessy
and J. D. Smith, Angew. Chem., Int. Ed., 1997, 36, 2815; (e) K. Yan,
B. M. Upton, A. Ellern and A. D. Sadow, J. Am. Chem. Soc., 2009, 131,
15110; (f) K.Yan, G. Schoendorff, B. M. Upton, A. Ellern, T. L. Windus
and A. D. Sadow, Organometallics, 2013, 32, 1300; (g) L. Hasinoff,
J. Takats, X. W. Zhang, A. H. Bond and R. D. Rogers, J. Am. Chem.
Soc., 1994, 116, 8833; (h) C. Eaborn, S. A. Hawkes, P. B. Hitchcock and
J. D. Smith, Chem. Commun., 1997, 1961; (i) M. Köhler, A. Koch,
H. Görls and M. Westerhausen, Organometallics, 2016, 35, 242;
(j)A. Koch, Q. Dufrois, M. Wirgenings, H. Görls, S. Krieck, M. Etienne,
G. Pohnert and M. Westerhausen, Chem. – Eur. J., 2018, 24, 16840;
(k) M. R. Crimmin, A. G. M. Barrett, M. S. Hill, D. J. MacDougall,
M. F. Mahon and P. A. Procopiou, Chem. – Eur. J., 2008, 14, 11292.
9 (a) F. Feil, C. Müller and S. Harder, J. Organomet. Chem., 2003, 683,
56; (b) F. Feil and S. Harder, Organometallics, 2000, 19, 5010;
(c) S. Harder, S. Müller and E. Hübner, Organometallics, 2004, 23, 178;
(d) S. Harder, Angew. Chem., Int. Ed., 2004, 43, 2714; (e) B. M. Wolf,
C. Stuhl and R. Anwander, Chem. Commun., 2018, 54, 8826;
(f) A. N. Selikhov, T. V. Mahrova, A. V. Cherkasov, G. K. Fukin,
L. Maron and A. A. Trifonov, Chem. – Eur. J., 2017, 23, 1436;
(g) D. O. Khristolyubov, D. M. Lyubov, A. V. Cherkasov, G. K. Fukin
and A. A. Trifonov, Mendeleev Commun., 2021, 31, 54.
In summary, new hydrocarbyl Ybii and Caii complexes
[(4-ButC6H4)2CR]2M(TMEDA) (M = Ca, R = Me; M = Yb,
R = Bui) have been synthesized and characterized. Complex 2b
is the first example of a hydrocarbyl derivative featuring
b-agostic interactions of Ybii with the protons of the methylene
group Yb–C–CH2. Despite the presence of hydrogen atoms in
the b-positions, the complexes are thermally stable up to 70 °C,
while further raising the temperature leads to decomposition and
the formation of the parent hydrocarbons. Complex 2b when
treated with toluene and thiophene readily undergoes
Yb–C bond protolysis accompanied by the elimination of
(4-ButC6H4)2CHBui.
10 M. M. Katzenmayer, B. M. Wolf, A. Mortis, C. Maichle-Mössmer and
R. Anwander, Chem. Commun., 2021, 57, 243.
11 B. M. Wolf, C. Stuhl, C. Maichle-Mössmer and R. Anwander, J. Am.
Chem. Soc., 2018, 140, 2373.
This work was supported by the Ministry of Science and
Higher Education of the Russian Federation (grant no. MK-
1688.2020.3) and Russian state assignment.
12 (a)
J. Spielmann and S. Harder, Chem. – Eur. J., 2007, 13, 8928;
(b) J. Spielmann, F. Buch and S. Harder, Angew. Chem., Int. Ed., 2008,
47, 9434; (c) X. Shi, C. Hou, L. Zhao, P. Deng and J. Cheng, Chem.
Commun., 2020, 56, 5162.
Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi: 10.1016/j.mencom.2021.05.017.
13 (a) A. N. Selikhov, G. S. Plankin, A. V. Cherkasov, A. S. Shavyrin,
E. Louyriac, L. Maron and A. A. Trifonov, Inorg. Chem., 2019, 58,
5325; (b) A. N. Selikhov, A. S. Shavyrin, A. V. Cherkasov, G. K. Fukin
and A. A. Trifonov, Organometallics, 2019, 38, 4615.
References
14 A. N. Selikhov, E. N. Boronin, A. V. Cherkasov, G. K. Fukin,
A. S. Shavyrin and A. A. Trifonov, Adv. Synth. Catal., 2020, 362, 5432.
15 C. Schade, W. Bauer and P. von Ragué Schleyer, J. Organomet. Chem.,
1985, 295, c25.
16 W. Scherer and G. S. McGrady, Angew. Chem., Int. Ed., 2004, 43, 1782.
17 M. Lein, Coord. Chem. Rev., 2009, 253, 625.
18 APEX3, Bruker AXS Inc., Madison, WI, 2018.
19 L. Krause, R. Herbst-Irmer, G. M. Sheldrick and D. Stalke, J. Appl.
Crystallogr., 2015, 48, 3.
20 G. M. Sheldrick, Acta Crystallogr., 2015, A71, 3.
1 (a) X. Liu, L. Xiang, E. Louyriac, L. Maron, X. Leng andY. Chen, J. Am.
Chem. Soc., 2019, 141, 138; (b) X. Liu, Q. Wen, L. Xiang, X. Leng and
Y. Chen, Chem. – Eur. J., 2020, 26, 5494; (c) Z. Hou, Y. Zhang,
M. Nishiura and Y. Wakatsuki, Organometallics, 2003, 22, 129;
(d) A. Pindwal, A. Ellern and A. D. Sadow, Organometallics, 2016, 35,
1674; (e) A. S. S. Wilson, M. S. Hill, M. F. Mahon, C. Dinoi and
L. Maron, Science, 2017, 358, 1168; (f) D. O. Khristolyubov,
D. M. Lyubov, A. S. Shavyrin, A. V. Cherkasov, G. K. Fukin and
A. A. Trifonov, Inorg. Chem. Front., 2020, 7, 2459.
2 D. O. Khristolyubov, D. M. Lyubov and A. A. Trifonov, Russ. Chem. Rev.,
2021, 90, doi: https://doi.org/10.1070/RCR4992.
21 O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard and
H. Puschmann, J. Appl. Crystallogr., 2009, 42, 339.
3 M. Nishiura, F. Guo and Z. Hou, Acc. Chem. Res., 2015, 48, 2209.
4 A. A. Trifonov and D. M. Lyubov, Coord. Chem. Rev., 2017, 340, 10.
5 (a) W. Huang, P. L. Diaconescu and P. J. Perez, Adv. Organomet. Chem.,
2015, 64, 41; (b) P. L. Arnold, M. W. McMullon, J. Rieb and
F. E. Kühn, Angew. Chem., Int. Ed., 2015, 54, 82.
6 R. D. Shannon, Acta Crystallogr., 1976, A32, 751.
Received: 18th January 2021; Com. 21/6422
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