Solution, Solid-State, and Computational Analysis of Agostic Interactions in a Coherent Set of Low-Coordinate Rhodium(III) and Iridium(III) Complexes

Article abstract of DOI:10.1002/chem.201705990

A homologous family of low-coordinate complexes of the formulation trans-[M(2,2′-biphenyl)(PR₃)₂][BAr^F₄] (M=Rh, Ir; R=Ph, Cy, iPr, iBu) has been prepared and extensively structurally characterised. Enabled through a comprehensive set of solution phase (VT ¹H and ³¹P NMR spectroscopy) and solid-state (single crystal X-ray diffraction) data, and analysis in silico (DFT-based NBO and QTAIM analysis), the structural features of the constituent agostic interactions have been systematically interrogated. The combined data substantiates the adoption of stronger agostic interactions for the Ir^III compared to Rh^III complexes and, with respect to the phosphine ligands, in the order PiBu₃>PCy₃>PiPr₃>PPh₃. In addition to these structure–property relationships, the effect of crystal packing on the agostic interactions was investigated in the tricyclohexylphosphine complexes. Compression of the associated cations, through inclusion of a more bulky solvent molecule (1,2-difluorobenzene vs. CH₂Cl₂) in the lattice or collection of data at very low temperature (25 vs. 150 K), lead to small but statistically significant shortening of the M?H?C distances.

Full text of DOI:10.1002/chem.201705990

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Accepted Article
Title: Solution, solid-state, and computational analysis of agostic
interactions in a coherent set of low-coordinate rhodium(III) and
iridium(III) complexes
Authors: Richard C Knighton, Jack Emerson-King, Jonathan P Rourke,
C André Ohlin, and Adrian B Chaplin
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To be cited as: Chem. Eur. J. 10.1002/chem.201705990
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10.1002/chem.201705990
Chemistry - A European Journal
Solution, solid-state, and computational analysis of agostic interactions in a
coherent set of low-coordinate rhodium(III) and iridium(III) complexes
Richard C. Knighton,a Jack Emerson-King,a Jonathan P. Rourke,a C. André Ohlinb and Adrian B. Chaplina,*
aDepartment of Chemistry, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK.
E-mail: a.b.chaplin@warwick.ac.uk
bDepartment of Chemistry, Umeå University, Linneausvag 6, 907 34 Umeå, Sweden.
TOC graphic
+
R2P
R
M
R
R2P
M = Rh, Ir
R = Ph, Cy, iPr, iBu
XRD
NMR
DFT (NBO, QTAIM)
Abstract
A homologous family of low-coordinate complexes of the formulation trans-[M(2,2’-biphenyl)(PR3)2][BArF ]
4
(M = Rh, Ir; R = Ph, Cy, iPr, iBu) has been prepared and extensively structurally characterised. Enabled
through a comprehensive set of solution (VT 1H and 31P NMR spectroscopy) and solid-state (single crystal X-
ray diffraction) data, and analysis in silico (DFT-based NBO and QTAIM analysis), the structural features of
the constituent agostic interactions have been systematically interrogated. The combined data
substantiates the adoption of stronger agostic interactions for the Ir(III) compared to Rh(III) complexes and,
with respect to the phosphine ligands, in the order PiBu3 > PCy3 > PiPr3 > PPh3. In addition to these
structure-property relationships, the effect of crystal packing on the agostic interactions was investigated in
the tricyclohexylphosphine complexes. Compression of the associated cations, through inclusion of a more
bulky solvent molecule (1,2-difluorobenzene vs CH2Cl2) in the lattice or collection of data at low
temperature (25 K vs 150 K), lead to small but statistically significant shortening of the M–H–C distances.
1
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Introduction
The coordination chemistry of C–H bonds is an important facet of contemporary organometallic
chemistry.1,2,3 Adoption of 3-centre-2-electron M–H–C bonds can help stabilise otherwise reactive low-
coordinate metal complexes that are implicated in many catalytic reactions, and from a fundamental
perspective represent an opportunity to gain insight into transition-metal-mediated C–H bond activation
reactions.4 As a consequence of the weakly interacting nature of C–H bonds, well-defined examples are
almost exclusively limited to intramolecular systems that are promoted through the chelate effect. As first
articulated by Brookhart and Green, the consistent interactions are termed “agostic” and typified by M–H–
C contacts of < ca 3 Å.1,5 The characterisation of alkane complexes is significantly more experimentally
demanding, but has been achieved in solution using time-resolved spectroscopic methods under low
temperature regimes,3,6 and recently in the solid-state by X-ray crystallography through application of
single-crystal to single-crystal transformations.7
Given that the development of C–H bond activation chemistry has been closely connected with the
organometallic chemistry of rhodium and iridium,8 it is perhaps unsurprising that a large number of well-
defined complexes of these group 9 metals featuring agostic interactions have been reported.9,10,11,12,13,14,15
Indeed amongst known examples a number of families can be identified, with M(III) complexes of the
formulation trans,cis-[ML2H2]+ (A: M = Rh, Ir; L = phosphine or NHC),9 [M(Binor-S)L]+ (B: Binor-S =
1,2,4,5,6,8-dimetheno-S-indacene; L = phosphine),10 and bearing cyclometalated ItBu (C, D) the most
outstanding (Chart 1).11 Structurally related clusters of this nature are of interest to gauge an understanding
of the effect of the metal alongside subtle variations of the ligand composition on the constituent agostic
interactions. Unfortunately, as they currently stand, neither the size nor specific membership of these three
families is well suited to an analysis of this nature.
Chart 1. Structurally related sets of low-coordinate Rh(III) and Ir(III) complexes featuring agostic
interactions (rmin = closest M–H–C contact) characterised in the solid-state by X-ray diffraction.
+
M = Rh; L = PiBu3, PtBuiBu2
rmin / Å = 2.81, 2.82
M = Rh; X = Cl, [PF6]
N
N
L
M
L
CH2
H
rmin / Å = 3.16, 3.27
H
X
CH2
H
A
C
M
H
CH2
H
M = Ir; L = PtBu2Ph, IPr, ItBu
rmin / Å = 2.81, 2.94, 2.65
M = Ir; X = Cl, [PF6]
N
N
tBu
rmin / Å = 2.79, 3.35
+
M = Rh
M = Rh; L = PiPr3, PCy3, PCy2Ph, PCyp3, PtBu2Me
rmin / Å = 2.90, 2.96, 3.34, 3.32, 3.31
N
N
tBu
rmin / Å = 2.70
Cl
H
CH2
H
B
D
M
M
L
H
CHR
M = Ir; L = PiPr3
rmin / Å = 2.95
M = Ir
N
N
tBu
rmin / Å = 2.70
Whilst it is conceivably possible to extend the membership of the aforementioned sets, the synthetic
chemistry underlying the isolation of these highly reactive organometallics presents a number of practical
2
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10.1002/chem.201705990
Chemistry - A European Journal
challenges. Recognising a degree of commonality amongst A – C and others,12 viz. sawhorse metal
geometries with high trans-influence ligands in the cis-equatorial positions, and building upon the previous
report of low-coordinate Rh(III) complex trans-[Rh(2,2’-biphenyl)(PiPr3)2][BArF ] (1c; ArF = 3,5-(CF3)2C6H3),13
4
we reasoned that utilising 2,2’-biphenyl as an ancillary ligand would be a straightforward means to gather a
set of solution and solid-state data for agostic interactions between phosphine ligand substituents and
Rh(III) and Ir(III) centres. To this end, and with a view to elucidating structure-property relationships within
such data, we report the synthesis and extensive characterisation of low-coordinate complexes of the
formulation trans-[M(2,2’-biphenyl)(PR3)2][BArF ] (M = Rh, 1; Ir, 2; R = Ph, a; Cy, b; iPr, c; iBu, d; Chart 2).
4
This series of complexes encompasses both aryl and alkyl phosphine ligands (i.e. Ph vs Cy), cyclic and acyclic
alkyl phosphine substituents (i.e. Cy vs iPr) and the possibility to adopt both γ– and δ–agostic interactions
(i.e. iPr vs iBu). DFT calculations have also been carried out to aid structural interrogation.
Chart 2. Low-coordinate rhodium(III) and iridium(III) complexes studied.
[BArF
]
R
Rh Ir
4
R2P
M
Ph 1a 2a
R
Cy 1b 2b
iPr 1c 2c
iBu 1d 2d
R
R2P
Results and discussion
1. Synthesis
The preparation of 1c has previously been achieved via oxidative addition of biphenylene to the latent low
coordinate complex [Rh(C6H5F)(PiPr3)2][BArF ].13 Guided by methodology developed by Jones and Crabtree
4
for the preparation of trans-[M(2,2’-biphenyl)(PPh3)2Cl] (M = Rh, 3a; Ir, 4a),16,17 we instead chose to employ
more systematic and synthetically robust protocols that proceed via facile substitution reactions of M(III)
precursors [Rh(2,2’-biphenyl)(dtbpm)Cl] (5; dtbpm = bis(di-tert-butylphosphino)methane) and [Ir(2,2’-
biphenyl)(COD)Cl]2 (6; COD = 1,5-cyclooctadiene) with the desired phosphine, followed by chloride
abstraction to afford low coordinate derivatives 1 and 2, respectively (Scheme 1). In this way, five-
coordinate intermediates 3 and 4 were readily obtained (37% – 83% isolated yield) and subsequently
treated with Na[BArF ] in CH2Cl2 at RT (3, 4b – d) or 50 °C (4a) to afford target complexes 1 and 2 that,
4
following filtration to remove insoluble sodium salts, were isolated by slow crystallisation from
CH2Cl2/pentane (liquid–liquid diffusion at RT; 32 – 80% isolated yields) and extensively characterised (vide
infra). Notably, samples of 1 and 2 obtained in this way were all suitable for interrogation in the solid-state
using X-ray diffraction.
3
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