Bimetallic μ-cyanoimide complexes prepared by NCN group transfer

Article abstract of DOI:10.1039/b006517j

A cyanoimide [NCN] transfer reagent has been developed and applied to the synthesis of the μ-NCN systems (μ²;η¹,η¹-NCN) {M[N(R)Ar]₃}₂ (Ar = C₆H₃Me₂-3,5; M = V or U, R

Full text of DOI:10.1039/b006517j

Bimetallic m-cyanoimide complexes prepared by NCN group transfer†
Daniel J. Mindiola, Yi-Chou Tsai, Ryuichiro Hara, Qinghao Chen, Karsten Meyer and
Christopher C. Cummins*‡
Department of Chemistry, Room 2-227, Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA.
E-mail: ccummins@mit.edu
Received (in Irvine, CA, USA) 8th August 2000, Accepted 9th October 2000
First published as an Advance Article on the web
A cyanoimide [NCN] transfer reagent has been developed
and applied to the synthesis of the m-NCN systems (m2;h1,h1-
NCN) {M[N(R)Ar]3}2 (Ar = C6H3Me2-3,5; M = V or U, R =
But; M = Mo, R = Pri).
visible spectrum of (m-NCN)[1]2 is assigned tentatively to a
fully allowed transition from the molecule’s eu HOMO to its eg
LUMO, assuming idealized D3d symmetry for the complex.
Calculation of the HOMO–LUMO gap for a model system,
using NH2 in place of N(Pri)Ar ligands, gave a value (1.38 eV)
well in accord with the experimentally observed quantity.3,7
Based on the orbital scheme developed for diamagnetic (m-
NCN)[1]2, an analogous divanadium complex was expected to
exist as a ground-state triplet. Such a complex was obtained
upon reaction of V[N(R)Ar]3 28,9 (R = But) with 0.5 equiv. of
NCdbabh (Scheme 1). The purple–black microcrystalline solid
thus obtained analyzed correctly for (m-NCN)[2]2, exhibited the
expected strong nNCN at 2037 cm21 {(m-N13CN)[2]2 had an IR
stretch at 1982 cm21}, and was found to possess a meff of 3.01
mB in solution at 25 °C.3 Purification of (m-NCN)[2]2 was
accomplished as for the molybdenum derivative, the isolated
yield being 91%. The divanadium complex (m-NCN)[2]2 shows
an interesting electrochemistry. Two reversible reduction
processes can be observed at 21.61 and 22.42 V (vs.
[FeCp2]0/+). An oxidation event at 20.68 V was found to be in
a range convenient for chemical oxidation. Oxidation of (m-
NCN)[2]2 thus was accomplished using silver triflate, a regimen
providing paramagnetic {(m-NCN)[2]2}[O3SCF3] in 56% yield.
Oxidized (m-NCN)[2]2 was characterized fully, including its
magnetochemistry and X-band EPR spectrum in solution.3
That complex (m-NCN)[1]2 contains a linear 5-atom MoN-
CNMo chain was confirmed by single-crystal X-ray crystallog-
raphy (Fig. 1(a)),§ the molecular structure in question being
found to incorporate a crystallographic center of inversion at the
cyanoimide carbon along with a Mo–N–C angle of 176.6(4)°
(two crystallographically independent molecules were confined
Small molecules or ions comprised of combinations of the
elements C, N and O hold inherent fascination due to their
simplicity and their importance with respect to the global cycles
that involve them. Studies in which reactive metal complexes
are employed for small molecule activation can shed light on
manifolds of processes that permit such substrates to be
manipulated. Recent reports involving dinitrogen, nitrous
oxide, carbon monoxide, and cyanate ion serve to exemplify
this approach. The present work is concerned with cyanoimide
ion, [NCN]22, the attributes of which as a ligand in coordination
chemistry currently are not well understood. To this end has
been developed a new reagent for the synthesis of cyanoimide
complexes,
a
reagent
based
on
the
bicyclic
amine
2,3+5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene
(Hdbabh). For references relating to this work see ESI.†
As reported by Carpino et al., the synthesis of Hbdabh
proceeds via an N-bromo derivative, Brdbabh or alternatively
from the hydrobromide salt BrH2dbabh.1,2 Treatment of
Brdbabh with trimethylsilyl cyanide in DMF has been found to
provide the desired N-cyano derivative NCdbabh in essentially
quantitative yield as colorless leaflets subsequent to re-
crystallization from diethyl ether.3 Cyano-dbabh thus prepared
constitutes a reagent for transfer of the neutral [NCN] fragment
to reducing metal complexes under mild conditions, the
reactions being two-electron redox events that occur with
concomitant release of anthracene.4 Methodology based on the
ability of the dbabh skeleton to effect group transfer coupled
with anthracene formation has been documented previously in
the case of [N]2 transfer to form chromium(VI) nitrido
complexes.5
Several metal complexes have been found to effect smooth
[NCN] abstraction from cyano-dbabh. Treatment of a molybde-
num metallaziridine–hydride complex known to serve as a
source of the relatively unhindered three-coordinate molybde-
num(III) fragment Mo[N(Pri)Ar]3 1 (Ar = C6H3Me2-3,5),6 with
0.5 equiv. of NCdbabh in diethyl ether at 25 °C led to rapid and
essentially quantitative formation of the diamagnetic dinuclear
m-cyanamide derivative (m-NCN)[1]2. Anthracene removal was
effected by filtration of the reaction mixture through activated
charcoal, and orange crystals of (m-NCN)[1]2 were obtained in
63% yield by storing a concentrated Et2O solution at 235 °C
(Scheme 1). Prepared analogously using the 13C-labeled reagent
N13Cdbabh3 was the isotopomer (m-N13CN)[1]2, an isotopomer
of interest due to its 13C NMR quintet (1JCN 24 Hz) at 176 ppm
and its strong IR nNCN stretch appearing at 2061 cm21, this
value being red-shifted by 61 cm21 relative to that for its
unlabeled counterpart.3 A low energy band at 475 nm in the
† Electronic supplementary information (ESI) available: introductory
section with associated references as well as synthetic, spectroscopic,
crystallographic and DFT calculation details for all complexes. See
http://www.rsc.org/suppdata/cc/b0/b006517j/
Scheme 1 Reaction scheme for 0.5 equiv. of NCdbabh with the metal
complexes M[N(R)Ar]3 to form the m-cyanoimide complexes of the type (m-
NCN)[M{N(R)Ar}3]2 (M = Mo, R = Pri; M = V or U, R = But).
Intermediate complexes have not been observed.
‡ Alfred P. Sloan Fellow, 1997–2000.
DOI: 10.1039/b006517j
Chem. Commun., 2001, 125–126
This journal is © The Royal Society of Chemistry 2001
125

represents a versatile platform for group transfer reactions, the
present work makes available for study a new class of
cyanoimide-bridged dimetal complexes. Mononuclear com-
plexes featuring a terminal cyanoimide ligand remain attractive
as synthetic targets.15,16
For financial support are gratefully acknowledged the
National Science Foundation (CAREER Award CHE-
9501992), the Alfred P. Sloan Foundation, the National Science
Board (1998 Alan T. Waterman award to C. C. C.), and the
Packard Foundation. K. M. thanks the DFG for a postdoctoral
stipend. We are grateful to Dr Jesper Bendix for helpful
comments and providing the EPR simulation program.
Notes and references
§ Crystallographic data: for (m-NCN)[1]2: C100.50H144Mo3N12, triclinic,
¯
space group P1, M = 1808.10, a = 15.219(2), b = 18.043(3), c =
20.510(3) Å, a
4972.1(12) Å3, Z = 2, T = 183(2) K, m(Mo-Ka) = 0.422 mm21, 15823
reflections measured, q range 2.41–20.75°, 10252 unique reflections, R1
0.0764, wR2 = 0.1894, GOF = 1.068, residuals based on I > 2s(I). The
residual peak and hole electron density was 1.478 and 21.390 e Å23. Two
crystallographically independent molecules were present in the asymmetric
unit with one of the chemically equivalent molecules possessing an
inversion center about the cyanoimide carbon.
For (m-NCN)[3]2: C73H108U2N8, monoclinic, space group P21/n, M =
1573.73, a = 10.9163(8), b = 19.6175(14), c = 17.4079(12) Å, b =
96.8390(10)°, U = 3701.4(5) Å3, Z = 2, T = 183(2) K, m(Mo-Ka) = 4.413
mm21, 12242 reflections measured, q range 2.10–21.25°, 4091 unique
reflections, R (based on F) = 0.0418, wR (based on F2) = 0.0770, GOF =
1.159, residuals based on I > 2s(I).
Single red-orange crystals of (m-NCN)[1]2 and yellow blocks of (m-
NCN)[3]2 were grown from a Et2O at 235 °C, mounted in inert oil
(paratone N oil from Exxon) and transferred to a cold stream of the
diffractometer. The structures were solved using direct methods and refined
by full-matrix least squares on F2. CCDC 182/1809.
= 63.998(2), b = 81.112(2), g = 80.611(2)°, U =
Fig. 1 (a) Structural diagram of (m-NCN)[1]2 with thermal ellipsoids at the
35% probability level. Selected distances (Å) and angles (°): Mo(3)–N(7)
1.947(6), Mo(3)–N(8) 1.994(6), Mo(3)–N(9) 1.960(7), Mo(3)–N(12)
1.852(7), N(12)–C(2) 1.233(7); Mo(3)–N(12)–N(2) 176.6(4). (b) Structural
diagram of (m-NCN)[3]2 with thermal ellipsoids at the 35% probability
level. Selected distances (Å) and angles (°): U–N(4) 2.226(7), U–N(1)
2.219(6), U–N(2) 2.225(6), U–N(3) 2.291(6), N(4)–C 1.189(8), U–C(21)
2.787(8); C–N(4)–U 162.6(5).
=
in the asymmetric unit, one of which possessed an inversion
center about the cyanoimide carbon). As for the m-nitrido
dimolybdenum complex (m-N)[1]2 studied previously,6 the six
isopropyl substituents are directed to the molecule’s interior, the
six aryl residues consequently occupying polar positions. Given
the properties of vanadium-containing (m-NCN)[2]2, its struc-
ture presumably likewise incorporates a linear 5-atom chain
with metal termini.3
In contrast is the structure of the diuranium derivative (m-
NCN)[3]2§ derived from the reaction of (THF)U[N(R)Ar]3
3-THF10,11 with 0.5 equiv. of NCdbabh (Scheme 1). X-Ray
crystallography revealed in this case a bent geometry at the
cyanoimide nitrogens [U–N–C 162.6(5)°], the molecule ex-
hibiting again, however, inversion symmetry at the cyanoimide
carbon atom (Fig. 1(b)).12 The bent geometry adopted by the
cyanoimide nitrogens in (m-NCN)[3]2, taken together with the
observation of a congruence in the U–Namide and U–Ncyanoimide
distances, may be indicative of relatively little p bonding in the
uranium–cyanoimide interaction. In contrast, the transition-
metal derivative (m-NCN)[1]2 exhibits a Mo–Ncyanoimide dis-
tance shorter by 0.115(6) Å than the Mo–Namide distance in the
same complex. Recently it has been suggested that the high
nodality of its valence 5 f orbitals renders mid-valent uranium
less effective at p bonding than its transition-metal counter-
parts.13,14 Using the 13C-labeled reagent N13Cdbabh, it was
found that the isotopomer (m-N13CN)[3]2 evinces a broad 13C
NMR resonance at 133 ppm.
1 L. A. Carpino, R. E. Padykula, D. E. Barr, F. H. Hall, J. G. Krause, R. F.
Dufresne and C. J. Thoman, J. Org. Chem., 1988, 53, 2565.
2 D. E. Barr, Ph.D. Thesis, University of Amherst, MA, 1965.
3 Experimental details are included as ESI†
4 For an extensive review on arene extrusion reactions, see: H. N. C.
Wong, T. K. Hg and T. Y. Wong, Heterocycles, 1983, 20, 1815.
5 D. J. Mindiola and C. C. Cummins, Angew. Chem., Int. Ed., 1998, 37,
945.
6 Y.-C. Tsai, M. J. A. Johnson, D. J. Mindiola, C. C. Cummins, W. T.
Klooster and T. F. Koetzle, J. Am. Chem. Soc., 1999, 121, 10426.
7 The HOMO and LUMO in question are p-symmetry orbitals extending
along the linear 5-atom MoNCNMo chain. The HOMO–LUMO gap
was calculated with the aid of DFT to be 1.38 eV, while the
experimentally observed value was 1.66 eV at 475 nm. The Eexcited
ground energy gap was calculated to be 1.42 eV.
–
E
8 M. G. Fickes, W. M. Davis and C. C. Cummins, J. Am. Chem. Soc.,
1995, 117, 6384.
9 M. G. Fickes, Ph.D. Thesis, Massachusetts Institute of Technology,
MA, 1998.
10 A. L. Odom, P. L. Arnold and C. C. Cummins, J. Am. Chem. Soc., 1998,
120, 5836.
11 A. L. Odom, Ph.D. Thesis, Massachusetts Institute of Technology, MA,
1997.
12 Experimentals and tables for bond lengths, angles, atomic coordinates
and anisotropic displacement parameters are included as ESI†
13 R. B. King, Inorg. Chem., 1992, 31, 1978.
14 P. L. Diaconescu, P. L. Arnold, T. A. Baker, D. J. Mindiola and C. C.
Cummins, J. Am. Chem. Soc., 2000, 122, in press.
15 A. J. L. Pombeiro, New. J. Chem., 1991, 45, 444.
16 Attempts to prepare such complexes, e.g. by reaction of an equimolar
amount of NCdbabh with an appropriate metal complex, gave rise only
to the dinculear complexes (m-NCN)[1]2, (m-NCN)[2]2 and (m-
NCN)[3]2, along with unreacted NCdbabh.
It is believed that the ultimate step in formation of these m-
cyanoimide systems is the combination of a putative terminal
cyanoimide complex with unreacted complex. Formation of the
bridged-cyanoimide complexes most likely occurs via mono-
nuclear reduction of NCdbabh, a process evidently being slow
relative to consumption of unreacted metal complex. This work
establishes NCdbabh as an efficient source of the [NCN]
moiety, its implementation having led to smooth assembly of
dinuclear cyanoimide-bridged complexes of vanadium, mo-
lybdenum and uranium. In addition to showing that dbabh
126
Chem. Commun., 2001, 125–126